The dystrophin glycoprotein complex: signaling strength and integrity for the sarcolemma

Calpains in muscle wasting

Calpains are intracellular nonlysosomal Ca(2+)-regulated cysteine proteases. They mediate regulatory cleavages of specific substrates in a large number of processes during the differentiation, life and death of the cell. The purpose of this review is to synthesize our current understanding of the participation of calpains in muscle atrophy. Muscle tissue expresses mainly three different calpains: the ubiquitous calpains and calpain 3. The participation of the ubiquitous calpains in the initial degradation of myofibrillar proteins occurring in muscle atrophy as well as in the necrosis process accompanying muscular dystrophies has been well characterized. Inactivating mutations in the calpain 3 gene are responsible for limb-girdle muscular dystrophy type 2A and calpain 3 has been found to be downregulated in different atrophic situations, suggesting that it has to be absent for the atrophy to occur. The fact that similar regulations of calpain activities occur during exercise as well as in atrophy led us to propose that the calpains control cytoskeletal modifications needed for muscle plasticity.

Changes in dysferlin, proteins from dystrophin glycoprotein complex, costameres, and cytoskeleton in human soleus and vastus lateralis muscles after a long-term bedrest with or without exercise

This study was designed to evaluate the effects of hypokinesia and hypodynamia on cytoskeletal and related protein contents in human skeletal muscles. Twelve proteins: dystrophin and its associated proteins (DGC), dysferlin, talin, vinculin and meta-vinculin, alpha-actinin, desmin, actin, and myosin, were quantitatively analyzed during an 84-day long-term bedrest (LTBR). The preventive or compensatory effects of maximal resistance exercise (MRE) as a countermeasure were evaluated. Most of these proteins are involved in several myopathies, and they play an important role in muscle structure, fiber cohesion, cell integrity maintenance, and force transmission. This is the first comparison of the cytoskeletal protein contents between slow postural soleus (SOL) and mixed poly-functional vastus lateralis (VL) human muscles. Protein contents were higher in VL than in SOL (from 12 to 94%). These differences could be mainly explained by the differential mechanical constraints imposed on the muscles, i.e., cytoskeletal protein contents increase with mechanical constraints. After LTBR, proteins belonging to the DGC, dysferlin, and proteins of the costamere exhibited large increases, higher in SOL (from 67 to 216%) than in VL (from 32 to 142%). Plasma membrane remodeling during muscle atrophy is probably one of the key points for interpreting these modifications, and mechanisms other than those involved in the resistance of the cytoskeleton to mechanical constraints may be implicated (membrane repair). MRE compensates the cytoskeletal changes induced by LTBR in SOL, except for gamma-sarcoglycan (+70%) and dysferlin (+108%). The exercise only partly compensated the DGC changes induced in VL, and, as for SOL, dysferlin remained largely increased (+132%). Moreover, vinculin and metavinculin, which exhibited no significant change in VL after LTBR, were increased with MRE during LTBR, reinforcing the pre-LTBR differences between SOL and VL. This knowledge will contribute to the development of efficient space flight countermeasures and rehabilitation methods in clinical situations where musculoskeletal unloading is a component.

Specific interaction of the actin-binding domain of dystrophin with intermediate filaments containing keratin 19

Cytokeratins 8 and 19 concentrate at costameres of striated muscle and copurify with the dystrophin-glycoprotein complex, perhaps through the interaction of the cytokeratins with the actin-binding domain of dystrophin. We overexpressed dystrophin's actin-binding domain (Dys-ABD), K8 and K19, as well as closely related proteins, in COS-7 cells to assess the basis and specificity of their interaction. Dys-ABD alone associated with actin microfilaments. Expressed with K8 and K19, which form filaments, Dys-ABD associated preferentially with the cytokeratins. This interaction was specific, as the homologous ABD of betaI-spectrin failed to interact with K8/K19 filaments, and Dys-ABD did not associate with desmin or K8/K18 filaments. Studies in COS-7 cells and in vitro showed that Dys-ABD binds directly and specifically to K19. Expressed in muscle fibers in vivo, K19 accumulated in the myoplasm in structures that contained dystrophin and spectrin and disrupted the organization of the sarcolemma. K8 incorporated into sarcomeres, with no effect on the sarcolemma. Our results show that dystrophin interacts through its ABD with K19 specifically and are consistent with the idea that cytokeratins associate with dystrophin at the sarcolemma of striated muscle.

