Endocrine and Transcriptome Changes Associated with Testicular Growth and Differentiation in Atlantic Salmon (Salmo salar L.)

Sexual maturation of Atlantic salmon males is marked by dramatic endocrine changes and rapid growth of the testes, resulting in an increase in the gonad somatic index (GSI). We examined the association of gonadal growth with serum sex steroids, as well as pituitary and testicular gene expression levels, which were assessed with a DNA oligonucleotide microarray. The testes transcriptome was stable in males with a GSI < 0.08% despite the large difference between the smallest and the largest gonads. Fish with a GSI ≥ 0.23% had 7-17 times higher serum levels of five male steroids and a 2-fold increase in progesterone, without a change in cortisol and related steroids. The pituitary transcriptome showed an upregulation of the hormone-coding genes that control reproduction and behavior, and structural rearrangement was indicated by the genes involved in synaptic transmission and the differentiation of neurons. The observed changes in the abundance of testicular transcripts were caused by the regulation of transcription and/or disproportional growth, with a greater increase in the germinative compartment. As these factors could not be separated, the transcriptome results are presented as higher or lower specific activities (HSA and LSA). LSA was observed in 4268 genes, including many genes involved in various immune responses and developmental processes. LSA also included genes with roles in female reproduction, germinal cell maintenance and gonad development, responses to endocrine and neural regulation, and the biosynthesis of sex steroids. Two functional groups prevailed among HSA: structure and activity of the cilia (95 genes) and meiosis (34 genes). The puberty of A. salmon testis is marked by the predominance of spermatogenesis, which displaces other processes; masculinization; and the weakening of external regulation. Results confirmed the known roles of many genes involved in reproduction and pointed to uncharacterized genes that deserve attention as possible regulators of sexual maturation.

Characterizing the relationship between gonadotropin releasing hormone (GnRH), kisspeptin, and RFamide related peptide 3 (RFRP-3) neurons in the equine hypothalamus across the estrous cycle and in the anovulatory seasons

To understand better the role that kisspeptin plays in regulating seasonal and estrous cycle changes in the mare, this study investigated the number, location and interactions between GnRH, kisspeptin and RFRP-3 neurons in the equine hypothalamus. Hypothalami were collected from mares during the non-breeding season, vernal transition and various stages of the breeding season. Fluorescent immunohistochemistry was used to label the neuropeptides of interest. GnRH cells were observed primarily in the arcuate nucleus (ARC), while very few labeled cells were identified in the pre-optic area (POA). Kisspeptin cells were identified primarily in the ARC, with a small number of cells observed dorsal to the ARC, surrounding the third ventricle (3V). The mean number of kisspeptin cells varied between animals and typically showed no pattern associated with season or stage of estrous cycle, but a seasonal difference was identified in the ARC population. Small numbers of RFRP-3 cells were observed in the ARC, ventromedial hypothalamus (VMH) and dorsomedial hypothalamus (DMH). The mean number of RFRP-3 cells appeared higher in pre-ovulatory animals compared to all other stages. The percentage of GnRH cell bodies with kisspeptin appositions did not change with season or stage of estrous cycle. The percentage of kisspeptin cells receiving inputs from RFRP-3 fibers did not vary with season or stage of estrous cycle. These interactions suggest the possibility of the presence of an ultra-short loop feedback system between these three peptides. The changes in RFRP-3 neurons suggest the possibility of a role in the regulation of reproduction in the horse, but it is unlikely to be as a gonadotropin inhibitory factor.

Functional genomics of reproduction in pigs: Are we there yet?

Reproductive failure is the main reason for culling females in swine herds and is both a financial and sustainability issue. Because reproductive traits are complex and lowly to moderately heritable, genomic selection within populations can achieve substantial genetic gain in reproductive efficiency. A better understanding of the physiological components affecting the expression of these traits will facilitate greater understanding of the genes affecting reproductive traits and is necessary to improve and optimize management strategies to maximize reproductive success of gilts and sows. Large-scale genotyping with single-nucleotide polymorphism (SNP) arrays are used for genome-wide association studies (GWAS) and have facilitated identification of positional candidate genes. Transcriptomic data can be used to weight SNP for GWAS and could lead to previously unidentified candidate genes. Resequencing and fine mapping of candidate genes are necessary to identify putative functional variants and some of these have been incorporated into new genotyping arrays. Sequence imputation and genotype by sequence are newer strategies that could reveal novel functional mutations. In this study, these approaches are discussed. Advantages and limitations are highlighted where additional research is needed.

