Force impairment in calpain 3-deficient mice is not correlated with mechanical disruption

Systemic delivery of AAVrh74.tMCK.hCAPN3 rescues the phenotype in a mouse model for LGMD2A/R1

Limb girdle muscular dystrophy (LGMD) 2A/R1, caused by mutations in the CAPN3 gene and CAPN3 loss of function, is known to play a role in disease pathogenicity. In this study, AAVrh74.tMCK.CAPN3 was delivered systemically to two different age groups of CAPN3 knockout (KO) mice; each group included two treatment cohorts receiving low (1.17 × 10¹⁴ vg/kg) and high (2.35 × 10¹⁴ vg/kg) doses of the vector and untreated controls. Treatment efficacy was tested 20 weeks after gene delivery using functional (treadmill), physiological (in vivo muscle contractility assay), and histopathological outcomes. AAV.CAPN3 gene therapy resulted in significant, robust improvements in functional outcomes and muscle physiology at low and high doses in both age groups. Histological analyses of skeletal muscle showed remodeling of muscle, a switch to fatigue-resistant oxidative fibers in females, and fiber size increases in both sexes. Safety studies revealed no organ tissue abnormalities; specifically, there was no histopathological evidence of cardiotoxicity. These results show that CAPN3 gene replacement therapy improved the phenotype in the CAPN3 KO mouse model at both doses independent of age at the time of vector administration. The improvements were supported by an absence of cardiotoxicity, showing the efficacy and safety of the AAV.CAPN3 vector as a potential gene therapy for LGMDR1.

Pharmacological blockade of TNFα prevents sarcopenia and prolongs survival in aging mice

Sarcopenia is a hallmark of aging. Inflammation due to increased generation of cytokines such as TNFα, IL-1β and IL-6 has been implicated in the pathogenesis of sarcopenia. In skeletal muscle of C57BL/6 mice from 12 until 28 months of age, we observed a progressive reduction of myofiber cross sectional area, loss of type II fibers and infiltration by inflammatory cells. Muscle strength decreased in parallel. Pharmacological TNFα blockade by weekly subcutaneous injection of Etanercept from 16 to 28 months of age prevented atrophy and loss of type II fibers, with significant improvements in muscle function and mice lifespan. The effects on leukocyte recruitment were limited. These results provide a proof of principle that endogenous TNFα is sufficient to cause sarcopenia and to reduce animal survival, and open a novel perspective on novel potential pharmacological treatment strategies based on TNFα blockade to prevent the noxious events associated with aging.

Gene Correction of LGMD2A Patient-Specific iPSCs for the Development of Targeted Autologous Cell Therapy

Limb girdle muscular dystrophy type 2A (LGMD2A), caused by mutations in the Calpain 3 (CAPN3) gene, is an incurable autosomal recessive disorder that results in muscle wasting and loss of ambulation. To test the feasibility of an autologous induced pluripotent stem cell (iPSC)-based therapy for LGMD2A, here we applied CRISPR-Cas9-mediated genome editing to iPSCs from three LGMD2A patients to enable correction of mutations in the CAPN3 gene. Using a gene knockin approach, we genome edited iPSCs carrying three different CAPN3 mutations, and we demonstrated the rescue of CAPN3 protein in myotube derivatives in vitro. Transplantation of gene-corrected LGMD2A myogenic progenitors in a novel mouse model combining immunodeficiency and a lack of CAPN3 resulted in muscle engraftment and rescue of the CAPN3 mRNA. Thus, we provide here proof of concept for the integration of genome editing and iPSC technologies to develop a novel autologous cell therapy for LGMD2A.

