The localization of human cyclins B1 and B2 determines CDK1 substrate specificity and neither enzyme requires MEK to disassemble the Golgi apparatus

Live-cell imaging defines a threshold in CDK activity at the G2/M transition

Cyclin-dependent kinase (CDK) determines the temporal ordering of the cell cycle phases. However, despite significant progress in studying regulators of CDK and phosphorylation patterns of CDK substrates at the population level, it remains elusive how CDK regulators coordinately affect CDK activity at the single-cell level and how CDK controls the temporal order of cell cycle events. Here, we elucidate the dynamics of CDK activity in fission yeast and mammalian cells by developing a CDK activity biosensor, Eevee-spCDK. We find that although CDK activity does not necessarily correlate with cyclin levels, it converges to the same level around mitotic onset in several mutant backgrounds, including pom1Δ cells and wee1 or cdc25 overexpressing cells. These data provide direct evidence that cells enter the M phase when CDK activity reaches a high threshold, consistent with the quantitative model of cell cycle progression in fission yeast.

Master mitotic kinases regulate viral genome delivery during papillomavirus cell entry

Mitosis induces cellular rearrangements like spindle formation, Golgi fragmentation, and nuclear envelope breakdown. Similar to certain retroviruses, nuclear delivery during entry of human papillomavirus (HPV) genomes is facilitated by mitosis, during which minor capsid protein L2 tethers viral DNA to mitotic chromosomes. However, the mechanism of viral genome delivery and tethering to condensed chromosomes is barely understood. It is unclear, which cellular proteins facilitate this process or how this process is regulated. This work identifies crucial phosphorylations on HPV minor capsid protein L2 occurring at mitosis onset. L2's chromosome binding region (CBR) is sequentially phosphorylated by the master mitotic kinases CDK1 and PLK1. L2 phosphorylation, thus, regulates timely delivery of HPV vDNA to mitotic chromatin during mitosis. In summary, our work demonstrates a crucial role of mitotic kinases for nuclear delivery of viral DNA and provides important insights into the molecular mechanism of pathogen import into the nucleus during mitosis.

Mad2 promotes Cyclin B2 recruitment to the kinetochore for guiding accurate mitotic checkpoint

Accurate mitotic progression relies on the dynamic phosphorylation of multiple substrates by key mitotic kinases. Cyclin-dependent kinase 1 is a master kinase that coordinates mitotic progression and requires its regulatory subunit Cyclin B to ensure full kinase activity and substrate specificity. The function of Cyclin B2, which is a closely related family member of Cyclin B1, remains largely elusive. Here, we show that Mad2 promotes the kinetochore localization of Cyclin B2 and that their interaction at the kinetochores guides accurate chromosome segregation. Our biochemical analyses have characterized the Mad2-Cyclin B2 interaction and delineated a novel Mad2-interacting motif (MIM) on Cyclin B2. The functional importance of the Cyclin B2-Mad2 interaction was demonstrated by real-time imaging in which MIM-deficient mutant Cyclin B2 failed to rescue the chromosomal segregation defects. Taken together, we have delineated a previously undefined function of Cyclin B2 at the kinetochore and have established, in human cells, a mechanism of action by which Mad2 contributes to the spindle checkpoint.

CRHBP is degraded via autophagy and exerts anti-hepatocellular carcinoma effects by reducing cyclin B2 expression and dissociating cyclin B2-CDK1 complex

Autophagy is the predominant self-eating catabolic pathway activated in response to nutrient starvation and hypoxia within the microenvironment of varied malignancies, including hepatocellular carcinoma (HCC). SQSTM1/p62 links its cargos to autophagosomes for degradation, and reportedly acts as a contributor for hepatocarcinogenesis. Five GEO gene microarrays identified corticotropin releasing hormone (CRH) binding protein (CRHBP) as a significantly downregulated gene in HCC (log2 Fold change < -3 and p < 0.001), and an earlier human interactome study indicated that CRHBP may interact with p62. This study aimed to explore (1) the role of CRHBP in HCC development, and (2) whether p62-mediated autophagy was responsible for low CRHBP expression within HCC tissue. Following functional experiments first revealed an anti-proliferative, anti-metastatic, and anti-angiogenic role of CRHBP in HCC cells (Huh-7, Li-7 and HCCLM3) and xenografts. CRHBP negatively regulated cyclin B2 expression, and dissociated cyclin B2-CDK1 complex in HCC cells, thereby leading to cell cycle arrest at G2 phase. To simulate HCC microenvironment in vitro, Huh-7 cells were incubated in Earle's Balanced Salt Solution (nutrient starvation) or exposed to 1% O₂ (hypoxic exposure). In addition to activating autophagy, nutrient starvation and hypoxic exposure also induced CRHBP degradation. Interestingly, CRHBP was demonstrated as a novel cargo targeted by p62 for degradation in autophagosomes. Blocking autophagy with 3-MA, chloroquine or siSQSTM1 prevented CRHBP degradation in HCC cells. Collectively, our study uncovers a role for CRHBP in retarding HCC development, reducing cyclin B2 expression and impairing cyclin B2-CDK1 interaction. CRHBP downregulation in HCC may attribute to p62-mediated autophagy.

B1-type cyclins control microtubule organization during cell division in Arabidopsis

Flowering plants contain a large number of cyclin families, each containing multiple members, most of which have not been characterized to date. Here, we analyzed the role of the B1 subclass of mitotic cyclins in cell cycle control during Arabidopsis development. While we reveal CYCB1;5 to be a pseudogene, the remaining four members were found to be expressed in dividing cells. Mutant analyses showed a complex pattern of overlapping, development-specific requirements of B1-type cyclins with CYCB1;2 playing a central role. The double mutant cycb1;1 cycb1;2 is severely compromised in growth, yet viable beyond the seedling stage, hence representing a unique opportunity to study the function of B1-type cyclin activity at the organismic level. Immunolocalization of microtubules in cycb1;1 cycb1;2 and treating mutants with the microtubule drug oryzalin revealed a key role of B1-type cyclins in orchestrating mitotic microtubule networks. Subsequently, we identified the GAMMA-TUBULIN COMPLEX PROTEIN 3-INTERACTING PROTEIN 1 (GIP1/MOZART) as an in vitro substrate of B1-type cyclin complexes and further genetic analyses support a potential role in the regulation of GIP1 by CYCB1s.

CDK1, the Other 'Master Regulator' of Autophagy

Autophagy and cap-dependent mRNA translation are tightly regulated by the mechanistic target of rapamycin complex 1 (mTORC1) signalling complex in response to nutrient availability. However, the regulation of these processes, and mTORC1 itself, is different during mitosis, and this has remained an area of significant controversy; for example, studies have argued that autophagy is either repressed or highly active during mitosis. Recent studies have shown that autophagy initiation is repressed, and cap-dependent mRNA translation is maintained during mitosis despite mTORC1 activity being repressed. This is achieved in large part by a switch from mTORC1- to cyclin-dependent kinase 1 (CDK1)-mediated regulation. Here, we review the history and recent advances and seek to present a unifying model to inform the future study of autophagy and mTORC1 during mitosis.

Transcriptome analysis revealed key prognostic genes and microRNAs in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is one of the most common cancers worldwide. However, the molecular mechanisms involved in HCC remain unclear and are in urgent need of elucidation. Therefore, we sought to identify biomarkers in the prognosis of HCC through an integrated bioinformatics analysis.

Methods

Messenger RNA (mRNA) expression profiles were obtained from the Gene Expression Omnibus database and The Cancer Genome Atlas-Liver Hepatocellular Carcinoma (TCGA-LIHC) for the screening of common differentially expressed genes (DEGs). Function and pathway enrichment analysis, protein-protein interaction network construction and key gene identification were performed. The significance of key genes in HCC was validated by overall survival analysis and immunohistochemistry. Meanwhile, based on TCGA data, prognostic microRNAs (miRNAs) were decoded using univariable and multivariable Cox regression analysis, and their target genes were predicted by miRWalk.

