Sstr5 encodes somatostatin receptor type 5, a class A (rhodopsin-like) Gi/o-coupled GPCR that preferentially binds somatostatin-28 with approximately 30-fold higher affinity than somatostatin-14. The receptor is prominently expressed in the rat anterior pituitary and small intestine, with lower levels in pancreatic islets. Upon somatostatin binding, SSTR5 activates pertussis toxin-sensitive Gi/o proteins to inhibit adenylyl cyclase, reducing intracellular cAMP and suppressing PKA-dependent signaling. Through this mechanism, SSTR5 is a primary negative regulator of insulin secretion from pancreatic beta cells and contributes to glucose homeostasis, as demonstrated by knockout mouse studies showing decreased blood glucose, hyperinsulinemia, and resistance to diet-induced insulin resistance. SSTR5 expression in pituitary is upregulated by glucocorticoids (dexamethasone), linking the hypothalamic-pituitary-adrenal axis to somatostatinergic tone. The receptor can heterodimerize with SSTR2, enhancing SSTR2-mediated growth inhibition. SSTR5 is palmitoylated at Cys-320 by the palmitoyltransferase ZDHHC5, which may regulate membrane localization and G-protein coupling.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0004994
somatostatin receptor activity
|
IDA
PMID:1362243 Molecular cloning and expression of a pituitary somatostatin... |
ACCEPT |
Summary: Directly demonstrated by radioligand binding in COS-7 cells transfected with the cloned rat SSTR5 cDNA. The receptor binds somatostatin-28 with ~30-fold preference over somatostatin-14. This is a core molecular function.
Reason: IDA from the original cloning paper. Specific binding of 125I-Tyr11-SRIF was demonstrated in membranes from transfected COS-7 cells.
Supporting Evidence:
PMID:1362243
Membranes prepared from COS-7 cells transfected with the rAP6-26 cDNA showed specific binding of 125I-Tyr11-SRIF, thus identifying the cDNA clone as a novel SRIF receptor
file:rat/Sstr5/Sstr5-deep-research-bioreason-sft.md
[BioReason correctly identifies] somatostatin receptor activity (GO:0004994) by binding somatostatin peptides at the extracellular N-terminus and transmembrane pocket
|
|
GO:0007193
adenylate cyclase-inhibiting G protein-coupled receptor signaling pathway
|
IDA
PMID:1362243 Molecular cloning and expression of a pituitary somatostatin... |
ACCEPT |
Summary: Directly demonstrated in COS-7 cells expressing cloned SSTR5. Somatostatin inhibited forskolin-induced cAMP accumulation, and GTP sensitivity indicated Gi coupling. This is a core signaling pathway for this receptor.
Reason: IDA showing functional coupling to adenylyl cyclase inhibition via Gi proteins, confirmed by pertussis toxin sensitivity in human ortholog studies.
Supporting Evidence:
PMID:1362243
forskolin-induced cAMP accumulation was inhibited by SRIF and SRIF-28, thus confirming that the rAP6-26 cDNA encodes a functional receptor protein...binding of 125I-Tyr11-SRIF was markedly reduced in the presence of Na+ ions and GTP, indicating coupling of rAP6-26 receptors to inhibitory G proteins
|
|
GO:0004994
somatostatin receptor activity
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Phylogenetically inferred somatostatin receptor activity, consistent with the IDA evidence from PMID:1362243. Redundant with the direct experimental evidence.
Reason: Correct annotation supported by both phylogenetic inference and direct experimental data.
|
|
GO:0004994
somatostatin receptor activity
|
IEA
GO_REF:0000002 |
ACCEPT |
Summary: InterPro2GO mapping from IPR000586 and IPR001184 (somatostatin receptor family/type 5). Correct and consistent with experimental evidence.
Reason: Automated annotation from domain signatures that is accurate for this well-characterized somatostatin receptor.
|
|
GO:0004930
G protein-coupled receptor activity
|
IEA
GO_REF:0000002 |
KEEP AS NON CORE |
Summary: InterPro2GO mapping from IPR000276 (rhodopsin-like GPCR family). Correct but less specific than GO:0004994 (somatostatin receptor activity).
Reason: Accurate but redundant with the more specific somatostatin receptor activity annotation. The parent GPCR activity term is implied by the child term.
|
|
GO:0005886
plasma membrane
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Phylogenetically inferred plasma membrane localization, consistent with this being a cell-surface GPCR. Supported by UniProt subcellular location annotation.
Reason: Correct localization for a seven-transmembrane receptor that binds extracellular somatostatin ligands.
Supporting Evidence:
PMID:1362243
Membranes prepared from COS-7 cells transfected with the rAP6-26 cDNA showed specific binding of 125I-Tyr11-SRIF
|
|
GO:0005886
plasma membrane
|
IEA
GO_REF:0000044 |
ACCEPT |
Summary: UniProt subcellular location mapping to plasma membrane. Correct and redundant with IBA evidence.
Reason: Consistent with GPCR biology; the receptor must be at the plasma membrane to bind extracellular somatostatin.
|
|
GO:0005886
plasma membrane
|
ISO
GO_REF:0000121 |
ACCEPT |
Summary: RGD ISO annotation from human and mouse orthologs. Correct and consistent with direct evidence for this 7TM receptor.
Reason: Accurate localization supported by multiple independent evidence lines.
|
|
GO:0043005
neuron projection
|
IBA
GO_REF:0000033 |
KEEP AS NON CORE |
Summary: Phylogenetically inferred localization to neuron projections. While other somatostatin receptors (SSTR1-4) are broadly expressed in brain neurons, SSTR5 in rat is notable for its absence from most brain regions by Northern blot and ISH -- expression is restricted to pituitary anterior lobe, not typical neurons. This annotation may be transferred from orthologs with broader neural expression.
Reason: SSTR5 mRNA was not detected in rat brain regions by Northern blot or ISH in the original studies. Expression is mainly pituitary and peripheral. While some neural expression cannot be excluded at low levels, this is not a core localization for rat SSTR5.
Supporting Evidence:
PMID:1362243
a approximately 2.6 kilobase mRNA encoding the receptor was present in the pituitary but not in the liver, small intestine, kidney, pancreas, cerebellum, or cortex. Lack of receptor mRNA expression in the brain was confirmed by in situ hybridization histochemical studies
|
|
GO:0042923
neuropeptide binding
|
IBA
GO_REF:0000033 |
KEEP AS NON CORE |
Summary: SSTR5 binds the neuropeptide somatostatin. This is correct but less specific than somatostatin receptor activity (GO:0004994).
