mdx(⁵cv) mice manifest more severe muscle dysfunction and diaphragm force deficits than do mdx Mice

Treatment With Full-Spectrum Cannabidiol Oil Improved the Pathological Findings of Dystrophic Mutant Mice

INTRODUCTION/AIMS

Duchenne muscular dystrophy (DMD) is caused by pathogenic variants in the DMD gene, making muscle fibers susceptible to contraction-induced membrane damage. Given the potential beneficial action of cannabidiol (CBD), we evaluated the in vitro effect of full-spectrum CBD oil on the viability of dystrophic muscle fibers and the in vivo effect on myopathy of the mdx mouse, a DMD model.

METHODS

In vitro, dystrophic cells from the mdx mouse were treated with full-spectrum CBD oil and assessed with cell viability and cytotoxic analyses. In vivo, fourteen-day-old mdx mice received 10 mg/kg/day of the full-spectrum CBD oil for 14 days. We analyzed creatine kinase (CK) levels, liver damage markers, and histopathology of the diaphragm (DIA) and quadriceps (QUA [myonecrotic fibers with positive IgG staining, regenerated fibers/central nuclei, the minimum Feret's diameter, the fibrosis area, the inflammatory area, the presence of macrophages, and NF-kappa B content]).

RESULTS

In vitro treatment with full-spectrum CBD oil showed a dose-dependent cytotoxic effect; however, in vivo 10 mg/kg treatment was safe and effectively improved DMD histopathological assessment parameters in DIA and QUA: reduction of central nuclei: 1.7% ± 2.0% versus 22.4% ± 5.3% and 11.1% ± 10.7% versus 32.3% ± 4.6%; reduction of IgG+ myofibers: 0.6% ± 0.7% versus 8.4% ± 1.6% and 0.9% ± 0.3% versus 7.5% ± 1.0%; increase in myofiber size: 85.2 ± 3.2 versus 64.3 ± 4.0 μm and 106.5 ± 8.6 versus 81.2 ± 4.8 μm; decrease in inflammatory area: 6.2% ± 2.7% versus 15.1% ± 2.6% and 5.3 ± 4.1 versus 17.3% ± 2.8%; reduced macrophage area: 0.05% ± 0.1% versus 10.8% ± 4.3% and 1.0% ± 0.7% versus 10.3% ± 4.9%; NF-κB levels: 0.6% ± 0.1% versus 1.7% ± 0.2% and 1.7% ± 0.1% versus 5.2% ± 2.1%; and fibrosis: 5.6% ± 1.8% versus 12.0% ± 3.7% and 1.3% ± 0.5% versus 4.7% ± 1.5%. It also reduced serum CK.

DISCUSSION

Full-spectrum CBD oil may represent a promising new approach to treating DMD, but its potential toxicity must be considered.

Generation of a novel mouse model of nemaline myopathy due to recurrent NEB exon 55 deletion

Biallelic pathogenic variants in the nebulin (NEB) gene lead to the congenital muscle disease nemaline myopathy. In-frame deletion of exon 55 (ΔExon55) is the most common disease-causing variant in NEB. Previously, a mouse model of NebΔExon55 was developed; however, it presented an uncharacteristically severe phenotype with a near complete reduction in Neb transcript expression that is not observed in NEB exon 55 patients. We identified by RNA sequencing that the cause of this unexpectedly severe presentation in mice is the generation of a pseudoexon containing two premature termination codons (and promoting nonsense mediated decay) at the Neb exon 55 deletion site. To prove that this is the cause of the loss of Neb transcript, and to generate a more faithful model of the human disease, we used CRISPR gene editing to remove the pseudoexon sequence and replace it with human intron 54 sequence containing a validated cas9 gRNA protospacer. The resulting "hmz" mice have a significant reduction in pseudoexon formation (93.6% reduction), and a re-introduction of stable Neb transcript expression. This new model has the characteristic features of nemaline myopathy at the physiological, histological, and molecular levels. Importantly, unlike the existing exon 55 deletion mice (which die by age 7 days), it survives beyond the first months and exhibits obvious signs of neuromuscular dysfunction. It thus provides a new, robust model for studying pathomechanisms and developing therapies for NEB related nemaline myopathy.

Conditional Dystrophin ablation in the skeletal muscle and brain causes profound effects on muscle function, neurobehavior, and extracellular matrix pathways

Duchenne muscular dystrophy (DMD) patients suffer from skeletal and cardiopulmonary weakness, and interestingly up to one third are diagnosed on the autism spectrum. Dystrophin is an essential protein for regulating the transmission of intracellular force to the extracellular matrix within the skeletal muscle, but also plays key roles in neurobehavior and cognitive function. The mouse dystrophin gene (also abbreviated Dmd) is X-linked and has several isoforms with tissue-specific expression, including the large Dp427m muscle transcript found in heart and skeletal muscle, and the Dp427c transcript that encodes the brain-specific dystrophin cerebellar protein. Understanding the functional requirements and pathways that are affected by dystrophin loss will impact dystrophin replacement gene therapy and exon-skipping correction strategies. We generated conditional Dystrophin knockout mice by targeting exon 52 of the mouse Dystrophin (Dmd flox52) locus. We generated dystrophin constitutive and inducible myofiber knockout (Dmd mKO) mice to evaluate the tissue-specific function of the large skeletal muscle dystrophin isoform. Constitutive embryonic deletion of the Dystrophin gene exclusively in skeletal myofibers resulted in a severe skeletal muscle myopathy, dystrophic histopathology, and functional deficits compared to the mdx mouse. Transcriptomic analysis of skeletal myofibers of the Dmd mKO mice revealed the dysregulation of key extracellular matrix and cytokine signaling pathways. Separately, we generated Purkinje neuron cerebellar dystrophin knockout (Dmd:Pcp2 KO) mice that displayed neurobehavioral deficits in social approach, social memory, and spatial navigation and working memory. These studies reveal the essential requirement for dystrophin expression in both the skeletal muscle and brain for normal physiological and neurobehavioral function.

Oestrogen suppresses the adipogenesis of fibro/adipogenic progenitors through reactivating the METTL3-ESR1-mediated loop in post-menopausal females

BACKGROUND

Post-menopausal women experience more severe muscular fatty infiltration, though the mechanisms remain unclear. The decline in estrogen levels is considered as a critical physiological alteration during post-menopause. Fibro/adipogenic progenitors (FAPs) are identified as major contributors to muscular fatty infiltration. This study aimed to investigate the detailed mechanism underlying the excessive muscular fatty infiltration in postmenopausal females.

METHODS

Supraspinatus muscle samples were collected from female patients with or without menopause, and from mice with or without ovariectomy (OVX), to evaluate muscular fatty infiltration and isolated FAPs. The expressions of (estrogen receptor 1) ESR1, methyltransferase-like 3 (METTL3), and adipogenesis ability in FAPs from post-menopausal women and OVX mice were investigated. RNA sequencing (RNA-Seq) was performed to explore the gene expression profiles and potential mechanisms in FAPs from Pdgfrα-CreERT2; Esr1 knockout (Esr1 KO) mice and Esr1 flox/flox (Esr1 f/f) mice. The interplay of the METTL3-ESR1 mediated loop and its role in regulating adipogenesis in FAPs were investigated using dual luciferase reporter assays, chromatin immunoprecipitation (ChIP), and protein and RNA stability assays. The effects of estrogen supplementation on muscular fatty infiltration and locomotor function in OVX mice were evaluated by immunofluorescent staining and functional analysis.

RESULTS

Decreased expression of ESR1/METTL3 and increased adipogenesis ability in FAPs was found in post-menopausal female. METTL3-mediated m6A methylation promoted ESR1 mRNA stability at the post-transcriptional level in FAPs. METTL3-mediated m6A modification promoted ESR1 expression by stabilizing ESR1 mRNA, while ESR1 acted as a transcription factor that enhanced METTL3 transcription in turn. ESR1 also suppressed the transcription of the adipogenic transcription factor peroxisome proliferator-activated receptor gamma (PPARγ), thereby inhibiting adipogenesis in FAPs. Reactivation of the METTL3-ESR1 mediated loop by estrogen alleviated excessive adipogenesis in FAPs from post-menopausal women, and it also reduced muscular fatty infiltration, and improved locomotor function in OVX mice.

CONCLUSION

Excessive muscular fatty infiltration in post-menopausal women arose from the disruption of the METTL3-ESR1 mediated loop of FAPs due to estrogen deficiency. Reactivation of the METTL3-ESR1 mediated loop by estrogen may serve as a novel intervention to inhibit excessive adipogenesis of post-menopausal female FAPs, thereby ameliorating muscular fatty infiltration and improving locomotor function in post-menopausal females.

KEY POINTS

Oestrogen insufficiency disrupted the METTL3ESR1 loop in post-menopausal FAPs, causing excessive muscular fatty infiltration. METTL3-mediated m6A modification stabilized ESR1 mRNA and enhanced ESR1 expression, while increased ESR1 further promoted METTL3 transcription. ESR1 inhibited the transcription of adipogenic factor PPARγ, ameliorating adipogenesis in FAPs. Reactivating the METTL3ESR1 loop via oestrogen in FAPs reduced muscular fatty infiltration and improved locomotor function.

