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In the sarcomeric M-band, the giant ruler proteins titin and obscurin, its small homologue obscurin-like-1 (obsl1), and the myosin cross-linking protein myomesin form a ternary complex that is crucial for the function of the M-band as a mechanical link. Mutations in the last titin immunoglobulin (Ig) domain M10, which interacts with the N-terminal Ig-domains of obscurin and obsl1, lead to hereditary muscle diseases. The M10 domain is unusual not only in that it is a frequent target of disease-linked mutations, but also in that it is the only currently known muscle Ig-domain that interacts with two ligands--obscurin and obsl1--in different sarcomeric subregions. Using x-ray crystallography, we show the structural basis for titin M10 interaction with obsl1 in a novel antiparallel Ig-Ig architecture and unravel the molecular basis of titin-M10 linked myopathies. The severity of these pathologies correlates with the disruption of the titin-obsl1/obscurin complex. Conserved signature residues at the interface account for differences in affinity that direct the cellular sorting in cardiomyocytes. By engineering the interface signature residues of obsl1 to obscurin, and vice versa, their affinity for titin can be modulated similar to the native proteins. In single-molecule force-spectroscopy experiments, both complexes yield at forces of around 30 pN, much lower than those observed for the mechanically stable Z-disk complex of titin and telethonin, suggesting why even moderate weakening of the obsl1/obscurin-titin links has severe consequences for normal muscle functions. immunoglobulin domain | protein complex | x-ray crystallography | mechanosensor | myopathy www.pnas.org/cgi/doi/10.1073/pnas.0913736107

Am J Physiol Regul Integr Comp Physiol 298: R96-R103, 2010. First published October 28, 2009; doi: 10.1152/ajpregu.00138.2009.--Hypoxia, or reduced oxygen, occurs in a variety of clinical and environmental situations. Hypoxic exposure is associated with decreased muscle mass and a concomitant reduction in exercise capacity, although the exact mechanisms are not completely understood. The activin type IIB receptor (ActRIIB) is a receptor for transforming growth factor-[beta] (TGF[beta]) superfamily members that are involved in the negative regulation of lean tissue mass. Given that hypoxia has negative effects on muscle mass and function and that modulation of the ActRIIB has been shown to increase muscle mass, we tested the hypothesis that pharmacological targeting of the ActRIIB for 2 wk would attenuate the loss of muscle mass and function in mice after exposure to normobaric hypoxia. ActRIIB modulation was achieved using a soluble activin receptor/Fc fusion protein (sActRIIB) in mice housed in a hypoxic chamber for 1 or 2 wk. Hypoxia induced a reduction in body weight in PBS-and sActRIIB-treated mice, although sActRIIB-treated mice remained larger throughout the hypoxic exposure. The absolute forces generated by extensor digitorum longus muscles were also significantly greater in sActRIIB-than PBS-treated mice and were more resistant to eccentric contraction-induced force drop after eccentric lengthening contractions. In summary, sActRIIB pretreatment attenuated hypoxia-induced muscle dysfunction. These data suggest that targeting the ActRIIB is an effective strategy to counter hypoxia-induced muscle dysfunction and to pre-acclimatize to hypoxia in clinical or high-altitude settings. activin receptor; myostatin; muscle wasting; muscle physiology

DeRuisseau LR, Recca DM, Mogle JA, Zoccolillo M, DeRuisseau KC. Metallothionein deficiency leads to soleus muscle contractile dysfunction following acute spinal cord injury in mice. Am J Physiol Regul Integr Comp Physiol 297: R1795-R1802, 2009. First published October 14, 2009; doi: 10.1152/ajpregu.00263.2009.--Metallothionein (MT) is a small molecular weight protein possessing metal binding and free radical scavenging properties. We hypothesized that MT-1/MT-2 null ([MT.sup.-/-]) mice would display exacerbated soleus muscle atrophy, oxidative injury, and contractile dysfunction compared with the response of wild-type (WT) mice following acute spinal cord transection (SCT). Four groups of mice were studied: WT laminectomy, WT transection, [MT.sup.-/-] laminectomy ([MT.sup.-/-] lami), and [MT.sup.-/-] transection ([MT.sup.-/-] trans). Laminectomy animals served as surgical controls. Mice in SCT groups experienced similar percent body mass (BM) losses at 7 days postinjury. Soleus muscle mass (MM) and MM-to-BM ratio were lower at 7 days postinjury in SCT vs. laminectomy mice, with no differences observed between strains. However, soleus muscles from [MT.sup.-/-] trans mice showed reduced maximal specific tension compared with [MT.sup.-/-] lami animals. Mean cross-sectional area ([[micro]m.sup.2]) of type I and type IIa fibers decreased similarly in SCT groups compared with laminectomy controls, and no difference in fiber distribution was observed. Lipid peroxidation (4-hydroxynoneal) was greater in [MT.sup.-/-] trans vs. [MT.sup.-/-] lami mice, but protein oxidation (protein carbonyls) was not altered by MT deficiency or SCT. Expression of key antioxidant proteins (catalase, manganese, and copper-zinc superoxide dismutase) was similar between the groups. In summary, MT deficiency did not impact soleus MM loss, but resulted in contractile dysfunction and increased lipid peroxidation following acute SCT. These findings suggest a role of MT in mediating protective adaptations in skeletal muscle following disuse mediated by spinal cord injury. muscle atrophy; oxidative stress; spinal cord transection; antioxidant; stress protein doi: 10.1152/ajpregu.00263.2009

