Direct visualization of the dystrophin network on skeletal muscle fiber membrane

A review on mechanistic insights into structure and function of dystrophin protein in pathophysiology and therapeutic targeting of Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is an X-linked recessive genetic disorder characterized by progressive and severe muscle weakening and degeneration. Among the various forms of muscular dystrophy, it stands out as one of the most common and impactful, predominantly affecting boys. The condition arises due to mutations in the dystrophin gene, a key player in maintaining the structure and function of muscle fibers. The manuscript explores the structural features of dystrophin protein and their pivotal roles in DMD. We present an in-depth analysis of promising therapeutic approaches targeting dystrophin and their implications for the therapeutic management of DMD. Several therapies aiming to restore dystrophin protein or address secondary pathology have obtained regulatory approval, and many others are ongoing clinical development. Notably, recent advancements in genetic approaches have demonstrated the potential to restore partially functional dystrophin forms. The review also provides a comprehensive overview of the status of clinical trials for major therapeutic genetic approaches for DMD. In addition, we have summarized the ongoing therapeutic approaches and advanced mechanisms of action for dystrophin restoration and the challenges associated with DMD therapeutics.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

Evolution and developmental functions of the dystrophin-associated protein complex: beyond the idea of a muscle-specific cell adhesion complex

The Dystrophin-Associated Protein Complex (DAPC) is a well-defined and evolutionarily conserved complex in animals. DAPC interacts with the F-actin cytoskeleton via dystrophin, and with the extracellular matrix via the membrane protein dystroglycan. Probably for historical reasons that have linked its discovery to muscular dystrophies, DAPC function is often described as limited to muscle integrity maintenance by providing mechanical robustness, which implies strong cell-extracellular matrix adhesion properties. In this review, phylogenetic and functional data from different vertebrate and invertebrate models will be analyzed and compared to explore the molecular and cellular functions of DAPC, with a specific focus on dystrophin. These data reveals that the evolution paths of DAPC and muscle cells are not intrinsically linked and that many features of dystrophin protein domains have not been identified yet. DAPC adhesive properties also are discussed by reviewing the available evidence of common key features of adhesion complexes, such as complex clustering, force transmission, mechanosensitivity and mechanotransduction. Finally, the review highlights DAPC developmental roles in tissue morphogenesis and basement membrane (BM) assembly that may indicate adhesion-independent functions.

Dystrophin missense mutations alter focal adhesion tension and mechanotransduction

Skeletal muscle is a mechanical organ that not only produces force but also uses mechanical stimuli as a signal to regulate cellular responses. Duchenne and Becker muscular dystrophy are lethal muscle wasting diseases that affect 1 in 3,500 boys and is caused by the absence or malfunction of dystrophin protein, respectively. There is a lack of understanding on how the integration of these mechanical signals is dysregulated in muscular dystrophy and how they may contribute to disease progression. In this study, we show that patient-relevant dystrophin mutations alter the mechanical signaling axis in muscle cells, leading to impaired migration. This work proposes dystrophin as a component of the cellular force-sensing machinery, furthering our knowledge in the pathomechanism of muscular dystrophy.

Dystrophin is an essential muscle protein that contributes to cell membrane stability by mechanically linking the actin cytoskeleton to the extracellular matrix via an adhesion complex called the dystrophin–glycoprotein complex. The absence or impaired function of dystrophin causes muscular dystrophy. Focal adhesions (FAs) are also mechanosensitive adhesion complexes that connect the cytoskeleton to the extracellular matrix. However, the interplay between dystrophin and FA force transmission has not been investigated. Using a vinculin-based bioluminescent tension sensor, we measured FA tension in transgenic C2C12 myoblasts expressing wild-type (WT) dystrophin, a nonpathogenic single nucleotide polymorphism (SNP) (I232M), or two missense mutations associated with Duchenne (L54R), or Becker muscular dystrophy (L172H). Our data revealed cross talk between dystrophin and FAs, as the expression of WT or I232M dystrophin increased FA tension compared to dystrophin-less nontransgenic myoblasts. In contrast, the expression of L54R or L172H did not increase FA tension, indicating that these disease-causing mutations compromise the mechanical function of dystrophin as an FA allosteric regulator. Decreased FA tension caused by these mutations manifests as defective migration, as well as decreased Yes-associated protein 1 (YAP) activation, possibly by the disruption of the ability of FAs to transmit forces between the extracellular matrix and cytoskeleton. Our results indicate that dystrophin influences FA tension and suggest that dystrophin disease-causing missense mutations may disrupt a cellular tension-sensing pathway in dystrophic skeletal muscle.

Essential roles of the dystrophin-glycoprotein complex in different cardiac pathologies

The dystrophin-glycoprotein complex (DGC), situated at the sarcolemma dynamically remodels during cardiac disease. This review examines DGC remodeling as a common denominator in diseases affecting heart function and health. Dystrophin and the DGC serve as broad cytoskeletal integrators that are critical for maintaining stability of muscle membranes. The presence of pathogenic variants in genes encoding proteins of the DGC can cause absence of the protein and/or alterations in other complex members leading to muscular dystrophies. Targeted studies have allowed the individual functions of affected proteins to be defined. The DGC has demonstrated its dynamic function, remodeling under a number of conditions that stress the heart. Beyond genetic causes, pathogenic processes also impinge on the DGC, causing alterations in the abundance of dystrophin and associated proteins during cardiac insult such as ischemia-reperfusion injury, mechanical unloading, and myocarditis. When considering new therapeutic strategies, it is important to assess DGC remodeling as a common factor in various heart diseases. The DGC connects the internal F-actin-based cytoskeleton to laminin-211 of the extracellular space, playing an important role in the transmission of mechanical force to the extracellular matrix. The essential functions of dystrophin and the DGC have been long recognized. DGC based therapeutic approaches have been primarily focused on muscular dystrophies, however it may be a beneficial target in a number of disorders that affect the heart. This review provides an account of what we now know, and discusses how this knowledge can benefit persistent health conditions in the clinic.

PDE10A Inhibition Reduces the Manifestation of Pathology in DMD Zebrafish and Represses the Genetic Modifier PITPNA

Duchenne muscular dystrophy (DMD) is a severe genetic disorder caused by mutations in the DMD gene. Absence of dystrophin protein leads to progressive degradation of skeletal and cardiac function and leads to premature death. Over the years, zebrafish have been increasingly used for studying DMD and are a powerful tool for drug discovery and therapeutic development.

