Regulation of ubiquitin–proteasome system, caspase enzyme activities, and extracellular proteinases in rat soleus muscle in response to unloading

Muscle unloading: A comparison between spaceflight and ground-based models

Prolonged unloading of skeletal muscle, a common outcome of events such as spaceflight, bed rest and hindlimb unloading, can result in extensive metabolic, structural and functional changes in muscle fibres. With advancement in investigations of cellular and molecular mechanisms, understanding of disuse muscle atrophy has significantly increased. However, substantial gaps exist in our understanding of the processes dictating muscle plasticity during unloading, which prevent us from developing effective interventions to combat muscle loss. This review aims to update the status of knowledge and underlying mechanisms leading to cellular and molecular changes in skeletal muscle during unloading. We have also discussed advances in the understanding of contractile dysfunction during spaceflights and in ground-based models of muscle unloading. Additionally, we have elaborated on potential therapeutic interventions that show promising results in boosting muscle mass and strength during mechanical unloading. Finally, we have identified key gaps in our knowledge as well as possible research direction for the future.

Muscle Atrophy Marker Expression Differs between Rotary Cell Culture System and Animal Studies

Muscular atrophy, defined as the loss of muscle tissue, is a serious issue for immobilized patients on Earth and for humans during spaceflight, where microgravity prevents normal muscle loading. In vitro modeling is an important step in understanding atrophy mechanisms and testing countermeasures before animal trials. The most ideal environment for modeling must be empirically determined to best mimic known responses in vivo. To simulate microgravity conditions, murine C2C12 myoblasts were cultured in a rotary cell culture system (RCCS). Alginate encapsulation was compared against polystyrene microcarrier beads as a substrate for culturing these adherent muscle cells. Changes after culture under simulated microgravity were characterized by assessing mRNA expression of MuRF1, MAFbx, Caspase 3, Akt2, mTOR, Ankrd1, and Foxo3. Protein concentration of myosin heavy chain 4 (Myh4) was used as a differentiation marker. Cell morphology and substrate structure were evaluated with brightfield and fluorescent imaging. Differentiated C2C12 cells encapsulated in alginate had a significant increase in MuRF1 only following simulated microgravity culture and were morphologically dissimilar to normal cultured muscle tissue. On the other hand, C2C12 cells cultured on polystyrene microcarriers had significantly increased expression of MuRF1, Caspase 3, and Foxo3 and easily identifiable multinucleated myotubes. The extent of differentiation was higher in simulated microgravity and protein synthesis more active with increased Myh4, Akt2, and mTOR. The in vitro microcarrier model described herein significantly increases expression of several of the same atrophy markers as in vivo models. However, unlike animal models, MAFbx and Ankrd1 were not significantly increased and the fold change in MuRF1 and Foxo3 was lower than expected. Using a standard commercially available RCCS, the substrates and culture methods described only partially model changes in mRNAs associated with atrophy in vivo.

Regulation of Akt-mTOR, ubiquitin-proteasome and autophagy-lysosome pathways in locomotor and respiratory muscles during experimental sepsis in mice

Sepsis induced loss of muscle mass and function contributes to promote physical inactivity and disability in patients. In this experimental study, mice were sacrificed 1, 4, or 7 days after cecal ligation and puncture (CLP) or sham surgery. When compared with diaphragm, locomotor muscles were more prone to sepsis-induced muscle mass loss. This could be attributed to a greater activation of ubiquitin-proteasome system and an increased myostatin expression. Thus, this study strongly suggests that the contractile activity pattern of diaphragm muscle confers resistance to atrophy compared to the locomotor gastrocnemius muscle. These data also suggest that a strategy aimed at preventing the activation of catabolic pathways and preserving spontaneous activity would be of interest for the treatment of patients with sepsis-induced neuromyopathy.

Proteomic analysis of mouse soleus muscles affected by hindlimb unloading and reloading

INTRODUCTION

Disuse muscle atrophy, induced by prolonged space flight, bed rest, or denervation, is a common process with obvious changes in slow-twitch soleus muscles.

METHODS

Proteomic analysis was performed on mouse soleus subjected to hindlimb unloading (HU) and hindlimb reloading (HR) to identify new dysregulated proteins.

