Targeting angiogenesis in Duchenne muscular dystrophy

Correction of human nonsense mutation via adenine base editing for Duchenne muscular dystrophy treatment in mouse

Duchenne muscular dystrophy (DMD) is the most prevalent herediatry disease in men, characterized by dystrophin deficiency, progressive muscle wasting, cardiac insufficiency, and premature mortality, with no effective therapeutic options. Here, we investigated whether adenine base editing can correct pathological nonsense point mutations leading to premature stop codons in the dystrophin gene. We identified 27 causative nonsense mutations in our DMD patient cohort. Treatment with adenine base editor (ABE) could restore dystrophin expression by direct A-to-G editing of pathological nonsense mutations in cardiomyocytes generated from DMD patient-derived induced pluripotent stem cells. We also generated two humanized mouse models of DMD expressing mutation-bearing exons 23 or 30 of human dystrophin gene. Intramuscular administration of ABE, driven by ubiquitous or muscle-specific promoters could correct these nonsense mutations in vivo, albeit with higher efficiency in exon 30, restoring dystrophin expression in skeletal fibers of humanized DMD mice. Moreover, a single systemic delivery of ABE with human single guide RNA (sgRNA) could induce body-wide dystrophin expression and improve muscle function in rotarod tests of humanized DMD mice. These findings demonstrate that ABE with human sgRNAs can confer therapeutic alleviation of DMD in mice, providing a basis for development of adenine base editing therapies in monogenic diseases.

Sodium hydrosulfide moderately alleviates the hallmark symptoms of Duchenne muscular dystrophy in mdx mice

Duchenne muscular dystrophy (DMD) is an incurable disease caused by mutations in the X-linked DMD gene that encodes a structural muscle protein, dystrophin. This, in turn, leads to progressive degeneration of the skeletal muscles and the heart. Hydrogen sulfide (H2S), the pleiotropic agent with antioxidant, anti-inflammatory, and pro-angiogenic activities, could be considered a promising therapeutic factor for DMD. In this work, we studied the effect of daily intraperitoneal administration of the H2S donor, sodium hydrosulfide (NaHS, 100 μmol/kg/day for 5 weeks) on skeletal muscle (gastrocnemius, diaphragm and tibialis anterior) pathology in dystrophin-deficient mdx mice, characterized by decreased expression of H2S-generating enzymes. NaHS reduced the level of muscle damage markers in plasma (creatine kinase, lactate dehydrogenase and osteopontin). It lowered oxidative stress by affecting the GSH/GSSG ratio, up-regulating the level of cytoprotective heme oxygenase-1 (HO-1) and down-regulating the NF-κB pathway. In the gastrocnemius muscle, it also increased angiogenic vascular endothelial growth factor (Vegf) and its receptor (Kdr) expression, accompanied by the elevated number of α-SMA/CD31/lectin-positive blood vessels. The expression of fibrotic regulators, like Tgfβ, Col1a1 and Fn1 was decreased by NaHS in the tibialis anterior, while the level of autophagy markers (AMPKα signalling and Atg genes), was mostly affected in the gastrocnemius. Histological and molecular analysis showed no effect of H2S donor on regeneration and the muscle fiber type composition. Overall, the H2S donor modified the gene expression and protein level of molecules associated with the pathophysiology of DMD, contributing to the regulation of oxidative stress, inflammation, autophagy, and angiogenesis.

ANT-dependent MPTP underlies necrotic myofiber death in muscular dystrophy

Mitochondrial permeability transition pore (MPTP) formation contributes to ischemia-reperfusion injury in the heart and several degenerative diseases, including muscular dystrophy (MD). MD is a family of genetic disorders characterized by progressive muscle necrosis and premature death. It has been proposed that the MPTP has two molecular components, the adenine nucleotide translocase (ANT) family of proteins and an unknown component that requires the chaperone cyclophilin D (CypD) to activate. This model was examined in vivo by deleting the gene encoding ANT1 (Slc25a4) or CypD (Ppif) in a δ-sarcoglycan (Sgcd) gene-deleted mouse model of MD, revealing that dystrophic mice lacking Slc25a4 were partially protected from cell death and MD pathology. Dystrophic mice lacking both Slc25a4 and Ppif together were almost completely protected from necrotic cell death and MD disease. This study provides direct evidence that ANT1 and CypD are required MPTP components governing in vivo cell death, suggesting a previously unrecognized therapeutic approach in MD and other necrotic diseases.

Characterization of the Ang/Tie2 Signaling Pathway in the Diaphragm Muscle of DMD Mice

In Duchenne muscular dystrophy (DMD), angiogenesis appears to be attenuated. Local administration of angiopoietin 1 (Ang1) has been shown to reduce inflammation, ischemia, and fibrosis in DMD mice. Ang1 is a vital vascular stabilizing factor that activates the endothelial cell receptor Tie2, leading to downstream pro-survival PI3K/Akt pathway activation and eNOS phosphorylation. In this study, we aimed to characterize the Ang/Tie2 signaling pathway within the diaphragm muscle of mouse models of DMD. Utilizing ELISA, immunoblots, and RT-qPCR, we demonstrated that Ang1 was downregulated, while the antagonist angiopoietin 2 (Ang2) was upregulated, leading to a decreased Ang1/Ang2 ratio. This correlated with a reduction in the phosphorylated Tie2/total Tie2 ratio. Interestingly, no significant differences in Akt or eNOS phosphorylation were observed, although DMD murine models did have elevated total Akt protein concentrations. These observations suggest that Ang1/Tie2 signaling may be dysregulated in the diaphragm muscle of DMD and further investigations may lead to new therapeutic interventions for DMD.

