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this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.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.
The requested target is unambiguously the human GHSR gene product, growth hormone secretagogue receptor type 1a (GHSR1a, ghrelin receptor; UniProt Q92847). Literature identity, organism, ligand, and molecular architecture all match the supplied record: GHSR1a is a 366-amino-acid, approximately 41-kDa, rhodopsin-like class-A seven-transmembrane G-protein-coupled receptor (GPCR). Its principal function is not enzymatic catalysis or transport, but conversion of extracellular energy-state signals—especially Ser3-octanoylated ghrelin—into intracellular G-protein and β-arrestin signaling. No conflicting similarly named gene or non-human protein was encountered. (bennett2009aninvestigationinto pages 18-21, yin2014thegrowthhormone pages 1-3, gross2023ghrelinreceptorsignaling pages 1-3)
The receptor’s best-established direct outputs are stimulation of the hypothalamic–pituitary growth hormone (GH) axis and activation of hypothalamic feeding circuits. At the molecular level it preferentially couples to Gq/11–PLC–IP3/Ca²⁺, but can also engage Gi/o, G12/13, Gβγ and β-arrestins. It is unusual among GPCRs because it exhibits basal, ligand-independent activity of roughly 50% of maximal agonist signaling in common recombinant assays. The endogenous peptide LEAP2 opposes both ghrelin-stimulated and constitutive receptor activity. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 3-4, polishchuk2025beyondhungerthe pages 21-23)
| Feature | Current annotation | Best evidence/qualification |
|---|---|---|
| Identity | Human GHSR encodes the growth hormone secretagogue receptor (ghrelin receptor); target accession UniProt Q92847. | Literature consistently identifies human GHSR/GHSR1a as the ghrelin-binding receptor originally cloned from pituitary and hypothalamus; no conflicting gene identity was found. (bennett2009aninvestigationinto pages 18-21, gross2023ghrelinreceptorsignaling pages 1-3) |
| Canonical and truncated isoforms | GHSR1a is the functional full-length receptor: 366 aa, approximately 41 kDa, with seven transmembrane helices. GHSR1b is a truncated splice product containing only TM1–TM5 and a unique 24-aa tail; it does not bind/respond normally to ghrelin and can reduce GHSR1a surface expression and signaling. | Sources disagree on the reported total length of GHSR1b (289 versus 298 aa), but agree on its five-TM, signaling-incompetent character; GHSR1a–GHSR1b heteromerization can stabilize a non-signaling state. (bennett2009aninvestigationinto pages 18-21, yin2014thegrowthhormone pages 1-3, gross2023ghrelinreceptorsignaling pages 15-16) |
| Molecular class and localization | Rhodopsin-like class A GPCR and integral plasma-membrane receptor with an extracellular N-terminus, seven-TM bundle, intracellular loops, and cytoplasmic C-terminus. | The UniProt domain assignments—GHS-R/MTLR, rhodopsin GPCR, and 7tm_1—align with structural literature. Activated receptors undergo GRK/β-arrestin-dependent desensitization and clathrin-mediated internalization; localization is dynamically regulated rather than exclusively surface-bound. (yin2014thegrowthhormone pages 1-3, gross2023ghrelinreceptorsignaling pages 1-3) |
| Endogenous agonist and specificity | Cognate agonist is acyl-ghrelin, a 28-aa peptide bearing an octanoyl group on Ser3. Its N-terminal five residues form the minimal activation region. Desacyl-ghrelin has affinity lower by several orders of magnitude and is generally inactive through GHSR1a at sub-micromolar concentrations. | Ser3 octanoylation is essential for high-affinity, nanomolar-range activation; GOAT installs the lipid modification. The peptide N-terminus and octanoyl chain occupy separate receptor cavities. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 3-4, gross2023ghrelinreceptorsignaling pages 6-8, gross2023ghrelinreceptorsignaling pages 1-3) |
