Slow-twitch Soleus Research Articles

The peak force-resting membrane potential relationships of mouse fast- and slow-twitch muscle.

We endeavored to understand the factors determining the peak force-resting membrane potential (EM) relationships of isolated slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles from mice (25°C), especially in relation to fatigue. Interrelationships between intracellular K+ activity ([Formula: see text]), extracellular K+ concentration ([K+]o), resting EM, action potentials, and force were studied. The large resting EM variation was mainly due to the variability of [Formula: see text]. Action potential overshoot-resting EM relationships determined at 4 and 8-10 mM [K+]o after short (<5 min) and prolonged (>50 min) depolarization periods revealed a constant overshoot from -90 to -70 mV providing a safety margin. Overshoot decline with depolarization beyond -70 mV was less after short than prolonged depolarization. Inexcitable fibers occurred only with prolonged depolarization. The overshoot decline during action potential trains (2 s) exceeded that during short depolarizations. Concomitant lower extracellular [Na+] and raised [K+]o depressed the overshoot in an additive manner and peak force in a synergistic manner. Raised [K+]o-induced force loss was exacerbated with transverse wire versus parallel plate stimulation in soleus, implicating action potential propagation failure in the surface membrane. Increasing stimulus pulse parameters restored tetanic force at 9-10 mM [K+]o in soleus but not EDL, indicative of action potential failure within trains. The peak tetanic force-resting EM relationships (determined with resting EM from deeper rather than surface fibers) were dynamic and showed pronounced force depression over -69 to -60 mV in both muscle types, implicating that such depolarization contributes to fatigue. The K+-Na+ interaction shifted this relationship toward less depolarized potentials, suggesting that the combined ionic effect is physiologically important during fatigue.

Bridging the muscle genome to phenome across multiple biological scales.

Muscle is highly hierarchically organized, with functions shaped by genetically controlled expression of protein ensembles with different isoform profiles at the sarcomere scale. However, it remains unclear how isoform profiles shape whole-muscle performance. We compared two mouse hindlimb muscles, the slow, relatively parallel-fibered soleus and the faster, more pennate-fibered tibialis anterior (TA), across scales: from gene regulation, isoform expression and translation speed, to force-length-velocity-power for intact muscles. Expression of myosin heavy-chain (MHC) isoforms directly corresponded with contraction velocity. The fast-twitch TA with fast MHC isoforms had faster unloaded velocities (actin sliding velocity, Vactin; peak fiber velocity, Vmax) than the slow-twitch soleus. For the soleus, Vactin was biased towards Vactin for purely slow MHC I, despite this muscle's even fast and slow MHC isoform composition. Our multi-scale results clearly identified a consistent and significant dampening in fiber shortening velocities for both muscles, underscoring an indirect correlation between Vactin and fiber Vmax that may be influenced by differences in fiber architecture, along with internal loading due to both passive and active effects. These influences correlate with the increased peak force and power in the slightly more pennate TA, leading to a broader length range of near-optimal force production. Conversely, a greater force-velocity curvature in the near-parallel fibered soleus highlights the fine-tuning by molecular-scale influences including myosin heavy and light chain expression along with whole-muscle characteristics. Our results demonstrate that the individual gene, protein and whole-fiber characteristics do not directly reflect overall muscle performance but that intricate fine-tuning across scales shapes specialized muscle function.

Exercise reduces intramuscular stress and counteracts muscle weakness in mice with breast cancer.

