The dystrophin glycoprotein complex: signaling strength and integrity for the sarcolemma

Long-term miR-669a therapy alleviates chronic dilated cardiomyopathy in dystrophic mice

BACKGROUND

Dilated cardiomyopathy (DCM) is a leading cause of chronic morbidity and mortality in muscular dystrophy (MD) patients. Current pharmacological treatments are not yet able to counteract chronic myocardial wastage, thus novel therapies are being intensely explored. MicroRNAs have been implicated as fine regulators of cardiomyopathic progression. Previously, miR-669a downregulation has been linked to the severe DCM progression displayed by Sgcb-null dystrophic mice. However, the impact of long-term overexpression of miR-669a on muscle structure and functionality of the dystrophic heart is yet unknown.

METHODS AND RESULTS

Here, we demonstrate that intraventricular delivery of adeno-associated viral (AAV) vectors induces long-term (18 months) miR-669a overexpression and improves survival of Sgcb-null mice. Treated hearts display significant decrease in hypertrophic remodeling, fibrosis, and cardiomyocyte apoptosis. Moreover, miR-669a treatment increases sarcomere organization, reduces ventricular atrial natriuretic peptide (ANP) levels, and ameliorates gene/miRNA profile of DCM markers. Furthermore, long-term miR-669a overexpression significantly reduces adverse remodeling and enhances systolic fractional shortening of the left ventricle in treated dystrophic mice, without significant detrimental consequences on skeletal muscle wastage.

CONCLUSIONS

Our findings provide the first evidence of long-term beneficial impact of AAV-mediated miRNA therapy in a transgenic model of severe, chronic MD-associated DCM.

Treatment with the cysteine precursor l-2-oxothiazolidine-4-carboxylate (OTC) implicates taurine deficiency in severity of dystropathology in mdx mice

Oxidative stress has been implicated in the pathology of the lethal skeletal muscle disease Duchenne muscular dystrophy (DMD), and various antioxidants have been investigated as a potential therapy. Recently, treatment of the mdx mouse model for DMD with the antioxidant and cysteine and glutathione (GSH) precursor n-acetylcysteine (NAC) was shown to decrease protein thiol oxidation and improve muscle pathology and ex vivo muscle strength. This study further investigates the mechanism for the benefits of NAC on dystrophic muscle by administering l-2-oxothiazolidine-4-carboxylate (OTC) which also upregulates intracellular cysteine and GSH, but does not directly function as an antioxidant. We observed that OTC, like NAC, decreases protein thiol oxidation, decreases pathology and increases strength, suggesting that the both NAC and OTC function via increasing cysteine and GSH content of dystrophic muscle. We demonstrate that mdx muscle is not deficient in either cysteine or GSH and that these are not increased by OTC treatment. However, we show that dystrophic muscle of 12 week old mdx mice is deficient in taurine, a by-product of disposal of excess cysteine, a deficiency that is ameliorated by OTC treatment. These data suggest that in dystrophic muscles, apart from the strong association of increased oxidative stress and protein thiol oxidation with dystropathology, another major issue is an insufficiency in taurine that can be corrected by increasing the availability of cysteine. This study provides new insight into the molecular mechanism underlying the benefits of NAC in muscular dystrophy and supports the use of OTC as an alternative drug for potential clinical applications to DMD.

Disruption of calcium homeostasis in cardiomyocytes underlies cardiac structural and functional changes in severe sepsis

Sepsis, a major cause of morbidity/mortality in intensive care units worldwide, is commonly associated with cardiac dysfunction, which worsens the prognosis dramatically for patients. Although in recent years the concept of septic cardiomyopathy has evolved, the importance of myocardial structural alterations in sepsis has not been fully explored. This study offers novel and mechanistic data to clarify subcellular events that occur in the pathogenesis of septic cardiomyopathy and myocardial dysfunction in severe sepsis. Cultured neonatal mice cardiomyocytes subjected to serum obtained from mice with severe sepsis presented striking increment of [Ca²⁺]_i and calpain-1 levels associated with decreased expression of dystrophin and disruption and derangement of F-actin filaments and cytoplasmic bleb formation. Severe sepsis induced in mice led to an increased expression of calpain-1 in cardiomyocytes. Moreover, decreased myocardial amounts of dystrophin, sarcomeric actin, and myosin heavy chain were observed in septic hearts associated with depressed cardiac contractile dysfunction and a very low survival rate. Actin and myosin from the sarcomere are first disassembled by calpain and then ubiquitinated and degraded by proteasome or sequestered inside specialized vacuoles called autophagosomes, delivered to the lysosome for degradation forming autophagolysosomes. Verapamil and dantrolene prevented the increase of calpain-1 levels and preserved dystrophin, actin, and myosin loss/reduction as well cardiac contractile dysfunction associated with strikingly improved survival rate. These abnormal parameters emerge as therapeutic targets, which modulation may provide beneficial effects on future vascular outcomes and mortality in sepsis. Further studies are needed to shed light on this mechanism, mainly regarding specific calpain inhibitors.

S151A δ-sarcoglycan mutation causes a mild phenotype of cardiomyopathy in mice

So far, the role of mutations in the δ-sarcogylcan (Sgcd) gene in causing autosomal dominant dilated cardiomyopathy (DCM) remains inconclusive. A p.S151A missense mutation in exon 6 of the Sgcd gene was reported to cause severe isolated autosomal dominant DCM without affecting skeletal muscle. This is controversial to our previous findings in a large consanguineous family where this p.S151A mutation showed no relevance for cardiac disease. In this study, the potential of the p.S151A mutation to cause DCM was investigated by using two different approaches: (1) engineering and characterization of heterozygous knock-in (S151A-) mice carrying the p.S151A mutation and (2) evaluation of the potential of adeno-associated virus (AAV) 9-based cardiac-specific transfer of p.S151A-mutated Sgcd cDNA to rescue the cardiac phenotype in Sgcd-deficient (Sgcd-null) mice as it has been demonstrated for intact, wild-type Sgcd cDNA. Heterozygous S151A knock-in mice developed a rather mild phenotype of cardiomyopathy. Increased heart to body weight suggests cardiac enlargement in 1-year-old S151A knock-in mice. However, at this age cardiac function, assessed by echocardiography, is maintained and histopathology completely absent. Myocardial expression of p.S151A cDNA, similar to intact Sgcd cDNA, restores cardiac function, although not being able to prevent myocardial histopathology in Sgcd-null mice completely. Our results suggest that the p.S151A mutation causes a mild, subclinical phenotype of cardiomyopathy, which is prone to be overseen in patients carrying such sequence variants. Furthermore, this study shows the suitability of an AAV-mediated cardiac gene transfer approach to analyze whether a sequence variant is a disease-causing mutation.

