Α-syntrophin modulates myogenin expression in differentiating myoblasts

LSD1 defines the fiber type-selective responsiveness to environmental stress in skeletal muscle

Skeletal muscle exhibits remarkable plasticity in response to environmental cues, with stress-dependent effects on the fast-twitch and slow-twitch fibers. Although stress-induced gene expression underlies environmental adaptation, it is unclear how transcriptional and epigenetic factors regulate fiber type-specific responses in the muscle. Here, we show that flavin-dependent lysine-specific demethylase-1 (LSD1) differentially controls responses to glucocorticoid and exercise in postnatal skeletal muscle. Using skeletal muscle-specific LSD1-knockout mice and in vitro approaches, we found that LSD1 loss exacerbated glucocorticoid-induced atrophy in the fast fiber-dominant muscles, with reduced nuclear retention of Foxk1, an anti-autophagic transcription factor. Furthermore, LSD1 depletion enhanced endurance exercise-induced hypertrophy in the slow fiber-dominant muscles, by induced expression of ERRγ, a transcription factor that promotes oxidative metabolism genes. Thus, LSD1 serves as an 'epigenetic barrier' that optimizes fiber type-specific responses and muscle mass under the stress conditions. Our results uncover that LSD1 modulators provide emerging therapeutic and preventive strategies against stress-induced myopathies such as sarcopenia, cachexia, and disuse atrophy.

α-Syntrophin alleviates ER stress to maintain protein homeostasis during myoblast differentiation

We have previously shown evidence that α-syntrophin plays an important role in myoblast differentiation. In this study, we focused on abnormal myotube formation of the α-syntrophin knockdown C2 cell line (SNKD). The overall amount of intracellular protein and muscle-specific proteins in SNKD cells were significantly lower than those in the control. Akt-mTOR signaling, an important pathway for protein synthesis and muscle hypertrophy, was downregulated. In addition, the levels of endoplasmic reticulum (ER) stress markers increased in SNKD cells. The decrease in intracellular protein synthesis and reduction in the myotube diameter in SNKD cells were restored by 4-phenylbutyric acid, a chemical chaperone, or overexpression of α-syntrophin. These results suggest a novel role for α-syntrophin in protein homeostasis during myoblast differentiation.

Astaxanthin Prevents Atrophy in Slow Muscle Fibers by Inhibiting Mitochondrial Reactive Oxygen Species via a Mitochondria-Mediated Apoptosis Pathway

Astaxanthin (AX) is a carotenoid that exerts potent antioxidant activity and acts in the lipid bilayer. This study aimed to investigate the effects of AX on muscle-atrophy-mediated disturbance of mitochondria, which have a lipid bilayer. Tail suspension was used to establish a muscle-atrophied mouse model. AX diet fed to tail-suspension mice prevented loss of muscle weight, inhibited the decrease of myofiber size, and restrained the increase of hydrogen peroxide (H₂O₂) production in the soleus muscle. Additionally, AX improved downregulation of mitochondrial respiratory chain complexes I and III in the soleus muscle after tail suspension. Meanwhile, AX promoted mitochondrial biogenesis by upregulating the expressions of adenosine 5′-monophosphate–activated protein kinase (AMPK) α-1, peroxisome proliferator–activated receptor (PPAR)-γ, and creatine kinase in mitochondrial (Ckmt) 2 in the soleus muscle of tail-suspension mice. To confirm the AX phenotype in the soleus muscle, we examined its effects on mitochondria using Sol8 myotubes derived from the soleus muscle. We found that AX was preferentially detected in the mitochondrial fraction; it significantly suppressed mitochondrial reactive oxygen species (ROS) production in Sol8 myotubes. Moreover, AX inhibited the activation of caspase 3 via inhibiting the release of cytochrome c into the cytosol in antimycin A–treated Sol8 myotubes. These results suggested that AX protected the functional stability of mitochondria, alleviated mitochondrial oxidative stress and mitochondria-mediated apoptosis, and thus, prevented muscle atrophy.

