Protein targeting to the plasma membrane of adult skeletal muscle fiber: An organized mosaic of functional domains

The plasma membrane of differentiated skeletal muscle fibers comprises the sarcolemma, the transverse (T) tubule network, and the neuromuscular and muscle-tendon junctions. We analyzed the organization of these domains in relation to defined surface markers, beta -dystroglycan, dystrophin, and caveolin-3, These markers were shown to exhibit highly organized arrays along the length of the fiber. Caveolin-3 and beta -dystroglycan/dystrophin showed distinct, but to some extent overlapping, labeling patterns and both markers left transverse tubule openings clear. This labeling pattern revealed microdomains over the entire plasma membrane with the exception of the neuromuscular and muscle-tendon junctions which formed distinct demarcated macrodomains. Our results suggest that the entire plasma membrane of mature muscle comprises a mosaic of T tubule domains together with sareolemmal caveolae and beta -dystroglycan domains. The domains identified with these markers were examined with respect to targeting of viral proteins and other expressed domain-specific markers, We found that each marker protein was targeted to distinct microdomains, The macrodomains were intensely labeled with all our markers. Replacing the cytoplasmic tail of the vesicular stomatitis virus glycoprotein with that of CD4 resulted in retargeting from one domain to another. The domain-specific protein distribution at the muscle cell surface may be generated by targeting pathways requiring specific sorting information but this trafficking is different from the conventional apical-basolateral division. (C) 2001 Academic Press.

Mutations in the human ortholog of Aristaless cause X-linked mental retardation and epilepsy

Mental retardation and epilepsy often occur together. They are both heterogeneous conditions with acquired and genetic causes. Where causes are primarily genetic, major advances have been made in unraveling their molecular basis. The human X chromosome alone is estimated to harbor more than 100 genes that, when mutated, cause mental retardation(1). At least eight autosomal genes involved in idiopathic epilepsy have been identified(2), and many more have been implicated in conditions where epilepsy is a feature. We have identified mutations in an X chromosome-linked, Aristaless-related, homeobox gene (ARX), in nine families with mental retardation (syndromic and nonspecific), various forms of epilepsy, including infantile spasms and myoclonic seizures, and dystonia. Two recurrent mutations, present in seven families, result in expansion of polyalanine tracts of the ARX protein. These probably cause protein aggregation, similar to other polyalanine(3) and polyglutamine(4) disorders. In addition, we have identified a missense mutation within the ARX homeodomain and a truncation mutation. Thus, it would seem that mutation of ARX is a major contributor to X-linked mental retardation and epilepsy.

Caveolae — from ultrastructure to molecular mechanisms

Almost 50 years after the first sighting of small pits that covered the surface of mammalian cells, investigators are now getting to grips with the detailed workings of these enigmatic structures that we now know as caveolae.

Atypical Phenotype in Two Patients with LAMA2 Mutations

Congenital muscular dystrophy type 1A is caused by mutations in the LAMA2 gene, which encodes the a2-chain of laminin. We report two patients with partial laminin-a2 deficiency and atypical phenotypes, one with almost exclusive central nervous system involvement (cognitive impairment and refractory epilepsy) and the second with marked cardiac dysfunction, rigid spine syndrome and limb-girdle weakness. Patients underwent clinical, histopathological, imaging and genetic studies. Both cases have two heterozygous LAMA2 variants sharing a potentially pathogenic missense mutation c.2461A>C (p.Thr821Pro) located in exon 18. Brain MRI was instrumental for the diagnosis, since muscular examination and motor achievements were normal in the first patient and there was a severe cardiac involvement in the second. The clinical phenotype of the patients is markedly different which could in part be explained by the different combination of mutations types (two missense versus a missense and a truncating mutation).

mRNA metabolism: nonsense mediated mRNA decay as a tool for gene therapy and the role of human DIS3L2 in transcript degradation

Tese de mestrado em Biologia Humana e Ambiente, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2015

Mdx myotubes have normal excitability but show reduced contraction-relaxation dynamics.

