Autoantibodies to a novel cell cycle-regulated protein that accumulates in the nuclear matrix during S phase and is localized in the kinetochores and spindle midzone during mitosis

The axis of complement C1 and nucleolus in antinuclear autoimmunity

Antinuclear autoantibodies (ANA) are heterogeneous self-reactive antibodies that target the chromatin network, the speckled, the nucleoli, and other nuclear regions. The immunological aberration for ANA production remains partially understood, but ANA are known to be pathogenic, especially, in systemic lupus erythematosus (SLE). Most SLE patients exhibit a highly polygenic disease involving multiple organs, but in rare complement C1q, C1r, or C1s deficiencies, the disease can become largely monogenic. Increasing evidence point to intrinsic autoimmunogenicity of the nuclei. Necrotic cells release fragmented chromatins as nucleosomes and the alarmin HMGB1 is associated with the nucleosomes to activate TLRs and confer anti-chromatin autoimmunogenecity. In speckled regions, the major ANA targets Sm/RNP and SSA/Ro contain snRNAs that confer autoimmunogenecity to Sm/RNP and SSA/Ro antigens. Recently, three GAR/RGG-containing alarmins have been identified in the nucleolus that helps explain its high autoimmunogenicity. Interestingly, C1q binds to the nucleoli exposed by necrotic cells to cause protease C1r and C1s activation. C1s cleaves HMGB1 to inactive its alarmin activity. C1 proteases also degrade many nucleolar autoantigens including nucleolin, a major GAR/RGG-containing autoantigen and alarmin. It appears that the different nuclear regions are intrinsically autoimmunogenic by containing autoantigens and alarmins. However, the extracellular complement C1 complex function to dampen nuclear autoimmunogenecity by degrading these nuclear proteins.

Clinical relevance of HEp-2 indirect immunofluorescent patterns: the International Consensus on ANA patterns (ICAP) perspective

The indirect immunofluorescence assay (IIFA) on HEp-2 cells is widely used for detection of antinuclear antibodies (ANA). The dichotomous outcome, negative or positive, is integrated in diagnostic and classification criteria for several systemic autoimmune diseases. However, the HEp-2 IIFA test has much more to offer: besides the titre or fluorescence intensity, it also provides fluorescence pattern(s). The latter include the nucleus and the cytoplasm of interphase cells as well as patterns associated with mitotic cells. The International Consensus on ANA Patterns (ICAP) initiative has previously reached consensus on the nomenclature and definitions of HEp-2 IIFA patterns. In the current paper, the ICAP consensus is presented on the clinical relevance of the 29 distinct HEp-2 IIFA patterns. This clinical relevance is primarily defined within the context of the suspected disease and includes recommendations for follow-up testing. The discussion includes how this information may benefit the clinicians in daily practice and how the knowledge can be used to further improve diagnostic and classification criteria.

The kinetochore proteins CENP-E and CENP-F directly and specifically interact with distinct BUB mitotic checkpoint Ser/Thr kinases

The segregation of chromosomes during cell division relies on the function of the kinetochores, protein complexes that physically connect chromosomes with microtubules of the spindle. The metazoan proteins, centromere protein E (CENP-E) and CENP-F, are components of a fibrous layer of mitotic kinetochores named the corona. Several of their features suggest that CENP-E and CENP-F are paralogs: they are very large (comprising ∼2700 and 3200 residues, respectively), contain abundant predicted coiled-coil structures, are C-terminally prenylated, and are endowed with microtubule-binding sites at their termini. Moreover, CENP-E contains an ATP-hydrolyzing motor domain that promotes microtubule plus end–directed motion. Here, we show that both CENP-E and CENP-F are recruited to mitotic kinetochores independently of the main corona constituent, the Rod/Zwilch/ZW10 (RZZ) complex. We identified specific interactions of CENP-F and CENP-E with budding uninhibited by benzimidazole 1 (BUB1) and BUB1-related (BUBR1) mitotic checkpoint Ser/Thr kinases, respectively, paralogous proteins involved in mitotic checkpoint control and chromosome alignment. Whereas BUBR1 was dispensable for kinetochore localization of CENP-E, BUB1 was stringently required for CENP-F localization. Through biochemical reconstitution, we demonstrated that the CENP-E/BUBR1 and CENP-F/BUB1 interactions are direct and require similar determinants, a dimeric coiled-coil in CENP-E or CENP-F and a kinase domain in BUBR1 or BUB1. Our findings are consistent with the existence of structurally similar BUB1/CENP-F and BUBR1/CENP-E complexes, supporting the notion that CENP-E and CENP-F are evolutionarily related.

Progerin impairs chromosome maintenance by depleting CENP-F from metaphase kinetochores in Hutchinson-Gilford progeria fibroblasts

Hutchinson-Gilford progeria syndrome (HGPS, OMIM 176670) is a rare premature aging disorder that leads to death at an average age of 14.7 years due to myocardial infarction or stroke. The most common mutation in HGPS is at position G608G (GGC>GGT) within exon 11 of the LMNA gene. This mutation results in the deletion of 50 amino acids at the carboxyl-terminal tail of prelamin A, producing a truncated farnesylated protein called progerin. Lamins play important roles in the organization and structure of the nucleus. The nuclear build-up of progerin causes severe morphological and functional changes in interphase HGPS cells. In this study, we investigated whether progerin elicits spatiotemporal deviations in mitotic processes in HGPS fibroblasts. We analyzed the nuclear distribution of endogenous progerin during mitosis in relation to components of the nuclear lamina, nuclear envelope (NE) and nuclear pores. We found that progerin caused defects in chromosome segregation as early as metaphase, delayed NE reformation and trapped lamina components and inner NE proteins in the endoplasmic reticulum at the end of mitosis. Progerin displaced the centromere protein F (CENP-F) from metaphase chromosome kinetochores, which caused increased chromatin lagging, binucleated cells and genomic instability. This accumulation of progerin-dependent defects with each round of mitosis predisposes cells to premature senescence.

Dr Eng M. Tan: a tribute to an enduring legacy in autoimmunity

At the age of ninety years, Dr Eng Meng Tan has had a remarkable impact on the accumulated knowledge of autoimmune diseases, including seminal findings in systemic lupus erythematosus (SLE) and a wide range of other autoimmune diseases. Dating to the first description of the Sm (Smith) autoantibody in SLE, his focus has been the use of autoantibodies as probes to identify and elucidate novel cellular molecules and then translating these discoveries into biomarkers and immunoassays for a wide range of these diseases and, later, cancer. He led efforts to standardize autoantibody nomenclature and testing protocols. Through his mentorship a great number of trainees and collaborators have had remarkably successful careers, and by that virtue he has garnered a remarkable continuing legacy.

