Background Experimentally upregulating compliant titins has been suggested as a therapeutic for lowering pathological diastolic stiffness levels. However, how increasing titin compliance impacts global cardiac function requires in-depth study. We investigate the effect of upregulating compliant titins in a novel mouse model with a genetically altered titin splicing factor; integrative approaches were used from intact cardiomyocyte mechanics to pressure(P)-volume(V) analysis and Doppler echocardiography. Methods and Results Compliant titins were upregulated through deletion of the RNA Recognition Motif of the splicing factor RBM20 (Rbm20ΔRRM mice). A genome-wide exon expression analysis and a candidate approach revealed that the phenotype is likely to be dominated by greatly increased lengths of titin’s spring-elements. At both cardiomyocyte and left ventricular (LV)chamber levels diastolic stiffness was reduced in heterozygous (+/−) Rbm20ΔRRM mice with a further reduction in homozygous (−/−) mice at only the intact myocyte level. Fibrosis was present in only −/− Rbm20ΔRRM hearts. The Frank-Starling Mechanism was reduced in a graded fashion in Rbm20ΔRRM mice, at both the cardiomyocyte and LV chamber levels. Exercise tests revealed an increase in exercise capacity in +/− mice. Conclusions Titin is not only important in diastolic but also in systolic cardiac function. Upregulating compliant titins reduces diastolic chamber stiffness due to increased compliance of myocytes but depresses end-systolic elastance; under conditions of exercise the beneficial effects on diastolic function dominate. Therapeutic manipulation of the RBM20-based splicing system might be able to minimize effects on fibrosis and systolic function while improving diastolic function of heart failure patients.

BACKGROUND Diastolic dysfunction is a poorly understood but clinically pervasive syndrome that is characterized by increased diastolic stiffness. Titin is the main determinant of cellular passive stiffness. However, the physiological role that titin’s tandem Ig segment plays in stiffness generation and whether shortening this segment is sufficient to cause diastolic dysfunction needs to be established. METHODS AND RESULTS We generated a mouse model in which nine immunoglobulin (Ig)-like domains (Ig3-11) were deleted from the proximal tandem Ig segment of titin’s spring region (IG KO). Exon microarray analysis revealed no adaptations in titin splicing, while novel phospho-specific antibodies did not detect changes in titin phosphorylation. Passive myocyte stiffness was increased in the IG KO and immunoelectron microscopy revealed increased extension of the remaining titin spring segments as the sole likely underlying mechanism. Diastolic stiffness was increased at the tissue and organ levels, with no consistent changes in ECM composition or ECM-based passive stiffness, supporting a titin-based mechanism for in-vivo diastolic dysfunction. Additionally, IG KO mice have a reduced exercise tolerance, a phenotype often associated with diastolic dysfunction. CONCLUSIONS Increased titin-based passive stiffness is sufficient to cause diastolic dysfunction with exercise intolerance.