Intravenous Administration of a MTMR2-Encoding AAV Vector Ameliorates the Phenotype of Myotubular Myopathy in Mice

X-linked myotubular myopathy (XLMTM) is a severe congenital disorder in male infants that leads to generalized skeletal muscle weakness and is frequently associated with fatal respiratory failure. XLMTM is caused by loss-of-function mutations in the MTM1 gene, which encodes myotubularin, the founder member of a family of 15 homologous proteins in mammals. We recently demonstrated the therapeutic efficacy of intravenous delivery of rAAV vectors expressing MTM1 in animal models of myotubular myopathy. Here, we tested whether the closest homologues of MTM1, MTMR1, and MTMR2 (the latter being implicated in Charcot-Marie-Tooth neuropathy type 4B1) are functionally redundant and could represent a therapeutic target for XLMTM. Serotype 9 recombinant AAV vectors encoding either MTM1, MTMR1, or MTMR2 were injected into the tibialis anterior muscle of Mtm1-deficient knockout mice. Two weeks after vector delivery, a therapeutic effect was observed with Mtm1 and Mtmr2, but not Mtmr1; with Mtm1 being the most efficacious transgene. Furthermore, intravenous administration of a single dose of the rAAV9-Mtmr2 vector in XLMTM mice improved the motor activity and muscle strength and prolonged survival throughout a 3-month study. These results indicate that strategies aiming at increasing MTMR2 expression levels in skeletal muscle may be beneficial in the treatment of myotubular myopathy.

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.

Reduced skeletal muscle fiber size following caloric restriction is associated with calpain-mediated proteolysis and attenuation of IGF-1 signaling

Caloric restriction decreases skeletal muscle mass in mammals, principally due to a reduction in fiber size. The effect of suboptimal nutrient intake on skeletal muscle metabolic properties in neonatal calves was examined. The longissimus muscle (LM) was collected after a control (CON) or caloric restricted (CR) diet was cosnumed for 8 wk and muscle fiber size, gene expression, and metabolic signal transduction activity were measured. Results revealed that CR animals had smaller (P < 0.05) LM fiber cross-sectional area than CON, as expected. Western blot analysis detected equivalent amounts of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) but reduced (P < 0.05) amounts of the splice-variant, PGC1α-4 in CR LM. Expression of IGF-1, a PGC1α-4 target gene, was 40% less (P < 0.05) in CR than CON. Downstream mediators of autocrine IGF-1 signaling also are attenuated in CR by comparison with CON. The amount of phosphorylated AKT1 was less (P < 0.05) in CR than CON. The ratio of p4EBP1^T37/46 to total 4EBP1, a downstream mediator of AKT1, did not differ between CON and CR. By contrast, protein lysates from CR LM contained less (P < 0.05) total glycogen synthase kinase-3β (GSK3β) and phosphorylated GSK3β than CON LM, suggesting blunted protein synthesis. Smaller CR LM fiber size associates with increased (P < 0.05) calpain 1 (CAPN1) activity coupled with lower (P < 0.05) expression of calpastatin, the endogenous inhibitor of CAPN1. Atrogin-1 and MuRF expression and autophagy components were unaffected by CR. Thus CR suppresses the hypertrophic PGC1α-4/IGF-1/AKT1 pathway while promoting activation of the calpain system.

Deletion of Mbtps1 (Pcsk8, S1p, Ski-1) Gene in Osteocytes Stimulates Soleus Muscle Regeneration and Increased Size and Contractile Force with Age

Conditional deletion of Mbtps1 (cKO) protease in bone osteocytes leads to an age-related increase in mass (12%) and in contractile force (30%) in adult slow twitch soleus muscles (SOL) with no effect on fast twitch extensor digitorum longus muscles. Surprisingly, bone from 10-12-month-old cKO animals was indistinguishable from controls in size, density, and morphology except for a 25% increase in stiffness. cKO SOL exhibited increased expression of Pax7, Myog, Myod1, Notch, and Myh3 and 6-fold more centralized nuclei, characteristics of postnatal regenerating muscle, but only in type I myosin heavy chain-expressing cells. Increased expression of gene pathways mediating EGF receptor signaling, circadian exercise, striated muscle contraction, and lipid and carbohydrate oxidative metabolism were also observed in cKO SOL. This muscle phenotype was not observed in 3-month-old mice. Although Mbtps1 mRNA and protein expression was reduced in cKO bone osteocytes, no differences in Mbtps1 or cre recombinase expression were observed in cKO SOL, explaining this age-related phenotype. Understanding bone-muscle cross-talk may provide a fresh and novel approach to prevention and treatment of age-related muscle loss.