Limb-girdle muscular dystrophy in childhood

LGMD refers to a class of muscular dystrophies with onset in the proximal muscles. They are genetically heterogeneous, with both autosomal recessive and dominant forms. The autosomal recessive forms are more common and in general follow a more severe course compared to the dominant forms. It is important to reach a specific genetic diagnosis beyond making a group diagnosis of LGMD to provide adequate genetic counseling, to predict risks for the patient such as the development of cardiomyopathy, and to be able to take advantage of specific treatments when they become available. Establishing a specific diagnosis requires knowledge about the individual clinical features, expert analysis of the muscule biopsy, and the guided initiation of appropriate genetic testing.

Proteome Dynamics during C2C12 Myoblast Differentiation

Mouse-derived C2C12 myoblasts serve as an experimentally tractable model system for investigating the molecular basis of skeletal muscle cell specification and development. To examine the biochemical adaptations associated with myocyte formation comprehensively, we used large scale gel-free tandem mass spectrometry to monitor global proteome alterations throughout a time course analysis of the myogenic C2C12 differentiation program. The relative abundance of approximately 1,800 high confidence proteins was tracked across multiple time points using capillary scale multidimensional liquid chromatography coupled to high throughput shotgun sequencing. Hierarchical clustering of the resulting profiles revealed differential waves of expression of proteins linked to intracellular signaling, transcription, cytoarchitecture, adhesion, metabolism, and muscle contraction across the early, mid, and late stages of differentiation. Several hundred previously uncharacterized proteins were likewise detected in a stage-specific manner, suggesting novel roles in myogenesis and/or muscle function. These proteomic data are complementary to recent microarray-based studies of gene expression patterns in developing myotubes and provide a holistic framework for understanding how diverse biochemical processes are coordinated at the cellular level during skeletal muscle development.

Reprogramming of the Human Atrial Transcriptome in Permanent Atrial Fibrillation

Atrial fibrillation is associated with increased expression of ventricular myosin isoforms in atrial myocardium, regarded as part of a dedifferentiation process. Whether reexpression of ventricular isoforms in atrial fibrillation is restricted to transcripts encoding for contractile proteins is unknown. Therefore, this study compares atrial mRNA expression in patients with permanent atrial fibrillation to atrial mRNA expression in patients with sinus rhythm and to ventricular gene expression using Affymetrix U133 arrays. In atrial myocardium, we identified 1434 genes deregulated in atrial fibrillation, the majority of which, including key elements of calcium-dependent signaling pathways, displayed downregulation. Functional classification based on Gene Ontology provided the specific gene sets of the interdependent processes of structural, contractile, and electrophysiological remodeling. In addition, we demonstrate for the first time a prominent upregulation of transcripts involved in metabolic activities, suggesting an adaptive response to increased metabolic demand in fibrillating atrial myocardium. Ventricular-predominant genes were 5 times more likely to be upregulated in atrial fibrillation (174 genes upregulated, 35 genes downregulated), whereas atrial-specific transcripts were predominantly downregulated (56 genes upregulated, 564 genes downregulated). Overall, in fibrillating atrial myocardium, functional classes of genes characteristic of ventricular myocardium were found to be upregulated (eg, metabolic processes), whereas functional classes predominantly expressed in atrial myocardium were downregulated (eg, signal transduction and cell communication). Therefore, dedifferentiation with adoption of a ventricular-like signature is a general feature of the fibrillating atrium.

