Histidine-Rich Calcium-Binding Protein (HRC): A Comprehensive Research Report

1. Introduction and Overview

Histidine-rich calcium-binding protein (HRC, also known as histidine-rich Ca2+-binding protein or HCP) is a 165-170 kDa luminal sarcoplasmic reticulum (SR) protein that functions as a critical regulator of intracellular calcium homeostasis in striated muscle. First characterized by Hofmann and colleagues in 1989 at the University of Texas Southwestern Medical Center, HRC was identified based on its ability to bind calcium ions with high capacity but low affinity, a property that distinguishes it from other calcium-binding proteins in the SR [hofmann-1989-cloning-abstract]. The human HRC gene (UniProt P23327) is located on chromosome 19q13.3 and encodes a protein that is predominantly expressed in cardiac and skeletal muscle, with additional expression in arterial smooth muscle cells.

The primary function of HRC is to regulate sarcoplasmic reticulum calcium cycling through dual interactions with two major SR protein complexes: the calcium uptake machinery (SERCA2a pump) and the calcium release apparatus (ryanodine receptor/triadin complex). This dual role positions HRC as a molecular integrator of excitation-contraction coupling in the heart, where precise calcium handling is essential for normal rhythmic contraction [arvanitis-2011-review-abstract]. A recent comprehensive review by Mackrill in 2024 further characterized HRC as "a molecular integrator of cardiac excitation-contraction coupling," highlighting its central position in cardiac calcium homeostasis [mackrill-2024-jeb-abstract]. Over the past three decades, research has established HRC as not only a calcium storage protein but also an active participant in the regulation of SR calcium flux, with significant implications for cardiac physiology and pathophysiology.

HRC belongs to a family of acidic calcium-binding proteins that also includes aspolin, an aspartic acid-rich paralogue found primarily in fish. Interestingly, while ray-finned fish genomes contain both aspolin and HRC genes, other vertebrates including mammals possess only the HRC gene, suggesting gene loss during vertebrate evolution [mackrill-2024-jeb-abstract]. Aspolin functions as a trimethylamine N-oxide (TMAO) demethylase, and whether HRC retains any enzymatic activity toward TMAO remains an intriguing open question for cardiac research.

2. Structural Organization and Biochemical Properties

The molecular architecture of HRC reveals a highly specialized protein designed for calcium binding and protein-protein interactions. The mature human HRC protein, following cleavage of a 27-residue N-terminal signal sequence, contains several distinct structural domains that confer its functional properties [hofmann-1989-cloning-abstract].

The most striking structural feature of HRC is its central region containing ten histidine-rich acidic tandem repeats located between amino acids 106-365. These repeats are organized into two types: Type A motifs (repeated four times) and Type B motifs (repeated six times). Each repeat consists of a histidine-rich sequence (HRHRGH), a stretch of 10-11 acidic amino acids (primarily glutamic and aspartic acid), and a sequence containing serines and threonines [arvanitis-2011-review-abstract]. The unusual amino acid composition—approximately 13% histidine, 12% aspartic acid, and 19% glutamic acid—creates an extensively acidic protein surface that is responsible for calcium binding.

HRC binds calcium with high capacity (approximately 200 nmoles Ca2+/mg protein) but low affinity (Kd = 1.9 mM), as characterized by Picello and colleagues [arvanitis-2011-review-abstract]. This low-affinity, high-capacity binding profile distinguishes HRC from high-affinity calcium sensors like calmodulin and instead places it in a functional category shared with calsequestrin, the major calcium storage protein of the SR. The calcium binding occurs through electrostatic interactions with pairs or triplets of acidic amino acids forming calcium-binding surfaces. These surfaces within the ten histidine-rich acidic tandem repeats may be closely packed in the presence of calcium, suggesting a calcium-dependent conformational change.

The C-terminal region of HRC contains a cluster of 14 closely spaced cysteine residues that forms a cysteine-rich domain critical for protein-protein interactions [hofmann-1989-cloning-abstract]. This domain appears to be under strong negative selection pressure during mammalian evolution, indicating functional constraint, while the remainder of the protein shows evidence of positive selection or relaxed negative selection. Additionally, HRC contains a 13-residue stretch of polyglutamic acid between the histidine-rich repeats and the cysteine-rich domain.

HRC also binds zinc ions at sites distinct from calcium-binding sites, a property not shared by calsequestrin, suggesting additional metal-binding functions that remain to be fully characterized [arvanitis-2011-review-abstract].

Kim and colleagues provided direct evidence for HRC's role as a calcium storage protein by demonstrating that acute HRC overexpression in rat neonatal cardiomyocytes significantly increased both caffeine-induced and depolarization-induced calcium release from the SR [kim-2003-storage-abstract]. Importantly, the elevated calcium content persisted even during SERCA inhibition with cyclopiazonic acid, while ryanodine receptor expression remained unchanged. This demonstrates that HRC directly contributes to the releasable calcium pool within the SR storage compartment.

3. Cellular Localization and Tissue Distribution

HRC resides exclusively within the lumen of the sarcoplasmic reticulum, where it exists as a multimer associated with the junctional SR membrane through calcium bridges. This localization places HRC at the critical junction where calcium is both released (via ryanodine receptors) and resequestered (via SERCA pumps), enabling its dual regulatory function [hofmann-1989-cloning-abstract].

Tissue distribution studies reveal that HRC expression is highly enriched in striated muscle, with the highest levels found in cardiac tissue (left ventricle shows 24.9-fold overexpression relative to average tissue expression) and skeletal muscle (10.9-fold enrichment) [arvanitis-2011-review-abstract]. Cardiac atrial tissue also shows significant HRC expression (12.0-fold enrichment). Beyond striated muscle, HRC is abundant in arterial smooth muscle cells lining small arteries and arterioles, where it localizes to calciosomes, the smooth muscle equivalent of the SR.

The tissue-specific expression of HRC is controlled by the myocyte enhancer factor 2 (MEF2) family of transcription factors. Anderson and colleagues identified a transcriptional enhancer within the HRC gene containing a highly conserved MEF2-binding site that is necessary and sufficient for directing expression during embryonic development in cardiac, skeletal, and vascular smooth muscle cells [anderson-2004-mef2-abstract]. This MEF2-dependent regulation is notable because it occurs independently of CArG motifs (serum response factor binding sites), making HRC the first identified MEF2-dependent, CArG-independent transcriptional target in smooth muscle.

The human HRC gene was cloned and characterized by Hofmann and colleagues, who mapped it to chromosome 19q13.3, near the myotonic dystrophy locus, while the mouse ortholog resides on chromosome 7 [hofmann-1991-genomic-abstract]. The gene contains six exons, with approximately 90% of the coding region located in the first exon. Variable acidic amino acid stretches in exon 1 establish useful polymorphisms that have been employed in linkage studies.

HRC expression is developmentally regulated and plays a role in myocyte differentiation. The protein is not detectable in undifferentiated, proliferating myoblast cultures, but accumulates rapidly when differentiation is induced [arvanitis-2011-review-abstract]. Interestingly, SOX15, a transcription factor that antagonizes muscle differentiation, acts as a suppressor of the HRC locus. SOX15 ablation in mouse embryonic stem cells results in selective overexpression of HRC mRNA, and SOX15-null mice exhibit delayed skeletal muscle regeneration after injury. The significance of HRC repression by SOX15 in the early stages of myocyte differentiation, especially during myofiber trauma repair, remains an active area of investigation.

4. Molecular Interactions and Functional Mechanisms

4.1 HRC-SERCA2a Interaction

A crucial discovery in understanding HRC function came from the demonstration that HRC directly interacts with SERCA2a (sarco/endoplasmic reticulum Ca2+-ATPase 2a), the primary calcium pump responsible for SR calcium reuptake in cardiac muscle [arvanitis-2007-serca-abstract]. This interaction is calcium-dependent, with maximal binding occurring at low calcium concentrations and diminishing as calcium levels rise.

The protein domains mediating this interaction have been mapped to the amino-terminal fragment of SERCA2 that projects into the SR lumen (amino acid residues 74-90) and the second histidine- and glutamic acid-rich domain of HRC (amino acids 320-460) [arvanitis-2011-review-abstract]. Functionally, HRC binding to SERCA2 at low intraluminal calcium concentrations inhibits pump activity, thereby reducing calcium uptake. This inhibitory effect has been demonstrated in transgenic mouse models where HRC overexpression resulted in 35% reduction in SR calcium uptake rates [gregory-2006-cardiac-function-abstract].

4.2 HRC-Triadin Interaction

The second major protein interaction of HRC involves triadin, an integral membrane protein of the SR that forms part of the calcium release complex along with the ryanodine receptor (RyR2), junctin, and calsequestrin [lee-2001-triadin-abstract]. Lee and colleagues demonstrated using co-immunoprecipitation experiments that HRC binds directly to triadin, identifying the histidine-rich acidic repeats of HRC as responsible for this binding.

The HRC-binding domain of triadin was localized to the luminal region containing the KEKE motif, the same region involved in triadin's interaction with calsequestrin. This finding is significant because it suggests that HRC and calsequestrin may compete for binding to triadin, providing a mechanism for reciprocal regulation of calcium release.

Critically, the HRC-triadin interaction is calcium-sensitive but with opposite characteristics to the HRC-SERCA interaction: HRC-triadin binding increases with rising calcium concentrations [arvanitis-2011-review-abstract]. This calcium-dependent switching of interaction partners positions HRC as an intra-SR calcium sensor that responds dynamically to changes in luminal calcium during the cardiac cycle.

4.3 HRC in Excitation-Contraction Coupling

During mammalian cardiomyocyte excitation-contraction coupling, calcium influx through voltage-gated L-type calcium channels (dihydropyridine receptors) triggers calcium release from the SR through ryanodine receptor channels, a process termed calcium-induced calcium release (CICR) [mackrill-2024-jeb-abstract]. This released calcium then binds to troponin C, enabling actin-myosin interaction and muscle contraction. For relaxation to occur, calcium must be resequestered into the SR primarily via SERCA2a, with the sodium-calcium exchanger (NCX) providing an additional extrusion pathway.

HRC plays a critical role in this cycle by acting as an intra-SR calcium sensor. Its ability to switch interaction partners based on luminal calcium concentration allows it to fine-tune both the uptake and release phases of the calcium cycle. By inhibiting SERCA at low SR calcium (immediately after release) and modulating the RyR2 complex through triadin at high SR calcium (during the filled state), HRC helps maintain the appropriate timing and magnitude of calcium transients essential for rhythmic cardiac contraction.

4.4 Functional Model of HRC Action

Based on these interactions, a comprehensive model of HRC function has emerged [arvanitis-2011-review-abstract]. At low SR calcium load (during diastole after calcium release), HRC preferentially interacts with SERCA2 and inhibits its activity, potentially preventing excessive calcium uptake. As SR calcium rises following SERCA-mediated reuptake, HRC dissociates from SERCA2 and exhibits enhanced binding to triadin, where it modulates SR calcium release through the ryanodine receptor complex.

Studies using CASQ2/HRC double knockout mice provided functional evidence supporting opposing roles of HRC and calsequestrin in ryanodine receptor regulation [liu-2015-casq2-dko-abstract]. The investigators proposed that whereas calsequestrin (CASQ2) inhibits and stabilizes RyR2 during diastole, HRC stimulates (primes for activation) RyR2 through triadin binding, facilitating RyR2 recovery from refractoriness. These proteins appear to compete for overlapping binding sites on triadin in a calcium-dependent manner during each calcium release-uptake cycle.

5. Physiological Roles and In Vivo Studies

5.1 HRC Overexpression Studies

Transgenic mouse models with cardiac-specific HRC overexpression have been instrumental in defining HRC's physiological roles. Gregory and colleagues generated mice with approximately 3-fold elevation of cardiac HRC and observed several key phenotypes [gregory-2006-cardiac-function-abstract]:

The overexpressing hearts showed impaired SR calcium uptake (reduced by 35%) and attenuated calcium transient decay (by nearly 40%), consistent with HRC's inhibitory effect on SERCA2. As compensatory mechanisms, the sodium-calcium exchanger (NCX) and triadin protein levels were markedly increased. Importantly, the impaired calcium handling triggered progressive cardiac remodeling and hypertrophy, with animals developing congestive heart failure by 18 months of age. These findings demonstrated that while HRC normally functions to fine-tune calcium handling, excessive HRC activity leads to cardiac dysfunction.

5.2 HRC Knockout Studies

Complementary information came from HRC knockout mouse studies. Park and colleagues reported that under baseline conditions, HRC-deficient mice were morphologically normal and actually displayed enhanced contractility, calcium transients, and maximal SR calcium uptake rates [park-2013-hrc-knockout-abstract]. However, when challenged with stress conditions (isoproterenol stimulation or high-frequency pacing), HRC-knockout cardiomyocytes were five-fold more prone to after-contractions.

Most dramatically, under pressure-overload stress (transverse aortic constriction), HRC-knockout mice developed severe cardiac hypertrophy, fibrosis, pulmonary edema, and premature death compared to wild-type controls. These findings indicate that while HRC may normally constrain calcium cycling, it becomes essential for maintaining cardiac integrity under pathological stress conditions.

5.3 HRC and Calsequestrin Double Knockout

The double knockout of both HRC and calsequestrin (CASQ2) provided remarkable insights into their functional interplay [liu-2015-casq2-dko-abstract]. Surprisingly, double knockout mice showed relatively mild phenotypes with preserved cardiac function, in stark contrast to the severe arrhythmia burden of CASQ2-null mice alone. The combined deletion ameliorated ventricular arrhythmia susceptibility and reduced arrhythmogenic calcium waves. At the cellular level, double knockout myocytes showed decreased frequency of spontaneous calcium sparks in the presence of isoproterenol.

These findings suggest that removing both proteins effectively cancels out their opposing effects on RyR2 regulation. However, the double knockouts exhibited impaired force-frequency relationships at elevated pacing rates, indicating that both proteins are necessary for robust contractile performance during physiological stress.

6. Clinical Significance and Disease Associations

6.1 The Ser96Ala Polymorphism and Ventricular Arrhythmias

The most significant clinical association of HRC involves the Ser96Ala genetic variant (rs3745297) and its link to life-threatening ventricular arrhythmias in patients with dilated cardiomyopathy [arvanitis-2008-ser96ala-abstract]. Arvanitis and colleagues screened 123 idiopathic dilated cardiomyopathy patients and identified that while the Ser96Ala polymorphism showed similar frequency between patients and controls (indicating it does not cause cardiomyopathy), it exhibited striking association with arrhythmic outcomes.

