Impaired diastolic relaxation, an important component of diastolic dysfunction, is present in nearly all patients with heart failure-both with reduced and with preserved ejection fraction- and is present in nearly 25% of asymptomatic individuals. Unfortunately, no treatments for impaired relaxation exist. Recently, my lab identified and defined Mechanical Control of Relaxation as a faster relaxation rate in response to the rate of a lengthening strain. In other words, the relaxation rate is sensitive to the strain rate of the myocardium. Our data demonstrate that this mechanical regulation of relaxation can increase the relaxation rate two-fold beyond the biochemical processes that limit myosin detachment from actin, including calcium removal and thin filament deactivation. Thus, diastolic dysfunction might result from two factors: i) a loss of the sensitivity of relaxation to strain rate and ii) an attenuation in strain, restricting the strain rate. The molecular mechanism underlying strain-rate sensitivity remains unknown, but our preliminary studies indicate that myosin detachment kinetics are key. Strain-sensitive myosin detachment is a poorly characterized biophysical property, especially in intact cardiac tissues. Our preliminary data further demonstrates that in vivo hemodynamics can alter myocardial strain. The global hypothesis of this proposal is that myosin-detachment kinetics biophysically regulates Mechanical Control of Relaxation. The goals of this project are to confirm this mechanism and to identify molecular and hemodynamic factors that regulate Mechanical Control of Relaxation.
Aim 1 will determine whether myosin detachment rate modifies the sensitivity of the relaxation rate to the strain rate. We hypothesize that both myosin isoforms and myosin activating drugs will modify the strain-sensitive detachment rate of myosin. Using myosin isoform altering treatments and myosin-specific activating drugs, we will evaluate Mechanical Control of Relaxation using intact cardiac trabeculae. Importantly, we will assess myosin head position using x-ray diffraction techniques.
Aim 2 will determine the role of titin based stiffness on Mechanical Control of Relaxation. Our preliminary studies suggest that high titin compliance eliminates a length (preload)-dependent change in myosin detachment. We hypothesize that titin-mediated thick filament extensibility is a mediator of relaxation and will test this hypothesis in trabeculae expressing altered titin isoforms using the same techniques as in Aim 1.
Aim 3 will determine how strain rate and/or the sensitivity of the relaxation rate to the strain rate is modified in vivo using i) the molecular modifications studied in Aims 1 and 2 and ii) a clinically relevant Fructose+High Salt model that replicates several markers of heart failure with preserved ejection fraction. The proposed methods uniquely consider how the myocardium moves (strains) throughout the cardiac cycle, an advance beyond standard methods (isolated myofibril, trabeculae) that are isometric. These studies will drive the discovery of novel targets to improve the treatment and diagnosis of impaired relaxation by isolating mechanisms underlying Mechanical Control of Relaxation.
The proposed research addresses the critical need to determine new molecular mechanisms that underlie how the heart relaxes, a key component of diastolic dysfunction and Heart Failure with preserved Ejection Fraction (HFpEF). We investigate a unique strain-rate (stretch) dependent mechanism to accelerate the relaxation of heart muscle. We utilize novel biomechanical tests to discover new drug targets and diagnostic indexes for diastolic dysfunction that can be translated into clinical practice.
