GHSR (Growth Hormone Secretagogue Receptor)

The ghrelin receptor (GHSR, GHS-R1a) is a class A GPCR expressed predominantly in the hypothalamus and pituitary that serves as the primary receptor for the hunger hormone ghrelin. It mediates pulsatile growth hormone (GH) release and appetite stimulation, and is the pharmacological target of MK-677 (ibutamoren). GHSR sits at the intersection of the GH-IGF-1 axis, appetite regulation, and nutrient-sensing — all of which decline with age — making it a key receptor in the biology of sarcopenia, anorexia of aging, and the contested longevity-paradox of GH stimulation.

Identity

UniProt: Q92847 (GHSR_HUMAN; Swiss-Prot manually curated entry)
NCBI Gene ID: 2693
HGNC symbol: GHSR
HGNC ID: 4267
Ensembl: ENSG00000121853
Mouse ortholog: Ghsr (one-to-one ortholog)
Length: 366 amino acids (isoform 1a, canonical signaling-active form)
Class: Class A (rhodopsin-like) GPCR; seven transmembrane helices
Chromosome: 3q26.31 (human)
GenAge entry: Not listed in GenAge Human or GenAge Models as of 2026-05 — GHSR aging-relevance is pathway-level rather than direct lifespan-genetic. genage-id: null.

Splice variants and isoforms

Two well-characterized isoforms arise from alternative splicing of the third intron:

Isoform	Length	Signaling	Notes
GHSR-1a (full-length)	366 aa / 7 TM helices	Active — couples to Gαq/11 and Gαs	Canonical receptor; expressed in hypothalamus and pituitary
GHSR-1b (truncated)	289 aa / 5 TM helices (predicted)	Inactive — does not bind ghrelin or signal independently	Functions as a decoy receptor; can heterodimerize with GHSR-1a and other GPCRs (dopamine D1R, serotonin 5-HT2C) to modulate signaling; may act as a dominant-negative in some tissues

The GHSR-1b isoform is broadly expressed across many tissues where GHSR-1a is absent ¹. The functional significance of GHSR-1a / GHSR-1b heterodimers in the context of aging is poorly characterized. no-mechanism

Structure and key features

Seven transmembrane helices with an extracellular N-terminus (Asn-13 and Asn-27 glycosylated) and intracellular C-terminus
Disulfide bond between Cys-116 and Cys-198 (extracellular loop — required for receptor folding and ligand access)
Octanoate (n-octanoyl) binding pocket — the acylation at Ser-3 of ghrelin engages the hydrophobic core of the binding pocket; deacylated ghrelin (desacyl ghrelin / UAG) does not bind GHSR-1a with significant affinity
Constitutive activity — GHSR-1a has high constitutive (ligand-independent) activity at Gαq/11, which is rare among GPCRs. This baseline activity is physiologically relevant: it maintains tonic appetite and GH tone even in the fasted state, and is a druggable handle for inverse agonists ². The quantitative extent of this constitutive activity relative to maximum agonist response is documented in Holst 2003; functional selectivity across ligand classes is described in Ramirez 2019 (closed-access, no-fulltext-access) ³.

Endogenous ligands

Ghrelin (primary agonist)

Ghrelin is a 28-amino acid peptide produced predominantly by X/A-like neuroendocrine cells of the gastric oxyntic mucosa. The n-octanoylation at Ser-3 (added post-translationally by ghrelin O-acyltransferase, GOAT) is required for GHSR-1a binding and GH-releasing activity ⁴. Acylated ghrelin (AG) circulates alongside the more abundant desacyl ghrelin (UAG / desacyl), which does not bind GHSR-1a but has biological activity through incompletely characterized receptors.