Expression Profile of Housekeeping Genes and Tissue-Specific Genes in Multiple Tissues of Pigs

Pigs have become an ideal model system for human disease research and development and an important farm animal that provides a valuable source of nutrition. To profile the all-sided gene expression and their biological functions across multiple tissues, we conducted a comprehensive analysis of gene expression on a large scale around the side of housekeeping genes (HKGs), tissue specific genes (TSGs), and the co-expressed genes in 14 various tissues. In this study, we identified 2351 HKGs and 3018 TSGs across tissues, among which 4 HKGs (COX1, UBB, OAZ1/NPFF) exhibited low variation and high expression levels, and 31 particular TSGs (e.g., PDC, FKBP6, STAT2, and COL1A1) were exclusively expressed in several tissues, including endocrine brain, ovaries, livers, backfat, jejunum, kidneys, lungs, and longissimus dorsi muscles. We also obtained 17 modules with 230 hub genes (HUBGs) by weighted gene co-expression network analysis. On the other hand, HKGs functions were enriched in the signaling pathways of the ribosome, spliceosome, thermogenesis, oxidative phosphorylation, and nucleocytoplasmic transport, which have been highly suggested to involve in the basic biological tissue activities. While TSGs were highly enriched in the signaling pathways that were involved in specific physiological processes, such as the ovarian steroidogenesis pathway in ovaries and the renin-angiotensin system pathway in kidneys. Collectively, these stable, specifical, and co-expressed genes provided useful information for the investigation of the molecular mechanism for an understanding of the genetic and biological processes of complex traits in pigs.

Hypothalamic kisspeptin and kisspeptin receptors: Species variation in reproduction and reproductive behaviours

Kisspeptin, encoded by the KISS1 gene, was first discovered as a potential metastasis suppressor gene. The prepro-kisspeptin precursor is cleaved into shorter mature bioactive peptides of varying sizes that bind to the G protein-coupled receptor GPR54 (=KISS1R). Over the last two decades, multiple types of Kiss and KissR genes have been discovered in mammalian and non-mammalian vertebrate species, but they are remarkably absent in birds. Kiss neuronal populations are distributed mainly in the hypothalamus. The KissRs are widely distributed in the brain, including the hypothalamic and non-hypothalamic regions, such as the hippocampus, amygdala, and habenula. The role of KISS1-KISS1R in humans and Kiss1-Kiss1R in rodents is associated with puberty, gonadal maturation, and the reproductive axis. However, recent gene deletion studies in zebrafish and medaka have provided controversial results, suggesting that the reproductive role of kiss is dispensable. This review highlights the evolutionary history, localisation, and significance of Kiss-KissR in reproduction and reproductive behaviours in mammalian and non-mammalian vertebrates.

The neuroendocrine pathways and mechanisms for the control of the reproduction in female pigs

Within the hypothalamic-pituitary-gonad (HPG) axis, the major hierarchical component is gonadotropin-releasing hormone (GnRH) neurons, which directly or indirectly receive regulatory inputs from a wide array of regulatory signals and pathways, involving numerous circulating hormones, neuropeptides, and neurotransmitters, and which operate as a final output for the brain control of reproduction. In recent years, there has been an increasing interest in neuropeptides that have the potential to stimulate or inhibit GnRH in the hypothalamus of pigs. Among them, Kisspeptin is a key component in the precise regulation of GnRH neuron secretion activity. Besides, other neuropeptides, including neurokinin B (NKB), neuromedin B (NMB), neuromedin S (NMS), α-melanocyte-stimulating hormone (α-MSH), Phoenixin (PNX), show potential for having a stimulating effect on GnRH neurons. On the contrary, RFamide-related peptide-3 (RFRP-3), endogenous opioid peptides (EOP), neuropeptide Y (NPY), and Galanin (GAL) may play an inhibitory role in the regulation of porcine reproductive nerves and may directly or indirectly regulate GnRH neurons. By combining data from suitable model species and pigs, we aim to provide a comprehensive summary of our current understanding of the neuropeptides acting on GnRH neurons, with a particular focus on their central regulatory pathways and underlying molecular basis.

