Wagging the dogma; tissue-specific cell cycle control in the mouse embryo

The family of cyclin-dependent kinases (Cdks) lies at the core of the machinery that drives the cell division cycle. Studies in cultured mammalian cells have provided insight into the cellular functions of many Cdks. Recent Cdk and cyclin knockouts in the mouse show that the functions of G1 cell cycle regulatory genes are often essential only in specific cell types, pointing to our limited understanding of tissue-specific expression, redundancy, and compensating mechanisms in the Cdk network.     

Interplay between the p53 tumor suppressor protein family and Cdk5: novel therapeutic approaches for the treatment of neurodegenerative diseases using selective Cdk inhibitors

Cyclin-dependent kinases (Cdks) play a key role in orchestrating the coordination of cell cycle progression in proliferating cells. The escape from the proper control of the cell cycle by the upregulation of cyclins or aberrant activation of Cdks leads to malignant transformation. In quiescent cells and/or terminally differentiated cells, the expression pattern and activity of Cdks is altered. In postmitotic neurons, expression of mitotic kinases is downregulated, whereas Cdk5 expression becomes upregulated. Similarly to other Cdks, free Cdk5 displays no enzymatic activity and requires complex formation with a specific regulatory subunit. Two activators of Cdk5 have been identified. p35 and its isoform p39 bind to, and thereby activate, Cdk5. Unlike mitotic kinases, Cdk5 does not require activating phosphorylation within the T-loop. Because p35 is a short-lived protein, the p35/Cdk5 complexes are unstable. The stability of the p35 protein is regulated by its Cdk5-mediated phosphorylation of p35. Activated p35/Cdk5 kinase phosphorylates numerous physiological targets. The proper phosphorylation of the most important substrates, such as tau protein and neurofilament H, is essential for the correct regulation of the cytoskeletal organization, thereby regulating cell adhesion, motility, and synaptic plasticity. Moreover, Cdk5 regulates the activity of the p53 tumor suppressor via phosphorylation. p53 is upregulated in multiple neuronal death paradigms, including hypoxia, ischemia, and excitotoxicity, and plays a key role in the induction of apoptosis. On the other hand, an abnormally high expression and elevated activity of Cdk5 was observed in neurodegenerative diseases, suggesting the application of Cdk inhibitors for their therapy. Considering the action of some Cdk inhibitors on the expression and activity of the p53 protein, their therapeutic efficacy must be carefully evaluated.     

Interplay between the p53 tumor suppressor protein family and Cdk5

Cyclin-dependent kinases (Cdks) play a key role in orchestrating the coordination of cell cycle progression in proliferating cells. The escape from the proper, control of the cell cycle by the upregulation of cyclins or aberrant activation of Cdks leads to malignant transformation. In quiescent cells and/or terminally differentiated cells, the expression pattern and activity of Cdks is altered. In postmitotic neurons, expression of mitotic kinases is downregulated, whereas Cdk5 expression becomes upregulated. Similarly to other Cdks, free Cdk5 displays no enzymatic activity and requires complex formation with a specific regulatory subunit. Two activators of Cdk5 have been identified. p35 and its isoform p39 bind to, and thereby activate, Cdk5. Unlike mitotic kinases, Cdk5 does not require activating phosphorylation within the T-loop. Because p35 is a short-lived protein, the p35/Cdk5 complexes are unstable. The stability of the p35 protein is regulated by its Cdk5-mediated phosphorylation of p35. Activated p35/Cdk5 kinase phosphorylates numerous physiological targets. The proper phosphorylation of the most important substrates, such as τ protein and neurofilament H, is essential for the correct regulation of the cytoskeletal organization, thereby regulating cell adhesion, motility, and synaptic plasticity. Moreover, Cdk5 regulates the activity of the p53 tumor suppressor via phosphorylation. p53 is upregulated in multiple neuronal death paradigms, including hypoxia, ischemia, and excitotoxicity, and plays a key role in the induction of apoptosis. On the other hand, an abnormally high expression and elevated activity of Cdk5 was observed in neurodegenerative diseases, suggesting the application of Cdk inhibitors for their therapy. Considering the action of some Cdk inhibitors on the expression and activity of the p53 protein, their therapeutic efficacy must be carefully evaluated.     

