Ca2+-promoted cyclin B1 degradation in mouse oocytes requires the establishment of a metaphase arrest

Abstract

CDK1-cyclin B1 is a universal cell cycle kinase required for mitotic/meiotic cell cycle entry and its activity needs to decline for mitotic/meiotic exit. During their maturation, mouse oocytes proceed through meiosis I and arrest at second meiotic metaphase with high CDK1-cyclin B1 activity. Meiotic arrest is achieved by the action of a cytostatic factor (CSF), which reduces cyclin B1 degradation. Meiotic arrest is broken by a Ca2+ signal from the sperm that accelerates it. Here we visualised degradation of cyclin B1::GFP in oocytes and found that its degradation rate was the same for both meiotic divisions. Ca2+ was the necessary and sufficient trigger for cyclin B1 destruction during meiosis II; but it played no role during meiosis I and furthermore could not accelerate cyclin B1 destruction during this time. The ability of Ca2+ to trigger cyclin B1 destruction developed in oocytes following a restabilisation of cyclin B1 levels at about 12 h of culture. This was independent of actual first polar body extrusion. Thus, in metaphase I arrested oocytes, Ca2+ would induce cyclin B1 destruction and the first polar body would be extruded. In contrast to some reports in lower species, we found no evidence that oocyte activation was associated with an increase in 26S proteasome activity. We therefore conclude that Ca2+ mediates cyclin B1 degradation by increasing the activity of an E3 ubiquitin ligase. However, this stimulation occurs only in the presence of the ubiquitin ligase inhibitor CSF. We propose a model in which Ca2+ directly stimulates destruction of CSF during mammalian fertilisation.

Establishment of the mammalian membrane block to polyspermy: evidence for calcium-dependent and -independent regulation

“…Three different cell cycle resumption endpoints were assessed, as was done previously (Ducibella et al 2002): (1) metaphase II arrest, (2) full progression to embryonic interphase, and (3) metaphase III which is an intermediate state in which the egg completes meiosis but does not continue to embryonic interphase (as a result of M-phase promoting factor (MPF) activity decreasing only transiently, due to insufficient [Ca 2C ] cyt increases, and then returning to high levels (Kubiak 1989, Ducibella et al 2002, Hyslop et al 2004). As the post-fertilization Ca 2C transients were increasingly attenuated, the proportion of eggs exiting metaphase II arrest gradually decreased, while the proportion of eggs remaining arrested at metaphase II increased (Supplementary Figure 1, which can be viewed online at http:// www.reproduction-online.org/supplemental/).…”

Effects of the conditioned medium of mesenchymal stem cells on mouse oocyte activation and development

“…Recent studies have demonstrated that [Ca 2+ ] i mediates the degradation of cyclin B1 by increasing the activity of an E3 ubiquitin ligase (29,30). Cyclin B1 is the regulatory subunit of M-phase promoting factor and its destruction is required for meiosis resumption (31,32 2+ ] i oscillations, was optimal for oocyte activation, and exhibited a better development than 1-and 2-h treatment, possibly reflecting the toxic effects of prolonged [Ca 2+ ] i oscillations.…”

The absence of a Ca2+ signal during mouse egg activation can affect parthenogenetic preimplantation development, gene expression patterns, and blastocyst quality

“…These sperminduced Ca 2C oscillations release the egg from meiotic arrest by targeting cyclin B for destruction by the proteosome (Hyslop et al 2004). Similar to meiotic resumption, the Ca 2C increase at fertilisation is responsible for triggering cortical granule exocytosis and pronuclear formation.…”