Nitrosative modifications of the Ca2+ release complex and actin underlie arthritis-induced muscle weakness

Dantrolene is an HDL-associated paraoxonase-1 activator with immunosuppressive and atheroprotective properties

Human paraoxonase 1 (PON1), an enzyme bound to high-density lipoprotein (HDL), hydrolyzes oxidized lipids and contributes to HDL atheroprotective functions. Decreased serum paraoxonase and arylesterase activities of PON1 have been reported in patients at increased atherosclerosis risk, such as rheumatoid arthritis patients, and associated with arthritis severity and cardiovascular risk. Agents that can modulate PON1 activity and HDL-mediated effects have not been discovered. Aiming to discover chemical tools that enhance PON1 activity, we screened a library of marketed drugs (956 compounds) to identify small molecules that can increase HDL-associated PON1 activity. Screening was performed by a kinetic absorbance assay using human HDL as a source of PON1, and paraoxon and phenyl acetate as substrates to measure paraoxonase and arylesterase activities, respectively. Screening identified the drug dantrolene as a potential PON1 activator, which was confirmed by enzymatic kinetic assays using recombinant wild-type PON1, as well as the PON1[L55M] variant displaying decreased enzyme activity in humans. Furthermore, we used the collagen-induced arthritis (CIA) mouse model to examine the effect of dantrolene on HDL properties and arthritis in vivo. Administration of dantrolene in CIA mice increased paraoxonase and arylesterase activities of PON1, as well as the antioxidant capacity of HDL, and reduced arthritis severity by inhibition of naïve CD4+ T cell differentiation to effector memory cells and generation of Th1 cells. Collectively, our in vitro and in vivo findings indicate using small molecules to enhance HDL-associated PON1 activity is a tractable approach that could lead to novel therapeutics targeting immune responses and atherosclerosis.

Methodology Development for Investigating Pathophysiological [18F]-FDG Muscle Uptake in Patients with Rheumatic Musculoskeletal Diseases

Objectives: This retrospective study explored the qualitative and quantitative assessment of F18-fluordeoxyglucose ([18F]-FDG) positron emission tomography and computed tomography (PET/CT) scans to assess pathophysiological muscle glucose uptake in patients with a rheumatic musculoskeletal disease (RMD). [18F]-FDG PET/CT detects metabolic activity via glucose uptake in tissues. This study aimed to determine the feasibility of quantitative assessment of [18F]-FDG uptake in muscles across three different RMDs compared to controls. Methods: In this study we analysed whole-body [18F]-FDG PET/CT scans from patients with rheumatoid arthritis (RA; n = 11), osteoarthritis (OA; n = 10), and idiopathic inflammatory myositis (IIM; n = 10), and non-RMD controls (n = 11), focusing on muscle-tracer uptake in specific muscle groups. Qualitative assessment visually identified regions with high [18F]-FDG uptake, followed by quantitative assessment using two methods: fixed volume-of-interest (VOI) and hotspot VOI. In the fixed VOI method, a VOI was placed in the respective muscle at a fixed position (50% height from proximal to distal end) on PET/CT images. In the hotspot VOI method, the VOI was placed at the site of the highest [18F]-FDG uptake observed during qualitative assessment. Standardised uptake values (SUVs) were determined for different muscle groups between RMDs and controls. Results: Qualitative assessment revealed a heterogenous uptake pattern of [18F]-FDG that was found in 93% of quadriceps and hamstring muscles, while other muscles displayed either heterogenous or homogenous patterns. A Bland-Altman analysis showed that the hotspot VOI method had a higher sensitivity in detecting differential [18F]-FDG uptake in muscles. Across all muscle groups, patients with IIM had the highest [18F]-FDG uptake, followed by patients with OA and RA, respectively. Conclusions: [18F]-FDG PET/CT enables qualitative and quantitative differentiation of muscle glucose uptake in patients with RA, OA, and IIM, at both individual muscle and patient group levels. The hotspot method and SUVpeak are recommended for quantitative assessment. High [18F]-FDG uptake in multiple muscle groups suggests pathophysiological glucose metabolism in RMD-affected muscles.

Stable oxidative posttranslational modifications alter the gating properties of RyR1

The ryanodine receptor type 1 (RyR1) is a Ca2+ release channel that regulates skeletal muscle contraction by controlling Ca2+ release from the sarcoplasmic reticulum (SR). Posttranslational modifications (PTMs) of RyR1, such as phosphorylation, S-nitrosylation, and carbonylation are known to increase RyR1 open probability (Po), contributing to SR Ca2+ leak and skeletal muscle dysfunction. PTMs on RyR1 have been linked to muscle dysfunction in diseases like breast cancer, rheumatoid arthritis, Duchenne muscle dystrophy, and aging. While reactive oxygen species (ROS) and oxidative stress induce PTMs, the impact of stable oxidative modifications like 3-nitrotyrosine (3-NT) and malondialdehyde adducts (MDA) on RyR1 gating remains unclear. Mass spectrometry and single-channel recordings were used to study how 3-NT and MDA modify RyR1 and affect Po. Both modifications increased Po in a dose-dependent manner, with mass spectrometry identifying 30 modified residues out of 5035 amino acids per RyR1 monomer. Key modifications were found in domains critical for protein interaction and channel activation, including Y808/3NT in SPRY1, Y1081/3NT and H1254/MDA in SPRY2&3, and Q2107/MDA and Y2128/3NT in JSol, near the binding site of FKBP12. Though these modifications did not directly overlap with FKBP12 binding residues, they promoted FKBP12 dissociation from RyR1. These findings provide detailed insights into how stable oxidative PTMs on RyR1 residues alter channel gating, advancing our understanding of RyR1-mediated Ca2+ release in conditions associated with oxidative stress and muscle weakness.

From amino-acid to disease: the effects of oxidation on actin-myosin interactions in muscle

Actin-myosin interactions form the basis of the force-producing contraction cycle within the sarcomere, serving as the primary mechanism for muscle contraction. Post-translational modifications, such as oxidation, have a considerable impact on the mechanics of these interactions. Considering their widespread occurrence, the explicit contributions of these modifications to muscle function remain an active field of research. In this review, we aim to provide a comprehensive overview of the basic mechanics of the actin-myosin complex and elucidate the extent to which oxidation influences the contractile cycle and various mechanical characteristics of this complex at the single-molecule, myofibrillar and whole-muscle levels. We place particular focus on amino acids shown to be vulnerable to oxidation in actin, myosin, and some of their binding partners. Additionally, we highlight the differences between in vitro environments, where oxidation is controlled and limited to actin and myosin and myofibrillar or whole muscle environments, to foster a better understanding of oxidative modification in muscle. Thus, this review seeks to encompass a broad range of studies, aiming to lay out the multi layered effects of oxidation in in vitro and in vivo environments, with brief mention of clinical muscular disorders associated with oxidative stress.

