Muscle

Background: Current understanding of skeletal muscle contraction is based on the sliding filament theory proposed independently by A.F. Huxley and H.E. Huxley (1954). The sliding filament theory very well explained shortening of skeletal muscle during contraction with reference to a single sarcomere with movement of Z discs toward center of that particular sarcomere. However, when we consider shortening of multiple sarcomeres in a myofibril arranged in series, the sliding filament theory fails to justify the movement of Z discs as each Z disc is being pulled in opposite direction by myosin heads of adjacent sarcomeres. Aims & Objectives: To find out alternate possible mechanism of Z disc movement on either side of A band if movement toward center by both sides is not possible. Materials and Methods: We have prepared a simulating model of sliding filament theory and observed mechanism of contraction on it. Results: The sliding filament theory fails to explain the movement of Z discs when all sarcomeres in a myofibril are contracting simultaneously. Therefore, we proposed a new hypothesis that Z discs and actin filaments in the half part of each sarcomere, which is toward origin end of muscle fiber, are not pulled toward center of myosin filament instead myosin filament slide toward origin end, and in the other half of each sarcomere (which is toward insertion end) actin filaments slide over myosin filament toward origin end of the muscle. Conclusion: This study adds a new insight into the sliding filament theory of muscle contraction and needs further confirmation on electron microscopic studies.

The effect of polylysine on the motility of outer hair cells and various muscle types was compared. Poly-L-lysine and its stereoisomer, poly-D-lysine, inhibited acoustically induced length changes of isolated outer hair cells from the guinea-pig hearing organ. The frequency specific displacements of the hearing organ in response to a tone stimulus are also inhibited to polylysine (Brundin et al. 1991). Poly-L-lysine, and its stereoisomer, irreversibly attenuated motile responses to transmural stimulation of guinea-pig ileum, vas deferens and taenia coli in a dose dependent manner, but were without significant effect on motile responses in skeletal and heart muscle. L-lysine, D-lysine, and the negatively charged polyaminoacid poly-L-aspartate, were without significant effect on outer hair cell and smooth muscle motility. The inhibitory effect of polylysine in smooth muscle is a direct effect on the muscle cell since polylysine attenuated acetylcholine- and adenosine triphosphate-induced contractions in the ileum, and ATP- or noradrenaline-induced contractions in the vas deferens. Pillar structures, believed to be of importance to excitation contraction coupling, were compared. In heart and skeletal muscle the pillars span the gap between sarcoplasmic reticulum and T-tubuli, deeply recessed into the muscle cell. In smooth muscle and outer hair cell the pillars are in closer relation to the cell exterior. The length of the pillars of the outer hair cells exceeds by two times that of smooth and skeletal muscle. The susceptibility of outer hair cells and smooth muscle tissue to the positively charged polylysine may indicate similarities in membrane or channel composition.

Highlights from the Literature

To the Editor: The data and analysis presented by Karatzaferi et al. ([6][1]) support a new paradigm for muscle contraction, which if correct demands a fundamental reassessment of decades' worth of muscle mechanics studies. At issue is how mechanics and chemistry are coupled in muscle. The

The ryanodine receptor type 2 (RyR-2) functions as a Ca2+-induced Ca2+ release (CICR) channel on intracellular Ca2+ stores and is distributed in most excitable cells with the exception of skeletal muscle cells. RyR-2 is abundantly expressed in cardiac muscle cells and is thought to mediate Ca2+ release triggered by Ca2+ influx through the voltage-gated Ca2+ channel to constitute the cardiac type of excitation-contraction (E-C) coupling. Here we report on mutant mice lacking RyR-2. The mutant mice died at approximately embryonic day (E) 10 with morphological abnormalities in the heart tube. Prior to embryonic death, large vacuolate sarcoplasmic reticulum (SR) and structurally abnormal mitochondria began to develop in the mutant cardiac myocytes, and the vacuolate SR appeared to contain high concentrations of Ca2+. Fluorometric Ca2+ measurements showed that a Ca2+ transient evoked by caffeine, an activator of RyRs, was abolished in the mutant cardiac myocytes. However, both mutant and control hearts showed spontaneous rhythmic contractions at E9.5. Moreover, treatment with ryanodine, which locks RyR channels in their open state, did not exert a major effect on spontaneous Ca2+ transients in control cardiac myocytes at E9.5-11.5. These results suggest no essential contribution of the RyR-2 to E-C coupling in cardiac myocytes during early embryonic stages. Our results from the mutant mice indicate that the major role of RyR-2 is not in E-C coupling as the CICR channel in embryonic cardiac myocytes but it is absolutely required for cellular Ca2+ homeostasis most probably as a major Ca2+ leak channel to maintain the developing SR.

