Heart failure is still a major cause for death and one of the most common reasons for hospital admission in the western world. It is known that proper calcium hemostasis and regular LTCC function play a key role in patients with dilated cardiomyopathy (DCM) and heart failure. In previous experiments we have shown that Myoscape - a member of the mammalian Striatin-interacting phosphatase and kinase (STRIPAK) complex – directly interacts with the L-type Ca²⁺channel (LTCC) to stabilize LTCC surface expression. Loss of Myoscape leads to endocytotic internalization of LTCC and subsequent heart failure in-vivo with damaged Ca²⁺ currents. Adenoviral overexpression of Myoscape/Strip2 in adult and neonatal rat ventricular cardiomyocytes (ARVCM and NRVCM) results in enhanced LTCC currents and increases contractility, as well as in inhibition of the Calcineurin NFAT axis to prevent cardiac hypertrophy and re-expression of the fetal gene program in vitro.

To elucidate a possibly protective role of Myoscape overexpression in vivo, we generated a transgenic Myoscape mouse model with cardiac restricted Myoscape/STRIP2 overexpression (Myoscape TG; up to 2.8fold; p<0.0001) and performed baseline characteristics and challenging experiments using two different hypertrophy models. While showing no obvious cardiac phenotype under baseline with similar morphometric and functional parameters, Myoscape transgenic mice also showed equivalent expression of hypertrophic genes (NPPA/NPPB) and various proteins involved in cardiac Calcium cycling (LTCC, Ryanodine receptor, SERCA2, Caveolin 3). 

But after transverse aortic constriction (TAC) in Myoscape transgenic mice, data analysis revealed that cardiac restricted overexpression prevented cardiac failure and that these mice showed preserved ejection fraction (36.63% vs. 23.74%, p<0,01) as well as stroke volume (33.49% vs 22.28µl; p<0,05) compared to wildtype mice after TAC. Moreover, Calcineurin-regulator RCAN1.4 protein-expression increased significantly (1.94-fold) in wild-type mice after TAC, as expected, and was significantly reduced by Myoscape overexpression in western blot analysis (1.94-fold vs. 1.41-fold; p<0.05). Nevertheless, transgenic Myoscape overexpression failed to prevent the development of cardiac hypertrophy in vivo. 

Beyond that, we crossbred Myoscape transgenic mice with Calcineurin transgenic mice that are known to develop severe hypertrophy and heart failure due to constantly activated cardiac Calcineurin expression. As expected the transgenic calcineurin animals showed reduced ejection fraction compared to wildtype littermates (39.46% vs 57.6%, p<0.0001). In contrast double-transgenic Calcineurin TG/Myoscape TG mice showed significantly improved ejection fraction when compared to Calcineurin transgenic mice (48.88 vs 39.46%, p<0.05). But again, additional Myoscape overexpression failed to prevent cardiac hypertrophy showing similar heart weight to tibia length ratios in both models, besides the fact that again, protein-expression RCAN1.4 was significantly lower with in double transgenic mice (86.5-fold vs 136-fold, p<0.05) when compared to Calcineurin TG alone, indicating significant but incomplete Calcineurin inhibition.

Taken together, Myoscape overexpression results in positive contractility and Calcineurin inhibition in vitro and prevents heart failure after TAC or due to persistent Calcineurin activation in vivo.