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

Defects of full-length dystrophin trigger retinal neuron damage and synapse alterations by disrupting functional autophagy

Dystrophin (dys) mutations predispose Duchenne muscular disease (DMD) patients to brain and retinal complications. Although different dys variants, including long dys products, are expressed in the retina, their function is largely unknown. We investigated the putative role of full-length dystrophin in the homeostasis of neuro-retina and its impact on synapsis stabilization and cell fate. Retinas of mdx mice, the most used DMD model which does not express the 427-KDa dys protein (Dp427), showed overlapped cell death and impaired autophagy. Apoptotic neurons in the outer plexiform/inner nuclear layer and the ganglion cell layer had an impaired autophagy with accumulated autophagosomes. The autophagy dysfunction localized at photoreceptor axonal terminals and bipolar, amacrine, and ganglion cells. The absence of Dp427 does not cause a severe phenotype but alters the neuronal architecture, compromising mainly the pre-synaptic photoreceptor terminals and their post-synaptic sites. The analysis of two dystrophic mutants of the fruit fly Drosophila melanogaster, the homozygous Dys^E17 and Dys^EP3397, lacking functional large-isoforms of dystrophin-like protein, revealed rhabdomere degeneration. Structural damages were evident in the internal network of retina/lamina where photoreceptors make the first synapse. Both accumulated autophagosomes and apoptotic features were detected and the visual system was functionally impaired. The reactivation of the autophagosome turnover by rapamycin prevented neuronal cell death and structural changes of mutant flies and, of interest, sustained autophagy ameliorated their response to light. Overall, these findings indicate that functional full-length dystrophin is required for synapsis stabilization and neuronal survival of the retina, allowing also proper autophagy as a prerequisite for physiological cell fate and visual properties.

Long-term human IgG treatment improves heart and muscle function in a mouse model of Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease caused by mutations in the dystrophin gene, which leads to structural instability of the dystrophin-glycoprotein-complex with subsequent muscle degeneration. In addition, muscle inflammation has been implicated in disease progression and therapeutically addressed with glucocorticosteroids. These have numerous adverse effects. Treatment with human immunoglobulin G (IgG) improved clinical and para-clinical parameters in the early disease phase in the well-established mdx mouse model. The aim of the present study was to confirm the efficacy of IgG in a long-term pre-clinical study in mdx mice.

METHODS

IgG (2 g/kg body weight) or NaCl solution as control was administered monthly over 18 months by intraperitoneal injection in mdx mice beginning at 3 weeks of age. Several clinical outcome measures including endurance, muscle strength, and echocardiography were assessed. After 18 months, the animals were sacrificed, blood was collected for analysis, and muscle samples were obtained for ex vivo muscle contraction tests, quantitative PCR, and histology.

RESULTS

IgG significantly improved the daily voluntary running performance (1.9 m more total daily running distance, P < 0.0001) and slowed the decrease in grip strength by 0.1 mN, (P = 0.018). IgG reduced fatigability of the diaphragm (improved ratio to maximum force by 0.09 ± 0.04, P = 0.044), but specific tetanic force remained unchanged in the ex vivo muscle contraction test. Cardiac function was significantly better after IgG, especially fractional area shortening (P = 0.012). These results were accompanied by a reduction in cardiac fibrosis and the infiltration of T cells (P = 0.0002) and macrophages (P = 0.0027). In addition, treatment with IgG resulted in a significant reduction of the infiltration of T cells (P ≤ 0.036) in the diaphragm, gastrocnemius, quadriceps, and a similar trend in tibialis anterior and macrophages (P ≤ 0.045) in gastrocnemius, quadriceps, tibialis anterior, and a similar trend in the diaphragm, as well as a decrease in myopathic changes as reflected by a reduced central nuclear index in the diaphragm, tibialis anterior, and quadriceps (P ≤ 0.002 in all).

CONCLUSIONS

The present study underscores the importance of an inflammatory contribution to the disease progression of DMD. The data demonstrate the long-term efficacy of IgG in the mdx mouse. IgG is well tolerated by humans and could preferentially complement gene therapy in DMD. The data call for a clinical trial with IgG in DMD.

Translating CRISPR-Cas Therapeutics: Approaches and Challenges

CRISPR-Cas clinical trials have begun, offering a first glimpse at how DNA and RNA targeting could enable therapies for many genetic and epigenetic human diseases. The speedy progress of CRISPR-Cas from discovery and adoption to clinical use is built on decades of traditional gene therapy research and belies the multiple challenges that could derail the successful translation of these new modalities. Here, we review how CRISPR-Cas therapeutics are translated from technological systems to therapeutic modalities, paying particular attention to the therapeutic cascade from cargo to delivery vector, manufacturing, administration, pipelines, safety, and therapeutic target profiles. We also explore potential solutions to some of the obstacles facing successful CRISPR-Cas translation. We hope to illuminate how CRISPR-Cas is brought from the academic bench toward use in the clinic.

Levelling the Translational Gap for Animal to Human Efficacy Data

Reports of a reproducibility crisis combined with a high attrition rate in the pharmaceutical industry have put animal research increasingly under scrutiny in the past decade. Many researchers and the general public now question whether there is still a justification for conducting animal studies. While criticism of the current modus operandi in preclinical research is certainly warranted, the data on which these discussions are based are often unreliable. Several initiatives to address the internal validity and reporting quality of animal studies (e.g., Animals in Research: Reporting In Vivo Experiments (ARRIVE) and Planning Research and Experimental Procedures on Animals: Recommendations for Excellence (PREPARE) guidelines) have been introduced but seldom implemented. As for external validity, progress has been virtually absent. Nonetheless, the selection of optimal animal models of disease may prevent the conducting of clinical trials, based on unreliable preclinical data. Here, we discuss three contributions to tackle the evaluation of the predictive value of animal models of disease themselves. First, we developed the Framework to Identify Models of Disease (FIMD), the first step to standardise the assessment, validation and comparison of disease models. FIMD allows the identification of which aspects of the human disease are replicated in the animals, facilitating the selection of disease models more likely to predict human response. Second, we show an example of how systematic reviews and meta-analyses can provide another strategy to discriminate between disease models quantitatively. Third, we explore whether external validity is a factor in animal model selection in the Investigator's Brochure (IB), and we use the IB-derisk tool to integrate preclinical pharmacokinetic and pharmacodynamic data in early clinical development. Through these contributions, we show how we can address external validity to evaluate the translatability and scientific value of animal models in drug development. However, while these methods have potential, it is the extent of their adoption by the scientific community that will define their impact. By promoting and adopting high quality study design and reporting, as well as a thorough assessment of the translatability of drug efficacy of animal models of disease, we will have robust data to challenge and improve the current animal research paradigm.

Desmin prevents muscle wasting, exaggerated weakness and fragility, and fatigue in dystrophic mdx mouse

KEY POINTS

Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix. Deletion of the desmin gene in mdx mice [double knockout (DKO) mice] induces marked muscle weakness and fatigue resistance compared to mdx mice. Muscle fragility (higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice. By contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy. Desmin cDNA transfer with adeno-associated virus in newborn mdx mice reduced muscle weakness. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic muscle.