PLEIAD/SIMC1/C5orf25, a novel autolysis regulator for a skeletal-muscle-specific calpain, CAPN3, scaffolds a CAPN3 substrate, CTBP1

CAPN3/p94/calpain-3 is a skeletal-muscle-specific member of the calpain protease family. Multiple muscle cell functions have been reported for CAPN3, and mutations in this protease cause limb-girdle muscular dystrophy type 2A. Little is known about the molecular mechanisms that allow CAPN3 to be so multifunctional. One hypothesis is that the very rapid and exhaustive autolytic activity of CAPN3 needs to be suppressed by dynamic molecular interactions for specific periods of time. The previously identified interaction between CAPN3 and connectin/titin, a giant molecule in muscle sarcomeres, supports this assumption; however, the regulatory mechanisms of non-sarcomere-associated CAPN3 are unknown. Here, we report that a novel CAPN3-binding protein, PLEIAD [Platform element for inhibition of autolytic degradation; originally called SIMC1/C5orf25 (SUMO-interacting motif containing protein 1/chromosome 5open reading frame 25)], suppresses the protease activity of CAPN3. Database analyses showed that PLEIAD homologs, like CAPN3 homologs, are evolutionarily conserved in vertebrates. Furthermore, we found that PLEIAD also interacts with CTBP1 (C-terminal binding protein 1), a transcriptional co-regulator, and CTBP1 is proteolyzed in COS7 cells expressing CAPN3. The identified cleavage sites in CTBP1 suggested that it undergoes functional modification upon its proteolysis by CAPN3, as well as by conventional calpains. These results indicate that PLEIAD can shift its major function from CAPN3 suppression to CAPN3-substrate recruitment, depending on the cellular context. Taken together, our data suggest that PLEIAD is a novel regulatory scaffold for CAPN3, as reflected in its name.

Animal models for metabolic, neuromuscular and ophthalmological rare diseases

Animal models are important tools in the discovery and development of treatments for rare diseases, particularly given the small populations of patients in which to evaluate therapeutic candidates. Here, we provide a compilation of mammalian animal models for metabolic, neuromuscular and ophthalmological orphan-designated conditions based on information gathered by the European Medicines Agency's Committee for Orphan Medicinal Products (COMP) since its establishment in 2000, as well as from a review of the literature. We discuss the predictive value of the models and their advantages and limitations with the aim of highlighting those that are appropriate for the preclinical evaluation of novel therapies, thereby facilitating further drug development for rare diseases.

Soluble activin receptor type IIB increases forward pulling tension in the mdx mouse

INTRODUCTION

In this study we investigated the action of RAP-031, a soluble activin receptor type IIB (ActRIIB) comprised of a form of the ActRIIB extracellular domain linked to a murine Fc, and the NF-κB inhibitor, ursodeoxycholic acid (UDCA), on the whole body strength of mdx mice.

METHODS

The whole body tension (WBT) method of assessing the forward pulling tension (FPT) exerted by dystrophic (mdx) mice was used.

RESULTS

RAP-031 produced a 41% increase in body mass and a 42.5% increase in FPT without altering the FPT normalized for body mass (WBT). Coadministration of RAP-031 with UDCA produced increases in FPT that were associated with an increase in WBT.

CONCLUSIONS

Myostatin inhibition increases muscle mass without altering the fundamental weakness characteristic of dystrophic muscle. Cotreatment with an NF-κB inhibitor potentiates the effects of myostatin inhibition in improving FPT in mdx mice.

Limb-girdle muscular dystrophy 2A

Limb-girdle muscular dystrophy type 2A (LGMD2A) is caused by mutations in the gene CAPN3 located in the chromosome region 15q15.1-q21.1. To date more than 300 mutations have been described. This gene encodes for a 94-kDa nonlysosomal calcium-dependent cysteine protease and its function in skeletal muscle is not fully understood. It seems that calpain-3 has an unusual zymogenic activation that involves, among other substrates, cytoskeletal proteins. Calpain-3 is thought to interact with titin and dysferlin. Calpain-3 deficiency produces abnormal sarcomeres that lead eventually to muscle fiber death. Hip adductors and gluteus maximus are the earliest clinically affected muscles. No clinical differences have been reported depending on the type of mutation in the CAPN3 gene. The muscle biopsy shows variability of fiber size, interstitial fibrosis, internal nuclei, lobulated fibers, and, in some cases, presence of eosinophils. Recent gene expression profiling studies have shown upregulation of interleukin-32 and immunoglobulin genes, which may explain the eosinophilic infiltration. Two mouse knockout models of CAPN3 have been characterized. There are no curative treatments for this disease. However, experimental therapeutics using mouse models conclude that adeno-associated virus (AAV) vectors seem to be one of the best approaches because of their efficiency and persistency of gene transfer.