Evidence that pubertal status impacts KNDy neurons in the gilt

Puberty onset is a complex physiological process which enables the capacity for reproduction through increased gonadotropin-releasing hormone (GnRH), and subsequently luteinizing hormone (LH), secretion. While cells that coexpress kisspeptin, neurokinin B (NKB), and dynorphin in the hypothalamic arcuate nucleus (ARC) are believed to govern the timing of puberty, the degree to which KNDy neurons exist and are regulated by pubertal status remains to be determined in the gilt. Hypothalamic tissue from prepubertal and postpubertal, early follicular phase gilts was used to determine the expression of kisspeptin, NKB, and dynorphin within the ARC. Fluorescent in situ hybridization revealed that the majority (> 74%) of ARC neurons that express mRNA for kisspeptin coexpressed mRNA for NKB and dynorphin. There were fewer ARC cells that expressed mRNA for dynorphin in postpubertal gilts compared to prepubertal gilts (P < 0.05), but the number of ARC cells expressing mRNA for kisspeptin or NKB was not different between groups. Within KNDy neurons, mRNA abundance for kisspeptin, NKB, and dynorphin of postpubertal gilts was the same as, less than, and greater than, respectively, prepubertal gilts. Immunostaining for kisspeptin did not differ between prepubertal and postpubertal gilts, but there were fewer NKB immunoreactive fibers in postpubertal gilts compared to prepubertal gilts (P < 0.05). Together, these data reveal novel information about KNDy neurons in gilts and supports the idea that NKB and dynorphin play a role in puberty onset in the female pig.

RFamide peptides, the novel regulators of mammalian HPG axis: A review

The RFamide-related peptides (RFRPs) are the group of neuropeptides synthesized predominantly from the hypothalamus that negatively affects the hypothalamo-hypophyseal-gonadal (hypothalamic–pituitary–gonadal [HPG]) axis. These peptides are first identified in quail brains and emerged as the mammalian orthologs of avian gonadotropin inhibitory hormones. The RFRP-3 neurons in the hypothalamus are present in several mammalian species. The action of RFRP-3 is mediated through a G-protein-coupled receptor called OT7T022. The predominant role of RFRP-3 is the inhibition of HPG axis with several other effects such as the regulation of metabolic activity, stress regulation, controlling of non-sexual motivated behavior, and sexual photoperiodicity in concert with other neuropeptides such as kisspeptin, neuropeptide-Y (NPY), pro-opiomelanocortin, orexin, and melanin. RFamide peptides synthesized in the granulosa cells, interstitial cells, and seminiferous tubule regulate steroidogenesis and gametogenesis in the gonads. The present review is intended to provide the recent findings that explore the role of RFRP-3 in regulating HPG axis and its potential applications in the synchronization of reproduction and its therapeutic interventions to prevent stress-induced amenorrhea.

Localization of kisspeptin, NKB, and NK3R in the hypothalamus of gilts treated with the progestin altrenogest

Mechanisms in the brain controlling secretion of gonadotropin hormones in pigs, particularly luteinizing hormone (LH), are poorly understood. Kisspeptin is a potent LH stimulant that is essential for fertility in many species, including pigs. Neurokinin B (NKB) acting through neurokinin 3 receptor (NK3R) is involved in kisspeptin-stimulated LH release, but organization of NKB and NK3R within the porcine hypothalamus is unknown. Hypothalamic tissue from ovariectomized (OVX) gilts was used to determine the distribution of immunoreactive kisspeptin, NKB, and NK3R cells in the arcuate nucleus (ARC). Almost all kisspeptin neurons coexpressed NKB in the porcine ARC. Immunostaining for NK3R was distributed throughout the preoptic area (POA) and in several hypothalamic areas including the periventricular and retrochiasmatic areas but was not detected within the ARC. There was no colocalization of NK3R with gonadotropin-releasing hormone (GnRH), but NK3R-positive fibers in the POA were in close apposition to GnRH neurons. Treating OVX gilts with the progestin altrenogest decreased LH pulse frequency and reduced mean circulating concentrations of LH compared with OVX control gilts (P < 0.01), but the number of kisspeptin and NKB cells in the ARC did not differ between treatments. The neuroanatomical arrangement of kisspeptin, NKB, and NK3R within the porcine hypothalamus confirm they are positioned to stimulate GnRH and LH secretion in gilts, though differences with other species exist. Altrenogest suppression of LH secretion in the OVX gilt does not appear to involve decreased peptide expression of kisspeptin or NKB.

RFamide-related peptides' gene expression, polymorphism, and their association with reproductive traits in chickens

RFamide-related peptides (RFRP) are synthesized by the hypothalamus and have a regulatory role in gonad development. The goal of this study was to investigate the association between SNP of the RFRP gene and the reproductive traits and hormone levels of Zhenning yellow chickens. The mRNA expression levels were detected based on different tissues, ages, and genotypes. Eleven mutation sites were detected in the RFRP gene, 4 of which were significantly related to reproductive traits and hormone levels. Association analysis revealed that A276G was associated with egg production at 300 d of age (EP300) and amount of prehierarchical follicles (P < 0.05). G1396A was associated with egg weight at 300 d of age and luteinizing hormone (LH) and prolactin levels (P < 0.05). G1694A showed significant associations with fertilization rate and LH levels (P < 0.05), and A2659G was associated with EP300 (P < 0.05). The results of expression analysis showed that the RFRP mRNA expression levels in the hypothalamus were higher than those in other tissues (P < 0.01). The expression in immature individuals was higher than that in mature ones (P < 0.01). There were also differences in mRNA expression levels between different genotypes (P < 0.05). In summary, the results of this study might provide potential markers and a theoretical basis for the improvement of chicken reproductive traits.

