Abnormal cardiac morphology, function and energy metabolism in the dystrophic mdx mouse: an MRI and MRS study

Cyclo His-Pro Attenuates Muscle Degeneration in Murine Myopathy Models

Among the inherited myopathies, a group of muscular disorders characterized by structural and metabolic impairments in skeletal muscle, Duchenne muscular dystrophy (DMD) stands out for its devastating progression. DMD pathogenesis is driven by the progressive degeneration of muscle fibers, resulting in inflammation and fibrosis that ultimately affect the overall muscle biomechanics. At the opposite end of the spectrum of muscle diseases, age-related sarcopenia is a common condition that affects an increasing proportion of the elderly. Although characterized by different pathological mechanisms, DMD and sarcopenia share the development of progressive muscle weakness and tissue inflammation. Here, the therapeutic effects of Cyclo Histidine-Proline (CHP) against DMD and sarcopenia are evaluated. In the mdx mouse model of DMD, it is shown that CHP restored muscle contractility and force production, accompanied by the reduction of fibrosis and inflammation in skeletal muscle. CHP furthermore prevented the development of cardiomyopathy and fibrosis in the diaphragm, the two leading causes of death for DMD patients. CHP also attenuated muscle atrophy and functional deterioration in a mouse model of age-related sarcopenia. These findings from two different models of muscle dysfunction hence warrant further investigation into the effects of CHP on muscle pathologies in animal models and eventually in patients.

Right ventricular preload and afterload challenge induces contractile dysfunction and arrhythmia in isolated hearts of dystrophin-deficient male mice

Duchenne muscular dystrophy (DMD) is an X-linked recessive myopathy due to mutations in the dystrophin gene. Diaphragmatic weakness in DMD causes hypoventilation and elevated afterload on the right ventricle (RV). Thus, RV dysfunction in DMD develops early in disease progression. Herein, we deliver a 30-min sustained RV preload/afterload challenge to isolated hearts of wild-type (Wt) and dystrophic (Dmdmdx-4Cv) mice at both young (2-6 month) and middle-age (8-12 month) to test the hypothesis that the dystrophic RV is susceptible to dysfunction with elevated load. Young dystrophic hearts exhibited greater pressure development than wild type under baseline (Langendorff) conditions, but following RV challenge exhibited similar contractile function as wild type. Following the RV challenge, young dystrophic hearts had an increased incidence of premature ventricular contractions (PVCs) compared to wild type. Hearts of middle-aged wild-type and dystrophic mice had similar contractile function during baseline conditions. After RV challenge, hearts of middle-aged dystrophic mice had severe RV dysfunction and arrhythmias, including ventricular tachycardia. Following the RV load challenge, dystrophic hearts had greater lactate dehydrogenase (LDH) release than wild-type mice indicative of damage. Our data indicate age-dependent changes in RV function with load in dystrophin deficiency, highlighting the need to avoid sustained RV load to forestall dysfunction and arrhythmia.

Mechanisms of reduced myocardial energetics of the dystrophic heart

Heart disease is a leading cause of death in patients with Duchenne muscular dystrophy (DMD), characterized by the progressive replacement of contractile tissue with scar tissue. Effective therapies for dystrophic cardiomyopathy will require addressing the disease before the onset of fibrosis, however, the mechanisms of the early disease are poorly understood. To understand the pathophysiology of DMD, we perform a detailed functional assessment of cardiac function of the mdx mouse, a model of DMD. These studies use a combination of functional, metabolomic, and spectroscopic approaches to fully characterize the contractile, energetic, and mitochondrial function of beating hearts. Through these innovative approaches, we demonstrate that the dystrophic heart has reduced cardiac reserve and is energetically limited. We show that this limitation does not result from poor delivery of oxygen. Using spectroscopic approaches, we provide evidence that mitochondria in the dystrophic heart have attenuated mitochondrial membrane potential and deficits in the flow of electrons in complex IV of the electron transport chain. These studies provide evidence that poor myocardial energetics precede the onset of significant cardiac fibrosis and likely results from mitochondrial dysfunction centered around complex IV and reduced membrane potential. The multimodal approach used here implicates specific molecular components in the etiology of reduced energetics. Future studies focused on these targets may provide therapies that improve the energetics of the dystrophic heart leading to improved resiliency against damage and preservation of myocardial contractile tissue.NEW & NOTEWORTHY Dystrophic hearts have poor contractile reserve that is associated with a reduction in myocardial energetics. We demonstrate that oxygen delivery does not contribute to the limited energy production of the dystrophic heart even with increased workloads. Cytochrome optical spectroscopy of the contracting heart reveals alterations in complex IV and evidence of depolarized mitochondrial membranes. We show specific alterations in the electron transport chain of the dystrophic heart that may contribute to poor myocardial energetics.

Receptor interacting protein kinase-3 mediates both myopathy and cardiomyopathy in preclinical animal models of Duchenne muscular dystrophy

BACKGROUND

Duchenne muscular dystrophy (DMD) is a progressive muscle degenerative disorder, culminating in a complete loss of ambulation, hypertrophic cardiomyopathy and a fatal cardiorespiratory failure. Necroptosis is the form of necrosis that is dependent upon the receptor-interacting protein kinase (RIPK) 3; it is involved in several inflammatory and neurodegenerative conditions. We previously identified RIPK3 as a key player in the acute myonecrosis affecting the hindlimb muscles of the mdx dystrophic mouse model. Whether necroptosis also mediates respiratory and heart disorders in DMD is currently unknown.

METHODS

Evidence of activation of the necroptotic axis was examined in dystrophic tissues from Golden retriever muscular dystrophy (GRMD) dogs and R-DMDdel52 rats. A functional assessment of the involvement of necroptosis in dystrophic animals was performed on mdx mice that were genetically depleted for RIPK3. Dystrophic mice aged from 12 to 18 months were analysed by histology and molecular biology to compare the phenotype of muscles from mdxRipk3+/+ and mdxRipk3-/- mice. Heart function was also examined by echocardiography in 40-week-old mice.

RESULTS

RIPK3 expression in sartorius and biceps femoris muscles from GRMD dogs positively correlated to myonecrosis levels (r = 0.81; P = 0.0076). RIPK3 was also found elevated in the diaphragm (P ≤ 0.05). In the slow-progressing heart phenotype of GRMD dogs, the phosphorylated form of RIPK1 at the Serine 161 site was dramatically increased in cardiomyocytes. A similar p-RIPK1 upregulation characterized the cardiomyocytes of the severe DMDdel52 rat model, associated with a marked overexpression of Ripk1 (P = 0.007) and Ripk3 (P = 0.008), indicating primed activation of the necroptotic pathway in the dystrophic heart. MdxRipk3-/- mice displayed decreased compensatory hypertrophy of the heart (P = 0.014), and echocardiography showed a 19% increase in the relative wall thickness (P < 0.05) and 29% reduction in the left ventricle mass (P = 0.0144). Besides, mdxRipk3-/- mice presented no evidence of a regenerative default or sarcopenia in skeletal muscles, moreover around 50% less affected by fibrosis (P < 0.05).

CONCLUSIONS

Our data highlight molecular and histological evidence that the necroptotic pathway is activated in degenerative tissues from dystrophic animal models, including the diaphragm and the heart. We also provide the genetic proof of concept that selective inhibition of necroptosis in dystrophic condition improves both histological features of muscles and cardiac function, suggesting that prevention of necroptosis is susceptible to providing multiorgan beneficial effects for DMD.

