Dystrophinopathy-associated dysfunction of Krebs cycle metabolism

Cardio-metabolic and cytoskeletal proteomic signatures differentiate stress hypersensitivity in dystrophin-deficient mdx mice

Extreme heterogeneity exists in the hypersensitive stress response exhibited by the dystrophin-deficient mdx mouse model of Duchenne muscular dystrophy. Because stress hypersensitivity can impact dystrophic phenotypes, this research aimed to understand the peripheral pathways driving this inter-individual variability. Male and female mdx mice were phenotypically stratified into "stress-resistant" or "stress-sensitive" groups based on their response to two laboratory stressors. Quantitative proteomics of striated muscle revealed that stress-resistant females were most dissimilar from all other groups, with over 250 proteins differentially regulated with stress hypersensitivity. Males showed less proteomic variation with stress hypersensitivity; however, these changes were associated with pathway enrichment. In the heart, stress-sensitive males had significant enrichment of pathways related to mitochondrial ATP synthesis, suggesting that increased cardio-metabolic capacity is associated with stress hypersensitivity in male mdx mice. In both sexes, stress hypersensitivity was associated with greater expression of beta-actin-like protein 2, indicative of altered cytoskeletal organisation. Despite identifying proteomic signatures associated with stress hypersensitivity, these did not correlate with differences in the serum metabolome acutely after a stressor. These data suggest that the heterogeneity in stress hypersensitivity in mdx mice is partially driven by cytoskeletal organisation, but that sex-specific cardio-metabolic reprogramming may also underpin this phenotype. SIGNIFICANCE: Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disease which is associated with a premature loss of ambulation and neurocognitive dysfunction. The hypersensitive stress response in DMD is a heterogeneous phenotype which is poorly understood. This study provided the first investigation of the peripheral mechanisms regulating the hypersensitive stress response by undertaking multi-omics analysis of phenotypically stratified mdx mice. Variations in behaviour and the striated muscle proteomic profiles suggest that cardio-metabolic remodelling and cytoskeletal organisation may contribute to this phenotype. This research offers significant insights into understanding how peripheral dystrophin deficiency relates to the cognitive abnormalities seen in patients with DMD.

The Gut Microbiota Involvement in the Panorama of Muscular Dystrophy Pathogenesis

Muscular dystrophies (MDs) are genetically heterogeneous diseases characterized by primary skeletal muscle atrophy. The collapse of muscle structure and irreversible degeneration of tissues promote the occurrence of comorbidities, including cardiomyopathy and respiratory failure. Mitochondrial dysfunction leads to inflammation, fibrosis, and adipogenic cellular infiltrates that exacerbate the symptomatology of MD patients. Gastrointestinal disorders and metabolic anomalies are common in MD patients and may be determined by the interaction between the intestine and its microbiota. Therefore, the gut-muscle axis is one of the actors involved in the spread of inflammatory signals to all muscles. In this review, we aim to examine in depth how intestinal dysbiosis can modulate the metabolic state, the immune response, and mitochondrial biogenesis in the course and progression of the most investigated MDs such as Duchenne Muscular Dystrophy (DMD) and Myotonic Dystrophy (MD1), to better identify gut microbiota metabolites working as therapeutic adjuvants to improve symptoms of MD.

Loss of endogenous estrogen alters mitochondrial metabolism and muscle clock-related protein Rbm20 in female mdx mice

