Activation at M-phase of a protein kinase encoded by a starfish homologue of the cell cycle control gene cdc2+

Japanese encephalitis virus NS1 and NS1' proteins induce vimentin rearrangement via the CDK1-PLK1 axis to promote viral replication

The host cytoskeleton plays crucial roles in various stages of virus infection, including viral entry, transport, replication, and release. However, the specific mechanisms by which intermediate filaments are involved in orthoflavivirus infection have not been well understood. In this study, we demonstrate that the Japanese encephalitis virus (JEV) remodels the vimentin network, resulting in the formation of cage-like structures that support viral replication. Mechanistically, JEV NS1 and NS1' proteins induce the translocation of CDK1 from the nucleus to the cytoplasm and interact with it, leading to the phosphorylation of vimentin at Ser56. This phosphorylation event recruits PLK1, which further phosphorylates vimentin at Ser83. Consequently, these phosphorylation modifications convert the typically filamentous vimentin into non-filamentous "particles" or "squiggles." These vimentin "particles" or "squiggles" are then transported retrogradely along microtubules to the endoplasmic reticulum, where they form cage-like structures. Notably, NS1' is more effective than NS1 in triggering the CDK1-PLK1 cascade response. Overall, our study provides new insights into how JEV NS1 and NS1' proteins manipulate the vimentin network to facilitate efficient viral replication.

IMPORTANCE

Japanese encephalitis virus (JEV) is a mosquito-borne orthoflavivirus that causes severe encephalitis in humans, particularly in Asia. Despite the availability of a safe and effective vaccine, JEV infection remains a significant public health threat due to limited vaccination coverage. Understanding the interactions between JEV and host proteins is essential for developing more effective antiviral strategies. In this study, we investigated the role of vimentin, an intermediate filament protein, in JEV replication. Our findings reveal that JEV NS1 and NS1' proteins induce vimentin rearrangement, resulting in the formation of cage-like structures that envelop the viral replication factories (RFs), thus facilitating efficient viral replication. Our research highlights the importance of the interplay between the cytoskeleton and orthoflavivirus, suggesting that targeting vimentin could be a promising approach for the development of antiviral strategies to inhibit JEV propagation.

Cyclins and cyclin-dependent kinases: from biology to tumorigenesis and therapeutic opportunities

The discussion on cell proliferation cannot be continued without taking a look at the cell cycle regulatory machinery. Cyclin-dependent kinases (CDKs), cyclins, and CDK inhibitors (CKIs) are valuable members of this system and their equilibrium guarantees the proper progression of the cell cycle. As expected, any dysregulation in the expression or function of these components can provide a platform for excessive cell proliferation leading to tumorigenesis. The high frequency of CDK abnormalities in human cancers, together with their druggable structure has raised the possibility that perhaps designing a series of inhibitors targeting CDKs might be advantageous for restricting the survival of tumor cells; however, their application has faced a serious concern, since these groups of serine-threonine kinases possess non-canonical functions as well. In the present review, we aimed to take a look at the biology of CDKs and then magnify their contribution to tumorigenesis. Then, by arguing the bright and dark aspects of CDK inhibition in the treatment of human cancers, we intend to reach a consensus on the application of these inhibitors in clinical settings.

Explaining Redundancy in CDK-Mediated Control of the Cell Cycle: Unifying the Continuum and Quantitative Models

In eukaryotes, cyclin-dependent kinases (CDKs) are required for the onset of DNA replication and mitosis, and distinct CDK–cyclin complexes are activated sequentially throughout the cell cycle. It is widely thought that specific complexes are required to traverse a point of commitment to the cell cycle in G1, and to promote S-phase and mitosis, respectively. Thus, according to a popular model that has dominated the field for decades, the inherent specificity of distinct CDK–cyclin complexes for different substrates at each phase of the cell cycle generates the correct order and timing of events. However, the results from the knockouts of genes encoding cyclins and CDKs do not support this model. An alternative “quantitative” model, validated by much recent work, suggests that it is the overall level of CDK activity (with the opposing input of phosphatases) that determines the timing and order of S-phase and mitosis. We take this model further by suggesting that the subdivision of the cell cycle into discrete phases (G0, G1, S, G2, and M) is outdated and problematic. Instead, we revive the “continuum” model of the cell cycle and propose that a combination with the quantitative model better defines a conceptual framework for understanding cell cycle control.

Cyclin B1-Cdk1 facilitates MAD1 release from the nuclear pore to ensure a robust spindle checkpoint

How the cell rapidly and completely reorganizes its architecture when it divides is a problem that has fascinated researchers for almost 150 yr. We now know that the core regulatory machinery is highly conserved in eukaryotes, but how these multiple protein kinases, protein phosphatases, and ubiquitin ligases are coordinated in space and time to remodel the cell in a matter of minutes remains a major question. Cyclin B1-Cdk is the primary kinase that drives mitotic remodeling; here we show that it is targeted to the nuclear pore complex (NPC) by binding an acidic face of the kinetochore checkpoint protein, MAD1, where it coordinates NPC disassembly with kinetochore assembly. Localized cyclin B1-Cdk1 is needed for the proper release of MAD1 from the embrace of TPR at the nuclear pore so that it can be recruited to kinetochores before nuclear envelope breakdown to maintain genomic stability.

Triggering mitosis

Entry into mitosis is triggered by the activation of cyclin-dependent kinase 1 (Cdk1). This simple reaction rapidly and irreversibly sets the cell up for division. Even though the core step in triggering mitosis is so simple, the regulation of this cellular switch is highly complex, involving a large number of interconnected signalling cascades. We do have a detailed knowledge of most of the components of this network, but only a poor understanding of how they work together to create a precise and robust system that ensures that mitosis is triggered at the right time and in an orderly fashion. In this review, we will give an overview of the literature that describes the Cdk1 activation network and then address questions relating to the systems biology of this switch. How is the timing of the trigger controlled? How is mitosis insulated from interphase? What determines the sequence of events, following the initial trigger of Cdk1 activation? Which elements ensure robustness in the timing and execution of the switch? How has this system been adapted to the high levels of replication stress in cancer cells?

