Ghrelin: The Acylated Gastric Peptide Driving Growth…

Introduction

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Ghrelin (PMID: 42709409) occupies a singular position in peptide biology: it is the only known endogenous peptide hormone that carries a post-translational fatty acid modification essential for receptor activation. Isolated from rat stomach extracts in 1999 by Masayasu Kojima and colleagues at the National Cardiovascular Center Research Institute in Osaka, ghrelin was identified as the endogenous ligand for the growth hormone secretagogue receptor (GHSR, now formally designated GHS-R1a) — an orphan G protein-coupled receptor that had been cloned three years earlier (Kojima et al., 1999, Nature).

The name "ghrelin" derives from the Proto-Indo-European root ghre-, meaning "to grow," reflecting its potent growth hormone (GH)-releasing activity. Yet the quarter-century since its discovery has revealed ghrelin to be far more than a GH secretagogue. Research now implicates the ghrelin system in energy homeostasis, appetite regulation, glucose metabolism, cardiovascular function, neurogenesis, and immune modulation — making GHSR one of the most intensively studied GPCRs in modern pharmacology.

This article provides a comprehensive, research-use-only overview of ghrelin's structure, its unique GOAT-mediated acylation, GHSR signaling architecture, the endogenous antagonist LEAP2, and the current landscape of ghrelin-related research tools.

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Structure and Biosynthesis

Primary Sequence and Gene

Ghrelin is a 28-amino-acid peptide derived from the GHRL gene (chromosome 3p25-26 in humans), which encodes a 117-amino-acid prepropeptide called preproghrelin. Proteolytic processing yields mature ghrelin with the sequence:

GSS(octanoyl)FLSPEHQRVQQRKESKKPPAKLQPR

The peptide is predominantly produced by X/A-like enteroendocrine cells in the oxyntic mucosa of the gastric fundus, though lower levels of expression have been detected in the hypothalamus, pituitary, intestine, pancreas, and other tissues (Date et al., 2000, Endocrinology).

The Octanoyl Modification: GOAT and Acylation

The defining structural feature of ghrelin is the n-octanoylation of serine-3 (Ser3) — the attachment of an eight-carbon (C8:0) fatty acid chain via an ester bond. This modification is absolutely required for GHSR activation and GH-releasing activity, and represents the only known example of a serine-octanoylated peptide hormone in vertebrates.

The enzyme responsible for this unique modification is ghrelin O-acyltransferase (GOAT), a member of the membrane-bound O-acyltransferase (MBOAT) family. GOAT was independently identified in 2008 by two research groups — Yang et al. and Gutierrez et al. — who demonstrated that it is the sole enzyme capable of octanoylating preproghrelin (Yang et al., 2008, Cell).

Key biochemical features of the GOAT-ghrelin interaction include:

•Substrate specificity: GOAT recognizes the first five N-terminal amino acids of ghrelin (GSSFL), with the Gly-Ser-Ser motif being critical for substrate recognition
•Catalytic mechanism: The conserved His-338 residue in the MBOAT catalytic core contacts the Ser3 hydroxyl of ghrelin as the peptide enters the ER lumen, while Asn-307 coordinates the octanoyl-CoA acyl donor from the cytoplasmic face
•Acyl donor: Octanoyl-CoA serves as the preferred acyl donor, linking dietary medium-chain fatty acid availability to ghrelin activation — a potential nutrient-sensing mechanism (Heppner et al., 2019, Biochemistry)
•Alternative acylation: While C8:0 predominates, GOAT can also transfer C6:0 (hexanoyl), C10:0 (decanoyl), and unsaturated acyl chains, though with reduced efficiency

The GOAT-ghrelin axis has attracted significant research interest as a potential control point: inhibiting GOAT would reduce acyl-ghrelin levels without affecting unacylated ghrelin, offering a selective pharmacological strategy for modulating GHSR signaling.

Des-Acyl Ghrelin (Unacylated Ghrelin)

Notably, the majority (~80–90%) of circulating ghrelin exists in the unacylated (des-acyl) form. Long dismissed as an inactive degradation product, des-acyl ghrelin (DAG) is now recognized as a biologically active peptide in its own right.

