Effects of prednisolone on skeletal muscle contractility in mdx mice

Ablation of skeletal muscle estrogen receptor alpha impairs contractility in male mice

Estradiol and estrogen receptor α (ERα) have been shown to be important for the maintenance of skeletal muscle strength in females; however, little is known about the roles of estradiol and ERα in male muscle. The purpose of this study was to determine if skeletal muscle ERα is required for optimal contractility in male mice. We hypothesize that reduced ERα in skeletal muscle impairs contractility in male mice. Skeletal muscle-specific knockout (skmERαKO) male mice exhibited reduced strength across multiple muscles and several contractile parameters related to force generation and kinetics compared with wild-type littermates (skmERαWT). Isolated EDL muscle-specific isometric tetanic force, peak twitch force, peak concentric and peak eccentric forces, as well as the maximal rates of force development and relaxation were 11%-21% lower in skmERαKO compared with skmERαWT mice. In contrast, isolated soleus muscles from skmERαKO mice were not affected. In vivo peak torque of the anterior crural muscles was 20% lower in skmERαKO compared with skmERαWT mice. Muscle masses, contractile protein contents, fiber types, phosphorylation of the myosin regulatory light chain, and caffeine-elicited force did not differ between muscles of skmERαKO and skmERαWT mice, suggesting that strength deficits were not due to size, composition, or calcium release components of muscle contraction. These results indicate that in male mice, reduced skeletal muscle ERα blunts contractility to a magnitude similar to that previously reported in females; however, the mechanism may be sexually dimorphic.NEW & NOTEWORTHY We comprehensively measured in vitro and in vivo contractility of leg muscles with reduced estrogen receptor α (ERα) in male mice and reported that force generation and contraction kinetics are impaired. In contrast to findings in females, phosphorylation of myosin regulatory light chain cannot account for low force production in male skeletal muscle ERα knockout mice. These results indicate that ERα is required for optimal contractility in males and females but via sexually dimorphic means.

Dimethyl fumarate modulates the dystrophic disease program following short-term treatment

New medicines are urgently required to treat the fatal neuromuscular disease Duchenne muscular dystrophy (DMD). Dimethyl fumarate (DMF) is a potent immunomodulatory small molecule nuclear erythroid 2-related factor 2 activator with current clinical utility in the treatment of multiple sclerosis and psoriasis that could be effective for DMD and rapidly translatable. Here, we tested 2 weeks of daily 100 mg/kg DMF versus 5 mg/kg standard-care prednisone (PRED) treatment in juvenile mdx mice with early symptomatic DMD. Both drugs modulated seed genes driving the DMD disease program and improved force production in fast-twitch muscle. However, only DMF showed pro-mitochondrial effects, protected contracting muscles from fatigue, improved histopathology, and augmented clinically compatible muscle function tests. DMF may be a more selective modulator of the DMD disease program than PRED, warranting follow-up longitudinal studies to evaluate disease-modifying impact.

Females display relatively preserved muscle quality compared with males during the onset and early stages of C26-induced cancer cachexia

Cancer cachexia is clinically defined by involuntary weight loss >5% in <6 mo, primarily affecting skeletal muscle. Here, we aimed to identify sex differences in the onset of colorectal cancer cachexia with specific consideration to skeletal muscle contractile and metabolic functions. Eight-weeks old BALB/c mice (69 males, 59 females) received subcutaneous C26 allografts or PBS vehicle. Tumors were developed for 10-, 15-, 20-, or 25 days. Muscles and organs were collected, in vivo muscle contractility, protein synthesis rate, mitochondrial function, and protein turnover markers were assessed. One-way ANOVA within sex and trend analysis between sexes were performed, P < 0.05. Gastrocnemius and tibialis anterior (TA) muscles became atrophic in male mice at 25 days, whereas female mice exhibited no significant differences in muscle weights at endpoints despite presenting hallmarks of cancer cachexia (fat loss, hepatosplenomegaly). We observed lowered muscle contractility and protein synthesis concomitantly to muscle mass decay in males, with higher proteolytic markers in muscles of both sexes. mRNA of Opa1 was lower in TA, whereas Bnip3 was higher in gastrocnemius after 25 days in male mice, with no significant effect in female mice. Our data suggest relative protections to skeletal muscle in females compared with males despite other canonical signs of cancer cachexia and increased protein degradation markers; suggesting we should place onus upon nonmuscle tissues during early stages of cancer cachexia in females. We noted potential protective mechanisms relating to skeletal muscle contractile and mitochondrial functions. Our findings underline possible heterogeneity in onset of cancer cachexia between biological sexes, suggesting the need for sex-specific approaches to treat cancer cachexia.NEW & NOTEWORTHY Our study demonstrates biological-sex differences in phenotypic characteristics of cancer cachexia between male and female mice, whereby females display many common characteristics of cachexia (gonadal fat loss and hepatosplenomegaly), protein synthesis markers alterations, and common catabolic markers in skeletal muscle despite relatively preserved muscle mass in early-stage cachexia compared with males. Mechanisms of cancer cachexia appear to differ between sexes. Data suggest need to place onus of early cancer cachexia detection and treatment on nonmuscle tissues in females.

ACTA1 H40Y mutant iPSC-derived skeletal myocytes display mitochondrial defects in an in vitro model of nemaline myopathy

Nemaline myopathies (NM) are a group of congenital myopathies that lead to muscle weakness and dysfunction. While 13 genes have been identified to cause NM, over 50% of these genetic defects are due to mutations in nebulin (NEB) and skeletal muscle actin (ACTA1), which are genes required for normal assembly and function of the thin filament. NM can be distinguished on muscle biopsies due to the presence of nemaline rods, which are thought to be aggregates of the dysfunctional protein. Mutations in ACTA1 have been associated with more severe clinical disease and muscle weakness. However, the cellular pathogenesis linking ACTA1 gene mutations to muscle weakness are unclear To evaluate cellular disease phenotypes, iPSC-derived skeletal myocytes (iSkM) harboring an ACTA1 H40Y point mutation were used to model NM in skeletal muscle. These were generated by Crispr-Cas9, and include one non-affected healthy control (C) and 2 NM iPSC clone lines, therefore representing isogenic controls. Fully differentiated iSkM were characterized to confirm myogenic status and subject to assays to evaluate nemaline rod formation, mitochondrial membrane potential, mitochondrial permeability transition pore (mPTP) formation, superoxide production, ATP/ADP/phosphate levels and lactate dehydrogenase release. C- and NM-iSkM demonstrated myogenic commitment as evidenced by mRNA expression of Pax3, Pax7, MyoD, Myf5 and Myogenin; and protein expression of Pax4, Pax7, MyoD and MF20. No nemaline rods were observed with immunofluorescent staining of NM-iSkM for ACTA1 or ACTN2, and these mRNA transcript and protein levels were comparable to C-iSkM. Mitochondrial function was altered in NM, as evidenced by decreased cellular ATP levels and altered mitochondrial membrane potential. Oxidative stress induction revealed the mitochondrial phenotype, as evidenced by collapsed mitochondrial membrane potential, early formation of the mPTP and increased superoxide production. Early mPTP formation was rescued with the addition of ATP to media. Together, these findings suggest that mitochondrial dysfunction and oxidative stress are disease phenotypes in the in vitro model of ACTA1 nemaline myopathy, and that modulation of ATP levels was sufficient to protect NM-iSkM mitochondria from stress-induced injury. Importantly, the nemaline rod phenotype was absent in our in vitro model of NM. We conclude that this in vitro model has the potential to recapitulate human NM disease phenotypes, and warrants further study.

CRABP1-CaMKII-Agrn regulates the maintenance of neuromuscular junction in spinal motor neuron

Cellular retinoic acid-binding protein 1 (CRABP1) binds retinoic acid (RA) specifically in the cytoplasm with unclear functions. CRABP1 is highly and specifically expressed in spinal motor neurons (MNs). Clinical and pre-clinical data reveal a potential link between CRABP1 and MN diseases, including the amyotrophic lateral sclerosis (ALS). We established a sequenced MN-muscle co-differentiation system to engineer an in vitro functional 3D NMJ model for molecular studies and demonstrated that CRABP1 in MNs contributes to NMJ formation and maintenance. Consistently, Crabp1 knockout (CKO) mice exhibited an adult-onset ALS-like phenotype with progressively deteriorated NMJs, characterized with behavioral, EchoMRI, electrophysiological, histological, and immunohistochemical studies at 2-20-months old. Mechanistically, CRABP1 suppresses CaMKII activation to regulate neural Agrn expression and downstream muscle LRP4-MuSK signaling, thereby maintaining NMJ. A proof-of-concept was provided by specific re-expression of CRABP1 to rescue Agrn expression and the phenotype. This study identifies CRABP1-CaMKII-Agrn signaling as a physiological pre-synaptic regulator in the NMJ. This study also highlights a potential protective role of CRABP1 in the progression of NMJ deficits in MN diseases.

Laminin α5_CD239_Spectrin is a candidate association that compensates the linkage between the basement membrane and cytoskeleton in skeletal muscle fibers

•

Laminin α5_CD239_spectrin complex is a candidate linkage in sarcolemma.