BackgroundPatients with breast cancer exhibit muscle weakness, which is associated with increased mortality risk and reduced quality of life. Muscle weakness is experienced even in the absence of loss of muscle mass in breast cancer patients, indicating intrinsic muscle dysfunction. Physical activity is correlated with reduced cancer mortality and disease recurrence. However, the molecular processes underlying breast cancer‐induced muscle weakness and the beneficial effect of exercise are largely unknown.MethodsEight‐week‐old breast cancer (MMTV‐PyMT, PyMT) and control (WT) mice had access to active or inactive in‐cage voluntary running wheels for 4 weeks. Mice were also subjected to a treadmill test. Muscle force was measured ex vivo. Tumour markers were determined with immunohistochemistry. Mitochondrial biogenesis and function were assessed with transcriptional analyses of PGC‐1α, the electron transport chain (ETC) and antioxidants superoxide dismutase (Sod) and catalase (Cat), combined with activity measurements of SOD, citrate synthase (CS) and β‐hydroxyacyl‐CoA‐dehydrogenase (βHAD). Serum and intramuscular stress levels were evaluated by enzymatic assays, immunoblotting, and transcriptional analyses of, for example, tumour necrosis factor‐α (TNF‐α) and p38 mitogen‐activated protein kinase (MAPK) signalling.ResultsPyMT mice endured shorter time and distance during the treadmill test (~30%, P < 0.05) and ex vivo force measurements revealed ~25% weaker slow‐twitch soleus muscle (P < 0.001). This was independent of cancer‐induced alteration of muscle size or fibre type. Inflammatory stressors in serum and muscle, including TNF‐α and p38 MAPK, were higher in PyMT than in WT mice (P < 0.05). Cancer‐induced decreases in ETC (P < 0.05, P < 0.01) and antioxidant gene expression were observed (P < 0.05). The exercise intervention counteracted the cancer‐induced muscle weakness and was accompanied by a less aggressive, differentiated tumour phenotype, determined by increased CK8 and reduced CK14 expression (P < 0.05). In PyMT mice, the exercise intervention led to higher CS activity (P = 0.23), enhanced β‐HAD and SOD activities (P < 0.05), and reduced levels of intramuscular stressors together with a normalization of the expression signature of TNFα‐targets and ETC genes (P < 0.05, P < 0.01). At the same time, the exercise‐induced PGC‐1α expression, and CS and β‐HAD activity was blunted in muscle from the PyMT mice as compared with WT mice, indicative that breast cancer interfere with transcriptional programming of mitochondria and that the molecular adaptation to exercise differs between healthy mice and those afflicted by disease.ConclusionsFour‐week voluntary wheel running counteracted muscle weakness in PyMT mice which was accompanied by reduced intrinsic stress and improved mitochondrial and antioxidant profiles and activities that aligned with muscles of healthy mice.

Involvement of neuronal and muscular Trk-fused gene (TFG) defects in the development of neurodegenerative diseases

Trk-fused gene (TFG) mutations have been identified in patients with several neurodegenerative diseases. In this study, we attempted to clarify the effects of TFG deletions in motor neurons and in muscle fibers, using tissue-specific TFG knockout (vMNTFG KO and MUSTFG KO) mice. vMNTFG KO, generated by crossing TFG floxed with VAChT-Cre, showed deterioration of motor function and muscle atrophy especially in slow-twitch soleus muscle, in line with the predominant Cre expression in slow-twitch fatigue-resistant (S) and fast-twitch fatigue-resistant (FR) motor neurons. Consistently, denervation of the neuromuscular junction (NMJ) was apparent in the soleus, but not in the extensor digitorum longus, muscle. Muscle TFG expressions were significantly downregulated in vMNTFG KO, presumably due to decreased muscle IGF-1 concentrations. However, interestingly, MUSTFG KO mice showed no apparent impairment of muscle movements, though a denervation marker, AChRγ, was elevated and Agrin-induced AChR clustering in C2C12 myotubes was inhibited. Our results clarify that loss of motor neuron TFG is sufficient for the occurrence of NMJ degeneration and muscle atrophy, though lack of muscle TFG may exert an additional effect. Reduced muscle TFG, also observed in aged mice, might be involved in age-related NMJ degeneration, and this issue merits further study.

Carotenoid transporter CD36 expression depends on hypoxia-inducible factor-1α in mouse soleus muscles.

