Skeletal muscle disuse induces fibre type-dependent enhancement of Na(+) channel expression

Changes in the skeletal muscle transcriptome due to the intramuscular administration of lidocaine in wether lambs

Lidocaine is a commonly used local anesthetic that blocks sodium channels in nociceptor neurons, preventing the transmission of pain signals to the brain. Lidocaine can be administered to reduce discomfort during tissue biopsies. Biopsy tissue may then be used to study the transcriptome under the assumption that the genomic activity of lidocaine-treated tissue accurately reflects that of untreated tissue. This study investigated how intramuscular lidocaine injection influenced skeletal muscle gene expression in sheep, aiming to understand how transcriptomic changes could affect data interpretation. Approximately 10 min before euthanasia, the left biceps brachii muscle from each of 6 wether lambs (48.7 ± 0.8 kg) was injected (IM; 20G hypodermic needle) at a depth of 3 cm with 2 mL of 2% lidocaine (20 mg/mL); the right biceps brachii was untreated. At necropsy, muscle samples were collected from the injection sites and contralateral limbs and flash-frozen. In an additional set of lambs, lidocaine-treated and untreated samples were collected from the biceps brachii of 4 lambs, and the vastus intermedius of 4 other lambs. RNA was isolated and mRNA sequenced to a targeted depth of 20 million reads per sample. Sequences were mapped and quantified; matched-pair analysis was performed in EdgeR. No genes were consistently differentially expressed due to treatment in both muscle types, perhaps reflecting their distinct physiological roles. Lidocaine did influence the transcriptome with anti-inflammatory effects evident in both muscle types, including the downregulation of immune-associated transcription factors and other genes. Lidocaine's influence varied on other broad categories of genes, including those associated with muscle contractility, tissue repair, and structural integrity, which could affect the interpretation of transcriptome data in studies of muscle growth and development. Pathway analysis revealed that lidocaine impacted signaling mechanisms for cellular connectivity and structure. This study demonstrates that intramuscular administration of lidocaine results in the alteration of tissue's gene expression profiles, highlighting the importance of considering lidocaine's influence in transcriptome analyses. Thus, the use of complementary physiological measures to validate transcriptomic findings is recommended to ensure observed gene expression changes are accurately attributed to experimental conditions rather than the effects of lidocaine.

The effects of environmental enrichment on voluntary physical activity and muscle mass gain in growing rats

Introduction: Environmental enrichment (EE) for rodents involves housing conditions that facilitate enhanced sensory, cognitive, and motor stimulation relative to standard housing conditions. A recent study suggested that EE induces muscle hypertrophy. However, it remains unclear whether muscle hypertrophy in EE is associated with voluntary physical activity, and the characteristics of muscle adaptation to EE remain unclarified. Therefore, this study investigated whether muscle adaptation to EE is associated with voluntary physical activity, and assessed the changes in the muscle fiber-type distribution and fiber-type-specific cross-sectional area in response to EE. Methods: Wistar rats (6 weeks of age) were randomly assigned to either the standard environment group (n = 10) or the EE group (n = 10). The voluntary physical activity of rats housed in EE conditions was measured using a recently developed three-axis accelerometer. After exposure to the standard or enriched environment for 30 days, the tibialis anterior, extensor digitorum longus, soleus, plantaris, and gastrocnemius muscles were removed and weighed. Immunohistochemistry analysis was performed on the surface (anterior) and deep (posterior) areas of the tibialis anterior and soleus muscles. Results and discussion: The EE group showed increased voluntary physical activity during the dark period compared with the standard environment group (p = 0.005). EE induced muscle mass gain in the soleus muscle (p = 0.002) and increased the slow-twitch muscle fiber cross-sectional area of the soleus muscle (p = 0.025). EE also increased the distribution of high-oxidative type IIa fibers of the surface area (p = 0.001) and type I fibers of the deep area (p = 0.037) of the tibialis anterior muscle. These findings suggest that EE is an effective approach to induce slow-twitch muscle fiber hypertrophy through increased daily voluntary physical activity.

Emergence and Progression of Behavioral Motor Deficits and Skeletal Muscle Atrophy across the Adult Lifespan of the Rat

The facultative loss of muscle mass and function during aging (sarcopenia) poses a serious threat to our independence and health. When activities of daily living are impaired (clinical phase), it appears that the processes leading to sarcopenia have been ongoing in humans for decades (preclinical phase). Here, we examined the natural history of sarcopenia in male outbred rats to compare the occurrence of motor behavioral deficits with the degree of muscle wasting and to explore the muscle-associated processes of the preclinical and clinical phases, respectively. Selected metrics were validated in female rats. We used the soleus muscle because of its long duty cycles and its importance in postural control. Results show that gait and coordination remain intact through middle age (40-60% of median lifespan) when muscle mass is largely preserved relative to body weight. However, the muscle shows numerous signs of remodeling with a shift in myofiber-type composition toward type I. As fiber-type prevalence shifted, fiber-type clustering also increased. The number of hybrid fibers, myofibers with central nuclei, and fibers expressing embryonic myosin increased from being barely detectable to a significant number (5-10%) at late middle age. In parallel, TGFβ1, Smad3, FBXO32, and MuRF1 mRNAs increased. In early (25-month-old) and advanced (30-month-old) aging, gait and coordination deteriorate with the progressive loss of muscle mass. In late middle age and early aging due to type II atrophy (>50%) followed by type I atrophy (>50%), the number of myofibers did not correlate with this process. In advanced age, atrophy is accompanied by a decrease in SCs and βCatenin mRNA, whereas several previously upregulated transcripts were downregulated. The re-expression of embryonic myosin in myofibers and the upregulation of mRNAs encoding the γ-subunit of the nicotinic acetylcholine receptor, the neuronal cell adhesion molecule, and myogenin that begins in late middle age suggest that one mechanism driving sarcopenia is the disruption of neuromuscular connectivity. We conclude that sarcopenia in rats, as in humans, has a long preclinical phase in which muscle undergoes extensive remodeling to maintain muscle mass and function. At later time points, these adaptive mechanisms fail, and sarcopenia becomes clinically manifest.

Neuromuscular recovery from botulism involves multiple forms of compensatory plasticity

Introduction

Botulinum neurotoxin (BoNT) causes neuroparalytic disease and death by blocking neuromuscular transmission. There are no specific therapies for clinical botulism and the only treatment option is supportive care until neuromuscular function spontaneously recovers, which can take weeks or months after exposure. The highly specialized neuromuscular junction (NMJ) between phrenic motor neurons and diaphragm muscle fibers is the main clinical target of BoNT. Due to the difficulty in eliciting respiratory paralysis without a high mortality rate, few studies have characterized the neurophysiological mechanisms involved in diaphragm recovery from intoxication. Here, we develop a mouse model of botulism that involves partial paralysis of respiratory muscles with low mortality rates, allowing for longitudinal analysis of recovery.

Methods and results

Mice challenged by systemic administration of 0.7 LD50 BoNT/A developed physiological signs of botulism, such as respiratory depression and reduced voluntary running activity, that persisted for an average of 8-12 d. Studies in isolated hemidiaphragm preparations from intoxicated mice revealed profound reductions in nerve-elicited, tetanic and twitch muscle contraction strengths that recovered to baseline 21 d after intoxication. Despite apparent functional recovery, neurophysiological parameters remained depressed for 28 d, including end plate potential (EPP) amplitude, EPP success rate, quantal content (QC), and miniature EPP (mEPP) frequency. However, QC recovered more quickly than mEPP frequency, which could explain the discrepancy between muscle function studies and neurophysiological recordings. Hypothesizing that differential modulation of voltage-gated calcium channels (VGCC) contributed to the uncoupling of QC from mEPP frequency, pharmacological inhibition studies were used to study the contributions of different VGCCs to neurophysiological function. We found that N-type VGCC and P/Q-type VGCC partially restored QC but not mEPP frequency during recovery from paralysis, potentially explaining the accelerated recovery of evoked release versus spontaneous release. We identified additional changes that presumably compensate for reduced acetylcholine release during recovery, including increased depolarization of muscle fiber resting membrane potential and increased quantal size.

Discussion

In addition to identifying multiple forms of compensatory plasticity that occur in response to reduced NMJ function, it is expected that insights into the molecular mechanisms involved in recovery from neuromuscular paralysis will support new host-targeted treatments for multiple neuromuscular diseases.

