Regulation of proteolysis during reloading of the unweighted soleus muscle

Bed-rest and exercise remobilization: Concurrent adaptations in muscle glucose and protein metabolism

BACKGROUND

Bed-rest (BR) of only a few days duration reduces muscle protein synthesis and induces skeletal muscle atrophy and insulin resistance, but the scale and juxtaposition of these events have not been investigated concurrently in the same individuals. Moreover, the impact of short-term exercise-supplemented remobilization (ESR) on muscle volume, protein turnover and leg glucose uptake (LGU) in humans is unknown.

METHODS

Ten healthy males (24 ± 1 years, body mass index 22.7 ± 0.6 kg/m2) underwent 3 days of BR, followed immediately by 3 days of ESR consisting of 5 × 30 maximal voluntary single-leg isokinetic knee extensions at 90°/s each day. An isoenergetic diet was maintained throughout the study (30% fat, 15% protein and 55% carbohydrate). Resting LGU was calculated from arterialized-venous versus venous difference across the leg and leg blood flow during the steady-state of a 3-h hyperinsulinaemic-euglycaemic clamp (60 mU/m2/min) measured before BR, after BR and after remobilization. Glycogen content was measured in vastus lateralis muscle biopsy samples obtained before and after each clamp. Leg muscle volume (LMV) was measured using magnetic resonance imaging before BR, after BR and after remobilization. Cumulative myofibrillar protein fractional synthetic rate (FSR) and whole-body muscle protein breakdown (MPB) were measured over the course of BR and remobilization using deuterium oxide and 3-methylhistidine stable isotope tracers that were administered orally.

RESULTS

Compared with before BR, there was a 45% decline in insulin-stimulated LGU (P < 0.05) after BR, which was paralleled by a reduction in insulin-stimulated leg blood flow (P < 0.01) and removal of insulin-stimulated muscle glycogen storage. These events were accompanied by a 43% reduction in myofibrillar protein FSR (P < 0.05) and a 2.5% decrease in LMV (P < 0.01) during BR, along with a 30% decline in whole-body MPB after 2 days of BR (P < 0.05). Myofibrillar protein FSR and LMV were restored by 3 days of ESR (P < 0.01 and P < 0.01, respectively) but not by ambulation alone. However, insulin-stimulated LGU and muscle glycogen storage were not restored by ESR.

CONCLUSIONS

Three days of BR caused concurrent reductions in LMV, myofibrillar protein FSR, myofibrillar protein breakdown and insulin-stimulated LGU, leg blood flow and muscle glycogen storage in healthy, young volunteers. Resistance ESR restored LMV and myofibrillar protein FSR, but LGU and muscle glycogen storage remained depressed, highlighting divergences in muscle fuel and protein metabolism. Furthermore, ambulation alone did not restore LMV and myofibrillar protein FSR in the non-exercised contralateral limb, emphasizing the importance of exercise rehabilitation following even short-term BR.

Gaining insight into the role of FoxO1 in the progression of disuse-induced skeletal muscle atrophy

Expression of FoxO transcription factors increases during certain forms of atrophy. In a dephosphorylated state, FoxOs participate in ubiquitin-mediated proteasomal degradation through the transcriptional activation of E3-ubiquitin ligases such as MAFbx/atrogin-1 and MuRF1. There is exhaustive research demonstrating that FoxO3a is sufficient to induce MAFbx/atrogin-1 and MuRF-1 expressions. In contrast, the data are conflicting on the requirement of FoxO1 signaling in the activation of the E3-ubiquitin ligases. Moreover, no reports currently exist on the particular role of FoxO1 in the molecular mechanisms involved in the progression of physiological muscle wasting. Here, we have applied the most extensively used rodent model of microgravity/functional unloading to stimulate disuse-induced skeletal muscle atrophy such as rat hindlimb suspension (HS). We showed that inhibition of FoxO1 activity by a selective inhibitor AS1842856 completely reversed an increase in expression of MuRF-1, but not MAFbx/atrogin-1, observed upon HS. Furthermore, we demonstrated that FoxO1 induced upregulation of another E3-ubiquitin-ligase of a MuRF protein family MuRF-2 in skeletal muscle subjected to disuse. Prevention of the MuRF increase upon HS impeded upregulation of transcript expression of a negative regulator of NFATc1 pathway calsarcin-2, which was associated with a partial reversion of MyHC-IId/x and MyHC-IIb mRNA expressions. Importantly, FoxO1 inhibition induced a marked increase in p70S6k phosphorylation, an important stage in the initiation of protein translation, concomitant with the restoration of global protein synthesis in the skeletal muscle of the HS rats. Examination of eIF3f expression and the eEF2k/eEF2 pathway, other factors controlling translation initiation and elongation respectively, did not reveal any impact of FoxO1 on their activity. Lastly, we observed a decrease in transcript levels of Sesn3, but not Sesn1 and Sesn2, upon disuse, which was completely reversed by FoxO1 inhibition. These data demonstrate that FoxO1 signaling contributes to the development of disuse-induced skeletal muscle atrophy, including slow to fast MyHC isoform shift, mostly through upregulation of MuRF-1 and MuRF-2 expression. Furthermore, FoxO1 inhibition is required to recover Sesn3 mRNA expression in atrophic conditions, which likely contributes to the enhanced p70S6k activity and restoration of the protein synthesis rate.

Skeletal Muscle Recovery from Disuse Atrophy: Protein Turnover Signaling and Strategies for Accelerating Muscle Regrowth

Skeletal muscle fibers have a unique capacity to adjust their metabolism and phenotype in response to alternations in mechanical loading. Indeed, chronic mechanical loading leads to an increase in skeletal muscle mass, while prolonged mechanical unloading results in a significant decrease in muscle mass (muscle atrophy). The maintenance of skeletal muscle mass is dependent on the balance between rates of muscle protein synthesis and breakdown. While molecular mechanisms regulating protein synthesis during mechanical unloading have been relatively well studied, signaling events implicated in protein turnover during skeletal muscle recovery from unloading are poorly defined. A better understanding of the molecular events that underpin muscle mass recovery following disuse-induced atrophy is of significant importance for both clinical and space medicine. This review focuses on the molecular mechanisms that may be involved in the activation of protein synthesis and subsequent restoration of muscle mass after a period of mechanical unloading. In addition, the efficiency of strategies proposed to improve muscle protein gain during recovery is also discussed.

