Recycling the cell cycle: cyclins revisited

Molecular mechanisms in gliomagenesis

Glioma, and in particular high-grade astrocytoma termed glioblastoma multiforme (GBM), is the most common primary tumor of the brain. Primarily because of its diffuse nature, there is no effective treatment for GBM, and relatively little is known about the processes by which it develops. Therefore, in order to design novel therapies and treatments for GBM, research has recently intensified to identify the cellular and molecular mechanisms leading to GBM formation. Modeling of astrocytomas by genetic manipulation of mice suggests that deregulation of the pathways that control gliogenesis during normal brain development, such as the differentiation of neural stem cells (NSCs) into astrocytes, might contribute to GBM formation. These pathways include growth factor-induced signal transduction routes and processes that control cell cycle progression, such as the p16-CDK4-RB and the ARF-MDM2-p53 pathways. The expression of several of the components of these signaling cascades has been found altered in GBM, and recent data indicate that combinations of mutations in these pathways may contribute to GBM formation, although the exact mechanisms are still to be uncovered. Use of novel techniques including large-scale genomics and proteomics in combination with relevant mouse models will most likely provide novel insights into the molecular mechanisms underlying glioma formation and will hopefully lead to development of treatment modalities for GBM.

Activation of a nuclear Cdc2-related kinase within a mitogen-activated protein kinase-like TDY motif by autophosphorylation and cyclin-dependent protein kinase-activating kinase

Male germ cell-associated kinase (MAK) and intestinal cell kinase (ICK) are nuclear Cdc2-related kinases with nearly identical N-terminal catalytic domains and more divergent C-terminal noncatalytic domains. The catalytic domain is also related to mitogen-activated protein kinases (MAPKs) and contains a corresponding TDY motif. Nuclear localization of ICK requires subdomain XI and interactions of the conserved Arg-272, but not kinase activity or, surprisingly, any of the noncatalytic domain. Further, nuclear localization of ICK is required for its activation. ICK is activated by dual phosphorylation of the TDY motif. Phosphorylation of Tyr-159 in the TDY motif requires ICK autokinase activity but confers only basal kinase activity. Full activation requires additional phosphorylation of Thr-157 in the TDY motif. Coexpression of ICK with constitutively active MEK1 or MEK5 fails to increase ICK phosphorylation or activity, suggesting that MEKs are not involved. ICK and MAK are related to Ime2p in budding yeast, and cyclin-dependent protein kinase-activating kinase Cak1p has been placed genetically upstream of Ime2p. Recombinant Cak1p phosphorylates Thr-157 in the TDY motif of recombinant ICK and activates its activity in vitro. Coexpression of ICK with wild-type CAK1 but not kinase-inactive CAK1 in cells also increases ICK phosphorylation and activity. Our studies establish ICK as the prototype for a new group of MAPK-like kinases requiring dual phosphorylation at TDY motifs.

Asymmetric cell divisions in flowering plants - one mother, "two-many" daughters

Plant development shows a fascinating range of asymmetric cell divisions. Over the years, however, cellular differentiation has been interpreted mostly in terms of a mother cell dividing mitotically to produce two daughter cells of different fates. This popular view has masked the significance of an entirely different cell fate specification pathway, where the mother cell first becomes a coenocyte and then cellularizes to simultaneously produce more than two specialized daughter cells. The "one mother - two different daughters" pathways rely on spindle-assisted mechanisms, such as translocation of the nucleus/spindle to a specific cellular site and orientation of the spindle, which are coordinated with cell-specific allocation of cell fate determinants and cytokinesis. By contrast, during "coenocyte-cellularization" pathways, the spindle-assisted mechanisms are irrelevant since cell fate specification emerges only after the nuclear divisions are complete, and the number of specialized daughter cells produced depends on the developmental context. The key events, such as the formation of a coenocyte and migration of the nuclei to specific cellular locations, are coordinated with cellularization by unique types of cell wall formation. Both one mother - two different daughters and the coenocyte-cellularization pathways are used by higher plants in precise spatial and time windows during development. In both the pathways, epigenetic regulation of gene expression is crucial not only for cell fate specification but also for its maintenance through cell lineage. In this review, the focus is on the coenocyte-cellularization pathways in the context of our current understanding of the asymmetric cell divisions. Instances where cell differentiation does not involve an asymmetric division are also discussed to provide a comprehensive account of cell differentiation.

Bub1 and aurora B cooperate to maintain BubR1-mediated inhibition of APC/CCdc20

The spindle checkpoint maintains genome stability by inhibiting Cdc20-mediated activation of the anaphase promoting complex/cyclosome (APC/C) until all the chromosomes correctly align on the microtubule spindle apparatus via their kinetochores. BubR1, an essential component of this checkpoint, localises to kinetochores and its kinase activity is regulated by the kinesin-related motor protein Cenp-E. BubR1 also inhibits APC/CCdc20 in vitro, thus providing a molecular link between kinetochore-microtubule interactions and the proteolytic machinery that regulates mitotic progression. Several other protein kinases, including Bub1 and members of the Ipl1/aurora family, also regulate anaphase onset. However, in human somatic cells Bub1 and aurora B kinase activity do not appear to be essential for spindle checkpoint function. Specifically, when Bub1 is inhibited by RNA interference, or aurora kinase activity is inhibited with the small molecule ZM447439, cells arrest transiently in mitosis following exposure to spindle toxins that prevent microtubule polymerisation. Here, we show that mitotic arrest of Bub1-deficient cells is dependent on aurora kinase activity, and vice versa. We suggest therefore that the checkpoint is composed of two arms, one dependent on Bub1, the other on aurora B. Analysis of BubR1 complexes suggests that both of these arms converge on the mitotic checkpoint complex (MCC), which includes BubR1, Bub3, Mad2 and Cdc20. Although it is known that MCC components can bind and inhibit the APC/C, we show here for the first time that the binding of the MCC to the APC/C is dependent on an active checkpoint signal. Furthermore, we show that both Bub1 and aurora kinase activity are required to promote binding of the MCC to the APC/C. These observations provide a simple explanation of why BubR1 and Mad2 are essential for checkpoint function following spindle destruction, yet Bub1 and aurora B kinase activity are not. Taken together with other observations, we suggest that these two arms respond to different spindle cues: whereas the Bub1 arm monitors kinetochore-microtubule attachment, the aurora B arm monitors biorientation. This bifurcation in the signalling mechanism may help explain why many tumour cells mount a robust checkpoint response following spindle damage, despite exhibiting chromosome instability.

