Relaxin gene family in teleosts: phylogeny, syntenic mapping, selective constraint, and expression analysis

Expression of Insl3 Protein in Adult Danio rerio

Insulin-like peptide 3 (INSL3) is a biomarker for Leydig cells in the testes of vertebrates, and it is principally involved in spermatogenesis through specific binding with the RXFP2 receptor. This study reports the insl3 gene transcript and the Insl3 prepropeptide expression in both non-reproductive and reproductive tissues of Danio rerio. An immunohistochemistry analysis shows that the hormone is present at a low level in the Leydig cells and germ cells at all stages of Danio rerio testis differentiation. Considering that the insl3 gene is transcribed in Leydig cells, our results highlight an autocrine and paracrine function of this hormone in the Danio rerio testis, adding new information on the Insl3 mode of action in reproduction. We also show that Insl3 and Rxfp2 belonging to Danio rerio and other vertebrate species share most of the amino acid residues involved in the ligand-receptor interaction and activation, suggesting a conserved mechanism of action during vertebrate evolution.

Expression of Relaxin 3 in the Ovarian Follicle of Japanese Quail

The relaxin (RLN) gene is expressed in the reproductive tracts, such as the ovary and uterus, of mammalian species. Although RLN expression is detected in the chicken ovary, detailed clarification of the physiological role of RLN has not yet been reported. To address this issue, in the present study we aimed to examine the spatiotemporal expression and hormonal control of RLN in Japanese quail. By performing semi-quantitative and quantitative reverse transcription-polymerase chain reaction analysis, we found that RLN mRNA was mainly expressed in the granulosa and theca layers of the ovary. The expression level in the granulosa layer increased with the stage of follicular development. Results from granulosa layer culture experiments revealed that RLN mRNA expression increased with the addition of estradiol-17β, whereas the addition of progesterone suppressed RLN transcription. More detailed analysis indicated that RLN expression was highest in the stigma region of the follicle but significantly decreased as the time of the expected luteinizing hormone (LH) surge approached. Together, our findings demonstrated that the granulosa cells in the mature preovulatory follicles constitute the main source of RLN in the Japanese quail. Because RLN expression was highest in the stigma region and the expression dramatically decreased following the LH surge, the results further suggest that RLN may be related to tissue remodeling for the ovulation process in birds.

Relaxin ligand/receptor systems in the developing teleost fish brain: Conserved features with mammals and a platform to address neuropeptide system functions

The relaxins (RLNs) are a group of peptide hormone/neuromodulators that can regulate a wide range of physiological processes ranging from reproduction to brain function. All the family members have originated from a RLN3-like ancestor via different rounds of whole genome and gene specific duplications during vertebrate evolution. In mammals, including human, the divergence of the different family members and the emergence of new members led to the acquisition of specific functions for the various relaxin family peptide and associated receptor genes. In particular, in mammals, it was shown, that the role of RLN3 is correlated to the modulation of arousal, stress responses, emotion, social recognition, and other brain functions, positioning this gene/peptide as a potential therapeutic target for neuropsychiatric disorders. This review highlights the evolutionary conservation of relaxin family peptide and receptor gene expression and their associated brain neural circuits. In the zebrafish, the expression pattern of the different relaxin family members has specific features that are conserved in higher species, including a likely similar functional role for the ancestral RLN3-like gene. The use of different model organisms, particularly the zebrafish, to explore the diversification and conservation of relaxin family ligands and receptor systems, provides a relatively high-throughput platform to identify their specific conserved or differential neuromodulatory roles in higher species including human.

Comparative Genomic Characterization of Relaxin Peptide Family in Cattle and Buffalo

Relaxin family peptides significantly regulate reproduction, nutrient metabolism, and immune response in mammals. The present study aimed to identify and characterize the relaxin family peptides in cattle and buffalo at the genome level. The genomic and proteomic sequences of cattle, buffalo, goat, sheep, horse, and camel were accessed through the NCBI database, and BLAST was performed. We identified four relaxin peptides genes (RLN3, INSL3, INSL5, and INSL6) in Bos taurus, whereas three relaxin genes (RLN3, INSL3, and INSL6) in Bubalus bubalis. Evolutionary analysis revealed the conserved nature of relaxin family peptides in buffalo and cattle. Physicochemical properties revealed that relaxin proteins were thermostable, hydrophilic, and basic peptides except for INSL5 which was an acidic peptide. Three nonsynonymous mutations (two in RLN3 at positions A16 > T and P29 > A, and one in INSL6 at position R32 > Q) in Bos taurus, whereas two nonsynonymous mutations (one in RLN3 at positions G105 > w and one in INSL3 at position G22 > R) in Bubalus bubalis, were identified. INSL3 had one indel (insertion) at position 55 in Bos taurus. Gene duplication analysis revealed predominantly segmental duplications (INSL5/RLN3 and INSL6/INSL3 gene pairs) that helped expand this gene family, whereas Bubalus bubalis showed primarily tandem duplication (INSL3/RLN3). Our study concluded that relaxin family peptides remained conserved during the evolution, and gene duplications might help to adapt and enrich specific functions like reproduction, nutrient metabolism, and immune response. Further, the nonsynonymous mutations identified potentially affect these functions in buffalo.

Insulin-like 3 affects zebrafish spermatogenic cells directly and via Sertoli cells

Pituitary hormones can use local signaling molecules to regulate target tissue functions. In adult zebrafish testes, follicle-stimulating hormone (Fsh) strongly increases the production of insulin-like 3 (Insl3), a Leydig cell-derived growth factor found in all vertebrates. Little information is available regarding Insl3 function in adult spermatogenesis. The Insl3 receptors Rxfp2a and 2b were expressed by type A spermatogonia and Sertoli and myoid cells, respectively, in zebrafish testis tissue. Loss of insl3 increased germ cell apoptosis in males starting at 9 months of age, but spermatogenesis appeared normal in fully fertile, younger adults. Insl3 changed the expression of 409 testicular genes. Among others, retinoic acid (RA) signaling was up- and peroxisome proliferator-activated receptor gamma (Pparg) signaling was down-regulated. Follow-up studies showed that RA and Pparg signaling mediated Insl3 effects, resulting in the increased production of differentiating spermatogonia. This suggests that Insl3 recruits two locally active nuclear receptor pathways to implement pituitary (Fsh) stimulation of spermatogenesis.

Characterization of the prohormone complement in Amphiprion and related fish species integrating genome and transcriptome assemblies

The Amphiprion (anemonefish or clownfish) family of teleost fish, which is not a common model species, exhibits multiple unique characteristics, including social control of body size and protandrous sex change. The social changes in sex and body size are modulated by neuropeptide signaling pathways. These neuropeptides are formed from complex processing from larger prohormone proteins; understanding the neuropeptide complement requires information on complete prohormones sequences. Genome and transcriptome information within and across 22 teleost fish species, including 11 Amphiprion species, were assembled and integrated to achieve the first comprehensive survey of their prohormone genes. This information enabled the identification of 175 prohormone isoforms from 159 prohormone proteins across all species. This included identification of 9 CART prepropeptide genes and the loss of insulin-like 5B and tachykinin precursor 1B genes in Pomacentridae species. Transcriptome assemblies generally detected most prohormone genes but provided fewer prohormone genes than genome assemblies due to the lack of expression of prohormone genes or specific isoforms and tissue sampled. Comparisons between duplicate genes indicated that subfunctionalization, degradation, and neofunctionalization may be occurring between all copies. Characterization of the prohormone complement lays the foundation for future peptidomic investigation of the molecular basis of social physiology and behavior in the teleost fish.