Reason: Accurate as somatostatin is a neuropeptide, but the more specific term GO:0004994 (somatostatin receptor activity) better represents the core function.
|
|
GO:0007218
neuropeptide signaling pathway
|
IBA
GO_REF:0000033 |
KEEP AS NON CORE |
Summary: SSTR5 participates in somatostatin (neuropeptide) signaling. Correct but less specific than GO:0038170 (somatostatin signaling pathway) and GO:0007193 (adenylate cyclase-inhibiting GPCR signaling pathway).
Reason: True at a general level -- somatostatin is a neuropeptide -- but the more specific somatostatin signaling pathway term is more informative for this receptor.
|
|
GO:0007186
G protein-coupled receptor signaling pathway
|
IEA
GO_REF:0000002 |
KEEP AS NON CORE |
Summary: InterPro2GO annotation for general GPCR signaling. Correct but less informative than the specific adenylate cyclase-inhibiting pathway annotation.
Reason: Subsumed by the more specific GO:0007193 annotation that is supported by IDA evidence.
|
|
GO:0016020
membrane
|
IEA
GO_REF:0000002 |
KEEP AS NON CORE |
Summary: Generic membrane localization from InterPro. Correct but uninformative given the more specific plasma membrane annotation.
Reason: Too general; plasma membrane (GO:0005886) is more appropriate and already annotated.
|
|
GO:0038170
somatostatin signaling pathway
|
IEA
GO_REF:0000108 |
ACCEPT |
Summary: Logically inferred from GO:0004994 (somatostatin receptor activity). This is a correct and informative annotation for this receptor.
Reason: SSTR5 is a somatostatin receptor, so involvement in the somatostatin signaling pathway is directly entailed. Supported by functional data from PMID:1362243.
Supporting Evidence:
PMID:1362243
forskolin-induced cAMP accumulation was inhibited by SRIF and SRIF-28
|
|
GO:0071385
cellular response to glucocorticoid stimulus
|
IEP
PMID:14512709 Role of glucocorticoids in the regulation of pituitary somat... |
ACCEPT |
Summary: SSTR5 mRNA is upregulated by dexamethasone in rat pituitary, both in vivo and in vitro. This is an expression pattern (IEP) annotation indicating that sst5 expression changes in response to glucocorticoids. The response is distinctive -- sst5 is the only subtype increased by DEX while all others decrease.
Reason: Well-supported IEP annotation. DEX treatment increases sst5 mRNA in rat pituitary cells at both in vivo and in vitro levels, demonstrating a cellular response.
Supporting Evidence:
PMID:14512709
High-dose DEX resulted in a decrease in sst1-sst4 mRNA and an increase in sst5 mRNA, independent of adrenal status
|
|
GO:0071385
cellular response to glucocorticoid stimulus
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Phylogenetically inferred, consistent with the IEP evidence from PMID:14512709.
Reason: Redundant with the directly supported IEP annotation but consistent.
|
|
GO:0071385
cellular response to glucocorticoid stimulus
|
IEA
GO_REF:0000117 |
ACCEPT |
Summary: ARBA machine learning annotation, consistent with IEP evidence.
Reason: Consistent with experimental IEP evidence from PMID:14512709.
|
|
GO:0050796
regulation of insulin secretion
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Phylogenetically inferred role in regulation of insulin secretion. Strongly supported by knockout mouse data showing SSTR5 mediates somatostatin inhibition of insulin secretion.
Reason: Core physiological function of SSTR5 demonstrated by knockout studies in mouse, with conservation expected in rat given high sequence identity.
Supporting Evidence:
PMID:12511609
sst(5) mediates SRIF inhibition of pancreatic insulin secretion and contributes to the regulation of glucose homeostasis and insulin sensitivity
|
|
GO:0050796
regulation of insulin secretion
|
ISO
GO_REF:0000121 |
ACCEPT |
Summary: RGD ISO annotation from mouse ortholog data. Well-supported by SSTR5 KO studies.
Reason: Consistent with multiple knockout studies demonstrating SSTR5 role in insulin secretion regulation.
|
|
GO:0042593
glucose homeostasis
|
ISO
GO_REF:0000121 |
ACCEPT |
Summary: RGD ISO from mouse data. SSTR5 KO mice show altered blood glucose levels and resistance to diet-induced insulin resistance, supporting a role in glucose homeostasis.
Reason: Supported by SSTR5 KO studies showing decreased blood glucose and improved insulin sensitivity.
Supporting Evidence:
PMID:12511609
sst(5) KO mice exhibited decreased blood glucose and plasma insulin levels...sst(5) KO mice displayed decreased susceptibility to high fat diet-induced insulin resistance
|
|
GO:0060124
positive regulation of growth hormone secretion
|
ISO
GO_REF:0000121 |
KEEP AS NON CORE |
Summary: RGD ISO annotation suggesting SSTR5 positively regulates growth hormone secretion. This is counterintuitive for a Gi-coupled receptor that generally inhibits secretion. However, the heterodimerization with SSTR2 modulates SSTR2 function in pituitary somatotropes, and the net effect on GH may be complex. The annotation is from RGD and may reflect indirect or context-dependent effects observed in specific model systems.
Reason: The relationship between SSTR5 and GH secretion is complex. SSTR5 is expressed in pituitary and somatostatin generally inhibits GH, but the positive regulation annotation likely reflects specific experimental contexts rather than the canonical inhibitory function of somatostatin receptors.
|
|
GO:0032467
positive regulation of cytokinesis
|
ISO
GO_REF:0000121 |
KEEP AS NON CORE |
Summary: RGD ISO annotation for positive regulation of cytokinesis. This is unexpected for SSTR5, which is generally associated with growth inhibition rather than promotion of cell division. May relate to SSTR2/SSTR5 heterodimerization effects on cell cycle or indirect downstream effects.
Reason: Not a well-characterized core function of SSTR5. The receptor is primarily known for Gi-mediated growth inhibition, not promotion of cytokinesis. This may reflect context-dependent or indirect effects.
|
Q: What is the relative contribution of SSTR5 versus SSTR2 to somatostatin-mediated inhibition of insulin secretion in rat pancreatic islets specifically?
Suggested experts: Strowski MZ, Brunicardi FC
Q: Does SSTR5 have significant expression in rat brain regions beyond the anterior pituitary, given improved detection methods since the original Northern blot studies?