Effects of short-term unloading and active recovery on human motor unit properties, neuromuscular junction transmission and transcriptomic profile

Electrophysiological alterations of the neuromuscular junction (NMJ) and motor unit potential (MUP) with unloading are poorly studied. We aimed to investigate these aspects and the underlying molecular mechanisms with short-term unloading and active recovery (AR). Eleven healthy males underwent a 10-day unilateral lower limb suspension (ULLS) period, followed by 21-day AR based on resistance exercise. Quadriceps femoris (QF) cross-sectional area (CSA) and isometric maximum voluntary contraction (MVC) were evaluated. Intramuscular electromyographic recordings were obtained during 10% and 25% MVC isometric contractions from the vastus lateralis (VL). Biomarkers of NMJ molecular instability (serum c-terminal agrin fragment, CAF), axonal damage (neurofilament light chain) and denervation status were assessed from blood samples and VL biopsies. NMJ and ion channel transcriptomic profiles were investigated by RNA-sequencing. QF CSA and MVC decreased with ULLS. Increased CAF and altered NMJ transcriptome with unloading suggested the emergence of NMJ molecular instability, which was not associated with impaired NMJ transmission stability. Instead, increased MUP complexity and decreased motor unit firing rates were found after ULLS. Downregulation of ion channel gene expression was found together with increased neurofilament light chain concentration and partial denervation. The AR period restored most of these neuromuscular alterations. In conclusion, the human NMJ is destabilized at the molecular level but shows functional resilience to a 10-day unloading period at least at relatively low contraction intensities. However, MUP properties are altered by ULLS, possibly due to alterations in ion channel dynamics and initial axonal damage and denervation. These changes are fully reversed by 21 days of AR. KEY POINTS: We used integrative electrophysiological and molecular approaches to comprehensively investigate changes in neuromuscular integrity and function after a 10-day unilateral lower limb suspension (ULLS), followed by 21 days of active recovery in young healthy men, with a particular focus on neuromuscular junction (NMJ) and motor unit potential (MUP) properties alterations. After 10-day ULLS, we found significant NMJ molecular alterations in the absence of NMJ transmission stability impairment. These findings suggest that the human NMJ is functionally resilient against insults and stresses induced by short-term disuse at least at relatively low contraction intensities, at which low-threshold, slow-type motor units are recruited. Intramuscular electromyography analysis revealed that unloading caused increased MUP complexity and decreased motor unit firing rates, and these alterations could be related to the observed changes in skeletal muscle ion channel pool and initial and partial signs of fibre denervation and axonal damage. The active recovery period restored these neuromuscular changes.

Metabolic Pathways and Ion Channels Involved in Skeletal Muscle Atrophy: A Starting Point for Potential Therapeutic Strategies

Skeletal muscle tissue has the important function of supporting and defending the organism. It is the largest apparatus in the human body, and its function is important for contraction and movements. In addition, it is involved in the regulation of protein synthesis and degradation. In fact, inhibition of protein synthesis and/or activation of catabolism determines a pathological condition called muscle atrophy. Muscle atrophy is a reduction in muscle mass resulting in a partial or complete loss of function. It has been established that many physiopathological conditions can cause a reduction in muscle mass. Nevertheless, it is not well known that the molecular mechanisms and signaling processes caused this dramatic event. There are multiple concomitant processes involved in muscle atrophy. In fact, the gene transcription of some factors, oxidative stress mechanisms, and the alteration of ion transport through specific ion channels may contribute to muscle function impairment. In this review, we focused on the molecular mechanisms responsible for muscle damage and potential drugs to be used to alleviate this disabling condition.

New Challenges Resulting From the Loss of Function of Nav1.4 in Neuromuscular Diseases

The voltage-gated sodium channel Na_v1.4 is a major actor in the excitability of skeletal myofibers, driving the muscle force in response to nerve stimulation. Supporting further this key role, mutations in SCN4A, the gene encoding the pore-forming α subunit of Na_v1.4, are responsible for a clinical spectrum of human diseases ranging from muscle stiffness (sodium channel myotonia, SCM) to muscle weakness. For years, only dominantly-inherited diseases resulting from Na_v1.4 gain of function (GoF) were known, i.e., non-dystrophic myotonia (delayed muscle relaxation due to myofiber hyperexcitability), paramyotonia congenita and hyperkalemic or hypokalemic periodic paralyses (episodic flaccid muscle weakness due to transient myofiber hypoexcitability). These last 5 years, SCN4A mutations inducing Na_v1.4 loss of function (LoF) were identified as the cause of dominantly and recessively-inherited disorders with muscle weakness: periodic paralyses with hypokalemic attacks, congenital myasthenic syndromes and congenital myopathies. We propose to name this clinical spectrum sodium channel weakness (SCW) as the mirror of SCM. Na_v1.4 LoF as a cause of permanent muscle weakness was quite unexpected as the Na⁺ current density in the sarcolemma is large, securing the ability to generate and propagate muscle action potentials. The properties of SCN4A LoF mutations are well documented at the channel level in cellular electrophysiological studies However, much less is known about the functional consequences of Na_v1.4 LoF in skeletal myofibers with no available pertinent cell or animal models. Regarding the therapeutic issues for Na_v1.4 channelopathies, former efforts were aimed at developing subtype-selective Na_v channel antagonists to block myofiber hyperexcitability. Non-selective, Na_v channel blockers are clinically efficient in SCM and paramyotonia congenita, whereas patient education and carbonic anhydrase inhibitors are helpful to prevent attacks in periodic paralyses. Developing therapeutic tools able to counteract Na_v1.4 LoF in skeletal muscles is then a new challenge in the field of Na_v channelopathies. Here, we review the current knowledge regarding Na_v1.4 LoF and discuss the possible therapeutic strategies to be developed in order to improve muscle force in SCW.

BCAAs and Di-Alanine supplementation in the prevention of skeletal muscle atrophy: preclinical evaluation in a murine model of hind limb unloading

Skeletal muscle atrophy occurs in response to various pathophysiological stimuli, including disuse, aging, and neuromuscular disorders, mainly due to an imbalance of anabolic/catabolic signaling. Branched Chain Amino Acids (BCAAs: leucine, isoleucine, valine) supplements can be beneficial for counteracting muscle atrophy, in virtue of their reported anabolic properties. Here, we carried out a proof-of-concept study to assess the in vivo/ex vivo effects of a 4-week treatment with BCAAs on disuse-induced atrophy, in a murine model of hind limb unloading (HU). BCAAs were formulated in drinking water, alone, or plus two equivalents of L-Alanine (2 ALA) or the dipeptide L-Alanyl-L-Alanine (Di-ALA), to boost BCAAs bioavailability. HU mice were characterized by reduction of body mass, decrease of soleus - SOL - muscle mass and total protein, alteration of postural muscles architecture and fiber size, dysregulation of atrophy-related genes (Atrogin-1, MuRF-1, mTOR, Mstn). In parallel, we provided new robust readouts in the HU murine model, such as impaired in vivo isometric torque and ex vivo SOL muscle contractility and elasticity, as well as altered immune response. An acute pharmacokinetic study confirmed that L-ALA, also as dipeptide, enhanced plasma exposure of BCAAs. Globally, the most sensitive parameters to BCAAs action were muscle atrophy and myofiber cross-sectional area, muscle force and compliance to stress, protein synthesis via mTOR and innate immunity, with the new BCAAs + Di-ALA formulation being the most effective treatment. Our results support the working hypothesis and highlight the importance of developing innovative formulations to optimize BCAAs biodistribution.

Master Regulators of Muscle Atrophy: Role of Costamere Components

The loss of muscle mass and force characterizes muscle atrophy in several different conditions, which share the expression of atrogenes and the activation of their transcriptional regulators. However, attempts to antagonize muscle atrophy development in different experimental contexts by targeting contributors to the atrogene pathway showed partial effects in most cases. Other master regulators might independently contribute to muscle atrophy, as suggested by our recent evidence about the co-requirement of the muscle-specific chaperone protein melusin to inhibit unloading muscle atrophy development. Furthermore, melusin and other muscle mass regulators, such as nNOS, belong to costameres, the macromolecular complexes that connect sarcolemma to myofibrils and to the extracellular matrix, in correspondence with specific sarcomeric sites. Costameres sense a mechanical load and transduce it both as lateral force and biochemical signals. Recent evidence further broadens this classic view, by revealing the crucial participation of costameres in a sarcolemmal “signaling hub” integrating mechanical and humoral stimuli, where mechanical signals are coupled with insulin and/or insulin-like growth factor stimulation to regulate muscle mass. Therefore, this review aims to enucleate available evidence concerning the early involvement of costamere components and additional putative master regulators in the development of major types of muscle atrophy.

