Molluscan GnRH associated with reproduction

Water temperature effects on embryonic development and GnRH-like expression in kisslip cuttlefish (Sepia lycidas) hinting the potential role of GnRH-like peptide

Gonadotropin-releasing hormone (GnRH)-like peptides are multifunctional neuropeptides involved in cardiac control, early ontogenesis, and reproduction in cephalopods. However, the precise role of GnRH-like peptides in embryonic development and juvenile growth in cephalopods remains unknown. In this study, we showed that GnRH-like peptides are involved in the embryonic development of kisslip cuttlefish (Sepia lycidas). We confirmed that higher water temperatures induced early hatching. Simultaneously, we found that brain GnRH-like peptide gene expression gradually increased with increasing hatching speed. However, the rise in water temperature within a suitable range had no effect on the juvenile sex ratio or early gonadal development. Our results indicate that GnRH-like peptides may play an accelerating role in embryonic development; however, they are not involved in sex determination or early gonadal development in kisslip cuttlefish.

Exploring the mechanism of nonylphenol-induced ovarian developmental delay of manila clams, Ruditapes philippinarum: Applying RNAi to toxicological analysis

Nonylphenol (NP) contamination in the coastal environment of China poses ecological risks to aquatic organisms. However, the endocrine disruptive impacts of NP on bivalves, particularly on ovarian development, remain poorly understood. In this study, Manila clams Ruditapes philippinarum at the developing stage of gonad were exposed to 1.0 μg/L NP for 21 days. Utilizing RNA interference (RNAi) to suppress ER gene expression, we observed a delay in ovarian development as evidenced by histological observations under both NP and NPRi (NP with ER-RNAi) treatment, with Vtg elevation exclusive to the NP group. Comprehensive analyses encompassing transcriptomics, real-time quantitative PCR, and steroid hormone measurement revealed significant alterations in aldosterone synthesis, estrogen signaling, and thyroid hormone synthesis. These pathways showed similar perturbations in both NP and NPRi groups compared to controls. Notably, the NPRi group exhibited distinct enrichment in PPAR and insulin signaling pathways, may implicating these in ER function suppression. Steroid hormone biosynthesis was notably reduced in both treatments, pointing to a profound impact on hormone synthesis. The contrast between in vivo and in vitro findings suggests that NP's detrimental effects on ovarian development may primarily involve neuroendocrine regulation of steroidogenesis. This investigation highlights the complex dynamics of NP-induced endocrine disruption in bivalves, emphasizing the pivotal role of ER and associated pathways.

Identification and characterization of gonadotropin-releasing hormone (GnRH) in Zhikong scallop Chlamys farreri during gonadal development

Gonadotropin-releasing hormone (GnRH) controls synthesis of sex steroid hormones through hypothalamic-pituitary-gonadal (HPG) axis in vertebrates. But in mollusks, research on neuroendocrine control of gonadal function, such as the function of GnRH during gonadal development is limited. In this study, we investigated the morphology and structure of the nerve ganglia of Zhikong scallop Chlamys farreri by physiological and histological observations. We also cloned the ORF and studied the expression patterns of GnRH in the scallop. Tissue expression analysis showed that GnRH was highly expressed in parietovisceral ganglion (PVG). The in situ hybridization result further confirmed that GnRH mRNA only distributed in some good-sized neurons in the posterior lobe (PL) and some pint-sized neurons in the lateral lobe (LL). In addition, by examining the expression of GnRH during gonadal development in ganglia, we found GnRH displayed higher expression in the female scallops, and showed significant high expression at the growing stage of female scallops in PVG. This study would contribute to gaining insight into the mechanism underlying reproduction regulation by GnRH in the scallop and help to provide a better understanding of reproductive neuroendocrine in mollusks.

Expression and functional analyses for estrogen receptor and estrogen related receptor of Yesso scallop, Patinopecten yessoensis

