Molecular mechanism of oocyte maturation

Maturation of vertebrate oocytes is regulated by maturation inducing hormone (MIH), which is progesterone in all vertebrates except in fish, where it is 17alpha, 20beta dihydroxy progesterone. Once the full growth of the oocytes is achieved, they arrest at prophase of meiosis I. MIH releases oocytes from this arrest. MIH promotes the formation of a dimeric protein kinase complex known as maturation promoting factor (MPF), the regulatory component of which is cyclin B and the catalytic component is cell division cycle (Cdc2) kinase. This complex is activated by phosphorylation at Thrl61 but remains inactive due to the inhibitory phosphorylation at Thrl4 and Tyrl5. MIH stimulates Cdc25, a dual specific phosphatase, that dephosphorylates both Thrl4 and Tyrl5 and converts pre- or inactive MPF to active MPF. Germinal vesicle break down (GVBD) is the marker of oocyte maturation. In an Indian freshwater perch, Anabas testudineus, MIH induced GVBD between 18-20 h. MIH induced oocytes extract in SDS-PAGE showed over-expression of a 30 kDa protein, which is confirmed to be cyclin B by using both monoclonal and polyclonal anti-cyclin B antibodies from various sources. The size of cyclin B in other vertebrates including mammals lies between 46-55 kDa. We have cloned cyclin B gene from perch oocyte and found it to contain the domains required for its function and immunological recognition. We also cloned Cdkl gene, which is very similar to other vertebrates Cdkl. Perch oocyte Cdc25 is overexpressed prior to GVBD converting inactive MPF to active MPF that affect GVBD. The objective of this overview is to deal with the molecular regulation of MPF activation which causes final maturation of oocytes.