Cutting edge: integration of human T lymphocyte cytoskeleton by the cytolinker plectin

Vimentin intermediate filament and plectin provide a scaffold for invadopodia, facilitating cancer cell invasion and extravasation for metastasis

To investigate the molecular mechanisms of cancer metastasis, we have isolated a high-metastatic bladder cancer cell subpopulation from a low-metastatic cell line by using an in vivo selection system. Cells in the subpopulation showed a high ability to form invadopodia, the filamentous actin (F-actin)-based membrane protrusions that play an essential role in cancer cell invasion. Analysis of the gene expression profile revealed that the expression of an intermediate filament (IF) protein, vimentin and a cytoskeletal linker protein, plectin was up-regulated in the high-metastatic subpopulation compared with the low metastatic cell line. Here we report a novel role of vimentin IF and plectin in metastasis. In invasive bladder cancer cells, the vimentin IF-plectin-invadopodia F-actin link was formed. Disruption of this link severely impaired invadopodia formation, reducing the capacities of extracellular matrix degradation, transendothelial migration and metastasis. In addition, the vimentin assembly into the filaments was required for invadopodia formation. Our results suggest that plectin anchoring invadopodia to vimentin IF scaffolds and stabilizes invadopodia, which is a critical molecular process for cancer cell invasion and extravasation for metastasis.

Plectin-intermediate filament partnership in skin, skeletal muscle, and peripheral nerve

Plectin is a large, 500-kDa, intermediate filament (IF)-associated protein. It acts as a cytoskeletal crosslinker and signaling scaffold, affecting mechanical as well as dynamic properties of the cytoskeleton. As a member of the plakin family of cytolinker proteins, plectin has a multidomain structure that is responsible for its vast binding portfolio. It not only binds to all types of IFs, actin filaments and microtubules, but also to transmembrane receptors, proteins of the subplasma membrane protein skeleton, components of the nuclear envelope, and several kinases with known roles in migration, proliferation, and energy metabolism of cells. Due to alternative splicing, plectin is expressed as various isoforms with differing N-terminal heads that dictate their differential subcellular targeting. Through specific interactions with other proteins at their target sites and their ability to bind to all types of IFs, plectin molecules provide strategically located IF anchorage sites within the cytoplasm of cells. In this review, we will present an overview of the structural features and functional properties of plectin and discuss recent progress in defining the role of its isoforms in stress-prone tissues and the implicated diseases, with focus on skin, skeletal muscle, and Schwann cells of peripheral nerve.

Cytoskeletal protein vimentin interacts with and regulates peroxisome proliferator-activated receptor gamma via a proteasomal degradation process

Peroxisome proliferators-activated receptor gamma (PPARγ) receptor is a transcription factor that is located in and functions primarily in the nucleus. PPARγ is exported from the nucleus upon mitogen and ligand stimulation under certain circumstances. However, a cytoplasmic PPARγ interacting protein and its function have not been previously identified. Here, we report for the first time that cytosolic PPARγ interacts directly with cytoskeletal vimentin. We performed PPARγ immunoprecipitation followed by mass spectrometry to identify the vimentin-PPARγ complex. This interaction was confirmed by reciprocal vimentin and PPARγ immunoprecipitation and co-immunofluorescence examination. We demonstrated that PPARγ colocalized with vimentin in certain organelles that is golgi, mitochondria, and endoplasmic reticulum. In cells depleted of vimentin, PPARγ was ubiquitinated and targeted to a proteasomal degradation pathway. Together, these findings indicate a direct interaction of PPARγ with vimentin in the cytosolic compartment, in which vimentin appears to play a role in regulating the turnover rate of PPARγ, which may further regulate genomic or non-genomic activities through the regulation of PPARγ protein degradation.

