Small-caliber skeletal muscle fibers do not suffer necrosis in mdx mouse dystrophy

Transcriptomic analysis of paired healthy human skeletal muscles to identify modulators of disease severity in DMD

Muscle damage and fibro-fatty replacement of skeletal muscles is a main pathologic feature of Duchenne muscular dystrophy (DMD) with more proximal muscles affected earlier and more distal affected later in the disease course, suggesting that different skeletal muscle groups possess distinctive characteristics that influence their susceptibility to disease. To explore transcriptomic factors driving differential gene expression and modulating DMD skeletal muscle severity, we characterized the transcriptome of vastus lateralis (VL), a more proximal and susceptible muscle, relative to tibialis anterior (TA), a more distal and protected muscle, in 15 healthy individuals using bulk RNA sequencing to identify gene expression differences that may mediate their relative susceptibility to damage with loss of dystrophin. Matching single nuclei RNA sequencing data was generated for 3 of the healthy individuals, to infer cell composition in the bulk RNA sequencing dataset and to improve mapping of differentially expressed genes to their cell source of expression. A total of 3,410 differentially expressed genes were identified and mapped to cell type using single nuclei RNA sequencing of muscle, including long non-coding RNAs and protein coding genes. There was an enrichment of genes involved in calcium release from the sarcoplasmic reticulum, particularly in the myofibers and these myofiber genes were higher in the VL. There was an enrichment of genes in "Collagen-Containing Extracellular Matrix" expressed by fibroblasts, endothelial, smooth muscle and pericytes, with most genes higher in the TA, as well as genes in "Regulation Of Apoptotic Process" expressed across all cell types. Previously reported genetic modifiers were also enriched within the differentially expressed genes. We also identify 6 genes with differential isoform usage between the VL and TA. Lastly, we integrate our findings with DMD RNA sequencing data from the TA, and identify "Collagen-Containing Extracellular Matrix" and "Negative Regulation Of Apoptotic Process" as differentially expressed between DMD compared to healthy. Collectively, these findings propose novel candidate mechanisms that may mediate differential muscle susceptibility in muscular dystrophies and provide new insight into potential therapeutic targets.

Modulating fast skeletal muscle contraction protects skeletal muscle in animal models of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal muscle disease caused by absence of the protein dystrophin, which acts as a structural link between the basal lamina and contractile machinery to stabilize muscle membranes in response to mechanical stress. In DMD, mechanical stress leads to exaggerated membrane injury and fiber breakdown, with fast fibers being the most susceptible to damage. A major contributor to this injury is muscle contraction, controlled by the motor protein myosin. However, how muscle contraction and fast muscle fiber damage contribute to the pathophysiology of DMD has not been well characterized. We explored the role of fast skeletal muscle contraction in DMD with a potentially novel, selective, orally active inhibitor of fast skeletal muscle myosin, EDG-5506. Surprisingly, even modest decreases of contraction (<15%) were sufficient to protect skeletal muscles in dystrophic mdx mice from stress injury. Longer-term treatment also decreased muscle fibrosis in key disease-implicated tissues. Importantly, therapeutic levels of myosin inhibition with EDG-5506 did not detrimentally affect strength or coordination. Finally, in dystrophic dogs, EDG-5506 reversibly reduced circulating muscle injury biomarkers and increased habitual activity. This unexpected biology may represent an important alternative treatment strategy for Duchenne and related myopathies.

Duchenne muscular dystrophy progression induced by downhill running is accompanied by increased endomysial fibrosis and oxidative damage DNA in muscle of mdx mice

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle necrosis. One of the major challenges for prescribing physical rehabilitation exercises for DMD patients is associated with the lack of a thorough knowledge of dystrophic muscle responsiveness to exercise. This study aims to understand the relationship between myogenic regulation, inflammation and oxidative stress parameters, and disease progression induced by downhill running in the skeletal muscle of an experimental model of DMD. Six-month-old C57BL/10 and C57BL/10-DMD^mdx male mice were distributed into three groups: Control (C), mdx, and mdx + Exercise (mdx + Ex). Animals were trained in a downhill running protocol for seven weeks. The gastrocnemius muscle was subjected to histopathology, muscle regeneration (myoD and myogenin), inflammation (COX-2), oxidative stress (8-OHdG) immunohistochemistry markers, and gene expression (qPCR) of NF-kB and NADP(H)Oxidase 2 (NOX-2) analysis. In the mdx + Ex group, the gastrocnemius muscle showed a higher incidence of endomysial fibrosis and a lower myonecrosis percentage area. Immunohistochemical analysis revealed decreased myogenin immunoexpression in the mdx group, as well as accentuated immunoexpression of nuclear 8-OHdG in both mdx groups and increase in cytoplasmic 8-OHdG only in the mdx + Ex. COX-2 immunoexpression was related to areas of regeneration process and inflammatory infiltrate in the mdx group, while associated with areas of muscle fibrosis in the mdx + Ex. Moreover, the NF-kB gene expression was not influenced by exercise; however, a NAD(P)HOxidase 2 increase was observed. Oxidative stress and oxidative DNA damage play a significant role in the DMD phenotype progression induced by exercise, compromising cellular patterns resulting in increased endomysial fibrosis.

Artificial intelligence workflow quantifying muscle features on Hematoxylin-Eosin stained sections reveals dystrophic phenotype amelioration upon treatment

Cell segmentation is a key step for a wide variety of biological investigations, especially in the context of muscle science. Currently, automated methods still struggle to perform skeletal muscle fiber quantification on Hematoxylin-Eosin (HE) stained histopathological whole slide images due to low contrast. On the other hand, the Deep Learning algorithm Cellpose offers new perspectives considering its increasing adoption for segmentation of a wide range of cells. Combining two open-source tools, Cellpose and QuPath, we developed MyoSOTHES, an automated Myofibers Segmentation wOrkflow Tuned for HE Staining. MyoSOTHES enables solving segmentation inconsistencies encountered by default Cellpose model in presence of large range size cells and provides information related to muscle Feret's diameter distribution and Centrally Nucleated Fibers, thus depicting muscle health and treatment effects. MyoSOTHES achieves high quality segmentation compared to baseline workflow with a detection F1-score increasing from 0.801 to 0.919 and a Root Mean Square Error (RMSE) on diameter improved by 31%. MyoSOTHES was validated on an animal study featuring gene transfer in [Formula: see text]-Sarcoglycanopathy, for which dose-response effect is visible and conclusions drawn are consistent with those previously published. MyoSOTHES thus paves the way for wide quantification of HE stained muscle sections and retrospective analysis of HE labeled slices used in laboratories for decades.

Taurine Administration Counteracts Aging-Associated Impingement of Skeletal Muscle Regeneration by Reducing Inflammation and Oxidative Stress

Sarcopenia, which occurs during aging, is characterized by the gradual loss of skeletal muscle mass and function, resulting in a functional decline in physical abilities. Several factors contribute to the onset of sarcopenia, including reduced regenerative capacity, chronic low-grade inflammation, mitochondrial dysfunction, and increased oxidative stress, leading to the activation of catabolic pathways. Physical activity and adequate protein intake are considered effective strategies able to reduce the incidence and severity of sarcopenia by exerting beneficial effects in improving the muscular anabolic response during aging. Taurine is a non-essential amino acid that is highly expressed in mammalian tissues and, particularly, in skeletal muscle where it is involved in the regulation of biological processes and where it acts as an antioxidant and anti-inflammatory factor. Here, we evaluated whether taurine administration in old mice counteracts the physiopathological effects of aging in skeletal muscle. We showed that, in injured muscle, taurine enhances the regenerative process by downregulating the inflammatory response and preserving muscle fiber integrity. Moreover, taurine attenuates ROS production in aged muscles by maintaining a proper cellular redox balance, acting as an antioxidant molecule. Although further studies are needed to better elucidate the molecular mechanisms responsible for the beneficial effect of taurine on skeletal muscle homeostasis, these data demonstrate that taurine administration ameliorates the microenvironment allowing an efficient regenerative process and attenuation of the catabolic pathways related to the onset of sarcopenia.