RESULTS

Following HU, the mass and cross-sectional area of muscle fibers decreased, but they recovered after HR. Proteomic analyses revealed 9 down-regulated and 7 up-regulated proteins in HU, and 2 down-regulated and 5 up-regulated proteins in HR. The dysregulated proteins were mainly involved in energy metabolism, protein degradation, and cytoskeleton stability. Among the dysregulated proteins were fatty acid binding protein 3, α-B crystalline, and transthyretin.

CONCLUSIONS

These results indicate that muscle atrophy induced by unloading is related to activation of proteolysis, metabolic alterations toward glycolysis, destruction of myofibrillar integrity, and dysregulation of heat shock proteins (HSPs). The dysregulated proteins may play a role in muscle atrophy and the recovery process.

Proteomic identification of hair cell repair proteins in the model sea anemone Nematostella vectensis

Sea anemones have an extraordinary capability to repair damaged hair bundles, even after severe trauma. A group of secreted proteins, named repair proteins (RPs), found in mucus covering sea anemones significantly assists the repair of damaged hair bundle mechanoreceptors both in the sea anemone Haliplanella luciae and the blind cavefish Astyanax hubbsi. The polypeptide constituents of RPs must be identified in order to gain insight into the molecular mechanisms by which repair of hair bundles is accomplished. In this study, several polypeptides of RPs were isolated from mucus using blue native PAGE and then sequenced using LC-MS/MS. Thirty-seven known polypeptides were identified, including Hsp70s, as well as many polypeptide subunits of the 20S proteasome. Other identified polypeptides included those involved in cellular stress responses, protein folding, and protein degradation. Specific inhibitors of Hsp70s and the 20S proteasome were employed in experiments to test their involvement in hair bundle repair. The results of those experiments suggested that repair requires biologically active Hsp70s and 20S proteasomes. A model is proposed that considers the function of extracellular Hsp70s and 20S proteasomes in the repair of damaged hair cells.

Trichostatin A, a histone deacetylase inhibitor, modulates unloaded-induced skeletal muscle atrophy

Skeletal muscle atrophy is commonly associated with immobilization, ageing, and catabolic diseases such as diabetes and cancer cachexia. Epigenetic regulation of gene expression resulting from chromatin remodeling through histone acetylation has been implicated in muscle disuse. The present work was designed to test the hypothesis that treatment with trichostatin A (TSA), a histone deacetylase inhibitor, would partly counteract unloading-induced muscle atrophy. Soleus muscle atrophy (-38%) induced by 14 days of rat hindlimb suspension was reduced to only 25% under TSA treatment. TSA partly prevented the loss of type I and IIa fiber size and reversed the transitions of slow-twitch to fast-twitch fibers in soleus muscle. Unloading or TSA treatment did not affect myostatin gene expression and follistatin protein. Soleus protein carbonyl content remained unchanged, whereas the decrease in glutathione vs. glutathione disulfide ratio and the increase in catalase activity (biomarkers of oxidative stress) observed after unloading were abolished by TSA treatment. The autophagy-lysosome pathway (Bnip3 and microtubule-associated protein 1 light chain 3 proteins, Atg5, Gabarapl1, Ulk1, and cathepsin B and L mRNA) was not activated by unloading or TSA treatment. However, TSA suppressed the rise in muscle-specific RING finger protein 1 (MuRF1) caused by unloading without affecting the forkhead box (Foxo3) transcription factor. Prevention of muscle atrophy by TSA might be due to the regulation of the skeletal muscle atrophy-related MuRF1 gene. Our findings suggest that TSA may provide a novel avenue to treat unloaded-induced muscle atrophy.

The delayed recovery of the remobilized rat tibialis anterior muscle reflects a defect in proliferative and terminal differentiation that impairs early regenerative processes

BACKGROUND

The immobilization-induced tibialis anterior (TA) muscle atrophy worsens after cast removal and is associated with altered extracellular matrix (ECM) composition. The secreted protein acidic and rich in cysteine (Sparc) is an ECM component involved in Akt activation and in β-catenin stabilization, which controls protein turnover and induces muscle regulatory factors (MRFs), respectively. We hypothesized that ECM alterations may influence these intracellular signalling pathways controlling TA muscle mass.

METHODS

Six-month-old Wistar rats were subjected to hindlimb cast immobilization for 8 days (I8) or not (I0) and allowed to recover for 1 to 10 days (R1-10).

