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

Evaluation of an AAV9-mini-dystrophin gene therapy candidate in a rat model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked disease caused by loss-of-function mutations in the dystrophin gene and is characterized by muscle wasting and early mortality. Adeno-associated virus-mediated gene therapy is being investigated as a treatment for DMD. In the nonclinical study documented here, we determined the effective dose of fordadistrogene movaparvovec, a clinical candidate adeno-associated virus serotype 9 vector carrying a human mini-dystrophin transgene, after single intravenous injection in a dystrophin-deficient (DMDmdx) rat model of DMD. Overall, we found that transduction efficiency, number of muscle fibers expressing the human mini-dystrophin polypeptide, improvement of the skeletal and cardiac muscle tissue architecture, correction of muscle strength and fatigability, and improvement of diastolic and systolic cardiac function were directly correlated with the amount of vector administered. The effective dose was then tested in older DMDmdx rats with a more dystrophic phenotype similar to the pathology observed in older patients with DMD. Except for a less complete rescue of muscle function in the oldest cohort, fordadistrogene movaparvovec was also found to be therapeutically effective in older DMDmdx rats, suggesting that this product may be appropriate for evaluation in patients with DMD at all stages of disease.

Biomarkers for Duchenne muscular dystrophy progression: impact of age in the mdx tongue spared muscle

BACKGROUND

Duchenne muscular dystrophy (DMD) is a severe form of muscular dystrophy without an effective treatment, caused by mutations in the DMD gene, leading to the absence of dystrophin. DMD results in muscle weakness, loss of ambulation, and death at an early age. Metabolomics studies in mdx mice, the most used model for DMD, reveal changes in metabolites associated with muscle degeneration and aging. In DMD, the tongue muscles exhibit unique behavior, initially showing partial protection against inflammation but later experiencing fibrosis and loss of muscle fibers. Certain metabolites and proteins, like TNF-α and TGF-β, are potential biomarkers for dystrophic muscle characterization.

METHODS

To investigate disease progression and aging, we utilized young (1 month old) and old (21-25 months old) mdx and wild-type tongue muscles. Metabolite changes were analyzed using 1H nuclear magnetic resonance, while TNF-α and TGF-β were assessed using Western blotting to examine inflammation and fibrosis. Morphometric analysis was conducted to assess the extent of myofiber damage between groups.

RESULTS

The histological analysis of the mid-belly tongue showed no differences between groups. No differences were found between the concentrations of metabolites from wild-type or mdx whole tongues of the same age. The metabolites alanine, methionine, and 3-methylhistidine were higher, and taurine and glycerol were lower in young tongues in both wild type and mdx (p < 0.001). The metabolites glycine (p < 0.001) and glutamic acid (p = 0.0018) were different only in the mdx groups, being higher in young mdx mice. Acetic acid, phosphocreatine, isoleucine, succinic acid, creatine, and the proteins TNF-α and TGF-β had no difference in the analysis between groups (p > 0.05).

CONCLUSIONS

Surprisingly, histological, metabolite, and protein analysis reveal that the tongue of old mdx remains partially spared from the severe myonecrosis observed in other muscles. The metabolites alanine, methionine, 3-methylhistidine, taurine, and glycerol may be effective for specific assessments, although their use for disease progression monitoring should be cautious due to age-related changes in the tongue muscle. Acetic acid, phosphocreatine, isoleucine, succinate, creatine, TNF-α, and TGF-β do not vary with aging and remain constant in spared muscles, suggesting their potential as specific biomarkers for DMD progression independent of aging.

Nanoparticles systemically biodistribute to regenerating skeletal muscle in DMD

Skeletal muscle disease severity can often progress asymmetrically across muscle groups and heterogeneously within tissues. An example is Duchenne Muscular Dystrophy (DMD) in which lack of dystrophin results in devastating skeletal muscle wasting in some muscles whereas others are spared or undergo hypertrophy. An efficient, non-invasive approach to identify sites of asymmetry and degenerative lesions could enable better patient monitoring and therapeutic targeting of disease. In this study, we utilized a versatile intravenously injectable mesoporous silica nanoparticle (MSNP) based nanocarrier system to explore mechanisms of biodistribution in skeletal muscle of mdx mouse models of DMD including wildtype, dystrophic, and severely dystrophic mice. Moreover, MSNPs could be imaged in live mice and whole muscle tissues enabling investigation of how biodistribution is altered by different types of muscle pathology such as inflammation or fibrosis. We found MSNPs were tenfold more likely to aggregate within select mdx muscles relative to wild type, such as gastrocnemius and quadriceps. This was accompanied by decreased biodistribution in off-target organs. We found the greatest factor affecting preferential delivery was the regenerative state of the dystrophic skeletal muscle with the highest MSNP abundance coinciding with the regions showing the highest level of embryonic myosin staining and intramuscular macrophage uptake. To demonstrate, muscle regeneration regulated MSNP distribution, we experimentally induced regeneration using barium chloride which resulted in a threefold increase of intravenously injected MSNPs to sites of regeneration 7 days after injury. These discoveries provide the first evidence that nanoparticles have selective biodistribution to skeletal muscle in DMD to areas of active regeneration and that nanoparticles could enable diagnostic and selective drug delivery in DMD skeletal muscle.

The Adiponectin Receptor Agonist, ALY688: A Promising Therapeutic for Fibrosis in the Dystrophic Muscle

Duchenne muscular dystrophy (DMD) is one of the most devastating myopathies, where severe inflammation exacerbates disease progression. Previously, we demonstrated that adiponectin (ApN), a hormone with powerful pleiotropic effects, can efficiently improve the dystrophic phenotype. However, its practical therapeutic application is limited. In this study, we investigated ALY688, a small peptide ApN receptor agonist, as a potential novel treatment for DMD. Four-week-old mdx mice were subcutaneously treated for two months with ALY688 and then compared to untreated mdx and wild-type mice. In vivo and ex vivo tests were performed to assess muscle function and pathophysiology. Additionally, in vitro tests were conducted on human DMD myotubes. Our results showed that ALY688 significantly improved the physical performance of mice and exerted potent anti-inflammatory, anti-oxidative and anti-fibrotic actions on the dystrophic muscle. Additionally, ALY688 hampered myonecrosis, partly mediated by necroptosis, and enhanced the myogenic program. Some of these effects were also recapitulated in human DMD myotubes. ALY688's protective and beneficial properties were mainly mediated by the AMPK-PGC-1α axis, which led to suppression of NF-κβ and TGF-β. Our results demonstrate that an ApN mimic may be a promising and effective therapeutic prospect for a better management of DMD.

Biotechnological advances and applications of human pluripotent stem cell-derived heart models

In recent years, significant biotechnological advancements have been made in engineering human cardiac tissues and organ-like models. This field of research is crucial for both basic and translational research due to cardiovascular disease being the leading cause of death in the developed world. Additionally, drug-associated cardiotoxicity poses a major challenge for drug development in the pharmaceutical and biotechnological industries. Progress in three-dimensional cell culture and microfluidic devices has enabled the generation of human cardiac models that faithfully recapitulate key aspects of human physiology. In this review, we will discuss 3D pluripotent stem cell (PSC)-models of the human heart, such as engineered heart tissues and organoids, and their applications in disease modeling and drug screening.

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.