Key findings in DAG research:

•GHSR-independent activity: DAG does not bind or activate GHS-R1a at physiological concentrations, yet it exerts measurable effects on insulin signaling, lipid metabolism, and skeletal muscle differentiation through a putative, still-unidentified receptor (Delhanty et al., 2010, PLoS ONE)
•Metabolic effects: In GHSR-knockout models, DAG rapidly modulates expression of lipogenic and insulin-signaling genes in adipose tissue, liver, and skeletal muscle, suggesting direct GHSR-independent metabolic activity
•Skeletal muscle: Both acyl and des-acyl ghrelin promote differentiation and fusion of C2C12 myoblasts in vitro, with DAG acting through a non-GHSR pathway (Filigheddu et al., 2007, Molecular Biology of the Cell)
•Circulating ratio: The acyl/des-acyl ghrelin ratio in plasma is dynamically regulated by nutritional state, GOAT expression, and esterase activity, and may serve as a metabolic biomarker

The existence of both acylated and unacylated forms adds a layer of complexity to the ghrelin system that is increasingly relevant to research design.

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The Ghrelin Receptor (GHSR)

Classification and Distribution

The growth hormone secretagogue receptor type 1a (GHS-R1a, or simply GHSR) is a class A (rhodopsin-family) GPCR encoded by the GHSR gene on chromosome 3q26. GHSR is expressed in the pituitary, hypothalamus (particularly the arcuate nucleus and ventromedial hypothalamus), hippocampus, ventral tegmental area, and substantia nigra, as well as peripheral tissues including the cardiovascular system, immune cells, and gastrointestinal tract.

A truncated splice variant, GHS-R1b (289 amino acids vs. 366 for GHS-R1a), is widely expressed but does not bind ghrelin and is pharmacologically inactive. However, GHS-R1b can form heterodimers with GHS-R1a and attenuate its signaling, potentially serving as a dominant-negative regulator.

Constitutive Activity

One of the most pharmacologically significant properties of GHSR is its remarkably high constitutive (basal) activity — meaning it signals even in the absence of ghrelin. GHSR exhibits approximately 50% of its maximal signaling capacity without any ligand bound, one of the highest constitutive activities reported for any wild-type GPCR (Holst et al., 2003, Molecular Endocrinology).

This constitutive activity has profound implications:

•Set-point regulation: Basal GHSR signaling may establish a tonic drive for food intake and GH secretion that operates independently of circulating ghrelin levels
•Fasting physiology: Constitutive GHSR activity has been shown to modulate the magnitude of compensatory hyperphagia after fasting in mouse models (Fernandez et al., 2018, Endocrinology)
•Pharmacological target: The constitutive activity makes GHSR uniquely susceptible to inverse agonists (compounds that suppress basal signaling below agonist-free levels), as distinct from neutral antagonists

Cryo-EM Structural Biology

The explosion of cryo-electron microscopy (cryo-EM) structures between 2020 and 2025 has transformed our understanding of GHSR at atomic resolution. Key structural milestones include:

•Antagonist-bound crystal structure (Shiimura et al., 2020): Revealed the inactive-state receptor architecture and the salt bridge between E124^3.33^ and R283^6.55^ that partitions the orthosteric binding pocket into two distinct cavities — a "bifurcated pocket" unique among peptide GPCRs ([Shiimura et al., 2020, Nature Communications]())
•Ghrelin-GHSR-Gq complex (Liu et al., 2021): Demonstrated how the octanoyl chain of ghrelin inserts into cavity I (a hydrophobic cleft between TM4, TM5, and TM6), while the peptide backbone engages cavity II, explaining the absolute requirement for Ser3 acylation ([Liu et al., 2021, Nature Communications]())
•Ghrelin-GHSR-Gi complex and Ibutamoren-GHSR-Gi complex (Qin et al., 2022): Showed how the non-peptide agonist ibutamoren (MK-677) occupies a completely different binding pose within the same bifurcated pocket, explaining its unique pharmacological profile as a peptidomimetic GH secretagogue ([Qin et al., 2022, Nature Communications]())
•Inverse agonist-bound crystal structure (Qin et al., 2022): The PF-05190457-bound GHSR structure revealed the molecular basis for inverse agonism — the compound stabilizes TM6 in an inward position, preventing the outward rotation required for G protein coupling ([Qin et al., 2022, Nature Communications]())
•Anamorelin-GHSR structures (Shiimura et al., 2025): Most recent structures from Nature Structural & Molecular Biology revealed how the superagonist anamorelin achieves greater efficacy than native ghrelin through optimized contacts across both cavities of the bifurcated pocket ([Shiimura et al., 2025, Nature Structural & Molecular Biology]())

Together, these structures establish a comprehensive framework for rational drug design targeting the ghrelin receptor.