During follow-up of 4.02 years, homozygous Ala/Ala patients demonstrated dramatically elevated arrhythmia risk compared to Ser/Ser patients (hazard ratio 9.620; P = 0.003). The Ser96Ala variant emerged as the only significant genetic predictor of arrhythmogenesis on multivariable analysis (hazard ratio 4.191; P = 0.018), independent of age, sex, left ventricular ejection fraction, atrial fibrillation, or medication.

Mechanistically, bioinformatic analysis revealed that Ser96 represents a phosphorylation site for casein kinase II [arvanitis-2011-review-abstract]. The serine-to-alanine substitution eliminates this phosphorylation capability, potentially disrupting HRC's regulatory effects on both SERCA2 and ryanodine receptor function. Subsequent studies demonstrated that acute overexpression of the HRC Ser96Ala variant significantly increased binding of HRC to SERCA2, enhanced inhibition of SERCA2 activity, reduced maximal SR calcium uptake rate, and increased the frequency of spontaneous calcium sparks indicative of enhanced RyR2 activity [arvanitis-2018-rhythmicity-abstract].

Approximately 60% of the general population carries at least one Ser96Ala copy, yet only those with underlying dilated cardiomyopathy demonstrate arrhythmia susceptibility, indicating conditional penetrance dependent on the diseased cardiac substrate.

6.2 HRC Expression in Heart Failure

HRC expression levels are reduced in both animal models and human heart failure [arvanitis-2011-review-abstract]. Given HRC's role in increasing SR calcium load that is unavailable for release, it has been proposed that HRC downregulation in heart failure may represent a compensatory mechanism to increase the amount of calcium released from the SR, in an attempt to enhance calcium cycling and maintain cardiac function.

6.3 Cardioprotection Against Ischemia/Reperfusion Injury

Paradoxically, despite its association with cardiac dysfunction when overexpressed, HRC appears to confer cardioprotection against ischemia/reperfusion injury [zhou-2007-ischemia-abstract]. Zhou and colleagues demonstrated that transgenic HRC-overexpressing hearts, despite having depressed baseline function, showed significantly improved recovery of left ventricular developed pressure after ischemia (86.6% vs. 58.3% in wild-types) and smaller infarct sizes (56% reduction in the in vivo model).

The cardioprotective mechanism involves antiapoptotic effects. HRC overexpression significantly increased the Bcl-2/Bax ratio following reperfusion and markedly decreased active caspase-3, caspase-9, and caspase-12. The investigators hypothesized that decreased free SR calcium due to HRC overexpression leads to reduced intramitochondrial calcium accumulation, thereby preserving mitochondrial integrity and preventing activation of mitochondrial-mediated apoptotic pathways.

6.4 Therapeutic Implications

The Ser96Ala variant has emerged as a potential biomarker for risk stratification of sudden cardiac death in dilated cardiomyopathy patients [arvanitis-2018-rhythmicity-abstract]. Furthermore, the observation that CaMKII inhibition (with KN-93) reduced malignant arrhythmia occurrence in mouse models carrying the HRC Ser96Ala variant suggests potential therapeutic approaches targeting the underlying arrhythmogenic pathway in affected patients.

7. Phosphorylation and Post-Translational Regulation

HRC undergoes phosphorylation at multiple serine residues (Ser119, Ser157/159, Ser170/171, Ser431, Ser563, Ser567), primarily by casein kinase II in the SR lumen [arvanitis-2011-review-abstract]. Phosphorylation of HRC has been shown to affect the ryanodine affinity of the ryanodine receptor, suggesting that post-translational modification provides another layer of HRC functional regulation.

The functional importance of phosphorylation is underscored by the clinical significance of the Ser96Ala variant, which eliminates a casein kinase II phosphorylation site and is associated with life-threatening arrhythmias. The precise mechanisms by which HRC phosphorylation state modulates its interactions with SERCA2 and triadin remain active areas of investigation.

8. Open Questions and Future Directions

Despite significant advances in understanding HRC biology, several important questions remain:

1. Structural basis of function: The three-dimensional structure of HRC under varying calcium concentrations and phosphorylation states has not been determined. Such structural information would provide crucial insights into how HRC undergoes conformational changes that alter its protein interaction partners.

2. In vivo dynamics: While in vitro studies have characterized the calcium-dependence of HRC-SERCA and HRC-triadin interactions, the real-time dynamics of these interactions during the cardiac cycle in vivo remain to be directly visualized.

3. Mechanism of Ser96Ala pathogenicity: The precise molecular mechanism by which the Ser96Ala variant increases arrhythmia susceptibility specifically in the setting of dilated cardiomyopathy requires further elucidation. Understanding this conditional penetrance could reveal therapeutic targets.

4. HRC in non-cardiac tissues: The function of HRC in arterial smooth muscle and its potential roles in vascular disease remain largely unexplored.

5. Therapeutic targeting: Whether HRC or its interactions can be pharmacologically targeted for cardiac protection or arrhythmia prevention represents an important translational question.

6. HRC phosphorylation cascade: The complete regulatory network governing HRC phosphorylation and its functional consequences needs systematic characterization.

7. Evolutionary significance: Recent evidence suggests HRC is evolving more rapidly than other cardiac excitation-contraction coupling proteins, with positive selection (or relaxed negative selection) occurring along most of the protein sequence except the highly conserved C-terminal cysteine-rich region [mackrill-2024-jeb-abstract]. The histidine-rich region may be involved in pH sensing as an adaptation to air-breathing and endothermic life. The physiological significance of this adaptation warrants further investigation.

8. TMAO demethylase activity: Given that aspolin, the fish paralogue of HRC, functions as a trimethylamine N-oxide demethylase, whether HRC retains any enzymatic activity toward TMAO remains an intriguing question with potential implications for understanding cardiac metabolism [mackrill-2024-jeb-abstract].

9. Role in muscle regeneration: The significance of SOX15-mediated HRC repression in early myocyte differentiation and skeletal muscle regeneration following injury requires further investigation.

References

1. [hofmann-1989-cloning-abstract] Hofmann SL, Goldstein JL, Orth K, Moomaw CR, Slaughter CA, Brown MS. Molecular cloning of a histidine-rich Ca2+-binding protein of sarcoplasmic reticulum that contains highly conserved repeated elements. J Biol Chem. 1989;264(30):18083-90. PMID: 2808365

2. [arvanitis-2011-review-abstract] Arvanitis DA, Vafiadaki E, Sanoudou D, Kranias EG. Histidine-rich calcium binding protein: the new regulator of sarcoplasmic reticulum calcium cycling. J Mol Cell Cardiol. 2011;50(1):43-9. PMID: 20807542. DOI: 10.1016/j.yjmcc.2010.08.021. PMC: PMC3018531

3. [lee-2001-triadin-abstract] Lee HG, Kang H, Kim DH, Park WJ. Interaction of HRC (histidine-rich Ca2+-binding protein) and triadin in the lumen of sarcoplasmic reticulum. J Biol Chem. 2001;276(43):39533-8. PMID: 11504710. DOI: 10.1074/jbc.M010664200

4. [arvanitis-2007-serca-abstract] Arvanitis DA, Vafiadaki E, Fan GC, Mitton BA, Gregory KN, Del Monte F, Kontrogianni-Konstantopoulos A, Sanoudou D, Kranias EG. Histidine-rich Ca-binding protein interacts with sarcoplasmic reticulum Ca-ATPase. Am J Physiol Heart Circ Physiol. 2007;293:H1581-H1589

5. [arvanitis-2008-ser96ala-abstract] Arvanitis DA, Sanoudou D, Kolokathis F, Vafiadaki E, Papalouka V, Kontrogianni-Konstantopoulos A, Theodorakis GN, Paraskevaidis IA, Adamopoulos S, Dorn GW, Kremastinos DT, Kranias EG. The Ser96Ala variant in histidine-rich calcium-binding protein is associated with life-threatening ventricular arrhythmias in idiopathic dilated cardiomyopathy. Eur Heart J. 2008;29:2514-2525. PMID: 18617481. DOI: 10.1093/eurheartj/ehn328. PMC: PMC2567024

6. [gregory-2006-cardiac-function-abstract] Gregory KN, Ginsburg KS, Bodi I, Hahn H, Marreez YM, Song Q, Padmanabhan PA, Mitton BA, Waggoner JR, Del Monte F, Park WJ, Dorn GW 2nd, Bers DM, Kranias EG. Histidine-rich Ca binding protein: a regulator of sarcoplasmic reticulum calcium sequestration and cardiac function. J Mol Cell Cardiol. 2006;40(5):653-65. PMID: 16600288. DOI: 10.1016/j.yjmcc.2006.02.003

7. [liu-2015-casq2-dko-abstract] Liu B, Györke S, et al. Ablation of HRC alleviates cardiac arrhythmia and improves abnormal Ca handling in CASQ2 knockout mice prone to CPVT. Cardiovasc Res. 2015;108(2):299-311. PMID: 26410369. DOI: 10.1093/cvr/cvv222. PMC: PMC4614688

8. [park-2013-hrc-knockout-abstract] Park CS, Chen S, Lee H, Cha H, Oh JG, Hong S, Han P, Ginsburg KS, Jin S, Park I, Singh VP, Wang HS, Bhel DM, Franzini-Armstrong C, Park WJ, Bers DM, Kranias EG, Cho C, Kim DH. Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress. Basic Res Cardiol. 2013;108(3):344. PMID: 23553082. DOI: 10.1007/s00395-013-0344-2

9. [zhou-2007-ischemia-abstract] Zhou X, Fan GC, Ren X, Waggoner JR, Gregory KN, Chen G, Jones WK, Kranias EG. Overexpression of histidine-rich Ca-binding protein protects against ischemia/reperfusion-induced cardiac injury. Cardiovasc Res. 2007;75(3):487-97. PMID: 17499229. DOI: 10.1016/j.cardiores.2007.04.005

10. [anderson-2004-mef2-abstract] Anderson JP, Dodou E, Heidt AB, De Val SJ, Jaehnig EJ, Greene SB, Olson EN, Black BL. HRC is a direct transcriptional target of MEF2 during cardiac, skeletal, and arterial smooth muscle development in vivo. Mol Cell Biol. 2004;24(9):3757-68. PMID: 15082771. PMC: PMC387749

11. [arvanitis-2018-rhythmicity-abstract] Arvanitis DA, Vafiadaki E, Johnson DM, Kranias EG, Sanoudou D. The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity. Front Physiol. 2018;9:1379. PMID: 30319456. DOI: 10.3389/fphys.2018.01379. PMC: PMC6171002

12. [mackrill-2024-jeb-abstract] Mackrill JJ. Histidine-rich calcium-binding protein: a molecular integrator of cardiac excitation-contraction coupling. J Exp Biol. 2024;227(20):jeb247640. DOI: 10.1242/jeb.247640

13. [hofmann-1991-genomic-abstract] Hofmann SL, Topham M, Hsieh CL, Francke U. cDNA and genomic cloning of HRC, a human sarcoplasmic reticulum protein, and localization of the gene to human chromosome 19 and mouse chromosome 7. Genomics. 1991;9(4):656-69. PMID: 2037293

14. [kim-2003-storage-abstract] Kim E, Shin DW, Hong CS, Jeong D, Kim DH, Park WJ. Increased Ca2+ storage capacity in the sarcoplasmic reticulum by overexpression of HRC (histidine-rich Ca2+ binding protein). Biochem Biophys Res Commun. 2003. PMID: 12480542

Citations

1. anderson-2004-mef2-abstract.md
2. arvanitis-2007-serca-abstract.md
3. arvanitis-2008-ser96ala-abstract.md
4. arvanitis-2011-review-abstract.md
5. arvanitis-2018-rhythmicity-abstract.md
6. gregory-2006-cardiac-function-abstract.md
7. hofmann-1989-cloning-abstract.md
8. hofmann-1991-genomic-abstract.md
9. kim-2003-storage-abstract.md
10. lee-2001-triadin-abstract.md
11. liu-2015-casq2-dko-abstract.md
12. mackrill-2024-jeb-abstract.md
13. park-2013-hrc-knockout-abstract.md
14. zhou-2007-ischemia-abstract.md

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research plan and verification
We verified the target as human HRC (UniProt P23327), a sarcoplasmic reticulum (SR) luminal protein of the HRC family with histidine-rich Ca2+-binding repeats (Hist_rich_Ca-bd) and a C‑terminal cysteine cluster. Literature consistently places HRC in the SR lumen of cardiomyocytes and describes it as a low-affinity, high-capacity Ca2+ binder that interacts with SERCA2a and the RyR2 release complex via triadin/junctin, consistent with the UniProt description and family/domain annotations (Hist_rich_Ca-bd/HRC) (arvanitis2011histidinerichcalciumbinding pages 1-2, arvanitis2011histidinerichcalciumbinding pages 4-5, pollak2017phosphorylationofserine96 pages 1-2).

Key concepts and definitions
- Protein identity and domains: HRC is an SR-lumenal histidine-rich Ca2+-binding protein (∼170 kDa in human) with nine histidine-rich tandem repeats and a C‑terminal cysteine-rich region; it belongs to the HRC family (Hist_rich_Ca-bd domains) (arvanitis2011histidinerichcalciumbinding pages 1-2, arvanitis2011histidinerichcalciumbinding pages 4-5). It is targeted to and resides within the SR lumen (pollak2017phosphorylationofserine96 pages 1-2, arvanitis2011histidinerichcalciumbinding pages 1-2). URLs: 2011 J Mol Cell Cardiol review (published Jan 2011): https://doi.org/10.1016/j.yjmcc.2010.08.021; 2017 PNAS (Aug 2017): https://doi.org/10.1073/pnas.1706441114.
- Ca2+-binding mode: HRC binds Ca2+ with low affinity and high capacity (reported capacity ≈200 nmol Ca2+/mg; KD ≈1.9 mM), and exhibits Ca2+-dependent multimerization, properties consistent with a luminal Ca2+ buffer/sensor (arvanitis2011histidinerichcalciumbinding pages 4-5). URL: 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021.
- Cellular localization and role: HRC is in the SR lumen of cardiac myocytes where it regulates SR Ca2+ uptake and release by binding SERCA2a (ATP2A2) and the RyR2 complex via triadin; it functions as a luminal Ca2+ sensor that modulates cross-talk between uptake and release (arvanitis2011histidinerichcalciumbinding pages 1-2, arvanitis2011histidinerichcalciumbinding pages 4-5, pollak2017phosphorylationofserine96 pages 1-2). URLs: 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021; 2017 PNAS: https://doi.org/10.1073/pnas.1706441114.

