Effect of chimeric relaxin-like gonad-stimulating peptides on oocyte maturation and ovulation in the starfish Asterias rubens and Aphelasterias japonica

Gonadotropic activity of a second relaxin-type peptide in starfish

In starfish, a relaxin-like gonad-stimulating peptide (RGP) acts as a gonadotropin that triggers gamete maturation and spawning. In common with other relaxin/insulin superfamily peptides, RGP consists of an A- and a B-chain, with cross-linkages mediated by one intra- and two inter-chain disulfide bonds. In this study, a second relaxin-like peptide (RLP2) was identified in starfish species belonging to the orders Valvatida, Paxillosida, and Forcipulatida. Like RGP, RLP2 precursors comprise a signal peptide and a C-peptide in addition to the A- and B-chains. However, a unique cysteine motif [CC-(3X)-C-(10X)-C] is present in the A-chain of RLP2, which contrasts with the cysteine motif in other members of the relaxin/insulin superfamily [CC-(3X)-C-(8X)-C]. Importantly, in vitro pharmacological tests revealed that Patiria pectinifera RLP2 (Ppe-RLP2) and Asterias rubens RLP2 (Aru-RLP2) trigger shedding of mature eggs from ovaries of P. pectinifera and A. rubens, respectively. Furthermore, the potencies of Ppe-RLP2 and Aru-RLP2 as gonadotropic peptides were similar to those of Ppe-RGP and Aru-RGP, respectively, and the effect of RLP2 exhibited partial species-specificity. These findings indicate that two relaxin-type peptides regulate spawning in starfish and therefore we propose that RGP and RLP2 are renamed RGP1 and RGP2, respectively.

Relaxin-like Gonad-Stimulating Peptides in Asteroidea

Starfish relaxin-like gonad-stimulating peptide (RGP) is the first identified peptide hormone with gonadotropin-like activity in invertebrates. RGP is a heterodimeric peptide, comprising A and B chains with disulfide cross-linkages. Although RGP had been named a gonad-stimulating substance (GSS), the purified peptide is a member of relaxin-type peptide family. Thus, GSS was renamed as RGP. The cDNA of RGP encodes not only the A and B chains, but also signal and C-peptides. After the rgp gene is translated as a precursor, mature RGP is produced by eliminating the signal and C-peptides. Hitherto, twenty-four RGP orthologs have been identified or predicted from starfish in the orders Valvatida, Forcipulatida, Paxillosida, Spinulosida, and Velatida. The molecular evolution of the RGP family is in good accordance with the phylogenetic taxonomy in Asteroidea. Recently, another relaxin-like peptide with gonadotropin-like activity, RLP2, was found in starfish. RGP is mainly present in the radial nerve cords and circumoral nerve rings, but also in the arm tips, the gonoducts, and the coelomocytes. RGP acts on ovarian follicle cells and testicular interstitial cells to induce the production of 1-methyladenine (1-MeAde), a starfish maturation-inducing hormone. RGP-induced 1-MeAde production is accompanied by an increase in intracellular cyclic AMP levels. This suggests that the receptor for RGP (RGPR) is a G protein-coupled receptor (GPCR). Two types of GPCRs, RGPR1 and RGPR2, have been postulated as candidates. Furthermore, 1-MeAde produced by RGP not only induces oocyte maturation, but also induces gamete shedding, possibly by stimulating the secretion of acetylcholine in the ovaries and testes. Thus, RGP plays an important role in starfish reproduction, but its secretion mechanism is still unknown. It has also been revealed that RGP is found in the peripheral adhesive papillae of the brachiolaria arms. However, gonads are not developed in the larvae before metamorphosis. It may be possible to discover new physiological functions of RGP other than gonadotropin-like activity.