Rln3a is a prerequisite for spermatogenesis and fertility in male fish

The essential roles of Relaxin3 (RLN3) in energy homeostasis had been well investigated, while the mechanisms of RLN3 regulating reproduction remain to be elusive in mammals. Although two rln3 paralogues have been characterized in several teleosts, their functions still remain largely unknown. In this study, two paralogous rln3 genes, represented as rln3a and rln3b, were identified from the testis of Nile tilapia (Oreochromis niloticus). Rln3a was dominantly expressed in testis, while the most abundant rln3b expression was in brain. In situ hybridization demonstrated that rln3a is abundantly expressed in the Leydig cells of the testis. To understand the role of Rln3 in the testicular development, homologous null-rln3a gene mutant line was constructed by CRISPR/Cas9 technology. Morphological observation demonstrated that null mutation of rln3a gene caused testicular hypertrophy and a significant increase of GSI. However, a significant decrease of spermatogenic cells at different phases, i.e. spermatogonia, spermatocytes, spermatids and sperms was found. Silencing of rln3a gene repressed the expression of key genes in germ cell and Leydig cell. Deficiency of Rln3a led to the significant decrease of 11-KT production, which stimulated the up-regulation of both FSH and LH production in the pituitary via a negative feedback manner possibly. Mutation of rln3a in XY fish led to the hypogonadism with sperm deformation, significant decrease of fertility, and sperm motility, revealing as the high mortality of the offspring obtained by crossing the wild type female and rln3a^-/- XY fish. Interestingly, recombinant human RLN3 injection significantly enhanced the sperm motility in rln3a^-/- XY fish. Moreover, hCG treatment stimulated the expression of steroidogenic enzyme genes and 11-KT production, which were repressed by rln3a mutation in XY fish. Taken together, this study, for the first time by using a gene knockout model, proved that Rln3a is an indispensable mediator for androgen production in testis via HPG axis, and plays an essential role in spermatogenesis, sperm motility and male fertility in fish.

Japanese medaka as a model for studying the relaxin family genes involved in neuroendocrine regulation: Insights from the expression of fish-specific rln3 and insl5 and rxfp3/4-type receptor paralogues

The goal of this paper is to establish Japanese medaka (Oryzias latipes) as a model for relaxin family peptide research, particularly for studying the functions of RLN3 and INSL5, hormones playing roles in neuroendocrine regulation. Medaka, like other teleosts, retained duplicate copies of rln3, insl5 and their rxfp3/4-type receptors following fish-specific whole genome duplication (WGD) and paralogous copies of these genes may have sub-functionalised providing an intuitive model for teasing apart the pleiotropic roles of the corresponding genes in mammals. To this end, we provide experimental evidence for the expression of the relaxin family genes in medaka that had previously only been identified in-silico, confirm the gene structure of five of the ligand genes, characterise gene expression across multiple tissues and during embryonic development, perform in situ hybridization with anti-sense insl5a on embryos and in adult brain and intestinal samples, and compare these results to the data available in zebrafish. We find broad similarities but also some differences in the expression of relaxin family genes in zebrafish versus medaka, and find support for the hypothesis that the rln3a/rln3b and insl5a/insl5b paralogues have been subfunctionalized. Given that medaka has a suite of relaxin family genes more similar to other teleosts, and has retained the gene for rxfp4 (which is lost in zebrafish), our results suggest that O. latipes may be a good model for delineating the ancestral function of the relaxin family genes involved in neuroendocrine regulation.

Fsh stimulates Leydig cell Wnt5a production, enriching zebrafish type A spermatogonia

Follicle-stimulating hormone (Fsh) modulates vertebrate spermatogenesis by regulating somatic cell functions in the testis. We have found previously that zebrafish Fsh stimulated the differentiating proliferation of type A undifferentiated spermatogonia (Aund) in an androgen-independent manner by regulating the production of growth factors and other signaling molecules in both Sertoli (SCs) and Leydig cells (LCs). For example, Fsh triggered the release of Igf3 that subsequently activated β-catenin signaling to promote the differentiating proliferation of Aund. In the present study, we report that Fsh moreover uses the non-canonical Wnt pathway to promote the proliferation and accumulation of Aund. Initially, we found that the stimulatory effect of Fsh on the proliferation activity of Aund was further strengthened when β-catenin signaling was inhibited, resulting in an accumulation of Aund. We then showed that this Fsh-induced accumulation of Aund was associated with increased transcript levels of the non-canonical Wnt ligand, wnt5a. In situ hybridization of insl3 mRNA, a gene expressed in LCs, combined with Wnt5a immunocytochemistry identified LCs as the cellular source of Wnt5a in the adult zebrafish testis. Addition of an antagonist of Wnt5a to incubations with Fsh decreased both the proliferation activity and the relative section area occupied by Aund, while an agonist of Wnt5a increased these same parameters for Aund. Taken together, our data suggest that Fsh triggered LCs to release Wnt5a, which then promoted the proliferation and accumulation of Aund. Hence, Fsh uses non-canonical Wnt signaling to ensure the production of Aund, while also triggering β-catenin signaling via Igf3 to ensure spermatogonial differentiation.

Identification of a Novel Zebrafish Mutant Line that Develops Testicular Germ Cell Tumors

Testicular tumors are the most common solid malignant tumors in men 20-35 years of age. Although most of testicular tumors are curable, current treatments still fail in 15%-20% of patients. However, insufficient understanding of the molecular basis and lack of animal models limit development of more effective treatments. This study reports the identification of a novel zebrafish mutant line, ns1402, which develops testicular germ cell tumors (TGCTs). While both male and female ns1402 mutants were fertile at young age, male ns1402 mutants became infertile as early as 9 months of age. This infertility was associated with progressive loss of mature sperm. Failure of spermatogenesis was, at least in part, explained by progressive loss of mature Leydig cells, a source of testosterone that is essential for spermatogenesis. Interestingly, TGCTs in ns1402 mutants contained a large number of Sertoli cells and gene expression profiles of Sertoli cells were altered before loss of mature Leydig cells. This suggests that changes in Sertoli cell properties happened first, followed by loss of mature Leydig cells and failure of spermatogenesis. Taken together, this study emphasizes the importance of cell-cell interactions and cell signaling in the testis for spermatogenesis and tissue homeostasis.

Igf3 activates β-catenin signaling to stimulate spermatogonial differentiation in zebrafish

Follicle-stimulating hormone (Fsh) is a major regulator of spermatogenesis, targeting somatic cell functions in the testes. We reported previously that zebrafish Fsh promoted the differentiation of type A undifferentiated spermatogonia (A_und) by stimulating the production of factors that advance germ cell differentiation, such as androgens, insulin-like peptide 3 (Insl3) and insulin-like growth factor 3 (Igf3). In addition, Fsh also modulated the transcript levels of several other genes, including some belonging to the Wnt signaling pathway. Here, we evaluated if and how Fsh utilizes part of the canonical Wnt pathway to regulate the development of spermatogonia. We quantified the proliferation activity and relative section areas occupied by A_und and type A differentiating (A_diff) spermatogonia and we analyzed the expression of selected genes in response to recombinant proteins and pharmacological inhibitors. We found that from the three downstream mediators of Fsh activity we examined, Igf3, but not 11-ketotestosterone or Insl3, modulated the transcript levels of two β-catenin sensitive genes (cyclinD1 and axin2). Using a zebrafish β-catenin signaling reporter line, we showed that Igf3 activated β-catenin signaling in type A spermatogonia and that this activation did not depend on the release of Wnt ligands. Pharmacological inhibition of the β-catenin or of the phosphoinositide 3-kinase (PI3K) pathways revealed that Igf3 activated β-catenin signaling in a manner involving PI3K to promote the differentiation of A_und to A_diff spermatogonia. This mechanism represents an intriguing example for a pituitary hormone like Fsh using Igf signaling to recruit the evolutionary conserved, local β-catenin signaling pathway to regulate spermatogenesis.