Intrinsic Muscle Stem Cell Dysfunction Contributes to Impaired Regeneration in the mdx Mouse

BACKGROUND

Duchenne muscular dystrophy (DMD) is a devastating disease characterized by progressive muscle wasting that leads to diminished lifespan. In addition to the inherent weakness of dystrophin-deficient muscle, the dysfunction of resident muscle stem cells (MuSC) significantly contributes to disease progression.

METHODS

Using the mdx mouse model of DMD, we performed an in-depth characterization of disease progression and MuSC function in dystrophin-deficient skeletal muscle using immunohistology, isometric force measurements, transcriptomic analysis and transplantation assays. We examined the architectural and functional changes in mdx skeletal muscle from 13 and 52 weeks of age and following acute cardiotoxin (CTX) injury. We also studied MuSC dynamics and function under homeostatic conditions, during regeneration post-acute injury, and following engraftment using a combination of histological and transcriptomic analyses.

RESULTS

Dystrophin-deficient skeletal muscle undergoes progressive changes with age and delayed regeneration in response to acute injury. Muscle hypertrophy, deposition of collagen and an increase in small myofibres occur with age in the tibialis anterior (TA) and diaphragm muscles in mdx mice. Dystrophic mdx mouse TA muscles become hypertrophic with age, whereas diaphragm atrophy is evident in 1-year-old mdx mice. Maximum tetanic force is comparable between genotypes in the TA, but maximum specific force is reduced by up to 38% between 13 and 52 weeks in the mdx mouse. Following acute injury, myofibre hyperplasia and hypotrophy and delayed recovery of maximum tetanic force occur in the mdx TA. We also find defective MuSC polarity and reduced numbers of myocytes in mdx muscle following acute injury. We observed a 50% and 30% decrease in PAX7+ and MYOG+ cells, respectively, at 5 days post CTX injury (5 dpi) in the mdx TA. A similar decrease in mdx progenitor cell proportion is observed by single cell RNA sequencing of myogenic cells at 5 dpi. The global expression of commitment-related genes is also reduced at 5 dpi. We find a 46% reduction in polarized PARD3 in mdx MuSCs. Finally, mdx MuSCs exhibit elevated PAX7+ cell engraftment with significantly fewer donor-derived myonuclei in regenerated myofibres.

CONCLUSIONS

Our study provides evidence that dystrophin deficiency in MuSCs and myofibres together contributes to progression of DMD. Ongoing muscle damage stimulates MuSC activation; however, aberrant intrinsic MuSC polarity and stem cell commitment deficits due to the loss of dystrophin impair muscle regeneration. Our study provides in vivo validation that dystrophin-deficient MuSCs undergo fewer asymmetric cell divisions, instead favouring symmetric expansion.

Generation of a novel mouse model of nemaline myopathy due to recurrent NEB exon 55 deletion

Biallelic pathogenic variants in the nebulin (NEB) gene lead to the congenital muscle disease nemaline myopathy. In-frame deletion of exon 55 (ΔExon55) is the most common disease-causing variant in NEB. Previously, a mouse model of Neb ΔExon55 was developed; however, it presented an uncharacteristically severe phenotype with a near complete reduction in Neb transcript expression that is not observed in NEB exon 55 patients. We identified by RNA sequencing that the cause of this unexpectedly severe presentation in mice is the generation of a pseudoexon containing two premature termination codons (and promoting nonsense mediated decay) at the Neb exon 55 deletion site. To prove that this is the cause of the loss of Neb transcript, and to generate a more faithful model of the human disease, we used CRISPR gene editing to remove the pseudoexon sequence and replace it with human intron 54 sequence containing a validated cas9 gRNA protospacer. The resulting "hmz" mice have a significant reduction in pseudoexon formation (93.6% reduction), and a re-introduction of stable Neb transcript expression. This new model has the characteristic features of nemaline myopathy at the physiological, histological, and molecular levels. Importantly, unlike the existing exon 55 deletion mice (which die by age 7 days), it survives beyond the first months and exhibits obvious signs of neuromuscular dysfunction. It thus provides a new, robust model for studying pathomechanisms and developing therapies for NEB related nemaline myopathy.

Impact of distinct dystrophin gene mutations on behavioral phenotypes of Duchenne muscular dystrophy

The severity of brain comorbidities in Duchenne muscular dystrophy (DMD) depends on the mutation position within the DMD gene and differential loss of distinct brain dystrophin isoforms (i.e. Dp427, Dp140, Dp71). Comparative studies of DMD mouse models with different mutation profiles may help to understand this genotype-phenotype relationship. The aim of this study was (1) to compare the phenotypes due to Dp427 loss in mdx5cv mice to those of mdx52 mice, which concomitantly lack Dp427 and Dp140; and (2) to evaluate replicability of phenotypes in separate laboratories. We show that mdx5cv mice displayed impaired fear conditioning and robust anxiety-related responses, the severity of which was higher in mdx52 mice. Depression-related phenotypes presented variably in these models and were difficult to replicate between laboratories. Recognition memory was unaltered or minimally affected in mdx5cv and mdx52 mice, at variance with the cognitive deficits described in the original Dp427-deficient mdx mouse, suggesting a difference related to its distinct genetic background. Our results confirm that Dp140 loss may increase the severity of emotional disturbances, and provide insights on the limits of the reproducibility of behavioral studies in DMD mouse models.

Distribution of MRI-derived T2 values as a biomarker for in vivo rapid screening of phenotype severity in mdx mice

BACKGROUND

The pathology in Duchenne muscular dystrophy (DMD) is characterized by degenerating muscle fibers, inflammation, fibro-fatty infiltrate, and edema, and these pathological processes replace normal healthy muscle tissue. The mdx mouse model is one of the most commonly used preclinical models to study DMD. Mounting evidence has emerged illustrating that muscle disease progression varies considerably in mdx mice, with inter-animal differences as well as intra-muscular differences in pathology in individual mdx mice. This variation is important to consider when conducting assessments of drug efficacy and in longitudinal studies. We developed a magnetic resonance imaging (MRI) segmentation and analysis pipeline to rapidly and non-invasively measure the severity of muscle disease in mdx mice.

METHODS

Wildtype and mdx mice were imaged with MRI and T2 maps were obtained axially across the hindlimbs. A neural network was trained to rapidly and semi-automatically segment the muscle tissue, and the distribution of resulting T2 values was analyzed. Interdecile range and Pearson Skew were identified as biomarkers to quickly and accurately estimate muscle disease severity in mice.

RESULTS

The semiautomated segmentation tool reduced image processing time approximately tenfold. Measures of Pearson skew and interdecile range based on that segmentation were repeatable and reflected muscle disease severity in healthy wildtype and diseased mdx mice based on both qualitative observation of images and correlation with Evans blue dye uptake.

CONCLUSION

Use of this rapid, non-invasive, semi-automated MR image segmentation and analysis pipeline has the potential to transform preclinical studies, allowing for pre-screening of dystrophic mice prior to study enrollment to ensure more uniform muscle disease pathology across treatment groups, improving study outcomes.

The multifaceted role of macrophages in homeostatic and injured skeletal muscle

Skeletal muscle is essential for body physical activity, energy metabolism, and temperature maintenance. It has excellent capabilities to maintain homeostasis and to regenerate after injury, which indispensably relies on muscle stem cells, satellite cells (MuSCs). The quiescence, activation, and differentiation of MuSCs are tightly regulated in homeostatic and regenerating muscles. Among the important regulators are intramuscular macrophages, which are functionally heterogeneous with different subtypes present in a spatiotemporal manner to regulate the balance of different MuSC statuses. During chronic injury and aging, intramuscular macrophages often undergo aberrant activation, which in turn disrupts muscle homeostasis and regenerative repair. Growing evidence suggests that the aberrant activation is mainly triggered by altered muscle microenvironment. The trained immunity that affects myeloid progenitors during hematopoiesis may also contribute. Aged immune system may contribute, in part, to the aging-related sarcopenia and compromised skeletal muscle injury repair. As macrophages are actively involved in the progression of many muscle diseases, manipulating their functional activation has become a promising therapeutic approach, which requires comprehensive knowledge of the cellular and molecular mechanisms underlying the diverse activation. To this end, we discuss here the current knowledge of multifaceted role of macrophages in skeletal muscle homeostasis, injury, and repair.

Genome-wide association studies of human and rat BMI converge on synapse, epigenome, and hormone signaling networks

A vexing observation in genome-wide association studies (GWASs) is that parallel analyses in different species may not identify orthologous genes. Here, we demonstrate that cross-species translation of GWASs can be greatly improved by an analysis of co-localization within molecular networks. Using body mass index (BMI) as an example, we show that the genes associated with BMI in humans lack significant agreement with those identified in rats. However, the networks interconnecting these genes show substantial overlap, highlighting common mechanisms including synaptic signaling, epigenetic modification, and hormonal regulation. Genetic perturbations within these networks cause abnormal BMI phenotypes in mice, too, supporting their broad conservation across mammals. Other mechanisms appear species specific, including carbohydrate biosynthesis (humans) and glycerolipid metabolism (rodents). Finally, network co-localization also identifies cross-species convergence for height/body length. This study advances a general paradigm for determining whether and how phenotypes measured in model species recapitulate human biology.