| Endogenous antagonist/inverse agonist | LEAP2 is an endogenous orthosteric antagonist and inverse agonist that opposes ghrelin and suppresses ligand-independent receptor activity. | LEAP2 is especially relevant in the fed state and positive energy balance, but its tissue-specific physiological contribution in humans remains less resolved than its cellular pharmacology. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 14-15, gross2023ghrelinreceptorsignaling pages 1-3) |
| Constitutive activity | GHSR1a has unusually high ligand-independent signaling—approximately 50% of the maximal ghrelin response in commonly used heterologous assays. | Basal activity is intrinsic to purified receptor but its magnitude is cell-context dependent; MRAP2, GHSR1b, and inverse agonists can suppress it. Human variants selectively reducing constitutive activity support physiological relevance to growth. (gross2023ghrelinreceptorsignaling pages 3-4, yin2014thegrowthhormone pages 12-15) |
| Major signaling | Preferential coupling is Gq/11 → PLC → IP3/DAG → intracellular Ca²⁺ → PKC/CaMKII/AMPK. Context-dependent coupling also occurs through Gi/o, G12/13, Gβγ, and β-arrestins; β-arrestin supports desensitization, trafficking, and signaling such as ERK/AKT. | Gq signaling is strongly linked to GH secretion and feeding; Gi/o contributes to pancreatic insulinostasis. Ligands, receptor variants, and accessory proteins can bias G-protein versus β-arrestin output. (gross2023ghrelinreceptorsignaling pages 3-4, polishchuk2025beyondhungerthe pages 21-23, gross2023ghrelinreceptorsignaling pages 18-23) |
| Structure | GHSR1a has a bifurcated orthosteric pocket separated by a Glu124–Arg283 ionic lock: cavity I recognizes ghrelin’s N-terminus and cavity II accommodates the Ser3-octanoyl group. Activation disrupts the lock, caps the pocket with ECL2, and opens the intracellular G-protein interface. | Structures include antagonist-bound GHSR1a at 3.3 Å, Gq complexes at 2.9–3.2 Å, and ghrelin/ibutamoren–Gi complexes at 2.7 Å. Agonism moves intracellular TM6 outward by about 12.1 Å; inverse agonism moves it inward by about 8.3 Å. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 8-9, gross2023ghrelinreceptorsignaling pages 9-11) |
| Principal cellular sites | Strongest established sites include anterior-pituitary somatotroph-associated signaling; hypothalamic arcuate AgRP/NPY and GHRH neurons; pancreatic islet δ cells, with lower expression in β and α cells; and dopaminergic VTA/SNc neurons. | Human tissue expression outside pituitary and selected endocrine tissues should be interpreted cautiously because some localization maps derive from rodents, reporter mice, or transcript detection rather than validated human receptor protein. (gross2023ghrelinreceptorsignaling pages 4-6, gross2023ghrelinreceptorsignaling pages 6-8, gross2023ghrelinreceptorsignaling pages 16-18) |
| Core physiological outputs | GHSR activation stimulates pulsatile GH release and downstream GH–IGF-1 physiology, drives hunger through arcuate feeding circuits, inhibits glucose-stimulated insulin secretion during energy deficit, promotes gastric emptying, and modulates reward-related dopamine signaling. | GH release and appetite are the best-established outputs. Pancreatic, gastrointestinal, and reward effects are pathway- and cell-context dependent; broad systemic claims should not automatically be assigned as direct receptor actions in every tissue. (gross2023ghrelinreceptorsignaling pages 4-6, gross2023ghrelinreceptorsignaling pages 6-8) |
| Human genetics | Rare loss-of-function or signaling-selective variants—including Ala204Glu/A203E-related nomenclature and Phe279Leu—have been associated with familial short stature and, in some reports, abnormal body weight. | Functional assays indicate that impaired constitutive signaling can occur despite retained ligand responsiveness, providing human evidence that basal GHSR activity contributes to growth regulation. Common GHSR variation has not shown a consistent obesity association. (yin2014thegrowthhormone pages 12-15, gross2023ghrelinreceptorsignaling pages 14-15) |
