Abstract
This study investigated the participation of MAPK in the resumption of meiosis [germinal vesicle breakdown (GVB)] in oocytes and cumulus expansion using oocyte-cumulus cell complexes (OCC) from Mos-null mice (Mos(tm1Ev)/Mos(tm1Ev), hereafter Mos(-/-)). MAPK activity was not detected in Mos(-/-) oocytes whether they matured in vivo or in vitro, with or without gonadotropin stimulation. Therefore, there are no pathways independent of MOS that activate MAPK during gonadotropin-induced maturation. In contrast, MAPK activity was always detected coincident with GVB in Mos(+/+) oocytes. Moreover, MAPK activity was detected in cumulus cells before gonadotropin-induced GVB in OCC regardless of genotype. A specific inhibitor (U0126) of MEK, a MAPKK required for MAPK activity, inhibited gonadotropin-induced GVB in OCC of both Mos(+/+) and Mos(-/-) mice. Activation of MAPK was downstream of elevation of cAMP. U0126 also inhibited cumulus expansion stimulated by FSH, epidermal growth factor, 8-bromo-cAMP, and recombinant growth differentiation factor-9. It is concluded that under the in vitro conditions used here, gonadotropin-induced GVB requires the participation of MAPK activity in the cumulus cells, but not in the oocyte. Moreover, the induction of cumulus expansion also requires the participation of MAPK, and this action is downstream of both elevation of cAMP and growth differentiation factor-9.
Highlights
FULLY GROWN mammalian oocytes are maintained in ovarian follicles at the diplotene stage of the first meiotic prophase, commonly known as the germinal vesicle (GV) stage, by meiosis-arresting factors such as cAMP and/or purines such as hypoxanthine [1,2,3,4,5,6,7]
In some special circumstances, MAPK is activated in mouse oocytes before germinal vesicle breakdown (GVB), for instance, when GVB is induced by okadaic acid or by MAPK kinase (MEK) or MOS RNA injection [27,28,29]
In the control groups none of the oocytes isolated from Mosϩ/ϩ or MosϪ/Ϫ mice underwent GVB at any time postsaline injection, and no active forms of MAPK were detected at any time in either Mosϩ/ϩ or MosϪ/Ϫ oocytes (Fig. 2, A and C)
Summary
FULLY GROWN mammalian oocytes are maintained in ovarian follicles at the diplotene stage of the first meiotic prophase, commonly known as the germinal vesicle (GV) stage, by meiosis-arresting factors such as cAMP and/or purines such as hypoxanthine [1,2,3,4,5,6,7]. No MAPK activity is detected in oocytes from mice carrying a Mos-null mutation (Mostm1Ev/Mostm1Ev, hereafter MosϪ/Ϫ) during spontaneous meiotic maturation, and GVB occurs normally both in vivo and in vitro in MosϪ/Ϫ oocytes (18 –23). This evidence suggests that MAPK activity in the oocyte is not essential for meiotic resumption. Denuded oocytes (DO) or oocyte-cumulus cell complexes (OCC) isolated from large antral follicles undergo GVB in vitro without hormonal stimulation [30, 31] This experimental paradigm is quite different from LHinduced GVB in vivo, which is probably mediated by signals
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