Regulation of vesicular trafficking in presence of mutations in contractile proteins

Abstract

The molecular and cellular mechanisms underlying disease phenotypes in dilated cardiomyopathy (DCM) are diverse and to date incompletely understood. Inherited mutations in genes encoding structural components of the sarcomere lead to DCM and have been studied previously. Moreover, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from patients carrying familial DCM mutations have been shown to recapitulate disease phenotypes such as sarcomere misalignment and reduced contractility. In addition, impaired β-adrenergic signaling was reported to represent an important diease phenotype in DCM. However, the consequences of inherited DCM mutations on molecular pathological signaling in cardiomyocytes are not yet completely clear. In presence of sarcomeric mutations, this study found disorganisation of sarcomeres to result in disturbed interactions between sarcomeric proteins, such as troponin T and tropomyosin. Moreover, interactions between sarcomeric proteins and proteins located at local microdomains, such as PKA (protein kinase A), were disturbed. Therefore, sarcomeric DCM mutations led to impaired local signaling and entailed reduced TnI phosphorylation upon β-adrenergic stimulation. Furthermore, interactions between sarcomeres and membrane-associated cytoskeleton-binding proteins, such as filament C and vinculin, were found to be disturbed. Together, these findings indicated that in presence of sarcomeric DCM mutations, defective interactions occur between sarcomeres and other cytoskeleton elements as well as the plasma membrane (PM). Defects in sarcomere-cytoskeleton-PM interactions were also discovered to lead to impaired actin polymerization at the PM and reduced plasma membrane PIP2 levels. This was discovered to further result in impaired cargo uptake and abnormal early endosome distribution. Disturbed uptake of cargo such as transferrin-bound iron caused decreased iron levels in the mitochondria and defective cardiomyocyte functions, such as reduced contractility. Moreover, Rho A activation was found to rescue these disease phenotypes, specifically the depletion of mitochondrial iron levels and the reduced contractility observed in presence of sarcomeric DCM mutations. This may represent a basis for potential future translational strategies. In addition, this study also showed that at cellular levels, replenishing intracellular iron could restore the depleted iron levels in mitochondria and rescue the impaired contractility in DCM iPSC-CMs. This provided further evidence for the benefits of iron supplementation, a treatment approach in patients with heart failure which is already in clinical use. Of note, left ventricular tissues from DCM patients with end-stage heart failure were found to display abnormal endosome distribution, compared to the left ventricular tissues from patients with preserved systolic left-ventricular function. This suggests that iron deficiency due to defective endocytosis may present a more general mechanism. Taken together, this study has discovered that disorganised sarcomere protein organization leads to impaired sarcomere- cytoskeleton-PM interactions, disturbed functions of cardiomyocyte signaling pathways, and defective uptake and distribution of cargo by clathrin-mediated endocytosis. This study provides potentially relevant directions for future translational therapeutic strategies.