Microtubule-actin interactions may regulate endothelial integrity and repair.

An important mechanism for the initiation and progression of atherosclerosis is the loss of endothelial integrity, which is required for normal blood vessel function. The important components of the endothelial cell cytoskeleton system that regulate endothelial integrity include actin microfilaments and microtubules, which are both associated with protein complexes that regulate cell-cell and cell-substratum adhesion. To date, studies have shown that microfilaments are essential in maintaining the structural integrity of the endothelium while microtubules regulate the directional cell migration during repair. When microtubules are disrupted at the onset of wounding, neither centrosome reorientation, which is essential for efficient endothelial cell wound repair, nor cell migration occurs. Disruption of microfilaments is also associated with inefficient endothelial cell migration and repair. How then might these systems be associated with one another? Linker proteins, which may facilitate interaction between microtubules and actin microfilaments, have recently been identified in nonendothelial systems. It is likely that microtubule-microfilament interactions are important in the complex regulation of endothelial integrity and repair especially as they relate to atherosclerotic plaque formation.     

Plectin deficiency on cytoskeletal disorganization and transformation of human liver cells in vitro

Plectin is a versatile cytoplasmic cross-linking protein that connects intermediate filaments to microfilaments, microtubules, and membrane adhesion sites. The cross-linking functions of plectin help organize the cytoskeleton into a stable meshwork important for maintaining uniformity in cell size and shape. As cells of hepatocellular carcinoma are morphologically different from normal human hepatocytes, we hypothesized that altered plectin expression and cytoskeletal organization underlies this pleomorphic transformation. To test this hypothesis, we analyzed expression levels and organization of all cytoskeletal elements, including intermediate filaments, microfilaments, and microtubules, after plectin knockdown in human Chang liver cells. We found that expression of cytokeratin 18, but not actin or tubulin, was downregulated by suppression of plectin protein. Furthermore, cytokeratin networks were partially collapsed and actin-rich stress fibers were increased. The organization of microtubule networks, by contrast, was unaltered. These findings support our hypothesis that, via effects on cytoskeletal organization, plectin deficiency might play an important role in the transformation of human liver cells.     

Effects of DNA synthesis inhibitors on early antigen expression following primary infection or superinfection by Epstein-Barr virus

Summary Seven lymphoid cell lines previously characterized with respect to their resident Epstein-Barr virus (EBV) genome content were infected or superinfected with concentrated EBV from supernatant of the P3 HR-1 cell line. Immunofluorescence assays were conducted on smears 48 hours after infection, using human sera containing antibodies to EBV early antigen (EA). Two EBV nuclear antigen (EBNA) negative cell lines containing no detectable resident EBV DNA and five EBNA positive cell lines containing EBV genomes were tested. The cell lines did not spontaneously express EBV EA (i.e., they were non-producers). All cell lines responded to infection or superinfection with EBV by expressing EA. Treatment of the cell lines with arabinosylcytosine (Ara-C) 10 µg/ml, at the time of infection resulted in significant decreases in the number of cells expressing detectable EA after drug treatment in all cell lines (72±5 percent inhibition of EA expression). Experiments were also conducted with hydroxyurea (HU) and phosphonoacetic acid (PAA). It was found that treatment with HU (100 µg/ml) inhibited EA production in cell lines containing EBV genome copies by 81 percent as compared to the superinfected cultures receiving no drug. In primary infection of EBNA negative cell lines, HU had minimal effects. PAA (100 µg/ml), on the other hand, had very little effect on EA expression following superinfection of cell lines harboring the EBV genome, but reduced the EA expression after primary infection of EBNA negative cell lines by 70 to 80 percent. All drugs were used at concentrations having little effect on RNA and protein synthesis. However, HU and Ara-C significantly reduced DNA synthesis and cell division in the treated cultures.With 2 Figures     

Penetration and damage of endothelial cells by Candida albicans.