Female carriers of a Duchenne muscular dystrophy (DMD) gene mutation manifest exercise intolerance and metabolic anomalies that may be exacerbated following menopause due to the loss of estrogen, a known regulator of skeletal muscle function and metabolism. Here, we studied the impact of estrogen depletion (via ovariectomy) on exercise tolerance and muscle mitochondrial metabolism in female mdx mice and the potential of estrogen replacement therapy (using estradiol) to protect against functional and metabolic perturbations. We also investigated the effect of estrogen depletion, and replacement, on the skeletal muscle proteome through an untargeted proteomic approach with TMT-labelling. Our study confirms that loss of estrogen in female mdx mice reduces exercise capacity, tricarboxylic acid cycle intermediates, and citrate synthase activity but that these deficits are offset through estrogen replacement therapy. Furthermore, ovariectomy downregulated protein expression of RNA-binding motif factor 20 (Rbm20), a critical regulator of sarcomeric and muscle homeostasis gene splicing, which impacted pathways involving ribosomal and mitochondrial translation. Estrogen replacement modulated Rbm20 protein expression and promoted metabolic processes and the upregulation of proteins involved in mitochondrial dynamics and metabolism. Our data suggest that estrogen mitigates dystrophinopathic features in female mdx mice and that estrogen replacement may be a potential therapy for post-menopausal DMD carriers.

Modeling Duchenne Muscular Dystrophy Cardiomyopathy with Patients' Induced Pluripotent Stem-Cell-Derived Cardiomyocytes

Duchenne muscular dystrophy (DMD) is an X-linked progressive muscle degenerative disease caused by mutations in the dystrophin gene, resulting in death by the end of the third decade of life at the latest. A key aspect of the DMD clinical phenotype is dilated cardiomyopathy, affecting virtually all patients by the end of the second decade of life. Furthermore, despite respiratory complications still being the leading cause of death, with advancements in medical care in recent years, cardiac involvement has become an increasing cause of mortality. Over the years, extensive research has been conducted using different DMD animal models, including the mdx mouse. While these models present certain important similarities to human DMD patients, they also have some differences which pose a challenge to researchers. The development of somatic cell reprograming technology has enabled generation of human induced pluripotent stem cells (hiPSCs) which can be differentiated into different cell types. This technology provides a potentially endless pool of human cells for research. Furthermore, hiPSCs can be generated from patients, thus providing patient-specific cells and enabling research tailored to different mutations. DMD cardiac involvement has been shown in animal models to include changes in gene expression of different proteins, abnormal cellular Ca²⁺ handling, and other aberrations. To gain a better understanding of the disease mechanisms, it is imperative to validate these findings in human cells. Furthermore, with the recent advancements in gene-editing technology, hiPSCs provide a valuable platform for research and development of new therapies including the possibility of regenerative medicine. In this article, we review the DMD cardiac-related research performed so far using human hiPSCs-derived cardiomyocytes (hiPSC-CMs) carrying DMD mutations.

"suMus," a novel digital system for arm movement metrics and muscle energy expenditure

Objective: In the field of non-treatable muscular dystrophies, promising new gene and cell therapies are being developed and are entering clinical trials. Objective assessment of therapeutic effects on motor function is mandatory for economical and ethical reasons. Main shortcomings of existing measurements are discontinuous data collection in artificial settings as well as a major focus on walking, neglecting the importance of hand and arm movements for patients' independence. We aimed to create a digital tool to measure muscle function with an emphasis on upper limb motility. Methods: suMus provides a custom-made App running on smartwatches. Movement data are sent to the backend of a suMus web-based platform, from which they can be extracted as CSV data. Fifty patients with neuromuscular diseases assessed the pool of suMus activities in a first orientation phase. suMus performance was hence validated in four upper extremity exercises based on the feedback of the orientation phase. We monitored the arm metrics in a cohort of healthy volunteers using the suMus application, while completing each exercise at low frequency in a metabolic chamber. Collected movement data encompassed average acceleration, rotation rate as well as activity counts. Spearman rank tests correlated movement data with energy expenditure from the metabolic chamber. Results: Our novel application "suMus," sum of muscle activity, collects muscle movement data plus Patient-Related-Outcome-Measures, sends real-time feedback to patients and caregivers and provides, while ensuring data protection, a long-term follow-up of disease course. The application was well received from the patients during the orientation phase. In our pilot study, energy expenditure did not differ between overnight fasted and non-fasted participants. Acceleration ranged from 1.7 ± 0.7 to 3.2 ± 0.5 m/sec² with rotation rates between 0.9 ± 0.5 and 2.0 ± 3.4 rad/sec. Acceleration and rotation rate as well as derived activity counts correlated with energy expenditure values measured in the metabolic chamber for one exercise (r = 0.58, p < 0.03). Conclusion: In the analysis of slow frequency movements of upper extremities, the integration of the suMus application with smartwatch sensors characterized motion parameters, thus supporting a use in clinical trial outcome measures. Alternative methodologies need to complement indirect calorimetry in validating accelerometer-derived energy expenditure data.

