Interhelical interactions within the STIM1 CC1 domain modulate CRAC channel activation

The calcium release activated calcium channel is activated by the endoplasmic reticulum-resident calcium sensor protein STIM1. On activation, STIM1 C terminus changes from an inactive, tight to an active, extended conformation. A coiled-coil clamp involving the CC1 and CC3 domains is essential in controlling STIM1 activation, with CC1 as the key entity. The nuclear magnetic resonance-derived solution structure of the CC1 domain represents a three-helix bundle stabilized by interhelical contacts, which are absent in the Stormorken disease-related STIM1 R304W mutant. Two interhelical sites between the CC1α1 and CC1α2 helices are key in controlling STIM1 activation, affecting the balance between tight and extended conformations. Nuclear magnetic resonance-directed mutations within these interhelical interactions restore the physiological, store-dependent activation behavior of the gain-of-function STIM1 R304W mutant. This study reveals the functional impact of interhelical interactions within the CC1 domain for modifying the CC1-CC3 clamp strength to control the activation of STIM1.

Yeast Beta-Glucan Supplementation Downregulates Markers of Systemic Inflammation after Heated Treadmill Exercise

Aerobic exercise and thermal stress instigate robust challenges to the immune system. Various attempts to modify or supplement the diet have been proposed to bolster the immune system responses. The purpose of this study was to identify the impact of yeast beta-glucan (Saccharomyces cerevisiae) supplementation on exercise-induced muscle damage and inflammation. Healthy, active men (29.6 ± 6.7 years, 178.1 ± 7.2 cm, 83.2 ± 11.2 kg, 49.6 ± 5.1 mL/kg/min, n = 16) and women (30.1 ± 8.9 years, 165.6 ± 4.1 cm, 66.7 ± 10.0 kg, 38.7 ± 5.8 mL/kg/min, n = 15) were randomly assigned in a double-blind and cross-over fashion to supplement for 13 days with either 250 mg/day of yeast beta-glucan (YBG) or a maltodextrin placebo (PLA). Participants arrived fasted and completed a bout of treadmill exercise at 55% peak aerobic capacity (VO_2Peak) in a hot (37.2 ± 1.8 °C) and humid (45.2 ± 8.8%) environment. Prior to and 0, 2, and 72 h after completing exercise, changes in white blood cell counts, pro- and anti-inflammatory cytokines, markers of muscle damage, markers of muscle function, soreness, and profile of mood states (POMS) were assessed. In response to exercise and heat, both groups experienced significant increases in white blood cell counts, plasma creatine kinase and myoglobin, and soreness along with reductions in peak torque and total work with no between-group differences. Concentrations of serum pro-inflammatory cytokines in YBG were lower than PLA for macrophage inflammatory protein 1β (MIP-1β) (p = 0.044) and tended to be lower for interleukin 8 (IL-8) (p = 0.079), monocyte chemoattractment protein 1 (MCP-1) (p = 0.095), and tumor necrosis factor α (TNF-α) (p = 0.085). Paired samples t-tests using delta values between baseline and 72 h post-exercise revealed significant differences between groups for IL-8 (p = 0.044, 95% Confidence Interval (CI): (0.013, 0.938, d = −0.34), MCP-1 (p = 0.038, 95% CI: 0.087, 2.942, d = −0.33), and MIP-1β (p = 0.010, 95% CI: 0.13, 0.85, d = −0.33). POMS outcomes changed across time with anger scores in PLA exhibiting a sharper decline than YBG (p = 0.04). Vigor scores (p = 0.04) in YBG remained stable while scores in PLA were significantly reduced 72 h after exercise. In conclusion, a 13-day prophylactic period of supplementation with 250 mg of yeast-derived beta-glucans invoked favorable changes in cytokine markers of inflammation after completing a prolonged bout of heated treadmill exercise.

CRAC channels and disease - From human CRAC channelopathies and animal models to novel drugs

Ca²⁺ release-activated Ca²⁺ (CRAC) channels are intimately linked with health and disease. The gene encoding the CRAC channel, ORAI1, was discovered in part by genetic analysis of patients with abolished CRAC channel function. And patients with autosomal recessive loss-of-function (LOF) mutations in ORAI1 and its activator stromal interaction molecule 1 (STIM1) that abolish CRAC channel function and store-operated Ca²⁺ entry (SOCE) define essential functions of CRAC channels in health and disease. Conversely, gain-of-function (GOF) mutations in ORAI1 and STIM1 are associated with tubular aggregate myopathy (TAM) and Stormorken syndrome due to constitutive CRAC channel activation. In addition, genetically engineered animal models of ORAI and STIM function have provided important insights into the physiological and pathophysiological roles of CRAC channels in cell types and organs beyond those affected in human patients. The picture emerging from this body of work shows CRAC channels as important regulators of cell function in many tissues, and as potential drug targets for the treatment of autoimmune and inflammatory disorders.