Human autoantibodies as reagents in biomedical research

Abstract  Autoantibodies are typically associated with autoimmune diseases. In some instances the association of specific autoantibodies to a specific autoimmune disease have made their detection invaluable in clinical diagnosis. However, certain autoantibodies show no specific disease association and therefore have limited clinical utility. Nevertheless, autoantibodies are powerful tools for identification, characterization, and functional studies of their cognate antoantigens. In addition, the study of autoantibodies and their cognate autoantigens in human disease and in experimental animal models can provide valuable insight into disease mechanisms and the factors that ameliorate or reverse disease. This review will focus on three specific sets of autoantibodies, which were initially selected for investigation purely on the basis of their novel patterns of reactivity. These were observed when they were applied to a diagnostic HEp-2 test slide for antinuclear antibody detection by indirect immunofluorescence. The target autoantigens were identified as the trans-Golgi network protein GOLGA4 (Golgin 245 or p230), the early endosome antigen-1 (EEA1) and a yet to be identified and fully characterized phosphoepitope(s) restricted to chromosomal arms of condensed mitotic/meiotic chromosomes (MCA1). This laboratory has exploited sera which are reactive to these autoantigens for their identification and characterization, and in functional studies. This review highlights the uses of autoantibodies that may have limited diagnostic or prognostic utility, but are nonetheless novel reagents in the prosecution of molecular cell biology.

Using immunoproteomics to identify tumor-associated antigens (TAAs) as biomarkers in cancer immunodiagnosis

Since intracellular proteins involved in carcinogenesis have been shown to provoke autoantibody responses, autoantibodies can be used as probes in immunoproteomics to isolate, identify, and characterize potential tumor-associated antigens (TAAs). Once a TAA is identified, several approaches will be used to comprehensively characterize and validate the identified TAA/anti-TAA systems that are potential biomarkers in certain types of cancer. Our ultimate goal is to establish rigorous criteria for designation of an autoantibody to a TAA as a cancer biomarker, examine candidate TAAs for sensitivity and specificity of anti-TAA antibody response, and further develop customized TAA arrays that can be used to enhance anti-TAA antibody detection in cancer. This review will mainly focus on the recent advances in our studies using immunoproteomic approach to identify and characterize TAAs as biomarkers in cancer.

Induction of cytoplasmic rods and rings structures by inhibition of the CTP and GTP synthetic pathway in mammalian cells

Background

Cytoplasmic filamentous rods and rings (RR) structures were identified using human autoantibodies as probes. In the present study, the formation of these conserved structures in mammalian cells and functions linked to these structures were examined.

Methodology/Principal Findings

Distinct cytoplasmic rods (∼3–10 µm in length) and rings (∼2–5 µm in diameter) in HEp-2 cells were initially observed in immunofluorescence using human autoantibodies. Co-localization studies revealed that, although RR had filament-like features, they were not enriched in actin, tubulin, or vimentin, and not associated with centrosomes or other known cytoplasmic structures. Further independent studies revealed that two key enzymes in the nucleotide synthetic pathway cytidine triphosphate synthase 1 (CTPS1) and inosine monophosphate dehydrogenase 2 (IMPDH2) were highly enriched in RR. CTPS1 enzyme inhibitors 6-diazo-5-oxo-L-norleucine and Acivicin as well as the IMPDH2 inhibitor Ribavirin exhibited dose-dependent induction of RR in >95% of cells in all cancer cell lines tested as well as mouse primary cells. RR formation by lower concentration of Ribavirin was enhanced in IMPDH2-knockdown HeLa cells whereas it was inhibited in GFP-IMPDH2 overexpressed HeLa cells. Interestingly, RR were detected readily in untreated mouse embryonic stem cells (>95%); upon retinoic acid differentiation, RR disassembled in these cells but reformed when treated with Acivicin.

Conclusions/Significance

RR formation represented response to disturbances in the CTP or GTP synthetic pathways in cancer cell lines and mouse primary cells and RR are the convergence physical structures in these pathways. The availability of specific markers for these conserved structures and the ability to induce formation in vitro will allow further investigations in structure and function of RR in many biological systems in health and diseases.

Bub1 and CENP-F can contribute to Kaposi's sarcoma-associated herpesvirus genome persistence by targeting LANA to kinetochores

The latency-associated nuclear antigen (LANA) encoded by Kaposi's sarcoma-associated herpesvirus (KSHV) is critical for segregation of viral episomes to progeny nuclei and allows for maintenance of the viral genome in newly divided daughter cells. LANA binds to KSHV terminal repeat (TR) DNA and simultaneously associates with chromatin-bound cellular proteins. This process tethers the viral episomes to host chromosomes. However, the mechanism of tethering is complex and involves multiple protein-protein interactions. Our previous proteomics studies which showed the association of LANA with centromeric protein F (CENP-F) prompted us to further study whether LANA targets centromeric proteins for persistence of KSHV episomes during cell division. Here we show that LANA colocalized with CENP-F as speckles, some of which are paired at centromeric regions of a subset of chromosomes in KSHV-infected JSC-1 cells. We also confirm that both the amino and carboxy termini of LANA can bind to CENP-F. Moreover, LANA associated with another kinetochore protein, Bub1 (budding uninhibited by benzimidazole 1), which is known to form a complex with CENP-F. Importantly, we demonstrated the dynamic association of LANA and Bub1/CENP-F and the colocalization between Bub1, LANA, and the KSHV episome tethered to the host chromosome using fluorescence in situ hybridization (FISH). Knockdown of Bub1 expression by lentivirus-delivered short hairpin RNA (shRNA) dramatically reduced the number of KSHV genome copies, whereas no dramatic effect was seen with CENP-F knockdown. Therefore, the interaction between LANA and the kinetochore proteins CENP-F and Bub1 is important for KSHV genome tethering and its segregation to new daughter cells, with Bub1 potentially playing a more critical role in the long-term persistence of the viral genome in the infected cell.

Identification of tumor-associated antigens as diagnostic and predictive biomarkers in cancer

Many studies demonstrated that cancer sera contain antibodies which react with autologous cellular antigens generally known as tumor-associated antigens (TAAs). In our laboratories, the approach used in the identification of TAAs has involved initially examining the sera of cancer patients using extracts of tissue culture cells as source of antigens in Western blotting and by indirect immunofluorescence on whole cells. With these two techniques, we identify sera which have high-titer fluorescent staining or strong signals to cell extracts on Western blotting and subsequently use these sera as probes in immunoscreening cDNA expression libraries, and also in proteomic approaches to isolate and identify targeted antigens which might potentially be involved in malignant transformation. In this manner, several novel TAAs including HCC1, p62, p90, and others have been identified. In extension of these studies, we evaluate the sensitivity and specificity of different antigen-antibody systems as markers in cancer in order to develop "tumor-associated antigen array" systems for cancer diagnosis, cancer prediction, and for following the response of patients to treatment.

Centromere protein H is a novel prognostic marker for nasopharyngeal carcinoma progression and overall patient survival

PURPOSE

The aim of the present study was to analyze the expression of Centromere protein H (CENP-H), one of the fundamental components of the human active kinetochore, in nasopharyngeal carcinoma (NPC) and to correlate it with clinicopathologic data, including patient survival.

EXPERIMENTAL DESIGN

Using reverse transcription-PCR and Western blot, we detected the expression of CENP-H in normal nasopharyngeal epithelial cells, immortalized nasopharyngeal epithelial cell lines, and NPC cell lines. Using immunohistochemistry, we analyzed CENP-H protein expression in 160 clinicopathologically characterized NPC cases. Statistical analyses were applied to test for prognostic and diagnostic associations.

