A.P.9: Elucidation of the mechanism of disease in BAG3-related myofibrillar myopathy

Abstract

Mutations in the co-chaperone Bcl2-associated athanogene 3 (BAG3) have been shown to cause myofibrillar myopathy (MFM). MFM is characterised at the cellular level by the formation of protein aggregates and structural failure of the Z-disk. In contrast to the other MFM causing proteins BAG3 has no structural role in the Z-disk but is involved in the regulation of autophagy and the degradation of misfolded proteins. Protein aggregation in BAG3-related MFM has therefore been proposed to result from a reduction in autophagic activity. To investigate the mechanism of disease in BAG3-related MFM we expressed the human wildtype BAG3 or the dominant myofibrillar myopathy causing mutant form BAG3^P209L in zebrafish muscle. The expression of BAG3^P209L resulted in protein aggregation and we performed time-lapse imaging to examine aggregate formation in vivo and examined their composition. To investigate the role of impaired autophagy in BAG related MFM we stimulated or inhibited autophagy in BAG3^wt or BAG3^P209L expressing embryos and examined its effect on protein aggregation. Our studies revealed that autophagy is active in BAG3^P209L expressing fish and that inhibition of autophagy is not sufficient to induce protein aggregation, suggesting that the pathology is not due to impaired autophagic activity. We did not observe fibre failure in the BAG3^P209L model fish and therefore examined the effect of knocking down BAG3 expression. Loss of BAG3 function resulted in fibre failure but not the formation of protein aggregates. Together our knockdown and overexpressing models demonstrate contraction-dependent fiber failure and the formation of protein aggregates. Furthermore, our experimental analysis of the transgenic and knockdown embryos demonstrates that protein aggregation is not due to an inhibition of autophagy and suggests a novel mechanism of disease with the cellular pathology caused by a combination of toxic gain of function and BAG3 insufficiency.