Estrogen receptors (ERs) were known as estrogen-activated transcription factors and function as major reproduction regulators in vertebrates. The presence of er genes had been reported in Molluscan cephalopods and gastropods. However, they were considered as constitutive activators with unknown biological functions since reporter assays for these ERs did not show a specific response to estrogens. In this study, we tried characterization of ER orthologues from the Yesso scallop, Patinopecten yessoensis, in which estrogens had been proven to be produced in the gonads and involved in the spermatogenesis and vitellogenesis. Identified ER and estrogen related receptor (ERR) of Yesso scallops, designated as py-ER and py-ERR, conserved specific domain structures for a nuclear receptor. Their DNA binding domains showed high similarities to those of vertebrate ER orthologues, while ligand binding domains had low similarities with them. Both the py-er and py-err expression levels decreased in the ovary at the mature stage while py-vitellogenin expression increased in the ovary by quantitative real-time RT-PCR. Also, the py-er and py-err showed higher expressions in the testis than ovary during the developing and mature period, suggesting both genes might function in the spermatogenesis and testis development. The py-ER showed binding affinities to vertebrate estradiol-17β (E₂). However, the intensity was weaker than the vertebrate ER, indicating scallops might exist endogenous estrogens with a different structure. On the other hand, the binding property of py-ERR to E₂ was not confirmed in this assay, speculating that py-ERR was a constitutive activator as other vertebrate ERRs. Further, the py-er was localized in the spermatogonia in the testis and in the auxiliary cells in the ovary by in situ hybridization, indicating its potential roles in promoting spermatogenesis and vitellogenesis. Taken together, the present study demonstrated that py-ER was an authentic E₂ receptor in the Yesso scallop and might have functions for the spermatogonia proliferation and vitellogenesis, while py-ERR was involved in the reproduction by undiscovered manners.

Concomitant downregulation of neuropeptide genes in a marine snail with consecutive sexual maturation after a nuclear disaster in Japan

Consecutive sexual maturation (CSM), an abnormal reproductive phenomenon of a marine snail, Reishia clavigera, has occurred since 2017 in the vicinity of the Fukushima Daiichi Nuclear Power Plant after the nuclear disaster there. We hypothesized that alterations in animal physiology mediated through genetic/epigenetic changes could sensitively reflect environmental pollution. Understanding the mechanism of this rapid biological response should enable us to quantitatively evaluate long-lasting effects of the nuclear disaster. To determine the molecular basis for CSM, we conducted transcriptome profiling in the ganglia of normal and CSM snails. We assembled the short-read cDNA sequences obtained by Illumina sequencing, and succeeded in characterizing more than 60,000 gene models that include 88 kinds of neuropeptide precursors by BLAST search and experimental curation. GO-enrichment analysis of the differentially expressed genes demonstrated that severe downregulation of neuropeptide-related genes occurred concomitantly with CSM. In particular, significant decreases of the transcripts of 37 genes among 88 neuropeptide precursor genes, including those for myomodulin, PentaFVamide, maturation-associated peptide-5A and conopressin, were commonly observed in female and male CSM snails. By contrast, microseminoprotein precursor was the only exceptional case where the expression was increased in CSM snails. These results indicate that down-regulation of neuropeptide precursors is a remarkable feature of CSM. We also found that factors involved in epigenetic modification rather than transcription factors showed altered patterns of expression upon CSM. Comprehensive expression panels of snail neuropeptide precursors made in this study will be useful tools for environmental assessment as well as for studying marine reproductive biology.

The Effects of Chronic Lead Exposure on Testicular Development of Japanese Quail (Coturnix japonica): Histopathological Damages, Oxidative Stress, Steroidogenesis Disturbance, and Hypothalamus-Pituitary-Testis Axis Disruption

Lead (Pb) becomes a global public health concern for its high toxicology. Birds are sensitive to environmental pollution and Pb contamination exerts multiple negative influences on bird life. Pb also impacts on avian reproductive system. Thus, in this study, we attempted to determine toxicological effects and possible mechanistic pathways of Pb on avian testicular development by using the model species-Japanese quail (Coturnix japonica). Male quail chicks of 1-week-old were exposed to 0, 50, 500, and 1000 ppm Pb concentrations in drinking water for 5 weeks when reaching sexual maturation. The results showed that high Pb doses (500 and 1000 ppm) induced testis atrophy and cloacal gland shrinkage. Microstructural damages of both hypothalamus and testis indicated the disruption of the hypothalamus-pituitary-gonadal (HPG) axis by Pb exposure. The decrease of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) and testosterone (T) may also imply HPG axis disruption. Moreover, excess testicular oxidative damages featured by increasing reactive oxygen species (ROS) and malondialdehyde (MDA) and decreasing catalase (CAT), glutathione (GSH), superoxide dismutase (SOD), glutathione-S-transferase (GST), and total antioxidant capacity (T-AOC) indicated increasing risks of reproductive dysfunction by Pb. Furthermore, increasing apoptosis and upregulation of gene expression associated with cell death suggested testicular abnormal development. In addition, molecular signaling involved with steroidogenesis in the testis was disturbed by Pb treatment. The study showed that Pb could impair testicular development and reproductive function by morphological and histological injury, hormone suppression, oxidative stress, cell death, and HPG axis disruption.

