Titin-Based Passive Tension is Increased in a Titin AI KO Mouse

Abstract

The giant sarcomeric protein titin spans the length of the half sarcomere and consists of an I-band region that functions as a molecular spring and a less well understood A-band region. Here we characterize a mouse model in which a portion of titin near the A-I junction has been removed(AIKO). We measured passive stiffness using a muscle mechanics stretch-hold-release protocol in which a skinned fiber from the left ventricle papillary was stretched to a given sarcomere length(SL) within the physiological range, held for 90 seconds, and subjected to a sinusoidal frequency sweep. Active tensions were measured at SL=2.0μm and SL=2.2μm in both tissue types. We found that both total and titin-based passive stiffness was significantly higher in the AI KO compared to WT; at SL=2.3μm peak total passive tension was 43.5+/8.3mN/mm2 in the KO compared to 23.5+/-3.6mN/mm2 in the WT while peak titin-based passive tension was 28.7+/-7.0mN/mm2 in the AI KO compared to 13.8+/-2.0mN/mm2 in the WT. A similar trend was observed for total and titin-based steady state passive tensions. No significant differences in active tensions were observed at either SL indicating no major changes to the thick filament or myofilament area; this suggests the passive tension difference is intrinsic to titin. The sinusoidal frequency sweep was used to quantify the vicious and elastic modulus. Viscous and elastic moduli are defined as (σ/e)sin(θ) and (σ/e)cos(θ) respectively where σ is stress, e is strain, and θ is the phase shift. We found that both the viscous modulus and the elastic modulus were higher in the AI KO at high frequencies. These results suggest that WT passive tension levels rely on an intact A-I junction; the removal of this region results in increased titin stiffness.