Identification of neuropeptides in the sea cucumber Holothuria leucospilota

Endocrine regulation of reproductive biology in echinoderms: An evolutionary perspective from closest marine invertebrate relatives to chordates

Echinoderms are a phylum of invertebrate deuterostomes, which contain echinoids, asteroids, holothuroids, crinoids, and ophiuroids. Echinoderms have special evolutionary position and unique characteristics, including pentamerous radial body structure, elaborate calcareous endoskeletons, and versatile water vascular system. Echinoderms exhibit extraordinarily diverse reproductive modes: asexual reproduction, sexual reproduction, sexual reversal, etc. Endocrine regulation plays important well-known roles in sex differentiation, gonadal development and maturation, gametogenesis, and reproductive behavior in vertebrates. However, the entire picture of reproductive endocrinology in echinoderms as an evolutionary model of the closest marine invertebrate relatives to chordates has not been revealed. Here, we reviewed previous and recent research progress on reproductive endocrinology in echinoderms, mainly including two sections: Sex steroids in echinoderms and neuropeptide regulation in echinoderm reproduction. This review introduces a variety of endocrine regulatory mechanisms in reproductive biology of echinoderms. It discusses the vertebrate-like sex steroids, putative steroidogenic pathway and metabolism, and reproduction-related neuropeptides. The review will provide a deeper understanding about endocrine regulatory mechanisms of gonadal development in lower deuterostomes and the application of endocrine control in economic echinoderm species in aquaculture.

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

Nervous System Development and Neuropeptides Characterization in Embryo and Larva: Insights from a Non-Chordate Deuterostome, the Sea Cucumber Apostichopus japonicus

Here, we described the complex nervous system at five early developmental stages (blastula, gastrula, auricularia, doliolaria and pentactula) of a holothurian species with highly economic value, Apostichopus japonicus. The results revealed that the nervous system of embryos and larvae is mainly distributed in the anterior apical region, ciliary bands or rings, and the feeding and attachment organs, and that serotonergic immunoreactivity was not observed until the embryo developed into the late gastrula; these are evolutionarily conserved features of echinoderm, hemichordate and protostome larvae. Furthermore, based on available transcriptome data, we reported the neuropeptide precursors profile at different embryonic and larval developmental stages. This analysis showed that 40 neuropeptide precursors present in adult sea cucumbers were also identified at different developmental stages of embryos and larvae, and only four neuropeptide precursors (SWYG precursor 2, GYWKDLDNYVKAHKT precursor, Neuropeptide precursor 14-like precursor, GLRFAmprecursor-like precursor) predicted in adults were absent in embryos and larvae. Combining the quantitative expression of ten specific neuropeptide precursor genes (NPs) by qRT-PCR, we revealed the potential important roles of neuropeptides in embryo development, feeding and attachment in A. japonicus larvae. In conclusion, this work provides novel perspectives on the diverse physiological functions of neuropeptides and contributes to understanding the evolution of neuropeptidergic systems in echinoderm embryos and larvae.

Molecular and functional characterization of the luqin-type neuropeptide signaling system in the sea cucumber Apostichopus japonicus

