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The regulation of autophagy-dependent lysosome homeostasis in vivo is unclear. We showed that the inositol polyphosphate 5-phosphatase INPP5K regulates autophagic lysosome reformation (ALR), a lysosome recycling pathway, in muscle. INPP5K hydrolyzes phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] to phosphatidylinositol 4-phosphate [PI(4)P], and INPP5K mutations cause muscular dystrophy by unknown mechanisms. We report that loss of INPP5K in muscle caused severe disease, autophagy inhibition, and lysosome depletion. Reduced PI(4,5)P2 turnover on autolysosomes in Inpp5k-/- muscle suppressed autophagy and lysosome repopulation via ALR inhibition. Defective ALR in Inpp5k-/- myoblasts was characterized by enlarged autolysosomes and the persistence of hyperextended reformation tubules, structures that participate in membrane recycling to form lysosomes. Reduced disengagement of the PI(4,5)P2 effector clathrin was observed on reformation tubules, which we propose interfered with ALR completion. Inhibition of PI(4,5)P2 synthesis or expression of WT INPP5K but not INPP5K disease mutants in INPP5K-depleted myoblasts restored lysosomal homeostasis. Therefore, bidirectional interconversion of PI(4)P/PI(4,5)P2 on autolysosomes was integral to lysosome replenishment and autophagy function in muscle. Activation of TFEB-dependent de novo lysosome biogenesis did not compensate for loss of ALR in Inpp5k-/- muscle, revealing a dependence on this lysosome recycling pathway. Therefore, in muscle, ALR is indispensable for lysosome homeostasis during autophagy and when defective is associated with muscular dystrophy.

New Findings What is the central question of this study? The Notch signalling pathway plays an important role in muscle regeneration, and activation of the pathway has been shown to enhance muscle regeneration in aged mice. It is unknown whether Notch activation will have a similarly beneficial effect on muscle regeneration in the context of Duchenne muscular dystrophy (DMD). What is the main finding and its importance? Although expression of Notch signalling components is altered in both mouse models of DMD and in human DMD patients, activation of the Notch signalling pathway does not confer any functional benefit on muscles from dystrophic mice, suggesting that other signalling pathways may be more fruitful targets for manipulation in treating DMD. Abstract In Duchenne muscular dystrophy (DMD), muscle damage and impaired regeneration lead to progressive muscle wasting, weakness and premature death. The Notch signalling pathway represents a central regulator of gene expression and is critical for cellular proliferation, differentiation and apoptotic signalling during all stages of embryonic muscle development. Notch activation improves muscle regeneration in aged mice, but its potential to restore regeneration and function in muscular dystrophy is unknown. We performed a comprehensive examination of several genes involved in Notch signalling in muscles from dystrophin-deficient mdx and dko (utrophin- and dystrophin-null) mice and DMD patients. A reduction of Notch1 and Hes1 mRNA in tibialis anterior muscles of dko mice and quadriceps muscles of DMD patients and a reduction of Hes1 mRNA in the diaphragm of the mdx mice were observed, with other targets being inconsistent across species. Activation and inhibition of Notch signalling, followed by measures of muscle regeneration and function, were performed in the mouse models of DMD. Notch activation had no effect on functional regeneration in C57BL/10, mdx or dko mice. Notch inhibition significantly depressed the frequency-force relationship in regenerating muscles of C57BL/10 and mdx mice after injury, indicating reduced force at each stimulation frequency, but enhanced the frequency-force relationship in muscles from dko mice. We conclude that while Notch inhibition produces slight functional defects in dystrophic muscle, Notch activation does not significantly improve muscle regeneration in murine models of muscular dystrophy. Furthermore, the inconsistent expression of Notch targets between murine models and DMD patients suggests caution when making interspecies comparisons.