| Clinical applications | Macimorelin, an oral GHSR agonist, is used as a GH-stimulation test for adult GH deficiency. Its pivotal phase 3 crossover study enrolled 157 participants and used 0.5 mg/kg with a 2.8 ng/mL peak-GH cutoff. A pediatric phase 3 study completed in June 2024, enrolling 101 participants and testing 1.0 mg/kg. Anamorelin has been evaluated at 100 mg/day for cancer cachexia in ROMANA 1 (484), ROMANA 2 (495), and ROMANA 3 (513) participants. | Macimorelin represents direct diagnostic implementation of receptor agonism. Anamorelin increased the clinical evidence base for appetite/lean-mass modulation, but functional outcomes such as handgrip strength were co-primary endpoints and therapeutic authorization varies by jurisdiction. (NCT02558829 chunk 1, NCT02558829 chunk 2, NCT04786873 chunk 1, NCT01395914 chunk 1, NCT01387282 chunk 1, NCT01387269 chunk 1) |
| Key uncertainty | The receptor’s precise β-arrestin-specific physiological outputs, significance of homo/heterodimers in native human tissues, regulation by GHSR1b, and activity in BBB-protected brain regions remain incompletely resolved. | Experts emphasize that signaling bias observed in recombinant systems may reflect ligand bias, receptor variants, or tissue-specific system bias; definitive human cell-type protein maps and biased-receptor structures are still needed. (gross2023ghrelinreceptorsignaling pages 3-4, gross2023ghrelinreceptorsignaling pages 18-23, gross2023ghrelinreceptorsignaling pages 9-11) |
Table: A compact evidence-based summary of human GHSR identity, molecular pharmacology, signaling, localization, physiology, genetics, and clinical translation. Qualifications distinguish well-established findings from isoform, tissue-localization, and pathway-specific uncertainties.
The human GHSR locus is reported at chromosome 3q26.2. Alternative transcript processing generates the full-length functional receptor GHSR1a and a truncated product, GHSR1b. GHSR1a has seven α-helical membrane-spanning segments, extracellular N-terminus and loops, and intracellular loops/C-terminus that contact signaling proteins. This agrees with the supplied InterPro/Pfam assignments—GHS-R/MTLR, GPCR_Rhodpsn, GPCR_Rhodpsn_7TM and 7tm_1—and establishes that Q92847 belongs to the rhodopsin-like class-A GPCR family. (bennett2009aninvestigationinto pages 18-21, yin2014thegrowthhormone pages 1-3, gross2023ghrelinreceptorsignaling pages 1-3)
The receptor was initially cloned as an orphan growth-hormone-secretagogue receptor from pituitary/hypothalamic tissue in 1996; the 1999 discovery of ghrelin established its endogenous ligand. The modern term “ghrelin receptor” and older names “GH-releasing peptide receptor” and “GHS-R” therefore refer to the same functional human receptor in this report. (bennett2009aninvestigationinto pages 18-21, gross2023ghrelinreceptorsignaling pages 14-15)
GHSR1a is the ligand-binding, signal-transducing 366-aa receptor. GHSR1b contains only the first five transmembrane segments followed by a unique 24-aa tail and does not normally bind/respond to ghrelin or synthetic secretagogues. Reports differ on its total length—289 versus 298 aa—so that value should not be used as a decisive identifier without specifying transcript annotation. The important consensus is that it is a truncated, five-TM, signaling-incompetent splice product. (bennett2009aninvestigationinto pages 18-21, yin2014thegrowthhormone pages 1-3)
GHSR1b can reduce GHSR1a cell-surface abundance and constitutive signaling in transfected cells. Biochemical work further supports GHSR1a–GHSR1b heterodimers stabilized in a non-signaling conformation, although the abundance and physiological importance of these complexes in native human tissues remain uncertain. (yin2014thegrowthhormone pages 1-3, gross2023ghrelinreceptorsignaling pages 3-4, gross2023ghrelinreceptorsignaling pages 15-16)