Changes in gonadotropin-releasing hormone in the cerebral ganglion of the Manila clam Ruditapes philippinarum during gonadal development

Gonadotropin-releasing hormone (GnRH) plays an important role in reproduction in both vertebrates and invertebrates; however, little is known about GnRH during gonadal development in bivalves. We developed a time-resolved fluoroimmunoassay (TR-FIA) for Manila clam Ruditapes philippinarum GnRH (rpGnRH) and measured the amount of rpGnRH in the cerebral ganglion (CG) and sex steroid hormones in the hemolymph during gonadal development. The cross-reactivity of the anti-rpGnRH antibody against other forms of GnRH was <0.15%, and the displacement curve obtained for serially diluted CG extracts was parallel to the rpGnRH standard curve, confirming the suitability of the TR-FIA system. Based on histological observation, gonadal development of the clams was classified into early developing (stage 1), late developing (stage 2), ripe (stage 3), and partially spent (stage 4). In female clams, rpGnRH levels in the CG peaked at stage 1, and 17β-estradiol (E2) levels in the hemolymph peaked at stage 2. The rpGnRH levels in males and hemolymph testosterone levels in both sexes did not differ significantly across stages. Hemolymph E2 levels in males were below the detection limit for the TR-FIA. These results suggest that rpGnRH and E2 secretion in females can activate ovarian development of the Manila clam at the early and late developing stages, respectively.

Possible roles of gonadotropin-releasing hormone (GnRH) and melatonin in the control of gonadal development of clam Ruditapes philippinarum

Gonadotropin-releasing Hormone (GnRH) is a key reproductive endocrine regulator, and melatonin is considered as a potent candidate in the regulation of photoperiod-related reproductive endocrinology. Nevertheless, their function during gonadal development of molluscs has not been uncovered yet. In the present study, RNAi of GnRH and melatonin injection were conducted on marine bivalve manila clam Ruditapes philippinarum. Tissue section showed that gonadal development was significantly inhibited in male clams injected with GnRH dsRNA for 21 days. For GnRH RNAi treatment group, the expression levels of steroid synthetic enzyme genes 3β-hydroxysteroid dehydrogenase (3β-HSD), 17β-hydroxysteroid dehydrogenase (17β-HSD), cytochrome P450 (CYP3A) and melatonin receptor homolog (MTNR) gene were significantly down-regulated in female clams while significantly up-regulated in male clams. In melatonin injection group, the expression of GnRH was significantly inhibited and the expression of 3β-HSD, 17β-HSD, CYP3A and MTNR genes also increased which was in line with the GnRH dsRNA injection group in male clams. These results suggest that melatonin may affect GnRH expression and both have effects on gonadal development of bivalves. This study provides evidence for understanding the effects of melatonin and GnRH on reproductive endocrinology and gonadal development in bivalve molluscs.

Molecular cloning, immunological characterization, and expression analysis of gonadotropin-releasing hormone (GnRH) in the brain of the Chinese alligator during different stages of reproductive cycle

The neurohormone gonadotropin-releasing hormone (GnRH) plays an essential role in the control of reproductive functions in vertebrates. However, the full-length complementary DNA (cDNA) encoding the GnRHs precursor and it role in the reproductive cycles regulating has not been illustrated in crocodilian species. In the present study, full-length cDNAs encoding GnRH1 forms, its predominant localization within brain and peripheral tissues, and GnRH1 peptide concentrations in the hypothalamus and pituitary in relation to seasonal gonadal development of Chinese alligator were investigated. The cDNA of GnRH1 is consisted of 282 bp open reading frame encoding 93 amino acids. The deduced amino acid sequence of alligator GnRH1 contains several conserved regions and shows a closer genetic relationship to the avian species than to other reptile species. The GnRH1 immunopositive cells were not only detected widely in cerebrum, diencephalon, medulla oblongata but also observed in peripheral tissues, these widespread distribution characteristics indicated that GnRH1 possibly possess the multi-functionality in Chinese Alligator. GnRH1 peptide concentration within hypothalamus were observed be the highest in RP group (P < 0.05), in association with an peak value in GSI and emerging of late vitellogenic follicles in the ovary. Taken together, our results suggested that GnRH1 was predominantly involved in the vitellogenesis process of seasonal gonadal development of Chinese Alligator.