Syntrophins entangled in cytoskeletal meshwork: Helping to hold it all together

Syntrophins are a family of 59 kDa peripheral membrane-associated adapter proteins, containing multiple protein-protein and protein-lipid interaction domains. The syntrophin family consists of five isoforms that exhibit specific tissue distribution, distinct sub-cellular localization and unique expression patterns implying their diverse functional roles. These syntrophin isoforms form multiple functional protein complexes and ensure proper localization of signalling proteins and their binding partners to specific membrane domains and provide appropriate spatiotemporal regulation of signalling pathways. Syntrophins consist of two PH domains, a PDZ domain and a conserved SU domain. The PH1 domain is split by the PDZ domain. The PH2 and the SU domain are involved in the interaction between syntrophin and the dystrophin-glycoprotein complex (DGC). Syntrophins recruit various signalling proteins to DGC and link extracellular matrix to internal signalling apparatus via DGC. The different domains of the syntrophin isoforms are responsible for modulation of cytoskeleton. Syntrophins associate with cytoskeletal proteins and lead to various cellular responses by modulating the cytoskeleton. Syntrophins are involved in many physiological processes which involve cytoskeletal reorganization like insulin secretion, blood pressure regulation, myogenesis, cell migration, formation and retraction of focal adhesions. Syntrophins have been implicated in various pathologies like Alzheimer's disease, muscular dystrophy, cancer. Their role in cytoskeletal organization and modulation makes them perfect candidates for further studies in various cancers and other ailments that involve cytoskeletal modulation. The role of syntrophins in cytoskeletal organization and modulation has not yet been comprehensively reviewed till now. This review focuses on syntrophins and highlights their role in cytoskeletal organization, modulation and dynamics via its involvement in different cell signalling networks.

Neuronal nitric oxide synthase (nNOS) splice variant function: Insights into nitric oxide signaling from skeletal muscle

Defects in neuronal nitric oxide synthase (nNOS) splice variant localization and signaling in skeletal muscle are a firmly established pathogenic characteristic of many neuromuscular diseases, including Duchenne and Becker muscular dystrophy (DMD and BMD, respectively). Therefore, substantial efforts have been made to understand and therapeutically target skeletal muscle nNOS isoform signaling. The purpose of this review is to summarize recent salient advances in understanding of the regulation, targeting, and function of nNOSμ and nNOSβ splice variants in normal and dystrophic skeletal muscle, primarily using findings from mouse models. The first focus of this review is how the differential targeting of nNOS splice variants creates spatially and functionally distinct nitric oxide (NO) signaling compartments at the sarcolemma, Golgi complex, and cytoplasm. Particular attention is given to the functions of sarcolemmal nNOSμ and limitations of current nNOS knockout models. The second major focus is to review current understanding of cGMP-mediated nNOS signaling in skeletal muscle and its emergence as a therapeutic target in DMD and BMD. Accordingly, we address the preclinical and clinical successes and setbacks with the testing of phosphodiesterase 5 inhibitors to redress nNOS signaling defects in DMD and BMD. In summary, this review of nNOS function in normal and dystrophic muscle aims to advance understanding how the messenger NO is harnessed for cellular signaling from a skeletal muscle perspective.

Deregulation of Nrf2/ARE signaling pathway causes susceptibility of dystrophin-deficient myotubes to menadione-induced oxidative stress

Duchenne muscular dystrophy (DMD) is an X chromosome-linked disorder caused by a mutation in the dystrophin gene. Many previous studies reported that the skeletal muscles of DMD patients were more susceptible to oxidative stress than those of healthy people. However, not much has been known about the responsible mechanism of the differential susceptibility. In this study, we established dystrophin knock-down (DysKD) cell lines by transfection of dystrophin shRNA lentiviral particles into C2 cells and found that DysKD myotubes are more vulnerable to menadione-induced oxidative stress than control myotubes. We focused on the nuclear erythroid 2-related factor 2 (Nrf2) which is a transcription factor that regulates the expression of phase II antioxidant enzymes by binding to the antioxidant response element (ARE). Under menadione-induced oxidative stress, the translocation of Nrf2 to the nucleus is significantly decreased in the DysKD myotubes. In addition, the binding of Nrf2 to ARE site of Bcl-2 gene as well as protein expression of Bcl-2 is decreased compared to the control cells. Interestingly, sulforaphane increased Akt activation and Nrf2 translocation to the nucleus in the DysKD myotubes. These results suggest that the Nrf2 pathway might be the responsible pathway to the oxidative stress-induced muscle damage in DMD.

