Association of intermediate filaments with vinculin-containing adhesion plaques of fibroblasts

Impairment of Assembly of the Vimentin Intermediate Filaments Leads to Suppression of Formation and Maturation of Focal Contacts and Alteration of the Type of Cellular Protrusions

Cell migration is largely determined by the type of protrusions formed by the cell. Mesenchymal migration is accomplished by formation of lamellipodia and/or filopodia, while amoeboid migration is based on bleb formation. Changing of migrational conditions can lead to alteration in the character of cell movement. For example, inhibition of the Arp2/3-dependent actin polymerization by the CK-666 inhibitor leads to transition from mesenchymal to amoeboid motility mode. Ability of the cells to switch from one type of motility to another is called migratory plasticity. Cellular mechanisms regulating migratory plasticity are poorly understood. One of the factors determining the possibility of migratory plasticity may be the presence and/or organization of vimentin intermediate filaments (VIFs). To investigate whether organization of the VIF network affects the ability of fibroblasts to form membrane blebs, we used rat embryo fibroblasts REF52 with normal VIF organization, fibroblasts with vimentin knockout (REF-/-), and fibroblasts with mutation inhibiting assembly of the full-length VIFs (REF117). Blebs formation was induced by treatment of cells with CK-666. Vimentin knockout did not lead to statistically significant increase in the number of cells with blebs. The fibroblasts with short fragments of vimentin demonstrate the significant increase in number of cells forming blebs both spontaneously and in the presence of CK-666. Disruption of the VIF organization did not lead to the significant changes in the microtubules network or the level of myosin light chain phosphorylation, but caused significant reduction in the focal contact system. The most pronounced and statistically significant decrease in both size and number of focal adhesions were observed in the REF117 cells. We believe that regulation of the membrane blebbing by VIFs is mediated by their effect on the focal adhesion system. Analysis of migration of fibroblasts with different organization of VIFs in a three-dimensional collagen gel showed that organization of VIFs determines the type of cell protrusions, which, in turn, determines the character of cell movement. A novel role of VIFs as a regulator of membrane blebbing, essential for manifestation of the migratory plasticity, is shown.

High salt diet impairs dermal tissue remodeling in a mouse model of IMQ induced dermatitis

Recent animal studies, as well as quantitative sodium MRI observations on humans demonstrated that remarkable amounts of sodium can be stored in the skin. It is also known that excess sodium in the tissues leads to inflammation in various organs, but its role in dermal pathophysiology has not been elucidated. Therefore, our aim was to study the effect of dietary salt loading on inflammatory process and related extracellular matrix (ECM) remodeling in the skin. To investigate the effect of high salt consumption on inflammation and ECM production in the skin mice were kept on normal (NSD) or high salt (HSD) diet and then dermatitis was induced with imiquimod (IMQ) treatment. The effect of high salt concentration on dermal fibroblasts (DF) and peripheral blood mononuclear cells (PBMC) was also investigated in vitro. The HSD resulted in increased sodium content in the skin of mice. Inflammatory cytokine Il17 expression was elevated in the skin of HSD mice. Expression of anti-inflammatory Il10 and Il13 decreased in the skin of HSD or HSD IMQ mice. The fibroblast marker Acta2 and ECM component Fn and Col1a1 decreased in HSD IMQ mice. Expression of ECM remodeling related Pdgfb and activation phosphorylated (p)-SMAD2/3 was lower in HSD IMQ mice. In PBMCs, production of IL10, IL13 and PDGFB was reduced due to high salt loading. In cultured DFs high salt concentration resulted in decreased cell motility and ECM production, as well. Our results demonstrate that high dietary salt intake is associated with increased dermal pro-inflammatory status. Interestingly, although inflammation induces the synthesis of ECM in most organs, the expression of ECM decreased in the inflamed skin of mice on high salt diet. Our data suggest that salt intake may alter the process of skin remodeling.

Expression and Role of E-Cadherin, β-Catenin, and Vimentin in Human Papillomavirus-Positive and Human Papillomavirus-Negative Oropharyngeal Squamous Cell Carcinoma

Oropharyngeal squamous cell carcinoma (OPSCC) is subclassified by the World Health Organization into two different entities: human papillomavirus (HPV)-positive and HPV-negative tumors. HPV infection promotes the epithelial-to-mesenchymal transition (EMT) and transformation of keratinocyte stem cells into cancer stem cells. EMT is a crucial process in the carcinogenesis of epithelial-derived malignancies, and we aimed to study the role of its markers in OPSCC. This study consists of 202 consecutive OPSCC patients diagnosed and treated with curative intent. We examined E-cadherin, β-catenin, and vimentin expression using immunohistochemistry and compared these with tumor and patient characteristics and treatment outcome. We found that the cell-membranous expression of β-catenin was stronger in HPV-positive than in HPV-negative tumors, and it was stronger in the presence of regional metastasis. The stromal vimentin expression was stronger among HPV-positive tumors. A high E-cadherin expression was associated with tumor grade. No relationship between these markers and survival emerged. In conclusion, β-catenin and vimentin seem to play different roles in OPSCC: the former in the tumor tissue itself, and the latter in the tumor stroma. HPV infection may exploit the β-catenin and vimentin pathways in carcinogenic process. More, β-catenin may serve as a marker for the occurrence of regional metastasis:

Unbiased pattern analysis reveals highly diverse responses of cytoskeletal systems to cyclic straining

In mammalian cells, actin, microtubules, and various types of cytoplasmic intermediate filaments respond to external stretching. Here, we investigated the underlying processes in endothelial cells plated on soft substrates from silicone elastomer. After cyclic stretch (0.13 Hz, 14% strain amplitude) for periods ranging from 5 min to 8 h, cells were fixed and double-stained for microtubules and either actin or vimentin. Cell images were analyzed by a two-step routine. In the first step, micrographs were segmented for potential fibrous structures. In the second step, the resulting binary masks were auto- or cross-correlated. Autocorrelation of segmented images provided a sensitive and objective measure of orientational and translational order of the different cytoskeletal systems. Aligning of correlograms from individual cells removed the influence of only partial alignment between cells and enabled determination of intrinsic cytoskeletal order. We found that cyclic stretching affected the actin cytoskeleton most, microtubules less, and vimentin mostly only via reorientation of the whole cell. Pharmacological disruption of microtubules had barely any influence on actin ordering. The similarity, i.e., cross-correlation, between vimentin and microtubules was much higher than the one between actin and microtubules. Moreover, prolonged cyclic stretching slightly decoupled the cytoskeletal systems as it reduced the cross-correlations in both cases. Finally, actin and microtubules were more correlated at peripheral regions of cells whereas vimentin and microtubules correlated more in central regions.

