Natural disease history of the D2-mdx mouse model for Duchenne muscular dystrophy

Reversal of electrophysiological and behavioral deficits mediated by 5-HT7 receptor upregulation following LP-211 treatment in an autistic-like rat model induced by prenatal valproic acid exposure

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by alterations and imbalances in multiple brain neurochemical systems, particularly the serotonergic neurotransmission. This includes changes in serotonin (5-HT) levels, aberrations in 5-HT transporter activity, and decreased synthesis and expression of 5-HT receptors (5-HT7Rs). The exact role of the brain 5-HT system in the development of ASD remains unclear, with conflicting evidence on its involvement. Recently, we have reported research has shown a significant decrease in serotonergic neurons originating from the raphe nuclei and projecting to the CA1 region of the dorsal hippocampus in autistic-like rats. Additionally, we have shown that chronic activation of 5-HT7Rs reverses the effects of autism induction on synaptic plasticity. However, the functional significance of 5-HT7Rs at the cellular level is still not fully understood. This study presents new evidence indicating an upregulation of 5-HT7R in the CA1 subregion of the hippocampus following the induction of autism. The present account also demonstrates that activation of 5-HT7R with its agonist LP-211 can reverse electrophysiological abnormalities in hippocampal pyramidal neurons in a rat model of autism induced by prenatal exposure to VPA. Additionally, in vivo administration of LP-211 resulted in improvements in motor coordination, novel object recognition, and a reduction in stereotypic behaviors in autistic-like offspring. The findings suggest that dysregulated expression of 5-HT7Rs may play a role in the pathophysiology of ASD, and that agonists like LP-211 could potentially be explored as a pharmacological treatment for autism spectrum disorder.

Respiratory performance in Duchenne muscular dystrophy: Clinical manifestations and lessons from animal models

Duchenne muscular dystrophy (DMD) is a fatal genetic neuromuscular disease. Lack of dystrophin in skeletal muscles leads to intrinsic weakness, injury, subsequent degeneration and fibrosis, decreasing contractile function. Dystropathology eventually presents in all inspiratory and expiratory muscles of breathing, severely curtailing their critical function. In people with DMD, premature death is caused by respiratory or cardiac failure. There is an urgent need to develop therapies that improve quality of life and extend life expectancy in DMD. Surprisingly, there is a dearth of information on respiratory control in animal models of DMD, and respiratory outcome measures are often limited or absent in clinical trials. Characterization of respiratory performance in murine and canine models has revealed extensive remodelling of the diaphragm, the major muscle of inspiration. However, significant compensation by extradiaphragmatic muscles of breathing is evident in early disease, contributing to preservation of peak respiratory system performance. Loss of compensation afforded by accessory muscles in advanced disease is ultimately associated with compromised respiratory performance. A new and potentially more translatable murine model of DMD, the D2.mdx mouse, has recently been developed. Respiratory performance in D2.mdx mice is yet to be characterized fully. However, based on histopathological features, D2.mdx mice might serve as useful preclinical models, facilitating the testing of new therapeutics that rescue respiratory function. This review summarizes the pathophysiological mechanisms associated with DMD both in humans and in animal models, with a focus on breathing. We consider the translational value of each model to human DMD and highlight the urgent need for comprehensive characterization of breathing in representative preclinical models to better inform human trials.

Muscle Organoid and Assembloid Systems

Skeletal muscle is one of the most complex and largest tissues that perform important processes in the body, including performing voluntary movements and maintaining body temperature. Disruption of muscle homeostasis results in the development of several disorders, including diabetes and sarcopenia. To study the developmental and regenerative dynamics of skeletal muscle and the mechanism behind muscle diseases, it is important to model skeletal muscle and diseases in vitro. Since skeletal muscle has a complex structure and interaction with other tissues and cells that are required to perform their function, conventional 2D cultures are not sufficient to model the skeletal muscle with their interactions. Advances in the field of organoids and assembloids will enable the establishment of more complex and realistic tissue or disease models which cannot be fully recapitulated in conventional 2D culture systems for use in several areas, including disease research, regenerative, and tissue biology. To overcome these limitations, 3D organoid systems and assembloid systems are promising because of their success in recapitulating the complex structural organization, function, and cellular interactions of skeletal muscle.

Cardiomyopathy in Duchenne Muscular Dystrophy and the Potential for Mitochondrial Therapeutics to Improve Treatment Response

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease caused by mutations to the dystrophin gene, resulting in deficiency of dystrophin protein, loss of myofiber integrity in skeletal and cardiac muscle, and eventual cell death and replacement with fibrotic tissue. Pathologic cardiac manifestations occur in nearly every DMD patient, with the development of cardiomyopathy-the leading cause of death-inevitable by adulthood. As early cardiac abnormalities are difficult to detect, timely diagnosis and appropriate treatment modalities remain a challenge. There is no cure for DMD; treatment is aimed at delaying disease progression and alleviating symptoms. A comprehensive understanding of the pathophysiological mechanisms is crucial to the development of targeted treatments. While established hypotheses of underlying mechanisms include sarcolemmal weakening, upregulation of pro-inflammatory cytokines, and perturbed ion homeostasis, mitochondrial dysfunction is thought to be a potential key contributor. Several experimental compounds targeting the skeletal muscle pathology of DMD are in development, but the effects of such agents on cardiac function remain unclear. The synergistic integration of small molecule- and gene-target-based drugs with metabolic-, immune-, or ion balance-enhancing compounds into a combinatorial therapy offers potential for treating dystrophin deficiency-induced cardiomyopathy, making it crucial to understand the underlying mechanisms driving the disorder.

Targeted Antisense Oligonucleotide-Mediated Skipping of Murine Postn Exon 17 Partially Addresses Fibrosis in D2.mdx Mice

Periostin, a multifunctional 90 kDa protein, plays a pivotal role in the pathogenesis of fibrosis across various tissues, including skeletal muscle. It operates within the transforming growth factor beta 1 (Tgf-β1) signalling pathway and is upregulated in fibrotic tissue. Alternative splicing of Periostin's C-terminal region leads to six protein-coding isoforms. This study aimed to elucidate the contribution of the isoforms containing the amino acids encoded by exon 17 (e17+ Periostin) to skeletal muscle fibrosis and investigate the therapeutic potential of manipulating exon 17 splicing. We identified distinct structural differences between e17+ Periostin isoforms, affecting their interaction with key fibrotic proteins, including Tgf-β1 and integrin alpha V. In vitro mouse fibroblast experimentation confirmed the TGF-β1-induced upregulation of e17+ Periostin mRNA, mitigated by an antisense approach that induces the skipping of exon 17 of the Postn gene. Subsequent in vivo studies in the D2.mdx mouse model of Duchenne muscular dystrophy (DMD) demonstrated that our antisense treatment effectively reduced e17+ Periostin mRNA expression, which coincided with reduced full-length Periostin protein expression and collagen accumulation. The grip strength of the treated mice was rescued to the wild-type level. These results suggest a pivotal role of e17+ Periostin isoforms in the fibrotic pathology of skeletal muscle and highlight the potential of targeted exon skipping strategies as a promising therapeutic approach for mitigating fibrosis-associated complications.

Functional cardiac consequences of β-adrenergic stress-induced injury in the mdx mouse model of Duchenne muscular dystrophy

Cardiomyopathy is the leading cause of death in Duchenne muscular dystrophy (DMD), however, in the mdx mouse model of DMD, the cardiac phenotype differs from that seen in DMD-associated cardiomyopathy. Although some have used pharmacologic stress to enhance the cardiac phenotype in the mdx model, many methods lead to high mortality, variable cardiac outcomes, and do not recapitulate the structural and functional cardiac changes seen in human disease. Here, we describe a simple and effective method to enhance the cardiac phenotype model in mdx mice using advanced 2D and 4D high-frequency ultrasound to monitor cardiac dysfunction progression in vivo. For our study, mdx and wild-type (WT) mice received daily low-dose (2 mg/kg/day) isoproterenol injections for 10 days. Histopathologic assessment showed that isoproterenol treatment increased myocyte injury, elevated serum cardiac troponin I levels, and enhanced fibrosis in mdx mice. Ultrasound revealed reduced ventricular function, decreased wall thickness, increased volumes, and diminished cardiac reserve in mdx mice compared to wild-type. Our findings highlight the utility of low-dose isoproterenol in mdx mice as a valuable model for exploring therapies targeting DMD-associated cardiac complications.

