Published online before print January 29, 2009, doi: 10.1161/CIRCRESAHA.108.193326
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
© 2009 American Heart Association, Inc.
Cellular Biology |
Hypophosphorylation of the Stiff N2B Titin Isoform Raises Cardiomyocyte Resting Tension in Failing Human Myocardium
From the Department of Physiology (A.B., I.F-P, L.v.H., N.H., J.v.d.V, G.J.M.S., W.J.P.) and Cardiology (J.G.F.B.), Institute for Cardiovascular Research, VU University Medical Center Amsterdam, The Netherlands; Institute of Cardiology (A.B., I.É., Z.P.), University of Debrecen Medical and Health Science Center, Debrecen, Hungary; and Department of Physiology (I.F-P., C.G., A.F.L-M.), Faculty of Medicine, University of Porto, Portugal.
Correspondence to Prof Dr Walter J. Paulus, MD, PhD, Laboratory of Physiology, VU University Medical Center Amsterdam, Van der Boechorststraat 7, 1081 BT Amsterdam, The Netherlands. E-mail wj.paulus{at}vumc.nl
 |
Abstract |
|---|
 Top Abstract IntroductionPatients and MethodsResultsDiscussionReferences |
|---|
High diastolic stiffness of failing myocardium results from interstitial fibrosis and elevated resting tension (Fpassive) 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 Fpassive of cardiomyocytes. In left ventricular biopsies of heart failure (HF) patients, aortic stenosis (AS) patients, and controls (CON), we therefore related Fpassive 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-µm sarcomere length to measure Fpassive. 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. Fpassive was higher in HF (6.1±0.4 kN/m2) than in CON (2.3±0.3 kN/m2; P<0.01) or in AS (2.2±0.2 kN/m2; 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 Fpassive of human HF cardiomyocytes.
Key Words: myocardium • heart failure • diastole • titin
|
Introduction |
|---|
|
TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
|---|
Diastolic left ventricular (LV) dysfunction importantly contributes to heart failure (HF) with either reduced LV ejection fraction (EF) (HFREF) or with normal LV ejection fraction (HFNEF).1 In HFREF, diastolic LV dysfunction correlates with exercise intolerance2; in HFNEF, diastolic LV dysfunction is an essential diagnostic feature.3,4 Diastolic LV dysfunction has usually been attributed to interstitial fibrosis because of an imbalance between matrix metalloproteinases and their tissue inhibitors.5,6 Recently high resting tension (Fpassive) of cardiomyocytes has also been implicated in diastolic LV dysfunction.7–9 Cardiomyocytes from patients with HFNEF have higher Fpassive than cardiomyocytes from patients with HFREF and cardiomyocytes from both HF groups have higher Fpassive than cardiomyocytes from controls (CON).7–9 In vitro administration of protein kinase (PK)A to HF cardiomyocytes lowers their elevated Fpassive to the level observed in CON cardiomyocytes,7,8 whereas Fpassive of CON cardiomyocytes does not respond to PKA.7
Alterations in cardiomyocyte Fpassive have been attributed to the giant cytoskeletal protein titin,8 which can modulate Fpassive through isoforms shifts10–12 and phosphorylation status.13–17 In explanted hearts of HFREF patients with ischemic or nonischemic cardiomyopathy,10–12 myocardial titin isoform expression shifts from the stiff N2B to the compliant N2BA isoform with a resultant rise of the N2BA/N2B ratio. In pooled endomyocardial biopsy samples of HFNEF patients, the N2BA/N2B ratio is lower than in HFREF patients,8 and the corresponding higher expression of the stiff N2B isoform could explain the higher Fpassive in HFNEF cardiomyocytes. Correction of the high Fpassive of skinned HF cardiomyocytes by PKA suggests a phosphorylation deficit of titin or of other myofilamentary proteins to be also involved. Phosphorylation of the cardiac-specific N2B spring element of titin by PKA reduces Fpassive of rat cardiomyocytes13 and of rat cardiac myofibrils.14 This phosphorylation-induced reduction of Fpassive is titin isoform–dependent, with the largest effect observed in rat ventricular myocardium, which has an N2BA/N2B ratio of 
0.1, and the smallest effect in bovine atrial myocardium, which has an N2BA/N2B ratio of 9.16 Apart from phosphorylation by PKA, the cardiac-specific N2B spring element of titin can also be phosphorylated by PKG, with a similar reduction of cardiomyofibrillar stiffness.15 Lower cardiomyocyte Fpassive after PKA or PKG can result not only from phosphorylation of titin but also from less diastolic actin–myosin interaction of weakly bound crossbridges. An effect of weakly bound crossbridges on cardiomyocyte Fpassive is unlikely in normal myocardium but possible in failing myocardium because of its increased myofilamentary calcium sensitivity.18,19
The present study investigates the importance for the high Fpassive in HF cardiomyocytes of: (1) titin isoform expression; (2) titin phosphorylation; (3) titin isoform phosphorylation; (4) phosphorylation of other myofilamentary proteins; and (5) formation of weakly bound crossbridges. All measurements were performed using human LV myocardial samples derived from transvascular biopsies, surgical biopsies, and explanted hearts. Transvascular procurement allowed HF patients with less advanced HF to be included in the study.