Sarcopenia and cardiovascular diseases: A systematic review and meta-analysis

Sarcopenia is an age-related disease and is often accompanied by other diseases. Now, many studies have shown that cardiovascular diseases (CVDs) may raise the incidence rate of sarcopenia. Therefore, the purpose of this study was to conduct a systematic review and meta-analysis to investigate the prevalence of sarcopenia in patients with CVDs compared with the general population, defined as relatively healthy non-hospitalized subjects. The databases of PubMed, Embase, Medline and Web of Science were searched for eligible studies published up to 12 November 2022. Two assessment tools were used to evaluate study quality and the risk of bias. Statistical analysis was conducted using STATA 14.0 and R Version 4.1.2. Thirty-eight out of the 89 629 articles retrieved were included in our review. The prevalence of sarcopenia ranged from 10.1% to 68.9% in patients with CVDs, and the pooled prevalence was 35% (95% confidence interval [95% CI]: 28-42%). The pooled prevalence of sarcopenia was 32% (95% CI: 23-41%) in patients with chronic heart failure (CHF), 61% (95% CI: 49-72%) in patients with acute decompensated heart failure (ADHF), 43% (95% CI: 2-85%) in patients with coronary artery disease, 30% (95% CI: 25-35%) in patients with cardiac arrhythmia (CA), 35% (95% CI: 10-59%) in patients with congenital heart disease and 12% (95% CI: 7-17%) in patients with unclassed CVDs. However, in the general population, the prevalence of sarcopenia varied from 2.9% to 28.6% and the pooled prevalence was 13% (95% CI: 9-17%), suggesting that the prevalence of sarcopenia in patients with CVDs was about twice compared with the general population. The prevalence of sarcopenia was significantly higher only in patients with ADHF, CHF and CA compared with the general population. There is a positive correlation between CVDs and sarcopenia. The prevalence of sarcopenia is higher in patients with CVDs than that in the general population. With global aging, sarcopenia has brought a heavy burden to individuals and society. Therefore, it is important to identify the populations with high-risk or probable sarcopenia in order to do an early intervention, such as exercise, to counteract or slow down the progress of sarcopenia.

Sphingomyelinase activity promotes atrophy and attenuates force in human muscle fibres and is elevated in heart failure patients

BACKGROUND

Activation of sphingomyelinase (SMase) as a result of a general inflammatory response has been implicated as a mechanism underlying disease-related loss of skeletal muscle mass and function in several clinical conditions including heart failure. Here, for the first time, we characterize the effects of SMase activity on human muscle fibre contractile function and assess skeletal muscle SMase activity in heart failure patients.

METHODS

The effects of SMase on force production and intracellular Ca²⁺ handling were investigated in single intact human muscle fibres. Additional mechanistic studies were performed in single mouse toe muscle fibres. RNA sequencing was performed in human muscle bundles exposed to SMase. Intramuscular SMase activity was measured from heart failure patients (n = 61, age 69 ± 0.8 years, NYHA III-IV, ejection fraction 25 ± 1.0%, peak VO₂ 14.4 ± 0.6 mL × kg × min) and healthy age-matched control subjects (n = 10, age 71 ± 2.2 years, ejection fraction 60 ± 1.2%, peak VO₂ 25.8 ± 1.1 mL × kg × min). SMase activity was related to circulatory factors known to be associated with progression and disease severity in heart failure.

RESULTS

Sphingomyelinase reduced muscle fibre force production (-30%, P < 0.05) by impairing sarcoplasmic reticulum (SR) Ca²⁺ release (P < 0.05) and reducing myofibrillar Ca²⁺ sensitivity. In human muscle bundles exposed to SMase, RNA sequencing analysis revealed 180 and 291 genes as up-regulated and down-regulated, respectively, at a FDR of 1%. Gene-set enrichment analysis identified 'proteasome degradation' as an up-regulated pathway (average fold-change 1.1, P = 0.008), while the pathway 'cytoplasmic ribosomal proteins' (average fold-change 0.8, P < 0.0001) and factors involving proliferation of muscle cells (average fold-change 0.8, P = 0.0002) where identified as down-regulated. Intramuscular SMase activity was ~20% higher (P < 0.05) in human heart failure patients than in age-matched healthy controls and was positively correlated with markers of disease severity and progression, and with several circulating inflammatory proteins, including TNF-receptor 1 and 2. In a longitudinal cohort of heart failure patients (n = 6, mean follow-up time 2.5 ± 0.2 years), SMase activity was demonstrated to increase by 30% (P < 0.05) with duration of disease.

CONCLUSIONS

The present findings implicate activation of skeletal muscle SMase as a mechanism underlying human heart failure-related loss of muscle mass and function. Moreover, our findings strengthen the idea that SMase activation may underpin disease-related loss of muscle mass and function in other clinical conditions, acting as a common patophysiological mechanism for the myopathy often reported in diseases associated with a systemic inflammatory response.

An increase in force after stretch of diaphragm fibers and myofibrils is accompanied by an increase in sarcomere length non-uniformities and Ca2+ sensitivity

When muscle fibers from limb muscles are stretched while activated, the force increases to a steady-state level that is higher than that produced during isometric contractions at a corresponding sarcomere length, a phenomenon known as residual force enhancement (RFE). The mechanisms responsible for the RFE are an increased stiffness of titin molecules which may lead to an increased Ca²⁺ sensitivity of the contractile apparatus,and the development of sarcomere length non-uniformities. RFE is not observed in cardiac muscles, which makes this phenomenon specific to certain preparations. The aim of this study was to investigate if the RFE is present in the diaphragm, and its potential association with an increased Ca²⁺ sensitivity and the development of sarcomere length non-uniformities. We used two preparations: single intact fibers and myofibrils isolated from the diaphragm from mice. We investigated RFE in a variety of lengths across the force-length relationship. RFE was observed in both preparations at all lengths investigated, and was larger with increasing magnitudes of stretch. RFE was accompanied by an increased Ca²⁺ sensitivity as shown by a change in the force-pCa²⁺-curve, and increased sarcomere length non-uniformities. Therefore, RFE is a phenomenon commonly observed in skeletal muscles, with mechanisms that are similar across preparations.

Role of nitration in control of phosphorylase and glycogenolysis in mouse skeletal muscle

Phosphorylase is one of the most carefully studied proteins in history, but knowledge of its regulation during intense muscle contraction is incomplete. Tyrosine nitration of purified preparations of skeletal muscle phosphorylase results in inactivation of the enzyme and this is prevented by antioxidants. Whether an altered redox state affects phosphorylase activity and glycogenolysis in contracting muscle is not known. Here, we investigate the role of the redox state in control of phosphorylase and glycogenolysis in isolated mouse fast-twitch (extensor digitorum longus, EDL) and slow-twitch (soleus) muscle preparations during repeated contractions. Exposure of crude muscle extracts to H₂O₂ had little effect on phosphorylase activity. However, exposure of extracts to peroxynitrite (ONOO^-), a nitrating/oxidizing agent, resulted in complete inactivation of phosphorylase (half-maximal inhibition at ∼200 µM ONOO^-), which was fully reversed by the presence of an ONOO^- scavanger, dithiothreitol (DTT). Incubation of isolated muscles with ONOO^- resulted in nitration of phosphorylase and marked inhibition of glycogenolysis during repeated contractions. ONOO^- also resulted in large decreases in high-energy phosphates (ATP and phosphocreatine) in the rested state and following repeated contractions. These metabolic changes were associated with decreased force production during repeated contractions (to ∼60% of control). In contrast, repeated contractions did not result in nitration of phosphorylase, nor did DTT or the general antioxidant N-acetylcysteine alter glycogenolysis during repeated contractions. These findings demonstrate that ONOO^- inhibits phosphorylase and glycogenolysis in living muscle under extreme conditions. However, nitration does not play a significant role in control of phosphorylase and glycogenolysis during repeated contractions.NEW & NOTEWORTHY Here we show that exogenous peroxynitrite results in nitration of phosphorylase as well as inhibition of glycogenolysis in isolated intact mouse skeletal muscle during short-term repeated contractions. However, repeated contractions in the absence of exogenous peroxynitrite do not result in nitration of phosphorylase or affect glycogenolysis, nor does the addition of antioxidants alter glycogenolysis during repeated contractions. Thus phosphorylase is not subject to redox control during repeated contractions.