APC/C: current understanding and future perspectives

The separation of sister chromatids at anaphase, which is regulated by an E3 ubiquitin ligase called the anaphase-promoting complex/cyclosome (APC/C), is arguably the most important irrevocable event during the cell cycle. The APC/C and cyclin-dependent kinase 1 (Cdk1) are just two of the many significant cell cycle regulators and exert control through ubiquitylation and phosphorylation, respectively. The temporal and spatial regulation of the APC/C is achieved by multiple mechanisms, including phosphorylation, interaction with the structurally related co-activators Cdc20 and Cdh1, loading of distinct E2 ubiquitin-conjugating enzymes, binding with inhibitors and differential affinities for various substrates. Since the discovery of APC/C 25 years ago, intensive studies have uncovered many aspects of APC/C regulation, but we are still far from a full understanding of this important cellular machinery. Recent high-resolution cryogenic electron microscopy analysis and reconstitution of the APC/C have greatly advanced our understanding of molecular mechanisms underpinning the enzymatic properties of APC/C. In this review, we will examine the historical background and current understanding of APC/C regulation.

Requirement for CCNB1 in mouse spermatogenesis

Spermatogenesis, which involves mitosis and meiosis of male germ cells, is a highly complicated and coordinately ordered process. Cyclin B1 (CCNB1), an important regulator in cell cycle machinery, is proved essential for mouse embryonic development. However, the role of CCNB1 in mammalian spermatogenesis remains unclear. Here we tested the requirement for CCNB1 using conditional knockout mice lacking CCNB1 in male germ cells. We found that ablation of CCNB1 in gonocytes and spermatogonia led to mouse sterile caused by the male germ cells’ depletion. Gonocyte and spermatogonia without CCNB1 is unable to proliferate normally and apoptosis increased. Moreover, CCNB1 ablation in spermatogonia may promote their differentiation by downregulating Lin28a and upregulating let-7 miRNA. However, ablation of CCNB1 in premeiotic male germ cells did not have an effect on meiosis of spermatocytes and male fertility, suggesting that CCNB1 may be dispensable for meiosis of spermatocytes. Collectively, these results indicate that CCNB1 is critically required for the proliferation of gonocytes and spermatogonia but may be redundant in meiosis of spermatocytes in mouse spermatogenesis.

Tangeretin induces cell cycle arrest and apoptosis through upregulation of PTEN expression in glioma cells

Tangeretin (TANG), present in peel of citrus fruits, has been shown to various medicinal properties such as chemopreventive and neuroprotective. However, the chemopreventive effect of TANG on glioblastoma cells has not been examined. The present study was designed to explore the anticancer potential of TANG in glioblastoma cells and to investigate the related mechanism. Human glioblastoma U-87MG and LN-18 cells were treated with 45μM concentration of TANG and cell growth was measured by MTT assay. The cell cycle distribution and cell death were measured by flow cytometry. The expression of cell cycle and apoptosis related genes were analyzed by quantitative RT-PCR and western blot. The cells treated with TANG were significantly increased cell growth suppression and cell death effects than vehicle treated cells. Further, TANG treatment increases G2/M arrest and apoptosis by modulating PTEN and cell-cycle regulated genes such as cyclin-D and cdc-2 mRNA and protein expressions. Moreover, the ability of TANG to decrease cell growth and to induce cell death was compromised when PTEN was knockdown by siRNA. Taken together, the chemopreventive effect of TANG is associated with regulation of cell-cycle and apoptosis in glioblastoma, thereby attenuating glioblastoma cell growth. Hence, the present findings suggest that TANG may be a therapeutic agent for glioblastoma treatment.

Targeting cyclin-dependent kinases in human cancers: from small molecules to Peptide inhibitors

Cyclin-dependent kinases (CDK/Cyclins) form a family of heterodimeric kinases that play central roles in regulation of cell cycle progression, transcription and other major biological processes including neuronal differentiation and metabolism. Constitutive or deregulated hyperactivity of these kinases due to amplification, overexpression or mutation of cyclins or CDK, contributes to proliferation of cancer cells, and aberrant activity of these kinases has been reported in a wide variety of human cancers. These kinases therefore constitute biomarkers of proliferation and attractive pharmacological targets for development of anticancer therapeutics. The structural features of several of these kinases have been elucidated and their molecular mechanisms of regulation characterized in depth, providing clues for development of drugs and inhibitors to disrupt their function. However, like most other kinases, they constitute a challenging class of therapeutic targets due to their highly conserved structural features and ATP-binding pocket. Notwithstanding, several classes of inhibitors have been discovered from natural sources, and small molecule derivatives have been synthesized through rational, structure-guided approaches or identified in high throughput screens. The larger part of these inhibitors target ATP pockets, but a growing number of peptides targeting protein/protein interfaces are being proposed, and a small number of compounds targeting allosteric sites have been reported.

Bromelain inhibits nuclear factor kappa-B translocation, driving human epidermoid carcinoma A431 and melanoma A375 cells through G(2)/M arrest to apoptosis

Bromelain, obtained from pineapple, is already in use clinically as adjunct in chemotherapy. Our objective was to test its ability to act as a sole anti-cancer agent. Therefore, we describe its anti-proliferative, anti-inflammatory and subsequent anti-cancer effects in vitro, against human epidermoid carcinoma-A431 and melanoma-A375 cells. Bromelain exhibited reduction in proliferation of both these cell-lines and suppressed their potential for anchorage-independent growth. Further, suppression of inflammatory signaling by bromelain was evident by inhibition of Akt regulated-nuclear factor-kappaB activation via suppression of inhibitory-kappaBα phosphorylation and concomitant reduction in cyclooxygenase-2. Since, the inflammatory cascade is well-known to be closely allied to cancer; we studied the effect of bromelain on events/molecules central to it. Bromelain caused depletion of intracellular glutathione and generation of reactive oxygen-species followed by mitochondrial membrane depolarization. This led to bromelain-induced cell-cycle arrest at G(2)/M phase which was mediated by modulation of cyclin B1, phospho-cdc25C, Plk1, phospho-cdc2, and myt1. This was subsequently followed by induction of apoptosis, indicated by membrane-blebbing, modulation of Bax-Bcl-2 ratio, Apaf-1, caspase-9, and caspase-3; chromatin-condensation, increase in caspase-activity and DNA-fragmentation. Bromelain afforded substantial anti-cancer potential in these settings; hence we suggest it as a potential prospect for anti-cancer agent besides only an additive in chemotherapy.