|
Patients and Methods |
|---|
|
TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
|---|
Patients
The HF group consisted of 48 patients and was composed of 2 subgroups: (1) 44 patients hospitalized for worsening HF (New York Heart Association [NYHA] class IV) and referred for cardiac catheterization and transvascular endomyocardial biopsy because of suspicion of infiltrative or inflammatory myocardial disease; and (2) 4 patients with end-stage dilated cardiomyopathy referred for cardiac transplantation. In all HF patients, coronary angiography revealed absence of significant (>50% luminal diameter reduction) coronary artery stenoses and histological examination of the biopsies or of the explanted hearts ruled out active inflammatory or infiltrative myocardial disease. Treatment consisted of angiotensin converting enzyme inhibitors (48/48), diuretics (48/48) and β-blockers (25/48). The 44 HF patients referred for cardiac catheterization and endomyocardial biopsy consisted of 27 HFREF patients and of 17 HFNEF patients. Their hemodynamic data are listed in the Table. The 4 HF patients referred for cardiac transplantation all had HFREF. Their hemodynamic data were not included in the Table because of the long time interval between hemodynamic evaluation and cardiac transplantation. All patients with HFREF had a LVEF <45%. All patients with HFNEF satisfied the criteria for the diagnosis of HFNEF as recently proposed in a consensus document of the Heart Failure and Echocardiography associations of the European Society of Cardiology.4
|
The CON group consisted of 15 patients and was composed of 3 subgroups: (1) 3 patients with normal LV function and major ventricular arrhythmias; (2) 4 patients with normal LV function and mitral stenosis; and (3) 8 explanted donor hearts. Transvascular LV biopsies were obtained in the first subgroup. In the second subgroup, perioperative transmural LV biopsies were obtained at the time of mitral valve replacement. The endomyocardial portion of the transmural biopsy was used to allow for comparison with the endomyocardial biopsies of the HF patients. No patient had significant (>50% luminal diameter reduction) coronary artery disease. Donors did not undergo hemodynamic evaluation before explantation and their hemodynamic data were therefore not included in the Table.
The AS group consisted of 25 patients with significant and symptomatic aortic valve stenosis (mean aortic valve area: 0.5±0.3 cm2). Patients had syncope, angina, and/or dyspnea (
NYHA class II). Their hemodynamic data are also listed in the Table. No patient had significant (>50% luminal diameter reduction) coronary artery disease. In the AS group, LV myocardial samples were procured perioperatively at the time of aortic valve replacement by transmural biopsy. The endomyocardial portion of the transmural biopsy was used to allow for comparison with the endomyocardial biopsies of the HF patients.
The local ethics committee approved the study protocol. Written informed consent was obtained, and there were no complications related to catheterization or biopsy procurement.
Fpassive Measurements in Isolated Cardiomyocytes
Force measurements were performed in single, mechanically isolated cardiomyocytes as described previously.7–9 Force measurements were obtained in cardiomyocytes isolated from transvascular biopsies (CON: n=3; HF: n=25) and perioperative biopsies (CON: n=4; AS: n=18). Biopsy samples (5 mg wet weight) were defrosted in relaxing solution, mechanically disrupted and incubated for 5 minutes in relaxing solution supplemented with 0.2% Triton X-100 to remove all membrane structures. Single cardiomyocytes were subsequently attached with silicone adhesive between a force transducer and a piezoelectric motor (2.7±0.4 cardiomyocytes per patient). Sarcomere length of isolated cardiomyocytes was adjusted to 2.2 µm. To assess the effect of PKA on the Fpassive, myocytes were incubated for 40 minutes in relaxing solution supplemented with the catalytic subunit of PKA (100 U/mL; Sigma, batch 12K7495) and dithiothreitol (6 mmol/L; Sigma). To study the effect of PKG, Fpassive measurements were also performed after incubation of HF cardiomyocytes in relaxing solution containing PKG1
(0.1 U/mL; Sigma, batch 034K1336), guanosine cGMP (10 µmol/L, Sigma) and dithiothreitol (6 mmol/L; Sigma). After 40 minutes of incubation with PKG, Fpassive measurements were repeated. Thereafter, cardiomyocytes were incubated with PKA, as described above, and their Fpassive were reassessed. To study the contribution of the thin filament to Fpassive, HF cardiomyocytes were incubated for 40 minutes with the actin-capping protein gelsolin (0.05 mg/mL; clone FX-45, kindly provided by Prof Dr Henk Granzier [University of Arizona, Tucson]20) and subsequently exposed to PKA. To investigate the involvement of weak crossbridge interaction, Fpassive of HF cardiomyocytes was also measured in relaxing solution containing 2,3-butanedione monoxime (BDM) (40 mmol/L for 5 minutes; Sigma). Force values were normalized for myocyte cross-sectional area.