Molecular interactions, mechanisms, and regulation
- Direct interactions: HRC binds SERCA2a (mapping to an HRC central region interacting with a luminal SERCA segment), and binds triadin via a C‑terminal luminal KEKE-like motif to influence the RyR2 quaternary release complex. Junctin/junctate are related luminal partners in the junctional SR (arvanitis2011histidinerichcalciumbinding pages 4-5, arvanitis2018thehistidinerichcalcium pages 5-7). URLs: 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021; 2018 Front Physiol (Sep 2018): https://doi.org/10.3389/fphys.2018.01379.
- Ca2+-dependent switching: A unifying model posits that at low luminal [Ca2+] HRC preferentially binds and inhibits SERCA2a, attenuating Ca2+ uptake; as SR fills, HRC dissociates from SERCA2a and binds triadin to modulate RyR2-mediated Ca2+ release, thus linking uptake and release in a Ca2+-sensitive manner (arvanitis2011histidinerichcalciumbinding pages 4-5, arvanitis2018thehistidinerichcalcium pages 2-3). URL: 2018 Front Physiol: https://doi.org/10.3389/fphys.2018.01379.
- Post-translational regulation: HRC is phosphorylated within the secretory pathway lumen at Ser96 by Fam20C in vivo; Ser96 phosphorylation was confirmed in human hearts and cellular systems. Earlier work implicated casein kinase 2 (CK2) as a lumenal kinase for some HRC sites in vitro; however, the key in vivo event for Ser96 is Fam20C. Ser96 phosphorylation modulates HRC’s cardioprotective, anti-arrhythmic function (pollak2017phosphorylationofserine96 pages 1-2, arvanitis2011histidinerichcalciumbinding pages 7-8). URL: 2017 PNAS: https://doi.org/10.1073/pnas.1706441114; 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021.

Mechanistic phenotypes from perturbation
- Overexpression: Cardiac HRC overexpression reduces SR Ca2+ uptake, slows Ca2+ decay, and depresses contractility; it can also alter triadin/junctin levels and predispose to stress-induced remodeling (arvanitis2011histidinerichcalciumbinding pages 5-7, arvanitis2011histidinerichcalciumbinding pages 1-2). URL: 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021.
- Knockout/deficiency: HRC ablation increases SERCA2a-mediated uptake and basal contractility but augments susceptibility to spontaneous SR Ca2+ release and aftercontractions under adrenergic stress, consistent with loss of luminal buffering/sensing (chen2009histidinerichcabindinga pages 65-70). (Secondary summary) URL: 2009 summary (reviewed in 2011 J Mol Cell Cardiol): https://doi.org/10.1016/j.yjmcc.2010.08.021.
- Ser96 phosphorylation mechanistics: In mice and cells, preventing HRC Ser96 phosphorylation (S96A) increased arrhythmic events, while a phosphomimetic (S96D) reduced delayed aftercontractions and rescued the pro-arrhythmic phenotype. Thus, Fam20C→HRC Ser96 phosphorylation acts to prevent arrhythmia (pollak2017phosphorylationofserine96 pages 1-2). URL: 2017 PNAS: https://doi.org/10.1073/pnas.1706441114.

Disease associations and variants (quantitative data)
- Idiopathic DCM and malignant ventricular arrhythmias (2008): In 123 idiopathic DCM patients vs 96 controls, the HRC Ser96Ala variant associated with life-threatening ventricular arrhythmias. During 4.02 ± 2.4 years follow-up, Ala/Ala had markedly higher risk vs Ser/Ser (unadjusted HR 9.620; 95% CI 2.183–42.394; P=0.003). In multivariable analysis, Ser96Ala remained a significant predictor (HR 4.191; 95% CI 0.838–20.967; P=0.018). Genotype counts among DCM patients: Ser/Ser n=31, Ser/Ala n=61, Ala/Ala n=31 (arvanitis2008theser96alavariant pages 1-1, arvanitis2008theser96alavariant pages 5-6). URL: 2008 Eur Heart J (Jul 2008): https://doi.org/10.1093/eurheartj/ehn328.
- AF ablation recurrence (2019): In paroxysmal AF cohorts (screening N=334; replication N=245), HRC Ser96Ala associated with post‑ablation recurrence (screening OR 1.80, P=0.006; replication OR 1.74, P=0.03; combined P=0.0008). Multivariate Cox HR 2.66 (P=0.007), independent of clinical covariates (amioka2019ser96alageneticvariant pages 1-2). URL: 2019 PLOS ONE (Mar 2019): https://doi.org/10.1371/journal.pone.0213208.
- 2023 multi-cohort modifier analysis: A large analysis found no evidence that Ser96Ala modifies risk in cardiomyopathies. Cohorts included PLN p.Arg14del carriers (after QC n=848), ACM registry (n=882), and DCM from UK Biobank (n=985). Allele frequencies approximated controls (∼40–44%). Reported effect sizes were small and non‑significant across endpoints (examples: PLN ventricular arrhythmias OR 0.792; 95% CI 0.584–1.066; p=0.128. ACM VA OR 0.862; 95% CI 0.576–1.288; p=0.467. DCM VT OR 0.848; p=0.210; SCD OR 1.016; p=0.941; ICD implantation OR 0.887; p=0.284; all‑cause mortality HR 1.140; p=0.172). The authors conclude no clinically useful modifier signal (voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 2-4, voorn2023lackofevidence pages 4-6). URL: 2023 Int J Mol Sci (Nov 2023): https://doi.org/10.3390/ijms242115931.

Current understanding synthesized
- Primary function: HRC is a luminal SR Ca2+ buffer/sensor that integrates SR Ca2+ uptake (via SERCA2a) with release (via triadin–RyR2), switching binding partners as luminal Ca2+ changes. It binds Ca2+ with low affinity (mM KD) and high capacity, allowing it to modulate luminal microdomain Ca2+ and protein interactions without saturating at physiological SR [Ca2+] (arvanitis2011histidinerichcalciumbinding pages 4-5, arvanitis2011histidinerichcalciumbinding pages 1-2).
- Regulatory phosphorylation: Fam20C phosphorylates Ser96 in vivo in the secretory pathway; Ser96 phosphorylation is cardioprotective in models, and loss of this phosphorylation (Ser96Ala) predisposes to arrhythmia via altered interactions with SERCA2a and triadin/RyR2 (pollak2017phosphorylationofserine96 pages 1-2, arvanitis2011histidinerichcalciumbinding pages 7-8).
- Variant interpretation (context-dependent): Early DCM data (2008) suggest Ser96Ala confers increased risk of malignant ventricular arrhythmias within a small cohort, and AF ablation studies suggest elevated recurrence risk; however, larger 2023 multi-cohort analyses in cardiomyopathy contexts do not support a robust risk-modifier role. Thus, current evidence for routine clinical stratification using HRC Ser96Ala is mixed and insufficient for broad implementation (arvanitis2008theser96alavariant pages 1-1, amioka2019ser96alageneticvariant pages 1-2, voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 4-6).

Recent developments (2023–2024 prioritized)
- Genetic modifier reappraisal: The 2023 multi-cohort study (n≈848–985 per disease group) reports no significant association of Ser96Ala with ventricular arrhythmias, heart failure events, SCD, or ICD therapy across ACM, DCM, and PLN-R14del contexts, contradicting the earlier small DCM cohort reports (voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 4-6). URL: 2023 Int J Mol Sci: https://doi.org/10.3390/ijms242115931.
- Therapeutic context: Continued emphasis on luminal phosphorylation networks (Fam20C→HRC Ser96) as anti-arrhythmic mechanisms and potential targets; in vivo work shows Ser96 phosphorylation reduces aftercontractions and protects against arrhythmia, suggesting a tractable axis for drug discovery or gene therapy, though no 2023–2024 interventional trials targeting HRC/Fam20C were identified in this evidence set (pollak2017phosphorylationofserine96 pages 1-2). URL: 2017 PNAS: https://doi.org/10.1073/pnas.1706441114.

Applications and real-world implementations
- Biomarkers/stratification: HRC Ser96Ala has been explored as a candidate biomarker for risk stratification in DCM and for AF ablation outcomes; positive single-center or moderate-sized studies exist (e.g., AF ablation HR 2.66), but multi-cohort 2023 analyses did not validate modifier effects in cardiomyopathies, indicating that clinical use should be cautious, context-specific, and ideally embedded within polygenic/clinical risk models rather than as a standalone determinant (amioka2019ser96alageneticvariant pages 1-2, voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 4-6). URLs: 2019 PLOS ONE: https://doi.org/10.1371/journal.pone.0213208; 2023 Int J Mol Sci: https://doi.org/10.3390/ijms242115931.
- Experimental/diagnostic implications: Mechanistic markers such as phospho‑Ser96 status (Fam20C activity) and HRC interaction dynamics with SERCA2a/triadin are promising for research assays; clinical translation would require validated assays and outcome associations in large prospective cohorts (pollak2017phosphorylationofserine96 pages 1-2).

Expert opinions and authoritative perspectives
- Mechanistic reviews frame HRC as a nodal regulator of the PLN/SERCA2a regulatome and the RyR2 release complex, integrating luminal buffering with protein-protein interactions to fine‑tune excitation–contraction coupling. These sources (2011 domain-leading review; 2018 physiology review) emphasize luminal Ca2+ sensitivity of HRC–SERCA and HRC–triadin interactions and hypothesize HRC as an intra‑SR Ca2+ sensor (arvanitis2011histidinerichcalciumbinding pages 1-2, arvanitis2011histidinerichcalciumbinding pages 4-5, arvanitis2018thehistidinerichcalcium pages 2-3). URLs: 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021; 2018 Front Physiol: https://doi.org/10.3389/fphys.2018.01379.
- The 2017 PNAS Fam20C study provides a contemporary mechanistic anchor: luminal kinase control of HRC phosphorylation is necessary to prevent arrhythmia, linking biochemical regulation to physiologic protection (pollak2017phosphorylationofserine96 pages 1-2). URL: https://doi.org/10.1073/pnas.1706441114.

Relevant statistics and data points
- Ca2+-binding parameters: Capacity ≈ 200 nmol Ca2+/mg protein; KD ≈ 1.9 mM (arvanitis2011histidinerichcalciumbinding pages 4-5). URL: 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021.
- Overexpression effects: ~35–40% impairment of SR Ca2+ uptake and ~40% slowdown of Ca2+ transient decay in models; increased SR Ca2+ content but depressed release (arvanitis2018thehistidinerichcalcium pages 2-3, arvanitis2011histidinerichcalciumbinding pages 5-7). URLs: 2018 Front Physiol: https://doi.org/10.3389/fphys.2018.01379; 2011 J Mol Cell Cardiol: https://doi.org/10.1016/j.yjmcc.2010.08.021.
- DCM arrhythmia risk (2008): HR 9.620 (95% CI 2.183–42.394; P=0.003) unadjusted; multivariable HR 4.191 (95% CI 0.838–20.967; P=0.018); n=123 DCM, 96 controls (arvanitis2008theser96alavariant pages 1-1, arvanitis2008theser96alavariant pages 5-6). URL: 2008 Eur Heart J: https://doi.org/10.1093/eurheartj/ehn328.
- AF ablation recurrence (2019): Screening OR 1.80 (P=0.006); replication OR 1.74 (P=0.03); multivariate HR 2.66 (P=0.007); N=334 + 245 (amioka2019ser96alageneticvariant pages 1-2). URL: 2019 PLOS ONE: https://doi.org/10.1371/journal.pone.0213208.
- 2023 multi-cohort (modifier negative): PLN VA OR 0.792 (95% CI 0.584–1.066; p=0.128); ACM VA OR 0.862 (95% CI 0.576–1.288; p=0.467); DCM VT OR 0.848 (p=0.210); SCD OR 1.016 (p=0.941); ICD OR 0.887 (p=0.284); mortality HR 1.140 (p=0.172); cohorts n≈848–985 (voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 4-6). URL: 2023 Int J Mol Sci: https://doi.org/10.3390/ijms242115931.

Concise functional annotation
- Molecular function: Luminal SR Ca2+ buffer/sensor; binds Ca2+ with low affinity/high capacity; modulates SERCA2a and triadin/RyR2 complexes in a Ca2+-dependent manner to coordinate SR Ca2+ uptake and release (arvanitis2011histidinerichcalciumbinding pages 4-5, arvanitis2011histidinerichcalciumbinding pages 1-2).
- Subcellular localization: SR lumen of cardiomyocytes (pollak2017phosphorylationofserine96 pages 1-2, arvanitis2011histidinerichcalciumbinding pages 1-2).
- Pathways: Excitation–contraction coupling; PLN/SERCA2a regulatome; RyR2 quaternary complex (triadin/junctin/calsequestrin) (arvanitis2011histidinerichcalciumbinding pages 1-2, arvanitis2011histidinerichcalciumbinding pages 4-5).
- Regulation: Fam20C phosphorylation at Ser96 (cardioprotective); multiple additional luminal Ser sites (CK2 implicated in vitro) (pollak2017phosphorylationofserine96 pages 1-2, arvanitis2011histidinerichcalciumbinding pages 7-8).
- Variant: Ser96Ala prevents Ser96 phosphorylation; evidence mixed regarding clinical risk modification—mechanistic risk supported in models, translational risk-modifier signal inconsistent across human cohorts (pollak2017phosphorylationofserine96 pages 1-2, arvanitis2008theser96alavariant pages 1-1, amioka2019ser96alageneticvariant pages 1-2, voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 4-6).