Objective: Musculoskeletal injuries represent a major public health problem and drugs that can improve muscle repair and restore function are needed for patients with these conditions and other related muscular pathologies. Increasing insulin-like growth factor-I (IGF-I) levels in skeletal muscle improves regeneration after myotoxic injury and while administration of IGF-I has a potential for accelerating healing after trauma, optimizing its method of delivery and obviating potential side-effects currently associated with recombinant human (rh) IGF-I, remain a hurdle., Design: We compared the treatment efficacy of rhIGF-I with a polyethylene glycol modified IGF-I (PEG-IGF-I) analog to improve functional repair of mouse tibialis anterior muscles after myotoxic injury, testing the hypothesis that PEG-IGF-I would exert greater beneficial effects on regenerating skeletal muscles than rhIGF-I due to improved pharmacokinetic properties. We also examined the relative efficacy of systemic versus local delivery of these IGF-I variants for improving functional muscle regeneration., Results: Local delivery of PEG-IGF-I, but not rhIGF-I, at 4 days post-injury significantly improved early functional recovery as evident by a 27% increase in normalized force compared with saline control (P<0.05), whereas systemic application of either IGF-I variant was not effective. The improved function with intramuscular PEG-IGF-I administration was attributed to a greater and prolonged residence within the regenerating muscles, resulting in increased Akt activation and a 13% larger fiber cross-sectional area compared with rhIGF-I (P<0.05)., Conclusions: These data support the hypothesis that PEG-IGF-I is more efficacious than rhIGF-I in hastening early fiber regeneration and improving muscle function after injury, highlighting its therapeutic potential for muscular pathologies., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Nitric oxide (NO) is an important signaling molecule produced in skeletal muscle primarily via the neuronal subtype of NO synthase (NOS1, or nNOS). While many studies have reported NO production to be important in muscle regeneration, none have examined the contribution of nNOS-derived NO to functional muscle regeneration (i.e., restoration of the muscle's ability to produce force) after acute myotoxic injury. In the present study, we tested the hypothesis that genetic deletion of nNOS would impair functional muscle regeneration after myotoxic injury in nNOS(-/-) mice. We found that nNOS(-/-) mice had lower body mass, lower muscle mass, and smaller myofiber cross-sectional area and that their tibialis anterior (TA) muscles produced lower absolute tetanic forces than those of wild-type littermate controls but that normalized or specific force was identical between the strains. In addition, muscles from nNOS(-/-) mice were more resistant to fatigue than those of wild-type littermates (P < 0.05). To determine whether deletion of nNOS affected muscle regeneration, TA muscles from nNOS(-/-) mice and wild-type littermates were injected with the myotoxin notexin to cause complete fiber degeneration, and muscle structure and function were assessed at 7 and 10 days postinjury. Myofiber cross-sectional area was lower in regenerating nNOS(-/-) mice than wild-type controls at 7 and 10 days postinjury; however, contrary to our original hypothesis, no difference in force-producing capacity of the TA muscle was evident between the two groups at either time point. Our findings reveal that nNOS is not essential for functional muscle regeneration after acute myotoxic damage.

Introduction: Musculoskeletal injuries represent a major global public health problem and muscle injury contributes significantly to the burden of disability and suffering. Drugs that can attenuate muscle trauma and/or hasten muscle repair to restore function can help reduce the economic burden and alleviate personal suffering and financial hardship., Areas Covered: This review provides an update on some emerging drugs with therapeutic potential for muscle injury including those that could attenuate damage or improve regeneration. Although there are few (if any) drugs in development specifically for muscle injury, there are numerous drugs in development for cardiovascular complications, such as ischemia-reperfusion, that might also have efficacy for promoting regeneration after similar events in skeletal muscle., Expert Opinion: Drugs in development for muscle wasting or inflammatory diseases should also be considered within the context of modulating the events associated with muscle degeneration and regeneration. More rigorous pre-clinical evaluations, especially of a drug's efficacy for improving function, would help minimize false leads and hasten development of effective approaches for treating muscle damage and promoting repair after injury.

Systemic administration of beta(2)-adrenoceptor agonists (beta(2)-agonists) can improve skeletal muscle regeneration after injury. However, therapeutic application of beta(2)-agonists for muscle injury has been limited by detrimental cardiovascular side effects. Intramuscular administration may obviate some of these side effects. To test this hypothesis, the right extensor digitorum longus (EDL) muscle from rats was injected with bupivacaine hydrochloride to cause complete muscle fiber degeneration. Five days after injury, half of the injured muscles received an intramuscular injection of formoterol (100 mug). Muscle function was assessed at 7, 10, and 14 days after injury. A single intramuscular injection of formoterol increased muscle mass and force-producing capacity at day 7 by 17 and 91%, respectively, but this effect was transient because these values were not different from control levels at day 10. A second intramuscular injection of formoterol at day 7 prolonged the increase in muscle mass and force-producing capacity. Importantly, single or multiple intramuscular injections of formoterol did not elicit cardiac hypertrophy. To characterize any potential cardiovascular effects of intramuscular formoterol administration, we instrumented a separate group of rats with indwelling radio telemeters. Following an intramuscular injection of formoterol, heart rate increased by 18%, whereas systolic and diastolic blood pressure decreased by 31 and 44%, respectively. These results indicate that intramuscular injection can enhance functional muscle recovery after injury without causing cardiac hypertrophy. Therefore, if the transient cardiovascular effects associated with intramuscular formoterol administration can be minimized, this form of treatment may have significant therapeutic potential for muscle-wasting conditions.