Vimentin Levels and Serine 71 Phosphorylation in the Control of Cell-Matrix Adhesions, Migration Speed, and Shape of Transformed Human Fibroblasts

Metastasizing tumor cells show increased expression of the intermediate filament (IF) protein vimentin, which has been used to diagnose invasive tumors for decades. Recent observations indicate that vimentin is not only a passive marker for carcinoma, but may also induce tumor cell invasion. To clarify how vimentin IFs control cell adhesions and migration, we analyzed the nanoscale (30-50 nm) spatial organization of vimentin IFs and cell-matrix adhesions in metastatic fibroblast cells, using three-color stimulated emission depletion (STED) microscopy. We also studied whether wild-type and phospho-deficient or -mimicking mutants of vimentin changed the size and lifetime of focal adhesions (FAs), cell shape, and cell migration, using live-cell total internal reflection imaging and confocal microscopy. We observed that vimentin exists in fragments of different lengths. Short fragments were mostly the size of a unit-length filament and were mainly localized close to small cell-matrix adhesions. Long vimentin filaments were found in the proximity of large FAs. Vimentin expression in these cells caused a reduction in FAs size and an elongated cell shape, but did not affect FA lifetime, or the speed or directionality of cell migration. Expression of a phospho-mimicking mutant (S71D) of vimentin increased the speed of cell migration. Taken together, our results suggest that in highly migratory, transformed mesenchymal cells, vimentin levels control the cell shape and FA size, but not cell migration, which instead is linked to the phosphorylation status of S71 vimentin. These observations are consistent with the possibility that not only levels, but also the assembly status of vimentin control cell migration.

Intermediate Filaments and the Plasma Membrane

A variety of intermediate filament (IF) types show intricate association with plasma membrane proteins, including receptors and adhesion molecules. The molecular basis of linkage of IFs to desmosomes at sites of cell-cell interaction and hemidesmosomes at sites of cell-matrix adhesion has been elucidated and involves IF-associated proteins. However, IFs also interact with focal adhesions and cell-surface molecules, including dystroglycan. Through such membrane interactions, it is well accepted that IFs play important roles in the establishment and maintenance of tissue integrity. However, by organizing cell-surface complexes, IFs likely regulate, albeit indirectly, signaling pathways that are key to tissue homeostasis and repair.

Tumor promoter PMA enhances kindlin-2 and decreases vimentin recruitment into cell adhesion sites

Phorbol diester PMA (phorbol 12-myristate 13-acetate) is a well-known promoter of tumor progression. PMA also regulates cell adhesion by several mechanisms including conformational activation of integrins and integrin clustering. Here, PMA was shown to induce lamellipodia formation and reorganization of the adhesion sites as well as actin and vimentin filaments independently of integrin preactivation. To further analyze the mechanism of PMA action, the protein composition in the α1β1 integrin/collagen IV adhesion sites was analyzed by mass spectrometry and proteomics. In four independent experiments we observed the reduced recruitment of vimentin in relation to integrin α1 subunit. This was in full agreement with the fact that we also detected the retraction of vimentin from cell adhesions by confocal microscopy. Furthermore, the accumulation of kindlin-2 into cell adhesions was significantly increased after PMA treatment. Kindlin-2 siRNA inhibited cell spreading as well as the formation of actin fibrils and cell adhesions, but did not prevent the effect of PMA on lamellipodia formation. Thus, kindlin-2 recruitment was considered to be a consequence rather than the primary cause for the loss of connection between vimentin and the adhesion sites.

Vimentin regulates lung cancer cell adhesion through a VAV2-Rac1 pathway to control focal adhesion kinase activity

Vimentin is an intermediate filament protein whose expression correlates with increased metastatic disease, reduced patient survival and poor prognosis across multiple tumor types. Despite these well-characterized correlations, the molecular role of vimentin in cancer cell motility remains undefined. To approach this, we used an unbiased phosphoproteomics screen in lung cancer cell lines to discover cell motility proteins that show significant changes in phosphorylation upon vimentin depletion. We identified the guanine nucleotide exchange factor (GEF), VAV2, as having the greatest loss of phosphorylation owing to vimentin depletion. Since VAV2 serves as a GEF for the small Rho GTPase Rac1, a key player in cell motility and adhesion, we explored the vimentin-VAV2 pathway as a potential novel regulator of lung cancer cell motility. We show that VAV2 localizes to vimentin-positive focal adhesions (FAs) in lung cancer cells and complexes with vimentin and FA kinase (FAK). Vimentin loss impairs both pY142-VAV2 and downstream pY397-FAK activity showing that vimentin is critical for maintaining VAV2 and FAK activity. Importantly, vimentin depletion reduces the activity of the VAV2 target, Rac1, and a constitutively active Rac1 rescues defects in FAK and cell adhesion when vimentin or VAV2 is compromised. Based upon this data, we propose a model whereby vimentin promotes FAK stabilization through VAV2-mediated Rac1 activation. This model may explain why vimentin expressing metastatic lung cancer cells are more motile and invasive.

A central role for vimentin in regulating repair function during healing of the lens epithelium

A unique ex vivo mock cataract surgery model is used to study the role of vimentin in repair cell function during wound healing within a clinically relevant setting. Vimentin is found to be critical for the function of repair cells in directing the collective migration of the epithelium during wound healing.

Mock cataract surgery provides a unique ex vivo model for studying wound repair in a clinically relevant setting. Here wound healing involves a classical collective migration of the lens epithelium, directed at the leading edge by an innate mesenchymal subpopulation of vimentin-rich repair cells. We report that vimentin is essential to the function of repair cells as the directors of the wound-healing process. Vimentin and not actin filaments are the predominant cytoskeletal elements in the lamellipodial extensions of the repair cells at the wound edge. These vimentin filaments link to paxillin-containing focal adhesions at the lamellipodial tips. Microtubules are involved in the extension of vimentin filaments in repair cells, the elaboration of vimentin-rich protrusions, and wound closure. The requirement for vimentin in repair cell function is revealed by both small interfering RNA vimentin knockdown and exposure to the vimentin-targeted drug withaferin A. Perturbation of vimentin impairs repair cell function and wound closure. Coimmunoprecipitation analysis reveals for the first time that myosin IIB is associated with vimentin, linking vimentin function in cell migration to myosin II motor proteins. These studies reveal a critical role for vimentin in repair cell function in regulating the collective movement of the epithelium in response to wounding.

Mechanosensing through focal adhesion-anchored intermediate filaments

Integrin-based mechanotransduction involves a complex focal adhesion (FA)-associated machinery that is able to detect and respond to forces exerted either through components of the extracellular matrix or the intracellular contractile actomyosin network. Here, we show a hitherto unrecognized regulatory role of vimentin intermediate filaments (IFs) in this process. By studying fibroblasts in which vimentin IFs were decoupled from FAs, either because of vimentin deficiency (V0) or loss of vimentin network anchorage due to deficiency in the cytolinker protein plectin (P0), we demonstrate attenuated activation of the major mechanosensor molecule FAK and its downstream targets Src, ERK1/2, and p38, as well as an up-regulation of the compensatory feedback loop acting on RhoA and myosin light chain. In line with these findings, we show strongly reduced FA turnover rates in P0 fibroblasts combined with impaired directional migration, formation of protrusions, and up-regulation of "stretched" high-affinity integrin complexes. By exploiting tension-independent conditions, we were able to mechanistically link these defects to diminished cytoskeletal tension in both P0 and V0 cells. Our data provide important new insights into molecular mechanisms underlying cytoskeleton-regulated mechanosensing, a feature that is fundamental for controlled cell movement and tumor progression.