By sensing intracellular energy levels, ATP-sensitive potassium ([K.sub.ATP]) channels help regulate vascular tone, glucose metabolism, and cardioprotection. SUR2 mutant mice lack full-length [K.sub.ATP] channels in striated and smooth muscle and display a complex phenotype of hypertension and coronary vasospasm. SUR2 mutant mice also display baseline cardioprotection and can withstand acute sympathetic stress better than normal mice. We now studied response to a form of chronic stress, namely that induced by 4 wk of daily exercise on SUR2 mutant mice. Control mice increased exercise capacity by 400% over the training period, while SUR2 mutant mice showed little increase in exercise capacity. Unexercised SUR2 mutant showed necrotic and regenerating fibers in multiple muscle skeletal muscles, including quadriceps, tibialis anterior, and diaphragm muscles. Unlike exercised control animals, SUR2 mutant mice did not lose weight, presumably due to less overall exertion. Unexercised SUR2 mutant mice showed a trend of mildly reduced cardiac function, measured by fractional shortening, (46 [+ or -] 4% vs. 57 [+ or -] 7% for SUR2 mutant and control, respectively), and this decrease was not exacerbated by chronic exercise exposure. Despite an improved response to acute sympathetic stress and baseline cardioprotection, exercise intolerance results from lack of SUR2 [K.sub.ATP] channels in mice. [K.sub.ATP] channel; sulfonylurea receptor; SUR2; skeletal myopathy; exercise intolerance doi: 10.1152/ajpregu.00081.2009

In addition to muscle weakness caused by injury to supraspinal centers, several mechanisms may contribute to motor impairment in the paretic lower limb following a stroke. Physiological changes in the [...]

Muscle wasting is a critical feature of patients afflicted by acquired immune deficiency syndrome (AIDS), cancer, or chronic inflammatory diseases. In a mouse model of muscle wasting, TNF-[alpha] induces oxidative stress and nitric oxide synthase-2 (NOS2) and decreases myogenin, Jun-D, and creatinine kinase muscle isoform (CKM) expression. Here, we studied 12 patients with muscle wasting due to cancer (N = 10) or AIDS (N = 2) and 4 control subjects. We show that in skeletal muscle of cachectic patients there is 1) increased expression and activity of the TNF-[alpha] signaling, including TNF-[alpha] mRNA, activation of TNFR1, and TNF-[alpha]-associated to TNFR1; 2) increased oxidative stress, as determined by the presence of malondialdehyde-lysine adducts; 3) increased NOS2 mRNA and protein; 4) decreased expression of Jun-D, myogenin, myosin, and CKM mRNA and protein; 5) impaired CKM-E box binding activities, associated with decreased Jun-D/myogenin activities; and 6) oxidative modification and ubiquitination of Jun-D. These studies show that these molecular pathways are modulated in association with muscle wasting in patients with cancer or AIDS, and whether or not they cause muscle wasting remains to be determined. muscle atrophy; creatinine kinase; nitric oxide synthase-2; Ref-1

Muscle fiber deterioration resulting in progressive skeletal muscle weakness, heart failure, and respiratory distress occurs in more than 20 inherited myopathies. As discussed in this Review, one of the newly identified myopathies is desminopathy, a disease caused by dysfunctional mutations in desmin, a type III intermediate filament protein, or [alpha]B-crystallin, a chaperone for desmin. The range of clinical manifestations in patients with desminopathy is wide and may overlap with those observed in individuals with other myopathies. Awareness of this disease needs to be heightened, diagnostic criteria reliably outlined, and molecular testing readily available; this would ensure prevention of sudden death from cardiac arrhythmias and other complications., Background Desminopathy was originally described as skeletal and cardiac myopathy characterized by bilateral weakness in distal leg muscles spreading proximally and leading eventually to tetraparesis and wheelchair dependence (1). Weakness [...]

It took more than 100 years before it was established that the proteins that form intermediate filaments (IFs) comprise a unified protein family, the members of which are ubiquitous in virtually all differentiated cells and present both in the cytoplasm and in the nucleus. However, during the past 2 decades, knowledge regarding the functions of these structures has been expanding rapidly. Many disease-related roles of IFs have been revealed. In some cases, the molecular mechanisms underlying these diseases reflect disturbances in the functions traditionally assigned to IFs, i.e., maintenance of structural and mechanical integrity of cells and tissues. However, many disease conditions seem to link to the nonmechanical functions of IFs, many of which have been defined only in the past few years., Introduction Intermediate filaments (IFs) represent one of the main cytoskeletal systems found in virtually all vertebrate cells. Depending upon the cell type, IFs are composed of different members of the [...]

Intermediate filaments (IFs) are encoded by the largest gene family among the three major cytoskeletal protein groups. Unique IF compliments are expressed in selective cell types, and this expression is reflected in their involvement, upon mutation, as a cause of or predisposition to more than 80 human tissue-specific diseases. This Review Series covers diseases and functional and structural aspects pertaining to IFs and highlights the molecular and functional consequences of IF-associated diseases (IF-pathies). Exciting challenges and opportunities face the IF field, including developing both a better understanding of the pathogenesis of IF-pathies and targeted therapeutic approaches., Intermediate filaments Intermediate filaments (IFs), microfilaments (MFs), and microtubules (MTs) are the major fibrillar cytoplasmic elements that make up what is referred to as the cytoskeleton. IFs consist of a [...]