Skeletal muscle redox signaling in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by synovitis and the presence of serum autoantibodies. In addition, skeletal muscle weakness is a common comorbidity that contributes to inability to work and reduced quality of life. Loss in muscle mass cannot alone account for the muscle weakness induced by RA, but instead intramuscular dysfunction appears as a critical factor underlying the decreased force generating capacity for patients afflicted by arthritis. Oxidative stress and associated oxidative post-translational modifications have been shown to contribute to RA-induced muscle weakness in animal models of arthritis and patients with RA. However, it is still unclear how and which sources of reactive oxygen and nitrogen species (ROS/RNS) that are involved in the oxidative stress that drives the progression toward decreased muscle function in RA. Nevertheless, mitochondria, NADPH oxidases (NOX), nitric oxide synthases (NOS) and phospholipases (PLA) have all been associated with increased ROS/RNS production in RA-induced muscle weakness. In this review, we aim to cover potential ROS sources and underlying mechanisms of oxidative stress and loss of force production in RA. We also addressed the use of antioxidants and exercise as potential tools to counteract oxidative stress and skeletal muscle weakness.

Fast skeletal muscle troponin activator CK-2066260 mitigates skeletal muscle weakness independently of the underlying cause

BACKGROUND

Muscle weakness is a common symptom in numerous diseases and a regularly occurring problem associated with ageing. Prolonged low-frequency force depression (PLFFD) is a form of exercise-induced skeletal muscle weakness observed after exercise. Three different intramuscular mechanisms underlying PLFFD have been identified: decreased sarcoplasmic reticulum Ca²⁺ release, decreased myofibrillar Ca²⁺ sensitivity, and myofibrillar dysfunction. We here used these three forms of PLFFD as models to study the effectiveness of a fast skeletal muscle troponin activator, CK-2066260, to mitigate muscle weakness.

METHODS

Experiments were performed on intact single muscle fibres or fibre bundles from mouse flexor digitorum brevis, which were stimulated with electrical current pulses, while force and the free cytosolic [Ca²⁺ ] ([Ca²⁺ ]_i ) were measured. PLFFD was induced by three different stimulation protocols: (i) repeated isometric contractions at low intensity (350 ms tetani given every 5 s for 100 contractions); (ii) repeated isometric contractions at high intensity (250 ms tetani given every 0.5 s for 300 contractions); and (iii) repeated eccentric contractions (350 ms tetani with 20% length increase given every 20 s for 10 contractions). The extent and cause of PLFFD were assessed by comparing the force-[Ca²⁺ ]_i relationship at low (30 Hz) and high (120 Hz) stimulation frequencies before (control) and 30 min after induction of PLFFD, and after an additional 5 min of rest in the presence of CK-2066260 (10 μM).

RESULTS

Prolonged low-frequency force depression following low-intensity and high-intensity fatiguing contractions was predominantly due to decreased sarcoplasmic reticulum Ca²⁺ release and decreased myofibrillar Ca²⁺ sensitivity, respectively. CK-2066260 exposure resulted in marked increases in 30 Hz force from 52 ± 16% to 151 ± 13% and from 6 ± 4% to 98 ± 40% of controls with low-intensity and high-intensity contractions, respectively. Following repeated eccentric contractions, PLFFD was mainly due to myofibrillar dysfunction, and it was not fully reversed by CK-2066260 with 30 Hz force increasing from 48 ± 8% to 76 ± 6% of the control.

CONCLUSIONS

The fast skeletal muscle troponin activator CK-2066260 effectively mitigates muscle weakness, especially when it is caused by impaired activation of the myofibrillar contractile machinery due to either decreased sarcoplasmic reticulum Ca²⁺ release or reduced myofibrillar Ca²⁺ sensitivity.

Redox modification of ryanodine receptor contributes to impaired Ca2+ homeostasis and exacerbates muscle atrophy under high altitude

At extreme altitude, prolonged and severe hypoxia menaces human function and survival, and also associated with profound loss of muscle mass which results into a debilitating critical illness of skeletal muscle atrophy. Hypobaric hypoxia altered redox homeostasis and impaired calcium ion handling in skeletal muscles. Dysregulated Ca²⁺ homeostasis and activated calpain is the prime stressor in high altitude hypoxia while the reason for subsequent abnormal release of pathological Ca²⁺ into cytoplasm is largely unexplored. The present study identified the redox remodeling in the Ca²⁺ release channel, Ryanodine Receptor (RyR1) owing to its hypernitrosylation state in skeletal muscles in chronic hypobaric hypoxia exposed rats. RyR1-hypernitrosylation decreases the binding of FKBP12/calstabin-1 and other complexes from the channel, causing "leakiness" in RyR1 ion-channel. A strong RyR1 stabilizer, S107 enhanced binding affinity of FKBP12 with hypernitrosylated RyR1, reduced Sarco(endo)plasmic reticulum (SR) Ca²⁺ leak and improved muscle strength and function under chronic hypoxia. Administration of S107 inhibited the skeletal muscle damage, maintained ultrastructure of sarcomere and sarcolemmal integrity. Histological analysis proved the increase in cross-sectional area of myofibers. Further, the number of apoptotic cells was also reduced by S107 treatment. Conclusively, we proposed that the redox remodeling of RyR1 (hypernitrosylated-RyR1) might be responsible for dysregulated Ca²⁺ homeostasis which consequently impaired muscle strength and function in response to chronic hypoxic stress. Reduced SR Ca²⁺ leak and enhanced binding affinity of FKBP12 may provide a novel therapeutic avenue in ameliorating skeletal muscle atrophy at high altitude.

Eccentric Resistance Training Ameliorates Muscle Weakness in a Mouse Model of Idiopathic Inflammatory Myopathies

OBJECTIVE

High-force eccentric contractions (ECCs) have traditionally been excluded from rehabilitation programs that include patients with idiopathic inflammatory myopathies (IIMs) due to unverified fear of causing muscle damage and inflammation. In an IIM animal model that used mice with experimental autoimmune myositis (EAM), we undertook this study to investigate whether ECC training can safely and effectively be used to counteract muscle weakness in IIM.

METHODS

EAM was induced in BALB/c mice by immunization with 3 injections of myosin emulsified in Freund's complete adjuvant. Controls (n = 12) and mice with EAM (n = 12) were exposed to either an acute bout of 100 ECCs or 4 weeks of ECC training (20 ECCs every other day). To induce ECCs, plantar flexor muscles were electrically stimulated while the ankle was forcibly dorsiflexed.

RESULTS

Less cell damage, as assessed by Evans blue dye uptake, was observed in the muscles of mice with EAM, compared to controls, after an acute bout of 100 ECCs (P < 0.05). Maximum Ca²⁺ -activated force was decreased in skinned gastrocnemius muscle fibers from mice with EAM, and this was accompanied by increased expression of endoplasmic reticulum (ER) stress proteins, including Gsp78 and Gsp94 (P < 0.05). ECC training prevented the decrease in force and the increase in ER stress proteins and also enhanced the expression and myofibrillar binding of small heat-shock proteins (HSPs) (P < 0.05), which can stabilize myofibrillar structure and function.

CONCLUSION

ECC training protected against the reduction in myofibrillar force-generating capacity in an IIM mouse model, and this occurred via inhibition of ER stress responses and small HSP-mediated myofibrillar stabilization.

Nitrosative Stress and Its Association with Cardiometabolic Disorders

Reactive nitrogen species (RNS) are formed when there is an abnormal increase in the level of nitric oxide (NO) produced by the inducible nitric oxide synthase (iNOS) and/or by the uncoupled endothelial nitric oxide synthase (eNOS). The presence of high concentrations of superoxide anions (O₂⁻) is also necessary for their formation. RNS react three times faster than O₂⁻ with other molecules and have a longer mean half life. They cause irreversible damage to cell membranes, proteins, mitochondria, the endoplasmic reticulum, nucleic acids and enzymes, altering their activity and leading to necrosis and to cell death. Although nitrogen species are important in the redox imbalance, this review focuses on the alterations caused by the RNS in the cellular redox system that are associated with cardiometabolic diseases. Currently, nitrosative stress (NSS) is implied in the pathogenesis of many diseases. The mechanisms that produce damage remain poorly understood. In this paper, we summarize the current knowledge on the participation of NSS in the pathology of cardiometabolic diseases and their possible mechanisms of action. This information might be useful for the future proposal of anti-NSS therapies for cardiometabolic diseases.