Proteomic profiling of endothelial invasion revealed receptor for activated C kinase 1 (RACK1) complexed with vimentin to regulate focal adhesion kinase (FAK)

Angiogenesis is critical for many physiological and pathological processes. To identify molecules relevant to angiogenesis, we performed a proteomic screen comparing invading versus non-invading endothelial cells in three-dimensional collagen matrices. We found up-regulated levels of receptor for activated C kinase 1 (RACK1) and the intermediate filament protein vimentin that correlated with increased endothelial cell invasion. Because both RACK1 and vimentin have been linked to focal adhesion kinase (FAK), we investigated whether this pathway regulated invasion. RACK1 depletion reduced invasion responses, and this was associated with attenuated activation of FAK. Knockdown of vimentin significantly decreased levels of phosphorylated and total FAK. Treatment with a pharmacological inhibitor of FAK dose-dependently reduced invasion, indicating a crucial role for FAK activity during invasion. Because RACK1 and vimentin were both up-regulated with sphingosine 1-phosphate treatment, required for invasion, and regulated FAK, we tested whether they complexed together. RACK1 complexed with vimentin, and growth factors enhanced this interaction. In addition, RACK1, vimentin, and FAK formed an intermolecular complex in invading endothelial cultures in three dimensions in response to stimulation by sphingosine 1-phosphate and growth factors. Moreover, depletion of RACK1 decreased the association of vimentin and FAK, suggesting that RACK1 was required for stabilizing vimentin-FAK interactions during sprouting. Silencing of vimentin and RACK1 decreased cell adhesion and focal contact formation. Taken together, these results demonstrate that proangiogenic signals converge to enhance expression and association of RACK1 and vimentin, which regulated FAK, resulting in successful endothelial sprout formation in three-dimensional collagen matrices.

Endoplasmic spreading requires coalescence of vimentin intermediate filaments at force-bearing adhesions

Interaction of vimentin filaments (vIFs) and force-bearing adhesions is essential for endoplasm spreading. For adhesions to be connected to a contractile network involved in endoplasm spreading, vIFs are needed. Thus endoplasm spreading and microtubule stabilization in the periphery require a multicomponent actin network anchored at adhesions.

For cells to develop long-range forces and carry materials to the periphery, the microtubule and organelle-rich region at the center of the cell—the endoplasm—needs to extend to near the cell edge. Depletion of the actin cross-linking protein filamin A (FlnA) causes a collapse of the endoplasm into a sphere around the nucleus of fibroblasts and disruption of matrix adhesions, indicating that FlnA is involved in endoplasmic spreading and adhesion growth. Here, we report that treatment with the calpain inhibitor N-[N-(N-acetyl-l-leucyl)-l-leucyl]-l-norleucine (ALLN) restores endoplasmic spreading as well as focal adhesion (FA) growth on fibronectin-coated surfaces in a Fln-depleted background. Addback of calpain-uncleavable talin, not full-length talin, achieves a similar effect in Fln-depleted cells and indicates a crucial role for talin in endoplasmic spreading. Because FA maturation involves the vimentin intermediate filament (vIF) network, we also examined the role of vIFs in endoplasmic spreading. Wild-type cells expressing a vimentin variant incapable of polymerization exhibit deficient endoplasmic spreading as well as defects in FA growth. ALLN treatment restores FA growth despite the lack of vIFs but does not restore endoplasmic spreading, implying that vIFs are essential for endoplasm spreading. Consistent with that hypothesis, vIFs are always displaced from adhesions when the endoplasm does not spread. In Fln-depleted cells, vIFs extend beyond adhesions, nearly to the cell edge. Finally, inhibiting myosin II–mediated contraction blocks endoplasmic spreading and adhesion growth. Thus we propose a model in which myosin II–mediated forces and coalescence of vIFs at mature FAs are required for endoplasmic spreading.

Role of vimentin in the inhibitory effects of low-molecular-weight heparin on PC-3M cell adhesion to, and migration through, endothelium

Low-molecular-weight heparin (LMWH) has been used in cancer patients with venous thromboembolic complications, resulting in a higher survival rate and an inhibitory action on experimental metastasis. In the present study, human umbilical vein endothelial cells (HUVECs) were treated with LMWH for 24 h. We found that the resulting HUVECs could significantly inhibit the highly metastatic human prostate cancer cell line (PC-3M) in terms of its adhesion to the endothelium and migration across the endothelium, according to scanning electron microscopy. We also determined the elevated levels of endothelial intercellular Ca(2+) concentration after the adhesion of PC-3M cells to HUVECs was greatly reduced by incubation with LMWH. Using proteomics, we surveyed the global protein changes in HUVECs after LMWH treatment and identified four down-regulated proteins that were possible isoforms of cytoskeletal vimentin intermediate filaments, cartilage-derived C-type lectin, and serine/threonine protein phosphatase 1β (PP-1B). LMWH affected the morphology of vimentin and the expression levels of α(v) integrin and PP-1B in HUVECs bound to PC-3M cells. Vimentin assists in the adhesion of PC-3M cells, which was confirmed by short interfering RNA experiments. Furthermore, the direct binding of purified vimentin protein with LMWH was detected with surface plasmon resonance methods. However, when we used fluorescence-labeled heparin for 24 h to identify whether this binding occurred within cells, heparin was distributed principally around endothelial cells. Taken together, these findings suggest that the monoincubation of LMWH with HUVECs could inhibit PC-3M cell adhesion to, and migration through, endothelium. LMWH's regulation of vimentin plays a role in the antimetastatic action.

Keeping the vimentin network under control: cell-matrix adhesion-associated plectin 1f affects cell shape and polarity of fibroblasts

Mature focal adhesions and fibrillar adhesions act as anchorage sites for vimentin filaments, with plectin isoform 1f being the crucial linker protein. Plectin serves as a nucleation and assembly center for the de novo formation of vimentin networks. Anchored vimentin creates a resilient cage-like core structure that affects cell shape.

Focal adhesions (FAs) located at the ends of actin/myosin-containing contractile stress fibers form tight connections between fibroblasts and their underlying extracellular matrix. We show here that mature FAs and their derivative fibronectin fibril-aligned fibrillar adhesions (FbAs) serve as docking sites for vimentin intermediate filaments (IFs) in a plectin isoform 1f (P1f)-dependent manner. Time-lapse video microscopy revealed that FA-associated P1f captures mobile vimentin filament precursors, which then serve as seeds for de novo IF network formation via end-to-end fusion with other mobile precursors. As a consequence of IF association, the turnover of FAs is reduced. P1f-mediated IF network formation at FbAs creates a resilient cage-like core structure that encases and positions the nucleus while being stably connected to the exterior of the cell. We show that the formation of this structure affects cell shape with consequences for cell polarization.