Nuclear Factor Erythroid 2-Related Factor 2 and Its Targets in Skeletal Muscle Repair and Regeneration

Significance: Skeletal muscles have a robust regenerative capacity in response to acute and chronic injuries. Muscle repair and redox homeostasis are intimately linked; increased generation of reactive oxygen species leads to cellular dysfunction and contributes to muscle wasting and progression of muscle diseases. In exemplary muscle disease, Duchenne muscular dystrophy (DMD), caused by mutations in the DMD gene that encodes the muscle structural protein dystrophin, the regeneration machinery is severely compromised, while oxidative stress contributes to the progression of the disease. The nuclear factor erythroid 2-related factor 2 (NRF2) and its target genes, including heme oxygenase-1 (HO-1), provide protective mechanisms against oxidative insults. Recent Advances: Relevant advances have been evolving in recent years in understanding the mechanisms by which NRF2 regulates processes that contribute to effective muscle regeneration. To this end, pathways related to muscle satellite cell differentiation, oxidative stress, mitochondrial metabolism, inflammation, fibrosis, and angiogenesis have been studied. The regulatory role of NRF2 in skeletal muscle ferroptosis has been also suggested. Animal studies have shown that NRF2 pathway activation can stop or reverse skeletal muscle pathology, especially when endogenous stress defence mechanisms are imbalanced. Critical Issues: Despite the growing recognition of NRF2 as a factor that regulates various aspects of muscle regeneration, the mechanistic impact on muscle pathology in various models of muscle injury remains imprecise. Future Directions: Further studies are necessary to fully uncover the role of NRF2 in muscle regeneration, both in physiological and pathological conditions, and to investigate the possibilities for development of new therapeutic modalities. Antioxid. Redox Signal. 38, 619-642.

Vascular therapy for Duchenne muscular dystrophy (DMD)

Duchenne muscular dystrophy (DMD) is a progressive disease characterized by the wasting of the muscles that eventually lead to difficulty moving and, ultimately, premature death from heart and respiratory complications. DMD deficiency is caused by mutations in the gene encoding dystrophin, which prevents skeletal muscle, cardiac muscle, and other cells from producing the functional protein. Located on the cytoplasmic face of the plasma membrane of muscle fibers, dystrophin serves as a component of the dystrophin glycoprotein complex (DGC), mechanically reinforces the sarcolemma, and stabilizes the DGC, preventing it from contraction-mediated muscle degradation. In DMD muscle, dystrophin deficiency leads to progressive fibrosis, myofiber damage, chronic inflammation, and dysfunction of the mitochondria and muscle stem cells. Currently, DMD is incurable, and treatment involves the administration of glucocorticoids in order to delay disease progression. In the presence of developmental delay, proximal weakness, and elevated serum creatine kinase levels, a definitive diagnosis can usually be made after an extensive review of the patient's history and physical examination, as well as confirmation through muscle biopsy or genetic testing. Current standards of care include the use of corticosteroids to prolong ambulation and delay the onset of secondary complications, including respiratory muscle and cardiac functions. However, different studies have been carried out to show the relationship between vascular density and impaired angiogenesis in the pathogenesis of DMD. Several recent studies on DMD management are vascular targeted and focused on ischemia as a culprit for the pathogenesis of DMD. This review critically discusses approaches-such as modulation of nitric oxide (NO) or vascular endothelial growth factor (VEGF)-related pathways-to attenuate the dystrophic phenotype and enhance angiogenesis.

Re-examination of therapeutic management of muscular dystrophies using a vascular smooth muscle-centered approach

In contrast to the long-standing focus on the pathophysiology of skeletal muscles in the hunt for a cure for Duchenne muscular dystrophy (DMD), we opine that the malfunctioning of dystrophin produced by vascular smooth muscle is a major contributor to the pathology of the illness. We believe that a biological response modifier glucan (BRMG), which has been shown in clinical studies of DMD to boost the expression of vascular smooth muscle dystrophin and provide anti-fibrotic and anti-inflammatory effects, may play a key role in reducing the pathogenesis of DMD. According to the evaluation of biomarkers, this BRMG, which is safe and side-effect-free, reduces the pathogenesis of DMD. We describe the possible mechanisms of action by which this BRMG helps in alleviating the symptoms of DMD by targeting smooth muscle dystrophin, in addition to its advantages over other therapeutic modalities, as well as how it can serve as a valuable adjunct to existing therapies. We suggest that using BRMG adjuncts that target smooth muscle dystrophin would be a potential therapeutic approach that prolongs the lifespan and extends the duration of ambulation from the onset of DMD. Further studies are needed to validate this hypothesis.

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.

