Thin filament regulation of striated muscle power output: implications for targets to improve failing human hearts.

Abstract

The heart's pumping capacity is determined by myofilament power generation. Power is work done per unit time and measured as the product of force and velocity. At a sarcomere level, these contractile properties are linked to the number of attached cross-bridges and their cycling rate; and many signaling pathways modulate one or both of these factors. We previously showed that rodent permeabilized cardiac myocyte power is increased following PKA-mediated phosphorylation of myofibrillar proteins (Herron, Korte, McDonald, Circ. Res. 2001). This increase in power demonstrates a capacity for improved myofilament function and this therapeutic potential adds significance for identification of the molecular specificity underlying enhanced power output. To address molecular specificity at the thin filament level, myofilament mechanical properties were measured before and after exchange of endogenous troponin with recombinant human Tn complex, which contained cardiac (c)TnT, cTnC and either WT cTnI or pseudo-phosphorylated cTnI at sites Ser23/24Asp, Y26E, or the combinatorial Ser23/24Asp and Y26E in single permeabilized rat slow-twitch skeletal muscle fibers. We found that cTnI Ser23/24Asp, Y26E, and combinatorial Ser23/24Asp and Y26E were sufficient for a ∼20% increase in power. We then tested the power reserve capacity in myofilaments from failing human hearts. We found that PKA increased power by ∼35% in permeabilized cardiac myocyte preparations from human failing hearts. Next, we determined if pseudo-phosphorylated cTnI at Ser23/24 was sufficient to increase power in cardiac myocytes from human failing hearts. Following cTn exchange that included cTnI Ser23/24Asp, power output increased ∼20% in permeabilized cardiac myocyte preparations from the left ventricle of human failing hearts. These results implicate cTnI N-terminal phosphorylation is a molecular regulator of myocyte power and could serve as a target to augment myocyte power reserve capacity in failing human hearts.