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

BCL2L13 at endoplasmic reticulum-mitochondria contact sites regulates calcium homeostasis to maintain skeletal muscle function

The physical connection between mitochondria and endoplasmic reticulum (ER) is an essential signaling hub to ensure organelle and cellular functions. In skeletal muscle, ER-mitochondria calcium (Ca2+) signaling is crucial to maintain cellular homeostasis during physical activity. High expression of BCL2L13, a member of the BCL-2 family, was suggested as an adaptive response in endurance-trained human subjects. In adult zebrafish, we found that the loss of Bcl2l13 impairs skeletal muscle structure and function. Ca2+ signaling is altered in Bcl2l13 knockout animals and mitochondrial complexes activity is decreased. Organelle fractioning in mammalian cells shows BCL2L13 at mitochondria, ER, and mitochondria-associated membranes. ER-mitochondria contact sites number is not modified by BCL2L13 modulation, but knockdown of BCL2L13 in C2C12 cells changes cytosolic Ca2+ release and mitochondrial Ca2+ uptake. This suggests that BCL2L13 interaction with mitochondria and ER, and its role in Ca2+ signaling, contributes to proper skeletal muscle function.

Loss of Calpain 3 dysregulates store-operated calcium entry and its exercise response in mice

Limb-Girdle Muscular Dystrophy R1/2A (LGMD R1/2A) is caused by mutations in the CAPN3 gene encoding Calpain 3, a skeletal-muscle specific, Ca2+-dependent protease. Localization of Calpain 3 within the triad suggests it contributes to Ca2+ homeostasis. Through live-cell Ca2+ measurements, muscle mechanics, immunofluorescence, and electron microscopy (EM) in Capn3 deficient (C3KO) and wild-type (WT) mice, we determined whether loss of Calpain 3 altered Store-Operated Calcium Entry (SOCE) activity. Direct Ca2+ influx measurements revealed loss of Capn3 elicits elevated resting SOCE and increased resting cytosolic Ca2+, supported by high incidence of calcium entry units (CEUs) observed by EM. C3KO and WT mice were subjected to a single bout of treadmill running to elicit SOCE. Within 1HR post-treadmill running, C3KO mice exhibited diminished force production in extensor digitorum longus muscles and a greater decay of Ca2+ transients in flexor digitorum brevis muscle fibers during repetitive stimulation. Striking evidence for impaired exercise-induced SOCE activation in C3KO mice included poor colocalization of key SOCE proteins, stromal-interacting molecule 1 (STIM1) and ORAI1, combined with disappearance of CEUs in C3KO muscles. These results demonstrate that Calpain 3 is a key regulator of SOCE in skeletal muscle and identify SOCE dysregulation as a contributing factor to LGMD R1/2A pathology.

Diagnosis of Two Unrelated Syndromes of Prader-Willi and Calpainopathy: Insight from Trio Whole Genome Analysis and Isodisomy Mapping

PURPOSE

An investigation for the co-occurrence of two unrelated genetic disorders of muscular dystrophy and Prader-Willi syndrome (PWS) (OMIM#176270) using joint whole genome sequencing (WGS).

METHODS

Trio WGS joint analysis was performed to investigate the genetic etiology in a proband with PWS, prolonged muscular hypotonia associated hyperCKemia, and early-onset obesity. The parents were unaffected.

RESULTS

Results showed maternal isodisomy uniparental disomy (UPD) in chromosome 15, expanding from 15q11.2 to 15q22.2, including PWS regions at 15q11.2-15q13. Maternal heterodisomy was detected from 15q22.2 to 15q26.3. A pathogenic variant, NM_000070.3(CAPN3):c.550del (p.Thr184fs), was identified at 15q15.1 in a heterozygous state in the mother that was homozygous in the proband due to maternal isodisomy.

CONCLUSION

This is the first study of the concurrent molecular etiology of PWS and calpainopathy (OMIM#253600) in the same patient. This report highlights the utility of joint analysis and the need for the assessment of autosomal recessive disease in regions of isodisomy in patients with complex and unexplained phenotypes.

Human calpain-3 and its structural plasticity: dissociation of a homohexamer into dimers on binding titin

Calpain-3 is an intracellular Ca2+-dependent cysteine protease abundant in skeletal muscle. Its physiological role in the sarcomere is thought to include removing damaged muscle proteins after exercise. Loss-of-function mutations in its single-copy gene cause a dystrophy of the limb-girdle muscles. These mutations, of which there are over 500 in humans, are spread all along this 94-kDa multi-domain protein that includes three 40+-residue sequences (NS, IS1, and IS2). The latter sequences are unique to this calpain isoform and are hypersensitive to proteolysis. To investigate the whole enzyme structure and how mutations might affect its activity, we produce the proteolytically more stable 85-kDa calpain-3 ΔNS ΔIS1 form with a C129A inactivating mutation as a recombinant protein in E. coli. During size-exclusion chromatography, this calpain-3 was consistently eluted as a much larger 0.5-MDa complex rather than the expected 170-kDa dimer. Its size, which was confirmed by SEC-MALS, Blue Native PAGE, and AUC, made the complex amenable to single-particle cryo-EM analysis. From two data sets, we obtained a 3.85-Å reconstruction map that shows the complex is a trimer of calpain-3 dimers with six penta-EF-hand domains at its core. Calpain-3 has been reported to bind the N2A region of the giant muscle protein titin. When this 37-kDa region of titin was co-expressed with calpain-3 the multimer was reduced to a 320-kDa particle, which appears to be the calpain dimer bound to several copies of the titin fragment. We suggest that newly synthesized calpain-3 is kept as an inactive hexamer until it binds the N2A region of titin in the sarcomere, whereupon it dissociates into functional dimers.

Novel Biomarkers for Limb Girdle Muscular Dystrophy (LGMD)

OBJECTIVE

To identify novel biomarkers as an alternative diagnostic tool for limb girdle muscular dystrophy (LGMD).

BACKGROUND

LGMD encompasses a group of muscular dystrophies characterized by proximal muscles weakness, elevated CK levels and dystrophic findings on muscle biopsy. Heterozygous CAPN3 mutations are associated with autosomal dominant LGMD-4, while biallelic mutations can cause autosomal recessive LGMD-1. Diagnosis is currently often based on invasive methods requiring muscle biopsy or blood tests. In most cases Western blotting (WB) analysis from muscle biopsy is essential for a diagnosis, as muscle samples are currently the only known tissues to express the full-length CAPN3 isoform.

METHODS

We analyzed CAPN3 in a cohort including 60 LGMD patients. Selected patients underwent a complete neurological examination, electromyography, muscle biopsy, and skin biopsies for primary fibroblasts isolation. The amount of CAPN3 was evaluated by WB analysis in muscle and skin tissues. The total RNA isolated from muscle, fibroblast and urine was processed, and cDNA was used for qualitative analysis. The expression of CAPN3 was investigated by qRT-PCR. The CAPN3 3D structure has been visualized and analyzed using PyMOL.