Skeletal muscle atrogenes: From rodent models to human pathologies

Skeletal muscle atrophy is a common side effect of most human diseases. Muscle loss is not only detrimental for the quality of life but it also dramatically impairs physiological processes of the organism and decreases the efficiency of medical treatments. While hypothesized for years, the existence of an atrophying programme common to all pathologies is still incompletely solved despite the discovery of several actors and key regulators of muscle atrophy. More than a decade ago, the discovery of a set of genes, whose expression at the mRNA levels were similarly altered in different catabolic situations, opened the way of a new concept: the presence of atrogenes, i.e. atrophy-related genes. Importantly, the atrogenes are referred as such on the basis of their mRNA content in atrophying muscles, the regulation at the protein level being sometimes more complicate to elucidate. It should be noticed that the atrogenes are markers of atrophy and that their implication as active inducers of atrophy is still an open question for most of them. While the atrogene family has grown over the years, it has mostly been incremented based on data coming from rodent models. Whether the rodent atrogenes are valid for humans still remain to be established. An "atrogene" was originally defined as a gene systematically up- or down-regulated in several catabolic situations. Even if recent works often restrict this notion to the up-regulation of a limited number of proteolytic enzymes, it is important to keep in mind the big picture view. In this review, we provide an update of the validated and potential rodent atrogenes and the metabolic pathways they belong, and based on recent work, their relevance in human physio-pathological situations. We also propose a more precise definition of the atrogenes that integrates rapid recovery when catabolic stimuli are stopped or replaced by anabolic ones.

Acute recovery from disuse atrophy: the role of stretch-activated ion channels in the activation of anabolic signaling in skeletal muscle

The aim of the study was to 1) measure time-course alternations in the rate of protein synthesis (PS) and phosphorylation status of the key anabolic markers, and 2) find out the role of stretch-activated ion channels (SACs) in the activation of anabolic signaling in the rat soleus during an acute reloading following disuse atrophy. Wistar rats were subjected to 14-day hindlimb suspension (HS) followed by 6, 12, and 24 h of reloading. To examine the role of SAC in the reloading-induced activation of anabolic signaling, the rats were treated with gadolinium (Gd³⁺), a SAC blocker. The content of signaling proteins was determined by Western blot. c-Myc mRNA expression was assessed by RT-PCR. After 24-h reloading, the PS rate was elevated by 44% versus control. After 6-h reloading, the p-70-kDa ribosomal protein S6 kinase (p70S6k) and translation initiation factor 4E-binding protein 1 (4E-BP1) did not differ from control; however, 12-h reloading resulted in an upregulation of both p70s6k and 4E-BP1 phosphorylation versus control. The phosphorylation of AKT (Ser473) and glycogen synthase kinase-3β (Ser9) was reduced after HS and then completely restored by 12-h reloading. c-Myc was significantly upregulated during the entire reloading. Gd³⁺ treatment during reloading (12 h) prevented a full phosphorylation of p70S6k, rpS6, 4E-BP1, as well as PS activation. The results of the study suggest that 1) enhanced PS during the acute recovery from HS may be associated with the activation of ribosome biogenesis as well as mammalian target of rapamycin complex 1 (mTORC1)-dependent signaling pathways, and 2) functional SACs are necessary for complete activation of mTORC1 signaling in rat soleus during acute recovery from HS.

Altered protein turnover signaling and myogenesis during impaired recovery of inflammation-induced muscle atrophy in emphysematous mice

Exacerbations in Chronic obstructive pulmonary disease (COPD) are often accompanied by pulmonary and systemic inflammation, and are associated with an increased susceptibility to weight loss and muscle wasting. As the emphysematous phenotype in COPD appears prone to skeletal muscle wasting, the aims of this study were to evaluate in emphysematous compared to control mice following repetitive exacerbations (1) changes in muscle mass and strength and, (2) whether muscle mass recovery and its underlying processes are impaired. Emphysema was induced by intra-tracheal (IT) elastase instillations, followed by three weekly IT-LPS instillations to mimic repetitive exacerbations. Loss of muscle mass and strength were measured, and related to analyses of muscle protein turnover and myogenesis signaling in tissue collected during and following recovery. Emphysematous mice showed impaired muscle mass recovery in response to pulmonary inflammation-induced muscle atrophy. Proteolysis and protein synthesis signaling remained significantly higher in emphysematous mice during recovery from LPS. Myogenic signaling in skeletal muscle was altered, and fusion capacity of cultured muscle cells treated with plasma derived from LPS-treated emphysematous mice was significantly decreased. In conclusion, repetitive cycles of pulmonary inflammation elicit sustained muscle wasting in emphysematous mice due to impaired muscle mass recovery, which is accompanied by aberrant myogenesis.

Protein translation, proteolysis and autophagy in human skeletal muscle atrophy after spinal cord injury

AIM

Spinal cord injury-induced loss of skeletal muscle mass does not progress linearly. In humans, peak muscle loss occurs during the first 6 weeks postinjury, and gradually continues thereafter. The aim of this study was to delineate the regulatory events underlying skeletal muscle atrophy during the first year following spinal cord injury.

METHODS

Key translational, autophagic and proteolytic proteins were analysed by immunoblotting of human vastus lateralis muscle obtained 1, 3 and 12 months following spinal cord injury. Age-matched able-bodied control subjects were also studied.

RESULTS

Several downstream targets of Akt signalling decreased after spinal cord injury in skeletal muscle, without changes in resting Akt Ser⁴⁷³ and Akt Thr³⁰⁸ phosphorylation or total Akt protein. Abundance of mTOR protein and mTOR Ser²⁴⁴⁸ phosphorylation, as well as FOXO1 Ser²⁵⁶ phosphorylation and FOXO3 protein, decreased in response to spinal cord injury, coincident with attenuated protein abundance of E3 ubiquitin ligases, MuRF1 and MAFbx. S6 protein and Ser^235/236 phosphorylation, as well as 4E-BP1 Thr^37/46 phosphorylation, increased transiently after spinal cord injury, indicating higher levels of protein translation early after injury. Protein abundance of LC3-I and LC3-II decreased 3 months postinjury as compared with 1 month postinjury, but not compared to able-bodied control subjects, indicating lower levels of autophagy. Proteins regulating proteasomal degradation were stably increased in response to spinal cord injury.

CONCLUSION

Together, these data provide indirect evidence suggesting that protein translation and autophagy transiently increase, while whole proteolysis remains stably higher in skeletal muscle within the first year after spinal cord injury.

Age-related deficits in skeletal muscle recovery following disuse are associated with neuromuscular junction instability and ER stress, not impaired protein synthesis

Age-related loss of muscle mass and strength can be accelerated by impaired recovery of muscle mass following a transient atrophic stimulus. The aim of this study was to identify the mechanisms underlying the attenuated recovery of muscle mass and strength in old rats following disuse-induced atrophy. Adult (9 month) and old (29 month) male F344BN rats underwent hindlimb unloading (HU) followed by reloading. HU induced significant atrophy of the hindlimb muscles in both adult (17-38%) and old (8-29%) rats, but only the adult rats exhibited full recovery of muscle mass and strength upon reloading. Upon reloading, total RNA and protein synthesis increased to a similar extent in adult and old muscles. At baseline and upon reloading, however, proteasome-mediated degradation was suppressed leading to an accumulation of ubiquitin-tagged proteins and p62. Further, ER stress, as measured by CHOP expression, was elevated at baseline and upon reloading in old rats. Analysis of mRNA expression revealed increases in HDAC4, Runx1, myogenin, Gadd45a, and the AChRs in old rats, suggesting neuromuscular junction instability/denervation. Collectively, our data suggests that with aging, impaired neuromuscular transmission and deficits in the proteostasis network contribute to defects in muscle fiber remodeling and functional recovery of muscle mass and strength.