Effects of multiple life stage exposure to the fungicide prochloraz in Xenopus laevis: Manifestations of antiandrogenic and other modes of toxicity

The Larval Amphibian Growth and Development Assay (LAGDA) is an internationally harmonized testing guideline for evaluating effects of chronic chemical exposure in amphibians. In order to evaluate the effects of chronic exposure to an antiandrogenic chemical in an amphibian model, prochloraz was tested using a variation of the LAGDA design. Exposure was initiated with <1d post-fertilization embryos at nominal concentrations of 0, 6.7, 20, 60 and 180 μg/L (0, 18, 53, 159, 478 nM) and continued in flow-through conditions until two months following the median time that controls completed metamorphosis. Growth, developmental rate, circulating thyroid hormone and thyroid gland histopathology were evaluated in a subsample at completion of metamorphosis. There were no effects on growth or development at this stage, but circulating thyroid hormone was elevated in the 20, 60 and 180 μg/L treatments and minimal to mild thyroid follicular cell hypertrophy was observed histologically in the 180 μg/L treatment. Growth, overt toxicity, and reproductive development were evaluated at test termination. There were no effects on growth in either gender, but livers and kidneys exhibited treatment-related pathologies consistent with organ toxicity related to metabolism and presumably impaired excretion of prochloraz metabolites. Histological assessments of female ovaries resulted in minimal pathologies only in the 180 μg/L treatment while male testes exhibited numerous treatment-related pathologies that are consistent with previously reported antiandrogenic effects of prochloraz in other species. The most severe testis pathologies occurred in the 180 μg/L treatment; however, incidences of treatment-related pathologies occurred in all prochloraz treatments. Müllerian duct regression in males was inhibited by prochloraz exposure while Müllerian duct maturation in females was accelerated, characteristic of a feminizing effect. Gene expression levels of potential biomarkers of testis function were also measured. Relative abundance of cyp17a1 transcripts was generally unaffected by prochloraz exposure whereas the Insl3 orthologue, rflcii, was elevated by 3 and >5-fold in the 60 and 180 μg/L treatments, respectively, indicating impaired Leydig cell maturation and testosterone signaling. Overall, prochloraz exposure caused effects characteristic of an antiandrogenic mode of action, which is consistent with previously reported results in other species and supports the utility of the LAGDA design for chemical testing.

Temporal and spatial expression of insulin-like peptide (insl5a and insl5b) paralog genes during the embryogenesis of Danio rerio

Relaxin (RLN) and insulin (INSL)-like peptides are member of the INSL/RLN superfamily, which are encoded by seven genes in humans and can activate the G-protein coupled receptor RXFP 1-4. These peptides evolved from a common ancestor, RLN3-like gene. Two rounds of whole genome duplication (WGD) in early vertebrate evolution, together with an additional WGD in the teleost lineage, caused an expansion of RLN genes set in the genome of Danio rerio. In particular, six RLN genes are present: a single copy of rln and insl3 genes, and two paralogs for the rln3 gene (rln3a and rln3b), and the insl5 gene (insl5a and insl5b). We have already reported the presence of rln3a and rln3b genes in the developing zebrafish brain, as well as the expression of rln gene in the developing zebrafish brain and extraneural territories, such as thyroid gland and pancreas. Here, we report for the first time the expression of the two parologs genes for insl5, insl5a, and insl5b in D. rerio embryonic development. The corresponding transcripts of both the paralogs are present in all embryonic stages analyzed by RT-qPCR. In situ hybridization analyses showed a restricted signal in intestinal cells and the pancreatic region at 72 hpf for insl5a, while at 96 hpf both genes are expressed in specific intestinal cells. Furthermore, in adult zebrafish intestine tissue, in situ hybridation experiments showed that insl5a transcript is specifically localized in the goblet cells, while insl5b transcript is in enteroendocrine cells. These data revealed a high degree of gene expression pattern conservation for such genes in vertebrate evolution.

Divergence of insulin superfamily ligands, receptors and Igf binding proteins in marine versus freshwater stickleback: Evidence of selection in known and novel genes

Three-spine stickleback (Gasterosteus aculeatus) is a teleost model for understanding genetic, physiological and morphological changes accompanying freshwater (FW) adaptation. There is growing evidence that the insulin superfamily plays important roles in traits involved in marine and FW adaptation. We performed a candidate gene analysis to look for evidence of selection on 33 insulin superfamily ligand-receptor genes and insulin-like growth factor binding proteins (Igfbp's) in stickleback. Using genotype data from 11 marine and 10 FW populations, we calculated the number of SNPs per site in regulatory and intronic regions, the number of synonymous and nonsynonymous mutations in coding regions, Wright's fixation index (Fst), and performed t-tests to identify SNPs with divergent genotype frequencies between marine/FW versus Atlantic/Pacific populations. Next, we analysed genome-wide transcriptome data from eight tissues to assess differential gene expression. Two Igfbp's (Igfbp2a and Igfbp5a) show evidence of divergent adaptation between life-history types, and a cluster of nonsynonymous mutations in Igfbp5a exhibit high Fst in exons apparently alternatively spliced in gill. We find evidence of selection on the relaxin family ligand-receptor gene pair, Insl3-Rxfp2, known to be involved in male spermatogenesis and bone metabolism, and in the 5' regulatory region of Igf2. We also confirmed the gene and coding sequence of two unannotated relaxin family ligands. These analyses underscore the utility of candidate gene studies and indicate directions for further exploration of the function of insulin superfamily genes in FW adaptation.

Distinct and Cooperative Roles of amh and dmrt1 in Self-Renewal and Differentiation of Male Germ Cells in Zebrafish

Spermatogenesis is a fundamental process in male reproductive biology and depends on precise balance between self-renewal and differentiation of male germ cells. However, the regulative factors for controlling the balance are poorly understood. In this study, we examined the roles of amh and dmrt1 in male germ cell development by generating their mutants with Crispr/Cas9 technology in zebrafish. Amh mutant zebrafish displayed a female-biased sex ratio, and both male and female amh mutants developed hypertrophic gonads due to uncontrolled proliferation and impaired differentiation of germ cells. A large number of proliferating spermatogonium-like cells were observed within testicular lobules of the amh-mutated testes, and they were demonstrated to be both Vasa- and PH3-positive. Moreover, the average number of Sycp3- and Vasa-positive cells in the amh mutants was significantly lower than in wild-type testes, suggesting a severely impaired differentiation of male germ cells. Conversely, all the dmrt1-mutated testes displayed severe testicular developmental defects and gradual loss of all Vasa-positive germ cells by inhibiting their self-renewal and inducing apoptosis. In addition, several germ cell and Sertoli cell marker genes were significantly downregulated, whereas a prominent increase of Insl3-positive Leydig cells was revealed by immunohistochemical analysis in the disorganized dmrt1-mutated testes. Our data suggest that amh might act as a guardian to control the balance between proliferation and differentiation of male germ cells, whereas dmrt1 might be required for the maintenance, self-renewal, and differentiation of male germ cells. Significantly, this study unravels novel functions of amh gene in fish.