The BALB/c.mdx62 mouse exhibits a dystrophic muscle pathology and is a model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating monogenic skeletal muscle-wasting disorder. Although many pharmacological and genetic interventions have been reported in preclinical studies, few have progressed to clinical trials with meaningful benefit. Identifying therapeutic potential can be limited by availability of suitable preclinical mouse models. More rigorous testing across models with varied background strains and mutations can identify treatments for clinical success. Here, we report the generation of a DMD mouse model with a CRISPR-induced deletion within exon 62 of the dystrophin gene (Dmd) and the first generated in BALB/c mice. Analysis of mice at 3, 6 and 12 months of age confirmed loss of expression of the dystrophin protein isoform Dp427 and resultant dystrophic pathology in limb muscles and the diaphragm, with evidence of centrally nucleated fibers, increased inflammatory markers and fibrosis, progressive decline in muscle function, and compromised trabecular bone development. The BALB/c.mdx62 mouse is a novel model of DMD with associated variations in the immune response and muscle phenotype, compared with those of existing models. It represents an important addition to the preclinical model toolbox for developing therapeutic strategies.

Sildenafil increases AAV9 transduction after a systemic administration and enhances AAV9-dystrophin therapeutic effect in mdx mice

Adeno-associated virus (AAV) vectors have been successfully used to deliver genes for treating rare diseases. However, the systemic administration of high AAV vector doses triggers several adverse effects, including immune response, the asymptomatic elevation of liver transaminase levels, and complement activation. Thus, improving AAV transduction and reducing AAV dosage for treatment is necessary. Recently, we found that a phosphodiesterase-5 inhibitor significantly promoted AAV9 transduction in vitro by regulating the caveolae and macropinocytosis pathways. When AAV9-Gaussian luciferase (AAV9-Gluc) and AAV9-green fluorescent protein (AAV9-GFP) were injected intravenously into mice pre-treated with sildenafil, the expressions of Gluc in the plasma and GFP in muscle tissues significantly increased (P < 0.05). Sildenafil also improved Evans blue permeation in tissues. Additionally, we found that sildenafil promoted Treg proliferation, inhibited B-cell activation, and decreased anti-AAV9 IgG levels (P < 0.05). Furthermore, sildenafil significantly promoted Duchenne muscular dystrophy gene therapy efficacy using AAV9 in mdx mice; it increased micro-dystrophin gene expression, forelimb grip strength, and time spent on the rotarod test, decreased serum creatine kinase levels, and ameliorated histopathology by improving muscle cell morphology and reducing fibrosis (P < 0.05). These results show that sildenafil significantly improved AAV transduction, suppressed the levels of anti-AAV9 IgG, and enhanced the efficacy of gene therapy.

Electrical impedance myography detects dystrophin-related muscle changes in mdx mice

BACKGROUND

The lack of functional dystrophin protein in Duchenne muscular dystrophy (DMD) causes chronic skeletal muscle inflammation and degeneration. Therefore, the restoration of functional dystrophin levels is a fundamental approach for DMD therapy. Electrical impedance myography (EIM) is an emerging tool that provides noninvasive monitoring of muscle conditions and has been suggested as a treatment response biomarker in diverse indications. Although magnetic resonance imaging (MRI) of skeletal muscles has become a standard measurement in clinical trials for DMD, EIM offers distinct advantages, such as portability, user-friendliness, and reduced cost, allowing for remote monitoring of disease progression or response to therapy. To investigate the potential of EIM as a biomarker for DMD, we compared longitudinal EIM data with MRI/histopathological data from an X-linked muscular dystrophy (mdx) mouse model of DMD. In addition, we investigated whether EIM could detect dystrophin-related changes in muscles using antisense-mediated exon skipping in mdx mice.

METHODS

The MRI data for muscle T2, the magnetic resonance spectroscopy (MRS) data for fat fraction, and three EIM parameters with histopathology were longitudinally obtained from the hindlimb muscles of wild-type (WT) and mdx mice. In the EIM study, a cell-penetrating peptide (Pip9b2) conjugated antisense phosphorodiamidate morpholino oligomer (PPMO), designed to induce exon-skipping and restore functional dystrophin production, was administered intravenously to mdx mice.

RESULTS

MRI imaging in mdx mice showed higher T2 intensity at 6 weeks of age in hindlimb muscles compared to WT mice, which decreased at ≥ 9 weeks of age. In contrast, EIM reactance began to decline at 12 weeks of age, with peak reduction at 18 weeks of age in mdx mice. This decline was associated with myofiber atrophy and connective tissue infiltration in the skeletal muscles. Repeated dosing of PPMO (10 mg/kg, 4 times every 2 weeks) in mdx mice led to an increase in muscular dystrophin protein and reversed the decrease in EIM reactance.

CONCLUSIONS

These findings suggest that muscle T2 MRI is sensitive to the early inflammatory response associated with dystrophin deficiency, whereas EIM provides a valuable biomarker for the noninvasive monitoring of subsequent changes in skeletal muscle composition. Furthermore, EIM reactance has the potential to monitor dystrophin-deficient muscle abnormalities and their recovery in response to antisense-mediated exon skipping.

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

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

Combining electromyographic and electrical impedance data sets through machine learning: A study in D2-mdx and wild-type mice

INTRODUCTION/AIMS

Needle impedance-electromyography (iEMG) assesses the active and passive electrical properties of muscles concurrently by using a novel needle with six electrodes, two for EMG and four for electrical impedance myography (EIM). Here, we assessed an approach for combining multifrequency EMG and EIM data via machine learning (ML) to discriminate D2-mdx muscular dystrophy and wild-type (WT) mouse skeletal muscle.

METHODS

iEMG data were obtained from quadriceps of D2-mdx mice, a muscular dystrophy model, and WT animals. EIM data were collected with the animals under deep anesthesia and EMG data collected under light anesthesia, allowing for limited spontaneous movement. Fourier transformation was performed on the EMG data to provide power spectra that were sampled across the frequency range using three different approaches. Random forest-based, nested ML was applied to the EIM and EMG data sets separately and then together to assess healthy versus disease category classification using a nested cross-validation procedure.

RESULTS

Data from 20 D2-mdx and 20 WT limbs were analyzed. EIM data fared better than EMG data in differentiating healthy from disease mice with 93.1% versus 75.6% accuracy, respectively. Combining EIM and EMG data sets yielded similar performance as EIM data alone with 92.2% accuracy.

DISCUSSION

We have demonstrated an ML-based approach for combining EIM and EMG data obtained with an iEMG needle. While EIM-EMG in combination fared no better than EIM alone with this data set, the approach used here demonstrates a novel method of combining the two techniques to characterize the full electrical properties of skeletal muscle.

T cell biology in neuromuscular disorders: a focus on Duchenne Muscular Dystrophy and Amyotrophic Lateral Sclerosis

Growing evidence demonstrates a continuous interaction between the immune system, the nerve and the muscle in neuromuscular disorders of different pathogenetic origins, such as Duchenne Muscular Dystrophy (DMD) and Amyotrophic Lateral Sclerosis (ALS), the focus of this review. Herein we highlight the complexity of the cellular and molecular interactions involving the immune system in neuromuscular disorders, as exemplified by DMD and ALS. We describe the distinct types of cell-mediated interactions, such as cytokine/chemokine production as well as cell-matrix and cell-cell interactions between T lymphocytes and other immune cells, which target cells of the muscular or nervous tissues. Most of these interactions occur independently of exogenous pathogens, through ligand-receptor binding and subsequent signal transduction cascades, at distinct levels of specificity. Although this issue reveals the complexity of the system, it can also be envisioned as a window of opportunity to design therapeutic strategies (including synthetic moieties, cell and gene therapy, as well as immunotherapy) by acting upon one or more targets. In this respect, we discuss ongoing clinical trials using VLA-4 inhibition in DMD, and in ALS, with a focus on regulatory T cells, both revealing promising results.