ABSTRACT

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by dystrophin deficiency. Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix that contributes to muscle function. In the present study, we attempted to provide further insight into the roles of desmin, for which the expression is increased in the muscle from the mouse mdx DMD model. We show that a deletion of the desmin gene (Des) in mdx mice [double knockout (DKO) mice, mdx:desmin-/-] induces a marked muscle weakness; namely, a reduced absolute maximal force production and increased fatigue compared to that in mdx mice. Fragility (i.e. higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice, despite the promotion of supposedly less fragile muscle fibres in DKO mice, and this worsening of fragility was related to a decreased muscle excitability. Moreover, in contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy, as indicated by smaller and fewer fibres, with a reduced percentage of centronucleated fibres, potentially explaining the severe muscle weakness. Notably, Desmin cDNA transfer with adeno-associated virus in newborn mdx mice improved specific maximal force normalized to muscle weight. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic mdx mice, which differ, at least in part, from those observed in healthy muscle.

Transport of Ca2+ and Ca2+-dependent permeability transition in heart mitochondria in the early stages of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive skeletal muscle disease that is associated with severe cardiac complications in the late stages. Significant mitochondrial dysfunction is reportedly responsible for the development of cardiomyopathy with age. At the same time, adaptive changes in mitochondrial metabolism in cardiomyocytes were identified in the early stages of DMD. In this work, we evaluate the functioning of calcium transport systems (MCU and NCLX), and MPT pore in the heart mitochondria of young dystrophin-deficient mice. As compared to wild-type animals, heart mitochondria of mdx mice have been found to be more efficient both in respect to Ca²⁺ uniport and Na⁺-dependent Ca²⁺ efflux. The data obtained indicate that the increased rate of Ca²⁺ uptake by heart mitochondria of mdx mice may be due to an increase in the ratio of MCU and MCUb subunits. In turn, an increase in the rate of Ca²⁺ efflux from organelles in DMD may be the result of a significant increase in the level of NCLX. Moreover, the heart mitochondria of mdx mice were more resistant to MPT pore opening, which may be due to an increase in the microviscosity of mitochondrial membranes of DMD mice. At the same time, the level of putative MPT pore proteins did not change. The paper discusses the effect of rearrangements of the mitochondrial proteome involved in the transport and accumulation of calcium on the adaptation of this organ to DMD.

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.

miR-379 links glucocorticoid treatment with mitochondrial response in Duchenne muscular dystrophy

Duchenne Muscular Dystrophy (DMD) is a lethal muscle disorder, caused by mutations in the DMD gene and affects approximately 1:5000-6000 male births. In this report, we identified dysregulation of members of the Dlk1-Dio3 miRNA cluster in muscle biopsies of the GRMD dog model. Of these, we selected miR-379 for a detailed investigation because its expression is high in the muscle, and is known to be responsive to glucocorticoid, a class of anti-inflammatory drugs commonly used in DMD patients. Bioinformatics analysis predicts that miR-379 targets EIF4G2, a translational factor, which is involved in the control of mitochondrial metabolic maturation. We confirmed in myoblasts that EIF4G2 is a direct target of miR-379, and identified the DAPIT mitochondrial protein as a translational target of EIF4G2. Knocking down DAPIT in skeletal myotubes resulted in reduced ATP synthesis and myogenic differentiation. We also demonstrated that this pathway is GC-responsive since treating mice with dexamethasone resulted in reduced muscle expression of miR-379 and increased expression of EIF4G2 and DAPIT. Furthermore, miR-379 seric level, which is also elevated in the plasma of DMD patients in comparison with age-matched controls, is reduced by GC treatment. Thus, this newly identified pathway may link GC treatment to a mitochondrial response in DMD.

Correction of muscular dystrophies by CRISPR gene editing

Muscular dystrophies are debilitating disorders that result in progressive weakness and degeneration of skeletal muscle. Although the genetic mutations and clinical abnormalities of a variety of neuromuscular diseases are well known, no curative therapies have been developed to date. The advent of genome editing technology provides new opportunities to correct the underlying mutations responsible for many monogenic neuromuscular diseases. For example, Duchenne muscular dystrophy, which is caused by mutations in the dystrophin gene, has been successfully corrected in mice, dogs, and human cells through CRISPR/Cas9 editing. In this Review, we focus on the potential for, and challenges of, correcting muscular dystrophies by editing disease-causing mutations at the genomic level. Ideally, because muscle tissues are extremely long-lived, CRISPR technology could offer a one-time treatment for muscular dystrophies by correcting the culprit genomic mutations and enabling normal expression of the repaired gene.

High prevalence of plasma lipid abnormalities in human and canine Duchenne and Becker muscular dystrophies depicts a new type of primary genetic dyslipidemia

BACKGROUND

Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are allelic X-linked recessive muscle diseases caused by mutations in the DMD gene, with DMD being the more severe form. We have recently shown that increased plasma low-density lipoprotein-associated cholesterol causes severe muscle wasting in the mdx mouse, a mild DMD model, which suggested that plasma lipids may play a critical role in DMD. We have also observed that loss of dystrophin in mice causes unexpected elevations in plasma lipoprotein levels.

OBJECTIVE

The objectives of the study were to determine whether patients with DMD and BMD also present with clinically relevant plasma lipoprotein abnormalities and to mitigate the presence of confounders (medications and lifestyle) by analyzing the plasma from patients with DMD/BMD and unmedicated dogs with DMD, the most relevant model of DMD.

METHODS

Levels of low-density lipoprotein-associated cholesterol, high-density lipoprotein cholesterol, and triglycerides were analyzed in patients with DMD and BMD and female carriers. Samples from unmedicated, ambulatory dogs with DMD, unaffected carriers, and normal controls were also analyzed.

RESULTS

We report that 97% and 64% of all pediatric patients with DMD (33 of 36) and BMD (6 of 11) are dyslipidemic, along with an unusually high incidence in adult patients with BMD. All dogs with DMD showed plasma lipid abnormalities that progressively worsened with age. Most strikingly, unaffected carrier dogs also showed plasma lipid abnormalities similar to affected dogs with DMD. Dyslipidemia is likely not secondary to liver damage as unaffected carriers showed no plasma aminotransferase elevation.

CONCLUSIONS

The high incidence of plasma lipid abnormalities in dystrophin-deficient plasma may depict a new type of genetic dyslipidemia. Abnormal lipid levels in dystrophinopathic samples in the absence of muscle damage suggest a primary state of dyslipidemia. Whether dyslipidemia plays a causal role in patients with DMD warrants further investigation, which could lead to new diagnostic and therapeutic options.

Pax7, Pax3 and Mamstr genes are involved in skeletal muscle impaired regeneration of dy2J/dy2J mouse model of Lama2-CMD

Congenital muscular dystrophy type-1A (Lama2-CMD) and Duchenne muscular dystrophy (DMD) result from deficiencies of laminin-α2 and dystrophin proteins, respectively. Although both proteins strengthen the sarcolemma, they are implicated in clinically distinct phenotypes. We used RNA-deep sequencing (RNA-Seq) of dy2J/dy2J, Lama2-CMD mouse model, skeletal muscle at 8 weeks of age to elucidate disease pathophysiology. This study is the first report of dy2J/dy2J model whole transcriptome profile. RNA-Seq of the mdx mouse model of DMD and wild-type (WT) mouse was carried as well in order to enable a novel comparison of dy2J/dy2J to mdx. A large group of shared differentially expressed genes (DEGs) was found in dy2J/dy2J and mdx models (1834 common DEGs, false discovery rate [FDR] < 0.05). Enrichment pathway analysis using ingenuity pathway analysis showed enrichment of inflammation, fibrosis, cellular movement, migration and proliferation of cells, apoptosis and necrosis in both mouse models (P-values 3E-10-9E-37). Via canonical pathway analysis, actin cytoskeleton, integrin, integrin-linked kinase, NF-kB, renin-angiotensin, epithelial-mesenchymal transition, and calcium signaling were also enriched and upregulated in both models (FDR < 0.05). Interestingly, significant downregulation of Pax7 was detected in dy2J/dy2J compared to upregulation of this key regeneration gene in mdx mice. Pax3 and Mamstr genes were also downregulated in dy2J/dy2J compared to WT mice. These results may explain the distinct disease course and severity in these models. While the mdx model at that stage shows massive regeneration, the dy2J/dy2J shows progressive dystrophic process. Our data deepen our understanding of the molecular pathophysiology and suggest new targets for additional therapies to upregulate regeneration in Lama2-CMD.

