Voltage-gated ion channel dysfunction in dystrophic muscle

Produced by a multitude of genetic mutations, the muscular dystrophies comprise a heterogeneous group of pathological conditions characterised by progressive muscle weakness with cycles of muscle necrosis and regeneration as the pathophysiological hallmarks of the disease. As muscle disease advances, muscle repair cannot adequately compensate for damage, and muscle fibres are gradually replaced by connective tissue and fat. This loss of muscle fibres impairs muscle function, and severe dystrophy types, such as Duchenne muscular dystrophy (DMD), are characterised by widespread muscle weakness that can lead to loss of ambulation, respiratory failure, cardiac failure, and premature death. To date, there is no effective therapy to stop the progression of DMD and other muscular dystrophy types. Anti-inflammatory glucocorticoids are currently used in the majority of DMD patients. These drugs, however, only slow the progression of the disease, and cause severe side effects. Research performed in the last couple of years has suggested that dysregulated expression and impaired function of voltage-gated ion channels are important features of dystrophic skeletal and cardiac muscle, which contribute to the pathology characterising the muscular dystrophies. Consequently, voltage-gated ion channels can be considered potential new drug targets, and drugs that modulate these channels may prove useful in future therapeutic strategies. The current knowledge in this science field is very limited so that it is difficult to judge the real dimension of the effects of an "impaired electrophysiology" on dystrophic muscle. Moreover, the existing knowledge is almost exclusively based on electrophysiological studies on the skeletal and cardiac muscle of the dystrophin-deficient mdx mouse, whose appropriateness as a model for human DMD has been seriously questioned. These circumstances, as well as the apparent lack of data on mouse models for other human dystrophy types, emphasise the urgent need for electrophysiological studies on proper dystrophy mouse models. The planned project intends to provide new information to fill the gaps of knowledge named. Therefore, we will study in detail the electrophysiological abnormalities of both dystrophic skeletal and cardiac muscle from mice out of an existing pool of mouse models for various types of the human muscular dystrophies. The main focus will be laid on cardiac muscle. The major aims are: 1) characterisation of electrophysiological impairments associated with mutations in genes encoding dystrophy-related proteins. This strategy should identify so far unknown interactions between voltage-gated ion channels and dystrophy-associated proteins or protein complexes; 2) exposure of the ion channel impairments common or specific for various types of the muscular dystrophies; 3) comparison between the electrophysiological abnormalities in dystrophic skeletal and cardiac myocytes; and 4) exposure of mechanisms by which impaired ion channels may contribute to the skeletal and cardiac muscle pathology of the muscular dystrophies. The expected results should provide a profound basis for future studies on the therapeutic effects of ion channel modulators on dystrophic skeletal and cardiac muscle.

The project studies have exposed significant cardiac ion channel abnormalities in dystrophic cardiomyocytes as potential new therapeutic targets for the treatment of the cardiovascular complications associated with the muscular dystrophies. Produced by a multitude of genetic mutations, the muscular dystrophies comprise a heterogeneous group of pathological conditions characterised by progressive skeletal muscle weakness and degeneration. In the most common and severe form, Duchenne muscular dystrophy (DMD), affected patients are faced with loss of ambulation, respiratory failure, and premature death. Besides skeletal muscle degeneration, DMD patients also show severe cardiac complications. Among those, cardiac arrhythmias and dilated cardiomyopathy development considerably contribute to the morbidity and mortality associated with the disease. Since the specific mechanisms causing these cardiac complications in DMD are hardly understood, current therapy approaches are not targeted, and their effectiveness is very limited. In the present project, we could show that dysregulated expression and abnormal function of cardiac ion channels are important features of dystrophic cardiomyocytes likely causative for the cardiac complications associated with DMD. This represents a starting point for the development of new targeted and evidence-based therapeutic strategies. We found that L-type Ca channel currents were significantly increased, and channel inactivation was substantially reduced in dystrophic cardiomyocytes. This was, at least in part, caused by impaired Ca channel regulation by neuronal nitric oxide synthase in dystrophic cells. Because gain of function in cardiac L-type Ca channels is known to be pro-arrhythmic, our results exposed a mechanism likely to generate cardiac arrhythmias in DMD patients. Further, Ca channel gain of function may also represent a trigger for cardiomyopathy development and as such a potential new drug target for disease prevention. By showing a significant reduction in IK1 potassium currents in dystrophic cardiomyocytes we uncovered another potential dystrophic cardiac arrhythmia source. Finally, we could clarify that significant abnormalities in cardiac ion channels do not represent a universal characteristic of all types of muscular dystrophy.Our basic research in the course of the present project is directly relevant for the clinic, because successful therapeutic target validation followed by promising animal treatment experiments could trigger the initiation of clinical studies, i.e. testing the application of Ca channel inhibitors for the treatment of young DMD patients prior to established clinical cardiomyopathy development, and/or for improved arrhythmia treatment. Besides clinical relevance, successful new drug development would also have considerable economic impact. Thus, a lot of money is currently used to care for DMD patients and to develop new therapies, as yet only associated with limited success.