Myostatin, a member of the TGF-[beta] family, has been identified as a powerful inhibitor of muscle growth. Absence or blockade of myostatin induces massive skeletal muscle hypertrophy that is widely attributed to proliferation of the population of muscle fiber-associated satellite cells that have been identified as the principle source of new muscle tissue during growth and regeneration. Postnatal blockade of myostatin has been proposed as a basis for therapeutic strategies to combat muscle loss in genetic and acquired myopathies. But this approach, according to the accepted mechanism, would raise the threat of premature exhaustion of the pool of satellite cells and eventual failure of muscle regeneration. Here, we show that hypertrophy in the absence of myostatin involves little or no input from satellite cells. Hypertrophic fibers contain no more myonuclei or satellite cells and myostatin had no significant effect on satellite cell proliferation in vitro, while expression of myostatin receptors dropped to the limits of detectability in postnatal satellite cells. Moreover, hypertrophy of dystrophic muscle arising from myostatin blockade was achieved without any apparent enhancement of contribution of myonuclei from satellite cells. These findings contradict the accepted model of myostatin-based control of size of postnatal muscle and reorient fundamental investigations away from the mechanisms that control satellite cell proliferation and toward those that increase myonuclear domain, by modulating synthesis and turnover of structural muscle fiber proteins. It predicts too that any benefits of myostatin blockade in chronic myopathies are unlikely to impose any extra stress on the satellite cells. muscle growth | muscular dystrophy | TGF-beta | muscle stem cells | myonuclear domain

Regulators of skeletal muscle mass are of interest, given the morbidity and mortality of muscle atrophy and myopathy. Four-and-a-half LIM protein 1 (FHL1) is mutated in several human myopathies, including reducing-body myopathy (RBM). The normal function of FHL1 in muscle and how it causes myopathy remains unknown. We find that FHL1 transgenic expression in mouse skeletal muscle promotes hypertrophy and an oxidative fiber-type switch, leading to increased whole-body strength and fatigue resistance. Additionally, FHL1 overexpression enhances myoblast fusion, resulting in hypertrophic myotubes in C2C12 cells, (a phenotype rescued by calcineurin inhibition). In FHL1 -RBM C2C12 cells, there are no hypertrophic myotubes. FHL1 binds with the calcineurinregulated transcription factor NFATc1 (nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1), enhancing NFATc1 transcriptional activity. Mutant RBM-FHL1 forms aggregate bodies in C2C12 cells, sequestering NFATc1 and resulting in reduced NFAT nuclear translocation and transcriptional activity. NFATc1 also colocalizes with mutant FHL1 to reducing bodies in RBM-afflicted skeletal muscle. Therefore, via NFATc1 signaling regulation, FHL1 appears to modulate muscle mass and strength enhancement.

Reducing body myopathy (RBM) is a rare disorder causing progressive muscular weakness characterized by aggresome-like inclusions in the myofibrils. Identification of genes responsible for RBM by traditional genetic approaches has been impossible due to the frequently sporadic occurrence in affected patients and small family sizes. As an alternative approach to gene identification, we used laser microdissection of intracytoplasmic inclusions identified in patient muscle biopsies, followed by nanoflow liquid chromatography--tandem mass spectrometry and proteomic analysis. The most prominent component of the inclusions was the Xq26.3-encoded four and a half LIM domain 1 (FHL1) protein, expressed predominantly in skeletal but also in cardiac muscle. Mutational analysis identified 4 FHL1 mutations in 2 sporadic unrelated females and in 2 families with severely affected boys and less-affected mothers. Transfection of kidney COS-7 and skeletal muscle C2C12 cells with mutant FHL1 induced the formation of aggresome-like inclusions that incorporated both mutant and wild-type FHL1 and trapped other proteins in a dominant-negative manner. Thus, a novel laser microdissection/proteomics approach has helped identify both inherited and de novo mutations in FHL1, thereby defining a new X-linked protein aggregation disorder of muscle., Introduction Reducing body myopathy (RBM) is a rare muscle disorder causing progressive muscular weakness and characteristic intracytoplasmic inclusions in myofibers, first described more than 30 years ago (1). Clinical presentations [...]

Over the past century, understanding the mechanisms underlying muscle fatigue and weakness has been the focus of much investigation. However, the dominant theory in the field, that lactic acidosis causes muscle fatigue, is unlikely to tell the whole story. Recently, dysregulation of sarcoplasmic reticulum (SR) [Ca.sup.2+] release has been associated with impaired muscle function induced by a wide range of stressors, from dystrophy to heart failure to muscle fatigue. Here, we address current understandings of the altered regulation of SR [Ca.sup.2+] release during chronic stress, focusing on the role of the SR [Ca.sup.2+] releases channel known as the type 1 ryanodine receptor., Introduction Skeletal muscle contraction is initiated by depolarization of the surface membrane (the sarcolemma) of muscle cells. Depolarization of the sarcolemma leads to the formation of cross-bridges between myosin and [...]

A large multigenerational Austrian family displaying a novel form of X-linked recessive myopathy is identified. It is found that a missense mutation within the four and a half LIM domain 1 (FHL1) gene causes the X-linked myopathy with a postural muscle atrophy (XMPMA).

The stress and strain in the vessel wall are important determinants of vascular physiology and pathophysiology. Vessels are constrained radially by the surrounding tissue. The hypothesis in this work is that the surrounding tissue takes up a considerable portion of the intravascular pressure and significantly reduces the wall strain and stress. Ten swine of either sex were used to test this hypothesis. An impedance catheter was inserted into the carotid or femoral artery, and after mechanical preconditioning pressure-cross-sectional area relations were obtained with the surrounding tissue intact and dissected away (untethered), respectively. The radial constraint of the surrounding tissue was quantified as an effective perivascular pressure on the outer surface of the vessel, which was estimated as 50% or more of the intravascular pressure. For carotid arteries at pressure of 100 mmHg, the circumferential wall stretch ratio in the intact state was ~20% lower than in the untethered state and the average circumferential stress was reduced by ~70%. For femoral arteries, the reductions were ~15% and 70%, respectively. These experimental data support the proposed hypothesis and suggest that in vitro and in vivo measurements of the mechanical properties of vessels must be interpreted with consideration of the constraint of the surrounding tissue. stress analysis; strain; impedance planimetry; carotid artery; femoral artery

Nearly four years ago, Hugh Rienhoff watched as his baby girl was pulled from a small incision in his wife's belly. It was their third child--the two boys had also [...]

