Stressed out: the skeletal muscle ryanodine receptor as a target of stress

Malignant hyperthermia: clinical and molecular aspects

CONTENT

Malignant hyperthermia (MH) is a potentially lethal pharmacogenetic disorder that affects genetically predisposed individuals. It manifests in susceptible individuals in response to exposure to Inhalant anesthetics, depolarizing muscle relaxants or extreme physical activity in hot environments. During exposure to these triggering agents, there is a rapid and sustained increase of myoplasmic calcium (Ca(2+)) concentration induced by hyperactivation of ryanodine receptor of skeletal muscle (RyR1), causing a profound change in Ca(2+) homeostasis, featuring a hypermetabolic state. RyR1, Ca(2+) release channels of sarcoplasmic reticulum, is the primary locus for MH susceptibility. Several mutations in the gene encoding the protein RyR1 have been identified; however, other genes may be involved. Actually, the standard method for diagnosing MH susceptibility is the muscle contracture test for exposure to halothane-caffeine (CHCT) and the only treatment is the use of dantrolene. However, with advances in molecular genetics, a full understanding of the disease etiology may be provided, favoring the development of an accurate diagnosis, less invasive, with DNA test, and also will provide the development of new therapeutic strategies for treatment of MH. Thus, this brief review aims to integrate molecular and clinical aspects of MH, gathering input for a better understanding of this channelopathy.

Epigallocatechin-3-gallate improves plantaris muscle recovery after disuse in aged rats

Aging exacerbates muscle loss and slows the recovery of muscle mass and function after disuse. In this study we investigated the potential that epigallocatechin-3-gallate (EGCg), an abundant catechin in green tea, would reduce signaling for apoptosis and promote skeletal muscle recovery in the fast plantaris muscle and the slow soleus muscle after hindlimb suspension (HLS) in senescent animals. Fischer 344 × Brown Norway inbred rats (age 34 months) received either EGCg (50 mg/kg body weight), or water daily by gavage. One group of animals received HLS for 14 days and a second group of rats received 14 days of HLS, then the HLS was removed and they recovered from this forced disuse for 2 weeks. Animals that received EGCg over the HLS followed by 14 days of recovery, had a 14% greater plantaris muscle weight (p<0.05) as compared to the animals treated with the vehicle over this same period. Plantaris fiber area was greater after recovery in EGCg (2715.2±113.8 μm(2)) vs. vehicle treated animals (1953.0±41.9 μm(2)). In addition, activation of myogenic progenitor cells was improved with EGCg over vehicle treatment (7.5% vs. 6.2%) in the recovery animals. Compared to vehicle treatment, the apoptotic index was lower (0.24% vs. 0.52%), and the abundance of pro-apoptotic proteins Bax (-22%), and FADD (-77%) was lower in EGCg treated plantaris muscles after recovery. While EGCg did not prevent unloading-induced atrophy, it improved muscle recovery after the atrophic stimulus in fast plantaris muscles. However, this effect was muscle specific because EGCg had no major impact in reversing HLS-induced atrophy in the slow soleus muscle of old rats.

Effect of Calstabin1 depletion on calcium transients and energy utilization in muscle fibers and treatment opportunities with RyR1 stabilizers

Depletion of calstabin1 (FKBP12) from the RyR1 channel and consequential calcium leakage from the sarcoplasmic reticulum (SR) is found in certain disease conditions such as dystrophy, aging or muscle overuse. Here, we first assessed the effect of calstabin1 depletion on resting Ca²⁺ levels and transients. We found that depletion of calstabin1 with the calstabin1-dissociation compound FK506 increased the release of calcium from the SR by 14 % during tetanic stimulation (50 Hz, 300 ms) and delayed cytosolic calcium removal. However, we did not find a significant increase in resting cytosolic Ca²⁺ levels. Therefore, we tested if increased SERCA activity could counterbalance calcium leakage. By measuring the energy utilization of muscle fibers with and without FK506 treatment, we observed that FK506-treatment increased oxygen consumption by 125% compared to baseline levels. Finally, we found that pretreatment of muscle fibers with the RyR1 stabilizer JTV-519 led to an almost complete normalization of calcium flux dynamics and energy utilization. We conclude that cytosolic calcium levels are mostly preserved in conditions with leaky RyR1 channels due to increased SERCA activity. Therefore, we suggest that RyR1 leakiness might lead to chronic metabolic stress, followed by cellular damage, and RyR1 stabilizers could potentially protect diseased muscle tissue.

Structure-activity relationship of non-coplanar polychlorinated biphenyls toward skeletal muscle ryanodine receptors in rainbow trout (Oncorhynchus mykiss)

Research addressing the health impacts of polychlorinated biphenyls (PCBs) has primarily focused on the effects of coplanar, or dioxin-like (DL), congeners, which is especially true for research assessing impacts in fish species. Ortho substituted non-coplanar, termed non-dioxin-like (NDL), PCBs have received less attention. In mammals, NDL PCBs enhance the activity of ryanodine receptors (RyR), calcium release channels necessary for engaging excitation-contraction (EC) coupling in striated muscle. We utilized in vitro receptor binding analysis to determine whether NDL PCB congeners detected in aquatic environments alter the activity of RyR isoform 1 (RyR1) found in the skeletal muscle of rainbow trout. Congeners 52, 95, 136, and149 were the most efficacious leading to an increase in receptor activity that was approximately 250% greater than that found under solvent control conditions. Other environmentally relevant congeners, namely PCB 153, 151 and 101, which all contain two or more chlorines in the ortho-position, enhanced receptor activity by greater than 160% of baseline. The mono-ortho congeners or the non-ortho PCB 77 had negligible impact on the RyR1. When combined, in binary or environmentally relevant mixtures, congeners shown to enhance receptor activity appeared to display additivity and when the active PCB 95 was present with the non-active congener PCB 77 the impact on receptor activity was reduced from 250% to 230%. The important role of the RyR and the demonstrated additive nature of NDL congeners toward altering channel function calls for further investigation into the ecological implications of altered RyR function in fish with high PCB burdens.