RESULTS

Reverse transcription-PCR and Western blot showed that the expression level of CENP-H was higher in NPC cell lines and in immortalized nasopharyngeal epithelial cells than in the normal nasopharyngeal epithelial cell line at both transcriptional and translational levels. By immunohistochemical analysis, we found that 76 of 160 (47.5%) paraffin-embedded archival NPC biopsies showed high expression of CENP-H. Statistical analysis showed that there was a significant difference of CENP-H expression in patients categorized according to clinical stage (P = 0.024) and T classification (P = 0.027). Patients with higher CENP-H expression had shorter overall survival time, whereas patients with lower CENP-H expression had better survival. A prognostic value of CENP-H was also found of the subgroup of N(0)-N(1) tumor classification. Multivariate analysis showed that CENP-H expression was an independent prognostic indicator for patient's survival.

CONCLUSIONS

Our results suggest that CENP-H protein is a valuable marker of NPC progression. High CENP-H expression is associated with poor overall survival in NPC patients.

Mini-array of multiple tumor-associated antigens to enhance autoantibody detection for immunodiagnosis of hepatocellular carcinoma

Liver cancer, especially hepatocellular carcinoma (HCC), is particularly prevalent in Africa and Asia. HCC affects the Hispanic population of the United States at a rate double that of the white population. The majority of people with HCC will die within 1 year of its detection. This high case-fatality rate can in part be attributed to lack of diagnostic methods that allow early detection. How to establish a methodology to identify the high-risk individuals for HCC remains to be investigated. The multi-factorial and multi-step nature in the molecular pathogenesis of human cancers must be taken into account in both the design and interpretation of studies to identify markers which will be useful for early detection of cancer. Our recent studies demonstrated that a mini-array of multiple tumor-associated antigens (TAAs) might enhance autoantibody detection for diagnosis of HCC, especially for the alpha fetoprotein (AFP)-negative cases. It also suggested that different types of cancer might require different panels of TAAs to achieve the sensitivity and specificity required to make immunodiagnosis a feasible adjunct to tumor diagnosis.

Autoantibodies to mitotic apparatus: association with other autoantibodies and their clinical significance

The most important mitotic apparatus (MA) antigens are centrosome (CE), nuclear mitotic apparatus (NuMA-1, NuMA-2), midbody, and centromere F (CENP-F). We studied associations of anti-MA antibodies with other autoantibodies and their clinical significance. A total of 6270 patients were studied for the presence of anti-MA antibodies on HEp-2 cells. Sera positive for anti-MA were tested for anti-extractable nuclear antigens (ENA) antibodies. Anti-MA antibodies were detected in 56 (45 females and 11 males) of 6270 sera (0.9%). Of these 56, NuMA-1 was found in 23, NuMA-2 in 7, CE in 20, CENP-F in 5, and CENP-F/centrosome in 1 case. Anti-NuMA-1 were associated with anti-ENA antibodies (p < 0.001). Diagnoses were established in 43/56 patients: 22 connective tissue diseases, 7 infections, 6 autoimmune hepatitis, 3 vasculitis, 3 primary antiphospholipid syndrome, 1 malignancy, and 1 fever of unknown origin. The differential diagnosis of anti-NuMA-1-positive patients must include Sjögren's syndrome, while patients with anti-CE antibodies must be observed for HCV infection.

CENP-F is a novel microtubule-binding protein that is essential for kinetochore attachments and affects the duration of the mitotic checkpoint delay

Centromeric protein F (CENP-F) is a 367-kDa human kinetochore protein that was identified a decade ago, but its function was only recently revealed by studies that used small interfering RNA to deplete the protein from cells. All studies showed that CENP-F is important for chromosome alignment, but these studies differed as to whether CENP-F is important to the mitotic checkpoint. We report here that CENP-F is essential for cells to sustain a prolonged mitotic delay in response to unattached kinetochores. Cells depleted of CENP-F exit mitosis in the presence of defective kinetochore attachments resulting from treatment with nocodazole, or the depletion of kinetochore proteins CENP-E and hSgo1. Kinetochores depleted of CENP-F exhibited a reduction in the amounts of the mitotic checkpoint proteins Mad1, Mad2, hBUBR1, hBUB1, and hMps1. We postulate that CENP-F is not an essential component of the mitotic checkpoint but facilitates the duration of the mitotic delay. Separately, we show that CENP-F is a novel microtubule-binding protein that possesses two microtubule-binding domains at opposite ends of the molecule. The C-terminal microtubule-binding domain was found to stimulate microtubule polymerization in vitro. These activities provide a biochemical explanation for how CENP-F contributes to kinetochore attachments in vivo.

Cenp-F (mitosin) is more than a mitotic marker

Cenp-F (mitosin) is a large coiled-coil protein whose function has remained obscure since its identification a decade ago. It has been suggested that the protein plays a role in the kinetochore-mediated mitotic functions but until recently there was little evidence to support this postulation. Recent results from five laboratories have given insights on how Cenp-F may participate in the regulation of cell division. In this mini-review, we will summarize the current data regarding the mitotic tasks of Cenp-F as well as discuss how it is used as a proliferation marker of malignant cell growth in the clinic. Also, the protein's post-translational modification by farnesylation and potential contribution to cell cycle effects of farnesyl transferase inhibitors will be addressed.

Silencing Cenp-F weakens centromeric cohesion, prevents chromosome alignment and activates the spindle checkpoint

Cenp-F is an unusual kinetochore protein in that it localizes to the nuclear matrix in interphase and the nuclear envelope at the G2/M transition; it is farnesylated and rapidly degraded after mitosis. We have recently shown that farnesylation of Cenp-F is required for G2/M progression, its localization to kinetochores, and its degradation. However, the role Cenp-F plays in mitosis has remained enigmatic. Here we show that, following repression of Cenp-F by RNA interference (RNAi), the processes of metaphase chromosome alignment, anaphase chromosome segregation and cytokinesis all fail. Although kinetochores attach to microtubules in Cenp-F-deficient cells, the oscillatory movements that normally occur following K-fibre formation are severely dampened. Consistently, inter-kinetochore distances are reduced. In addition, merotelic associations are observed, suggesting that whereas kinetochores can attach microtubules in the absence of Cenp-F, resolving inappropriate interactions is inhibited. Repression of Cenp-F does not appear to compromise the spindle checkpoint. Rather, the chromosome alignment defect induced by Cenp-F RNA interference is accompanied by a prolonged mitosis, indicating checkpoint activation. Indeed, the prolonged mitosis induced by Cenp-F RNAi is dependent on the spindle checkpoint kinase BubR1. Surprisingly, chromosomes in Cenp-F-deficient cells frequently show a premature loss of chromatid cohesion. Thus, in addition to regulating kinetochore-microtubule interactions, Cenp-F might be required to protect centromeric cohesion prior to anaphase commitment. Intriguingly, whereas most of the sister-less kinetochores cluster near the spindle poles, some align at the spindle equator, possibly through merotelic or lateral orientations.

