Dysferlin-deficient immortalized human myoblasts and myotubes as a useful tool to study dysferlinopathy

A Novel Assay Reveals the Early Setting-Up of Membrane Repair Machinery in Human Skeletal Muscle Cells

Defect in membrane repair contributes to the development of muscular dystrophies such as limb girdle muscular dystrophy (LGMD) type R2 or R12. Nevertheless, many other muscular dystrophies may also result from a defect in this process. Identifying these pathologies requires the development of specific methods to inflict sarcolemma damage on a large number of cells and rapidly analyze their response. We adapted a protocol hitherto used to study the behavior of cancer cells to mechanical constraint. This method is based on forcing the passage of cells through a thin needle, which induces shear stress. Due to size considerations, this method requires working with mononuclear muscle cells instead of myotubes or muscle fibers. Although functional sarcolemma repair was thought to be restricted to myotubes and muscle fibers, we show here that 24h-differentiated myoblasts express a complete machinery capable of addressing membrane damage. At this stage, muscle cells do not yet form myotubes, revealing that the membrane repair machinery is set up early throughout the differentiation process. When submitted to the shear-stress assay, these cells were observed to repair membrane damage in a Ca2+-dependent manner, as previously reported. We show that this technique is able to identify the absence of membrane resealing in muscle cells from patient suffering from LGMDR2. The proposed technique provides therefore a suitable method for identifying cellular dysregulations in membrane repair of dystrophic human muscle cells.

Bioengineered Model of Human LGMD2B Skeletal Muscle Reveals Roles of Intracellular Calcium Overload in Contractile and Metabolic Dysfunction in Dysferlinopathy

Dysferlin is a multi-functional protein that regulates membrane resealing, calcium homeostasis, and lipid metabolism in skeletal muscle. Genetic loss of dysferlin results in limb girdle muscular dystrophy 2B/2R (LGMD2B/2R) and other dysferlinopathies - rare untreatable muscle diseases that lead to permanent loss of ambulation in humans. The mild disease severity in dysferlin-deficient mice and diverse genotype-phenotype relationships in LGMD2B patients have prompted the development of new in vitro models for personalized studies of dysferlinopathy. Here the first 3-D tissue-engineered hiPSC-derived skeletal muscle ("myobundle") model of LGMD2B is described that exhibits compromised contractile function, calcium-handling, and membrane repair, and transcriptomic changes indicative of impaired oxidative metabolism and mitochondrial dysfunction. In response to the fatty acid (FA) challenge, LGMD2B myobundles display mitochondrial deficits and intracellular lipid droplet (LD) accumulation. Treatment with the ryanodine receptor (RyR) inhibitor dantrolene or the dissociative glucocorticoid vamorolone restores LGMD2B contractility, improves membrane repair, and reduces LD accumulation. Lastly, it is demonstrated that chemically induced chronic RyR leak in healthy myobundles phenocopies LGMD2B contractile and metabolic deficit, but not the loss of membrane repair capacity. Together, these results implicate intramyocellular Ca2+ leak as a critical driver of dysferlinopathic phenotype and validate the myobundle system as a platform to study LGMD2B pathogenesis.

Skeletal Muscle Cells Derived from Induced Pluripotent Stem Cells: A Platform for Limb Girdle Muscular Dystrophies

Limb girdle muscular dystrophies (LGMD), caused by mutations in 29 different genes, are the fourth most prevalent group of genetic muscle diseases. Although the link between LGMD and its genetic origins has been determined, LGMD still represent an unmet medical need. Here, we describe a platform for modeling LGMD based on the use of human induced pluripotent stem cells (hiPSC). Thanks to the self-renewing and pluripotency properties of hiPSC, this platform provides a renewable and an alternative source of skeletal muscle cells (skMC) to primary, immortalized, or overexpressing cells. We report that skMC derived from hiPSC express the majority of the genes and proteins that cause LGMD. As a proof of concept, we demonstrate the importance of this cellular model for studying LGMDR9 by evaluating disease-specific phenotypes in skMC derived from hiPSC obtained from four patients.