Muscle Damage in Dystrophic mdx Mice Is Influenced by the Activity of Ca2+-Activated KCa3.1 Channels

Duchenne muscular dystrophy (DMD) is an X-linked disease, caused by a mutant dystrophin gene, leading to muscle membrane instability, followed by muscle inflammation, infiltration of pro-inflammatory macrophages and fibrosis. The calcium-activated potassium channel type 3.1 (K_Ca3.1) plays key roles in controlling both macrophage phenotype and fibroblast proliferation, two critical contributors to muscle damage. In this work, we demonstrate that pharmacological blockade of the channel in the mdx mouse model during the early degenerative phase favors the acquisition of an anti-inflammatory phenotype by tissue macrophages and reduces collagen deposition in muscles, with a concomitant reduction of muscle damage. As already observed with other treatments, no improvement in muscle performance was observed in vivo. In conclusion, this work supports the idea that K_Ca3.1 channels play a contributing role in controlling damage-causing cells in DMD. A more complete understanding of their function could lead to the identification of novel therapeutic approaches.

Caenorhabditis elegans as a Model System for Duchenne Muscular Dystrophy

The nematode worm Caenorhabditis elegans has been used extensively to enhance our understanding of the human neuromuscular disorder Duchenne Muscular Dystrophy (DMD). With new arising clinically relevant models, technologies and treatments, there is a need to reconcile the literature and collate the key findings associated with this model.

Ixazomib, an oral proteasome inhibitor, exhibits potential effect in dystrophin-deficient mdx mice

Dystrophin deficiency makes the sarcolemma fragile and susceptible to degeneration in Duchenne muscular dystrophy. The proteasome is a multimeric protease complex and is central to the regulation of cellular proteins. Previous studies have shown that proteasome inhibition improved pathological changes in mdx mice. Ixazomib is the first oral proteasome inhibitor used as a therapy in multiple myeloma. This study investigated the effects of ixazomib on the dystrophic muscle of mdx mice. MDX mice were treated with ixazomib (7.5 mg/kg/wk by gavage) or 0.2 mL of saline for 12 weeks. The Kondziela test was performed to measure muscle strength. The tibialis anterior (TA) and diaphragm (DIA) muscles were used for morphological analysis, and blood samples were collected for biochemical measurement. We observed maintenance of the muscle strength in the animals treated with ixazomib. Treatment with ixazomib had no toxic effect on the mdx mouse. The morphological analysis showed a reduction in the inflammatory area and fibres with central nuclei in the TA and DIA muscles and an increase in the number of fibres with a diameter of 20 µm2 in the DIA muscle after treatment with ixazomib. There was an increase in the expression of dystrophin and utrophin in the TA and DIA muscles and a reduction in the expression of osteopontin and TGF-β in the DIA muscle of mdx mice treated with ixazomib. Ixazomib was thus shown to increase the expression of dystrophin and utrophin associated with improved pathological and functional changes in the dystrophic muscles of mdx mice.

Histological features of masticatory muscles after botulinum toxin A injection into the right masseter muscle of dystrophin deficient (mdx-) mice

OBJECTIVE/BACKGROUND

The most frequently used animal model for human DMD (Duchenne muscular dystrophy) research is the mdx mouse. In both species, characteristic histological changes like inflammation, muscle fiber degeneration and fibrosis are the same, but in contrast to humans, in mdx mice, phases of muscle fiber degeneration are compensated by regeneration processes.

AIM

Therefore, the interest of this study was to evaluate histological features in masticatory muscles after BTX-A injection into the right masseter muscle of wild type and dystrophic (mdx) mice, illustrating de- and regeneration processes induced by this substance.

MATERIAL AND METHODS

The right masseter muscle of 100 days old healthy and mdx mice were selectively paralyzed by a single intramuscular BTX-A injection. Masseter as well as temporal muscle of injection and non-injection side were carefully dissected 21 days and 42 days after injection, respectively, and fiber diameter, cell nuclei position, necrosis and collagen content were analyzed histomorphologically in order to evaluate de- and regeneration processes in these muscles. Statistical analysis was performed using SigmaStat Software and Mann Whitney U-test (significance level: p < 0.05).

RESULTS

At both investigation periods and in both mouse strains fiber diameter was significantly reduced and collagen content was significantly increased in the right injected masseter muscle whereas fiber diameters in mdx mice were much smaller, and these differences were even more apparent at the second investigation period. Necrosis and central located nuclei could generally be found in all mdx mice muscles investigated with an amount of centronucleation exceeding 60%, and a significant increase of necrosis six weeks after injection. In wild type mice central located nuclei could primarily be found in the treated masseter muscle with a portion of 2.7%, and this portion decreased after six weeks, whereas in mdx mice a decrease could also be seen in the non-injected muscles. In contrast, in wild type mice necrosis was not apparent at any time and in all muscles investigated.

CONCLUSION

From our results it can be concluded that in mdx mice masticatory muscles de- and regeneration processes were extended, triggered by a selective BTX-A injection, or mdx mice at this age, independently of BTX-A treatment, went through another cycle of de- and regeneration as a characteristic of this disease.

Mitochondrial oxidative stress impairs contractile function but paradoxically increases muscle mass via fibre branching

BACKGROUND

Excess reactive oxygen species (ROS) and muscle weakness occur in parallel in multiple pathological conditions. However, the causative role of skeletal muscle mitochondrial ROS (mtROS) on neuromuscular junction (NMJ) morphology and function and muscle weakness has not been directly investigated.

METHODS

We generated mice lacking skeletal muscle-specific manganese-superoxide dismutase (mSod2KO) to increase mtROS using a cre-Lox approach driven by human skeletal actin. We determined primary functional parameters of skeletal muscle mitochondrial function (respiration, ROS, and calcium retention capacity) using permeabilized muscle fibres and isolated muscle mitochondria. We assessed contractile properties of isolated skeletal muscle using in situ and in vitro preparations and whole lumbrical muscles to elucidate the mechanisms of contractile dysfunction.

RESULTS

The mSod2KO mice, contrary to our prediction, exhibit a 10-15% increase in muscle mass associated with an ~50% increase in central nuclei and ~35% increase in branched fibres (P < 0.05). Despite the increase in muscle mass of gastrocnemius and quadriceps, in situ sciatic nerve-stimulated isometric maximum-specific force (N/cm² ), force per cross-sectional area, is impaired by ~60% and associated with increased NMJ fragmentation and size by ~40% (P < 0.05). Intrinsic alterations of components of the contractile machinery show elevated markers of oxidative stress, for example, lipid peroxidation is increased by ~100%, oxidized glutathione is elevated by ~50%, and oxidative modifications of myofibrillar proteins are increased by ~30% (P < 0.05). We also find an approximate 20% decrease in the intracellular calcium transient that is associated with specific force deficit. Excess superoxide generation from the mitochondrial complexes causes a deficiency of succinate dehydrogenase and reduced complex-II-mediated respiration and adenosine triphosphate generation rates leading to severe exercise intolerance (~10 min vs. ~2 h in wild type, P < 0.05).