One function--multiple mechanisms: the manifold activities of p53 as a transcriptional repressor

Maintenance of genome integrity is a dynamic process involving complex regulation systems. Defects in one or more of these pathways could result in cancer. The most important tumor-suppressor is the transcription factor p53, and its functional inactivation is frequently observed in many tumor types. The tumor suppressive function of p53 is mainly attributed to its ability to regulate numerous target genes at the transcriptional level. While the mechanism of transcriptional induction by p53 is well characterized, p53-dependent repression is not understood in detail. Here, we review the manifold mechanisms of p53 as a transcriptional repressor. We classify two different categories of repressed genes based on the underlying mechanism, and novel mechanisms which involve regulation through noncoding RNAs are discussed. The complete elucidation of p53 functions is important for our understanding of its tumor-suppressor activity and, therefore, represents the key for the development of novel therapeutic approaches.

New Cdc2 Tyr 4 phosphorylation by dsRNA-activated protein kinase triggers Cdc2 polyubiquitination and G2 arrest under genotoxic stresses

Cell division cycle 2 (Cdc2) protein is an essential subunit of M-phase kinase (MPK), which has a key role in G2/M transition. Even though the control of MPK activity has been well established with regard to the phosphorylation of Cdc2 at Thr 14 and/or Tyr 15 and Thr 161, little is known about the proteolytic control of Cdc2. In this study, we observed that Cdc2 was downregulated under genotoxic stresses and that double-stranded RNA-activated protein kinase (PKR) was involved in the process. The PKR-mediated Tyr4 phosphorylation triggered Cdc2 ubiquitination. Phospho-mimic mutations at the Tyr 4 residue (Y4D or Y4E) caused significant ubiquitination of Cdc2 even in the absence of PKR. Our findings demonstrate that (i) PKR, Ser/Thr kinase, phosphorylates its new substrate Cdc2 at the Tyr 4 residue, (ii) PKR-mediated Tyr 4-phosphorylation facilitates Cdc2 ubiquitination and proteosomal degradation, (iii) unphosphorylated Tyr 4 prevents Cdc2 ubiquitination, and (iv) downstream from p53, PKR has a crucial role in G2 arrest and triggers Cdc2 downregulation under genotoxic conditions.

Regulation of signaling pathways involved in lupeol induced inhibition of proliferation and induction of apoptosis in human prostate cancer cells

Prostate cancer (PCa) is the most frequently diagnosed noncutaneous cancer and the leading cause of cancer related deaths in men in the United States and many other Asian countries. Dietary factors are considered as a strategic agent to control the risk of PCa. Lupeol, a triterpene, present in fruits and medicinal plants, has been shown to possess many pharmacological properties including anticancer effects. Here, effect of lupeol on cell proliferation and cell death was evaluated using human PCa cells, PC-3. In MTT assay, lupeol inhibited the cell proliferation (12-71%) in dose (50-800 microM) and time dependent manner. Flow-cytometric analysis of cell-cycle revealed that an antiproliferative effect of lupeol (400-600 microM) is associated with an increase in G(2)/M-phase arrest (34-58%). RT-PCR analysis showed that lupeol-induced G2/M-phase arrest was mediated through the inhibition of cyclin regulated signaling pathway. Lupeol inhibited the expression of cyclin B, cdc25C, and plk1 but induced the expression of 14-3-3sigma genes. However no changes were observed in the expression of gadd45, p21(waf1/cip1) and cdc2 genes. Results of western blot showed that lupeol regulates the phosphorylation of cdc2 (Tyr15) and cdc25C (Ser198). Further, on increase of lupeol exposure to PC-3 cells an induction of apoptosis was recorded, which was associated with upregulation of bax, caspase-3, -9, and apaf1 genes and down regulation of antiapoptotic bcl-2 gene. The role of caspase-induced apoptosis was confirmed by increase in reactive oxygen species, loss of mitochondrial membrane potential followed by DNA fragmentation. Thus, our study suggests that lupeol possess novel antiproliferative and apoptotic potential against PCa.

Mitotic CDKs control the metaphase-anaphase transition and trigger spindle elongation

Mitotic cyclin-dependent kinases (CDKs) control entry into mitosis, but their role during mitotic progression is less well understood. Here we characterize the functions of CDK activity associated with the mitotic cyclins Clb1, Clb2, and Clb3. We show that Clb-CDKs are important for the activation of the ubiquitin ligase Anaphase-Promoting Complex/Cyclosome (APC/C)-Cdc20 that triggers the metaphase-anaphase transition. Furthermore, we define an essential role for Clb-CDK activity in anaphase spindle elongation. Thus, mitotic CDKs serve not only to initiate M phase, but are also needed continuously throughout mitosis to trigger key mitotic events such as APC/C activation and anaphase spindle elongation.

Comparison of bulk enucleation methods for porcine oocytes

Cloning of mammalian oocytes requires that the recipient oocyte is enucleated to remove all genetic material associated with the chromosomes. The procedure currently used in most species requires careful micromanipulation of oocytes treated with cytochalasin B to prevent structural damage. Although functional, this procedure requires time and limits the number of oocytes available for cloning, and our ability to understand the mechanisms of nuclear reprogramming. Therefore, this study aimed at evaluating different procedures to enucleate large pools of oocytes in a time-efficient manner. Two different approaches were tested. The first approach involved centrifugation of zona-free oocytes through a percoll gradient to separate the portion containing the chromatin from the cytoplasmic portion. The second used etoposide to prevent chromatin segregation at first metaphase and resulting in the expulsion of all chromosomes in the polar body. Using the chemical approach an average enucleation rate of 39.4 +/- 7.5% was obtained, while the centrifugation approach resulted in an average enucleation rate of 66.9 +/- 6. In terms of time efficiency, the control manipulation method takes 0.11 min and the centrifugation took an average of 0.52 min per oocyte. The MPF activity at the end of procedure was estimated through the measurement of H1 activity and as expected, the etoposide-cycloheximide treated oocytes had lower H1 activity which was restored by further incubation in the maturation medium for 5 hr while the centrifugation gave a nonsignificant intermediary result. In conclusion, the results presented suggest that both the chemical and the mechanical methods are usable alternatives to micromanipulation of oocytes to generate a large number of chromosome free cytoplasm for biochemical analysis. Mol. Reprod. Dev. 67: 70-76, 2004.