Prevalence of anticentromere F protein autoantibodies in 347 patients with non-Hodgkin's lymphoma

An association between autoimmunity and hematological malignancies has been reported including the detection of antinuclear autoantibodies (ANAs) in patients suffering from non-Hodgkin's lymphoma (NHL), with a high prevalence of ANAs directed to components of the mitotic apparatus or the mitosis-associated proteins. Previous studies have demonstrated that one of the targets of such ANAs could be the CENP-F protein, especially in some carcinomas. The prevalence and specificity of anti-CENP-F autoantibodies (aAbs) thus were analyzed in 347 patients with different histological subgroups of NHL before any treatment of NHL, along with 150 controls. The detection of these aAbs was performed using two techniques: a radioimmunological assay (RIA) and an indirect immunofluorescence technique (IIF). Twenty-five (7.2%) NHL patients and 2 (1.3%) control patients displayed anti-CENP-F aAbs using RIA. This difference between the two groups was found to be significant (P < 0.01), with a higher prevalence of aAbs in the follicular (13%) and in the marginal zone B and MALT (10.2%) lymphoma subgroups. By IIF, 10 (2.9%) patients with NHL displayed aAbs with a CENP-F or CENP-F-like pattern, whereas none of the control group did. In conclusion, these data demonstrate that a significant incidence of anti-CENP-F aAbs is observed, before any treatment, in some histological subgroups of NHL patients. In addition to the usefulness of anti-CENP-F aAbs as a marker for some NHL subgroups, prospective studies may be important to evaluate the predictive value of anti-CENP-F aAbs for the development of carcinomas.

Immunohistochemical detection of cdc2 is useful in predicting survival in patients with mantle cell lymphoma

Recent cDNA microarray studies have reported the prognostic value of several genes in mantle cell lymphoma patients. We aimed to validate the prognostic significance of three of these genes: alpha-tubulin, cdc2, and CENP-F. The protein expression of alpha-tubulin, cdc2, and CENP-F was assessed using immunohistochemistry. Their immunoreactivity in 48 formalin-fixed/paraffin-embedded mantle cell lymphoma tumors was determined by estimating the percentage of positive cells. These results were correlated with the expression of proliferation marker Ki67 and survival. Of these 48 mantle cell lymphoma patients, 41 were men and seven were women. The median age at time of diagnosis was 64.5 years, and the overall median survival was 40 months. In benign lymph nodes, the expression of cdc2 and alpha-tubulin was restricted to the germinal centers; mantle zones were negative. Expression of CENP-F was more uniformly distributed. In mantle cell lymphoma, Ki67 significantly correlated with all three markers (P<0.05, Spearman), but only Ki67 (>50%) and cdc2 (>25%) significantly correlated with shorter survival (P<0.0006, Spearman). Of several clinical parameters examined, international prognostic index of >or=2 correlated with worse clinical outcome, and high clinical stage (ie 4 vs <or=3) showed a trend for shorter survival. The prognostic significance of cdc2 and Ki67 was independent of international prognostic index and clinical stage. We have validated the prognostic value of cdc2, and confirmed that of Ki67, in a cohort of mantle cell lymphoma patients. Immunohistochemical detection of cdc2 and Ki67 may be a useful and simple method in evaluating the prognosis of mantle cell lymphoma patients.

Hotelling's T2 multivariate profiling for detecting differential expression in microarrays

The most widely used statistical methods for finding differentially expressed genes (DEGs) are essentially univariate. In this study, we present a new T(2) statistic for analyzing microarray data. We implemented our method using a multiple forward search (MFS) algorithm that is designed for selecting a subset of feature vectors in high-dimensional microarray datasets. The proposed T2 statistic is a corollary to that originally developed for multivariate analyses and possesses two prominent statistical properties. First, our method takes into account multidimensional structure of microarray data. The utilization of the information hidden in gene interactions allows for finding genes whose differential expressions are not marginally detectable in univariate testing methods. Second, the statistic has a close relationship to discriminant analyses for classification of gene expression patterns. Our search algorithm sequentially maximizes gene expression difference/distance between two groups of genes. Including such a set of DEGs into initial feature variables may increase the power of classification rules. We validated our method by using a spike-in HGU95 dataset from Affymetrix. The utility of the new method was demonstrated by application to the analyses of gene expression patterns in human liver cancers and breast cancers. Extensive bioinformatics analyses and cross-validation of DEGs identified in the application datasets showed the significant advantages of our new algorithm.

GW182 is critical for the stability of GW bodies expressed during the cell cycle and cell proliferation

A novel cytoplasmic compartment referred to as GW bodies was initially identified using human autoantibodies to a 182 kDa protein named GW182. GW bodies are small, generally spherical, cytoplasmic domains that vary in number and size in several mammalian cell types examined to date. Based on our earlier studies, GW bodies were proposed to be cytoplasmic sites for mRNA storage and/or degradation. In the present study, immunogold electron microscopy identified electron dense structures of 100-300 nm diameter devoid of a lipid bilayer membrane. These structures appeared to comprise clusters of electron dense strands of 8-10 nm in diameter. By costaining with CENP-F and PCNA, and employing a double-thymidine block to synchronize HeLa cells, GW bodies were observed to be small in early S phase and larger during late S and G2 phases of the cell cycle. The majority of GW bodies disassembled prior to mitosis and small GW bodies reassembled in early G1. The analysis of GW bodies in two experimental models of cell proliferation using reversal of 3T3/serum-starvation and concanavalin A stimulation of mouse splenocytes and T cells, revealed that proliferating cells contained larger, brighter, and more numerous GW bodies as well as up to a fivefold more total GW182 protein than quiescent cells. In vitro gene knockdown of GW182 led to the disappearance of GW bodies demonstrating that GW182 is a critical component of GW bodies. The incremental expression of the GW182 protein in cells induced to proliferate and the cyclic formation and breakdown of GW bodies during mitosis are intriguing in view of the notion that GW bodies are specialized centers involved in maintaining stability and/or controlling degradation of mRNA.

Immunohistochemical analysis of centromere protein F expression in buccal and gingival squamous cell carcinoma

Centromere protein F (CENP-F) expression (localization and characteristics) in relation to tumor clinicopathological parameters was immunohistochemically examined and evaluated in 47 archival biopsy specimens of buccal and gingival squamous cell carcinomas (SCC). Centromere protein F expression was detected in 79% of the samples. An increase in the labeling index (LI) with WHO grading was obtained (P < 0.05). Correlations were obtained between the CENP-F LI and tumor size (P < 0.05). Immunoelectron microscopy showed CENP-F nuclear staining as punctate or fine dots. The present study shows that CENP-F expression and detection of a more specific cell subpopulation presents a theoretical advantage for the analysis of the precise cell cycle of G2 to M cells, compared to Ki-67.

Farnesylation of Cenp-F is required for G2/M progression and degradation after mitosis

Farnesyl transferase inhibitors induce G2/M cell cycle delays that cannot be explained by inhibition of the Ras GTPase. Recently, the kinetochore protein Cenp-F has been shown to be farnesylated. Here, we show that ectopic expression of the kinetochore targeting domain of Cenp-F delays progression through G2/M. Significantly, this is dependent on the CAAX farnesylation motif. We also show that localisation of Cenp-F to the nuclear envelope at G2/M and kinetochores in prometaphase is dependent both on its CAAX motif and farnesyl transferase activity. Strikingly, farnesyl transferase activity is also required for Cenp-F degradation after mitosis. Thus, these observations suggest that farnesylation of Cenp-F is required not only for its localisation to the nuclear envelope and kinetochores but also for timely progression through G2/M and its degradation after mitosis. In addition, these observations raise the possibility that the anti-proliferative effects induced by farnesyl transferase inhibitors may be due to inhibition of Cenp-F function and/or turnover.