RESULTS

The TA atrophy during remobilization correlated with reduced fibre cross-sectional area and thickening of endomysium. mRNA levels for Sparc increased during remobilization until R10 and for integrin-α7 and -β1 at I8 and R1. Integrin-linked kinase protein levels increased during immobilization and remobilization until R10. This was inversely correlated with changes in Akt phosphorylation. β-Catenin protein levels increased in the remobilized TA at R1 and R10. mRNA levels of the proliferative MRFs (Myf5 and MyoD) increased at I8 and R1, respectively, without changes in Myf5 protein levels. In contrast, myogenin mRNA levels (a terminal differentiation MRF) decreased at R1, but only increased at R10 in remobilized muscles, as for protein levels.

CONCLUSIONS

Altogether, this suggests that the TA inefficiently attempted to preserve regeneration during immobilization by increasing transcription of proliferative MRFs, and that the TA could engage recovery during remobilization only when the terminal differentiation step of regeneration is enhanced.

Impaired exercise training-induced muscle fiber hypertrophy and Akt/mTOR pathway activation in hypoxemic patients with COPD

Exercise training (ExTr) is largely used to improve functional capacity in patients with chronic obstructive pulmonary disease (COPD). However, ExTr only partially restores muscle function in patients with COPD, suggesting that confounding factors may limit the efficiency of ExTr. In the present study, we hypothesized that skeletal muscle adaptations triggered by ExTr could be compromised in hypoxemic patients with COPD. Vastus lateralis muscle biopsies were obtained from patients with COPD who were either normoxemic (n = 15, resting arterial Po2 = 68.5 ± 1.5 mmHg) or hypoxemic (n = 8, resting arterial Po2 = 57.0 ± 1.0 mmHg) before and after a 2-mo ExTr program. ExTr induced a significant increase in exercise capacity both in normoxemic and hypoxemic patients with COPD. However, ExTr increased citrate synthase and lactate dehydrogenase enzyme activities only in skeletal muscle of normoxemic patients. Similarly, muscle fiber cross-sectional area and capillary-to-fiber ratio were increased only in patients who were normoxemic. Expression of atrogenes (MuRF1, MAFbx/Atrogin-1) and autophagy-related genes (Beclin, LC3, Bnip, Gabarapl) remained unchanged in both groups. Phosphorylation of Akt (Ser473), GSK-3β (Ser9), and p70S6k (Thr389) was nonsignificantly increased in normoxemic patients in response to ExTr, but it was significantly decreased in hypoxemic patients. We further showed on C2C12 myotubes that hypoxia completely prevented insulin-like growth factor-1-induced phosphorylation of Akt, GSK-3β, and p70S6K. Together, our observations suggest a role for hypoxemia in the adaptive response of skeletal muscle of patients with COPD in an ExTr program.

Myostatin gene inactivation prevents skeletal muscle wasting in cancer

Cachexia is a muscle-wasting syndrome that contributes significantly to morbidity and mortality of many patients with advanced cancers. However, little is understood about how the severe loss of skeletal muscle characterizing this condition occurs. In the current study, we tested the hypothesis that the muscle protein myostatin is involved in mediating the pathogenesis of cachexia-induced muscle wasting in tumor-bearing mice. Myostatin gene inactivation prevented the severe loss of skeletal muscle mass induced in mice engrafted with Lewis lung carcinoma (LLC) cells or in Apc(Min) (/+) mice, an established model of colorectal cancer and cachexia. Mechanistically, myostatin loss attenuated the activation of muscle fiber proteolytic pathways by inhibiting the expression of atrophy-related genes, MuRF1 and MAFbx/Atrogin-1, along with autophagy-related genes. Notably, myostatin loss also impeded the growth of LLC tumors, the number and the size of intestinal polyps in Apc(Min) (/+) mice, thus strongly increasing survival in both models. Gene expression analysis in the LLC model showed this phenotype to be associated with reduced expression of genes involved in tumor metabolism, activin signaling, and apoptosis. Taken together, our results reveal an essential role for myostatin in the pathogenesis of cancer cachexia and link this condition to tumor growth, with implications for furthering understanding of cancer as a systemic disease.