Extracellular Matrix Proteomics: The mdx-4cv Mouse Diaphragm as a Surrogate for Studying Myofibrosis in Dystrophinopathy

The progressive degeneration of the skeletal musculature in Duchenne muscular dystrophy is accompanied by reactive myofibrosis, fat substitution, and chronic inflammation. Fibrotic changes and reduced tissue elasticity correlate with the loss in motor function in this X-chromosomal disorder. Thus, although dystrophinopathies are due to primary abnormalities in the DMD gene causing the almost-complete absence of the cytoskeletal Dp427-M isoform of dystrophin in voluntary muscles, the excessive accumulation of extracellular matrix proteins presents a key histopathological hallmark of muscular dystrophy. Animal model research has been instrumental in the characterization of dystrophic muscles and has contributed to a better understanding of the complex pathogenesis of dystrophinopathies, the discovery of new disease biomarkers, and the testing of novel therapeutic strategies. In this article, we review how mass-spectrometry-based proteomics can be used to study changes in key components of the endomysium, perimysium, and epimysium, such as collagens, proteoglycans, matricellular proteins, and adhesion receptors. The mdx-4cv mouse diaphragm displays severe myofibrosis, making it an ideal model system for large-scale surveys of systematic alterations in the matrisome of dystrophic fibers. Novel biomarkers of myofibrosis can now be tested for their appropriateness in the preclinical and clinical setting as diagnostic, pharmacodynamic, prognostic, and/or therapeutic monitoring indicators.

Partial restoration of brain dystrophin by tricyclo-DNA antisense oligonucleotides alleviates emotional deficits in mdx52 mice

The mdx52 mouse model recapitulates a frequent mutation profile associated with brain involvement in Duchenne muscular dystrophy. Deletion of exon 52 impedes expression of two dystrophins (Dp427, Dp140) expressed in brain, and is eligible for therapeutic exon-skipping strategies. We previously showed that mdx52 mice display enhanced anxiety and fearfulness, and impaired associative fear learning. In this study, we examined the reversibility of these phenotypes using exon 51 skipping to restore exclusively Dp427 expression in the brain of mdx52 mice. We first show that a single intracerebroventricular administration of tricyclo-DNA antisense oligonucleotides targeting exon 51 restores 5%-15% of dystrophin protein expression in the hippocampus, cerebellum, and cortex, at stable levels between 7 and 11 week after injection. Anxiety and unconditioned fear were significantly reduced in treated mdx52 mice and acquisition of fear conditioning appeared fully rescued, while fear memory tested 24 h later was only partially improved. Additional restoration of Dp427 in skeletal and cardiac muscles by systemic treatment did not further improve the unconditioned fear response, confirming the central origin of this phenotype. These findings indicate that some emotional and cognitive deficits associated with dystrophin deficiency may be reversible or at least improved by partial postnatal dystrophin rescue.

BMP-7 Attenuates Sarcopenia and Adverse Muscle Remodeling in Diabetic Mice via Alleviation of Lipids, Inflammation, HMGB1, and Pyroptosis

Diabetic myopathy involves hyperglycemia, oxidative stress, and inflammation. However, the role of hypercholesterolemia-induced inflammation-mediated pathological mechanisms leading to fibrosis, sarcopenia, deterioration of muscle, and muscle dysfunction in diabetes is not well understood. In this study, we investigated the novel role of bone morphogenetic protein-7 (BMP-7) in ameliorating metabolic alterations, inflammation, pyroptosis, TGF-β/SMAD cell signaling mechanisms, and progression of diabetic myopathy. C57BL/6J mice were treated with saline, streptozotocin (STZ), or STZ+BMP-7. Diabetes was confirmed by increased fasting glucose levels and a glucose tolerance test. Gastrocnemius muscle and blood samples were collected for lipid and tissue analysis using various methods. A significant increase in hyperglycemia resulted in an increase in lipid accumulation, monocyte infiltration, and inflammation, as well as an increase in pyroptotic markers and signaling markers in diabetic muscle myocytes. A structural analysis showed significant muscle loss, and increased muscle deterioration and fibrosis leading to muscle dysfunction. BMP-7 attenuated pathological processes that resulted in significantly improved muscle function. We report, for the first time, that increased hyperlipidemia aggravates inflammation-induced pyroptosis, resulting in significant muscle loss, sarcopenia, and adverse skeletal muscle remodeling in diabetic muscle myopathy. Interventional treatment with BMP-7 attenuates hypercholesterolemia-induced inflammation-mediated sarcopenia and adverse muscle remodeling, suggesting BMP-7 could be a potential treatment option for diabetic muscle myopathy.

Eccentric contraction-induced strength loss in dystrophin-deficient muscle: Preparations, protocols, and mechanisms

The absence of dystrophin hypersensitizes skeletal muscle of lower and higher vertebrates to eccentric contraction (ECC)-induced strength loss. Loss of strength can be accompanied by transient and reversible alterations to sarcolemmal excitability and disruption, triad dysfunction, and aberrations in calcium kinetics and reactive oxygen species production. The degree of ECC-induced strength loss, however, appears dependent on several extrinsic and intrinsic factors such as vertebrate model, skeletal muscle preparation (in vivo, in situ, or ex vivo), skeletal muscle hierarchy (single fiber versus whole muscle and permeabilized versus intact), strength production, fiber branching, age, and genetic background, among others. Consistent findings across research groups show that dystrophin-deficient fast(er)-twitch muscle is hypersensitive to ECCs relative to wildtype muscle, but because preparations are highly variable and sensitivity to ECCs are used repeatedly to determine efficacy of many preclinical treatments, it is critical to evaluate the impact of skeletal muscle preparations on sensitivity to ECC-induced strength loss in dystrophin-deficient skeletal muscle. Here, we review and discuss variations in skeletal muscle preparations to evaluate the factors responsible for variations and discrepancies between research groups. We further highlight that dystrophin-deficiency, or loss of the dystrophin-glycoprotein complex in skeletal muscle, is not a prerequisite for accelerated strength loss-induced by ECCs.

Systemic γ-sarcoglycan AAV gene transfer results in dose-dependent correction of muscle deficits in the LGMD 2C/R5 mouse model

Limb-girdle muscular dystrophy (LGMD) type 2C/R5 results from mutations in the γ-sarcoglycan (SGCG) gene and is characterized by muscle weakness and progressive wasting. Loss of functional γ-sarcoglycan protein in the dystrophin-associated protein complex destabilizes the sarcolemma, leading to eventual myofiber death. The SGCG knockout mouse (SGCG ^-/-) has clinical-pathological features that replicate the human disease, making it an ideal model for translational studies. We designed a self-complementary rAAVrh74 vector containing a codon-optimized human SGCG transgene driven by the muscle-specific MHCK7 promoter (SRP-9005) to investigate adeno-associated virus (AAV)-mediated SGCG gene transfer in SGCG ^-/- mice as proof of principle for LGMD 2C/R5. Gene transfer therapy resulted in widespread transgene expression in skeletal muscle and heart, improvements in muscle histopathology characterized by decreased central nuclei and fibrosis, and normalized fiber size. Histopathologic improvements were accompanied by functional improvements, including increased ambulation and force production and resistance to injury of the tibialis anterior and diaphragm muscles. This study demonstrates successful systemic delivery of the hSGCG transgene in SGCG ^-/- mice, with functional protein expression, reconstitution of the sarcoglycan complex, and corresponding physiological and functional improvements, which will help establish a minimal effective dose for translation of SRP-9005 gene transfer therapy in patients with LGMD 2C/R5.

Diverse effector and regulatory functions of fibro/adipogenic progenitors during skeletal muscle fibrosis in muscular dystrophy

Fibrosis is a prominent pathological feature of skeletal muscle in Duchenne muscular dystrophy (DMD). The commonly used disease mouse model, mdx ^5cv , displays progressive fibrosis in the diaphragm but not limb muscles. We use single-cell RNA sequencing to determine the cellular expression of the genes involved in extracellular matrix (ECM) production and degradation in the mdx ^5cv diaphragm and quadriceps. We find that fibro/adipogenic progenitors (FAPs) are not only the primary source of ECM but also the predominant cells that express important ECM regulatory genes, including Ccn2, Ltbp4, Mmp2, Mmp14, Timp1, Timp2, and Loxs. The effector and regulatory functions are exerted by diverse FAP clusters which are different between diaphragm and quadriceps, indicating their activation by different tissue microenvironments. FAPs are more abundant in diaphragm than in quadriceps. Our findings suggest that the development of anti-fibrotic therapy for DMD should target not only the ECM production but also the pro-fibrogenic regulatory functions of FAPs.

The Many Roles of Macrophages in Skeletal Muscle Injury and Repair

Skeletal muscle is essential to physical activity and energy metabolism. Maintaining intact functions of skeletal muscle is crucial to health and wellbeing. Evolutionarily, skeletal muscle has developed a remarkable capacity to maintain homeostasis and to regenerate after injury, which indispensably relies on the resident muscle stem cells, satellite cells. Satellite cells are largely quiescent in the homeostatic steady state. They are activated in response to muscle injury. Activated satellite cells proliferate and differentiate into myoblasts. Myoblasts fuse to form myotubes which further grow and differentiate into mature myofibers. This process is tightly regulated by muscle microenvironment that consists of multiple cellular and molecular components, including macrophages. Present in both homeostatic and injured muscles, macrophages contain heterogeneous functional subtypes that play diverse roles in maintaining homeostasis and promoting injury repair. The spatial-temporal presence of different functional subtypes of macrophages and their interactions with myogenic cells are vital to the proper regeneration of skeletal muscle after injury. However, this well-coordinated process is often disrupted in a chronic muscle disease, such as muscular dystrophy, leading to asynchronous activation and differentiation of satellite cells and aberrant muscle regeneration. Understanding the precise cellular and molecular processes regulating interactions between macrophages and myogenic cells is critical to the development of therapeutic manipulation of macrophages to promote injury repair. Here, we review the current knowledge of the many roles played by macrophages in the regulation of myogenic cells in homeostatic, regenerating, and dystrophic skeletal muscles.

