Talin: role at sites of cell-substratum adhesion

Cell budding from pre-invasive tumors: Intrinsic precursor of invasive breast lesions?

Our previous studies showed that in patients with ductal carcinoma in situ (DCIS) of the breast, the tumor cells that overlie focal myoepithelial cell layer disruptions (FMCLDs) are generally arranged as finger-like projections that bud into the stroma. These budding cells have significantly more genetic instability and invasion-related gene expression, and less estrogen receptor (ER) expression, than their epithelial cell counterparts. This study aimed to assess these cells for potential molecular markers that are uniquely associated with cell adhesion and motility. Seventeen ER-positive DCIS cases were screened by immunostaining for ER, and 7 cases which harbored FMCLD lesions were used to examine the expression of the potential markers. Two cases with both DCIS and invasive lesions were selected for comparing the differences in molecular expression between these lesion types. The results showed that expression levels of talin, E-cadherin and focal adhesion kinase (FAK) in tumor cells overlying FMCLDs were higher than those within the corresponding duct. Integrin β1 staining was detected only in a small number of the tumor cells overlying the FMCLDs. Vinculin staining was weak (18%) or not detected (82%), and no expression was found in the tumor cells within the corresponding duct or in the pure isolated DCIS. By contrast, the expression levels of talin, vinculin and integrin β1 in the invasive tumors were distinctly higher than those in DCIS, and the expression of FAK and E-cadherin was lower. Using electron microscopy, we found that the tight junctions between tumor cells overlying the FMCLDs were reduced compared to the adjacent tumor cells in the lumen. These results indicate that the tumor cells overlying FMCLDs are likely to represent the specific precursors of invasive breast lesions. Our findings may also facilitate the identification of specific targets for further molecular profiling, which will more completely characterize this important cell population.

Ovarian hormones regulate expression of the focal adhesion proteins, talin and paxillin, in rat uterine luminal but not glandular epithelial cells

During early pregnancy in the rat, focal adhesions disassemble in uterine luminal epithelial cells at the time of implantation to facilitate their removal so that the implanting blastocyst can invade into the underlying endometrial decidual cells. This study investigated the effect of ovarian hormones on the distribution and protein expression of two focal adhesion proteins, talin and paxillin, in rat uterine luminal and glandular epithelial cells under various hormone regimes. Talin and paxillin showed a major distributional change between different hormone regimes. Talin and paxillin were highly concentrated along the basal cell surface of uterine luminal epithelial cells in response to oestrogen treatment. However, this prominent staining of talin and paxillin was absent and also a corresponding reduction of paxillin expression was demonstrated in response to progesterone alone or progesterone in combination with oestrogen, which is also observed at the time of implantation. In contrast, the distribution of talin and paxillin in uterine glandular epithelial cells was localised on the basal cell surface and remained unchanged in all hormone regimes. Thus, not all focal adhesions are hormonally dependent in the rat uterus; however, the dynamics of focal adhesion in uterine luminal epithelial cells is tightly regulated by ovarian hormones. In particular, focal adhesion disassembly in uterine luminal epithelial cells, a key component to establish successful implantation, is predominantly under the influence of progesterone.

Focal adhesions disassemble during early pregnancy in rat uterine epithelial cells

During early pregnancy in rodents, invasion of the blastocyst into the endometrial decidual cells is accompanied by the removal of uterine epithelial cells around the implantation sites. The present study investigated the distribution and expression of two focal adhesion proteins, namely talin and paxillin, in rat uterine epithelial cells during early pregnancy and their role in the loss of these cells at the time of implantation. A major distributional change of talin and paxillin was demonstrated in uterine epithelial cells during early pregnancy. From a highly concentrated expression along the basal cell surface on Day 1 of pregnancy, talin and paxillin were lost from the basal cell surface at the time of implantation. There was also a corresponding statistically significant decrease in paxillin seen through western blotting analysis. Together, these observations suggest that uterine epithelial cells are less adherent to the underlying basal lamina due to the disassembly of talin and paxillin from focal adhesions, facilitating removal of these cells at the time of implantation. This phenomenon was restricted to the period of receptivity because talin and paxillin reappeared along the basal cell surface soon after implantation.

Vinculin and talin: kinetics of entry and exit from the cytoskeletal pool

Vinculin and talin, two major components of focal contacts, exist in cytosolic and cytoskeletal pools. The kinetics of entry and exit of the two proteins between the two pools were investigated in normal and transformed cells. In cultured chick embryo fibroblasts, a fraction (2-5%) of the newly synthesized vinculin and talin reached maximal levels in the cytoskeleton in 30-45 min. Both proteins had 2-3 times shorter half-lives in the cytoskeletal pool (t1/2 = 6-7 h) than in the cytosolic pool (t1/2 = 14-15 h), which suggests that the incorporation of cytosolic vinculin and talin into the cytoskeleton does not involve a simple equilibrium between the two pools. However, after disruption of cell-to-substrate adhesion by trypsinization, an equilibrium in the incorporation between the two pools was transiently established, resulting in the use of the preexisting cytosolic pools of the two proteins during re-establishment of cell-to-matrix contacts. Viral transformation did not cause a significant change in the incorporation rates into the cytoskeleton. However, it decreased the half-lives of both proteins in the cytoskeletal pool (t1/2 = approximately 4 h) and in the cytosolic pool (t1/2 = 9-10 h). The increased turn-over rates of vinculin and talin in the cytoskeletal pool in transformed cells may contribute to the enhanced motility of transformed cells.

Immunofluorescence distribution of actin-associated proteins in human seminiferous tubules of adolescent testes, normal and pathologic

The aim of our study on human seminiferous tubules of adolescent testes was to study the localization of two actin-associated proteins of the adherens junctions, such as vinculin and talin, and to verify if there were modifications in their pattern in varicocele, a frequent disease of the testis in adolescent age. The study group consisted of 8 biopsies from normal testes (i.e., adolescents operated on for hydrocele or inguinal hernia) and 20 biopsies from pathological testes (i.e., adolescents operated on for idiophatic left varicocele). Biopsies were evaluated by indirect immunofluorescence using anti-human vinculin and anti-human talin antibodies. Observation was recorded with a Leica TCS 4D upright confocal microscope. In the normal testes, there was a strong positive immunoreactivity for vinculin, which was localized in the interstitial cells of Leydig, and both basal pole and lateral cell surface of Sertoli cells; the pattern of talin immunoreactivity was the same except that the lateral cell surface of Sertoli cells was not stained. In the varicocele group the pattern was different. Vinculin immunoreactivity showed small patches of fluorescence only in the cytoplasm of Sertoli cells while talin immunoreactivity showed a scanty distribution at the basal surface of Sertoli cells. These results confirm that, similarly to other tissues, vinculin is expressed at cell-cell and cell-matrix adherens junctions, while talin is present at cell-matrix adherens junctions in human seminiferous tubules of normal adolescents. Varicocele alters the patterns of these two proteins both quantitatively and qualitatively.

Integrins and development: how might these receptors regulate differentiation of the lens

Integrins transduce both internal signals and signals from the matrix. These interactions between integrins, their extracellular matrix ligands, and their cytoskeletal partners play an important role in the regulation of cellular differentiation. We have shown them to be important in lens cell differentiation. In the lens capsule there is a compartmentalization of matrix components with fibronectin, primarily localized to the anterior capsule, and tenascin in the posterior capsule. Integrins are developmentally regulated in the lens. alpha 5 beta 1 integrin, like fibronectin, is primarily associated with the lens epithelial cells, where together they are likely to be important in regulation of adhesion and proliferation. alpha 6A beta 1, the integrin laminin receptor, is expressed at its highest levels in the equatorial epithelium and the peripheral fiber cells, both migratory populations. Because laminin is uniformly distributed in the lens capsule, such changes in alpha 6A integrin expression are likely critical to the cell's ability to regulate its response to laminin in the matrix. The organization of cytoskeletal molecules associated with the integrin cytoplasmic face also changes with development. In the epithelial regions of the lens, where the initiation of lens cell differentiation occurs, expression of the cytoskeletal proteins involved in cell-substrate interactions, talin, alpha-actinin, and the signaling proteins, are high. In the fiber cell region of the lens, where the cells establish stable cell-cell contacts, vinculin predominates and becomes highly associated with the cytoskeletal fraction. The role of integrins in lens development is not only regulated by changes in the expression of different integrin receptors but is also closely correlated with the expression and organization of the molecules with which they associate.

