37 Effect of in vivo and in vitro heat stress on DNA methylation and DNA hydroxymethylation of bovine oocytes and embryos

Abstract

Reduced reproductive performance is one of the main effects caused by heat stress in cattle. Its negative effects have been observed at the transcriptional, biochemical, morphological, and developmental levels on the oocyte and embryo. There are no studies evaluating the effect of heat stress on the epigenetic profile of bovine oocytes and early embryos. The objective of this study was to evaluate the effect of in vivo and in vitro heat stress on DNA methylation and DNA hydroxymethylation in bovine MII oocytes, pronuclear, and 2- to 4-cell stage embryos. Seven Bos taurus crossbred nonpregnant, non-lactating beef cows located in Saint Gabriel, Louisiana (30.269746, −91.103357) were used for oocyte collection. Dominant follicle removal was performed 5 days before oocyte collection. Cumulus–oocyte complexes were collected by ovum pickup from follicles >2 mm. Samples were collected during the summer (August) and winter (February) (5 collections each). Three treatments were utilised: in vivo heat stress (August samples), in vitro heat stress (February samples subjected to 41°C during the first 12 h of IVM and then to 38.5°C during the next 12 h of IVM), and control (February samples IVM at 38.5°C). All oocytes collected per treatment were assigned to 3 developmental stages: MII oocytes, pronuclear, and 2- to 4-cell stage embryos. Embryos were obtained through standard IVF. DNA methylation and DNA hydroxymethylation was assessed by fluorescence immunohistochemistry utilising primary antibodies against 5′-methylcytosine and 5′-hydromethylcytosine and secondary antibodies Alexa Fluor 488 and Alexa Fluor 546, respectively. Samples were visualised with a fluorescence deconvolution microscope, and immunofluorescence data were expressed as corrected relative fluorescence per nucleus. Results were analysed by the Type III test of fixed effects and Tukey media separation utilising the Proc Glimmix of SAS 9.4 (P < 0.05). Maturation rate, 2 pronuclei (2PN) rate, cleavage rate, and 2- to 4-cell rate were analysed by Chi-square. There was no difference in maturation rate (88.19 ± 7.57, 82.91 ± 5.18, 94.51 ± 5.04; P = 0.2516), 2PN rate (79.34 ± 10.23, 93.75 ± 7.21, 81.74 ± 12.53; P = 0.1757), cleavage rate (79.26 ± 2.69, 70.65 ± 7.22, 81.85 ± 16.65; P = 0.2388) and 2- to 4-cell rate (69.38 ± 7.83, 81.25 ± 10.34, 61.11 ± 11.69; P = 0.4392) between in vivo and in vitro heat stress compared with control, respectively. No difference was found in DNA methylation (P = 0.0537) or DNA hydroxymethylation (P = 0.4632) between treatments in MII oocytes. When evaluating the paternal and maternal pronuclei, there was no difference in DNA methylation (P = 0.9766; P = 0.1954, respectively) or DNA hydroxymethylation (P = 0.6440; P = 0.1932, respectively) between in vivo and in vitro heat stress compared with control. Similarly, there was no difference in DNA methylation (P = 0.0903) or DNA hydroxymethylation (P = 0.2452) between treatments when evaluating the 2- to 4-cell embryos. In conclusion, we detected no effect of in vivo or in vitro heat stress on MII oocytes and early embryos when evaluating global DNA methylation and hydroxymethylation through fluorescence immunohistochemistry.