Mitosin/CENP-F as a negative regulator of activating transcription factor-4

Contribution of CENP-F to FOXM1-Mediated Discordant Centromere and Kinetochore Transcriptional Regulation

Proper chromosome segregation is required to ensure chromosomal stability. The centromere (CEN) is a unique chromatin domain defined by CENP-A and is responsible for recruiting the kinetochore (KT) during mitosis, ultimately regulating microtubule spindle attachment and mitotic checkpoint function. Upregulation of many CEN/KT genes is commonly observed in cancer. Here, we show that although FOXM1 occupies promoters of many CEN/KT genes with MYBL2, FOXM1 overexpression alone is insufficient to drive the FOXM1-correlated transcriptional program. CENP-F is canonically an outer kinetochore component; however, it functions with FOXM1 to coregulate G2/M transcription and proper chromosome segregation. Loss of CENP-F results in altered chromatin accessibility at G2/M genes and reduced FOXM1-MBB complex formation. We show that coordinated CENP-FFOXM1 transcriptional regulation is a cancer-specific function. We observe a small subset of CEN/KT genes including CENP-C, that are not regulated by FOXM1. Upregulation of CENP-C in the context of CENP-A overexpression leads to increased chromosome missegregation and cell death suggesting that escape of CENP-C from FOXM1 regulation is a cancer survival mechanism. Together, we show that FOXM1 and CENP-F coordinately regulate G2/M genes, and this coordination is specific to a subset of genes to allow for maintenance of chromosome instability levels and subsequent cell survival.

Contribution of CENP-F to FOXM1-mediated discordant centromere and kinetochore transcriptional regulation

Proper chromosome segregation is required to ensure genomic and chromosomal stability. The centromere is a unique chromatin domain present throughout the cell cycle on each chromosome defined by the CENP-A nucleosome. Centromeres (CEN) are responsible for recruiting the kinetochore (KT) during mitosis, ultimately regulating spindle attachment and mitotic checkpoint function. Upregulation of many genes that encode the CEN/KT proteins is commonly observed in cancer. Here, we show although that FOXM1 occupies the promoters of many CEN/KT genes with MYBL2, occupancy is insufficient alone to drive the FOXM1 correlated transcriptional program. We show that CENP-F, a component of the outer kinetochore, functions with FOXM1 to coregulate G2/M transcription and proper chromosome segregation. Loss of CENP-F results in alteration of chromatin accessibility at G2/M genes, including CENP-A, and leads to reduced FOXM1-MBB complex formation. The FOXM1-CENP-F transcriptional coordination is a cancer-specific function. We observed that a few CEN/KT genes escape FOXM1 regulation such as CENP-C which when upregulated with CENP-A, leads to increased chromosome misegregation and cell death. Together, we show that the FOXM1 and CENP-F coordinately regulate G2/M gene expression, and this coordination is specific to a subset of genes to allow for proliferation and maintenance of chromosome stability for cancer cell survival.

Proteome balance in ciliopathies: the OFD1 protein example

The balance of protein synthesis and degradation is finely regulated and influences cellular homeostasis and biological processes (e.g., embryonic development and neuronal plasticity). Recent data demonstrated that centrosomal/ciliary proteins enable proteome control in response to spatial or microenvironmental stimuli. Here, we discuss recent discoveries regarding the role in the balance of the proteome of centrosomal/ciliary proteins associated with genetic disorders known as ciliopathies. In particular, OFD1 was the first example of a ciliopathy protein controlling both protein expression and autophagic/proteasomal degradation. Understanding the role of proteome balance in the pathogenesis of the clinical manifestations of ciliopathies may pave the way to the identification of a wide range of putative novel therapeutic targets for these conditions.