The functional characteristics of neuropeptides in marine invertebrates have attracted significant attention recently although functional studies of luqin-type neuropeptides are still very limited, especially in deuterostomes. The sea cucumber, Apostichopus japonicus, is a representative species of deuterostomian Holothurian invertebrates. The species has high nutritional and medicinal value in China. In this study, we report the first comprehensive histological, biochemical and pharmacological characterization of luqin-type neuropeptide signaling in the sea cucumber A. japonicus. The A. japonicus luqin-like neuropeptide precursor (AjLQP) contains a single typical deuterostomian luqin-like neuropeptide AjLQ with an xFxRWamide motif. AjLQ was identified as the ligand for a luqin-type neuropeptide receptor AjLQR, that was previously predicted to be a tachykinin-type receptor, and triggers a rapid intracellular mobilization of Ca²⁺, followed by receptor internalization and a transient increase in ERK1/2 phosphorylation. In situ hybridization, immunohistochemistry and qRT-PCR analysis revealed extensive expression of AjLQP and AjLQ in A. japonicus tissues, especially in locomotion-related organs. In vitro pharmacological tests revealed that AjLQ caused 12.69% ± 1.99% (p < 0.01) relaxation of longitudinal muscle preparations at 10^-7 M concentration. Furthermore, we observed significantly increased expression of AjLQP (about 17.63 fold, p < 0.01) in intestine of deeply aestivating sea cucumbers, which suggests that AjLQ might be involved in feeding inhibition during aestivation. The present study provides a first insight into the experimental characterization of luqin-type neuropeptide signaling in a sea cucumber. The results will broaden our understanding of the potential function of neuropeptides during important biological processes in marine invertebrates and provide theoretical support for optimizing sea cucumber aquaculture technology.

Receptor deorphanization in an echinoderm reveals kisspeptin evolution and relationship with SALMFamide neuropeptides

BACKGROUND

Kisspeptins are neuropeptides that regulate reproductive maturation in mammals via G-protein-coupled receptor-mediated stimulation of gonadotropin-releasing hormone secretion from the hypothalamus. Phylogenetic analysis of kisspeptin-type receptors indicates that this neuropeptide signaling system originated in a common ancestor of the Bilateria, but little is known about kisspeptin signaling in invertebrates.

RESULTS

Contrasting with the occurrence of a single kisspeptin receptor in mammalian species, here, we report the discovery of an expanded family of eleven kisspeptin-type receptors in a deuterostome invertebrate - the starfish Asterias rubens (phylum Echinodermata). Furthermore, neuropeptides derived from four precursor proteins were identified as ligands for six of these receptors. One or more kisspeptin-like neuropeptides derived from two precursor proteins (ArKPP1, ArKPP2) act as ligands for four A. rubens kisspeptin-type receptors (ArKPR1,3,8,9). Furthermore, a family of neuropeptides that act as muscle relaxants in echinoderms (SALMFamides) are ligands for two A. rubens kisspeptin-type receptors (ArKPR6,7). The SALMFamide neuropeptide S1 (or ArS1.4) and a 'cocktail' of the seven neuropeptides derived from the S1 precursor protein (ArS1.1-ArS1.7) act as ligands for ArKPR7. The SALMFamide neuropeptide S2 (or ArS2.3) and a 'cocktail' of the eight neuropeptides derived from the S2 precursor protein (ArS2.1-ArS2.8) act as ligands for ArKPR6.

CONCLUSIONS

Our findings reveal a remarkable diversity of neuropeptides that act as ligands for kisspeptin-type receptors in starfish and provide important new insights into the evolution of kisspeptin signaling. Furthermore, the discovery of the hitherto unknown relationship of kisspeptins with SALMFamides, neuropeptides that were discovered in starfish prior to the identification of kisspeptins in mammals, presents a radical change in perspective for research on kisspeptin signaling.

Intracellular Pathways of Holothuroid Oocyte Maturation Induced by the Thioredoxin Trx-REES

In holothuroids, oocyte maturation is stopped in ovaries at the prophase I stage of meiosis. In natural conditions, the blockage is removed during the spawning by an unknown mechanism. When oocytes are isolated by dissection, the meiotic release can be successfully induced by a natural inducer, the REES (i.e., Rough Extract of Echinoid Spawn) that is used in aquaculture to obtain viable larvae in mass. A thioredoxin has recently been identified in the REES as the molecule responsible for holothuroid oocyte maturation. As a redox-active protein, thioredoxin is thought to reduce target proteins within the oocyte membrane and initiate an intracellular reaction cascade that leads to the unblocking of the oocyte meiosis. Our results allow us to understand additional steps in the intracellular reaction cascade induced by the action of thioredoxin on oocytes. Pharmacological agents known to have activating or inhibiting actions on oocyte maturation have been used (Forskolin, Isobutylmethylxanthine, Hypoxanthine, 6-dimethyaminopurine, Lavendustin, Genistein, Roscovitine, Cycloheximide). The effects of these agents were analysed on oocytes of the holothuroid Holothuria tubulosa incubated with or without REES and were compared to those obtained with another reducing agent, the dithiothreitol. Our results demonstrated that, at the opposite of dithiothreitol-induced oocyte maturation, thioredoxin-induced oocyte maturation is cAMP independent, but dependent of the presence of calcium in the seawater. Both pathways of induction require the activation of protein serine/threonine kinases.