Altered phosphorylation and distribution status of vimentin in rat seminiferous epithelium following 17β-estradiol treatment

Vimentin, type III intermediate filament, has stage-specific localization in the Sertoli cell. In the rat, during stages I-V and XI-XIV of the seminiferous epithelium, vimentin is localized in the perinuclear area with filaments projecting into the apical region toward the developing germ cells. These filaments decrease in length at stages VI-VII with perinuclear staining in stages VIII-IX, when spermiation occurs. Our earlier studies following 17β-estradiol treatment to adult male rats demonstrated an increase in germ cell apoptosis, spermiation failure and disruption of Sertoli cell microfilaments and microtubules. The present study was undertaken to determine the stage-specific distribution of vimentin and its involvement in spermiation failure and germ cell apoptosis. Immunofluorescence studies revealed that in contrast to the perinuclear localization with small extensions in control stages VII-IX, long extensions radiating apically to the spermatids in deep recess were observed in the treated group. Immunoprecipitation studies showed marked absence of phosphorylated vimentin in stages VII-VIII in the treated group. Further, localization of plectin, cytoskeletal linker protein, showed decrease in all the stages of spermatogenesis following estradiol treatment. Interestingly, for the first time the localization of plectin in the tubulobulbar complex was observed. In conclusion, the study suggests that estradiol treatment leads to an effect on vimentin phosphorylation, which could have inhibited the disassembly of vimentin leading to retention of apical projection in stages VII-VIII. These effects could be presumably due to a decrease in plectin, affecting the reorganization of vimentin and therefore the apical movement of spermatids, leading to spermiation failure.

Plectin isoforms as organizers of intermediate filament cytoarchitecture

Intermediate filaments (IFs) form cytoplamic and nuclear networks that provide cells with mechanical strength. Perturbation of this structural support causes cell and tissue fragility and accounts for a number of human genetic diseases. In recent years, important additional roles, nonmechanical in nature, were ascribed to IFs, including regulation of signaling pathways that control survival and growth of the cells, and vectorial processes such as protein targeting in polarized cellular settings. The cytolinker protein plectin anchors IF networks to junctional complexes, the nuclear envelope and cytoplasmic organelles and it mediates their cross talk with the actin and tubulin cytoskeleton. These functions empower plectin to wield significant influence over IF network cytoarchitecture. Moreover, the unusual diversity of plectin isoforms with different N termini and a common IF-binding (C-terminal) domain enables these isoforms to specifically associate with and thereby bridge IF networks to distinct cellular structures. Here we review the evidence for IF cytoarchitecture being controlled by specific plectin isoforms in different cell systems, including fibroblasts, endothelial cells, lens fibers, lymphocytes, myocytes, keratinocytes, neurons and astrocytes, and discuss what impact the absence of these isoforms has on IF cytoarchitecture-dependent cellular functions.

Plectin gene defects lead to various forms of epidermolysis bullosa simplex

Plectin is an important organizer of the keratin filament cytoskeleton in basal keratinocytes. It is essential for anchoring these filaments to the extracellular matrix via hemidesmosomal integrins. Loss of plectin or incorrect function of the protein due to mutations in its gene can lead to various forms of the skin blistering disease, epidermolysis bullosa simplex. Severity and subtype of the disease is dependent on the specific mutation and can be associated with (late-onset) muscular dystrophy or pyloric atresia. Mouse models mimicking the human phenotypes allow detailed study of plectin function.

Prefrontal cortex shotgun proteome analysis reveals altered calcium homeostasis and immune system imbalance in schizophrenia

Schizophrenia is a complex disease, likely to be caused by a combination of serial alterations in a number of genes and environmental factors. The dorsolateral prefrontal cortex (Brodmann's Area 46) is involved in schizophrenia and executes high-level functions such as working memory, differentiation of conflicting thoughts, determination of right and wrong concepts and attitudes, correct social behavior and personality expression. Global proteomic analysis of post-mortem dorsolateral prefrontal cortex samples from schizophrenia patients and non-schizophrenic individuals was performed using stable isotope labeling and shotgun proteomics. The analysis resulted in the identification of 1,261 proteins, 84 of which showed statistically significant differential expression, reinforcing previous data supporting the involvement of the immune system, calcium homeostasis, cytoskeleton assembly, and energy metabolism in schizophrenia. In addition a number of new potential markers were found that may contribute to the understanding of the pathogenesis of this complex disease.