RANKL Inhibition Reduces Cardiac Hypertrophy in mdx Mice and Possibly in Children with Duchenne Muscular Dystrophy

Cardiomyopathy has become one of the leading causes of death in patients with Duchenne muscular dystrophy (DMD). We recently reported that the inhibition of the interaction between the receptor activator of nuclear factor κB ligand (RANKL) and receptor activator of nuclear factor κB (RANK) significantly improves muscle and bone functions in dystrophin-deficient mdx mice. RANKL and RANK are also expressed in cardiac muscle. Here, we investigate whether anti-RANKL treatment prevents cardiac hypertrophy and dysfunction in dystrophic mdx mice. Anti-RANKL treatment significantly reduced LV hypertrophy and heart mass, and maintained cardiac function in mdx mice. Anti-RANKL treatment also inhibited NFκB and PI3K, two mediators implicated in cardiac hypertrophy. Furthermore, anti-RANKL treatment increased SERCA activity and the expression of RyR, FKBP12, and SERCA2a, leading possibly to an improved Ca2+ homeostasis in dystrophic hearts. Interestingly, preliminary post hoc analyses suggest that denosumab, a human anti-RANKL, reduced left ventricular hypertrophy in two patients with DMD. Taken together, our results indicate that anti-RANKL treatment prevents the worsening of cardiac hypertrophy in mdx mice and could potentially maintain cardiac function in teenage or adult patients with DMD.

Evaluation of Cardiac Function in Young Mdx Mice Using MRI with Feature Tracking and Self-Gated Magnetic Resonance Cine Imaging

This study aimed to evaluate cardiac function in a young mouse model of Duchenne muscular dystrophy (mdx) using cardiac magnetic resonance imaging (MRI) with feature tracking and self-gated magnetic resonance cine imaging. Cardiac function was evaluated in mdx and control mice (C57BL/6JJmsSlc mice) at 8 and 12 weeks of age. Preclinical 7-T MRI was used to capture short-axis, longitudinal two-chamber view and longitudinal four-chamber view cine images of mdx and control mice. Strain values were measured and evaluated from cine images acquired using the feature tracking method. The left ventricular ejection fraction was significantly less (p < 0.01 each) in the mdx group at both 8 (control, 56.6 ± 2.3% mdx, 47.2 ± 7.4%) and 12 weeks (control, 53.9 ± 3.3% mdx, 44.1 ± 2.7%). In the strain analysis, all strain value peaks were significantly less in mdx mice, except for the longitudinal strain of the four-chamber view at both 8 and 12 weeks of age. Strain analysis with feature tracking and self-gated magnetic resonance cine imaging is useful for assessing cardiac function in young mdx mice.

Cardiac 31P MR spectroscopy: development of the past five decades and future vision-will it be of diagnostic use in clinics?

In the past five decades, the use of the magnetic resonance (MR) technique for cardiovascular diseases has engendered much attention and raised the opportunity that the technique could be useful for clinical applications. MR has two arrows in its quiver: One is magnetic resonance imaging (MRI), and the other is magnetic resonance spectroscopy (MRS). Non-invasively, highly advanced MRI provides unique and profound information about the anatomical changes of the heart. Excellently developed MRS provides irreplaceable and insightful evidence of the real-time biochemistry of cardiac metabolism of underpinning diseases. Compared to MRI, which has already been successfully applied in routine clinical practice, MRS still has a long way to travel to be incorporated into routine diagnostics. Considering the exceptional potential of ³¹P MRS to measure the real-time metabolic changes of energetic molecules qualitatively and quantitatively, how far its powerful technique should be waited before a successful transition from "bench-to-bedside" is enticing. The present review highlights the seminal studies on the chronological development of cardiac ³¹P MRS in the past five decades and the future vision and challenges to incorporating it for routine diagnostics of cardiovascular disease.

Effect of Large-Conductance Calcium-Dependent K+ Channel Activator NS1619 on Function of Mitochondria in the Heart of Dystrophin-Deficient Mice

Dystrophin-deficient muscular dystrophy (Duchenne dystrophy) is characterized by impaired ion homeostasis, in which mitochondria play an important role. In the present work, using a model of dystrophin-deficient mdx mice, we revealed decrease in the efficiency of potassium ion transport and total content of this ion in the heart mitochondria. We evaluated the effect of chronic administration of the benzimidazole derivative NS1619, which is an activator of the large-conductance Ca2+-dependent K+ channel (mitoBK_Ca), on the structure and function of organelles and the state of the heart muscle. It was shown that NS1619 improves K+ transport and increases content of the ion in the heart mitochondria of mdx mice, but this is not associated with the changes in the level of mitoBK_Ca protein and expression of the gene encoding this protein. The effect of NS1619 was accompanied by the decrease in the intensity of oxidative stress, assessed by the level of lipid peroxidation products (MDA products), and normalization of the mitochondrial ultrastructure in the heart of mdx mice. In addition, we found positive changes in the tissue manifested by the decrease in the level of fibrosis in the heart of dystrophin-deficient animals treated with NS1619. It was noted that NS1619 had no significant effect on the structure and function of heart mitochondria in the wild-type animals. The paper discusses mechanisms of influence of NS1619 on the function of mouse heart mitochondria in Duchenne muscular dystrophy and prospects for applying this approach to correct pathology.

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.

Givinostat as metabolic enhancer reverting mitochondrial biogenesis deficit in Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a rare disorder characterized by progressive muscle wasting, weakness, and premature death. Remarkable progress has been made in genetic approaches, restoring dystrophin, or its function. However, the targeting of secondary pathological mechanisms, such as increasing muscle blood flow or stopping fibrosis, remains important to improve the therapeutic benefits, that depend on tackling both the genetic disease and the downstream consequences. Mitochondrial dysfunctions are one of the earliest deficits in DMD, arise from multiple cellular stressors and result in less than 50% of ATP content in dystrophic muscles. Here we establish that there are two temporally distinct phases of mitochondrial damage with depletion of mitochondrial mass at early stages and an accumulation of dysfunctional mitochondria at later stages, leading to a different oxidative fibers pattern, in young and adult mdx mice. We also observe a progressive mitochondrial biogenesis impairment associated with increased deacetylation of peroxisome proliferator-activated receptor-gamma coactivator 1 α (PGC-1α) promoter. Such histone deacetylation is inhibited by givinostat that positively modifies the epigenetic profile of PGC-1α promoter, sustaining mitochondrial biogenesis and oxidative fiber type switch. We, therefore, demonstrate that givinostat exerts relevant effects at mitochondrial level, acting as a metabolic remodeling agent capable of efficiently promoting mitochondrial biogenesis in dystrophic muscle.

Cardioprotective Effect of Whole Body Periodic Acceleration in Dystrophic Phenotype mdx Rodent

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle wasting and the development of a dilated cardiomyopathy (DCM), which is the leading cause of death in DMD patients. Despite knowing the cause of DMD, there are currently no therapies which can prevent or reverse its inevitable progression. We have used whole body periodic acceleration (WBPA) as a novel tool to enhance intracellular constitutive nitric oxide (NO) production. WBPA adds small pulses to the circulation to increase pulsatile shear stress, thereby upregulating endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) and subsequently elevating the production of NO. Myocardial cells from dystrophin-deficient 15-month old mdx mice have contractile deficiency, which is associated with elevated concentrations of diastolic Ca²⁺ ([Ca²⁺]_d), Na⁺ ([Na⁺]_d), and reactive oxygen species (ROS), increased cell injury, and decreased cell viability. Treating 12-month old mdx mice with WBPA for 3 months reduced cardiomyocyte [Ca²⁺]_d and [Na⁺]_d overload, decreased ROS production, and upregulated expression of the protein utrophin resulting in increased cell viability, reduced cardiomyocyte damage, and improved contractile function compared to untreated mdx mice.

A 1H-NMR approach to myocardial energetics

Understanding the energetic state of the heart is essential for unraveling the central tenets of cardiac physiology. The heart uses a tremendous amount of energy and reductions in that energy supply can have lethal consequences. While ischemic events clearly result in significant metabolic perturbations, heart failure with both preserved and reduced ejection fraction display reductions in energetic status. To date, most cardiac energetics have been performed using ³¹P-NMR, which requires dedicated access to a specialized NMR spectrometer. This has limited the availability of this method to a handful of centers around the world. Here we present a method of assessing myocardial energetics in the isolated mouse heart using ¹H-NMR spectrometers that are widely available in NMR core facilities. In addition, this methodology provides information on many other important metabolites within the heart, including unique metabolic differences between the hypoxic and ischemic hearts. Furthermore, we demonstrate the correlation between myocardial energetics and measures of contractile function in the mouse heart. These methods will allow a broader examination of myocardial energetics providing a valuable tool to aid in the understanding of the nature of these energetic deficits and to develop therapies directed at improving myocardial energetics in failing hearts.

