Effects of abnormal cytoskeletal structure on erythrocyte membrane mechanical properties

Membrane Tension and the Role of Ezrin During Phagocytosis

During phagocytosis, there is an apparent expansion of the plasma membrane to accommodate the target within a phagosome. This is accompanied (or driven by) a change in membrane tension. It is proposed that the wrinkled topography of the phagocyte surface, by un-wrinkling, provides the additional available membrane and that this explains the changes in membrane tension. There is no agreement as to the mechanism by which unfolding of cell surface wrinkles occurs during phagocytosis, but there is a good case building for the involvement of the actin-plasma membrane crosslinking protein ezrin. Not only have direct measurements of membrane tension strongly implicated ezrin as the key component in establishing membrane tension, but the cortical location of ezrin changes at the phagocytic cup, suggesting that it is locally signalled. This chapter therefore attempts to synthesise our current state of knowledge about ezrin and membrane tension with phagocytosis to provide a coherent hypothesis.

Unfolding of membrane ruffles of in situ chondrocytes under compressive loads

Impact loading results in chondrocyte death. Previous studies implicated high tensile strain rates in chondrocyte membranes as the cause of impact-induced cell deaths. However, this hypothesis relies on the untested assumption that chondrocyte membranes unfold in vivo during physiological tissue compression, but do not unfold during impact loading. Although membrane unfolding has been observed in isolated chondrocytes during osmotically induced swelling and mechanical compression, it is not known if membrane unfolding also occurs in chondrocytes embedded in their natural extracellular matrix. This study was aimed at quantifying changes in membrane morphology of in situ superficial zone chondrocytes during slow physiological cartilage compression. Bovine cartilage-bone explants were loaded at 5 μm/s to nominal compressive strains ranging from 0% to 50%. After holding the final strains for 45 min, the loaded cartilage was chemically pre-fixed for 12 h. The cartilage layer was post-processed for visualization of cell ultrastructure using electron microscopy. The changes in membrane morphology in superficial zone cells were quantified from planar electron micrographs by measuring the roughness and the complexity of the cell surfaces. Qualitatively, the cell surface ruffles that existed before loading disappeared when cartilage was loaded. Quantitatively, the roughness and complexity of cell surfaces decreased with increasing load magnitudes, suggesting a load-dependent use of membrane reservoirs. Chondrocyte membranes unfold in a load-dependent manner when cartilage is compressed. Under physiologically meaningful loading conditions, the cells likely expand their surface through unfolding of the membrane ruffles, and therefore avoid direct stretch of the cell membrane. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:304-310, 2017.

MD/DPD Multiscale Framework for Predicting Morphology and Stresses of Red Blood Cells in Health and Disease

Healthy red blood cells (RBCs) have remarkable deformability, squeezing through narrow capillaries as small as 3 microns in diameter without any damage. However, in many hematological disorders the spectrin network and lipid bilayer of diseased RBCs may be significantly altered, leading to impaired functionality including loss of deformability. We employ a two-component whole-cell multiscale model to quantify the biomechanical characteristics of the healthy and diseased RBCs, including Plasmodium falciparum-infected RBCs (Pf-RBCs) and defective RBCs in hereditary disorders, such as spherocytosis and elliptocytosis. In particular, we develop a two-step multiscale framework based on coarse-grained molecular dynamics (CGMD) and dissipative particle dynamics (DPD) to predict the static and dynamic responses of RBCs subject to tensile forcing, using experimental information only on the structural defects in the lipid bilayer, cytoskeleton, and their interaction. We first employ CGMD on a small RBC patch to compute the shear modulus, bending stiffness, and network parameters, which are subsequently used as input to a whole-cell DPD model to predict the RBC shape and corresponding stress field. For Pf-RBCs at trophozoite and schizont stages, the presence of cytoadherent knobs elevates the shear response in the lipid bilayer and stiffens the RBC membrane. For RBCs in spherocytosis and elliptocytosis, the bilayer-cytoskeleton interaction is weakened, resulting in substantial increase of the tensile stress in the lipid bilayer. Furthermore, we investigate the transient behavior of stretching deformation and shape relaxation of the normal and defective RBCs. Different from the normal RBCs possessing high elasticity, our simulations reveal that the defective RBCs respond irreversibly, i.e., they lose their ability to recover the normal biconcave shape in successive loading cycles of stretching and relaxation. Our findings provide fundamental insights into the microstructure and biomechanics of RBCs, and demonstrate that the two-step multiscale framework presented here can be used effectively for in silico studies of hematological disorders based on first principles and patient-specific experimental input at the protein level.

