Calpain-3 is autolyzed and hence activated in human skeletal muscle 24 h following a single bout of eccentric exercise

Protein Turnover in Skeletal Muscle: Looking at Molecular Regulation towards an Active Lifestyle

Skeletal muscle is a highly plastic tissue, able to change its mass and functional properties in response to several stimuli. Skeletal muscle mass is influenced by the balance between protein synthesis and breakdown, which is regulated by several signaling pathways. The relative contribution of Akt/mTOR signaling, ubiquitin-proteasome pathway, autophagy among other signaling pathways to protein turnover and, therefore, to skeletal muscle mass, differs depending on the wasting or loading condition and muscle type. By modulating mitochondria biogenesis, PGC-1α has a major role in the cell's bioenergetic status and, thus, on protein turnover. In fact, rates of protein turnover regulate differently the levels of distinct protein classes in response to atrophic or hypertrophic stimuli. Mitochondrial protein turnover rates may be enhanced in wasting conditions, whereas the increased turnover of myofibrillar proteins triggers muscle mass gain. The present review aims to update the knowledge on the molecular pathways implicated in the regulation of protein turnover in skeletal muscle, focusing on how distinct muscle proteins may be modulated by lifestyle interventions with emphasis on exercise training. The comprehensive analysis of the anabolic effects of exercise programs will pave the way to the tailored management of muscle wasting conditions.

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

Many neuromuscular disorders can have a differential impact on a specific myofibre type, forming the central premise of this review. The many different skeletal muscles in mammals contain a spectrum of slow- to fast-twitch myofibres with varying levels of protein isoforms that determine their distinctive contractile, metabolic, and other properties. The variations in functional properties across the range of classic 'slow' to 'fast' myofibres are outlined, combined with exemplars of the predominantly slow-twitch soleus and fast-twitch extensor digitorum longus muscles, species comparisons, and techniques used to study these properties. Other intrinsic and extrinsic differences are discussed in the context of slow and fast myofibres. These include inherent susceptibility to damage, myonecrosis, and regeneration, plus extrinsic nerves, extracellular matrix, and vasculature, examined in the context of growth, ageing, metabolic syndrome, and sexual dimorphism. These many differences emphasise the importance of carefully considering the influence of myofibre-type composition on manifestation of various neuromuscular disorders across the lifespan for both sexes. Equally, understanding the different responses of slow and fast myofibres due to intrinsic and extrinsic factors can provide deep insight into the precise molecular mechanisms that initiate and exacerbate various neuromuscular disorders. This focus on the influence of different myofibre types is of fundamental importance to enhance translation for clinical management and therapies for many skeletal muscle disorders.

Muscle calcium stress cleaves junctophilin1, unleashing a gene regulatory program predicted to correct glucose dysregulation

Calcium ion movements between cellular stores and the cytosol govern muscle contraction, the most energy-consuming function in mammals, which confers skeletal myofibers a pivotal role in glycemia regulation. Chronic myoplasmic calcium elevation ("calcium stress"), found in malignant hyperthermia-susceptible (MHS) patients and multiple myopathies, has been suggested to underlie the progression from hyperglycemia to insulin resistance. What drives such progression remains elusive. We find that muscle cells derived from MHS patients have increased content of an activated fragment of GSK3β - a specialized kinase that inhibits glycogen synthase, impairing glucose utilization and delineating a path to hyperglycemia. We also find decreased content of junctophilin1, an essential structural protein that colocalizes in the couplon with the voltage-sensing CaV1.1, the calcium channel RyR1 and calpain1, accompanied by an increase in a 44 kDa junctophilin1 fragment (JPh44) that moves into nuclei. We trace these changes to activated proteolysis by calpain1, secondary to increased myoplasmic calcium. We demonstrate that a JPh44-like construct induces transcriptional changes predictive of increased glucose utilization in myoblasts, including less transcription and translation of GSK3β and decreased transcription of proteins that reduce utilization of glucose. These effects reveal a stress-adaptive response, mediated by the novel regulator of transcription JPh44.

Mechanisms of eccentric contraction-induced muscle damage and nutritional supplementations for mitigating it

Eccentric contraction (ECC) often results in large and long-lasting force deficits accompanied by muscle soreness, primarily due to muscle damage. In this sense, exercises that involve ECC are less desirable. Paradoxically, exercise training that includes a substantial eccentric phase leads to a more powerful activation of the genes responsible for skeletal muscle remodeling (e.g., hypertrophy) than other types of training that emphasize a concentric or isometric phase. Therefore, effective strategies that lessen ECC-induced muscle damage will be of interest and importance to many individuals. The purpose of this brief review is to highlight the published literature on the effects of ECC and/or nutritional supplementations on proteins, lipids, metabolic and ionic changes, and enzyme activities in skeletal muscles subjected to an acute bout of ECC. First, we discuss the potential mechanisms by which ECC causes muscle damage. Previous findings implicate a Ca²⁺ overload-oxidative modification pathway as one possible mechanism contributing to muscle damage. Thereafter, the efficacy of two nutritional supplementations, i.e., L-arginine and antioxidant, is discussed because L-arginine and antioxidant would be expected to ameliorate the adverse effects of Ca²⁺ overload and oxidative modification, respectively. Of these, L-arginine ingestion before ECC seems likely to be the effective strategy for mitigating ECC-related proteolysis. More studies are needed to establish the effectiveness of antioxidant ingestion. The application of effective strategies against muscle damage may contribute to improvements in health and fitness, muscle function, and sports performance.

Ryanodine receptors mediate high intracellular Ca2+ and some myocyte damage following eccentric contractions in rat fast-twitch skeletal muscle

Eccentric contractions (ECC) facilitate cytosolic calcium ion (Ca2+) release from the sarcoplasmic reticulum (SR) and Ca2+ influx from the extracellular space. Ca2+ is a vital signaling messenger that regulates multiple cellular processes via its spatial and temporal concentration ([Ca2+]i) dynamics. We hypothesized that 1) a specific pattern of spatial/temporal intramyocyte Ca2+ dynamics portends muscle damage following ECC and 2) these dynamics would be regulated by the ryanodine receptor (RyR). [Ca2+]i in the tibialis anterior muscles of anesthetized adult Wistar rats was measured by ratiometric (i.e., ratio, R, 340/380 nm excitation) in vivo bioimaging with Fura-2 pre-ECC and at 5 and 24 h post-ECC (5 × 40 contractions). Separate groups of rats received RyR inhibitor dantrolene (DAN; 10 mg/kg ip) immediately post-ECC (+DAN). Muscle damage was evaluated by histological analysis on hematoxylin-eosin stained muscle sections. Compared with control (CONT, no ECC), [Ca2+]i distribution was heterogeneous with increased percent total area of high [Ca2+]i sites (operationally defined as R ≥ 1.39, i.e., ≥1 SD of mean control) 5 h post-ECC (CONT, 14.0 ± 8.0; ECC5h: 52.0 ± 7.4%, P < 0.01). DAN substantially reduced the high [Ca2+]i area 5 h post-ECC (ECC5h + DAN: 6.4 ± 3.1%, P < 0.01) and myocyte damage (ECC24h, 63.2 ± 1.0%; ECC24h + DAN: 29.1 ± 2.2%, P < 0.01). Temporal and spatially amplified [Ca2+]i fluctuations occurred regardless of DAN (ECC vs. ECC + DAN, P > 0.05). These results suggest that the RyR-mediated local high [Ca2+]i itself is related to the magnitude of muscle damage, whereas the [Ca2+]i fluctuation is an RyR-independent phenomenon.

