Immunoelectron microscope studies of membrane-microfilament interactions: distributions of alpha-actinin, tropomyosin, and vinculin in intestinal epithelial brush border and chicken gizzard smooth muscle cells

Myosin rings and spreading in mouse blastomeres

The relationship between myosin organization and cell spreading in the preimplantation mouse embryo was studied by indirect immunofluorescence in embryos cultured on lectin-coated substrates. Binding of cell surface polysaccharides to substrate-bound concanavalin A and wheat germ agglutinin induced changes in myosin distribution that resembled those which occur during cell-cell contact interaction. This involved an initial loss of myosin from the contact region that was associated with the development of stable cell-substrate attachments. In addition, a ring of myosin was formed along the edge of the cells' contact to the substrate. The presence of such a ring may be related to the potential for subsequent cell spreading. A myosin ring was also identified in the apical junctional region of the outer morula cells where it similarly separated the cell periphery into contacted and free peripheral domains. Following these changes in myosin organization the embryos spread on the substrate by extension of lamellipodia. These movements were coupled to the dissolution of the myosin ring and the reorganization of myosin into filament bundles. The sequence of changes in the pattern of myosin distribution suggests that contact regulation of myosin organization plays an important role in controlling the spreading behavior of blastomeres and perhaps more generally in the organization of cells into epithelia.

An improved procedure for immunoelectron microscopy: ultrathin plastic embedding of immunolabeled ultrathin frozen sections

Ultrathin frozen sections are ideal substrates with which to carry out immunolabeling experiments in electron microscopy. However, the ultrastructural delineation in positively stained frozen sections has not been as detailed as in conventionally osmium-stained and plastic-embedded sections. We now describe a simple technique in which immunolabeled ultrathin frozen sections are subsequently treated with osmium tetroxide, dehydrated, and then embedded in plastic by impregnation with a monomer to the thickness of the section, followed by polymerization of the monomer. By this technique ultrastructural definition as good as that of conventional plastic sections is achieved, while the high density and specificity of immunolabeling characteristic of ultrathin frozen sections are retained.

Microfilament-organizing centers in areas of cell contact: cytoskeletal interactions during cell attachment and locomotion

In this article we discuss three aspects of cell contact formation: (a) the molecular architecture of the cytomatrix in cell-to-substrate focal contacts, (b) the dynamic properties of membrane- and microfilament-associated proteins in the contact areas, and (c) the involvement of microtubules in the coordinated and directed formation of new substrate contacts during cell locomotion. We show that different microfilament-associated proteins exhibit distinct patterns of association with focal contacts: some proteins are specifically associated with focal contacts (vinculin and talin); alpha-actinin is enriched in the contact areas but also is present along the stress fibers and in the lamellipodium; actin and filamin are detected throughout the contact areas but in apparently reduced amounts compared with the associated stress fibers; and tropomyosin, myosin, and spectrin are either absent from the endofacial surfaces of contact areas or are present in only very small amounts. Fluorescence photobleaching recovery analyses performed with living cells microinjected with fluorescently labeled actin, vinculin, and alpha-actinin indicate that each of these proteins maintains a dynamic equilibrium between a soluble cytoplasmic pool and a membrane-bound fraction. Correlation of the distribution of vinculin and tubulin in motile fibroblasts to local movements of the leading edge of the same cells indicates that free-end microtubules extend into actively ruffling areas along the lamellipodium and that new vinculin-containing contacts are preferentially formed in these protruding regions.

Biosynthesis of microvillar proteins

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Zonulae occludentes in junctional complex-enriched fractions from mouse liver: preliminary morphological and biochemical characterization

A bile canaliculus-derived preparation containing junctional complexes has been obtained from mouse livers using subcellular fractionation techniques. The junctional complexes include structurally intact zonulae occludentes (ZOs). Extraction of this preparation with the anionic detergent sodium deoxycholate (DOC) left junctional ribbons, the detergent-insoluble zonular remnants of the junctional complexes. When visualized in negative stain electron microscopy, each of these ribbons contained a branching and anastomosing network of fibrils which appears similar to that of ZOs in freeze-fractured whole liver. Comparative measurements of freeze-fracture and negative stain fibril diameters and network densities support this relationship. SDS polyacrylamide gel analysis shows the DOC-insoluble junctional ribbons to be characterized by major polypeptides at 37,000 and at 48,000, with minor bands at 34,000, 41,000, 71,000, 86,000, 92,000, and 102,000. The ZO-containing membrane fractions have been isolated in the presence of EGTA in concentrations and under conditions shown by others to disrupt normal ZO morphology and physiology in whole living epithelia. The network of fibrils visualized in these fractions by negative staining is structurally resistant to treatment with DOC, but is either solubilized or disrupted by N-lauroylsarcosine.

Identification and isolation of vinculin from platelets

A vinculin-like protein was identified in chicken as well as in bovine platelets by ELISA competitive binding assay using antibodies against vinculin from chicken gizzard. By a modified procedure (J. Biol. Chem. (1980) 255, 1194-1199) we succeeded in isolating bovine platelet vinculin to apparent homogeneity. The structural identity of platelet and chicken gizzard vinculin was demonstrated by circular dichroism analysis. It was also shown that platelet vinculin induces a significant decrease in the low shear viscosity of F-actin. Vinculin, in all probability, plays an important role in the organization of actin filaments in platelets, especially in the linkages of microfilaments to the membrane.

