Calpain 3 and CaMKIIβ signaling are required to induce HSP70 necessary for adaptive muscle growth after atrophy

Identification and functional characterization of a novel heterozygous splice‑site mutation in the calpain 3 gene causes rare autosomal dominant limb‑girdle muscular dystrophy

Limb-girdle muscular dystrophies are a group of extremely heterogenous neuromuscular disorders that manifest with gradual and progressive weakness of both proximal and distal muscles. Autosomal dominant limb-girdle muscular dystrophy (LGMDD4) or calpainopathy is a very rare form of myopathy characterized by weakness and atrophy of both proximal and distal muscles with a variable age of onset. LGMDD4 is caused by germline heterozygous mutations of the calpain 3 (CAPN3) gene. Patients with LGMDD4 often show extreme phenotypic heterogeneity; however, most patients present with gait difficulties, increased levels of serum creatine kinase, myalgia and back pain. In the present study, a 16-year-old male patient, clinically diagnosed with LGMDD4, was investigated. The proband had been suffering from weakness and atrophy of both of their proximal and distal muscles, and had difficulty walking and standing independently. The serum creatine kinase levels (4,754 IU/l; normal, 35-232 IU/l) of the patient were markedly elevated. The younger sister and mother of the proband were also clinically diagnosed with LGMDD4, while the father was phenotypically normal. Whole exome sequencing identified a heterozygous novel splice-site (c.2440-1G>A) mutation in intron 23 of the CAPN3 gene in the proband. Sanger sequencing confirmed that this mutation was also present in both the younger sister and mother of the proband, but the father was not a carrier of this mutation. This splice-site (c.2440-1G>A) mutation causes aberrant splicing of CAPN3 mRNA, leading to the skipping of the last exon (exon 24) of CAPN3 mRNA and resulting in the removal of eight amino acids from the C-terminal of domain IV of the CAPN3 protein. Hence, this splice site mutation causes the formation of a truncated CAPN3 protein (p.Trp814*) of 813 amino acids instead of the wild-type CAPN3 protein that consists of 821 amino acids. This mutation causes partial loss of domain IV (PEF domain) in the CAPN3 protein, which is involved in calcium binding and homodimerization; therefore, this is a loss-of-function mutation. Relative expression of the mutated CAPN3 mRNA was reduced in comparison with the wild-type CAPN3 mRNA in the proband, and their younger sister and mother. This mutation was also not present in 100 normal healthy control individuals of the same ethnicity. The present study reported the first case of CAPN3 gene-associated LGMDD4 in the Chinese population.

Novel mutations in the SGCA gene in unrelated Vietnamese patients with limb-girdle muscular dystrophies disease

Background: Limb-girdle muscular dystrophy (LGMD) is a group of inherited neuromuscular disorders characterized by atrophy and weakness in the shoulders and hips. Over 30 subtypes have been described in five dominant (LGMD type 1 or LGMDD) and 27 recessive (LGMD type 2 or LGMDR). Each subtype involves a mutation in a single gene and has high heterogeneity in age of onset, expression, progression, and prognosis. In addition, the lack of understanding of the disease and the vague, nonspecific symptoms of LGMD subtypes make diagnosis difficult. Even as next-generation sequencing (NGS) genetic testing has become commonplace, some patients remain undiagnosed for many years. Methods: To identify LGMD-associated mutations, Targeted sequencing was performed in the patients and Sanger sequencing was performed in patients and family members. The in silico analysis tools such as Fathmm, M-CAP, Mutation Taster, PolyPhen 2, PROVEAN, REVEL, SIFT, MaxEntScan, Spliceailookup, Human Splicing Finder, NetGene2, and Fruitfly were used to predict the influence of the novel mutations. The pathogenicity of the mutation was interpreted according to the ACMG guidelines. Results: In this study, six patients from four different Vietnamese families were collected for genetic analysis at The Center for Gene and Protein Research and The Department of Molecular Pathology Faculty of Medical Technology, Hanoi Medical University, Hanoi, Vietnam. Based on clinical symptoms and serum creatine kinase (CK) levels, the patients were diagnosed with limb-girdle muscular dystrophies. Five mutations, including four (c.229C>T, p.Arg77Cys; exon one to three deletion; c.983 + 5G>C; and c.257_258insTGGCT, p.Phe88Leufs*125) in the SGCA gene and one (c.946-4_946-1delACAG) in the CAPN3 gene, were detected in six LGMD patients from four unrelated Vietnamese families. Two homozygous mutations (c.983 + 5G>C and c.257_258insTGGCT) in the SGCA gene were novel. These mutations were identified as the cause of the disease in the patients. Conclusion: Our results contribute to the general understanding of the etiology of the disease and provide the basis for definitive diagnosis and support genetic counseling and prenatal screening.

