Impaired recovery of dysferlin-null skeletal muscle after contraction-induced injury in vivo

Mechanotransduction and Skeletal Muscle Atrophy: The Interplay Between Focal Adhesions and Oxidative Stress

Mechanical unloading leads to profound musculoskeletal degeneration, muscle wasting, and weakness. Understanding the specific signaling pathways involved is essential for uncovering effective interventions. This review provides new perspectives on mechanotransduction pathways, focusing on the critical roles of focal adhesions (FAs) and oxidative stress in skeletal muscle atrophy under mechanical unloading. As pivotal mechanosensors, FAs integrate mechanical and biochemical signals to sustain muscle structural integrity. When disrupted, these complexes impair force transmission, activating proteolytic pathways (e.g., ubiquitin-proteasome system) that accelerate atrophy. Oxidative stress, driven by mitochondrial dysfunction and NADPH oxidase-2 (NOX2) hyperactivation, exacerbates muscle degeneration through excessive reactive oxygen species (ROS) production, impaired repair mechanisms, and dysregulated redox signaling. The interplay between FA dysfunction and oxidative stress underscores the complexity of muscle atrophy pathogenesis: FA destabilization heightens oxidative damage, while ROS overproduction further disrupts FA integrity, creating a self-amplifying vicious cycle. Therapeutic strategies, such as NOX2 inhibitors, mitochondrial-targeted antioxidants, and FAK-activating compounds, promise to mitigate muscle atrophy by preserving mechanotransduction signaling and restoring redox balance. By elucidating these pathways, this review advances the understanding of muscle degeneration during unloading and identifies promising synergistic therapeutic targets, emphasizing the need for combinatorial approaches to disrupt the FA-ROS feedback loop.

Fish Oil Supplement Mitigates Muscle Injury In Vivo and In Vitro: A Preliminary Report

Background: Following injury, older adults exhibit slow recovery of muscle function. Age-related impairment of sarcolemmal membrane repair may contribute to myocyte death, increasing the need for myogenesis and prolonging recovery. Dietary fish oil (FO) is a common nutritional supplement that may alter plasma membrane composition to enhance the response to membrane injury. Methods: We assessed effects of an 8-week dietary intervention on muscle contractile recovery in aged (22 mo.) rats on control (n = 5) or FO (control + 33 g/kg FO (45% eicosapentaenoic acid; 10% docosahexaenoic acid); n = 5) diets 1-week after contusion injury, as well as adult (8 mo., n = 8) rats on the control diet. Results: Recovery was reduced in aged rats on the control diet vs. adults (63 vs. 80%; p = 0.042), while those on the FO diet recovered similarly to (78%) adults. To directly assess sarcolemma injury, C2C12 cells were cultured in media with and without FO (1, 10, and 100 μg/mL; 24 or 48 h) and injured with an infrared laser in medium containing FM4-64 dye as a marker of sarcolemmal injury. FO reduced the area under the FM4-64 fluorescence-time curve at all concentrations after both 24 and 48 h supplementation. Conclusions: These preliminary data suggest FO might aid recovery of muscle function following injury in older adults by enhancing membrane resealing and repair.

Bioengineered Model of Human LGMD2B Skeletal Muscle Reveals Roles of Intracellular Calcium Overload in Contractile and Metabolic Dysfunction in Dysferlinopathy

Dysferlin is a multi-functional protein that regulates membrane resealing, calcium homeostasis, and lipid metabolism in skeletal muscle. Genetic loss of dysferlin results in limb girdle muscular dystrophy 2B/2R (LGMD2B/2R) and other dysferlinopathies - rare untreatable muscle diseases that lead to permanent loss of ambulation in humans. The mild disease severity in dysferlin-deficient mice and diverse genotype-phenotype relationships in LGMD2B patients have prompted the development of new in vitro models for personalized studies of dysferlinopathy. Here the first 3-D tissue-engineered hiPSC-derived skeletal muscle ("myobundle") model of LGMD2B is described that exhibits compromised contractile function, calcium-handling, and membrane repair, and transcriptomic changes indicative of impaired oxidative metabolism and mitochondrial dysfunction. In response to the fatty acid (FA) challenge, LGMD2B myobundles display mitochondrial deficits and intracellular lipid droplet (LD) accumulation. Treatment with the ryanodine receptor (RyR) inhibitor dantrolene or the dissociative glucocorticoid vamorolone restores LGMD2B contractility, improves membrane repair, and reduces LD accumulation. Lastly, it is demonstrated that chemically induced chronic RyR leak in healthy myobundles phenocopies LGMD2B contractile and metabolic deficit, but not the loss of membrane repair capacity. Together, these results implicate intramyocellular Ca2+ leak as a critical driver of dysferlinopathic phenotype and validate the myobundle system as a platform to study LGMD2B pathogenesis.

Novel five nucleotide deletion in dysferlin leads to autosomal recessive limb-girdle muscular dystrophy

Muscular dystrophy (MD) is a genetic disorder that causes progressive muscle weakness and degeneration. Limb-girdle muscular dystrophy (LGMD) is a type of MD that mainly causes muscle atrophy within the shoulder and pelvic girdles. LGMD is classified into autosomal dominant (LGMD-D) and autosomal recessive (LGMD-R) inheritance patterns. Mutations in the Dysferlin gene (DYSF) are common causes of LGMD-R. However, genetic screening of DYSF mutations is rare in Taiwan. Herein, we identified a novel c.2867_2871del ACCAG deletion and a previously reported c.937+1G>A mutation in DYSF from a Taiwanese family with LGMD. The primary symptoms of both siblings were difficulty climbing stairs, walking on the toes, and gradually worsening weakness in the proximal muscles and increased creatine kinase level. Through pedigree analysis and sequencing, two siblings from this family were found to have compound heterozygous DYSF mutations (c. 937+1G>A and c. 2867_2871del ACCAG) within the separated alleles. These mutations induced early stop codons; if translated, truncated DYSF proteins will be expressed. Or, the mRNA products of these two mutations will merit the nonsense-mediated decay, might result in no dysferlin protein expressed. To our knowledge, this is the first report of a novel c.2867_2871del ACCAG deletion in DYSF. Further research is required to examine the effects of the novel DYSF mutation in Taiwanese patients with LGMD.

Tetraspanin CD82 Associates with Trafficking Vesicle in Muscle Cells and Binds to Dysferlin and Myoferlin

Tetraspanins organize protein complexes at the cell membrane and are responsible for assembling diverse binding partners in changing cellular states. Tetraspanin CD82 is a useful cell surface marker for prospective isolation of human myogenic progenitors and its expression is decreased in Duchenne muscular dystrophy (DMD) cell lines. The function of CD82 in skeletal muscle remains elusive, partly because the binding partners of this tetraspanin in muscle cells have not been identified. CD82-associated proteins are sought to be identified in human myotubes via mass spectrometry proteomics, which identifies dysferlin and myoferlin as CD82-binding partners. In human dysferlinopathy (Limb girdle muscular dystrophy R2, LGMDR2) myogenic cell lines, expression of CD82 protein is near absent in two of four patient samples. In the cell lines where CD82 protein levels are unaffected, increased expression of the ≈72 kDa mini-dysferlin product is identified using an antibody recognizing the dysferlin C-terminus. These data demonstrate that CD82 binds dysferlin/myoferlin in differentiating muscle cells and its expression can be affected by loss of dysferlin in human myogenic cells.