VEGF-A and FGF4 Engineered C2C12 Myoblasts and Angiogenesis in the Chick Chorioallantoic Membrane

Angiogenesis is the formation of new blood vessels from pre-existing vessels. Adequate oxygen transport and waste removal are necessary for tissue homeostasis. Restrictions in blood supply can lead to ischaemia which can contribute to disease pathology. Vascular endothelial growth factor (VEGF) is essential in angiogenesis and myogenesis, making it an ideal candidate for angiogenic and myogenic stimulation in muscle. We established C2C12 mouse myoblast cell lines which stably express elevated levels of (i) human VEGF-A and (ii) dual human FGF4-VEGF-A. Both stably transfected cells secreted increased amounts of human VEGF-A compared to non-transfected cells, with the latter greater than the former. In vitro, conditioned media from engineered cells resulted in a significant increase in endothelial cell proliferation, migration, and tube formation. In vivo, this conditioned media produced a 1.5-fold increase in angiogenesis in the chick chorioallantoic membrane (CAM) assay. Delivery of the engineered myoblasts on Matrigel demonstrated continued biological activity by eliciting an almost 2-fold increase in angiogenic response when applied directly to the CAM assay. These studies qualify the use of genetically modified myoblasts in therapeutic angiogenesis for the treatment of muscle diseases associated with vascular defects.

Single-nucleus cross-tissue molecular reference maps toward understanding disease gene function

Understanding gene function and regulation in homeostasis and disease requires knowledge of the cellular and tissue contexts in which genes are expressed. Here, we applied four single-nucleus RNA sequencing methods to eight diverse, archived, frozen tissue types from 16 donors and 25 samples, generating a cross-tissue atlas of 209,126 nuclei profiles, which we integrated across tissues, donors, and laboratory methods with a conditional variational autoencoder. Using the resulting cross-tissue atlas, we highlight shared and tissue-specific features of tissue-resident cell populations; identify cell types that might contribute to neuromuscular, metabolic, and immune components of monogenic diseases and the biological processes involved in their pathology; and determine cell types and gene modules that might underlie disease mechanisms for complex traits analyzed by genome-wide association studies.

Multiple LEDT wavelengths modulate the Akt signaling pathways and attenuate pathological events in mdx dystrophic muscle cells

This study is aimed at investigating the effects of LEDT, at multiple wavelengths, on intracellular calcium concentration; on transient receptor potential canonical channels; on calcium-binding protein; on myogenic factors; on myosin heavy chains; on Akt signaling pathway; on inflammatory markers; and on the angiogenic-inducing factor in dystrophic muscle cell culture experimental model. Dystrophic primary muscle cells were submitted to LEDT, at multiple wavelengths (420 nm, 470 nm, 660 nm, and 850 nm), and evaluated after 48 h for cytotoxic effects and intracellular calcium content. TRPC-1, TRPC-6, Calsequestrin, MyoD, Myogenin, MHC-slow, MHC-fast, p-AKT, p-mTOR, p-FoxO1, Myostatin, NF-κB, TNF-α, and VEGF levels were evaluated in dystrophic primary muscle cells by western blotting. The LEDT, at multiple wavelengths, treated-mdx muscle cells showed no cytotoxic effect and significant lower levels in [Ca2 +]i. The mdx muscle cells treated with LEDT showed a significant reduction of TRPC-1, NF-κB, TNF-α and MyoD levels and a significant increase of Myogenin, MHC-slow, p-AKT, p-mTOR, p-FoxO1 levels, and VEGF levels. Our findings suggest that different LEDT wavelengths modulate the Akt-signaling pathways and attenuate pathological events in dystrophic muscle cells, and a combined multiwavelength irradiation protocol may even provide a potentially therapeutic strategy for muscular dystrophies.

miR-378 affects metabolic disturbances in the mdx model of Duchenne muscular dystrophy

Although Duchenne muscular dystrophy (DMD) primarily affects muscle tissues, the alterations to systemic metabolism manifested in DMD patients contribute to the severe phenotype of this fatal disorder. We propose that microRNA-378a (miR-378) alters carbohydrate and lipid metabolism in dystrophic mdx mice. In our study, we utilized double knockout animals which lacked both dystrophin and miR-378 (mdx/miR-378^-/-). RNA sequencing of the liver identified 561 and 194 differentially expressed genes that distinguished mdx versus wild-type (WT) and mdx/miR-378^-/- versus mdx counterparts, respectively. Bioinformatics analysis predicted, among others, carbohydrate metabolism disorder in dystrophic mice, as functionally proven by impaired glucose tolerance and insulin sensitivity. The lack of miR-378 in mdx animals mitigated those effects with a faster glucose clearance in a glucose tolerance test (GTT) and normalization of liver glycogen levels. The absence of miR-378 also restored the expression of genes regulating lipid homeostasis, such as Acly, Fasn, Gpam, Pnpla3, and Scd1. In conclusion, we report for the first time that miR-378 loss results in increased systemic metabolism of mdx mice. Together with our previous finding, demonstrating alleviation of the muscle-related symptoms of DMD, we propose that the inhibition of miR-378 may represent a new strategy to attenuate the multifaceted symptoms of DMD.

