Unravelling the role of mitochondrial dysfunction in core myopathies

Core myopathies are a diverse group of congenital muscle diseases, which typically present with proximal muscle weakness, hypotonia and delayed motor milestones. Biopsies from affected individuals display ‘core’ areas that lack mitochondrial staining; despite cores being a pathological hallmark for these diseases, their formation and development is not well understood. In this thesis I have explored the involvement of mitochondrial dysfunction in core myopathy pathology by studying different models with mutations of proteins involved in intracellular calcium (Ca2+) homeostasis. The first model discussed is a new mouse sub-line carrying the I4898T mutation in RYR1. RYR1 codes for a Ca2+ release channel on the sarcoplasmic reticulum (SR) called Ryanodine Receptor 1 (RyR1), and given that mitochondria localise close to the SR, a faulty RyR1 is likely to upset the Ca2+ balance that is required for mitochondrial function. In contrast to studies of other RyR1 I4898T models, I did not observe reduced SR-stimulated Ca2+ release in myotubes or myofibres isolated from the heterozygous mice. Consequently, no defects in mitochondrial localisation, respiration or biogenesis were identified. The second model discussed is fibroblasts from a patient with a mutation in STAC3, which codes for an adaptor protein that is thought to be involved in excitation-contraction coupling. While histamine-induced Ca2+ signals were elevated in the patient fibroblasts, resting mitochondrial membrane potential was reduced. As STAC3 is predominantly expressed in skeletal muscle, these results are difficult to interpret and highlight the need for a future skeletal muscle model. The final model consists of fibroblasts from individuals that have loss-offunction mutations in MICU1, which codes for a Ca2+ sensor that regulates the opening of the Mitochondrial Ca2+ Uniporter. The mitochondria in these MICU1-deficient fibroblasts are fragmented, exhibit accelerated mitochondrial Ca2+ uptake and are Ca2+ -loaded at rest. While mitochondrial membrane potential and respiration appear normal, there is evidence for a futile Ca2+ cycle across the mitochondrial membrane, which illustrates the bioenergetic cost of mitochondrial Ca2+ accumulation