In our study, a birefringence screening assay led to identification of phosphodiesterase 10A (PDE10A) inhibitors that reduced the manifestation of dystrophic muscle phenotype in dystrophin-deficient sapje-like zebrafish larvae. PDE10A has been validated as a therapeutic target by pde10a morpholino-mediated reduction in muscle pathology and improvement in locomotion, muscle, and vascular function as well as long-term survival in sapje-like larvae. PDE10A inhibition in zebrafish and DMD patient-derived myoblasts were also associated with reduction of PITPNA expression that has been previously identified as a protective genetic modifier in two exceptional dystrophin-deficient golden retriever muscular dystrophy (GRMD) dogs that escaped the dystrophic phenotype. The combination of a phenotypic assay and relevant functional assessments in the sapje-like zebrafish enhances the potential for the prospective discovery of DMD therapeutics. Indeed, our results suggest a new application for a PDE10A inhibitor as a potential DMD therapeutic to be investigated in a mouse model of DMD.

Microutrophin expression in dystrophic mice displays myofiber type differences in therapeutic effects

Gene therapy approaches for DMD using recombinant adeno-associated viral (rAAV) vectors to deliver miniaturized (or micro) dystrophin genes to striated muscles have shown significant progress. However, concerns remain about the potential for immune responses against dystrophin in some patients. Utrophin, a developmental paralogue of dystrophin, may provide a viable treatment option. Here we examine the functional capacity of an rAAV-mediated microutrophin (μUtrn) therapy in the mdx4cv mouse model of DMD. We found that rAAV-μUtrn led to improvement in dystrophic histopathology & mostly restored the architecture of the neuromuscular and myotendinous junctions. Physiological studies of tibialis anterior muscles indicated peak force maintenance, with partial improvement of specific force. A fundamental question for μUtrn therapeutics is not only can it replace critical functions of dystrophin, but whether full-length utrophin impacts the therapeutic efficacy of the smaller, highly expressed μUtrn. As such, we found that μUtrn significantly reduced the spacing of the costameric lattice relative to full-length utrophin. Further, immunostaining suggested the improvement in dystrophic pathophysiology was largely influenced by favored correction of fast 2b fibers. However, unlike μUtrn, μdystrophin (μDys) expression did not show this fiber type preference. Interestingly, μUtrn was better able to protect 2a and 2d fibers in mdx:utrn-/- mice than in mdx4cv mice where the endogenous full-length utrophin was most prevalent. Altogether, these data are consistent with the role of steric hindrance between full-length utrophin & μUtrn within the sarcolemma. Understanding the stoichiometry of this effect may be important for predicting clinical efficacy.

Multi-Compartment, Early Disruption of cGMP and cAMP Signalling in Cardiac Myocytes from the mdx Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is the most frequent and severe form of muscular dystrophy. The disease presents with progressive body-wide muscle deterioration and, with recent advances in respiratory care, cardiac involvement is an important cause of morbidity and mortality. DMD is caused by mutations in the dystrophin gene resulting in the absence of dystrophin and, consequently, disturbance of other proteins that form the dystrophin-associated protein complex (DAPC), including neuronal nitric oxide synthase (nNOS). The molecular mechanisms that link the absence of dystrophin with the alteration of cardiac function remain poorly understood but disruption of NO-cGMP signalling, mishandling of calcium and mitochondrial disturbances have been hypothesized to play a role. cGMP and cAMP are second messengers that are key in the regulation of cardiac myocyte function and disruption of cyclic nucleotide signalling leads to cardiomyopathy. cGMP and cAMP signals are compartmentalised and local regulation relies on the activity of phosphodiesterases (PDEs). Here, using genetically encoded FRET reporters targeted to distinct subcellular compartments of neonatal cardiac myocytes from the DMD mouse model mdx, we investigate whether lack of dystrophin disrupts local cyclic nucleotide signalling, thus potentially providing an early trigger for the development of cardiomyopathy. Our data show a significant alteration of both basal and stimulated cyclic nucleotide levels in all compartments investigated, as well as a complex reorganization of local PDE activities.

Annexins and plasma membrane repair

Plasma membrane wound repair is a cell-autonomous process that is triggered by Ca²⁺ entering through the site of injury and involves membrane resealing, i.e., re-establishment of a continuous plasma membrane, as well as remodeling of the cortical actin cytoskeleton. Among other things, the injury-induced Ca²⁺ elevation initiates the wound site recruitment of Ca²⁺-regulated proteins that function in the course of repair. Annexins are a class of such Ca²⁺-regulated proteins. They associate with acidic phospholipids of cellular membranes in their Ca²⁺ bound conformation with Ca²⁺ sensitivities ranging from the low to high micromolar range depending on the respective annexin protein. Annexins accumulate at sites of plasma membrane injury in a temporally controlled manner and are thought to function by controlling membrane rearrangements at the wound site, most likely in conjunction with other repair proteins such as dysferlin. Their role in membrane repair, which has been evidenced in several model systems, will be discussed in this chapter.

Pericytes in Muscular Dystrophies

The muscular dystrophies are an heterogeneous group of inherited myopathies characterised by the progressive wasting of skeletal muscle tissue. Pericytes have been shown to make muscle in vitro and to contribute to skeletal muscle regeneration in several animal models, although recent data has shown this to be controversial. In fact, some pericyte subpopulations have been shown to contribute to fibrosis and adipose deposition in muscle. In this chapter, we explore the identity and the multifaceted role of pericytes in dystrophic muscle, potential therapeutic applications and the current need to overcome the hurdles of characterisation (both to identify pericyte subpopulations and track cell fate), to prevent deleterious differentiation towards myogenic-inhibiting subpopulations, and to improve cell proliferation and engraftment efficacy.

Dystrophin As a Molecular Shock Absorber

Dystrophin is the largest protein isoform (427 kDa) expressed from the gene defective in Duchenne muscular dystrophy, a lethal muscle-wasting and genetically inherited disease. Dystrophin, localized within a cytoplasmic lattice termed costameres, connects the intracellular cytoskeleton of a myofiber through the cell membrane (sarcolemma) to the surrounding extracellular matrix. In spite of its mechanical regulation roles in stabilizing the sarcolemma during muscle contraction, the underlying molecular mechanism is still elusive. Here, we systematically investigated the mechanical stability and kinetics of the force-bearing central domain of human dystrophin that contains 24 spectrin repeats using magnetic tweezers. We show that the stochastic unfolding and refolding of central domain of dystrophin is able to keep the forces below 25 pN over a significant length change up to ∼800 nm in physiological level of pulling speeds. These results suggest that dystrophin may serve as a molecular shock absorber that defines the physiological level of force in the dystrophin-mediated force-transmission pathway during muscle contraction/stretch, thereby stabilizing the sarcolemma.