Androgen Action via the Androgen Receptor in Neurons Within the Brain Positively Regulates Muscle Mass in Male Mice

Although it is well established that exogenous androgens have anabolic effects on skeletal muscle mass in humans and mice, data from muscle-specific androgen receptor (AR) knockout (ARKO) mice indicate that myocytic expression of the AR is dispensable for hind-limb muscle mass accrual in males. To identify possible indirect actions of androgens via the AR in neurons to regulate muscle, we generated neuron-ARKO mice in which the dominant DNA binding-dependent actions of the AR are deleted in neurons of the cortex, forebrain, hypothalamus, and olfactory bulb. Serum testosterone and luteinizing hormone levels were elevated twofold in neuron-ARKO males compared with wild-type littermates due to disruption of negative feedback to the hypothalamic-pituitary-gonadal axis. Despite this increase in serum testosterone levels, which was expected to increase muscle mass, the mass of the mixed-fiber gastrocnemius (Gast) and the fast-twitch fiber extensor digitorum longus hind-limb muscles was decreased by 10% in neuron-ARKOs at 12 weeks of age, whereas muscle strength and fatigue of the Gast were unaffected. The mass of the soleus muscle, however, which consists of a high proportion of slow-twitch fibers, was unaffected in neuron-ARKOs, demonstrating a stimulatory action of androgens via the AR in neurons to increase the mass of fast-twitch hind-limb muscles. Furthermore, neuron-ARKOs displayed reductions in voluntary and involuntary physical activity by up to 60%. These data provide evidence for a role of androgens via the AR in neurons to positively regulate fast-twitch hind-limb muscle mass and physical activity in male mice.

Drosophila studies support a role for a presynaptic synaptotagmin mutation in a human congenital myasthenic syndrome

During chemical transmission, the function of synaptic proteins must be coordinated to efficiently release neurotransmitter. Synaptotagmin 2, the Ca²⁺ sensor for fast, synchronized neurotransmitter release at the human neuromuscular junction, has recently been implicated in a dominantly inherited congenital myasthenic syndrome associated with a non-progressive motor neuropathy. In one family, a proline residue within the C₂B Ca²⁺-binding pocket of synaptotagmin is replaced by a leucine. The functional significance of this residue has not been investigated previously. Here we show that in silico modeling predicts disruption of the C₂B Ca²⁺-binding pocket, and we examine the in vivo effects of the homologous mutation in Drosophila. When expressed in the absence of native synaptotagmin, this mutation is lethal, demonstrating for the first time that this residue plays a critical role in synaptotagmin function. To achieve expression similar to human patients, the mutation is expressed in flies carrying one copy of the wild type synaptotagmin gene. We now show that Drosophila carrying this mutation developed neurological and behavioral manifestations similar to those of human patients and provide insight into the mechanisms underlying these deficits. Our Drosophila studies support a role for this synaptotagmin point mutation in disease etiology.

York platelet syndrome is a CRAC channelopathy due to gain-of-function mutations in STIM1

Store-operated Ca(2+) entry is the major route of replenishment of intracellular Ca(2+) in animal cells in response to the depletion of Ca(2+) stores in the endoplasmic reticulum. It is primarily mediated by the Ca(2+)-selective release-activated Ca(2+) (CRAC) channel, which consists of the pore-forming subunits ORAI1-3 and the Ca(2+) sensors, STIM1 and STIM2. Recessive loss-of-function mutations in STIM1 or ORAI1 result in immune deficiency and nonprogressive myopathy. Heterozygous gain-of-function mutations in STIM1 cause non-syndromic myopathies as well as syndromic forms of miosis and myopathy with tubular aggregates and Stormorken syndrome; some of these syndromic forms are associated with thrombocytopenia. Increased concentration of Ca(2+) as a result of store-operated Ca(2+) entry is essential for platelet activation. The York Platelet syndrome (YPS) is characterized by thrombocytopenia, striking ultrastructural platelet abnormalities including giant electron-opaque organelles and massive, multilayered target bodies and deficiency of platelet Ca(2+) storage in delta granules. We present clinical and molecular findings in 7 YPS patients from 4 families, demonstrating that YPS patients have a chronic myopathy associated with rimmed vacuoles and heterozygous gain-of-function STIM1 mutations. These findings expand the phenotypic spectrum of STIM1-related human disorders and define the molecular basis of YPS.

[Pompe disease is a differential diagnosis in case of reduced physical capacity and abnormal muscular fatigue]

Late-onset Pompe disease is an inherited metabolic myopathy with low activity of alpha glucosidase and variable clinical symptoms. In this case report we describe a woman with long standing muscular fatigue and malaise with the diagnosis initially established by pathologic findings in the muscle biopsy. Enzyme replacement therapy is now a treatment option, and a prompt diagnosis is therefore relevant. This disease should be considered in patients with unexplained fatigue and reduced physical capacity, especially in case of concurrent elevated levels of creatine kinase and liver enzymes.

Clinical manifestations of highly prevalent corticosteroid-binding globulin mutations in a village in southern Italy

CONTEXT

Corticosteroid-binding globulin (CBG) is the binding protein for cortisol. Rare kindreds with CBG mutations reducing CBG levels or altering binding affinity have been described, along with clinical manifestations encompassing fatigue, chronic pain, and hypotension. The largest kindred, exhibiting two mutations (null and Lyon) were Australian immigrants from Italy.

OBJECTIVE

Our objective was to determine the prevalence of the null/Lyon mutations in the village where the original null/Lyon family emigrated and compare subjects with and without CBG mutations, without previous knowledge of their mutation status.

DESIGN, SETTING, AND PARTICIPANTS

We conducted a survey field study that included 495 adult residents.

MAIN OUTCOMES

We assessed clinical history, CBG mutation analysis, plasma CBG, salivary cortisol, body mass index, waist circumference, blood pressure, and the Krupp fatigue scale.

RESULTS

Eighteen of 495 participants (3.6%, seven males and 11 females) had one of two function-altering CBG mutations. All were heterozygous for the null (n = 6) or Lyon mutations (n = 12). Of 12 Lyon participants (four males and eight females), eight (two males and six females) had chronic widespread pain and five osteoarthritis with associated pain (one male and four females). Of six null participants (three males and three females), three (one male and two females) had chronic pain and four osteoarthritis with associated pain (two males and two females).