Maintenance of skeletal and cardiac muscle structure and function requires precise control of the synthesis, assembly, and turnover of contractile proteins of the sarcomere. Abnormalities in accumulation of sarcomere proteins [...]

Prolonged or unaccustomed exercise leads to muscle cell membrane damage, detectable as release of the intracellular enzyme lactic acid dehydrogenase (LDH). This is correlated to excitation-induced influx of [Ca.sup.2+], but it cannot be excluded that mechanical stress contributes to the damage. We here explore this question using N-benzyl-p-toluene sulfonamide (BTS), which specifically blocks muscle contraction. Extensor digitorum longus muscles were prepared from 4-wk-old rats and mounted on holders for isometric contractions. Muscles were stimulated intermittently at 40 Hz for 15-60 min or exposed to the [Ca.sup.2+] ionophore A23187. Electrical stimulation increased [sup.45]Ca influx 3-5 fold. This was followed by a progressive release of LDH, which was correlated to the influx of [Ca.sup.2+]. BTS (50 [micro]M) caused a 90% inhibition of contractile force but had no effect on the excitation-induced [sup.45]Ca influx. After stimulation, ATP and creatine phosphate levels were higher in BTS-treated muscles, most likely due to the cessation of ATP-utilization for cross-bridge cycling, indicating a better energy status of these muscles. No release of LDH was observed in BTS-treated muscles. However, when exposed to anoxia, electrical stimulation caused a marked increase in LDH release that was not suppressed by BTS but associated with a decrease in the content of ATP. Dynamic passive stretching caused no increase in muscle [Ca.sup.2+] content and only a minor release of LDH, whereas treatment with A23187 markedly increased LDH release both in control and BTStreated muscles. In conclusion, after isometric contractions, muscle cell membrane damage depends on [Ca.sup.2+] influx and energy status and not on mechanical stress. N-benzyl-p-toluene sulfonamide; [Ca.sup.2+]; muscle damage; passive stretch doi:10.1152/ajpregu.00415.2006

Downhill running is associated with fiber damage, inflammation, delayed-onset muscle soreness, and various functional deficits. Curcumin, a constituent of the Indian spice turmeric has been investigated for its anti-inflammatory activity and may offset some of the damage and functional deficits associated with downhill running. This study examined the effects of curcumin on inflammation and recovery of running performance following downhill running in mice. Male mice were assigned to downhill placebo (Down-Plac), downhill curcumin (Down-Cur), uphill placebo (Up-Plac), or uphill curcumin (Up-Cur) groups and run on a treadmill at 22 m/min at -14% or +14% grade, for 150 min. At 48 h or 72 h after the up/downhill run, mice (experiment 1) underwent a treadmill performance run to fatigue. Another subset of mice was placed in voluntary activity wheel cages following the up/downhill run (experiment 2) and their voluntary activity (distance, time and peak speed) was recorded. Additional mice (experiment 3) were killed at 24 h and 48 h following the up/downhill run, and the soleus muscle was harvested for analysis of inflammatory cytokines (IL-1[beta], IL-6, and TNF-[alpha]), and plasma was collected for creatine kinase analysis. Downhill running decreased both treadmill run time to fatigue (48 h and 72 h) and voluntary activity (24 h) (P < 0.05), and curcumin feedings offset these effects on running performance. Downhill running was also associated with an increase in inflammatory cytokines (24 h and 48 h) and creatine kinase (24 h) (P < 0.05) that were blunted by curcumin feedings. These results support the hypothesis that curcumin can reduce inflammation and offset some of the performance deficits associated with eccentric exercise-induced muscle damage. inflammatory cytokines; fatigue; mice; nutraceutical; phytochemicals doi: 10.1152/ajpregu.00858.2006

The prion protein (PrP) level in muscle has been reported to be elevated in patients with inclusion-body myositis, polymyositis, dermatomyositis, and neurogenic muscle atrophy, but it is not clear whether the elevated PrP accumulation in the muscles is sufficient to cause muscle diseases. We have generated transgenic mice with muscle-specific expression of PrP under extremely tight regulation by doxycycline, and we have demonstrated that doxycycline-induced overexpression of PrP strictly limited to muscles leads to a myopathy characterized by increased variation of myofiber size, centrally located nuclei, and endomysial fibrosis, in the absence of intracytoplasmic inclusions, rimmed vacuoles, or any evidence of a neurogenic disorder. The PrP-induced myopathy correlates with accumulation of an N-terminal truncated PrP fragment in the muscle, and the muscular PrP displayed consistent mild resistance to protease digestion. Our findings indicate that overexpression of wild-type PrP in skeletal muscles is sufficient to cause a primary myopathy with no signs of peripheral neuropathy, possibly due to accumulation of a cytotoxic truncated form of PrP and/or PrP aggregation. doxycycline | inducible transgenic mice | muscle disease

