Nicotinamide Phosphoribosyltransferase-elevated NAD+ biosynthesis prevents muscle disuse atrophy by reversing mitochondrial dysfunction.

BACKGROUND: It is well known that muscle disuse atrophy is associated with mitochondrial dysfunction, which is implicated in reduced nicotinamide adenine dinucleotide (NAD+ ) levels. Nicotinamide phosphoribosyltransferase (NAMPT), a rate-limiting enzyme in NAD+ biosynthesis, may serve as a novel strategy to treat muscle disuse atrophy by reversing mitochondrial dysfunction. METHODS: To investigate the effects of NAMPT on the prevention of disuse atrophy of skeletal muscles predominantly composed of slow-twitch (type I) or fast-twitch (type II) fibres, rabbit models of rotator cuff tear-induced supraspinatus muscle atrophy and anterior cruciate ligament (ACL) transection-induced extensor digitorum longus (EDL) atrophy were established and then administered NAMPT therapy. Muscle mass, fibre cross-sectional area (CSA), fibre type, fatty infiltration, western blot, and mitochondrial function were assayed to analyse the effects and molecular mechanisms of NAMPT in preventing muscle disuse atrophy. RESULTS: Acute disuse of the supraspinatus muscle exhibited significant loss of mass (8.86 ± 0.25 to 5.10 ± 0.79 g; P < 0.001) and decreased fibre CSA (3939.6 ± 136.1 to 2773.4 ± 217.6 µm2 , P < 0.001), which were reversed by NAMPT (muscle mass 6.17 ± 0.54 g, P = 0.0033; fibre CSA, 3219.8 ± 289.4 µm2 , P = 0.0018). Disuse-induced impairment of mitochondrial function were significantly improved by NAMPT, including citrate synthase activity (40.8 ± 6.3 to 50.5 ± 5.6 nmol/min/mg, P = 0.0043), and NAD+ biosynthesis (279.9 ± 48.7 to 392.2 ± 43.2 pmol/mg, P = 0.0023). Western blot revealed that NAMPT increases NAD+ levels by activating NAMPT-dependent NAD+ salvage synthesis pathway. In supraspinatus muscle atrophy due to chronic disuse, a combination of NAMPT injection and repair surgery was more effective than repair in reversing muscle atrophy. Although the predominant composition of EDL muscle is fast-twitch (type II) fibre type that differ from supraspinatus muscle, its mitochondrial function and NAD+ levels are also susceptible to disuse. Similar to the supraspinatus muscle, NAMPT-elevated NAD+ biosynthesis was also efficient in preventing EDL disuse atrophy by reversing mitochondrial dysfunction. CONCLUSIONS: NAMPT-elevated NAD+ biosynthesis can prevent disuse atrophy of skeletal muscles that predominantly composed with either slow-twitch (type I) or fast-twitch (type II) fibres by reversing mitochondrial dysfunction.

Disuse muscle atrophy-improving effect of ninjin'yoeito in a mouse model.

Disuse syndrome indicates psychosomatic hypofunction caused by excess rest and motionless and muscle atrophy is termed disuse muscle atrophy. Disuse muscle atrophy-induced muscle weakness and hypoactivity further induces muscle atrophy, leading to a vicious cycle, and this is considered a factor causing secondary sarcopenia and subsequently frailty. Since frailty finally leads to a bedridden state requiring nursing, in facing a super-aging society, intervention for a risk factor of frailty, disuse muscle atrophy, is important. However, the main treatment of disuse muscle atrophy is physical therapy and there are fewer effective preventive and therapeutic drugs. The objective of this study was to search for Kampo medicine with a disuse muscle atrophy-improving effect. Ninjin'yoeito is classified as a qi-blood sohozai (dual supplement) in Chinese herbal medicine, and it has an action supplementing the spleen related to muscle. In addition, improvement of muscle mass and muscle weakness by ninjin'yoeito in a clinical study has been reported. In this study, the effect of ninjin'yoeito on disuse muscle atrophy was investigated. A disuse muscle atrophy model was prepared using male ICR mice. After surgery applying a ring for tail suspension, a 1-week recovery period was set. Ninjin'yoeito was administered by mixing it in the diet for 1 week after the recovery period, followed by tail suspension for 14 days. Ninjin'yoeito administration was continued until autopsy including the hindlimb suspension period. The mice were euthanized and autopsied immediately after completion of tail suspension, and the hindlimb muscles were collected. The food and water intakes during the hindlimb unloaded period, wet weight of the collected muscle, and muscle synthesis and muscle degradation-related factors in blood and muscle were evaluated. Ingestion of ninjin'yoeito inhibited tail suspension-induced reduction of the soleus muscle wet weight. In addition, an increase in the blood level of a muscle synthesis-related factor, IGF-1, and promotion of phosphorylation of mTOR and 4E-BP1 in the soleus muscle were observed. It was suggested that ninjin'yoeito has a disuse muscle atrophy-improving action. Promotion of the muscle synthesis pathway was considered the action mechanism of this.