Therapeutic Benefit of Galectin-1: Beyond Membrane Repair, a Multifaceted Approach to LGMD2B

Two of the main pathologies characterizing dysferlinopathies are disrupted muscle membrane repair and chronic inflammation, which lead to symptoms of muscle weakness and wasting. Here, we used recombinant human Galectin-1 (rHsGal-1) as a therapeutic for LGMD2B mouse and human models. Various redox and multimerization states of Gal-1 show that rHsGal-1 is the most effective form in both increasing muscle repair and decreasing inflammation, due to its monomer-dimer equilibrium. Dose-response testing shows an effective 25-fold safety profile between 0.54 and 13.5 mg/kg rHsGal-1 in Bla/J mice. Mice treated weekly with rHsGal-1 showed downregulation of canonical NF-κB inflammation markers, decreased muscle fat deposition, upregulated anti-inflammatory cytokines, increased membrane repair, and increased functional movement compared to non-treated mice. Gal-1 treatment also resulted in a positive self-upregulation loop of increased endogenous Gal-1 expression independent of NF-κB activation. A similar reduction in disease pathologies in patient-derived human cells demonstrates the therapeutic potential of Gal-1 in LGMD2B patients.

Functions of Vertebrate Ferlins

Ferlins are multiple-C2-domain proteins involved in Ca²⁺-triggered membrane dynamics within the secretory, endocytic and lysosomal pathways. In bony vertebrates there are six ferlin genes encoding, in humans, dysferlin, otoferlin, myoferlin, Fer1L5 and 6 and the long noncoding RNA Fer1L4. Mutations in DYSF (dysferlin) can cause a range of muscle diseases with various clinical manifestations collectively known as dysferlinopathies, including limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy. A mutation in MYOF (myoferlin) was linked to a muscular dystrophy accompanied by cardiomyopathy. Mutations in OTOF (otoferlin) can be the cause of nonsyndromic deafness DFNB9. Dysregulated expression of any human ferlin may be associated with development of cancer. This review provides a detailed description of functions of the vertebrate ferlins with a focus on muscle ferlins and discusses the mechanisms leading to disease development.

AMPK Complex Activation Promotes Sarcolemmal Repair in Dysferlinopathy

Mutations in dysferlin are responsible for a group of progressive, recessively inherited muscular dystrophies known as dysferlinopathies. Using recombinant proteins and affinity purification methods combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS), we found that AMP-activated protein kinase (AMPK)γ1 was bound to a region of dysferlin located between the third and fourth C2 domains. Using ex vivo laser injury experiments, we demonstrated that the AMPK complex was vital for the sarcolemmal damage repair of skeletal muscle fibers. Injury-induced AMPK complex accumulation was dependent on the presence of Ca²⁺, and the rate of accumulation was regulated by dysferlin. Furthermore, it was found that the phosphorylation of AMPKα was essential for plasma membrane repair, and treatment with an AMPK activator rescued the membrane-repair impairment observed in immortalized human myotubes with reduced expression of dysferlin and dysferlin-null mouse fibers. Finally, it was determined that treatment with the AMPK activator metformin improved the muscle phenotype in zebrafish and mouse models of dysferlin deficiency. These findings indicate that the AMPK complex is essential for plasma membrane repair and is a potential therapeutic target for dysferlinopathy.

Defects in G-Actin Incorporation into Filaments in Myoblasts Derived from Dysferlinopathy Patients Are Restored by Dysferlin C2 Domains