L-carnitine L-tartrate (LCLT) supplementation beneficially affects markers of postexercise metabolic stress and muscle damage. However, to date, no study has determined the dose response of LCLT to elicit such responses. Therefore, the purpose of this study was to determine the effects of different doses of LCLT on criterion variables previously shown to be responsive to LCLT supplementation. Eight healthy men (22 [+ or -] 3 y, 174 [+ or -] 5 cm, 83.0 [+ or -] 15.3 kg) were supplemented with 0 g, 1 g, and 2 g of LCLT for 3 weeks and then performed a bout of resistance exercise (5 sets of 15-20 repetition maximum with a 2-min rest between sets) with associated blood draws. This procedure was performed in a balanced, randomized, repeated measures design. Serum carnitine concentrations increased (p [less than or equal to] 0.05) following the 1 g and 2 g doses, with the 2-g dose providing the highest carnitine concentrations. The 1 and 2-g doses reduced postexercise serum hypoxanthine, serum xanthine oxidase, serum myoglobin, and perceived muscle soreness. In conclusion, both the 1- and 2-g doses were effective in mediating various markers of metabolic stress and of muscle soreness. Use of LCLT appears to attenuate metabolic stress and the hypoxic chain of events leading to muscle damage after exercise. KEY WORDS. ergogenic aids, metabolic stress, performance, recovery, sports supplements, strength training

We hypothesized that severe hypoxia limits exercise performance via decreased contractility of limb locomotor muscles. Nine male subjects [mean [+ or -] SE maximum [O.sub.2] uptake ([Vo.sub.2max]) = 56.5 [+ or -] 2.7 ml*[kg.sup.-1]*[min.sup.-1]] cycled at [greater than or equal to] 90% [Vo.sub.2max] to exhaustion in normoxia [NORM-EXH; inspired [O.sub.2] fraction ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]) = 0.21, arterial [O.sub.2] saturation ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]) = 93 [+ or -] 1%] and hypoxia (HYPOX-EXH; [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] = 0.13, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] = 76 [+ or -] 1%). The subjects also exercised in normoxia for a time equal to that achieved in hypoxia (NORM-CTRL; [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] = 96 [+ or -] 1%). Quadriceps twitch force, in response to supramaximal single (nonpotentiated and potentiated 1 Hz) and paired magnetic stimuli of the femoral nerve (10-100 Hz), was assessed pre- and at 2.5, 35, and 70 min postexercise. Hypoxia exacerbated exercise-induced peripheral fatigue, as evidenced by a greater decrease in potentiated twitch force in HYPOX-EXH vs. NORM-CTRL (-39 [+ or -] 4 vs. -24 [+ or -] 3%, P < 0.01). Time to exhaustion was reduced by more than two-thirds in HYPOX-EXH vs. NORM-EXH (4.2 [+ or -] 0.5 vs. 13.4 [+ or -] 0.8 min, P < 0.01); however, peripheral fatigue was not different in HYPOX-EXH vs. NORM-EXH (-34 [+ or -] 4 vs. -39 [+ or -] 4%, P > 0.05). Blood lactate concentration and perceptions of limb discomfort were higher throughout HYPOX-EXH vs. NORM-CTRL but were not different at end-exercise in HYPOX-EXH vs. NORM-EXH. We conclude that severe hypoxia exacerbates peripheral fatigue of limb locomotor muscles and that this effect may contribute, in part, to the early termination of exercise. hypoxemia; muscle fatigue; exercise performance

The sixth thin-filament-related nemaline myopathy (NM) gene called CFL2 is identified encoding the actin-binding protein muscle cofilin-2, which is mutated in two siblings with congenital myopathy. It is found that cofilin-2 levels are significantly lower in the proband's muscle and the mutant protein is less soluble when expressed in Escherichia coli, suggesting that deficiency of cofilin-2 may result in reduced depolymerization of actin filaments.

Muscle degeneration and myotonia are clinical hallmarks of myotonic dystrophy type 1 (DM1), a multisystemic disorder caused by a CTG repeat expansion in the 3' untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Transgenic mice engineered to express mRNA with expanded [(CUG).sub.250] repeats ([HSA.sup.LR] mice) exhibit prominent myotonia and altered splicing of muscle chloride channel gene (Clcn1) transcripts. We used whole-cell patch clamp recordings and nonstationary noise analysis to compare and biophysically characterize the magnitude, kinetics, voltage dependence, and single channel properties of the skeletal muscle chloride channel (ClC-1) in individual flexor digitorum brevis (FDB) muscle fibers isolated from 1-3-wk-old wild-type and [HSA.sup.LR] mice. The results indicate that peak ClC-1 current density at -140 mV is reduced >70% (-48.5 [+ or -] 3.6 and -14.0 [+ or -] 1.6 pA/pF, respectively) and the kinetics of channel deactivation increased in FDB fibers obtained from 18-20-d-old [HSA.sup.LR] mice. Nonstationary noise analysis revealed that the reduction in ClC-1 current density in [HSA.sup.LR] FDB fibers results from a large reduction in ClC-1 channel density (170 [+ or -] 21 and 58 [+ or -] 11 channels/pF in control and [HSA.sup.LR] fibers, respectively) and a modest decrease in maximal channel open probability (0.91 [+ or -] 0.01 and 0.75 [+ or -] 0.03, respectively). Qualitatively similar results were observed for ClC-1 channel activity in knockout mice for muscleblind-like 1 ([Mbnl1.sup.[DELTA]E3/ [DELTA]E3]), a second murine model of DM1 that exhibits prominent myotonia and altered Clcn1 splicing (Kanadia et al., 2003). These results support a molecular mechanism for myotonia in DM1 in which a reduction in both the number of tunctional sarcolemmal ClC-1 and maximal channel open probability, as well as an acceleration in the kinetics of channel deactivation, results from CUG repeat-containing mRNA molecules sequestering Mbnl1 proteins required for proper CLCN1 pre-mRNA splicing and chloride channel function.