Induction of p19INK4d in response to ultraviolet light improves DNA repair and confers resistance to apoptosis in neuroblastoma cells

The genetic instability driving tumorigenesis is fueled by DNA damage and by errors made by the DNA replication. Upon DNA damage the cell organizes an integrated response not only by the classical DNA repair mechanisms but also involving mechanisms of replication, transcription, chromatin structure dynamics, cell cycle progression, and apoptosis. In the present study, we investigated the role of p19INK4d in the response driven by neuroblastoma cells against DNA injury caused by UV irradiation. We show that p19INK4d is the only INK4 protein whose expression is induced by UV light in neuroblastoma cells. Furthermore, p19INK4d translocation from cytoplasm to nucleus is observed after UV irradiation. Ectopic expression of p19INK4d clearly reduces the UV-induced apoptosis as well as enhances the cellular ability to repair the damaged DNA. It is clearly shown that DNA repair is the main target of p19INK4d effect and that diminished apoptosis is a downstream event. Importantly, experiments performed with CDK4 mutants suggest that these p19INK4d effects would be independent of its role as a cell cycle checkpoint gene. The results presented herein uncover a new role of p19INK4d as regulator of DNA-damage-induced apoptosis and suggest that it protects cells from undergoing apoptosis by allowing a more efficient DNA repair. We propose that, in addition to its role as cell cycle inhibitor, p19INK4d is involved in maintenance of DNA integrity and, therefore, would contribute to cancer prevention.

Perturbations in O-linked beta-N-acetylglucosamine protein modification cause severe defects in mitotic progression and cytokinesis

The dynamic modification of nuclear and cytoplasmic proteins with O-linked beta-N-acetylglucosamine (O-GlcNAc) is a regulatory post-translational modification that is rapidly responsive to morphogens, hormones, nutrients, and cellular stress. Here we show that O-GlcNAc is an important regulator of the cell cycle. Increased O-GlcNAc (pharmacologically or genetically) results in growth defects linked to delays in G2/M progression, altered mitotic phosphorylation, and cyclin expression. Overexpression of O-GlcNAcase, the enzyme that removes O-GlcNAc, induces a mitotic exit phenotype accompanied by a delay in mitotic phosphorylation, altered cyclin expression, and pronounced disruption in nuclear organization. Overexpression of the O-GlcNAc transferase, the enzyme that adds O-GlcNAc, results in a polyploid phenotype with faulty cytokinesis. Notably, O-GlcNAc transferase is concentrated at the mitotic spindle and midbody at M phase. These data suggest that dynamic O-GlcNAc processing is a pivotal regulatory component of the cell cycle, controlling cell cycle progression by regulating mitotic phosphorylation, cyclin expression, and cell division.

Structure of the Mediator Subunit Cyclin C and its Implications for CDK8 Function

Cyclin C binds the cyclin-dependent kinases CDK8 and CDK3, which regulate mRNA transcription and the cell cycle, respectively. The crystal structure of cyclin C reveals two canonical five-helix repeats and a specific N-terminal helix. In contrast to other cyclins, the N-terminal helix is short, mobile, and in an exposed position that allows for interactions with proteins other than the CDKs. A model of the CDK8/cyclin C pair reveals two regions in the interface with apparently distinct roles. A conserved region explains promiscuous binding of cyclin C to CDK8 and CDK3, and a non-conserved region may be responsible for discrimination of CDK8 against other CDKs involved in transcription. A conserved and cyclin C-specific surface groove may recruit substrates near the CDK8 active site. Activation of CDKs generally involves phosphorylation of a loop at a threonine residue. In CDK8, this loop is longer and the threonine is absent, suggesting an alternative mechanism of activation that we discuss based on a CDK8-cyclin C model.

Atypical regulation of a green lineage-specific B-type cyclin-dependent kinase

Cyclin-dependent kinases (CDKs) are the main regulators of cell cycle progression in eukaryotes. The role and regulation of canonical CDKs, such as the yeast (Saccharomyces cerevisiae) Cdc2 or plant CDKA, have been extensively characterized. However, the function of the plant-specific CDKB is not as well understood. Besides being involved in cell cycle control, Arabidopsis (Arabidopsis thaliana) CDKB would integrate developmental processes to cell cycle progression. We investigated the role of CDKB in Ostreococcus (Ostreococcus tauri), a unicellular green algae with a minimal set of cell cycle genes. In this primitive alga, at the basis of the green lineage, CDKB has integrated two levels of regulations: It is regulated by Tyr phosphorylation like cdc2/CDKA and at the level of synthesis-like B-type CDKs. Furthermore, Ostreococcus CDKB/cyclin B accounts for the main peak of mitotic activity, and CDKB is able to rescue a yeast cdc28(ts) mutant. By contrast, Ostreococcus CDKA is not regulated by Tyr phosphorylation, and it exhibits a low and steady-state activity from DNA replication to exit of mitosis. This suggests that from a major role in the control of mitosis in green algae, CDKB has evolved in higher plants to assume other functions outside the cell cycle.

Formation of the head organizer in hydra involves the canonical Wnt pathway

Stabilization of β-catenin by inhibiting the activity of glycogen synthase kinase-3β has been shown to initiate axis formation or axial patterning processes in many bilaterians. In hydra, the head organizer is located in the hypostome, the apical portion of the head. Treatment of hydra with alsterpaullone, a specific inhibitor of glycogen synthase kinase-3β,results in the body column acquiring characteristics of the head organizer, as measured by transplantation experiments, and by the expression of genes associated with the head organizer. Hence, the role of the canonical Wnt pathway for the initiation of axis formation was established early in metazoan evolution.

A structural perspective of CTD function

The C-terminal domain (CTD) of RNA polymerase II (Pol II) integrates nuclear events by binding proteins involved in mRNA biogenesis. CTD-binding proteins recognize a specific CTD phosphorylation pattern, which changes during the transcription cycle, due to the action of CTD-modifying enzymes. Structural and functional studies of CTD-binding and -modifying proteins now reveal some of the mechanisms underlying CTD function. Proteins recognize CTD phosphorylation patterns either directly, by contacting phosphorylated residues, or indirectly, without contact to the phosphate. The catalytic mechanisms of CTD kinases and phosphatases are known, but the basis for CTD specificity of these enzymes remains to be understood.

Structure-guided design of pyrazolo[1,5-a]pyrimidines as inhibitors of human cyclin-dependent kinase 2

The protein structure guided design of a series of pyrazolo[1,5-a]pyrimidines with high potency for human cyclin-dependent kinase 2 (CDK2) is described. Some examples were shown to inhibit the growth of human colon tumour cells, were equipotent for CDK1 and were selective against GSK-3beta and other kinases.

Swe1 regulation and transcriptional control restrict the activity of mitotic cyclins toward replication proteins in Saccharomyces cerevisiae

Cyclin-dependent kinases (CDKs) drive the cell cycle through the phosphorylation of substrates that function in genome duplication and cell division. The existence of multiple cyclin subunits and their distinct cell cycle-regulated expression suggests that cyclins impart unique specificities to CDK-substrate interactions that are critical for normal cellular function. This study shows that the combination of early cell cycle expression and deletion of the CDK inhibitor Saccharomyces Wee1 (Swe1) enables the mitotic B-type (Clb) cyclins Clb2, Clb3, and Clb4 of Saccharomyces cerevisiae to initiate S phase with similar effectiveness as the S-phase cyclin Clb5. Although in vivo analysis indicates preferential phosphorylation of a replication substrate by Clb5-Cdk1, this difference is relatively minor compared with the impact of transcriptional control and Swe1 regulation. Indeed, early expressed Clb2-Cdk1 can activate all essential Clb-Cdk substrates in a strain lacking all other Clbs and Swe1. Thus, Swe1 regulation and expression timing are key mechanisms that sequester the broad activity of Clb2-Cdk1 from critical substrates. Furthermore, the ability of Swe1 to inhibit the activity of different B-type cyclins in replication initiation correlates with the normal expression timing of those cyclins, with no apparent in vivo inhibition of Clb5 and Clb6, moderate inhibition of Clb3 and Clb4, and strong inhibition of Clb2. Hence, Swe1 appears to reinforce the temporal activity of cyclins established through transcriptional control. The conserved nature of CDK function suggests that similar mechanisms regulate CDK specificity in multicellular organisms.

