Pedal peptide/orcokinin‐type neuropeptide signaling in a deuterostome: The anatomy and pharmacology of starfish myorelaxant peptide in Asterias rubens

Abstract

Pedal peptide (PP) and orcokinin (OK) are related neuropeptides that were discovered in protostomian invertebrates (mollusks, arthropods). However, analysis of genome/transcriptome sequence data has revealed that PP/OK‐type neuropeptides also occur in a deuterostomian phylum—the echinoderms. Furthermore, a PP/OK‐type neuropeptide (starfish myorelaxant peptide, SMP) was recently identified as a muscle relaxant in the starfish Patiria pectinifera. Here mass spectrometry was used to identify five neuropeptides (ArPPLN1a‐e) derived from the SMP precursor (PP‐like neuropeptide precursor 1; ArPPLNP1) in the starfish Asterias rubens. Analysis of the expression of ArPPLNP1 and neuropeptides derived from this precursor in A. rubens using mRNA in situ hybridization and immunohistochemistry revealed a widespread pattern of expression, with labeled cells and/or processes present in the radial nerve cords, circumoral nerve ring, digestive system (e.g., cardiac stomach) and body wall‐associated muscles (e.g., apical muscle) and appendages (e.g., tube feet and papulae). Furthermore, our data provide the first evidence that neuropeptides are present in the lateral motor nerves and in nerve processes innervating interossicular muscles. In vitro pharmacological tests with SMP (ArPPLN1b) revealed that it causes dose‐dependent relaxation of apical muscle, tube foot and cardiac stomach preparations from A. rubens. Collectively, these anatomical and pharmacological data indicate that neuropeptides derived from ArPPLNP1 act as inhibitory neuromuscular transmitters in starfish, which contrasts with the myoexcitatory actions of PP/OK‐type neuropeptides in protostomian invertebrates. Thus, the divergence of deuterostomes and protostomes may have been accompanied by an inhibitory–excitatory transition in the roles of PP/OK‐type neuropeptides as regulators of muscle activity.

Functional characterization of a second pedal peptide/orcokinin‐type neuropeptide signaling system in the starfish Asterias rubens

“…Consistent with this pattern of expression, intense immunostaining was also observed in the lateral motor nerves, components of the starfish nervous system that were first described based on analysis of histochemical staining (Smith, ). Investigation of the in vitro pharmacological effects of ArPPLN1b revealed that it causes dose‐dependent relaxation of three preparations from A. rubens —apical muscle, tube feet and cardiac stomach (Lin, Egertová et al, ), consistent with previous findings from P. pectinifera (Kim et al, ). Thus, collectively the data obtained from experimental studies on SMP/PPLN1‐type neuropeptides in P. pectinifera and A. rubens indicate that a physiological role of these peptides is to act as inhibitory neuromuscular transmitters or modulators in starfish.…”

“…The structures of neuropeptides derived from ArPPLNP2 were determined by MS/MS analysis of A. rubens radial nerve cords extracted in 90% methanol/9% acetic acid, employing use of methods described previously (Lin, Egertová et al, ; Lin, Mita et al, ).…”

“…Recently, we reported a detailed analysis of the anatomy of ArPPLNP1 expression in A. rubens using mRNA in situ hybridization and immunohistochemistry (Lin, Egertová, Zampronio, Jones, & Elphick, ). Consistent with qPCR data from P. pectinifera (Kim et al, ), a widespread pattern of expression of ArPPLNP1 and the neuropeptides derived from this precursor was revealed in A. rubens .…”

“…Consistent with qPCR data from P. pectinifera (Kim et al, ), a widespread pattern of expression of ArPPLNP1 and the neuropeptides derived from this precursor was revealed in A. rubens . Thus, cells expressing ArPPLNP1 transcripts were detected, for example, in the radial nerve cords, circumoral nerve ring, esophagus, cardiac stomach, pyloric stomach, pyloric caeca, tube feet, and body wall (Lin, Egertová et al, ). Immunohistochemical analysis using antibodies to one of the neuropeptides derived from ArPPLNP1 (ArPPLN1b or ArSMP) revealed a pattern of immunostained cells consistent with data obtained using mRNA in situ hybridization.…”

“…Immunohistochemical analysis using antibodies to one of the neuropeptides derived from ArPPLNP1 (ArPPLN1b or ArSMP) revealed a pattern of immunostained cells consistent with data obtained using mRNA in situ hybridization. Furthermore, immunohistochemistry revealed extensive networks of immunostained processes in the radial nerve cords, circumoral nerve ring, digestive system, tube feet, coelomic lining, apical muscle, and body wall (Lin, Egertová, et al, ). One of the most striking features of the immunostaining was the presence of labeled processes in close association with interossicular muscles that link the calcite ossicles of the endoskeleton.…”