Localization of talin in skeletal and cardiac muscles

Cooperation between myofibril growth and costamere maturation in human cardiomyocytes

Costameres, as striated muscle-specific cell adhesions, anchor both M-lines and Z-lines of the sarcomeres to the extracellular matrix. Previous studies have demonstrated that costameres intimately participate in the initial assembly of myofibrils. However, how costamere maturation cooperates with myofibril growth is still underexplored. In this work, we analyzed zyxin (costameres), α-actinin (Z-lines) and myomesin (M-lines) to track the behaviors of costameres and myofibrils within the cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CMs). We quantified the assembly and maturation of costameres associated with the process of myofibril growth within the hiPSC-CMs in a time-dependent manner. We found that asynchrony existed not only between the maturation of myofibrils and costameres, but also between the formation of Z-costameres and M-costameres that associated with different structural components of the sarcomeres. This study helps us gain more understanding of how costameres assemble and incorporate into the cardiomyocyte sarcomeres, which sheds a light on cardiomyocyte mechanobiology.

The Cryogenic Electron Microscopy Structure of the Cell Adhesion Regulator Metavinculin Reveals an Isoform-Specific Kinked Helix in Its Cytoskeleton Binding Domain

Vinculin and its heart-specific splice variant metavinculin are key regulators of cell adhesion processes. These membrane-bound cytoskeletal proteins regulate the cell shape by binding to several other proteins at cell-cell and cell-matrix junctions. Vinculin and metavinculin link integrin adhesion molecules to the filamentous actin network. Loss of both proteins prevents cell adhesion and cell spreading and reduces the formation of stress fibers, focal adhesions, or lamellipodia extensions. The binding of talin at cell-matrix junctions or of α-catenin at cell-cell junctions activates vinculin and metavinculin by releasing their autoinhibitory head-tail interaction. Once activated, vinculin and metavinculin bind F-actin via their five-helix bundle tail domains. Unlike vinculin, metavinculin has a 68-amino-acid insertion before the second α-helix of this five-helix F-actin-binding domain. Here, we present the full-length cryogenic electron microscopy structure of metavinculin that captures the dynamics of its individual domains and unveiled a hallmark structural feature, namely a kinked isoform-specific α-helix in its F-actin-binding domain. Our identified conformational landscape of metavinculin suggests a structural priming mechanism that is consistent with the cell adhesion functions of metavinculin in response to mechanical and cellular cues. Our findings expand our understanding of metavinculin function in the heart with implications for the etiologies of cardiomyopathies.

A short review: Doxorubicin and its effect on cardiac proteins

Doxorubicin (DOX) is a boon for cancer-suffering patients. However, the undesirable effect on health on vital organs, especially the heart, is a limiting factor, resulting in an increased number of patients with cardiac dysfunction. The present review focuses on the contractile machinery and associated factors, which get affected due to DOX toxicity in chemo-patients for which they are kept under life-long investigation for cardiac function. DOX-induced oxidative stress disrupts the integrity of cardiac contractile muscle proteins that alter the rhythmic mechanism and oxygen consumption rate of the heart. DOX is an oxidant and it is further discussed that oxidative stress prompts the damage of contractile components and associated factors, which include Ca²⁺ load through Ca²⁺ ATPase, SERCA, ryanodine receptor-2, phospholamban, and calsequestrin, which ultimately results in left ventricular ejection and dilation. Based on data and evidence, the associated proteins can be considered as clinical markers to develop medications for patients. Even with the advancement of various diagnosing tools and modified drugs to mitigate DOX-induced cardiotoxicity, the risk could not be surmounted with survivors of cancer.

The intercalated disc: a mechanosensing signalling node in cardiomyopathy

Cardiomyocytes, the cells generating contractile force in the heart, are connected to each other through a highly specialised structure, the intercalated disc (ID), which ensures force transmission and transduction between neighbouring cells and allows the myocardium to function in synchrony. In addition, cardiomyocytes possess an intrinsic ability to sense mechanical changes and to regulate their own contractile output accordingly. To achieve this, some of the components responsible for force transmission have evolved to sense changes in tension and to trigger a biochemical response that results in molecular and cellular changes in cardiomyocytes. This becomes of particular importance in cardiomyopathies, where the heart is exposed to increased mechanical load and needs to adapt to sustain its contractile function. In this review, we will discuss key mechanosensing elements present at the intercalated disc and provide an overview of the signalling molecules involved in mediating the responses to changes in mechanical force.

Ankyrin Repeat Domain 1 Protein: A Functionally Pleiotropic Protein with Cardiac Biomarker Potential

The ankyrin repeat domain 1 (ANKRD1) protein is a cardiac-specific stress-response protein that is part of the muscle ankyrin repeat protein family. ANKRD1 is functionally pleiotropic, playing pivotal roles in transcriptional regulation, sarcomere assembly and mechano-sensing in the heart. Importantly, cardiac ANKRD1 has been shown to be highly induced in various cardiomyopathies and in heart failure, although it is still unclear what impact this may have on the pathophysiology of heart failure. This review aims at highlighting the known properties, functions and regulation of ANKRD1, with focus on the underlying mechanisms that may be involved. The current views on the actions of ANKRD1 in cardiovascular disease and its utility as a candidate cardiac biomarker with diagnostic and/or prognostic potential are also discussed. More studies of ANKRD1 are warranted to obtain deeper functional insights into this molecule to allow assessment of its potential clinical applications as a diagnostic or prognostic marker and/or as a possible therapeutic target.

Talin1 is required for cardiac Z-disk stabilization and endothelial integrity in zebrafish

Talin (tln) binds and activates integrins to couple extracellular matrix-bound integrins to the cytoskeleton; however, its role in heart development is not well characterized. We identified the defective gene and the resulting cardiovascular phenotypes in zebrafish tln1(fl02k) mutants. The ethylnitrosourea-induced fl02k mutant showed heart failure, brain hemorrhage, and diminished cardiac and vessel lumens at 52 h post fertilization. Positional cloning revealed a nonsense mutation of tln1 in this mutant. tln1, but neither tln2 nor -2a, was dominantly expressed in the heart and vessels. Unlike tln1 and -2 in the mouse heart, the unique tln1 expression in the heart enabled us, for the first time, to determine the critical roles of Tln1 in the maintenance of cardiac sarcomeric Z-disks and endothelial/endocardial cell integrity, partly through regulating F-actin networks in zebrafish. The similar expression profiles of tln1 and integrin β1b (itgb1b) and synergistic function of the 2 genes revealed that itgb1b is a potential partner for tln1 in the stabilization of cardiac Z-disks and vessel lumens. Taken together, the results of this work suggest that Tln1-mediated Itgβ1b plays a crucial role in maintaining cardiac sarcomeric Z-disks and endothelial/endocardial cell integrity in zebrafish and may also help to gain molecular insights into congenital heart diseases.