RESULTS

Among our patients, seven different CAPN3 mutations were detected, of which two were novel. After sequencing CAPN3 transcripts from fibroblast and urine, we detected different CAPN3 isoforms surprisingly including the full-length transcript. We found comparable protein levels from fibroblasts and muscle tissue; in particular, patients harboring a novel CAPN3 mutation showed a 30% reduction in protein compared to controls from both tissues.

CONCLUSIONS

Our findings showed for the first time the presence of the CAPN3 full-length transcript in urine and skin samples. Moreover, we demonstrated surprisingly comparable CAPN3 protein levels between muscle and skin samples, thus allowing us to hypothesize the use of skin biopsy and probably of urine samples as an alternative less invasive method to assess the amount of CAPN3 when molecular diagnosis turns out to be inconclusive.

Management of a 25-Year-Old Female Patient With Limb-Girdle Muscular Dystrophy With Physiotherapy: A Case Report

Limb-girdle muscular dystrophy (LGMD) is a collection of neuromuscular diseases that develop gradually and are rare, genetically, and clinically diverse. The weakness in muscles affecting the shoulder and pelvic girdles is a defining feature of LGMD. Calpainopathy is another name for limb-girdle muscular dystrophy type 2A (LGMD2A). Limb-girdle muscular dystrophy type 2A results from alterations in the calpain-3 (CAPN3) gene, which results in a CAPN3 protein shortage. Gower's sign is most commonly found in LGMD2A. The prevalence ranges from one person in every 14,500 to one in every 123,000. We present a case of a 25-year-old hypotensive female patient who complained of weakness in all four limbs and easy fatigue with a positive Gower's sign. For subsequent management, the neurologist referred the patient to the physical therapy department. The physical therapy goals included enhanced muscle strength, increased joint mobility, reduced fatigue, normalizing gait, and building dynamic balance and postural stability. Diagnosing LGMD clinical variability is important, emphasizing the importance of precise subtype identification and tailoring therapy. Tackling specific muscular deficits and functional restrictions emerges as a critical component in the holistic care of LGMD by physiotherapists. Continuous monitoring and evaluation using appropriate scales and measurements are essential for tracking performance and tailoring treatment strategies. Regular follow-up consultations with the physiotherapist are needed to identify changes in an individual's health and alter the treatment plan accordingly.

Alternations of NF-κB Signaling by Natural Compounds in Muscle-Derived Cancers

The NF-κB-signaling pathway plays a crucial role in cancer progression, including muscle-derived cancers such as rhabdomyosarcoma or sarcoma. Several natural compounds have been studied for their ability to alter NF-κB signaling in these types of cancers. This review paper summarizes the current knowledge on the effects of natural compounds, including curcumin, resveratrol, quercetin, epigallocatechin-3-gallate, and berberine, on NF-κB signaling in muscle-derived cancers. These compounds have been shown to inhibit NF-κB signaling in rhabdomyosarcoma cells through various mechanisms, such as inhibiting the activation of the IKK complex and the NF-κB transcription factor. These findings suggest that natural compounds could be potential therapeutic agents for muscle-derived cancers. However, further research is needed to fully understand their mechanisms of action and potential clinical applications.

Modeling Patient-Specific Muscular Dystrophy Phenotypes and Therapeutic Responses in Reprogrammed Myotubes Engineered on Micromolded Gelatin Hydrogels

In vitro models of patient-derived muscle allow for more efficient development of genetic medicines for the muscular dystrophies, which often present mutation-specific pathologies. One popular strategy to generate patient-specific myotubes involves reprogramming dermal fibroblasts to a muscle lineage through MyoD induction. However, creating physiologically relevant, reproducible tissues exhibiting multinucleated, aligned myotubes with organized striations is dependent on the introduction of physicochemical cues that mimic the native muscle microenvironment. Here, we engineered patient-specific control and dystrophic muscle tissues in vitro by culturing and differentiating MyoD–directly reprogrammed fibroblasts isolated from one healthy control subject, three patients with Duchenne muscular dystrophy (DMD), and two Limb Girdle 2A/R1 (LGMD2A/R1) patients on micromolded gelatin hydrogels. Engineered DMD and LGMD2A/R1 tissues demonstrated varying levels of defects in α-actinin expression and organization relative to control, depending on the mutation. In genetically relevant DMD tissues amenable to mRNA reframing by targeting exon 44 or 45 exclusion, exposure to exon skipping antisense oligonucleotides modestly increased myotube coverage and alignment and rescued dystrophin protein expression. These findings highlight the value of engineered culture substrates in guiding the organization of reprogrammed patient fibroblasts into aligned muscle tissues, thereby extending their value as tools for exploration and dissection of the cellular and molecular basis of genetic muscle defects, rescue, and repair.

Molecular Diagnosis of Muscular Dystrophy Patients in Western Indian Population: A Comprehensive Mutation Analysis Using Amplicon Sequencing

Muscular Dystrophies (MDs) are a group of inherited diseases and heterogeneous in nature. To date, 40 different genes have been reported for the occurrence and/or progression of MDs. This study was conducted to demonstrate the application of next-generation sequencing (NGS) in developing a time-saving and cost-effective diagnostic method to detect single nucleotide variants (SNVs) and copy number variants (CNVs) in a single test. A total of 123 cases clinically suspected of MD were enrolled in this study. Amplicon panel-based diagnosis was carried out for 102 (DMD/BMD) cases and the results were further screened using multiplex ligation-dependent probe amplification (MLPA). Whilst in the case of LGMD (N = 19) and UMD (N = 2), only NGS panel-based analysis was carried out. We identified the large deletions in 74.50% (76/102) of the cases screened with query DMD or BMD. Further, the large deletion in CAPN3 gene (N = 3) and known SNV mutations (N = 4) were identified in LGMD patients. Together, the total diagnosis rate for this amplicon panel was 70.73% (87/123) which demonstrated the utility of panel-based diagnosis for high throughput, affordable, and time-saving diagnostic strategy. Collectively, present study demonstrates that the panel based NGS sequencing could be superior over to MLPA.

Targeted Next-Generation Sequencing Reveals Mutations in Non-coding Regions and Potential Regulatory Sequences of Calpain-3 Gene in Polish Limb-Girdle Muscular Dystrophy Patients

Limb–girdle muscular dystrophy type R1 (LGMDR1) is caused by mutations in CAPN3 and is the most common type of recessive LGMD. Even with the use of whole-exome sequencing (WES), only one mutant allele of CAPN3 is found in a significant number of LGMDR patients. This points to a role of non-coding, intronic or regulatory, sequence variants in the disease pathogenesis. Targeted sequencing of the whole CAPN3 gene including not only intronic, 3′ and 5′ UTRs but also potential regulatory regions was performed in 27 patients suspected with LGMDR1. This group included 13 patients with only one mutated CAPN3 allele detected previously with exome sequencing. A second rare variant in the non-coding part of CAPN3 was found in 11 of 13 patients with previously identified single mutation. Intronic mutations were found in 10 cases, with c.1746-20C>G variant present in seven patients. In addition, a large deletion of exons 2–8 was found in one patient. In the patients with no causative mutation previously found, we detected rare CAPN3 variants in 5 out of 10 patients and in two of them in a compound heterozygous state. Rare variants within putative regulatory sequences distant from the CAPN3 gene were found in 15 patients, although in 11 of these cases, other variants are deemed causative. The results indicate that intronic mutations are common in Polish LGMDR patients, and testing for non-coding mutations in CAPN3 should be performed in apparently single heterozygous patients.