Recruitment of vimentin to the cell surface by beta3 integrin and plectin mediates adhesion strength

Much effort has been expended on analyzing how microfilament and microtubule cytoskeletons dictate the interaction of cells with matrix at adhesive sites called focal adhesions (FAs). However, vimentin intermediate filaments (IFs) also associate with the cell surface at FAs in endothelial cells. Here, we show that IF recruitment to FAs in endothelial cells requires beta3 integrin, plectin and the microtubule cytoskeleton, and is dependent on microtubule motors. In CHO cells, which lack beta3 integrin but contain vimentin, IFs appear to be collapsed around the nucleus, whereas in CHO cells expressing beta3 integrin (CHOwtbeta3), vimentin IFs extend to FAs at the cell periphery. This recruitment is regulated by tyrosine residues in the beta3 integrin cytoplasmic tail. Moreover, CHOwtbeta3 cells exhibit significantly greater adhesive strength than CHO or CHO cells expressing mutated beta3 integrin proteins. These differences require an intact vimentin network. Therefore, vimentin IF recruitment to the cell surface is tightly regulated and modulates the strength of adhesion of cells to their substrate.

Plectin contributes to mechanical properties of living cells

Plectin is a 500-kDa cross-linking protein that plays important roles in a number of cell functions including migration and wound healing. We set out to characterize the role of plectin in mechanical properties of living cells. Plectin(-/-) cells were less stiff than plectin(+/+) cells, but the slopes of the two power laws in response to loading frequencies (0.002-1,000 Hz) were similar. Plectin(-/-) cells lost the capacity to propagate mechanical stresses to long distances in the cytoplasm; traction forces in plectin(-/-) cells were only half of those in plectin(+/+) cells, suggesting that plectin deficiency compromised prestress generation, which, in turn, resulted in the inhibition of long distance stress propagation. Both plectin(+/+) and plectin(-/-) cells exhibited nonlinear stress-strain relationships. However, plectin(+/+) cells, but not plectin(-/-) cells, further stiffened in response to lysophosphatidic acid (LPA). Dynamic fluorescence resonance energy transfer analysis revealed that RhoA GTPase proteins were activated in plectin(+/+) cells but not in plectin(-/-) cells after treatment with LPA. Expression in plectin(-/-) cells of constitutively active RhoA (RhoA-V14) but not a dominant negative mutant of RhoA (RhoA-N19) or an empty vector restored the long distance force propagation behavior, suggesting that plectin is important in normal functions of RhoA. Our findings underscore the importance of plectin for mechanical properties, stress propagation, and prestress of living cells, thereby influencing their biological functions.

Novel functions of vimentin in cell adhesion, migration, and signaling

Vimentin is the major intermediate filament (IF) protein of mesenchymal cells. It shows dynamically altered expression patterns during different developmental stages and high sequence homology throughout all vertebrates, suggesting that the protein is physiologically important. Still, until recently, the real tasks of vimentin have been elusive, primarily because the vimentin-deficient mice were originally characterized as having a very mild phenotype. Recent studies have revealed several key functions for vimentin that were not obvious at first sight. Vimentin emerges as an organizer of a number of critical proteins involved in attachment, migration, and cell signaling. The highly dynamic and complex phosphorylation of vimentin seems to be a likely regulator mechanism for these functions. The implicated novel vimentin functions have broad ramifications into many different aspects of cell physiology, cellular interactions, and organ homeostasis.

Intermediate filament associated proteins

Intermediate filament associated proteins (IFAPs) coordinate interactions between intermediate filaments (IFs) and other cytoskeletal elements and organelles, including membrane-associated junctions such as desmosomes and hemidesmosomes in epithelial cells, costameres in striated muscle, and intercalated discs in cardiac muscle. IFAPs thus serve as critical connecting links in the IF scaffolding that organizes the cytoplasm and confers mechanical stability to cells and tissues. However, in recent years it has become apparent that IFAPs are not limited to structural crosslinkers and bundlers but also include chaperones, enzymes, adapters, and receptors. IF networks can therefore be considered scaffolding upon which associated proteins are organized and regulated to control metabolic activities and maintain cell homeostasis.

Effect of Vimentin on Reactive Gliosis:In VitroandIn VivoAnalysis

Reactive gliosis is a prominent result of many types of insult to the central nervous system (CNS) and leads to the formation of glial scar that impedes the regeneration of axons. The intermediate filament protein vimentin is found in pathology of the CNS, mainly in the vicinity of injuries to the CNS. In the present study we investigated the role of vimentin in the formation of glial scars in vitro and in vivo by using immunohistochemistry, Western blot analysis, and in situ hybridization. In vitro experiments showed that the intensity of immunofluorescent labeling for vimentin and glial fibrillary acidic protein (GFAP) was consistently decreased in astrocytes after transfection with a retrovirus carrying antisense complementary DNA (cDNA) for vimentin. Transfection also inhibited the growth of astrocytes and decreased the expression of vimentin mRNA. In vivo studies demonstrated that transfection with the retrovirus carrying the antisense cDNA vimentin inhibited the upregulation of vimentin and GFAP in stab wounds in rat cerebrum. These results suggest that vimentin may play a key role in the formation of glial scars in the CNS.

The vimentin cytoskeleton regulates focal contact size and adhesion of endothelial cells subjected to shear stress

Recently, we reported that vimentin-type intermediate filaments, in addition to microfilaments, associate with αvβ3 integrin-positive focal contacts in endothelial cells. To gain insight into intermediate filament-focal contact interaction, we induced expression of yellow fluorescent protein (YFP)-integrin β3 and cyan fluorescent protein (CFP)-vimentin protein in endothelial cells. At least 50% of the YFP-β3 integrin-labeled focal contacts associated with CFP-labeled vimentin intermediate filaments in live cells. Moreover, focal contacts and intermediate filaments moved in concert in the plane of the membrane and assembling focal contacts were sites of vimentin filament assembly. When endothelial cells were subjected to flow, large focal contacts assembled and associated with thick vimentin bundles. These large focal contacts showed minimal dynamic activity. Cells in which vimentin expression had been inhibited by RNA interference assembled smaller than normal focal contacts. More dramatically, such cells showed decreased adhesion to the substratum. These data provide evidence that the vimentin cytoskeleton regulates focal contact size and helps stabilize cell-matrix adhesions in endothelial cells.

Vimentin intermediate filament reorganization by Cdc42: involvement of PAK and p70 S6 kinase

Rho family GTPases play a major role in actin cytoskeleton reorganization. Recent studies have shown that the activation of Rho family GTPases also induces collapse of the vimentin intermediate filament (IF) network in fibroblasts. Here, we report that Cdc42V12 induces the reorganization of vimentin IFs in Hela cells, and such reorganization is independent of actin and microtubule status. We analyzed the involvement of three serine/threonine kinase effectors, MRCK, PAK and p70 S6K in the Cdc42-induced vimentin reorganization. Surprisingly, the ROK-related MRCK is not involved in this IF reorganization. We detected phosphorylation of vimentin Ser72, a site phosphorylated by PAK, after Cdc42 activation. PAK inhibition partially blocked Cdc42-induced vimentin IF collapse suggesting the involvement of other effectors. We report that p70 S6 kinase (S6K)1 participates in this IF rearrangement since the inhibitor rapamycin or a dominant inhibitory S6K could reduce the Cdc42V12 or bradykinin-induced vimentin collapse. Further, inhibition of PAK and S6K in combination very effectively prevents Cdc42-induced vimentin IF collapse. Conversely, only in combination active PAK and S6K could induce a vimentin IF rearrangement that mimics the Cdc42 effect. Thus, Cdc42-induced vimentin reorganization involves PAK and, in a novel cytoskeletal role, p70 S6K.