The Donnan-dominated resting state of skeletal muscle fibers contributes to resilience and longevity in dystrophic fibers

Duchenne muscular dystrophy (DMD) is an X-linked dystrophin-minus muscle-wasting disease. Ion homeostasis in skeletal muscle fibers underperforms as DMD progresses. But though DMD renders these excitable cells intolerant of exertion, sodium overloaded, depolarized, and spontaneously contractile, they can survive for several decades. We show computationally that underpinning this longevity is a strikingly frugal, robust Pump-Leak/Donnan (P-L/D) ion homeostatic process. Unlike neurons, which operate with a costly “Pump-Leak–dominated” ion homeostatic steady state, skeletal muscle fibers operate with a low-cost “Donnan-dominated” ion homeostatic steady state that combines a large chloride permeability with an exceptionally small sodium permeability. Simultaneously, this combination keeps fiber excitability low and minimizes pump expenditures. As mechanically active, long-lived multinucleate cells, skeletal muscle fibers have evolved to handle overexertion, sarcolemmal tears, ischemic bouts, etc.; the frugality of their Donnan dominated steady state lets them maintain the outsized pump reserves that make them resilient during these inevitable transient emergencies. Here, P-L/D model variants challenged with DMD-type insult/injury (low pump-strength, overstimulation, leaky Nav and cation channels) show how chronic “nonosmotic” sodium overload (observed in DMD patients) develops. Profoundly severe DMD ion homeostatic insult/injury causes spontaneous firing (and, consequently, unwanted excitation–contraction coupling) that elicits cytotoxic swelling. Therefore, boosting operational pump-strength and/or diminishing sodium and cation channel leaks should help extend DMD fiber longevity.

Hydrogen sulfide as a therapeutic option for the treatment of Duchenne muscular dystrophy and other muscle-related diseases

Hydrogen sulfide (H₂S) has been known for years as a poisoning gas and until recently evoked mostly negative associations. However, the discovery of its gasotransmitter functions suggested its contribution to various physiological and pathological processes. Although H₂S has been found to exert cytoprotective effects through modulation of antioxidant, anti-inflammatory, anti-apoptotic, and pro-angiogenic responses in a variety of conditions, its role in the pathophysiology of skeletal muscles has not been broadly elucidated so far. The classical example of muscle-related disorders is Duchenne muscular dystrophy (DMD), the most common and severe type of muscular dystrophy. Mutations in the DMD gene that encodes dystrophin, a cytoskeletal protein that protects muscle fibers from contraction-induced damage, lead to prominent dysfunctions in the structure and functions of the skeletal muscle. However, the main cause of death is associated with cardiorespiratory failure, and DMD remains an incurable disease. Taking into account a wide range of physiological functions of H₂S and recent literature data on its possible protective role in DMD, we focused on the description of the 'old' and 'new' functions of H₂S, especially in muscle pathophysiology. Although the number of studies showing its essential regulatory action in dystrophic muscles is still limited, we propose that H₂S-based therapy has the potential to attenuate the progression of DMD and other muscle-related disorders.

Disrupted Calcium Homeostasis in Duchenne Muscular Dystrophy: A Common Mechanism behind Diverse Consequences

Duchenne muscular dystrophy (DMD) leads to disability and death in young men. This disease is caused by mutations in the DMD gene encoding diverse isoforms of dystrophin. Loss of full-length dystrophins is both necessary and sufficient for causing degeneration and wasting of striated muscles, neuropsychological impairment, and bone deformities. Among this spectrum of defects, abnormalities of calcium homeostasis are the common dystrophic feature. Given the fundamental role of Ca2+ in all cells, this biochemical alteration might be underlying all the DMD abnormalities. However, its mechanism is not completely understood. While abnormally elevated resting cytosolic Ca2+ concentration is found in all dystrophic cells, the aberrant mechanisms leading to that outcome have cell-specific components. We probe the diverse aspects of calcium response in various affected tissues. In skeletal muscles, cardiomyocytes, and neurons, dystrophin appears to serve as a scaffold for proteins engaged in calcium homeostasis, while its interactions with actin cytoskeleton influence endoplasmic reticulum organisation and motility. However, in myoblasts, lymphocytes, endotheliocytes, and mesenchymal and myogenic cells, calcium abnormalities cannot be clearly attributed to the loss of interaction between dystrophin and the calcium toolbox proteins. Nevertheless, DMD gene mutations in these cells lead to significant defects and the calcium anomalies are a symptom of the early developmental phase of this pathology. As the impaired calcium homeostasis appears to underpin multiple DMD abnormalities, understanding this alteration may lead to the development of new therapies. In fact, it appears possible to mitigate the impact of the abnormal calcium homeostasis and the dystrophic phenotype in the total absence of dystrophin. This opens new treatment avenues for this incurable disease.

Simvastatin does not alleviate muscle pathology in a mouse model of Duchenne muscular dystrophy

Background

Duchenne muscular dystrophy (DMD) is an incurable disease, caused by the mutations in the DMD gene, encoding dystrophin, an actin-binding cytoskeletal protein. Lack of functional dystrophin results in muscle weakness, degeneration, and as an outcome cardiac and respiratory failure. As there is still no cure for affected individuals, the pharmacological compounds with the potential to treat or at least attenuate the symptoms of the disease are under constant evaluation. The pleiotropic agents, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, known as statins, have been suggested to exert beneficial effects in the mouse model of DMD. On the other hand, they were also reported to induce skeletal-muscle myopathy. Therefore, we decided to verify the hypothesis that simvastatin may be considered a potential therapeutic agent in DMD.

Methods

Several methods including functional assessment of muscle function via grip strength measurement, treadmill test, and single-muscle force estimation, enzymatic assays, histological analysis of muscle damage, gene expression evaluation, and immunofluorescence staining were conducted to study simvastatin-related alterations in the mdx mouse model of DMD.

