167 EFFECT OF DNA METHYLATION INHIBITOR ON HETEROCHROMATIN IN BOVINE EMBRYOS DERIVED FROM HEAT-SHOCKED OOCYTES

The cell response to stress involves epigenetic modifications in order to regulate the gene expression, which is dependent of chromatin structure and DNA methylation status. On the other hand, changes on DNA methylation can have an effect on chromatin organisation (Espada and Esteller 2010 Semin. Cell. Dev. Biol. 21, 238–246). In this study we evaluated the effect of 5-aza-2′-deoxycytidine (5-aza; Sigma, St. Louis, MO, USA), a DNA methylation inhibitor, on heterochromatin 1 β formation of bovine pre-implantation embryos derived from oocytes that did or did not undergo heat shock during in vitro maturation (IVM). Oocytes were IVM under 38.5°C for 24 h (non-heat-shock: NHS group) or under 41.5°C for 12 h followed by 38.5°C for 12 h (heat-shock: HS group). Oocytes were IVF for 20 h and the denuded presumptive zygotes from NHS or HS groups were cultured with 0 (nontreated controls) or 10 nM of 5-aza for 24 h or 48 h in CR2aa plus 2.5% FCS at 38.5°C with 5% CO2, 5% O2 and 90% N2. Embryos with 4–7 cells at 44 h post-insemination (hpi) and embryos with 8–16 cells at 68 hpi were fixed in 4% paraformaldehyde and stained with anti-mouse HP1β first antibody, then immunofluorescence was evaluated by confocal microscopy (Leica TCS SP5II) and images were processed by ImageJ 1.49 (NIH, Bethesda, MD, USA). Fluorescence of nuclei and of background area (fluorescence/unit area) were measured and then the corrected relative fluorescence per nucleus was calculated. We analysed 129 and 149 nuclei at 44 hpi from 29 and 34 embryos as well as 268 and 182 nuclei at 68 hpi from 37 and 22 embryos of the NHS and HS groups, respectively, obtained from 3 replicates. Data underwent log-transformation and was analysed by ANOVA, and means compared by Student-Newman-Keuls. Embryos with 8–16 cells derived from NHS oocytes and treated with 5-aza for 24 h or for 48 h had nuclei with lower HP1 fluorescence than their respective NHS (nontreated) control (P < 0.01). In contrast, 8–16-cells embryos derived from HS and treated with 5-aza displayed nuclei with the same HP1 fluorescence of their respective HS control (P > 0.05). When embryos derived from HS and NHS (nontreated) control groups were compared, higher HP1 fluorescence was found in those with 4–7 cells of HS group (P < 0.05); however, embryos with 8–16 cells displayed similar HP1 fluorescence between both NHS and HS control groups (P > 0.05). There was no difference on HP1 fluorescence between nuclei of embryos with 4–7 cells treated with 5-aza for 24 h and control (nontreated) in both HS and NHS groups. These data suggest that embryos derived from heat-shocked oocytes can accumulate more heterochromatin at earlier stages than those from non-heat-shocked oocytes and that the effect of DNA methylation inhibition by 5-aza on embryo heterochromatin can vary accordingly to the exposure of the oocyte to heat shock during in vitro maturation. Financial support from CNPq, Fapemig, and CAPES is acknowledged.