Are Melanocortin Receptors Present in Extant Protochordates?

Functional melanocortin receptor (MCR) genes have been identified in the genomes of early chordates, e.g., the cyclostomata. Whether they appear in the most ancient chordates such as cephalochordate and urochordata, however, remains unclear due to missing genetic data. Herein, we studied five putative (from NCBI database), sequence-based predicted MCR-like receptors from urochordata and cephalochordate, including Styela clava, Ciona intestinalis, Branchiostoma floridae, and Branchiostoma belcheri. The BLAST and phylogenetic analyses suggested a relationship between these specific receptors and vertebrate MCRs. However, several essential residues for MCR functions in vertebrates were missing in these putative chordata MCRs. To test receptor functionality, several experimental studies were conducted. Binding assays and functional analyses showed no specific binding and no ligand-induced cAMP or ERK1/2 signaling (with either endogenous α-MSH or synthetic ligands for MC4R), despite successfully expressing four receptors in HEK 293T cells. These four receptors showed high basal cAMP signaling, likely mediated by ligand-independent Gs coupling. In summary, our results suggest that the five predicted MCR-like receptors are, indeed, class A G protein-coupled receptors (GPCRs), which in four cases show high constitutive activity in the Gs-cAMP signaling pathway but are not MCR-like receptors in terms of ligand recognition of known MCR ligands. These receptors might be ancient G protein-coupled receptors with so far unidentified ligands.