Identification and isolation of vinculin from platelets

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.

Cytoskeletal assembly and vinculin-cytoskeleton interaction in different phases of the activation of bovine platelets

Vinculin is an Mr 130 kDa protein that has been implicated in membrane-cytoskeleton interaction in various cell types. It has been demonstrated that vinculin is not a cytoskeletal component in resting platelets, but part of it becomes associated with the cytoskeleton during thrombin-induced activation. In this study, using a quantitative immunoblotting technique, the relation of vinculin to the cytoskeleton in different phases of activation of bovine platelets was explored, and the process of incorporation of vinculin into the cytoskeleton was related to that of cytoskeletal assembly. The assembly of cytoskeleton proceeded at a significantly faster rate than the association of vinculin with it, which shows that the latter process is not due to passive trapping of vinculin into the Triton-insoluble residue, but certain biochemical changes had to occur before such an interaction became possible. When the formation of pseudopodia was prevented by cytochalasin B, but neither aggregation nor the release reaction induced by thrombin were inhibited, the recovery of vinculin in the Triton-insoluble residue even increased. In both time- and thrombin-concentration-dependent studies, poor correlation was found between vinculin-cytoskeleton association and the extent of aggregation. Activation with phorbol-myristate-acetate, which is a strong stimulus for aggregation but produces only a slight release in the granular content, resulted in the association of only a negligible amount of vinculin with the cytoskeletal fraction. The incorporation of vinculin into the cytoskeletal fraction of thrombin activated platelets started with the release reaction but still proceeded, and the greatest part of the reaction occurred after secretion had gone to completion.(ABSTRACT TRUNCATED AT 250 WORDS)

Vinculin in relation to stress fibers in spread platelets

To investigate the function of vinculin in blood platelets, we studied its localization in relation to other cytoskeletal proteins as well as its state of phosphorylation in platelets allowed to spread on fibrinogen-coated surfaces. By 5 minutes after loading the platelets onto the surfaces the 47 and 20 kDa polypeptides became phosphorylated, indicating activation. By 30 minutes, platelets formed small, typical bundles of fibers which stained brilliantly with rhodamine phalloidin. Myosin and tropomyosin, detected with specific antibodies, were localized in periodic arrays along these bundles. By indirect immunofluorescence, a discrete patch of vinculin was observed at each end of every actin-containing bundle. Vinculin phosphorylation was not detected in immunoprecipitates protected against phosphatases. Interference reflection images showed that regions of close binding to the substratum (adhesion plaques) closely matched the vinculin staining sites. Talin appeared diffusely localized. It could be shown to be present in the plaques when platelets were stabilized with ZnCl2 by the method of Geiger and then sonicated to remove some of the surface membrane. Localizations of vinculin and myosin were unaltered by this treatment. Talin phosphorylation or proteolysis could not account for vinculin translocation. We conclude that platelets, in response to an appropriate physiological surface, form typical actin bundles with vinculin at the termination of each bundle, in close relation to adhesion plaques. The signal for this translocation does not appear to depend on phosphorylation of vinculin or on phosphorylation or proteolysis of talin. Our findings support the conclusion that in platelets, as in nucleated cells, vinculin serves as at least part of the connection between bundled actin fibers and the extracellular matrix. Such a connection seems required for platelets' known ability to exert tension on surfaces.

Vinculin is a permanent component of the membrane skeleton and is incorporated into the (re)organising cytoskeleton upon platelet activation

Vinculin, a 130-kDa protein discovered in chicken gizzard smooth-muscle cells and subsequently also described in platelets, is believed to be involved in membrane-cytoskeleton interactions. In this study we investigated vinculin distribution in human blood platelets. Two skeletal fractions and a remaining cytosolic fraction were prepared with a recently described Triton X-100 lysis buffer causing minimal post-lysis breakdown by proteolysis. The presence of vinculin was demonstrated in the membrane skeleton and cytosol of resting and thrombin-activated human platelets. Upon thrombin stimulation vinculin also appeared in the cytoskeleton. this cytoskeletal incorporation was completed during the early stages of platelet aggregation and secretion, when the uptake of myosin, actin-binding protein and talin was still not maximal. We conclude therefore, that vinculin may play an important role in the structural (re)organisation of the human platelet cytoskeleton upon platelet activation.

