Dystrophin is required for the normal function of the cardio-protective K(ATP) channel in cardiomyocytes

Cardiomyopathy in Duchenne Muscular Dystrophy and the Potential for Mitochondrial Therapeutics to Improve Treatment Response

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disease caused by mutations to the dystrophin gene, resulting in deficiency of dystrophin protein, loss of myofiber integrity in skeletal and cardiac muscle, and eventual cell death and replacement with fibrotic tissue. Pathologic cardiac manifestations occur in nearly every DMD patient, with the development of cardiomyopathy-the leading cause of death-inevitable by adulthood. As early cardiac abnormalities are difficult to detect, timely diagnosis and appropriate treatment modalities remain a challenge. There is no cure for DMD; treatment is aimed at delaying disease progression and alleviating symptoms. A comprehensive understanding of the pathophysiological mechanisms is crucial to the development of targeted treatments. While established hypotheses of underlying mechanisms include sarcolemmal weakening, upregulation of pro-inflammatory cytokines, and perturbed ion homeostasis, mitochondrial dysfunction is thought to be a potential key contributor. Several experimental compounds targeting the skeletal muscle pathology of DMD are in development, but the effects of such agents on cardiac function remain unclear. The synergistic integration of small molecule- and gene-target-based drugs with metabolic-, immune-, or ion balance-enhancing compounds into a combinatorial therapy offers potential for treating dystrophin deficiency-induced cardiomyopathy, making it crucial to understand the underlying mechanisms driving the disorder.

Cardioprotective Effects of Hydrogen Sulfide and Its Potential Therapeutic Implications in the Amelioration of Duchenne Muscular Dystrophy Cardiomyopathy

Hydrogen sulfide (H2S) belongs to the family of gasotransmitters and can modulate a myriad of biological signaling pathways. Among others, its cardioprotective effects, through antioxidant, anti-inflammatory, anti-fibrotic, and proangiogenic activities, are well-documented in experimental studies. Cardiorespiratory failure, predominantly cardiomyopathy, is a life-threatening complication that is the number one cause of death in patients with Duchenne muscular dystrophy (DMD). Although recent data suggest the role of H2S in ameliorating muscle wasting in murine and Caenorhabditis elegans models of DMD, possible cardioprotective effects have not yet been addressed. In this review, we summarize the current understanding of the role of H2S in animal models of cardiac dysfunctions and cardiac cells. We highlight that DMD may be amenable to H2S supplementation, and we suggest H2S as a possible factor regulating DMD-associated cardiomyopathy.

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.

Ion Channels of the Sarcolemma and Intracellular Organelles in Duchenne Muscular Dystrophy: A Role in the Dysregulation of Ion Homeostasis and a Possible Target for Therapy

Duchenne muscular dystrophy (DMD) is caused by the absence of the dystrophin protein and a properly functioning dystrophin-associated protein complex (DAPC) in muscle cells. DAPC components act as molecular scaffolds coordinating the assembly of various signaling molecules including ion channels. DMD shows a significant change in the functioning of the ion channels of the sarcolemma and intracellular organelles and, above all, the sarcoplasmic reticulum and mitochondria regulating ion homeostasis, which is necessary for the correct excitation and relaxation of muscles. This review is devoted to the analysis of current data on changes in the structure, functioning, and regulation of the activity of ion channels in striated muscles in DMD and their contribution to the disruption of muscle function and the development of pathology. We note the prospects of therapy based on targeting the channels of the sarcolemma and organelles for the correction and alleviation of pathology, and the problems that arise in the interpretation of data obtained on model dystrophin-deficient objects.