Suggested experts: O'Carroll AM, Patel YC
Experiment: Perform ChIP-seq for glucocorticoid receptor in rat pituitary cells treated with dexamethasone, focusing on the Sstr5 locus to determine whether GR directly binds the promoter region or whether the effect is indirect.
Hypothesis: SSTR5 expression in rat pituitary is regulated at the transcriptional level by glucocorticoid receptor binding to the Sstr5 promoter.
Type: ChIP-seq
Experiment: Use proximity ligation assays (PLA) or BRET in primary rat pituitary cell cultures to detect SSTR2-SSTR5 heterodimers in situ, and correlate with GH secretion measurements in response to somatostatin analogs.
Hypothesis: SSTR5/SSTR2 heterodimerization occurs in rat pituitary somatotropes and modulates growth hormone secretion.
Type: Proximity ligation assay / BRET
---
provider: bioreason-pro
model: SFT
source: huggingface.co/datasets/wanglab/protein_catalogue
date: '2026-04-04'
uniprot_id: P30938
gene: Sstr5
organism: Rattus norvegicus (Rat)
---
## Thinking Trace
I begin with the InterPro architecture. Multiple overlapping family and domain signatures define a class A (rhodopsin-like) G protein–coupled receptor (GPCR) with somatostatin receptor 5 (SSTR5) specificity. Short N-terminal motifs IPR001184 (Somatostatin receptor 5 family, residues 3–16 and 33–47) and broader family marks IPR000586 (Somatostatin receptor family, residues 36–48) indicate an extracellular N-terminus tailored for peptide binding. The core seven-transmembrane (7TM) region is captured by IPR017452 (GPCR, rhodopsin-like, 7TM domain, residues 54–304), with flanking family annotations IPR000276 (G protein-coupled receptor, rhodopsin-like family, residues 39–63, 48–319, 54–304, 72–93, 117–139, 123–139, 153–174, 201–224, 246–270, 286–312) that collectively specify the canonical class A GPCR fold and its intracellular coupling surfaces. Within this 7TM scaffold, SSTR-specific family signatures recur at key helices: IPR000586 (Somatostatin receptor family, residues 91–99, 111–120, 225–234, 235–248, 268–276, 306–318) and SSTR5-specific marks IPR001184 (Somatostatin receptor 5 family, residues 226–240, 272–288, 326–340). These motifs map to intracellular loops and the C-terminal tail that determine coupling to Gi/o proteins and receptor regulation. The ensemble of overlapping family entries (including IPR000276 and IPR000586) across the 7TM region establishes a peptide-activated GPCR that preferentially engages Gi/o.
This architecture causes somatostatin receptor activity (GO:0004994) by binding somatostatin peptides at the extracellular N-terminus and transmembrane pocket, then transmitting the signal through the 7TM core to intracellular loops that recruit heterotrimeric G proteins. The SSTR5-specific signatures imply high-affinity recognition of somatostatin-14 and -28, consistent with the observed preference.
Gi/o engagement by this receptor inhibits adenylate cyclase, lowering cAMP and dampening PKA signaling. This places the protein squarely in the adenylate cyclase-inhibiting G protein–coupled receptor signaling pathway (GO:0007193). Through cAMP/PKA and downstream effectors (e.g., CREB, CRTC/TORC transcriptional regulators), SSTR5 signaling modulates transcriptional programs that intersect with stress hormone pathways; such cAMP-dependent control provides a mechanistic route by which SSTR signaling shapes the cellular response to glucocorticoid stimulus (GO:0071385), for example by tuning glucocorticoid receptor–dependent gene expression.
The 7TM domain and multi-pass topology dictate residence in the plasma membrane (GO:0005886), where ligand access and G protein coupling occur. Internalization motifs and scaffold interactions commonly drive ligand-induced endocytosis and recycling, explaining a cytoplasmic (GO:0005737) pool associated with trafficking and signaling complexes. The subcellular description of a cell membrane, multi-pass protein aligns with this GPCR life cycle.
Mechanistically, somatostatin-14 binding stabilizes an active 7TM conformation that exposes intracellular interfaces for Gi/o. Gαi/o inhibits adenylate cyclase, while Gβγ can engage ion channels and kinases, collectively suppressing cAMP and modulating neuronal excitability and endocrine secretion. The receptor likely forms transient complexes with Gi α subunits (G(i) α-1 and α-2) and may hetero-oligomerize with other somatostatin receptors to fine-tune ligand affinity and signaling bias. Crosstalk with appetite-regulatory systems (appetite-regulating hormone; C-flanking peptide of NPY) is expected at the level of shared neuronal circuits and convergent control of cAMP/PKA and CREB-dependent transcription, providing a route to coordinate feeding behavior and stress-axis outputs.
## Functional Summary
A seven-transmembrane, cell-surface receptor in rat that binds somatostatin peptides with a strong preference for the 14–residue form and transduces the signal through inhibitory G proteins to suppress adenylate cyclase and reduce cAMP. This signaling dampens PKA-dependent transcriptional programs and intersects with stress hormone pathways, shaping cellular responses to glucocorticoids. The receptor resides at the plasma membrane and cycles through cytoplasmic trafficking pools, where it assembles with Gi proteins and likely coordinates with other neuropeptidergic systems to regulate excitability, secretion, and endocrine tone.
## UniProt Summary
Receptor for somatostatin with a high affinity binding for somatostatin-14 and-28.
