91 Effect of heat stress during mice germ-cell DNA methylation programming on oocyte developmental competence: A preliminary study

Abstract

Heat stress (HS) is characterised by an elevation in body temperature that ultimately undermines organism physiology. Most livestock production occurs in tropical regions under potential HS conditions that diminish productive and reproductive potential. Despite extensive evidence of HS-mediated effects in cell function, stage-specific detrimental effects of HS during oogenesis remain elusive. Mouse models represent an attractive alternative for faster interrogation of stage-specific phenomena during oogenesis. Therefore, the aim of the study was to determine the effects of HS exposure during the major window of female mice germ-cell DNA methylation programming. CD1/Swiss female mice with litters (F₀ progeny) at postnatal Day 10 (P10) were randomly allocated to HS (35°C/12-h light; 21°C/12-h dark) or control (CTL: 21°C/24 h) for 11 days. The F₀ progeny were weaned at P21 and superovulated after reaching puberty at P35. F₀ females were superovulated by intraperitoneal injections with 5.0 IU of equine chorionic gonadotrophin (PMSG) followed by 5.0 IU of human chorionic gonadotrophin (hCG) within a 48-h interval. Pre-implantation embryos were harvested at Day E3.5 in M2 medium under a stereomicroscope. One F₀ female per litter was randomly mated to control mice when it reached 6 weeks of age. Data were subjected to least-squares analysis of variance using the General Linear Models procedure of SAS (SAS Institute Inc.). The experiment was replicated twice (CTL: n = 4 F₀ females and HS: n = 4 F₀ females). Preliminary results are given as LSM ± s.e.m. There was no effect of heat stress on the number of embryos collected per female (CTL: 9.75 ± 4.87 vs. HS: 11.25 ± 4.81) or the percentage of non-viable embryos (CTL: 25.0 ± 0.23% vs. HS: 42.5 ± 0.25%). However, heat stress tended (P = 0.07) to reduce the percentage of embryos that reached the morula stage from 63.5 ± 0.08% for CTL to 35.1 ± 0.09% for HS. The percentage of blastocysts collected (CTL: 11.45 ± 0.18% vs. HS: 22.32 ± 0.19%) and litter size of F₀ females (CTL: 7.47 ± 1.76 vs. HS: 7.66 ± 1.47) was not affected by treatment. In conclusion, exposure of female mice to 11-day HS during the major wave of de novo DNA methylation during oocyte growth tended to reduce subsequent pre-implantation embryonic development, although it did not affect full-term development after natural mating.