In vitro assay for the efficacy assessment of AAV vectors expressing microdystrophin

AAV-delivered microdystrophin genes hold great promise for Duchenne muscular dystrophy (DMD) treatment. It is anticipated that the optimization of engineered dystrophin genes will be required to increase the efficacy and reduce the immunogenicity of transgenic proteins. An in vitro system is required for the efficacy testing of genetically engineered dystrophin genes. We report here on the proof of concept for an in vitro assay based on the assessment of sarcolemma damage after repetitively applied electrical stimuli. The primary cell culture of myoblasts was established from wild-type C57BL/10ScSnJ and dystrophin-deficient mdx mice. The preparation parameters and the differentiation of contractile myotubes were optimized. DAPI and TO-PRO-3 dyes were used to assess myotubular membrane permeability in response to electrical pulse stimulation (EPS). Myotubes derived from mdx mice exhibited a greater increase in membrane damage, as assessed by TO-PRO-3-measured permeability after EPS, than was exhibited by the healthy control myotubes. AAV-DJ particles carrying the microdystrophin gene were used to transduce mdx-derived differentiated myotubes. Microdystrophin delivery ameliorated the disease phenotype and reduced the EPS-induced membrane damage to a level comparable to that of the healthy controls. Thus, the in vitro system was shown to be capable of supporting studies on DMD gene therapy.

A 96-well culture platform enables longitudinal analyses of engineered human skeletal muscle microtissue strength

Three-dimensional (3D) in vitro models of human skeletal muscle mimic aspects of native tissue structure and function, thereby providing a promising system for disease modeling, drug discovery or pre-clinical validation, and toxicity testing. Widespread adoption of this research approach is hindered by the lack of easy-to-use platforms that are simple to fabricate and that yield arrays of human skeletal muscle micro-tissues (hMMTs) in culture with reproducible physiological responses that can be assayed non-invasively. Here, we describe a design and methods to generate a reusable mold to fabricate a 96-well platform, referred to as MyoTACTIC, that enables bulk production of 3D hMMTs. All 96-wells and all well features are cast in a single step from the reusable mold. Non-invasive calcium transient and contractile force measurements are performed on hMMTs directly in MyoTACTIC, and unbiased force analysis occurs by a custom automated algorithm, allowing for longitudinal studies of function. Characterizations of MyoTACTIC and resulting hMMTs confirms the capability of the device to support formation of hMMTs that recapitulate biological responses. We show that hMMT contractile force mirrors expected responses to compounds shown by others to decrease (dexamethasone, cerivastatin) or increase (IGF-1) skeletal muscle strength. Since MyoTACTIC supports hMMT long-term culture, we evaluated direct influences of pancreatic cancer chemotherapeutics agents on contraction competent human skeletal muscle myotubes. A single application of a clinically relevant dose of Irinotecan decreased hMMT contractile force generation, while clear effects on myotube atrophy were observed histologically only at a higher dose. This suggests an off-target effect that may contribute to cancer associated muscle wasting, and highlights the value of the MyoTACTIC platform to non-invasively predict modulators of human skeletal muscle function.

The Dog Model in the Spotlight: Legacy of a Trustful Cooperation

Dogs have long been used as a biomedical model system and in particular as a preclinical proof of concept for innovative therapies before translation to humans. A recent example of the utility of this animal model is the promising myotubularin gene delivery in boys affected by X-linked centronuclear myopathy after successful systemic, long-term efficient gene therapy in Labrador retrievers. Mostly, this is due to unique features that make dogs an optimal system. The continuous emergence of spontaneous inherited disorders enables the identification of reliable complementary molecular models for human neuromuscular disorders (NMDs). Dogs’ characteristics including size, lifespan and unprecedented medical care level allow a comprehensive longitudinal description of diseases. Moreover, the highly similar pathogenic mechanisms with human patients yield to translational robustness. Finally, interindividual phenotypic heterogeneity between dogs helps identifying modifiers and anticipates precision medicine issues.

This review article summarizes the present list of molecularly characterized dog models for NMDs and provides an exhaustive list of the clinical and paraclinical assays that have been developed. This toolbox offers scientists a sensitive and reliable system to thoroughly evaluate neuromuscular function, as well as efficiency and safety of innovative therapies targeting these NMDs. This review also contextualizes the model by highlighting its unique genetic value, shaped by the long-term coevolution of humans and domesticated dogs. Because the dog is one of the most protected research animal models, there is considerable opposition to include it in preclinical projects, posing a threat to the use of this model. We thus discuss ethical issues, emphasizing that unlike many other models, the dog also benefits from its contribution to comparative biomedical research with a drastic reduction in the prevalence of morbid alleles in the breeding stock and an improvement in medical care.

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.

CRISPR-Generated Animal Models of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a fatal X-linked recessive neuromuscular disorder most commonly caused by mutations disrupting the reading frame of the dystrophin (DMD) gene. DMD codes for dystrophin, which is critical for maintaining the integrity of muscle cell membranes. Without dystrophin, muscle cells receive heightened mechanical stress, becoming more susceptible to damage. An active body of research continues to explore therapeutic treatments for DMD as well as to further our understanding of the disease. These efforts rely on having reliable animal models that accurately recapitulate disease presentation in humans. While current animal models of DMD have served this purpose well to some extent, each has its own limitations. To help overcome this, clustered regularly interspaced short palindromic repeat (CRISPR)-based technology has been extremely useful in creating novel animal models for DMD. This review focuses on animal models developed for DMD that have been created using CRISPR, their advantages and disadvantages as well as their applications in the DMD field.

Canine Models of Inherited Musculoskeletal and Neurodegenerative Diseases

Mouse models of human disease remain the bread and butter of modern biology and therapeutic discovery. Nonetheless, more often than not mouse models do not reproduce the pathophysiology of the human conditions they are designed to mimic. Naturally occurring large animal models have predominantly been found in companion animals or livestock because of their emotional or economic value to modern society and, unlike mice, often recapitulate the human disease state. In particular, numerous models have been discovered in dogs and have a fundamental role in bridging proof of concept studies in mice to human clinical trials. The present article is a review that highlights current canine models of human diseases, including Alzheimer's disease, degenerative myelopathy, neuronal ceroid lipofuscinosis, globoid cell leukodystrophy, Duchenne muscular dystrophy, mucopolysaccharidosis, and fucosidosis. The goal of the review is to discuss canine and human neurodegenerative pathophysiologic similarities, introduce the animal models, and shed light on the ability of canine models to facilitate current and future treatment trials.

Myosoft: An automated muscle histology analysis tool using machine learning algorithm utilizing FIJI/ImageJ software

METHODS

Muscle sections were stained for cell boundary (laminin) and myofiber type (myosin heavy chain isoforms). Myosoft, running in the open access software platform FIJI (ImageJ), was used to analyze myofiber size and type in transverse sections of entire gastrocnemius/soleus muscles.

RESULTS

Myosoft provides an accurate analysis of hundreds to thousands of muscle fibers within 25 minutes, which is >10-times faster than manual analysis. We demonstrate that Myosoft is capable of handling high-content images even when image or staining quality is suboptimal, which is a marked improvement over currently available and comparable programs.