D-type cyclins and G1 progression during liver development in the rat

Initiation and progression through G1 requires the activity of signaling complexes containing cyclins (D- or E-type) and cyclin-dependent kinases (CDK4/6 and CDK2, respectively). We set out to identify the G1-phase cyclins and CDKs that are operative during late gestation liver development in the rat. This is a period during which hepatocytes show a high rate of proliferation that is, at least in part, independent of the mitogenic signaling pathways that are functional in mature hepatocytes. RNase protection assay and Western immunoblotting indicated that cyclin D1 is expressed at similar levels in fetal and adult liver. When cyclin D1 was induced after partial hepatectomy, its predominant CDK-binding partner was CDK4. In contrast, cyclins D2 and D3 predominated in fetal liver and were complexed with both CDK4 and CDK6. Little CDK6 protein was expressed in quiescent or regenerating adult liver. Cyclins E1 and E2 were both transcriptionally up-regulated in fetal liver. Activity of complexes containing cyclins E1 and E2 was higher in fetal liver, as was content of the cell cycle regulator, Rb. In fetal liver, Rb was highly phosphorylated at both cyclin D- and cyclin E-dependent sites. In conclusion, liver development is associated with a switch from cyclin D2/D3-containing complexes to cyclin D1:CDK4 complexes. We speculate that the switch in D-type cyclins may be associated with the dependence on mitogenic signaling that develops as hepatocytes mature.

Circadian G2 Arrest as Related to Circadian Gating of Cell Population Growth in Euglena

Cell population growth is gated to occur in particular circadian phases, which has been known for over four decades in various organisms including cyanobacteria and human. However, little is known as to which cell cycle phases from G1 to M are primarily regulated by the circadian rhythm or when in a circadian cycle this primary regulation takes place. We report here that in the flagellate alga Euglena gracilis grown photoautotrophically, the circadian rhythm primarily prevented developmentally matured G2 cells from progressing to mitosis, such that cell population growth occurred only during subjective night. In addition, we found that the circadian rhythm also arrests G1-to-S and S-to-G2 transitions at particular circadian phases.

Cyclins and cdks in development and cancer: a perspective

A fundamental aspect of cancer is dysregulated cell cycle control. Unlike normal cells that only proliferate when compelled to do so by developmental or other mitogenic signals in response to tissue growth needs, the proliferation of cancer cells proceeds essentially unchecked. This does not mean that cancer cell cycles are necessarily different from those found in normal cycling cells, but rather implies that cancer cells proliferate because they are no longer subject to proliferation-inhibitory influences arising from the stroma or from gene expression pattern changes consequent to 'terminal' differentiation, nor do they necessarily require extrinsic growth factors to recruit them into or maintain their proliferative state. Finally, cancer cells have also often avoided normal controls linked to cell cycle progression that halt proliferation in the presence of damaged DNA or other physiological insults. The result of these alterations is the inappropriate proliferation commonly associated with cancerous tumor formation. This review will summarize the current understanding of dysregulation of the G0/G1-to-S-phase transition in cancer cells, with particular emphasis on recent in vivo studies that suggest a need to rethink existing models of cell cycle control in development and tumorigenesis.

p38 MAP kinase inhibition enables proliferation of adult mammalian cardiomyocytes

Adult mammalian cardiomyocytes are considered terminally differentiated and incapable of proliferation. Consequently, acutely injured mammalian hearts do not regenerate, they scar. Here, we show that adult mammalian cardiomyocytes can divide. One important mechanism used by mammalian cardiomyocytes to control cell cycle is p38 MAP kinase activity. p38 regulates expression of genes required for mitosis in cardiomyocytes, including cyclin A and cyclin B. p38 activity is inversely correlated with cardiac growth during development, and its overexpression blocks fetal cardiomyocyte proliferation. Activation of p38 in vivo by MKK3bE reduces BrdU incorporation in fetal cardiomyocytes by 17.6%. In contrast, cardiac-specific p38alpha knockout mice show a 92.3% increase in neonatal cardiomyocyte mitoses. Furthermore, inhibition of p38 in adult cardiomyocytes promotes cytokinesis. Finally, mitosis in adult cardiomyocytes is associated with transient dedifferentiation of the contractile apparatus. Our findings establish p38 as a key negative regulator of cardiomyocyte proliferation and indicate that adult cardiomyocytes can divide.

Mitotic-Exit Control as an Evolved Complex System

The exit from mitosis is the last critical decision during a cell-division cycle. A complex regulatory system has evolved to evaluate the success of mitotic events and control this decision. Whereas outstanding genetic work in yeast has led to rapid discovery of a large number of interacting genes involved in the control of mitotic exit, it has also become increasingly difficult to comprehend the logic and mechanistic features embedded in the complex molecular network. Our view is that this difficulty stems in part from the attempt to explain mitotic-exit control using concepts from traditional top-down engineering design, and that exciting new results from evolutionary engineering design applied to networks and electronic circuits may lend better insights. We focus on four particularly intriguing features of the mitotic-exit control system and attempt to examine these features from the perspective of evolutionary design and complex system engineering.

Evaluating putative mechanisms of the mitotic spindle checkpoint

The mitotic spindle checkpoint halts the cell cycle until all chromosomes are attached to the mitotic spindles. Evidence suggests that the checkpoint prevents cell-cycle progression by inhibiting the activity of the APC-Cdc20 complex, but the precise mechanism underlying this inhibition is not yet known. Here, we use mathematical modeling to compare several mechanisms that could account for this inhibition. We describe the interplay between the capacities to strongly inhibit cell-cycle progression before spindle attachment on one hand and to rapidly resume cell-cycle progression once the last kinetochore is attached on the other hand. We find that inhibition that is restricted to the kinetochore region is not sufficient for supporting both requirements when realistic diffusion constants are considered. A mechanism that amplifies the checkpoint signal through autocatalyzed inhibition is also insufficient. In contrast, amplifying the signal through the release of a diffusible inhibitory complex can support reliable checkpoint function. Our results suggest that the design of the spindle checkpoint network is limited by physical constraints imposed by realistic diffusion constants and the relevant spatial and temporal dimensions where computation is performed.