Removal of the calpain 3 protease reverses the myopathology in a mouse model for titinopathies

The dominant tibial muscular dystrophy (TMD) and recessive limb-girdle muscular dystrophy 2J are allelic disorders caused by mutations in the C-terminus of titin, a giant sarcomeric protein. Both clinical presentations were initially identified in a large Finnish family and linked to a founder mutation (FINmaj). To further understand the physiopathology of these two diseases, we generated a mouse model carrying the FINmaj mutation. In heterozygous mice, dystrophic myopathology appears late at 9 months of age in few distal muscles. In homozygous (HO) mice, the first signs appear in the Soleus at 1 month of age and extend to most muscles at 6 months of age. Interestingly, the heart is also severely affected in HO mice. The mutation leads to the loss of the very C-terminal end of titin and to a secondary deficiency of calpain 3, a partner of titin. By crossing the FINmaj model with a calpain 3-deficient model, the TMD phenotype was corrected, demonstrating a participation of calpain 3 in the pathogenesis of this disease.

AAV-directed muscular dystrophy gene therapy

IMPORTANCE OF THE FIELD

Muscle-directed gene therapy for genetic muscle diseases can be performed by the recombinant adeno-associated viral (rAAV) vector delivery system to achieve long-term therapeutic gene transfer in all affected muscles.

AREAS COVERED IN THIS REVIEW

Recent progress in rAAV-vector-mediated muscle-directed gene transfer and associated techniques for the treatment of muscular dystrophies (MD). The review covers literature from the past 2 - 3 years.

WHAT THE READER WILL GAIN

rAAV-directed muscular dystrophy gene therapy can be achieved by mini-dystrophin replacement and exon-skipping strategies. The additional strategies of enhancing muscle regeneration and reducing inflammation in the muscle micro-environment should be useful to optimize therapeutic efficacy. This review compares the merits and shortcomings of different administration methods, promoters and experimental animals that will guide the choice of the appropriate strategy for clinical trials.

TAKE HOME MESSAGE

Restoration of muscle histopathology and function has been performed using rAAV systemic gene delivery. In addition, the combination of gene replacement and adjuvant therapies in the future may be beneficial with regard to improving muscle regeneration and decreasing myofiber necrosis. The challenges faced by large animal model studies and in human trials arise from gene transfer efficiency and immune response, which may be overcome by optimizing the rAAV vectors utilized and the administration methods.

A simple protocol for assessing inter-trial and inter-examiner reliability for two noninvasive measures of limb muscle strength

Noninvasive measures of limb muscle strength are quite useful in preclinical translational studies that use mouse models of muscle disease, peripheral nerve disease, and movement disorders. The present study uses a simple protocol for assessing both inter-trial and inter-examiner reliability for two noninvasive methods of assessing limb strength in dystrophic (mdx) and wild type mice. One method, termed the whole body tension (WBT) method or escape test, measures the total phasic pulling tension exerted by the fore- and hindlimbs while a mouse attempts to escape into a darkened tube. Another procedure, termed the four limb wire grid holding test, measures the minimal amount of sustained tension (physical impulse) exerted by the fore- and hindlimbs while the mouse hangs suspended in an upside-down position. A comparison of the two methods revealed significant inter-trial and inter-examiner correlations in each procedure, although the WBT procedure consistently produced higher correlations than the four limb wire grid holding test. Inter-trial reliability for each test was higher than inter-examiner reliability, indicating that each longitudinal series of tests is best performed by a single investigator. The holding test also did not consistently detect differences between wild type and mdx populations at ages greater than 4 months. These results demonstrate the utility of a simple protocol for assessing the reliability of noninvasive tests that measure limb strength, and should be useful in comparing different functional measures in a broad range of translational studies.

