Functional characterisation of neuropeptides that act as ligands for both calcitonin-type and pigment-dispersing factor-type receptors in a deuterostome

Abstract

The calcitonin (CT) family of related peptides exert their diverse physiological effects in mammals via two G-protein coupled receptors, CTR and the CTR-like receptor CLR. Analysis of the phylogenetic distribution of CT-type signaling has revealed the occurrence of CT-type peptides and CTR/CLR-type proteins in deuterostome and protostome invertebrates. Furthermore, experimental studies have revealed that in the protostomeDrosophila melanogasterthe CT-like peptide DH31can act as a ligand for a CTR/CLR-type receptor and a pigment-dispersing factor (PDF) receptor. Here we investigated the signaling mechanisms and functions of CT-type neuropeptides in a deuterostome invertebrate, the sea cucumberApostichopus japonicus(phylum Echinodermata). InA. japonicus,a single gene encodes two CT-type peptides (AjCT1 and AjCT2), both of which act as ligands for a CTR/CLR-type receptor (AjCTR) and two PDF-type receptors (AjPDFR1, AjPDFR2), but with differential activation of downstream cAMP/PKA, Gαq/Ca2+/PKC and ERK1/2 signaling pathways. Analysis of the expression of the gene encoding AjCT1 and AjCT2 revealed transcripts in a variety of organ systems, but with highest expression in the circumoral nervous system.In vitropharmacological experiments revealed that AjCT1 and/or AjCT2 cause dose-dependent relaxation of longitudinal body wall muscle and intestine preparations. Furthermore,in vivopharmacological experiments and loss-of-function tests revealed a potential physiological role for AjCT2 signaling in promoting feeding and growth inA. japonicus. This is the first study to obtain evidence that CT-type peptides can act as ligands for both CTR/CLR-type and PDF-type receptors in a deuterostome. Furthermore, because of the economic importance ofA. japonicusas a foodstuff, discovery of the potential role for CT-type peptides as regulators of feeding and growth in this species may provide a basis for practical applications in aquaculture.Graphical abstractA schematic showing proposed molecular mechanisms by which CT-type neuropeptides regulate feeding and growth inA. japonicus. AjCT1 and AjCT2 are represented by purple and yellow circles, respectively. For cell signaling, black arrows indicate that this process is activated, whilst the pink arrow indicates that the ERK1/2 is activated in the AjCT1/AjPDFR1, AjCT2/AjPDFR1 and AjCT2/AjPDFR2 signaling pathways.