Chronic Ouabain Prevents Na,K-ATPase Dysfunction and Targets AMPK and IL-6 in Disused Rat Soleus Muscle.

Sustained sarcolemma depolarization due to loss of the Na,K-ATPase function is characteristic for skeletal muscle motor dysfunction. Ouabain, a specific ligand of the Na,K-ATPase, has a circulating endogenous analogue. We hypothesized that the Na,K-ATPase targeted by the elevated level of circulating ouabain modulates skeletal muscle electrogenesis and prevents its disuse-induced disturbances. Isolated soleus muscles from rats intraperitoneally injected with ouabain alone or subsequently exposed to muscle disuse by 6-h hindlimb suspension (HS) were studied. Conventional electrophysiology, Western blotting, and confocal microscopy with cytochemistry were used. Acutely applied 10 nM ouabain hyperpolarized the membrane. However, a single injection of ouabain (1 µg/kg) prior HS was unable to prevent the HS-induced membrane depolarization. Chronic administration of ouabain for four days did not change the α1 and α2 Na,K-ATPase protein content, however it partially prevented the HS-induced loss of the Na,K-ATPase electrogenic activity and sarcolemma depolarization. These changes were associated with increased phosphorylation levels of AMP-activated protein kinase (AMPK), its substrate acetyl-CoA carboxylase and p70 protein, accompanied with increased mRNA expression of interleikin-6 (IL-6) and IL-6 receptor. Considering the role of AMPK in regulation of the Na,K-ATPase, we suggest an IL-6/AMPK contribution to prevent the effects of chronic ouabain under skeletal muscle disuse.

Allopurinol partially prevents disuse muscle atrophy in mice and humans.

Disuse muscle wasting will likely affect everyone in his or her lifetime in response to pathologies such as joint immobilization, inactivity or bed rest. There are no good therapies to treat it. We previously found that allopurinol, a drug widely used to treat gout, protects muscle damage after exhaustive exercise and results in functional gains in old individuals. Thus, we decided to test its effect in the prevention of soleus muscle atrophy after two weeks of hindlimb unloading in mice, and lower leg immobilization following ankle sprain in humans (EudraCT: 2011-003541-17). Our results show that allopurinol partially protects against muscle atrophy in both mice and humans. The protective effect of allopurinol is similar to that of resistance exercise which is the best-known way to prevent muscle mass loss in disuse human models. We report that allopurinol protects against the loss of muscle mass by inhibiting the expression of ubiquitin ligases. Our results suggest that the ubiquitin-proteasome pathway is an appropriate therapeutic target to inhibit muscle wasting and emphasizes the role of allopurinol as a non-hormonal intervention to treat disuse muscle atrophy.

The complex of PAMAM-OH dendrimer with Angiotensin (1-7) prevented the disuse-induced skeletal muscle atrophy in mice.