Multicellularity, stem cells, and the neoblasts of the planarian Schmidtea mediterranea

All multicellular organisms depend on stem cells for their survival and perpetuation. Their central role in reproductive, embryonic, and post-embryonic processes, combined with their wide phylogenetic distribution in both the plant and animal kingdoms intimates that the emergence of stem cells may have been a prerequisite in the evolution of multicellular organisms. We present an evolutionary perspective on stem cells and extend this view to ascertain the value of current comparative studies on various invertebrate and vertebrate somatic and germ line stem cells. We suggest that somatic stem cells may be ancestral, with germ line stem cells being derived later in the evolution of multicellular organisms. We also propose that current studies of stem cell biology are likely to benefit from studying the somatic stem cells of simple metazoans. Here, we present the merits of neoblasts, a largely unexplored, yet experimentally accessible population of stem cells found in the planarian Schmidtea mediterranea. We introduce what we know about the neoblasts, and posit some of the questions that will need to be addressed in order to better resolve the relationship between planarian somatic stem cells and those found in other organisms, including humans.

Regulation of vascular smooth muscle proliferation by heparin: inhibition of cyclin-dependent kinase 2 activity by p27(kip1)

Uncontrolled proliferation of vascular smooth muscle cells (VSMCs) contribute to intimal hyperplasia during atherosclerosis and restenosis. Heparin is an antiproliferative agent for VSMCs and has been shown to block VSMC proliferation both in tissue culture systems and in animals. Despite the well documented antiproliferative actions of heparin, its cellular targets largely remain unknown. In an effort to characterize the mechanism of the antiproliferative property of heparin, we have analyzed the effect of heparin on cell cycle in VSMC. Our results indicate that the heparin-induced block in G(1) to S phase transition is imposed by p27(kip1)-mediated inhibition of cyclin-dependent kinase 2 activity. Further analysis of p27(kip1) mRNA levels showed that the increase in p27(kip1) protein levels in heparin-treated VSMC occurs at posttranscriptional levels. We present evidence that heparin causes stabilization of p27(kip1) protein during G(1) phase and thereby prevents activation of cyclin-dependent kinase 2.

Differential localization and expression of the Cdk9 42k and 55k isoforms

Cdk9, a member of the cyclin-dependent kinase family, is the catalytic subunit of P-TEFb, a protein kinase complex that stimulates transcriptional elongation. Cdk9, complexed with its regulatory partner cyclin T1, serves as the cellular mediator of the transactivation function of the HIV Tat protein. There are two known isoforms of Cdk9: a 42 kDa protein (42k, originally identified as PITALRE) and a more recently identified 55 kDa form (55k). To investigate possible functional differences between the two isoforms, we examined their kinase activities, their subcellular distributions, and their expression levels in primary cells relevant to HIV infection. Both isoforms were found to hyper-phosphorylate the carboxyl-terminal domain of the largest subunit of RNA polymerase II and displayed identical phosphorylation patterns with 144 peptide substrates. Epitope-tagged transiently-expressed Cdk9 42k localized diffusely in the nucleoplasm, while Cdk9 55k accumulated in the nucleolus. In primary undifferentiated monocytes, Cdk9 55k expression was not detected although 42k was present at high levels; however, 55k expression was induced upon macrophage differentiation. In primary lymphocytes, the levels of 55k decreased or remained steady following activation, while the levels of 42k increased. The promoter for 42k was significantly stronger than that of 55k in HeLa cells, and only the 42k promoter was responsive to activation signals in primary lymphocytes. These results indicate that expression of the 42k and 55k isoforms is differentially regulated and suggest that functional differences between the 42k and 55k isoforms of Cdk9 are likely to depend on access to substrates based on their differential subcellular localization and expression patterns.

Control of cell proliferation and apoptosis by mob as tumor suppressor, mats

Appropriate cell number and organ size in a multicellular organism are determined by coordinated cell growth, proliferation, and apoptosis. Disruption of these processes can cause cancer. Recent studies have identified the Large tumor suppressor (Lats)/Warts (Wts) protein kinase as a key component of a pathway that controls the coordination between cell proliferation and apoptosis. Here we describe growth inhibitory functions for a Mob superfamily protein, termed Mats (Mob as tumor suppressor), in Drosophila. Loss of Mats function results in increased cell proliferation, defective apoptosis, and induction of tissue overgrowth. We show that mats and wts function in a common pathway. Mats physically associates with Wts to stimulate the catalytic activity of the Wts kinase. A human Mats ortholog (Mats1) can rescue the lethality associated with loss of Mats function in Drosophila. As Mats1 is mutated in human tumors, Mats-mediated growth inhibition and tumor suppression is likely conserved in humans.

The retinoblastoma gene pathway regulates the postmitotic state of hair cells of the mouse inner ear

Precursors of cochlear and vestibular hair cells of the inner ear exit the cell cycle at midgestation. Hair cells are mitotically quiescent during late-embryonic differentiation stages and postnatally. We show here that the retinoblastoma gene Rb and the encoded protein pRb are expressed in differentiating and mature hair cells. In addition to Rb, the cyclin dependent kinase inhibitor (CKI) p21 is expressed in developing hair cells, suggesting that p21 is an upstream effector of pRb activity. p21 apparently cooperates with other CKIs, as p21-null mice exhibited an unaltered inner ear phenotype. By contrast, Rb inactivation led to aberrant hair cell proliferation, as analysed at birth in a loss-of-function/transgenic mouse model. Supernumerary hair cells expressed various cell type-specific differentiation markers, including components of stereocilia. The extent of alterations in stereociliary bundle morphology ranged from near-normal to severe disorganization. Apoptosis contributed to the mutant phenotype, but did not compensate for the production of supernumerary hair cells, resulting in hyperplastic sensory epithelia. The Rb-null-mediated proliferation led to a distinct pathological phenotype, including multinucleated and enlarged hair cells, and infiltration of hair cells into the mesenchyme. Our findings demonstrate that the pRb pathway is required for hair cell quiescence and that manipulation of the cell cycle machinery disrupts the coordinated development within the inner ear sensory epithelia.

A-432411, a novel indolinone compound that disrupts spindle pole formation and inhibits human cancer cell growth

Microtubules are among the most successful targets for anticancer therapies and for the development of new anticancer drugs. A-432411 is a novel small molecule that destabilizes microtubules at high concentration and disrupts normal spindle formation at low concentration. A-432411 is an indolinone that is structurally different from other known synthetic microtubule inhibitors. This compound is efficacious against a variety of human cancer cell lines including drug-resistant HCT-15 that overexpresses Pgp170. Biochemical studies show that A-432411 competes with the colchicine-binding site on tubulin and inhibits microtubule polymerization. Fluorescence-activated cell sorting analysis indicates that A-432411 causes G2-M arrest and induces apoptosis. Cells treated with A-432411 have increased level of phospho-histone H3 at Ser10 and decreased level of phospho-cdc2 at Tyr15. Concurrently, securin and cyclin B1 expression levels remain the same, indicating the activation of the spindle checkpoint. Immunocytochemistry and fluorescence microscopy experiments reveal that 1 micromol/L A-432411 destabilizes microtubules in cells. At 0.1 micromol/L, the compound disrupts normal spindle pole formation possibly through stabilization of microtubule dynamic. Both structural and cellular properties of A-432411 make it an attractive candidate for further development.