## InterPro Domains
- IPR001184: Somatostatin receptor 5 (family) [3-16]
- IPR001184: Somatostatin receptor 5 (family) [33-47]
- IPR000586: Somatostatin receptor family (family) [36-48]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [39-63]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [48-319]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [54-304]
- IPR017452: GPCR, rhodopsin-like, 7TM (domain) [54-304]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [72-93]
- IPR000586: Somatostatin receptor family (family) [91-99]
- IPR000586: Somatostatin receptor family (family) [111-120]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [117-139]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [123-139]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [153-174]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [201-224]
- IPR000586: Somatostatin receptor family (family) [225-234]
- IPR001184: Somatostatin receptor 5 (family) [226-240]
- IPR000586: Somatostatin receptor family (family) [235-248]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [246-270]
- IPR000586: Somatostatin receptor family (family) [268-276]
- IPR001184: Somatostatin receptor 5 (family) [272-288]
- IPR000276: G protein-coupled receptor, rhodopsin-like (family) [286-312]
- IPR000586: Somatostatin receptor family (family) [306-318]
- IPR001184: Somatostatin receptor 5 (family) [326-340]
## GO Term Predictions
### Molecular Function
### Biological Process
### Cellular Component
PMID:1362243 O'Carroll et al. (1992) Mol Pharmacol. Cloned the receptor from rat pituitary cDNA library. Originally designated "SSTR4" in this paper (nomenclature was later revised to SSTR5). Key findings:
- "SRIF-28 was the most potent competitor of 125I-Tyr11-SRIF binding, with a approximately 30-fold greater affinity for the receptor than that of SRIF"
- "binding of 125I-Tyr11-SRIF was markedly reduced in the presence of Na+ ions and GTP, indicating coupling of rAP6-26 receptors to inhibitory G proteins"
- "forskolin-induced cAMP accumulation was inhibited by SRIF and SRIF-28, thus confirming that the rAP6-26 cDNA encodes a functional receptor protein"
- "a approximately 2.6 kilobase mRNA encoding the receptor was present in the pituitary but not in the liver, small intestine, kidney, pancreas, cerebellum, or cortex"
PMID:7908405 Panetta et al. (1994) Mol Pharmacol. Cloned the human SSTR5. Key for rat:
- Confirmed preferential affinity for somatostatin-28 over somatostatin-14 (12.6-fold, Ki = 0.19 nM vs 2.24 nM for human)
- "hSSTR5 is coupled to pertussis toxin-sensitive G proteins" confirming Gi/o coupling
- SSTR5 mRNA "selectively localized in the anterior lobe" of rat pituitary by ISH
- Corrected the C-terminal sequence of the rat receptor (363 aa, not 383 aa)
PMID:14512709 Park et al. (2003) Neuroendocrinology. This is the basis for the IEP annotation for GO:0071385 (cellular response to glucocorticoid stimulus):
- "High-dose DEX resulted in a decrease in sst1-sst4 mRNA and an increase in sst5 mRNA, independent of adrenal status"
- "DEX also decreased sst2, sst3 and sst4 mRNA levels and increased sst5 mRNA levels by short-term in vitro application (10 nM, 4 h) in primary rat pituitary cell cultures"
- The sst5 increase is distinctive: all other subtypes decreased. This supports the IEP annotation.
- Mechanism: partially indirect via somatostatin-mediated effects rather than purely direct glucocorticoid action.
PMID:21820437 Kokkola et al. (2011) FEBS Lett:
- "ZDHHC5 and SSTR5 are colocalized at the plasma membrane and can be efficiently coimmunoprecipitated from transfected cells"
- "Coexpression of ZDHHC5 in HEK293 cells increased palmitoylation of SSTR5 whereas knock-down of endogenous ZDHHC5 by siRNAs decreased it"
- This was the first palmitoyltransferase identified for any GPCR.
PMID:12511609 Strowski et al. (2003) Mol Endocrinol. sst5 KO mice:
- "sst(5) KO mice exhibited decreased blood glucose and plasma insulin levels and increased leptin and glucagon concentrations"
- "sst(5) KO mice displayed decreased susceptibility to high fat diet-induced insulin resistance"
- "sst(5) mediates SRIF inhibition of pancreatic insulin secretion and contributes to the regulation of glucose homeostasis and insulin sensitivity"
- This is in mouse, but directly supports the ISO annotations for rat (regulation of insulin secretion, glucose homeostasis)
PMID:16026801 Wang et al. (2005) J Surg Res. Aging SSTR5 KO mice:
- At 12 months, "SSTR5-/- mice had basal hypoglycemia and improved glucose intolerance associated with hyperinsulinemia"
- "SSTR5 and SSTR1 play a pivotal role in insulin secretion and glucose regulation in mice"
PMID:15349106 Wang et al. (2004) Surgery. SSTR1/5 double KO:
- "SSTR1/5 -/- mice also had significant increase of both basal and glucose-stimulated insulin levels in vitro"
- Confirms SSTR5 as negative regulator of insulin secretion
PMID:22669743 Zhou et al. (2012) Mol Endocrinol:
- "SSTR5 is a negative regulator for PDX-1 expression"
- "SSTR5 may mediate the inhibitory effects of somatostatin and its analogs on insulin expression/secretion and cell proliferation via down-regulating PDX-1"
PMID:18653781 Grant et al. (2008) Mol Endocrinol:
- "SSTR2 and SSTR5 heterodimerize" (confirmed by co-IP and photobleaching FRET)
- "The SSTR2-selective agonist L-779,976 is more efficacious at inhibiting adenylate cyclase, activating ERK1/2, and inducing the cyclin-dependent kinase inhibitor p27(Kip1) in cells expressing both SSTR2 and SSTR5 compared with SSTR2 alone"
- "cell growth inhibition by L-779,976 treatment was markedly extended in coexpressing cells"
- This supports the UniProt comment about heterodimerization with SSTR2 enhancing growth inhibition.
PMID:33434183 Jepsen et al. (2021) JCI Insight:
- "the selective SSTR5 antagonist (SSTR5a) stimulated glucose-induced GLP-1 secretion"
- "mice lacking the SSTR5R showed increased glucose-induced GLP-1 secretion"
- SSTR5 modulates glucose homeostasis in part through the intestinal GLP-1 system
The BioReason SFT report (Sstr5-deep-research-bioreason.md) contains several errors:
Ligand preference reversed: The functional summary states "a strong preference for the 14-residue form" -- this is WRONG. SSTR5 has preferential affinity for somatostatin-28 (28-residue), not somatostatin-14. This is clearly stated in the original cloning paper PMID:1362243 which found ~30-fold preference for SRIF-28 over SRIF-14.
Cytoplasmic localization claim: The report claims "The receptor resides at the plasma membrane and cycles through cytoplasmic trafficking pools" and predicts GO:0005737 (cytoplasm). While internalization/recycling is a general GPCR property, there is no specific evidence for SSTR5 cytoplasmic pools in the existing GOA annotations or literature for rat.
Appetite-regulatory system crosstalk: The report speculates about "Crosstalk with appetite-regulatory systems (appetite-regulating hormone; C-flanking peptide of NPY)" -- this is unsupported speculation, not grounded in the SSTR5 rat literature.
No additional GO predictions: The GO Term Predictions sections are empty, despite the model claiming various functions in its reasoning trace. This suggests the model could not produce confident term predictions.
