Ubiquitin targeting of rat muscle proteins during short periods of unloading

Losartan treatment attenuates hindlimb unloading-induced atrophy in the soleus muscle of female rats via canonical TGF-β signaling

We investigated the protective effect of losartan, an angiotensin II type 1 receptor blocker, on soleus muscle atrophy. Age-matched male and female Wistar rats were subjected to hindlimb unloading, and the soleus muscle was removed on days 1 and 7 for analysis. Females showed greater reductions in relative weight and myofiber cross-sectional area of the soleus muscle than males on day 7 post-hindlimb unloading. Losartan partially protected females against muscle atrophy. Activation of the canonical TGF-β signaling pathway, assessed via Smad2/3 phosphorylation, was lower in females following losartan treatment and associated with lower levels of protein ubiquitination after 1 (myofibril) and 7 (cytosol) days of unloading. However, no effect was observed in non-canonical TGF-β signaling (p44/p42 and p38 MAPK phosphorylation) in males or females during unloading. Our results suggest that losartan provides partial protection against hindlimb unloading-induced soleus muscle atrophy in female rats, possibly associated with decreased canonical TGF-β signaling.

Long-term physical inactivity exacerbates hindlimb unloading-induced muscle atrophy in young rat soleus muscle

This study investigated the effects of long-term physical inactivity in adolescent on subsequent hindlimb unloading-induced muscle atrophy in rat soleus muscle. First, 3-wk-old male Wistar rats were assigned to an age-matched control (n = 6) or a physical inactivity (n = 8) group. Rats in the physical inactivity group were housed in narrow cages with approximately half the usual floor space for 8 wk to limit range of movement. Whole body energy consumption was measured, and the blood, organs, femoral bone, and hindlimb muscles were removed. We found that long-term physical inactivity did not affect the metabolic and physiological characteristics of growing rats. Then, fifty-six 3-wk-old male Wistar rats were assigned randomly into control (n = 28) and physical inactivity (n = 28) groups. After 8 wk, the rats in both groups underwent hindlimb unloading. The soleus muscles were removed before unloading (0 day), and 1, 3, and 7 days after unloading (n = 7 for each). Although the soleus muscle weight was significantly decreased after 7 days of hindlimb unloading in both groups, the decrease was drastic in the inactive group. A significant interaction between inactivity and unloading (P < 0.01) was observed according to the 4-hydroxynonenal-conjugated protein levels and the histone deacetylase 4 (HDAC4) and NF-κB protein levels. HDAC4 and NF-κB p65 protein levels in the physical inactivity group increased significantly 1 day after hindlimb unloading, along with the mRNA levels of their downstream targets myogenin and muscle RING finger protein 1 (MuRF1). Subsequent protein ubiquitination was upregulated by long-term physical inactivity (P < 0.05).NEW & NOTEWORTHY Long-term physical inactivity exacerbates hindlimb unloading-induced disuse muscle atrophy in young rat soleus muscles, possibly mediated by oxidative stress-induced protein ubiquitination via HDAC4- and NF-κB p65-induced MuRF1 mRNA upregulation.

Exercise preconditioning attenuates hind limb unloading-induced gastrocnemius muscle atrophy possibly via the HDAC4/Gadd45 axis in old rats

The mechanisms involved in unloading-induced skeletal muscle loss may be age-specific, and the evidence for exercise preconditioning-induced protection against disuse muscle atrophy in aged rats is limited. Therefore, in this study, we investigated age-related differences in the activation of the HDAC4/Gadd45α pathway following hindlimb unloading (HU). We also assessed the protective effect of preconditioning exercise on this pathway in young and old rat gastrocnemius muscle. Three-month-old (young, n = 18) and 24-month-old (old, n = 18) male Wistar rats were assigned to the following groups: control group (n = 6), seven days of HU group (n = 6), and a bout of exercise preconditioning prior to HU (Ex+HU) group (n = 6). Rats of both ages in the Ex + HU group ran continuously on a motor-driven treadmill (0° slope, 20 m/min, 15 min) prior to HU. The gastrocnemius muscles were removed after 7 days of HU and analyzed for protein content and mRNA expression. Gastrocnemius muscle weight was significantly higher in the Ex+HU group than in the HU group of old rats, but not in young rats. Levels of HDAC4 protein and mRNA were significantly increased in the old HU group. However, the increase was significantly suppressed in the old Ex+HU group. Moreover, the protective effect of exercise preconditioning had a positive effect on Gadd45α mRNA and protein levels only in the old Ex+HU group. No exercise preconditioning-related protection was observed in the young rats. Our data indicated that a single bout of preconditioning exercise prior to HU may exert a protective effect in disuse muscle atrophy in old rats and that these effects may be partially mediated by the HDAC4/Gadd45α axis.