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 Effect of Uridine on the State of Skeletal Muscles and the Functioning of Mitochondria in Duchenne Dystrophy

Duchenne muscular dystrophy is caused by the loss of functional dystrophin that secondarily causes systemic metabolic impairment in skeletal muscles and cardiomyocytes. The nutraceutical approach is considered as a possible complementary therapy for this pathology. In this work, we have studied the effect of pyrimidine nucleoside uridine (30 mg/kg/day for 28 days, i.p.), which plays an important role in cellular metabolism, on the development of DMD in the skeletal muscles of dystrophin deficient mdx mice, as well as its effect on the mitochondrial dysfunction that accompanies this pathology. We found that chronic uridine administration reduced fibrosis in the skeletal muscles of mdx mice, but it had no effect on the intensity of degeneration/regeneration cycles and inflammation, pseudohypetrophy, and muscle strength of the animals. Analysis of TEM micrographs showed that uridine also had no effect on the impaired mitochondrial ultrastructure of mdx mouse skeletal muscle. The administration of uridine was found to lead to an increase in the expression of the Drp1 and Parkin genes, which may indicate an increase in the intensity of organelle fission and the normalization of mitophagy. Uridine had little effect on OXPHOS dysfunction in mdx mouse mitochondria, and moreover, it was suppressed in the mitochondria of wild type animals. At the same time, uridine restored the transport of potassium ions and reduced the production of reactive oxygen species; however, this had no effect on the impaired calcium retention capacity of mdx mouse mitochondria. The obtained results demonstrate that the used dose of uridine only partially prevents mitochondrial dysfunction in skeletal muscles during Duchenne dystrophy, though it mitigates the development of destructive processes in skeletal muscles.

Bioenergetic and Metabolic Impairments in Induced Pluripotent Stem Cell-Derived Cardiomyocytes Generated from Duchenne Muscular Dystrophy Patients

Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene and dilated cardiomyopathy (DCM) is a major cause of morbidity and mortality in DMD patients. We tested the hypothesis that DCM is caused by metabolic impairments by employing induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) generated from four DMD patients; an adult male, an adult female, a 7-year-old (7y) male and a 13-year-old (13y) male, all compared to two healthy volunteers. To test the hypothesis, we measured the bioenergetics, metabolomics, electrophysiology, mitochondrial morphology and mitochondrial activity of CMs, using respirometry, LC–MS, patch clamp, electron microscopy (EM) and confocal microscopy methods. We found that: (1) adult DMD CMs exhibited impaired energy metabolism and abnormal mitochondrial structure and function. (2) The 7y CMs demonstrated arrhythmia-free spontaneous firing along with “healthy-like” metabolic status, normal mitochondrial morphology and activity. In contrast, the 13y CMs were mildly arrhythmogenic and showed adult DMD-like bioenergetics deficiencies. (3) In DMD adult CMs, mitochondrial activities were attenuated by 45–48%, whereas the 7y CM activity was similar to that of healthy CMs. (4) In DMD CMs, but not in 7y CMs, there was a 75% decrease in the mitochondrial ATP production rate compared to healthy iPSC-CMs. In summary, DMD iPSC-CMs exhibit bioenergetic and metabolic impairments that are associated with rhythm disturbances corresponding to the patient’s phenotype, thereby constituting novel targets for alleviating cardiomyopathy in DMD patients.

