P123The rescue of Ca2+ cycling abnormalities conferred by HCM-causing mutations with analogues of the green tea polyphenol epigallocatechin-3-gallate.
Robinson P., Liu X., Zhang YH., Khandelwal A., Blagg B., Casadei B., Watkins H., Redwood C.
Mutations in thin filament regulatory proteins that cause hypertrophic cardiomyopathy (HCM) confer distinct primary alterations of cardiac contractility. We have shown that altered Ca2+-buffering by mutant thin filaments leads to altered Ca2+ handling and results in stimulation of Ca2+-dependent signalling pathways. To do this we have used adenoviral mediated expression of cTnT R92Q, cTnI R145G and α-TM D175N in adult guinea pig cardiomyocytes at a ratio of 1:1 with the endogenous protein. Simultaneous measurement of unloaded sarcomere-shortening and Ca2+ transients using fura-2 loading, showed the HCM mutations caused a significant decrease in the basal sarcomere length coupled with an increase in the diastolic Ca2+ concentration. The mechanism of alterations to EC-coupling was also investigated using tetracaine and caffeine challenging combined with simultaneous whole cell patch clamping. HCM mutant cells displayed reduced SR load (~1.4fold), slowed NCX calcium extrusion (~2.5fold), unchanged SERCA2 activity, increased ryanodine receptor leak (~5fold) and increased calcium buffering (~3fold). This was coupled to an increase in Ca2+ dependent NFAT nuclear localisation. Further studies using the green tea catechin, epigallocatechin gallate (EGCg) and sister compound epicatechin-3-gallate (ECG), have shown that the compounds can partiality reverse the increase in diastolic Ca2+ observed in cardiomyocytes containing HCM causing mutations. The mechanism is thought to be via an interaction with cTnC (measured using intrinsic cTnC tyrosine fluorescence) which in turn causes a reduction of in vitro thin filament Ca2+ affinity (measured using cTnC labelled with IAANS fluorophore at Cys 35). Data acquired so far suggests that catechins and their derivative compounds may provide a possible therapeutic approach for correcting Ca2+ regulation and Ca2+ dependent remodelling in HCM. We have recently obtained library of 37 EGCg analogues which we have screened for cTnC affinity and thin filament Ca2+ affinity. We have now identified several catechins with potentially greater efficacy than the parent compound, and plan to test their ability to rescue the cellular HCM phenotype characterised above.