Investigating the Effect of Cardiomyopathy-Causing Mutations in Cardiac Troponin-T on Calcium Buffering In Situ

Abstract

In vitro investigation of the effects of cardiomyopathy-causing mutations in thin filament regulatory proteins has demonstrated that hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are caused by distinct primary alterations of cardiac contractility and myofilament calcium affinity. We hypothesise that chronically altered calcium-buffering by mutant thin filaments leads to altered calcium handling and, via calcium-dependent signalling pathways, contributes to disease pathogenesis. We aim to study the in situ effect on calcium flux of a HCM and a DCM causing mutation in human cardiac troponin T (cTnT) (R92Q, R131W respectively), by adenoviral mediated expression of mutant protein in adult guinea pig cardiomyocytes. The adenoviral vectors co-express GFP and western blot analysis of FACS-sorted, GFP-expressing cells showed that recombinant cTnT comprised 45-50% of the total cTnT in these cardiomyocytes, 48 hours after infection. Analysis of unloaded sarcomere shortening showed that at an excitation frequency of 2 Hertz, cardiomyocytes infected with R131W cTnT elongated the time to 50% relaxation and reduced the magnitude of contraction, whilst R92Q cTnT reduced the time to 50% relaxation and increased the contractile magnitude compared to wild type. Analysis of calcium transients of the same cells using fura-2 loading, indicates that the R92Q mutation reduces calcium transient amplitude, whilst the R131W mutation increases the time to complete calcium reuptake, with no change to the transient amplitude, despite the observed decrease in contraction. We are currently assessing the caffeine transients of these cells to investigate alterations to overall SR load and measuring alterations to components of calcium-dependent signalling cascades which may link the acute effects of cTnT mutations to macroscopic remodelling observed in the pathological disease states of HCM and DCM.