Calpains, skeletal muscle function and exercise

1. Skeletal muscle fibres contain ubiquitous (mu-calpain and m-calpain) and muscle-specific (calpain-3) Ca(2+)-dependent proteases. The physiological roles of the calpains are not well understood, although ubiquitous calpains have been associated with apoptosis and myogenesis and calpain-3 is likely involved in sarcomeric remodelling. A defect in the expression of calpain-3 results in limb-girdle muscular dystrophy Type 2A. 2. At resting [Ca(2+)](i), calpains are present predominantly in their full-length, unautolysed/unactivated forms. Once activated, mu-calpain and calpain-3 appear in their autolysed forms and this measurement can be used to determine when in vivo activation occurs. Endogenously expressed mu-calpain and calpain-3 are activated within a physiological [Ca(2+)] range in a Ca(2+)- and time-dependent manner. 3. In skeletal muscle, mu-calpain is a freely diffusible protein that binds rapidly when [Ca(2+)](i) is increased. Calpain-3 is tightly bound in skeletal muscle fibres at the N2A line of the large elastic protein titin. 4. Overall, neither mu-calpain nor calpain-3 are activated immediately following sprint, endurance or eccentric exercise, despite the frequent episodes of high cytoplasmic [Ca(2+)] that would occur during these types of muscle contractions. Importantly, however, a substantial proportion of calpain-3, but not mu-calpain, is activated 24 h after a single bout of eccentric exercise. 5. In vitro studies have shown that calpain-3 becomes activated if exposed for a prolonged period of time (> 1 h) to resting cytoplasmic [Ca(2+)] that are approximately two- to fourfold higher than normal. This suggests that the small but sustained increase in [Ca(2+)](i) that likely occurs after eccentric contractions is both high and long enough to result in calpain-3 activation and supports the role for calpain-3 in sarcomeric remodelling.

Therapeutic possibilities in the autosomal recessive limb-girdle muscular dystrophies

Fourteen years ago, the first disease-causing mutation in a form of autosomal recessive limb-girdle muscular dystrophy was reported. Since then the number of genes has been extended to at least 14 and the phenotypic spectrum has been broadened. The generation of mouse models helped to improve our understanding of the pathogenesis of the disease and also served to study therapeutic possibilities. All autosomal recessive limb-girdle muscular dystrophies are rare diseases, which is one reason why there have been so very few controlled clinical trials. Other reasons are insufficient natural history data and the lack of standardized assessment criteria and validated outcome measures. Currently, therapeutic possibilities are mainly restricted to symptomatic treatment and the treatment of disease complications. On the other hand, new efforts in translational research and the development of molecular therapeutic approaches suggest that more promising clinical trials will be carried out in autosomal recessive limb-girdle muscular dystrophy in the next several years.

AAV-mediated intramuscular delivery of myotubularin corrects the myotubular myopathy phenotype in targeted murine muscle and suggests a function in plasma membrane homeostasis

Myotubular myopathy (XLMTM, OMIM 310400) is a severe congenital muscular disease due to mutations in the myotubularin gene (MTM1) and characterized by the presence of small myofibers with frequent occurrence of central nuclei. Myotubularin is a ubiquitously expressed phosphoinositide phosphatase with a muscle-specific role in man and mouse that is poorly understood. No specific treatment exists to date for patients with myotubular myopathy. We have constructed an adeno-associated virus (AAV) vector expressing myotubularin in order to test its therapeutic potential in a XLMTM mouse model. We show that a single intramuscular injection of this vector in symptomatic Mtm1-deficient mice ameliorates the pathological phenotype in the targeted muscle. Myotubularin replacement in mice largely corrects nuclei and mitochondria positioning in myofibers and leads to a strong increase in muscle volume and recovery of the contractile force. In addition, we used this AAV vector to overexpress myotubularin in wild-type skeletal muscle and get insight into its localization and function. We show that a substantial proportion of myotubularin associates with the sarcolemma and I band, including triads. Myotubularin overexpression in muscle induces the accumulation of packed membrane saccules and presence of vacuoles that contain markers of sarcolemma and T-tubules, suggesting that myotubularin is involved in plasma membrane homeostasis of myofibers. This study provides a proof-of-principle that local delivery of an AAV vector expressing myotubularin can improve the motor capacities of XLMTM muscle and represents a novel approach to study myotubularin function in skeletal muscle.