To investigate thin filament regulation of force activation in smooth muscle, we recorded force and stiffness of alpha-toxin-permeabilized single smooth muscle cells. At pCa 9, the rigor state was characterized by high in-phase stiffness, low force, and low quadrature stiffness, suggesting that the attachment of rigor cross bridges does not depend on either Ca2+ or myosin light chain (MLC) phosphorylation, and cross bridges can enter a rigor state without producing force. At pCa 4, 20 microM ATP increased force, in-phase stiffness, and quadrature stiffness, while 20 microM CTP did not increase any of these parameters, suggesting that although MLC phosphorylation is not required for the formation of rigor cross bridges, MLC phosphorylation is required for detached cross bridges to attach to actin and undergo a force-producing isomerization. These results also suggest that for smooth muscle, force activation is regulated by myosin light-chain kinase. From rigor, 20 microM ATP (pCa 9) increased force and quadrature without changing in-phase stiffness. This force increase could be explained if in rigor solution both actomyosin (AM) and AM.ADP cross bridges exist (2, 32), and ATP-induced detachment of AM cross bridges is accompanied by AM.ADP cross bridges undergoing a force-producing isomerization in combination with cooperative cross-bridge reattachment (36). Thus results of our experiments suggest that thin filament-based regulation of force activation is not essential in smooth muscle, and a population of cross bridges must begin in an attached state for force to be produced in the absence of MLC phosphorylation.

Comparative Mechanisms for Contraction of Cardiac and Skeletal Muscle

Vinculin localizes to membrane adhesion junctions where it links actin filaments to the extracellular matrix by binding to the integrin-binding protein talin at its head domain (Vh) and to actin filaments at its tail domain (Vt). Vinculin can assume an inactive (closed) conformation in which Vh and Vt bind to each other, masking the binding sites for actin and talin, and an active (open) conformation in which the binding sites for talin and actin are exposed. We hypothesized that the contractile activation of smooth muscle tissues might regulate the activation of vinculin and thereby contribute to the regulation of contractile tension. Stimulation of tracheal smooth muscle tissues with acetylcholine (ACh) induced the recruitment of vinculin to cell membrane and its interaction with talin and increased the phosphorylation of membrane-localized vinculin at the C-terminal Tyr-1065. Expression of recombinant vinculin head domain peptide (Vh) in smooth muscle tissues, but not the talin-binding deficient mutant head domain, VhA50I, inhibited the ACh-induced recruitment of endogenous vinculin to the membrane and the interaction of vinculin with talin and also inhibited vinculin phosphorylation. Expression of Vh peptide also inhibited ACh-induced smooth muscle contraction and inhibited ACh-induced actin polymerization; however, it did not affect myosin light chain phosphorylation, which is necessary for cross-bridge cycling. Inactivation of RhoA inhibited vinculin activation in response to ACh. We conclude that ACh stimulation regulates vinculin activation in tracheal smooth muscle via RhoA and that vinculin activation contributes to the regulation of active tension by facilitating connections between actin filaments and talin-integrin adhesion complexes and by mediating the initiation of actin polymerization.

SKELETAL MUSCLES adjust their contractile output over an exceptionally wide dynamic range, as needed to produce movements ranging from sitting and standing, to athletics and dance. At the single fiber level, the force generated by skeletal muscles is determined, ultimately, by the formation of cycling cross bridges between actin and myosin. This interaction requires Ca 2 and is governed by the simple relationship: force is proportional to intracellular ionized Ca 2 . Cross-bridge formation is inhibited in resting muscle when Ca 2 is sequestered within the sarcoplasmic reticulum (SR); it is activated when stored Ca 2 is released into the cytosol, allowing actin-myosin interactions to proceed. Contraction stops as Ca 2 is actively resequestered by the Ca 2 -ATPase of the SR (SERCA1). Thus Ca 2 is the central messenger in muscle activation, and the amplitude and time course of the intracellular Ca 2 change is the major determinant of contractile output. The key mechanisms that activate Ca 2 release from the SR are now well defined. This sequence, termed excitation-contraction coupling, begins when an excitatory input from the motor nerve triggers a propagating action potential on the muscle membrane. The action potential rapidly depolarizes the entire sarcolemma, including transverse tubules, which invaginate from the surface membrane and form specialized junctions (triads) with the SR along much of their length. The triad junction serves as a platform for assembling sarcolemmal calcium

anyone contemplating dynamic roentgenograms of hollow viscera must be impressed that the length changes in the smooth muscles that comprise their walls are larger than the threefold range produced by skeletal muscle having a fixed filament array ([1][1]), and studies of muscles isolated from these