G Protein Coupling and Biased Signaling

GHSR signals through multiple G protein pathways, with Gq/11 as the primary canonical pathway:

•Gq/11 → PLC → IP3/DAG → Ca²⁺ mobilization: The dominant pathway mediating GH release from pituitary somatotrophs
•Gi/o → inhibition of cAMP: Particularly relevant in hypothalamic neurons
•G13 → Rho/ROCK pathway: Implicated in cytoskeletal remodeling
•β-arrestin recruitment: Mediates receptor internalization and potentially signals through MAPK cascades

Critically, different GHSR ligands exhibit distinct signaling bias — preferentially activating one pathway over others. This biased agonism has been demonstrated for synthetic secretagogues like GHRP-6, ibutamoren, and hexarelin, each of which displays a unique signaling fingerprint compared to native ghrelin (M'Kadmi et al., 2015, Trends in Pharmacological Sciences). Lipid composition of the cell membrane also allosterically modulates GHSR signaling bias, adding another dimension of regulation ([Damian et al., 2021, Nature Communications]()).

Receptor Heterodimerization

GHSR does not operate in isolation. It forms functionally significant heterodimers with several other GPCRs, resulting in modified signaling pharmacology:

•GHSR-DRD1 (dopamine D1 receptor): Heterodimer amplifies dopamine signaling in the hippocampus, potentially linking appetite and reward circuitry
•GHSR-DRD2 (dopamine D2 receptor): Modifies dopaminergic tone in the ventral tegmental area
•GHSR-MC3R (melanocortin-3 receptor): Interaction in the hypothalamic arcuate nucleus may integrate orexigenic (ghrelin) and anorexigenic (melanocortin) signals
•GHSR-5-HT2C (serotonin 2C receptor): Attenuates GHSR constitutive activity, providing a serotonergic brake on basal appetite signaling

These heterodimer interactions suggest that GHSR function is highly context-dependent, varying by tissue, receptor expression profile, and local signaling environment (Schellekens et al., 2022, Journal of Neuroendocrinology).

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LEAP2: The Endogenous Ghrelin Counterbalance

One of the most significant discoveries in ghrelin biology since the hormone itself was the 2018 identification of liver-expressed antimicrobial peptide 2 (LEAP2) as an endogenous GHSR antagonist and inverse agonist (Ge et al., 2018, Cell Metabolism).

Mechanism of Action

LEAP2 is a 40-amino-acid hepatokine originally identified in the context of innate immunity. Its dual mechanism at GHSR includes:

1. Competitive antagonism: LEAP2 competes with ghrelin for orthosteric GHSR binding, blocking ghrelin-stimulated Gq signaling, Ca²⁺ mobilization, and inositol phosphate production

2. Inverse agonism: Beyond blocking ghrelin, LEAP2 actively suppresses GHSR constitutive activity below basal levels — a critical distinction from neutral antagonists (M'Kadmi et al., 2019, Journal of Medicinal Chemistry)

Physiological Regulation

The ghrelin/LEAP2 ratio is dynamically regulated by nutritional state in a reciprocal manner:

•Fasting: Ghrelin rises, LEAP2 falls → net GHSR activation → increased appetite and GH secretion
•Fed state: Ghrelin falls, LEAP2 rises → GHSR suppression → reduced appetite signaling
•Obesity: Chronically elevated LEAP2 and suppressed ghrelin, potentially contributing to GH hyposecretion

This reciprocal regulation establishes a push-pull system for metabolic homeostasis, with the ghrelin/LEAP2 ratio functioning as an integrated nutritional sensor.

Research Applications

LEAP2 has rapidly become a tool compound in ghrelin research. When combined with GLP-1 receptor agonists in preclinical models, LEAP2 co-administration produced sustained weight reduction exceeding either agent alone (Yang et al., 2025, Diabetes). This synergistic approach has generated interest in LEAP2-based strategies for cardiometabolic research.