Sex differences in forkhead box O3a signaling response to hindlimb unloading in rat soleus muscle

We tested the hypothesis that there are sex differences in hindlimb unloading-induced activation of the forkhead box subfamily O3a (FoxO3a) signaling pathway in rat soleus muscle. Age-matched male and female Wistar rats were subjected to hindlimb unloading, and the soleus muscle was removed before or 1 or 7 days after unloading. Female rats showed greater percent changes in relative soleus muscle weight than males. FoxO3a phosphorylation was lower in females than in males and was associated with higher levels of protein ubiquitination 7 days after unloading. Heat shock protein 72 (Hsp72) levels were lower in female rats and increased in males during unloading. Female rats showed slightly higher myostatin levels, which showed a non-significant decline in male rats following unloading. Thus, males and females show different responses to the FoxO3a/ubiquitin-proteasome pathway following hindlimb unloading in rat soleus muscle, which may be associated with differences in Hsp72 expression and myostatin signaling.

Muscle-specific activation of calpain system in hindlimb unloading rats and hibernating Daurian ground squirrels: a comparison between artificial and natural disuse

To determine whether the regulation of calpain system is involved in non-hibernators and hibernators in disused condition, the soleus (SOL) and extensor digitorum longus (EDL) muscles were used for investigating the muscle mass, the ratio of muscle wet weight/body weight (MWW/BW), fiber-type distribution, fiber cross-sectional area (CSA), and the protein expression of MuRF1, calpain-1, calpain-2, calpastatin, desmin, troponin T, and troponin C in hindlimb unloading rats and hibernating Daurian ground squirrels. The muscle mass, MWW/BW, and fiber CSA were found significantly decreased in SOL and EDL of hindlimb unloading rats, but unchanged in hibernating ground squirrels. The MuRF1 expression was increased in both SOL and EDL of unloading rats, while it was only increased in SOL, but maintained in EDL of hibernating ground squirrels. The expression levels of calpain-1 and calpain-2 were increased in different degrees in unloaded SOL and EDL in rats, while they were maintained in EDL and even reduced in SOL of hibernating ground squirrels. Besides, the expression of calpastatin was decreased in unloaded rats, but increased in hibernating ground squirrels. The desmin expression was decreased in unloaded rats, but maintained in hibernating squirrels. Interestingly, the levels of troponin T and troponin C were decreased in both SOL and EDL of unloaded rats, but increased in hibernating ground squirrels with muscle-type specificity. In conclusion, differential calpain activation and substrate-selective degradation in slow and fast muscles are involved in the mechanisms of muscle atrophy of unloaded rats and remarkable ability of muscle maintenance of hibernating ground squirrels.

Dietary astaxanthin supplementation attenuates disuse-induced muscle atrophy and myonuclear apoptosis in the rat soleus muscle

