Ancient role of vasopressin/oxytocin-type neuropeptides as regulators of feeding revealed in an echinoderm

Abstract

Background: Vasopressin/oxytocin (VP/OT)-type neuropeptides are well known for their roles as regulators of diuresis, reproductive physiology and social behaviour. However, our knowledge of their functions is largely based on findings from studies on vertebrates and selected protostomian invertebrates. Little is known about the roles of VP/OT-type neuropeptides in deuterostomian invertebrates, which are more closely related to vertebrates than protostomes. Results: Here, we have identified and functionally characterised a VP/OT-type signalling system comprising the neuropeptide asterotocin and its cognate G-protein coupled receptor in the starfish (sea star) Asterias rubens, a deuterostomian invertebrate belonging to the phylum Echinodermata. Analysis of the distribution of asterotocin and the asterotocin receptor in A. rubens using mRNA in situ hybridisation and immunohistochemistry revealed expression in the central nervous system (radial nerve cords and circumoral nerve ring), the digestive system (including the cardiac stomach) and the body wall and associated appendages. Informed by the anatomy of asterotocin signalling, in vitro pharmacological experiments revealed that asterotocin acts as a muscle relaxant in starfish, contrasting with the myotropic actions of VP/OT-type neuropeptides in vertebrates. Furthermore, in vivo injection of asterotocin had a striking effect on starfish behaviour-triggering fictive feeding where eversion of the cardiac stomach and changes in body posture resemble the unusual extra-oral feeding behaviour of starfish. Conclusions:We provide a comprehensive characterisation of VP/OT-type signalling in an echinoderm, including a detailed anatomical analysis of the expression of both the VP/OT-type neuropeptide asterotocin and its cognate receptor. Our discovery that asterotocin triggers fictive feeding in starfish provides important new evidence of an evolutionarily ancient role of VP/OT-type neuropeptides as regulators of feeding in animals.

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

“…Therefore, here, we investigated the effect of in vivo injection of ArSS2 in A. rubens and found that it causes partial eversion of the cardiac stomach. Our previous studies have revealed that both S2 and asterotocin also cause cardiac stomach eversion in A. rubens but, consistent with their relative efficacy as cardiac stomach relaxants in vitro, asterotocin is much more effective and potent than S2 in triggering cardiac stomach eversion [67]. Accordingly, here, we found that asterotocin is much more effective than ArSS2 in triggering cardiac stomach eversion, consistent with their relative effectiveness as cardiac stomach relaxants in vitro.…”

“…At 10 −6 M, there was no significant difference in the efficacy of ArSS2 and S2, but the efficacy of these peptides was approximately 50% of that of asterotocin ( figure 11). A previous study revealed that asterotocin is much more effective as a cardiac stomach relaxant than S2 [67] and here, we show that asterotocin is more effective than both S2 and ArSS2 as a cardiac stomach relaxant when tested at 10 −7 or 10 −6 M.…”

Comparative and Evolutionary Physiology of Vasopressin/ Oxytocin-Type Neuropeptide Signaling in Invertebrates

“…Moreover, the effect of asterotocin on body posture was so powerful that it adversely affected the ability of starfish to right themselves when upturned. Collectively, the findings of this study indicate that asterotocin signaling has an important physiological/behavioral role in triggering the unusual extra-oral feeding behavior of the starfish A. rubens (131). Expression of the asterotocin precursor has also been investigated in the free-swimming larvae of A. rubens, with transcripts detected during the brachiolaria larval stage in cells located at the tips of the brachia and associated with the adhesive disk-structures that mediate larval attachment to the substratum prior to metamorphosis (132).…”

“…Sequencing of the neural transcriptome of the starfish A. rubens enabled identification of a transcript encoding a VP/OT-type precursor protein comprising the neuropeptide "asterotocin" (130) and the structure of this peptide (CLVQDCPEG-NH 2 ) has been confirmed using mass spectrometry (131). An unusual characteristic of asterotocin is that it has acidic residues at positions 5 and 8, which are usually occupied by a basic or hydrophobic residue in other VP/OT-type peptides ( Table 1).…”

“…An unusual characteristic of asterotocin is that it has acidic residues at positions 5 and 8, which are usually occupied by a basic or hydrophobic residue in other VP/OT-type peptides ( Table 1). The asterotocin receptor has also been identified in A. rubens and shown to be selectively activated by asterotocin and not by mammalian VP or OT (131). To gain insights into the physiological roles of asterotocin signaling in starfish, the expression of asterotocin and the asterotocin receptor in A. rubens has been examined using both mRNA in situ hybridization and immunohistochemical techniques.…”