Alpha-actinin-3 deficiency results in reduced glycogen phosphorylase activity and altered calcium handling in skeletal muscle.

Approximately one billion people worldwide are homozygous for a stop codon polymorphism in the ACTN3 gene (R577X) which results in complete deficiency of the fast fibre muscle protein alpha-actinin-3. ACTN3 genotype is associated with human athletic performance and alpha-actinin-3 deficient mice [Actn3 knockout (KO) mice] have a shift in the properties of fast muscle fibres towards slower fibre properties, with increased activity of multiple enzymes in the aerobic metabolic pathway and slower contractile properties. alpha-Actinins have been shown to interact with a number of muscle proteins including the key metabolic regulator glycogen phosphorylase (GPh). In this study, we demonstrated a link between alpha-actinin-3 and glycogen metabolism which may underlie the metabolic changes seen in the KO mouse. Actn3 KO mice have higher muscle glycogen content and a 50% reduction in the activity of GPh. The reduction in enzyme activity is accompanied by altered post-translational modification of GPh, suggesting that alpha-actinin-3 regulates GPh activity by altering its level of phosphorylation. We propose that the changes in glycogen metabolism underlie the downstream metabolic consequences of alpha-actinin-3 deficiency. Finally, as GPh has been shown to regulate calcium handling, we examined calcium handling in KO mouse primary mouse myoblasts and find changes that may explain the slower contractile properties previously observed in these mice. We propose that the alteration in GPh activity in the absence of alpha-actinin-3 is a fundamental mechanistic link in the association between ACTN3 genotype and human performance.

Regulation of TRPC1 and TRPC4 cation channels requires an alpha1-syntrophin-dependent complex in skeletal mouse myotubes.

The dystrophin-associated protein complex (DAPC) is essential for skeletal muscle, and the lack of dystrophin in Duchenne muscular dystrophy results in a reduction of DAPC components such as syntrophins and in fiber necrosis. By anchoring various molecules, the syntrophins may confer a role in cell signaling to the DAPC. Calcium disorders and abnormally elevated cation influx in dystrophic muscle cells have suggested that the DAPC regulates some sarcolemmal cationic channels. We demonstrated previously that mini-dystrophin and alpha1-syntrophin restore normal cation entry in dystrophin-deficient myotubes and that sarcolemmal TRPC1 channels associate with dystrophin and the bound PDZ domain of alpha1-syntrophin. This study shows that small interfering RNA (siRNA) silencing of alpha1-syntrophin dysregulated cation influx in myotubes. Moreover, deletion of the PDZ-containing domain prevented restoration of normal cation entry by alpha1-syntrophin transfection in dystrophin-deficient myotubes. TRPC1 and TRPC4 channels are expressed at the sarcolemma of muscle cells; forced expression or siRNA silencing showed that cation influx regulated by alpha1-syntrophin is supported by TRPC1 and TRPC4. A molecular association was found between TRPC1 and TRPC4 channels and the alpha1-syntrophin-dystrophin complex. TRPC1 and TRPC4 channels may form sarcolemmal channels anchored to the DAPC, and alpha1-syntrophin is necessary to maintain the normal regulation of TRPC-supported cation entry in skeletal muscle. Cation channels with DAPC form a signaling complex that modulates cation entry and may be crucial for normal calcium homeostasis in skeletal muscles.

Intranuclear rod myopathy: molecular pathogenesis and mechanisms of weakness.

OBJECTIVE: Mutations in the alpha-skeletal actin gene (ACTA1) result in a variety of inherited muscle disorders characterized by different pathologies and variable clinical phenotypes. Mutations at Val163 in ACTA1 result in pure intranuclear rod myopathy; however, the molecular mechanisms by which mutations at Val163 lead to intranuclear rod formation and muscle weakness are unknown. METHODS AND RESULTS: We investigated the effects of the Val163Met mutation in ACTA1 in tissue culture and Drosophila models, and in patient muscle. In cultured cells, the mutant actin tends to aggregate rather than incorporate into cytoplasmic microfilaments, and it affects the dynamics of wild-type actin, causing it to accumulate with the mutant actin in the nucleus. In Drosophila, the Val163Met mutation severely disrupts the structure of the muscle sarcomere. The intranuclear aggregates in patient muscle biopsies impact on nuclear structure and sequester normal Z-disc-associated proteins within the nucleus; however, the sarcomeric structure is relatively well preserved, with evidence of active regeneration. By mass spectrometry, the levels of mutant protein are markedly reduced in patient muscle compared with control. INTERPRETATION: Data from our tissue culture and Drosophila models show that the Val163Met mutation in alpha-skeletal actin can affect the dynamics of other actin isoforms and severely disrupt sarcomeric structure, processes that can contribute to muscle weakness. However, in human muscle, there is evidence of regeneration, and the mutant protein tends to aggregate rather than incorporate into cytoplasmic microfilaments in cells. These are likely compensatory processes that ameliorate the effects of the mutant actin and contribute to the milder clinical and pathological disease phenotype.

Regulation of capacitative calcium entries by alpha1-syntrophin: association of TRPC1 with dystrophin complex and the PDZ domain of alpha1-syntrophin.