Altered Synaptic Transmission and Excitability of Cerebellar Nuclear Neurons in a Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is generally regarded as a muscle-wasting disease. However, human patients and animal models of DMD also frequently display non-progressive cognitive deficits and high comorbidity with neurodevelopmental disorders, suggesting impaired central processing. Previous studies have identified the cerebellar circuit, and aberrant inhibitory transmission in Purkinje cells, in particular, as a potential site of dysfunction in the central nervous system (CNS). In this work, we investigate potential dysfunction in the output of the cerebellum, downstream of Purkinje cell (PC) activity. We examined synaptic transmission and firing behavior of excitatory projection neurons of the cerebellar nuclei, the primary output of the cerebellar circuit, in juvenile wild-type and mdx mice, a common mouse model of DMD. Using immunolabeling and electrophysiology, we found a reduced number of PC synaptic contacts, but no change in postsynaptic GABAA receptor expression or clustering in these cells. Furthermore, we found that the replenishment rate of synaptic vesicles in Purkinje terminals is reduced in mdx neurons, suggesting that dysfunction at these synapses may be primarily presynaptic. We also found changes in the excitability of cerebellar nuclear neurons. Specifically, we found greater spontaneous firing but reduced evoked firing from a hyperpolarized baseline in mdx neurons. Analysis of action potential waveforms revealed faster repolarization and greater after-hyperpolarization of evoked action potentials in mdx neurons, suggesting an increased voltage- or calcium- gated potassium current. We did not find evidence of dystrophin protein or messenger RNA (mRNA) expression in wild-type nuclear neurons, suggesting that the changes observed in these cells are likely due to the loss of dystrophin in presynaptic PCs. Together, these data suggest that the loss of dystrophin reduces the dynamic range of synaptic transmission and firing in cerebellar nuclear neurons, potentially disrupting the output of the cerebellar circuit to other brain regions and contributing to cognitive and neurodevelopmental deficits associated with DMD.

Emotional behavior and brain anatomy of the mdx52 mouse model of Duchenne muscular dystrophy

The exon-52-deleted mdx52 mouse is a critical model of Duchenne muscular dystrophy (DMD), as it features a deletion in a hotspot region of the DMD gene, frequently mutated in patients. Deletion of exon 52 impedes expression of several brain dystrophins (Dp427, Dp260 and Dp140), thus providing a key model for studying the cognitive impairment associated with DMD and testing rescuing strategies. Here, using in vivo magnetic resonance imaging and neurohistology, we found no gross brain abnormalities in mdx52 mice, suggesting that the neural dysfunctions in this model are likely at the level of brain cellular functionalities. Then, we investigated emotional behavior and fear learning performance of mdx52 mice compared to mdx mice that only lack Dp427 to focus on behavioral phenotypes that could be used in future comparative preclinical studies. mdx52 mice displayed enhanced anxiety and a severe impairment in learning an amygdala-dependent Pavlovian association. These replicable behavioral outcome measures are reminiscent of the internalizing problems reported in a quarter of DMD patients, and will be useful for preclinical estimation of the efficacy of treatments targeting brain dysfunctions in DMD.

VEGFR-1/Flt-1 inhibition increases angiogenesis and improves muscle function in a mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy is characterized by structural degeneration of muscle, which is exacerbated by localized functional ischemia due to loss of nitric oxide synthase-induced vasodilation. Treatment strategies aimed at increasing vascular perfusion have been proposed. Toward this end, we have developed monoclonal antibodies (mAbs) that bind to the vascular endothelial growth factor (VEGF) receptor VEGFR-1 (Flt-1) and its soluble splice variant isoform (sFlt-1) leading to increased levels of free VEGF and proangiogenic signaling. The lead chimeric mAb, 21B3, had high affinity and specificity for both human and mouse sFlt-1 and inhibited VEGF binding to sFlt-1 in a competitive manner. Proof-of-concept studies in the mdx mouse model of Duchenne muscular dystrophy showed that intravenous administration of 21B3 led to elevated VEGF levels, increased vascularization and blood flow to muscles, and decreased fibrosis after 6-12 weeks of treatment. Greater muscle strength was also observed after 4 weeks of treatment. A humanized form of the mAb, 27H6, was engineered and demonstrated a comparable pharmacologic effect. Overall, administration of anti-Flt-1 mAbs in mdx mice inhibited the VEGF:Flt-1 interaction, promoted angiogenesis, and improved muscle function. These studies suggest a potential therapeutic benefit of Flt-1 inhibition for patients with Duchenne muscular dystrophy.

Muscular dystrophy: Experimental animal models and therapeutic approaches (Review)

The muscular dystrophies are a heterogeneous group of genetically inherited diseases characterized by muscle weakness and progressive wasting, which can cause premature death in severe forms. Although >30 years have passed since the identification of the first protein involved in a type of muscular dystrophy, there is no effective treatment for these disabling disorders. In the last decade, several novel therapeutic approaches have been developed and investigated as promising therapeutic approaches aimed to ameliorate the dystrophic phenotype either by restoring dystrophin expression or by compensating for dystrophin deficiency. Concurrently, with the development of therapeutic approaches, in addition to naturally occurring animal models, a wide range of genetically engineered animal models has been generated. The use of animals as models of muscular dystrophies has greatly improved the understanding of the pathogenicity of these diseases and has proven useful in gene therapy studies. In this review, we summarize these latest innovative therapeutic approaches to muscular dystrophies and the usefulness of the various most common experimental animal models.

Amelioration of diabetes-induced inflammation mediated pyroptosis, sarcopenia, and adverse muscle remodelling by bone morphogenetic protein-7

BACKGROUND

Diabetic myopathy involves hyperglycaemia and inflammation that causes skeletal muscle dysfunction; however, the potential cellular mechanisms that occur between hyperglycaemia and inflammation, which induces sarcopenia, and muscle dysfunction remain unknown. In this study, we investigated hyperglycaemia-induced inflammation mediating high-mobility group box 1 activation, which is involved in a novel form of cell death, pyroptosis, diabetic sarcopenia, atrophy, and adverse muscle remodelling. Furthermore, we investigated the therapeutic potential of bone morphogenetic protein-7 (BMP-7), an osteoporosis drug, to treat pyroptosis, and diabetic muscle myopathy.

METHODS

C57BL6 mice were treated with saline (control), streptozotocin (STZ), or STZ + BMP-7 to generate diabetic muscle myopathy. Diabetes was established by determining the increased levels of glucose. Then, muscle function was examined, and animals were sacrificed. Gastrocnemius muscle or blood samples were analysed for inflammation, pyroptosis, weight loss, muscle atrophy, and adverse structural remodelling of gastrocnemius muscle using histology, enzyme-linked immunosorbent assay, immunohistochemistry, western blotting, and reverse transcription polymerase chain reaction.

RESULTS

A significant (P < 0.05) increase in hyperglycaemia leads to an increase in inflammasome (high-mobility group box 1, toll-like receptor-4, and nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain containing protein 3) formation in diabetic muscle cells. Further analysis showed an up-regulation of the downstream pyroptotic pathway with significant (P < 0.05) number of positive muscle cells expressing pyroptosis-specific markers [caspase-1, interleukin (IL)-1β, IL-18, and gasdermin-D]. Pyroptotic cell death is involved in further increasing inflammation by releasing pro-inflammatory cytokine IL-6. Structural analysis showed the loss of muscle weight, decreased myofibrillar area, and increased fibrosis leading to muscle dysfunction. Consistent with this finding, BMP-7 attenuated hyperglycaemia (~50%), pyroptosis, inflammation, and diabetic adverse structural modifications as well as improved muscle function.

CONCLUSIONS

In conclusion, we report for the first time that increased hyperglycaemia and inflammation involve cellular pyroptosis that induces significant muscle cell loss and adverse remodelling in diabetic myopathy. We also report that targeting pyroptosis with BMP-7 improves diabetic muscle pathophysiology and muscle function. These findings suggest that BMP-7 could be a potential therapeutic option to treat diabetic myopathy.

