Antioxidants stimulate meiosis resumption, but inhibit mitogen-activated protein kinase phosphorylation and further cell cycle progression in porcine oocytes

Inhibition of germinal vesicle breakdown by antioxidants and the roles of signaling pathways related to nitric oxide and cGMP during meiotic resumption in oocytes of a marine worm

In mammalian oocytes, cAMP elevations prevent the resumption of meiotic maturation and thereby block nuclear disassembly (germinal vesicle breakdown (GVBD)), whereas nitric oxide (NO) and its downstream mediator cGMP can either inhibit or induce GVBD. Alternatively, some invertebrate oocytes use cAMP to stimulate, rather than inhibit, GVBD, and in such cases, the effects of NO/cGMP signaling on GVBD remain unknown. Moreover, potential interactions between NO/cGMP and AMP-activated kinase (AMPK) have not been assessed during GVBD. Thus, this study analyzed intraoocytic signaling pathways related to NO/cGMP in a marine nemertean worm that uses cAMP to induce GVBD. For such tests, follicle-free nemertean oocytes were stimulated to mature by seawater (SW) and cAMP elevators. Based on immunoblots and NO assays of maturing oocytes, SW triggered AMPK deactivation, NO synthase (NOS) phosphorylation, and an NO elevation. Accordingly, SW-induced GVBD was blocked by treatments involving the AMPK agonist AICAR, antioxidants, the NO scavenger carboxy-PTIO, NOS inhibitors, and cGMP antagonists that target the NO-stimulated enzyme, soluble guanylate cyclase (sGC). Conversely, SW solutions combining NO/cGMP antagonists with a cAMP elevator restored GVBD. Similarly, AICAR plus a cAMP-elevating drug reestablished GVBD while deactivating AMPK and phosphorylating NOS. Furthermore, sGC stimulators and 8-Br-cGMP triggered GVBD. Such novel results indicate that NO/cGMP signaling can upregulate SW-induced GVBD and that cAMP-elevating drugs restore GVBD by overriding the inhibition of various NO/cGMP downregulators, including AMPK. Moreover, considering the opposite effects of intraoocytic cAMP in nemerteans vs mammals, these data coincide with previous reports that NO/cGMP signaling blocks GVBD in rats.

Mechanisms Regulating Oocyte Meiotic Resumption: Roles of Mitogen-Activated Protein Kinase

Oocyte meiotic maturation is one of the important physiological requirements for species survival. However, little is known about the detailed events occurring during this process. A number of studies have demonstrated that MAPK plays a pivotal role in the regulation of meiotic cell cycle progression in oocytes, but controversial findings have been reported in both lower vertebrates and mammals. In this review, we summarized the roles of MAPK cascade and related signal pathways in oocyte meiotic reinitiation in both lower vertebrates and mammals. We also tried to reconcile the paradoxical results and highlight the new findings concerning the function of MAPK in both oocytes and the surrounding follicular somatic cells. The unresolved questions and future research directions regarding the role of MAPK in meiotic resumption are addressed.

Regulation of mitogen-activated protein kinase phosphorylation, microtubule organization, chromatin behavior, and cell cycle progression by protein phosphatases during pig oocyte maturation and fertilization in vitro

We used okadaic acid (OA), a potent inhibitor of protein phosphatases 1 and 2A, to study the regulatory effects of protein phosphatases on mitogen-activated protein (MAP) kinase phosphorylation, morphological changes in the nucleus, and microtubule assembly during pig oocyte maturation and fertilization in vitro. When germinal vesicle (GV) stage oocytes were exposed to OA, MAP kinase phosphorylation was greatly accelerated, being fully activated at 10 min. However, MAP kinase was dephosphorylated by long-term (>20 h) exposure to OA. Correspondingly, premature chromosome condensation and GV breakdown were accelerated, whereas meiotic spindle assembly and meiotic progression beyond metaphase I stage were inhibited. OA also quickly reversed the inhibitory effects of butyrolactone I, a specific inhibitor of maturation-promoting factor (MPF), on MAP kinase phosphorylation and meiosis resumption. Treatment of metaphase II oocytes triggered metaphase II spindle elongation and disassembly as well as chromosome alignment disruption. OA treatment of fertilized eggs resulted in prompt phosphorylation of MAP kinase, disassembly of microtubules around the pronuclear area, chromatin condensation, and pronuclear membrane breakdown, but inhibited further cleavage. Our results suggest that inhibition of protein phosphatases promptly phosphorylates MAP kinase, induces premature chromosome condensation and meiosis resumption as well as pronucleus breakdown, but inhibits spindle organization and suppresses microtubule assembly by sperm centrosomes in pig oocytes and fertilized eggs.