Nitric oxide reverses prednisolone-induced inactivation of muscle satellite cells

Impaired muscle stem cell function and abnormal myogenesis in acquired myopathies

Skeletal muscle possesses a high plasticity and a remarkable regenerative capacity that relies mainly on muscle stem cells (MuSCs). Molecular and cellular components of the MuSC niche, such as immune cells, play key roles to coordinate MuSC function and to orchestrate muscle regeneration. An abnormal infiltration of immune cells and/or imbalance of pro- and anti-inflammatory cytokines could lead to MuSC dysfunctions that could have long lasting effects on muscle function. Different genetic variants were shown to cause muscular dystrophies that intrinsically compromise MuSC function and/or disturb their microenvironment leading to impaired muscle regeneration that contributes to disease progression. Alternatively, many acquired myopathies caused by comorbidities (e.g., cardiopulmonary or kidney diseases), chronic inflammation/infection, or side effects of different drugs can also perturb MuSC function and their microenvironment. The goal of this review is to comprehensively summarize the current knowledge on acquired myopathies and their impact on MuSC function. We further describe potential therapeutic strategies to restore MuSC regenerative capacity.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

Muscle proteolysis via ubiquitin-proteasome system (UPS) is activated by BthTx-I Lys49 PLA2 but not by BthTx-II Asp49 PLA2 and Bothrops jararacussu venom

Bites by viperid snakes belonging to Bothrops genus produce fast and intense local edema, inflammation, bleeding and myonecrosis. In this study, we investigated the role of Myogenic Regulatory Factors (MRFs: MyoD; Myog), negatively regulated by GDF-8 (Myostatin), and ubiquitin-proteasome system pathway (UPS: MuRF-1; Fbx-32) in gastrocnemius muscle regeneration after Bothrops jararacussu snake venom (Bjussu) or its isolated phospholipase A₂ myotoxins, BthTx-I (Lys-49 PLA₂) and BthTx-II (Asp-49 PLA₂) injection. Male Swiss mice received a single intra-gastrocnemius injection of crude Bjussu, at a dose/volume of 0.83 mg/kg/20 μl, and BthTx-I or BthTx-II, at a dose/volume of 2.5 mg/kg/20 μl. Control mice (Sham) received an injection of sterile saline solution (NaCl 0.9%; 20 μl). At 24, 48, 72 and 96 h post injection, right gastrocnemius was collected for protein expression analyses. Based on the temporal expressional dynamics of MyoD, Myog and GDF-8/Myostatin, it was possible to propose that the myogenesis pathway was impacted most badly by BthTx-II followed by BthTx-I and lastly by B. jararacussu venom, thus suggesting that catalytic activity has likely inhibitory role on the satellite cells-mediated reparative myogenesis pathway. Inversely, the catalytic activity seems to be not a determinant for the activation of proteins ubiquitination by MuRF-1 and Fbx-32/Atrogin-1 E3 proteasome ligases, given proteolysis pathway through UPS was activated neither after Bjussu, nor after BthTx-II, but just after the catalytically-inactive BthTx-I Lys-49 PLA₂-homologue exposure. The findings of this study disclose interesting perspective for further mechanistic studies about pathways that take part in the atrophy and repair after permanent damage induced by bothropic snakebites.

Cortisone and dexamethasone inhibit myogenesis by modulating the AKT/mTOR signaling pathway in C2C12

Myogenesis occurs in both the prenatal and postnatal periods and the prenatal myogenesis is related to the postnatal myogenesis and the incidence of disease later in life. Glucocorticoids used as therapeutic agents for many diseases, but cause adverse effects on muscle homeostasis, including defects in fetal muscle development. The action of glucocorticoids on differentiated skeletal muscle was well studied, but their effects on myotube formation have not been well investigated. Dexamethasone (DEX) and cortisone (COR), two synthetic therapeutic glucocorticoids, suppress myotube formation in C2C12 cells. Both COR and DEX attenuated myotube formation through modulation of myogenic regulatory factors. In addition, they affected the IGF/PI3K/AKT/mTOR signaling pathway, resulting in increased proteolytic protein (atrogin-1 and MURF1) for muscle degradation and decreased ribosomal S6 phosphorylation. The current results conclude that COR and DEX inhibit myotube formation in C2C12 cells by modulating both the myogenic program via MRFs and protein metabolism via IGF/PI3K/AKT/mTOR signaling pathway.