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

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

The super-healing MRL strain promotes muscle growth in muscular dystrophy through a regenerative extracellular matrix

Genetic background shifts the severity of muscular dystrophy. In mice, the DBA/2J strain confers a more severe muscular dystrophy phenotype, whereas the Murphy's Roth Large (MRL) strain has "super-healing" properties that reduce fibrosis. A comparative analysis of the Sgcg null model of Limb Girdle Muscular Dystrophy in the DBA/2J versus MRL strain showed the MRL background was associated with greater myofiber regeneration and reduced structural degradation of muscle. Transcriptomic profiling of dystrophic muscle in the DBA/2J and MRL strains indicated strain-dependent expression of the extracellular matrix (ECM) and TGF-β signaling genes. To investigate the MRL ECM, cellular components were removed from dystrophic muscle sections to generate decellularized "myoscaffolds". Decellularized myoscaffolds from dystrophic mice in the protective MRL strain had significantly less deposition of collagen and matrix-bound TGF-β1 and TGF-β3 throughout the matrix, and dystrophic myoscaffolds from the MRL background were enriched in myokines. C2C12 myoblasts were seeded onto decellularized matrices from Sgcg-/- MRL and Sgcg-/- DBA/2J matrices. Acellular myoscaffolds from the dystrophic MRL background induced myoblast differentiation and growth compared to dystrophic myoscaffolds from the DBA/2J matrices. These studies establish that the MRL background also generates its effect through a highly regenerative ECM, which is active even in muscular dystrophy.

Biomarkers for Duchenne muscular dystrophy progression: impact of age in the mdx tongue spared muscle

Background: Duchenne muscular dystrophy (DMD) is a severe form of muscular dystrophy without an effective treatment, caused by mutations in the DMD gene, leading to the absence of dystrophin. DMD results in muscle weakness, loss of ambulation and death at an early age. Metabolomics studies in mdx mice, the most used model for DMD, reveal changes in metabolites associated with muscle degeneration and aging. In DMD, the tongue muscles exhibit unique behavior, initially showing partial protection against inflammation but later experiencing fibrosis and loss of muscle fibers. Certain metabolites and proteins, like TNF-α and TGF-β, are potential biomarkers for dystrophic muscle characterization. Methods: To investigate disease progression and aging, we utilized young (1-month old) and old (21-25 months old) mdx and wild-type mice. Metabolite changes were analyzed using 1-H Nuclear Magnetic Resonance, while TNF-α and TGF-β were assessed using Western blotting to examine inflammation, and fibrosis. Morphometric analysis was conducted to assess the extent of myofiber damage between groups. Results: The histological analysis of the tongue showed no differences between groups. No differences were found between the concentrations of metabolites from wild type or mdx animals of the same age. The metabolites alanine, methionine, 3-methylhistidine were higher, and taurine and glycerol were lower in young animals in both wild type and mdx (p < 0.001). The metabolites glycine (p < 0.001) and glutamic acid (p = 0.0018) were different only in the mdx groups, being higher in young mdx mice. Acetic acid, phosphocreatine, isoleucine, succinic acid, creatine and the proteins TNF-α and TGF-β had no difference in the analysis between groups (p > 0.05). Conclusions: Surprisingly, histological and protein analysis reveals that the tongue of young and old mdx animals is protected from severe myonecrosis observed in other muscles. The metabolites alanine, methionine, 3-methylhistidine, taurine, and glycerol may be effective for specific assessments, although their use for disease progression monitoring should be cautious due to age-related changes. Acetic acid, phosphocreatine, isoleucine, succinate, creatine, TNF-α, and TGF-β do not vary with aging and remain constant in spared muscles, suggesting their potential as specific biomarkers for DMD progression independent of aging.

Inhibition of ATP synthase reverse activity restores energy homeostasis in mitochondrial pathologies

The maintenance of cellular function relies on the close regulation of adenosine triphosphate (ATP) synthesis and hydrolysis. ATP hydrolysis by mitochondrial ATP Synthase (CV) is induced by loss of proton motive force and inhibited by the mitochondrial protein ATPase inhibitor (ATPIF1). The extent of CV hydrolytic activity and its impact on cellular energetics remains unknown due to the lack of selective hydrolysis inhibitors of CV. We find that CV hydrolytic activity takes place in coupled intact mitochondria and is increased by respiratory chain defects. We identified (+)-Epicatechin as a selective inhibitor of ATP hydrolysis that binds CV while preventing the binding of ATPIF1. In cells with Complex-III deficiency, we show that inhibition of CV hydrolytic activity by (+)-Epichatechin is sufficient to restore ATP content without restoring respiratory function. Inhibition of CV-ATP hydrolysis in a mouse model of Duchenne Muscular Dystrophy is sufficient to improve muscle force without any increase in mitochondrial content. We conclude that the impact of compromised mitochondrial respiration can be lessened using hydrolysis-selective inhibitors of CV.

Lifelong Outcomes of Systemic Adeno-Associated Virus Micro-Dystrophin Gene Therapy in a Murine Duchenne Muscular Dystrophy Model

Adeno-associated virus (AAV)-mediated systemic micro-dystrophin (μDys) therapy is currently in clinical trials. The hope is to permanently improve the life quality of Duchenne muscular dystrophy (DMD) patients. Numerous preclinical studies have been conducted to support these trials. However, none examined whether a single therapy at a young age can lead to lifelong disease amelioration. To address this critical question, we injected 1 × 1013 vg particles/mouse of an AAV serotype-9 μDys vector to 3-month-old mdx mice through the tail vein. Therapeutic outcomes were evaluated at the age of 11 months (adulthood, 8 months postinjection) and 21 months (terminal age, 18 months postinjection). Immunostaining and Western blot showed saturated supraphysiological levels of μDys expression in skeletal muscle and heart till the end of the study. Treatment significantly improved grip force and treadmill running, and significantly reduced the serum creatine kinase level at both time points. Since cardiac death is a major threat in late-stage patients, we evaluated cardiac electrophysiology and hemodynamics by ECG and the closed-chest cardiac catheter assay, respectively. Significant improvements were observed in these assays. Importantly, many ECG and hemodynamic parameters (heart rate, PR interval, QRS duration, QTc interval, end-diastolic/systolic volume, dP/dt max and min, max pressure, and ejection fraction) were completely normalized at 21 months of age. Our results have provided direct evidence that a single systemic AAV μDys therapy has the potential to provide lifelong benefits in the murine DMD model.

Duchenne Muscular Dystrophy Gene Therapy in 2023: Status, Perspective, and Beyond

Duchenne muscular dystrophy (DMD) was named more than 150 years ago. About four decades ago, the DMD gene was discovered, and the reading frame shift was determined as the genetic underpinning. These pivotal findings significantly changed the landscape of DMD therapy development. Restoration of dystrophin expression with gene therapy became a primary focus. Investment in gene therapy has led to the approval of exon skipping by regulatory agencies, multiple clinical trials of systemic microdystrophin therapy using adeno-associated virus vectors, and revolutionary genome editing therapy using the CRISPR technology. However, many important issues surfaced during the clinical translation of DMD gene therapy (such as low efficiency of exon skipping, immune toxicity-induced serious adverse events, and patient death). In this issue of Human Gene Therapy, several research articles highlighted some of the latest developments in DMD gene therapy. Importantly, a collection of articles from experts in the field reviewed the progress, major challenges, and future directions of DMD gene therapy. These insightful discussions have significant implications for gene therapy of other neuromuscular diseases.