Comparison of Acellular Solutions for Ex-situ Perfusion of Amputated Limbs

INTRODUCTION

Hypothermic ex-situ machine perfusion (MP) has been shown to be a promising alternative to static cold storage (SCS) for preservation of solid organs for transplantation and vascularized composite allotransplantation. Perfusion with blood-based perfusion solutions in austere environments is problematic due to their need for appropriate storage and short shelf life, making it impractical for military and emergency use. Acellular perfusion has been shown to be effective, but the ideal perfusate solution for MP of amputated limbs is yet to be determined. The purpose of this study is to evaluate the efficacy of alternative perfusate solutions, such as dextran-enriched Phoxilium, Steen, and Phoxilium in ex-vivo hypothermic MP of amputated limbs in a porcine model.

MATERIALS AND METHODS

Amputated forelimbs from Yorkshire pigs (n = 8) were preserved either in SCS (n = 2) at 4°C for 12 hours or machine-perfused at 10°C for 12 hours with oxygenated perfusion solutions (n = 6) at a constant flow rate. The perfusates used include modified Steen-solution, Phoxilium (PHOX), or Phoxilium enriched with dextran-40 (PHODEX). The perfusate was exchanged after 1 and 6 hours of perfusion. Machine data were recorded continuously. Perfusate samples for clinical chemistry, blood gas analysis, and muscle biopsies were procured at specific timepoints and subsequently analyzed. In this semi in-vivo study, limb replantation has not been performed.

RESULTS

After amputation, every limb was successfully transferred and connected to our perfusion device. The mean total ischemia time was 77.5 ± 5.24 minutes. The temperature of the perfusion solution was maintained at 10.18 ± 2.01°C, and perfusion pressure at 24.48 ± 10.72 mmHg. Limb weight increased by 3% in the SCS group, 36% in the PHODEX group, 25% in the Steen group, and 58% in the PHOX group after 12 hours. This increase was significant in the PHOX group compared with the SCS group. All perfusion groups showed a pressure increase of 10.99 mmHg over time due to edema. The levels of HIF-1a decreased over time in all groups except the Steen and the PHODEX group. The biomarkers of muscle injury in the perfusate samples, such as creatine kinase and lactate-dehydrogenase, showed a significant difference between groups, with highest values in the PHODEX group. No significant differences were found in the results of the blood gas analysis.

CONCLUSION

With the exception of significantly higher levels of creatine kinase and lactate dehydrogenase, MP with dextran-enriched Phoxilium provides similar results as that of the commercially available perfusates such as Steen, without the need for cold storage, and at circa 5% of the cost of the Steen solution. Further large-scale replantation studies are necessary to evaluate the efficacy of dextran-enriched Phoxilium as an alternate perfusate solution.

Can Exercise-Induced Muscle Damage Be a Good Model for the Investigation of the Anti-Inflammatory Properties of Diet in Humans?

Subclinical, low-grade, inflammation is one of the main pathophysiological mechanisms underlying the majority of chronic and non-communicable diseases. Several methodological approaches have been applied for the assessment of the anti-inflammatory properties of nutrition, however, their impact in human body remains uncertain, because of the fact that the majority of the studies reporting anti-inflammatory effect of dietary patterns, have been performed under laboratory settings and/or in animal models. Thus, the extrapolation of these results to humans is risky. It is therefore obvious that the development of an inflammatory model in humans, by which we could induce inflammatory responses to humans in a regulated, specific, and non-harmful way, could greatly facilitate the estimation of the anti-inflammatory properties of diet in a more physiological way and mechanistically relevant way. We believe that exercise-induced muscle damage (EIMD) could serve as such a model, either in studies investigating the homeostatic responses of individuals under inflammatory stimuli or for the estimation of the anti-inflammatory or pro-inflammatory potential of dietary patterns, foods, supplements, nutrients, or phytochemicals. Thus, in this review we discuss the possibility of exercise-induced muscle damage being an inflammation model suitable for the assessment of the anti-inflammatory properties of diet in humans.

The Role of Calpains in Skeletal Muscle Remodeling with Exercise and Inactivity-induced Atrophy

Calpains are cysteine proteases expressed in skeletal muscle fibers and other cells. Although calpain was first reported to act as a kinase activating factor in skeletal muscle, the consensus is now that calpains play a canonical role in protein turnover. However, recent evidence reveals new and exciting roles for calpains in skeletal muscle. This review will discuss the functions of calpains in skeletal muscle remodeling in response to both exercise and inactivity-induced muscle atrophy. Calpains participate in protein turnover and muscle remodeling by selectively cleaving target proteins and creating fragmented proteins that can be further degraded by other proteolytic systems. Nonetheless, an often overlooked function of calpains is that calpain-mediated cleavage of proteins can result in fragmented proteins that are biologically active and have the potential to actively influence cell signaling. In this manner, calpains function beyond their roles in protein turnover and influence downstream signaling effects. This review will highlight both the canonical and noncanonical roles that calpains play in skeletal muscle remodeling including sarcomere transformation, membrane repair, triad junction formation, regulation of excitation-contraction coupling, protein turnover, cell signaling, and mitochondrial function. We conclude with a discussion of key unanswered questions regarding the roles that calpains play in skeletal muscle.

The ERG1a potassium channel increases basal intracellular calcium concentration and calpain activity in skeletal muscle cells

Background

Skeletal muscle atrophy is the net loss of muscle mass that results from an imbalance in protein synthesis and protein degradation. It occurs in response to several stimuli including disease, injury, starvation, and normal aging. Currently, there is no truly effective pharmacological therapy for atrophy; therefore, exploration of the mechanisms contributing to atrophy is essential because it will eventually lead to discovery of an effective therapeutic target. The ether-a-go-go related gene (ERG1A) K⁺ channel has been shown to contribute to atrophy by upregulating ubiquitin proteasome proteolysis in cachectic and unweighted mice and has also been implicated in calcium modulation in cancer cells.

Methods

We transduced C₂C₁₂ myotubes with either a human ERG1A encoded adenovirus or an appropriate control virus. We used fura-2 calcium indicator to measure intracellular calcium concentration and Calpain-Glo assay kits (ProMega) to measure calpain activity. Quantitative PCR was used to monitor gene expression and immunoblot evaluated protein abundances in cell lysates. Data were analyzed using either a Student’s t test or two-way ANOVAs and SAS software as indicated.

Results

Expression of human ERG1A in C₂C₁₂ myotubes increased basal intracellular calcium concentration 51.7% (p < 0.0001; n = 177). Further, it increased the combined activity of the calcium-activated cysteine proteases, calpain 1 and 2, by 31.9% (p < 0.08; n = 24); these are known to contribute to degradation of myofilaments. The increased calcium levels are likely a contributor to the increased calpain activity; however, the change in calpain activity may also be attributable to increased calpain protein abundance and/or a decrease in levels of the native calpain inhibitor, calpastatin. To explore the enhanced calpain activity further, we evaluated expression of calpain and calpastatin genes and observed no significant differences. There was no change in calpain 1 protein abundance; however, calpain 2 protein abundance decreased 40.7% (p < 0.05; n = 6). These changes do not contribute to an increase in calpain activity; however, we detected a 31.7% decrease (p < 0.05; n = 6) in calpastatin which could contribute to enhanced calpain activity.