CONCLUSIONS

A high combined prevalence (3.6%) of these two CBG mutations was detected. The presence of either mutation conferred a propensity to chronic pain. In other communities, individuals with the same genetic background complain more of fatigue than pain, suggesting an environmental effect on the phenotype. These findings, combined with animal CBG gene knockout and human CBG single-nucleotide polymorphism haplotype studies, suggest that CBG influences the endocrine and neurobehavioral response to stress, including the development of pain/fatigue syndromes.

The effect of skeletal myosin light chain kinase gene ablation on the fatigability of mouse fast muscle

Contraction-induced activation of a skeletal muscle specific Ca(2+) and calmodulin dependent myosin light chain kinase (skMLCK) catalyzes phosphorylation of the myosin regulatory light chain (RLC), a reaction that potentiates twitch force. The purpose of this study was to test the effect of skMLCK gene ablation on the fatigability of mouse extensor digitorum longus (EDL) muscle (in vitro at 25°C). Muscles were isolated from wildtype (WT, n = 10-12) and skeletal MLCK knockout (skMLCK KO, n = 10-12) mice and fatigued using a protocol consisting of 5 min of repeated tetanic stimulation (150 Hz for 1000 ms every 5 s). Both twitch (P(t)) and tetanic (P(o)) force as well as unloaded shortening velocity (V(o)) were assessed before, during and after fatiguing stimulation. Fatiguing stimulation increased RLC phosphorylation in WT but not skMLCK KO muscles (16 ± 0.01-0.63 ± 0.02 and 0.07 ± 0.02-0.08 ± 0.02 mol phos mol RLC, respectively). Although P(t) was potentiated above baseline in both WT and KO muscles, this increase was greater in WT than in KO muscles (to 1.37 ± 0.05 vs. 1.14 ± 0.02 of unpotentiated values, respectively). The difference in P(t) persisted until P(o) had been diminished to ~60% of baseline and thereafter P(t) declined to similar levels in both WT and KO muscles (to ~35% of initial). Overall, the time-course and decline in P(o) for WT and KO was similar (reduced to 0.20 ± 0.01 and 0.20 ± 0.01 of baseline, respectively) (P < 0.05). Initial values for V(o) were similar between WT and KO muscles and, moreover, the fatigue related decline in Vo was similar for both muscle genotypes (P < 0.05). Thus, our results demonstrate that skMLCK--catalyzed RLC phosphorylation augments isometric twitch force during moderate, but not severe, levels of fatigue.

Androgen signaling in myocytes contributes to the maintenance of muscle mass and fiber type regulation but not to muscle strength or fatigue

Muscle frailty is considered a major cause of disability in the elderly and chronically ill. However, the exact role of androgen receptor (AR) signaling in muscle remains unclear. Therefore, a postmitotic myocyte-specific AR knockout (mARKO) mouse model was created and investigated together with a mouse model with ubiquitous AR deletion. Muscles from mARKO mice displayed a marked reduction in AR protein (60-88%). Interestingly, body weights and lean body mass were lower in mARKO vs. control mice (-8%). The weight of the highly androgen-sensitive musculus levator ani was significantly reduced (-46%), whereas the weights of other peripheral skeletal muscles were not or only slightly reduced. mARKO mice had lower intra-abdominal fat but did not demonstrate a cortical or trabecular bone phenotype, indicating that selective ablation of the AR in myocytes affected male body composition but not skeletal homeostasis. Furthermore, muscle contractile performance in mARKO mice did not differ from their controls. Myocyte-specific AR ablation resulted in a conversion of fast toward slow fibers, without affecting muscle strength or fatigue. Similar results were obtained in ubiquitous AR deletion, showing lower body weight, whereas some but not all muscle weights were reduced. The percent slow fibers was increased, but no changes in muscle strength or fatigue could be detected. Together, our findings show that myocyte AR signaling contributes to the maintenance of muscle mass and fiber type regulation but not to muscle strength or fatigue. The levator ani weight remains the most sensitive and specific marker of AR-mediated anabolic action on muscle.

Isotonic fatigue in laminin alpha2-deficient dy/dy dystrophic mouse diaphragm

Laminin alpha2 deficiency causes approximately 50% of human congenital muscular dystrophies. Muscle in the corresponding dy/dy mouse model has reduced force but increased fatigue resistance during isometric contractions. To determine whether a similar pattern of alterations is present during isotonic contractions, dy/dy diaphragm was studied in vitro. During 20% load, dystrophic diaphragm had significantly reduced shortening, shortening velocity, work and power deficits, which persisted during the fatigue-inducing stimulation. In contrast, during 40% load, isotonic contractile performance of diseased muscle was impaired only mildly and only for some contractile parameters. At both loads, rate of isotonic fatigue when expressed relative to initial contractile values was similar for dystrophic and normal muscle, or in some instances slightly higher for dystrophic muscle. Therefore, fatigue resistance is considerably impaired during isotonic contractions relative to that reported previously for isometric contractions. This has important implications for increased susceptibility to respiratory failure in laminin alpha2-deficient muscular dystrophy.