Unlike cardiac and skeletal muscle, little is known about vascular smooth muscle mitochondrial respiration. Therefore, the present study examined mitochondrial respiratory rates in smooth muscle of healthy human feed arteries and compared with that of healthy cardiac and skeletal muscles. Cardiac, skeletal, and smooth muscles were harvested from a total of 22 subjects (53 ± 6 yr), and mitochondrial respiration was assessed in permeabilized fibers. Complex I + II, state 3 respiration, an index of oxidative phosphorylation capacity, fell progressively from cardiac to skeletal to smooth muscles (54 ± 1, 39 ± 4, and 15 ± 1 pmol·s(-1)·mg(-1), P < 0.05, respectively). Citrate synthase (CS) activity, an index of mitochondrial density, also fell progressively from cardiac to skeletal to smooth muscles (222 ± 13, 115 ± 2, and 48 ± 2 μmol·g(-1)·min(-1), P < 0.05, respectively). Thus, when respiration rates were normalized by CS (respiration per mitochondrial content), oxidative phosphorylation capacity was no longer different between the three muscle types. Interestingly, complex I state 2 normalized for CS activity, an index of nonphosphorylating respiration per mitochondrial content, increased progressively from cardiac to skeletal to smooth muscles, such that the respiratory control ratio, state 3/state 2 respiration, fell progressively from cardiac to skeletal to smooth muscles (5.3 ± 0.7, 3.2 ± 0.4, and 1.6 ± 0.3 pmol·s(-1)·mg(-1), P < 0.05, respectively). Thus, although oxidative phosphorylation capacity per mitochondrial content in cardiac, skeletal, and smooth muscles suggest all mitochondria are created equal, the contrasting respiratory control ratio and nonphosphorylating respiration highlight the existence of intrinsic functional differences between these muscle mitochondria. This likely influences the efficiency of oxidative phosphorylation and could potentially alter ROS production.