Is inflammatory signaling involved in disease-related muscle wasting? Evidence from osteoarthritis, chronic obstructive pulmonary disease and type II diabetes

Muscle loss is an important feature that occurs in multiple pathologies including osteoarthritis (OA), chronic obstructive pulmonary disease (COPD) and type II diabetes (T2D). Despite differences in pathogenesis and disease-related complications, there are reasons to believe that some fundamental underlying mechanisms are inherent to the muscle wasting process, irrespective of the pathology. Recent evidence shows that inflammation, either local or systemic, contributes to the modulation of muscle mass and/or muscle strength, via an altered molecular profile in muscle tissue. However, it remains ambiguous to which extent and via which mechanisms inflammatory signaling affects muscle mass in disease. Therefore, the objective of the present review is to discuss the role of inflammation on skeletal muscle anabolism, catabolism and functionality in three pathologies that are characterized by an eventual loss in muscle mass (and muscle strength), i.e. OA, COPD and T2D. In OA and COPD, most rodent models confirmed that systemic (COPD) or muscle (OA) inflammation directly induces muscle loss or muscle dysfunctionality. However, in a patient population, the association between inflammation and muscular maladaptations are more ambiguous. For example, in T2D patients, systemic inflammation is associated with muscle loss whereas in OA patients this link has not consistently been established. T2D rodent models revealed that increased levels of advanced glycation end-products (AGEs) and a decreased mTORC1 activation play a key role in muscle atrophy, but it remains to be elucidated whether AGEs and mTORC1 are interconnected and contribute to muscle loss in T2D patients. Generally, if any, associations between inflammation and muscle are mainly based on observational and cross-sectional data. There is definitely a need for longitudinal evidence through well-powered randomized control trials that take into account confounders such as age, disease-phenotypes, comorbidities, physical (in) activity etc. This will allow to improve our understanding of the complex interaction between inflammatory signaling and muscle mass loss and hence contribute to the development of therapeutic strategies to combat muscle wasting in these diseases.

Intramuscular mechanisms of overtraining

Strenuous exercise is a potent stimulus to induce beneficial skeletal muscle adaptations, ranging from increased endurance due to mitochondrial biogenesis and angiogenesis, to increased strength from hypertrophy. While exercise is necessary to trigger and stimulate muscle adaptations, the post-exercise recovery period is equally critical in providing sufficient time for metabolic and structural adaptations to occur within skeletal muscle. These cyclical periods between exhausting exercise and recovery form the basis of any effective exercise training prescription to improve muscle endurance and strength. However, imbalance between the fatigue induced from intense training/competitions, and inadequate post-exercise/competition recovery periods can lead to a decline in physical performance. In fact, prolonged periods of this imbalance may eventually lead to extended periods of performance impairment, referred to as the state of overreaching that may progress into overtraining syndrome (OTS). OTS may have devastating implications on an athlete's career and the purpose of this review is to discuss potential underlying mechanisms that may contribute to exercise-induced OTS in skeletal muscle. First, we discuss the conditions that lead to OTS, and their potential contributions to impaired skeletal muscle function. Then we assess the evidence to support or refute the major proposed mechanisms underlying skeletal muscle weakness in OTS: 1) glycogen depletion hypothesis, 2) muscle damage hypothesis, 3) inflammation hypothesis, and 4) the oxidative stress hypothesis. Current data implicates reactive oxygen and nitrogen species (ROS) and inflammatory pathways as the most likely mechanisms contributing to OTS in skeletal muscle. Finally, we allude to potential interventions that can mitigate OTS in skeletal muscle.

Force generated by myosin cross-bridges is reduced in myofibrils exposed to ROS/RNS

Skeletal muscle weakness is associated with oxidative stress and oxidative posttranslational modifications on contractile proteins. There is indirect evidence that reactive oxygen/nitrogen species (ROS/RNS) affect skeletal muscle myofibrillar function, although the details of the acute effects of ROS/RNS on myosin-actin interactions are not known. In this study, we examined the effects of peroxynitrite (ONOO−) on the contractile properties of individual skeletal muscle myofibrils by monitoring myofibril-induced displacements of an atomic force cantilever upon activation and relaxation. The isometric force decreased by ~50% in myofibrils treated with the ONOO− donor (SIN-1) or directly with ONOO−, which was independent of the cross-bridge abundancy condition (i.e., rigor or relaxing condition) during SIN-1 or ONOO− treatment. The force decrease was attributed to an increase in the cross-bridge detachment rate ( gapp) in combination with a conservation of the force redevelopment rate (kTr) and hence, an increase in the population of cross-bridges transitioning from force-generating to non-force-generating cross-bridges during steady-state. Taken together, the results of this study provide important information on how ROS/RNS affect myofibrillar force production which may be of importance for conditions where increased oxidative stress is part of the pathophysiology.

A Mechanism for Statin-Induced Susceptibility to Myopathy

This study aimed to identify a mechanism for statin-induced myopathy that explains its prevalence and selectivity for skeletal muscle, and to understand its interaction with moderate exercise. Statin-associated adverse muscle symptoms reduce adherence to statin therapy; this limits the effectiveness of statins in reducing cardiovascular risk. The issue is further compounded by perceived interactions between statin treatment and exercise. This study examined muscles from individuals taking statins and rats treated with statins for 4 weeks. In skeletal muscle, statin treatment caused dissociation of the stabilizing protein FK506 binding protein (FKBP12) from the sarcoplasmic reticulum (SR) calcium (Ca2+) release channel, the ryanodine receptor 1, which was associated with pro-apoptotic signaling and reactive nitrogen species/reactive oxygen species (RNS/ROS)-dependent spontaneous SR Ca2+ release events (Ca2+ sparks). Statin treatment had no effect on Ca2+ spark frequency in cardiac myocytes. Despite potentially deleterious effects of statins on skeletal muscle, there was no impact on force production or SR Ca2+ release in electrically stimulated muscle fibers. Statin-treated rats with access to a running wheel ran further than control rats; this exercise normalized FKBP12 binding to ryanodine receptor 1, preventing the increase in Ca2+ sparks and pro-apoptotic signaling. Statin-mediated RNS/ROS-dependent destabilization of SR Ca2+ handling has the potential to initiate skeletal (but not cardiac) myopathy in susceptible individuals. Importantly, although exercise increases RNS/ROS, it did not trigger deleterious statin effects on skeletal muscle. Indeed, our results indicate that moderate exercise might benefit individuals who take statins.

Eccentric training enhances the αB-crystallin binding to the myofibrils and prevents skeletal muscle weakness in adjuvant-induced arthritis rat

Patients with rheumatoid arthritis (RA) frequently suffer from muscle weakness. We examined whether eccentric training prevents skeletal muscle weakness in adjuvant-induced arthritis (AIA) rat, a widely used animal model for RA. AIA was induced in the knees of Wistar rats by injection of complete Freund’s adjuvant. To induce eccentric contractions (ECCs), neuromuscular electrical stimulation (45 V) was applied to the plantar flexor muscles simultaneously with forced dorsiflexion of the ankle joint (0–40°) and was given every 6 s. ECC exercise was applied every other day for a total of 11 sessions and consisted of 4 sets of 5 contractions. There was a significant reduction in in vitro maximum Ca2+-activated force in skinned fibers in gastrocnemius muscle from AIA rats. These changes were associated with reduced expression levels of contractile proteins (i.e., myosin and actin), increased levels of inflammation redox stress-related biomarkers (i.e., TNF-α, malondialdehyde-protein adducts, NADPH oxidase 2, and neuronal nitric oxide synthase), and autolyzed active calpain-1 in AIA muscles. ECC training markedly enhanced the steady-state levels of αB-crystallin, a small heat shock protein, and its binding to the myofibrils and prevented the AIA-induced myofibrillar dysfunction, reduction in contractile proteins, and inflammation-oxidative stress insults. Our findings demonstrate that ECC training preserves myofibrillar function without muscle damage in AIA rats, which is at least partially attributable to the protective effect of αB-crystallin on the myofibrils against oxidative stress-mediated protein degeneration. Thus ECC training can be a safe and effective intervention, counteracting the loss of muscle strength in RA patients.