AMP deaminase deficiency is associated with lower sprint cycling performance in healthy subjects

AMP deaminase (AMPD) deficiency is an inherited disorder of skeletal muscle found in ∼2% of the Caucasian population. Although most AMPD-deficient individuals are asymptomatic, a small subset has exercise-related cramping and pain without any other identifiable neuromuscular complications. This heterogeneity has raised doubts about the physiological significance of AMPD in skeletal muscle, despite evidence for disrupted adenine nucleotide catabolism during exercise in deficient individuals. Previous studies have evaluated the effect of AMPD deficiency on exercise performance with mixed results. This study was designed to circumvent the perceived limitations in previous reports by measuring exercise performance during a 30-s Wingate test in 139 healthy, physically active subjects of both sexes, with different AMPD1 genotypes, including 12 AMPD-deficient subjects. Three of the deficient subjects were compound heterozygotes characterized by the common c.34C>T mutation in one allele and a newly discovered AMPD1 mutation, c.404delT, in the other. While there was no significant difference in peak power across AMPD1 genotypes, statistical analysis revealed a faster power decrease in the AMPD-deficient group and a difference in mean power across the genotypes ( P = 0.0035). This divergence was most striking at 15 s of the 30-s cycling. Assessed by the fatigue index, the decrease in power output at 15 s of exercise was significantly greater in the deficient group compared with the other genotypes ( P = 0.0006). The approximate 10% lower mean power in healthy AMPD-deficient subjects during a 30-s Wingate cycling test reveals a functional role for the AMPD1 enzyme in sprint exercise.

Muscle Na+-K+-ATPase activity and isoform adaptations to intense interval exercise and training in well-trained athletes

The Na+-K+-ATPase enzyme is vital in skeletal muscle function. We investigated the effects of acute high-intensity interval exercise, before and following high-intensity training (HIT), on muscle Na+-K+-ATPase maximal activity, content, and isoform mRNA expression and protein abundance. Twelve endurance-trained athletes were tested at baseline, pretrain, and after 3 wk of HIT (posttrain), which comprised seven sessions of 8 × 5-min interval cycling at 80% peak power output. Vastus lateralis muscle was biopsied at rest (baseline) and both at rest and immediately postexercise during the first (pretrain) and seventh (posttrain) training sessions. Muscle was analyzed for Na+-K+-ATPase maximal activity (3- O-MFPase), content ([3H]ouabain binding), isoform mRNA expression (RT-PCR), and protein abundance (Western blotting). All baseline-to-pretrain measures were stable. Pretrain, acute exercise decreased 3- O-MFPase activity [12.7% (SD 5.1), P < 0.05], increased α1, α2, and α3 mRNA expression (1.4-, 2.8-, and 3.4-fold, respectively, P < 0.05) with unchanged β-isoform mRNA or protein abundance of any isoform. In resting muscle, HIT increased ( P < 0.05) 3- O-MFPase activity by 5.5% (SD 2.9), and α3 and β3 mRNA expression by 3.0- and 0.5-fold, respectively, with unchanged Na+-K+-ATPase content or isoform protein abundance. Posttrain, the acute exercise induced decline in 3- O-MFPase activity and increase in α1 and α3 mRNA each persisted ( P < 0.05); the postexercise 3- O-MFPase activity was also higher after HIT ( P < 0.05). Thus HIT augmented Na+-K+-ATPase maximal activity despite unchanged total content and isoform protein abundance. Elevated Na+-K+-ATPase activity postexercise may contribute to reduced fatigue after training. The Na+-K+-ATPase mRNA response to interval exercise of increased α- but not β-mRNA was largely preserved posttrain, suggesting a functional role of α mRNA upregulation.

Haploinsufficiency of the SERPINA6 gene is associated with severe muscle fatigue: A de novo mutation in corticosteroid-binding globulin deficiency

Corticosteroid-binding globulin (SERPINA6) deficiency is an extremely rare hereditary disorder characterized by reduced corticosteroid-binding capacity with normal or low plasma corticosteroid-binding globulin concentration, and normal or low basal cortisol levels associated with hypo-/hypertension and muscle fatigue. Here, we present a patient with severe muscle fatigue, normal blood pressure, and abnormal high saliva cortisol levels following a standardized stress test. This patient was found heterozygous for a de novo 367 asparagine-encoding variant of the corticosteroid-binding globulin gene, previously described as "transcortin Lyon". Both parents were homozygous for the ("wildtype") 367 aspartate-encoding allele. To the best of our knowledge, this case represents the first de novo mutation reported for corticosteroid-binding globulin deficiency, implicating a pathogenic role of variants of SERPINA6 in some cases of muscle fatigue.

Role of brain IL-1β on fatigue after exercise-induced muscle damage

Brain cytokines, induced by various inflammatory challenges, have been linked to sickness behaviors, including fatigue. However, the relationship between brain cytokines and fatigue after exercise is not well understood. Delayed recovery of running performance after muscle-damaging downhill running is associated with increased brain IL-1β concentration compared with uphill running. However, there has been no systematic evaluation of the direct effect of brain IL-1β on running performance after exercise-induced muscle damage. This study examined the specific role of brain IL-1β on running performance (either treadmill or wheel running) after uphill and downhill running by manipulating brain IL-1β activity via intracerebroventricular injection of either IL-1 receptor antagonist (ra; downhill runners) or IL-1β (uphill runners). Male C57BL/6 mice were assigned to the following groups: uphill-saline, uphill-IL-1β, downhill-saline, or downhill-IL-1ra. Mice initially ran on a motor-driven treadmill at 22 m/min and −14% or +14% grade for 150 min. After the run, at 8 h (wheel cage) or 22 h (treadmill), uphill mice received intracerebroventricular injections of IL-1β (900 pg in 2 μl saline) or saline (2 μl), whereas downhill runners received IL-1ra (1.8 μg in 2 μl saline) or saline (2 μl). Later (2 h), running performance was measured (wheel running activity and treadmill run to fatigue). Injection of IL-1β significantly decreased wheel running activity in uphill runners ( P < 0.01), whereas IL-1ra improved wheel running in downhill runners ( P < 0.05). Similarly, IL-1β decreased and Il-1ra increased run time to fatigue in the uphill and downhill runners, respectively ( P < 0.01). These results support the hypothesis that increased brain IL-1β plays an important role in fatigue after muscle-damaging exercise.