17-(allylamino)-17-demethoxygeldanamycin drives Hsp70 expression but fails to improve morphological or functional recovery in injured skeletal muscle

The stress inducible 70 kDa heat shock protein (Hsp70) is instrumental to efficient morphological and functional recovery following skeletal muscle injury because of its roles in protein quality control and molecular signalling. Therefore, in attempt to improve recovery, Hsp70 expression was increased with 17-(allylamino)-17-demethoxygeldanamycin (17-AAG) prior to and following an intramuscular injection of barium chloride (BaCl2) into the tibialis anterior (TA) of healthy young mice. To assess recovery, regenerating fibre cross-sectional area (CSA) of the TA and in vivo peak isometric torque produced by the anterior crural muscles (TA, extensor digitorum longus and extensor hallucis muscles) were analyzed for up to 3 weeks after the injury. Because treatment of 17-AAG and Hsp70 are known to influence inflammatory and myogenic signalling, tumor necrosis factor-α (TNF-α) and myogenin content were also assessed. This study reports that 17-AAG was effective at up-regulating Hsp70 expression, increasing content fivefold in the uninjured muscle. However, this significant increase in Hsp70 content did not enhance morphological or functional recovery following the injury, as the return of regenerating fibre CSA and in vivo peak isometric torque did not differ compared to that of the injured muscle from the vehicle treated mice. Treatment with 17-AAG also altered TNF-α and myogenin content, increasing both to a greater extent after the injury. Together, these findings demonstrate that although 17-AAG may alter molecular makers of regeneration, it does not improve recovery following BaCl2-induced skeletal muscle injury in healthy young mice.

Absence of Dystrophin Disrupts Skeletal Muscle Signaling: Roles of Ca2+, Reactive Oxygen Species, and Nitric Oxide in the Development of Muscular Dystrophy

Dystrophin is a long rod-shaped protein that connects the subsarcolemmal cytoskeleton to a complex of proteins in the surface membrane (dystrophin protein complex, DPC), with further connections via laminin to other extracellular matrix proteins. Initially considered a structural complex that protected the sarcolemma from mechanical damage, the DPC is now known to serve as a scaffold for numerous signaling proteins. Absence or reduced expression of dystrophin or many of the DPC components cause the muscular dystrophies, a group of inherited diseases in which repeated bouts of muscle damage lead to atrophy and fibrosis, and eventually muscle degeneration. The normal function of dystrophin is poorly defined. In its absence a complex series of changes occur with multiple muscle proteins showing reduced or increased expression or being modified in various ways. In this review, we will consider the various proteins whose expression and function is changed in muscular dystrophies, focusing on Ca(2+)-permeable channels, nitric oxide synthase, NADPH oxidase, and caveolins. Excessive Ca(2+) entry, increased membrane permeability, disordered caveolar function, and increased levels of reactive oxygen species are early changes in the disease, and the hypotheses for these phenomena will be critically considered. The aim of the review is to define the early damage pathways in muscular dystrophy which might be appropriate targets for therapy designed to minimize the muscle degeneration and slow the progression of the disease.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Nuclear recruitment of neuronal nitric-oxide synthase by α-syntrophin is crucial for the induction of mitochondrial biogenesis

Neuronal nitric-oxide synthase (nNOS) has various splicing variants and different subcellular localizations. nNOS can be found also in the nucleus; however, its exact role in this compartment is still not completely defined. In this report, we demonstrate that the PDZ domain allows the recruitment of nNOS to nuclei, thus favoring local NO production, nuclear protein S-nitrosylation, and induction of mitochondrial biogenesis. In particular, overexpression of PDZ-containing nNOS (nNOSα) increases S-nitrosylated CREB with consequent augmented binding on cAMP response element consensus sequence on peroxisome proliferator-activated receptor γ co-activator (PGC)-1α promoter. The resulting PGC-1α induction is accompanied by the expression of mitochondrial genes (e.g., TFAM, MtCO1) and increased mitochondrial mass. Importantly, full active nNOS lacking PDZ domain (nNOSβ) does not localize in nuclei and fails in inducing the expression of PGC-1α. Moreover, we substantiate that the mitochondrial biogenesis normally accompanying myogenesis is associated with nuclear translocation of nNOS. We demonstrate that α-Syntrophin, which resides in nuclei of myocytes, functions as the upstream mediator of nuclear nNOS translocation and nNOS-dependent mitochondrial biogenesis. Overall, our results indicate that altered nNOS splicing and nuclear localization could be contributing factors in human muscular diseases associated with mitochondrial impairment.