Ghrelin and aging: circulating total ghrelin changes with age are inconsistent across studies — some report decreased fasting ghrelin in older adults, others unchanged or increased levels. The acyl-to-desacyl ratio may shift with age, affecting functional GHSR-1a engagement. contradictory-evidence

LEAP2 (endogenous antagonist/inverse agonist)

Liver-Expressed Antimicrobial Peptide 2 (LEAP2) was identified as an endogenous antagonist of GHSR-1a ⁵. LEAP2 rises with positive energy balance (fed state, obesity) and suppresses ghrelin-induced GH secretion and appetite. Two recruiting clinical trials (NCT06013592, NCT07171723) are studying LEAP2/ghrelin balance in obesity and during semaglutide treatment (2026). The LEAP2 axis is emerging as a second endogenous regulator of GHSR tone, but its age-specific dynamics have not been characterized. needs-human-replication

Signaling

GHSR-1a couples primarily through Gαq/11, with secondary Gαs coupling:

Gαq/11 → PLC-β → IP3/DAG → PKC + intracellular Ca2+ — rapid signaling; drives GH vesicle exocytosis from somatotrophs; drives appetite signaling in hypothalamic NPY/AgRP neurons
Gαs → adenylyl cyclase → cAMP → PKA — amplifies the GH-secretory response; relevant in pituitary somatotrophs
β-arrestin recruitment — GHSR-1a displays signaling bias: some ligands (inverse agonists, partial agonists) shift the balance toward or away from β-arrestin internalization vs G-protein-mediated signaling. This functional selectivity is being exploited to design ligands that dissociate appetite effects from GH effects ³.

Constitutive activity drives high basal Gαq/11 signaling independently of ligand ². The constitutive activity is sensitive to inverse agonists (which reduce signaling below baseline) and is responsible for maintaining tonic GH pulsatility in the fasted state. The specific quantitative level relative to maximum agonist response is not independently confirmed here (Ramirez 2019 closed-access; no-fulltext-access).

Tissue expression

From GTEx v10 (ENSG00000121853.4):

Tissue	Expression (TPM, approx.)	Relevance
Pituitary	~24 (highest)	Somatotroph GH release; primary endocrine site
Hypothalamus	~18	Arcuate NPY/AgRP neurons; appetite regulation
Nucleus accumbens	~1.9	Reward-motivated feeding; mesolimbic dopamine integration
Testis	~1.2	Reproductive axis cross-talk
Whole blood	~0.9	Immune cell expression; significance unclear
Other peripheral tissues	<0.5	Includes stomach (local paracrine role), pancreas, heart

UniProt also documents expression in the stomach (paracrine ghrelin/GHSR axis), cardiac tissue (cardioprotective GHSR signaling in animal models), and pancreas (glucose homeostasis).

Role in aging

Somatopause and GHSR signaling decline

From approximately age 30, pulsatile GH release declines by ~14% per decade (somatopause). By age 70, mean 24-hour GH secretion is ~5–10% of young-adult levels. Contributing mechanisms include:

Increased somatostatinergic tone (inhibiting GH release)
Reduced GHRH secretion from the hypothalamus
Possible reduced GHSR-1a responsiveness at the pituitary level (direct receptor downregulation is debated; the predominant mechanism appears to be upstream hypothalamic)
Increased adiposity (which further blunts GH pulsatility via free fatty acid-mediated inhibition)

The net consequence is secondary: decreased IGF-1, contributing to sarcopenia, adiposity, reduced bone density, and sleep-architecture deterioration. This positions GHSR-1a as a proximal drug target for pharmacological restoration of GH pulsatility — the rationale for mk-677.

GHSR and anorexia of aging

GHSR-1a signaling in hypothalamic arcuate NPY/AgRP neurons drives the orexigenic (appetite-stimulating) response to ghrelin. Age-related decline in appetite (anorexia of aging) — distinct from somatopause — is multifactorial and involves gastrointestinal hormonal changes. Picca et al. 2022 reviewed metabolic and hormonal biomarker candidates for anorexia of aging, including ghrelin, CCK, leptin, and inflammatory mediators ⁶. Importantly, that review notes that plasma concentrations of total ghrelin and acyl-ghrelin do not show consistent variation with aging per se; ghrelin-pathway dysregulation may occur via altered secretion timing, receptor responsiveness, or acyl/desacyl ratio rather than bulk circulating levels. contradictory-evidence