The actin cytoskeleton in T cell activation

T cell cytoarchitecture differs dramatically depending on whether the cell is circulating within the bloodstream, migrating through tissues, or interacting with antigen-presenting cells. The transition between these states requires important signaling-dependent changes in actin cytoskeletal dynamics. Recently, analysis of actin-regulatory proteins associated with T cell activation has provided new insights into how T cells control actin dynamics in response to external stimuli and how actin facilitates downstream signaling events and effector functions. Among the actin-regulatory proteins that have been identified are nucleation-promoting factors such as WASp, WAVE2, and HS1; severing proteins such as cofilin; motor proteins such as myosin II; and linker proteins such as ezrin and moesin. We review the current literature on how signaling pathways leading from diverse cell surface receptors regulate the coordinated activity of these and other actin-regulatory proteins and how these proteins control T cell function.

Plectin regulates the signaling and trafficking of the HIV-1 co-receptor CXCR4 and plays a role in HIV-1 infection

The CXC chemokine CXCL12 and its cognate receptor CXCR4 play an important role in inflammation, human immunodeficiency virus (HIV) infection and cancer metastasis. The signal transduction and intracellular trafficking of CXCR4 are involved in these functions, but the underlying mechanisms remain incompletely understood. In the present study, we demonstrated that the CXCR4 formed a complex with the cytolinker protein plectin in a ligand-dependent manner in HEK293 cells stably expressing CXCR4. The glutathione-S-transferase (GST)-CXCR4 C-terminal fusion proteins co-precipitated with the full-length and the N-terminal fragments of plectin isoform 1 but not with the N-terminal deletion mutants of plectin isoform 1, thereby suggesting an interaction between the N-terminus of plectin and the C-terminus of CXCR4. This interaction was confirmed by confocal microscopic reconstructions showing co-distribution of these two proteins in the internal vesicles after ligand-induced internalization of CXCR4 in HEK293 cells stably expressing CXCR4. Knockdown of plectin with RNA interference (RNAi) significantly inhibited ligand-dependent CXCR4 internalization and attenuated CXCR4-mediated intracellular calcium mobilization and activation of extracellular signal regulated kinase 1/2 (ERK1/2). CXCL12-induced chemotaxis of HEK293 cells stably expressing CXCR4 and of Jurkat T cells was inhibited by the plectin RNAi. Moreover, CXCR4 tropic HIV-1 infection in MAGI (HeLa-CD4-LTR-Gal) cells was inhibited by the RNAi of plectin. Thus, plectin appears to interact with CXCR4 and plays an important role in CXCR4 signaling and trafficking and HIV-1 infection.

Regulation of T-cell activation by the cytoskeleton

To become activated, T cells must efficiently recognize antigen-presenting cells or target cells through several complex cytoskeleton-dependent processes, including integrin-mediated adhesion, immunological-synapse formation, cellular polarization, receptor sequestration and signalling. The actin and microtubule systems provide the dynamic cellular framework that is required to orchestrate these processes and ultimately contol T-cell activation. Here, we discuss recent advances that have furthered our understanding of the crucial importance of the T-cell cytoskeleton in controlling these aspects of T-cell immune recognition.