Conclusions

Human ERG1A expression increases both intracellular calcium concentration and combined calpain 1 and 2 activity. The increased calpain activity is likely a result of the increased calcium levels and decreased calpastatin abundance.

l-arginine ingestion inhibits eccentric contraction-induced proteolysis and force deficit via S-nitrosylation of calpain

It has been shown that calpains are involved in the proteolysis of muscle proteins that occurs with eccentric contraction (ECC) and that exogenously applied nitric oxide decreases the calpain‐mediated proteolysis. The aim of this study was to examine the effects of ingestion of l‐arginine (ARG), a nitric oxide precursor, on ECC‐related calpain activation. In the first and second experiments, male Wistar rats were given ARG in water for 7 days starting from 3 days before the ECC protocol (average ingestion, ~600 mg kg‐body wt⁻¹ day⁻¹). Tibialis anterior muscles underwent 200 repeated ECCs and, subsequently, were excised 3 days later. Whole muscle analyses (the first experiment) revealed that ARG attenuated ECC‐induced force deficit and autolysis of calpain‐1, and increased the amounts of S‐nitrosylated calpain‐1. Regarding ryanodine receptor (RyR) and dihydropyridine receptor (DHPR), ECC‐induced proteolysis was completely inhibited by ARG, whereas the inhibition was partial for junctophilin‐1 (JP1). Skinned fiber analyses (the second experiment) showed that ARG also inhibited ECC‐elicited reductions in the ratio of depolarization‐induced to maximum Ca²⁺‐activated force. In the third experiment, homogenates of rested muscles were treated with S‐nitrosylating agent, S‐nitrosoglutathione (GSNO), and/or high Ca²⁺ concentration ([Ca²⁺]). Treatment with high [Ca²⁺] and without GSNO produced proteolysis of RyR, DHPR, and JP1. On the other hand, treatment with high [Ca²⁺] and GSNO caused complete inhibition of RyR and DHPR proteolysis and partial inhibition of JP1 proteolysis. These results indicate that ARG ingestion can attenuate ECC‐induced proteolysis of Ca²⁺ regulatory proteins and force deficit by decreasing calpain activation via S‐nitrosylation.

Assessing the Role of Muscle Protein Breakdown in Response to Nutrition and Exercise in Humans

Muscle protein breakdown (MPB) is an important metabolic component of muscle remodeling, adaptation to training, and increasing muscle mass. Degradation of muscle proteins occurs via the integration of three main systems—autophagy and the calpain and ubiquitin-proteasome systems. These systems do not operate independently, and the regulation is complex. Complete degradation of a protein requires some combination of the systems. Determination of MPB in humans is technically challenging, leading to a relative dearth of information. Available information on the dynamic response of MPB primarily comes from stable isotopic methods with expression and activity measures providing complementary information. It seems clear that resistance exercise increases MPB, but not as much as the increase in muscle protein synthesis. Both hyperaminoacidemia and hyperinsulinemia inhibit the post-exercise response of MPB. Available data do not allow a comprehensive examination of the mechanisms behind these responses. Practical nutrition recommendations for interventions to suppress MPB following exercise are often made. However, it is likely that some degree of increased MPB following exercise is an important component for optimal remodeling. At this time, it is not possible to determine the impact of nutrition on any individual muscle protein. Thus, until we can develop and employ better methods to elucidate the role of MPB following exercise and the response to nutrition, recommendations to optimize post exercise nutrition should focus on the response of muscle protein synthesis. The aim of this review is to provide a comprehensive examination of the state of knowledge, including methodological considerations, of the response of MPB to exercise and nutrition in humans.

Insertion sequence 1 from calpain-3 is functional in calpain-2 as an internal propeptide

Calpains are intracellular, calcium-activated cysteine proteases. Calpain-3 is abundant in skeletal muscle, where its mutation-induced loss of function causes limb-girdle muscular dystrophy type 2A. Unlike the small subunit-containing calpain-1 and -2, the calpain-3 isoform homodimerizes through pairing of its C-terminal penta-EF-hand domain. It also has two unique insertion sequences (ISs) not found in the other calpains: IS1 within calpain-3's protease core and IS2 just prior to the penta-EF-hand domain. Production of either native or recombinant full-length calpain-3 to characterize the function of these ISs is challenging. Therefore, here we used recombinant rat calpain-2 as a stable surrogate and inserted IS1 into its equivalent position in the protease core. As it does in calpain-3, IS1 occupied the catalytic cleft and restricted the enzyme's access to substrate and inhibitors. Following activation by Ca²⁺, IS1 was rapidly cleaved by intramolecular autolysis, permitting the enzyme to freely accept substrate and inhibitors. The surrogate remained functional until extensive intermolecular autoproteolysis inactivated the enzyme, as is typical of calpain-2. Although the small-molecule inhibitors E-64 and leupeptin limited intermolecular autolysis of the surrogate, they did not block the initial intramolecular cleavage of IS1, establishing its role as a propeptide. Surprisingly, the large-molecule calpain inhibitor, calpastatin, completely blocked enzyme activity, even with IS1 intact. We suggest that calpastatin is large enough to oust IS1 from the catalytic cleft and take its place. We propose an explanation for why calpastatin can inhibit calpain-2 bearing the IS1 insertion but cannot inhibit WT calpain-3.

Minimal Evidence for a Secondary Loss of Strength After an Acute Muscle Injury: A Systematic Review and Meta-Analysis

BACKGROUND

An immediate loss of strength follows virtually all types of muscle injury but there is debate whether the initial strength loss is maximal or if a secondary loss of strength occurs during the first 3 days post-injury.

OBJECTIVE

The objective of this analysis was to conduct a systematic review and meta-analysis of the research literature to determine if a secondary loss of strength occurs after an injurious initiating event.

METHODS

Literature searches were performed using eight electronic databases (e.g., PubMed, Cochrane Library). Search terms included skeletal muscle AND (injur* OR damage*) AND (strength OR force OR torque). The extracted strength data were converted to a standard format by calculating the standardized mean difference, which is reported as the effect size (ES) along with its 95 % confidence interval (CI). The calculation of ES was designed so that a negative ES that was statistically less than zero would be interpreted as indicating a secondary loss of strength.

RESULTS

A total of 223 studies with over 4000 human and animal subjects yielded data on 262 independent groups and a total of 936 separate ESs. Our overall meta-analysis yielded a small-to-medium, positive overall ES that was statistically greater than zero (overall ES = +0.34, 95 % CI 0.27-0.40; P < 0.00000001). Considerable variation in ES was observed among studies (I ² = 86 %), which could be partially explained by the research group conducting the study, sex of the subject, day of post-injury strength assessment, whether fatigue was present immediately post-injury, and the muscle group injured. From the subgroup meta-analyses probing these variables, 36 subgroup ESs were calculated and none were statistically less than zero.

CONCLUSION

Overall, our findings do not support the presence of a secondary loss of strength following an acute muscle injury, and strongly suggest that strength, on average, recovers steadily over the first 3 days post-injury.

Past injurious exercise attenuates activation of primary calcium-dependent injury pathways in skeletal muscle during subsequent exercise

Past contraction‐induced skeletal muscle injury reduces the degree of subsequent injury; this phenomenon is called the “repeated bout effect (RBE).” This study addresses the mechanisms underlying the RBE, focusing on primary calcium‐dependent injury pathways. Wistar rats were subdivided into single injury (SI) and repeated injury (RI) groups. At age 10 weeks, the right gastrocnemius muscle in each rat in the RI group was subjected to strenuous eccentric contractions (ECs). Subsequently, mild ECs were imposed on the same muscle of each rat at 14 weeks of age in both groups. One day after the exercise, the RI group showed a lower strength deficit than did the SI group, and neither group manifested any increase in membrane permeability. The concentration of protein carbonyls and activation of total calpain increased after ECs given at the age of 14 weeks. Nonetheless, these increases were lower in the RI group than in the SI group. Furthermore, calcium‐dependent autolysis of calpain‐1 and calpain‐3 in the RI group was diminished as compared with that in the SI group. Although peak ankle joint torque and total force generation during ECs at the age of 14 weeks were similar between the two groups, phosphorylation of JNK (Thr¹⁸³/Tyr¹⁸⁵), an indicator of mechanical stress applied to a muscle, was lower in the RI group than in the SI group. These findings suggest that activation of the primary calcium‐dependent injury pathways is attenuated by past injurious exercise, and mechanical stress applied to muscle fibers during ECs may decrease in the RBE.