Characterization of acellular dermal matrices (ADMs) prepared by two different methods

The efficacy of acellular dermal matrix (ADM) in the treatment of full-thickness skin injuries as a dermal substitute depends on its low antigenicity, capacity for rapid vascularization, and stability as a dermal template. These properties will be determined largely by the final composition of the ADM. We have treated human skin with either Dispase followed by Triton X-100 detergent or NaCl followed by SDS detergent, cryosectioned the resulting ADMs, and then characterized them immunohistochemically. Staining for cell-associated antigens (HLA-ABC, HLA-DR, vimentin, desmin, talin), extracellular matrix components (chondroitin sulfate, fibronectin, laminin, vitronectin, hyaluronic acid), elastin, and collagen type VII was dramatically reduced or absent from ADMs prepared by both methods. However, significant amounts of elastin, keratan sulfate, laminin, and collagen types III and IV were still observed in both ADMs. Both methods of ADM preparation resulted in extensive extraction of both cellular and extracellular components of the skin but retention of the basic dermal architecture. In general, ADM prepared by the NaCl-SDS method retained larger amounts of each antigen than did that prepared by the Dispase-Triton method. This was most evident for laminin and type VII collagen but larger amounts of type IV collagen, fibronectin, desmin, elastin, and HLA-DR were also evident in the NaCl-SDS ADM.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Talin-null cells of Dictyostelium are strongly defective in adhesion to particle and substrate surfaces and slightly impaired in cytokinesis

Dictyostelium discoideum contains a full-length homologue of talin, a protein implicated in linkage of the actin system to sites of cell-to-substrate adhesion in fibroblasts and neuronal growth cones. Gene replacement eliminated the talin homologue in Dictyostelium and led to defects in phagocytosis and cell-to-substrate interaction of moving cells, two processes dependent on a continuous cross talk between the cell surface and underlying cytoskeleton. The uptake rate of yeast particles was reduced, and only bacteria devoid of the carbohydrate moiety of cell surface lipopolysaccharides were adhesive enough to be recruited by talin-null cells in suspension and phagocytosed. Cell-to-cell adhesion of undeveloped cells was strongly impaired in the absence of talin, in contrast with the cohesion of aggregating cells mediated by the phospholipid-anchored contact site A glycoprotein, which proved to be less talin dependent. The mutant cells were still capable of moving and responding to a chemoattractant, although they attached only loosely to a substrate via small areas of their surface. With their high proportion of binucleated cells, the talin-null mutants revealed interactions of the mitotic apparatus with the cell cortex that were not obvious in mononucleated cells.

Growth cone interactions with purified cell and substrate adhesion molecules visualized by interference reflection microscopy

The migration of growth cones on substrates consisting of naturally occurring cell adhesion molecules has been extensively studied in cell culture. However, relatively little is known about how growth cones contact the substrate or how the patterns of contact change as growth cones move forward. We have examined the interactions of chick retinal ganglion cell growth cones with laminin, merosin, N-cadherin, L1 and poly-L-lysine by time-lapse interference reflection microscopy (IRM) using a laser scanning confocal microscope. In images obtained by IRM, areas of a cell that are closely apposed to the substrate appear dark whereas areas that are farther away appear light. Growth cones on Jaminin and merosin were almost uniformly light, indicating that very little of the membrane was in close contact with the substrate. Growth cones on N-cadherin had a mottled appearance with some relatively large dark gray areas. The proximal portions of filopodia often were dark, in contrast to those on laminin and merosin which were light. In addition, growth cones on N-cadherin had numerous dark gray punctate regions of close association with the substrate. Growth cones on L1 had darker regions than growth cones on other substrates and these comprised a larger fraction of their area. There also were differences in the temporal dynamics of growth cone interactions with different substrates and these differences correlated with differences in rates of growth. None of the contacts observed in growth cones were as dark or stable as focal contacts of fibroblasts.

Perineurium talin immunoreactivity decreases in diabetic neuropathy

We studied the immunolocalization of Dp116 (a 116 kDa protein product of the dystrophin gene), vinculin, talin, vimentin, desmin, spectrin and titin in the sural nerve biopsies of 25 patients with peripheral neuropathies of different origin. 4 patients presented with HMSN type 1, 4 with HMSN type 2, 2 with HNPP, 4 with CIDP, 5 with chronic axonal neuropathy of unknown origin, 3 with vasculitic neuropathy, 3 with diabetic neuropathy. Expression and localization of Dp116, vinculin, vimentin, desmin, spectrin and titin did not differ from normal control cases. Spectrin and titin immunoreactivities were absent and desmin was occasionally found in few epineurial vessels. A thin rim of Dp116 binding surrounded the outermost layer of myelin sheaths. Perineurium and epineurial vessels stained deeply for vinculin. Vimentin immunoreactivity was seen in all endoneurial, perineurial and epineurial cells. Immunoreactivity for talin was normally found at endoneurial and epineurial vessel walls, perineurial cells and epineurial fibroblasts in all the sural nerves except diabetic nerves. In the latter, whereas talin binding was normal in the vessel walls and epineurial fibroblasts, it was markedly reduced in the perineurium. On immunoblot, two bands at 235 and 190 kDa were found in the sural nerves with the antibody anti-talin, and both were reduced only in the patients with diabetic neuropathy. We postulate that decreased perineurium talin in diabetic polyneuropathy may be related to the known alterations of the tight junctions of the perineurial cells, which have been proposed to be a contributory factor to impaired permeability barrier properties.

Tyrosine phosphorylation of pp125FAK and paxillin in aortic endothelial cells induced by mechanical strain

The objective of this study was to determine whether focal adhesion proteins pp125FAK (focal adhesion kinase) and paxillin are phosphorylated on tyrosine and might play a role in the morphological change and cell migration induced by strain. Bovine aortic endothelial cells (EC) were subjected to 10% average strain at 60 cycles/min. Cyclic strain increased the tyrosine phosphorylation of pp125FAK at 30 min (3.4-fold) and 4 h (5.9-fold) and the tyrosine phosphorylation of paxillin at 4 h (2.0-fold). Confocal microscopy showed that, after 4-h exposure to strain, EC began to elongate and F-actin, pp125FAK, and paxillin aligned, although they randomly distributed in static condition. Tyrosine kinase inhibitor tyrphostin A25 (100 microM) inhibited not only the tyrosine phosphorylation of pp125FAK and paxillin but also the redistribution of pp125FAK and paxillin, morphological change, and migration of EC induced by strain. These data demonstrate that cyclic strain induced tyrosine phosphorylation and reorganization of pp125FAK and paxillin and suggest that these focal adhesion proteins play a specific role in cyclic strain-induced morphological change and migration.

Developmental expression of dystrophin, dystrophin-associated glycoproteins and other membrane cytoskeletal proteins in human skeletal and heart muscle

Dystrophin, utrophin and the dystrophin-associated glycoproteins, beta-dystroglycan and adhalin, were analyzed, together with the membrane cytoskeletal proteins beta-spectrin, vinculin and talin, and adult and fetal myosin heavy chains, in 25 normal human fetuses from 8 to 24 weeks of gestation. Dystrophin was present in heart and skeletal muscle from 8 weeks although in the latter was mainly in the cytoplasm at this stage. Utrophin expression increased until around gestational weeks 19/21, but by 24 weeks immunostaining and immunoblot band intensities had reduced. Beta-dystroglycan was scarce in skeletal muscle at 8 weeks, increased with maturation and was more abundant in heart of the same age. Adhalin appeared later than beta-dystroglycan on skeletal muscle fiber surfaces, positivity became more intense as the fibers matured. In heart adhalin was detectable only in groups of cells at 12-16 weeks. From 8 weeks all fetal myotubes expressed beta-spectrin on their surfaces, while vinculin and talin positivity was mainly at the periphery of the fascicles, increasing with age. Adult slow myosin was seen in most myotubes at 10 weeks. Secondary myotubes then formed which increasingly expressed adult fast myosin, while still retaining fetal myosin. By 24 weeks most fibers expressing adult slow myosin had lost fetal myosin and were more mature in the expression of most membrane proteins. Muscle membrane organization during human fetal development is a complex process and takes place earlier in heart than skeletal muscle.