Large1 gene transfer in older myd mice with severe muscular dystrophy restores muscle function and greatly improves survival

Muscular dystrophy is a progressive and ultimately lethal neuromuscular disease. Although gene editing and gene transfer hold great promise as therapies when administered before the onset of severe clinical symptoms, it is unclear whether these strategies can restore muscle function and improve survival in the late stages of muscular dystrophy. Largemyd/Largemyd (myd) mice lack expression of like-acetylglucosaminyltransferase-1 (Large1) and exhibit severe muscle pathophysiology, impaired mobility, and a markedly reduced life span. Here, we show that systemic delivery of AAV2/9 CMV Large1 (AAVLarge1) in >34-week-old myd mice with advanced disease restores matriglycan expression on dystroglycan, attenuates skeletal muscle pathophysiology, improves motor and respiratory function, and normalizes systemic metabolism, which collectively and markedly extends survival. Our results in a mouse model of muscular dystrophy demonstrate that skeletal muscle function can be restored, illustrating its remarkable plasticity, and that survival can be greatly improved even after the onset of severe muscle pathophysiology.

Biomarkers in Duchenne Muscular Dystrophy

PURPOSE OF REVIEW

This review highlights the key studies investigating various types of biomarkers in Duchenne muscular dystrophy (DMD).

RECENT FINDINGS

Several proteomic and metabolomic studies have been undertaken in both human DMD patients and animal models of DMD that have identified potential biomarkers in DMD. Although there have been a number of proteomic and metabolomic studies that have identified various potential biomarkers in DMD, more definitive studies still need to be undertaken in DMD patients to firmly correlate these biomarkers with diagnosis, disease progression, and monitoring the effects of novel treatment strategies being developed.

Sensitivity to behavioral stress impacts disease pathogenesis in dystrophin-deficient mice

Mutation to the gene encoding dystrophin can cause Duchenne muscular dystrophy (DMD) and increase the sensitivity to stress in vertebrate species, including the mdx mouse model of DMD. Behavioral stressors can exacerbate some dystrophinopathy phenotypes of mdx skeletal muscle and cause hypotension-induced death. However, we have discovered that a subpopulation of mdx mice present with a wildtype-like response to mild (forced downhill treadmill exercise) and moderate (scruff restraint) behavioral stressors. These "stress-resistant" mdx mice are more physically active, capable of super-activating the hypothalamic-pituitary-adrenal and renin-angiotensin-aldosterone pathways following behavioral stress and they express greater levels of mineralocorticoid and glucocorticoid receptors in striated muscle relative to "stress-sensitive" mdx mice. Stress-resistant mdx mice also presented with a less severe striated muscle histopathology and greater exercise and skeletal muscle oxidative capacity at rest. Most interestingly, female mdx mice were more physically active following behavioral stressors compared to male mdx mice; a response abolished after ovariectomy and rescued with estradiol. We demonstrate that the response to behavioral stress greatly impacts disease severity in mdx mice suggesting the management of stress in patients with DMD be considered as a therapeutic approach to ameliorate disease progression.

Complexity of skeletal muscle degeneration: multi-systems pathophysiology and organ crosstalk in dystrophinopathy

Duchenne muscular dystrophy is a highly progressive muscle wasting disorder due to primary abnormalities in one of the largest genes in the human genome, the DMD gene, which encodes various tissue-specific isoforms of the protein dystrophin. Although dystrophinopathies are classified as primary neuromuscular disorders, the body-wide abnormalities that are associated with this disorder and the occurrence of organ crosstalk suggest that a multi-systems pathophysiological view should be taken for a better overall understanding of the complex aetiology of X-linked muscular dystrophy. This article reviews the molecular and cellular effects of deficiency in dystrophin isoforms in relation to voluntary striated muscles, the cardio-respiratory system, the kidney, the liver, the gastrointestinal tract, the nervous system and the immune system. Based on the establishment of comprehensive biomarker signatures of X-linked muscular dystrophy using large-scale screening of both patient specimens and genetic animal models, this article also discusses the potential usefulness of novel disease markers for more inclusive approaches to differential diagnosis, prognosis and therapy monitoring that also take into account multi-systems aspects of dystrophinopathy. Current therapeutic approaches to combat muscular dystrophy are summarised.