Distinct and Cooperative Roles of amh and dmrt1 in Self-Renewal and Differentiation of Male Germ Cells in Zebrafish

Spermatogenesis is a fundamental process in male reproductive biology and depends on precise balance between self-renewal and differentiation of male germ cells. However, the regulative factors for controlling the balance are poorly understood. In this study, we examined the roles of amh and dmrt1 in male germ cell development by generating their mutants with Crispr/Cas9 technology in zebrafish. Amh mutant zebrafish displayed a female-biased sex ratio, and both male and female amh mutants developed hypertrophic gonads due to uncontrolled proliferation and impaired differentiation of germ cells. A large number of proliferating spermatogonium-like cells were observed within testicular lobules of the amh-mutated testes, and they were demonstrated to be both Vasa- and PH3-positive. Moreover, the average number of Sycp3- and Vasa-positive cells in the amh mutants was significantly lower than in wild-type testes, suggesting a severely impaired differentiation of male germ cells. Conversely, all the dmrt1-mutated testes displayed severe testicular developmental defects and gradual loss of all Vasa-positive germ cells by inhibiting their self-renewal and inducing apoptosis. In addition, several germ cell and Sertoli cell marker genes were significantly downregulated, whereas a prominent increase of Insl3-positive Leydig cells was revealed by immunohistochemical analysis in the disorganized dmrt1-mutated testes. Our data suggest that amh might act as a guardian to control the balance between proliferation and differentiation of male germ cells, whereas dmrt1 might be required for the maintenance, self-renewal, and differentiation of male germ cells. Significantly, this study unravels novel functions of amh gene in fish.

Progressive erosion of the Relaxin1 gene in bovids

The relaxin/insulin-like (RLN/INSL) gene family is a group of genes that encode peptide hormones involved in a variety of physiological functions related to reproduction. Previous studies have shown that relaxin plays a key role in widening of the pubic bone during labor and in gamete maturation. Because of these functions, studying the evolution of RLN1, the gene encoding for relaxin, is relevant in livestock species, most of which belong in the group Laurasiatheria, which includes cow, pig, horse, goat, and sheep in addition to bats, cetaceans and carnivores. Experimental evidence suggests that cows do not synthesize relaxin, but respond to it, and sheep apparently have a truncated RLN1 gene. Thus, we made use of genome sequence data to characterize the genomic locus of the RLN1 gene in Laurasiatherian mammals to better understand how cows lost the ability to synthesize this peptide. We found that all ruminants in our study (cow, giraffe, goat, sheep and Tibetan antelope) lack a functional RLN1 gene, and document the progressive loss of RLN1 in the lineage leading to cows. Our analyses indicate that 1 - all ruminants have lost all key regulatory elements upstream of the first exon, 2 - giraffe, goat, sheep and Tibetan antelope have multiple inactivating mutations in the RLN1 pseudogene, and 3 - the cow genome has lost all traces of RLN1. The 5' regulatory sequence plays a key role in activating expression, and the loss of this sequence would impair synthesis of mRNA. Our results suggest that changes in regulatory sequence preceded mutations in coding sequence and highlight the importance of these regions in maintaining proper gene function. In addition, we found that all bovids examined posses copies of the relaxin receptors, which explains why they are able to respond to relaxin despite their inability to produce it.

From molecule to behavior: Brain aromatase (cyp19a1b) characterization, expression analysis and its relation with social status and male agonistic behavior in a Neotropical cichlid fish

The enzyme aromatase, responsible for the conversion of C19 androgens to C18 estrogens, exists as two paralogue copies in teleost fish: Cyp19a1a mostly expressed in the gonads, referred as gonadal aromatase, and Cyp19a1b, mostly expressed in the brain, accordingly known as brain aromatase. The neural localization of Cyp19a1b is greatly contained within the social behavior network and mesolimbic reward system in fish, suggesting a strong role of estrogen synthesis in the regulation of social behavior. In this work we aimed to analyze the variation in cyp19a1b expression in brain and pituitary of males of a highly social cichlid, Cichlasoma dimerus (locally known as chanchita), and its relation with inter-individual variability in agonistic behavior in a communal social environment. We first characterized chanchita's cyp19a1b mRNA and deduced amino acid sequence, which showed a high degree of conservation when compared to other teleost brain aromatase sequences, and its tissue expression patterns. Within the brain, Cyp19a1b was solely detected at putative radial glial cells of the forebrain, close to the brain ventricles. We then studied the relative expression levels of cyp19a1b by Real Time PCR in the brain and pituitary of males of different social status, territorial vs. non-territorial, and its relationship with an index of agonistic behavior. We found that even though, brain aromatase expression did not differ between types of males, pituitary cyp19a1b expression levels positively correlated with the index of agonistic behavior. This suggests a novel role of the pituitary in the regulation of social behavior by local estrogen synthesis.

Expression profiling identifies Sertoli and Leydig cell genes as Fsh targets in adult zebrafish testis

Spermatogonial stem cells are quiescent, undergo self-renewal or differentiating divisions, thereby forming the cellular basis of spermatogenesis. This cellular development is orchestrated by follicle-stimulating hormone (FSH), through the production of Sertoli cell-derived factors, and by Leydig cell-released androgens. Here, we investigate the transcriptional events induced by Fsh in a steroid-independent manner on the restart of zebrafish (Danio rerio) spermatogenesis ex vivo, using testis from adult males where type A spermatogonia were enriched by estrogen treatment in vivo. Under these conditions, RNA sequencing preferentially detected differentially expressed genes in somatic/Sertoli cells. Fsh-stimulated spermatogonial proliferation was accompanied by modulating several signaling systems (i.e. Tgf-β, Hedgehog, Wnt and Notch pathways). In silico protein-protein interaction analysis indicated a role for Hedgehog family members potentially integrating signals from different pathways during fish spermatogenesis. Moreover, Fsh had a marked impact on metabolic genes, such as lactate and fatty acid metabolism, or on Sertoli cell barrier components. Fish Leydig cells express the Fsh receptor and one of the most robust Fsh-responsive genes was insulin-like 3 (insl3), a Leydig cell-derived growth factor. Follow-up work showed that recombinant zebrafish Insl3 mediated pro-differentiation effects of Fsh on spermatogonia in an androgen-independent manner. Our experimental approach allowed focusing on testicular somatic genes in zebrafish and showed that the activity of signaling systems known to be relevant in stem cells was modulated by Fsh, providing promising leads for future work, as exemplified by the studies on Insl3.

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

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 factor-type 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 hormone-type, 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.