The influence of micro-topography on cellular response and the implications for silicone implants

Tissue attachment to substratum surfaces is of central importance to the in vivo performance of prosthetic implant materials. It is not yet understood why connective tissue does not attach to the surface of silicone or any other polymeric material. Recently the authors have conclusively demonstrated that micro-range surface roughness modifies cellular responses in cell culture and modifies biocompatibility and tissue attachment in vivo significantly. In order to better understand the basic interactions between living cells or tissues on one hand and man-made substratum surfaces on the other hand, the germane literature is reviewed here. Cells adhere to substratum surfaces mainly through focal adhesions which are a complex of intracellular transmembrane and extracellular proteins. Adhesion is facilitated and modified by proteins adsorbed to the substratum surface. Protein adsorption in turn is modified by the underlying substratum surface properties including surface chemistry, charge, and free energy. When silicone and other polymeric implants having well-defined surface topographic features including pores, pillars, or grooves were implanted, the tissue response to these implants was strongly influenced by the dimensions of these features as well as by other geometric details. Highest biocompatibility along with tissue attachment was seen when topographic features had dimensions of 1-3 microns and a uniform distribution. Cell culture studies revealed that topographic features affect cellular alignment, direction of proliferation, cellular attachment, growth rate, metabolism, and cytoskeletal arrangement. Since discontinuities or curvatures associated with topographic features may represent local changes in surface free energy, it is hypothesized that these discontinuities trigger changes in protein adsorption, protein configuration, and cellular response.

The Dictyostelium cytoskeleton

New avenues of cytoskeleton research in Dictyostelium discoideum have opened up with the cloning of the alpha- and beta-tubulin genes and the characterization of kinesins and cytoplasmic dynein. Much research, however, continues to focus on the actin cytoskeleton and its dynamics during chemotaxis, morphogenesis, and other motile processes. New actin-associated proteins are being identified and characterized by biochemical means and through isolation of mutants lacking individual components. This work is shedding light on the roles of specific actin assemblies in various biological processes.

Down regulation of talin alters cell adhesion and the processing of the alpha 5 beta 1 integrin

The role of talin was addressed by down regulating its expression using an antisense RNA strategy. HeLa cells were transfected with a talin 5′ cDNA fragment under the control of the inducible human metallothionein promotor. Isolated clones displayed a decrease in talin level down to 10% of control. The reduction in talin expression dramatically slowed down the kinetics of cell spreading. Mock-transfected cells, spread out onto fibronectin, exhibited large peripheral adhesion plaques. In contrast, cells with reduced talin expression showed smaller focal contacts localized all over the ventral face, and displayed a marked decrease in the number of stress fibers. Immunoprecipitation experiments carried out with a polyclonal antibody on surface-labeled receptor indicated a shift in the mobility for both alpha 5 and beta 1 subunits. Surprisingly, beta 1 integrin chains could not be detected by indirect immunofluorescence using monoclonal antibodies in talin deficient clones. Western blot analysis indicated the presence of two forms of beta 1. We analyzed the processing of beta 1 in normal and talin deficient cells using pulse chase experiments. Normal cells required a minimum of 5 hours for the processing of mature beta 1, while the talin deficient AT22 clone showed that the beta 1 precursor was slowly converted into a very low molecular mass product. Our data demonstrate that talin plays a central role in the establishment of cell-matrix contacts. In addition, down regulation of talin impairs the folding and processing of beta 1 integrins.

Interaction of the 47-kDa talin fragment and the 32-kDa vinculin fragment with acidic phospholipids: a computer analysis

In recent in vitro experiments, it has been demonstrated that the 47-kDa fragment of the talin molecule and the 32-kDa fragment of the vinculin molecule interact with acidic phospholipids. By using a computer analysis method, we determined the hydrophobic and amphipathic stretches of these fragments and, by applying a purpose-written matrix method, we ascertained the molecular amphipathic structure of alpha-helices. Calculations for the 47-kDa mouse talin fragment (residues 1-433; NH2-terminal region) suggest specific interactions of residues 21-39, 287-342, and 385-406 with acidic phospholipids and a general lipid-binding domain for mouse talin (primary amino acid sequence 385-401) and for Dictyostelium talin (primary amino acid sequence 348-364). Calculations for the 32-kDa chicken embryo vinculin fragment (residues 858-1066; COOH-terminal region) and from nematode vinculin alignment indicate for chicken embryo vinculin residues 935-978 and 1020-1040 interactions with acidic phospholipids. Experimental confirmation has been given for vinculin (residues 916-970), and future detailed experimental analyses are now needed to support the remaining computational data.

A talin homologue of Dictyostelium rapidly assembles at the leading edge of cells in response to chemoattractant

In an attempt to identify unknown actin-binding proteins in cells of Dictyostelium discoideum that may be involved in the control of cell motility and chemotaxis, monoclonal antibodies were raised against proteins that had been enriched on an F-actin affinity matrix. One antibody recognized a protein distinguished by its strong accumulation at the tips of filopods. These cell-surface extensions containing a core of bundled actin filaments are rapidly protruded and retracted by cells in the growth-phase stage. The protein of 269 kD turned out to resemble mouse fibroblast talin (Rees et al., 1990) in its primary structure. The fit is best among the first 400-amino acid residues of the NH2-terminal region where identity between the two proteins is 44% and the last 200-amino acid residues of the COOH-terminal region with 36% identity. In the elongated cells of the aggregation stage the Dictyostelium talin is accumulated at the entire front where also F-actin is enriched. Since this protein exists in a soluble state in the cytoplasm, mechanisms are predicted that cause accumulation at sites of the cell where a front is established. Evidence for receptor-mediated accumulation was obtained by local stimulation of cells with cAMP. When a new front was induced by the chemoattractant, the talin accumulated there within half a minute, indicating a signal cascade in Dictyostelium responsible for assembly of the talin beneath sites of the plasma membrane where chemoattractant receptors are strongly activated. The ordered assembly of the talin homologue together with actin and a series of other proteins is considered to play a key role in chemotactic orientation.

The effects of mechanical stimulation on the distribution of beta 1 integrin and expression of beta 1-integrin mRNA in TE-85 human osteosarcoma cells

Mechanical stimulation of the skeleton alters the metabolism of bone cells, but the effects of mechanical strain on the cytoskeleton of osteoblasts are poorly understood. While changes in the distribution of the cytoskeleton in mechanically strained cells have been reported, little is known about the pathways by which these changes are transduced into cell functions. Human osteosarcoma (HOS) TE-85 cells were cultured in Dubelcco's modified Eagle's medium/F-12 and grown to confluency in Flexercell type I dishes in a humidified incubator with 5% CO2 and 95% air. Intermittent strain (3 cycles/min) was applied to the cells for periods of 15 and 30 min, 2, 4 and 24 h, and 3, 5, 7, 10, 14, 20 and 28 days. Unstrained cells were used as controls. The distribution of beta 1 integrin was studied immunocytochemically. Total RNA was isolated at every period of time and Northern blots were used to study the effects of strain on the levels of beta 1-integrin expression. The results indicated that mechanical strain increased the synthesis of beta 1 integrin. Northern blots showed that beta 1 mRNA expression was increased significantly (p < 0.005) at 30 min and 3 days of strain application. Strain also affected beta 1 distribution markedly in 24-h cultures. The response of HOS cells to mechanical strain demonstrates that the cytoskeleton of the osteoblast adapts to strain through the stimulation of specific cytoskeletal and receptor proteins. These results suggest a pathway through which mechanical strain is transmitted to the osteoblastic-like cells.

