Postmitotic Expression of SOD1(G93A) Gene Affects the Identity of Myogenic Cells and Inhibits Myoblasts Differentiation.

To determine the role of mutant SOD1 gene (SOD1(G93A)) on muscle cell differentiation, we derived C2C12 muscle cell lines carrying a stably transfected SOD1(G93A) gene under the control of a myosin light chain (MLC) promoter-enhancer cassette. Expression of MLC/SOD1(G93A) in C2C12 cells resulted in dramatic inhibition of myoblast differentiation. Transfected SOD1(G93A) gene expression in postmitotic skeletal myocytes downregulated the expression of relevant markers of committed and differentiated myoblasts such as MyoD, Myogenin, MRF4, and the muscle specific miRNA expression. The inhibitory effects of SOD1(G93A) gene on myogenic program perturbed Akt/p70 and MAPK signaling pathways which promote differentiation cascade. Of note, the inhibition of the myogenic program, by transfected SOD1(G93A) gene expression, impinged also the identity of myogenic cells. Expression of MLC/SOD1(G93A) in C2C12 myogenic cells promoted a fibro-adipogenic progenitors (FAPs) phenotype, upregulating HDAC4 protein and preventing the myogenic commitment complex BAF60C-SWI/SNF. We thus identified potential molecular mediators of the inhibitory effects of SOD1(G93A) on myogenic program and disclosed potential signaling, activated by SOD1(G93A), that affect the identity of the myogenic cell population.

Local expression of mIgf-1 modulates ubiquitin, caspase and CDK5 expression in skeletal muscle of an ALS mouse model.

OBJECTIVE: The functional connection between muscle and nerve is often altered in several neuromuscular diseases, including amyotrophic lateral sclerosis (ALS). Knowledge about the molecular and cellular mechanisms involved in the restorative reactions is important to our understanding of the processes involved in neuromuscular maintenance. We previously reported that muscle-restricted expression of a localized Igf-1 isoform maintained muscle integrity, stabilized neuromuscular junctions, reduced inflammation in the spinal cord and enhanced motor neuronal survival in SOD(G93A) mice, delaying the onset and progression of the disease. In this study, we analysed potential molecular pathways that are modulated by mIgf-1 to counteract muscle wasting and to preserve motor neurons activity. METHODS: We performed molecular and morphologic analysis to address the specific proposed questions. RESULTS AND DISCUSSION: Ubiquitin expression and caspase activity resulted markedly increased in SOD(G93A) muscle but maintained at very low levels in the SOD(G93A) x MLC/mIgf-1 (SOD(G93A)/mIgf-1) transgenic muscle. In addition, CDK5 expression, a serine-threonine protein kinase that has been implicated in a number of physiologic processes in nerve and muscle cells, was reduced in SOD(G93A) muscle but increased in SOD(G93A)/mIgf-1 muscle. Notably, while the toxic p25 protein accumulated in SOD(G93A) muscle, no accumulation was evident in the SOD(G93A)/mIgf-1 muscle. The maintenance of muscle phenotype was also associated with maintenance of a normal peripheral nerve, and a greater number of myelinated axons. CONCLUSION: These observations offer novel insights into the role of mIgf-1 in the attenuation of muscle wasting in the mouse model of ALS disease.

Muscle atrophy induced by SOD1G93A expression does not involve the activation of caspase in the absence of denervation.