Calcium dysregulation, functional calpainopathy, and endoplasmic reticulum stress in sporadic inclusion body myositis

Sporadic inclusion body myositis (IBM) is the most common primary myopathy in the elderly, but its pathoetiology is still unclear. Perturbed myocellular calcium (Ca²⁺) homeostasis can exacerbate many of the factors proposed to mediate muscle degeneration in IBM, such as mitochondrial dysfunction, protein aggregation, and endoplasmic reticulum stress. Ca²⁺ dysregulation may plausibly be initiated in IBM by immune-mediated membrane damage and/or abnormally accumulating proteins, but no studies to date have investigated Ca²⁺ regulation in IBM patients. We first investigated protein expression via immunoblot in muscle biopsies from IBM, dermatomyositis, and non-myositis control patients, identifying several differentially expressed Ca²⁺-regulatory proteins in IBM. Next, we investigated the Ca²⁺-signaling transcriptome by RNA-seq, finding 54 of 183 (29.5%) genes from an unbiased list differentially expressed in IBM vs. controls. Using an established statistical approach to relate genes with causal transcription networks, Ca²⁺ abundance was considered a significant upstream regulator of observed whole-transcriptome changes. Post-hoc analyses of Ca²⁺-regulatory mRNA and protein data indicated a lower protein to transcript ratio in IBM vs. controls, which we hypothesized may relate to increased Ca²⁺-dependent proteolysis and decreased protein translation. Supporting this hypothesis, we observed robust (4-fold) elevation in the autolytic activation of a Ca²⁺-activated protease, calpain-1, as well as increased signaling for translational attenuation (eIF2α phosphorylation) downstream of the unfolded protein response. Finally, in IBM samples we observed mRNA and protein under-expression of calpain-3, the skeletal muscle-specific calpain, which broadly supports proper Ca²⁺ homeostasis. Together, these data provide novel insight into mechanisms by which intracellular Ca²⁺ regulation is perturbed in IBM and offer evidence of pathological downstream effects.

Reduced skeletal muscle fiber size following caloric restriction is associated with calpain-mediated proteolysis and attenuation of IGF-1 signaling

Caloric restriction decreases skeletal muscle mass in mammals, principally due to a reduction in fiber size. The effect of suboptimal nutrient intake on skeletal muscle metabolic properties in neonatal calves was examined. The longissimus muscle (LM) was collected after a control (CON) or caloric restricted (CR) diet was cosnumed for 8 wk and muscle fiber size, gene expression, and metabolic signal transduction activity were measured. Results revealed that CR animals had smaller (P < 0.05) LM fiber cross-sectional area than CON, as expected. Western blot analysis detected equivalent amounts of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) but reduced (P < 0.05) amounts of the splice-variant, PGC1α-4 in CR LM. Expression of IGF-1, a PGC1α-4 target gene, was 40% less (P < 0.05) in CR than CON. Downstream mediators of autocrine IGF-1 signaling also are attenuated in CR by comparison with CON. The amount of phosphorylated AKT1 was less (P < 0.05) in CR than CON. The ratio of p4EBP1^T37/46 to total 4EBP1, a downstream mediator of AKT1, did not differ between CON and CR. By contrast, protein lysates from CR LM contained less (P < 0.05) total glycogen synthase kinase-3β (GSK3β) and phosphorylated GSK3β than CON LM, suggesting blunted protein synthesis. Smaller CR LM fiber size associates with increased (P < 0.05) calpain 1 (CAPN1) activity coupled with lower (P < 0.05) expression of calpastatin, the endogenous inhibitor of CAPN1. Atrogin-1 and MuRF expression and autophagy components were unaffected by CR. Thus CR suppresses the hypertrophic PGC1α-4/IGF-1/AKT1 pathway while promoting activation of the calpain system.

Calpain-3-mediated regulation of the Na⁺-Ca²⁺ exchanger isoform 3

Ca²⁺ disturbances are observed when Ca²⁺-dependent cysteine proteases malfunction, causing muscle weakness and wasting. For example, loss of calpain-3 (CAPN3) activity leads to limb-girdle muscular dystrophy 2A (LGMD2A). In neuronal excitotoxicity, the cleavage of the Na⁺-Ca²⁺ exchanger isoform 3 (NCX3) has been associated with an increase in activity and elevation of the Ca²⁺ content in the endoplasmic reticulum (ER). Since NCX3 is expressed in skeletal muscle, we evaluated the cleavage of different NCX3 splice variants by CAPN1 and CAPN3. Using Fura-2-based cellular Ca²⁺ imaging, we showed for the first time that CAPN3 increases NCX3 activity and that only NCX3-AC, the variant predominantly expressed in skeletal muscle, is sensitive to calpain. The silencing of the endogenous CAPN1 and the expression of the inactive form of CAPN3 (C129S CAPN3) confirmed the specificity for CAPN1 and CAPN3. Functional studies revealed that cellular Ca²⁺ uptake through the reverse mode of NCX3 was significantly increased independently of the mode of activation of the exchanger by either a rise in intracellular Ca²⁺ ([Ca²⁺]_i) or Na⁺ ([Na⁺]_i). Subsequently, the sensitivity to CAPN1 and CAPN3 could be abrogated by removal of the six residues coded in exon C of NCX3-AC. Additionally, mutation of the Leu-600 and Leu-601 suggested the presence of a cleavage site at Leu-602. The increased Ca²⁺ uptake of NCX3 might participate in the Ca²⁺ refilling of the sarcoplasmic reticulum (SR) after the excitation-contraction uncoupling following exercise and therefore be implicated in the impaired reticular Ca²⁺ storage observed in LGMD2A.

Changes in α7β1 integrin signaling after eccentric exercise in heat-shocked rat soleus

INTRODUCTION

α7β1 integrin links the extracellular matrix to the focal adhesion (FA) in skeletal muscle and serves as a stabilizing and signal relayer. Heat shock (HS) induces expression of proteins that interact with the FA.

METHODS

Male Wistar rats were assigned to 1 of 3 groups: control (CON); eccentric exercise (EE); or EE+HS (HS). Soleus muscle was analyzed at 2 h and 48 h post-exercise.

RESULTS

The 120-kDa α7 integrin decreased in the EE and HS groups, and the 70-kDa peptide decreased in the EE group at 2 h post-exercise. Total expression of focal adhesion kinase (FAK) and RhoA were decreased in EE and HS at 2 h post-exercise. Expression of phosphorylated FAK(397) decreased in the EE group but not the HS group at 2 h post-exercise.

CONCLUSIONS

Long-duration EE may cause alterations in the FA in rat soleus muscle through the α7 integrin subunit and FAK.

Altered cross-bridge properties in skeletal muscle dystrophies

Force and motion generated by skeletal muscle ultimately depends on the cyclical interaction of actin with myosin. This mechanical process is regulated by intracellular Ca²⁺ through the thin filament-associated regulatory proteins i.e.; troponins and tropomyosin. Muscular dystrophies are a group of heterogeneous genetic affections characterized by progressive degeneration and weakness of the skeletal muscle as a consequence of loss of muscle tissue which directly reduces the number of potential myosin cross-bridges involved in force production. Mutations in genes responsible for skeletal muscle dystrophies (MDs) have been shown to modify the function of contractile proteins and cross-bridge interactions. Altered gene expression or RNA splicing or post-translational modifications of contractile proteins such as those related to oxidative stress, may affect cross-bridge function by modifying key proteins of the excitation-contraction coupling. Micro-architectural change in myofilament is another mechanism of altered cross-bridge performance. In this review, we provide an overview about changes in cross-bridge performance in skeletal MDs and discuss their ultimate impacts on striated muscle function.