CONCLUSIONS

Increased skeletal muscle mtROS is sufficient to elicit NMJ disruption and contractile abnormalities, but not muscle atrophy, suggesting new roles for mitochondrial oxidative stress in maintenance of muscle mass through increased fibre branching.

Cycles of myofiber degeneration and regeneration lead to remodeling of the neuromuscular junction in two mammalian models of Duchenne muscular dystrophy

Mice lacking the sarcolemmal protein dystrophin, designated mdx, have been widely used as a model of Duchenne muscular dystrophy. Dystrophic mdx mice as they mature develop notable morphological abnormalities to their neuromuscular junctions, the peripheral cholinergic synapses responsible for activating muscle fibers. Most obviously the acetylcholine receptor aggregates are fragmented into small non-continuous, islands. This contrasts with wild type mice whose acetylcholine receptor aggregates are continuous and pretzel-shaped in appearance. We show here that these abnormalities in mdx mice are also present in a canine model of Duchenne muscular dystrophy and provide additional evidence to support the hypothesis that NMJ remodeling occurs due to myofiber degeneration and regeneration. Using a method to investigate synaptic AChR replacement, we show that neuromuscular junction remodeling in mdx animals is caused by muscle fiber degeneration and regeneration at the synaptic site and is mimicked by deliberate myofiber injury in wild type mice. Importantly, the innervating motor axon plays a crucial role in directing the remodeling of the neuromuscular junction in dystrophy, as has been recorded in aging and deliberate muscle fiber injury in wild type mice. The remodeling occurs repetitively through the life of the animal and the changes in junctions become greater with age.

The dual CCR2/CCR5 chemokine receptor antagonist Cenicriviroc reduces macrophage infiltration and disease severity in Duchenne muscular dystrophy (Dmdmdx-4Cv) mice

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle weakness which is ultimately fatal, most often due to involvement of the diaphragm. Macrophage infiltration of dystrophic muscles has been strongly linked to muscle damage and fibrosis in DMD. We hypothesized that cenicriviroc (CVC), a dual chemokine receptor (CCR2/CCR5) antagonist currently under clinical evaluation for other diseases, could prevent macrophage accumulation and blunt disease progression in the diaphragms of mdx mice (genetic homologue of DMD). Treatment with CVC (20 mg/kg/day intraperitoneally) or vehicle was initiated in mdx mice at 2 weeks of age (prior to the onset of muscle necrosis) and continued for 4 weeks. Flow cytometry to assess inflammatory cell subsets as well as histological and force generation parameters were determined in mdx diaphragms at the conclusion of the treatment. CVC therapy induced a major (3.9-fold) reduction in total infiltrating macrophages, whereas total numbers of neutrophils and T lymphocytes (CD4+ and CD8+) were unaffected. No changes in macrophage polarization status (inflammatory versus anti-inflammatory skewing based on iNOS and CD206 expression) were observed. Muscle fiber size and fibrosis were not altered by CVC, whereas a significant reduction in centrally nucleated fibers was found suggesting a decrease in prior necrosis-regeneration cycles. In addition, maximal isometric force production by the diaphragm was increased by CVC therapy. These results suggest that CVC or other chemokine receptor antagonists which reduce pathological macrophage infiltration may have the potential to slow disease progression in DMD.

Repurposing Dantrolene for Long-Term Combination Therapy to Potentiate Antisense-Mediated DMD Exon Skipping in the mdx Mouse

Duchenne muscular dystrophy (DMD) is caused by mutations in DMD, resulting in loss of dystrophin, which is essential to muscle health. DMD “exon skipping” uses anti-sense oligo-nucleotides (AONs) to force specific exon exclusion during mRNA processing to restore reading frame and rescue of partially functional dystrophin protein. Although exon-skipping drugs in humans show promise, levels of rescued dystrophin protein remain suboptimal. We previously identified dantrolene as a skip booster when combined with AON in human DMD cultures and short-term mdx dystrophic mouse studies. Here, we assess the effect of dantrolene/AON combination on DMD exon-23 skipping over long-term mdx treatment under conditions that better approximate potential human dosing. To evaluate the dantrolene/AON combination treatment effect on dystrophin induction, we assayed three AON doses, with and without oral dantrolene, to assess multiple outcomes across different muscles. Meta-analyses of the results of statistical tests from both the quadriceps and diaphragm assessing contributions of dantrolene beyond AON, across all AON treatment groups, provide strong evidence that dantrolene modestly boosts exon skipping and dystrophin rescue while reducing muscle pathology in mdx mice (p < 0.0087). These findings support a trial of combination dantrolene/AON to increase exon-skipping efficacy and highlight the value of combinatorial approaches and Food and Drug Administration (FDA) drug re-purposing for discovery of unsuspected therapeutic application and rapid translation.

Preaged remodeling of myofibrillar cytoarchitecture in skeletal muscle expressing R349P mutant desmin

The majority of hereditary and acquired myopathies are clinically characterized by progressive muscle weakness. We hypothesized that ongoing derangement of skeletal muscle cytoarchitecture at the single fiber level may precede and be responsible for the progressive muscle weakness. Here, we analyzed the effects of aging in wild-type (wt) and heterozygous (het) and homozygous (hom) R349P desmin knock-in mice. The latter harbor the ortholog of the most frequently encountered human R350P desmin missense mutation. We quantitatively analyzed the subcellular cytoarchitecture of fast- and slow-twitch muscles from young, intermediate, and aged wt as well as desminopathy mice. We recorded multiphoton second harmonic generation and nuclear fluorescence signals in single muscle fibers to compare aging-related effects in all genotypes. The analysis of wt mice revealed that the myofibrillar cytoarchitecture remained stable with aging in fast-twitch muscles, whereas slow-twitch muscle fibers displayed structural derangements during aging. In contrast, the myofibrillar cytoarchitecture and nuclear density were severely compromised in fast- and slow-twitch muscle fibers of hom R349P desmin mice at all ages. Het mice only showed a clear degradation in their fiber structure in fast-twitch muscles from the adult to the presenescent age bin. Our study documents distinct signs of normal and R349P mutant desmin-related remodeling of the 3D myofibrillar architecture during aging, which provides a structural basis for the progressive muscle weakness.

Stem Cells and Tissue Niche: Two Faces of the Same Coin of Muscle Regeneration

Capacity of adult muscle to regenerate in response to injury stimuli represents an important homeostatic process. Regeneration is a highly coordinated program that partially recapitulates the embryonic developmental program. However, muscle regeneration is severely compromised in several pathological conditions. It is likely that the restricted tissue repair program under pathological conditions is due to either progressive loss of stem cell populations or to missing signals that limit the damaged tissues to efficiently activate a regenerative program. It is therefore plausible that loss of control over these cell fates might lead to a pathological cell transdifferentiation, limiting the ability of a pathological muscle to sustain an efficient regenerative process. The critical role of microenvironment on stem cells activity and muscle regeneration is discussed.

Duane retraction syndrome in a patient with Duchenne muscular dystrophy

PURPOSE

We describe the clinical features of a boy with bilateral Duane retraction syndrome (DRS), Duchenne muscular dystrophy (DMD), and other medical problems.

METHODS

The child was followed-up for five years; his chart was reviewed, including the results of a muscle biopsy and genetic testing. Multiplex ligation-dependent probe amplification (MLPA) was used to interrogate deletions/duplications in the dystrophin gene.

RESULTS

The proband had bilateral DRS with otherwise normal ocular motility; he also had developmental delay, mild mental retardation, and seizures. Clinical diagnosis of DMD included progressive proximal weakness, highly elevated creatine kinase levels, and a muscle biopsy showing significant dystrophic changes including contracted, degenerative, and regenerative fibers, and negative dystrophin immunostaining. MLPA documented duplication of exons 3 and 4 of the dystrophin gene.