Autoimmune responses to mRNA binding proteins p62 and Koc in diverse malignancies

Two tumor-associated antigens, p62 and Koc, are insulin-like growth factor II (IGF-II) messenger RNA binding proteins. Autoantibodies to p62 have been detected in cancer sera but have not been reported for Koc. This study determined the extent and frequency of autoantibodies to p62 and Koc in diverse malignancies, the epitopes on the antigens, and the presence or absence of cross-reactive antibodies. Recombinant polypeptides were expressed from full-length and partial cDNA constructs and used as antigens in Western blotting, enzyme-linked immunoassay, and immunoprecipitation. After identifying the epitopes, cross-absorption with recombinant polypeptides was used to determine specificity. Sera from 777 patients with 10 different types of malignancy were analyzed. Autoantibodies to p62 were found in 11.6% and to Koc in 12.2% and cumulatively to both antigens in 20.5%, with significant difference from the control populations consisting of normal subjects and autoimmune disease patients (P < 0.01). The immunodominant epitopes were at the amino termini of both antigens and absorption studies showed that the majority of autoantibodies were not cross-reactive. Autoantibodies to p62 and Koc were present in approximately similar frequencies in a variety of malignancies and the immune responses appeared to be independent of each other. The immune responses might be related to overexpression or dysregulation of p62 and Koc in some tumors.

De-novo humoral immune responses to cancer-associated autoantigens during transition from chronic liver disease to hepatocellular carcinoma

A feature of hepatocellular carcinoma (HCC) is that antecedent liver cirrhosis and chronic hepatitis are common precursor conditions and during transition to malignancy some patients develop autoantibodies which were not present during the preceding chronic liver disease phase. Serum samples from such patients can be used to immunoscreen cDNA expression libraries to identify genes encoding the new autoantigens. We demonstrate here the de novo appearance of antibodies to p62, a cytoplasmic protein which has been shown to bind to a developmentally regulated fetal species of insulin-like growth factor II (IGF-II) mRNA. Another antibody appearing during the transition period was against CENP-F, a cell cycle-related nuclear protein with maximum expression in the G2 and M phases of the cell cycle and previously shown to have a high association with malignancy. In three additional patients in whom serial serum samples were examined, new appearance of anti-p62 was detected in two patients and anti-CENP-F in one patient. This study demonstrates that transition to malignancy can be associated with autoantibody responses to certain cellular proteins which might have some role in tumorigenesis.

CENP-F gene amplification and overexpression in head and neck squamous cell carcinomas

BACKGROUND

Antibodies against cancer-related genes have been detected in human cancers including head and neck cancers. High titers of c-Myc autoantibodies have been linked to gene amplification and tumor progression. Centromere protein-F (CENP-F) autoantibodies have been detected in patients with various cancers, suggesting similar gene alteration.

METHODS

CENP-F and c-MYC amplification was assessed in 72 head and neck squamous cell carcinoma (HNSCC) patients. Tumor and matched mucosa from 22 patients were analyzed for CENP-F mRNA levels by RT-PCR.

RESULTS

The larynx was the site most altered by amplification of either gene. CENP-F and c-MYC were amplified in 11% and 17% of the tumors, respectively. Coamplification was found in 7% of the tumors, most of which showed regional node involvement. CENP-F mRNA was overexpressed in 36% of tumors, and 23% of paired mucosa.

CONCLUSION

Our results provide the first evidence that CENP-F gene is amplified and overexpressed in HNSCC. No correlation was noted between CENP-F amplification and clinicopathologic parameters. However, CENP-F overexpression correlated with nodal metastasis.

Skeletor, a novel chromosomal protein that redistributes during mitosis provides evidence for the formation of a spindle matrix

A spindle matrix has been proposed to help organize and stabilize the microtubule spindle during mitosis, though molecular evidence corroborating its existence has been elusive. In Drosophila, we have cloned and characterized a novel nuclear protein, skeletor, that we propose is part of a macromolecular complex forming such a spindle matrix. Skeletor antibody staining shows that skeletor is associated with the chromosomes at interphase, but redistributes into a true fusiform spindle structure at prophase, which precedes microtubule spindle formation. During metaphase, the spindle, defined by skeletor antibody labeling, and the microtubule spindles are coaligned. We find that the skeletor-defined spindle maintains its fusiform spindle structure from end to end across the metaphase plate during anaphase when the chromosomes segregate. Consequently, the properties of the skeletor-defined spindle make it an ideal substrate for providing structural support stabilizing microtubules and counterbalancing force production. Furthermore, skeletor metaphase spindles persist in the absence of microtubule spindles, strongly implying that the existence of the skeletor-defined spindle does not require polymerized microtubules. Thus, the identification and characterization of skeletor represents the first direct molecular evidence for the existence of a complete spindle matrix that forms within the nucleus before microtubule spindle formation.

Mammalian centromeres: DNA sequence, protein composition, and role in cell cycle progression

The centromere is a specialized region of the eukaryotic chromosome that is responsible for directing chromosome movements in mitosis and for coordinating the progression of mitotic events at the crucial transition between metaphase and anaphase. In this review, we will focus on recent advances in the understanding of centromere composition at the protein and DNA level and of the role of centromeres in sister-chromatid cohesion and mitotic checkpoint control.

INCENP centromere and spindle targeting: identification of essential conserved motifs and involvement of heterochromatin protein HP1

The inner centromere protein (INCENP) has a modular organization, with domains required for chromosomal and cytoskeletal functions concentrated near the amino and carboxyl termini, respectively. In this study we have identified an autonomous centromere- and midbody-targeting module in the amino-terminal 68 amino acids of INCENP. Within this module, we have identified two evolutionarily conserved amino acid sequence motifs: a 13-amino acid motif that is required for targeting to centromeres and transfer to the spindle, and an 11-amino acid motif that is required for transfer to the spindle by molecules that have targeted previously to the centromere. To begin to understand the mechanisms of INCENP function in mitosis, we have performed a yeast two-hybrid screen for interacting proteins. These and subsequent in vitro binding experiments identify a physical interaction between INCENP and heterochromatin protein HP1(Hsalpha). Surprisingly, this interaction does not appear to be involved in targeting INCENP to the centromeric heterochromatin, but may instead have a role in its transfer from the chromosomes to the anaphase spindle.