Impact of estrogen deficiency on diaphragm and leg muscle contractile function in female mdx mice

Female carriers of Duchenne muscular dystrophy (DMD) presenting with DMD symptomology similar to males with DMD, such as skeletal muscle weakness and cardiomyopathy, are termed manifesting carriers. There is phenotypic variability among manifesting carriers including the age of onset, which can range from the first to fourth decade of life. In females, estrogen levels typically begin to decline during the fourth decade of life and estrogen deficiency contributes to loss of muscle strength and recovery of strength following injury. Thus, we questioned whether the decline of estrogen impacts the development of DMD symptoms in females. To address this question, we studied 6-8 month-old homozygous mdx female mice randomly assigned to a sham or ovariectomy (OVX) surgical group. In vivo whole-body plethysmography assessed ventilatory function and diaphragm muscle strength was measured in vitro before and after fatigue. Anterior crural muscles were analyzed in vivo for contractile function, fatigue, and in response to eccentric contraction (ECC)-induced injury. For the latter, 50 maximal ECCs were performed by the anterior crural muscles to induce injury. Body mass, uterine mass, hypoxia-hypercapnia ventilatory response, and fatigue index were analyzed by a pooled unpaired t-test. A two-way ANOVA was used to analyze ventilatory measurements. Fatigue and ECC-injury recovery experiments were analyzed by a two-way repeated-measures ANOVA. Results show no differences between sham and OVX mdx mice in ventilatory function, strength, or recovery of strength after fatigue in the diaphragm muscle or anterior crural muscles (p ≥ 0.078). However, OVX mice had significantly greater eccentric torque loss and blunted recovery of strength after ECC-induced injury compared to sham mice (p ≤ 0.019). Although the results show that loss of estrogen has minimal impact on skeletal muscle contractile function in female mdx mice, a key finding suggests that estrogen is important in muscle recovery in female mdx mice after injury.

Preclinical Systemic Delivery of Adeno-Associated α-Sarcoglycan Gene Transfer for Limb-Girdle Muscular Dystrophy

Limb-girdle muscular dystrophy type 2D/R3 (LGMD2D/R3) is a progressive muscular dystrophy that manifests with muscle weakness, respiratory abnormalities, and in rare cases cardiomyopathy. LGMD2D/R3 is caused by mutations in the SGCA gene resulting in loss of protein and concomitant loss of some or all components of the dystrophin-associated glycoprotein complex. The sgca-null (sgca^−/−) mouse recapitulates the clinical phenotype of patients with LGMD2D/R3, including dystrophic features such as muscle necrosis and fibrosis, elevated serum creatine kinase (CK), and reduction in the generation of absolute muscle force and locomotor activity. Thus, sgca^−/− mice provide a relevant model to test the safety and efficacy of gene transfer. We designed a self-complementary AAVrh74 vector containing a codon-optimized full-length human SGCA (hSGCA) transgene driven by a muscle-specific promoter, shortened muscle creatine kinase (tMCK). In this report, we test the efficacy and safety of scAAVrh74.tMCK.hSGCA in sgca^−/− mice using a dose-escalation design to evaluate a single systemic injection of 1.0 × 10¹², 3.0 × 10¹², and 6.0 × 10¹² vg total dose compared with vehicle-treatment and wild-type mice. In sgca^−/− mice, treatment with scAAVrh74.tMCK.hSGCA resulted in robust expression of α-sarcoglycan protein at the sarcolemma membrane in skeletal muscle at all doses tested. In addition, scAAVrh74.tMCK.hSGCA was effective in improving the histopathology of limb and diaphragm muscle of sgca^−/− mice, as indicated by reductions in fibrosis, central nucleation, and normalization of myofiber size. These molecular changes were concomitant with significant increases in specific force generation in the diaphragm and tibialis anterior muscle, protection against eccentric force loss, and reduction in serum CK. Locomotor activity was improved at all doses of vector-treated compared with vehicle-treated sgca^−/− mice. Lastly, vector toxicity was not detected in a serum chemistry panel and by gross necropsy. Collectively, these findings provide support for a systemic delivery of scAAVrh74.tMCK.hSGCA in a clinical setting for the treatment of LGMD2D/R3.

Dose-Escalation Study of Systemically Delivered rAAVrh74.MHCK7.micro-dystrophin in the mdx Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a rare, X-linked, fatal, degenerative neuromuscular disease caused by mutations in the DMD gene. More than 2,000 mutations of the DMD gene are responsible for progressive loss of muscle strength, loss of ambulation, and generally respiratory and cardiac failure by age 30. Recently, gene transfer therapy has received widespread interest as a disease-modifying treatment for all patients with DMD. We designed an adeno-associated virus vector (rAAVrh74) containing a codon-optimized human micro-dystrophin transgene driven by a skeletal and cardiac muscle-specific promoter, MHCK7. To test the efficacy of rAAVrh74.MHCK7.micro-dystrophin, we evaluated systemic injections in mdx (dystrophin-null) mice at low (2 × 10¹² vector genome [vg] total dose, 8 × 10¹³ vg/kg), intermediate (6 × 10¹² vg total dose, 2 × 10¹⁴ vg/kg), and high doses (1.2 × 10¹³ vg total dose, 6 × 10¹⁴ vg/kg). Three months posttreatment, specific force increased in the diaphragm (DIA) and tibialis anterior muscle, with intermediate and high doses eliciting force outputs at wild-type (WT) levels. Histological improvement included reductions in fibrosis and normalization of myofiber size, specifically in the DIA, where results for low and intermediate doses were not significantly different from the WT. Significant reduction in central nucleation was also observed, although complete normalization to WT was not seen. No vector-associated toxicity was reported either by clinical or organ-specific laboratory assessments or following formal histopathology. The findings in this preclinical study provided proof of principle for safety and efficacy of systemic delivery of rAAVrh74.MHCK7.micro-dystrophin at high vector titers, supporting initiation of a Phase I/II safety study in boys with DMD.

Behavioural Characterisation of Macrod1 and Macrod2 Knockout Mice

Adenosine diphosphate ribosylation (ADP-ribosylation; ADPr), the addition of ADP-ribose moieties onto proteins and nucleic acids, is a highly conserved modification involved in a wide range of cellular functions, from viral defence, DNA damage response (DDR), metabolism, carcinogenesis and neurobiology. Here we study MACROD1 and MACROD2 (mono-ADP-ribosylhydrolases 1 and 2), two of the least well-understood ADPr-mono-hydrolases. MACROD1 has been reported to be largely localized to the mitochondria, while the MACROD2 genomic locus has been associated with various neurological conditions such as autism, attention deficit hyperactivity disorder (ADHD) and schizophrenia; yet the potential significance of disrupting these proteins in the context of mammalian behaviour is unknown. Therefore, here we analysed both Macrod1 and Macrod2 gene knockout (KO) mouse models in a battery of well-defined, spontaneous behavioural testing paradigms. Loss of Macrod1 resulted in a female-specific motor-coordination defect, whereas Macrod2 disruption was associated with hyperactivity that became more pronounced with age, in combination with a bradykinesia-like gait. These data reveal new insights into the importance of ADPr-mono-hydrolases in aspects of behaviour associated with both mitochondrial and neuropsychiatric disorders.

DOCK3 is a dosage-sensitive regulator of skeletal muscle and Duchenne muscular dystrophy-associated pathologies

DOCK3 is a member of the DOCK family of guanine nucleotide exchange factors that regulate cell migration, fusion and viability. Previously, we identified a dysregulated miR-486/DOCK3 signaling cascade in dystrophin-deficient muscle, which resulted in the overexpression of DOCK3; however, little is known about the role of DOCK3 in muscle. Here, we characterize the functional role of DOCK3 in normal and dystrophic skeletal muscle. Utilizing Dock3 global knockout (Dock3 KO) mice, we found that the haploinsufficiency of Dock3 in Duchenne muscular dystrophy mice improved dystrophic muscle pathologies; however, complete loss of Dock3 worsened muscle function. Adult Dock3 KO mice have impaired muscle function and Dock3 KO myoblasts are defective for myogenic differentiation. Transcriptomic analyses of Dock3 KO muscles reveal a decrease in myogenic factors and pathways involved in muscle differentiation. These studies identify DOCK3 as a novel modulator of muscle health and may yield therapeutic targets for treating dystrophic muscle symptoms.

Duchenne muscular dystrophy hiPSC-derived myoblast drug screen identifies compounds that ameliorate disease in mdx mice

Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy. In the present study, when human induced pluripotent stem cells (hiPSCs) were differentiated into myoblasts, the myoblasts derived from DMD patient hiPSCs (DMD hiPSC-derived myoblasts) exhibited an identifiable DMD-relevant phenotype: myogenic fusion deficiency. Based on this model, we developed a DMD hiPSC-derived myoblast screening platform employing a high-content imaging (BD Pathway 855) approach to generate parameters describing morphological as well as myogenic marker protein expression. Following treatment of the cells with 1524 compounds from the Johns Hopkins Clinical Compound Library, compounds that enhanced myogenic fusion of DMD hiPSC-derived myoblasts were identified. The final hits were ginsenoside Rd and fenofibrate. Transcriptional profiling revealed that ginsenoside Rd is functionally related to FLT3 signaling, while fenofibrate is linked to TGF-β signaling. Preclinical tests in mdx mice showed that treatment with these 2 hit compounds can significantly ameliorate some of the skeletal muscle phenotypes caused by dystrophin deficiency, supporting their therapeutic potential. Further study revealed that fenofibrate could inhibit mitochondrion-induced apoptosis in DMD hiPSC-derived cardiomyocytes. We have developed a platform based on DMD hiPSC-derived myoblasts for drug screening and identified 2 promising small molecules with in vivo efficacy.