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Research Domains

Growth Hormone and the Somatotropic Axis

Ghrelin's original and most well-characterized function is its potent stimulation of GH release from anterior pituitary somatotrophs. This occurs through:

•Direct GHSR activation on somatotrophs → Gq/11-mediated Ca²⁺ influx → GH vesicle exocytosis
•Synergistic amplification with GHRH signaling — co-administration of ghrelin and GHRH produces supraadditive GH release
•Modulation of somatostatin tone in the hypothalamus

The ghrelin-GH axis is central to understanding several synthetic GH secretagogues used in research, including GHRP-6, ipamorelin, and MK-677 (ibutamoren). These compounds were developed as GHSR agonists before ghrelin itself was discovered, and their signaling profiles can now be mapped onto the structural framework provided by cryo-EM studies.

Energy Homeostasis and Appetite Regulation

Ghrelin is the only known orexigenic (appetite-stimulating) gut hormone, and its levels exhibit a characteristic preprandial rise and postprandial decline. The orexigenic circuit involves:

•Circulating ghrelin activating GHSR on neuropeptide Y (NPY) and agouti-related peptide (AgRP) neurons in the arcuate nucleus of the hypothalamus
•Downstream activation of orexin neurons in the lateral hypothalamus
•Integration with reward circuitry via mesolimbic dopamine signaling in the ventral tegmental area

The GOAT-ghrelin-GHSR-LEAP2 axis is now recognized as a complete endocrine circuit regulating energy balance, with multiple pharmacological intervention points relevant to metabolic research.

Neuroprotection and Hippocampal Neurogenesis

A rapidly growing body of literature positions ghrelin as a neurotropic and neuroprotective factor, particularly in the hippocampus:

•Neurogenesis: Ghrelin directly stimulates proliferation and differentiation of adult hippocampal progenitor cells in the subgranular zone (SGZ) of the dentate gyrus. Ghrelin-knockout models show reduced progenitor cell numbers that are restored by exogenous ghrelin treatment (Li et al., 2013, Endocrinology)
•Synaptic plasticity: GHSR activation enhances long-term potentiation (LTP) in the hippocampus, a cellular correlate of learning and memory
•Neuroprotection: Ghrelin signaling in hippocampal neurons activates PI3K/Akt and ERK1/2 survival pathways, reducing apoptosis in models of excitotoxicity and oxidative stress (Moon et al., 2009, Journal of Endocrinological Investigation)
•Caloric restriction link: The cognitive benefits of caloric restriction may be partially mediated through elevated ghrelin levels and enhanced hippocampal GHSR signaling

These findings have positioned ghrelin alongside Semax, Selank, and Dihexa as peptides of interest in cognitive and neuroprotection research.

Cardiovascular Research

GHSR expression in cardiomyocytes, endothelial cells, and vascular smooth muscle underlies ghrelin's cardiovascular research profile:

•Cardiac function: In preclinical models of chronic heart failure, ghrelin administration improved left ventricular function and attenuated cardiac remodeling (Nagaya et al., 2001, Circulation)
•Anti-cachexia: Ghrelin counteracted cardiac cachexia through both GH-dependent (anabolic) and GH-independent (anti-inflammatory, appetite-stimulating) mechanisms (Nagaya et al., 2005, Internal Medicine)
•Vasodilation: Ghrelin decreases systemic vascular resistance independently of GH release, through endothelial nitric oxide production

The cardiovascular profile of ghrelin intersects with the apelin-APJ receptor system as both represent cardioactive peptide systems under active investigation.

Immune Modulation

Emerging research highlights GHSR expression on T cells, monocytes, and dendritic cells. Ghrelin signaling in immune cells has been associated with:

•Suppression of pro-inflammatory cytokine production (IL-1β, IL-6, TNF-α)
•Inhibition of NF-κB signaling in macrophages
•Modulation of T helper cell differentiation

These immunomodulatory properties place ghrelin research in dialogue with other peptides possessing immune regulatory activity, such as thymosin alpha-1 and KPV.

