Skeletal muscle-specific ablation of raptor, but not of rictor, causes metabolic changes and results in muscle dystrophy.

Mammalian target of rapamycin (mTOR) is a central controller of cell growth. mTOR assembles into two distinct multiprotein complexes called mTOR complex 1 (mTORC1) and mTORC2. Here we show that the mTORC1 component raptor is critical for muscle function and prolonged survival. In contrast, muscles lacking the mTORC2 component rictor are indistinguishable from wild-type controls. Raptor-deficient muscles become progressively dystrophic, are impaired in their oxidative capacity, and contain increased glycogen stores, but they express structural components indicative of oxidative muscle fibers. Biochemical analysis indicates that these changes are probably due to loss of activation of direct downstream targets of mTORC1, downregulation of genes involved in mitochondrial biogenesis, including PGC1alpha, and hyperactivation of PKB/Akt. Finally, we show that activation of PKB/Akt does not require mTORC2. Together, these results demonstrate that muscle mTORC1 has an unexpected role in the regulation of the metabolic properties and that its function is essential for life.

Congenital myopathies: diseases of the actin cytoskeleton.

Congenital myopathies are clinical and genetic heterogeneous disorders characterized by skeletal muscle weakness ranging in severity. Three major forms have been identified: actin myopathy, intranuclear rod myopathy, and nemaline myopathy. Nemaline myopathy is the most common of these myopathies and is further subdivided into seven groups according to severity, progressiveness, and age of onset. At present, five genes have been linked to congenital myopathies. These include alpha-actin (ACTA1), alpha- and beta-tropomyosin (TPM3 and TPM2), troponin T (TNNT1), and nebulin (NEB). Their protein products are all components of the thin filament of the sarcomere. The mutations identified within these genes have varying impacts on protein structure and give rise to different forms of congenital myopathies. Greater understanding of muscle formation and cause of disease can be established through the study of the effect of mutations on the functional proteins. However, a major limitation in the understanding of congenital myopathies is the lack of correlation between the degree of sarcomeric disruption and disease severity. Consequently, great difficulty may be encountered when diagnosing patients and predicting the progression of the disorders. There are no existing cures for congenital myopathies, although improvements can be made to both the standard of living and the life expectancy of the patient through various therapies.

Myopathy mutations in alpha-skeletal-muscle actin cause a range of molecular defects.

Mutations in the gene encoding alpha-skeletal-muscle actin, ACTA1, cause congenital myopathies of various phenotypes that have been studied since their discovery in 1999. Although much is now known about the clinical aspects of myopathies resulting from over 60 different ACTA1 mutations, we have very little evidence for how mutations alter the behavior of the actin protein and thus lead to disease. We used a combination of biochemical and cell biological analysis to classify 19 myopathy mutants and found a range of defects in the actin. Using in vitro expression systems, we probed actin folding and actin's capacity to interact with actin-binding proteins and polymerization. Only two mutants failed to fold; these represent recessive alleles, causing severe myopathy, indicating that patients produce nonfunctional actin. Four other mutants bound tightly to cyclase-associated protein, indicating a possible instability in the nucleotide-binding pocket, and formed rods and aggregates in cells. Eleven mutants showed defects in the ability to co-polymerize with wild-type actin. Some of these could incorporate into normal actin structures in NIH 3T3 fibroblasts, but two of the three tested also formed aggregates. Four mutants showed no defect in vitro but two of these formed aggregates in cells, indicating functional defects that we have not yet tested for. Overall, we found a range of defects and behaviors of the mutants in vitro and in cultured cells, paralleling the complexity of actin-based muscle myopathy phenotypes.