The mechanisms of phagocytosis of Candida albicans by human vascular endothelial cells and subsequent endothelial cell injury were examined in vitro. Both live and killed C. albicans cells were phagocytized by endothelial cells. This organism specifically induced endothelial cell phagocytosis because neither Candida tropicalis nor Torulopsis glabrata was ingested. Endothelial cell microfilaments polymerized around C. albicans as the organisms were phagocytized. Cytochalasin D inhibited this polymerization of microfilaments around C. albicans and blocked phagocytosis. The blocking of actin depolymerization with phalloidin had no effect on microfilament condensation around the organism, indicating that the microfilaments surrounding C. albicans are formed from a pool of G-actin. Intact microtubules were also necessary for the phagocytosis of C. albicans, since the depolymerizing of endothelial cell microtubules with nocodazole prevented the condensation of actin filaments around the organisms and inhibited phagocytosis. In contrast, microtubule depolymerization was not required for microfilament function because the blocking of microtubule depolymerization with taxol had no effect on microfilament condensation around C. albicans. The phagocytosis of C. albicans was pivotal in the induction of endothelial cell damage, since the blocking of candidal internalization significantly reduced endothelial cell injury. Endothelial cells were not damaged by phagocytosis of dead organisms, indicating that injury was caused by a factor associated with viable organisms. Therefore, C. albicans is uniquely able to induce endothelial cell phagocytosis by comparison with non-albicans species of Candida. Furthermore, at least two components of the endothelial cytoskeleton, microfilaments and microtubules, are necessary for the phagocytosis of C. albicans.     

Modulation of the expression of connective tissue growth factor (CTGF) by alterations of the cytoskeleton

Introduction In closing wounds or developing fibrotic tissue, fibroblasts are exposed to mechanical stress, which leads to alterations in cell morphology and reorganization of the cytoskeleton. The impact of morphological changes on gene expression was investigated in the present study. As a model system, we used a human renal fibroblast cell line and studied the expression of connective tissue growth factor (CTGF). CTGF is a downstream mediator of TGF‐β mediating many of the pro‐fibrotic actions of this growth factor and has also been shown to be up‐regulated by static or dynamic pressure. The molecular mechanisms regulating CTGF expression related to stress are not yet known. Materials and methods A human renal fibroblast cell line was kindly provided by Dr Müller, Göttingen, Germany. These cells express CTGF upon treatment with soluble stimuli ( Heusinger‐Ribeiro et al. 2001 ; Eberlein et al. 2001 ). CTGF expression was detected by northern and Western blot analysis. The actin cytoskeleton was visualized by rhodamine phalloidin staining, and microtubules were detected by immunocytochemistry. Results Low concentrations of the microtubule disrupting agents nocodazole and colchicine strongly up‐regulated CTGF mRNA and protein expression in the human renal fibroblast cell line TK173. The up‐regulation was prevented by stabilization of the microtubules by paclitaxel (taxol). As a consequence of microtubule disruption, the small GTPase RhoA was activated and the actin stress fibers were stabilized. Both effects were related to CTGF induction: interference with RhoA signalling by simvastatin, toxin B and Y27632 prevented up‐regulation of CTGF. The important role of RhoA was supported by an increased CTGF expression upon overexpression of constitutively active RhoA. Direct disassembly of the actin cytoskeleton by latrunculin B interfered with colchicine‐mediated up‐regulation of CTGF expression. Disassembly of actin fibers by cytochalasin D unexpectedly increased CTGF expression. This indicated that the content of F‐actin per se was not the major determinant for CTGF gene expression. It has been shown, however, that cytochalasin D sequesters G‐actin, whereas latrunculin increases the level of G‐actin. Our data are thus in accordance with an inverse correlation between G‐actin levels and CTGF expression. Discussion These data link alterations in the microtubule and actin cytoskeleton to the expression of CTGF. Recently, decreased levels of G‐actin were observed in vascular smooth muscle cells in response to increased vascular pressure ( Cipolla et al. 2002 ). Our findings thus provide a molecular basis for the observation that CTGF is up‐regulated in cells exposed to mechanical stress.     

Respiratory syncytial virus infection in human bone marrow stromal cells

Respiratory syncytial virus (RSV) is the most common respiratory pathogen in infants and young children. The pathophysiology of this infection in the respiratory system has been studied extensively, but little is known about its consequences in other systems. We studied whether RSV infects human bone marrow stromal cells (BMSCs) in vitro and in vivo, and investigated whether and how this infection affects BMSC structure and hematopoietic support function. Primary human BMSCs were infected in vitro with recombinant RSV expressing green fluorescent protein. In addition, RNA from naive BMSCs was amplified by PCR, and the products were sequenced to confirm homology with the RSV genome. The BMSC cytoskeleton was visualized by immunostaining for actin. Finally, we analyzed infected BMSCs for the expression of multiple cytokines and chemokines, evaluated their hematopoietic support capacity, and measured their chemotactic activity for both lymphoid and myeloid cells. We found that BMSCs support RSV replication in vitro with efficiency that varies among cell lines derived from different donors; furthermore, RNA sequences homologous to the RSV genome were found in naive primary human BMSCs. RSV infection disrupted cytoskeletal actin microfilaments, altered cytokine/chemokine expression patterns, decreased the ability of BMSCs to support B cell maturation, and modulated local chemotaxis. Our data indicate that RSV infects human BMSCs in vitro, and this infection has important structural and functional consequences that might affect hematopoietic and immune functions. Furthermore, we have amplified viral RNA from naive primary BMSCs, suggesting that in vivo these cells provide RSV with an extrapulmonary target.     

