Review: kisspeptin and reproduction in the pig

Clustering of multi-tissue transcriptomes in gilts with normal cyclicity or delayed puberty reveals genes related to pubertal development†

In gilts, puberty is marked by standing estrus in the presence of a boar. Delayed puberty (DP; failure to display pubertal estrus) is a major reason for gilt removal. To investigate the physiological determinants underlying DP in gilts, transcriptomic data from tissues relevant to estrus and puberty, such as mediobasal hypothalamus, anterior pituitary gland, ovarian cortex, olfactory bulb, amygdala, and hippocampus, were obtained from age-matched DP (n = 8) and cyclic control gilts at follicular phase (n = 8) and luteal phase (n = 8) of the estrous cycle. A gene expression module analysis via three-way gene × individual × tissue clustering using tensor decomposition identified pituitary and ovary gene modules contributing to regulation of pubertal development. Analysis of gene expression in the hypothalamic-pituitary-ovary axis identified reduced expression of hypothalamic genes critical for stimulating gonadotropin secretion (KISS1 and TAC3) and reduced expression of LHB in the anterior pituitary of DP gilts compared with their cyclic counterparts. Consequently, luteinizing hormone-induced genes in the ovary important for folliculogenesis (OXTR, RUNX2, and PTX3) were less expressed in DP gilts. Other intrafollicular genes (AHR, PTGS2, PTGFR, and IGFBP7) and genes in the steroidogenesis pathways (STAR and CYP11A1) necessary to complete the ovulatory cascade were also less expressed in DP gilts. This is the first clustering of multi-tissue expression data from DP and cyclic gilts to identify genes differentially expressed in gilts of similar ages but at different levels of sexual development. A critical lack of gonadotropin support and reduced ovarian responsiveness underlie DP in gilts.

Pathogenesis of the crosstalk between reproductive function and stress in animals-part 1: Hypothalamo-pituitary-adrenal axis, sympatho-adrenomedullary system and kisspeptin

Stress is defined as a disruption of the body homeostasis in response to modest as well as perceived challenge. Two main physiological routes, the hypothalamic-pituitary-adrenal system (HPA) and the sympatho-adrenomedullary system (SAM), aim to maintain or restore homeostasis by mutual interaction. SAM is quickly-reacting as it primarily works through the nervous system-the sympathetic nervous system. In response to stress, signals are sent to activate the adrenal medulla which releases catecholamines (primarily adrenaline and norepinephrine). The catecholamines have a momentary effect on the body's organs that are prepared for a fight situation. At the same time, the stressor activates the HPA axis by signals from the brain causing secretion of the pituitary hormone adrenocorticotropic hormone (ACTH). ACTH acts on the adrenal cortex, which secretes glucocorticoids, including cortisol. Since HPA primarily works through hormones, the system is slightly slower than SAM and gives rise to a metabolic effect. While short-term stress response is an adaptive and beneficial process, chronic or excessive stress can lead to a range of negative health outcomes including reproductive disorders and infertility. Several mechanisms have been proposed to explain the link between stress and reproduction. This includes in particular kisspeptin, which is closely related to reproduction, as it is a powerful stimulator of the Hypothalamic-pituitary-gonadal (HPG) system. The present review, through current knowledge in various male and female species, deals with the role of the SAM and the HPA, including the major action of kisspeptin and glucocorticoids that trigger the consequences of psychological or physiological stress on reproductive function.

Follicle development in pigs: State of the art

Understanding the factors and pathways involved with recruitment, atresia, and selection of follicles in the pig, may provide insight into approaches to limit fertility failures. Antral follicles depend upon FSH to the 2-3 mm stage, become codependent upon LH at 4-5 mm, and rely on LH when >5 mm. Within the follicle, gonadotropin binding, steroids, growth factors, and inhibin interact to determine the fate of the follicle. Continuous recruitment appears likely for follicles, and once >1 mm, they may have a limited period for survival, before selection or atresia. If true, then the number of healthy follicles that can respond to a hormone signal for selection, could vary by size and development stage. Which follicles are selected may depend upon their age, numbers of capillaries, granulosa and thecal cells, and FSH and LH receptors. This might also suggest that factors such as management, nutrition, and stress in prior weeks, could affect different cohorts of follicles to determine which of those from the ovarian population will be selected.