Duchenne muscular dystrophy animal models for high-throughput drug discovery and precision medicine

Introduction: Duchenne muscular dystrophy (DMD) is an X-linked handicapping disease due to the loss of an essential muscle protein dystrophin. Dystrophin-null animals have been extensively used to study disease mechanisms and to develop experimental therapeutics. Despite decades of research, however, treatment options for DMD remain very limited.Areas covered: High-throughput high-content screening and precision medicine offer exciting new opportunities. Here, the authors review animal models that are suitable for these studies.Expert opinion: Nonmammalian models (worm, fruit fly, and zebrafish) are particularly attractive for cost-effective large-scale drug screening. Several promising lead compounds have been discovered using these models. Precision medicine for DMD aims at developing mutation-specific therapies such as exon-skipping and genome editing. To meet these needs, models with patient-like mutations have been established in different species. Models that harbor hotspot mutations are very attractive because the drugs developed in these models can bring mutation-specific therapies to a large population of patients. Humanized hDMD mice carry the entire human dystrophin gene in the mouse genome. Reagents developed in the hDMD mouse-based models are directly translatable to human patients.

Necroptosis mediates myofibre death in dystrophin-deficient mice

Duchenne muscular dystrophy (DMD) is a severe degenerative disorder caused by mutations in the dystrophin gene. Dystrophin-deficient muscles are characterised by progressive myofibre necrosis in which inflammation plays a deleterious role. However, the molecular mechanisms underlying inflammation-induced necrosis in muscle cells are unknown. Here we show that necroptosis is a mechanism underlying myofibre death in dystrophin-deficient muscle. RIPK1, RIPK3 and MLKL are upregulated in dystrophic mouse myofibres. In human DMD samples, there is strong immunoreactivity to RIPK3 and phospho-MLKL in myofibres. In vitro, TNFα can elicit necroptosis in C2C12 myoblasts, and RIPK3 overexpression sensitises myoblasts to undergo TNF-induced death. Furthermore, genetic ablation of Ripk3 in mdx mice reduces myofibre degeneration, inflammatory infiltrate, and muscle fibrosis, and eventually improves muscle function. These findings provide the first evidence of necroptotic cell death in a disease affecting skeletal muscle and identify RIPK3 as a key player in the degenerative process in dystrophin-deficient muscles.

Biological scaffold-mediated delivery of myostatin inhibitor promotes a regenerative immune response in an animal model of Duchenne muscular dystrophy

Recent studies have reported that the immune system significantly mediates skeletal muscle repair and regeneration. Additionally, biological scaffolds have been shown to play a role in polarizing the immune microenvironment toward pro-myogenic outcomes. Moreover, myostatin inhibitors are known to promote muscle regeneration and ameliorate fibrosis in animal models of Duchenne muscular dystrophy (DMD), a human disease characterized by chronic muscle degeneration. Biological scaffolds and myostatin inhibition can potentially influence immune-mediated regeneration in the dystrophic environment, but have not been evaluated together. Toward this end, here we created an injectable biological scaffold composed of hyaluronic acid and processed skeletal muscle extracellular matrix. This material formed a cytocompatible hydrogel at physiological temperatures in vitro When injected subfascially above the tibialis anterior muscles of both WT and dystrophic mdx-5^Cv mice, a murine model of DMD, the hydrogel spreads across the entire muscle before completely degrading at 3 weeks in vivo We found that the hydrogel is associated with CD206⁺ pro-regenerative macrophage polarization and elevated anti-inflammatory cytokine expression in both WT and dystrophic mice. Co-injection of both hydrogel and myostatin inhibitor significantly increased FoxP3⁺ regulatory T cell modulation and Foxp3 gene expression in the scaffold immune microenvironment. Finally, delivery of myostatin inhibitor with the hydrogel increased its bioactivity in vivo, and transplantation of immortalized human myoblasts with the hydrogel promoted their survival in vivo This study identifies a key role for biological scaffolds and myostatin inhibitors in modulating a pro-regenerative immune microenvironment in dystrophic muscle.

Pifithrin-μ modulates microglial activation and promotes histological recovery following spinal cord injury

BACKGROUND

Treatments immediately after spinal cord injury (SCI) are anticipated to decrease neuronal death, disruption of neuronal connections, demyelination, and inflammation, and to improve repair and functional recovery. Currently, little can be done to modify the acute phase, which extends to the first 48 hours post-injury. Efforts to intervene have focused on the subsequent phases - secondary (days to weeks) and chronic (months to years) - to both promote healing, prevent further damage, and support patients suffering from SCI.

METHODS

We used a contusion model of SCI in female mice, and delivered a small molecule reagent during the early phase of injury. Histological and behavioral outcomes were assessed and compared.

RESULTS

We find that the reagent Pifithrin-μ (PFT-μ) acts early and directly on microglia in vitro, attenuating their activation. When administered during the acute phase of SCI, PFT-μ resulted in reduced lesion size during the initial inflammatory phase, and reduced the numbers of pro-inflammatory microglia and macrophages. Treatment with PFT-μ during the early stage of injury maintained a stable anti-inflammatory environment.

CONCLUSIONS

Our results indicate that a small molecule reagent PFT-μ has sustained immunomodulatory effects following a single dose after injury.

α-smooth muscle actin is not a marker of fibrogenic cell activity in skeletal muscle fibrosis

α-Smooth muscle actin (α-SMA) is used as a marker for a subset of activated fibrogenic cells, myofibroblasts, which are regarded as important effector cells of tissue fibrogenesis. We address whether α-SMA-expressing myofibroblasts are detectable in fibrotic muscles of mdx^5cv mice, a mouse model for Duchenne muscular dystrophy (DMD), and whether the α-SMA expression correlates with the fibrogenic function of intramuscular fibrogenic cells. α-SMA immunostaining signal was not detected in collagen I (GFP)-expressing cells in fibrotic muscles of ColI-GFP/mdx^5cv mice, but it was readily detected in smooth muscle cells lining intramuscular blood vessel walls. α-SMA expression was detected by quantitative RT-PCR and Western blot in fibrogenic cells sorted from diaphragm and quadriceps muscles of the ColI-GFP/mdx^5cv mice. Consistent with the more severe fibrosis in the ColI-GFP/mdx^5cv diaphragm, the fibrogenic cells in the diaphragm exerted a stronger fibrogenic function than the fibrogenic cells in the quadriceps as gauged by their extracellular matrix gene expression. However, both gene and protein expression of α-SMA was lower in the diaphragm fibrogenic cells than in the quadriceps fibrogenic cells in the ColI-GFP/mdx^5cv mice. We conclude that myofibroblasts are present in fibrotic skeletal muscles, but their expression of α-SMA is not detectable by immunostaining. The level of α-SMA expression by intramuscular fibrogenic cells does not correlate positively with the level of collagen gene expression or the severity of skeletal muscle fibrosis in the mdx^5cv mice. α-SMA is not a functional marker of fibrogenic cells in skeletal muscle fibrosis associated with muscular dystrophy.

Systemic AAV-Mediated β-Sarcoglycan Delivery Targeting Cardiac and Skeletal Muscle Ameliorates Histological and Functional Deficits in LGMD2E Mice

Limb-girdle muscular dystrophy type 2E (LGMD2E), resulting from mutations in β-sarcoglycan (SGCB), is a progressive dystrophy with deteriorating muscle function, respiratory failure, and cardiomyopathy in 50% or more of LGMD2E patients. SGCB knockout mice share many of the phenotypic deficiencies of LGMD2E patients. To investigate systemic SGCB gene transfer to treat skeletal and cardiac muscle deficits, we designed a self-complementary AAVrh74 vector containing a codon-optimized human SGCB transgene driven by a muscle-specific promoter. We delivered scAAV.MHCK7.hSGCB through the tail vein of SGCB^-/- mice to provide a rationale for a clinical trial that would lead to clinically meaningful results. This led to 98.1% transgene expression across all muscles that was accompanied by improvements in histopathology. Serum creatine kinase (CK) levels were reduced following treatment by 85.5%. Diaphragm force production increased by 94.4%, kyphoscoliosis of the spine was significantly reduced by 48.1%, overall ambulation increased by 57%, and vertical rearing increased dramatically by 132% following treatment. Importantly, no adverse effects were seen in muscle of wild-type mice injected systemically with scAAV.hSGCB. In this well-defined model of LGMD2E, we have demonstrated the efficacy and safety of systemic scAAV.hSGCB delivery, and these findings have established a path for clinically beneficial AAV-mediated gene therapy for LGMD2E.

Identification and Function of Fibrocytes in Skeletal Muscle Injury Repair and Muscular Dystrophy

We identified and characterized the function of CD45⁺/collagen I⁺ fibrocytes in acutely injured skeletal muscle of wild-type (WT) and Ccr2^-/- mice, and in quadriceps and diaphragm muscles of mdx^5cv mice, a mouse model for Duchenne muscular dystrophy. Fibrocytes were not detected in peripheral blood of WT mice after acute muscle injury or mdx^5cv mice. Fibrocytes were detected in acutely injured muscles and in mdx^5cv quadriceps and diaphragm muscles. These cells expressed F4/80 and CCR2, and they were mostly Ly6C^lo They expressed a low level of collagens but a high level of profibrotic growth factors as compared with i.m. fibroblasts. Fibrocyte expression of collagens and profibrotic growth factors was not increased in Ccr2^-/- mice as compared with WT controls. Fibrocyte expression of both proinflammatory and profibrotic cytokines was significantly higher in mdx^5cv diaphragm than in mdx^5cv quadriceps. In cocultures, fibrocytes from the mdx^5cv diaphragm stimulated a higher level of fibroblast expression of extracellular matrix genes than did those from the mdx^5cv quadriceps. Our findings suggest that i.m. fibrocytes most likely originate from infiltrating monocytes/macrophages and differentiate within injured muscles. They likely contribute to the normal muscle injury repair by producing growth factors. They do not appear to contribute to the persistent muscle fibrosis associated with poor injury repair in Ccr2^-/- mice. However, they likely contribute to the persistent inflammation and progressive fibrosis in the mdx^5cv diaphragm.

