Truncated dystrophin ameliorates the dystrophic phenotype of mdx mice by reducing sarcolipin-mediated SERCA inhibition.

Duchenne muscular dystrophy (DMD) and the less severe Becker muscular dystrophy (BMD) are due to mutations in the DMD gene. Previous reports show that in-frame deletion of exons 45-55 produces an internally shorted, but functional, dystrophin protein resulting in a very mild BMD phenotype. In order to elucidate the molecular mechanism leading to this phenotype, we generated exon 45-55 deleted dystrophin transgenic/mdx (Tg/mdx) mice. Muscular function of Tg/mdx mice was restored close to that of wild type (WT) mice but the localization of the neuronal type of nitric oxide synthase was changed from the sarcolemma to the cytosol. This led to hyper-nitrosylation of the ryanodine receptor 1 causing increased Ca2+ release from the sarcoplasmic reticulum. On the other hand, Ca2+ reuptake by the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) was restored to the level of WT mice, suggesting that the Ca2+ dysregulation had been compensated by SERCA activation. In line with this, expression of sarcolipin (SLN), a SERCA-inhibitory peptide, was upregulated in mdx mice, but strongly reduced in Tg/mdx mice. Furthermore, knockdown of SLN ameliorated the cytosolic Ca2+ homeostasis and the dystrophic phenotype in mdx mice. These findings suggest that SLN may be a novel target for DMD therapy.

Low-Intensity Training and the C5a Complement Antagonist NOX-D21 Rescue the mdx Phenotype through Modulation of Inflammation.

Inflammatory events occurring in dystrophic muscles contribute to the progression of Duchenne muscular dystrophy (DMD). Low-intensity training (LIT) attenuates the phenotype of mdx mice, an animal model for DMD. Therefore, we postulated that LIT could have anti-inflammatory properties. We assessed levels of inflammatory cytokines and infiltrated immune cells in gastrocnemius muscle of mdx mice after LIT. We detected high levels of complement component C5a, chemokine ligand (CCL) 2, CD68+ monocytes/macrophages, and proinflammatory M1 macrophages in muscles of mdx mice. LIT decreased CCL2 levels, increased CD68+ cell numbers, and shifted the macrophage population to the regenerative M2 type. We investigated whether inhibition of C5a or CCL2 with L-aptamers could mimic the effects of LIT. Although no effect of CCL2 inhibition was detected, treatment with the C5a inhibitor, NOX-D21, rescued the phenotype of nonexercised mdx mice, but not of exercised ones. In both cases, the level of CD68+ cells increased and macrophage populations leaned toward the inflammatory M1 type. In muscles of nonexercised treated mice, the level of IL-1 receptor antagonist increased, damage decreased, and fibers were switched toward the glycolytic fast type; in muscles of exercised mice, fibers were switched to the oxidative slow type. These results reveal the effects of LIT on the inflammatory status of mdx mice and suggest that NOX-D21 could be an anti-inflammatory drug for DMD.

ATP-Induced Increase in Intracellular Calcium Levels and Subsequent Activation of mTOR as Regulators of Skeletal Muscle Hypertrophy.

Intracellular signaling pathways, including the mammalian target of rapamycin (mTOR) and the mitogen-activated protein kinase (MAPK) pathway, are activated by exercise, and promote skeletal muscle hypertrophy. However, the mechanisms by which these pathways are activated by physiological stimulation are not fully understood. Here we show that extracellular ATP activates these pathways by increasing intracellular Ca2+ levels ([Ca2+]i), and promotes muscle hypertrophy. [Ca2+]i in skeletal muscle was transiently increased after exercise. Treatment with ATP induced the increase in [Ca2+]i through the P2Y2 receptor/inositol 1,4,5-trisphosphate receptor pathway, and subsequent activation of mTOR in vitro. In addition, the ATP-induced increase in [Ca2+]i coordinately activated Erk1/2, p38 MAPK and mTOR that upregulated translation of JunB and interleukin-6. ATP also induced an increase in [Ca2+]i in isolated soleus muscle fibers, but not in extensor digitorum longus muscle fibers. Furthermore, administration of ATP led to muscle hypertrophy in an mTOR- and Ca2+-dependent manner in soleus, but not in plantaris muscle, suggesting that ATP specifically regulated [Ca2+]i in slow muscles. These findings suggest that ATP and [Ca2+]i are important mediators that convert mechanical stimulation into the activation of intracellular signaling pathways, and point to the P2Y receptor as a therapeutic target for treating muscle atrophy.

