Abnormal lipid metabolism in skeletal muscle tissue of patients with muscular dystrophy: In vitro, high-resolution NMR spectroscopy based observation in early phase of the disease

The role of microRNAs in dexamethasone-induced skeletal muscle atrophy

Muscle atrophy is characterized by a decrease in muscle mass, strength, and activity. Recently, it was determined that microRNAs (miRNAs) can regulate muscle atrophy and that dexamethasone (Dex), an allergy and autoimmune disorder treatment that can induce muscle atrophy. Therefore, this study was designed to identify miRNAs expressed in Dex-induced muscle atrophy in mice using small RNA sequencing. A total of 820 miRNAs were identified, with 58 miRNAs expressed explicitly in atrophic muscles. Dex-induced muscle atrophy miRNAs clustered separately from the differential miRNAs in aging, disuse, and cancer-induced muscle atrophy models. The target genes of Dex-induced muscle atrophy miRNAs were independently enriched in inositol phosphate metabolism, hypoxia-inducible factor-1 signaling pathway, etc. Of note, there was a significant increase in the volume of fat cells and adipose weight in the Dex group, suggesting that fat deposition during Dex-induced skeletal muscle atrophy is a unique and typical feature. SIMPLE SUMMARY: Dexamethasone (Dex) is a glucocorticoid used to treat allergic and autoimmune diseases, but excessive use can lead to skeletal muscle atrophy. We used dexamethasone (Dex) to build a muscle atrophy model in mice, and obvious changes had taken place in mouse body weight, muscle tissue morphology and related genes. A large number of microRNAs were found to be differentially expressed, and their functions were enriched in pathways related to muscle development. At the same time, we compared the similarities and differences of microRNAs and their functions between Dex induced muscle atrophy model and other muscle atrophy models. Finally, we were surprised to find that Dex induced muscle atrophy is specifically accompanied by the accumulation of body fat.

Metabolic dysregulation contributes to the development of dysferlinopathy

Dysferlin is a transmembrane protein that plays a prominent role in membrane repair of damaged muscle fibers. Accordingly, mutations in the dysferlin gene cause progressive muscular dystrophies, collectively referred to as dysferlinopathies for which no effective treatment exists. Unexpectedly, experimental approaches that successfully restore membrane repair fail to prevent a dystrophic phenotype, suggesting that additional, hitherto unknown dysferlin-dependent functions contribute to the development of the pathology. Our experiments revealed an altered metabolic phenotype in dysferlin-deficient muscles, characterized by (1) mitochondrial abnormalities and elevated death signaling and (2) increased glucose uptake, reduced glycolytic protein levels, and pronounced glycogen accumulation. Strikingly, elevating mitochondrial volume density and muscle glycogen accelerates disease progression; whereas, improvement of mitochondrial function and recruitment of muscle glycogen with exercise ameliorated functional parameters in a mouse model of dysferlinopathy. Collectively, our results not only shed light on a metabolic function of dysferlin but also imply new therapeutic avenues aimed at promoting mitochondrial function and normalizing muscle glycogen to ameliorate dysferlinopathies, complementing efforts that target membrane repair.

Skeletal muscle disorders as risk factors for type 2 diabetes

The incidence and prevalence of muscular disorders and of type 2 diabetes (T2D) is increasing and both represent highly significant healthcare problems, both economically and compromising quality of life. Interestingly, skeletal muscle dysfunction and T2D share some commonalities including dysregulated glucose homeostasis, increased oxidative stress, dyslipidemia, and cytokine alterations. Several lines of evidence have hinted to a relationship between skeletal muscle dysfunction and T2D. For instance, T2D affects skeletal muscle morphology, functionality, and overall health through altered protein metabolism, impaired mitochondrial function, and ultimately cell viability. Conversely, humans suffering from myopathies and their experimental models demonstrated increased incidence of T2D through altered muscle glucose disposal function due to abnormal calcium homeostasis, compromised mitochondrial function, dyslipidemia, increased inflammatory cytokines and fiber size alterations and disproportions. Lifestyle modifications are essential for improving and maintaining mobility and metabolic health in individuals suffering from myopathies along with T2D. In this review, we updated current literature evidence on clinical incidence of T2D in inflammatory, mitochondrial, metabolic myopathies, and muscular dystrophies and further discussed the molecular basis of these skeletal muscle disorders leading to T2D.

Comparative lipidomic and metabolomic profiling of mdx and severe mdx-apolipoprotein e-null mice

Despite its notoriously mild phenotype, the dystrophin-deficient mdx mouse is the most common model of Duchenne muscular dystrophy (DMD). By mimicking a human DMD-associated metabolic comorbidity, hyperlipidemia, in mdx mice by inactivating the apolipoprotein E gene (mdx-ApoE) we previously reported severe myofiber damage exacerbation via histology with large fibro-fatty infiltrates and phenotype humanization with ambulation dysfunction when fed a cholesterol- and triglyceride-rich Western diet (mdx-ApoEW). Herein, we performed comparative lipidomic and metabolomic analyses of muscle, liver and serum samples from mdx and mdx-ApoEW mice using solution and high-resolution-magic angle spinning (HR-MAS) 1H-NMR spectroscopy. Compared to mdx and regular chow-fed mdx-ApoE mice, we observed an order of magnitude increase in lipid deposition in gastrocnemius muscle of mdx-ApoEW mice including 11-fold elevations in -CH3 and -CH2 lipids, along with pronounced elevations in serum cholesterol, fatty acid, triglyceride and phospholipids. Hepatic lipids were also elevated but did not correlate with the extent of muscle lipid infiltration or differences in serum lipids. This study provides the first lipometabolomic signature of severe mdx lesions exacerbated by high circulating lipids and lends credence to claims that the liver, the main regulator of whole-body lipoprotein metabolism, may play only a minor role in this process.