Dietary β-carotene induces muscle hypertrophy and prevents muscle atrophy in red slow-twitch soleus muscles, but not in white fast-twitch extensor digitorum longus (EDL) muscles and gastrocnemius muscles. However, it remains unclear why these beneficial effects of β-carotene are elicited in soleus muscles. To address this issue, we focused on carotenoid transporters in skeletal muscles. In mice, Cd36 mRNA levels were higher in red muscle than in white muscle. The siRNA-mediated knockdown of CD36 decreased β-carotene uptake in C2C12 myotubes. In soleus muscles, CD36 knockdown inhibited β-carotene-induced increase in muscle mass. Intravenous injection of the hypoxia marker pimonidazole produced more pimonidazole-bound proteins in soleus muscles than in EDL muscles, and the hypoxia-inducible factor-1 (HIF-1) α protein level was higher in soleus muscles than in EDL muscles. In C2C12 myotubes, hypoxia increased the expression of CD36 and HIF-1α at the protein and mRNA levels, and HIF-1α knockdown reduced hypoxia-induced increase in Cd36 mRNA level. In soleus muscles, HIF-1α knockdown reduced Cd36 mRNA level. These results indicate that CD36 is predominantly involved in β-carotene-induced increase in soleus muscle mass of mice. Furthermore, we demonstrate that CD36 expression depends on HIF-1α in the soleus muscles of mice, even under normal physiological conditions.

Variation in muscle and neuromuscular junction morphology between atrophy-resistant and atrophy-prone muscles supports failed re-innervation in aging muscle atrophy.

In advanced age, there is an accelerated decline in skeletal muscle mass that appears to be secondary to repeated cycles of denervation-reinnervation and eventually, failed reinnervation. However, whether variation in reinnervation capacity explains why some muscles are less vulnerable to age-related atrophy has not been addressed. In this study we examined changes in neuromuscular junction (NMJ) morphology, fiber cross-sectional area (CSA) and fiber type, accumulation of severely atrophied myofibers, and expression of a marker of denervation in four muscles that exhibit differences in the degree of age-related atrophy and which span the extremes of fiber type composition in 8 mo old (8M) and 34 mo old (34M) male Fischer 344 Brown Norway F1 hybrid rats. Aging muscle atrophy was most pronounced in the fast twitch gastrocnemius (Gas; 25%) and similar between extensor digitorum longus (EDL) and slow-twitch soleus (Sol) muscle (14-15%), whereas the slow-twitch adductor longus (AL) increased in mass by 21% between 8M and 34M (P<0.05 for all). Only the Sol exhibited significant alterations in fiber type with aging, and there was a decrease in fiber CSA in the Gas, EDL, and Sol (P<0.05) with aging that was not seen in the AL. Muscles that atrophied had an increased fraction of severely atrophic myofibers (P<0.05), but this was not observed in the AL. The Gas and EDL both demonstrated a similar degree of age-related remodeling of pre- and post-synaptic NMJ components. On the other hand, pre- and post-synaptic morphology underwent greater changes with aging in the AL, and many of these same morphological variables were already greater in the Sol vs AL at 8M, suggesting the Sol had already undergone substantial remodeling and may be nearing its adaptive limits. Consistent with this idea, analysis of NMJ morphology in Sol from 3M rats exhibited similar values as 8M AL, and the Sol demonstrated greater expression of the denervation marker neural cell adhesion molecule (NCAM) compared to the AL at 34M. Collectively, our results are consistent with NMJ remodeling capacity being finite with aging and that maintained remodeling potential confers atrophy protection in aging skeletal muscle by reducing the degree of persistent denervation.

Assessment of Calf Skeletal Muscle in Male Type 2 Diabetes Mellitus Patients With Different Courses Using T1ρ Mapping.

The current clinical methods for detecting skeletal muscle complications of type 2 diabetes mellitus (T2DM) are invasive and insensitive. There is an urgent need for noninvasive assessment of skeletal muscle microstructure changes during the disease progression and treatment to assist the clinical management. This work aimed to investigate the T2DM caused changes in the fast-twitch tibialis anterior (TA) and slow-twitch soleus (SOL) skeletal muscles using T1ρ magnetic resonance imaging (MRI). This cross-sectional study took place from December 2014 to December 2020 at Zhongda Hospital Southeast University. A total of 26 new-onset and 15 long-term T2DM patients were enrolled, with the addition of 20 young and 13 older healthy volunteers as age-matched controls. T1ρ relaxation times of SOL and TA muscles in different groups were measured. Parametric and nonparametric tests were used to analyze the relationship between the T1ρ values in SOL and TA muscles and the length of illness, level of fasting blood glucose, and status of homeostasis model assessment of insulin resistance (HOMA-IR). T1ρ relaxation times of SOL and TA muscles both of new-onset and long-term T2DM patients were significantly higher than those of the young (P < .01, P < .05) and older healthy controls (P < .05, P < .01). Positive correlations were observed between the T1ρ relaxation times of the TA or SOL and the duration of T2DM (R2 = 0.420, R2 = 0.326), the level of fasting blood glucose (R2 = 0.253, R2 = 0.071) and HOMA-IR (R2 = 0.232, R2 = 0.414). Quantitative MRI measurement of T1ρ provides a noninvasive tool to assess T2DM-induced changes in the skeletal muscles of T2DM patients.