Recycling the cell cycle: cyclins revisited

I discuss advances in the cell cycle in the 21 years since cyclin was discovered. The surprising redundancy amongst the classical cyclins (A, B, and E) and cyclin-dependent kinases (Cdk1 and Cdk2) show that the important differences between these proteins are when and where they are expressed rather than the proteins they phosphorylate. Although the broad principles of the cell cycle oscillator are widely accepted, we are surprisingly ignorant of its detailed mechanism. This is especially true of the anaphase promoting complex (APC), the machine that triggers chromosome segregation and the exit of mitosis by targeting securin and mitotic cyclins for destruction. I discuss how a cyclin/Cdk-based engine could have evolved to assume control of the cell cycle from other, older protein kinases.

Inhibition of MEK or cdc2 kinase parthenogenetically activates mouse eggs and yields the same phenotypes as Mos(-/-) parthenogenotes

Mammalian eggs are arrested in metaphase II of meiosis until fertilization. Arrest is maintained by cytostatic factor (CSF) activity, which is dependent on the MOS-MEK-MAPK pathway. Inhibition of MEK1/2 with a specific inhibitor, U0126, parthenogenetically activated mouse eggs, producing phenotypes similar to Mos(-/-) parthenogenotes (premature, unequal cleavages and large polar bodies). U0126 inactivated MAPK in eggs within 1 h, in contrast to the 5 h required after fertilization, while the time course of MPF inactivation was similar in U0126-activated and fertilized eggs. We also found that inactivation of MPF by the cdc2 kinase inhibitor roscovitine induced parthenogenetic activation. Inactivation of MPF by roscovitine resulted in the subsequent inactivation of MAPK with a time course similar to that following fertilization. Notably, roscovitine also produced some Mos(-/-)-like phenotypes, indistinguishable from U0126 parthenogenotes. Simultaneous inhibition of both MPF and MAPK in eggs treated with roscovitine and U0126 produced a very high proportion of eggs with the more severe phenotype. These findings confirm that MEK is a required component of CSF in mammalian eggs and imply that the sequential inactivation of MPF followed by MAPK inactivation is required for normal spindle function and polar body emission.

From Cdc2 to Cdk1: when did the cell cycle kinase join its cyclin partner?

The idea that Cdc2 and cyclins play a key role in the control of the G2/M transition of the cell cycle came largely from genetic analysis of fission yeast and physiological studies of clam, frog, sea urchin and starfish eggs and oocytes. However, it took a long time to realise that Cdc2 and cyclins form a stoichiometric complex and that a cyclin subunit is necessary for the Cdc2 subunit to gain its protein kinase activity.

Cyclins were first recognized as proteins whose abundance oscillates during the early cell cycles of marine invertebrate eggs and their connection with MPF (maturation-promoting factor), the entity defined in frog and starfish oocytes whose activity controls entry into M phase, was far from clear at first. Indeed, it was a long time before MPF was shown to be a protein kinase,and direct proof that MPF is a heterodimer comprising one molecule of cyclin and one molecule of Cdc2 was finally obtained only when the Cdc2-associated component of purified starfish MPF was sequenced and found to be cyclin B. When this fundamental discovery was confirmed in vertebrates and mammalian members of the Cdc2 family were also shown to bind cyclins, Cdc2 became Cdk1,the first cyclin-dependent protein kinase.

Differential regulation of growth and checkpoint control mediated by a Cdc25 mitotic phosphatase from Pneumocystis carinii

Pneumocystis carinii is an opportunistic fungal pathogen phylogenetically related to the fission yeast Schizosaccharomyces pombe. P. carinii causes severe pneumonia in immunocompromised patients with AIDS and malignancies. Although the life cycle of P. carinii remains poorly characterized, morphologic studies of infected lung tissue indicate that P. carinii alternates between numerous small trophic forms and fewer large cystic forms. To understand further the molecular mechanisms that regulate progression of the cell cycle of P. carinii, we have sought to identify and characterize genes in P. carinii that are important regulators of eukaryotic cell cycle progression. In this study, we have isolated a cDNA from P. carinii that exhibits significant homology, but unique functional characteristics, to the mitotic phosphatase Cdc25 found in S. pombe. P. carinii Cdc25 was shown to rescue growth of the temperature-sensitive S. pombe cdc25-22 strain and thus provides an additional tool to investigate the unique P. carinii life cycle. Although P. carinii Cdc25 could also restore the DNA damage checkpoint in cdc25-22 cells, it was unable to restore fully the DNA replication checkpoint. The dissociation of checkpoint control at the level of Cdc25 indicates that Cdc25 may be under distinct regulatory control in mediating checkpoint signaling.

Regulation of the meiosis-inhibited protein kinase, a p38(MAPK) isoform, during meiosis and following fertilization of seastar oocytes

A p38(MAPK) homolog Mipk (meiosis-inhibited protein kinase) was cloned from seastar oocytes. This 40-kDa protein shares approximately 65% amino acid identity with mammalian p38-alpha isoforms. Mipk was one of the major tyrosine-phosphorylated proteins in immature oocytes arrested at the G(2)/M transition of meiosis I. The tyrosine phosphorylation of Mipk was increased in response to anisomycin, heat, and osmotic shock of oocytes. During 1-methyladenine-induced oocyte maturation, Mipk underwent tyrosine dephosphorylation and remained dephosphorylated in mature oocytes and during the early mitotic cell divisions until approximately 12 h after fertilization. At the time of differentiation and acquisition of G phases in the developing embryos, Mipk was rephosphorylated on tyrosine. In oocytes that were microinjected with Mipk antisense oligonucleotides and subsequently were allowed to mature and become fertilized, differentiation was blocked. Because MipK antisense oligonucleotides and a dominant-negative (K62R)Mipk when microinjected into immature oocytes failed to induce germinal vesicle breakdown, inhibition of Mipk function was not sufficient by itself to cause oocyte maturation. These findings point to a putative role for Mipk in cell cycle control as a G-phase-promoting factor.

Cell-anchorage, cell cytoskeleton, and Rho-GTPase family in regulation of cell cycle progression

It has been well known that cell-anchorage and the cell cytoskeleton are deeply involved in the regulation of cell proliferation and cell cycle. However, the precise molecular mechanism involved in cell-anchorage and the cell cytoskeleton have remained be to elucidated. The recent great volume of information regarding cell cycle regulators such as cyclin, cyclin-dependent kinases (CDKs) and CDK inhibitors (CKI) has facilitated the understanding of the cell cycle in mammalian cells. In this review, we will focus on these cell cycle regulators to discuss the regulation of cell proliferation controlled by cell-anchorage and the cytoskeleton, and especially the roles of Rho family GTPases.

