Background— Heart failure with preserved left ventricular (LV) ejection fraction (EF) is increasingly recognized and usually referred to as diastolic heart failure (DHF). Its pathogenetic mechanism remains unclear, partly because of a lack of myocardial biopsy material. Endomyocardial biopsy samples obtained from DHF patients were therefore analyzed for collagen volume fraction (CVF) and sarcomeric protein composition and compared with control samples. Single cardiomyocytes were isolated from these biopsy samples to assess cellular contractile performance. Methods and Results— DHF patients (n=12) had an LVEF of 71±11%, an LV end-diastolic pressure (LVEDP) of 28±4 mm Hg, and no significant coronary artery stenoses. DHF patients had higher CVFs (7.5±4.0%, P<0.05) than did controls (n=8, 3.8±2.0%), and no conspicuous changes in sarcomeric protein composition were detected. Cardiomyocytes, mechanically isolated and treated with Triton X-100 to remove all membranes, were stretched to a sarcomere length of 2.2 ...

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Cardiomyocyte Stiffness in Diastolic Heart Failure

Abnormalities of diastolic function are common to virtually all forms of cardiac failure. However, their underlying mechanisms, precise role in the generation and phenotypic expression of heart failure, and value as specific therapeutic targets remain poorly understood. A growing proportion of heart failure patients, particularly among the elderly, have apparently preserved systolic function, and this is fueling interest for better understanding and treating diastolic abnormalities. Much of the attention in clinical and experimental studies has focused on relaxation and filling abnormalities of the heart, whereas chamber stiffness has been less well studied, particularly in humans. Nonetheless, new insights from basic and clinical research are helping define the regulators of diastolic dysfunction and illuminate novel targets for treatment. This review puts these developments into perspective with the major aim of highlighting current knowledge gaps and controversies.

What Mechanisms Underlie Diastolic Dysfunction in Heart Failure

Cardiac troponin I (cTnI) is a key regulatory protein in cardiac muscle contraction and relaxation, linking Ca(2+)-troponin C binding with activation of crossbridge reactions with the thin filament. In recent years, it has become increasingly apparent that myofilament properties as well as changes in intracellular Ca(2+) have a major role in the dynamic modulation of contractile function. The phosphorylation of specific serine and threonine residues on cTnI by several different kinases represents a major physiological mechanism for alteration of myofilament properties. Furthermore, altered thin filament function plays an important role in the contractile dysfunction associated with heart failure. Modification of cTnI by protein kinases A and C has been extensively studied with especially useful information deriving from (a) in vitro studies in reconstituted detergent-skinned fibre bundles in which endogenous cTnI was replaced with various targeted cTnI mutants and (b) transgenic animals in which endogenous cTnI was similarly manipulated through overexpression of cardiomyocyte-targeted cTnI mutants. cTnI may also be specifically modified by protein kinase G, p21-activated kinases and by dephosphorylation. This review focuses on recent advances in understanding the mechanisms of cTnI modification by these kinases and the consequent functional effects both under physiological conditions and in pathophysiological settings.

Regulation of cardiac contractile function by troponin I phosphorylation

High diastolic stiffness of failing myocardium results from interstitial fibrosis and elevated resting tension (F(passive)) of cardiomyocytes. A shift in titin isoform expression from N2BA to N2B isoform, lower overall phosphorylation of titin, and a shift in titin phosphorylation from N2B to N2BA isoform can raise F(passive) of cardiomyocytes. In left ventricular biopsies of heart failure (HF) patients, aortic stenosis (AS) patients, and controls (CON), we therefore related F(passive) of isolated cardiomyocytes to expression of titin isoforms and to phosphorylation of titin and titin isoforms. Biopsies were procured by transvascular technique (44 HF, 3 CON), perioperatively (25 AS, 4 CON), or from explanted hearts (4 HF, 8 CON). None had coronary artery disease. Isolated, permeabilized cardiomyocytes were stretched to 2.2-microm sarcomere length to measure F(passive). Expression and phosphorylation of titin isoforms were analyzed using gel electrophoresis with ProQ Diamond and SYPRO Ruby stains and reported as ratio of titin (N2BA/N2B) or of phosphorylated titin (P-N2BA/P-N2B) isoforms. F(passive) was higher in HF (6.1+/-0.4 kN/m(2)) than in CON (2.3+/-0.3 kN/m(2); P<0.01) or in AS (2.2+/-0.2 kN/m(2); P<0.001). Titin isoform expression differed between HF (N2BA/N2B=0.73+/-0.06) and CON (N2BA/N2B=0.39+/-0.05; P<0.001) and was comparable in HF and AS (N2BA/N2B=0.59+/-0.06). Overall titin phosphorylation was also comparable in HF and AS, but relative phosphorylation of the stiff N2B titin isoform was significantly lower in HF (P-N2BA/P-N2B=0.77+/-0.05) than in AS (P-N2BA/P-N2B=0.54+/-0.05; P<0.01). Relative hypophosphorylation of the stiff N2B titin isoform is a novel mechanism responsible for raised F(passive) of human HF cardiomyocytes.