Mitochondrial and sarcoplasmic reticulum abnormalities in cancer cachexia: altered energetic efficiency?

BACKGROUND

Cachexia is a wasting condition that manifests in several types of cancer, and the main characteristic is the profound loss of muscle mass.

METHODS

The Yoshida AH-130 tumor model has been used and the samples have been analyzed using transmission electronic microscopy, real-time PCR and Western blot techniques.

RESULTS

Using in vivo cancer cachectic model in rats, here we show that skeletal muscle loss is accompanied by fiber morphologic alterations such as mitochondrial disruption, dilatation of sarcoplasmic reticulum and apoptotic nuclei. Analyzing the expression of some factors related to proteolytic and thermogenic processes, we observed in tumor-bearing animals an increased expression of genes involved in proteolysis such as ubiquitin ligases Muscle Ring Finger 1 (MuRF-1) and Muscle Atrophy F-box protein (MAFBx). Moreover, an overexpression of both sarco/endoplasmic Ca(2+)-ATPase (SERCA1) and adenine nucleotide translocator (ANT1), both factors related to cellular energetic efficiency, was observed. Tumor burden also leads to a marked decreased in muscle ATP content.

CONCLUSIONS

In addition to muscle proteolysis, other ATP-related pathways may have a key role in muscle wasting, both directly by increasing energetic inefficiency, and indirectly, by affecting the sarcoplasmic reticulum-mitochondrial assembly that is essential for muscle function and homeostasis.

GENERAL SIGNIFICANCE

The present study reports profound morphological changes in cancer cachectic muscle, which are visualized mainly in alterations in sarcoplasmic reticulum and mitochondria. These alterations are linked to pathways that can account for energy inefficiency associated with cancer cachexia.

Skeletal muscle function during exercise-fine-tuning of diverse subsystems by nitric oxide

Skeletal muscle is responsible for altered acute and chronic workload as induced by exercise. Skeletal muscle adaptations range from immediate change of contractility to structural adaptation to adjust the demanded performance capacities. These processes are regulated by mechanically and metabolically induced signaling pathways, which are more or less involved in all of these regulations. Nitric oxide is one of the central signaling molecules involved in functional and structural adaption in different cell types. It is mainly produced by nitric oxide synthases (NOS) and by non-enzymatic pathways also in skeletal muscle. The relevance of a NOS-dependent NO signaling in skeletal muscle is underlined by the differential subcellular expression of NOS1, NOS2, and NOS3, and the alteration of NO production provoked by changes of workload. In skeletal muscle, a variety of highly relevant tasks to maintain skeletal muscle integrity and proper signaling mechanisms during adaptation processes towards mechanical and metabolic stimulations are taken over by NO signaling. The NO signaling can be mediated by cGMP-dependent and -independent signaling, such as S-nitrosylation-dependent modulation of effector molecules involved in contractile and metabolic adaptation to exercise. In this review, we describe the most recent findings of NO signaling in skeletal muscle with a special emphasis on exercise conditions. However, to gain a more detailed understanding of the complex role of NO signaling for functional adaptation of skeletal muscle (during exercise), additional sophisticated studies are needed to provide deeper insights into NO-mediated signaling and the role of non-enzymatic-derived NO in skeletal muscle physiology.

The Munich Triathlon Heart Study: ventricular function, myocardial velocities, and two-dimensional strain in healthy children before and after endurance stress

Intense exercise has been shown to have negative effects on systolic and diastolic ventricular function in adults. Very little is known about the normal reaction of the growing heart to endurance stress. For this study, 26 healthy children (18 males) with a mean age of 12.61 years (range, 7.92-16.42 years) took part in an age-adapted triathlon circuit. The athletes were investigated by two-dimensional (2D) echocardiographic/speckle tracking, M-mode, pulse-wave Doppler, color Doppler, and color-coded tissue Doppler at 2-4 weeks before and immediately after the race. After the competition, cardiac output increased, mediated by an increase in heart rate and not by an elevated preload, according the Frank-Starling mechanism. Two-dimensional speckle tracking showed a reduced longitudinal strain in the right and left ventricles and additionally reduced circumferential strain in the left ventricle. The late diastolic inflow velocities were increased in both ventricles, indicating reduced diastolic function due to an impairment of myocardial relaxation. Immediately after endurance exercise, systolic and diastolic functions were attenuated in children and adolescents. In contrast to adult studies, this study could show a heart rate-mediated increase in cardiac output. The sequelae of these alterations are unclear, and the growing heart especially may be more susceptible to myocardial damage caused by intense endurance stress.

Triclosan impairs swimming behavior and alters expression of excitation-contraction coupling proteins in fathead minnow (Pimephales promelas)

Triclosan (TCS), a high volume chemical widely used in consumer products, is a known aquatic contaminant found in fish inhabiting polluted watersheds. Mammalian studies have recently demonstrated that TCS disrupts signaling between the ryanodine receptor (RyR) and the dihydropyridine receptor (DHPR), two proteins essential for excitation-contraction (EC) coupling in striated muscle. We investigated the swimming behavior and expression of EC coupling proteins in larval fathead minnows (Pimephales promelas) exposed to TCS for up to 7 days. Concentrations as low as 75 μg L(-1) significantly altered fish swimming activity after 1 day; which was consistent after 7 days of exposure. The mRNA transcription and protein levels of RyR and DHPR (subunit CaV1.1) isoforms changed in a dose and time dependent manner. Crude muscle homogenates from exposed larvae did not display any apparent changes in receptor affinity toward known radioligands. In nonexposed crude muscle homogenates, TCS decreased the binding of [(3)H]PN20-110 to the DHPR and decreased the binding of [(3)H]-ryanodine to the RyR, demonstrating a direct impact at the receptor level. These results support TCS's impact on muscle function in vertebrates further exemplifying the need to re-evaluate the risks this pollutant poses to aquatic environments.