Effects of microcystin-LR on actin and the actin-associated proteins alpha-actinin and talin in hepatocytes

Microcystin-LR (MCLR) is a commonly encountered blue-green algal hepatotoxin and a known inhibitor of cellular protein phosphatase types 1 and 2A. The toxin causes alterations in, and redistribution of, intermediate filaments, microtubules, and actin microfilaments (MFs) in affected cells. In this study, the effect of MCLR on the sequence of alterations in MFs and actin-associated proteins (AAPs) of isolated hepatocytes was examined in an effort to determine whether morphologic changes induced in MFs by microcystins are a result of prior dislocation of AAPs. We studied the effects of MCLR exposure on alpha-actinin and talin, two AAPs that play a role in the orientation of the MFs. Primary hepatocytes were incubated with 10 microns MCLR for 0-64 min. The distribution of actin, alpha-actinin, and talin were examined using fluorescence microscopy. MCLR induced similar changes in the distribution of actin and the AAPs. Actin filament redistribution was first observed after 12 min of MCLR exposure, and was characterized by detachment of MFs from the cell periphery, followed by condensation at distinct focal points and progressive collapse into the interior of affected cells. Changes in alpha-actinin and talin distribution were first observed after 20 min of toxin exposure. The AAPs appeared to detach from focal contacts on the cytoplasmic surface of the plasma membrane, condense into cytoplasmic aggregates, and ultimately collapse into a juxtanuclear bundle. The results of this study indicate that, in hepatocytes exposed to MCLR, the collapse of actin MFs occurs prior to the dislocation of alpha-actinin and talin. Changes in these actin associated proteins are not likely to account for the initial changes in actin MFs.

Disruption of microtubules in vivo by vincristine induces large membrane complexes and other cytoplasmic abnormalities in megakaryocytes and platelets of normal rats like those in human and Wistar Furth rat hereditary macrothrombocytopenias

Abnormal organization of platelet microtubules is associated with abnormal platelet formation in hereditary macrothrombocytopenias such as the gray platelet syndrome, May-Hegglin anomaly, and Epstein's syndrome, and that of the Wistar Furth rat, suggesting that aberrant microtubule organization may contribute to defective platelet formation in these clinical entities. Here, we examined the consequence of microtubule disruption on the organization of megakaryocyte cytoplasmic organelles using the microtubule depolymerizing agent, vincristine (VCR). Wistar rat bone marrow was fixed and processed for transmission electron microscopy after VCR administration alone, after 5-fluorouracil (5-FU) administration alone, or after 5-FU followed by intravenous injection of 0.1-1.0 mg/kg VCR for intervals of 30 min to 8 hr. 5-FU was given to increase megakaryocyte frequency to facilitate ultrastructural evaluations. VCR alone or in combination with 5-FU caused formation of large membrane complexes in the cytoplasm of Wistar rat megakaryocytes at all dosages studied, identical to those found in megakaryocytes of human hereditary macrothrombocytopenias and the Wistar Furth rat. The proportion of megakaryocytes with these large membrane complexes increased with time after 5-FU and VCR, and was maximal (approximately two-thirds of megakaryocytes) at VCR dosages of 0.75-1.0 mg/kg. The majority of megakaryocytes displayed other abnormalities, including blebbing of plasma membranes, an increased number of dense compartments, dilated demarcation membrane (DMS) channels, which contained dense material immunocytochemically identified as secreted alpha-granule proteins, and an increased incidence of emperipolesis. Rats administered 5-FU alone did not demonstrate these abnormalities, with the exception of an increase in dense compartments. Platelets from rats treated with VCR alone or 5-FU and VCR also showed abnormalities including membrane complexes, rounded shape, formation of tubulin paracrystals, development of membrane blebs, and the presence of proteinaceous material within the cisternae of the surface-connected canalicular system (SCCS). The membrane complexes in platelets of 5-FU-, VCR-treated Wistar rats as well as untreated Wistar Furth rats were composed of elements of both the SCCS and dense tubular system; membrane complexes in megakaryocytes of 5-FU-, VCR-treated rats were composed of both DMS and smooth endoplasmic reticulum. We conclude that intact microtubules play a major role in the organization of the megakaryocyte DMS and may contribute to the stability of megakaryocyte alpha-granules.

Platelet talin is phosphorylated by calyculin A

Calyculin A and okadaic acid, potent and cell permeable inhibitors of type 1 and type 2A protein phosphatases, inhibit platelet aggregation and secretion. However, the relationship between phosphatase inhibition and inhibition of platelet function is not well understood. We found that in unstimulated platelets, talin (P235) was phosphorylated at threonine residues by calyculin A. Furthermore, the extent of talin phosphorylation by calyculin A was closely correlated with its inhibition of thrombin-induced platelet aggregation. Since the binding of talin to platelet glycoprotein IIb/IIIa complex has been shown to be affected by its phosphorylation, these results suggest that type 1 and/or type 2A protein phosphatases may play a role in the regulation of membrane-cytoskeleton interaction through dephosphorylation of talin.

Focal contact assembly through cytoskeletal polymerization: steady state analysis

For many cell types, initial receptor-mediated attachment to a ligand-coated surface is followed by the formation of focal contacts--strong, specialized, discrete adhesive connections between cell and substrate in which receptors are clustered and simultaneously linked to extracellular ligand and cytoskeletal proteins. Since adhesion affects many aspects of cellular physiology including growth, differentiation, and motility, understanding the biochemical factors which regulate focal contact assembly should enhance our understanding of these phenomena. In this paper, we present a mathematical model to examine how receptor-ligand, receptor-cytoskeleton, and cytoskeleton-cytoskeleton interactions affect the formation of receptor clusters which serve as precursors to mature focal contacts. Receptor clustering is presumed to occur through self-recognition of cytoskeletal elements which induce the polymerization of ligand-receptor-cytoskeleton complexes. Polymerization only occurs when the ligand density is above a critical value and a decrease in the receptor-ligand affinity shifts the critical ligand density to higher values. While cytoskeletal protein expression and receptor-cytoskeleton affinity influence the concentration of monomeric complexes, the formation of polymeric ligand-receptor-cytoskeleton aggregates is most sensitive to changes in the self-association affinity between cytoskeletal proteins. We find that a 100-fold enhancement in the affinity between cytoskeletal elements can produce a substantial increase in the total fraction of adhesion receptors associated with focal contact precursors (from 5% to over 90%). Our results suggest that under physiological conditions, cellular control of focal contact assembly most likely occurs through modulation of specific cytoskeletal proteins to solidify cytoskeleton-cytoskeleton connections within precursor focal contact structures.

Effects of beta subunit cytoplasmic domain deletions on the recruitment of the integrin alpha v beta 6 to focal contacts

The integrin alpha v beta 6 is expressed primarily in epithelial cells and mediates attachment to the extracellular matrix protein fibronectin. We recently observed that alpha v beta 6 localizes to focal contacts when cells that express the receptor are plated on fibronectin. To determine which regions of the beta 6 cytoplasmic domain are required for recruitment to focal contacts, the wild type human beta 6 cytoplasmic domain are required for recruitment to focal contacts, the wild type human beta 6 subunit and mutants containing selective deletions within the beta 6 cytoplasmic domain were stably expressed in Chinese Hamster Ovary cells. The beta 6 cytoplasmic domain is composed of a 41 amino acid region that is highly conserved among integrin beta subunits, and a unique 11 amino acid carboxy terminal extension. The 11 amino acid extension was not required for localization to focal contacts; in fact, its removal appeared to increase receptor localization. However, removal of any of the 3 conserved regions previously identified as important for the localization of beta 1 integrins to focal contacts (Reszka, et al., 1992, J. Cell Biol. 117:1321-30) eliminated recruitment of beta 6 to focal contacts. These data suggest that, as with the integrin beta 1 subunit, multiple regions within the first 41 amino acids of the beta 6 cytoplasmic domain are necessary for recruitment of alpha v beta 6 to focal contacts. However, the unique 11 amino acid carboxy terminus of beta 6 is not required for this recruitment, and may actually play a negative regulatory role.

