Immunolocalization of the costameres in human skeletal muscle fibers: confocal scanning laser microscope investigations

Paxillin and focal adhesion kinase colocalise in human skeletal muscle and its associated microvasculature

Focal adhesion kinase (FAK) and paxillin are functionally linked hormonal- and mechano-sensitive proteins. We aimed to describe paxillin's subcellular distribution using widefield and confocal immunofluorescence microscopy and test the hypothesis that FAK and paxillin colocalise in human skeletal muscle and its associated microvasculature. Percutaneous muscle biopsies were collected from the m. vastus lateralis of seven healthy males, and 5-μm cryosections were stained with anti-paxillin co-incubated with anti-dystrophin to identify the sarcolemma, anti-myosin heavy chain type I for fibre-type differentiation, anti-dihydropyridine receptor to identify T-tubules, lectin UEA-I to identify the endothelium of microvessels and anti-α-smooth muscle actin to identify vascular smooth muscle cells (VSMC). Colocalisation of anti-paxillin with anti-dystrophin or anti-FAK was quantified using Pearson's correlation coefficient on confocal microscopy images. Paxillin was primarily present in (sub)sarcolemmal regions of skeletal muscle fibres where it colocalised with dystrophin (r = 0.414 ± 0.026). The (sub)sarcolemmal paxillin immunofluorescence intensity was ~2.4-fold higher than in sarcoplasmic regions (P < 0.001) with sarcoplasmic paxillin immunofluorescence intensity ~10 % higher in type I than in type II fibres (P < 0.01). In some longitudinally orientated fibres, paxillin formed striations that corresponded to the I-band region. Paxillin immunostaining was highest in endothelial and VSMC and distributed heterogeneously in both cell types. FAK and paxillin colocalised at (sub)sarcolemmal regions and within the microvasculature (r = 0.367 ± 0.036). The first images of paxillin in human skeletal muscle suggest paxillin is present in (sub)sarcolemmal and I-band regions of muscle fibres and within the microvascular endothelium and VSMC. Colocalisation of FAK and paxillin supports their suggested role in hormonal and mechano-sensitive signalling.

Immunolocalisation of intermediate filament proteins in porcine meat. Fibre type and muscle-specific variations during conditioning

Two intermediate filament proteins, desmin and vinculin, were immunofluorescently localised in porcine longissimus dorsi(∗∗) and iliocostalis muscles up to 7 days post mortem. Changes in tenderness of the m. longissimus and the water-holding capacity of both muscles were also monitored throughout this storage period. Longissimus muscle shows a rapid decline in the overall intensity of labelling for both desmin and vinculin. In contrast to the vinculin labelling, desmin labelling is preferentially lost from type IIB muscle fibres in the longissimus muscle. In the redder iliocostalis muscle, the loss of desmin and vinculin labelling was less rapid and did not show an obvious relation to muscle fibre type. In one sample with very high water loss, there were indications of greater extracellular space development and delayed loss of intermediate filament labelling. The time course of diminishing intermediate filament labelling is consistent with previous suggestions that degradation of these proteins is involved in post-mortem conditioning. The variations within and between muscles shown here may explain some of the variability in their mechanical properties. Additionally, it is suggested that intermediate filament integrity is necessary for the expulsion of water from the muscle cells during drip formation.

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.

Structural and functional roles of desmin in mouse skeletal muscle during passive deformation

Mechanical interactions between desmin and Z-disks, costameres, and nuclei were measured during passive deformation of single muscle cells. Image processing and continuum kinematics were used to quantify the structural connectivity among these structures. Analysis of both wild-type and desmin-null fibers revealed that the costamere protein talin colocalized with the Z-disk protein alpha-actinin, even at very high strains and stresses. These data indicate that desmin is not essential for mechanical coupling of the costamere complex and the sarcomere lattice. Within the sarcomere lattice, significant differences in myofibrillar connectivity were revealed between passively deformed wild-type and desmin-null fibers. Connectivity in wild-type fibers was significantly greater compared to desmin-null fibers, demonstrating a significant functional connection between myofibrils that requires desmin. Passive mechanical analysis revealed that desmin may be partially responsible for regulating fiber volume, and consequently, fiber mechanical properties. Kinematic analysis of alpha-actinin strain fields revealed that knockout fibers transmitted less shear strain compared to wild-type fibers and experienced a slight increase in fiber volume. Finally, linkage of desmin intermediate filaments to muscle nuclei was strongly suggested based on extensive loss of nuclei positioning in the absence of desmin during passive fiber loading.

