Fig. 3

Mechanism of cAMP-regulated oocyte MI arrest and resumption. (A) Oocytes are arrested at prophase I by the following two mechanisms. (1) First, cAMP levels in oocytes are increased by exogenous cAMP from granulosa cells and cAMP generated in oocytes via the self-activated GPCRs-Gs-AC cascade. (2) Second, cGMP inhibits the hydrolytic activity of PDE3A, which in turn inhibits cAMP hydrolysis in oocytes. These two pathways work together to arrest oocytes at the diplotene stage of prophase of MI until the LH peak re-initiates meiosis. (B) Downregulation of cAMP levels in oocytes is induced by an increase in LH secretion. In the presence of low cAMP levels, Wee1B kinase remains inactive by activating PKA, while CDC25B phosphatase activity is increased, thereby activating MPF, which acts on the phosphorylation pathways of other meiosis-associated proteins to accelerate meiotic resumption