Filamin C is a multi-domain, actin-binding and actin-crosslinking protein,

specifically expressed in cardiac and skeletal muscles. Multiple mutation points within FlnC have be identified and related to cardiac diseases, such as dilated cardiomyopathy, hypertrophic cardiomyopathy and restrictive cardiomyopathy. As one of the cytoskeleton proteins in the cardiomyocytes, FlnC has also been reported to play an important role in maintaining the integrity and stability of the sarcomere structure and a linkage to the sarcolemma. However, the precise role of FlnC in the cardiomyocytes and the underlying mechanisms by which FlnC deficiency causes cardiomyopathy is still unclear. Previous studies have revealed that truncation mutation of FlnC in mouse model resulted in embryonic lethality without significant cardiac phenotypes, and loss of FlnC in fish model resulted in heart ruptures. In this study, by generating a true null FlnC mouse model, we located the place where the cardiac structural integrity was largely alternated. We also visualized the rupture by 3D model. We will look into the specific mechanism which leads the rupture in the future.

Cardiomyocytes are under constant mechanical and metabolic stress, which makes functional protein quality control (PQC) systems particularly important. Molecular chaperones and co-chaperones are essential components of a functioning PQC. Mutation and downregulation of the co-chaperone protein BCL-2–associated athanogene 3 (BAG3) are associated with cardiomyopathy and heart failure. It has been reported that a BAG3 P209L missense mutation leads to the development of myofibrillar myopathy with severe cardiac defects. However, the mechanisms by which the P209L mutation leads to cardiomyopathy remain obscure. In our present study, we found that BAG3 P209L mutation in mice did not cause cardiomyopathy. In the molecular level, the levels of ATP-dependent and ATP-independent heat shock proteins were not changed in cardiac tissues from BAG3 P209L mutant mice. Further experiments revealed that the P209L mutation did not influence the levels of autophagy or global ubiquitination in cardiac tissues. Together, these observations suggest that the BAG3 209L mutation alone may not cause cardiomyopathy in mice.

Alpha Protein Kinase 3 (ALPK3) is an alpha kinase highly expressed in skeletal and cardiac muscles. Recent studies reveal that the biallelic truncating mutations in ALPK3 are associated with pediatric cardiomyopathy in human patients, suggesting that ALPK3 plays an essential role in cardiac development and function. However, no in vivo studies have elucidated the cardiac function of ALPK3; the subcellular localization of ALPK3 in cardiomyocyte is unknown, and whether the putative kinase activity is essential for the function of ALPK3 remains to be investigated. Utilizing ALPK3 global and cardiac specific knock out mouse models, we show that both the global and cardiac specific loss of ALPK3 leads to progressive dilated cardiomyopathy and premature lethality in mice. To investigate whether the putative kinase activity of ALPK3 is essential for its cardiac function, we mutate the invariant catalytic lysine residue in the alpha kinase domain of ALPK3 in mice and show that mice harboring the homozygous lysine mutation do not develop cardiac defects. Our in vitro and in vivo data further indicate that ALPK3 localizes in the nucleus of cardiomyocyte. In summary, our present study suggests that ALPK3 is a nucleus-localized protein indispensable for cardiac development and function, and that ALPK3 functions by protein-protein interactions independently of its putative kinase activity in heart.

Cardiolipin, the signature phospholipid of mitochondria, is crucial for mitochondrial function and architecture in the heart. Defects in cardiolipin remodeling processes and metabolism lead to cardiomyopathy, as seen in patients with Barth Syndrome (BTHS). Tafazzin is a major acyltransferase in CL remodeling that transfers linoleoyl groups to monolyso-CL until the final symmetric acyl composition is achieved. Mutations in Taz that result in BTHS cause inefficient transacylation between phosphoplipid-lysophospholipids, leading to cardiomyopathy. However, little is known as to the detailed molecular mechanisms by which Taz deficiency and consequent CL absnormalities lead to the progression of cardiomyopathy. In our present study, we found that loss of Tafazzin results in increased levels of cardiac stress markers, Anf and Bnp. Histological analysis demonstrated that mice with Taz deletions had thinner left ventricular walls and dilated left ventricular chambers relative to the control mice. Echocardiography analysis also showed that loss of Tafazzin causes decreased cardiac function. Together, these observations suggest that deletion of Tafazzin in cardiomyocytes results in dilated cardiomyopathy as seen in Barth Syndrome.

Neuronal precursor cell-expressed developmentally downregulated 4 (NEDD4) is a member of the NEDD4 family of HECT E3 ubiquitin ligases. NEDD4 is highly expressed in cardiomyocytes, but its specific function in adult cardiomyocytes is unknown. To study the role of NEDD4 in adult cardiomyocytes, we have generated a Nedd4 floxed mouse model and crossed it with tamoxifen-inducible aMHC-MerCreMer mice to generate an inducible Nedd4 cardiomyocyte-specific knockout mouse model (Nedd4 icKO). In this study, we performed comprehensive molecular and cardiac physiological studies on Nedd4 icKO mice. Our results show that after performing age-dependent studies, cardiac function and structure were not dramatically altered in Nedd4 icKO mice, suggesting that the loss of NEDD4 did not cause cardiomyopathy. Further experiments revealed that the levels of chaperone proteins and the levels of global ubiquitination were not changed within the cardiac tissue of Nedd4 icKO mice. Together, these results suggest that deletion of NEDD4 in adult cardiomyocytes does not cause overt cardiomyopathy.