Actin scaffolding by clathrin heavy chain is required for skeletal muscle sarcomere organization

Clathrin heavy chain contributes to the formation and maintenance of the contractile apparatus in skeletal muscle through interactions with costameric proteins.

The ubiquitous clathrin heavy chain (CHC), the main component of clathrin-coated vesicles, is well characterized for its role in intracellular membrane traffic and endocytosis from the plasma membrane (PM). Here, we demonstrate that in skeletal muscle CHC regulates the formation and maintenance of PM–sarcomere attachment sites also known as costameres. We show that clathrin forms large coated lattices associated with actin filaments and the muscle-specific isoform of α-actinin at the PM of differentiated myotubes. Depletion of CHC in myotubes induced a loss of actin and α-actinin sarcomeric organization, whereas CHC depletion in vivo induced a loss of contractile force due to the detachment of sarcomeres from the PM. Our results suggest that CHC contributes to the formation and maintenance of the contractile apparatus through interactions with costameric proteins and highlight an unconventional role for CHC in skeletal muscle that may be relevant to pathophysiology of neuromuscular disorders.

Arginylation-dependent regulation of a proteolytic product of talin is essential for cell-cell adhesion

Talin is a large scaffolding molecule that plays a major role in integrin-dependent cell-matrix adhesion. A role for talin in cell-cell attachment through cadherin has never been demonstrated, however. Here, we identify a novel calpain-dependent proteolytic cleavage of talin that results in the release of a 70-kD C-terminal fragment, which serves as a substrate of posttranslational arginylation. The intracellular levels of this fragment closely correlated with the formation of cell-cell adhesions, and this fragment localized to cadherin-containing cell-cell contacts. Moreover, reintroduction of this fragment rescued the cell-cell adhesion defects in arginyltransferase (Ate1) knockout cells, which normally have a very low level of this fragment. Arginylation of this fragment further enhanced its ability to rescue cell-cell adhesion formation. In addition, arginylation facilitated its turnover, suggesting a dual role of arginylation in its intracellular regulation. Thus, our work identifies a novel proteolytic product of talin that is regulated by arginylation and a new role of talin in cadherin-dependent cell-cell adhesion.

Costameric proteins in human skeletal muscle during muscular inactivity

Costameres are regions that are associated with the sarcolemma of skeletal muscle fibres and comprise proteins of the dystrophin-glycoprotein complex and vinculin-talin-integrin system. Costameres play both a mechanical and a signalling role, transmitting force from the contractile apparatus to the extracellular matrix in order to stabilize skeletal muscle fibres during contraction and relaxation. Recently, it was shown that bidirectional signalling occurs between sarcoglycans and integrins, with muscle agrin potentially interacting with both types of protein to enable signal transmission. Although numerous studies have been carried out on skeletal muscle diseases, such as Duchenne muscular dystrophy, recessive autosomal muscular dystrophies and other skeletal myopathies, insufficient data exist on the relationship between costameres and the pathology of the second motor nerve and between costameric proteins and muscle agrin in other conditions in which skeletal muscle atrophy occurs. Previously, we carried out a preliminary study on skeletal muscle from patients with sensitive-motor polyneuropathy, in which we analysed the distribution of sarcoglycans, integrins and agrin by immunostaining only. In the present study, we have examined the skeletal muscle fibres of ten patients with sensitive-motor polyneuropathy. We used immunofluorescence and reverse transcriptase PCR to examine the distribution of vinculin, talin and dystrophin, in addition to that of those proteins previously studied. Our aim was to characterize in greater detail the distribution and expression of costameric proteins and muscle agrin during this disease. In addition, we used transmission electron microscopy to evaluate the structural damage of the muscle fibres. The results showed that immunostaining of alpha 7B-integrin, beta 1D-integrin and muscle agrin appeared to be severely reduced, or almost absent, in the muscle fibres of the diseased patients, whereas staining of alpha 7A-integrin appeared normal, or slightly increased, compared with that in normal skeletal muscle fibres. We also observed a lower level of alpha 7B- and beta 1D-integrin mRNA and a normal, or slightly higher than normal, level of alpha 7A-integrin mRNA in the skeletal muscle fibres of the patients with sensitive-motor polyneuropathy, compared with those in the skeletal muscle of normal patients. Additionally, transmission electron microscopy of transverse sections of skeletal muscle fibres indicated that the normal muscle fibre architecture was disrupted, with no myosin present inside the actin hexagons. Based on our results, we hypothesize that skeletal muscle inactivity, such as that found after denervation, could result in a reorganization of the costameres, with alpha 7B-integrin being replaced by alpha 7A-integrin. In this way, the viability of the skeletal muscle fibre is maintained. It will be interesting to clarify, by future experimentation, the mechanisms that lead to the down-regulation of integrins and agrin in muscular dystrophies.

Identification of a repeated domain within mammalian alpha-synemin that interacts directly with talin

The type VI intermediate filament (IF) protein synemin is a unique member of the IF protein superfamily. Synemin associates with the major type III IF protein desmin forming heteropolymeric intermediate filaments (IFs) within developed mammalian striated muscle cells. These IFs encircle and link all adjacent myofibrils together at their Z-lines, as well as link the Z-lines of the peripheral layer of cellular myofibrils to the costameres located periodically along and subjacent to the sarcolemma. Costameres are multi-protein assemblies enriched in the cytoskeletal proteins vinculin, alpha-actinin, and talin. We report herein a direct interaction of human alpha-synemin with the cytoskeletal protein talin by protein-protein interaction assays. The 312 amino acid insert (SNTIII) present only within alpha-synemin binds to the rod domain of talin in vitro and co-localizes with talin at focal adhesion sites within mammalian muscle cells. Confocal microscopy studies showed that synemin co-localizes with talin within the costameres of human skeletal muscle cells. Analysis of the primary sequences of human alpha- and beta-synemins revealed that SNTIII is composed of seven tandem repeats, each containing a specific Ser/Thr-X-Arg-His/Gln (S/T-X-R-H/Q) motif. Our results suggest human alpha-synemin plays an essential role in linking the heteropolymeric IFs to adherens-type junctions, such as the costameres within mammalian striated muscle cells, via its interaction with talin, thereby helping provide mechanical integration for the muscle cell cytoskeleton.