Gene Therapy Rescues Cardiac Dysfunction in Duchenne Muscular Dystrophy Mice by Elevating Cardiomyocyte Deoxy-Adenosine Triphosphate

Mutations in the gene encoding for dystrophin leads to structural and functional deterioration of cardiomyocytes and is a hallmark of cardiomyopathy in Duchenne muscular dystrophy (DMD) patients. Administration of recombinant adeno-associated viral vectors delivering microdystrophin or ribonucleotide reductase (RNR), under muscle-specific regulatory control, rescues both baseline and high workload-challenged hearts in an aged, DMD mouse model. However, only RNR treatments improved both systolic and diastolic function under those conditions. Cardiac-specific recombinant adeno-associated viral treatment of RNR holds therapeutic promise for improvement of cardiomyopathy in DMD patients.

Analysis of skeletal-muscle condition after excessive loading of the lower legs by sequential magnetic resonance imaging

OBJECTIVES

To determine the changes of muscle conditions of lower leg after stretch shortening cycle exercises by MRI.

SUBJECTS AND METHODS

This study included 20 healthy male adult volunteers. After conducting control MRI, each subject performed 3 sets of exercise loading, each set consisting of 100 repetitions of rebound jumping on one foot. MRIs were performed immediately after exercise loading (0 h), 6, 30, and 168 h later. After constructing T1/T2/Fractional Anisotropy (FA)/Apparent Diffusion Coefficient (ADC) maps, the changes of three skeletal muscles of the leg (the tibialis anterior [TA], soleus [SOL], and gastrocnemius [GA]) were quantitatively evaluated in each map at each time point.

RESULTS

The T1 and T2 values were prolonged after exercise loading, and there was a delay in the recovery of T1 at 6 and 30 h after exercise loading, as compared to those of T2 values over time. The ADC values were elevated in all three muscles immediately after exercise loading, then recovered more slowly than T1 and T2, and still had not returned to baseline 168 h after exercise loading. The FA value decreased in all three muscles after exercise loading, with the greatest decrease occurring immediately after exercise loading. As with the ADC values, the FA values were slow to recover from the decrease, and had not returned to baseline levels 168 h post-loading.

CONCLUSION

The delay of T1 value recovery suggested that the T1 value may reflect the muscle condition like fatigue and damage. Changes in the ADC and FA values over time suggested that structural changes such as minute muscular injuries can be detected by diffusion-weighted MRI. Meanwhile, the changes observed in the T1 and T2 values suggested that the measured relaxation time data reflected not only the water volume in the muscle, but also the muscle condition after exercise loading.

Cardiac Pathophysiology and the Future of Cardiac Therapies in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a devastating disease featuring skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. Historically, respiratory failure has been the leading cause of mortality in DMD, but recent improvements in symptomatic respiratory management have extended the life expectancy of DMD patients. With increased longevity, the clinical relevance of heart disease in DMD is growing, as virtually all DMD patients over 18 year of age display signs of cardiomyopathy. This review will focus on the pathophysiological basis of DMD in the heart and discuss the therapeutic approaches currently in use and those in development to treat dystrophic cardiomyopathy. The first section will describe the aspects of the DMD that result in the loss of cardiac tissue and accumulation of fibrosis. The second section will discuss cardiac small molecule therapies currently used to treat heart disease in DMD, with a focus on the evidence supporting the use of each drug in dystrophic patients. The final section will outline the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, or repair. There are several new and promising therapeutic approaches that may protect the dystrophic heart, but their limitations suggest that future management of dystrophic cardiomyopathy may benefit from combining gene-targeted therapies with small molecule therapies. Understanding the mechanistic basis of dystrophic heart disease and the effects of current and emerging therapies will be critical for their success in the treatment of patients with DMD.

Is Exercise the Right Medicine for Dystrophic Muscle?

INTRODUCTION

Duchenne muscular dystrophy (DMD) is a neuromuscular disease caused by a dystrophin protein deficiency. Dystrophin functions to stabilize and protect the muscle fiber during muscle contraction; thus, the absence of functional dystrophin protein leads to muscle injury. DMD patients experience progressive muscle necrosis, loss of function, and ultimately succumb to respiratory failure or cardiomyopathy. Exercise is known to improve muscle health and strength in healthy individuals as well as positively affect other systems. Because of this, exercise has been investigated as a potential therapeutic approach for DMD.

METHODS

This review aims to provide a concise presentation of the exercise literature with a focus on dystrophin-deficient muscle. Our intent was to identify trends and gaps in knowledge with an appreciation of exercise modality.

RESULTS

After compiling data from mouse and human studies, it became apparent that endurance exercises such as a swimming and voluntary wheel running have therapeutic potential in limb muscles of mice and respiratory training was beneficial in humans. However, in the comparatively few long-term investigations, the effect of low-intensity training on cardiac and respiratory muscles was contradictory. In addition, the effect of exercise on other systems is largely unknown.

CONCLUSIONS

To safely prescribe exercise as a therapy to DMD patients, multisystemic investigations are needed including the evaluation of respiratory and cardiac muscle.

Voluntary exercise improves muscle function and does not exacerbate muscle and heart pathology in aged Duchenne muscular dystrophy mice

Duchenne muscular dystrophy is a severe muscle wasting disease, characterized by a severely reduced lifespan in which cardiomyopathy is one of the leading causes of death. Multiple therapies aiming at dystrophin restoration have been approved. It is anticipated that these therapies will maintain muscle function for longer and extend the ambulatory period, which in turn will increase the cardiac workload which could be detrimental for cardiac function. We investigated the effects of voluntary running exercise in combination with low dystrophin levels on function and pathology of skeletal muscle and heart. We divided 15.5-month old female mdx (no dystrophin), mdx-Xist^Δhs (varying low dystrophin levels) and wild type mice (BL10-WT and Xist^Δhs-WT) to either a sedentary or voluntary wheel running regime and assessed muscle function at 17.5 months of age. Thereafter, a cardiac MRI was obtained, and muscle and heart histopathology were assessed. We show that voluntary exercise is beneficial to skeletal muscle and heart function in dystrophic mice while not affecting muscle pathology. Low amounts of dystrophin further improve skeletal muscle and cardiac function. These findings suggest that voluntary exercise may be beneficial for skeletal muscle and heart in DMD patients, especially in conjunction with low amounts of dystrophin.

Data on triiodothyronine treated peroxisome proliferator-activated receptor-alpha-null mouse hearts using magnetic resonance imaging and magnetic resonance spectroscopy

This data contain left ventricular end-diastolic volumes, end-systolic volumes, stroke volumes, ejection fractions, cardiac outputs, heart rates, phosphocreatine concentrations, adenosine 5’-triphosphate (ATP) concentrations, total creatine concentrations, citrate synthase activities and heart weights for wild-type and peroxisome proliferator-activated receptor-alpha-null mouse hearts without and with triiodothyronine treatment.