Exploration of key regulators driving primary feather follicle induction in goose skin

The primary feather follicles are universal skin appendages widely distributed in the skin of feathered birds. The morphogenesis and development of the primary feather follicles in goose skin remain largely unknown. Here, the induction of primary feather follicles in goose embryonic skin (pre-induction vs induction) was investigated by de novo transcriptome analyses to reveal 409 differentially expressed genes (DEGs). The DEGs were characterized to potentially regulate the de novo formation of feather follicle primordia consisting of placode (4 genes) and dermal condensate (12 genes), and the thickening of epidermis (5 genes) and dermal fibroblasts (17 genes), respectively. Further analyses enriched DEGs into GO terms represented as cell adhesion and KEGG pathways including Wnt and Hedgehog signaling pathways that are highly correlated with cell communication and molecular regulation. Six selected Wnt pathway genes were detected by qPCR with up-regulation in goose skin during the induction of primary feather follicles. The localization of WNT16, SFRP1 and FRZB by in situ hybridization showed weak expression in the primary feather primordia, whereas FZD1, LEF1 and DKK1 were expressed initially in the inter-follicular skin and feather follicle primordia, then mainly restricted in the feather primordia. The spatial-temporal expression patterns indicate that Wnt pathway genes DKK1, FZD1 and LEF1 are the important regulators functioned in the induction of primary feather follicle in goose skin. The dynamic molecular changes and specific gene expression patterns revealed in this report provide the general knowledge of primary feather follicle and skin development in waterfowl, and contribute to further understand the diversity of hair and feather development beyond the mouse and chicken models.

Regulation of Cenp-F localization to nuclear pores and kinetochores

In metazoans, the assembly of kinetochores on centrometric chromatin and the dismantling of nuclear pore complexes are processes that have to be tightly coordinated to ensure the proper assembly of the mitotic spindle and a successful mitosis. It is therefore noteworthy that these two macromolecular assemblies share a subset of constituents. One of these multifaceted components is Cenp-F, a protein implicated in cancer and developmental pathologies. During the cell cycle, Cenp-F localizes in multiple cellular structures including the nuclear envelope in late G2/early prophase and kinetochores throughout mitosis. We recently characterized the molecular determinants of Cenp-F interaction with Nup133, a structural nuclear pore constituent. In parallel with two other independent studies, we further elucidated the mechanisms governing Cenp-F kinetochore recruitment that mainly relies on its interaction with Bub1, with redundant contribution of Cenp-E upon acute microtubule depolymerisation. Here we synthesize the current literature regarding the dual location of Cenp-F at nuclear pores and kinetochores and extend our discussion to the regulation of these NPC and kinetochore localizations by mitotic kinase and spindle microtubules.

Disentangling the molecular determinants for Cenp-F localization to nuclear pores and kinetochores

Cenp-F is a multifaceted protein implicated in cancer and developmental pathologies. The Cenp-F C-terminal region contains overlapping binding sites for numerous proteins that contribute to its functions throughout the cell cycle. Here, we focus on the nuclear pore protein Nup133 that interacts with Cenp-F both at nuclear pores in prophase and at kinetochores in mitosis, and on the kinase Bub1, known to contribute to Cenp-F targeting to kinetochores. By combining in silico structural modeling and yeast two-hybrid assays, we generate an interaction model between a conserved helix within the Nup133 β-propeller and a short leucine zipper-containing dimeric segment of Cenp-F. We thereby create mutants affecting the Nup133/Cenp-F interface and show that they prevent Cenp-F localization to the nuclear envelope, but not to kinetochores. Conversely, a point mutation within an adjacent leucine zipper affecting the kinetochore targeting of Cenp-F KT-core domain impairs its interaction with Bub1, but not with Nup133, identifying Bub1 as the direct KT-core binding partner of Cenp-F. Finally, we show that Cenp-E redundantly contributes together with Bub1 to the recruitment of Cenp-F to kinetochores.

PARP inhibition protects mitochondria and reduces ROS production via PARP-1-ATF4-MKP-1-MAPK retrograde pathway