Distinct patterns of viral antigen expression in Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus coinfected body-cavity-based lymphoma cell lines: potential switches in latent gene expression due to coinfection.

Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV), also referred to as human herpesvirus 8 (HHV-8), are human gammaherpesviruses associated with numerous lymphomas and proliferative diseases in humans. We were interested in the protein expression patterns of specific latent and lytic proteins from the EBV genome in two body-cavity-based lymphoma cell lines, BC-1 and BC-2, which are coinfected with EBV and KSHV. BC-1 and BC-2 were analyzed using specific antibodies to latent proteins known to be essential for EBV immortalization of human primary B-lymphocytes in vitro and lytic antigens important for EBV replication and production of viral progeny. The coinfected cell lines are compared with two singly infected KSHV cell lines to determine whether antibodies against EBV-specific proteins cross-reacted against KSHV antigens. All the KSHV-infected cell lines express the KSHV-specific latency-associated nuclear antigen (LANA) with a specific pattern in the nucleus. This staining was distinct from that seen for EBNA1 in the EBV coinfected lines BC-1 and BC-2 staining the nucleus as a diffused pattern throughout the nucleus with denser staining in some regions. The coinfected cell lines all express EBNA1 and LMP1 at lower levels compared with singly infected EBV lymphoblastoid cell lines (LCLs). However, the essential latent antigens EBNA2, EBNA3A, and EBNA3C are not expressed in BC-1 and BC-2. This indicates potential regulation of EBV latent gene expression by KSHV-encoded viral or KSHV-induced cellular gene products. Additionally, lytic gene expression analysis demonstrated that BZLF1 and BMRF1 are expressed along with other early antigens (EA-D). A specific protein is detected in a singly infected KSHV cell line with cross-reactivity to antibodies that detected the EA-D complex. Moreover, in all the cell lines infected with EBV, KSHV, or EBV and KSHV, human serum with antibodies against KSHV antigens recognizes specific viral antigens approximately 110 and 41-42 kDa, suggesting that human antibodies against KSHV-specific antigens can cross-react with similar EBV antigens. Therefore these data suggest that the EBV pattern of gene expression in the coinfected cell lines is a type II pattern of latency also seen in other human tumors including nasopharyngeal carcinoma and Hodgkin's lymphoma. This distinct pattern of latent and lytic gene expression in these cell lines may provide clues as to the selection for coinfection in these body cavity based lymphomas in immunocompromised hosts.     

Expression of Epstein-Barr virus nuclear antigens 3, 4, and 6 are altered in cell lines containing B-type virus

A high proportion of HIV-positive sera were found to react with 130- and 180-kDa antigens which were present in the Jijoye cell line. The majority of the HIV-positive sera which detected these antigens also contained antibodies to Epstein-Barr virus (EBV) nuclear antigen 2B (EBNA2B) suggesting a relationship between B-type EBV strains and the expression of the 130K/180K antigens. Cell lines were established by infection of B lymphocytes with different A- and B-type strains of EBV. Incubation of these lines with sera from individuals infected with either A-type or B-type EBV strains demonstrated that the 130K and 180K antigens were only expressed by cell lines containing B-type virus. Sera from individuals infected with A-type EBV did not react with the 180K antigen in any cell lines but could detect EBNAs 3, 4, and 6 antigens in the A-type cell lines. Restriction enzyme analysis of the BamHI E region of the EBV genome revealed marked differences between the A and B types of the virus. These results demonstrate that expression of antigens encoded from the BamHI E region of EBV (EBNAs 3, 4, and 6) are altered in cell lines transformed by B-type strains of EBV.     

