A potential feeding regulation strategy during aestivation: Relaxation of intestine mediated by pedal peptide/orcokinin-type neuropeptides in sea cucumber Apostichopus japonicus

Abstract

Aestivation is an important survival strategy that is used by a wide variety of organisms to cope with high environmental temperatures and/or arid conditions. Summer aestivation also occurs among marine animals living in shallow inshore waters. Sea cucumbers, Apostichopus japonicus, are an economically important species that are farmed extensively in shallow in-shore waters across Asia. In recent years, this species has also been developed as an important model for studying temperature-induced aestivation in the marine environment. Over about 100 days of summer aestivation A. japonicus stops feeding, experiences severe degeneration of the intestine and substantial body mass reduction, which seriously impacts production efficiency of sea cucumbers in aquaculture. Our previous studies of A. japonicus applied LC-MS/MS to reveal changes in neuropeptide expression during aestivation and identified pedal peptide/orcokinin (PP/OK)-type neuropeptides, an evolutionarily ancient regulator of muscle relaxation/contraction, as significantly up-regulated in the circumoral nerve ring (CNR) during A. japonicus aestivation. Based on this, we proposed that PP/OK-type neuropeptides are potentially key to regulate feeding and intestinal integrity during aestivation by regulating the mechanical behavior of intestine. The present study investigated PP/OK-type neuropeptides in A. japonicus tissues via histological, functional and ecological analyses. A widespread expression pattern, especially in the CNR, radial nerve cord (RNC) and intestine was revealed using real-time quantitative PCR and immunohistochemistry. Our findings show a key role for the expression of the neuropeptide, A. japonicus pedal peptide-type precursor 2 (AjPPLNP2), during the early stage of aestivation that increased significantly in the CNR and intestine as compared with the non-aestivating condition. Furthermore, in vitro pharmacological tests showed that three PP/OK-type neuropeptides confirmed by MS displayed relaxing effects on intestine, suggesting that PP/OK-type neuropeptides have a key involvement in A. japonicus aestivation by regulating relaxation of the intestine that halts normal peristalsis and inhibits feeding. In conclusion, our results suggest a mechanism for feeding inhibition controlled by neuropeptides and provide a potential regulatory strategy for counteracting intestine atrophy and body mass reduction in sea cucumber aquaculture by controlling the action of neuropeptides.