Mechanism of concanavalin A-induced anchorage of the major cell surface glycoproteins to the submembrane cytoskeleton in 13762 ascites mammary adenocarcinoma cells

Concanavalin A (Con A)-induced anchorage of the major cell surface sialoglycoprotein component complex (ASGP-1/ASGP-2) was studied in 13762 rat mammary adenocarcinoma sublines with mobile (MAT-B1 subline) and immobile (MAT-C1 subline) cell surface Con A receptors. Treatment of cells, isolated microvilli, or microvillar membranes with Con A resulted in marked retention of ASGP-1 and ASGP-2, a Con A-binding protein, in cytoskeletal residues of both sublines obtained by extraction with Triton X-100 in PBS. When Con A-treated microvillar membranes were extracted with a buffer containing Triton X-100, the sialoglycoprotein complex was found associated in the residues with a transmembrane complex composed of actin, a 58,000-dalton polypeptide, and a cytoskeleton-associated glycoprotein (CAG), also a Con A-binding protein, in MAT-C1 membranes, and of actin and CAG in MAT-B1 membranes. Untreated membrane Triton residues retained very little ASGP-1/ASGP-2 complex. Association of the sialoglycomembrane complex and the transmembrane complex was also demonstrated in Con A-treated, but not untreated, microvilli by their comigration on CsCl gradients. Association of both complexes with the cytoskeleton of microvilli was shown by sucrose density gradient centrifugation. A fraction of the polymerized actin comigrated with the transmembrane complex alone in the absence of Con A and with both the transmembrane complex and the sialoglycoprotein complex in the presence of Con A. From these results we propose that anchorage of the sialoglycoprotein complex to the cytoskeleton on Con A treatment occurs by cross-linking ASGP-2, the major cell surface Con A-binding component, to CAG of the transmembrane complex, which is natively linked to the cytoskeleton via its actin component. Since Con A-induced anchorage occurs in sublines with mobile and immobile receptors, the anchorage process cannot be responsible for the differences in receptor mobility between the sublines.

Cytoskeletal studies on Lowicryl K4M embedded and Affi-Gel 731 attached rat peritoneal mast cells

The subplasmalemmal network in mast cells consists of irregularly arranged 6-7 nm filaments (actin) connected by thinner filaments. In places oblique filaments with crossbridges or short, perpendicular filaments (11-12 nm) connect cell and granule membrane. Filaments attaching subplasmalemmal network to cell membrane divide like a Y and attach cell membrane end-on with a conical, hook-like bending. Each granule is surrounded by a regular network of filaments.

Spreading of non-transformed and transformed cells

Mechanisms of cellular reactions responsible for the spreading of non-transformed cultured tissue cells on the surface of various substrata and relationships of these reactions to the control of cell proliferation are reviewed; the special role of the membrane-cytoskeleton interactions leading to extension and attachment of pseudopods is stressed. Transition of cells from non-transformed to transformed phenotype is characterized by decreased spreading and by decreased dependence of proliferation on spreading. Manifestations of both of these spreading-associated changes are reviewed and their possible mechanisms are discussed. It is suggested that cell transition to transformed phenotype involves shift of an equilibrium between the reactions induced by the two groups of membrane-bound ligands: those attached and those not attached to the substratum.

Phosphorylation controls brush border motility by regulating myosin structure and association with the cytoskeleton

The intestinal epithelial cell brush border (BB) is a useful model for nonmuscle cell motility. We studied regulation of BB motility by analyzing myosin phosphorylation and its association with the cytoskeleton. Our results demonstrate that myosin associates with the cytoskeleton only when it is dephosphorylated. Myosin light chain kinase substrates release myosin, phosphorylated and in the form of filaments, from the cytoskeleton. Although ITP and GTP serve as myosin ATPase substrates, they do not cause BB contraction, myosin release, or phosphorylation. Brush border contraction occurs with ATP or with a mixture of ITP and ATP gamma S. Therefore, phosphorylation regulates myosin association with the cytoskeleton, myosin is not bound at the actin-myosin binding site, and when phosphorylated, myosin forms filaments for movement.

Vinculin is a component of an extensive network of myofibril-sarcolemma attachment regions in cardiac muscle fibers

Immunofluorescent staining of bovine and avian cardiac tissue with affinity-purified antibody to chicken gizzard vinculin reveals two new sites of vinculin reactivity. First, vinculin is organized at the sarcolemma in a striking array of rib-like bands, or costameres. The costameres encircle the cardiac muscle cell perpendicular to the long axis of the fiber and overlie the I bands of the immediately subjacent sarcomeres. The second new site of vinculin reactivity is found in bovine cardiocytes at tubular invaginations of the plasma membrane. The frequency and location of these invaginations correspond to the known frequency and distribution of the transverse tubular system in bovine atrial, ventricular, and Purkinje fibers. We do not detect tubular invaginations that stain with antivinculin in avian cardiocytes and, in fact, a transverse tubular system has not been found in avian cardiac fibers. Apparent lateral Z-line attachments to the sarcolemma and its invaginations have been observed in cardiac muscle by electron microscopy in the same regions where we find vinculin. On the basis of these previous ultrastructural findings and our published evidence for a physical connection between costameres and the underlying myofibrils in skeletal muscle, we interpret the immunofluorescence data of this study to mean that, in cardiac muscle, vinculin is a component of an extensive system of lateral attachment of myofibrils to the plasma membrane and its invaginations.