Extended periods of skeletal muscle disuse results in muscle atrophy and weakness. Currently, no therapeutic treatment is available for the prevention of this problem. Nonetheless, growing evidence suggests that prevention of disuse-induced oxidative stress in inactive muscle fibers can delay inactivity-induced muscle wasting. Therefore, this study tested the hypothesis that dietary supplementation with the antioxidant astaxanthin would protect against disuse muscle atrophy, in part, by prevention of myonuclear apoptosis. Wistar rats (8 weeks old) were divided into control (CT, n = 9), hindlimb unloading (HU, n = 9), and hindlimb unloading with astaxanthin (HU + AX, n = 9) groups. Following 2 weeks of dietary supplementation, rats in the HU and HU + AX groups were exposed to unloading for 7 days. Seven-day unloading resulted in reduced soleus muscle weight and myofiber cross-sectional area (CSA) by ~30 and ~47 %, respectively. Nonetheless, relative muscle weights and CSA of the soleus muscle in the HU + AX group were significantly greater than those of the HU group. Moreover, astaxanthin prevented disuse-induced increase in the number of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive nuclei. We conclude that astaxanthin supplementation prior to and during hindlimb unloading attenuates soleus muscle atrophy, in part, by suppressing myonuclear apoptosis.

The response of apoptotic and proteolytic systems to repeated heat stress in atrophied rat skeletal muscle

We examined the effect of repeated heat stress on muscle atrophy, and apoptotic and proteolytic regulation in unloaded rat slow- and fast-type skeletal muscles. Forty male Wistar rats (11 week-old) were divided into control (CT), hindlimb unweighting (HU), intermittent weight-bearing during HU (HU + IWB), and intermittent weight-bearing with heat stress during HU (41–41.5°C for 30 min; HU + IWB + HS) groups. The HU + IWB + HS and HU + IWB groups were released from unloading for 1 h every second day, during which the HU + IWB + HS group underwent the heating. Our results revealed that repeated bouts of heat stress resulted in protection against disuse muscle atrophy in both soleus and plantaris muscles. This heat stress–induced protection against disuse-induced muscular atrophy may be partially due to reduced apoptotic activation in both muscles, and decreased ubiquitination in only the soleus muscle. We concluded that repeated heat stress attenuated skeletal muscle atrophy via suppressing apoptosis but the response to proteolytic systems depend on the muscle phenotype.

Electrical stimulation using sine waveform prevents unloading-induced muscle atrophy in the deep calf muscles of rat

The aim of this study was to compare the effects of electrical stimulation by using rectangular and sine waveforms in the prevention of deep muscle atrophy in rat calf muscles. Rats were randomly divided into the following groups: control, hindlimb unloading (HU), and HU plus electrical stimulation (ES). The animals in the ES group were electrically stimulated using rectangular waveform (RS) on the left calves and sine waveform (SS) on the right calves, twice a day, for 2 weeks during unloading. HU for 2 weeks resulted in a loss of the muscle mass, a decrease in the cross-sectional area of the muscle fibers, and overexpression of ubiquitinated proteins in the gastrocnemius and soleus muscles. In contrast, electrical stimulation with RS attenuated the HU-induced reduction in the cross-sectional area of muscle fibers and the increase of ubiquitinated proteins in the gastrocnemius muscle. However, electrical stimulation with RS failed to prevent muscle atrophy in the deep portion of the gastrocnemius and the soleus muscles. Nevertheless, electrical stimulation with SS attenuated the HU-induced muscle atrophy and the up-regulation of ubiquitinated proteins in both gastrocnemius and soleus muscles. This indicates that SS was more effective in the prevention of deep muscle atrophy than RS. Since the skin muscle layers act like the plates of a capacitor, separated by the subcutaneous adipose layer, the SS can pass through this capacitor more easily than the RS. Hence, SS can prevent the progressive loss of muscle fibers in the deep portion of the calf muscles.

