Effect of Shortening Titin's Proximal IG Segment on Passive Cellular Mechanics

Abstract

Titin is widely considered the main contributor to passive stiffness of cardiac myocytes. To test this we studied a novel mouse model, in which a large fraction of titin's proximal tandem Ig segment is deleted (we refer to the model as IG KO; for details on model, see Chung et al poster). The proximal tandem Ig segment extends greatly under physiological conditions and shortening the tandem Ig segment is predicted to increase titin-based passive tension. We first studied passive tension in skinned cardiac myocytes by stretching cells in relaxing solution with a velocity of 1 base-length/s (∼in vivo speed during diastole), and determining stiffness from the slope of the passive stress - SL relation in the physiological SL range of 1.95-2.10 μm. Cellular stiffness was significantly increased in the IG KO by ∼50%. Using a sinusoidal analysis, the elastic moduli at physiological SLs were found to be increased in the IG KO but the viscous moduli were not different. This shows that the tandem Ig segment is a pure elastic spring. Currently we are investigating intact myocytes using a carbon-fiber based system for attaching cells and measuring force. We conclude that theproximal Ig segment of titin's I band region is a pure elastic spring and that shortening of this spring element increases titin-based cellular stiffness in the physiological SL range. Increased titin-based stiffness is found in patients with heart failure with preserved ejection fraction (HFpEF) and the IGKO is well-suited to study the effect of increased titin-based stiffness on heart function (see poster by Chung et al).