Short-Term Sensorimotor Deprivation Impacts Feedforward and Feedback Processes of Motor Control

Sensory loss involves irreversible behavioral and neural changes. Paradigms of short-term limb immobilization mimic deprivation of proprioceptive inputs and motor commands, which occur after the loss of limb use. While several studies have shown that short-term immobilization induced motor control impairments, the origin of such modifications is an open question. A Fitts’ pointing task was conducted, and kinematic analyses were performed to assess whether the feedforward and/or feedback processes of motor control were impacted. The Fitts’ pointing task specifically required dealing with spatial and temporal aspects (speed-accuracy trade-off) to be as fast and as accurate as possible. Forty trials were performed on two consecutive days by Control and Immobilized participants who wore a splint on the right arm during this 24 h period. The immobilization modified the motor control in a way that the full spatiotemporal structure of the pointing movements differed: A global slowdown appeared. The acceleration and deceleration phases were both longer, suggesting that immobilization impacted both the early impulse phase based on sensorimotor expectations and the later online correction phase based on feedback use. First, the feedforward control may have been less efficient, probably because the internal model of the immobilized limb would have been incorrectly updated relative to internal and environmental constraints. Second, immobilized participants may have taken more time to correct their movements and precisely reach the target, as the processing of proprioceptive feedback might have been altered.

Effects of Resting, Consecutive, Long-Duration Water Immersions on Neuromuscular Endurance in Well-Trained Males

Purpose: This study examined the effects of repeated long-duration water immersions (WI)s at 1.35 atmospheres absolute (ATA) on neuromuscular endurance performance. We hypothesized that, following 5 days of consecutive, resting, long-duration WIs, neuromuscular endurance performance would decrease.

Methods: Fifteen well-trained, male subjects completed five consecutive 6-h resting WIs with 18-h surface intervals during the dive week while breathing compressed air at 1.35 ATA. Skeletal muscle endurance performance was assessed before and after each WI, and 24 and 72 h after the final WI. Muscular endurance assessments included 40% maximum handgrip endurance (MHE) and 50-repetition maximal isokinetic knee extensions. Near infrared spectroscopy was used to measure muscle oxidative capacity of the vastus lateralis and localized muscle tissue oxygenation of the vastus lateralis and flexor carpi radialis. Simultaneously, brachioradialis neuromuscular activation was measured by surface electromyography.

Results: A 24.9% increase (p = 0.04) in the muscle oxidative capacity rate constant (k) occurred on WI 4 compared to baseline. No changes occurred in 40% MHE time to exhaustion or rate of fatigue or total work performed for the 50-repetition maximal isokinetic knee extension. The first quartile of deoxygenated hemoglobin concentration showed a 6 and 35% increase on WIs 3 and 5 (p = 0.026) with second quartile increases of 9 and 32% on WIs 3 and 5 (p = 0.049) during the 40% MHE testing when compared to WI 1.

Conclusion: Our specific WI protocol resulted in no change to muscular endurance and oxygen kinetics in load bearing and non-load bearing muscles.

Effects of repeated long-duration water immersions on skeletal muscle performance in well-trained male divers

PURPOSE

The objective of this study was to examine the effects of repeated long-duration water immersions (WI)s at 1.35 atmospheres absolute (ATA) on neuromuscular performance in load bearing and non-load bearing muscle groups.

METHODS

During a dive week (DW), fifteen well-trained male divers completed five consecutive 6-h resting dives with 18-h surface intervals while breathing compressed air at 1.35 ATA. Skeletal muscle performance assessments occurred immediately before and after each WI, and 24 and 72 h after the final WI. Exercise assessments included maximum voluntary isometric contraction (MVIC), maximal isokinetic (IK) contraction, maximum handgrip strength (MHG). Surface electromyography measured neuromuscular activation of the quadriceps, biceps brachii (BB), and brachioradialis.

RESULTS

MVIC torque of knee extensors and BB decreased by 6% (p = 0.001) and 2% (p = 0.014), respectively, by WI 3. Maximal IK torque of knee extensors increased by 11 and 5% post-WI on WIs 3 and 5 (p < 0.001) with greater neuromuscular activation post-WI than pre-WI (p < 0.001). Maximum IK elbow flexion torque did not change throughout the DW with BB neuromuscular activation greater post-WI than pre-WI (p < 0.001). MHG force output was 4% greater post-WI than pre-WI (p < 0.001) with increased brachioradialis activation through 72-h post-WI (p < 0.001). All muscle performance metrics returned baseline levels by 72-h post-WI.

CONCLUSION

Our findings indicate that repeated WIs caused noticeable decrements in neuromuscular activation and performance of load bearing muscles on WI 3 while full recovery was observed by 72-h post-WI.

Membrane lipid rafts are disturbed in the response of rat skeletal muscle to short-term disuse

Marked loss of skeletal muscle mass occurs under various conditions of disuse, but the molecular and cellular mechanisms leading to atrophy are not completely understood. We investigate early molecular events that might play a role in skeletal muscle remodeling during mechanical unloading (disuse). The effects of acute (6-12 h) hindlimb suspension on the soleus muscles from adult rats were examined. The integrity of plasma membrane lipid rafts was tested utilizing cholera toxin B subunit or fluorescent sterols. In addition, resting intracellular Ca²⁺ level was analyzed. Acute disuse disturbed the plasma membrane lipid-ordered phase throughout the sarcolemma and was more pronounced in junctional membrane regions. Ouabain (1 µM), which specifically inhibits the Na-K-ATPase α2 isozyme in rodent skeletal muscles, produced similar lipid raft changes in control muscles but was ineffective in suspended muscles, which showed an initial loss of α2 Na-K-ATPase activity. Lipid rafts were able to recover with cholesterol supplementation, suggesting that disturbance results from cholesterol loss. Repetitive nerve stimulation also restores lipid rafts, specifically in the junctional sarcolemma region. Disuse locally lowered the resting intracellular Ca²⁺ concentration only near the neuromuscular junction of muscle fibers. Our results provide evidence to suggest that the ordering of lipid rafts strongly depends on motor nerve input and may involve interactions with the α2 Na-K-ATPase. Lipid raft disturbance, accompanied by intracellular Ca²⁺ dysregulation, is among the earliest remodeling events induced by skeletal muscle disuse.

Specialized Functional Diversity and Interactions of the Na,K-ATPase

Na,K-ATPase is a protein ubiquitously expressed in the plasma membrane of all animal cells and vitally essential for their functions. A specialized functional diversity of the Na,K-ATPase isozymes is provided by molecular heterogeneity, distinct subcellular localizations, and functional interactions with molecular environment. Studies over the last decades clearly demonstrated complex and isoform-specific reciprocal functional interactions between the Na,K-ATPase and neighboring proteins and lipids. These interactions are enabled by a spatially restricted ion homeostasis, direct protein-protein/lipid interactions, and protein kinase signaling pathways. In addition to its "classical" function in ion translocation, the Na,K-ATPase is now considered as one of the most important signaling molecules in neuronal, epithelial, skeletal, cardiac and vascular tissues. Accordingly, the Na,K-ATPase forms specialized sub-cellular multimolecular microdomains which act as receptors to circulating endogenous cardiotonic steroids (CTS) triggering a number of signaling pathways. Changes in these endogenous cardiotonic steroid levels and initiated signaling responses have significant adaptive values for tissues and whole organisms under numerous physiological and pathophysiological conditions. This review discusses recent progress in the studies of functional interactions between the Na,K-ATPase and molecular microenvironment, the Na,K-ATPase-dependent signaling pathways and their significance for diversity of cell function.