Early changes in costameric and mitochondrial protein expression with unloading are muscle specific

We hypothesised that load-sensitive expression of costameric proteins, which hold the sarcomere in place and position the mitochondria, contributes to the early adaptations of antigravity muscle to unloading and would depend on muscle fibre composition and chymotrypsin activity of the proteasome. Biopsies were obtained from vastus lateralis (VL) and soleus (SOL) muscles of eight men before and after 3 days of unilateral lower limb suspension (ULLS) and subjected to fibre typing and measures for costameric (FAK and FRNK), mitochondrial (NDUFA9, SDHA, UQCRC1, UCP3, and ATP5A1), and MHCI protein and RNA content. Mean cross-sectional area (MCSA) of types I and II muscle fibres in VL and type I fibres in SOL demonstrated a trend for a reduction after ULLS (0.05 ≤ P < 0.10). FAK phosphorylation at tyrosine 397 showed a 20% reduction in VL muscle (P = 0.029). SOL muscle demonstrated a specific reduction in UCP3 content (−23%; P = 0.012). Muscle-specific effects of ULLS were identified for linear relationships between measured proteins, chymotrypsin activity and fibre MCSA. The molecular modifications in costamere turnover and energy homoeostasis identify that aspects of atrophy and fibre transformation are detectable at the protein level in weight-bearing muscles within 3 days of unloading.

Dynamic patterns of ventricular remodeling and apoptosis in hearts unloaded by heterotopic transplantation

BACKGROUND

Mechanical unloading of failing hearts can trigger functional recovery but results in progressive atrophy and possibly detrimental adaptation. In an unbiased approach, we examined the dynamic effects of unloading duration on molecular markers indicative of myocardial damage, hypothesizing that potential recovery may be improved by optimized unloading time.

METHODS

Heterotopically transplanted normal rat hearts were harvested at 3, 8, 15, 30, and 60 days. Forty-seven genes were analyzed using TaqMan-based microarray, Western blot, and immunohistochemistry.

RESULTS

In parallel with marked atrophy (22% to 64% volume loss at 3 respectively 60 days), expression of myosin heavy-chain isoforms (MHC-α/-β) was characteristically switched in a time-dependent manner. Genes involved in tissue remodeling (FGF-2, CTGF, TGFb, IGF-1) were increasingly upregulated with duration of unloading. A distinct pattern was observed for genes involved in generation of contractile force; an indiscriminate early downregulation was followed by a new steady-state below normal. For pro-apoptotic transcripts bax, bnip-3, and cCasp-6 and -9 mRNA levels demonstrated a slight increase up to 30 days unloading with pronunciation at 60 days. Findings regarding cell death were confirmed on the protein level. Proteasome activity indicated early increase of protein degradation but decreased below baseline in unloaded hearts at 60 days.

CONCLUSIONS

We identified incrementally increased apoptosis after myocardial unloading of the normal rat heart, which is exacerbated at late time points (60 days) and inversely related to loss of myocardial mass. Our findings suggest an irreversible detrimental effect of long-term unloading on myocardium that may be precluded by partial reloading and amenable to molecular therapeutic intervention.

Regulation of Akt-mTOR, ubiquitin-proteasome and autophagy-lysosome pathways in response to formoterol administration in rat skeletal muscle

Administration of β2-agonists triggers skeletal muscle anabolism and hypertrophy. We investigated the time course of the molecular events responsible for rat skeletal muscle hypertrophy in response to 1, 3 and 10 days of formoterol administration (i.p. 2000μg/kg/day). A marked hypertrophy of rat tibialis anterior muscle culminated at day 10. Phosphorylation of Akt, ribosomal protein S6, 4E-BP1 and ERK1/2 was increased at day 3, but returned to control level at day 10. This could lead to a transient increase in protein translation and could explain previous studies that reported increase in protein synthesis following β2-agonist administration. Formoterol administration was also associated with a significant reduction in MAFbx/atrogin-1 mRNA level (day 3), suggesting that formoterol can also affect protein degradation of MAFbx/atrogin1 targeted substrates, including MyoD and eukaryotic initiation factor-3f (eIF3-f). Surprisingly, mRNA level of autophagy-related genes, light chain 3 beta (LC3b) and gamma-aminobutyric acid receptor-associated protein-like 1 (Gabarapl1), as well as lysosomal hydrolases, cathepsin B and cathepsin L, was significantly and transiently increased after 1 and/or 3 days, suggesting that autophagosome formation would be increased in response to formoterol administration. However, this has to be relativized since the mRNA level of Unc-51-like kinase1 (Ulk1), BCL2/adenovirus E1B interacting protein3 (Bnip3), and transcription factor EB (TFEB), as well as the protein content of Ulk1, Atg13, Atg5-Atg12 complex and p62/Sqstm1 remained unchanged or was even decreased in response to formoterol administration. These results demonstrate that the effects of formoterol are mediated, in part, through the activation of Akt-mTOR pathway and that other signaling pathways become more important in the regulation of skeletal muscle mass with chronic administration of β2-agonists.