Immunolocalization of meta-vinculin in human smooth and cardiac muscles

Meta-vinculin, a vinculin-related protein, has been isolated from human uterus smooth muscle. Specific antibodies to meta-vinculin, which distinguish between meta-vinculin and vinculin, were prepared by absorption of anti-meta-vinculin serum on vinculin coupled to nitrocellulose. Meta-vinculin specific antibody demonstrates only smooth and cardiac muscle specificity and is able to cross-react with a small 21-kD fragment of the meta-vinculin polypeptide chain. This antibody does not interact with protease resistant 95-kD core shared by vinculin and meta-vinculin. Meta-vinculin specific antibody was used for the localization of meta-vinculin in smooth and cardiac muscles by the indirect immunofluorescence method. At the light microscopy resolution level it was found that meta-vinculin and vinculin are localized in the same cellular adhesive structures. Meta-vinculin is present in membrane-associated microfilament-bound plaques of smooth muscle, in intercalated discs and costameres of cardiac muscle. In primary culture of smooth muscle cells from human aorta, meta-vinculin and vinculin were found to be present in focal contacts of the cells. During the cultivation of smooth muscle cells, the quantity of meta-vinculin decreased progressively and finally meta-vinculin completely disappeared from the focal contacts. The data show that in smooth and cardiac muscles meta-vinculin could be a structural component of microfilament-membrane attachment sites, defined earlier by the localization of vinculin.

Platelet vinculin: a substrate of activated factor XIII

In addition to plasma, Factor XIII of blood coagulation (FXIII) is also present in the cytosol of platelets, monocytes and macrophages. However, its intracellular function has not yet been revealed. Activated Factor XIII (FXIIIa) is a transglutaminase (protein-glutamine: amine gamma-glutamyltransferase, EC 2.3.2.13) of highly restricted substrate specificity with only a few known protein substrates. In this report, we showed that FXIIIa can link dansylcadaverine, radiolabelled histamine and putrescine to vinculin. Quantitative determinations revealed that in the vinculin molecule a single glutamine residue can serve as acyl donor for the incorporation of small-molecular-weight amines. Vinculin could not be crosslinked to another vinculin molecule. It could be covalently bound, however, to fibrinogen, which indicates that the acyl donor glutamine residue can be engaged in an epsilon-(gamma-glutamyl)lysyl crosslink formation. Since it has been shown that platelet actin and myosin, two main components of cytoskeleton, are also substrates for FXIIIa, and that vinculin is associated to the cytoskeleton during platelet activation, the involvement of FXIII in the stabilization of cytoskeleton at certain phases of cellular function is a likely possibility.

Interaction of iodinated vinculin, metavinculin and alpha-actinin with cytoskeletal proteins

Iodinated vinculin, metavinculin and alpha-actinin were used to probe the interaction of these proteins with electrophoretically separated cytoskeletal proteins. Using the gel overlay technique, we detected strong binding of 125I-vinculin and 125I-metavinculin to alpha-actinin, 175 kDa polypeptide, talin, vinculin and metavinculin themselves, and moderate binding to actin. 125I-alpha-actinin was capable of interacting with vinculin and metavinculin. The specific binding of 125-I-alpha-actinin to vinculin and metavinculin immobilized on a polysterene surface was also demonstrated. We suggest that the ability of vinculin and alpha-actinin to form a complex may be realized in microfilament-membrane linkages.

Metavinculin and vinculin from mammalian smooth muscle: bulk isolation and characterization

Metavinculin, a vinculin related protein found only in muscle, has been prepared in bulk amounts from porcine stomach by a new procedure: the same procedure is applicable to the purification of vinculin from porcine stomach and avian gizzard. A comparison of the mammalian and avian proteins by peptide mapping showed them all to contain a common protease resistant 90 kDa core; however both avian and mammalian vinculins were notably more resistant to proteolysis down to this core than their respective metavinculins. Despite the close similarities in the peptide maps, in molecular weight and amino acid composition neither of the mammalian proteins exhibited the head and tail morphology formerly described for gizzard vinculin and metavinculin; both porcine proteins appeared globular under the electron microscope. From the gross variability in the ratios of metavinculin to vinculin among smooth muscles of different origin as well as from the common detergent-independent solubility properties of both proteins during isolation, it is concluded that vinculin and metavinculin perform duplicatory roles as peripheral membrane components. No definitive evidence for the interaction of either protein with actin filaments was obtained.

Association of vinculin to the platelet cytoskeleton during thrombin-induced aggregation

Vinculin is a protein generally believed to be involved in membrane-cytoskeleton interaction, and its presence in platelets has been verified earlier. Here we show that in resting bovine platelets, vinculin is not associated with the Triton-insoluble cytoskeletal fraction but becomes incorporated into it during the thrombin-induced activation process. The incorporation starts around the same time as the release reaction and only after the shape change and the first phase of aggregation have taken place. Its time course parallels the cytoskeletal association of actin and certain other contractile proteins. Vinculin is a minor component of platelet cytoskeleton and only about 10% of the total platelet vinculin becomes incorporated into the Triton X-100 residue.