Angiotensin (1-7) (Ang-(1-7)) is a bioactive heptapeptide with a short half-life and has beneficial effects in several tissues - among them, skeletal muscle - by preventing muscle atrophy. Dendrimers are promising vehicles for the protection and transport of numerous bioactive molecules. This work explored the use of a neutral, non-cytotoxic hydroxyl-terminated poly(amidoamine) (PAMAM-OH) dendrimer as an Ang-(1-7) carrier. Bioinformatics analysis showed that the Ang-(1-7)-binding capacity of the dendrimer presented a 2:1 molar ratio. Molecular dynamics simulation analysis revealed the capacity of neutral PAMAM-OH to protect Ang-(1-7) and form stable complexes. The peptide coverage ability of the dendrimer was between ~50% and 65%. Furthermore, an electrophoretic mobility shift assay demonstrated that neutral PAMAM-OH effectively bonded peptides. Experimental results showed that the Ang-(1-7)/PAMAM-OH complex, but not Ang-(1-7) alone, had an anti-atrophic effect when administered intraperitoneally, as evaluated by muscle strength, fiber diameter, myofibrillar protein levels, and atrogin-1 and MuRF-1 expressions. The results of the Ang-(1-7)/PAMAM-OH complex being intraperitoneally injected were similar to the results obtained when Ang-(1-7) was systemically administered through mini-osmotic pumps. Together, the results suggest that Ang-(1-7) can be protected for PAMAM-OH when this complex is intraperitoneally injected. Therefore, the Ang-(1-7)/PAMAM-OH complex is an efficient delivery method for Ang-(1-7), since it improves the anti-atrophic activity of this peptide in skeletal muscle.

Beta-agonist stimulation ameliorates the phenotype of spinal and bulbar muscular atrophy mice and patient-derived myotubes.

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease characterized by the loss of lower motor neurons. SBMA is caused by expansions of a polyglutamine tract in the gene coding for androgen receptor (AR). Expression of polyglutamine-expanded AR causes damage to motor neurons and skeletal muscle cells. Here we investigated the effect of ß-agonist stimulation in SBMA myotube cells derived from mice and patients, and in knock-in mice. We show that treatment of myotubes expressing polyglutamine-expanded AR with the ß-agonist clenbuterol increases their size. Clenbuterol activated the phosphatidylinositol-3-kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) pathway and decreased the accumulation of polyglutamine-expanded AR. Treatment of SBMA knock-in mice with clenbuterol, which was started at disease onset, ameliorated motor function and extended survival. Clenbuterol improved muscle pathology, attenuated the glycolytic-to-oxidative metabolic alterations occurring in SBMA muscles and induced hypertrophy of both glycolytic and oxidative fibers. These results indicate that ß-agonist stimulation is a novel therapeutic strategy for SBMA.

Androgens have antiresorptive effects on trabecular disuse osteopenia independent from muscle atrophy.

Aging hypogonadal men are at increased risk of osteoporosis and sarcopenia. Testosterone is a potentially appealing strategy to prevent simultaneous bone and muscle loss. The androgen receptor (AR) mediates antiresorptive effects on trabecular bone via osteoblast-lineage cells, as well as muscle-anabolic actions. Sex steroids also modify the skeletal response to mechanical loading. However, it is unclear whether the effects of androgens on bone remain effective independent of mechanical stimulation or rather require indirect androgen effects via muscle. This study aims to characterize the effects and underlying mechanisms of androgens on disuse osteosarcopenia. Adult male mice received a unilateral botulinum toxin (BTx) injection, and underwent sham surgery or orchidectomy (ORX) without or with testosterone (ORX+T) or dihydrotestosterone (ORX+DHT) replacement. Compared to the contralateral internal control hindlimb, acute trabecular number and bone volume loss was increased by ORX and partially prevented DHT. T was more efficient and increased BV/TV in both hindlimbs over sham values, although it did not reduce the detrimental effect of BTx. Both androgens and BTx regulated trabecular osteoclast surface as well as tartrate-resistant acid phosphatase expression. Androgens also prevented BTx-induced body weight loss but did not significantly influence paralysis or muscle atrophy. BTx and ORX both reduced cortical thickness via endosteal expansion, which was prevented by T but not DHT. In long-term follow-up, the residual trabecular bone volume deficit in sham-BTx hindlimbs was prevented by DHT but T restored it more efficiently to pre-treatment levels. Conditional AR deletion in late osteoblasts and osteocytes or in the satellite cell lineage increased age-related trabecular bone loss in both hindlimbs without influencing the effect of BTx on trabecular osteopenia. We conclude that androgens have antiresorptive effects on trabecular disuse osteopenia which do not require AR actions on bone via muscle or via osteocytes.

A small-molecule Nrf1 and Nrf2 activator mitigates polyglutamine toxicity in spinal and bulbar muscular atrophy.