Minocycline Treatment Reduces Mass and Force Output From Fast-Twitch Mouse Muscles and Inhibits Myosin Production in C2C12 Myotubes.

Minocycline, a tetracycline-class of antibiotic, has been tested with mixed effectiveness on neuromuscular disorders such as amyotrophic lateral sclerosis, autoimmune neuritis and muscular dystrophy. The independent effect of minocycline on skeletal muscle force production and signalling remain poorly understood. Our aim here is to investigate the effects of minocycline on muscle mass, force production, myosin heavy chain abundance and protein synthesis. Mice were injected with minocycline (40 mg/kg i.p.) daily for 5 days and sacrificed at day six. Fast-twitch EDL, TA muscles and slow-twitch soleus muscles were dissected out, the TA muscle was snap-frozen and the remaining muscles were attached to force transducer whilst maintained in an organ bath. In C2C12 myotubes, minocycline was applied to the media at a final concentration of 10 μg/mL for 48 h. In minocycline treated mice absolute maximal force was lower in fast-twitch EDL while in slow-twitch soleus there was an increase in the time to peak and relaxation of the twitch. There was no effect of minocycline treatment on the other contractile parameters measured in isolated fast- and slow-twitch muscles. In C2C12 cultured cells, minocycline treatment significantly reduced both myosin heavy chain content and protein synthesis without visible changes to myotube morphology. In the TA muscle there was no significant changes in myosin heavy chain content. These results indicate that high dose minocycline treatment can cause a reduction in maximal isometric force production and mass in fast-twitch EDL and impair protein synthesis during myogenesis in C2C12 cultured cells. These findings have important implications for future studies investigating the efficacy of minocycline treatment in neuromuscular or other muscle-atrophy inducing conditions.

Chloride channel inhibition improves neuromuscular function under conditions mimicking neuromuscular disorders.

The skeletal muscle Cl- channels, the ClC-1 channels, stabilize the resting membrane potential and dampen muscle fibre excitability. This study explored whether ClC-1 inhibition can recover nerve-stimulated force in isolated muscle under conditions of compromised neuromuscular transmission akin to disorders of myasthenia gravis and Lambert-Eaton syndrome. Nerve-muscle preparations were isolated from rats. Preparations were exposed to pre-or post-synaptic inhibitors (ω-agatoxin, elevated extracellular Mg2+ , α-bungarotoxin or tubocurarine). The potential of ClC-1 inhibition (9-AC or reduced extracellular Cl- ) to recover nerve-stimulated force under these conditions was assessed. ClC-1 inhibition recovered force in both slow-twitch soleus and fast-twitch EDL muscles exposed to 0.2µmol/L tubocurarine or 3.5mmol/L Mg2+ . Similarly, ClC-1 inhibition recovered force in soleus muscles exposed to α-bungarotoxin or ω-agatoxin. Moreover, the concentrations of tubocurarine and Mg2+ required for reducing force to 50% rose from 0.14±0.02µmol/L and 4.2±0.2mmol/L in control muscles to 0.45±0.03µmol/L and 4.7±0.3mmol/L in muscles with 9-AC respectively (P<.05, paired T test). Inhibition of acetylcholinesterase (neostigmine) and inhibition of voltage-gated K+ channels (4-AP) relieve symptoms in myasthenia gravis and Lambert-Eaton syndrome, respectively. Neostigmine and 9-AC additively increased the tubocurarine concentration required to reduce nerve-stimulated force to 50% (0.56±0.05µmol/L with 9-AC and neostigmine) and, similarly, 4-AP and 9-AC additively increased the Mg2+ concentration required to reduce nerve-stimulated force to 50% (6.5±0.2mmol/L with 9-AC and 4-AP). This study shows that ClC-1 inhibition can improve neuromuscular function in pharmacological models of compromised neuromuscular transmission.