Sstr5 encodes somatostatin receptor type 5, a Gi/o-coupled GPCR that preferentially binds somatostatin-28 with high affinity. It is prominently expressed in the rat pituitary anterior lobe and small intestine, with lower levels in pancreatic islets. Upon ligand binding, it inhibits adenylyl cyclase, reducing intracellular cAMP. Through this signaling, SSTR5 acts as a negative regulator of insulin secretion from pancreatic beta cells and participates in glucose homeostasis. SSTR5 expression in pituitary is upregulated by glucocorticoids (dexamethasone), providing a link between the stress axis and somatostatinergic tone. The receptor can heterodimerize with SSTR2, enhancing the growth-inhibitory signaling of SSTR2. SSTR5 is palmitoylated at Cys-320 by the palmitoyltransferase ZDHHC5, which may regulate its membrane localization and G-protein coupling efficiency.
Source: Sstr5-deep-research-bioreason-sft.md
The BioReason SFT functional summary correctly identifies SSTR5 as a seven-transmembrane cell-surface receptor that binds somatostatin and transduces signals through inhibitory G proteins to suppress adenylate cyclase and reduce cAMP. It also correctly notes intersection with stress hormone pathways and glucocorticoid responses. These core claims are accurate.
However, the summary contains a significant factual error regarding ligand preference:
"a strong preference for the 14-residue form"
This is incorrect. SSTR5 has well-documented preferential affinity for somatostatin-28 (the 28-residue form), not somatostatin-14. The original cloning paper (PMID:1362243) demonstrated approximately 30-fold greater affinity for SRIF-28 over SRIF-14, and the human ortholog study (PMID:7908405) confirmed 12.6-fold SST-28 selectivity (Ki = 0.19 nM vs 2.24 nM). This SST-28 preference is in fact the defining pharmacological characteristic of SSTR5 and is reflected in the protein name ("preferential affinity for somatostatin-28"). Reversing this specificity is a meaningful biological error.
The thinking trace also contains this reversal, stating "The SSTR5-specific signatures imply high-affinity recognition of somatostatin-14 and -28," which suggests the model conflated SSTR5 with other subtypes.
Additionally, the summary claims:
"The receptor resides at the plasma membrane and cycles through cytoplasmic trafficking pools"
While GPCR internalization is a general phenomenon, the claim of a specific "cytoplasmic pool" for SSTR5 is unsupported by the rat literature. There is no GO annotation for cytoplasmic localization in the curated data.
The thinking trace speculates about:
"Crosstalk with appetite-regulatory systems (appetite-regulating hormone; C-flanking peptide of NPY)"
This appears to be hallucinated from general neuroendocrine context rather than grounded in SSTR5-specific literature.
Missing biology: The summary does not mention several well-established functions:
- Role as a negative regulator of insulin secretion (a primary physiological role supported by multiple KO studies: PMID:12511609, PMID:16026801)
- Contribution to glucose homeostasis
- Heterodimerization with SSTR2 to enhance growth inhibition (PMID:18653781)
- Palmitoylation by ZDHHC5 (PMID:21820437)
- Tissue-specific expression pattern (mainly pituitary anterior lobe and small intestine, not broad brain)
The InterPro2GO annotations (GO_REF:0000002) for SSTR5 include:
- GO:0004930 (G protein-coupled receptor activity) from IPR000276
- GO:0004994 (somatostatin receptor activity) from IPR000586/IPR001184
- GO:0007186 (G protein-coupled receptor signaling pathway) from IPR000276
- GO:0016020 (membrane) from multiple InterPro entries
BioReason's functional summary largely recapitulates what interpro2go provides: a rhodopsin-class GPCR with somatostatin receptor activity that couples to Gi/o to inhibit adenylyl cyclase. The biological insight beyond interpro2go is limited to:
1. Mention of glucocorticoid pathway intersection (correct)
2. General statements about PKA/CREB signaling (correct but generic)
3. Speculation about appetite-regulatory crosstalk (unsupported)
The model does NOT add the key biological insight that distinguishes SSTR5 from other somatostatin receptors: its role in insulin secretion regulation and glucose homeostasis. This is arguably the most important biological function unique to SSTR5 and is well-supported by knockout studies. The BioReason output also fails to capture the SSTR2/SSTR5 heterodimerization biology.
Overall, BioReason provides a moderately accurate but incomplete picture that is essentially an elaborated version of interpro2go with some correct but generic signaling context, marred by the significant SST-28 vs SST-14 preference reversal.
The thinking trace demonstrates reasonable structural biology reasoning about the 7TM architecture and Gi-coupling from InterPro domains. However:
- The model appears to derive functional claims primarily from domain architecture rather than literature, leading to generic GPCR biology without SSTR5-specific insights.
- The somatostatin-14 vs -28 error likely stems from the model defaulting to the more common somatostatin-14 rather than recognizing SSTR5's distinctive SST-28 preference.
- The GO Term Predictions sections (MF, BP, CC) are completely empty, suggesting the upstream ESM predictor did not generate confident predictions for this protein, or the model chose not to report them.
id: P30938
gene_symbol: Sstr5
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:10116
label: Rattus norvegicus
description: >-
Sstr5 encodes somatostatin receptor type 5, a class A (rhodopsin-like) Gi/o-coupled GPCR that
preferentially binds somatostatin-28 with approximately 30-fold higher affinity than somatostatin-14.
The receptor is prominently expressed in the rat anterior pituitary and small intestine, with lower
levels in pancreatic islets. Upon somatostatin binding, SSTR5 activates pertussis toxin-sensitive
Gi/o proteins to inhibit adenylyl cyclase, reducing intracellular cAMP and suppressing PKA-dependent
signaling. Through this mechanism, SSTR5 is a primary negative regulator of insulin secretion from
pancreatic beta cells and contributes to glucose homeostasis, as demonstrated by knockout mouse
studies showing decreased blood glucose, hyperinsulinemia, and resistance to diet-induced insulin
resistance. SSTR5 expression in pituitary is upregulated by glucocorticoids (dexamethasone), linking
the hypothalamic-pituitary-adrenal axis to somatostatinergic tone. The receptor can heterodimerize
with SSTR2, enhancing SSTR2-mediated growth inhibition. SSTR5 is palmitoylated at Cys-320 by the
palmitoyltransferase ZDHHC5, which may regulate membrane localization and G-protein coupling.
references:
- id: PMID:1362243
title: Molecular cloning and expression of a pituitary somatostatin receptor with
preferential affinity for somatostatin-28.
findings:
- statement: SSTR5 (originally called SSTR4) was cloned from rat pituitary and shown to bind
somatostatin-28 with ~30-fold higher affinity than somatostatin-14, and to couple to
inhibitory G proteins to inhibit adenylyl cyclase.