Expression profile of GnRH-like peptide during gonadal sex differentiation in the cephalopod kisslip cuttlefish, Sepia lycidas

Gonadotropin-releasing hormone (GnRH) is one of the most important neuroendocrine regulators for animal reproduction. GnRH-like peptide (GnRH-like) has recently been shown to play a critical reproductive role mainly in gametogenesis or steroidogenesis in the gonads of some molluscs, including cephalopods. However, its involvement in gonadal sex differentiation remains unknown. Here, we show the expression profile of GnRH-like in the brain of the cephalopod kisslip cuttlefish, Sepia lycidas, throughout gonadal sex differentiation, by quantitative real time RT-PCR and immunohistochemistry. We found that GnRH-like could be detected in the brain at a sexually undifferentiated stage, and its expression level significantly increased upon initiation of gonadal sex differentiation. However, no significant difference in GnRH-like expression levels was observed between sexes during gonadal sex differentiation. Additionally, we demonstrated immunoreactivity of GnRH-like in glial cells or immature neurons, which are mainly distributed in the non-reproductive related area of the cephalopod brain, suggesting the immature function of the reproductive endocrine axis during early ontogenesis. Our results demonstrate for the first time, the expression profile of GnRH-like during early ontogenesis in cephalopods.

Potential GnRH and steroidogenesis pathways in the scallop Patinopecten yessoensis

Gonadotropin-releasing hormone (GnRH) controls synthesis of sex steroid hormones through hypothalamic-pituitary-gonadal (HPG) axis in vertebrates. But in mollusks, research on GnRH and steroidogenesis pathways is still limited. In this study, we first identified two gonadotropin receptor like genes (LGR and LGR5L) and four steroidogenesis-related genes (CYP17A, HSD17B12, HSD3B1 and HSD3B2) in the scallop Patinopecten yessoensis. By examining the expression of 11 genes in the ganglia and/or gonad as well as the concentration of progesterone, testosterone and estradiol in the gonad, we postulate that a potential GnRH signaling pathway (GnRH-GnRHR-GPB5-LGR/LGR5L) in the cerebral and pedal ganglia (CPG) and steroidogenesis pathway (CYP17A, HSD17B12 and HSD3B1) in the gonad are involved in regulating sex steroid hormones. E₂/T index that indicates aromatase activity is higher in the ovary than testis and is positively correlated with the expression of FOXL2 in the gonad, implying the presence of aromatase in the scallop. In addition, we confirmed that expression of most of the downstream genes in the two pathways was significantly elevated after injection of mature py-GnRH peptide. This study would contribute to a new understanding of the molecular basis underlying reproduction regulation by GnRH in mollusks.

Manganese exposure caused reproductive toxicity of male mice involving activation of GnRH secretion in the hypothalamus by prostaglandin E2 receptors EP1 and EP2

Exposure to manganese (Mn) can cause male reproductive damage and lead to abnormal secretion of sex hormones. Gonadotropin-releasing hormone (GnRH) plays an important role in the neuromodulation of vertebrate reproduction. Astrocytes can indirectly regulate the secretion of GnRH by binding paracrine prostaglandin E₂ (PGE₂) specifically to the EP1 and EP2 receptors on GnRH neurons. Prior studies assessed the abnormal secretion of GnRH caused by Mn exposure, but the specific mechanism has not been reported in detail. This study investigated the effects of Mn exposure on the reproductive system of male mice to clarify the role of PGE₂ in the abnormal secretion of GnRH in the hypothalamus caused by exposure to Mn. Our data demonstrate that antagonizing the EP1 and EP2 receptors of PGE₂ can restore abnormal levels of GnRH caused by Mn exposure. Mn exposure causes reduced sperm count and sperm shape deformities. These findings suggest that EP1 and EP2, the receptors of PGE₂, may be the key to abnormal GnRH secretion caused by Mn exposure. Antagonizing the PGE₂ receptors may reduce reproductive damage caused by Mn exposure.

Effect of gonadotropin releasing hormone on the expression of luteinizing hormone and estrogen in the nerve ganglia and ovary of a tropical abalone, Haliotis asinina Linnaeus

Gonadotropin releasing hormone (GnRH) is a peptide brain hormone that is involved in the regulation of reproduction in vertebrates via stimulation of the secretion of the pituitary hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which in their turn stimulate sexual development and sex steroid hormone secretion by the gonads. The tropical abalone, Haliotis asinina, in common with many other invertebrates contains a peptide with a similar structure to GnRH. This study looks at its possible involvement in reproduction by injecting groups of one-year-old female abalone at the mature phase by injecting them with synthetic H. asinina (Has) GnRH at doses of 0, 250 and 500 ng/g and then measuring the amount of material in nerve ganglia, ovary and hemolymph that cross-reacted with enzyme-linked immunosorbent assays (ELISA) for vertebrate LH and steroid, estradiol. Immunohistochemistry, using antibodies for the same two compounds, was also carried out to examine the location of immunoactivity in the tissues of the animals. There were slight (in some cases statistically significant) increases in LH-immunoactivity and estradiol in the hemolymph and tissues. However, this applied to the lower dose only (i.e the dose-response relationship was non-monotonic). Using immunohistochemistry, LH-immunoreactive cells were observed in types 1 and 2 neurosecretory (NS1 and NS2) cells within the cerebral and pleuropedal ganglia of H. asinina. In addition, LH-immunoreactive nerve fiber bundles were strongly detected in both ganglia. The immunoactivity against the estrogen appeared to be localized in the granulated cells within the connective tissue and trabeculae of the mature ovary. There was no positive staining in the cytoplasm of any stage of the germ cells. The interpretation of these findings is presently hindered by the fact that the homologous gene for vertebrate LH has not yet been identified in the genomes of any mollusks (so the cause of the immunostaining is as yet unknown) and also by the fact that mollusks are known to readily absorb steroids from the environment and store them long-term in the form of fatty acid esters. More work, involving identification of the protein that cross-reacts with the LH antiserum and also exclusion of the possibility that the estradiol is of exogenous origin, will have to be carried out before these findings can be used to manipulate reproduction in this species.