CONCLUSIONS

Myosoft is a reliable, accurate, high-throughput, and convenient tool to analyze high-content muscle histology. Myosoft is freely available to download from Github at https://github.com/Hyojung-Choo/Myosoft/tree/Myosoft-hub.

Mouse models for muscular dystrophies: an overview

Muscular dystrophies (MDs) encompass a wide variety of inherited disorders that are characterized by loss of muscle tissue associated with a progressive reduction in muscle function. With a cure lacking for MDs, preclinical developments of therapeutic approaches depend on well-characterized animal models that recapitulate the specific pathology in patients. The mouse is the most widely and extensively used model for MDs, and it has played a key role in our understanding of the molecular mechanisms underlying MD pathogenesis. This has enabled the development of therapeutic strategies. Owing to advancements in genetic engineering, a wide variety of mouse models are available for the majority of MDs. Here, we summarize the characteristics of the most commonly used mouse models for a subset of highly studied MDs, collated into a table. Together with references to key publications describing these models, this brief but detailed overview would be useful for those interested in, or working with, mouse models of MD.

Lack of PKCθ Promotes Regenerative Ability of Muscle Stem Cells in Chronic Muscle Injury

Duchenne muscular dystrophy (DMD) is a genetic disease characterized by muscle wasting and chronic inflammation, leading to impaired satellite cells (SCs) function and exhaustion of their regenerative capacity. We previously showed that lack of PKCθ in mdx mice, a mouse model of DMD, reduces muscle wasting and inflammation, and improves muscle regeneration and performance at early stages of the disease. In this study, we show that muscle regeneration is boosted, and fibrosis reduced in mdxθ^−/− mice, even at advanced stages of the disease. This phenotype was associated with a higher number of Pax7 positive cells in mdxθ^−/− muscle compared with mdx muscle, during the progression of the disease. Moreover, the expression level of Pax7 and Notch1, the pivotal regulators of SCs self-renewal, were upregulated in SCs isolated from mdxθ^−/− muscle compared with mdx derived SCs. Likewise, the expression of the Notch ligands Delta1 and Jagged1 was higher in mdxθ^−/− muscle compared with mdx. The expression level of Delta1 and Jagged1 was also higher in PKCθ^−/− muscle compared with WT muscle following acute injury. In addition, lack of PKCθ prolonged the survival and sustained the differentiation of transplanted myogenic progenitors. Overall, our results suggest that lack of PKCθ promotes muscle repair in dystrophic mice, supporting stem cells survival and maintenance through increased Delta-Notch signaling.

An improved method for studying mouse diaphragm function

Dysfunction in the contractile properties of the diaphragm muscle contributes to the morbidity and mortality in many neuromuscular and respiratory diseases. Methods that can accurately quantify diaphragm function in mouse models are essential for preclinical studies. Diaphragm function is usually measured using the diaphragm strip. Two methods have been used to attach the diaphragm strip to the force transducer. The suture method is easy to adopt but it cannot maintain the physiological orientation of the muscle fibers. Hence, results may not accurately reflect diaphragm contractility. The clamp method can better maintain diaphragm muscle fiber orientation but is used less often because detailed information on clamp fabrication and application has never been published. Importantly, a side-by-side comparison of the two methods is lacking. To address these questions, we engineered diaphragm clamps using mechanically highly durable material. Here, we present a detailed and ready-to-use protocol on the design and manufacture of diaphragm clamps. Also, we present a step by step protocol on how to mount the diaphragm strip to the clamp and then to the muscle force measurement system. We compared the diaphragm force from the same mouse with both suture and clamp methods. We found the clamp method yielded a significantly higher muscle force. Finally, we validated the utility of the clamp method in the mdx model of Duchenne muscular dystrophy. In summary, the clamp method described in this paper yields reliable and consistent diaphragm force data. This method will be useful to any laboratory interested in performing mouse diaphragm function assay.

Gut microbiotas and immune checkpoint inhibitor therapy response: a causal or coincidental relationship?

As the largest immune organ, human gut microbiome could influence the efficacy of immune checkpoint inhibitor therapy (ICI). However, identifying contributory microbes from over 35,000 species is virtually impossible and the identified microbes are not consistent among studies. The reason for the disparity may be that the microbes found in feces are markers of other factors that link immune response and microbiotas. Notably, gut microbiome is influenced by stool consistency, diet and other lifestyle factors. Therefore, the ICI and microbiotas relationship must be adjusted for potential confounders and analyzed longitudinally. Moreover, a recent study where 11 low-abundance commensal bacteria induced interferon-γ-producing CD8 T cells, challenges the validity of the abundance-oriented microbiotas investigations. This study also confirmed the hierarchy in immunogenic roles among microbiotas. Fecal transplantation trials in germ-free mice provided "the proof of principle" that germ-free mice reproduce the donor's microbiome and corresponding ICI efficacy. However, species-specific biological differences prevent direct extrapolation between the results in murine and human models. Fecal transplantation or supplementation with microbes found in ICI responders requires caution due to potential adverse events.

Proteomic and cell biological profiling of the renal phenotype of the mdx-4cv mouse model of Duchenne muscular dystrophy

The X-linked inherited muscle wasting disease Duchenne muscular dystrophy, which is caused by primary abnormalities in the membrane cytoskeletal protein dystrophin, is a multi-system disorder. Highly progressive forms of dystrophinopathy are associated with a complex secondary pathophysiology, including renal dysfunction. It was therefore of interest to carry out a systematic survey of potential proteome-wide changes in the kidney of the established mdx-4cv mouse model of dystrophinopathy. Of 5878 mass spectrometrically identified kidney proteins, 82 versus 142 proteins were shown to be decreased or increased, respectively, in association with muscular dystrophy. The most decreased versus increased protein species are the ACSM3 isoform of mitochondrial acyl-coenzyme A synthetase and the FABP1 isoform of fatty acid binding protein, respectively. Both proteomic findings were verified by immunofluorescence microscopy and immunoblot analysis. Interestingly, haematoxylin/eosin staining indicated diffuse whitish deposits in the mdx-4cv kidney, and an increased intensity of Sudan Black labelling of kidney cells revealed ectopic fat deposition. Although the proteomic results and cell biological findings do not demonstrate a direct functional link between increased FABP1 and fat accumulation, the results suggest that the up-regulation of FABP1 may be related to abnormal fat metabolism. This makes FABP1 potentially a novel pathobiochemical indicator for studying kidney abnormalities in the mdx-4cv model of dystrophinopathy.

A customizable microfluidic platform for medium-throughput modeling of neuromuscular circuits

Neuromuscular circuits (NMCs) are vital for voluntary movement, and effective models of NMCs are needed to understand the pathogenesis of, as well as to identify effective treatments for, multiple diseases, including Duchenne’s muscular dystrophy and amyotrophic lateral sclerosis. Microfluidics are ideal for recapitulating the central and peripheral compartments of NMCs, but myotubes often detach before functional NMCs are formed. In addition, microfluidic systems are often limited to a single experimental unit, which significantly limits their application in disease modeling and drug discovery. Here, we developed a microfluidic platform (MFP) containing over 100 experimental units, making it suitable for medium-throughput applications. To overcome detachment, we incorporated a reactive polymer surface allowing customization of the environment to culture different cell types. Using this approach, we identified conditions that enable long-term co-culture of human motor neurons and myotubes differentiated from human induced pluripotent stem cells inside our MFP. Optogenetics demonstrated the formation of functional NMCs. Furthermore, we developed a novel application of the rabies tracing assay to efficiently identify NMCs in our MFP. Therefore, our MFP enables large-scale generation and quantification of functional NMCs for disease modeling and pharmacological drug targeting.