Molecular definition of heterogeneous nuclear ribonucleoprotein R (hnRNP R) using autoimmune antibody: immunological relationship with hnRNP P

Serum from a patient showing symptoms related to autoimmunity was found to contain autoantibodies to the nuclear mitotic apparatus (NuMA) protein and to several novel nuclear antigens with estimated molecular weights of 40, 43, 72, 74 and 82 kDa. Using this serum for screening a human cDNA expression library a 2.5 kb cDNA clone was isolated which encoded the complete sequence of a protein of 633 amino acids. Sequence analysis revealed a modular structure of the protein: an acidic N-terminal region of approximately 150 amino acids was followed by three adjacent consensus sequence RNA binding domains located in the central part of the protein. In the C-terminal portion a nuclear localization signal and an octapeptide (PPPRMPPP) with similarity to a major B cell epitope of the snRNP core protein B were identified. This was followed by a glycine- and arginine-rich section of approximately 120 amino acids forming another type of RNA binding motif, a RGG box. Interestingly, three copies of a tyrosine-rich decapeptide were found interspersed in the RGG box region. The major in vitro translation product of the cDNA co-migrated in SDS-PAGE with the 82 kDa polypeptide that was recognized by autoantibodies. The structural motifs as well as the immunofluorescence pattern generated by anti-82 kDa antibodies suggested that the antigen was one of the proteins of the heterogeneous nuclear ribonucleoprotein (hnRNP) complex. Subsequently the 82 kDa antigen was identified as hnRNP R protein by its presence in immunoprecipitated hnRNP complexes and co-migration of the recombinant protein with this hitherto uncharacterized hnRNP constituent in two-dimensional gel electrophoresis. The concomitant autoimmune response to a hnRNP component of the pre-mRNA processing machinery and to NuMA, a protein engaged in mitotic events and reported to be associated with mRNA splicing complexes in interphase, may indicate physical and functional association of these antigens. Support for this notion comes from observations that concomitant or coupling of autoantibody responses to proteins which are associated with each other as components of subcellular particles are often found in autoimmune diseases.

Diversity of antinuclear antibody responses in hepatocellular carcinoma

BACKGROUND/AIMS

The development of antinuclear antibodies (ANA) in malignancies has been described but its mechanism is still not understood. The aim of this study was to examine ANA specificities in hepatocellular carcinoma to further understand autoimmunity in cancer.

METHODS

Two hundred and four hepatocellular carcinoma patients were compared with 68 chronic hepatitis C, with 126 chronic hepatitis B and with 30 alcoholic liver cirrhosis patients, as well as with 87 healthy donors. Indirect immunofluorescence, immunoblotting, and immunoprecipitation were used to study ANA reactivities.

RESULTS

Hepatocellular carcinoma had a significantly higher frequency of ANA using HEp-2 cells as substrate (31%) than chronic hepatitis C (10%), chronic hepatitis B (9.5%), alcoholic liver cirrhosis (10%) or healthy donors (4.5%). A great diversity of ANA specificities was found in hepatocellular carcinoma. Three hepatoma sera had antibodies that co-localized with non-snRNP splicing factor SC35, suggesting that the antigenic targets might be involved in mRNA splicing. We identified antibodies to two known nuclear autoantigens: fibrillarin and p330d/CENP-F. These autoantigens are involved in the 5' processing of precursor ribosomal RNA transcripts and in mitotic functions, respectively.

CONCLUSIONS

Diversity was found in the autoantibody specificity, in contrast to the specific subsets of autoantibodies seen in several systemic rheumatic autoimmune diseases. Our data suggest that immune response in hepatocellular carcinoma targets important proteins involved in cellular biosynthetic or proliferative functions.

Chromosomal proteins and cytokinesis: patterns of cleavage furrow formation and inner centromere protein positioning in mitotic heterokaryons and mid-anaphase cells

After the separation of sister chromatids in anaphase, it is essential that the cell position a cleavage furrow so that it partitions the chromatids into two daughter cells of roughly equal size. The mechanism by which cells position this cleavage furrow remains unknown, although the best current model is that furrows always assemble midway between asters. We used micromanipulation of human cultured cells to produce mitotic heterokaryons with two spindles fused in a V conformation. The majority (15/19) of these cells cleaved along a single plane that transected the two arms of the V at the position where the metaphase plate had been, a result at odds with current views of furrow positioning. However, four cells did form an additional ectopic furrow between the spindle poles at the open end of the V, consistent with the established view. To begin to address the mechanism of furrow assembly, we have begun a detailed study of the properties of the chromosome passenger inner centromere protein (INCENP) in anaphase and telophase cells. We found that INCENP is a very early component of the cleavage furrow, accumulating at the equatorial cortex before any noticeable cortical shape change and before any local accumulation of myosin heavy chain. In mitotic heterokaryons, INCENP was detected in association with spindle midzone microtubules beneath sites of furrowing and was not detected when furrows were absent. A functional role for INCENP in cytokinesis was suggested in experiments where a nearly full-length INCENP was tethered to the centromere. Many cells expressing the chimeric INCENP failed to complete cytokinesis and entered the next cell cycle with daughter cells connected by a large intercellular bridge with a prominent midbody. Together, these results suggest that INCENP has a role in either the assembly or function of the cleavage furrow.

Nuclear components with microtubule-organizing properties in multicellular eukaryotes: functional and evolutionary considerations

The nucleus and the microtubular cytoskeleton of eukaryotic cells appear to be structurally and functionally interrelated. Together they constitute a "cell body". One of the most important components of this body is a primary microtubule-organizing center (MTOC-I) located on or near the nuclear surface and composed of material that, in addition to constitutive centrosomal material, also comprises some nuclear matrix components. The MTOC-I shares a continuity with the mitotic spindle and, in animal cells, with the centrosome also. Secondary microtubule-organizing centers (MTOC-IIs) are a special feature of walled plant cells and are found at the plasma membrane where they organize arrays of cortical MTs that are essential for ordered cell wall synthesis and hence for cellular morphogenesis. MTOC-IIs are held to be similar in origin to the MTOC-I, but their material has been translocated to the cell periphery, perhaps by MTs organized and radiating from the MTOC-I. Many intranuclear, matrix-related components have been identified to participate in MT organization during mitosis and cytokinesis; some of them also seem to be related to the condensation and decondensation of chromatin during the mitotic chromosome cycle.

Two major autoantigen-antibody systems of the mitotic spindle apparatus

OBJECTIVE

To characterize human autoantigen-antibody systems related to the mitotic poles and spindles.

METHODS

Thirty-seven human sera with autoantibodies staining mitotic poles and spindles in indirect immunofluorescence (IIF) studies were further characterized by immunofluorescence on mitotic cells and by immunoblotting and immunoprecipitation. Clinical diagnoses meeting the American College of Rheumatology criteria were based on chart review and interview with the corresponding physicians.

RESULTS

Two autoantibody systems reactive with mitotic poles and spindles were defined. Type 1 nuclear mitotic apparatus (NuMA-1) antibodies were identified in the serum of 30 patients. Interphase cells showed a fine, speckled, nuclear staining, while mitotic cells had bright staining of the rim of the centrosomes and light staining of the spindles proximal to the centrosomes. In telophase, the staining shifted from the centrosomes to the reforming nuclei. On immunoblotting, anti-NuMA-1 sera reacted with a 210-kd protein. The reactivity of these sera was identified (with the aid of reference antibodies) as the previously described NuMA antigen-antibody system. Clinical information was available for only 17 of the 30 patients with anti-NuMA-1; of these, 17 (53%) had clinical and lip biopsy findings that met the criteria for Sjögren's syndrome. NuMA-2 antibodies were found in the sera of 7 patients. Interphase cells showed no nuclear or cytoplasmic staining, but mitotic cells had brightly stained poles and spindles. At anaphase/telophase, staining shifted to the midbody and the intercellular bridge. Anti-NuMA-2 sera immunoprecipitated a protein of 116 kd. This group of patients was more heterogeneous and had both systemic and organ-specific autoimmune diseases.