ICU-acquired weakness: mechanisms of disability

PURPOSE OF REVIEW

ICU-acquired weakness (ICUAW) is now recognized as a major complication of critical illness. There is no doubt that ICUAW is prevalent - some might argue ubiquitous - after critical illness, but its true role, the interaction with preexisting nerve and muscle lesions as well as its contribution to long-term functional disability, remains to be elucidated.

RECENT FINDINGS

In this article, we review the current state-of-the-art of the basic pathophysiology of nerve and muscle weakness after critical illness and explore the current literature on ICUAW with a special emphasis on the most important mechanisms of weakness.

SUMMARY

Variable contributions of structural and functional changes likely contribute to both early and late myopathy and neuropathy, although the specifics of the temporality of both processes, and the influence patient comorbidities, age, and nature of the ICU insult have on them, remain to be determined.

Ryanodine receptor patents

Research over the past two decades has implicated dysfunction of the ryanodine receptor (RyR), a Ca(2+) release channel on the sarcoplasmic reticulum (SR) required for excitation-contraction (EC) coupling, in the pathogenesis of cardiac and skeletal myopathies. These discoveries have led to the development of novel drugs, screening tools, and research methods. The patents associated with these advances tell the story of the initial discovery of RyRs as a target for plant alkaloids, to their central role in cardiac and skeletal muscle excitation-contraction coupling, and ongoing clinical trials with a novel class of drugs called RycalsTM that inhibit pathological intracellular Ca(2+) leak. Additionally, these patents highlight questions, controversies, and future directions of the RyR field.

Intensive care unit-acquired weakness: clinical phenotypes and molecular mechanisms

Intensive care unit-acquired weakness (ICUAW) begins within hours of mechanical ventilation and may not be completely reversible over time. It represents a major functional morbidity of critical illness and is an important patient-centered outcome with clear implications for quality of life and resumption of prior work and lifestyle. There is heterogeneity in functional outcome related to ICUAW across various patient populations after an episode of critical illness. This state-of-the art review argues that this observed heterogeneity may represent a clinical spectrum of disability in which there are recognizable clinical phenotypes for outcome according to age, burden of comorbid illness, and ICU length of stay. It further argues that these functional outcomes are modified by mood, cognition, and caregiver physical and mental health. This proposed construct of clinical phenotypes will be used as a framework for a review of the current literature on the molecular biology of muscle and nerve injury. This translational approach for the development of models pairing clinical phenotypes for different functional outcomes after critical illness with molecular mechanism of injury may offer unique insights into the diagnosis and treatment of muscle and nerve lesions.

Intense resistance exercise induces early and transient increases in ryanodine receptor 1 phosphorylation in human skeletal muscle

Background

While ryanodine receptor 1 (RyR1) critically contributes to skeletal muscle contraction abilities by mediating Ca²⁺ion oscillation between sarcoplasmatic and myofibrillar compartments, AMP-activated protein kinase (AMPK) senses contraction-induced energetic stress by phosphorylation at Thr¹⁷². Phosphorylation of RyR1 at serine²⁸⁴³ (pRyR1Ser²⁸⁴³) results in leaky RyR1 channels and impaired Ca²⁺homeostasis. Because acute resistance exercise exerts decreased contraction performance in skeletal muscle, preceded by high rates of Ca²⁺-oscillation and energetic stress, intense myofiber contractions may induce increased RyR1 and AMPK phosphorylation. However, no data are available regarding the time-course and magnitude of early RyR1 and AMPK phosphorylation in human myofibers in response to acute resistance exercise.

Purpose

Determine the effects and early time-course of resistance exercise on pRyR1Ser²⁸⁴³ and pAMPKThr¹⁷² in type I and II myofibers.

Methods

7 male subjects (age 23±2 years, height: 185±7 cm, weight: 82±5 kg) performed 3 sets of 8 repetitions of maximum eccentric knee extensions. Muscle biopsies were taken at rest, 15, 30 and 60 min post exercise. pRyR1Ser²⁸⁴³ and pAMPKThr¹⁷² levels were determined by western blot and semi-quantitative immunohistochemistry techniques.

Results

While total RyR1 and total AMPK levels remained unchanged, RyR1 was significantly more abundant in type II than type I myofibers. pRyR1Ser²⁸⁴³ increased 15 min and peaked 30 min (p<0.01) post exercise in both myofiber types. Type I fibers showed relatively higher increases in pRyR1Ser²⁸⁴³ levels than type II myofibers and remained elevated up to 60 min post resistance exercise (p<0.05). pAMPKThr¹⁷² also increased 15 to 30 min post exercise (p<0.01) in type I and II myofibers and in whole skeletal muscle.

Conclusion

Resistance exercise induces acutely increased pRyR1Ser²⁸⁴³ and concomitantly pAMPKThr¹⁷² levels for up to 30 min in resistance exercised myofibers. This provides a time-course by which pRyR1Ser²⁸⁴³ can mechanistically impact Ca²⁺handling properties and consequently induce reduced myofiber contractility beyond immediate fatiguing mechanisms.