Orientation of the Dysferlin C2A Domain is Responsive to the Composition of Lipid Membranes

Dysferlin is a 230 kD protein that plays a critical function in the active resealing of micron-sized injuries to the muscle sarcolemma by recruiting vesicles to patch the injured site via vesicle fusion. Muscular dystrophy is observed in humans when mutations disrupt this repair process or dysferlin is absent. While lipid binding by dysferlin's C2A domain (dysC2A) is considered fundamental to the membrane resealing process, the molecular mechanism of this interaction is not fully understood. By applying nonlinear surface-specific vibrational spectroscopy, we have successfully demonstrated that dysferlin's N-terminal C2A domain (dysC2A) alters its binding orientation in response to a membrane's lipid composition. These experiments reveal that dysC2A utilizes a generic electrostatic binding interaction to bind to most anionic lipid surfaces, inserting its calcium binding loops into the lipid surface while orienting its β-sheets 30-40° from surface normal. However, at lipid surfaces, where PI(4,5)P2 is present, dysC2A tilts its β-sheets more than 60° from surface normal to expose a polybasic face, while it binds to the PI(4,5)P2 surface. Both lipid binding mechanisms are shown to occur alongside dysC2A-induced lipid clustering. These different binding mechanisms suggest that dysC2A could provide a molecular cue to the larger dysferlin protein as to signal whether it is bound to the sarcolemma or another lipid surface.

Elevated Ca2+ at the triad junction underlies dysregulation of Ca2+ signaling in dysferlin-null skeletal muscle

Dysferlin-null A/J myofibers generate abnormal Ca2+ transients that are slightly reduced in amplitude compared to controls. These are further reduced in amplitude by hypoosmotic shock and often appear as Ca2+ waves (Lukyanenko et al., J. Physiol., 2017). Ca2+ waves are typically associated with Ca2+-induced Ca2+ release, or CICR, which can be myopathic. We tested the ability of a permeable Ca2+ chelator, BAPTA-AM, to inhibit CICR in injured dysferlin-null fibers and found that 10-50 nM BAPTA-AM suppressed all Ca2+ waves. The same concentrations of BAPTA-AM increased the amplitude of the Ca2+ transient in A/J fibers to wild type levels and protected transients against the loss of amplitude after hypoosmotic shock, as also seen in wild type fibers. Incubation with 10 nM BAPTA-AM led to intracellular BAPTA concentrations of ∼60 nM, as estimated with its fluorescent analog, Fluo-4AM. This should be sufficient to restore intracellular Ca2+ to levels seen in wild type muscle. Fluo-4AM was ∼10-fold less effective than BAPTA-AM, however, consistent with its lower affinity for Ca2+. EGTA, which has an affinity for Ca2+ similar to BAPTA, but with much slower kinetics of binding, was even less potent when introduced as the -AM derivative. By contrast, a dysferlin variant with GCaMP6fu in place of its C2A domain accumulated at triad junctions, like wild type dysferlin, and suppressed all abnormal Ca2+ signaling. GCaMP6fu introduced as a Venus chimera did not accumulate at junctions and failed to suppress abnormal Ca2+ signaling. Our results suggest that leak of Ca2+ into the triad junctional cleft underlies dysregulation of Ca2+ signaling in dysferlin-null myofibers, and that dysferlin's C2A domain suppresses abnormal Ca2+ signaling and protects muscle against injury by binding Ca2+ in the cleft.

The C2 domains of dysferlin: roles in membrane localization, Ca2+ signalling and sarcolemmal repair

Dysferlin is an integral membrane protein of the transverse tubules of skeletal muscle that is mutated or absent in limb girdle muscular dystrophy 2B and Miyoshi myopathy. Here we examine the role of dysferlin's seven C2 domains, C2A through C2G, in membrane repair and Ca2+ release, as well as in targeting dysferlin to the transverse tubules of skeletal muscle. We report that deletion of either domain C2A or C2B inhibits membrane repair completely, whereas deletion of C2C, C2D, C2E, C2F or C2G causes partial loss of membrane repair that is exacerbated in the absence of extracellular Ca2+ . Deletion of C2C, C2D, C2E, C2F or C2G also causes significant changes in Ca2+ release, measured as the amplitude of the Ca2+ transient before or after hypo-osmotic shock and the appearance of Ca2+ waves. Most deletants accumulate in endoplasmic reticulum. Only the C2A domain can be deleted without affecting dysferlin trafficking to transverse tubules, but Dysf-ΔC2A fails to support normal Ca2+ signalling after hypo-osmotic shock. Our data suggest that (i) every C2 domain contributes to repair; (ii) all C2 domains except C2B regulate Ca2+ signalling; (iii) transverse tubule localization is insufficient for normal Ca2+ signalling; and (iv) Ca2+ dependence of repair is mediated by C2C through C2G. Thus, dysferlin's C2 domains have distinct functions in Ca2+ signalling and sarcolemmal membrane repair and may play distinct roles in skeletal muscle. KEY POINTS: Dysferlin, a transmembrane protein containing seven C2 domains, C2A through C2G, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage-induced Ca2+ transients and participates in sarcolemmal membrane repair. Each of dysferlin's C2 domains except C2B regulate Ca2+ signalling. Localization of dysferlin variants to the transverse tubules is not sufficient to support normal Ca2+ signalling or membrane repair. Each of dysferlin's C2 domains contributes to sarcolemmal membrane repair. The Ca2+ dependence of membrane repair is mediated by C2C through C2G. Dysferlin's C2 domains therefore have distinct functions in Ca2+ signalling and sarcolemmal membrane repair.

The Role of Calpains in Skeletal Muscle Remodeling with Exercise and Inactivity-induced Atrophy

Calpains are cysteine proteases expressed in skeletal muscle fibers and other cells. Although calpain was first reported to act as a kinase activating factor in skeletal muscle, the consensus is now that calpains play a canonical role in protein turnover. However, recent evidence reveals new and exciting roles for calpains in skeletal muscle. This review will discuss the functions of calpains in skeletal muscle remodeling in response to both exercise and inactivity-induced muscle atrophy. Calpains participate in protein turnover and muscle remodeling by selectively cleaving target proteins and creating fragmented proteins that can be further degraded by other proteolytic systems. Nonetheless, an often overlooked function of calpains is that calpain-mediated cleavage of proteins can result in fragmented proteins that are biologically active and have the potential to actively influence cell signaling. In this manner, calpains function beyond their roles in protein turnover and influence downstream signaling effects. This review will highlight both the canonical and noncanonical roles that calpains play in skeletal muscle remodeling including sarcomere transformation, membrane repair, triad junction formation, regulation of excitation-contraction coupling, protein turnover, cell signaling, and mitochondrial function. We conclude with a discussion of key unanswered questions regarding the roles that calpains play in skeletal muscle.

The effects of concentric and eccentric training in murine models of dysferlin-associated muscular dystrophy

INTRODUCTION

Dysferlin-deficient murine muscle sustains severe damage after repeated eccentric contractions.

METHODS

With a robotic dynamometer, we studied the response of dysferlin-sufficient and dysferlin-deficient mice to 12 weeks of concentrically or eccentrically biased contractions. We also studied whether concentric contractions before or after eccentric contractions reduced muscle damage in dysferlin-deficient mice.

RESULTS

After 12 weeks of concentric training, there was no net gain in contractile force in dysferlin-sufficient or dysferlin-deficient mice, whereas eccentric training produced a net gain in force in both mouse strains. However, eccentric training induced more muscle damage in dysferlin-deficient vs dysferlin-sufficient mice. Although concentric training produced minimal muscle damage in dysferlin-deficient mice, it still led to a prominent increase in centrally nucleated fibers. Previous exposure to concentric contractions conferred slight protection on dysferlin-deficient muscle against damage from subsequent injurious eccentric contractions.

DISCUSSION

Concentric contractions may help dysferlin-deficient muscle derive the benefits of exercise without inducing damage.

