Modulation of Crossbridge Cycling Kinetics and Length Dependent Calcium Sensitivity by Titin and Nebulin

Abstract

The contractility of striated muscle is modulated by various modifications in titin and nebulin. Several hypotheses have been proposed in order to explain observations but clear mechanisms of dynamic action of these proteins in the 3-D saromere lattice are still lacking. In order to quantitatively assess the effect of modulations in these sarcomeric proteins on muscle functions, we employed the computational platform, MUSICO(MUscle SImulation COde). This platform incorporates explicit 3-D sarcomere structure, extensible actin and myosin filaments, various actomyosin cycles, thin filament regulation via continuous flexible chain (CFC), elasticity of titin elastomer and structural changes in actin filaments in presence and absence of nebulin. Simulations for cardiac muscle predicted a leftward shift in force-pCa relationship depending on the degree of titin-dependent passive force. Titin-dependent tension decreased active force in muscle when shortens below slack length and increased the active force when stretched beyond slack length. This behavior is caused by partial redistribution of the muscle load between active muscle force and titin-dependent passive force, and also by redistribution of stretch along the thin filament, causing significant changes in TnI-actin bond forces. These changes modulate the release of TnI from actin, i.e. activation of the thin filament. The simulations also showed severely decreased active force and reduced the rate of force redevelopment in sarcomeres with altered filament lengths, observed in nebulin deficient fibers. This behavior is caused by a combination of altered crossbridge kinetics and thin filament stretch dependent activation. Our results support the hypothesis that titin and nebulin both play distinct roles in the Frank-Starling mechanism of the contracting heart, but mechanisms of their actions are distinctly different. Supported by: NIH R01 AR048776, R01 DC 011528, and Serbian Ministry of Science grants III41007 and OI174028.