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.

Mechanisms Applied by Protein Inhibitors to Inhibit Cysteine Proteases

Protein inhibitors of proteases are an important tool of nature to regulate and control proteolysis in living organisms under physiological and pathological conditions. In this review, we analyzed the mechanisms of inhibition of cysteine proteases on the basis of structural information and compiled kinetic data. The gathered structural data indicate that the protein fold is not a major obstacle for the evolution of a protease inhibitor. It appears that nature can convert almost any starting fold into an inhibitor of a protease. In addition, there appears to be no general rule governing the inhibitory mechanism. The structural data make it clear that the "lock and key" mechanism is a historical concept with limited validity. However, the analysis suggests that the shape of the active site cleft of proteases imposes some restraints. When the S1 binding site is shaped as a pocket buried in the structure of protease, inhibitors can apply substrate-like binding mechanisms. In contrast, when the S1 binding site is in part exposed to solvent, the substrate-like inhibition cannot be employed. It appears that all proteases, with the exception of papain-like proteases, belong to the first group of proteases. Finally, we show a number of examples and provide hints on how to engineer protein inhibitors.

Mutational spectrum of autosomal recessive limb-girdle muscular dystrophies in a cohort of 112 Iranian patients and reporting of a possible founder effect

Background

Limb-girdle muscular dystrophies are a group of genetically heterogeneous diseases that are inherited in both autosomal dominant (LGMDD) and autosomal recessive forms (LGMDR), the latter is more common especially in populations with high consanguineous marriages like Iran. In the present study, we aimed to investigate the genetic basis of patients who are suspicious of being affected by LGMDR.

DNA samples of 60 families suspected of LGMD were extracted from their whole blood. Four short tandem repeat (STR) markers for each candidate genes related to LGMD R1 (calpain3 related)- R6 (δ-sarcoglycan-related) were selected, and all these 24 STRs were applied in two sets of multiplex PCR. After autozygosity mapping, Sanger sequencing and variant analysis were done. Predicting identified variants’ effect was performed using in-silico tools, and they were interpreted according to the American College of Medical Genomics and Genetics (ACMG) guideline. MLPA was used for those patients who had large deletions.

Fresh muscle specimens were taken from subjects and were evaluated using the conventional panel of histochemical stains.

Results

forty out of sixty families showed homozygote haplotypes in CAPN3, DYSF, SGCA, and SGCB genes. The exons and intron-exon boundaries of the relevant genes were sequenced and totally 38 mutations including CAPN3 (n = 15), DYSF (n = 9), SGCB (n = 11), and SGCA (n = 3) were identified. Five out of them were novel. The most prevalent form of LGMDs in our study was calpainopathy followed by sarcoglycanopathy in which beta-sarcoglycanopathy was the most common form amongst them. Exon 2 deletion in the SGCB gene was the most frequent mutation in this study.

We also reported evidence of a possible founder effect in families with mutations in DYSF and SGCB genes. We also detected a large consanguineous family suffered from calpainopathy who showed allelic heterogeneity.

Conclusions

This study can expand our knowledge about the genetic spectrum of LGMD in Iran, and also suggest the probable founder effects in some Iranian subpopulations which confirming it with more sample size can facilitate our genetic diagnosis and genetic counseling.

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.

Divergent Features of Mitochondrial Deficiencies in LGMD2A Associated With Novel Calpain-3 Mutations

Limb girdle muscular dystrophy type 2A (LGMD2A) is an autosomal recessive disorder characterized by progressive muscle weakness and wasting. LGMD2A is caused by mutations in the calpain-3 gene (CAPN3) that encodes a Ca2+-dependent cysteine protease predominantly expressed in the skeletal muscle. Underlying pathological mechanisms have not yet been fully elucidated. Mitochondrial abnormalities have been variably reported in human subjects with LGMD2A and were more systematically evaluated in CAPN3-knocked out mouse models. We have combined histochemical, immunohistochemical, molecular, biochemical, and ultrastructural analyses in our study in order to better outline mitochondrial features in 2 LGMD2A patients with novel CAPN3-associated mutations. Both patients underwent detailed clinical evaluations, followed by muscle biopsies from the quadriceps muscles. The diagnosis of LGMD2A in both patients was first suspected on the basis of a typical clinical localization of the muscle weakness, and confirmed by molecular investigations. Two novel homozygous mutations, c.2242C>G (p.Arg748Gly) and c.291C>A (p.Phe97Leu) were identified: c.2242C>G (p.Arg748Gly) mutation was associated with a significant mitochondrial mass depletion and myofibrillar disruption in the first patient, while c.291C>A (p.Phe97Leu) mutation was accompanied by reactive mitochondrial proliferation with ragged-red fibers in the second patient. Our results delineate CAPN3 mutation-specific patterns of mitochondrial dysfunction and their ultrastructural characteristics in LGMD2A.

[A male patient with adult-onset sporadic calpainopathy presenting with hypertrophy of the upper extremities]

A 33-year-old man presented with slowly progressive weakness in the lower extremities over 8 years. At the age of 16 years, the elevation of serum creatine kinase level was detected. Physical examination revealed scapular winging, exaggerated lumbar lordosis and tendoachilles contracture. Gowers sign was positive and proximal dominant limb weakness was noted. Hypertrophy was observed in the upper limbs such as the biceps brachii and forearm flexor muscles. Muscle biopsy showed distinct differences in size of muscle fibers and regenerating and necrotic muscle fibers. A histological study revealed decreased calpain3 expression. Gene analysis of CAPN3 revealed two known gene mutations, leading to a diagnosis of calpainopathy (limb girdle muscular dystrophy 2A; LGMD2A). We here report our patient to discuss findings of upper limb hypertrophy, which are frequently missed compared to the lower limb, but important clinical findings.

Calcium Mechanisms in Limb-Girdle Muscular Dystrophy with CAPN3 Mutations

Limb-girdle muscular dystrophy recessive 1 (LGMDR1), previously known as LGMD2A, is a rare disease caused by mutations in the CAPN3 gene. It is characterized by progressive weakness of shoulder, pelvic, and proximal limb muscles that usually appears in children and young adults and results in loss of ambulation within 20 years after disease onset in most patients. The pathophysiological mechanisms involved in LGMDR1 remain mostly unknown, and to date, there is no effective treatment for this disease. Here, we review clinical and experimental evidence suggesting that dysregulation of Ca2+ homeostasis in the skeletal muscle is a significant underlying event in this muscular dystrophy. We also review and discuss specific clinical features of LGMDR1, CAPN3 functions, novel putative targets for therapeutic strategies, and current approaches aiming to treat LGMDR1. These novel approaches may be clinically relevant not only for LGMDR1 but also for other muscular dystrophies with secondary calpainopathy or with abnormal Ca2+ homeostasis, such as LGMD2B/LGMDR2 or sporadic inclusion body myositis.