Predictive model identifies key network regulators of cardiomyocyte mechano-signaling

Mechanical strain is a potent stimulus for growth and remodeling in cells. Although many pathways have been implicated in stretch-induced remodeling, the control structures by which signals from distinct mechano-sensors are integrated to modulate hypertrophy and gene expression in cardiomyocytes remain unclear. Here, we constructed and validated a predictive computational model of the cardiac mechano-signaling network in order to elucidate the mechanisms underlying signal integration. The model identifies calcium, actin, Ras, Raf1, PI3K, and JAK as key regulators of cardiac mechano-signaling and characterizes crosstalk logic imparting differential control of transcription by AT1R, integrins, and calcium channels. We find that while these regulators maintain mostly independent control over distinct groups of transcription factors, synergy between multiple pathways is necessary to activate all the transcription factors necessary for gene transcription and hypertrophy. We also identify a PKG-dependent mechanism by which valsartan/sacubitril, a combination drug recently approved for treating heart failure, inhibits stretch-induced hypertrophy, and predict further efficacious pairs of drug targets in the network through a network-wide combinatorial search.

Common stresses such as high blood pressure or heart attack can lead to heart failure, which afflicts over 25 million people worldwide. These stresses cause cardiomyocytes to grow and remodel, which may initially be beneficial but ultimately worsen heart function. Current heart failure drugs such as beta-blockers counteract biochemical cues prompting cardiomyocyte growth, yet mechanical cues to cardiomyocytes such as stretch are just as important in driving cardiac dysfunction. However, no pharmacological treatments have yet been approved that specifically target mechano-signaling, in part because it is not clear how cardiomyocytes integrate signals from multiple mechano-responsive sensors and pathways into their decision to grow. To address this challenge, we built a systems-level computational model that represents 125 interactions between 94 stretch-responsive signaling molecules. The model correctly predicts 134 of 172 previous independent experimental observations, and identifies the key regulators of stretch-induced cardiomyocyte remodeling. Although cardiomyocytes have many mechano-signaling pathways that function largely independently, we find that cooperation between them is necessary to cause growth and remodeling. We identify mechanisms by which a recently approved heart failure drug pair affects mechano-signaling, and we further predict additional pairs of drug targets that could be used to help reverse heart failure.

Dystrophic Cardiomyopathy-Potential Role of Calcium in Pathogenesis, Treatment and Novel Therapies

Duchenne muscular dystrophy (DMD) is caused by defects in the DMD gene and results in progressive wasting of skeletal and cardiac muscle due to an absence of functional dystrophin. Cardiomyopathy is prominent in DMD patients, and contributes significantly to mortality. This is particularly true following respiratory interventions that reduce death rate and increase ambulation and consequently cardiac load. Cardiomyopathy shows an increasing prevalence with age and disease progression, and over 95% of patients exhibit dilated cardiomyopathy by the time they reach adulthood. Development of the myopathy is complex, and elevations in intracellular calcium, functional muscle ischemia, and mitochondrial dysfunction characterise the pathophysiology. Current therapies are limited to treating symptoms of the disease and there is therefore an urgent need to treat the underlying genetic defect. Several novel therapies are outlined here, and the unprecedented success of phosphorodiamidate morpholino oligomers (PMOs) in preclinical and clinical studies is overviewed.

In vivo gene editing in dystrophic mouse muscle and muscle stem cells

Frame-disrupting mutations in the DMD gene, encoding dystrophin, compromise myofiber integrity and drive muscle deterioration in Duchenne muscular dystrophy (DMD). Removing one or more exons from the mutated transcript can produce an in-frame mRNA and a truncated, but still functional, protein. In this study, we developed and tested a direct gene-editing approach to induce exon deletion and recover dystrophin expression in the mdx mouse model of DMD. Delivery by adeno-associated virus (AAV) of clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 endonucleases coupled with paired guide RNAs flanking the mutated Dmd exon23 resulted in excision of intervening DNA and restored the Dmd reading frame in myofibers, cardiomyocytes, and muscle stem cells after local or systemic delivery. AAV-Dmd CRISPR treatment partially recovered muscle functional deficiencies and generated a pool of endogenously corrected myogenic precursors in mdx mouse muscle.

Filamentous structures in skeletal muscle: anchors for the subsarcolemmal space

In skeletal muscle fibers, intermediate filaments and actin filaments provide structural support to the myofibrils and the sarcolemma. For many years, it was poorly understood from ultrastructural observations that how these filamentous structures were kept anchored. The present study was conducted to determine the architecture of filamentous anchoring structures in the subsarcolemmal space and the intermyofibrils. The diaphragms (Dp) of adult wild type and mdx mice (mdx is a model for Duchenne muscular dystrophy) were subjected to tension applied perpendicular to the long axis of the muscle fibers, with or without treatment with 1% Triton X-100 or 0.03% saponin. These experiments were conducted to confirm the presence and integrity of the filamentous anchoring structures. Transmission electron microscopy revealed that these structures provide firm transverse connections between the sarcolemma and peripheral myofibrils. Most of the filamentous structures appeared to be inserted into subsarcolemmal densities, forming anchoring connections between the sarcolemma and peripheral myofibrils. In some cases, actin filaments were found to run longitudinally in the subsarcolemmal space to connect to the sarcolemma or in some cases to connect to the intermyofibrils as elongated thin filaments. These filamentous anchoring structures were less common in the mdx Dp. Our data suggest that the transverse and longitudinal filamentous structures form an anchoring system in the subsarcolemmal space and the intermyofibrils.

