Résumé :
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Protein aggregates or rods are the primary pathological feature in nemaline myopathy. Mutations in the gene encoding skeletal muscle ?-actin (ACTA1) are responsible for about 20% of nemaline myopathy cases associated with cytoplasmic rods, as well as cases of intranuclear rod myopathy. “Nemaline” rods also occur as a secondary feature in some mitochondrial disorders, in particular in association with complex I deficiency. The mechanisms of rod formation are not well understood – particularly when they can occur in diverse disorders with very different structural and metabolic defects. Therefore we sought to determine their composition and structure. We have developed a tissue culture model in which to study mutations in ACTA1 identified in patients. The expression of mutant actin–EGFP leads to formation of cytoplasmic or intranuclear rod-like structures in muscle cells, similar to those observed in patient muscle. We are also able to induce rod formation in cells transfected with wild-type actin-EGFP by depleting ATP, as a model of oxidative stress (and mitochondrial myopathy). We have determined that these different rods have altered biochemical properties, varying in their protein composition and actin conformation. Using Fluorescence Recovery After Photobleaching (FRAP) we have also seen a difference in actin turnover in different rods suggesting that they have very different actin dynamics. In summary, we have demonstrated that rods form secondary to different pathogenic processes (mutations in ACTA1 and ischemia/ATP depletion) have different structural properties and biochemical dynamics. Characterisation of these different rods will ultimately help in understanding the mechanism of their formation and also the impact they have on cellular function in disease.
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