Cardiac titin: A multifunctional giant

Titin is responsible for the passive and restoring force of the cardiac sarcomere and makes a major contribution to the diastolic wall stress of the LV, the level of which can be tuned through differential splicing and phosphorylation. PKA and PKG phosphorylation lower stress, and PKC increases it. Changes in titin phosphorylation and titin splicing occur in cardiac disease, in addition to mutations in the titin gene. A host of titin-binding proteins have been discovered that implicate titin as a key player in the organization and development of the sarcomere, in protein turnover, and in sensing mechanical stress. Several stress-sensing signalosomes along the molecule have been discovered, of which only the FHL-based signalosome binds to a spring element (N2B). This N2B-FHL signalosome is ideally situated to sense sarcomere strain and link diastolic dysfunction to hypertrophy signaling.