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.

Sarcoglycans in human skeletal muscle and human cardiac muscle: a confocal laser scanning microscope study

Sarcoglycans are a subcomplex of transmembrane proteins which are part of the dystrophin-glycoprotein complex. They are expressed in the skeletal, cardiac and smooth muscle. Although numerous studies have been conducted on the sarcoglycan subcomplex in skeletal and cardiac muscle, the manner of the distribution and localization of these proteins along the nonjunctional sarcolemma is not clear. We therefore carried out an indirect immunofluorescence study on surgical biopsies of normal human skeletal muscle and of healthy human atrial myocardium biopsies of patients affected by valvulopathy. Our results indicate that, in skeletal muscle, sarcoglycans have a costameric distribution and all colocalize with each other. Only in a few cases did the alpha-sarcoglycan not colocalize with other sarcoglycans. In addition, these glycoproteins can be localized in different fibers either in the regions of the sarcolemma over band I or band A. In cardiac muscle, our results show a costameric distribution of all proteins examined and, unlike in skeletal muscle, they show a constant colocalization of all sarcoglycans with each other, along with a consistent localization of these proteins in the region of the sarcolemma over band I. In our opinion, this situation seems to confirm the hypothesis of a correlation between the region of the sarcolemma occupied by costameric proteins and the metabolic type, fast or slow, of the muscular fibers. These data, besides opening a new line of research in understanding interactions between the sarcoglycans and other transmembrane proteins, could also be extended to skeletal and cardiac muscles affected by neuromuscular and cardiovascular pathologies to understand possible structural alterations.

Assembly of myofibrils in cardiac muscle cells

How do myofibrils assemble in cardiac muscle cells? When does titin first assemble into myofibrils? What is the role of titin in the formation of myofibrils in cardiac muscle cells? This chapter reviews when titin is first detected in cultured cardiomyocytes that have been freshly isolated from embryonic avian hearts. Our results support a model for myofibrillogenesis that involves three stages of assembly: premyofibrils, nascent myofibrils and mature myofibrils. Titin and muscle thick filaments were first detected associated with the nascent myofibrils. The Z-band targeting site for titin is localized in the N-terminus of titin. This region of titin binds alpha-actinin and less avidly vinculin. Thus the N-terminus of titin via its binding to alpha-actinin, and vinculin could also help mediate the costameric attachment of the Z-bands of mature myofibrils to the nearest cell surfaces.

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

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

Transmission of forces within mammalian skeletal muscles

Most models of in vivo musculoskeletal function fail to take into account the diversity of force trajectories defined by muscle fiber architecture. It has been shown for many muscles, across species, that muscle fibers commonly end within muscle fascicles without reaching a myotendinous junction, and that many of these fibers show a progressive decline in cross-sectional area along the length of the muscle. The significance of these anatomical observations is that the tapering would seem to preclude forces generated at the largest cross-sectional area of the fibers being transmitted to the sarcomeres toward the ends of the tapered fiber. If all of the forces are transmitted via the sarcomeres arranged in series, those few sarcomeres at the smaller ends of the fibers must tolerate the stress exerted by the more numerous sarcomeres arranged in parallel at the portions of the fiber with larger cross-sectional areas. A logical alternative would be for forces to be transmitted laterally along the length of a fiber to the cell membrane and the extracellular matrix. Such a structural arrangement would permit an alternative force transmission vector and minimize the necessity for a precise level of force to be generated along the entire length of a fiber. There are cytoarchitectural and biochemical data demonstrating the presence of a subcellular network which is appropriately located to transmit forces from the active intracellular contractile elements to the extracellular intramuscular connective tissues. However, to fully comprehend how forces are transmitted from individual cross bridges to the tendon, it will be necessary to understand the interactions of all of the components of the muscle tendon complex from the molecular to the multicellular level. It is insufficient to know the physiology of the individual components in a restricted experimental paradigm and assume that these conditions account for the functional characteristics in vivo. Thus, the challenge is to understand how the sarcomeres and all of the associated structures transmit the forces of the whole muscle to its attachments.