Plectin rodless isoform expression and its detection in mouse brain

The widely expressed cytolinker protein plectin shows extensive isoform diversity both at the N-terminus and in the central part of the molecule. Judged on mRNA data, plectin variants lacking the central rod domain are expressed at a approximately 20-fold lower level than full-length proteins and their detection on the protein level can be difficult. Here we present data on the expression of plectin rodless isoforms in mouse brain and in rat glioma C6 cells on RNA and protein levels. Our data indicate that among the rodless variants expressed in neuronal tissues, those starting with exon 1c (plectin 1c) seem to be the most prominent ones. In addition, we show that similar to other monoclonal antibodies reported in the literature, the widely used mAb 7A8 recognizes an epitope within plectin's rod domain and therefore is unsuited to detect rodless variants of plectin.

A role for the neuronal protein collapsin response mediator protein 2 in T lymphocyte polarization and migration

The semaphorin-signaling transducer collapsin response mediator protein 2 (CRMP2) has been identified in the nervous system where it mediates Sema3A-induced growth cone navigation. In the present study, we provide first evidence that CRMP2 is present in the immune system and plays a critical role in T lymphocyte function. CRMP2 redistribution at the uropod in polarized T cells, a structural support of lymphocyte motility, suggests that it may regulate T cell migration. This was evidenced in primary T cells by small-interfering RNA-mediated CRMP2 gene silencing and blocking Ab, as well as CRMP2 overexpression in Jurkat T cells tested in a chemokine- and semaphorin-mediated transmigration assay. Expression analysis in PBMC from healthy donors showed that CRMP2 is enhanced in cell subsets bearing the activation markers CD69+ and HLA-DR+. Heightened expression in T lymphocytes of patients suffering from neuroinflammatory disease with enhanced T cell-transmigrating activity points to a role for CRMP2 in pathogenesis. The elucidation of the signals and mechanisms that control this pathway will lead to a better understanding of T cell trafficking in physiological and pathological situations.

Targeted ablation of plectin isoform 1 uncovers role of cytolinker proteins in leukocyte recruitment

Plectin, a typical cytolinker protein, is essential for skin and skeletal muscle integrity. It stabilizes cells mechanically, regulates cytoskeleton dynamics, and serves as a scaffolding platform for signaling molecules. A variety of isoforms expressed in different tissues and cell types account for this versatility. To uncover the role of plectin 1, the major isoform expressed in tissues of mesenchymal origin, against the background of all other variants, we raised plectin isoform 1-specific antibodies and generated isoform-deficient mice. In contrast to plectin-null mice (lacking all plectin isoforms), which die shortly after birth because of severe skin blistering, plectin isoform 1-deficient mice were viable at birth, had a normal lifespan, and did not display the skin blistering phenotype. However, dermal fibroblasts isolated from plectin 1-deficient mice exhibited abnormalities in their actin cytoskeleton and impaired migration potential. Similarly, plectin 1-deficient T cells isolated from nymph nodes showed diminished chemotactic migration in vitro. Most strikingly, in vivo we found that leukocyte infiltration during wound healing was reduced in the mutant mice. These data show a specific role of a cytolinker protein in immune cell motility. Single isoform-deficient mice thus represent a powerful tool to unravel highly specific functions of plectin variants.

Dynamic modes of the cortical actomyosin gel during cell locomotion and division

Tight regulation of the contractility of the actomyosin cortex is essential for proper cell locomotion and division. Enhanced contractility leads, for example, to aberrations in the positioning of the mitotic spindle or to anomalous migration modes that allow tumor cells to escape anti-dissemination treatments. Spherical membrane protrusions called blebs occasionally appear during cell migration, cell division or apoptosis. We have shown that the cortex ruptures at sites where actomyosin cortical contractility is increased, leading to the formation of blebs. Here, we propose that bleb formation, which releases cortical tension, can be used as a reporter of cortical contractility. We go on to analyze the implications of spontaneous cortical contractile behaviors on cell locomotion and division and we particularly emphasize that variations in actomyosin contractility can account for a variety of migration modes.