•

The linkage molecules are expressed in embryonic and regenerative muscle fibers.

•

CD239 expression is upregulated by steroid therapy for muscular dystrophy.

•

The compensatory linkage may be a therapeutic target for muscular dystrophy.

The linkage between the basement membrane (BM) and cytoskeleton is crucial for muscle fiber stability and signal transduction. Mutations in the linkage molecules can cause various types of muscular dystrophies. The different severities and times of onset suggest that compensatory linkages occur at the sarcolemma. Cluster of differentiation 239 (CD239) binds to the α5 subunit of laminin-511 extracellularly and is connected to spectrin intracellularly, resulting in a linkage between the BM and cytoskeleton. In this study, we explored the linkage of laminin α5_CD239_spectrin in skeletal muscles. Although laminin α5, CD239, and spectrin were present in embryonic skeletal muscles, they disappeared in adult skeletal muscle tissues, except for the soleus and diaphragm. Laminin α5_CD239_spectrin was localized in the skeletal muscle tissues of Duchenne muscular dystrophy and congenital muscular dystrophy mouse models. The experimental regeneration of skeletal muscle increased the CD239-mediated linkage, indicating that it responds to regeneration, but not to genetic influence. Furthermore, in silico analysis showed that laminin α5_CD239_spectrin was upregulated by steroid therapy for muscular dystrophy. Therefore, CD239-mediated linkage may serve as a therapeutic target to prevent the progression of muscular dystrophy.

Muscle miR-16 deletion results in impaired insulin sensitivity and contractile function in a sex-dependent manner

microRNAs (miRs) are linked to various human diseases including type 2 diabetes mellitus (T2DM) and emerging evidence suggests that miRs may serve as potential therapeutic targets. Lower miR-16 content is consistent across different models of T2DM; however, the role of miR-16 in muscle metabolic health is still elusive. Therefore, the purpose of this study was to investigate how deletion of miR-16 in mice affects skeletal muscle metabolic health and contractile function in both sexes. This study was conducted using both 1) in vitro and 2) in vivo experiments. In in vitro experiments, we used C2C12 myoblasts to test if inhibition or overexpression of miR-16 affected insulin-mediated glucose handling. In in vivo experiments, we generated muscle-specific miR-16 knockout (KO) mice fed a high-fat diet (HFD) to assess how miR-16 content impacts metabolic and contractile properties including glucose tolerance, insulin sensitivity, muscle contractile function, protein anabolism, and mitochondrial network health. In in vitro experiments, although inhibition of miR-16 induced impaired insulin signaling (P = 0.002) and glucose uptake (P = 0.014), overexpression of miR-16 did not attenuate lipid overload-induced insulin resistance using the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol. In in vivo experiments, miR-16 deletion induced both impaired muscle contractility (P = 0.031-0.033), and mitochondrial network health (P = 0.008-0.018) in both sexes. However, although males specifically exhibited impaired insulin sensitivity following miR-16 deletion (P = 0.030), female KO mice showed pronounced glucose intolerance (P = 0.046), corresponding with lower muscle weights (P = 0.015), and protein hyperanabolism (P = 0.023). Our findings suggest distinct sex differences in muscle adaptation in response to miR-16 deletion and miR-16 may serve as a key regulator for metabolic dysregulation in T2DM.NEW & NOTEWORTHY We set to investigate the role of miR-16 in skeletal muscle during diet-induced insulin resistance. Our data provide novel evidence that the lack of miR-16 induced multiple aberrations in insulin sensitivity, muscle contractility, mitochondrial network health, and protein turnover in a sex-dependent manner. Interestingly, miR-16 deletion leads to insulin resistance in males and exacerbated glucose intolerance in females, suggesting different mechanisms of metabolic dysregulation with a lack of miR-16 between sexes.

Intermittent glucocorticoid treatment enhances skeletal muscle performance through sexually dimorphic mechanisms

Glucocorticoid steroids are commonly prescribed for many inflammatory conditions, but chronic daily use produces adverse effects, including muscle wasting and weakness. In contrast, shorter glucocorticoid pulses may improve athletic performance, although the mechanisms remain unclear. Muscle is sexually dimorphic and comparatively little is known about how male and female muscles respond to glucocorticoids. We investigated the impact of once-weekly glucocorticoid exposure on skeletal muscle performance comparing male and female mice. One month of once-weekly glucocorticoid dosing improved muscle specific force in both males and females. Transcriptomic profiling of isolated myofibers identified a striking sexually dimorphic response to weekly glucocorticoids. Male myofibers had increased expression of genes in the IGF1/PI3K pathway and calcium handling, while female myofibers had profound upregulation of lipid metabolism genes. Muscles from weekly prednisone–treated males had improved calcium handling, while comparably treated female muscles had reduced intramuscular triglycerides. Consistent with altered lipid metabolism, weekly prednisone–treated female mice had greater endurance relative to controls. Using chromatin immunoprecipitation, we defined a sexually dimorphic chromatin landscape after weekly prednisone. These results demonstrate that weekly glucocorticoid exposure elicits distinct pathways in males versus females, resulting in enhanced performance.

In vivo methodologies to assist preclinical development of topical fixed-dose combinations for pain management

The combination in a fixed dose of two or more active pharmaceutical ingredients in the same pharmaceutical dosage form is an approach that has been used successfully in the treatment of several pathologies, including pain. In the preclinical development of a topical fixed-dose combination product with analgesic and anti-inflammatory activities for pain management, the main objective is to establish the nature of the interaction between the different active pharmaceutical ingredients while obtaining data on the medicinal product safety and efficacy. Despite the improvement of in vitro assays, animal models remain a fundamental strategy to characterise the interaction, efficacy and safety of active pharmaceutical ingredients at the physiological level, which cannot be reached by in vitro assays. Thus, the main goal of this review is to systematise the available animal models to evaluate the efficacy and safety of a new fixed-dose combination product for topical administration indicated for pain management. Particular emphasis is given to animal models that are accepted for regulatory purposes.

Inhibition of sphingolipid de novo synthesis counteracts muscular dystrophy

Duchenne muscular dystrophy (DMD), the most common muscular dystrophy, is a severe muscle disorder, causing muscle weakness, loss of independence, and premature death. Here, we establish the link between sphingolipids and muscular dystrophy. Transcripts of sphingolipid de novo biosynthesis pathway are up-regulated in skeletal muscle of patients with DMD and other muscular dystrophies, which is accompanied by accumulation of metabolites of the sphingolipid pathway in muscle and plasma. Pharmacological inhibition of sphingolipid synthesis by myriocin in the mdx mouse model of DMD ameliorated the loss in muscle function while reducing inflammation, improving Ca²⁺ homeostasis, preventing fibrosis of the skeletal muscle, heart, and diaphragm, and restoring the balance between M1 and M2 macrophages. Myriocin alleviated the DMD phenotype more than glucocorticoids. Our study identifies inhibition of sphingolipid synthesis, targeting multiple pathogenetic pathways simultaneously, as a strong candidate for treatment of muscular dystrophies.

Resolvin-D2 targets myogenic cells and improves muscle regeneration in Duchenne muscular dystrophy

Lack of dystrophin causes muscle degeneration, which is exacerbated by chronic inflammation and reduced regenerative capacity of muscle stem cells in Duchenne Muscular Dystrophy (DMD). To date, glucocorticoids remain the gold standard for the treatment of DMD. These drugs are able to slow down the progression of the disease and increase lifespan by dampening the chronic and excessive inflammatory process; however, they also have numerous harmful side effects that hamper their therapeutic potential. Here, we investigated Resolvin-D2 as a new therapeutic alternative having the potential to target multiple key features contributing to the disease progression. Our in vitro findings showed that Resolvin-D2 promotes the switch of macrophages toward their anti-inflammatory phenotype and increases their secretion of pro-myogenic factors. Moreover, Resolvin-D2 directly targets myogenic cells and promotes their differentiation and the expansion of the pool of myogenic progenitor cells leading to increased myogenesis. These effects are ablated when the receptor Gpr18 is knocked-out, knocked-down, or blocked by the pharmacological antagonist O-1918. Using different mouse models of DMD, we showed that Resolvin-D2 targets both inflammation and myogenesis leading to enhanced muscle function compared to glucocorticoids. Overall, this preclinical study has identified a new therapeutic approach that is more potent than the gold-standard treatment for DMD.

Obestatin signalling counteracts glucocorticoid-induced skeletal muscle atrophy via NEDD4/KLF15 axis

BACKGROUND

A therapeutic approach for the treatment of glucocorticoid-induced skeletal muscle atrophy should be based on the knowledge of the molecular mechanisms determining the unbalance between anabolic and catabolic processes and how to re-establish this balance. Here, we investigated whether the obestatin/GPR39 system, an autocrine signalling system acting on myogenesis and with anabolic effects on the skeletal muscle, could protect against chronic glucocorticoid-induced muscle atrophy.

METHODS

In this study, we used an in vivo model of muscle atrophy induced by the synthetic glucocorticoid dexamethasone to examine the liaison molecules that define the interaction between the glucocorticoid receptor and the obestatin/GPR39 systems. The findings were extended to in vitro effects on human atrophy using human KM155C25 myotubes.