Cardiac and respiratory dysfunction in Duchenne muscular dystrophy and the role of second messengers

Duchenne muscular dystrophy (DMD) affects young boys and is characterized by the absence of dystrophin, a large cytoskeletal protein present in skeletal and cardiac muscle cells and neurons. The heart and diaphragm become necrotic in DMD patients and animal models of DMD, resulting in cardiorespiratory failure as the leading cause of death. The major consequences of the absence of dystrophin are high levels of intracellular Ca(2+) and the unbalanced production of NO that can finally trigger protein degradation and cell death. Cytoplasmic increase in Ca(2+) concentration directly and indirectly triggers different processes such as necrosis, fibrosis, and activation of macrophages. The absence of the neuronal isoform of nitric oxide synthase (nNOS) and the overproduction of NO by the inducible isoform (iNOS) further increase the intracellular Ca(2+) via a hypernitrosylation of the ryanodine receptor. NO overproduction, which further induces the expression of iNOS but decreases the expression of the endothelial isoform (eNOS), deregulates the muscle tissue blood flow creating an ischemic situation. The high levels of Ca(2+) in dystrophic muscles and the ischemic state of the muscle tissue would culminate in a positive feedback loop. While efforts continue toward optimizing cardiac and respiratory care of DMD patients, both Ca(2+) and NO in cardiac and respiratory muscle pathways have been shown to be important to the etiology of the disease. Understanding the mechanisms behind the fine regulation of Ca(2+) -NO may be important for a noninterventional and noninvasive supportive approach to treat DMD patients, improving the quality of life and natural history of DMD patients.

Isolation of sarcolemmal plasma membranes by mechanically skinning rat skeletal muscle fibers for phospholipid analysis

Membrane phospholipid (PL) composition has been shown to affect cellular function by altering membrane physical structure. The sarcolemma plasma membrane (SLpm) is integral to skeletal muscle function and health. Previous studies assessing SLpm PL composition have demonstrated contamination from transverse (t)-tubule, sarcoplasmic reticulum, and nuclear membranes. This study assessed the possibility of isolating SL by mechanically skinning skeletal muscle fiber segments for the analysis of SLpm PL composition. Mechanically skinned SLpm from rat extensor digitorum longus (EDL) muscle fibers underwent Western blot analysis to assess contamination from t-tubule, sarcoplasmic reticulum, nuclear and mitochondrial membranes. The results indicate that isolated SLpm had minimal nuclear and mitochondrial membrane contamination and was void of contamination from sarcoplasmic reticulum and t-tubule membranes. After performing both high-performance thin layer chromatography and gas chromatography, we found that the SLpm obtained by mechanical skinning had higher sphingomyelin and total fatty acid saturation and lower phosphatidylcholine when compared to previous literature. Thus, by avoiding the use of various chemical treatments and membrane fractionation, we present data that may truly represent the SLpm and future studies can use this technique to assess potential changes under various perturbations and disease conditions such as insulin resistance and muscular dystrophy.

Skeletal muscle degenerative diseases and strategies for therapeutic muscle repair

Skeletal muscle is a highly specialized, postmitotic tissue that must withstand chronic mechanical and physiological stress throughout life to maintain proper contractile function. Muscle damage or disease leads to progressive weakness and disability, and manifests in more than 100 different human disorders. Current therapies to treat muscle degenerative diseases are limited mostly to the amelioration of symptoms, although promising new therapeutic directions are emerging. In this review, we discuss the pathological basis for the most common muscle degenerative diseases and highlight new and encouraging experimental and clinical opportunities to prevent or reverse these afflictions.

The carboxy-terminal third of dystrophin enhances actin binding activity

Dystrophin is an actin binding protein that is thought to stabilize the cardiac and skeletal muscle cell membranes during contraction. Here, we investigated the contributions of each dystrophin domain to actin binding function. Cosedimentation assays and pyrene-actin fluorescence experiments confirmed that a fragment spanning two-thirds of the dystrophin molecule [from N-terminal actin binding domain (ABD) 1 through ABD2] bound actin filaments with high affinity and protected filaments from forced depolymerization, but was less effective in both assays than full-length dystrophin. While a construct encoding the C-terminal third of dystrophin displayed no specific actin binding activity or competition with full-length dystrophin, our data show that it confers an unexpected regulation of actin binding by the N-terminal two-thirds of dystrophin when present in cis. Time-resolved phosphorescence anisotropy experiments demonstrated that the presence of the C-terminal third of dystrophin in cis also influences actin interaction by restricting actin rotational amplitude. We propose that the C-terminal region of dystrophin allosterically stabilizes an optimal actin binding conformation of dystrophin.

Aquaporin 4 Expression in the mdx Mouse Diaphragm

Expression of aquaporin (AQP) 4 in the surface membranes of skeletal myofibers is well established; however, its functional significance is still unknown. The alterations of AQP4 expressions in dystrophic muscles at RNA and protein levels have been reported in various dystrophic muscles such as dystrophinopathy, dysferlinopathy, and sarcoglycanopathy. We are interested in the relationship between the severity of dystrophic muscle degeneration and the expression of AQP4. Here we compared the AQP4 expression of the limb muscles with that of diaphragms in both mdx and control mice. The dystrophic muscle degeneration, such as rounding profile of cross sectional myofiber shape, dense eosin staining, central nuclei, and endomysial fibrosis in mdx mice, were more marked in diaphragms than in limb muscles. The decrease of AQP4 expression at protein level was more marked in diaphragms than in the limb muscles of mdx mice. However, the expression of AQP4 mRNA in the diaphragms of mdx mice was not reduced in comparison with limb muscles of mdx mice. The present study revealed that AQP4 expression at protein level was correlated with the severity of dystrophic changes in muscle tissues of mdx mice.

Biomechanics of the sarcolemma and costameres in single skeletal muscle fibers from normal and dystrophin-null mice

We studied the biomechanical properties of the sarcolemma and its links through costameres to the contractile apparatus in single mammalian myofibers of Extensor digitorum longus muscles isolated from wild (WT) and dystrophin-null (mdx) mice. Suction pressures (P) applied through a pipette to the sarcolemma generated a bleb, the height of which increased with increasing P. Larger increases in P broke the connections between the sarcolemma and myofibrils and eventually caused the sarcolemma to burst. We used the values of P at which these changes occurred to estimate the tensions and stiffness of the system and its individual elements. Tensions of the whole system and the sarcolemma, as well as the maximal tension sustained by the costameres, were all significantly lower (1.8-3.3 fold) in muscles of mdx mice compared to WT. Values of P at which separation and bursting occurred, as well as the stiffness of the whole system and of the isolated sarcolemma, were ~2-fold lower in mdx than in WT. Our results indicate that the absence of dystrophin reduces muscle stiffness, increases sarcolemmal deformability, and compromises the mechanical stability of costameres and their connections to nearby myofibrils.