Dysferlin is a transmembrane C-2 domain-containing protein involved in vesicle trafficking and membrane remodeling in skeletal muscle cells. However, the mechanism by which dysferlin regulates these cellular processes remains unclear. Since actin dynamics is critical for vesicle trafficking and membrane remodeling, we studied the role of dysferlin in Ca²⁺-induced G-actin incorporation into filaments in four different immortalized myoblast cell lines (DYSF2, DYSF3, AB320, and ER) derived from patients harboring mutations in the dysferlin gene. As compared with immortalized myoblasts obtained from a control subject, dysferlin expression and G-actin incorporation were significantly decreased in myoblasts from dysferlinopathy patients. Stable knockdown of dysferlin with specific shRNA in control myoblasts also significantly reduced G-actin incorporation. The impaired G-actin incorporation was restored by the expression of full-length dysferlin as well as dysferlin N-terminal or C-terminal regions, both of which contain three C2 domains. DYSF3 myoblasts also exhibited altered distribution of annexin A2, a dysferlin partner involved in actin remodeling. However, dysferlin N-terminal and C-terminal regions appeared to not fully restore such annexin A2 mislocation. Then, our results suggest that dysferlin regulates actin remodeling by a mechanism that does to not involve annexin A2.

Serum exosomes can restore cellular function in vitro and be used for diagnosis in dysferlinopathy

Purpose: It is challenging to deliver the full-length dysferlin gene or protein to restore cellular functions of dysferlin-deficient (DYSF^-/-) myofibres in dysferlinopathy, a disease caused by the absence of dysferlin, which is currently without effective treatment. Exosomes, efficient membranous nanoscale carriers of biological cargoes, could be useful.

Experimental design: Myotube- and human serum-derived exosomes were investigated for their capabilities of restoring dysferlin protein and cellular functions in murine and human DYSF^-/- cells. Moreover, dysferlinopathic patient serum- and urine-derived exosomes were assessed for their abilities as diagnostic tools for dysferlinopathy.

Results: Here we show that exosomes from dysferlin-expressing myotubes carry abundant dysferlin and enable transfer of full-length dysferlin protein to DYSF^-/- myotubes. Exogenous dysferlin correctly localizes on DYSF^-/- myotube membranes, enabling membrane resealing in response to injury. Human serum exosomes also carry dysferlin protein and improve membrane repair capabilities of human DYSF^-/- myotubes irrespective of mutations. Lack of dysferlin in dysferlinopathic patient serum and urine exosomes enables differentiation between healthy controls and dysferlinopathic patients.

Conclusions: Our findings provide evidence that exosomes are efficient carriers of dysferlin and can be employed for the treatment and non-invasive diagnosis of dysferlinopathy.

Cell Membrane Repair Assay Using a Two-photon Laser Microscope

Numerous pathophysiological insults can cause damage to cell membranes and, when coupled with innate defects in cell membrane repair or integrity, can result in disease. Understanding the underlying molecular mechanisms surrounding cell membrane repair is, therefore, an important objective to the development of novel therapeutic strategies for diseases associated with dysfunctional cell membrane dynamics. Many in vitro and in vivo studies aimed at understanding cell membrane resealing in various disease contexts utilize two-photon laser ablation as a standard for determining functional outcomes following experimental treatments. In this assay, cell membranes are subjected to wounding with a two-photon laser, which causes the cell membrane to rupture and fluorescent dye to infiltrate the cell. The intensity of fluorescence within the cell can then be monitored to quantify the cell's ability to reseal itself. There are several alternative methods for assessing cell membrane response to injury, as well as great variation in the two-photon laser wounding approach itself, therefore, a single, unified model of cell wounding would beneficially serve to decrease the variation between these methodologies. In this article, we outline a simple two-photon laser wounding protocol for assessing cell membrane repair in vitro in both healthy and dysferlinopathy patient fibroblast cells transfected with or without a full-length dysferlin plasmid.