Accelerated de novo sarcomere assembly by electric pulse stimulation in C2C12 myotubes

The assembly of sarcomeres, the smallest contractile units in striated muscle, is a complex and highly coordinated process that relies on spatio-temporal organization of sarcomeric proteins, a process requiring spontaneous Ca(2+) transients. To investigate the relationship between Ca(2+) transients and sarcomere assembly in C2C12 myotubes, we employed electric pulse stimulation (EPS), which allows the frequency of Ca(2+) transients to be manipulated. We monitored contractile activity as a means of evaluating functional sarcomere establishment using the differential image subtraction (DIS) method. C2C12 myotubes initially displayed no contractility with EPS, due to a lack of sarcomere architecture. However, C2C12 myotubes showed remarkable contractile activity with EPS-induced repetitive Ca(2+) transients (1 Hz) within only 2 h. This activity was concurrent with the development of sarcomere structure. Importantly, the period required for the acquisition of contractile activity in response to excitation was dependent upon the frequency of Ca(2+) oscillations, but a sustained increase in intracellular Ca(2+) (not oscillatory) by high-frequency EPS (10 Hz) was incapable of conferring either contractility or sarcomere assembly on the myotubes. The EPS-facilitated de novo functional sarcomere assembly appeared to require calpain-mediated proteolysis. In addition, modulation of integrin signals, by adding collagen IV or RGD-peptide, significantly affected the EPS-induced development of contractility. Taken together, these observations indicate that the frequency of the Ca(2+) oscillation determines the time required to establish functionally active sarcomere assembly and also suggest that the Ca(2+) oscillatory signal may be decoded through reorganization of the integrin-cytoskeletal protein complex via calpain-mediated proteolysis.

Talin2 is induced during striated muscle differentiation and is targeted to stable adhesion complexes in mature muscle

The cytoskeletal protein talin serves as an essential link between integrins and the actin cytoskeleton in several similar, but functionally distinct, adhesion complexes, including focal adhesions, costameres, and intercalated disks. Vertebrates contain two talin genes, TLN1 and TLN2, but the different roles of Talin1 and Talin2 in cell adhesion are unclear. In this report we have analyzed Talin1 and Talin2 in striated muscle. Using isoform-specific antibodies, we found that Talin2 is highly expressed in mature striated muscle. Using mouse C2C12 cells and primary human skeletal muscle myoblasts as models of muscle differentiation, we show that Talin1 is expressed in undifferentiated myoblasts and that Talin2 expression is upregulated during muscle differentiation at both the mRNA and protein levels. We have also identified regulatory sequences that may be responsible for the differential expression of Talin1 and Talin2. Using GFP-tagged Talin1 and Talin2 constructs, we found that GFP-Talin1 targets to focal adhesions while GFP-Talin2 targets to abnormally large adhesions in myoblasts. We also found that ectopic expression of Talin2 in myoblasts, which do not contain appreciable levels of Talin2, dysregulates the actin cytoskeleton. Finally we demonstrate that Talin2, but not Talin1, localizes to costameres and intercalated disks, which are stable adhesions required for the assembly of mature striated muscle. Our results suggest that Talin1 is the primary link between integrins and actin in dynamic focal adhesions in undifferentiated, motile cells, but that Talin2 may serve as the link between integrins and the sarcomeric cytoskeletonin stable adhesion complexes in mature striated muscle.

Culture of human skeletal muscle myoblasts: timing appearance and localization of dystrophin-glycoprotein complex and vinculin-talin-integrin complex

The dystrophin-glycoprotein complex together with the vinculin-talin-integrin complex plays an important role in muscle function; in fact the mutations of their elements lead to diverse forms of muscular dystrophies. The relationship between the elements of dystrophin-glycoprotein complex and vinculin-talin-integrin and the time course of their formation are still not known in detail. In order to better understand this relationship we studied their expression during development in normal human skeletal muscle culture. Using a standardized muscle cell culture procedure, this study was performed to analyze the timing, appearance and the localization of some proteins of the dystrophin-glycoprotein complex and vinculin-talin-integrin complex during cellular proliferation (myoblast) and differentiation (4, 7, 15 and 21 days). The indirect immunofluorescence technique was used and cells were examined using a Meta Zeiss LSM510 confocal laser scanning inverted microscope. We examined the progressive appearance of the following proteins: alpha, beta, gamma, delta-sarcoglycans, beta-dystroglycan, dystrophin, talin, vinculin and integrin isoform alpha7/beta1. Immunofluorescence of these proteins, in satellite cells entering myogenic differentiation, revealed different patterns of localization depending on the time of culture. We showed that nondifferentiated cultures of human myoblasts expressed a perinuclear distribution of all proteins tested. During myoblast differentiation into myotubes (4 days) immunofluorescence gradually increased and was located in the whole cytoplasm. Subsequently, at day 7, a strong and homogeneous cytoplasmic labelling of all proteins was seen. At 15 days the distribution of the proteins was on the membrane. At this time some myotubes displayed a significant degree of precostameric banding pattern. As fusion proceeded at 21 days, the cytodistribution progressively changed and appeared along fibrillar longitudinal structures, and myotubes showed a clear periodic distribution (costameres). In conclusion, in normal human muscle cultures DGC and vinculin-talin-integrin proteins are first localized in the perinuclear region, then they diffuse in the cytoplasm and finally form at the plasma membrane into typical rib-like structures that are sarcolemma-associated.

Targeted disruption of N-RAP gene function by RNA interference: a role for N-RAP in myofibril organization

N-RAP is a muscle-specific protein concentrated in myofibril precursors during sarcomere assembly and at intercalated disks in adult heart. We used RNA interference to achieve a targeted decrease in N-RAP transcript and protein levels in primary cultures of embryonic mouse cardiomyocytes. N-RAP transcript levels were decreased by approximately 70% within 2 days following transfection with N-RAP specific siRNA. N-RAP protein levels steadily decreased over several days, reaching approximately 50% of control levels within 6 days. N-RAP protein knockdown was associated with decreased myofibril assembly, as assessed by alpha-actinin organization into mature striations. Transcripts encoding N-RAP binding proteins associated with assembling or mature myofibrils, such as alpha-actinin, Krp1, and muscle LIM protein, were expressed at normal levels during N-RAP protein knockdown, and alpha-actinin and Krp-1 protein levels were also unchanged. Transcripts encoding muscle myosin heavy chain and nonmuscle myosin heavy chain IIB were also expressed at relatively normal levels. However, decreased N-RAP protein levels were associated with dramatic changes in the encoded myosin proteins, with muscle myosin heavy chain levels increasing and nonmuscle myosin heavy chain IIB decreasing. N-RAP transcript and protein levels recovered to normal by days 6 and 7, respectively, and the changes in myofibril organization and myosin heavy chain isoform levels were reversed. Our data indicate that we can achieve transient N-RAP protein knockdown using the RNA interference technique and that alpha-actinin organization into myofibrils in cardiomyocytes is closely linked to N-RAP protein levels. Finally, N-RAP protein levels regulate the balance between nonmuscle myosin IIB and muscle myosin by post-trancriptional mechanisms.