Inhibition of contractile activity during postconditioning enhances cardioprotection by restoring sarcolemmal dystrophin through phosphatidylinositol 3-kinase

BACKGROUND

Although ischemic postconditioning (IPost) confers cardioprotection by protecting the mitochondria though the activation of phosphatidylinositol 3-kinase (PI3K), a potential drawback of IPost is impairment of aerobic ATP generation during reperfusion by repeated ischemia. This decrease in ATP might inhibit the restoration of sarcolemmal dystrophin, which is translocated during ischemia, and render cardiomyocytes susceptible to contraction-induced oncosis.

METHODS AND RESULTS

Isolated rat hearts were subjected to 30 min ischemia and 120 min reperfusion. IPost induced by 20 cycles of 10-s reperfusion and 10-s ischemia enhanced the activation of PI3K as evidenced by the increased phosphorylation of Akt, but had no effect on myocardial ATP, restoration of sarcolemmal dystrophin, or cardiomyocyte oncosis during IPost. Administration of the contractile blocker, 2,3-butanedione monoxim (BDM), during IPost increased myocardial ATP and facilitated the redistribution of dystrophin to the sarcolemma. This led to reduced cardiomyocyte oncosis and infarct size, and improved the left ventricular function. The anti-oncotic effect of BDM occurred without changing the anti-apoptotic effect of IPost. The PI3K inhibitor, LY294002, prevented the phosphorylation of Akt, decreased the recovery of ATP and restoration of sarcolemmal dystrophin, and blocked the anti-oncotic and anti-apoptotic effects of IPost.

CONCLUSIONS

These results suggest that the inhibition of contractile activity during IPost prevents cardiomyocyte oncosis and enhances cardioprotection through PI3K-dependent restoration of sarcolemmal dystrophin.

Disease-causing missense mutations in actin binding domain 1 of dystrophin induce thermodynamic instability and protein aggregation

Mutations in the dystrophin gene cause Duchenne muscular dystrophy (DMD) most commonly through loss of protein expression. In a small subpopulation of patients, missense mutations can cause DMD, Becker muscular dystrophy, or X-linked cardiomyopathy. Nearly one-half of disease-causing missense mutations are located in actin-binding domain 1 (ABD1) of dystrophin. To test the hypothesis that ABD1 missense mutations cause disease by impairing actin-binding activity, we engineered the K18N, L54R, D165V, A168D, L172H, and Y231N mutations into the full-length dystrophin cDNA and characterized the biochemical properties of each mutant protein. The K18N and L54R mutations are associated with the most severe diseases in humans and each caused a small but significant 4-fold decrease in actin-binding affinity, while the affinities of the other four mutant proteins were not significantly different from WT dystrophin. More interestingly, WT dystrophin was observed to unfold in a single-step, highly cooperative manner. In contrast, all six mutant proteins were significantly more prone to thermal denaturation and aggregation. Our results suggest that missense mutations in ABD1 may all cause loss of dystrophin function via protein instability and aggregation rather than through loss of ligand binding function. However, more severe disease progressions may be due to the combinatorial effects of some mutations on both protein aggregation and impaired actin-binding activity.

Disruption of sarcolemmal dystrophin and beta-dystroglycan may be a potential mechanism for myocardial dysfunction in severe sepsis

Evidence from our laboratory has shown alterations in myocardial structure in severe sepsis/septic shock. The morphological alterations are heralded by sarcolemmal damage, characterized by increased plasma membrane permeability caused by oxidative damage to lipids and proteins. The critical importance of the dystrophin-glycoprotein complex (DGC) in maintaining sarcolemmal stability led us to hypothesize that loss of dystrophin and associated glycoproteins could be involved in early increased sarcolemmal permeability in experimentally induced septic cardiomyopathy. Male C57Bl/6 mice were subjected to sham operation and moderate (MSI) or severe (SSI) septic injury induced by cecal ligation and puncture (CLP). Using western blot and immunofluorescence, a downregulation of dystrophin and beta-dystroglycan expression in both severe and moderate injury could be observed in septic hearts. The immunofluorescent and protein amount expressions of laminin-alpha2 were similar in SSI and sham-operated hearts. Consonantly, the evaluation of plasma membrane permeability by intracellular albumin staining provided evidence of severe injury of the sarcolemma in SSI hearts, whereas antioxidant treatment significantly attenuated the loss of sarcolemmal dystrophin expression and the increased membrane permeability. This study offers novel and mechanistic data to clarify subcellular events in the pathogenesis of cardiac dysfunction in severe sepsis. The main finding was that severe sepsis leads to a marked reduction in membrane localization of dystrophin and beta-dystroglycan in septic cardiomyocytes, a process that may constitute a structural basis of sepsis-induced cardiac depression. In addition, increased sarcolemmal permeability suggests functional impairment of the DGC complex in cardiac myofibers. In vivo observation that antioxidant treatment significantly abrogated the loss of dystrophin expression and plasma membrane increased permeability supports the hypothesis that oxidative damage may mediate the loss of dystrophin and beta-dystroglycan in septic mice. These abnormal parameters emerge as therapeutic targets and their modulation may provide beneficial effects on future cardiovascular outcomes and mortality in sepsis.

Dystrophin and utrophin have distinct effects on the structural dynamics of actin

We have used time-resolved spectroscopy to investigate the structural dynamics of actin interaction with dystrophin and utrophin in relationship to the pathology of muscular dystrophy. Dystrophin and utrophin bind actin in vitro with similar affinities, but the molecular contacts of these two proteins with actin are different. It has been hypothesized that the presence of two low-affinity actin-binding sites in dystrophin allows more elastic response of the actin-dystrophin-sarcolemma linkage to muscle stretches, compared with utrophin, which binds via one contiguous actin-binding domain. We have directly tested this hypothesis by determining the effects of dystrophin and utrophin on the microsecond rotational dynamics of a phosphorescent dye attached to C374 on actin, as detected by transient phosphorescence anisotropy (TPA). Binding of dystrophin or utrophin to actin resulted in significant changes in the TPA decay, increasing the final anisotropy (restricting the rotational amplitude) and decreasing the rotational correlation times (increasing the rotational rates and the torsional flexibility). This paradoxical combination of effects on actin dynamics (decreased amplitude but increased rate) has not been observed for other actin-binding proteins. Thus, when dystrophin or utrophin binds, actin becomes less like cast iron (strong but brittle) and more like steel (stronger and more resilient). At low levels of saturation, the binding of dystrophin and utrophin has similar effects, but at higher levels, utrophin caused much greater restrictions in amplitude and increases in rate. The effects of dystrophin and utrophin on actin dynamics provide molecular insight into the pathology of muscular dystrophy.