Oligonucleotide-microarray analysis of peripheral-blood lymphocytes in severe asthma

CD4 + lymphocytes play a key role in asthma pathogenesis, but much remains unknown about the genetic mechanisms that affect disease severity. In this study we sought to investigate global patterns of gene expression in CD4 + lymphocytes isolated from subjects with severe asthma through the use of microarray technology. CD4 + lymphocytes were separated from peripheral blood, total RNA was purified, and biotinylated complementary RNA was prepared and hybridized to Affymetrix HU133 chips (Affymetrix, Santa Clara, Calif). Using the robust multi-chip average procedure, we compared the messenger RNA expression profiles of more than 33,000 genes of CD4 + lymphocytes in subjects with severe ( n = 5) and mild ( n = 5) asthma. Forty genes had 2-fold mean expression differences or greater. Thirty-seven genes were up-regulated, including transforming growth factor-beta and those involved in T-cell activation, proliferation, and cytoskeletal changes. Three genes were down-regulated, including the T-cell-receptor delta locus. This study demonstrates a method by which CD4 + lymphocytes can be extracted from blood for the purpose of microarray analysis. Furthermore, we show that T-lymphocytes from the peripheral blood of subjects with severe and mild asthma differ in their gene-expression profiles, supporting the view that asthma is a systemic disease. These differentially expressed genes identify potential molecular targets for preventive and therapeutic options for severe asthma.

Apoptosis and estrogen deficiency in primary Sjögren syndrome

PURPOSE OF REVIEW

Primary Sjögren syndrome is an autoimmune disorder characterized by lymphocytic infiltrates and destruction of the salivary and lacrimal glands, and systemic production of autoantibodies to the ribonucleoprotein particles SS-A/Ro and SS-B/La. The purpose of this review is to discuss recent advances in the pathogenesis of primary Sjögren syndrome.

RECENT FINDINGS

Although several candidate autoantigens including alpha-fodrin have been reported in Sjögren syndrome, the pathogenic roles of the autoantigens in initiation and progression of SS are still unclear. It is possible that individual T cells activated by an appropriate self antigen can proliferate and form a restricted clone. Recent evidence suggests that the apoptotic pathway plays a central role in tolerizing T cells to tissue-specific self antigen, and may drive the autoimmune phenomenon. Cleavage of certain autoantigens during apoptosis may reveal immunocryptic epitopes that could potentially induce autoimmune response. The studies reviewed imply that Fas-mediated cytotoxicity and caspase-mediated alpha-fodrin proteolysis are involved in the progression of tissue destruction in Sjögren syndrome. Fas ligand (FasL), and its receptor Fas are essential in the homeostasis of the peripheral immune system. It can be considered that a defect in activation-induced cell death of effector T cells may result in the development of autoimmune exocrinopathy in Sjögren syndrome.

SUMMARY

Although the mechanisms by which estrogen deficiency influences autoimmune lesions remain unclear, it is possible that antiestrogenic actions might be a potent factor in the formation of pathogenic autoantigens.

Cytoplasmic intermediate filaments revealed as dynamic and multipurpose scaffolds

Intermediate filaments are cytoskeletal polymers encoded by a large family of differentially expressed genes that provide crucial structural support in the cytoplasm and nucleus of higher eukaryotes. Perturbation of their function accounts for several genetically determined diseases in which fragile cells cannot sustain mechanical and non-mechanical stresses. Recent studies shed light on how this structural support is modulated to meet the changing needs of cells, and reveal a novel role whereby intermediate filaments influence cell growth and death through dynamic interactions with non-structural proteins.

Role of the cytoskeleton during leukocyte responses

The cytoskeleton is a cellular network of structural, adaptor and signalling molecules that regulates most cellular functions that are related to the immune response, including migration, extravasation, antien recognition, activation and phagocytosis by different subsets of leukocytes. Recently, a large number of regulatory elements and structural constituents of the leukocyte cytoskeleton have been identified. In this review, we discuss the composition and regulation of the different cytoskeletal elements and their role in immune responses.