UBE2D2 is not involved in MuRF1-dependent muscle wasting during hindlimb suspension

The Ubiquitin Proteasome System (UPS) is mainly responsible for the increased protein breakdown observed in muscle wasting. The E3 ligase MuRF1 is so far the only enzyme known to direct the main contractile proteins for degradation (i.e. troponin I, myosin heavy chains and actin). However, MuRF1 does not possess any catalytic activity and thus depends on the presence of a dedicated E2 for catalyzing the covalent binding of polyubiquitin (polyUb) chains on the substrates. The E2 enzymes belonging to the UBE2D family are commonly used for in vitro ubiquitination assays but no experimental data suggesting their physiological role as bona fide MuRF1-interacting E2 enzymes are available. In this work, we first found that the mRNA levels of critical E3 enzymes implicated in the atrophying program (MuRF1, MAFbx, Nedd4 and to a lesser extent Mdm2) are tightly and rapidly controlled during the atrophy (up regulation) and recovery (down regulation) phases in the soleus muscle from hindlimb suspended rats. By contrast, E3 ligases (Ozz, ASB2β and E4b) implicated in other processes (muscle development or regeneration) poorly responded to atrophy and recovery. UBE2B, an E2 enzyme systematically up regulated in various catabolic situations, was controlled at the mRNA levels like the E3s implicated in the atrophying process. By contrast, UBE2D2 was progressively repressed during atrophy and recovery, which makes it a poor candidate for a role during muscle atrophy. In addition, UBE2D2 did not exhibit any affinity with MuRF1 using either yeast two-hybrid or Surface Plasmon Resonance (SPR) approaches. Finally, UBE2D2 was unable to promote the degradation of the MuRF1 substrate α-actin in HEK293T cells, suggesting that no functional interaction exists between these enzymes within a cellular context. Altogether, our data strongly suggest that UBE2D2 is not the cognate ubiquitinating enzyme for MuRF1 and that peculiar properties of UBE2D enzymes may have biased in vitro ubiquitination assays.

UBE2B is implicated in myofibrillar protein loss in catabolic C2C12 myotubes

BACKGROUND

Skeletal muscle protein loss is an adaptive response to various patho-physiological situations, and the ubiquitin proteasome system (UPS) is responsible for the degradation of the bulk of muscle proteins. The role of E2 ubiquitin-conjugating enzymes is still poorly understood in skeletal muscle.

METHODS

We screened for E2s expression levels in C2C12 myotubes submitted to the catabolic glucocorticoid dexamethasone (Dex).

RESULTS

One micromolar Dex induced an accumulation of proteasome substrates (polyUb conjugates) and an overexpression of the muscle-specific E3 ligase MuRF1 and of six E2 enzymes, UBE2A, UBE2B, UBE2D1, UBE2D2, UBE2G1, and UBE2J1. However, only MuRF1 and UBE2B were sensitive to mild catabolic conditions (0.16 μM Dex). UBE2B knockdown induced a sharp decrease of total (-18%) and K48 (-28%) Ub conjugates, that is, proteasome substrates, indicating an important role of UBE2B in the overall protein breakdown in catabolic myotubes.

CONCLUSIONS

Interestingly, these results indicate an important role of UBE2B on muscle protein homeostasis during catabolic conditions.

Role of E2-Ub-conjugating enzymes during skeletal muscle atrophy

The Ubiquitin Proteasome System (UPS) is a major actor of muscle wasting during various physio-pathological situations. In the past 15 years, increasing amounts of data have depicted a picture, although incomplete, of the mechanisms implicated in myofibrillar protein degradation, from the discovery of muscle-specific E3 ligases to the identification of the signaling pathways involved. The targeting specificity of the UPS relies on the capacity of the system to first recognize and then label the proteins to be degraded with a poly-ubiquitin (Ub) chain. It is fairly assumed that the recognition of the substrate is accomplished by the numerous E3 ligases present in mammalian cells. However, most E3s do not possess any catalytic activity and E2 enzymes may be more than simple Ub-providers for E3s since they are probably important actors in the ubiquitination machinery. Surprisingly, most authors have tried to characterize E3 substrates, but the exact role of E2s in muscle protein degradation is largely unknown. A very limited number of the 35 E2s described in humans have been studied in muscle protein breakdown experiments and the vast majority of studies were only descriptive. We review here the role of E2 enzymes in skeletal muscle and the difficulties linked to their study and provide future directions for the identification of muscle E2s responsible for the ubiquitination of contractile proteins.

Muscle-specific Drp1 overexpression impairs skeletal muscle growth via translational attenuation

Mitochondrial fission and fusion are essential processes in the maintenance of the skeletal muscle function. The contribution of these processes to muscle development has not been properly investigated in vivo because of the early lethality of the models generated so far. To define the role of mitochondrial fission in muscle development and repair, we have generated a transgenic mouse line that overexpresses the fission-inducing protein Drp1 specifically in skeletal muscle. These mice displayed a drastic impairment in postnatal muscle growth, with reorganisation of the mitochondrial network and reduction of mtDNA quantity, without the deficiency of mitochondrial bioenergetics. Importantly we found that Drp1 overexpression activates the stress-induced PKR/eIF2α/Fgf21 pathway thus leading to an attenuated protein synthesis and downregulation of the growth hormone pathway. These results reveal for the first time how mitochondrial network dynamics influence muscle growth and shed light on aspects of muscle physiology relevant in human muscle pathologies.

Reloading functionally ameliorates disuse-induced muscle atrophy by reversing mitochondrial dysfunction, and similar benefits are gained by administering a combination of mitochondrial nutrients

We previously found that mitochondrial dysfunction occurs in disuse-induced muscle atrophy. However, the mitochondrial remodeling that occurs during reloading, an effective approach for rescuing unloading-induced atrophy, remains to be investigated. In this study, using a rat model of 3-week hindlimb unloading plus 7-day reloading, we found that reloading protected mitochondria against dysfunction, including mitochondrial loss, abnormal mitochondrial morphology, inhibited biogenesis, and activation of mitochondria-associated apoptotic signaling. Interestingly, a combination of nutrients, including α-lipoic acid, acetyl-L-carnitine, hydroxytyrosol, and CoQ10, which we designed to target mitochondria, was able to efficiently rescue muscle atrophy via a reloading-like action. It is suggested that reloading ameliorates skeletal muscle atrophy through the activation of mitochondrial biogenesis and the amelioration of oxidative stress. Nutrient administration acted similarly in unloaded rats. Here, the study of mitochondrial remodeling in rats during unloading and reloading provides a more detailed picture of the pathology of muscle atrophy.

Nutritional strategies to counteract muscle atrophy caused by disuse and to improve recovery

Periods of immobilisation are often associated with pathologies and/or ageing. These periods of muscle disuse induce muscle atrophy which could worsen the pathology or elderly frailty. If muscle mass loss has positive effects in the short term, a sustained/uncontrolled muscle mass loss is deleterious for health. Muscle mass recovery following immobilisation-induced atrophy could be critical, particularly when it is uncompleted as observed during ageing. Exercise, the best way to recover muscle mass, is not always applicable. So, other approaches such as nutritional strategies are needed to limit muscle wasting and to improve muscle mass recovery in such situations. The present review discusses mechanisms involved in muscle atrophy following disuse and during recovery and emphasises the effect of age in these mechanisms. In addition, the efficiency of nutritional strategies proposed to limit muscle mass loss during disuse and to improve protein gain during recovery (leucine supplementation, whey proteins, antioxidants and anti-inflammatory compounds, energy intake) is also discussed.

