Regulation of parthenogenetic activation of metaphase II mouse oocytes by pyruvate

We report that parthenogenetic activation (pronuclear formation) is induced during in vitro culture of recently ovulated (13-14 hr post-hCG) mouse oocytes in pyruvate deficient medium. Pronuclear formation occurred when oocytes were cultured in medium containing 1/10X (Pyr-) or lower concentrations of pyruvate but failed to occur either in oocytes cultured in the presence of 0.47 mM (1X, Pyr+) or 1/2X pyruvate or in oocytes cultured in the absence of pyruvate but with cumulus cells. Pronuclear formation was evident within 8 hr of culture and completed by 16 hr and remained intact during continuous culture in Pyr- medium. Transfer of pronuclear oocytes to Pyr+ medium resulted in pronuclear membrane disassembly and further parthenogenetic development. A similar incidence of parthenogenetic activation occurred when recently ovulated oocytes were cultured in the presence of cycloheximide but not following ethanol or hyaluronidase treatment. However, both ethanol and hyaluronidase induced pronuclear formation in in vivo aged oocytes. Results suggest that the type of activation induced varies with the age of the oocyte and the nature of the stimulus. Amino acid uptake ([35S]methionine) by oocytes was unaffected by Pyr- culture whereas incorporation into protein was markedly inhibited. Gel electrophoretic analysis of labeled egg extracts revealed a marked inhibition of egg protein synthesis after 4 hr of culture in Pyr-. The occurrence of a cortical reaction was monitored by binding of fluorescent labeled lectin to the oocyte surface. A cortical reaction occurred in response to ethanol treatment of freshly ovulated and in vivo aged oocytes cultured in Pyr+ medium but not in pronucleate oocytes induced by Pyr- culture. Suppression of ethanol-induced cortical reaction by Pyr- culture was restored following transfer of oocytes to Pyr+ medium. Results demonstrate that nuclear events as well as plasma membrane events can be simply regulated by controlling the amount of energy substrate available to the germ cell. Effects of Pyr- culture in inducing pronuclear formation appear to be mediated in a large part via inhibition of protein synthesis.