Spinal and bulbar muscular atrophy (SBMA, also known as Kennedy's disease) is one of nine neurodegenerative disorders that are caused by expansion of polyglutamine-encoding CAG repeats. Intracellular accumulation of abnormal proteins in these diseases, a pathological hallmark, is associated with defects in protein homeostasis. Enhancement of the cellular proteostasis capacity with small molecules has therefore emerged as a promising approach to treatment. Here, we characterize a novel curcumin analog, ASC-JM17, as an activator of central pathways controlling protein folding, degradation and oxidative stress resistance. ASC-JM17 acts on Nrf1, Nrf2 and Hsf1 to increase the expression of proteasome subunits, antioxidant enzymes and molecular chaperones. We show that ASC-JM17 ameliorates toxicity of the mutant androgen receptor (AR) responsible for SBMA in cell, fly and mouse models. Knockdown of the Drosophila Nrf1 and Nrf2 ortholog cap 'n' collar isoform-C, but not Hsf1, blocks the protective effect of ASC-JM17 on mutant AR-induced eye degeneration in flies. Our observations indicate that activation of the Nrf1/Nrf2 pathway is a viable option for pharmacological intervention in SBMA and potentially other polyglutamine diseases.

Signal transducer and activator of transcription 3 signaling as a potential target to treat muscle wasting diseases.

PURPOSE OF REVIEW: The review summarizes our current knowledge of the role of signal transducer and activator of transcription 3 (STAT3) signaling in skeletal muscle regeneration and the maintenance of muscle mass. RECENT FINDINGS: STAT3 signaling plays a pivotal role in regulating the function of multiple cell types in skeletal muscle. This includes muscle stem cells, myofibers, and macrophages. It regulates muscle stem cell function by antagonizing self-renewal. STAT3 also functions in myofibers to regulate skeletal muscle mass. This is highly relevant under pathological conditions where STAT3 activation promotes protein degradation and muscle atrophy. Transient pharmacological inhibition of STAT3 partially prevents muscle wasting. However, the mechanisms responsible for the improvement of muscle condition are not currently well understood. This is because of the complexity of the system, as STAT3 has a critical role in regulating the function of several cell types residing in skeletal muscle. SUMMARY: Muscle wasting is associated with several human diseases such as muscle dystrophies or cancer cachexia. However, currently there are no effective treatments for this condition, and there is a critical need to identify new potential targets for the development of efficient therapeutic approaches.

Angiotensin-(1-7) attenuates disuse skeletal muscle atrophy in mice via its receptor, Mas.

Immobilization is a form of disuse characterized by a loss of strength and muscle mass. Among the main features are decreased IGF-1/Akt signalling and increased ubiquitin-proteasome pathway signalling, which induce greater myosin heavy chain degradation. Activation of the classical renin-angiotensin system (RAS) causes deleterious effects in skeletal muscle, including muscle wasting. In contrast, angiotensin-(1-7) [Ang-(1-7)], a peptide of the non-classical RAS, produces beneficial effects in skeletal muscle. However, the role of Ang-(1-7) in skeletal muscle disuse atrophy and independent of classical RAS activation has not been evaluated. Therefore, we assessed the functions of Ang-(1-7) and the Mas receptor in disuse muscle atrophyin vivousing unilateral cast immobilization of the hind limb in male, 12-week-old wild-type (WT) and Mas-knockout (Mas KO) mice for 1 and 14 days. Additionally, we evaluated the participation of IGF-1/IGFR-1/Akt signalling and ubiquitin-proteasome pathway expression on the effects of Ang-(1-7) immobilization-induced muscle atrophy. Our results found that Ang-(1-7) prevented decreased muscle strength and reduced myofiber diameter, myosin heavy chain levels, and the induction of atrogin-1 and MuRF-1 expressions, all of which normally occur during immobilization. Analyses indicated that Ang-(1-7) increases IGF-1/IGFR-1/Akt pathway signalling through IGFR-1 and Akt phosphorylation, and the concomitant activation of two downstream targets of Akt, p70S6K and FoxO3. These anti-atrophic effects of Ang-(1-7) were not observed in Mas KO mice, indicating crucial participation of the Mas receptor. This report is the first to propose anti-atrophic effects of Ang-(1-7) via the Mas receptor and the participation of the IGF-1/IGFR-1/Akt/p70S6K/FoxO3 mechanism in disuse skeletal muscle atrophy.