Repurposing the Selective Oestrogen Receptor Modulator Tamoxifen for the Treatment of Duchenne Muscular Dystrophy.

Drug discovery is a long, expensive and risky process. Evaluating drugs that have already been proved safe for use in humans and testing them for a new indication greatly reduces the time and monetary costs involved in finding treatments for life-threatening conditions. Here tamoxifen, a drug that is used for the treatment of breast cancer, is investigated in a mouse model of Duchenne muscular dystrophy. Tamoxifen was efficacious in countering the symptoms of the disease without affecting the underlying genetic cause. Based on these results, tamoxifen has been tested in other forms of muscle disease with success. Drug repurposing may not only be a cost-effective manner for treating a variety of diseases, it may also help us uncover common mechanisms between conditions that were previously thought to be unrelated.

The Swiss Society of Experimental Pharmacology in Times of Change.

Experimental pharmacology is undergoing fundamental changes. This article describes the challenges and opportunities associated with these changes from the perspective of the Swiss Society of Pharmacology (SSEP), the society which aims to advance experimental pharmacology in Switzerland and abroad.

The anticancer drug tamoxifen counteracts the pathology in a mouse model of duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is a severe disorder characterized by progressive muscle wasting,respiratory and cardiac impairments, and premature death. No treatment exists so far, and the identification of active substances to fight DMD is urgently needed. We found that tamoxifen, a drug used to treat estrogen-dependent breast cancer, caused remarkable improvements of muscle force and of diaphragm and cardiac structure in the mdx(5Cv) mouse model of DMD. Oral tamoxifen treatment from 3 weeks of age for 15 months at a dose of 10 mg/kg/day stabilized myofiber membranes, normalized whole body force, and increased force production and resistance to repeated contractions of the triceps muscle above normal values. Tamoxifen improved the structure of leg muscles and diminished cardiac fibrosis by~ 50%. Tamoxifen also reduced fibrosis in the diaphragm, while increasing its thickness,myofiber count, and myofiber diameter, thereby augmenting by 72% the amount of contractile tissue available for respiratory function. Tamoxifen conferred a markedly slower phenotype to the muscles.Tamoxifen and its metabolites were present in nanomolar concentrations in plasma and muscles,suggesting signaling through high-affinity targets. Interestingly, the estrogen receptors ERa and ERb were several times more abundant in dystrophic than in normal muscles, and tamoxifen normalized the relative abundance of ERb isoforms. Our findings suggest that tamoxifen might be a useful therapy for DMD.

Trpc1 ion channel modulates phosphatidylinositol 3-kinase/Akt pathway during myoblast differentiation and muscle regeneration.

We previously showed in vitro that calcium entry through Trpc1 ion channels regulates myoblast migration and differentiation. In the present work, we used primary cell cultures and isolated muscles from Trpc1(-/-) and Trpc1(+/+) murine model to investigate the role of Trpc1 in myoblast differentiation and in muscle regeneration. In these models, we studied regeneration consecutive to cardiotoxin-induced muscle injury and observed a significant hypotrophy and a delayed regeneration in Trpc1(-/-) muscles consisting in smaller fiber size and increased proportion of centrally nucleated fibers. This was accompanied by a decreased expression of myogenic factors such as MyoD, Myf5, and myogenin and of one of their targets, the developmental MHC (MHCd). Consequently, muscle tension was systematically lower in muscles from Trpc1(-/-) mice. Importantly, the PI3K/Akt/mTOR/p70S6K pathway, which plays a crucial role in muscle growth and regeneration, was down-regulated in regenerating Trpc1(-/-) muscles. Indeed, phosphorylation of both Akt and p70S6K proteins was decreased as well as the activation of PI3K, the main upstream regulator of the Akt. This effect was independent of insulin-like growth factor expression. Akt phosphorylation also was reduced in Trpc1(-/-) primary myoblasts and in control myoblasts differentiated in the absence of extracellular Ca(2+) or pretreated with EGTA-AM or wortmannin, suggesting that the entry of Ca(2+) through Trpc1 channels enhanced the activity of PI3K. Our results emphasize the involvement of Trpc1 channels in skeletal muscle development in vitro and in vivo, and identify a Ca(2+)-dependent activation of the PI3K/Akt/mTOR/p70S6K pathway during myoblast differentiation and muscle regeneration.

