Naturally occurring steroids in Xenopus oocyte during meiotic maturation. Unexpected presence and role of steroid sulfates.

In the ovary, oocytes are surrounded by follicle cells and arrested in prophase of meiosis I. Although steroidogenic activity of follicle cells is involved in oogenesis regulation, clear qualitative and quantitative data about the steroid content of follicles are missing. We measured steroid levels of Xenopus oocytes and follicles by gas chromatography-mass spectrometry.

Differential regulation of Cdc2 and Aurora-A in Xenopus oocytes: a crucial role of phosphatase 2A.

The success of cell division relies on the activation of its master regulator Cdc2-cyclin B, and many other kinases controlling cellular organization, such as Aurora-A. Most of these kinase activities are regulated by phosphorylation. Despite numerous studies showing that okadaic acid-sensitive phosphatases regulate both Cdc2 and Aurora-A activation, their identity has not yet been established in Xenopus oocytes and the importance of their regulation has not been evaluated.

How does Xenopus oocyte acquire its competence to undergo meiotic maturation?

During Xenopus oogenesis, the follicle-enclosed oocyte, arrested at the diplotene stage of meiotic prophase, accumulates pre-MPF. Pre-MPF is an heterodimer formed of cyclin B2 and Cdc2 protein kinase, which is maintained inactive by inhibitory phosphorylations on Thr14 and Tyr15. When the oocyte reaches its full size, it becomes competent to respond to progesterone and to activate MPF through a positive feedback loop.

Polo-like kinase confers MPF autoamplification competence to growing Xenopus oocytes.

During oogenesis, the Xenopus oocyte is blocked in prophase of meiosis I. It becomes competent to resume meiosis in response to progesterone at the end of its growing period (stage VI of oogenesis). Stage IV oocytes contain a store of inactive pre-MPF (Tyr15-phosphorylated Cdc2 bound to cyclin B2); the Cdc25 phosphatase that catalyzes Tyr15 dephosphorylation of Cdc2 is also present.

Xenopus H-RasV12 promotes entry into meiotic M phase and cdc2 activation independently of Mos and p42(MAPK).

In the Xenopus oocyte, progesterone triggers M phase Promoting Factor (MPF) activation in a protein synthesis dependent manner. Although the synthesis of the p42(MAPK) activator Mos appears to be required for MPF activation, p42(MAPK) activity has been shown to be dispensable. To clarify this paradox, we attempted to activate the p42(MAPK) pathway independently of Mos synthesis by cloning and using Xenopus H-Ras in the oocyte.

Mos is not required for the initiation of meiotic maturation in Xenopus oocytes.

In Xenopus oocytes, the c-mos proto-oncogene product has been proposed to act downstream of progesterone to control the entry into meiosis I, the transition from meiosis I to meiosis II, which is characterized by the absence of S phase, and the metaphase II arrest seen prior to fertilization. Here, we report that inhibition of Mos synthesis by morpholino antisense oligonucleotides does not prevent the progesterone-induced initiation of Xenopus oocyte meiotic maturation, as previously thought. Mos-depleted oocytes complete meiosis I but fail to arrest at metaphase II, entering a series of embryonic-like cell cycles accompanied by oscillations of Cdc2 activity and DNA replication.

Thr-161 phosphorylation of monomeric Cdc2. Regulation by protein phosphatase 2C in Xenopus oocytes.

Fully grown Xenopus oocyte is arrested at prophase I of meiosis. Re-entry into meiosis depends on the activation of MPF (M-phase promoting factor or cyclin B.Cdc2 complex), triggered by progesterone.