Pioglitazone suppresses neuronal and muscular degeneration caused by polyglutamine-expanded androgen receptors.

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by the expansion of a CAG repeat in the androgen receptor (AR) gene. Mutant AR has been postulated to alter the expression of genes important for mitochondrial function and induce mitochondrial dysfunction. Here, we show that the expression levels of peroxisome proliferator-activated receptor-γ (PPARγ), a key regulator of mitochondrial biogenesis, were decreased in mouse and cellular models of SBMA. Treatment with pioglitazone (PG), an activator of PPARγ, improved the viability of the cellular model of SBMA. The oral administration of PG also improved the behavioral and histopathological phenotypes of the transgenic mice. Furthermore, immunohistochemical and biochemical analyses demonstrated that the administration of PG suppressed oxidative stress, nuclear factor-κB (NFκB) signal activation and inflammation both in the spinal cords and skeletal muscles of the SBMA mice. These findings suggest that PG is a promising candidate for the treatment of SBMA.

Cell biological approaches to investigate polyglutamine-expanded AR metabolism.

Spinal and bulbar muscular atrophy (SBMA) is a late-onset neurodegenerative disease caused by a polyglutamine expansion in the androgen receptor (AR). In vivo and in vitro studies have suggested that some steps of normal AR function and metabolism, such as hormone binding and nuclear translocation of the AR, are necessary for toxicity and aggregation of the mutant protein. Mutation of discreet functional domains of the AR and sites of posttranslational modification enable the detailed analysis of the role of AR function and metabolism in toxicity and aggregation of polyglutamine-expanded AR. This analysis could potentially lead to the development of targeted therapy for the treatment of SBMA. We have developed a stably transfected, tetracycline-inducible, cell model that replicates many of the hallmarks of disease, including ligand-dependent aggregation and toxicity, and provides a relatively quick system for the reliable expression and analysis of the mutated polyglutamine-expanded AR. Multiple cell lines, each expressing the androgen receptor with a distinct functional mutation, can be created and the dose of tetracycline modulated to produce equal protein expression across lines in order to evaluate the structural and functional requirements of AR toxicity and aggregation. Results from these studies can then be validated in a disease-relevant cell type, spinal motor neurons, using viral delivery of the gene of interest into dissociated spinal cord cultures. Utilization of these cell models provides a relatively rapid, cost-effective experimental pathway to analyze the role of distinct steps in AR metabolism in disease pathogenesis using in vitro systems.

Pharmacological protein targets in polyglutamine diseases: mutant polypeptides and their interactors.

Polyglutamine diseases are a group of pathologies affecting different parts of the brain and causing dysfunction and atrophy of certain neural cell populations. These diseases stem from mutations in various cellular genes that result in the synthesis of proteins with extended polyglutamine tracts. In particular, this concerns huntingtin, ataxins, and androgen receptor. These mutant proteins can form oligomers, aggregates, and, finally, aggresomes with distinct functions and different degrees of cytotoxicity. In this review, we analyze the effects of different forms of polyQ proteins on other proteins and their functions, which are considered as targets for therapeutic intervention.

Prevention of disuse muscle atrophy by dietary ingestion of 8-prenylnaringenin in denervated mice.

Flavonoids have attracted considerable attention in relation to their effects upon health. 8-Prenylnaringenin (8-PN) is found in the common hop (Humulus lupulus) and assumed to be responsible for the health impact of beer consumption. We wanted to clarify the effects of prenylation on the physiological functions of dietary flavonoids by comparing the effects of 8-PN with that of intact naringenin in the prevention of disuse muscle atrophy using a model of denervation in mice. Consumption of 8-PN (but not naringenin) prevented loss of weight in the gastrocnemius muscle further supported by the lack of induction of the protein content of a key ubiquitin ligase involved in muscle atrophy, atrogin-1, and by the activation of Akt phosphorylation. 8-PN content in the gastrocnemius muscle was tenfold higher than that of naringenin. These results suggested that, compared with naringenin, 8-PN was effectively concentrated into skeletal muscle to exert its preventive effects upon disuse muscle atrophy. It is likely that prenylation generates novel functions for 8-PN by enhancing its accumulation into muscle tissue through dietary intake.