Readthrough compounds for nonsense mutations: bridging the translational gap

Approximately 10% of all pathological mutations are nonsense mutations that are responsible for several severe genetic diseases for which no treatment regimens are currently available. The most widespread strategy for treating nonsense mutations is by enhancing ribosomal readthrough of premature termination codons (PTCs) to restore the production of the full-length protein. In the past decade several compounds with readthrough potential have been identified. However, although preclinical results on these compounds are promising, clinical studies have not yielded positive outcomes. We review preclinical and clinical research related to readthrough compounds and characterize factors that contribute to the observed translational gap.

Brain magnetic resonance imaging in the DE50-MD dog model of Duchenne muscular dystrophy reveals regional reductions in cerebral gray matter

BACKGROUND

Duchenne muscular dystrophy is a X-linked disease characterized by severe and progressive muscle weakness, alongside cognitive impairment and a range of neurobehavioral disorders secondary to brain dystrophin deficiency. Duchenne muscular dystrophy patients have reduced cerebral gray matter and altered white matter ultrastructure (detected by magnetic resonance imaging) compared to age-matched controls.

METHODS

We studied the DE50-MD canine model of Duchenne muscular dystrophy, which is deficient in full length brain dystrophin (Dp427) isoforms and has a neurocognitive phenotype. Eight DE50-MD and 6 age-matched littermate wild type male dogs underwent serial brain magnetic resonance imaging from 14 to 33 months of age.

RESULTS

Reduced regional gray matter was detected in DE50-MD dogs compared with wildtype, including the piriform lobe, hippocampus and cingulate gyrus. Lateral ventricle volume was larger in DE50-MD dogs. Differences did not progress over time. White matter volume did not differ between DE50-MD and wildtype dogs. There was no difference in brain nor cranial vault volume between DE50-MD and wildtype dogs.

CONCLUSION

Dystrophin deficiency in the canine brain results in structural changes that likely contribute to the neurocognitive phenotype.

Magnetic resonance quantification of skeletal muscle lipid infiltration in a humanized mouse model of Duchenne muscular dystrophy

Rodent models of Duchenne muscular dystrophy (DMD) often do not recapitulate the severity of muscle wasting and resultant fibro-fatty infiltration observed in DMD patients. Having recently documented severe muscle wasting and fatty deposition in two preclinical models of muscular dystrophy (Dysferlin-null and mdx mice) through apolipoprotein E (ApoE) gene deletion without and with cholesterol-, triglyceride-rich Western diet supplementation, we sought to determine whether magnetic resonance imaging and spectroscopy (MRI and MRS, respectively) could be used to detect, characterize, and compare lipid deposition in mdx-ApoE knockout with mdx mice in a diet-dependent manner. MRI revealed that both mdx and mdx-ApoE mice exhibited elevated proton relaxation time constants (T2 ) in their lower hindlimbs irrespective of diet, indicating both chronic muscle damage and fatty tissue deposition. The mdx-ApoE mice on a Western diet (mdx-ApoEW ) presented with greatest fatty tissue infiltration in the posterior compartment of the hindlimb compared with other groups, as detected by MRI/MRS. High-resolution magic angle spinning confirmed elevated lipid deposition in the posterior compartments of mdx-ApoEW mice in vivo and ex vivo, respectively. In conclusion, the mdx-ApoEW model recapitulates some of the extreme fatty tissue deposition observed clinically in DMD muscle but typically absent in mdx mice. This preclinical model will help facilitate the development of new imaging modalities directly relevant to the image contrast generated in DMD, and help to refine MR-based biomarkers and their relationship to tissue structure and disease progression.

PCYT2-regulated lipid biosynthesis is critical to muscle health and ageing

Muscle degeneration is the most prevalent cause for frailty and dependency in inherited diseases and ageing. Elucidation of pathophysiological mechanisms, as well as effective treatments for muscle diseases, represents an important goal in improving human health. Here, we show that the lipid synthesis enzyme phosphatidylethanolamine cytidyltransferase (PCYT2/ECT) is critical to muscle health. Human deficiency in PCYT2 causes a severe disease with failure to thrive and progressive weakness. pcyt2-mutant zebrafish and muscle-specific Pcyt2-knockout mice recapitulate the participant phenotypes, with failure to thrive, progressive muscle weakness and accelerated ageing. Mechanistically, muscle Pcyt2 deficiency affects cellular bioenergetics and membrane lipid bilayer structure and stability. PCYT2 activity declines in ageing muscles of mice and humans, and adeno-associated virus-based delivery of PCYT2 ameliorates muscle weakness in Pcyt2-knockout and old mice, offering a therapy for individuals with a rare disease and muscle ageing. Thus, PCYT2 plays a fundamental and conserved role in vertebrate muscle health, linking PCYT2 and PCYT2-synthesized lipids to severe muscle dystrophy and ageing.

Histone Deacetylases: Molecular Mechanisms and Therapeutic Implications for Muscular Dystrophies

Histone deacetylases (HDACs) are enzymes that regulate the deacetylation of numerous histone and non-histone proteins, thereby affecting a wide range of cellular processes. Deregulation of HDAC expression or activity is often associated with several pathologies, suggesting potential for targeting these enzymes for therapeutic purposes. For example, HDAC expression and activity are higher in dystrophic skeletal muscles. General pharmacological blockade of HDACs, by means of pan-HDAC inhibitors (HDACi), ameliorates both muscle histological abnormalities and function in preclinical studies. A phase II clinical trial of the pan-HDACi givinostat revealed partial histological improvement and functional recovery of Duchenne Muscular Dystrophy (DMD) muscles; results of an ongoing phase III clinical trial that is assessing the long-term safety and efficacy of givinostat in DMD patients are pending. Here we review the current knowledge about the HDAC functions in distinct cell types in skeletal muscle, identified by genetic and -omic approaches. We describe the signaling events that are affected by HDACs and contribute to muscular dystrophy pathogenesis by altering muscle regeneration and/or repair processes. Reviewing recent insights into HDAC cellular functions in dystrophic muscles provides new perspectives for the development of more effective therapeutic approaches based on drugs that target these critical enzymes.

Urine titin as a novel biomarker for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most severe form of muscular dystrophy that is caused by lack of dystrophin, a critical structural protein in skeletal muscle. DMD treatments, and quantitative biomarkers to assess the efficacy of potential treatments, are urgently needed. Previous evidence has shown that titin, a muscle cell protein, is increased in the urine of patients with DMD, suggesting its usefulness as a DMD biomarker. Here, we demonstrated that the elevated titin in urine is directly associated with the lack of dystrophin and urine titin responses to drug treatment. We performed a drug intervention study using mdx mice, a DMD mouse model. We showed that mdx mice, which lack dystrophin due to a mutation in exon 23 of the Dmd gene, have elevated urine titin. Treatment with an exon skipper that targets exon 23 rescued muscle dystrophin level and dramatically decreased urine titin in mdx mice and correlates with dystrophin expression. We also demonstrated that titin levels were significantly increased in the urine of patients with DMD. This suggests that elevated urine titin level might be a hallmark of DMD and a useful pharmacodynamic marker for therapies designed to restore dystrophin levels.