BKCa Activator NS1619 Improves the Structure and Function of Skeletal Muscle Mitochondria in Duchenne Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive hereditary disease caused by the absence of the dystrophin protein. This is secondarily accompanied by a dysregulation of ion homeostasis, in which mitochondria play an important role. In the present work, we show that mitochondrial dysfunction in the skeletal muscles of dystrophin-deficient mdx mice is accompanied by a reduction in K+ transport and a decrease in its content in the matrix. This is associated with a decrease in the expression of the mitochondrial large-conductance calcium-activated potassium channel (mitoBKCa) in the muscles of mdx mice, which play an important role in cytoprotection. We observed that the BKCa activator NS1619 caused a normalization of mitoBKCa expression and potassium homeostasis in the muscle mitochondria of these animals, which was accompanied by an increase in the calcium retention capacity, mitigation of oxidative stress, and improvement in mitochondrial ultrastructure. This effect of NS1619 contributed to the reduction of degeneration/regeneration cycles and fibrosis in the skeletal muscles of mdx mice as well as a normalization of sarcomere size, but had no effect on the leakage of muscle enzymes and muscle strength loss. In the case of wild-type mice, we noted the negative effect of NS1619 manifested in the inhibition of the functional activity of mitochondria and disruption of their structure, which, however, did not significantly affect the state of the skeletal muscles of the animals. This article discusses the role of mitoBKCa in the development of DMD and the prospects of the approach associated with the correction of its function in treatments of this secondary channelopathy.

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

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

A Long-Term Study Evaluating the Effects of Nicorandil Treatment on Duchenne Muscular Dystrophy-Associated Cardiomyopathy in mdx Mice

BACKGROUND

Duchenne muscular dystrophy (DMD) is a neuromuscular disease caused by dystrophin gene mutations affecting striated muscle. Due to advances in skeletal muscle treatment, cardiomyopathy has emerged as a leading cause of death. Previously, nicorandil, a drug with antioxidant and nitrate-like properties, ameliorated cardiac damage and improved cardiac function in young, injured mdx mice. Nicorandil mitigated damage by stimulating antioxidant activity and limiting pro-oxidant expression. Here, we examined whether nicorandil was similarly cardioprotective in aged mdx mice.

METHODS AND RESULTS

Nicorandil (6 mg/kg) was given over 15 months. Echocardiography of mdx mice showed some functional defects at 12 months compared to wild-type (WT) mice, but not at 15 months. Disease manifestation was evident in mdx mice via treadmill assays and survival, but not open field and grip strength assays. Cardiac levels of SOD2 and NOX4 were decreased in mdx vs. WT. Nicorandil increased survival in mdx but did not alter cardiac function, fibrosis, diaphragm function or muscle fatigue.

CONCLUSIONS

In contrast to our prior work in young, injured mdx mice, nicorandil did not exert cardioprotective effects in 15 month aged mdx mice. Discordant findings may be explained by the lack of cardiac disease manifestation in aged mdx mice compared to WT, whereas significant cardiac dysfunction was previously seen with the sub-acute injury in young mice. Therefore, we are not able to conclude any cardioprotective effects with long-term nicorandil treatment in aging mdx mice.

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

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

Cardioprotective effect of nicorandil on isoproterenol induced cardiomyopathy in the Mdx mouse model

BACKGROUND

Duchenne muscular dystrophy (DMD) associated cardiomyopathy is a major cause of morbidity and mortality. In an in vitro DMD cardiomyocyte model, nicorandil reversed stress-induced cell injury through multiple pathways implicated in DMD. We aimed to test the efficacy of nicorandil on the progression of cardiomyopathy in mdx mice following a 10-day treatment protocol.

METHODS

A subset of mdx mice was subjected to low-dose isoproterenol injections over 5 days to induce a cardiac phenotype and treated with vehicle or nicorandil for 10 days. Baseline and day 10 echocardiograms were obtained to assess cardiac function. At 10 days, cardiac tissue was harvested for further analysis, which included histologic analysis and assessment of oxidative stress. Paired student's t test was used for in group comparison, and ANOVA was used for multiple group comparisons.

RESULTS

Compared to vehicle treated mice, isoproterenol decreased ejection fraction and fractional shortening on echocardiogram. Nicorandil prevented isoproterenol induced cardiac dysfunction. Isoproterenol increased cardiac fibrosis, which nicorandil prevented. Isoproterenol increased gene expression of NADPH oxidase, which decreased to baseline with nicorandil treatment. Superoxide dismutase 2 protein expression increased in those treated with nicorandil, and xanthine oxidase activity decreased in mice treated with nicorandil during isoproterenol stress compared to all other groups.

CONCLUSIONS

In conclusion, nicorandil is cardioprotective in mdx mice and warrants continued investigation as a therapy for DMD associated cardiomyopathy.