In vivo longitudinal study of rodent skeletal muscle atrophy using ultrasonography

Muscle atrophy is a widespread ill condition occurring in many diseases, which can reduce quality of life and increase morbidity and mortality. We developed a new method using non-invasive ultrasonography to measure soleus and gastrocnemius lateralis muscle atrophy in the hindlimb-unloaded rat, a well-accepted model of muscle disuse. Soleus and gastrocnemius volumes were calculated using the conventional truncated-cone method and a newly-designed sinusoidal method. For Soleus muscle, the ultrasonographic volume determined in vivo with either method was linearly correlated to the volume determined ex-vivo from excised muscles as muscle weight-to-density ratio. For both soleus and gastrocnemius muscles, a strong linear correlation was obtained between the ultrasonographic volume and the muscle fiber cross-sectional area determined ex-vivo on muscle cryosections. Thus ultrasonography allowed the longitudinal in vivo evaluation of muscle atrophy progression during hindlimb unloading. This study validates ultrasonography as a powerful method for the evaluation of rodent muscle atrophy in vivo, which would prove useful in disease models and therapeutic trials.

Distinct α2 Na,K-ATPase membrane pools are differently involved in early skeletal muscle remodeling during disuse

Location, location, location. The Na-K pump of skeletal muscle is regulated differently at neuromuscular junctions.

The Na,K-ATPase is essential for the contractile function of skeletal muscle, which expresses the α1 and α2 subunit isoforms of Na,K-ATPase. The α2 isozyme is predominant in adult skeletal muscles and makes a greater contribution in working compared with noncontracting muscles. Hindlimb suspension (HS) is a widely used model of muscle disuse that leads to progressive atrophy of postural skeletal muscles. This study examines the consequences of acute (6–12 h) HS on the functioning of the Na,K-ATPase α1 and α2 isozymes in rat soleus (disused) and diaphragm (contracting) muscles. Acute disuse dynamically and isoform-specifically regulates the electrogenic activity, protein, and mRNA content of Na,K-ATPase α2 isozyme in rat soleus muscle. Earlier disuse-induced remodeling events also include phospholemman phosphorylation as well as its increased abundance and association with α2 Na,K-ATPase. The loss of α2 Na,K-ATPase activity results in reduced electrogenic pump transport and depolarized resting membrane potential. The decreased α2 Na,K-ATPase activity is caused by a decrease in enzyme activity rather than by altered protein and mRNA content, localization in the sarcolemma, or functional interaction with the nicotinic acetylcholine receptors. The loss of extrajunctional α2 Na,K-ATPase activity depends strongly on muscle use, and even the increased protein and mRNA content as well as enhanced α2 Na,K-ATPase abundance at this membrane region after 12 h of HS cannot counteract this sustained inhibition. In contrast, additional factors may regulate the subset of junctional α2 Na,K-ATPase pool that is able to recover during HS. Notably, acute, low-intensity muscle workload restores functioning of both α2 Na,K-ATPase pools. These results demonstrate that the α2 Na,K-ATPase in rat skeletal muscle is dynamically and acutely regulated by muscle use and provide the first evidence that the junctional and extrajunctional pools of the α2 Na,K-ATPase are regulated differently.

Taurine: the appeal of a safe amino acid for skeletal muscle disorders

Taurine is a natural amino acid present as free form in many mammalian tissues and in particular in skeletal muscle. Taurine exerts many physiological functions, including membrane stabilization, osmoregulation and cytoprotective effects, antioxidant and anti-inflammatory actions as well as modulation of intracellular calcium concentration and ion channel function. In addition taurine may control muscle metabolism and gene expression, through yet unclear mechanisms. This review summarizes the effects of taurine on specific muscle targets and pathways as well as its therapeutic potential to restore skeletal muscle function and performance in various pathological conditions. Evidences support the link between alteration of intracellular taurine level in skeletal muscle and different pathophysiological conditions, such as disuse-induced muscle atrophy, muscular dystrophy and/or senescence, reinforcing the interest towards its exogenous supplementation. In addition, taurine treatment can be beneficial to reduce sarcolemmal hyper-excitability in myotonia-related syndromes. Although further studies are necessary to fill the gaps between animals and humans, the benefit of the amino acid appears to be due to its multiple actions on cellular functions while toxicity seems relatively low. Human clinical trials using taurine in various pathologies such as diabetes, cardiovascular and neurological disorders have been performed and may represent a guide-line for designing specific studies in patients of neuromuscular diseases.

Effects of Nandrolone in the Counteraction of Skeletal Muscle Atrophy in a Mouse Model of Muscle Disuse: Molecular Biology and Functional Evaluation

Muscle disuse produces severe atrophy and a slow-to-fast phenotype transition in the postural Soleus (Sol) muscle of rodents. Antioxidants, amino-acids and growth factors were ineffective to ameliorate muscle atrophy. Here we evaluate the effects of nandrolone (ND), an anabolic steroid, on mouse skeletal muscle atrophy induced by hindlimb unloading (HU). Mice were pre-treated for 2-weeks before HU and during the 2-weeks of HU. Muscle weight and total protein content were reduced in HU mice and a restoration of these parameters was found in ND-treated HU mice. The analysis of gene expression by real-time PCR demonstrates an increase of MuRF-1 during HU but minor involvement of other catabolic pathways. However, ND did not affect MuRF-1 expression. The evaluation of anabolic pathways showed no change in mTOR and eIF2-kinase mRNA expression, but the protein expression of the eukaryotic initiation factor eIF2 was reduced during HU and restored by ND. Moreover we found an involvement of regenerative pathways, since the increase of MyoD observed after HU suggests the promotion of myogenic stem cell differentiation in response to atrophy. At the same time, Notch-1 expression was down-regulated. Interestingly, the ND treatment prevented changes in MyoD and Notch-1 expression. On the contrary, there was no evidence for an effect of ND on the change of muscle phenotype induced by HU, since no effect of treatment was observed on the resting gCl, restCa and contractile properties in Sol muscle. Accordingly, PGC1α and myosin heavy chain expression, indexes of the phenotype transition, were not restored in ND-treated HU mice. We hypothesize that ND is unable to directly affect the phenotype transition when the specialized motor unit firing pattern of stimulation is lacking. Nevertheless, through stimulation of protein synthesis, ND preserves protein content and muscle weight, which may result advantageous to the affected skeletal muscle for functional recovery.

Clinical, Molecular, and Functional Characterization of CLCN1 Mutations in Three Families with Recessive Myotonia Congenita

Myotonia congenita (MC) is an inherited muscle disease characterized by impaired muscle relaxation after contraction, resulting in muscle stiffness. Both recessive (Becker’s disease) or dominant (Thomsen’s disease) MC are caused by mutations in the CLCN1 gene encoding the voltage-dependent chloride ClC-1 channel, which is quite exclusively expressed in skeletal muscle. More than 200 CLCN1 mutations have been associated with MC. We provide herein a detailed clinical, molecular, and functional evaluation of four patients with recessive MC belonging to three different families. Four CLCN1 variants were identified, three of which have never been characterized. The c.244A>G (p.T82A) and c.1357C>T (p.R453W) variants were each associated in compound heterozygosity with c.568GG>TC (p.G190S), for which pathogenicity is already known. The new c.809G>T (p.G270V) variant was found in the homozygous state. Patch-clamp studies of ClC-1 mutants expressed in tsA201 cells confirmed the pathogenicity of p.G270V, which greatly shifts the voltage dependence of channel activation toward positive potentials. Conversely, the mechanisms by which p.T82A and p.R453W cause the disease remained elusive, as the mutated channels behave similarly to WT. The results also suggest that p.G190S does not exert dominant-negative effects on other mutated ClC-1 subunits. Moreover, we performed a RT-PCR quantification of selected ion channels transcripts in muscle biopsies of two patients. The results suggest gene expression alteration of sodium and potassium channel subunits in myotonic muscles; if confirmed, such analysis may pave the way toward a better understanding of disease phenotype and a possible identification of new therapeutic options.