Biomarkers of peripheral muscle fatigue during exercise

BACKGROUND

Biomarkers of peripheral muscle fatigue (BPMFs) are used to offer insights into mechanisms of exhaustion during exercise in order to detect abnormal fatigue or to detect defective metabolic pathways. This review aims at describing recent advances and future perspectives concerning the most important biomarkers of muscle fatigue during exercise.

RESULTS

BPMFs are classified according to the mechanism of fatigue related to adenosine-triphosphate-metabolism, acidosis, or oxidative-metabolism. Muscle fatigue is also related to an immunological response. impaired calcium handling, disturbances in bioenergetic pathways, and genetic responses. The immunological and genetic response may make the muscle susceptible to fatigue but may not directly cause muscle fatigue. Production of BPMFs is predominantly dependent on the type of exercise. BPMFs need to change as a function of the process being monitored, be stable without appreciable diurnal variations, correlate well with exercise intensity, and be present in detectable amounts in easily accessible biological fluids. The most well-known BPMFs are serum lactate and interleukin-6. The most widely applied clinical application is screening for defective oxidative metabolism in mitochondrial disorders by means of the lactate stress test. The clinical relevance of most other BPMFs, however, is under debate, since they often depend on age, gender, physical fitness, the energy supply during exercise, the type of exercise needed to produce the BPMF, and whether healthy or diseased subjects are investigated.

CONCLUSIONS

Though the role of BPMFs during fatigue is poorly understood, measuring BPMFs under specific, standardised conditions appears to be helpful for assessing biological states or processes during exercise and fatigue.

Role of leaky neuronal ryanodine receptors in stress-induced cognitive dysfunction

The type 2 ryanodine receptor/calcium release channel (RyR2), required for excitation-contraction coupling in the heart, is abundant in the brain. Chronic stress induces catecholamine biosynthesis and release, stimulating β-adrenergic receptors and activating cAMP signaling pathways in neurons. In a murine chronic restraint stress model, neuronal RyR2 were phosphorylated by protein kinase A (PKA), oxidized, and nitrosylated, resulting in depletion of the stabilizing subunit calstabin2 (FKBP12.6) from the channel complex and intracellular calcium leak. Stress-induced cognitive dysfunction, including deficits in learning and memory, and reduced long-term potentiation (LTP) at the hippocampal CA3-CA1 connection were rescued by oral administration of S107, a compound developed in our laboratory that stabilizes RyR2-calstabin2 interaction, or by genetic ablation of the RyR2 PKA phosphorylation site at serine 2808. Thus, neuronal RyR2 remodeling contributes to stress-induced cognitive dysfunction. Leaky RyR2 could be a therapeutic target for treatment of stress-induced cognitive dysfunction.

Illicit organogenesis: Methods and substances of doping and manipulation

Doping and manipulation are undesirable companions of professional and amateur sport. Numerous adverse analytical findings as well as confessions of athletes have demonstrated the variety of doping agents and methods as well as the inventiveness of cheating sportsmen. Besides 'conventional' misuse of drugs such as erythropoietin and insulins, experts fear that therapeutics that are currently undergoing clinical trials might be part of current or future doping regimens, which aim for an increased functionality and performance or organs and tissues. Emerging drugs such as selective androgen receptor modulators (SARMs), hypoxia-inducible factor (HIF) complex stabilizers or modulators of muscle fiber calcium channels are considered relevant for current and future doping controls due to their high potential for misuse in sports.

Variable phenotype of del45-55 Becker patients correlated with nNOSμ mislocalization and RYR1 hypernitrosylation

Duchenne and Becker muscular dystrophies (DMD and BMD) are muscle-wasting diseases caused by mutations in the DMD gene-encoding dystrophin. Usually, out-of-frame deletions give rise to DMD, whereas in-frame deletions result in BMD. BMD patients exhibit a less severe disease because an abnormal but functional dystrophin is produced. This is the rationale for attempts to correct the reading frame by using an exon-skipping strategy. In order to apply this approach to a larger number of patients, a multi-exon skipping strategy of exons 45-55 has been proposed, because it should correct the mRNA reading frame in almost 75% of DMD patients with a deletion. The resulting dystrophin lacks part of the binding site for the neuronal nitric oxide synthase (nNOSμ), which normally binds to spectrin-like repeats 16 and 17 of the dystrophin. Since these domains are encoded by exons 42-45, we investigated the nNOSμ status in muscle biopsies from 12 BMD patients carrying spontaneous deletions spaning exons 45-55. We found a wide spectrum of nNOSμ expression and localization. The strictly cytosolic mislocalization of nNOSμ was associated with the more severe phenotypes. Cytosolic NO production correlated with both hypernitrosylation of the sarcoplasmic reticulum calcium-release-channel ryanodine receptor type-1 (RyR1) and release of calstabin-1, a central hub of Ca(2+) signaling and contraction in muscle. Finally, this study shows that the terminal truncation of the nNOS-binding domain in the 'therapeutic' del45-55 dystrophin is not innocuous, since it can perturb the nNOS-dependent stability of the RyR1/calstabin-1 complex.