IRS4, a novel modulator of BMP/Smad and Akt signalling during early muscle differentiation

Elaborate regulatory networks of the Bone Morphogenetic Protein (BMP) pathways ensure precise signalling outcome during cell differentiation and tissue homeostasis. Here, we identified IRS4 as a novel regulator of BMP signal transduction and provide molecular insights how it integrates into the signalling pathway. We found that IRS4 interacts with the BMP receptor BMPRII and specifically targets Smad1 for proteasomal degradation consequently leading to repressed BMP/Smad signalling in C2C12 myoblasts while concomitantly activating the PI3K/Akt axis. IRS4 is present in human and primary mouse myoblasts, the expression increases during myogenic differentiation but is downregulated upon final commitment coinciding with Myogenin expression. Functionally, IRS4 promotes myogenesis in C2C12 cells, while IRS4 knockdown inhibits differentiation of myoblasts. We propose that IRS4 is particularly critical in the myoblast stage to serve as a molecular switch between BMP/Smad and Akt signalling and to thereby control cell commitment. These findings provide profound understanding of the role of BMP signalling in early myogenic differentiation and open new ways for targeting the BMP pathway in muscle regeneration.

Skeletal muscle characteristics are preserved in hTERT/cdk4 human myogenic cell lines

Background

hTERT/cdk4 immortalized myogenic human cell lines represent an important tool for skeletal muscle research, being used as therapeutically pertinent models of various neuromuscular disorders and in numerous fundamental studies of muscle cell function. However, the cell cycle is linked to other cellular processes such as integrin regulation, the PI3K/Akt pathway, and microtubule stability, raising the question as to whether genetic modification related to the cell cycle results in secondary effects that could undermine the validity of these cell models.

Results

Here we subjected five healthy and disease muscle cell isolates to transcriptomic analysis, comparing immortalized lines with their parent primary populations in both differentiated and undifferentiated states, and testing their myogenic character by comparison with non-myogenic (CD56-negative) cells. Principal component analysis of global gene expression showed tight clustering of immortalized myoblasts to their parent primary populations, with clean separation from the non-myogenic reference. Comparison was made to publicly available transcriptomic data from studies of muscle human pathology, cell, and animal models, including to derive a consensus set of genes previously shown to have altered regulation during myoblast differentiation. Hierarchical clustering of samples based on gene expression of this consensus set showed that immortalized lines retained the myogenic expression patterns of their parent primary populations. Of 2784 canonical pathways and gene ontology terms tested by gene set enrichment analysis, none were significantly enriched in immortalized compared to primary cell populations. We observed, at the whole transcriptome level, a strong signature of cell cycle shutdown associated with senescence in one primary myoblast population, whereas its immortalized clone was protected.

Conclusions

Immortalization had no observed effect on the myogenic cascade or on any other cellular processes, and it was protective against the systems level effects of senescence that are observed at higher division counts of primary cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0115-5) contains supplementary material, which is available to authorized users.

Hybrid Randomly Electrospun Poly(lactic-co-glycolic acid):Poly(ethylene oxide) (PLGA:PEO) Fibrous Scaffolds Enhancing Myoblast Differentiation and Alignment

Cellular responses are regulated by their microenvironments, and engineered synthetic scaffolds can offer control over different microenvironment properties. This important relationship can be used as a tool to manipulate cell fate and cell responses for different biomedical applications. We show for the first time in this study how blending of poly(ethylene oxide) (PEO) to poly(lactic-co-glycolic acid) (PLGA) fibers to yield hybrid scaffolds changes the physical and mechanical properties of PLGA fibrous scaffolds and in turn affects cellular response. For this purpose we employed electrospinning to create fibrous scaffolds mimicking the basic structural properties of the native extracellular matrix. We introduced PEO to PLGA electrospun fibers by spinning a blend of PLGA:PEO polymer solutions in different ratios. PEO served as a sacrificial component within the fibers upon hydration, leading to pore formation in the fibers, fiber twisting, increased scaffold disintegration, and hydrophilicity, decreased Young's modulus, and significantly improved strain at break of initially electrospun scaffolds. We observed that the blended PLGA:PEO fibrous scaffolds supported myoblast adhesion and proliferation and resulted in increased myotube formation and self-alignment, when compared to PLGA-only scaffolds, even though the scaffolds were randomly oriented. The 50:50 PLGA:PEO blended scaffold showed the most promising results in terms of mechanical properties, myotube formation, and alignment, suggesting an optimal microenvironment for myoblast differentiation from the PLGA:PEO blends tested. The explored approach for tuning fiber properties can easily extend to other polymeric scaffolds and provides a valuable tool to engineer fibrillar microenvironments for several biomedical applications.