Identification of Circular RNAs in Hypothalamus of Gilts during the Onset of Puberty

The disorders of puberty have shown negative outcomes on health of mammals, and the hypothalamus is thought to be the main regulator of puberty by releasing GnRH. Many studies show that the circular RNAs (circRNAs) might be implicated in the timing of puberty in mammals. However, the circRNAs in the hypothalamus of gilts have not been explored. To profile the changes and biological functions of circRNAs in the hypothalamus during the onset of puberty, RNA-seq was utilized to establish pre-, in-, and post-pubertal hypothalamic circRNAs profiles. In this study, the functions of hypothalamic circRNAs were enriched in the signaling pathway of neurotrophin, progesterone-mediated oocyte maturation, oocyte meiosis, insulin, ErbB, and mTOR, which have been highly suggested to be involved in the timing of puberty. Furthermore, 53 circRNAs were identified to be putative hypothalamus-specific expressed circRNAs, and some of them were exclusively expressed in the one of three pubertal stages. Moreover, 22 differentially expressed circRNAs were identified and chosen to construct the circRNA-miRNA-gene network. Moreover, 10 circRNAs were found to be driven by six puberty-related genes (ESR1, NF1, APP, ENPP2, ARNT, and DICER1). Subsequently, the expression changes of several circRNAs were confirmed by RT-qPCR. Collectively, the preliminary results of hypothalamic circRNAs provided useful information for the investigation of the molecular mechanism for the timing of puberty in gilts.

Gonadotropin Inhibitory Hormone and Its Receptor: Potential Key to the Integration and Coordination of Metabolic Status and Reproduction

Since its discovery as a novel gonadotropin inhibitory peptide in 2000, the central and peripheral roles played by gonadotropin-inhibiting hormone (GnIH) have been significantly expanded. This is highlighted by the wide distribution of its receptor (GnIH-R) within the brain and throughout multiple peripheral organs and tissues. Furthermore, as GnIH is part of the wider RF-amide peptides family, many orthologues have been characterized across vertebrate species, and due to the promiscuity between ligands and receptors within this family, confusion over the nomenclature and function has arisen. In this review, we intend to first clarify the nomenclature, prevalence, and distribution of the GnIH-Rs, and by reviewing specific localization and ligand availability, we propose an integrative role for GnIH in the coordination of reproductive and metabolic processes. Specifically, we propose that GnIH participates in the central regulation of feed intake while modulating the impact of thyroid hormones and the stress axis to allow active reproduction to proceed depending on the availability of resources. Furthermore, beyond the central nervous system, we also propose a peripheral role for GnIH in the control of glucose and lipid metabolism at the level of the liver, pancreas, and adipose tissue. Taken together, evidence from the literature strongly suggests that, in fact, the inhibitory effect of GnIH on the reproductive axis is based on the integration of environmental cues and internal metabolic status.

Physiological and genomic insight into neuroendocrine regulation of puberty in gilts

The timing of pubertal attainment in gilts is a critical factor for pork production and is an early indicator of future reproductive potential. Puberty, defined as age at first standing estrus in the presence of a boar, is brought about by an escape from estrogen inhibition of the GnRH pulse generator, which allows for increasing LH pulses leading to the onset of cyclicity. The biological mechanisms that control the timing of these events is related to decreasing inhibitory signals with a concomitant increase in stimulatory signals within the hypothalamus. The roles of gamma-aminobutyric acid, endogenous opioid peptides, and gonadotropin-inhibitory hormone in negatively regulating gonadotropin secretion in gilts is explored. Developmental changes in stimulatory mechanisms of glutamatergic and kisspeptin neurons are important for increased LH pulsatility required for the occurrence of puberty in pigs. Age at first estrus of gilts is metabolically gated, and numerous metabolites, metabolic hormones, and appetite-regulating neurotransmitters have been implicated in the nutritional regulation of gonadotropin secretion. Leptin is an important metabolic signal linking body energy reserves with age at puberty in gilts. Leptin acting through neuropeptide Y and proopiomelanocortin neurons in the hypothalamus has important impacts on the function of the reproductive neurosecretory axis of gilts. Age at puberty in swine is heritable, and genomic analyses reveal it to be a polygenic trait. Genome-wide association studies for pubertal age in gilts have revealed several genomic regions in common with those identified for age at menarche in humans. Candidate genes have been identified that have important functions in growth and adiposity. Numerous genes regulating hypothalamic neuronal function, gonadotropes in the adenohypophysis, and ovarian follicular development have been identified and illustrate the complex maturational changes occurring in the hypothalamic-pituitary-ovarian axis during puberty in gilts.