Apolipoprotein E knockout, but not cholesteryl ester transfer protein (CETP)-associated high-density lipoprotein cholesterol (HDL-C) lowering, exacerbates muscle wasting in dysferlin-null mice

BACKGROUND

Dysferlin-deficient limb-girdle muscular dystrophy type 2B (Dysf) mice are notorious for their mild phenotype. Raising plasma total cholesterol (CHOL) via apolipoprotein E (ApoE) knockout (KO) drastically exacerbates muscle wasting in Dysf mice. However, dysferlinopathic patients have abnormally reduced plasma high-density lipoprotein cholesterol (HDL-C) levels. The current study aimed to determine whether HDL-C lowering can exacerbate the mild phenotype of dysferlin-null mice.

METHODS

Human cholesteryl ester transfer protein (CETP), a plasma lipid transfer protein not found in mice that reduces HDL-C, and/or its optimal adapter protein human apolipoprotein B (ApoB), were overexpressed in Dysf mice. Mice received a 2% cholesterol diet from 2 months of age and characterized through ambulatory and hanging functional tests, plasma analyses, and muscle histology.

RESULTS

CETP/ApoB expression in Dysf mice caused reduced HDL-C (54.5%) and elevated ratio of CHOL/HDL-C (181.3%) compared to control Dysf mice in plasma, but without raising CHOL. Compared to the severe muscle pathology found in high CHOL Dysf/ApoE double knockout mice, Dysf/CETP/ApoB mice did not show significant changes in ambulation, hanging capacity, increases in damaged area, collagen deposition, or decreases in cross-sectional area and healthy myofibre coverage.

CONCLUSIONS

CETP/ApoB over-expression in Dysf mice decreases HDL-C without increasing CHOL or exacerbating muscle pathology. High CHOL or nonHDL-C caused by ApoE KO, rather than low HDL-C, likely lead to rodent muscular dystrophy phenotype humanization.

Caveolin and NOS in the Development of Muscular Dystrophy

Caveolin is a structural protein within caveolae that may be involved in transmembrane molecular transport and/or various intercellular interactions within cells. Specific mutations of caveolin-3 in muscle fibers are well known to cause limb-girdle muscular dystrophy. Altered expression of caveolin-3 has also been detected in Duchenne muscular dystrophy, which may be a part of the pathological process leading to muscle weakness. Interestingly, it has been shown that the renovation of nitric oxide synthase (NOS) in sarcolemma with muscular dystrophy could improve muscle health, suggesting that NOS may be involved in the pathology of muscular dystrophy. Here, we summarize the notable function of caveolin and/or NOS in skeletal muscle fibers and discuss their involvement in the pathology as well as possible tactics for the innovative treatment of muscular dystrophies.

Astaxanthin Ameliorates Worsened Muscle Dysfunction of MDX Mice Fed with a High-Fat Diet through Reducing Lipotoxicity and Regulating Gut Microbiota

Duchenne muscular dystrophy (DMD), a severe X-linked inherited neuromuscular disease, has a high prevalence of obesity. Obesity exacerbates muscle damage and results in adverse clinical outcomes. Preventing obesity helps DMD patients delay disease progression and improve quality of life. Astaxanthin (AX) is a kind of carotenoid which has antioxidant and anti-adipogenesis effects. In this study, male C57BL/10ScSnDmdmdx/J mice were fed with a normal diet, a high-fat diet (HFD), and an HFD containing AX for 16 weeks, respectively. The results showed that AX significantly increased gastrocnemius fiber cross-section area and grip strength, improved treadmill endurance test and mitochondrial morphology, and reduced muscle triglyceride and malonaldehyde levels compared to the HFD. Lipidomic analysis revealed that AX decreased high levels of triglyceride, diglyceride, ceramides, and wax ester induced by HFD. Gut microbiota analysis indicated that AX supplementation failed to alleviate abnormal microbiota diversity, but increased the relative abundances of Akkermansia, Bifidobacterium, Butyricicoccus, and Staphylococcus. In conclusion, AX was expected to alleviate disease progression associated with obesity in DMD patients by reducing lipotoxicity and increasing the abundance of beneficial bacteria.

A Dysferlin Exon 32 Nonsense Mutant Mouse Model Shows Pathological Signs of Dysferlinopathy

Dysferlinopathies are a group of autosomal recessive muscular dystrophies caused by pathogenic variants in the DYSF gene. While several animal models of dysferlinopathy have been developed, most of them involve major disruptions of the Dysf gene locus that are not optimal for studying human dysferlinopathy, which is often caused by single nucleotide substitutions. In this study, the authors describe a new murine model of dysferlinopathy that carries a nonsense mutation in Dysf exon 32, which has been identified in several patients with dysferlinopathy. This mouse model, called Dysf ^{p.Y1159X/p.Y1159X}, displays several molecular, histological, and functional defects observed in dysferlinopathy patients and other published mouse models. This mutant mouse model is expected to be useful for testing various therapeutic approaches such as termination codon readthrough, pharmacological approaches, and exon skipping. Therefore, the data presented in this study strongly support the use of this animal model for the development of preclinical strategies for the treatment of dysferlinopathies.

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.