Decoding the transcriptome of Duchenne muscular dystrophy to the single nuclei level reveals clinical-genetic correlations

Duchenne muscular dystrophy is a genetic disease produced by mutations in the dystrophin gene characterized by early onset muscle weakness leading to severe and irreversible disability. The cellular and molecular consequences of the lack of dystrophin in humans are only partially known, which is crucial for the development of new therapies aiming to slow or stop the progression of the disease. Here we have analyzed quadriceps muscle biopsies of seven DMD patients aged 2 to 4 years old and five age and gender matched controls using single nuclei RNA sequencing (snRNAseq) and correlated the results obtained with clinical data. SnRNAseq identified significant differences in the proportion of cell population present in the muscle samples, including an increase in the number of regenerative fibers, satellite cells, and fibro-adipogenic progenitor cells (FAPs) and a decrease in the number of slow fibers and smooth muscle cells. Muscle samples from the younger patients with stable mild weakness were characterized by an increase in regenerative fibers, while older patients with moderate and progressive weakness were characterized by loss of muscle fibers and an increase in FAPs. An analysis of the gene expression profile in muscle fibers identified a strong regenerative signature in DMD samples characterized by the upregulation of genes involved in myogenesis and muscle hypertrophy. In the case of FAPs, we observed upregulation of genes involved in the extracellular matrix regeneration but also several signaling pathways. Indeed, further analysis of the potential intercellular communication profile showed a dysregulation of the communication profile in DMD samples identifying FAPs as a key regulator of cell signaling in DMD muscle samples. In conclusion, our study has identified significant differences at the cellular and molecular levels in the different cell populations present in skeletal muscle samples of patients with DMD compared to controls.

Spatial transcriptomics reveal markers of histopathological changes in Duchenne muscular dystrophy mouse models

Duchenne muscular dystrophy is caused by mutations in the DMD gene, leading to lack of dystrophin. Chronic muscle damage eventually leads to histological alterations in skeletal muscles. The identification of genes and cell types driving tissue remodeling is a key step to developing effective therapies. Here we use spatial transcriptomics in two Duchenne muscular dystrophy mouse models differing in disease severity to identify gene expression signatures underlying skeletal muscle pathology and to directly link gene expression to muscle histology. We perform deconvolution analysis to identify cell types contributing to histological alterations. We show increased expression of specific genes in areas of muscle regeneration (Myl4, Sparc, Hspg2), fibrosis (Vim, Fn1, Thbs4) and calcification (Bgn, Ctsk, Spp1). These findings are confirmed by smFISH. Finally, we use differentiation dynamic analysis in the D2-mdx muscle to identify muscle fibers in the present state that are predicted to become affected in the future state.

Characterization of the Cardiac Structure and Function of Conscious D2.B10-Dmdmdx/J (D2-mdx) mice from 16-17 to 24-25 Weeks of Age

Duchenne muscular dystrophy (DMD) is the most common form of muscle degenerative hereditary disease. Muscular replacement by fibrosis and calcification are the principal causes of progressive and severe musculoskeletal, respiratory, and cardiac dysfunction. To date, the D2.B10-Dmd^mdx/J (D2-mdx) model is proposed as the closest to DMD, but the results are controversial. In this study, the cardiac structure and function was characterized in D2-mdx mice from 16-17 up to 24-25 weeks of age. Echocardiographic assessment in conscious mice, gross pathology, and histological and cardiac biomarker analyses were performed. At 16-17 weeks of age, D2-mdx mice presented mild left ventricular function impairment and increased pulmonary vascular resistance. Cardiac fibrosis was more extended in the right ventricle, principally on the epicardium. In 24-25-week-old D2-mdx mice, functional and structural alterations increased but with large individual variation. High-sensitivity cardiac Troponin T, but not N-terminal pro-atrial natriuretic peptide, plasma levels were increased. In conclusion, left ventricle remodeling was mild to moderate in both young and adult mice. We confirmed that right ventricle epicardial fibrosis is the most outstanding finding in D2-mdx mice. Further long-term studies are needed to evaluate whether this mouse model can also be considered a model of DMD cardiomyopathy.

What Nutraceuticals Can Do for Duchenne Muscular Dystrophy: Lessons Learned from Amino Acid Supplementation in Mouse Models

Duchenne muscular dystrophy (DMD), the severest form of muscular dystrophy, is characterized by progressive muscle weakness with fatal outcomes most often before the fourth decade of life. Despite the recent addition of molecular treatments, DMD remains a disease without a cure, and the need persists for the development of supportive therapies aiming to help improve patients' quality of life. This review focuses on the therapeutical potential of amino acid and derivative supplements, summarizing results obtained in preclinical studies in murine disease models. Several promising compounds have emerged, with L-arginine, N-acetylcysteine, and taurine featuring among the most intensively investigated. Their beneficial effects include reduced inflammatory, oxidative, fibrotic, and necrotic damage to skeletal muscle tissues. Improvement of muscle strength and endurance have been reported; however, mild side effects have also surfaced. More explorative, placebo-controlled and long-term clinical trials would need to be conducted in order to identify amino acid formulae that are safe and of true benefit to DMD patients.

Altered muscle niche contributes to myogenic deficit in the D2-mdx model of severe DMD

Lack of dystrophin expression is the underlying genetic basis for Duchenne muscular dystrophy (DMD). However, disease severity varies between patients, based on specific genetic modifiers. D2-mdx is a model for severe DMD that exhibits exacerbated muscle degeneration and failure to regenerate even in the juvenile stage of the disease. We show that poor regeneration of juvenile D2-mdx muscles is associated with an enhanced inflammatory response to muscle damage that fails to resolve efficiently and supports the excessive accumulation of fibroadipogenic progenitors (FAPs), leading to increased fibrosis. Unexpectedly, the extent of damage and degeneration in juvenile D2-mdx muscle is significantly reduced in adults, and is associated with the restoration of the inflammatory and FAP responses to muscle injury. These improvements enhance regenerative myogenesis in the adult D2-mdx muscle, reaching levels comparable to the milder B10-mdx model of DMD. Ex vivo co-culture of healthy satellite cells (SCs) with juvenile D2-mdx FAPs reduces their fusion efficacy. Wild-type juvenile D2 mice also manifest regenerative myogenic deficit and glucocorticoid treatment improves their muscle regeneration. Our findings indicate that aberrant stromal cell responses contribute to poor regenerative myogenesis and greater muscle degeneration in juvenile D2-mdx muscles and reversal of this reduces pathology in adult D2-mdx muscle, identifying these responses as a potential therapeutic target for the treatment of DMD.

New semi-automated tool for the quantitation of MR imaging to estimate in vivo muscle disease severity in mice

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. Magnetic resonance imaging (MRI) is a non-invasive method that can be used qualitatively or quantitatively to measure muscle disease progression in the clinic and in preclinical models. Although MR imaging is highly sensitive, image acquisition and analysis can be time intensive. The purpose of this study was to develop a semi-automated muscle segmentation and quantitation pipeline that can quickly and accurately estimate muscle disease severity in mice. Herein, we show that the newly developed segmentation tool accurately divides muscle. We show that measures of skew and interdecile range based on segmentation sufficiently estimate muscle disease severity in healthy wildtype and diseased mdx mice. Moreover, the semi-automated pipeline reduced analysis time by nearly 10-fold. Use of this rapid, non-invasive, semi-automated MR imaging 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.

Microdystrophin Gene Addition Significantly Improves Muscle Functionality and Diaphragm Muscle Histopathology in a Fibrotic Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a rare neuromuscular disease affecting 1:5000 newborn males. No cure is currently available, but gene addition therapy, based on the adeno-associated viral (AAV) vector-mediated delivery of microdystrophin transgenes, is currently being tested in clinical trials. The muscles of DMD boys present significant fibrotic and adipogenic tissue deposition at the time the treatment starts. The presence of fibrosis not only worsens the disease pathology, but also diminishes the efficacy of gene therapy treatments. To gain an understanding of the efficacy of AAV-based microdystrophin gene addition in a relevant, fibrotic animal model of DMD, we conducted a systemic study in juvenile D2.mdx mice using the single intravenous administration of an AAV8 system expressing a sequence-optimized murine microdystrophin, named MD1 (AAV8-MD1). We mainly focused our study on the diaphragm, a respiratory muscle that is crucial for DMD pathology and that has never been analyzed after treatment with AAV-microdystrophin in this mouse model. We provide strong evidence here that the delivery of AAV8-MD1 provides significant improvement in body-wide muscle function. This is associated with the protection of the hindlimb muscle from contraction-induced damage and the prevention of fibrosis deposition in the diaphragm muscle. Our work corroborates the observation that the administration of gene therapy in DMD is beneficial in preventing muscle fibrosis.

