Prophase I arrest and progression to metaphase I in mouse oocytes: comparison of resumption of meiosis and recovery from G2-arrest in somatic cells

Abstract

Mammalian oocytes are arrested at prophase I until puberty when luteinizing hormone (LH) induces resumption of meiosis of follicle-enclosed oocytes. Resumption of meiosis is tightly coupled with regulating cyclin-dependent kinase 1 (CDK1) activity. Prophase I arrest depends on inhibitory phosphorylation of CDK1 and anaphase-promoting complex-(APC-CDH1)-mediated regulation of cyclin B levels. Prophase I arrest is maintained by endogenously produced cyclic adenosine monophosphate (cAMP), which activates protein kinase A (PKA) that in turn phosphorylates (and activates) the nuclear kinase WEE2. In addition, PKA-mediated phosphorylation of the phosphatase CDC25B results in its cytoplasmic retention. The combined effect maintains low levels of CDK1 activity that are not sufficient to initiate resumption of meiosis. LH triggers synthesis of epidermal growth factor-like factors in mural granulosa cells and leads to reduced cGMP transfer from cumulus cells to oocytes via gap junctions that couple the two cell types. cGMP inhibits oocyte phosphodiesterase 3A (PDE3A) and a decline in oocyte cGMP results in increased PDE3A activity. The ensuing decrease in oocyte cAMP triggers maturation by alleviating the aforementioned phosphorylations of WEE2 and CDC25B. As a direct consequence CDC25B translocates into the nucleus. The resulting activation of CDK1 also promotes extrusion of WEE2 from the nucleus thereby providing a positive amplification mechanism for CDK1 activation. Other kinases, e.g. protein kinase B, Aurora kinase A and polo-like kinase 1, also participate in resumption of meiosis. Mechanisms governing meiotic prophase I arrest and resumption of meiosis share common features with DNA damage-induced mitotic G2-checkpoint arrest and checkpoint recovery, respectively. These common features include CDC14B-dependent activation of APC-CDH1 in prophase I arrested oocytes or G2-arrested somatic cells, and CDC25B-dependent cell cycle resumption in both oocytes and somatic cells.

Figure 1

CDK1 and its regulators during resumption of meiosis. (A) Inhibition of CDK1 during prophase I arrest and CDK1 activation during resumption of meiosis. (B) Localization of cyclin B (CCNB1), phosphatases CDC25A and CDC25B, kinases WEE2 and MYT1 during prophase I arrest and CDC25B and CCNB1 nuclear translocation and partial WEE2 nuclear extrusion shortly before GVBD. The dependency of the changes in localization of cAMP and CDK1 is marked.

Figure 2

Effect of genetic disruption of molecules involved in oocytes cAMP production (GPR3 or ADCY3) or cAMP destruction (PDE3A) on the maintenance of prophase I arrest at oocytes in early antral follicles and during LH-induced resumption of meiosis, respectively.

Figure 3

Regulation of anaphase-promoting complex with CDH1 co-activator (APC–CDH1) during prophase I arrest. Molecules inhibiting resumption of meiosis are in red and those stimulating resumption of meiosis are green. PTTG1, pituitary tumour-transforming gene (securin); CDC14B, phosphatase; EMI1, early mitotic inhibitor 1; CCNB1, cyclin B; BUBR1, cell cycle spindle check point protein kinase.

Figure 4

Role of AURKA and PLK1 during resumption of meiosis. (A) Effect of pharmacological inhibition of AURKA by MLN8054 (Hoar et al., 2007) and PLK1 by BI2536 (Lenart et al., 2007) on the kinetics of GVBD. The indicated times are in hours, and ON indicates that the oocytes were cultured overnight for 18 h. (B) PLK1 is not responsible for nuclear CDC25B translocation. Shown are oocytes expressing a small amount of exogenous GFP-CDC25B. In the control group and experimental group, in which PLK1 is inhibited with BI2536, shortly before GVBD there is a clear translocation of CDC25B to the nucleus. Note that in prophase I arrested oocytes with a high cAMP level that CDC25B is strictly cytoplasmic (Fig. 1B).

Figure 5

Schematic diagram depicting signalling pathway in the follicle after LH peak leading to the resumption of meiosis in oocytes. AREG, amphiregulin; EREG, epiregulin; MAPK, mitogen-activated protein kinase.