The tumor necrosis factor-[alpha] (TNF[alpha])-mediated inhibition of myogenic differentiation is found to be dependent on p53 through a PW1-mediated pathway. The p53 plays a novel role in mediating muscle stem cell behavior and muscle atrophy, and has pointed towards new targets for the therapeutic treatment of muscle wasting (cachexia).

A frequent finding in patients surviving critical illness myopathy is chronic muscle dysfunction. Its pathogenesis is mostly unknown; one explanation could be that muscle regeneration, which normally follows myopathy, is insufficient in these patients because of a high glucocorticoid level in their blood. Glucocorticoids can prevent stimulatory effects of proinflammatory factors on the interleukin (IL)-6 secretion, diminishing in this way the autocrine and paracrine IL-6 actions known to stimulate proliferation at the earliest, myoblast stage of muscle formation. To test this hypothesis, we compared the effects of major proinflammatory agents [tumor necrosis factor (TNF)-[alpha] and endotoxin lipopolysaccharide (LPS)] on the IL-6 secretion from the muscle precursors and then studied the influence of dexamethasone (Dex) on these effects. Mononuclear myoblasts, which still proliferate, were compared with myotubes in which this capacity is already lost. For correct interpretation of results, cultures were examined for putative apoptosis and necrosis. We found that constitutive secretion of IL-6 did not differ significantly between myoblasts and myotubes; however, the TNF-[alpha]- and LPS-stimulated IL-6 release was more pronounced (P sepsis-induced myopathy; critical illness myopathy; myoblasts; myotubes; cytokines; glucocorticoids

Muscle inflammation is a relatively common pathological process in childhood. The diagnosis of the underlying cause relies on an appreciation of the pattern of clinical features, as well as the results of biochemical, histological and radiological investigations. Often the clinical and biochemical features are non-specific and insensitive. Consequently, the radiological abnormalities are very important in establishing a diagnosis and an understanding of the imaging features of muscle inflammatory disorders in childhood is needed. Some of the imaging protocols needed to investigate a variety of muscle and soft-tissue inflammatory conditions in childhood are reviewed in this article. Those features that are helpful in narrowing the differential diagnosis are indicated and a logical approach to the investigation of affected children is provided. The value of MR imaging is highlighted. Keywords Muscle * Inflammation * MRI * Children

Increases in [P.sub.i] combined with decreases in myoplasmic [Ca.sup.2+] are believed to cause a significant portion of the decrease in muscular force during fatigue. To investigate this further, we determined the effect of 30 mM [P.sub.i] on the force-[Ca.sup.2+] relationship of chemically skinned single muscle fibers at near-physiological temperature (30[degrees]C). Fibers isolated from rat soleus (slow) and gastrocnemius (fast) muscle were subjected to a series of solutions with an increasing free [Ca.sup.2+] concentration in the presence and absence of 30 mM [P.sub.i] at both low (15[degrees]C) and high (30[degrees]C) temperature. In slow fibers, 30 mM [P.sub.i] significantly increased the [Ca.sup.2+] required to elicit measurable force, referred to as the activation threshold at both low and high temperatures; however, the effect was twofold greater at the higher temperature. In fast fibers, the activation threshold was unaffected by elevating [P.sub.i] at 15[degrees]C but was significantly increased at 30[degrees]C. At both low and high temperatures, 30 mM [P.sub.i] increased the [Ca.sup.2+] required to elicit half-maximal force (p[Ca.sub.50]) in both slow and fast fibers, with the effect of Pa twofold greater at the higher temperature. These data suggest that during fatigue, reductions in the myoplasmic [Ca.sup.2+] and increases in Pi act synergistically to reduce muscular force. Consequently, the combined changes in these ions likely account for a greater portion of fatigue than previously predicted based on studies at lower temperatures or high temperatures at saturating [Ca.sup.2+] levels. force-pCa relationship; phosphate; fatigue

ATP-sensitive [K.sup.+] channels ([K.sub.ATP]) are an octameric complex of inwardly rectifying [K.sup.+] channels (Kir6.1 and Kir6.2) and sulfonylurea receptors (SUR1 and SUR2A/B), which are involved in several diseases. The tissue-selective expression of the subunits leads to different channels; however, the composition and role of the functional channel in native muscle fibers is not known. In this article, the properties of [K.sub.ATP] channels of fast-twitch and slow-twitch muscles were compared by combining patch-clamp experiments with measurements of gene expression. We found that the density of [K.sub.ATP] currents/area was muscle-type specific, being higher in fast-twitch muscles compared with the slow-twitch muscle. The density of KATP currents/area was correlated with the level of Kir6.2 expression. SUR2A was the most abundant subunit expressed in all muscles, whereas the vascular SUR2B subunit was expressed but at lower levels. A significant expression of the pancreatic SUR1 was also found in fast-twitch muscles. Pharmacological experiments showed that the channel response to the SUR1 agonist diazoxide, SUR2A/B agonist cromakalim, SUR1 antagonist tolbutamide, and the SUR1/SUR2A/B-antagonist glibenclamide matched the SURs expression pattern. Muscle-specific [K.sub.ATP] subunit compositions contribute to the physiological performance of different muscle fiber types and determine the pharmacological actions of drugs modulating [K.sub.ATP] activity in muscle diseases. physiology | pharmacology | skeletal muscle

Myostatin is a secreted protein that normally functions as a negative regulator of muscle growth. Agents capable of blocking the myostatin signaling pathway could have important applications for treating human muscle degenerative diseases as well as for enhancing livestock production. Here we describe a potent myostatin inhibitor, a soluble form of the activin type IIB receptor (ACVR2B), which can cause dramatic increases in muscle mass (up to 60% in 2 weeks) when injected into wild-type mice. Furthermore, we show that the effect of the soluble receptor is attenuated but not eliminated in [Mstn.sup.-/-] mice, suggesting that at least one other ligand in addition to myostatin normally functions to limit muscle growth. Finally, we provide genetic evidence that these ligands signal through both activin type II receptors, ACVR2 and ACVR2B, to regulate muscle growth in vivo. myostatin | TGF-[beta]