Detection of vinculin-binding proteins with an 125I-vinculin gel overlay technique

Vinculin is an adhesion plaque component localized on the cytoplasmic side of the cell membrane where stress fibers end. To detect vinculin-binding proteins, we have developed an 125I-vinculin gel overlay method. SDS PAGE was used to separate different protein preparations. After fixing the proteins in the gel with methanol-acetic acid, the SDS was removed with ethanol and the proteins renatured in buffer. The gel was then incubated with 125I-vinculin. After extensive washing to remove nonspecifically associated label, the gel was dried and autoradiographed. Chick embryo fibroblasts, their Rous sarcoma virus transformants, and HeLa cells were found to contain two proteins (Mr 220,000 and 130,000) that bound 125I-vinculin strongly and another (Mr 42,000) that bound it moderately. The 130,000-mol-wt protein was identified as vinculin itself, which suggests that it may self-associate. The 42,000-mol-wt protein was identified as actin with which vinculin is known to interact. The identity of the 220,000-mol-wt protein is not known. It is not cellular fibronectin, myosin, or filamin. When fibroblast proteins were separated into Triton X-100 soluble and insoluble fractions, most of the vinculin and the 220,000-mol-wt protein was found to be in the soluble fraction. Chicken gizzard also contained these vinculin-binding proteins along with three others of Mr 190,000, 170,000, and 100,000.

Ca2+ regulation of tight-junction permeability and structure in Necturus gallbladder

To explore the role of Ca2+ in tight-junction permeability, the Necturus gallbladder was exposed to varying Ca2+ concentrations and to the Ca2+ ionophore A23187 added to the mucosal side (1.9 X 10(-6) to 6.8 X 10(-5) M). Electrophysiological parameters measured in an Ussing-type chamber were correlated with tight-junction morphology revealed by freeze-fracture electron microscopy. In Ca2+-free bathing media, transepithelial resistance decreases and tight-junctional ultrastructure is fragmented. In 1.8 mM Ca2+ media, A23187 induces an initial drop in transepithelial resistance, followed by an increase in transepithelial resistance to a value 20% above base line. At peak response to A23187, NaCl diffusion potentials decrease. Freeze-fracture replicas reveal that the number of junctional strands increase pari passu with junctional depth. Both physiological and morphological changes were partially reversible. The initial decrease in transepithelial resistance coincided with a persistent hyperpolarization of the mucosal cell membrane potential difference and a decrease in the mucosal-to-serosal cell membrane resistance ratio. Thus A23187 alters both the transcellular and paracellular pathway, resulting in opposing effects on transepithelial resistance.

Structural and functional properties of the non-muscle tropomyosins

The non-muscle tropomyosins (TMs), isolated from such tissues as platelets, brain and thyroid, are structurally very similar to the muscle TMs, being composed of two highly alpha-helical subunits wound around each other to form a rod-like molecule. The non-muscle TMs are shorter than the muscle TMs; sequence analysis demonstrates that each subunit of equine platelet TM consists of 247 amino acids, 37 fewer than for skeletal muscle TM. The major differences in sequence between platelet and skeletal muscle TM are found near the amino and carboxyl terminal ends of the proteins. Probably as the result of such alterations, the non-muscle TMs aggregate in a linear end-to-end manner much more weakly than do the muscle TMs. Since end-to-end interactions are responsible for the highly cooperative manner in which TM binds to actin, the non-muscle TMs have a lower affinity for actin filaments than do the muscle TMs. However, the attachment of other proteins to actin (e.g. the Tn-I subunit of skeletal muscle troponin or the S-1 subfragment of skeletal muscle myosin) can increase the affinity of actin filaments for non-muscle TM. The non-muscle TMs interact functionally with the Tn-I component of skeletal muscle troponin to inhibit the ATPase activity of muscle actomyosin and with whole troponin to regulate the muscle actomyosin ATPase in a Ca++-dependent manner, even though one of the binding sites for troponin on skeletal TM is missing in non-muscle TM. A novel actomyosin regulatory system can be produced using Tn-I, calmodulin and non-muscle TM; in this case inhibition is released when the non-muscle TM detaches from the actin filament in the presence of Ca++. Although it has not yet been demonstrated that the non-muscle TMs participate in a Ca++-dependent contractile regulatory system in vivo it does appear that they are associated with actin filaments in vivo.

Vinculin localization in cardiac muscle

Vinculin isolated from chicken cardiac muscle crossreacts with antibodies against smooth muscle vinculin. Antibodies to vinculin were used for localization of vinculin in cardiac muscle by indirect immunofluorescence method. In cardiac muscle vinculin was localized in intercalated discs and near plasma membrane at the cell periphery between external myofibrils and sarcolemma. It was suggested that vinculin plays an important role in myofibril-sarcolemma interaction in cardiac muscle.

