Previously, the connection between cell proliferation and Wnt signaling focused on transcriptional activation of cyclin D1 and c-myc, which control the G1/S transition of the cell cycle. In this issue of Developmental Cell, the Niehrs group demonstrates mitotic activation of Wnt signaling by a novel Cdk/cyclin complex containing Cdk14 (PFTK1) and cyclin Y.

Cdk2 was once believed to play an essential role in cell cycle progression, but cdk2-/- mice have minimal phenotypic abnormalities. In this study, we examined the role of cdk2 in hepatocyte proliferation, centrosome duplication and survival. Cdk2-/- hepatocytes underwent mitosis and had normal centrosome content after mitogen stimulation. Unlike wild-type cells, cdk2-/- liver cells failed to under

Early studies on cell cycle regulation were based on experiments in model systems (Yeast, Xenopus, Starfish, Drosophila) and have shaped the way we understand many events that control the cell cycle. Although these model systems are of great value, the last decade was highlighted by studies done in human cells and using in vivo mouse models. Mouse models are irreplaceable tools for understanding t

After a decade of extensive work on gene knockout mouse models of cell-cycle regulators, the classical model of cell-cycle regulation was seriously challenged. Several unexpected compensatory mechanisms were uncovered among cyclins and Cdks in these studies. The most astonishing observation is that Cdk2 is dispensable for the regulation of the mitotic cell cycle with both Cdk4 and Cdk1 covering fo

Once it was believed that Cdk2 was the master regulator of S phase entry. Gene knockout mouse studies of cell cycle regulators revealed that Cdk2 is dispensable for S phase initiation and progression whereby Cdk1 can compensate for the loss of Cdk2. Nevertheless, recent evidence indicates that Cdk2 is involved in cell cycle independent functions such as DNA damage repair. Whether these properties

The loss of p53 induces spontaneous tumors in mice, and p53 mutations are found in approximately 50% of human tumors. These tumors are generally caused by a number of events, including genomic instability, checkpoint defects, mitotic defects, deregulation of transcriptional targets, impaired apoptosis, and G 1 deregulation or a combination of these effects. In order to determine the role of protei

The G1/S-phase transition is a well-toned switch in the mammalian cell cycle. Cdk2, Cdk4, and the rate-limiting tumor suppressor retinoblastoma protein (Rb) have been studied in separate animal models, but interactions between the kinases and Rb in vivo have yet to be investigated. To further dissect the regulation of the G1 to S-phase progression, we generated Cdk2-/-Cdk4-/-Rb-/- (TKO) mutant mic

It was believed that Cdk2-cyclin E complexes are essential to drive cells through the G1-S phase transition. However, it was discovered recently that the mitotic kinase Cdk1 (Cdc2a) compensates for the loss of Cdk2. In the present study, we tested whether Cdk2 can compensate for the loss of Cdk1. We generated a knockin mouse in which the Cdk2 cDNA was knocked into the Cdk1 locus (Cdk1Cdk2KI). Subs

Germ-line mutations in BRCA2 have been linked to early-onset familial breast cancer. BRCA2 is known to play a key role in repairing double-strand breaks. Here,we describe the involvement of BRCA2 in O6-alkylguanine DNA alkyltransferase (AGT)-mediated repair of O6-methylguanine adducts. We show that BRCA2 physically associates and undergoes repair-mediated degradation with AGT. In contrast, BRCA2 w

Ischemia/reperfusion (I/R) injury to the heart is accompanied by the upregulation and posttranslational modification of a number of proteins normally involved in regulating cell cycle progression. Two such proteins, cyclin-dependent kinase-2 (Cdk2) and its downstream target, the retinoblastoma gene product (Rb), also play a critical role in the control of apoptosis. Myocardial ischemia activates C

The p53 tumor suppressor restricts tumorigenesis through the transcriptional activation of target genes involved in cell-cycle arrest and apoptosis. Here, we identify Prl-3 (phosphatase of regenerating liver-3) as a p53-inducible gene. Whereas previous studies implicated Prl-3 in metastasis because of its overexpression in metastatic human colorectal cancer and its ability to promote invasiveness

Cdk1 was proposed to compensate for the loss of Cdk2. Here we present evidence that this is possible due to premature translocation of Cdk1 from the cytoplasm to the nucleus in the absence of Cdk2. We also investigated the consequence of loss of Cdk2 on the maintenance of the G1/S DNA damage checkpoint. Cdk2-/- mouse embryonic fibroblasts in vitro as well as regenerating liver cells after partial

Granule neurons of the dentate gyrus (DG) of the hippocampus undergo continuous renewal throughout life. Among cell cycle regulators, cyclin-dependent kinase 2 (Cdk2) is considered as a major regulator of S-phase entry. We used Cdk2-deficient mice to decipher the requirement of Cdk2 for the generation of new neurons in the adult hippocampus. The quantification of cell cycle markers first revealed

We investigated the function of cyclin-dependent kinase 2 (Cdk2) in neural progenitor cells during postnatal development. Chondroitin sulfate proteoglycan (NG2)-expressing progenitor cells of the subventricular zone (SVZ) show no significant difference in density and proliferation between Cdk2-/- and wild-type mice at perinatal ages and are reduced only in adult Cdk2 -/- mice. Adult Cdk2-/- SVZ ce

In humans, the molecular mechanisms underlying ovarian follicle endowment and activation, which are closely related to the control of female reproduction, occurrence of menopause, and related diseases such as premature ovarian failure, are poorly understood. In the current study, we provide several lines of genetic evidence that the cyclin-dependent kinase (Cdk) inhibitor 1B (commonly known as p27

Cell cycle regulation is essential for proper homeostasis of hematopoietic cells. Cdk2 is a major regulator of S phase entry, is activated by mitogenic cytokines, and has been suggested to be involved in antigen-induced apoptosis of T lymphocytes. The role of Cdk2 in hematopoietic cells and apoptosis in vivo has not yet been addressed. To determine whether Cdk2 plays a role in these cells, we perf

Inactivation of cyclin-dependent kinases (Cdks) and/or cyclins in mice has changed our view of cell cycle regulation. In general, cells are far more resistant to the loss of Cdks than originally anticipated, suggesting widespread compensation among the Cdks. Early embryonic cells are, so far, not sensitive to the lack of multiple Cdks or cyclins. In contrast, differentiated cells are more dependen

Centrosomes organize the microtubule cytoskeleton for both interphase and mitotic functions. They are implicated in cell-cycle progression but the mechanism is unknown. Here, we show that depletion of 14 out of 15 centrosome proteins arrests human diploid cells in G1 with reduced Cdk2-cyclin A activity and that expression of a centrosome-disrupting dominant-negative construct gives similar results

How extracellular signals communicate with the cell cycle is poorly understood. In this issue, two papers (Grimmler et al., 2007; Chu et al., 2007) address this problem by reporting phosphorylation of the cyclin-dependent kinase inhibitor p27Kip1 on a tyrosine residue by nonreceptor tyrosine kinases, which decreases p27 stability. This new mechanism could explain how cells enter the cell cycle fro