GHSR1a is the high-affinity receptor for acyl-ghrelin, a 28-aa peptide produced predominantly by gastric enteroendocrine cells. Ghrelin O-acyltransferase (GOAT) installs an octanoyl group on Ser3 in the endoplasmic reticulum. This modification is a direct receptor-contact determinant: acyl-ghrelin binds GHSR1a with nanomolar affinity, whereas desacyl-ghrelin has affinity lower by several orders of magnitude and is generally inactive through GHSR1a at sub-micromolar concentrations. Claims that desacyl-ghrelin acts through GHSR1a are assay-dependent and should not be treated as equivalent to canonical acyl-ghrelin signaling. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 3-4, gross2023ghrelinreceptorsignaling pages 1-3)
The minimal signaling-competent ghrelin region is its N-terminal five-residue motif, Gly-Ser-Ser(octanoyl)-Phe-Leu. Both the free N-terminal region and lipidated Ser3 are required for productive binding and activation. Consequently, GHSR is best annotated as an acyl-peptide hormone receptor with strict dependence on ghrelin’s N-terminal lipidation rather than as a general receptor for unmodified ghrelin-family peptides. (polishchuk2025beyondhungerthe pages 21-23, gross2023ghrelinreceptorsignaling pages 6-8)
LEAP2 is a cognate endogenous peptide antagonist/inverse agonist. It competes with ghrelin at GHSR1a and suppresses ligand-independent signaling. LEAP2 therefore helps set the effective receptor tone jointly with acyl-ghrelin, receptor abundance and accessory proteins; its relative influence tends to increase in the fed/positive-energy state. The cellular pharmacology is well supported, but its quantitative, tissue-specific contribution in humans remains less settled. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 14-15, gross2023ghrelinreceptorsignaling pages 1-3)
Clinically or experimentally important agonists include macimorelin, anamorelin, ibutamoren/MK-677, relamorelin and GHRP-6. Antagonists and inverse agonists include PF-05190457/PF-5190457-class compounds. These ligand classes are not interchangeable: antagonists prevent agonist-driven deviation from basal activity, whereas inverse agonists actively reduce GHSR1a’s high constitutive output. (liu2021structuralbasisof pages 1-2, polishchuk2025beyondhungerthe pages 23-24, gross2023ghrelinreceptorsignaling pages 9-11)
The dominant pathway is:
acyl-ghrelin → GHSR1a → Gαq/11 → PLCβ → IP3 + DAG → cytosolic Ca²⁺ elevation → PKC/CaMKII and context-dependent AMPK signaling.
This pathway is especially important for hypothalamic feeding responses and GH secretion. In anterior-pituitary somatotroph-associated signaling, increased IP3/Ca²⁺ promotes GH exocytosis; hypothalamic GHSR1a on GHRH neurons provides an additional upstream route. (polishchuk2025beyondhungerthe pages 21-23, gross2023ghrelinreceptorsignaling pages 4-6)
GHSR1a can also recruit Gi/o, G12/13 and β-arrestins, but coupling depends on ligand, receptor conformation, accessory proteins and cell type. Gi/o-dependent signaling is implicated in pancreatic suppression of glucose-stimulated insulin secretion. β-arrestins terminate or spatially reorganize signaling and can scaffold ERK/AKT pathways; β-arrestin-2 is also implicated in dopamine-associated reward behavior in animal models. GHSR1a does not appear to use Gs as a principal direct transducer. (gross2023ghrelinreceptorsignaling pages 3-4, polishchuk2025beyondhungerthe pages 21-23, gross2023ghrelinreceptorsignaling pages 4-6, gross2023ghrelinreceptorsignaling pages 6-8)
“Biased signaling” means that different ligands or receptor contexts disproportionately favor only a subset of these outputs. ICL2 and ECL2 mutations can strongly separate Gq from β-arrestin signaling, while the accessory protein MRAP2 suppresses basal activity and biases ghrelin responses toward Gq. Experts regard this as a promising route to retain desired appetite, prokinetic or neuroprotective actions while avoiding unwanted metabolic/reward effects, but no biased GHSR therapy has yet validated that promise clinically. (gross2023ghrelinreceptorsignaling pages 3-4, gross2023ghrelinreceptorsignaling pages 18-23)