Indicators of increased ER stress and UPR in aged D2-mdx and human dystrophic skeletal muscles

Duchenne muscular dystrophy (DMD) is a progressive muscle disease that results in muscle wasting, wheelchair dependence, and eventual death due to cardiac and respiratory complications. In addition to muscle fragility, dystrophin deficiency also results in multiple secondary dysfunctions, which may lead to the accumulation of unfolded proteins causing endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). The purpose of this investigation was to understand how ER stress and the UPR are modified in muscle from D2-mdx mice, an emerging DMD model, and from humans with DMD. We hypothesized that markers of ER stress and the UPR are upregulated in D2-mdx and human dystrophic muscles compared to their healthy counterparts. Immunoblotting in diaphragms from 11-month-old D2-mdx and DBA mice indicated increased ER stress and UPR in dystrophic diaphragms compared to healthy, including increased relative abundance of ER stress chaperone CHOP, canonical ER stress transducers ATF6 and pIRE1α S724, and transcription factors that regulate the UPR such as ATF4, XBP1s, and peIF2α S51. The publicly available Affymetrix dataset (GSE38417) was used to analyze the expression of ER stress and UPR-related transcripts and processes. Fifty-eight upregulated genes related to ER stress and the UPR in human dystrophic muscles suggest pathway activation. Further, based on analyses using iRegulon, putative transcription factors that regulate this upregulation profile were identified, including ATF6, XBP1, ATF4, CREB3L2, and EIF2AK3. This study adds to and extends the emerging knowledge of ER stress and the UPR in dystrophin deficiency and identifies transcriptional regulators that may be responsible for these changes and be of therapeutic interest.

Growth hormone secretagogues modulate inflammation and fibrosis in mdx mouse model of Duchenne muscular dystrophy

Introduction

Growth hormone secretagogues (GHSs) exert multiple actions, being able to activate GHS-receptor 1a, control inflammation and metabolism, to enhance GH/insulin-like growth factor-1 (IGF-1)-mediated myogenesis, and to inhibit angiotensin-converting enzyme. These mechanisms are of interest for potentially targeting multiple steps of pathogenic cascade in Duchenne muscular dystrophy (DMD).

Methods

Here, we aimed to provide preclinical evidence for potential benefits of GHSs in DMD, via a multidisciplinary in vivo and ex vivo comparison in mdx mice, of two ad hoc synthesized compounds (EP80317 and JMV2894), with a wide but different profile. 4-week-old mdx mice were treated for 8 weeks with EP80317 or JMV2894 (320 µg/kg/d, s.c.).

Results

In vivo, both GHSs increased mice forelimb force (recovery score, RS towards WT: 20% for EP80317 and 32% for JMV2894 at week 8). In parallel, GHSs also reduced diaphragm (DIA) and gastrocnemius (GC) ultrasound echodensity, a fibrosis-related parameter (RS: ranging between 26% and 75%). Ex vivo, both drugs ameliorated DIA isometric force and calcium-related indices (e.g., RS: 40% for tetanic force). Histological analysis highlighted a relevant reduction of fibrosis in GC and DIA muscles of treated mice, paralleled by a decrease in gene expression of TGF-β1 and Col1a1. Also, decreased levels of pro-inflammatory genes (IL-6, CD68), accompanied by an increment in Sirt-1, PGC-1α and MEF2c expression, were observed in response to treatments, suggesting an overall improvement of myofiber metabolism. No detectable transcript levels of GHS receptor-1a, nor an increase of circulating IGF-1 were found, suggesting the presence of a novel receptor-independent mechanism in skeletal muscle. Preliminary docking studies revealed a potential binding capability of JMV2894 on metalloproteases involved in extracellular matrix remodeling and cytokine production, such as ADAMTS-5 and MMP-9, overactivated in DMD.

Discussion

Our results support the interest of GHSs as modulators of pathology progression in mdx mice, disclosing a direct anti-fibrotic action that may prove beneficial to contrast pathological remodeling.

Altered muscle niche contributes to myogenic deficit in the D2- mdx model of severe DMD

Lack of dystrophin is the genetic basis for the Duchenne muscular dystrophy (DMD). However, disease severity varies between patients, based on specific genetic modifiers. D2- mdx is a model for severe DMD that exhibits exacerbated muscle degeneration and failure to regenerate even in the juvenile stage of the disease. We show that poor regeneration of juvenile D2- mdx muscles is associated with enhanced inflammatory response to muscle damage that fails to resolve efficiently and supports excessive accumulation of fibroadipogenic progenitors (FAPs). Unexpectedly, the extent of damage and degeneration of juvenile D2- mdx muscle is reduced in adults and is associated with the restoration of the inflammatory and FAP responses to muscle injury. These improvements enhance myogenesis in the adult D2- mdx muscle, reaching levels comparable to the milder (B10- mdx ) mouse model of DMD. Ex vivo co-culture of healthy satellite cells (SCs) with the juvenile D2- mdx FAPs reduced their fusion efficacy and in vivo glucocorticoid treatment of juvenile D2 mouse improved muscle regeneration. Our findings indicate that aberrant stromal cell response contributes to poor myogenesis and greater muscle degeneration in dystrophic juvenile D2- mdx muscles and reversal of this reduces pathology in adult D2- mdx mouse muscle, identifying these as therapeutic targets to treat dystrophic DMD muscles.

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.

Electrical Impedance Myography Correlates with Functional Measures of Disease Progression in D2-mdx Mice and Boys with Duchenne Muscular Dystrophy

BACKGROUND

Sensitive, objective, and longitudinal outcome measures applicable to both pre-clinical and clinical interventions are needed to assess muscle health in Duchenne muscular dystrophy (DMD). Electrical impedance myography (EIM) has the potential to non-invasively measure disease progression in mice and boys with DMD.

OBJECTIVE

We sought to evaluate how electrical impedance values (i.e., phase, reactance, and resistance) correlate to established measures of disease in both D2-mdx and wild type (WT) mice and boys with and without DMD.

METHODS

Histological, functional, and EIM data collected from previous studies of WT and D2-mdx mice at 6, 13, 21 and 43 weeks of age were reanalyzed. In parallel, previously collected functional outcome measures and EIM values were reanalyzed from boys with and without DMD at four different age groups from 2 to 14 years old.

RESULTS

In mice, disease progression as detected by histological, functional, and EIM measures, was appreciable over this time period and grip strength best correlated to longitudinal phase and reactance impedance values. In boys, disease progression quantified through commonly utilized functional outcome measures was significant and longitudinal phase demonstrated the strongest correlation with functional outcome measures.

CONCLUSION

Similar changes in EIM values, specifically in longitudinal reactance and phase, were found to show significant correlations to functional measures in both mice and boys. Thus, EIM demonstrates applicability in both pre-clinical and clinical settings and can be used as a safe, non-invasive, and longitudinal proxy biomarker to assess muscle health in DMD.

On the use of D2.B10-Dmdmdx/J (D2.mdx) Versus C57BL/10ScSn-Dmdmdx/J (mdx) Mouse Models for Preclinical Studies on Duchenne Muscular Dystrophy: A Cautionary Note from Members of the TREAT-NMD Advisory Committee on Therapeutics

The C57BL/10ScSn-Dmdmdx/J (mdx) mouse model has been used by researchers for decades as a model to study pathology of and develop therapies for Duchenne muscular dystrophy. However, the model is relatively mildly affected compared to the human situation. Recently, the D2.B10-Dmdmdx/J (D2.mdx) mouse model was suggested as a more severely affected and therefore better alternative. While the pathology of this model is indeed more pronounced early in life, it is not progressive, and increasing evidence suggest that it actually partially resolves with age. As such, caution is needed when using this model. However, as preclinical experts of the TREAT-NMD advisory committee for therapeutics (TACT), we frequently encounter study designs that underestimate this caveat. We here provide context for how to best use the two models for preclinical studies at the current stage of knowledge.