One Health challenges for pig reproduction

The current state of the world challenges pig reproduction as an important part of One Health, which involves interrelationships between animal, human and environmental health. The One Health concept underlines a comparative aspect in reproductive physiology and disease occurrence, bridging knowledge from one species to another. Seasonal changes in the environment affect pig reproduction and climate change may further strengthen those effects. Endocrine-disrupting chemicals (EDCs), and specifically phthalates and heavy metals, interfere with endocrine function, and thereby sexual behavior, fertilization capacity and steroidogenesis. Reproductive infections and extended semen storage are important indications for antimicrobial use. Innovative solutions are needed to explore alternatives to antimicrobials. Efforts to ensure reproductive efficiency have prolonged farrowing as litter size has doubled over the past three decades, compromising immune transfer and welfare. Physiological, metabolic and programming related events around parturition are key areas for future One Health research in pig reproduction. In conclusion, climate change challenges reproductive management and breeding. More resilient pigs that can tolerate harsh environment but maintain high reproductive performance are needed. EDCs continue to grow as an environmental challenge for reproductive management and alternatives to antibiotics will be required.

KNDy neurons as the GnRH pulse generator: Recent studies in ruminants

This review considers three aspects of recent work on the role of KNDy neurons in GnRH pulse generation in ruminants. First, work on basic mechanisms of pulse generation includes several tests of this hypothesis, all of which support it, and evidence that Kiss1r-containing neurons form a positive feedback circuit with the KNDy neural network that strengthen the activity of this network. The second section on pathways mediating external inputs focuses on the influence of nutrition and photoperiod, and describes the evidence supporting roles for proopiomelanocortin (POMC) and agouti-related peptide (AgRP) afferents to KNDy cells in each of these. Finally, we review studies exploring the potential applications of manipulating signaling by kisspeptin, and the other KNDy peptides, to control reproductive function in domestic animals and conclude that, although these approaches show some promise, they do not have major advantages over current practices at this time.

CRISPR/Cas9-editing of KISS1 to generate pigs with hypogonadotropic hypogonadism as a castration free trait

Introduction: Most male pigs are surgically castrated to avoid puberty-derived boar taint and aggressiveness. However, this surgical intervention represents a welfare concern in swine production. Disrupting porcine KISS1 is hypothesized to delay or abolish puberty by inducing variable hypogonadotropism and thus preventing the need for castration. Methods: To test this hypothesis, we generated the first KISS1-edited large animal using CRISPR/Cas9-ribonucleoproteins and single-stranded donor oligonucleotides. The targeted region preceded the sequence encoding a conserved core motif of kisspeptin. Genome editors were intracytoplasmically injected into 684 swine zygotes and transferred to 19 hormonally synchronized surrogate sows. In nine litters, 49 American Yorkshire and 20 Duroc liveborn piglets were naturally farrowed. Results: Thirty-five of these pigs bore KISS1-disruptive alleles ranging in frequency from 5% to 97% and did not phenotypically differ from their wild-type counterparts. In contrast, four KISS1-edited pigs (two boars and two gilts) with disruptive allele frequencies of 96% and 100% demonstrated full hypogonadotropism, infantile reproductive tracts, and failed to reach sexual maturity. Change in body weight during development was unaffected by editing KISS1. Founder pigs partially carrying KISS1-disruptive alleles were bred resulting in a total of 53 KISS1 ^+/+, 60 KISS1 ^+/-, and 34 KISS1 ^-/- F1 liveborn piglets, confirming germline transmission. Discussion: Results demonstrate that a high proportion of KISS1 alleles in pigs must be disrupted before variation in gonadotropin secretion is observed, suggesting that even a small amount of kisspeptin ligand is sufficient to confer proper sexual development and puberty in pigs. Follow-on studies will evaluate fertility restoration in KISS1 KO breeding stock to fully realize the potential of KISS1 gene edits to eliminate the need for surgical castration.