Chemokine stimulation of human peripheral blood T lymphocytes induces rapid dephosphorylation of ERM proteins, which facilitates loss of microvilli and polarization

Lymphocyte microvilli mediate initial rolling-adhesion along endothelium but are lost during transmigration from circulation to tissue. However, the mechanism for resorption of lymphocyte microvilli remains unexplored. We show that chemokine stimulation of human peripheral blood T (PBT) cells is sufficient to induce rapid resorption of microvilli. Microvilli in other cells are regulated by ezrin/radixin/moesin (ERM) proteins, which link the plasma membrane to the cortical F-actin cytoskeleton; maintenance of these linkages requires ERM activation, reflected by phosphorylation at a specific carboxy-terminal threonine residue. Carboxyphosphorylated-ERM (cpERM) proteins in resting PBT cells show a punctate peripheral distribution consistent with localization to microvilli. cpERM dephosphorylation begins within seconds of stimulation by chemokines (stromal derived factor 1 alpha [SDF-1 alpha] or secondary lymphoid tissue cytokine), and ERM proteins lose their punctate distribution with kinetics paralleling the loss of microvilli. The cpERM proteins are preferentially associated with the cytoskeleton at rest and this association is lost with chemokine-induced dephosphorylation. Transfection studies show that a dominant-negative ERM construct destroys microvilli, whereas a construct mimicking cpERM facilitates formation of microvilli, retards chemokine-induced loss of microvilli, and markedly impairs chemokine-induced polarization. Thus, chemokine induces rapid dephosphorylation and inactivation of cpERM, which may in turn facilitate 2 aspects of cytoskeletal reorganization involved in lymphocyte recruitment: loss of microvilli and polarization.

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.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Expression of plectin in muscle fibers with cytoarchitectural abnormalities

Plectin is a protein belonging to the cytoskeletal anchoring system, concentrated at sites of mechanical stress in different cell types. In normal skeletal muscle, plectin is located at level of Z-discs, sarcolemma, post-synaptic membrane, and intermyofibrillar network. We investigated plectin immunocytochemistry in lobulated fibers, fibers with tubular aggregates, target fibers, central core disease and centronuclear myopathy. Thirty to forty percent of lobulated fibers had patchy increase of plectin immunoreactivity at sarcolemmal level with focal subsarcolemmal increases. Tubular aggregates revealed a low binding for plectin. Ten percent of central cores exhibited faint focal increase of plectin immunoreactivity. Target formations had a normal plectin pattern. In centronuclear myopathy, plectin immunoreactivity was increased around the centrally located nuclei in 8-12% of the fibers, at the sarcolemma of 50% of type 2 fibers, and at the membrane of small vacuoles located peripherally around the central nuclei. We postulate that plectin may play a role in the subsarcolemmal aggregation of mitochondria in the lobulated fibers, and in the central position of nuclei as well as in shape formation, positioning and moving of the vacuoles in centronuclear myopathy.

Role of Rho-family GTPase Cdc42 in polarized expression of lymphocyte appendages

Lymphocytes polarize for motility by developing a broad anterior, where lamellipodia arise, and a simple stalk-like posterior appendage, the uropod. Through time-lapse analysis of normal and leukemic human T cells, it was found that this polarized form is maintained by a mechanism that excludes lamellipodia from the uropod. Lamellipodia regularly traveled rearward to encroach upon the uropod but disassembled abruptly at the uropod border. This exclusion of lamellipodia from the uropod required the Rho-family guanosine triphosphatase Cdc42. Reduction of Cdc42 activity by expression of dominant-negative Cdc42 resulted in "two headed" cells in which lamellipodia persisted at the distal end of the uropod. Random and chemotactic motility were impaired. Increased Cdc42 activity, induced by expression of activated, mutant Cdc42, was accompanied by a general loss of lamellipodia. The results suggest that one role of Cdc42 in lymphocyte motility is to preserve polarity by concentrating lamellipodial disassembly signals in the uropod.