Novel Missense CAPN3 Mutation Responsible for Adult-Onset Limb Girdle Muscular Dystrophy with Calves Hypertrophy

CAPN3 gene encodes for calpain-3; this protein is a calcium-dependent intracellular protease. Deficiency of this enzyme leads to weakness of the proximal limb muscles and pelvic and shoulder girdles, the so-called limb-girdle muscular dystrophy type 2A (LGMD2A). Here, we reported the case of a Tunisian patient with LGMD2A associated with a novel missense mutation (c.T1681C/p.Y561H). A 61-year-old man, with consanguineous parents, was referred for gait difficulties and slowly progressive proximal weakness of the four limbs associated with moderate hypertrophy of the calves but his facial muscles were unaffected. Electromyography showed that the profile was myopathic pattern and creatine kinase (CK) level was high. Muscle biopsy processing included routine histological, immunohistochemical, and Western Blot reactions, using a panel of antibodies directed against dystrophin, dysferlin, calpain-3, sarcoglycan α, β, γ, and δ. For mutation analysis, we designed an NGS-based screening. Immunological analyses demonstrated a total deficiency in calpain-3 and δ-sarcoglycan, and a reduced expression of dysferlin. The genetic study yielded a homozygous missense mutation (c.T1681C) of the 13th exon of the CAPN3 gene. The mutation found in our patient (c.T1681C/p.Y561H) has not been previously reported. It is responsible for complete calpain-3 and δ-sarcoglycan deficiency and reduced dysferlin expression. The genetic study is mandatory in such cases with multiple-protein deficiency and ambiguous results of immune-histology and Western Blot studies.

Prevalence, pathological mechanisms, and genetic basis of limb-girdle muscular dystrophies: A review

Limb-girdle muscular dystrophies (LGMDs) are a highly heterogeneous group of neuromuscular disorders that are associated with weakness and wasting of muscles in legs and arms. Signs and symptoms may begin at any age and usually worsen by time. LGMDs are autosomal disorders with different types and their prevalence is not the same in different areas. New technologies such as next-generation sequencing can accelerate their diagnosis. Several important pathological mechanisms that are involved in the pathology of the LGMD include abnormalities in dystrophin-glycoprotein complex, the sarcomere, glycosylation of dystroglycan, vesicle and molecular trafficking, signal transduction pathways, and nuclear functions. Here, we provide a comprehensive review that integrates LGMD clinical manifestations, prevalence, and some pathological mechanisms involved in LGMDs.

Insertion sequence 1 from calpain-3 is functional in calpain-2 as an internal propeptide

Calpains are intracellular, calcium-activated cysteine proteases. Calpain-3 is abundant in skeletal muscle, where its mutation-induced loss of function causes limb-girdle muscular dystrophy type 2A. Unlike the small subunit-containing calpain-1 and -2, the calpain-3 isoform homodimerizes through pairing of its C-terminal penta-EF-hand domain. It also has two unique insertion sequences (ISs) not found in the other calpains: IS1 within calpain-3's protease core and IS2 just prior to the penta-EF-hand domain. Production of either native or recombinant full-length calpain-3 to characterize the function of these ISs is challenging. Therefore, here we used recombinant rat calpain-2 as a stable surrogate and inserted IS1 into its equivalent position in the protease core. As it does in calpain-3, IS1 occupied the catalytic cleft and restricted the enzyme's access to substrate and inhibitors. Following activation by Ca²⁺, IS1 was rapidly cleaved by intramolecular autolysis, permitting the enzyme to freely accept substrate and inhibitors. The surrogate remained functional until extensive intermolecular autoproteolysis inactivated the enzyme, as is typical of calpain-2. Although the small-molecule inhibitors E-64 and leupeptin limited intermolecular autolysis of the surrogate, they did not block the initial intramolecular cleavage of IS1, establishing its role as a propeptide. Surprisingly, the large-molecule calpain inhibitor, calpastatin, completely blocked enzyme activity, even with IS1 intact. We suggest that calpastatin is large enough to oust IS1 from the catalytic cleft and take its place. We propose an explanation for why calpastatin can inhibit calpain-2 bearing the IS1 insertion but cannot inhibit WT calpain-3.

Molecular genetic study of Calpainopathy in Iran

INTRODUCTION

Calpainopathy is an autosomal recessive form of limb girdle muscular dystrophies (LGMDs) caused by mutations in the CAPN3 gene. CAPN3 is a Ca²⁺-dependent cystein protease consisting of 821 amino acids. LGMD is a highly heterogeneous disorder and mutation identification of this disease by Sanger sequencing of all genes is expensive and time consuming. Using autozygosity mapping is an effective approach to address this issue.

METHODS

We used two sets of multiplex STR (Short tandem repeat) markers linked to CAPN3, DYSF, SGCA, SGCB, SGCG, SGCD genes following sequencing of the CAPN3 gene. In silico analysis and mutation detection in one hundred ethnically matched healthy individuals were carried out to determine the pathogenicity of novel mutations. Sequence variant interpretation was performed using the American College of Medical Genetics and Genomics (ACMG) guideline.

RESULTS

Sixteen out of 50 families linked to the CAPN3 gene. In this study, mutations were found in 14 out of 16 families including 4 novel (c.1894A > T, c.567delG, c.2254-2256delAAC, and c.2373C > T) and 9 previously reported mutations consisting of 5 missense (c.2105C > T, c.2243G > A, c.1714C > T, c.291C > A, c.956C > T), 3 splice site (c.2380 + 2 T > G, c.946-2A > G, c.380G > A), and one indel (c.2257delinsAA) mutations.

DISCUSSION

The c.2105C > T was found to be the most frequent mutation in this study. The results of this study revealed that most cases with splicing, frame shift and nonsense mutations experienced more severe clinical manifestations. Nonetheless, this should be confirmed by further studies on larger sample size.

Structures of human calpain-3 protease core with and without bound inhibitor reveal mechanisms of calpain activation

Limb-girdle muscular dystrophy type 2a arises from mutations in the Ca2+-activated intracellular cysteine protease calpain-3. This calpain isoform is abundant in skeletal muscle and differs from the main isoforms, calpain-1 and -2, in being a homodimer and having two short insertion sequences. The first of these, IS1, interrupts the protease core and must be cleaved for activation and substrate binding. Here, to learn how calpain-3 can be regulated and inhibited, we determined the structures of the calpain-3 protease core with IS1 present or proteolytically excised. To prevent intramolecular IS1 autoproteolysis, we converted the active-site Cys to Ala. Small-angle X-ray scattering (SAXS) analysis of the C129A mutant suggested that IS1 is disordered and mobile enough to occupy several locations. Surprisingly, this was also true for the apo version of this mutant. We therefore concluded that IS1 might have a binding partner in the sarcomere and is unstructured in its absence. After autoproteolytic IS1 removal from the active Cys129 calpain-3 protease core, we could solve its crystal structures with and without the cysteine protease inhibitors E-64 and leupeptin covalently bound to the active-site cysteine. In each structure, the active state of the protease core was assembled by the cooperative binding of two Ca2+ ions to the equivalent sites used in calpain-1 and -2. These structures of the calpain-3 active site with residual IS1 and with bound E-64 and leupeptin may help guide the design of calpain-3-specific inhibitors.