Structure and function of a vimentin-associated matrix adhesion in endothelial cells

The alpha4 laminin subunit is a component of endothelial cell basement membranes. An antibody (2A3) against the alpha4 laminin G domain stains focal contact-like structures in transformed and primary microvascular endothelial cells (TrHBMECs and HMVECs, respectively), provided the latter cells are activated with growth factors. The 2A3 antibody staining colocalizes with that generated by alphav and beta3 integrin antibodies and, consistent with this localization, TrHBMECs and HMVECs adhere to the alpha4 laminin subunit G domain in an alphavbeta3-integrin-dependent manner. The alphavbeta3 integrin/2A3 antibody positively stained focal contacts are recognized by vinculin antibodies as well as by antibodies against plectin. Unusually, vimentin intermediate filaments, in addition to microfilament bundles, interact with many of the alphavbeta3 integrin-positive focal contacts. We have investigated the function of alpha4-laminin and alphavbeta3-integrin, which are at the core of these focal contacts, in cultured endothelial cells. Antibodies against these proteins inhibit branching morphogenesis of TrHBMECs and HMVECs in vitro, as well as their ability to repopulate in vitro wounds. Thus, we have characterized an endothelial cell matrix adhesion, which shows complex cytoskeletal interactions and whose assembly is regulated by growth factors. Our data indicate that this adhesion structure may play a role in angiogenesis.

Impaired wound healing in embryonic and adult mice lacking vimentin

It is generally assumed that the vimentin intermediate filament network present in most mesenchymally-derived cells is in part responsible for the strength and integrity of these cells, and necessary for any tissue movements that require the generation of significant tractional forces. Surprisingly, we have shown that transgenic KO mice deficient for vimentin are apparently able to undergo embryonic development absolutely normally and go onto develop into adulthood and breed without showing any obvious phenotype. However, fibroblasts derived from these mice are mechanically weak and severely disabled in their capacity to migrate and to contract a 3-D collagen network. To assess whether these functions are necessary for more challenging tissue movements such as those driving in vivo tissue repair processes, we have analysed wound healing ability in wild-type versus vimentin-deficient embryos and adult mice. Wounds in vimentin-deficient adult animals showed delayed migration of fibroblasts into the wound site and subsequently retarded contraction that correlated with a delayed appearance of myofibroblasts at the wound site. Wounds made to vimentin-deficient embryos also failed to heal during the 24 hour culture period it takes for wild-type embryos to fully heal an equivalent wound. By DiI marking the wound mesenchyme and following its fate during the healing process we showed that this impaired healing is almost entirely due to a failure of mesenchymal contraction at the embryonic wound site. These observations reveal an in vivo phenotype for the vimentin-deficient mouse, and challenge the dogma that key morphogenetic events occurring during development require generation of significant tractional forces by mesenchymal cells.

Vimentin contributes to human mammary epithelial cell migration

Vimentin expression in human mammary epithelial MCF10A cells was examined as a function of their migratory status using an in vitro wound-healing model. Analysis of the trajectories of the cells and their migratory speeds by time lapse-video microscopy revealed that vimentin mRNA and protein expression were exclusively induced in cells at the wound's edge which were actively migrating towards the center of the lesion. Actin labeling showed the reorganization of actin filaments in cells at the wound's edge which confirmed the migratory phenotype of this cell subpopulation. Moreover, the vimentin protein disappeared when the cells became stationary after wound closure. Using cells transfected with the vimentin promoter controlling the green fluorescent protein gene, we also demonstrated the specific activation of the vimentin promoter in the migratory cells at the wound's edge. Transfection of the antisense vimentin cDNA into MCF10A cells clearly reduced both their ability to express vimentin and their migratory speed. Taken together, these observations demonstrate that vimentin is transiently associated with, and could be functionally involved in, the migratory status of human epithelial cells.

Integrating the actin and vimentin cytoskeletons. adhesion-dependent formation of fimbrin-vimentin complexes in macrophages

Cells adhere to the substratum through specialized structures that are linked to the actin cytoskeleton. Recent studies report that adhesion also involves the intermediate filament (IF) and microtubule cytoskeletons, although their mechanisms of interaction are unknown. Here we report evidence for a novel adhesion-dependent interaction between components of the actin and IF cytoskeletons. In biochemical fractionation experiments, fimbrin and vimentin coprecipitate from detergent extracts of macrophages using vimentin- or fimbrin-specific antisera. Fluorescence microscopy confirms the biochemical association. Both proteins colocalized to podosomes in the earliest stages of cell adhesion and spreading. The complex is also found in filopodia and retraction fibers. After detergent extraction, fimbrin and vimentin staining of podosomes, filopodia, and retraction fibers are lost, confirming that the complex is localized to these structures. A 1:4 stoichiometry of fimbrin binding to vimentin and a low percentage (1%) of the extracted vimentin suggest that fimbrin interacts with a vimentin subunit. A fimbrin-binding site was identified in the NH(2)-terminal domain of vimentin and the vimentin binding site at residues 143-188 in the CH1 domain of fimbrin. Based on these observations, we propose that a fimbrin-vimentin complex may be involved in directing the assembly of the vimentin cytoskeleton at cell adhesion sites.

Distribution of cytoskeletal proteins in neomycin-induced protrusions of human fibroblasts

The organization of actin, tubulin, and vimentin was studied in protruding lamellae of human fibroblasts induced by the aminoglycoside antibiotic neomycin, an inhibitor of the phosphatidylinositol cycle. Neomycin stimulates the simultaneous protrusion of lamellae in all treated cells, and the lamellae remain extended for about 15-20 min, before gradually withdrawing. The pattern and distribution of actin, tubulin, and vimentin during neomycin stimulation were analyzed by fluorescence and electron microscopy. F-actin in the newly formed lamellae is localized in a marginal band at the leading edge. Tubulin is colocalized with F-actin in the marginal band, but the newly formed lamellae are initially devoid of microtubules. Over a period of 10 to 20 min after the addition of neomycin, microtubules grow into the lamellae from the adjacent cytoplasm, while the intensity of tubulin staining of the marginal band decreases. Distribution of vimentin remains unchanged in neomycin-treated cells and vimentin filaments do not enter the new protrusions. Treatment of cells with colchicine and Taxol do not inhibit neomycin-induced protrusion but protrusions are no longer localized at the ends of cell processes and occur all around the cell periphery. We conclude that actin filaments are the major component of the cytoskeleton involved in generating protrusions. Microtubules and, possibly, intermediate filaments control the pattern of protrusions by their interaction with actin filaments.