Heat Stress during Summer Attenuates Expression of the Hypothalamic Kisspeptin, an Upstream Regulator of the Hypothalamic-Pituitary-Gonadal Axis, in Domestic Sows

The release of reproductive hormones in the hypothalamic-pituitary-gonadal (HPG) axis is regulated by its upstream regulator, kisspeptin, and influenced by external stresses, including heat stress. Since the effect of heat stress (summer infertility) on hypothalamic kisspeptin expression in domestic sows is not yet understood, the present study attempted to identify changes in kisspeptin expression in different seasons (summer and spring). The high atmospheric temperature in summer decreased the pregnancy rate and litter size and increased stress-related hormones as a chronic stressor to domestic sows. The hypothalamic kisspeptin expression in summer was decreased regardless of the estrus phase and negatively correlated with atmospheric temperature, indicating that high temperature decreased kisspeptin. When the activity of hypothalamic kisspeptin neurons in the follicular phase was assessed using c-Fos staining, a decreased number of kisspeptin neurons coexpressing c-Fos was observed in domestic sows in summer. Accordingly, lower expression of kisspeptin induced decreased levels of HPG axis-related reproductive hormones, such as gonadotropins and estrogen, and fewer large ovarian follicles. In conclusion, the present study demonstrated that reduced kisspeptin expression and its neuronal activity in the hypothalamus under heat stress in summer induced downregulation of the HPG axis and caused summer infertility in domestic sows.

Effects of kisspeptin-10 on the reproductive performance of sows in a fixed-time artificial insemination programme

Kisspeptin (KP) is a major positive regulator of the hypothalamo-pituitary-gonadal axis and affects female reproductive cyclicity in mammals. It offers an attractive alternative strategy to control reproduction in fixed-time artificial insemination (FTAI) protocols. We aimed to evaluate the effects of different doses of kisspeptin-10 (KP-10) on sow reproductive performance in FTAI protocols. One hundred ninety-eight weaned sows were divided into three groups at random. A FTAI-GnRH group of sows (n = 98) received 100 µg (2 mL) gonadotropin-releasing hormone (GnRH; gonadorelin) by intramuscular injection at 96 h after weaning (t = 0 h); FTAI-KPL (KPL: low-dose KP-10, n = 50), and FTAI-KPH groups of sows (KPH: high-dose KP-10, n = 50) received 0.5 or 1 mg KP-10 (2 mL) respectively at 96 h after weaning. Sows were checked twice daily for oestrus. Ultrasonographic evaluations were performed to determine the follicular diameter and time of ovulation; blood samples were collected immediately before injection (t₀ = 0 min) and at 15, 30, 45, 60, 75, 90 min, 24 and 48 h postinjection. Sows were inseminated at 112 and 132 h after weaning. The oestrus rates (96 vs 92%; 96 vs 88%) and weaning-to-oestrus intervals (98.9 vs 98.6 h; 98.9 vs 97.1 h) were not affected by treatment, but oestrus in the FTAI-KPL group was significantly longer than in the FTAI-GnRH group (38.7 vs 30.0 h; P < 0.05). The peak LH concentrations were 1.29 times greater than at t₀ = 0 in the FTAI-GnRH group, and 1.45 and 1.44 times greater than at t₀ = 0 in the FTAI-KPL and FTAI-KPH groups, respectively. Follicular diameters and pregnancy rates (86 vs 88%, 86 vs 80%, respectively) did not differ between the treatments. Moreover, the total numbers of piglets born and those born alive did not differ among the three groups. These findings suggested that 0.5 mg KP-10 given at 96 h after weaning could be used in FTAI programmes to manage batch farrowing in sows.

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.