Motor protein function in skeletal abdominal muscle of cachectic cancer patients

Cachexia presents with ongoing muscle wasting, altering quality of life in cancer patients. Cachexia is a limiting prognostic factor for patient survival and health care costs. Although animal models and human trials have shown mechanisms of motorprotein proteolysis, not much is known about intrinsic changes of muscle functionality in cancer patients suffering from muscle cachexia, and deeper insights into cachexia pathology in humans are needed. To address this question, rectus abdominis muscle samples were collected from several surgical control, non-cachectic and cachectic cancer patients and processed for skinned fibre biomechanics, molecular in vitro motility assays, myosin isoform protein compositions and quantitative ubiquitin polymer protein analysis. In pre-cachectic and cachectic cancer patient samples, maximum force was significantly compromised compared with controls, but showed an unexpected increase in myofibrillar Ca²⁺ sensitivity consistent with a shift from slow to fast myosin isoform expression seen in SDS-PAGE analysis and in vitro motility assays. Force deficit was specific for ‘cancer’, but not linked to presence of cachexia. Interestingly, quantitative ubiquitin immunoassays revealed no major changes in static ubiquitin polymer protein profiles, whether cachexia was present or not and were shown to mirror profiles in control patients. Our study on muscle function in cachectic patients shows that abdominal wall skeletal muscle in cancer cachexia shows signs of weakness that can be partially attributed to intrinsic changes to contractile motorprotein function. On protein levels, static ubiquitin polymeric distributions were unaltered, pointing towards evenly up-regulated ubiquitin protein turnover with respect to ubiquitin conjugation, proteasome degradation and de-ubiquitination.

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.

Seven days of muscle re-loading and voluntary wheel running following hindlimb suspension in mice restores running performance, muscle morphology and metrics of fatigue but not muscle strength

In this study, we examined the effects of 2-week hindlimb un-loading in mice followed by re-ambulation with voluntary access to running wheels. The recovery period was terminated at a time point when physical performance--defined by velocity, time, and distance ran per day--of the suspended group matched that of an unsuspended group. Mice were assigned to one of four groups: unsuspended non-exercise (Control), 14 days of hindlimb suspension (HS), 7 days of access to running wheels (E7), or 14 days of HS plus 7 days access to running wheels (HSE7). HS resulted in significant decreases in body and muscle mass, hindlimb strength, soleus force, soleus specific force, fatigue resistance, and fiber cross sectional area (CSA). Seven days of re-ambulation with access to running wheels following HS recovered masses to Control values, increased fiber CSA, increased resistance to fatigue and improved recovery from fatigue in the soleus. HS resulted in a myosin heavy chain (MHC) phenotype shift from slow toward fast-twitch fibers, though running alone did not influence the expression of MHC fibers. Compared to the Control group, HSE7 mice did not recover functional hindlimb strength as assessed through measurements either in vivo or ex vivo. Results from this study demonstrate that 7 days of muscle re-loading with access to wheel-running following HS can stimulate muscle to regain mass and fiber CSA and exhibit improved metrics of fatigue resistance and recovery, yet muscles remain impaired in regard to strength. Understanding this mismatch between muscle morphology and strength may prove of value in designing effective exercise protocols for disuse muscle atrophy rehabilitation.

Aging impairs the expression of the catalytic subunit of glutamate cysteine ligase in soleus muscle under stress

This study investigated the mechanisms responsible for the disrupted homeostasis of reduced glutathione (GSH) in aging muscles with stress (14 days of hind-limb unloading [HU]). Adult and old rats were randomized into four groups: weight bearing and 3, 7, and 14 days of HU. Soleus muscles were harvested to investigate the activity or content of enzymes involved in GSH metabolism (utilization and synthesis). The activities of glutathione S transferase, glutathione reductase, gamma-glutamyl transpeptidase, and glutamate cysteine ligase (GCL) were determined. The protein content of the two subunits of GCL, catalytic subunit (GCLC) and modifier subunit (GCLM), were evaluated. The major results, failure to maintain the accelerated GCLC production and GCL activity, are associated with the GSH depletion in aging muscles with 14 days of HU. The results suggest that the regulation of GCL, especially the catalytic subunit, with stress may be compromised in aging muscles.