CRISPR applications for Duchenne muscular dystrophy: From animal models to potential therapies

CRISPR gene-editing technology creates precise and permanent modifications to DNA. It has significantly advanced our ability to generate animal disease models for use in biomedical research and also has potential to revolutionize the treatment of genetic disorders. Duchenne muscular dystrophy (DMD) is a monogenic muscle-wasting disease that could potentially benefit from the development of CRISPR therapy. It is commonly associated with mutations that disrupt the reading frame of the DMD gene that encodes dystrophin, an essential scaffolding protein that stabilizes striated muscles and protects them from contractile-induced damage. CRISPR enables the rapid generation of various animal models harboring mutations that closely simulates the wide variety of mutations observed in DMD patients. These models provide a platform for the testing of sequence-specific interventions like CRISPR therapy that aim to reframe or skip DMD mutations to restore functional dystrophin expression. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics.

Cardiac and Skeletal Muscle Pathology in the D2/mdx Mouse Model and Caveats Associated with the Quantification of Utrophin

Duchenne muscular dystrophy (DMD) (the most common form of muscular dystrophy) is caused by a lack of dystrophin protein. Currently, although many therapeutic strategies are under investigation, there is no cure for DMD and unfortunately, patients succumb to respiratory and/or cardiac failure in their second or third decade of life. Preclinical work has focused on the mouse model C57BL/10ScSn-Dmd^mdx/J (BL10/mdx), which does not exhibit a robust pathophenotype. More recently, the D2.B10-Dmd^mdx/J (D2/mdx) mouse has been utilized, which presents a more severe pathology and therefore more closely mimics the human pathophenotype, particularly in the heart. Here, we outline important considerations when utilizing the D2/mdx model by highlighting the differences between these models in addition to describing histological and immunohistochemical methods utilized in Kennedy et al. (Mol Ther Methods Clin Dev 11:92-105, 2018) for both cardiac and skeletal muscle, which can quantify these differences. These considerations are particularly important when investigating treatment strategies that may be affected by regeneration; such is the case for upregulation of the dystrophin paralogue, utrophin.

Humanization of the mdx Mouse Phenotype for Duchenne Muscular Dystrophy Modeling: A Metabolic Perspective

Duchenne muscular dystrophy (DMD) is a severe form of muscular dystrophy (MD) that is characterized by early muscle wasting and lethal cardiorespiratory failure. While the mdx mouse is the most common model of DMD, it fails to replicate the severe loss of muscle mass and other complications observed in patients, in part due to the multiple rescue pathways found in mice. This led to several attempts at improving DMD animal models by interfering with these rescue pathways through double transgenic approaches, resulting in more severe phenotypes with mixed relevance to the human pathology. As a growing body of literature depicts DMD as a multi-system metabolic disease, improvements in mdx-based modeling of DMD may be achieved by modulating whole-body metabolism instead of muscle homeostasis. This review provides an overview of the established dual-transgenic approaches that exacerbate the mild mdx phenotype by primarily interfering with muscle homeostasis and highlights how advances in DMD modeling coincide with inducing whole-body metabolic changes. We focus on the DBA2/J strain-based D2.mdx mouse with heightened transforming growth factor (TGF)-β signaling and the dyslipidemic mdx/apolipoprotein E (mdx/ApoE) knock-out (KO) mouse, and summarize how these novel models emulate the metabolic changes observed in DMD.

Transplantation of Differentiated Tonsil-Derived Mesenchymal Stem Cells Ameliorates Murine Duchenne Muscular Dystrophy via Autophagy Activation

BACKGROUND

Skeletal muscles play many important roles in the human body and any malfunction or disorder of the skeletal muscles can lead to a reduced quality of life. Some skeletal dysfunctions are acquired, such as sarcopenia but others are congenital. Duchenne muscular dystrophy (DMD) is one of the most common forms of hereditary muscular dystrophy and is caused by a deficiency of the protein, Dystrophin. Currently, there is no clear treatment for DMD, there are only methods that can alleviate the symptoms of the disease. Mesenchymal stem cells, including tonsil-derived mesenchymal stem cells (TMSCs) have been shown to differentiate into skeletal muscle cells (TMSC-myocyte) and can be one of the resources for the treatment of DMD. Skeletal muscle cell characteristics of TMSC-myocytes have been confirmed through changes in morphology and expression of skeletal muscle markers such as Myogenin, Myf6, and MYH families after differentiation.

MEOTHDS

Based on these characteristics, TMSC-myocytes have been transplanted into mdx mice, a mouse model of DMD, to investigate whether they can help improve the symptoms of DMD. The red fluorescent protein gene was transduced into TMSC (TMSC-R) for tracking transplanted cells.

RESULTS

Prior to transplantation (TP), it was confirmed whether TMSC-R-myocytes had the same differentiation potential as TMSC-myocytes. Increased expression of dystrophin and autophagy markers in the TP group compared with the sham group was confirmed in the gastrocnemius muscle 12 weeks after TP.

CONCLUSION

These results demonstrate muscle regeneration and functional recovery of mdx via autophagy activation following TMSC-myocyte TP.

Dystrophin Restoration after Adeno-Associated Virus U7-Mediated Dmd Exon Skipping Is Modulated by Muscular Exercise in the Severe D2-Mdx Duchenne Muscular Dystrophy Murine Model

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by Dmd mutations, resulting in the absence of dystrophin in skeletal muscle, and a greater susceptibility to damage during contraction (exercise). The current study evaluated whether voluntary exercise impacts a Dmd exon skipping and muscle physiology in a severe DMD murine model. D2-mdx mice were intramuscularly injected with an adeno-associated virus (AAV) U7 snRNA to correct Dmd reading frame, and allowed to voluntary run on a wheel for 1 month. Voluntary running did not induce muscle fiber regeneration, as indicated by the percentage of centronucleated fibers, Myh3 and Myh4 expression, and maximal force production, and thus possibly did not compromise the gene therapy approach. Voluntary running did not impact the number of viral genomes and the expression of U7 and Dmd 1 month after injection of AAV-U7 injected just before exercise initiation, but reduced the amount of dystrophin in dystrophin-expressing fibers from 80% to 65% of the muscle cross-sectional area. In conclusion, voluntary running did not induce muscle damage and had no drastic detrimental effect on the AAV gene therapy exon skipping approach in a severe murine DMD model. Moreover, these results suggest considering exercise as an additional element in the design and conception of future therapeutic approaches for DMD.

Collagen cross-links scale with passive stiffness in dystrophic mouse muscles, but are not altered with administration of a lysyl oxidase inhibitor

In Duchenne muscular dystrophy (DMD), a lack of functional dystrophin leads to myofiber instability and progressive muscle damage that results in fibrosis. While fibrosis is primarily characterized by an accumulation of extracellular matrix (ECM) components, there are changes in ECM architecture during fibrosis that relate more closely to functional muscle stiffness. One of these architectural changes in dystrophic muscle is collagen cross-linking, which has been shown to increase the passive muscle stiffness in models of fibrosis including the mdx mouse, a model of DMD. We tested whether the intraperitoneal injections of beta-aminopropionitrile (BAPN), an inhibitor of the cross-linking enzyme lysyl oxidase, would reduce collagen cross-linking and passive stiffness in young and adult mdx mice compared to saline-injected controls. We found no significant differences between BAPN treated and saline treated mice in collagen cross-linking and stiffness parameters. However, we observed that while collagen cross-linking and passive stiffness scaled positively in dystrophic muscles, collagen fiber alignment scaled with passive stiffness distinctly between muscles. We also observed that the dystrophic diaphragm showed the most dramatic fibrosis in terms of collagen content, cross-linking, and stiffness. Overall, we show that while BAPN was not effective at reducing collagen cross-linking, the positive association between collagen cross-linking and stiffness in dystrophic muscles still show cross-linking as a viable target for reducing passive muscle stiffness in DMD or other fibrotic muscle conditions.