The use of genetically humanized animal models for personalized medicine approaches

For many genetic diseases, researchers are developing personalized medicine approaches. These sometimes employ custom genetic interventions such as antisense-mediated exon skipping or genome editing, aiming to restore protein function in a mutation-specific manner. Animal models can facilitate the development of personalized medicine approaches; however, given that they target human mutations and therefore human genetic sequences, scientists rely on the availability of humanized animal models. Here, we outline the usefulness, caveats and potential of such models, using the example of the hDMDdel52/mdx model, a humanized model recently generated for Duchenne muscular dystrophy (DMD).

Laminin-111 protein therapy enhances muscle regeneration and repair in the GRMD dog model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating X-linked disease affecting ~1 in 5000 males. DMD patients exhibit progressive muscle degeneration and weakness, leading to loss of ambulation and premature death from cardiopulmonary failure. We previously reported that mouse Laminin-111 (msLam-111) protein could reduce muscle pathology and improve muscle function in the mdx mouse model for DMD. In this study, we examined the ability of msLam-111 to prevent muscle disease progression in the golden retriever muscular dystrophy (GRMD) dog model of DMD. The msLam-111 protein was injected into the cranial tibial muscle compartment of GRMD dogs and muscle strength and pathology were assessed. The results showed that msLam-111 treatment increased muscle fiber regeneration and repair with improved muscle strength and reduced muscle fibrosis in the GRMD model. Together, these findings support the idea that Laminin-111 could serve as a novel protein therapy for the treatment of DMD.

miR-146a deficiency does not aggravate muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is a genetic disease evoked by a mutation in the dystrophin gene. It is associated with progressive muscle degeneration and increased inflammation. Up to this date, mainly anti-inflammatory treatment is available for patients suffering from DMD. miR-146a is known to diminish inflammation and fibrosis in different tissues by downregulating the expression of proinflammatory cytokines. However, its role in DMD has not been studied so far.

In our work, we have generated mice globally lacking both dystrophin and miR-146a (miR-146a^−/−mdx) and examined them together with wild-type, single miR-146a knockout and dystrophic (mdx—lacking dystrophin) mice in a variety of aspects associated with DMD pathophysiology (muscle degeneration, inflammatory reaction, muscle satellite cells, muscle regeneration, and fibrosis).

We have shown that miR-146a level is increased in dystrophic muscles in comparison to wild-type mice. Its deficiency augments the expression of proinflammatory cytokines (IL-1β, CCL2, TNFα). However, muscle degeneration was not significantly worsened in mdx mice lacking miR-146a up to 24 weeks of age, although some aggravation of muscle damage and inflammation was evident in 12-week-old animals, though no effect of miR-146a deficiency was visible on quantity, proliferation, and in vitro differentiation of muscle satellite cells isolated from miR-146a^−/−mdx mice vs. mdx. Similarly, muscle regeneration and collagen deposition were not changed by miR-146a deficiency. Nevertheless, the lack of miR-146a is associated with decreased Vegfa and increased Tgfb1.

Overall, the lack of miR-146a did not aggravate significantly the dystrophic conditions in mdx mice, but its effect on DMD in more severe conditions warrants further investigation.

Natural History of Cardiomyopathy in Adult Dogs With Golden Retriever Muscular Dystrophy

Background

Duchenne muscular dystrophy (DMD) is an X‐linked disease that causes progressive muscle weakness. Affected boys typically die from respiratory or cardiac failure. Golden retriever muscular dystrophy (GRMD) is genetically homologous with DMD and causes analogous skeletal and cardiac muscle disease. Previous studies have detailed features of GRMD cardiomyopathy in mostly young dogs. Cardiac disease is not well characterized in adult GRMD dogs, and cardiac magnetic resonance (CMR) imaging studies have not been completed.

Methods and Results

We evaluated echocardiography and CMR in 24 adult GRMD dogs at different ages. Left ventricular systolic and diastolic functions, wall thickness, and myocardial strain were assessed with echocardiography. Features evaluated with CMR included left ventricular function, chamber size, myocardial mass, and late gadolinium enhancement. Our results largely paralleled those of DMD cardiomyopathy. Ejection fraction and fractional shortening correlated well with age, with systolic dysfunction occurring at ≈30 to 45 months. Circumferential strain was more sensitive than ejection fraction in early disease detection. Evidence of left ventricular chamber dilatation provided proof of dilated cardiomyopathy. Late gadolinium enhancement imaging showed DMD‐like left ventricular lateral wall lesions and earlier involvement of the anterior septum. Multiple functional indexes were graded objectively and added, with and without late gadolinium enhancement, to give cardiac and cardiomyopathy scores of disease severity. Consistent with DMD, there was parallel skeletal muscle involvement, as tibiotarsal joint flexion torque declined in tandem with cardiac function.

Conclusions

This study established parallels of progressive cardiomyopathy between dystrophic dogs and boys, further validating GRMD as a model of DMD cardiac disease.

Skeletal muscle cell transplantation: models and methods

Xenografts of skeletal muscle are used to study muscle repair and regeneration, mechanisms of muscular dystrophies, and potential cell therapies for musculoskeletal disorders. Typically, xenografting involves using an immunodeficient host that is pre-injured to create a niche for human cell engraftment. Cell type and method of delivery to muscle depend on the specific application, but can include myoblasts, satellite cells, induced pluripotent stem cells, mesangioblasts, immortalized muscle precursor cells, and other multipotent cell lines delivered locally or systemically. Some studies follow cell engraftment with interventions to enhance cell proliferation, migration, and differentiation into mature muscle fibers. Recently, several advances in xenografting human-derived muscle cells have been applied to study and treat Duchenne muscular dystrophy and Facioscapulohumeral muscular dystrophy. Here, we review the vast array of techniques available to aid researchers in designing future experiments aimed at creating robust muscle xenografts in rodent hosts.

Sarcolipin overexpression impairs myogenic differentiation in Duchenne muscular dystrophy

Reduction in the expression of sarcolipin (SLN), an inhibitor of sarco(endo)plasmic reticulum (SR) Ca²⁺-ATPase (SERCA), ameliorates severe muscular dystrophy in mice. However, the mechanism by which SLN inhibition improves muscle structure remains unclear. Here, we describe the previously unknown function of SLN in muscle differentiation in Duchenne muscular dystrophy (DMD). Overexpression of SLN in C₂C₁₂ resulted in decreased SERCA pump activity, reduced SR Ca²⁺ load, and increased intracellular Ca²⁺ (Cai2+) concentration. In addition, SLN overexpression resulted in altered expression of myogenic markers and poor myogenic differentiation. In dystrophin-deficient dog myoblasts and myotubes, SLN expression was significantly high and associated with defective Cai2+ cycling. The dystrophic dog myotubes were less branched and associated with decreased autophagy and increased expression of mitochondrial fusion and fission proteins. Reduction in SLN expression restored these changes and enhanced dystrophic dog myoblast fusion during differentiation. In summary, our data suggest that SLN upregulation is an intrinsic secondary change in dystrophin-deficient myoblasts and could account for the Cai2+ mishandling, which subsequently contributes to poor myogenic differentiation. Accordingly, reducing SLN expression can improve the Cai2+ cycling and differentiation of dystrophic myoblasts. These findings provide cellular-level supports for targeting SLN expression as a therapeutic strategy for DMD.