Spinal and bulbar muscular atrophy (SBMA) is an adult-onset motor neuron disease caused by the expansion of a trinucleotide CAG repeat encoding the polyglutamine tract in the first exon of the androgen receptor gene (AR). The pathogenic, polyglutamine-expanded AR protein accumulates in the cell nucleus in a ligand-dependent manner and inhibits transcription by interfering with transcriptional factors and coactivators. Heat-shock proteins (HSPs) are stress-induced chaperones that facilitate the refolding and, thus, the degradation of abnormal proteins. Geranylgeranylacetone (GGA), a nontoxic antiulcer drug, has been shown to potently induce HSP expression in various tissues, including the central nervous system. In a cell model of SBMA, GGA increased the levels of Hsp70, Hsp90, and Hsp105 and inhibited cell death and the accumulation of pathogenic AR. Oral administration of GGA also up-regulated the expression of HSPs in the central nervous system of SBMA-transgenic mice and suppressed nuclear accumulation of the pathogenic AR protein, resulting in amelioration of polyglutamine-dependent neuromuscular phenotypes. These observations suggest that, although a high dose appears to be needed for clinical effects, oral GGA administration is a safe and promising therapeutic candidate for polyglutamine-mediated neurodegenerative diseases, including SBMA. spinal and bulbar muscular atrophy | geranylgeranylacetone | androgen receptor | heat-shock factor-1

Desmin is the major intermediate filament (IF) protein of muscle. Recently, mutations of the desmin gene have been reported to cause familial or sporadic forms of human skeletal, as well as cardiac, myopathy, termed desmin-related myopathy (DRM). The impact of any of these mutations on filament assembly and integration into the cytoskeletal network of myocytes is currently not understood, despite the fact that all cause the same histopathological defect, i.e., desmin aggregation. To gain more insight into the molecular basis of this process, we investigated how mutations within the s-helical rod domain of desmin affect both the assembly of the recombinant protein in vitro as well as the filament-forming capacity in cDNA-transfected cells. Whereas 6 of 14 mutants assemble into seemingly normal IFs in the test tube, the other mutants interfere with the assembly process at distinct stages, i.e., tetramer formation, unit-length filament (ULF) formation, filament elongation, and IF maturation. Correspondingly, the mutants with in vitro assembly defects yield dot-like aggregates in transfected cells, whereas the mutants that form IFs constitute a seemingly normal IF cytoskeleton in the cellular context. At present, it is entirely unclear why the latter mutant proteins also lead to aggregate formation in myocytes. Hence, these findings may be a starting point to dissect the contribution of the individual subdomains for desmin pathology and, eventually, the development of therapeutic interventions. desmin-related myopathy | intermediate filaments

Accurate and rapid identification of perturbed pathways through the analysis of genome-wide expression profiles facilitates the generation of biological hypotheses. We propose a statistical framework for determining whether a specified group of genes for a pathway has a coordinated association with a phenotype of interest. Several issues on proper hypothesis-testing procedures are clarified. In particular, it is shown that the differences in the correlation structure of each set of genes can lead to a biased comparison among gene sets unless a normalization procedure is applied. We propose statistical tests for two important but different aspects of association for each group of genes. This approach has more statistical power than currently available methods and can result in the discovery of statistically significant pathways that are not detected by other methods. This method is applied to data sets involving diabetes, inflammatory myopathies, and Alzheimer's disease, using gene sets we compiled from various public databases. In the case of inflammatory myopathies, we have correctly identified the known cytotoxic T lymphocyte-mediated autoimmunity in inclusion body myositis. Furthermore, we predicted the presence of dendritic cells in inclusion body myositis and of an IFN-[alpha]/[beta] response in dermatomyositis, neither of which was previously described. These predictions have been subsequently corroborated by immunohistochemistry. microarrays | gene ontology | normalization | correlated data | inflammatory myopathies

Bone marrow stromal cells (MSCs) have great potential as therapeutic agents. We report a method for inducing skeletal muscle lineage cells from human and rat general adherent MSCs with an efficiency of 89%. Induced cells differentiated into muscle fibers upon transplantation into degenerated muscles of rats and mdx-nude mice. The induced population contained Pax7-positive cells that contributed to subsequent regeneration of muscle upon repetitive damage without additional transplantation of cells. These MSCs represent a more ready supply of myogenic cells than do the rare myogenic stem cells normally found in muscle and bone marrow., Cell transplantation therapy offers hope for the treatment of intractable muscle degenerative disorders. Embryonic stem (ES) cells and stem cells derived from muscle have been considered as candidates for transplantation [...]