Oxidative stress and pathology in muscular dystrophies: focus on protein thiol oxidation and dysferlinopathies

The muscular dystrophies comprise more than 30 clinical disorders that are characterized by progressive skeletal muscle wasting and degeneration. Although the genetic basis for many of these disorders has been identified, the exact mechanism for pathogenesis generally remains unknown. It is considered that disturbed levels of reactive oxygen species (ROS) contribute to the pathology of many muscular dystrophies. Reactive oxygen species and oxidative stress may cause cellular damage by directly and irreversibly damaging macromolecules such as proteins, membrane lipids and DNA; another major cellular consequence of reactive oxygen species is the reversible modification of protein thiol side chains that may affect many aspects of molecular function. Irreversible oxidative damage of protein and lipids has been widely studied in Duchenne muscular dystrophy, and we have recently identified increased protein thiol oxidation in dystrophic muscles of the mdx mouse model for Duchenne muscular dystrophy. This review evaluates the role of elevated oxidative stress in Duchenne muscular dystrophy and other forms of muscular dystrophies, and presents new data that show significantly increased protein thiol oxidation and high levels of lipofuscin (a measure of cumulative oxidative damage) in dysferlin-deficient muscles of A/J mice at various ages. The significance of this elevated oxidative stress and high levels of reversible thiol oxidation, but minimal myofibre necrosis, is discussed in the context of the disease mechanism for dysferlinopathies, and compared with the situation for dystrophin-deficient mdx mice.

Genetic mutations and mechanisms in dilated cardiomyopathy

Genetic mutations account for a significant percentage of cardiomyopathies, which are a leading cause of congestive heart failure. In hypertrophic cardiomyopathy (HCM), cardiac output is limited by the thickened myocardium through impaired filling and outflow. Mutations in the genes encoding the thick filament components myosin heavy chain and myosin binding protein C (MYH7 and MYBPC3) together explain 75% of inherited HCMs, leading to the observation that HCM is a disease of the sarcomere. Many mutations are "private" or rare variants, often unique to families. In contrast, dilated cardiomyopathy (DCM) is far more genetically heterogeneous, with mutations in genes encoding cytoskeletal, nucleoskeletal, mitochondrial, and calcium-handling proteins. DCM is characterized by enlarged ventricular dimensions and impaired systolic and diastolic function. Private mutations account for most DCMs, with few hotspots or recurring mutations. More than 50 single genes are linked to inherited DCM, including many genes that also link to HCM. Relatively few clinical clues guide the diagnosis of inherited DCM, but emerging evidence supports the use of genetic testing to identify those patients at risk for faster disease progression, congestive heart failure, and arrhythmia.

Respiratory management strategies for Duchenne muscular dystrophy: practice variation amongst Canadian sub-specialists

PURPOSE

Respiratory management of Duchenne muscular dystrophy (DMD) is not well studied and may vary across centers and practitioners. Our objective was to describe and compare the respiratory management practices of Canadian Pediatric Respirologists and Neuromuscular specialists for children with DMD.

METHODS

A web-based survey was sent to all 56 practicing Canadian Pediatric Respirologists and to all 24 members of the Canadian Pediatric Neuromuscular Group (CPNG) who follow children with neuromuscular diseases. The survey included 28 questions about timing and indications for respiratory consultation, sleep disordered breathing (SDB) assessments, and treatments.

RESULTS

Thirty eight (68%) pediatric respirologists and 17 (71%) CPNG members responded. Respirologists provide initial consultation after a patient's first admission to hospital with respiratory complications (14/38, 37%) and when symptoms of SDB are present (14/38, 37%). Half of the CPNG members request initial Respirology consultation at the time of DMD diagnosis. Both groups request routine pulmonary function tests. Ninety-six percent of respirologists use maximal inspiratory (MIP) and expiratory pressures (MEP) to assess respiratory muscle strength, whereas 82% of CPNG members additionally use peak cough flow. Assessment for SDB is requested by both groups when pulmonary function is abnormal or patients are symptomatic. Respirologists favor polysomnography, whereas CPNG members use overnight pulse oximetry. Nocturnal non-invasive ventilation and lung volume recruitment (LVR) are used in a minority of patients.

CONCLUSIONS

Respirologists and CPNG members provide similar respiratory management of DMD patients, but differ in timing of consultation and choice of tests for pulmonary function and SDB. Canadian practices differ from the American Thoracic Society and Centre for Disease Control guidelines.

Altered mitogen-activated protein kinase signaling in dystrophic (mdx) muscle

INTRODUCTION

Duchenne muscular dystrophy (DMD) results from a deficiency in the protein, dystrophin. Dystrophic myotubes are susceptible to stressful stimuli. This may be partly due to altered regulation of pro-survival signaling pathways, but a role for mitogen-activated protein (MAP) kinases has not been investigated.

METHODS

We examined patterns of phosphorylation of key MAP kinase proteins in cultured myotubes responding to oxidative stress, and in muscle tissue in vivo.

RESULTS

Dystrophic (mdx) myotubes have an increased susceptibility to oxidant-induced death compared with wild-type (C57Bl/10ScSn) myotubes. This correlates with late phosphorylation of c-Jun N-terminal kinase (JNK), and persistently high p38 MAP kinase phosphorylation in mdx myotubes. JNK and extracellular signal-regulated kinase 1/2 (ERK1/2) also showed altered phosphorylation levels in mdx muscle tissue.