Dysferlin function in skeletal muscle: Possible pathological mechanisms and therapeutical targets in dysferlinopathies

Mutations in the dysferlin gene are linked to a group of muscular dystrophies known as dysferlinopathies. These myopathies are characterized by progressive atrophy. Studies in muscle tissue from dysferlinopathy patients or dysferlin-deficient mice point out its importance in membrane repair. However, expression of dysferlin homologous proteins that restore sarcolemma repair function in dysferlinopathy animal models fail to arrest muscle wasting, therefore suggesting that dysferlin plays other critical roles in muscle function. In the present review, we discuss dysferlin functions in the skeletal muscle, as well as pathological mechanisms related to dysferlin mutations. Particular focus is presented related the effect of dysferlin on cell membrane related function, which affect its repair, vesicle trafficking, as well as Ca(2+) homeostasis. Such mechanisms could provide accessible targets for pharmacological therapies.

Membrane repair of human skeletal muscle cells requires Annexin-A5

Defect in membrane repair contributes to the development of limb girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy. In healthy skeletal muscle, unraveling membrane repair mechanisms requires to establish an exhaustive list of the components of the resealing machinery. Here we show that human myotubes rendered deficient for Annexin-A5 (AnxA5) suffer from a severe defect in membrane resealing. This defect is rescued by the addition of recombinant AnxA5 while an AnxA5 mutant, which is unable to form 2D protein arrays, has no effect. Using correlative light and electron microscopy, we show that AnxA5 binds to the edges of the torn membrane, as early as a few seconds after sarcolemma injury, where it probably self-assembles into 2D arrays. In addition, we observed that membrane resealing is associated with the presence of a cluster of lipid vesicles at the wounded site. AnxA5 is present at the surface of these vesicles and may thus participate in plugging the cell membrane disruption. Finally, we show that AnxA5 behaves similarly in myotubes from a muscle cell line established from a patient suffering from LGMD2B, a myopathy due to dysferlin mutations, which indicates that trafficking of AnxA5 during sarcolemma damage is independent of the presence of dysferlin.

The absence of dysferlin induces the expression of functional connexin-based hemichannels in human myotubes

Background

Mutations in the gene encoding for dysferlin cause recessive autosomal muscular dystrophies called dysferlinopathies. These mutations induce several alterations in skeletal muscles, including, inflammation, increased membrane permeability and cell death. Despite the fact that the etiology of dysferlinopathies is known, the mechanism that explains the aforementioned alterations is still elusive. Therefore, we have now evaluated the potential involvement of connexin based hemichannels in the pathophysiology of dysferlinopathies.

Results

Human deltoid muscle biopsies of 5 Chilean dysferlinopathy patients exhibited the presence of muscular connexins (Cx40.1, Cx43 and Cx45). The presence of these connexins was also observed in human myotubes derived from immortalized myoblasts derived from other patients with mutated forms of dysferlin. In addition to the aforementioned connexins, these myotubes expressed functional connexin based hemichannels, evaluated by ethidium uptake assays, as opposed to myotubes obtained from a normal human muscle cell line, RCMH. This response was reproduced in a knock-down model of dysferlin, by treating RCMH cell line with small hairpin RNA specific for dysferlin (RCMH-sh Dysferlin). Also, the presence of P2X₇ receptor and the transient receptor potential channel, TRPV2, another Ca²⁺ permeable channels, was detected in the myotubes expressing mutated dysferlin, and an elevated resting intracellular Ca²⁺ level was found in the latter myotubes, which was in turn reduced to control levels in the presence of the molecule D4, a selective Cx HCs inhibitor.

Conclusions

The data suggests that dysferlin deficiency, caused by mutation or downregulation of dysferlin, promotes the expression of Cx HCs. Then, the de novo expression Cx HC causes a dysregulation of intracellular free Ca²⁺ levels, which could underlie muscular damage associated to dysferlin mutations. This mechanism could constitute a potential therapeutical target in dysferlinopathies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-016-0096-6) contains supplementary material, which is available to authorized users.