Mini-dCas13X-mediated RNA editing restores dystrophin expression in a humanized mouse model of Duchenne muscular dystrophy

Approximately 10% of monogenic diseases are caused by nonsense point mutations that generate premature termination codons (PTCs), resulting in a truncated protein and nonsense-mediated decay of the mutant mRNAs. Here, we demonstrate a mini-dCas13X-mediated RNA adenine base editing (mxABE) strategy to treat nonsense mutation-related monogenic diseases via A-to-G editing in a genetically humanized mouse model of Duchenne muscular dystrophy (DMD). Initially, we identified a nonsense point mutation (c.4174C>T, p.Gln1392*) in the DMD gene of a patient and validated its pathogenicity in humanized mice. In this model, mxABE packaged in a single adeno-associated virus (AAV) reached A-to-G editing rates up to 84% in vivo, at least 20-fold greater than rates reported in previous studies using other RNA editing modalities. Furthermore, mxABE restored robust expression of dystrophin protein to over 50% of WT levels by enabling PTC read-through in multiple muscle tissues. Importantly, systemic delivery of mxABE by AAV also rescued dystrophin expression to averages of 37%, 6%, and 54% of WT levels in the diaphragm, tibialis anterior, and heart muscle, respectively, as well as rescued muscle function. Our data strongly suggest that mxABE-based strategies may be a viable new treatment modality for DMD and other monogenic diseases.

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.

Osteoimmunology: The effect of autoimmunity on fracture healing and skeletal analysis

Understanding factors that affect bone response to trauma is integral to forensic skeletal analysis. It is essential in forensic anthropology to identify if impaired fracture healing impacts assessment of post-traumatic time intervals and whether a correction factor is required. This paper presents a synthetic review of the intersection of the literature on the immune system, bone biology, and osteoimmunological research to present a novel model of interactions that may affect fracture healing under autoimmune conditions. Results suggest that autoimmunity likely impacts fracture healing, the pathogenesis however, is under researched, but likely multifactorial. With autoimmune diseases being relatively common, significant clinical history should be incorporated when assessing skeletal remains. Future research includes the true natural healing rate of bone; effect of autoimmunity on this rate; variation of healing with different autoimmune diseases; and if necessary, development of a correction factor on the natural healing rate to account for impairment in autoimmunity.

Identifying FDA-Approved Drugs that Upregulate Utrophin A as a Therapeutic Strategy for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a neuromuscular disease caused by mutations and deletions within the DMD gene, which result in a lack of dystrophin protein at the sarcolemma of skeletal muscle fibers. The absence of dystrophin fragilizes the sarcolemma and compromises its integrity during cycles of muscle contraction, which, progressively, leads to reductions in muscle mass and function. DMD is thus a progressive muscle-wasting disease that results in a loss of ambulation, cardiomyopathy , respiratory impairment, and death. Although there is presently no cure for DMD, recent advances have led to many promising treatments. One such approach entails increasing expression of a homologous protein to dystrophin, named utrophin A, which is endogenously expressed in both healthy and DMD muscle fibers. Upregulation of utrophin A all along the sarcolemma of DMD muscle fibers can, in part, compensate for the absence of dystrophin. Over the years, our laboratory has focused a significant portion of our efforts in identifying and characterizing drugs and small molecules for their ability to target utrophin A and cause its overexpression. As part of these efforts, we have recently developed a novel ELISA-based high-throughput drug screen, to identify FDA-approved drugs that increase the expression of utrophin A in muscle cells in culture as well as in dystrophic mice. Here, we describe our overall strategy to identify and characterize several FDA-approved drugs that upregulate utrophin A expression and provide details on all experimental approaches. Such strategy has the potential to lead to the rapid development of novel therapeutics for DMD.

MRI Evaluation of Gene Therapy in the Canine Model of Duchenne Muscular Dystrophy

Magnetic resonance imaging (MRI) is a well-established and widely used technique to characterize and quantify skeletal and cardiac muscle changes in Duchenne muscular dystrophy (DMD). Recently, MRI has been explored to study disease progression and response to gene therapy in the canine DMD model. Using traditional sequences, delayed gadolinium enhancement, novel sequences, and spectroscopy, investigators have begun to (i) establish the baseline MRI characteristics of the muscles in normal and affected dogs and (ii) evaluate gene therapy outcomes in treated dogs. As a noninvasive assay, MRI offers an excellent opportunity to study longitudinal muscle changes in long-term gene therapy studies in the canine model. In this chapter, we outline the MRI method used to study DMD in the canine model.

Histological Assessment of Gene Therapy in the Canine DMD Model

Assessing histological changes is essential for characterizing muscle disease progression and for studying the response to therapies in Duchenne muscular dystrophy (DMD), an X-linked progressive muscle-wasting disease caused by the loss of the dystrophin protein. Canine models are by far the best-characterized large animal models for DMD. In this chapter, we describe methods for muscle tissue collection and storage, hematoxylin and eosin staining for studying general muscle morphology, and special staining protocols for evaluating fibrosis, calcification, and neuronal nitric oxide synthase (nNOS) activity. We also provide immunofluorescence staining protocols that are often used to characterize the expression and localization of dystrophin and components of the dystrophin-associated glycoprotein complex. Lastly, we presented immunohistochemical staining protocols that we use to assess muscle inflammation and immune responses.

In Vivo Evaluation of Exon 51 Skipping in hDMD/Dmd-null Mice

Duchenne muscular dystrophy (DMD) is a fatal X-linked condition that affects 1 in 3500-6000 newborn boys a year. An out-of-frame mutation in the DMD gene typically causes the condition. Exon skipping therapy is an emerging approach that uses antisense oligonucleotides (ASOs), short synthetic DNA-like molecules that can splice out mutated or frame-disrupting mRNA fragments, to restore the reading frame. The restored reading frame will be in-frame and will produce a truncated, yet functional protein. ASOs called phosphorodiamidate morpholino oligomers (PMO), including eteplirsen, golodirsen, and viltolarsen, have recently been approved by the US Food and Drug Administration as the first ASO-based drugs for DMD. ASO-facilitated exon skipping has been extensively studied in animal models. An issue that arises with these models is that the DMD sequence differs from the human DMD sequence. A solution to this issue is to use double mutant hDMD/Dmd-null mice, which only carry the human DMD sequence and are null for the mouse Dmd sequence. Here, we describe intramuscular and intravenous injections of an ASO to skip exon 51 in hDMD/Dmd-null mice, and the evaluation of its efficacy in vivo.

Gene therapy review: Duchenne muscular dystrophy case study

Gene therapy, i.e., any therapeutic approach involving the use of genetic material as a drug and more largely altering the transcription or translation of one or more genes, covers a wide range of innovative methods for treating diseases, including neurological disorders. Although they share common principles, the numerous gene therapy approaches differ greatly in their mechanisms of action. They also differ in their maturity for some are already used in clinical practice while others have never been used in humans. The aim of this review is to present the whole range of gene therapy techniques through the example of Duchenne muscular dystrophy (DMD). DMD is a severe myopathy caused by mutations in the dystrophin gene leading to the lack of functional dystrophin protein. It is a disease known to all neurologists and in which almost all gene therapy methods were applied. Here we discuss the mechanisms of gene transfer techniques with or without viral vectors, DNA editing with or without matrix repair and those acting at the RNA level (RNA editing, exon skipping and STOP-codon readthrough). For each method, we present the results obtained in DMD with a particular focus on clinical data. This review aims also to outline the advantages, limitations and risks of gene therapy related to the approach used.