Hypoxia-induced cardiac injury in dystrophic mice

Duchenne muscular dystrophy (DMD) is a disease of progressive destruction of striated muscle, resulting in muscle weakness with progressive respiratory and cardiac failure. Respiratory and cardiac disease are the leading causes of death in DMD patients. Previous studies have suggested an important link between cardiac dysfunction and hypoxia in the dystrophic heart; these studies aim to understand the mechanism underlying this connection. Here we demonstrate that anesthetized dystrophic mice display significant mortality following acute exposure to hypoxia. This increased mortality is associated with a significant metabolic acidosis, despite having significantly higher levels of arterial Po2 Chronic hypoxia does not result in mortality, but rather is characterized by marked cardiac fibrosis. Studies in isolated hearts reveal that the contractile function of dystrophic hearts is highly susceptible to short bouts of ischemia, but these hearts tolerate prolonged acidosis better than wild-type hearts, indicating an increased sensitivity of the dystrophic heart to hypoxia. Dystrophic hearts display decreased cardiac efficiency and oxygen extraction. Isolated dystrophic cardiomyocytes and hearts have normal levels of FCCP-induced oxygen consumption, and mitochondrial morphology and content are normal in the dystrophic heart. These studies demonstrate reductions in cardiac efficiency and oxygen extraction of the dystrophic heart. The underlying cause of this reduced oxygen extraction is not clear; however, the current studies suggest that large disruptions of mitochondrial respiratory function or coronary flow regulation are not responsible. This finding is significant, as hypoxia is a common and largely preventable component of DMD that may contribute to the progression of the cardiac disease in DMD patients.

The Effects of Experimental Sleep Apnea on Cardiac and Respiratory Functions in 6 and 18 Month Old Dystrophic (mdx) Mice

Duchenne muscular dystrophy (DMD) is a fatal disease where over 90% of patients succumb to respiratory or cardiac failure. Sleep apnea and sleep disordered breathing (SDB) are noted in a plurality of DMD patients, and the resulting nocturnal episodic hypoxia (EH) cannot be ruled out as a contributing factor to cardiac and respiratory dysfunction. In this study, we investigated the impact of long-term episodic hypoxia, which mimics the cyclic hypoxia seen in sleep apnea, on cardiac and respiratory function in a murine model of DMD (mdx mice). Since the severity and prevalence of sleep apnea in DMD increases with age, we studied the impact of EH on young (6-month) and on older (18-month) mdx mice. Mice were either exposed for 12 weeks to EH (8 hours/day, 5 days/week) or to room air. We noted a significant increase in left ventricular (LV) dilatation (transthoracic echocardiography) on EH exposure in both age groups, but reduced LV contractility was seen only in 6-month old mice. With EH exposure, an increased fibrosis (hydroxyproline) was noted in both cardiac and diaphragm muscle in 18-month but not 6-month old mice. No significant change in relative diaphragm strength (in-vitro) was noted on EH exposure in 18-month old mice. In contrast, EH exposed 6-month old mice showed a significant increase in relative diaphragm strength. EH exposure did not result in any significant change in ventilatory parameters (barometric plethysmography) in awake 6-month old mdx mice. In contrast, 18-month old mdx mice showed considerable ventilatory dysfunction, consistent with reduced ventilatory reserve. Our findings highlight that sleep apnea impacts respiratory and cardiac function in muscular dystrophy, and that EH can have divergent effects on both systems. To our knowledge, this is the first comprehensive study to investigate the impact of EH on cardiac and respiratory function in mdx mice.

Crosstalk between RyR2 oxidation and phosphorylation contributes to cardiac dysfunction in mice with Duchenne muscular dystrophy

BACKGROUND

Patients with Duchenne muscular dystrophy (DMD) are at risk of developing cardiomyopathy and cardiac arrhythmias. Studies in a mouse model of DMD revealed that enhanced sarcoplasmic reticulum (SR) Ca(2+) leak contributes to the pathogenesis of cardiac dysfunction. In view of recent data suggesting the involvement of altered phosphorylation and oxidation of the cardiac ryanodine receptor (RyR2)/Ca(2+) release channel, we hypothesized that inhibition of RyR2 phosphorylation in a mouse model of DMD can prevent SR Ca(2+) leak by reducing RyR2 oxidation.

METHODS AND RESULTS

Confocal Ca(2+) imaging and single RyR2 channel recordings revealed that both inhibition of S2808 or S2814 phosphorylation, and inhibition of oxidation could normalize RyR2 activity in mdx mice. Moreover, Western blotting revealed that genetic inhibition of RyR2 phosphorylation at S2808 or S2814 reduced RyR2 oxidation. Production of reactive oxygen species (ROS) in myocytes from mdx mice was reduced by both inhibition of RyR2 phosphorylation or the ROS scavenger 2-mercaptopropionyl glycine (MPG). Finally, it was shown that ROS production in mdx mice is proportional to the activity of RyR2-mediated SR Ca(2+) leak, and likely generated by Nox2.

CONCLUSIONS

Increased ROS production in the hearts of mdx mice drives the progression of cardiac dysfunction. Inhibition of RyR2 phosphorylation can suppress SR Ca(2+) leak in mdx mouse hearts in part by reducing RyR2 oxidation.

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.

Early right ventricular fibrosis and reduction in biventricular cardiac reserve in the dystrophin-deficient mdx heart

Duchenne muscular dystrophy (DMD) is a progressive disease of striated muscle deterioration. Respiratory and cardiac muscle dysfunction are particularly clinically relevant because they result in the leading causes of death in DMD patients. Despite the clinical and physiological significance of these systems, little has been done to understand the cardiorespiratory interaction in DMD. We show here that prior to the onset of global cardiac dysfunction, dystrophin-deficient mdx mice have increased cardiac fibrosis with the right ventricle being particularly affected. Using a novel biventricular cardiac catheterization technique coupled with cardiac stress testing, we demonstrate that both the right and left ventricles have significant reductions in both systolic and diastolic function in response to dobutamine. Unstimulated cardiac function is relatively normal except for a significant reduction in the ventricular pressure transient duration compared with controls. These biventricular analyses also reveal the absence of a dobutamine-induced increase in isovolumic relaxation in the right ventricle of control hearts. Simultaneous assessment of biventricular pressure demonstrates a dobutamine-dependent enhancement of coupling between the ventricles in control mice, which is absent in mdx mice. Furthermore, studies probing the passive-extension properties of the left ventricle demonstrate that the mdx heart is significantly more compliant compared with age-matched C57BL/10 hearts, which have an age-dependent stiffening that is completely absent from dystrophic hearts. These new results indicate that right ventricular fibrosis is an early indicator of the development of dystrophic cardiomyopathy, suggesting a mechanism by which respiratory insufficiency may accelerate the development of heart failure in DMD.

Non-invasive MRI and spectroscopy of mdx mice reveal temporal changes in dystrophic muscle imaging and in energy deficits

In Duchenne muscular dystrophy (DMD), a genetic disruption of dystrophin protein expression results in repeated muscle injury and chronic inflammation. Magnetic resonance imaging shows promise as a surrogate outcome measure in both DMD and rehabilitation medicine that is capable of predicting clinical benefit years in advance of functional outcome measures. The mdx mouse reproduces the dystrophin deficiency that causes DMD and is routinely used for preclinical drug testing. There is a need to develop sensitive, non-invasive outcome measures in the mdx model that can be readily translatable to human clinical trials. Here we report the use of magnetic resonance imaging and spectroscopy techniques for the non-invasive monitoring of muscle damage in mdx mice. Using these techniques, we studied dystrophic mdx muscle in mice from 6 to 12 weeks of age, examining both the peak disease phase and natural recovery phase of the mdx disease course. T2 and fat-suppressed imaging revealed significant levels of tissue with elevated signal intensity in mdx hindlimb muscles at all ages; spectroscopy revealed a significant deficiency of energy metabolites in 6-week-old mdx mice. As the mdx mice progressed from the peak disease stage to the recovery stage of disease, each of these phenotypes was either eliminated or reduced, and the cross-sectional area of the mdx muscle was significantly increased when compared to that of wild-type mice. Histology indicates that hyper-intense MRI foci correspond to areas of dystrophic lesions containing inflammation as well as regenerating, degenerating and hypertrophied myofibers. Statistical sample size calculations provide several robust measures with the ability to detect intervention effects using small numbers of animals. These data establish a framework for further imaging or preclinical studies, and they support the development of MRI as a sensitive, non-invasive outcome measure for muscular dystrophy.