Spermatozoa motility in bivalves: Signaling, flagellar beating behavior, and energetics

Though bivalve mollusks are keystone species and major species groups in aquaculture production worldwide, gamete biology is still largely unknown. This review aims to provide a synthesis of current knowledge in the field of sperm biology, including spermatozoa motility, flagellar beating, and energy metabolism; and to illustrate cellular signaling controlling spermatozoa motility initiation in bivalves. Serotonin (5-HT) induces hyper-motility in spermatozoa via a 5-HT receptor, suggesting a serotoninergic system in the male reproductive tract that might regulate sperm physiology. Acidic pH and high concentration of K⁺ are inhibitory factors of spermatozoa motility in the testis. Motility is initiated at spawning by a Na⁺-dependent alkalization of intracellular pH mediated by a Na⁺/H⁺ exchanger. Increase of 5-HT in the testis and decrease of extracellular K⁺ when sperm is released in seawater induce hyperpolarization of spermatozoa membrane potential mediated by K⁺ efflux and associated with an increase in intracellular Ca²⁺ via opening of voltage-dependent Ca²⁺ channels under alkaline conditions. These events activate dynein ATPases and Ca²⁺/calmodulin-dependent proteins resulting in flagellar beating. It may be possible that 5-HT is also involved in intracellular cAMP rise controlling cAMP-dependent protein kinase phosphorylation in the flagellum. Once motility is triggered, flagellum beats in asymmetric wave pattern leading to circular trajectories of spermatozoa. Three different flagellar wave characteristics are reported, including "full", "twitching", and "declining" propagation of wave, which are described and illustrated in the present review. Mitochondrial respiration, ATP content, and metabolic pathways producing ATP in bivalve spermatozoa are discussed. Energy metabolism of Pacific oyster spermatozoa differs from previously studied marine species since oxidative phosphorylation synthetizes a stable level of ATP throughout 24-h motility period and the end of movement is not explained by a low intracellular ATP content, revealing different strategy to improve oocyte fertilization success. Finally, our review highlights physiological mechanisms that require further researches and points out some advantages of bivalve spermatozoa to extend knowledge on mechanisms of motility.

Phenotypic Stability of Sex and Expression of Sex Identification Markers in the Adult Yesso Scallop Mizuhopecten yessoensis throughout the Reproductive Cycle

Simple Summary

Bivalve sex is thought to fluctuate depending on environmental conditions. So far, there has been no investigation on the phenotypic stability of sex in the commercially important Yesso scallop Mizuhopecten yessoensis. The present study revealed that the sex of the Yesso scallop is stable after initial sex differentiation and that this species maintains a sex-stable maturation system throughout its life. In addition, gonad differentiation for each sex was precisely characterized by using molecular markers throughout the maturational cycle.

Abstract

The objective of the present study was to analyze the phenotypic stability of sex after sex differentiation in the Yesso scallop, which is a gonochoristic species that has been described as protandrous. So far, no study has investigated in detail the sexual fate of the scallop after completion of sex differentiation, although bivalve species often show annual sex change. In the present study, we performed a tracking experiment to analyze the phenotypic stability of sex in scallops between one and two years of age. We also conducted molecular marker analyses to describe sex differentiation and gonad development. The results of the tracking experiment revealed that all scallops maintained their initial sex phenotype, as identified in the last reproductive period. Using molecular analyses, we characterized my-dmrt2 and my-foxl2 as sex identification markers for the testis and ovary, respectively. We conclude by proposing that the Yesso scallop is a sex-stable bivalve after its initial sex differentiation and that it maintains a sex-stable maturation system throughout its life. The sex-specific molecular markers identified in this study are useful tools to assess the reproductive status of the Yesso scallop.

