Component Of Dystrophin-glycoprotein Complex Research Articles

Systemic delivery of full-length dystrophin in Duchenne muscular dystrophy mice

Current gene therapy for Duchenne muscular dystrophy (DMD) utilizes adeno-associated virus (AAV) to deliver micro-dystrophin (µDys), which does not provide full protection for striated muscles as it lacks many important functional domains of full-length (FL) dystrophin. Here we develop a triple vector system to deliver FL-dystrophin into skeletal and cardiac muscles. We split FL-dystrophin into three fragments linked to two orthogonal pairs of split intein, allowing efficient assembly of FL-dystrophin. The three fragments packaged in myotropic AAV (MyoAAV4A) restore FL-dystrophin expression in both skeletal and cardiac muscles in male mdx4cv mice. Dystrophin-glycoprotein complex components are also restored at the sarcolemma of dystrophic muscles. MyoAAV4A-delivered FL-dystrophin significantly improves muscle histopathology, contractility, and overall strength comparable to µDys, but unlike µDys, it also restores defective cavin 4 localization and associated signaling in mdx4cv heart. Therefore, our data support the feasibility of a mutation-independent FL-dystrophin gene therapy for DMD, warranting further clinical development.

Signaling through the dystrophin glycoprotein complex affects the stress-dependent transcriptome in Drosophila.

Deficiencies in the human dystrophin glycoprotein complex (DGC), which links the extracellular matrix with the intracellular cytoskeleton, cause muscular dystrophies, a group of incurable disorders associated with heterogeneous muscle, brain and eye abnormalities. Stresses such as nutrient deprivation and aging cause muscle wasting, which can be exacerbated by reduced levels of the DGC in membranes, the integrity of which is vital for muscle health and function. Moreover, the DGC operates in multiple signaling pathways, demonstrating an important function in gene expression regulation. To advance disease diagnostics and treatment strategies, we strive to understand the genetic pathways that are perturbed by DGC mutations. Here, we utilized a Drosophila model to investigate the transcriptomic changes in mutants of four DGC components under temperature and metabolic stress. We identified DGC-dependent genes, stress-dependent genes and genes dependent on the DGC for a proper stress response, confirming a novel function of the DGC in stress-response signaling. This perspective yields new insights into the etiology of muscular dystrophy symptoms, possible treatment directions and a better understanding of DGC signaling and regulation under normal and stress conditions.

Enterovirus 71 infection induced Aquaporin-4 depolarization by increasing matrix metalloproteinase-9 activity

Aquaporin-4 (AQP4) is the key water channel protein that regulates brain water homeostasis. Polarized expression of AQP4 on the astroglial endfeet facilitates its role in bi-directional brain water flux control. In the current study, we found that enterovirus 71 (EV71) infection induced depolarization of AQP4 in mouse brain, and demonstrated that β-dystroglycan (β-DG), the key component of dystrophin glycoprotein complex (DGC) that anchors AQP4 to the astroglial endfeet, was degraded upon infection. Elevated activity or expression of matrix metalloproteinase 9 (MMP9) upon infection was found in both mouse brains and patient cerebrospinal fluid (CSF) samples. Inhibiting MMP9 activity by SB-3CT rescued the decay of β-DG and reduced the depolarization of AQP4. Brain edema induced by viral infection was also ameliorated by SB-3CT treatment in mice.

Loss of sarcospan exacerbates pathology in mdx mice, but does not affect utrophin amelioration of disease.

