Timing of calcium and protein synthesis requirements for the first mitotic cell cycle in fertilised Xenopus eggs

Punctuated cyclin synthesis drives early embryonic cell cycle oscillations

Cyclin B activates cyclin-dependent kinase 1 (CDK1) at mitosis, but conflicting views have emerged on the dynamics of its synthesis during embryonic cycles, ranging from continuous translation to rapid synthesis during mitosis. Here we show that a CDK1-mediated negative-feedback loop attenuates cyclin production before mitosis. Cyclin B plateaus before peak CDK1 activation, and proteasome inhibition caused minimal accumulation during mitosis. Inhibiting CDK1 permitted continual cyclin B synthesis, whereas adding nondegradable cyclin stalled it. Cycloheximide treatment before mitosis affected neither cyclin levels nor mitotic entry, corroborating this repression. Attenuated cyclin production collaborates with its destruction, since excess cyclin B1 mRNA accelerated cyclin synthesis and caused incomplete proteolysis and mitotic arrest. This repression involved neither adenylation nor the 3' untranslated region, but it corresponded with a shift in cyclin B1 mRNA from polysome to nonpolysome fractions. A pulse-driven CDK1-anaphase-promoting complex (APC) model corroborated these results, revealing reduced cyclin levels during an oscillation and permitting more effective removal. This design also increased the robustness of the oscillator, with lessened sensitivity to changes in cyclin synthesis rate. Taken together, the results of this study underscore that attenuating cyclin synthesis late in interphase improves both the efficiency and robustness of the CDK1-APC oscillator.

Temporal regulation of the first mitosis in Xenopus and mouse embryos

Cell cycle regulation in Eukaryotes is based on common molecular actors and mechanisms. However, the canonical cell cycle is modified in certain cells. Such modifications play a key role in oocyte maturation and embryonic development. They can be achieved either by introduction of new components, pathways, substrates, changed interactions between them, or by elimination of some factors inherited by the cells from previous developmental stages. Here we discuss a particular temporal regulation of the first embryonic M-phase of Xenopus and mouse embryos. These two examples help to understand better the general regulation of M-phase of the cell cycle.

Pre-M phase-promoting factor associates with annulate lamellae in Xenopus oocytes and egg extracts

We have used complementary biochemical and in vivo approaches to study the compartmentalization of M phase-promoting factor (MPF) in prophase Xenopus eggs and oocytes. We first examined the distribution of MPF (Cdc2/CyclinB2) and membranous organelles in high-speed extracts of Xenopus eggs made during mitotic prophase. These extracts were found to lack mitochondria, Golgi membranes, and most endoplasmic reticulum (ER) but to contain the bulk of the pre-MPF pool. This pre-MPF could be pelleted by further centrifugation along with components necessary to activate it. On activation, Cdc2/CyclinB2 moved into the soluble fraction. Electron microscopy and Western blot analysis showed that the pre-MPF pellet contained a specific ER subdomain comprising "annulate lamellae" (AL): stacked ER membranes highly enriched in nuclear pores. Colocalization of pre-MPF with AL was demonstrated by anti-CyclinB2 immunofluorescence in prophase oocytes, in which AL are positioned close to the vegetal surface. Green fluorescent protein-CyclinB2 expressed in oocytes also localized at AL. These data suggest that inactive MPF associates with nuclear envelope components just before activation. This association may explain why nuclei and centrosomes stimulate MPF activation and provide a mechanism for targeting of MPF to some of its key substrates.