The C-terminally amidated relaxin-like gonad-stimulating peptide in the starfish Astropecten scoparius

A relaxin-like gonad-stimulating peptide (RGP) in starfish was the first identified invertebrate gonadotropin, consisting of A- and B-chain. Recently, an RGP ortholog (Asc-RGP) from Astropecten scoparius in the order Paxillosida was found to harbor an amidation signal (Gly-Arg) at the C-terminus of the B-chain (Mita et al., 2020a). Two cleavage sites were also predicted within the signal peptide of the Asc-RGP precursor. Thus, four kinds of analogs (Asc-RGP-NH₂(S), Asc-RGP-GR(S), Asc-RGP- NH₂(L), Asc-RGP-GR(L) were hypothesized as natural Asc-RGPs. To identify the natural Asc-RGP, an extract of radial nerve cords from A. scoparius was analyzed using reverse-phase high-performance liquid chromatography and MALDI-TOF-mass spectrometry. The molecular weight of Asc-RGP was 4585.3, and those of A- and B-chains were 2511.8 and 2079.8, respectively. This strongly suggests that natural RGP in A. scoparius is Asc-RGP-NH₂(S). Asc-RGP-NH₂(S) stimulated 1-methyladenine and cyclic AMP production in isolated ovarian follicle cells of A. scoparius. On the other hand, the concentrations of four synthetic Asc-RGP analogs required for the induction of spawning in 50% of ovarian fragments were almost the same. The size and C-terminal amidation of the B-chain might not be important for spawning-inducing activity. C-terminally amidated RGPs in the B-chain were also observed in other species of starfish belonging to the order Paxillosida, particularly the family Astropectinidae, but not the family Luidiidae.

Comparative Transcriptome Reveals the Potential Modulation Mechanisms of Spdsx Affecting Ovarian Development of Scylla paramamosain

In previous study, we reported the identification, tissue distribution, and the roles of Spdsx played in the testis, androgenic gland, and ovary in Scylla paramamosain. Here, we primally identify its potential target genes in the ovary with RNAi and RNA-Seq technology. By comparing the transcriptome data of two groups (ovaries that injected with dsRNA for EGFP and Dsx), we found that 6520 Unigenes were differentially expressed, including a plenty of conserved crucial genes involved in ovarian development, such as vitellogenin (vtg), vtg receptor (vtgR), apolipoprotein D, adenylate cyclase 3, adenylate cyclase 5, cyclin A, cyclin B, and cell division cycle 2 (cdc2). In addition, these DEGs were also enriched in pathways related to ovary development, including PI3K-Akt signaling pathway, MAPK signaling pathway, insulin signaling pathway, Wnt signaling pathway, relaxin signaling pathway, estrogen signaling pathway, progesterone-mediated oocyte maturation, ovarian steroidogenesis, and oocyte meiosis. Moreover, several genes were selected for qRT-PCR to validate the accuracy of the bioinformatic result. According to current transcriptome result, we speculate that the Spdsx is a crucial regulator of ovary development in S. paramamosain. To the best of our knowledge, the current study was the first report about dsx function through comparative transcriptome analysis in crustacean species, which not only identified relevant genes and pathways involved in ovarian development of S. paramamosain, but also shed light on the regulatory mechanisms of dsx at the molecular level in crustacean.

Characterization and localization of relaxin-like gonad-stimulating peptide in the crown-of-thorns starfish, Acanthaster cf. solaris

In starfish, a relaxin-like gonad-stimulating peptide (RGP) is the gonadotropin responsible for final gamete maturation. RGP comprises two different peptides, A- and B-chains with two interchain and one intrachain disulfide bonds. The existence of two isomers of RGP in the crown-of-thorns starfish, Acanthaster planci, has been reported previously, but it was recently shown that A. planci represents a species complex with four different species. Here we elucidated the authentic sequence of the Pacific species, Acanthaster cf. solaris, RGP (Aso-RGP). The Aso-RGP precursor encoded by a 354 base pair open reading frame was composed of 117 amino acids (aa). The amino acid identity of Aso-RGP to Patiria pectinifera RGP (Ppe-RGP) and Asterias amurensis RGP (Aam-RGP) was 74% and 60%, respectively. Synthetic Aso-RGP induced spawning of ovarian fragments from A. cf. solaris. Ppe-RGP and Aam-RGP also induced spawning by A. cf. solaris ovaries. In contrast, Ppe-RGP and Aso-RGP induced spawning by P. pectinifera ovaries, but Aam-RGP was inactive. Notably, anti-Ppe-RGP antibodies recognized Aso-RGP as well as Ppe-RGP. Localization of Aso-RGP was observed immunohistochemically using anti-Ppe-RGP antibodies, showing that Aso-RGP was mainly present in the radial nerve cords of A. cf. solaris. Aso-RGP was distributed not only in the epithelium of the ectoneural region but also in the neuropile of the ectoneural region. These results suggest that Aso-RGP is synthesized in the epithelium of the ectoneural region, then transferred to fibers in the neuropile of the ectoneural region in radial nerve cords.