Immunochemical localization of NADP-specific isocitrate dehydrogenase in Escherichia coli

The intracellular localization of isocitrate dehydrogenase was determined by immunochemical techniques with ultrathin sections of Escherichia coli. The thin sections, which were obtained by ultracryomicrotomy, were incubated first with antiserum specific for the enzyme and then with a protein A-gold complex. Transmission electron microscopy showed that the gold label was dispersed mainly in the cytoplasm.

Polar redistribution of Na+K+ATPase in aggregating MDCK cells

The distribution of Na+K+ATPase was examined immunoelectronmicroscopically along the plasma membranes of aggregating MDCK cells grown in tissue culture. Na+K+ATPase was localized with the use of affinity-purified antibodies employing the double-antibody immunoperoxidase technique. Na+K+ATPase-like immunoreactivity was uniformly distributed along the plasmalemma of cells grown in suspension. Once attached to the substratum, individual MDCK cells exhibited Na+K+ATPase immunoreactivity limited to the microvilli-laden mucosal surface. This exclusive mucosal distribution of Na+K+ATPase disappeared and a basolateral immunoreactivity became apparent once intercellular contacts and the formation of tight junctions occurred between neighboring cells. Thus, both cellular attachment to the substratum, and tight junction formation between aggregating MDCK cells are essential to the genesis of Na+K+ATPase polar distribution. These two events, however, appear to induce opposing distribution patterns for Na+K+ATPase.

Actin-independent association of vinculin with the cytoplasmic aspect of the plasma membrane in cell-contact areas

We investigated the mode of association of vinculin with areas of contact between the termini of microfilament bundles and the cell membrane in sites of focal contact with the substrate by selective removal of actin from these areas. Opened-up substrate-attached membranes of chick fibroblasts as well as detergent-permeabilized cells were treated with fragmin from Physarum in the presence of Ca+2. This treatment removed actin filaments from the cytoplasmic faces of the membranes, along with several actin-associated proteins (alpha-actinin, tropomyosin, myosin, and filamin). Vinculin distribution was not affected by treatment. Moreover, rhodamine- or fluorescein-conjugated vinculin, when added to these preparations, became specifically associated with the focal contacts regardless of whether the latter were pretreated with fragmin or not. We conclude that the association of vinculin with focal contacts is largely actin-independent. We discuss the implications of these findings in the molecular mechanisms of microfilament membrane association in areas of cell contact.

Immunoelectron microscopic studies of desmin (skeletin) localization and intermediate filament organization in chicken cardiac muscle

We studied the localization of desmin (skeletin), the major protein subunit of muscle-type intermediate filaments, in adult chicken cardiac muscle by high resolution immunoelectron microscopic labeling of ultrathin frozen sections of the intact fixed tissues. We carried out single labeling for desmin and double labeling for both desmin and either vinculin or alpha-actinin. In areas removed from the intercalated disk membranes, we observed desmin labeling between adjacent Z-bands in every interfibrillar space. Where these spaces were wide and contained mitochondria, convoluted strands of desmin labeling bridged between the periphery of neighboring Z-bands and the mitochondria. The intermediate filaments appeared to be organized in a more three-dimensional manner within the interfibrillar spaces of cardiac as compared to skeletal muscle. Near the intercalated disks, desmin labeling was intense within the interfibrillar spaces, but was completely segregated from the microfilament attachment sites (fascia adherens) where vinculin and alpha-actinin were localized. Desmin therefore appears to play no role in the attachment of microfilaments to the intercalated disk membrane. We discuss the role of intermediate filaments in the organization of cardiac and skeletal striated muscle in the light of these and other results.

Mechanism of brush border contractility studied by the quick-freeze, deep-etch method

We have analyzed terminal web contraction in sheets of glycerinated chicken small intestine epithelium and in isolated intestinal brush borders using a quick-freeze, deep-etch, rotary shadow replication technique. In the presence of Mg-ATP at 37 degrees C, the terminal web region of each cell in the glycerinated sheet and of each isolated brush border became severely constricted at the level of its zonula adherens (ZA). Consequently, the individual brush borders rounded up, splaying out their microvilli in fanlike patterns. The most prominent ultrastructural changes that occurred during terminal web contraction were a dramatic decrease in the diameter of the circumferential ring composed of a bundle of 8-9-nm filaments adjacent to the zonula adherens and a decrease in the number of cross-linkers between the microvillus rootlets. Microvilli were not retracted into the terminal web. We have used myosin S1 decoration to demonstrate that most of the circumferential bundle filaments are actin and that the actin filaments are arranged in the bundle with mixed polarity. Some filaments within the bundle did not decorate with myosin S1 and had tiny projections that appeared to be attached to adjacent actin filaments. Because of their morphology and immunofluorescent localization of myosin within this region of the terminal web, we propose that these undecorated filaments are myosin. From these results, we conclude that brush border contraction is caused primarily by an active sliding of actin and myosin filaments within the circumferential bundle of filaments associated with the ZA.