Eosinophils Do Not Drive Acute Muscle Pathology in the mdx Mouse Model of Duchenne Muscular Dystrophy

Eosinophils are present in muscle lesions associated with Duchenne muscular dystrophy and dystrophin-deficient mdx mice that phenocopy this disorder. Although it has been hypothesized that eosinophils promote characteristic inflammatory muscle damage, this has not been fully examined. In this study, we generated mice with the dystrophin mutation introduced into PHIL, a strain with a transgene that directs lineage-specific eosinophil ablation. We also explored the impact of eosinophil overabundance on dystrophinopathy by introducing the dystrophin mutation into IL-5 transgenic mice. We evaluated the degree of eosinophil infiltration in association with myofiber size distribution, centralized nuclei, serum creatine kinase, and quantitative histopathology scores. Among our findings, eosinophils were prominent in the quadriceps muscles of 4-wk-old male mdx mice but no profound differences were observed in the quantitative measures of muscle damage when comparing mdx versus mdx.PHIL versus mdx.IL5tg mice, despite dramatic differences in eosinophil infiltration (CD45+CD11c-Gr1-MHC class IIloSiglecF+ eosinophils at 1.2 ± 0.34% versus <0.1% versus 20 ± 7.6% of total cells, respectively). Further evaluation revealed elevated levels of eosinophil chemoatttractants eotaxin-1 and RANTES in the muscle tissue of all three dystrophin-deficient strains; eotaxin-1 concentration in muscle correlated inversely with age. Cytokines IL-4 and IL-1R antagonist were also detected in association with eosinophils in muscle. Taken together, our findings challenge the long-held perception of eosinophils as cytotoxic in dystrophin-deficient muscle; we show clearly that eosinophil infiltration is not a driving force behind acute muscle damage in the mdx mouse strain. Ongoing studies will focus on the functional properties of eosinophils in this unique microenvironment.

What is the level of dystrophin expression required for effective therapy of Duchenne muscular dystrophy?

Duchenne muscular dystrophy (DMD) is a fatal X-linked muscle wasting disease. The disease is due to mutations in the DMD gene that encodes for a large intracellular protein called dystrophin. Dystrophin plays a critical role in linking the internal cytoskeleton of the striated muscle cell with the extracellular matrix as well as having cell signalling functions. In its absence muscle contraction is associated with cycles of damage, repair, inflammation and fibrosis with eventual loss of muscle and replacement with fat. Experiments in animal models of DMD have generated a number of different approaches to the induction of dystrophin including viral vector mediated delivery of a recombinant dystrophin gene, antisense oligonucleotide mediated exon-skipping to restore the open reading frame in the dystrophin mRNA, read-through of premature stop mutations, genome modification using CRISPR-Cas9 or cell based transfer of a functional dystrophin gene. In all cases, it will be important to understand how much dystrophin expression is required for a clinically effective therapy and this review examines the data from humans and animal models to estimate the percentage of endogenous dystrophin that is likely to have significant clinical benefit. While there are a number of important caveats to consider, including the appropriate outcome measures, this review suggests that approximately 20% of endogenous levels uniformly distributed within the skeletal muscles and the heart may be sufficient to largely prevent disease progression.

Comparative study of calcium and calcium-related enzymes with differentiation markers in different ages and muscle types in mdx mice

Sarcolemma instability and increased calcium influx in muscle fibers are characteristics of the Duchenne muscular dystrophy. Excessive calcium activates calcium-dependent enzymes, such as calpains (CAPN) and matrix metalloproteases (MMP). Here, we analyzed calcium deposits, the activity of CAPN and MMP and the expression of Myh, SERCA and myogenic regulatory factors in different skeletal muscles during myonecrosis (4-weeks) and regeneration (12-weeks) phases of the mdx muscular pathology. Alizarin red staining was used to assess calcium deposits, casein and gelatin zymography were performed to evaluate CAPN and MMP activity, and qPCR was used to evaluate the expression of Myh, Capn, Atp2a1 and Atp2a2, Myod1 and Myog. We observed the following characteristics in mdx muscles: (i) calcium deposits almost exclusively in mdx muscles, (ii) lower CAPN1 activity in mdx muscles, (iii) higher CAPN2 activity in mdx muscles (only at 12 wks), (iv) autolyzed CAPN activity exclusively in mdx muscles, (v) lower expression of Capn1 and higher expression of Capn2 in mdx muscles; (vi) lower expression of Atp2a1 and Atp2a2 in mdx muscles, (vii) higher MMP (pre pro MMP2, pro MMP2, MMP2 and MMP9) activity in mdx muscles, (viii) MMP2 activity exclusively in mdx muscles at 12 wks, (ix) MMP9 activity exclusively in mdx muscles, (x) higher expression of Myog in mdx muscles at 12 wks, and (xi) lower expression of Myh (Myh7, Myh2, Myh1, Myh4) in mdx muscles, particularly Myh7 and Myh2. The collection of our results provides valuable information for a better characterization of mdx pathology phenotype.

A standardised framework to identify optimal animal models for efficacy assessment in drug development

INTRODUCTION

Poor translation of efficacy data derived from animal models can lead to clinical trials unlikely to benefit patients-or even put them at risk-and is a potential contributor to costly and unnecessary attrition in drug development.

OBJECTIVES

To develop a tool to assess, validate and compare the clinical translatability of animal models used for the preliminary assessment of efficacy.

DESIGN AND RESULTS

We performed a scoping review to identify the key aspects used to validate animal models. Eight domains (Epidemiology, Symptomatology and Natural History-SNH, Genetic, Biochemistry, Aetiology, Histology, Pharmacology and Endpoints) were identified. We drafted questions to evaluate the different facets of human disease simulation. We designed the Framework to Identify Models of Disease (FIMD) to include standardised instructions, a weighting and scoring system to compare models as well as factors to help interpret model similarity and evidence uncertainty. We also added a reporting quality and risk of bias assessment of drug intervention studies in the Pharmacological Validation domain. A web-based survey was conducted with experts from different stakeholders to gather input on the framework. We conducted a pilot study of the validation in two models for Type 2 Diabetes (T2D)-the ZDF rat and db/db mouse. Finally, we present a full validation and comparison of two animal models for Duchenne Muscular Dystrophy (DMD): the mdx mouse and GRMD dog. We show that there are significant differences between the mdx mouse and the GRMD dog, the latter mimicking the human epidemiological, SNH, and histological aspects to a greater extent than the mouse despite the overall lack of published data.

CONCLUSIONS

FIMD facilitates drug development by serving as the basis to select the most relevant model that can provide meaningful data and is more likely to generate translatable results to progress drug candidates to the clinic.

Dystrophin is expressed in smooth muscle and afferent nerve fibers in the rat urinary bladder

INTRODUCTION

With increasing life expectancy, comorbidities become overt in Duchenne muscular dystrophy (DMD). Although micturition problems are common, bladder function is poorly understood in DMD. We studied dystrophin expression and multiple isoform involvement in the bladder during maturation to gain insights into their roles in micturition.

METHODS

Dystrophin distribution was evaluated in rat bladders by immunohistochemical colocalization with smooth muscle, interstitial, urothelial, and neuronal markers. Protein levels of Dp140, Dp71, and smooth muscle were quantitated by Western blotting of neonatal to adult rat bladders.

RESULTS

Dystrophin colocalized with smooth muscle cells and afferent nerve fibers. Dp71 was expressed two- to threefold higher compared with Dp140, independently of age. Age-related muscle mass changes did not influence isoform expression levels.

DISCUSSION

Dystrophin is expressed in smooth muscle cells and afferent nerve fibers in the urinary bladder, which underscores that micturition problems in DMD may have not solely a myogenic but also a neurogenic origin. Muscle Nerve 60: 202-210, 2019.