Comparison of the cytoskeleton in aortic endothelial cells in situ and in vitro

Using fluorescence microscopy and a previously developed technique for making en face preparations of aortic endothelial cells (EC), we have compared the distribution of microtubules, centriolar regions, and microfilaments in porcine EC in situ with that in EC from the same source grown in monolayers in vitro. The results show that the cytoskeleton of EC undergoes considerable reorganization when EC are removed from the blood vessel and cultured. In vivo the microtubules run helically along the longitudinal axis of EC. It is not clear that all of these microtubules originate from the centriolar region which is located at one end of the nucleus, usually toward the heart. Prominent bundles of microfilaments extend almost the full length of the cell in a direction parallel to blood flow. In contrast, in EC in culture almost all of the microtubules radiate from a single prominent perinuclear microtubule-organizing center toward the cell periphery. In adjacent cells this microtubule-organizing center, which contains the centrioles, is randomly oriented with respect to the nucleus. The microfilaments in EC in vitro form a much more complex pattern in which circumferentially, radially, and longitudinally oriented bundles are evident. These results indicate that a dramatic rearrangement in these components of the cytoskeleton takes place when EC from the aorta are cultured in vitro and that care should be exercised in applying results of in vitro observations on EC to the cells of the endothelium lining blood vessels.     

Cell biology of leukocyte abnormalities--membrane and cytoskeletal function in normal and defective cells. A review.

In this review I have attempted to explain the processes of chemotaxis, phagocytosis, oxidant generation, and lysosomal degranulation in normal and genetically abnormal human PMN. In my view these leukocyte functions are most importantly dependent on the integrity of three cellular components: the plasma membrane, the submembranous microfilaments, and the cytoplasmic microtubules. These components are often discussed in isolation, and the biochemical and pharmacological aspects of their function are analyzed separately here. However, PMN motile and bactericidal activities require the interdependent functioning of membranes, microtubules, and microfilaments. I have therefore tried to provide an integrated view of cytoskeleton-membrane organization and function in human PMN. I have particularly emphasized dynamic aspects of the cytoskeleton and membranes, eg, the induction of microtubule assembly and membrane enzyme activation by surface ligands and the reorganization of microfilaments in response to the same ligands. With this background established, I have selected for discussion a series of diseases in which abnormalities of chemotaxis, phagocytosis, lysosomal degranulation, and/or oxidant generation can be explained directly or indirectly by abnormalities in dynamic properties of PMN membranes, microtubules, or microfilaments. I emphasize that even preliminary insight into the basis of these disorders has sometimes been sufficient to suggest useful clinical approaches to the management of patients. In several of these neutrophil abnormalities, ie, neutrophil actin dysfunction, Chédiak-Higashi syndrome, and its "antithesis" described by Gallin and co-workers, the cellular dysfunctions were well documented but the molecular basis was completely obscure prior to cell biologic analysis. Snyderman and Pike 159 and Chusid and co-workers 160 emphasized the existence of a large number of other neutrophil bactericidal abnormalities resulting from as yet unexplained cellular defects. Further analyses of the functional interactions between membranes and cytoskeletal components in neutrophils may not only clarify the molecular bases of the disorders described here but also may provide insight into the origins and proper therapeutic approach to other granulocyte dysfunctions.     

Integration of Epstein-Barr virus into chromosome 6q15 of Burkitt lymphoma cell line (Raji) induces loss of BACH2 expression

Epstein-Barr virus (EBV) initially isolated from cultured Burkitt lymphoma (BL) cells, is a well-known oncogenic virus. The Raji cell line was established from BL tissue and used for research worldwide. Previous study showed that each Raji cell contains an average of 50-60 EBV genome equivalents, and a significant proportion of the EBV genome is linearly integrated into host genome through BamHI-W close to the BamHI-Y fragment. However, a definitive EBV integration site in the chromosome has not been identified as yet. In this study, direct evidence that EBV DNA is integrated into the host genome was provided through cloning of the fragments containing nucleotide sequence of Raji integration sites. Integrated EBV DNA consisted of the BamHI-W fragment at one end and BamHI-D fragment at another end. Both junction sites were highly guanine/cytosine-rich. The BamHI-W fragment and the adjacent part of chromosome 6 showed 70% homology, while no homology was found between the BamHI-D and adjacent host sequences. EBV was present at intron 1 of the BACH2 gene located on chromosome 6q15. BACH2 was not expressed in the Raji cell line. Because BACH2 is a putative tumor suppressor gene, loss of its expression through EBV integration might contribute to lymphomagenesis.     

