Role of damage and management in muscle hypertrophy: Different behaviors of muscle stem cells in regeneration and hypertrophy

Skeletal muscle is a dynamic tissue with two unique abilities; one is its excellent regenerative ability, due to the activity of skeletal muscle–resident stem cells named muscle satellite cells (MuSCs); and the other is the adaptation of myofiber size in response to external stimulation, intrinsic factors, or physical activity, which is known as plasticity. Low physical activity and some disease conditions lead to the reduction of myofiber size, called atrophy, whereas hypertrophy refers to the increase in myofiber size induced by high physical activity or anabolic hormones/drugs. MuSCs are essential for generating new myofibers during regeneration and the increase in new myonuclei during hypertrophy; however, there has been little investigation of the molecular mechanisms underlying MuSC activation, proliferation, and differentiation during hypertrophy compared to those of regeneration. One reason is that ‘degenerative damage’ to myofibers during muscle injury or upon hypertrophy (especially overloaded muscle) is believed to trigger similar activation/proliferation of MuSCs. However, evidence suggests that degenerative damage of myofibers is not necessary for MuSC activation/proliferation during hypertrophy. When considering MuSC-based therapy for atrophy, including sarcopenia, it will be indispensable to elucidate MuSC behaviors in muscles that exhibit non-degenerative damage, because degenerated myofibers are not present in the atrophied muscles. In this review, we summarize recent findings concerning the relationship between MuSCs and hypertrophy, and discuss what remains to be discovered to inform the development and application of relevant treatments for muscle atrophy.     

CD90-positive cells, an additional cell population, produce laminin alpha2 upon transplantation to dy(3k)/dy(3k) mice

Laminin alpha2 is a component of skeletal and cardiac muscle basal lamina. A defect of the laminin alpha2 chain leads to severe congenital muscular dystrophy (MDC1A) in humans and dy/dy mice. Myogenic cells including myoblasts, myotubes, and myofibers in skeletal muscle are a possible source of the laminin alpha2 chain, and myogenic cells are thus proposed as a cell source for congenital muscular dystrophy therapy. However, we observed production of laminin alpha2 in non-myogenic cells of normal mice, and we could enrich these laminin alpha2-producing cells in CD90(+) cell fractions. Intriguingly, the number of CD90(+) cells increased dramatically during skeletal muscle regeneration in mice. This fraction did not include myogenic cells but exhibited a fibroblast-like phenotype. Moreover, these cells were resident in skeletal muscle, not derived from bone marrow. Finally, the production of laminin alpha2 in CD90(+) cells was not dependent on fusion with myogenic cells. Thus, CD90(+) cells are a newly identified additional cell fraction that increased during skeletal muscle regeneration in vivo and could be another cell source for therapy for lama2-deficient muscular dystrophy.     

Emerin increase in regenerating muscle fibers

The fate of emerin during skeletal muscle regeneration was investigated in an animal model by means of crush injury. Immunofluorescence, immunoblotting and mRNA analysis demonstrated that emerin level is increased in regenerating rat muscle fibers with respect to normal mature myofibers. This finding suggests an involvement of emerin during the muscle fiber regeneration process, in analogy with its reported involvement in muscle cell differentiation in vitro. The impairment of skeletal muscle physiological regeneration or reorganization could be a possible pathogenetic mechanism for Emery Dreifuss muscular dystrophy.     

Secreted Protein Acidic and Rich in Cysteine (SPARC) in Human Skeletal Muscle

Secreted protein acidic and rich in cysteine (SPARC)/osteonectin is expressed in different tissues during remodeling and repair, suggesting a function in regeneration. Several gene expression studies indicated that SPARC was expressed in response to muscle damage. Studies on myoblasts further indicated a function of SPARC in skeletal muscle. We therefore found it of interest to study SPARC expression in human skeletal muscle during development and in biopsies from Duchenne and Becker muscular dystrophy and congenital muscular dystrophy, congenital myopathy, inclusion body myositis, and polymyositis patients to analyze SPARC expression in a selected range of inherited and idiopathic muscle wasting diseases. SPARC-positive cells were observed both in fetal and neonatal muscle, and in addition, fetal myofibers were observed to express SPARC at the age of 15–16 weeks. SPARC protein was detected in the majority of analyzed muscle biopsies (23 of 24), mainly in mononuclear cells of which few were pax7 positive. Myotubes and regenerating myofibers also expressed SPARC. The expression-degree seemed to reflect the severity of the lesion. In accordance with these in vivo findings, primary human-derived satellite cells were found to express SPARC both during proliferation and differentiation in vitro. In conclusion, this study shows SPARC expression both during muscle development and in regenerating muscle. The expression is detected both in satellite cells/myoblasts and in myotubes and muscle fibers, indicating a role for SPARC in the skeletal muscle compartment. (J Histochem Cytochem 57:29–39, 2009)     