CONCLUSIONS

We show altered patterns of MAP kinase protein phosphorylation in dystrophic muscle in vitro and in vivo. These pathways may be novel pharmacological targets for treating DMD.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

Expression of sarcoglycans in the human cerebral cortex: an immunohistochemical and molecular study

The sarcoglycan (SG) complex (SGC) is a subcomplex within the dystrophin-glycoprotein complex (DGC) and is composed of several transmembrane proteins (α, β, δ, γ, ε and ζ). The DGC supplies a transmembranous connection between the subsarcolemmal cytoskeleton networks and the basal lamina in order to protect the lipid bilayer and to provide a scaffold for signaling molecules in all muscle cells. In addition to its role in muscle tissue, dystrophin and some DGC components are expressed in neurons and glia. Very little is known about the SG subunits in the central nervous system (CNS) and some data suggested the presence of ε and ζ subunits only. In fact, mutations in the ε-SG gene cause myoclonus-dystonia, indicating its importance for brain function. To determine the presence and localization of SGC in the human cerebral cortex, we performed an investigation using immunofluorescence, immunoblotting and reverse transcriptase polymerase chain reaction. The results showed that all SG subunits are expressed in the human cerebral cortex, particularly in large neurons but also in astrocytes. These data suggest that the SG subcomplex may be involved in the organization of CNS synapses.

Successful regional delivery and long-term expression of a dystrophin gene in canine muscular dystrophy: a preclinical model for human therapies

Duchenne muscular dystrophy (DMD) is a fatal, X-linked muscle disease caused by mutations in the dystrophin gene. Adeno-associated viral (AAV) vector-mediated gene replacement strategies hold promise as a treatment. Studies in animal models and human trials suggested that immune responses to AAV capsid proteins and transgene products prevented efficient gene therapy. In this study, we used widespread intramuscular (i.m.) injection to deliver AAV6-canine micro-dystrophin (c-µdys) throughout a group of skeletal muscles in dystrophic dogs given a brief course of commonly used immunosuppressants. Robust c-µdys expression was obtained for at least two years and was associated with molecular reconstitution of the dystrophin-glycoprotein complex (DGC) at the muscle membrane. Importantly, c-µdys expression was maintained for at least 18 months after discontinuing immunosuppression. The results obtained in a relevant preclinical model of DMD demonstrate feasibility of widespread AAV-mediated muscle transduction and transgene expression in the presence of transient immunosuppression to achieve molecular reconstitution that can be directly translated to human trials.

Dp71 gene disruption alters the composition of the dystrophin-associated protein complex and neuronal nitric oxide synthase expression in the hypothalamic supraoptic and paraventricular nuclei

DP71 is the major cerebral dystrophin isoform and exerts its multiple functions via the dystrophin-associated protein complex (DAPC), also comprised of β-dystroglycan (β-DG) and α1-syntrophin (α1-Syn). Since DP71 disruption leads to impairment in the central control of the osmoregulatory axis, we investigated: 1) the DAPC composition in the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) of Dp71-null mice; and 2) the expression and activity of neuronal nitric oxide synthase (nNOS), because it is a potential partner of the DAPC and a functional index of osmoregulatory axis activity. In wild-type mice, dystrophins and their autosomal homologs the utrophins, β-DG, and α1-Syn were localized in astrocyte end feet. In Dp71-null mice, the levels of β-DG and α1-Syn were lower and utrophin expression did not change. The location of the DAPC in astrocytic end feet suggests that it could be involved in hypothalamic osmosensitivity, which adapts the osmotic response. The altered composition of the DAPC in Dp71-null mice could thus explain why these mice manifest an hypo-osmolar status. In the SON and PVN neurons of Dp71-null mice, nNOS expression and activity were increased. Although we previously established that DP140 is expressed de novo in these neurons, the DAPC remained incomplete due to the low levels of β-DG and α1-Syn produced in these cells. Our data reveal the importance of DP71 for the constitution of a functional DAPC in the hypothalamus. Such DAPC disorganization may lead to modification of the microenvironment of the SON and PVN neurons and thus may result in a perturbed osmoregulation.

Detection of caveolin-3/caveolin-1/P2X7R complexes in mice atrial cardiomyocytes in vivo and in vitro

Caveolae and caveolins, structural components of caveolae, are associated with specific ion channels in cardiac myocytes. We have previously shown that P2X purinoceptor 7 (P2X7R), a ligand-gated ion channel, is increased in atrial cardiomyocytes of caveolin-1 knockout mice; however, the specific biochemical relationship of P2X7R with caveolins in the heart is not clear. The aim of this work was to study the presence of the P2X7R in atrial cardiomyocytes and its biochemical relationship to caveolin-1 and caveolin-3. Caveolin isoforms and P2X7R were predominantly localized in buoyant membrane fractions (lipid rafts/caveolae) prepared from hearts using detergent-free sucrose gradient centrifugation. Caveolin-1 knockout mice showed normal distribution of caveolin-3 and P2X7R to buoyant membranes indicating the importance of caveolin-3 to formation of caveolae. Using clear native-PAGE, we showed that caveolin-1, -3 and P2X7R contribute to the same protein complex in the membranes of murine cardiomyocytes and in the immortal cardiomyocyte cell line HL-1. Western blot analysis revealed increased caveolin-1 and -3 proteins in tissue homogenates of P2X7R knockout mice. Finally, tissue homogenates of atrial tissues from caveolin-3 knockout mice showed elevated mRNA for P2X7R in atria. The colocalization of caveolins with P2X7R in a biochemical complex and compensated upregulation of P2X7R or caveolins in the absence of any component of the complex suggests P2X7R and caveolins may serve an important regulatory control point for disease pathology in the heart.

Interplay between heart and skeletal muscle disease in heart failure: the 2011 George E. Brown Memorial Lecture

The study of single gene disorders often provides insight for more complex human disease. Mutations in the genes encoding the dystrophin protein complex cause muscular dystrophy and cardiomyopathy by destabilizing the plasma membrane of skeletal myofibers and cardiomyocytes. In these diseases, progressive skeletal muscle degeneration and weakness contribute to cardiac dysfunction. Moreover, the pace and pattern of muscle weakness, along with onset of cardiomyopathy, is highly variable even when associated with the same identical mutation. Using a mouse model of muscular dystrophy and cardiomyopathy, we identified genetic loci that modify muscle pathology and cardiac fibrosis. Distinct genetic modifiers were identified for diaphragm and abdominal musculature, and these genetic intervals differ from those that regulate pathology in the skeletal muscle of the limbs and the heart. One modifier gene was identified and highlights the importance of the transforming growth factor-β pathway in the pathogenesis of muscular dystrophy and cardiomyopathy. We determined that canonical transforming growth factor-β signaling contributes to heart and muscle dysfunction using a Drosophila model. Together, these studies demonstrate the value of using a genetically sensitized model to uncover pathways that regulate heart failure and muscle weakness.