Cell surface localization and tissue distribution of a hepatocyte cell-cell adhesion glycoprotein (cell-CAM 105)

We recently identified a 105,000-dalton plasma membrane glycoprotein, denoted cell-CAM 105 (CAM, cell adhesion molecule), that is involved in intercellular adhesion of reaggregating rat hepatocytes (Ocklind, C., and B. Obrink, 1982, J. Biol. Chem., 257:6788-6795). In this communication we used a monospecific rabbit antiserum against cell-CAM 105 to localize the antigen by indirect immunofluorescence on isolated rat cells and on frozen rat tissue sections. This antiserum stained the surface of freshly isolated hepatocytes. In liver sections, however, the fluorescence seemed to be located exclusively along the bile canaliculi. In addition, cell-CAM 105 showed a very specific tissue distribution. Thus a specific fluorescence was seen only in the epithelia of the stomach, the small intestine, the large intestine, the glandular epithelium of the parotid gland, and the tubules of the kidney. No specific fluorescence was found in variety of other tissues, including cartilage, interstitial connective tissue, smooth muscle, skeletal muscle, heart muscle, eye, brain, skin, the epithelia of oesophagus, bladder, uterin mucosa, thyroid follicles, prostate gland, or collecting ducts of the kidney. In the simple epithelia of the intestine and the kidney tubules the fluorescence was confined to the apical, luminal portion. Thus, both in these epithelia and in liver, cell-CAM 105 was confined to the apical, luminal portion. Thus, both in these epithelia and in liver, cell-CAM 105 was located where the typical junctional complexes between cells are found. These findings taken together with the fact that cell-CAM 105 is involved in intercellular adhesion between hepatocytes suggest with the fac that cell-CAM 105 is involved in intercellular adhesion between hepatocytes suggest that cell-CAM 105 is a member of the junctional complexes of hepatocytes and some simple epithelia.

Antibodies to epithelial desmosomes show wide tissue and species cross-reactivity

Many workers regard cell adhesion as a highly specific phenomenon, believing that different molecular mechanisms are involved in the adhesion of cells of different tissues and different species. We believe that the evidence from cell behaviour is against this view and that cells share common adhesion mechanisms (for reviews see refs 1, 2); however, molecular evidence is lacking. As an approach to providing such evidence we have begun to study desmosomes, the cell-surface organelles responsible for strong intercellular adhesion in epithelia. We have raised antisera against each of five high-molecular weight (MW) desmosomal components. Having determined the specificity of our antisera by immunoblotting, we show here that each gives a staining pattern corresponding to the distribution of desmosomes in a range of tissues from different vertebrate species, demonstrating that desmosomal components are widely shared and highly conserved.

Effects of extracellular calcium depletion on membrane topography and occluding junctions of mammary epithelial cells in culture

Ca2+ dependence of occluding junction structure and permeability, well documented in explanted or cultured epithelial sheets, presumably reflects inherent control mechanisms. As an approach to identification of these mechanisms, we induced disassembly of zonulae occludentes in confluent monolayers of mouse mammary epithelial cells by exposure to low concentrations of the chelators, EGTA or sodium citrate. Stages in disassembly were monitored during treatment by phase-contrast microscopy and prepared for transmission and scanning electron microscopy. Cellular response included several events affecting occluding junctions: (a) Centripetal cytoplasmic contraction created tension on junction membranes and displaced intramembrane strands along lines determined by the axis of tension. (b) Destabilization of junction position, probably through increased membrane fluidity, augmented tension-induced movement of strands, resulting in fragmentation of the junction belt. (c) Active ruffling and retraction of freed peripheral membranes remodeled cell borders to produce many filopodia, distally attached by occluding-junction fragments to neighboring cell membranes. Filopodia generally persisted until mechanically ruptured, when endocytosis of the junction and adhering cytoplasmic bleb ensued. Junction disassembly thus resulted from mechanical tensions generated by initial centripetal contraction and subsequent peripheral cytoskeletal activity, combined with destabilization of the junction's intramembrane strand pattern.

Biochemical and immunological characterization of desmoplakins I and II, the major polypeptides of the desmosomal plaque

Epithelial cells contain complexes of cytokeratin filaments (tonofilaments) with specific domains of the plasma membrane that appear as symmetric junctions, i.e. desmosomes, or as asymmetric hemi-desmosomes. These regions of filament-membrane-attachment are characterized by 14 to 20 nm thick dense plaques (desmosomal plaque). In isolated desmosome-tonofilament complexes or other desmosomal fractions from various stratified squamous epithelia (e.g. bovine muzzle epidermis and tongue mucosa) desmosomal plaque structures are recognized and show a relatively high resistance to various extraction buffers and detergents. Such fractions enriched in desmosomal plaque material are also enriched in two prominent polypeptide bands of apparent molecular weights 250,000 (desmoplakin I) and 215,000 (desmoplakin II) which appear, on two-dimensional gel electrophoresis, as two distinct polypeptides isoelectric near neutral pH. These two polypeptides are present in almost equimolar amounts and each of them appears as a series of isoelectric variants, including some labeled by [32P]phosphate in tissue slices. The two desmoplakin polypeptides are closely related as shown by tryptic peptide map analysis and are different from keratin-like proteins and other major polypeptides of desmosome-rich fractions. Guinea pig antibodies raised against desmoplakins and specific for these proteins do not cross-react with other desmosomal antigen(s) or constituents of other types of junctions. Using desmoplakin antibodies we have identified desmoplakins as the major constituents of the desmosomal plaques present in epithelial and myocardiac cells of diverse species. The significance of this group of cell type-specific membrane-associated cytoskeletal proteins and their possible cytoskeletal functions are discussed.