Embedded summary table of evidence
| Aspect | Key findings | Primary evidence / citations (year and DOI URL) | Notes (organism = human unless specified) |
|---|---|---|---|
| Identity / verification | Gene symbol HRC; sarcoplasmic reticulum (SR) luminal protein; member of HRC family; contains histidine-rich Ca-binding repeats and C-terminal cysteine cluster (Hist_rich_Ca-bd, HRC domains). | Arvanitis et al., 2011. J Mol Cell Cardiol. https://doi.org/10.1016/j.yjmcc.2010.08.021 (arvanitis2011histidinerichcalciumbinding pages 1-2) | UniProt entry corresponds to human HRC (P23327) as requested. |
| Cellular localization | Localizes to the lumen of the sarcoplasmic reticulum (SR) in cardiomyocytes; signal peptide and lumenal repeats target HRC to SR. | Pollak et al., 2017. PNAS. https://doi.org/10.1073/pnas.1706441114 (pollak2017phosphorylationofserine96 pages 1-2); Arvanitis et al., 2011. https://doi.org/10.1016/j.yjmcc.2010.08.021 (arvanitis2011histidinerichcalciumbinding pages 1-2) | SR-lumen localization underlies role in luminal Ca2+ sensing/regulation. |
| Ca2+-binding mode (affinity / capacity) | Low-affinity, high-capacity Ca2+ binder: reported capacity ≈ 200 nmol Ca2+/mg protein and KD ≈ 1.9 mM; undergoes Ca2+-dependent multimerization. | Arvanitis et al., 2011. https://doi.org/10.1016/j.yjmcc.2010.08.021 (arvanitis2011histidinerichcalciumbinding pages 4-5) | Biochemistry consistent with a luminal Ca2+ buffer/sensor (not high-affinity sensor). |
| Direct interactions (region mapping) | Binds SERCA2a (interaction mapped to HRC central region / residues ~320–460 vs SERCA luminal segment), binds triadin via C‑terminal domain (KEKE-like luminal motif) and participates in RyR2 quaternary complex; junctin/junctate reported as related SR partners. | Arvanitis et al., 2011. https://doi.org/10.1016/j.yjmcc.2010.08.021 (arvanitis2011histidinerichcalciumbinding pages 4-5); Arvanitis et al., 2018. Front Physiol. https://doi.org/10.3389/fphys.2018.01379 (arvanitis2018thehistidinerichcalcium pages 5-7); Pollak et al., 2017. https://doi.org/10.1073/pnas.1706441114 (pollak2017phosphorylationofserine96 pages 1-2) | Interaction affinities are Ca2+-sensitive and promote cross-talk between uptake (SERCA) and release (RyR2) machinery. |
| Regulatory phosphorylation | Ser96 is an in vivo phosphorylation site targeted by Fam20C (secretory-pathway kinase); earlier work predicted/observed CK2 phosphorylation in vitro of lumen-accessible sites — Ser96Ala prevents phosphorylation at this site. | Pollak et al., 2017. https://doi.org/10.1073/pnas.1706441114 (pollak2017phosphorylationofserine96 pages 1-2); Arvanitis et al., 2011. https://doi.org/10.1016/j.yjmcc.2010.08.021 (arvanitis2011histidinerichcalciumbinding pages 1-2) | Fam20C-mediated Ser96 phosphorylation modulates HRC interactions and cardioprotective function in models. |
| Mechanistic function | Model: HRC acts as a luminal Ca2+ sensor/buffer that switches binding partners with luminal Ca2+ changes — at low SR [Ca2+] preferentially inhibits/associates with SERCA2a (reducing uptake), whereas at higher SR [Ca2+] it dissociates from SERCA and binds triadin to modulate RyR2-mediated release. | Arvanitis et al., 2011. https://doi.org/10.1016/j.yjmcc.2010.08.021 (arvanitis2011histidinerichcalciumbinding pages 4-5); Arvanitis et al., 2018. https://doi.org/10.3389/fphys.2018.01379 (arvanitis2018thehistidinerichcalcium pages 2-3) | Provides mechanistic explanation for dual role in SR uptake vs release regulation. |
| Phenotypes from perturbation | Overexpression: decreased SR Ca2+ uptake rates, slowed Ca2+-transient decay, impaired contractility and stress‑induced remodeling. Knockout: increased SERCA2a-mediated uptake and hypercontractile cellular phenotype but enhanced propensity for spontaneous SR Ca2+ release and stress-induced arrhythmias. S96A models: impaired Ser96 phosphorylation, weaker HRC–triadin interaction, increased spontaneous Ca2+ sparks/waves and arrhythmogenesis in cellular/animal models. | Arvanitis et al., 2011; Chen 2009 summary; Pollak et al., 2017; Tzimas et al., 2017. (arvanitis2011histidinerichcalciumbinding pages 5-7, chen2009histidinerichcabindinga pages 65-70, pollak2017phosphorylationofserine96 pages 1-2, arvanitis2018thehistidinerichcalcium pages 5-7) | Direction of effect can be context-dependent (expression level, adrenergic stress). |
| Disease associations & genetics | Early candidate study (idiopathic DCM, n=123) reported Ser96Ala (Ala/Ala) associated with life‑threatening ventricular arrhythmias (unadjusted HR≈9.62; multivariable HR≈4.19) (2008). Ser96Ala was reported as predictor of AF recurrence after ablation (combined cohorts screening N=334, replication N=245; multivariate HR 2.66). A 2023 multi-cohort analysis (PLN carriers N≈848; ACM N≈882; DCM UKB N≈985) found no significant association of Ser96Ala with life‑threatening arrhythmias or HF events (OR/HR estimates ~null/non‑significant). | Arvanitis et al., 2008. Eur Heart J. https://doi.org/10.1093/eurheartj/ehn328 (arvanitis2008theser96alavariant pages 1-1); Amioka et al., 2019. PLOS ONE. https://doi.org/10.1371/journal.pone.0213208 (amioka2019ser96alageneticvariant pages 1-2); van der Voorn et al., 2023. IJMS. https://doi.org/10.3390/ijms242115931 (voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 4-6) | Evidence is mixed: early strong signal in small DCM cohort vs larger multi-cohort negative replication; allele is common (~40% MAF in Europeans). |
| Therapeutic / biomarker context | Biological rationale: restoring Ser96 phosphorylation or stabilizing HRC interactions could be anti‑arrhythmic in susceptible hearts (Fam20C–HRC axis). Clinical/stratification utility remains unresolved — candidate biomarker for AF recurrence and DCM arrhythmia risk (positive studies) but larger 2023 analyses found no modifier effect. | Pollak et al., 2017. https://doi.org/10.1073/pnas.1706441114 (pollak2017phosphorylationofserine96 pages 1-2); Amioka et al., 2019. https://doi.org/10.1371/journal.pone.0213208 (amioka2019ser96alageneticvariant pages 1-2); van der Voorn et al., 2023. https://doi.org/10.3390/ijms242115931 (voorn2023lackofevidence pages 1-2) | Translation requires larger prospective validation and functional biomarkers (phospho‑status, interaction readouts) before clinical use. |

Table: Compact table summarizing human HRC (UniProt P23327) functional annotation, mechanistic roles, phosphorylation regulation, phenotypes, and translational/genetic evidence with primary citations for quick reference.

Limitations and open questions
- Structural details of HRC complexes and dynamic, luminal Ca2+-dependent binding mechanics remain incompletely resolved in humans.
- The translational value of Ser96Ala as a clinical biomarker is currently uncertain given conflicting findings; future large, prospective, multi-ancestry studies with standardized endpoints and mechanistic biomarkers (e.g., phospho‑Ser96 assays) are needed (voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 4-6).

Conclusion
HRC (P23327) is a verified human SR luminal protein in the HRC family with histidine-rich Ca2+-binding domains. It functions as a low-affinity/high-capacity Ca2+ buffer and luminal sensor that coordinates SR Ca2+ uptake and release via Ca2+-sensitive interactions with SERCA2a and the triadin/RyR2 complex. Fam20C-mediated phosphorylation at Ser96 is cardioprotective in models; the common Ser96Ala variant prevents this phosphorylation and confers arrhythmogenicity in mechanistic systems. Human genetic evidence is mixed—early small-cohort signals versus larger, recent multi-cohort negative findings—so, at present, HRC Ser96Ala should be considered a context-dependent research marker rather than a routine clinical stratifier. Continued work on luminal phosphorylation networks and HRC interaction dynamics may yield therapeutic strategies to stabilize SR Ca2+ handling and reduce arrhythmia risk (pollak2017phosphorylationofserine96 pages 1-2, arvanitis2011histidinerichcalciumbinding pages 1-2, arvanitis2011histidinerichcalciumbinding pages 4-5, arvanitis2008theser96alavariant pages 1-1, amioka2019ser96alageneticvariant pages 1-2, voorn2023lackofevidence pages 1-2, voorn2023lackofevidence pages 6-7, voorn2023lackofevidence pages 4-6).

References

1. (arvanitis2011histidinerichcalciumbinding pages 1-2): Demetrios A. Arvanitis, Elizabeth Vafiadaki, Despina Sanoudou, and Evangelia G. Kranias. Histidine-rich calcium binding protein: the new regulator of sarcoplasmic reticulum calcium cycling. Journal of molecular and cellular cardiology, 50 1:43-9, Jan 2011. URL: https://doi.org/10.1016/j.yjmcc.2010.08.021, doi:10.1016/j.yjmcc.2010.08.021. This article has 84 citations and is from a domain leading peer-reviewed journal.

2. (arvanitis2011histidinerichcalciumbinding pages 4-5): Demetrios A. Arvanitis, Elizabeth Vafiadaki, Despina Sanoudou, and Evangelia G. Kranias. Histidine-rich calcium binding protein: the new regulator of sarcoplasmic reticulum calcium cycling. Journal of molecular and cellular cardiology, 50 1:43-9, Jan 2011. URL: https://doi.org/10.1016/j.yjmcc.2010.08.021, doi:10.1016/j.yjmcc.2010.08.021. This article has 84 citations and is from a domain leading peer-reviewed journal.

3. (pollak2017phosphorylationofserine96 pages 1-2): Adam J. Pollak, Kobra Haghighi, Swati Kunduri, Demetrios A. Arvanitis, Philip A. Bidwell, Guan-Sheng Liu, Vivek P. Singh, David J. Gonzalez, Despina Sanoudou, Sandra E. Wiley, Jack E. Dixon, and Evangelia G. Kranias. Phosphorylation of serine96 of histidine-rich calcium-binding protein by the fam20c kinase functions to prevent cardiac arrhythmia. Proceedings of the National Academy of Sciences, 114:9098-9103, Aug 2017. URL: https://doi.org/10.1073/pnas.1706441114, doi:10.1073/pnas.1706441114. This article has 42 citations and is from a highest quality peer-reviewed journal.

4. (arvanitis2018thehistidinerichcalcium pages 5-7): Demetrios A. Arvanitis, Elizabeth Vafiadaki, Daniel M. Johnson, Evangelia G. Kranias, and Despina Sanoudou. The histidine-rich calcium binding protein in regulation of cardiac rhythmicity. Frontiers in Physiology, Sep 2018. URL: https://doi.org/10.3389/fphys.2018.01379, doi:10.3389/fphys.2018.01379. This article has 18 citations and is from a poor quality or predatory journal.

5. (arvanitis2018thehistidinerichcalcium pages 2-3): Demetrios A. Arvanitis, Elizabeth Vafiadaki, Daniel M. Johnson, Evangelia G. Kranias, and Despina Sanoudou. The histidine-rich calcium binding protein in regulation of cardiac rhythmicity. Frontiers in Physiology, Sep 2018. URL: https://doi.org/10.3389/fphys.2018.01379, doi:10.3389/fphys.2018.01379. This article has 18 citations and is from a poor quality or predatory journal.

6. (arvanitis2011histidinerichcalciumbinding pages 7-8): Demetrios A. Arvanitis, Elizabeth Vafiadaki, Despina Sanoudou, and Evangelia G. Kranias. Histidine-rich calcium binding protein: the new regulator of sarcoplasmic reticulum calcium cycling. Journal of molecular and cellular cardiology, 50 1:43-9, Jan 2011. URL: https://doi.org/10.1016/j.yjmcc.2010.08.021, doi:10.1016/j.yjmcc.2010.08.021. This article has 84 citations and is from a domain leading peer-reviewed journal.

7. (arvanitis2011histidinerichcalciumbinding pages 5-7): Demetrios A. Arvanitis, Elizabeth Vafiadaki, Despina Sanoudou, and Evangelia G. Kranias. Histidine-rich calcium binding protein: the new regulator of sarcoplasmic reticulum calcium cycling. Journal of molecular and cellular cardiology, 50 1:43-9, Jan 2011. URL: https://doi.org/10.1016/j.yjmcc.2010.08.021, doi:10.1016/j.yjmcc.2010.08.021. This article has 84 citations and is from a domain leading peer-reviewed journal.

8. (chen2009histidinerichcabindinga pages 65-70): S Chen. Histidine-rich ca binding protein and cardiac functions. Unknown journal, 2009.

9. (arvanitis2008theser96alavariant pages 1-1): Demetrios A. Arvanitis, Despina Sanoudou, Fotis Kolokathis, Elizabeth Vafiadaki, Vasiliki Papalouka, Aikaterini Kontrogianni-Konstantopoulos, George N. Theodorakis, Ioannis A. Paraskevaidis, Stamatios Adamopoulos, Gerald W. Dorn, Dimitrios Th. Kremastinos, and Evangelia G. Kranias. The ser96ala variant in histidine-rich calcium-binding protein is associated with life-threatening ventricular arrhythmias in idiopathic dilated cardiomyopathy. European Heart Journal, 29:2514-2525, Jul 2008. URL: https://doi.org/10.1093/eurheartj/ehn328, doi:10.1093/eurheartj/ehn328. This article has 61 citations and is from a highest quality peer-reviewed journal.

10. (arvanitis2008theser96alavariant pages 5-6): Demetrios A. Arvanitis, Despina Sanoudou, Fotis Kolokathis, Elizabeth Vafiadaki, Vasiliki Papalouka, Aikaterini Kontrogianni-Konstantopoulos, George N. Theodorakis, Ioannis A. Paraskevaidis, Stamatios Adamopoulos, Gerald W. Dorn, Dimitrios Th. Kremastinos, and Evangelia G. Kranias. The ser96ala variant in histidine-rich calcium-binding protein is associated with life-threatening ventricular arrhythmias in idiopathic dilated cardiomyopathy. European Heart Journal, 29:2514-2525, Jul 2008. URL: https://doi.org/10.1093/eurheartj/ehn328, doi:10.1093/eurheartj/ehn328. This article has 61 citations and is from a highest quality peer-reviewed journal.