Low human dystrophin levels prevent cardiac electrophysiological and structural remodelling in a Duchenne mouse model

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder caused by loss of dystrophin. This lack also affects cardiac structure and function, and cardiovascular complications are a major cause of death in DMD. Newly developed therapies partially restore dystrophin expression. It is unclear whether this will be sufficient to prevent or ameliorate cardiac involvement in DMD. We here establish the cardiac electrophysiological and structural phenotype in young (2-3 months) and aged (6-13 months) dystrophin-deficient mdx mice expressing 100% human dystrophin (hDMD), 0% human dystrophin (hDMDdel52-null) or low levels (~ 5%) of human dystrophin (hDMDdel52-low). Compared to hDMD, young and aged hDMDdel52-null mice displayed conduction slowing and repolarisation abnormalities, while only aged hDMDdel52-null mice displayed increased myocardial fibrosis. Moreover, ventricular cardiomyocytes from young hDMDdel52-null animals displayed decreased sodium current and action potential (AP) upstroke velocity, and prolonged AP duration at 20% and 50% of repolarisation. Hence, cardiac electrical remodelling in hDMDdel52-null mice preceded development of structural alterations. In contrast to hDMDdel52-null, hDMDdel52-low mice showed similar electrophysiological and structural characteristics as hDMD, indicating prevention of the cardiac DMD phenotype by low levels of human dystrophin. Our findings are potentially relevant for the development of therapeutic strategies aimed at restoring dystrophin expression in DMD.

The Experimental Pathology at Ancona: 50 Years of Exciting and Pioneering Research on Human Pathology

Half century ago, a few academic pioneers founded the laboratories of experimental and ultrastructural pathology in Ancona. From this origin, a new phase of experimental studies developed aimed at translational and clinical research up to the present, when our group is internationally recognized for its fundamental contributions in gerontological research and molecular diagnostic pathology. Since the desire of immortality and of eternal youth seems to be as old as mankind, in the future we plan to focus our scientific research on Regenerative Medicine and Rejuvenation strategies. This is the most ambitious aim in the framework of the world aging population. We do not know whether we would achieve these results by ourselves. We are confident that, as in the past, new generations of scientist of the school of experimental pathology at Ancona will get the baton by the older one and lead the future with the same enthusiasm, love and commitment.

Voltage-Dependent Sarcolemmal Ion Channel Abnormalities in the Dystrophin-Deficient Heart

Mutations in the gene encoding for the intracellular protein dystrophin cause severe forms of muscular dystrophy. These so-called dystrophinopathies are characterized by skeletal muscle weakness and degeneration. Dystrophin deficiency also gives rise to considerable complications in the heart, including cardiomyopathy development and arrhythmias. The current understanding of the pathomechanisms in the dystrophic heart is limited, but there is growing evidence that dysfunctional voltage-dependent ion channels in dystrophin-deficient cardiomyocytes play a significant role. Herein, we summarize the current knowledge about abnormalities in voltage-dependent sarcolemmal ion channel properties in the dystrophic heart, and discuss the potentially underlying mechanisms, as well as their pathophysiological relevance.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

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.

BDNF modulates heart contraction force and long-term homeostasis through truncated TrkB.T1 receptor activation

BDNF exerts inotropic function in the adult mammalian heart through TrkB.T1 receptor and loss of this ligand/receptor system in cardiomyocytes impairs calcium signaling and causes cardiomyopathy, suggesting an essential physiological role for this pathway in cardiac function.

Brain-derived neurotrophic factor (BDNF) is critical for mammalian development and plasticity of neuronal circuitries affecting memory, mood, anxiety, pain sensitivity, and energy homeostasis. Here we report a novel unexpected role of BDNF in regulating the cardiac contraction force independent of the nervous system innervation. This function is mediated by the truncated TrkB.T1 receptor expressed in cardiomyocytes. Loss of TrkB.T1 in these cells impairs calcium signaling and causes cardiomyopathy. TrkB.T1 is activated by BDNF produced by cardiomyocytes, suggesting an autocrine/paracrine loop. These findings unveil a novel signaling mechanism in the heart that is activated by BDNF and provide evidence for a global role of this neurotrophin in the homeostasis of the organism by signaling through different TrkB receptor isoforms.