Skeletal muscle phenotype signaling with ex vivo endurance-type dynamic contractions in rat muscle.

Skeletal muscle phenotype may influence the response sensitivity of myocellular regulatory mechanisms to contractile activity. To examine this, we employed an ex vivo endurance-type dynamic contraction model to evaluate skeletal muscle phenotype-specific protein signaling responses in rat skeletal muscle. Preparations of slow-twitch soleus and fast-twitch extensor digitorum longus skeletal muscle from 4-wk-old female Wistar rats were exposed to an identical ex vivo dynamic endurance-type contraction paradigm consisting of 40 min of stretch-shortening contractions under simultaneous low-frequency electrostimulation delivered in an intermittent pattern. Phosphorylation of proteins involved in metabolic signaling and signaling for translation initiation was evaluated at 0, 1, and 4 h after stimulation by immunoblotting. For both muscle phenotypes, signaling related to metabolic events was upregulated immediately after stimulation, with concomitant absence of signaling for translation-initiation. Signaling for translation-initiation was then activated in both muscle phenotypes at 1-4 h after stimulation, coinciding with attenuated metabolic signaling. The recognizable pattern of signaling responses support how our ex vivo dynamic muscle contraction model can be utilized to infer a stretch-shortening contraction pattern resembling stretch-shortening contraction of in vivo endurance exercise. Moreover, using this model, we observed that some specific signaling proteins adhering to metabolic events or to translation-initiation exhibited phosphorylation changes in a phenotype-dependent manner, whereas other signaling proteins exhibited phenotype-independent changes. These findings may aid the interpretation of myocellular signaling outcomes adhering to mixed muscle samples collected during human experimental trials.NEW & NOTEWORTHY The application of cyclic ex vivo dynamic muscle contractions delivered in an intermittent pattern may be suitable for the exploration of skeletal muscle regulatory responses to endurance-type contractile activity. In the present study, it is demonstrated that the response to such stimulus of some nodal myocellular signaling proteins related to either metabolic or anabolic signaling events may be influenced by muscle phenotype, whereas the response of others appears to be independent of phenotype.

Skeletal muscle and fiber type-specific intramyocellular lipid accumulation in obese mice.

ABSTRACTIn obesity, accumulation of lipid droplets in skeletal muscle fibers and a shift towards fast muscle fiber types can both contribute to insulin resistance. However, it is not yet clear how intramyocellular lipid accumulation and fiber type changes are associated. Therefore, we investigated to what extent the lipids accumulated in a fiber type-specific manner in the functionally similar fast-, intermediate- and slow-twitch gastrocnemius, plantaris, and soleus muscles, respectively, in high-fat diet-induced obese 54-week-old female C57BL/6JOlaHsd mice (n = 9) compared to control standard-diet-treated lean mice (n = 9). A high-fat diet was administered for 26 weeks. Fiber-type specific intramyocellular lipid content analysis and muscle fiber typing were performed using histochemical analysis of lipids with Sudan Black and immunohistochemical analysis of myosin heavy chains on serial sections of skeletal muscles. Compared to the lean mice, the lipid accumulation was most prominent in types 2a and 2x/d fibers (p < 0.05) of fast-twitch gastrocnemius and intermediate plantaris muscles in the obese mice, while in slow-twitch soleus muscle, there was no significant lipid accumulation in the obese animals. Furthermore, the slow-twitch soleus muscle of the obese mice with no significant change in muscle fiber diameters exhibited the most pronounced shift towards fast-type myosin heavy chain isoform expression (p < 0.05). In contrast, the fast-twitch and intermediate-twitch gastrocnemius and plantaris muscles, respectively, in which the muscle fiber diameters increased (p < 0.05), were more resistant toward myosin heavy chain expression changes. In conclusion, we demonstrated both muscle- and fiber-type specificity in intramyocellular lipid accumulation in obese mice, suggesting that in obesity, similar muscle fiber types in different muscles accumulate lipids differentially.