Non-dependence of cyclin E/Cdk2 kinase activity on the initiation of oocyte maturation in goldfish

Cdk2 kinase activity increases during oocyte maturation but neither cyclin A nor B is associated with Cdk2 in mature oocytes in goldfish. As a potential Cdk2 partner in meiosis, a cyclin E homolog was isolated from a goldfish oocyte cDNA library. A monoclonal antibody was raised against bacterially produced full-length goldfish cyclin E. Both cyclin E and Cdk2 were already present in immature oocytes and their protein levels did not change remarkably during oocyte maturation. Cyclin E formed a complex mainly with Cdk2 just at the time of germinal vesicle breakdown (GVBD) in association with the increase in Cdk2 kinase activity, although a fraction of cyclin E bound to Cdk(s) other than Cdk2 and Cdc2. Ectopic activation of cyclin E/Cdk2 by the injection of cyclin E messenger RNA (mRNA) into immature oocytes did not induce maturation-promoting factor (MPF) activation and GVBD. Furthermore, inhibition of cyclin E/Cdk2 kinase activity by the injection of p21SDI1 into the oocytes treated with 17alpha,20beta-dihydroxy-4-pregnen-3-one had no effect on MPF activation and GVBD. These results indicate that cyclin E/Cdk2 kinase activity is insufficient and unnecessary for initiating goldfish oocyte maturation.

Intra-M phase-promoting factor phosphorylation of cyclin B at the prophase/metaphase transition

Activation of Cdc2-cyclin B (or M phase-promoting factor (MPF)) at the prophase/metaphase transition proceeds in two steps: dephosphorylation of Cdc2 and phosphorylation of cyclin B. We here investigated the regulation of cyclin B phosphorylation using the starfish oocyte model. Cyclin B phosphorylation is not required for Cdc2 kinase activity; both the prophase complex dephosphorylated on Cdc2 with Cdc25 and the metaphase complex dephosphorylated on cyclin B with protein phosphatase 2A display high kinase activities. An in vitro assay of cyclin B kinase activity closely mimics in vivo phosphorylation as shown by phosphopeptide maps of in vivo and in vitro phosphorylated cyclin B. We demonstrate that Cdc2 itself is the cyclin B kinase; cyclin B phosphorylation requires Cdc2 activity both in vivo (sensitivity to vitamin K3, a Cdc25 inhibitor) and in vitro (copurification with Cdc2-cyclin B, requirement of Cdc2 dephosphorylation, and sensitivity to chemical inhibitors of cyclin-dependent kinases). Furthermore, cyclin B phosphorylation occurs as an intra-M phase-promoting factor reaction as shown by the following: 1) active Cdc2 is unable to phosphorylate cyclin B associated to phosphorylated Cdc2, and 2) cyclin B phosphorylation is insensitive to enzyme/substrate dilution. We conclude that, at the prophase/metaphase transition, cyclin B is mostly phosphorylated by its own associated Cdc2 subunit.

Partial purification of maturation-promoting factor from catfish, Clarias batrachus: identification as the histone H1 kinase and its periodic activation

Maturation-promoting factor (MPF) has been demonstrated in the 100,000 g supernatant of 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17 alpha, 20 beta-DP)-induced catfish, Clarias batrachus oocytes using DEAE-cellulose and sephadex G-200 chromatography. Partially purified MPF molecule eluted as a single peak on sephadex G-200 with molecular mass of approximately 200 kDa in native PAGE. SDS-PAGE analysis showed the presence of five proteins of 32, 34, 45, 46 and 48 kDa. Antibody against the PSTAIR sequence of p34cdc2 recognized 32 and 34 kDa proteins, whereas rabbit anti-cyclin B1 and B2 crossreacted with 46 and 48 kDa proteins, respectively. Cyclin B was absent in immature oocytes and appeared after 7 h of 17 alpha, 20 beta-DP stimulation, coinciding with the histone H1 kinase (HH1K) activity and start of germinal vesicle breakdown (GVBD). Our data indicate that C. batrachus MPF is a complex of cdc2 kinase and cyclin B molecules. A close relationship between HH1K activity and catfish oocyte maturation has been demonstrated using cycloheximide, cytochalasin B and colchicine. HH1K activation was inhibited by cycloheximide, while cytochalasin B and colchicine were ineffective. These finding suggests that the activation of HH1K depends on protein synthesis, whereas disruption of microfilaments influences only nucleus migration without effect on GVBD or HH1K activation. An increase of phosphorylated proteins after activation of catfish oocytes with 17 alpha, 20 beta-DP has also been observed.

Growth promotion by homocysteine but not by homocysteic acid: a role for excessive growth in homocystinuria or proliferation in hyperhomocysteinemia?

Excessive growth of long bones in patients with homocystinuria is still unexplained and previous work incriminating homocysteic acid could not be confirmed by others. In vitro studies from our laboratory showed that homocysteine stimulated growth in a clonogenic assay. This observation made us study plasma cyclin dependent kinase (CDK), homocyst(e)ine and homocysteic acid in 10 patients with homocystinuria and 20 controls. In addition, homocysteine and homocysteic acid were tested in a clonogenic assay to correlate the growth promoting activity with CDK. Plasma CDK (protein) correlated strongly with homocysteine (r=0.84) but not with homocysteic acid. Supernatants of the clonogenic assay samples showed up to three times higher CDK levels in the presence of homocyst(e)ine but not homocysteic acid. In vitro data and the strong correlation between homocysteine and CDK suggest a role for homocysteine stimulating CDK, the starter of mitosis, with subsequent stimulation of growth.

The oocyte metaphase arrest

Usually, oocyte meiosis reinitiation appears as a two step process during which release from the prophase block is followed by a second arrest in metaphase I or II. In this review, we will examine the mechanisms required to maintain the metaphase arrest and stabilize MPF activity at this stage. Then, we will analyse the processes required to exit from the metaphase block. These may drive the cells forward to the metaphase-anaphase transition, as a result of fertilization, activation or protein synthesis inhibition. Instead, inhibiting protein phosphorylation drives the oocyte back to interphase. All these treatments result in derepression of DNA synthesis.