Dysferlin-deficiency has greater impact on function of slow muscles, compared with fast, in aged BLAJ mice

Dysferlinopathies are a form of muscular dystrophy caused by gene mutations resulting in deficiency of the protein dysferlin. Symptoms manifest later in life in a muscle specific manner, although the pathomechanism is not well understood. This study compared the impact of dysferlin-deficiency on in vivo and ex vivo muscle function, and myofibre type composition in slow (soleus) and fast type (extensor digitorum longus; EDL) muscles using male dysferlin-deficient (dysf^-/-) BLAJ mice aged 10 months, compared with wild type (WT) C57Bl/6J mice. There was a striking increase in muscle mass of BLAJ soleus (+25%) (p<0.001), with no strain differences in EDL mass, compared with WT. In vivo measures of forelimb grip strength and wheel running capacity showed no strain differences. Ex vivo measures showed the BLAJ soleus had faster twitch contraction (-21%) and relaxation (-20%) times, and delayed post fatigue recovery (ps<0.05); whereas the BLAJ EDL had a slower relaxation time (+11%) and higher maximum rate of force production (+25%) (ps<0.05). Similar proportions of MHC isoforms were evident in the soleus muscles of both strains (ps>0.05); however, for the BLAJ EDL, there was an increased proportion of type IIx MHC isoform (+5.5%) and decreased type IIb isoform (-5.5%) (ps<0.01). This identification of novel differences in the impact of dysferlin-deficiency on slow and fast twitch muscles emphasises the importance of evaluating myofibre type specific effects to provide crucial insight into the mechanisms responsible for loss of function in dysferlinopathies; this is critical for the development of targeted future clinical therapies.

Diltiazem improves contractile properties of skeletal muscle in dysferlin-deficient BLAJ mice, but does not reduce contraction-induced muscle damage

B6.A-Dysfprmd /GeneJ (BLAJ) mice model human limb-girdle muscular dystrophy 2B (LGMD2B), which is linked to mutations in the dysferlin (DYSF) gene. We tested the hypothesis that, the calcium ion (Ca2+ ) channel blocker diltiazem (DTZ), reduces contraction-induced skeletal muscle damage, in BLAJ mice. We randomly assigned mice (N = 12; 3-4 month old males) to one of two groups - DTZ (N = 6) or vehicle (VEH, distilled water, N = 6). We conditioned mice with either DTZ or VEH for 1 week, after which, their tibialis anterior (TA) muscles were tested for contractile torque and susceptibility to injury from forced eccentric contractions. We continued dosing with DTZ or VEH for 3 days following eccentric contractions, and then studied torque recovery and muscle damage. We analyzed contractile torque before eccentric contractions, immediately after eccentric contractions, and at 3 days after eccentric contractions; and counted damaged fibers in the injured and uninjured TA muscles. We found that DTZ improved contractile torque before and immediately after forced eccentric contractions, but did not reduce delayed-onset muscle damage that was observed at 3 days after eccentric contractions.

Effect of Ibuprofen on Skeletal Muscle of Dysferlin-Null Mice

Ibuprofen, a nonsteroidal anti-inflammatory drug, and nitric oxide (NO) donors have been reported to reduce the severity of muscular dystrophies in mice associated with the absence of dystrophin or α-sarcoglycan, but their effects on mice that are dystrophic due to the absence of dysferlin have not been examined. We have tested ibuprofen, as well as isosorbide dinitrate (ISDN), a NO donor, to learn whether used alone or together they protect dysferlin-null muscle in A/J mice from large strain injury (LSI) induced by a series of high strain lengthening contractions. Mice were maintained on chow containing ibuprofen and ISDN for 4 weeks. They were then subjected to LSI and maintained on the drugs for 3 additional days. We measured loss of torque immediately following injury and at day 3 postinjury, fiber necrosis, and macrophage infiltration at day 3 postinjury, and serum levels of the drugs at the time of euthanasia. Loss of torque immediately after injury was not altered by the drugs. However, the torque on day 3 postinjury significantly decreased as a function of ibuprofen concentration in the serum (range, 0.67-8.2 µg/ml), independent of ISDN. The effects of ISDN on torque loss at day 3 postinjury were not significant. In long-term studies of dysferlinopathic BlAJ mice, lower doses of ibuprofen had no effects on muscle morphology, but reduced treadmill running by 40%. Our results indicate that ibuprofen can have deleterious effects on dysferlin-null muscle and suggest that its use at pharmacological doses should be avoided by individuals with dysferlinopathies.

AAV-mediated transfer of FKRP shows therapeutic efficacy in a murine model but requires control of gene expression

Limb Girdle Muscular Dystrophies type 2I (LGMD2I), a recessive autosomal muscular dystrophy, is caused by mutations in the Fukutin Related Protein (FKRP) gene. It has been proposed that FKRP, a ribitol-5-phosphate transferase, is a participant in α-dystroglycan (αDG) glycosylation, which is important to ensure the cell/matrix anchor of muscle fibers. A LGMD2I knock-in mouse model was generated to express the most frequent mutation (L276I) encountered in patients. The expression of FKRP was not altered neither at transcriptional nor at translational levels, but its function was impacted since abnormal glycosylation of αDG was observed. Skeletal muscles were functionally impaired from 2 months of age and a moderate dystrophic pattern was evident starting from 6 months of age. Gene transfer with a rAAV2/9 vector expressing Fkrp restored biochemical defects, corrected the histological abnormalities and improved the resistance to eccentric stress in the mouse model. However, injection of high doses of the vector induced a decrease of αDG glycosylation and laminin binding, even in WT animals. Finally, intravenous injection of the rAAV-Fkrp vector into a dystroglycanopathy mouse model due to Fukutin (Fktn) knock-out indicated a dose-dependent toxicity. These data suggest requirement for a control of FKRP expression in muscles.

Coupling of excitation to Ca2+ release is modulated by dysferlin

KEY POINTS

Dysferlin, the protein missing in limb girdle muscular dystrophy 2B and Miyoshi myopathy, concentrates in transverse tubules of skeletal muscle, where it stabilizes voltage-induced Ca²⁺ transients against loss after osmotic shock injury (OSI). Local expression of dysferlin in dysferlin-null myofibres increases transient amplitude to control levels and protects them from loss after OSI. Inhibitors of ryanodine receptors (RyR1) and L-type Ca²⁺ channels protect voltage-induced Ca²⁺ transients from loss; thus both proteins play a role in injury in dysferlin's absence. Effects of Ca²⁺ -free medium and S107, which inhibits SR Ca²⁺ leak, suggest the SR as the primary source of Ca²⁺ responsible for the loss of the Ca²⁺ transient upon injury. Ca²⁺ waves were induced by OSI and suppressed by exogenous dysferlin. We conclude that dysferlin prevents injury-induced SR Ca²⁺ leak.

ABSTRACT

Dysferlin concentrates in the transverse tubules of skeletal muscle and stabilizes Ca²⁺ transients when muscle fibres are subjected to osmotic shock injury (OSI). We show here that voltage-induced Ca²⁺ transients elicited in dysferlin-null A/J myofibres were smaller than control A/WySnJ fibres. Regional expression of Venus-dysferlin chimeras in A/J fibres restored the full amplitude of the Ca²⁺ transients and protected against OSI. We also show that drugs that target ryanodine receptors (RyR1: dantrolene, tetracaine, S107) and L-type Ca²⁺ channels (LTCCs: nifedipine, verapamil, diltiazem) prevented the decrease in Ca²⁺ transients in A/J fibres following OSI. Diltiazem specifically increased transients by ∼20% in uninjured A/J fibres, restoring them to control values. The fact that both RyR1s and LTCCs were involved in OSI-induced damage suggests that damage is mediated by increased Ca²⁺ leak from the sarcoplasmic reticulum (SR) through the RyR1. Congruent with this, injured A/J fibres produced Ca²⁺ sparks and Ca²⁺ waves. S107 (a stabilizer of RyR1-FK506 binding protein coupling that reduces Ca²⁺ leak) or local expression of Venus-dysferlin prevented OSI-induced Ca²⁺ waves. Our data suggest that dysferlin modulates SR Ca²⁺ release in skeletal muscle, and that in its absence OSI causes increased RyR1-mediated Ca²⁺ leak from the SR into the cytoplasm.