Genome-wide prediction and comparative transcriptomic analysis reveals the G protein-coupled receptors involved in gonadal development of Apostichopus japonicus

The sea cucumber Apostichopus japonicus is dioecious, with seasonal reproduction. G protein-coupled receptor (GPCR)-mediated signaling systems might play critical roles in the reproductive control of A. japonicus. Here, we classified GPCR from the genome in silico and used transcriptomic analyses to further mine those that function in gonadal-development control. Totally, 487 GPCRs were predicted from A. japonicus, and 183 of these were further annotated to molecular pathways. Transcriptome analysis revealed 327 GPCRs expressed in gonads, and these were classified into four families and 19 subfamilies. Three pathways were apparently associated with reproduction, including neuroactive ligand-receptor interaction, the mTOR and Wnt signaling pathways. Seven and eight ovary- and testis-specific GPCRs were filtered, and the gene expression profiles were determined in multiple tissues and gonads at different developmental stages by qPCR. These results provide new insights into the discovery of GPCR-mediated signaling control in sea cucumber reproduction, especially in gonadal development control.

Evolution and Comparative Physiology of Luqin-Type Neuropeptide Signaling

Luqin is a neuropeptide that was discovered and named on account of its expression in left upper quadrant cells of the abdominal ganglion in the mollusc Aplysia californica. Subsequently, luqin-type peptides were identified as cardio-excitatory neuropeptides in other molluscs and a cognate receptor was discovered in the pond snail Lymnaea stagnalis. Phylogenetic analyses have revealed that orthologs of molluscan luqin-type neuropeptides occur in other phyla; these include neuropeptides in ecdysozoans (arthropods, nematodes) that have a C-terminal RYamide motif (RYamides) and neuropeptides in ambulacrarians (echinoderms, hemichordates) that have a C-terminal RWamide motif (RWamides). Furthermore, precursors of luqin-type neuropeptides typically have a conserved C-terminal motif containing two cysteine residues, although the functional significance of this is unknown. Consistent with the orthology of the neuropeptides and their precursors, phylogenetic and pharmacological studies have revealed that orthologous G-protein coupled receptors (GPCRs) mediate effects of luqin-type neuropeptides in spiralians, ecdysozoans, and ambulacrarians. Luqin-type signaling originated in a common ancestor of the Bilateria as a paralog of tachykinin-type signaling but, unlike tachykinin-type signaling, luqin-type signaling was lost in chordates. This may largely explain why luqin-type signaling has received less attention than many other neuropeptide signaling systems. However, insights into the physiological actions of luqin-type neuropeptides (RYamides) in ecdysozoans have been reported recently, with roles in regulation of feeding and diuresis revealed in insects and roles in regulation of feeding, egg laying, locomotion, and lifespan revealed in the nematode Caenorhabditis elegans. Furthermore, characterization of a luqin-type neuropeptide in the starfish Asterias rubens (phylum Echinodermata) has provided the first insights into the physiological roles of luqin-type signaling in a deuterostome. In conclusion, although luqin was discovered in Aplysia over 30 years ago, there is still much to be learnt about luqin-type neuropeptide signaling. This will be facilitated in the post-genomic era by the emerging opportunities for experimental studies on a variety of invertebrate taxa.