Genetic ablation of acetylcholinesterase alters muscle function in mice

Although acetylcholinesterase (AChE) knockout mice survive, they have abnormal neuromuscular function. We analysed further the effects of the mutation on hind limb muscle contractile properties. Tibialis anterior muscle from AChE KO mice is unable to maintain tension during a short period of repetitive nerve stimulation (tetanic fade) and has an increased twitch tension in response to a single nerve electric stimulation. In response to direct muscle stimulation, we found that maximal velocity of shortening of soleus muscle is increased and maximum tetanic force is decreased in AchE KO mice versus control animals. As the contractile properties of the soleus muscle were altered by AChE ablation, our results suggest cellular and molecular changes in AChE ablated muscle containing both fast and slow muscle fibres.

Structural and functional alterations of muscle fibres in the novel mouse model of facioscapulohumeral muscular dystrophy

We recently generated a mouse model of facioscapulohumeral muscular dystrophy (FSHD) by selectively overexpressing FRG1, a candidate gene for FSHD, in skeletal muscle. The muscles of the FRG-1 mice did not show any plasmamembrane defect suggesting a novel pathogenetic mechanism for FSHD. Here, we study structure and function of muscle fibres from three lines of mice overexpressing FRG1 at different levels: FRG1-low, FRG1-med, FRG1-high. Cross-sectional area (CSA), specific force (Po/CSA) and maximum shortening velocity (V(o)) of identified types of muscle fibres from FRG1-low and FRG1-med mice were analysed and found to be lower than in WT mice. Fast fibres and especially type 2B fibres (the fastest type) were preferentially involved in the dystrophic process showing a much larger force deficit than type 1 (slow) fibres. Consistent with the latter observation, the MHC isoform distribution of several muscles of the three FRG1 lines showed a shift towards slower MHC isoforms in comparison to WT muscle. Moreover, fast muscles showed a more evident histological deterioration, a larger atrophy and a higher percentage of centrally nucleated fibres than the soleus, the slowest muscle in mice. Interestingly, loss in CSA, Po/CSA and V(o) of single muscle fibres and MHC isoform shift towards a slower phenotype can be considered early signs of muscular dystrophy (MD). They were, in fact, found also in FRG1-low mice which did not show any impairment of function in vivo and of muscle size in vitro and in soleus muscles, which had a completely preserved morphology. This study provides a detailed characterization of structure and function of muscle fibres in a novel murine model of one of the main human MDs and suggests that fundamental features of the dystrophic process, common to most MDs, such as the intrinsic loss of contractile strength of muscle fibres, the preferential involvement of fast fibres and the shift towards a slow muscle phenotype can occur independently from obvious alterations of the plasma membrane.

Calpain-3 is autolyzed and hence activated in human skeletal muscle 24 h following a single bout of eccentric exercise

The function and normal regulation of calpain-3, a muscle-specific Ca(2+)-dependent protease, is uncertain, although its absence leads to limb-girdle muscular dystrophy type 2A. This study examined the effect of eccentric exercise on calpain-3 autolytic activation, because such exercise is known to damage sarcomeric structures and to trigger adaptive changes that help prevent such damage on subsequent exercise. Six healthy human subjects performed a 30-min bout of one-legged, eccentric, knee extensor exercise. Torque measurements, vastus lateralis muscle biopsies, and venous blood samples were taken before and up to 7 days following the exercise. Peak isometric muscle torque was depressed immediately and at 3 h postexercise and recovered by 24 h, and serum creatine kinase concentration peaked at 24 h postexercise. The amount of autolyzed calpain-3 was unchanged immediately and 3 h after exercise, but increased markedly (from approximately 16% to approximately 35% of total) 24 h after the exercise, and returned to preexercise levels within 7 days. In contrast, the eccentric exercise produced little autolytic activation of the ubiquitous Ca(2+)-activated protease, mu-calpain. Eccentric exercise is the first physiological circumstance shown to result in calpain-3 activation in vivo.