Results

Eleven hub genes (upregulated ASPM, AURKA, CCNB2, CDC20, PRC1 and TOP2A and downregulated AOX1, CAT, CYP2E1, CYP3A4 and HP) with the most interactions were considered as potential biomarkers in HCC and confirmed by overall survival analysis. Moreover, AURKA, PRC1, TOP2A, AOX1, CYP2E1, and CYP3A4 were considered candidate liver-biopsy markers for high risk of developing HCC and poor prognosis in HCC. Upregulation of hsa-mir-1269b, hsa-mir-518d, hsa-mir-548aq, hsa-mir-548f-1, and hsa-mir-6728, and downregulation of hsa-mir-139 and hsa-mir-4800 were determined to be risk factors of poor prognosis, and most of these miRNAs have strong potential to help regulate the expression of key genes.

Conclusions

This study undertook the first large-scale integrated bioinformatics analysis of the data from Illumina BeadArray platforms and the TCGA database. With a comprehensive analysis of transcriptional alterations, including mRNAs and miRNAs, in HCC, our study presented candidate biomarkers for the surveillance and prognosis of the disease, and also identified novel therapeutic targets at the molecular and pathway levels.

The Hydrophobic Patch Directs Cyclin B to Centrosomes to Promote Global CDK Phosphorylation at Mitosis

The cyclin-dependent kinases (CDKs) are the major cell-cycle regulators that phosphorylate hundreds of substrates, controlling the onset of S phase and M phase [1, 2, 3]. However, the patterns of substrate phosphorylation increase are not uniform, as different substrates become phosphorylated at different times as cells proceed through the cell cycle [4, 5]. In fission yeast, the correct ordering of CDK substrate phosphorylation can be established by the activity of a single mitotic cyclin-CDK complex [6, 7]. Here, we investigate the substrate-docking region, the hydrophobic patch, on the fission yeast mitotic cyclin Cdc13 as a potential mechanism to correctly order CDK substrate phosphorylation. We show that the hydrophobic patch targets Cdc13 to the yeast centrosome equivalent, the spindle pole body (SPB), and disruption of this motif prevents both centrosomal localization of Cdc13 and the onset of mitosis but does not prevent S phase. CDK phosphorylation in mitosis is compromised for approximately half of all mitotic CDK substrates, with substrates affected generally being those that require the highest levels of CDK activity to become phosphorylated and those that are located at the SPB. Our experiments suggest that the hydrophobic patch of mitotic cyclins contributes to CDK substrate selection by directing the localization of Cdc13-CDK to centrosomes and that this localization of CDK contributes to the CDK substrate phosphorylation necessary to ensure proper entry into mitosis. Finally, we show that mutation of the hydrophobic patch prevents cyclin B1 localization to centrosomes in human cells, suggesting that this mechanism of cyclin-CDK spatial regulation may be conserved across eukaryotes.

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The hydrophobic patch of human and yeast cyclin B directs it to the centrosome

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Loss of the yeast cyclin B hydrophobic patch allows S phase but prevents mitosis

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Compartmentalized mitotic CDK phosphorylation relies on the hydrophobic patch

Mitotic kinase anchoring proteins: the navigators of cell division

The coordinated activities of many protein kinases, acting on multiple protein substrates, ensures the error-free progression through mitosis of eukaryotic cells. Enormous research effort has thus been devoted to studying the roles and regulation of these mitotic kinases, and to the identification of their physiological substrates. Central for the timely deployment of specific protein kinases to their appropriate substrates during the cell division cycle are the many anchoring proteins, which serve critical regulatory roles. Through direct association, anchoring proteins are capable of modulating the catalytic activity and/or sub-cellular distribution of the mitotic kinases they associate with. The key roles of some anchoring proteins in cell division are well-established, whilst others are still being unearthed. Here, we review the current knowledge on anchoring proteins for some mitotic kinases, and highlight how targeting anchoring proteins for inhibition, instead of the mitotic kinases themselves, could be advantageous for disrupting the cell division cycle.

Nuclear Export of Cyclin B Mediated by the Nup62 Complex Is Required for Meiotic Initiation in Drosophila Males

BACKGROUND

The central channel of the nuclear pore complex plays an important role in the selective transport of proteins between the nucleus and cytoplasm. Previous studies have demonstrated that the depletion of the Nup62 complex, constructing the nuclear pore channel in premeiotic Drosophila cells, resulted in the absence of meiotic cells. We attempted to understand the mechanism underlying the cell cycle arrest before meiosis.

METHODS

We induced dsRNAs against the nucleoporin mRNAs using the Gal4/UAS system in Drosophila.

RESULTS

The cell cycle of the Nup62-depleted cells was arrested before meiosis without CDK1 activation. The ectopic over-expression of CycB, but not constitutively active CDK1, resulted in partial rescue from the arrest. CycB continued to exist in the nuclei of Nup62-depleted cells and cells depleted of exportin encoded by emb. Protein complexes containing CycB, Emb, and Nup62 were observed in premeiotic spermatocytes. CycB, which had temporally entered the nucleus, was associated with Emb, and the complex was transported back to the cytoplasm through the central channel, interacting with the Nup62 complex. Conclusion: We proposed that CycB is exported with Emb through the channel interacting with the Nup62 complex before the onset of meiosis. The nuclear export ensures the modification and formation of sufficient CycB-CDK1 in the cytoplasm.

Identification of key protein-coding genes in lung adenocarcinomas based on bioinformatic analysis

Background

Lung cancer is one of the most common cancers and the primary cause of cancer-related deaths in the world. The 5-year survival of lung cancer patients is lower than 15%. As a common subtype of lung cancer, lung adenocarcinoma still has a high morbidity and mortality, although many strategies have been made, such as surgical operation, chemotherapy, targeted therapy. The use of gene expression microarray has provided a feasible and effective approach for the study on lung cancer. However, the biomarkers and potential therapeutic targets of lung adenocarcinomas are still not completely identified. Our study is aimed to find biomarkers and therapeutic targets of lung adenocarcinomas by identifying the key protein-coding gene in lung adenocarcinomas by bioinformatical approaches.

Methods

We selected and obtained messenger RNA microarray datasets from Gene Expression Omnibus database to identify differentially expressed genes between lung adenocarcinomas and normal lung tissue. The differentially expressed genes were clarified by Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, the protein-protein interaction (PPI) network and statistical analyses. Subsequently, quantitative real-time PCR was used to verify the results of bioinformatic analysis.

Results

We obtained 1,264, 896 and 408 differentially expressed genes from GSE32863, GSE43458 and GSE63459, respectively. The 242 common differentially expressed genes in three datasets were related to cell adhesion molecules, ECM-receptor interaction, leukocyte transendothelial migration according to KEGG analysis. GO analysis showed that these common differentially expressed genes were enriched in tumor-related functions. ASPM, CCNB2, CDC20, CDC45, MELK, TOP2A and UBE2T and KIAA0101 have the strongest protein-protein interaction relationships based on protein-protein interaction networks. Survival analysis showed that these nine genes were closely related to the survival of lung adenocarcinomas. The further qRT-PCR assays indicated that seven key genes (ASPM, CCNB2, CDC20, CDC45, MELK, TOP2A and UBE2T) display differential profile between clinical lung adenocarcinoma specimens and their matched normal tissues.

Conclusions

ASPM, CCNB2, CDC20, CDC45, MELK, TOP2A and UBE2T may be key protein coding genes in lung adenocarcinoma, and deserve further study to verify their feasibility and effectiveness as biomarkers and therapeutic targets for lung adenocarcinomas.