CDC2/CDK1 expression in esophageal adenocarcinoma and precursor lesions serves as a diagnostic and cancer progression marker and potential novel drug target

Esophageal adenocarcinoma arises through well-defined precursor lesions (Barrett esophagus), although only a subset of these lesions advances to invasive adenocarcinoma. The lack of markers predicting progression in Barrett esophagus, typical presentation at advanced stage, and limitations of conventional chemotherapy result in >90% mortality for Barrett-associated adenocarcinomas. To identify potential prognostic markers and therapeutic targets, we compared gene expression profiles from Barrett-associated esophageal adenocarcinoma cell lines (BIC1, SEG1, KYAE, OE33) and normal esophageal epithelial scrapings utilizing the Affymetrix U133_A gene expression platform. We identified 560 transcripts with >3-fold up-regulation in the adenocarcinoma cell lines compared with normal epithelium. Utilizing tissue microarrays composed of normal esophageal squamous mucosa (n = 20), Barrett esophagus (n = 10), low-grade dysplasia (n = 14), high-grade dysplasia (n = 27), adenocarcinoma (n = 59), and node metastases (n = 27), we confirmed differential up-regulation of three proteins (Cdc2/Cdk1, Cdc5, and Igfbp3) in adenocarcinomas and Barrett lesions. Protein expression mirrored histologic progression; thus, 87% of low-grade dysplasias had at least focal surface Cdc2/Cdk1 and 20% had >5% surface staining; 96% of high-grade dysplasias expressed abundant surface Cdc2/Cdk1, while invasive adenocarcinoma and metastases demonstrated ubiquitous expression. Esophageal adenocarcinoma cell lines treated with the novel CDC2/CDK1 transcriptional inhibitor, tetra-O-methyl nordihydroguaiaretic acid (EM-1421, formerly named M4N) demonstrated a dose-dependent reduction in cell proliferation, paralleling down-regulation of CDC2/CDK1 transcript and protein levels. These findings suggest a role for CDC2/CDK1 in esophageal adenocarcinogenesis, both as a potential histopathologic marker of dysplasia and a putative treatment target.

The hedgehog-PKA pathway regulates two distinct steps of the differentiation of retinal ganglion cells: the cell-cycle exit of retinoblasts and their neuronal maturation

In the developing zebrafish retina, neurogenesis is initiated in cells adjacent to the optic stalk and progresses to the entire neural retina. It has been reported that hedgehog (Hh) signalling mediates the progression of the differentiation of retinal ganglion cells (RGCs) in zebrafish. However, the progression of neurogenesis seems to be only mildly delayed by genetic or chemical blockade of the Hh signalling pathway. Here, we show that cAMP-dependent protein kinase (PKA) effectively inhibits the progression of retinal neurogenesis in zebrafish. Almost all retinal cells continue to proliferate when PKA is activated, suggesting that PKA inhibits the cell-cycle exit of retinoblasts. A cyclin-dependent kinase (cdk) inhibitor p27 inhibits the PKA-induced proliferation, suggesting that PKA functions upstream of cyclins and cdk inhibitors. Activation of the Wnt signalling pathway induces the hyperproliferation of retinal cells in zebrafish. The blockade of Wnt signalling inhibits the PKA-induced proliferation, but the activation of Wnt signalling promotes proliferation even in the absence of PKA activity. These observations suggest that PKA inhibits exit from the Wnt-mediated cell cycle rather than stimulates Wnt-mediated cell-cycle progression. PKA is an inhibitor of Hh signalling, and Hh signalling molecule morphants show severe defects in cell-cycle exit of retinoblasts. Together, these data suggest that Hh acts as a short-range signal to induce the cell-cycle exit of retinoblasts. The pulse inhibition of Hh signalling revealed that Hh signalling regulates at least two distinct steps of RGC differentiation: the cell-cycle exit of retinoblasts and RGC maturation. This dual requirement of Hh signalling in RGC differentiation implies that the regulation of a neurogenic wave is more complex in the zebrafish retina than in the Drosophila eye.

Genetic interactions between Cdk1-CyclinB and the Separase complex in Drosophila

Cdk1-CycB plays a key role in regulating many aspects of cell-cycle events, such as cytoskeletal dynamics and chromosome behavior during mitosis. To investigate how Cdk1-CycB controls the coordination of these events, we performed a dosage-sensitive genetic screen, which is based on the observations that increased maternal CycB (four extra gene copies) leads to higher Cdk1-CycB activity in early Drosophila embryos, delays anaphase onset, and generates a sensitized non-lethal phenotype at the blastoderm stage (defined as six cycB phenotype). Here, we report that mutations in the gene three rows (thr) enhance, while mutations in pimples (pim, encoding Drosophila Securin) or separase (Sse) suppress, the sensitized phenotype. In Drosophila, both Pim and Thr are known to regulate Sse activity, and activated Sse cleaves a Cohesin subunit to initiate anaphase. Compared with the six cycB embryos, reducing Thr in embryos with more CycB further delays the initiation of anaphase, whereas reducing either Pim or Sse has the opposite effect. Furthermore, nuclei move slower during cortical migration in embryos with higher Cdk1-CycB activity, whereas reducing either Pim or Sse suppresses this phenotype by causing a novel nuclear migration pattern. Therefore, our genetic screen has identified all three components of the complex that regulates sister chromatid separation, and our observations indicate that interactions between Cdk1-CycB and the Pim-Thr-Sse complex are dosage sensitive.

Xenopus polo-like kinase Plx1 regulates XErp1, a novel inhibitor of APC/C activity

Metaphase-to-anaphase transition is a fundamental step in cell cycle progression where duplicated sister-chromatids segregate to the future daughter cells. The anaphase-promoting complex/cyclosome (APC/C) is a highly regulated ubiquitin-ligase that triggers anaphase onset and mitotic exit by targeting securin and mitotic cyclins for destruction. It was previously shown that the Xenopus polo-like kinase Plx1 is essential to activate APC/C upon release from cytostatic factor (CSF) arrest in Xenopus egg extract. Although the mechanism by which Plx1 regulates APC/C activation remained unclear, the existence of a putative APC/C inhibitor was postulated whose activity would be neutralized by Plx1 upon CSF release. Here we identify XErp1, a novel Plx1-regulated inhibitor of APC/C activity, and we demonstrate that XErp1 is required to prevent anaphase onset in CSF-arrested Xenopus egg extract. Inactivation of XErp1 leads to premature APC/C activation. Conversely, addition of excess XErp1 to Xenopus egg extract prevents APC/C activation. Plx1 phosphorylates XErp1 in vitro at a site that targets XErp1 for degradation upon CSF release. Thus, our data lead to a model of APC/C activation in Xenopus egg extract in which Plx1 targets the APC/C inhibitor XErp1 for degradation.