Focal adhesion kinase is essential for costamerogenesis in cultured skeletal muscle cells

A central question in muscle biology is how costameres are formed and become aligned with underlying myofibrils in mature tissues. Costameres are composed of focal adhesion proteins, including vinculin and paxillin, and anchor myofibril Z-bands to the sarcolemma. In the present study, we investigated the process of costamere formation ("costamerogenesis") in differentiating primary mouse myoblasts. Using vinculin and paxillin as costameric markers, we found that two additional focal adhesion components, alpha5beta1 integrin and focal adhesion kinase (FAK), are associated with costameres. We have characterized costamerogenesis as occurring in three distinct stages based on the organizational pattern of these costameric proteins. We show that both costamerogenesis and myofibrillogenesis are initiated at sites of membrane contacts with the extracellular matrix and that their maturation is tightly coupled. To test the importance of FAK signaling in these processes, we analyzed cells expressing a dominant negative form of FAK (dnFAK). When cells expressing dnFAK were induced to differentiate, both costamerogenesis and myofibrillogenesis were disrupted although the expression of constituent proteins was not inhibited. Likewise, inhibiting FAK activity by reducing FAK levels using an siRNA approach also resulted in an inhibition of costamerogenesis and myofibrillogenesis. The relationship between costamere and myofibril formation was tested further by treating myotube cultures with potassium or tetrodotoxin to block contraction and disrupt myofibril organization. This also resulted in inhibition of costamere maturation. We present a model of costamerogenesis whereby signaling through FAK is essential for both normal costamerogenesis and normal myofibrillogenesis which are tightly coupled during skeletal myogenesis.

The activity of the vinculin binding sites in talin is influenced by the stability of the helical bundles that make up the talin rod

The talin rod contains approximately 11 vinculin binding sites (VBSs), each defined by hydrophobic residues in a series of amphipathic helices that are normally buried within the helical bundles that make up the rod. Consistent with this, talin failed to compete for binding of the vinculin Vd1 domain to an immobilized talin polypeptide containing a constitutively active VBS. However, talin did bind to GST-Vd1 in pull-down assays, and isothermal titration calorimetry measurements indicate a K(d) of approximately 9 mum. Interestingly, Vd1 binding exposed a trypsin cleavage site in the talin rod between residues 898 and 899, indicating that there are one or more active VBSs in the N-terminal part of the talin rod. This region comprises a five helix bundle (residues 482-655) followed by a seven-helix bundle (656-889) and contains five VBSs (helices 4, 6, 9, 11, and 12). The single VBS within 482-655 is cryptic at room temperature. In contrast, talin 482-889 binds Vd1 with high affinity (K(d) approximately 0.14 mum), indicating that one or more of the four VBSs within 656-889 are active, and this likely represents the vinculin binding region in intact talin. In support of this, hemagglutinin-tagged talin 482-889 localized efficiently to focal adhesions, whereas 482-655 did not. Differential scanning calorimetry showed a strong negative correlation between Vd1 binding and helical bundle stability, and a 755-889 mutant with a more stable fold bound Vd1 much less well than wild type. We conclude that the stability of the helical bundles that make up the talin rod is an important factor determining the activity of the individual VBSs.

Dystrophin and the cardiomyocyte membrane cytoskeleton in the healthy and failing heart

The cardiomyocyte membrane cytoskeleton consists of the costameric proteins that mediate force transduction from the cell to the extracellular matrix, and a sub-membrane network composed of dystrophin and associated proteins. Studies of the precise cellular distribution of dystrophin and of the consequences of genetic mutations leading to abnormal expression of the dystrophin molecule, as occurs in Duchenne and Becker's muscular dystrophies, highlight potential functional roles of this sub-membrane protein complex in cardiomyocytes. Detailed investigation of dystrophin distribution using the complementary cell imaging techniques of immunoconfocal microscopy and freeze-fracture cytochemistry at the electron-microscopical level show that, in contrast to rat cardiomyocytes, the dystrophin network in human cardiomyocytes is locally enriched at costameres. Thus located, the dystrophin network appears to have a mechanical role, involving stabilization of the peripheral plasma membrane during the repetitive distortion associated with cardiac contraction and, in the human myocyte, contributing to lateral force-transduction. Evidence from animal models of muscular dystrophy and from investigation of the interactions of the sub-membrane cytoskeleton with other membrane-associated proteins including ion channels, receptors and enzymes, further suggests a role for dystrophin in organization and regulation of membrane domains. The relative preservation of the membrane cytoskeleton in non-dystrophic dilated cardiomyopathy and in ischemic cardiomyopathy, conditions in which the myocyte contractile apparatus and internal desmin-based cytoskeleton are commonly disrupted, emphasizes the vital role of the membrane cytoskeleton in cell survival. Continued cardiomyocyte survival despite loss of contractile protein organization has implications in the potential for reversibility of left ventricular remodeling that can be achieved in the clinical setting.

Characterization of an actin-binding site within the talin FERM domain

Talin is a large cytoskeletal protein that couples integrins to F-actin. Three actin-binding sites (ABS1-3) have been reported: one in the N-terminal head, and two in the C-terminal rod domain. Although the C-terminal ABS3 has been partially characterized, the presence and properties of ABS1 within the talin head are less well defined. We show here that the talin head binds F-actin in vitro and in vivo at a specific site within the actin filament. Thus, purified talin head liberated from gizzard talin by calpain cleavage cosediments with F-actin in a low salt buffer at pH 6.4 (conditions that are optimal for binding intact talin), and using recombinant polypeptides, we have mapped ABS1 to the FERM domain within the talin head. Both the F2 and F3 FERM subdomains contribute to binding, and EGFP-tagged FERM subdomains colocalize with actin stress fibers when expressed in COS cells. High-resolution electron microscopy of actin filaments decorated with F2F3 localizes binding to a site that is distinct from that recognized by members of the calponin-homology superfamily. Finally, we show that the FERM domain can couple F-actin to PIPkin, and by inference to integrins, since they bind to the same pocket in the F3 subdomain. This suggests that the talin FERM domain functions as a linker between PIPkin or integrins and F-actin at sites of cell-matrix adhesions.

Distribution and Localization of Vinculin-Talin-Integrin System and Dystrophin-Glycoprotein Complex in Human Skeletal Muscle

The vinculin-talin-integrin system and the dystrophin-glycoprotein complex (DGC) are two protein systems with structural and signaling functions, allowing interaction between muscle fibers and extracellular matrix. Although numerous studies have been conducted on these systems, their localization and distribution patterns along the nonjunctional sarcolemma are not clear. On this basis, we carried out an indirect immunofluorescence study on the vastus lateralis muscle of human adults not affected by neuromuscular diseases to better define these patterns. Our results showed that all tested proteins of the two systems have a costameric distribution; all tested proteins of the two systems colocalize with each other (about 90-95% of the cases); only alpha-sarcoglycan in a few cases (about 6%) does not colocalize with other proteins; in about 9-10% of the cases, dystrophin and beta-dystroglycan colocalize partially with other proteins; all tested proteins can be localized in different fibers, both in the region of the sarcolemma over I or A bands. The colocalization between the vinculin-talin-integrin and DGC systems may imply their functional interaction involving the structural aspect, by providing a stronger adhesion between sarcolemma and extracellular matrix in well-defined regions of the muscle fiber. Besides, their colocalization may suggest the existence of a mechanism of mutual modulation of the transmitted signals. This reciprocal control may determine, in different conditions, the prevalence of one system over another with a consequent transmission of different messages to the sarcolemma-associated cytoskeleton.

