Structural basis of human ghrelin receptor signaling by ghrelin and the synthetic agonist ibutamoren

Ghrelin is a central orexigenic peptide hormone in human energy homeostasis that is also known as 'hunger hormone' and signals through its GPCR, GHSR. Here, the authors present the cryo-EM structures of the human GHSR-Gi signaling complex with bound ghrelin and the synthetic non-peptide agonist ibutamoren that are of interest for drug design. The hunger hormone ghrelin activates the ghrelin receptor GHSR to stimulate food intake and growth hormone secretion and regulate reward signaling. Acylation of ghrelin at Ser3 is required for its agonistic action on GHSR. Synthetic agonists of GHSR are under clinical evaluation for disorders related to appetite and growth hormone dysregulation. Here, we report high-resolution cryo-EM structures of the GHSR-G(i) signaling complex with ghrelin and the non-peptide agonist ibutamoren as an investigational new drug. Our structures together with mutagenesis data reveal the molecular basis for the binding of ghrelin and ibutamoren. Structural comparison suggests a salt bridge and an aromatic cluster near the agonist-binding pocket as important structural motifs in receptor activation. Notable structural variations of the G(i) and GHSR coupling are observed in our cryo-EM analysis. Our results provide a framework for understanding GHSR signaling and developing new GHSR agonist drugs.