The neuropeptide transcriptome of a model echinoderm, the sea urchin Strongylocentrotus purpuratus

Neuronal secretion of peptide signaling molecules (neuropeptides) is an evolutionarily ancient feature of nervous systems. Here we report the identification of 20 cDNAs encoding putative neuropeptide precursors in the sea urchin Strongylocentrotus purpuratus (Phylum Echinodermata), providing new insights on the evolution and diversity of neuropeptides. Identification of a gonadotropin-releasing hormone-like peptide precursor (SpGnRHP) is consistent with the widespread phylogenetic distribution of GnRH-type neuropeptides in the bilateria. A protein (SpTRHLP) comprising multiple copies of peptides that share structural similarity with thyrotropin-releasing hormone (TRH) is the first TRH-like precursor to be identified in an invertebrate. SpCTLP is the first calcitonin-like peptide with two N-terminally located cysteine residues to be found in a non-chordate species. Discovery of two proteins (SpPPLNP1, SpPPLNP2) comprising homologs of molluscan pedal peptides and arthropod orcokinins indicates the existence of a bilaterian family of pedal peptide/orcokinin-type neuropeptides. Other proteins identified contain peptides that do not share apparent sequence similarity with known neuropeptides. These include Spnp5, which comprises multiple copies of C-terminally amidated peptides that have an N-terminal Ala-Asn motif (AN peptides), and Spnp9, Spnp10 and Spnp12, which contain putative neuropeptides with a C-terminal Phe-amide, Ser-amide or Pro-amide, respectively. Several proteins (Spnp11, 14, 15, 16, 17, 18, 19 and 20) contain putative neuropeptides with multiple cysteine residues (2, 6 or 8), which may mediate formation of intramolecular or intermolecular disulphide bridges. Looking ahead, the identification of these neuropeptide precursors in S. purpuratus has provided a strong basis for a comprehensive analysis of neuropeptide function in this model echinoderm species.