Impaired regeneration in calpain-3 null muscle is associated with perturbations in mTORC1 signaling and defective mitochondrial biogenesis

Background

Previous studies in patients with limb-girdle muscular dystrophy type 2A (LGMD2A) have suggested that calpain-3 (CAPN3) mutations result in aberrant regeneration in muscle.

Methods

To gain insight into pathogenesis of aberrant muscle regeneration in LGMD2A, we used a paradigm of cardiotoxin (CTX)-induced cycles of muscle necrosis and regeneration in the CAPN3-KO mice to simulate the early features of the dystrophic process in LGMD2A. The temporal evolution of the regeneration process was followed by assessing the oxidative state, size, and the number of metabolic fiber types at 4 and 12 weeks after last CTX injection. Muscles isolated at these time points were further investigated for the key regulators of the pathways involved in various cellular processes such as protein synthesis, cellular energy status, metabolism, and cell stress to include Akt/mTORC1 signaling, mitochondrial biogenesis, and AMPK signaling. TGF-β and microRNA (miR-1, miR-206, miR-133a) regulation were also assessed. Additional studies included in vitro assays for quantifying fusion index of myoblasts from CAPN3-KO mice and development of an in vivo gene therapy paradigm for restoration of impaired regeneration using the adeno-associated virus vector carrying CAPN3 gene in the muscle.

Results

At 4 and 12 weeks after last CTX injection, we found impaired regeneration in CAPN3-KO muscle characterized by excessive numbers of small lobulated fibers belonging to oxidative metabolic type (slow twitch) and increased connective tissue. TGF-β transcription levels in the regenerating CAPN3-KO muscles were significantly increased along with microRNA dysregulation compared to wild type (WT), and the attenuated radial growth of muscle fibers was accompanied by perturbed Akt/mTORC1 signaling, uncoupled from protein synthesis, through activation of AMPK pathway, thought to be triggered by energy shortage in the CAPN3-KO muscle. This was associated with failure to increase mitochondria content, PGC-1α, and ATP5D transcripts in the regenerating CAPN3-KO muscles compared to WT. In vitro studies showed defective myotube fusion in CAPN3-KO myoblast cultures. Replacement of CAPN3 by gene therapy in vivo increased the fiber size and decreased the number of small oxidative fibers.

Conclusion

Our findings provide insights into understanding of the impaired radial growth phase of regeneration in calpainopathy.

Electronic supplementary material

The online version of this article (10.1186/s13395-017-0146-6) contains supplementary material, which is available to authorized users.

Treadmill Training with HAL Exoskeleton-A Novel Approach for Symptomatic Therapy in Patients with Limb-Girdle Muscular Dystrophy-Preliminary Study

Purpose: Exoskeletons have been developed for rehabilitation of patients with walking impairment due to neurological disorders. Recent studies have shown that the voluntary-driven exoskeleton HAL® (hybrid assistive limb) can improve walking functions in spinal cord injury and stroke. The aim of this study was to assess safety and effects on walking function of HAL® supported treadmill therapy in patients with limb-girdle muscular dystrophy (LGMD).

Materials and Methods: Three LGMD patients received 8 weeks of treadmill training with HAL® 3 times a week. Outcome parameters were 10-meter walk test (10 MWT), 6-minute walk test, and timed-up-and-go test (TUG). Parameters were assessed pre and post training and 6 weeks later (follow-up).

Results: All patients completed the therapy without adverse reactions and reported about improvement in endurance. Improvements in outcome parameters after 8 weeks could be demonstrated. Persisting effects were observed after 6 weeks for the 10 MWT and TUG test (follow-up).

Conclusions: HAL® treadmill training in LGMD patients can be performed safely and enables an intensive highly repetitive locomotor training. All patients benefitted from this innovative method. Upcoming controlled studies with larger cohorts should prove its effects in different types of LGMD and other myopathies.

Calcium dysregulation, functional calpainopathy, and endoplasmic reticulum stress in sporadic inclusion body myositis

Sporadic inclusion body myositis (IBM) is the most common primary myopathy in the elderly, but its pathoetiology is still unclear. Perturbed myocellular calcium (Ca²⁺) homeostasis can exacerbate many of the factors proposed to mediate muscle degeneration in IBM, such as mitochondrial dysfunction, protein aggregation, and endoplasmic reticulum stress. Ca²⁺ dysregulation may plausibly be initiated in IBM by immune-mediated membrane damage and/or abnormally accumulating proteins, but no studies to date have investigated Ca²⁺ regulation in IBM patients. We first investigated protein expression via immunoblot in muscle biopsies from IBM, dermatomyositis, and non-myositis control patients, identifying several differentially expressed Ca²⁺-regulatory proteins in IBM. Next, we investigated the Ca²⁺-signaling transcriptome by RNA-seq, finding 54 of 183 (29.5%) genes from an unbiased list differentially expressed in IBM vs. controls. Using an established statistical approach to relate genes with causal transcription networks, Ca²⁺ abundance was considered a significant upstream regulator of observed whole-transcriptome changes. Post-hoc analyses of Ca²⁺-regulatory mRNA and protein data indicated a lower protein to transcript ratio in IBM vs. controls, which we hypothesized may relate to increased Ca²⁺-dependent proteolysis and decreased protein translation. Supporting this hypothesis, we observed robust (4-fold) elevation in the autolytic activation of a Ca²⁺-activated protease, calpain-1, as well as increased signaling for translational attenuation (eIF2α phosphorylation) downstream of the unfolded protein response. Finally, in IBM samples we observed mRNA and protein under-expression of calpain-3, the skeletal muscle-specific calpain, which broadly supports proper Ca²⁺ homeostasis. Together, these data provide novel insight into mechanisms by which intracellular Ca²⁺ regulation is perturbed in IBM and offer evidence of pathological downstream effects.

MicroRNA signatures predict dysregulated vitamin D receptor and calcium pathways status in limb girdle muscle dystrophies (LGMD) 2A/2B

miRNA expression profile and predicted pathways involved in selected limb-girdle muscular dystrophy (LGMD)2A/2B patients were investigated. A total of 187 miRNAs were dysregulated in all patients, with six miRNAs showing opposite regulation in LGMD2A versus LGMD2B patients. Silico analysis evidence: (1) a cluster of the dysregulated miRNAs resulted primarily involved in inflammation and calcium metabolism, and (2) two genes predicted as controlled by calcium-assigned miRNAs (Vitamin D Receptor gene and Guanine Nucleotide Binding protein beta polypeptide 1gene) showed an evident upregulation in LGMD2B patients, in accordance with miRNA levels. Our data support alterations in calcium pathway status in LGMD 2A/B, suggesting myofibre calcium imbalance as a potential therapeutic target. Copyright © 2016 John Wiley & Sons, Ltd.