Titin Isoform Separation
Titin isoform separation was performed in myocardial samples of 10 HF patients, 9 AS patients, and 8 CON subjects. For 6 HF patients, the myocardial sample was procured by transvascular biopsy and for 4 HF patients the myocardial sample was derived from an explanted heart. For all CON subjects, the myocardial sample was derived from an explanted donor heart. Tissue samples were homogenized in 100 to 200 µL Tris–sodium dodecyl sulfate buffer (pH 6.8) containing 8 µg/mL leupeptin (Peptin-Institute, Japan) and phosphatase inhibitor cocktails (PIC I [P2850] and PIC II [P5726], 10 µL/mL from each; Sigma). Samples were heated (3 minutes at 99°C) and centrifuged (10 minutes, 13.000g at 0°C). Each sample (40 µg dry weight) was applied on agarose-strengthened 2% sodium dodecyl sulfate–polyacrylamide gels. The gel was run at 4-mA constant current for 16 hours. Gels were washed and stained with SYPRO Ruby (Molecular Probes, Eugene, Ore) according to the instructions of the manufacturer. Staining was analyzed with a LAS-3000 system (Fuji Science Imaging Systems) and AIDA Image analyzer software (Isotopenmeßgeräte GmbH, Staubenhardt, Germany).21
Myofilamentary Protein and Titin Isoform Phosphorylation
Myofilamentary protein separation and phosphorylation were determined in myocardial samples procured by transvascular biopsy in 8 HF patients and by perioperative biopsy in 7 AS patients. Tissue was dissolved in 1D sample buffer containing 100 mmol/L dithiothreitol, PIC I, PIC II, and protease inhibitor cocktail (10 µL/mL, P 8340, Sigma), heated (3 minutes at 99°C) and centrifuged (10 minutes, 13.000 g at 0°C), and a sample (35 µg dry weight in 25 µL) was applied on a 3% to 8% gradient gel (Criterion, Bio-Rad). The gel was run at 100 V for 30 minutes followed by 200 V for 50 minutes. The gels were stained for 90 minutes with ProQ Diamond (Molecular Probes). Thereafter, the gels were washed and subsequently stained with SYPRO Ruby (Molecular Probes).21 The intensity of the myosin binding protein-C band was used to normalize for differences in protein loading.
Titin isoform phosphorylation was determined in perioperative biopsies of 9 AS patients and in transvascular biopsies of 6 HF patients. Tissue samples were handled as for titin isoform separation. The gels were subsequently stained with ProQ Diamond and with SYPRO Ruby and analyzed as described above.
Statistical Analysis
Values are given as means±SEM. Statistical significance was set at P<0.05 and was obtained for multiple comparisons between groups by ANOVA followed by a Bonferroni test. Relationships between 2 continuous variables were assessed with linear regression analysis. Statistical analysis was performed with SPSS (version 9.0, SPSS Inc, Chicago, Ill).
|
Results |
|---|
|
TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
|---|
Fpassive of Isolated Cardiomyocytes
Cardiomyocytes were isolated from myocardial samples of the CON (n=7), AS (n=18), and HF (n=25) groups and stretched to an identical sarcomere length of 2.2 µm before measuring Fpassive (Figure 1a). Fpassive in HF cardiomyocytes (6.1±0.4 kN/m2) was higher than in CON cardiomyocytes (2.3±0.3 kN/m2; P<0.01) or in AS cardiomyocytes (2.2±0.2 kN/m2; P<0.001) (Figure 1b). Following administration of PKA, the high Fpassive of HF cardiomyocytes fell (Figure 1c). The high Fpassive of HF cardiomyocytes also fell on administration of PKG and remained unaltered on subsequent treatment with PKA (Figure 1d). The high Fpassive of HF cardiomyocytes was unaltered after 40 minutes of exposure to gelsolin despite maximal Ca2+ activated force falling below 10% of the baseline value. Subsequent administration of PKA continued to induce a significant fall in Fpassive (P<0.05; n=4). Gelsolin exposure confirms involvement of titin in the high Fpassive because it removes the thin filament from the cardiomyocytes.20 A contribution of crossbridge cycling to Fpassive was ruled out by experiments exposing HF cardiomyocytes to BDM. In these experiments BDM failed to alter cardiomyocyte Fpassive (n=4).
|
P<0.01 vs CON) and AS patients (
P<0.001 vs AS). c, After in vitro administration of PKA, Fpassive fell in HF cardiomyocytes but not in CON and AS cardiomyocytes (#P<0.0001 vs HF). d, PKG administration also significantly lowered Fpassive of HF cardiomyocytes (##P<0.0001 vs HF; n=18); however, no further decrease in Fpassive was detected on subsequent PKA treatment.