Molecular events and signalling pathways involved in skeletal muscle disuse-induced atrophy and the impact of countermeasures

Disuse-induced skeletal muscle atrophy occurs following chronic periods of inactivity such as those involving prolonged bed rest, trauma and microgravity environments. Deconditioning of skeletal muscle is mainly characterized by a loss of muscle mass, decreased fibre cross-sectional area, reduced force, increased fatigability, increased insulin resistance and transitions in fibre types. A description of the role of specific transcriptional mechanisms contributing to muscle atrophy by altering gene expression during muscle disuse has recently emerged and focused primarily on short period of inactivity. A better understanding of the transduction pathways involved in activation of proteolytic and apoptotic pathways continues to represent a major objective, together with the study of potential cross-talks in these cellular events. In parallel, evaluation of the impact of countermeasures at the cellular and molecular levels in short- and long-term disuse experimentations or microgravity environments should undoubtedly and synergistically increase our basic knowledge in attempts to identify new physical, pharmacological and nutritional targets to counteract muscle atrophy. These investigations are important as skeletal muscle atrophy remains an important neuromuscular challenge with impact in clinical and social settings affecting a variety of conditions such as those seen in aging, cancer cachexia, muscle pathologies and long-term space exploration.

Alterations of protein turnover underlying disuse atrophy in human skeletal muscle

Unloading-induced atrophy is a relatively uncomplicated form of muscle loss, dependent almost solely on the loss of mechanical input, whereas in disease states associated with inflammation (cancer cachexia, AIDS, burns, sepsis, and uremia), there is a procatabolic hormonal and cytokine environment. It is therefore predictable that muscle loss mainly due to disuse alone would be governed by mechanisms somewhat differently from those in inflammatory states. We suggest that in vivo measurements made in human subjects using arterial-venous balance, tracer dilution, and tracer incorporation are dynamic and thus robust by comparison with static measurements of mRNA abundance and protein expression and/or phosphorylation in human muscle. In addition, measurements made with cultured cells or in animal models, all of which have often been used to infer alterations of protein turnover, appear to be different from results obtained in immobilized human muscle in vivo. In vivo measurements of human muscle protein turnover in disuse show that the primary variable that changes facilitating the loss of muscle mass is protein synthesis, which is reduced in both the postabsorptive and postprandial states; muscle proteolysis itself appears not to be elevated. The depressed postprandial protein synthetic response (a phenomenon we term "anabolic resistance") may even be accompanied by a diminished suppression of proteolysis. We therefore propose that most of the loss of muscle mass during disuse atrophy can be accounted for by a depression in the rate of protein synthesis. Thus the normal diurnal fasted-to-fed cycle of protein balance is disrupted and, by default, proteolysis becomes dominant but is not enhanced.

Large-scale mRNA analysis of female skeletal muscles during 60 days of bed rest with and without exercise or dietary protein supplementation as countermeasures

Microgravity has a dramatic impact on human physiology, illustrated in particular, with skeletal muscle impairment. A thorough understanding of the mechanisms leading to loss of muscle mass and structural disorders is necessary for defining efficient clinical and spaceflight countermeasures. We investigated the effects of long-term bed rest on the transcriptome of soleus (SOL) and vastus lateralis (VL) muscles in healthy women (BRC group, n = 8), and the potential beneficial impact of protein supplementation (BRN group, n = 8) and of a combined resistance and aerobic training (BRE group, n = 8). Gene expression profiles were obtained using a customized microarray containing 6,681 muscles-relevant genes. A two-class statistical analysis was applied on 2,103 genes with consolidated expression in BRC, BRN, and BRE groups. We identified 472 and 207 mRNAs whose expression was modified in SOL and VL from BRC group, respectively. Further clustering analysis, identifying relevant biological mechanisms and pathways, reported five main subclusters. Three are composed of upregulated mRNAs involved mainly in nucleic acid and protein metabolism, and two made up of downregulated transcripts encoding components involved in energy metabolism. Exercise countermeasure demonstrated drastic compensatory effects, decreasing the number of differentially expressed mRNAs by 89 and 96% in SOL and VL, respectively. In contrast, nutrition countermeasure had moderate effects and decreased the number of differentially-expressed transcripts by 40 and 25% in SOL and VL. Together, these data present a systematic, global and comprehensive view of the adaptive response of female muscle to long-term atrophy.