Attractin deficiency causes metabolic and morphological abnormalities in slow-twitch muscle

Skeletal muscle fibers are classified as slow-twitch and fast-twitch fibers, which have different reactive oxygen species (ROS) metabolism and mitochondrial biogenesis. Recently, Attractin (Atrn), which encodes secreted (sAtrn) and transmembrane (mAtrn)-type proteins, has been shown to be involved in free radical scavenging. Although Atrn has been found in skeletal muscle, little is known about the expression levels and function of Atrn in each muscle fiber type. Therefore, we investigatesAtrn and mAtrn expression levels in the slow-twitch soleus (sol) and fast-twitch extensor digitorum longus (EDL) muscles as well as the morphology and expression levels of antioxidant enzymes and functional mitochondrial markers using Atrn-deficient muscles. Both types of Atrn were expressed in the sol and EDL. mAtrn was mainly expressed in the adult sol, whereas sAtrn expression levels did not differ between muscle types. Moreover, mAtrn in the sol was abundantly localized in the subsarcolemmal area, especially in the myoplasm near mitochondria. Atrn-deficient Zitter rats showed muscle fiber atrophy, myofibril misalignment, mitochondrial swelling and vacuolation in the sol but not EDL. Furthermore, the Atrn-deficient sol exhibited a marked reduction in antioxidant enzyme SOD1, GPx1, catalaseand Prx6and mitochondrial functional protein, UCP2, expression. Even Atrn-deficient EDL showed a significant reduction in Prx3, Prx6, UCP2and UCP3 expression. These data indicate that Atrn-deficiency disturbs ROS metabolism in skeletal muscles. In particular, mAtrn is involved in metabolism in the slow-twitch sol muscle and mAtrn-deficiency may cause ROS imbalance, resulting in morphological abnormalities in the muscle.

Muscle fiber type specific alterations of mitochondrial respiratory function and morphology in aged female mice

Mitochondrial dysfunction is considered to be a major cause of sarcopenia, defined as age-related muscle fiber atrophy and muscle weakness, as reduced mitochondrial respiration and morphological changes such as ragged red fibers (RRFs) are observed in aging muscles. However, the role of mitochondrial dysfunction in sarcopenia is not fully elucidated. Although previous studies have suggested that aging has a fiber type-specific effect on mitochondrial function, little is known about mitochondrial changes in individual fiber types. Here, we used C57BL/6NCr female mice to identify fiber type-specific pathological changes, examine the significance of pathological changes in sarcopenia, and identify possible mechanisms behind mitochondrial changes in slow-twitch soleus muscle (SOL) and fast-twitch extensor digitorum longus muscle (EDL). We observed reduced type I fiber-specific mitochondrial respiratory enzyme activity, impaired respiration, and subsarcolemmal mitochondrial accumulation in aged SOL, which was different from RRFs. These pathological alterations were not directly associated with fiber atrophy. Additionally, we found increased oxidative stress markers in aged SOL, suggesting that oxidative stress is involved in the pathological and functional changes in mitochondria. Meanwhile, obvious mitochondrial changes were not seen in aged EDL. Thus, age-related mitochondrial dysfunction is specific to the fiber type and may correlate with the muscle quality rather than the muscle mass.

Paxilline Prevents the Onset of Myotonic Stiffness in Pharmacologically Induced Myotonia: A Preclinical Investigation