Inhibiting the inflammasome with MCC950 counteracts muscle pyroptosis and improves Duchenne muscular dystrophy

Background

Duchenne muscular dystrophy (DMD) is the most common inherited human myopathy. Typically, the secondary process involving severe inflammation and necrosis exacerbate disease progression. Previously, we reported that the NLRP3 inflammasome complex plays a crucial role in this disorder. Moreover, pyroptosis, a form of programmed necrotic cell death, is triggered by NLRP3 via gasdermin D (GSDMD). So far, pyroptosis has never been described either in healthy muscle or in dystrophic muscle. The aim of this study was to unravel the role of NLRP3 inflammasome in DMD and explore a potentially promising treatment with MCC950 that selectively inhibits NLRP3.

Methods

Four-week-old mdx mice (n=6 per group) were orally treated for 2 months with MCC950 (mdx-T), a highly potent, specific, small-molecule inhibitor of NLRP3, and compared with untreated (mdx) and wild-type (WT) mice. In vivo functional tests were carried out to measure the global force and endurance of mice. Ex vivo biochemical and molecular analyses were performed to evaluate the pathophysiology of the skeletal muscle. Finally, in vitro tests were conducted on primary cultures of DMD human myotubes.

Results

After MCC950 treatment, mdx mice exhibited a significant reduction of inflammation, macrophage infiltration and oxidative stress (-20 to -65%, P<0.05 vs untreated mdx). Mdx-T mice displayed considerably less myonecrosis (-54%, P<0.05 vs mdx) and fibrosis (-75%, P<0.01 vs mdx). Moreover, a more mature myofibre phenotype, characterized by larger-sized fibres and higher expression of mature myosin heavy chains 1 and 7 was observed. Mdx-T also exhibited enhanced force and resistance to fatigue (+20 to 60%, P<0.05 or less). These beneficial effects resulted from MCC950 inhibition of both active caspase-1 (-46%, P=0.075) and cleaved gasdermin D (N-GSDMD) (-42% in medium-sized-fibres, P<0.001). Finally, the anti-inflammatory action and the anti-pyroptotic effect of MCC950 were also recapitulated in DMD human myotubes.

Conclusion

Specific inhibition of the NLRP3 inflammasome can significantly attenuate the dystrophic phenotype. A novel finding of this study is the overactivation of GSDMD, which is hampered by MCC950. This ultimately leads to less inflammation and pyroptosis and to a better muscle maturation and function. Targeting NLRP3 might lead to an effective therapeutic approach for a better management of DMD.

CRISPR-mediated correction of skeletal muscle Ca2+ handling in a novel DMD patient-derived pluripotent stem cell model

Mutations in the dystrophin gene cause the most common and currently incurable Duchenne muscular dystrophy (DMD) characterized by progressive muscle wasting. Although abnormal Ca²⁺ handling is a pathological feature of DMD, mechanisms underlying defective Ca²⁺ homeostasis remain unclear. Here we generate a novel DMD patient-derived pluripotent stem cell (PSC) model of skeletal muscle with an isogenic control using clustered regularly interspaced short palindromic repeat (CRISPR)-mediated precise gene correction. Transcriptome analysis identifies dysregulated gene sets in the absence of dystrophin, including genes involved in Ca²⁺ handling, excitation-contraction coupling and muscle contraction. Specifically, analysis of intracellular Ca²⁺ transients and mathematical modeling of Ca²⁺ dynamics reveal significantly reduced cytosolic Ca²⁺ clearance rates in DMD-PSC derived myotubes. Pharmacological assays demonstrate Ca²⁺ flux in myotubes is determined by both intracellular and extracellular sources. DMD-PSC derived myotubes display significantly reduced velocity of contractility. Compared with a non-isogenic wildtype PSC line, these pathophysiological defects could be rescued by CRISPR-mediated precise gene correction. Our study provides new insights into abnormal Ca²⁺ homeostasis in DMD and suggests that Ca²⁺ signaling pathways amenable to pharmacological modulation are potential therapeutic targets. Importantly, we have established a human physiology-relevant in vitro model enabling rapid pre-clinical testing of potential therapies for DMD.

The importance of dystrophin and the dystrophin associated proteins in vascular smooth muscle

This review details the role of dystrophin and the dystrophin associated proteins (DAPs) in the vascular smooth muscle. Dystrophin is most comprehensively studied in the skeletal muscle due to serious symptoms found related to the skeletal muscle of patients with muscular dystrophy. Mutations in the dystrophin gene, or DAPs genes, result in a wide range of muscular dystrophies. In skeletal muscle, dystrophin is known to act to as a cytoskeletal stabilization protein and protects cells against contraction-induced damage. In skeletal muscle, dystrophin stabilizes the plasma membrane by transmitting forces generated by sarcomeric contraction to the extracellular matrix (ECM). Dystrophin is a scaffold that binds the dystroglycan complex (DGC) and has many associated proteins (DAPs). These DAPs include sarcoglycans, syntrophins, dystroglycans, dystrobrevin, neuronal nitric oxide synthase, and caveolins. The DAPs provide biomechanical support to the skeletal or cardiac plasma membrane during contraction, and loss of one or several of these DAPs leads to plasma membrane fragility. Dystrophin is expressed near the plasma membrane of all muscles, including cardiac and vascular smooth muscle, and some neurons. Dystrophic mice have noted biomechanical irregularities in the carotid arteries and spontaneous motor activity in portal vein altered when compared to wild type mice. Additionally, some studies suggest the vasculature of patients and animal models with muscular dystrophy is abnormal. Although the function of dystrophin and the DAPs in vascular smooth muscle is not thoroughly established in the field, this review makes the point that these proteins are expressed, and important and further study is warranted.

A deep redox proteome profiling workflow and its application to skeletal muscle of a Duchenne Muscular Dystrophy model

Perturbation to the redox state accompanies many diseases and its effects are viewed through oxidation of biomolecules, including proteins, lipids, and nucleic acids. The thiol groups of protein cysteine residues undergo an array of redox post-translational modifications (PTMs) that are important for regulation of protein and pathway function. To better understand what proteins are redox regulated following a perturbation, it is important to be able to comprehensively profile protein thiol oxidation at the proteome level. Herein, we report a deep redox proteome profiling workflow and demonstrate its application in measuring the changes in thiol oxidation along with global protein expression in skeletal muscle from mdx mice, a model of Duchenne Muscular Dystrophy (DMD). In-depth coverage of the thiol proteome was achieved with >18,000 Cys sites from 5,608 proteins in muscle being quantified. Compared to the control group, mdx mice exhibit markedly increased thiol oxidation, where a ∼2% shift in the median oxidation occupancy was observed. Pathway analysis for the redox data revealed that coagulation system and immune-related pathways were among the most susceptible to increased thiol oxidation in mdx mice, whereas protein abundance changes were more enriched in pathways associated with bioenergetics. This study illustrates the importance of deep redox profiling in gaining greater insight into oxidative stress regulation and pathways/processes that are perturbed in an oxidizing environment.