The pathogenesis of Duchenne muscular dystrophy (DMD), characterised by lack of the cytoskeletal protein dystrophin, is not completely understood. An early event in the degenerative process of DMD muscle could be a rise in cytosolic calcium concentration. In order to investigate whether this leads to alterations of contractile behaviour, we studied the excitability and contractile properties of cultured myotubes from control (C57BL/10) and mdx mice, an animal model for DMD. The myotubes were stimulated electrically and their motion was recorded photometrically. No significant differences were found between control and mdx myotubes with respect to the following parameters: chronaxy and rheobase (0.33 +/- 0.03 ms and 23 +/- 4 V vs. 0.39 +/- 0.07 ms and 22 +/- 2 V for C57 and mdx myotubes, respectively), tetanisation frequency (a similar distribution pattern was found between 5 and 30 Hz), fatigue during tetanus (found in 35% of both types of myotubes) and post-tetanic contracture. In contrast, contraction and relaxation times were longer (P < 0.005) in mdx (36 +/- 2 and 142 +/- 13 ms, respectively) than in control myotubes (26 +/- 1 and 85 +/- 9 ms, respectively). Together with our earlier findings, these results suggest a decreased capacity for calcium removal in mdx cells leading, in particular, to alterations of muscle relaxation.

Long-term miR-669a therapy alleviates chronic dilated cardiomyopathy in dystrophic mice.

BACKGROUND: Dilated cardiomyopathy (DCM) is a leading cause of chronic morbidity and mortality in muscular dystrophy (MD) patients. Current pharmacological treatments are not yet able to counteract chronic myocardial wastage, thus novel therapies are being intensely explored. MicroRNAs have been implicated as fine regulators of cardiomyopathic progression. Previously, miR-669a downregulation has been linked to the severe DCM progression displayed by Sgcb-null dystrophic mice. However, the impact of long-term overexpression of miR-669a on muscle structure and functionality of the dystrophic heart is yet unknown. METHODS AND RESULTS: Here, we demonstrate that intraventricular delivery of adeno-associated viral (AAV) vectors induces long-term (18 months) miR-669a overexpression and improves survival of Sgcb-null mice. Treated hearts display significant decrease in hypertrophic remodeling, fibrosis, and cardiomyocyte apoptosis. Moreover, miR-669a treatment increases sarcomere organization, reduces ventricular atrial natriuretic peptide (ANP) levels, and ameliorates gene/miRNA profile of DCM markers. Furthermore, long-term miR-669a overexpression significantly reduces adverse remodeling and enhances systolic fractional shortening of the left ventricle in treated dystrophic mice, without significant detrimental consequences on skeletal muscle wastage. CONCLUSIONS: Our findings provide the first evidence of long-term beneficial impact of AAV-mediated miRNA therapy in a transgenic model of severe, chronic MD-associated DCM.

Transcriptional activation of the utrophin promoter B by a constitutively active Ets-transcription factor.

Duchenne muscular dystrophy is an X-linked genetic disease caused by the absence of functional dystrophin. Pharmacological upregulation of utrophin, the autosomal homologue of dystrophin, offers a potential therapeutic approach to treat Duchenne patients. Full-length utrophin mRNA is transcribed from two alternative promoters, called A and B. In contrast to the utrophin promoter A, little is known about the factors regulating the activity of the utrophin promoter B. Computer analysis of this second promoter revealed the presence of several conserved binding motives for Ets-transcription factors. Using electrotransfer of cDNA into mouse muscles, we demonstrate that a genetically modified beta-subunit of the Ets-transcription factor GA-binding protein potently activates a utrophin promoter B reporter construct in innervated muscle fibers in vivo. These results make the GA-binding protein and the signaling cascade regulating its activity in muscle cells, potential targets for the pharmacological modulation of utrophin expression in Duchenne patients.