Reduced Cl− conductance causes inhibited muscle relaxation after forceful voluntary contraction due to muscle membrane hyperexcitability. This represents the pathomechanism of myotonia congenita. Due to the prevailing data suggesting that an increased potassium level is a main contributor, we studied the effect of a modulator of a big conductance Ca2+- and voltage-activated K+ channels (BK) modulator on contraction and relaxation of slow- and high-twitch muscle specimen before and after the pharmacological induction of myotonia. Human and murine muscle specimens (wild-type and BK−/−) were exposed to anthracene-9-carboxylic acid (9-AC) to inhibit CLC-1 chloride channels and to induce myotonia in-vitro. Functional effects of BK-channel activation and blockade were investigated by exposing slow-twitch (soleus) and fast-twitch (extensor digitorum longus) murine muscle specimens or human musculus vastus lateralis to an activator (NS1608) and a blocker (Paxilline), respectively. Muscle-twitch force and relaxation times (T90/10) were monitored. Compared to wild type, fast-twitch muscle specimen of BK−/− mice resulted in a significantly decreased T90/10 in presence of 9-AC. Paxilline significantly shortened T90/10 of murine slow- and fast-twitch muscles as well as human vastus lateralis muscle. Moreover, twitch force was significantly reduced after application of Paxilline in myotonic muscle. NS1608 had opposite effects to Paxilline and aggravated the onset of myotonic activity by prolongation of T90/10. The currently used standard therapy for myotonia is, in some individuals, not very effective. This in vitro study demonstrated that a BK channel blocker lowers myotonic stiffness and thus highlights its potential therapeutic option in myotonia congenital (MC).

MUSCLE FUNCTION & HOMEOSTASIS / MOLECULAR THERAPEUTIC APPROACHES: P.72 Importance of myofibre-type specific differences in disease manifestation for muscular dystrophies: focus on dysferlinopathy

Dysferlinopathies are a form of limb girdle muscular dystrophy (LGMDR2 dysferlin-related) caused by gene mutations resulting in deficiency of the membrane-associated protein dysferlin. Symptoms manifest post growth in a muscle specific manner, although the pathomechanism is not well understood. One feature is many lipid droplets present in slow-twitch dysferlin-deficient myofibres of humans and mice, and more recently we identified an increased capacity for fatty acid esterification to triglycerides in the predominantly slow-twitch soleus of young adult (3 months) dysferlin-deficient BLAJ, compared with wild type (WT) mice.

Thyroid hormone receptor α in skeletal muscle is essential for T3-mediated increase in energy expenditure.

Thyroid hormones are important for homeostatic control of energy metabolism and body temperature. Although skeletal muscle is considered a key site for thyroid action, the contribution of thyroid hormone receptor signaling in muscle to whole‐body energy metabolism and body temperature has not been resolved. Here, we show that T3‐induced increase in energy expenditure requires thyroid hormone receptor alpha 1 (TRα1) in skeletal muscle, but that T3‐mediated elevation in body temperature is achieved in the absence of muscle‐TRα1. In slow‐twitch soleus muscle, loss‐of‐function of TRα1 (TRαHSACre) alters the fiber‐type composition toward a more oxidative phenotype. The change in fiber‐type composition, however, does not influence the running capacity or motivation to run. RNA‐sequencing of soleus muscle from WT mice and TRαHSACre mice revealed differentiated transcriptional regulation of genes associated with muscle thermogenesis, such as sarcolipin and UCP3, providing molecular clues pertaining to the mechanistic underpinnings of TRα1‐linked control of whole‐body metabolic rate. Together, this work establishes a fundamental role for skeletal muscle in T3‐stimulated increase in whole‐body energy expenditure.

Muscle fiber-type specific terminal Schwann cell pathology leads to sprouting deficits following partial denervation in SOD1G93A mice

Amyotrophic lateral sclerosis (ALS) is an adult-onset disease characterized by the progressive death of motoneurons and denervation of muscle fibers. To restore motor function, surviving motoneurons in partially denervated muscles typically sprout axons to reinnervate denervated endplates. However, studies on the SOD1G93A rodent models of ALS indicate that sprouting is significantly limited in fast, but not slow, twitch muscles after disease onset. This limitation hastens the rate of muscle weakness and loss of motor function. The causes of this limitation are currently unknown. Sprouting could be limited because the SOD1G93A mutation weakens motoneurons making them incapable of expanding their field of innervation. Alternatively, motoneurons may be capable of sprouting, but unable to do so due to the loss of a permissive sprouting environment. To distinguish between the two possibilities, we compared the sprouting capacity of motoneuron subtypes by partially denervating the fast twitch plantaris (composed of type IIa/IIb muscle fibers) and slow twitch soleus muscles (type I/IIa fibers) prior to disease onset and weakening in SOD1G93A and WT mice. We found that only motoneurons innervating the SOD1G93A plantaris had a limited sprouting capacity. This was correlated with the selective loss of terminal Schwann cells (TSCs) at IIb fibers and an increase in macrophage infiltration. Treating SOD1G93A mice with the tyrosine kinase inhibitor, masitinib, significantly reduced infiltration, prevented TSC loss, and increased the sprouting capacity to near normal. These results suggest that TSCs at denervated type IIb muscle fibers are aberrantly targeted by infiltrating macrophages in SOD1G93A mice, and their loss accounts, at least in part, for the compromised sprouting capacity of the largest motoneurons during early stages of ALS.