Neuromuscular electrical stimulation reduces skeletal muscle protein degradation and stimulates insulin-like growth factors in an age- and current-dependent manner: a randomized, controlled clinical trial in major abdominal surgical patients

OBJECTIVE

To investigate the effect of neuromuscular electrical stimulation (NMES) on skeletal muscle metabolism after major abdominal surgery.

SUMMARY BACKGROUND DATA

Protein catabolism associated with surgical interventions leads to reduced muscle strength, increased clinical complications and prolonged convalescence. Immobilization is suggested as a major stimulus for muscle wasting. This study investigates the potency of NMES on skeletal muscle growth factors and degradation processes in surgical patients.

METHODS

This observer blind study included 26 patients after major abdominal surgery mainly due to cancer aged 60 +/- 10 years. Starting on the first postoperative day, 1 randomly assigned thigh of each patient was treated on 4 consecutive days with NMES, whereas the other leg was used as sham-stimulated control. Thereafter, muscle biopsies from both legs were performed. Differences in mRNA level, protein expression, and enzyme activity between legs were analyzed by cross-over analysis of variance (Clinical Trial Registration Number: NCT00635440).

RESULTS

NMES significantly increased total RNA content and total sarcoplasmatic protein content. NMES significantly reduced ubiquitin-conjugated sarcoplasmatic proteins and proteasome activity. The mechano growth factor mRNA level correlated positively with the applied current and negatively with the body mass index of the patients. The increase in insulin like growth factor-1Ea mRNA after NMES correlated negatively with the age of the patients.

CONCLUSIONS

This study shows that NMES significantly increases total RNA content and reduces protein degradation in postoperative patients. Moreover, the induction of growth factors by NMES reveals dependency on body mass index, age, and applied current. We conclude that NMES is a useful clinical tool to reduce protein catabolism in postoperative patients.

Carbonic anhydrase III and four-and-a-half LIM protein 1 are preferentially oxidized with muscle unloading

The identities of proteins that show disuse-related changes in the content of oxidative modification are unknown. Furthermore, it is unknown whether the global accumulation of oxidized proteins is greater in aged animals with muscle disuse. The purposes of this study are 1) to identify the exact proteins that show disuse-related changes in oxidation levels and 2) to test the hypothesis that the global accumulation of oxidized proteins with muscle disuse would be greater in aged animals. Adult and old rats were randomized into four groups: weight bearing and 3, 7, or 14 days of hindlimb unloading. Soleus muscles were harvested to investigate the protein oxidation with unloading. Slot blot, SDS-PAGE, and Western blot analyses were used to detect the accumulation of 4-hydroxy-2-nonenol (HNE)- and nitrotyrosine (NT)-modified proteins. Matrix-assisted laser desorption ionization-time of flight and tandem mass spectroscopy were used to identify modified proteins. We found that global HNE- and NT-modified proteins accumulated significantly with aging but not with muscle unloading. Two HNE and NT target proteins, four-and-a-half LIM protein 1 (FHL1) and carbonic anhydrase III (CAIII), showed changes in the oxidation levels with muscle unloading. The changes in the oxidation levels happened to adult rats but not old rats. However, old rats had higher baseline levels of HNE-modified FHL1. In summary, the data suggest that the muscle unloading-related changes of protein oxidation are more significant in specific proteins and that the changes are age related.

Muscle disuse: adaptation of antioxidant systems is age dependent

This study investigated the age effect on antioxidant adaptation to muscle disuse. Adult and old rats were randomized into 4 groups: weight bearing (control), 3 days of hind-limb unloading (HU), 7 days of HU, and 14 days of HU. Activities of Cu-Zn superoxide dismutase (SOD), catalase, and glutathione (GSH), as well as GSH peroxidase levels were measured in the soleus. Neither disuse nor aging changed the activity of Cu-Zn SOD. The old rats had greater GSH peroxidase activity, whereas the activity of catalase had a compensatory increase with disuse, independent of age. Reduced GSH level and total glutathione (tGSH) level had age-related change with disuse. In old rats, the GSH and tGSH levels were lower with disuse, whereas the levels remained stable with disuse in adult rats. The depletion of intracellular GSH and tGSH levels of muscles from aged animals with disuse may make aged muscles more susceptible to oxidative damage.