Actions and Roles of FSH in Germinative Cells

Follicle stimulating hormone (FSH) is produced by the pituitary gland in a coordinated hypothalamic-pituitary-gonadal (HPG) axis event, plays important roles in reproduction and germ cell development during different phases of reproductive development (fetal, neonatal, puberty, and adult life), and is consequently essential for fertility. FSH is a heterodimeric glycoprotein hormone of two dissociable subunits, α and β. The FSH β-subunit (FSHβ) function starts upon coupling to its specific receptor: follicle-stimulating hormone receptor (FSHR). FSHRs are localized mainly on the surface of target cells on the testis and ovary (granulosa and Sertoli cells) and have recently been found in testicular stem cells and extra-gonadal tissue. Several reproduction disorders are associated with absent or low FSH secretion, with mutation of the FSH β-subunit or the FSH receptor, and/or its signaling pathways. However, the influence of FSH on germ cells is still poorly understood; some studies have suggested that this hormone also plays a determinant role in the self-renewal of germinative cells and acts to increase undifferentiated spermatogonia proliferation. In addition, in vitro, together with other factors, it assists the process of differentiation of primordial germ cells (PGCLCs) into gametes (oocyte-like and SSCLCs). In this review, we describe relevant research on the influence of FSH on spermatogenesis and folliculogenesis, mainly in the germ cell of humans and other species. The possible roles of FSH in germ cell generation in vitro are also presented.

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.

Alternative Splicing Dynamics of the Hypothalamus-Pituitary-Ovary Axis During Pubertal Transition in Gilts

The timing of puberty in mammals marks the point at which reproduction becomes possible. Abnormalities in the timing of puberty may exert a series of negative effects on subsequent health outcomes. Alternative splicing (AS) has not only emerged as a significant factor in the transcription of genes but it is also reported to play a role in the timing of puberty. However, to date, the changes and dynamics of AS during the onset of puberty is extremely seldom explored. In the present study, we used gilts as a research model to investigated the dynamics of AS and differentially expressed AS (DEAS) events within the hypothalamus-pituitary-ovary (HPO) axis across pre-, in-, and post-puberty. We detected 3,390, 6,098, and 9,085 DEAS events in the hypothalamus, pituitary, and ovary when compared across pre-, in-, and post-pubertal stages, respectively. Within the entire HPO axis, we also identified 22,889, 22,857, and 21,055 DEAS events in the pre-, in-, and post-pubertal stages, respectively. Further analysis revealed that the differentially spliced genes (DSGs) associated with staged DEAS events were likely to be enriched in the oxytocin signaling pathway, thyroid hormone signaling pathway, GnRH signaling pathway, and oocyte meiosis signaling pathway. The DSGs associated with DEAS events across the entire HPO axis were enriched in endocytosis signaling pathway, the MAPK signaling pathway, and the Rap1 signaling pathway. Moreover. the ASs of TAC1, TACR3, CYP19A1, ESR1, ESRRA, and FSHR were likely to regulate the functions of the certain HPO tissues during the onset of puberty. Collectively, the AS dynamics and DEAS events were comprehensively profiled in hypothalamus, pituitary, and ovary across the pre-, in-, and post-pubertal stages in pigs. These findings may enhance our knowledge of how puberty is regulated by AS and shed new light on the molecular mechanisms underlying the timing of puberty in mammals.

Peripheral action of kisspeptin at reproductive tissues-role in ovarian function and embryo implantation and relevance to assisted reproductive technology in livestock: a review

Kisspeptin (KISS1) is encoded by the KISS1 gene and was initially found to be a repressor of metastasis. Natural mutations in the KISS1 receptor gene (KISS1R) were subsequently shown to be associated with idiopathic hypothalamic hypogonadism and impaired puberty. This led to interest in the role of KISS1 in reproduction. It was established that KISS1 had a fundamental role in the control of gonadotropin releasing hormone (GnRH) secretion. KISS1 neurons have receptors for leptin and estrogen receptor α (ERα), which places KISS1 at the gateway of metabolic (leptin) and gonadal (ERα) regulation of GnRH secretion. More recently, KISS1 has been shown to act at peripheral reproductive tissues. KISS1 and KISS1R genes are expressed in follicles (granulosa, theca, oocyte), trophoblast, and uterus. KISS1 and KISS1R proteins are found in the same tissues. KISS1 appears to have autocrine and paracrine actions in follicle and oocyte maturation, trophoblast development, and implantation and placentation. In some studies, KISS1 was beneficial to in vitro oocyte maturation and blastocyst development. The next phase of KISS1 research will explore potential benefits on embryo survival and pregnancy. This will likely involve longer-term KISS1 treatments during proestrus, early embryo development, trophoblast attachment, and implantation and pregnancy. A deeper understanding of the direct action of KISS1 at reproductive tissues could help to achieve the next step change in embryo survival and improvement in the efficiency of assisted reproductive technology.

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.