Low atmospheric oxygen avoids maturation, senescence and cell death of murine mesencephalic neural precursors

The efficient generation of specific brain cells in vitro may serve as a source of cells for brain repair in several devastating neurological diseases. Production of dopaminergic neurons from precursor cells for transplantation in Parkinson's disease has become a major research goal. We found that murine mesencephalic neurospheres were viable and proliferated, preserved telomerase activity, pluripotency and dopaminergic commitment for many weeks when cultured in 3% O2, whereas exposing these cells to 21% oxygen prohibited long-term expansion. Microarray data suggest that a variety of genes related to the cell cycle, cell maturation and apoptosis are differentially regulated in midbrain-derived precursors cultured in 3 versus 21% oxygen after 1-2 months. Taken together, we hypothesize that sustained high oxygen has deleterious effects on the self-renewal capacity of mesencephalic neural precursors, possibly accelerating maturation and senescence resulting in overall cell loss. Gene regulation governed by low oxygen tension may be relevant to the normal development and survival of midbrain neurons.

Cyclic mechanical strain inhibits skeletal myogenesis through activation of focal adhesion kinase, Rac-1 GTPase, and NF-kappaB transcription factor

Myogenesis is a multistep developmental program that generates and regenerates skeletal muscles. Several extracellular factors have been identified that participate in the regulation of myogenesis. Although skeletal muscles are always subjected to mechanical stress in vivo, the role of mechanical forces in the regulation of myogenesis remains unknown. We have investigated the molecular mechanisms by which cyclic mechanical strain modulates myogenesis. Application of cyclic mechanical strain using the computer-controlled Flexcell Strain Unit increased the proliferation of C2C12 cells and inhibited their differentiation into myotubes. Cyclic strain increased the activity of cyclin-dependent kinase 2 (cdk2) and the cellular level of cyclin A, and inhibited the expression of myosin heavy chain and formation of myotubes in C2C12 cultures. The activity of nuclear factor-kappa B (NF-kappaB) transcription factor and the expression of NF-kappaB-regulated genes, cyclin D1 and IL-6, were augmented in response to mechanical strain. Cyclic strain also increased the activity of Rho GTPases, especially Rac-1. The inhibition of Rho GTPases activity, by overexpression of Rho GDP dissociation inhibitor (Rho-GDI), inhibited the strain-induced activation of NF-kappaB in C2C12 cells. Overexpression of either NF-kappaB inhibitory protein IkappaBalphaDeltaN (a degradation resistant mutant IkappaBalpha) or Rho-GDI blocked the strain-induced proliferation of C2C12 cells. Furthermore, overexpression of FRNK, a dominant negative mutant of focal adhesion kinase (FAK), inhibited the strain-induced proliferation of C2C12 cells. Our study demonstrates that cyclic mechanical strain inhibits myogenesis through the activation of FAK, Rac-1, and NF-kappaB.

Genetic replacement of cyclin D1 function in mouse development by cyclin D2

D cyclins (D1, D2, and D3) are components of the core cell cycle machinery in mammalian cells. It is unclear whether each of the D cyclins performs unique, tissue-specific functions or the three proteins have virtually identical functions and differ mainly in their pattern of expression. We previously generated mice lacking cyclin D1, and we observed that these animals displayed hypoplastic retinas and underdeveloped mammary glands and a presented developmental neurological abnormality. We now asked whether the specific requirement for cyclin D1 in these tissues reflected a unique pattern of D cyclin expression or the presence of specialized functions for cyclin D1 in cyclin D1-dependent compartments. We generated a knock-in strain of mice expressing cyclin D2 in place of D1. Cyclin D2 was able to drive nearly normal development of retinas and mammary glands, and it partially replaced cyclin D1's function in neurological development. We conclude that the differences between these two D cyclins lie mostly in the tissue-specific pattern of their expression. However, we propose that subtle differences between the two D cyclins do exist and they may allow D cyclins to function in a highly optimized fashion. We reason that the acquisition of multiple D cyclins may allow mammalian cells to drive optimal proliferation of a diverse array of cell types.

Cdk1 and Cdk2 complexes (cyclin dependent kinases) in apoptosis: a role beyond the cell cycle

The family of cyclin-dependent kinase complexes (Cdks) are well known for their role in the cell division cycle. What is less well known, however, is that Cdks also participate in a subset of apoptosis programs. Evidence for the role of Cdks in apoptosis comes from a variety of experimental approaches, including studies using genetic mutants, protein inhibitors, and chemical inhibitors of protein kinase activity. The precise role of Cdks in apoptosis remains to be defined, although one promising approach to clarify this question is to identify Cdk protein substrates during apoptosis. Currently a number of Cdk inhibitors are being tested in clinical trials. By understanding how Cdks function during apoptosis it may be possible to optimise the use of these inhibitors in treating human tumours by blocking proliferation but permitting apoptosis.

Integration of GO annotations in Correspondence Analysis: facilitating the interpretation of microarray data

MOTIVATION

The functional interpretation of microarray datasets still represents a time-consuming and challenging task. Up to now functional categories that are relevant for one or more experimental context(s) have been commonly extracted from a set of regulated genes and presented in long lists.

RESULTS

To facilitate interpretation, we integrated Gene Ontology (GO) annotations into Correspondence Analysis to display genes, experimental conditions and gene-annotations in a single plot. The position of the annotations in these plots can be directly used for the functional interpretation of clusters of genes or experimental conditions without the need for comparing long lists of annotations. Correspondence Analysis is not limited in the number of experimental conditions that can be compared simultaneously, allowing an easy identification of characterizing annotations even in complex experimental settings. Due to the rapidly increasing amount of annotation data available, we apply an annotation filter. Hereby the number of displayed annotations can be significantly reduced to a set of descriptive ones, further enhancing the interpretability of the plot. We validated the method on transcription data from Saccharomyces cerevisiae and human pancreatic adenocarcinomas.

AVAILABILITY

The M-CHiPS software is accessible for collaborators at http://www.mchips.org

Evaluation of cyclin D1 expression and its subcellular distribution in mouse tissues

Cyclin D1 is a key cell-cycle regulatory protein required for the cell to progress through G1 to S phase. We have shown by Western blot analysis that cyclin D1 has a wide distribution in adult mouse tissues, with its level of expression being tissue-dependent. Immunohistochemistry has also shown that cyclin D1 may be present in the cytoplasm, in the nucleus or in both these cell compartments: cytoplasmic staining was observed in both proliferating cells (e.g. kidney, intestine, stomach and salivary gland) and in the non-dividing cells (the mature neurons of adult brain), while nuclear staining was seen in the neurons of the embryonic nervous system. Immunoelectron microscopy results indicate that, in tissues where cyclin D1 is present in both compartments (e.g. intestinal enterocytes), it may move via nuclear pores from the nucleus to the cytoplasm, and vice versa. The findings as a whole suggest that cyclin D1 may play multiple roles within specific tissues, probably by interacting with different substrates, and that its transit between nuclear and cytoplasmic compartments may help maintain cell homeostasis.

