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Submitted on August 2, 2004
Accepted on November 23, 2004
Department of Biological Regulation, The Weizmann Institute of Science, Rehovot 76100, Israel,; Department of Neurobiology, Institute of Life Sciences, Hebrew University, Jerusalem 91904, Israel
* To whom correspondence should be addressed. E-mail: nava.dekel{at}weizmann.ac.il.
Resumption of meiosis, induced by LH (LH), is preceded by the breakdown of gap junctional communication, which terminates the supply of cAMP from the somatic cells of the ovarian follicle to the oocyte. It has recently been shown that LH-induced reinitiation of meiosis is mediated by mitogen-activated protein kinase (MAPK); however, the underlying molecular mechanism involved in the action of this enzyme remains unknown. We hypothesized that activation of MAPK interrupts junctional communication within the ovarian follicle, leading, in turn, to oocyte maturation. To test this hypothesis, we blocked the activation of MAPK by UO126, which specifically inhibits the MAPK signaling pathway. We analyzed junctional communication using three complementary methods: (1) patch-clamp analysis, which determined changes in the electrical coupling between two adjacent granulosa cells, (2) the scrape-loading technique, which monitored the spread of dyes through a granulosa cell layer, and (3) a metabolic coupling assay, which evaluated the transfer of radiolabeled uridine from the cumulus cells to the oocyte. We show, herein, that the somatic follicle cells, rather than the oocyte, activate MAPK immediately after their exposure to LH. Moreover, inhibition of LH-induced MAPK activation not only prevents oocyte maturation but also blocks the reduction in junctional communication. In addition, the appearance of the two phosphorylated forms of the gap junction protein, connexin 43 (Cx43), in response to LH, was avoided by UO126. We concluded that MAPK mediates LH-induced oocyte maturation by interrupting cell-to-cell communication within the ovarian follicle, possibly through phosphorylation of Cx43.
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