Transcriptomic identification of starfish neuropeptide precursors yields new insights into neuropeptide evolution

Abstract

Neuropeptides are evolutionarily ancient mediators of neuronal signalling in nervous systems. With recent advances in genomics/transcriptomics, an increasingly wide range of species has become accessible for molecular analysis. The deuterostomian invertebrates are of particular interest in this regard because they occupy an 'intermediate' position in animal phylogeny, bridging the gap between the well-studied model protostomian invertebrates (e.g. Drosophila melanogaster, Caenorhabditis elegans) and the vertebrates. Here we have identified 40 neuropeptide precursors in the starfish Asterias rubens, a deuterostomian invertebrate from the phylum Echinodermata. Importantly, these include kisspeptin-type and melanin-concentrating hormone-type precursors, which are the first to be discovered in a non-chordate species. Starfish tachykinin-type, somatostatin-type, pigment-dispersing factortype and corticotropin-releasing hormone-type precursors are the first to be discovered in the echinoderm/ambulacrarian clade of the animal kingdom. Other precursors identified include vasopressin/oxytocin-type, gonadotropin-releasing hormone-type, thyrotropin-releasing hormonetype, calcitonin-type, cholecystokinin/gastrin-type, orexin-type, luqin-type, pedal peptide/orcokinin-type, glycoprotein hormone-type, bursicon-type, relaxin-type and insulin-like growth factor-type precursors. This is the most comprehensive identification of neuropeptide precursor proteins in an echinoderm to date, yielding new insights into the evolution of neuropeptide signalling systems. Furthermore, these data provide a basis for experimental analysis of neuropeptide function in the unique context of the decentralized, pentaradial echinoderm bauplan.

Localization of Neuropeptide Gene Expression in Larvae of an Echinoderm, the Starfish Asterias rubens

“…The sequences of cDNAs encoding six of the seven other neuropeptide precursors were determined here for the first time, with the exception of the ArGnRH precursor cDNA, which was reported recently (Tian et al, 2016). The sequences of cDNAs encoding the F-type SALMFamide (S2) precursor (Supplementary Figure 2; GenBank accession number KP330476), asterotocin precursor (Supplementary Figure 3; GenBank accession number KT601711), NGFFYamide precursor (Supplementary Figure 4; GenBank accession number KC977457), ArGnRH precursor (Supplementary Figure 6; GenBank accession number KT601712), ArCT precursor (Supplementary Figure 7; GenBank accession number KT601715), and ArCRH precursor (Supplementary Figure 8; GenBank accession number KT601710) were found to be identical to the assembled transcript sequences reported previously (Semmens et al, 2013, 2016). However, sequencing of a cDNA encoding the A. rubens TRH-type precursor (ArTRHP) revealed a difference to the predicted transcript sequence reported previously (Semmens et al, 2016).…”

“…The sequences of cDNAs encoding the F-type SALMFamide (S2) precursor (Supplementary Figure 2; GenBank accession number KP330476), asterotocin precursor (Supplementary Figure 3; GenBank accession number KT601711), NGFFYamide precursor (Supplementary Figure 4; GenBank accession number KC977457), ArGnRH precursor (Supplementary Figure 6; GenBank accession number KT601712), ArCT precursor (Supplementary Figure 7; GenBank accession number KT601715), and ArCRH precursor (Supplementary Figure 8; GenBank accession number KT601710) were found to be identical to the assembled transcript sequences reported previously (Semmens et al, 2013, 2016). However, sequencing of a cDNA encoding the A. rubens TRH-type precursor (ArTRHP) revealed a difference to the predicted transcript sequence reported previously (Semmens et al, 2016). The cloned cDNA comprises a protein-coding region of 768 bases encoding a 256-residue protein (Supplementary Figure 5), which is longer than the 675 base protein-coding region encoding a 225-residue protein in the transcript sequence predicted from assembled radial nerve cord transcriptome sequence data (see Figure S11 in Semmens et al, 2016; GenBank accession number KT601714).…”

“…The sequences of transcripts encoding A. rubens neuropeptide precursors have been identified previously by analysis of radial nerve cord transcriptome sequence data from this species (Semmens et al, 2013, 2016). Here, the expression of eight of these neuropeptide precursors was investigated in A. rubens larvae using whole-mount mRNA in situ hybridization.…”

“…Investigation of the phylogenetic distribution and relationships of neuropeptide signaling systems has revealed that the evolutionary origin of many neuropeptides can be traced to the common ancestor of the Bilateria (Jékely, 2013; Mirabeau and Joly, 2013). Thus, orthologs of vertebrate neuropeptides have been identified in deuterostomian and protostomian invertebrates (Hewes and Taghert, 2001; Veenstra, 2010, 2011; Semmens et al, 2016). Peptidergic signaling systems have also been identified in the nervous systems of the Cnidaria (e.g., sea anemones), which are a sister group to the Bilateria (Galliot and Quiquand, 2011), and the origins of some peptide signaling systems may even predate the evolution of nervous systems (Schuchert, 1993; Jékely, 2013; Nikitin, 2015).…”

“…The foundation for the present study was the recent identification of 40 neuropeptide precursor transcript sequences in the starfish A. rubens (Semmens et al, 2013, 2016), which represents the most comprehensive resource to date for echinoderm neuropeptide research. These neuropeptide precursors were identified by analysis of the transcriptome of the radial nerve cords from adult starfish.…”