Physical view on migration modes

Cellular motility is essential for many processes such as embryonic development, wound healing processes, tissue assembly and regeneration, immune cell trafficing and diseases such as cancer. The migration efficiency and the migratory potential depend on the type of migration mode. The previously established migration modes such as epithelial (non-migratory) and mesenchymal (migratory) as well as amoeboid (squeezing motility) relay mainly on phenomenological criteria such as cell morphology and molecular biological criteria such as gene expression. However, the physical view on the migration modes is still not well understood. As the process of malignant cancer progression such as metastasis depends on the migration of single cancer cells and their migration mode, this review focuses on the different migration strategies and discusses which mechanical prerequisites are necessary to perform a special migration mode through a 3-dimensional microenvironment. In particular, this review discusses how cells can distinguish and finally switch between the migration modes and what impact do the physical properties of cells and their microenvironment have on the transition between the novel migration modes such as blebbing and protrusive motility.

Nanomechanical and surface properties of rMSCs post-exposure to CAP treated UHMWPE wear particles

Wear debris generated by ultra-high molecular weight polyethylene (UHMWPE) used in joint replacement devices has been of concern due to reductions of the implant longevity. Cold atmospheric plasma (CAP) has been used to improve the wear performance of UHMWPE. Our aim was to investigate the elastic and adhesive properties of rat mesenchymal stem cells (rMSCs), through AFM, after exposure to UHMWPE wear debris pre- and post-CAP treatment. The results indicated that the main changes in cell elasticity and spring constant of MSC exposed to wear particles occurred in the first 24 h of contact and the particle concentration from 0.5 to 50 mg/l did not play a significant role. For UHMWPE treated for 7.5 min, with progression of the wear simulation the results of the CAP treated samples were getting closer to the result of untreated samples; while with longer CAP treatment this was not observed.

From the Clinical Editor

Joint replacements are now common clinical practice. However, the use of ultra-high molecular weight polyethylene (UHMWPE) still poses a concern, due to the presence of wear debris. The authors here investigated the effects of wear debris after cold atmospheric plasma treatment on rat mesenchymal stem cells. The positive results provided new strategies in future design of joint replacement materials.

Ezrin is a Major Regulator of Membrane Tension in Epithelial Cells

Plasma membrane tension is responsible for a variety of cellular functions such as motility, cell division, and endocytosis. Since membrane tension is dominated by the attachment of the actin cortex to the inner leaflet of the plasma membrane, we investigated the importance of ezrin, a major cross-linker of the membrane-cytoskeleton interface, for cellular mechanics of confluent MDCK II cells. For this purpose, we carried out ezrin depletion experiments and also enhanced the number of active ezrin molecules at the interface. Mechanical properties were assessed by force indentation experiments followed by membrane tether extraction. PIP₂ micelles were injected into individual living cells to reinforce the linkage between plasma membrane and actin-cortex, while weakening of this connection was reached by ezrin siRNA and administration of the inhibitors neomycin and NSC 668394, respectively. We observed substantial stiffening of cells and an increase in membrane tension after addition of PIP₂ micelles. In contrast, reduction of active ezrin led to a decrease of membrane tension accompanied by loss of excess surface area, increase in cortical tension, remodelling of actin cytoskeleton, and reduction of cell height. The data confirm the importance of the ezrin-mediated connection between plasma membrane and cortex for cellular mechanics and cell morphology.