Interplay Between the p53 Tumor Suppressor Protein Family and Cdk5: Novel Therapeutic Approaches for the Treatment of Neurodegenerative Diseases Using Selective Cdk Inhibitors.

Cyclin-dependent kinases (Cdks) play a key role in orchestrating the coordination of cell cycle progression in proliferating cells. The escape from the proper control of the cell cycle by the upregulation of cyclins or aberrant activation of Cdks leads to malignant transformation. In quiescent cells and/or terminally differentiated cells, the expression pattern and activity of Cdks is altered. In postmitotic neurons, expression of mitotic kinases is downregulated, whereas Cdk5 expression becomes upregulated. Similarly to other Cdks, free Cdk5 displays no enzymatic activity and requires complex formation with a specific regulatory subunit. Two activators of Cdk5 have been identified. p35 and its isoform p39 bind to, and thereby activate, Cdk5. Unlike mitotic kinases, Cdk5 does not require activating phosphorylation within the T-loop. Because p35 is a short-lived protein, the p35/Cdk5 complexes are unstable. The stability of the p35 protein is regulated by its Cdk5-mediated phosphorylation of p35. Activated p35/Cdk5 kinase phosphorylates numerous physiological targets. The proper phosphorylation of the most important substrates, such as tau protein and neurofilament H, is essential for the correct regulation of the cytoskeletal organization, thereby regulating cell adhesion, motility, and synaptic plasticity. Moreover, Cdk5 regulates the activity of the p53 tumor suppressor via phosphorylation. p53 is upregulated in multiple neuronal death paradigms, including hypoxia, ischemia, and excitotoxicity, and plays a key role in the induction of apoptosis. On the other hand, an abnormally high expression and elevated activity of Cdk5 was observed in neurodegenerative diseases, suggesting the application of Cdk inhibitors for their therapy. Considering the action of some Cdk inhibitors on the expression and activity of the p53 protein, their therapeutic efficacy must be carefully evaluated.     

Preferences for phosphorylation sites in the retinoblastoma protein of D-type cyclin-dependent kinases, Cdk4 and Cdk6, in vitro

D-type cyclin-dependent kinases (Cdk4 and Cdk6) regulate the G1 to S phase progression of the mammalian cell cycle. It has been suggested that Cdk4 and Cdk6 may have distinct functions in vivo, even though they are indistinguishable biochemically. Here we show that although these Cdks phosphorylate multiple residues in pRB, they do so with different residue selectivities in vitro; Thr821 and Thr826 are preferentially phosphorylated by Cdk6 and Cdk4, respectively. This raises the possibility different substrate specificities lead to their different roles in the regulation of cellular events. Furthermore, our results indicate the new concept that Cdk itself contributes to substrate recognition.     

F-box proteins: more than baits for the SCF?

Progression through the mammalian cell cycle is associated with the activity of four cyclin dependent kinases (Cdc2/Cdk1, Cdk2, Cdk4, and Cdk6). Knockout mouse models have provided insight into the interplay of these Cdks. Most of these models do not exhibit major cell cycle defects revealing redundancies, and suggesting that a single Cdk might be sufficient to drive the cell cycle, similar as in yeast. Recent work on Cdk2/Cdk4 double knockouts has indicated that these two Cdks are required to phosphorylate Rb during late embryogenesis. The lack of Rb phosphorylation is progressive and associated with reduced E2F-inducible gene expression. Cdk2 and Cdk4 share the essential function of coupling the G1/S transition with mitosis. However, proliferation in early embryogenesis appears to be independent of Cdk2 and Cdk4. We discuss these observations and propose molecular mechanisms that establish the requirement for Cdk2 and Cdk4 at the G1/S transition. We are considering that the balance between proliferation and differentiation is disturbed, which affects especially heart development and leads to embryonic lethality in Cdk2 ^-/- Cdk4 ^-/- mutants. We also discuss the specific functions of Cdk4 and Cdk6, which ironically do not compensate for each other.     