Repression of miR-142 by p300 and MAPK is required for survival signalling via gp130 during adaptive hypertrophy

An increase in cardiac workload, ultimately resulting in hypertrophy, generates oxidative stress and therefore requires the activation of both survival and growth signal pathways. Here, we wanted to characterize the regulators, targets and mechanistic roles of miR-142, a microRNA (miRNA) negatively regulated during hypertrophy. We show that both miRNA-142-3p and -5p are repressed by serum-derived growth factors in cultured cardiac myocytes, in models of cardiac hypertrophy in vivo and in human cardiomyopathic hearts. Levels of miR-142 are inversely related to levels of acetyltransferase p300 and MAPK activity. When present, miR-142 inhibits both survival and growth pathways by directly targeting nodal regulators p300 and gp130. MiR-142 also potently represses multiple components of the NF-κB pathway, preventing cytokine-mediated NO production and blocks translation of α-actinin. Forced expression of miR-142 during hypertrophic growth induced extensive apoptosis and cardiac dysfunction; conversely, loss of miR-142 fully rescued cardiac function in a murine heart failure model. Downregulation of miR-142 is required to enable cytokine-mediated survival signalling during cardiac growth in response to haemodynamic stress and is a critical element of adaptive hypertrophy.

Talin, vinculin and nestin expression in orofacial muscles of dystrophin deficient mdx mice

The activity of cytoskeletal proteins like talin, vinculin and nestin increases in muscle that regenerates. Little is known about their role or at least their expression in the process of regeneration in masticatory muscles of mdx mice, a model of Duchenne muscular dystrophy. To determine a potential role of cytoskeletal proteins in the regeneration process of mdx masticatory muscles, we examined the expression of talin 1, talin 2, vinculin and nestin in 100-day-old control and mdx mice using quantitative RT-PCR, Western blot analyses and histochemistry. The protein expression of talin 1, talin 2, nestin and vinculin in mdx muscles remained unchanged as compared with normal mice. However, in mdx masseter it was found a relative increase of nestin compared to controls. The protein expression of talin 1 and vinculin tended to be increased in mdx tongue and talin 2 to diminish in mdx masseter and temporal muscle. In mdx mice, we found significantly lower percentage of transcripts coding for nestin, talin 1, talin 2 and vinculin in masseter (p < 0.05) and temporal muscle (p < 0.001). In contrast, the mRNA expression of nestin was found to be increased in mdx tongue. Activated satellite cells, myoblasts and immature regenerated muscle fibres in mdx masseter and temporal revealed positive staining for nestin. The findings of the presented work suggest dystrophin-lack-associated changes in the expression of cytoskeletal proteins in mdx masticatory muscles could be compensatory for dystrophin absence. The expression of nestin may serve as an indicator for the regeneration in the orofacial muscles.

The atypical calpains: evolutionary analyses and roles in Caenorhabditis elegans cellular degeneration

The calpains are physiologically important Ca²⁺-activated regulatory proteases, which are divided into typical or atypical sub-families based on constituent domains. Both sub-families are present in mammals, but our understanding of calpain function is based primarily on typical sub-family members. Here, we take advantage of the model organism Caenorhabditis elegans, which expresses only atypical calpains, to extend our knowledge of the phylogenetic evolution and function of calpains. We provide evidence that a typical human calpain protein with a penta EF hand, detected using custom profile hidden Markov models, is conserved in ancient metazoans and a divergent clade. These analyses also provide evidence for the lineage-specific loss of typical calpain genes in C. elegans and Ciona, and they reveal that many calpain-like genes lack an intact catalytic triad. Given the association between the dysregulation of typical calpains and human degenerative pathologies, we explored the phenotypes, expression profiles, and consequences of inappropriate reduction or activation of C. elegans atypical calpains. These studies show that the atypical calpain gene, clp-1, contributes to muscle degeneration and reveal that clp-1 activity is sensitive to genetic manipulation of [Ca²⁺]_i. We show that CLP-1 localizes to sarcomeric sub-structures, but is excluded from dense bodies (Z-disks). We find that the muscle degeneration observed in a C. elegans model of dystrophin-based muscular dystrophy can be suppressed by clp-1 inactivation and that nemadipine-A inhibition of the EGL-19 calcium channel reveals that Ca²⁺ dysfunction underlies the C. elegans MyoD model of myopathy. Taken together, our analyses highlight the roles of calcium dysregulation and CLP-1 in muscle myopathies and suggest that the atypical calpains could retain conserved roles in myofilament turnover.

Calpains are calcium activated non-lysosomal proteases that cleave proteins with exquisite selectivity. Proteins can be activated by calpain cleavage, because they are released from inhibitory constraints, or they can be targeted for further degradation to facilitate their normal physiological turnover or to promote cellular remodelling. Inappropriate calpain activity can lead to degenerative pathologies and cancers. Our understanding of calpain function is based primarily on typical calpains, which carry EF hand motifs that bind Ca²⁺ or mediate dimerization; however, typical and atypical calpains, which lack EF hand motifs, are both present in mammals. Hence, any therapeutic intervention designed to suppress degenerative conditions, particularly those caused by elevated Ca²⁺ levels, should also consider the potential involvement of atypical calpains. We have taken advantage of the model organism C. elegans, which only encodes atypical calpain proteins, to gain an understanding of the evolution and activities of these proteins. We show that the CLP-1 atypical calpain is normally expressed in muscle and localizes to sarcomeric sub-structures. We find that CLP-1 contributes to the muscle degeneration observed in a model of Duchenne muscular dystrophy. Our studies also highlight the importance of calcium dysregulation in promoting CLP-1 activity and muscle degeneration.

Ephrin-B1 Is a Novel Specific Component of the Lateral Membrane of the Cardiomyocyte and Is Essential for the Stability of Cardiac Tissue Architecture Cohesion

Rationale:

Cardiac tissue cohesion relying on highly ordered cardiomyocytes (CM) interactions is critical because most cardiomyopathies are associated with tissue remodeling and architecture alterations.

