Abstract 315: Mice With Cardiac-restricted Overexpression Of Myozap Are Sensitized To Biomechanical Stress And Develop A Protein-aggregate Associated Cardiomyopathy

Abstract

The intercalated disc (ID) is a major component of the cell-cell contact structures of cardiomyocytes ensuring their electro-mechanical coupling. In previous work, we could identify and characterize Myozap, a novel cardiac-enriched ID protein, which interacts with several other ID proteins. We could now show in cardiomyocytes that overexpression of Myozap activates the Rho-dependent SRF pathway. Consistently, RNAi-mediated Myozap knockdown blocked Rho-mediated SRF activation linking the ID to gene regulation and actin dynamics. For further analyses of Myozap’s functions in vivo, we generated a mouse model with cardiac overexpression of Myozap cDNA (αMHC promoter). 1 year old mice developed cardiomyopathy with significant hypertrophy (heart weight/tibia length +43%, p<0.01, n=8-10) as well as LV dilation (LVEDD +20%, p<0.01) and a trend towards LV dysfunction (fractional shortening 50% vs. 42%, n=10, p=0.07). Consistently, these mice displayed upregulation of the hypertrophy-associated gene program (nppa 4.3-fold, p<0.001, and nppb 2.2-fold, p<0.01, n=7-9). On the ultrastructural level, we could detect bulky aggregates containing Myozap, desmoplakin and other ID proteins. This aggregate-associated pathology closely resembled the changes in the hearts of patients suffering from desminopathies. Interestingly, desmin was not detectable in the aggregates in MyozapTG mice, but appeared to be displaced from the ID. Further molecular analyses in the aged mice also revealed dysregulation of typical members of the unfolded protein response (UPR) associated with upregulation of UPR-related apoptosis (cleaved caspase 12). Functionally, pressure overload via transverse aortic constriction (TAC) caused exaggerated cardiac hypertrophy (heart/body weight 7.9 mg/g in Myozap TG-TAC vs. 5.6 mg/g in WT-TAC, n=11 each group, p=0.002) and loss of contractility (fractional shortening of 15.3% in Myozap TG-TAC vs. 33.0% in WT-TAC, n=11 each group, p<0.001). Similarly, a physiological stimulus (voluntary running) led to a less dramatic but significant premature LV dysfunction. In conclusion, Myozap overexpression offers new insights in the pathogenesis of protein aggregate-associated cardiomyopathies.