GHSR1a’s basal signaling reaches approximately 50% of maximal ligand-driven response in commonly used heterologous systems. Purified receptor studies show that high basal activity is intrinsic, although its magnitude varies by membrane composition and cellular context. GHSR1b, MRAP2 and LEAP2 reduce this basal tone. Human variants selectively impairing constitutive activity provide evidence that this property is physiologically meaningful rather than merely a recombinant-expression artifact. (gross2023ghrelinreceptorsignaling pages 3-4, yin2014thegrowthhormone pages 12-15)
Human structures now cover antagonist-, agonist- and inverse-agonist-bound states. The orthosteric site contains two cavities separated by a Glu124–Arg283 salt bridge: cavity I recognizes ghrelin’s N-terminal peptide, while cavity II accommodates the Ser3 octanoyl group. ECL2 caps the extracellular site in active conformations. (gross2023ghrelinreceptorsignaling pages 23-25, gross2023ghrelinreceptorsignaling pages 8-9)
Agonist engagement disrupts the ionic lock and triggers conserved class-A GPCR microswitches, including DRY, CWxP, PIF and NPxxY-related rearrangements. Intracellular TM6 moves outward to expose the G-protein-binding interface, while extracellular TM7 moves inward and ECL2 closes over the ligand. Quantitatively, agonism produces an approximately 12.1-Å outward displacement of intracellular TM6; inverse agonism produces an approximately 8.3-Å inward displacement, while extracellular TM7 shifts inward by about 6.5 Å in the agonist state. (gross2023ghrelinreceptorsignaling pages 8-9, gross2023ghrelinreceptorsignaling pages 9-11)
Key structures include the antagonist-bound receptor at 3.3 Å; ghrelin/GHRP-6–Gq complexes at 2.9 and 3.2 Å; and ghrelin- or ibutamoren-bound Gi complexes at 2.7 Å. Coupling to Gq, Gi and Go preserves the broad activation mechanism but changes octanoyl-chain orientation and intracellular contacts, offering a structural explanation for pathway-dependent efficacy. Cholesterol and PIP2 can additionally modulate receptor activity allosterically. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 8-9, gross2023ghrelinreceptorsignaling pages 9-11, gross2023ghrelinreceptorsignaling pages 16-18)
Functional GHSR1a acts at the plasma membrane, where extracellular acyl-ghrelin reaches its orthosteric pocket and intracellular receptor surfaces engage G proteins. Agonist-stabilized or constitutively active GPCR conformations are phosphorylated by GRKs, increasing β-arrestin binding, desensitization, clathrin-mediated endocytosis and endosomal trafficking. Thus, GHSR1a dynamically cycles between the cell surface and intracellular trafficking compartments rather than occupying a single static location. (gross2023ghrelinreceptorsignaling pages 1-3)
The truncated GHSR1b product reduces GHSR1a surface expression in heterologous cells. Evidence for predominant GHSR1b endoplasmic-reticulum retention exists in the literature, but the retrieved full-text evidence was insufficient to quantify that localization or establish its extent in native human cells; the conservative annotation is therefore “intracellularly retained/truncated regulator with demonstrated reduction of GHSR1a surface signaling.” (yin2014thegrowthhormone pages 1-3, gross2023ghrelinreceptorsignaling pages 3-4)
The strongest functional sites are the anterior pituitary and hypothalamus. In the arcuate nucleus, high receptor expression occurs in AgRP/NPY neurons, which regulate homeostatic feeding, adiposity and energy use, and in GHRH neurons, which regulate pituitary GH release. Circulating acyl-ghrelin has best access to receptors near circumventricular organs and fenestrated capillaries, including the median eminence and area postrema; penetration into BBB-protected regions is limited and varies with species, brain region and energy state. (gross2023ghrelinreceptorsignaling pages 4-6, gross2023ghrelinreceptorsignaling pages 15-16)