Novel Homozygous Missense Mutation in CAPN3 Gene Detected in a Saudi Arabian Family With Limb-Girdle Muscular Dystrophy Type 2A

More than 300 mutations were identified in Calpainopathy (CAPN3) gene in limb-girdle muscular dystrophy type 2A (LGMD2A) patients. LGMD2A type is also known as Calpainopathy, which is characterized by selective atrophy and weakness of proximal limb muscles. We report a Saudi Arabian family with weakness in limb-girdle distribution: waddling gait, positive Gowers' sign, and marked muscle atrophy in the shoulder and pelvic girdle muscles. We sequenced all exonic and intronic regions of the CAPN3 gene and identified c.1699 G>A variant as a novel variant not previously described in other patients. In silico predictions indicate that this is probably a disease-causing mutation. Here, for the first time, we report this c.1699 G>A new variant in the CAPN3 gene that can be considered as a robust genetics factor causing LGMD2A disease.

Failure to up-regulate transcription of genes necessary for muscle adaptation underlies limb girdle muscular dystrophy 2A (calpainopathy)

Limb girdle muscular dystrophy 2A is due to loss-of-function mutations in the Calpain 3 (CAPN3) gene. Our previous data suggest that CAPN3 helps to maintain the integrity of the triad complex in skeletal muscle. In Capn3 knock-out mice (C3KO), Ca²⁺ release and Ca²⁺/calmodulin kinase II (CaMKII) signaling are attenuated. We hypothesized that calpainopathy may result from a failure to transmit loading-induced Ca²⁺-mediated signals, necessary to up-regulate expression of muscle adaptation genes. To test this hypothesis, we compared transcriptomes of muscles from wild type (WT) and C3KO mice subjected to endurance exercise. In WT mice, exercise induces a gene signature that includes myofibrillar, mitochondrial and oxidative lipid metabolism genes, necessary for muscle adaptation. C3KO muscles fail to activate the same gene signature. Furthermore, in agreement with the aberrant transcriptional profile, we observe a commensurate functional defect in lipid metabolism whereby C3KO muscles fail to release fatty acids from stored triacylglycerol. In conjunction with the defects in oxidative metabolism, C3KO mice demonstrate reduced exercise endurance. Failure to up-regulate genes in C3KO muscles is due, in part, to decreased levels of PGC1α, a transcriptional co-regulator that orchestrates the muscle adaptation response. Destabilization of PGC1α is attributable to decreased p38 MAPK activation via diminished CaMKII signaling. Thus, we elucidate a pathway downstream of Ca²⁺-mediated CaMKII activation that is dysfunctional in C3KO mice, leading to reduced transcription of genes involved in muscle adaptation. These studies identify a novel mechanism of muscular dystrophy: a blunted transcriptional response to muscle loading resulting in chronic failure to adapt and remodel.

Clinical and Pathological Heterogeneity of Korean Patients with CAPN3 Mutations

PURPOSE

This study was designed to investigate the characteristics of Korean patients with calpainopathy.

MATERIALS AND METHODS

Thirteen patients from ten unrelated families were diagnosed with calpainopathy via direct or targeted sequencing of the CAPN3 gene. Clinical, mutational, and pathological spectra were then analyzed.

RESULTS

Nine different mutations, including four novel mutations (NM_000070: c.1524+1G>T, c.1789_1790inA, c.2184+1G>T, and c.2384C>T) were identified. The median age at symptom onset was 22 (interquartile range: 15-28). Common clinical findings were joint contracture in nine patients, winged scapula in four, and lordosis in one. However, we also found highly variable clinical features including early onset joint contractures, asymptomatic hyperCKemia, and heterogeneous clinical severity in three members of the same family. Four of nine muscle specimens revealed lobulated fibers, but three showed normal skeletal muscle histology.

CONCLUSION

We identified four novel CAPN3 mutations and demonstrated clinical and pathological heterogeneity in Korean patients with calpainopathy.

Genetic profile for suspected dysferlinopathy identified by targeted next-generation sequencing

Objective:

To investigate the genetic causes of suspected dysferlinopathy and to reveal the genetic profile for myopathies with dysferlin deficiency.

Methods:

Using next-generation sequencing, we analyzed 42 myopathy-associated genes, including DYSF, in 64 patients who were clinically or pathologically suspected of having dysferlinopathy. Putative pathogenic mutations were confirmed by Sanger sequencing. In addition, copy-number variations in DYSF were investigated using multiplex ligation-dependent probe amplification. We also analyzed the genetic profile for 90 patients with myopathy with dysferlin deficiency, as indicated by muscle specimen immunohistochemistry, including patients from a previous cohort.

Results:

We identified putative pathogenic mutations in 38 patients (59% of all investigated patients). Twenty-three patients had DYSF mutations, including 6 novel mutations. The remaining 16 patients, including a single patient who also carried the DYSF mutation, harbored putative pathogenic mutations in other genes. The genetic profile for 90 patients with dysferlin deficiency revealed that 70% had DYSF mutations (n = 63), 10% had CAPN3 mutations (n = 9), 2% had CAV3 mutations (n = 2), 3% had mutations in other genes (in single patients), and 16% did not have any identified mutations (n = 14).

Conclusions:

This study clarified the heterogeneous genetic profile for myopathies with dysferlin deficiency. Our results demonstrate the importance of a comprehensive analysis of related genes in improving the genetic diagnosis of dysferlinopathy as one of the most common subtypes of limb-girdle muscular dystrophy. Unresolved diagnoses should be investigated using whole-genome or whole-exome sequencing.

The limb-girdle muscular dystrophies

A collection of more than 30 genetic muscle diseases that share certain key features, limb-girdle muscular dystrophies are characterized by progressive weakness and muscle atrophy of the hips, shoulders, and proximal extremity muscles with postnatal onset. This article discusses clinical, laboratory, and histologic features of the 6 most prevalent limb-girdle dystrophies. In this large group of disorders, certain distinctive features often can guide clinicians to a correct diagnosis.