Titin Isoform Expression
Titin isoform expression was determined in myocardial samples of CON (n=8), AS (n=9) and HF patients (n=10) (Figure 2a). HF patients (0.73±0.06; P<0.001) and AS patients (0.59±0.06; P<0.05) had an N2BA/N2B ratio higher than the CON group (0.39±0.05) (Figure 2b). For 6 HF and all AS patients, myocardial biopsy occurred, respectively, at the time of hemodynamic data acquisition or shortly thereafter. Correlations between N2BA/N2B ratio and hemodynamic variables were limited to this group. A significant correlation (r=0.71; P<0.01) was observed between N2BA/N2B ratio and LV end-diastolic wall stress (LVEDWS) (Figure 2c).
|
Overall Titin Phosphorylation and Titin Isoform Phosphorylation
Because titin isoform expression differed between HF and CON, unequal titin isoform expression could have contributed to the difference in Fpassive between HF and CON cardiomyocytes. However, because titin isoform expression was comparable in HF and AS, mechanisms other than a titin isoform shift had to account for raising Fpassive in HF cardiomyocytes compared to AS cardiomyocytes. Overall phosphorylation of titin was therefore compared between HF and AS patients (Figure 3a). The ratio of phosphorylated titin to total titin was similar in HF and AS samples (0.29±0.02 versus 0.30±0.02; P=0.84). Similar ratios of phosphorylated to total protein were also observed for myosin binding protein-C (0.21±0.02 versus 0.26±0.03; P=0.23), desmin (0.35±0.05 versus 0.34±0.05; P=0.93), troponin T (0.48±0.04 versus 0.55±0.05; P=0.32), and myosin light chain-2 (0.27±0.02 versus 0.31±0.01; P=0.09) (values indicate HF versus AS, respectively) (Figure 3a). Only troponin I had a lower ratio of phosphorylated to total protein in HF compared to AS (0.12±0.01 versus 0.16±0.02; P=0.048).
|
Because titin isoform expression and overall titin phosphorylation were comparable in HF and AS patients, the phosphorylation status of the respective N2BA and N2B titin isoforms was subsequently investigated in myocardial samples of HF and AS patients (Figure 3b). The P-N2BA/P-N2B ratio was higher in HF (0.77±0.05) than in AS (0.54±0.05; P<0.01) (Figure 3c), consistent with lower phosphorylation of the stiff N2B and higher phosphorylation of the compliant N2BA titin isoform. Lower phosphorylation of N2B explains the elevated Fpassive of HF cardiomyocytes because titin phosphorylation was previously shown to lower Fpassive in an isoform-dependent manner, with a larger fall in Fpassive when more of the stiff N2B isoform was phosphorylated.16 The P-N2BA/P-N2B ratio correlated with the N2BA/N2B ratio in AS patients (r=0.68; P<0.05) (Figure 3d). This correlation was absent in HF patients.
HFNEF Versus HFREF
In previous studies, Fpassive was especially elevated in cardiomyocytes isolated from LV myocardium of HFNEF patients.7–9 Fpassive, titin isoform expression and titin isoform phosphorylation were therefore compared in subgroups of HFNEF and HFREF patients (Figure 4a through 4e), whose hemodynamic characteristics are shown in the Table. HFREF patients had the highest N2BA/N2B ratio (0.77±0.07) (Figure 4b). LVEDWS was also highest in HFREF (8.7±0.7 kN/m2) (Figure 4c). Fpassive (7.4±0.7 kN/m2) and the PKA-induced decrease in Fpassive (–3.2±0.6 kN/m2) were largest in HFNEF (Figure 4d). Fpassive and the PKA-induced absolute change in Fpassive rose from AS to HFREF and to HFNEF (Figure 4e) (P<0.05 for trend). The deficit of N2B titin isoform phosphorylation followed a similar course also rising from AS to HFREF and to HFNEF (Figure 4e) (P<0.05 for trend). The deficit was expressed as the difference between observed P-N2BA/P-N2B ratio and predicted P-N2BA/P-N2B ratio. The predicted P-N2BA/P-N2B ratio was derived from the observed N2BA/N2B ratio and the equation describing the linear curve fit of Figure 3d. A larger N2B titin isoform phosphorylation deficit in HFNEF patients explains both their higher Fpassive and their larger PKA-induced fall of Fpassive.
