No abstract

Recent experimental evidence indicates that progesterone acts at the cell surface to trigger protein synthesis and to reinitiate the first meiotic division in Xenopus laevis oocytes. The steroid hormone is physiologically released by follicle cells surrounding oocytes in the ovaries, and this naturally occurring event can be reproduced in vitro by adding progesterone to the incubation medium. Recently, cyclic AMP has been implicated in the mechanism of progesterone action in oocytes; there was an almost immediate decrease in cyclic AMP concentration in oocytes after addition of progesterone in vitro, whether or not the oocytes were pretreated with cholera toxin. Adenylate cyclase in X. laevis oocytes is compartmentalized, greater than 50% soluble and approximately 30% is found in the plasma membrane-containing fraction. We report here that physiological concentrations of progesterone selectively inhibit membrane-bound adenylate cyclase activity, after addition in intact oocytes or in cell-free experiments; this specificity confirms the proposed membrane site of action for the hormone when reinitiating meiosis and is the first example of a 'direct' enzymatic action of a steroid (not by protein synthesis) related to a physiological function.

Insulin can induce meiotic division in Xenopus laevis oocytes. This effect shows the specificity expected of a receptor-mediated mechanism. It is potentiated by ethynylestradiol, a steroid antagonist of pregesterone (the natural hormone that provokes meiosis). The Xenopus laevis oocytes may serve as a model for the study of the poorly understood effect of insulin on cell division.

No abstract

Full-grown mouse oocytes spontaneously resume meiosis in vitro when released from their follicular environment. By contrast, growing oocytes are not competent to resume meiosis; the molecular basis of meiotic competence is not known. Entry into M phase of the eukaryotic cell cycle is controlled by MPF, a catalytically active complex comprising p34cdc2 kinase and cyclin B. Incompetent oocytes contain levels of cyclin B comparable to those in competent oocytes, while their level of p34cdc2 is markedly lower; p34cdc2 accumulates abruptly at the end of oocyte growth, at the time of meiotic competence acquisition. We show here that this change in p34cdc2 concentration is not secondary to a corresponding change in the concentration of the cognate mRNA, indicating that translational control may be involved. Microinjection of translatable p34cdc2 mRNA into incompetent oocytes yielded high levels of the protein, but it did not lead to resumption of meiosis. Similarly, microinjection of cyclin B1 mRNA resulted in accumulation of the protein, but not in the acquisition of meiotic competence. By contrast, the microinjection of both p34cdc2 and cyclin B1 mRNAs in incompetent oocytes induced histone H1 and MAP kinase activation, germinal vesicle breakdown, and entry into M-phase including the translational activation of a dormant mRNA. Thus, endogenous cyclin B1 in incompetent oocytes is not available for interaction with p34cdc2, suggesting that a posttranslational event must occur to achieve meiotic competence. Microinjection of either p34cdc2 or cyclin B1 mRNAs accelerated meiotic reinitiation of okadaic acid-treated incompetent oocytes. Taken together, these results suggest that acquisition of meiotic competence by mouse oocytes is regulated at both translational and posttranslational levels.