Membrane-stabilizing copolymers confer marked protection to dystrophic skeletal muscle in vivo

Duchenne muscular dystrophy (DMD) is a fatal disease of striated muscle deterioration. A unique therapeutic approach for DMD is the use of synthetic membrane stabilizers to protect the fragile dystrophic sarcolemma against contraction-induced mechanical stress. Block copolymer-based membrane stabilizer poloxamer 188 (P188) has been shown to protect the dystrophic myocardium. In comparison, the ability of synthetic membrane stabilizers to protect fragile DMD skeletal muscles has been less clear. Because cardiac and skeletal muscles have distinct structural and functional features, including differences in the mechanism of activation, variance in sarcolemma phospholipid composition, and differences in the magnitude and types of forces generated, we speculated that optimized membrane stabilization could be inherently different. Our objective here is to use principles of pharmacodynamics to evaluate membrane stabilization therapy for DMD skeletal muscles. Results show a dramatic differential effect of membrane stabilization by optimization of pharmacodynamic-guided route of poloxamer delivery. Data show that subcutaneous P188 delivery, but not intravascular or intraperitoneal routes, conferred significant protection to dystrophic limb skeletal muscles undergoing mechanical stress in vivo. In addition, structure-function examination of synthetic membrane stabilizers further underscores the importance of copolymer composition, molecular weight, and dosage in optimization of poloxamer pharmacodynamics in vivo.

Membrane Repair: Mechanisms and Pathophysiology

Eukaryotic cells have been confronted throughout their evolution with potentially lethal plasma membrane injuries, including those caused by osmotic stress, by infection from bacterial toxins and parasites, and by mechanical and ischemic stress. The wounded cell can survive if a rapid repair response is mounted that restores boundary integrity. Calcium has been identified as the key trigger to activate an effective membrane repair response that utilizes exocytosis and endocytosis to repair a membrane tear, or remove a membrane pore. We here review what is known about the cellular and molecular mechanisms of membrane repair, with particular emphasis on the relevance of repair as it relates to disease pathologies. Collective evidence reveals membrane repair employs primitive yet robust molecular machinery, such as vesicle fusion and contractile rings, processes evolutionarily honed for simplicity and success. Yet to be fully understood is whether core membrane repair machinery exists in all cells, or whether evolutionary adaptation has resulted in multiple compensatory repair pathways that specialize in different tissues and cells within our body.

GRAF1 deficiency blunts sarcolemmal injury repair and exacerbates cardiac and skeletal muscle pathology in dystrophin-deficient mice

Background

The plasma membranes of striated muscle cells are particularly susceptible to rupture as they endure significant mechanical stress and strain during muscle contraction, and studies have shown that defects in membrane repair can contribute to the progression of muscular dystrophy. The synaptotagmin-related protein, dysferlin, has been implicated in mediating rapid membrane repair through its ability to direct intracellular vesicles to sites of membrane injury. However, further work is required to identify the precise molecular mechanisms that govern dysferlin targeting and membrane repair. We previously showed that the bin–amphiphysin–Rvs (BAR)–pleckstrin homology (PH) domain containing Rho-GAP GTPase regulator associated with focal adhesion kinase-1 (GRAF1) was dynamically recruited to the tips of fusing myoblasts wherein it promoted membrane merging by facilitating ferlin-dependent capturing of intracellular vesicles. Because acute membrane repair responses involve similar vesicle trafficking complexes/events and because our prior studies in GRAF1-deficient tadpoles revealed a putative role for GRAF1 in maintaining muscle membrane integrity, we postulated that GRAF1 might also play an important role in facilitating dysferlin-dependent plasma membrane repair.

Methods

We used an in vitro laser-injury model to test whether GRAF1 was necessary for efficient muscle membrane repair. We also generated dystrophin/GRAF1 doubledeficient mice by breeding mdx mice with GRAF1 hypomorphic mice. Evans blue dye uptake and extensive morphometric analyses were used to assess sarcolemmal integrity and related pathologies in cardiac and skeletal muscles isolated from these mice.

Results

Herein, we show that GRAF1 is dynamically recruited to damaged skeletal and cardiac muscle plasma membranes and that GRAF1-depleted muscle cells have reduced membrane healing abilities. Moreover, we show that dystrophin depletion exacerbated muscle damage in GRAF1-deficient mice and that mice with dystrophin/GRAF1 double deficiency phenocopied the severe muscle pathologies observed in dystrophin/dysferlin-double null mice. Consistent with a model that GRAF1 facilitates dysferlin-dependent membrane patching, we found that GRAF1 associates with and regulates plasma membrane deposition of dysferlin.

Conclusions

Overall, our work indicates that GRAF1 facilitates dysferlin-dependent membrane repair following acute muscle injury. These findings indicate that GRAF1 might play a role in the phenotypic variation and pathological progression of cardiac and skeletal muscle degeneration in muscular dystrophy patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-015-0054-6) contains supplementary material, which is available to authorized users.

Myofiber damage precedes macrophage infiltration after in vivo injury in dysferlin-deficient A/J mouse skeletal muscle

Mutations in the dysferlin gene (DYSF) lead to human muscular dystrophies known as dysferlinopathies. The dysferlin-deficient A/J mouse develops a mild myopathy after 6 months of age, and when younger models the subclinical phase of the human disease. We subjected the tibialis anterior muscle of 3- to 4-month-old A/J mice to in vivo large-strain injury (LSI) from lengthening contractions and studied the progression of torque loss, myofiber damage, and inflammation afterward. We report that myofiber damage in A/J mice occurs before inflammatory cell infiltration. Peak edema and inflammation, monitored by magnetic resonance imaging and by immunofluorescence labeling of neutrophils and macrophages, respectively, develop 24 to 72 hours after LSI, well after the appearance of damaged myofibers. Cytokine profiles 72 hours after injury are consistent with extensive macrophage infiltration. Dysferlin-sufficient A/WySnJ mice show much less myofiber damage and inflammation and lesser cytokine levels after LSI than do A/J mice. Partial suppression of macrophage infiltration by systemic administration of clodronate-incorporated liposomes fails to suppress LSI-induced damage or to accelerate torque recovery in A/J mice. The findings from our studies suggest that, although macrophage infiltration is prominent in dysferlin-deficient A/J muscle after LSI, it is the consequence and not the cause of progressive myofiber damage.

A comparison of AAV strategies distinguishes overlapping vectors for efficient systemic delivery of the 6.2 kb Dysferlin coding sequence

Recombinant adeno-associated virus (rAAV) is currently the best vector for gene delivery into the skeletal muscle. However, the 5-kb packaging size of this virus is a major obstacle for large gene transfer. This past decade, many different strategies were developed to circumvent this issue (concatemerization-splicing, overlapping vectors, hybrid dual or fragmented AAV). Loss of function mutations in the DYSF gene whose coding sequence is 6.2kb lead to progressive muscular dystrophies (LGMD2B: OMIM_253601; MM: OMIM_254130; DMAT: OMIM_606768). In this study, we compared large gene transfer techniques to deliver the DYSF gene into the skeletal muscle. After rAAV8s intramuscular injection into dysferlin deficient mice, we showed that the overlap strategy is the most effective approach to reconstitute a full-length messenger. After systemic administration, the level of dysferlin obtained on different muscles corresponded to 0.5- to 2-fold compared to the normal level. We further demonstrated that the overlapping vector set was efficient to correct the histopathology, resistance to eccentric contractions and whole body force in the dysferlin deficient mice. Altogether, these data indicate that using overlapping vectors could be a promising approach for a potential clinical treatment of dysferlinopathies.

A stepwise procedure to test contractility and susceptibility to injury for the rodent quadriceps muscle

In patients with muscle injury or muscle disease, assessment of muscle damage is typically limited to clinical signs, such as tenderness, strength, range of motion, and more recently, imaging studies. Biological markers can also be used in measuring muscle injury, such as increased creatine kinase levels in the blood, but these are not always correlated with loss in muscle function (i.e. loss of force production). This is even true of histological findings from animals, which provide a "direct measure" of damage, but do not account for loss of function. The most comprehensive measure of the overall health of the muscle is contractile force. To date, animal models testing contractile force have been limited to the muscle groups moving the ankle. Here we describe an in vivo animal model for the quadriceps, with abilities to measure torque, produce a reliable muscle injury, and follow muscle recovery within the same animal over time. We also describe a second model used for direct measurement of force from an isolated quadriceps muscle in situ.