The worsening of tibialis anterior muscle atrophy during recovery post-immobilization correlates with enhanced connective tissue area, proteolysis, and apoptosis

Sustained muscle wasting due to immobilization leads to weakening and severe metabolic consequences. The mechanisms responsible for muscle recovery after immobilization are poorly defined. Muscle atrophy induced by immobilization worsened in the lengthened tibialis anterior (TA) muscle but not in the shortened gastrocnemius muscle. Here, we investigated some mechanisms responsible for this differential response. Adult rats were subjected to unilateral hindlimb casting for 8 days (I8). Casts were removed at I8, and animals were allowed to recover for 10 days (R1 to R10). The worsening of TA atrophy following immobilization occurred immediately after cast removal at R1 and was sustained until R10. This atrophy correlated with a decrease in type IIb myosin heavy chain (MyHC) isoform and an increase in type IIx, IIa, and I isoforms, with muscle connective tissue thickening, and with increased collagen (Col) I mRNA levels. Increased Col XII, Col IV, and Col XVIII mRNA levels during TA immobilization normalized at R6. Sustained enhanced peptidase activities of the proteasome and apoptosome activity contributed to the catabolic response during the studied recovery period. Finally, increased nuclear apoptosis prevailed only in the connective tissue compartment of the TA. Altogether, the worsening of the TA atrophy pending immediate reloading reflects a major remodeling of its fiber type properties and alterations in the structure/composition of the extracellular compartment that may influence its elasticity/stiffness. The data suggest that sustained enhanced ubiquitin-proteasome-dependent proteolysis and apoptosis are important for these adaptations and provide some rationale for explaining the atrophy of reloaded muscles pending immobilization in a lengthened position.

The time course of the adaptations of human muscle proteome to bed rest and the underlying mechanisms

In order to get a comprehensive picture of the complex adaptations of human skeletal muscle to disuse and further the understanding of the underlying mechanisms, we participated in two bed rest campaigns, one lasting 35 days and one 24 days. In the first bed rest (BR) campaign, myofibrillar proteins, metabolic enzymes and antioxidant defence systems were found to be down-regulated both post-8 days and post-35 days BR by proteomic analysis of vastus lateralis muscle samples from nine subjects. Such profound alterations occurred early (post-8 days BR), before disuse atrophy developed, and persisted through BR (post-35 days BR). To understand the mechanisms underlying the protein adaptations observed, muscle biopsies from the second bed rest campaign (nine subjects) were used to evaluate the adaptations of master controllers of the balance between muscle protein breakdown and muscle protein synthesis (MuRF-1 and atrogin-1; Akt and p70S6K), of autophagy (Beclin-1, p62, LC3, bnip3, cathepsin-L), of expression of antioxidant defence systems (NRF2) and of energy metabolism (PGC-1α, SREBP-1, AMPK). The results indicate that: (i) redox imbalance and remodelling of muscle proteome occur early and persist through BR; (ii) impaired energy metabolism is an early and persistent phenomenon comprising both the oxidative and glycolytic one; (iii) although both major catabolic systems, ubiquitin proteasome and autophagy, could contribute to the progression of atrophy late into BR, a decreased protein synthesis cannot be ruled out; (iv) a decreased PGC-1α, with the concurrence of SREBP-1 up-regulation, is a likely trigger of metabolic impairment, whereas the AMPK pathway is unaltered.