NADPH oxidase-2 inhibition restores contractility and intracellular calcium handling and reduces arrhythmogenicity in dystrophic cardiomyopathy

Duchenne muscular dystrophy may affect cardiac muscle, producing a dystrophic cardiomyopathy in humans and the mdx mouse. We tested the hypothesis that oxidative stress participates in disrupting calcium handling and contractility in the mdx mouse with established cardiomyopathy. We found increased expression (fivefold) of the NADPH oxidase (NOX) 2 in the mdx hearts compared with wild type, along with increased superoxide production. Next, we tested the impact of NOX2 inhibition on contractility and calcium handling in isolated cardiomyocytes. Contractility was decreased in mdx myocytes compared with wild type, and this was restored toward normal by pretreating with apocynin. In addition, the amplitude of evoked intracellular Ca(2+) concentration transients that was diminished in mdx myocytes was also restored with NOX2 inhibition. Total sarcoplasmic reticulum (SR) Ca(2+) content was reduced in mdx hearts and normalized by apocynin treatment. Additionally, NOX2 inhibition decreased the production of spontaneous diastolic calcium release events and decreased the SR calcium leak in mdx myocytes. In addition, nitric oxide (NO) synthase 1 (NOS-1) expression was increased eightfold in mdx hearts compared with wild type. Nevertheless, cardiac NO production was reduced. To test whether this paradox implied NOS-1 uncoupling, we treated cardiac myocytes with exogenous tetrahydrobioterin, along with the NOX inhibitor VAS2870. These agents restored NO production and phospholamban phosphorylation in mdx toward normal. Together, these results demonstrate that, in mdx hearts, NOX2 inhibition improves the SR calcium handling and contractility, partially by recoupling NOS-1. These findings reveal a new layer of nitroso-redox imbalance in dystrophic cardiomyopathy.

Proteomic profiling of the dystrophin-deficient mdx phenocopy of dystrophinopathy-associated cardiomyopathy

Cardiorespiratory complications are frequent symptoms of Duchenne muscular dystrophy, a neuromuscular disorder caused by primary abnormalities in the dystrophin gene. Loss of cardiac dystrophin initially leads to changes in dystrophin-associated glycoproteins and subsequently triggers secondarily sarcolemmal disintegration, fibre necrosis, fibrosis, fatty tissue replacement, and interstitial inflammation. This results in progressive cardiac disease, which is the cause of death in a considerable number of patients afflicted with X-linked muscular dystrophy. In order to better define the molecular pathogenesis of this type of cardiomyopathy, several studies have applied mass spectrometry-based proteomics to determine proteome-wide alterations in dystrophinopathy-associated cardiomyopathy. Proteomic studies included both gel-based and label-free mass spectrometric surveys of dystrophin-deficient heart muscle from the established mdx animal model of dystrophinopathy. Comparative cardiac proteomics revealed novel changes in proteins associated with mitochondrial energy metabolism, glycolysis, signaling, iron binding, antibody response, fibre contraction, basal lamina stabilisation, and cytoskeletal organisation. This review summarizes the importance of studying cardiomyopathy within the field of muscular dystrophy research, outlines key features of the mdx heart and its suitability as a model system for studying cardiac pathogenesis, and discusses the impact of recent proteomic findings for exploring molecular and cellular aspects of cardiac abnormalities in inherited muscular dystrophies.

Ultrastructural and functional alterations of EC coupling elements in mdx cardiomyocytes: an analysis from membrane surface to depth

A dilated cardiomyopathy (DCM) is associated with Duchenne muscular dystrophy (DMD). The loss of dystrophin leads to membrane instability and calcium dysregulation in skeletal muscle but effects of such a loss are not elucidated at cardiomyocytes level. We sought to examine whether membrane and transverse tubules damages occur in ventricular myocytes from mdx mouse model of DMD and how they impact the function of single excitation-contraction coupling elements. Scanning ion conductance microscopy (SICM) was used to characterize the integrity loss of living mdx cardiomyocytes surface. 2D Fourier transform analysis of labeled internal networks (transverse tubules, alpha-actinin, dihydropyridine receptors, ryanodine receptors) was performed to evaluate internal alterations. During calcium measurements, "smart microperfusions" of depolarizing solutions were applied through SICM nanopipette, stimulating single tubules elements. These approaches revealed structural membrane surface (39% decrease for Z-groove ratio) and transverse tubules disorganization (21% transverse tubules ratio decrease) in mdx as compared to control. These disruptions were associated with functional alterations (sixfold increase of calcium signal duration and twofold increase of sparks frequency). In DCM associated with DMD, myocytes display evident membrane alterations at the surface level but also in the cell depth with a disruption of transverse tubules network as observed in other cases of heart failure. These ultrastructural changes are associated with changes in the function of some coupling elements. Thus, these profound disruptions may play a role in calcium dysregulation through excitation-contraction coupling elements perturbation and suggest a transverse tubules stabilizing role for dystrophin.

Murine cardiosphere-derived cells are impaired by age but not by cardiac dystrophic dysfunction

To be clinically relevant as a therapy for heart failure, endogenous progenitor cells must be isolated and expanded from aged and/or diseased tissue. Here, we investigated the effect of age and cardiac impairment resulting from lack of dystrophin on murine cardiosphere-derived cells (CDCs). CDCs were isolated and expanded from atrial biopsies from wild-type mice aged 1.5, 6, 18, and 24 months and from mdx mice aged 6 and 18 months. Cardiac function was measured in mdx mice and age-matched wild-type mice using high-resolution cine magnetic resonance imaging. CDCs could be isolated and expanded from all mice, however, the number of cells obtained, and their regenerative potential, decreased with age, as demonstrated by decreased expression of stem cell markers, c-kit and Sca-1, and decreased cell proliferation, migration, clonogenicity, and differentiation. Six-month-old mdx mice showed right ventricular (RV) dilation and reduced RV ejection fraction (EF) in comparison to wild-type mice. Older mdx mice displayed significant RV and left ventricular dilation and decreased EF in both ventricles, compared with age-matched wild-type mice. Mdx mouse hearts contained significantly more fibrotic tissue than age-matched wild-type mouse hearts. However, CDCs isolated from mice aged 6 and 18 months had the same number and regenerative potential from mdx mice and age-matched wild-type mice. Thus, the cardiac progenitor cell population is impaired by age but is not substantially altered by the progressive deterioration in function of the dystrophic heart.

Low dystrophin levels in heart can delay heart failure in mdx mice

Duchenne muscular dystrophy is caused by mutations that prevent synthesis of functional dystrophin. All patients develop dilated cardiomyopathy. Promising therapeutic approaches are underway that successfully restore dystrophin expression in skeletal muscle. However, their efficiency in the heart is limited. Improved quality and function of only skeletal muscle potentially accelerate the development of cardiomyopathy. Our study aimed to elucidate which dystrophin levels in the heart are required to prevent or delay cardiomyopathy in mice. Heart function and pathology assessed with magnetic resonance imaging and histopathological analysis were compared between 2, 6 and 10-month-old female mdx-Xist(Δhs) mice, expressing low dystrophin levels (3-15%) in a mosaic manner based on skewed X-inactivation, dystrophin-negative mdx mice, and wild type mice of corresponding genetic backgrounds and gender. With age mdx mice developed dilated cardiomyopathy and hypertrophy, whereas the onset of heart pathology was delayed and function improved in mdx-Xist(Δhs) mice. The ejection fraction, the most severely affected parameter for both ventricles, correlated to dystrophin expression and the percentage of fibrosis. Fibrosis was partly reduced from 9.8% in mdx to 5.4% in 10 month old mdx-Xist(Δhs) mice. These data suggest that mosaic expression of 4-15% dystrophin in the heart is sufficient to delay the onset and ameliorate cardiomyopathy in mice.