|
P-N2BA/P-N2B) (open squares) and the PKA-induced fall in Fpassive (filled circles). |
Discussion |
|---|
|
TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
|---|
High Fpassive of HF Cardiomyocytes and Titin
In the present study, Fpassive was twice as high in human HF cardiomyocytes than in human AS cardiomyocytes. The high Fpassive of HF cardiomyocytes fell after in vitro administration of PKA or PKG but remained unchanged after in vitro administration of BDM or gelsolin. The PKA- and PKG-induced decrease in Fpassive suggested a phosphorylation deficit of myofilamentary proteins, which are a target for both PKA and PKG. Only troponin I and titin have so far been identified as myofilamentary proteins targeted by both PKA and PKG.15,22 The present study observed a phosphorylation deficit of troponin I in HF myocardium. The concomitant increase in myofilamentary calcium sensitivity could promote diastolic actin–myosin interaction and thereby raise Fpassive in HF cardiomyocytes.19 Such an effect of weakly bound crossbridges on Fpassive was however ruled out by the unchanged Fpassive following exposure to BDM, which inhibits crossbridge cycling. Involvement of the giant cytoskeletal protein titin in the high Fpassive of HF cardiomyocytes was suggested by unaltered Fpassive after gelsolin, which removes the thin filament from the cardiomyocytes.20,23,24 Effective thin filament removal was evident from the fall in maximal Ca2+ activated force to less than 10% of its baseline value. The ability of PKA to lower Fpassive after thin filament removal implied phosphorylation of titin to mediate the PKA-induced fall in Fpassive.
The present study observed titin isoform expression to be comparable in HF and AS myocardium. Mechanisms other than a titin isoform shift therefore had to account for the difference in Fpassive between HF and AS cardiomyocytes. Overall titin phosphorylation was also similar in HF and AS myocardium. However, relative titin isoform phosphorylation was different, with less N2B isoform phosphorylated in HF than in AS myocardium. Less N2B titin isoform phosphorylation was previously shown to raise Fpassive,16 and the observed hypophosphorylation of the stiff N2B titin isoform relative to the compliant N2BA titin isoform could therefore explain the higher Fpassive in HF cardiomyocytes.
Titin Isoform Expression
In a subgroup of HF and AS patients, hemodynamic data were obtained either at the time of transvascular biopsy or shortly before perioperative biopsy. In these patient groups, myocardial titin isoform expression could accurately be correlated with hemodynamic variables. The closest correlation observed was between N2BA/N2B ratio and LVEDWS (Figure 2c). The higher expression of the compliant N2BA titin isoform at higher LVEDWS implies titin isoform shifts to be a compensatory mechanism whereby higher expression of the compliant N2BA titin isoform compensates for an elevated LVEDWS. A compensatory role of titin isoform shifts has previously also been suggested in patients with dilated cardiomyopathy, in whom the N2BA/N2B ratio correlated with the LV end-diastolic volume/pressure ratio, a measure of end-diastolic LV distensibility, and with peak oxygen consumption during exercise.11 The time span over which LVEDWS rises is probably an important determinant for shifts in titin isoform expression because 2 canine dilated cardiomyopathy models with rapid elevation of LVEDWS induced by pacing tachycardia or by intracoronary microembolizations revealed an opposite shift with overexpression of the stiff N2B titin isoform.25–27 Furthermore, the correlation observed in the present study between N2BA/N2B ratio and LVEDWS is also consistent with stress-sensing abilities of titin. Stress-sensing abilities of titin reside both in its kinase domain at the M-line28 and in the titin/Tcap/muscle LIM protein complex at the Z-disc.29
Titin Isoform Phosphorylation
Higher Fpassive in HF than in AS cardiomyocytes was attributed to a shift in isoform phosphorylation with more of the N2BA and less of the N2B titin isoform phosphorylated in HF myocardium. Previous studies indeed demonstrated less reduction in Fpassive when the compliant N2BA titin isoform is phosphorylated than when the stiff N2B titin isoform is phosphorylated.16 The present study not only observed higher N2BA titin isoform phosphorylation in HF myocardium but also a loss in HF myocardium of the close relationship between titin isoform expression and titin isoform phosphorylation, which was present in AS myocardium (Figure 3d). The uniformly low Fpassive observed in AS cardiomyocytes despite the N2BA/N2B titin isoform ratio ranging from 0.4 to 0.9 (Figure 3d), probably resulted from the concordant changes in titin isoform expression and phosphorylation in AS myocardium, whereby higher N2B titin isoform phosphorylation offsets a rise of Fpassive induced by higher N2B titin isoform expression. In HF myocardium, this concordance between titin isoform expression and phosphorylation was absent. This could be of special relevance to HFNEF myocardium, which had higher N2B titin isoform expression (Figure 4b) and a larger N2B titin isoform phosphorylation deficit (Figure 4e). The latter explained both the larger Fpassive and the larger response of Fpassive to PKA in HFNEF cardiomyocytes (Figure 4e).