An EBNA-negative, EBV-genome-positive human lymphoblast cell line in which superinfecting EBV DNA is not maintained

A human B-lymphoid cell line, designated TG8, which does not express detectable levels of the EBV (Epstein-Barr virus)-associated nuclear antigen (EBNA), yet carries an average of one to two plasmid copies of the P3HR-1 EBV genome has been identified. TG8 can be superinfected by B95-8 EBV, resulting in up to 60-70% of the population becoming EBNA-positive and 20-30% of the incoming EBV genomes becoming circular by 48 hr postinfection. Neither EBNA expression nor the superinfecting viral DNA is maintained in the population. It is concluded that (1) superinfection of this EBV-genome-positive lymphoblast cell line leads to detectable EBNA expression and circularization of the incoming viral genome and (2) the incoming viral genome and detectable EBNA expression are selectively lost, whereas the endogenous viral plasmid DNA is maintained.     

The spreading of B lymphocytes induced by CD44 cross-linking requires actin, tubulin, and vimentin rearrangements

CD44 is a polymorphic family of adhesion molecules widely distributed on cells and tissues. CD44 is up-regulated on activated lymphocytes, and it can function as a receptor, mediating rolling and migration. Although it has been demonstrated that anti-CD44 antibodies bound to tissue-culture plates induce multidirectional emission of retractile dendrites ("spreading") in activated murine B lymphocytes, the involvement of cytoskeleton elements in this phenomenon is largely unknown. In this work, it is shown that the generation of dendrites induced by CD44 cross-linking in activated B cells depends on actin, microtubules, and vimentin reorganization. Immunofluorescence analysis showed that dendrite formation began with actin polymerization, and its extension was favored by microtubules and intermediate filaments of vimentin oriented to the polymerized actin. Pretreatment of activated B lymphocytes with cytochalasin E inhibited the dendrites formation; moreover, when cells were treated with this drug at different time points during the dendrite formation process, the stability of the dendrites was affected. In contrast, although the treatment with colchicine and nocodazole (tubulin polymerization inhibitors) inhibited the dendrites formation, it did not inhibit the initial phase of actin polymerization. According to these results, B cell spreading and dendrite formation induced by anti-CD44 antibodies require coordinated rearrangements of actin, microtubules, and vimentin, being the actin cytoskeleton, the most important element that confers stability and drives the morphological changes during B cell spreading, conceivably preparing B lymphocytes for locomotion.     

Optimal development of Chlamydophila psittaci in L929 fibroblast and BGM epithelial cells requires the participation of microfilaments and microtubule-motor proteins

The cytoskeleton is involved in several cellular activities, including internalization and transport of foreign particles. Although particular functions to each cytoskeleton component have been described, interactions between those components seem to occur. The involvement of the different host cell cytoskeletal components in uptake and development of Chlamydophila psittaci is incompletely understood. In this study, the participation of the microfilament network along with the kinesin and dynein microtubule motor proteins in the internalization and further development of Chlamydophila psittaci were investigated in L929 fibroblast and BGM epithelial cells. Cytochalasin D disruption of actin filaments, and blockage of the motor proteins through the introduction of monoclonal antibodies into the host cells were carried out, either single or combined, at different moments around bacterial inoculation, and Chlamydophila infectivity determined 24 h post- inoculation by direct immunofluorescence. Our results show that, although Chlamydophila Ipsittaci can make use of both microfilament-dependent and independent entry pathways in both cell types, Chlamydophila internalization and development in the fibroblast cells mainly concerned processes mediated by microfilaments while in the epithelial cells mechanisms that require microtubule motor proteins were the ones predominantly involved. Evidence that mutual participation of the actin and tubulin networks in both host cells are required for optimal growth of Chlamydophila psittaci is also presented.     

NIH 3T3细胞转化前后细胞骨架及细胞表面纤维粘连蛋白的免疫荧光观察

本实验用抗纤维粘连蛋白(FN)亲和层析纯抗体和抗管蛋白抗体及鬼笔环肽,以免疫荧光组织化学方法,对NIH3T3细胞转染人肺腺癌细胞系AGZY基因组DNA前后细胞表面FN及细胞内骨架系统进行染色观察。结果表明,细胞在发生转化后,微丝及微管均表现出明显受损,细胞骨架结构不清,呈现为弥散样荧光;细胞表面FN大量减少,仅及正常NIH3T3细胞的1/9,其分布也由细丝形成的网状,变成斑点或斑块状。这一结果进一步证实,细胞恶变是涉及到细胞骨架系统及膜表面糖蛋白变化的复杂过程,并预示这些变化可能就是导致细胞形态发生变化、细胞失去正常生长调控的原因之一。     