CONCLUSIONS

This boy is the third patient to be reported with DRS and DMD, the second with bilateral DRS and the only one with other neurologic features. Mutated dystrophin is present in extraocular muscles and in the central nervous system (CNS) in DMD, leaving open the question of whether this co-occurrence is the result of the genetic muscle abnormality, CNS effects caused by dystrophin mutations, or chance.

Isolation of Mouse Periocular Tissue for Histological and Immunostaining Analyses of the Extraocular Muscles and Their Satellite Cells

The extraocular muscles (EOMs) comprise a group of highly specialized skeletal muscles controlling eye movements. Although a number of unique features of EOMs including their sparing in Duchenne muscular dystrophy have drawn a continuous interest, knowledge about these hard to reach muscles is still limited. The goal of this chapter is to provide detailed methods for the isolation and histological analysis of mouse EOMs. We first introduce in brief the basic anatomy and established nomenclature of the extraocular primary and accessory muscles. We then provide a detailed description with step-by-step images of our procedure for isolating (and subsequently cryosectioning) EOMs while preserving the integrity of their original structural organization. Next, we present several useful histological protocols frequently used by us, including: (1) a method for highlighting the general organization of periocular tissue, using the MyoD(Cre) × R26(mTmG) reporter mouse that elegantly distinguishes muscle (MyoD(Cre)-driven GFP(+)) from the non-myogenic constituents (Tomato(+)); (2) analysis by H&E staining, allowing for example, detection of the pathological features of the dystrophin-null phenotype in affected limb and diaphragm muscles that are absent in EOMs; (3) detection of the myogenic progenitors (i.e., satellite cells) in their native position underneath the myofiber basal lamina using Pax7/laminin double immunostaining. The EOM tissue harvesting procedure described here can also be adapted for isolating and studying satellite cells and other cell types. Overall, the methods described in this chapter should provide investigators the necessary tools for entering the EOM research field and contribute to a better understanding of this highly specialized muscle group and its complex micro-anatomy.

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.

Inflammatory monocytes promote progression of Duchenne muscular dystrophy and can be therapeutically targeted via CCR2

Myofiber necrosis and fibrosis are hallmarks of Duchenne muscular dystrophy (DMD), leading to lethal weakness of the diaphragm. Macrophages (MPs) are required for successful muscle regeneration, but the role of inflammatory monocyte (MO)-derived MPs in either promoting or mitigating DMD is unclear. We show that DMD (mdx) mouse diaphragms exhibit greatly increased expression of CCR2 and its chemokine ligands, along with inflammatory (Ly6C(high)) MO recruitment and accumulation of CD11b(high) MO-derived MPs. Loss-of-function of CCR2 preferentially reduced this CD11b(high) MP population by impeding the release of Ly6C(high) MOs from the bone marrow but not the splenic reservoir. CCR2 deficiency also helped restore the MP polarization balance by preventing excessive skewing of MPs toward a proinflammatory phenotype. These effects were linked to amelioration of histopathological features and increased muscle strength in the diaphragm. Chronic inhibition of CCR2 signaling by mutated CCL2 secreted from implanted mesenchymal stem cells resulted in similar improvements. These data uncover a previously unrecognized role of inflammatory MOs in DMD pathogenesis and indicate that CCR2 inhibition could offer a novel strategy for DMD management.

Dystroglycanopathy muscles lacking functional glycosylation of alpha-dystroglycan retain regeneration capacity

In dystroglycanopathies, lack of glycosylated alpha-dystroglycan (α-DG) alters membrane fragility leading to fiber damage and repetitive cycles of muscle degeneration and regeneration. However the effect of the glycosylation of α-DG on muscle regeneration is not clearly understood. In this study, we examined the regenerative capacity of dystrophic muscles in vivo in FKRP mutant and LARGE(myd) mice with little and complete lack of functionally glycosylated α-DG (F-α-DG) respectively. The number of regenerating fibers expressing embryonic myosin heavy chain (eMyHC) in the diseased muscles up to the age of 10 months is higher than or at similar levels to wild type muscle after notexin and polyethyleminine insults. The process of fiber maturation is not significantly affected by the lack of F-α-DG assessed by size distribution. The earlier appearance of a larger number of regenerating fibers after injury is consistent with the observation that the populations of myogenic satellite cells are increased and being readily activated in the dystroglycanopathy muscles. F-α-DG is expressed at trace amounts in undifferentiated myoblasts, but increases in differentiated myotubes in vitro. We therefore conclude that muscle regeneration is not impaired in the early stage of the dystroglycanopathies, and F-α-DG does not play a significant role in myogenic cell proliferation and fiber formation and maturation.

Pharmacologic management of Duchenne muscular dystrophy: target identification and preclinical trials

Duchenne muscular dystrophy (DMD) is an X-linked human disorder in which absence of the protein dystrophin causes degeneration of skeletal and cardiac muscle. For the sake of treatment development, over and above definitive genetic and cell-based therapies, there is considerable interest in drugs that target downstream disease mechanisms. Drug candidates have typically been chosen based on the nature of pathologic lesions and presumed underlying mechanisms and then tested in animal models. Mammalian dystrophinopathies have been characterized in mice (mdx mouse) and dogs (golden retriever muscular dystrophy [GRMD]). Despite promising results in the mdx mouse, some therapies have not shown efficacy in DMD. Although the GRMD model offers a higher hurdle for translation, dogs have primarily been used to test genetic and cellular therapies where there is greater risk. Failed translation of animal studies to DMD raises questions about the propriety of methods and models used to identify drug targets and test efficacy of pharmacologic intervention. The mdx mouse and GRMD dog are genetically homologous to DMD but not necessarily analogous. Subcellular species differences are undoubtedly magnified at the whole-body level in clinical trials. This problem is compounded by disparate cultures in clinical trials and preclinical studies, pointing to a need for greater rigor and transparency in animal experiments. Molecular assays such as mRNA arrays and genome-wide association studies allow identification of genetic drug targets more closely tied to disease pathogenesis. Genes in which polymorphisms have been directly linked to DMD disease progression, as with osteopontin, are particularly attractive targets.

Toll-like receptor 4 ablation in mdx mice reveals innate immunity as a therapeutic target in Duchenne muscular dystrophy

Toll-like receptor 4 (TLR4) recognizes specific structural motifs associated with microbial pathogens and also responds to certain endogenous host molecules associated with tissue damage. In Duchenne muscular dystrophy (DMD), inflammation plays an important role in determining the ultimate fate of dystrophic muscle fibers. In this study, we used TLR4-deficient dystrophic mdx mice to assess the role of TLR4 in the pathogenesis of DMD. TLR4 expression was increased and showed enhanced activation following agonist stimulation in mdx diaphragm muscle. Genetic ablation of TLR4 led to significantly increased muscle force generation in dystrophic diaphragm muscle, which was associated with improved histopathology including decreased fibrosis, as well as reduced pro-inflammatory gene expression and macrophage infiltration. TLR4 ablation in mdx mice also altered the phenotype of muscle macrophages by inducing a shift toward a more anti-inflammatory (iNOS(neg) CD206(pos)) profile. In vitro experiments confirmed that lack of TLR4 is sufficient to influence macrophage activation status in response to classical polarizing stimuli such as IFN-gamma and IL-4. Finally, treatment of dystrophic mice with glycyrrhizin, an inhibitor of the endogenous TLR4 ligand, high mobility group box (HMGB1), also pointed to involvement of the HMGB1-TLR4 axis in promoting dystrophic diaphragm pathology. Taken together, our findings reveal TLR4 and the innate immune system as important players in the pathophysiology of DMD. Accordingly, targeting either TLR4 or its endogenous ligands may provide a new therapeutic strategy to slow disease progression.