Gonadotropin-releasing hormone by any other name would smell as sweet

The goal of this article is to discuss the nomenclature of members of the gonadotropin-releasing hormone (GnRH) superfamily. This superfamily currently consists of 5 families: (1) vertebrate GnRH, (2) adipokinetic hormone, (3) corazonin, (4) adipokinetic hormone/corazonin-related peptide and (5) invertebrate GnRH (or GnRH/corazonin). The naming of some of these peptides, especially members of the invertebrate GnRH family, may not have reflected their true evolutionary origin, leading to some confusion and controversy. Using a few examples from the invertebrate GnRH family, this article proposes several peptide-naming criteria and discusses naming challenges and problem cases. It is recommended that the invertebrate GnRH family be renamed based on the naming criteria of (1) mature peptide structure, (2) prepropeptide phylogeny, and (3) receptor phylogeny. Following this approach, the names of the peptides should reflect their phylogeny, and if possible, delineate a monophyletic group.

Identification of jellyfish neuropeptides that act directly as oocyte maturation-inducing hormones

Oocyte meiotic maturation is crucial for sexually reproducing animals, and its core cytoplasmic regulators are highly conserved between species. By contrast, the few known maturation-inducing hormones (MIHs) that act on oocytes to initiate this process are highly variable in their molecular nature. Using the hydrozoan jellyfish species Clytia and Cladonema, which undergo oocyte maturation in response to dark-light and light-dark transitions, respectively, we deduced amidated tetrapeptide sequences from gonad transcriptome data and found that synthetic peptides could induce maturation of isolated oocytes at nanomolar concentrations. Antibody preabsorption experiments conclusively demonstrated that these W/RPRPamide-related neuropeptides account for endogenous MIH activity produced by isolated gonads. We show that the MIH peptides are synthesised by neural-type cells in the gonad, are released following dark-light/light-dark transitions, and probably act on the oocyte surface. They are produced by male as well as female jellyfish and can trigger both sperm and egg release, suggesting a role in spawning coordination. We propose an evolutionary link between hydrozoan MIHs and the neuropeptide hormones that regulate reproduction upstream of MIHs in bilaterian species.

Neuropeptides encoded within a neural transcriptome of the giant triton snail Charonia tritonis, a Crown-of-Thorns Starfish predator

Neuropeptides represent a diverse class of signaling molecules originating from neural tissues. These chemical modulators orchestrate complex physiological events including those associated with growth and reproduction. De novo transcriptome sequencing of a cerebral ganglion library of the endangered giant triton snail (Charonia tritonis) was undertaken in an effort to identify key neuropeptides that control or influence its physiology. The giant triton snail is considered a primary predator of the corallivore Acanthaster planci (Crown-of-Thorns Starfish) that is responsible for a significant loss in coral cover on reefs in the Indo-Pacific. The transcriptome library was assembled into contigs, and then bioinformatic analysis was used to identify a repertoire of 38 giant triton snail neuropeptide precursor genes, and various isoforms, that encode conserved molluscan neuropeptides. C. tritonis neuropeptides show overall precursor organisation consistent with those of other molluscs. These include those neuropeptides associated with mollusc reproduction such as the APGWamide, buccalin, conopressin, gonadotropin-releasing hormone (GnRH), NKY and egg-laying hormone. These data provide a foundation for further studies targeted towards the functional characterisation of neuropeptides to further understand aspects of the biology of the giant triton snail, such as elucidating its reproductive neuroendocrine pathway to allow the development of knowledge based captive breeding programs.

Sex Reversal and Analyses of Possible Involvement of Sex Steroids in Scallop Gonadal Development in Newly Established Organ-Culture Systems

Many molluscs perform sex reversal, and sex hormones may be involved in the process. In adult scallops, Patinopecten yessoensis, gonadotropin releasing hormone and 17β-estradiol (E₂) are involved in male sexual maturation, however, little is known about the effects of E₂ and testosterone (T) on the gonadal differentiation in young scallops. In the present study, scallop gonadal development was analyzed to determine the sex reversal stage in Funka bay, and effects of E₂ and T were examined. In Funka bay, almost all scallops were male at month 12. Scallops equipped with ambiguous gonads were 61.1% at month 16 and disappeared at month 18. Therefore, sex reversal in Funka bay occurs at around month 16. For establishment of organ culture systems for bivalves, Manila clam gonads were cultured in 15% L-15 medium diluted with HBSS containing 10% KSR on agarose gel at 10°C, and the gonads survived for 14 days. Scallop gonads were also able to be cultured in 30% L15 medium diluted with ASW containing 10% KSR on agarose gel for seven days. At mature stage, Foxl2 and Tesk were predominantly expressed in ovary and testis, respectively. When scallop gonads at sex reversal stage were organ-cultured, sex steroid treatment decreased Tesk expression in the majority of scallop gonads at sex reversal stage. However, no obvious change in Foxl2 and Tesk expression was detected in mature gonads in response to either E₂ or T in culture, suggesting sex steroid treatment might affect gonadal development at sex reversal stage.