AAV-mediated delivery of a mutated myostatin propeptide ameliorates calpain 3 but not α-sarcoglycan deficiency

Myostatin is a negative regulator of muscle mass whose inhibition has been proposed as a therapeutic strategy for muscle-wasting conditions. Indeed, blocking myostatin action through different strategies has proved beneficial for the pathophysiology of the dystrophin-deficient mdx mouse. In this report, we tested the inhibition of myostatin by AAV-mediated expression of a mutated propeptide in animal models of two limb-girdle muscular dystrophies: LGMD2A caused by mutations in the calpain 3 (CAPN3) gene and LGMD2D caused by mutations in the alpha-sarcoglycan gene (SGCA). In the highly regenerative Sgca-null mice, survival of the alpha-sarcoglycan-deficient muscle fibers did not improve after transfer of the myostatin propeptide. In calpain 3-deficient mice, a boost in muscle mass and an increase in absolute force were obtained, suggesting that myostatin inhibition could constitute a therapeutic strategy in this predominantly atrophic disorder.

A third of LGMD2A biopsies have normal calpain 3 proteolytic activity as determined by an in vitro assay

Limb-girdle muscular dystrophy type 2A (LGMD2A) is an autosomal recessive muscular disorder caused by mutations in the gene coding for calpain 3, a calcium-dependent protease. We developed an in vitro assay that can detect the proteolytic activity of calpain 3 in a muscle sample. This assay is based on the use of an inactive calpain 3 as a substrate for active calpain 3 molecules. A total of 79 human biopsies have been analysed using an unbiased single blind method. Results were confronted with the molecular diagnosis for confirmation. Proteolytic activity was either reduced or absent in 68% of LGMD2A biopsies. In the remaining 32%, normal proteolytic activity was found despite the presence of calpain 3 mutation(s), suggesting that other calpain 3 properties might be impaired to give rise to the LGMD2A phenotype. Our assay is easily adaptable to routine and appears to be more sensitive than common analysis by immunodetection.

Calpains and human disease

Calpains, particularly conventional dimeric calpains, have claimed to be involved in the cell degeneration processes that characterize numerous disease conditions linked to dysfunctions of cellular Ca2+ homeostasis. The evidence supporting their involvement has traditionally been indirect and circumstantial, but recent work has added more solid evidence supporting the role of ubiquitous dimeric calpains in the process of neurodegeneration. The only disease condition in which a calpain defect has been conclusively involved concerns an atypical monomeric calpain: the muscle specific calpain-3, also known as p94. Inactivating defects in its gene cause a muscular dystrophy termed LGMD-2A. The molecular mechanism by which the absence of the proteolytic activity of calpain-3 causes the dystrophic process is unknown. Another atypical calpain, which has been characterized recently as a Ca2(+)-dependent protease, calpain 10, appears To be involved in the etiology of type 2 diabetes. The involvement has been inferred essentially from genetic evidence. Also in the case of type 2 diabetes the molecular mechanisms that could link the disease to calpain 10 are unknown.

Phenotypic Correction of α-Sarcoglycan Deficiency by Intra-arterial Injection of a Muscle-specific Serotype 1 rAAV Vector

alpha-Sarcoglycanopathy (limb-girdle muscular dystrophy type 2D, LGMD2D) is a recessive muscular disorder caused by deficiency in alpha-sarcoglycan, a transmembrane protein part of the dystrophin-associated complex. To date, no treatment exists for this disease. We constructed recombinant pseudotype-1 adeno-associated virus (rAAV) vectors expressing the human alpha-sarcoglycan cDNA from a ubiquitous or a muscle-specific promoter. Evidence of specific immune response leading to disappearance of the vector was observed with the ubiquitous promoter. In contrast, efficient and sustained transgene expression with correct sarcolemmal localization and without evident toxicity was obtained with the muscle-specific promoter after intra-arterial injection into the limbs of an LGMD2D murine model. Transgene expression resulted in restoration of the sarcoglycan complex, histological improvement, membrane stabilization, and correction of pseudohypertrophy. More importantly, alpha-sarcoglycan transfer produced full rescue of the contractile force deficits and stretch sensibility and led to an increase of the global activity of the animals when both posterior limbs are injected. Our results establish the feasibility for AAV-mediated alpha-sarcoglycan gene transfer as a therapeutic approach.