L-type Ca2+ channel function is linked to dystrophin expression in mammalian muscle

Background

In dystrophic mdx skeletal muscle, aberrant Ca²⁺ homeostasis and fibre degeneration are found. The absence of dystrophin in models of Duchenne muscular dystrophy (DMD) has been connected to altered ion channel properties e.g. impaired L-type Ca²⁺ currents. In regenerating mdx muscle, ‘revertant’ fibres restore dystrophin expression. Their functionality involving DHPR-Ca²⁺-channels is elusive.

Methods and Results

We developed a novel ‘in-situ’ confocal immuno-fluorescence and imaging technique that allows, for the first time, quantitative subcellular dystrophin-DHPR colocalization in individual, non-fixed, muscle fibres. Tubular DHPR signals alternated with second harmonic generation signals originating from myosin. Dystrophin-DHPR colocalization was substantial in wt fibres, but diminished in most mdx fibres. Mini-dystrophin (MinD) expressing fibres successfully restored colocalization. Interestingly, in some aged mdx fibres, colocalization was similar to wt fibres. Most mdx fibres showed very weak membrane dystrophin staining and were classified ‘mdx-like’. Some mdx fibres, however, had strong ‘wt-like’ dystrophin signals and were identified as ‘revertants’. Split mdx fibres were mostly ‘mdx-like’ and are not generally ‘revertants’. Correlations between membrane dystrophin and DHPR colocalization suggest a restored putative link in ‘revertants’. Using the two-micro-electrode-voltage clamp technique, Ca²⁺-current amplitudes (i_max) showed very similar behaviours: reduced amplitudes in most aged mdx fibres (as seen exclusively in young mdx mice) and a few mdx fibres, most likely ‘revertants’, with amplitudes similar to wt or MinD fibres. Ca²⁺ current activation curves were similar in ‘wt-like’ and ‘mdx-like’ aged mdx fibres and are not the cause for the differences in current amplitudes. i_max amplitudes were fully restored in MinD fibres.

Conclusions

We present evidence for a direct/indirect DHPR-dystrophin interaction present in wt, MinD and ‘revertant’ mdx fibres but absent in remaining mdx fibres. Our imaging technique reliably detects single isolated ‘revertant’ fibres that could be used for subsequent physiological experiments to study mechanisms and therapy concepts in DMD.

Ischemic preconditioning: from molecular mechanisms to therapeutic opportunities

Ischemia/reperfusion (I/R) injury is a major contributory factor to cardiac dysfunction and infarct size that determines patient prognosis after acute myocardial infarction. Considerable interest exists in harnessing the heart's endogenous capacity to resist I/R injury, known as ischemic preconditioning (IPC). The IPC research has contributed to uncovering the pathophysiology of I/R injury on a molecular and cellular basis and to invent potential therapeutic means to combat such damage. However, the translation of basic research findings learned from IPC into clinical practice has often been inadequate because the majority of basic research findings have stemmed from young and healthy animals. Few if any successful implementations of IPC have occurred in the diseased hearts that are the primary target of viable therapies activating cardioprotective mechanisms to limit cardiac dysfunction and infarct size. Therefore, the first purpose of this review is to facilitate understanding of pathophysiology of I/R injury and the mechanisms of cardioprotection afforded by IPC in the normal heart. Then I focus on the problems and opportunities for successful bench-to-bedside translation of IPC in the diseased hearts.

Biology of the striated muscle dystrophin-glycoprotein complex

Since its first description in 1990, the dystrophin-glycoprotein complex has emerged as a critical nexus for human muscular dystrophies arising from defects in a variety of distinct genes. Studies in mammals widely support a primary role for the dystrophin-glycoprotein complex in mechanical stabilization of the plasma membrane in striated muscle and provide hints for secondary functions in organizing molecules involved in cellular signaling. Studies in model organisms confirm the importance of the dystrophin-glycoprotein complex for muscle cell viability and have provided new leads toward a full understanding of its secondary roles in muscle biology.

Poloxamer 188 failed to prevent exercise-induced membrane breakdown in mdx skeletal muscle fibers

We sought to determine the effectiveness of poloxamer 188 (P188) in protecting dystrophin-deficient, mdx skeletal muscle fiber membrane against exercise-induced breaches. mdx mice were treated with either P188 or placebo via intraperitoneal injections and run on a treadmill for 60-90 min. Membrane breakdown was quantified in cross-sections of rectus femoris muscle pretreated with Evans blue dye (in vivo). The mean % dye-penetrated muscle in the P188 and placebo groups was not significantly different in each of three trials. These results contrast with a recent report of P188 being highly effective in protecting the stretch- and dobutamine-stressed mdx heart muscle. The most likely explanations for the disparity are: (1) the exercise stress we used was beyond the protective range of P188, (2) P188 delivery and serum concentration were sub-optimal, or (3) the mdx skeletal myopathy and cardiomyopathy have fundamentally different responses to treatment.

Challenges and opportunities in dystrophin-deficient cardiomyopathy gene therapy

The last decade has evidenced unprecedented progress in gene therapy of Duchenne and Becker muscular dystrophy (DMD and BMD) skeletal muscle disease. Cardiomyopathy is a leading cause of morbidity and mortality in both patients and carriers of DMD, BMD and X-linked dilated cardiomyopathy. However, there is little advance in heart gene therapy. The gene, the vector, vector delivery, the target tissue and animal models are five fundamental components in developing an effective gene therapy. Intensive effort has been made in optimizing gene transfer vectors and methods. Systemic and/or local delivery of recombinant adeno-associated viral vector have resulted in widespread transduction in the rodent heart. The current challenge is to define other parameters that are essential for a successful gene therapy such as the best candidate gene(s), the optimal expression level and the target tissue. This review focuses on these long-ignored aspects and points out future research directions. In particular, we need to address whether all or only some of the recently developed mini- and microgenes are protective in the heart, whether partial correction can lead to whole heart function improvement, whether over-expression is hazardous and whether correcting skeletal muscle disease can slow down or stop the progression of cardiomyopathy. Discussion is also made on whether the current mouse models can meet these research needs.