SMASH - semi-automatic muscle analysis using segmentation of histology: a MATLAB application

Background

Histological assessment of skeletal muscle tissue is commonly applied to many areas of skeletal muscle physiological research. Histological parameters including fiber distribution, fiber type, centrally nucleated fibers, and capillary density are all frequently quantified measures of skeletal muscle. These parameters reflect functional properties of muscle and undergo adaptation in many muscle diseases and injuries. While standard operating procedures have been developed to guide analysis of many of these parameters, the software to freely, efficiently, and consistently analyze them is not readily available. In order to provide this service to the muscle research community we developed an open source MATLAB script to analyze immunofluorescent muscle sections incorporating user controls for muscle histological analysis.

Results

The software consists of multiple functions designed to provide tools for the analysis selected. Initial segmentation and fiber filter functions segment the image and remove non-fiber elements based on user-defined parameters to create a fiber mask. Establishing parameters set by the user, the software outputs data on fiber size and type, centrally nucleated fibers, and other structures. These functions were evaluated on stained soleus muscle sections from 1-year-old wild-type and mdx mice, a model of Duchenne muscular dystrophy. In accordance with previously published data, fiber size was not different between groups, but mdx muscles had much higher fiber size variability. The mdx muscle had a significantly greater proportion of type I fibers, but type I fibers did not change in size relative to type II fibers. Centrally nucleated fibers were highly prevalent in mdx muscle and were significantly larger than peripherally nucleated fibers.

Conclusions

The MATLAB code described and provided along with this manuscript is designed for image processing of skeletal muscle immunofluorescent histological sections. The program allows for semi-automated fiber detection along with user correction. The output of the code provides data in accordance with established standards of practice. The results of the program have been validated using a small set of wild-type and mdx muscle sections. This program is the first freely available and open source image processing program designed to automate analysis of skeletal muscle histological sections.

Impaired structural and functional regeneration of skeletal muscles from β2-adrenoceptor knockout mice

Aims

β2-adrenergic stimulation causes beneficial effects on structure and function of regenerating muscles; thus, the β2-adrenoceptor may play an important role in the muscle regenerative process. Here, we investigated the role of the β₂-adrenoceptor in skeletal muscle regeneration.

Methods

Tibialis anterior (TA) muscles from β2-adrenoceptor knockout (β₂KO) mice were cryolesioned and analysed after 1, 3, 10 and 21 days. The role of β2-adrenoceptor on regenerating muscles was assessed through the analysis of morphological and contractile aspects, M1 and M2 macrophage profile, cAMP content, and activation of TGF-β signalling elements.

Results

Regenerating muscles from β₂KO mice showed decreased calibre of regenerating myofibres and reduced muscle contractile function at 10 days when compared with those from wild type. The increase in cAMP content in muscles at 10 days post-cryolesion was attenuated in the absence of the β₂-adrenoceptor. Furthermore, there was an increase in inflammation and in the number of macrophages in regenerating muscles lacking the β2-adrenoceptor at 3 and 10 days, a predominance of M1 macrophage phenotype, a decrease in TβR-I/Smad2/3 activation, and in the Smad4 expression at 3 days, while akirin1 expression increased at 10 days in muscles from β₂KO mice when compared to those from wild type.

Conclusions

Our results suggest that the β2-adrenoceptor contributes to the regulation of the initial phases of muscle regeneration, especially in the control of macrophage recruitment in regenerating muscle through activation of TβR-I/Smad2/3 and reduction in akirin1 expression. These findings have implications for the future development of better therapeutic approaches to prevent or treat muscle injuries.

Autologous minced muscle grafts: a tissue engineering therapy for the volumetric loss of skeletal muscle

Volumetric muscle loss (VML) results in a large void deficient in the requisite materials for regeneration for which there is no definitive clinical standard of care. Autologous minced muscle grafts (MG), which contain the essential components for muscle regeneration, may embody an ideal tissue engineering therapy for VML. The purpose of this study was to determine if orthotopic transplantation of MG acutely after VML in the tibialis anterior muscle of male Lewis rats promotes functional tissue regeneration. Herein we report that over the first 16 wk postinjury, MG transplantation 1) promotes remarkable regeneration of innervated muscle fibers within the defect area (i.e., de novo muscle fiber regeneration); 2) reduced evidence of chronic injury in the remaining muscle mass compared with nonrepaired muscles following VML (i.e., transplantation attenuated chronically upregulated transforming growth factor-β1 gene expression and the presence of centrally located nuclei in 30% of fibers observed in nonrepaired muscles); and 3) significantly improves net torque production (i.e., ∼55% of the functional deficit in nonrepaired muscles was restored). Additionally, voluntary wheel running was shown to reduce the heightened accumulation of extracellular matrix deposition observed within the regenerated tissue of MG-repaired sedentary rats 8 wk postinjury (collagen 1% area: sedentary vs. runner, ∼41 vs. 30%), which may have been the result of an augmented inflammatory response [i.e., M1 (CCR7) and M2 (CD163) macrophage expression was significantly greater in runner than sedentary MG-repaired muscles 2 wk postinjury]. These findings support further exploration of autologous minced MGs for the treatment of VML.

Why short stature is beneficial in Duchenne muscular dystrophy

INTRODUCTION

Duchenne muscular dystrophy (DMD) is caused by a genetic defect resulting in absent dystrophin, yet children are able to walk when small and young but lose this ability as they grow. The mdx mouse has absent dystrophin yet does not exhibit significant disability.

METHODS

Allometric modeling of linearly increasing load per muscle fiber and stress on the sarcolemma with growth and exponential decline associated with loss of muscle fibers correlated with case studies and animal models of DMD.

RESULTS

Smaller species or breeds are predictably less affected than large as follows: mdx mice < small golden retriever muscular dystrophy (GRMD) dogs < large GRMD dogs < humans. Case reports of combined growth hormone and dystrophin deficiency show a relatively benign course of disease.

CONCLUSIONS

Future therapeutic trials in DMD might include specific growth inhibitors in combination with standard of care treatments to delay the clinical onset and reduce the severity of disease and disability.

The role of Pitx2 in maintaining the phenotype of myogenic precursor cells in the extraocular muscles

Many differences exist between extraocular muscles (EOM) and non-cranial skeletal muscles. One striking difference is the sparing of EOM in various muscular dystrophies compared to non-cranial skeletal muscles. EOM undergo continuous myonuclear remodeling in normal, uninjured adults, and distinct transcription factors are required for the early determination, development, and maintenance of EOM compared to limb skeletal muscle. Pitx2, a bicoid-like homeobox transcription factor, is required for the development of EOM and the maintenance of characteristic properties of the adult EOM phenotype, but is not required for the development of limb muscle. We hypothesize that these unique properties of EOM contribute to the constitutive differences between EOM and non-craniofacial skeletal muscles. Using flow cytometry, CD34(+)/Sca1(-/)CD45(-/)CD31(-) cells (EECD34 cells) were isolated from extraocular and limb skeletal muscle and in vitro, EOM EECD34 cells proliferated faster than limb muscle EECD34 cells. To further define these myogenic precursor cells from EOM and limb skeletal muscle, they were analyzed for their expression of Pitx2. Western blotting and immunohistochemical data demonstrated that EOM express higher levels of Pitx2 than limb muscle, and 80% of the EECD34 cells expressed Pitx2. siRNA knockdown of Pitx2 expression in EECD34 cells in vitro decreased proliferation rates and impaired the ability of EECD34 cells to fuse into multinucleated myotubes. High levels of Pitx2 were retained in dystrophic and aging mouse EOM and the EOM EECD34 cells compared to limb muscle. The differential expression of Pitx2 between EOM and limb skeletal muscle along with the functional changes in response to lower levels of Pitx2 expression in the myogenic precursor cells suggest a role for Pitx2 in the maintenance of constitutive differences between EOM and limb skeletal muscle that may contribute to the sparing of EOM in muscular dystrophies.