Archvillin, a muscle-specific isoform of supervillin, is an early expressed component of the costameric membrane skeleton

The membrane skeleton protein supervillin binds tightly to both F-actin and membranes and can potentiate androgen receptor activity in non-muscle cells. We report that muscle, which constitutes the principal tissue source for supervillin sequences, contains a approximately 250 kDa isoform of supervillin that localizes within nuclei and with dystrophin at costameres, regions of F-actin membrane attachment in skeletal muscle. The gene encoding this protein, 'archvillin' (Latin, archi; Greek, árchos; 'principal' or 'chief'), contains an evolutionarily conserved, muscle-specific 5' leader sequence. Archvillin cDNAs also contain four exons that encode approximately 47 kDa of additional muscle-specific protein sequence in the form of two inserts within the function-rich N-terminus of supervillin. The first of these muscle-specific inserts contains two conserved nuclear targeting signals in addition to those found in sequences shared with supervillin. Archvillin, like supervillin, binds directly to radiolabeled F-actin and co-fractionates with plasma membranes. Colocalization of archvillin with membrane-associated actin filaments, non-muscle myosin II, and--to a lesser extent--vinculin was observed in myoblasts. Striking localizations of archvillin protein and mRNA were observed at the tips of differentiating myotubes. Transfected protein chimeras containing archvillin insert sequences inhibited myotube formation, consistent with a dominant-negative effect during early myogenesis. These data suggest that archvillin is among the first costameric proteins to assemble during myogenesis and that it contributes to myogenic membrane structure and differentiation.

Striated muscle cytoarchitecture: an intricate web of form and function

Striated muscle is an intricate, efficient, and precise machine that contains complex interconnected cytoskeletal networks critical for its contractile activity. The individual units of the sarcomere, the basic contractile unit of myofibrils, include the thin, thick, titin, and nebulin filaments. These filament systems have been investigated intensely for some time, but the details of their functions, as well as how they are connected to other cytoskeletal elements, are just beginning to be elucidated. These investigations have advanced significantly in recent years through the identification of novel sarcomeric and sarcomeric-associated proteins and their subsequent functional analyses in model systems. Mutations in these cytoskeletal components account for a large percentage of human myopathies, and thus insight into the normal functions of these proteins has provided a much needed mechanistic understanding of these disorders. In this review, we highlight the components of striated muscle cytoarchitecture with respect to their interactions, dynamics, links to signaling pathways, and functions. The exciting conclusion is that the striated muscle cytoskeleton, an exquisitely tuned, dynamic molecular machine, is capable of responding to subtle changes in cellular physiology.

Plectin tethers desmin intermediate filaments onto subsarcolemmal dense plaques containing dystrophin and vinculin

Plectin is a versatile cytoskeletal linker protein that preferentially localizes at interfaces between intermediate filaments and the plasma membrane in muscle, epithelial cells, and other tissues. Its deficiency causes muscular dystrophy with epidermolysis bullosa simplex. To better understand the functional roles of plectin beneath the sarcolemma of skeletal muscles and to gain some insights into the underlying mechanism of plectin-deficient muscular dystrophy, we studied in vivo structural and molecular relationships of plectin to subsarcolemmal cytoskeletal components, such as desmin, dystrophin, and vinculin, in rat skeletal muscles. Immunogold electron microscopy revealed that plectin fine threads tethered desmin intermediate filaments onto subsarcolemmal dense plaques overlying Z-lines and I-bands. These dense plaques were found to contain dystrophin and vinculin, and thus may be the structural basis of costameres. The in vivo association of plectin with desmin, (meta-)vinculin, dystrophin, and actin was demonstrated by immunoprecipitation experiments. Treatment of plectin immunoprecipitates with gelsolin reduced actin, dystrophin, and (meta-)vinculin but not desmin, implicating that subsarcolemmal actin could partly mediate the interaction between plectin and dystrophin or (meta-)vinculin. Altogether, our data suggest that plectin, along with desmin intermediate filaments, might serve a vital structural role in the stabilization of the subsarcolemmal cytoskeleton.

The structure and functional significance of variations in the connective tissue within muscle

The amount of intramuscular connective tissue (IMCT) and its morphological distribution is highly variable between muscles of differing function. The functional roles of this component of muscle have been poorly understood, but a picture is gradually emerging of the central role this component has in growth, transmission of mechanical signals to muscle cells and co-ordination of forces between fibres within a muscle. The aim of this review is to highlight recent advances that begin to show the functional significance of some of the variability in IMCT. IMCT has a number of clearly defined roles. It patterns muscle development and innervation, and mechanically integrates the tissue. In developing muscles, proliferation and growth of muscle cells is stimulated and guided by cell-matrix interactions. Recent work has shown that the topography of collagen fibres is an important signal. The timing and rates of expression of connective tissue proteins also show differences between muscles. Discussion of mechanical roles for IMCT has traditionally been limited to the passive elastic response of muscle. However, it is now clear that IMCT provides a matrix to integrate the contractile function of the whole tissue. Mechanical forces are co-ordinated and passed between adjacent muscle cells via cell-matrix interactions and the endomysial connective tissue that links the cells together. An emerging concept is that division of a muscle into fascicles by the perimysial connective tissue is related to the need to accommodate shear strains as muscles change shape during contraction and extension.

Role of N-cadherin- and integrin-based costameres in the development of rat cardiomyocytes

Costameres, vinculin-containing structures found in skeletal and cardiac muscle, are thought to anchor the Z-discs of the peripheral myofibrils to the sarcolemma. Several lines of evidence indicate that two different sets of costameres, integrin- and N-cadherin-based, are present in cardiac muscles. In this study, immunoblot analysis was used to study the expression of N-cadherin, alpha-catenin, beta-catenin, vinculin, talin, and laminin in rat cardiac muscles at embryonic days 15 and 19, the day of birth (postnatal day 0), postnatal weeks 1, 2, 3, and 4, and in the adult. Double immunofluorescence microscopy was performed to study the spatial and temporal distribution of these two sets of costameres in rat cardiomyocytes. Costameric staining for N-cadherin, codistributed with beta-catenin, was strong from embryonic day 15 up to postnatal week 2, gradually decreased after postnatal week 3, and was undetectable at postnatal week 4 and in the adult. Confocal microscopy showed that N-cadherin colocalized with alpha-actinin at cortical myofibrils. Double-labeling of beta-catenin and talin indicated the coexistence of N-cadherin/catenin- and integrin/talin-based costameres in rat cardiac muscle. Although beta-catenin and vinculin were co-localized at the costamere of cardiomyocytes from embryonic day 15 to postnatal week 3, staining for beta-catenin or talin was mutually exclusive at all stages examined. These results demonstrate the simultaneous, but mutually exclusive, existence of N-cadherin/catenin- and integrin/talin-based costameres in rat cardiomyocytes between late embryonic stages and postnatal week 3, while only integrin/talin-based costameres were found in adult rats. The N-cadherin/catenin-based costameres in rat cardiac muscles may play a role in myofibrillogenesis similar to that of their counterparts in cultured cardiomyocytes.