Oxidative stress induces DNA breaks and PARP-1 activation which initiates mitochondrial reactive oxygen species (ROS) production and cell death through pathways not yet identified. Here, we show the mechanism by which PARP-1 influences these processes via PARylation of activating transcription factor-4 (ATF4) responsible for MAP kinase phosphatase-1 (MKP-1) expression and thereby regulates MAP kinases. PARP inhibitor, or silencing, of PARP induced MKP-1 expression by ATF4-dependent way, and inactivated JNK and p38 MAP kinases. Additionally, it induced ATF4 expression and binding to cAMP-response element (CRE) leading to MKP-1 expression and the inactivation of MAP kinases. In contrast, PARP-1 activation induced the PARylation of ATF4 and reduced its binding to CRE sequence in vitro. CHIP-qPCR analysis showed that PARP inhibitor increased the ATF4 occupancy at the initiation site of MKP-1. In oxidative stress, PARP inhibition reduced ROS-induced cell death, suppressed mitochondrial ROS production and protected mitochondrial membrane potential on an ATF4 and MKP-1 dependent way. Basically identical results were obtained in WRL-68, A-549 and T24/83 human cell lines indicating that the aforementioned mechanism can be universal. Here, we provide the first description of PARP-1-ATF4-MKP-1-JNK/p38 MAPK retrograde pathway, which is responsible for the regulation of mitochondrial integrity, ROS production and cell death in oxidative stress, and may represent a new mechanism of PARP in cancer therapy since cancer stem cells development is JNK-dependent.

The integrated stress response

In response to diverse stress stimuli, eukaryotic cells activate a common adaptive pathway, termed the integrated stress response (ISR), to restore cellular homeostasis. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) by one of four members of the eIF2α kinase family, which leads to a decrease in global protein synthesis and the induction of selected genes, including the transcription factor ATF4, that together promote cellular recovery. The gene expression program activated by the ISR optimizes the cellular response to stress and is dependent on the cellular context, as well as on the nature and intensity of the stress stimuli. Although the ISR is primarily a pro-survival, homeostatic program, exposure to severe stress can drive signaling toward cell death. Here, we review current understanding of the ISR signaling and how it regulates cell fate under diverse types of stress.

Autoantibodies directed to centromere protein F in a patient with BRCA1 gene mutation

Background

Autoantibodies directed to centromere protein F were first reported in 1993 and their association with malignancy has been well documented.

Case

We present the case of a 48-year-old Caucasian female with a BRCA1 gene mutation associated with bilateral breast cancer. Antinuclear autoantibody immunofluorescence performed for workup of possible inflammatory arthropathy showed a high titre cell cycle related nuclear speckled pattern, with subsequent confirmation by addressable laser bead immunoassay of the target antigen as an immunodominant epitope at the C-terminus of centromere protein F.

Conclusion

Here we review the current literature on centromere protein F, its association with breast cancer and present the first case of this antibody being identified in a person with a BRCA1 gene mutation.

Correlation between centromere protein-F autoantibodies and cancer analyzed by enzyme-linked immunosorbent assay

Background

Centromere protein-F (CENP-F) is a large nuclear protein of 367 kDa, which is involved in multiple mitosis-related events such as proper assembly of the kinetochores, stabilization of heterochromatin, chromosome alignment and mitotic checkpoint signaling. Several studies have shown a correlation between CENP-F and cancer, e.g. the expression of CENP-F has been described to be upregulated in cancer cells. Furthermore, several studies have described a significant correlation between the expression of autoantibodies to CENP-F and cancer.

Methods

Autoantibodies to CENP-F were detected in a small number of samples during routine indirect immunofluorescence (IIF) analysis for anti-nuclear antibodies (ANA) using HEp-2 cells as substrate. Using overlapping synthetic peptides covering a predicted structural maintenance of chromosomes (SMC) domain, we developed an enzyme-linked immunosorbent assay (ELISA) for detection of CENP-F antibodies.

Results

Analyzing the reactivity of the sera positive in IIF for CENP-F antibodies to overlapping CENP-F peptides, we showed that autoantibodies to several peptides correlate with the presence of antibodies to CENP-F and a diagnosis of cancer, as increased CENP-F antibody expression specific for malignant cancer patients to five peptides was found (A9, A12, A14, A16, A27). These antibodies to CENP-F in clinical samples submitted for ANA analysis were found to have a positive predictive value for cancer of 50%. Furthermore, the expression of cancer-correlated CENP-F antibodies seemed to increase as a function of time from diagnosis.

Conclusion

These results conform to previous findings that approximately 50% of those patients clinically tested for ANA analyses who express CENP-F antibodies are diagnosed with cancer, confirming that these antibodies may function as circulating tumor markers. Thus, a peptide-based CENP-F ELISA focused on the SMC domain may aid in identifying individuals with a potential cancer.