The dystrophin-glycoprotein complex (DGC) is a membrane adhesion complex that provides structural stability at the sarcolemma by linking the myocyte's internal cytoskeleton and external extracellular matrix. In Duchenne muscular dystrophy (DMD), the absence of dystrophin leads to the loss of the DGC at the sarcolemma, resulting in sarcolemmal instability and progressive muscle damage. Utrophin (UTRN), an autosomal homolog of dystrophin, is upregulated in dystrophic muscle and partially compensates for the loss of dystrophin in muscle from patients with DMD. Here, we examine the interaction between Utr and sarcospan (SSPN), a small transmembrane protein that is a core component of both UTRN-glycoprotein complex (UGC) and DGC. We show that additional loss of SSPN causes an earlier onset of disease in dystrophin-deficient mdx mice by reducing the expression of the UGC at the sarcolemma. In order to further evaluate the role of SSPN in maintaining therapeutic levels of Utr at the sarcolemma, we tested the effect of Utr transgenic overexpression in mdx mice lacking SSPN (mdx:SSPN -/-:Utr-Tg). We found that overexpression of Utr restored SSPN to the sarcolemma in mdx muscle but that the ablation of SSPN in mdx muscle reduced Utr at the membrane. Nevertheless, Utr overexpression reduced central nucleation and improved grip strength in both lines. These findings demonstrate that high levels of Utr transgenic overexpression ameliorate the mdx phenotype independently of SSPN expression but that loss of SSPN may impair Utr-based mechanisms that rely on lower levels of Utr protein.

DMD – ANIMAL MODELS & PRECLINICAL TREATMENT: P.218 A novel in-frame deletion of exons 52-55 DMD mouse model preserves muscle function

Duchenne muscular dystrophy (DMD) is a life-limiting X-linked recessive neuromuscular disorder, affecting approximately 1 in 5000 boys. DMD is caused by mutations in the DMD gene encoding dystrophin, a member of the dystrophin glycoprotein complex (DGC) which protects muscle fibers from contraction-induced injury. DMD is marked by absent dystrophin expression, often caused by frame-shift mutations. Among the dystrophinopathies, Becker muscular dystrophy (BMD) is a milder disorder typically caused by in-frame DMD mutations, resulting in shortened yet partly functional dystrophin formation. Both the clinical presentation and prognosis of BMD patients are improved compared to those with DMD. Therefore, conversion of out-of-frame Dmd mutations into in-frame mutations by means of exon-skipping, is appealing as a potential therapeutic option. This study first created a DMD mouse model mirroring a patient deletion of exons 52-54 (Dmd Δ52-54). Such a mutation may be amenable to exon skipping to create a deletion of exons 52-55. We then generated a mouse model of the in-frame deletion mutation Dmd Δ52-55, recapitulating the result of exon skipping in our DMD mouse. To date, Dmd Δ52-55 is the first BMD mouse model generated. Both mouse models were subsequently subjected to histopathological and functional analysis. Dmd Δ52-54 mice lacked dystrophin expression, exhibited a muscular dystrophy phenotype, and had compromised motor and cardiac function. In contrast, the Dmd Δ52-55 mice showed expression of a truncated dystrophin which localized correctly to the sarcolemma and recruited DGC components, which were absent in Dmd Δ52-54. Furthermore, this shortened dystrophin maintained muscle integrity and protected from exercise-induced damage similar to wildtype dystrophin. Thus, our results indicate that therapies targeting the conversion of DMD-causing frameshift mutations into BMD-like in-frame mutations may restore shortened dystrophin expression and improve dystrophic phenotype.

Regulation of the dystrophin-associated glycoprotein complex composition by the metabolic properties of muscle fibres

The dystrophin-glycoprotein complex (DGC) links the muscle cytoskeleton to the extracellular matrix and is responsible for force transduction and protects the muscle fibres from contraction induced damage. Mutations in components of the DGC are responsible for muscular dystrophies and congenital myopathies. Expression of DGC components have been shown to be altered in many myopathies. In contrast we have very little evidence of whether adaptive changes in muscle impact on DGC expression. In this study we investigated connection between muscle fibre phenotype and the DGC. Our study reveals that the levels of DGC proteins at the sarcolemma differ in highly glycolytic muscle compared to wild-type and that these changes can be normalised by the super-imposition of an oxidative metabolic programme. Importantly we show that the metabolic properties of the muscle do not impact on the total amount of DGC components at the protein level. Our work shows that the metabolic property of a muscle fibre is a key factor in regulating the expression of DGC proteins at the sarcolemma.