supporting_text: >-
SRIF-28 was the most potent competitor of 125I-Tyr11-SRIF binding, with a approximately
30-fold greater affinity for the receptor than that of SRIF...binding of 125I-Tyr11-SRIF
was markedly reduced in the presence of Na+ ions and GTP, indicating coupling of rAP6-26
receptors to inhibitory G proteins...forskolin-induced cAMP accumulation was inhibited by
SRIF and SRIF-28
- statement: SSTR5 mRNA is expressed in pituitary but not brain, liver, kidney, pancreas,
cerebellum, or cortex by Northern blot.
supporting_text: >-
a approximately 2.6 kilobase mRNA encoding the receptor was present in the pituitary
but not in the liver, small intestine, kidney, pancreas, cerebellum, or cortex
- id: PMID:7908405
title: Molecular cloning, functional characterization, and chromosomal localization of a
human somatostatin receptor (somatostatin receptor type 5) with preferential affinity
for somatostatin-28.
findings:
- statement: Human SSTR5 confirmed as SST-28-selective receptor coupled to pertussis
toxin-sensitive G proteins, with SSTR5 mRNA selectively localized in rat anterior pituitary.
supporting_text: >-
hSSTR5 bound SST-28 with a 12.6-fold greater affinity (Ki = 0.19 nM), compared with
SST-14 (Ki = 2.24 nM)...hSSTR5 is coupled to pertussis toxin-sensitive G proteins
...In situ hybridization of the rat pituitary showed that SSTR5 mRNA is selectively
localized in the anterior lobe
- id: PMID:14512709
title: 'Role of glucocorticoids in the regulation of pituitary somatostatin receptor
subtype (sst1-sst5) mRNA levels: evidence for direct and somatostatin-mediated effects.'
findings:
- statement: Dexamethasone increases sst5 mRNA levels in rat pituitary while decreasing
sst1-sst4, both in vivo and in vitro.
supporting_text: >-
High-dose DEX resulted in a decrease in sst1-sst4 mRNA and an increase in sst5 mRNA,
independent of adrenal status...DEX also decreased sst2, sst3 and sst4 mRNA levels and
increased sst5 mRNA levels by short-term in vitro application (10 nM, 4 h) in primary
rat pituitary cell cultures
- id: PMID:21820437
title: Somatostatin receptor 5 is palmitoylated by the interacting ZDHHC5 palmitoyltransferase.
findings:
- statement: SSTR5 is palmitoylated by the palmitoyltransferase ZDHHC5, the first
palmitoyltransferase identified for any GPCR.
supporting_text: >-
Coexpression of ZDHHC5 in HEK293 cells increased palmitoylation of SSTR5 whereas
knock-down of endogenous ZDHHC5 by siRNAs decreased it
- id: PMID:12511609
title: Somatostatin receptor subtype 5 regulates insulin secretion and glucose homeostasis.
findings:
- statement: SSTR5 knockout mice show decreased blood glucose, altered insulin secretion,
and resistance to diet-induced insulin resistance, demonstrating SSTR5 mediates
somatostatin inhibition of insulin secretion.
supporting_text: >-
sst(5) KO mice exhibited decreased blood glucose and plasma insulin levels and increased
leptin and glucagon concentrations...sst(5) KO mice displayed decreased susceptibility
to high fat diet-induced insulin resistance
- id: PMID:18653781
title: Cell growth inhibition and functioning of human somatostatin receptor type 2 are
modulated by receptor heterodimerization.
findings:
- statement: SSTR2 and SSTR5 heterodimerize, and SSTR5 enhances SSTR2-mediated adenylyl
cyclase inhibition, ERK1/2 activation, p27 induction, and growth inhibition.
supporting_text: >-
SSTR2 and SSTR5 heterodimerize...The SSTR2-selective agonist L-779,976 is more
efficacious at inhibiting adenylate cyclase, activating ERK1/2, and inducing the
cyclin-dependent kinase inhibitor p27(Kip1) in cells expressing both SSTR2 and SSTR5
compared with SSTR2 alone
- id: GO_REF:0000002
title: Gene Ontology annotation through association of InterPro records with GO terms
findings: []
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings: []
- id: GO_REF:0000044
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
findings: []
- id: GO_REF:0000108
title: Automatic assignment of GO terms using logical inference, based on inter-ontology
links
findings: []
- id: GO_REF:0000117
title: Electronic Gene Ontology annotations created by ARBA machine learning models
findings: []
- id: GO_REF:0000121
title: RGD ISO annotations to rat from other mammalian species
findings: []
- id: file:rat/Sstr5/Sstr5-deep-research-bioreason-sft.md
title: BioReason-Pro SFT reasoning trace for Sstr5
findings:
- statement: BioReason correctly identifies the core GPCR/Gi-coupled signaling and adenylyl
cyclase inhibition but incorrectly states preference for somatostatin-14 over somatostatin-28.
existing_annotations:
- term:
id: GO:0004994
label: somatostatin receptor activity
evidence_type: IDA
original_reference_id: PMID:1362243
review:
summary: Directly demonstrated by radioligand binding in COS-7 cells transfected with
the cloned rat SSTR5 cDNA. The receptor binds somatostatin-28 with ~30-fold preference
over somatostatin-14. This is a core molecular function.
action: ACCEPT
reason: IDA from the original cloning paper. Specific binding of 125I-Tyr11-SRIF was
demonstrated in membranes from transfected COS-7 cells.
supported_by:
- reference_id: PMID:1362243
supporting_text: >-
Membranes prepared from COS-7 cells transfected with the rAP6-26 cDNA showed specific
binding of 125I-Tyr11-SRIF, thus identifying the cDNA clone as a novel SRIF receptor
- reference_id: file:rat/Sstr5/Sstr5-deep-research-bioreason-sft.md
supporting_text: >-
[BioReason correctly identifies] somatostatin receptor activity (GO:0004994) by binding
somatostatin peptides at the extracellular N-terminus and transmembrane pocket
- term:
id: GO:0007193
label: adenylate cyclase-inhibiting G protein-coupled receptor signaling pathway
evidence_type: IDA
original_reference_id: PMID:1362243
review:
summary: Directly demonstrated in COS-7 cells expressing cloned SSTR5. Somatostatin
inhibited forskolin-induced cAMP accumulation, and GTP sensitivity indicated Gi coupling.