CDK4 regulation by TNFR1 and JNK is required for NF-kappaB-mediated epidermal growth control

Nuclear factor kappaB (NF-kappaB) mediates homeostatic growth inhibition in the epidermis, and a loss of NF-kappaB function promotes proliferation and oncogenesis. To identify mechanisms responsible for these effects, we impaired NF-kappaB action in the epidermis by three different genetic approaches, including conditional NF-kappaB blockade. In each case, epidermal hyperplasia was accompanied by an increase in both protein levels and tissue distribution of the G1 cell cycle kinase, CDK4. CDK4 up-regulation required intact TNFR1 and c-Jun NH2-terminal kinase (JNK) function. Cdk4 gene deletion concomitant with conditional NF-kappaB blockade demonstrated that CDK4 is required for growth deregulation. Therefore, epidermal homeostasis depends on antagonist regulation of CDK4 expression by NF-kappaB and TNFR1/JNK.

APC/C and SCF: controlling each other and the cell cycle

Regulated protein degradation has emerged as a key recurring theme in multiple aspects of cell-cycle regulation. Importantly, the irreversible nature of proteolysis makes it an invaluable complement to the intrinsically reversible regulation through phosphorylation and other post-translational modifications. Consequently, ubiquitin-protein ligases, the protagonists of regulated protein destruction, have gained prominence that compares to that of the cyclin-dependent kinases (Cdks) in driving the eukaryotic cell-cycle clock. This review will focus on the two main players, the related ubiquitin-protein ligases APC/C and SCF, and how they control cell-cycle progression. I will also try to delineate the regulation and interplay of these destruction mechanisms, which are intricately connected to the kinase network as well as to extrinsic signals. Moreover, cell-cycle ubiquitin-protein ligases are themselves subject to proteolytic control in cis as well as in trans. Finally, a careful comparison of the functions and regulation of APC/C and SCF shows that, in certain aspects, their logic of action is fundamentally different.

Linking tumor suppression, DNA damage and the anaphase-promoting complex

A recent study shows that the RASSF1A tumor suppressor functions as a regulator of the ordered proteolytic steps that organize mitosis. By controlling the stability of microtubules and the activity of the anaphase-promoting complex (APC), RASSF1A might provide a crucial link between mechanisms of tumor suppression and mitotic cell division. Furthermore, another recent study shows that protein kinase A, which is a key growth regulator, inhibits the APC during mitosis in yeast.

Genome-wide annotation and expression profiling of cell cycle regulatory genes in Chlamydomonas reinhardtii

Eukaryotic cell cycles are driven by a set of regulators that have undergone lineage-specific gene loss, duplication, or divergence in different taxa. It is not known to what extent these genomic processes contribute to differences in cell cycle regulatory programs and cell division mechanisms among different taxonomic groups. We have undertaken a genome-wide characterization of the cell cycle genes encoded by Chlamydomonas reinhardtii, a unicellular eukaryote that is part of the green algal/land plant clade. Although Chlamydomonas cells divide by a noncanonical mechanism termed multiple fission, the cell cycle regulatory proteins from Chlamydomonas are remarkably similar to those found in higher plants and metazoans, including the proteins of the RB-E2F pathway that are absent in the fungal kingdom. Unlike in higher plants and vertebrates where cell cycle regulatory genes have undergone extensive duplication, most of the cell cycle regulators in Chlamydomonas have not. The relatively small number of cell cycle genes and growing molecular genetic toolkit position Chlamydomonas to become an important model for higher plant and metazoan cell cycles.

p21 stability: linking chaperones to a cell cycle checkpoint

Progression through the cell cycle is regulated by numerous proteins, one of which is the cyclin-dependent kinase inhibitor, p21. A new study identifies a novel protein complex that stabilizes p21. The stability of this complex is critical in effecting the p53-mediated cell cycle checkpoint.

The Division of Drosophila Germline Stem Cells and Their Precursors Requires a Specific Cyclin

A fundamental yet essentially unexplored question in stem cell biology is whether the stem cell cycle has specific features. Three B-cyclins in Drosophila, Cyclins (Cyc) A, B, and B3, associate with CDK1 and play partially redundant roles in embryogenic mitosis . Here, we show that the division of Drosophila GSCs and their precursors, the primordial germ cells (PGCs), specifically requires CycB. CycB is ubiquitously expressed in both germline and somatic lineages. However, CycB mutation does not have obvious effect on somatic development but causes PGCs to severely under proliferate. Moreover, both female and male CycB mutant GSCs fail to be maintained properly. Removing Cyclin B specifically from female GSCs causes the same defect, confirming the direct and cell-autonomous function of Cyclin B for GSC division. In contrast, two other G2 cyclins, CycA and CycB3, are also expressed in PGCs and GSCs, but overexpressing CycA cannot rescue the CycB mutant defects. These results indicate that the requirement of CycB for PGC and GSC divisions unlikely reflects the insufficient level of G2 cyclins in the CycB mutant but is in favor of a distinct function of CycB in these cells. Our results indicate that stem cells may use specific cell cycle regulators for their division.

Positive and negative regulation of c-Myb by cyclin D1, cyclin-dependent kinases, and p27 Kip1

The c-Myb transcription factor controls differentiation and proliferation in hematopoietic and other cell types and has latent transforming activity, but little is known about its regulation during the cell cycle. Here, c-Myb was identified as part of a protein complex from human T cells containing the cyclin-dependent kinase (CDK) CDK6. Assays using model reporter constructs as well as endogenous target genes showed that the activity of c-Myb was inhibited by cyclin D1 plus CDK4 or CDK6 but stimulated by expression of the CDK inhibitors p16 Ink4a, p21 Cip1, or p27 Kip1. Mapping experiments identified a highly conserved region in c-Myb which, when transferred to the related A-Myb transcription factor, also rendered it responsive to CDKs and p27. The results suggest that c-Myb activity is directly regulated by cyclin D1 and CDKs and imply that c-Myb activity is regulated during the cell cycle in hematopoietic cells.

The rate of bipolar spindle assembly depends on the microtubule-gliding velocity of the mitotic kinesin Eg5

During early embryonic cycles, the time required for mitotic spindle assembly must match the autonomous cell cycle oscillations because a lack of coordination between these two processes will result in chromosome segregation errors. Members of the widely conserved BimC kinesin family are essential for spindle formation in all eukaryotes, and complete loss of BimC function results in monopolar spindles that have two spindle poles that are not separated. However, the precise roles of BimC motor activity in the spindle assembly process are not known. To examine the contribution of BimC kinesin's motor activity to spindle assembly, we generated and characterized mutants of Eg5, a vertebrate BimC kinesin, with reduced in vitro microtubule-gliding velocities. In Xenopus egg extracts, we replaced endogenous Eg5 with recombinant wild-type or mutant motor proteins. By using centrosome-dependent and centrosome-independent spindle assembly assays, we found that mechanisms that determine spindle size and shape were robust to approximately 6-fold reductions in Eg5 motility. However, the spindle assembly process was slower when Eg5 motor function was impaired. This role of Eg5 was independent of its contribution to centrosome separation. We provide evidence that Eg5 is a rate-limiting component of the cellular machinery that drives spindle assembly in vertebrates.