Myelination is delayed during postnatal brain development in the mdx mouse model of Duchenne muscular dystrophy

BackgroundIn Duchenne muscular dystrophy (DMD), the loss of the dystrophin component of the dystrophin-glycoprotein complex (DGC) compromises plasma membrane integrity in skeletal muscle, resulting in extensive muscle degeneration. In addition, many DMD patients exhibit brain deficits in which the cellular etiology remains poorly understood. We recently found that dystroglycan, a receptor component of the DGC that binds intracellularly to dystrophin, regulates the development of oligodendrocytes, the myelinating glial cells of the brain.ResultsWe investigated whether dystrophin contributes to oligodendroglial function and brain myelination. We found that oligodendrocytes express up to three dystrophin isoforms, in conjunction with classic DGC components, which are developmentally regulated during differentiation and in response to extracellular matrix engagement. We found that mdx mice, a model of DMD lacking expression of the largest dystrophin isoform, have delayed myelination and inappropriate oligodendrocyte progenitor proliferation in the cerebral cortex. When we prevented the expression of all oligodendroglial dystrophin isoforms in cultured oligodendrocytes using RNA interference, we found that later stages of oligodendrocyte maturation were significantly delayed, similar to mdx phenotypes in the developing brain.ConclusionsWe find that dystrophin is expressed in oligodendrocytes and influences developmental myelination, which provides new insight into potential cellular contributors to brain dysfunction associated with DMD.

Abstract 78: Hippo Pathway and Dystrophin Glycoprotein Complex Regulate Cardiomyocyte Proliferation

Regeneration of mammalian heart is limited due to the extremely low renewal rate of cardiomyocytes and their inability to reenter the cell cycle. The Hippo pathway controls heart size during development and represses postnatal heart regeneration by repressing cardiomyocyte proliferation. Our approach for activating adult heart regeneration is to uncover the mechanisms responsible for repression of cardiomyocyte proliferation. We have previously found that deletion of Salv, a modulator of the Hippo pathway, results in myocardial damage repair in postnatal and adult hearts. Deletion of Salv results in activation of the transcription factor, Yap, which positively regulates cytoskeleton and cell cycle genes. We also found that the components of dystrophin glycoprotein complex (DGC) are the target of Yap and DGC regulates heart regeneration. The dystrophin glycoprotein complex (DGC) is essential for muscle maintenance by anchoring the cytoskeleton and extracellular matrix. Disruption of the DGC results in muscular dystrophies, including Duchenne muscular dystrophy, resulting in both skeletal and cardiac myopathies. To explore the connection between DGC and the Hippo pathway, we conditionally deleted Salv in the mdx background, a mouse model of muscular dystrophy. We found that simultaneous disruption of the DGC and the Hippo pathway leads an increased cardiomyocyte proliferation after heart damage. This is associated with increased activity of Yap, suggesting DGC negatively regulate Yap to repress proliferation. We also found that one of the components DGC, dystroglycan directly binds Yap and anchors to the membrane. Our findings provide new insights into the mechanisms leading to heart repair through proliferation of endogenous cardiomyocytes.

ABC of multifaceted dystrophin glycoprotein complex (DGC).

Dystrophin protein in association with several other cellular proteins and glycoproteins leads to the formation of a large multifaceted protein complex at the cell membrane referred to as dystrophin glycoprotein complex (DGC), that serves distinct functions in cell signaling and maintaining the membrane stability as well as integrity. In accordance with this, several findings suggest exquisite role of DGC in signaling pathways associated with cell development and/or maintenance of homeostasis. In the present review, we summarize the established facts about the various components of this complex with emphasis on recent insights into specific contribution of the DGC in cell signaling at the membrane. We have also discussed the recent advances made in exploring the molecular associations of DGC components within the cells and the functional implications of these interactions. Our review would help to comprehend the composition, role, and functioning of DGC and may lead to a deeper understanding of its role in several human diseases.