RESULTS

KLF15 and FoxO transcription factors were identified as direct targets of obestatin signalling in the control of proteostasis in skeletal muscle. The KLF15-triggered gene expression program, including atrogenes and FoxOs, was regulated via KLF15 ubiquitination by the E3 ubiquitin ligase NEDD4. Additionally, a specific pattern of FoxO post-translational modification, including FoxO4 phosphorylation by Akt pathway, was critical in the regulation of the ubiquitin-proteasome system. The functional cooperativity between Akt and NEDD4 in the regulation of FoxO and KLF15 provides integrated cues to counteract muscle proteostasis and re-establish protein synthesis.

CONCLUSIONS

The effective control of FoxO activity in response to glucocorticoid is critical to counteract muscle-related pathologies. These results highlight the potential of the obestatin/GPR39 system to fine-tune the effects of glucocorticoids on skeletal muscle wasting.

Recent advances of glucocorticoids in the treatment of Duchenne muscular dystrophy (Review)

Duchenne muscular dystrophy (DMD) is the most common degenerative neuromuscular disease. The incidence of DMD in live births is 1/3,600-1/6,000. Although glucocorticoid-dependent medication is the mainstay treatment option for DMD, a standard treatment regimen has yet to be determined. The present review discusses the literature on the timing, methods and courses of glucocorticoid treatment for DMD. The review highlights the importance of the immediate commencement of glucocorticoid treatment following the diagnosis of DMD, with weekend-only administration being advantageous. Adherence to long-term single-glucocorticoid therapy can delay the loss of ambulation ability, and the side effects of the treatment are controllable. However, the standard medication for patients of different ages and stages of disease development, and the use of combination therapy require further investigation.

Lifelong Ulk1-Mediated Autophagy Deficiency in Muscle Induces Mitochondrial Dysfunction and Contractile Weakness

The accumulation of damaged mitochondria due to insufficient autophagy has been implicated in the pathophysiology of skeletal muscle aging. Ulk1 is an autophagy-related kinase that initiates autophagosome assembly and may also play a role in autophagosome degradation (i.e., autophagy flux), but the contribution of Ulk1 to healthy muscle aging is unclear. Therefore, the purpose of this study was to investigate the role of Ulk1-mediated autophagy in skeletal muscle aging. At age 22 months (80% survival rate), muscle contractile and metabolic function were assessed using electrophysiology in muscle-specific Ulk1 knockout mice (MKO) and their littermate controls (LM). Specific peak-isometric torque of the ankle dorsiflexors (normalized by tibialis anterior muscle cross-sectional area) and specific force of the fast-twitch extensor digitorum longus muscles was reduced in MKO mice compared to LM mice (p < 0.03). Permeabilized muscle fibers from MKO mice had greater mitochondrial content, yet lower mitochondrial oxygen consumption and greater reactive oxygen species production compared to fibers from LM mice (p ≤ 0.04). Alterations in neuromuscular junction innervation patterns as well as changes to autophagosome assembly and flux were explored as possible contributors to the pathological features in Ulk1 deficiency. Of primary interest, we found that Ulk1 phosphorylation (activation) to total Ulk1 protein content was reduced in older muscles compared to young muscles from both human and mouse, which may contribute to decreased autophagy flux and an accumulation of dysfunctional mitochondria. Results from this study support the role of Ulk1-mediated autophagy in aging skeletal muscle, reflecting Ulk1′s dual role in maintaining mitochondrial integrity through autophagosome assembly and degradation.

Mechanisms and Clinical Applications of Glucocorticoid Steroids in Muscular Dystrophy

Glucocorticoid steroids are widely used as immunomodulatory agents in acute and chronic conditions. Glucocorticoid steroids such as prednisone and deflazacort are recommended for treating Duchenne Muscular Dystrophy where their use prolongs ambulation and life expectancy. Despite this benefit, glucocorticoid use in Duchenne Muscular Dystrophy is also associated with significant adverse consequences including adrenal suppression, growth impairment, poor bone health and metabolic syndrome. For other forms of muscular dystrophy like the limb girdle dystrophies, glucocorticoids are not typically used. Here we review the experimental evidence supporting multiple mechanisms of glucocorticoid action in dystrophic muscle including their role in dampening inflammation and myofiber injury. We also discuss alternative dosing strategies as well as novel steroid agents that are in development and testing, with the goal to reduce adverse consequences of prolonged glucocorticoid exposure while maximizing beneficial outcomes.

Estrogen Regulates the Satellite Cell Compartment in Females

Skeletal muscle mass, strength, and regenerative capacity decline with age, with many measures showing a greater deterioration in females around the time estrogen levels decrease at menopause. Here, we show that estrogen deficiency severely compromises the maintenance of muscle stem cells (i.e., satellite cells) as well as impairs self-renewal and differentiation into muscle fibers. Mechanistically, by hormone replacement, use of a selective estrogen-receptor modulator (bazedoxifene), and conditional estrogen receptor knockout, we implicate 17β-estradiol and satellite cell expression of estrogen receptor α and show that estrogen signaling through this receptor is necessary to prevent apoptosis of satellite cells. Early data from a biopsy study of women who transitioned from peri- to post-menopause are consistent with the loss of satellite cells coincident with the decline in estradiol in humans. Together, these results demonstrate an important role for estrogen in satellite cell maintenance and muscle regeneration in females.

Collins et al. show the loss of estrogen in female mice and post-menopausal women leads to a decrease in skeletal muscle stem cells. Using muscle stem cell-specific mutants, it was demonstrated that ERα is necessary for satellite cell maintenance, self-renewal, and protection from apoptosis, thereby promoting optimal muscle regeneration.

Mitochondrial dysfunction in skeletal muscle of fukutin-deficient mice is resistant to exercise- and 5-aminoimidazole-4-carboxamide ribonucleotide-induced rescue

NEW FINDINGS

What is the central question of this study? Does fukutin deficiency in skeletal muscle cause mitochondrial dysfunction, and if so, can AMP-activated protein kinase (AMPK) stimulation via 5-aminoimidazole-4-carboxamide ribonucleotide attenuate this through regulation of mitochondrial biogenesis and autophagy? What is the main finding and its importance? Mitochondrial dysfunction is associated with fukutin deficiency and AMPK stimulation may benefit muscle contractility to a greater extent than mitochondrial function.

ABSTRACT

Disruptions in the dystrophin-glycoprotein complex (DGC) are clearly the primary basis underlying various forms of muscular dystrophies and dystroglycanopathies, but the cellular consequences of DGC disruption are still being investigated. Mitochondrial abnormalities are becoming an apparent consequence and contributor to dystrophy disease pathology. Herein, we demonstrate that muscle-specific deletion of the fukutin gene (Myf5/fktn-KO mice (Fktn KO)), a model of secondary dystroglycanopathy, results in ∼30% lower muscle strength (P < 0.001) and 16% lower mitochondrial respiratory function (P = 0.002) compared to healthy littermate controls (LM). We also observed ∼80% lower expression of the gene for peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) (P = 0.004), a primary transcription factor for mitochondrial biogenesis, in Fktn KO mice that likely contributes to the mitochondrial defects. PGC-1α is post-translationally regulated via phosphorylation by AMP-activated protein kinase (AMPK). Treatment with the AMPK agonist 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) failed to rescue mitochondrial deficits in Fktn KO mice (P = 0.458) but did have beneficial (∼30% greater) effects on recovery of muscle contractility following injury in both LM and Fktn KO mice compared to saline treatment (P = 0.006). The beneficial effects of AMPK stimulation via AICAR on muscle contractile function may be partially explained by AMPK's other role of regulating skeletal muscle autophagy, a cellular process critical for clearance of damaged and/or dysfunctional organelles. Two primary conclusions can be drawn from this data: (1) fukutin deletion produces intrinsic muscular metabolic defects that likely contribute to dystroglycanopathy disease pathology, and (2) AICAR treatment accelerates recovery of muscle contractile function following injury suggesting AMPK signalling as a possible target for therapeutic strategies.

Glucocorticoids counteract hypertrophic effects of myostatin inhibition in dystrophic muscle

Duchenne muscular dystrophy (DMD) is a devastating genetic muscle disease resulting in progressive muscle degeneration and wasting. Glucocorticoids, specifically prednisone/prednisolone and deflazacort, are commonly used by DMD patients. Emerging DMD therapeutics include those targeting the muscle-wasting factor, myostatin (Mstn). The aim of this study was to investigate how chronic glucocorticoid treatment impacts the efficacy of Mstn inhibition in the D2.mdx mouse model of DMD. We report that chronic treatment of dystrophic mice with prednisolone (Pred) causes significant muscle wasting, entailing both activation of the ubiquitin-proteasome degradation pathway and inhibition of muscle protein synthesis. Combining Pred with Mstn inhibition, using a modified Mstn propeptide (dnMstn), completely abrogates the muscle hypertrophic effects of Mstn inhibition independently of Mstn expression or SMAD3 activation. Transcriptomic analysis identified that combining Pred with dnMstn treatment affects gene expression profiles associated with inflammation, metabolism, and fibrosis. Additionally, we demonstrate that Pred-induced muscle atrophy is not prevented by Mstn ablation. Therefore, glucocorticoids interfere with potential muscle mass benefits associated with targeting Mstn, and the ramifications of glucocorticoid use should be a consideration during clinical trial design for DMD therapeutics. These results have significant implications for past and future Mstn inhibition trials in DMD.