Isometric skeletal muscle contractile properties in common strains of male laboratory mice

Assessing contractile function of skeletal muscle in murine models is a commonly employed laboratory technique that investigators utilize to measure the impact of genetic manipulations, drug efficacy, or other therapeutic interventions. Often overlooked is the potential for the strain of the mouse to influence the functional properties of the skeletal muscle. Thus, we sought to characterize commonly assessed isometric force measures in the hindlimb muscles across a variety of mouse strains. Using 6-8-week-old male mice, we measured isometric force, fatigue susceptibility, relaxation kinetics, muscle mass, myofiber cross-sectional area, and fiber type composition of the extensor digitorum longus (EDL) and soleus muscles in C57BL/6NJ, BALB/cJ, FVB/NJ, C57BL/6J, and C57BL/10 mice. The data demonstrate both unique differences and a number of similarities between both muscles in the various genetic backgrounds of mice. Soleus muscle specific force (i.e., force per unit size) exhibited higher variation across strains while specific force of the EDL muscle exhibited minimal variation. In contrast, absolute force differed only in a few mouse strains whereas analysis of muscle morphology revealed many distinctions when compared across all the groups. Collectively, the data suggest that the strain of the mouse can potentially influence the measured biological outcome and may possibly promote a synergistic effect with any genetic manipulation or therapeutic intervention. Thus, it is critical for the investigator to carefully consider the genetic background of the mouse used in the experimental design and precisely document the strain of mouse employed during publication.

Next-Generation SINE Compound KPT-8602 Ameliorates Dystrophic Pathology in Zebrafish and Mouse Models of DMD

Duchenne muscular dystrophy (DMD) is a progressive, X-linked childhood neuromuscular disorder that results from loss-of-function mutations in the DYSTROPHIN gene. DMD patients exhibit muscle necrosis, cardiomyopathy, respiratory failure, and loss of ambulation. One of the major driving forces of DMD disease pathology is chronic inflammation. The current DMD standard of care is corticosteroids; however, there are serious side effects with long-term use, thus identifying novel anti-inflammatory and anti-fibrotic treatments for DMD is of high priority. We investigated the next-generation SINE compound, KPT-8602 (eltanexor) as an oral therapeutic to alleviate dystrophic symptoms. We performed pre-clinical evaluation of the effects of KPT-8602 in DMD zebrafish (sapje) and mouse (D2-mdx) models. KPT-8602 improved dystrophic skeletal muscle pathologies, muscle architecture and integrity, and overall outcomes in both animal models. KPT-8602 treatment ameliorated DMD pathology in D2-mdx mice, with increased locomotor behavior and improved muscle histology. KPT-8602 altered the immunological profile of the dystrophic mice, and reduced circulating osteopontin serum levels. These findings demonstrate KPT-8602 as an effective therapeutic in DMD through by promotion of an anti-inflammatory environment and overall improvement of DMD pathological outcomes.

Evaluation of the DBA/2J mouse as a potential background strain for genetic models of cardiomyopathy

The potential use of the D2.mdx mouse (the mdx mutation on the DBA/2J genetic background) as a preclinical model of the cardiac aspects of Duchenne muscular dystrophy (DMD) has been criticized based on speculation that the DBA/2J genetic background displays an inherent hypertrophic cardiomyopathy (HCM) phenotype. Accordingly, the goal of the current study was to further examine the cardiac status of this mouse strain over a 12-month period to determine if observable signs of HCM develop, including histopathology and pathological enlargement of the myocardium. Previous reports have documented heightened TGFβ signaling in the DBA2/J striated muscles, as compared to the C57 background, which, as expected, is manifested as increased cardiomyocyte size, wall thickness, and heart mass as compared to the C57 background. While normalized heart mass is larger in the DBA/2J mice, compared to age-matched C57/BL10 mice, both strains similarly increase in size from 4 to 12 months of age. We also report that DBA/2J mice contain equivalent amounts of left ventricular collagen as healthy canine and human samples. In a longitudinal echocardiography study, neither sedentary nor exercised DBA/2J mice demonstrated left ventricular wall thickening or cardiac functional deficits. In summary, we find no evidence of HCM, nor any other cardiac pathology, and thus propose that it is an appropriate background strain for genetic modeling of cardiac diseases, including the cardiomyopathy associated with DMD.

Sarco(endo)plasmic reticulum Ca2+-ATPase function is impaired in skeletal and cardiac muscles from young DBA/2J mdx mice

The DBA/2J (D2) mdx mouse is a more severe model of Duchenne muscular dystrophy when compared to the traditional C57BL/10 (C57) mdx mouse. Here, we questioned whether sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) function would differ in muscles from young D2 and C57 mdx mice. Both D2 and C57 mdx mice exhibited signs of impaired Ca²⁺ uptake in the gastrocnemius, diaphragm, and left ventricle; however, the level of impairment was more severe in D2 mdx mice. Reductions in maximal SERCA activity were also more prominent in the D2 mdx gastrocnemius and diaphragm when compared to those from C57 mdx mice; however, there were no differences detected in the left ventricle. Across all muscles, D2 mdx mice had the highest levels of oxidative stress as indicated by protein nitrosylation and/or nitration. In conclusion, our study shows that SERCA function is more impaired in young D2 mdx mice compared with age-matched C57 mdx mice.

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Ca²⁺ uptake is severely impaired in muscles from young DBA/2J (D2) mdx mice

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Maximal SERCA activity is lowered to a greater degree in muscles from D2 mdx mice

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Muscles from young D2 mdx mice have higher levels of oxidative/nitrosative stress

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Worsened SERCA function may contribute to worsened muscle pathology in D2 mdx mice

Comparison of Quantitative Ultrasound Methods to Classify Dystrophic and Obese Models of Skeletal Muscle

In this study, we compared multiple quantitative ultrasound metrics for the purpose of differentiating muscle in 20 healthy, 10 dystrophic and 10 obese mice. High-frequency ultrasound scans were acquired on dystrophic (D2-mdx), obese (db/db) and control mouse hindlimbs. A total of 248 image features were extracted from each scan, using brightness-mode statistics, Canny edge detection metrics, Haralick features, envelope statistics and radiofrequency statistics. Naïve Bayes and other classifiers were trained on single and pairs of features. The a parameter from the Homodyned K distribution at 40 MHz achieved the best univariate classification (accuracy = 85.3%). Maximum classification accuracy of 97.7% was achieved using a logistic regression classifier on the feature pair of a² (K distribution) at 30 MHz and brightness-mode variance at 40MHz. Dystrophic and obese mice have muscle with distinct acoustic properties and can be classified to a high level of accuracy using a combination of multiple features.

Rescue of a familial dysautonomia mouse model by AAV9-Exon-specific U1 snRNA

Familial dysautonomia (FD) is a currently untreatable, neurodegenerative disease caused by a splicing mutation (c.2204+6T>C) that causes skipping of exon 20 of the elongator complex protein 1 (ELP1) pre-mRNA. Here, we used adeno-associated virus serotype 9 (AAV9-U1-FD) to deliver an exon-specific U1 (ExSpeU1) small nuclear RNA, designed to cause inclusion of ELP1 exon 20 only in those cells expressing the target pre-mRNA, in a phenotypic mouse model of FD. Postnatal systemic and intracerebral ventricular treatment in these mice increased the inclusion of ELP1 exon 20. This also augmented the production of functional protein in several tissues including brain, dorsal root, and trigeminal ganglia. Crucially, the treatment rescued most of the FD mouse mortality before one month of age (89% vs 52%). There were notable improvements in ataxic gait as well as renal (serum creatinine) and cardiac (ejection fraction) functions. RNA-seq analyses of dorsal root ganglia from treated mice and human cells overexpressing FD-ExSpeU1 revealed only minimal global changes in gene expression and splicing. Overall then, our data prove that AAV9-U1-FD is highly specific and will likely be a safe and effective therapeutic strategy for this debilitating disease.

Kynurenine metabolism is altered in mdx mice: A potential muscle to brain connection

NEW FINDINGS

Research has shown that promoting muscle health with regular aerobic exercise can improve mental health through a kynurenine metabolic pathway. With regular aerobic exercise, kynurenine metabolism is favourably altered towards a neuroprotective pathway that promotes kynurenic acid production through increased expression of PGC-1α, kynurenine amino acid transferase (KAT) enzymes, and lowered inflammation. Whether conditions of muscle disease such as muscular dystrophy can negatively influence this pathway remains unknown. We show that the DBA/2J mdx model of Duchenne muscular dystrophy exhibit altered kynurenine metabolism with less KYNA and PGC-1α and the highest level of TNF-a mRNA - results associated with anxiety-like behaviour.