Objective:

Eph/ephrin system constitutes a ubiquitous system coordinating cellular communications which recently emerged as a major regulator in adult organs. We examined if eph/ephrin could participate in cardiac tissue cyto-organization.

Methods and Results:

We reported the expression of cardiac ephrin-B1 in both endothelial cells and for the first time in CMs where ephrin-B1 localized specifically at the lateral membrane. Ephrin-B1 knock-out (KO) mice progressively developed cardiac tissue disorganization with loss of adult CM rod-shape and sarcomeric and intercalated disk structural disorganization confirmed in CM-specific ephrin-B1 KO mice. CMs lateral membrane exhibited abnormal structure by electron microscopy and notably increased stiffness by atomic force microscopy. In wild-type CMs, ephrin-B1 interacted with claudin-5/ZO-1 complex at the lateral membrane, whereas the complex disappeared in KO/CM-specific ephrin-B1 KO mice. Ephrin-B1 deficiency resulted in decreased mRNA expression of CM basement membrane components and disorganized fibrillar collagen matrix, independently of classical integrin/dystroglycan system. KO/CM-specific ephrin-B1 KO mice exhibited increased left ventricle diameter and delayed atrioventricular conduction. Under pressure overload stress, KO mice were prone to death and exhibited striking tissue disorganization. Finally, failing CMs displayed downregulated ephrin-B1/claudin-5 gene expression linearly related to the ejection fraction.

Conclusions:

Ephrin-B1 is necessary for cardiac tissue architecture cohesion by stabilizing the adult CM morphology through regulation of its lateral membrane. Because decreased ephrin-B1 is associated with molecular/functional cardiac defects, it could represent a new actor in the transition toward heart failure.

Neural maintenance roles for the matrix receptor dystroglycan and the nuclear anchorage complex in Caenorhabditis elegans

Recent studies in Caenorhabditis elegans have revealed specific neural maintenance mechanisms that protect soma and neurites against mispositioning due to displacement stresses, such as muscle contraction. We report that C. elegans dystroglycan (DG) DGN-1 functions to maintain the position of lumbar neurons during late embryonic and larval development. In the absence of DGN-1 the cell bodies of multiple lumbar neuron classes are frequently displaced anterior of their normal positions. Early but not later embryonic panneural expression of DGN-1 rescues positional maintenance, suggesting that dystroglycan is required for establishment of a critical maintenance pathway that persists throughout later developmental stages. Lumbar neural maintenance requires only a membrane-tethered N-terminal domain of DGN-1 and may involve a novel extracellular partner for dystroglycan. A genetic screen for similar lumbar maintenance mutants revealed a role for the nesprin/SYNE family protein ANC-1 as well as for the extracellular protein DIG-1, previously implicated in lumbar neuron maintenance. The involvement of ANC-1 reveals a previously unknown role for nucleus-cytoskeleton interactions in neural maintenance. Genetic analysis indicates that lumbar neuron position is maintained in late embryos by parallel DGN-1/DIG-1 and ANC-1-dependent pathways, and in larvae by separate DGN-1 and ANC-1 pathways. The effect of muscle paralysis on late embryonic- or larval-stage maintenance defects in mutants indicates that lumbar neurons are subject to both muscle contraction-dependent and contraction-independent displacement stresses, and that different maintenance pathways may protect against specific types of displacement stress.

Pathophysiology and therapy of cardiac dysfunction in Duchenne muscular dystrophy

Cardiac dysfunction is a frequent manifestation of Duchenne muscular dystrophy (DMD) and a common cause of death for individuals with this condition. Early diastolic dysfunction and focal fibrosis proceed to dilated cardiomyopathy (DCM), complicated by heart failure and arrhythmia in most patients. Improvements in the management of respiratory insufficiency in DMD have improved lifespan and overall prognosis, but heart failure and sudden death continue to impact survival and quality of life for people with DMD. Since the specific mechanisms resulting in heart failure for people with DMD are poorly understood, current treatments are not targeted, but rely on approaches that are considered standard for DCM. These approaches include angiotensin-converting enzyme (ACE) inhibitors and β-adrenoceptor antagonists. Data from one trial in DMD support the use of ACE inhibitors before the onset of left ventricular dysfunction. Angiotensin receptor blockers have shown similar efficacy to ACE inhibitors in numerous studies of dilated cardiomyopathy, and are a good choice for patients who cannot tolerate ACE inhibition. The pathogenesis of DMD-associated cardiomyopathy may be similar to other genetic disorders of the cytoskeletal complex of ventricular myocytes, though unique features offer targeted opportunities to impact treatment. Novel areas of investigation are focused on the regulatory role of dystrophin in relation to neuronal nitric oxide synthase (nNOS) and transient receptor potential canonical channels (TRPC). Inhibition of phosphodiesterase-5 (PDE5) addresses several aspects of regulatory dysfunction induced by dystrophin deficiency, and studies with PDE5-inhibitors have shown benefits in murine models of DMD. PDE5-inhibitors are currently under investigation in at least one study in humans. This article focuses on mechanisms of cardiac dysfunction, as well as potential targets for pharmacologic manipulation to prevent or improve cardiomyopathy in DMD.

In vivo MRI characterization of progressive cardiac dysfunction in the mdx mouse model of muscular dystrophy

AIMS

The mdx mouse has proven to be useful in understanding the cardiomyopathy that frequently occurs in muscular dystrophy patients. Here we employed a comprehensive array of clinically relevant in vivo MRI techniques to identify early markers of cardiac dysfunction and follow disease progression in the hearts of mdx mice.

METHODS AND RESULTS

Serial measurements of cardiac morphology and function were made in the same group of mdx mice and controls (housed in a non-SPF facility) using MRI at 1, 3, 6, 9 and 12 months after birth. Left ventricular (LV) and right ventricular (RV) systolic and diastolic function, response to dobutamine stress and myocardial fibrosis were assessed. RV dysfunction preceded LV dysfunction, with RV end systolic volumes increased and RV ejection fractions reduced at 3 months of age. LV ejection fractions were reduced at 12 months, compared with controls. An abnormal response to dobutamine stress was identified in the RV of mdx mice as early as 1 month. Late-gadolinium-enhanced MRI identified increased levels of myocardial fibrosis in 6, 9 and 12-month-old mdx mice, the extent of fibrosis correlating with the degree of cardiac remodeling and hypertrophy.