Cardiac-specific deletion of the microtubule-binding protein CENP-F causes dilated cardiomyopathy

CENP-F is a large multifunctional protein with demonstrated regulatory roles in cell proliferation, vesicular transport and cell shape through its association with the microtubule (MT) network. Until now, analysis of CENP-F has been limited to in vitro analysis. Here, using a Cre-loxP system, we report the in vivo disruption of CENP-F gene function in murine cardiomyocytes, a cell type displaying high levels of CENP-F expression. Loss of CENP-F function in developing myocytes leads to decreased cell division, blunting of trabeculation and an initially smaller, thin-walled heart. Still, embryos are born at predicted mendelian ratios on an outbred background. After birth, hearts lacking CENP-F display disruption of their intercalated discs and loss of MT integrity particularly at the costamere; these two structures are essential for cell coupling/electrical conduction and force transduction in the heart. Inhibition of myocyte proliferation and cell coupling as well as loss of MT maintenance is consistent with previous reports of generalized CENP-F function in isolated cells. One hundred percent of these animals develop progressive dilated cardiomyopathy with heart block and scarring, and there is a 20% mortality rate. Importantly, although it has long been postulated that the MT cytoskeleton plays a role in the development of heart disease, this study is the first to reveal a direct genetic link between disruption of this network and cardiomyopathy. Finally, this study has broad implications for development and disease because CENP-F loss of function affects a diverse array of cell-type-specific activities in other organs.

Negative regulation of IRF7 activation by activating transcription factor 4 suggests a cross-regulation between the IFN responses and the cellular integrated stress responses

Cells react to viral infection by exhibiting IFN-based innate immune responses and integrated stress responses, but little is known about the interrelationships between the two. In this study, we report a linkage between these two host-protective cellular mechanisms. We found that IFN regulatory factor (IRF)7, the master regulator of type I IFN gene expression, interacts with activating transcription factor (ATF)4, a key component of the integrated stress responses whose translation is induced by viral infection and various stresses. We have demonstrated that IRF7 upregulates ATF4 activity and expression, whereas ATF4 in return inhibits IRF7 activation, suggesting a cross-regulation between the IFN response and the cellular integrated stress response that controls host innate immune defense against viral infection.

Involvement of Cenp-F in interphase chromatin organization possibly through association with DNA-dependent protein kinase

Cenp-F (also named mitosin) is a 350-kDa human kinetochore protein important for the mitotic progression. It is also a nuclear matrix protein in interphase cells. Here, we showed that overexpression of N-terminal deletion mutants of Cenp-F containing the C-terminal 112 residues induced chromatin condensation into numerous aggregates of varying sizes in interphase nucleus, colocalizing with the exogenous proteins. In situ hybridization using whole chromosome painting probes indicated that the chromatin aggregates were not prematurely condensed individual chromosomes. Neither were they due to apoptosis. We provided evidence showing association of Cenp-F with certain regions of interphase chromatin fibers. Cenp-F associated with the DNA-dependent protein kinase (DNA-PK), a trimeric protein complex critical for genome homeostasis. Moreover, the DNA-PK association activity of Cenp-F mutants correlated with their ability to induce chromatin aggregation. These results imply a role of Cenp-F in organization of interphase chromatin through association and possibly regulation of DNA-PK.

Involvement of CENP-F in histone methylation

CENP-F (also named mitosin) is a multifunctional protein of 350 kDa. In interphase, it is a nuclear protein, whereas in M phase it localizes to the kinetochore, the major microtubule-binding structure on chromosomes essential for chromosome segregation. CENP-F is also critical for myocyte differentiation through the interaction with Rb. It binds to ATF4 and negatively regulates the transcriptional activity of ATF4. It is also important for mitotic progression. Here we show that depletion of CENP-F by RNAi markedly downregulated the methylation of histone H3 at K4 and K9. Consistently, association of HP1a with mitotic chromosomes was largely decreased. These results uncover a novel role of CENP-F in regulation of epigenetic modification on histone H3.