Degradation of CCNB1 mediated by APC11 through UBA52 ubiquitination promotes cell cycle progression and proliferation of non-small cell lung cancer cells

OBJECTIVE

Mechanism by which CCNB1 regulates the cell cycle progression and its prognostic function in non-squamous non-small cell lung cancer (NSCLC) are necessary to be further elucidated.

METHODS

Data retrieved from gene expression omnibus (GEO) and cancer genome atlas (TCGA) combined with clinical data were used. Survival analysis was conducted in public datasets. Proteomics and co-immunoprecipation assays were designed to unravel proteins with interaction with CCNB1. Short hairpin RNA and small interfering RNA as well as overexpressing genes of interest were used.

RESULTS

CCNB1 was not implicated in apoptosis, migration and invasion of NSCLC cells. After either knockdown or overexpression of CCNB1, the occurrence of cell cycle arrest in G2/M phase, fewer cloning formation and diminished dimension of xenograft tumors were observed. CCNB1 expression level was clinically associated with several clinicopathological parameters including gender, smoking, T stage and N stage. Survival analysis showed that the higher level of CCNB1, the more dismal outcome in overall survival as well as in disease-free survival. Mechanistically, we confirmed that the role of CCNB1 on cell cycle and cloning formation was dependent on UBA52, which was able to promote degradation of CCNB1; nevertheless, this consequence relied on APC11. Knockdown of APC11 led to cell cycle arrest in G2/M and less cloning formation even in the presence of overexpressed UBA52. Following upregulation of APC11, the protein of CCNB1 degraded with resultant cell cycle progression and more cloning formation.

CONCLUSION

Degradation of CCNB1 by APC11 via UBA52 ubiquitylation was critical in cell cycle progression and proliferation of NSCLC cell lines.

Cell cycle regulation and hematologic malignancies

A complex network precisely regulates the cell cycle through the G₁, S, G₂, and M phases and is the basis for cell division under physiological and pathological conditions. On the one hand, the transition from one phase to another as well as the progression within each phase is driven by the specific cyclin-dependent kinases (CDKs; e.g., CDK1, CDK2, CDK4, CDK6, and CDK7), together with their exclusive partner cyclins (e.g., cyclin A1, B1, D1–3, and E1). On the other hand, these phases are negatively regulated by endogenous CDK inhibitors such as p16^ink4a, p18^ink4c, p19^ink4d, p21^cip1, and p27^kip1. In addition, several checkpoints control the commitment of cells to replicate DNA and undergo mitosis, thereby avoiding the passage of genomic errors to daughter cells. CDKs are often constitutively activated in cancer, which is characterized by the uncontrolled proliferation of transformed cells, due to genetic and epigenetic abnormalities in the genes involved in the cell cycle. Moreover, several oncogenes and defective tumor suppressors promote malignant changes by stimulating cell cycle entry and progression or disrupting DNA damage responses, including the cell cycle checkpoints, DNA repair mechanisms, and apoptosis. Thus, genes or proteins related to cell cycle regulation remain the main targets of interest in the treatment of various cancer types, including hematologic malignancies. In this context, advances in the understanding of the cell cycle regulatory machinery provide a basis for the development of novel therapeutic approaches. The present article summarizes the pathways as well as their genetic and epigenetic alterations that regulate the cell cycle; moreover, it discusses the various approved or potential therapeutic targets associated with the cell cycle, focusing on hematologic malignancies.

MAD1-dependent recruitment of CDK1-CCNB1 to kinetochores promotes spindle checkpoint signaling

Cyclin B-dependent kinase (CDK1-CCNB1) promotes entry into mitosis. Additionally, it inhibits mitotic exit by activating the spindle checkpoint. This latter role is mediated through phosphorylation of the checkpoint kinase MPS1 and other spindle checkpoint proteins. We find that CDK1-CCNB1 localizes to unattached kinetochores and like MPS1 is lost from these structures upon microtubule attachment. This suggests that CDK1-CCNB1 is an integral component and not only an upstream regulator of the spindle checkpoint pathway. Complementary proteomic and cell biological analysis demonstrate that the spindle checkpoint protein MAD1 is one of the major components of CCNB1 complexes, and that CCNB1 is recruited to unattached kinetochores in an MPS1-dependent fashion through interaction with the first 100 amino acids of MAD1. This MPS1 and MAD1-dependent pool of CDK1-CCNB1 creates a positive feedback loop necessary for timely recruitment of MPS1 to kinetochores during mitotic entry and for sustained spindle checkpoint arrest. CDK1-CCNB1 is therefore an integral component of the spindle checkpoint, ensuring the fidelity of mitosis.

Lack of Cyclin B1 in zebrafish causes lengthening of G2 and M phases

An essential part of the Mitosis Promoting Factor, Cyclin B1 is indispensable for cells to enter mitosis. We report here that the zebrafish early arrest mutant specter is a loss-of-function mutation in the сyclin B1 gene. cyclin B1 is maternally transcribed in zebrafish, and the zygotic phenotype is apparent by early segmentation. Lack of zygotic Cyclin B1 does not stop cells from dividing, rather it causes an abnormal and elongated progression through the G2 and M phases of the cell cycle. Many mutant cells show signs of chromosomal instability or enter apoptosis. Using CRISPR-mediated gene editing, we produced a more severe gain-of-function mutation confirming that specter is the result of nonfunctional Cyclin B1. Although also a recessive phenotype, this new mutation produces an alternative splice-form of cyclin B1 mRNA, whose product lacks several key components for Cyclin B1, but not the Cdk1-binding domain. This mutant form of Cyclin B1 completely prevents cell division. We conclude that, although Cyclin B1 is critical for cells to enter mitosis, another cell cycle protein may be cooperating with Cdk1 at the G2/M checkpoint to sustain a partly functional Mitosis Promoting Factor.

The Emerging Role of Cyclin-Dependent Kinases (CDKs) in Pancreatic Ductal Adenocarcinoma

The family of cyclin-dependent kinases (CDKs) has critical functions in cell cycle regulation and controlling of transcriptional elongation. Moreover, dysregulated CDKs have been linked to cancer initiation and progression. Pharmacological CDK inhibition has recently emerged as a novel and promising approach in cancer therapy. This idea is of particular interest to combat pancreatic ductal adenocarcinoma (PDAC), a cancer entity with a dismal prognosis which is owed mainly to PDAC's resistance to conventional therapies. Here, we review the current knowledge of CDK biology, its role in cancer and the therapeutic potential to target CDKs as a novel treatment strategy for PDAC.

CRISPR/Cas9-mediated genome editing induces gene knockdown by altering the pre-mRNA splicing in mice

Background

Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated protein 9 (CRISPR/Cas9) has been wildly used to generate gene knockout models through inducing indels causing frame-shift. However, there are few studies concerning the post-transcript effects caused by CRISPR-mediated genome editing.

Results

In the present study, we showed that gene knockdown model also could be generated using CRISPR-mediated gene editing by disrupting the boundary of exon and intron in mice (C57BL/6 J). CRISPR induced indel at the boundary of exon and intron (5′ splice site) caused alternative splicing and produced multiple different mRNAs, most of these mRNAs introduced premature termination codon causing down expression of the gene.

Conclusions

These results showed that alternative splicing mutants were able to generate through CRISPR-mediated genome editing by deleting the boundary of exon and intron causing disruption of 5′ splice site. Although alternative splicing was an unexpected outcome, this finding could be developed as a technology to generate gene knockdown models or to investigate pre-mRNA splicing.

Electronic supplementary material

The online version of this article (10.1186/s12896-018-0472-8) contains supplementary material, which is available to authorized users.

