Length-dependent activation is modulated by cardiac troponin I bisphosphorylation at Ser23 and Ser24 but not by Thr143 phosphorylation

Abstract

Frank-Starling's law reflects the ability of the heart to adjust the force of its contraction to changes in ventricular filling, a property based on length-dependent myofilament activation (LDA). The threonine at amino acid 143 of cardiac troponin I (cTnI) is prerequisite for the length-dependent increase in Ca²⁺ sensitivity. Thr143 is a known target of protein kinase C (PKC) whose activity is increased in cardiac disease. Thr143 phosphorylation may modulate length-dependent myofilament activation in failing hearts. Therefore, we investigated if pseudo-phosphorylation at Thr143 modulates length dependence of force using troponin exchange experiments in human cardiomyocytes. In addition, we studied effects of protein kinase A (PKA)-mediated cTnI phosphorylation at Ser23/24, which has been reported to modulate LDA. Isometric force was measured at various Ca²⁺ concentrations in membrane-permeabilized cardiomyocytes exchanged with recombinant wild-type (WT) troponin or troponin mutated at the PKC site Thr143 or Ser23/24 into aspartic acid (D) or alanine (A) to mimic phosphorylation and dephosphorylation, respectively. In troponin-exchanged donor cardiomyocytes experiments were repeated after incubation with exogenous PKA. Pseudo-phosphorylation of Thr143 increased myofilament Ca²⁺ sensitivity compared with WT without affecting LDA in failing and donor cardiomyocytes. Subsequent PKA treatment enhanced the length-dependent shift in Ca²⁺ sensitivity after WT and 143D exchange. Exchange with Ser23/24 variants demonstrated that pseudo-phosphorylation of both Ser23 and Ser24 is needed to enhance the length-dependent increase in Ca²⁺ sensitivity. cTnI pseudo-phosphorylation did not alter length-dependent changes in maximal force. Thus phosphorylation at Thr143 enhances myofilament Ca²⁺ sensitivity without affecting LDA, while Ser23/24 bisphosphorylation is needed to enhance the length-dependent increase in myofilament Ca²⁺ sensitivity.

frank-starling's law of the heart describes the ability of the heart to adjust the force of its contraction (stroke volume) to changes in ventricular filling (end-diastolic volume), a property based on length-dependent activation (LDA) of the myofilaments (7, 8). An increase in sarcomere length within the working range of the heart enhances the maximal force-generating capacity and the sensitivity of the myofilaments to calcium. Length-dependent myofilament activation thus represents an important cellular mechanism to adjust cardiac performance in response to increased preload of the heart.

Cardiac troponin I (cTnI) is an important regulator of LDA of the myofilaments. This has been elegantly demonstrated via replacement of endogenous cTnI by slow skeletal troponin I (ssTnI) in cardiac muscle, which significantly reduced the increase in myofilament Ca²⁺ sensitivity upon an increase in sarcomere length (2, 16, 33). More recently, it was demonstrated that the threonine residue 143 (Thr144 in rat and mice, Thr143 in human) of the inhibitory region of cTnI is essential for length-dependent alterations in myofilament Ca²⁺ sensitivity. Substitution of the threonine for a proline at 143 (cTnI_Thr143Pro), as present in ssTnI and fast skeletal TnI, largely ablated the length-dependent increase in myofilament Ca²⁺ sensitivity in cardiac muscle (33).

Interestingly, Thr143 is a well-known target of protein kinase C (PKC), and its phosphorylation has been implicated in heart failure (25, 32, 43) where an increase in PKC-mediated cTnI phosphorylation has been found together with a decreased phosphorylation of the protein kinase A (PKA) sites (43). PKC-mediated Thr143 phosphoryation may, either alone or in concert with downregulated PKA-mediated phosphorylation, underlie changes in LDA in the heart. In the present study we therefore first investigated the effect of Thr143 phosphorylation on LDA in human failing cardiomyocytes. Troponin exchange experiments were used to study the functional effects of pseudo-phosphorylated cTnI at Thr143 (mimicked by aspartic acid) compared with unphosphorylated (mimicked by alanine) and wild-type (WT) cTnI.