CCR2 deficiency does not provide sustained improvement of muscular dystrophy in mdx5cv mice

Genetic ablation or pharmacologic inhibition of CC chemokine receptor type 2 (CCR2) reduced macrophage (MP) infiltration and improved muscle pathology and function in mdx diaphragm muscle at early stages. We addressed whether CCR2 deficiency resulted in sustained improvement of mdx^5cv-Ccr2^-/- diaphragm. Compared to mdx^5cv controls, CCR2 deficiency in mdx^5cv-Ccr2^-/- mice markedly reduced intramuscular Ly6C^hi MPs at all stages, but it reduced Ly6C^low MPs only at early stages (4 and 9 wk). CCR2 deficiency reduced quadriceps and diaphragm muscle damage and fibrosis at 14 wk but not at 6 mo, and it improved diaphragm muscle regeneration and respiratory function at 14 wk but not at 6 mo. Intramuscular MPs in mdx^5cv-Ccr2^-/- diaphragm expressed a low level of IL-1β, IL-6, and IFN-γ genes, a similar level of TNF-α, TGF-β1, and platelet-derived growth factor α genes, and a high level of IGF-1 and osteopontin genes compared to mdx^5cv controls. Diaphragm fibroblasts at 14 wk showed a similar cell number with a similar level of collagen and profibrogenic growth factor gene expression in mdx^5cv-Ccr2^-/- and mdx^5cv mice. Diaphragm MPs from both mdx^5cv-Ccr2^-/- and mdx^5cv mice stimulated collagen gene expression by cocultured fibroblasts. The findings suggest that CCR2 deficiency does not provide a sustained benefit and that Ly6C^low MPs may contribute to the progressive fibrosis and dysfunction of mdx^5cv diaphragm.-Zhao, W., Wang, X., Ransohoff, R. M., Zhou, L. CCR2 deficiency does not provide sustained improvement of muscular dystrophy in mdx^5cv mice.

Protein-Anchoring Therapy of Biglycan for Mdx Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a devastating muscle disease caused by loss-of-function mutations in DMD encoding dystrophin. No rational therapy is currently available. Utrophin is a paralog of dystrophin and is highly expressed at the neuromuscular junction. In mdx mice, utrophin is naturally upregulated throughout the muscle fibers, which mitigates muscular dystrophy. Protein-anchoring therapy was previously reported, in which a recombinant extracellular matrix (ECM) protein is delivered to and anchored to a specific target using its proprietary binding domains. Being prompted by a report that intramuscular and intraperitoneal injection of an ECM protein, biglycan, upregulates expression of utrophin and ameliorates muscle pathology in mdx mice, protein-anchoring therapy was applied to mdx mice. Recombinant adeno-associated virus serotype 8 (rAAV8) carrying hBGN encoding human biglycan was intravenously injected into 5-week-old mdx mice. The rAAV8-hBGN treatment improved motor deficits and decreased plasma creatine kinase activities. In muscle sections of treated mice, the number of central myonuclei and the distribution of myofiber sizes were improved. The treated mice increased gene expressions of utrophin and β1-syntrophin, as well as protein expressions of biglycan, utrophin, γ-sarcoglycan, dystrobrevin, and α1-syntrophin. The expression of hBGN in the skeletal muscle of the treated mice was 1.34-fold higher than that of the native mouse Bgn (mBgn). The low transduction efficiency and improved motor functions suggest that biglycan expressed in a small number of muscle fibers was likely to have been secreted and anchored to the cell surface throughout the whole muscular fibers. It is proposed that the protein-anchoring strategy can be applied not only to deficiency of an ECM protein as previously reported, but also to augmentation of a naturally induced ECM protein.

Defective membrane fusion and repair in Anoctamin5-deficient muscular dystrophy

Limb-girdle muscular dystrophies are a genetically diverse group of diseases characterized by chronic muscle wasting and weakness. Recessive mutations in ANO5 (TMEM16E) have been directly linked to several clinical phenotypes including limb-girdle muscular dystrophy type 2L and Miyoshi myopathy type 3, although the pathogenic mechanism has remained elusive. ANO5 is a member of the Anoctamin/TMEM16 superfamily that encodes both ion channels and regulators of membrane phospholipid scrambling. The phenotypic overlap of ANO5 myopathies with dysferlin-associated muscular dystrophies has inspired the hypothesis that ANO5, like dysferlin, may be involved in the repair of muscle membranes following injury. Here we show that Ano5-deficient mice have reduced capacity to repair the sarcolemma following laser-induced damage, exhibit delayed regeneration after cardiotoxin injury and suffer from defective myoblast fusion necessary for the proper repair and regeneration of multinucleated myotubes. Together, these data suggest that ANO5 plays an important role in sarcolemmal membrane dynamics. Genbank Mouse Genome Informatics accession no. 3576659.

Animal models of Duchenne muscular dystrophy: from basic mechanisms to gene therapy

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disorder. It is caused by loss-of-function mutations in the dystrophin gene. Currently, there is no cure. A highly promising therapeutic strategy is to replace or repair the defective dystrophin gene by gene therapy. Numerous animal models of DMD have been developed over the last 30 years, ranging from invertebrate to large mammalian models. mdx mice are the most commonly employed models in DMD research and have been used to lay the groundwork for DMD gene therapy. After ~30 years of development, the field has reached the stage at which the results in mdx mice can be validated and scaled-up in symptomatic large animals. The canine DMD (cDMD) model will be excellent for these studies. In this article, we review the animal models for DMD, the pros and cons of each model system, and the history and progress of preclinical DMD gene therapy research in the animal models. We also discuss the current and emerging challenges in this field and ways to address these challenges using animal models, in particular cDMD dogs.

In vivo assessment of contractile strength distinguishes differential gene function in skeletal muscle of zebrafish larvae

The accessible genetics and extensive skeletal musculature of the zebrafish make it a versatile and increasingly used model for studying muscle contraction. We here describe the development of an in vivo assay for measuring the contractile force of intact zebrafish at the larval stage. In addition, as proof of applicability, we have used this assay to quantify contractile strength of zebrafish larvae in a morphant model of deranged rbfox function. Average maximum tetanic (180 Hz) whole body forces produced by wild-type larvae at 2, 3, 4, and 5 days postfertilization amounted to 3.0, 7.2, 9.1, and 10.8 mN, respectively. To compare at potentially different stages of muscle development, we developed an immunohistological assay for empirically determining the cross-sectional area of larval trunk skeletal muscle to quantify muscle-specific force per cross-sectional area. At 4-5 days postfertilization, specific force amounts to ∼ 300 mN/mm(2), which is similar to fully developed adult mammalian skeletal muscle. We used these assays to measure contractile strength in zebrafish singly or doubly deficient for two rbfox paralogs, rbfox1l and rbfox2, which encode RNA-binding factors shown previously to modulate muscle function and muscle-specific splicing. We found rbfox2 morphants produce maximal tetanic forces similar to wild-type larvae, whereas rbfox1l morphants demonstrate significantly impaired function. rbfox1l/rbfox2 morphants are paralyzed, and their lack of contractile force production in our assay suggests that paralysis is a muscle-autonomous defect. These quantitative functional results allow measurement of muscle-specific phenotypes independent of neural input.

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.

AAV.Dysferlin Overlap Vectors Restore Function in Dysferlinopathy Animal Models

OBJECTIVE

Dysferlinopathies are a family of untreatable muscle disorders caused by mutations in the dysferlin gene. Lack of dysferlin protein results in progressive dystrophy with chronic muscle fiber loss, inflammation, fat replacement, and fibrosis; leading to deteriorating muscle weakness. The objective of this work is to demonstrate efficient and safe restoration of dysferlin expression following gene therapy treatment.

METHODS

Traditional gene therapy is restricted by the packaging capacity limit of adeno-associated virus (AAV), however, use of a dual vector strategy allows for delivery of over-sized genes, including dysferlin. The two vector system (AAV.DYSF.DV) packages the dysferlin cDNA utilizing AAV serotype rh.74 through the use of two discrete vectors defined by a 1 kb region of homology. Delivery of AAV.DYSF.DV via intramuscular and vascular delivery routes in dysferlin deficient mice and nonhuman primates was compared for efficiency and safety.

RESULTS

Treated muscles were tested for dysferlin expression, overall muscle histology, and ability to repair following injury. High levels of dysferlin overexpression was shown for all muscle groups treated as well as restoration of functional outcome measures (membrane repair ability and diaphragm specific force) to wild-type levels. In primates, strong dysferlin expression was demonstrated with no safety concerns.

INTERPRETATION

Treated muscles showed high levels of dysferlin expression with functional restoration with no evidence of toxicity or immune response providing proof of principle for translation to dysferlinopathy patients.

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.

What do mouse models of muscular dystrophy tell us about the DAPC and its components?