Sulfur amino acid supplementation displays therapeutic potential in a C. elegans model of Duchenne muscular dystrophy

Mutations in the dystrophin gene cause Duchenne muscular dystrophy (DMD), a common muscle disease that manifests with muscle weakness, wasting, and degeneration. An emerging theme in DMD pathophysiology is an intramuscular deficit in the gasotransmitter hydrogen sulfide (H₂S). Here we show that the C. elegans DMD model displays reduced levels of H₂S and expression of genes required for sulfur metabolism. These reductions can be offset by increasing bioavailability of sulfur containing amino acids (L-methionine, L-homocysteine, L-cysteine, L-glutathione, and L-taurine), augmenting healthspan primarily via improved calcium regulation, mitochondrial structure and delayed muscle cell death. Additionally, we show distinct differences in preservation mechanisms between sulfur amino acid vs H₂S administration, despite similarities in required health-preserving pathways. Our results suggest that the H₂S deficit in DMD is likely caused by altered sulfur metabolism and that modulation of this pathway may improve DMD muscle health via multiple evolutionarily conserved mechanisms.

A C. elegans model of Duchenne muscular dystrophy reveals a potential role for disrupted sulfur metabolism in the disease and thus the therapeutic potential of sulfur amino acid supplementation.

Nanomedicine for Treating Muscle Dystrophies: Opportunities, Challenges, and Future Perspectives

Muscular dystrophies are a group of genetic muscular diseases characterized by impaired muscle regeneration, which leads to pathological inflammation that drives muscle wasting and eventually results in weakness, functional dependency, and premature death. The most known causes of death include respiratory muscle failure due to diaphragm muscle decay. There is no definitive treatment for muscular dystrophies, and conventional therapies aim to ameliorate muscle wasting by promoting physiological muscle regeneration and growth. However, their effects on muscle function remain limited, illustrating the requirement for major advancements in novel approaches to treatments, such as nanomedicine. Nanomedicine is a rapidly evolving field that seeks to optimize drug delivery to target tissues by merging pharmaceutical and biomedical sciences. However, the therapeutic potential of nanomedicine in muscular dystrophies is poorly understood. This review highlights recent work in the application of nanomedicine in treating muscular dystrophies. First, we discuss the history and applications of nanomedicine from a broader perspective. Second, we address the use of nanoparticles for drug delivery, gene regulation, and editing to target Duchenne muscular dystrophy and myotonic dystrophy. Next, we highlight the potential hindrances and limitations of using nanomedicine in the context of cell culture and animal models. Finally, the future perspectives for using nanomedicine in clinics are summarized with relevance to muscular dystrophies.

Loss of full-length dystrophin expression results in major cell-autonomous abnormalities in proliferating myoblasts

Duchenne muscular dystrophy (DMD) affects myofibers and muscle stem cells, causing progressive muscle degeneration and repair defects. It was unknown whether dystrophic myoblasts-the effector cells of muscle growth and regeneration-are affected. Using transcriptomic, genome-scale metabolic modelling and functional analyses, we demonstrate, for the first time, convergent abnormalities in primary mouse and human dystrophic myoblasts. In Dmdmdx myoblasts lacking full-length dystrophin, the expression of 170 genes was significantly altered. Myod1 and key genes controlled by MyoD (Myog, Mymk, Mymx, epigenetic regulators, ECM interactors, calcium signalling and fibrosis genes) were significantly downregulated. Gene ontology analysis indicated enrichment in genes involved in muscle development and function. Functionally, we found increased myoblast proliferation, reduced chemotaxis and accelerated differentiation, which are all essential for myoregeneration. The defects were caused by the loss of expression of full-length dystrophin, as similar and not exacerbated alterations were observed in dystrophin-null Dmdmdx-βgeo myoblasts. Corresponding abnormalities were identified in human DMD primary myoblasts and a dystrophic mouse muscle cell line, confirming the cross-species and cell-autonomous nature of these defects. The genome-scale metabolic analysis in human DMD myoblasts showed alterations in the rate of glycolysis/gluconeogenesis, leukotriene metabolism, and mitochondrial beta-oxidation of various fatty acids. These results reveal the disease continuum: DMD defects in satellite cells, the myoblast dysfunction affecting muscle regeneration, which is insufficient to counteract muscle loss due to myofiber instability. Contrary to the established belief, our data demonstrate that DMD abnormalities occur in myoblasts, making these cells a novel therapeutic target for the treatment of this lethal disease.

The skeletal muscle phenotype of the DE50-MD dog model of Duchenne muscular dystrophy

Background: Animal models of Duchenne muscular dystrophy (DMD) are essential to study disease progression and assess efficacy of therapeutic intervention, however dystrophic mice fail to display a clinically relevant phenotype, limiting translational utility. Dystrophin-deficient dogs exhibit disease similar to humans, making them increasingly important for late-stage preclinical evaluation of candidate therapeutics. The DE50-MD canine model of DMD carries a mutation within a human 'hotspot' region of the dystrophin gene, amenable to exon-skipping and gene editing strategies. As part of a large natural history study of disease progression, we have characterised the DE50-MD skeletal muscle phenotype to identify parameters that could serve as efficacy biomarkers in future preclinical trials. Methods: Vastus lateralis muscles were biopsied from a large cohort of DE50-MD dogs and healthy male littermates at 3-monthly intervals (3-18 months) for longitudinal analysis, with multiple muscles collected post-mortem to evaluate body-wide changes. Pathology was characterised quantitatively using histology and measurement of gene expression to determine statistical power and sample sizes appropriate for future work. Results: DE50-MD skeletal muscle exhibits widespread degeneration/regeneration, fibrosis, atrophy and inflammation. Degenerative/inflammatory changes peak during the first year of life, while fibrotic remodelling appears more gradual. Pathology is similar in most skeletal muscles, but in the diaphragm, fibrosis is more prominent, associated with fibre splitting and pathological hypertrophy. Picrosirius red and acid phosphatase staining represent useful quantitative histological biomarkers for fibrosis and inflammation respectively, while qPCR can be used to measure regeneration ( MYH3, MYH8), fibrosis ( COL1A1), inflammation ( SPP1), and stability of DE50-MD dp427 transcripts. Conclusion: The DE50-MD dog is a valuable model of DMD, with pathological features similar to young, ambulant human patients. Sample size and power calculations show that our panel of muscle biomarkers are of strong pre-clinical value, able to detect therapeutic improvements of even 25%, using trials with only six animals per group.