Molecular Signatures of Membrane Protein Complexes Underlying Muscular Dystrophy

Mutations in genes encoding components of the sarcolemmal dystrophin-glycoprotein complex (DGC) are responsible for a large number of muscular dystrophies. As such, molecular dissection of the DGC is expected to both reveal pathological mechanisms, and provides a biological framework for validating new DGC components. Establishment of the molecular composition of plasma-membrane protein complexes has been hampered by a lack of suitable biochemical approaches. Here we present an analytical workflow based upon the principles of protein correlation profiling that has enabled us to model the molecular composition of the DGC in mouse skeletal muscle. We also report our analysis of protein complexes in mice harboring mutations in DGC components. Bioinformatic analyses suggested that cell-adhesion pathways were under the transcriptional control of NFκB in DGC mutant mice, which is a finding that is supported by previous studies that showed NFκB-regulated pathways underlie the pathophysiology of DGC-related muscular dystrophies. Moreover, the bioinformatic analyses suggested that inflammatory and compensatory mechanisms were activated in skeletal muscle of DGC mutant mice. Additionally, this proteomic study provides a molecular framework to refine our understanding of the DGC, identification of protein biomarkers of neuromuscular disease, and pharmacological interrogation of the DGC in adult skeletal muscle https://www.mda.org/disease/congenital-muscular-dystrophy/research.

HSPB7 interacts with dimerized FLNC and its absence results in progressive myopathy in skeletal muscles.

ABSTRACTHSPB7 belongs to the small heat-shock protein (sHSP) family, and its expression is restricted to cardiac and skeletal muscles from embryonic stages to adulthood. Here, we found that skeletal-muscle-specific ablation of the HspB7 does not affect myogenesis during embryonic stages to postnatal day 1 (P1), but causes subsequent postnatal death owing to a respiration defect, with progressive myopathy phenotypes in the diaphragm. Deficiency of HSPB7 in the diaphragm muscle resulted in muscle fibrosis, sarcomere disarray and sarcolemma integrity loss. We identified dimerized filamin C (FLNC) as an interacting partner of HSPB7. Immunofluorescence studies demonstrated that the aggregation and mislocalization of FLNC occurred in the muscle of HspB7 mutant adult mice. Furthermore, the components of dystrophin glycoprotein complex, γ- and δ-sarcoglycan, but not dystrophin, were abnormally upregulated and mislocalized in HSPB7 mutant muscle. Collectively, our findings suggest that HSPB7 is essential for maintaining muscle integrity, which is achieved through its interaction with FLNC, in order to prevent the occurrence and progression of myopathy.

The role of MICAL2 gene in myogenic differentiation

The dystrophin-glycoprotein complex (DGC) is composed of several transmembrane and peripheral components localized in the sarcolemma of skeletal muscle. Mutations in genes that encode DGC components lead to the loss of either expression and/or function of the DGC in muscle. As DGC complex interacts with F-actin it is reasonable that the multidomain F-actin binding protein MICAL2 that transduces semaphorin/plexin external signaling into cytoskeletal modifications, might interact either directly or indirectly with the DGC complex. MICAL2 is indeed expressed in skeletal and cardiac muscles and drosophila Mical mutants reveal that the architecture of contractile muscle filaments is negatively affected. We focus here on the role of MICALs in myogenic differentiation. The rationale to investigate MICAL2 in muscle differentiation is also highlighted in a paper regarding a complex muscle genome-wide expression profiling during the disease evolution in mdx mice, a mouse model of Duchenne muscular dystrophy (1). In this study the authors found MICAL2 among a set of totally ten functionally linked genes involved in the decline of muscle necrosis in mdx mice. For this purpose MICAL2 gain and loss of function studies have been performed in myogenic cell line and compared to in vivo analysis of MICAL2 expressions in acute and chronic muscle degeneration. Recently we showed that that differential myogenic propensity influences the commitment of isogenic induce pluripotent stem cells and a specifically isolated pool of mesodermal iPSC-derived progenitors (MiPs) toward the striated muscle lineages (2). Analysis of MICAL2 expression in MiPs is currently under investigation. Taken together modulation of MICAL2 has an impact on skeletal muscle commitment and could be considered a potential therapeutic target for Duchenne patients. This work was supported with the contribution of “Opening The Future” Campaign [EJJ-OPTFUT-02010] CARIPLO 2015_0634, FWO (#G088715N, #G060612N, #G0A8813N), and IUAP-VII/07 (EJJ-C4851-17/07-P) grants.