Regulation of Cdc2 activity by phosphorylation at T14/Y15

The highly conserved Cdc2 serine/threonine kinase plays a central role in cell cycle progression. Although Cdc2 levels remain constant throughout the cell cycle, Cdc2 kinase activity peaks at the G2/M boundary, in order to drive entry into mitosis. In the model organism Schizosaccharomysces pombe, potentially active Cdc2/Cdc13 kinase complex accumulates throughout the S and G2 phases of the cell cycle. This complex, however, is maintained in an active state by Wee1/Mik1-mediated phosphorylation at Y15 (and, possibly, T14). At the G2/M boundary, the Cdc25 protein phosphatase is activated to dephosphorylate the Cdc2/Cdc13 complex, resulting in abrupt activation of Cdc2 kinase activity and entry into mitosis.

Homocysteine increases cyclin-dependent kinase in aortic rat tissue

BACKGROUND

Hyperhomocyst(e)inemia is strongly associated with occlusive arterial disease. A direct effect of homocysteine on the proliferation of smooth muscle cells was proposed recently. This observation led us to examine the effect of homocysteine on cyclin-dependent kinase, the starter of mitosis and reflecting proliferation.

METHODS AND RESULTS

Seventy Him:OFA rats were divided into seven groups. For 12 weeks, 10 rats were fed homocysteine 25 mg/kg body weight per day, 10 were fed 50 mg/kg body wt per day, and 10 were fed 100 mg/kg body weight per day; 10 were given homocysteic acid 100 mg/kg body weight per day, 10 were administered cysteine 100 mg/kg body weight per day, and 10 were given ascorbic acid 270 mg/kg body weight per day. Ten remained untreated and served as controls. Aortic cyclin-dependent kinase was determined at the transcriptional (mRNA) and protein levels. Phosphokinase C and aortic homocyst(e)ine also were evaluated in aortic tissue. Aortic cyclin-dependent kinase protein was significantly (P = .0001) elevated in the three homocysteine-treated groups, and mRNA cyclin-dependent kinase levels were significantly elevated in the rats given the 50 and 100 mg/kg body weight per day protocol. Endothelial damage was shown at higher homocysteine doses as reflected by circulating ACE and von Willebrand factor changes. Proliferation of cells of the aortic wall by bromodeoxyuridine incorporation could be shown in the high-dose homocysteine group only.

CONCLUSIONS

Our findings indicate that homocysteine specifically stimulates aortic cyclin-dependent kinase at the transcriptional level, with the possible consequence of proliferation of aortic cells as revealed by incorporation of bromodeoxyuridine in the aortic wall.

Decreased cyclin dependent kinase in brain of patients with Down syndrome

In order to study whether phosphokinases might be involved in the neuropathology of Down Syndrome (DS) and Alzheimer disease (AD), cyclin dependent kinase (CDK) activity and protein, phosphokinase C (PKC) and phosphokinase A (PKA) activities have been determined in frontal lobes of DS, AD and control brains. An enzyme linked immunosorbent assay (ELISA) technique for CDK protein, and commercially available enzyme assays for CDK, PKC and PKA activities have been used. The major finding of our study was the remarkable and significant decrease of CDK protein and activity in DS brains in comparison to AD and controls. PKC and PKA were unaffected in both, AD and DS. As CDK controls cell division and differentiation, lowered CDK levels could reflect impaired proliferation and differentiation in DS.

Identification and characterization of maturation-promoting factor from catfish,Clarias batrachus

Maturation-promoting factor (MPF) extracted from maturing oocytes of catfishes (Clarias batrachus andHeteropneustes fossilis) and carp (Labeo rohita) induces 100% germinal vesicle breakdown (GVBD) when microinjected intoClarias immature unstimulated oocytes. The presence of a similar MPF activity has also been demonstrated in the active fractions collected after superose 12. SDS-PAGE analyses of cytosolic extracts (CE) prepared from immature and mature oocytes revealed the presence of 34- and 46 kDa proteins apart from a few others. Antibody against the PSTAIR sequence of p34(cdc2) recognized 32- and 34 kDa proteins of immature as well as mature oocytes while, 46 kDa protein of mature oocytes was recognized by anti-cyclin B1 antibody. Moreover, labelling of [(35)S]methionine was observed mainly in the region of 46 kDa protein band indicatingde novo synthesis of this particular protein. Anti-cyclin A antibody did not recognize any proteins of immature or mature oocytes. Cyclin B1 was absent in immature oocytes and ovulated eggs. These findings indicate the presence of p34(cdc2) homologs and cyclin B in the MPF of the catfishes and carp oocytes.

Purification and characterization of a p13suc1-bound serine/threonine kinase that is expressed in mature rat brain

We previously reported that the histone-H1 kinase activity bound to p13suc1 increased dramatically during development of the rat brain. In the present work, an in situ kinase assay in an SDS/polyacrylamide gel that contained substrate proteins was employed to characterize the enzyme. Two major proteins of 45 kDa and 100 kDa were found to have p13suc1-bound histone-H1 kinase activity. The former (p45) exhibited strong activity towards histone H1 and had weak autophosphorylation activity, whereas the latter (p100) acted on myelin basic protein or histone H1, and underwent autophosphorylation. p45 was further purified from the nuclear-enriched fraction of rat brain to near homogeneity through sequential column chromatographies. The purified enzyme retained its ability to bind specifically to p13suc1, which suggests that this binding does not require a cofactor. The immunochemical and enzymatic properties of p45 revealed that it differs from Cdk that are known to bind to p13suc1 with high affinity. However, in vitro p45 acted on the peptide motif that is conserved among substrates for cyclin-dependent kinases (Cdk) and mitogen-activated protein kinases, which implies that this protein might belong to the large family of proline-directed kinases. The evidence obtained in this study suggest that p45 is a nuclear p13suc1-bound kinase that has unique functions in the mature brain.