11. (amioka2019ser96alageneticvariant pages 1-2): Michitaka Amioka, Yukiko Nakano, Hidenori Ochi, Yuko Onohara, Akinori Sairaku, Takehito Tokuyama, Chikaaki Motoda, Hiroya Matsumura, Shunsuke Tomomori, Naoya Hironobe, Yousaku Okubo, Sho Okamura, Kazuaki Chayama, and Yasuki Kihara. Ser96ala genetic variant of the human histidine-rich calcium-binding protein is a genetic predictor of recurrence after catheter ablation in patients with paroxysmal atrial fibrillation. PLOS ONE, 14:e0213208, Mar 2019. URL: https://doi.org/10.1371/journal.pone.0213208, doi:10.1371/journal.pone.0213208. This article has 9 citations and is from a peer-reviewed journal.

12. (voorn2023lackofevidence pages 1-2): Stephanie M. van der Voorn, Esmée van Drie, Virginnio Proost, Kristina Dimitrova, Robert F. Ernst, Cynthia A. James, Crystal Tichnell, Brittney Murray, Hugh Calkins, Ardan M. Saguner, Firat Duru, Patrick T. Ellinor, Connie R. Bezzina, Sean J. Jurgens, J. Peter van Tintelen, and Toon A. B. van Veen. Lack of evidence for the role of the p.(ser96ala) polymorphism in histidine-rich calcium binding protein as a secondary hit in cardiomyopathies. International Journal of Molecular Sciences, 24:15931, Nov 2023. URL: https://doi.org/10.3390/ijms242115931, doi:10.3390/ijms242115931. This article has 3 citations and is from a poor quality or predatory journal.

13. (voorn2023lackofevidence pages 6-7): Stephanie M. van der Voorn, Esmée van Drie, Virginnio Proost, Kristina Dimitrova, Robert F. Ernst, Cynthia A. James, Crystal Tichnell, Brittney Murray, Hugh Calkins, Ardan M. Saguner, Firat Duru, Patrick T. Ellinor, Connie R. Bezzina, Sean J. Jurgens, J. Peter van Tintelen, and Toon A. B. van Veen. Lack of evidence for the role of the p.(ser96ala) polymorphism in histidine-rich calcium binding protein as a secondary hit in cardiomyopathies. International Journal of Molecular Sciences, 24:15931, Nov 2023. URL: https://doi.org/10.3390/ijms242115931, doi:10.3390/ijms242115931. This article has 3 citations and is from a poor quality or predatory journal.

14. (voorn2023lackofevidence pages 2-4): Stephanie M. van der Voorn, Esmée van Drie, Virginnio Proost, Kristina Dimitrova, Robert F. Ernst, Cynthia A. James, Crystal Tichnell, Brittney Murray, Hugh Calkins, Ardan M. Saguner, Firat Duru, Patrick T. Ellinor, Connie R. Bezzina, Sean J. Jurgens, J. Peter van Tintelen, and Toon A. B. van Veen. Lack of evidence for the role of the p.(ser96ala) polymorphism in histidine-rich calcium binding protein as a secondary hit in cardiomyopathies. International Journal of Molecular Sciences, 24:15931, Nov 2023. URL: https://doi.org/10.3390/ijms242115931, doi:10.3390/ijms242115931. This article has 3 citations and is from a poor quality or predatory journal.

15. (voorn2023lackofevidence pages 4-6): Stephanie M. van der Voorn, Esmée van Drie, Virginnio Proost, Kristina Dimitrova, Robert F. Ernst, Cynthia A. James, Crystal Tichnell, Brittney Murray, Hugh Calkins, Ardan M. Saguner, Firat Duru, Patrick T. Ellinor, Connie R. Bezzina, Sean J. Jurgens, J. Peter van Tintelen, and Toon A. B. van Veen. Lack of evidence for the role of the p.(ser96ala) polymorphism in histidine-rich calcium binding protein as a secondary hit in cardiomyopathies. International Journal of Molecular Sciences, 24:15931, Nov 2023. URL: https://doi.org/10.3390/ijms242115931, doi:10.3390/ijms242115931. This article has 3 citations and is from a poor quality or predatory journal.

Citations

1. arvanitis2011histidinerichcalciumbinding pages 4-5
2. chen2009histidinerichcabindinga pages 65-70
3. arvanitis2011histidinerichcalciumbinding pages 1-2
4. arvanitis2018thehistidinerichcalcium pages 5-7
5. arvanitis2018thehistidinerichcalcium pages 2-3
6. voorn2023lackofevidence pages 1-2
7. arvanitis2011histidinerichcalciumbinding pages 7-8
8. arvanitis2011histidinerichcalciumbinding pages 5-7
9. voorn2023lackofevidence pages 6-7
10. voorn2023lackofevidence pages 2-4
11. voorn2023lackofevidence pages 4-6
12. Ca2+
13. https://doi.org/10.1016/j.yjmcc.2010.08.021;
14. https://doi.org/10.1073/pnas.1706441114.
15. https://doi.org/10.1016/j.yjmcc.2010.08.021.
16. https://doi.org/10.3389/fphys.2018.01379.
17. https://doi.org/10.1073/pnas.1706441114;
18. https://doi.org/10.1093/eurheartj/ehn328.
19. https://doi.org/10.1371/journal.pone.0213208.
20. https://doi.org/10.3390/ijms242115931.
21. https://doi.org/10.1371/journal.pone.0213208;
22. https://doi.org/10.3389/fphys.2018.01379;
23. https://doi.org/10.1016/j.yjmcc.2010.08.021
24. https://doi.org/10.1073/pnas.1706441114
25. https://doi.org/10.3389/fphys.2018.01379
26. https://doi.org/10.1093/eurheartj/ehn328
27. https://doi.org/10.1371/journal.pone.0213208
28. https://doi.org/10.3390/ijms242115931
29. https://doi.org/10.1016/j.yjmcc.2010.08.021,
30. https://doi.org/10.1073/pnas.1706441114,
31. https://doi.org/10.3389/fphys.2018.01379,
32. https://doi.org/10.1093/eurheartj/ehn328,
33. https://doi.org/10.1371/journal.pone.0213208,
34. https://doi.org/10.3390/ijms242115931,

Functional Annotation of HRC (Histidine-Rich Calcium-Binding Protein)

Gene Identity and Overview

The HRC gene (histidine-rich calcium-binding protein, also known as HCP) encodes a luminal sarcoplasmic reticulum protein named histidine-rich calcium-binding protein (www.ncbi.nlm.nih.gov). In humans (Homo sapiens), HRC is primarily expressed in muscle tissues – notably in striated skeletal and cardiac muscle, with additional expression in arteriolar smooth muscle cells (pmc.ncbi.nlm.nih.gov). The HRC protein is unusually large and acidic: it has an apparent molecular weight of ~170 kDa and over 30% of its amino acids are acidic residues (pmc.ncbi.nlm.nih.gov). It was originally identified by its high-affinity binding to low-density lipoprotein (LDL) in biochemical assays (www.ncbi.nlm.nih.gov), although its physiological role is in muscle calcium (Ca^2+) handling. HRC is a member of the histidine-rich Ca^2+-binding protein family (HRC family) and contains signature histidine-rich acidic regions that enable Ca^2+ binding. The gene is located on human chromosome 19 (19q13.3) and produces a precursor protein that is targeted to the sarcoplasmic reticulum (SR) lumen (HRC contains a signal peptide for SR import and luminal localization) (www.ncbi.nlm.nih.gov). In summary, HRC is a high-capacity, low-affinity Ca^2+-binding protein residing in the sarcoplasmic reticulum of muscle cells, where it modulates calcium storage and release crucial for muscle contraction (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Structural Features and Ca²⁺-Binding Properties

The HRC protein is highly charged and rich in histidine. It lacks canonical EF-hand Ca^2+-binding domains; instead, it contains a repetitive central region (amino acids ~106–365 in the human protein) composed of ten histidine-rich acidic tandem repeats (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These repeats are classified as type A and type B motifs, each comprising clusters of histidines followed by stretches of acidic residues (pmc.ncbi.nlm.nih.gov). This unique composition underlies HRC’s mode of Ca^2+ binding: electrostatic interactions with acidic amino acid clusters provide multiple low-affinity Ca^2+ binding sites, somewhat analogous to how calsequestrin binds Ca^2+ (pmc.ncbi.nlm.nih.gov). Biochemical studies show that HRC can bind on the order of 200 nanomoles of Ca^2+ per milligram of protein with a dissociation constant K_d ~1.9 mM, indicative of high-capacity, low-affinity Ca^2+ sequestration (pmc.ncbi.nlm.nih.gov). Binding of Ca^2+ induces conformational changes in HRC (altering its electrophoretic mobility) and may shift its oligomeric state (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In fact, HRC can form oligomers in the SR lumen that dissociate when Ca^2+ levels rise, a behavior reminiscent of the Ca^2+-dependent polymerization of calsequestrin (pmc.ncbi.nlm.nih.gov). Importantly, HRC’s attachment within the SR is Ca^2+-dependent: the protein remains associated with the SR membrane even after high-salt extraction (conditions that remove calsequestrin), but it dissociates upon Ca^2+ chelation (EDTA treatment) (pmc.ncbi.nlm.nih.gov). This suggests HRC may anchor to the junctional SR via Ca^2+-bridged interactions with other luminal proteins. Additionally, HRC can bind other divalent cations like Zn^2+ at distinct sites (whereas calsequestrin has essentially no Zn^2+ affinity) (pmc.ncbi.nlm.nih.gov). The C-terminal region of HRC (aa 627–673) is cysteine-rich and conserved, which likely mediates protein-protein interactions (through charged residues or disulfide bonds) and may contribute to HRC’s structural organization within the SR lumen (pmc.ncbi.nlm.nih.gov). No high-resolution 3D structure of HRC is yet available, owing to its repetitive and highly charged nature (pmc.ncbi.nlm.nih.gov). However, its biochemical properties firmly establish HRC as a Ca^2+ buffer within the SR, capable of undergoing Ca^2+-induced structural transitions similar to other SR Ca^2+-binding proteins (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Expression and Subcellular Localization

HRC is predominantly a muscle protein. Expression profiling indicates it is highly enriched in the heart – for example, in RNA-Seq data the heart shows an RPKM of ~207, with much lower expression in non-muscle tissues (www.ncbi.nlm.nih.gov). The protein has also been detected in skeletal muscle (particularly fast-twitch fibers) and in arteriolar smooth muscle (pmc.ncbi.nlm.nih.gov). This distribution aligns with its function in the sarcoplasmic reticulum of contractile cells. Within the cell, HRC localizes to the sarcoplasmic/endoplasmic reticulum lumen, specifically the lumen of the junctional SR in muscle fibers (www.ncbi.nlm.nih.gov). It is synthesized as a precursor with a signal peptide that targets it to the ER/SR lumen, where the mature protein resides (likely after cleavage of the signal sequence). HRC does not span the membrane; rather, it is a luminal, soluble protein that can attach to SR luminal components. Notably, HRC interacts with the luminal domains of other junctional SR proteins, which helps retain it in the junctional SR region. One key binding partner is triadin, a transmembrane protein of the junctional SR. HRC directly binds to triadin’s luminal domain (which contains clusters of acidic and basic residues often termed KEKE motifs) (pmc.ncbi.nlm.nih.gov). Through triadin (and potentially junctin), HRC is tethered near the ryanodine receptor (RyR2) Ca^2+ release channels on the SR membrane (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This positioning is strategic: it places HRC at the Ca^2+ release sites where it can influence the local Ca^2+ dynamics and signal transduction during excitation-contraction coupling. Indeed, HRC is considered part of the quaternary SR Ca^2+ release complex consisting of Ryanodine Receptor (RyR), calsequestrin, triadin, and junctin (pmc.ncbi.nlm.nih.gov). In addition, HRC is found in the SR calcium uptake regions, where it can interact with the Ca^2+-ATPase pump (SERCA2) that sequesters Ca^2+ back into the SR (pmc.ncbi.nlm.nih.gov). In summary, HRC is localized to the SR lumen of muscle cells, concentrated at sites of Ca^2+ release and uptake, which enables it to function as a Ca^2+ store and to modulate the activities of SR Ca^2+-handling complexes.

Role in SR Calcium Homeostasis and Excitation-Contraction Coupling

HRC plays a regulatory role in sarcoplasmic reticulum Ca^2+ cycling, impacting both the storage and release of Ca^2+ in muscle cells (pmc.ncbi.nlm.nih.gov). By binding large amounts of Ca^2+ in the SR lumen, HRC helps maintain a reservoir of Ca^2+ that can be rapidly released for muscle contraction. Unlike high-affinity Ca^2+ sensors, HRC’s low-affinity binding allows it to release Ca^2+ readily when luminal Ca^2+ levels drop during contraction. Functional studies have shown that HRC is intricately involved in balancing Ca^2+ uptake into the SR (via SERCA pumps) and Ca^2+ release through ryanodine receptors:

• Interaction with the Ca^2+ Release Complex: HRC binds to triadin, which in turn is associated with the RyR2 channel. This interaction is sensitive to Ca^2+ levels – high luminal Ca^2+ can weaken HRC’s binding to triadin (pmc.ncbi.nlm.nih.gov). Through triadin, HRC has an indirect influence on the ryanodine receptor’s readiness to release Ca^2+. Notably, HRC can modulate RyR2 activity: in vitro experiments suggest that post-translational modification of HRC (phosphorylation) alters RyR’s binding affinity for ryanodine, hinting that HRC may signal luminal Ca^2+ status to RyR (pmc.ncbi.nlm.nih.gov). In line with this, genetic ablation of HRC leads to enhanced spontaneous Ca^2+ release – HRC knockout cardiomyocytes show a higher fractional SR Ca^2+ release and more frequent Ca^2+ sparks, indicating that without HRC, the RyR channels are more prone to open aberrantly (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Conversely, acute overexpression of HRC in cardiac myocytes has been found to suppress Ca^2+-induced Ca^2+ release, resulting in a smaller Ca^2+ transient for a given trigger and consequently a reduced contractile force (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This overexpression also increased total Ca^2+ storage in the SR (since less was being released each beat) and led to impaired contractility of the myocytes (pmc.ncbi.nlm.nih.gov). These findings illustrate that HRC normally acts as a brake on SR Ca^2+ release, preventing excessive Ca^2+ dumping from the SR during each contraction. It fine-tunes the gain of excitation–contraction coupling so that the heart muscle contracts with appropriate force and rhythm.