[Cellular regulation of anabolism and catabolism in skeletal muscle during immobilisation, aging and critical illness]

Skeletal muscle atrophy is associated with situations of acute and chronical illness, such as sepsis, surgery, trauma and immobility. Additionally, it is a common problem during the physiological process of aging. The myofibrillar proteins myosin and actin, which are essential for muscle contraction, are the major targets during the process of protein degradation. This leads to a general loss of muscle mass, muscle strength and to increased muscle fatigue. In critically ill or immobile patients skeletal muscle atrophy is accompanied by enhanced inflammation, reduced wound healing, weaning complications and difficulties in mobilisation. During aging it results in falls, fractures, physical injuries and loss of mobility. Relating to the primary stimulators - hormones, muscle lengthening, stress, inflammation, neuronal activity - research is now focusing on the investigation of the signal transduction pathways, which influence protein synthesis and protein degradation during skeletal muscle atrophy.

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

In the present study, we determined the impact of 5 and 10 days of muscle deconditioning induced by hindlimb suspension (HS) on the ubiquitin-proteasome system of protein degradation and caspase enzyme activities in rat soleus muscles. A second goal was to determine whether activities of matrix metalloproteinase-2/9 (MMP-2/9) and urokinase-type/tissue-type plasminogen activator (PAs) were responsive to HS. As expected, HS led to a pronounced atrophy of soleus muscle. Level of ubiquitinated proteins, chymotrypsin-like activity of 20S proteasome, and Bcl-2-associated gene product-1 protein level were all transitory increased in response to 5 days of HS. These changes may thus potentially account for the decrease in muscle mass observed in response to 5 days of HS. Caspase-3 activity was significantly increased throughout the experimental period, whereas activities of caspase-6, another effector caspase, and caspase-9, the mitochondrial-dependent activator of both caspase-3 and -6, were only increased in response to 10 days of HS. This suggests that caspase-3 may be regulated through mitochondrial-independent and mitochondrial-dependent mechanisms in response to HS. Finally, MMP-2/9 activities remained unchanged, whereas PAs activities were increased after 5 days of HS. Overall, these data suggest that time-dependent regulation of intracellular and extracellular proteinases are important in setting the new phenotype of rat soleus muscle in response to HS.

Role for IκBα, but not c-Rel, in skeletal muscle atrophy

Skeletal muscle atrophy is associated with a marked and sustained activation of nuclear factor-κB (NF-κB) activity. Previous work showed that p50 is one of the NF-κB family members required for this activation and for muscle atrophy. In this work, we tested whether another NF-κB family member, c-Rel, is required for atrophy. Because endogenous inhibitory factor κBα (IκBα) was activated (i.e., decreased) at 3 and 7 days of muscle disuse (i.e., hindlimb unloading), we also tested if IκBα, which binds and retains Rel proteins in the cytosol, is required for atrophy and intermediates of the atrophy process. To do this, we electrotransferred a dominant negative IκBα (IκBαΔN) in soleus muscles, which were either unloaded or weight bearing. IκBαΔN expression abolished the unloading-induced increase in both NF-κB activation and total ubiquitinated protein. IκBαΔN inhibited unloading-induced fiber atrophy by 40%. The expression of certain genes known to be upregulated with atrophy were significantly inhibited by IκBαΔN expression during unloading, including MAFbx/atrogin-1, Nedd4, IEX, 4E-BP1, FOXO3a, and cathepsin L, suggesting these genes may be targets of NF-κB transcription factors. In contrast, c-Rel was not required for atrophy because the unloading-induced markers of atrophy were the same in c-rel−/−and wild-type mice. Thus IκBα degradation is required for the unloading-induced decrease in fiber size, the increase in protein ubiquitination, activation of NF-κB signaling, and the expression of specific atrophy genes, but c-Rel is not. These data represent a significant advance in our understanding of the role of NF-κB/IκB family members in skeletal muscle atrophy, and they provide new candidate NF-κB target genes for further study.