NEW & NOTEWORTHY Eccentric contractions (ECCs) are regarded as an effective way to increase muscle strength. No studies, however, assess safety and effectiveness of ECC training on muscle weakness associated with rheumatoid arthritis. Here, we used adjuvant-induced arthritis (AIA) rats to demonstrate that ECC training prevents intrinsic contractile dysfunction without muscle damage in AIA rats, which may be attributed to the protective effect of αB-crystallin on the myofibrils against inflammation-oxidative stress insults.

Oxidative hotspots on actin promote skeletal muscle weakness in rheumatoid arthritis

Skeletal muscle weakness in patients suffering from rheumatoid arthritis (RA) adds to their impaired working abilities and reduced quality of life. However, little molecular insight is available on muscle weakness associated with RA. Oxidative stress has been implicated in the disease pathogenesis of RA. Here we show that oxidative post-translational modifications of the contractile machinery targeted to actin result in impaired actin polymerization and reduced force production. Using mass spectrometry, we identified the actin residues targeted by oxidative 3-nitrotyrosine (3-NT) or malondialdehyde adduct (MDA) modifications in weakened skeletal muscle from mice with arthritis and patients afflicted by RA. The residues were primarily located to three distinct regions positioned at matching surface areas of the skeletal muscle actin molecule from arthritis mice and RA patients. Moreover, molecular dynamic simulations revealed that these areas, here coined "hotspots", are important for the stability of the actin molecule and its capacity to generate filaments and interact with myosin. Together, these data demonstrate how oxidative modifications on actin promote muscle weakness in RA patients and provide novel leads for targeted therapeutic treatment to improve muscle function.

Redox Status and Muscle Pathology in Rheumatoid Arthritis: Insights from Various Rat Hindlimb Muscles

Due to atrophy, muscle weakness is a common occurrence in rheumatoid arthritis (RA). The majority of human studies are conducted on the vastus lateralis muscle—a muscle with mixed fiber type—but little comparative data between multiple muscles in either rodent or human models are available. The current study therefore assessed both muscle ultrastructure and selected redox indicators across various muscles in a model of collagen-induced rheumatoid arthritis in female Sprague-Dawley rats. Only three muscles, the gastrocnemius, extensor digitorum longus (EDL), and soleus, had lower muscle mass (38%, 27%, and 25% loss of muscle mass, respectively; all at least P < 0.01), while the vastus lateralis muscle mass was increased by 35% (P < 0.01) in RA animals when compared to non-RA controls. However, all four muscles exhibited signs of deterioration indicative of rheumatoid cachexia. Cross-sectional area was similarly reduced in gastrocnemius, EDL, and soleus (60%, 58%, and 64%, respectively; all P < 0.001), but vastus lateralis (22% smaller, P < 0.05) was less affected, while collagen deposition was significantly increased in muscles. This pathology was associated with significant increases in tissue levels of reactive oxygen species (ROS) in all muscles except the vastus lateralis, while only the gastrocnemius had significantly increased levels of lipid peroxidation (TBARS) and antioxidant activity (FRAP). Current data illustrates the differential responses of different skeletal muscles of the hindlimb to a chronic inflammatory challenge both in terms of redox changes and resistance to cachexia.

The effects of fatigue and oxidation on contractile function of intact muscle fibers and myofibrils isolated from the mouse diaphragm

The goal of this study was to investigate the effects of repetitive stimulation and the oxidant H₂O₂ on fatigue of diaphragm intact fibers and in myofibrils measured with different Ca²⁺ concentrations. Intact fibers were isolated from mice diaphragm, and twitch and tetanic contractions (500 ms duration) were performed at different frequencies of stimulation ranging from 15 Hz to 150 Hz to establish a force-frequency relation before and after a fatigue and recovery protocol, without or after a treatment with H₂O₂. Fatigue was induced with isometric contractions (500 ms, 40 Hz) evoked every 0.8 seconds, with a total of 625 tetani. After the fatigue, the force recovery was followed by invoking tetanic contractions (500 ms, 40 Hz) every 1 min, with a total duration of 30 min. Individual myofibrils were also isolated from the mouse diaphragm and were tested for isometric contractions before and after treatment with H₂O₂ and NAC. In a second series of experiments, myofibrils were activated at different pCa (pCa = -log₁₀ [Ca²⁺]), before and after H₂O₂ treatment. After 15 minutes of H₂O₂ treatment, the myofibrillar force was decreased to 54 ± 12% of its control, maximal value, and a result that was reversed by NAC treatment. The force was also decreased after myofibrils were treated with H₂O₂ and activated in pCa ranging between 4.5 and 5.7. These results suggest that fatigue in diaphragm intact fibers and at the myofibrils level is caused partially by oxidation of the contractile proteins that may be responsible for changing the force in various levels of Ca²⁺ activation.

Temporomandibular Disorders and Oral Features in Early Rheumatoid Arthritis Patients: An Observational Study

Aims: Temporomandibular disorders (TMD) represent a heterogeneous group of inflammatory or degenerative diseases of the stomatognatic system, with algic and/or dysfunctional clinical features involving temporomandibular joint (TMJ) and related masticatory muscles. Rheumatoid Arthritis (RA) is an autoimmune polyarthritis characterized by the chronic inflammation of synovial joints and oral implications such as hyposalivation, difficulty in swallowing and phoning, feeling of burning mouth, increased thirst, loss of taste or unpleasant taste and smell, dental sensitivity. The aim of this observational study was to investigate the prevalence of TMD symptoms and signs as well as oral implications in patients with Early Rheumatoid Arthritis (ERA), that is a RA diagnosed within 12 months, compared with a control group. Methods: The study group included 52 ERA patients (11 men, 41 women) diagnosed according to the 2010 ACR/EULAR Classification Criteria for Rheumatoid Arthritis. A randomly selected group of 52 patients not affected by this disease, matched by sex and age, served as the control group. The examination for TMD signs and symptoms was based on the standardized Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) by means of a questionnaire and through clinical examination. Results: Regarding the oral kinematics, the left lateral excursion of the mandible was restricted in statistically significant way in ERA patients (p=0.017). The endfeel values were significantly increased in ERA group (p=0.0017), thus showing the presence of a higher muscle contracture. On the other side, the study group complained less frequently (67.3%) of TDM symptoms (muscle pain on chewing, pain in the neck and shoulders muscles, difficulty in mouth opening, arthralgia of TMJ, tinnitus) than controls (90.4%) (χ²= 8.301 p=0.0039). The presence of TMJ noises was significantly lower in the study group (χ²= 3.869 p=0.0049), as well as presence of opening derangement (χ²= 14.014 p=0.0002). The salivary flow was significantly decreased in the study group respect to the control one (p<0.0001). Conclusions: The data collected show a weak TMJ kinematic impairment, a paucisymptomatic muscle contracture (positive endfeel) and a remarkable reduction of salivary flow in ERA patients. Myofacial pain (MP) evoked by palpation was more frequent and severe in the control group than in the study one, this result being highly significant.

