NEW INSIGHTS INTO CELLULAR FUNCTIONS: O.9Dysfunctional mitophagy: a potential therapeutic target in inclusion body myositis

Abstract

Inclusion body myositis (IBM) is the commonest acquired myopathy in adults over 50 years of age. There is no established treatment. Aetiology and pathogenesis are unknown. Diagnosis is based on clinical presentation supported by pathological findings. The combination of protein aggregates and mitochondrial changes in the muscle in IBM suggest that autophagy, a process for recycling damaged cellular components, is defective. Autophagy that recycles mitochondria is called mitophagy. To test our hypothesis that dysregulated mitophagy plays a key role in IBM we studied samples from patients with IBM (n=26) and aged-matched controls(n=39). Muscle mitochondrial DNA (mtDNA) copy number and mtDNA deletions were assessed by Q-PCR and long-range PCR. Fibroblasts were studied using high content imaging (INCell 1000). Energetic stress was achieved by using low glucose media. Mitochondrial membrane potential was assessed using tetramethyrhodamine dye. Mitochondria were evaluated by staining with antibodies to the outer mitochondrial membrane protein TOM-20. Mitophagy was determined by the colocalisation of TOM-20 and autophagosome marker LC3-II. We found that mtDNA copy number was lower in patients with IBM than controls. IBM samples with the lowest mtDNA copy number had greater numbers of cytochrome oxidase negative fibres and more variable mtDNA deletions. High content imaging revealed that fibroblasts from patients with IBM grown under energetically stressful conditions had i) reduced mitochondrial membrane potential, ii) increased mitochondrial volume, and iii) increased colocalisation of mitochondria and autophagosomes. Our findings of increased numbers of damaged and dysfunctional mitochondria in fibroblasts from patients with IBM indicate that mitophagy may be dysregulated. More work is needed to confirm these findings. However, if confirmed, it is possible that drug modulators of mitophagy may improve mitochondrial quality and affect disease progression in IBM.