Additional reported sites include VTA and substantia-nigra dopaminergic neurons, pancreatic islets—especially somatostatin-positive δ cells, with lower expression in β and α cells—and gastrointestinal, cardiovascular and immune tissues. However, much of the fine-resolution neuroanatomy derives from rodent in-situ hybridization, reporter mice or pharmacology. Human transcript detection should not automatically be interpreted as validated receptor protein at the cell surface, especially in low-expression peripheral tissues. (gross2023ghrelinreceptorsignaling pages 4-6, gross2023ghrelinreceptorsignaling pages 6-8, gross2023ghrelinreceptorsignaling pages 16-18)
GHSR1a stimulates GH secretion through direct pituitary signaling and activation of hypothalamic GHRH neurons. GH then promotes hepatic IGF-1 production and anabolic growth responses. During caloric deficiency, this axis also supports glucose availability through GH-associated gluconeogenic and insulin-antagonistic effects. This endocrine function is supported by receptor cloning, pharmacological GH stimulation in humans, loss-of-function genetics and the clinical performance of GHSR agonists as GH-stimulation tests. (yin2014thegrowthhormone pages 12-15, gross2023ghrelinreceptorsignaling pages 4-6)
In arcuate AgRP/NPY neurons, Gq-linked GHSR1a activation increases orexigenic drive. Peripheral ghrelin rises during negative energy balance and reaches accessible hypothalamic/brainstem sites; constitutive receptor signaling may contribute where ligand access is poor. In a foundational rat experiment, intracerebroventricular ghrelin increased hypothalamic NPY mRNA to approximately 160% of vehicle, and an NPY-Y1 antagonist dose-dependently abolished feeding, supporting an NPY-linked mechanism. This is strong circuit-level evidence but not a direct quantitative human result. (gross2023ghrelinreceptorsignaling pages 4-6)
In islets, GHSR signaling can inhibit glucose-stimulated insulin secretion through δ-cell Gq/Ca²⁺/somatostatin and β-cell Gi/o-related mechanisms. The resulting acute hyperglycemic response may preserve glucose for the brain during fasting. Conversely, antagonists and inverse agonists can be insulinotropic and improve glucose tolerance in experimental systems. Human ghrelin administration has also suppressed glucose-stimulated insulin secretion and worsened glucose tolerance, supporting relevance beyond rodents. (gross2023ghrelinreceptorsignaling pages 6-8, gross2023ghrelinreceptorsignaling pages 16-18)
GHSR agonism accelerates gastric emptying in healthy people and diabetic gastroparesis. Pharmacological evidence suggests this prokinetic output may depend more on β-arrestin, Gi/o or G13 than on Gq. In VTA/SNc circuits, receptor activation increases dopamine-neuron firing and striatal dopamine release, influencing food/drug reward and locomotor behavior; these latter assignments are primarily supported by animal models. (gross2023ghrelinreceptorsignaling pages 4-6, gross2023ghrelinreceptorsignaling pages 6-8)
Rare human variants provide mechanistically informative evidence. Ala204Glu—also discussed using adjacent-numbering nomenclature in some model systems—selectively impairs constitutive signaling and has been associated with familial short stature. Phe279Leu, affecting a residue important for basal activation, has been reported with short stature and obesity. These observations suggest that ligand-independent receptor tone contributes to normal growth, although variant penetrance and phenotype breadth are not fully defined. (yin2014thegrowthhormone pages 12-15, gross2023ghrelinreceptorsignaling pages 14-15)
Common variation does not establish GHSR as a major general obesity gene. In 15,854 middle-aged Danes, seven common variants/haplotypes showed no consistent association with obesity. A rare −151C/T promoter variant increased transcription in vitro but had a minor-allele frequency of approximately 0.01% and no demonstrated population-level obesity association. This cautions against extrapolating rare functional variants to common obesity risk.