CAPN3: A muscle‑specific calpain with an important role in the pathogenesis of diseases (Review)

Calpains are a family of Ca²⁺‑dependent cysteine proteases that participate in various cellular processes. Calpain 3 (CAPN3) is a classical calpain with unique N‑terminus and insertion sequence 1 and 2 domains that confer characteristics such as rapid autolysis, Ca²⁺‑independent activation and Na⁺ activation of the protease. CAPN3 is the only muscle‑specific calpain that has important roles in the promotion of calcium release from skeletal muscle fibers, calcium uptake of sarcoplasmic reticulum, muscle formation and muscle remodeling. Studies have indicated that recessive mutations in CAPN3 cause limb‑girdle muscular dystrophy (MD) type 2A and other types of MD; eosinophilic myositis, melanoma and epilepsy are also closely related to CAPN3. In the present review, the characteristics of CAPN3, its biological functions and roles in the pathogenesis of a number of disorders are discussed.

Skeletal muscle atrophy: From mechanisms to treatments

Skeletal muscle is a crucial tissue for movement, gestural assistance, metabolic homeostasis, and thermogenesis. It makes up approximately 40% of the total body weight and 50% of total protein. However, several pathological abnormalities (e.g., chronic diseases, cancer, long-term infection, aging) can induce an imbalance in skeletal muscle protein synthesis and degradation, which triggers muscle wasting and even leads to atrophy. Skeletal muscle atrophy is characterized by weakening, shrinking, and decreasing muscle mass and fiber cross-sectional area at the histological level. It manifests as a reduction in force production, easy fatigue and decreased exercise capability, along with a lower quality of life. Mechanistically, there are several pathophysiological processes involved in skeletal muscle atrophy, including oxidative stress and inflammation, which then activate signal transduction, such as the ubiquitin proteasome system, autophagy lysosome system, and mTOR. Considering the great economic and social burden that muscle atrophy can inflict, effective prevention and treatment strategies are essential but still limited. Exercise is widely acknowledged as the most effective therapy for skeletal muscle atrophy; unfortunately, it is not applicable for all patients. Several active substances for skeletal muscle atrophy have been discovered and evaluated in clinical trials, however, they have not been marketed to date. Knowledge is being gained on the underlying mechanisms, highlighting more promising treatment strategies in the future. In this paper, the mechanisms and treatment strategies for skeletal muscle atrophy are briefly reviewed.

Calpains for dummies: What you need to know about the calpain family

This review was written in memory of our late friend, Dr. Hiroyuki Sorimachi, who, following the steps of his mentor Koichi Suzuki, a pioneer in calpain research, has made tremendous contributions to the field. During his career, Hiro also wrote several reviews on calpain, the last of which, published in 2016, was comprehensive. In this manuscript, we decided to put together a review with the basic information a novice may need to know about calpains. We also tried to avoid similarities with previous reviews and reported the most significant new findings, at the same time highlighting Hiro's contributions to the field. The review will cover a short history of calpain discovery, the presentation of the family, the life of calpain from transcription to activity, human diseases caused by calpain mutations and therapeutic perspectives.

Effects of functionally diverse calpain system on immune cells

Calpains are a family of nonlysosomal cysteine proteases, which play important roles in numerous physiological and pathological processes. Locations of them dictates the functions so that they are classified as ubiquitously expressed calpains and tissue-specific calpains. Recent studies are mainly focused on conventional calpains (calpain-1,2) in development and diseases, and increasing people pay attention to other subtypes of calpains but may not been summarized appropriately. Growing evidence suggests that calpains are also involved in immune regulation. However, seldom articles review the regulation of calpains on immune cells. The aim of this article is to review the research progress of each calpain isozyme and the effect of calpains on immune cells, especially the promotion effect of calpains on the immune response of macrophage, neutrophils, dendritic cells, mast cells, natural killed cells, and lymphocytes. These effects would hold great promise for the clinical application of calpains as a practicable therapeutic option in the treatment of immune related diseases.