The role of proteases in excitation-contraction coupling failure in muscular dystrophy

Duchenne muscular dystrophy (DMD) is one of the most frequent types of muscular dystrophy. Alterations in intracellular calcium (Ca(2+)) handling are thought to contribute to the disease severity in DMD, possibly due to the activation of Ca(2+)-activated proteases. The purpose of this study was twofold: 1) to determine whether prolonged excitation-contraction (E-C) coupling disruption following repeated contractions is greater in animals lacking both dystrophin and utrophin (mdx/Utr(-/-)) compared with mice lacking only dystrophin (mdx); and 2) to assess whether protease inhibition can prevent E-C coupling failure following repeated tetani in these dystrophic mouse models. Excitation-contraction coupling was assessed using Fura-2 ratio, as an index of intracellular free Ca(2+) concentration, in response to electrical stimulation of single muscle fibers from the flexor digitorum brevis muscle. Resting Fura-2 ratio was higher in dystrophic compared with control (Con) fibers, but peak Fura-2 ratios during stimulation were similar in dystrophic and Con fibers. One hour after a series of repeated tetani, peak Fura-2 ratios were reduced by 30 ± 5.6%, 23 ± 2%, and 36 ± 3.1% in mdx, mdx/Utr(+/-), and mdx/Utr(-/-), respectively, with the greatest reduction in mdx/Utr(-/-) fibers (P < 0.05). Protease inhibition attenuated this decrease in peak Fura-2 ratio. These data indicate that E-C coupling impairment after repeated contractions is greatest in fibers lacking both dystrophin and utrophin and that prevention of protease activation can mitigate the prolonged E-C coupling impairment. These data further suggest that acute protease inhibition may be useful in reducing muscle weakness in DMD.

Redox state and mitochondrial respiratory chain function in skeletal muscle of LGMD2A patients

Background

Calpain-3 deficiency causes oxidative and nitrosative stress-induced damage in skeletal muscle of LGMD2A patients, but mitochondrial respiratory chain function and anti-oxidant levels have not been systematically assessed in this clinical population previously.

Methods

We identified 14 patients with phenotypes consistent with LGMD2A and performed CAPN3 gene sequencing, CAPN3 expression/autolysis measurements, and insilico predictions of pathogenicity. Oxidative damage, anti-oxidant capacity, and mitochondrial enzyme activities were determined in a subset of muscle biopsies.

Results

Twenty-one disease-causing variants were detected along the entire CAPN3 gene, five of which were novel (c.338 T>C, c.500 T>C, c.1525-1 G>T, c.2115+4 T>G, c.2366 T>A). Protein- and mRNA-based tests confirmed insilico predictions and the clinical diagnosis in 75% of patients. Reductions in antioxidant defense mechanisms (SOD-1 and NRF-2, but not SOD-2), coupled with increased lipid peroxidation and protein ubiquitination, were observed in calpain-3 deficient muscle, indicating a redox imbalance primarily affecting non-mitochondrial compartments. Although ATP synthase levels were significantly lower in LGMD2A patients, citrate synthase, cytochrome c oxidase, and complex I+III activities were not different from controls.

Conclusions

Despite significant oxidative damage and redox imbalance in cytosolic/myofibrillar compartments, mitochondrial respiratory chain function is largely maintained in skeletal muscle of LGMD2A patients.

Unaccustomed eccentric contractions impair plasma K+ regulation in the absence of changes in muscle Na+,K+-ATPase content

The Na⁺,K⁺-ATPase (NKA) plays a fundamental role in the regulation of skeletal muscle membrane Na⁺ and K⁺ gradients, excitability and fatigue during repeated intense contractions. Many studies have investigated the effects of acute concentric exercise on K⁺ regulation and skeletal muscle NKA, but almost nothing is known about the effects of repeated eccentric contractions. We therefore investigated the effects of unaccustomed maximal eccentric knee extensor contractions on K⁺ regulation during exercise, peak knee extensor muscle torque, and vastus lateralis muscle NKA content and 3-O-MFPase activity. Torque measurements, muscle biopsies, and venous blood samples were taken before, during and up to 7 days following the contractions in six healthy adults. Eccentric contractions reduced peak isometric muscle torque immediately post-exercise by 26±11% and serum creatine kinase concentration peaked 24 h post-exercise at 339±90 IU/L. During eccentric contractions, plasma [K⁺] rose during Set 1 and remained elevated at ∼4.9 mM during sets 4–10; this was despite a decline in work output by Set 4, which fell by 18.9% at set 10. The rise in plasma [K⁺].work⁻¹ ratio was elevated over Set 2 from Set 4– Set 10. Eccentric contractions had no effect on muscle NKA content or maximal in-vitro 3-O-MFPase activity immediately post- or up to 7 d post-exercise. The sustained elevation in plasma [K⁺] despite a decrease in work performed by the knee extensor muscles suggests an impairment in K⁺ regulation during maximal eccentric contractions, possibly due to increased plasma membrane permeability or to excitation-contraction uncoupling.

Acute change of titin at mid-sarcomere remains despite 8 wk of plyometric training

The purpose of this study was to investigate skeletal muscle changes induced by an acute bout of plyometric exercise (PlyEx) both before and after PlyEx training, to understand if titin is affected differently after PlyEx training. Healthy untrained individuals ( N = 11) completed the 1stPlyEx (10 × 10 squat-jumps, 1-min rest). Thereafter, six subjects completed 8 wk of PlyEx, while five controls abstained from any jumping activity. Seven days after the last training session, all subjects completed the 2ndPlyEx. Blood samples were collected before and 6 h and 1, 2, 3, and 4 days after each acute bout of PlyEx, and muscle biopsies 4 days before and 3 days after each acute bout of PlyEx. The 1stPlyEx induced an increase in circulating myoglobin concentration. Muscle sample analysis revealed Z-disk streaming, a stretch or a fragmentation of titin (immunogold), and increased calpain-3 autolysis. After training, 2ndPlyEx did not induce Z-disk streaming or calpain-3 activation. The previously observed post-1stPlyEx positional change of the titin COOH terminus was still present pre-2ndPlyEx, in all trained and all control subjects. Only two controls presented with Z-disk streaming after 2ndPlyEx, while calpain-3 activation was absent in all controls. Eccentric explosive exercise induced a stretch or fragmentation of titin, which presented as a positional change of the COOH terminus. Calpain-3 activation does not occur when titin is already stretched before explosive jumping. Enzymatic digestion results in titin fragmentation, but since an increase in calpain-3 autolysis was visible only after the 1stPlyEx acute bout, fragmentation cannot explain the prolonged positional change.

L-arginine supplementation protects exercise performance and structural integrity of muscle fibers after a single bout of eccentric exercise in rats

Eccentric exercise is known to disrupt sarcolemmal integrity and induce damage of skeletal muscle fibers. We hypothesized that L-arginine (L-Arg; nitric oxide synthase (NOS) substrate) supplementation prior to a single bout of eccentric exercise would diminish exercise-induced damage. In addition, we used N-nitro-L-arginine methyl ester hydrochloride (L-NAME; NOS inhibitor) to clarify the role of native NOS activity in the development of exercise-induced muscle damage. Rats were divided into four groups: non-treated control (C), downhill running with (RA) or without (R) L-Arg supplementation and downhill running with L-NAME supplementation (RN). Twenty four hours following eccentric exercise seven rats in each group were sacrificed and soleus muscles were dissected and frozen for further analysis. The remaining seven rats in each group were subjected to the exercise performance test. Our experiments showed that L-Arg supplementation prior to a single bout of eccentric exercise improved subsequent exercise performance capacity tests in RA rats when compared with R, RN and C rats by 37%, 27% and 13%, respectively. This outcome is mediated by L-Arg protection against post-exercise damage of sarcolemma (2.26- and 0.87-fold less than R and RN groups, respectively), reduced numbers of damaged muscle fibers indicated by the reduced loss of desmin content in the muscle (15% and 25% less than R and RN groups, respectively), and diminished µ-calpain mRNA up-regulation (42% and 30% less than R and RN groups, respectively). In conclusion, our study indicates that L-Arg supplementation prior to a single bout of eccentric exercise alleviates muscle fiber damage and preserves exercise performance capacity.