Coronary adventitial cells are linked to perivascular cardiac fibrosis via TGFβ1 signaling in the mdx mouse model of Duchenne muscular dystrophy

In Duchenne muscular dystrophy (DMD), progressive accumulation of cardiac fibrosis promotes heart failure. While the cellular origins of fibrosis in DMD hearts remain enigmatic, fibrotic tissue conspicuously forms near the coronary adventitia. Therefore, we sought to characterize the role of coronary adventitial cells in the formation of perivascular fibrosis. Utilizing the mdx model of DMD, we have identified a population of Sca1+, PDGFRα+, CD31-, and CD45- coronary adventitial cells responsible for perivascular fibrosis. Histopathology of dystrophic hearts revealed that Sca1+ cells extend from the adventitia and occupy regions of perivascular fibrosis. The number of Sca1+ adventitial cells increased two-fold in fibrotic mdx hearts vs. age matched wild-type hearts. Moreover, relative to Sca1-, PDGFRα+, CD31-, and CD45- cells and endothelial cells, Sca1+ adventitial cells FACS-sorted from mdx hearts expressed the highest level of Collagen1α1 and 3α1, Connective tissue growth factor, and Tgfβr1 transcripts. Surprisingly, mdx endothelial cells expressed the greatest level of the Tgfβ1 ligand. Utilizing Collagen1α1-GFP reporter mice, we confirmed that the majority of Sca1+ adventitial cells expressed type I collagen, an abundant component of cardiac fibrosis, in both wt (71%±4.1) and mdx (77%±3.5) hearts. In contrast, GFP+ interstitial fibroblasts were PDGFRα+ but negative for Sca1. Treatment of cultured Collagen1α1-GFP+ adventitial cells with TGFβ1 resulted in increased collagen synthesis, whereas pharmacological inhibition of TGFβR1 signaling reduced the fibrotic response. Therefore, perivascular cardiac fibrosis by coronary adventitial cells may be mediated by TGFβ1 signaling. Our results implicate coronary endothelial cells in mediating cardiac fibrosis via transmural TGFβ signaling, and suggest that the coronary adventitia is a promising target for developing novel anti-fibrotic therapies.

Proteomic profiling of cardiomyopathic tissue from the aged mdx model of Duchenne muscular dystrophy reveals a drastic decrease in laminin, nidogen and annexin

The majority of patients afflicted with Duchenne muscular dystrophy develop cardiomyopathic complications, warranting large-scale proteomic studies of global cardiac changes for the identification of new protein markers of dystrophinopathy. The aged heart from the X-linked dystrophic mdx mouse has been shown to exhibit distinct pathological aspects of cardiomyopathy. In order to establish age-related alterations in the proteome of dystrophin-deficient hearts, cardiomyopathic tissue from young versus aged mdx mice was examined by label-free LC-MS/MS. Significant age-dependent alterations were established for 67 proteins, of which 28 proteins were shown to exhibit a lower abundance and 39 proteins were found to be increased in their expression levels. Drastic changes were demonstrated for 17 proteins, including increases in Ig chains and transferrin, and drastic decreases in laminin, nidogen and annexin. An immunblotting survey of young and old wild-type versus mdx hearts confirmed these proteomic findings and illustrated the effects of natural aging versus dystrophin deficiency. These proteome-wide alterations suggest a disintegration of the basal lamina structure and cytoskeletal network in dystrophin-deficient cardiac fibres, increased levels of antibodies in a potential autoimmune reaction of the degenerating heart, compensatory binding of excess iron and a general perturbation of metabolic pathways in dystrophy-associated cardiomyopathy.

Ibuprofen plus isosorbide dinitrate treatment in the mdx mice ameliorates dystrophic heart structure

BACKGROUND

Co-administration of ibuprofen (IBU) and isosorbide dinitrate (ISDN) provides synergistic beneficial effects on dystrophic skeletal muscle. Whether this treatment has also cardioprotective effects in this disease was still unknown.

AIMS

To evaluate the effects of co-administration of IBU and ISDN (a) on left ventricular (LV) structure and function, and (b) on cardiac inflammatory response and fibrosis in mdx mice.

METHODS

Three groups of mice were studied: mdx mice treated with IBU (50 mg kg⁻¹)+ISDN (30 mg kg⁻¹) administered daily in the diet, mdx mice that received standard diet without drugs and wild type aged-matched mice. Animals were analysed after 10-11 months of treatment. Structural and functional parameters were evaluated by echocardiography while histological analyses were performed to evaluate inflammatory response, collagen deposition, cardiomyocyte number and area.

RESULTS

Treatment for 10-11 months with IBU+ISDN preserved LV wall thickness and LV mass. Drug treatment also preserved the total number of cardiomyocytes in the LV and attenuated the increase in cardiomyocyte size, when compared to untreated mdx mice. Moreover, a trend towards a decreased number of inflammatory cells, a reduced LV myocardial interstitial fibrosis and an enhanced global LV function response to stress was observed in treated mdx mice.

CONCLUSIONS

Treatment for 10-11 months with IBU+ISDN is effective in preventing the alterations in LV morphology of mdx mice while not reaching statistical significance on LV function and cardiac inflammation.

Cardiac and respiratory dysfunction in Duchenne muscular dystrophy and the role of second messengers

Duchenne muscular dystrophy (DMD) affects young boys and is characterized by the absence of dystrophin, a large cytoskeletal protein present in skeletal and cardiac muscle cells and neurons. The heart and diaphragm become necrotic in DMD patients and animal models of DMD, resulting in cardiorespiratory failure as the leading cause of death. The major consequences of the absence of dystrophin are high levels of intracellular Ca(2+) and the unbalanced production of NO that can finally trigger protein degradation and cell death. Cytoplasmic increase in Ca(2+) concentration directly and indirectly triggers different processes such as necrosis, fibrosis, and activation of macrophages. The absence of the neuronal isoform of nitric oxide synthase (nNOS) and the overproduction of NO by the inducible isoform (iNOS) further increase the intracellular Ca(2+) via a hypernitrosylation of the ryanodine receptor. NO overproduction, which further induces the expression of iNOS but decreases the expression of the endothelial isoform (eNOS), deregulates the muscle tissue blood flow creating an ischemic situation. The high levels of Ca(2+) in dystrophic muscles and the ischemic state of the muscle tissue would culminate in a positive feedback loop. While efforts continue toward optimizing cardiac and respiratory care of DMD patients, both Ca(2+) and NO in cardiac and respiratory muscle pathways have been shown to be important to the etiology of the disease. Understanding the mechanisms behind the fine regulation of Ca(2+) -NO may be important for a noninterventional and noninvasive supportive approach to treat DMD patients, improving the quality of life and natural history of DMD patients.

Hierarchical accumulation of RyR post-translational modifications drives disease progression in dystrophic cardiomyopathy

AIMS

Duchenne muscular dystrophy (DMD) is a muscle disease with serious cardiac complications. Changes in Ca(2+) homeostasis and oxidative stress were recently associated with cardiac deterioration, but the cellular pathophysiological mechanisms remain elusive. We investigated whether the activity of ryanodine receptor (RyR) Ca(2+) release channels is affected, whether changes in function are cause or consequence and which post-translational modifications drive disease progression.

METHODS AND RESULTS

Electrophysiological, imaging, and biochemical techniques were used to study RyRs in cardiomyocytes from mdx mice, an animal model of DMD. Young mdx mice show no changes in cardiac performance, but do so after ∼8 months. Nevertheless, myocytes from mdx pups exhibited exaggerated Ca(2+) responses to mechanical stress and 'hypersensitive' excitation-contraction coupling, hallmarks of increased RyR Ca(2+) sensitivity. Both were normalized by antioxidants, inhibitors of NAD(P)H oxidase and CaMKII, but not by NO synthases and PKA antagonists. Sarcoplasmic reticulum Ca(2+) load and leak were unchanged in young mdx mice. However, by the age of 4-5 months and in senescence, leak was increased and load was reduced, indicating disease progression. By this age, all pharmacological interventions listed above normalized Ca(2+) signals and corrected changes in ECC, Ca(2+) load, and leak.