Comparative Transcriptome Reveals the Potential Modulation Mechanisms of Spfoxl-2 Affecting Ovarian Development of Scylla paramamosain

Previously, we reported the identification, tissue distribution and confirmed the roles of Spfoxl-2 in regulating vitellogenin (vtg) expression in Scylla paramamosain. Here, we primally analyzed its potential target genes in the ovary with RNAi and RNA-Seq technology. By comparing the transcriptome data of two groups (ovaries that injected with EGFP and Foxl-2 siRNA, respectively), we found 645 DEGs (differentially expressed genes), including several conserved crucial genes involved in ovarian development, such as vtg, vitellogenin receptor (vtgR), adenylate cyclase (AC), cyclinB, and cell division cycle 2 (cdc2). In addition, these DEGs were also enriched in pathways related to ovary development, including relaxin signaling pathway, ovarian steroidogenesis, and progesterone-mediated oocyte maturation. Moreover, several genes were selected for qRT-PCR to validate the accuracy of the bioinformatic result. To the best of our knowledge, the current study was the first report about foxl-2 function through comparative transcriptome analysis in crustacean species, which identified not only relevant genes and pathways involved in ovarian development of S. paramamosain, but also provided new insights into the regulatory mechanisms of foxl-2 at the molecular level in crustacean.

A relaxin-like gonad-stimulating peptide identified from the starfish Astropecten scoparius

A relaxin-like gonad-stimulating peptide (RGP) in starfish was the first identified invertebrate gonadotropin responsible for final gamete maturation. An RGP ortholog was newly identified from Astropecten scoparius of the order Paxillosida. The A. scoparius RGP (AscRGP) precursor is encoded by a 354 base pair open reading frame and is a 118 amino acid (aa) protein consisting of a signal peptide (26 aa), B-chain (21 aa), C-peptide (47 aa), and A-chain (24 aa). There are three putative processing sites (Lys-Arg) between the B-chain and C-peptide, between the C-peptide and A-chain, and within the C-peptide. This structural organization revealed that the mature AscRGP is composed of A- and B-chains with two interchain disulfide bonds and one intrachain disulfide bond. The C-terminal residues of the B-chain are Gln-Gly-Arg, which is a potential substrate for formation of an amidated C-terminal Gln residue. Non-amidated (AscRGP-GR) and amidated (AscRGP-NH₂ ) peptides were chemically synthesized and their effect on gamete shedding activity was examined using A. scoparius ovaries. Both AscRGP-GR and AscRGP-NH₂ induced oocyte maturation and ovulation in similar dose-dependent manners. This is the first report on a C-terminally amidated functional RGP. Collectively, these results suggest that AscRGP-GR and AscRGP-NH₂ act as a natural gonadotropic hormone in A. scoparius.

A novel G protein-coupled receptor for starfish gonadotropic hormone, relaxin-like gonad-stimulating peptide

Gonadotropic hormones play important regulatory roles in reproduction. Relaxin-like gonad-stimulating peptide (RGP) is a gonadotropin-like hormone in starfish. However, a receptor for RGP remains to be identified. Here, we describe the identification of an authentic receptor for RGP (RGPR) in the starfish, Patiria pectinifera. A binding assay using radioiodinated P. pectinifera RGP (PpeRGP) revealed that RGPR was expressed in ovarian follicle cells. A RGPR candidate was identified by homology-searching of transcriptome data of P. pectinifera follicle cells. Based on the contig sequences, a putative 947-amino acid PpeRGPR was cloned from follicle cells. Like the vertebrate relaxin family peptide receptors (RXFP 1 and 2), PpeRGPR was a G protein-coupled receptor that harbored a low-density lipoprotein-receptor class A motif and leucine-rich repeat sequences in the extracellular domain of the N-terminal region. Sf9 cells transfected with Gαq16-fused PpeRGPR activated calcium ion mobilization in response to PpeRGP, but not to RGP of another starfish Asterias amurensis, in a dose-dependent fashion. These results confirmed the species-specific reactivity of RGP and the cognate receptor. Thus, the present study provides evidence that PpeRGPR is a specific receptor for PpeRGP. To the best of our knowledge, this is the first report on the identification of a receptor for echinoderm RGP.