GHSR and sarcopenia

Ghrelin/GHSR signaling intersects with skeletal muscle maintenance via two mechanisms:

Indirect (GH-IGF-1-dependent): GHSR-1a agonism drives GH, which drives hepatic IGF-1 production, which activates pi3k-akt-pathway → mtor → muscle protein synthesis. This is the primary mechanism exploited by MK-677 in sarcopenia trials.
Direct (GH-IGF-1-independent): Evidence from Sun 2020 and Pradhan 2013 suggests ghrelin (including desacyl ghrelin, which does not bind GHSR-1a) promotes C2C12 myoblast differentiation and fusion and attenuates muscle atrophy ⁷⁸. The direct myogenic effect may not be fully captured by GHSR-1a agonism alone — UAG has activity not mediated through this receptor.

The GH-IGF-1 longevity paradox

GHSR-1a occupies a critical position in one of the central paradoxes of aging biology: interventions that reduce GH-IGF-1 signaling extend lifespan in model organisms, while short-term GH-axis stimulation partially reverses age-associated functional decline in humans. See mk-677 § Aging-relevance paradox for the full framing. Key points:

Ames and Snell dwarf mice (GH-deficient) live substantially longer than WT controls ⁹
Heterozygous Igf1r knockout mice have extended lifespan ⁹
Human centenarian cohorts show enrichment of low-IGF-1 pathway variants
Against this: somatopause in humans produces sarcopenia, frailty, and functional decline — the short-term trade-off is visible even if the long-term effect on human lifespan is unknown

The current resolution is unresolved. Chronic GHSR-1a agonism (e.g., MK-677) likely improves specific healthspan metrics while the net effect on human lifespan is unknown and theoretically adverse per the model-organism data. See deregulated-nutrient-sensing for pathway-level context.

Dimension	Status	Notes
Pathway conserved in humans?	yes	GHSR cloned from human pituitary; human vs swine type Ia ~93% identical, ~98% similar at amino acid level ¹; rat ortholog not quantified in Howard 1996
Somatopause phenotype conserved?	yes	GH/IGF-1 decline with age is well-documented across mammalian species
Longevity paradox replicated in humans?	partial	Longevity associations exist but no controlled human lifespan data

needs-human-replication — the causal contribution of GHSR-1a signaling changes to human aging (vs. the correlative somatopause decline) is not established.

Pharmacology

Agonists (GH secretagogues)

Compound	Route	Class	Aging indication
mk-677 (ibutamoren)	Oral	Non-peptide spiroindoline	Phase 2 (sarcopenia, frailty, fracture recovery)
GHRP-6	Injection	Hexapeptide	Preclinical; research tool
GHRP-2	Injection	Hexapeptide	Preclinical; research tool
Hexarelin	Injection	Hexapeptide; cardioprotective in animal models	Preclinical
Ipamorelin	Injection	Pentapeptide; high GH selectivity	Preclinical / compounded

MK-677 is the only orally active, non-peptide full agonist with multi-year Phase 2 human data in older adults.

Inverse agonists / antagonists

GHSR-1a constitutive activity is targetable by inverse agonists (reduce signaling below basal). These are being developed for obesity (suppress appetite + GH-driven weight gain) ¹⁰. Not currently in aging-longevity indication.

Druggability-tier 2 rationale: MK-677 is a full agonist with well-characterized Phase 2 human data in aging-relevant populations (elderly, sarcopenia, hip-fracture recovery). However, no aging-indication FDA approval exists, Merck discontinued the program, and MK-677 is now in compounded-drug limbo under FDA 503A. Multiple agonists and inverse agonists in development confirm high chemical tractability. Tier 1 would require an FDA-approved drug for an aging indication — not met. Tier 2 reflects high-quality preclinical + advanced-clinical tooling without approved aging use. Consistent with the aging-context convention in CLAUDE.md.