Dysferlin at transverse tubules regulates Ca(2+) homeostasis in skeletal muscle

The class of muscular dystrophies linked to the genetic ablation or mutation of dysferlin, including Limb Girdle Muscular Dystrophy 2B (LGMD2B) and Miyoshi Myopathy (MM), are late-onset degenerative diseases. In lieu of a genetic cure, treatments to prevent or slow the progression of dysferlinopathy are of the utmost importance. Recent advances in the study of dysferlinopathy have highlighted the necessity for the maintenance of calcium handling in altering or slowing the progression of muscular degeneration resulting from the loss of dysferlin. This review highlights new evidence for a role for dysferlin at the transverse (t-) tubule of striated muscle, where it is involved in maintaining t-tubule structure and function.

Mechanisms of muscle injury, repair, and regeneration

Skeletal muscle continuously adapts to changes in its mechanical environment through modifications in gene expression and protein stability that affect its physiological function and mass. However, mechanical stresses commonly exceed the parameters that induce adaptations, producing instead acute injury. Furthermore, the relatively superficial location of many muscles in the body leaves them further vulnerable to acute injuries by exposure to extreme temperatures, contusions, lacerations or toxins. In this article, the molecular, cellular, and mechanical factors that underlie muscle injury and the capacity of muscle to repair and regenerate are presented. Evidence shows that muscle injuries that are caused by eccentric contractions result from direct mechanical damage to myofibrils. However, muscle pathology following other acute injuries is largely attributable to damage to the muscle cell membrane. Many feaures in the injury-repair-regeneration cascade relate to the unregulated influx of calcium through membrane lesions, including: (i) activation of proteases and hydrolases that contribute muscle damage, (ii) activation of enzymes that drive the production of mitogens and motogens for muscle and immune cells involved in injury and repair, and (iii) enabling protein-protein interactions that promote membrane repair. Evidence is also presented to show that the myogenic program that is activated by acute muscle injury and the inflammatory process that follows are highly coordinated, with myeloid cells playing a central role in modulating repair and regeneration. The early-invading, proinflammatory M1 macrophages remove debris caused by injury and express Th1 cytokines that play key roles in regulating the proliferation, migration, and differentiation of satellite cells. The subsequent invasion by anti-inflammatory, M2 macrophages promotes tissue repair and attenuates inflammation. Although this system provides an effective mechanism for muscle repair and regeneration following acute injury, it is dysregulated in chronic injuries. In this article, the process of muscle injury, repair and regeneration that occurs in muscular dystrophy is used as an example of chronic muscle injury, to highlight similarities and differences between the injury and repair processes that occur in acutely and chronically injured muscle.

Dysferlin stabilizes stress-induced Ca2+ signaling in the transverse tubule membrane

Dysferlinopathies, most commonly limb girdle muscular dystrophy 2B and Miyoshi myopathy, are degenerative myopathies caused by mutations in the DYSF gene encoding the protein dysferlin. Studies of dysferlin have focused on its role in the repair of the sarcolemma of skeletal muscle, but dysferlin's association with calcium (Ca(2+)) signaling proteins in the transverse (t-) tubules suggests additional roles. Here, we reveal that dysferlin is enriched in the t-tubule membrane of mature skeletal muscle fibers. Following experimental membrane stress in vitro, dysferlin-deficient muscle fibers undergo extensive functional and structural disruption of the t-tubules that is ameliorated by reducing external [Ca(2+)] or blocking L-type Ca(2+) channels with diltiazem. Furthermore, we demonstrate that diltiazem treatment of dysferlin-deficient mice significantly reduces eccentric contraction-induced t-tubule damage, inflammation, and necrosis, which resulted in a concomitant increase in postinjury functional recovery. Our discovery of dysferlin as a t-tubule protein that stabilizes stress-induced Ca(2+) signaling offers a therapeutic avenue for limb girdle muscular dystrophy 2B and Miyoshi myopathy patients.

Full-length dysferlin expression driven by engineered human dystrophic blood derived CD133+ stem cells

The protein dysferlin is abundantly expressed in skeletal and cardiac muscles, where its main function is membrane repair. Mutations in the dysferlin gene are involved in two autosomal recessive muscular dystrophies: Miyoshi myopathy and limb-girdle muscular dystrophy type 2B. Development of effective therapies remains a great challenge. Strategies to repair the dysferlin gene by skipping mutated exons, using antisense oligonucleotides (AONs), may be suitable only for a subset of mutations, while cell and gene therapy can be extended to all mutations. AON-treated blood-derived CD133+ stem cells isolated from patients with Miyoshi myopathy led to partial dysferlin reconstitution in vitro but failed to express dysferlin after intramuscular transplantation into scid/blAJ dysferlin null mice. We thus extended these experiments producing the full-length dysferlin mediated by a lentiviral vector in blood-derived CD133+ stem cells isolated from the same patients. Transplantation of engineered blood-derived CD133+ stem cells into scid/blAJ mice resulted in sufficient dysferlin expression to correct functional deficits in skeletal muscle membrane repair. Our data suggest for the first time that lentivirus-mediated delivery of full-length dysferlin in stem cells isolated from Miyoshi myopathy patients could represent an alternative therapeutic approach for treatment of dysferlinopathies.

The phenotype of dysferlin-deficient mice is not rescued by adeno-associated virus-mediated transfer of anoctamin 5

Mutations in dysferlin and anoctamin 5 are the cause of muscular disorders, with the main presentations as limb-girdle muscular dystrophy or Miyoshi type of distal myopathy. Both these proteins have been implicated in sarcolemmal resealing. On the basis of similarities in associated phenotypes and protein functions, we tested the hypothesis that ANO5 protein could compensate for dysferlin absence. We first defined that the main transcript of ANO5 expressed in skeletal muscle is the 22-exon full-length isoform, and we demonstrated that dysferlin-deficient (Dysf (prmd)) mice have lower Ano5 expression levels, an observation that further enhanced the rational of the tested hypothesis. We then showed that AAV-mediated transfer of human ANO5 (hANO5) did not lead to apparent toxicity in wild-type mice. Finally, we demonstrated that AAV-hANO5 injection was not able to compensate for dysferlin deficiency in the Dysf (prmd) mouse model or improve the membrane repair defect seen in the absence of dysferlin. Consequently, overexpressing hANO5 does not seem to provide a valuable therapeutic strategy for dysferlin deficiency.

X-ROS signaling in the heart and skeletal muscle: stretch-dependent local ROS regulates [Ca²⁺]i

X-ROS signaling is a novel redox signaling pathway that links mechanical stress to changes in [Ca(2+)]i. This pathway is activated rapidly and locally within a muscle cell under physiological conditions, but can also contribute to Ca(2+)-dependent arrhythmia in the heart and to the dystrophic phenotype in the heart and skeletal muscle. Upon physiologic cellular stretch, microtubules serve as mechanotransducers to activate NADPH oxidase 2 in the transverse tubules and sarcolemmal membranes to produce reactive oxygen species (ROS). In the heart, the ROS acts locally to activate ryanodine receptor Ca(2+) release channels in the junctional sarcoplasmic reticulum, increasing the Ca(2+) spark rate and "tuning" excitation-contraction coupling. In the skeletal muscle, where Ca(2+) sparks are not normally observed, the X-ROS signaling process is muted. However in muscular dystrophies, such as Duchenne Muscular Dystrophy and dysferlinopathy, X-ROS signaling operates at a high level and contributes to myopathy. Importantly, Ca(2+) permeable stretch-activated channels are activated by X-ROS and contribute to skeletal muscle pathology. Here we review X-ROS signaling and mechanotransduction in striated muscle, and highlight important questions to drive future work on stretch-dependent signaling. We conclude that X-ROS provides an exciting mechanism for the mechanical control of redox and Ca(2+) signaling, but much work is needed to establish its contribution to physiologic and pathophysiologic processes in diverse cell systems.