CONCLUSIONS

MRI could identify cardiac abnormalities in the RV of mdx mice as young as 1 month, and detected myocardial fibrosis at 6 months. We believe these to be the earliest MRI measurements of cardiac function reported for any mice, and the first use of late-gadolinium-enhancement in a mouse model of congenital cardiomyopathy. These techniques offer a sensitive and clinically relevant in vivo method for assessment of cardiomyopathy caused by muscular dystrophy and other diseases.

N-Acetylcysteine treatment of dystrophic mdx mice results in protein thiol modifications and inhibition of exercise induced myofibre necrosis

Oxidative stress is implicated as a factor that increases necrosis of skeletal muscles in Duchenne Muscular Dystrophy (DMD) and the dystrophic mdx mouse. Consequently, drugs that minimize oxidative stress are potential treatments for muscular dystrophy. This study examined the in vivo benefits to mdx mice of an antioxidant treatment with the cysteine precursor N-acetylcysteine (NAC), administered in drinking water. NAC was completely effective in preventing treadmill exercise-induced myofibre necrosis (assessed histologically) and the increased blood creatine kinase levels (a measure of sarcolemma leakiness) following exercise were significantly lower in the NAC treated mice. While NAC had no effect on malondialdehyde level or protein carbonylation (two indicators of irreversible oxidative damage), treatment with NAC for one week significantly decreased the oxidation of glutathione and protein thiols, and enhanced muscle protein thiol content. These data provide in vivo evidence for protective benefits of NAC treatment on dystropathology, potentially via protein thiol modifications.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Alpha sarcoglycan is required for FGF-dependent myogenic progenitor cell proliferation in vitro and in vivo

Mice deficient in α-sarcoglycan (Sgca-null mice) develop progressive muscular dystrophy and serve as a model for human limb girdle muscular dystrophy type 2D. Sgca-null mice suffer a more severe myopathy than that of mdx mice, the model for Duchenne muscular dystrophy. This is the opposite of what is observed in humans and the reason for this is unknown. In an attempt to understand the cellular basis of this severe muscular dystrophy, we isolated clonal populations of myogenic progenitor cells (MPCs), the resident postnatal muscle progenitors of dystrophic and wild-type mice. MPCs from Sgca-null mice generated much smaller clones than MPCs from wild-type or mdx dystrophic mice. Impaired proliferation of Sgca-null myogenic precursors was confirmed by single fiber analysis and this difference correlated with Sgca expression during MPC proliferation. In the absence of dystrophin and associated proteins, which are only expressed after differentiation, SGCA complexes with and stabilizes FGFR1. Deficiency of Sgca leads to an absence of FGFR1 expression at the membrane and impaired MPC proliferation in response to bFGF. The low proliferation rate of Sgca-null MPCs was rescued by transduction with Sgca-expressing lentiviral vectors. When transplanted into dystrophic muscle, Sgca-null MPCs exhibited reduced engraftment. The reduced proliferative ability of Sgca-null MPCs explains, at least in part, the severity of this muscular dystrophy and also why wild-type donor progenitor cells engraft efficiently and consequently ameliorate disease.

Vinculin and talin: focus on the myocardium

Cardiomyopathy is a heart muscle disease caused by decreased contractility of the ventricles leading to heart failure and premature death. Multiple conditions like ischemic heart disease (atherosclerosis), hypertension, diabetes, viral infection, alcohol abuse, obesity and genetic mutations can lead to cardiomyopathy. Single gene mutations in sarcomeric proteins, Z-disk-associated proteins, membrane/associated proteins, intermediate filaments, calcium cycle proteins as well as in modifier genes have been linked to cardiomyopathy. Clinical practice guidelines have been formulated by the American Heart Association and the Heart Failure Association of America on how to genetically evaluate patients with cardiomyopathy. To illustrate the concept that alterations in genes cause cardiovascular disease, this review will focus on two membrane-associated proteins, vinculin and talin. We will discuss the general function of vinculin/metavinulin as well as talin1 and talin2, with emphasis on what is understood about their role in the cardiac myocyte and in whole heart.

Osteopontin-stimulated expression of matrix metalloproteinase-9 causes cardiomyopathy in the mdx model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, is a common and lethal form of muscular dystrophy. With progressive disease, most patients succumb to death from respiratory or heart failure, or both. However, the mechanisms, especially those governing cardiac inflammation and fibrosis in DMD, remain less understood. Matrix metalloproteinase (MMPs) are a group of extracellular matrix proteases involved in tissue remodeling in both physiologic and pathophysiologic conditions. Previous studies have shown that MMP-9 exacerbates myopathy in dystrophin-deficient mdx mice. However, the role and the mechanisms of action of MMP-9 in cardiac tissue and the biochemical mechanisms leading to increased levels of MMP-9 in mdx mice remain unknown. Our results demonstrate that the levels of MMP-9 are increased in the heart of mdx mice. Genetic ablation of MMP-9 attenuated cardiac injury, left ventricle dilation, and fibrosis in 1-y-old mdx mice. Echocardiography measurements showed improved heart function in Mmp9-deficient mdx mice. Deletion of the Mmp9 gene diminished the activation of ERK1/2 and Akt kinase in the heart of mdx mice. Ablation of MMP-9 also suppressed the expression of MMP-3 and MMP-12 in the heart of mdx mice. Finally, our experiments have revealed that osteopontin, an important immunomodulator, contributes to the increased amounts of MMP-9 in cardiac and skeletal muscle of mdx mice. This study provides a novel mechanism for development of cardiac dysfunction and suggests that MMP-9 and OPN are important therapeutic targets to mitigating cardiac abnormalities in patients with DMD.