This is a core signaling pathway for this receptor.
action: ACCEPT
reason: IDA showing functional coupling to adenylyl cyclase inhibition via Gi proteins,
confirmed by pertussis toxin sensitivity in human ortholog studies.
supported_by:
- reference_id: PMID:1362243
supporting_text: >-
forskolin-induced cAMP accumulation was inhibited by SRIF and SRIF-28, thus confirming
that the rAP6-26 cDNA encodes a functional receptor protein...binding of
125I-Tyr11-SRIF was markedly reduced in the presence of Na+ ions and GTP, indicating
coupling of rAP6-26 receptors to inhibitory G proteins
- term:
id: GO:0004994
label: somatostatin receptor activity
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: Phylogenetically inferred somatostatin receptor activity, consistent with the
IDA evidence from PMID:1362243. Redundant with the direct experimental evidence.
action: ACCEPT
reason: Correct annotation supported by both phylogenetic inference and direct experimental
data.
- term:
id: GO:0004994
label: somatostatin receptor activity
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: InterPro2GO mapping from IPR000586 and IPR001184 (somatostatin receptor family/type 5).
Correct and consistent with experimental evidence.
action: ACCEPT
reason: Automated annotation from domain signatures that is accurate for this well-characterized
somatostatin receptor.
- term:
id: GO:0004930
label: G protein-coupled receptor activity
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: InterPro2GO mapping from IPR000276 (rhodopsin-like GPCR family). Correct but
less specific than GO:0004994 (somatostatin receptor activity).
action: KEEP_AS_NON_CORE
reason: Accurate but redundant with the more specific somatostatin receptor activity
annotation. The parent GPCR activity term is implied by the child term.
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: Phylogenetically inferred plasma membrane localization, consistent with this
being a cell-surface GPCR. Supported by UniProt subcellular location annotation.
action: ACCEPT
reason: Correct localization for a seven-transmembrane receptor that binds extracellular
somatostatin ligands.
supported_by:
- reference_id: PMID:1362243
supporting_text: >-
Membranes prepared from COS-7 cells transfected with the rAP6-26 cDNA showed specific
binding of 125I-Tyr11-SRIF
- term:
id: GO:0005886
label: plasma membrane
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: UniProt subcellular location mapping to plasma membrane. Correct and redundant
with IBA evidence.
action: ACCEPT
reason: Consistent with GPCR biology; the receptor must be at the plasma membrane to
bind extracellular somatostatin.
- term:
id: GO:0005886
label: plasma membrane
evidence_type: ISO
original_reference_id: GO_REF:0000121
review:
summary: RGD ISO annotation from human and mouse orthologs. Correct and consistent with
direct evidence for this 7TM receptor.
action: ACCEPT
reason: Accurate localization supported by multiple independent evidence lines.
- term:
id: GO:0043005
label: neuron projection
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: Phylogenetically inferred localization to neuron projections. While other somatostatin
receptors (SSTR1-4) are broadly expressed in brain neurons, SSTR5 in rat is notable for
its absence from most brain regions by Northern blot and ISH -- expression is restricted
to pituitary anterior lobe, not typical neurons. This annotation may be transferred from
orthologs with broader neural expression.
action: KEEP_AS_NON_CORE
reason: SSTR5 mRNA was not detected in rat brain regions by Northern blot or ISH in the
original studies. Expression is mainly pituitary and peripheral. While some neural
expression cannot be excluded at low levels, this is not a core localization for rat SSTR5.
supported_by:
- reference_id: PMID:1362243
supporting_text: >-
a approximately 2.6 kilobase mRNA encoding the receptor was present in the pituitary
but not in the liver, small intestine, kidney, pancreas, cerebellum, or cortex. Lack
of receptor mRNA expression in the brain was confirmed by in situ hybridization
histochemical studies
- term:
id: GO:0042923
label: neuropeptide binding
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: SSTR5 binds the neuropeptide somatostatin. This is correct but less specific
than somatostatin receptor activity (GO:0004994).
action: KEEP_AS_NON_CORE
reason: Accurate as somatostatin is a neuropeptide, but the more specific term
GO:0004994 (somatostatin receptor activity) better represents the core function.
- term:
id: GO:0007218
label: neuropeptide signaling pathway
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: SSTR5 participates in somatostatin (neuropeptide) signaling. Correct but less
specific than GO:0038170 (somatostatin signaling pathway) and GO:0007193 (adenylate
cyclase-inhibiting GPCR signaling pathway).
action: KEEP_AS_NON_CORE
reason: True at a general level -- somatostatin is a neuropeptide -- but the more specific
somatostatin signaling pathway term is more informative for this receptor.
- term:
id: GO:0007186
label: G protein-coupled receptor signaling pathway
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: InterPro2GO annotation for general GPCR signaling. Correct but less informative
than the specific adenylate cyclase-inhibiting pathway annotation.
action: KEEP_AS_NON_CORE
reason: Subsumed by the more specific GO:0007193 annotation that is supported by IDA evidence.
- term:
id: GO:0016020
label: membrane
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: Generic membrane localization from InterPro. Correct but uninformative given the
more specific plasma membrane annotation.
action: KEEP_AS_NON_CORE
reason: Too general; plasma membrane (GO:0005886) is more appropriate and already annotated.
- term:
id: GO:0038170
label: somatostatin signaling pathway
evidence_type: IEA
original_reference_id: GO_REF:0000108
review:
summary: Logically inferred from GO:0004994 (somatostatin receptor activity). This is a
correct and informative annotation for this receptor.
action: ACCEPT
reason: SSTR5 is a somatostatin receptor, so involvement in the somatostatin signaling
pathway is directly entailed. Supported by functional data from PMID:1362243.
supported_by:
- reference_id: PMID:1362243
supporting_text: >-
forskolin-induced cAMP accumulation was inhibited by SRIF and SRIF-28
- term:
id: GO:0071385
label: cellular response to glucocorticoid stimulus
evidence_type: IEP
original_reference_id: PMID:14512709
review:
summary: SSTR5 mRNA is upregulated by dexamethasone in rat pituitary, both in vivo and
in vitro. This is an expression pattern (IEP) annotation indicating that sst5 expression
changes in response to glucocorticoids. The response is distinctive -- sst5 is the only
subtype increased by DEX while all others decrease.
action: ACCEPT
reason: Well-supported IEP annotation. DEX treatment increases sst5 mRNA in rat pituitary
cells at both in vivo and in vitro levels, demonstrating a cellular response.
supported_by:
- reference_id: PMID:14512709
supporting_text: >-
High-dose DEX resulted in a decrease in sst1-sst4 mRNA and an increase in sst5 mRNA,
independent of adrenal status
- term:
id: GO:0071385
label: cellular response to glucocorticoid stimulus
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: Phylogenetically inferred, consistent with the IEP evidence from PMID:14512709.
action: ACCEPT
reason: Redundant with the directly supported IEP annotation but consistent.