ABSTRACT

Regular exercise can direct muscle kynurenine (KYN) metabolism toward the neuroprotective branch of the kynurenine pathway thereby limiting the accumulation of neurotoxic metabolites in the brain and contributing to mental resilience. However, the effect of muscle disease on KYN metabolism has not yet been investigated. Previous work has highlighted anxiety-like behaviors in approximately 25% of patients with Duchenne muscular dystrophy (DMD), possibly due to altered KYN metabolism. Here, we characterized KYN metabolism in mdx mouse models of DMD. Young (8-10 week old) DBA/2J (D2) mdx mice, but not age-matched C57BL/10 (C57) mdx mice, had lower levels of circulating KYNA and KYNA:KYN ratio compared with their respective wild-type (WT) controls. While both C57 and D2 mdx mice displayed signs of anxiety-like behaviour, spending more time in the corners of the arena during a novel object recognition test, this effect was more prominent in D2 mdx mice. Correlational analysis detected a significant negative association between KYNA:KYN levels and time spent in corners in D2 mice, but not C57 mice. In extensor digitorum longus muscles from D2 mdx mice, but not C57 mdx mice, we found lowered protein levels of peroxisome proliferator-activated receptor-gamma coactivator 1-alpha and kynurenine amino transferase-1 enzyme when compared with WT. Furthermore, D2 mdx quadricep muscles had the highest level of TNF-α expression, which is suggestive of enhanced inflammation. Thus, our pilot work shows that KYN metabolism is altered in D2 mdx mice, with a potential contribution from altered muscle health. This article is protected by copyright. All rights reserved.

Natural history study and statistical modelling of disease progression in a preclinical model of myotubular myopathy

Generating reliable preclinical data in animal models of disease is essential in therapy development. Here we perform statistical analysis and joint longitudinal-survival modelling of the progressive phenotype observed in Mtm1-/y knock-out mice, a faithful model for myotubular myopathy (XLMTM). Analysis of historical data was used to generate a model for phenotype progression, which was then confirmed with phenotypic data from a new colony of mice derived via in vitro fertilization in an independent animal house, highlighting the reproducibility of disease phenotype in Mtm1-/y mice. This combined data was then used to refine the phenotypic parameters analyzed in these mice, and improve the model generated for expected disease progression. The disease progression model was then used to test therapeutic efficacy of Dnm2 targeting. Dnm2 reduction by antisense oligonucleotides blocked or postponed disease development, and resulted in a significant dose-dependent improvement outside the expected disease progression in untreated Mtm1-/y mice. This provides an example of optimizing disease analysis and testing therapeutic efficacy in a preclinical model, that can be applied by scientists testing therapeutic approaches using neuromuscular disease models in different laboratories.

On the measurement of skeletal muscle anisotropic permittivity property with a single cross-shaped needle insertion

Application of minimally invasive methods to enable the measurement of tissue permittivity in the neuromuscular clinic remain elusive. This paper provides a theoretical and modeling study on the measurement of the permittivity of two-dimensional anisotropic tissues such as skeletal muscle with a multi-electrode cross-shaped needle. For this, we design a novel cross-shaped needle with multiple-electrodes and analyse apparent impedance corresponding to the measured impedance. In addition, we propose three methods of estimate anisotropic muscle permittivity. Compared to existing electrical impedance-based needle methods that we have developed, the new needle design and numerical methods associated enable estimating in vivo muscle permittivity values with only a single needle insertion. Being able to measure muscle permittivity directly with a single needle insertion could open up an entirely new area of research with direct clinical application, including using these values to assist in neuromuscular diagnosis and to assess subtle effects of therapeutic intervention on muscle health.

Paeonia lactiflora extract improves the muscle function of mdx mice, an animal model of Duchenne muscular dystrophy, via downregulating the high mobility group box 1 protein

ETHNOPHARMACOLOGICAL RELEVANCE

Paeonia lactiflora Pall. is an ethnopharmacological medicine with a long history of human use for treating various inflammatory diseases in many Asian countries.

AIM OF THE STUDY

Duchenne muscular dystrophy (DMD) is an X-linked degenerative muscle disease affecting 1 in 3500 males and is characterized by severe muscle inflammation and a progressive decline in muscle function. This study aimed to elucidate the effects of an ethanol extract of the root of Paeonia lactiflora Pall. (PL) on the muscle function in the muscular dystrophy X-linked (mdx) mouse, the most commonly used animal model of DMD.

MATERIALS AND METHODS

Male mdx mice and wild-type controls aged 5 weeks were orally treated with PL for 4 weeks. The corticosteroid prednisolone was used as a comparator drug. Muscle strength and motor coordination were assessed via the grip-strength and rotarod tests, respectively. Muscle damage was evaluated via histological examination and assessment of plasma creatine-kinase activity. Proteomic analyses were conducted to identify the muscle proteins whose levels were significantly affected by PL (ProteomeXchange identifier: PXD028886). Muscle and plasma levels of these proteins, and their corresponding mRNAs were measured using western blotting and ELISA, and quantitative reverse transcription-polymerase chain reaction, respectively.

RESULTS

The muscle strength and motor coordination of mdx mice were significantly increased by the oral treatment of PL. PL significantly reduced the histological muscle damage and plasma creatine-kinase activity. Proteomic analyses of the muscle showed that PL significantly downregulated the high mobility group box 1 (HMGB1) protein and Toll-like receptor (TLR) 4, thus suppressing the HMGB1-TLR4-NF-κB signaling, in the muscle of mdx mice. Consequently, the muscle levels of proinflammatory cytokines/chemokines, which play crucial roles in inflammation, were downregulated.

CONCLUSION

PL improves the muscle function and reduces the muscle damage in mdx mice via suppressing the HMGB1-TLR4-NF-κB signaling and downregulating proinflammatory cytokines/chemokines.

Preserved Left Ventricular Function despite Myocardial Fibrosis and Myopathy in the Dystrophin-Deficient D2.B10-Dmdmdx/J Mouse

Duchenne muscular dystrophy involves an absence of dystrophin, a cytoskeletal protein which supports cell structural integrity and scaffolding for signalling molecules in myocytes. Affected individuals experience progressive muscle degeneration that leads to irreversible loss of ambulation and respiratory diaphragm function. Although clinical management has greatly advanced, heart failure due to myocardial cell loss and fibrosis remains the major cause of death. We examined cardiac morphology and function in D2.B10-Dmd^mdx/J (D2-mdx) mice, a relatively new mouse model of muscular dystrophy, which we compared to their wild-type background DBA/2J mice (DBA/2). We also tested whether drug treatment with a specific blocker of mitochondrial permeability transition pore opening (Debio-025), or ACE inhibition (Perindopril), had any effect on dystrophy-related cardiomyopathy. D2-mdx mice were treated for six weeks with Vehicle control, Debio-025 (20 mg/kg/day), Perindopril (2 mg/kg/day), or a combination (n = 8/group). At 18 weeks, compared to DBA/2, D2-mdx hearts displayed greater ventricular collagen, lower cell density, greater cell diameter, and greater protein expression levels of IL-6, TLR4, BAX/Bcl2, caspase-3, PGC-1α, and notably monoamine oxidases A and B. Remarkably, these adaptations in D2-mdx mice were associated with preserved resting left ventricular function similar to DBA/2 mice. Compared to vehicle, although Perindopril partly attenuated the increase in heart weight and collagen at 18 weeks, the drug treatments had no marked impact on dystrophic cardiomyopathy.

A Long-Term Study Evaluating the Effects of Nicorandil Treatment on Duchenne Muscular Dystrophy-Associated Cardiomyopathy in mdx Mice

BACKGROUND

Duchenne muscular dystrophy (DMD) is a neuromuscular disease caused by dystrophin gene mutations affecting striated muscle. Due to advances in skeletal muscle treatment, cardiomyopathy has emerged as a leading cause of death. Previously, nicorandil, a drug with antioxidant and nitrate-like properties, ameliorated cardiac damage and improved cardiac function in young, injured mdx mice. Nicorandil mitigated damage by stimulating antioxidant activity and limiting pro-oxidant expression. Here, we examined whether nicorandil was similarly cardioprotective in aged mdx mice.

METHODS AND RESULTS

Nicorandil (6 mg/kg) was given over 15 months. Echocardiography of mdx mice showed some functional defects at 12 months compared to wild-type (WT) mice, but not at 15 months. Disease manifestation was evident in mdx mice via treadmill assays and survival, but not open field and grip strength assays. Cardiac levels of SOD2 and NOX4 were decreased in mdx vs. WT. Nicorandil increased survival in mdx but did not alter cardiac function, fibrosis, diaphragm function or muscle fatigue.