A new web-based method for automated analysis of muscle histology

Background

Duchenne Muscular Dystrophy is an inherited degenerative neuromuscular disease characterised by rapidly progressive muscle weakness. Currently, curative treatment is not available. Approaches for new treatments that improve muscle strength and quality of life depend on preclinical testing in animal models. The mdx mouse model is the most frequently used animal model for preclinical studies in muscular dystrophy research. Standardised pathology-relevant parameters of dystrophic muscle in mdx mice for histological analysis have been developed in international, collaborative efforts, but automation has not been accessible to most research groups. A standardised and mainly automated quantitative assessment of histopathological parameters in the mdx mouse model is desirable to allow an objective comparison between laboratories.

Methods

Immunological and histochemical reactions were used to obtain a double staining for fast and slow myosin. Additionally, fluorescence staining of the myofibre membranes allows defining the minimal Feret’s diameter. The staining of myonuclei with the fluorescence dye bisbenzimide H was utilised to identify nuclei located internally within myofibres. Relevant structures were extracted from the image as single objects and assigned to different object classes using web-based image analysis (MyoScan). Quantitative and morphometric data were analysed, e.g. the number of nuclei per fibre and minimal Feret’s diameter in 6 month old wild-type C57BL/10 mice and mdx mice.

Results

In the current version of the module “MyoScan”, essential parameters for histologic analysis of muscle sections were implemented including the minimal Feret’s diameter of the myofibres and the automated calculation of the percentage of internally nucleated myofibres. Morphometric data obtained in the present study were in good agreement with previously reported data in the literature and with data obtained from manual analysis.

Conclusions

A standardised and mainly automated quantitative assessment of histopathological parameters in the mdx mouse model is now available. Automated analysis of histological parameters is more rapid and less time-consuming. Moreover, results are unbiased and more reliable. Efficacy of therapeutic interventions, e.g. within the scope of a drug screening or therapeutic exon skipping, can be monitored. The automatic analysis system MyoScan used in this study is not limited exclusively to dystrophin-deficient mice but also represents a useful tool for applications in the research of other dystrophic pathologies, various other skeletal muscle diseases and degenerative neuromuscular disorders.

Extensive inflammatory cell infiltration in human skeletal muscle in response to an ultraendurance exercise bout in experienced athletes

The impact of a 24-h ultraendurance exercise bout on systemic and local muscle inflammatory reactions was investigated in nine experienced athletes. Blood and muscle biopsies were collected before (Pre), immediately after the exercise bout (Post), and after 28 h of recovery (Post28). Circulating blood levels of leukocytes, creatine kinase (CK), C-reactive protein (CRP), and selected inflammatory cytokines were assessed together with the evaluation of the occurrence of inflammatory cells (CD3(+), CD8(+), CD68(+)) and the expression of major histocompatibility complex class I (MHC class I) in skeletal muscle. An extensive inflammatory cell infiltration occurred in all athletes, and the number of CD3(+), CD8(+), and CD68(+) cells were two- to threefold higher at Post28 compared with Pre (P < 0.05). The inflammatory cell infiltration was associated with a significant increase in the expression of MHC class I in muscle fibers. There was a significant increase in blood leukocyte count, IL-6, IL-8, CRP, and CK at Post. At Post28, total leukocytes, IL-6, and CK had declined, whereas IL-8 and CRP continued to increase. Increases in IL-1β and TNF-α were not significant. There were no significant associations between the magnitude of the systemic and local muscle inflammatory reactions. Signs of muscle degenerative and regenerative events were observed in all athletes with various degrees of severity and were not affected by the 24-h ultraendurance exercise bout. In conclusion, a low-intensity but very prolonged single-endurance exercise bout can generate a strong inflammatory cell infiltration in skeletal muscle of well-trained experienced ultraendurance athletes, and the amplitude of the local reaction is not proportional to the systemic inflammatory response.

Serum cholinesterases are differentially regulated in normal and dystrophin-deficient mutant mice

The cholinesterases, acetylcholinesterase (AChE), and butyrylcholinesterase (BChE) (pseudocholinesterase), are abundant in the nervous system and in other tissues. The role of AChE in terminating transmitter action in the peripheral and central nervous system is well understood. However, both knowledge of the function(s) of the cholinesterases in serum, and of their metabolic and endocrine regulation under normal and pathological conditions, is limited. This study investigates AChE and BChE in sera of dystrophin-deficient mdx mutant mice, an animal model for the human Duchenne muscular dystrophy (DMD) and in control healthy mice. The data show systematic and differential variations in the concentrations of both enzymes in the sera, and specific changes dictated by alteration of hormonal balance in both healthy and dystrophic mice. While AChE in mdx-sera is elevated, BChE is markedly diminished, resulting in an overall cholinesterase decrease compared to sera of healthy controls. The androgen testosterone (T) is a negative modulator of BChE, but not of AChE, in male mouse sera. T-removal elevated both BChE activity and the BChE/AChE ratio in mdx male sera to values resembling those in healthy control male mice. Mechanisms of regulation of the circulating cholinesterases and their impairment in the dystrophic mice are suggested, and clinical implications for diagnosis and treatment are considered.

AMPK activation stimulates autophagy and ameliorates muscular dystrophy in the mdx mouse diaphragm

Duchenne muscular dystrophy (DMD) is characterized by myofiber death from apoptosis or necrosis, leading in many patients to fatal respiratory muscle weakness. Among other pathological features, DMD muscles show severely deranged metabolic gene regulation and mitochondrial dysfunction. Defective mitochondria not only cause energetic deficiency, but also play roles in promoting myofiber atrophy and injury via opening of the mitochondrial permeability transition pore. Autophagy is a bulk degradative mechanism that serves to augment energy production and eliminate defective mitochondria (mitophagy). We hypothesized that pharmacological activation of AMP-activated protein kinase (AMPK), a master metabolic sensor in cells and on-switch for the autophagy-mitophagy pathway, would be beneficial in the mdx mouse model of DMD. Treatment of mdx mice for 4 weeks with an established AMPK agonist, AICAR (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside), potently triggered autophagy in the mdx diaphragm without inducing muscle fiber atrophy. In AICAR-treated mdx mice, the exaggerated sensitivity of mdx diaphragm mitochondria to calcium-induced permeability transition pore opening was restored to normal levels. There were associated improvements in mdx diaphragm histopathology and in maximal force-generating capacity, which were not linked to increased mitochondrial biogenesis or up-regulated utrophin expression. These findings suggest that agonists of AMPK and other inducers of the autophagy-mitophagy pathway can help to promote the elimination of defective mitochondria and may thus serve as useful therapeutic agents in DMD.