Background. There is growing evidence for the significant involvement of inflammatory processes in the development of muscle wasting in old age. Therefore, any disease accompanied by inflammation can be threatening to the muscle function in geriatric patients. Methods. Sixty-three hospitalized geriatric patients (42 female, 21 male; mean age 84.2 [+ or -] 5.7 years) were monitored weekly for muscle function (grip strength, fatigue resistance, shoulder extension strength, and hip extension strength) and for concentration of circulating C-reactive protein (CRP), fibrinogen, interleukin 6 (IL-6), and tumor necrosis factor-[alpha] alpha (TNF-[alpha]). Results. On the basis of circulating CRP and fibrinogen concentrations, 42 patients were categorized on admission as inflammatory and 21 as noninflammatory. Inflammatory patients presented significantly weaker grip strength, shoulder extension strength, and a worse fatigue resistance than did noninflammatory patients. These muscle functions were negatively correlated with the concentrations of circulating CRP and IL-6, but not with fibrinogen or TNF-[alpha]. In noninflammatory patients, the fatigue resistance improved significantly during the first week of hospitalization. In patients admitted with inflammation, no improvement of muscle function was observed. Patients who remained inflammatory for 2 weeks or more presented a significant worsening of fatigue resistance. Conclusions. Geriatric hospitalized patients presenting with inflammation show significantly worse muscle functions, which do not improve during hospitalization despite adequate treatment of the primary disease. Reduced strength and fatigue resistance are significantly related to the concentration of circulating CRP and IL-6. Standard treatment of the underlying illness and classic physical therapy are not sufficient to normalize the skeletal muscle strength and fatigue resistance in these hospitalized patients.

Background. Several studies have documented the substantial health and economic burdens associated with sarcopenia among the elderly, but there has been no systematic study among Asians. A cross-sectional survey of elderly community-dwelling Chinese volunteers (262 men and 265 women), aged 70 years and older, was undertaken in Hong Kong. The purposes of this study were to evaluate the prevalence of and risk factors for sarcopenia in elderly Chinese, and to compare these observations with those in white persons. Methods. Muscle mass was estimated by dual-energy X-ray absorptiometry, in this study, sarcopenia was defined as a total adjusted skeletal muscle mass two standard deviations or more below the normal mean for young Asian men and women in this study. The relationship between risk factors (alcohol consumption, cigarette smoking, regular exercise, body mass index, medical conditions) and sarcopenia was studied by multiple logistic regression. Results. The prevalence of sarcopenia was 12.3% in Chinese men and 7.6% in Chinese women aged 70 years and older, which was slightly lower than figures observed in white persons. Being underweight was a significant risk factor in both men (odds ratio, 39.1; 95% confidence interval, 11.3 to 134.6) and women (odds ratio, 9.7; 95% confidence interval, 2.8 to 33.8). No other risk factors were found in Chinese men or women. Conclusions. Sarcopenia exists among elderly Chinese men and women, albeit at a lower rate than in white persons. This may be due to the lower muscle mass among young men and women or to an attenuated rate of loss in muscle mass with aging in the Chinese elderly. Being underweight is a major risk factor for sarcopenia in both sexes.

Intravenous Fe is widely used to treat anemia in renal disease patients. However, concerns of potential Fe toxicity exist. To more fully define its spectrum, this study tested Fe's impact on systemic inflammation following either endotoxemia or the induction of direct tissue damage (glycerol-mediated rhabdomyolysis). The inflammatory response was gauged by tissue TNF-[alpha] message expression and plasma TNF-[alpha] levels. CD-1 mice received either intravenous Fe sucrose, -gluconate, or -dextran (FeS, FeG, or FeD, respectively; 2 mg), followed by either endotoxin (LPS) or glycerol injection 0-48 h later. Plasma TNF-[alpha] was assessed by ELISA 2-3 h after the LPS or glycerol challenge. TNF-[alpha] mRNA expression (RT-PCR) was measured in the kidney, heart, liver, lung, and spleen with Fe [+ or -] LPS treatment. Finally, the relative impacts of intramuscular vs. intravenous Fe and of glutathione (GSH) on Fe/ LPS- induced TNF-[alpha] generation were assessed. Each Fe preparation significantly enhanced LPS- or muscle injury-mediated TNF-[alpha] generation. This effect was observed for at least 48 h post-Fe injection, a time at which plasma iron levels were increased by levels insufficient to fully saturate transferrin. Fe did not independently increase plasma TNF-[alpha] or tissue mRNA. However, it potentiated postinjury-induced TNF-[alpha] mRNA increments and did so in an organ-specific fashion (kidney, heart, and lung; but not in liver or spleen). Intramuscular administration, but not GSH treatment, negated Fe's ability to synergize LPS-mediated TNF-[alpha] release. We conclude 1) intravenous Fe can enhance TNF-[alpha] generation during LPS- or glycerol-induced tissue damage; 2) increased TNF-[alpha] gene transcription in the kidney, heart, and lung may contribute to this result; and 3) intramuscular administration, but not GSH, might potentially mitigate some of Fe's systemic toxic effects. endotoxemia; rhabdomyolysis; end-stage renal disease; iron deficiency; glutathione

Serum response factor (SRF) controls the transcription of muscle genes by recruiting a variety of partner proteins, including members of the myocardin family of transcriptional coactivators. Mice lacking SRF fail to form mesoderm and die before gastrulation, precluding an analysis of the roles of SRF in muscle tissues. To investigate the functions of SRF in skeletal muscle development, we conditionally deleted the Srf gene in mice by skeletal muscle-specific expression of Cre recombinase. In mice lacking skeletal muscle SRF expression, muscle fibers formed, but failed to undergo hypertrophic growth after birth. Consequently, mutant mice died during the perinatal period from severe skeletal muscle hypoplasia. The myopathic phenotype of these mutant mice resembled that of mice expressing a dominant negative mutant of a myocardin family member in skeletal muscle. These findings reveal an essential role for the partnership of SRF and myocardin-related transcription factors in the control of skeletal muscle growth and maturation in vivo. hypertrophy | myocardin-related transcription factor | myofiber | myopathy

Byline: Kathleen Patterson (1) Author Affiliation: (1) University of Washington School of Medicine and Children's Hospital and Regional Medical Center, 4800 Sandpoint Way NE, Seattle, WA, 98105, USA Article History: Registration Date: 01/01/2004 Received Date: 24/05/2004 Accepted Date: 26/07/2004 Online Date: 06/12/2004