Using SNP Weights Derived From Gene Expression Modules to Improve GWAS Power for Feed Efficiency in Pigs

The "large p small n" problem has posed a significant challenge in the analysis and interpretation of genome-wide association studies (GWAS). The use of prior information to rank genomic regions and perform SNP selection could increase the power of GWAS. In this study, we propose the use of gene expression data from RNA-Seq of multiple tissues as prior information to assign weights to SNP, select SNP based on a weight threshold, and utilize weighted hypothesis testing to conduct a GWAS. RNA-Seq libraries from hypothalamus, duodenum, ileum, and jejunum tissue of 30 pigs with divergent feed efficiency phenotypes were sequenced, and a three-way gene x individual x tissue clustering analysis was performed, using constrained tensor decomposition, to obtain a total of 10 gene expression modules. Loading values from each gene module were used to assign weights to 49,691 commercial SNP markers, and SNP were selected using a weight threshold, resulting in 10 SNP sets ranging in size from 101 to 955 markers. Weighted GWAS for feed intake in 4,200 pigs was performed separately for each of the 10 SNP sets. A total of 36 unique significant SNP associations were identified across the ten gene modules (SNP sets). For comparison, a standard unweighted GWAS using all 49,691 SNP was performed, and only 2 SNP were significant. None of the SNP from the unweighted analysis resided in known QTL related to swine feed efficiency (feed intake, average daily gain, and feed conversion ratio) compared to 29 (80.6%) in the weighted analyses, with 9 SNP residing in feed intake QTL. These results suggest that the heritability of feed intake is driven by many SNP that individually do not attain genome-wide significance in GWAS. Hence, the proposed procedure for prioritizing SNP based on gene expression data across multiple tissues provides a promising approach for improving the power of GWAS.

Regulation of LH secretion by RFRP-3 - From the hypothalamus to the pituitary

RFamide-related peptides (RFRPs) have long been identified as inhibitors of the hypothalamus-pituitary-gonad axis in mammals. However, less progress has been made in the detailed roles of RFRPs in the control of LH secretion. Recent studies have suggested that RFRP-3 neurons in the hypothalamus can regulate the secretion of LH at different levels, including kisspeptin neurons, GnRH neurons, and the pituitary. Additionally, conflicting results regarding the effects of RFRP-3 on these levels exist. In this review, we collect the latest evidence related to the effects of RFRP-3 neurons in regulating LH secretion by acting on kisspeptin neurons, GnRH neurons, and the pituitary and discuss the potential role of the timely reduction of RFRP-3 signaling in the modulation of the preovulatory LH surge.

The roles of RFamide-related peptides (RFRPs), mammalian gonadotropin-inhibitory hormone (GnIH) orthologues in female reproduction

OBJECTIVES

To benefit from reproduction and deal with challenges in the environmental conditions, animals must adapt internal physiology to maximize the reproduction rate. Maladaptive variations in the neurochemical systems and reproductive system can lead to manifestation of several significant mammalian reprocesses, including mammalian ovarian lifespan. RFamide-related peptide (RFRP, Rfrp), mammalian orthologues of gonadotropin-inhibitory hormone (GnIH), which is a regulator to prevent the gonadotropin-releasing hormone (GnRH) neural activity, is known to be related to reproductive traits. This review aimed to summarize recent five-year observations to outline historic insights and novel perspectives into the functions of RFRPs in coding the mammalian reproductive physiology, especially highlight recent advances in the impact on RFRPs in regulating mammalian ovary lifespan.

MATERIALS AND METHODS

We reviewed the recent five-year important findings of RFRP system involved in mammalian ovary development. Data for this review were collected from Google Scholar and PubMed using the RFRP keyword combined with the keywords related to physiological or pathological reproductive functions.

RESULTS

Recent discoveries are focused on three major fronts in research on RFRP role in female reproduction including reproductive functions, energy balance, and stress regulation. The roles of RFRPs in various development phases of mammal reproduction including prepuberty, puberty, estrous cycle, pregnancy, milking, menopause, and/or ovarian diseases have been shown.

CONCLUSION

Overall, these recent advances demonstrate that RFRPs serve as critical mediators in mammalian ovarian development.