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Oocyte Wake-Up Call

The signals that tell an oocyte to complete meiosis in preparation for fertilization are coming into focus, and a new study in rats hones in on a key component: dephosphorylation of a protein that manufactures cGMP, a factor responsible for maintaining meiotic arrest. In the pre-ovulatory follicle, cGMP is provided by the granulosa cells surrounding the developing oocyte. cGMP in the granulosa cells is produced by natriuretic peptide receptor 2 (NPR2), a guanyl cyclase. Previous studies in mice have shown that luteinizing hormone, the hormonal trigger for oocyte maturation, results in a rapid (within 20 minutes) reduction in the guanyl cyclase activity of NPR2. But exactly how NPR2 is regulated has been unclear. Using isolated preovulatory follicles from rats, Jeremy Egbert et al. [1] now look closely at this system. They were able to closely examine the phosphorylation status of NPR2 and other proteins because they could collect an order of magnitude more protein from each follicle in rats, compared with mice. The researchers found that, within ten minutes of LH signaling, NPR2 was dephosphorylated and inactivated— leading to a reduction in cGMP levels in the oocyte. The researchers provided evidence that inactivation of NPR2 occurs thorough phosphoprotein phosphatase (PPPF)-family members—and they honed in on three likely candidates in this family. These findings fit with previous studies showing that LH activation leads to activation of PPP2 (via G s and Protein Kinase A). Other molecular events also contribute to cGMP degradation. For instance, consistent with other studies, Egbert et al. found that follicular levels of an NPR2 agonist, C-type natriuretic peptide, decrease slowly (over several hours) in response to luteinizing hormone. REFERENCES