Open Targets identifies GHSR as the approved human target ENSG00000121853 and records the strongest directly relevant disease association with “short stature due to GHSR deficiency”; broader cancer or fibromyalgia associations are heterogeneous target-level evidence and do not prove that GHSR is a causal driver in those diseases. (OpenTargets Search: -GHSR)
Macimorelin is an orally active GHSR agonist implemented as a GH-stimulation test for adult GH deficiency. The pivotal randomized crossover phase-3 study, NCT02558829, enrolled 157 adults and compared a single 0.5 mg/kg oral dose with insulin tolerance testing. The predefined macimorelin peak-GH cutoff was 2.8 ng/mL, with measurements over 90 minutes. The associated clinical publication appeared on 1 August 2018: Garcia et al., Journal of Clinical Endocrinology & Metabolism, DOI 10.1210/jc.2018-00665. (NCT02558829 chunk 1, NCT02558829 chunk 2)
A major 2024 development was completion of the pediatric phase-3 DETECT study (NCT04786873) on 13 June 2024. It enrolled 101 participants aged 2–17 years, used macimorelin 1.0 mg/kg, repeated the test to assess reproducibility, and compared it with arginine and clonidine stimulation. The registry indicates completion, but the retrieved record did not yet provide final diagnostic-accuracy results; efficacy should therefore not be inferred from completion alone. ClinicalTrials.gov record. (NCT04786873 chunk 1, NCT04786873 chunk 2, NCT04786873 chunk 3)
The oral agonist anamorelin has been evaluated for NSCLC-associated anorexia/cachexia. ROMANA 1 and 2 were randomized, double-blind phase-3 trials of 100 mg once daily for 12 weeks, enrolling 484 and 495 patients; co-primary outcomes were lean body mass and handgrip strength. ROMANA 3 extended safety evaluation in 513 patients. The principal ROMANA 1/2 report was published 20 February 2016: Temel et al., Lancet Oncology, DOI 10.1016/S1470-2045(15)00558-6. (NCT01395914 chunk 1, NCT01387282 chunk 1, NCT01387282 chunk 2, NCT01387269 chunk 1)
These programs validate that pharmacological GHSR activation can increase appetite/body-mass-related measures, but improvement in muscle mass does not necessarily imply restoration of muscle function; handgrip strength was specifically tested because that distinction matters clinically. Regulatory availability differs by jurisdiction, so anamorelin should not be described as a universally approved therapy. (NCT01387282 chunk 1, NCT01387269 chunk 1)
Other active translational directions include relamorelin-like prokinetic agonists for gastroparesis; inverse agonists for obesity, type-2 diabetes or alcohol-use disorder; LEAP2 analogues; and Gq- or β-arrestin-biased ligands intended to separate appetite/GH, gastrointestinal, metabolic and reward effects. These remain investigational. The leading expert assessment is that tissue-dependent “system bias” and an incomplete definition of β-arrestin-specific physiology presently limit rational clinical translation. (gross2023ghrelinreceptorsignaling pages 6-8, gross2023ghrelinreceptorsignaling pages 18-23, gross2023ghrelinreceptorsignaling pages 9-11)
The most authoritative recent synthesis retrieved was Gross et al., February 2023, Trends in Endocrinology & Metabolism, DOI 10.1016/j.tem.2022.12.001, with the manuscript becoming openly available in PMC in February 2024. Its central interpretation is that GHSR1a should no longer be viewed as a simple ghrelin-on/Gq-on switch: ligand structure, receptor variants, dimerization, MRAP2, membrane lipids and cell-specific transducer abundance can generate different signaling profiles. (gross2023ghrelinreceptorsignaling pages 3-4, gross2023ghrelinreceptorsignaling pages 1-3)
That review also identifies major unresolved questions: whether β-arrestin-coupled receptor structures differ materially from G-protein complexes; whether feeding, GH release, insulinostasis and neuroprotection are truly β-arrestin-independent; and whether biased drugs can achieve tissue selectivity without metabolic adverse effects. These are expert hypotheses and development priorities, not established clinical benefits. (gross2023ghrelinreceptorsignaling pages 18-23)
Mechanistic advances immediately preceding this window remain foundational: 2021 cryo-EM structures resolved ghrelin/ibutamoren–Gi complexes at 2.7 Å, and 2022 work directly contrasted agonist and inverse-agonist conformations. The relative scarcity of new 2023–2024 primary structures means these studies remain the current mechanistic backbone rather than being superseded by newer evidence. (liu2021structuralbasisof pages 1-2, gross2023ghrelinreceptorsignaling pages 9-11, gross2023ghrelinreceptorsignaling pages 16-18)
The high-confidence functional annotation is: GHSR1a is a cell-surface, constitutively active, class-A GPCR that recognizes Ser3-octanoylated ghrelin and signals primarily through Gq/11–PLC–IP3/Ca²⁺ to stimulate hypothalamic feeding circuits and hypothalamic/pituitary GH release; it can additionally engage Gi/o, G12/13 and β-arrestin pathways, and is antagonized/inversely agonized by LEAP2.