Catechins attenuate eccentric exercise-induced inflammation and loss of force production in muscle in senescence-accelerated mice

Catechins have a great variety of biological actions. We evaluated the potential benefits of catechin ingestion on muscle contractile properties, oxidative stress, and inflammation following downhill running, which is a typical eccentric exercise, in senescence-accelerated prone mice (SAMP). Downhill running (13 m/min for 60 min; 16° decline) induced a greater decrease in the contractile force of soleus muscle and in Ca(2+)-ATPase activity in SAMP1 compared with the senescence-resistant mice (SAMR1). Moreover, compared with SAMR1, SAMP1 showed greater downhill running-induced increases in plasma CPK and LDH activity, malondialdehyde, and carbonylated protein as markers of oxidative stress; and in protein and mRNA expression levels of the inflammatory mediators such as tumor necrosis factor-α and monocyte chemoattractant protein-1 in muscle. SAMP1 exhibited aging-associated vulnerability to oxidative stress and inflammation in muscle induced by downhill running. Long-term (8 wk) catechin ingestion significantly attenuated the downhill running-induced decrease in muscle force and the increased inflammatory mediators in both plasma and gastrocnemius muscle. Furthermore, catechins significantly inhibited the increase in oxidative stress markers immediately after downhill running, accompanied by an increase in glutathione reductase activity. These findings suggest that long-term catechin ingestion attenuates the aging-associated loss of force production, oxidative stress, and inflammation in muscle after exercise.

iNOS ablation does not improve specific force of the extensor digitorum longus muscle in dystrophin-deficient mdx4cv mice

Nitrosative stress compromises force generation in Duchenne muscular dystrophy (DMD). Both inducible nitric oxide synthase (iNOS) and delocalized neuronal NOS (nNOS) have been implicated. We recently demonstrated that genetic elimination of nNOS significantly enhanced specific muscle forces of the extensor digitorum longus (EDL) muscle of dystrophin-null mdx4cv mice (Li D et al J. Path. 223:88-98, 2011). To determine the contribution of iNOS, we generated iNOS deficient mdx4cv mice. Genetic elimination of iNOS did not alter muscle histopathology. Further, the EDL muscle of iNOS/dystrophin DKO mice yielded specific twitch and tetanic forces similar to those of mdx4cv mice. Additional studies suggest iNOS ablation did not augment nNOS expression neither did it result in appreciable change of nitrosative stress markers in muscle. Our results suggest that iNOS may play a minor role in mediating nitrosative stress-associated force reduction in DMD.

Tea catechins prevent contractile dysfunction in unloaded murine soleus muscle: a pilot study

OBJECTIVE

Extended periods of muscle disuse, physical inactivity, immobilization, and bedrest result in a loss of muscle mass and a decrease in muscle force, which are accompanied by an increase in oxidative stress. We investigated the effects of the intake of green tea catechins on unloading-induced muscle dysfunction in tail-suspended mice.

METHODS

Ten-week-old male BALB/c mice were fed a purified control diet or a diet containing 0.5% tea catechins for 14 d. Thereafter, the mice were subjected to continuous tail suspension for 10 d. On the final day, muscle mass, contractile force production, antioxidant potential, and carbonylated protein levels were evaluated.

RESULTS

Hind limb unloading caused a loss of soleus muscle weight and muscle force. Intake of tea catechins significantly inhibited the unloading-induced decrease in force in isolated soleus muscle by 19% compared with the control group, although tea catechins did not affect muscle weight. In addition, intake of tea catechins suppressed the decrease in antioxidant potential and the increase in carbonyl myofibrillar protein.

CONCLUSION

Ingestion of tea catechins minimized contractile dysfunction in skeletal muscle and muscle atrophy in unloaded muscle. This effect might be partly due to the lower oxidative modification of myofibrillar protein through the antioxidant activity of tea catechins.

Exercise and Duchenne muscular dystrophy: toward evidence-based exercise prescription

To develop a rational framework for answering questions about the role of exercise in Duchenne muscular dystrophy (DMD), we focused on five pathophysiological mechanisms and offer brief hypotheses regarding how exercise may beneficially modulate pertinent cellular and molecular pathways. We aimed to provide an integrative overview of mechanisms of DMD pathology that may improve or worsen as a result of exercise. We also sought to stimulate discussion of what outcomes/dependent variables most appropriately measure these mechanisms, with the purpose of defining criteria for well-designed, controlled studies of exercise in DMD. The five mechanisms include pathways that are both intrinsic and extrinsic to the diseased muscle cells.

Mechanisms of skeletal muscle weakness

Skeletal muscle weakness is an important feature of numerous -pathological conditions and it may also be a component in normal ageing. Decreased muscular strength can be due to decreased muscle mass and/or intrinsic defects in the muscle cells. In this chapter we will discuss decreased force production due to mechanisms intrinsic to skeletal muscle cells. We will mainly use data from mouse disease models to exemplify defects at various sites in the cellular activation-contraction pathway. We will show that depending on the underlying problem, muscle weakness can be due decreased Ca²(+) release from the sarcoplasmic reticulum, reduced myofibrillar Ca²(+) sensitivity and/or decreased ability of the cross-bridges to generate force.

Nitrosative stress elicited by nNOSµ delocalization inhibits muscle force in dystrophin-null mice

The mechanism of force reduction is not completely understood in Duchenne muscular dystrophy (DMD), a dystrophin-deficient lethal disease. Nitric oxide regulates muscle force. Interestingly, neuronal nitric oxide synthase µ (nNOSµ), a major source of muscle nitric oxide, is lost from the sarcolemma in DMD muscle. We hypothesize that nNOSµ delocalization contributes to force reduction in DMD. To test this hypothesis, we generated dystrophin/nNOSµ double knockout mice. Genetic elimination of nNOSµ significantly enhanced force in dystrophin-null mice. Pharmacological inhibition of nNOS yielded similar results. To further test our hypothesis, we studied δ-sarcoglycan-null mice, a model of limb-girdle muscular dystrophy. These mice had minimal sarcolemmal nNOSµ delocalization and muscle force was less compromised. Annihilation of nNOSµ did not improve their force either. To determine whether nNOSµ delocalization itself inhibited force, we corrected muscle disease in dystrophin-null mice with micro-dystrophins that either restored or did not restore sarcolemmal nNOSµ. Similar muscle force was obtained irrespective of nNOSµ localization. Additional studies suggest that nNOSµ delocalization selectively inhibits muscle force in dystrophin-null mice via nitrosative stress. In summary, we have demonstrated for the first time that nitrosative stress elicited by nNOSµ delocalization is an important mechanism underlying force loss in DMD.