Are movements necessary for the sense of body ownership? Evidence from the rubber hand illusion in pure hemiplegic patients

A question still debated within cognitive neuroscience is whether signals present during actions significantly contribute to the emergence of human’s body ownership. In the present study, we aimed at answer this question by means of a neuropsychological approach. We administered the classical rubber hand illusion paradigm to a group of healthy participants and to a group of neurological patients affected by a complete left upper limb hemiplegia, but without any propriceptive/tactile deficits. The illusion strength was measured both subjectively (i.e., by a self-report questionnaire) and behaviorally (i.e., the location of one’s own hand is shifted towards the rubber hand). We aimed at examining whether, and to which extent, an enduring absence of movements related signals affects body ownership. Our results showed that patients displayed, respect to healthy participants, stronger illusory effects when the left (affected) hand was stimulated and no effects when the right (unaffected) hand was stimulated. In other words, hemiplegics had a weaker/more flexible sense of body ownership for the affected hand, but an enhanced/more rigid one for the healthy hand. Possible interpretations of such asymmetrical distribution of body ownership, as well as limits of our results, are discussed. Broadly speaking, our findings suggest that the alteration of the normal flow of signals present during movements impacts on human’s body ownership. This in turn, means that movements have a role per se in developing and maintaining a coherent body ownership.

The effects of knee injury on skeletal muscle function, Na+, K+-ATPase content, and isoform abundance

While training upregulates skeletal muscle Na(+), K(+)-ATPase (NKA), the effects of knee injury and associated disuse on muscle NKA remain unknown. This was therefore investigated in six healthy young adults with a torn anterior cruciate ligament, (KI; four females, two males; age 25.0 ± 4.9 years; injury duration 15 ± 17 weeks; mean ± SD) and seven age- and BMI-matched asymptomatic controls (CON; five females, two males). Each participant underwent a vastus lateralis muscle biopsy, on both legs in KI and one leg in CON. Muscle was analyzed for muscle fiber type and cross-sectional area (CSA), NKA content ([(3)H]ouabain binding), and α1-3 and β1-2 isoform abundance. Participants also completed physical activity and knee function questionnaires (KI only); and underwent quadriceps peak isometric strength, thigh CSA and postural sway assessments in both injured and noninjured legs. NKA content was 20.1% lower in the knee-injured leg than the noninjured leg and 22.5% lower than CON. NKA α2 abundance was 63.0% lower in the knee-injured leg than the noninjured leg, with no differences in other NKA isoforms. Isometric strength and thigh CSA were 21.7% and 7.1% lower in the injured leg than the noninjured leg, respectively. In KI, postural sway did not differ between legs, but for two-legged standing was 43% higher than CON. Hence, muscle NKA content and α2 abundance were reduced in severe knee injury, which may contribute to impaired muscle function. Restoration of muscle NKA may be important in rehabilitation of muscle function after knee and other lower limb injury.

Isoform-specific Na,K-ATPase alterations precede disuse-induced atrophy of rat soleus muscle

This study examines the isoform-specific effects of short-term hindlimb suspension (HS) on the Na,K-ATPase in rat soleus muscle. Rats were exposed to 24–72 h of HS and we analyzed the consequences on soleus muscle mass and contractile parameters; excitability and the resting membrane potential (RMP) of muscle fibers; the electrogenic activity, protein, and mRNA content of the α1 and α2 Na,K-ATPase; the functional activity and plasma membrane localization of the α2 Na,K-ATPase. Our results indicate that 24–72 h of HS specifically decreases the electrogenic activity of the Na,K-ATPase α2 isozyme and the RMP of soleus muscle fibers. This decrease occurs prior to muscle atrophy or any change in contractile parameters. The α2 mRNA and protein content increased after 24 h of HS and returned to initial levels at 72 h; however, even the increased content was not able to restore α2 enzyme activity in the disused soleus muscle. There was no change in the membrane localization of α2 Na,K-ATPase. The α1 Na,K-ATPase electrogenic activity, protein and mRNA content did not change. Our findings suggest that skeletal muscle use is absolutely required for α2 Na,K-ATPase transport activity and provide the first evidence that Na,K-ATPase alterations precede HS-induced muscle atrophy.

Action potential-evoked calcium release is impaired in single skeletal muscle fibers from heart failure patients

BACKGROUND

Exercise intolerance in chronic heart failure (HF) has been attributed to abnormalities of the skeletal muscles. Muscle function depends on intact excitation-contraction coupling (ECC), but ECC studies in HF models have been inconclusive, due to deficiencies in the animal models and tools used to measure calcium (Ca2+) release, mandating investigations in skeletal muscle from HF patients. The purpose of this study was to test the hypothesis that Ca2+ release is significantly impaired in the skeletal muscle of HF patients in whom exercise capacity is severely diminished compared to age-matched healthy volunteers.

METHODS AND FINDINGS

Using state-of-the-art electrophysiological and optical techniques in single muscle fibers from biopsies of the locomotive vastus lateralis muscle, we measured the action potential (AP)-evoked Ca2+ release in 4 HF patients and 4 age-matched healthy controls. The mean peak Ca2+ release flux in fibers obtained from HF patients (10±1.2 µM/ms) was markedly (2.6-fold) and significantly (p<0.05) smaller than in fibers from healthy volunteers (28±3.3 µM/ms). This impairment in AP-evoked Ca2+ release was ubiquitous and was not explained by differences in the excitability mechanisms since single APs were indistinguishable between HF patients and healthy volunteers.

CONCLUSIONS

These findings prove the feasibility of performing electrophysiological experiments in single fibers from human skeletal muscle, and offer a new approach for investigations of myopathies due to HF and other diseases. Importantly, we have demonstrated that one step in the ECC process, AP-evoked Ca2+ release, is impaired in single muscle fibers in HF patients.

Protein kinase C theta (PKCθ) modulates the ClC-1 chloride channel activity and skeletal muscle phenotype: a biophysical and gene expression study in mouse models lacking the PKCθ

In skeletal muscle, the resting chloride conductance (gCl), due to the ClC-1 chloride channel, controls the sarcolemma electrical stability. Indeed, loss-of-function mutations in ClC-1 gene are responsible of myotonia congenita. The ClC-1 channel can be phosphorylated and inactivated by protein kinases C (PKC), but the relative contribution of each PKC isoforms is unknown. Here, we investigated on the role of PKCθ in the regulation of ClC-1 channel expression and activity in fast- and slow-twitch muscles of mouse models lacking PKCθ. Electrophysiological studies showed an increase of gCl in the PKCθ-null mice with respect to wild type. Muscle excitability was reduced accordingly. However, the expression of the ClC-1 channel, evaluated by qRT-PCR, was not modified in PKCθ-null muscles suggesting that PKCθ affects the ClC-1 activity. Pharmacological studies demonstrated that although PKCθ appreciably modulates gCl, other isoforms are still active and concur to this role. The modification of gCl in PKCθ-null muscles has caused adaptation of the expression of phenotype-specific genes, such as calcineurin and myocyte enhancer factor-2, supporting the role of PKCθ also in the settings of muscle phenotype. Importantly, the lack of PKCθ has prevented the aging-related reduction of gCl, suggesting that its modulation may represent a new strategy to contrast the aging process.

Paracrine effects of IGF-1 overexpression on the functional decline due to skeletal muscle disuse: molecular and functional evaluation in hindlimb unloaded MLC/mIgf-1 transgenic mice

Slow-twitch muscles, devoted to postural maintenance, experience atrophy and weakness during muscle disuse due to bed-rest, aging or spaceflight. These conditions impair motion activities and can have survival implications. Human and animal studies demonstrate the anabolic role of IGF-1 on skeletal muscle suggesting its interest as a muscle disuse countermeasure. Thus, we tested the role of IGF-1 overexpression on skeletal muscle alteration due to hindlimb unloading (HU) by using MLC/mIgf-1 transgenic mice expressing IGF-1 under the transcriptional control of MLC promoter, selectively activated in skeletal muscle. HU produced atrophy in soleus muscle, in terms of muscle weight and fiber cross-sectional area (CSA) reduction, and up-regulation of atrophy gene MuRF1. In parallel, the disuse-induced slow-to-fast fiber transition was confirmed by an increase of the fast-type of the Myosin Heavy Chain (MHC), a decrease of PGC-1α expression and an increase of histone deacetylase-5 (HDAC5). Consistently, functional parameters such as the resting chloride conductance (gCl) together with ClC-1 chloride channel expression were increased and the contractile parameters were modified in soleus muscle of HU mice. Surprisingly, IGF-1 overexpression in HU mice was unable to counteract the loss of muscle weight and the decrease of fiber CSA. However, the expression of MuRF1 was recovered, suggesting early effects on muscle atrophy. Although the expression of PGC-1α and MHC were not improved in IGF-1-HU mice, the expression of HDAC5 was recovered. Importantly, the HU-induced increase of gCl was fully contrasted in IGF-1 transgenic mice, as well as the changes in contractile parameters. These results indicate that, even if local expression does not seem to attenuate HU-induced atrophy and slow-to-fast phenotype transition, it exerts early molecular effects on gene expression which can counteract the HU-induced modification of electrical and contractile properties. MuRF1 and HDAC5 can be attractive therapeutic targets for pharmacological countermeasures and then deserve further investigations.

