The switch from cAMP-independent to cAMP-dependent arrest of meiotic prophase is associated with coordinated GPR3 and CDK1 expression in mouse oocytes

Mammalian oocytes are arrested in meiotic prophase from around the time of birth until just before ovulation. Following an extended period of growth, they are stimulated to mature to the metaphase II stage by a preovulatory luteinizing hormone (LH) surge that occurs with each reproductive cycle. Small, growing oocytes are not competent to mature into fertilizable eggs because they do not possess adequate amounts of cell cycle regulatory proteins, particularly cyclin-dependent kinase 1 (CDK1). As oocytes grow, they synthesize CDK1 and acquire the ability to mature. After oocytes achieve meiotic competence, meiotic arrest at the prophase stage is dependent on high levels of cAMP that are generated in the oocyte under the control of the constitutively active G(s)-coupled receptor, GPR3. In this study, we examined the switch between GPR3-independent and GPR3-dependent meiotic arrest. We found that the ability of oocytes to mature, as well as oocyte CDK1 levels, were dependent on follicle size, but CDK1 expression in oocytes from preantral follicles was not acutely altered by the activity of follicle stimulating hormone (FSH). Gpr3 was expressed and active in incompetent oocytes within early stage follicles, well before cAMP is required to maintain meiotic arrest. Oocytes from Gpr3(-/-) mice were less competent to mature than oocytes from Gpr3(+/+) mice, as assessed by the time course of germinal vesicle breakdown. Correspondingly, Gpr3(-/-) oocytes contained significantly lower CDK1 levels than their Gpr3(+/+) counterparts that were at the same stage of follicle development. These results demonstrate that GPR3 potentiates meiotic competence, most likely by raising cAMP.