Dilated Cardiomyopathy Mutations in Thin Filament Regulatory Proteins Reduce Contractility, Suppress Systolic Ca2+ & Activate NFAT & AKT Signalling.
Robinson P., Sparrow AJ., Patel S., Malinowska M., Reilly SN., Zhang YH., Casadei B., Watkins H., Redwood C.
Dilated cardiomyopathy (DCM) is clinically characterised by dilated ventricular cavities and reduced ejection fraction, leading to heart failure and increased thromboembolic risk. Mutations in thin filament regulatory proteins can cause DCM and have been shown in vitro to reduce contractility and myofilament Ca2+-affinity. In this work we have studied the functional consequences of mutations in cardiac troponin T (R131W), cardiac troponin I (K36Q) and α-tropomyosin (E40K) using adenovirally transduced isolated guinea pig left ventricular cardiomyocytes. We find significantly reduced fractional shortening with reduced systolic Ca2+. We also observe slowed contraction and Ca2+-reuptake times, which contrast with some findings in murine models of myofilament Ca2+-desensitisation. We also observe increased sarco-endoplasmic reticulum (SR) Ca2+ load and smaller fractional SR Ca2+ release. This corresponds to a reduction in SR Ca2+-ATPase activity and increase in Sodium-Calcium Exchanger activity. The disequilibrium of Ca2+ handling promotes dephosphorylation and nuclear translocation of the Nuclear-Factor-of-Activated T-cells (NFAT), with concordant RAC-alpha serine/threonine-protein kinase (AKT) phosphorylation but no change to Extracellular-Signal-Regulated Kinase activation in chronically paced cardiomyocytes expressing DCM mutations. These changes in Ca2+-handling and signalling are common to all three mutations, indicating a common pathway of disease pathogenesis in thin filament sarcomeric DCM. Previous work has shown changes to myofilament Ca2+ sensitivity caused by DCM mutations are qualitatively opposite those resulting from hypertrophic cardiomyopathy (HCM) mutations. However, we find several common pathways such as increased relaxation times and NFAT activation that are also hallmarks of HCM. This suggests more complex intracellular signalling underpinning DCM, driven by the primary mutation.