- term:
id: GO:0071385
label: cellular response to glucocorticoid stimulus
evidence_type: IEA
original_reference_id: GO_REF:0000117
review:
summary: ARBA machine learning annotation, consistent with IEP evidence.
action: ACCEPT
reason: Consistent with experimental IEP evidence from PMID:14512709.
- term:
id: GO:0050796
label: regulation of insulin secretion
evidence_type: IBA
original_reference_id: GO_REF:0000033
review:
summary: Phylogenetically inferred role in regulation of insulin secretion. Strongly
supported by knockout mouse data showing SSTR5 mediates somatostatin inhibition of
insulin secretion.
action: ACCEPT
reason: Core physiological function of SSTR5 demonstrated by knockout studies in mouse,
with conservation expected in rat given high sequence identity.
supported_by:
- reference_id: PMID:12511609
supporting_text: >-
sst(5) mediates SRIF inhibition of pancreatic insulin secretion and contributes to
the regulation of glucose homeostasis and insulin sensitivity
- term:
id: GO:0050796
label: regulation of insulin secretion
evidence_type: ISO
original_reference_id: GO_REF:0000121
review:
summary: RGD ISO annotation from mouse ortholog data. Well-supported by SSTR5 KO studies.
action: ACCEPT
reason: Consistent with multiple knockout studies demonstrating SSTR5 role in insulin
secretion regulation.
- term:
id: GO:0042593
label: glucose homeostasis
evidence_type: ISO
original_reference_id: GO_REF:0000121
review:
summary: RGD ISO from mouse data. SSTR5 KO mice show altered blood glucose levels and
resistance to diet-induced insulin resistance, supporting a role in glucose homeostasis.
action: ACCEPT
reason: Supported by SSTR5 KO studies showing decreased blood glucose and improved
insulin sensitivity.
supported_by:
- reference_id: PMID:12511609
supporting_text: >-
sst(5) KO mice exhibited decreased blood glucose and plasma insulin levels...sst(5)
KO mice displayed decreased susceptibility to high fat diet-induced insulin resistance
- term:
id: GO:0060124
label: positive regulation of growth hormone secretion
evidence_type: ISO
original_reference_id: GO_REF:0000121
review:
summary: RGD ISO annotation suggesting SSTR5 positively regulates growth hormone secretion.
This is counterintuitive for a Gi-coupled receptor that generally inhibits secretion.
However, the heterodimerization with SSTR2 modulates SSTR2 function in pituitary
somatotropes, and the net effect on GH may be complex. The annotation is from RGD
and may reflect indirect or context-dependent effects observed in specific model systems.
action: KEEP_AS_NON_CORE
reason: The relationship between SSTR5 and GH secretion is complex. SSTR5 is expressed
in pituitary and somatostatin generally inhibits GH, but the positive regulation
annotation likely reflects specific experimental contexts rather than the canonical
inhibitory function of somatostatin receptors.
- term:
id: GO:0032467
label: positive regulation of cytokinesis
evidence_type: ISO
original_reference_id: GO_REF:0000121
review:
summary: RGD ISO annotation for positive regulation of cytokinesis. This is unexpected
for SSTR5, which is generally associated with growth inhibition rather than promotion
of cell division. May relate to SSTR2/SSTR5 heterodimerization effects on cell cycle
or indirect downstream effects.
action: KEEP_AS_NON_CORE
reason: Not a well-characterized core function of SSTR5. The receptor is primarily known
for Gi-mediated growth inhibition, not promotion of cytokinesis. This may reflect
context-dependent or indirect effects.
core_functions:
- description: >-
SSTR5 functions as a Gi/o-coupled somatostatin receptor at the plasma membrane of
pituitary and neuroendocrine cells, preferentially binding somatostatin-28 to inhibit
adenylyl cyclase and reduce cAMP, thereby suppressing hormone secretion.
molecular_function:
id: GO:0004994
label: somatostatin receptor activity
directly_involved_in:
- id: GO:0007193
label: adenylate cyclase-inhibiting G protein-coupled receptor signaling pathway
- id: GO:0038170
label: somatostatin signaling pathway
locations:
- id: GO:0005886
label: plasma membrane
supported_by:
- reference_id: PMID:1362243
supporting_text: >-
SRIF-28 was the most potent competitor...forskolin-induced cAMP accumulation was
inhibited by SRIF and SRIF-28...coupling of rAP6-26 receptors to inhibitory G proteins
- description: >-
SSTR5 mediates somatostatin inhibition of insulin secretion from pancreatic beta cells
and contributes to glucose homeostasis, as demonstrated by knockout mouse studies.
molecular_function:
id: GO:0004994
label: somatostatin receptor activity
directly_involved_in:
- id: GO:0050796
label: regulation of insulin secretion
- id: GO:0042593
label: glucose homeostasis
locations:
- id: GO:0005886
label: plasma membrane
supported_by:
- reference_id: PMID:12511609
supporting_text: >-
sst(5) mediates SRIF inhibition of pancreatic insulin secretion and contributes to
the regulation of glucose homeostasis and insulin sensitivity
suggested_questions:
- question: What is the relative contribution of SSTR5 versus SSTR2 to somatostatin-mediated
inhibition of insulin secretion in rat pancreatic islets specifically?
experts:
- Strowski MZ
- Brunicardi FC
- question: Does SSTR5 have significant expression in rat brain regions beyond the anterior
pituitary, given improved detection methods since the original Northern blot studies?
experts:
- O'Carroll AM
- Patel YC
suggested_experiments:
- hypothesis: SSTR5 expression in rat pituitary is regulated at the transcriptional level
by glucocorticoid receptor binding to the Sstr5 promoter.
description: Perform ChIP-seq for glucocorticoid receptor in rat pituitary cells treated
with dexamethasone, focusing on the Sstr5 locus to determine whether GR directly binds
the promoter region or whether the effect is indirect.
experiment_type: ChIP-seq
- hypothesis: SSTR5/SSTR2 heterodimerization occurs in rat pituitary somatotropes and
modulates growth hormone secretion.
description: Use proximity ligation assays (PLA) or BRET in primary rat pituitary cell
cultures to detect SSTR2-SSTR5 heterodimers in situ, and correlate with GH secretion
measurements in response to somatostatin analogs.
experiment_type: Proximity ligation assay / BRET