Autolytic activation of calpain 3 proteinase is facilitated by calmodulin protein

Calpains are broadly distributed, calcium-dependent enzymes that induce limited proteolysis in a wide range of substrates. Mutations in the gene encoding the muscle-specific family member calpain 3 (CAPN3) underlie limb-girdle muscular dystrophy 2A. We have shown previously that CAPN3 knockout muscles exhibit attenuated calcium release, reduced calmodulin kinase (CaMKII) signaling, and impaired muscle adaptation to exercise. However, neither the precise role of CAPN3 in these processes nor the mechanisms of CAPN3 activation in vivo have been fully elucidated. In this study, we identify calmodulin (CaM), a known transducer of the calcium signal, as the first positive regulator of CAPN3 autolytic activity. CaM was shown to bind CAPN3 at two sites located in the C2L domain. Biochemical studies using muscle extracts from transgenic mice overexpressing CAPN3 or its inactive mutant revealed that CaM binding enhanced CAPN3 autolytic activation. Furthermore, CaM facilitated CAPN3-mediated cleavage of its in vivo substrate titin in tissue extracts. Therefore, these studies reveal a novel interaction between CAPN3 and CaM and identify CaM as the first positive regulator of CAPN3 activity.

Dysregulation of calcium homeostasis in muscular dystrophies

Muscular dystrophies are a group of diseases characterised by the primary wasting of skeletal muscle, which compromises patient mobility and in the most severe cases originate a complete paralysis and premature death. Existing evidence implicates calcium dysregulation as an underlying crucial event in the pathophysiology of several muscular dystrophies, such as dystrophinopathies, calpainopathies or myotonic dystrophy among others. Duchenne muscular dystrophy is the most frequent myopathy in childhood, and calpainopathy or LGMD2A is the most common form of limb-girdle muscular dystrophy, whereas myotonic dystrophy is the most frequent inherited muscle disease worldwide. In this review, we summarise recent advances in our understanding of calcium ion cycling through the sarcolemma, the sarcoplasmic reticulum and mitochondria, and its involvement in the pathogenesis of these dystrophies. We also discuss some of the clinical implications of recent findings regarding Ca2+ handling as well as novel approaches to treat muscular dystrophies targeting Ca2+ regulatory proteins.

Redox state and mitochondrial respiratory chain function in skeletal muscle of LGMD2A patients

Background

Calpain-3 deficiency causes oxidative and nitrosative stress-induced damage in skeletal muscle of LGMD2A patients, but mitochondrial respiratory chain function and anti-oxidant levels have not been systematically assessed in this clinical population previously.

Methods

We identified 14 patients with phenotypes consistent with LGMD2A and performed CAPN3 gene sequencing, CAPN3 expression/autolysis measurements, and insilico predictions of pathogenicity. Oxidative damage, anti-oxidant capacity, and mitochondrial enzyme activities were determined in a subset of muscle biopsies.

Results

Twenty-one disease-causing variants were detected along the entire CAPN3 gene, five of which were novel (c.338 T>C, c.500 T>C, c.1525-1 G>T, c.2115+4 T>G, c.2366 T>A). Protein- and mRNA-based tests confirmed insilico predictions and the clinical diagnosis in 75% of patients. Reductions in antioxidant defense mechanisms (SOD-1 and NRF-2, but not SOD-2), coupled with increased lipid peroxidation and protein ubiquitination, were observed in calpain-3 deficient muscle, indicating a redox imbalance primarily affecting non-mitochondrial compartments. Although ATP synthase levels were significantly lower in LGMD2A patients, citrate synthase, cytochrome c oxidase, and complex I+III activities were not different from controls.

Conclusions

Despite significant oxidative damage and redox imbalance in cytosolic/myofibrillar compartments, mitochondrial respiratory chain function is largely maintained in skeletal muscle of LGMD2A patients.

Mitochondrial dysfunction in neuromuscular disorders

This review deciphers aspects of mitochondrial (mt) dysfunction among nosologically, pathologically, and genetically diverse diseases of the skeletal muscle, lower motor neuron, and peripheral nerve, which fall outside the traditional realm of mt cytopathies. Special emphasis is given to well-characterized mt abnormalities in collagen VI myopathies (Ullrich congenital muscular dystrophy and Bethlem myopathy), megaconial congenital muscular dystrophy, limb-girdle muscular dystrophy type 2 (calpainopathy), centronuclear myopathies, core myopathies, inflammatory myopathies, spinal muscular atrophy, Charcot-Marie-Tooth neuropathy type 2, and drug-induced peripheral neuropathies. Among inflammatory myopathies, mt abnormalities are more prominent in inclusion body myositis and a subset of polymyositis with mt pathology, both of which are refractory to corticosteroid treatment. Awareness is raised about instances of phenotypic mimicry between cases harboring primary mtDNA depletion, in the context of mtDNA depletion syndrome, and established neuromuscular disorders such as spinal muscular atrophy. A substantial body of experimental work, derived from animal models, attests to a major role of mitochondria (mt) in the early process of muscle degeneration. Common mechanisms of mt-related cell injury include dysregulation of the mt permeability transition pore opening and defective autophagy. The therapeutic use of mt permeability transition pore modifiers holds promise in various neuromuscular disorders, including muscular dystrophies.

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.

Selective pseudohypertrophy of vastus medialis muscles associated with calpain 3 deficiency

INTRODUCTION

Calpain 3 deficiency causes limb girdle muscular dystrophy type 2A, which is one of the most common forms of limb girdle muscular dystrophy. Nevertheless, calpainopathy is not always associated with mutations in the specific gene and secondary reduction in protein expression has been described.

CASE REPORT

We report a case of a 43-year-old man who complained of thigh muscle stiffness and had muscle hypertrophy of both vastus medialis with prolonged myotonic contraction by percussion. A muscle biopsy showed dystrophic features and calpain 3 deficiency was shown by immunoblot analysis although mutations in the specific gene were not found. Known cases of secondary calpain 3 protein deficiency were ruled out and mutations in MD1 and MD2 genes were excluded.

CONCLUSIONS

This patient represents the first case of calpain 3 deficiency with selective pseudohypertrophy of vastus medialis muscles.

C3KO mouse expression analysis: downregulation of the muscular dystrophy Ky protein and alterations in muscle aging

Mutations in CAPN3 gene cause limb-girdle muscular dystrophy type 2A (LGMD2A) characterized by muscle wasting and progressive degeneration of scapular and pelvic musculature. Since CAPN3 knockout mice (C3KO) display features of muscle pathology similar to those features observed in the earliest-stage or preclinical LGMD2A patients, gene expression profiling analysis in C3KO mice was performed to gain insight into mechanisms of disease. Two different comparisons were carried out in order to determine, first, the differential gene expression between wild-type (WT) and C3KO soleus and, second, to identify the transcripts differentially expressed in aging muscles of WT and C3KO mice. The up/downregulation of two genes, important for normal muscle function, was identified in C3KO mice: the Ky gene, encoding a protease implicated in muscle development, and Park2 gene encoding an E3 ubiquitin ligase (parkin). The Ky gene was downregulated in C3KO muscles suggesting that Ky protease may play a complementary role in regulating muscle cytoskeleton homeostasis in response to changes in muscle activity. Park2 was upregulated in the aged WT muscles but not in C3KO muscles. Taking into account the known functions of parkin E3 ligase, it is possible that it plays a role in ubiquitination and degradation of atrophy-specific and damaged proteins that are necessary to avoid cellular toxicity and a cellular stress response in aging muscles.