Time course of diaphragm function recovery after controlled mechanical ventilation in rats

Controlled mechanical ventilation (CMV) is known to result in rapid and severe diaphragmatic dysfunction, but the recovery response of the diaphragm to normal function after CMV is unknown. Therefore, we examined the time course of diaphragm function recovery in an animal model of CMV. Healthy rats were submitted to CMV for 24-27 h (n = 16), or to 24-h CMV followed by either 1 h (CMV + 1 h SB, n = 9), 2 h (CMV + 2 h SB, n = 9), 3 h (CMV + 3 h SB, n = 9), or 4-7 h (CMV + 4-7 h SB, n = 9) of spontaneous breathing (SB). At the end of the experiment, the diaphragm muscle was excised for functional and biochemical analysis. The in vitro diaphragm force was significantly improved in the CMV + 3 h SB and CMV + 4-7 h SB groups compared with CMV (maximal tetanic force: +27%, P < 0.05, and +59%, P < 0.001, respectively). This was associated with an increase in the type IIx/b fiber dimensions (P < 0.05). Neutrophil influx was increased in the CMV + 4-7 h SB group (P < 0.05), while macrophage numbers remained unchanged. Markers of protein synthesis (phosphorylated Akt and eukaryotic initiation factor 4E binding protein 1) were significantly increased (±40%, P < 0.001, and ±52%, P < 0.01, respectively) in the CMV + 3 h SB and CMV + 4-7 h SB groups and were positively correlated with diaphragm force (P < 0.05). Finally, also the maximal specific force generation of skinned single diaphragm fibers was increased in the CMV + 4-7 h SB group compared with CMV (+45%, P < 0.05). In rats, reloading the diaphragm for 3 h after CMV is sufficient to improve diaphragm function, while complete recovery occurs after longer periods of reloading. Enhanced muscle fiber dimensions, increased protein synthesis, and improved intrinsic contractile properties of diaphragm muscle fibers may have contributed to diaphragm function recovery.

Disuse-induced muscle wasting

Loss of skeletal muscle mass occurs frequently in clinical settings in response to joint immobilization and bed rest, and is induced by a combination of unloading and inactivity. Disuse-induced atrophy will likely affect every person in his or her lifetime, and can be debilitating especially in the elderly. Currently there are no good therapies to treat disuse-induced muscle atrophy, in part, due to a lack of understanding of the cellular and molecular mechanisms responsible for the induction and maintenance of muscle atrophy. Our current understanding of disuse atrophy comes from the investigation of a variety of models (joint immobilization, hindlimb unloading, bed rest, spinal cord injury) in both animals and humans. Under conditions of unloading, it is widely accepted that there is a decrease in protein synthesis, however, the role of protein degradation, especially in humans, is debated. This review will examine the current understanding of the molecular and cellular mechanisms regulating muscle loss under disuse conditions, discussing the similarities and areas of dispute between the animal and human literature. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Deciphering the ubiquitin proteome: Limits and advantages of high throughput global affinity purification-mass spectrometry approaches

Ubiquitination is a posttranslational modification of proteins that involves the covalent attachment of ubiquitin, either as a single moiety or as polymers. This process controls almost every cellular metabolic pathway through a variety of combinations of linkages. Mass spectrometry now allows high throughput approaches for the identification of the thousands of ubiquitinated proteins and of their ubiquitination sites. Despite major technological improvements in mass spectrometry in terms of sensitivity, resolution and acquisition speed, the use of efficient purification methods of ubiquitinated proteins prior to mass spectrometry analysis is critical to achieve an efficient characterization of the ubiquitome. This critical step is achieved using different approaches that possess advantages and pitfalls. Here, we discuss the limits that can be encountered when deciphering the ubiquitome. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Contrarily to whey and high protein diets, dietary free leucine supplementation cannot reverse the lack of recovery of muscle mass after prolonged immobilization during ageing

During ageing, immobilization periods increase and are partially responsible of sarcopaenia by inducing a muscle atrophy which is hardly recovered from. Immobilization-induced atrophy is due to an increase of muscle apoptotic and proteolytic processes and decreased protein synthesis. Moreover, previous data suggested that the lack of muscle mass recovery might be due to a defect in protein synthesis response during rehabilitation. This study was conducted to explore protein synthesis during reloading and leucine supplementation effect as a nutritional strategy for muscle recovery. Old rats (22–24 months old) were subjected to unilateral hindlimb casting for 8 days (I8) and allowed to recover for 10–40 days (R10–R40). They were fed a casein (±leucine) diet during the recovery. Immobilized gastrocnemius muscles atrophied by 20%, and did not recover even at R40. Amount of polyubiquitinated conjugates and chymotrypsin- and trypsin-like activities of the 26S proteasome increased. These changes paralleled an ‘anabolic resistance' of the protein synthesis at the postprandial state (decrease of protein synthesis, P-S6 and P-4E-BP1). During the recovery, proteasome activities remained elevated until R10 before complete normalization and protein synthesis was slightly increased. With free leucine supplementation during recovery, if proteasome activities were normalized earlier and protein synthesis was higher during the whole recovery, it nevertheless failed in muscle mass gain. This discrepancy could be due to a ‘desynchronization' between the leucine signal and the availability of amino acids coming from casein digestion. Thus, when supplemented with leucine-rich proteins (i.e. whey) and high protein diets, animals partially recovered the muscle mass loss.

Muscle actin is polyubiquitinylated in vitro and in vivo and targeted for breakdown by the E3 ligase MuRF1

Muscle atrophy prevails in numerous diseases (cancer cachexia, renal failure, infections, etc.), mainly results from elevated proteolysis, and is accelerated by bed rest. This largely contributes to increased health costs. Devising new strategies to prevent muscle wasting is a major clinical challenge. The ubiquitin proteasome system (UPS) degrades myofibrillar proteins, but the precise mechanisms responsible for actin breakdown are surprisingly poorly characterized. We report that chimeric flag-actin was destabilized and polyubiquitinylated in stably transfected C2C12 myotubes treated with the catabolic agent dexamethasone (1 μM) and that only proteasome inhibitors blocked its breakdown. Actin polyubiquitinylation was also detected in wild-type C2C12 myotubes and human muscle biopsies from control participants and patients with cancer. The muscle-specific E3 ubiquitin ligase MuRF1 is up-regulated in catabolic conditions and polyubiquitinylates components of the thick filament. We also demonstrate that recombinant GST-MuRF1 physically interacted and polyubiquitinylated actin in vitro and that MuRF1 is a critical component for actin breakdown, since MuRF1 siRNA stabilized flag-actin. These data identify unambiguously the abundant contractile protein actin as a target of the UPS in skeletal muscle both in vitro and in vivo, further supporting the need for new strategies blocking specifically the activation of this pathway in muscle wasting conditions.

