A zinc-dependent mechanism regulates meiotic progression in mammalian oocytes.

Precise coordination of meiotic progression is a critical determinant of an egg's capacity to be fertilized successfully, and zinc has emerged as a key regulatory element in this process. An early manifestation of a regulatory role for this transition metal is the significant increase in total intracellular zinc. This accumulation is essential for meiotic progression beyond telophase I and the establishment of meiotic arrest at metaphase II. The subsequent developmental event, fertilization, induces a rapid expulsion of labile zinc that is a hallmark event in meiotic resumption. In the present study, we show that the zinc fluxes work, in part, by altering the activity of the cytostatic factor (CSF), the cellular activity required for the establishment and maintenance of metaphase II arrest in the mature, unfertilized egg. We propose a model in which zinc exerts concentration-dependent regulation of meiosis through the CSF component EMI2, a zinc-binding protein. Together, the data support the conclusion that zinc itself, through its interaction with EMI2, is a central component of the CSF.

c-Mos proteolysis is independent of the CA(2+) rise induced by 6-DMAP in Xenopus oocytes.

In Xenopus oocytes, metaphase II arrest is due to a cytostatic factor (CSF) that involves c-Mos, maintaining a high MPF (cdk1/cyclin B) activity in the cell. At fertilization, a rise in intracellular calcium triggers the proteolysis of both cyclin B and c-Mos. The kinase inhibitor 6-dimethylaminopurine (6-DMAP) is also able to release matured Xenopus oocytes from metaphase II block. This is characterized by c-Mos proteolysis without degradation of cyclin B. We hypothesized that 6-DMAP induced an increase in intracellular calcium. Using the calcium-sensitive fluorescent dye Fura-2, we observed a systematic increase in intracellular calcium following 6-DMAP application. In matured oocytes previously microinjected with the calcium chelator BAPTA, no calcium changes occurred after 6-DMAP addition; however, c-Mos was still proteolysed. In oocytes at the GVBD stage, c-Mos proteolysis occurred in response to 6-DMAP but not to calcium ionophore treatment. We suggest that c-Mos proteolysis is rather controlled by a phosphorylation-dependent process.

Enhancement of mouse egg activation by the kinase inhibitor, 6-dimethylaminopurine (6-DMAP).

Metaphase arrest in vertebrate eggs is maintained by the action of cytostatic factor (CSF) on maturation promoting factor (MPF). In amphibian eggs, the kinase inhibitor, 6-dimethylaminopurine (6-DMAP) inactivates both CSF and MPF, resulting in the release from metaphase and entry into interphase. In the mouse, 6-DMAP induces nuclear formation in maturing oocytes, but not in eggs at metaphase II. We found that 6-DMAP accelerated the transition to interphase in mouse eggs treated with the parthenogenetic agents A23187, cycloheximide, or phorbol 12-myristate 13-acetate (PMA). Newly ovulated eggs, refractory to full activation by A23187, entered interphase when treated with A23187 and 6-DMAP in combination. Occasional batches of eggs formed nuclei in response to 6-DMAP alone. These results are discussed with regard to the possible effects of 6-DMAP on MPF and CSF.