Talin, vinculin and DRP (utrophin) concentrations are increased at mdx myotendinous junctions following onset of necrosis

Duchenne muscular dystrophy (DMD) and the myopathy seen in the mdx mouse both result from absence of the protein dystrophin. Structural similarities between dystrophin and other cytoskeletal proteins, its enrichment at myotendinous junctions, and its indirect association with laminin mediated by a transmembrane glycoprotein complex suggest that one of dystrophin's functions in normal muscle is to form one of the links between the actin cytoskeleton and the extracellular matrix. Unlike Duchenne muscular dystrophy patients, mdx mice suffer only transient muscle necrosis, and are able to regenerate damaged muscle tissue. The present study tests the hypothesis that mdx mice partially compensate for dystrophin's absence by upregulating one or more dystrophin-independent mechanisms of cytoskeleton-membrane association. Quantitative analysis of immunoblots of adult mdx muscle samples showed an increase of approximately 200% for vinculin and talin, cytoskeletal proteins that mediate thin filament-membrane interactions at myotendinous junctions. Blots also showed an increase (143%) in the dystrophin-related protein called utrophin, another myotendinous junction constituent, which may be able to substitute for dystrophin directly. Muscle samples from 2-week-old animals, a period immediately preceding the onset of muscle necrosis, showed no significant differences in protein concentration between mdx and controls. Quantitative analyses of confocal images of myotendinous junctions from mdx and control muscles show significantly higher concentrations of talin and vinculin at the myotendinous junctions of mdx muscle. These findings indicate that mdx mice may compensate in part for the absence of dystrophin by increased expression of other molecules that subsume dystrophin's mechanical function.

Rearrangement of mRNAs for costamere proteins during costamere development in cultured skeletal muscle from chicken

Mature skeletal myofibrils are surrounded by costameres, ribs of metavinculin, vinculin, intermediate filaments, and other proteins that connect the myofibril to the extracellular matrix. Costameres have recently been shown to be the sites at which the forces generated by the myofibril are transduced laterally into the extracellular matrix. We observed costameres developing in cultured skeletal muscles, grown in micromass culture from cells taken from embryonic chicken leg. We detected proteins by immunofluorescence and mRNA by in situ hybridization. Antibody and probe signals were imaged by laser scanning confocal microscopy. Antibody to vimentin protein is first detected in stripes in register with the Z line of the myofibril, at approximately day 12 after fusion; soon thereafter probe to vimentin mRNA is also detected in the same stripes. Optical sections indicate that vimentin mRNA and protein are very close, no more than 0.1 mm apart and possibly in immediate contact. Antibody to vimentin is detected in stripes only in cells that twitch spontaneously. Antibodies and probes to desmin and vinculin protein and mRNA are next detected in stripes of the same periodicity, at approximately day 17 after fusion. Vinculin protein (but not mRNA) is detected at focal contacts much earlier in development. Controls for bleed through of fluorescence, RNase H sensitivity, hybridization without probe, and binding to the myofibril all gave appropriate results. Probes to glyceraldehyde-3-phosphate dehydrogenase, a glycolytic enzyme, stained diffusely and did not associate with the myofibril. These results show that components of the costamere arrive at the structure in a defined sequence, and that mRNA organization is a conspicuous, precise and temporally controlled aspect of costamere development. These results may have wider implications. In these cells, some mRNAs are positioned with submicrometer precision in space and differentially over time. Particular mRNAs differ in the time and place of such positioning. This implies both that cellular structures provide physical cues for such positioning and that mRNA contains information that interacts with such cues in a message-specific manner. If such precision in mRNA location is found in other somatic cells, it could have significant implications for the ways in which cells generate and maintain cellular structures.

Biochemical and molecular characterization of the chicken cysteine-rich protein, a developmentally regulated LIM-domain protein that is associated with the actin cytoskeleton

LIM domains are present in a number of proteins including transcription factors, a proto-oncogene product, and the adhesion plaque protein zyxin. The LIM domain exhibits a characteristic arrangement of cysteine and histidine residues and represents a novel zinc binding sequence (Michelsen et al., 1993). Previously, we reported the identification of a 23-kD protein that interacts with zyxin in vitro (Sadler et al., 1992). In this report, we describe the purification and characterization of this 23-kD zyxin-binding protein from avian smooth muscle. Isolation of a cDNA encoding the 23-kD protein has revealed that it consists of 192 amino acids and exhibits two copies of the LIM motif. The 23-kD protein is 91% identical to the human cysteine-rich protein (hCRP); therefore we refer to it as the chicken cysteine-rich protein (cCRP). Examination of a number of chick embryonic tissues by Western immunoblot analysis reveals that cCRP exhibits tissue-specific expression. cCRP is most prominent in tissues that are enriched in smooth muscle cells, such as gizzard, stomach, and intestine. In primary cell cultures derived from embryonic gizzard, differentiated smooth muscle cells exhibit the most striking staining with anti-cCRP antibodies. We have performed quantitative Western immunoblot analysis of cCRP, zyxin, and alpha-actinin levels during embryogenesis. By this approach, we have demonstrated that the expression of cCRP is developmentally regulated.

Occurrence of fibers and their association with talin in the cleavage furrows of PtK2 cells

PtK2 cells of exceptionally large size were microinjected with fluorescently labeled probes for actin, myosin, filamin, and talin in order to follow the assembly of the contractile proteins into the cleavage furrows. Whereas in cells of normal size, there is usually a diffuse pattern of localization of proteins in the cleavage furrow, in these large, flat cells the labeled proteins localized in fibers in the cleavage furrow. Often, the fibers were striated in a pattern comparable to that measured in the stress fibers of the same cell type. The presence of talin in discrete plaques along fibers in the cleavage furrows of the large cells suggests a further similarity between cleavage furrow and stress fiber structure. The presence of filamin in the cleavage furrows also suggests the possibility of an overlapping mechanism in addition to that of a talin mediated mechanism for the attachment of actin filaments to the cell surfaces in the cleavage furrow. A model is presented that emphasizes the interrelationships between stress fibers, myofibrils, and cleavage furrows.

Molecular and cellular concepts in atherosclerosis

Atherosclerosis is a complex disease of uncertain cause. Its pathobiology is believed to represent an abnormal expression of the processes of vascular healing. Etiologic models derive from a 'response to injury' paradigm and can be divided into three separate disease stages: endothelial dysfunction, smooth muscle proliferation and architectural disruption. The initiating event of endothelial dysfunction is unknown, but is believed to be related to low-density lipoproteins and/or their oxidized derivatives. Endothelial injury is signalled to the smooth muscle cells of the media by three routes: direct cell-cell interaction, secretion of soluble growth factors and monocyte-derived cytokines. Monocytes are recruited by the endothelium and invade the subintimal space by a complex interaction of a variety of adhesion proteins and receptors on both cell types. Smooth muscle cell proliferation is initiated by a change in phenotype expression from 'contractile' to 'synthetic' resulting from the binding of fibronectin to specific integrin receptors. Three functionally distinct activities may represent separate subtypes of the 'synthetic phenotype': migration from the media to the intima, increased proliferation and inappropriate extracellular matrix synthesis. The loss of normal regulatory control and anchorage independence of proliferation suggest a relationship to oncogenic transformation. Both migration and proliferation result from the binding of platelet-derived growth factor-like factors to smooth muscle cell receptors, which initiates a cascade of intracellular molecular events leading either to cytoskeletal locomotory restructuring or cell cycle activation. Both pathways also appear to be coregulated by integrin receptors and both depend upon phosphorylation of cell membrane, cytosolic and nuclear regulatory proteins. Clinical expression of atherosclerosis may follow sudden loss of architectural integrity of the intimal plaque by three different mechanisms: plaque fissuring, intraluminal plaque rupture or intramural hemorrhage related to abnormal vessel wall stress and/or biochemistry.

