Mst4, a novel cardiac STRIPAK complex associated kinase, regulates cardiomyocyte growth and survival and is upregulated in human cardiomyopathy

Myocardial failure is associated with adverse remodeling which includes apoptotic loss of cardiomyocytes, hypertrophy as well as alterations in cell-cell contacts. Striatin-interacting phosphatase and kinase (STRIPAK) complexes and their kinase Mst4 have been linked to the development of different diseases. The role and targets of Mst4 in cardiomyocytes have not been investigated, yet.Multi tissue immunoblot experiments show highly enriched Mst4-expression in rodent hearts. Analyses of human biopsy samples from patients suffering from dilated cardiomyopathy revealed that Mst4 is upregulated (5,8-fold p<0.001) compared with non-failing controls. Increased abundance of Mst4 could also be detected in mouse models of cardiomyopathy. We confirmed that Mst4 interacts with STRIPAK components in neonatal rat ventricular cardiomyocytes, indicating that STRIPAK is present in the heart. Immunofluorescence stainings and molecular interaction studies revealed that Mst4 is localized to the intercalated disc and interacts with several intercalated disc proteins. Overexpression of Mst4 in neonatal rat cardiomyocytes results in hypertrophy compared with controls. In adult rat cardiomyocytes, Mst4-overexpression increases cellular and sarcomeric fractional shortening (p<0.05), indicating enhanced contractility. Overexpression of Mst4 also inhibits apoptotic cell death shown by reduction of cleaved Caspase3 (-69%, p<0.0001) and Caspase7 (-80%, p<0.0001) as well as cleaved Parp1 (-27%, p<0.001). To elucidate potential Mst4 targets, we performed phosphoproteomics analyses in neonatal rat cardiomyocytes after Mst4 overexpression and inhibition. The results revealed several target candidates of Mst4 at the intercalated disc and sarcolemma. Conclusion We identified Mst4 as a novel cardiac kinase that is upregulated in human cardiomyopathy and regulates cardiomyocyte growth and survival.