An inhibitor of p34cdc2/cyclin B that regulates the G2/M transition in Xenopus extracts

The activity of maturation-promoting factor (MPF), a protein kinase complex composed of p34cdc2 and cyclin B, is undetectable during interphase but rises abruptly at the G2/M transition to induce mitosis. After the synthesis of cyclin B, the suppression of MPF activity before mitosis has been attributed to the phosphorylation of p34cdc2 on sites (threonine-14 and tyrosine-15) that inhibit its catalytic activity. We previously showed that the activity of the mitotic p34cdc2/cyclin B complex is rapidly suppressed when added to interphase Xenopus extracts that lack endogenous cyclin B. Here we show that a mutant of p34cdc2 that cannot be inhibited by phosphorylation (threonine-14-->alanine, tyrosine-15-->phenylalanine) is also susceptible to inactivation, demonstrating that inhibitory mechanisms independent of threonine-14 and tyrosine-15 phosphorylation must exist. We have partially characterized this inhibitory pathway as one involving a reversible binding inhibitor of p34cdc2/cyclin B that is tightly associated with cell membranes. Kinetic analysis suggests that this inhibitor, in conjunction with the kinases that mediate the inhibitory phosphorylations on p34cdc2, maintains the interphase state in Xenopus; it may play an important role in the exact timing of the G2/M transition.

Presence of maturation-promoting factor in 17α,20β-dihydroxy-4-pregnen-3-one-induced oocytes of catfish, Clarias batrachus

Full-grown immature Clarias batrachus oocytes respond in vitro to exogenous 17α,20β-dihydroxy-4-preg-nen-3-one ( 17α,20β-DP) by undergoing germinal vesicle breakdown (GVBD). Cytosolic extract (CE) prepared from 17α,20β-DP-induced oocytes has been shown to produce similar effect when microinjected into unstimulated immature oocytes of the same fish. A dose of ∼ 50 nl is enough to cause 100% GVBD after 4 h. Maturation-promoting factor was investigated from 17α,20β-DP-induced, immature and cycloheximide treated oocytes incubated in presence of [(35)S] methionine. When the proteins were extracted and analyzed on SDS-PAGE, two prominent bands corresponding to molecular weight 34- and 46-kDa were detected in the CE of mature oocytes. However, labelling of [(35)S] methionine was observed mainly in the region of 46 kDa protein band indicating de novo synthesis of this particular protein during l7α,20β-DP-induction. Further, immunoblotting study by using rabbit anti-cyclin B1 antibody has clearly demonstrated that the protein which is newly synthesized is highly homologous to Xenopus cyclin B1 and goldfish cyclin B.

Cdk family genes are expressed not only in dividing but also in terminally differentiated mouse germ cells, suggesting their possible function during both cell division and differentiation

The roles of the cyclin dependent kinase (Cdk) family in murine germ cell development have been examined by studying the expression of five Cdk family genes (Cdc2, Cdk2, Cdk4, Pctaire-1, and Pctaire-3) in mouse reproductive organs. Northern blot and in situ hybridization analyses revealed distinctive expression patterns of these genes with striking cellular, lineage, and developmental stage specificity. We observed Cdk expression in cell types with proliferative activity: Cdc2 and Cdk2 expression in premeiotic spermatocytes in the testis, and Cdc2, Cdk2, and Cdk4 expression in granulosa cells of ovarian follicles. Cdc2 transcripts were most abundant in late pachytene to diplotene spermatocytes, soon to undergo meiosis. Surprisingly, we also observed expression of Cdk family genes in non-proliferating cell types. All five Cdk family genes examined were expressed in Sertoli cells of the adult testis, which are no longer mitotically active. With regard to Pctaire-1 and Pctaire-3, the highest levels of expression were observed in postmeiotic spermatids. Immunoblot analysis also revealed the presence of high levels of Pctaire-1 in postmeiotic germ cells. These results suggest that Cdk family kinases may exhibit various functions in germinal and somatic cells during gametogenesis, not only in the cell cycle but also in other regulatory processes, including differentiation.

UV irradiation of chromosomal DNA and its effect upon MPF and meiosis in mammalian oocytes

Bovine oocytes were irradiated at germinal vesicle (GV) or metaphase II (MII) stage, after Hoechst staining, with chromosomally focused UV-C (254 nm) or UV-A ( > 330 nm). UV-C irradiation at GV stage did not inhibit germinal vesicle breakdown (GVBD) or chromosomal condensation; spindle formation was abolished and maturation promoting factor (MPF) levels failed to increase. UV-A irradiation at GV stage caused meiotic arrest at anaphase I; MPF levels were lower than control. UV-C irradiation at MII stage led to subsequent abnormal parthenogenetic activation when MPF levels failed to decrease. A normal male but no female pronucleus was formed at fertilization. UV-A irradiation at MII stage also caused abnormal activation; MPF levels declined normally. A normal male and abnormal female pronucleus formed at fertilization. UV-A irradiation results have implications for oocyte evaluation during development using Hoechst staining. UV-C irradiation is a potential means for oocyte enucleation in nuclear transfer.

Cortical changes in starfish (Asterina pectinifera) oocytes during 1-methyladenine-induced maturation and fertilisation/activation

Maturation of the starfish oocyte cortex to produce an effective cortical granule reaction and fertilisation envelope is believed to develop in three phases: (1) pre-methyladenine (1-MA) stimulation; (2) post-1-MA stimulation, pregerminal vesicle breakdown; and (3) post-germinal vesicle breakdown. The present study was initiated to identify what each of these phases may encompass, specifically with respect to structures associated with the oocyte cortex, including cortical granules, microvilli and vitelline layer. 1-MA treatment brought about an orientation of cortical granules such that they became positioned perpendicular to the oocyte surface, and an approximately 4-fold decrease in microvillar length. A-23187 activation of immature oocytes treated with (10 min; pregerminal vesicle breakdown) or without 1-MA resulted in a reduction in cortical granule number of 21% and 41%, respectively (mature oocytes underwent a 96% reduction in cortical granules). Elevation of the fertilisation envelope in both cases was significantly retarded compared with activated mature oocytes. In activated mature oocytes, the vitelline layer elevated 20.0 +/- 5.4 mu m from the egg's surface, whereas in immature oocytes treated with just A-23187 or with 1-MA (10 min) and A-23187, it lifted 0.35 +/- 0.1 and 0.17 +/- 0.04 mu m, respectively. The fertilisation envelopes of activated (or fertilised) immature oocytes also differed morphologically from those of mature oocytes. In activated, immature oocytes, the fertilisation envelope was not uniform in its thickness and possessed thick and thin regions as well as fenestrations. Additionally, it lacked a complete electron-dense stratum that characterised the fertilisation envelopes of mature oocytes. The nascent perivitelline space of immature oocytes was also distinguished by the presence of numerous vesicles which appeared to be derived from microvilli. Differences in the morphology of cortices from activated (fertilised) and non-activated, immature and mature oocytes substantiate previous investigations demonstrating three phases of cortical maturation, and are consistent with physiological changes that occur during oocyte maturation, involving ionic conductance of the plasma membrane, establishment of slow and fast blocks to polyspermy and elevation of a fertilisation envelope.

