Amior, N;
(2018)
Developing models to study the mechanisms of weakness and myotonia in Periodic Paralysis.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Periodic paralysis (PP) is a disorder characterised by episodic attacks of paralysis, caused by mutations of skeletal muscle voltage gated ion channels. Although episodes eventually subside, patients develop progressive muscle weakness and frequently, myopathy. The relationship between this progression and the associated mutations is not understood. I propose that the longer term defect might result from disordered calcium signalling secondary to altered excitability, and its impact on mitochondrial function. I sought models where these aspects of muscle signalling could be studied. These were: A genetic model derived from patients: patient derived fibroblasts were virally transduced with MyoD to generate myoblasts, which were differentiated into myotubes with patient specific gene mutations. A pharmacological model: generated by treating neonatal rat myotube cultures with barium (an inhibitor of potassium channels) and low extracellular potassium to simulate attacks of PP. Treated cultures displayed more frequent spontaneous calcium fluctuations. Mitochondrial membrane potential was not affected by the treatment, but expression of TFAM (mitochondrial transcription factor A; a regulator of mitochondrial transcription and biogenesis) was upregulated, suggesting activation of retrograde signalling pathways. A mouse model: collaborators at MRC Harwell generated mice carrying a mutation (c.1744A>G; p.Ile582Val) equivalent to a novel point mutation in SCN4A, one of the ion channel genes associated with PP. Measurements in vivo established that affected mice show muscle weakness and delayed fatigue during tetanic responses. Calcium handling and mitochondrial function were analysed in single isolated myofibres. Calcium handling was not affected, however mitochondrial membrane potential was reduced in fibres from the PP mice and distribution was also affected, with fewer intermyofibrillar mitochondria, indicating altered mitochondrial bioenergetics. Thus I describe several approaches to investigate mechanisms that cause progressive weakness and myopathy in PP, and assess the relative merits of each approach. Furthermore, results suggest that a shift toward a more oxidative phenotype is taking place.
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