Date of Award
Doctor of Philosophy
J. Lannett Edwards
Cheryl Kojima, Arnold Saxton, F. Neal Schrick, Neal Stewart
Heat-induced reductions in developmental competence after direct exposure of oocytes to 41ºC have been coincident with reduced protein synthesis. Since heat stress perturbs RNA integrity and polyadenylation in somatic cells, it was hypothesized that heat stress during meiotic maturation may alter RNA within oocytes and/or their surrounding cumulus to account for some of the reductions in development. Initial efforts utilized microcapillary electrophoresis to examine oocyte and cumulus RNA without heat stress as a first step toward transcriptome profile analysis. Size distribution of RNA, rRNA ratio, and other related endpoints differed for oocyte RNA compared to cumulus, and were conserved across other mammalian species. Size distribution of polyadenylated RNA after amplification was similar for oocytes and cumulus.
Effects of heat stress on total and polyadenylated RNA, RNA size distribution, and individual transcripts important for meiotic maturation and response to heat stress were examined in oocytes and cumulus during maturation, and resultant embryos after fertilization. There was no impact of heat stress during the first 12 h of maturation to alter size distribution of RNA, rRNA ratio, and other endpoints in oocytes or cumulus. Heat stress perturbed the abundance of polyadenylated RNA in oocytes in one study. Abundance of eight examined transcripts was not altered after exposure to elevated temperature, suggesting that the impact of heat stress on oocyte RNA, if any, was subtle.
Consequences of elevated temperature exposure during maturation on transcriptomes of oocytes and their surrounding cumulus vestment were investigated using microarray technology. Thousands of transcripts changed in oocytes and surrounding cumulus over meiotic maturation, some in a stage-specific and amplification-dependent manner. Culture at elevated temperature for the first 12 h of meiotic maturation impacted a small proportion of transcripts in matured oocytes and cumulus at 24 h. Alterations suggested perturbations in oocyte mitochondrial function, and intracellular signaling and extracellular matrix production in cumulus. Heat-induced alterations in oocyte mitochondria and cumulus expansion are supported by existing literature. The findings discussed here are informative of heat-induced molecular alterations in oocytes and cumulus and may prove useful for development of strategies to mitigate negative impacts of heat stress on fertility.
Payton, Rebecca R., "Direct Effects of Heat Stress During Meiotic Maturation on Bovine Oocyte and Cumulus RNA. " PhD diss., University of Tennessee, 2009.