Calcium, calmodulin and cell cycle progression

Proliferation of mammalian cells both in vivo and in vitro is dependent upon physiological concentrations of extracellular Ca2+. Growth factor stimulation of quiescent cells at the G0/G1 border usually results in a rapid mobilization of Ca2+ from both intra- and extracellular pools. However, Ca2+ influx is also required for later phases of cell cycle transition, especially in the late G1 phase for initiation of DNA synthesis. Available evidence indicates that calmodulin plays the major and essential roles in the Ca(2+)-dependent regulation of cell proliferation. Ca2+ and calmodulin act at multiple points in the cell cycle, including the initiation of the S phase and both initiation and completion of the M phase. Ca2+ and calmodulin stimulate the expression of genes involved in the cell cycle progression, leading to activation of cyclin-dependent kinases p33cdk2 and p34cdc2. Ca2+ and calmodulin are also involved in activation of enzymes participating in nucleotide metabolism and DNA replication, as well as nuclear envelope breakdown and cytokinesis. Ca2+/calmodulin-dependent protein kinase II and protein phosphatase calcineurin are both involved in the Ca2+ and calmodulin-mediated signalling of growth regulation. As compared to normal cells, growth of transformed cells is independent of extracellular Ca2+ and much less sensitive to calmodulin antagonists, suggesting the existence of derangements in the Ca2+ and calmodulin-mediated growth regulation mechanisms.

Histone H1 kenase activity during in vitro fertilization of pig follicular oocytes matured in vitro

The present study was conducted to clarify the relationship between histone H1 kinase (H1K) activity and events associated with in vitro fertilization of pig follicular oocytes matured in vitro. Histone H1 kinase has been shown to be homologous with a maturation promoting factor (MPF). Cumulus-oocyte complexes obtained from prepubertal gilts were cultured for 46 h in a modified Waymouth's MB752/1 medium and were then inseminated in vitro with frozen-thawed and preincubated epididymal boar spermatozoa. At 4, 6, 8 and 10 h post insemination, the oocytes were stained with 10 microg/ml Hoechst-33342 and examined under a fluorescent microscope for the stage of fertilization, according to morphological changes of oocyte nuclear chromatin and the extent of sperm penetration. Sperm penetration was observed to occur within 4 h post insemination (20.5%), and the percentage of fertilized oocytes increased (P < 0.01) to 72.9% at 8 h post insemination. Pronuclear formation was observed from 6 h post insemination (3.3%) and the percentage increased (P < 0.01) to 46.8% at 10 h post insemination. In each examination period, H1K activities in unfertilized oocytes at metaphase-II remained unchanged (112.0 fmol/h/oocyte) and were higher (P < 0.01) than those in fertilized oocytes (30.1 fmol/h/oocyte). The H1K activity in fertilized oocytes such as oocytes emitting a second polar body, oocytes with an enlarging sperm head(s) and oocytes with multiple pronuclei did not differ significantly. These results suggest that MPF in pig oocytes is inactivated shortly after sperm penetration and is maintained at the basal level throughout pronuclear formation.

Inactivation of a Cdk2 inhibitor during interleukin 2-induced proliferation of human T lymphocytes

Peripheral blood T lymphocytes require two sequential mitogenic signals to reenter the cell cycle from their natural, quiescent state. One signal is provided by stimulation of the T-cell antigen receptor, and this induces the synthesis of both cyclins and cyclin-dependent kinases (CDKs) that are necessary for progression through G1. Antigen receptor stimulation alone, however, is insufficient to promote activation of G1 cyclin-Cdk2 complexes. This is because quiescent lymphocytes contain an inhibitor of Cdk2 that binds directly to this kinase and prevents its activation by cyclins. The second mitogenic signal, which can be provided by the cytokine interleukin 2, leads to inactivation of this inhibitor, thereby allowing Cdk2 activation and progression into S phase. Enrichment of the Cdk2 inhibitor from G1 lymphocytes by cyclin-CDK affinity chromatography indicates that it may be p27Kip1. These observations show how sequentially acting mitogenic signals can combine to promote activation of cell cycle proteins and thereby cause cell proliferation to start. CDK inhibitors have been shown previously to be induced by signals that negatively regulate cell proliferation. Our new observations show that similar proteins are down-regulated by positively acting signals, such as interleukin 2. This finding suggests that both positive and negative growth signals converge on common targets which are regulators of G1 cyclin-CDK complexes. Inactivation of G1 cyclin-CDK inhibitors by mitogenic growth factors may be one biochemical pathway underlying cell cycle commitment at the restriction point in G1.

Inactivation of a Cdk2 inhibitor during interleukin 2-induced proliferation of human T lymphocytes

Peripheral blood T lymphocytes require two sequential mitogenic signals to reenter the cell cycle from their natural, quiescent state. One signal is provided by stimulation of the T-cell antigen receptor, and this induces the synthesis of both cyclins and cyclin-dependent kinases (CDKs) that are necessary for progression through G1. Antigen receptor stimulation alone, however, is insufficient to promote activation of G1 cyclin-Cdk2 complexes. This is because quiescent lymphocytes contain an inhibitor of Cdk2 that binds directly to this kinase and prevents its activation by cyclins. The second mitogenic signal, which can be provided by the cytokine interleukin 2, leads to inactivation of this inhibitor, thereby allowing Cdk2 activation and progression into S phase. Enrichment of the Cdk2 inhibitor from G1 lymphocytes by cyclin-CDK affinity chromatography indicates that it may be p27Kip1. These observations show how sequentially acting mitogenic signals can combine to promote activation of cell cycle proteins and thereby cause cell proliferation to start. CDK inhibitors have been shown previously to be induced by signals that negatively regulate cell proliferation. Our new observations show that similar proteins are down-regulated by positively acting signals, such as interleukin 2. This finding suggests that both positive and negative growth signals converge on common targets which are regulators of G1 cyclin-CDK complexes. Inactivation of G1 cyclin-CDK inhibitors by mitogenic growth factors may be one biochemical pathway underlying cell cycle commitment at the restriction point in G1.