Photobiomodulation Therapy for Attenuating the Dystrophic Phenotype of Mdx Mice

This study analyzed photobiomodulation therapy (PBMT) effects on regenerative, antioxidative, anti-inflammatory and angiogenic markers in the dystrophic skeletal muscle of mdx mice, the experimental model of Duchenne muscular dystrophy (DMD), during the acute phase of dystrophy disease. The following groups were set up: Ctrl (control group of normal wild-type mice; C57BL/10); mdx (untreated mdx mice); mdxPred (mdx mice treated with prednisolone) and mdxLA (mdx mice treated with PBMT). The PBMT was carried out using an Aluminum Gallium Arsenide (AIGaAs; IBRAMED® laserpulse) diode, 830 nm wavelength, applied on the dystrophic quadriceps muscle. The mdxLA group showed a degenerative and regenerative area reduction simultaneously with a MyoD level increase, ROS production and inflammatory marker reduction and up-regulation in the VEGF factor. In addition, PBMT presented similar effects to prednisolone treatment in most of the parameters analyzed. In conclusion, our results indicate that PBMT in the parameters selected attenuated the dystrophic phenotype of mdx mice, improving skeletal muscle regeneration; reducing the oxidative stress and inflammatory process; and up-regulating the angiogenic marker.

Glycine administration attenuates progression of dystrophic pathology in prednisolone-treated dystrophin/utrophin null mice

Duchenne muscular dystrophy (DMD) is an X-linked genetic disease characterized by progressive muscle wasting and weakness and premature death. Glucocorticoids (e.g. prednisolone) remain the only drugs with a favorable impact on DMD patients, but not without side effects. We have demonstrated that glycine preserves muscle in various wasting models. Since glycine effectively suppresses the activity of pro-inflammatory macrophages, we investigated the potential of glycine treatment to ameliorate the dystrophic pathology. Dystrophic mdx and dystrophin-utrophin null (dko) mice were treated with glycine or L-alanine (amino acid control) for up to 15 weeks and voluntary running distance (a quality of life marker and strong correlate of lifespan in dko mice) and muscle morphology were assessed. Glycine increased voluntary running distance in mdx mice by 90% (P < 0.05) after 2 weeks and by 60% (P < 0.01) in dko mice co-treated with prednisolone over an 8 week treatment period. Glycine treatment attenuated fibrotic deposition in the diaphragm by 28% (P < 0.05) after 10 weeks in mdx mice and by 22% (P < 0.02) after 14 weeks in dko mice. Glycine treatment augmented the prednisolone-induced reduction in fibrosis in diaphragm muscles of dko mice (23%, P < 0.05) after 8 weeks. Our findings provide strong evidence that glycine supplementation may be a safe, simple and effective adjuvant for improving the efficacy of prednisolone treatment and improving the quality of life for DMD patients.

Gastrointestinal colonisation and systemic spread of Candida albicans in mice treated with antibiotics and prednisolone

Normally, Candida albicans is a commensal microbe that resides in the human oral cavity, gut and vagina. However, the fungus can cause mucosal and systemic infections in immunocompromised individuals. The mechanism by which local mucosal infections progress to systemic candidiasis is poorly understood. Here, a murine model of gastrointestinal (GI) candidiasis was developed by inoculation of the oral cavity, followed by treatment with tetracycline (TC) and prednisolone (PSL). Temporal progression from a local infection of the oral cavity to a systemic infection was then monitored. Histological analysis of tissues from mice treated with both TC and PSL revealed massive infiltration of the tongue and stomach by hyphae. PSL increased the fungal burden in the tongue, stomach and small intestine, and facilitated dissemination to the spleen, kidney and liver within 3 days post-infection. Treatment with both TC and PSL supressed interferon (IFN)-γ and interleukin (IL)-17 (cytokines that play key roles in host defence against fungal infection) levels in the tongue, which were induced by C. albicans infection. In addition, the mucosal layer of the small intestine of mice treated with both TC and PSL was almost destroyed by the fungal infection; this may be a critical event that allows passage of the fungus across the mucosa and into the systemic circulation. Thus, this mouse model is useful for studying mechanisms underlying progression of C. albicans from a local infection of the oral cavity to a systemic infection in immunocompromised individuals.

Deletion of estrogen receptor α in skeletal muscle results in impaired contractility in female mice

Estradiol deficiency in females can result in skeletal muscle strength loss, and treatment with estradiol mitigates the loss. There are three primary estrogen receptors (ERs), and estradiol elicits effects through these receptors in various tissues. Ubiquitous ERα-knockout mice exhibit numerous biological disorders, but little is known regarding the specific role of ERα in skeletal muscle contractile function. The purpose of this study was to determine the impact of skeletal muscle-specific ERα deletion on contractile function, hypothesizing that ERα is a main receptor through which estradiol affects muscle strength in females. Deletion of ERα specifically in skeletal muscle (skmERαKO) did not affect body mass compared with wild-type littermates (skmERαWT) until 26 wk of age, at which time body mass of skmERαKO mice began to increase disproportionally. Overall, skmERαKO mice had low strength demonstrated in multiple muscles and by several contractile parameters. Isolated extensor digitorum longus muscles from skmERαKO mice produced 16% less eccentric and 16-26% less submaximal and maximal isometric force, and isolated soleus muscles were more fatigable, with impaired force recovery relative to skmERαWT mice. In vivo maximal torque productions by plantarflexors and dorsiflexors were 16% and 12% lower in skmERαKO than skmERαWT mice, and skmERαKO muscles had low phosphorylation of myosin regulatory light chain. Plantarflexors also generated 21-32% less power, submaximal isometric and peak concentric torques. Data support the hypothesis that ablation of ERα in skeletal muscle results in muscle weakness, suggesting that the beneficial effects of estradiol on muscle strength are receptor mediated through ERα. NEW & NOTEWORTHY We comprehensively measured in vitro and in vivo skeletal muscle contractility in female estrogen receptor α (ERα) skeletal muscle-specific knockout mice and report that force generation is impaired across multiple parameters. These results support the hypothesis that a primary mechanism through which estradiol elicits its effects on strength is mediated by ERα. Evidence is presented that estradiol signaling through ERα appears to modulate force at the molecular level via posttranslational modifications of myosin regulatory light chain.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Intermittent glucocorticoid steroid dosing enhances muscle repair without eliciting muscle atrophy

Glucocorticoid steroids such as prednisone are prescribed for chronic muscle conditions such as Duchenne muscular dystrophy, where their use is associated with prolonged ambulation. The positive effects of chronic steroid treatment in muscular dystrophy are paradoxical because these steroids are also known to trigger muscle atrophy. Chronic steroid use usually involves once-daily dosing, although weekly dosing in children has been suggested for its reduced side effects on behavior. In this work, we tested steroid dosing in mice and found that a single pulse of glucocorticoid steroids improved sarcolemmal repair through increased expression of annexins A1 and A6, which mediate myofiber repair. This increased expression was dependent on glucocorticoid response elements upstream of annexins and was reinforced by the expression of forkhead box O1 (FOXO1). We compared weekly versus daily steroid treatment in mouse models of acute muscle injury and in muscular dystrophy and determined that both regimens provided comparable benefits in terms of annexin gene expression and muscle repair. However, daily dosing activated atrophic pathways, including F-box protein 32 (Fbxo32), which encodes atrogin-1. Conversely, weekly steroid treatment in mdx mice improved muscle function and histopathology and concomitantly induced the ergogenic transcription factor Krüppel-like factor 15 (Klf15) while decreasing Fbxo32. These findings suggest that intermittent, rather than daily, glucocorticoid steroid regimen promotes sarcolemmal repair and muscle recovery from injury while limiting atrophic remodeling.

Skeletal muscle metabolic adaptations to endurance exercise training are attainable in mice with simvastatin treatment

We tested the hypothesis that a 6-week regimen of simvastatin would attenuate skeletal muscle adaptation to low-intensity exercise. Male C57BL/6J wildtype mice were subjected to 6-weeks of voluntary wheel running or normal cage activities with or without simvastatin treatment (20 mg/kg/d, n = 7-8 per group). Adaptations in in vivo fatigue resistance were determined by a treadmill running test, and by ankle plantarflexor contractile assessment. The tibialis anterior, gastrocnemius, and plantaris muscles were evaluated for exercised-induced mitochondrial adaptations (i.e., biogenesis, function, autophagy). There was no difference in weekly wheel running distance between control and simvastatin-treated mice (P = 0.51). Trained mice had greater treadmill running distance (296%, P<0.001), and ankle plantarflexor contractile fatigue resistance (9%, P<0.05) compared to sedentary mice, independent of simvastatin treatment. At the cellular level, trained mice had greater mitochondrial biogenesis (e.g., ~2-fold greater PGC1α expression, P<0.05) and mitochondrial content (e.g., 25% greater citrate synthase activity, P<0.05), independent of simvastatin treatment. Mitochondrial autophagy-related protein contents were greater in trained mice (e.g., 40% greater Bnip3, P<0.05), independent of simvastatin treatment. However, Drp1, a marker of mitochondrial fission, was less in simvastatin treated mice, independent of exercise training, and there was a significant interaction between training and statin treatment (P<0.022) for LC3-II protein content, a marker of autophagy flux. These data indicate that whole body and skeletal muscle adaptations to endurance exercise training are attainable with simvastatin treatment, but simvastatin may have side effects on muscle mitochondrial maintenance via autophagy, which could have long-term implications on muscle health.