Use of Serum Circulating Ccnb2 in Cancer Surveillance

Cyclin B2 (CCNB2), a member of the cyclin protein family, has been found to be up-regulated in human cancers. To evaluate the potential use of circulating CCNB2 in serum in cancer surveillance, we examined relative expression levels of serum circulating CCNB2 mRNA in 103 cancer patients, 19 normal controls, and 40 benign disease patients using real-time quantitative reverse transcriptase polymerase chain reaction. We found that the relative expression level of circulating CCNB2 mRNA in cancer patients was significantly higher (p<0.0001) than that in normal controls and benign diseases group. Circulating CCNB2 mRNA level was significantly (p<0.001) correlated with cancer stage and metastasis status. Receiver operating characteristic (ROC) analysis showed an area under the curve (AUC) of 0.87 and 0.83 (p<0.05) in identifying cancer patients' metastasis status in lung and digestive tract cancer, respectively. Moreover, we observed that expression levels of circulating CCNB2 mRNA in cancer patients significantly decreased (p=0.0084) after their therapeutic treatments. These data suggest that detection of serum circulating CCNB2 mRNA may have potential clinical applications in screening and monitoring of metastasis and therapeutic treatments.

Cyclin B1 is essential for mitosis in mouse embryos, and its nuclear export sets the time for mitosis

There is remarkable redundancy between the Cyclin-Cdk complexes that comprise the cell cycle machinery. None of the mammalian A-, D-, or E-type cyclins are required in development until implantation, and only Cdk1 is essential for early cell divisions. Cyclin B1 is essential for development, but whether it is required for cell division is contentious. Here, we used a novel imaging approach to analyze Cyclin B1-null embryos from fertilization onward. We show that Cyclin B1-/- embryos arrest in G2 phase after just two divisions. This is the earliest arrest of any Cyclin known and places Cyclin B1 with cdk1 as the essential regulators of the cell cycle. We reintroduced mutant proteins into this genetically null background to determine why Cyclin B1 is constantly exported from the nucleus. We found that Cyclin B1 must be exported from the nucleus for the cell to prevent premature entry to mitosis, and retaining Cyclin B1-Cdk1 at the plasma membrane precludes entry to mitosis.

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.

The history and future of targeting cyclin-dependent kinases in cancer therapy

Cancer represents a pathological manifestation of uncontrolled cell division; therefore, it has long been anticipated that our understanding of the basic principles of cell cycle control would result in effective cancer therapies. In particular, cyclin-dependent kinases (CDKs) that promote transition through the cell cycle were expected to be key therapeutic targets because many tumorigenic events ultimately drive proliferation by impinging on CDK4 or CDK6 complexes in the G1 phase of the cell cycle. Moreover, perturbations in chromosomal stability and aspects of S phase and G2/M control mediated by CDK2 and CDK1 are pivotal tumorigenic events. Translating this knowledge into successful clinical development of CDK inhibitors has historically been challenging, and numerous CDK inhibitors have demonstrated disappointing results in clinical trials. Here, we review the biology of CDKs, the rationale for therapeutically targeting discrete kinase complexes and historical clinical results of CDK inhibitors. We also discuss how CDK inhibitors with high selectivity (particularly for both CDK4 and CDK6), in combination with patient stratification, have resulted in more substantial clinical activity.

Exploiting holistic approaches to model specificity in protein phosphorylation

Phosphate plays a chemically unique role in shaping cellular signaling of all current living systems, especially eukaryotes. Protein phosphorylation has been studied at several levels, from the near-site context, both in sequence and structure, to the crowded cellular environment, and ultimately to the systems-level perspective. Despite the tremendous advances in mass spectrometry and efforts dedicated to the development of ad hoc highly sophisticated methods, phosphorylation site inference and associated kinase identification are still unresolved problems in kinome biology. The sequence and structure of the substrate near-site context are not sufficient alone to model the in vivo phosphorylation rules, and they should be integrated with orthogonal information in all possible applications. Here we provide an overview of the different contexts that contribute to protein phosphorylation, discussing their potential impact in phosphorylation site annotation and in predicting kinase-substrate specificity.

Cyclin B2 and p53 control proper timing of centrosome separation

Cyclins B1 and B2 are frequently elevated in human cancers and are associated with tumour aggressiveness and poor clinical outcome; however, whether and how B-type cyclins drive tumorigenesis is unknown. Here we show that cyclin B1 and B2 transgenic mice are highly prone to tumours, including tumour types where B-type cyclins serve as prognosticators. Cyclins B1 and B2 both induce aneuploidy when overexpressed but through distinct mechanisms, with cyclin B1 inhibiting separase activation, leading to anaphase bridges, and cyclin B2 triggering aurora-A-mediated Plk1 hyperactivation, resulting in accelerated centrosome separation and lagging chromosomes. Complementary experiments revealed that cyclin B2 and p53 act antagonistically to control aurora-A-mediated centrosome splitting and accurate chromosome segregation in normal cells. These data demonstrate a causative link between B-type cyclin overexpression and tumour pathophysiology, and uncover previously unknown functions of cyclin B2 and p53 in centrosome separation that may be perturbed in many human cancers.

The kinetics of G2 and M transitions regulated by B cyclins

B cyclins regulate G2-M transition. Because human somatic cells continue to cycle after reduction of cyclin B1 (cycB1) or cyclin B2 (cycB2) by RNA interference (RNAi), and because cycB2 knockout mice are viable, the existence of two genes should be an optimization. To explore this idea, we generated HeLa BD™ Tet-Off cell lines with inducible cyclin B1- or B2-EGFP that were RNAi resistant. Cultures were treated with RNAi and/or doxycycline (Dox) and bromodeoxyuridine. We measured G2 and M transit times and 4C cell accumulation. In the absence of ectopic B cyclin expression, knockdown (kd) of either cyclin increased G2 transit. M transit was increased by cycB1 kd but decreased by cycB2 depletion. This novel difference was further supported by time-lapse microscopy. This suggests that cycB2 tunes mitotic timing, and we speculate that this is through regulation of a Golgi checkpoint. In the presence of endogenous cyclins, expression of active B cyclin-EGFPs did not affect G2 or M phase times. As previously shown, B cyclin co-depletion induced G2 arrest. Expression of either B cyclin-EGFP completely rescued knockdown of the respective endogenous cyclin in single kd experiments, and either cyclin-EGFP completely rescued endogenous cyclin co-depletion. Most of the rescue occurred at relatively low levels of exogenous cyclin expression. Therefore, cycB1 and cycB2 are interchangeable for ability to promote G2 and M transition in this experimental setting. Cyclin B1 is thought to be required for the mammalian somatic cell cycle, while cyclin B2 is thought to be dispensable. However, residual levels of cyclin B1 or cyclin B2 in double knockdown experiments are not sufficient to promote successful mitosis, yet residual levels are sufficient to promote mitosis in the presence of the dispensible cyclin B2. We discuss a simple model that would explain most data if cyclin B1 is necessary.

Golgi-associated RhoBTB3 targets cyclin E for ubiquitylation and promotes cell cycle progression

The Golgi protein RhoBTB3 in complex with CUL3 and RBX1 promotes Cyclin E ubiquitylation to allow its turnover during S phase and progression through the cell cycle.