Muscle

dystrophin-glycoprotein complex The sarcolemma (muscle plasma membrane) plays a central role in skeletal muscle structure and function (1Engel A.G. Franzini-Armstrong C. Myology. 2nd Ed. McGraw-Hill, New York1994Google Scholar). In addition to the housekeeping functions of a cell plasma membrane, the sarcolemma is directly involved in synaptic transmission, action potential propagation, and excitation-contraction coupling (1Engel A.G. Franzini-Armstrong C. Myology. 2nd Ed. McGraw-Hill, New York1994Google Scholar). Besides these well established physiological functions, the sarcolemma, subsarcolemma cytoskeleton, and surrounding basement membrane (extracellular matrix) play an essential structural role in skeletal muscle (1Engel A.G. Franzini-Armstrong C. Myology. 2nd Ed. McGraw-Hill, New York1994Google Scholar, 2Clark K.A. McElhinny A.S. Beckerle M.C. Gregorio C.C. Annu. Rev. Cell Biol. 2002; 18: 637-706Crossref Scopus (474) Google Scholar, 3Colognato H. Yurchenco P.D. Dev. Dyn. 2000; 218: 213-234Crossref PubMed Scopus (1052) Google Scholar). The biological importance of the basement membrane-sarcolemma-cytoskeleton in skeletal muscle is underscored by the number of inherited muscle diseases caused by mutations in components of the basement membrane, or cytoskeleton, or the sarcolemma protein complexes that link the basement membrane to the cytoskelton (4Cohn R.D. Campbell K.P. Muscle Nerve. 2000; 23: 1456-1471Crossref PubMed Scopus (426) Google Scholar, 5Blake D.F. Weir A. Newey S.E. Davies K.E. Physiol. Rev. 2002; 82: 291-329Crossref PubMed Scopus (925) Google Scholar, 6Spence H.J. Chen Y.J. Winder S.J. Bioessays. 2002; 24: 542-552Crossref PubMed Scopus (57) Google Scholar, 7Brown S.C. Muntoni F. Sewer C.A. Brain Pathol. 2002; 11: 193-205Crossref Scopus (17) Google Scholar, 8Durbeej M. Campbell K.P. Curr. Opin. Genet. Dev. 2002; 9: 2019-2027Google Scholar). The minireview in this and the following issues updates our understanding of the structure and function of the basement membrane, cytoskeletal costameres, and the major trans-sarcolemma links (integrins and dystroglycan) in skeletal muscle. The basement membrane surrounds skeletal muscle fibers and is now known to be critical in muscle fiber structure and function. The skeletal muscle basement membrane is composed of the basal lamina and the reticular lamina. Basal lamina is directly linked to the sarcolemma. Genetic studies of muscular dystrophy patients and animal models of muscular dystrophy have demonstrated the importance of the basement membrane in maintenance of muscle integrity. In addition to maintenance of muscle integrity, the basement membrane is essential in the promotion of myogenesis and muscle development. Muscle regeneration is also a process that depends on the skeletal muscle basement membrane. Satellite cells (endogenous stem cells of skeletal muscle) reside between the muscle fiber and the basal lamina. Following injury, new muscle fibers regenerate within a basement membrane tube, which is believed to act as a mechanical barrier to limit migration of satellite cells, and a scaffold to orient myotube regeneration. Finally, the basement membrane is structurally and functionally specialized in areas of neuromuscular and myotendinous junctions and is required for the assembly of these structures. In the first minireview of this series entitled "The Basement Membrane/Basal Lamina of Skeletal Muscle," Joshua R. Sanes reviews the structure and function of the skeletal muscle basement membrane. In particular, he focuses on recent molecular studies that have led to a better understanding of the function of the basement membrane in skeletal muscle physiology and pathophysiology (9Sanes J.R. J. Biol. Chem. 2003; 278: 12601-12604Abstract Full Text Full Text PDF PubMed Scopus (305) Google Scholar). A major cytoskeletal structure in muscle that has the unique role of connecting the sarcomere to the basement membrane is the costamere. Costameres were originally described as subsarcolemma protein complexes that align in register with the Z-disk and are physically coupled to the sarcomeres. Costameres may be equivalent to focal adhesions that are expressed in non-muscle cells and are believed to be involved in the lateral transmission of contractile forces from sarcomeres across the sarcolemma to the basement membrane. In the second minireview, entitled "Costameres: the Achilles Heel of Herculean Muscle," James M. Ervasti reviews the structure and function of the striated muscle costamere (10Ervasti J.M. J. Biol. Chem. 2003; 278: 13591-13594Abstract Full Text Full Text PDF PubMed Scopus (333) Google Scholar). As with the basement membrane, the importance of the costamere for normal muscle function has been revealed by genetic studies of muscular dystrophies and dilated cardiomyopathies. Dystrophin is known to be enriched in costameres, but dystrophin is not required for costamere assembly. In the absence of dystrophin, there is a disorganization of the costameric lattice, as well as disruption of the sarcolemma integrity. Disruption of the costameric lattice correlates with functional studies that show reduction of contractile force in muscles lacking dystrophin. Ervasti's minireview provides insights into the growing costameric protein network and illustrates how these proteins can interact with many components of both the sarcolemma and cytoskeleton. In addition, the newly identified proteins suggest a role for costameric proteins in converting mechanical stimuli to alterations in cell signaling and gene expression. Finally, Ervasti discusses non-sarcolemmal mechanical defects associated with the loss of costameric proteins. It is well recognized that the function and maintenance of skeletal muscle cell integrity is dependent upon interactions of the muscle cell with the surrounding basement membrane and underlying cytoskeleton. Trans-sarcolemma receptors are known to be involved in providing critical mechanical links between the basement membrane and the cytoskeleton. In addition, recent data suggest that these receptors transmit signals from the basement membrane into the muscle cell. Over the past 10 years there has been great progress in the identification and characterization of the two sarcolemma protein complexes that connect cytoskeleton to the basement membrane in skeletal muscle: integrins and the dystrophin-glycoprotein complex (DGC).1 Integrins form a large family of cell surface receptors that mediate cell-extracellular matrix interactions. In the third minireview, entitled "Integrins, Redundant or Important Players in Skeletal Muscle?," Ulrike Mayer focuses on the role of integrins in skeletal muscle (11Mayer U. J. Biol. Chem. 2003; 278: 14587-14590Abstract Full Text Full Text PDF PubMed Scopus (144) Google Scholar). Of the current members of this family, a subset is expressed in skeletal muscle, in particular, at the sarcolemma, neuromuscular junction, myotendinous junction, and costameres. Expression of the integrin family is regulated during skeletal muscle development. Integrins play a major role in muscle differentiation, and α7β1 integrin is a critical receptor for myoblast migration. In adult skeletal muscle, integrins are concentrated at the neuromuscular junction and the myotendinous junction and provide an important link to the basement membrane in these specialized regions of muscle. α7β1 integrin is the major form found in adult skeletal muscle. Integrins have also been implicated in skeletal muscle diseases, and the absence of α7 integrin leads to a mild muscular dystrophy in both mice and humans. Myotendinous junctions were severely disturbed in mouse models with α7integrin deficiency. Both mouse and human studies suggest that muscle weakness arises from destruction of the myotendinous junction, rather than from compromised sarcolemma integrity. Finally, α7β1 integrin is reduced in several muscular dystrophies, and α7β1 integrin overexpression may provide a possible therapeutic approach for Duchenne muscular dystrophy. In the final minireview, entitled "Dystrophin-Glycoprotein Complex: Post-translational Processing and Dystroglycan Function," Daniel E. Michele and Kevin P. Campbell focus on the major sarcolemma membrane complex in adult skeletal muscle that links the cytoskeleton to the basement membrane (12Michele D.E. Campbell K.P. J. Biol. Chem. 2003; (January 29, 10.1074/jbc.R200031200)Google Scholar). DGC is a large oligomeric complex of proteins in the sarcolemma of skeletal muscle. The DGC is composed of both integral and peripheral membrane proteins and provides a structural connection between the basement membrane and the actin cytoskeleton and has been hypothesized to protect the sarcolemma from mechanical damage during muscle contraction. Several forms of muscular dystrophy arise from primary mutations in genes encoding components of the DGC. The DGC is grouped into three subcomplexes, dystroglycan (α- and β-dystroglycan), the sarcoglycan-sarcospan subcomplex, and the cytoskeletal components dystrophin, syntrophyin, and dystrobrevin. Michele and Campbell review the current status of our understanding of the DGC, and in particular focus on the structure and post-translational processing of dystroglycan. Interestingly, recent genetic data have demonstrated that proteins with homology similar to glycosyltransferases are linked to muscular dystrophy and appear to preferentially or exclusively modify α-dystroglycan. The role of glycosylation in the function of dystroglycan is discussed, and the mechanisms whereby the loss of functional dystroglycan leads to clinical symptoms, including muscular dystrophy, and abnormal central nervous system development and function are presented. Finally, new insights into dystroglycan function revealed from the studies of mouse models and patients with incomplete glycosylation (dystroglycanopathies) are reviewed. Much progress has been made in our understanding of the role of basement membrane-sarcolemma-cytoskeleton interactions in the development, structure, and function of striated muscle. The field continues to be a dynamic arena for future research, in which new functional molecules, their interactions, and their functional significance, continue to be identified from molecular genetics of human diseases, biochemistry, cell biology, and gene targeting in the mouse. Given the essential role of these molecules in human disease, understanding the interactions of the skeletal muscle basement membrane-sarcolemma-cytoskeleton will hopefully lead to a better understanding of disease pathogenesis and therapeutic opportunity.