Three calpain isoforms are autolyzed in rat fast-twitch muscle after eccentric contractions

The present study investigated changes in autolysis of three calpain isoforms in skeletal muscles undergoing eccentric contractions (ECC), leading to prolonged force deficits. Rat extensor digitorum longus and tibialis anterior muscles were exposed to 200-repeated ECC in situ, excised immediately after or 3 or 6 days after cessation of ECC, and used for measures of force output and for biochemical analyses. Full restoration of tetanic force in ECC-treated muscles was not attained until 6 days of recovery. Maximal calpain activity determined by a fluorogenic substrate was unaltered immediately after ECC, but increased to 313 and 450 % after 3 and 6 days, respectively. Increases in the amount of autolyzed calpain-3 were apparent immediately and developed progressively with recovery time, whereas elevations of autolyzed μ- and m-calpain occurred after 3 and 6 days, respectively. The protein content was augmented only in m-calpain. It is suggested that the three calpain isoforms may be involved in the dismantling, repair, remodeling and/or regeneration processes in ECC-treated muscles.

Serological muscle loss biomarkers: an overview of current concepts and future possibilities

BACKGROUND

The skeletal muscle mass is the largest organ in the healthy body, comprising 30-40 % of the body weight of an adult man. It confers protection from trauma, locomotion, ventilation, and it represents a "sink" in glucose metabolism and a reservoir of amino acids to other tissues such as the brain and blood cells. Naturally, loss of muscle has dire consequences for health as well as functionality. Muscle loss is a natural consequence of especially aging, inactivity, and their associated metabolic dysfunction, but it is strongly accelerated in critical illness such as organ failure, sepsis, or cancer. Whether this muscle loss is considered a primary or secondary condition, it is known that muscle loss is a symptom that predicts morbidity and mortality and one that is known to impact quality of life and independence. Therefore, monitoring of muscle mass is relevant in a number of pathologies as well as in clinical trials as measures of efficacy as well as safety.

METHODS AND RESULTS

Existing biomarkers of muscle mass or muscle loss have shown to be either too unreliable or too impractical in relation to the perceived clinical benefit to reach regular clinical research or use. We suggest serological neoepitope biomarkers as a possible technology to address some of these problems. Blood biomarkers of this kind have previously been shown to respond with high sensitivity and shorter time to minimum significant change than available biomarkers of muscle mass. We provide brief reviews of existing muscle mass or function biomarker technologies, muscle protein biology, and existing neoepitope biomarkers and proceed to present tentative recommendations on how to select and detect neoepitope biomarkers.

CONCLUSION

We suggest that serological peptide biomarkers whose tissue and pathology specificity are derived from post-translational modification of proteins in tissues of interest, presenting so-called neoepitopes, represents an exciting candidate technology to fill out an empty niche in biomarker technology.

Pathways of Ca²⁺ entry and cytoskeletal damage following eccentric contractions in mouse skeletal muscle

Muscles that are stretched during contraction (eccentric contractions) show deficits in force production and a variety of structural changes, including loss of antibody staining of cytoskeletal proteins. Extracellular Ca(2+) entry and activation of calpains have been proposed as mechanisms involved in these changes. The present study used isolated mouse extensor digitorum longus (EDL) muscles subjected to 10 eccentric contractions and monitored force production, immunostaining of cytoskeletal proteins, and resting stiffness. Possible pathways for Ca(2+) entry were tested with streptomycin (200 μM), a blocker of stretch-activated channels, and with muscles from mice deficient in the transient receptor potential canonical 1 gene (TRPC1 KO), a candidate gene for stretch-activated channels. At 30 min after the eccentric contractions, the isometric force was decreased to 75 ± 3% of initial control and this force loss was reduced by streptomycin but not in the TRPC1 KO. Desmin, titin, and dystrophin all showed patchy loss of immunostaining 30 min after the eccentric contractions, which was substantially reduced by streptomycin and in the TRPC1 KO muscles. Muscles showed a reduction of resting stiffness following eccentric contractions, and this reduction was eliminated by streptomycin and absent in the TRPC1 KO muscles. Calpain activation was determined by the appearance of a lower molecular weight autolysis product and μ-calpain was activated at 30 min, whereas the muscle-specific calpain-3 was not. To test whether the loss of stiffness was caused by titin cleavage, protein gels were used but no significant titin cleavage was detected. These results suggest that Ca(2+) entry following eccentric contractions is through a stretch-activated channel that is blocked by streptomycin and encoded or modulated by TRPC1.

Activation of skeletal muscle calpain-3 by eccentric exercise in humans does not result in its translocation to the nucleus or cytosol

The skeletal muscle-specific calpain-3 protease is likely involved in muscle repair, although the mechanism is not known. Physiological activation of calpain-3 occurs 24 h following eccentric exercise in humans. Functional consequences of calpain-3 activation are not known; however, calpain-3 has been suggested to be involved in nuclear signaling via NF-κB. To test this and help identify how/where calpain-3 acts, we investigated whether calpain-3 autolysis (hence, activation) following eccentric exercise results in translocation from its normal myofibrillar location to the nucleus or the cytosol. In resting human skeletal muscle, the majority (87%) of calpain-3 was present in myofibrillar fractions, with only a small proportion (<10%) in an autolyzed state. Enriched nuclear fractions contained ∼8% of the total calpain-3, which was present in a predominantly (>80%) autolyzed state. Using freshly dissected human muscle fibers to identify freely diffusible proteins, we showed that only ∼5% of the total calpain-3 pool was cytosolic. At 3 and 24 h following eccentric step exercise, there was an ∼70% increase in autolysis in whole muscle samples ( n = 11, P < 0.05, by 1-way ANOVA with repeated measures and Newman-Keuls post hoc analysis). This exercise-induced autolysis was attributed to myofibrillar-bound calpain-3, since neither the amount of calpain-3 nor the proportion autolyzed was significantly changed in enriched nuclear or cytosolic fractions following the exercise intervention. We present a model for calpain-3 localization at rest and following activation in human skeletal muscle and suggest that the functional importance of calpain-3 remains predominantly tightly associated with its localization within the myofibrillar compartment.

Proteolysis in illness-associated skeletal muscle atrophy: from pathways to networks

Improvements in health in the past decades have resulted in increased numbers of the elderly in both developed and developing regions of the world. Advances in therapy have also increased the prevalence of patients with chronic and degenerative diseases. Muscle wasting, a feature of most chronic diseases, is prominent in the elderly and contributes to both morbidity and mortality. A major research goal has been to identify the proteolytic system(s) that is responsible for the degradation of proteins that occurs in muscle atrophy. Findings over the past 20 years have clearly confirmed an important role of the ubiquitin proteasome system in mediating muscle proteolysis, particularly that of myofibrillar proteins. However, recent observations have provided evidence that autophagy, calpains and caspases also contribute to the turnover of muscle proteins in catabolic states, and furthermore, that these diverse proteolytic systems interact with each other at various levels. Importantly, a number of intracellular signaling pathways such as the IGF1/AKT, myostatin/Smad, PGC1, cytokine/NFκB, and AMPK pathways are now known to interact and can regulate some of these proteolytic systems in a coordinated manner. A number of loss of function studies have identified promising therapeutic approaches to the prevention and treatment of wasting. However, additional biomarkers and other approaches to improve early identification of patients who would benefit from such treatment need to be developed. The current data suggests a network of interacting proteolytic and signaling pathways in muscle. Future studies are needed to improve understanding of the nature and control of these interactions and how they work to preserve muscle function under various states of growth and atrophy.