BACKGROUND: The most remarkable feature of skeletal muscle is the capacity to adapt its morphological, biochemical and molecular properties in response to several factors. Nonetheless, under pathological conditions, skeletal muscle loses its adaptability, leading to atrophy or wasting. Several signals might function as physiopathological triggers of muscle atrophy. However, the specific mechanisms underlying the atrophic phenotype under different pathological conditions remain to be fully elucidated. In this paper, we address the involvement of caspases in the induction of muscle atrophy in experimental models of amyotrophic lateral sclerosis (ALS) expressing the mutant SOD1G93A transgene either locally or ubiquitously. RESULTS: We demonstrate that SOD1G93A-mediated muscle atrophy is independent from caspase activity. In particular, the expression of SOD1G93A promotes a reduction of the phosphatidylinositol 3-kinase/Akt pathway associated with activation of forkhead box O3. In contrast, the activation of caspases occurs later and is causally linked to motor neuron degeneration, which is associated with exacerbation of the atrophic phenotype and a shift in fiber-type composition. CONCLUSION: This study suggests that muscle atrophy induced by the toxic effect of SOD1G93A is independent from the activation of apoptotic markers and that caspase-mediated apoptosis is a process activated upon muscle denervation.

Impact of ageing on muscle cell regeneration.

Skeletal muscle regeneration is a coordinate process in which several factors are sequentially activated to maintain and preserve muscle structure and function. The major role in the growth, remodeling and regeneration is played by satellite cells, a quiescent population of myogenic cells that reside between the basal lamina and plasmalemma and are rapidly activated in response to appropriate stimuli. However, in several muscle conditions, including aging, the capacity of skeletal muscle to sustain an efficient regenerative pathway is severely compromised. Nevertheless, if skeletal muscle possesses a stem cell compartment it is not clear why the muscle fails to regenerate under pathological conditions. Either the resident muscle stem cells are too rare or intrinsically incapable of repairing major damage, or perhaps the injured/pathological muscle is a prohibitive environment for stem cell activation and function. Although we lack definitive answers, recent experimental evidences suggest that the mere presence of endogenous stem cells may not be sufficient to guarantee muscle regeneration, and that the presence of appropriate stimuli and factors are necessary to provide a permissive environment that permits stem cell mediated muscle regeneration and repair. In this review we discuss the molecular basis of muscle regeneration and how aging impacts stem cell mediated muscle regeneration and repair.

Localized accumulation of oxidative stress causes muscle atrophy through activation of an autophagic pathway.

A crucial system severely affected in different chronic diseases is the antioxidative defense, leading to accumulation of reactive oxygen species (ROS). The discovery that deletion in the antioxidant genes shortens significantly the mouse life span, and that mutation in the major antioxidant enzyme SOD1 is associated with neurodegenerative diseases, has placed oxidative stress as a central mechanism in the pathogenesis of many pathological conditions. However, how such an oxidative insult plays a role in the disease-related decrease of muscle performance and mass remains largely unknown. We recently demonstrated that autophagy plays a dominant role in the promotion of muscle atrophy associated with local alteration in the activity of the antioxidant enzyme SOD1. In particular, transcription of autophagy-related genes, such as those encoding LC3, Cathepsin-L and Bnip3, is activated in response to localized accumulation of oxidative stress and is mediated by FoxO3. In addition, our study documents how the T-tubule might be the potential donor of membrane that forms sequestering autophagic vesicles. Here we discuss the sequence of events leading to muscle atrophy.

Skeletal muscle is a primary target of SOD1G93A-mediated toxicity.

The antioxidant enzyme superoxide dismutase 1 (SOD1) is a critical player of the antioxidative defense whose activity is altered in several chronic diseases, including amyotrophic lateral sclerosis. However, how oxidative insult affects muscle homeostasis remains unclear. This study addresses the role of oxidative stress on muscle homeostasis and function by the generation of a transgenic mouse model expressing a mutant SOD1 gene (SOD1(G93A)) selectively in skeletal muscle. Transgenic mice developed progressive muscle atrophy, associated with a significant reduction in muscle strength, alterations in the contractile apparatus, and mitochondrial dysfunction. The analysis of molecular pathways associated with muscle atrophy revealed that accumulation of oxidative stress served as signaling molecules to initiate autophagy, one of the major intracellular degradation mechanisms. These data demonstrate that skeletal muscle is a primary target of SOD1(G93A) -mediated toxicity and disclose the molecular mechanism whereby oxidative stress triggers muscle atrophy.