Identification of prohormones and pituitary neuropeptides in the African cichlid, Astatotilapia burtoni

Background

Cichlid fishes have evolved remarkably diverse reproductive, social, and feeding behaviors. Cell-to-cell signaling molecules, notably neuropeptides and peptide hormones, are known to regulate these behaviors across vertebrates. This class of signaling molecules derives from prohormone genes that have undergone multiple duplications and losses in fishes. Whether and how subfunctionalization, neofunctionalization, or losses of neuropeptides and peptide hormones have contributed to fish behavioral diversity is largely unknown. Information on fish prohormones has been limited and is complicated by the whole genome duplication of the teleost ancestor. We combined bioinformatics, mass spectrometry-enabled peptidomics, and molecular techniques to identify the suite of neuropeptide prohormones and pituitary peptide products in Astatotilapia burtoni, a well-studied member of the diverse African cichlid clade.

Results

Utilizing the A. burtoni genome, we identified 148 prohormone genes, with 21 identified as a single copy and 39 with at least 2 duplicated copies. Retention of prohormone duplicates was therefore 41 %, which is markedly above previous reports for the genome-wide average in teleosts. Beyond the expected whole genome duplication, differences between cichlids and mammals can be attributed to gene loss in tetrapods and additional duplication after divergence. Mass spectrometric analysis of the pituitary identified 620 unique peptide sequences that were matched to 120 unique proteins. Finally, we used in situ hybridization to localize the expression of galanin, a prohormone with exceptional sequence divergence in cichlids, as well as the expression of a proopiomelanocortin, prohormone that has undergone an additional duplication in some bony fish lineages.

Conclusion

We characterized the A. burtoni prohormone complement. Two thirds of prohormone families contain duplications either from the teleost whole genome duplication or a more recent duplication. Our bioinformatic and mass spectrometric findings provide information on a major vertebrate clade that will further our understanding of the functional ramifications of these prohormone losses, duplications, and sequence changes across vertebrate evolution. In the context of the cichlid radiation, these findings will also facilitate the exploration of neuropeptide and peptide hormone function in behavioral diversity both within A. burtoni and across cichlid and other fish species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2914-9) contains supplementary material, which is available to authorized users.

A relaxin-like gonad-stimulating peptide from the starfish Aphelasterias japonica

Relaxin-like gonad-stimulating peptide (RGP) in starfish is the first identified invertebrate gonadotropin responsible for final gamete maturation. In this study, a new ortholog RGP was identified from Aphelasterias japonica. The DNA sequence encoding A. japonica RGP (AjaRGP) consists of 342 base pairs with an open reading frame encoding a peptide of 113 amino acids (aa), including a signal peptide (26aa), B-chain (20aa), C-peptide (42aa), and A-chain (25aa). AjaRGP is a heterodimeric peptide with disulfide cross-linkages. Comparing with Asterias amurensis RGP (AamRGP) and Patiria (=Asterina) pectinifera RGP (PpeRGP), the amino acid identity levels of AjaRGP with respect to AamRGP and PpeRGP are 84% and 58% for the A-chain and 90% and 68% for the B-chain, respectively. This suggests that AjaRGP is closer to AmaRGP rather than PpeRGP. Although chemical synthetic AjaRGP can induce gamete spawning and oocyte maturation in ovarian fragments of A. japonica, the ovary of P. pectinifera fails to respond to AjaRGP. This suggests that AjaRGP acts species-specifically.

A new relaxin-like gonad-stimulating peptide identified in the starfish Asterias amurensis

Relaxin-like gonad-stimulating peptide (RGP) of starfish Asterina pectinifera was the first invertebrate gonadotropin to have its chemical structure identified. However, it is unclear whether gonadotropic hormones in other species starfish are relaxin-like peptides. Thus, this study tried to identify the molecular structure of gonadotropic hormone in Asterias amurensis. As a result, we identified A. amurensis gonadotropic hormone as the RGP (AamRGP). The DNA sequence encoding AamRGP consisted of 330 base pairs with an open reading frame encoding a peptide of 109 amino acids (aa), including a signal peptide (26 aa), B-chain (20 aa), C-peptide (38 aa) and A-chain (25 aa). Comparing with A. pectinifera RGP (ApeRGP), the amino acid identity levels between AmaRGP and ApeRGP were 58% for the A-chain and 73% for the B-chain. Furthermore, chemical synthetic AamRGP induced gamete spawning and oocyte maturation in ovarian fragments of A. amurensis. In contrast, the ovary of A. pectinifera failed to respond to the AamRGP. This suggested that AamRGP is a new relaxin-like peptide.

Expression pattern of zebrafish rxfp2 homologue genes during embryonic development

RXFP2 is one of the 4 receptors for relaxin insulin-like peptides, in particular it binds with high affinity the INSL3 peptide. INSL3/RXFP2 pair is essential for testicular descent during placental mammalian development. The evolutionary history of this ligand/receptor pair has received much attention, since its function in vertebrate species lacking testicular descent, such as the fishes, remains elusive. Herein, we analyzed the expression pattern of three rxfp2 homologue genes in zebrafish embryonic development. For all the three rxfp2 genes (rxfp2a, rxfp2b, and rxfp2-like) we showed the presence of maternally derived transcripts. Later in the development, rxfp2a is only expressed at larval stage, whereas rxfp2b is expressed in all the analyzed stage with highest level in the larvae. The rxfp2-like gene is expressed in all the analyzed stage with a transcript level that increased starting at early pharyngula stage. The spatial localization analysis of rxfp2-like gene showed that it is expressed in many cell clusters in the developing brain. In addition, other rxfp2-like-expressing cells were identified in the retina and oral epithelium. This analysis provides new insights to elucidate the evolution of rxfp2 genes in vertebrate lineage and lays the foundations to study their role in vertebrate embryonic development.

Expression profiles of relaxin family peptides and their receptors indicate their influence on spermatogenesis in the domestic cat (Felis catus)

Disturbed spermatogenesis is a common problem in felines. Studying spermatogenesis in the domestic cat can improve the understanding of the biological background and help to counteract fertility problems in other feline species. Here, we analyzed 3 relaxin family peptides (relaxin, relaxin-3, and INSL3) and their receptors (RXFP1, RXFP2, and RXFP3) as potential spermatogenic factors involving their expression in the testis at different stages of its development. It may be concluded from its stage-dependent expression that relaxin, together with RXFP1, appears to be involved in the first stage of spermatogenesis, whereas relaxin-3 via binding to RXFP3 influences spermiogenesis. Furthermore, correlations were observed between relaxin, relaxin-3, RXFP1, RXFP2 and RXFP3 messenger RNA expression, and the relative numbers of haploid cells in testes. The peptide INSL3 was highly expressed at all testis development stages. Because of the low and stage-independent expression of its receptor RXFP2, an auto- and/or paracrine function of INSL3 in spermatogenesis seems unlikely. In the adult testis, messenger RNA expression of relaxin, RXFP1, and RXFP3 predominantly occurs in the tubular testis compartment, whereas INLS3 is mainly expressed in the interstitium.