Treatment with nitric oxide donors diminishes hyperinfection by Strongyloides venezuelensis in mice treated with dexamethasone

The effects of using nitric oxide (NO) donors and inhibitors in experimental strongyloidiasis were showed using, both naïve and dexamethasone immunosuppressed BALB/c mice infected with Strongyloides venezuelensis. Aminoguanidine, an inhibitor of inducible NO synthase and LA419 a NO donor, were administered. Dexamethasone was used to induce immunosuppression. The study in BALB/c mice revealed increases in counts of fecal eggs, larvae in lungs and parasitic females following treatment with aminoguanidine, while mice treated with LA419 had limited egg output with low larval and adult recoveries. Mice immunosuppressed with dexamethasone developed hyperinfection with high long lasting fecal egg emission, high numbers of larvae in lungs and high numbers of parasitic females in the intestine even when the infection had already been cleared in non-immunosuppressed infected controls. Mice treated with dexamethasone and aminoguanidine had the highest egg output and the highest larva and parasitic female recovery showing a severe hyperinfection syndrome. In contrast, treatment with dexamenthasone and LA419 resulted in a controlled hyperinfection syndrome and these mice were able to eliminate the parasite. Therefore, NO modulation appears to be a determinant factor in severe strongyloidiasis and further studies should be conducted to confirm in other experimental models.

Inhibition of glycogen synthase kinase-3β attenuates glucocorticoid-induced suppression of myogenic differentiation in vitro

Glucocorticoids are the only therapy that has been demonstrated to alter the progress of Duchenne muscular dystrophy (DMD), the most common muscular dystrophy in children. However, glucocorticoids disturb skeletal muscle metabolism and hamper myogenesis and muscle regeneration. The mechanisms involved in the glucocorticoid-mediated suppression of myogenic differentiation are not fully understood. Glycogen synthase kinase-3β (GSK-3β) is considered to play a central role as a negative regulator in myogenic differentiation. Here, we showed that glucocorticoid treatment during the first 48 h in differentiation medium decreased the level of phosphorylated Ser9-GSK-3β, an inactive form of GSK-3β, suggesting that glucocorticoids affect GSK-3β activity. We then investigated whether GSK-3β inhibition could regulate glucocorticoid-mediated suppression of myogenic differentiation in vitro. Two methods were employed to inhibit GSK-3β: pharmacological inhibition with LiCl and GSK-3β gene knockdown. We found that both methods resulted in enhanced myotube formation and increased levels of muscle regulatory factors and muscle-specific protein expression. Importantly, GSK-3β inhibition attenuated glucocorticoid-induced suppression of myogenic differentiation. Collectively, these data suggest the involvement of GSK-3β in the glucocorticoid-mediated impairment of myogenic differentiation. Therefore, the inhibition of GSK-3β may be a strategy for preventing glucocorticoid-induced muscle degeneration.

Requirement of inducible nitric oxide synthase for skeletal muscle regeneration after acute damage

Adult skeletal muscle regeneration results from activation, proliferation, and fusion of muscle stem cells, such as myogenic precursor cells. Macrophages are consistently present in regenerating skeletal muscles and participate into the repair process. The signals involved in the cross-talk between various macrophage populations and myogenic precursor cells have been only partially identified. In this study, we show a key role of inducible NO synthase (iNOS), expressed by classically activated macrophages in the healing of skeletal muscle. We found that, after sterile injury, iNOS expression is required for effective regeneration of the tissue, as myogenic precursor cells in the muscle of injured iNOS(-/-) mice fail to proliferate and differentiate. We also found that iNOS modulates inflammatory cell recruitment: damaged muscles of iNOS(-/-) animals express significantly higher levels of chemokines such as MIP2, MCP1, MIP-1α, and MCP1, and display more infiltrating neutrophils after injury and a persistence of macrophages at later time points. Finally, we found that iNOS expression in the injured muscle is restricted to infiltrating macrophages. To our knowledge, these data thus provide the first evidence that iNOS expression by infiltrating macrophages contributes to muscle regeneration, revealing a novel mechanism of inflammation-dependent muscle healing.

Nitric oxide sustains long-term skeletal muscle regeneration by regulating fate of satellite cells via signaling pathways requiring Vangl2 and cyclic GMP