CRISPR-Based Therapeutic Gene Editing for Duchenne Muscular Dystrophy: Advances, Challenges and Perspectives

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease arising from loss-of-function mutations in the dystrophin gene and characterized by progressive muscle degeneration, respiratory insufficiency, cardiac failure, and premature death by the age of thirty. Albeit DMD is one of the most common types of fatal genetic diseases, there is no curative treatment for this devastating disorder. In recent years, gene editing via the clustered regularly interspaced short palindromic repeats (CRISPR) system has paved a new path toward correcting pathological mutations at the genetic source, thus enabling the permanent restoration of dystrophin expression and function throughout the musculature. To date, the therapeutic benefits of CRISPR genome-editing systems have been successfully demonstrated in human cells, rodents, canines, and piglets with diverse DMD mutations. Nevertheless, there remain some nonignorable challenges to be solved before the clinical application of CRISPR-based gene therapy. Herein, we provide an overview of therapeutic CRISPR genome-editing systems, summarize recent advancements in their applications in DMD contexts, and discuss several potential obstacles lying ahead of clinical translation.

p-TAK1 acts as a switch between myoblast proliferation phase and differentiation phase in mdx mice via regulating HO-1 expression

Skeletal muscle transforming growth factor-β-activated kinase 1 (TAK1) continuous excessive phosphorylation was observed in Duchenne muscular dystrophy (DMD) patients and mdx mice. Inhibiting TAK1 phosphorylation ameliorated fibrosis and muscular atrophy, while TAK1 knockout also impaired muscle regeneration. The definite effect and mechanism of p-TAK1 in muscle regeneration disorder is still obscure. In this study, BaCl₂-induced acute muscle injury model was used to investigate the role of p-TAK1 in myoblast proliferation and differentiation phase. The results showed that TAK1 phosphorylation was significantly up-regulated in proliferation phase along with Keap1/Nrf2/HO-1 signaling pathway activation, which was down-regulated in differentiation phase yet. In C2C12 cells, inhibiting TAK1 phosphorylation markedly suppressed the expression of heme oxygenase-1 (HO-1), and both myoblast proliferation and differentiation were inhibited. As for activation, p-TAK1 promoted myoblast proliferation via up-regulating HO-1 level. However, excessive TAK1 phosphorylation (induced by 20 ng·mL^-1 TGF-β1) notably up-regulated HO-1 expression, inhibiting myogenic differentiation antigen (MyOD) and myogenic differentiation. A mild p-TAK1 level (induced by 5 or 10 ng·mL^-1 TGF-β1) was beneficial for myoblast differentiation. In mdx mice, robust myoblast proliferation and differentiation arrest were observed with high p-TAK1 level in skeletal muscle. HO-1 expression was significantly up-regulated. TAK1 phosphorylation inhibitor NG25 (N-[4-[(4-ethylpiperazin-1-yl)methyl]-3-(trifluoromethyl)phenyl]-4-methyl-3-(1H-pyrrolo[2,3-b]pyridin-4-yloxy)benzamide) significantly inhibited HO-1 expression, relieved excessive myoblast proliferation and differentiation arrest, promoted new myofiber formation, and eventually improved muscle function. In conclusion, p-TAK1 acted as "a switch" between proliferation and differentiation phase. Mitigating p-TAK1 level transformed myoblast excessive proliferation phase into differentiation phase in mdx mouse via regulating HO-1 expression.

In vivo restoration of dystrophin expression in mdx mice using intra-muscular and intra-arterial injections of hydrogel microsphere carriers of exon skipping antisense oligonucleotides

Duchenne muscular dystrophy (DMD) is a genetic disease caused by a mutation in the X-linked Dytrophin gene preventing the expression of the functional protein. Exon skipping therapy using antisense oligonucleotides (AONs) is a promising therapeutic strategy for DMD. While benefits of AON therapy have been demonstrated, some challenges remain before this strategy can be applied more comprehensively to DMD patients. These include instability of AONs due to low nuclease resistance and poor tissue uptake. Delivery systems have been examined to improve the availability and stability of oligonucleotide drugs, including polymeric carriers. Previously, we showed the potential of a hydrogel-based polymeric carrier in the form of injectable PEG-fibrinogen (PF) microspheres for delivery of chemically modified 2'-O-methyl phosphorothioate (2OMePs) AONs. The PF microspheres proved to be cytocompatible and provided sustained release of the AONs for several weeks, causing increased cellular uptake in mdx dystrophic mouse cells. Here, we further investigated this delivery strategy by examining in vivo efficacy of this approach. The 2OMePS/PEI polyplexes loaded in PF microspheres were delivered by intramuscular (IM) or intra-femoral (IF) injections. We examined the carrier biodegradation profiles, AON uptake efficiency, dystrophin restoration, and muscle histopathology. Both administration routes enhanced dystrophin restoration and improved the histopathology of the mdx mice muscles. The IF administration of the microspheres improved the efficacy of the 2OMePS AONs over the IM administration. This was demonstrated by a higher exon skipping percentage and a smaller percentage of centered nucleus fibers (CNF) found in H&E-stained muscles. The restoration of dystrophin expression found for both IM and IF treatments revealed a reduced dystrophic phenotype of the treated muscles. The study concludes that injectable PF microspheres can be used as a carrier system to improve the overall therapeutic outcomes of exon skipping-based therapy for treating DMD.

Modelling Neurological Diseases in Large Animals: Criteria for Model Selection and Clinical Assessment

Issue: The impact of neurological disorders is recognised globally, with one in six people affected in their lifetime and few treatments to slow or halt disease progression. This is due in part to the increasing ageing population, and is confounded by the high failure rate of translation from rodent-derived therapeutics to clinically effective human neurological interventions. Improved translation is demonstrated using higher order mammals with more complex/comparable neuroanatomy. These animals effectually span this translational disparity and increase confidence in factors including routes of administration/dosing and ability to scale, such that potential therapeutics will have successful outcomes when moving to patients. Coupled with advancements in genetic engineering to produce genetically tailored models, livestock are increasingly being used to bridge this translational gap. Approach: In order to aid in standardising characterisation of such models, we provide comprehensive neurological assessment protocols designed to inform on neuroanatomical dysfunction and/or lesion(s) for large animal species. We also describe the applicability of these exams in different large animals to help provide a better understanding of the practicalities of cross species neurological disease modelling. Recommendation: We would encourage the use of these assessments as a reference framework to help standardise neurological clinical scoring of large animal models.

Long-term culture of patient-derived cardiac organoids recapitulated Duchenne muscular dystrophy cardiomyopathy and disease progression

Duchenne Muscular Dystrophy (DMD) is an X-linked neuromuscular disease which to date is incurable. The major cause of death is dilated cardiomyopathy however, its pathogenesis is unclear as existing cellular and animal models do not fully recapitulate the human disease phenotypes. In this study, we generated cardiac organoids from patient-derived induced pluripotent stem cells (DMD-COs) and isogenic-corrected controls (DMD-Iso-COs) and studied if DMD-related cardiomyopathy and disease progression occur in the organoids upon long-term culture (up to 93 days). Histological analysis showed that DMD-COs lack initial proliferative capacity, displayed a progressive loss of sarcoglycan localization and high stress in endoplasmic reticulum. Additionally, cardiomyocyte deterioration, fibrosis and aberrant adipogenesis were observed in DMD-COs over time. RNA sequencing analysis confirmed a distinct transcriptomic profile in DMD-COs which was associated with functional enrichment in hypertrophy/dilated cardiomyopathy, arrhythmia, adipogenesis and fibrosis pathways. Moreover, five miRNAs were identified to be crucial in this dysregulated gene network. In conclusion, we generated patient-derived cardiac organoid model that displayed DMD-related cardiomyopathy and disease progression phenotypes in long-term culture. We envision the feasibility to develop a more complex, realistic and reliable in vitro 3D human cardiac-mimics to study DMD-related cardiomyopathies.