Genetic associations

GHSR mutations and isolated partial GH deficiency (GHDP): Missense variants in GHSR that reduce constitutive receptor activity (without abolishing ghrelin responsiveness) cause short stature / growth delay. Documented in UniProt Q92847 disease annotations.
GWAS and longevity: No strong direct GHSR GWAS hits for longevity in LongevityMap as of 2026 literature. IGF-1 pathway GWAS hits exist but are not GHSR-specific.
MR evidence: No published Mendelian randomization study specifically uses GHSR variants as instruments for aging outcomes. mr-causal-evidence: not-tested. needs-human-replication

Key interactors

growth-hormone — downstream effector; GHSR-1a drives pulsatile GH release from anterior pituitary somatotrophs
igf-1 — secondary downstream; GH drives hepatic IGF-1 production
igf1r — anabolic signaling receptor downstream of IGF-1
Ghrelin — primary endogenous agonist (no dedicated wiki page yet — implicit stub)
LEAP2 — endogenous inverse agonist/antagonist (no dedicated wiki page yet — implicit stub)
NPY / AgRP neurons — GHSR-1a expression on arcuate nucleus neurons drives appetite; NPY/AgRP are downstream effectors (no dedicated wiki pages — implicit stubs)
Somatostatin — tonically inhibits GH release; opposes the GHSR-1a signal at the pituitary level (no dedicated wiki page — implicit stub)
GOAT (ghrelin O-acyltransferase) — enzyme that octanoylates ghrelin, enabling GHSR-1a binding (no dedicated wiki page — implicit stub)

Cross-references

Parent compound page: mk-677 — full human-trial evidence, pharmacokinetics, safety profile, and the aging-paradox framing
Downstream pathway: insulin-igf1 — GH-IGF-1 signaling axis; GHSR-1a agonism feeds into this pathway
Downstream pathway: mtor — IGF-1 activates PI3K-AKT-mTOR; mTOR drives protein synthesis (anabolic arm) but also suppresses autophagy (longevity-antagonistic arm)
Key hallmark: deregulated-nutrient-sensing — the GH-IGF-1 axis is a canonical nutrient/anabolic-sensing pathway; GHSR sits at its apex
Key phenotype: sarcopenia — primary clinical aging phenotype motivating GHSR-1a agonist development
Related protein: growth-hormone — GHSR-1a’s primary secretagogue target

Limitations and gaps

#gap/needs-gtex-aging-correlation — GTEx v10 does not provide per-age-group GHSR expression summary statistics at the tissue level in the standard API endpoint; an age-stratified analysis would require sample-level metadata. The aging-correlation direction (up/down with age in hypothalamus or pituitary) is unknown from GTEx alone.
#gap/needs-human-replication — MR-based causal evidence linking GHSR signaling changes to human aging outcomes has not been published.
#gap/contradictory-evidence — circulating ghrelin levels with aging show inconsistent direction across studies; the acyl/desacyl ratio with aging is not well characterized.
#gap/no-mechanism — GHSR-1b / GHSR-1a heterodimer function in aging context is not characterized.
#gap/long-term-unknown — whether chronic GHSR-1a agonism (e.g., >2 years in older adults) has net beneficial or adverse effects on lifespan is unknown. The model-organism longevity literature argues against it; the human healthspan literature argues for short-term use in specific populations.
genage-id: null — GHSR is not listed in GenAge Human or GenAge Models; no direct lifespan-manipulation study using GHSR-specific genetic perturbation in a model organism has been published that meets GenAge inclusion criteria.