Loss of emerin alters myogenic signaling and miRNA expression in mouse myogenic progenitors

Emerin is an integral membrane protein of the inner nuclear membrane. Mutations in emerin cause X-linked Emery-Dreifuss muscular dystrophy (EDMD), a disease characterized by skeletal muscle wasting and dilated cardiomyopathy. Current evidence suggests the muscle wasting phenotype of EDMD is caused by defective myogenic progenitor cell differentiation and impaired muscle regeneration. We obtained genome-wide expression data for both mRNA and micro-RNA (miRNA) in wildtype and emerin-null mouse myogenic progenitor cells. We report here that emerin-null myogenic progenitors exhibit differential expression of multiple signaling pathway components required for normal muscle development and regeneration. Components of the Wnt, IGF-1, TGF-β, and Notch signaling pathways are misexpressed in emerin-null myogenic progenitors at both the mRNA and protein levels. We also report significant perturbations in the expression and activation of p38/Mapk14 in emerin-null myogenic progenitors, showing that perturbed expression of Wnt, IGF-1, TGF-β, and Notch signaling components disrupts normal downstream myogenic signaling in these cells. Collectively, these data support the hypothesis that emerin is essential for proper myogenic signaling in myogenic progenitors, which is necessary for myogenic differentiation and muscle regeneration.     

Regulation and dysregulation of fibrosis in skeletal muscle

In response to skeletal muscle injury, distinct cellular pathways are activated to repair the damaged tissue. Activation and restriction of these pathways must be temporally coordinated in a precise sequence as regeneration progresses if muscle integrity and homeostasis are to be restored. However, if tissue injury persists, as in severe muscular dystrophies, the repair process becomes uncontrolled leading to the substitution of myofibers by a non-functional mass of fibrotic tissue. In this review, we provide an overview of how muscle responds to damage and aging, with special emphasis on the cellular effectors and the regulatory and inflammatory pathways that can shift normal muscle repair to fibrosis development.     

调控骨骼肌纤维类型转化的信号通路

骨骼肌由异质性的肌纤维组成,不同类型的肌纤维具有不同的形态、代谢、生理和生化特性.根据不同肌纤维中表达的特异肌球蛋白重链亚型可将成体哺乳动物骨骼肌纤维分为4类,即Ⅰ,Ⅱa,Ⅱx和Ⅱb型.骨骼肌保持高度可塑性,当机体受到某些生理或病理刺激时,骨骼肌为了适应需要,通过激活胞内相关信号通路改变肌纤维特异基因的表达从而诱发肌纤维类型的转化.本文综述了细胞内参与调控肌纤维类型转化的多条重要信号通路,如Ca2+信号通路,Ras/MAPK信号通路及多种转录调节因子,辅激活因子和抑制子等,为改善肉类品质,提高运动训练效果及治疗肌肉相关疾病奠定了理论基础.     

Connexin- and Pannexin-Based Channels in Normal Skeletal Muscles and Their Possible Role in Muscle Atrophy

Precursor cells of skeletal muscles express connexins 39, 43 and 45 and pannexin1. In these cells, most connexins form two types of membrane channels, gap junction channels and hemichannels, whereas pannexin1 forms only hemichannels. All these channels are low-resistance pathways permeable to ions and small molecules that coordinate developmental events. During late stages of skeletal muscle differentiation, myofibers become innervated and stop expressing connexins but still express pannexin1 hemichannels that are potential pathways for the ATP release required for potentiation of the contraction response. Adult injured muscles undergo regeneration, and connexins are reexpressed and form membrane channels. In vivo, connexin reexpression occurs in undifferentiated cells that form new myofibers, favoring the healing process of injured muscle. However, differentiated myofibers maintained in culture for 48 h or treated with proinflammatory cytokines for less than 3 h also reexpress connexins and only form functional hemichannels at the cell surface. We propose that opening of these hemichannels contributes to drastic changes in electrochemical gradients, including reduction of membrane potential, increases in intracellular free Ca²⁺ concentration and release of diverse metabolites (e.g., NAD⁺ and ATP) to the extracellular milieu, contributing to multiple metabolic and physiologic alterations that characterize muscles undergoing atrophy in several acquired and genetic human diseases. Consequently, inhibition of connexin hemichannels expressed by injured or denervated skeletal muscles might reduce or prevent deleterious changes triggered by conditions that promote muscle atrophy.     

