The Functional Role of the N2B Region Within the Elastic Sarcomeric Protein Titin

Abstract

Utilizing a N2B knockout (KO) mouse model in which the exon that encodes the cardiac-specific N2B unique sequence (N2B-Us) spring element (exon 49) has been deleted, I investigated the mechanical role of the N2B-Us, one of the three extensible regions of cardiac titin. I was able to show that the extensibility provided by the N2B-Us limits energy loss during stretch and shortening cycles of the heart (i.e., during diastole and systole). In a range of conditions, KO mice showed significant increases in hysteresis, a measure of energy loss determined from the area between the stretch and release force-sarcomere length curves, over wild type (WT) mice. Most prominently, hysteresis increased significantly from 320±46 pJ/mm2 /sarcomere in WT tissue to 650±94 pJ/mm2 /sarcomere in KO tissue that had been preconditioned with a physiological stretch-release protocol (p < 0.005). To complement this KO model, oxidative stress was used to mechanically inactivate portions of the N2B-Us of WT titin through cysteine crosslinking. This inactivation displayed a greater increase in hysteresis response in WT compared to KO tissue, (32.3±5% vs. 12.9±2.2%, p-value < 0.05). The results of this study support the concept that the mechanical function of the N2B-Us of titin is to reduce hysteresis and increase the efficiency of the cardiac cycle.