CONCLUSION

Our findings suggest that increased RyR Ca(2+) sensitivity precedes and presumably drives the progression of dystrophic cardiomyopathy, with oxidative stress initiating its development. RyR oxidation followed by phosphorylation, first by CaMKII and later by PKA, synergistically contributes to cardiac deterioration.

Cardiac phenotype of Duchenne Muscular Dystrophy: insights from cellular studies

Dilated cardiomyopathy is a serious and almost inevitable complication of Duchenne Muscular Dystrophy, a devastating and fatal disease of skeletal muscle resulting from the lack of functional dystrophin, a protein linking the cytoskeleton to the extracellular matrix. Ultimately, it leads to congestive heart failure and arrhythmias resulting from both cardiac muscle fibrosis and impaired function of the remaining cardiomyocytes. Here we summarize findings obtained in several laboratories, focusing on cellular mechanisms that result in degradation of cardiac functions in dystrophy.

The mdx mouse as a model for carnitine deficiency in the pathogenesis of Duchenne muscular dystrophy

INTRODUCTION

Muscle and cardiac metabolism are dependent on the oxidation of fats and glucose for adenosine triphosphate production, for which L-carnitine is an essential cofactor.

METHODS

We measured muscle carnitine concentrations in skeletal muscles, diaphragm, and ventricles of C57BL/10ScSn-DMDmdx/J mice (n = 10) and compared them with wild-type C57BL/6J (n = 3), C57BL/10 (n = 10), and C3H (n = 12) mice. Citrate synthase (CS) activity was measured in quadriceps/gluteals and ventricles of mdx and wild-type mice.

RESULTS

We found significantly lower tissue carnitine in quadriceps/gluteus (P < 0.05) and ventricle (P < 0.05), but not diaphragm of mdx mice, when compared with controls. CS activity was increased in mdx quadriceps/gluteus (P < 0.03) and ventricle (P < 0.02). This suggests compensatory mitochondrial biogenesis.

CONCLUSIONS

Decreased tissue carnitine has implications for reduced fatty acid and glucose oxidation in mdx quadriceps/gluteus and ventricle. The mdx mouse may be a useful model for studying the role of muscle carnitine deficiency in DMD bioenergetic insufficiency and providing a targeted and timed rationale for L-carnitine therapy.

In vivo MRI characterization of progressive cardiac dysfunction in the mdx mouse model of muscular dystrophy

AIMS

The mdx mouse has proven to be useful in understanding the cardiomyopathy that frequently occurs in muscular dystrophy patients. Here we employed a comprehensive array of clinically relevant in vivo MRI techniques to identify early markers of cardiac dysfunction and follow disease progression in the hearts of mdx mice.

METHODS AND RESULTS

Serial measurements of cardiac morphology and function were made in the same group of mdx mice and controls (housed in a non-SPF facility) using MRI at 1, 3, 6, 9 and 12 months after birth. Left ventricular (LV) and right ventricular (RV) systolic and diastolic function, response to dobutamine stress and myocardial fibrosis were assessed. RV dysfunction preceded LV dysfunction, with RV end systolic volumes increased and RV ejection fractions reduced at 3 months of age. LV ejection fractions were reduced at 12 months, compared with controls. An abnormal response to dobutamine stress was identified in the RV of mdx mice as early as 1 month. Late-gadolinium-enhanced MRI identified increased levels of myocardial fibrosis in 6, 9 and 12-month-old mdx mice, the extent of fibrosis correlating with the degree of cardiac remodeling and hypertrophy.

CONCLUSIONS

MRI could identify cardiac abnormalities in the RV of mdx mice as young as 1 month, and detected myocardial fibrosis at 6 months. We believe these to be the earliest MRI measurements of cardiac function reported for any mice, and the first use of late-gadolinium-enhancement in a mouse model of congenital cardiomyopathy. These techniques offer a sensitive and clinically relevant in vivo method for assessment of cardiomyopathy caused by muscular dystrophy and other diseases.

Assessment of cardiac function in three mouse dystrophinopathies by magnetic resonance imaging

Lack of dystrophin results in skeletal muscle dystrophy and dilated cardiomyopathy in humans and animal models. To achieve a basic understanding of the natural development of cardiomyopathy in different dystrophinopathy mouse models, left and right ventricular heart function was assessed at different ages in three dystrophinopathy mouse models (mdx, mdx/utrn(+/-) model and mdx/utrn(-/-)) using magnetic resonance imaging. Left ventricular function was significantly decreased, already at 2months in the most severely affected mdx/utrn(-/-) mice. Furthermore, whereas heart function was stable in wild-type mice over time, both mdx and mdx/utrn(+/-) showed a clear decrease at 10months of age, most prominently in the right ventricle. Therefore magnetic resonance imaging is an adequate technique to determine heart function in dystrophinopathy mouse models and can be used to assess the effect of potential therapies.

INTERCOMPARISON OF PERFORMANCE OF RF COIL GEOMETRIES FOR HIGH FIELD MOUSE CARDIAC MRI

Multi-turn spiral surface coils are constructed in flat and cylindrical arrangements and used for high field (7.1 T) mouse cardiac MRI. Their electrical and imaging performances, based on experimental measurements, simulations, and MRI experiments in free space, and under phantom, and animal loading conditions, are compared with a commercially available birdcage coil. Results show that the four-turn cylindrical spiral coil exhibits improved relative SNR (rSNR) performance to the flat coil counterpart, and compares fairly well with a commercially available birdcage coil. Phantom experiments indicate a 50% improvement in the SNR for penetration depths ≤ 6.1 mm from the coil surface compared to the birdcage coil, and an increased penetration depth at the half-maximum field response of 8 mm in the 4-spiral cylindrical coil case, in contrast to 2.9 mm in the flat 4-turn spiral case. Quantitative comparison of the performance of the two spiral coil geometries in anterior, lateral, inferior, and septal regions of the murine heart yield maximum mean percentage rSNR increases of the order of 27-167% in vivo post-mortem (cylindrical compared to flat coil). The commercially available birdcage outperforms the cylindrical spiral coil in rSNR by a factor of 3-5 times. The comprehensive approach and methodology adopted to accurately design, simulate, implement, and test radiofrequency coils of any geometry and type, under any loading conditions, can be generalized for any application of high field mouse cardiac MRI.

Proteomic assessment of the acute phase of dystrophin deficiency in mdx mice

Duchenne muscular dystrophy (DMD) is caused by the absence of a functional dystrophin protein and is modeled by the mdx mouse. The mdx mouse suffers an early necrotic bout in the hind limb muscles lasting from approximately 4 to 7 weeks. The purpose of this investigation was to determine the extent to which dystrophin deficiency changed the proteome very early in the disease process. In order to accomplish this, proteins from gastrocnemius from 6-week-old C57 (n = 6) and mdx (n = 6) mice were labeled with fluorescent dye and subjected to two-dimensional differential in-gel electrophoresis (2D-DIGE). Resulting differentially expressed spots were excised and protein identity determined via MALDI-TOF followed by database searching using MASCOT. Proteins of the immediate energy system and glycolysis were generally down-regulated in mdx mice compared to C57 mice. Conversely, expression of proteins involved in the Kreb's cycle and electron transport chain were increased in dystrophin-deficient muscle compared to control. Expression of cytoskeletal components, including tubulins, vimentin, and collagen, were increased in mdx mice compared to C57 mice. Importantly, these changes are occurring at only 6 weeks of age and are caused by acute dystrophin deficiency rather than more chronic injury. These data may provide insight regarding early pathologic changes occurring in dystrophin-deficient skeletal muscle.