Satellite cells are myogenic precursors that proliferate, activate, and differentiate on muscle injury to sustain the regenerative capacity of adult skeletal muscle; in this process, they self-renew through the return to quiescence of the cycling progeny. This mechanism, while efficient in physiological conditions does not prevent exhaustion of satellite cells in pathologies such as muscular dystrophy where numerous rounds of damage occur. Here, we describe a key role of nitric oxide, an important signaling molecule in adult skeletal muscle, on satellite cells maintenance, studied ex vivo on isolated myofibers and in vivo using the α-sarcoglycan null mouse model of dystrophy and a cardiotoxin-induced model of repetitive damage. Nitric oxide stimulated satellite cells proliferation in a pathway dependent on cGMP generation. Furthermore, it increased the number of Pax7⁺/Myf5⁻ cells in a cGMP-independent pathway requiring enhanced expression of Vangl2, a member of the planar cell polarity pathway involved in the Wnt noncanonical pathway. The enhanced self-renewal ability of satellite cells induced by nitric oxide is sufficient to delay the reduction of the satellite cell pool during repetitive acute and chronic damages, favoring muscle regeneration; in the α-sarcoglycan null dystrophic mouse, it also slowed disease progression persistently. These results identify nitric oxide as a key messenger in satellite cells maintenance, expand the significance of the Vangl2-dependent Wnt noncanonical pathway in myogenesis, and indicate novel strategies to optimize nitric oxide-based therapies for muscular dystrophy. Stem Cells 2012; 30:197–209.

Exercise-induced muscle growth is muscle-specific and age-dependent

INTRODUCTION

Sarcopenia, and the importance of satellite cells (SCs) in muscle growth led us to examine the effects of exercise and age on SC activation and gene expression.

METHODS

Eight- and 18-month-old mice were either sedentary or underwent 3 weeks of exercise (N = 24). Body mass, distance traveled, and grip strength were recorded at weekly intervals. The extensor digitorum longus (EDL), tibialis anterior (TA), gastrocnemius (GAST), and quadriceps (QUAD) muscles were analyzed along with muscle fiber area, SC activation, neuronal nitric oxide synthase (NOS-I), MyoD, and myostatin protein content.

RESULTS

Older mice demonstrated decreased body mass, grip strength, and fiber area, but these changes were not affected by exercise. The QUAD muscle from young mice demonstrated an exercise-induced increase in SC activation and NOS-I and downregulation of myostatin.

CONCLUSIONS

Exercise-induced activation of SCs and regulation of gene expression are muscle-specific and age-dependent. Perturbed sensitivity to exercise in older mice provides insight into sarcopenia and potential treatments.

Expression of glucocorticoid receptors in the regenerating human skeletal muscle

Many stress conditions are accompanied by skeletal muscle dysfunction and regeneration, which is essentially a recapitulation of the embryonic development. However, regeneration usually occurs under conditions of hypothalamus-pituitary-adrenal gland axis activation and therefore increased glucocorticoid (GC) levels. Glucocorticoid receptor (GR), the main determinant of cellular responsiveness to GCs, exists in two isoforms (GRalpha and GRbeta) in humans. While the role of GRalpha is well characterized, GRbeta remains an elusive player in GC signalling. To elucidate basic characteristics of GC signalling in the regenerating human skeletal muscle we assessed GRalpha and GRbeta expression pattern in cultured human myoblasts and myotubes and their response to 24-hour dexamethasone (DEX) treatment. There was no difference in GRalpha mRNA and protein expression or DEX-mediated GRalpha down-regulation in myoblasts and myotubes. GRbeta mRNA level was very low in myoblasts and remained unaffected by differentiation and/or DEX. GRbeta protein could not be detected. These results indicate that response to GCs is established very early during human skeletal muscle regeneration and that it remains practically unchanged before innervation is established. Very low GRbeta mRNA expression and inability to detect GRbeta protein suggests that GRbeta is not a major player in the early stages of human skeletal muscle regeneration.

Steroid myopathy: some unresolved issues

Steroid myopathy is a non-inflammatory toxic myopathy that occurs as side effect of exogenous and endogenous glucocorticoid excess. The purpose of this review is to examine issues that limit our understanding of this myopathy with respect to nosology, etiopathogenesis, conditioning factors, and muscle fiber selectivity. We suggest that if more data were available on these issues, the understanding of steroid myopathy would be enhanced substantially, thus allowing an early detection of its occurrence (before the appearance of clinical or laboratory signs) and a proper treatment of the patients.

Musculoskeletal complications of fluoroquinolones: guidelines and precautions for usage in the athletic population

Fluoroquinolone antibiotics are associated with a wide spectrum of musculoskeletal complications that involve not only tendon but also cartilage, bone, and muscle. Insights into the pathoetiology of fluoroquinolone toxicity on musculoskeletal tissues have been evolving over recent years. Although the pathoetiology is certainly multifactorial, alterations in cell signaling proteins and direct toxic effects on musculoskeletal tissues have been strongly implicated. Increasing age and concomitant systemic corticosteroid use appear to significantly increase the risk of adverse events. The purpose of this article is to review the musculoskeletal complications associated with use of fluoroquinolone antibiotics by adults; identify risk factors associated with fluoroquinolone toxicity; explore the possible pathoetiology of fluoroquinolone toxicity on tendon, cartilage, bone, and muscle; and offer recommendations regarding evaluation and treatment of fluoroquinolone-associated musculoskeletal complications. In addition, this review will provide recommendations regarding fluoroquinolone use in athletes and return to play after fluoroquinolone exposure.