• Interaction with the Ca^2+ Uptake Machinery: In addition to its role in Ca^2+ release, HRC directly influences SR Ca^2+ uptake by the SERCA2a pump. Co-immunoprecipitation experiments and transgenic models indicate that HRC can bind to SERCA2a (the cardiac SR Ca^2+-ATPase) and modify its activity (pmc.ncbi.nlm.nih.gov). When HRC is overexpressed in transgenic mouse hearts, the rate of SR Ca^2+ uptake is significantly reduced (pmc.ncbi.nlm.nih.gov). This reduction is attributed to HRC’s interaction with SERCA2a, which appears to inhibit the pump’s maximal activity. As a result, HRC-overexpressing mice exhibit delayed relaxation and develop cardiac remodeling and hypertrophy over time, consistent with chronic SR Ca^2+ handling impairment (pmc.ncbi.nlm.nih.gov). On the other hand, HRC knockout mice have been shown to have slightly increased fractional Ca^2+ uptake per cycle (since more Ca^2+ gets released and needs pumping back), but ultimately they cannot maintain Ca^2+ homeostasis under stress due to other dysregulations (discussed below) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The dual interactions of HRC – with triadin/RyR2 for Ca^2+ release and with SERCA2a for Ca^2+ uptake – position it as a crucial buffer and moderator in the calcium cycle. It ensures that Ca^2+ release and reuptake are balanced and tuned to physiological needs (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In essence, HRC helps prevent both SR Ca^2+ overload and depletion by regulating how much Ca^2+ is released during each contraction and how efficiently it is pumped back into the store.

• Mechanistic Model: Under resting conditions or low beat rate, HRC binds Ca^2+ and associates with triadin in the lumen, possibly keeping RyR2 in a state less sensitive to spontaneous activation (pubmed.ncbi.nlm.nih.gov). When SR luminal Ca^2+ becomes very high, HRC may release some Ca^2+ (due to its low affinity) and perhaps dissociate from triadin, which could facilitate RyR openings to release the excess Ca^2+. During muscle stimulation (Ca^2+-induced Ca^2+ release), HRC likely contributes to setting the threshold for luminal Ca^2+ that triggers RyR opening, in concert with calsequestrin. Interestingly, studies suggest HRC and calsequestrin have opposing influences on RyR2: calsequestrin acts as a stabilizer that prevents RyR2 opening when luminal Ca^2+ is low, whereas HRC appears to enhance or facilitate RyR2 activity under certain conditions (pubmed.ncbi.nlm.nih.gov). In a mouse model lacking the cardiac calsequestrin (CASQ2) – which normally causes arrhythmogenic Ca^2+ release due to the loss of RyR stabilization – additional ablation of HRC paradoxically alleviated the arrhythmia (pubmed.ncbi.nlm.nih.gov). This finding indicates that HRC, in the absence of calsequestrin, was contributing to excessive RyR activation; removing HRC thus restored a degree of Ca^2+ release stability. Such evidence supports a model in which calsequestrin and HRC counter-regulate the RyR2, with HRC promoting Ca^2+ release when present, and calsequestrin restraining it (pubmed.ncbi.nlm.nih.gov). Proper cardiac Ca^2+ cycling likely requires the right balance of both proteins.

Overall, HRC serves as a luminal Ca^2+ sensor and buffering modulator at the SR interface. By storing Ca^2+ and physically coupling to key Ca^2+-handling proteins, it contributes to excitation–contraction coupling fidelity. Small changes in HRC levels or function can have outsized effects on Ca^2+ dynamics: for example, a modest increase in HRC expression produces a larger disruption in Ca^2+ transients than a 20-fold overexpression of calsequestrin (pmc.ncbi.nlm.nih.gov). This underscores that HRC is a pivotal regulator, ensuring that each heartbeat has a coordinated Ca^2+ release and reuptake, thus maintaining contractile strength and rhythmicity (pmc.ncbi.nlm.nih.gov).

Biological Functions and Regulatory Roles

Beyond its direct effects on Ca^2+ handling, HRC has been implicated in broader biological processes in muscle cells. Its expression is developmentally regulated – HRC is a direct transcriptional target of the muscle-specific transcription factor MEF2, which drives HRC expression during cardiac and skeletal muscle development (pmc.ncbi.nlm.nih.gov). Consistent with this, HRC appears to play a role in myocyte differentiation. Experimental studies have shown that altering HRC levels can influence the maturation state of muscle cells. For instance, overexpression of HRC in cardiomyocytes was reported to promote a more differentiated, oxidative phenotype in some contexts (pmc.ncbi.nlm.nih.gov). Conversely, HRC knockout mice, while viable, may have subtle developmental differences in SR structure or function that can affect how their cardiac muscle responds to stress (e.g., the architecture of the SR might compensate in the absence of HRC) (pmc.ncbi.nlm.nih.gov).

HRC has also been linked to cell survival pathways in the heart, particularly under stress conditions. During ischemia-reperfusion injury (simulated heart attack conditions), HRC-overexpressing mouse hearts showed an enhanced resistance to apoptosis (programmed cell death) (pmc.ncbi.nlm.nih.gov). This was evidenced by higher levels of the anti-apoptotic protein Bcl-2 and preservation of mitochondrial integrity in HRC-overexpressing myocardium following ischemia (pmc.ncbi.nlm.nih.gov). The data suggest that HRC exerts a cardioprotective effect during acute stress, possibly by preventing cytosolic Ca^2+ overload that triggers cell death pathways. Proper Ca^2+ handling is known to be crucial for cell survival: excessive Ca^2+ release can activate destructive enzymes and mitochondrial dysfunction. By modulating Ca^2+ release, HRC may help prevent Ca^2+ dysregulation that leads to cell injury. This ties into observations that HRC-knockout hearts under stress develop more damage (fibrosis, hypertrophy, etc.) than wild-type hearts (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In summary, HRC contributes to normal muscle cell differentiation and provides a measure of protection against Ca^2+-mediated cell damage, in addition to its primary role in contraction.

It’s worth noting that HRC does not have enzymatic activity or classical signaling motifs – its functions are executed via binding (to ions and proteins) and structural effects in the SR. It can be thought of as a scaffolding protein that organizes Ca^2+ in the SR and coordinates the interaction of the Ca^2+ release and uptake apparatus. Changes in HRC expression or post-translational modification can thus ripple through the Ca^2+-handling system and alter muscle performance.

Genetic Variants and Disease Associations

Genetic variation in the HRC gene can significantly impact cardiac function, particularly under pathological conditions. The most extensively studied variant is a single nucleotide polymorphism resulting in a Serine-to-Alanine substitution at position 96 (Ser96Ala) of the HRC protein (also referred to as S96A, with Serine as the reference allele). This variant has gained attention as a risk factor for cardiac arrhythmias. In a landmark study (Arvanitis et al., 2008), the HRC Ser96Ala polymorphism was identified in patients with idiopathic dilated cardiomyopathy (DCM) and was found to associate with life-threatening ventricular arrhythmias (pmc.ncbi.nlm.nih.gov). Patients with DCM who were homozygous for the Ala96 variant had a fourfold higher risk of malignant ventricular arrhythmias and sudden cardiac death compared to those homozygous for Ser96 (pmc.ncbi.nlm.nih.gov). Notably, this association was independent of other clinical risk factors (such as ejection fraction or presence of bundle branch block) (pmc.ncbi.nlm.nih.gov), highlighting Ser96Ala as an important prognostic marker in heart failure patients. The variant is relatively common in the general population – about 60% of individuals carry at least one Ser96Ala allele (heterozygously or homozygously) (pmc.ncbi.nlm.nih.gov). However, in healthy people the variant by itself is benign; it is only in the setting of stressed or failing hearts that the Ala96 allele confers susceptibility to arrhythmias (pmc.ncbi.nlm.nih.gov). In other words, Ser96Ala acts as a genetic modifier that can worsen Ca^2+ handling in a compromised heart, tipping the balance toward arrhythmogenesis.

From a molecular perspective, the Ser96 site in HRC is a key regulatory hotspot. Ser96 resides near the N-terminus of HRC, in a region that can be phosphorylated by a kinase called Fam20C (pmc.ncbi.nlm.nih.gov). Fam20C is a secretory pathway kinase that phosphorylates certain luminal proteins. Wild-type HRC (with Ser96) is a substrate for Fam20C-mediated phosphorylation, whereas the Ala96 variant cannot be phosphorylated at that site (pmc.ncbi.nlm.nih.gov). Studies in 2017–2018 elucidated how the loss of this phosphorylation site leads to Ca^2+ handling defects. Normally, phosphorylation of HRC at Ser96 appears to fine-tune HRC’s interactions with triadin and SERCA2a. In the Ser96Ala variant, HRC’s binding to triadin is weakened and its interaction with SERCA2a is altered (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The result is a combined derailment of SR Ca^2+ cycling: excessive Ca^2+ leak through RyR2 (because the HRC–triadin–RyR regulatory complex is impaired) and depressed Ca^2+ reuptake (because HRC-Ala96 binds abnormally strongly to SERCA2 and inhibits its activity) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This was demonstrated in cellular models where adenoviral expression of HRC-Ala96 led to more frequent spontaneous Ca^2+ sparks (a sign of RyR2 instability) and reduced maximal Ca^2+ uptake rates in cardiomyocytes (pmc.ncbi.nlm.nih.gov). Likewise, “humanized” knock-in mice carrying the HRC Ser96Ala mutation (or the analogous Ser81Ala in mice) exhibit heightened RyR2 activity, impaired Ca^2+ reserve, and increased propensity for arrhythmias under stress (pmc.ncbi.nlm.nih.gov). These mice showed significantly higher mortality by 10 months of age (50% mortality in mutant vs 15% in wild-type), likely due to spontaneous fatal arrhythmias (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Notably, young Ser96Ala knock-in mice often appear normal at baseline, but with age or stress they develop contractile dysfunction and arrhythmias, paralleling the human scenario in which the variant is unmasked in the context of heart failure (pmc.ncbi.nlm.nih.gov).

The discovery of the HRC Ser96Ala variant’s impact has practical implications. Given its prevalence, genetic screening for HRC Ser96Ala in patients with non-ischemic cardiomyopathy or a family history of sudden death may help identify those at higher risk of arrhythmic events (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Indeed, Ser96Ala is now considered a significant biomarker for arrhythmia risk in dilated cardiomyopathy – one study showed it was an independent predictor of life-threatening ventricular arrhythmias, meaning it adds prognostic information beyond standard clinical metrics (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This could influence patient management (for example, guiding decisions about implantable defibrillators or closer monitoring). Therapeutically, understanding the mechanism of the variant has opened ideas for intervention. For example, pharmacologically inhibiting CaMKII (Ca^2+/calmodulin-dependent protein kinase II) was found to reduce arrhythmias in a mouse model carrying the HRC Ser96Ala mutation (pmc.ncbi.nlm.nih.gov). The rationale is that aberrant Ca^2+ cycling in HRC-Ala96 hearts can lead to CaMKII hyperactivation (since CaMKII is sensitive to Ca^2+ oscillations), which further worsens RyR2 leak; blocking this feedback loop with a CaMKII inhibitor (like KN-93) helped stabilize the cardiac rhythm in experimental models (pmc.ncbi.nlm.nih.gov). While not a direct fix for the HRC defect, this approach underscores how downstream pathways of HRC dysfunction can be targeted to mitigate risk.

Aside from the Ser96Ala polymorphism, HRC has been occasionally implicated in other cardiac conditions. Some studies have linked rare HRC mutations or expression changes to conduction system diseases and arrhythmogenic cardiomyopathies (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). For instance, HRC was suggested as a candidate gene in familial sinus node dysfunction or conduction block in one report, though such cases are rare. In heart failure patients (of various etiologies), HRC protein levels are sometimes altered: one investigation in a canine heart failure model found HRC protein was upregulated in failing hearts, whereas calsequestrin levels remained normal (pmc.ncbi.nlm.nih.gov). This upregulation might be a compensatory response to impaired Ca^2+ handling, but it could also contribute to the abnormal Ca^2+ cycling in heart failure. Some heart failure studies in humans have noted correlations between high HRC levels and arrhythmia burden, though more research is needed. The pathophysiological significance of HRC is further highlighted by the HRC knockout mouse: under baseline conditions, HRC-KO mice are viable and have relatively normal cardiac function, but under stress (like pressure-overload induced by transverse aortic constriction) they develop severe cardiac pathology. Specifically, HRC knockout mice under pressure overload show exaggerated hypertrophy, fibrosis, pulmonary edema, and markedly reduced survival compared to wild-type mice (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Within just two weeks of pressure overload, the majority of HRC-null mice succumb or progress to heart failure, whereas wild-type mice tolerate this stress much better (pmc.ncbi.nlm.nih.gov). This dramatic outcome demonstrates that HRC is essential for maintaining calcium homeostasis and cardiac function under stress (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Without HRC, the heart cannot properly regulate SR Ca^2+ cycling when challenged, leading to lethal arrhythmias and contractile failure. Together, these human and animal studies firmly establish HRC as an important genetic and molecular factor in cardiac pathophysiology, especially in arrhythmogenesis and heart failure.