Cdc20 is required for the post-anaphase, KEN-dependent degradation of centromere protein F

Progression through mitosis and cytokinesis requires the sequential proteolysis of several cell-cycle regulators. This proteolysis is mediated by the ubiquitin-proteasome system, with the E3 ligase being the anaphase-promoting complex, also known as the cyclosome (APC/C). The APC/C is regulated by two activators, namely Cdc20 and Cdh1. The current view is that prior to anaphase, the APC/C is activated by Cdc20, but that following anaphase, APC/C switches to Cdh1-dependent activation. However, here we present an analysis of the kinetochore protein Cenp-F that is inconsistent with this notion. Although it has long been appreciated that Cenp-F is degraded sometime during or after mitosis, exactly when and how has not been clear. Here we show that degradation of Cenp-F initiates about six minutes after anaphase, and that this is dependent on a C-terminal KEN-box. Although these two observations are consistent with Cenp-F being a substrate of Cdh1-activated APC/C, Cenp-F is degraded normally in Cdh1-null cells. By contrast, RNAi-mediated repression of APC/C subunits or Cdc20 does inhibit Cenp-F degradation. These findings therefore suggest that the APC/C does not simply 'switch' upon anaphase onset; rather, our observations indicate that Cdc20 also contributes to post-anaphase activation of the APC/C. We also show that the post-anaphase, KEN-box-dependent degradation of Cenp-F requires it to be farnesylated, a post-translational modification usually linked to membrane association. Because so many of the behaviours of Cenp-F are farnesylation-dependent, we suggest that this modification plays a more global role in Cenp-F function.

Murine CENP-F regulates centrosomal microtubule nucleation and interacts with Hook2 at the centrosome

The microtubule (MT) network is essential in a broad spectrum of cellular functions. Many studies have linked CENP-F to MT-based activities as disruption of this protein leads to major changes in MT structure and function. Still, the basis of CENP-F regulation of the MT network remains elusive. Here, our studies reveal a novel and critical localization and role for CENP-F at the centrosome, the major MT organizing center (MTOC) of the cell. Using a yeast two-hybrid screen, we identify Hook2, a linker protein that is essential for regulation of the MT network at the centrosome, as a binding partner of CENP-F. With recently developed immunochemical reagents, we confirm this interaction and reveal the novel localization of CENP-F at the centrosome. Importantly, in this first report of CENP-F(-/-) cells, we demonstrate that ablation of CENP-F protein function eliminates MT repolymerization after standard nocodazole treatment. This inhibition of MT regrowth is centrosome specific because MT repolymerization is readily observed from the Golgi in CENP-F(-/-) cells. The centrosome-specific function of CENP-F in the regulation of MT growth is confirmed by expression of truncated CENP-F containing only the Hook2-binding domain. Furthermore, analysis of partially reconstituted MTOC asters in cells that escape complete repolymerization block shows that disruption of CENP-F function impacts MT nucleation and anchoring rather than promoting catastrophe. Our study reveals a major new localization and function of CENP-F at the centrosome that is likely to impact a broad array of MT-based actions in the cell.

Murine CENPF interacts with syntaxin 4 in the regulation of vesicular transport

Syntaxin 4 is a component of the SNARE complex that regulates membrane docking and fusion. Using a yeast two-hybrid screen, we identify a novel interaction between syntaxin 4 and cytoplasmic murine CENPF, a protein previously demonstrated to associate with the microtubule network and SNAP-25. The binding domain for syntaxin 4 in CENPF was defined by yeast two-hybrid assay and co-immunoprecipitation. Confocal analyses in cell culture reveal a high degree of colocalization between endogenously expressed proteins in interphase cells. Additionally, the endogenous SNARE proteins can be isolated as a complex with CENPF in immunoprecipitation experiments. Further analyses demonstrate that murine CENPF and syntaxin 4 colocalize with components of plasma membrane recycling: SNAP-25 and VAMP2. Depletion of endogenous CENPF disrupts GLUT4 trafficking whereas expression of a dominant-negative form of CENPF inhibits cell coupling. Taken together, these studies demonstrate that CENPF provides a direct link between proteins of the SNARE system and the microtubule network and indicate a diverse role for murine CENPF in vesicular transport.