Various jobs of proteolytic enzymes in skeletal muscle during unloading: facts and speculations

Skeletal muscles, namely, postural muscles, as soleus, suffer from atrophy under disuse. Muscle atrophy development caused by unloading differs from that induced by denervation or other stimuli. Disuse atrophy is supposed to be the result of shift of protein synthesis/proteolysis balance towards protein degradation increase. Maintaining of the balance involves many systems of synthesis and proteolysis, whose activation leads to muscle adaptation to disuse rather than muscle degeneration. Here, we review recent data on activity of signaling systems involved in muscle atrophy development under unloading and muscle adaptation to the lack of support.

Disuse atrophy of human skeletal muscle: cell signaling and potential interventions

In response to atrophic stimuli, physical alterations include decreases in fiber diameter and contractile protein content. Despite the fact that these phenotypical alterations have been well characterized, the signaling pathways that mediate these adaptations are still under investigation. There have been significant advances in the past few years delineating signal transduction pathways that regulate protein turnover. In the process of evaluating the effect of various atrophy-inducing stimuli on signal transduction pathways in skeletal muscle, it is apparent that differences do exist concerning both transcriptional and translational adaptations. To this end, it is hypothesized that the processes responsible for invoking skeletal muscle atrophy are unique, despite similar upstream signals and downstream phenotypical adaptations. If this is the case, countermeasures to attenuate atrophy may be more effective if they are designed to accommodate molecular alterations specific to the atrophic stimulus. The aim of this review was to characterize the recent work in humans elucidating the molecular basis of skeletal muscle atrophy in response to immobilization, unloading, spinal cord injury, and detraining to highlight the possibility that all skeletal muscle atrophy is not the same. With an increased understanding of the unique signaling pathways that regulate skeletal muscle protein turnover in the face of various atrophy models, it is possible to exploit these pathways to develop countermeasures to prevent or attenuate atrophy. Eugenics, gene therapy, pharmacology, nutritional, and physical countermeasures are discussed concerning their potential to treat or mitigate atrophy.

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. β5 proteasome activity tended to increase in HU (+16.7 ± 6.1%, P = 0.086). Neither β1 and β2 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.

Lack of muscle recovery after immobilization in old rats does not result from a defect in normalization of the ubiquitin-proteasome and the caspase-dependent apoptotic pathways

Immobilization periods increase with age because of decreased mobility and/or because of increased pathological episodes that require bed-rest. Then, sarcopaenia might be partially explained by an impaired recovery of skeletal muscle mass after a catabolic state due to an imbalance of muscle protein metabolism, apoptosis and cellular regeneration. Mechanisms involved during muscle recovery have been little studied and in elderly they remain almost unknown. We show, in rats, that a short immobilization period during ageing initiated muscle atrophy that was indeed not recovered after 40 days. Immobilization was associated with an activation of both the ubiquitin-proteasome and the mitochondria-associated apoptotic pathways and the inflammatory and redox processes, and a decrease of cellular regeneration. We show that the lack of muscle recovery during ageing is not due to a defect in proteolysis or apoptosis down-regulation. These observations lead us to hypothesize that muscle protein synthesis activation after immobilization was altered during ageing.

Oxidative stress, apoptosis, and proteolysis in skeletal muscle repair after unloading

Although several lines of evidence link muscle-derived oxidants and inflammation to skeletal muscle wasting via regulation of apoptosis and proteolysis, little information is currently available on muscle repair. The present work was designed to study oxidative stress response, inflammatory cytokines, apoptotic, or proteolytic pathways during the early (1 and 5 days) and later (14 days) stages of the regrowth process subsequent to 14 days of hindlimb unloading. During the early stages of reloading, muscle mass recovery ( day 5) was facilitated by transcriptional downregulation ( day 1) of pathways involved in muscle proteolysis [μ-calpain, atrogin-1/muscle atrophy F-box (MAFbx), and muscle RING finger-1/(MuRF1) mRNA] and upregulation of an autophagy-related protein Beclin-1 ( day 5). At the same time, oxidative stress (glutathione vs. glutathione disulfide ratio, superoxide dismutase, catalase activities) remained still enhanced, whereas the increased uncoupling protein 3 gene expression recovered. Increased caspase-9 (mitochondrial-driven apoptosis) and decreased caspase-12 (sarcoplasmic reticulum-mediated apoptosis) activation was also normalized at early stages ( day 5). Conversely, the receptor-mediated apoptotic pathway initiated by ligand-induced (tumor necrosis factor-α, TNF-α) binding and promoting the activation of caspase-8 remained elevated until 14 days. Our data suggest that at early stages, muscle repair is mediated via the modulation of mitochondrial-driven apoptosis and muscle proteolysis. Despite full muscle mass recovery, oxidative stress and TNF-α-mediated apoptotic pathway are still activated till later stages of muscle remodeling.