Dystrophin, its interactions with other proteins, and implications for muscular dystrophy

Duchenne muscular dystrophy is the most prevalent and severe form of human muscular dystrophy. Investigations into the molecular basis for Duchenne muscular dystrophy were greatly facilitated by seminal studies in the 1980s that identified the defective gene and its major protein product, dystrophin. Biochemical studies revealed its tight association with a multi-subunit complex, the so-named dystrophin-glycoprotein complex. Since its description, the dystrophin-glycoprotein complex has emerged as an important structural unit of muscle and also as a critical nexus for understanding a diverse array of muscular dystrophies arising from defects in several distinct genes. The dystrophin homologue utrophin can compensate at the cell/tissue level for dystrophin deficiency, but functions through distinct molecular mechanisms of protein-protein interaction.

High-resolution en-face visualization of the cardiomyocyte plasma membrane reveals distinctive distributions of spectrin and dystrophin

The actin-binding proteins, spectrin and dystrophin, are key components of the plasma membrane-associated cytoskeleton of the cardiac muscle cell. From confocal immunofluorescence studies, the distribution of spectrin appears to overlap with that of dystrophin, but the precise functional differentiation, molecular distributions and spatial relationship of these two cytoskeletal systems remain unclear. Freeze-fracture replica immuno-electron microscopy, in parallel with immunofluorescence/confocal microscopy, were applied to examine at high resolution the spatial relationships between the spectrin and dystrophin membrane-associated cytoskeleton systems in cardiac muscle. Application of freeze-fracture replica cytochemistry, with single and double immunogold labeling, permitted simultaneous examination of the organization of spectrin and dystrophin in en-face views of the plasma membrane at high resolution. In contrast to the close spatial relationship previously demonstrated for dystrophin and beta-dystroglycan, no association between the gold label marking dystrophin and that marking spectrin was observed. Our freeze-fracture cytochemical results suggest that the two membrane skeletal networks formed by dystrophin and spectrin in cardiac muscle are independently organized, implying that whatever overlap of function (e.g., in structural support to the plasma membrane) may exist between them, the two systems may each have additional distinctive roles.

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.

Regulation of store-operated calcium entries and mitochondrial uptake by minidystrophin expression in cultured myotubes

Defective expression of dystrophin in muscle cells is the primary feature of Duchenne muscular dystrophy (DMD), which is accompanied by fiber necrosis and intracellular calcium mishandling. These features led to the hypothesis that dystrophin could control calcium movements. Calcium mishandling in human DMD myotubes is dependent on contraction and/or calcium release activity, suggesting the involvement of channels being activated during these processes. Forced expression of minidystrophin at the plasma membrane of dystrophin-deficient Sol8 myotubes reactivates appropriate sarcolemmal expression of dystrophin-associated proteins and results in normal calcium homeostasis. In active dystrophic myotubes, store-operated calcium channels could be responsible for a sustained calcium influx in muscle cells. We show here that depletion of calcium stores (sarcoplasmic reticulum) by repetitive activation of calcium release and blockade of SERCA leads to a calcium influx. In myotubes expressing recombinant minidystrophin, these store-dependent influxes were reduced to a level similar to that observed in myotubes expressing native dystrophin. High store-dependent calcium influxes in dystrophin-deficient myotubes were associated with sustained cytosolic calcium transients and high intramitochondrial entries, while lower store-dependent calcium influx in myotubes expressing minidystrophin resulted in shorter calcium transients and reduced calcium uptake into mitochondria. We propose that minidystrophin negatively regulates sarcolemmal store-dependent calcium channels, which reduces store-dependent calcium influx, as well as its mitochondrial uptake. Forced expression of minidystrophin in dystrophic cells might restore the regulation of sarcolemmal store-dependent channels, which could protect against calcium mishandling.

Postnatal changes in sarcolemmal organization in the mdx mouse

The tibialis anterior muscles of mdx mice degenerate between 3 and 4 weeks after birth and then partially recover. We show that the membrane cytoskeleton at the mdx sarcolemma is disorganized at 18-days postnatal, and becomes more disorganized at 4 weeks compared to earlier or later times. Mdx muscle at 18 days have few central nuclei, suggesting that it has not yet sustained significant damage. The variance of myofiber diameter confirms that the mdx pathology is greatest at 4 weeks, when the sarcolemma is most disorganized. Sarcolemmal disorganization in the mdx does not involve contractile structures, nor is it seen in age-matched controls. In revertant mdx fibers expressing dystrophin, sarcolemmal organization is similar to controls. Our results suggest that the absence of dystrophin results in the disorganization of the sarcolemma, even in 18-day-old mice, and that the extent of disorganization is greater when the myopathy is most severe.

Muscle regeneration in dystrophin-deficient mdx mice studied by gene expression profiling

Background

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is lethal. In contrast, dystrophin-deficient mdx mice recover due to effective regeneration of affected muscle tissue. To characterize the molecular processes associated with regeneration, we compared gene expression levels in hindlimb muscle tissue of mdx and control mice at 9 timepoints, ranging from 1–20 weeks of age.

Results

Out of 7776 genes, 1735 were differentially expressed between mdx and control muscle at at least one timepoint (p < 0.05 after Bonferroni correction). We found that genes coding for components of the dystrophin-associated glycoprotein complex are generally downregulated in the mdx mouse. Based on functional characteristics such as membrane localization, signal transduction, and transcriptional activation, 166 differentially expressed genes with possible functions in regeneration were analyzed in more detail. The majority of these genes peak at the age of 8 weeks, where the regeneration activity is maximal. The following pathways are activated, as shown by upregulation of multiple members per signalling pathway: the Notch-Delta pathway that plays a role in the activation of satellite cells, and the Bmp15 and Neuregulin 3 signalling pathways that may regulate proliferation and differentiation of satellite cells. In DMD patients, only few of the identified regeneration-associated genes were found activated, indicating less efficient regeneration processes in humans.

Conclusion

Based on the observed expression profiles, we describe a model for muscle regeneration in mdx mice, which may provide new leads for development of DMD therapies based on the improvement of muscle regeneration efficacy.