Molecular, physiological, and motor performance defects in DMSXL mice carrying >1,000 CTG repeats from the human DM1 locus

Myotonic dystrophy type 1 (DM1) is caused by an unstable CTG repeat expansion in the 3′UTR of the DM protein kinase (DMPK) gene. DMPK transcripts carrying CUG expansions form nuclear foci and affect splicing regulation of various RNA transcripts. Furthermore, bidirectional transcription over the DMPK gene and non-conventional RNA translation of repeated transcripts have been described in DM1. It is clear now that this disease may involve multiple pathogenic pathways including changes in gene expression, RNA stability and splicing regulation, protein translation, and micro–RNA metabolism. We previously generated transgenic mice with 45-kb of the DM1 locus and >300 CTG repeats (DM300 mice). After successive breeding and a high level of CTG repeat instability, we obtained transgenic mice carrying >1,000 CTG (DMSXL mice). Here we described for the first time the expression pattern of the DMPK sense transcripts in DMSXL and human tissues. Interestingly, we also demonstrate that DMPK antisense transcripts are expressed in various DMSXL and human tissues, and that both sense and antisense transcripts accumulate in independent nuclear foci that do not co-localize together. Molecular features of DM1-associated RNA toxicity in DMSXL mice (such as foci accumulation and mild missplicing), were associated with high mortality, growth retardation, and muscle defects (abnormal histopathology, reduced muscle strength, and lower motor performances). We have found that lower levels of IGFBP-3 may contribute to DMSXL growth retardation, while increased proteasome activity may affect muscle function. These data demonstrate that the human DM1 locus carrying very large expansions induced a variety of molecular and physiological defects in transgenic mice, reflecting DM1 to a certain extent. As a result, DMSXL mice provide an animal tool to decipher various aspects of the disease mechanisms. In addition, these mice can be used to test the preclinical impact of systemic therapeutic strategies on molecular and physiological phenotypes.

Myotonic dystrophy type 1 (DM1) is caused by the abnormal expansion of a CTG repeat located in the DM protein kinase (DMPK) gene. DMPK transcripts carrying CUG expansions form toxic nuclear foci that affect other RNAs. DM1 involve multiple pathogenic pathways including changes in gene expression, RNA stability and splicing regulation, protein translation, and micro–RNA metabolism. We previously generated transgenic mice carrying the human DM1 locus and very large expansions >1,000 CTG (DMSXL mice). Here we described for the first time, the expression pattern of the DMPK sense transcripts in DMSXL and human tissues. We also demonstrate that DMPK antisense transcripts are expressed in various tissues from DMSXL mice and human. Both sense and antisense transcripts form nuclear foci. DMSXL mice showed molecular DM1 features such as foci and mild splicing defects as well as muscles defects, reduced muscle strength, and lower motor performances. These mice recapitulate some molecular features of DM1 leading to physiological abnormalities. DMSXL are not only a tool to decipher various mechanisms involved in DM1 but also to test the preclinical impact of systemic therapeutic strategies.

Lack of correlation between outcomes of membrane repair assay and correction of dystrophic changes in experimental therapeutic strategy in dysferlinopathy

Mutations in the dysferlin gene are the cause of Limb-girdle Muscular Dystrophy type 2B and Miyoshi Myopathy. The dysferlin protein has been implicated in sarcolemmal resealing, leading to the idea that the pathophysiology of dysferlin deficiencies is due to a deficit in membrane repair. Here, we show using two different approaches that fullfiling membrane repair as asseyed by laser wounding assay is not sufficient for alleviating the dysferlin deficient pathology. First, we generated a transgenic mouse overexpressing myoferlin to test the hypothesis that myoferlin, which is homologous to dysferlin, can compensate for the absence of dysferlin. The myoferlin overexpressors show no skeletal muscle abnormalities, and crossing them with a dysferlin-deficient model rescues the membrane fusion defect present in dysferlin-deficient mice in vitro. However, myoferlin overexpression does not correct muscle histology in vivo. Second, we report that AAV-mediated transfer of a minidysferlin, previously shown to correct the membrane repair deficit in vitro, also fails to improve muscle histology. Furthermore, neither myoferlin nor the minidysferlin prevented myofiber degeneration following eccentric exercise. Our data suggest that the pathogenicity of dysferlin deficiency is not solely related to impairment in sarcolemmal repair and highlight the care needed in selecting assays to assess potential therapies for dysferlinopathies.

Myogenesis in dysferlin-deficient myoblasts is inhibited by an intrinsic inflammatory response

Limb-girdle muscular dystrophy type 2B results from mutations in dysferlin, a membrane-associated protein involved in cellular membrane repair. Primary myoblast cultures derived from dysferlinopathy patients show reduced myogenic potential, suggesting that dysferlin may regulate myotube fusion and be required for muscle regeneration. These observations contrast with the findings that muscle develops normally in pre-symptomatic dysferlinopathy patients. To better understand the role of dysferlin in myogenesis, we investigated this process in vitro using cells derived from two mouse models of dysferlinopathy: SJL/J and A/J mice. We observed that myotubes derived from dysferlin-deficient muscle were of significantly smaller diameters, contained fewer myonuclei, and displayed reduced myogenic gene expression compared to dysferlin-sufficient cells. Together, these findings suggest that the absence of dysferlin from myoblasts is detrimental to myogenesis. Pro-inflammatory NFκB signaling was upregulated in dysferlin-deficient myotubes; the anti-inflammatory agent celastrol reduced the NFκB activation and improved myogenesis in dysferlin-deficient cultures. The results suggest that decreased myotube fusion in dysferlin deficiency is attributable to intrinsic inflammatory activation and can be improved using anti-inflammatory mediators.

Distinct effects of contraction-induced injury in vivo on four different murine models of dysferlinopathy

Mutations in the DYSF gene, encoding dysferlin, cause muscular dystrophies in man. We compared 4 dysferlinopathic mouse strains: SJL/J and B10.SJL-Dysf^im/AwaJ (B10.SJL), and A/J and B6.A-Dysf^prmd/GeneJ (B6.A/J). The former but not the latter two are overtly myopathic and weaker at 3 months of age. Following repetitive large-strain injury (LSI) caused by lengthening contractions, all except B6.A/J showed ~40% loss in contractile torque. Three days later, torque in SJL/J, B10.SJL and controls, but not A/J, recovered nearly completely. B6.A/J showed ~30% torque loss post-LSI and more variable recovery. Pre-injury, all dysferlinopathic strains had more centrally nucleated fibers (CNFs) and all but A/J showed more inflammation than controls. At D3, all dysferlinopathic strains showed increased necrosis and inflammation, but not more CNFs; controls were unchanged. Dystrophin-null DMD^mdx mice showed more necrosis and inflammation than all dysferlin-nulls. Torque loss and inflammation on D3 across all strains were linearly related to necrosis. Our results suggest that (1) dysferlin is not required for functional recovery 3 days after LSI; (2) B6.A/J mice recover from LSI erratically; (3) SJL/J and B10.SJL muscles recover rapidly, perhaps due to ongoing myopathy; (4) although they recover function to different levels, all 4 dysferlinopathic strains show increased inflammation and necrosis 3 days after LSI.

Genetic deletion of trkB.T1 increases neuromuscular function

Neurotrophin-dependent activation of the tyrosine kinase receptor trkB.FL modulates neuromuscular synapse maintenance and function; however, it is unclear what role the alternative splice variant, truncated trkB (trkB.T1), may have in the peripheral neuromuscular axis. We examined this question in trkB.T1 null mice and demonstrate that in vivo neuromuscular performance and nerve-evoked muscle tension are significantly increased. In vitro assays indicated that the gain-in-function in trkB.T1(-/-) animals resulted specifically from an increased muscle contractility, and increased electrically evoked calcium release. In the trkB.T1 null muscle, we identified an increase in Akt activation in resting muscle as well as a significant increase in trkB.FL and Akt activation in response to contractile activity. On the basis of these findings, we conclude that the trkB signaling pathway might represent a novel target for intervention across diseases characterized by deficits in neuromuscular function.