A real time in vitro assay for studying leukocyte transendothelial migration under physiological flow conditions

The mechanisms underlying leukocyte migration across endothelial barriers are largely elusive. Most of the current knowledge on transendothelial migration (TEM) of leukocytes has been derived from in vitro modified Boyden chamber transfilter migration assays. In these assays, leukocyte migration towards chemokine gradients constructed across the endothelial barrier is measured under shear-free conditions. These assays do not incorporate the contribution of shear flow to leukocyte adherence and migration across the endothelial barrier. Furthermore, transfilter assays do not reconstitute the physiological distribution of endothelial chemokines shown to be displayed in vivo at high levels on vessel walls. To overcome these two drawbacks, we have recently developed a novel in vitro assay to follow real time leukocyte migration across endothelial barriers under physiological flow conditions. Using this assay, we have found that apically displayed endothelial chemokines could trigger robust lymphocyte TEM through signaling to lymphocyte-expressed G-protein coupled receptors. This migration required continuous exposure of lymphocytes, adherent to the endothelial barrier, to fluid shear, but did not require a chemotactic gradient across the barrier. In the present review, we describe this new flow-based migration assay and discuss future applications for investigating TEM processes of different types of leukocytes across distinct endothelial barriers.

Actin cytoskeletal dynamics in T lymphocyte activation and migration

Dynamic rearrangements of the actin cytoskeleton are crucial for the function of numerous cellular elements including T lymphocytes. They are required for migration of T lymphocytes through the body to scan for the presence of antigens, as well as for the formation and stabilization of the immunological synapse at the interface between antigen-presenting cells and T lymphocytes. Supramolecular activation clusters within the immunological synapse play an important role for the initiation of T cell responses and for the execution of T cell effector functions. In addition to the T cell receptor/CD3 induced actin nucleation via Wasp/Arp2/3-activation, signals through accessory receptors of the T cell (i.e., costimulation) regulate actin cytoskeletal dynamics. In this regard, the actin-binding proteins cofilin and L-plastin represent prominent candidates linking accessory receptor stimulation to the rearrangement of the actin cytoskeleton. Cofilin enhances actin polymerization via its actin-severing activity, and as a long-lasting effect, cofilin generates novel actin monomers through F-actin depolymerization. L-plastin stabilizes actin filament structures by means of its actin-bundling activity.

The leukocyte cytoskeleton in cell migration and immune interactions

Leukocyte migration is crucial during the development of the immune system and in the responses to infection, inflammation, and tumor rejection. The migratory behavior of leukocytes under physiological and pathological conditions as well as the extracellular cues and intracellular machinery that control and guide migration have been studied thoroughly. The cytoskeleton of leukocytes is extremely versatile, bearing characteristic features that enable these cells to migrate under conditions of flow through narrow spaces and onto target tissues. What makes the cytoskeleton machinery so extraordinary is not so much its molecular composition, but its flexibility which allows it to display a unique combination of responses to the extracellular medium and a rapid regulation of the architecture of its components. This review focuses on the cytoskeleton of the leukocyte. Its molecular components and the regulation of their assembly and organization are discussed. Furthermore, it highlights aspects of the regulation of the leukocyte cytoskeleton that confer flexibility to these cells in order to perform their specific tasks. Finally, different subcellular structures such as the immunological synapse, the uropod of migrating leukocytes, and the phagosome displayed by phagocytic cells are discussed in detail. The relationship of the leukocyte with its environment occurs through different kinds of receptors that interact with ligands that are soluble, fixed on the membrane of other cells, or immobilized on the extracellular matrix. The impact of receptor-ligand binding on the functional responses and the rearrangement of the cytoskeleton is also examined.