Functional Characterization of Paralogous Gonadotropin-Releasing Hormone-Type and Corazonin-Type Neuropeptides in an Echinoderm

Homologs of the vertebrate neuropeptide gonadotropin-releasing hormone (GnRH) have been identified in invertebrates, including the insect neuropeptide corazonin (CRZ). Recently, we reported the discovery of GnRH-type and CRZ-type signaling systems in an echinoderm, the starfish Asterias rubens, demonstrating that the evolutionary origin of paralogous GnRH-type and CRZ-type neuropeptides can be traced back to the common ancestor of protostomes and deuterostomes. Here, we have investigated the physiological roles of the GnRH-type (ArGnRH) and the CRZ-type (ArCRZ) neuropeptides in A. rubens, using mRNA in situ hybridization, immunohistochemistry and in vitro pharmacology. ArGnRH precursor (ArGnRHP)-expressing cells and ArGnRH-immunoreactive cells and/or processes are present in the radial nerve cords, circumoral nerve ring, digestive system (e.g., cardiac stomach and pyloric stomach), body wall-associated muscle (apical muscle), and appendages (tube feet, terminal tentacle). The general distribution of ArCRZ precursor (ArCRZP)-expressing cells is similar to that of ArGnRHP, but with specific local differences. For example, cells expressing ArGnRHP are present in both the ectoneural and hyponeural regions of the radial nerve cords and circumoral nerve ring, whereas cells expressing ArCRZP were only observed in the ectoneural region. In vitro pharmacological experiments revealed that both ArGnRH and ArCRZ cause contraction of cardiac stomach, apical muscle, and tube foot preparations. However, ArGnRH was more potent/effective than ArCRZ as a contractant of the cardiac stomach, whereas ArCRZ was more potent/effective than ArGnRH as a contractant of the apical muscle. These findings demonstrate that both ArGnRH and ArCRZ are myoexcitatory neuropeptides in starfish, but differences in their expression patterns and pharmacological activities are indicative of distinct physiological roles. This is the first study to investigate the physiological roles of both GnRH-type and CRZ-type neuropeptides in a deuterostome, providing new insights into the evolution and comparative physiology of these paralogous neuropeptide signaling systems in the Bilateria.

Characterisation of Reproduction-Associated Genes and Peptides in the Pest Land Snail, Theba pisana

Increased understanding of the molecular components involved in reproduction may assist in understanding the evolutionary adaptations used by animals, including hermaphrodites, to produce offspring and retain a continuation of their lineage. In this study, we focus on the Mediterranean snail, Theba pisana, a hermaphroditic land snail that has become a highly invasive pest species within agricultural areas throughout the world. Our analysis of T. pisana CNS tissue has revealed gene transcripts encoding molluscan reproduction-associated proteins including APGWamide, gonadotropin-releasing hormone (GnRH) and an egg-laying hormone (ELH). ELH isoform 1 (ELH1) is known to be a potent reproductive peptide hormone involved in ovulation and egg-laying in some aquatic molluscs. Two other non-CNS ELH isoforms were also present in T. pisana (Tpi-ELH2 and Tpi-ELH3) within the snail dart sac and mucous glands. Bioactivity of a synthetic ELH1 on sexually mature T. pisana was confirmed through bioassay, with snails showing ELH1-induced egg-laying behaviours, including soil burrowing and oviposition. In summary, this study presents a detailed molecular analysis of reproductive neuropeptide genes in a land snail and provides a foundation for understanding ELH function.

Characterization of GnRH-like peptides from the nerve ganglia of Yesso scallop, Patinopecten yessoensis

