Roger J. Hajjar

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

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 (≈3 MDa) N2B-isoform and longer (3.20 to 3.35 MDa) N2BA-isoforms, but the average N2BA:N2B-protein ratio was shifted from ≈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 ≈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.

Type 1 Phosphatase, a Negative Regulator of Cardiac Function

ABSTRACT Increases in type 1 phosphatase (PP1) activity have been observed in end stage human heart failure, but the role of this enzyme in cardiac function is unknown. To elucidate the functional significance of increased PP1 activity, we generated models with (i) overexpression of the catalytic subunit of PP1 in murine hearts and (ii) ablation of the PP1-specific inhibitor. Overexpression of PP1 (threefold) was associated with depressed cardiac function, dilated cardiomyopathy, and premature mortality, consistent with heart failure. Ablation of the inhibitor was associated with moderate increases in PP1 activity (23%) and impaired β-adrenergic contractile responses. Extension of these findings to human heart failure indicated that the increased PP1 activity may be partially due to dephosphorylation or inactivation of its inhibitor. Indeed, expression of a constitutively active inhibitor was associated with rescue of β-adrenergic responsiveness in failing human myocytes. Thus, PP1 is an important regulator of cardiac function, and inhibition of its activity may represent a novel therapeutic target in heart failure.

Calcium Cycling in Synthetic and Contractile Phasic or Tonic Vascular Smooth Muscle Cells

© 2012 Lipskaia et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Calcium Cycling in Synthetic and Contractile Phasic or Tonic Vascular Smooth Muscle Cells