Binding Domain Characterization of Growth Hormone Secretagogue Receptor.
This study investigated how synthetic and endogenous ligands bind to and activate the growth hormone secretagogue receptor (GHS-R), a G-protein coupled receptor involved in growth hormone (GH) release and appetite regulation. Using radiolabeled ligand-binding assays, calcium-response (aequorin-based) assays, GH release assays in mice, receptor chimeras (human/puffer fish domain swaps), and site-directed mutagenesis, researchers characterized the structural basis of ligand-receptor interactions. The study found that synthetic agonists MK-0677 and GHS-25 displayed high binding affinity and, notably, greater in vivo GH secretagogue activity compared to the endogenous peptide ghrelin. GHS-R knockout mice showed complete abolition of activity, confirming receptor specificity. Chimera analysis identified the C-terminal region, particularly transmembrane domain 6 (TM6), as critical for ligand-dependent activation. Site-directed mutagenesis pinpointed residues D99 and W276 as essential for ligand binding, while E124 was selectively important for MK-0677, and F279 was preferentially involved in ghrelin and GHS-25 interactions. The study is primarily mechanistic and largely preclinical (in vitro and animal models), limiting direct translation to human therapeutic contexts. Its findings advance structural understanding of GHS-R and may inform future drug design efforts.