The class II PI 3-kinase, PI3KC2α, links platelet internal membrane structure to shear-dependent adhesive function

PI3KC2α is a broadly expressed lipid kinase with critical functions during embryonic development but poorly defined roles in adult physiology. Here we utilize multiple mouse genetic models to uncover a role for PI3KC2α in regulating the internal membrane reserve structure of megakaryocytes (demarcation membrane system) and platelets (open canalicular system) that results in dysregulated platelet adhesion under haemodynamic shear stress. Structural alterations in the platelet internal membrane lead to enhanced membrane tether formation that is associated with accelerated, yet highly unstable, thrombus formation in vitro and in vivo. Notably, agonist-induced 3-phosphorylated phosphoinositide production and cellular activation are normal in PI3KC2α-deficient platelets. These findings demonstrate an important role for PI3KC2α in regulating shear-dependent platelet adhesion via regulation of membrane structure, rather than acute signalling. These studies provide a link between the open canalicular system and platelet adhesive function that has relevance to the primary haemostatic and prothrombotic function of platelets.

The properties of chondrocyte membrane reservoirs and their role in impact-induced cell death

Impact loading of articular cartilage causes extensive chondrocyte death. Cell membranes have a limited elastic range of 3-4% strain but are protected from direct stretch during physiological loading by their membrane reservoir, an intricate pattern of membrane folds. Using a finite-element model, we suggested previously that access to the membrane reservoir is strain-rate-dependent and that during impact loading, the accessible membrane reservoir is drastically decreased, so that strains applied to chondrocytes are directly transferred to cell membranes, which fail when strains exceed 3-4%. However, experimental support for this proposal is lacking. The purpose of this study was to measure the accessible membrane reservoir size for different membrane strain rates using membrane tethering techniques with atomic force microscopy. We conducted atomic force spectroscopy on isolated chondrocytes (n = 87). A micron-sized cantilever was used to extract membrane tethers from cell surfaces at constant pulling rates. Membrane tethers could be identified as force plateaus in the resulting force-displacement curves. Six pulling rates were tested (1, 5, 10, 20, 40, and 80 μm/s). The size of the membrane reservoir, represented by the membrane tether surface areas, decreased exponentially with increasing pulling rates. The current results support our theoretical findings that chondrocytes exposed to impact loading die because of membrane ruptures caused by high tensile membrane strain rates.

The role and regulation of blebs in cell migration

Blebs are cellular protrusions that have been shown to be instrumental for cell migration in development and disease. Bleb expansion is driven by hydrostatic pressure generated in the cytoplasm by the contractile actomyosin cortex. The mechanisms of bleb formation thus fundamentally differ from the actin polymerization-based mechanisms responsible for lamellipodia expansion. In this review, we summarize recent findings relevant for the mechanics of bleb formation and the underlying molecular pathways. We then review the processes involved in determining the type of protrusion formed by migrating cells, in particular in vivo, in the context of embryonic development. Finally, we discuss how cells utilize blebs for their forward movement in the presence or absence of strong substrate attachment.

Control of cortical rigidity by the cytoskeleton: emerging roles for septins

The cortex is the outermost region of the cell, comprising all of the elements from the plasma membrane to the cortical actin cytoskeleton that cooperate to maintain the cell's shape and topology. In eukaryotes without cell walls, this cortex governs the contact between their plasma membranes and the environment and thereby influences cell shape, motility, and signaling. It is therefore of considerable interest to understand how cells control their cortices, both globally and with respect to small subdomains. Here we review the current understanding of this control, including the regulation of cell shape by balances of outward hydrostatic pressure and cortical tension. The actomyosin cytoskeleton is the canonical regulator of cortical rigidity and indeed many would consider the cortex to comprise the actin cortex nearly exclusively. However, this actomyosin array is intimately linked to the membrane, for example via ERM and PIP2 proteins. Additionally, the lipid membrane likely undergoes rigidification by other players, such as Bin-Amphiphysin-Rvs proteins. Recent data also indicates that the septin cytoskeleton may play a formidable and more direct role in stabilization of membranes, particularly in contexts where cells receive limited external stabilization from their environments. Here, we review how septins may play this role, drawing on their physical form, their ability to directly bind and modify membranes and actomyosin, and their interactions with vesicular machinery. Deficiencies and alterations in the nature of the septin cytoskeleton may thus be relevant in multiple disease settings.