Skeletal Muscle Recovery from Disuse Atrophy: Protein Turnover Signaling and Strategies for Accelerating Muscle Regrowth

Skeletal muscle fibers have a unique capacity to adjust their metabolism and phenotype in response to alternations in mechanical loading. Indeed, chronic mechanical loading leads to an increase in skeletal muscle mass, while prolonged mechanical unloading results in a significant decrease in muscle mass (muscle atrophy). The maintenance of skeletal muscle mass is dependent on the balance between rates of muscle protein synthesis and breakdown. While molecular mechanisms regulating protein synthesis during mechanical unloading have been relatively well studied, signaling events implicated in protein turnover during skeletal muscle recovery from unloading are poorly defined. A better understanding of the molecular events that underpin muscle mass recovery following disuse-induced atrophy is of significant importance for both clinical and space medicine. This review focuses on the molecular mechanisms that may be involved in the activation of protein synthesis and subsequent restoration of muscle mass after a period of mechanical unloading. In addition, the efficiency of strategies proposed to improve muscle protein gain during recovery is also discussed.

A Small-Molecule Approach to Restore a Slow-Oxidative Phenotype and Defective CaMKIIβ Signaling in Limb Girdle Muscular Dystrophy

Mutations in CAPN3 cause limb girdle muscular dystrophy R1 (LGMDR1, formerly LGMD2A) and lead to progressive and debilitating muscle wasting. Calpain 3 deficiency is associated with impaired CaMKIIβ signaling and blunted transcriptional programs that encode the slow-oxidative muscle phenotype. We conducted a high-throughput screen on a target of CaMKII (Myl2) to identify compounds to override this signaling defect; 4 were tested in vivo in the Capn3 knockout (C3KO) model of LGMDR1. The leading compound, AMBMP, showed good exposure and was able to reverse the LGMDR1 phenotype in vivo, including improved oxidative properties, increased slow fiber size, and enhanced exercise performance. AMBMP also activated CaMKIIβ signaling, but it did not alter other pathways known to be associated with muscle growth. Thus, AMBMP treatment activates CaMKII and metabolically reprograms skeletal muscle toward a slow muscle phenotype. These proof-of-concept studies lend support for an approach to the development of therapeutics for LGMDR1.

Role of CaMKII and sarcolipin in muscle adaptations to strength training with different levels of fatigue in the set

Strength training promotes a IIX-to-IIA shift in myosin heavy chain (MHC) composition, likely due to changes in sarcoplasmic [Ca²⁺ ] which are sensed by CaMKII. Sarcoplasmic [Ca²⁺ ] is in part regulated by sarcolipin (SLN), a small protein that when overexpressed in rodents stimulates mitochondrial biogenesis and a fast-to-slow fiber type shift. The purpose of this study was to determine whether CaMKII and SLN are involved in muscle phenotype and performance changes elicited by strength training. Twenty-two men followed an 8-week velocity-based resistance training program using the full squat exercise while monitoring repetition velocity. Subjects were randomly assigned to two resistance training programs differing in the repetition velocity loss allowed in each set: 20% (VL20) vs 40% (VL40). Strength training caused muscle hypertrophy, improved 1RM and increased total CaMKII protein expression, particularly of the δ_D isoform. Phospho-Thr²⁸⁷ -CaMKII δ_D expression increased only in VL40 (+89%), which experienced greater muscle hypertrophy, and a reduction in MHC-IIX percentage. SLN expression was increased in VL20 (+33%) remaining unaltered in VL40. The changes in phospho-Thr²⁸⁷ -CaMKII δ_D were positively associated with muscle hypertrophy and the number of repetitions during training, and negatively with the changes in MHC-IIX and SLN. Most OXPHOS proteins remained unchanged, except for NDUFB8 (Complex I), which was reduced after training (-22%) in both groups. The amount of fatigue allowed in each set critically influences muscle CaMKII and SLN responses and determines muscle phenotype changes. With lower intra-set fatigue, the IIX-to-IIA MHC shift is attenuated.

