Passive Stiffness Changes Caused by Upregulation of Compliant Titin Isoforms in Human Dilated Cardiomyopathy Hearts

Abstract

In the pathogenesis of dilated cardiomyopathy, cytoskeletal proteins play an important role. In this study, we analyzed titin expression in left ventricles of 19 control human donors and 9 severely diseased (nonischemic) dilated cardiomyopathy (DCM) transplant-patients, using gel-electrophoresis, immunoblotting, and quantitative RT-PCR. Both human-heart groups coexpressed smaller (approximately 3 MDa) N2B-isoform and longer (3.20 to 3.35 MDa) N2BA-isoforms, but the average N2BA:N2B-protein ratio was shifted from approximately 30:70 in controls to 42:58 in DCM hearts, due mainly to increased expression of N2BA-isoforms >3.30 MDa. Titin per unit tissue was decreased in some DCM hearts. The titin-binding protein obscurin also underwent isoform-shifting in DCM. Quantitative RT-PCR revealed a 47% reduction in total-titin mRNA levels in DCM compared with control hearts, but no differences in N2B, all-N2BA, and individual-N2BA transcripts. The reduction in total-titin transcripts followed from a decreased area occupied by myocytes and increased connective tissue in DCM hearts, as detected by histological analysis. Force measurements on isolated cardiomyofibrils showed that sarcomeric passive tension was reduced on average by 25% to 30% in DCM, a reduction readily predictable with a model of wormlike-chain titin elasticity. Passive-tension measurements on human-heart fiber bundles, before and after titin proteolysis, revealed a much-reduced relative contribution of titin to total passive stiffness in DCM. Results suggested that the titin-isoform shift in DCM depresses the proportion of titin-based stiffness by approximately 10%. We conclude that a lower-than-normal proportion of titin-based stiffness in end-stage failing hearts results partly from loss of titin and increased fibrosis, partly from titin-isoform shift. The titin-isoform shift may be beneficial for myocardial diastolic function, but could impair the contractile performance in systole.

Transmural Heterogeneity of Cellular Level Power Output Is Reduced in Human Heart Failure

“…This probably reflects a compensatory effect of titin isoform expression. Consistent with previous studies [15,16], samples from failing hearts had a greater relative content of the large N2BA isoform of titin (p=0.047, Figure S3B). This isoform shift tends to decrease passive stiffness and seems to counteract the mechanical effect of increased fibrosis in the samples from the failing hearts.…”

Impaired myocardial performance in a normotensive rat model of intrauterine growth restriction

“…The somewhat stronger increase of N2BA expression compared with N2B in our IUGR rats together with the significantly elevated values of LVEDD and LVESD in these animals supports the hypothesis of a more distensible myocardium post-IUGR. According to this, an overexpression of N2BA could be found in the myocardium of patients with dilated cardiomyopathy (23).…”

Nuances of electrophoresis study of titin/connectin

“…These gels showed that the T1 mobility varied greatly between skeletal and cardiac muscles from different mammals Trombitás et al 2000;Neagoe et al 2003;Warren et al 2003a, b;Lahmers et al 2004;Makarenko et al 2004;Udaka et al 2008;Guo et al 2013). The major T1 bands were ascribed to the titin isoforms N2B and N2BA in cardiac muscle and the titin isoform N2A in skeletal muscles.…”