Outcome of acetylcholinesterase deficiency for neuromuscular functioning

Acetylcholinesterase (AChE) plays an essential role in neuromuscular transmission, therefore it is surprising that AChE knockout (KO) mice could live to the adulthood. Neuromuscular functioning in KO and normal (wild type, WT) mice were studied, at different age (1.5-, 4- and 9-month-old). Hindlimb muscle force productions in response to nerve or muscle electric stimulation were recorded in situ and in vitro. Our results show that contrary to WT mice, 1.5-, 4- and 9-month-old KO mice exhibited a decreased in tetanic force during short periods (500 ms) of repetitive nerve stimulations (tetanic fade). Nevertheless submaximal muscle forces in response to single or repetitive nerve stimulation were increased (potentiation) in 1.5-, 4- and 9-month-old KO mice as compared to WT mice (p<0.05). Tetanic fade and potentiation were absent when muscles were directly stimulated, indicating neuromuscular transmission alterations in KO mice. Contrary to younger mice, muscle weight and maximal tetanic force in response to repetitive nerve stimulation were not reduced in 4- and 9-month-old KO mice as compared to WT mice (p>0.05). In conclusion AChE deficit leads to marked neuromuscular alterations in hind limb muscle functioning and a prominent symptom is the lack of resistance to fatigue.

Gene deletional strategies reveal novel physiological roles for myoglobin in striated muscle

Myoglobin is an abundant hemoprotein that is expressed in cardiomyocytes and oxidative skeletal myofibers of vertebrates. Elegant studies using physiological, biochemical and spectroscopic analyses support a role for myoglobin in facilitated oxygen transport and as a reservoir for oxygen in muscle of diving and hypoxia-adapted animals. In contrast, the functional role of myoglobin in terrestrial animals that function at ambient oxygen levels is a subject of debate. This debate was further fueled by the observation that genetically engineered mice that lack myoglobin are viable and capable of withstanding the hemodynamic stress associated with reproduction. Analysis of the myoglobin mutant striated muscle reveals a spectrum of adaptive mechanisms that partially compensate for the absence of myoglobin and further supports an important function for this hemoprotein in the maintenance of contractile function during exercise under ambient and hypoxic conditions. Future studies utilizing transgenic and gene deletional strategies will further enhance our understanding of myoglobin function under normoxic and hypoxic conditions and will impact our understanding of exercise physiology.

Increasing cannabinoid levels by pharmacological and genetic manipulation delay disease progression in SOD1 mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the selective loss of motoneurons in the spinal cord, brain stem, and motor cortex. However, despite intensive research, an effective treatment for this disease remains elusive. In this study we show that treatment of postsymptomatic, 90-day-old SOD1G93A mice with a synthetic cannabinoid, WIN55,212-2, significantly delays disease progression. Furthermore, genetic ablation of the Faah enzyme, which results in raised levels of the endocannabinoid anandamide, prevented the appearance of disease signs in 90-day-old SOD1G93A mice. Surprisingly, elevation of cannabinoid levels with either WIN55,212-2 or Faah ablation had no effect on life span. Ablation of the CB1 receptor, in contrast, had no effect on disease onset in SOD1(G93A) mice but significantly extended life span. Together these results show that cannabinoids have significant neuroprotective effects in this model of ALS and suggest that these beneficial effects may be mediated by non-CB1 receptor mechanisms.

Selective vulnerability and pruning of phasic motoneuron axons in motoneuron disease alleviated by CNTF

Neurodegenerative diseases can have long preclinical phases and insidious progression patterns, but the mechanisms of disease progression are poorly understood. Because quantitative accounts of neuronal circuitry affected by disease have been lacking, it has remained unclear whether disease progression reflects processes of stochastic loss or temporally defined selective vulnerabilities of distinct synapses or axons. Here we derive a quantitative topographic map of muscle innervation in the hindlimb. We show that in two mouse models of motoneuron disease (G93A SOD1 and G85R SOD1), axons of fast-fatiguable motoneurons are affected synchronously, long before symptoms appear. Fast-fatigue-resistant motoneuron axons are affected at symptom-onset, whereas axons of slow motoneurons are resistant. Axonal vulnerability leads to synaptic vesicle stalling and accumulation of BC12a1-a, an anti-apoptotic protein. It is alleviated by ciliary neurotrophic factor and triggers proteasome-dependent pruning of peripheral axon branches. Thus, motoneuron disease involves predictable, selective vulnerability patterns by physiological subtypes of axons, episodes of abrupt pruning in the target region and compensation by resistant axons.

Increase of AMPA receptor glutamate receptor 1 subunit and B-cell receptor-associated protein 31 gene expression in hippocampus of fatigued mice

Central fatigue is an indispensable biosignal for maintaining life, but the neuronal and molecular mechanisms involved remain unclear. In this study, we searched for genes differentially expressed in the hippocampus of fatigued mice to elucidate the mechanisms underlying fatigue. Mice were forced to swim in an adjustable-current water pool, and the maximum swimming time (endurance) until fatigue was measured thrice. Fatigued and nonfatigued mice with equal swimming capacity and body weight were compared. We found that the genes of GluR1 and B-cell receptor-associated protein 31 (Bap31), which acts as a transport molecule in the secretory pathway or as a mediator of apoptosis, were upregulated in the hippocampus of fatigued mice, and increases of GluR1 and Bap31 were confirmed by Northern blotting and real-time PCR. No change of gene expression of AMPA receptor subunits other than GluR1 was observed. These results suggest that a compositional change of AMPA receptor (increase of GluR1) and upregulation of the Bap31 gene may be implicated in fatigue in mice.