Association of actin and 10 nm filaments with the dense body in smooth muscle cells of the chicken gizzard

Association of actin filaments and intermediate, 10 nm filaments with the dense bodies in smooth muscle cells of the chicken gizzard was studied by thin-section and freeze-etch-replica electron microscopy. For thin-section electron microscopy we used the isolated dense bodies with attached filaments. Actin filaments appeared to be inserted into both ends (poles) of individual oblong dense bodies in such a way that arrowheads with HMM S-1 pointed away from the dense body. 10 nm filaments were attached laterally to the dense body in a side-to-side fashion. This site-specific association of actin and 10 nm filaments with the dense body was confirmed by the freeze-etch replica observations on Triton-treated smooth muscles.

A vinculin-containing cortical lattice in skeletal muscle: transverse lattice elements ("costameres") mark sites of attachment between myofibrils and sarcolemma

We have found that vinculin is localized at the sarcolemma of skeletal muscle cells in a two-dimensional orthogonal lattice. Perpendicular to the longitudinal axis of the cell, bands of vinculin encircle the muscle cell and repeat along its length with a periodicity corresponding to the subjacent sarcomeres. Because of their appearance and probable function, we call the transverse elements of the lattice "costameres" (Latin costa, rib; Greek meros, part). Costameres have a substructure consisting of densely clustered patches of vinculin; the patches are segregated into two rows which flank the Z line and overlie the I band of the underlying sarcomere. It is likely that the costameres are physically coupled to the underlying myofibrils because: (i) the costameres broaden and narrow in concert with the underlying I band in stretched and contracted muscle, and (ii) adjacent but misaligned myofibrils are mirrored by corresponding discontinuities in the overlying costameres. We hypothesize that the sarcolemmal lattice, detected because vinculin is one of its molecular components, integrates the contractile apparatus with the sarcolemma during lengthening and shortening of the muscle cells.

The dynamic interrelationships of actin and vinculin in cultured cells

The dynamic state of cytoskeletal proteins actin and vinculin was studied in living cells using microinjection of fluorescently-labeled proteins combined with fluorescence photobleaching recovery (FPR). It is shown that both proteins maintain a dynamic equilibrium between their diffusible pools in the cytoplasms and their "organized" cytoskeletal fraction. These interrelationships could be simulated in model systems consisting of isolated substrate attached membranes. It was demonstrated that fluorophore bound vinculin was incorporated into the exposed focal contacts and that this binding was largely actin independent. These results are in line with the hypothesis that local contacts induce binding of vinculin to the endofacial surface of the membranes and that this region serves as a nucleation center for the assembly of actin bundles.

Ultrastructural localization of actin in muscle, epithelial and secretory cells by applying the protein A-gold immunocytochemical technique

Actin-immunoreactive sites have been localized at the electron microscope level by the protein A-gold technique in striated and smooth muscle cells as well as in epithelial and secretory cells. The combination of the highly sensitive protein A-gold technique with the good ultrastructural preservation and retention of antigenicity obtained using low-temperature embedding conditions has allowed a very precise identification of the labelled structures with high resolution. In striated muscle cells the labelling was obtained over the myofilaments and the Z-band, mainly at its periphery. Labelling was also observed at the edge of the intercalated discs of the cardiac muscle cells. In smooth muscle cells the labelling was present over the myofilaments; the dense plaques associated with the plasma membrane were labelled at their periphery where actin filaments have been reported to anchor. In epithelial cells of the duodenum and the renal convoluted proximal tubule, the labelling occurred over the filamentous core of the microvilli and over the cell web. Gold particles were often present over, or closely associated with, the cell membrane at the tip of the microvilli or of invaginations and vesicular structures. At the level of the junctional complexes the gold particles were aligned at the edge of the dense zones. In pancreatic endocrine and exocrine secretory cells, actin-immunoreactive sites were revealed over the Golgi apparatus, mainly at the level of the inner cisternae in the maturing face over or closely associated with the membranes of the condensing vacuoles and secretory granules, and also over the plasma membrane. Microvilli and cell web were also labelled. Finally, in fibroblasts, gold particles were associated with the membrane of vesicular structures. The consistent finding of actin-immunoreactive sites closely associated with membranes of secretory granules and vesicular structures brings support to the proposal that contractile proteins might play an important role in transcellular transport and protein secretion.

Interaction of the plasma membrane with the cytoskeleton: an overview

The intent of this review was to point out the diversity of cellular functions thought to be mediated by PM-cytoskeleton interactions. Based upon possible molecular mechanism, the functions were categorized into those involving PM proteins which are dispersed and those involving clustered proteins. Functions associated with dispersed proteins are thought to mediate the stabilization and shape of the PM. Clustering of PM proteins provides the driving force inducing their interaction with the cytoskeleton. Clustering by external ligands, pH or ionic exchanges, etc., is also a means of transmembrane signalling. Various methods used to explore cytoskeletal-PM mediated functions were evaluated. The methods were considered separately under biophysical, morphological and biochemical headings. This made it easier to point out current and potential values of the methods as well as their limitations. Each method taken separately is insufficient to elucidate molecular mechanisms regulating cytoskeletal-PM reactions, but combined they hold great promise of future solutions.