Distinct membrane mechanical properties of human mesenchymal stem cells determined using laser optical tweezers

The therapeutic efficacy of mesenchymal stem cells (MSCs) in tissue engineering and regenerative medicine is determined by their unique biological, mechanical, and physicochemical characteristics, which are yet to be fully explored. Cell membrane mechanics, for example, has been shown to critically influence MSC differentiation. In this study, we used laser optical tweezers to measure the membrane mechanics of human MSCs and terminally differentiated fibroblasts by extracting tethers from the outer cell membrane. The average tether lengths were 10.6+/-1.1 microm (hMSC) and 3.0+/-0.5 microm (fibroblasts). The tether extraction force did not increase during tether formation, which suggests existence of a membrane reservoir intended to buffer membrane tension fluctuations. Cytoskeleton disruption resulted in a fourfold tether length increase in fibroblasts but had no effect in hMSCs, indicating weak association between the cell membrane and hMSC actin cytoskeleton. Cholesterol depletion, known to decrease lipid bilayer stiffness, caused an increase in the tether length both in fibroblasts and hMSCs, as does the treatment of cells with DMSO. We postulate that whereas fibroblasts use both the membrane rigidity and membrane-cytoskeleton association to regulate their membrane reservoir, hMSC cytoskeleton has only a minor impact on stem cell membrane mechanics.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Characteristics of a membrane reservoir buffering membrane tension

When membrane-attached beads are pulled vertically by a laser tweezers, a membrane tube of constant diameter (tether) is formed. We found that the force on the bead (tether force) did not depend on tether length over a wide range of tether lengths, which indicates that a previously unidentified reservoir of membrane and not stretch of the plasma membrane provides the tether membrane. Plots of tether force vs. tether length have an initial phase, an elongation phase, and an exponential phase. During the major elongation phase, tether force is constant, buffered by the "membrane reservoir." Finally, there is an abrupt exponential rise in force that brings the tether out of the trap, indicating depletion of the membrane reservoir. In chick embryo fibroblasts and 3T3 fibroblasts, the maximum tether lengths that can be pulled at a velocity of 4 microm/s are 5.1 +/- 0. 3 and 5.0 +/- 0.2 microm, respectively. To examine the importance of the actin cytoskeleton, we treated cells with cytochalasin B or D and found that the tether lengths increased dramatically to 13.8 +/- 0.8 and 12.0 +/- 0.7 microm, respectively. Similarly, treatment of the cells with colchicine and nocodazole results in more than a twofold increase in tether length. We found that elevation of membrane tension (through osmotic pressure, a long-term elevation of tether force, or a number of transitory increases) increased reservoir size over the whole cell. Using a tracking system to hold tether force on the bead constant near its maximal length in the exponential phase, the rate of elongation of the tethers was measured as a function of tether force (membrane tension). The rate of elongation of tethers was linearly dependent on the tether force and reflected an increase in size of the reservoir. Increases in the reservoir caused by tension increases on one side of the cell caused increases in reservoir size on the other side of the cell. Thus, we suggest that cells maintain a plasma membrane reservoir to buffer against changes in membrane tension and that the reservoir is increased with membrane tension or disruption of the cytoskeleton.

Calculation of a Gap restoration in the membrane skeleton of the red blood cell: possible role for myosin II in local repair

Human red blood cells contain all of the elements involved in the formation of nonmuscle actomyosin II complexes (V. M. Fowler. 1986. J. Cell. Biochem. 31:1-9; 1996. Curr. Opin. Cell Biol. 8:86-96). No clear function has yet been attributed to these complexes. Using a mathematical model for the structure of the red blood cell spectrin skeleton (M. J. Saxton. 1992. J. Theor. Biol. 155:517-536), we have explored a possible role for myosin II bipolar minifilaments in the restoration of the membrane skeleton, which may be locally damaged by major mechanical or chemical stress. We propose that the establishment of stable links between distant antiparallel actin protofilaments after a local myosin II activation may initiate the repair of the disrupted area. We show that it is possible to define conditions in which the calculated number of myosin II minifilaments bound to actin protofilaments is consistent with the estimated number of myosin II minifilaments present in the red blood cells. A clear restoration effect can be observed when more than 50% of the spectrin polymers of a defined area are disrupted. It corresponds to a significant increase in the spectrin density in the protein free region of the membrane. This may be involved in a more complex repair process of the red blood cell membrane, which includes the vesiculation of the bilayer and the compaction of the disassembled spectrin network.