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Synthetic GH Secretagogues and the Ghrelin Receptor Ligand Landscape

The discovery of ghrelin retrospectively illuminated the mechanism of action for an entire class of synthetic compounds — the growth hormone secretagogues (GHSs) — that had been developed empirically from the 1970s onward:

Compound	Type	Key Feature
GHRP-6	Peptide agonist	First potent synthetic GHS; also activates ghrelin receptor
GHRP-2	Peptide agonist	Higher potency than GHRP-6
Hexarelin	Peptide agonist	GH-releasing and cardioprotective properties
Ipamorelin	Peptide agonist	Highly selective for GH release without cortisol/prolactin effects
MK-677 (Ibutamoren)	Non-peptide agonist	Orally active; occupies different binding pose in GHSR
Anamorelin	Non-peptide agonist	Superagonist; approved for cachexia research
PF-05190457	Non-peptide inverse agonist	Tool compound for constitutive activity studies
LEAP2	Endogenous antagonist/inverse agonist	Hepatokine counterbalancing ghrelin
[D-Lys3]-GHRP-6	Peptide antagonist	Research tool for blocking GHSR

Understanding these compounds within the context of GHSR structural biology enables researchers to design experiments with appropriate pharmacological controls and interpret results within a mechanistic framework.

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Methodological Considerations for Ghrelin Research

Peptide Handling

Ghrelin presents specific challenges for laboratory handling:

•Acyl-group lability: The Ser3 octanoyl ester bond is susceptible to hydrolysis, particularly at alkaline pH and elevated temperatures. Acyl-ghrelin should be stored lyophilized at -20°C to -80°C and reconstituted in mildly acidic buffer (pH 4–5) immediately before use. For community feedback on which vendors reliably maintain cold-chain integrity for ghrelin shipments, see researcher supplier reviews
•Esterase degradation: Plasma and tissue esterases rapidly cleave the octanoyl group. Use our reconstitution calculator for precise mixing ratios. For ex vivo studies, protease/esterase inhibitor cocktails and acidification of samples are essential to preserve acyl-ghrelin levels
•Measurement: Sandwich ELISAs specific for acyl-ghrelin (requiring anti-octanoyl antibodies) versus total ghrelin (detecting both forms) are commercially available, but sample handling dramatically affects results

Distinguishing Acyl vs. Des-Acyl Effects

Given that both ghrelin forms are biologically active through different mechanisms, research protocols should specify:

1. Which form (acyl-ghrelin, des-acyl ghrelin, or total ghrelin) is being studied

2. Whether GHSR-dependent or GHSR-independent pathways are relevant

3. GOAT inhibitors can be used to selectively reduce acyl-ghrelin while preserving des-acyl levels, enabling pathway dissection

Assessing GHSR Constitutive Activity

The high basal signaling of GHSR requires careful experimental design:

•Neutral antagonists (block agonist binding but do not affect constitutive activity) vs. inverse agonists (suppress constitutive signaling) produce different phenotypes
•Overexpression systems amplify constitutive activity artifacts — physiological expression levels should be used when possible
•The GHSR A204E mutant, identified in short stature cohorts, selectively impairs constitutive activity while preserving ghrelin-stimulated signaling, providing a genetic tool for dissecting constitutive vs. ligand-dependent functions

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Current Research Frontiers

Several active areas are shaping the next phase of ghrelin research:

1. GOAT inhibitors as pharmacological tools: Small molecule and peptide-based GOAT inhibitors are being developed to selectively reduce acyl-ghrelin while maintaining des-acyl ghrelin levels, enabling dissection of acylation-dependent functions

2. Biased agonist development: Structural insights from cryo-EM are enabling rational design of GHSR ligands with tailored signaling profiles — for example, compounds that activate Gq without recruiting β-arrestin, or that selectively couple to Gi

3. LEAP2-GLP-1 synergy: The combination of LEAP2 (GHSR inverse agonist) with GLP-1 receptor agonists represents an emerging strategy in metabolic research, addressing potential rebound appetite signaling through GHSR blockade

4. Ghrelin-immune axis: Understanding GHSR signaling in T cell differentiation and tumor microenvironment biology is an area of expanding investigation

5. GHSR heterodimer pharmacology: Compounds that selectively modulate GHSR-DRD1 or GHSR-MC3R heterodimer function could enable tissue-selective ghrelin pathway modulation

6. Des-acyl ghrelin receptor identification: Despite two decades of research, the receptor mediating DAG's GHSR-independent effects remains unidentified — its discovery would be transformative for the field

