Arg92Leu-cTnT Alters the cTnC-cTnI Interface Disrupting PKA-Mediated Relaxation

BACKGROUND: Impaired left ventricular relaxation, high filling pressures, and dysregulation of Ca2+ homeostasis are common findings contributing to diastolic dysfunction in hypertrophic cardiomyopathy (HCM). Studies have shown that impaired relaxation is an early observation in the sarcomere-gene-positive preclinical HCM cohort, which suggests the potential involvement of myofilament regulators in relaxation. A molecular-level understanding of mechanism(s) at the level of the myofilament is lacking. We hypothesized that mutation-specific, allosterically mediated, changes to the cTnC (cardiac troponin C)-cTnI (cardiac troponin I) interface can account for the development of early-onset diastolic dysfunction via decreased PKA accessibility to cTnI. METHODS: HCM mutations R92L-cTnT (cardiac troponin T; Arg92Leu) and Δ160E-cTnT (Glu160 deletion) were studied in vivo, in vitro, and in silico via 2-dimensional echocardiography, Western blotting, ex vivo hemodynamics, stopped-flow kinetics, time-resolved fluorescence resonance energy transfer, and molecular dynamics simulations. RESULTS: The HCM-causative mutations R92L-cTnT and Δ160E-cTnT result in different time-of-onset diastolic dysfunction. R92L-cTnT demonstrated early-onset diastolic dysfunction accompanied by a localized decrease in phosphorylation of cTnI. Constitutive phosphorylation of cTnI (cTnI-D23D24) was sufficient to recover diastolic function to non-Tg levels only for R92L-cTnT. Mutation-specific changes in Ca2+ dissociation rates associated with R92L-cTnT reconstituted with cTnI-D23D24 led us to investigate potential involvement of structural changes in the cTnC-cTnI interface as an explanation for these observations. We probed the interface via time-resolved fluorescence resonance energy transfer revealing a repositioning of the N-terminus of cTnI, closer to cTnC, and concomitant decreases in distance distributions at sites flanking the PKA consensus sequence. Implementing time-resolved fluorescence resonance energy transfer distances as constraints into our atomistic model identified additional electrostatic interactions at the consensus sequence. CONCLUSIONS: These data show that the early diastolic dysfunction observed in a subset of HCM is attributable to allosterically mediated structural changes at the cTnC-cTnI interface that impair accessibility of PKA, thereby blunting β-adrenergic responsiveness and identifying a potential molecular target for therapeutic intervention.