Nonmechanical Roles of Dystrophin and Associated Proteins in Exercise, Neuromuscular Junctions, and Brains.

Dystrophin-glycoprotein complex (DGC) is an important structural unit in skeletal muscle that connects the cytoskeleton (f-actin) of a muscle fiber to the extracellular matrix (ECM). Several muscular dystrophies, such as Duchenne muscular dystrophy, Becker muscular dystrophy, congenital muscular dystrophies (dystroglycanopathies), and limb-girdle muscular dystrophies (sarcoglycanopathies), are caused by mutations in the different DGC components. Although many early studies indicated DGC plays a crucial mechanical role in maintaining the structural integrity of skeletal muscle, recent studies identified novel roles of DGC. Beyond a mechanical role, these DGC members play important signaling roles and act as a scaffold for various signaling pathways. For example, neuronal nitric oxide synthase (nNOS), which is localized at the muscle membrane by DGC members (dystrophin and syntrophins), plays an important role in the regulation of the blood flow during exercise. DGC also plays important roles at the neuromuscular junction (NMJ) and in the brain. In this review, we will focus on recently identified roles of DGC particularly in exercise and the brain.

611. Alpha-Dystrobrevin-3 Prevents Myopathy and Restores Diaphragm Function in the Alpha-Dystrobrevin Null Mouse

The dystrophin-glycoprotein complex (DGC) primarily functions as a structural element enabling a linkage between the cytoskeleton and the extracellular matrix. Alpha-dystrobrevin (α-Db), a dystrophin subfamily member, is a component of the DGC within striated muscles. Decreasing amounts of α-Db from the sarcolemma contributes to severity of disease in several muscular dystrophies including Duchenne muscular dystrophy (DMD). The functional roles of α-Db1 and α-Db2 in muscle have been well studied. However, the role of a-Db3 has largely been ignored because it lacks many of the known functional domains present in α-Db1 and α-Db2. Importantly, we demonstrate by whole-body adeno-associated viral vector gene transfer into α-dystrobrevin-null mice, that α-DB3 becomes integrated within the DGC, localizes to costameres, prevents muscle degeneration, synapse fragmentation, myotendinous junction defects and corrects diaphragm functional capacity. Further, we demonstrate α-Db3 can bind the sarcoglycan complex in vitro. Thus, the association of α-Db3 within the DGC appears essential for its function, providing a direct link similar to dystrophin, with transmembrane and cytoplasmic DGC components whose absence likely contributes to sarcolemma fragility and/or disruption of signaling pathways in multiple muscular dystrophies. View Large Image | Download PowerPoint Slide

Abstract 15657: Regulation of Cardiac Regeneration by Hippo Pathway and Dystrophin Glycoprotein Complex

Regeneration of the mammalian heart is limited in adults. In rodents, endogenous regenerative capacity exists during development and in neonate but is rapidly repressed after birth. We are elucidating the mechanisms responsible for regenerative repression and applying this knowledge to reactivate cardiac regeneration in adult hearts. We have previously shown that the Hippo pathway is responsive for regenerative repression, however, the molecular and cellular mechanism responsible remain unclear. The Hippo pathway controls heart size by repressing myocardial cell proliferation during development. By deleting Salv, a modulator of Hippo pathway, we found myocardial damage in the postnatal and adult heart was repaired anatomically and functionally. This heart repair occurred primarily through proliferation of preexisting cardiomyocyte. We observed that cardiomyocytes in border the zone protrude and fill the damage area during Hippo-mediated cardiac regeneration and thus preventing formation of fibrotic scars. The molecular analysis identified components of dystrophin glycoprotein complex (DGC) as downstream targets of Hippo pathway. The DGC anchors the cytoskeleton and extracellular matrix and is involved in cell migration. The studies using the muscular dystrophy mouse model, mdx, reveals that DGC is required for endogenous cardiac regeneration and cardiomyocyte protrusion. Taken together, we show that cardiomyocyte protrusion is an essential process for cardiac regeneration and the Hippo pathway regulates it through regulating DGC. Our studies provide insights into the mechanisms leading to repair of damaged hearts from endogenous cardiomyocytes and novel information into DGC function.

