Molecular and functional characterization of somatostatin-type signalling in a deuterostome invertebrate

Abstract

Somatostatin (SS) and allatostatin-C (ASTC) are structurally and evolutionarily related neuropeptides that act as inhibitory regulators of physiological processes in mammals and insects, respectively. Here, we report the first molecular and functional characterization of SS/ASTC-type signalling in a deuterostome invertebrate—the starfish Asterias rubens (phylum Echinodermata). Two SS/ASTC-type precursors were identified in A. rubens (ArSSP1 and ArSSP2) and the structures of neuropeptides derived from these proteins (ArSS1 and ArSS2) were analysed using mass spectrometry. Pharmacological characterization of three cloned A. rubens SS/ASTC-type receptors (ArSSR1–3) revealed that ArSS2, but not ArSS1, acts as a ligand for all three receptors. Analysis of ArSS2 expression in A. rubens using mRNA in situ hybridization and immunohistochemistry revealed stained cells/fibres in the central nervous system, the digestive system (e.g. cardiac stomach) and the body wall and its appendages (e.g. tube feet). Furthermore, in vitro pharmacological tests revealed that ArSS2 causes dose-dependent relaxation of tube foot and cardiac stomach preparations, while injection of ArSS2 in vivo causes partial eversion of the cardiac stomach. Our findings provide new insights into the molecular evolution of SS/ASTC-type signalling in the animal kingdom and reveal an ancient role of SS-type neuropeptides as inhibitory regulators of muscle contractility.

Evolutionarily ancient role of cholecystokinin-type neuropeptide signalling as an inhibitory regulator of feeding-related processes revealed in an echinoderm

“…Thus, the axons of hyponeural motor neurons project around the tube feet and coalesce as a fibre bundle known as the lateral motor nerve (Lin et al 2017a, Smith 1937. Consistent, with the expression of ArCCKP in hyponeural cells, ArCCK1-ir fibres can be seen in the lateral motor nerves, as has been previously reported for other A. rubens neuropeptides (Cai et al 2018, Lin et al 2017a, Lin et al 2018, Zhang et al 2020). Branches of the lateral motor nerves project into the coelomic lining of the body wall in starfish (Smith 1937(Smith , 1950 and accordingly ArCCK1-ir fibres were observed in coelomic lining of the body wall in A. rubens.…”

“…This revealed a widespread pattern of expression, including the central nervous system (CNS), tube feet and other body wall associated structures and the digestive system, as discussed below. This pattern of expression can be interpreted with reference to the anatomy of the starfish nervous system (Cobb 1970, Mashanov et al 2016, Smith 1937, digestive system (Anderson 1953(Anderson , 1954 and body wall (Blowes et al 2017) and in comparison with the distribution of other neuropeptides in A. rubens (Cai et al 2018, Elphick et al 1995, Lin et al 2017a, Lin et al 2018, Moore and Thorndyke 1993, Odekunle et al 2019, Tinoco et al 2018, Yáñez-Guerra et al 2018, Zhang et al 2020).…”

“…The starfish CNS consists of radial nerve cords and a circumoral nerve ring, where cells expressing ArCCKP/ArCCK1 were revealed in both the ectoneural and hyponeural regions in common with other neuropeptides such as the calcitonin-type neuropeptide ArCT (Cai et al 2018), pedal peptide/orcokinin-type peptides (Lin et al 2017a, Lin et al 2018; the gonadotropin-releasing hormone-type peptide ArGnRH (Tian et al 2017); and the somatostatin-type peptide ArSS2 (Zhang et al 2020). The presence of an extensive network of immunostained fibres in the neuropile of the ectoneural region is consistent with neuronal expression of ArCCKP/ArCCK1.…”

“…Then when external digestion and ingestion of prey tissue is completed, contraction of the musculature enables cardiac stomach retraction (Anderson 1954). Previous studies have identified neuropeptides that cause relaxation of the cardiac stomach in vitro and trigger cardiac stomach eversion when injected in vivo; for example, the SALMFamide neuropeptide S2 (Melarange et al 1999), the vasopressin/oxytocin-type neuropeptide asterotocin (Odekunle et al 2019) and the somatostatin-type neuropeptide ArSS2 (Zhang et al 2020). Conversely, NGFFYamide has been identified as a neuropeptide in A.…”

“…However, functional characterisation of native CCK-type peptides and receptors has yet to be reported for an echinoderm or hemichordate species. We have established the common European starfish Asterias rubens as an experimental model for molecular and functional characterisation of neuropeptides, obtaining novel insights into the evolution and comparative physiology of several neuropeptide signalling systems (Cai et al 2018, Lin et al 2017a, Lin et al 2018, Odekunle et al 2019, Semmens and Elphick 2017, Tian et al 2016, Tinoco et al 2018, Yáñez-Guerra et al 2018, Zhang et al 2020. Accordingly, here we used A. rubens to enable the first detailed molecular, anatomical and pharmacological analysis of CCK-type signalling in an echinoderm.…”