Longitudinal metabolomic analysis of plasma enables modeling disease progression in Duchenne muscular dystrophy mouse models

Duchenne muscular dystrophy is a severe pediatric neuromuscular disorder caused by the lack of dystrophin. Identification of biomarkers is needed to support and accelerate drug development. Alterations of metabolites levels in muscle and plasma have been reported in pre-clinical and clinical cross-sectional comparisons. We present here a 7-month longitudinal study comparing plasma metabolomic data in wild-type and mdx mice. A mass spectrometry approach was used to study metabolites in up to five time points per mouse at 6, 12, 18, 24 and 30 weeks of age, providing an unprecedented in depth view of disease trajectories. A total of 106 metabolites were studied. We report a signature of 31 metabolites able to discriminate between healthy and disease at various stages of the disease, covering the acute phase of muscle degeneration and regeneration up to the deteriorating phase. We show how metabolites related to energy production and chachexia (e.g. glutamine) are affected in mdx mice plasma over time. We further show how the signature is connected to molecular targets of nutraceuticals and pharmaceutical compounds currently in development as well as to the nitric oxide synthase pathway (e.g. arginine and citrulline). Finally, we evaluate the signature in a second longitudinal study in three independent mouse models carrying 0, 1 or 2 functional copies of the dystrophin paralog utrophin. In conclusion, we report an in-depth metabolomic signature covering previously identified associations and new associations, which enables drug developers to peripherally assess the effect of drugs on the metabolic status of dystrophic mice.

Striated muscle activator of Rho signalling (STARS) overexpression in the mdx mouse enhances muscle functional capacity and regulates the actin cytoskeleton and oxidative phosphorylation pathways

NEW FINDINGS

What is the central question of this study? Striated muscle activator of rho signalling (STARS) is an actin-binding protein that regulates transcriptional pathways controlling muscle function, growth and myogenesis, processes that are impaired in dystrophic muscle: what is the regulation of the STARS pathway in Duchenne muscular dystrophy (DMD)? What is the main finding and its importance? Members of the STARS signalling pathway are reduced in the quadriceps of patients with DMD and in mouse models of muscular dystrophy. Overexpression of STARS in the dystrophic deficient mdx mouse model increased maximal isometric specific force and upregulated members of the actin cytoskeleton and oxidative phosphorylation pathways. Regulating STARS may be a therapeutic approach to enhance muscle health.

ABSTRACT

Duchenne muscular dystrophy (DMD) is characterised by impaired cytoskeleton organisation, cytosolic calcium handling, oxidative stress and mitochondrial dysfunction. This results in progressive muscle damage, wasting and weakness and premature death. The striated muscle activator of rho signalling (STARS) is an actin-binding protein that activates the myocardin-related transcription factor-A (MRTFA)/serum response factor (SRF) transcriptional pathway, a pathway regulating cytoskeletal structure and muscle function, growth and repair. We investigated the regulation of the STARS pathway in the quadriceps muscle from patients with DMD and in the tibialis anterior (TA) muscle from the dystrophin-deficient mdx and dko (utrophin and dystrophin null) mice. Protein levels of STARS, SRF and RHOA were reduced in patients with DMD. STARS, SRF and MRTFA mRNA levels were also decreased in DMD muscle, while Stars mRNA levels were decreased in the mdx mice and Srf and Mrtfa mRNAs decreased in the dko mice. Overexpressing human STARS (hSTARS) in the TA muscles of mdx mice increased maximal isometric specific force by 13% (P < 0.05). This was not associated with changes in muscle mass, fibre cross-sectional area, fibre type, centralised nuclei or collagen deposition. Proteomics screening followed by pathway enrichment analysis identified that hSTARS overexpression resulted in 31 upregulated and 22 downregulated proteins belonging to the actin cytoskeleton and oxidative phosphorylation pathways. These pathways are impaired in dystrophic muscle and regulate processes that are vital for muscle function. Increasing the STARS protein in dystrophic muscle improves muscle force production, potentially via synergistic regulation of cytoskeletal structure and energy production.

