Filamin C is a highly dynamic protein associated with fast repair of myofibrillar microdamage

Abstract

Filamin c (FLNc) is a large dimeric actin-binding protein located at premyofibrils,myofibrillar Z-discs and myofibrillar attachment sites of striatedmuscle cells, where it is involved inmechanical stabilization, mechanosensation and intracellular signaling. Mutations in the gene encoding FLNc give rise to skeletalmuscle diseases and cardiomyopathies. Here, we demonstrate by fluorescence recovery after photobleaching that a large fraction of FLNc is highlymobile in cultured neonatalmouse cardiomyocytes and in cardiac and skeletal muscles of live transgenic zebrafish embryos. Analysis of cardiomyocytes fromXirp1 and Xirp2 deficient animals indicates that both Xin actin-binding repeat-containing proteins stabilize FLNc selectively in premyofibrils. Using a novel assay to analyzemyofibrillarmicrodamage and subsequent repair in cultured contracting cardiomyocytes by live cell imaging, we demonstrate that repair of damagedmyofibrils is achieved within only 4h, even in the absence of de novo protein synthesis. FLNc is immediately recruited to these sarcomeric lesions together with its binding partner aciculin and precedes detectable assembly of filamentous actin and recruitment of other myofibrillar proteins. These data disclose an unprecedented degree of flexibility of the almost crystalline contractilemachinery and imply FLNc as a dynamic signaling hub, rather than a primarily structural protein. Ourmyofibrillar damage/repairmodel illustrates how (cardio)myocytes are kept functional in theirmechanically andmetabolically strained environment. Our results help to better understand the pathomechanisms and pathophysiology of early stages of FLNcrelatedmyofibrillarmyopathy and skeletal and cardiac diseases preceding pathological protein aggregation.