Cyclin E regulates the cell cycle transition from G1 to S phase and is degraded before entry into G2 phase. Here we show that RhoBTB3, a Golgi-associated, Rho-related ATPase, regulates the S/G2 transition of the cell cycle by targeting Cyclin E for ubiquitylation. Depletion of RhoBTB3 arrested cells in S phase, triggered Golgi fragmentation, and elevated Cyclin E levels. On the Golgi, RhoBTB3 bound Cyclin E as part of a Cullin3 (CUL3)-dependent RING–E3 ubiquitin ligase complex comprised of RhoBTB3, CUL3, and RBX1. Golgi association of this complex was required for its ability to catalyze Cyclin E ubiquitylation and allow normal cell cycle progression. These experiments reveal a novel role for a Ras superfamily member in catalyzing Cyclin E turnover during S phase, as well as an unexpected, essential role for the Golgi as a ubiquitylation platform for cell cycle control.

Multi-step down-regulation of the secretory pathway in mitosis: a fresh perspective on protein trafficking

The secretory pathway delivers proteins synthesized at the rough endoplasmic reticulum (RER) to various subcellular locations via the Golgi apparatus. Currently, efforts are focused on understanding the molecular machineries driving individual processes at the RER and Golgi that package, modify and transport proteins. However, studies are routinely performed using non-dividing cells. This obscures the critical issue of how the secretory pathway is affected by cell division. Indeed, several studies have indicated that protein trafficking is down-regulated during mitosis. Moreover, the RER and Golgi apparatus exhibit gross reorganization in mitosis. Here I provide a relatively neglected perspective of how the mitotic cyclin-dependent kinase (CDK1) could regulate various stages of the secretory pathway. I highlight several aspects of the mitotic control of protein trafficking that remain unresolved and suggest that further studies on how the mitotic CDK1 influences the secretory pathway are necessary to obtain a deeper understanding of protein transport.

Mitosis is a source of potential markers for screening and survival and therapeutic targets in cervical cancer

The effect of preventive human papillomavirus (HPV) vaccination on the reduction of the cervical cancer (CC) burden will not be known for 30 years. Therefore, it's still necessary to improve the procedures for CC screening and treatment. The objective of this study was to identify and characterize cellular targets that could be considered potential markers for screening or therapeutic targets. A pyramidal strategy was used. Initially the expression of 8,638 genes was compared between 43 HPV16-positive CCs and 12 healthy cervical epitheliums using microarrays. A total of 997 genes were deregulated, and 21 genes that showed the greatest deregulation were validated using qRT-PCR. The 6 most upregulated genes (CCNB2, CDC20, PRC1, SYCP2, NUSAP1, CDKN3) belong to the mitosis pathway. They were further explored in 29 low-grade cervical intraepithelial neoplasias (CIN1) and 21 high-grade CIN (CIN2/3) to investigate whether they could differentiate CC and CIN2/3 (CIN2+) from CIN1 and controls. CCNB2, PRC1, and SYCP2 were mostly associated with CC and CDC20, NUSAP1, and CDKN3 were also associated with CIN2/3. The sensitivity and specificity of CDKN3 and NUSAP1 to detect CIN2+ was approximately 90%. The proteins encoded by all 6 genes were shown upregulated in CC by immunohistochemistry. The association of these markers with survival was investigated in 42 CC patients followed up for at least 42 months. Only CDKN3 was associated with poor survival and it was independent from clinical stage (HR = 5.9, 95%CI = 1.4-23.8, p = 0.01). CDKN3 and NUSAP1 may be potential targets for the development of screening methods. Nevertheless, further studies with larger samples are needed to define the optimal sensitivity and specificity. Inhibition of mitosis is a well-known strategy to combat cancers. Therefore, CDKN3 may be not only a screening and survival marker but a potential therapeutic target in CC. However, whether it's indispensable for tumor growth remains to be demonstrated.

Recent insights into the complexity of Tank-binding kinase 1 signaling networks: the emerging role of cellular localization in the activation and substrate specificity of TBK1

Tank-binding kinase 1 (TBK1) serves as an important component of multiple signaling pathways. While the majority of research on TBK1 has focused on its role in innate immunity, critical functions for TBK1 in autophagy and cancer are beginning to emerge. This review highlights recent structural and biochemical studies that provide insights into the molecular mechanism of TBK1 activation and summarizes what is known to date about TBK1 substrate selection. Growing evidence suggests that both processes rely on TBK1 subcellular localization, with a variety of adaptor proteins each directing TBK1 to discrete signaling complexes for different cellular responses. Further study of TBK1-mediated pathways will require careful consideration of TBK1 mechanisms of activation and specificity for proper dissection of these distinct signaling cascades.

MEK1 inactivates Myt1 to regulate Golgi membrane fragmentation and mitotic entry in mammalian cells

The pericentriolar stacks of Golgi cisternae are separated from each other in G2 and fragmented extensively during mitosis. MEK1 is required for Golgi fragmentation in G2 and for the entry of cells into mitosis. We now report that Myt1 mediates MEK1's effects on the Golgi complex. Knockdown of Myt1 by siRNA increased the efficiency of Golgi complex fragmentation by mitotic cytosol in permeabilized and intact HeLa cells. Myt1 knockdown eliminated the requirement of MEK1 in Golgi fragmentation and alleviated the delay in mitotic entry due to MEK1 inhibition. The phosphorylation of Myt1 by MEK1 requires another kinase but is independent of RSK, Plk, and CDK1. Altogether our findings reveal that Myt1 is inactivated by MEK1 mediated phosphorylation to fragment the Golgi complex in G2 and for the entry of cells into mitosis. It is known that Myt1 inactivation is required for CDK1 activation. Myt1 therefore is an important link by which MEK1 dependent fragmentation of the Golgi complex in G2 is connected to the CDK1 mediated breakdown of Golgi into tubules and vesicles in mitosis.

"The Octet": Eight Protein Kinases that Control Mammalian DNA Replication

Development of a fertilized human egg into an average sized adult requires about 29 trillion cell divisions, thereby producing enough DNA to stretch to the Sun and back 200 times (DePamphilis and Bell, 2011)! Even more amazing is the fact that throughout these mitotic cell cycles, the human genome is duplicated once and only once each time a cell divides. If a cell accidentally begins to re-replicate its nuclear DNA prior to cell division, checkpoint pathways trigger apoptosis. And yet, some cells are developmentally programmed to respond to environmental cues by switching from mitotic cell cycles to endocycles, a process in which multiple S phases occur in the absence of either mitosis or cytokinesis. Endocycles allow production of viable, differentiated, polyploid cells that no longer proliferate. What is surprising is that among the 516 (Manning et al., 2002) to 557 (BioMart web site) protein kinases encoded by the human genome, only eight regulate nuclear DNA replication directly. These are Cdk1, Cdk2, Cdk4, Cdk6, Cdk7, Cdc7, Checkpoint kinase-1 (Chk1), and Checkpoint kinase-2. Even more remarkable is the fact that only four of these enzymes (Cdk1, Cdk7, Cdc7, and Chk1) are essential for mammalian development. Here we describe how these protein kinases determine when DNA replication occurs during mitotic cell cycles, how mammalian cells switch from mitotic cell cycles to endocycles, and how cancer cells can be selectively targeted for destruction by inducing them to begin a second S phase before mitosis is complete.

Gene expression profiling of nephrotoxicity from copper nanoparticles in rats after repeated oral administration

The goal of this study was to investigate the mechanisms of nanocopper-induced nephrotoxicity by analyzing renal gene expression profiles phenotypically anchored to conventional toxicological outcomes. Male Wistar rats were given nanocopper (50, 100, 200 mg/kg) and microcopper (200 mg/kg) at different doses for 5 days. We found nanocopper can induce widespread renal proximal tubule necrosis in rat kidneys with blood urea nitrogen and creatinine increase. Whole genome transcriptome profiling of rat kidneys revealed significant alterations in the expression of many genes involved in valine, leucine, and isoleucine degradation, complement and coagulation cascades, oxidative phosphorylation, cell cycle, mitogen-activated protein kinase signaling pathway, glutathione metabolism, and others may be involved in the development of these phenotypes. Results from this study provide new insights into the nephrotoxicity of copper nano-particles and illustrate how toxicogenomic approaches are providing an unprecedented amount of mechanistic information on molecular responses to nanocopper and how they are likely to impact hazard and risk assessment.