Ultrastructural and biochemical localization of N-RAP at the interface between myofibrils and intercalated disks in the mouse heart

N-RAP is a recently discovered muscle-specific protein found at cardiac intercalated disks. Double immunogold labeling of mouse cardiac muscle reveals that vinculin is located immediately adjacent to the fascia adherens region of the intercalated disk membrane, while N-RAP extends approximately 100 nm further toward the interior of the cell. We partially purified cardiac intercalated disks using low- and high-salt extractions followed by density gradient centrifugation. Immunoblots show that this preparation is highly enriched in desmin and junctional proteins, including N-RAP, talin, vinculin, beta1-integrin, N-cadherin, and connexin 43. Electron microscopy and immunolabeling demonstrate that N-RAP and vinculin are associated with the large fragments of intercalated disks that are present in this preparation, which also contains numerous membrane vesicles. Detergent treatment of the partially purified intercalated disks removed the membrane vesicles and extracted vinculin and beta1-integrin. Further separation on a sucrose gradient removed residual actin and myosin and yielded a fraction morphologically similar to fasciae adherentes that was highly enriched in N-RAP, N-cadherin, connexin 43, talin, desmin, and alpha-actinin. The finding that N-RAP copurifies with detergent-extracted intercalated disk fragments even though beta-integrin and vinculin have been completely removed suggests that N-RAP association with the adherens junction region is mediated by the cadherin system. Consistent with this hypothesis, we found that recombinant N-RAP fragments bind alpha-actinin in a gel overlay assay. In addition, immunofluorescence shows that N-RAP remains bound at the ends of isolated, detergent-treated cardiac myofibrils. These results demonstrate that N-RAP remains tightly bound to myofibrils and fasciae adherentes during biochemical purification and may be a key constituent in the mechanical link between these two structures.

Ischemic loss of sarcolemmal dystrophin and spectrin: correlation with myocardial injury

Sarcolemmal blebbing and rupture are prominent features of irreversible ischemic myocardial injury. Dystrophin and spectrin are sarcolemmal structural proteins. Dystrophin links the transmembrane dystroglycan complex and extracellular laminin receptors to intracellular F-actin. Spectrin forms the backbone of the membrane skeleton conferring an elastic modulus to the sarcolemmal membrane. An ischemic loss of membrane dystrophin and spectrin, in ischemically pelleted rabbit cardiomyocytes or in vivo 30--45 min permanently ischemic, LAD-ligated hearts, was detected by immunofluorescence with monoclonal antibodies. Western blots of light and heavy microsomal vesicles and Triton-extracted membrane fractions from ischemic myocytes demonstrated a rapid loss of dystrophin coincident with sub-sarcolemmal bleb formation, subsequent to a hypotonic challenge. The loss of spectrin from purified sarcolemma of autolysed rabbit heart, and both isolated membrane vesicles and Triton solubilized membrane fractions of ischemic cardiomyocytes correlated linearly with the onset of osmotic fragility as assessed by membrane rupture, subsequent to a hypotonic challenge. In contrast to the ischemic loss of dystrophin and spectrin from the membrane, the dystrophin-associated proteins, alpha-sarcoglycan and beta-dystroglycan and the integral membrane protein, sodium-calcium exchanger, were maintained in the membrane fraction of ischemic cells as compared to oxygenated cells. Preconditioning protected cells, but did not significantly alter ischemic dystrophin or spectrin translocation. This previously unrecognized loss of sarcolemmal dystrophin and spectrin may be the molecular basis for sub-sarcolemmal bleb formation and membrane fragility during the transition from reversible to irreversible ischemic myocardial injury.

Irregular costameres represent nitric oxide synthase-1-positive sarcolemma invaginations enriched in contracted skeletal muscle fibres

NADPH diaphorase histochemistry and NOS-1 immunohistochemistry on 60 microm thick frozen sections of rat extensor digitorum longus muscles led to the detection of prominent rings clearly encompassing the surface of the muscle fibres. These so far unknown costameres were usually found as doublets flanking a space of about 2 microm width. Because these costameric doublets did not appear in regular periods, we designate them irregular costameres to discriminate them from regular ones with a 1 microm periodicity overlying Z-discs and M-lines. Irregular costameres were thicker than the regular ones and free of intercostameres. Immunohistochemistry demonstrated that NOS-1 was co-localized with integral (beta-dystroglycan, alpha-sarcoglycan) and peripheral (caveolin-3, dystrophin) members of the enlarged dystrophin complex in the irregular costameres but not with non-sarcolemmal organized proteins (myosin heavy chain, alpha-actinin, desmin and sarcoplasmic reticulum-located Ca2+-dependent ATPase-1). Invaginations of the sarcolemma to form irregular costameres were observed. In teased myofibres the sarcolemma between two following irregular costameres was ballooned, while the irregular costameres themselves clamped the fibres together. Finally, the number of detectable irregular costameres was significantly increased in maximally contracted extensor digitorum longus muscles generated by electric stimulation but decreased in mechanically stretched ones. Combining these observations, we hypothesize that irregular costameres belong to a reserve zone for the sarcolemma necessary for the contraction/relaxation cycle in myofibres.

Vinculin, Talin, Integrin alpha6beta1 and laminin can serve as components of attachment complex mediating contraction force transmission from cardiomyocytes to extracellular matrix

Recently, we reported that cardiomyocytes adhere to extracellular matrix at costameres, the striated distribution of vinculin between Z-lines and the sarcolemma, where transmission of contraction forces from myofibrils to the extracellular matrix occurs. To identify other molecules involved in force transmission at costameres, we examined adult rat and embryonic chick cardiomyocytes cultured on coverslips or flexible thin silicone rubber substrata. Immunolocalization of talin showed a costameric, striated distribution, which corresponded to dark contacts with interference reflection microscopy. The molecules involved in substrate adhesion were cross-linked with the non-penetrating cross-linking agent Bis(sulfosuccinimidyl)-suberate and detected by immunohistochemical staining with anti-alpha6, alpha3, alphav, or beta1 integrin antibodies. Both alpha6 and beta1 showed costameric distributions, but alpha3 and alpha(v) did not. The distribution of laminin after cross-linking and extraction also showed a costameric distribution. When anti-integrin beta1 antibody was added to live cardiomyocytes grown on the silicone rubber substratum, the transmission of contraction forces was inhibited. These findings suggest that vinculin, talin, integrin alpha6beta1 and laminin system can be involved in transmission of contraction force to the extracellular matrix.