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

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

Limb-girdle muscular dystrophy type 2B causes HDL-C abnormalities in patients and statin-resistant muscle wasting in dysferlin-deficient mice

Limb-girdle muscular dystrophy (MD) type 2B (LGMD2B) and Duchenne MD (DMD) are caused by mutations to the Dysferlin and Dystrophin genes, respectively. We have recently demonstrated in typically mild dysferlin- and dystrophin-deficient mouse models that increased plasma cholesterol levels severely exacerbate muscle wasting, and that DMD patients display primary dyslipidemia characterized by elevated plasma cholesterol and triglycerides. Herein, we investigate lipoprotein abnormalities in LGMD2B and if statin therapy protects dysferlin-deficient mice (Dysf) from muscle damage. Herein, lipoproteins and liver enzymes from LGMD2B patients and dysferlin-null (Dysf) mice were analyzed. Simvastatin, which exhibits anti-muscle wasting effects in mouse models of DMD and corrects aberrant expression of key markers of lipid metabolism and endogenous cholesterol synthesis, was tested in Dysf mice. Muscle damage and fibrosis were assessed by immunohistochemistry and cholesterol signalling pathways via Western blot. LGMD2B patients show reduced serum high-density lipoprotein cholesterol (HDL-C) levels compared to healthy controls and exhibit a greater prevalence of abnormal total cholesterol (CHOL)/HDL-C ratios despite an absence of liver dysfunction. While Dysf mice presented with reduced CHOL and associated HDL-C and LDL-C-associated fractions, simvastatin treatment did not prevent muscle wasting in quadriceps and triceps muscle groups or correct aberrant low-density lipoprotein receptor (LDLR) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) protein expression. LGMD2B patients present with reduced serum concentrations of HDL-C, a major metabolic comorbidity, and as a result, statin therapy is unlikely to prevent muscle wasting in this population. We propose that like DMD, LGMD2B should be considered as a new type of genetic dyslipidemia.

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.

The metabolic landscape in chronic rotator cuff tear reveals tissue-region-specific signatures

BACKGROUND

Degeneration of shoulder muscle tissues often result in tearing, causing pain, disability and loss of independence. Differential muscle involvement patterns have been reported in tears of shoulder muscles, yet the molecules involved in this pathology are poorly understood. The spatial distribution of biomolecules across the affected tissue can be accurately obtained with matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). The goal of this pilot study was to decipher the metabolic landscape across shoulder muscle tissues and to identify signatures of degenerated muscles in chronic conditions.

METHODS

Paired biopsies of two rotator cuff muscles, torn infraspinatus and intact teres minor, together with an intact shoulder muscle, the deltoid, were collected during an open tendon transfer surgery. Five patients, average age 65.2 ± 3.8 years, were selected for spatial metabolic profiling using high-spatial resolution (MALDI-TOF) and high-mass resolution (MALDI-FTICR) MSI in negative or positive ion mode. Metabolic signatures were identified using data-driven analysis. Verifications of spatial localization for selected metabolic signatures were carried out using antibody immunohistology.

RESULTS

Data-driven analysis revealed major metabolic differences between intact and degenerated regions across all muscles. The area of degenerated regions, encompassed of fat, inflammation and fibrosis, significantly increased in both rotator cuff muscles, teres minor (27.9%) and infraspinatus (22.8%), compared with the deltoid (8.7%). The intact regions were characterized by 49 features, among which lipids were recognized. Several of the identified lipids were specifically enriched in certain myofiber types. Degenerated regions were specifically marked by the presence of 37 features. Heme was the most abundant metabolite in degenerated regions, whereas Heme oxygenase-1 (HO-1), which catabolizes heme, was found in intact regions. Higher HO-1 levels correlated with lower heme accumulation.

CONCLUSIONS

Degenerated regions are distinguished from intact regions by their metabolome profile. A muscle-specific metabolome profile was not identified. The area of tissue degeneration significantly differs between the three examined muscles. Higher HO-1 levels in intact regions concurred with lower heme levels in degenerated regions. Moreover, HO-1 levels discriminated between dysfunctional and functional rotator cuff muscles. Additionally, the enrichment of specific lipids in certain myofiber types suggests that lipid metabolism differs between myofiber types. The signature metabolites can open options to develop personalized treatments for chronic shoulder muscles degeneration.

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.

Effect of Dysferlin Deficiency on Atherosclerosis and Plasma Lipoprotein Composition Under Normal and Hyperlipidemic Conditions

Dysferlinopathies are a group of muscle disorders caused by mutations to dysferlin, a transmembrane protein involved in membrane patching events following physical damage to skeletal myofibers. We documented dysferlin expression in vascular tissues including non-muscle endothelial cells, suggesting that blood vessels may have an endogenous repair system that helps promote vascular homeostasis. To test this hypothesis, we generated dysferlin-null mice lacking apolipoprotein E (ApoE), a common model of atherosclerosis, dyslipidemia and endothelial injury when stressed with a high fat, and cholesterol-rich diet. Despite high dysferlin expression in mouse and human atheromatous plaques, loss of dysferlin did not affect atherosclerotic burden as measured in the aortic root, arch, thoracic, and abdominal aortic regions. Interestingly, we observed that dysferlin-null mice exhibit lower plasma high-density lipoprotein cholesterol (HDL-C) levels than their WT controls at all measured stages of the disease process. Western blotting revealed abundant dysferlin expression in protein extracts from mouse livers, the main regulator of plasma lipoprotein levels. Despite abnormal lipoprotein levels, Dysf/ApoE double knockout mice responded to cholesterol absorption blockade with lower total cholesterol and blunted atherosclerosis. Our study suggests that dysferlin does not protect against atherosclerosis or participate in cholesterol absorption blockade but regulates basal plasma lipoprotein composition. Dysferlinopathic patients may be dyslipidemic without greater atherosclerotic burden while remaining responsive to cholesterol absorption blockade.

Altered cholesterol homeostasis in critical illness-induced muscle weakness: effect of exogenous 3-hydroxybutyrate

Background

Muscle weakness is a complication of critical illness which hampers recovery. In critically ill mice, supplementation with the ketone body 3-hydroxybutyrate has been shown to improve muscle force and to normalize illness-induced hypocholesterolemia. We hypothesized that altered cholesterol homeostasis is involved in development of critical illness-induced muscle weakness and that this pathway can be affected by 3-hydroxybutyrate.