Talin distribution and phosphorylation in thrombin-activated platelets

We have previously demonstrated that the subcellular distribution of the adhesion plaque protein, talin, changes dramatically in human platelets in response to platelet activation (Beckerle et al., J. Cell Biol. 109, 3333–3346, 1989). Talin is uniformly distributed throughout the cytoplasm of resting platelets. However, when platelets are stimulated to become activated and adhesive, a significant amount of the talin population rapidly redistributes to a peripheral, submembranous location. In the present study we have examined talin phosphorylation and proteolytic cleavage as possible mechanisms by which talin's subcellular distribution could be regulated in platelets. We have found that thrombin activation of platelets leads to a fourfold increase in talin phosphorylation. Proteolytic cleavage of talin, however, is not detected in washed platelets activated with thrombin for as long as 30 minutes. Because talin moves to a submembranous location upon platelet activation and has been shown to interact with integrins in vitro, we also investigated whether the major platelet integrin, GPIIb-IIIa, is required for talin redistribution. Using Glanzmann thrombasthenic platelets, which are deficient in GPIIb-IIIa, we found that talin redistribution occurs even in the absence of GPIIb-IIIa. Collectively, our studies suggest that neither proteolytic cleavage of talin nor interactions between talin and GPIIb-IIIa is required for the regulated redistribution of talin in thrombin-activated platelets. Phosphorylation of talin in response to thrombin activation may, however, be one mechanism utilized by platelets to regulate talin distribution and function in human platelets.

Talin: adherens junction protein is localized at the epidermal-dermal interface in skin

The interaction between cells of the epidermis and the basal lamina is important for the integrity of the skin. Several hereditary and acquired diseases show changes at the dermal-epidermal interface due to loss of adhesion between basal cells and the basement membrane. The structures mediating this interaction are hemidesmosomes, which have been extensively characterized by biochemical, molecular biologic, and morphologic techniques. Recently, however, a group of adhesion molecules that are distinct from hemidesmosomes and that mediate cell-matrix interactions was described in cultured fibroblasts, keratinocytes, and skin. These adhesion molecules, beta 1 integrins, have been shown to be present in the focal adhesion, a cell-matrix contact associated with microfilaments rather than intermediate filaments characteristic of hemidesmosomes. In cultured cells, integrins of the beta 1 family have been shown to be linked by a protein complex to actin filaments. In this study we describe the localization of talin, the binding protein for beta 1 integrins, and vinculin at the dermal-epidermal interface in skin with immunofluorescence and immunoblotting techniques. These data suggest the presence of a link between the cytoplasmic actin filament system in basal keratinocytes and the extracellular matrix.

A review of the role of platelet membrane glycoproteins in the platelet-vessel wall interaction

This review concerns our understanding of the molecular basis of platelet function in haemostasis. In particular, we indicate how research into platelet membrane glycoprotein (GP) receptors is yielding vital information on the mechanisms of platelet adhesion and aggregation. These receptors, nearly always complexes of two or more subunits, are now known to belong to distinct gene families, some of which are unique to platelets while others are widely distributed in mammalian tissues. GP Ib-IX complexes are responsible for the high-shear-rate-dependent adhesion of platelets to von Willebrand factor (vWF) exposed within the subendothelium of damaged vessels. Other adhesion receptors include members of the VLA subclass of the integrin family: VLA-2, VLA-5 and VLA-6, which mediate platelet adhesion to collagen, fibronectin and laminin, respectively. Platelet aggregation is initiated by distinct populations of receptors specific for each physiological agonist. Many of these receptors, including the highly important and recently cloned thrombin receptor, have seven transmembrane domains and possess highly selective agonist-binding determinants. Finally, we highlight platelet aggregation and the role of GP IIb-IIIa complexes which, following platelet activation, bind fibrinogen and other adhesive proteins. The latter, through being polyvalent for GP IIb-IIIa, then form the bridges linking adjoining platelets. The 'ligand-binding pocket' of GP IIb-IIIa contains at least three sequences essential for ligand binding; fibrinogen also binds to the activated complex through identified domains, one of which, the Arg-Gly-Asp (RGD) sequence, is also found in vWF and the other adhesive proteins able to support platelet aggregation. Finally, we further describe how these, and other glycoproteins in both surface and internal membrane systems, constitute a complex receptor network capable of translocation and reorganization after platelet activation. In cardiovascular disease, platelets accumulate within arteries whose luminal surface has been modified through atherosclerosis. Recent molecular advances are yielding exciting opportunities for the development of new, and more powerful, drugs acting as specific inhibitors of thrombotic processes.

Influence of cold preservation on the cytoskeleton of cultured pulmonary arterial endothelial cells

In donor lungs preserved for transplantation, pulmonary arterial endothelial cells become thin and partially detached from the basement membrane at 4 degrees C, recovering slowly after transplantation. These changes have now been modeled in vitro. Porcine pulmonary arterial endothelial cell monolayers were incubated at 4 degrees C for 2 or 4 h, rewarmed to 37 degrees C, and incubated for up to 24 h. Responses were studied using wound healing assays, bead phagocytosis, immunostaining of cytoskeletal components, and quantification of actin by SDS-PAGE and immunoblotting. Cooling caused cessation of cell movement and phagocytosis associated with depolymerization of the cytoskeleton. Depolymerization of microtubules was complete after 2 h but 14.6% of actin filaments remained (SDS-PAGE) after 4 h at 4 degrees C. Loss of actin stress fibers paralleled the disappearance of vinculin/talin co-labeling focal adhesions. However, a fiber network at the inner surface of the cell membrane labeling for talin was stable at 4 degrees C. After rewarming, the rate of cell movement and phagocytosis immediately returned to normal. Actin filaments and thin stress fibers were present by 1 h, although poorly organized, and actin had increased to 68.2% of control. Many small vinculin/talin focal adhesions had formed. Microtubules redeveloped by 1 h. The cytoskeleton of cultured human pulmonary arterial endothelial cells showed similar changes. In conclusion, the cytoskeletal changes help explain in vivo observations. On rewarming, the endothelial cells appeared to recover rapidly, but the abnormal appearance of the reformed cytoskeleton suggests an interim period of instability which may have metabolic and mechanical consequences.

Alpha 3 beta 1 integrin is moved into focal contacts in kidney mesangial cells

The movement of integrins into focal adhesive structures accompanies cell attachment to extracellular matrix. The kinetics of incorporation of integrins into focal contacts was studied during attachment to matrix of mesangial cells of the kidney glomerulus. On collagen, fibronectin, laminin and vitronectin, the number and intensity of talin-focal contacts increased with time. Talin-containing focal contacts were present in mesangial cells within 2 h of plating and in control cells (HT1080 and Rugli) within 1 h. Integrin alpha-chains colocalized with talin, dependent on the matrix substrate. The attachment, spreading and organization of integrin into focal contacts was not affected when endogenous protein synthesis was suppressed with cycloheximide. In Rugli, alpha 1 beta 1 organized into focal contacts on collagen and laminin, while in HT1080 alpha 2 beta 1 organized on collagen type I, alpha 5 beta 1 on fibronectin, alpha 6 beta 1 on laminin, and alpha 3 beta 1 and alpha 4 beta 1 were diffusely distributed on all substrates. These distributions mirrored the usage and expression patterns previously established for integrins in these cells and was as predicted from the literature. In mesangial cells, however, alpha 3 beta 1 was also organized into prominent focal contact arrays on collagen, fibronectin, EHS and human placental laminins, but not on vitronectin, while alpha 6 beta 1 was not organized. Initial attachment and spreading of mesangial cells was absolutely dependent on divalent cations. Mg2+ and Mn2+ supported attachment on all substrates, while Ca2+ stimulated attachment on laminin (E8), fibronectin and vitronectin. The data suggest that the functional integrins on mesangial cells include alpha 1 beta 1 (on collagen and laminin) alpha 2 beta 1 (on collagen), alpha 5 beta 1 (on fibronectin) and alpha V beta 3 (on vitronectin). However, mesangial cells do not use alpha 6 beta 1 on laminin, and the data support a role for alpha 3 beta 1 as putative receptor for fibronectin, collagen and laminin.

