Titin Isoforms and Titin-Based Stiffness in Diastolic Heart Failure

Abstract

Diastolic heart failure (DHF) is a common heart disease characterized, e.g., by delayed relaxation, impaired left ventricular (LV) filling and increased LV stiffness. Titin is an established contributor to LV stiffness, but little is known about the protein's contribution to altered diastolic function in DHF. We investigated LV tissue samples of several animal models of DHF, as well as interventricular septum samples of aortic stenosis (AS) patients, for titin-isoform composition by loose-gel electrophoresis and titin-based stiffness by skinned-fiber mechanics. Induction of diastolic dysfunction in a small animal model, the “two kidney one clip” (2K1C) rat, which develops LV hypertrophy due to chronic afterload increase, caused no significant changes in the titin-isoform expression pattern, both 6 weeks and 8 months following surgery (∼6% N2BA in both 2K1C and SHAM-operated LV, the remainder being N2B isoform; N2B contains a stiffer, N2BA a more compliant titin spring). Similarly, in a volume-overload mouse model created by aortocaval fistula surgery, cardiac titin isoforms remained unaltered compared to SHAM-operated animals (18.5% vs. 19.8% mean N2BA). However, in an old dog model (8-12 years) made hypertensive by bilateral renal wrapping, the cardiac N2BA proportion was significantly lower (41.6±4.9%; mean±SD) than in normal old dog LV (46.2±4.2%; p<0.020). Mechanical measurements revealed passive-stiffness modulations consistent with the magnitude of titin-isoform switching. In contrast, in human AS samples, the titin isoform composition showed 42.0±4.0% N2BA, significantly higher than in location-matched normal donor hearts (37.5±5.0%; p<0.025). We conclude that diastolic dysfunction is associated with changes in cardiac titin isoform composition in a large animal model and in humans. The direction and the magnitude of the isoform shift appear to be determined by multiple factors not excluding, but clearly not restricted to, hemodynamic overload.