175 EFFECT OF HEAT STRESS ON GENE EXPRESSION OF IN VITRO-PRODUCED BOVINE EMBRYOS (BOS TAURUS VS. BOS INDICUS)

Heat stress (HS) reduces the production of bovine embryos, especially taurine embryos, which are not adapted to heat. However, little is known about the competence of embryos produced under HS in breeds adapted or not adapted to heat. The aim of this study was to compare the gene expression of PLAC8, HSF1, COX2 and CDX2, related to competence and implantation, in bovine in vitro-produced embryos (Bos taurus vs Bos indicus), submitted or not submitted to HS. Oocytes from Nelore (zebu) and Jersey (taurine) cows were aspirated by ovum pickup, in vitro-matured in TCM-199 medium with bicarbonate containing 10% FCS, 2 μg mL–1 of pyruvate, 75 μg mL–1 of gentamicin, 20 μg mL–1 of FSH and 10 IU mL–1 of LH for 22 h at 38.5°C in 5% CO2 in air. Matured oocytes were fertilized with semen from Nelore (n = 6) and Jersey (n = 6) bulls, respectively, at 38.5°C in 5% CO2 in air. The fertilization medium was TALP-IVF supplemented with 6 mg mL–1 of fatty acid-free BSA, 2 μL mL–1 of pyruvate, 75 μg mL–1 of gentamicin, 11 μg mL–1 of heparin and 44 μL mL–1 of penicillamine, hypotaurine and epinephrine. The day of fertilization was considered Day 0. Twelve hours post-insemination, presumptive zygotes were denuded and randomly divided into 2 groups, nonstressed or stressed and both were in vitro cultured at 38.5°C in 90% N2, 5% CO2 and 5% O2 in SOFaaci medium supplemented with 5% FCS, 5% BSA and 0,2% sodium pyruvate. In the stressed group, 96-h post-insemination embryos were subjected to HS of 41°C for 6 consecutive hours and then returned to 38.5°C. On Day 7, pools with 5 blastocysts [Nelore (n = 9); Nelore HS (n = 7); Jersey (n = 5); Jersey HS (n = 5)] were subjected to RNA extraction (RNeasy, Qiagen Inc., Valencia, CA, USA). The expression of target genes was analysed by real-time reverse transcription PCR with oligo-dT in reverse transcription and bovine specific-primers in PCR. The expression of cyclophilin A was used as an internal control. The mean mRNA levels of target genes among groups were compared by parametric ANOVA, followed by orthogonal contrast. Heat stress reduced (P < 0.05) mRNA expression of CDX2 and PLAC8 in both breeds; additionally, the expression of these genes was higher in the zebu breed when compared with the taurine breed. Messenger RNA expression of COX2 did not differ between groups, under HS or not, in both the Jersey and Nelore breeds. Moreover, HS reduced the mRNA expression of HSF1 (P < 0.05) in Nelore groups, but not in Jersey groups. The highest levels of PLAC8 and CDX2 in nonstressed Nelore embryos indicate better competence and a higher capacity of implantation of these embryos when compared with Jersey and HS embryos in both breeds. Moreover, low HSF1 levels in stressed Nelore embryos indicate the thermotolerance ability of this breed. In conclusion, the data indicate that HS alters the pattern of gene expression in Nelore and Jersey in vitro-produced bovine embryos. This research was supported by FAPESP.