Location of and post-mortem changes in some cytoskeletal proteins in pork and cod muscle

The cytoskeletal proteins actin, nebulin, spectrin, desmin, vinculin and talin were labelled immunohistochemically in sections of muscle from commercially available pigs and cod (Gadus morhua) taken pre-rigor and from samples stored for several days. Actin, nebulin and spectrin gave similar labelling patterns in both pork and cod muscle which remained the same in stored samples. Desmin was intensely labelled at the cell boundaries and within the body of the cells in both pork and cod in the initial and the stored samples. Vinculin was readily labelled in pork muscle but showed only diffuse labelling in fish. Labelling for talin in pork muscle was intense at the sarcolemma but was not present in samples stored for 4 days. In contrast, the label for talin was concentrated at the myotendinous junction of the cod muscle throughout the storage period. These are the first reports of the detection and location of spectrin and vinculin in fish muscle and of the location of talin. The results are discussed in terms of muscle structure, function and post-mortem tenderisation. © 2000 Society of Chemical Industry.

Integrin beta cytoplasmic domains differentially bind to cytoskeletal proteins

Integrin cytoplasmic domains connect these receptors to the cytoskeleton. Furthermore, integrin-cytoskeletal interactions involve ligand binding (occupancy) to the integrin extracellular domain and clustering of the integrin. To construct mimics of the cytoplasmic face of an occupied and clustered integrin, we fused the cytoplasmic domains of integrin beta subunits to an N-terminal sequence containing four heptad repeat sequences. The heptad repeats form coiled coil dimers in which the cytoplasmic domains are parallel dimerized and held in an appropriate vertical stagger. In these mimics we found 1) that both conformation and protein binding properties are altered by insertion of Gly spacers C-terminal to the heptad repeat sequences; 2) that the cytoskeletal proteins talin and filamin are among the polypeptides that bind to the integrin beta1A tail. Filamin, but not talin binding, is enhanced by the insertion of Gly spacers; 3) binding of both cytoskeletal proteins to beta1A is direct and specific, since it occurs with purified talin and filamin and is inhibited in a point mutant (beta1A(Y788A)) or in splice variants (beta1B, beta1C) known to disrupt cytoskeletal associations of beta1 integrins; 4) that the muscle-specific splice variant, beta1D, binds talin more tightly than beta1A and is therefore predicted to form more stable cytoskeletal associations; and 5) that the beta7 cytoplasmic domain binds filamin better than beta1A. The structural specificity of these associations suggests that these mimics offer a useful approach for the analysis of the interactions and structure of the integrin cytoplasmic face.

Muscle beta1D integrin reinforces the cytoskeleton-matrix link: modulation of integrin adhesive function by alternative splicing

Expression of muscle-specific beta1D integrin with an alternatively spliced cytoplasmic domain in CHO and GD25, beta1 integrin-minus cells leads to their phenotypic conversion. beta1D-transfected nonmuscle cells display rounded morphology, lack of pseudopodial activity, retarded spreading, reduced migration, and significantly enhanced contractility compared with their beta1A-expressing counterparts. The transfected beta1D is targeted to focal adhesions and efficiently displaces the endogenous beta1A and alphavbeta3 integrins from the sites of cell-matrix contact. This displacement is observed on several types of extracellular matrix substrata and leads to elevated stability of focal adhesions in beta1D transfectants. Whereas a significant part of cellular beta1A integrin is extractable in digitonin, the majority of the transfected beta1D is digitonin-insoluble and is strongly associated with the detergent-insoluble cytoskeleton. Increased interaction of beta1D integrin with the actin cytoskeleton is consistent with and might be mediated by its enhanced binding to talin. In contrast, beta1A interacts more strongly with alpha-actinin, than beta1D. Inside-out driven activation of the beta1D ectodomain increases ligand binding and fibronectin matrix assembly by beta1D transfectants. Phenotypic effects of beta1D integrin expression in nonmuscle cells are due to its enhanced interactions with both cytoskeletal and extracellular ligands. They parallel the transitions that muscle cells undergo during differentiation. Modulation of beta1 integrin adhesive function by alternative splicing serves as a physiological mechanism reinforcing the cytoskeleton- matrix link in muscle cells. This reflects the major role for beta1D integrin in muscle, where extremely stable association is required for contraction.

Spatial and temporal expression of the beta1D integrin during mouse development

The beta1D protein is a recently characterized isoform of the integrin beta1 subunit that is present in cardiac and skeletal muscles. In this study, we have examined the expression of beta1D in different types of skeletal muscle and in cardiac muscle and studied its distribution during mouse development, using new monoclonal antibodies specific for beta1D. Immunoprecipitation studies revealed that, while beta1A is strongly expressed in proliferating C2C12 myoblasts, beta1D is only expressed after their differentiation to myotubes. In these myotubes, beta1D is associated with different alpha subunits, namely alpha3A, alpha5, alpha7A, or alpha7B. Initially, during embryogenesis, the alpha1A subunit is the only beta1 variant expressed in skeletal and cardiac muscle. The beta1D subunit is first detected in skeletal muscle at E17.5, whereas in cardiac muscle its expression begins around the time of birth. Later the expression of beta1A in skeletal and cardiac muscle becomes restricted to capillary cells, whereas beta1D eventually becomes the only variant expressed in adult cardiac and skeletal muscle cells. The switch from the beta1A to the beta1D subunit in cardiac muscle cells coincides with the expression of alpha7. In adults there is a distinct concentration of beta1D at the myotendinous junctions of muscle fibers and at costameres in both cardiac and skeletal muscle. In addition, beta1D is present at intercalated discs in cardiac muscle and at neuromuscular junctions in skeletal muscle cells. The amount of beta1D in different types of skeletal muscle (fast, slow, and mixed-type) was similar, but cardiac muscle expressed almost five times as much of this protein. We suggest that beta1D plays a role in the maintenance of the cytoarchitecture of mature muscle and in the functional integrity of the muscle cells.