A novel paradigm for therapeutic basis of advanced heart failure--assessment by gene therapy

The precise mechanism(s) of the progression of advanced heart failure (HF) should be determined to establish strategies for its treatment or prevention. Based on pathological, molecular, and physiological findings in 3 animal models and human cases, we propose a novel scheme that a vicious cycle formed by increased sarcolemma (SL) permeability, preferential activation of calpain over calpastatin, and translocation and cleavage of dystrophin (Dys) commonly lead to advanced HF. The aim of this article was to assess our recent paradigm that disruption of myocardial Dys is a final common pathway to advanced HF, irrespective of its hereditary or acquired origin, but not intended to provide a comprehensive overview of the various factors that may be involved in the course of HF in different clinical settings. In addition, each component of Dys-associated proteins (DAP) was heterogeneously degraded in vivo and in vitro, i.e. Dys and alpha-sarcoglycan (SG) were markedly destroyed using isolated calpain 2, while delta-SG was not degraded at all. The up-regulation of calpain 2 was confirmed through previously published data that remain insufficient for precise evaluation, supporting our new scheme that the activation of calpain(s) is involved in the steady process of Dys cleavage. In addition, somatic gene therapy is discussed as a potential option to ameliorate the physiological/metabolic indices and to improve the prognosis.

Muscle-specific BCL2 expression ameliorates muscle disease in laminin {alpha}2-deficient, but not in dystrophin-deficient, mice

To examine the role of apoptosis in neuromuscular disease progression, we have determined whether pathogenesis in dystrophin-deficient (mdx) and laminin alpha2-deficient (Lama2-null) mice is ameliorated by overexpression of the anti-apoptosis protein BCL2 in diseased muscles. The mdx mice are a model for the human disease, Duchenne muscular dystrophy (DMD), and the Lama2-null mice are a model for human congenital muscular dystrophy type 1A (MDC1A). For these studies, we generated transgenic mice that overexpressed human BCL2 under control of muscle-specific MyoD or MRF4 promoter fragments. We then used cross-breeding to introduce the transgenes into diseased mdx or Lama2-null mice. In mdx mice, we found that overexpression of BCL2 failed to produce any significant differences in muscle pathology. In contrast, in the Lama2-null mice, we found that muscle-specific expression of BCL2 led to a several-fold increase in lifespan and an increased growth rate. Thus, BCL2-mediated apoptosis appears to play a significant role in pathogenesis of laminin alpha2 deficiency, but not of dystrophin deficiency, suggesting that therapies designed to ameliorate disease by inhibition of apoptosis are more likely to succeed in MDC1A than in DMD.

Stem cell activation sustains hereditary hypertrophy in hamster cardiomyopathy

Recent studies have documented the presence of stem cells within the myocardium and their role in the repair of ischaemic injury. Nevertheless, the pathogenic role of stem cells in non-ischaemic myocardial diseases, as well as the factors potentially responsible for their activation, is still under debate. The present study demonstrates the presence of an increased number of c-kit positive, MDR-positive, and Sca-1-positive stem cells within the myocardium of hereditary delta-SG null hamsters, a spontaneously occurring model of hypertrophic cardiomyopathy. When hamsters are 80 days old, ie at the 'hypertrophic' stage of the disease, but without haemodynamic overload, these cells associate with a multitude of cells co-expressing c-kit, cMet, GATA4, or MEF-2, and proliferating myocytes co-expressing myosin heavy chain, telomerase, ki67 and cyclin B. Furthermore, at the same animal age, the number of myocardial cells co-expressing c-kit and Flk-1, and the number of capillary vessels, is also amplified. In order to identify factors potentially responsible for stem cell activation, the myocardial expression of HGF and cMet and HGF plasma levels were evaluated, demonstrating their increase in 80-day-old delta-SG null hamsters. To demonstrate the possible ability of HGF to induce stem cell differentiation, bone-marrow-derived mesenchymal stem cells were challenged with HGF at the same plasma concentration observed in vivo. HGF induced cMet phosphorylation, and caused loss of stem cell features and overexpression of MEF-2, TEF1, and MHC. Our results demonstrate that stem cell activation occurs within the cardiomyopathic myocardium, very likely to maintain an efficient cardiac architecture. In this context, elevated levels of HGF might play a role in induction of stem cell commitment to the cardiomyocyte lineage and in cardioprotection through its anti-apoptotic action. Consistently, when cytokine levels declined to physiological concentrations, as in 150-day-old cardiomyopathic animals, myocardial apoptosis prevailed, prejudicing cardiac function.