Altered activity of signaling pathways in diaphragm and tibialis anterior muscle of dystrophic mice

Duchenne muscular dystrophy is a musculoskeletal disease caused by mutations in the dystrophin gene. The purpose of this study was to use the mouse model of muscular dystrophy (mdx) to determine if the progression of the dystrophic phenotype in the diaphragm (costal) versus limb skeletal muscle (tibialis anterior) is associated with specific changes in extracellular regulated kinase (ERK1/2), p70 S6 kinase (p70(S6k)), or p38 signaling pathways. The studies detected that consistent with an earlier dystrophic phenotype, phosphorylation of p70(S6k) is elevated by 40% in the diaphragm with no change in limb muscle. In addition, phosphorylation of p38 kinase was decreased by 33% in the mdx diaphragm muscle. Levels of ERK1/2 as well as phosphorylation states were elevated in the diaphragm and limb muscle of mdx mice compared with age-matched control muscles. These results indicate that distinct signaling pathways are differentially activated in skeletal muscle of mdx mice. The specificity of these responses, particularly in the diaphragm, provides insight for potential targets for blunting the progression of the muscular dystrophy phenotype.     

Muscular Dystrophy in mdx Mice Despite Lack of Neuronal Nitric Oxide Synthase

Abstract: Neuronal nitric oxide synthase (nNOS) is a component of the dystrophin complex in skeletal muscle. The absence of dystrophin protein in Duchenne muscular dystrophy and in mdx mouse causes a redistribution of nNOS from the plasma membrane to the cytosol in muscle cells. Aberrant nNOS activity in the cytosol can induce free radical oxidation, which is toxic to myofibers. To test the hypothesis that derangements in nNOS disposition mediate muscle damage in Duchenne dystrophy, we bred dystrophin-deficient mdx male mice and female mdx heterozygote mice that lack nNOS. We found that genetic deletion of nNOS does not itself cause detectable pathology and that removal of nNOS does not influence the extent of increased sarcolemmal permeability in dystrophin-deficient mice. Thus, histological analyses of nNOS-dystrophin double mutants show pathological changes similar to the dystrophin mutation alone. Taken together, nNOS defects alone do not produce muscular dystrophy in the mdx model.     

nNOS regulation of skeletal muscle fatigue and exercise performance

Neuronal nitric oxide synthases (nNOS) are Ca²⁺/calmodulin-activated enzymes that synthesize the gaseous messenger nitric oxide (NO). nNOSμ and the recently described nNOSβ, both spliced nNOS isoforms, are important enzymatic sources of NO in skeletal muscle, a tissue long considered to be a paradigmatic system for studying NO-dependent redox signaling. nNOS is indispensable for skeletal muscle integrity and contractile performance, and deregulation of nNOSμ signaling is a common pathogenic feature of many neuromuscular diseases. Recent evidence suggests that both nNOSμ and nNOSβ regulate skeletal muscle size, strength, and fatigue resistance, making them important players in exercise performance. nNOSμ acts as an activity sensor and appears to assist skeletal muscle adaptation to new functional demands, particularly those of endurance exercise. Prolonged inactivity leads to nNOS-mediated muscle atrophy through a FoxO-dependent pathway. nNOS also plays a role in modulating exercise performance in neuromuscular disease. In the mdx mouse model of Duchenne muscular dystrophy, defective nNOS signaling is thought to restrict contractile capacity of working muscle in two ways: loss of sarcolemmal nNOSμ causes excessive ischemic damage while residual cytosolic nNOSμ contributes to hypernitrosylation of the ryanodine receptor, causing pathogenic Ca²⁺ leak. This defect in Ca²⁺ handling promotes muscle damage, weakness, and fatigue. This review addresses these recent advances in the understanding of nNOS-dependent redox regulation of skeletal muscle function and exercise performance under physiological and neuromuscular disease conditions.     

Stem cell based therapy for skeletal muscle diseases

The use of stem cells to repair and replace damaged skeletal muscle cells in chronic, debilitating muscle diseases such as the muscular dystrophies holds great promise. Different stem cell populations, both of embryonic and adult origin display the potential to generate skeletal muscle cells and have been studied in animal models of muscular dystrophy. These include muscle derived satellite cells; bone marrow derived mesenchymal stem cells, muscle or bone marrow side population cells, circulating CD133+ cells and cells derived from blood vessel walls such as mesoangioblasts or pericytes. The design of effective stem cell based therapies requires a detailed understanding of the molecules and signaling pathways which determine myogenic lineage commitment and differentiation. We discuss the great strides that have been made in delineating these pathways and how a better understanding of muscle stem cell biology has the potential to lead to more effective stem cell based therapies for skeletal muscle regeneration for devastating muscle diseases.     

Functional characteristics of dystrophic skeletal muscle: insights from animal models.

