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

Medaka (Oryzias latipes) Dmrt3a Is Involved in Male Fertility

Research across various species has demonstrated that the doublesex and mab-3-related transcription factor 3 (dmrt3) plays pivotal roles in testis development. However, the precise molecular mechanisms of dmrt3 remain unclear. In this study, we investigated the role of dmrt3 (dmrt3a) in testis development using the model organism medaka (Oryzias latipes). SqRT-PCR and ISH analyses revealed that dmrt3a is predominantly expressed in the testis, especially in the spermatid and spermatozoon. Using CRISPR/Cas9, we generated two dmrt3a homozygous mutants (-8 bp and -11 bp), which exhibited significantly reduced fertilization rates and embryo production. Additionally, the number of germ cells and sperm motility were markedly decreased in the dmrt3a mutants, manifesting as the symptoms of asthenozoospermia and oligozoospermia. Interestingly, RNA-Seq analysis showed that the deficiency of dmrt3a could lead to a significant downregulation of numerous genes related to gonadal development and severe disruptions in mitochondrial function. These results suggested that dmrt3a is essential for spermatogenesis and spermatozoa energy production. This paper provides new insights and perspectives for further exploring the molecular mechanisms underlying spermatogenesis and addressing male reproductive issues.

Hypogonadotropic hypogonadism in male tilapia lacking a functional rln3b gene

Relaxin 3 is a neuropeptide that plays a crucial role in reproductive functions of mammals. Previous studies have confirmed that rln3a plays an important role in the male reproduction of tilapia. To further understand the significance of its paralogous gene rln3b in male fertility, we generated a homozygous mutant line of rln3b in Nile tilapia. Our findings indicated that rln3b mutation delayed spermatogenesis and led to abnormal testes structure. Knocking out rln3b gene resulted in a decrease in sperm count, sperm motility and male fish fertility. TUNEL detection revealed a small amount of apoptosis in the testes of rln3b-/- male fish at 390 days after hatching (dah). RT-qPCR analysis demonstrated that mutation of rln3b gene caused a significant downregulation of steroid synthesis-related genes such as cyp17a1, cyp11b2, germ cell marker gene, Vasa, and gonadal somatic cell marker genes of amh and amhr2. Furthermore, we found a significant down-regulation of hypothalamic-pituitary-gonadal (HPG) axis-related genes, while a significantly up-regulation of the dopamine synthetase gene in the rln3b-/- male fish. Taken together, our data strongly suggested that Rln3b played a crucial role in the fertility of XY tilapia by regulating HPG axis genes.

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.

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.

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.