Perceived Versus Performance Fatigability in Patients With Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic, inflammatory disease that affects 1% of the general population. Fatigue is a common complaint of patients with RA, however their perceived fatigue may be more exacerbated than objective measures of fatigue may indicate. The assessment of fatigue is made complex due to inconsistent and vague terms used to define fatigue, and the task dependence of fatigability. Fatigue is defined as a state of exhaustion and decreased strength, while fatigability indicates an individual's susceptibility to fatigue. In order to offer some clarity to the manifestation of fatigue in clinical populations, in this review we outline that fatigue should be described with subsections that are related to the symptom, such as: perceived fatigability and performance fatigability. Where perceived fatigability indicates the subjective state of the individual and thus involves the individual's subjective measure of fatigue, performance fatigability would be measured through clinical and laboratory-based assessments that quantify the functional decline in performance. This review describes RA and the various neuromuscular changes associated with the disease that can lead to alterations in both perceived and performance fatigue. From there, we discuss fatigue and RA, how fatigue can be assessed, effects of exercise interventions on RA symptoms and fatigue, and recommendations for future studies investigating subjective and objective measures of fatigability.

Oxidative Stress Induces Disruption of the Axon Initial Segment

The axon initial segment (AIS), the domain responsible for action potential initiation and maintenance of neuronal polarity, is targeted for disruption in a variety of central nervous system pathological insults. Previous work in our laboratory implicates oxidative stress as a potential mediator of structural AIS alterations in two separate mouse models of central nervous system inflammation, as these effects were attenuated following reactive oxygen species scavenging and NADPH oxidase-2 ablation. While these studies suggest a role for oxidative stress in modulation of the AIS, the direct effects of reactive oxygen and nitrogen species (ROS/RNS) on the stability of this domain remain unclear. Here, we demonstrate that oxidative stress, as induced through treatment with 3-morpholinosydnonimine (SIN-1), a spontaneous ROS/RNS generator, drives a reversible loss of AIS protein clustering in primary cortical neurons in vitro. Pharmacological inhibition of both voltage-dependent and intracellular calcium (Ca²⁺) channels suggests that this mechanism of AIS disruption involves Ca²⁺ entry specifically through L-type voltage-dependent Ca²⁺ channels and its release from IP₃-gated intracellular stores. Furthermore, ROS/RNS-induced AIS disruption is dependent upon activation of calpain, a Ca²⁺-activated protease previously shown to drive AIS modulation. Overall, we demonstrate for the first time that oxidative stress, as induced through exogenously applied ROS/RNS, is capable of driving structural alterations in the AIS complex.

Role of defective Ca2+ signaling in skeletal muscle weakness: Pharmacological implications

The misbehaving attitude of Ca²⁺ signaling pathways could be the probable reason in many muscular disorders such as myopathies, systemic disorders like hypoxia, sepsis, cachexia, sarcopenia, heart failure, and dystrophy. The present review throws light upon the calcium flux regulating signaling channels like ryanodine receptor complex (RyR1), SERCA (Sarco-endoplasmic Reticulum Calcium ATPase), DHPR (Dihydropyridine Receptor) or Cav1.1 and Na+/Ca²⁺ exchange pump in detail and how remodelling of these channels contribute towards disturbed calcium homeostasis. Understanding these pathways will further provide an insight for establishing new therapeutic approaches for the prevention and treatment of muscle atrophy under stress conditions, targeting calcium ion channels and associated regulatory proteins.

Very Low Volume High-Intensity Interval Exercise Is More Effective in Young Than Old Women

We investigated the acute neuromuscular and stress responses to three different high-intensity interval training sessions in young (age 19.5 ± 1.3 years) and older (age 65.7 ± 2.8 years) women. Cycling exercise comprised either 6 × 5 s or 3 × 30 s all-out, or 3 × 60 s submaximal, efforts each performed 5 weeks apart in randomized order. Peak and average power was higher in young than in older women and was largest during the 6 × 5 s strategy in both groups (p < 0.05). The decrease in the ratio of torques evoked by 20 and 100 Hz electrical stimulation, representing low-frequency fatigue, was more evident after the 3 × 30 and 3 × 60 s than the 6 × 5 s bout in both groups and was larger in young than in older women (p < 0.05). Both groups preferred 6 × 5 s cycling for further training. In conclusion, in young women, very low volume (6 × 5 s) all-out exercise induces significant physiological stress and seems to be an effective means of training. For older women, longer exercise sessions (3 × 60 s) are more stressful than shorter ones but are still tolerable psychologically.

Intramuscular Contributions to Low-Frequency Force Potentiation Induced by a High-Frequency Conditioning Stimulation

Electrically-evoked low-frequency (submaximal) force is increased immediately following high-frequency stimulation in human skeletal muscle. Although central mechanisms have been suggested to be the major cause of this low-frequency force potentiation, intramuscular factors might contribute. Thus, we hypothesized that two intramuscular Ca²⁺-dependent mechanisms can contribute to the low-frequency force potentiation: increased sarcoplasmic reticulum Ca²⁺ release and increased myofibrillar Ca²⁺ sensitivity. Experiments in humans were performed on the plantar flexor muscles at a shortened, intermediate, and long muscle length and electrically evoked contractile force and membrane excitability (i.e., M-wave amplitude) were recorded during a stimulation protocol. Low-frequency force potentiation was assessed by stimulating with a low-frequency tetanus (25 Hz, 2 s duration), followed by a high-frequency tetanus (100 Hz, 2 s duration), and finally followed by another low-frequency (25 Hz, 2 s duration) tetanus. Similar stimulation protocols were performed on intact mouse single fibers from flexor digitorum brevis muscle, whereby force and myoplasmic free [Ca²⁺] ([Ca²⁺]_i) were assessed. Our data show a low-frequency force potentiation that was not muscle length-dependent in human muscle and it was not accompanied by any increase in M-wave amplitude. A length-independent low-frequency force potentiation could be replicated in mouse single fibers, supporting an intramuscular mechanism. We show that at physiological temperature (31°C) this low-frequency force potentiation in mouse fibers corresponded with an increase in sarcoplasmic reticulum (SR) Ca²⁺ release. When mimicking the slower contractile properties of human muscle by cooling mouse single fibers to 18°C, the low-frequency force potentiation was accompanied by minimally increased SR Ca²⁺ release and hence it could be explained by increased myofibrillar Ca²⁺ sensitivity. Finally, introducing a brief 200 ms pause between the high- and low-frequency tetanus in human and mouse muscle revealed that the low-frequency force potentiation is abolished, arguing that increased myofibrillar Ca²⁺ sensitivity is the main intramuscular mechanism underlying the low-frequency force potentiation in humans.

Mechanical isolation, and measurement of force and myoplasmic free [Ca2+] in fully intact single skeletal muscle fibers

Mechanical dissection of single intact mammalian skeletal muscle fibers permits real-time measurement of intracellular properties and contractile function of living fibers. A major advantage of mechanical over enzymatic fiber dissociation is that single fibers can be isolated with their tendons remaining attached, which allows contractile forces (in the normal expected range of 300-450 kN/m²) to be measured during electrical stimulation. Furthermore, the sarcolemma of single fibers remains fully intact after mechanical dissection, and hence the living fibers can be studied with intact intracellular milieu and normal function and metabolic properties, as well as ionic control. Given that Ca²⁺ is the principal regulator of the contractile force, measurements of myoplasmic free [Ca²⁺] ([Ca²⁺]_i) can be used to further delineate the intrinsic mechanisms underlying changes in skeletal muscle function. [Ca²⁺]_i measurements are most commonly performed in intact single fibers using ratiometric fluorescent indicators such as indo-1 or fura-2. These Ca²⁺ indicators are introduced into the fiber by pressure injection or by using the membrane-permeable indo-1 AM, and [Ca²⁺]_i is measured by calculating a ratio of the fluorescence at specific wavelengths emitted for the Ca²⁺-free and Ca²⁺-bound forms of the dye. We describe here the procedures for mechanical dissection, and for force and [Ca²⁺]_i measurement in intact single fibers from mouse flexor digitorum brevis (FDB) muscle, which is the most commonly used muscle in studies using intact single fibers. This technique can also be used to isolate intact single fibers from various muscles and from various species. As an alternative to Ca²⁺ indicators, single fibers can also be loaded with fluorescent indicators to measure, for instance, reactive oxygen species, pH, and [Mg²⁺], or they can be injected with proteins to change functional properties. The entire protocol, from dissection to the start of an experiment on a single fiber, takes ∼3 h.