Impaired mechanical stability, migration and contractile capacity in vimentin-deficient fibroblasts

Loss of a vimentin network due to gene disruption created viable mice that did not differ overtly from wild-type littermates. Here, primary fibroblasts derived from vimentin-deficient (-/-) and wild-type (+/+) mouse embryos were cultured, and biological functions were studied in in vitro systems resembling stress situations. Stiffness of -/- fibroblasts was reduced by 40% in comparison to wild-type cells. Vimentin-deficient cells also displayed reduced mechanical stability, motility and directional migration towards different chemo-attractive stimuli. Reorganization of collagen fibrils and contraction of collagen lattices were severely impaired. The spatial organization of focal contact proteins, as well as actin microfilament organization was disturbed. Thus, absence of a vimentin filament network does not impair basic cellular functions needed for growth in culture, but cells are mechanically less stable, and we propose that therefore they are impaired in all functions depending upon mechanical stability.

The association of tau-like proteins with vimentin filaments in cultured cells

There is increasing evidence that the different polymers that constitute the cytoskeleton are interconnected to form a three-dimensional network. The macromolecular interaction patterns that stabilize this network and its intrinsic dynamics are the basis for numerous cellular processes. Within this context, in vitro studies have pointed to the existence of specific associations between microtubules, microfilaments, and intermediate filaments. It has also been postulated that microtubule-associated proteins (MAPs) are directly involved in mediating these interactions. The interactions of tau with vimentin filaments, and its relationships with other filaments of the cytoskeletal network, were analyzed in SW-13 adenocarcinoma cells, through an integrated approach that included biochemical and immunological studies. This cell line has the advantage of presenting a wild-type clone (vim+) and a mutant clone (vim-) which is deficient in vimentin expression. We analyzed the cellular roles of tau, focusing on its interactions with vimentin filaments, within the context of its functional aspects in the organization of the cytoskeletal network. Cosedimentation experiments of microtubular protein with vimentin in cell extracts enriched in intermediate filaments, combined with studies on the direct interaction of tau with nitrocellulose-bound vimentin and analysis of tau binding to vimentin immobilized in single-strand DNA affinity columns, indicate that tau interacts with the vimentin network. These studies were confirmed by a quantitative analysis of the immunofluorescence patterns of cytoskeleton-associated tubulin, tau, and vimentin using flow cytometry. In this regard, a decrease in the levels of tau associated to the cytoskeletal network in the vim- cell mutant compared with the wild-type clones was observed. However, immunofluorescence data on SW-13 cells suggest that the absence of a structured network of vimentin in the mutant vim- cells does not affect the cytoplasmic organization formed by microtubules and actin filaments, when compared with the wild-type vim+ cells. These studies suggest that tau associates with vimentin filaments and that these interactions may play a structural role in cells containing these filaments.

Splice variant-specific interaction of the NMDA receptor subunit NR1 with neuronal intermediate filaments

NMDA receptors are excitatory neurotransmitter receptors critical for synaptic plasticity and neuronal development in the mammalian brain. These receptors are found highly concentrated in the postsynaptic membrane of glutamatergic synapses. To investigate the molecular mechanisms underlying NMDA receptor localization, we used the yeast two-hybrid system to identify proteins expressed in the brain that interact with the NMDA receptor subunit NR1. Here we report that the 68 kDa neurofilament subunit NF-L directly interacts with the NR1 subunit. This interaction occurs between the cytoplasmic C-terminal domain of NR1 and the rod domain of NF-L. However, NR1 splice variants lacking the first C-terminal exon cassette (C1) failed to associate with NF-L. Immunogold electron microscopy revealed a preferential localization of NR1 at the ends of in vitro-assembled neurofilaments. Overexpression of C1 cassette-containing NR1 constructs in fibroblast cells disrupted the assembly of recombinant neurofilaments. In addition, NR1 and NF-L cofractionated in detergent-treated rat brain synaptic plasma membranes. Furthermore, NR1 and NF-L colocalize in the dendrites and growth cones of cultured hippocampal neurons. These results demonstrate the splice variant-specific association of NR1 with neurofilaments and suggest a possible mechanism for anchoring or localizing NMDA receptors in the neuronal plasma membrane.

Tensegrity: the architectural basis of cellular mechanotransduction

Physical forces of gravity, hemodynamic stresses, and movement play a critical role in tissue development. Yet, little is known about how cells convert these mechanical signals into a chemical response. This review attempts to place the potential molecular mediators of mechanotransduction (e.g. stretch-sensitive ion channels, signaling molecules, cytoskeleton, integrins) within the context of the structural complexity of living cells. The model presented relies on recent experimental findings, which suggests that cells use tensegrity architecture for their organization. Tensegrity predicts that cells are hard-wired to respond immediately to mechanical stresses transmitted over cell surface receptors that physically couple the cytoskeleton to extracellular matrix (e.g. integrins) or to other cells (cadherins, selectins, CAMs). Many signal transducing molecules that are activated by cell binding to growth factors and extracellular matrix associate with cytoskeletal scaffolds within focal adhesion complexes. Mechanical signals, therefore, may be integrated with other environmental signals and transduced into a biochemical response through force-dependent changes in scaffold geometry or molecular mechanics. Tensegrity also provides a mechanism to focus mechanical energy on molecular transducers and to orchestrate and tune the cellular response.

Plectin sidearms mediate interaction of intermediate filaments with microtubules and other components of the cytoskeleton

By immunogold labeling, we demonstrate that "millipede-like" structures seen previously in mammalian cell cytoskeletons after removal of actin by treatment with gelsolin are composed of the cores of vimentin IFs with sidearms containing plectin. These plectin sidearms connect IFs to microtubules, the actin-based cytoskeleton and possibly membrane components. Plectin binding to microtubules was significantly increased in cells from transgenic mice lacking IFs and was reversed by microinjection of exogenous vimentin. These results suggest the existence of a pool of plectin which preferentially associates with IFs but may also be competed for by microtubules. The association of IFs with microtubules did not show a preference for Glu-tubulin. Nor did it depend upon the presence of MAP4 since plectin links were retained after specific immunodepletion of MAP4. The association of IFs with stress fibers survived actin depletion by gelsolin suggesting that myosin II minifilaments or components closely associated with them may play a role as plectin targets. Our results provide direct structural evidence for the hypothesis that plectin cross-links elements of the cytoskeleton thus leading to integration of the cytoplasm.