Pédiatrie: nouveautés en médecine 2007 [Pediatrics]

Cette année, nous avons résumé les avances dans deux pathologies très fréquentes et deux plus rares : 1) la cryptorchidie est opérée plus précocement sans utiliser des hormones. Ce traitement vise à assurer la fertilité et à éviter le développement de tumeurs testiculaires. 2) Dans le traitement de la pharyngite streptococcique, les céphalosporines orales durant quatre à cinq jours, peuvent remplacer la pénicilline. 3) Le carvédilol (bêtabloquant), le lévosimendan (sensibilisateur du calcium) et le nésiritide (analogue du peptide natriurétique) permettent une approche neurohormonale à l'insuffisance cardiaque. 4) Les glucocorticoïdes administrés tôt et une prise en charge multidisciplinaire ralentissent le cours de la dystrophie musculaire de Duchenne. This article summarizes the medical progress achieved in 2 frequent and 2 rare pathologies: 1. Cryptorchidism should be operated around 12 months of age and hormonal treatment abandoned in order to maintain fertility and avoid development of testicular tumors. 2. For the treatment of streptococcal pharyngitis oral cephalosporins for 4 to 5 days are equivalent to a Penicillin treatment of 10 days. 3. Thanks to carvedilol (a beta-blocker agent), levosimendan (a calcium sensibiliser) and nesiritide (an analog to the natriuretic peptide) a new hormonal approach to cardiac failure is possible. 4. Corticosteroids allow to improve quality of live and life expectancy in Duchenne muscular dystrophy, provided treatment starts early and a multidisciplinary approach is assured

Combining MAR elements and transposable vectors for more reliable transgene integration and expression

Integration without cytotoxic effects and long-term expression of a transgene constitutes a major challenge in gene therapy and biotechnology applications. In this context, transposons represent an attractive system for gene transfer because of their ability to promote efficient integration of a transgene in a variety of cell lines. However, the transgene integration can lead to insertional mutagenesis and/or unstable transgene expression by epigenetic modifications. These unwanted events may be limited by the use of chromatin control elements called MARs (matrix attachment regions). Indeed, the insertion of these DNA elements next to the transgene usually results in higher and more stable expression by maintaining transgene chromatin in an active configuration and preventing gene silencing. In this study, we tested if the inclusion of the MAR 1-68 in the piggyBac transposon system may lead to efficient and safer transgene integration and ensure reliable stable and long-term expression of a transgene. The MAR-containing transposon construct was tested in CHO cells, for biotechnology applications, and in mesoangioblast cells that can differentiate into muscle cells and are important candidates for potential stem cell therapies of myopathies. We showed that the addition of the MAR 1 -68 in the piggyBac transposon did not interfere with transposition, thereby maintaining high frequency of transgene integrations in these cells. Moreover, the MAR allowed higher transgene expression from fewer transposon integration events. We also found that enriched transgene-expressing cell populations could be obtained without the need of selection pressure. Since antibiotic-enforced selection protocols often result in a higher integrated copy number and mosaic expression patterns, this strategy could benefit many applications in which a low copy number of integrated transgenes and antibiotic-free conditions are desired. In addition, the intramuscular transplantation of mouse tibialis anterior muscles with mesoangioblasts containing the transposon led to widespread and sustained myofiber transgene expression after differentiation of these cells in vivo. These findings indicated that piggyBac vectors may provide a viable approach to achieve stable gene transfer in the context of Duchenne muscular dystrophy therapy. - L'intégration sans effets cytotoxiques et l'expression à long terme d'un transgène constituent un défi majeur en thérapie génique et en biotechnologie. Dans ce contexte, les transposons représentent un système attrayant pour le transfert de gènes en raison de leur capacité à promouvoir l'intégration efficace d'un transgène dans une variété de lignées cellulaires. Toutefois, l'intégration d'un transgène peut conduire à une mutagénèse insertionnelle et/ou à une expression instable due au silençage du transgène suite à des modifications épigénétiques. Ces événements indésirables de silençage génique peuvent être diminués par l'utilisation d'éléments de contrôle de la chromatine appelés MAR (matrix attachment region). En effet, l'insertion de ces éléments d'ADN à proximité du transgène se traduit généralement par une expression plus élevée et plus stable de celui-ci, en permettant le maintien d'une chromatine dans une configuration active autour du transgène et en empêchant l'inactivation du gène. Dans cette étude, nous avons testé si l'inclusion du MAR 1-68 dans le système transposon piggyBac peut améliorer l'efficacité d'intégration de façon sécuritaire et l'expression à long terme d'un transgène. Le transposon contenant l'élément MAR a été testé dans les cellules CHO, couramment utilisées en biotechnologie, et dans des cellules progénitrices appelées mésoangioblastes, qui peuvent se différencier en cellules musculaires, et qui constituent ainsi des candidats prometteurs pour la thérapie à partir de cellules souches de patients souffrant de myopathie. Nous avons montré que l'addition du MAR 1-68 dans le transposon piggyBac n'interfère pas avec la transposition et permet de maintenir une fréquence élevée d'intégration du transgène dans ces deux types cellulaires. De plus, il semble que cette association mène à une meilleure expression du transgène à partir de peu d'événements d'intégration du transposon. En outre, ces populations enrichies en cellules exprimant de façon stable le transgène ont pu être obtenues sans avoir recours à une pression de sélection. Etant donné que les protocoles de sélection basée sur l'utilisation d'antibiotiques conduisent souvent à un nombre plus élevé de copies intégrées et à la variégation de l'expression du transgène et qu'ils impliquent une longue culture in vitro, cette stratégie pourrait profiter à des applications pour lesquelles on souhaite un faible nombre de copies intégrées et/ou l'utilisation d'antibiotiques n'est pas souhaitable. De plus, la transplantation intramusculaire de mésoangioblastes contenant le transposon dans le muscle tibial antérieur de souris a conduit, après la différentiation de ces cellules in vivo, à une expression constante et étendue du transgène dans les myofibres. Ces résultats indiquent que les vecteurs piggyBac pourraient fournir une approche viable pour assurer un transfert de gènes stables dans le contexte d'un traitement de la dystrophic musculaire de Duchenne.