Cell cycle: driving the centrosome cycle.

Cyclin-dependent kinases (Cdks) control major transitions as cells pass through the cell cycle. It has recently been shown that centrosome duplication in vertebrates requires Cdk2 activity and can be driven solely by Cdk2-cyclin E complexes.     

Cdk2 knockout mice are viable

BACKGROUND: Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2/cyclin E complexes are thought to be required because they phosphorylate the retinoblastoma protein and drive cells through the G1/S transition into the S phase of the cell cycle. In addition, Cdk2 associates with cyclin A, which itself is essential for cell proliferation during early embryonic development. RESULTS: In order to study the functions of Cdk2 in vivo, we generated Cdk2 knockout mice. Surprisingly, these mice are viable, and therefore Cdk2 is not an essential gene in the mouse. However, Cdk2 is required for germ cell development; both male and female Cdk2(-/-) mice are sterile. Immunoprecipitates of cyclin E1 complexes from Cdk2(-/-) spleen extracts displayed no activity toward histone H1. Cyclin A2 complexes were active in primary mouse embryonic fibroblasts (MEFs), embryo extracts and in spleen extracts from young animals. In contrast, there was little cyclin A2 kinase activity in immortalized MEFs and spleen extracts from adult animals. Cdk2(-/-) MEFs proliferate but enter delayed into S phase. Ectopic expression of Cdk2 in Cdk2(-/-) MEFs rescued the delayed entry into S phase. CONCLUSIONS: Although Cdk2 is not an essential gene in the mouse, it is required for germ cell development and meiosis. Loss of Cdk2 affects the timing of S phase, suggesting that Cdk2 is involved in regulating progression through the mitotic cell cycle.     

Differential requirement of cyclin-dependent kinase 2 for oligodendrocyte progenitor cell proliferation and differentiation

ABSTRACT: Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2-cyclin E complexes, phosphorylating the retinoblastoma protein, drive cells through the G1/S transition into the S phase of the cell cycle. Despite its fundamental role, Cdk2 was found to be indispensable only in specific cell types due to molecular redundancies in its function. Converging studies highlight involvement of Cdk2 and associated cell cycle regulatory proteins in oligodendrocyte progenitor cell proliferation and differentiation. Giving the contribution of this immature cell type to brain plasticity and repair in the adult, this review will explore the requirement of Cdk2 for oligodendrogenesis, oligodendrocyte progenitor cells proliferation and differentiation during physiological and pathological conditions.     

Structure and function of cyclin-dependent Pho85 kinase of Saccharomyces cerevisiae

Yeast Saccharomyces cerevisiae has five cyclin-dependent protein kinases (Cdks), Cdc28, Srb10, Kin28, Ctk1, and Pho85. Any of these Cdks requires a cyclin partner for its kinase activity and a Cdk/cyclin complex, thus produced, phosphorylates a set of specific substrate proteins to exert its function. The cyclin partners of Srb10, Kin28, and Ctk1 are Srb11, Ccl1, and Ctk2, respectively. In contrast to the fact that each of Srb10, Kin28, and Ctk1 has a single cyclin partner, Cdc28 and Pho85 are polygamous; Cdc28 has 9 cyclins and Pho85 has 10 cyclins. Among these Cdks, Kin28 and Cdc28 are essential Cdks and it is well known that Cdc28 kinase plays a major role in regulating cell cycle progression. Pho85 is a non-essential Cdk but its absence causes a broad spectrum of phenotypes such as constitutive expression of PHO5, inability to utilize non-fermentable carbon sources, defects in cell cycle progression, and so on. Pho85 homologues are expanding to higher eukaryotes. Pho85 is most closely related with Cdk5 in terms of the amino acid sequence. The functional analysis of the domains of Pho85 also supports the close relationship between Pho85 and Cdk5, in which it was shown that the method of regulation of these two kinases is similar. Furthermore, forced expression of the mammalian CDK5 gene in a pho85Delta strain canceled a part of the pho85 defects. In this review, we summarize the functions of both Pho85/cyclin kinase and emphasize yeast Pho85 as valuable model systems to elucidate the functions of their homologues in other organisms.     