Differential localization of autolyzed calpains 1 and 2 in slow and fast skeletal muscles in the early phase of atrophy

Calpains have been proposed to be involved in the cytoskeletal remodeling and wasting of skeletal muscle. However, limited data are available about the specific involvement of each calpain in the early stages of muscle atrophy. The aims of this study were to determine whether calpains 1 and 2 are autolyzed after a short period of muscle disuse, and, if so, where in the myofibers the autolyzed products are localized. In the rat soleus muscle, 5 days of immobilization increased autolyzed calpain 1 in the particulate and not the soluble fraction. Conversely, autolyzed calpain 2 was not found in the particulate fraction, whereas it was increased in the soluble fraction after immobilization. In the less atrophied plantaris muscle, no difference was noted between the control and immobilized groups whatever the fraction or calpain. Other proteolytic pathways were also investigated. The ubiquitin-proteasome pathway was activated in both skeletal muscles, and caspase 3 was activated only in the soleus muscle. Taken together, our data suggest that calpains 1 and 2 are involved in atrophy development in slow type muscle exclusively and that they have different regulation and protein targets. Moreover, the activation of proteolytic pathways appears to differ in slow and fast muscles, and the proteolytic mechanisms involved in fast-type muscle atrophy remain unclear.

Isometric resistance exercise fails to counteract skeletal muscle atrophy processes during the initial stages of unloading

This study tested the hypothesis that an isometric resistance training paradigm targeting the medial gastrocnemius of adult rodents is effective in preventing muscle atrophy during the early stages of hindlimb unloading by maintaining normal activation of the insulin receptor substrate-1 (IRS-1)/phosphoinositide-3 kinase (PI3K)/Akt signaling pathway. This pathway has been shown to simultaneously create an anabolic response while inhibiting processes upregulating catabolic processes involving expression of key enzymes in the ubiquitination of proteins for degradation. The findings show that during the 5 days of unloading 1) absolute medial gastrocnemius muscle weight reduction occurred by ∼20%, but muscle weight corrected to body weight was not different from normal weight-bearing controls ( P < 0.05); 2) normalized myofibril fraction concentration and content were decreased; and 3) a robust isometric training program, known to induce a hypertrophy response, failed to maintain the myofibril protein content. This response occurred despite fully blunting the increases in the mRNA for of atrogin-1, MURF-1, and myostatin, e.g., sensitive gene markers of an activated catabolic state. Analyses of the IRS-1/PI3K/Akt markers indicated that abundance of IRS-1 and phosphorylation state of Akt and p70S6 kinase were decreased relative to normal control rats, and the resistance training failed to maintain these signaling markers at normal regulatory level. Our findings suggest that to fully prevent muscle atrophy responses affecting the myofibril system during unloading, the volume of mechanical stress must be augmented sufficiently to maintain optimal activity of the IRS-1/PI3K/Akt pathway to provide an effective anabolic stimulus on the muscle.

Determinants of Disuse-Induced Skeletal Muscle Atrophy: Exercise and Nutrition Countermeasures to Prevent Protein Loss

Muscle atrophy results from a variety of conditions such as disease states, neuromuscular injuries, disuse, and aging. Absence of gravitational loading during spaceflight or long-term bed rest predisposes humans to undergo substantial loss of muscle mass and, consequently, become unfit and/or unhealthy. Disuse- or inactivity-induced skeletal muscle protein loss takes place by differential modulation of proteolytic and synthetic systems. Transcriptional, translational, and posttranslational events are involved in the regulation of protein synthesis and degradation in myofibers, and these regulatory events are known to be responsive to contractile activity. However, regardless of the numerous studies which have been performed, the intracellular signals that mediate skeletal muscle wasting due to muscular disuse are not completely comprehended. Understanding the triggers of atrophy and the mechanisms that regulate protein loss in unloaded muscles may lead to the development of effective countermeasures such as exercise and dietary intervention. The objective of the present review is to provide a window into the molecular processes that underlie skeletal muscle remodeling and to examine what we know about exercise and nutrition countermeasures designed to minimize muscle atrophy.