Muscle Weakness in Rheumatoid Arthritis: The Role of Ca2+ and Free Radical Signaling

In addition to the primary symptoms arising from inflammatory processes in the joints, muscle weakness is commonly reported by patients with rheumatoid arthritis (RA). Muscle weakness not only reduces the quality of life for the affected patients, but also dramatically increases the burden on society since patients' work ability decreases. A 25–70% reduction in muscular strength has been observed in pateints with RA when compared with age-matched healthy controls. The reduction in muscle strength is often larger than what could be explained by the reduction in muscle size in patients with RA, which indicates that intracellular (intrinsic) muscle dysfunction plays an important role in the underlying mechanism of muscle weakness associated with RA. In this review, we highlight the present understanding of RA-associated muscle weakness with special focus on how enhanced Ca^2 + release from the ryanodine receptor and free radicals (reactive oxygen/nitrogen species) contributes to muscle weakness, and recent developments of novel therapeutic interventions.

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Muscle weakness is commonly reported by patients with rheumatoid arthritis (RA).

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Intrinsic muscle weakness is important in the underlying mechanisms of muscle weakness associated with rheumatoid arthritis.

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Enhanced Ca^2 + release and peroxynitrite-induced stress contributes to RA-induced muscle weakness.

Neuromuscular electrical stimulation prevents skeletal muscle dysfunction in adjuvant-induced arthritis rat

Skeletal muscle weakness is a prominent feature in patients with rheumatoid arthritis (RA). In this study, we investigated whether neuromuscular electrical stimulation (NMES) training protects against skeletal muscle dysfunction in rats with adjuvant-induced arthritis (AIA). AIA was produced by intraarticular injection of complete Freund’s adjuvant into the knees of Wistar rats. For NMES training, dorsiflexor muscles were stimulated via a surface electrode (0.5 ms pulse, 50 Hz, 2 s on/4 s off). NMES training was performed every other day for three weeks and consisted of three sets produced at three min intervals. In each set, the electrical current was set to achieve 60% of the initial maximum isometric torque and the current was progressively increased to maintain this torque; stimulation was stopped when the 60% torque could no longer be maintained. After the intervention period, extensor digitorum longus (EDL) muscles were excised and used for physiological and biochemical analyses. There was a reduction in specific force production (i.e. force per cross-sectional area) in AIA EDL muscles, which was accompanied by aggregation of the myofibrillar proteins actin and desmin. Moreover, the protein expressions of the pro-oxidative enzymes NADPH oxidase, neuronal nitric oxide synthase, p62, and the ratio of the autophagosome marker LC3bII/LC3bI were increased in AIA EDL muscles. NMES training prevented all these AIA-induced alterations. The present data suggest that NMES training prevents AIA-induced skeletal muscle weakness presumably by counteracting the formation of actin and desmin aggregates. Thus, NMES training can be an effective treatment for muscle dysfunction in patients with RA.

GSNOR Deficiency Enhances In Situ Skeletal Muscle Strength, Fatigue Resistance, and RyR1 S-Nitrosylation Without Impacting Mitochondrial Content and Activity

AIM

Nitric oxide (NO) plays important, but incompletely defined roles in skeletal muscle. NO exerts its regulatory effects partly though S-nitrosylation, which is balanced by denitrosylation by enzymes such as S-nitrosoglutathione reductase (GSNOR), whose functions in skeletal muscle remain to be fully deciphered.

RESULTS

GSNOR null (GSNOR^-/-) tibialis anterior (TA) muscles showed normal growth and were stronger and more fatigue resistant than controls in situ. However, GSNOR^-/- lumbrical muscles showed normal contractility and Ca²⁺ handling in vitro, suggesting important differences in GSNOR function between muscles or between in vitro and in situ environments. GSNOR^-/- TA muscles exhibited normal mitochondrial content, and capillary densities, but reduced type IIA fiber content. GSNOR inhibition did not impact mitochondrial respiratory complex I, III, or IV activities. These findings argue that enhanced GSNOR^-/- TA contractility is not driven by changes in mitochondrial content or activity, fiber type, or blood vessel density. However, loss of GSNOR led to RyR1 hypernitrosylation, which is believed to increase muscle force output under physiological conditions. cGMP synthesis by soluble guanylate cyclase (sGC) was decreased in resting GSNOR^-/- muscle and was more responsive to agonist (DETANO, BAY 41, and BAY 58) stimulation, suggesting that GSNOR modulates cGMP production in skeletal muscle.

INNOVATION

GSNOR may act as a "brake" on skeletal muscle contractile performance under physiological conditions by modulating nitrosylation/denitrosylation balance.

CONCLUSIONS

GSNOR may play important roles in skeletal muscle contractility, RyR1 S-nitrosylation, fiber type specification, and sGC activity. Antioxid. Redox Signal. 26, 165-181.

The Role of Reactive Oxygen Species in β-Adrenergic Signaling in Cardiomyocytes from Mice with the Metabolic Syndrome

The metabolic syndrome is associated with prolonged stress and hyperactivity of the sympathetic nervous system and afflicted subjects are prone to develop cardiovascular disease. Under normal conditions, the cardiomyocyte response to acute β-adrenergic stimulation partly depends on increased production of reactive oxygen species (ROS). Here we investigated the interplay between beta-adrenergic signaling, ROS and cardiac contractility using freshly isolated cardiomyocytes and whole hearts from two mouse models with the metabolic syndrome (high-fat diet and ob/ob mice). We hypothesized that cardiomyocytes of mice with the metabolic syndrome would experience excessive ROS levels that trigger cellular dysfunctions. Fluorescent dyes and confocal microscopy were used to assess mitochondrial ROS production, cellular Ca2+ handling and contractile function in freshly isolated adult cardiomyocytes. Immunofluorescence, western blot and enzyme assay were used to study protein biochemistry. Unexpectedly, our results point towards decreased cardiac ROS signaling in a stable, chronic phase of the metabolic syndrome because: β-adrenergic-induced increases in the amplitude of intracellular Ca2+ signals were insensitive to antioxidant treatment; mitochondrial ROS production showed decreased basal rate and smaller response to β-adrenergic stimulation. Moreover, control hearts and hearts with the metabolic syndrome showed similar basal levels of ROS-mediated protein modification, but only control hearts showed increases after β-adrenergic stimulation. In conclusion, in contrast to the situation in control hearts, the cardiomyocyte response to acute β-adrenergic stimulation does not involve increased mitochondrial ROS production in a stable, chronic phase of the metabolic syndrome. This can be seen as a beneficial adaptation to prevent excessive ROS levels.

