Cell cycle progression during mitosis and meiosis is known to be regulated by the M-phase promoting factor (MPF). However, recent findings revealed that mitogen-activated protein kinase (MAPK) also plays an important regulatory role during transition through the cell cycle. At fertilization the activity of MAPK drops shortly after MPF inactivation; the objective of this study was to investigate the dynamics of MAPK activity in pig oocytes after different activation methods. In vitro-matured oocytes were allocated to 3 groups. In group 1 (EP), the oocytes were activated by 2 DC pulses of 1.2 kV cm-1, 60 �s each. In the second group (EP + BU), the oocytes were electroporated and incubated for 4 h in 100 �M butyrolactone I (BU, an inhibitor of cdc2 kinase). In group 3 (EP + CHX), the oocytes were electroporated and treated for 5 h with 10 �g mL-1 cycloheximide (CHX, a protein synthesis inhibitor). After electroporation all oocytes were incubated in 7.5 �g mL-1 cytochalasin B for 4 h. Some oocytes were used to determine MAPK activity at 0, 1, 2, 3, 4, 5, and 6 h after electroporation using a MAPK assay kit. The assay measures MAPK activity by determining the phosphorylation of myelin basic protein by MAPK using the transfer of the γ-phosphate of [γ-32P] ATP. Pronuclear formation was evaluated at 6 h after electroporation; blastocyst formation and total cell numbers per embryo were determined after a 7-day culture in PZM-3 medium. Pronuclear formation was compared by the chi-square test, blastocyst formation was assessed using ANOVA, and the kinase activity was evaluated using the Student t-test. Pronuclear formation was highest in the combined methods [69.39% (EP) vs. 86.32% (EP + BU) and 87.56 % (EP + CHX); P < 0.05]. Similarly, the combined methods supported better development to the blastocyst stage [25.06 � 7.96% (EP), 58.32 � 7.62% (EP + BU), and 63.91 � 6.35% (EP + CHX); P < 0.05], whereas the average cell numbers of the blastocysts did not differ (47.11 � 3.12, 46.56 � 2.33, and 44.04 � 1.86, respectively). The initial MAPK activity was 0.123 � 0.017 pmol/min/oocyte which, after 1 h, dropped in all cases to values of 0.069 � 0.009 (EP), 0.072 � 0.007 (EP + BU), and 0.077 � 0.012 (EP + CHX) pmol/min/oocyte (P < 0.05). The MAPK activity in the EP group reached its lowest level at 3 h (0.057 � 0.007 pmol/min/oocyte); however, at 4 h it started to recover and by 6 h the activity (0.079 � 0.022 pmol/min/oocyte) did not differ from that of the non-activated oocytes. In the other groups, MAPK activity stayed low, and by the end of the experimental period it was significantly lower than that in the nontreated metaphase II oocytes (P < 0.05). The results indicate that electroporation followed by protein kinase inhibition or protein synthesis inhibition leads to the efficient inactivation of MAPK activity, and confirm our earlier findings that these combined treatments support superior embryo development after oocyte activation.
Mitogen-activated protein kinase kinase activity is required for the G2/M transition of the cell cycle in mammalian fibroblasts