Methods

In both human critically ill patients and septic mice, the association between circulating cholesterol concentrations and muscle weakness was assessed. In septic mice, the impact of 3-hydroxybutyrate supplementation on cholesterol homeostasis was evaluated with use of tracer technology and through analysis of markers of cholesterol metabolism and downstream pathways.

Results

Serum cholesterol concentrations were lower in weak than in non-weak critically ill patients, and in multivariable analysis adjusting for baseline risk factors, serum cholesterol was inversely correlated with weakness. In septic mice, plasma cholesterol correlated positively with muscle force. In septic mice, exogenous 3-hydroxybutyrate increased plasma cholesterol and altered cholesterol homeostasis, by normalization of plasma mevalonate and elevation of muscular, but not hepatic, expression of cholesterol synthesis genes. In septic mice, tracer technology revealed that 3-hydroxybutyrate was preferentially taken up by muscle and metabolized into cholesterol precursor mevalonate, rather than TCA metabolites. The 3-hydroxybutyrate protection against weakness was not related to ubiquinone or downstream myofiber mitochondrial function, whereas cholesterol content in myofibers was increased.

Conclusions

These findings point to a role for low cholesterol in critical illness-induced muscle weakness and to a protective mechanism-of-action for 3-hydroxybutyrate supplementation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03688-1.

Plasma lipidomic analysis shows a disease progression signature in mdx mice

Duchenne muscular dystrophy (DMD) is a rare genetic disorder affecting paediatric patients. The disease course is characterized by loss of muscle mass, which is rapidly substituted by fibrotic and adipose tissue. Clinical and preclinical models have clarified the processes leading to muscle damage and myofiber degeneration. Analysis of the fat component is however emerging as more evidence shows how muscle fat fraction is associated with patient performance and prognosis. In this article we aimed to study whether alterations exist in the composition of lipids in plasma samples obtained from mouse models. Analysis of plasma samples was performed in 4 mouse models of DMD and wild-type mice by LC–MS. Longitudinal samplings of individual mice covering an observational period of 7 months were obtained to cover the different phases of the disease. We report clear elevation of glycerolipids and glycerophospholipids families in dystrophic mice compared to healthy mice. Triacylglycerols were the strongest contributors to the signatures in mice. Annotation of individual lipids confirmed the elevation of lipids belonging to these families as strongest discriminants between healthy and dystrophic mice. A few sphingolipids (such as ganglioside GM2, sphingomyelin and ceramide), sterol lipids (such as cholesteryl oleate and cholesteryl arachidonate) and a fatty acyl (stearic acid) were also found to be affected in dystrophic mice. Analysis of serum and plasma samples show how several lipids are affected in dystrophic mice affected by muscular dystrophy. This study sets the basis to further investigations to understand how the lipid signature relates to the disease biology and muscle performance.

Cholesterol metabolism is a potential therapeutic target in Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is a lethal muscle disease detected in approximately 1:5000 male births. DMD is caused by mutations in the DMD gene, encoding a critical protein that links the cytoskeleton and the extracellular matrix in skeletal and cardiac muscles. The primary consequence of the disrupted link between the extracellular matrix and the myofibre actin cytoskeleton is thought to involve sarcolemma destabilization, perturbation of Ca2+ homeostasis, activation of proteases, mitochondrial damage, and tissue degeneration. A recently emphasized secondary aspect of the dystrophic process is a progressive metabolic change of the dystrophic tissue; however, the mechanism and nature of the metabolic dysregulation are yet poorly understood. In this study, we characterized a molecular mechanism of metabolic perturbation in DMD.

METHODS

We sequenced plasma miRNA in a DMD cohort, comprising 54 DMD patients treated or not by glucocorticoid, compared with 27 healthy controls, in three groups of the ages of 4-8, 8-12, and 12-20 years. We developed an original approach for the biological interpretation of miRNA dysregulation and produced a novel hypothesis concerning metabolic perturbation in DMD. We used the mdx mouse model for DMD for the investigation of this hypothesis.

RESULTS

We identified 96 dysregulated miRNAs (adjusted P-value <0.1), of which 74 were up-regulated and 22 were down-regulated in DMD. We confirmed the dysregulation in DMD of Dystro-miRs, Cardio-miRs, and a large number of the DLK1-DIO3 miRNAs. We also identified numerous dysregulated miRNAs yet unreported in DMD. Bioinformatics analysis of both target and host genes for dysregulated miRNAs predicted that lipid metabolism might be a critical metabolic perturbation in DMD. Investigation of skeletal muscles of the mdx mouse uncovered dysregulation of transcription factors of cholesterol and fatty acid metabolism (SREBP-1 and SREBP-2), perturbation of the mevalonate pathway, and the accumulation of cholesterol in the dystrophic muscles. Elevated cholesterol level was also found in muscle biopsies of DMD patients. Treatment of mdx mice with Simvastatin, a cholesterol-reducing agent, normalized these perturbations and partially restored the dystrophic parameters.

CONCLUSIONS

This investigation supports that cholesterol metabolism and the mevalonate pathway are potential therapeutic targets in DMD.