Results

In our study, simvastatin treatment of mdx mice did not result in improved running performance, grip strength, or specific force of the single muscle. Creatine kinase and lactate dehydrogenase activity, markers of muscle injury, were also unaffected by simvastatin delivery in mdx mice. Furthermore, no significant changes in inflammation, fibrosis, and angiogenesis were noted. Despite the decreased percentage of centrally nucleated myofibers in gastrocnemius muscle after simvastatin delivery, no changes were noticed in other regeneration-related parameters. Of note, even an increased rate of necrosis was found in simvastatin-treated mdx mice.

Conclusion

In conclusion, our study revealed that simvastatin does not ameliorate DMD pathology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-021-00276-3.

Generation of microRNA-378a-deficient hiPSC as a novel tool to study its role in human cardiomyocytes

microRNA-378a (miR-378a) is one of the most highly expressed microRNAs in the heart. However, its role in the human cardiac tissue has not been fully understood. It was observed that miR-378a protects cardiomyocytes from hypertrophic growth by regulation of IGF1R and the expression of downstream kinases. Increased levels of miR-378a were reported in the serum of Duchenne muscular dystrophy (DMD) patients and female carriers of DMD gene-associated mutations with developed cardiomyopathy. In order to shed more light on the role of miR-378a in human cardiomyocytes and its potential involvement in DMD-related cardiomyopathy, we generated two human induced pluripotent stem cell (hiPSC) models; one with deletion of miR-378a and the second one with deletion of DMD exon 50 leading to the DMD phenotype. Our results indicate that lack of miR-378a does not influence the pluripotency of hiPSC and their ability to differentiate into cardiomyocytes (hiPSC-CM). miR-378a-deficient hiPSC-CM exhibited, however, significantly bigger size compared to the isogenic control cells, indicating the role of this miRNA in the hypertrophic growth of human cardiomyocytes. In accordance, the level of NFATc3, phosphoAKT, phosphoERK and ERK was higher in these cells compared to the control counterparts. A similar effect was achieved by silencing miR-378a with antagomirs. Of note, the percentage of cells with nuclear localization of NFATc3 was higher in miR-378a-deficient hiPSC-CM. Analysis of electrophysiological properties and Ca²⁺ oscillations revealed the decrease in the spike slope velocity and lower frequency of calcium spikes in miR-378a-deficient hiPSC-CM. Interestingly, the level of miR-378a increased gradually during cardiac differentiation of hiPSC. Of note, it was low until day 15 in differentiating DMD-deficient hiPSC-CM and then rose to a similar level as in the isogenic control counterparts. In summary, our findings confirmed the utility of hiPSC-based models for deciphering the role of miR-378a in the control and diseased human cardiomyocytes.

Hypoxia and Hypoxia-Inducible Factor Signaling in Muscular Dystrophies: Cause and Consequences

Muscular dystrophies (MDs) are a group of inherited degenerative muscle disorders characterized by a progressive skeletal muscle wasting. Respiratory impairments and subsequent hypoxemia are encountered in a significant subgroup of patients in almost all MD forms. In response to hypoxic stress, compensatory mechanisms are activated especially through Hypoxia-Inducible Factor 1 α (HIF-1α). In healthy muscle, hypoxia and HIF-1α activation are known to affect oxidative stress balance and metabolism. Recent evidence has also highlighted HIF-1α as a regulator of myogenesis and satellite cell function. However, the impact of HIF-1α pathway modifications in MDs remains to be investigated. Multifactorial pathological mechanisms could lead to HIF-1α activation in patient skeletal muscles. In addition to the genetic defect per se, respiratory failure or blood vessel alterations could modify hypoxia response pathways. Here, we will discuss the current knowledge about the hypoxia response pathway alterations in MDs and address whether such changes could influence MD pathophysiology.

The multifaceted view of heart problem in Duchenne muscular dystrophy

Dystrophin is a large protein serving as local scaffolding repetitively bridging cytoskeleton and the outside of striated muscle cell. As such dystrophin is a critical brick primarily in dystrophin-associated protein complex (DAGC) and in a larger submembranous unit, costamere. Accordingly, the lack of functional dystrophin laying at the root of Duchenne muscular dystrophy (DMD) drives sarcolemma instability. From this point on, the cascade inevitably leading to the death of myocyte begins. In cardiomyocytes, intracellular calcium overload and related mitochondrial-mediated cell death mainly contribute to myocardial dysfunction and dilation while other protein dysregulation and/or mislocalization may affect electrical conduction system and favor arrhythmogenesis. Although clinically DMD manifests as progressive muscle weakness and skeletal muscle symptoms define characteristic of DMD, it is the heart problem the biggest challenge that most often develop in the form of dilated cardiomyopathy (DCM). Current standards of treatment and recent progress in respiratory care, introduced in most settings in the 1990s, have improved quality of life and median life expectancy to 4th decade of patient's age. At the same time, cardiac causes of death related to DMD increases. Despite preventive and palliative cardiac treatments available, the prognoses remain poor. Direct therapeutic targeting of dystrophin deficiency is critical, however, hindered by the large size of the dystrophin cDNA and/or stochastic, often extensive genetic changes in DMD gene. The correlation between cardiac involvement and mutations affecting specific dystrophin isoforms, may provide a mutation-specific cardiac management and novel therapeutic approaches for patients with CM. Nonetheless, the successful cardiac treatment poses a big challenge and may require combined therapy to combat dystrophin deficiency and its after-effects (critical in DMD pathogenesis). This review locates the multifaceted heart problem in the course of DMD, balancing the insights into basic science, translational efforts and clinical manifestation of dystrophic heart disease.