There is yet no firm experimental evidence that the evolutionary ancient gonadotropin-releasing hormone GnRH (i.e., GnRH1) also acts in invertebrate gametogenesis. The objective of this paper is to characterize candidate invGnRH peptides of Yesso scallop Patinopecten yessoensis (i.e., peptide identification, immunohistochemical localization, and immunoquantification) in order to reveal their bioactive form in bivalves. Using mass spectrometry (MS), we identified two invGnRH (py-GnRH) peptides from the scallop nerve ganglia: a precursor form of py-GnRH peptide (a non-amidated dodecapeptide; py-GnRH12aa-OH) and a mature py-GnRH peptide (an amidated undecapeptide; py-GnRH11aa-NH2). Immunohistochemical staining allowed the localization of both py-GnRH peptides in the neuronal cell bodies and fibers of the cerebral and pedal ganglia (CPG) and the visceral ganglion (VG). We found that the peptides showed a dimorphic distribution pattern. Notably, the broad distribution of mature py-GnRH in neuronal fibers elongating to peripheral organs suggests that it is multi-functional. Time-resolved fluorescent immunoassays (TR-FIA) enabled the quantification of each py-GnRH form in the single CPG or VG tissue obtained from one individual. In addition, we observed greater abundance of mature py-GnRH in VG compared with its level in CPG, suggesting that VG is the main producing organ of mature py-GnRH peptide and that py-GnRH may play a central regulatory role in neurons of scallops. Our study provides evidence, for the first time, for the presence of precursor and mature forms of invGnRH peptides in the nerve ganglia of an invertebrate.

In Vivo Administration of Scallop GnRH-Like Peptide Influences on Gonad Development in the Yesso Scallop, Patinopecten yessoensis

Existing research on the role of gonadotropin-releasing hormone (GnRH) in bivalve reproduction is inadequate, even though a few bivalve GnRH orthologs have been cloned. The objective of this paper was to elucidate the in vivo effect of GnRH administration in Yesso scallop reproduction. We performed in vivo administration of scallop GnRH (py-GnRH) synthetic peptide into the developing gonad, and analyzed its effect on gonad development for 6 weeks during the reproductive season. The resulting sex ratio in the GnRH administered (GnRH(+)) group might be male biased, whereas the control (GnRH(-)) group had an equal sex ratio throughout the experiment. The gonad index (GI) of males in the GnRH(+) group increased from week 2 to 24.8% at week 6. By contrast the GI of the GnRH(-) group peaked in week 4 at 16.6%. No significant difference was seen in female GI between the GnRH(+) and GnRH(-) groups at any sampling point. Oocyte diameter in the GnRH(+) group remained constant (about 42–45 μm) throughout the experiment, while in the GnRH(-) group it increased from 45 to 68 μm i.e. normal oocyte growth. The number of spermatogonia in the germinal acini of males in the GnRH(+) group increased from week 4 to 6. Hermaphrodites appeared in the GnRH(+) group in weeks 2 and 4. Their gonads contained many apoptotic cells including oocytes. In conclusion, this study suggests that py-GnRH administration could have a potential to accelerate spermatogenesis and cause an inhibitory effect on oocyte growth in scallops.

Expression analysis of gnrh1 and gnrhr1 in spermatogenic cells of rat

Hypothalamic Gonadotropin Releasing Hormone (GnRH), via GnRH receptor (GnRHR), is the main actor in the control of reproduction, in that it induces the biosynthesis and the release of pituitary gonadotropins, which in turn promote steroidogenesis and gametogenesis in both sexes. Extrabrain functions of GnRH have been extensively described in the past decades and, in males, local GnRH activity promotes the progression of spermatogenesis and sperm functions at several levels. The canonical localization of Gnrh1 and Gnrhr1 mRNA is Sertoli and Leydig cells, respectively, but ligand and receptor are also expressed in germ cells. Here, we analysed the expression rate of Gnrh1 and Gnrhr1 in rat testis (180 days old) by quantitative real-time PCR (qPCR) and by in situ hybridization we localized Gnrh1 and Gnrhr1 mRNA in different spermatogenic cells of adult animals. Our data confirm the testicular expression of Gnrh1 and of Gnrhr1 in somatic cells and provide evidence that their expression in the germinal compartment is restricted to haploid cells. In addition, not only Sertoli cells connected to spermatids in the last steps of maturation but also Leydig and peritubular myoid cells express Gnrh1.

Reconciling the deep homology of neuromodulation with the evolution of behavior

The evolution of behavior seems inconsistent with the deep homology of neuromodulatory signaling. G protein coupled receptors (GPCRs) evolved slowly from a common ancestor through a process involving gene duplication, neofunctionalization, and loss. Neuropeptides co-evolved with their receptors and exhibit many conserved functions. Furthermore, brain areas are highly conserved with suggestions of deep anatomical homology between arthropods and vertebrates. Yet, behavior evolved more rapidly; even members of the same genus or species can differ in heritable behavior. The solution to the paradox involves changes in the compartmentalization, or subfunctionalization, of neuromodulation; neurons shift their expression of GPCRs and the content of monoamines and neuropeptides. Furthermore, parallel evolution of neuromodulatory signaling systems suggests a route for repeated evolution of similar behaviors.