Early myopathy in Duchenne muscular dystrophy is associated with elevated mitochondrial H2 O2 emission during impaired oxidative phosphorylation

BACKGROUND

Muscle wasting and weakness in Duchenne muscular dystrophy (DMD) causes severe locomotor limitations and early death due in part to respiratory muscle failure. Given that current clinical practice focuses on treating secondary complications in this genetic disease, there is a clear need to identify additional contributions in the aetiology of this myopathy for knowledge-guided therapy development. Here, we address the unresolved question of whether the complex impairments observed in DMD are linked to elevated mitochondrial H2 O2 emission in conjunction with impaired oxidative phosphorylation. This study performed a systematic evaluation of the nature and degree of mitochondrial-derived H2 O2 emission and mitochondrial oxidative dysfunction in a mouse model of DMD by designing in vitro bioenergetic assessments that attempt to mimic in vivo conditions known to be critical for the regulation of mitochondrial bioenergetics.

METHODS

Mitochondrial bioenergetics were compared with functional and histopathological indices of myopathy early in DMD (4 weeks) in D2.B10-DMDmdx /2J mice (D2.mdx)-a model that demonstrates severe muscle weakness. Adenosine diphosphate's (ADP's) central effect of attenuating H2 O2 emission while stimulating respiration was compared under two models of mitochondrial-cytoplasmic phosphate exchange (creatine independent and dependent) in muscles that stained positive for membrane damage (diaphragm, quadriceps, and white gastrocnemius).

RESULTS

Pathway-specific analyses revealed that Complex I-supported maximal H2 O2 emission was elevated concurrent with a reduced ability of ADP to attenuate emission during respiration in all three muscles (mH2 O2 : +17 to +197% in D2.mdx vs. wild type). This was associated with an impaired ability of ADP to stimulate respiration at sub-maximal and maximal kinetics (-17 to -72% in D2.mdx vs. wild type), as well as a loss of creatine-dependent mitochondrial phosphate shuttling in diaphragm and quadriceps. These changes largely occurred independent of mitochondrial density or abundance of respiratory chain complexes, except for quadriceps. This muscle was also the only one exhibiting decreased calcium retention capacity, which indicates increased sensitivity to calcium-induced permeability transition pore opening. Increased H2 O2 emission was accompanied by a compensatory increase in total glutathione, while oxidative stress markers were unchanged. Mitochondrial bioenergetic dysfunctions were associated with induction of mitochondrial-linked caspase 9, necrosis, and markers of atrophy in some muscles as well as reduced hindlimb torque and reduced respiratory muscle function.

CONCLUSIONS

These results provide evidence that Complex I dysfunction and loss of central respiratory control by ADP and creatine cause elevated oxidant generation during impaired oxidative phosphorylation. These dysfunctions may contribute to early stage disease pathophysiology and support the growing notion that mitochondria are a potential therapeutic target in this disease.

Human induced pluripotent stem cell models for the study and treatment of Duchenne and Becker muscular dystrophies

Duchenne and Becker muscular dystrophies are the most common muscle diseases and are both currently incurable. They are caused by mutations in the dystrophin gene, which lead to the absence or reduction/truncation of the encoded protein, with progressive muscle degeneration that clinically manifests in muscle weakness, cardiac and respiratory involvement and early death. The limits of animal models to exactly reproduce human muscle disease and to predict clinically relevant treatment effects has prompted the development of more accurate in vitro skeletal muscle models. However, the challenge of effectively obtaining mature skeletal muscle cells or satellite stem cells as primary cultures has hampered the development of in vitro models. Here, we discuss the recently developed technologies that enable the differentiation of skeletal muscle from human induced pluripotent stem cells (iPSCs) of Duchenne and Becker patients. These systems recapitulate key disease features including inflammation and scarce regenerative myogenic capacity that are partially rescued by genetic and pharmacological therapies and can provide a useful platform to study and realize future therapeutic treatments. Implementation of this model also takes advantage of the developing genome editing field, which is a promising approach not only for correcting dystrophin, but also for modulating the underlying mechanisms of skeletal muscle development, regeneration and disease. These data prove the possibility of creating an accurate Duchenne and Becker in vitro model starting from iPSCs, to be used for pathogenetic studies and for drug screening to identify strategies capable of stopping or reversing muscular dystrophinopathies and other muscle diseases.

Questions Answered and Unanswered by the First CRISPR Editing Study in a Canine Model of Duchenne Muscular Dystrophy

Clustered regularly interspaced short palindromic repeats (CRISPR) editing is being considered as a potential gene repair therapy to treat Duchenne muscular dystrophy, a dystrophin-deficient lethal muscle disease affecting all muscles in the body. A recent preliminary study from the Olson laboratory (Amoasii et al. Science 2018;362:89-91) showed robust dystrophin restoration in a canine Duchenne muscular dystrophy model following intramuscular or intravenous delivery of the CRISPR editing machinery by adeno-associated virus serotype 9. Despite the limitation of the small sample size, short study duration, and the lack of muscle function data, the Olson lab findings have provided important proof of principle for scaling up CRISPR therapy from rodents to large mammals. Future large-scale, long-term, and comprehensive studies are warranted to establish the safety and efficacy of CRISPR editing therapy in large mammals.

CRISPR/Cas9-generated mouse model of Duchenne muscular dystrophy recapitulating a newly identified large 430 kb deletion in the human DMD gene

Exon skipping is a promising strategy for Duchenne muscular dystrophy (DMD) disease-modifying therapy. To make this approach safe, ensuring that excluding one or more exons will restore the reading frame and that the resulting protein will retain critical functions of the full-length dystrophin protein is necessary. However, in vivo testing of the consequences of skipping exons that encode the N-terminal actin-binding domain (ABD) has been confounded by the absence of a relevant animal model. We created a mouse model of the disease recapitulating a novel human mutation, a large de novo deletion of exons 8-34 of the DMD gene, found in a Russian DMD patient. This mutation was achieved by deleting exons 8-34 of the X-linked mouse D md gene using CRISPR/Cas9 genome editing, which led to a reading frame shift and the absence of functional dystrophin production. Male mice carrying this deletion display several important signs of muscular dystrophy, including a gradual age-dependent decrease in muscle strength, increased creatine kinase, muscle fibrosis and central nucleation. The degrees of these changes are comparable to those observed in mdx mice, a standard laboratory model of DMD. This new model of DMD will be useful for validating therapies based on skipping exons that encode the N-terminal ABD and for improving our understanding of the role of the N-terminal domain and central rod domain in the biological function of dystrophin. Simultaneous skipping of exons 6 and 7 should restore the gene reading frame and lead to the production of a protein that might retain functionality despite the partial deletion of the ABD.