Assessing dystrophies and other muscle diseases at the nanometer scale by atomic force microscopy.

AIM: Atomic force microscopy nanoindentation of myofibers was used to assess and quantitatively diagnose muscular dystrophies from human patients. MATERIALS & METHODS: Myofibers were probed from fresh or frozen muscle biopsies from human dystrophic patients and healthy volunteers, as well as mice models, and Young's modulus stiffness values were determined. RESULTS: Fibers displaying abnormally low mechanical stability were detected in biopsies from patients affected by 11 distinct muscle diseases, and Young's modulus values were commensurate to the severity of the disease. Abnormal myofiber resistance was also observed from consulting patients whose muscle condition could not be detected or unambiguously diagnosed otherwise. DISCUSSION & CONCLUSION: This study provides a proof-of-concept that atomic force microscopy yields a quantitative read-out of human muscle function from clinical biopsies, and that it may thereby complement current muscular dystrophy diagnosis.

O-mannosyl phosphorylation of alpha-dystroglycan is required for laminin binding.

Alpha-dystroglycan (alpha-DG) is a cell-surface glycoprotein that acts as a receptor for both extracellular matrix proteins containing laminin-G domains and certain arenaviruses. Receptor binding is thought to be mediated by a posttranslational modification, and defective binding with laminin underlies a subclass of congenital muscular dystrophy. Using mass spectrometry- and nuclear magnetic resonance (NMR)-based structural analyses, we identified a phosphorylated O-mannosyl glycan on the mucin-like domain of recombinant alpha-DG, which was required for laminin binding. We demonstrated that patients with muscle-eye-brain disease and Fukuyama congenital muscular dystrophy, as well as mice with myodystrophy, commonly have defects in a postphosphoryl modification of this phosphorylated O-linked mannose, and that this modification is mediated by the like-acetylglucosaminyltransferase (LARGE) protein. These findings expand our understanding of the mechanisms that underlie congenital muscular dystrophy.

Early onset collagen VI myopathies: Genetic and clinical correlations.