Insulinlike growth factor (IGF)-1 administration ameliorates disease manifestations in a mouse model of spinal and bulbar muscular atrophy.

Spinal and bulbar muscular atrophy is an X-linked motor neuron disease caused by polyglutamine expansion in the androgen receptor. Patients develop slowly progressive proximal muscle weakness, muscle atrophy and fasciculations. Affected individuals often show gynecomastia, testicular atrophy and reduced fertility as a result of mild androgen insensitivity. No effective disease-modifying therapy is currently available for this disease. Our recent studies have demonstrated that insulinlike growth factor (IGF)-1 reduces the mutant androgen receptor toxicity through activation of Akt in vitro, and spinal and bulbar muscular atrophy transgenic mice that also overexpress a noncirculating muscle isoform of IGF-1 have a less severe phenotype. Here we sought to establish the efficacy of daily intraperitoneal injections of mecasermin rinfabate, recombinant human IGF-1 and IGF-1 binding protein 3, in a transgenic mouse model expressing the mutant androgen receptor with an expanded 97 glutamine tract. The study was done in a controlled, randomized, blinded fashion, and, to reflect the clinical settings, the injections were started after the onset of disease manifestations. The treatment resulted in increased Akt phosphorylation and reduced mutant androgen receptor aggregation in muscle. In comparison to vehicle-treated controls, IGF-1-treated transgenic mice showed improved motor performance, attenuated weight loss and increased survival. Our results suggest that peripheral tissue can be targeted to improve the spinal and bulbar muscular atrophy phenotype and indicate that IGF-1 warrants further investigation in clinical trials as a potential treatment for this disease.

Developing treatment for spinal and bulbar muscular atrophy.

Spinal and bulbar muscular atrophy is unique among the polyglutamine diseases in that the toxicity of the mutant protein, the androgen receptor, is ligand-dependent. In cell culture and animal models the mutant androgen receptor causes protein aggregation and alterations in transcriptional regulation, axonal transport, and mitochondrial function. Various therapeutic approaches have shown efficacy in mouse models, including androgen reduction and agents that alter the processing and degradation of the mutant androgen receptor protein, such as HSP90 inhibitors, IGF-1, and ASC-J9. Clinical trials of androgen-reducing agents have shown indications of efficacy but not proof of clinically meaningful benefit to date. This trial experience has set the stage for future clinical studies of other agents that have been found to be beneficial in transgenic animal models.

Losartan restores skeletal muscle remodeling and protects against disuse atrophy in sarcopenia.

Sarcopenia, a critical loss of muscle mass and function because of the physiological process of aging, contributes to disability and mortality in older adults. It increases the incidence of pathologic fractures, causing prolonged periods of hospitalization and rehabilitation. The molecular mechanisms underlying sarcopenia are poorly understood, but recent evidence suggests that increased transforming growth factor-ß (TGF-ß) signaling contributes to impaired satellite cell function and muscle repair in aged skeletal muscle. We therefore evaluated whether antagonism of TGF-ß signaling via losartan, an angiotensin II receptor antagonist commonly used to treat high blood pressure, had a beneficial impact on the muscle remodeling process of sarcopenic mice. We demonstrated that mice treated with losartan developed significantly less fibrosis and exhibited improved in vivo muscle function after cardiotoxin-induced injury. We found that losartan not only blunted the canonical TGF-ß signaling cascade but also modulated the noncanonical TGF-ß mitogen-activated protein kinase pathway. We next assessed whether losartan was able to combat disuse atrophy in aged mice that were subjected to hindlimb immobilization. We showed that immobilized mice treated with losartan were protected against loss of muscle mass. Unexpectedly, this protective mechanism was not mediated by TGF-ß signaling but was due to an increased activation of the insulin-like growth factor 1 (IGF-1)/Akt/mammalian target of rapamycin (mTOR) pathway. Thus, blockade of the AT1 (angiotensin II type I) receptor improved muscle remodeling and protected against disuse atrophy by differentially regulating the TGF-ß and IGF-1/Akt/mTOR signaling cascades, two pathways critical for skeletal muscle homeostasis. Thus, losartan, a Food and Drug Administration-approved drug, may prove to have clinical benefits to combat injury-related muscle remodeling and provide protection against disuse atrophy in humans with sarcopenia.