Mice lacking COX10 in skeletal muscle recapitulate the phenotype of progressive mitochondrial myopathies associated with cytochrome c oxidase deficiency

We have created a mouse model with an isolated cytochrome c oxidase (COX) deficiency by disrupting the COX10 gene in skeletal muscle. Missense mutations in COX10 have been previously associated with mitochondrial disorders. Cox10p is a protoheme:heme-O-farnesyl transferase required for the synthesis of heme a, the prosthetic group of the catalytic center of COX. COX10 conditional knockout mice were generated by crossing a LoxP-tagged COX10 mouse with a transgenic mouse expressing cre recombinase under the myosin light chain 1f promoter. The COX10 knockout mice were healthy until approximately 3 months of age when they started developing a slowly progressive myopathy. Surprisingly, even though COX activity in COX10 KO muscles was <5% of control muscle at 2.5 months, these muscles were still able to contract at 80-100% of control maximal forces and showed only a 10% increase in fatigability, and no signs of oxidative damage or apoptosis were detected. However, the myopathy worsened with time, particularly in female animals. This COX10 KO mouse allowed us to correlate the muscle function with residual COX activity, an estimate that can help predict the progression pattern of human mitochondrial myopathies.

Defective maintenance of intracellular Ca2+ homeostasis is linked to increased muscle fatigability in the MG29 null mice

Mitsugumin 29 (MG29) is a transmembrane protein that is normally found in the triad junction of skeletal muscle. Our previous studies have shown that targeted deletion of mg29 from the skeletal muscle resulted in abnormality of the triad junction structure, and also increased susceptibility to muscle fatigue. To elucidate the basis of these effects, we investigated the properties of Ca2+-uptake and -release in toxin-skinned Extensor Digitorium Longus (EDL) muscle fibers from control and mg29 knockout mice. Compared with the control muscle, submaximal Ca2+-uptake into the sarcoplasmic reticulum (SR) was slower and the storage of Ca2+ inside the SR was less in the mutant muscle, due to increased leakage process of Ca2+ movement across the SR. The leakage pathway is associated with the increased sensitivity of Ca2+/caffeine -induced Ca2+ release to myoplasmic Ca2+. Therefore, the increased fatigability of mutant EDL muscles can result from a combination of a slowing of Ca2+ uptake, modification of Ca2+-induced Ca2+ release (CICR), and a reduction in total SR Ca2+ content.

Angiotension-converting enzyme gene I/D polymorphism in patients with angina and normal coronary arteriograms

A polymorphism of the human angiotensin-converting enzyme (ACE) gene has been identified in which the insertion (I) rather than the deletion (D) variant is associated with lower circulating and tissue ACE activity. ACE I allele is associated with resistance and endurance performance. Skeletal muscle metabolic efficiency is reduced in patients with heart failure and is improved by ACE inhibition. Profound muscle fatigue is a predominant and debilitating symptom in a proportion or patients with angina and normal coronary arteriograms (ANCA), and we postulated that the gene D allele might be associated with the presence of fatigue in ANCA patients. We studied 33 consecutive patients with typical ANCA who completed a validated fatigue questionnaire, and found an excess of the D allele frequency in patients with the highest fatigue scores compared to those with the lowest (64% vs. 36%; p=0.027).

Interleukin-6 (IL6) Genotype Is Associated With Fat-Free Mass in Men But Not Women

We studied the association of the G-174C promoter polymorphism in the interleukin-6 gene (IL6) with total body fat and fat-free mass (FFM) in 242 men and women (IL6 genotypes: G/G, n = 87; G/C, n = 100; C/C, n = 55) across the adult age span (21-92 years). In men, but not women (significant genotype by sex interactions; p =.023-.048), the C/C group exhibited significantly lower total FFM than the G/G group (54.7 +/- 0.8 kg vs 57.2 +/- 0.7 kg, respectively, p =.020), as well as significantly lower FFM of the lower limbs compared with the G/G group (18.4 +/- 0.3 kg vs 19.8 +/- 0.3 kg, respectively, p =.004). No significant genotype differences were observed in total body fat mass in either men or women. The results indicate that the IL6 G-174C polymorphism is significantly associated with FFM in men but not women.

Breakdown of adenine nucleotide pool in fatiguing skeletal muscle in McArdle's disease: a noninvasive 31P-MRS and EMG study

Energy metabolism and electrical muscle activity were studied in the calf muscles of 19 patients with proven McArdle's disease and in 25 healthy subjects. Phosphorus magnetic resonance spectroscopy and surface electromyography (S-EMG) were performed during two isometric muscle contractions of 3 min at 30% maximum voluntary contraction, one performed during normal perfusion and the other during applied ischemia. After about 1 min of ischemic muscle contraction in diseased muscle a significant acceleration in phosphocreatine breakdown was observed, along with a significant decrease in adenosine triphosphate. During both contractions the absence of glycolysis was shown by a significant alkalinization. Furthermore, in patients we observed a greater increase in the S-EMG amplitude than in control subjects. We conclude that early on during moderate exercise, a small number of muscle fibers reach metabolic depletion, indicated by a reduction in the adenine nucleotide pool. An increasing number of motor units, which are still in a high-energy state, are continuously recruited to compensate for muscle fatigue. This functional compartmentation may contribute to the pathophysiology of exercise intolerance in McArdle's disease.