INSL3 stimulates spermatogonial differentiation in testis of adult zebrafish (Danio rerio)

INSL3 (insulin-like peptide 3) is a relaxin peptide family member expressed by Leydig cells in the vertebrate testis. In mammals, INSL3 mediates testicular descent during embryogenesis but information on its function in adults is limited. In fish, the testes remain in the body cavity, although the insl3 gene is still expressed, suggesting yet undiscovered, evolutionary older functions. Anti-Müllerian hormone (Amh), in addition to inhibiting spermatogonial differentiation and androgen release, inhibits the Fsh (follicle-stimulating hormone)-induced increase in insl3 transcript levels in zebrafish testis. Therefore, the two growth factors might have antagonistic effects. We examine human INSL3 (hINSL3) effects on zebrafish germ cell proliferation/differentiation and androgen release by using a testis tissue culture system. hINSL3 increases the proliferation of type A undifferentiated (A_und) but not of type A differentiating (A_diff) spermatogonia, while reducing the proliferation of Sertoli cells associated with proliferating A_und. Since the area occupied by A_und decreases and that of A_diff increases, we conclude that hINSL3 recruits A_und into differentiation; this is supported by the hINSL3-induced down-regulation of nanos2 transcript levels, a marker of single A_und spermatogonia in zebrafish and other vertebrates. Pulse-chase experiments with a mitosis marker also indicate that hINSL3 promotes spermatogonial differentiation. However, hINSL3 does not modulate basal or Fsh-stimulated androgen release or growth factor transcript levels, including those of amh. Thus, hINSL3 seems to recruit A_und spermatogonia into differentiation, potentially mediating an Fsh effect on spermatogenesis.

The relaxin family peptide receptors and their ligands: new developments and paradigms in the evolution from jawless fish to mammals

Relaxin family peptide receptors (Rxfps) and their ligands, relaxin (Rln) and insulin-like (Insl) peptides, are broadly implicated in the regulation of reproductive and neuroendocrine processes in mammals. Most placental mammals harbour genes for four receptors, namely rxfp1, rxfp2, rxfp3 and rxfp4. The number and identity of rxfps in other vertebrates are immensely variable, which is probably attributable to intraspecific variation in reproductive and neuroendocrine regulation. Here, we highlight several interesting, but greatly overlooked, aspects of the rln/insl-rxfp evolutionary history: the ancient origin, recruitment of novel receptors, diverse roles of selection, differential retention and lineage-specific loss of genes over evolutionary time. The tremendous diversity of rln/insl and rxfp genes appears to have arisen from two divergent receptors and one ligand that were duplicated by whole genome duplications (WGD) in early vertebrate evolution, although several genes, notably relaxin in mammals, were also duplicated via small scale duplications. Duplication and loss of genes have varied across lineages: teleosts retained more WGD-derived genes, dominated by those thought to be involved in neuroendocrine regulation (rln3, insl5 and rxfp 3/4 genes), while eutherian mammals witnessed the diversification and rapid evolution of genes involved in reproduction (rln/insl3). Several genes that arose early in evolutionary history were lost in most mammals, but retained in teleosts and, to a lesser extent, in early diverging tetrapods. To elaborate on their evolutionary history, we provide updated phylogenies of the Rxfp1/2 and Rxfp3/4 receptors and their ligands, including new sequences from early diverging vertebrate taxa such as coelacanth, skate, spotted gar, and lamprey. We also summarize the recent progress made towards understanding the functional biology of Rxfps in non-mammalian taxa, providing a new conceptual framework for research on Rxfp signaling across vertebrates.

Relaxin-related gene expression differs between anadromous and stream-resident stickleback (Gasterosteus aculeatus) following seawater transfer

Relaxin (RLN) is a hormone that was originally identified as a regulator of pregnancy and reproduction. However, recent mammalian studies have demonstrated that relaxins also have potent osmoregulatory actions. In mammals, six relaxin family peptides have been identified: RLN1/2, RLN3, insulin-like peptide (INSL) 3, INSL4, INSL5, and INSL6. Previous genome database searches have revealed that teleosts also possess multiple relaxin family genes. However, the functions of these relaxin family peptides in teleosts remain unclear. In order to gain insight into the osmoregulatory functions of teleost relaxins, we studied the relaxin family peptides in euryhaline three-spined sticklebacks (Gasterosteus aculeatus), which have diversified into a variety of ecotypes. Rln3a, rln3b, and rln transcripts were abundant in the stickleback brain, whereas insl5b transcript levels were highest in the intestine among tissues. Seawater challenge experiments showed that transcript levels of rln3a, rln3b, and rln in the brain changed significantly after seawater transfer. Particularly, rln3b showed different patterns of temporal changes between anadromous and stream-resident morphs. The transcript levels of relaxin family peptide receptors, rxfp1, rxfp2b, rxfp3-2a, and rxfp3-2b, did not exhibit substantial changes in the brain, although these were constantly higher in the anadromous morph than the stream-resident morph. These results suggest that stickleback relaxin systems are differentially regulated by salinity signals, at least at the transcriptional level, and anadromous and stream-resident morphs differ in relaxin signaling pathways. The differences in the expression of relaxin-related genes between these two morphs provide a foundation for further exploration of the osmoregulatory function of relaxins in teleosts.

Cyp17a1 and Cyp19a1 in the zebrafish testis are differentially affected by oestradiol

Oestrogens can affect expression of genes encoding steroidogenic enzymes in fish gonads. However, little information is available on their effects at the protein level. In this context, we first analysed the expression of key steroidogenic enzyme genes and proteins in zebrafish testis, paying attention also to other cell types than Leydig cells. Gene expression was analysed by quantitative PCR on fluorescence-activated cell-sorting fractions coupled or not to differential plating, while protein synthesis was studied by immunohistochemistry using specific antibodies against zebrafish Cyp17a1, Cyp19a1a and Cyp19a1b. Furthermore, we have evaluated the effect of oestrogen treatment (17β-oestradiol (E(2)), 10 nM) on the localization of these enzymes after 7 and 14 days of in vivo exposure in order to study how oestrogen-mediated modulation of their expression is linked to oestrogen effects on spermatogenesis. The major outcomes of this study are that Leydig cells express Cyp17a1 and Cyp19a1a, while testicular germ cells express Cyp17a1 and both, Cyp19a1a and Cyp19a1b. As regards Cyp17a1, both protein and mRNA seem to be quantitatively dominating in Leydig cells. Moreover, E(2) exposure specifically affects only Leydig cell Cyp17a1 synthesis, preceding the disruption of spermatogenesis. The oestrogen-induced suppression of the androgen production capacity in Leydig cells is a major event in altering spermatogenesis, while germ cell steroidogenesis may have to be fuelled by precursors from Leydig cells. Further studies are needed to elucidate the functionality of steroidogenic enzymes in germ cells and their potential role in testicular physiology.

Relaxin-like gonad-stimulating substance in an echinoderm, the starfish: a novel relaxin system in reproduction of invertebrates

Gonad-stimulating substance (GSS) in starfish is the only known invertebrate peptide hormone responsible for final gamete maturation, rendering it functionally analogous to gonadotropins in vertebrates. Recently, GSS was purified from the radial nerves of the starfish Asterina pectinifera and its chemical structure determined. This review summarizes the chemical structure of relaxin-like peptide, GSS, from a starfish as the first identified gonadotropin in invertebrates and its hormonal action on reproduction. The starfish GSS is a relaxin-like heterodimeric peptide composed of two peptides (A- and B-chains) with disulfide cross-linkages. Chemically synthesized GSS induced oocyte maturation and ovulation in vitro and an unique spawning behavior followed by release of gametes in vivo. GSS is a first trigger for oocyte maturation in starfish, but its effect is indirect because GSS acts on the ovary to produce a second mediator, 1-methyladenine (1-MeAde), as a maturation-inducing hormone of starfish. The action of GSS on ovarian follicle cells to produce 1-MeAde is mediated through the activation of its receptor, G-protein, and adenylyl cyclase. In contrast to follicle cells in a fully grown state, GSS fails to induce 1-MeAde production in growing follicle cells because of a lack of Gs-proteins. Thus, relaxin-like GSS is a major factor in the neuroendocrine cascade controlling reproduction in starfish.