The distal pole complex: a novel membrane domain distal to the immunological synapse

While much interest has focused on the finding that T cell-antigen presenting cell (APC) interaction induces the recruitment of proteins to the immunological synapse (IS), we have recently discovered that APC binding induces the formation of a novel protein complex distal to the site of T-cell receptor ligation. This 'distal pole complex' (DPC) is important for appropriate T-cell activation, functioning either to remove proteins from the synapse or as a signaling complex in its own right. The first component of the DPC to be identified was CD43, a cell-surface mucin that has been proposed to function as a negative regulator of T-cell signaling. CD43 movement was found to depend on ezrin and moesin, members of the ERM family, which serve to link CD43 and other cargo molecules to the actin cytoskeleton. ERM proteins interact with several other important surface receptors and cytoplasmic signaling molecules, some of which we have identified as additional components of the DPC. Disruption of the DPC leaves early T-cell activation events intact but affects cytokine expression. Here, we review what is currently known about the formation and function of the DPC and speculate on how this novel protein complex serves to facilitate T-cell activation.

The regulation of actin remodeling during T-cell-APC conjugate formation

The T-cell cytoskeleton is intimately involved in determining the efficiency and fidelity of the immune response. During T-cell interactions with antigen-presenting cells (APCs), dynamic remodeling of the actin cytoskeleton is particularly important for stabilizing long-lived integrin-dependent adhesive interactions. In addition, actin remodeling is important for facilitating the sustained signaling required for full T-cell activation. Although the relationship between T-cell signaling and cytoskeletal remodeling is complex, new molecular genetic tools are making it possible to investigate individual molecular interactions in the context of bona fide conjugate formation. We describe here the progress from our laboratory toward defining the pathways required for actin remodeling during conjugate formation. Our studies show that engagement of T-cell receptor (TCR) and leukocyte functional antigen-1 (LFA-1) leads to distinct effects on the remodeling of individual cytoskeletal elements. Downstream of TCR, we find that p56Lck (Lck) plays a critical role in integrin-dependent adhesion independent of its ability to activate zeta-associated protein of 70 kDa (ZAP-70). TCR engagement also results in the assembly of a signaling complex that facilitates the activation of Wiskott-Aldrich syndrome protein (WASP) by colocalization with Cdc42-GTP. These events, together with other parallel actin regulatory pathways, induce localized actin polymerization at the site of APC binding.

Prevention and induction of autoimmune exocrinopathy is dependent on pathogenic autoantigen cleavage in murine Sjögren's syndrome

The in vivo role of autoantigen cleavage during apoptosis in autoimmune diseases remains unclear. Previously, we found a cleavage product of 120-kDa alpha-fodrin as an important autoantigen in the pathogenesis of primary Sjögren's syndrome (SS). In the murine primary SS model, tissue-infiltrating CD4(+) T cells purified from the salivary glands bear a large proportion of Fas ligand, and the salivary gland duct cells constitutively possess Fas. Infiltrating CD4(+) T cells, but not CD8(+) T cells, identified significant (51)Cr release against mouse salivary gland cells. In vitro studies demonstrated that apoptotic mouse salivary gland cells result in a specific alpha-fodrin cleavage into 120 kDa and that preincubation with caspase inhibitor peptides blocked alpha-fodrin cleavage. In vivo treatment with caspase inhibitors N-benzyloxycarbonyl-Val-Ala-Asp fluoromethyl ketone and N-acetyl-Asp-Glu-Val-Asp-al-CHO into the murine model results in dramatic inhibitory effects on the development of autoimmune lesions and in restoration of sicca syndrome. Furthermore, we found that immunization with recombinant alpha-fodrin protein identical with an autoantigen into normal recipients induced autoimmune lesions similar to SS. These data indicate that prevention and induction of autoimmune exocrinopathy is dependent on autoantigen cleavage via caspase cascade and that caspase inhibitors might provide a new therapeutic option directed at reducing tissue damage in the murine model for SS.