TRB3 protects cells against the growth inhibitory and cytotoxic effect of ATF4

Tribbles homolog 3 (TRB3) is a pseudokinase the level of which is increased in response to various stresses. We and other researchers have previously shown that TRB3 interacts with activating transcription factor 4 (ATF4) and may function as a negative feedback regulator of ATF4. In the present study, we investigate the effect of ATF4 and TRB3 on cell growth and viability, using both the enforced expression and silencing of the genes. HEK293 cells overexpressing ATF4 show retarded growth in the complete medium and decreased viability in the glucose-free medium. The enforced expression of ATF4 increases the level of reactive oxygen species (ROS) and the supplementation of the medium with ROS scavenging and reducing compounds supports the growth and survival of cells overexpressing ATF4. The deleterious effects of elevated ATF4 are suppressed by the coexpression of TRB3, which downregulates ATF4 transcriptional activity and results in the decrease of intracellular ROS. Also, the coexpression of TRB3 rescues postmitotic neuronally differentiated PC12 cells from the apoptosis evoked by ATF4 overexpression. The silencing of ATF4 and TRB3 genes by RNA interference reveals that endogenous ATF4 promotes and TRB3 suppresses the death of glucose-deprived SaOS2 cells. Together, the results indicate that TRB3 protects cells against the growth inhibitory and cytotoxic effect of ATF4.

Nudel modulates kinetochore association and function of cytoplasmic dynein in M phase

The microtubule-based motor cytoplasmic dynein/dynactin is a force generator at the kinetochore. It also transports proteins away from kinetochores to spindle poles. Regulation of such diverse functions, however, is poorly understood. We have previously shown that Nudel is critical for dynein-mediated protein transport, whereas mitosin, a kinetochore protein that binds Nudel, is involved in retention of kinetochore dynein/dynactin against microtubule-dependent stripping. Here we demonstrate that Nudel is required for robust localization of dynein/dynactin at the kinetochore. It localizes to kinetochores after nuclear envelope breakdown, depending mostly ( approximately 78%) on mitosin and slightly on dynein/dynactin. Depletion of Nudel by RNA interference (RNAi) or overexpression of its mutant incapable of binding either Lis1 or dynein heavy chain abolishes the kinetochore protein transport and mitotic progression. Similar to mitosin RNAi, Nudel RNAi also leads to increased stripping of kinetochore dynein/dynactin in the presence of microtubules. Taking together, our results suggest a dual role of kinetochore Nudel: it activates dynein-mediated protein transport and, when interacting with both mitosin and dynein, stabilizes kinetochore dynein/dynactin against microtubule-dependent stripping to facilitate the force generation function of the motor.

CENP-F expression is associated with poor prognosis and chromosomal instability in patients with primary breast cancer

DNA microarrays have the potential to classify tumors according to their transcriptome. Tissue microarrays (TMAs) facilitate the validation of biomarkers by offering a high-throughput approach to sample analysis. We reanalyzed a high profile breast cancer DNA microarray dataset containing 96 tumor samples using a powerful statistical approach, between group analyses. Among the genes we identified was centromere protein-F (CENP-F), a gene associated with poor prognosis. In a published follow-up breast cancer DNA microarray study, comprising 295 tumour samples, we found that CENP-F upregulation was significantly associated with worse overall survival (p<0.001) and reduced metastasis-free survival (p<0.001). To validate and expand upon these findings, we used 2 independent breast cancer patient cohorts represented on TMAs. CENP-F protein expression was evaluated by immunohistochemistry in 91 primary breast cancer samples from cohort I and 289 samples from cohort II. CENP-F correlated with markers of aggressive tumor behavior including ER negativity and high tumor grade. In cohort I, CENP-F was significantly associated with markers of CIN including cyclin E, increased telomerase activity, c-Myc amplification and aneuploidy. In cohort II, CENP-F correlated with VEGFR2, phosphorylated Ets-2 and Ki67, and in multivariate analysis, was an independent predictor of worse breast cancer-specific survival (p=0.036) and overall survival (p=0.040). In conclusion, we identified CENP-F as a biomarker associated with poor outcome in breast cancer and showed several novel associations of biological significance.