A shift in titin isoform phosphorylation, as demonstrated in the present study in HF myocardium, can be explained by spatial separation of phosphorylation sites on the N2BA and N2B titin isoforms and by changes in compartmentalized activities of protein kinases and phosphodiesterases.30,31 The N2B unique sequence contains the phosphorylation site, which mediates the fall in Fpassive.13,15,32 Immunoelectron micrographs with antibody labeling C-terminal N2B unique sequence revealed the site to be closer to the Z-disc in the N2BA isoform than in the N2B isoform.26 PKA is targeted by A-kinase anchoring proteins (AKAP) to transverse tubules,30 which are also adjacent to the Z disc. In HF, this targeting of PKA by AKAP is disrupted,33 and this leads to a fall in phosphorylation of surrounding myofilamentary proteins.30 The shift in titin isoform phosphorylation in HF myocardium could result from altered local availability of not only cAMP but also cGMP because in mouse, cardiac muscle phosphodiesterase 5 activity is also highly compartmentalized.34
Conclusions
Because of comparable expression of titin isoforms and similar overall phosphorylation of titin, the 2-fold rise in Fpassive of HF cardiomyocytes compared to AS cardiomyocytes was attributed to a shift in titin isoform phosphorylation with relative hypophosphorylation of the stiff N2B titin isoform. Relative hypophosphorylation of the stiff N2B titin isoform is a novel mechanism responsible for raising Fpassive in human HF cardiomyocytes. High cardiomyocyte Fpassive is an important contributor to the high diastolic LV stiffness of the failing heart, and the present findings could therefore have important therapeutic implications.
|
Acknowledgments |
|---|
Sources of Funding
This study was supported by a grant from the Dutch Heart Foundation (2006B035). A.B. is a recipient of a Mecenatúra Grant from the University of Debrecen Medical and Health Science Center, Hungary (Mec-4/2008) and of a basic research fellowship grant from the Heart Failure Association of the European Society of Cardiology (Nice, France). I.F-P. is a recipient of a grant from the Portuguese Foundation for Science and Technology (SFRH/BD/19538/2004).
Disclosures
None.
|
Footnotes |
|---|
*Both authors contributed equally to this study.
Original received August 12, 2008; resubmission received December 23, 2008; accepted January 21, 2009.
|
References |
|---|
|
TopAbstractIntroductionPatients and MethodsResultsDiscussionReferences |
|---|
1. Kass DA, Bronzwaer JGF, Paulus WJ. What mechanisms underlie diastolic dysfunction in heart failure? Circ Res. 2004; 94: 1533–1542.
2. Skaluba SJ, Litwin SE. Mechanisms of exercise intolerance. Insights from Tissue Doppler Imaging. Circulation. 2004; 109: 972–977.
3. Zile MR, Baicu CF, Gaasch WH. Diastolic heart failure – abnormalities in active relaxation and passive stiffness of the left ventricle. N Engl J Med. 2004; 350: 1953–1959.
4. Paulus WJ, Tschöpe C, Sanderson JE, Rusconi C, Flachskampf FA, Rademakers FE, Marino P, Smiseth OA, De Keulenaer G, Leite-Moreira AF, Borbély A, Édes I, Handoko ML, Heymans S, Pezzali N, Pieske B, Dickstein K, Fraser AG, Brutsaert DL. How to diagnose diastolic heart failure? A consensus statement on the diagnosis of heart failure with normal left ventricular ejection fraction (HFNEF) by the Heart Failure and Echocardiography Associations of the European Society of Cardiology. Eur Heart J. 2007; 28: 2539–2550.
5. Ahmed SH, Clark LL, Pennington WR, Webb CS, Bonnema DD, Leonardi AH, McClure CD, Spinale FG, Zile MR. Matrix metalloproteinases/tissue inhibitors of metalloproteinases. Relationship between changes in proteolytic determinants of matrix composition and structural, functional, and clinical manifestations of hypertensive heart disease. Circulation. 2006; 113: 2089–2096.
