Molecular and morphological analyses of the initiation of cyclin B translation during oocyte maturation

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Oocyte maturation is triggered by the maturation-promoting factor (also known as the M-phase-promoting factor) (MPF) that consists of Cdc2 and Cyclin B. Synthesis of Cyclin B by translational activation of the dormant mRNA stored in oocytes is required for normal progression of maturation. In this study, we investigated the signal transduction pathways to cyclin B translation, the mechanisms of translational activation, and the action mechanisms of MPF.We examined the involvement of phosphatidylinositol 3-kinase (PI3K) and protein kinase B (PKB) in progesterone-induced Rana oocyte maturation. The obtained results suggest that PI3K, but not PKB, functions during the signal transduction pathway. To know the initiation mechanisms of cyclin B translation, we investigated Pumilio and cytoplasmic polyadenylation element-binding protein (CPEB), and suggested that Pumilio acts as a specific regulator that determines the timing of translational activation of cyclin B mRNA by its release from ph … More osphorylated CPEB during oocyte maturation.Investigations into the functions of Cyclins B1 and B2 using Rana oocytes provide evidence of the requirement of Cyclin B2, but not Cyclin B1, for organizing the bipolar spindle, though either Cyclin B1 or B2 is redundant for inducing germinal vesicle breakdown and chromosome condensation. MPF and mitogen-activated protein kinase (MAPK) are involved in the morphological changes that occur during maturation, but their precise roles are unknown. Using Rana oocytes, we clarified the functions of MPF and MAPK in each process of sequential morphological changes during oocyte maturation.Gamma-tubulin is essential for nucleation of microtubules at centrosomes, but little is known about its role during spindle formation in oocytes. We observed the behavior of gamma-tubulin during spindle formation in Xenopus oocytes, and found that gamma-tubulin is probably translocated by dynein to regulate microtubule organization leading to spindle formation. Significance of gamma-tubulin in spindle pole formation is also demonstrated by overexpression experiments. Less