Increased metabolic muscle fatigue is caused by some but not all mitochondrial mutations

CONTEXT

Excessive muscle fatigue occurs in patients with a mitochondrial encephalomyopathy (MEM), but it is also a frequent problem in patients with other neuromuscular disorders (ONMD).

OBJECTIVE

To determine whether, and to what extent, metabolic muscle fatigue specifically occurs in patients with an MEM.

DESIGN

Metabolic muscle fatigue was assessed in a series of 21 patients with an MEM, including 13 patients with chronic progressive external ophthalmoplegia and 8 patients with various mitochondrial point mutations; 27 patients with ONMD; and 25 healthy controls. Isometric twitch force of the ankle dorsiflexors was measured after supramaximal stimulation of peroneal nerves. Six trains of stimuli (of 1 minute's duration with rates from 0.2 to 5 stimuli per second) were given to each subject.

RESULTS

An abnormal decrement of the twitch amplitude that occurred during a stimulation train was found in patients with MEM and in those with ONMD. The decrement of the twitch amplitude of controls and of patients with ONMD was strongly influenced by their muscle force (P<.001). After subtraction of the influence of the muscle force, specific fatigue was notably higher in patients with chronic progressive external ophthalmoplegia than in patients with ONMD and in controls, and it correlated well with elevations of serum lactate. Specific fatigue was also abnormal in a patient with a mitochondrial G7497A mutation, but normal in patients with an A3243G or a G11778A mutation. The heteroplasmy of mitochondrial DNA in muscle correlated neither with the force measures nor with the serum lactate levels.

CONCLUSIONS

Generally, metabolic muscle fatigue accompanies muscular weakness. Specifically, some but not all mitochondrial mutations cause excessive metabolic muscle fatigue.

Skeletal muscle of mice with a mutation in slow alpha-tropomyosin is weaker at lower lengths

Skeletal muscle function was measured in anaesthetised transgenic mice having a mutation in the TPM3 gene (slow alpha-tropomyosin), a similar mutation as found in some patients with nemaline myopathy, and was compared with control muscles. Measurements of isometric and dynamic muscle performance were done with electrical nerve stimulation at physiological temperatures. No muscle weakness was found in the transgenic muscles when performance was measured at muscle optimum length. This was true not only with full activation but also at lower activation levels, indicating that calcium sensitivity was not affected at this length. Also, fatigability was not affected in these conditions. However, isometric force of the muscles with the mutation in TPM3 was lower at lengths below optimum, with more impairment at decreasing length. As the muscles are active over a large range of different muscle lengths during daily activities, this finding may explain, at least in part, the muscle weakness experienced by patients with nemaline myopathy.

Exercise-induced muscle "burning," fatigue, and hyper-CKemia: mtDNA T10010C mutation in tRNA(Gly)

A 42-year-old woman presented with myopathy and without a family history of neuromuscular disorder. Muscle biopsy showed ragged red fibers and reduced activities of mitochondrial respiratory chain enzyme complexes I, III, and IV. Analysis of mitochondrial DNA revealed a heteroplasmic T10010C mutation in the transfer RNA glycine gene.

Functional properties of skeletal muscle from transgenic animals with upregulated heat shock protein 70

The influence of inducible heat stress proteins on protecting contracting skeletal muscle against fatigue-induced injury was investigated. A line of transgenic mice overexpressing the inducible form of the 72-kDa heat shock protein (HSP72) in skeletal muscles was used. We examined the relationship between muscle contractility and levels of the constitutive (HSC73) and inducible (HSP72) forms of the 72-kDa heat shock protein in intact, mouse extensor digitorum longus (EDL), soleus (SOL), and the diaphragm (DPH). In all transgenic muscles, HSP72 was expressed at higher levels compared with transgene-negative controls, where HSP72 was below the level of detection. At the same time, HSC73 levels were downregulated in all transgenic muscle types. Shipment-related stress caused an elevation in the levels of HSP72 in all muscles for 1 wk after arrival of the animals. We also found that, although no statistical differences in response to intermittent fatiguing stimulation in the contractile properties of intact transgene-positive muscles compared with their transgene-negative counterparts were observed, the response of intact transgene-positive EDL muscles to caffeine was enhanced. These findings demonstrate that elevated HSP72 does not protect EDL, SOL, or DPH muscles from the effects of intermittent fatiguing stimulation. However, HSP72 may influence the excitation-contraction coupling (ECC) process, either directly or indirectly, in EDL muscle. If the effects on ECC were indirect, then these results would suggest that manipulation of a specific gene might cause functional effects that seem independent of the manipulated gene/protein.

Increased susceptibility to fatigue of slow- and fast-twitch muscles from mice lacking the MG29 gene

Mitsugumin 29 (MG29), a major protein component of the triad junction in skeletal muscle, has been identified to play roles in the formation of precise junctional membrane structures important for efficient signal conversion in excitation-contraction (E-C) coupling. We carried out several experiments to not only study the role of MG29 in normal muscle contraction but also to determine its role in muscle fatigue. We compared the in vitro contractile properties of three muscles types, extensor digitorum longus (EDL) (fast-twitch muscle), soleus (SOL) (slow-twitch muscle), and diaphragm (DPH) (mixed-fiber muscle), isolated from mice lacking the MG29 gene and wild-type mice prior to and after fatigue. Our results indicate that the mutant EDL and SOL muscles, but not DPH, are more susceptible to fatigue than the wild-type muscles. The mutant muscles not only fatigued to a greater extent but also recovered significantly less than the wild-type muscles. Following fatigue, the mutant EDL and SOL muscles produced lower twitch forces than the wild-type muscles; in addition, fatiguing produced a downward shift in the force-frequency relationship in the mutant mice compared with the wild-type controls. Our results indicate that fatiguing affects the E-C components of the mutant EDL and SOL muscles, and the effect of fatigue in these mutant muscles could be primarily due to an alteration in the intracellular Ca homeostasis.