Transcriptome and translational signaling following endurance exercise in trained skeletal muscle: impact of dietary protein

Postexercise protein feeding regulates the skeletal muscle adaptive response to endurance exercise, but the transcriptome guiding these adaptations in well-trained human skeletal muscle is uncharacterized. In a crossover design, eight cyclists ingested beverages containing protein, carbohydrate and fat (PTN: 0.4, 1.2, 0.2 g/kg, respectively) or isocaloric carbohydrate and fat (CON: 1.6, 0.2 g/kg) at 0 and 1 h following 100 min of cycling. Biopsies of the vastus lateralis were collected at 3 and 48 h following to determine the early and late transcriptome and regulatory signaling responses via microarray and immunoblot. The top gene ontology enriched by PTN were: muscle contraction, extracellular matrix--signaling and structure, and nucleoside, nucleotide, and nucleic acid metabolism (3 and 48 h); developmental processes, immunity, and defense (3 h); glycolysis, lipid and fatty acid metabolism (48 h). The transcriptome was also enriched within axonal guidance, actin cytoskeletal, Ca2+, cAMP, MAPK, and PPAR canonical pathways linking protein nutrition to exercise-stimulated signaling regulating extracellular matrix, slow-myofibril, and metabolic gene expression. At 3 h, PTN attenuated AMPKα1Thr172 phosphorylation but increased mTORC1Ser2448, rps6Ser240/244, and 4E-BP1-γ phosphorylation, suggesting increased translation initiation, while at 48 h AMPKα1Thr172 phosphorylation and PPARG and PPARGC1A expression increased, supporting the late metabolic transcriptome, relative to CON. To conclude, protein feeding following endurance exercise affects signaling associated with cell energy status and translation initiation and the transcriptome involved in skeletal muscle development, slow-myofibril remodeling, immunity and defense, and energy metabolism. Further research should determine the time course and posttranscriptional regulation of this transcriptome and the phenotype responding to chronic postexercise protein feeding.

Association analysis of UBE3C polymorphisms in Korean aspirin-intolerant asthmatic patients

BACKGROUND

Aspirin-intolerant asthma (AIA), as an asthma phenotype that involves the upper or lower airways, occurs from excessive leukotriene production on administration of nonsteroidal anti-inflammatory drugs. The UBE3C gene on chromosome 7 is a member of the E3 ligase enzymes and is implicated in the ubiquitin-proteasome pathway. This pathway is involved in immune responses to inflammation, including asthma.

OBJECTIVE

To investigate whether the UBE3C polymorphisms are associated with the risk of AIA.

METHODS

Twenty-four nonmonomorphic genetic variants of UBE3C were genotyped in 163 patients with AIA and 429 controls with aspirin-tolerant asthma. After genotyping, logistic analyses were performed and haplotypes of each individual were inferred using the PHASE algorithm.

RESULTS

Logistic analyses revealed that 2 polymorphisms (rs3802122 and rs6979947) in the intron showed significant associations with risk of AIA (P < .001 and P(corr) = .002 in both single nucleotide polymorphisms; odds ratios, 0.61 and 0.60, respectively). In associations with haplotypes, haplotype 2, which contains all the significantly associated single nucleotide polymorphisms and was infrequent in AIA compared with aspirin-tolerant asthma, was associated with aspirin hypersensitivity in asthmatic patients (P = .003 and P(corr) = .03; odds ratio, 0.64; 95% confidence interval, 0.47-0.86).

CONCLUSIONS

The rs3802122 and rs6979947 polymorphisms were significantly associated with the risk of AIA. However, further studies are required to establish the underlying mechanism by which UBE3C and its polymorphisms affect the risk of AIA.

Tearin' up my heart: proteolysis in the cardiac sarcomere

Proteolysis within the cardiac sarcomere is a constantly evolving area of research. Three major pathways of proteolysis have been identified as being active within the cardiac sarcomere, namely the ubiquitin-proteasome system, autophagy, and the calpain system. The role of ubiquitin-proteasome system-mediated proteolysis in cardiovascular health and disease has been known for some time; however, it is now apparent that other proteolytic systems also aid in the stabilization of cardiac sarcomere structure and function. This minireview focuses on the individual as well as cooperative involvement of each of these three major pathways of proteolysis within the cardiac sarcomere.

Mechanical ventilation-induced oxidative stress in the diaphragm: role of heme oxygenase-1

BACKGROUND

Prolonged mechanical ventilation (MV) results in a rapid onset of diaphragmatic atrophy that is primarily due to increased proteolysis. Although MV-induced protease activation can involve several factors, it is clear that oxidative stress is a required signal for protease activation in the diaphragm during prolonged MV. However, the oxidant-producing pathways in the diaphragm that contribute to MV-induced oxidative stress remain unknown. We have demonstrated that prolonged MV results in increased diaphragmatic expression of a key stress-sensitive enzyme, heme oxygenase (HO)-1. Paradoxically, HO-1 can function as either a pro-oxidant or an antioxidant, and the role that HO-1 plays in MV-induced diaphragmatic oxidative stress is unknown. We tested the hypothesis that HO-1 acts as a pro-oxidant in the diaphragm during prolonged MV.

METHODS

To determine whether HO-1 functions as a pro-oxidant or an antioxidant in the diaphragm during MV, we assigned rats into three experimental groups: (1) a control group, (2) a group that received 18 h of MV and saline solution, and (3) a group that received 18 h of MV and was treated with a selective HO-1 inhibitor. Indices of oxidative stress, protease activation, and fiber atrophy were measured in the diaphragm.

RESULTS

Inhibition of HO-1 activity did not prevent or exacerbate MV-induced diaphragmatic oxidative stress (as indicated by biomarkers of oxidative damage). Further, inhibition of HO-1 activity did not influence MV-induced protease activation or myofiber atrophy in the diaphragm.

CONCLUSIONS

Our results indicate that HO-1 is neither a pro-oxidant nor an antioxidant in the diaphragm during MV. Furthermore, our findings reveal that HO-1 does not play an important role in MV-induced protease activation and diaphragmatic atrophy.

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.

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.