Calcium mishandling in Duchenne dystrophic muscle suggested that dystrophin, a membrane-associated cytoskeleton protein, might regulate calcium signaling cascade such as calcium influx pathway. It was previously shown that abnormal calcium entries involve uncontrolled stretch-activated currents and store-operated Ca2+ currents supported by TRPC1 channels. Moreover, our recent work demonstrated that reintroduction of minidystrophin in dystrophic myotubes restores normal capacitative calcium entries (CCEs). However, until now, no molecular link between the dystrophin complex and calcium entry channels has been described. This study is the first to show by coimmunoprecipitation assays the molecular association of TRPC1 with dystrophin and alpha1-syntrophin in muscle cells. TRPC1 was also associated with alpha1-syntrophin in dystrophic muscle cells independently of dystrophin. Furthermore, glutathione S-transferase (GST) pull-down assays showed that TRPC1 binds to the alpha1-syntrophin PDZ domain. Transfected recombinant alpha1-syntrophin formed a complex with TRPC1 channels and restored normal CCEs in dystrophic muscle cells. We suggest that normal regulation of CCEs in skeletal muscle depends on the association between TRPC1 channels and alpha1-syntrophin that may anchor the store-operated channels to the dystrophin-associated protein complex (DAPC). The loss of this molecular association could participate in the calcium alterations observed in dystrophic muscle cells. This study provides a new model for the regulation of calcium influx by interaction with the scaffold of the DAPC in muscle cells.

Congenital muscular dystrophy type 1D (MDC1D) due to a large intragenic insertion/deletion, involving intron 10 of the LARGE gene.

Mutation of the LARGE gene is the rarest of the six known genetic causes of α-dystroglycanopathy. We report further a family with MDC1D due to a complex genomic rearrangement that was not apparent on standard sequencing of LARGE. Two sisters in a consanguineous family had moderate mental retardation and cerebellar malformations, together with dystrophic changes and markedly reduced α-dystroglycan glycosylation staining on muscle biopsy. There was homozygous linkage to the LARGE locus but sequencing of LARGE coding regions was normal. Analysis of LARGE cDNA showed an abnormal sequence inserted between exons 10 and 11, in most of the transcripts, predicted to introduce a premature stop codon. The abnormal sequence mapped to a spliced EST (DA935254) of unknown function, normally located at 100 kb centromeric of LARGE on chromosome 22q12.3. Quantitative PCR analysis of the EST and adjacent regions showed twice the normal copy number in patients' genomic DNA samples, consistent with a large intra-chromosomal duplication inserted into intron 10 of LARGE in a homozygous state. This insertion was associated with deletion of a central region of intron 10, but the exact break points of the deletion/duplication were not found, suggesting that an even more complex rearrangement may have occurred. The exact function of LARGE, a golgi protein, remains uncertain. POMT and POMGnT enzyme activities were normal in patients' lymphoblast cells, suggesting that defects in LARGE do not affect the initiation of O-mannosyl glycans.

Glycosaminoglycan mimetics trigger IP3-dependent intracellular calcium release in myoblasts.

Glycosaminoglycans (GAG) are sulfated polysaccharides that play an important role in regulating cell functions. GAG mimetics called RGTAs (for ReGeneraTing Agents) have been shown to stimulate tissue repair. In particular they accelerate myogenesis, in part via their heparin-mimetic property towards growth factors. RGTAs also increase activity of calcium-dependent intracellular protease suggesting an effect on calcium cellular homeostasis. This effect was presently investigated on myoblasts in vitro using one member of the RGTA family molecule named OTR4120. We have shown that OTR4120 or heparin induced transient increases of intracellular calcium concentration ([Ca(2+)]i) in pre-fusing myoblasts from both mouse SolD7 cell line and rat skeletal muscle satellite cells grown in primary culture by mobilising sarcoplasmic reticulum store. This [Ca(2+)]i was not mediated by ryanodine receptors but instead resulted from stimulation of the Inositol-3 phosphate-phospholipase C activation pathway. OTR4120-induced calcium transient was not mediated through an ATP, nor a tyrosine kinase, nor an acetylcholine receptor but principally through serotonin 5-HT2A receptor. This original finding shows that the GAG mimetic can induce calcium signal through serotonin receptors and the IP3 pathway may be relevant to its ability to favour myoblast differentiation. It supports a novel and unexpected function of GAGs in the regulation of calcium homeostasis.

In vitro analysis of rod composition and actin dynamics in inherited myopathies.

Rods are the pathological hallmark of nemaline myopathy, but they can also occur as a secondary phenomenon in other disorders, including mitochondrial myopathies such as complex I deficiency. The mechanisms of rod formation are not well understood, particularly when rods occur in diverse disorders with very different structural and metabolic defects. We compared the characteristics of rods associated with abnormalities in structural components of skeletal muscle thin filament (3 mutations in the skeletal actin gene ACTA1) with those of rods induced by the metabolic cell stress of adenosine triphosphate depletion. C2C12 and NIH/3T3 cell culture models and immunocytochemistry were used to study rod composition and conformation. Fluorescent recovery after photobleaching was used to measure actin dynamics inside the rods. We demonstrate that not all rods are the same. Rods formed under different conditions contain a unique fingerprint of actin-binding proteins (cofilin and alpha-actinin) and display differences in actin dynamics that are specific to the mutation, to the cellular location of the rods (intranuclear vs cytoplasmic), and/or to the underlying pathological process (i.e. mutant actin or adenosine triphosphate depletion). Thus, rods likely represent a common morphological end point of a variety of different pathological processes, either structural or metabolic.