Living adult rat cardiomyocytes in culture: evidence for dissociation of costameric distribution of vinculin from costameric distributions of attachments

Adult rat cardiomyocytes were placed in tissue culture to determine the relationships of their vinculin positive costameres, their attachments associated with the costameres, the fate of their myofibrils. The costameric structures were detected using interference contrast microscopy and the visualization of the fluorescently labeled vinculin and alpha-actinin molecules. The cardiomyocytes isolated from the heart retained their myofibrils upon attachment to the cell surfaces. One group of cells then rounded up, only to respread after 6 days in culture. These cells initially demonstrated costameric distributions of attachments and vinculin. These relationships were lost during the rounding-up process only to be regained as the cells respread. The second group of freshly isolated cardiomyocytes did not round up but began to spread on the substratum by sending out lamellipodia from their rectangularly shaped body. These newly cultured cardiomyocytes initially exhibited costameric distributions of close attachments detected by interference microscopy. Over the next 3 days although the cells remain attached to the substratum, the costameric attachments were gradually lost. Nevertheless, when similar cells were injected with fluorescently labeled vinculin, costameric distributions of vinculin could be detected in the absence of costameric attachments. Cardiomyocytes, injected with fluorescent alpha-actinin, revealed that during the first few days in culture the existing myofibrils disassembled from the edges of the cell towards the middle. The center group of myofibrils was retained as the cells began to spread. Our observations of living cells support the hypothesis that proteins in addition to vinculin are needed for cardiomyocytes to generate costameric attachments to the cell surfaces. We speculate that the ability of the vinculin-attached Z-lines of adult cardiomyocytes to dissociate from the extracellular matrix may aid in the remodeling of the adult heart in the repair process after myocardial infarction and also in stress induced hypertrophic growth.

Microfilament-membrane interactions in Xenopus myocytes

We used quick-freeze, deep-etch, rotary-replication transmission electron microscopy to determine at molecular resolution the organization of microfilaments at the cytoplasmic surface of the sarcolemma of Xenopus myocytes. We demonstrate that actin microfilaments interact with the sarcolemma in two distinct ways. In one, which resembled focal contacts in Xenopus fibroblasts [Samuelsson et al., 1993: J. Cell Biol. 122:485-496], bundles of microfilaments approached the sarcolemma at sites containing aggregates of membrane-associated particles. Immunogold cytochemistry showed that these particle aggregates contained vinculin, talin and beta 1-integrin. In the second, which covered most of the cytoplasmic surface of the sarcolemma, individual actin microfilaments formed an extensive, lattice-like array. Particle aggregates associated with this array of actin microfilaments also labeled with antibodies to vinculin, talin and beta 1-integrin. The unique, lattice-like association of actin microfilaments with the membrane in Xenopus myocytes suggests that the organization of actin filaments over most of the sarcolemma is distinct from focal contacts, mediating widespread associations of the actin cytoskeleton with the cytoplasmic membrane face.

Immunocytochemistry of the muscle cell cytoskeleton

The muscle cell cytoskeleton is defined for this review as any structure or protein primarily involved in linking or connecting protein filaments to each other or to anchoring sites. In striated muscle, the M line connects thick filaments at their centers to adjacent thick filaments. Titin forms elastic filaments that extend from the M line to the Z line and may contribute to the resting tension properties of striated muscle. Nebulin forms inextensible filaments in skeletal muscle that are closely associated with thin filaments and that may provide a length template for thin filaments. Z lines anchor thin filaments from adjacent sarcomeres via the actin-binding function of alpha-actinin. Other proteins located at the Z line include Cap Z, Z-nin, Z protein, and zeugmatin. Intermediate filaments connect myofibrils to each other at the level of the Z line and to the sarcolemma at the Z- and possibly the M-line levels. Immunolocalization has identified the adhesion plaque proteins spectrin, vinculin, dystrophin, ankyrin, and talin at subsarcolemmal sites where they may be involved with filament attachment. Smooth muscle cell cytoskeletons are believed to include membrane associated dense bodies (MADBs), intermediate filaments, cytoplasmic dense bodies (CDBs), and perhaps a subset of actin filaments. MADBs contain a menu of attachment plaque proteins and anchor both thin filaments and intermediate filaments to the sarcolemma. CDBs are intracellular analogs of striated muscle Z lines and anchor thin filaments and intermediate filaments.

Enzymatic activity and in vivo distribution of 5'-nucleotidase, an extracellular matrix binding glycoprotein, during the development of chicken striated muscle

The ecto-enzyme 5'-nucleotidase isolated from chicken gizzard has previously been shown to be a potent ligand of two glycoproteins of the extracellular matrix, namely fibronectin and laminin. Using immunofluorescent labeling techniques we observed that 5'-nucleotidase codistributed with laminin during the development of chicken striated muscle. In contrast, ecto-5'-nucleotidase was only faintly detectable on cells surrounded by a matrix expressing high levels of fibronectin. This distribution pattern distinguished 5'-nucleotidase from the pluripotent extracellular matrix receptors, chicken beta 1-integrins, which are expressed equally well in muscle and connective tissue. In addition, the specific activity of striated muscle ecto-5'-nucleotidase was stable during development and increased markedly posthatching. At each age considered, this specific activity corresponded to an 80-kDa enzyme which was inhibited by alpha,beta-methyleneadenosine diphosphate or by a monoclonal antibody directed against the smooth muscle isoform of the enzyme. Previous in vitro studies have revealed that 5'-nucleotidase is involved in the spreading of various mesenchyme-derived cells, such as chicken embryonic fibroblasts and myoblasts, on a laminin substrate. A prerequisite to examining a potential in vivo role for 5'-nucleotidase as an extracellular matrix ligand was to study its distribution. In adult muscle, 5'-nucleotidase displayed a more restricted distribution than in embryo. Results show that, in vivo, 5'-nucleotidase is revealed by immunofluorescent labeling using poly- and monoclonal antibodies to chicken gizzard 5'-nucleotidase in two structures, the costameres and myotendinous junctions, which are closely related to the focal adhesion sites observed in cell culture.

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

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

Talin dynamics in living microinjected nonmuscle cells

To investigate the role of talin in the anchoring of actin-containing stress fibers to the cell membrane of nonmuscle cells, a fluorescent analog of the adhesion plaque protein talin was developed, characterized, and microinjected into living cells. Purified chicken gizzard talin was covalently labeled with the fluorescent dye lissamine rhodamine B sulfonyl chloride. The fluorescently labeled protein was then chromatographed on Sephadex G-25 and DEAE-cellulose in order to remove free dye and denatured protein. The fluorescent talin was able to bind purified vinculin and was localized in adhesion plaques, membrane ruffles, microspikes, and polygonal networks in acetone-permeabilized nonmuscle cells. In cells that were double-stained with fluorescent talin and an affinity-purified anti-talin antibody, a one-to-one correspondence of adhesion plaque staining was seen. Living epithelial cells (PtK2) were microinjected during interphase with fluorescent talin. Computer-enhanced video microscopy was used to document adhesion plaque dynamics such as 1) changes in plaque shape, 2) alterations in plaque positions, and 3) the appearance, growth, and dissolution of plaques. In cells that were followed during mitosis, the adhesion plaques disappeared during cell rounding and then subsequently reappeared upon spreading of the two daughter cells. Treatment of microinjected cells with DMSO in order to disassemble stress fibers resulted in an altered localization of the fluorescent talin. Upon recovery of the cell from the drug, the talin was visualized in its characteristic submembraneous position. These results are the first to document the role and distribution of talin in dynamic processes occurring in living microinjected nonmuscle cells.

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.