The scaffolding protein calpain-3 has multiple distributions in embryonic chick muscle cells and it is essential for the formation of muscle fibers

CAPN3 is a muscle-specific and an intrinsically disordered protein. Thus, as a scaffolding protein CAPN3 could play a role during early stages of myogenesis. To test this hypothesis, we studied the distribution and function of CAPN3 during myogenesis using embryonic chick muscle cells grown in vitro. Super-resolution microscopy showed CAPN3 distribution in (i) amorphous patches in myoblasts, (ii) a region near the nuclei of myotubes; (iii) adhesion plaques in myotubes, (iv) stress fiber-like structures in myotubes, and (v) filaments in fibroblasts. Downregulation of CAPN3 induced a decrease in the number of muscle cells and in the size of myotubes formed. These data show a diverse intracellular distribution of CAPN3, compatible with a scaffolding protein, and suggest a multitude of different interactions of CAPN3 with other partners during muscle formation.

Myostatin inhibition promotes fast fibre hypertrophy but causes loss of AMP-activated protein kinase signalling and poor exercise tolerance in a model of limb-girdle muscular dystrophy R1/2A

KEY POINTS

Limb-girdle muscular dystrophy R1 (LGMD R1) is caused by mutations in the CAPN3 gene and is characterized by progressive muscle loss, impaired mitochondrial function and reductions in the slow oxidative gene expression programme. Myostatin is a negative regulator of muscle growth, and its inhibition improves the phenotype in several muscle wasting disorders. The effect of genetic and pharmacological inhibition of myostatin signalling on the disease phenotype in a mouse model of LGMD R1 (CAPN3 knockout mouse-C3KO) was studied. Inhibition of myostatin signalling in C3KO muscles resulted in significant muscle hypertrophy; however, there were no improvements in muscle strength and exacerbation of exercise intolerance concomitant with further reduction of muscle oxidative capacity was observed. Inhibition of myostatin signalling is unlikely to be a valid therapeutic strategy for LGMD R1.

ABSTRACT

Limb-girdle muscular dystrophy R1 (LGMD R1) is caused by mutations in the CAPN3 gene and is characterized by progressive muscle loss, impaired mitochondrial function and reductions in the slow oxidative gene expression programme. There are currently no therapies available to patients. We sought to determine if induction of muscle growth, through myostatin inhibition, represents a viable therapeutic strategy for this disease. Myostatin is a negative regulator of muscle growth, and its inhibition improves the phenotype in several muscle wasting disorders. However, the effect of myostatin depends on the genetic and pathophysiological context and may not be efficacious in all contexts. We found that genetic inhibition of myostatin through overexpression of follistatin (an endogenous inhibitor of myostatin) in our LGMD R1 model (C3KO) resulted in 1.5- to 2-fold increase of muscle mass for the majority of limb muscles. However, muscle strength was not improved and exercise intolerance was exacerbated. Pharmacological inhibition of myostatin, using an anti-myostatin antibody, resulted in statistically significant increases in muscle mass; however, functional testing did not reveal changes in muscle strength nor endurance in treated C3KO mice. Histochemical and biochemical evaluation of follistatin overexpressing mice revealed a reduction in the percentage of oxidative fibres and decreased activation of AMP-activated protein kinase signalling in transgenics compared to C3KO muscles. Our data suggest that muscle hypertrophy, induced by myostatin inhibition, leads to loss of oxidative capacity, which further compromises metabolically impaired C3KO muscles and thus is unlikely to be a valid strategy for treatment of LGMD R1.

The ties that bind: functional clusters in limb-girdle muscular dystrophy

The limb-girdle muscular dystrophies (LGMDs) are a genetically pleiomorphic class of inherited muscle diseases that are known to share phenotypic features. Selected LGMD genetic subtypes have been studied extensively in affected humans and various animal models. In some cases, these investigations have led to human clinical trials of potential disease-modifying therapies, including gene replacement strategies for individual subtypes using adeno-associated virus (AAV) vectors. The cellular localizations of most proteins associated with LGMD have been determined. However, the functions of these proteins are less uniformly characterized, thus limiting our knowledge of potential common disease mechanisms across subtype boundaries. Correspondingly, broad therapeutic strategies that could each target multiple LGMD subtypes remain less developed. We believe that three major "functional clusters" of subcellular activities relevant to LGMD merit further investigation. The best known of these is the glycosylation modifications associated with the dystroglycan complex. The other two, mechanical signaling and mitochondrial dysfunction, have been studied less systematically but are just as promising with respect to the identification of significant mechanistic subgroups of LGMD. A deeper understanding of these disease pathways could yield a new generation of precision therapies that would each be expected to treat a broader range of LGMD patients than a single subtype, thus expanding the scope of the molecular medicines that may be developed for this complex array of muscular dystrophies.