Effects of lactate on the voltage-gated sodium channels of rat skeletal muscle: modulating current opinion

During muscle contraction, lactate production and translocation across the membrane increase. While it has recently been shown that lactate anion acts on chloride channel, less is known regarding a potential effect on the voltage-gated sodium channel (Nav) of skeletal muscle. The electrophysiological properties of muscle Nav were studied in the absence and presence of lactate (10 mM) by using the macropatch-clamp method in dissociated fibers from rat peroneus longus (PL). Lactate in the external medium (petri dish + pipette) increases the maximal sodium current, while the voltage dependence of activation and fast inactivation are shifted toward the hyperpolarized potentials. Lactate induces a leftward shift in the relationship between the kinetic parameters and the imposed potentials, resulting in an earlier recruitment of muscle Nav. In addition, lactate significantly decreases the time constant of activation at voltages more negative than −10 mV, corresponding to an acceleration of Nav activation. The slow inactivation process is decreased by lactate, corresponding to an enhancement in the number of excitable Nav. In an additional series of experiments, lactate (10 mM) was only added to the petri dish, while the pipette remained sealed on the membrane area. With this approach, the electrophysiological properties of Nav were unaffected by lactate compared with the control condition. Altogether, these data indicate that lactate modulates muscle Nav properties by an extracellular pathway. These effects are consistent with an enhancement in excitability, providing new insights into the role of lactate in muscle physiology.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Na v1.4 and Na v1.5 are modulated differently during muscle immobilization and contractile phenotype conversion

Muscle immobilization leads to modification in its fast/slow contractile phenotype. Since the properties of voltage-gated sodium channels (Na(v)) are different between "fast" and "slow" muscles, we studied the effects of immobilization on the contractile properties and the Na(v) of rat peroneus longus (PL). The distal tendon of PL was cut and fixed to the adjacent bone at neutral muscle length. After 4 or 8 wk of immobilization, the contractile and the Na(v) properties were studied and compared with muscles from control animals (Student's t-test). After 4 wk of immobilization, PL showed a faster phenotype with a rightward shift of the force-frequency curve and a decrease in both the Burke's index of fatigability and the tetanus-to-twitch ratio. These parameters showed opposite changes between 4 and 8 wk of immobilization. The maximal sodium current in 4-wk immobilized fibers was higher compared with that of control fibers (11.5 ± 1.2 vs. 7.8 ± 0.8 nA, P = 0.008), with partial recovery to the control values in 8-wk immobilized fibers (8.6 ± 0.7 nA, P = 0.48). In the presence of tetrodotoxin, the maximal residual sodium current decreased continuously throughout immobilization. Using the Western blot analysis, Na(v)1.4 expression showed a transient increase in 4-wk muscle, whereas Na(v)1.5 expression decreased during immobilization. Our results indicate that a muscle immobilized at optimal functional length with the preservation of neural inputs exhibits a transient fast phenotype conversion. Na(v)1.4 expression and current are related to the contractile phenotype variation.

Gating behaviour of sodium currents in adult mouse muscle recorded with an improved two-electrode voltage clamp

Muscle contraction is triggered by the spread of an action potential along the fibre. The ionic current to generate the action potential is conducted through voltage-activated sodium channels, and mutations of these channels are known to cause several human muscle disorders. Mouse models have been created by introducing point mutations into the sodium channel gene. This achievement has created the need for a high-fidelity technique to record sodium currents from intact mouse muscle fibres. We have optimized a two-electrode voltage clamp, using sharp microelectrodes to preserve the myoplasmic contents. The voltage-dependent behaviour of sodium channel activation, inactivation and slow-inactivation were characterized. The voltage range for these gating behaviours was remarkably hyperpolarized, in comparison to studies in artificial expression systems. These results provide normative data for sodium channels natively expressed in mouse muscle and illustrate the need to modify model simulations of muscle excitability to account for the hyperpolarized shift.

Influence of 14-day hind limb unloading on isolated muscle spindle activity in rats

During hind limb unloading (HU), the soleus is often in a shortened position and the natural physiological stimulus of muscle spindles is altered, such that muscle spindle activity also changes. Using isolated spindle conditions, the present study investigates the electrophysiological activity and ultrastructure of muscle spindles following HU. Results show that muscle spindle discharges fall into either of two main patterns, single spikes or spike clusters in shortened positions, with a steady frequency of 18-38 spikes/s (mean 29.08 +/- 2.45) in an extended position. Following 14-day HU, afferent discharge activity was significantly altered in soleus muscle spindles. Duration of individual spikes was significantly prolonged, from 0.54 +/- 0.05 ms for control rats to 1.53 +/- 0.25 ms for rats in the HU group. In a shortened position, regular rhythm afferent discharges were obviously depressed, and the majority of muscle spindles became silent, while in an extended position, the discharges remained continuous but with decreased frequency. Results also show that the ultrastructure of muscle spindles experience degenerative changes during HU. Altered muscle spindle afference could possibly modify the activity of motor neurons and further affect the activity of extrafusal fibers.

Sodium channel Na(V)1.5 expression is enhanced in cultured adult rat skeletal muscle fibers

This study analyzes changes in the distribution, electrophysiological properties, and proteic composition of voltage-gated sodium channels (Na(V)) in cultured adult rat skeletal muscle fibers. Patch clamp and molecular biology techniques were carried out in flexor digitorum brevis (FDB) adult rat skeletal muscle fibers maintained in vitro after cell dissociation with collagenase. After 4 days of culture, an increase of the Na(V)1.5 channel type was observed. This was confirmed by an increase in TTX-resistant channels and by Western blot test. These channels exhibited increased activation time constant (tau(m)) and reduced conductance, similar to what has been observed in denervated muscles in vivo, where the density of Na(V)1.5 was increasing progressively after denervation. By real-time polymerase chain reaction, we found that the expression of beta subunits was also modified, but only after 7 days of culture: increase in beta(1) without beta(4) modifications. beta(1) subunit is known to induce a negative shift of the inactivation curve, thus reducing current amplitude and duration. At day 7, tau(h) was back to normal and tau(m) still increased, in agreement with a decrease in sodium current and conductance at day 4 and normalization at day 7. Our model is a useful tool to study the effects of denervation in adult muscle fibers in vitro and the expression of sodium channels. Our data evidenced an increase in Na(V)1.5 channels and the involvement of beta subunits in the regulation of sodium current and fiber excitability.

Antioxidant treatment of hindlimb-unloaded mouse counteracts fiber type transition but not atrophy of disused muscles

Oxidative stress was proposed as a trigger of muscle impairment in various muscle diseases. The hindlimb-unloaded (HU) rodent is a model of disuse inducing atrophy and slow-to-fast transition of postural muscles. Here, mice unloaded for 14 days were chronically treated with the selective antioxidant trolox. After HU, atrophy was more pronounced in the slow-twitch soleus muscle (Sol) than in the fast-twitch gastrocnemius and tibialis anterior muscles, and was absent in extensor digitorum longus muscle. In accord with the phenotype transition, HU Sol showed a reduced expression of myosin heavy chain type 2A (MHC-2A) and increase in MHC-2X and MHC-2B isoforms. In parallel, HU Sol displayed an increased sarcolemma chloride conductance related to an increased expression of ClC-1 channels, changes in excitability parameters, a positive shift of the mechanical threshold, and a decrease of the resting cytosolic calcium concentration. Moreover, the level of lipoperoxidation increased proportionally to the degree of atrophy of each muscle type. As expected, trolox treatment fully prevented oxidative stress in HU mice. Atrophy was not prevented but the drug significantly attenuated Sol phenotypic transition and excitability changes. Trolox treatment had no effect on control mice. These results suggest possible benefits of antioxidants in protecting muscle against disuse.