Roles and potential therapeutic targets of the ubiquitin proteasome system in muscle wasting

Muscle wasting, characterized by the loss of protein mass in myofibers, is in most cases largely due to the activation of intracellular protein degradation by the ubiquitin proteasome system (UPS). During the last decade, mechanisms contributing to this activation have been unraveled and key mediators of this process identified. Even though much remains to be understood, the available information already suggests screens for new compounds inhibiting these mechanisms and highlights the potential for pharmaceutical drugs able to treat muscle wasting when it becomes deleterious. This review presents an overview of the main pathways contributing to UPS activation in muscle and describes the present state of efforts made to develop new strategies aimed at blocking or slowing muscle wasting.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

Cathepsin D and MMP-9 activity increase following a high intensity exercise in hind limb muscles of young rats

The influence of an intensive exercise regime on cathepsin D and MMP-9 activity in hind limb muscles was investigated. We hypothesized that high-intensity exercise would increase the number of these proteins, indicating their involvement in the pathogenesis of exercise-induced muscle injury. Muscle fibers from the gastrocnemius and soleus were used from young (6-mo-old) female rats (n = 6) who completed 10 consecutive days of treadmill running at high intensity (34 m min(-1) gradually up to 40 min per day), compared with nonrunning, age and sex-matched rats (n = 6). After a high-intensity exercise regime, cathepsin D activity significantly increased in the gastrocnemius (from 6.6 x 10(-3) to 10.7 x 10(-3) or 61% nM tyrosine x mg-1 protein x min-1) and the soleus (from 5.9 x 10(-3) to 8.9 x 10(-3) or 66%). The activity level of mRNA MMP-9, expressed as ng mg(-1) protein, increased in both muscles subjected to intensity running. The results of this study suggest that high-intensity running results in an elevation in the activity of lysosomal enzymes involved in matrix protein degradation.

Calpain/calpastatin activities and substrate depletion patterns during hindlimb unweighting and reweighting in skeletal muscle

Unloading of skeletal muscle by hindlimb unweighting (HU) is characterized by atrophy, protein loss, and an elevation in intracellular Ca(2+) levels that may be sufficient to activate Ca(2+)-dependent proteases (calpains). In this study, we investigated the time course of calpain activation and the depletion pattern of a specific structural protein (desmin) with unloading and subsequent reweighting. Rats underwent 12 h, 24 h, 72 h or 9 days of HU, followed by reweighting for either 0, 12 or 24 h. Total calpain-like activity was elevated with HU in skeletal muscle (P < 0.05) and was further enhanced with reweighting (P < 0.05). The increases in calpain-like activity were associated with a proportional increase in activity of the particulate fraction (P < 0.05). Activity of the mu-calpain isoform was elevated with 12 and 24 h of HU (P < 0.05) and returned to control levels thereafter. With reweighting, activities of mu-calpain were elevated above control levels for all HU groups except 9 days (P < 0.05). In contrast, minimal changes in m-calpain and calpastatin activity were observed with HU and reweighting. Although desmin depletion levels did not reach statistical significance, a significant inverse relationship was found between the mu-calpain/calpastatin ratio and the amount of desmin in isolated myofibrils (R = -0.83, P < 0.001). The results suggest that calpain activation is an early event during unloading in skeletal muscle, and that the majority of the increase in calpain activity can be attributed to the micro-isoform.

Alterations in mRNA expression and protein products following spinal cord injury in humans

We examined the effects of spinal cord injury (SCI) on alterations in gene expression and respective protein products in human skeletal muscle 2 days and 5 days post-SCI. Biopsies were taken from skeletal muscle of 9 men and 1 woman (n = 10) (43.9 +/- 6.7 years) 2 days and 5 days post-SCI and from 5 healthy young men who served as controls (20.4 +/- 0.5 years). Global changes in gene expression were analysed using Affymetrix GeneChips on a subsample of subjects (n = 3). Candidate genes were then pursued via qRT-PCR. Western blotting (WB) was used to quantify protein products of candidate genes. Immunohistochemistry (IHC) was used to localize proteins. Groups of transcripts showing the greatest percentage of altered expression, the most robust fold-changes, and indicative of involvement of an entire pathway using the GeneChip included genes involved in the ubiquitin proteasome pathway (UPP), metallothionein function, and protease inhibition. qRT-PCR analysis confirmed increases in gene expression for UPP components (UBE3C, Atrogin-1, MURF1, and PSMD11), the metallothioneins (MT1A, MT1F, MT1H), and the protease inhibitor, SLPI (P < 0.05) at 2 days and 5 days post-SCI. Protein levels of the proteasome subunit (PSMD11) and the metallothioneins were increased 5 days post-SCI. Protein levels of UBE3C, Atrogin-1, MURF1 and SLPI were unchanged (P > 0.05). IHC showed increased staining for PSMD11 and the metallothioneins 5 days post-SCI, along the peripheral region of the cells. IHC also showed altered staining for Atrogin-1 at 5 days post-SCI along the membrane region. Thus, there was a profound increase in gene expression of UPP components, the metallothioneins, and the protease inhibitor, SLPI, within 5 days of SCI. Increased protein levels for PSMD11 and the metallothioneins 5 days post-SCI, specifically along the cell periphery, indicate that proteins in this region may be early targets for degradation post-SCI.

Mechanisms of skeletal muscle atrophy

PURPOSE OF REVIEW

Recent clinical and mechanistic studies have shown that increased proteolysis is a major determinant of muscle wasting in numerous catabolic states and of alterations in myopathies or dystrophies. The implications of these observations for improving muscle mass and function are discussed.

RECENT FINDINGS

Several proteolytic systems (i.e. the ubiquitin-proteasome system, the lysosomal, the Ca-dependent, and the caspase systems) are responsible for muscle wasting. The Ca-dependent and caspase systems may initiate myofibrillar proteolysis. The ubiquitin-proteasome system is believed to degrade actin and myosin heavy chain and, consequently, plays a major role in muscle wasting. Multiple steps in the ubiquitin-proteasome system (ubiquitination, deubiquitination, proteasome activities) are upregulated in muscle wasting diseases. Few key components of the ubiquitin-proteasome system that are strictly necessary for muscle wasting have been so far characterized. Recent studies have led to the elucidation of various signaling pathways of the ubiquitin-proteasome system that are activated in muscle wasting conditions.

SUMMARY

Although the precise role of the different muscle proteolytic machineries is still largely unknown, current studies are leading to new pharmacologic approaches that can be useful in blocking or partially preventing muscle wasting or improving muscle function in human patients.

Differential response of heat shock proteins to hindlimb unloading and reloading in the soleus

Hindlimb unloading (HU) results in oxidative stress, skeletal muscle atrophy, and increased damage upon reloading. Heat shock proteins (HSPs) protect against oxidative stress. However, it is unknown whether HSPs are depressed with long-term unloading (28 days) or reloading. We tested the hypotheses that long-term HU would depress Hsp70 and Hsp25 pathways, whereas reloading would allow recovery in the soleus. Adult Sprague-Dawley rats were divided into three groups: controls; HU for 28 days; and HU + 7 days of reloading (HU-R). Soleus mass decreased with HU, and did not recover to control values with reloading. Hsp70 decreased with HU (-78.5%) and did not recover with HU-R (-81.4%). Upstream heat shock factor-1 was depressed with HU and HU-R. Hsp25 was reduced with HU, but recovered with reloading. Downstream of Hsp25, NADP-specific isocitrate dehydrogenase and glutathione peroxidase decreased with unloading, but only NADP-specific isocitrate dehydrogenase recovered with HU-R. Lipid peroxidation increased in both HU and HU-R. These data indicate that prolonged unloading and subsequent reloading results in complex, differential regulation of Hsp70 and Hsp25 pathways in the rat soleus muscle. Thus dysregulation and uncoupling of the Hsp70 and Hsp25 pathways may lead not only to muscle atrophy with prolonged unloading, but also impaired recovery of muscle mass during early reloading.

