Cyclin A2/cyclin-dependent kinase 1-dependent phosphorylation of Top2a is required for S phase entry during retinal development in zebrafish

Cyclin-dependent kinase 1 (CDK1) plays an essential role in cell cycle regulation.However,as mouse Cdk1embryos die early,the role of CDK1 in regulating the cell cycle and embryo development remains unclear.Here,we showed that zebrafish cdk1~(-/-)embryos exhibit severe microphthalmia accompanied by multiple defects in S phase entry,M phase progression,and cell differentiation but not in interkinetic nuclear migration.We identified Top2a as a potential downstream target and cyclin A2 and cyclin B1 as partners of Cdk1 in cell cycle regulation via an in silico analysis.While depletion of either cyclin A2 or Top2a led to the decreased S phase entry in zebrafish retinal cells,the depletion of cyclin B1 led to M phase arrest.Moreover,phosphorylation of Top2a at serine 1213 (S1213) was nearly abolished in both cdk1 and ccna2mutants,but not in ccnb1 mutants.Furthermore,overexpression of TOP2A~(S1213D),the phosphomimetic form of human TOP2A,rescued S phase entry and alleviated the microphthalmia defects in both cdk1~(-/-)and ccna2~(-/-)embryos.Taken together,our data suggest that Cdk1 interacts with cyclin A2 to regulate S phase entry partially through Top2a phosphorylation and interacts with cyclin B1 to regulate M phase progression.     

Roles of cyclin D1 and related genes in growth inhibition, senescence and apoptosis

It is now apparent that apoptosis is closely linked to the control of cell cycle progression. During the G1 to S progression, cyclin D1, p53, and the cyclin dependent kinase inhibitors p21^WAF1 and p27^kip1 can play roles in induction of apoptosis. During the G2 and M phases, premature activation of Cdk1 can cause cells to enter mitotic catastrophe, which results in apoptosis. In this review we focus on factors acting during G1 and S, particularly cyclin D1, and their effects on cell growth, senescence and apoptosis. We emphasize that cyclin D1 can have diverse effects on cells depending on its level of expression, the specific cell type, the cell context and other factors. Possible mechanisms by which cyclin D1 exerts these diverse effects, via cyclin dependent kinase-dependent and -independent pathways, are discussed.     

Patterns of expression of cyclins A, B1, D, E and cdk 2 in preimplantation mouse embryos

Cell cycle regulatory proteins have been characterized in somatic cells and exhibit phase-specific expression patterns. Changes in expression of these regulatory proteins have not been clearly characterized in early preimplantation mouse embryos. This study utilized indirect immunofluorescence to determine the expression pattern of G1/S phase cyclins D and E; S, G2/M phase cyclins A and B1, and cdk 2 during the first three cell cycles of mouse embryo development. Cyclin D demonstrated low expression throughout the first cell cycle but had a somatic-like pattern of expression in cycles 2 and 3 with peak expression at G1 declining through S phase to a low during G2. Cyclin E was present at peak levels in G1 declining through S to a low in G2 during both the first and third cell cycles, but remained at moderate levels during the entire second cell cycle. Cyclin A was maintained at moderate levels throughout the first two cell cycles but showed a somatic-like pattern with a low level in G1 increasing during S phase with peak levels during G2 of the third cell cycle. Cyclin B consistently demonstrated a pattern opposite to a somatic G2/M cyclin, with peak levels in G1 declining through S phase to a low in G2 during each of the three cell cycles examined. Cdk 2 was present at consistent levels during G1 and S phases of all three cell cycles declining slightly in G2.     

Dominant negative E2F inhibits progression of the cell cycle after the midblastula transition in Xenopus

The cleavage cycle, which is initiated by fertilization, consists of only S and M phases, and the gap phases (G1 and G2) appear after the midblastula transition (MBT) in the African clawed frog, Xenopus laevis. During early development in Xenopus, we examined the E2F activity, which controls transition from the G1 to S phase in the somatic cell cycle. Gel retardation and transactivation assays revealed that, although the E2F protein was constantly present throughout early development, the E2F transactivation activity was induced in a stage-specific manner, that is, low before MBT and rapidly increased after MBT. Introduction of the recombinant dominant negative E2F (dnE2F), but not the control, protein into the 2-cell stage embryos specifically suppressed E2F activation after MBT. Cells in dnE2F-injected embryos appeared normal before MBT, but ceased to proliferate and eventually died at the gastrula. These cells contained decreased cdk activity with enhanced inhibitory phosphorylation of Cdc2 at Tyr15. Thus, E2F activity is required for cell cycle progression and cell viability after MBT, but not essential for MBT transition and developmental progression during the cleavage stage.