Impact of Heat Stress Exposure During Meiotic Maturation on Oocyte, Surrounding Cumulus Cell, and Embryo RNA Populations

To determine if reductions in developmental competence related to heat stress exposure were correlated with perturbations in certain RNA populations, poly(A) RNA, total RNA, RNA size distribution, and the abundance of transcripts (cyclin B1, GDF9, BMP15, poly(A) polymerase, HSP70, 18S & 28S rRNA) were examined in oocytes matured at 38.5 or 41 C. Performing in vitro fertilization resulted in embryos for examining RNA. Relative to germinal vesicle-stage oocytes, total amount of poly(A) RNA decreased similarly in oocytes matured at 38.5 or 41 C. Total RNA did not change during meiotic maturation or up through the 4 to 8-cell stage of embryonic development. Blastocyst-stage embryos had more total RNA; those originating from heat-stressed oocytes had more than those from nonheat-stressed oocytes. Oocytes and 4 to 8-cell embryos had similar RIN values and ratios for rRNA, 18S/fast region, and 18S/inter region. Values obtained for blastocyst-stage embryos were similar to those obtained for cumulus cell RNA, which did not change during maturation. Culture at 41 C for the first 12 h of meiotic maturation had no impact on RNA size distribution or transcripts examined from oocytes, surrounding cumulus or resultant 4 to 8-cell embryos. Interestingly, however, RNA from blastocysts originating from heat-stressed oocytes had lower 18S/fast region and 18S/inter region ratios compared to other developmental stages and cumulus cells. Although biological significance of these RNA changes is unclear, differences at the molecular level in embryos from heat-stressed oocytes emphasize the importance of minimizing stress exposure during meiotic maturation, if the intent is to obtain developmentally-competent embryos.