Two close, too close: sarcoplasmic reticulum-mitochondrial crosstalk and cardiomyocyte fate

Mitochondria are key organelles in cell life whose dysfunction is associated with a variety of diseases. Their crucial role in intermediary metabolism and energy conversion makes them a preferred target in tissues, such as the heart, where the energetic demands are very high. In the cardiomyocyte, the spatial organization of mitochondria favors their interaction with the sarcoplasmic reticulum, thereby offering a mechanism for Ca(2+)-mediated crosstalk between these 2 organelles. Recently, the molecular basis for this interaction has begun to be unraveled, and we are learning how endoplasmic reticulum-mitochondrial interactions are often exploited by death signals, such as proapoptotic Bcl-2 family members, to amplify the cell death cascade. Here, we review our present understanding of the structural basis and the functional consequences of the close interaction between sarcoplasmic reticulum and mitochondria on cardiomyocyte function and death.

Ryanodine receptor channelopathies

Ryanodine receptors (RyR) are intracellular Ca2+-permeable channels that provide the sarcoplasmic reticulum Ca2+ release required for skeletal and cardiac muscle contractions. RyR1 underlies skeletal muscle contraction, and RyR2 fulfills this role in cardiac muscle. Over the past 20 years, numerous mutations in both RyR isoforms have been identified and linked to skeletal and cardiac diseases. Malignant hyperthermia, central core disease, and catecholaminergic polymorphic ventricular tachycardia have been genetically linked to mutations in either RyR1 or RyR2. Thus, RyR channelopathies are both of interest because they cause significant human diseases and provide model systems that can be studied to elucidate important structure-function relationships of these ion channels.

Fixing ryanodine receptor Ca leak - a novel therapeutic strategy for contractile failure in heart and skeletal muscle

A critical component in regulating cardiac and skeletal muscle contractility is the release of Ca(2+) via ryanodine receptor (RyR) Ca(2+) release channels in the sarcoplasmic reticulum (SR). In heart failure and myopathy, the RyR has been found to be excessively phosphorylated or nitrosylated and depleted of the RyR-stabilizing protein calstabin (FK506 binding protein 12/12.6). This remodeling of the RyR channel complex results in an intracellular SR Ca(2+) leak and impaired contractility. Despite recent advances in heart failure treatment, there are still devastatingly high mortality rates with this disease. Moreover, pharmacological treatment for muscle weakness and myopathy is nearly nonexistent. A novel class of RyR-stabilizing drugs, rycals, which reduce Ca(2+) leak by stabilizing the RyR channels due to preservation of the RyR-calstabin interaction, have recently been shown to improve contractile function in both heart and skeletal muscle. This opens up a novel therapeutic strategy for the treatment of contractile failure in the cardiac and skeletal muscle.

Aerobic exercise training improves Ca2+ handling and redox status of skeletal muscle in mice

Exercise training is known to promote relevant changes in the properties of skeletal muscle contractility toward powerful fibers. However, there are few studies showing the effect of a well-established exercise training protocol on Ca2+ handling and redox status in skeletal muscles with different fiber-type compositions. We have previously standardized a valid and reliable protocol to improve endurance exercise capacity in mice based on maximal lactate steady-state workload (MLSSw). The aim of this study was to investigate the effect of exercise training, performed at MLSSw, on the skeletal muscle Ca2+ handling-related protein levels and cellular redox status in soleus and plantaris. Male C57BL/6J mice performed treadmill training at MLSSw over a period of eight weeks. Muscle fiber-typing was determined by myosin ATPase histochemistry, citrate synthase activity by spectrophotometric assay, Ca2+ handling-related protein levels by Western blot and reduced to oxidized glutathione ratio (GSH:GSSG) by high-performance liquid chromatography. Trained mice displayed higher running performance and citrate synthase activity compared with untrained mice. Improved running performance in trained mice was paralleled by fast-to-slow fiber-type shift and increased capillary density in both plantaris and soleus. Exercise training increased dihydropyridine receptor (DHPR) α2 subunit, ryanodine receptor and Na+/Ca2+ exchanger levels in plantaris and soleus. Moreover, exercise training elevated DHPR β1 subunit and sarcoplasmic reticulum Ca2+-ATPase (SERCA) 1 levels in plantaris and SERCA2 levels in soleus of trained mice. Skeletal muscle GSH content and GSH:GSSG ratio was increased in plantaris and soleus of trained mice. Taken together, our findings indicate that MLSSw exercise-induced better running performance is, in part, due to increased levels of proteins involved in skeletal muscle Ca2+ handling, whereas this response is partially dependent on specificity of skeletal muscle fiber-type composition. Finally, we demonstrated an augmented cellular redox status and GSH antioxidant capacity in trained mice.

Ryanodine receptor studies using genetically engineered mice

Ryanodine receptors (RyR) regulate intracellular Ca(2+) release in many cell types and have been implicated in a number of inherited human diseases. Over the past 15 years genetically engineered mouse models have been developed to elucidate the role that RyRs play in physiology and pathophysiology. To date these models have implicated RyRs in fundamental biological processes including excitation-contraction coupling and long term plasticity as well as diseases including malignant hyperthermia, cardiac arrhythmias, heart failure, and seizures. In this review we summarize the RyR mouse models and how they have enhanced our understanding of the RyR channels and their roles in cellular physiology and disease.

Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Homeostatic control of the endoplasmic reticulum (ER) both as the site for protein handling (synthesis, folding, trafficking, disaggregation and degradation) and as a Ca2+ store is of crucial importance for correct functioning of the cell. Disturbance of the homeostatic control mechanisms leads to a vast array of severe pathologies. The Ca2+ content of the ER is a dynamic equilibrium between active uptake via Ca2+ pumps and Ca2+ release by a number of highly regulated Ca2+-release channels. Regulation of the Ca2+-release channels is very complex and several mechanisms are still poorly understood or controversial. There is increasing evidence that a number of unrelated proteins, either by themselves or in association with other Ca2+ channels, can provide additional Ca2+-leak pathways. The ER is a dynamic organelle and changes in its size and components have been described, either as a result of (de)differentiation processes affecting the secretory capacity of cells, or as a result of adaptation mechanisms to diverse stress conditions such as the unfolded protein response and autophagy. In this review we want to give an overview of the current knowledge of the (short-term) regulatory mechanisms that affect Ca2+-release and Ca2+-leak pathways and of the (long-term) adaptations in ER size and capacity. Understanding of the consequences of these mechanisms for cellular Ca2+ signaling could provide a huge therapeutic potential.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

Mass spectrometry-based characterization of new drugs and methods of performance manipulation in doping control analysis

Efficient and comprehensive sports drug testing necessitates frequent updating and proactive, preventive anti-doping research, and the early implementation of new, emerging drugs into routine doping controls is an essential aspect. Several new drugs and drug candidates with potential for abuse, including so-called Rycals (ryanodine receptor calstabin complex stabilizers, for example, S-107), hypoxia-inducible factor (HIF) stabilizers, and peroxisome-proliferator-activated receptor (PPAR) delta agonists (for example, GW1516), were studied using different mass spectrometry- and ion mobility-based approaches, and their gas phase dissociation behaviors were elucidated. The detailed knowledge of fragmentation routes allows a more rapid identification of metabolites and structurally related, presumably "tailor-made", analogs potentially designed for doping purposes. The utility of product ion characterization is demonstrated in particular with GW1516, for which oxidation products were readily identified in urine samples by means of diagnostic fragment ions as measured using high resolution/high accuracy mass spectrometry and higher energy collision-induced dissociation (HCD).

Toxicology in the fast lane: application of high-throughput bioassays to detect modulation of key enzymes and receptors

BACKGROUND

Legislation at state, federal, and international levels is requiring rapid evaluation of the toxicity of numerous chemicals. Whole-animal toxicologic studies cannot yield the necessary throughput in a cost-effective fashion, leading to a critical need for a faster and more cost-effective toxicologic evaluation of xenobiotics.

OBJECTIVES

We tested whether mechanistically based screening assays can rapidly provide information on the potential for compounds to affect key enzymes and receptor targets, thus identifying those compounds requiring further in-depth analysis.

METHODS

A library of 176 synthetic chemicals was prepared and examined in a high-throughput screening (HTS) manner using nine enzyme-based and five receptor-based bioassays.

RESULTS

All the assays have high Z' values, indicating good discrimination among compounds in a reliable fashion, and thus are suitable for HTS assays. On average, three positive hits were obtained per assay. Although we identified compounds that were previously shown to inhibit a particular enzyme class or receptor, we surprisingly discovered that triclosan, a microbiocide present in personal care products, inhibits carboxylesterases and that dichlone, a fungicide, strongly inhibits the ryanodine receptors.

CONCLUSIONS

Considering the need to rapidly screen tens of thousands of anthropogenic compounds, our study shows the feasibility of using combined HTS assays as a novel approach toward obtaining toxicologic data on numerous biological end points. The HTS assay approach is very useful to quickly identify potentially hazardous compounds and to prioritize them for further in-depth studies.

Automated drug screening with contractile muscle tissue engineered from dystrophic myoblasts

Identification of factors that improve muscle function in boys with Duchenne muscular dystrophy (DMD) could lead to an improved quality of life. To establish a functional in vitro assay for muscle strength, mdx murine myoblasts, the genetic homologue of DMD, were tissue engineered in 96-microwell plates into 3-dimensional muscle constructs with parallel arrays of striated muscle fibers. When electrically stimulated, they generated tetanic forces measured with an automated motion tracking system. Thirty-one compounds of interest as potential treatments for patients with DMD were tested at 3 to 6 concentrations. Eleven of the compounds (insulin-like growth factor-1, creatine, beta-hydroxy-beta-methylbutyrate, trichostatin A, lisinopril, and 6 from the glucocorticoid family) significantly increased tetanic force relative to placebo-treated controls. The glucocorticoids methylprednisolone, deflazacort, and prednisone increased tetanic forces at low doses (EC(50) of 6, 19, and 56 nM, respectively), indicating a direct muscle mechanism by which they may be benefitting DMD patients. The tetanic force assay also identified beneficial compound interactions (arginine plus deflazacort and prednisone plus creatine) as well as deleterious interactions (prednisone plus creatine inhibited by pentoxifylline) of combinatorial therapies taken by some DMD patients. Since mdx muscle in vivo and DMD patients respond in a similar manner to many of these compounds, the in vitro assay will be a useful tool for the rapid identification of new potential treatments for muscle weakness in DMD and other muscle disorders.