The subcellular localization of cyclin B2 is required for bipolar spindle formation during Xenopus oocyte maturation

Cyclins B1 and B2 are subtypes of cyclin B, a regulatory subunit of a maturation/M-phase promoting factor, and they are also highly conserved in many vertebrate species. Cyclin B1 is essential for mitosis, whereas cyclin B2 is regarded as dispensable. However, the overexpression of the cyclin B2 N-terminus containing the cytoplasmic retention signal, but not cyclin B1, inhibits bipolar spindle formation in Xenopus oocytes and embryos. Here we show that endogenous cyclin B2 was localized in and around the germinal vesicle. The perinuclear localization of cyclin B2 was perturbed by the overexpression of its N-terminus containing the cytoplasmic retention signal, which resulted in a spindle defect. This spindle defect was rescued by the overexpression of bipolar kinesin Eg5, which is located at the perinuclear region in the proximity of endogenous cyclin B2. These results demonstrate that the proper localization of cyclin B2 is essential for bipolar spindle formation in Xenopus oocytes.

Among B-type cyclins only CLB5 and CLB6 promote premeiotic S phase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae cyclin Clb5 is required for premeiotic S phase, meiotic recombination, and successful progression through meiosis. Clb5 is not essential for mitotic proliferation because Clb1-Clb4 can support DNA replication in clb5 clb6 mutants. Clb1, Clb3, and Clb4 accumulate in clb5 clb6 cells during meiotic differentiation yet fail to promote premeiotic DNA replication. When expressed under the regulation of the CLB5 promoter, Clb1 and Clb3 accumulate and are active in the early stages of meiotic differentiation but cannot induce premeiotic DNA replication, suggesting that they do not target Cdk1 to the necessary substrates. The Clb5 hydrophobic patch (HP) residues are important for Clb5 function but this motif alone does not provide the specificity required for Clb5 to induce premeiotic S phase. Domain exchange experiments demonstrated that the amino terminus of Clb5 when fused to Clb3 confers upon Clb3 the ability to induce premeiotic S phase. Chimeric cyclins containing smaller regions of the Clb5 amino terminus displayed reduced ability to activate premeiotic DNA replication despite being more abundant and having greater associated histone H1 kinase activity than endogenous Clb5. These observations suggest that Clb5 has a unique ability to trigger premeiotic S phase and that the amino-terminal region of Clb5 contributes to its specificity and regulates the functions performed by the cyclin-Cdk complex.

Cyclin-specific docking motifs promote phosphorylation of yeast signaling proteins by G1/S Cdk complexes

BACKGROUND

The eukaryotic cell cycle begins with a burst of cyclin-dependent kinase (Cdk) phosphorylation. In budding yeast, several Cdk substrates are preferentially phosphorylated at the G1/S transition rather than later in the cell cycle when Cdk activity levels are high. These early Cdk substrates include signaling proteins in the pheromone response pathway. Two such proteins, Ste5 and Ste20, are phosphorylated only when Cdk is associated with the G1/S cyclins Cln1 and Cln2 and not G1, S, or M cyclins. The basis of this cyclin specificity is unknown.

RESULTS

Here we show that Ste5 and Ste20 have recognition sequences, or "docking" sites, for the G1/S cyclins. These docking sites, which are distinct from Clb5/cyclin A-binding "RXL" motifs, bind preferentially to Cln2. They strongly enhance Cln2-driven phosphorylation of each substrate in vivo and function largely independent of position and distance to the Cdk sites. We exploited this functional independence to rewire a Cdk regulatory circuit in a way that changes the target of Cdk inhibition in the pheromone response pathway. Furthermore, we uncover functionally active Cln2 docking motifs in several other Cdk substrates. The docking motifs drive cyclin-specific phosphorylation, and the cyclin preference can be switched by using a distinct motif.

CONCLUSIONS

Our findings indicate that some Cdk substrates are intrinsically capable of being phosphorylated by several different cyclin-Cdk forms, but they are inefficiently phosphorylated in vivo without a cyclin-specific docking site. Docking interactions may play a prevalent but previously unappreciated role in driving phosphorylation of select Cdk substrates preferentially at the G1/S transition.

Quantitative proteomics reveals the basis for the biochemical specificity of the cell-cycle machinery

Cyclin-dependent kinases comprise the conserved machinery that drives progress through the cell cycle, but how they do this in mammalian cells is still unclear. To identify the mechanisms by which cyclin-cdks control the cell cycle, we performed a time-resolved analysis of the in vivo interactors of cyclins E1, A2, and B1 by quantitative mass spectrometry. This global analysis of context-dependent protein interactions reveals the temporal dynamics of cyclin function in which networks of cyclin-cdk interactions vary according to the type of cyclin and cell-cycle stage. Our results explain the temporal specificity of the cell-cycle machinery, thereby providing a biochemical mechanism for the genetic requirement for multiple cyclins in vivo and reveal how the actions of specific cyclins are coordinated to control the cell cycle. Furthermore, we identify key substrates (Wee1 and c15orf42/Sld3) that reveal how cyclin A is able to promote both DNA replication and mitosis.

Spatial exclusivity combined with positive and negative selection of phosphorylation motifs is the basis for context-dependent mitotic signaling

The timing and localization of events during mitosis are controlled by the regulated phosphorylation of proteins by the mitotic kinases, which include Aurora A, Aurora B, Nek2 (never in mitosis kinase 2), Plk1 (Polo-like kinase 1), and the cyclin-dependent kinase complex Cdk1/cyclin B. Although mitotic kinases can have overlapping subcellular localizations, each kinase appears to phosphorylate its substrates on distinct sites. To gain insight into the relative importance of local sequence context in kinase selectivity, identify previously unknown substrates of these five mitotic kinases, and explore potential mechanisms for substrate discrimination, we determined the optimal substrate motifs of these major mitotic kinases by positional scanning oriented peptide library screening (PS-OPLS). We verified individual motifs with in vitro peptide kinetic studies and used structural modeling to rationalize the kinase-specific selection of key motif-determining residues at the molecular level. Cross comparisons among the phosphorylation site selectivity motifs of these kinases revealed an evolutionarily conserved mutual exclusion mechanism in which the positively and negatively selected portions of the phosphorylation motifs of mitotic kinases, together with their subcellular localizations, result in proper substrate targeting in a coordinated manner during mitosis.

The APC/C recruits cyclin B1-Cdk1-Cks in prometaphase before D box recognition to control mitotic exit

Prior associations with the APC/C complex during prometaphase makes cyclin B1 a better substrate for the cell cycle–regulating ubiquitin ligase in metaphase (see also a related paper by Di Fiore et al. in this issue).

The ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C) is activated at prometaphase by mitotic phosphorylation and binding of its activator, Cdc20. This initiates cyclin A degradation, whereas cyclin B1 is stabilized by the spindle checkpoint. Upon checkpoint release, the RXXL destruction box (D box) was proposed to direct cyclin B1 to core APC/C or Cdc20. In this study, we report that endogenous cyclin B1–Cdk1 is recruited to checkpoint-inhibited, phosphorylated APC/C in prometaphase independently of Cdc20 or the cyclin B1 D box. Like cyclin A, cyclin B1 binds the APC/C by the Cdk cofactor Cks and the APC3 subunit. Prior binding to APC/C^Cdc20 makes cyclin B1 a better APC/C substrate in metaphase, driving mitotic exit and cytokinesis. We conclude that in prometaphase, the phosphorylated APC/C can recruit both cyclin A and cyclin B1 in a Cks-dependent manner. This suggests that the spindle checkpoint blocks D box recognition of APC/C-bound cyclin B1, whereas distinctive complexes between the N terminus of cyclin A and Cdc20 evade checkpoint control.