The effect of taurine and β-alanine supplementation on taurine transporter protein and fatigue resistance in skeletal muscle from mdx mice

This study investigated the effect of taurine and β-alanine supplementation on muscle function and muscle taurine transporter (TauT) protein expression in mdx mice. Wild-type (WT) and mdx mice (5 months) were supplemented with taurine or β-alanine for 4 weeks, after which in vitro contractile properties, fatigue resistance and force recovery, and the expression of the TauT protein and proteins involved in excitation-contraction (E-C) coupling were examined in fast-twitch muscle. There was no difference in basal TauT protein expression or basal taurine content between mdx than WT muscle. Supplementation with taurine and β-alanine increased and reduced taurine content, respectively, in muscle from WT and mdx mice but had no effect of TauT protein. Taurine supplementation reduced body and muscle mass, and enhanced fatigue resistance and force recovery in mdx muscle. β-Alanine supplementation enhanced fatigue resistance in WT and mdx muscle. There was no difference in the basal expression of key E-C coupling proteins [ryanodine receptor 1 (RyR1), dihydropyridine receptor (DHPR), sarco(endo)plasmic reticulum Ca²⁺-ATPase 1 (SERCA1) or calsequestrin 1 (CSQ1)] between WT and mdx mice, and the expression of these proteins was not altered by taurine or β-alanine supplementation. These findings suggest that TauT protein expression is relatively insensitive to changes in muscle taurine content in WT and mdx mice, and that taurine and β-alanine supplementation may be viable therapeutic strategies to improve fatigue resistance of dystrophic skeletal muscle.

BMP4 inhibits myogenic differentiation of bone marrow-derived mesenchymal stromal cells in mdx mice

BACKGROUND AIMS

Bone marrow-derived mesenchymal stromal cells (BMSCs) are a promising therapeutic option for treating Duchenne muscular dystrophy (DMD). Myogenic differentiation occurs in the skeletal muscle of the mdx mouse (a mouse model of DMD) after BMSC transplantation. The transcription factor bone morphogenic protein 4 (BMP4) plays a crucial role in growth regulation, differentiation and survival of many cell types, including BMSCs. We treated BMSCs with BMP4 or the BMP antagonist noggin to examine the effects of BMP signaling on the myogenic potential of BMSCs in mdx mice.

METHODS

We added BMP4 or noggin to cultured BMSCs under myogenic differentiation conditions. We then injected BMP4- or noggin-treated BMSCs into the muscles of mdx mice to determine their myogenic potential.

RESULTS

We found that the expression levels of desmin and myosin heavy chain decreased after treating BMSCs with BMP4, whereas the expression levels of phosphorylated Smad, a downstream target of BMP4, were higher in these BMSCs than in the controls. Mdx mouse muscles injected with BMSCs pretreated with BMP4 showed decreased dystrophin expression and increased phosphorylated Smad levels compared with muscles injected with non-treated BMSCs. The opposite effects were seen after pretreatment with noggin, as expected.

CONCLUSIONS

Our results identified BMP/Smad signaling as an essential negative regulator of promyogenic BMSC activity; inhibition of this pathway improved the efficiency of BMSC myogenic differentiation, which suggests that this pathway might serve as a target to regulate BMSC function for better myogenic differentiation during treatment of DMD and degenerative skeletal muscle diseases.

Influence of ovarian hormones on strength loss in healthy and dystrophic female mice

PURPOSE

The primary objective of this study was to determine whether strength loss and recovery after eccentric contractions are impaired in healthy and dystrophic female mice with low levels of ovarian hormones.

METHODS

Female C57BL/6 (wild-type) or mdx mice were randomly assigned to ovarian-intact (Sham) and ovariectomized (Ovx) groups. Anterior crural muscles were tested for susceptibility to injury from 150 or 50 eccentric contractions in wild-type and mdx mice, respectively. An additional experiment challenged mdx mice with a 2-wk treadmill running protocol followed by an eccentric contraction injury to posterior crural muscles. Functional recovery from injury was evaluated in wild-type mice by measuring isometric torque 3, 7, 14, or 21 d after injury.

RESULTS

Ovarian hormone deficiency in wild-type mice did not affect susceptibility to injury because the ∼50% isometric torque loss after eccentric contractions did not differ between Sham and Ovx mice (P = 0.121). Similarly, in mdx mice, hormone deficiency did not affect the percent of preinjury isometric torque lost by anterior crural muscles after eccentric contractions (P = 0.952), but the percent of preinjury torque in posterior crural muscles was lower in Ovx than in Sham mice (P = 0.014). Recovery from injury in wild-type mice was affected by hormone deficiency. Sham mice recovered preinjury isometric strength by 14 d (96% ± 2%), whereas Ovx mice maintained deficits at 14 and 21 d after injury (80% ± 3% and 84% ± 2%, P < 0.001).

CONCLUSIONS

Ovarian hormone status did not affect the vulnerability of skeletal muscle to strength loss after eccentric contractions. However, ovarian hormone deficiency did impair the recovery of muscle strength in female mice.

Pathways Implicated in Tadalafil Amelioration of Duchenne Muscular Dystrophy

Numerous therapeutic approaches for Duchenne and Becker Muscular Dystrophy (DMD and BMD), the most common X-linked muscle degenerative disease, have been proposed. So far, the only one showing a clear beneficial effect is the use of corticosteroids. Recent evidence indicates an improvement of dystrophic cardiac and skeletal muscles in the presence of sustained cGMP levels secondary to a blocking of their degradation by phosphodiesterase five (PDE5). Due to these data, we performed a study to investigate the effect of the specific PDE5 inhibitor, tadalafil, on dystrophic skeletal muscle function. Chronic pharmacological treatment with tadalafil has been carried out in mdx mice. Behavioral and physiological tests, as well as histological and biochemical analyses, confirmed the efficacy of the therapy. We then performed a microarray-based genomic analysis to assess the pattern of gene expression in muscle samples obtained from the different cohorts of animals treated with tadalafil. This scrutiny allowed us to identify several classes of modulated genes. Our results show that PDE5 inhibition can ameliorate dystrophy by acting at different levels. Tadalafil can lead to (1) increased lipid metabolism; (2) a switch towards slow oxidative fibers driven by the up-regulation of PGC-1α; (3) an increased protein synthesis efficiency; (4) a better actin network organization at Z-disk.

Membrane-stabilizing copolymers confer marked protection to dystrophic skeletal muscle in vivo

Duchenne muscular dystrophy (DMD) is a fatal disease of striated muscle deterioration. A unique therapeutic approach for DMD is the use of synthetic membrane stabilizers to protect the fragile dystrophic sarcolemma against contraction-induced mechanical stress. Block copolymer-based membrane stabilizer poloxamer 188 (P188) has been shown to protect the dystrophic myocardium. In comparison, the ability of synthetic membrane stabilizers to protect fragile DMD skeletal muscles has been less clear. Because cardiac and skeletal muscles have distinct structural and functional features, including differences in the mechanism of activation, variance in sarcolemma phospholipid composition, and differences in the magnitude and types of forces generated, we speculated that optimized membrane stabilization could be inherently different. Our objective here is to use principles of pharmacodynamics to evaluate membrane stabilization therapy for DMD skeletal muscles. Results show a dramatic differential effect of membrane stabilization by optimization of pharmacodynamic-guided route of poloxamer delivery. Data show that subcutaneous P188 delivery, but not intravascular or intraperitoneal routes, conferred significant protection to dystrophic limb skeletal muscles undergoing mechanical stress in vivo. In addition, structure-function examination of synthetic membrane stabilizers further underscores the importance of copolymer composition, molecular weight, and dosage in optimization of poloxamer pharmacodynamics in vivo.

Glucocorticoids and Skeletal Muscle

Glucocorticoids are known to regulate protein metabolism in skeletal muscle, producing a catabolic effect that is opposite that of insulin. In many catabolic diseases, such as sepsis, starvation, and cancer cachexia, endogenous glucocorticoids are elevated contributing to the loss of muscle mass and function. Further, exogenous glucocorticoids are often given acutely and chronically to treat inflammatory conditions such as asthma, chronic obstructive pulmonary disease, and rheumatoid arthritis, resulting in muscle atrophy. This chapter will detail the nature of glucocorticoid-induced muscle atrophy and discuss the mechanisms thought to be responsible for the catabolic effects of glucocorticoids on muscle.