Characterisation of Progressive Skeletal Muscle Fibrosis in the Mdx Mouse Model of Duchenne Muscular Dystrophy: An In Vivo and In Vitro Study

Duchenne muscular dystrophy (DMD) is a rare genetic disease leading to progressive muscle wasting, respiratory failure, and cardiomyopathy. Although muscle fibrosis represents a DMD hallmark, the organisation of the extracellular matrix and the molecular changes in its turnover are still not fully understood. To define the architectural changes over time in muscle fibrosis, we used an mdx mouse model of DMD and analysed collagen and glycosaminoglycans/proteoglycans content in skeletal muscle sections at different time points during disease progression and in comparison with age-matched controls. Collagen significantly increased particularly in the diaphragm, quadriceps, and gastrocnemius in adult mdx, with fibrosis significantly correlating with muscle degeneration. We also analysed collagen turnover pathways underlying fibrosis development in cultured primary quadriceps-derived fibroblasts. Collagen secretion and matrix metalloproteinases (MMPs) remained unaffected in both young and adult mdx compared to wt fibroblasts, whereas collagen cross-linking and tissue inhibitors of MMP (TIMP) expression significantly increased. We conclude that, in the DMD model we used, fibrosis mostly affects diaphragm and quadriceps with a higher collagen cross-linking and inhibition of MMPs that contribute differently to progressive collagen accumulation during fibrotic remodelling. This study offers a comprehensive histological and molecular characterisation of DMD-associated muscle fibrosis; it may thus provide new targets for tailored therapeutic interventions.

Simultaneous assessment of lung morphology and respiratory motion in retrospectively gated in-vivo microCT of free breathing anesthetized mice

Retrospective gating (RG) is a well established technique in preclinical computed tomography (CT) to assess 3D morphology of the lung. In RG additional angular projections are recorded typically by performing multiple rotations. Consequently, the projections are sorted according to the expansion state of the chest and those sets are then reconstructed separately. Thus, the breathing motion artefacts are suppressed at a cost of strongly elevated X-ray dose levels. Here we propose to use the entire raw data to assess respiratory motion in addition to retrospectively gated 3D reconstruction that visualize anatomical structures of the lung. Using this RG based X-ray respiratory motion measurement approach, which will be referred to as RG based X-ray lung function measurement (rgXLF) on the example of the mdx mouse model of Duchenne muscle dystrophy (mdx) we accurately obtained both the 3D anatomical morphology of the lung and the thoracic bones as well as functional temporal parameters of the lung. Thus, rgXLF will remove the necessity for separate acquisition procedures by being able to reproduce comparable results to the previously established planar X-ray based lung function measurement approach in a single low dose CT scan.

Structural and ultrastructural changes in the skeletal muscles of dysferlin-deficient mice during postnatal ontogenesis

A number of sarcolemma proteins are responsible for muscle fiber repair. Dysferlin encoded by the DYSF gene is one of these proteins. Dysferlin promotes membrane repair in striated muscle fibers (MFs). Mutations in DYSF lead to loss of or decreased dysferlin expression, impaired membrane repair in MF, and its destruction, clinically manifesting as dysferlinopathy. Preclinical studies of cell and gene therapies aimed at restoring impaired muscle regeneration require well-characterized small animal models. Our investigation aimed to distinguish the histopathological features of a mouse strain lacking dysferlin expression (Bla/J strain). Ultrastructural changes in the sarcolemma, mitochondria and contractile apparatus were observed. It was shown that postnatal histogenesis of skeletal muscles in genetically determined dysferlin deficiency is characterized by a higher proportion of necrotic muscle fibers, compensatory hypertrophy of muscle fibers with their subsequent atrophy, and decreases in proliferative activity and the level of myogenic differentiation of myogenic progenitor cells compared to wild-type mice (C57Bl/6).

Mitochondrial Stress Responses in Duchenne muscular dystrophy: Metabolic Dysfunction or Adaptive Reprogramming?

Mitochondrial stress may be a secondary contributor to muscle weakness in inherited muscular dystrophies. Duchenne muscular dystrophy has received the majority of attention whereby most discoveries suggest mitochondrial ATP synthesis may be reduced. However, not all studies support this finding. Furthermore, some studies have reported increased mitochondrial reactive oxygen species and propensity for permeability transition pore formation as an inducer of apoptosis, although divergent findings have also been described. A closer examination of the literature suggests the degree and direction of mitochondrial stress responses may depend on the progression of the disease, the muscle type examined, the mouse model employed with regards to pre-clinical research, the precise metabolic pathways in consideration, and in some cases the in vitro technique used to assess a given mitochondrial bioenergetic function. One intent of this review is to provide careful considerations for future experimental designs to resolve the heterogeneous nature of mitochondrial stress during the progression of Duchenne muscular dystrophy. Such considerations have implications for other muscular dystrophies as well which are addressed briefly herein. A renewed perspective of the term 'mitochondrial dysfunction' is presented whereby stress responses might be re-explored in future investigations as direct contributors to myopathy vs an adaptive 'reprogramming' intended to maintain homeostasis in the face of disease stressors themselves. In so doing, the prospective development of mitochondrial enhancement therapies can be driven by advances in perspectives as much as experimental approaches when resolving the precise relationships between mitochondrial remodelling and muscle weakness in Duchenne and, indeed, other muscular dystrophies.

The predictive value of models of neuromuscular disorders to potentiate clinical translation

Neuromuscular disorders (NMDs) are a heterogenous group of rare inherited diseases that compromise the function of peripheral nerves and/or muscles. With limited treatment options available, there is a growing need to design effective preclinical studies that can lead to greater success in clinical trials for novel therapeutics. Here, I discuss recent advances in modelling NMDs to improve preclinical studies as well as two articles from this issue that work in parallel to enable a deeper understanding of a particularly rare NMD, known as X-linked myotubular myopathy.

Metabolic reprogramming of skeletal muscle by resident macrophages points to CSF1R inhibitors as muscular dystrophy therapeutics

The role of tissue-resident macrophages during tissue regeneration or fibrosis is not well understood, mainly due to the lack of a specific marker for their identification. Here, we identified three populations of skeletal muscle-resident myelomonocytic cells: a population of macrophages positive for lymphatic vessel endothelial receptor 1 (LYVE1) and T cell membrane protein 4 (TIM4 or TIMD4), a population of LYVE1^-TIM4^- macrophages, and a population of cells likely representing dendritic cells that were positive for CD11C and major histocompatibility complex class II (MHCII). Using a combination of parabiosis and lineage-tracing experiments, we found that, at steady state, TIM4^- macrophages were replenished from the blood, whereas TIM4⁺ macrophages locally self-renewed [self-renewing resident macrophages (SRRMs)]. We further showed that Timd4 could be reliably used to distinguish SRRMs from damage-induced infiltrating macrophages. Using a colony-stimulating factor 1 receptor (CSF1R) inhibition/withdrawal approach to specifically deplete SRRMs, we found that SRRMs provided a nonredundant function in clearing damage-induced apoptotic cells early after extensive acute injury. In contrast, in chronic mild injury as seen in a mouse model of Duchenne muscular dystrophy, depletion of both TIM4^-- and TIM4⁺-resident macrophage populations through long-term CSF1R inhibition changed muscle fiber composition from damage-sensitive glycolytic fibers toward damage-resistant glycolytic-oxidative fibers, thereby protecting muscle against contraction-induced injury both ex vivo and in vivo. This work reveals a previously unidentified role for resident macrophages in modulating tissue metabolism and may have therapeutic potential given the ongoing clinical testing of CSF1R inhibitors.