Preventing phosphorylation of dystroglycan ameliorates the dystrophic phenotype in mdx mouse

Loss of dystrophin protein due to mutations in the DMD gene causes Duchenne muscular dystrophy. Dystrophin loss also leads to the loss of the dystrophin glycoprotein complex (DGC) from the sarcolemma which contributes to the dystrophic phenotype. Tyrosine phosphorylation of dystroglycan has been identified as a possible signal to promote the proteasomal degradation of the DGC. In order to test the role of tyrosine phosphorylation of dystroglycan in the aetiology of DMD, we generated a knock-in mouse with a phenylalanine substitution at a key tyrosine phosphorylation site in dystroglycan, Y890. Dystroglycan knock-in mice (Dag1(Y890F/Y890F)) had no overt phenotype. In order to examine the consequence of blocking dystroglycan phosphorylation on the aetiology of dystrophin-deficient muscular dystrophy, the Y890F mice were crossed with mdx mice an established model of muscular dystrophy. Dag1(Y890F/Y890F)/mdx mice showed a significant improvement in several parameters of muscle pathophysiology associated with muscular dystrophy, including a reduction in centrally nucleated fibres, less Evans blue dye infiltration and lower serum creatine kinase levels. With the exception of dystrophin, other DGC components were restored to the sarcolemma including α-sarcoglycan, α-/β-dystroglycan and sarcospan. Furthermore, Dag1(Y890F/Y890F)/mdx showed a significant resistance to muscle damage and force loss following repeated eccentric contractions when compared with mdx mice. While the Y890F substitution may prevent dystroglycan from proteasomal degradation, an increase in sarcolemmal plectin appeared to confer protection on Dag1(Y890F/Y890F)/mdx mouse muscle. This new model confirms dystroglycan phosphorylation as an important pathway in the aetiology of DMD and provides novel targets for therapeutic intervention.

Expression of Sarcoglycans in the Human Cerebral Cortex: An Immunohistochemical and Molecular Study

The sarcoglycan (SG) complex (SGC) is a subcomplex within the dystrophin-glycoprotein complex (DGC) and is composed of several transmembrane proteins (α, β, δ, γ, ε and ζ). The DGC supplies a transmembranous connection between the subsarcolemmal cytoskeleton networks and the basal lamina in order to protect the lipid bilayer and to provide a scaffold for signaling molecules in all muscle cells. In addition to its role in muscle tissue, dystrophin and some DGC components are expressed in neurons and glia. Very little is known about the SG subunits in the central nervous system (CNS) and some data suggested the presence of ε and ζ subunits only. In fact, mutations in the ε-SG gene cause myoclonus-dystonia, indicating its importance for brain function. To determine the presence and localization of SGC in the human cerebral cortex, we performed an investigation using immunofluorescence, immunoblotting and reverse transcriptase polymerase chain reaction. The results showed that all SG subunits are expressed in the human cerebral cortex, particularly in large neurons but also in astrocytes. These data suggest that the SG subcomplex may be involved in the organization of CNS synapses.