β-Glucans as Dietary Supplement to Improve Locomotion and Mitochondrial Respiration in a Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular childhood disorder that causes progressive muscle weakness and degeneration. A lack of dystrophin in DMD leads to inflammatory response, autophagic dysregulation, and oxidative stress in skeletal muscle fibers that play a key role in the progression of the pathology. β-glucans can modulate immune function by modifying the phagocytic activity of immunocompetent cells, notably macrophages. Mitochondrial function is also involved in an important mechanism of the innate and adaptive immune responses, owing to high need for energy of immune cells. In the present study, the effects of 1,3-1,6 β-glucans on five-day-old non-dystrophic and dystrophic (sapje) zebrafish larvae were investigated. The effects of the sonication of β-glucans and the dechorionation of embryos were also evaluated. The results showed that the incidence of dystrophic phenotypes was reduced when dystrophic embryos were exposed to 2 and 4 mg L^-1 of 1,3-1,6 β-glucans. Moreover, when the dystrophic larvae underwent 8 mg L^-1 treatment, an improvement of the locomotor performances and mitochondrial respiration were observed. In conclusion, the observed results demonstrated that 1,3-1,6 β-glucans improve locomotor performances and mitochondrial function in dystrophic zebrafish. Therefore, for ameliorating their life quality, 1,3-1,6 β-glucans look like a promising diet supplement for DMD patients, even though further investigations are required.

Single-cell RNA sequencing and lipidomics reveal cell and lipid dynamics of fat infiltration in skeletal muscle

BACKGROUND

Ageing is accompanied by sarcopenia and intramuscular fat (IMAT) infiltration. In skeletal muscle, fat infiltration is a common feature in several myopathies and is associated with muscular dysfunction and insulin resistance. However, the cellular origin and lipidomic and transcriptomic changes during fat infiltration in skeletal muscle remain unclear.

METHODS

In the current study, we generated a high IMAT-infiltrated skeletal muscle model by glycerol (GLY) injection. Single-cell RNA sequencing and lineage tracing were performed on GLY-injured skeletal muscle at 5 days post-injection (DPI) to identify the cell origins and dynamics. Lipidomics and RNA sequencing were performed on IMAT-infiltrated skeletal muscle at 14 DPI (or 17 DPI for the cold treatment) to analyse alterations of lipid compositions and gene expression levels.

RESULTS

We identified nine distinct major clusters including myeloid-derived cells (52.13%), fibroblast/fibro/adipogenic progenitors (FAPs) (23.24%), and skeletal muscle stem cells (2.02%) in GLY-injured skeletal muscle. Clustering and pseudotemporal trajectories revealed six subpopulations in fibroblast/FAPs and 10 subclusters in myeloid-derived cells. A subpopulation of myeloid-derived cells expressing adipocyte-enriched genes and Pdgfra- /Cd68+ cells displayed lipid droplets upon adipogenic induction, indicating their adipogenic potential. Lipidomic analysis revealed the changes of overall lipid classes composition (e.g. triglycerides (TAGs) increased by 19.3 times, P = 0.0098; sulfoquinovosyl diacylglycerol decreased by 83%, P = 0.0056) and in the distribution of lipids [e.g. TAGs (18:2/18:2/22:6) increased by 181.6 times, P = 0.021] between GLY-group and saline control. RNA-seq revealed 1847 up-regulated genes and 321 down-regulated genes and significant changes in lipid metabolism-related pathways (e.g. glycerolipid pathway and glycerophospholipid pathway) in our model of GLY-injured skeletal muscle. Notably, short-term cold exposure altered fatty acid composition (e.g. saturated fatty acid decreased by 6.4%, P = 0.058) in fat-infiltrated muscles through directly affecting lipid metabolism pathways including PI3K-AKT and MAPK signalling pathway.

CONCLUSIONS

Our results showed that a subpopulation of myeloid-derived cells may contribute to IMAT infiltration. GLY-induced IMAT infiltration changed the lipid composition and gene expression profiles. Short-term cold exposure might regulate lipid metabolism and its related signalling pathways in fat-infiltrated muscle. Our study provides a comprehensive resource describing the molecular signature of fat infiltration in skeletal muscle.

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.

Relationship between markers of disease activity and progression in skeletal muscle of GNE myopathy patients using quantitative nuclear magnetic resonance imaging and 31P nuclear magnetic resonance spectroscopy

Background

Quantitative nuclear magnetic resonance imaging (NMRI) is an objective and precise outcome measure for evaluating disease progression in neuromuscular disorders. We aimed to investigate predictive 'disease activity' NMR indices, including water T₂ and ³¹P NMR spectroscopy (NMRS), and its relation to NMR markers of 'disease progression', such as the changes in fat fraction (ΔFat%) and contractile cross-sectional area (ΔcCSA), in GNE myopathy (GNEM) patients.

Methods

NMR was performed on a 3T clinical scanner, at baseline and at a 1-year interval, in 10 GNEM patients and 29 age-matched controls. Dixon-based fat-water imaging and water T₂ mapping were acquired in legs and thighs, and in the dominant forearm. ³¹P NMRS was performed at the level of quadriceps and hamstring. Water T₂ and ³¹P NMRS indices were determined for all muscle groups and visits. Correlations were performed with 'disease progression' indices ΔFat%, ΔcCSA and the muscle fat transformation rate (R_{muscle_transf}).

Results

In quadriceps, known to be relatively preserved in GNEM, water T₂ at baseline was significantly higher compared to controls, and correlated strongly with the one-year evolution of Fat% and cCSA and R_{muscle_transf}. Various ³¹P NMRS indices showed significant differences in quadriceps and hamstring compared to controls and correlations existed between these indices and ΔFat%, ΔcCSA and R_{muscle_transf}.

Conclusions

This study demonstrates that disease activity indices such as water T₂ and ³¹P NMRS may predict disease progression in skeletal muscles of GNEM patients, and suggests that these measures may be considered to be valuable surrogate endpoints in the assessment of GNEM disease progression.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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 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.