New insights into ligand-receptor pairing and coevolution of relaxin family peptides and their receptors in teleosts

Relaxin-like peptides (RLN/INSL) play diverse roles in reproductive and neuroendocrine processes in placental mammals and are functionally associated with two distinct types of receptors (RXFP) for each respective function. The diversification of RLN/INSL and RXFP gene families in vertebrates was predominantly driven by whole genome duplications (2R and 3R). Teleosts preferentially retained duplicates of genes putatively involved in neuroendocrine regulation, harboring a total of 10-11 receptors and 6 ligand genes, while most mammals have equal numbers of ligands and receptors. To date, the ligand-receptor relationships of teleost Rln/Insl peptides and their receptors have largely remained unexplored. Here, we use selection analyses based on sequence data from 5 teleosts and qPCR expression data from zebrafish to explore possible ligand-receptor pairings in teleosts. We find support for the hypothesis that, with the exception of RLN, which has undergone strong positive selection in mammalian lineages, the ligand and receptor genes shared between mammals and teleosts appear to have similar pairings. On the other hand, the teleost-specific receptors show evidence of subfunctionalization. Overall, this study underscores the complexity of RLN/INSL and RXFP ligand-receptor interactions in teleosts and establishes theoretical background for further experimental work in nonmammals.

Using paleogenomics to study the evolution of gene families: origin and duplication history of the relaxin family hormones and their receptors

Recent progress in the analysis of whole genome sequencing data has resulted in the emergence of paleogenomics, a field devoted to the reconstruction of ancestral genomes. Ancestral karyotype reconstructions have been used primarily to illustrate the dynamic nature of genome evolution. In this paper, we demonstrate how they can also be used to study individual gene families by examining the evolutionary history of relaxin hormones (RLN/INSL) and relaxin family peptide receptors (RXFP). Relaxin family hormones are members of the insulin superfamily, and are implicated in the regulation of a variety of primarily reproductive and neuroendocrine processes. Their receptors are G-protein coupled receptors (GPCR's) and include members of two distinct evolutionary groups, an unusual characteristic. Although several studies have tried to elucidate the origins of the relaxin peptide family, the evolutionary origin of their receptors and the mechanisms driving the diversification of the RLN/INSL-RXFP signaling systems in non-placental vertebrates has remained elusive. Here we show that the numerous vertebrate RLN/INSL and RXFP genes are products of an ancestral receptor-ligand system that originally consisted of three genes, two of which apparently trace their origins to invertebrates. Subsequently, diversification of the system was driven primarily by whole genome duplications (WGD, 2R and 3R) followed by almost complete retention of the ligand duplicates in most vertebrates but massive loss of receptor genes in tetrapods. Interestingly, the majority of 3R duplicates retained in teleosts are potentially involved in neuroendocrine regulation. Furthermore, we infer that the ancestral AncRxfp3/4 receptor may have been syntenically linked to the AncRln-like ligand in the pre-2R genome, and show that syntenic linkages among ligands and receptors have changed dynamically in different lineages. This study ultimately shows the broad utility, with some caveats, of incorporating paleogenomics data into understanding the evolution of gene families.

An evolutionary comparison of leucine-rich repeat containing G protein-coupled receptors reveals a novel LGR subtype

Leucine-rich repeat containing G protein-coupled receptors or LGRs are receptors with important functions in development and reproduction. Belonging to this evolutionarily conserved group of receptors are the well-studied glycoprotein hormone receptors and relaxin receptors in mammals, as well as the bursicon receptor, which triggers cuticle hardening and tanning in freshly enclosed insects. In this study, the numerous LGR sequences in different animal phyla are analyzed and compared. Based on these data a phylogenetic tree was generated. This information sheds new light on structural and evolutionary aspects regarding this receptor group. Apart from vertebrates and insects, LGRs are also present in early chordates (Urochordata, Cephalochordata and Hyperoartia) and other arthropods (Arachnida and Branchiopoda) as well as in Mollusca, Echinodermata, Hemichordata, Nematoda, and even in ancient animal life forms, such as Cnidaria and Placozoa. Three distinct types of LGR exist, distinguishable by their number of leucine-rich repeats (LRRs), their type-specific hinge region and the presence or absence of an LDLa motif. Type C LGRs containing only one LDLa (C1 subtype) appear to be present in nearly all animal phyla. We here describe a second subtype, C2, containing multiple LDLa motifs, which was discovered in echinoderms, mollusks and in one insect species (Pediculus humanis corporis). In addition, eight putative LGRs can be predicted from the genome data of the placozoan species Trichoplax adhaerens. They may represent an ancient form of the LGRs, however, more genomic data will be required to confirm this hypothesis.

Gene turnover and differential retention in the relaxin/insulin-like gene family in primates

The relaxin/insulin-like gene family is related to the insulin gene family, and includes two separate types of peptides: relaxins (RLNs) and insulin-like peptides (INSLs) that perform a variety of physiological roles including testicular descent, growth and differentiation of the mammary glands, trophoblast development, and cell differentiation. In vertebrates, these genes are found on three separate genomic loci, and in mammals, variation in the number and nature of genes in this family is mostly restricted to the Relaxin Family Locus B. For example, this locus contains a single copy of RLN in platypus and opossum, whereas it contains copies of the INSL6, INSL4, RLN2 and RLN1 genes in human and chimp. The main objective of this research is to characterize changes in the size and membership composition of the RLN/INSL gene family in primates, reconstruct the history of the RLN/INSL genes of primates, and test competing evolutionary scenarios regarding the origin of INSL4 and of the duplicated copies of the RLN gene of apes. Our results show that the relaxin/INSL-like gene family of primates has had a more dynamic evolutionary history than previously thought, including several examples of gene duplications and losses which are consistent with the predictions of the birth-and-death model of gene family evolution. In particular, we found that the differential retention of relatively old paralogs played a key role in shaping the gene complement of this family in primates. Two examples of this phenomenon are the origin of the INSL4 gene of catarrhines (the group that includes Old World monkeys and apes), and of the duplicate RLN1 and RLN2 paralogs of apes. In the case of INSL4, comparative genomics and phylogenetic analyses indicate that the origin of this gene, which was thought to represent a catarrhine-specific evolutionary innovation, is as old as the split between carnivores and primates, which took place approximately 97 million years ago. In addition, in the case of the RLN1 and RLN2 genes of apes our phylogenetic trees and topology tests indicate that the duplication that gave rise to these two genes maps to the last common ancestor of anthropoid primates. All these genomic changes in gene complement, which are particularly prevalent among anthropoid primates, might be linked to the many physiological and anatomical changes found in this group. Given the various roles of members of the RLN/INSL-like gene family in reproductive biology, it might be that changes in this gene family are associated to changes in reproductive traits.