The KATP channel is a molecular sensor of atrophy in skeletal muscle

The involvement of ATP-sensitive K(+) (K(ATP)) channels in the atrophy of slow-twitch (MHC-I) soleus (SOL) and fast-twitch (MHC-IIa) flexor digitorum brevis (FDB) muscles was investigated in vivo in 14-day-hindlimb-unloaded (14-HU) rats, an animal model of disuse, and in vitro in drug-induced muscle atrophy. Patch-clamp and gene expression experiments were performed in combination with measurements of fibre diameters used as an index of atrophy, and with MHC labelling in 14-HU rats and controls. A down-regulation of K(ATP) channel subunits Kir6.2, SUR1 and SUR2B with marked atrophy and incomplete phenotype transition were observed in SOL of 14-HU rats. The observed changes in K(ATP) currents were well correlated with changes in fibre diameters and SUR1 expression, as well as with MHC-IIa expression. Half of the SOL fibres of 14-HU rats had reduced diameter and K(ATP) currents and were labelled by MHC-I antibodies. Non-atrophic fibres were labelled by MHC-IIa (22%) antibodies and had enhanced K(ATP) currents, or were labelled by MHC-I (28%) antibodies but had normal current. FDB was not affected in 14-HU rats and this is related to the high expression/activity of Kir6.2/SUR1 subunits characterizing this muscle phenotype. The long-term incubation of the control muscles in vitro with the K(ATP) channel blocker glibenclamide (10(6)m) reduced the K(ATP) currents with atrophy and these effects were prevented by the K(ATP) channel opener diazoxide (10(4)m). The in vivo down-regulation of SUR1, and possibly of Kir6.2 and SUR2B, or their in vitro pharmacological blockade activates atrophic signalling in skeletal muscle. All these findings suggest a new role for the K(ATP) channel as a molecular sensor of atrophy.

Differences in sodium voltage-gated channel properties according to myosin heavy chain isoform expression in single muscle fibres

The myosin heavy chain (MHC) isoform determines the characteristics and shortening velocity of muscle fibres. The functional properties of the muscle fibre are also conditioned by its membrane excitability through the electrophysiological properties of sodium voltage-gated channels. Macropatch-clamp is used to study sodium channels in fibres from peroneus longus (PL) and soleus (Sol) muscles (Wistar rats, n = 8). After patch-clamp recordings, single fibres are identified by SDS-PAGE electrophoresis according to their myosin heavy chain isoform (slow type I and the three fast types IIa, IIx, IIb). Characteristics of sodium currents are compared (Student's t test) between fibres exhibiting only one MHC isoform. Four MHC isoforms are identified in PL and only type I in Sol single fibres. In PL, maximal sodium current (I(max)), maximal sodium conductance (g(Na,max)) and time constants of activation and inactivation ((m) and (h)) increase according to the scheme I-->IIa-->IIx-->IIb (P < 0.05). (m) values related to sodium channel type and/or function, are similar in Sol I and PL IIb fibres (P = 0.97) despite different contractile properties. The voltage dependence of activation (V(a,1/2)) shows a shift towards positive potentials from Sol type I to IIa, IIx and finally IIb fibres from PL (P < 0.05). These data are consistent with the earlier recruitment of slow fibres in a fast-mixed muscle like PL, while slow fibres of postural muscle such as soleus could be recruited in the same ways as IIb fibres in a fast muscle.

Disuse of rat muscle in vivo reduces protein kinase C activity controlling the sarcolemma chloride conductance

Muscle disuse produced by hindlimb unloading (HU) induces severe atrophy and slow-to-fast fibre type transition of the slow-twitch soleus muscle (Sol). After 2 weeks HU, the resting ClC-1 chloride conductance (g(Cl)) of sarcolemma, which controls muscle excitability, increases in Sol toward a value typical of the fast-twitch EDL muscle. After 3 days of HU, the g(Cl) increases as well before initiation of fibre type transition. Since ClC-1 channels are acutely silenced by PKC-dependent phosphorylation, we studied the modulation of g(Cl) by PKC and serine-threonine phosphatase in Sol during HU, using a number of pharmacological tools. We show that a fraction of ClC-1 channels of control Sol are maintained in an inactive state by PKC basal activity, which contributes to the lower g(Cl) in control Sol compared to EDL. After 14 days of HU, PKC/phosphatase manipulation produces effects on Sol g(Cl) that corroborate the partial slow-to-fast transition. After 3 days of HU, the early increase of g(Cl) in Sol is entirely attributable to a reduction of PKC activity and/or activation of phosphatase, maintaining ClC-1 channels in a fully active state. Accordingly, we found that HU reduces expression of PKCalpha, epsilon, and isoenzymes in Sol and EDL muscles and reduces total PKC activity. Moreover, we show that the rheobase current is increased in Sol muscle fibres as soon as after 3 days of HU, most probably in relation to the increased g(Cl). In conclusion, Sol muscle disuse is characterized by a rapid reduction of PKC activity, which reduces muscle excitability and is likely to contribute to disuse-induced muscle impairment.

Modulation of angiogenesis by dithiolethione-modified NSAIDs and valproic acid

BACKGROUND AND PURPOSE

Angiogenesis involves multiple signaling pathways that must be considered when developing agents to modulate pathological angiogenesis. Because both cyclooxygenase inhibitors and dithioles have demonstrated anti-angiogenic properties, we investigated the activities of a new class of anti-inflammatory drugs containing dithiolethione moieties (S-NSAIDs) and S-valproate.

EXPERIMENTAL APPROACH

Anti-angiogenic activities of S-NSAIDS, S-valproate, and the respective parent compounds were assessed using umbilical vein endothelial cells, muscle and tumor tissue explant angiogenesis assays, and developmental angiogenesis in Fli:EGFP transgenic zebrafish embryos.

KEY RESULTS

Dithiolethione derivatives of diclofenac, valproate, and sulindac inhibited endothelial cell proliferation and induced Ser(78) phosphorylation of hsp27, a known molecular target of anti-angiogenic signaling. The parent drugs lacked this activity, but dithiolethiones were active at comparable concentrations. Although dithiolethiones can potentially release hydrogen sulphide, NaSH did not reproduce some activities of the S-NSAIDs, indicating that the dithioles regulate angiogenesis through mechanisms other than release of H(2)S. In contrast to the parent drugs, S-NSAIDs, S-valproate, NaSH, and dithiolethiones were potent inhibitors of angiogenic responses in muscle and HT29 tumor explants assessed by 3-dimensional collagen matrix assays. Dithiolethiones and valproic acid were also potent inhibitors of developmental angiogenesis in zebrafish embryos, but the S-NSAIDs, remarkably, lacked this activity.

CONCLUSIONS AND IMPLICATION

S-NSAIDs and S-valproate have potent anti-angiogenic activities mediated by their dithiole moieties. The novel properties of S-NSAIDs and S-valproate to inhibit pathological versus developmental angiogenesis suggest that these agents may have a role in cancer treatment.

Is spinal cord isolation a good model of muscle disuse?

The patterns of normal daily activity that are required to maintain normal skeletal muscle properties remain unknown. The present study was designed to determine whether spinal cord isolation can be used as a reliable experimental model of neuromuscular inactivity, that is, as a baseline for the absence of activity. Electromyograms (EMGs) were recorded from selected hindlimb muscles of unanesthetized rats over 24-hour periods before and 7, 30, 60, and 90 days after surgical isolation of the lumbar spinal cord. Our data indicate that some rat slow muscle fibers pre-surgery were activated for less than 3 hours per day. Spinal cord isolation (SI) reduced the mean daily integrated EMG (IEMG) and daily EMG duration in the primary slow extensor muscle (soleus) to <1% of control, and in the primary fast extensor muscles [medial gastrocnemius (MG) and vastus lateralis (VL)] to <2% of control. These parameters were decreased to <8% and 3% of control, respectively, in a primary fast flexor muscle, the tibialis anterior (TA). From 30 to 90 days post-SI, the mean amplitudes of the spontaneous EMG bursts were relatively normal in the soleus, increased approximately 2-fold in the MG and VL, and increased approximately 4-fold in the TA. Some evidence of the normal antagonistic flexor-extensor relationship was apparent in the brief periods of recorded activity post-SI. These results indicate that SI eliminates nearly all of the normal EMG activity in the hindlimb muscles in the presence of relatively normal muscle innervation and functional intraspinal neural circuitry.