Effect of flywheel-based resistance exercise on processes contributing to muscle atrophy during unloading in adult rats

Flywheel-based resistance exercise (RE) attenuates muscle atrophy during hindlimb suspension. We have previously shown that protein synthesis is elevated in response to RE, but the effect on protein degradation, cell proliferation, or apoptosis was not investigated. We hypothesized that, in addition to affecting protein synthesis, RE inhibits processes that actively contribute to muscle atrophy during hindlimb suspension. Male rats were housed in regular cages (control), tail suspended for 2 wk (HS), or HS with RE every other day for 2 wk (HSRE). Although RE attenuated soleus muscle atrophy during HS, the observed fivefold elevation in apoptosis and the 53% decrease in cell proliferation observed with HS were unaffected by RE. Expression of genes encoding components of the ubiquitin-proteasome pathway of protein degradation were elevated with HS, including ubiquitin, MAFbx, Murf-1, Nedd4, and XIAP, and proteasome subunits C2 and C9. Total ubiquitinated protein was increased with HS, but proteasome activity was not different from control. RE selectively altered the expression of different components of this pathway: MAFbx, Murf-1, and ubiquitin mRNA abundance were downregulated, whereas C2 and C9 subunits remained elevated. Similarly, Nedd4 and XIAP continued to be upregulated, potentially accounting for the observed augmentation in total ubiquitinated protein with RE. Thus a different constellation of proteins is likely ubiquitinated with RE due to altered ubiquitin ligase composition. In summary, the flywheel-based resistance exercise paradigm used in this study is associated with the inhibition of some mechanisms associated with muscle atrophy, such as the increase in MAFbx and Murf-1, but not with others, such as proteasome subunit remodeling, apoptosis, and decreased proliferation, potentially accounting for the inability to completely restore muscle mass. Identifying specific exercise parameters that affect these latter processes may be useful in designing effective exercise strategies in the elderly or during spaceflight.

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.

Trim32 is a Ubiquitin Ligase Mutated in Limb Girdle Muscular Dystrophy Type 2H that Binds to Skeletal Muscle Myosin and Ubiquitinates Actin

Trim32 belongs to the tripartite motif (TRIM) protein family, which is characterized by a common domain structure composed of a RING-finger, a B-box, and a coiled-coil motif. In addition to these motifs, Trim32 possesses six C-terminal NHL-domains. A point mutation in one NHL domain (D487N) has been linked to two forms of muscular dystrophy called limb girdle muscular dystrophy type 2H and sarcotubular myopathy. In the present study we demonstrate that Trim32 is an E3 ubiquitin ligase that acts in conjunction with ubiquitin-conjugating enzymes UbcH5a, UbcH5c, and UbcH6. Western blot analysis showed that Trim32 is expressed primarily in skeletal muscle, and revealed its differential expression from one muscle to another. The level of Trim32 expression was elevated significantly in muscle undergoing remodeling due to changes in weight bearing. Furthermore, expression of Trim32 was induced in myogenic differentiation. Thus, variability in Trim32 expression in different skeletal muscles could be due to induction of Trim32 expression upon changes in physiological conditions. We show that Trim32 associates with skeletal muscle thick filaments, interacting directly with the head and neck region of myosin. Our data indicate that myosin is not a substrate of Trim32; however, Trim32 was found to ubiquitinate actin in vitro and to cause a decrease in the level of endogenous actin when transfected into HEK293 cells. In conclusion, our results demonstrate that Trim32 is a ubiquitin ligase that is expressed in skeletal muscle, can be induced upon muscle unloading and reloading, associates with myofibrils and is able to ubiquitinate actin, suggesting its likely participation in myofibrillar protein turnover, especially during muscle adaptation.

Ubiquitin-proteasome-dependent proteolytic activity remains elevated after zymosan-induced sepsis in rats while muscle mass recovers

We studied the role of the ubiquitin-proteasome system in rat skeletal muscle during sepsis and subsequent recovery. Sepsis was induced with intraperitoneal zymosan injections. This model allows one to study a sustained and reversible catabolic phase and mimics the events that prevail in septic and subsequently recovering patients. In addition, the role of the ubiquitin-proteasome system during muscle recovery is poorly documented. There was a trend for increased ubiquitin-conjugate formation in the muscle wasting phase, which was abolished during the recovery phase. The trypsin- and chymotrypsin-like peptidase activities of the 20S proteasome peaked at day 6 following zymosan injection (i.e. when both muscle mass and muscle fiber cross-sectional area were reduced the most), but remained elevated when muscle mass and muscle fiber cross-sectional area were recovering (11 days). This clearly suggests a role for the ubiquitin-proteasome pathway in the muscle remodeling and/or recovery process. Protein levels of 19S complex and 20S proteasome subunits did not increase throughout the study, pointing to alternative mechanisms regulating proteasome activities. Overall these data support a role for ubiquitin-proteasome dependent proteolysis in the zymosan septic model, in both the catabolic and muscle recovery phases.

The ubiquitin–proteasome system and skeletal muscle wasting

The ubiquitin–proteasome system (UPS) is believed to degrade the major contractile skeletal muscle proteins and plays a major role in muscle wasting. Different and multiple events in the ubiquitination, deubiquitination and proteolytic machineries are responsible for the activation of the system and subsequent muscle wasting. However, other proteolytic enzymes act upstream (possibly m-calpain, cathepsin L, and/or caspase 3) and downstream (tripeptidyl-peptidase II and aminopeptidases) of the UPS, for the complete breakdown of the myofibrillar proteins into free amino acids. Recent studies have identified a few critical proteins that seem necessary for muscle wasting {i.e. the MAFbx (muscle atrophy F-box protein, also called atrogin-1) and MuRF-1 [muscle-specific RING (really interesting new gene) finger 1] ubiquitin–protein ligases}. The characterization of their signalling pathways is leading to new pharmacological approaches that can be useful to block or partially prevent muscle wasting in human patients.

Transcriptional reprogramming during reloading of atrophied rat soleus muscle

The hypothesis was tested that differential, coregulated transcriptional adaptations of various cellular pathways would occur early with increased mechanical loading of atrophied skeletal muscle and relate to concurrent damage of muscle fibers. Atrophy and slow-to-fast fiber transformation of rat soleus muscle was provoked by 14 days of hindlimb suspension (HS). Subsequent reloading of hindlimbs caused a fourfold increase in the percentage of muscle fibers, demonstrating endomysial tenascin-C staining. Five days after reloading, when 10% of the fibers were damaged, the normal muscle weight and slow-type fiber percentage were reestablished. Microarray analysis revealed major, biphasic patterns of gene expressional alterations with reloading that distinguish between treatments and gene ontologies. Transcript levels of factors involved in protein synthesis and certain proteasomal mRNAs were increased after 1 day of reloading and correlated to the percentage of fibers surrounded by tenascin-C. By contrast, levels of gene messages for fatty acid transporters, respiratory chain constituents, and voltage-gated cation channels were transiently reduced after 1 day of muscle loading and associated with the number of damaged fibers and the regain in muscle weight. This coregulation points toward important retooling of oxidative metabolism and the T- and SR-tubular systems with rebuilding of slow fibers. The observations demonstrate that early nuclear reprogramming with reloading of atrophic soleus muscle is coordinated and links to the processes involved in mechanical damage and regeneration of muscle fibers.