Footnotes

doi:10.1126/science.273.5277.974 · Howard AD et al. · Science 1996 · n=N/A (molecular cloning) · in-vitro / molecular biology · cloned GHS-R from swine then human pituitary cDNA libraries; type Ia (366 aa, 7 TM) and type Ib (289 aa, 5 predicted TM); Gα11 coupling required (other Gα subunits inactive); in situ hybridization detected mRNA in pituitary and arcuate/infundibular hypothalamus; human vs swine type Ia ~93% identical, ~98% similar at amino acid level · locally available ↩ ↩²
doi:10.1210/me.2003-0069 · Holst B et al. · Mol Endocrinol 2003 · in-vitro (COS-7 / HEK293) · demonstrated high constitutive (ligand-independent) Gαq/11 signaling of GHSR; identified [D-Arg1,D-Phe5,D-Trp7,9,Leu11]-substance P as a full inverse agonist (EC50 ~5.2 nM) capable of reducing constitutive signaling to untransfected-cell baseline; concluded GHSR is highly constitutively active with physiological relevance for GH secretion and appetite control ↩ ↩²
doi:10.1096/fj.201800655R · Ramirez VT et al. · FASEB Journal 2019 · in-vitro · characterized functional selectivity of GHSR-1a ligands (antagonists and inverse agonists); demonstrated G-protein vs β-arrestin signaling bias across compound classes · not locally available (not_oa in archive; no-fulltext-access — constitutive activity quantification and β-arrestin bias claims from this source cannot be independently confirmed) ↩ ↩²
doi:10.1038/45230 · Kojima M et al. · Nature 1999 · n=N/A (peptide purification) · in-vivo/in-vitro · purified and identified ghrelin from rat stomach as the endogenous GHS-R ligand; octanoylation at Ser-3 required for activity; 8399 citations · not locally available (not_oa in archive) ↩
doi:10.1016/j.cmet.2017.10.016 · Ge X et al. · Cell Metabolism 2018 · in-vitro/in-vivo · identified LEAP2 (liver-expressed antimicrobial peptide 2) as an endogenous antagonist of GHSR-1a; LEAP2 suppresses ghrelin-induced GH secretion and food intake; rises with positive energy balance · primary source for LEAP2 as endogenous GHSR antagonist ↩
doi:10.2147/CIA.S325008 · Picca A et al. · Clin Interv Aging 2022 · review · reviewed metabolic and hormonal biomarkers for anorexia of aging including CCK, ghrelin, leptin, and inflammatory mediators; notes plasma total ghrelin and acyl-ghrelin concentrations do not show consistent variation with aging per se; ghrelin agonists (anamorelin) proposed as therapeutic strategy warranting further investigation · locally available ↩
sun-2020-ghrelin-muscle-aging · PMID 34368393 · PMC8341557 · Sun Y · J Aging Science 2020;8(Suppl 3):005 · review · both AG and UAG promote differentiation and fusion of C2C12 myoblasts; both attenuate fasting- or denervation-induced muscle atrophy in aging mice; UAG lacks GHSR-1a binding (no octanoylation) and avoids GH elevation + adiposity side effects; ghrelin gene deficiency increases vulnerability to fasting-induced muscle loss in aging mice · abstract-only confirmed (no local PDF; open-access via PMC) ↩
doi:10.1097/MCO.0b013e328365b9be · Pradhan G, Samson SL, Sun Y · Curr Opin Clin Nutr Metab Care 2013 · review · summarized ghrelin signaling roles in glucose homeostasis, cardioprotection, muscle atrophy, and bone; ghrelin prevents muscle atrophy by inducing differentiation and fusion · not locally available (green OA but download failed — 0 candidate URLs; no-fulltext-access — direct myogenic mechanism claims from this source unconfirmed) ↩
doi:10.2337/db11-1300 · Barzilai N et al. · Diabetes 2012 · review (Perspectives in Diabetes) · framed paradox of GH/IGF-1 axis in aging: IIS attenuation extends lifespan in model organisms (yeast, nematodes, flies, mice); Ames and Snell dwarf mice plus GH receptor KO mice (citing Brown 2001/Coschigano 2003 as primary sources, refs 31/32 in paper); heterozygous Igf1r KO also extends lifespan; paper does NOT itself provide primary lifespan extension percentages for dwarf mice — these come from cited primary studies · locally available ↩ ↩²
doi:10.1111/dom.13020 · Abegg K et al. · Diabetes Obes Metab 2017 · in-vivo (rodent) · two GHSR-1a inverse agonists reduced food intake and body weight while improving glucose tolerance and hepatic steatosis; exploited constitutive receptor activity ↩

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