Muscular dystrophies are a clinically and genetically heterogeneous group of disorders that show myofiber degeneration and regeneration. Identification of animal models of muscular dystrophy has been instrumental in research on the pathogenesis, pathophysiology, and treatment of these disorders. We review our understanding of the functional status of dystrophic skeletal muscle from selected animal models with a focus on 1) the mdx mouse model of Duchenne muscular dystrophy, 2) the Bio 14.6 delta-sarcoglycan-deficient hamster model of limb-girdle muscular dystrophy, and 3) transgenic null mutant murine lines of sarcoglycan (alpha, beta, delta, and gamma) deficiencies. Although biochemical data from these models suggest that the dystrophin-sarcoglycan-dystroglycan-laminin network is critical for structural integrity of the myofiber plasma membrane, emerging studies of muscle physiology suggest a more complex picture, with specific functional deficits varying considerably from muscle to muscle and model to model. It is likely that changes in muscle structure and function, downstream of the specific, primary biochemical deficiency, may alter muscle contractile properties.     

Genetic disruption of calcineurin improves skeletal muscle pathology and cardiac disease in a mouse model of limb-girdle muscular dystrophy

Calcineurin (Cn) is a Ca(2+)/calmodulin-dependent serine/threonine phosphatase that regulates differentiation-specific gene expression in diverse tissues, including the control of fiber-type switching in skeletal muscle. Recent studies have implicated Cn signaling in diminishing skeletal muscle pathogenesis associated with muscle injury or disease-related muscle degeneration. For example, use of the Cn inhibitor cyclosporine A has been shown to delay muscle regeneration following toxin-induced injury and inhibit regeneration in the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy. In contrast, transgenic expression of an activated mutant of Cn in skeletal muscle was shown to increase utrophin expression and reduce overall disease pathology in mdx mice. Here we examine the effect of altered Cn activation in the context of the delta-sarcoglycan-null (scgd(-/-)) mouse model of limb-girdle muscular dystrophy. In contrast to results discussed in mdx mice, genetic deletion of a loxP-targeted calcineurin B1 (CnB1) gene using a skeletal muscle-specific cre allele in the scgd(-/-) background substantially reduced skeletal muscle degeneration and histopathology compared with the scgd(-/-) genotype alone. A similar regression in scgd-dependent disease manifestation was also observed in calcineurin Abeta (CnAbeta) gene-targeted mice in both skeletal muscle and heart. Conversely, increased Cn expression using a muscle-specific transgene increased cardiac fibrosis, decreased cardiac ventricular shortening, and increased muscle fiber loss in the quadriceps. Our results suggest that inhibition of Cn may benefit select types of muscular dystrophy.     

Signaling pathways controlling skeletal muscle mass

Abstract

The molecular mechanisms underlying skeletal muscle maintenance involve interplay between multiple signaling pathways. Under normal physiological conditions, a network of interconnected signals serves to control and coordinate hypertrophic and atrophic messages, culminating in a delicate balance between muscle protein synthesis and proteolysis. Loss of skeletal muscle mass, termed “atrophy”, is a diagnostic feature of cachexia seen in settings of cancer, heart disease, chronic obstructive pulmonary disease, kidney disease, and burns. Cachexia increases the likelihood of death from these already serious diseases. Recent studies have further defined the pathways leading to gain and loss of skeletal muscle as well as the signaling events that induce differentiation and post-injury regeneration, which are also essential for the maintenance of skeletal muscle mass. In this review, we summarize and discuss the relevant recent literature demonstrating these previously undiscovered mediators governing anabolism and catabolism of skeletal muscle.     

Human intronless genes: functional groups, associated diseases, evolution, and mRNA processing in absence of splicing

Duchenne muscular dystrophy is the most prevalent inheritable muscle disease. Transplantation of autologous stem cells with gene direction is an ideal therapeutic approach for the disease. The current study aimed to investigate the restoration of myofibers in mdx mice after mdx bone marrow-derived mesenchymal stem cell (mMSC) transplantation with human microdystrophin delivery. Possible mechanisms of action were also studied. In our research, mMSCs were successfully transduced by retrovirus carrying a functional human microdystrophin gene. Transplantation of transduced mMSCs enabled persistent dystrophin restoration in the skeletal muscle of mdx mice up to the 12th week after transplantation. Simultaneous coexpression of human microdystrophin and desmin showed that implanted mMSCs are capable of long-term survival as muscle satellite cells.     

失神经骨骼肌萎缩的研究现状及进展

当今骨科领域,周围神经损伤一直影响着患者疗效。肌萎缩的发生,细胞凋亡导致骨骼肌萎缩,神经-肌肉接头处营养因子的代谢发生障碍,肌卫星细胞的减少,生长因子以及线粒体和各种酶的变化都是失神经骨骼肌萎缩的机制。电刺激法,保护神经元,生长因子,神经植入提高神经再生速度以及被动活动可以有效治疗患者。失神经骨骼肌萎缩的研究进展也趋于完善。