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Connections to Other Research Peptides

The ghrelin system intersects with multiple peptide research areas covered on this platform:

•GH Secretagogues: GHRP-6, Ipamorelin, MK-677, CJC-1295 DAC, and Sermorelin all modulate the somatotropic axis that ghrelin physiologically regulates
•Neuroprotective Peptides: Ghrelin's hippocampal neurogenesis effects parallel those studied with Semax, Selank, and Dihexa, though through distinct receptor mechanisms
•Metabolic Peptides: The ghrelin-LEAP2 axis provides a counterpoint to the GLP-1 agonist family, and understanding both systems is essential for metabolic research design
•Cardioactive Peptides: Ghrelin's cardiovascular profile complements research on the apelin-APJ system and VIP

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Summary

Ghrelin stands as one of the most structurally unique and functionally pleiotropic peptides in contemporary research. Its octanoyl modification — catalyzed by GOAT and required for GHSR activation — has no parallel in vertebrate peptide biology. The discovery of LEAP2 as an endogenous counter-regulatory signal has completed the picture of a sophisticated homeostatic circuit. And the structural revolution provided by cryo-EM has opened GHSR to rational drug design at a level previously impossible.

For researchers working with growth hormone secretagogues, metabolic peptides, or neuroprotective compounds, understanding the ghrelin system provides essential mechanistic context. As the only known orexigenic gut hormone with direct CNS actions spanning appetite, cognition, and cardiovascular function, ghrelin remains at the center of multiple high-impact research programs.

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References

1. Kojima M, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656-660. PubMed

2. Date Y, et al. Ghrelin, a novel growth hormone-releasing acylated peptide, is synthesized in a distinct endocrine cell type in the gastrointestinal tracts of rats and humans. Endocrinology. 2000;141(11):4255-4261. PubMed

3. Yang J, et al. Identification of the acyltransferase that octanoylates ghrelin, an appetite-stimulating peptide hormone. Cell. 2008;132(3):387-396. PubMed

4. Heppner KM, et al. Ghrelin octanoylation by ghrelin O-acyltransferase: Unique protein biochemistry underlying metabolic signaling. Biochemistry. 2019;58(6):453-462. PubMed

5. Filigheddu N, et al. Ghrelin and des-acyl ghrelin promote differentiation and fusion of C2C12 skeletal muscle cells. Mol Biol Cell. 2007;18(3):986-994. PubMed

6. Delhanty PJ, et al. Unacylated ghrelin rapidly modulates lipogenic and insulin signaling pathway gene expression in metabolically active tissues of GHSR deleted mice. PLoS ONE. 2010;5(7):e11749. PubMed

7. Holst B, et al. High constitutive signaling of the ghrelin receptor — identification of a potent inverse agonist. Mol Endocrinol. 2003;17(11):2201-2210. PubMed

8. Fernandez G, et al. Evidence supporting a role for constitutive ghrelin receptor signaling in fasting-induced hyperphagia. Endocrinology. 2018;159(2):1021-1034. PubMed

13. Ge X, et al. LEAP2 is an endogenous antagonist of the ghrelin receptor. Cell Metab. 2018;27(2):461-469. PubMed

14. M'Kadmi C, et al. N-terminal LEAP2 region exhibits inverse agonist activity toward the ghrelin receptor. J Med Chem. 2019;62(2):965-973. PubMed

15. Li E, et al. Ghrelin directly stimulates adult hippocampal neurogenesis. Endocrinology. 2013;154(12):4962-4970. PubMed

16. Moon M, et al. Ghrelin regulates hippocampal neurogenesis in adult mice. Endocr J. 2009;56(3):525-531. PubMed

17. Nagaya N, et al. Chronic administration of ghrelin improves left ventricular dysfunction and attenuates development of cardiac cachexia in rats with heart failure. Circulation. 2001;104(12):1430-1435. PubMed

19. Schellekens H, et al. G protein-coupled receptor interactions and modification of signalling involving the ghrelin receptor, GHSR1a. J Neuroendocrinol. 2022;34(2):e13072. PubMed

This article is for research and educational purposes only. All compounds discussed are research chemicals intended for laboratory investigation. No information herein should be construed as guidance for use outside of controlled research settings.

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