Zinc at the crossroads of exercise and proteostasis

Zinc is an essential element for all forms of life, and one in every ten human proteins is a zinc protein. Zinc has catalytic, structural and signalling functions and its correct homeostasis affects many cellular processes. Zinc deficiency leads to detrimental consequences, especially in tissues with high demand such as skeletal muscle. Zinc cellular homeostasis is tightly regulated by different transport and buffer protein systems. Specifically, in skeletal muscle, zinc has been found to affect myogenesis and muscle regeneration due to its effects on muscle cell activation, proliferation and differentiation. In relation to skeletal muscle, exercise has been shown to modulate zinc serum and urinary levels and could directly affect cellular zinc transport. The oxidative stress induced by exercise may provide the basis for the mild zinc deficiency observed in athletes and could have severe consequences on health and sport performance. Proteostasis is induced during exercise and zinc plays an essential role in several of the associated pathways.

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Zinc deficiency could be a crucial issue in sport performance for athletes.

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Exercise could modulate zinc serum and cellular homeostasis.

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Zinc is part of proteostatic systems critical during exercise.

A muscle-specific calpain, CAPN3, forms a homotrimer

Calpain-3 (CAPN3), a 94-kDa member of the calpain protease family, is abundant in skeletal muscle. Mutations in the CAPN3 gene cause limb girdle muscular dystrophy type 2A, indicating that CAPN3 plays important roles in muscle physiology. CAPN3 has several unique features. A crystallographic study revealed that its C-terminal penta-EF-hand domains form a homodimer, suggesting that CAPN3 functions as a homodimeric protease. To analyze complex formation of CAPN3 in a more convenient manner, we performed blue native polyacrylamide gel electrophoresis and found that the observed molecular weight of native CAPN3, as well as recombinant CAPN3, was larger than 240 kDa. Further analysis by cross-linking and sequential immunoprecipitation revealed that CAPN3 in fact forms a homotrimer. Trimer formation was abolished by the deletion of the PEF domain, but not the CAPN3-specific insertion sequences NS, IS1, and IS2. The PEF domain alone formed a homodimer, as reported, but addition of the adjacent CBSW domain to its N-terminus reinforced the trimer-forming property. Collectively, these results suggest that CAPN3 forms a homotrimer in which the PEF domain's dimer-forming ability is influenced by other domains.

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.

Hyperglycemia Induces Osteoclastogenesis and Bone Destruction Through the Activation of Ca2+/Calmodulin-Dependent Protein Kinase II

Hyperglycemia induces osteoclastogenesis and bone resorption through complicated, undefined mechanisms. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) promotes osteoclastogenesis, and could be activated by hyperglycemia. Here, we investigated whether CaMKII is involved in hyperglycemia-induced osteoclastogenesis and subsequent bone resorption. Osteoclast formation, bone resorption, CaMKII expression and phosphorylation were measured under high glucose in vitro and in streptozotocin-induced hyperglycemia rats with or without CaMKII inhibitor KN93. The results showed that 25 mmol/L high glucose in vitro promoted cathepsin K and tartrate-resistant acid phosphatase expression (p < 0.05) and osteoclast formation (p < 0.01) associated with enhancing β isoform expression (p < 0.05) and CaMKII phosphorylation (p < 0.001). Hyperglycemia promoted the formation of osteoclasts and resorption of trabecular and alveolar bone, and inhibited sizes of femur and mandible associated with enhanced CaMKII phosphorylation (p < 0.001) in rats. All these changes could be alleviated by KN93. These findings imply that CaMKII participates not only in hyperglycemia-induced osteoclastogenesis and subsequent bone resorption, but also in the hyperglycemia-induced developmental inhibition of bone.