CONCLUSIONS

NuMA protein (here called NuMA-1) and a 116-kd protein (here called NuMA-2) are the major targets of the autoimmune response in the mitotic apparatus, since most of the selected sera (based on IIF staining of the mitotic spindles and poles) recognized 1 of these 2 antigens.

Treatment with the anti-tumor drugs, cis-platin and mafosfamide, does not affect the structure of prekinetochores in a human breast cancer cell line. An immunofluorescence study using human anti-centromere autoantibodies

The goal of the present article was to determine whether a nuclear parameter, centromere structure of interphase cells, could serve as an indicator to assess cellular damage caused by anti-tumor drugs. These were cis-platin and mafosfamide, which are widely used for the management of solid tumors. To visualize the centromeres, we probed treated and untreated cells of a human breast cancer cell line, MX-1, with a human anti-centromere serum. The serum was obtained from a scleroderma patient and detects antigens associated with prekinetochores of the decondensed chromosomes. The DNA was simultaneously displayed by a specific fluorescent dye. The cells were grown on coverslips, incubated for 1 h in a drug-containing medium, transferred into a drug-free medium and observed 24 h later. Since the efficiency of many anti-tumor drugs increases with the temperature, two different temperatures, 37 and 42 degrees C, were used. The analysis revealed that the treatment did not visibly alter the labeling pattern. We conclude that chromosome structure remains largely intact and is not suitable for the cytological evaluation of the efficiency of anti-tumor drugs. This is in contrast with, for example, the microtubular cytoskeleton and mitochondria, which were extensively damaged under the conditions applied here (compare Wolf et al. 1995). Independent of the drug and the temperature selected, the nuclear lumen of mononucleated and multinucleated cells contained small fluorescent spots. Double dots corresponding to the sister centromeres in the G2 phase of the cell cycle were rare. In addition to the voluminous nuclei, some cells possessed micronuclei in the lateral cytoplasm and these were regularly labeled by the autoantibodies. A small subset of the mononucleated MX-1 cells had unusually large nuclei. It is reasonable to assume that they are polyploid. The fluorescent spots marking the prekinetochores were very large in these cells. This may indicate that the chromosomes remain associated after replication.

Specific interaction between human kinetochore protein CENP-C and a nucleolar transcriptional regulator

CENP-C is a human kinetochore protein that was originally identified as a chromosomal autoantigen in patients with scleroderma spectrum disease. To begin to establish a comprehensive protein map of the human centromere, affinity chromatography was used to identify nuclear proteins that specifically interact with CENP-C. Whereas a number of polypeptides appeared to interact with the full-length CENP-C protein, only a pair of similarly sized proteins of approximately 100 kDa specifically interacted with the isolated carboxyl-terminal third of the CENP-C protein. Neither protein of the doublet bound to control affinity columns. Affinity purification and microsequence analysis of the proteins in the doublet identified them as the two highly related nucleolar transcription factors, UBF1 and UBF2 (also known as the nucleolar autoantigen NOR-90). Immunoblot analysis confirmed that both proteins also interacted with the full-length CENP-C polypeptide with similar affinities. Double indirect immunofluorescence using monospecific antibodies demonstrated that a subset of CENP-C and UBF/NOR-90 is colocalized at nucleoli of interphase HeLa cells, suggesting that the in vitro interaction detected by affinity chromatography may reflect an interaction that occurs in vivo. We discuss the implications of these findings in terms of the properties of interphase centromeres and the role of the nucleolus in scleroderma autoimmunity.

The centromere kinesin-like protein, CENP-E. An autoantigen in systemic sclerosis

OBJECTIVE

Autoantibodies directed against centromere proteins (CENPs) are a serologic feature in some patients with systemic sclerosis (SSc). Previous studies have focused on autoantibodies to CENPs A, B, and C. CENP-E is a recently described 312-kd protein that also localizes to the centromere. Therefore, we studied the presence of autoantibodies to recombinant CENP-E in patients with SSc.

METHODS

Sixty sera from patients with the SSc spectrum of diseases were screened for the presence of autoantibodies against CENP-E, by indirect immunofluorescence and immunoblotting using recombinant CENP-E protein. HLA class II alleles were determined by DNA oligotyping.

RESULTS

Among the SSc sera, 15 of 60 (25%) demonstrated antibody reactivity with recombinant CENP-E, and 14 of these 15 sera (93%) had antibodies directed against another CENP. Anti-CENP-E was seen in 13 of 30 sera with anti-CENP (43%). All patients with anti-CENP-E had a limited form of SSc, known as the CREST variant (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasias). When patients with anti-CENPs A, B, or C were compared with patients with anti-CENP-E, no unique clinical features in the anti-CENP-E positive group were identified. Ninety-three percent of the patients with anti-CENP-E had HLA-DQB1 alleles that had polar amino acids at position 26 (primarily DQB1*05), similar to patients with other CENP autoantibodies.

CONCLUSION

Antibodies to CENP-E are common in patients with SSc, and are seen in higher frequency in sera from patients with a limited form, or CREST variant, of the disease.

Dynamic continuity of nuclear and mitotic matrix proteins in the cell cycle

The eukaryotic cell nucleus is a membrane-enclosed compartment containing the genome and associated molecules supported by a highly insoluble filamentous network known as the nucleoskeleton or nuclear matrix. The nuclear matrix is believed to play roles in maintaining nuclear architecture and organizing nuclear metabolism. Recently, advances in microscopic techniques and the availability of new molecular probes have made it possible to localize functional domains within the nuclear matrix and demonstrate dynamic interactions between both soluble and insoluble components involved in the control of multiple nuclear transactions. Like the cytoplasm and its skeleton, the nucleoplasm is highly structured and very crowded with an equally complex skeletal framework. In fact, there is growing evidence that the two skeletal systems are functionally contiguous, providing a dynamic cellular matrix connecting the cell surface with the genome. If we impose cell cycle dynamics upon this skeletal organization, it is obvious that the genome and associated nuclear matrix must undergo a major structural transition during mitosis, being disassembled and/or reorganized in late G2 and reassembled again in daughter nuclei. However, recent evidence from our laboratory and elsewhere suggests that much of the nuclear matrix is used to form the mitotic apparatus (MA). Indeed, both facultative and constitutive matrix-associated proteins such as NuMA, CENP-B, CENP-F, and the retinoblastoma protein (Rb) associate within and around the MA. During mitosis, the nuclear matrix proteins may either become inert "passengers" or assume critical functions in partitioning the genome into newly formed G1 nuclei. Therefore, we support the view that the nuclear matrix exists as a dynamic architectural continuum, embracing the genome and maintaining cellular regulation throughout the cell cycle.

Association of Lyn tyrosine kinase with the nuclear matrix and cell-cycle-dependent changes in matrix-associated tyrosine kinase activity

The nuclear matrix isolated from HeLa cells and Rat2 fibroblasts harbors tyrosine kinase and tyrosine phosphatase activities. Polypeptides of 53, 56 and 60 kDa, associated with this subnuclear structure, were phosphorylated at tyrosine in vivo. By immunoblot and immunolabelling experiments, we identified one of the nuclear-matrix-associated tyrosine kinases as Lyn, a Src family member. Lyn was distributed as foci throughout the matrix. The p56 and p53 isoforms of Lyn remained firmly associated with the nuclear matrix after a variety of matrix preparation procedures, and were not detectable in the chromatin fraction of the nucleus. The tyrosine kinase activity associated with the nuclear matrix showed cell-cycle-dependent changes, maximum activity being observed at the G1/S transition phase. Polyoma-virus-transformed rat fibroblast cells showed sixfold higher tyrosine kinase activity in the nuclear matrix preparations compared to that in untransformed cells. These observations are consistent with the suggestion that tyrosine kinase activity associated with the nuclear matrix may be an important determinant of cellular proliferation.