Biological Role of Dystroglycan in Schwann Cell Function and Its Implications in Peripheral Nervous System Diseases

Dystroglycan is a central component of the dystrophin-glycoprotein complex (DGC) that links extracellular matrix with cytoskeleton, expressed in a variety of fetal and adult tissues. Dystroglycan plays diverse roles in development and homeostasis including basement membrane formation, epithelial morphogenesis, membrane stability, cell polarization, and cell migration. In this paper, we will focus on biological role of dystroglycan in Schwann cell function, especially myelination. First, we review the molecular architecture of DGC in Schwann cell abaxonal membrane. Then, we will review the loss-of-function studies using targeted mutagenesis, which have revealed biological functions of each component of DGC in Schwann cells. Based on these findings, roles of dystroglycan in Schwann cell function, in myelination in particular, and its implications in diseases will be discussed in detail. Finally, in view of the fact that understanding the role of dystroglycan in Schwann cells is just beginning, future perspectives will be discussed.

Murine mammary tumor growth is blunted in dystrophin‐deficient mdx mice

Breast cancer (BC) is the second leading cause of cancer death in women in the US. Changes to the dystrophin glycoprotein complex (DGC), a multi‐protein structure that likely plays mechanical and signaling roles, have been reported in BC cells. For example, expression of α‐ and β‐dystroglycan appears to be inversely related to tumor stage. However, systemic absence of the DGC, as seen in the mdx mouse model of Duchenne muscular dystrophy, does not appear to induce spontaneous mammary tumor formation, indicating that presence of the DGC may be necessary for breast cancer initiation. We therefore hypothesized that breast tumor growth would be altered in mdx mice compared to wild type (C57BL/6) mice. We injected E0771 murine mammary adenocarcinoma cells into mdx and C57BL/6 mice. After 3 weeks of tumor growth, blood, skeletal muscles and tumors were collected and analyzed. Growth of E0771 tumors and serum content of migration and invasion chemokine markers, RANTES and MCP‐1, was dramatically blunted in mdx mice. The dystrophin protein was not detectable in E0771 cells or tumors, suggesting that its expression, like that of other DGC components, may be suppressed or altered in this cancer cell line. The exact mechanism(s) of tumor inhibition in mdx mice is not presently known; however, our results suggest a mechanism for suppressing BC growth and progression may depend on the absence of one or more proteins in the DGC.

Impact of sarcoglycan complex on mechanical signal transduction in murine skeletal muscle

Loss of the dystrophin glycoprotein complex (DGC) or a subset of its components can lead to muscular dystrophy. However, the patterns of symptoms differ depending on which proteins are affected. Absence of dystrophin leads to loss of the entire DGC and is associated with susceptibility to contractile injury. In contrast, muscles lacking gamma-sarcoglycan (gamma-SG) display little mechanical fragility and still develop severe pathology. Animals lacking dystrophin or gamma-SG were used to identify DGC components critical for sensing dynamic mechanical load. Extensor digitorum longus muscles from 7-wk-old normal (C57), dystrophin- null (mdx), and gamma-SG-null (gsg(-/-)) mice were subjected to a series of eccentric contractions, after which ERK1/2 phosphorylation levels were determined. At rest, both dystrophic strains had significantly higher ERK1 phosphorylation, and gsg(-/-) muscle also had heightened ERK2 phosphorylation compared with wild-type controls. Eccentric contractions produced a significant and transient increase in ERK1/2 phosphorylation in normal muscle, whereas the mdx strain displayed no significant proportional change of ERK1/2 phosphorylation after eccentric contraction. Muscles from gsg(-/-) mice had no significant increase in ERK1 phosphorylation; however, ERK2 phosphorylation was more robust than in C57 controls. The reduction in mechanically induced ERK1 phosphorylation in gsg(-/-) muscle was not dependent on age or severity of phenotype, because muscle from both young and old (age 20 wk) animals exhibited a reduced response. Immunoprecipitation experiments revealed that gamma-SG was phosphorylated in normal muscle after eccentric contractions, indicating that members of the DGC are modified in response to mechanical perturbation. This study provides evidence that the SGs are involved in the transduction of mechanical information in skeletal muscle, potentially unique from the entire DGC.