Heat-induced increases in body temperature in lactating dairy cows: impact on the cumulus and granulosa cell transcriptome of the periovulatory follicle.

Cows acutely heat stressed after a pharmacologically induced luteinizing hormone (LH) surge had periovulatory changes in the follicular fluid proteome that may potentiate ovulation and impact oocyte developmental competence. Because the cellular origins of differentially abundant proteins were not known, we have examined the cumulus and granulosa cell transcriptomes from the periovulatory follicle in cows exhibiting varying levels of hyperthermia when occurring after the LH surge. After pharmacological induction of a dominant follicle, lactating dairy cows were administered gonadotropin releasing hormone (GnRH) and maintained in thermoneutral conditions (~67 temperature-humidity index [THI]) or heat stress conditions where THI was steadily increased for ~12 h (71 to 86 THI) and was sufficient to steadily elevate rectal temperatures. Cumulus-oocyte complexes and mural granulosa cells were recovered by transvaginal aspiration of dominant follicle content ~16 h after GnRH. Rectal temperature was used as a continuous, independent variable to identify differentially expressed genes (DEGs) increased or decreased per each 1 °C change in temperature. Cumulus (n = 9 samples) and granulosa (n = 8 samples) cells differentially expressed (false discovery rate [FDR] < 0.05) 25 and 87 genes, respectively. The majority of DEGs were upregulated by hyperthermia. Steady increases in THI are more like the "turning of a dial" than the "flipping of a switch." The moderate but impactful increases in rectal temperature induced modest fold changes in gene expression (<2-fold per 1 °C change in rectal temperature). Identification of cumulus DEGs involved in cell junctions, plasma membrane rafts, and cell-cycle regulation are consistent with marked changes in the interconnectedness and function of cumulus after the LH surge. Depending on the extent to which impacts may be occurring at the junctional level, cumulus changes may have indirect but impactful consequences on the oocyte as it undergoes meiotic maturation. Two granulosa cell DEGs have been reported by others to promote ovulation. Based on what is known, several other DEGs are suggestive of impacts on collagen formation or angiogenesis. Collectively these and other findings provide important insight regarding the extent to which the transcriptomes of the components of the periovulatory follicle (cumulus and mural granulosa cells) are affected by varying degrees of hyperthermia.