Nuclear matrix and the cell cycle

The facts that the nuclear matrix represents a structural framework of the cell nucleus and that nuclear events, such as DNA replication, transcription, and DNA repair, are associated with this skeletal structure suggest that its components are subject to cell cycle-regulatory mechanisms. Cell cycle regulation has been shown for nuclear lamina assembly and disassembly during mitosis and chromatin reorganization. Little attention has so far been paid to internal nuclear matrix proteins and matrix-associated proteins with respect to the cell cycle. This survey attempts to summarize available data and presents experimental evidence that important metabolic functions of the nucleus are regulated by the transient, cell cycle-dependent attachment of enzymes and regulatory proteins to the nuclear matrix. Results on thymidine kinase and RNA polymerase during the synchronous cell cycle of Physarum polycephalum demonstrate that reversible binding to the nuclear matrix represents an additional level of regulation for nuclear processes.

Nuclear matrix proteins as structural and functional components of the mitotic apparatus

The eukaryotic nucleus is a membrane-enclosed compartment containing the genome and associated organelles supported by a complex matrix of nonhistone proteins. Identified as the nuclear matrix, this component maintains spatial order and provides the structural framework needed for DNA replication, RNA synthesis and processing, nuclear transport, and steroid hormone action. During mitosis, the nucleoskeleton and associated chromatin is efficiently dismantled, packaged, partitioned, and subsequently reassembled into daughter nuclei. The dramatic dissolution of the nucleus is accompanied by the assembly of a mitotic apparatus required to facilitate the complex events associated with nuclear division. Until recently, little was known about the fate or disposition of nuclear matrix proteins during mitosis. The availability of specific molecular probes and imaging techniques, including confocal microscopy and improved immunoelectron microscopy using resinless sections and related procedures, has enabled investigators to identify and map the distribution of nuclear matrix proteins throughout the cell cycle. This chapter will review the structure, function, and distribution of the protein NuMA (nuclear matrix mitotic apparatus) and other nuclear matrix proteins that depart the nucleus during the interphase/mitosis transition to become structural and functional components within specific domains of the mitotic apparatus.

Characterization of a cell cycle-dependent nuclear autoantigen

Analysis of reactivity to nuclear antigens in autoimmune sera revealed a serum that produced a previously undescribed cell cycle-dependent immunofluorescence staining pattern. By indirect immunofluorescence using HEp-2 cells as substrate, the serum generated a speckled and nucleolar pleomorphic staining pattern. This characteristic immunofluorescence pattern was detected in different cell lines from various species indicating that the antigen was highly conserved. This serum immunoprecipitated a 85 kDa protein using an extract from [35S]-labeled HeLa cells. Indirect immunofluorescence of proliferating mouse 3T3 cells displayed the characteristic pleomorphic staining which was not observed in serum-starved cells. Resting human and mouse peripheral blood lymphocytes were negative in immunofluorescence while mitogen-stimulated lymphocytes were positive. Germinal centers of mice two weeks after immunization with 2-phenyl-oxazolone showed speckled immunofluorescence staining in the dark zones whereas unimmunized mice were completely negative. Cell synchronization experiments showed a characteristic sequence of locations of the antigen during the cell cycle. In G1, cells were completely negative. In late G1, G1/S and S phase, speckles were visible. In early G2, speckles were visible, and later in G2, the nucleoli were positive. During mitosis chromosomes were stained. Further characterization of this antibody specificity and cloning of cDNA are in progress.

The centromere: hub of chromosomal activities

Centromeres are the structures that direct eukaryotic chromosome segregation in mitosis and meiosis. There are two major classes of centromeres. Point centromeres, found in the budding yeasts, are compact loci whose constituent proteins are now beginning to yield to biochemical analysis. Regional centromeres, best described in the fission yeast Schizosaccharomyces pombe, encompass many kilobases of DNA and are packaged into heterochromatin. Their associated proteins are as yet poorly understood. In addition to providing the site for microtubule attachment, centromeres also have an important role in checkpoint regulation during mitosis.

The C terminus of mitosin is essential for its nuclear localization, centromere/kinetochore targeting, and dimerization

Mitosin is a novel 350-kDa nuclear phosphoprotein that dramatically relocates from the evenly nuclear distribution in S phase to the centromere/kinetochore and mitotic apparatus in M phase. The dynamic relocalization of mitosin is accompanied by the phosphorylation of itself, suggesting that mitosin plays a role in mitotic progression. The molecular basis of nuclear localization and targeting of mitosin to the centromere/kinetochore were characterized using a set of epitope-tagged deletion mutants. The data indicate that the extreme C terminus (amino acids 2,487-3,113) of mitosin has both an independent centromere/kinetochore targeting domain and an unusually spaced bipartite nuclear localization signal. Moreover, the same centromere/kinetochore targeting domain was shown to be essential for the ability of mitosin to bind to itself or other putative mitosin-associated proteins through use of the yeast two-hybrid system. These results suggest that the C terminus of the mitosin is essential for its role in influencing cell cycle progression.

Autoimmunity to the cell cycle-dependent centromere protein p330d/CENP-F in disorders associated with cell proliferation

p330d/CENP-F is a novel proliferation-associated and cell cycle-dependent centromere autoantigen which appears to play a very important role in mitotic progression. As an initial step in exploring the clinical and biological significance of autoantibodies to this protein, we evaluated the clinical histories of 26 patients producing these antibodies. The antibodies were detected by both indirect immunofluorescence microscopy (IIF) and Western blotting. All the sera contained anti-p330d/CENP-F IgG antibodies, with an average titer by IIF of 1:6,917 (range 1:160 to 1:20,480). Most of the patients had disorders associated with abnormal or increased cell proliferation at the time the anti-p330d/CENP-F antibodies were detected. These included cancers of various types (14), chronic liver disease (3), chronic rejection of renal allografts (2), and Crohn's disease (1). The average IIF titer of the anti-p330d/CENP-F antibodies in the patients with cancer, 1:10,103, was significantly higher than the average titer in non-cancer patients, 1:3,200 (P = 0.008). Autoimmunity to p330d/CENP-F appeared not to be associated with rheumatic diseases, in particular scleroderma, since only three of the 26 patients had rheumatic disease and the antibodies were not detected by IIF in a group of 351 patients with scleroderma and related disorders. Our findings, although retrospective and limited to a relatively small number of patients, point to the hypothesis that autoimmunity to p330d/CENP-F could be related to events involving increased or abnormal cell proliferation.

New horizons for cytokinesis

The mechanism of cytokines is an old problem in cell biology that has received fresh attention recently with a large variety of powerful approaches and experimental systems. Significant advances have been made on the structure of the cortical cytoskeleton, the identification of proteins and genes involved, and the regulatory mechanism. Many surprises have surfaced within the past two years, leading us toward a major revision in our understanding of this important process.