Tetrahydrobiopterin synthesis and metabolism is impaired in dystrophin-deficient mdx mice and humans

AIM

Loss of dystrophin causes oxidative stress and affects nitric oxide synthase-mediated vascular function in striated muscle. Because tetrahydrobiopterin is an antioxidant and co-factor for nitric oxide synthase, we tested the hypothesis that tetrahydrobiopterin would be low in mdx mice and humans deficient for dystrophin.

METHODS

Tetrahydrobiopterin and its metabolites were measured at rest and in response to exercise in Duchenne and Becker muscular dystrophy patients, age-matched male controls as well as wild-type, mdx and mdx mice transgenically overexpressing skeletal muscle-specific dystrophins. Mdx mice were also supplemented with tetrahydrobiopterin and pathophysiology was assessed.

RESULTS

Duchenne muscular dystrophy patients had lower urinary dihydrobiopterin + tetrahydrobiopterin/specific gravity1.020 compared to unaffected age-matched males and Becker muscular dystrophy patients. Mdx mice had low urinary and skeletal muscle dihydrobiopterin + tetrahydrobiopterin compared to wild-type mice. Overexpression of dystrophins that localize neuronal nitric oxide synthase restored dihydrobiopterin + tetrahydrobiopterin in mdx mice to wild-type levels while utrophin overexpression did not. Mdx mice and Duchenne muscular dystrophy patients did not increase tetrahydrobiopterin during exercise and in mdx mice tetrahydrobiopterin deficiency was likely because of lower levels of sepiapterin reductase in skeletal muscle. Tetrahydrobiopterin supplementation improved skeletal muscle strength, resistance to fatiguing and injurious contractions in vivo, increased utrophin and capillary density of skeletal muscle and lowered cardiac muscle fibrosis and left ventricular wall thickness in mdx mice.

CONCLUSION

These data demonstrate that impaired tetrahydrobiopterin synthesis is associated with dystrophin loss and treatment with tetrahydrobiopterin improves striated muscle histopathology and skeletal muscle function in mdx mice.

Muscle metabolic remodelling patterns in Duchenne muscular dystrophy revealed by ultra-high-resolution mass spectrometry imaging

Duchenne muscular dystrophy (DMD) is a common and severe X-linked myopathy, characterized by muscle degeneration due to altered or absent dystrophin. DMD has no effective cure, and the underlying molecular mechanisms remain incompletely understood. The aim of this study is to investigate the metabolic changes in DMD using mass spectrometry-based imaging. Nine human muscle biopsies from DMD patients and nine muscle biopsies from control individuals were subjected to untargeted MSI using matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry. Both univariate and pattern recognition techniques have been used for data analysis. This study revealed significant changes in 34 keys metabolites. Seven metabolites were decreased in the Duchenne biopsies compared to control biopsies including adenosine triphosphate, and glycerophosphocholine. The other 27 metabolites were increased in the Duchenne biopsies, including sphingomyelin, phosphatidylcholines, phosphatidic acids and phosphatidylserines. Most of these dysregulated metabolites are tightly related to energy and phospholipid metabolism. This study revealed a deep metabolic remodelling in phospholipids and energy metabolism in DMD. This systems-based approach enabled exploring the metabolism in DMD in an unprecedented holistic and unbiased manner with hypothesis-free strategies.

