139. Award Talk: Integrative Physiology and Functional Genomics Reveal Novel Mechanisms Regulating Pubertal Development in Gilts.

Abstract

Gilts reaching puberty early have greater lifetime productivity but 10-20% are acyclic before first breeding, presenting a major management challenge. Puberty is a complex maturational process, involving molecular and cellular events that are influenced by the environment, nutrition, and genetics. Initiation of puberty results from increasing episodic release of GnRH from the hypothalamus that drives increased pulsatile secretion of LH from the anterior pituitary gland. Through classical endocrine studies and precision gene editing we have shown that kisspeptin is a master regulator of gonadotropin secretion in pigs and is essential for puberty. Virtually all hypothalamic kisspeptin neurons in the pig coexpress neurokinin B and dynorphin (KNDy neurons). These KNDy neurons are thought to regulate GnRH release in an auto/paracrine fashion and kisspeptin knockout pigs have been used to demonstrate this. We have described the spatial organization of KNDy cells and related receptors within the gilt hypothalamus that show similarities to other species and differences unique to pigs. Pubertal development and acyclicity in gilts have been linked to developmental changes in KNDy expression. We have shown how KNDy cells and functions are affected by nutrition and steroid feedback. Because age at puberty is the earliest indicator of future reproductive potential of a gilt and because it is moderately heritable, identification of genomic markers can support marker-assisted selection to improve gilt development. Loci on Sus scrofa chromosomes (SCC) 1, 2, 9, and 14, identified through genome-wide association studies (GWAS) reveal age at first estrus is polygenic, controlled by many genes with small effects (typical allelic affects between 1-2 d). Several QTL associated with age at first estrus in gilts contain candidate genes associated with age at menarche in humans (RORA, AQP8, ANGPT2) and that overlap with correlated human traits of body mass index, height, and obesity (e.g., CRCT1, ADAMTSL3). GWAS for age at puberty in gilts identified genes associated with gonadotropin hormones (NHLH2, TAC1, LHX6, FOXD1, CABP7), and ovarian function (PAPPA, IGF1R, ESSRG, SLIT2, CRTC1). The QTL on SSC2 harbored putative olfactory receptor genes (OR2M4) necessary for response to boars and we showed that stage of the estrous cycle has large effects on expression of putative olfactory receptors in the olfactory epithelium of gilts. Several QTL contained candidate genes with pleotropic effects on traits of reproduction (GAS2L1, MEF2C, LIF), and growth and metabolism (ACACA, PIAS1, FEM1B, NUCB2). Treating gilts with Nesfatin-1 (NUCB2 gene) altered feed intake and increased LH release, demonstrating biological function of this puberty linked gene. Putative functional polymorphisms (94 variants in 33 genes) associated with age at puberty have been identified with fine SNP mapping and, more recently, imputation to whole genome sequence. Functional variants in genes related to social and sexual behavior (AVPR1A), ovarian function (BMP4, SOSTDC1, AHR, ANKRA2) and gonadotropin secretion (NPFFR2) have been identified. Although the NPFF2 gene was differentially expressed in the anterior pituitary gland of cyclic vs acyclic gilts, its ligand, a putative mammalian gonadotropin-inhibitory hormone, did not affect LH secretion. Functional variants in AHR were confirmed with allelic specific effects on expression of target genes (CYP1A2) and AHR was differently expressed in the ovary and pituitary of cyclic vs acyclic gilts. Using whole transcriptome sequencing we have identified over 1500 genes across seven tissues (amygdala, hippocampus, olfactory bulb, olfactory epithelium, hypothalamus, pituitary, and ovary) that are differentially expressed between gilts with normal versus delayed puberty. Tensor decomposition was used to identify gene modules in the pituitary, amygdala, and ovary that explained significant portions of the variation in phenotype, and 57 of the differentially expressed genes in the ovary and pituitary are in QTL for age at puberty. Intra pituitary pathways involving TSHR and IL6R and ovarian pathways involving AHR signaling affecting steroidogenesis were revealed. These studies provide a greater understanding of the complex physiological and genetic components affecting pubertal development in gilts. This will lead to improving and optimizing management strategies to maximize reproductive success of gilts.