Myocardial titin and collagen in cardiac diastolic dysfunction: partners in crime.

High myocardial diastolic stiffness has usually been attributed to excessive myocardial collagen deposition. Over the last decennium, stiff cardiomyocytes were also identified as important contributors to high myocardial diastolic stiffness, especially in heart failure (HF) with preserved ejection fraction (HFPEF).1–3 Cardiomyocyte stiffness relates to elasticity of the giant cytoskeletal protein titin, which spans the sarcomere from the Z disk to the M line and functions as a bidirectional spring responsible for early diastolic recoil and late diastolic distensibility of cardiomyocytes.4 In HFPEF patients and in HFPEF animal models,5 the observed increase in cardiomyocyte stiffness was always accompanied by increased deposition of collagen; therefore, it remained unclear whether impaired elasticity of titin could be solely responsible for high myocardial diastolic stiffness and HFPEF. In this issue of Circulation , however, Chung et al6 provide compelling evidence for titin being the sole perpetrator in the diastolic left ventricular (LV) dysfunction of an HFPEF mouse model. They generated mice with a deletion of nine immunoglobulin (Ig)-like domains from the proximal tandem Ig segment of the titin spring region (IG KO). This deletion extended the remaining titin spring segments and increased overall titin stiffness. Despite unaltered myocardial collagen content or composition, the IG KO mice developed HFPEF, evident from a reduced exercise tolerance, an enlarged left atrium, and a steeper LV end-diastolic pressure-volume relationship. The elegant study by Chung et al therefore clearly establishes myocardial titin to be able to sufficiently compromise diastolic LV function to induce HFPEF. Article see p 19 Titin, with a molecular mass of up to 3800 kDa, spans half-sarcomeres from the Z disk to the M band and contains a molecular spring segment, the I-band region, that supports early diastolic recoil and late diastolic resistance to stretch (Figure 1A). The I-band region has …