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

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

Combined gene therapy via VEGF and mini-dystrophin synergistically improves pathologies in temporalis muscle of dystrophin/utrophin double knockout mice

Duchenne muscular dystrophy (DMD) is a severe X-linked inherited muscular disorder characterized by the loss of dystrophin. We have previously shown that monogene therapy using the mini-dystrophin gene improves muscle function in DMD. However, chronic inflammation plays an important role in progressive muscle degeneration in DMD as well. Vascular endothelial growth factor (VEGF) has been used to enhance muscle vasculature, reduce local inflammation and improve DMD muscle function. Temporalis muscles are the key skeletal muscles for mastication and loss of their function negatively affects DMD patient quality of life by reducing nutritional intake, but little is known about the pathology and treatment of the temporalis muscle in DMD. In this work, we tested the hypothesis that the combined delivery of the human mini-dystrophin and human VEGF genes to the temporalis muscles using separate recombinant adeno-associated viral (rAAV) vectors will synergistically improve muscle function and pathology in adult male dystrophin/utrophin double-knockout (mdx/utrn+/-) mice. The experimental mice were divided into four groups including: dystrophin + VEGF combined, dystrophin only, VEGF only and PBS control. After 2 months, gene expression and histological analysis of the temporalis muscles showed a synergistic improvement in temporalis muscle pathology and function coincident with increased restoration of dystrophin-associated protein complexes and nNOS in the dystrophin + VEGF combined group. We also observed significantly reduced inflammatory cell infiltration, central nucleation, and fibrosis in the dystrophin + VEGF combined group. We have demonstrated the efficacy of combined rAAV-mediated dystrophin and VEGF treatment of temporalis muscles in a DMD mouse model.

Age-Dependent Dysregulation of Muscle Vasculature and Blood Flow Recovery after Hindlimb Ischemia in the mdx Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD), caused by a lack of functional dystrophin, is characterized by progressive muscle degeneration. Interestingly, dystrophin is also expressed in endothelial cells (ECs), and insufficient angiogenesis has already been hypothesized to contribute to DMD pathology, however, its status in mdx mice, a model of DMD, is still not fully clear. Our study aimed to reveal angiogenesis-related alterations in skeletal muscles of mdx mice compared to wild-type (WT) counterparts. By investigating 6- and 12-week-old mice, we sought to verify if those changes are age-dependent. We utilized a broad spectrum of methods ranging from gene expression analysis, flow cytometry, and immunofluorescence imaging to determine the level of angiogenic markers and to assess muscle blood vessel abundance. Finally, we implemented the hindlimb ischemia (HLI) model, more biologically relevant in the context of functional studies evaluating angiogenesis/arteriogenesis processes. We demonstrated that both 6- and 12-week-old dystrophic mice exhibited dysregulation of several angiogenic factors, including decreased vascular endothelial growth factor A (VEGF) in different muscle types. Nonetheless, in younger, 6-week-old mdx animals, neither the abundance of CD31⁺α-SMA⁺ double-positive blood vessels nor basal blood flow and its restoration after HLI was affected. In 12-week-old mdx mice, although a higher number of CD31⁺α-SMA⁺ double-positive blood vessels and an increased percentage of skeletal muscle ECs were found, the abundance of pericytes was diminished, and blood flow was reduced. Moreover, impeded perfusion recovery after HLI associated with a blunted inflammatory and regenerative response was evident in 12-week-old dystrophic mice. Hence, our results reinforce the hypothesis of age-dependent angiogenic dysfunction in dystrophic mice. In conclusion, we suggest that older mdx mice constitute an appropriate model for preclinical studies evaluating the effectiveness of vascular-based therapies aimed at the restoration of functional angiogenesis to mitigate DMD severity.

Oxidative Stress, Inflammation, and Activators of Mitochondrial Biogenesis: Tempol Targets in the Diaphragm Muscle of Exercise Trained-mdx Mice

The mdx mouse phenotype aggravated by chronic exercise on a treadmill makes this murine model more reliable for the study of muscular dystrophy. Thus, to better assess the Tempol effect on dystrophic pathways, the analyses in this study were performed in the blood samples and diaphragm muscle from treadmill trained adult (7–11-weeks old) mdx animals. The mdx mice were divided into three groups: mdxSed, sedentary controls (n = 28); mdxEx, exercise-trained animals (n = 28); and mdxEx+T, exercise-trained animals with the Tempol treatment (n = 28). The results demonstrated that the Tempol treatment promoted muscle strength gain, prevented muscle damage, reduced the inflammatory process, oxidative stress, and angiogenesis regulator, and up regulated the activators of mitochondrial biogenesis. The main new findings of this study are that Tempol reduced the NF-κB and increased the PGC1-α and PPARδ levels in the exercise-trained-mdx mice, which are probably related to the ability of this antioxidant to scavenge excessive ROS. These results reinforce the use of Tempol as a potential therapeutic strategy in DMD.