Increased fatigability of external anal sphincter in inflammatory bowel disease: significance in fecal urgency and incontinence

BACKGROUND AND AIMS

Fatigability of external anal sphincter (EAS) has not been studied in inflammatory bowel disease (IBD) patients. We evaluated EAS fatigability in IBD patients with and without fecal incontinence (FI) and urgency, and correlated fatigability with demographic and clinical factors, and EAS endosonography.

METHODS

Fifty-eight consecutive IBD cases and 14 healthy volunteers completed Bristol stool form and a FI severity scale. Groups I, II and III included 27 patients with urgency including 13 with concomitant FI, 31 patients without FI or urgency, and 14 controls, respectively. We performed stationary pull-through manometry with an 8-channel water-perfused catheter. Fatigue rate (FR) was calculated by linear regression during a 20-s anal squeeze, and fatigue rate index (FRI) as the ratio of squeeze pressure increment to FR. EAS thickness and deficits were evaluated with an endoanal 10-MHz probe. Patients underwent sigmoidoscopy.

RESULTS

Group I demonstrated a higher Bristol score, more frequent defecations, and more EAS defects compared to group II. Resting, peak squeeze pressures and EAS thickness did not differ between groups. FR was increased in group I versus II, and in group II versus III; FRI was decreased in group I versus II and in group II versus III (p<0.001, adjusting for age and BMI). Gender, oral glucocorticoids, presence of proctitis, perianal disease and EAS defects did not interact with group membership on FR or FRI.

CONCLUSIONS

IBD is associated with increased fatigue rate and decreased fatigue rate index. These differences were even more striking in patients with incontinence or urgency.

Co-administration of ibuprofen and nitric oxide is an effective experimental therapy for muscular dystrophy, with immediate applicability to humans

BACKGROUND AND PURPOSE

Current therapies for muscular dystrophy are based on corticosteroids. Significant side effects associated with these therapies have prompted several studies aimed at identifying possible alternative strategies. As inflammation and defects of nitric oxide (NO) generation are key pathogenic events in muscular dystrophies, we have studied the effects of combining the NO donor isosorbide dinitrate (ISDN) and the non-steroidal anti-inflammatory drug ibuprofen.

EXPERIMENTAL APPROACH

alpha-Sarcoglycan-null mice were treated for up to 8 months with ISDN (30 mg.kg(-1)) plus ibuprofen (50 mg.kg(-1)) administered daily in the diet. Effects of ISDN and ibuprofen alone were assessed in parallel. Drug effects on animal motility and muscle function, muscle damage, inflammatory infiltrates and cytokine levels, as well as muscle regeneration including assessment of endogenous stem cell pool, were measured at selected time points.

KEY RESULTS

Combination of ibuprofen and ISDN stimulated regeneration capacity, of myogenic precursor cells, reduced muscle necrotic damage and inflammation. Muscle function in terms of free voluntary movement and resistance to exercise was maintained throughout the time window analysed. The effects of ISDN and ibuprofen administered separately were transient and significantly lower than those induced by their combination.

CONCLUSIONS AND IMPLICATIONS

Co-administration of NO and ibuprofen provided synergistic beneficial effects in a mouse model of muscular dystrophy, leading to an effective therapy. Our results open the possibility of immediate clinical testing of a combination of ISDN and ibuprofen in dystrophic patients, as both components are approved for use in humans, with a good safety profile.

Glucocorticoid-induced leucine zipper (GILZ) and long GILZ inhibit myogenic differentiation and mediate anti-myogenic effects of glucocorticoids

Myogenesis is a process whereby myoblasts differentiate and fuse into multinucleated myotubes, the precursors of myofibers. Various signals and factors modulate this process, and glucocorticoids (GCs) are important regulators of skeletal muscle metabolism. We show that glucocorticoid-induced leucine zipper (GILZ), a GC-induced gene, and the newly identified isoform long GILZ (L-GILZ) are expressed in skeletal muscle tissue and in C2C12 myoblasts where GILZ/L-GILZ maximum expression occurs during the first few days in differentiation medium. Moreover, we observed that GC treatment of myoblasts, which increased GILZ/L-GILZ expression, resulted in reduced myotube formation, whereas GILZ and L-GILZ silencing dampened GC effects. Inhibition of differentiation caused by GILZ/L-GILZ overexpression correlated with inhibition of MyoD function and reduced expression of myogenin. Notably, results indicate that GILZ and L-GILZ bind and regulate MyoD/HDAC1 transcriptional activity, thus mediating the anti-myogenic effect of GCs.