Specific interaction of the actin-binding domain of dystrophin with intermediate filaments containing keratin 19

Cytokeratins 8 and 19 concentrate at costameres of striated muscle and copurify with the dystrophin-glycoprotein complex, perhaps through the interaction of the cytokeratins with the actin-binding domain of dystrophin. We overexpressed dystrophin's actin-binding domain (Dys-ABD), K8 and K19, as well as closely related proteins, in COS-7 cells to assess the basis and specificity of their interaction. Dys-ABD alone associated with actin microfilaments. Expressed with K8 and K19, which form filaments, Dys-ABD associated preferentially with the cytokeratins. This interaction was specific, as the homologous ABD of betaI-spectrin failed to interact with K8/K19 filaments, and Dys-ABD did not associate with desmin or K8/K18 filaments. Studies in COS-7 cells and in vitro showed that Dys-ABD binds directly and specifically to K19. Expressed in muscle fibers in vivo, K19 accumulated in the myoplasm in structures that contained dystrophin and spectrin and disrupted the organization of the sarcolemma. K8 incorporated into sarcomeres, with no effect on the sarcolemma. Our results show that dystrophin interacts through its ABD with K19 specifically and are consistent with the idea that cytokeratins associate with dystrophin at the sarcolemma of striated muscle.

Identification of spectrin-like repeats required for high affinity utrophin-actin interaction

Most studies aimed at characterizing the utrophinactin interaction have focused on the amino-terminal tandem calponin homology domain. However, we recently reported evidence suggesting that spectrin-like repeats of utrophin also participate in binding to actin. Here we expressed several recombinant fragments encoding the utrophin amino-terminal domain alone or in combination with various numbers of spectrin-like repeats. We further quantitatively characterized the actin binding properties of each recombinant utrophin fragment using a high-speed sedimentation assay. To evaluate the capacity of each protein to stabilize actin filaments, we compared the effect of utrophin recombinant fragments and full-length utrophin on 6-propionyl-2-(N,N-dimethylamino)naphthalene actin depolymerization. Our results suggest that, whereas the amino-terminal domain is essential for primary interaction between utrophin and actin, spectrin-like repeats have additive effects on the affinity and stoichiometry of binding. Our data indicate that the amino-terminal domain and first 10 consecutive spectrin-like repeats recapitulate the actin binding activity of full-length utrophin more faithfully than the amino-terminal domain alone. These findings support the model for lateral association of utrophin along the actin filament and provide the molecular basis for designing the most effective utrophin "mini-genes" for treatment of dystrophinopathies.

Temporary blockade of contractility during reperfusion elicits a cardioprotective effect of the p38 MAP kinase inhibitor SB-203580

p38 MAP kinase activation is known to be deleterious not only to mitochondria but also to contractile function. Therefore, p38 MAP kinase inhibition therapy represents a promising approach in preventing reperfusion injury in the heart. However, reversal of p38 MAP kinase-mediated contractile dysfunction may disrupt the fragile sarcolemma of ischemic-reperfused myocytes. We, therefore, hypothesized that the beneficial effect of p38 MAP kinase inhibition during reperfusion can be enhanced when contractility is simultaneously blocked. Isolated and perfused rat hearts were paced at 330 rpm and subjected to 20 min of ischemia followed by reperfusion. p38 MAP kinase was activated after ischemia and early during reperfusion (<30 min). Treatment with the p38 MAP kinase inhibitor SB-203580 (10 μM) for 30 min during reperfusion, but not the c-Jun NH2-terminal kinase inhibitor SP-600125 (10 μM), improved contractility but increased creatine kinase release and infarct size. Cotreatment with SB-203580 and the contractile blocker 2,3-butanedione monoxime (BDM, 20 mM) or the ultra-short-acting β-blocker esmorol (0.15 mM) for the first 30 min during reperfusion significantly reduced creatine kinase release and infarct size. In vitro mitochondrial ATP generation and myocardial ATP content were significantly increased in the heart cotreated with SB-203580 and BDM during reperfusion. Dystrophin was translocated from the sarcolemma during ischemia and reperfusion. SB-203580 increased accumulation of Evans blue dye in myocytes depleted of sarcolemmal dystrophin during reperfusion, whereas cotreatment with BDM facilitated restoration of sarcolemmal dystrophin and mitigated sarcolemmal damage after withdrawal of BDM. These results suggest that treatment with SB-203580 during reperfusion aggravates myocyte necrosis but concomitant blockade of contractile force unmasks cardioprotective effects of SB-203580.

Spectrin, alpha-actinin, and dystrophin

Spectrin family proteins represent an important group of actin-bundling and membrane-anchoring proteins found in diverse structures from yeast to man. Arising from a common ancestral alpha-actinin gene through duplications and rearrangements, the family has increased to include the spectrins and dystrophin/utrophin. The spectrin family is characterized by the presence of spectrin repeats, actin binding domains, and EF hands. With increasing divergence, new domains and functions have been added such that spectrin and dystrophin also contain specialized protein-protein interaction motifs and regions for interaction with membranes and phospholipids. The acquisition of new domains also increased the functional complexity of the family such that the proteins perform a range of tasks way beyond the simple bundling of actin filaments by alpha-actinin in S. pombe. We discuss the evolutionary, structural, functional, and regulatory roles of the spectrin family of proteins and describe some of the disease traits associated with loss of spectrin family protein function.

Ischemic preconditioning-mediated restoration of membrane dystrophin during reperfusion correlates with protection against contraction-induced myocardial injury

Dystrophin is an integral membrane protein involved in the stabilization of the sarcolemmal membrane in cardiac muscle. We hypothesized that the loss of membrane dystrophin during ischemia and reperfusion is responsible for contractile force-induced myocardial injury and that cardioprotection afforded by ischemic preconditioning (IPC) is related to the preservation of membrane dystrophin. Isolated and perfused rat hearts were subjected to 30 min of global ischemia, followed by reperfusion with or without the contractile blocker 2,3-butanedione monoxime (BDM). IPC was introduced by three cycles of 5-min ischemia and 5-min reperfusion before the global ischemia. Dystrophin was distributed exclusively in the membrane of myocytes in the normally perfused heart but was redistributed to the myofibril fraction after 30 min of ischemia and was lost from both of these compartments during reperfusion in the presence or absence of BDM. The loss of dystrophin preceded uptake of the membrane-impermeable Evans blue dye by myocytes that occurred after the withdrawal of BDM and was associated with creatine kinase release and the development of contracture. Although IPC did not alter the redistribution of membrane dystrophin induced by 30 min of ischemia, it facilitated the restoration of membrane dystrophin during reperfusion. Also, myocyte necrosis was not observed when BDM was withdrawn after complete restoration of membrane dystrophin. These results demonstrate that IPC-mediated restoration of membrane dystrophin during reperfusion correlates with protection against contractile force-induced myocardial injury and suggest that the cardioprotection conferred by IPC can be enhanced by the temporary blockade of contractile activity until restoration of membrane dystrophin during reperfusion.

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.