Age-matched comparison reveals early electrocardiography and echocardiography changes in dystrophin-deficient dogs

The absence of dystrophin in the heart leads to Duchenne cardiomyopathy. Dystrophin-deficient dogs represent a critical model to translate novel therapies developed in mice to humans. Unfortunately, little is known about cardiophysiology changes in these dogs. We performed prospective electrocardiographic and echocardiographic examinations at 3, 6 and 12 months of age in four normal and three affected dogs obtained from the same litter. Affected dogs showed growth retardation and serum creatine kinase elevation. Necropsy confirmed cardiac dystrophin deficiency and histopathology. Q/R ratio elevation and diastolic left ventricular (LV) internal diameter reduction were the most consistent findings in affected dogs at all ages. At 6 and 12 months, dystrophic dogs also showed significant reduction of PR intervals, LV end diastolic/systolic volumes and systolic LV internal diameters. Epicardial and endocardial slope times were significantly reduced in affected dogs at 12 months. These results establish the baseline for evaluating experimental therapies in the future.

Cardiac assessment in duchenne and becker muscular dystrophies

Mutations in the dystrophin gene cause Duchenne and Becker muscular dystrophies. In addition to muscle disease, there nearly always is an associated cardiomyopathy in Duchenne or Becker muscular dystrophy. In these muscular dystrophies, the severity of cardiomyopathy and congestive heart failure may not parallel the severity of skeletal muscle disease. Loss of normal dystrophin function in the heart produces four-chamber dilation and reduction in left ventricular function that develop after the onset of muscle weakness. Arrhythmias affecting both atrial and ventricular rhythms occur and may be life threatening. The degree to which hypoventilation and pulmonary dysfunction are present also directly affect cardiac function in muscular dystrophy. Care guidelines recently were issued to outline surveillance and treatment strategies for the younger patient with Duchenne muscular dystrophy. Herein, we review those guidelines, and additionally, provide recommendations for monitoring and treating cardiac disease in the populations of advanced Duchenne and Becker muscular dystrophies.

Mechanotransduction: the role of mechanical stress, myocyte shape, and cytoskeletal architecture on cardiac function

Mechanotransduction refers to the conversion of mechanical forces into biochemical or electrical signals that initiate structural and functional remodeling in cells and tissues. The heart is a kinetic organ whose form changes considerably during development and disease, requiring cardiac myocytes to be mechanically durable and capable of fusing a variety of environmental signals on different time scales. During physiological growth, myocytes adaptively remodel to mechanical loads. Pathological stimuli can induce maladaptive remodeling. In both of these conditions, the cytoskeleton plays a pivotal role in both sensing mechanical stress and mediating structural remodeling and functional responses within the myocyte.

Effective myotube formation in human adipose tissue-derived stem cells expressing dystrophin and myosin heavy chain by cellular fusion with mouse C2C12 myoblasts

Stem cell therapy for muscular dystrophies requires stem cells that are able to participate in the formation of new muscle fibers. However, the differentiation steps that are the most critical for this process are not clear. We investigated the myogenic phases of human adipose tissue-derived stem cells (hASCs) step by step and the capability of myotube formation according to the differentiation phase by cellular fusion with mouse myoblast C2C12 cells. In hASCs treated with 5-azacytidine and fibroblast growth factor-2 (FGF-2) for 1 day, the early differentiation step to express MyoD and myogenin was induced by FGF-2 treatment for 6 days. Dystrophin and myosin heavy chain (MyHC) expression was induced by hASC conditioned medium in the late differentiation step. Myotubes were observed only in hASCs undergoing the late differentiation step by cellular fusion with C2C12 cells. In contrast, hASCs that were normal or in the early stage were not involved in myotube formation. Our results indicate that stem cells expressing dystrophin and MyHC are more suitable for myotube formation by co-culture with myoblasts than normal or early differentiated stem cells expressing MyoD and myogenin.

Cardiomyopathy in α-Kinase 3 (ALPK3)–Deficient Mice

Cardiomyopathy developed in mice deficient for α-kinase 3 (ALPK3), a nuclear kinase previously implicated in the differentiation of cardiomyocytes. Alpk3–/– mice were produced according to normal Mendelian ratios and appeared normal except for a nonprogressive cardiomyopathy that had features of both hypertrophic and dilated forms of cardiomyopathy. Cardiac hypertrophy in Alpk3–/– mice was characterized by increased thickness of both left and right ventricular (LV and RV) walls and by markedly increased heart weight and increased heart weight/body weight and heart weight/tibia length ratios. Magnetic resonance imaging studies confirmed the increased thickness in both septal and LV free walls at end-diastole, although there was no significant change in LV wall thickness at end-systole. Myocardial hypertrophy was the predominant feature in Alpk3–/– mice, but several changes more typically associated with dilated cardiomyopathy included a marked increase in end-diastolic and end-systolic LV volume, as well as reduced cardiac output, stroke volume, and ejection fractions, suggesting LV chamber dilation. Magnetic resonance imaging showed a 50% reduction in both septal and free wall LV contractility in Alpk3–/– mice. Interstitial fibrosis and inflammation were notably absent in Alpk3–/– mice; however, light and electron microscopy revealed altered cardiomyocyte architecture, characterized by reduced numbers of abnormal intercalated discs being associated with mild disarray of myofibrils. These lesions could account for the impaired contractility of the myofibrillar apparatus and contribute to the pathogenesis of cardiomyopathy in Alpk3–/– mice.

The role of α-dystrobrevin in striated muscle

Muscular dystrophies are a group of diseases that primarily affect striated muscle and are characterized by the progressive loss of muscle strength and integrity. Major forms of muscular dystrophies are caused by the abnormalities of the dystrophin glycoprotein complex (DGC) that plays crucial roles as a structural unit and scaffolds for signaling molecules at the sarcolemma. α-Dystrobrevin is a component of the DGC and directly associates with dystrophin. α-Dystrobrevin also binds to intermediate filaments as well as syntrophin, a modular adaptor protein thought to be involved in signaling. Although no muscular dystrophy has been associated within mutations of the α-dystrobrevin gene, emerging findings suggest potential significance of α-dystrobrevin in striated muscle. This review addresses the functional role of α-dystrobrevin in muscle as well as its possible implication for muscular dystrophy.

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.