Calpain 3 participates in sarcomere remodeling by acting upstream of the ubiquitin-proteasome pathway

Mutations in the non-lysosomal cysteine protease calpain 3 cause limb-girdle muscular dystrophy type 2A (LGMD2A). Our previous studies of the calpain 3 knockout mouse (C3KO) suggested a role for calpain 3 in sarcomere formation and remodeling. Calpain 3 may mediate remodeling by cleavage and release of myofibrillar proteins, targeting them for ubiquitination and proteasomal degradation. Loss of proper protein turnover may be the basis for this muscle disease. To test this hypothesis in vivo, we used an experimental model of hindlimb unloading and reloading that has been shown to induce sarcomere remodeling. We showed that the rate of atrophy and especially the rate of growth are decreased in C3KO muscles under conditions promoting sarcomere remodeling. In wild-type mice, an elevated level of ubiquitinated proteins was observed during muscle reloading, which is presumably necessary to remove atrophy-specific and damaged proteins. This increase in ubiquitination correlated with an increase in calpain 3 expression. C3KO muscles did not show any increase in ubiquitination at the reloading stage, suggesting that calpain 3 is necessary for ubiquitination and that it acts upstream of the ubiquitination machinery. We found upregulation of heat shock proteins in C3KO muscles following challenge with a physiological condition that requires highly increased protein degradation. Furthermore, old C3KO mice show evidence of insoluble protein aggregate formation in skeletal muscles. These studies suggest that accumulation of aged and damaged proteins can lead to cellular toxicity and a cell stress response in C3KO muscles, and that these characteristics are pathological features of LGMD2A.

Response of the ubiquitin-proteasome pathway to changes in muscle activity

The ubiquitin-proteasome pathway plays a critical role in the adaptation of skeletal muscle to persistent decreases or increases in muscle activity. This article outlines the basics of pathway function and reviews what we know about pathway responses to altered muscle use. The ubiquitin-proteasome pathway regulates proteolysis in mammalian cells by attaching ubiquitin polymers to damaged proteins; this targets the protein for degradation via the 26S proteasome. The pathway is constitutively active in muscle and continually regulates protein turnover. Conditions of decreased muscle use, e.g., unloading, denervation, or immobilization, stimulate general pathway activity. This activity increase is caused by upregulation of regulatory components in the pathway and leads to accelerated proteolysis, resulting in net loss of muscle protein. Pathway activity is also increased in response to exercise, a two-phase response. An immediate increase in selective ubiquitin conjugation by constitutive pathway components contributes to exercise-stimulated signal transduction. Over hours-to-days, exercise also stimulates a delayed increase in general ubiquitin conjugating activity by inducing expression of key components in the pathway. This increase mediates a late-phase rise in protein degradation that is required for muscle adaptation to exercise. Thus the ubiquitin-proteasome pathway functions as an essential mediator of muscle remodeling, both in atrophic states and exercise training.

Recovery of the soleus muscle after short- and long-term disuse induced by hindlimb unloading: effects on the electrical properties and myosin heavy chain profile

The hindlimb unloading (HU) rat is a model of muscle disuse characterized by atrophy and slow-to-fast phenotype transition of the postural muscles, such as the soleus. We previously found that the resting sarcolemmal chloride conductance (gCl) that is typically lower in slow-twitch myofibers than in fast ones increased in soleus fibers following 1 to 3 weeks of HU in accord with the slow-to-fast transition of myosin heavy chain (MHC) isoforms. Nevertheless, the gCl already raised after a 3-day HU, whereas no change in MHC expression was detected. The present work evaluates the ability of soleus muscle to recover on return to normal load after a short (3 days) or long (2 weeks) disuse period. The changes observed after a 2-week HU were slowly reversible, since 3-4 weeks of reloading were needed to completely recover gCl, fiber diameter, MHC expression pattern, as well as the mechanical threshold Rheobase, an index of calcium homeostasis. After 3-day HU, the gCl increased homogeneously in most of the soleus muscle fibers and gCl recovery was rapidly completed after 4-day reloading. These results suggest different induction mechanisms for gCl augmentation after the short and long HU periods, as well as a possible role for gCl in the slow muscle adaptation to disuse.

Control of ubiquitination in skeletal muscle wasting

The ubiquitin proteasome system is now well recognized to play a role in mediating skeletal muscle protein wasting. Ubiquitin exerts its effects by covalent attachment to other proteins. Increased ubiquitination of muscle proteins has been observed in a number of conditions of atrophy suggesting that flux through the pathway may be regulated by controlling availability of ubiquitinated substrates for the proteasome. Therefore the enzymes that control ubiquitination of proteins likely play critical roles in regulating flux through the pathway, are sites of activation by catabolic stimuli and potentially good drug targets in the search for therapies for wasting disorders. In this article, the enzymes that can modulate ubiquitination are briefly reviewed and the current data regarding regulation of these enzymes in skeletal muscle are described. Physiological regulators of muscle size appear to modulate many of these enzymes and several of these regulators appear to do so via signaling pathways that involve Akt or NFkappaB. Further work needs to be done to identify all the enzymes that are involved in controlling ubiquitination in muscle, to characterize their regulation by non-transcriptional mechanisms also, and most importantly to identify their target substrates and to determine how these various pathways of ubiquitination work together to mediate the catabolic stimulus.

Transcriptional reprogramming and ultrastructure during atrophy and recovery of mouse soleus muscle

This study investigated the use of the hindlimb suspension (HS) and reloading model of mice for the mapping of ultrastructural and gene expressional alterations underlying load-dependent muscular adaptations. Mice were hindlimb suspended for 7 days or kept as controls (n = 12). Soleus muscles were harvested after HS (HS7, n = 23) or after resuming ambulatory cage activity (reloading) for either 1 day (R1, n = 13) or 7 days (R7, n = 9). Using electron microscopy, a reduction in mean fiber area (-37%) and in capillary-to-fiber ratio (from 1.83 to 1.42) was found for HS7. Subsequent reloading caused an increase in interstitial cells (+96%) and in total capillary length (+57%), whereas mean fiber area and capillary-to-fiber ratio did not significantly change compared with HS. Total RNA in the soleus muscle was altered with both HS (-63%) and reloading (+108% in R7 compared with control). This is seen as an important adaptive mechanism. Gene expression alterations were assessed by a muscle-specific low-density cDNA microarray. The transcriptional adjustments indicate an early increase of myogenic factors during reloading together with an overshoot of contractile (MyHC I and IIa) and metabolic (glycolytic and oxidative) mRNA amounts and suggest mechano-sensitivity of factors keeping the sarcomeres in register (desmin, titin, integrin-beta1). Important differences to published data from former rat studies were found with the mouse HS model for contractile and glycolytic enzyme expression. These species-specific differences need to be considered when transgenic mice are used for the elucidation of monogenetic factors in mechano-dependent muscle plasticity.