Pharmacological prospects in the treatment of Duchenne muscular dystrophy.

PURPOSE OF REVIEW: The most encouraging recent advances regarding pharmacological agents for treating Duchenne muscular dystrophy (DMD) are summarized. Emphasis is given to compounds acting downstream of dystrophin, the protein lacking in DMD, on cellular pathways leading to pathological consequences. The author highlights the progress that may have the greatest potential for clinical use in DMD. RECENT FINDINGS: Modifying the transcripts of the mutated gene by exon skipping has led to expression of shortened dystrophins in DMD patients. Currently, the most promising potential drugs are the exon-skipping agents eteplirsen and drisapersen. Biglycan and SMTC1100 upregulate utrophin. The steroid receptor modulating compounds VBP15 and tamoxifen, and specific antioxidants appear promising agents for symptomatic therapy. SUMMARY: The past 18 months have seen a strong increase in the number of exciting reports on novel therapeutic agents for DMD. Exon-skipping agents have been fine-tuned to improve tissue delivery and stability. Impressive discoveries regarding pathogenic events in cellular signalling have revealed targets that were unknown in the context of DMD, thus enabling approaches that limit inflammation, fibrosis and necrosis. The targets are nuclear hormone receptors, NADPH-oxidases and Ca channels. Inhibition of NF-KB, transforming growth factor-alpha (TGF-α) and transforming growth factor-beta (TGF-ß)/myostatin production or action are also promising routes in counteracting the complex pathogenesis of DMD.

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.

Quantitative evaluation of the beneficial effects in the mdx mouse of epigallocatechin gallate, an antioxidant polyphenol from green tea.

In two separate previous studies, we reported that subcutaneous (sc) or oral administration of (-)-epigallocatechin-3-gallate (EGCG) limited the development of muscle degeneration of mdx mice, a mild phenotype model for Duchenne muscular dystrophy (DMD). However, it was not possible to conclude which was the more efficient route of EGCG administration because different strains of mdx mice, periods of treatment and methods of assessment were used. In this study, we investigated which administration routes and dosages of EGCG are the most effective for limiting the onset of dystrophic lesions in the same strain of mdx mice and applying the same methods of assessment. Three-week-old mdx mice were injected sc for 5 weeks with either saline or a daily average of 3 or 6 mg/kg EGCG. For comparison, age-matched mdx mice were fed for 5 weeks with either a diet containing 0.1% EGCG or a control diet. The effects of EGCG were assessed quantitatively by determining the activities of serum muscle-derived creatine kinase, isometric contractions of triceps surae muscles, integrated spontaneous locomotor activities, and oxidative stress and fibrosis in selected muscles. Oral administration of 180 mg/kg/day EGCG in the diet was found the most effective for significantly improving several parameters associated with muscular dystrophy. However, the improvements were slightly less than those observed previously for sc injection started immediately after birth. The efficacy of EGCG for limiting the development of dystrophic muscle lesions in mice suggests that EGCG may be of benefit for DMD patients.

Melatonin improves muscle function of the dystrophic mdx5Cv mouse, a model for Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is a severe X-linked muscle-wasting disease caused by the absence of the cytoskeletal protein dystrophin. In addition to abnormal calcium handling, numerous studies point to a crucial role of oxidative stress in the pathogenesis of the disease. Considering the impressive results provided by antioxidants on dystrophic muscle structure and function, we investigated whether melatonin can protect the mdx(5Cv) mouse, an animal model for DMD. Male mdx(5Cv) mouse pups were treated with melatonin by daily intraperitoneal (i.p.) injection (30 mg/kg body weight) or by subcutaneous (s.c.) implant(s) (18 or 54 mg melatonin as Melovine® implants) from 17/18 to 28/29 days of age. Isometric force of the triceps surae was recorded at the end of the treatment. The i.p. treatment increased the phasic twitch tension of mdx(5Cv) mice. The maximal tetanic tension was ameliorated by 18 mg s.c. and 30 mg/kg i.p. treatments. Melatonin caused the dystrophic muscle to contract and relax faster. The force-frequency relationship of melatonin-treated dystrophic mice was shifted to the right. In accordance with improved muscle function, melatonin decreased plasma creatine kinase activity, a marker for muscle injury. Melatonin treatment increased total glutathione content and lowered the oxidized/reduced glutathione ratio, indicating a better redox status of the muscle. In light of the present investigation, the therapeutic potential of melatonin should be further considered for patients with DMD.