Oral Administration of Succinoglycan Riclin Improves Diet-Induced Hypercholesterolemia in Mice

Epidemiological studies have demonstrated that hypercholesterolemia is associated with an elevated risk of atherosclerosis and cardiovascular diseases. In addition to the available cholesterol-lowering drugs, nutritionally balanced diets containing functional foods have attracted much interest as potential candidates to improve hypercholesterolemia. In the study, we demonstrated that dietary succinoglycan riclin effectively alleviated diet-induced hypercholesterolemia. Compared with the high-cholesterol-diet (HCD) group, the high-riclin group significantly decreased levels of the serum total cholesterol, low-density lipoprotein cholesterol (LDL-C), and hepatic cholesterol (34, 40, and 51%, respectively), consequently improving hepatic steatosis and reducing proinflammatory cytokine expressions. ¹H nuclear magnetic resonance (NMR)-based lipidomics and metabolomics analyses revealed that the riclin group partially reversed metabolic profile changes induced by the HCD, approaching that of the normal diet (ND) group. Riclin has no direct effects on cholesterol metabolism-related gene expression among the three HCD model groups. Basically, riclin increased the solution viscosity and interfered in the process of bile acid-cholesterol emulsification, decreasing cholesterol digestion and promoting cholesterol and bile acid excretion in the feces. These results suggested potential therapeutic utility of succinoglycan riclin as a food additive for people suffering from hypercholesterolemia and related diseases.

Proceedings of the Ninth HDL (High-Density Lipoprotein) Workshop: Focus on Cardiovascular Disease

The HDL (high-density lipoprotein) Workshop was established in 2009 as a forum for candid discussions among academic basic scientists, clinical investigators, and industry researchers about the role of HDL in cardiovascular disease. This ninth HDL Workshop was held on May 16 to 17, 2019 in Boston, MA, and included outstanding oral presentations from established and emerging investigators. The Workshop featured 5 sessions with topics that tackled the role of HDL in the vasculature, its structural complexity, its role in health and disease states, and its interaction with the intestinal microbiome. The highlight of the program was awarding the Jack Oram Award to the distinguished professor emeritus G.S. Getz from the University of Chicago. The tenth HDL Workshop will be held on May 2020 in Chicago and will continue the focus on intellectually stimulating presentations by established and emerging investigators on novel roles of HDL in cardiovascular and noncardiovascular health and disease states.

Age-dependent changes in metabolite profile and lipid saturation in dystrophic mice

Duchenne Muscular Dystrophy (DMD) is a fatal X-linked genetic disorder. In DMD, the absence of the dystrophin protein causes decreased sarcolemmal integrity resulting in progressive replacement of muscle with fibrofatty tissue. The effects of lacking dystrophin on muscle and systemic metabolism are still unclear. Therefore, to determine the impact of the absence of dystrophin on metabolism, we investigated the metabolic and lipid profile at two different, well-defined stages of muscle damage and stabilization in mdx mice. We measured NMR-detectable metabolite and lipid profiles in the serum and muscles of mdx mice at 6 and 24 weeks of age. Metabolites were determined in muscle in vivo using ¹ H MRI/MRS, in isolated muscles using ¹ H-HR-MAS NMR, and in serum using high resolution ¹ H/¹³ C NMR. Dystrophic mice were found to have a unique lipid saturation profile compared with control mice, revealing an age-related metabolic change. In the 6-week-old mdx mice, serum lipids were increased and the degree of lipid saturation changed between 6 and 24 weeks. The serum taurine-creatine ratio increased over the life span of mdx, but not in control mice. Furthermore, the saturation index of lipids increased in the serum but decreased in the tissue over time. Finally, we demonstrated associations between MRI-T₂ , a strong indicator of inflammation/edema, with tissue and serum lipid profiles. These results indicate the complex temporal changes of metabolites in the tissue and serum during repetitive bouts of muscle damage and regeneration that occur in dystrophic muscle.

Metabolomic Analyses Reveal Extensive Progenitor Cell Deficiencies in a Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a musculoskeletal disorder that causes severe morbidity and reduced lifespan. Individuals with DMD have an X-linked mutation that impairs their ability to produce functional dystrophin protein in muscle. No cure exists for this disease and the few therapies that are available do not dramatically delay disease progression. Thus, there is a need to better understand the mechanisms underlying DMD which may ultimately lead to improved treatment options. The muscular dystrophy (MDX) mouse model is frequently used to explore DMD disease traits. Though some studies of metabolism in dystrophic mice exist, few have characterized metabolic profiles of supporting cells in the diseased environment. Using nontargeted metabolomics we characterized metabolic alterations in muscle satellite cells (SCs) and serum of MDX mice. Additionally, live-cell imaging revealed MDX-derived adipose progenitor cell (APC) defects. Finally, metabolomic studies revealed a striking elevation of acylcarnitines in MDX APCs, which we show can inhibit APC proliferation. Together, these studies highlight widespread metabolic alterations in multiple progenitor cell types and serum from MDX mice and implicate dystrophy-associated metabolite imbalances in APCs as a potential contributor to adipose tissue disequilibrium in DMD.

Cytotoxic Swelling of Sick Excitable Cells - Impaired Ion Homeostasis and Membrane Tension Homeostasis in Muscle and Neuron

When they become simultaneously leaky to both Na⁺ and Cl^-, excitable cells are vulnerable to potentially lethal cytotoxic swelling. Swelling ensues in spite of an isosmotic milieu because the entering ions add osmolytes to the cytoplasm's high concentration of impermeant anionic osmolytes. An influx of osmotically-obliged water is unavoidable. A cell that cannot stanch at least one the leaks will succumb to death by Donnan effect. "Sick excitable cells" are those injured through ischemia, trauma, inflammation, hyperactivity, genetically-impaired membrane skeletons and other insults, all of which foster bleb-damage to regions of the plasma membrane. Nav channels resident in damaged membrane exhibit left-shifted kinetics; the corresponding Nav window conductance constitutes a Na⁺-leak. In cortical neurons, sustained depolarization to ∼-20mV elicits a sustained lethal gCl. Underlying V_rest in skeletal muscle is a constitutively active gCl; not surprisingly therefore, dystrophic muscle fibers, which are prone to bleb damage and which exhibit Nav-leak and Na⁺-overload, are prone to cytotoxic swelling. To restore viability in cytotoxically swelling neurons and muscle, the imperative of fully functional ion homeostasis is well-recognized. However, as emphasized here, in a healthy excitable cell, fully functional membrane tension homeostasis is also imperative. ATPase-pumps keep plasma membrane batteries charged, and ATPase-motor proteins maintain membrane tone. In sick excitable cells, neither condition prevails.