Calpain 3: a key regulator of the sarcomere?

Calpain 3 is a 94-kDa calcium-dependent cysteine protease mainly expressed in skeletal muscle. In this tissue, it localizes at several regions of the sarcomere through binding to the giant protein, titin. Loss-of-function mutations in the calpain 3 gene have been associated with limb-girdle muscular dystrophy type 2A (LGMD2A), a common form of muscular dystrophy found world wide. Recently, significant progress has been made in understanding the mode of regulation and the possible function of calpain 3 in muscle. It is now well accepted that it has an unusual zymogenic activation and that cytoskeletal proteins are one class of its substrates. Through the absence of cleavage of these substrates, calpain 3 deficiency leads to abnormal sarcomeres, impairment of muscle contractile capacity, and death of the muscle fibers. These data indicate a role for calpain 3 as a chef d'orchestre in sarcomere remodeling and suggest a new category of LGMD2 pathological mechanisms.

Molecular and cellular basis of calpainopathy (limb girdle muscular dystrophy type 2A)

Limb girdle muscular dystrophy type 2A results from mutations in the gene encoding the calpain 3 protease. Mutations in this disease are inherited in an autosomal recessive fashion and result in progressive proximal skeletal muscle wasting but no cardiac abnormalities. Calpain 3 has been shown to proteolytically cleave a wide variety of cytoskeletal and myofibrillar proteins and to act upstream of the ubiquitin-proteasome pathway. In this review, we summarize the known biochemical and physiological features of calpain 3 and hypothesize why mutations result in disease.

Safety and efficacy of AAV-mediated calpain 3 gene transfer in a mouse model of limb-girdle muscular dystrophy type 2A

Calpainopathy (limb-girdle muscular dystrophy type 2A, LGMD2A) is a recessive muscular disorder caused by deficiency in the calcium-dependent cysteine protease calpain 3. To date, no treatment exists for this disease. We evaluated the potential of recombinant adeno-associated virus (rAAV) vectors for gene therapy in a murine model for LGMD2A. To drive the expression of calpain 3, we used rAAV2/1 pseudotyped vectors and muscle-specific promoters to avoid calpain 3 cell toxicity. We report efficient and stable transgene expression in muscle with restoration of the proteolytic activity and without evident toxicity. In addition, calpain 3 was correctly targeted to the sarcomere. Moreover, its presence resulted in improvement of the histological features and in therapeutic efficacy at the physiological levels, including correction of atrophy and full rescue of the contractile force deficits. Our results establish the feasibility of AAV-mediated calpain 3 gene transfer as a therapeutic approach.

Calpain 3 is activated through autolysis within the active site and lyses sarcomeric and sarcolemmal components

Calpain 3 (Capn3) is known as the skeletal muscle-specific member of the calpains, a family of intracellular nonlysosomal cysteine proteases. This enigmatic protease has many unique features among the calpain family and, importantly, mutations in Capn3 have been shown to be responsible for limb girdle muscular dystrophy type 2A. Here we demonstrate that the Capn3 activation mechanism is similar to the universal activation of caspases and corresponds to an autolysis within the active site of the protease. We undertook a search for substrates in immature muscle cells, as several lines of evidence suggest that Capn3 is mostly in an inactive state in muscle and needs a signal to be activated. In this model, Capn3 proteolytic activity leads to disruption of the actin cytoskeleton and disorganization of focal adhesions through cleavage of several endogenous proteins. In addition, we show that titin, a previously identified Capn3 partner, and filamin C are further substrates of Capn3. Finally, we report that Capn3 colocalizes in vivo with its substrates at various sites along cytoskeletal structures. We propose that Capn3-mediated cleavage produces an adaptive response of muscle cells to external and/or internal stimuli, establishing Capn3 as a muscle cytoskeleton regulator.