Therapeutic aspects of cell signaling and communication in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating chromosome X-linked disease that manifests predominantly in progressive skeletal muscle wasting and dysfunctions in the heart and diaphragm. Approximately 1/5000 boys and 1/50,000,000 girls suffer from DMD, and to date, the disease is incurable and leads to premature death. This phenotypic severity is due to mutations in the DMD gene, which result in the absence of functional dystrophin protein. Initially, dystrophin was thought to be a force transducer; however, it is now considered an essential component of the dystrophin-associated protein complex (DAPC), viewed as a multicomponent mechanical scaffold and a signal transduction hub. Modulating signal pathway activation or gene expression through epigenetic modifications has emerged at the forefront of therapeutic approaches as either an adjunct or stand-alone strategy. In this review, we propose a broader perspective by considering DMD to be a disease that affects myofibers and muscle stem (satellite) cells, as well as a disorder in which abrogated communication between different cell types occurs. We believe that by taking this systemic view, we can achieve safe and holistic treatments that can restore correct signal transmission and gene expression in diseased DMD tissues.

Targeting Nrf2 for the treatment of Duchenne Muscular Dystrophy

Imbalances in redox homeostasis can result in oxidative stress, which is implicated in various pathological conditions including the fatal neuromuscular disease Duchenne Muscular Dystrophy (DMD). DMD is a complicated disease, with many druggable targets at the cellular and molecular level including calcium-mediated muscle degeneration; mitochondrial dysfunction; oxidative stress; inflammation; insufficient muscle regeneration and dysregulated protein and organelle maintenance. Previous investigative therapeutics tended to isolate and focus on just one of these targets and, consequently, therapeutic activity has been limited. Nuclear erythroid 2-related factor 2 (Nrf2) is a transcription factor that upregulates many cytoprotective gene products in response to oxidants and other toxic stressors. Unlike other strategies, targeted Nrf2 activation has the potential to simultaneously modulate separate pathological features of DMD to amplify therapeutic benefits. Here, we review the literature providing theoretical context for targeting Nrf2 as a disease modifying treatment against DMD.

Potential cross-talk between muscle and tendon in Duchenne muscular dystrophy

PURPOSE

To describe potential signaling (cross-talk) between dystrophic skeletal muscle and tendon in Duchenne muscular dystrophy.

MATERIALS AND METHODS

Review of Duchenne muscular dystrophy and associated literature relevant to muscle-tendon cross-talk.

RESULTS AND CONCLUSIONS

Duchenne muscular dystrophy results from the absence of the protein dystrophin and the associated dystrophin - glycoprotein complex, which are thought to provide both structural support and signaling functions for the muscle fiber. In addition, there are other potential signal pathways that could represent cross-talk between muscle and tendon, particularly at the myotendinous junction. Duchenne muscular dystrophy is characterized by multiple pathophysiologic mechanisms. Herein, we explore three of these: (1) the extracellular matrix, fibrosis, and fat deposition; (2) satellite cells; and (3) tensegrity. A key signaling protein that emerged in each was transforming growth factor - beta one (TGF-β1).].

The role of Nrf2 in acute and chronic muscle injury

The nuclear factor erythroid 2-related factor 2 (Nrf2) is considered as a master cytoprotective factor regulating the expression of genes encoding anti-oxidant, anti-inflammatory, and detoxifying proteins. The role of Nrf2 in the pathophysiology of skeletal muscles has been evaluated in different experimental models, however, due to inconsistent data, we aimed to investigate how Nrf2 transcriptional deficiency (Nrf2tKO) affects muscle functions both in an acute and chronic injury. The acute muscle damage was induced in mice of two genotypes-WT and Nrf2tKO mice by cardiotoxin (CTX) injection. To investigate the role of Nrf2 in chronic muscle pathology, mdx mice that share genetic, biochemical, and histopathological features with Duchenne muscular dystrophy (DMD) were crossed with mice lacking transcriptionally active Nrf2 and double knockouts (mdx/Nrf2tKO) were generated. To worsen the dystrophic phenotype, the analysis of disease pathology was also performed in aggravated conditions, by applying a long-term treadmill test. We have observed slightly increased muscle damage in Nrf2tKO mice after CTX injection. Nevertheless, transcriptional ablation of Nrf2 in mdx mice did not significantly aggravate the most deleterious, pathological hallmarks of DMD related to degeneration, inflammation, fibrotic scar formation, angiogenesis, and the number and proliferation of satellite cells in non-exercised conditions. On the other hand, upon chronic exercises, the degeneration and inflammatory infiltration of the gastrocnemius muscle, but not the diaphragm, turned to be increased in Nrf2tKOmdx in comparison to mdx mice. In conclusion, the lack of transcriptionally active Nrf2 influences moderately muscle pathology in acute CTX-induced muscle injury and chronic DMD mouse model, without affecting muscle functionality. Hence, in general, we demonstrated that the deficiency of Nrf2 transcriptional activity has no profound impact on muscle pathology in various models of muscle injury.

Muscle and cardiac therapeutic strategies for Duchenne muscular dystrophy: past, present, and future

BACKGROUND

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular childhood disorder that causes progressive muscle weakness and degeneration and results in functional decline, loss of ambulation and early death of young men due to cardiac or respiratory failure. Although the major cause of the disease has been known for many years-namely mutation in the DMD gene encoding dystrophin, one of the largest human genes-DMD is still incurable, and its treatment is challenging.

METHODS

A comprehensive and systematic review of literature on the gene, cell, and pharmacological experimental therapies aimed at restoring functional dystrophin or to counteract the associated processes contributing to disease progression like inflammation, fibrosis, calcium signaling or angiogenesis was carried out.