Structural and biological consequences of increased vimentin expression in simple epithelial cell types

Cytoskeletal intermediate filaments (IFs) constitute a diverse family of proteins whose members are expressed in tissue-specific patterns. Although vimentin IFs are normally restricted to mesenchyme, a variety of cell types express vimentin alone or together with cell-specific IFs during growth, differentiation, and neoplasia. In this study, we have investigated the influence of increased vimentin expression on the simple epithelial cell phenotype. An expression vector encoding a human vimentin cDNA was transfected into murine HR9 endoderm and F9 embryonal carcinoma cell lines, which serve as models for early extraembryonic epithelial differentiation. Stable clones that expressed varying levels of human vimentin were characterized by human vimentin were characterized by immunofluorescence and biochemical analysis. A relatively high level of vimentin expression in HR9 and differentiated F9 epithelial cells resulted in aberrant vimentin structures with co-collapse of keratin K8/K18 filaments and lowered amounts of keratin protein. In F9 epithelial cells, the desmosomal proteins DP I/II did not appear to localize to cell surface desmosome s but rather but rather co-aggregated with the perturbed IFs. Although overall cell morphology was not dramatically altered, individual nuclei were distorted by excess intracellular vimentin. Furthermore, cell proliferation as well as the cell spreading response time were slowed. Ther appears to be a threshold effect regarding overall vimentin levels as cells that expressed lower amounts of the human vimentin exhibited no obvious structural nor biological effects. Our results demonstrate that wild-type vimentin can act as a "mutant" protein when present at high intracellular levels, inducing a variety of phenotypic changes.

Vimentin's tail interacts with actin-containing structures in vivo

The tail domain of the intermediate filament (IF) protein vimentin is unnecessary for IF assembly in vitro. To study the role of vimentin's tail in vivo, we constructed a plasmid that directs the synthesis of a ‘myc-tagged’ version of the Xenopus vimentin-1 tail domain in bacteria. This polypeptide, mycVimTail, was purified to near homogeneity and injected into cultured Xenopus A6 cells. In these cells the tail polypeptide co-localized with actin even in the presence of cytochalasin. Two myc-tagged control polypeptides argue for the specificity of this interaction. First, a similarly myc-tagged lamin tail domain localizes to the nucleus, indicating that the presence of the myc tag did not itself confer the ability to co-localize with actin (Hennekes and Nigg (1994) J. Cell Sci. 107, 1019–1029). Second, a myc-tagged polypeptide with a molecular mass and net charge at physiological pH (i.e. -4) similar to that of the mycVimTail polypeptide, failed to show any tendency to associate with actin-containing structures, indicating that the interaction between mycVimTail and actin-containing structures was not due to a simple ionic association. Franke (1987; Cell Biol. Int. Rep. 11, 831) noted a similarity in the primary sequence between the tail of the type I keratin DG81A and vimentin. To test whether the DG81A tail interacted with actin-containing structures, we constructed and purified myc-tagged DG81A tail polypeptides. Unexpectedly, these keratin tail polypeptides were largely insoluble under physiological conditions and formed aggregates at the site of injection. While this insolubility made it difficult to determine if they associated with actin-containing structures, it does provide direct evidence that the tails of vimentin and DG81A differ dramatically in their physical properties. Our data suggest that vimentin's tail domain has a highly extended structure, binds to actin-containing structures and may mediate the interaction between vimentin filaments and microfilaments involved in the control of vimentin filament organization (Hollenbeck et al. (1989) J. Cell Sci. 92, 621; Tint et al. (1991) J. Cell Sci. 98, 375).

12(S)-HETE-induced microvascular endothelial cell retraction results from PKC-dependent rearrangement of cytoskeletal elements and alpha V beta 3 integrins

12(S)-HETE, a lipoxygenase metabolite of arachidonic acid, has been demonstrated to induce a reversible retraction of vascular endothelial cells (EC). 12(S)-HETE-induced microvascular EC retraction was blocked by a selective protein kinase C inhibitor, calphostin C, but not by the protein kinase A inhibitor, H8. EC exposed to 12(S)-HETE demonstrated a gradual dissolution of actin microfilaments and a decrease of vinculin-containing focal adhesions. The intermediate filaments, vimentin, also underwent extensive reorganization (i.e., filament bundling and enrichment to the cell filapodia) following 12(S)-HETE treatment. In vivo phosphorylation studies revealed that 12(S)-HETE induced a hyperphosphorylation of several major cytoskeletal proteins including myosin light chain, actin, and vimentin. The increased phosphorylation of these cytoskeletal proteins following 12(S)-HETE stimulation was abolished by calphostin C but not by H8. Confluent EC express alpha v beta 3 in focal adhesions at both the cell body and the cell-cell borders. 12(S)-HETE induced a sequential rearrangement of the alpha v beta 3-containing focal adhesions, resulting in a general decrease in alpha v beta 3 integrin receptors, especially in those retracted EC. 12(S)-HETE-induced rearrangement of alpha v beta 3 was inhibited by calphostin C but not by H8. In contrast to alpha v beta 3, confluent EC enrich alpha 5 beta 1 integrin receptors primarily at the cell-cell borders, colocalizing with extracellular fibronectin and cell cortical microfilaments. 12(S)-HETE treatment also disrupted the cell-border distribution pattern of alpha 5 beta 1 as EC retracted, but no distinct alterations (such as time-related redistribution and quantitative differences) in alpha 5 beta 1 were observed.

Motility of intracellular particles in rat fibroblasts is greatly enhanced by phorbol ester and by over-expression of normal p21N-ras

Particle motility in cultured rat fibroblasts was studied using video-enhanced differential interference contrast microscopy. The average velocity of large bright particles (apparent diameter about 0.5-0.7 micron) was measured in control cells and in cells treated with agents which affected targets related to signal transduction pathways. A Rat-2-derived fibroblast line transfected with a construct containing multiple copies of the N-ras proto-oncogene under the control of dexamethasone-sensitive promoter was used as a main experimental model. Dexamethasone treatment was shown to induce high levels of N-ras expression in these cells. This treatment greatly increased the average particle velocity. At the same time dexamethasone did not influence the particle motility in the non-transfected parent cells and in the cells transfected with a construct which did not contain N-ras. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), also induced an approximate eightfold increase in the particle rate after several hours of incubation, while sphingosine, an inhibitor of PKC, prevented this activation. Sphingosine alone reduced the particle motility after a 20 min incubation. The particle movements were inhibited also by colcemid. These data show that the activation of N-ras and PKC produced dramatic activation of microtubule-dependent particle motility. A possible role of this activation in signal-induced alterations of cell morphology is discussed.

Reversible structural alterations of undifferentiated and differentiated human neuroblastoma cells induced by phorbol ester

Morphological alterations in the structure of undifferentiated and morphologically differentiated human neuroblastoma cells induced by phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, were examined by video microscopy and immunomorphology. In undifferentiated cells, PMA induced the formation of motile actin-rich lamellas and of stable cylindrical processes rich in microtubules. Formation of stable processes resulted either from the collapse of lamellas or the movement of the cell body away from the base of a process. In differentiated cells, PMA induced the rapid extension of small lamellas and subsequent formation of short-lived elongated processes from the lateral edges of neurites. Additionally, growth cones exhibited enhanced modulation in shape after PMA treatment. These reversible reorganizations were similar to the actinoplast-tubuloplast transformations exhibited by PMA-treated fibroblasts. We suggest that actinoplast-tubuloplast reorganizations play essential roles in morphogenesis where stable cytoplasmic extensions are induced by external stimuli. In particular, PMA-induced reorganizations of neural cells in culture may be a model for morphological modulations that occur in nerve tissue.