Influence of network topology on the elasticity of the red blood cell membrane skeleton

A finite-element network model is used to investigate the influence of the topology of the red blood cell membrane skeleton on its macroscopic mechanical properties. Network topology is characterized by the number of spectrin oligomers per actin junction (phi a) and the number of spectrin dimers per self-association junction (phi s). If it is assumed that all associated spectrin is in tetrameric form, with six tetramers per actin junction (i.e., phi a = 6.0 and phi s = 2.0), then the topology of the skeleton may be modeled by a random Delaunay triangular network. Recent images of the RBC membrane skeleton suggest that the values for these topological parameters are in the range of 4.2 < phi a < 5.5 and 2.1 < phi s < 2.3. Model networks that simulate these realistic topologies exhibit values of the shear modulus that vary by more than an order of magnitude relative to triangular networks. This indicates that networks with relatively sparse nontriangular topologies may be needed to model the RBC membrane skeleton accurately. The model is also used to simulate skeletal alterations associated with hereditary spherocytosis and Southeast Asian ovalocytosis.

Kinematics of red cell aspiration by fluorescence-imaged microdeformation

Maps of fluorescing red cell membrane components on a pipette-aspirated projection are quantitated in an effort to elucidate and unify the heterogeneous kinematics of deformation. Transient gradients of diffusing fluorescent lipid first demonstrate the fluidity of an otherwise uniform-density bilayer and corroborate a "universal" calibration scale for relative surface density. A steep but smooth and stable gradient in the densities of the skeleton components spectrin, actin, and protein 4.1 is used to estimate large elastic strains along the aspirated skeleton. The deformation fields are argued to be an unhindered response to loading in the surface normal direction. Density maps intermediate to those of the compressible skeleton and fluid bilayer are exhibited by particular transmembrane proteins (e.g., Band 3) and yield estimates for the skeleton-connected fractions. Such connected proteins appear to occupy a significant proportion of the undeformed membrane surface and can lead to steric exclusion of unconnected integral membrane proteins from regions of network condensation. Consistent with membrane repatterning kinematics in reversible deformation, final vesiculation of the projection tip produces a cell fragment concentrated in freely diffusing proteins but depleted of skeleton.

Mechanochemistry of protein 4.1's spectrin-actin-binding domain: ternary complex interactions, membrane binding, network integration, structural strengthening

Mechanical strength of the red cell membrane is dependent on ternary interactions among the skeletal proteins, spectrin, actin, and protein 4.1. Protein 4.1's spectrin-actin-binding (SAB) domain is specified by an alternatively spliced exon encoding 21 amino acid (aa) and a constitutive exon encoding 59 aa. A series of truncated SAB peptides were engineered to define the sequences involved in spectrin-actin interactions, and also membrane strength. Analysis of in vitro supramolecular assemblies showed that gelation activity of SAB peptides correlates with their ability to recruit a critical amount of spectrin into the complex to cross-link actin filaments. Also, several SAB peptides appeared to exhibit a weak, cooperative actin-binding activity which mapped to the first 26 residues of the constitutive 59 aa. Fluorescence-imaged microdeformation was used to show SAB peptide integration into the elastic skeletal network of spectrin, actin, and protein 4.1. In situ membrane-binding and membrane-strengthening abilities of the SAB peptides correlated with their in vitro gelation activity. The findings imply that sites for strong spectrin binding include both the alternative 21-aa cassette and a conserved region near the middle of the 59 aa. However, it is shown that only weak SAB affinity is necessary for physiologically relevant action. Alternatively spliced exons can thus translate into strong modulation of specific protein interactions, economizing protein function in the cell without, in and of themselves, imparting unique function.