CONCLUSIONS

In contrast to our prior work in young, injured mdx mice, nicorandil did not exert cardioprotective effects in 15 month aged mdx mice. Discordant findings may be explained by the lack of cardiac disease manifestation in aged mdx mice compared to WT, whereas significant cardiac dysfunction was previously seen with the sub-acute injury in young mice. Therefore, we are not able to conclude any cardioprotective effects with long-term nicorandil treatment in aging mdx mice.

Systemic delivery of a DUX4-targeting antisense oligonucleotide to treat facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most prevalent skeletal muscle dystrophies. Skeletal muscle pathology in individuals with FSHD is caused by inappropriate expression of the transcription factor DUX4, which activates different myotoxic pathways. At the moment there is no molecular therapy that can delay or prevent skeletal muscle wasting in FSHD. In this study, a systemically delivered antisense oligonucleotide (ASO) targeting the DUX4 transcript was tested in vivo in ACTA1-MCM;FLExDUX4 mice that express DUX4 in skeletal muscles. We show that the DUX4 ASO was well tolerated and repressed the DUX4 transcript, DUX4 protein, and mouse DUX4 target gene expression in skeletal muscles. In addition, the DUX4 ASO alleviated the severity of skeletal muscle pathology and partially prevented the dysregulation of inflammatory and extracellular matrix genes. DUX4 ASO-treated ACTA1-MCM;FLExDUX4 mice performed better on a treadmill; however, the hanging grid and four-limb grip strength tests were not improved compared to control ASO-treated ACTA1-MCM;FLExDUX4 mice. This study shows that systemic delivery of ASOs targeting DUX4 is a promising therapeutic strategy for FSHD and strategies that further improve the ASO efficacy in skeletal muscle are warranted.

Advanced models of human skeletal muscle differentiation, development and disease: Three-dimensional cultures, organoids and beyond

Advanced in vitro models of human skeletal muscle tissue are increasingly needed to model complex developmental dynamics and disease mechanisms not recapitulated in animal models or in conventional monolayer cell cultures. There has been impressive progress towards creating such models by using tissue engineering approaches to recapitulate a range of physical and biochemical components of native human skeletal muscle tissue. In this review, we discuss recent studies focussed on developing complex in vitro models of human skeletal muscle beyond monolayer cell cultures, involving skeletal myogenic differentiation from human primary myoblasts or pluripotent stem cells, often in the presence of structural scaffolding support. We conclude with our outlook on the future of advanced skeletal muscle three-dimensional cultures (e.g. organoids and biofabrication) to produce physiologically and clinically relevant platforms for disease modelling and therapy development in musculoskeletal and neuromuscular disorders.

Neuromuscular Development and Disease: Learning From in vitro and in vivo Models

The neuromuscular junction (NMJ) is a specialized cholinergic synaptic interface between a motor neuron and a skeletal muscle fiber that translates presynaptic electrical impulses into motor function. NMJ formation and maintenance require tightly regulated signaling and cellular communication among motor neurons, myogenic cells, and Schwann cells. Neuromuscular diseases (NMDs) can result in loss of NMJ function and motor input leading to paralysis or even death. Although small animal models have been instrumental in advancing our understanding of the NMJ structure and function, the complexities of studying this multi-tissue system in vivo and poor clinical outcomes of candidate therapies developed in small animal models has driven the need for in vitro models of functional human NMJ to complement animal studies. In this review, we discuss prevailing models of NMDs and highlight the current progress and ongoing challenges in developing human iPSC-derived (hiPSC) 3D cell culture models of functional NMJs. We first review in vivo development of motor neurons, skeletal muscle, Schwann cells, and the NMJ alongside current methods for directing the differentiation of relevant cell types from hiPSCs. We further compare the efficacy of modeling NMDs in animals and human cell culture systems in the context of five NMDs: amyotrophic lateral sclerosis, myasthenia gravis, Duchenne muscular dystrophy, myotonic dystrophy, and Pompe disease. Finally, we discuss further work necessary for hiPSC-derived NMJ models to function as effective personalized NMD platforms.

Characterization of Alzheimer's disease-like neuropathology in Duchenne's muscular dystrophy using the DBA/2J mdx mouse model

Duchenne muscular dystrophy (DMD) is a progressive muscle wasting disorder caused by a mutation in the dystrophin gene. In addition to muscle pathology, some patients with DMD will exhibit cognitive impairments with severity being linked to age and type of genetic mutation. Likewise, some studies have shown that mdx mice display impairments in spatial memory compared with wild‐type (WT) controls, while others have not observed any such effect. Most studies have utilized the traditional C57BL/10 (C57) mdx mouse, which exhibits a mild disease phenotype. Recently, the DBA/2J (D2) mdx mouse has emerged as a more severe and perhaps clinically relevant DMD model; however, studies examining cognitive function in these mice are limited. Thus, in this study we examined cognitive function in age‐matched C57 and D2 mdx mice along with their respective WT controls. Our findings show that 8‐ to 12‐week‐old C57 mdx mice did not display any differences in exploration time when challenged with a novel object recognition test. Conversely, age‐matched D2 mdx mice spent less time exploring objects in total as a well as less time exploring the novel object, suggestive of impaired recognition memory. Biochemical analyses of the D2 mdx brain revealed higher soluble amyloid precursor protein β (APPβ) and APP in the prefrontal cortex of mdx mice compared with WT, and lower soluble APPα in the hippocampus, suggestive of a shift towards amyloidogenesis and a similar pathogenesis to Alzheimer's disease. Furthermore, our study demonstrates the utility of the D2 mdx model in studying cognitive impairment.

Systemically Administered Homing Peptide Targets Dystrophic Lesions and Delivers Transforming Growth Factor-β (TGFβ) Inhibitor to Attenuate Murine Muscular Dystrophy Pathology

Muscular dystrophy is a progressively worsening and lethal disease, where accumulation of functionality-impairing fibrosis plays a key pathogenic role. Transforming growth factor-β1 (TGFβ1) is a central signaling molecule in the development of fibrosis in muscular dystrophic humans and mice. Inhibition of TGFβ1 has proven beneficial in mouse models of muscular dystrophy, but the global strategies of TGFβ1 inhibition produce significant detrimental side effects. Here, we investigated whether murine muscular dystrophy lesion-specific inhibition of TGFβ1 signaling by the targeted delivery of therapeutic decorin (a natural TGFβ inhibitor) by a vascular homing peptide CAR (CARSKNKDC) would reduce skeletal muscle fibrosis and pathology and increase functional characteristics of skeletal muscle. We demonstrate that CAR peptide homes to dystrophic lesions with specificity in two muscular dystrophy models. Recombinant fusion protein consisting of CAR peptide and decorin homes selectively to sites of skeletal muscle damage in mdxDBA2/J and gamma-sarcoglycan deficient DBA2/J mice. This targeted delivery reduced TGFβ1 signaling as demonstrated by reduced nuclear pSMAD staining. Three weeks of targeted decorin treatment decreased both membrane permeability and fibrosis and improved skeletal muscle function in comparison to control treatments in the mdxD2 mice. These results show that selective delivery of decorin to the sites of skeletal muscle damage attenuates the progression of murine muscular dystrophy.

Macrophages in Skeletal Muscle Dystrophies, An Entangled Partner

While skeletal muscle remodeling happens throughout life, diseases that result in its dysfunction are accountable for many deaths. Indeed, skeletal muscle is exceptionally capable to respond to stimuli modifying its homeostasis, such as in atrophy, hypertrophy, regeneration and repair. In particular conditions such as genetic diseases (muscular dystrophies), skeletal muscle’s capacity to remodel is strongly affected and undergoes continuous cycles of chronic damage. This induces scarring, fatty infiltration, as well as loss of contractibility and of the ability to generate force. In this context, inflammation, primarily mediated by macrophages, plays a central pathogenic role. Macrophages contribute as the primary regulators of inflammation during skeletal muscle regeneration, affecting tissue-resident cells such as myogenic cells and endothelial cells, but also fibro-adipogenic progenitors, which are the main source of the fibro fatty scar. During skeletal muscle regeneration their function is tightly orchestrated, while in dystrophies their fate is strongly disturbed, resulting in chronic inflammation. In this review, we will discuss the latest findings on the role of macrophages in skeletal muscle diseases, and how they are regulated.