Prevention of muscle disuse atrophy by MG132 proteasome inhibitor.

INTRODUCTION: Our goal was to determine whether in vivo administration of the proteasome inhibitor MG132 can prevent muscle atrophy caused by hindlimb unloading (HU). METHODS: Twenty-seven NMRI mice were assigned to a weight-bearing control, a 6-day HU, or a HU+MG132 (1 mg/kg/48 h) treatment group. RESULTS: Gastrocnemius wasting was significantly less in HU+MG132 mice (-6.7 ± 2.0%) compared with HU animals (-12.6 ± 1.1%, P = 0.011). HU was also associated with an increased expression of MuRF-1 (P = 0.006), MAFbx (P = 0.001), and USP28 (P = 0.027) mRNA, whereas Nedd4, E3α, USP19, and UBP45 mRNA did not change significantly. Increases in MuRF-1, MAFbx, and USP28 mRNA were largely repressed after MG132 administration. ß5 proteasome activity tended to increase in HU (+16.7 ± 6.1%, P = 0.086). Neither ß1 and ß2 proteasome activities nor ubiquitin-conjugated proteins were changed by HU. CONCLUSIONS: Our results indicate that in vivo administration of MG132 partially prevents muscle atrophy associated with disuse and highlight an unexpected regulation of MG132 proteasome inhibitor on ubiquitin-ligases.

Are antioxidants useful for treating skeletal muscle atrophy?

Changes in the skeletal muscle protein mass frequently occur in both physiological and pathological states. Muscle hypotrophy, in particular, is commonly observed during aging and is characteristic of several pathological conditions such as neurological diseases, cancer, diabetes, and sepsis. The skeletal muscle protein content depends on the relative rates of synthesis and degradation, which must be coordinately regulated to maintain the equilibrium. Pathological muscle depletion is characterized by a negative nitrogen balance, which results from disruption of this equilibrium due to reduced synthesis, increased breakdown, or both. The current view, mainly based on experimental data, considers hypercatabolism as the major cause of muscle protein depletion. Several signaling pathways that probably contribute to muscle atrophy have been identified, and there is increasing evidence that oxidative stress, due to reactive oxygen species production overwhelming the intracellular antioxidant systems, plays a role in causing muscle depletion both during aging and in chronic pathological states. In particular, oxidative stress has been proposed to enhance protein breakdown, directly or by interacting with other factors. This review focuses on the possibility of using antioxidant treatments to target molecular pathways involved in the pathogenesis of skeletal muscle wasting.

The potential and the pitfalls of beta-adrenoceptor agonists for the management of skeletal muscle wasting.

The beta-adrenergic signaling pathway represents a novel therapeutic target for skeletal muscle wasting and weakness due to its role in the mechanisms controlling protein synthesis and degradation and in modulating fiber type. Stimulation of the pathway with beta-adrenoceptor agonists (beta-agonists) has therapeutic potential for muscle wasting disorders including: sarcopenia, cancer cachexia, disuse and inactivity, unloading or microgravity, sepsis and other metabolic disorders, denervation, burns, HIV-AIDS, chronic kidney or heart failure, and neuromuscular diseases. However, there are also pitfalls associated with beta-agonist administration and clinical applications have so far been limited, largely because of cardiovascular side effects. In rats and mice, newer generation beta-agonists (such as formoterol) can elicit an anabolic response in skeletal muscle even at very low doses, with reduced effects on the heart and cardiovascular system compared with older generation beta-agonists (such as fenoterol and clenbuterol). However, the potentially deleterious cardiovascular side effects of beta-agonists have not been obviated completely and so it is important to refine their development and therapeutic approach in order to overcome these obstacles. This review describes the therapeutic potential of stimulating the beta-adrenergic signaling pathway with beta-agonists, highlighting the beneficial effects on skeletal muscle structure and function and identifying some of the pitfalls associated with short- and long-term beta-agonist administration. The review also identifies some important, but as yet unanswered questions, regarding the importance of beta-adrenoceptor signaling in muscle health and disease and the strategies needed to improve the efficacy and safety of beta-agonists for muscle wasting disorders.