Lateral transmission of force is impaired in skeletal muscles of dystrophic mice and very old rats

The dystrophin–glycoprotein complex (DGC) provides an essential link from the muscle fibre cytoskeleton to the extracellular matrix. In dystrophic humans and mdx mice, mutations in the dystrophin gene disrupt the structure of the DGC causing severe damage to muscle fibres. In frog muscles, transmission of force laterally from an activated fibre to the muscle surface occurs without attenuation, but lateral transmission of force has not been demonstrated in mammalian muscles. A unique ‘yoke' apparatus was developed that attached to the epimysium of muscles midway between the tendons and enabled the measurement of lateral force. We now report that in muscles of young wild-type (WT) mice and rats, compared over a wide range of longitudinal forces, forces transmitted laterally showed little or no decrement. In contrast, for muscles of mdx mice and very old rats, forces transmitted laterally were impaired severely. Muscles of both mdx mice and very old rats showed major reductions in the expression of dystrophin. We conclude that during contractions, forces developed by skeletal muscles of young WT mice and rats are transmitted laterally from fibre to fibre through the DGC without decrement. In contrast, in muscles of dystrophic or very old animals, disruptions in DGC structure and function impair lateral transmission of force causing instability and increased susceptibility of fibres to contraction-induced injury.

Physiology, structure, and susceptibility to injury of skeletal muscle in mice lacking keratin 19-based and desmin-based intermediate filaments

Intermediate filaments, composed of desmin and of keratins, play important roles in linking contractile elements to each other and to the sarcolemma in striated muscle. Our previous results show that the tibialis anterior (TA) muscles of mice lacking keratin 19 (K19) lose costameres, accumulate mitochondria under the sarcolemma, and generate lower specific tension than controls. Here we compare the physiology and morphology of TA muscles of mice lacking K19 with muscles lacking desmin or both proteins [double knockout (DKO)]. K19-/- mice and DKO mice showed a threefold increase in the levels of creatine kinase (CK) in the serum. The absence of desmin caused a larger change in specific tension (-40%) than the absence of K19 (-19%) and played the predominant role in contractile function (-40%) and decreased tolerance to exercise in the DKO muscle. By contrast, the absence of both proteins was required to obtain a significantly greater loss of contractile torque after injury (-48%) compared with wild type (-39%), as well as near-complete disruption of costameres. The DKO muscle also showed a significantly greater misalignment of myofibrils than either mutant alone. In contrast, large subsarcolemmal gaps and extensive accumulation of mitochondria were only seen in K19-null TA muscles, and the absence of both K19 and desmin yielded milder phenotypes. Our results suggest that keratin filaments containing K19- and desmin-based intermediate filaments can play independent, complementary, or antagonistic roles in the physiology and morphology of fast-twitch skeletal muscle.

Congenital muscular dystrophies

Congenital muscular dystrophies (CMDs) are a heterogeneous group of disorders characterized by muscle weakness from birth, or shortly after, and variable clinical manifestations of the eye and central nervous system. Some of these disorders are fatal in the first years of life, whereas others have a milder course, with survival into adulthood. The CMDs were initially classified by clinical features and country of origin; however, with new molecular techniques it is now possible to classify these patients better. More than 10 genes have been identified to date that cause forms of CMD. However, even with current molecular diagnostic techniques, only approximately 25-50% of patients with CMD have an identifiable genetic mutation. In addition, some phenotypic classifications have been attempted. There is significant overlap between the phenotypic and molecular classifications, making diagnosis within this heterogeneous group of disorders difficult.

SMAD signaling drives heart and muscle dysfunction in a Drosophila model of muscular dystrophy

Loss-of-function mutations in the genes encoding dystrophin and the associated membrane proteins, the sarcoglycans, produce muscular dystrophy and cardiomyopathy. The dystrophin complex provides stability to the plasma membrane of striated muscle during muscle contraction. Increased SMAD signaling due to activation of the transforming growth factor-β (TGFβ) pathway has been described in muscular dystrophy; however, it is not known whether this canonical TGFβ signaling is pathogenic in the muscle itself. Drosophila deleted for the γ/δ-sarcoglycan gene (Sgcd) develop progressive muscle and heart dysfunction and serve as a model for the human disorder. We used dad-lacZ flies to demonstrate the signature of TGFβ activation in response to exercise-induced injury in Sgcd null flies, finding that those muscle nuclei immediately adjacent to muscle injury demonstrate high-level TGFβ signaling. To determine the pathogenic nature of this signaling, we found that partial reduction of the co-SMAD Medea, homologous to SMAD4, or the r-SMAD, Smox, corrected both heart and muscle dysfunction in Sgcd mutants. Reduction in the r-SMAD, MAD, restored muscle function but interestingly not heart function in Sgcd mutants, consistent with a role for activin but not bone morphogenic protein signaling in cardiac dysfunction. Mammalian sarcoglycan null muscle was also found to exhibit exercise-induced SMAD signaling. These data demonstrate that hyperactivation of SMAD signaling occurs in response to repetitive injury in muscle and heart. Reduction of this pathway is sufficient to restore cardiac and muscle function and is therefore a target for therapeutic reduction.

Distinct genetic regions modify specific muscle groups in muscular dystrophy

Phenotypic expression in the muscular dystrophies is variable, even with the identical mutation, providing strong evidence that genetic modifiers influence outcome. To identify genetic modifier loci, we used quantitative trait locus mapping in two differentially affected mouse strains with muscular dystrophy. Using the Sgcg model of limb girdle muscular dystrophy that lacks the dystrophin-associated protein γ-sarcoglycan, we evaluated chromosomal regions that segregated with two distinct quantifiable characteristics of muscular dystrophy, membrane permeability and fibrosis. We previously identified a single major locus on murine chromosome 7 that influences both traits of membrane permeability and fibrosis in the quadriceps muscle. Using a larger cohort, we now found that this same interval strongly associated with both traits in all limb skeletal muscle groups studied, including the gastrocnemius/soleus, gluteus/hamstring, and triceps muscles. In contrast, the muscles of the trunk were modified by distinct genetic loci, possibly reflecting the embryological origins and physiological stressors unique to these muscle groups. A locus on chromosome 18 was identified that modified membrane permeability of the abdominal muscles, and a locus on chromosome 3 was found that regulated diaphragm and abdominal muscle fibrosis. Fibrosis in the heart associated with a region on chromosome 9 and likely reflects differential function between cardiac and skeletal muscle. These data underscore the complexity of inheritance and penetrance of single-gene disorders.