Length-dependent activation in human myocardium

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ABSTRACT This translational project uses human biospecimens procured from organ donors and patients undergoing cardiac transplant to advance understanding of a cellular-level mechanism that underpins the Frank-Starling relationship. Specifically, the project focuses on length-dependent activation, defined as the increased maximum force and Ca2+ sensitivity of contraction induced by myocardial stretch. The mechanisms that underlie length- dependent activation remain unclear, but may involve thick-filament regulation and transitions between the newly discovered OFF and ON states of myosin. Co-PI Campbell has spent a decade building a biobank that now contains >10,000 myocardial specimens from >360 patients. Pilot experiments performed by Co-PI Tanner with these samples show that length-dependent changes in Ca2+ sensitivity are eliminated in myocardium from patients who have non-ischemic heart failure, but preserved in myocardium from organ donors and patients who have ischemic heart failure. New computer modeling predicts that these functional effects reflect destabilization of the myosin OFF state in patients who have non-ischemic heart failure. This hypothesis is supported by additional experiments that used fluorescent polarization techniques to assess OFF/ON dynamics in the thick filaments of human myocardium. Further pilot studies tested the effects of peptides targeted to the thick filament. Peptides that stabilize the OFF state reduced the Ca2+ sensitivity of contraction at long sarcomere length while destabilizing peptides enhanced Ca2+ sensitivity at short length. The length-dependence of these effects was predicted by our computer modeling. The project builds on these data from human biospecimens and integrates the skills and resources of five cardiovascular researchers, a statistician, and a physician-scientist who specializes in advanced heart failure. The Aims explore the global hypothesis that length-dependent activation is reduced in patients who have non- ischemic heart failure because their cardiac thick filaments are biased towards the ON state. Aim 1: Test the hypothesis that length-dependent changes in Ca2+ sensitivity are reduced in myocardium from patients who have non-ischemic heart failure. Aim 2: Test the hypothesis that the OFF state of the thick filament is destabilized in myocardium from patients who have non-ischemic heart failure. Aim 3: Target OFF/ON transitions to manipulate the Ca2+ sensitivity of human myocardium.