Multiomics analysis of the mdx/mTR mouse model of Duchenne muscular dystrophy

PURPOSE/AIM

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease characterized by extensive muscle weakness. Patients with DMD lack a functional dystrophin protein, which transmits force and organizes the cytoskeleton of skeletal muscle. Multiomic studies have been proposed as a way to obtain novel insight about disease processes from preclinical models, and we used this approach to study pathological changes in dystrophic muscles.

MATERIALS AND METHODS

We evaluated hindlimb muscles of male mdx/mTR mice, which lack a functional dystrophin protein and have deficits in satellite cell abundance and proliferative capacity. Wild type (WT) C57BL/6 J mice served as controls. Muscle fiber contractility was measured, along with changes in the transcriptome using RNA sequencing, and in the proteome, metabolome, and lipidome using mass spectrometry.

RESULTS

While mdx/mTR mice displayed gross pathological changes and continued cycles of degeneration and regeneration, we found no differences in permeabilized fiber contractility between strains. However, there were numerous changes in the transcriptome and proteome related to protein balance, contractile elements, extracellular matrix, and metabolism. There was only a 53% agreement in fold-change data between the proteome and transcriptome. Numerous changes in markers of skeletal muscle metabolism were observed, with dystrophic muscles exhibiting elevated glycolytic metabolites such as 6-phosphoglycerate, fructose-6-phosphate and glucose-6-phosphate, fructose bisphosphate, phosphorylated hexoses, and phosphoenolpyruvate.

CONCLUSIONS

These findings highlight the utility of multiomics in studying muscle disease, and provide additional insight into the pathological changes in dystrophic muscles that might help to indirectly guide evidence-based nutritional or exercise prescription in DMD patients.

Complementary NAD+ replacement strategies fail to functionally protect dystrophin-deficient muscle

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive muscle wasting disorder stemming from a loss of functional dystrophin. Current therapeutic options for DMD are limited, as small molecule modalities remain largely unable to decrease the incidence or mitigate the consequences of repetitive mechanical insults to the muscle during eccentric contractions (ECCs).

METHODS

Using a metabolomics-based approach, we observed distinct and transient molecular phenotypes in muscles of dystrophin-deficient MDX mice subjected to ECCs. Among the most chronically depleted metabolites was nicotinamide adenine dinucleotide (NAD), an essential metabolic cofactor suggested to protect muscle from structural and metabolic degeneration over time. We tested whether the MDX muscle NAD pool can be expanded for therapeutic benefit using two complementary small molecule strategies: provision of a biosynthetic precursor, nicotinamide riboside, or specific inhibition of the NAD-degrading ADP-ribosyl cyclase, CD38.

RESULTS

Administering a novel, potent, and orally available CD38 antagonist to MDX mice successfully reverted a majority of the muscle metabolome toward the wildtype state, with a pronounced impact on intermediates of the pentose phosphate pathway, while supplementing nicotinamide riboside did not significantly affect the molecular phenotype of the muscle. However, neither strategy sustainably increased the bulk tissue NAD pool, lessened muscle damage markers, nor improved maximal hindlimb strength following repeated rounds of eccentric challenge and recovery.

CONCLUSIONS

In the absence of dystrophin, eccentric injury contributes to chronic intramuscular NAD depletion with broad pleiotropic effects on the molecular phenotype of the tissue. These molecular consequences can be more effectively overcome by inhibiting the enzymatic activity of CD38 than by supplementing nicotinamide riboside. However, we found no evidence that either small molecule strategy is sufficient to restore muscle contractile function or confer protection from eccentric injury, undermining the modulation of NAD metabolism as a therapeutic approach for DMD.