Reliable and versatile immortal muscle cell models from healthy and myotonic dystrophy type 1 primary human myoblasts

Primary human skeletal muscle cells (hSkMCs) are invaluable tools for deciphering the basic molecular mechanisms of muscle-related biological processes and pathological alterations. Nevertheless, their use is quite restricted due to poor availability, short life span and variable purity of the cells during in vitro culture. Here, we evaluate a recently published method of hSkMCs immortalization, relying on ectopic expression of cyclin D1 (CCND1), cyclin-dependent kinase 4 (CDK4) and telomerase (TERT) in myoblasts from healthy donors (n=3) and myotonic dystrophy type 1 (DM1) patients (n=2). The efficacy to maintain the myogenic and non-transformed phenotype, as well as the main pathogenetic hallmarks of DM1, has been assessed. Combined expression of the three genes i) maintained the CD56(NCAM)-positive myoblast population and differentiation potential; ii) preserved the non-transformed phenotype and iii) maintained the CTG repeat length, amount of nuclear foci and aberrant alternative splicing in immortal muscle cells. Moreover, immortal hSkMCs displayed attractive additional features such as structural maturation of sarcomeres, persistence of Pax7-positive cells during differentiation and complete disappearance of nuclear foci following (CAG)7 antisense oligonucleotide (ASO) treatment. Overall, the CCND1, CDK4 and TERT immortalization yields versatile, reliable and extremely useful human muscle cell models to investigate the basic molecular features of human muscle cell biology, to elucidate the molecular pathogenetic mechanisms and to test new therapeutic approaches for DM1 in vitro.

Dysferlinopathy Fibroblasts Are Defective in Plasma Membrane Repair

Background: Dysferlin is a sarcolemmal protein that is defective in Miyoshi myopathy and limb-girdle muscular dystrophy type 2B, and is involved in sarcolemmal repair. Primary cultured myoblasts and myotubes established from patient muscle biopsies have been widely utilized to explore the molecular mechanism of dysferlinopathy.

Objectives: The purpose of this study was to explore the possible utility of dermal fibroblasts from dysferlin-deficient patients and SJL mice as a tool for studying dysferlinopathy.

Methods: Dysferlin protein expression in fibroblasts from dysferlin-deficient patients and SJL mice was analyzed by immunoblotting and immunocytochemistry. The membrane wound-repair assay was performed on the fibroblasts using a confocal microscope equipped with a UV-laser. The membrane blebbing assay using hypotonic shock, in which normal membrane blebbing is detected only in the presence of dysferlin, was also performed using human and mouse fibroblasts.

Results: Mis-sense mutated dysferlin was expressed at a very low level in fibroblasts from a dysferlinopathy patient, and lower expression level of truncated dysferlin was observed in SJL mouse fibroblast. Fibroblasts from patients with dysferlinopathy and SJL mice showed attenuated membrane repair and did not form membrane blebs in response to hypoosmotic shock. Proteosomal inhibitior increased mis-sense mutated or truncated dysferlin levels, and restored membrane blebbing, however, proteosomal inhibition failed to improve levels of dysferlin with non-sense or frame-shift mutation.

Conclusion: Fibroblasts from dysferlinopathy patients and SJL mice showed attenuated plasma membrane repair, and could be a tool for studying dysferlinopathy.

Cavin 1 function does not follow caveolar morphology

The function of caveolae, small invaginations of the plasma membrane, remains a matter of debate. We discuss endocytosis and compartmentalization of metabolic and signaling pathways. Caveolin 3 (CAV3) and polymerase I and transcript release factor (PTRF) are important proteins that ensure shaping of caveolae in muscle cells. We investigated caveolae morphologically by electron microscopy in myotubes obtained from patients with CAV3 mutations and performed functional analyses in fibroblasts from a patient with a mutation in PTRF. Despite the complete clinical picture of a caveolinopathy, we found that caveolae in the CAV3-deficient myotubes were normal in shape and number. Furthermore, we found a difference in uptake of cholera toxin B between PTRF-deficient fibroblasts devoid of caveolae and normal fibroblasts. However, after caveolae were rescued by transfection of PTRF, cholera toxin B uptake did not normalize. We conclude that the presence of caveolae as an anatomic structure is not sufficient to ensure their proper function. Alternatively, the functional properties assigned to caveolae might be mediated by different mechanisms that have yet to be resolved.