The roles of cyclin A2, B1, and B2 in early and late mitotic events

This paper presents evidence that chromatin condensation, like nuclear envelope breakdown, is brought about through the combined effects of cyclins A2 and B1, and that cyclins B1 and B2 are largely responsible for maintenance of a spindle assembly checkpoint arrest.

Here we have used siRNAs and time-lapse epifluorescence microscopy to examine the roles of various candidate mitotic cyclins in chromatin condensation in HeLa cells. Knocking down cyclin A2 resulted in a substantial (∼7 h) delay in chromatin condensation and histone H3 phosphorylation, and expressing an siRNA-resistant form of cyclin A2 partially rescued chromatin condensation. There was no detectable delay in DNA replication in the cyclin A2 knockdowns, arguing that the delay in chromatin condensation is not secondary to a delay in S-phase completion. Cyclin A2 is required for the activation and nuclear accumulation of cyclin B1-Cdk1, raising the possibility that cyclin B1-Cdk1 mediates the effects of cyclin A2. Consistent with this possibility, we found that chromatin condensation was tightly associated temporally with the redistribution of cyclin B1 to the nucleus. Moreover, a constitutively nuclear cyclin B1 rescued chromatin condensation in cyclin A2 knockdown cells. On the other hand, knocking down cyclin B1 delayed chromatin condensation by only about one hour. Our working hypothesis is that active, nuclear cyclin B1-Cdk1 normally cooperates with cyclin A2 to bring about early mitotic events. Because cyclin A2 is present only during the early stages of mitosis, we asked whether cyclin B knockdown might have more dramatic defects on late mitotic events. Consistent with this possibility, we found that cyclin B1- and cyclin B1/B2-knockdown cells had difficulty in maintaining a mitotic arrest in the presence of nocodazole. Taken together, these data suggest that cyclin A2 helps initiate mitosis, in part through its effects on cyclin B1, and that cyclins B1 and B2 are particularly critical for the maintenance of the mitotic state.

Lipopolysaccharide-induced inflammation attenuates taste progenitor cell proliferation and shortens the life span of taste bud cells

Background

The mammalian taste bud, a complex collection of taste sensory cells, supporting cells, and immature basal cells, is the structural unit for detecting taste stimuli in the oral cavity. Even though the cells of the taste bud undergo constant turnover, the structural homeostasis of the bud is maintained by balancing cell proliferation and cell death. Compared with nongustatory lingual epithelial cells, taste cells express higher levels of several inflammatory receptors and signalling proteins. Whether inflammation, an underlying condition in some diseases associated with taste disorders, interferes with taste cell renewal and turnover is unknown. Here we report the effects of lipopolysaccharide (LPS)-induced inflammation on taste progenitor cell proliferation and taste bud cell turnover in mouse taste tissues.

Results

Intraperitoneal injection of LPS rapidly induced expression of several inflammatory cytokines, including tumor necrosis factor (TNF)-α, interferon (IFN)-γ, and interleukin (IL)-6, in mouse circumvallate and foliate papillae. TNF-α and IFN-γ immunoreactivities were preferentially localized to subsets of cells in taste buds. LPS-induced inflammation significantly reduced the number of 5-bromo-2'-deoxyuridine (BrdU)-labeled newborn taste bud cells 1-3 days after LPS injection, suggesting an inhibition of taste bud cell renewal. BrdU pulse-chase experiments showed that BrdU-labeled taste cells had a shorter average life span in LPS-treated mice than in controls. To investigate whether LPS inhibits taste cell renewal by suppressing taste progenitor cell proliferation, we studied the expression of Ki67, a cell proliferation marker. Quantitative real-time RT-PCR revealed that LPS markedly reduced Ki67 mRNA levels in circumvallate and foliate epithelia. Immunofluorescent staining using anti-Ki67 antibodies showed that LPS decreased the number of Ki67-positive cells in the basal regions surrounding circumvallate taste buds, the niche for taste progenitor cells. PCR array experiments showed that the expression of cyclin B2 and E2F1, two key cell cycle regulators, was markedly downregulated by LPS in the circumvallate and foliate epithelia.

Conclusions

Our results show that LPS-induced inflammation inhibits taste progenitor cell proliferation and interferes with taste cell renewal. LPS accelerates cell turnover and modestly shortens the average life span of taste cells. These effects of inflammation may contribute to the development of taste disorders associated with infections.

Activation of cyclin B1-Cdk1 synchronizes events in the nucleus and the cytoplasm at mitosis

The cyclin B-Cdk1 kinase triggers mitosis in most eukaryotes. In animal cells, cyclin B shuttles between the nucleus and cytoplasm in interphase before rapidly accumulating in the nucleus at prophase, which promotes disassembly of the nuclear lamina and nuclear envelope breakdown (NEBD). What triggers the nuclear accumulation of cyclin B1 is presently unclear, although the prevailing view is that the Plk1 kinase inhibits its nuclear export. In this study, we use a biosensor specific for cyclin B1-Cdk1 activity to show that activating cyclin B1-Cdk1 immediately triggers its rapid accumulation in the nucleus through a 40-fold increase in nuclear import that remains dependent on Cdk1 activity until NEBD. Nevertheless, a substantial proportion of cyclin B1-Cdk1 remains in the cytoplasm. The increase in nuclear import is driven by changes in the nuclear import machinery that require neither Plk1 nor inhibition of nuclear export. Thus, the intrinsic link between cyclin B1-Cdk1 activation and its rapid nuclear import inherently coordinates the reorganization of the nucleus and the cytoplasm at mitotic entry.

Progressive activation of CyclinB1-Cdk1 coordinates entry to mitosis

The CyclinB1-Cdk1 kinase is the catalytic activity at the heart of mitosis-promoting factor (MPF), yet fundamental questions concerning its role in mitosis remained unresolved. It is not known when and how rapidly CyclinB1-Cdk1 is activated in mammalian cells, nor how its activation coordinates the substantial changes in the cell at mitosis. Here, we have developed a FRET biosensor specific for CyclinB1-Cdk1 that enables us to assay its activity with very high temporal precision in living human cells. We show that CyclinB1-Cdk1 is inactive in G2 phase and activated at a set time before nuclear envelope breakdown, thereby initiating the events of prophase. CyclinB1-Cdk1 levels rise to their maximum extent over the course of approximately 30 min, and we demonstrate that different levels of CyclinB1-Cdk1 kinase activity trigger different mitotic events, thus revealing how the remarkable reorganization of the cell is coordinated at mitotic entry.

Quantitative site-specific phosphorylation dynamics of human protein kinases during mitotic progression

Reversible protein phosphorylation is a key regulatory mechanism of mitotic progression. Importantly, protein kinases themselves are also regulated by phosphorylation-dephosphorylation processes; hence, phosphorylation dynamics of kinases hold a wealth of information about phosphorylation networks. Here, we investigated the site-specific phosphorylation dynamics of human kinases during mitosis using synchronization of HeLa suspension cells, kinase enrichment, and high resolution mass spectrometry. In biological triplicate analyses, we identified 206 protein kinases and more than 900 protein kinase phosphorylation sites, including 61 phosphorylation sites on activation segments, and quantified their relative abundances across three specific mitotic stages. Around 25% of the kinase phosphorylation site ratios were found to be changed by at least 50% during mitotic progression. Further network analysis of jointly regulated kinase groups suggested that Cyclin-dependent kinase- and mitogen-activated kinase-centered interaction networks are coordinately down- and up-regulated in late mitosis, respectively. Importantly, our data cover most of the already known mitotic kinases and, moreover, identify attractive candidates for future studies of phosphorylation-based mitotic signaling. Thus, the results of this study provide a valuable resource for cell biologists and provide insight into the system properties of the mitotic phosphokinome.