Long-term blocking of calcium channels in mdx mice results in differential effects on heart and skeletal muscle

The disease mechanisms underlying dystrophin-deficient muscular dystrophy are complex, involving not only muscle membrane fragility, but also dysregulated calcium homeostasis. Specifically, it has been proposed that calcium channels directly initiate a cascade of pathological events by allowing calcium ions to enter the cell. The objective of this study was to investigate the effect of chronically blocking calcium channels with the aminoglycoside antibiotic streptomycin from onset of disease in the mdx mouse model of Duchenne muscular dystrophy (DMD). Treatment in utero onwards delayed onset of dystrophic symptoms in the limb muscle of young mdx mice, but did not prevent degeneration and regeneration events occurring later in the disease course. Long-term treatment had a positive effect on limb muscle pathology, reduced fibrosis, increased sarcolemmal stability, and promoted muscle regeneration in older mice. However, streptomycin treatment did not show positive effects in diaphragm or heart muscle, and heart pathology was worsened. Thus, blocking calcium channels even before disease onset does not prevent dystrophy, making this an unlikely treatment for DMD. These findings highlight the importance of analyzing several time points throughout the life of the treated mice, as well as analyzing many tissues, to get a complete picture of treatment efficacy.

Diaphragm rescue alone prevents heart dysfunction in dystrophic mice

Duchenne muscular dystrophy (DMD) is an X-linked recessive disease caused, in most cases, by the complete absence of the 427 kDa cytoskeletal protein, dystrophin. There is no effective treatment, and affected individuals die from respiratory failure and cardiomyopathy by age 30. Here, we investigated whether cardiomyopathy could be prevented in animal models of DMD by increasing diaphragm utrophin or dystrophin expression and thereby restoring diaphragm function. In a transgenic mdx mouse, where utrophin was over expressed in the skeletal muscle and the diaphragm, but not in the heart, we found cardiac function, specifically right and left ventricular ejection fraction as measured using in vivo magnetic resonance imaging, was restored to wild-type levels. In mdx mice treated with a peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO) that resulted in high levels of dystrophin restoration in the skeletal muscle and the diaphragm only, cardiac function was also restored to wild-type levels. In dystrophin/utrophin-deficient double-knockout (dKO) mice, a more severely affected animal model of DMD, treatment with a PPMO again produced high levels of dystrophin only in the skeletal muscle and the diaphragm, and once more restored cardiac function to wild-type levels. In the dKO mouse, there was no difference in heart function between treatment of the diaphragm plus the heart and treatment of the diaphragm alone. Restoration of diaphragm and other respiratory muscle function, irrespective of the method used, was sufficient to prevent cardiomyopathy in dystrophic mice. This novel mechanism of treating respiratory muscles to prevent cardiomyopathy in dystrophic mice warrants further investigation for its implications on the need to directly treat the heart in DMD.

Timeline of cardiac dystrophy in 3-18-month-old MDX mice

The dystrophin-deficient (mdx) mouse remains the most commonly used model for Duchenne muscular dystrophy (DMD). Mdx mice show a predominantly covert cardiomyopathy, the hallmark of which is fibrosis. We compared mdx and normal mice at six ages (3, 6, 9, 12, 15, and 18 months) using in vivo assessment of cardiac function, selective collagen staining, and measures of TGF-β mRNA, Evans blue dye infiltration, macrophage infiltration, and aortic wall thickness. Clear temporal progression was demonstrated, including early fragility of cardiomyocyte membranes, which has an unrelated impact on cardiac function but is associated with macrophage infiltration and fibrosis. Aortic wall thickness is less in older mdx mice. Mdx mice display impaired responses to inotropic challenge from a young age; this is indicative of altered adrenoreceptor function. We draw attention to the paradox of ongoing fibrosis in mdx hearts without a strong molecular signature (in the form of TGF-β mRNA expression).

Magnetic resonance imaging assessment of cardiac dysfunction in δ-sarcoglycan null mice

Delta-sarcoglycan (δ-sarcoglycan) null, Scgd(-/-), mice develop cardiac and skeletal muscle histopathological alterations similar to those in humans with limb-girdle muscular dystrophy. The objective of this study was to assess the feasibility of using MRI to investigate cardiac dysfunction in Scgd(-/-) mice. Cardiac MRI of 8 month old Scgd(-/-) and wild type (WT) mice was performed. Compared to WT, Scgd(-/-) mice had significantly lower LV ejection fraction (44±5% vs. 66±4%, p=0.014), lower RV ejection fraction (25±2% vs. 51±3%, p<0.001) lower myocardial circumferential strain, (15.0±0.3% vs. 16.9±0.3%, p=0.007) and RV dilatation (54±3 μL vs. 40±3 μL, p=0.007). The regional circumferential strain also demonstrated significant temporal dyssynchrony between opposing regions of the Scgd(-/-) LV. Our results demonstrate severe cardiac dysfunction in Scgd(-/-) mice at 8 months. The study identifies a set of non-invasive markers that could be used to study efficacy of novel therapeutic agents in dystrophic mice.

Transcriptomic analysis of dystrophin RNAi knockdown reveals a central role for dystrophin in muscle differentiation and contractile apparatus organization

BACKGROUND

Duchenne muscular dystrophy (DMD) is a fatal muscle wasting disorder caused by mutations in the dystrophin gene. DMD has a complex and as yet incompletely defined molecular pathophysiology hindering development of effective ameliorative approaches. Transcriptomic studies so far conducted on dystrophic cells and tissues suffer from non-specific changes and background noise due to heterogeneous comparisons and secondary pathologies. A study design in which a perfectly matched control cell population is used as reference for transcriptomic studies will give a much more specific insight into the effects of dystrophin deficiency and DMD pathophysiology.

RESULTS

Using RNA interference (RNAi) to knock down dystrophin in myotubes from C57BL10 mice, we created a homogenous model to study the transcriptome of dystrophin-deficient myotubes. We noted significant differences in the global gene expression pattern between these myotubes and their matched control cultures. In particular, categorical analyses of the dysregulated genes demonstrated significant enrichment of molecules associated with the components of muscle cell contractile unit, ion channels, metabolic pathways and kinases. Additionally, some of the dysregulated genes could potentially explain conditions and endophenotypes associated with dystrophin deficiency, such as dysregulation of calcium homeostasis (Pvalb and Casq1), or cardiomyopathy (Obscurin, Tcap). In addition to be validated by qPCR, our data gains another level of validity by affirmatively reproducing several independent studies conducted previously at genes and/or protein levels in vivo and in vitro.

CONCLUSION

Our results suggest that in striated muscles, dystrophin is involved in orchestrating proper development and organization of myofibers as contractile units, depicting a novel pathophysiology for DMD where the absence of dystrophin results in maldeveloped myofibers prone to physical stress and damage. Therefore, it becomes apparent that any gene therapy approaches for DMD should target early stages in muscle development to attain a maximum clinical benefit. With a clear and specific definition of the transcriptome of dystrophin deficiency, manipulation of identified dysregulated molecules downstream of dystrophin may lead to novel ameliorative approaches for DMD.

Proteomic Profiling of the Dystrophin-Deficient MDX Heart Reveals Drastically Altered Levels of Key Metabolic and Contractile Proteins

Although Duchenne muscular dystrophy is primarily classified as a neuromuscular disease, cardiac complications play an important role in the course of this X-linked inherited disorder. The pathobiochemical steps causing a progressive decline in the dystrophic heart are not well understood. We therefore carried out a fluorescence difference in-gel electrophoretic analysis of 9-month-old dystrophin-deficient versus age-matched normal heart, using the established MDX mouse model of muscular dystrophy-related cardiomyopathy. Out of 2,509 detectable protein spots, 79 2D-spots showed a drastic differential expression pattern, with the concentration of 3 proteins being increased, including nucleoside diphosphate kinase and lamin-A/C, and of 26 protein species being decreased, including ATP synthase, fatty acid binding-protein, isocitrate dehydrogenase, NADH dehydrogenase, porin, peroxiredoxin, adenylate kinase, tropomyosin, actin, and myosin light chains. Hence, the lack of cardiac dystrophin appears to trigger a generally perturbed protein expression pattern in the MDX heart, affecting especially energy metabolism and contractile proteins.