6. Martos R, Baugh J, Ledwidge M, O'Loughlin C, Conlon C, Patle A, Donnelly SC, Mc Donald K. Diastolic heart failure: evidence of increased myocardial collagen turnover linked to diastolic dysfunction. Circulation. 2007; 115: 888–895.
7. Borbély A, van der Velden J, Papp Z, Bronzwaer JGF, Edes I, Stienen GJ, Paulus WJ. Cardiomyocyte stiffness in diastolic heart failure. Circulation. 2005; 111: 774–781.
8. Van Heerebeek L, Borbély A, Niessen HW, Bronzwaer JGF, van der Velden J, Stienen GJ, Linke WA, Laarman GJ, Paulus WJ. Myocardial structure and function differ in systolic and diastolic heart failure. Circulation. 2006; 113: 1966–1973.
9. Van Heerebeek L, Hamdani N, Handoko L, Falcao-Pires I, Musters RJ, Kupreishvili K, Ijsselmuiden AJJ, Schalkwijk CG, Bronzwaer JGF, Diamant M, Borbély A, van der Velden J, Stienen GJM, Laarman GJ, Niessen HWM, Paulus WJ. Diastolic stiffness of the failing diabetic heart: importance of fibrosis, advanced glycation endproducts and myocyte resting tension. Circulation. 2008; 117: 52–60.
10. Makarenko I, Opitz CA, Leake MC, Kulke M, Gwathmey JK, del Monte F, Hajjar RJ, Linke WA. Passive stiffness changes caused by upregulation of compliant titin isoforms in human dilated cardiomyopathy hearts. Circ Res. 2004; 95: 708–716.
11. Nagueh SF, Shah G, Wu Y, Torre-Amione G, King NM, Lahmers S, Witt CC, Becker K, Labeit S, Granzier HL. Altered titin expression, myocardial stiffness, and left ventricular function in patients with dilated cardiomyopathy. Circulation. 2004; 110: 155–162.
12. Neagoe C, Kulke M, del Monte F, Gwathmey JK, de Tombe PP, Hajjar RJ, Linke WA. Titin isoform switch in ischemic human heart disease. Circulation. 2002; 106: 1333–1341.
13. Yamasaki R, Wu Y, McNabb M, Greaser M, Labeit S, Granzier H. Protein kinase A phosphorylates titin’s cardiac-specific N2B domain and reduces passive tension in rat cardiac myocytes. Circ Res. 2002; 90: 1181–1188.
14. Krüger M, Linke WA. Protein kinase-A phosphorylates titin in human heart muscle and reduces myofibrillar passive tension. J Muscle Res Cell Motil. 2006; 27: 435–444.[CrossRef][Medline] [Order article via Infotrieve]
15. Krüger M, Kötter S, Grützner A, Lang P, Andresen C, Redfield MM, Butt E, dos Remedios C, Linke WA. Protein kinase G modulates human myocardial passive stiffness by phosphorylation of the titin springs. Circ Res. 2009; 104: 87–94.
16. Fukuda N, Wu Y, Nair P, Granzier HL. Phosphorylation of titin modulates passive stiffness of cardiac muscle in a titin isoform-dependent manner. J Gen Physiol. 2005; 125: 257–271.
17. Borbély A, van Heerebeek L, Paulus WJ. Transcriptional and posttranslational modifications of titin: implications for diastole. Circ Res. 2009; 104: 12–14.
18. Van der Velden J, Papp Z, Zaremba R, Boontje NM, de Jong JW, Owen VJ, Burton PB, Goldmann P, Jaquet K, Stienen GJM. Increased Ca2+-sensitivity of the contractile apparatus in end-stage human heart failure results from altered phosphorylation of contractile proteins. Cardiovasc Res. 2003; 57: 37–47.
19. Flagg TP, Cazorla O, Remedi MS, Haim TE, Tones MA, Bahinski A, Numann RE, Kovacs A, Schaffer JE, Nichols CG, Nerbonne JM. Ca2+-independent alterations in diastolic sarcomere length and relaxation kinetics in a mouse model of lipotoxic diabetic cardiomyopathy. Circ Res. 2009; 104: 95–103.
20. Trombitás K, Granzier H. Actin removal from cardiac myocytes shows that near Z line titin attaches to actin while under tension. Am J Physiol. 1997; 273: C662–C670.[Medline] [Order article via Infotrieve]
21. Zaremba R, Merkus D, Lamers JML, Paulus WJ, dos Remedios C, Duncker DJ, Stienen GJM, van der Velden J. Quantitative analysis of myofilament protein phosphorylation in small cardiac biopsies. Proteomics Clin Appl. 2007; 1: 1285–1290.[CrossRef]
22. Layland J, Li JM, Shah AM. Role of cyclic GMP-dependent protein kinase in the contractile response to exogenous nitric oxide in rat cardiac myocytes. J Physiol. 2002; 540: 457–467.