Evidence for the selective association of a subpopulation of GPIIb-IIIa with the actin cytoskeletons of thrombin-activated platelets

Activation of blood platelets triggers a series of responses leading to the formation and retraction of blood clots. Among these responses is the establishment of integrin-mediated transmembrane connections between extracellular matrix components and the actin cytoskeleton of the platelet. Here we report that a specific subpopulation of the major platelet integrin, glycoprotein IIb-IIIa (GPIIb-IIIa) (also referred to as alpha IIb beta 3 integrin), becomes incorporated into the detergent-insoluble actin cytoskeleton of platelets during the platelet activation response. The cytoskeletal association of GPIIb-IIIa is independent of platelet aggregation and fibrin sedimentation and is sensitive to cytochalasin D treatment. As determined by Western immunoblot analysis, approximately 22% of the total cellular GPIIb-IIIa becomes associated with the actin cytoskeleton upon thrombin activation in a manner that is independent of the detection of talin, alpha-actinin, or vinculin in the complex. We found that the cytoskeleton-associated GPIIb-IIIa is derived from an intracellular source since it is not available for lactoperoxidase-catalyzed radioiodination before platelet activation. Two intracellular sources of GPIIb-IIIa are present in resting platelets: GPIIb-IIIa associated with the alpha-granule secretory compartment as well as surface-inaccessible domains of the surface-connected canalicular system. Interestingly, alpha-granule secretion, which occurs in thrombin-activated platelets and results in the translocation of intracellular GPIIb-IIIa to the plasma membrane, appears to be required for the cytoskeleton incorporation of GPIIb-IIIa that we observe. Collectively, our data provide evidence that a subpopulation of GPIIb-IIIa derived from an intracellular source is selectively linked to the actin cytoskeleton of platelets upon thrombin activation in the absence of platelet aggregation.

Zyxin and cCRP: two interactive LIM domain proteins associated with the cytoskeleton

Interaction with extracellular matrix can trigger a variety of responses by cells including changes in specific gene expression and cell differentiation. The mechanism by which cell surface events are coupled to the transcriptional machinery is not understood, however, proteins localized at sites of cell-substratum contact are likely to function as signal transducers. We have recently purified and characterized a low abundance adhesion plaque protein called zyxin (Crawford, A. W., and M. C. Beckerle. 1991. J. Biol. Chem. 266:5847-5853; Crawford, A. W., J. W. Michelsen, and M. C. Beckerle. 1992. J. Cell Biol. 116:1381-1393). We have now isolated and sequenced zyxin cDNA and we report here that zyxin exhibits an unusual proline-rich NH2-terminus followed by three tandemly arrayed LIM domains. LIM domains have previously been identified in proteins that play important roles in transcriptional regulation and cellular differentiation. LIM domains have been proposed to coordinate metal ions and we have demonstrated by atomic absorption spectroscopy that purified zyxin binds zinc, a result consistent with the idea that zyxin has zinc fingers. In addition, we have discovered that zyxin interacts in vitro with a 23-kD protein that also exhibits LIM domains. Microsequence analysis has revealed that the 23-kD protein (or cCRP) is the chicken homologue of the human cysteine-rich protein (hCRP). By double-label indirect immunofluorescence, we found that zyxin and cCRP are extensively colocalized in chicken embryo fibroblasts, consistent with the idea that they interact in vivo. We conclude that LIM domains are zinc-binding sequences that may be involved in protein-protein interactions. The demonstration that two cytoskeletal proteins, zyxin and cCRP, share a sequence motif with proteins important for transcriptional regulation raises the possibility that zyxin and cCRP are components of a signal transduction pathway that mediates adhesion-stimulated changes in gene expression.

Cytoskeleton--plasma membrane interactions

Proteins at the boundary between the cytoskeleton and the plasma membrane control cell shape, delimit specialized membrane domains, and stabilize attachments to other cells and to the substrate. These proteins also regulate cell locomotion and cytoplasmic responses to growth factors and other external stimuli. This diversity of cellular functions is matched by the large number of biochemical mechanisms that mediate the connections between membrane proteins and the underlying cytoskeleton, the so-called membrane skeleton. General organizational themes are beginning to emerge from examination of this biochemical diversity.

Costameres are sites of force transmission to the substratum in adult rat cardiomyocytes

Costameres, the vinculin-rich, sub-membranous transverse ribs found in many skeletal and cardiac muscle cells (Pardo, J. V., J. D. Siciliano, and S. W. Craig. 1983. Proc. Natl. Acad. Sci. USA. 80:363-367.) are thought to anchor the Z-lines of the myofibrils to the sarcolemma. In addition, it has been postulated that costameres provide mechanical linkage between the cells' internal contractile machinery and the extracellular matrix, but direct evidence for this supposition has been lacking. By combining the flexible silicone rubber substratum technique (Harris, A. K., P. Wild, and D. Stopak. 1980. Science (Wash. DC). 208:177-179.) with the microinjection of fluorescently labeled vinculin and alpha-actinin, we have been able to correlate the distribution of costameres in adult rat cardiac myocytes with the pattern of forces these cells exert on the flexible substratum. In addition, we used interference reflection microscopy to identify areas of the cells which are in close contact to the underlying substratum. Our results indicate that, in older cell cultures, costameres can transmit forces to the extracellular environment. We base this conclusion on the following observations: (a) adult rat heart cells, cultured on the silicone rubber substratum for 8 or more days, produce pleat-like wrinkles during contraction, which diminish or disappear during relaxation; (b) the pleat-like wrinkles form between adjacent alpha-actinin-positive Z-lines; (c) the presence of pleat-like wrinkles is always associated with a periodic, "costameric" distribution of vinculin in the areas where the pleats form; and (d) a banded or periodic pattern of dark gray or close contacts (as determined by interference reflection microscopy) has been observed in many cells which have been in culture for eight or more days, and these close contacts contain vinculin. A surprising finding is that vinculin can be found in a costameric pattern in cells which are contracting, but not producing pleat-like wrinkles in the substratum. This suggests that additional proteins or posttranslational modifications of known costamere proteins are necessary to form a continuous linkage between the myofibrils and the extracellular matrix. These results confirm the hypothesis that costameres mechanically link the myofibrils to the extracellular matrix. We put forth the hypothesis that costameres are composite structures, made up of many protein components; some of these components function primarily to anchor myofibrils to the sarcolemma, while others form transmembrane linkages to the extracellular matrix.

Probing actin and liposome interaction of talin and talin-vinculin complexes: a kinetic, thermodynamic and lipid labeling study

Talin purified from human platelets and chicken gizzard smooth muscle is an actin and lipid binding protein. Here, we have investigated the effect of vinculin on (a) talin-nucleated actin polymerization and (b) insertion of talin into lipid bilayers. Calorimetric data show ternary complex formation between talin, vinculin, and actin. Actin-talin, actin-vinculin and actin-(talin-vinculin) binding and rate constants as well as actin polymerization rates for all three protein species have been determined by steady state titration, stopped-flow, and fluorescence assay. In contrast to an increase of the polymerization rate by a factor of less than 2 for actin-talin and actin-(talin-vinculin) when lowering the temperature, we measured a decrease in rates for actin alone and actin-vinculin. The overall equilibrium constants (Keq) in the van't Hoff plot proved linear and were of one-step reactions. Thermodynamic data exhibited signs of van der Waal's binding forces. Using the photoactivatable lipid analogue [3H]PTPC/11, which selectively labels membrane-embedded hydrophobic domains of proteins, we also show that talin partially inserts into the hydrophobic bilayer of liposomes. This insertion occurs in a similar manner irrespective of preincubation with vinculin.