Malignant hyperthermia: fatigue characteristics of skeletal muscle

Although the defects in cellular Ca(2+) homeostasis associated with malignant hyperthermia (MH) have been extensively studied, the functional consequences of the MH mutation are not clear. We used continuous and intermittent high-frequency stimulation to determine whether this mutation might alter the fatigue resistance of muscle from MH susceptible (MHS) pigs. Force decline with 10 s continuous stimulation (150 Hz) was significantly less in MHS muscle (58.4 +/- 1.0%) than in normal muscle (50.5 +/- 3.0%). With intermittent stimulation, there was no significant difference in tension decline between muscle types. Post-stimulation twitch and tetanus responses were similar in MHS and normal muscles except: 1) twitch potentiation was significantly greater in normal muscle after continuous stimulation, and 2) recovery of tetanic tension was slowed in MHS muscle. Although the MH defect does not cause major functional abnormalities, subtle differences in MHS muscle response to fatiguing stimulation are apparent. Therefore, it is unlikely the work capacity of MH patients would be limited by any MH associated defect within the muscle.

A missense mutation W797R in the myophosphorylase gene in a Spanish patient with McArdle's disease

We identified a novel missense mutation in the myophosphorylase gene (PYGM) in a Spanish patient with McArdle's disease. This homozygous T-to-C transition results in the replacement of a highly conserved tryptophan at amino acid position (aa) 797 with an arginine in the C-terminal domain of the PYGM protein. The lack of enzyme activity in the proband's muscle is consistent with a crucial role of the aa 797 in the normal function of the PYGM protein. Our data further expand the genetic heterogeneity in patients with McArdle's disease.

Exercise intolerance due to mutations in the cytochrome b gene of mitochondrial DNA

BACKGROUND

The mitochondrial myopathies typically affect many organ systems and are associated with mutations in mitochondrial DNA (mtDNA) that are maternally inherited. However, there is also a sporadic form of mitochondrial myopathy in which exercise intolerance is the predominant symptom. We studied the biochemical and molecular characteristics of this sporadic myopathy.

METHODS

We sequenced the mtDNA cytochrome b gene in blood and muscle specimens from five patients with severe exercise intolerance, lactic acidosis in the resting state (in four patients), and biochemical evidence of complex III deficiency. We compared the clinical and molecular features of these patients with those previously described in four other patients with mutations in the cytochrome b gene.

RESULTS

We found a total of three different nonsense mutations (G15084A, G15168A, and G15723A), one missense mutation (G14846A), and a 24-bp deletion (from nucleotide 15498 to 15521) in the cytochrome b gene in the five patients. Each of these mutations impairs the enzymatic function of the cytochrome b protein. In these patients and those previously described, the clinical manifestations included progressive exercise intolerance, proximal limb weakness, and in some cases, attacks of myoglobinuria. There was no maternal inheritance and there were no mutations in tissues other than muscle. The absence of these findings suggests that the disorder is due to somatic mutations in myogenic stem cells after germ-layer differentiation. All the point mutations involved the substitution of adenine for guanine, but all were in different locations.

CONCLUSIONS

The sporadic form of mitochondrial myopathy is associated with somatic mutations in the cytochrome b gene of mtDNA. This myopathy is one cause of the common and often elusive syndrome of exercise intolerance.

African runners exhibit greater fatigue resistance, lower lactate accumulation, and higher oxidative enzyme activity

Nine African and eight Caucasian 10-km runners resident at sea level volunteered. Maximal O2 consumption and peak treadmill velocity (PTV) were measured by using a progressive test, and fatigue resistance [time to fatigue (TTF)] was measured by using a newly developed high-intensity running test: 5 min at 72, 80, and 88% of individual PTV followed by 92% PTV to exhaustion. Skeletal muscle enzyme activities were determined in 12 runners and 12 sedentary control subjects. In a comparison of African and Caucasian runners, mean 10-km race time, maximal O2 consumption, and PTV were similar. In African runners, TTF was 21% longer (P < 0.01), plasma lactate accumulation after 5 min at 88% PTV was 38% lower (P < 0.05), and citrate synthase activity was 50% higher (27.9 +/- 7.5 vs. 18.6 +/- 2.1 micromol. g wet wt-1. min-1, P = 0.02). Africans accumulated lactate at a slower rate with increasing exercise intensity (P < 0.05). Among the entire group of runners, a higher citrate synthase activity was associated with a longer TTF (r = 0.70, P < 0.05), a lower plasma lactate accumulation (r = -0.73, P = 0.01), and a lower respiratory exchange ratio (r = -0.63, P < 0.05). We conclude that the African and Caucasian runners in the present study differed with respect to oxidative enzyme activity, rate of lactate accumulation, and their ability to sustain high-intensity endurance exercise.

A nonsense mutation (G15059A) in the cytochrome b gene in a patient with exercise intolerance and myoglobinuria

We describe a new mitochondrial DNA mutation in the cytochrome b gene in a patient presenting with progressive exercise intolerance and myoglobinuria associated with complex III deficiency in muscle. The point mutation results in the replacement of a glycine at amino acid position 190 with a stop codon. This change predicts premature termination of translation, leading to a truncated protein missing 244 amino acids at the C-terminus of cytochrome b. The mutation fulfills all the accepted criteria for pathogenicity, suggesting that this is the primary cause of the myopathy in the patient.