Transcriptomic discovery and comparative analysis of neuropeptide precursors in sea cucumbers (Holothuroidea)

Neuropeptides synthesized and released by neuronal cells play important roles in the regulation of many processes, e.g. growth, feeding, reproduction, and behavior. In the past decade, next-generation sequencing technologies have helped to facilitate the identification of multiple neuropeptide genes in a variety of taxa, including arthropods, molluscs and echinoderms. In this study, we extend these studies to Holothuria scabra, a sea cucumber species that is widely cultured for human consumption. In silico analysis of H. scabra neural and gonadal transcriptomes enabled the identification of 28 transcripts that encode a total of 26 bilaterian and echinoderm-specific neuropeptide precursors. Furthermore, publicly available sequence data from another sea cucumber, Holothuria glaberrima, allowed a more in-depth comparative investigation. Interestingly, two isoforms of a calcitonin-type peptide precursor (CTPP) were deduced from the H. scabra transcriptome - HscCTPP-long and HscCTPP-short, likely the result of alternative splicing. We also identified a sea cucumber relaxin-type peptide precursor, which is of interest because relaxin-type peptides have been shown to act as gonadotropic hormones in starfish. Two neuropeptides that appear to be holothurian-specific are GLRFA, and GN-19. In H. scabra, the expression of GLRFA was restricted to neural tissues, while GN-19 expression was additionally found in the longitudinal muscle and intestinal tissues. In conclusion, we have obtained new insights into the neuropeptide signaling systems of holothurians, which will facilitate physiological studies that may enable advances in the aquaculture of sea cucumbers.