Effects of Pre-ovulatory Follicle Physiological Status on Oocyte Metabolic Capacity

Fixed time artificial insemination (FTAI) has many benefits, however a subset of cattle in inseminated during these protocols rely on the administration of gonadotropin releasing hormone (GnRH) to induce ovulation. Inducing ovulation prior to the animal’s endogenous gonadotropin surge results in reduced pregnancy rates, embryo survival, day seven embryo quality, and successful embryo cleavage in beef cows undergoing FTAI. RNA-sequencing of oocytes and associated cumulus cells collected from follicles prematurely exposed to a gonadotropin surge suggested reduced capacity for glucose metabolism in cumulus cells of follicles ≤11.7 mm. Based on this information, we designed a series of studies to investigate the relationship between physiological status of the pre-ovulatory follicle at exposure to an exogenous gonadotropin surge and the metabolic capacity of the oocyte and follicular environment. We synchronized the development of a pre-ovulatory follicle in 319 lactating, Angus beef cattle and collected the follicular contents approximately 20 hours after administration of GnRH to induce the pre-ovulatory gonadotropin surge. Metabolomics analysis of 43 follicular fluid samples identified 18 metabolites with a significant, positive correlation to pre-ovulatory follicle diameter. Individual and pathway enrichment analysis of significantly correlated metabolites suggested that altered glucose and amino acid metabolism likely contribute to reduced developmental competence of oocytes when small pre-ovulatory follicles undergo induced ovulation. To further relate oocyte metabolic capacity to follicle status, we developed and validated procedures to measure both mitochondrial DNA (mtDNA) copy number and intraoocyte ATP in individual oocytes. We next determined the relationship between follicle physiological status and oocyte metabolic competence and follicular fluid metabolome profiles. Intraoocyte ATP content was significantly related to serum estradiol at GnRH2 and time between GnRH2 and follicle aspiration. Serum estradiol concentration at GnRH2 and follicular fluid progesterone concentration were positively correlated with the abundance of 22 and 61 follicular fluid metabolites, respectively. Pathway enrichment analysis of the significant metabolites suggested altered proteinogenesis, citric acid cycle, and pyrimidine metabolism. Collectively, our studies show that premature exposure to an exogenous gonadotropin surge results in reduced metabolic capacity of the oocyte as well as an altered follicular microenvironment.