There are over 30 mouse models with mutations or inactivations in the dystrophin-associated protein complex. This complex is thought to play a crucial role in the functioning of muscle, as both a shock absorber and signalling centre, although its role in the pathogenesis of muscular dystrophy is not fully understood. The first mouse model of muscular dystrophy to be identified with a mutation in a component of the dystrophin-associated complex (dystrophin) was the mdx mouse in 1984. Here, we evaluate the key characteristics of the mdx in comparison with other mouse mutants with inactivations in DAPC components, along with key modifiers of the disease phenotype. By discussing the differences between the individual phenotypes, we show that the functioning of the DAPC and consequently its role in the pathogenesis is more complicated than perhaps currently appreciated.

MicroRNA-486-dependent modulation of DOCK3/PTEN/AKT signaling pathways improves muscular dystrophy-associated symptoms

Duchenne muscular dystrophy (DMD) is caused by mutations in the gene encoding dystrophin, which results in dysfunctional signaling pathways within muscle. Previously, we identified microRNA-486 (miR-486) as a muscle-enriched microRNA that is markedly reduced in the muscles of dystrophin-deficient mice (Dmdmdx-5Cv mice) and in DMD patient muscles. Here, we determined that muscle-specific transgenic overexpression of miR-486 in muscle of Dmdmdx-5Cv mice results in reduced serum creatine kinase levels, improved sarcolemmal integrity, fewer centralized myonuclei, increased myofiber size, and improved muscle physiology and performance. Additionally, we identified dedicator of cytokinesis 3 (DOCK3) as a miR-486 target in skeletal muscle and determined that DOCK3 expression is induced in dystrophic muscles. DOCK3 overexpression in human myotubes modulated PTEN/AKT signaling, which regulates muscle hypertrophy and growth, and induced apoptosis. Furthermore, several components of the PTEN/AKT pathway were markedly modulated by miR-486 in dystrophin-deficient muscle. Skeletal muscle-specific miR-486 overexpression in Dmdmdx-5Cv animals decreased levels of DOCK3, reduced PTEN expression, and subsequently increased levels of phosphorylated AKT, which resulted in an overall beneficial effect. Together, these studies demonstrate that stable overexpression of miR-486 ameliorates the disease progression of dystrophin-deficient skeletal muscle.

Prednisolone attenuates improvement of cardiac and skeletal contractile function and histopathology by lisinopril and spironolactone in the mdx mouse model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an inherited disease that causes striated muscle weakness. Recently, we showed therapeutic effects of the combination of lisinopril (L), an angiotensin converting enzyme (ACE) inhibitor, and spironolactone (S), an aldosterone antagonist, in mice lacking dystrophin and haploinsufficient for utrophin (utrn(+/-);mdx, het mice); both cardiac and skeletal muscle function and histology were improved when these mice were treated early with LS. It was unknown to what extent LS treatment is effective in the most commonly used DMD murine model, the mdx mouse. In addition, current standard-of-care treatment for DMD is limited to corticosteroids. Therefore, potentially useful alternative or additive drugs need to be both compared directly to corticosteroids and tested in presence of corticosteroids. We evaluated the effectiveness of this LS combination in the mdx mouse model both compared with corticosteroid treatment (prednisolone, P) or in combination (LSP). We tested the additional combinatorial treatment containing the angiotensin II receptor blocker losartan (T), which is widely used to halt and treat the developing cardiac dysfunction in DMD patients as an alternative to an ACE inhibitor. Peak myocardial strain rate, assessed by magnetic resonance imaging, showed a negative impact of P, whereas in both diaphragm and extensor digitorum longus (EDL) muscle contractile function was not significantly impaired by P. Histologically, P generally increased cardiac damage, estimated by percentage area infiltrated by IgG as well as by collagen staining. In general, groups that only differed in the presence or absence of P (i.e. mdx vs. P, LS vs. LSP, and TS vs. TSP) demonstrated a significant detrimental impact of P on many assessed parameters, with the most profound impact on cardiac pathology.

Dystrophic muscle improvement in zebrafish via increased heme oxygenase signaling

Duchenne muscular dystrophy (DMD) is caused by a lack of the dystrophin protein and has no effective treatment at present. Zebrafish provide a powerful in vivo tool for high-throughput therapeutic drug screening for the improvement of muscle phenotypes caused by dystrophin deficiency. Using the dystrophin-deficient zebrafish, sapje, we have screened a total of 2640 compounds with known modes of action from three drug libraries to identify modulators of the disease progression. Six compounds that target heme oxygenase signaling were found to rescue the abnormal muscle phenotype in sapje and sapje-like, while upregulating the inducible heme oxygenase 1 (Hmox1) at the protein level. Direct Hmox1 overexpression by injection of zebrafish Hmox1 mRNA into fertilized eggs was found to be sufficient for a dystrophin-independent restoration of normal muscle via an upregulation of cGMP levels. In addition, treatment of mdx(5cv) mice with the PDE5 inhibitor, sildenafil, which was one of the six drugs impacting the Hmox1 pathway in zebrafish, significantly increased the expression of Hmox1 protein, thus making Hmox1 a novel target for the improvement of dystrophic symptoms. These results demonstrate the translational relevance of our zebrafish model to mammalian models and support the use of zebrafish to screen for new drugs to treat human DMD. The discovery of a small molecule and a specific therapeutic pathway that might mitigate DMD disease progression could lead to significant clinical implications.

The anticancer drug tamoxifen counteracts the pathology in a mouse model of duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a severe disorder characterized by progressive muscle wasting,respiratory and cardiac impairments, and premature death. No treatment exists so far, and the identification of active substances to fight DMD is urgently needed. We found that tamoxifen, a drug used to treat estrogen-dependent breast cancer, caused remarkable improvements of muscle force and of diaphragm and cardiac structure in the mdx(5Cv) mouse model of DMD. Oral tamoxifen treatment from 3 weeks of age for 15 months at a dose of 10 mg/kg/day stabilized myofiber membranes, normalized whole body force, and increased force production and resistance to repeated contractions of the triceps muscle above normal values. Tamoxifen improved the structure of leg muscles and diminished cardiac fibrosis by~ 50%. Tamoxifen also reduced fibrosis in the diaphragm, while increasing its thickness,myofiber count, and myofiber diameter, thereby augmenting by 72% the amount of contractile tissue available for respiratory function. Tamoxifen conferred a markedly slower phenotype to the muscles.Tamoxifen and its metabolites were present in nanomolar concentrations in plasma and muscles,suggesting signaling through high-affinity targets. Interestingly, the estrogen receptors ERa and ERb were several times more abundant in dystrophic than in normal muscles, and tamoxifen normalized the relative abundance of ERb isoforms. Our findings suggest that tamoxifen might be a useful therapy for DMD.

Muscle side population cells from dystrophic or injured muscle adopt a fibro-adipogenic fate

Muscle side population (SP) cells are rare multipotent stem cells that can participate in myogenesis and muscle regeneration upon transplantation. While they have been primarily studied for the development of cell-based therapies for Duchenne muscular dystrophy, little is known regarding their non-muscle lineage choices or whether the dystrophic muscle environment affects their ability to repair muscle. Unfortunately, the study of muscle SP cells has been challenged by their low abundance and the absence of specific SP cell markers. To address these issues, we developed culture conditions for the propagation and spontaneous multi-lineage differentiation of muscle SP cells. Using this approach, we show that SP cells from wild type muscle robustly differentiate into satellite cells and form myotubes without requiring co-culture with myogenic cells. Furthermore, this myogenic activity is associated with SP cells negative for immune (CD45) and vascular (CD31) markers but positive for Pax7, Sca1, and the mesenchymal progenitor marker PDGFRα. Additionally, our studies revealed that SP cells isolated from dystrophic or cardiotoxin-injured muscle fail to undergo myogenesis. Instead, these SP cells rapidly expand giving rise to fibroblast and adipocyte progenitors (FAPs) and to their differentiated progeny, fibroblasts and adipocytes. Our findings indicate that muscle damage affects the lineage choices of muscle SP cells, promoting their differentiation along fibro-adipogenic lineages while inhibiting myogenesis. These results have implications for a possible role of muscle SP cells in fibrosis and fat deposition in muscular dystrophy. In addition, our studies provide a useful in vitro system to analyze SP cell biology in both normal and pathological conditions.

The force-temperature relationship in healthy and dystrophic mouse diaphragm; implications for translational study design

In the field of muscular dystrophy, striated muscle function is often assessed in vitro in dystrophin-deficient mdx mice in order to test the impact of a potential treatment strategy. Although many past studies have assessed diaphragm contractile function at or near room temperature, the diaphragm performs in vivo at 37°C. To improve translation of bench-top results to possible clinical application, we studied temperature-dependence of contractile performance in wild-type (C57BL/10) and mdx muscle strips at temperatures from 25°C to 37°C. Maximal tetanic force in wild-type muscles was higher at 37°C (198 ± 11 vs. 155 ± 9 mN/mm² at 25°C), while the difference between wild-type and mdx was extremely similar: wild-type muscles produced 45.9% and 45.1% more force at 25°C and 37°C respectively. At 37°C twitch contraction kinetics and 50% rise time to tetanic plateau were slower in mdx diaphragm. A fatigue/injury protocol indicated 2-fold fatigue/contraction-induced force deficit in mdx muscles. We conclude that assessment of diaphragm muscle strips can be reliably and reproducibly performed at 37°C.