The paradox of muscle hypertrophy in muscular dystrophy

Mutations in the dystrophin gene cause Duchenne and Becker muscular dystrophy in humans and syndromes in mice, dogs, and cats. Affected humans and dogs have progressive disease that leads primarily to muscle atrophy. Mdx mice progress through an initial phase of muscle hypertrophy followed by atrophy. Cats have persistent muscle hypertrophy. Hypertrophy in humans has been attributed to deposition of fat and connective tissue (pseudohypertrophy). Increased muscle mass (true hypertrophy) has been documented in animal models. Muscle hypertrophy can exaggerate postural instability and joint contractures. Deleterious consequences of muscle hypertrophy should be considered when developing treatments for muscular dystrophy.

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.

β1D chain increases α7β1 integrin and laminin and protects against sarcolemmal damage in mdx mice

The dystrophin-glycoprotein complex connects myofibers with extracellular matrix laminin. In Duchenne muscular dystrophy, this linkage system is absent and the integrity of muscle fibers is compromised. One potential therapy for addressing muscular dystrophy is to augment the amount of α7β1 integrin, the major laminin-binding integrin in skeletal muscle. Whereas transgenic over-expression of α7 chain may alleviate development of muscular dystrophy and extend the lifespan of severely dystrophic mdx/utrn(-/-) mice, further enhancing levels of α7 chain provided little additional membrane integrin and negligible additional improvement in mdx mice. We demonstrate here that normal levels of β1 chain limit formation of integrin heterodimer and that increasing β1D chain in mdx mice results in more functional integrin at the sarcolemma, more matrix laminin and decreased damage of muscle fibers. Moreover, increasing the amount of β1D chain in vitro enhances transcription of α7 integrin and α2 laminin genes and the amounts of these proteins. Thus manipulation of β1D integrin expression offers a novel approach to enhance integrin-mediated therapy for muscular dystrophy.

Changes in aging mouse neuromuscular junctions are explained by degeneration and regeneration of muscle fiber segments at the synapse

Vertebrate neuromuscular junctions are highly stable synapses, retaining the morphology they achieve in early postnatal development throughout most of life. However, these synapses undergo dramatic change during aging. The acetylcholine receptors (AChRs) change from smooth gutters into fragmented islands, and the nerve terminals change similarly to be varicosities apposed to these islands. These changes have been attributed to a slow deterioration in mechanisms maintaining the synapse. We have used repeated, vital imaging to investigate how these changes occur in the sternomastoid muscle of aging mice. We have found, contrary to expectation, that individual junctions change infrequently, but change, when it occurs, is sudden and dramatic. The change mimics that reported previously for cases in which muscle fibers are deliberately damaged: most of the AChRs present disappear rapidly and are replaced by a new set of receptors that become fragmented. The fiber segment underneath the synapse has centrally located nuclei, showing that this segment has undergone necrosis, quickly regenerated, and been reinnervated with an altered synapse. We show that necrotic events are common in aged muscle and have likely been missed previously as a cause of the alterations in aging because central nuclei are a transient phenomenon and the necrotic events at the junction infrequent. However, the changes are permanent and accumulate over time. Interventions to reduce the neuromuscular changes during aging should likely focus on making muscle fibers resistant to injury.

The β2-adrenoceptor agonist formoterol improves structural and functional regenerative capacity of skeletal muscles from aged rat at the early stages of postinjury

Skeletal muscles from old rats fail to completely regenerate following injury. This study investigated whether pharmacological stimulation of β2-adrenoceptors in aged muscles following injury could improve their regenerative capacity, focusing on myofiber size recovery. Young and aged rats were treated with a subcutaneous injection of β2-adrenergic agonist formoterol (2 μg/kg/d) up to 10 and 21 days after soleus muscle injury. Formoterol-treated muscles from old rats evaluated at 10 and 21 days postinjury showed reduced inflammation and connective tissue but a similar number of regenerating myofibers of greater caliber when compared with their injured controls. Formoterol minimized the decrease in tetanic force and increased protein synthesis and mammalian target of rapamycin phosphorylation in old muscles at 10 days postinjury. Our results suggest that formoterol improves structural and functional regenerative capacity of regenerating skeletal muscles from aged rats by increasing protein synthesis via mammalian target of rapamycin activation. Furthermore, formoterol may have therapeutic benefits in recovery following muscle damage in senescent individuals.

Deficiency of α-actinin-3 is associated with increased susceptibility to contraction-induced damage and skeletal muscle remodeling

Sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-disk in skeletal muscle, where they crosslink actin and other structural proteins to maintain an ordered myofibrillar array. Homozygosity for the common null polymorphism (R577X) in ACTN3 results in the absence of fast fiber-specific α-actinin-3 in ∼20% of the general population. α-Actinin-3 deficiency is associated with decreased force generation and is detrimental to sprint and power performance in elite athletes, suggesting that α-actinin-3 is necessary for optimal forceful repetitive muscle contractions. Since Z-disks are the structures most vulnerable to eccentric damage, we sought to examine the effects of α-actinin-3 deficiency on sarcomeric integrity. Actn3 knockout mouse muscle showed significantly increased force deficits following eccentric contraction at 30% stretch, suggesting that α-actinin-3 deficiency results in an increased susceptibility to muscle damage at the extremes of muscle performance. Microarray analyses demonstrated an increase in muscle remodeling genes, which we confirmed at the protein level. The loss of α-actinin-3 and up-regulation of α-actinin-2 resulted in no significant changes to the total pool of sarcomeric α-actinins, suggesting that alterations in fast fiber Z-disk properties may be related to differences in functional protein interactions between α-actinin-2 and α-actinin-3. In support of this, we demonstrated that the Z-disk proteins, ZASP, titin and vinculin preferentially bind to α-actinin-2. Thus, the loss of α-actinin-3 changes the overall protein composition of fast fiber Z-disks and alters their elastic properties, providing a mechanistic explanation for the loss of force generation and increased susceptibility to eccentric damage in α-actinin-3-deficient individuals.

Carbamylated erythropoietin does not alleviate signs of dystrophy in mdx mice

Erythropoietin promotes myoblast proliferation and inhibits fibrosis and thus it could impede the pathogenesis of muscle degenerative diseases. However, its stimulation of erythropoiesis limits its use as a therapeutic agent. An erythropoietin analog, carbamylated erythropoietin (C-EPO), retains these protective actions, yet it does not interact with the erythropoietin receptor. To determine whether treatment with C-EPO alleviates the signs of muscular dystrophy in an animal model of Duchenne muscular dystrophy, we treated mdx mice with intraperitoneal injections of 50 μg/kg and 100 μg/kg C-EPO for 4 and 12 weeks, and we monitored weight, serum creatine kinase levels, and changes in muscle histology. Moderate histological improvement was observed at 4 weeks, which did not translate into a significantly decreased level of serum creatine kinase. At the doses tested, C-EPO is not an effective therapeutic for the treatment of a mouse model of Duchenne muscular dystrophy.

Impact of ageing on muscle cell regeneration

Skeletal muscle regeneration is a coordinate process in which several factors are sequentially activated to maintain and preserve muscle structure and function. The major role in the growth, remodeling and regeneration is played by satellite cells, a quiescent population of myogenic cells that reside between the basal lamina and plasmalemma and are rapidly activated in response to appropriate stimuli. However, in several muscle conditions, including aging, the capacity of skeletal muscle to sustain an efficient regenerative pathway is severely compromised. Nevertheless, if skeletal muscle possesses a stem cell compartment it is not clear why the muscle fails to regenerate under pathological conditions. Either the resident muscle stem cells are too rare or intrinsically incapable of repairing major damage, or perhaps the injured/pathological muscle is a prohibitive environment for stem cell activation and function. Although we lack definitive answers, recent experimental evidences suggest that the mere presence of endogenous stem cells may not be sufficient to guarantee muscle regeneration, and that the presence of appropriate stimuli and factors are necessary to provide a permissive environment that permits stem cell mediated muscle regeneration and repair. In this review we discuss the molecular basis of muscle regeneration and how aging impacts stem cell mediated muscle regeneration and repair.