RESULTS

Although some therapies lead to satisfying effects in skeletal muscle, they are highly ineffective in the heart; therefore, targeting defective cardiac and respiratory systems is vital in DMD patients. Unfortunately, most of the pharmacological compounds treat only the symptoms of the disease. Some drugs addressing the underlying cause, like eteplirsen, golodirsen, and ataluren, have recently been conditionally approved; however, they can correct only specific mutations in the DMD gene and are therefore suitable for small sub-populations of affected individuals.

CONCLUSION

In this review, we summarize the possible therapeutic options and describe the current status of various, still imperfect, strategies used for attenuating the disease progression.

Lack of miR-378 attenuates muscular dystrophy in mdx mice

The severity of Duchenne muscular dystrophy (DMD), an incurable disease caused by the lack of dystrophin, might be modulated by different factors, including miRNAs. Among them, miR-378 is considered of high importance for muscle biology, but intriguingly, its role in DMD and its murine model (mdx mice) has not been thoroughly addressed so far. Here, we demonstrate that dystrophic mice additionally globally lacking miR-378 (double-KO [dKO] animals) exhibited better physical performance and improved absolute muscle force compared with mdx mice. Accordingly, markers of muscle damage in serum were significantly decreased in dKO mice, accompanied by diminished inflammation, fibrosis, and reduced abundance of regenerating fibers within muscles. The lack of miR-378 also normalized the aggravated fusion of dystrophin-deficient muscle satellite cells (mSCs). RNA sequencing of gastrocnemius muscle transcriptome revealed fibroblast growth factor 1 (Fgf1) as one of the most significantly downregulated genes in mice devoid of miR-378, indicating FGF1 as one of the mediators of changes driven by the lack of miR-378. In conclusion, we suggest that targeting miR-378 has the potential to ameliorate DMD pathology.

HIF-hypoxia signaling in skeletal muscle physiology and fibrosis

Hypoxia refers to the decrease in oxygen tension in the tissues, and the central effector of the hypoxic response is the transcription factor Hypoxia-Inducible Factor α (HIF1-α). Transient hypoxia in acute events, such as exercising or regeneration after damage, play an important role in skeletal muscle physiology and homeostasis. However, sustained activation of hypoxic signaling is a feature of skeletal muscle injury and disease, which can be a consequence of chronic damage but can also increase the severity of the pathology and worsen its outcome. Here, we review evidence that supports the idea that hypoxia and HIF-1α can contribute to the establishment of fibrosis in skeletal muscle through its crosstalk with other profibrotic factors, such as Transforming growth factor β (TGF-β), the induction of profibrotic cytokines expression, as is the case of Connective Tissue Growth Factor (CTGF/CCN2), or being the target of the Renin-angiotensin system (RAS).

Metabolic Alterations in Cardiomyocytes of Patients with Duchenne and Becker Muscular Dystrophies

Duchenne and Becker muscular dystrophies (DMD/BMD) result in progressive weakness of skeletal and cardiac muscles due to the deficiency of functional dystrophin. Respiratory failure is a leading cause of mortality in DMD patients; however, improved management of the respiratory symptoms have increased patients' life expectancy, thereby also increasing the clinical relevance of heart disease. In fact, the prevalence of cardiomyopathy, which significantly contributes to mortality in DMD patients, increases with age and disease progression, so that over 95% of adult patients has cardiomyopathy signs. We here review the current literature featuring the metabolic alterations observed in the dystrophic heart of the mdx mouse, i.e., the best-studied animal model of the disease, and discuss their pathophysiological role in the DMD heart. It is well assessed that dystrophin deficiency is associated with pathological alterations of lipid metabolism, intracellular calcium levels, neuronal nitric oxide (NO) synthase localization, and NO and reactive oxygen species production. These metabolic stressors contribute to impair the function of the cardiac mitochondrial bulk, which has a relevant pathophysiological role in the development of cardiomyopathy. In fact, mitochondrial dysfunction becomes more severe as the dystrophic process progresses, thereby indicating it may be both the cause and the consequence of the dystrophic process in the DMD heart.

Photobiomodulation Therapy for Attenuating the Dystrophic Phenotype of Mdx Mice

This study analyzed photobiomodulation therapy (PBMT) effects on regenerative, antioxidative, anti-inflammatory and angiogenic markers in the dystrophic skeletal muscle of mdx mice, the experimental model of Duchenne muscular dystrophy (DMD), during the acute phase of dystrophy disease. The following groups were set up: Ctrl (control group of normal wild-type mice; C57BL/10); mdx (untreated mdx mice); mdxPred (mdx mice treated with prednisolone) and mdxLA (mdx mice treated with PBMT). The PBMT was carried out using an Aluminum Gallium Arsenide (AIGaAs; IBRAMED® laserpulse) diode, 830 nm wavelength, applied on the dystrophic quadriceps muscle. The mdxLA group showed a degenerative and regenerative area reduction simultaneously with a MyoD level increase, ROS production and inflammatory marker reduction and up-regulation in the VEGF factor. In addition, PBMT presented similar effects to prednisolone treatment in most of the parameters analyzed. In conclusion, our results indicate that PBMT in the parameters selected attenuated the dystrophic phenotype of mdx mice, improving skeletal muscle regeneration; reducing the oxidative stress and inflammatory process; and up-regulating the angiogenic marker.

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.