Downregulation of Akt/mammalian target of rapamycin pathway in skeletal muscle is associated with increased REDD1 expression in response to chronic hypoxia

Although it is well established that chronic hypoxia leads to an inexorable loss of skeletal muscle mass in healthy subjects, the underlying molecular mechanisms involved in this process are currently unknown. Skeletal muscle atrophy is also an important systemic consequence of chronic obstructive pulmonary disease (COPD), but the role of hypoxemia in this regulation is still debated. Our general aim was to determine the molecular mechanisms involved in the regulation of skeletal muscle mass after exposure to chronic hypoxia and to test the biological relevance of our findings into the clinical context of COPD. Expression of positive and negative regulators of skeletal muscle mass were explored 1) in the soleus muscle of rats exposed to severe hypoxia (6,300 m) for 3 wk and 2) in vastus lateralis muscle of nonhypoxemic and hypoxemic COPD patients. In rodents, we observed a marked inhibition of the mammalian target of rapamycin (mTOR) pathway together with a strong increase in regulated in development and DNA damage response 1 (REDD1) expression and in its association with 14-3-3, a mechanism known to downregulate the mTOR pathway. Importantly, REDD1 overexpression in vivo was sufficient to cause skeletal muscle fiber atrophy in normoxia. Finally, the comparative analysis of skeletal muscle in hypoxemic vs. nonhypoxemic COPD patients confirms that hypoxia causes an inhibition of the mTOR signaling pathway. We thus identify REDD1 as a negative regulator of skeletal muscle mass during chronic hypoxia. Translation of this fundamental knowledge into the clinical investigation of COPD shows the interest to develop therapeutic strategies aimed at inhibiting REDD1.

Protective effects of exercise preconditioning on hindlimb unloading-induced atrophy of rat soleus muscle

AIM

A chronic decrease in the activation and loading levels of skeletal muscles as occurs with hindlimb unloading (HU) results in a number of detrimental changes. Several proteolytic pathways are involved with an increase in myofibrillar protein degradation associated with HU. Exercise can be used to counter this increase in proteolytic activity and, thus, may be able to protect against some of the detrimental changes associated with chronic decreased use. The purpose of the present study was to determine the potential of a single bout of preconditioning endurance exercise in attenuating the effects of 2 weeks of HU on the mass, phenotype and force-related properties of the soleus muscle in adult rats.

METHODS

Male Wistar rats were subjected to HU for 2 weeks. One half of the rats performed a single bout of treadmill exercise for 25 min immediately prior to the 2 weeks of HU.

RESULTS

Soleus mass, maximum tetanic tension, myofibrillar protein content, fatigue resistance and percentage of type I (slow) myosin heavy chain were decreased in HU rats. In addition, markers for the cathepsin, calpain, caspase and ATP-ubiquitin-proteasome proteolytic pathways were increased. The preconditioning endurance exercise bout attenuated all of the detrimental changes associated with HU, and increased HSP72 mRNA expression and protein levels.

CONCLUSION

These findings indicate that exercise preconditioning may be an effective countermeasure to the detrimental effects of chronic decreases in activation and loading levels on skeletal muscles and that an elevation in HSP72 may be one of the mechanisms associated with these responses.

Down-regulation of Akt/mammalian target of rapamycin signaling pathway in response to myostatin overexpression in skeletal muscle