Unmasking potential intracellular roles for dysferlin through improved immunolabeling methods

Mutations in the DYSF gene that severely reduce the levels of the protein dysferlin are implicated in muscle-wasting syndromes known as dysferlinopathies. Although studies of its function in skeletal muscle have focused on its potential role in repairing the plasma membrane, dysferlin has also been found, albeit inconsistently, in the sarcoplasm of muscle fibers. The aim of this article is to study the localization of dysferlin in skeletal muscle through optimized immunolabeling methods. We studied the localization of dysferlin in control rat skeletal muscle using several different methods of tissue collection and subsequent immunolabeling. We then applied our optimized immunolabeling methods on human cadaveric muscle, control and dystrophic human muscle biopsies, and control and dysferlin-deficient mouse muscle. Our data suggest that dysferlin is present in a reticulum of the sarcoplasm, similar but not identical to those containing the dihydropyridine receptors and distinct from the distribution of the sarcolemmal protein dystrophin. Our data illustrate the importance of tissue fixation and antigen unmasking for proper immunolocalization of dysferlin. They suggest that dysferlin has an important function in the internal membrane systems of skeletal muscle, involved in calcium homeostasis and excitation-contraction coupling.

Dystrophin deficiency exacerbates skeletal muscle pathology in dysferlin-null mice

Background

Mutations in the genes coding for either dystrophin or dysferlin cause distinct forms of muscular dystrophy. Dystrophin links the cytoskeleton to the sarcolemma through direct interaction with β-dystroglycan. This link extends to the extracellular matrix by β-dystroglycan's interaction with α-dystroglycan, which binds extracellular matrix proteins, including laminin α2, agrin and perlecan, that possess laminin globular domains. The absence of dystrophin disrupts this link, leading to compromised muscle sarcolemmal integrity. Dysferlin, on the other hand, plays an important role in the Ca²⁺-dependent membrane repair of damaged sarcolemma in skeletal muscle. Because dysferlin and dystrophin play different roles in maintaining muscle cell integrity, we hypothesized that disrupting sarcolemmal integrity with dystrophin deficiency would exacerbate the pathology in dysferlin-null mice and allow further characterization of the role of dysferlin in skeletal muscle.

Methods

To test our hypothesis, we generated dystrophin/dysferlin double-knockout (DKO) mice by breeding mdx mice with dysferlin-null mice and analyzed the effects of a combined deficiency of dysferlin and dystrophin on muscle pathology and sarcolemmal integrity.

Results

The DKO mice exhibited more severe muscle pathology than either mdx mice or dysferlin-null mice, and, importantly, the onset of the muscle pathology occurred much earlier than it did in dysferlin-deficient mice. The DKO mice showed muscle pathology of various skeletal muscles, including the mandible muscles, as well as a greater number of regenerating muscle fibers, higher serum creatine kinase levels and elevated Evans blue dye uptake into skeletal muscles. Lengthening contractions caused similar force deficits, regardless of dysferlin expression. However, the rate of force recovery within 45 minutes following lengthening contractions was hampered in DKO muscles compared to mdx muscles or dysferlin-null muscles, suggesting that dysferlin is required for the initial recovery from lengthening contraction-induced muscle injury of the dystrophin-glycoprotein complex-compromised muscles.

Conclusions

The results of our study suggest that dysferlin-mediated membrane repair helps to limit the dystrophic changes in dystrophin-deficient skeletal muscle. Dystrophin deficiency unmasks the function of dysferlin in membrane repair during lengthening contractions. Dystrophin/dysferlin-deficient mice provide a very useful model with which to evaluate the effectiveness of therapies designed to treat dysferlin deficiency.

Determinants of the repeated-bout effect after lengthening contractions

OBJECTIVE

Stresses to skeletal muscle often result in injury. A subsequent bout of the same activity performed days or even weeks after an initial bout results in significantly less damage. The underlying causes of this phenomenon, termed the "repeated-bout effect" (RBE), are unclear. This study compared the protective effect of two different injury protocols on the ankle dorsiflexors in the rat. We hypothesized that the RBE would occur soon after the initial injury and persist for several weeks and that the RBE would occur even if the second injury was performed under different biomechanical conditions than the first.

DESIGN

In this controlled laboratory study, the dorsiflexor muscles in the left hind limbs of adult male Sprague-Dawley rats (N = 75) were subjected to ten repetitions of large-strain lengthening contractions or 150 repetitions of small-strain lengthening contractions.

RESULTS

Both protocols induced a significant (P < 0.001) and similar loss of isometric torque (approximately 50%) after the first bout of contractions. The RBE occurred as early as 2 days after the injury and remained high for 14 days (P < 0.001) but diminished by 28 days and was lost by 42 days. The small-strain contractions offered a protective effect against a subsequent large-strain contraction, but not vice versa. Although the RBE did not occur sooner than day 2, the early recovery after a second large-strain injury performed 8 hrs after the first was 2-fold greater than after a single injury.

CONCLUSIONS

The RBE is both rapid in onset and prolonged, and some, but not all, injuries can protect against different types of subsequent injury.

GREG cells, a dysferlin-deficient myogenic mouse cell line

The dysferlinopathies (e.g. LGMD2b, Myoshi myopathy) are progressive, adult-onset muscle wasting syndromes caused by mutations in the gene coding for dysferlin. Dysferlin is a large (~200kDa) membrane-anchored protein, required for maintenance of plasmalemmal integrity in muscle fibers. To facilitate analysis of dysferlin function in muscle cells, we have established a dysferlin-deficient myogenic cell line (GREG cells) from the A/J mouse, a genetic model for dysferlinopathy. GREG cells have no detectable dysferlin expression, but proliferate normally in growth medium and fuse into functional myotubes in differentiation medium. GREG myotubes exhibit deficiencies in plasma membrane repair, as measured by laser wounding in the presence of FM1-43 dye. Under the wounding conditions used, the majority (~66%) of GREG myotubes lack membrane repair capacity, while no membrane repair deficiency was observed in dysferlin-normal C2C12 myotubes, assayed under the same conditions. We discuss the possibility that the observed heterogeneity in membrane resealing represents genetic compensation for dysferlin deficiency.

The dystrophin-glycoprotein complex in the prevention of muscle damage

Muscular dystrophies are genetically diverse but share common phenotypic features of muscle weakness, degeneration, and progressive decline in muscle function. Previous work has focused on understanding how disruptions in the dystrophin-glycoprotein complex result in muscular dystrophy, supporting a hypothesis that the muscle sarcolemma is fragile and susceptible to contraction-induced injury in multiple forms of dystrophy. Although benign in healthy muscle, contractions in dystrophic muscle may contribute to a higher degree of muscle damage which eventually overwhelms muscle regeneration capacity. While increased susceptibility of muscle to mechanical injury is thought to be an important contributor to disease pathology, it is becoming clear that not all DGC-associated diseases share this supposed hallmark feature. This paper outlines experimental support for a function of the DGC in preventing muscle damage and examines the evidence that supports novel functions for this complex in muscle that when impaired, may contribute to the pathogenesis of muscular dystrophy.