Impaired calcium calmodulin kinase signaling and muscle adaptation response in the absence of calpain 3

Mutations in the non-lysosomal, cysteine protease calpain 3 (CAPN3) result in the disease limb girdle muscular dystrophy type 2A (LGMD2A). CAPN3 is localized to several subcellular compartments, including triads, where it plays a structural, rather than a proteolytic, role. In the absence of CAPN3, several triad components are reduced, including the major Ca(2+) release channel, ryanodine receptor (RyR). Furthermore, Ca(2+) release upon excitation is impaired in the absence of CAPN3. In the present study, we show that Ca-calmodulin protein kinase II (CaMKII) signaling is compromised in CAPN3 knockout (C3KO) mice. The CaMK pathway has been previously implicated in promoting the slow skeletal muscle phenotype. As expected, the decrease in CaMKII signaling that was observed in the absence of CAPN3 is associated with a reduction in the slow versus fast muscle fiber phenotype. We show that muscles of WT mice subjected to exercise training activate the CaMKII signaling pathway and increase expression of the slow form of myosin; however, muscles of C3KO mice do not exhibit these adaptive changes to exercise. These data strongly suggest that skeletal muscle's adaptive response to functional demand is compromised in the absence of CAPN3. In agreement with our mouse studies, RyR levels were also decreased in biopsies from LGMD2A patients. Moreover, we observed a preferential pathological involvement of slow fibers in LGMD2A biopsies. Thus, impaired CaMKII signaling and, as a result, a weakened muscle adaptation response identify a novel mechanism that may underlie LGMD2A and suggest a pharmacological target that should be explored for therapy.

CAPN3 mRNA processing alteration caused by splicing mutation associated with novel genomic rearrangement of Alu elements

Recessive mutations of CAPN3 gene are reported to be responsible for limb girdle muscular dystrophy type 2A (LGMD2A). In all, 15-25% of intronic nucleotide changes identified in this gene were investigated by in silico analysis, but occasionally supported by experimental data or reported in some cases as a polymorphism. We report here genetic and transcriptional analyses in three Tunisian patients belonging to the same consanguineous family sharing the same mutation c.1194-9 A>G and Alu repeats insertion in intron 7 of CAPN3 gene. Reverse transcriptase-PCR experiments performed on total RNA from the patient's muscle biopsy showed retention of the eight last nucleotides of intron 9 in the CAPN3 transcript lacking the first seven exons. Our results provide evidence regarding the potential involvement of Alu elements in aberrant processing of pre-mRNA owing to the disruption of pre-existing intronic splicing regulatory elements. We also demonstrated variable mRNA alternative splicing among tissues and between LGMD2A patients. A deep intronic variation and rearrangement have been reported in the literature as causing genetic diseases in humans. However, this is the first report on a potential pathogenic CAPN3 gene mutation resulting from an Alu insertion.

Pathogenity of some limb girdle muscular dystrophy mutations can result from reduced anchorage to myofibrils and altered stability of calpain 3

Calpain 3 (CAPN3) is a muscle-specific, calcium-dependent proteinase that is mutated in Limb Girdle Muscle Dystrophy type 2A. Most pathogenic missense mutations in LGMD2A affect CAPN3's proteolytic activity; however, two mutations, D705G and R448H, retain activity but nevertheless cause muscular dystrophy. Previously, we showed that D705G and R448H mutations reduce CAPN3s ability to bind to titin in vitro. In this investigation, we tested the consequence of loss of titin binding in vivo and examined whether this loss can be an underlying pathogenic mechanism in LGMD2A. To address this question, we created transgenic mice that express R448H or D705G in muscles, on wild-type (WT) CAPN3 or knock-out background. Both mutants were readily expressed in insect cells, but when D705G was expressed in skeletal muscle, it was not stable enough to study. Moreover, the D705G mutation had a dominant negative effect on endogenous CAPN3 when expressed on a WT background. The R448H protein was stably expressed in muscles; however, it was more rapidly degraded in muscle extracts compared with WT CAPN3. Increased degradation of R448H was due to non-cysteine, cellular proteases acting on the autolytic sites of CAPN3, rather than autolysis. Fractionation experiments revealed a significant decrease of R448H from the myofibrillar fraction, likely due to the mutant's inability to bind titin. Our data suggest that R448H and D705G mutations affect both CAPN3s anchorage to titin and its stability. These studies reveal a novel mechanism by which mutations that spare enzymatic activity can still lead to calpainopathy.

Nuclear membrane receptors for ET-1 in cardiovascular function

Plasma membrane endothelin type A (ETA) receptors are internalized and recycled to the plasma membrane, whereas endothelin type B (ETB) receptors undergo degradation and subsequent nuclear translocation. Recent studies show that G protein-coupled receptors (GPCRs) and ion transporters are also present and functional at the nuclear membranes of many cell types. Similarly to other GPCRs, ETA and ETB are present at both the plasma and nuclear membranes of several cardiovascular cell types, including human cardiac, vascular smooth muscle, endocardial endothelial, and vascular endothelial cells. The distribution and density of ETARs in the cytosol (including the cell membrane) and the nucleus (including the nuclear membranes) differ between these cell types. However, the localization and density of ET-1 and ETB receptors are similar in these cell types. The extracellular ET-1-induced increase in cytosolic ([Ca]c) and nuclear ([Ca]n) free Ca2+ is associated with an increase of cytosolic and nuclear reactive oxygen species. The extracellular ET-1-induced increase of [Ca]c and [Ca]n as well as intracellular ET-1-induced increase of [Ca]n are cell-type dependent. The type of ET-1 receptor mediating the extracellular ET-1-induced increase of [Ca]c and [Ca]n depends on the cell type. However, the cytosolic ET-1-induced increase of [Ca]n does not depend on cell type. In conclusion, nuclear membranes' ET-1 receptors may play an important role in overall ET-1 action. These nuclear membrane ET-1 receptors could be targets for a new generation of antagonists.

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.

Down-regulation of MyoD by calpain 3 promotes generation of reserve cells in C2C12 myoblasts

Calpain 3 is a calcium-dependent cysteine protease that is primarily expressed in skeletal muscle and is implicated in limb girdle muscular dystrophy type 2A. To date, its best characterized function is located within the sarcomere, but this protease is found in other cellular compartments, which suggests that it exerts multiple roles. Here, we present evidence that calpain 3 is involved in the myogenic differentiation process. In the course of in vitro culture of myoblasts to fully differentiated myotubes, a population of quiescent undifferentiated "reserve cells" are maintained. These reserve cells are closely related to satellite cells responsible for adult muscle regeneration. In the present work, we observe that reserve cells express higher levels of endogenous Capn3 mRNA than proliferating myoblasts. We show that calpain 3 participates in the establishment of the pool of reserve cells by decreasing the transcriptional activity of the key myogenic regulator MyoD via proteolysis independently of the ubiquitin-proteasome degradation pathway. Our results identify calpain 3 as a potential new player in the muscular regeneration process by promoting renewal of the satellite cell compartment.