Role of TRPC1 channel in skeletal muscle function.

Skeletal muscle contraction is reputed not to depend on extracellular Ca2+. Indeed, stricto sensu, excitation-contraction coupling does not necessitate entry of Ca2+. However, we previously observed that, during sustained activity (repeated contractions), entry of Ca2+ is needed to maintain force production. In the present study, we evaluated the possible involvement of the canonical transient receptor potential (TRPC)1 ion channel in this entry of Ca2+ and investigated its possible role in muscle function. Patch-clamp experiments reveal the presence of a small-conductance channel (13 pS) that is completely lost in adult fibers from TRPC1(-/-) mice. The influx of Ca2+ through TRPC1 channels represents a minor part of the entry of Ca(2+) into muscle fibers at rest, and the activity of the channel is not store dependent. The lack of TRPC1 does not affect intracellular Ca2+ concentration ([Ca2+](i)) transients reached during a single isometric contraction. However, the involvement of TRPC1-related Ca2+ entry is clearly emphasized in muscle fatigue. Indeed, muscles from TRPC1(-/-) mice stimulated repeatedly progressively display lower [Ca2+](i) transients than those observed in TRPC1(+/+) fibers, and they also present an accentuated progressive loss of force. Interestingly, muscles from TRPC1(-/-) mice display a smaller fiber cross-sectional area, generate less force per cross-sectional area, and contain less myofibrillar proteins than their controls. They do not present other signs of myopathy. In agreement with in vitro experiments, TRPC1(-/-) mice present an important decrease of endurance of physical activity. We conclude that TRPC1 ion channels modulate the entry of Ca(2+) during repeated contractions and help muscles to maintain their force during sustained repeated contractions.

Phospholipase A2-derived lysophosphatidylcholine triggers Ca2+ entry in dystrophic skeletal muscle fibers.

Duchenne muscular dystrophy is an inherited disease caused by the absence of dystrophin, a structural protein normally located under the sarcolemma of skeletal muscle fibers. Muscle degeneration occurring in this disease is thought to be partly caused by increased Ca(2+) entry through sarcolemmal cationic channels. Using the Mn(2+) quench method, we show here that Mn(2+) entry triggered by Ca(2+) store depletion but not basal Mn(2+) entry relies on Ca(2+)-independent PLA(2) (iPLA(2)) activity in dystrophic fibers isolated from a murine model of Duchenne muscular dystrophy, the mdx(5cv) mouse. iPLA(2) was found to be localized in the vicinity of the sarcolemma and consistently, the iPLA(2) lipid product lysophosphatidylcholine was found to trigger Ca(2+) entry through sarcolemmal channels, suggesting that it acts as an intracellular messenger responsible for store-operated channels opening in dystrophic fibers. Our results suggest that inhibition of iPLA(2) and lysophospholipid production may be of interest to reduce Ca(2+) entry and subsequent degeneration of dystrophic muscle.

Gene-mediated restoration of normal myofiber elasticity in dystrophic muscles.

Dystrophin mediates a physical link between the cytoskeleton of muscle fibers and the extracellular matrix, and its absence leads to muscle degeneration and dystrophy. In this article, we show that the lack of dystrophin affects the elasticity of individual fibers within muscle tissue explants, as probed using atomic force microscopy (AFM), providing a sensitive and quantitative description of the properties of normal and dystrophic myofibers. The rescue of dystrophin expression by exon skipping or by the ectopic expression of the utrophin analogue normalized the elasticity of dystrophic muscles, and these effects were commensurate to the functional recovery of whole muscle strength. However, a more homogeneous and widespread restoration of normal elasticity was obtained by the exon-skipping approach when comparing individual myofibers. AFM may thus provide a quantification of the functional benefit of gene therapies from live tissues coupled to single-cell resolution.

Beta1 integrins regulate myoblast fusion and sarcomere assembly.

The mechanisms that regulate the formation of multinucleated muscle fibers from mononucleated myoblasts are not well understood. We show here that extracellular matrix (ECM) receptors of the beta1 integrin family regulate myoblast fusion. beta1-deficient myoblasts adhere to each other, but plasma membrane breakdown is defective. The integrin-associated tetraspanin CD9 that regulates cell fusion is no longer expressed at the cell surface of beta1-deficient myoblasts, suggesting that beta1 integrins regulate the formation of a protein complex important for fusion. Subsequent to fusion, beta1 integrins are required for the assembly of sarcomeres. Other ECM receptors such as the dystrophin glycoprotein complex are still expressed but cannot compensate for the loss of beta1 integrins, providing evidence that different ECM receptors have nonredundant functions in skeletal muscle fibers.