Assembly and Signaling of Adhesion Complexes

Cell-extracellular matrix (ECM) adhesion is crucial for control of cell behavior. It connects the ECM to the intracellular cytoskeleton and transduces bidirectional signals between the extracellular and intracellular compartments. The subcellular machinery that mediates cell-ECM adhesion and signaling is complex. It consists of transmembrane proteins (e.g., integrins) and at least several dozens of membrane-proximal proteins that assemble into a network through multiple protein interactions. Furthermore, despite sharing certain common components, cell-ECM adhesions exhibit considerable heterogeneity in different types of cells (e.g., the cell-ECM adhesions in cardiac myocytes are considerably different from those in fibroblasts). Here, we will first briefly describe the general properties of the integrin-mediated cell-ECM adhesion and signal transduction. Next, we will focus on one of the recently discovered cell-ECM adhesion protein complexes consisting of PINCH, integrin-linked kinase (ILK), and Parvin and use it as an example to illustrate the molecular basis underlying the assembly and functions of cell-ECM adhesions. Finally, we will discuss in detail the structure and regulation of cell-ECM adhesion complexes in cardiac myocytes, which illustrate the importance and complexity of the cell-ECM adhesion structures in organogenesis and diseases.

Cardiac involvement in muscular dystrophies: Molecular mechanisms

In this review, we draw attention to the multiple mechanisms responsible for the pathogenesis of cardiomyopathies in patients with muscular dystrophies. More than one single mechanism is likely to be involved in the development of skeletal and cardiac muscle pathology even when there is a single protein defect. The best example is dystrophin deficiency, in which increased sarcolemmal permeability following eccentric exercise, reduced force generation, and abnormal signaling are all likely to contribute to the progressive muscle damage observed. In other conditions, such as the sarcoglycanopathies, a protein deficiency both in the striated cardiomyocte and the vascular smooth muscle appears to play a significant role. An entirely different mechanism of disease is likely in defects of nuclear envelope proteins, although the precise pathogenesis of this group of conditions is still not clear. Differences between the organization of skeletal and cardiac muscle protein complex are also only starting to emerge and will very likely be the focus of future research.

The dystroglycan complex: From biology to cancer

Dystroglycan (DG), a non-integrin adhesion molecule, is a pivotal component of the dystrophin-glycoprotein complex, that is expressed in skeletal muscle and in a wide variety of tissues at the interface between the basement membrane (BM) and the cell membrane. DG has been mainly studied for its role in skeletal muscle cell stability and its alterations in muscular diseases, such as dystrophies. However, accumulating evidence have implicated DG in a variety of other biological functions, such as maturation of post-synaptic elements in the central and peripheral nervous system, early morphogenesis, and infective pathogens targeting. Moreover, DG has been reported to play a role in regulating cytoskeletal organization, cell polarization, and cell growth in epithelial cells. Recent studies also indicate that abnormalities in the expression of DG frequently occur in human cancers and may play a role in both the process of tumor progression and in the maintenance of the malignant phenotype. This paper reviews the available information on the biology of DG, the abnormalities found in human cancers, and the implications of these findings with respect to our understanding of cancer pathogenesis and to the development of novel strategies for a better management of cancer patients.