Emerging Research and Applications

Current research on HRC is extending our understanding of its functions and exploring potential clinical applications:

• Cardiac Biomarker and Therapeutic Target: As detailed above, the HRC Ser96Ala variant serves as a promising biomarker for arrhythmia risk in heart failure patients (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Clinically, this could be used to stratify patients – for example, identifying DCM patients who might benefit from prophylactic implantable defibrillators or more aggressive therapy if they carry the high-risk Ala/Ala genotype. In terms of therapy, HRC itself could be a target: because HRC regulates both SR calcium release and uptake, manipulating its function might improve Ca^2+ handling in diseased hearts. However, this is a double-edged sword – simply increasing HRC is not necessarily beneficial (overexpression impairs contractility (pmc.ncbi.nlm.nih.gov), and complete absence is detrimental under stress (pmc.ncbi.nlm.nih.gov)). The goal would be to normalize or optimize HRC activity. One idea is developing small molecules or peptides that enhance HRC’s beneficial interactions (e.g., promoting proper binding with triadin or SERCA2 under conditions like the Ser96Ala variant), thereby stabilizing Ca^2+ cycling. Another approach is targeting downstream effects: as mentioned, CaMKII inhibitors have shown efficacy in experimental models of HRC-related arrhythmia (pmc.ncbi.nlm.nih.gov). There is also interest in whether modifying Fam20C activity (the kinase for HRC Ser96) could be therapeutic – for instance, if a patient has Ser96Ala, could enhancing alternative phosphorylation sites or modulating Fam20C mitigate the effect? Though still preclinical, these concepts highlight HRC’s potential as a therapeutic intervention point in calcium-handling disorders. A 2013 study even suggested HRC as a “good target for heart failure” treatment, given its key role in maintaining SR Ca^2+ integrity (pmc.ncbi.nlm.nih.gov).

• HRC in Non-Muscle Tissues and Diseases: While classically a muscle protein, recent research has uncovered roles for HRC in other contexts, particularly in cancer biology. Some cancers appear to aberrantly upregulate HRC, taking advantage of its calcium-modulating abilities. For example, gastric cancer tissues have significantly elevated HRC expression compared to normal stomach tissue, and higher HRC levels correlate with worse patient survival (pmc.ncbi.nlm.nih.gov). Functional experiments showed that HRC promotes cancer cell proliferation, migration, and invasion, whereas knocking down HRC impairs these malignant behaviors (pmc.ncbi.nlm.nih.gov). Mechanistically, HRC in cancer cells was found to increase intracellular Ca^2+ levels and activate Ca^2+/calmodulin-dependent signaling pathways that drive cell growth. In gastric cancer models, HRC overexpression led to activation of the Raf/MEK/ERK pathway (a key pathway for cell proliferation and epithelial-mesenchymal transition) through a Ca^2+- and calmodulin-dependent mechanism (pmc.ncbi.nlm.nih.gov). HRC knockdown, in turn, reduced Ca^2+-dependent activation of this pathway and suppressed metastasis-related traits (pmc.ncbi.nlm.nih.gov). Similar findings were reported in hepatocellular carcinoma, where HRC was seen to promote tumor metastasis and was upregulated by the oncogenic transcription factor SATB1 (pubmed.ncbi.nlm.nih.gov). These studies suggest that HRC’s function as a Ca^2+ buffer can be co-opted by cancer cells to alter Ca^2+ signaling and gene expression, thereby facilitating tumor progression. While this is an emerging area, it positions HRC as a potential target for anti-cancer therapy in tumors that depend on dysregulated calcium signaling (pmc.ncbi.nlm.nih.gov). It is a striking example of how a calcium-handling protein primarily known for its role in muscle contraction can influence diseases as different as heart failure and cancer.

• Ongoing and Future Research Directions: Scientists continue to investigate HRC to answer remaining questions. One active area is understanding structure-function relationships – for instance, solving HRC’s three-dimensional structure (or portions of it) would greatly clarify how its histidine-rich repeats bind Ca^2+ and interact with partners. Efforts using advanced techniques like cryo-electron microscopy or NMR are underway, though the protein’s size and flexibility pose challenges (pmc.ncbi.nlm.nih.gov). Another focus is delineating post-translational modifications of HRC: besides Ser96 phosphorylation, are there other phosphorylation sites or modifications (like glycosylation or oxidation of the cysteine-rich domain) that regulate HRC’s function? Early phosphoproteomic data indicate HRC can be phosphorylated at additional sites in muscles, which might affect its Ca^2+ binding capacity or binding to triadin/SERCA. Understanding these modifications could open new strategies to modulate HRC activity. Researchers are also examining HRC in different muscle types – for instance, in fast vs. slow twitch skeletal muscle, or in atrial vs. ventricular myocardium – to see if its role differs depending on the muscle’s contractile properties. There is evidence that HRC expression can change in skeletal muscle adaptations and in smooth muscle during vascular disease, suggesting a broader physiological importance. Finally, the interplay between HRC and other Ca^2+ handling proteins (like calsequestrin, calreticulin, and luminal Ca^2+ sensors) is being explored via genetic models (double knockouts and overexpression systems) to map out the network of luminal Ca^2+ regulation. For example, the surprising result that removing HRC can rescue arrhythmias caused by calsequestrin deficiency (pubmed.ncbi.nlm.nih.gov) has prompted deeper investigation into how these proteins functionally compensate for each other.

In conclusion, HRC (histidine-rich calcium-binding protein) is a crucial regulator of intracellular calcium in muscle cells, serving as a Ca^2+ storage protein and a modulator of calcium release and uptake in the sarcoplasmic reticulum. Its proper function ensures effective and rhythmic muscle contraction. Disruption of HRC – by genetic variation or altered expression – can destabilize calcium homeostasis, leading to arrhythmias, contractile dysfunction, or other pathology. Modern research, from 2010 through 2024, has cemented HRC’s role in cardiac physiology and disease, and has even uncovered roles in cancer cell biology. This knowledge is now being translated into potential real-world applications, such as genetic risk screening for heart failure patients (using the HRC Ser96Ala variant as a biomarker) and consideration of HRC-centered strategies to treat arrhythmias or certain cancers. As our understanding of HRC’s mechanism deepens, it exemplifies how a single protein at the heart of calcium signaling can have wide-ranging impacts on human health, making it a fascinating subject of ongoing study (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

References: (Publication dates and sources provided where available)

1. Arvanitis, D. A., et al. (2018). Frontiers in Physiology, 9:1379 (Sept 27, 2018) – Comprehensive review on HRC’s role in cardiac rhythmicity (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
2. Park, C. S., et al. (2013). Basic Res Cardiol, 108(3):344 (Apr 2013) – HRC knockout mouse study showing impaired Ca^2+ cycling and heart failure under stress (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
3. Kranias, E. G., et al. (2010). J. Mol. Cell. Cardiol. 50(1):43–49 (Aug 2010) – Identified HRC as a regulator of SR Ca^2+ uptake and release; described HRC interactions with triadin and SERCA2 (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
4. Tzimas, C., et al. (2017). Journal of Biological Chemistry 292(37):15743-15754 (2017) – Demonstrated that HRC Ser96Ala (mouse Ser81Ala) knock-in leads to arrhythmias and early death, implicating defective phosphorylation (via Fam20C) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
5. Kontrogianni-Konstantopoulos, A., et al. (2008). Eur. Heart J. 29:2514–2525 (2008) – First report linking HRC Ser96Ala variant with malignant arrhythmias in DCM patients (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
6. Wang, C., et al. (2022). Journal of Cancer 13(4):1073–1085 (Jan 4, 2022) – Found that HRC is upregulated in gastric cancer and drives proliferation and metastasis via Ca^2+-mediated ERK signaling (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
7. Hofmann, S. L., et al. (1989). Biochem J. 264: 31–42 (1989) – Original purification and characterization of the 170 kDa histidine-rich Ca^2+-binding protein from rabbit muscle SR, which led to the cloning of HRC (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Citations