Localization of diversified relaxin gene transcripts in the brain of eels

Relaxin 3 (RLN3) is a newly-discovered member of the insulin superfamily. We isolated three RLN3-like cDNAs from the brain of the Japanese eel (Anguilla japonica). The deduced amino acid sequences of the RLN3-like cDNAs contained the two-chain structure common to relaxin including a RXXXRXXI/V motif in the B-chain. Phylogenetic analysis assigned the two prepropeptides into teleost/mammalian RLN3 group, which are a pair of duplicates generated by the teleost-specific third-round whole genome duplication, and the other one into teleost RLN group. Therefore, they have been named eel rln3a, rln3b and rln. rln3a transcripts were abundant in the middle-posterior region of the brain and detected at lower levels in the gills, head kidney and kidney. rln3b transcripts were also detected in the middle-posterior region of the brain, but the expression levels were lower than those of rln3a. Low levels of rln transcripts were detected in all brain areas, pituitary, digestive tract and gonad. Quantitative PCR analysis did not detect differences in expression of any rln3 or rln gene between freshwater- and seawater-acclimated eels. In situ hybridization showed that rln3a was expressed in neurons of the lateral lemniscus of the midbrain and of the griseum centrale (GC) of the hindbrain, while low amounts of rln transcripts were found in neurons of the periventricular nucleus of the posterior tuberculum of the diencephalon and the GC. These results suggest that the multiple RLN3-like peptides may play regulatory roles in the brain of euryhaline fish.

Characterization and developmental expression pattern of the relaxin receptor rxfp1 gene in zebrafish

We report the gene characterization, the cDNA cloning and the temporal and spatial expression pattern of the relaxin receptor rxfp1 gene in the zebrafish Danio rerio. The zebrafish rxfp1 gene has the same syntenic genomic organization, and a similar exon-intron structure to the homologue human gene. Furthermore, the deduced Rxfp1 protein sequence shows a high degree of amino acid similarity when compared with the human protein and the conservation of all amino acid identity necessary for the binding with relaxin. Our results show that rxfp1 gene is active either during embryogenesis or in the adult organism, showing a wide expression pattern. Moreover, we provide the first description of rxfp1 spatial expression pattern during embryo development, showing that the transcript is already present at the early developmental stage and is distributed in all of the embryonic cells until somitogenesis. Starting at the pharyngula stage the gene expression becomes mainly restricted in the brain territories. In fact, at the larval stage, the transcript is detectable in the epiphysis, postoptic region, posterior tuberculum, hypothalamus, optic tectum, tegmentum/pons, medulla and also in the structure of a peripheral nervous system, the terminal nerve. The rxfp1 expression pattern in Danio rerio embryos is very similar to that reported in the adult mammalian brain, suggesting a pivotal role of this receptor in the neurophysiology processes already at very early developmental stages.

Spermatogonial stem cell niche and spermatogonial stem cell transplantation in zebrafish

BACKGROUND

Spermatogonial stem cells (SSCs) are the foundation of spermatogenesis, and reside within a specific microenvironment in the testes called "niche" which regulates stem cell properties, such as, self-renewal, pluripotency, quiescence and their ability to differentiate.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we introduce zebrafish as a new model for the study of SSCs in vertebrates. Using 5'-bromo-2'-deoxyuridine (BrdU), we identified long term BrdU-retaining germ cells, type A undifferentiated spermatogonia as putative stem cells in zebrafish testes. Similar to rodents, these cells were preferentially located near the interstitium, suggesting that the SSC niche is related to interstitial elements and might be conserved across vertebrates. This localization was also confirmed by analyzing the topographical distribution of type A undifferentiated spermatogonia in normal, vasa::egfp and fli::egfp zebrafish testes. In the latter one, the topographical arrangement suggested that the vasculature is important for the SSC niche, perhaps as a supplier of nutrients, oxygen and/or signaling molecules. We also developed an SSC transplantation technique for both male and female recipients as an assay to evaluate the presence, biological activity, and plasticity of the SSC candidates in zebrafish.

CONCLUSIONS/SIGNIFICANCE

We demonstrated donor-derived spermato- and oogenesis in male and female recipients, respectively, indicating the stemness of type A undifferentiated spermatogonia and their plasticity when placed into an environment different from their original niche. Similar to other vertebrates, the transplantation efficiency was low. This might be attributed to the testicular microenvironment created after busulfan depletion in the recipients, which may have caused an imbalance between factors regulating self-renewal or differentiation of the transplanted SSCs.

Evolution of the regulatory control of vertebrate striated muscle: the roles of troponin I and myosin binding protein-C

Troponin I (TnI) and myosin binding protein-C (MyBP-C) are key regulatory proteins of contractile function in vertebrate muscle. TnI modulates the Ca2+ activation signal, while MyBP-C regulates cross-bridge cycling kinetics. In vertebrates, each protein is distributed as tissue-specific paralogs in fast skeletal (fs), slow skeletal (ss), and cardiac (c) muscles. The purpose of this study is to characterize how TnI and MyBP-C have changed during the evolution of vertebrate striated muscle and how tissue-specific paralogs have adapted to different physiological conditions. To accomplish this we have completed phylogenetic analyses using the amino acid sequences of all known TnI and MyBP-C isoforms. This includes 99 TnI sequences (fs, ss, and c) from 51 different species and 62 MyBP-C sequences from 26 species, with representatives from each vertebrate group. Results indicate that the role of protein kinase A (PKA) and protein kinase C (PKC) in regulating contractile function has changed during the evolution of vertebrate striated muscle. This is reflected in an increased number of phosphorylatable sites in cTnI and cMyBP-C in endothermic vertebrates and the loss of two PKC sites in fsTnI in a common ancestor of mammals, birds, and reptiles. In addition, we find that His132, Val134, and Asn141 in human ssTnI, previously identified as enabling contractile function during cellular acidosis, are present in all vertebrate cTnI isoforms except those from monotremes, marsupials, and eutherian mammals. This suggests that the replacement of these residues with alternative residues coincides with the evolution of endothermy in the mammalian lineage.

Studies in zebrafish reveal unusual cellular expression patterns of gonadotropin receptor messenger ribonucleic acids in the testis and unexpected functional differentiation of the gonadotropins

This study aimed to improve, using the zebrafish model, our understanding of the distinct roles of pituitary gonadotropins FSH and LH in regulating testis functions in teleost fish. We report, for the first time in a vertebrate species, that zebrafish Leydig cells as well as Sertoli cells express the mRNAs for both gonadotropin receptors (fshr and lhcgr). Although Leydig cell fshr expression has been reported in other piscine species and may be a common feature of teleost fish, Sertoli cell lhcgr expression has not been reported previously and might be related to the undifferentiated gonochoristic mode of gonadal sex differentiation in zebrafish. Both recombinant zebrafish (rzf) gonadotropins (i.e. rzfLH and rzfFSH) stimulated androgen release in vitro and in vivo, with rzfFSH being significantly more potent than rzfLH. Forskolin-induced adenylate cyclase activation mimicked, whereas the protein kinase A inhibitor H-89 significantly reduced, the gonadotropin-stimulated androgen release. Therefore, we conclude that both FSH receptor and LH/choriogonadotropin receptor signaling are predominantly mediated through the cAMP/protein kinase A pathway to promote steroid production. Despite this similarity, other downstream mechanisms seem to differ. For example, rzfFSH up-regulated the testicular mRNA levels of a number of steroidogenesis-related genes both in vitro and in vivo, whereas rzfLH or human chorionic gonadotropin did not. Although not fully understood at present, these differences could explain the capacity of FSH to support both steroidogenesis and spermatogenesis on a long-term basis, whereas LH-stimulated steroidogenesis might be a more acute process, possibly restricted to periods during which peak steroid levels are required.