Confidence is lower for receptor-protein localization in many peripheral human tissues, native GHSR1a heteromers, the physiological role of GHSR1b, and assignment of broad protective or inflammatory effects directly to GHSR rather than to indirect endocrine pathways. Much localization and pathway-selectivity evidence comes from recombinant cells or rodents. Accordingly, human pituitary/GH physiology, ligand specificity, receptor structure and macimorelin pharmacology should receive greater annotation weight than broad pleiotropic claims. (gross2023ghrelinreceptorsignaling pages 3-4, gross2023ghrelinreceptorsignaling pages 4-6, gross2023ghrelinreceptorsignaling pages 18-23)
References
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(gross2023ghrelinreceptorsignaling pages 15-16): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
(gross2023ghrelinreceptorsignaling pages 6-8): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
(gross2023ghrelinreceptorsignaling pages 14-15): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
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(gross2023ghrelinreceptorsignaling pages 18-23): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
(gross2023ghrelinreceptorsignaling pages 8-9): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
(gross2023ghrelinreceptorsignaling pages 9-11): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
(gross2023ghrelinreceptorsignaling pages 4-6): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
(gross2023ghrelinreceptorsignaling pages 16-18): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
(NCT02558829 chunk 1): Validation of Macimorelin as a Test for Adult Growth Hormone Deficiency. AEterna Zentaris. 2015. ClinicalTrials.gov Identifier: NCT02558829
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(NCT04786873 chunk 1): A Research Study of How Well Macimorelin Works to Find Out if Children Have a Lack of Growth Hormone and How Safe it is. AEterna Zentaris. 2021. ClinicalTrials.gov Identifier: NCT04786873
(NCT01395914 chunk 1): Anamorelin HCl in the Treatment of Non-Small Cell Lung Cancer-Cachexia (NSCLC-C): An Extension Study (ROMANA 3). Helsinn Therapeutics (U.S.), Inc. 2011. ClinicalTrials.gov Identifier: NCT01395914
(NCT01387282 chunk 1): Safety and Efficacy of Anamorelin HCl in Patients With Non-Small Cell Lung Cancer-Cachexia (ROMANA 2). Helsinn Therapeutics (U.S.), Inc. 2011. ClinicalTrials.gov Identifier: NCT01387282
(NCT01387269 chunk 1): Safety and Efficacy of Anamorelin HCl in Patients With Non-Small Cell Lung Cancer-Cachexia (ROMANA 1). Helsinn Therapeutics (U.S.), Inc. 2011. ClinicalTrials.gov Identifier: NCT01387269
(polishchuk2025beyondhungerthe pages 23-24): Hlafira Polishchuk, Krzysztof Guzik, and Tomasz Kantyka. Beyond hunger: the structure, signaling, and systemic roles of ghrelin. International Journal of Molecular Sciences, Nov 2025. URL: https://doi.org/10.3390/ijms262210996, doi:10.3390/ijms262210996. This article has 12 citations.
(gross2023ghrelinreceptorsignaling pages 23-25): Joshua D. Gross, Yang Zhou, Lawrence S. Barak, and Marc G. Caron. Ghrelin receptor signaling in health and disease: a biased view. Trends in Endocrinology & Metabolism, 34:106-118, Feb 2023. URL: https://doi.org/10.1016/j.tem.2022.12.001, doi:10.1016/j.tem.2022.12.001. This article has 48 citations and is from a domain leading peer-reviewed journal.
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(NCT04786873 chunk 2): A Research Study of How Well Macimorelin Works to Find Out if Children Have a Lack of Growth Hormone and How Safe it is. AEterna Zentaris. 2021. ClinicalTrials.gov Identifier: NCT04786873
(NCT04786873 chunk 3): A Research Study of How Well Macimorelin Works to Find Out if Children Have a Lack of Growth Hormone and How Safe it is. AEterna Zentaris. 2021. ClinicalTrials.gov Identifier: NCT04786873
(NCT01387282 chunk 2): Safety and Efficacy of Anamorelin HCl in Patients With Non-Small Cell Lung Cancer-Cachexia (ROMANA 2). Helsinn Therapeutics (U.S.), Inc. 2011. ClinicalTrials.gov Identifier: NCT01387282