23. Linke WA, Ivemeyer M, Labeit S, Hinssen H, Rüegg JC, Gautel M. Actin-titin interaction in cardiac myofibrils: probing a physiological role. Biophys J. 1997; 73: 905–919.[Medline] [Order article via Infotrieve]
24. Linke WA, Fernandez JM. Cardiac titin: molecular basis of elasticity and cellular contribution to elastic and viscous stiffness components in myocardium. J Muscle Res Cell Motil. 2002; 23: 483–497.[CrossRef][Medline] [Order article via Infotrieve]
25. Jaber WA, Maniu C, Krysiak J, Shapiro BP, Meyer DM, Linke WA, Redfield MM. Titin isoforms, extracellular matrix, and global chamber remodeling in experimental dilated cardiomyopathy: functional implications and mechanistic insights. Circ Heart Fail. 2008; 1: 192–199.
26. Wu Y, Bell SP, Trombitás K, Witt CC, Labeit S, LeWinter MM, Granzier H. Changes in titin isoform expression in pacing-induced cardiac failure give rise to increased passive muscle stiffness. Circulation. 2002; 106: 1384–1389.
27. Rastogi S, Mishra S, Zaca V, Alesh I, Gupta RC, Goldstein S, Sabbah HN. Effect of long-term monotherapy with the aldosterone receptor blocker eplerenone on cytoskeletal proteins and matrix metalloproteinases in dogs with heart failure. Cardiovasc Drugs Ther. 2007; 21: 415–422.[CrossRef][Medline] [Order article via Infotrieve]
28. Peng J, Raddatz K, Molkentin JD, Wu Y, Labeit S, Granzier H, Gotthardt M. Cardiac hypertrophy and reduced contractility in hearts deficient in the titin kinase region. Circulation. 2007; 115: 743–751.
29. Hayashi T, Arimura T, Itoh-Satoh M, Ueda K, Hohda S, Inagaki N, Takahashi M, Hori H, Yasunami M, Nishi H, Koga Y, Nakamura H, Matsuzaki M, Choi BY, Bae SW, You CW, Han KH, Park JE, Knoll R, Hoshijima M, Chien KR, Kimura A. Tcap gene mutations in hypertrophic cardiomyopathy and dilated cardiomyopathy. J Am Coll Cardiol. 2004; 44: 2192–2201.
30. Fink MA, Zakhary DR, Mackey JA, Desnoyer RW, Apperson-Hansen C, Damron DS, Bond M. AKAP-mediated targeting of protein kinase A regulates contractility in cardiac myocytes. Circ Res. 2001; 88: 291–297.
31. Mongillo M, McSorley T, Evellin S, Sood A, Lissandron V, Terrin A, Huston E, Hannawacker A, Lohse MJ, Pozzan T, Houslay MD, Zaccolo M. Fluorescence resonance energy transfer-based analysis of cAMP dynamics in live neonatal rat cardiac myocytes reveals distinct functions of compartmentalized phosphodiesterases. Circ Res. 2004; 95: 67–75.
32. Trombitás K, Wu Y, Labeit D, Labeit S, Granzier H. Cardiac titin isoforms are coexpressed in the half-sarcomere and extend independently. Am J Physiol. 2001; 281: H1793–H1799.
33. Zakhary DR, Moravec CS, Bond M. Regulation of PKA binding to AKAPs in the heart. Alterations in human heart failure. Circulation. 2000; 101: 1459–1464.
34. Takimoto E, Belardi D, Tocchetti CG, Vahebi S, Cormaci G, Ketner EA, Moens AL, Champion HC, Kass DA. Compartmentalization of cardiac β-adrenergic inotropy modulation by phosphodiesterase type 5. Circulation. 2007; 115: 2159–2167.
This article has been cited by other articles:
 |
|
 |
|
  S. H. Ahmed and M. L. Lindsey Titin Phosphorylation: Myocardial Passive Stiffness Regulated by the Intracellular Giant Circ. Res., September 25, 2009; 105(7): 611 - 613. [Full Text] [PDF]
|
|
|
|
|
|
C. Hidalgo, B. Hudson, J. Bogomolovas, Y. Zhu, B. Anderson, M. Greaser, S. Labeit, and H. Granzier PKC Phosphorylation of Titin's PEVK Element: A Novel and Conserved Pathway for Modulating Myocardial Stiffness Circ. Res., September 25, 2009; 105(7): 631 - 638. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Tschope and W. J. Paulus Doppler Echocardiography Yields Dubious Estimates of Left Ventricular Diastolic Pressures Circulation, September 1, 2009; 120(9): 810 - 820. [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||