Organization and function of structural elements in focal contacts of tissue culture cells

The role of structural elements in the organization and maintenance of focal contacts was studied by microinjecting into tissue culture cells specific probes which interfere with filamentous actin or with vinculin: actin interaction. Injection of actin capping proteins from Physarum and brain resulted in breakdown of microfilament bundles starting at their distal ends and in loss of focal contacts. This process was fully reversible. Injection of a high affinity antibody against chicken gizzard vinculin led to partial breakdown of microfilament bundles con-concomitant with disruption of focal contacts with vinculin remaining at the plasma membrane. This process was irreversible.

Specific attachment of desmin filaments to desmosomal plaques in cardiac myocytes

Intercellular junctions which are similar in ultrastructure and protein composition to typical desmosomes have so far only been found in epithelial cells and in heart tissue, specifically in the intercalated disks of cardiac myocytes and at cell boundaries between Purkinje fiber cells. In epithelial cells the cytoplasmic side of desmosomes, the 'desmosomal plaque', represents a specific attachment structure for the anchorage of intermediate filaments (IF) of the cytokeratin type. Cardiac myocytes do not contain cytokeratin filaments. In primary cultures of rat cardiac myocytes, we have examined by immunofluorescence and electron microscopy, using single and double label techniques, whether other types of IF are attached to the desmosomal plaques of the heart. Antibodies to desmoplakin, the major protein of the desmosomal plaque, have been used to label specifically the desmosomal plaques. It is shown that the desmoplakin-containing structures are often associated with IF stained by antibodies to desmin, i.e., the characteristic type of IF present in these cells. Like cytokeratin filaments in epithelial cells, desmin filaments attach laterally to the desmosomal plaque. They also remain attached to these plaques after endocytotic internalization of desmosomal domains by treatment of the cells with EGTA. These desmin filaments do not appear to attach to junctions of the fascia adherens type and to nexuses (gap junctions). These observations show that anchorage at desmosomal plaques is not restricted to IF of the cytokeratin type and that IF composed of either cytokeratin or desmin, specifically attach, in a lateral fashion, to desmoplakin-containing regions of the plasma membrane. We conclude that special domains exist in these two IF proteins that are involved in binding to the desmosomal plaque.

Reactivation of intestinal epithelial cell brush border motility: ATP-dependent contraction via a terminal web contractile ring

Various models have been put forward suggesting ways in which brush borders from intestinal epithelial cells may be motile. Experiments documenting putative brush border motility have been performed on isolated brush borders and have generated models suggesting microvillar retraction or microvillar rootlet interactions. The reported Ca++ ATP-induced retraction of microvilli has been shown, instead, to be microvillar dissolution in response to Ca++ and not active brush border motility. I report here studies on the reactivation of motility in intact sheets of isolated intestinal epithelium. Whole epithelial sheets were glycerinated, which leaves the brush border and intercellular junctions intact, and then treated with ATP, PPi, ITP, ADP, GTP, or delta S-ATP. Analysis by video enhanced differential interference-contrast microscopy and thin-section transmission electron microscopy reveals contractions in the terminal web region causing microvilli to be fanned apart in response to ATP and delta S-ATP but not in response to ADP, PPi, ITP, or GTP. Electron microscopy reveals that the contractions occur at the level of the intermediate junction in a circumferential constriction which can pull cells completely apart. This constriction occurs in a location occupied by an actin-containing circumferential band of filaments, as demonstrated by S-1 binding, which completely encircles the terminal web at the level of the intermediate junction. Upon contraction, this band becomes denser and thicker. Since myosin, alpha-actinin and tropomyosin, in addition to actin, have been localized to this region of the terminal web, it is proposed that the intestinal epithelial cell can be motile via a circumferential terminal web contractile ring analogous to the contractile ring of dividing cells.

Desmoplakins of epithelial and myocardial desmosomes are immunologically and biochemically related

Guinea pig antibodies against desmoplakins from bovine muzzle epidermis showed specific reaction in several epithelial tissues with desmoplakin I (Mr 250,000) and desmoplakin II (Mr 215,000). By immunofluorescence microscopy, prominent punctate staining was observed in various lines of cultured epithelial cells, revealing desmosomal junctions at sites of established cell-to-cell contacts as well as hemidesmosomes and internalized desmosome-derived membrane domains. On frozen tissue sections punctate staining was observed along plasma membranes of epithelial cells, and electron microscopy using the immunoperoxidase technique revealed that the antibodies were specifically localized at the plaques associated with desmosomes and hemidesmosomes. Of a large number of non-epithelial cells examined positive staining was only observed on desmosome-like junctions of myocardial cells and Purkinje fiber cells. In both epithelial and myocardial tissues the antibodies showed a broad range of cross-reactivity between diverse vertebrate species such as man, cow, rodent, and chicken, indicating that desmoplakins contain determinants strongly conserved during evolution. When binding of these antibodies to cytoskeletal polypeptides separated by gel electrophoresis and blotted on nitrocellulose paper sheets was examined, specific reaction was noted with desmoplakin I and, to a variable degree, also desmoplakin II from various epithelial cells. Reaction was also observed with a myocardial polypeptide from bovine and human hearts which had a similar Mr value (250,000) and isoelectric pH range as desmoplakin I. We conclude that desmoplakins are the major proteins present in the desmosomal plaques of both epithelial and myocardial cells and that the desmoplakin polypeptides present in these two different cell types are very similar, if not identical.