Therapeutic effects of exon skipping and losartan on skeletal muscle of mdx mice

Various attempts have been made to find treatments for Duchenne muscular dystrophy (DMD) patients. Exon skipping is one of the promising technologies for DMD treatment by restoring dystropin protein, which is one of the muscle components. It is well known that losartan, an angiotensin II type1 receptor blocker, promotes muscle regeneration and differentiation by lowering the level of transforming growth factor-beta1 signaling. In this study, we illustrated the combined effects of exon skipping and losartan on skeletal muscle of mdx mice. We supplied mdx mice with losartan for 2 weeks before exon skipping treatment. The losartan with the exon skipping group showed less expression of myf5 than the losartan treated group. Also the losartan with exon skipping group recovered normal muscle architecture, in contrast to the losartan group which still showed many central nuclei. However, the exon skipping efficiency and the restoration of dystrophin protein were lower in the losartan with exon skipping group compared to the exon skipping group. We reveal that losartan promotes muscle regeneration and shortens the time taken to restore normal muscle structure when combined with exon skipping. However, combined treatment of exon skipping and losartan decreases the restoration of dystrophin protein meaning decrease of exon skipping efficiency.

What has the mdx mouse model of Duchenne muscular dystrophy contributed to our understanding of this disease?

Duchenne muscular dystrophy (DMD) is a fatal X-chromosome linked recessive disorder caused by the truncation or deletion of the dystrophin gene. The most widely used animal model of this disease is the dystrophin-deficient mdx mouse which was first discovered 30 years ago. Despite its extensive use in DMD research, no effective treatment has yet been developed for this devastating disease. This review explores what we have learned from this mouse model regarding the pathophysiology of DMD and asks if it has a future in providing a better more thorough understanding of this disease or if it will bring us any closer to improving the outlook for DMD patients.

Transplantation of human umbilical cord-derived mesenchymal stems cells for the treatment of Becker muscular dystrophy in affected pedigree members

The regeneration of muscle tissue has been achieved using multipotent mesenchymal stem cells in mouse models of injured skeletal muscle. In the present study, the utility of multipotent human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) in the treatment of Becker muscular dystrophy (BMD), a genetic disease where muscle tissue fails to regenerate, was examined in members from a pedigree affected by BMD. The disease status was evaluated in 4 affected pedigree members (II1, II2, II3 and III2; aged 50, 46, 42 and 6 years, respectively). The transplantation of the hUC‑MSCs (performed on 3 patients, I2, II3 and III2) was performed by infusion with an intravenous drip over a 30‑min period, and the patients were evaluated at 1, 3, 4 and 12 weeks following the procedure. The evaluation was based on physical characteristics, as well as on molecular testing for serum creatine kinase (CK) and lactate dehydrogenase (LDH) levels and a histological examination of muscle biopsies. The patients suffered no adverse reactions in response to the transplantation of the hUC‑MSCs. At 1 week following transplantation all 3 patients showed improvement in the muscle force of the limbs, muscle size and daily activity. The walking gait of patient III2 had improved by 1 week post-transplantation and reached a normal status by 12 weeks. Serum CK and LDH levels were decreased relative to the baseline levels. A histological examination of muscle biopsies displayed no obvious tissue regeneration. In conclusion, the treatment of patients with BMD using hUC-MSCs was safe and of therapeutic benefit that lasted for up to 12 weeks. hUC-MSCs are, therefore, a potential cell therapy-based treatment option for patients with muscular dystrophies.

The myosin super-relaxed state is disrupted by estradiol deficiency

We have used quantitative epifluorescence microscopy of fluorescent ATP to measure single-nucleotide turnover in skinned skeletal muscle fibers from mouse models of female aging and hormone treatment. Aging causes declines in muscle strength, often leading to frailty, disability, and loss of independence for the elderly. Female muscle is additionally affected by age due to reduction of ovarian hormone production with menopause. Estradiol (E2) is the key hormonal signal to skeletal muscle in females, and strength loss is attenuated by E2 treatment. To investigate E2 mechanisms on skeletal muscle, single fibers were isolated from sham-operated or ovariectomized (OVX) mice, with or without E2 treatment, and were incubated with 2'-(or-3')-O-(N-methylanthraniloyl) adenosine 5'-triphosphate (mantATP). We measured decay of mantATP fluorescence in an ATP-chase experiment, as pioneered by Cooke and coworkers, who unveiled a novel regulated state of muscle myosin characterized by slow nucleotide turnover on the order of minutes, termed the super-relaxed state (SRX). We detected a slow phase of nucleotide turnover in a portion of the myosin heads from sham fibers, consistent with SRX. Turnover was substantially faster in OVX fibers, with a turnover time constant for the slow phase of 65 ± 8s as compared to 102 ± 7s for sham fibers. 60-days E2 treatment in OVX mice substantially reversed this effect on SRX, while acute exposure of isolated muscles from OVX mice to E2 had no effect. We conclude that E2-mediated signaling reversibly regulates slow ATP turnover by myosin. Age- and hormone-related muscle functional losses may be targetable at the level of myosin structure/function for strategies to offset weakness and metabolic changes that occur with age.

Low intensity, high frequency vibration training to improve musculoskeletal function in a mouse model of Duchenne muscular dystrophy

The objective of the study was to determine if low intensity, high frequency vibration training impacted the musculoskeletal system in a mouse model of Duchenne muscular dystrophy, relative to healthy mice. Three-week old wildtype (n = 26) and mdx mice (n = 22) were randomized to non-vibrated or vibrated (45 Hz and 0.6 g, 15 min/d, 5 d/wk) groups. Invivo and exvivo contractile function of the anterior crural and extensor digitorum longus muscles, respectively, were assessed following 8 wks of vibration. Mdx mice were injected 5 and 1 days prior to sacrifice with Calcein and Xylenol, respectively. Muscles were prepared for histological and triglyceride analyses and subcutaneous and visceral fat pads were excised and weighed. Tibial bones were dissected and analyzed by micro-computed tomography for trabecular morphometry at the metaphysis, and cortical geometry and density at the mid-diaphysis. Three-point bending tests were used to assess cortical bone mechanical properties and a subset of tibiae was processed for dynamic histomorphometry. Vibration training for 8 wks did not alter trabecular morphometry, dynamic histomorphometry, cortical geometry, or mechanical properties (P≥0.34). Vibration did not alter any measure of muscle contractile function (P≥0.12); however the preservation of muscle function and morphology in mdx mice indicates vibration is not deleterious to muscle lacking dystrophin. Vibrated mice had smaller subcutaneous fat pads (P = 0.03) and higher intramuscular triglyceride concentrations (P = 0.03). These data suggest that vibration training at 45 Hz and 0.6 g did not significantly impact the tibial bone and the surrounding musculature, but may influence fat distribution in mice.

Microtubule binding distinguishes dystrophin from utrophin

Dystrophin and utrophin are highly similar proteins that both link cortical actin filaments with a complex of sarcolemmal glycoproteins, yet localize to different subcellular domains within normal muscle cells. In mdx mice and Duchenne muscular dystrophy patients, dystrophin is lacking and utrophin is consequently up-regulated and redistributed to locations normally occupied by dystrophin. Transgenic overexpression of utrophin has been shown to significantly improve aspects of the disease phenotype in the mdx mouse; therefore, utrophin up-regulation is under intense investigation as a potential therapy for Duchenne muscular dystrophy. Here we biochemically compared the previously documented microtubule binding activity of dystrophin with utrophin and analyzed several transgenic mouse models to identify phenotypes of the mdx mouse that remain despite transgenic utrophin overexpression. Our in vitro analyses revealed that dystrophin binds microtubules with high affinity and pauses microtubule polymerization, whereas utrophin has no activity in either assay. We also found that transgenic utrophin overexpression does not correct subsarcolemmal microtubule lattice disorganization, loss of torque production after in vivo eccentric contractions, or physical inactivity after mild exercise. Finally, our data suggest that exercise-induced inactivity correlates with loss of sarcolemmal neuronal NOS localization in mdx muscle, whereas loss of in vivo torque production after eccentric contraction-induced injury is associated with microtubule lattice disorganization.

Adaptations of mouse skeletal muscle to low-intensity vibration training

PURPOSE

We tested the hypothesis that low-intensity vibration training in mice improves contractile function of hindlimb skeletal muscles and promotes exercise-related cellular adaptations.

METHODS

We subjected C57BL/6J mice to 6 wk, 5 d·wk, 15 min·d of sham or low-intensity vibration (45 Hz, 1.0g) while housed in traditional cages (Sham-Active, n = 8; Vibrated-Active, n = 10) or in small cages to restrict physical activity (Sham-Restricted, n = 8; Vibrated-Restricted, n = 8). Contractile function and resistance to fatigue were tested in vivo (anterior and posterior crural muscles) and ex vivo on the soleus muscle. Tibialis anterior and soleus muscles were evaluated histologically for alterations in oxidative metabolism, capillarity, and fiber types. Epididymal fat pad and hindlimb muscle masses were measured. Two-way ANOVAs were used to determine the effects of vibration and physical inactivity.

RESULTS

Vibration training resulted in a 10% increase in maximal isometric torque (P = 0.038) and 16% faster maximal rate of relaxation (P = 0.030) of the anterior crural muscles. Posterior crural muscles were unaffected by vibration, except greater rates of contraction in Vibrated-Restricted mice compared with Vibrated-Active and Sham-Restricted mice (P = 0.022). Soleus muscle maximal isometric tetanic force tended to be greater (P = 0.057), and maximal relaxation was 20% faster (P = 0.005) in vibrated compared with sham mice. The restriction of physical activity induced muscle weakness but was not required for vibration to be effective in improving strength or relaxation. Vibration training did not affect muscle fatigability or any indicator of cellular adaptation investigated (P ≥ 0.431). Fat pad but not hindlimb muscle masses were affected by vibration training.