Non-Glycanated Biglycan and LTBP4: Leveraging the extracellular matrix for Duchenne Muscular Dystrophy therapeutics

The extracellular matrix (ECM) plays key roles in normal and diseased skeletal and cardiac muscle. In healthy muscle the ECM is essential for transmitting contractile force, maintaining myofiber integrity and orchestrating cellular signaling. Duchenne Muscular Dystrophy (DMD) is caused by loss of dystrophin, a cytosolic protein that anchors a transmembrane complex and serves as a vital link between the actin cytoskeleton and the basal lamina. Loss of dystrophin leads to membrane fragility and impaired signaling, resulting in myofiber death and cycles of inflammation and regeneration. Fibrosis is also a cardinal feature of DMD. In this review, we will focus on two cases where understanding the normal function and regulation of ECM in muscle has led to the discovery of candidate therapeutics for DMD. Biglycan is a small leucine rich repeat ECM protein present as two glycoforms in muscle that have dramatically different functions. One widely expressed form is biglycan proteoglycan (PG) that bears two chondroitin sulfate GAG chains (typically chondroitin sulfate) and two N-linked carbohydrates. The second glycoform, referred to as 'NG' (non-glycanated) biglycan, lacks the GAG side chains. NG, but not PG biglycan recruits utrophin, an autosomal paralog of dystrophin, and an NOS-containing signaling complex to the muscle cell membrane. Recombinant NG biglycan can be systemically delivered to dystrophic mice where it upregulates utrophin at the membrane and improves muscle health and function. An optimized version of NG biglycan, 'TVN-102', is under development as a candidate therapeutic for DMD. A second matrix-embedded protein being evaluated for therapeutic potential is latent TGFβ binding protein 4 (LTBP4). Identified in a genomic screen for modifiers of muscular dystrophy, LTBP4 binds both TGFβ and myostatin. Genetic studies identified the hinge region of LTBP4 as linked to TGFβ release and contributing to the "hyper-TGFβ" signaling state that promotes fibrosis in muscular dystrophy. This hinge region can be stabilized by antibodies directed towards this domain. Stabilizing the hinge region of LTBP4 is expected to reduce latent TGFβ release and thus reduce fibrosis.

MR Imaging of Atraumatic Muscle Disorders

Atraumatic disorders of skeletal muscles include congenital variants; inherited myopathies; acquired inflammatory, infectious, or ischemic disorders; neoplastic diseases; and conditions leading to muscle atrophy. These have overlapping appearances at magnetic resonance (MR) imaging and are challenging for the radiologist to differentiate. The authors organize muscle disorders into four MR imaging patterns: (a) abnormal anatomy with normal signal intensity, (b) edema/inflammation, (c) mass, and (d) atrophy, highlighting each of their key clinical and imaging findings. Anatomic muscle variants, while common, do not produce signal intensity alterations and therefore are easily overlooked. Muscle edema is the most common pattern but is nonspecific, with a broad differential diagnosis. Autoimmune, paraneoplastic, and drug-induced myositis tend to be symmetric, whereas infection, radiation-induced injury, and myonecrosis are focal asymmetric processes. Architectural distortion in the setting of muscle edema suggests one of these latter processes. Intramuscular masses include primary neoplasms, metastases, and several benign masslike lesions that simulate malignancy. Some lesions, such as lipomas, low-flow vascular malformations, fibromatoses, and subacute hematomas, are distinctive, but many intramuscular masses ultimately require a biopsy for definitive diagnosis. Atrophy is the irreversible end result of any muscle disease of sufficient severity and is the dominant finding in disorders such as the muscular dystrophies, denervation myopathy, and sarcopenia. This imaging-based classification, in correlation with clinical and laboratory data, will aid the radiologist in interpreting MR imaging findings in patients with atraumatic muscle disorders. ^©RSNA, 2018.

Increased nonHDL cholesterol levels cause muscle wasting and ambulatory dysfunction in the mouse model of LGMD2B

Progressive limb and girdle muscle atrophy leading to loss of ambulation is a hallmark of dysferlinopathies, which include limb-girdle muscular dystrophy type 2B and Miyoshi myopathy. However, animal models fail to fully reproduce the disease severity observed in humans, with dysferlin-null (Dysf^-/-) mice exhibiting minor muscle damage and weakness without dramatic ambulatory dysfunction. As we have previously reported significant Dysf expression in blood vessels, we investigated the role of vascular function in development of muscle pathology by generating a Dysf-deficient mouse model with vascular disease. This was achieved by crossing Dysf^-/- mice with ApoE^-/- mice, which have high levels of nonHDL-associated cholesterol. Double-knockout Dysf^-/-ApoE^-/- mice exhibited severe ambulatory dysfunction by 11 months of age. In limb-girdle muscles, histology confirmed dramatic muscle wasting, fibrofatty replacement, and myofiber damage in Dysf^-/-ApoE^-/- mice without affecting the ratio of centrally nucleated myofibers. Although there were no major changes in ex vivo diaphragm and soleus muscle function, histological analyses revealed these muscles to be untouched by damage and remodelling. In all, these data suggest that cholesterol may be deleterious to dysferlinopathic muscle and lead to ambulatory dysfunction. Moreover, differences in plasma lipid handling between mice and humans could be a key factor affecting dysferlinopathy severity.