Exploring the lncRNAs Related to Skeletal Muscle Fiber Types and Meat Quality Traits in Pigs.

The alteration in skeletal muscle fiber is a critical factor affecting livestock meat quality traits and human metabolic diseases. Long non-coding RNAs (lncRNAs) are a diverse class of non-coding RNAs with a length of more than 200 nucleotides. However, the mechanisms underlying the regulation of lncRNAs in skeletal muscle fibers remain elusive. To understand the genetic basis of lncRNA-regulated skeletal muscle fiber development, we performed a transcriptome analysis to identify the key lncRNAs affecting skeletal muscle fiber and meat quality traits on a pig model. We generated the lncRNA expression profiles of fast-twitch Biceps femoris (Bf) and slow-twitch Soleus (Sol) muscles and identified the differentially expressed (DE) lncRNAs using RNA-seq and performed bioinformatics analyses. This allowed us to identify 4581 lncRNA genes among six RNA libraries and 92 DE lncRNAs between Bf and Sol which are the key candidates for the conversion of skeletal muscle fiber types. Moreover, we detected the expression patterns of lncRNA MSTRG.42019 in different tissues and skeletal muscles of various development stages. In addition, we performed a correlation analyses between the expression of DE lncRNA MSTRG.42019 and meat quality traits. Notably, we found that DE lncRNA MSTRG.42019 was highly expressed in skeletal muscle and its expression was significantly higher in Sol than in Bf, with a positive correlation with the expression of Myosin heavy chain 7 (MYH7) (r = 0.6597, p = 0.0016) and a negative correlation with meat quality traits glycolytic potential (r = −0.5447, p = 0.0130), as well as drip loss (r = −0.5085, p = 0.0221). Moreover, we constructed the lncRNA MSTRG.42019–mRNAs regulatory network for a better understanding of a possible mechanism regulating skeletal muscle fiber formation. Our data provide the groundwork for studying the lncRNA regulatory mechanisms of skeletal muscle fiber conversion, and given the importance of skeletal muscle fiber types in muscle-related diseases, our data may provide insight into the treatment of muscular diseases in humans.

Effects of ten-week 30% caloric restriction on metabolic health and skeletal muscles of adult and old C57BL/6J mice

Caloric restriction (CR) can improve health, but the benefits are age-dependant. We studied effects of ten-week 30 % CR on skeletal muscles of adult (7-month old) and old (24-month old) C57BL/6 J mice. Old mice were heavier than adult mice (36.1 ± 4.0 g versus 32.9 ± 2.3 g, p < 0.05, respectively), but lost more weight (34.7 ± 6.0 % versus 23.9 ± 3.3 %, p < 0.001, respectively) during CR. Old mice did not differ from adult mice in extent of hind-limb muscle wasting or improvement in glucose tolerance after CR. Ageing and CR had an additive effect on increase in percentage of type 1 fibres in the soleus (SOL) muscle. CR was associated with greater atrophy of fast-twitch extensor digitorum longus (EDL) compared to slow-twitch SOL muscle. Old mice showed reduced gene expression of lysosomal markers, p62 and LC3B, while CR tended to upregulate the proteolysis genes. CR was also associated with increase in specific force of EDL muscle, but did not affect it in SOL muscle. In summary, ten-week CR induces only limited improvements in skeletal muscle function, but leads to significant muscle wasting and weakness in both adult and old mice.

Muscle BDNF: a potential therapeutic target for Kennedy's disease.

Muscle BDNF: a potential therapeutic target for Kennedy's disease.