The effects of eccentric contraction on myofibrillar proteins in rat skeletal muscle

The present study investigated the effects of eccentric muscle contractions (ECC) on the content of myofibrillar proteins (my-proteins) and the catalytic activity of myofibrillar ATPase (my-ATPase) in skeletal muscles. Rat extensor digitorum longus and tibialis anterior muscles were exposed to 200-repeated ECC or isometric contractions (ISC) and used for measures of force output and for biochemical analyses, respectively. Whereas in ISC-treated muscles, full restoration of tetanic force was attained after 2 days of recovery, force developed by ECC-treated muscles remained depressed (P < 0.05) after 6 days. The total my-protein content and the relative content of myosin heavy chain (MHC) in total my-proteins were unaltered during 4 days of recovery after ECC, but fell (P < 0.05) to 55.9 and 63.4% after 6 days of recovery, respectively. my-ATPase activity expressed on a my-protein weight basis was unaltered immediately after ECC. However, it decreased (P < 0.05) to 75.3, 45.3, and 49.3% after 2, 4 and 6 days of recovery, respectively. Total maximal calpain activity measured at 5 mM Ca(2+) was significantly augmented (P < 0.05) after 2 days of recovery, reaching a level of threefold higher after 6 days. These alterations were specific for ECC and not observed for ISC. These results suggest that depressions in my-ATPase activity contribute to ECC-induced decreases in force and power which can take a number of days to recover.

Dynamic distribution of muscle-specific calpain in mice has a key role in physical-stress adaptation and is impaired in muscular dystrophy

Limb-girdle muscular dystrophy type 2A (LGMD2A) is a genetic disease that is caused by mutations in the calpain 3 gene (CAPN3), which encodes the skeletal muscle-specific calpain, calpain 3 (also known as p94). However, the precise mechanism by which p94 functions in the pathogenesis of this disease remains unclear. Here, using p94 knockin mice (termed herein p94KI mice) in which endogenous p94 was replaced with a proteolytically inactive but structurally intact p94:C129S mutant protein, we have demonstrated that stretch-dependent p94 distribution in sarcomeres plays a crucial role in the pathogenesis of LGMD2A. The p94KI mice developed a progressive muscular dystrophy, which was exacerbated by exercise. The exercise-induced muscle degeneration in p94KI mice was associated with an inefficient redistribution of p94:C129S in stretched sarcomeres. Furthermore, the p94KI mice showed impaired adaptation to physical stress, which was accompanied by compromised upregulation of muscle ankyrin-repeat protein-2 and hsp upon exercise. These findings indicate that the stretch-induced dynamic redistribution of p94 is dependent on its protease activity and essential to protect muscle from degeneration, particularly under conditions of physical stress. Furthermore, our data provide direct evidence that loss of p94 protease activity can result in LGMD2A and molecular insight into how this could occur.

Changes in calpain activity, muscle structure, and function after eccentric exercise

PURPOSE

The aim of this study was to investigate changes in muscle function, muscle structure, and calpain activity after high-force eccentric exercise.

METHODS

Eleven healthy males performed 300 maximal voluntary eccentric actions with knee extensors in one leg. Maximal force-generating capacity was measured before exercise and regularly during the next 7 d. Biopsies from musculus vastus lateralis were taken in both control and exercised legs 0.5, 4, 8, 24, 96, and 168 h after exercise for evaluation of myofibrillar structure, extracellular matrix proteins, and calpain activity.

RESULTS

In the exercised leg, peak torque was reduced by 47 +/- 5% during exercise and was still 22 +/- 5% lower than baseline 4 d after the exercise. Calpain activity was three times higher in the exercised leg compared with the control leg 30 min after exercise. Myofibrillar disruptions were observed in 36 +/- 6% of all fibers in exercised muscle and in 2 +/- 1% of fibers in control muscle. The individual reductions in peak torque correlated with the proportion of fibers with myofibrillar disruptions (r = 0.89). The increase in calpain activity was not correlated to the proportion of fibers with myofibrillar disruptions. Nevertheless, the characteristics of the myofibrillar disruptions mimicked calpain-mediated degradation of myofibrils. Tenascin-C and the N-terminal propeptide of procollagen type III showed increased staining intensity on cross-sections 4-7 d after the exercise.

CONCLUSIONS

Myofibrillar disruptions seem to be a main cause for the long-lasting reduction in force-generating capacity after high-force eccentric exercise. The increase in calpain activity, but the lack of a relationship between calpain activity and the amount of muscle damage, suggests multiple roles of calpain in the damage and repair process.

Molt cycle regulation of protein synthesis in skeletal muscle of the blackback land crab, Gecarcinus lateralis, and the differential expression of a myostatin-like factor during atrophy induced by molting or unweighting

In decapod crustaceans, claw muscle undergoes atrophy in response to elevated ecdysteroids while thoracic muscle undergoes atrophy in response to unweighting. The signaling pathways that regulate muscle atrophy in crustaceans are largely unknown. Myostatin is a negative regulator of muscle growth in mammals, and a myostatin-like cDNA is preferentially expressed in muscle of the land crab, Gecarcinus lateralis (Gl-Mstn). Contrary to prediction, levels of Gl-Mstn mRNA decreased dramatically in both the claw closer and weighted thoracic muscles when molting was induced by either eyestalk ablation (ESA) or multiple limb autotomy (MLA). However, the effect of molt induction was greater in the claw muscle. By late premolt, Gl-Mstn mRNA in the claw muscle decreased 81% and 94% in ESA and MLA animals, respectively, and was negatively correlated with ecdysteroids. Gl-Mstn mRNA in thoracic muscle decreased 68% and 82% in ESA and MLA animals, respectively, but was only weakly correlated with ecdysteroid. Claw and thoracic muscles also differed to varying extents in the expression of ecdysteroid receptor (Gl-EcR and Gl-RXR), elongation factor-2 (Gl-EF-2), and calpain T (Gl-CalpT) in response to molt induction, but levels of the four transcripts were not correlated with ecdysteroid. The downregulation of Gl-Mstn expression in premolt claw muscle coincided with 11- and 13-fold increases in protein synthesis in the myofibrillar and soluble protein fractions, respectively. Furthermore, the rate of the increase in the synthesis of soluble proteins was greater than that of myofibrillar proteins during early premolt (1.4:1, soluble:myofibrillar), but the two were equivalent during late premolt. By contrast, Gl-Mstn mRNA increased 3-fold and Gl-CalpT mRNA decreased 40% in unweighted thoracic muscle; there was little or no effect on Gl-EF-2, Gl-EcR, and Gl-RXR mRNA levels. These data indicate that Gl-Mstn expression is negatively regulated by both ecdysteroids and load-bearing contractile activity. The downregulation of Gl-Mstn in claw muscle may induce the elevated protein turnover associated with remodeling of the contractile apparatus during molt-induced atrophy. The upregulation of Gl-Mstn in unweighted thoracic muscle suggests that this factor is also involved in disuse atrophy when hemolymph ecdysteroid levels are low.

Eccentric contractions lead to myofibrillar dysfunction in muscular dystrophy

It is commonly accepted that skeletal muscles from dystrophin-deficient mdx mice are more susceptible than those from wild-type mice to damage from eccentric contractions. However, the downstream mechanisms involved in this enhanced force drop remain controversial. We studied the reduction of contractile force induced by eccentric contractions elicited in vivo in the gastrocnemius muscle of wild-type mice and three distinct models of muscle dystrophy: mdx, alpha-sarcoglycan (Sgca)-null, and collagen 6A1 (Col6a1)-null mice. In mdx and Sgca-null mice, force decreased 35% compared with 14% in wild-type mice. Drop of force in Col6a1-null mice was comparable to that in wild-type mice. To identify the determinants of the force drop, we measured force generation in permeabilized fibers dissected from gastrocnemius muscle that had been exposed in vivo to eccentric contractions and from the contralateral unstimulated muscle. A force loss in skinned fibers after in vivo eccentric contractions was detectable in fibers from mdx and Sgca-null, but not wild-type and Col6a1-null, mice. The enhanced force reduction in mdx and Sgca-null mice was observed only when eccentric contractions were elicited in vivo, since eccentric contractions elicited in vitro had identical effects in wild-type and dystrophic skinned fibers. These results suggest that 1) the enhanced force loss is due to a myofibrillar impairment that is present in all fibers, and not to individual fiber degeneration, and 2) the mechanism causing the enhanced force reduction is active in vivo and is lost after fiber permeabilization.