Physiological and histological changes in skeletal muscle following in vivo gene transfer by electroporation

Electroporation (EP) is used to transfect skeletal muscle fibers in vivo, but its effects on the structure and function of skeletal muscle tissue have not yet been documented in detail. We studied the changes in contractile function and histology after EP and the influence of the individual steps involved to determine the mechanism of recovery, the extent of myofiber damage, and the efficiency of expression of a green fluorescent protein (GFP) transgene in the tibialis anterior (TA) muscle of adult male C57Bl/6J mice. Immediately after EP, contractile torque decreased by ∼80% from pre-EP levels. Within 3 h, torque recovered to ∼50% but stayed low until day 3. Functional recovery progressed slowly and was complete at day 28. In muscles that were depleted of satellite cells by X-irradiation, torque remained low after day 3, suggesting that myogenesis is necessary for complete recovery. In unirradiated muscle, myogenic activity after EP was confirmed by an increase in fibers with central nuclei or developmental myosin. Damage after EP was confirmed by the presence of necrotic myofibers infiltrated by CD68+ macrophages, which persisted in electroporated muscle for 42 days. Expression of GFP was detected at day 3 after EP and peaked on day 7, with ∼25% of fibers transfected. The number of fibers expressing green fluorescent protein (GFP), the distribution of GFP+ fibers, and the intensity of fluorescence in GFP+ fibers were highly variable. After intramuscular injection alone, or application of the electroporating current without injection, torque decreased by ∼20% and ∼70%, respectively, but secondary damage at D3 and later was minimal. We conclude that EP of murine TA muscles produces variable and modest levels of transgene expression, causes myofiber damage due to the interaction of intramuscular injection with the permeabilizing current, and that full recovery requires myogenesis.

A single 30 min treadmill exercise session is suitable for 'proof-of concept studies' in adult mdx mice: a comparison of the early consequences of two different treadmill protocols

The extent of muscle pathology in sedentary adult mdx mice is very low and treadmill exercise is often used to increase myofibre necrosis; however, the early events in dystrophic muscle and blood in response to treadmill exercise (leading to myofibre necrosis) are unknown. This study describes in detail two standardised protocols for the treadmill exercise of mdx mice and profiles changes in molecular and cellular events after a single 30 min treadmill session (Protocol A) or after 4 weeks of (twice weekly) treadmill exercise (Protocol B). Both treadmill protocols increased multiple markers of muscle damage. We conclude that a single 30 min treadmill exercise session is a sufficient and conveniently fast screening test and could be used in 'proof-of-concept' studies to evaluate the benefits of pre-clinical drugs in vivo. Myofibre necrosis, blood serum CK and oxidative stress (specifically the ratio of oxidised to reduced protein thiols) are reliable markers of muscle damage after exercise; many parameters demonstrated high biological variation including changes in mRNA levels for key inflammatory cytokines in muscle. The sampling (sacrifice and tissue collection) time after exercise for these parameters is critical. A more precise understanding of the changes in dystrophic muscle after exercise aims to identify biomarkers and new potential therapeutic drug targets for Duchenne Muscular Dystrophy.

An in vivo rodent model of contraction-induced injury and non-invasive monitoring of recovery

Muscle strains are one of the most common complaints treated by physicians. A muscle injury is typically diagnosed from the patient history and physical exam alone, however the clinical presentation can vary greatly depending on the extent of injury, the patient's pain tolerance, etc. In patients with muscle injury or muscle disease, assessment of muscle damage is typically limited to clinical signs, such as tenderness, strength, range of motion, and more recently, imaging studies. Biological markers, such as serum creatine kinase levels, are typically elevated with muscle injury, but their levels do not always correlate with the loss of force production. This is even true of histological findings from animals, which provide a "direct measure" of damage, but do not account for all the loss of function. Some have argued that the most comprehensive measure of the overall health of the muscle in contractile force. Because muscle injury is a random event that occurs under a variety of biomechanical conditions, it is difficult to study. Here, we describe an in vivo animal model to measure torque and to produce a reliable muscle injury. We also describe our model for measurement of force from an isolated muscle in situ. Furthermore, we describe our small animal MRI procedure.

Translational research and therapeutic perspectives in dysferlinopathies

Dysferlinopathies are autosomal recessive disorders caused by mutations in the dysferlin (DYSF) gene, encoding the dysferlin protein. DYSF mutations lead to a wide range of muscular phenotypes, with the most prominent being Miyoshi myopathy (MM) and limb girdle muscular dystrophy type 2B (LGMD2B) and the second most common being LGMD. Symptoms generally appear at the end of childhood and, although disease progression is typically slow, walking impairments eventually result. Dysferlin is a modular type II transmembrane protein for which numerous binding partners have been identified. Although dysferlin function is only partially elucidated, this large protein contains seven calcium sensor C2 domains, shown to play a key role in muscle membrane repair. On the basis of this major function, along with detailed clinical observations, it has been possible to design various therapeutic approaches for dysferlin-deficient patients. Among them, exon-skipping and minigene transfer strategies have been evaluated at the preclinical level and, to date, represent promising approaches for clinical trials. This review aims to summarize the pathophysiology of dysferlinopathies and to evaluate the therapeutic potential for treatments currently under development.

Muscle membrane repair and inflammatory attack in dysferlinopathy

Repair of plasma membrane tears is an important normal physiological process that enables the cells to survive a variety of physiological and pathological membrane lesions. Dysferlin was the first protein reported to play a crucial role in this repair process in muscle, and recently, several other proteins including Mitsugumin 53 (MG53), annexin and calpain were also found to participate. These findings have now established the framework of the membrane repair mechanism. Defective membrane repair in dysferlin-deficient muscle leads to the development of muscular dystrophy associated with remarkable muscle inflammation. Recent studies have demonstrated a crosstalk between defective membrane repair and immunological attack, thus unveiling a new pathophysiological mechanism of dysferlinopathy. Here I summarize and discuss the latest progress in the molecular mechanisms of membrane repair and the pathogenesis of dysferlinopathy. Discussion about potential therapeutic applications of these findings is also provided.

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.

Ferlin proteins in myoblast fusion and muscle growth

Myoblast fusion contributes to muscle growth in development and during regeneration of mature muscle. Myoblasts fuse to each other as well as to multinucleate myotubes to enlarge the myofiber. The molecular mechanisms of myoblast fusion are incompletely understood. Adhesion, apposition, and membrane fusion are accompanied by cytoskeletal rearrangements. The ferlin family of proteins is implicated in human muscle disease and has been implicated in fusion events in muscle, including myoblast fusion, vesicle trafficking and membrane repair. Dysferlin was the first mammalian ferlin identified and it is now known that there are six different ferlins. Loss-of-function mutations in the dysferlin gene lead to limb girdle muscular dystrophy and the milder disorder Miyoshi Myopathy. Dysferlin is a membrane-associated protein that has been implicated in resealing disruptions in the muscle plasma membrane. Newer data supports a broader role for dysferlin in intracellular vesicular movement, a process also important for resealing. Myoferlin is highly expressed in myoblasts that undergoing fusion, and the absence of myoferlin leads to impaired myoblast fusion. Myoferlin also regulates intracellular trafficking events, including endocytic recycling, a process where internalized vesicles are returned to the plasma membrane. The trafficking role of ferlin proteins is reviewed herein with a specific focus as to how this machinery alters myogenesis and muscle growth.

Genetic ablation of complement C3 attenuates muscle pathology in dysferlin-deficient mice

Mutations in the dysferlin gene underlie a group of autosomal recessive muscle-wasting disorders denoted as dysferlinopathies. Dysferlin has been shown to play roles in muscle membrane repair and muscle regeneration, both of which require vesicle-membrane fusion. However, the mechanism by which muscle becomes dystrophic in these disorders remains poorly understood. Although muscle inflammation is widely recognized in dysferlinopathy and dysferlin is expressed in immune cells, the contribution of the immune system to the pathology of dysferlinopathy remains to be fully explored. Here, we show that the complement system plays an important role in muscle pathology in dysferlinopathy. Dysferlin deficiency led to increased expression of complement factors in muscle, while muscle-specific transgenic expression of dysferlin normalized the expression of complement factors and eliminated the dystrophic phenotype present in dysferlin-null mice. Furthermore, genetic disruption of the central component (C3) of the complement system ameliorated muscle pathology in dysferlin-deficient mice but had no significant beneficial effect in a genetically distinct model of muscular dystrophy, mdx mice. These results demonstrate that complement-mediated muscle injury is central to the pathogenesis of dysferlinopathy and suggest that targeting the complement system might serve as a therapeutic approach for this disease.