Reactive oxygen/nitrogen species and contractile function in skeletal muscle during fatigue and recovery

The production of reactive oxygen/nitrogen species (ROS/RNS) is generally considered to increase during physical exercise. Nevertheless, direct measurements of ROS/RNS often show modest increases in ROS/RNS in muscle fibres even during intensive fatiguing stimulation, and the major source(s) of ROS/RNS during exercise is still being debated. In rested muscle fibres, mild and acute exposure to exogenous ROS/RNS generally increases myofibrillar submaximal force, whereas stronger or prolonged exposure has the opposite effect. Endogenous production of ROS/RNS seems to preferentially decrease submaximal force and positive effects of antioxidants are mainly observed during fatigue induced by submaximal contractions. Fatigued muscle fibres frequently enter a prolonged state of reduced submaximal force, which is caused by a ROS/RNS-dependent decrease in sarcoplasmic reticulum Ca(2+) release and/or myofibrillar Ca(2+) sensitivity. Increased ROS/RNS production during exercise can also be beneficial and recent human and animal studies show that antioxidant supplementation can hamper the beneficial effects of endurance training. In conclusion, increased ROS/RNS production have both beneficial and detrimental effects on skeletal muscle function and the outcome depends on a combination of factors: the type of ROS/RNS; the magnitude, duration and location of ROS/RNS production; and the defence systems, including both endogenous and exogenous antioxidants.

Absence of Dystrophin Disrupts Skeletal Muscle Signaling: Roles of Ca2+, Reactive Oxygen Species, and Nitric Oxide in the Development of Muscular Dystrophy

Dystrophin is a long rod-shaped protein that connects the subsarcolemmal cytoskeleton to a complex of proteins in the surface membrane (dystrophin protein complex, DPC), with further connections via laminin to other extracellular matrix proteins. Initially considered a structural complex that protected the sarcolemma from mechanical damage, the DPC is now known to serve as a scaffold for numerous signaling proteins. Absence or reduced expression of dystrophin or many of the DPC components cause the muscular dystrophies, a group of inherited diseases in which repeated bouts of muscle damage lead to atrophy and fibrosis, and eventually muscle degeneration. The normal function of dystrophin is poorly defined. In its absence a complex series of changes occur with multiple muscle proteins showing reduced or increased expression or being modified in various ways. In this review, we will consider the various proteins whose expression and function is changed in muscular dystrophies, focusing on Ca(2+)-permeable channels, nitric oxide synthase, NADPH oxidase, and caveolins. Excessive Ca(2+) entry, increased membrane permeability, disordered caveolar function, and increased levels of reactive oxygen species are early changes in the disease, and the hypotheses for these phenomena will be critically considered. The aim of the review is to define the early damage pathways in muscular dystrophy which might be appropriate targets for therapy designed to minimize the muscle degeneration and slow the progression of the disease.

Critical Role of Intracellular RyR1 Calcium Release Channels in Skeletal Muscle Function and Disease

The skeletal muscle Ca²⁺ release channel, also known as ryanodine receptor type 1 (RyR1), is the largest ion channel protein known and is crucial for effective skeletal muscle contractile activation. RyR1 function is controlled by Ca_v1.1, a voltage gated Ca²⁺ channel that works mainly as a voltage sensor for RyR1 activity during skeletal muscle contraction and is also fine-tuned by Ca²⁺, several intracellular compounds (e.g., ATP), and modulatory proteins (e.g., calmodulin). Dominant and recessive mutations in RyR1, as well as acquired channel alterations, are the underlying cause of various skeletal muscle diseases. The aim of this mini review is to summarize several current aspects of RyR1 function, structure, regulation, and to describe the most common diseases caused by hereditary or acquired RyR1 malfunction.

Nitric oxide mediates stretch-induced Ca2+ oscillation in smooth muscle

The stretching of smooth muscle tissue modulates contraction via augmentation of Ca2+ transients, but the mechanism underlying stretch-induced Ca2+ transients is still unknown. We found that mechanical stretching and maintenance of mouse urinary bladder smooth muscle strips and single myocytes at the initial length of 30% and 18%, respectively, resulted in Ca2+ oscillations. Experiments indicated that mechanical stretching remarkably increases the production of nitric oxide (NO) as well as the amplitude and duration of muscle contraction. Stretch-induced Ca2+ oscillations and contractility increases were completely abolished by NO inhibitor L-NAME or eNOS gene inactivation. Moreover, exposure of eNOS knockout myocytes to exogenous NO donor induced Ca2+ oscillations. The stretch-induced Ca2+ oscillations were greatly inhibited by selective IP3R inhibitor xestospongin C and partially inhibited by ryanodine. Moreover, the stretch-induced Ca2+ oscillations were also suppressed by LY294002, but not by the soluble guanylyl cyclase (sGC) inhibitor ODQ. These results suggest that myocytes stretching and maintenance at a certain length resulted in Ca2+ oscillations that is NO dependent and sGC/cGMP independent and results from the activation of PI(3)K in smooth muscle.

Ryanodine receptor fragmentation and sarcoplasmic reticulum Ca2+ leak after one session of high-intensity interval exercise

High-intensity interval training (HIIT) is a time-efficient way of improving physical performance in healthy subjects and in patients with common chronic diseases, but less so in elite endurance athletes. The mechanisms underlying the effectiveness of HIIT are uncertain. Here, recreationally active human subjects performed highly demanding HIIT consisting of 30-s bouts of all-out cycling with 4-min rest in between bouts (≤3 min total exercise time). Skeletal muscle biopsies taken 24 h after the HIIT exercise showed an extensive fragmentation of the sarcoplasmic reticulum (SR) Ca(2+) release channel, the ryanodine receptor type 1 (RyR1). The HIIT exercise also caused a prolonged force depression and triggered major changes in the expression of genes related to endurance exercise. Subsequent experiments on elite endurance athletes performing the same HIIT exercise showed no RyR1 fragmentation or prolonged changes in the expression of endurance-related genes. Finally, mechanistic experiments performed on isolated mouse muscles exposed to HIIT-mimicking stimulation showed reactive oxygen/nitrogen species (ROS)-dependent RyR1 fragmentation, calpain activation, increased SR Ca(2+) leak at rest, and depressed force production due to impaired SR Ca(2+) release upon stimulation. In conclusion, HIIT exercise induces a ROS-dependent RyR1 fragmentation in muscles of recreationally active subjects, and the resulting changes in muscle fiber Ca(2+)-handling trigger muscular adaptations. However, the same HIIT exercise does not cause RyR1 fragmentation in muscles of elite endurance athletes, which may explain why HIIT is less effective in this group.

Muscle dysfunction associated with adjuvant-induced arthritis is prevented by antioxidant treatment

Background

In addition to the primary symptoms arising from inflamed joints, muscle weakness is prominent and frequent in patients with rheumatoid arthritis (RA). Here, we investigated the mechanisms of arthritis-induced muscle dysfunction in rats with adjuvant-induced arthritis (AIA).

Methods

AIA was induced in the knees of rats by injection of complete Freund’s adjuvant and was allowed to develop for 21 days. Muscle contractile function was assessed in isolated extensor digitorum longus (EDL) muscles. To assess mechanisms underlying contractile dysfunction, we measured redox modifications, redox enzymes and inflammatory mediators, and activity of actomyosin ATPase and sarcoplasmic reticulum (SR) Ca²⁺-ATPase.

Results

EDL muscles from AIA rats showed decreased tetanic force per cross-sectional area and slowed twitch contraction and relaxation. These contractile dysfunctions in AIA muscles were accompanied by marked decreases in actomyosin ATPase and SR Ca²⁺-ATPase activities. Actin aggregates were observed in AIA muscles, and these contained high levels of 3-nitrotyrosine and malondialdehyde-protein adducts. AIA muscles showed increased protein expression of NADPH oxidase 2/gp91^phox, neuronal nitric oxide synthase, tumor necrosis factor α (TNF-α), and high-mobility group box 1 (HMGB1). Treatment of AIA rats with EUK-134 (3 mg/kg/day), a superoxide dismutase/catalase mimetic, prevented both the decrease in tetanic force and the formation of actin aggregates in EDL muscles without having any beneficial effect on the arthritis development.

Conclusions

Antioxidant treatment prevented the development of oxidant-induced actin aggregates and contractile dysfunction in the skeletal muscle of AIA rats. This implies that antioxidant treatment can be used to effectively counteract muscle weakness in inflammatory conditions.