Reductions of erythrocyte membrane viscoelastic coefficients reflect spectrin deficiencies in hereditary spherocytosis

Hereditary spherocytosis is a common hemolytic anemia associated with deficiencies in spectrin, the principal structural protein of the erythrocyte membrane-skeleton. We have examined 20 different individuals from 10 spherocytosis kindreds and 2 elliptocytosis kindreds to determine the effects of different levels of spectrin deficiency on the viscoelastic properties of the erythrocyte membrane. Micropipettes were used to perform single-cell micromechanical measurements of approximately 1,000 individual cells to determine the membrane elastic shear modulus, the apparent membrane bending stiffness, and whole cell recovery time constant for the different cell populations. The membrane viscosity was calculated by the product of the shear modulus and the recovery time constant. Results show correlation between the fractional reduction in shear modulus and the fractional reduction in spectrin content (determined by spectrin radioimmunoassay) and spectrin density (determined by the ratios of spectrin to band 3 on electrophoresis gels) suggesting that membrane shear elasticity is directly proportional to the surface density of spectrin on the membrane (P less than 0.001). The apparent membrane bending stiffness is also reduced in proportion to the density of spectrin (P less than 0.001). The membrane viscosity is reduced relative to control (P less than 0.001), but the nature of the relationship between spectrin density and membrane viscosity is less clearly defined. These studies document striking relationships between partial deficiencies of erythrocyte spectrin and specific viscoelastic properties of the mutant membranes.

Abnormal oxidant sensitivity and beta-chain structure of spectrin in hereditary spherocytosis associated with defective spectrin-protein 4.1 binding

Hereditary spherocytosis (HS) is an inherited disorder of erythrocyte shape associated with spectrin deficiency and hemolytic anemia. In a subset of patients with the autosomal dominant form of HS, spectrin displays a reduced capacity to bind protein 4.1 and, therefore, actin; both functions that are critical to the membrane skeleton. A specific structural defect has not been identified in the spectrin from these patients. Chymotryptic digestion of the isolated spectrin chains shows impaired cleavage of the distal peptide of the beta subunit, the beta IV domain. In previous work, we have shown that mild oxidation markedly diminishes the binding capacity of normal spectrin for protein 4.1. Here we observe that chemical reduction of freshly isolated, untreated HS spectrin dramatically improves its function. Thus, a primary structural defect in the beta subunit of spectrin in this subtype of HS may lead to oxidant sensitivity, and secondarily, to a functional defect in the binding of spectrin to protein 4.1 and actin.

Effects of inherited membrane abnormalities on the viscoelastic properties of erythrocyte membrane

Several workers have identified molecular abnormalities associated with inherited blood disorders. The present work examines how these alterations in molecular structure affect the viscoelastic properties of the red blood cell membrane. Changes in the membrane shear modulus, the membrane viscosity, and the apparent membrane bending stiffness were observed in cells of eight patients having a variety of disorders: Two had reductions in the number of high-affinity ankyrin binding sites, two had abnormalities associated with the protein band 4.1, and six were known to be deficient in spectrin. The data suggest that the membrane shear modulus is proportional to the density of spectrin on the membrane and support the view that spectrin is primarily responsible for membrane shear elasticity. Although membranes having abnormalities associated with the function of ankyrin or band 4.1 exhibited reduced elasticity, the degree of mechanical dysfunction was quantitatively inconsistent with the extent of the molecular abnormality. This indicates that these skeletal components do not play a primary role in determining membrane shear elasticity. The membrane viscosity was reduced in seven of the eight patients studied. The reduction in viscosity was usually greater than the reduction in shear modulus, but the degree of reduction in viscosity was variable and did not correlate well with the degree of molecular abnormality.

Talin at myotendinous junctions

Junctions formed by skeletal muscles where they adhere to tendons, called myotendinous junctions, are sites of tight adhesion and where forces generated by the cell are placed on the substratum. In this regard, myotendinous junctions and focal contacts of fibroblasts in vitro are analogues. Talin is a protein located at focal contacts that may be involved in force transmission from actin filaments to the plasma membrane. This study investigates whether talin is also found at myotendinous junctions. Protein separations on SDS polyacrylamide gels and immunolabeling procedures show that talin is present in skeletal muscle. Immunofluorescence microscopy using anti-talin indicates that talin is found concentrated at myotendinous junctions and in lesser amounts in periodic bands over nonjunctional regions. Electron microscopic immunolabeling shows talin is a component of the digitlike processes of muscle cells that extend into tendons at myotendinous junctions. These findings indicate that there may be similarities in the molecular composition of focal contacts and myotendinous junctions in addition to functional analogies.