Molecular motions of human cyclin-dependent kinase 2

We present a comprehensive description of the dynamic behavior of CDK2 in complex with cyclin A, arrived at by analysis of a total of 0.25 micros of solvated molecular dynamics trajectories and 42 deposited CDK2 structures, and refined using other protein simulation algorithms. The CDK2-cyclin A dimer is a dynamic complex of 6 subdomains. Thermal motions are dominated by a relative twisting of the two monomers. The predominant motion within CDK2 is a "breathing" of the N-terminal and C-terminal lobes. The N-terminal lobe of cyclin A is tightly linked to the "PSTAIRE" helix of CDK2 to provide a rigid nucleus to the complex. By contrast, the "CDK-insert" region (residues 219-251) sometimes becomes highly mobile, a behavior that is observed in crystallographic analyses of CDK2 structures and that may relate to its role in recognizing diverse binding partners. We find that the three arginines that anchor phosphothreonine 160 of fully active CDK2 do not contribute equally to structural stabilization. This observation is supported by a survey of protein kinase sequences. We have also explored the physical basis of the role of the phosphate moiety in signaling by artificially modifying the charge of phosphothreonine 160 in molecular dynamics simulations. We find that phosphothreonine binding involves an active process of attraction in which both the receptor site (the arginine triad), and the phosphothreonine have a higher charge than is required to maintain an active conformation once formed. We have deposited our dynamics data to aid protein kinase inhibitor design.

Invadolysin: a novel, conserved metalloprotease links mitotic structural rearrangements with cell migration

The cell cycle is widely known to be regulated by networks of phosphorylation and ubiquitin-directed proteolysis. Here, we describe IX-14/invadolysin, a novel metalloprotease present only in metazoa, whose activity appears to be essential for mitotic progression. Mitotic neuroblasts of Drosophila melanogaster IX-14 mutant larvae exhibit increased levels of nuclear envelope proteins, monopolar and asymmetric spindles, and chromosomes that appear hypercondensed in length with a surrounding halo of loosely condensed chromatin. Zymography reveals that a protease activity, present in wild-type larval brains, is missing from homozygous tissue, and we show that IX-14/invadolysin cleaves lamin in vitro. The IX-14/invadolysin protein is predominantly found in cytoplasmic structures resembling invadopodia in fly and human cells, but is dramatically relocalized to the leading edge of migrating cells. Strikingly, we find that the directed migration of germ cells is affected in Drosophila IX-14 mutant embryos. Thus, invadolysin identifies a new family of conserved metalloproteases whose activity appears to be essential for the coordination of mitotic progression, but which also plays an unexpected role in cell migration.

High-throughput screening for inhibitors of the e3 ubiquitin ligase APC

The anaphase-promoting complex (APC) is an E3 ubiquitin ligase that mediates the ubiquitination and degradation of the securin protein and mitotic cyclins, resulting in the regulation of the onset of sister-chromatid separation and mitotic exit. In an effort to identify novel therapeutic compounds that modulate cell proliferation and, therefore, have potential applications in oncology, a plate-based in vitro ubiquitination assay that uses recombinant purified E1, E2 (UbcH5c), E3 (APC11/APC2), and Flag-ubiquitin has been established and used to screen for small molecule inhibitors of APC E3 ligase activity. In this assay, APC2/APC11 is immobilized on the plate, and its E3 ligase activity (i.e., the incorporation of Flag-tagged polyubiquitin chain onto APC2/APC11 as a result of auto-ubiquitination) is detected with anti-Flag-horseradish peroxidase-conjugated antibody by monitoring the luminescence signal from the plate. Here we describe in detail the protocol for high-throughput screening of APC, including expression and purification of the individual proteins, assay development, and optimization. This assay has been validated in a 96-well plate format and successfully implemented to identify novel small molecule compounds that potently inhibit APC2/APC11 ligase activity.

Expression of p16INK4A variants in senescent human fibroblasts independent of protein phosphorylation

Upregulation of the p16 tumor suppressor is a hallmark of senescence in human fibroblasts. In this study, we investigated potential protein modification of p16 in senescent human fibroblasts using 2D SDS-PAGE analysis. Three distinct p16 variants with isoelectric points of 5.2, 5.4, and 5.6, were consistently detected in normal human IMR90 fibroblasts that had undergone senescence due to forced expression of oncogenic H-ras or culture passage. Moreover, in contrast to short-term serum starvation, which induces quiescence, IMR90 fibroblasts cultured in low serum for a prolonged period exhibited senescent phenotypes and expression of the three p16 variants. All three p16 variants are unlikely phosphoproteins since they failed to react with antibodies against phospho-serine, and were resistant to the treatment with phosphatases. Functionally, co-immunoprecipitation assays using antibodies against cdk4 and/or cdk6 revealed that only the two most acidic p16 variants associated with cdk4/6. Moreover, senescence induced by the forced expression of p16 in early passage IMR90 fibroblasts or osteosarcoma U2OS cells was accompanied by expression of the two most acidic p16 variants, which also associated with cdk4/6. In summary, we report that prolonged serum starvation-induced senescence may provide an additional model for studying biochemical changes in senescence, including p16 regulation. Furthermore, induction of endogenous p16 in senescent human fibroblasts correlates with the expression of three distinct p16 variants independent of protein phosphorylation. Lastly, expression of the two cdk-bound variants is sufficient to induce senescence in human cells.

Lithium-mediated downregulation of PKB/Akt and cyclin E with growth inhibition in hepatocellular carcinoma cells

We studied in vitro effects of glycogen synthase kinase 3beta (GSK3beta)-inhibitor lithium on the growth of hepatocellular carcinoma (HCC) cells. Lithium induced strong growth inhibition (> 70%) in 75% (n = 9 of 12) of cell lines, apparently independent from the status of major genes that are mutated in HCC including p53, p16(INK4a), beta-catenin and Axin1. Comparative studies with a growth-sensitive Huh7 and growth-resistant Hep40 cell lines showed that lithium induces growth arrest in Huh7 cells but not in Hep40 cells. Lithium induced the accumulation of N-terminally phosphorylated inactive form of GSK3beta with concomitant increase in beta-catenin and beta-catenin/TCF transcriptional activity in both cell lines. This suggests that lithium-mediated HCC growth inhibition is independent of its well-known stimulatory effect on Wnt-beta-catenin signaling. The main differences between Huh7 and Hep40 responses to lithium treatment were observed at the levels PKB/Akt and cyclin E proteins. Lithium induced depletion of both proteins in growth-sensitive Huh7, but not in growth-resistant Hep40 cells. PKB/Akt and Cyclin E are 2 major proteins that are known to be constitutively active in HCC. The targeting of both proteins with lithium may be the main reason why most HCC cells are responsive to lithium-mediated growth inhibition, independent of their p53, retinoblastoma and Wnt-beta-catenin pathways. The exploration of molecular mechanisms involved in lithium-mediated growth inhibition in relation with PKB/Akt and cyclin E downregulation may provide new insights for therapy of liver tumors.