Fluorenone-azomethines, a novel class of microtubule inhibitors that specifically affect cell proliferation

The effects of various azomethine derivatives on microtubule (MT) assembly in vitro as well as on cell proliferation, cell shape, and the cytoskeleton of some cultured murine cell lines were studied. 3 of them, the alpha-diphenylene-N-(p-[bis-(beta-hydroxyethyl)-amino]-phenyl)-nit rone (DHPN), alpha-diphenylene-N-(p-[N-(hydroxyethyl)-N-(gamma-hydroxypropyl)- amino]-phenyl)-nitrone, and alpha-diphenylene-N-(p-diethylaminophenyl)-nitrone, strongly inhibit the assembly of microtubules (MTs) in vitro (50% inhibition at 4 to 7 mumol/l). The same compounds are also able to disrupt preformed microtubules. Moreover, they were found to inhibit proliferation of leukaemia L 1210, melanoma B16 K, fibroblast L 929, and embryo fibroblast cells down to 1 to 10 mumol/l, completely. Immunofluorescence microscopy revealed that DHPN, used as a representative of the active azomethines, causes a reversible destruction of the microtubule part of the cytoskeleton. Apparently resulting from microtubule disruption, the intermediate filament system collapsed whereas the microfilament system remained unaffected. The results indicate that the antiproliferative action of the azomethines is based, at least partially, on their ability to attack microtubules.

Fine structure of the human synovial lining cell in osteoarthritis: its prominent cytoskeleton

The cytoskeleton of the human osteoarthritic synovial lining cell (SLC) consists of an extensive number of vimentin intermediate filaments (IFs) in addition to microfilaments and microtubules. The IFs are especially prevalent in the SLC processes, but are commonly seen in a paranuclear arrangement. Processes, ending in numerous microvilli and blebs, project into the joint space. Scanning electron microscopy (SEM) further reveals the processes that may parallel the synovium surface for a short distance. IFs extend to the termination of such Numerous pinocytotic vesicles and extensive rough endoplasmic reticulum (rER) are characteristic of the type B cells. Lysosomes and long microvilli identify the type A cell. Punctate adherens, gap junctions, and cilia are the cell membrane specializations of the osteoarthritis (OA) synovium. A comparison with synovium from rheumatoid arthritis (RA) patients is made in order to assess the effect o this inflammatory disease on the SLC cytoskeleton, cell type relationship, and cell arrangement. The prominent cytoskeleton appears to play an important role in the architecture of the synovium. Our findings are further presented in the form of a drawing which in some aspects could describe the morphology of the normal synovium.

Post-translational modification of microtubules is a component of synergic alterations of cytoskeleton leading to formation of cytoplasmic processes in fibroblasts

The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) induces rapid and reversible shape changes in cultured fibroblasts: extension of motile lamellas is followed by transformation of these lamellas into nonmotile stalklike processes. This "lamella-to-stalk" transformation was found to be associated with the formation of microtubules enriched in detyrosinated alpha-tubulin. This change was local: microtubules in motile lamellas at the distal ends of the processes and in the cell bodies were not enriched in detyrosinated alpha-tubulin. Detyrosinated microtubules in the processes were more resistant to Colcemid treatment than other microtubules of PMA-treated and control cells. The effects of PMA were reversible and could be abolished by sphingosine, a specific inhibitor of protein kinase C. Besides modification of microtubules, lamella-to-stalk transformation is associated with the ingrowth of intermediate filaments into the extensions. Earlier it was found that this transformation is also associated with the profound reorganization of the system of actin microfilaments. Thus, all three cytoskeletal systems are altered simultaneously during PMA-induced formation of processes. Similar "cytoskeletal synergies" may play essential roles in many morphogenetic processes--e.g., in the growth of neurites.

Deep-etching immunogold replica electron microscopy of cytoskeletal elements in cultured hamster heart cells

A procedure has been developed for the three-dimensional immunoelectron microscopic localization of cytoskeletal filaments by a deep-etching replica method in combination with immunogold labeling and/or myosin subfragment 1 (S1) decoration techniques. Neonatal hamster heart cells grown on glass coverslips were extracted with Triton X-100 or physically permeabilized by breaking open the cell membranes. S1 decoration was performed on some specimens immediately after the permeabilization. After prefixation in formaldehyde, samples were immunostained with poly- or monoclonal antibodies to desmin or vimentin, and indirectly tagged with colloidal gold probes by the biotin-streptavidin method. After postfixation with glutaraldehyde, tannic acid and osmium tetroxide, the cells were freeze-etched and rotary-replicated with platinum and carbon in a freeze-fracture apparatus. Replicas were viewed with a transmission electron microscope using a tilting specimen stage to obtain stereo images. The procedure made it possible to identify the specific filaments within the complex cytoskeletal networks in cultured hamster heart muscle and nonmuscle cells at high resolution and in three dimensions. The method has advantages in its three-dimensionality and feasibility to evaluate the data by comparing them with those obtained by alternative light microscopic methods. Details of the protocol and a description of the results of using three different antibodies are given.

Cytokeratin filament modulation in pulmonary microvessel endothelial cells by vasoactive agents and culture confluency

Recently, bovine pulmonary microvascular endothelial cells (PMV) were shown to contain cytokeratin 8 and 19 intermediate filaments (Patton et al., 1990). In this study, we examine the effect of culture contiguity and vasoactive agents on the content and assembly of cytokeratins in PMV. Immunofluorescent staining of PMV cultures show a progressive increase in cytokeratin filament assembly. In freshly plated PMV, keratin appears as hazy staining (less than 4 hr) and later organizes into keratin 'plaques' (4 days) associated with cell-cell contacts; post confluent (greater than 7 days) PMV cultures contain fully assembled cytokeratin filaments which extend to the cell periphery and approach filaments in apposed cells. Vimentin filaments are also present in freshly plated PMV cultures but unlike cytokeratins, become less filamentous at confluency. This cell density-dependent modulation of cytokeratins is also demonstrated by densitometric analysis of autoradiographs of 35S-methionine labeled keratins in which PMV keratin content is elevated at high cell densities, while vimentin content remains constant. Desmoplakins I and II, components of desmosomes, could not be demonstrated in PMV by immunoblotting. PMV treated with permeability modulating agents (4 x 10(-3) M EGTA, 1 microM cytochalasin B, 1 microM bradykinin, 1 microM A23187, and 1 microM PMA) exhibit border retraction and altered keratin filament staining. From these studies we conclude: 1) cytokeratin 8 and 19 containing intermediate filaments are present in confluent PMV cultures with vimentin but without desmosomes, 2) the state of assembly of PMV cytokeratin and vimentin filaments appears to be oppositely affected by culture contiguity, and 3) treatment of monolayers with vasoactive agents alters the state of assembly of cytokeratin filaments. We speculate that modulation of cytokeratin assembly in PMV may be involved in regulation of pulmonary microvascular structure and function.