Dysferlin rescue by spliceosome-mediated pre-mRNA trans-splicing targeting introns harbouring weakly defined 3' splice sites

The modification of the pre-mRNA cis-splicing process employing a pre-mRNA trans-splicing molecule (PTM) is an attractive strategy for the in situ correction of genes whose careful transcription regulation and full-length expression is determinative for protein function, as it is the case for the dysferlin (DYSF, Dysf) gene. Loss-of-function mutations of DYSF result in different types of muscular dystrophy mainly manifesting as limb girdle muscular dystrophy 2B (LGMD2B) and Miyoshi muscular dystrophy 1 (MMD1). We established a 3' replacement strategy for mutated DYSF pre-mRNAs induced by spliceosome-mediated pre-mRNA trans-splicing (SmaRT) by the use of a PTM. In contrast to previously established SmaRT strategies, we particularly focused on the identification of a suitable pre-mRNA target intron other than the optimization of the PTM design. By targeting DYSF pre-mRNA introns harbouring differentially defined 3' splice sites (3' SS), we found that target introns encoding weakly defined 3' SSs were trans-spliced successfully in vitro in human LGMD2B myoblasts as well as in vivo in skeletal muscle of wild-type and Dysf(-/-) mice. For the first time, we demonstrate rescue of Dysf protein by SmaRT in vivo. Moreover, we identified concordant qualities among the successfully targeted Dysf introns and targeted endogenous introns in previously reported SmaRT approaches that might facilitate a selective choice of target introns in future SmaRT strategies.

Small molecules dorsomorphin and LDN-193189 inhibit myostatin/GDF8 signaling and promote functional myoblast differentiation

GDF8, or myostatin, is a member of the TGF-β superfamily of secreted polypeptide growth factors. GDF8 is a potent negative regulator of myogenesis both in vivo and in vitro. We found that GDF8 signaling was inhibited by the small molecule ATP competitive inhibitors dorsomorphin and LDN-193189. These compounds were previously shown to be potent inhibitors of BMP signaling by binding to the BMP type I receptors ALK1/2/3/6. We present the crystal structure of the type II receptor ActRIIA with dorsomorphin and demonstrate that dorsomorphin or LDN-193189 target GDF8 induced Smad2/3 signaling and repression of myogenic transcription factors. As a result, both inhibitors rescued myogenesis in myoblasts treated with GDF8. As revealed by quantitative live cell microscopy, treatment with dorsomorphin or LDN-193189 promoted the contractile activity of myotubular networks in vitro. We therefore suggest these inhibitors as suitable tools to promote functional myogenesis.

Dysferlin regulates cell membrane repair by facilitating injury-triggered acid sphingomyelinase secretion

Dysferlin deficiency compromises the repair of injured muscle, but the underlying cellular mechanism remains elusive. To study this phenomenon, we have developed mouse and human myoblast models for dysferlinopathy. These dysferlinopathic myoblasts undergo normal differentiation but have a deficit in their ability to repair focal injury to their cell membrane. Imaging cells undergoing repair showed that dysferlin-deficit decreased the number of lysosomes present at the cell membrane, resulting in a delay and reduction in injury-triggered lysosomal exocytosis. We find repair of injured cells does not involve formation of intracellular membrane patch through lysosome-lysosome fusion; instead, individual lysosomes fuse with the injured cell membrane, releasing acid sphingomyelinase (ASM). ASM secretion was reduced in injured dysferlinopathic cells, and acute treatment with sphingomyelinase restored the repair ability of dysferlinopathic myoblasts and myofibers. Our results provide the mechanism for dysferlin-mediated repair of skeletal muscle sarcolemma and identify ASM as a potential therapy for dysferlinopathy.