Syntrophin proteins as Santa Claus: role(s) in cell signal transduction

Syntrophins are a family of cytoplasmic membrane-associated adaptor proteins, characterized by the presence of a unique domain organization comprised of a C-terminal syntrophin unique (SU) domain and an N-terminal pleckstrin homology (PH) domain that is split by insertion of a PDZ domain. Syntrophins have been recognized as an important component of many signaling events, and they seem to function more like the cell's own personal 'Santa Claus' that serves to 'gift' various signaling complexes with precise proteins that they 'wish for', and at the same time care enough for the spatial, temporal control of these signaling events, maintaining overall smooth functioning and general happiness of the cell. Syntrophins not only associate various ion channels and signaling proteins to the dystrophin-associated protein complex (DAPC), via a direct interaction with dystrophin protein but also serve as a link between the extracellular matrix and the intracellular downstream targets and cell cytoskeleton by interacting with F-actin. They play an important role in regulating the postsynaptic signal transduction, sarcolemmal localization of nNOS, EphA4 signaling at the neuromuscular junction, and G-protein mediated signaling. In our previous work, we reported a differential expression pattern of alpha-1-syntrophin (SNTA1) protein in esophageal and breast carcinomas. Implicated in several other pathologies, like cardiac dys-functioning, muscular dystrophies, diabetes, etc., these proteins provide a lot of scope for further studies. The present review focuses on the role of syntrophins in membrane targeting and regulation of cellular proteins, while highlighting their relevance in possible development and/or progression of pathologies including cancer which we have recently demonstrated.

The activation of satellite cells by nanofibrous poly ε-caprolacton constructs

Nanoscale patterning of scaffolds provides broad surface for adhesion and differentiation of stem cells. As we know, the combination of tissue engineering with stem cells technology hold the key for regeneration of damaged tissues for example skeletal muscle tissues. On the other hand, the mechanical assessments of poly ε-caprolacton nanofibers determined the required features of biomedical scaffold for skeletal muscle tissue. In this study, skeletal muscle satellite cells as the main group of stem cells were cultivated on the electrospun poly ε-caprolacton nanofibers. Our results indicated that in comparison with tissue culture polystyrene, the nanoscale of scaffolds provided more induction to matured cells of skeletal muscle. Moreover, the immobilization of cells by collagen on poly ε-caprolacton nanofibers significantly improved the differential potency of satellite cells.

Myocilin interacts with syntrophins and is member of dystrophin-associated protein complex

Genetic studies have linked myocilin to open angle glaucoma, but the functions of the protein in the eye and other tissues have remained elusive. The purpose of this investigation was to elucidate myocilin function(s). We identified α1-syntrophin, a component of the dystrophin-associated protein complex (DAPC), as a myocilin-binding candidate. Myocilin interacted with α1-syntrophin via its N-terminal domain and co-immunoprecipitated with α1-syntrophin from C2C12 myotubes and mouse skeletal muscle. Expression of 15-fold higher levels of myocilin in the muscles of transgenic mice led to the elevated association of α1-syntrophin, neuronal nitric-oxide synthase, and α-dystroglycan with DAPC, which increased the binding of laminin to α-dystroglycan and Akt signaling. Phosphorylation of Akt and Forkhead box O-class 3, key regulators of muscle size, was increased more than 3-fold, whereas the expression of muscle-specific RING finger protein-1 and atrogin-1, muscle atrophy markers, was decreased by 79 and 88%, respectively, in the muscles of transgenic mice. Consequently, the average size of muscle fibers of the transgenic mice was increased by 36% relative to controls. We suggest that intracellular myocilin plays a role as a regulator of muscle hypertrophy pathways, acting through the components of DAPC.

α-Syntrophin is required for the hepatocyte growth factor-induced migration of cultured myoblasts

Syntrophins are adaptor proteins that link intracellular signaling molecules to the dystrophin based scaffold. In this study, we investigated the function of syntrophins in cell migration, one of the early steps in myogenic differentiation and in regeneration of adult muscle. Hepatocyte growth factor (HGF) stimulates migration and lamellipodia formation in cultured C2 myoblasts. In the migrating cells, syntrophin concentrated in the rear-lateral region of the cell, opposite of the lamellipodia, instead of being diffusely present throughout the cytoplasm of non-migrating cells. When the expression of α-syntrophin, the major syntrophin isoform of skeletal muscle, was reduced by transfection with the α-syntrophin-specific siRNA, HGF stimulation of lamellipodia formation was prevented. Likewise, migration of myoblasts from α-syntrophin knockout mice could not be stimulated by HGF. However, HGF-induced migration was restored in myoblasts isolated from a transgenic mouse expressing α-syntrophin only in muscle cells. Treatment of C2 myoblasts with inhibitors of PI3-kinase not only reduced the rate of cell migration, but also impaired the accumulation of syntrophins in the rear-lateral region of the migrating cells. Phosphorylation of Akt was reduced in the α-syntrophin siRNA-treated C2 cells. These results suggest that α-syntrophin is required for HGF-induced migration of myoblasts and for proper PI3-kinase/Akt signaling.