Mammalian target of rapamycin complex 1 is involved in differentiation of regenerating myofibers in vivo

This work was undertaken to provide further insight into the role of mammalian target of rapamycin complex 1 (mTORC1) in skeletal muscle regeneration, focusing on myofiber size recovery. Rats were treated or not with rapamycin, an mTORC1 inhibitor. Soleus muscles were then subjected to cryolesion and analyzed 1, 10, and 21 days later. A decrease in soleus myofiber cross-section area on post-cryolesion days 10 and 21 was accentuated by rapamycin, which was also effective in reducing protein synthesis in these freeze-injured muscles. The incidence of proliferating satellite cells during regeneration was unaltered by rapamycin, although immunolabeling for neonatal myosin heavy chain (MHC) was weaker in cryolesion+rapamycin muscles than in cryolesion-only muscles. In addition, the decline in tetanic contraction of freeze-injured muscles was accentuated by rapamycin. This study indicates that mTORC1 plays a key role in the recovery of muscle mass and the differentiation of regenerating myofibers, independently of necrosis and satellite cell proliferation mechanisms.

Age- and gender-related changes in contractile properties of non-atrophied EDL muscle

Background

In humans, ageing causes skeletal muscles to become atrophied, weak, and easily fatigued. In rodent studies, ageing has been associated with significant muscle atrophy and changes in the contractile properties of the muscles. However, it is not entirely clear whether these changes in contractile properties can occur before there is significant atrophy, and whether males and females are affected differently.

Methods and Results

We investigated various contractile properties of whole isolated fast-twitch EDL muscles from adult (2–6 months-old) and aged (12–22 months-old) male and female mice. Atrophy was not present in the aged mice. Compared with adult mice, EDL muscles of aged mice had significantly lower specific force, longer tetanus relaxation times, and lower fatiguability. In the properties of absolute force and muscle relaxation times, females were affected by ageing to a greater extent than males. Additionally, EDL muscles from a separate group of male mice were subjected to eccentric contractions of 15% strain, and larger force deficits were found in aged than in adult mice.

Conclusion

Our findings provide further insight into the muscle atrophy, weakness and fatiguability experienced by the elderly. We have shown that even in the absence of muscle atrophy, there are definite alterations in the physiological properties of whole fast-twitch muscle from ageing mice, and for some of these properties the alterations are more pronounced in female mice than in male mice.

Fiber type composition of the sternomastoid and diaphragm muscles of dystrophin-deficient mdx mice

The muscle fiber phenotype is mainly determined by motoneuron innervation and changes in neuromuscular interaction alter the muscle fiber type. In dystrophin-deficient mdx mice, changes in the molecular assembly of the neuromuscular junction and in nerve terminal sprouting occur in the sternomastoid (STN) muscle during early stages of the disease. In this study, we were interested to see whether early changes in neuromuscular assembly are correlated with alterations in fiber type in dystrophic STN at 2 months of age. A predominance of hybrid fast myofibers (about 52% type IIDB) was observed in control (C57Bl/10) STN. In mdx muscle, the lack of dystrophin did not change this profile (about 54% hybrid type IIDB). Pure fast type IID fibers predominated in normal and dystrophic diaphragm (DIA; about 39% in control and 30% in mdx muscle) and a population of slow Type I fibers was also present (about 10% in control and 13% in mdx muscle). In conclusion, early changes in neuromuscular assembly do not affect the fiber type composition of dystrophic STN. In contrast to the pure fast fibers of the more affected DIA, the hybrid phenotype of the STN may permit dynamic adaptations during progression of the disease.

Effect of dystrophin deficiency on selected intrinsic laryngeal muscles of the mdx mouse

BACKGROUND

Intrinsic laryngeal muscles (ILM) show biological differences from the broader class of skeletal muscles. Yet most research regarding ILM specialization has been completed on a few muscles, most notably the thyroarytenoid and posterior cricoarytenoid. Little information exists regarding the biology of other ILM. Early evidence suggests that the interarytenoid (IA) and cricothyroid (CT) may be more similar to classic skeletal muscle than their associated laryngeal muscles. Knowledge of the IA and CT's similarity or dissimilarity to typical skeletal muscle may hold implications for the treatment of dysphonia.

PURPOSE

The purpose of this study was to further define IA and CT biology by examining their response to the biological challenge of dystrophin deficiency.

METHOD

Control and dystrophin-deficient superior cricoarytenoid (SCA; mouse counterpart of IA) and CT muscles were examined for fiber morphology, sarcolemmal integrity, and immunohistochemical detection of dystrophin.

RESULTS

Despite the absence of dystrophin, experimental muscles did not show disease markers.

CONCLUSIONS

The SCA and the CT appear spared in dystrophin-deficient mouse models. These laryngeal muscles possess specializations that separate them from typical skeletal muscle. Considered in light of previous research, the CT and IA may represent transitional form of muscle, evidencing properties of typical and specialized skeletal muscle.

Use of Evans blue dye to compare limb muscles in exercised young and old mdx mice

Evans blue dye (EBD) is used to mark damaged and permeable muscle fibers in mouse models of muscular dystrophy and as an endpoint in therapeutic trials. We counted EBD-positive muscle fibers and extracted EBD from muscles sampled throughout the hindlimbs in young adult and old mdx mice to determine if the natural variability in morphology would allow measurement of a functional improvement in one limb compared to the contralateral limb. Following one bout of rotarod or treadmill exercise that greatly increased serum creatine kinase levels, the number of EBD(+) muscle fibers in 12-19-month-old mdx mice increased 3-fold, EBD in the muscles increased, and, importantly, contralateral pairs of muscles contained similar amounts of EBD. In contrast, the intra- and interlimb amounts of EBD in 2-7-month-old mdx mice were much too variable. A therapeutic effect can more readily be measured in old mdx mice. These results will be useful in the design of therapy protocols using the mdx mouse.

Effect of VEGF on the regenerative capacity of muscle stem cells in dystrophic skeletal muscle

We have isolated a population of muscle-derived stem cells (MDSCs) that, when compared with myoblasts, display an improved regeneration capacity, exhibit better cell survival, and improve myogenesis and angiogenesis. In addition, we and others have observed that the origin of the MDSCs may reside within the blood vessel walls (endothelial cells and pericytes). Here, we investigated the role of vascular endothelial growth factor (VEGF)-mediated angiogenesis in MDSC transplantation-based skeletal muscle regeneration in mdx mice (an animal model of muscular dystrophy). We studied MDSC and MDSC transduced to overexpress VEGF; no differences were observed in vitro in terms of phenotype or myogenic differentiation. However, after in vivo transplantation, we observe an increase in angiogenesis and endogenous muscle regeneration as well as a reduction in muscle fibrosis in muscles transplanted with VEGF-expressing cells when compared to control cells. In contrast, we observe a significant decrease in vascularization and an increase in fibrosis in the muscles transplanted with MDSCs expressing soluble forms-like tyrosine kinase 1 (sFlt1) (VEGF-specific antagonist) when compared to control MDSCs. Our results indicate that VEGF-expressing cells do not increase the number of dystrophin-positive fibers in the injected mdx muscle, when compared to the control MDSCs. Together the results suggest that the transplantation of VEGF-expressing MDSCs improved skeletal muscle repair through modulation of angiogenesis, regeneration and fibrosis in the injected mdx skeletal muscle.