ALTERED CALCIUM REGULATION IN CARDIOMYOPATHIES

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DESCRIPTION (the applicant's description verbatim): Mutations in cardiac thin filament protein troponin I (cTnI) have been identified as causal in some forms of familial hypertrophic cardiomyopathy (FHC). The overall goal of this proposal is to characterize functional consequences of these six disease-related mutations in the C-terminus of cTnI. Specific predictions about their effects on steady-state force-pCa relationship derive from the localization of mutations in binding of TnI's C-terminus to: (i) the N-terminus of cTnC (R145G and R145Q mutations in the inhibitory peptide and R162W); or (ii) actin-Tm (R145G and R145Q mutations in the inhibitory peptide which is part of actin-Tm binding site I and K183delta which is part of actin-Tm binding site II). In addition, C-terminal truncation studies of TnI predict that R162W, K183delta, G203S and K206Q could all compromise the ability of cTn to relax the myocardium during diastole. Recombinant cTnI constructs will be altered with mutations found in FHC. Mutant and wild type (WT) constructs will be incorporated into permeabilized muscle preparations-for measurements of sarcomere mechanics-and into regulated actin filaments-for measurements on single actin filaments using in vitro motility assays. Mutant proteins will be tested, first by complete substitution of mutant for WT, secondly in varying proportions of WT and mutant as occurs in the diseased myocardium, and thirdly with additional mutations that mimic phosphorylation of Ser22 & Ser23 or Thr143 (introduction of acidic residues). For each mutation, we will determine the effects on Ca2+ sensitivity of steady-state isometric force, filament sliding, and the rate of tension redevelopment (kTR; a parameter that is important for evaluating cardiac function). The results will aid understanding normal Ca2+ regulation of the heart, pathological mechanism(s) of hypertrophy, and the assays will he useful for identifying treatments.