Assessing Mitochondrial Activity in Embryos from Heat-Stressed Ova

Elevated ambient temperatures negatively impacts pregnancy rates in agriculturally important females. An oocyte undergoing meiotic maturation exposed to a physiologically-relevant heat stress results in alterations of developmentally important processes and reportedly decreases subsequent developmental potential after fertilization. To address problems of reduced competence after exposure to heat stress for the first 12 h of meiotic maturation (hIVM), Payton (2009) assessed the transcriptome profiles of oocytes and associated cumulus cells by microarray analysis and reported heat-induced perturbations at the molecular level in the oocyte. Specifically, 20 transcripts involved in mitochondrial function exhibited alterations in relative abundance of heat-stressed oocytes which coincided with significantly more ATP content at 24 hIVM compared to non-heat-stressed controls (Payton et al., unpublished). These findings of altered transcripts and elevated ATP production suggest heat-induced perturbations of mitochondrial function. To further discern whether these heat-induced alterations in mitochondria persists in subsequent embryos after fertilization, the present study measured ATP content in oocytes matured at 38.5°C for 24 h or 41°C for the first 12 h followed by 38.5°C for the remaining 12 h, resultant cleavage, and blastocyst-stage embryos. Results demonstrated a significant increase in ATP content in heat-stressed matured oocytes (P = 0.0148) and of resultant 8-to-16-cell (P = 0.0323) stage embryos but was not evident in blastocyst stage embryos. Here we provide information that heat stress alters mitochondrial function in oocytes which persists following fertilization, which may ultimately contribute to a reduced developmental potential of embryos from heat-stressed oocytes. A second objective was to validate microarray findings via qPCR of: 1) electron transport chain transcripts, COQ3 and NDUFC2, in the oocyte and 2) IHH of the Hedgehog Pathway in the surrounding cumulus cells. Results agreed with the previous microarray data (Payton, 2009) reporting heat-induced decreases of NDUFC2 (P ≤ 0.002) and COQ3 (P = 0.02) in oocytes and heat-induced increases of IHH (P = 0.007) in cumulus cells. While functional significance of these findings remains unclear, heat-induced perturbations at the molecular level in the oocyte are evident and may provide explanation for reductions seen in developmental competence of resultant embryos.