Adenosine triphosphate-sensitive potassium channels and cardiomyopathies (Review)

Cardiomyopathies have been indicated to be one of the leading causes of heart failure. Though it was indicated that genetic defects, viral infection and trace element deficiency were among the causes of cardiomyopathy, the etiology has remained to be fully elucidated. Cardiomyocytes require large amounts of energy to maintain their normal biological functions. Adenosine triphosphate-sensitive potassium channels (KATP), composed of inward-rectifier potassium ion channel and sulfonylurea receptor subunits, are present on the cell surface and mitochondrial membrane of cardiac muscle cells. As metabolic sensors sensitive to changes in intracellular energy levels, KATP adapt electrical activities to metabolic challenges, maintaining normal biological functions of myocytes. It is implied that malfunctions, mutations and altered expression of KATP are associated with the pathogenesis of conditions including c hypertrophy, diabetes as well as dilated, ischemic and endemic cardiomyopathy. However, the current knowledge is only the tip of the iceberg and the roles of KATP in cardiomyopathies largely remain to be elucidated in future studies.

Subcellular localization of dystrophin isoforms in cardiomyocytes and phenotypic analysis of dystrophin-deficient mice reveal cardiac myopathy is predominantly caused by a deficiency in full-length dystrophin

Duchenne muscular dystrophy (DMD) is an X-linked recessive progressive muscle degenerative disorder that causes dilated cardiomyopathy in the second decade of life in affected males. Dystrophin, the gene responsible for DMD, encodes full-length dystrophin and various short dystrophin isoforms. In the mouse heart, full-length dystrophin Dp427 and a short dystrophin isoform, Dp71, are expressed. In this study, we intended to clarify the functions of these dystrophin isoforms in DMD-related cardiomyopathy. We used two strains of mice: mdx mice, in which Dp427 was absent but Dp71 was present, and DMD-null mice, in which both were absent. By immunohistochemical staining and density-gradient centrifugation, we found that Dp427 was located in the cardiac sarcolemma and also at the T-tubules, whereas Dp71 was specifically located at the T-tubules. In order to determine whether T tubule-associated Dp71 was involved in DMD-related cardiac disruption, we compared the cardiac phenotypes between DMD-null mice and mdx mice. Both DMD-null mice and mdx mice exhibited severe necrosis, which was followed by fibrosis in cardiac muscle. However, we could not detect a significant difference in myocardial fibrosis between mdx mice and DMD-null mice. Based on the present results, we have shown that cardiac myopathy is caused predominantly by a deficiency of full-length dystrophin Dp427.

Heterogeneity of ATP-sensitive K+ channels in cardiac myocytes: enrichment at the intercalated disk

Ventricular ATP-sensitive potassium (K(ATP)) channels link intracellular energy metabolism to membrane excitability and contractility. Our recent proteomics experiments identified plakoglobin and plakophilin-2 (PKP2) as putative K(ATP) channel-associated proteins. We investigated whether the association of K(ATP) channel subunits with junctional proteins translates to heterogeneous subcellular distribution within a cardiac myocyte. Co-immunoprecipitation experiments confirmed physical interaction between K(ATP) channels and PKP2 and plakoglobin in rat heart. Immunolocalization experiments demonstrated that K(ATP) channel subunits (Kir6.2 and SUR2A) are expressed at a higher density at the intercalated disk in mouse and rat hearts, where they co-localized with PKP2 and plakoglobin. Super-resolution microscopy demonstrate that K(ATP) channels are clustered within nanometer distances from junctional proteins. The local K(ATP) channel density, recorded in excised inside-out patches, was larger at the cell end when compared with local currents recorded from the cell center. The K(ATP) channel unitary conductance, block by MgATP and activation by MgADP, did not differ between these two locations. Whole cell K(ATP) channel current density (activated by metabolic inhibition) was ∼40% smaller in myocytes from mice haploinsufficient for PKP2. Experiments with excised patches demonstrated that the regional heterogeneity of K(ATP) channels was absent in the PKP2 deficient mice, but the K(ATP) channel unitary conductance and nucleotide sensitivities remained unaltered. Our data demonstrate heterogeneity of K(ATP) channel distribution within a cardiac myocyte. The higher K(ATP) channel density at the intercalated disk implies a possible role at the intercellular junctions during cardiac ischemia.