Calpains, skeletal muscle function and exercise

1. Skeletal muscle fibres contain ubiquitous (mu-calpain and m-calpain) and muscle-specific (calpain-3) Ca(2+)-dependent proteases. The physiological roles of the calpains are not well understood, although ubiquitous calpains have been associated with apoptosis and myogenesis and calpain-3 is likely involved in sarcomeric remodelling. A defect in the expression of calpain-3 results in limb-girdle muscular dystrophy Type 2A. 2. At resting [Ca(2+)](i), calpains are present predominantly in their full-length, unautolysed/unactivated forms. Once activated, mu-calpain and calpain-3 appear in their autolysed forms and this measurement can be used to determine when in vivo activation occurs. Endogenously expressed mu-calpain and calpain-3 are activated within a physiological [Ca(2+)] range in a Ca(2+)- and time-dependent manner. 3. In skeletal muscle, mu-calpain is a freely diffusible protein that binds rapidly when [Ca(2+)](i) is increased. Calpain-3 is tightly bound in skeletal muscle fibres at the N2A line of the large elastic protein titin. 4. Overall, neither mu-calpain nor calpain-3 are activated immediately following sprint, endurance or eccentric exercise, despite the frequent episodes of high cytoplasmic [Ca(2+)] that would occur during these types of muscle contractions. Importantly, however, a substantial proportion of calpain-3, but not mu-calpain, is activated 24 h after a single bout of eccentric exercise. 5. In vitro studies have shown that calpain-3 becomes activated if exposed for a prolonged period of time (> 1 h) to resting cytoplasmic [Ca(2+)] that are approximately two- to fourfold higher than normal. This suggests that the small but sustained increase in [Ca(2+)](i) that likely occurs after eccentric contractions is both high and long enough to result in calpain-3 activation and supports the role for calpain-3 in sarcomeric remodelling.

Calcium-dependent plasma membrane repair requires m- or mu-calpain, but not calpain-3, the proteasome, or caspases

Mechanically damaged plasma membrane undergoes rapid calcium-dependent resealing that appears to depend, at least in part, on calpain-mediated cortical cytoskeletal remodeling. Cells null for Capns1, the non-catalytic small subunit present in both m- and mu-calpains, do not undergo calcium-mediated resealing. However, it is not known which of these calpains is needed for repair, or whether other major cytosolic proteinases may participate. Utilizing isozyme-selective siRNAs to decrease expression of Capn1 or Capn2, catalytic subunits of mu- and m-calpains, respectively, in a mouse embryonic fibroblast cell line, we now show that substantial loss of both activities is required to compromise calcium-mediated survival after cell scrape-damage. Using skeletal myotubes derived from Capn3-null mice, we were unable to demonstrate loss of sarcolemma resealing after needle scratch or laser damage. Isolated muscle fibers from Capn3 knockout mice also efficiently repaired laser damage. Employing either a cell line expressing a temperature sensitive E1 ubiquitin ligase, or lactacystin, a specific proteasome inhibitor, it was not possible to demonstrate an effect of the proteasome on calcium-mediated survival after injury. Moreover, several cell-permeant caspase inhibitors were incapable of significantly decreasing survival or inhibiting membrane repair. Taken together with previous studies, the results show that m- or mu-calpain can facilitate repair of damaged plasma membrane. While there was no evidence for the involvement of calpain-3, the proteasome or caspases in early events of plasma membrane repair, our studies do not rule out their participation in downstream events that may link plasma membrane repair to adaptive remodeling after injury.

Mechanisms of excitation-contraction uncoupling relevant to activity-induced muscle fatigueThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

If the free [Ca2+] in the cytoplasm of a skeletal muscle fiber is raised substantially for a period of seconds to minutes or to high levels just briefly, it leads to disruption of the normal excitation-contraction (E-C) coupling process and a consequent long-lasting decrease in force production. It appears that the disruption to the coupling occurs at the triad junction, where the voltage-sensor molecules (dihydropyridine receptors) normally interact with and open the Ca2+ release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum (SR). This disruption results in inadequate release of SR Ca2+ upon stimulation. Such E-C uncoupling may underlie the long-duration low-frequency fatigue that can occur after various types of exercise, as well as possibly being a contributing factor to the muscle weakness in certain muscle diseases. The process or processes causing the disruption of the coupling between the voltage sensors and the release channels is not known with certainty, but might be associated with structural changes at the triad junction, possibly caused by activation of the Ca2+-dependent protease, µ-calpain.

Taurine supplementation increases skeletal muscle force production and protects muscle function during and after high-frequency in vitro stimulation

Recent studies report that depletion and repletion of muscle taurine (Tau) to endogenous levels affects skeletal muscle contractility in vitro. In this study, muscle Tau content was raised above endogenous levels by supplementing male Sprague-Dawley rats with 2.5% (wt/vol) Tau in drinking water for 2 wk, after which extensor digitorum longus (EDL) muscles were examined for in vitro contractile properties, fatigue resistance, and recovery from fatigue after two different high-frequency stimulation bouts. Tau supplementation increased muscle Tau content by approximately 40% and isometric twitch force by 19%, shifted the force-frequency relationship upward and to the left, increased specific force by 4.2%, and increased muscle calsequestrin protein content by 49%. Force at the end of a 10-s (100 Hz) continuous tetanic stimulation was 6% greater than controls, while force at the end of the 3-min intermittent high-frequency stimulation bout was significantly higher than controls, with a 12% greater area under the force curve. For 1 h after the 10-s continuous stimulation, tetanic force in Tau-supplemented muscles remained relatively stable while control muscle force gradually deteriorated. After the 3-min intermittent bout, tetanic force continued to slowly recover over the next 1 h, while control muscle force again began to decline. Tau supplementation attenuated F(2)-isoprostane production (a sensitive indicator of reactive oxygen species-induced lipid peroxidation) during the 3-min intermittent stimulation bout. Finally, Tau transporter protein expression was not altered by the Tau supplementation. Our results demonstrate that raising Tau content above endogenous levels increases twitch and subtetanic and specific force in rat fast-twitch skeletal muscle. Also, we demonstrate that raising Tau protects muscle function during high-frequency in vitro stimulation and the ensuing recovery period and helps reduce oxidative stress during prolonged stimulation.

Involvement of calpains in Ca2+-induced disruption of excitation-contraction coupling in mammalian skeletal muscle fibers

In skeletal muscle fibers, the coupling between excitation of the surface membrane and the release of Ca2+ from the sarcoplasmic reticulum is irreversibly disrupted if cytoplasmic Ca2+ concentration ([Ca2+]) is raised to micromolar levels for a prolonged period. This excitation-contraction (EC) uncoupling may contribute to muscle weakness after some types of exercise and in certain muscle diseases and has been linked to structural alteration of the triad junctions, but its molecular basis is unclear. Both μ-calpain, a ubiquitous Ca2+-activated protease, and muscle-specific calpain-3 become autolytically activated at micromolar Ca2+ and have been suggested to be responsible for the uncoupling. This study used controlled Ca2+ exposure in mechanically skinned fibers from extensor digitorum longus muscle to show that EC uncoupling still occurs in muscle fibers of calpain-3-deficient mice, with a Ca2+ dependence indistinguishable from that in normal mice and rats. Western blotting of muscle fibers that had been partially EC uncoupled by exposure to an intermediate Ca2+ level (∼5 μM Ca2+ for 3 min, no ATP) showed the presence of autolytic activation of a proportion of the μ-calpain present, but with little or no activation of calpain-3. Homogenates of normal and calpain-3-deficient muscles exposed to micromolar Ca2+ displayed similar levels of diffusible proteolytic activity, as gauged by the rate of decline of passive force in stretched, skinned muscle fibers. Exogenously added μ-calpain, preactivated by elevated [Ca2+] and applied in the presence of 1 μM Ca2+, disrupted EC coupling in a manner similar to raised [Ca2+]. We conclude that calpain-3 is not responsible for Ca2+-induced disruption of EC coupling, but that μ-calpain is a plausible candidate.