Microtubule-associated protein 2 and the organization of cellular microtubules

Summary Microtubule-associated proteins (MAPs) are prominent components of the neuronal cytoskeleton that can promote microtubule formation and whose expression is under strong developmental regulation. They are thought to be involved in organizing the structure of microtubule fascicles in axons and dendrites, although whether they form active cross-links between microtubules or serve as strut-like spacer elements has yet to be resolved. In the experiments reported here we explored their influence on microtubules by expressing them in non-neuronal cells using DNA transfection techniques. We confirm earlier reports that microtubule-associated proteins of the MAP2/tau class can induce bundling of microtubules. In addition we find that MAP2 causes the rearrangement of microtubules in the cytoplasm in a manner that is dependent on the length of the microtubule bundles. Short bundles are straight and run across the cytoplasm whereas long bundles form a marginal band-like array at the periphery. We suggest that the latter arrangement is produced when microtubule bundles that are too long to fit inside the diameter of the cell bend under the restraining influence of the cortical cytoskeleton. In confirmation of this, we show that when the cortical actin network is depolymerized by cytochalasin B the MAP2-containing microtubule bundles push out cylindrical extensions from the cell surface. These results suggest that the induction of stiff microtubules bundles by MAP2, coupled with a breach in the cortical actin network, can confer two of the properties characteristic of neuronal processes; their cylindrical form and the presence of fasciculated microtubules.     

Subpopulations of Human Lymphoblastoid Cell Lines

Forty lymphoblast cell lines derived from normal subjects, patients with infectious mononucleosis, leukemia, and Burkitt's lymphoma have been studied for surface receptors including surface Ig, complement receptors by the EAC rosette and fluorescent (Raji cell) techniques, and Fc (aggregate) receptor by direct and indirect immunofluorescence. Because of the B-cell tropism of the Epstein-Barr virus (EBV), an effort was made to correlate the expresion of various surface properties of lymphoblastoid cell lines with the content of EBV viral DNA as determined by complementary RNA-DNA (cNRA-DNA) hybridization on membrane filters or by DNA-DNA renaturation kinetic analysis. The only correlation established was with the Fc receptor determined by direct immunofluorescence. No correlation of EBV genome equivalents per cell with complement receptor or surface Ig was noted, suggesting that the expression of these receptors is not influenced by EBV viral DNA content. Subgroups of lymphoblastoid cell lines were on the basis of variable expression of surface receptors, designated B1, B2, B3, B4, and T. The distribution of lymphoblastoid cell lines into these subgroups were in the ratio of 14:4:1:4:1. The B1, B2, and B4 cell lines (except Molt 4F) were found to contain EBV. The B3 subgroup, for wich cell line 698 was the sole example, expressed surface immunoglobulins but no other B-cell characteristics, and H.S.B., a T-cell line, lacked detectable EBV.     

Mode of integration of Epstein-Barr virus genome into host DNA in Burkitt lymphoma cells.

The ebv dna integrated into the host genome from cell of an African Burkitt lymphoma biopsy has been compared to the corresponding fraction of EBV DNA from the cell line SU-AmB-2 which is of American Burkitt lymphoma origin. It is shown that while, in the case of the African biopsy, large pieces of EBV DNA sequences are integrated into the host DNA, much smaller pieces of virus DNA are integrated into the DNA of the SU-AmB-2 cells. The possibility that this difference might be related to the fact that EBV is rarely associated with Burkitt lymphomas outside East Africa is discussed.     

Involvement of the cytoskeletal elements in articular cartilage homeostasis and pathology

The cytoskeleton of all cells is a three-dimensional network comprising actin microfilaments, tubulin microtubules and intermediate filaments. Studies in many cell types have indicated roles for these cytoskeletal proteins in many diverse cellular processes including alteration of cell shape, movement of organelles, migration, endocytosis, secretion, cell division and extracellular matrix assembly. The cytoskeletal networks are highly organized in structure enabling them to fulfil their biological functions. This review will primarily focus on the organization and function of the three major cytoskeletal networks in articular cartilage chondrocytes. Articular cartilage is a major load-bearing tissue of the synovial joint; it is well known that the cytoskeleton acts as a physical interface between the chondrocytes and the extracellular matrix in 'sensing' mechanical stimuli. The effect of mechanical load on cytoskeletal element expression and organization will also be reviewed. Abnormal mechanical load is widely believed to be a risk factor for the development of osteoarthritis. Several studies have intimated that the major cytoskeletal networks are disorganized or often absent in osteoarthritic cartilage chondrocytes. The implications and possible reasoning for this are more widely discussed and placed into context with their potential relevance to disease and therapeutic strategies.