Hypophosphorylation of the Stiff N2B Titin Isoform Raises Cardiomyocyte Resting Tension in Failing Human Myocardium

Background— Mutations in the MYBPC3 gene, encoding cardiac myosin-binding protein C (cMyBP-C), are a frequent cause of familial hypertrophic cardiomyopathy. In the present study, we investigated whether protein composition and function of the sarcomere are altered in a homogeneous familial hypertrophic cardiomyopathy patient group with frameshift mutations in MYBPC3 (MYBPC3mut). Methods and Results— Comparisons were made between cardiac samples from MYBPC3 mutant carriers (c.2373dupG, n=7; c.2864_2865delCT, n=4) and nonfailing donors (n=13). Western blots with the use of antibodies directed against cMyBP-C did not reveal truncated cMyBP-C in MYBPC3mut. Protein expression of cMyBP-C was significantly reduced in MYBPC3mut by 33±5%. Cardiac MyBP-C phosphorylation in MYBPC3mut samples was similar to the values in donor samples, whereas the phosphorylation status of cardiac troponin I was reduced by 84±5%, indicating divergent phosphorylation of the 2 main contractile target proteins of the β-adrenergic pathwa...

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Cardiac Myosin-Binding Protein C Mutations and Hypertrophic Cardiomyopathy : Haploinsufficiency, Deranged Phosphorylation, and Cardiomyocyte Dysfunction

Objective: The alterations in contractile proteins underlying enhanced Ca2+-sensitivity of the contractile apparatus in end-stage failing human myocardium are still not resolved. In the present study an attempt was made to reveal to what extent protein alterations contribute to the increased Ca2+-responsiveness in human heart failure. Methods: Isometric force and its Ca2+-sensitivity were studied in single left ventricular myocytes from non-failing donor (n=6) and end-stage failing (n=10) hearts. To elucidate which protein alterations contribute to the increased Ca2+-responsiveness isoform composition and phosphorylation status of contractile proteins were analysed by one- and two-dimensional gel electrophoresis and Western immunoblotting. Results: Maximal tension did not differ between myocytes obtained from donor and failing hearts, while Ca2+-sensitivity of the contractile apparatus (pCa50) was significantly higher in failing myocardium (ΔpCa50=0.17). Protein analysis indicated that neither re-expression of atrial light chain 1 and fetal troponin T (TnT) nor degradation of myosin light chains and troponin I (TnI) are responsible for the observed increase in Ca2+-responsiveness. An inverse correlation was found between pCa50 and percentage of phosphorylated myosin light chain 2 (MLC-2), while phosphorylation of MLC-1 and TnT did not differ between donor and failing hearts. Incubation of myocytes with protein kinase A decreased Ca2+-sensitivity to a larger extent in failing (ΔpCa50=0.20) than in donor (ΔpCa50=0.03) myocytes, abolishing the difference in Ca2+-responsiveness. An increased percentage of dephosphorylated TnI was found in failing hearts, which significantly correlated with the enhanced Ca2+-responsiveness. Conclusions: The increased Ca2+-responsiveness of the contractile apparatus in end-stage failing human hearts cannot be explained by a shift in contractile protein isoforms, but results from the complex interplay between changes in the phosphorylation status of MLC-2 and TnI.

P. Goldmann

Papers

Increased Ca2+-sensitivity of the contractile apparatus in end-stage human heart failure results from altered phosphorylation of contractile proteins