Targeting angiogenesis in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) represents one of the most devastating types of muscular dystrophies which affect boys already at early childhood. Despite the fact that the primary cause of the disease, namely the lack of functional dystrophin is known already for more than 30 years, DMD still remains an incurable disease. Thus, an enormous effort has been made during recent years to reveal novel mechanisms that could provide therapeutic targets for DMD, especially because glucocorticoids treatment acts mostly symptomatic and exerts many side effects, whereas the effectiveness of genetic approaches aiming at the restoration of functional dystrophin is under the constant debate. Taking into account that dystrophin expression is not restricted to muscle cells, but is present also in, e.g., endothelial cells, alterations in angiogenesis process have been proposed to have a significant impact on DMD progression. Indeed, already before the discovery of dystrophin, several abnormalities in blood vessels structure and function have been revealed, suggesting that targeting angiogenesis could be beneficial in DMD. In this review, we will summarize current knowledge about the angiogenesis status both in animal models of DMD as well as in DMD patients, focusing on different organs as well as age- and sex-dependent effects. Moreover, we will critically discuss some approaches such as modulation of vascular endothelial growth factor or nitric oxide related pathways, to enhance angiogenesis and attenuate the dystrophic phenotype. Additionally, we will suggest the potential role of other mediators, such as heme oxygenase-1 or statins in those processes.

rAAVrh74.MCK.GALGT2 Protects against Loss of Hemodynamic Function in the Aging mdx Mouse Heart

Dilated cardiomyopathy is a common cause of death in patients with Duchenne muscular dystrophy (DMD). Gene therapies for DMD must, therefore, have a therapeutic impact in cardiac as well as skeletal muscles. Our previous studies have shown that GALGT2 overexpression in mdx skeletal muscles can prevent muscle damage. Here we have tested whether rAAVrh74.MCK.GALGT2 gene therapy in mdx cardiac muscle can prevent the loss of heart function. Treatment of mdx hearts with rAAVrh74.MCK.GALGT2 1 day after birth did not negatively alter hemodynamic function, tested at 3 months of age, and it prevented early left ventricular remodeling and expression of fibrotic gene markers. Intravenous treatment of mdx mice with rAAVrh74.MCK.GALGT2 at 2 months of age significantly improved stroke volume and cardiac output compared to mock-treated mice analyzed at 17 months, both at rest and after stimulation with dobutamine. rAAVrh74.MCK.GALGT2 treatment of mdx heart correlated with increased glycosylation of α-dystroglycan with the CT glycan and increased utrophin protein expression. These data provide the first demonstration that GALGT2 overexpression can inhibit the loss of cardiac function in the dystrophin-deficient heart and, thus, may benefit both cardiac and skeletal muscles in DMD patients.

Impairments in left ventricular mitochondrial bioenergetics precede overt cardiac dysfunction and remodelling in Duchenne muscular dystrophy

KEY POINTS

Ninety-eight per cent of patients with Duchenne muscular dystrophy (DMD) develop cardiomyopathy, with 40% developing heart failure. While increased propensity for mitochondrial induction of cell death has been observed in left ventricle, it remains unknown whether this is linked to impaired mitochondrial respiratory control and elevated H₂ O₂ emission prior to the onset of cardiomyopathy. Classic mouse models of DMD demonstrate hyper-regeneration in skeletal muscle which may mask mitochondrial abnormalities. Using a model with less regenerative capacity that is more akin to DMD patients, we observed elevated left ventricular mitochondrial H₂ O₂ and impaired oxidative phosphorylation in the absence of cardiac remodelling or overt cardiac dysfunction at 4 weeks. These impairments were associated with dysfunctions at complex I, governance by ADP and creatine-dependent phosphate shuttling, which results in a less efficient response to energy demands. Mitochondria may be a therapeutic target for the treatment of cardiomyopathy in DMD.

ABSTRACT

In Duchenne muscular dystrophy (DMD), mitochondrial dysfunction is predicted as a response to numerous cellular stressors, yet the contribution of mitochondria to the onset of cardiomyopathy remains unknown. To resolve this uncertainty, we designed in vitro assessments of mitochondrial bioenergetics to model mitochondrial control parameters that influence cardiac function. Both left ventricular mitochondrial responsiveness to the central bioenergetic controller ADP and the ability of creatine to facilitate mitochondrial-cytoplasmic phosphate shuttling were assessed. These measurements were performed in D2.B10-DMD^mdx /2J mice - a model that demonstrates skeletal muscle atrophy and weakness due to limited regenerative capacities and cardiomyopathy more akin to people with DMD than classic models. At 4 weeks of age, there was no evidence of cardiac remodelling or cardiac dysfunction despite impairments in ADP-stimulated respiration and ADP attenuation of H₂ O₂ emission. These impairments were seen at both submaximal and maximal ADP concentrations despite no reductions in mitochondrial content markers. The ability of creatine to enhance ADP's control of mitochondrial bioenergetics was also impaired, suggesting an impairment in mitochondrial creatine kinase-dependent phosphate shuttling. Susceptibly to permeability transition pore opening and the subsequent activation of cell death pathways remained unchanged. Mitochondrial H₂ O₂ emission was elevated despite no change in markers of irreversible oxidative damage, suggesting alternative redox signalling mechanisms should be explored. These findings demonstrate that selective mitochondrial dysfunction precedes the onset of overt cardiomyopathy in D2.mdx mice, suggesting that improving mitochondrial bioenergetics by restoring ADP, creatine-dependent phosphate shuttling and complex I should be considered for treating DMD patients.

Reduced mitochondrial respiration and increased calcium deposits in the EDL muscle, but not in soleus, from 12-week-old dystrophic mdx mice

Mitochondria play an important role in providing ATP for muscle contraction. Muscle physiology is compromised in Duchenne muscular dystrophy (DMD) and several studies have shown the involvement of bioenergetics. In this work we investigated the mitochondrial physiology in fibers from fast-twitch muscle (EDL) and slow-twitch muscle (soleus) in the mdx mouse model for DMD and in control C57BL/10J mice. In our study, multiple mitochondrial respiratory parameters were investigated in permeabilized muscle fibers from 12-week-old animals, a critical age where muscle regeneration is observed in the mdx mouse. Using substrates of complex I and complex II from the electron transport chain, ADP and mitochondrial inhibitors, we found in the mdx EDL, but not in the mdx soleus, a reduction in coupled respiration suggesting that ATP synthesis is affected. In addition, the oxygen consumption after addition of complex II substrate is reduced in mdx EDL; the maximal consumption rate (measured in the presence of uncoupler) also seems to be reduced. Mitochondria are involved in calcium regulation and we observed, using alizarin stain, calcium deposits in mdx muscles but not in control muscles. Interestingly, more calcium deposits were found in mdx EDL than in mdx soleus. These data provide evidence that in 12-week-old mdx mice, calcium is accumulated and mitochondrial function is disturbed in the fast-twitch muscle EDL, but not in the slow-twitch muscle soleus.

Physical exertion exacerbates decline in the musculature of an animal model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a genetic disorder caused by loss of the protein dystrophin. In humans, DMD has early onset, causes developmental delays, muscle necrosis, loss of ambulation, and death. Current animal models have been challenged by their inability to model the early onset and severity of the disease. It remains unresolved whether increased sarcoplasmic calcium observed in dystrophic muscles follows or leads the mechanical insults caused by the muscle's disrupted contractile machinery. This knowledge has important implications for patients, as potential physiotherapeutic treatments may either help or exacerbate symptoms, depending on how dystrophic muscles differ from healthy ones. Recently we showed how burrowing dystrophic (dys-1) C. elegans recapitulate many salient phenotypes of DMD, including loss of mobility and muscle necrosis. Here, we report that dys-1 worms display early pathogenesis, including dysregulated sarcoplasmic calcium and increased lethality. Sarcoplasmic calcium dysregulation in dys-1 worms precedes overt structural phenotypes (e.g., mitochondrial, and contractile machinery damage) and can be mitigated by reducing calmodulin expression. To learn how dystrophic musculature responds to altered physical activity, we cultivated dys-1 animals in environments requiring high intensity or high frequency of muscle exertion during locomotion. We find that several muscular parameters (e.g., size) improve with increased activity. However, longevity in dystrophic animals was negatively associated with muscular exertion, regardless of effort duration. The high degree of phenotypic conservation between dystrophic worms and humans provides a unique opportunity to gain insight into the pathology of the disease as well as the initial assessment of potential treatment strategies.