CONCLUSION

Vibration training in mice improved muscle contractility, specifically strength and relaxation rates, with no indication of adverse effects to muscle function or cellular adaptations.

Musculoskeletal response of dystrophic mice to short term, low intensity, high frequency vibration

OBJECTIVES

We aimed to identify parameters of low-intensity vibration that initiate the greatest osteogenic response in dystrophin-deficient mice and determine vibration safety for diseased muscle in three separate studies.

METHODS

Study1: Mdx mice were randomized into seven vibration treatments and 14 d later, plasma osteocalcin and tibial osteogenic gene expression were compared among treatments. Study2: Three days of vibration was compared to other modalities known to elicit muscle injury in mdx mice. Study3: Dystrophic mice with more severe phenotypes due to altered utrophin were subjected to 7 d vibration to determine if muscle injury was induced. Muscle torque and genes associated with inflammation and myogenesis were assessed in Studies 2-3.

RESULTS

Two sets of parameters (45 Hz 0.6 g and 90 Hz 0.6 g) evoked osteogenic responses. 45 Hz upregulated alkaline phosphatase and tended to upregulate osteoprotegerin without altering RANKL, and 90 Hz simultaneously upregulated osteprotegerin and RANKL. Thus, subsequent muscle studies utilized 45 Hz. Vibration for 3 or 7 d was not injurious to dystrophic muscle as shown by the lack of differences between vibrated and non-vibrated mice in torque and gene expression.

CONCLUSIONS

Results indicate that vibration at 45 Hz and 0.6 g is safe for dystrophic muscle and may be a therapeutic modality to improve musculoskeletal health in DMD.

A new immuno-, dystrophin-deficient model, the NSG-mdx(4Cv) mouse, provides evidence for functional improvement following allogeneic satellite cell transplantation

Transplantation of a myogenic cell population into an immunodeficient recipient is an excellent way of assessing the in vivo muscle-generating capacity of that cell population. To facilitate both allogeneic and xenogeneic transplantations of muscle-forming cells in mice, we have developed a novel immunodeficient muscular dystrophy model, the NSG-mdx(4Cv) mouse. The IL2Rg mutation, which is linked to the Dmd gene on the X chromosome, simultaneously depletes NK cells and suppresses thymic lymphomas, issues that limit the utility of the SCID/mdx model. The NSG-mdx(4Cv) mouse presents a muscular dystrophy of similar severity to the conventional mdx mouse. We show that this animal supports robust engraftment of both pig and dog muscle mononuclear cells. The question of whether satellite cells prospectively isolated by flow cytometry can confer a functional benefit upon transplantation has been controversial. Using allogeneic Pax7-ZsGreen donors and NSG-mdx(4Cv) recipients, we demonstrate definitively that as few as 900 FACS-isolated satellite cells can provide functional regeneration in vivo, in the form of an increased mean maximal force-generation capacity in cell-transplanted muscles, compared to a sham-injected control group. These studies highlight the potency of satellite cells to improve muscle function and the utility of the NSG-mdx(4Cv) model for studies on muscle regeneration and Duchenne muscular dystrophy therapy.

Micro-dystrophin and follistatin co-delivery restores muscle function in aged DMD model

Pharmacologic strategies have provided modest improvement in the devastating muscle-wasting disease, Duchenne muscular dystrophy (DMD). Pre-clinical gene therapy studies have shown promise in the mdx mouse model; however, studies conducted after disease onset fall short of fully correcting muscle strength or protecting against contraction-induced injury. Here we examine the treatment effect on muscle physiology in aged dystrophic mice with significant disease pathology by combining two promising therapies: micro-dystrophin gene replacement and muscle enhancement with follistatin, a potent myostatin inhibitor. Individual treatments with micro-dystrophin and follistatin demonstrated marked improvement in mdx mice but were insufficient to fully restore muscle strength and response to injury to wild-type levels. Strikingly, when combined, micro-dystrophin/follistatin treatment restored force generation and conferred resistance to contraction-induced injury in aged mdx mice. Pre-clinical studies with miniature dystrophins have failed to demonstrate full correction of the physiological defects seen in mdx mice. Importantly, the addition of a muscle enhancement strategy with delivery of follistatin in combination with micro-dystrophin gene therapy completely restored resistance to eccentric contraction-induced injury and improved force. Eccentric contraction-induced injury is a pre-clinical parameter relevant to the exercise induced injury that occurs in DMD patients, and herein, we demonstrate compelling evidence for the therapeutic potential of micro-dystrophin/follistatin combinatorial therapy.

Exercise training improves plantar flexor muscle function in mdx mice

PURPOSE

We tested the hypothesis that low-intensity exercise in mdx mice improves plantar flexor muscle contractile function, resistance to fatigue, and mitochondrial adaptations without exacerbating muscular dystrophy.

METHODS

We subjected mdx mice to 12 wk of voluntary low-resistance wheel running (Run, n = 17) or normal cage activities (sedentary (Sed), n = 16) followed by in vivo analyses for plantar flexor torque generation and fatigue resistance or running capacity on a treadmill. Gastrocnemius muscles were further evaluated for exercise-induced mitochondrial adaptations and fiber type distribution and central nuclei. t-tests were used to determine differences between the Sed and Run groups.

RESULTS

Plantar flexor submaximal isometric torques and maximal isometric torque at multiple ankle joint angles and resistance to fatigue were greater in Run compared with Sed mdx mice (P G 0.05). Citrate synthase and A-hydroxyacyl-CoA dehydrogenase enzyme activities and cytochrome c oxidase IV protein expression in gastrocnemius muscles were greater in Run than in Sed mdx mice(P e 0.04), along with a trend of fiber type transformation from Type IIb to Type IIx fibers. Exercise training in mdx mice did not elevate serum creatine kinase levels but led to a significant reduction of centrally nucleated myofibers.

CONCLUSIONS

Voluntary low-resistance wheel running in mdx mice can result in skeletal muscle adaptation, leading to improved contractile function and reduced fatigability,with no indication that exercise was detrimental. This study supports the need for further investigation of low-intensity exercise as an early therapeutic intervention in ambulatory boys with Duchenne muscular dystrophy.

Acute failure of action potential conduction in mdx muscle reveals new mechanism of contraction-induced force loss

A primary feature of skeletal muscle lacking the protein dystrophin, as occurring in Duchenne muscular dystrophy, is a hypersensitivity to contraction-induced strength loss. We tested the hypothesis that the extensive strength loss results from an impairment in the electrophysiological function of the plasmalemma specifically impaired action potential development. Anterior crural muscles from mdx and wildtype mice performed a single bout of 100 electrically stimulated eccentric contractions in vivo. Electromyography, specifically the M-wave, was analysed during muscle contraction to assess the ability of the tibialis anterior muscle plasmalemma to generate and conduct action potentials. During eccentric contractions, wildtype mice exhibited a 36% loss in torque about the ankle but mdx mice exhibited a greater torque loss of 73% (P < 0.001). Despite the loss of torque, there was no reduction in M-wave root mean square (RMS) for wildtype mice, which was in stark contrast to mdx mice that had a 55% reduction in M-wave RMS (P < 0.001). This impairment resolved within 24 h and coincided with a significant improvement in strength and membrane integrity. Intracellular measurements of resting membrane potential (RMP) in uninjured and injured extensor digitorum longus muscles were made to determine if a chronic depolarization had occurred, which could lead to impaired fibre excitability and/or altered action potential conduction properties. The distributions of RMP were not different between wildtype uninjured and injured muscle cells (median: -73.2 mV vs. -72.7 mV, P = 0.46) whereas there was a significant difference between mdx uninjured and injured cells (median: -71.5 mV vs. -56.6 mV, P < 0.001). These data show that mdx muscle fibres are depolarized after an injurious bout of eccentric contractions. These findings (i) suggest a major plasmalemma-based mechanism of strength loss underlying contraction-induced injury in Duchenne muscular dystrophy distinctly different from that for healthy muscle, and (ii) demonstrate dystrophin is critical for maintaining action potential generation and conduction after eccentric contractions.