The vinculin/sarcomeric-alpha-actinin/alpha-actin nexus in cultured cardiac myocytes

Experiments are described supporting the proposition that the assembly of stress fibers in non-muscle cells and the assembly of myofibrils in cardiac cells share conserved mechanisms. Double staining with a battery of labeled antibodies against membrane-associated proteins, myofibrillar proteins, and stress fiber proteins reveals the following: (a) dissociated, cultured cardiac myocytes reconstitute intercalated discs consisting of adherens junctions (AJs) and desmosomes at sites of cell-cell contact and sub-sarcolemmal adhesion plaques (SAPs) at sites of cell-substrate contact; (b) each AJ or SAP associates proximally with a striated myofibril, and conversely every striated myofibril is capped at either end by an AJ or a SAP; (C) the invariant association between a given myofibril and its SAP is especially prominent at the earliest stages of myofibrillogenesis; nascent myofibrils are capped by oppositely oriented SAPs; (d) the insertion of nascent myofibrils into AJs or into SAPs invariably involves vinculin, alpha-actin, and sarcomeric alpha-actinin (s-alpha-actinin); (e) AJs are positive for A-CAM but negative for talin and integrin; SAPs lack A-CAM but are positive for talin and integrin; (f) in cardiac cells all alpha-actinin-containing structures invariably are positive for the sarcomeric isoform, alpha-actin and related sarcomeric proteins; they lack non-s-alpha-actinin, gamma-actin, and caldesmon; (g) in fibroblasts all alpha-actinin-containing structures are positive for the non-sarcomeric isoform, gamma-actin, and related non-sarcomeric proteins, including caldesmon; and (h) myocytes differ from all other types of adherent cultured cells in that they do not assemble authentic stress fibers; instead they assemble stress fiber-like structures of linearly aligned I-Z-I-like complexes consisting exclusively of sarcomeric proteins.

An interaction between zyxin and alpha-actinin

Zyxin is an 82-kD protein first identified as a component of adhesion plaques and the termini of stress fibers near where they associate with the cytoplasmic face of the adhesive membrane. We report here that zyxin interacts with the actin cross-linking protein alpha-actinin. Zyxin cosediments with filamentous actin in an alpha-actinin-dependent manner and an association between zyxin and alpha-actinin is observed in solution by analytical gel filtration. The specificity of the interaction between zyxin and alpha-actinin was demonstrated by blot overlay experiments in which 125I-zyxin recognizes most prominently alpha-actinin among a complex mixture of proteins extracted from avian smooth muscle. By these blot overlay binding studies, we determined that zyxin interacts with the NH2-terminal 27-kD domain of alpha-actinin, a region that also contains the actin binding site. Solid phase binding assays were performed to evaluate further the specificity of the binding and to determine the affinity of the zyxin-alpha-actinin interaction. By these approaches we have demonstrated a specific, saturable, moderate-affinity interaction between zyxin and alpha-actinin. Furthermore, double-label immunofluorescence reveals that zyxin and alpha-actinin exhibit extensive overlap in their subcellular distributions in both chicken embryo fibroblasts and pigmented retinal epithelial cells. The significant colocalization of the two proteins is consistent with the possibility that the interaction between zyxin and alpha-actinin has a biologically relevant role in coordinating membrane-cytoskeletal interactions.

Identification of a new hemidesmosomal protein, HD1: a major, high molecular mass component of isolated hemidesmosomes

Hemidesmosomes (HDs) mediate cell adhesion to the extracellular matrix and have morphological association with intermediate-sized filaments (IFs) through cytoplasmic plaques. Though several proteins have been located in HDs, most of them have not been well characterized, with the exception of the 230-kD antigen of bullous pemphigoid (BP), an autoimmune skin blistering disease. Only recently we have succeeded in isolating HDs from bovine corneal epithelial cells and in identifying five major components on SDS-PAGE (Owaribe K., Y. Nishizawa, and W. W. Franke. 1991. Exp. Cell Res. 192:622-630). In this study we report on immunological characterization of one of the major components, termed HD1, with an apparent molecular mass of 500 kD. Immunofluorescence microscopy showed colocalization of HD1 with BP antigen at the basement membrane zone of those tissues that have typical HDs, including skin epidermis, corneal and tracheal epithelia, and myoepithelium. In cultured keratinocytes, HD1 demonstrated colocalization with BP antigen in the precise way, while being absent from focal adhesions. Immunoelectron microscopy revealed that an epitope of HD1 was located on the cytoplasmic side of HDs. Taking all these results together, we conclude that HD1 is a new hemidesmosomal component. Interestingly, HD1 also exists in endothelial and glial cells, which lack typical HDs.

Dystrophin at the plasma membrane of human muscle fibers shows a costameric localization

We studied the distribution of dystrophin at the sarcolemma of normal human muscle fibers using high resolution immunofluorescence and confocal laser scanning optical microscopy (CLSOM). We found that the dystrophin lattice is organized at the muscle plasma membrane in an array of thick bands interconnected by a finer network. The bands encircle the muscle fiber perpendicular to the long axis of the fiber and they matched the sites of attachment of the sarcomeres to the plasma membrane. Dystrophin co-localized with vinculin, and dystrophin and vinculin co-localized with alpha-actinin at the region of the I-band. Dystrophin may be one of the proteins involved in the linkage of the sarcomeres to the extracellular matrix.

Junctional epidermolysis bullosa keratinocytes in culture display adhesive, structural, and functional abnormalities

An unusual, elongated, refractile cell morphology was observed in keratinocytes cultured from three patients with non-lethalis forms of junctional epidermolysis bullosa (JEB). To determine whether these changes might be related to altered cell adhesion, keratinocyte strains established from one patient were examined for adhesive, structural, and functional characteristics. JEB keratinocytes expressed keratin tonofilaments, as determined by staining with AE1 monoclonal antibodies and direct observation of tonofilaments by electron microscopy. JEB keratinocytes showed diminished cell-substratum adhesions, judged by interference reflection microscopy. Areas of diminished cell-substratum adhesion corresponded to F-actin-rich cell adhesions (focal adhesions) and not to cellular areas that abundantly express hemidesmosomal antigens. Analysis of cell-substratum adhesion by electron microscopy revealed extensive areas of cell-substratum separation in JEB keratinocytes that were not present in normal keratinocytes maintained in serum-free medium. Normal keratinocytes displayed numerous regions of focal contact between the ventral plasma membrane and the culture substratum, but these structures were not seen in JEB keratinocytes. Bundled actin filaments (stress fibers) were greatly diminished in expected regions of cell-substratum adhesion in JEB keratinocytes and, instead, displayed disorganized individual filaments. The growth rate of JEB keratinocytes was quite slow in culture, with a population doubling time of 2.7 d versus 1.5 d for normal keratinocytes under identical conditions. JEB keratinocytes also displayed a reduced ability to aggregate into colonies upon exposure to medium with increased extracellular calcium. JEB keratinocytes thus display adhesive, structural, and functional abnormalities that suggest this cell type may be central to the pathogenesis of junctional epidermolysis bullosa. Study of affected keratinocytes could be important to characterize associated molecular pathologies.

Integrins as mechanochemical transducers

A recent resurgence of interest in mechanical forces and cell shape as biological regulators has revealed extracellular matrix as the site at which forces are transmitted both to and from cells. at the same time, great advances have been made in terms of defining cell-surface integrin receptors as transmembrane molecules that mediate cell attachment and physically interlink extracellular matrix with the intracellular cytoskeleton. Convergence of these two lines of research has begun to elucidate the molecular mechanism by which cells sense physical forces and transduce mechanical signals into a biochemical response.

Talin binds to actin and promotes filament nucleation

Platelet talin binds to actin in vitro and hence is an actin binding protein. By four different non-interfering assay conditions (fluorescence, fluorescence recovery after photobleaching, (FRAP), dynamic light scattering and DNase-I inhibition) we show that talin promotes filament nucleation, raises the filament number concentration and increases the net rate of actin polymerization but has no inhibitory effect on filament elongation. Binding of talin to actin occurs at a maximal molar ratio of 1:3 as determined by fluorescencetitration under G-buffer conditions. The overall binding constant was approximately 0.25 microM.