Altered smooth muscle development and innervation in the lower genitourinary and gastrointestinal tract of the male human fetus with myelomeningocele.

To examine the role of AMP-activated protein kinase (AMPK) in muscle glucose transport, we generated muscle-specific transgenic mice (TG) carrying cDNAs of inactive alpha2 (alpha2i TG) and alpha1 (alpha1i TG) catalytic subunits. Extensor digitorum longus (EDL) muscles from wild type and TG mice were isolated and subjected to a series of in vitro incubation experiments. In alpha2i TG mice basal alpha2 activity was barely detectable, whereas basal alpha1 activity was only partially reduced. Known AMPK stimuli including 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR), rotenone (a Complex I inhibitor), dinitrophenol (a mitochondrial uncoupler), muscle contraction, and sorbitol (producing hyperosmolar shock) did not increase AMPK alpha2 activity in alpha2i TG mice, whereas alpha1 activation was attenuated by only 30-50%. Glucose transport was measured in vitro using isolated EDL muscles from alpha2i TG mice. AICAR- and rotenone-stimulated glucose transport was fully inhibited in alpha2i TG mice; however, the lack of AMPK alpha2 activity had no effect on contraction- or sorbitol-induced glucose transport. Similar to these observations in vitro, contraction-stimulated glucose transport, assessed in vivo by 2-deoxy-d-[(3)H]glucose incorporation into EDL, tibialis anterior, and gastrocnemius muscles, was normal in alpha2i TG mice. Thus, AMPK alpha2 activation is essential for some, but not all, insulin-independent glucose transport. Muscle contraction- and hyperosmolarity-induced glucose transport may be regulated by a redundant mechanism in which AMPK alpha2 is one of multiple signaling pathways.