Modulation of angiogenesis by dithiolethione-modified NSAIDs and valproic acid

BACKGROUND AND PURPOSE

Angiogenesis involves multiple signaling pathways that must be considered when developing agents to modulate pathological angiogenesis. Because both cyclooxygenase inhibitors and dithioles have demonstrated anti-angiogenic properties, we investigated the activities of a new class of anti-inflammatory drugs containing dithiolethione moieties (S-NSAIDs) and S-valproate.

EXPERIMENTAL APPROACH

Anti-angiogenic activities of S-NSAIDS, S-valproate, and the respective parent compounds were assessed using umbilical vein endothelial cells, muscle and tumor tissue explant angiogenesis assays, and developmental angiogenesis in Fli:EGFP transgenic zebrafish embryos.

KEY RESULTS

Dithiolethione derivatives of diclofenac, valproate, and sulindac inhibited endothelial cell proliferation and induced Ser(78) phosphorylation of hsp27, a known molecular target of anti-angiogenic signaling. The parent drugs lacked this activity, but dithiolethiones were active at comparable concentrations. Although dithiolethiones can potentially release hydrogen sulphide, NaSH did not reproduce some activities of the S-NSAIDs, indicating that the dithioles regulate angiogenesis through mechanisms other than release of H(2)S. In contrast to the parent drugs, S-NSAIDs, S-valproate, NaSH, and dithiolethiones were potent inhibitors of angiogenic responses in muscle and HT29 tumor explants assessed by 3-dimensional collagen matrix assays. Dithiolethiones and valproic acid were also potent inhibitors of developmental angiogenesis in zebrafish embryos, but the S-NSAIDs, remarkably, lacked this activity.

CONCLUSIONS AND IMPLICATION

S-NSAIDs and S-valproate have potent anti-angiogenic activities mediated by their dithiole moieties. The novel properties of S-NSAIDs and S-valproate to inhibit pathological versus developmental angiogenesis suggest that these agents may have a role in cancer treatment.

Hybrid assemblies of ATP-sensitive K+ channels determine their muscle-type-dependent biophysical and pharmacological properties

ATP-sensitive K(+) channels (K(ATP)) are an octameric complex of inwardly rectifying K(+) 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(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(ATP) currents/area was muscle-type specific, being higher in fast-twitch muscles compared with the slow-twitch muscle. The density of K(ATP) 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(ATP) subunit compositions contribute to the physiological performance of different muscle fiber types and determine the pharmacological actions of drugs modulating K(ATP) activity in muscle diseases.

Phenotype-dependent functional and pharmacological properties of BK channels in skeletal muscle: effects of microgravity

We investigated the involvement of calcium-activated potassium channel (BK) in skeletal muscle phenotype determination and response to acetazolamide, a BK opener. The BKs of slow-twitching soleus (SOL) and fast-twitching flexor digitorum brevis (FDB) muscles of the rat were investigated by patch-clamp technique. The changes of BK properties following muscle disuse were investigated in the hindlimb-unloaded (HU) rat, an animal model of disuse/microgravity. Two functionally different BKs were found in skeletal muscle. The BK of FDB was sensitive to calcium and to acetazolamide, in contrast the BK of SOL was less sensitive to calcium and was resistant to acetazolamide. After 3-14 days HU, in parallel with the slow-to-fast phenotype transition of the fibers, the BK of SOL acquired properties similar to those of FDB. In skeletal muscle, the BK plays muscle-specific roles contributing to the calcium-dependent phenotype determination/adaptation to disuse. The phenotype specificity of acetazolamide has implications for drug-based therapy of neuromuscular disorders associated to disuse.

Immobilization effects in young and older adults

This experiment compared the effects of disuse on the adductor pollicis (AP) muscle in young (YM) and old (OM) men. The AP of the YM and OM was assessed for strength (MVC), compound muscle action potential (CMAP), and volume, and then immobilized for 2 weeks. MVC decreased approximately 22% in the YM, and OM (P<0.001). AP volume was 4.1% (not significant) and 9.5% (P<0.05) less in the YM and OM, respectively. CMAP increased in the OM 0, 24, and 48 h post-immobilization, and did not change in the YM. However, the YM showed a greater decrease in specific force as compared to the OM. YM and OM experienced similar losses in strength, yet muscle volume loss was only significant in OM. Although OM are more susceptible to immediate losses in muscle volume, muscle activation strategies appear to preserve strength during atrophy.

Transcriptional reprogramming during reloading of atrophied rat soleus muscle

The hypothesis was tested that differential, coregulated transcriptional adaptations of various cellular pathways would occur early with increased mechanical loading of atrophied skeletal muscle and relate to concurrent damage of muscle fibers. Atrophy and slow-to-fast fiber transformation of rat soleus muscle was provoked by 14 days of hindlimb suspension (HS). Subsequent reloading of hindlimbs caused a fourfold increase in the percentage of muscle fibers, demonstrating endomysial tenascin-C staining. Five days after reloading, when 10% of the fibers were damaged, the normal muscle weight and slow-type fiber percentage were reestablished. Microarray analysis revealed major, biphasic patterns of gene expressional alterations with reloading that distinguish between treatments and gene ontologies. Transcript levels of factors involved in protein synthesis and certain proteasomal mRNAs were increased after 1 day of reloading and correlated to the percentage of fibers surrounded by tenascin-C. By contrast, levels of gene messages for fatty acid transporters, respiratory chain constituents, and voltage-gated cation channels were transiently reduced after 1 day of muscle loading and associated with the number of damaged fibers and the regain in muscle weight. This coregulation points toward important retooling of oxidative metabolism and the T- and SR-tubular systems with rebuilding of slow fibers. The observations demonstrate that early nuclear reprogramming with reloading of atrophic soleus muscle is coordinated and links to the processes involved in mechanical damage and regeneration of muscle fibers.

Extension and magnitude of denervation in skeletal muscle from ageing mice

In this work we hypothesized that denervation in flexor digitorum brevis (FDB) muscle from ageing mice is more extensive than predicted by standard functional and structural assays used in the past. In addition, we asked whether denervation is a fully or partially developed process. Despite the reported alteration in skeletal muscle innervation, the quantification of the extension and magnitude of denervation in ageing rodents has remained elusive. To address these two questions we utilized a combination of electrophysiological and immunohistochemical assays directed to detecting the expression of tetrodotoxin (TTX)-resistant sodium channels (Na(v)1.5) in FDB muscles from young-adult and senescent mice. Sodium current density measured with the macropatch cell-attached technique did not show significant differences between FDB fibres from young and old mice. The TTX dose-response curve, using the whole cell voltage-clamp technique, showed three populations of fibres in senescent mice, one similar to fibres from young mice (TTX sensitive), another one similar to fibres from experimentally denervated muscle (TTX resistant), and a third group intermediate between these two. Partially and fully denervated fibres added up to approximately 50% of the total number of fibres tested, a number that concurs with the percentage of fibres positive for the Na(v)1.5 channel by specific immunostaining.

Recovery of the soleus muscle after short- and long-term disuse induced by hindlimb unloading: effects on the electrical properties and myosin heavy chain profile

The hindlimb unloading (HU) rat is a model of muscle disuse characterized by atrophy and slow-to-fast phenotype transition of the postural muscles, such as the soleus. We previously found that the resting sarcolemmal chloride conductance (gCl) that is typically lower in slow-twitch myofibers than in fast ones increased in soleus fibers following 1 to 3 weeks of HU in accord with the slow-to-fast transition of myosin heavy chain (MHC) isoforms. Nevertheless, the gCl already raised after a 3-day HU, whereas no change in MHC expression was detected. The present work evaluates the ability of soleus muscle to recover on return to normal load after a short (3 days) or long (2 weeks) disuse period. The changes observed after a 2-week HU were slowly reversible, since 3-4 weeks of reloading were needed to completely recover gCl, fiber diameter, MHC expression pattern, as well as the mechanical threshold Rheobase, an index of calcium homeostasis. After 3-day HU, the gCl increased homogeneously in most of the soleus muscle fibers and gCl recovery was rapidly completed after 4-day reloading. These results suggest different induction mechanisms for gCl augmentation after the short and long HU periods, as well as a possible role for gCl in the slow muscle adaptation to disuse.