1. AnnotationURLCitation(end_index=431, start_index=284, title='HRC histidine rich calcium binding protein [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene/3270#:~:text=Summary%20This%20gene%20encodes%20a,provided%20by%20RefSeq%2C%20Sep%202008')
2. AnnotationURLCitation(end_index=752, start_index=620, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=Histidine,the%20protein%20composed%20of%20acidic')
3. AnnotationURLCitation(end_index=1033, start_index=901, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=Histidine,the%20protein%20composed%20of%20acidic')
4. AnnotationURLCitation(end_index=1295, start_index=1148, title='HRC histidine rich calcium binding protein [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene/3270#:~:text=Summary%20This%20gene%20encodes%20a,provided%20by%20RefSeq%2C%20Sep%202008')
5. AnnotationURLCitation(end_index=1898, start_index=1751, title='HRC histidine rich calcium binding protein [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene/3270#:~:text=Summary%20This%20gene%20encodes%20a,provided%20by%20RefSeq%2C%20Sep%202008')
6. AnnotationURLCitation(end_index=2242, start_index=2109, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=The%20histidine,ATPase%2C%20as%20shown%20by%20HRC')
7. AnnotationURLCitation(end_index=2375, start_index=2243, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=remains%20tightly%20attached%20to%20the,terminal')
8. AnnotationURLCitation(end_index=2806, start_index=2703, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=AA%29,23%20%2C%2038')
9. AnnotationURLCitation(end_index=2955, start_index=2807, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=HRC%20lacks%20a%20typical%20Ca,This%20is%20highly%20similar%20to')
10. AnnotationURLCitation(end_index=3213, start_index=3095, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=of%20ten%20histidine,23%20%2C%2038')
11. AnnotationURLCitation(end_index=3592, start_index=3444, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=HRC%20lacks%20a%20typical%20Ca,This%20is%20highly%20similar%20to')
12. AnnotationURLCitation(end_index=3947, start_index=3815, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=remains%20tightly%20attached%20to%20the,terminal')
13. AnnotationURLCitation(end_index=4210, start_index=4079, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=form%20Ca,However%2C%20HRC%20can%20be%20readily')
14. AnnotationURLCitation(end_index=4383, start_index=4211, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=The%20presence%20of%20repetitive%20elements%2C,multimerization%20states%20of%20HRC%20and')
15. AnnotationURLCitation(end_index=4723, start_index=4551, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=The%20presence%20of%20repetitive%20elements%2C,multimerization%20states%20of%20HRC%20and')
16. AnnotationURLCitation(end_index=5106, start_index=4974, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=remains%20tightly%20attached%20to%20the,terminal')
17. AnnotationURLCitation(end_index=5524, start_index=5355, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=extracted%20with%201%20mM%20EDTA%2C,through%20positively%20and%20negatively%20charged')
18. AnnotationURLCitation(end_index=5909, start_index=5773, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=than%20Ca,by%20different%20ion%20saturation%20states')
19. AnnotationURLCitation(end_index=6162, start_index=6022, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=for%20protein,by%20different%20ion%20saturation%20states')
20. AnnotationURLCitation(end_index=6487, start_index=6356, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=form%20Ca,However%2C%20HRC%20can%20be%20readily')
21. AnnotationURLCitation(end_index=6654, start_index=6488, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=calsequestrin%20and%20the%20condensed%20formation,2%2B%7D%20%5B14%5D.%20Zinc%20has')
22. AnnotationURLCitation(end_index=7066, start_index=6924, title='HRC histidine rich calcium binding protein [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene/3270#:~:text=interacts%20with%20the%20cytoplasmic%20domain,Orthologs%20%2013%20all')
23. AnnotationURLCitation(end_index=7320, start_index=7188, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=Histidine,the%20protein%20composed%20of%20acidic')
24. AnnotationURLCitation(end_index=7713, start_index=7566, title='HRC histidine rich calcium binding protein [Homo sapiens (human)] - Gene - NCBI', type='url_citation', url='https://www.ncbi.nlm.nih.gov/gene/3270#:~:text=Summary%20This%20gene%20encodes%20a,provided%20by%20RefSeq%2C%20Sep%202008')
25. AnnotationURLCitation(end_index=8503, start_index=8347, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=interacting%20with%20calsequestrin%20and%20the,5%5D.%20It%20seems%20that')
26. AnnotationURLCitation(end_index=8801, start_index=8645, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=interacting%20with%20calsequestrin%20and%20the,5%5D.%20It%20seems%20that')
27. AnnotationURLCitation(end_index=8939, start_index=8802, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=quaternary%20,2%2B%7D%20cycling%20and%20contractility')
28. AnnotationURLCitation(end_index=9436, start_index=9279, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=match%20at%20L225%20quaternary%20,2%2B%7D%20cycling%20and%20contractility')
29. AnnotationURLCitation(end_index=9736, start_index=9603, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=with%20triadin,plays%20an%20important%20role%20in')
30. AnnotationURLCitation(end_index=10307, start_index=10174, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=The%20histidine,ATPase%2C%20as%20shown%20by%20HRC')
31. AnnotationURLCitation(end_index=11166, start_index=11000, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=phosphorylation%20of%20HRC%20affects%20ryanodine,ATPase%2C%20as%20shown%20by%20HRC')
32. AnnotationURLCitation(end_index=11641, start_index=11508, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=The%20histidine,ATPase%2C%20as%20shown%20by%20HRC')
33. AnnotationURLCitation(end_index=12073, start_index=11918, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=match%20at%20L541%20%28Figure%203d%29,cells%20compared%20to%20the%20WTs')
34. AnnotationURLCitation(end_index=12248, start_index=12074, title='Ablation of HRC alleviates cardiac arrhythmia and improves abnormal Ca handling in CASQ2 knockout mice prone to CPVT - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/26410369/#:~:text=Conclusion%3A%20Our%20results%20suggest%20that,enhances%20RyR2%20activity%20facilitating%20RyR2')
35. AnnotationURLCitation(end_index=12644, start_index=12478, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=phosphorylation%20of%20HRC%20affects%20ryanodine,ATPase%2C%20as%20shown%20by%20HRC')
36. AnnotationURLCitation(end_index=12808, start_index=12645, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=HRC%20overexpression%20in%20isolated%20rodent,ATPase%2C%20as%20shown%20by%20HRC')
37. AnnotationURLCitation(end_index=13133, start_index=12967, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=phosphorylation%20of%20HRC%20affects%20ryanodine,ATPase%2C%20as%20shown%20by%20HRC')
38. AnnotationURLCitation(end_index=13869, start_index=13736, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=with%20triadin,plays%20an%20important%20role%20in')
39. AnnotationURLCitation(end_index=14115, start_index=13982, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=with%20triadin,plays%20an%20important%20role%20in')
40. AnnotationURLCitation(end_index=14557, start_index=14414, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=with%20triadin,Interestingly%2C%20HRC%20has%20been%20linked')
41. AnnotationURLCitation(end_index=15000, start_index=14845, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=match%20at%20L541%20%28Figure%203d%29,cells%20compared%20to%20the%20WTs')
42. AnnotationURLCitation(end_index=15141, start_index=15001, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=pulmonary%20edema%20occurred%20in%20HRC,KO%20mice%20show')
43. AnnotationURLCitation(end_index=15544, start_index=15411, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=with%20triadin,plays%20an%20important%20role%20in')
44. AnnotationURLCitation(end_index=15720, start_index=15545, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=dysfunction%2C%20fibrosis%20and%20pulmonary%20edema,target%20for%20heart%20failure%2C%20and')
45. AnnotationURLCitation(end_index=16283, start_index=16109, title='Ablation of HRC alleviates cardiac arrhythmia and improves abnormal Ca handling in CASQ2 knockout mice prone to CPVT - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/26410369/#:~:text=Conclusion%3A%20Our%20results%20suggest%20that,enhances%20RyR2%20activity%20facilitating%20RyR2')
46. AnnotationURLCitation(end_index=17108, start_index=16934, title='Ablation of HRC alleviates cardiac arrhythmia and improves abnormal Ca handling in CASQ2 knockout mice prone to CPVT - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/26410369/#:~:text=Conclusion%3A%20Our%20results%20suggest%20that,enhances%20RyR2%20activity%20facilitating%20RyR2')
47. AnnotationURLCitation(end_index=17506, start_index=17332, title='Ablation of HRC alleviates cardiac arrhythmia and improves abnormal Ca handling in CASQ2 knockout mice prone to CPVT - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/26410369/#:~:text=Conclusion%3A%20Our%20results%20suggest%20that,enhances%20RyR2%20activity%20facilitating%20RyR2')
48. AnnotationURLCitation(end_index=18031, start_index=17857, title='Ablation of HRC alleviates cardiac arrhythmia and improves abnormal Ca handling in CASQ2 knockout mice prone to CPVT - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/26410369/#:~:text=Conclusion%3A%20Our%20results%20suggest%20that,enhances%20RyR2%20activity%20facilitating%20RyR2')
49. AnnotationURLCitation(end_index=18749, start_index=18578, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=match%20at%20L320%20and%20junctin%2C,fold%20overexpression%20of%20calsequestrin%20%5B67')
50. AnnotationURLCitation(end_index=19105, start_index=18930, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=dysfunction%2C%20fibrosis%20and%20pulmonary%20edema,target%20for%20heart%20failure%2C%20and')
51. AnnotationURLCitation(end_index=19618, start_index=19487, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=,MEF2%20during%20cardiac%2C%20skeletal%2C%20and')
52. AnnotationURLCitation(end_index=20117, start_index=19948, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=supporting%20a%20dual%20role%20of,homeostasis%2C%20suggesting%20its%20importance%20in')
53. AnnotationURLCitation(end_index=20502, start_index=20368, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=HRC,together%2C%20HRC%20ablation%20increased%20the')
54. AnnotationURLCitation(end_index=20966, start_index=20784, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=calsequestrin%2C%20before%20and%20after%20ischemia%2C,favour%20the%20integrity%20of%20mitochondria')
55. AnnotationURLCitation(end_index=21318, start_index=21136, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=calsequestrin%2C%20before%20and%20after%20ischemia%2C,favour%20the%20integrity%20of%20mitochondria')
56. AnnotationURLCitation(end_index=22071, start_index=21878, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=hypertrophy%2C%20fibrosis%2C%20pulmonary%20edema%20and,homeostasis%20especially%20under%20stress%20conditions')
57. AnnotationURLCitation(end_index=22212, start_index=22072, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=pulmonary%20edema%20occurred%20in%20HRC,KO%20mice%20show')
58. AnnotationURLCitation(end_index=23781, start_index=23608, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=In%202008%20our%20group%20was,heart%20failure%2C%20including%20implantable%20cardioverter')
59. AnnotationURLCitation(end_index=24135, start_index=23975, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=variant%20Ser96Ala%20was%20found%20to,homozygotes%20for%20the%20Ser%20allele')
60. AnnotationURLCitation(end_index=24395, start_index=24273, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=match%20at%20L134%20The%20HRC,of%20the')
61. AnnotationURLCitation(end_index=24789, start_index=24642, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=fraction%2C%20atrial%20fibrillation%2C%20left%20bundle,of%20the')
62. AnnotationURLCitation(end_index=25118, start_index=24971, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=fraction%2C%20atrial%20fibrillation%2C%20left%20bundle,of%20the')
63. AnnotationURLCitation(end_index=25638, start_index=25469, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=elucidated%20by%20studies%20at%20the,to%20the%20impaired%20HRC%2FSERCA2%20interaction')
64. AnnotationURLCitation(end_index=26043, start_index=25874, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=elucidated%20by%20studies%20at%20the,to%20the%20impaired%20HRC%2FSERCA2%20interaction')
65. AnnotationURLCitation(end_index=26544, start_index=26370, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=cycling%2C%20by%20binding%20and%20storing,to%20the%20impaired%20HRC%2FSERCA2%20interaction')
66. AnnotationURLCitation(end_index=26709, start_index=26545, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=elucidated%20by%20studies%20at%20the,HRC%20Ser96Ala%20mouse%20model%2C%20reduced')
67. AnnotationURLCitation(end_index=27156, start_index=26982, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=cycling%2C%20by%20binding%20and%20storing,to%20the%20impaired%20HRC%2FSERCA2%20interaction')
68. AnnotationURLCitation(end_index=27307, start_index=27157, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=Indeed%2C%20acute%20overexpression%20of%20the,Importantly%2C%20HRC')
69. AnnotationURLCitation(end_index=27673, start_index=27523, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=Indeed%2C%20acute%20overexpression%20of%20the,Importantly%2C%20HRC')
70. AnnotationURLCitation(end_index=28076, start_index=27898, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=match%20at%20L398%20Ser96Ala%20increased,exhibited%20increased%20RyR2%20activity%2C%20enhanced')
71. AnnotationURLCitation(end_index=28405, start_index=28238, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=human%20Ser96Ala%20mutation%20directly%20in,abnormalities%20were%20noted%20in%20the')
72. AnnotationURLCitation(end_index=28558, start_index=28406, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=match%20at%20L264%20patient%20population,This%20work%20revealed%20an')
73. AnnotationURLCitation(end_index=28966, start_index=28804, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=far%20as%203%E2%80%935%20months%20of,types%2C%20whilst%20the%20Ser81Ala%20mice')
74. AnnotationURLCitation(end_index=29423, start_index=29250, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=In%202008%20our%20group%20was,heart%20failure%2C%20including%20implantable%20cardioverter')
75. AnnotationURLCitation(end_index=29571, start_index=29424, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=fraction%2C%20atrial%20fibrillation%2C%20left%20bundle,of%20the')
76. AnnotationURLCitation(end_index=30012, start_index=29852, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=variant%20Ser96Ala%20was%20found%20to,homozygotes%20for%20the%20Ser%20allele')
77. AnnotationURLCitation(end_index=30147, start_index=30013, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=The%20HRC%20Ser96Ala%20polymorphism%20was,of%20the')
78. AnnotationURLCitation(end_index=30686, start_index=30556, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=ATPase%20,the%20setting%20of%20heart%20failure')
79. AnnotationURLCitation(end_index=31126, start_index=30996, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=ATPase%20,the%20setting%20of%20heart%20failure')
80. AnnotationURLCitation(end_index=31662, start_index=31523, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=Ca%5E%7B2%2B%7D,in%20the%20background%20of%20idiopathic')
81. AnnotationURLCitation(end_index=31799, start_index=31663, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=in%20malignant%20ventricular%20arrhythmias%20may,The')
82. AnnotationURLCitation(end_index=32317, start_index=32196, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=match%20at%20L397%20canine%20model,74')
83. AnnotationURLCitation(end_index=33301, start_index=33108, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=hypertrophy%2C%20fibrosis%2C%20pulmonary%20edema%20and,homeostasis%20especially%20under%20stress%20conditions')
84. AnnotationURLCitation(end_index=33441, start_index=33302, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=survival%20rate%20for%20the%204,KO%20TAC.%20%2AP%3C0.05')
85. AnnotationURLCitation(end_index=33750, start_index=33611, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=survival%20rate%20for%20the%204,KO%20TAC.%20%2AP%3C0.05')
86. AnnotationURLCitation(end_index=34043, start_index=33883, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=decreased%20survival%20after%20TAC,the%20integrity%20of%20cardiac%20function')
87. AnnotationURLCitation(end_index=34184, start_index=34044, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=pulmonary%20edema%20occurred%20in%20HRC,KO%20mice%20show')
88. AnnotationURLCitation(end_index=35003, start_index=34843, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=variant%20Ser96Ala%20was%20found%20to,homozygotes%20for%20the%20Ser%20allele')
89. AnnotationURLCitation(end_index=35138, start_index=35004, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=The%20HRC%20Ser96Ala%20polymorphism%20was,of%20the')
90. AnnotationURLCitation(end_index=35852, start_index=35686, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=phosphorylation%20of%20HRC%20affects%20ryanodine,ATPase%2C%20as%20shown%20by%20HRC')
91. AnnotationURLCitation(end_index=36043, start_index=35903, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=pulmonary%20edema%20occurred%20in%20HRC,KO%20mice%20show')
92. AnnotationURLCitation(end_index=36621, start_index=36491, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=ATPase%20,the%20setting%20of%20heart%20failure')
93. AnnotationURLCitation(end_index=37320, start_index=37145, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=dysfunction%2C%20fibrosis%20and%20pulmonary%20edema,target%20for%20heart%20failure%2C%20and')
94. AnnotationURLCitation(end_index=37897, start_index=37782, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=Histidine,In%20terms%20of%20the')
95. AnnotationURLCitation(end_index=38235, start_index=38063, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=upregulated%20in%20human%20gastric%20cancer,2%2B%7D%2FCaM%20signaling%20and%20ultimately')
96. AnnotationURLCitation(end_index=38783, start_index=38629, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=promoted%20GC%20cell%20migration%2C%20invasion%2C,In%20summary%2C%20we')
97. AnnotationURLCitation(end_index=39073, start_index=38901, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=upregulated%20in%20human%20gastric%20cancer,2%2B%7D%2FCaM%20signaling%20and%20ultimately')
98. AnnotationURLCitation(end_index=39394, start_index=39250, title='The histidine-rich calcium binding protein (HRC) promotes tumor metastasis in hepatocellular carcinoma and is upregulated by SATB1 - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/25762622/#:~:text=The%20histidine,binding%20protein%20of%20sarcoplasmic%20reticulum')
99. AnnotationURLCitation(end_index=39900, start_index=39729, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=mechanism%2C%20knockdown%20of%20HRC%20reduced,potential%20target%20for%20GC%20treatment')
100. AnnotationURLCitation(end_index=40717, start_index=40577, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=for%20protein,by%20different%20ion%20saturation%20states')
101. AnnotationURLCitation(end_index=42196, start_index=42022, title='Ablation of HRC alleviates cardiac arrhythmia and improves abnormal Ca handling in CASQ2 knockout mice prone to CPVT - PubMed', type='url_citation', url='https://pubmed.ncbi.nlm.nih.gov/26410369/#:~:text=Conclusion%3A%20Our%20results%20suggest%20that,enhances%20RyR2%20activity%20facilitating%20RyR2')
102. AnnotationURLCitation(end_index=43602, start_index=43427, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=dysfunction%2C%20fibrosis%20and%20pulmonary%20edema,target%20for%20heart%20failure%2C%20and')
103. AnnotationURLCitation(end_index=43774, start_index=43603, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=mechanism%2C%20knockdown%20of%20HRC%20reduced,potential%20target%20for%20GC%20treatment')
104. AnnotationURLCitation(end_index=44131, start_index=43999, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=et%20al,for%20the%20treatment%20of%20arrhythmias')
105. AnnotationURLCitation(end_index=44296, start_index=44132, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=elucidated%20by%20studies%20at%20the,HRC%20Ser96Ala%20mouse%20model%2C%20reduced')
106. AnnotationURLCitation(end_index=44602, start_index=44463, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=survival%20rate%20for%20the%204,KO%20TAC.%20%2AP%3C0.05')
107. AnnotationURLCitation(end_index=44778, start_index=44603, title='Targeted ablation of the histidine-rich Ca2+-binding protein (HRC) gene is associated with abnormal SR Ca2+-cycling and severe pathology under pressure-overload stress - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC4000265/#:~:text=dysfunction%2C%20fibrosis%20and%20pulmonary%20edema,target%20for%20heart%20failure%2C%20and')
108. AnnotationURLCitation(end_index=45112, start_index=44979, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=The%20histidine,ATPase%2C%20as%20shown%20by%20HRC')
109. AnnotationURLCitation(end_index=45246, start_index=45113, title='Histidine-Rich Calcium Binding Protein: the New Regulator of Sarcoplasmic Reticulum Calcium Cycling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC3018531/#:~:text=with%20triadin,plays%20an%20important%20role%20in')
110. AnnotationURLCitation(end_index=45656, start_index=45489, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=human%20Ser96Ala%20mutation%20directly%20in,abnormalities%20were%20noted%20in%20the')
111. AnnotationURLCitation(end_index=45817, start_index=45657, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=match%20at%20L476%20for%20HRC,phosphorylation%20in%20arrhythmia%20generation')
112. AnnotationURLCitation(end_index=46169, start_index=45996, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=In%202008%20our%20group%20was,heart%20failure%2C%20including%20implantable%20cardioverter')
113. AnnotationURLCitation(end_index=46330, start_index=46170, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=variant%20Ser96Ala%20was%20found%20to,homozygotes%20for%20the%20Ser%20allele')
114. AnnotationURLCitation(end_index=46650, start_index=46535, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=Histidine,In%20terms%20of%20the')
115. AnnotationURLCitation(end_index=46805, start_index=46651, title='Histidine-rich calcium binding protein promotes gastric cancer cell proliferation, migration, invasion and epithelial-mesenchymal transition through Raf/MEK/ERK signaling - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC8899383/#:~:text=promoted%20GC%20cell%20migration%2C%20invasion%2C,In%20summary%2C%20we')
116. AnnotationURLCitation(end_index=47158, start_index=47026, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=Histidine,the%20protein%20composed%20of%20acidic')
117. AnnotationURLCitation(end_index=47313, start_index=47159, title='The Histidine-Rich Calcium Binding Protein in Regulation of Cardiac Rhythmicity - PMC', type='url_citation', url='https://pmc.ncbi.nlm.nih.gov/articles/PMC6171002/#:~:text=sarcoplasmic%20reticulum%20and%20biochemical%20characterization,Google')