Effect of tropomyosin on the interactions of actin with actin-binding proteins isolated from pig platelets

Several non-muscle tropomyosins have been reported to lack the ability to polymerize in a head-to-tail manner [Dabrowska, R. et al. (1983) J. Muscle Res. Cell Motil. 1, 83-92; Côté, G.P. (1983) Mol. Cell. Biochem. 57, 127-146]. Unlike rabbit skeletal muscle tropomyosin, these proteins could therefore not protect the F-actin microfilaments neither from disassembly or from cross-linking by the other actin-associating factors. However, we have provided evidence that, in vitro, pig platelet tropomyosin, although shorter in molecular length, exhibits the same properties as the muscle protein: it self-associates and forms a 1:6 complex with platelet filamentous actin under physiological conditions [Prulière et al. (1984) J. Muscle Res. Cell Motil. 6, 126]. In this paper, we examine the effects of several other actin-binding proteins on the microfilaments saturated with this non-muscle tropomyosin. Since contractile proteins often vary with the cell type and may require different conditions for their interactions, we have developed a procedure which allows the parallel purification of actin-binding protein (ABP), vinculin, alpha-actinin, gelsolin as well as actin and tropomyosin from the same batch of cells. Thus, using an homogeneous system, we show by viscometry, sedimentation and densitometry, and by electron microscopy, that pig platelet tropomyosin can protect the structure of the microfilaments from the action of the modulating factors to the same extent as rabbit skeletal muscle alpha-tropomyosin. Our data suggest that interaction of ABP, vinculin or alpha-actinin can occur only with the ends of the filaments when F-actin is saturated with tropomyosin, while cross-linking takes place by interactions with sites localized along the entire length of F-actin in the absence of tropomyosin. Moreover, the presence of tropomyosin on F-actin leads to the total inhibition of gelsolin severing activity, although it did not prevent the binding of gelsolin to the F-actin--tropomyosin complex. This suggests that pig platelet as well as skeletal muscle tropomyosins have the ability to increase the strength of the interaction between actin monomers within the filament. This also suggests that the binding sites of gelsolin along the filaments are not localized in the groove of the F-actin helix.

Effect of membrane-interacting amphiphiles on association of membrane glycoproteins with assembled cytoskeletal proteins in concanavalin A-activated rabbit platelets

Membrane-interacting amphiphiles, lysophosphatidylcholine, cepharanthine and chlorpromazine, inhibited concanavalin A (Con A)-induced platelet activation in a dose-dependent manner, as judged by the inhibition of serotonin release. These amphiphiles did not influence the binding of Con A to surface membrane glycoproteins. Marked increase in the amount of the cytoskeletal proteins, myosin, actin and actin-binding protein, in the Triton-insoluble residue of the Con A-activated platelets, as well as in the surface membrane glycoproteins with molecular weights of 224,000, 201,000, 119,000 and 92,000 found in the same residue, was inhibited by any of the three amphiphiles in a dose-dependent manner. Such inhibitory effect, however, was abolished when the amphiphiles were washed out from the platelets before the activation. These findings suggest that these membrane-interacting amphiphiles may inhibit the Con A-induced assembly of the cytoskeletal proteins and their association with surface membrane glycoproteins, probably by physically altering the membrane properties.

Isolation and partial characterization of human platelet vinculin

A 130,000 Mr protein was isolated from human platelets by sequential DEAE-Sephacel and Sepharose Cl-4B chromatography. Low shear viscometric measurements showed that the enriched protein after DEAE-Sephacel chromatography inhibited actin polymerization. This effect was somewhat greater in the presence of EGTA than in the presence of calcium. Further purification by Sepharose Cl-4B chromatography resulted in a complete loss of this inhibitory effect. Studies with fluorescent actin detected no nucleation or "+" end capping activity in either the DEAE-Sephacel- or Sepharose Cl-4B-purified vinculin. Antibodies raised in mice against the 130,000-mol-wt protein were shown to cross-react with chicken gizzard vinculin and a similar molecular weight protein was detected in WI38 cells and, Madin-Darby canine kidney cells. Lysis experiments with the Madin-Darby canine kidney cells indicated that most of the vinculin was soluble in Triton X-100, although some was found associated with the insoluble cytoskeletal residue. By immunofluorescence, vinculin in WI38 cells was localized to adhesion plaques as described by others. Discrete localization in platelets was also detected and appeared to depend on their state of adhesion and spreading. The results of these experiments suggest that human platelets contain a protein similar to vinculin. It is not clear if platelet vinculin is associated with structures analogous to adhesion plaques found in other cell types. The data indicate that the previously reported effects of nonmuscle vinculins on actin polymerization may be due to a contaminant or contaminants.