Specific deletion of CMF1 nuclear localization domain causes incomplete cell cycle withdrawal and impaired differentiation in avian skeletal myoblasts

CMF1 is a protein expressed in embryonic striated muscle with onset of expression preceding that of contractile proteins. Disruption of CMF1 in myoblasts disrupts muscle-specific protein expression. Preliminary studies indicate both nuclear and cytoplasmic distribution of CMF1 protein, suggesting functional roles in both cellular compartments. Here we examine the nuclear function of CMF1, using a newly characterized antibody generated against the CMF1 nuclear localization domain and a CMF1 nuclear localization domain-deleted stable myocyte line. The antibody demonstrates nuclear distribution of the CMF1 protein both in vivo and in cell lines, with clustering of CMF1 protein around chromatin during mitosis. In more differentiated myocytes, the protein shifts to the cytoplasm. The CMF1 NLS-deleted cell lines have markedly impaired capacity to differentiate. Specifically, these cells express less contractile protein than wild-type or full-length CMF1 stably transfected cells, and do not fuse properly into multinucleate syncytia with linear nuclear alignment. In response to low serum medium, a signal to differentiate, CMF1 NLS-deleted cells enter G0, but continue to express proliferation markers and will reenter the cell cycle when stimulated by restoring growth medium. These data suggest that CMF1 is involved in regulation the transition from proliferation to differentiation in embryonic muscle.

The centrosomal protein nephrocystin-6 is mutated in Joubert syndrome and activates transcription factor ATF4

The molecular basis of nephronophthisis, the most frequent genetic cause of renal failure in children and young adults, and its association with retinal degeneration and cerebellar vermis aplasia in Joubert syndrome are poorly understood. Using positional cloning, we here identify mutations in the gene CEP290 as causing nephronophthisis. It encodes a protein with several domains also present in CENPF, a protein involved in chromosome segregation. CEP290 (also known as NPHP6) interacts with and modulates the activity of ATF4, a transcription factor implicated in cAMP-dependent renal cyst formation. NPHP6 is found at centrosomes and in the nucleus of renal epithelial cells in a cell cycle-dependent manner and in connecting cilia of photoreceptors. Abrogation of its function in zebrafish recapitulates the renal, retinal and cerebellar phenotypes of Joubert syndrome. Our findings help establish the link between centrosome function, tissue architecture and transcriptional control in the pathogenesis of cystic kidney disease, retinal degeneration, and central nervous system development.

Mitosin/CENP-F in mitosis, transcriptional control, and differentiation

Mitosin/CENP-F is a large nuclear/kinetochore protein containing multiple leucine zipper motifs potentially for protein interactions. Its expression levels and subcellular localization patterns are regulated in a cell cycle-dependent manner. Recently, accumulating lines of evidence have suggested it a multifunctional protein involved in mitotic control, microtubule dynamics, transcriptional regulation, and muscle cell differentiation. Consistently, it is shown to interact directly with a variety of proteins including CENP-E, NudE/Nudel, ATF4, and Rb. Here we review the current progress and discuss possible mechanisms through which mitosin may function.

LEK1 protein expression in normal and dysregulated cardiomyocyte mitosis

A defining characteristic of embryonic cells is their ability to divide rapidly, even in tissues such as cardiac muscle, which cannot divide once fully differentiated. This suggests that regulators of cell division differ in embryonic and differentiated cells. LEK1 is a member of an emerging family of proteins with diverse functions but shared structural domains, including numerous leucine zippers, a nuclear localization site, and a functional Rb-binding domain. LEK1 is expressed ubiquitously in the developing mouse embryo from the earliest stages of differentiation through birth. It is absent in adult tissues, even those that maintain active cell division. We hypothesize that LEK1 is a regulator of mitosis restricted to the developing embryo and early neonate. Here, using BrdU incorporation, we show that LEK1 protein downregulation in cardiac myocytes correlates directly with cessation of DNA synthesis between neonatal days 6 and 10. In contrast, in an immortalized cardiac cell line (HL1 cells), both BrdU incorporation and LEK1 protein expression persist, and actively dividing cells express LEK1. However, BrdU incorporation can be decreased in these cells by treatment with a morpholino targeting LEK1 mRNA. These data suggest a role for LEK1 in regulating the normal embryonic cardiomyocyte cell cycle and in promoting continued mitosis in transformed, abnormally dividing cardiomyocytes.