Cdk2 acts upstream of mitochondrial permeability transition during paclitaxel-induced apoptosis

Sequential activation of cyclin-dependent kinases (Cdks) controls mammalian cell cycle. Here we demonstrate that the upregulation of cyclin-dependent kinase 2 (Cdk2) activity coincides with the loss of mitochondrial membrane potential (MMP) in paclitaxel-induced apoptosis. Ectopic expression of the dominant negative Cdk2 (Cdk2-dn) and a specific Cdk2 inhibitor, p21^WAF1/CIP1, effectively suppresses the loss of MMP, the release of cytochrome c, and subsequent activation of caspase-3 in paclitaxel-treated cells. Whereas forced activation of Cdk2 by overexpression of cyclin A dramatically promotes these events. We further show that Cdk2 activation status does not interfere with a procedure that lies downstream of cytochrome c release induced by Bax protein. These findings suggest that Cdk2 kinase can regulate apoptosis at earlier stages than mitochondrial permeability transition and cytochrome c release.     

Cell-cycle regulation and mammalian gametogenesis: a lesson from the unexpected

The progression of mammalian gametogenesis requires a precise balance between cell-cycle activities and elimination of defective gametogenic cells to ensure the perpetuation of species. Both spermatogonia and oogonia are stem cell populations committed to meiosis with the aim of generating haploid gametes for fertilization. At puberty, mitotically dividing spermatogonial cell cohorts maintain the ability of cell renewal and occupy niches in the seminiferous tubule. In contrast, mitotically dividing oogonial cell cohorts produced in the fetal ovary, are exclusively committed to meiosis and produce primordial follicles housing a primary oocyte surrounded by somatic follicular cells. A consistent physiological event during mammalian gametogenesis is the disposal of spermatogenic cells by apoptosis and ovarian follicles by atresia. Cyclin-dependent kinases (Cdks) and their cyclin partners coordinate the activities of the cell cycle. An additional cell-cycle regulatory component is the centrosome. The centrosome harbors regulatory proteins controlling the normal progression of the cell cycle. Changes in individual centrosome proteins can lead to cell-cycle arrest and a decrease in the genomic protective function of p53 that promotes apoptosis. Disruption of cyclin A1, Cdk2, and Cdk4 expression in transgenic mice results in infertility and gonadal atrophy. Cdk-cyclin complexes interact with regulatory proteins, which may fine-tune the activities of the complex. One of the many regulatory proteins is p12, a 115 amino acid growth suppressor polypeptide designated p12(CDK2AP1), partner of Cdk2 and with binding affinity to DNA polymerase alpha/primase. Overexpression of p12 is associated with testicular and ovarian atrophy without affecting fertility. Ectopic expression of p12 was driven by the keratin 14 promoter. Keratin 14 is the pairing partner of keratin 5 and both keratins are expressed in testis. The efficiency of keratin promoters in driving ectopic gonadal gene expression, the association of gonadal atrophy with the ectopic expression of a Cdk2 regulatory protein and the centrosome, as a reservoir of cell-cycle regulatory proteins, open new experimental opportunities to address still lingering questions concerning cell differentiation and division during mammalian gametogenesis.