Spatial distribution of proteins specific for desmosomes and adhaerens junctions in epithelial cells demonstrated by double immunofluorescence microscopy

The spatial relationships between the protein constituents to two junctional structures, adhaerens junctions and desmosomes, were determined by double immunofluorescence microscopy using marker proteins specific for these structures. Adhaerens junctions were visualized by immunofluorescent labeling for the membrane-associated protein vinculin and by their association with actin filaments. Desmosomal components were identified by labeling with antibodies to a group of minor desmosomal plaque proteins (DP1 antigens) and their association with filaments stained by cytokeratin antibodies. Double immunofluorescence microscopy of these components was performed in several tissues and cultured cells, including intact intestine, dissociated intestinal cells, and two morphologically different types of epithelial cells, cultured bovine kidney (MDBK), and mammary gland (BMGE) epithelial cells. This allowed the direct demonstration that each filament system is associated exclusively with its specific membrane-bound junctional protein. Vinculin and DP1-protein were found in distinct sites in the subapical intercellular junctional complex of intestinal epithelium and MDBK cells. Cell-substrate focal contacts contained vinculin and actin and showed no apparent relationships to the tonofilament system whereas intercellular contacts of BMGE cells were characterized by positive staining for DP1-protein and associated cytokeratin filaments. Immunolabeling of the cultured cells at different intervals after plating for the cytoskeletal elements and their membrane anchorage proteins was used to determine the temporal sequence of their organization. We propose that this approach may be used for the molecular definition and identification of cellular contacts and junctions as well as for studies of junction topology, dynamics and junction-cytoskeleton interactions, and junction biogenesis.

Dense bodies and actin polarity in vertebrate smooth muscle

The arrangement of cytoplasmic dense bodies in vertebrate smooth muscle and their relationship to the thin filaments was studied in cells from rabbit vas deferens and portal vein which were made hyperpermeable (skinned) with saponin and incubated with myosin subfragment 1 (S-1). The dense bodies were obliquely oriented, elongated structures sometimes appearing as chains up to 1.5 microns in length; they were often continuous across the cell for 200 to 300 nm and were interconnected by an oblique network of 10-nm filaments. The arrowheads, formed by S-1 decoration of actins, which inserted into both the sides and ends of dense bodies, always pointed away from the dense body, similar to the polarity of the thin filaments at the Z-bands of skeletal muscle. These results show that the cytoplasmic dense bodies function as anchoring sites for the thin filaments and indicate that the thin filaments, thick filaments, and dense bodies constitute a contractile unit.

Immunoelectron microscopic studies of the sites of cell-substratum and cell-cell contacts in cultured fibroblasts

Our object was to obtain information about the molecular structures present at cell-substratum and cell-cell contact sites formed by cultured fibroblasts. We have carried out double immunoelectron-microscopic labeling experiments on ultrathin frozen sections cut through such contact sites to determine the absolute and relative dispositions of the three proteins fibronectin, vinculin, and alpha-actinin with respect to these sites. (a) Three types of cell-substratum and cell-cell contact sites familiar from plastic sections could also be discriminated in the frozen sections by morphological criteria alone, i.e., the gap distances between the two surfaces, and the presence of submembranous densities. These types were: (i) focal adhesions (FA); (ii) close contacts (CC); and (iii) extracellular matrix contacts (ECM). This morphological typing of the contact sites allowed us to recognize and assign distinctive immunolabeling patterns for the three proteins to each type of site on the frozen sections. (b) FA sites were immunolabeled intracellularly for vinculin and alpha-actinin, with vinculin labeling situated closer to the membrane than alpha-actinin. Fibronectin was not labeled in the narrow gap between the cell surface and the substratum, or between two cells, at FA sites. Control experiments showed that this could not be ascribed to inaccessibility of the FA narrow gap to the immunolabeling reagents but indicated an absence or severe depletion of fibronectin from these sites. (c) CC sites were labeled intracellularly for alpha-actinin but not vinculin and were labeled extracellularly for fibronectin. (d) ECM sites were characterized by large separations (often greater than 100 nm) between the cell and substratum or between two cells, which were connected by long cables of extracellular matrix components, including fibronectin. In late (24-36 h) cultures, ECM contacts predominated over the other types. ECM sites appeared to be of two kinds, one labeled intracellularly for both alpha-actinin and vinculin, the other for alpha-actinin alone. (e) From these and other results, a coherent but tentative scheme is proposed for the molecular ultrastructure of these contacts sites, and specific functional roles are suggested for fibronectin, vinculin, and alpha-actinin in cell adhesion and in the linkage of intracellular microfilaments to membranes at the different types of contact sites.