Increased plasma lipid levels exacerbate muscle pathology in the mdx mouse model of Duchenne muscular dystrophy

Background

Duchenne muscular dystrophy (DMD) is caused by loss of dystrophin expression and leads to severe ambulatory and cardiac function decline. However, the dystrophin-deficient mdx murine model of DMD only develops a very mild form of the disease. Our group and others have shown vascular abnormalities in animal models of MD, a likely consequence of the fact that blood vessels express the same dystrophin-associated glycoprotein complex (DGC) proteins as skeletal muscles.

Methods

To test the blood vessel contribution to muscle damage in DMD, mdx^4cv mice were given elevated lipid levels via apolipoprotein E (ApoE) gene knockout combined with normal chow or lipid-rich Western diets. Ambulatory function and heart function (via echocardiogram) were assessed at 4 and 7 months of age. After sacrifice, muscle histology and aortic staining were used to assess muscle pathology and atherosclerosis development, respectively. Plasma levels of total cholesterol, high-density lipoprotein (HDL), triglycerides, and creatine kinase (CK) were also measured.

Results

Although there was an increase in left ventricular heart volume in mdx-ApoE mice compared to that in mdx mice, parameters of heart function were not affected. Compared with wild-type and ApoE-null, only mdx-ApoE KO mice showed significant ambulatory dysfunction. Despite no significant difference in plasma CK, histological analyses revealed that elevated plasma lipids in chow- and Western diet-fed mdx-ApoE mice was associated with severe exacerbation of muscle pathology compared to mdx mice: significant increase in myofiber damage and fibrofatty replacement in the gastrocnemius and triceps brachii muscles, more reminiscent of human DMD pathology. Finally, although both ApoE and mdx-ApoE groups displayed increased plasma lipids, mdx-ApoE exhibited atherosclerotic plaque deposition equal to or less than that of ApoE mice.

Conclusions

Since others have shown that lipid abnormalities correlate with DMD severity, our data suggest that plasma lipids could be primary contributors to human DMD severity and that the notoriously mild phenotype of mdx mice might be attributable in part to their endogenously low plasma lipid profiles. Hence, DMD patients may benefit from lipid-lowering and vascular-targeted therapies.

Electronic supplementary material

The online version of this article (10.1186/s13395-017-0135-9) contains supplementary material, which is available to authorized users.

Non-Targeted Metabolomics Analysis of Golden Retriever Muscular Dystrophy-Affected Muscles Reveals Alterations in Arginine and Proline Metabolism, and Elevations in Glutamic and Oleic Acid In Vivo

BACKGROUND

Like Duchenne muscular dystrophy (DMD), the Golden Retriever Muscular Dystrophy (GRMD) dog model of DMD is characterized by muscle necrosis, progressive paralysis, and pseudohypertrophy in specific skeletal muscles. This severe GRMD phenotype includes moderate atrophy of the biceps femoris (BF) as compared to unaffected normal dogs, while the long digital extensor (LDE), which functions to flex the tibiotarsal joint and serves as a digital extensor, undergoes the most pronounced atrophy. A recent microarray analysis of GRMD identified alterations in genes associated with lipid metabolism and energy production.

METHODS

We, therefore, undertook a non-targeted metabolomics analysis of the milder/earlier stage disease GRMD BF muscle versus the more severe/chronic LDE using GC-MS to identify underlying metabolic defects specific for affected GRMD skeletal muscle.

RESULTS

Untargeted metabolomics analysis of moderately-affected GRMD muscle (BF) identified eight significantly altered metabolites, including significantly decreased stearamide (0.23-fold of controls, p = 2.89 × 10-3), carnosine (0.40-fold of controls, p = 1.88 × 10-2), fumaric acid (0.40-fold of controls, p = 7.40 × 10-4), lactamide (0.33-fold of controls, p = 4.84 × 10-2), myoinositol-2-phosphate (0.45-fold of controls, p = 3.66 × 10-2), and significantly increased oleic acid (1.77-fold of controls, p = 9.27 × 10-2), glutamic acid (2.48-fold of controls, p = 2.63 × 10-2), and proline (1.73-fold of controls, p = 3.01 × 10-2). Pathway enrichment analysis identified significant enrichment for arginine/proline metabolism (p = 5.88 × 10-4, FDR 4.7 × 10-2), where alterations in L-glutamic acid, proline, and carnosine were found. Additionally, multiple Krebs cycle intermediates were significantly decreased (e.g., malic acid, fumaric acid, citric/isocitric acid, and succinic acid), suggesting that altered energy metabolism may be underlying the observed GRMD BF muscle dysfunction. In contrast, two pathways, inosine-5'-monophosphate (VIP Score 3.91) and 3-phosphoglyceric acid (VIP Score 3.08) mainly contributed to the LDE signature, with two metabolites (phosphoglyceric acid and inosine-5'-monophosphate) being significantly decreased. When the BF and LDE were compared, the most significant metabolite was phosphoric acid, which was significantly less in the GRMD BF compared to control and GRMD LDE groups.

CONCLUSIONS

The identification of elevated BF oleic acid (a long-chain fatty acid) is consistent with recent microarray studies identifying altered lipid metabolism genes, while alterations in arginine and proline metabolism are consistent with recent studies identifying elevated L-arginine in DMD patient sera as a biomarker of disease. Together, these studies demonstrate muscle-specific alterations in GRMD-affected muscle, which illustrate previously unidentified metabolic changes.