Establishment of a human skeletal muscle-derived cell line: biochemical, cellular and electrophysiological characterization

Excitation-contraction coupling is the physiological mechanism occurring in muscle cells whereby an electrical signal sensed by the dihydropyridine receptor located on the transverse tubules is transformed into a chemical gradient (Ca2+ increase) by activation of the ryanodine receptor located on the sarcoplasmic reticulum membrane. In the present study, we characterized for the first time the excitation-contraction coupling machinery of an immortalized human skeletal muscle cell line. Intracellular Ca2+ measurements showed a normal response to pharmacological activation of the ryanodine receptor, whereas 3D-SIM (super-resolution structured illumination microscopy) revealed a low level of structural organization of ryanodine receptors and dihydropyridine receptors. Interestingly, the expression levels of several transcripts of proteins involved in Ca2+ homoeostasis and differentiation indicate that the cell line has a phenotype closer to that of slow-twitch than fast-twitch muscles. These results point to the potential application of such human muscle-derived cell lines to the study of neuromuscular disorders; in addition, they may serve as a platform for the development of therapeutic strategies aimed at correcting defects in Ca2+ homoeostasis due to mutations in genes involved in Ca2+ regulation.

Altered expression of cyclin A 1 in muscle of patients with facioscapulohumeral muscle dystrophy (FSHD-1)

Objectives

Cyclin A1 regulates cell cycle activity and proliferation in somatic and germ-line cells. Its expression increases in G1/S phase and reaches a maximum in G2 and M phases. Altered cyclin A1 expression might contribute to clinical symptoms in facioscapulohumeral muscular dystrophy (FSHD).

Methods

Muscle biopsies were taken from the Vastus lateralis muscle for cDNA microarray, RT-PCR, immunohistochemistry and Western blot analyses to assess RNA and protein expression of cyclin A1 in human muscle cell lines and muscle tissue. Muscle fibers diameter was calculated on cryosections to test for hypertrophy.

Results

cDNA microarray data showed specifically elevated cyclin A1 levels in FSHD vs. other muscular disorders such as caveolinopathy, dysferlinopathy, four and a half LIM domains protein 1 deficiency and healthy controls. Data could be confirmed with RT-PCR and Western blot analysis showing up-regulated cyclin A1 levels also at protein level. We found also clear signs of hypertrophy within the Vastus lateralis muscle in FSHD-1 patients.

Conclusions

In most somatic human cell lines, cyclin A1 levels are low. Overexpression of cyclin A1 in FSHD indicates cell cycle dysregulation in FSHD and might contribute to clinical symptoms of this disease.

Dysferlin-peptides reallocate mutated dysferlin thereby restoring function

Mutations in the dysferlin gene cause the most frequent adult-onset limb girdle muscular dystrophy, LGMD2B. There is no therapy. Dysferlin is a membrane protein comprised of seven, beta-sheet enriched, C2 domains and is involved in Ca²⁺dependent sarcolemmal repair after minute wounding. On the protein level, point mutations in DYSF lead to misfolding, aggregation within the endoplasmic reticulum, and amyloidogenesis. We aimed to restore functionality by relocating mutant dysferlin. Therefore, we designed short peptides derived from dysferlin itself and labeled them to the cell penetrating peptide TAT. By tracking fluorescently labeled short peptides we show that these dysferlin-peptides localize in the endoplasmic reticulum. There, they are capable of reducing unfolded protein response stress. We demonstrate that the mutant dysferlin regains function in membrane repair in primary human myotubes derived from patients’ myoblasts by the laser wounding assay and a novel technique to investigate membrane repair: the interventional atomic force microscopy. Mutant dysferlin abuts to the sarcolemma after peptide treatment. The peptide-mediated approach has not been taken before in the field of muscular dystrophies. Our results could redirect treatment efforts for this condition.