Myostatin, a member of the TGF-beta family, has been identified as a master regulator of embryonic myogenesis and early postnatal skeletal muscle growth. However, cumulative evidence also suggests that alterations in skeletal muscle mass are associated with dysregulation in myostatin expression and that myostatin may contribute to muscle mass loss in adulthood. Two major branches of the Akt pathway are relevant for the regulation of skeletal muscle mass, the Akt/mammalian target of rapamycin (mTOR) pathway, which controls protein synthesis, and the Akt/forkhead box O (FOXO) pathway, which controls protein degradation. Here, we provide further insights into the mechanisms by which myostatin regulates skeletal muscle mass by showing that myostatin negatively regulates Akt/mTOR signaling pathway. Electrotransfer of a myostatin expression vector into the tibialis anterior muscle of Sprague Dawley male rats increased myostatin protein level and decreased skeletal muscle mass 7 d after gene electrotransfer. Using RT-PCR and immunoblot analyses, we showed that myostatin overexpression was ineffective to alter the ubiquitin-proteasome pathway. By contrast, myostatin acted as a negative regulator of Akt/mTOR pathway. This was supported by data showing that the phosphorylation of Akt on Thr308, tuberous sclerosis complex 2 on Thr1462, ribosomal protein S6 on Ser235/236, and 4E-BP1 on Thr37/46 was attenuated 7 d after myostatin gene electrotransfer. The data support the conclusion that Akt/mTOR signaling is a key target that accounts for myostatin function during muscle atrophy, uncovering a novel role for myostatin in protein metabolism and more specifically in the regulation of translation in skeletal muscle.

The ubiquitin-proteasome and the mitochondria-associated apoptotic pathways are sequentially downregulated during recovery after immobilization-induced muscle atrophy

Immobilization produces morphological, physiological, and biochemical alterations in skeletal muscle leading to muscle atrophy and long periods of recovery. Muscle atrophy during disuse results from an imbalance between protein synthesis and proteolysis but also between apoptosis and regeneration processes. This work aimed to characterize the mechanisms underlying muscle atrophy and recovery following immobilization by studying the regulation of the mitochondria-associated apoptotic and the ubiquitin-proteasome-dependent proteolytic pathways. Animals were subjected to hindlimb immobilization for 4-8 days (I4 to I8) and allowed to recover after cast removal for 10-40 days (R10 to R40). Soleus and gastrocnemius muscles atrophied from I4 to I8 to a greater extent than extensor digitorum longus and tibialis anterior muscles. Gastrocnemius muscle atrophy was first stabilized at R10 before being progressively reduced until R40. Polyubiquitinated proteins accumulated from I4, whereas the increased ubiquitination rates and chymotrypsin-like activity of the proteasome were detectable from I6 to I8. Apoptosome and caspase-3 or -9 activities increased at I6 and I8, respectively. The ubiquitin-proteasome-dependent pathway was normalized early when muscle stops to atrophy (R10). By contrast, the mitochondria-associated apoptotic pathway was first downregulated below basal levels when muscle started to recover at R15 and completely normalized at R20. Myf 5 protein levels decreased from I4 to I8 and were normalized at R10. Altogether, our results suggest a two-stage process in which the ubiquitin-proteasome pathway is rapidly up- and downregulated when muscle atrophies and recovers, respectively, whereas apoptotic processes may be involved in the late stages of atrophy and recovery.

Time-dependent changes in caspase-3 activity and heat shock protein 25 after spinal cord transection in adult rats

Chronic reductions in muscle activation and loading are associated with decreased heat shock protein 25 (Hsp25) expression and phosphorylation (pHsp25) which, in turn, may contribute to elevated caspase-3-mediated muscle protein breakdown. Thus, the purpose of the present study was to determine whether there are any changes in Hsp25, pHsp25 and caspase-3 activity among rat muscles having different fibre type compositions and functions [soleus, adductor longus (AL), plantaris and tibialis anterior (TA)] at 0 (control), 1, 8 or 28 days after a complete spinal cord transection (ST). The Hsp25 levels were unaffected on days 1 and 8 in all muscles, except for a significant reduction on day 8 in plantaris. The Hsp25 levels were lower than control values in all muscles except TA on day 28. The pHsp25 levels were lower than control values after 8 and 28 days in plantaris and AL and after 28 days in soleus, but higher than control in TA after 8 and 28 days. Caspase-3 activity was higher in ST than control rats on day 8 in all muscles except TA. Caspase-3 activity was negatively correlated with muscle mass for all muscles. In plantaris, Hsp25 and pHsp25 were negatively correlated with caspase-3 activity and Hsp25 was correlated with muscle mass. These relationships were not observed in other muscles. Thus, the effects of ST on Hsp25 and caspase-3 are muscle specific and time dependent, factors that should be considered in developing any intervention to maintain muscle mass after a spinal cord injury.