The ghrelin receptor GHSR has two efficient agonists in an ancient fish species

The gastric peptide ghrelin and its receptor GHSR have important functions in energy metabolism. Recently, liver-expressed antimicrobial peptide 2 (LEAP2) was identified as an endogenous antagonist for GHSR. Ghrelin, LEAP2, and GHSR are ubiquitously present from fishes to mammals and are highly conserved in evolution. However, our recent study suggested that GHSRs from the Actinopterygii fish Danio rerio (zebrafish) and Larimichthys crocea (large yellow croaker) have lost their binding to ghrelin, despite binding normally to LEAP2. Do these fish GHSRs use another peptide as their agonist? To answer this question, in the present study, we tested to two fish motilins that are closely related to ghrelin. In ligand binding and activation assays, the fish GHSRs from D. rerio and L. crocea displayed no detectable or very low binding to all tested motilins; however, the GHSR from the Sarcopterygii fish Latimeria chalumnae (coelacanth) bound to its motilin with high affinity and was efficiently activated by it. Therefore, it seemed that motilin is not a ligand for GHSR in D. rerio and L. crocea, but is an efficient agonist for GHSR in L. chalumnae, which is known as a living fossil and is believed to be one of the closest fish ancestors of tetrapods. The results of present study suggested that in ancient fishes, GHSR had two efficient agonists, ghrelin and motilin; however, this feature might be only preserved in some extant fishes with ancient evolutionary origins. Our present work shed new light on the ligand usage of GHSR in different fish species and in evolution.