GHRP-6 and MK-0677 Activate Conserved Pufferfish GHS-R Homolog Across 400 Million Years
Background
The growth hormone secretagogue receptor (GHS-R) is an orphan G-protein-coupled receptor (GPCR) mediating growth hormone release in response to synthetic peptides like GHRP-6 and nonpeptide ligands like MK-0677. Unlike most GPCRs, GHS-R does not cluster within any established GPCR subfamily, making identification of related family members and endogenous ligands a critical unresolved problem. Without knowing the full receptor family or its native ligands, the physiological role of GH secretagogue signaling remains incomplete. Identifying family members and endogenous ligands is essential for understanding whether GHS-R-targeted compounds like GHRP-6 or MK-0677 have off-target activity at related receptors and for mapping the full scope of the GH-axis pathway.
Study Design
Investigators screened a human genomic library using low-stringency hybridization to isolate GHS-R homologs, and separately screened a Pufferfish (Fugu rubripes) genomic library — chosen for its compact genome — to identify additional family members. Isolated receptor sequences were expressed in HEK293 cells and challenged with GHRP-6 and MK-0677 to assess functional activation. Endogenous ligand searches were conducted for both the orphan GHS-R and the newly identified human homolog GPR38.
Results
From the human genomic library, one homolog designated GPR38 was isolated, sharing 52% amino acid identity with GHS-R. From the Pufferfish library, three family members were identified. The Pufferfish gene with 58% identity to GHS-R, upon expression in HEK293 cells, was functionally activated by both GHRP-6 and MK-0677, confirming authentic receptor-ligand engagement across species.
The GHS-R has been conserved for at least 400 million years, as evidenced by functional activation of the Pufferfish ortholog by the same synthetic ligands active at the human receptor.
For endogenous ligand identity, adenosine was shown to act as a partial agonist of the GHS-R, while motilin was confirmed as the endogenous ligand for GPR38. Separately, ghrelin was confirmed as a full agonist of the GHS-R, aligning with its established role as the primary endogenous activator of this receptor system.
Key Findings
- Human homolog GPR38 shares 52% amino acid identity with GHS-R.
- Pufferfish GHS-R homolog (58% identity) activated by both GHRP-6 and MK-0677 in HEK293 cells.
- Three GHS-R family members isolated from the compact Pufferfish genome.
- GHS-R conservation spans at least 400 million years of evolution.
- Motilin confirmed as endogenous ligand for GPR38; ghrelin confirmed as full agonist of GHS-R.
Why It Matters
The identification of GPR38 as a GHS-R family member with motilin as its ligand reveals that GH secretagogue pharmacology is entangled with gut-motility signaling — a consideration for clinicians and researchers using GHRP-6 or MK-0677 who may not anticipate motilin-pathway cross-talk. The 400-million-year conservation of the GHS-R suggests its endogenous ligand system is under strong evolutionary pressure, reinforcing ghrelin-axis targeting as pharmacologically robust. For protocol designers, the partial agonism of adenosine at GHS-R raises questions about endogenous tone modulation that could interact with secretagogue dosing strategies. Clinical translation of selective GHS-R vs. GPR38 ligands now has a clearer receptor-family map to guide specificity optimization.