Skeletal muscle contractile function and neuromuscular performance in Zmpste24 -/- mice, a murine model of human progeria

Human progeroid syndromes and premature aging mouse models present as segmental, accelerated aging because some tissues and not others are affected. Skeletal muscle is detrimentally changed by normal aging but whether it is an affected tissue in progeria has not been resolved. We hypothesized that mice which mimic Hutchinson-Gilford progeria syndrome would exhibit age-related alterations of skeletal muscle. Zmpste24 (-/-) and Zmpste24 (+/+) littermates were assessed for skeletal muscle functions, histo-morphological characteristics, and ankle joint mechanics. Twenty-four-hour active time, ambulation, grip strength, and whole body tension were evaluated as markers of neuromuscular performance, each of which was at least 33% lower in Zmpste24 (-/-) mice compared with littermates (p < 0.06). Contractile capacity of the posterior leg muscles were not affected in Zmpste24 (-/-) mice, but muscles of the anterior leg were 30-90% weaker than those of Zmpste24 (+/+) mice (p < 0.01). Leg muscles were 32-47% smaller in the Zmpste24 (-/-) mice and contained ~60% greater collagen relative to littermates (p < 0.01). Soleus and extensor digitorum longus muscles of Zmpste24 (-/-) mice had excessive myonuclei and altered fiber size distributions but, otherwise, appeared normal. Ankle range of motion was 70% lower and plantar- and dorsiflexion passive torques were nearly 3-fold greater in Zmpste24 (-/-) than Zmpste24 (+/+) mice (p ≤ 0.01). The combined factors of muscle atrophy, collagen accumulation, and perturbed joint mechanics likely contributed to poor neuromuscular performance and selective muscle weakness displayed by Zmpste24 (-/-)mice. In summary, these characteristics are similar to those of aged mice indicating accelerated aging of skeletal muscle in progeria.

Sildenafil reduces respiratory muscle weakness and fibrosis in the mdx mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy caused by mutations in the dystrophin gene. Loss of dystrophin initiates a progressive decline in skeletal muscle integrity and contractile capacity which weakens respiratory muscles including the diaphragm, culminating in respiratory failure, the leading cause of morbidity and mortality in DMD patients. At present, corticosteroid treatment is the primary pharmacological intervention in DMD, but has limited efficacy and adverse side effects. Thus, there is an urgent need for new safe, cost-effective, and rapidly implementable treatments that slow disease progression. One promising new approach is the amplification of nitric oxide-cyclic guanosine monophosphate (NO-cGMP) signalling pathways with phosphodiesterase 5 (PDE5) inhibitors. PDE5 inhibitors serve to amplify NO signalling that is attenuated in many neuromuscular diseases including DMD. We report here that a 14-week treatment of the mdx mouse model of DMD with the PDE5 inhibitor sildenafil (Viagra(®), Revatio(®)) significantly reduced mdx diaphragm muscle weakness without impacting fatigue resistance. In addition to enhancing respiratory muscle contractility, sildenafil also promoted normal extracellular matrix organization. PDE5 inhibition slowed the establishment of mdx diaphragm fibrosis and reduced matrix metalloproteinase-13 (MMP-13) expression. Sildenafil also normalized the expression of the pro-fibrotic (and pro-inflammatory) cytokine tumour necrosis factor α (TNFα). Sildenafil-treated mdx diaphragms accumulated significantly less Evans Blue tracer dye than untreated controls, which is also indicative of improved diaphragm muscle health. We conclude that sildenafil-mediated PDE5 inhibition significantly reduces diaphragm respiratory muscle dysfunction and pathology in the mdx mouse model of Duchenne muscular dystrophy. This study provides new insights into the therapeutic utility of targeting defects in NO-cGMP signalling with PDE5 inhibitors in dystrophin-deficient muscle.

Prednisolone treatment and restricted physical activity further compromise bone of mdx mice

OBJECTIVES

The purpose of this study was to determine the extent to which prednisolone treatment and restricted physical activity caused deleterious changes in inherently compromised mdx bone.

METHODS

Four week-old male mdx mice (n=36) were treated for 8-wk either with or without prednisolone (0.8-1.3 mg/kg/d) and were housed in traditional or small cages (restricted activity). Tibial bone strength, geometry, and intrinsic material properties were assessed at the mid-shaft by three-point bending and micro-computed tomography (μCT).

RESULTS

Three-point bending results showed that both prednisolone and restricted activity reduced bone strength (7%), however stiffness was only reduced in restricted-activity mice. μCT analyses showed that cortical bone area and cortical thickness were 13% smaller in restricted-activity mice, and may have accounted for their compromised bone strength. Intrinsic material properties, including volumetric bone mineral density (vBMD) and modulus of elasticity, were not impacted by either treatment, however, vBMD tended to be lower in restricted-activity mice (p=0.06).

CONCLUSIONS

These data show that prednisolone treatment and restricted physical activity independently accentuate reductions in the strength and geometry of mdx bone, but do not influence intrinsic material properties.

Prednisolone treatment does not interfere with 2'-O-methyl phosphorothioate antisense-mediated exon skipping in Duchenne muscular dystrophy

In Duchenne muscular dystrophy (DMD), dystrophin deficiency leading to progressive muscular degeneration is caused by frame-shifting mutations in the DMD gene. Antisense oligonucleotides (AONs) aim to restore the reading frame by skipping of a specific exon(s), thereby allowing the production of a shorter, but semifunctional protein, as is found in the mostly more mildly affected patients with Becker muscular dystrophy. AONs are currently being investigated in phase 3 placebo-controlled clinical trials. Most of the participating patients are treated symptomatically with corticosteroids (mainly predniso[lo]ne) to stabilize the muscle fibers, which might affect the uptake and/or efficiency of AONs. Therefore the effect of prednisolone on 2'-O-methyl phosphorothioate AON efficacy in patient-derived cultured muscle cells and the mdx mouse model (after local and systemic AON treatment) was assessed in this study. Both in vitro and in vivo skip efficiency and biomarker expression were comparable between saline- and prednisolone-cotreated cells and mice. After systemic exon 23-specific AON (23AON) treatment for 8 weeks, dystrophin was detectable in all treated mice. Western blot analyses indicated slightly higher dystrophin levels in prednisolone-treated mice, which might be explained by better muscle condition and consequently more target dystrophin pre-mRNA. In addition, fibrotic and regeneration biomarkers were normalized to some extent in prednisolone- and/or 23AON-treated mice. Overall these results show that the use of prednisone forms no barrier to participation in clinical trials with AONs.

Motor performance of young dystrophic mdx mice treated with long-circulating prednisolone liposomes

For Duchenne muscular dystrophy (DMD), a common myopathy that leads to severe disability, no causal therapy is available. Glucocorticosteroids improve patients' muscle strength, but their long-term use is limited by negative side effects. Thus, pharmacological modifications of glucocorticosteroids are required to increase the efficacy by drug targeting. Liposomal encapsulation augments systemic half-life and local tissue concentrations of glucocorticosteroids and, at the same time, reduces systemic side effects. In this study, the efficacy of novel, long-circulating, polyethylene-glycol-coated liposomes encapsulating prednisolone was compared with free prednisolone in the treatment of mdx mice, a well-established animal model for DMD. Using an objective and sensitive computerized 24-hr detection system of voluntary wheel-running in single cages, we demonstrate a significant impairment of the running performance in mdx compared with black/10 control mice aged 3-6 weeks. Treatment with liposomal or free prednisolone did not improve running performance compared with saline control or empty liposomes. Histopathological parameters, including the rate of internalized nuclei and fiber size variation, and mRNA and protein expression levels of transforming growth factor (TGF)-β and monocytes chemotactic protein (MCP)-1 also remained unchanged. Bioactivity in skeletal muscle of liposomal and free prednisolone was demonstrated by elevated mRNA expression of muscle ring finger protein 1 (MuRF1), a mediator of muscle atrophy, and its forkhead box transcription factors (Foxo1/3). Our data support the assessment of voluntary running to be a robust and reproducible outcome measure of skeletal muscle performance during the early disease course of mdx mice and suggest that liposomal encapsulation is not superior in treatment efficacy compared with conventional prednisolone. Our study helps to improve the future design of experimental treatment in animal models of neuromuscular diseases.

Adaptive strength gains in dystrophic muscle exposed to repeated bouts of eccentric contraction

The objective of this study was to determine the functional recovery and adaptation of dystrophic muscle to multiple bouts of contraction-induced injury. Because lengthening (i.e., eccentric) contractions are extremely injurious for dystrophic muscle, it was considered that repeated bouts of such contractions would exacerbate the disease phenotype in mdx mice. Anterior crural muscles (tibialis anterior and extensor digitorum longus) and posterior crural muscles (gastrocnemius, soleus, and plantaris) from mdx mice performed one or five repeated bouts of 100 electrically stimulated eccentric contractions in vivo, and each bout was separated by 10-18 days. Functional recovery from one bout was achieved 7 days after injury, which was in contrast to a group of wild-type mice, which still showed a 25% decrement in electrically stimulated isometric torque at that time point. Across bouts there was no difference in the immediate loss of strength after repeated bouts of eccentric contractions for mdx mice (-70%, P = 0.68). However, after recovery from each bout, dystrophic muscle had greater torque-generating capacity such that isometric torque was increased ∼38% for both anterior and posterior crural muscles at bout 5 compared with bout 1 (P < 0.001). Moreover, isolated extensor digitorum longus muscles excised from in vivo-tested hindlimbs 14-18 days after bout 5 had greater specific force than contralateral control muscles (12.2 vs. 10.4 N/cm(2), P = 0.005) and a 20% greater maximal relaxation rate (P = 0.049). Additional adaptations due to the multiple bouts of eccentric contractions included rapid recovery and/or sparing of contractile proteins, enhanced parvalbumin expression, and a decrease in fiber size variability. In conclusion, eccentric contractions are injurious to dystrophic skeletal muscle; however, the muscle recovers function rapidly and adapts to repeated bouts of eccentric contractions by improving strength.