Melatonin prevents oxidative stress-mediated mitochondrial permeability transition and death in skeletal muscle cells.

Oxidative stress-induced mitochondrial dysfunction plays a crucial role in the pathogenesis of a wide range of diseases including muscle disorders. In this study, we demonstrate that melatonin readily rescued mitochondria from oxidative stress-induced dysfunction and effectively prevented subsequent apoptosis of primary muscle cultures prepared from C57BL/6J mice. In particular, melatonin (10(-4)-10(-6) m) fully prevented myotube death induced by tert-butylhydroperoxide (t-BHP; 10 microm-24 hr) as assessed by acid phosphatase, caspase-3 activities and cellular morphological changes. Using fluorescence imaging, we showed that the mitochondrial protection provided by melatonin was associated with an inhibition of t-BHP-induced reactive oxygen species generation. In line with this observation, melatonin prevented t-BHP-induced mitochondrial depolarization and mitochondrial permeability transition pore (PTP) opening. This was associated with a highly reduced environment as reflected by an increased glutathione content and an increased ability to maintain mitochondrial pyridine nucleotides and glutathione in a reduced state. Using isolated mitochondria, in a similar manner as cyclosporin A, melatonin (10(-8)-10(-6) m) desensitized the PTP to Ca(2+) and prevented t-BHP-induced mitochondrial swelling, pyridine nucleotide and glutathione oxidation. In conclusion, our findings suggest that inhibition of the PTP essentially contributes to the protective effect of melatonin against oxidative stress in myotubes.

Protection of dystrophic muscle cells with polyphenols from green tea correlates with improved glutathione balance and increased expression of 67LR, a receptor for (-)-epigallocatechin gallate.

Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disease caused by the absence of the protein dystrophin. Because oxidative stress contributes to the pathogenesis of DMD, we investigated if a green tea polyphenol blend (GTP) and its major polyphenol (-)-epigallocatechin gallate (EGCg), could protect muscle cell primary cultures from oxidative damage induced by hydrogen peroxide (H(2)O(2)) in the widely used mdx mouse model. On-line fluorimetric measurements using an H(2)O(2)-sensitive probe indicated that GTP and EGCg scavenged peroxide in a concentration-dependent manner. A 48 h exposure to EGCg increased glutathione content but did not alter the expression of proteins involved in membrane stabilization and repair. Pretreatment of dystrophic cultures with GTP or EGCg 48 h before exposure to H(2)O(2) improved cell survival. Normal cultures were protected by GTP but not by EGCg. 67LR, a receptor for EGCg, was seven times more abundant in dystrophic compared with normal cultures. Altogether our results demonstrate that GTP and EGCg protect muscle cells by scavenging H(2)O(2) and by improving the glutathione balance. In addition, the higher levels of 67LR in dystrophic muscle cells compared with normal ones likely contribute to EGCg-mediated survival.

Bcl-2 overexpression prevents calcium overload and subsequent apoptosis in dystrophic myotubes.

Duchenne muscular dystrophy (DMD) is a lethal disease caused by the lack of the cytoskeletal protein dystrophin. Altered calcium homoeostasis and increased calcium concentrations in dystrophic fibres may be responsible for the degeneration of muscle occurring in DMD. In the present study, we used subsarcolemmal- and mitochondrial-targeted aequorin to study the effect of the antiapoptotic Bcl-2 protein overexpression on carbachol-induced near-plasma membrane and mitochondrial calcium responses in myotubes derived from control C57 and dystrophic (mdx) mice. We show that Bcl-2 overexpression decreases subsarcolemmal and mitochondrial calcium overload that occurs during activation of nicotinic acetylcholine receptors in dystrophic myotubes. Moreover, our results suggest that overexpressed Bcl-2 protein may prevent near-plasma membrane and mitochondrial calcium overload by inhibiting IP3Rs (inositol 1,4,5-trisphosphate receptors), which we have shown previously to be involved in abnormal calcium homoeostasis in dystrophic myotubes. Most likely as a consequence, the inhibition of IP3R function by Bcl-2 also inhibits calcium-dependent apoptosis in these cells.