Molecular mechanisms of spontaneous activation in rat eggs

Abstract

The aim of this research was to identify the molecular mechanisms that promote spontaneous activation in rat eggs after their recovery from the oviduct. Typically, mammalian eggs await fertilisation arrested at the second metaphase II of meiosis. However, ovulated rat eggs spontaneously enter anaphase II when exposed to in vitro culture. After extrusion of the second polar body, these spontaneously activated eggs do not proceed to interphase but become arrested at metaphase III stage with chromatids scattered in the egg cytoplasm. This instability may be one factor that has made it more difficult to establish reliable protocols for somatic cell nuclear transfer in rats. The triggers of spontaneous activation and signalling pathways leading to the metaphase III progression are largely unknown. Analyses of signalling pathways that are involved in the regulation of final stages of meiosis during fertilisation revealed several anomalies that were associated with spontaneous activation and the transition from metaphase II to metaphase III. Metaphase II arrested eggs usually exhibit an increased level of maturation promoting factor (MPF) activity. Spontaneous activation in rat eggs was associated with a drop in MPF activity at the time of the second polar body extrusion. MPF is composed of a catalytic subunit, CDK1, and a regulatory subunit, cyclin B1. Interestingly, the level of cyclin B1 was stable throughout spontaneous activation. Post-translational modifications of CDK1 can influence MPF activity: whereas no inhibitory phosphorylation on Tyr15 of CDK1 was found; a decrease in activating Thr161 phosphorylation of CDK1 was associated with the time of the second polar body extrusion, and hence could contribute to the transient MPF inactivation. MAPK (p42/p44) activity has been shown to decrease during egg activation in fertilisation. By contrast, during spontaneous activation, MAPK (p42/p44) remained active and thus resembled the profile usually found between two meiotic divisions (metaphase I to metaphase II). Securin, a protein which prevents premature chromatid separation, was degraded in eggs going through spontaneous activation. Cytostatic factor (CSF) is a biochemical activity, which enables stable metaphase II arrest in ovulated eggs of vertebrates. Recently, the endogenous meiotic inhibitor 2, EMI2, was confirmed as the major component of CSF. For egg activation to occur, the CSF must be destroyed. At the beginning of egg activation, Ca2+/calmodulin kinase (CaMKII) promotes posttranslational modifications of EMI2, leading to its degradation. In the rat, inhibition of CaMKII activity stably prevented the onset of spontaneous activation in a subset of metaphase II eggs. However, no degradation of EMI2 protein was found at the start of abortive metaphase II exit. This finding revealed that one of the central elements of the CSF pathway, EMI2, could be preserved in the rat eggs going through spontaneous activation. In order to study the mechanisms regulating EMI2 stability in rat oocyte maturation and spontaneous activation, functional analysis of ectopically expressed synthetic mRNA was performed. The mechanism enabling EMI2 degradation became active 12 hours after the start of oocyte maturation. The C-terminal fragment of EMI2, known to be non-degradable in Xenopus oocyte maturation, was significantly more stable than the full-length counterpart in matured rat eggs but not during oocyte maturation. Interestingly, C-terminal EMI2 became degraded in parthenogenetic rat embryos. This indicated that additional not previously reported mechanisms responsible for EMI2 degradation might exist in the rat. The microinjection of metaphase II rat eggs with the C-terminal fragment of EMI2 or IVT full-length EMI2 protein had little effect on the progression of spontaneous activation. Taken together, these observations suggest that abortive spontaneous activation in rat eggs was a result of incomplete engagement of signalling pathways normally triggered in fertilisation or parthenogenetic activation. Activation of CaMKII initiated pathways that allowed anaphase entry and chromatid segregation. At the same time, not all pathways normally triggered during fertilisation or parthenogenetic activation were fully engaged, possibly due to the presence of non-degraded component of CSF. Abortive incomplete activation results in the re-establishment of high level of MPF activity in metaphase III eggs. Early prevention of CaMKII activation, perhaps by blocking [Ca2+ i] signalling, may provide a means of holding ovulated eggs at metaphase II prior to enucleation and somatic cell nuclear transfer.