Mitotic division of the mammalian Golgi apparatus

Successful cell reproduction requires faithful duplication and proper segregation of cellular contents, including not only the genome but also intracellular organelles. Since the Golgi apparatus is an essential organelle of the secretory pathway, its accurate inheritance is therefore of importance to sustain cellular function. Regulation of Golgi division and its coordination with cell cycle progression involves a series of sequential events that are subjected to a precise spatiotemporal control. Here, we summarize the current knowledge about the underlying mechanisms, the molecular players and the biological relevance of this process, particularly in mammalian cells, and discuss the unsolved problems and future perspectives opened by the recent studies.

Global gene expression profiling of human pleural mesotheliomas: identification of matrix metalloproteinase 14 (MMP-14) as potential tumour target

BACKGROUND

The goal of our study was to molecularly dissect mesothelioma tumour pathways by mean of microarray technologies in order to identify new tumour biomarkers that could be used as early diagnostic markers and possibly as specific molecular therapeutic targets.

METHODOLOGY

We performed Affymetrix HGU133A plus 2.0 microarray analysis, containing probes for about 39,000 human transcripts, comparing 9 human pleural mesotheliomas with 4 normal pleural specimens. Stringent statistical feature selection detected a set of differentially expressed genes that have been further evaluated to identify potential biomarkers to be used in early diagnostics. Selected genes were confirmed by RT-PCR. As reported by other mesothelioma profiling studies, most of genes are involved in G2/M transition. Our list contains several genes previously described as prognostic classifier. Furthermore, we found novel genes, never associated before to mesotheliom that could be involved in tumour progression. Notable is the identification of MMP-14, a member of matrix metalloproteinase family. In a cohort of 70 mesothelioma patients, we found by a multivariate Cox regression analysis, that the only parameter influencing overall survival was expression of MMP14. The calculated relative risk of death in MM patients with low MMP14 expression was significantly lower than patients with high MMp14 expression (P = 0.002).

CONCLUSIONS

Based on the results provided, this molecule could be viewed as a new and effective therapeutic target to test for the cure of mesothelioma.

Mammalian cell-cycle regulation: several Cdks, numerous cyclins and diverse compensatory mechanisms

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 for Cdk2's functions. Similar to yeast, it was recently discovered that Cdk1 alone can drive the mammalian cell cycle, indicating that the regulation of the mammalian cell cycle is highly conserved. Nevertheless, cell-cycle-independent functions of Cdks and cyclins such as in DNA damage repair are still under investigation. Here we review the compensatory mechanisms among major cyclins and Cdks in mammalian cell-cycle regulation.

Cyclin Y, a novel membrane-associated cyclin, interacts with PFTK1

A novel cyclin, CCNY, was identified as a PFTK1 interacting protein in a yeast two-hybrid screen. The cyclin box in CCNY and the PFTAIRE motif in PFTK1 are both required for the interaction which was confirmed by in vivo and in vitro assays. Two transcripts (4 and 2kb), of CCNY were detected by Northern blot analysis and CCNY was enriched at the plasma membrane due to an N-terminal myristoylation signal. We propose that binding of CCNY to PFTK1 enhances PFTK1 kinase activity and changes its intracellular location.

Fertilization differently affects the levels of cyclin B1 and M-phase promoting factor activity in maturing and metaphase II mouse oocytes

Fertilization affects levels of cyclin B1 and M-phase promoting factor (MPF) activity in maturing and metaphase II mouse oocytes in two distinct ways. In metaphase II oocytes, it leads to a Ca2+-dependent, continuous degradation of cyclin B1 and inactivation of cyclin dependent kinase (CDC2A)–cyclin B1 complex (MPF). In this paper, we show that neither mono- nor polyspermic fertilization of prometaphase I and metaphase I oocytes triggered degradation of cyclin B1. However, polyspermic fertilization of prometaphase I oocytes led to a transient decrease in MPF activity that lasted for 2 h. The inactivation of MPF in polyspermic prometaphase I oocytes did not depend on the fertilization-induced increase in the cytoplasmic concentration of free Ca2+ions, but was caused, at least in part, by dephosphorylation of CDC2A at threonine 161 (Thr161). We found that polyspermic fertilization did not affect glutathione levels in prometaphase I oocytes, and concluded that the decrease in MPF activity and dephosphorylation of CDC2A at Thr161 in polyspermic prometaphase I oocytes were not caused by a change in the redox status of the cell induced by an introduction of excessive amount of sperm protamines. Instead, we propose that inactivation of MPF activity in polyspermic maturing oocytes is caused by a change in nucleo-cytoplasmic ratio that leads to a ‘titration’ of kinases and phosphatases responsible for keeping MPF in an active state. This idea is supported by the finding that oocytes fused with thymocytes rather than spermatozoa also showed a transient decrease in MPF activity.

The yeast LATS/Ndr kinase Cbk1 regulates growth via Golgi-dependent glycosylation and secretion

Saccharomyces cerevisiae Cbk1 is a LATS/Ndr protein kinase and a downstream component of the regulation of Ace2 and morphogenesis (RAM) signaling network. Cbk1 and the RAM network are required for cellular morphogenesis, cell separation, and maintenance of cell integrity. Here, we examine the phenotypes of conditional cbk1 mutants to determine the essential function of Cbk1. Cbk1 inhibition severely disrupts growth and protein secretion, and triggers the Swe1-dependent morphogenesis checkpoint. Cbk1 inhibition also delays the polarity establishment of the exocytosis regulators Rab-GTPase Sec4 and its exchange factor Sec2, but it does not interfere with actin polarity establishment. Cbk1 binds to and phosphorylates Sec2, suggesting that it regulates Sec4-dependent exocytosis. Intriguingly, Cbk1 inhibition causes a >30% decrease in post-Golgi vesicle accumulation in late secretion mutants, indicating that Cbk1 also functions upstream of Sec2-Sec4, perhaps at the level of the Golgi. In agreement, conditional cbk1 mutants mislocalize the cis-Golgi mannosyltransferase Och1, are hypersensitive to the aminoglycoside hygromycin B, and exhibit diminished invertase and Sim1 glycosylation. Significantly, the conditional lethality and hygromycin B sensitivity of cbk1 mutants are suppressed by moderate overexpression of several Golgi mannosyltransferases. These data suggest that an important function for Cbk1 and the RAM signaling network is to regulate growth and secretion via Golgi and Sec2/Sec4-dependent processes.

Grapes(Chk1) prevents nuclear CDK1 activation by delaying cyclin B nuclear accumulation

Entry into mitosis is characterized by a dramatic remodeling of nuclear and cytoplasmic compartments. These changes are driven by cyclin-dependent kinase 1 (CDK1) activity, yet how cytoplasmic and nuclear CDK1 activities are coordinated is unclear. We injected cyclin B (CycB) into Drosophila melanogaster embryos during interphase of syncytial cycles and monitored effects on cytoplasmic and nuclear mitotic events. In untreated embryos or embryos arrested in interphase with a protein synthesis inhibitor, injection of CycB accelerates nuclear envelope breakdown and mitotic remodeling of the cytoskeleton. Upon activation of the Grapes(checkpoint kinase 1) (Grp(Chk1))-dependent S-phase checkpoint, increased levels of CycB drives cytoplasmic but not nuclear mitotic events. Grp(Chk1) prevents nuclear CDK1 activation by delaying CycB nuclear accumulation through Wee1-dependent and independent mechanisms.