OBJECTIVE: Mutations in the genes encoding the extracellular matrix protein collagen VI (ColVI) cause a spectrum of disorders with variable inheritance including Ullrich congenital muscular dystrophy, Bethlem myopathy, and intermediate phenotypes. We extensively characterized, at the clinical, cellular, and molecular levels, 49 patients with onset in the first 2 years of life to investigate genotype-phenotype correlations. METHODS: Patients were classified into 3 groups: early-severe (18%), moderate-progressive (53%), and mild (29%). ColVI secretion was analyzed in patient-derived skin fibroblasts. Chain-specific transcript levels were quantified by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), and mutation identification was performed by sequencing of complementary DNA. RESULTS: ColVI secretion was altered in all fibroblast cultures studied. We identified 56 mutations, mostly novel and private. Dominant de novo mutations were detected in 61% of the cases. Importantly, mutations causing premature termination codons (PTCs) or in-frame insertions strikingly destabilized the corresponding transcripts. Homozygous PTC-causing mutations in the triple helix domains led to the most severe phenotypes (ambulation never achieved), whereas dominant de novo in-frame exon skipping and glycine missense mutations were identified in patients of the moderate-progressive group (loss of ambulation). INTERPRETATION: This work emphasizes that the diagnosis of early onset ColVI myopathies is arduous and time-consuming, and demonstrates that quantitative RT-PCR is a helpful tool for the identification of some mutation-bearing genes. Moreover, the clinical classification proposed allowed genotype-phenotype relationships to be explored, and may be useful in the design of future clinical trials.

Urocortins improve dystrophic skeletal muscle structure and function through both PKA- and Epac-dependent pathways.

In Duchenne muscular dystrophy, the absence of dystrophin causes progressive muscle wasting and premature death. Excessive calcium influx is thought to initiate the pathogenic cascade, resulting in muscle cell death. Urocortins (Ucns) have protected muscle in several experimental paradigms. Herein, we demonstrate that daily s.c. injections of either Ucn 1 or Ucn 2 to 3-week-old dystrophic mdx(5Cv) mice for 2 weeks increased skeletal muscle mass and normalized plasma creatine kinase activity. Histological examination showed that Ucns remarkably reduced necrosis in the diaphragm and slow- and fast-twitch muscles. Ucns improved muscle resistance to mechanical stress provoked by repetitive tetanizations. Ucn 2 treatment resulted in faster kinetics of contraction and relaxation and a rightward shift of the force-frequency curve, suggesting improved calcium homeostasis. Ucn 2 decreased calcium influx into freshly isolated dystrophic muscles. Pharmacological manipulation demonstrated that the mechanism involved the corticotropin-releasing factor type 2 receptor, cAMP elevation, and activation of both protein kinase A and the cAMP-binding protein Epac. Moreover, both STIM1, the calcium sensor that initiates the assembly of store-operated channels, and the calcium-independent phospholipase A(2) that activates these channels were reduced in dystrophic muscle by Ucn 2. Altogether, our results demonstrate the high potency of Ucns for improving dystrophic muscle structure and function, suggesting that these peptides may be considered for treatment of Duchenne muscular dystrophy.

Cholesterol and Fatty Acids Regulate Dynamic Caveolin Trafficking through the Golgi Complex and between the Cell Surface and Lipid Bodies

Caveolins are a crucial component of plasma membrane (PM) caveolae but have also been localized to intracellular compartments, including the Golgi complex and lipid bodies. Mutant caveolins associated with human disease show aberrant trafficking to the PM and Golgi accumulation. We now show that the Golgi pool of mainly newly synthesized protein is detergent-soluble and predominantly in a monomeric state, in contrast to the surface pool. Caveolin at the PM is not recognized by specific caveolin antibodies unless PM cholesterol is depleted. Exit from the Golgi complex of wild-type caveolin-1 or -3, but not vesicular stomatitis virus-G protein, is modulated by changing cellular cholesterol levels. In contrast, a muscular dystrophy-associated mutant of caveolin-3, Cav3P104L, showed increased accumulation in the Golgi complex upon cholesterol treatment. In addition, we demonstrate that in response to fatty acid treatment caveolin can follow a previously undescribed pathway from the PM to lipid bodies and can move from lipid bodies to the PM in response to removal of fatty acids. The results suggest that cholesterol is a rate-limiting component for caveolin trafficking. Changes in caveolin flux through the exocytic pathway can therefore be an indicator of cellular cholesterol and fatty acid levels.