Proteomic profiling of fatty acid binding proteins in muscular dystrophy

Introduction: Duchenne muscular dystrophy is a neuromuscular disorder, which is caused by abnormalities in the DMD gene that encodes the membrane cytoskeletal protein dystrophin. Besides progressive skeletal muscle wasting, dystrophinopathy also affects non-skeletal muscle tissues, including cells in the cardio-respiratory system, the central nervous system, the liver and the kidney.Areas covered: This review summarizes the proteomic characterization of a key class of lipid chaperones, the large family of fatty acid binding proteins, and their potential role in muscular dystrophy. Recent proteomic surveys using animal models and patient specimens are reviewed. Pathobiochemical changes in specific proteoforms of fatty acid binding protein in the multi-system pathology of dystrophinopathy are discussed.Expert opinion: The mass spectrometric identification of distinct changes in fatty acid binding proteins in muscle, heart, liver, kidney and serum demonstrates that considerable alterations occur in key steps of metabolite transport and fat metabolism in muscular dystrophy. These new findings might be helpful to further develop a comprehensive biomarker signature of metabolic changes in X-linked muscular dystrophy, which should improve (i) our understanding of complex pathobiochemical changes due to dystrophin deficiency, (ii) the identification of novel therapeutic targets, and (iii) the design of differential diagnostic, prognostic and therapy-monitoring approaches.

Metabolic reprogramming of fibro/adipogenic progenitors facilitates muscle regeneration

High-fat diet ameliorates muscle dystrophic phenotype by promoting the FAP-dependent myogenesis of satellite cells.

In Duchenne muscular dystrophy (DMD), the absence of the dystrophin protein causes a variety of poorly understood secondary effects. Notably, muscle fibers of dystrophic individuals are characterized by mitochondrial dysfunctions, as revealed by a reduced ATP production rate and by defective oxidative phosphorylation. Here, we show that in a mouse model of DMD (mdx), fibro/adipogenic progenitors (FAPs) are characterized by a dysfunctional mitochondrial metabolism which correlates with increased adipogenic potential. Using high-sensitivity mass spectrometry–based proteomics, we report that a short-term high-fat diet (HFD) reprograms dystrophic FAP metabolism in vivo. By combining our proteomic dataset with a literature-derived signaling network, we revealed that HFD modulates the β-catenin–follistatin axis. These changes are accompanied by significant amelioration of the histological phenotype in dystrophic mice. Transplantation of purified FAPs from HFD-fed mice into the muscles of dystrophic recipients demonstrates that modulation of FAP metabolism can be functional to ameliorate the dystrophic phenotype. Our study supports metabolic reprogramming of muscle interstitial progenitor cells as a novel approach to alleviate some of the adverse outcomes of DMD.

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

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

Discovery of potential urine-accessible metabolite biomarkers associated with muscle disease and corticosteroid response in the mdx mouse model for Duchenne

Urine is increasingly being considered as a source of biomarker development in Duchenne Muscular Dystrophy (DMD), a severe, life-limiting disorder that affects approximately 1 in 4500 boys. In this study, we considered the mdx mice-a murine model of DMD-to discover biomarkers of disease, as well as pharmacodynamic biomarkers responsive to prednisolone, a corticosteroid commonly used to treat DMD. Longitudinal urine samples were analyzed from male age-matched mdx and wild-type mice randomized to prednisolone or vehicle control via liquid chromatography tandem mass spectrometry. A large number of metabolites (869 out of 6,334) were found to be significantly different between mdx and wild-type mice at baseline (Bonferroni-adjusted p-value < 0.05), thus being associated with disease status. These included a metabolite with m/z = 357 and creatine, which were also reported in a previous human study looking at serum. Novel observations in this study included peaks identified as biliverdin and hypusine. These four metabolites were significantly higher at baseline in the urine of mdx mice compared to wild-type, and significantly changed their levels over time after baseline. Creatine and biliverdin levels were also different between treated and control groups, but for creatine this may have been driven by an imbalance at baseline. In conclusion, our study reports a number of biomarkers, both known and novel, which may be related to either the mechanisms of muscle injury in DMD or prednisolone treatment.