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 file:rat/Sstr5/Sstr5-deep-research-falcon.md SSTR5 has been reported to bind both SST forms, with **~10-fold higher affinity for SST-28 than SST-14**, a property often invoked to explain subtype-selective physiological regulation. |
| 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 file:rat/Sstr5/Sstr5-deep-research-falcon.md Across subtypes, SSTR activation is canonically **Gi/Go-coupled** (pertussis-toxin-sensitive), producing **inhibition of adenylyl cyclase** and decreased intracellular **cAMP**, often accompanied by reduced Ca2+ signaling and suppression of secretion. file:rat/Sstr5/Sstr5-deep-research-falcon.md A major 2024 advance is the **cryo-EM solution of SSTR5–Gi complexes** bound to cyclic peptide agonists **cortistatin-17** and **octreotide** at **2.7–2.9 Å resolution**, enabling residue-level mapping of agonist recognition and activation. |
| 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. The original rat cloning study reported absence of SSTR5 mRNA from most brain regions by Northern blot and ISH, with expression concentrated in pituitary anterior lobe. However, the falcon deep research notes that later synthesis describes SSTR5 as moderately expressed throughout the brain with brain expression higher in rats than humans, so low-level neural expression cannot be excluded. Regardless, neuron projection is a subcellular-localization (CC) claim that has not been directly demonstrated for rat SSTR5; the annotation is most plausibly a phylogenetic transfer and is not a core localization. Reason: Neuron projection localization is not directly demonstrated for rat SSTR5. The original studies emphasized pituitary/peripheral expression, while later reviews suggest some rat brain expression. Even if SSTR5 is expressed in some neurons, the specific sub-compartmental neuron-projection localization is an IBA transfer, not a core, directly supported localization. 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 file:rat/Sstr5/Sstr5-deep-research-falcon.md SSTR5 is described as **moderately expressed throughout the brain**, and brain SSTR5 expression is stated to be **higher in rats than humans**, supporting biological relevance for rat neuroendocrine regulation. |
| 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 file:rat/Sstr5/Sstr5-deep-research-falcon.md Somatostatin receptors (SSTR1–5) are **class A/rhodopsin-like GPCRs** with a **canonical 7TM topology**. They mediate the inhibitory endocrine and neuromodulatory actions of the peptide hormone **somatostatin** (SST). |
| 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 file:rat/Sstr5/Sstr5-deep-research-falcon.md Sstr5 acts as an inhibitory GPCR node in paracrine/endocrine circuits where SST restrains secretion (e.g., pancreatic islet hormone release, pituitary hormone output) and can modulate growth-related signaling via phosphatases and MAPK branches. |
| 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 file:rat/Sstr5/Sstr5-deep-research-falcon.md genetic SSTR5 deletion and an orally delivered selective antagonist (compound-1) lowered glycemic markers and improved insulin sensitivity indices. |
| GO:0060124 positive regulation of growth hormone secretion | ISO GO_REF:0000121 | MARK AS OVER ANNOTATED | Summary: RGD ISO annotation asserting SSTR5 positively regulates growth hormone secretion. This directly contradicts the canonical biology of SSTR5 as a Gi/Go-coupled inhibitory receptor through which somatostatin SUPPRESSES pituitary hormone output. The falcon deep research describes SSTR5 as an inhibitory GPCR node that restrains secretion (including pituitary hormone output), the opposite of positive regulation. The directionality of this ISO annotation is most likely erroneous or reflects a narrow indirect/context-dependent observation, not a core function. Reason: The "positive" directionality conflicts with the established inhibitory function of somatostatin receptors on pituitary hormone secretion. No direct rat experimental evidence supports SSTR5 increasing GH release; the canonical role of somatostatin/SSTR5 in the pituitary is inhibitory. This ISO annotation over-annotates SSTR5 to a process whose direction is inconsistent with its core Gi-coupled inhibitory pharmacology. Propagation Review Root cause: PROPAGATION BAD Failure modes: REGULATORY SIGN INVERSION CONTEXT OR TISSUE MISMATCH Sources checked: RGD:14121952 · human SSTR5 SUPPORTS SOURCE BUT NOT TARGET The donor is the human SSTR5 gene record; a positive-direction growth-hormone-secretion term conflicts with the Gi-coupled inhibitory pharmacology shared by human and rat SSTR5, so the direction does not transfer. Supporting Evidence: file:rat/Sstr5/Sstr5-deep-research-falcon.md Sstr5 acts as an inhibitory GPCR node in paracrine/endocrine circuits where SST restrains secretion (e.g., pancreatic islet hormone release, pituitary hormone output) and can modulate growth-related signaling via phosphatases and MAPK branches. |
| GO:0032467 positive regulation of cytokinesis | ISO GO_REF:0000121 | MARK AS OVER ANNOTATED | Summary: RGD ISO annotation for positive regulation of cytokinesis. This conflicts with the well-established antiproliferative/growth-inhibitory role of SSTR5, which signals via Gi to activate protein tyrosine phosphatases and induces cell-cycle inhibitors (e.g., p27) in the SSTR2/SSTR5 heterodimer context. The falcon deep research describes SSTR5-associated modulation of growth-related signaling via phosphatases and MAPK branches in the direction of restraint, not promotion of cell division. Promotion of cytokinesis is not a documented core function of this receptor. Reason: SSTR5 is canonically growth-inhibitory (Gi-coupled, phosphatase-activating, p27-inducing); a "positive regulation of cytokinesis" annotation is inconsistent with this core pharmacology and lacks direct rat experimental support. This is an over-annotation transferred by ISO that does not represent the receptor's core function. Propagation Review Root cause: PROPAGATION BAD Failure modes: REGULATORY SIGN INVERSION Sources checked: UniProtKB:P35346 · human SSTR5 SUPPORTS SOURCE BUT NOT TARGET Human SSTR5 signaling is growth-inhibitory (Gi-coupled, phosphatase-activating, p27-inducing), so a positive regulation of cytokinesis term is sign-inverted relative to the donor's own pharmacology. Supporting Evidence: file:rat/Sstr5/Sstr5-deep-research-falcon.md Sstr5 acts as an inhibitory GPCR node in paracrine/endocrine circuits where SST restrains secretion (e.g., pancreatic islet hormone release, pituitary hormone output) and can modulate growth-related signaling via phosphatases and MAPK branches. |
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Download this section (compressed HTML)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
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