Emerging drugs: mechanism of action, mass spectrometry and doping control analysis

The number of compounds and doping methods in sports is in a state of constant flux. In addition to 'traditional' doping agents, such as anabolic androgenic steroids or erythropoietin, new therapeutics and emerging drugs have considerable potential for misuse in elite sport. Such compounds are commonly based on new chemical structures, and the mechanisms underlying their modes of action represent new therapeutic approaches arising from recent advances in medical research; therefore, sports drug testing procedures need to be continuously modified and complementary methods developed, preferably based on mass spectrometry, to enable comprehensive doping controls. This tutorial not only discusses emerging drugs that can be categorized as anabolic agents (selective androgen receptor modulators, SARMs), gene doping [hypoxia-inducible factor stabilizers, peroxisome-proliferator-activated receptor (PPAR)delta-agonists] and erythropoietin-mimetics (Hematide) but also compounds with potentially performance-enhancing properties that are not classified in the current list of the World Anti-Doping Agency. Compounds such as ryanodine-calstabin-complex modulators (benzothiazepines) are included, their mass spectrometric properties discussed, and current approaches in sports drug testing outlined.

Doping control analysis of emerging drugs in human plasma - identification of GW501516, S-107, JTV-519, and S-40503

An important aspect of preventive doping research is the rapid implementation of tests for emerging drugs with potential for misuse into routine doping control assays. New therapeutics of different classes such as PPARdelta-agonists (e.g. GW501516), ryanodine-calstabin-complex stabilizers (e.g. S-107 and JTV-519), and selective androgen receptor modulators (SARMs, e.g. S-40503) are currently used for the treatment of particular medical conditions such as metabolic syndrome, cardiac arrhythmia, debilitating diseases and osteoporosis, respectively. Due to their being at an early stage of clinical trials and the limited availability of data on the metabolism and possible renal elimination of the active drugs, the development of protocols for doping control analyses of plasma specimens could be an option for the detection of the circulating agents. The mass spectrometric fragmentation of four emerging drug candidates (GW501516, S-107, JTV-519, and S-40503) was elucidated by positive electrospray ionization and collision-induced dissociation using a high resolution/high accuracy mass spectrometer. A screening and confirmation procedure was established based on liquid chromatography/tandem mass spectrometry requiring a volume of 100 microL of plasma. Proteins were precipitated using acetonitrile, the specimens were centrifuged and the supernatant analyzed using a triple-quadrupole mass spectrometer employing multiple reaction monitoring of diagnostic ion transitions. The method was validated with regard to specificity, limits of detection (0.4-8.3 ng/mL), recoveries (72-98%), intraday and interday precisions (12-21%), and ion suppression/enhancement effects.

Enantiomeric specificity of (-)-2,2',3,3',6,6'-hexachlorobiphenyl toward ryanodine receptor types 1 and 2

Polychlorinated biphenyls (PCBs) with unsymmetrical chlorine substitutions and multiple orthosubstitutions that restrict rotation around the biphenyl bond may exist in two stable enantiomeric forms.Stereospecific binding and functional modification of specific biological signaling targets have not been previously described for PCB atropisomers. We report that (-)-2,2',3,3',6,6'-hexachlorobiphenyl [(-)-PCB 136] enhances the binding of [3H]ryanodine to high-affinity sites on ryanodine receptors type 1(RyR1) and type 2 (RyR2) (EC50 values ~0.95 microM), whereas (+)-PCB 136 is inactive at < or =10 microM.(-)-PCB 136 induces a rapid release of Ca2+ from microsomal vesicles by selective sensitization of RyRs, an effect not antagonized by (+)-PCB 136. (-)-PCB 136 (500nM) enhances the activity of reconstituted RyR1 channels 3-fold by stabilizing the open and destabilizing the closed conformational states. The enantiomeric specificity is also demonstrated in intact HEK 293 cells expressing RyR1 where exposure to (-)-PCB 136 (100 nM; 12 h) sensitizes responses to caffeine, whereas (+)-PCB 136 does not. These data show enantiomeric specificity of (-)-PCB 136 toward a broadly expressed family of microsomal Ca2+ channels that may extend to other chiral noncoplanar PCBs and related structures.Evidence for enantioselective enrichment of PCBs in biological tissues that express RyR1 and RyR2channels may provide new mechanistic leads about their toxicological impacts on human health

Genetic calcium signaling abnormalities in the central nervous system: seizures, migraine, and autism

The calcium ion is one of the most versatile, ancient, and universal of biological signaling molecules, known to regulate physiological systems at every level from membrane potential and ion transporters to kinases and transcription factors. Disruptions of intracellular calcium homeostasis underlie a host of emerging diseases, the calciumopathies. Cytosolic calcium signals originate either as extracellular calcium enters through plasma membrane ion channels or from the release of an intracellular store in the endoplasmic reticulum (ER) via inositol triphosphate receptor and ryanodine receptor channels. Therefore, to a large extent, calciumopathies represent a subset of the channelopathies, but include regulatory pathways and the mitochondria, the major intracellular calcium repository that dynamically participates with the ER stores in calcium signaling, thereby integrating cellular energy metabolism into these pathways, a process of emerging importance in the analysis of the neurodegenerative and neuropsychiatric diseases. Many of the calciumopathies are common complex polygenic diseases, but leads to their understanding come most prominently from rare monogenic channelopathy paradigms. Monogenic forms of common neuronal disease phenotypes-such as seizures, ataxia, and migraine-produce a constitutionally hyperexcitable tissue that is susceptible to periodic decompensations. The gene families and genetic lesions underlying familial hemiplegic migraine, FHM1/CACNA1A, FHM2/ATP1A2, and FHM3/SCN1A, and monogenic mitochondrial migraine syndromes, provide a robust platform from which genes, such as CACNA1C, which encodes the calcium channel mutated in Timothy syndrome, can be evaluated for their role in autism and bipolar disease.