The effect of Thr143 phosphorylation may depend on the phosphorylation status of Ser23/24, since it has been demonstrated that PKA-mediated cTnI phosphorylation enhances the length-dependent increase in myofilament Ca²⁺ sensitivity (16). Therefore, secondary experiments were performed with and without exogenous PKA incubation after cTn exchange in nonfailing donor cardiomyocytes. Also, as previous experiments showed that PKA-bisphosphorylation at Ser23 and Ser24 is required to decrease Ca²⁺ sensitivity (39, 44), we investigated effects of pseudo-mono- and pseudo-bisphosphorylation at cTnI-Ser23/24. To study the effect of Ser23/24 phosphorylation on LDA, experiments were performed in failing cardiomyocytes that have a low baseline level of cTnI-Ser23/24 phosphorylation.

Our data show that pseudo-phosphorylation of Thr143 in human cardiomyocytes increases myofilament Ca²⁺ sensitivity without affecting LDA of the sarcomeres. Moreover, in human myocardium Ser23/24 bisphosphorylation is required to increase the length-dependent shift in myofilament Ca²⁺ sensitivity.

MATERIALS AND METHODS

Human myocardial tissue.

Troponin exchange experiments were performed in cardiomyocytes from end-stage failing idiopathic dilated cardiomyopathy (IDCM) hearts (2 males/1 female; left ventricular ejection fraction 16.7 ± 4.4%; age: 54.3 ± 1.9 yr) or from nonfailing donor myocardium, obtained during heart transplantation surgery. The tissue was collected in cardioplegic solution and stored in liquid nitrogen. Samples were obtained after informed consent and with approval of the Human Research Ethics Committee of The University of Sydney (#7326). The investigation conforms with the principles outlined in the Declaration of Helsinki (1997). The human cardiac samples used were extensively characterized (cardiomyocyte force characteristics and cTnI phosphorylation) in a previous study (10).

Expression and purification of recombinant troponin subunits.

Recombinant human troponin complex was produced as described in detail previously (39). Shortly, six different cardiac troponin I (cTnI) forms were made via site-directed mutations of Thr143 and Ser23/Ser24 into aspartic acid (D) to mimic phosphorylation or into alanine (A) to mimic dephosphorylation: 143D, 143A, 23D/24D, 23D/24A, 23A/24D, and 23A/24A. cDNA encoding human cardiac isoforms [troponin C (cTnC), myc-tag labeled cTnT (cTnT-myc), WT cTnI and cTnI mutants] were transformed in Escherichia coli Rosetta2 (22) and cultured under carbenicillin/chloramphenicol selection in Overnight Express TB medium (EMD Biosciences). Cultures were harvested by centrifugation, resuspended in PBS, and centrifuged at 10,000 g. Pellets were stored at −80°C until use.

Troponin subunits were purified using fast protein liquid chromatography (AKTA-FPLC System; Amersham Biosciences) as described previously (22).

Reconstitution of troponin complexes.

Fractions containing equal ratios of cTnT, cTnC, and cTnI subunits were pooled, subsequently purified by AKTA-FPLC chromatography using a Resource Q to remove residual uncomplexed troponin subunits and finally dialyzed against 10 mM imidazole, 200 mM KCl, 5 mM MgCl₂, 2.5 mM EGTA, 1 mM DTT, and 0.1 mM PMSF (pH 6.9; 2 times, 1 liter each) before concentrating the complexes to a final concentration of >2 mg/ml by centrifugation using Centriprep YM-10 centrifugal filters (Millipore).

Exchange of human troponin complex.

Exchange of recombinant cTn in human cardiomyocytes was done as described previously (39). Briefly, single cardiomyocytes were mechanically isolated with a glass tissue homogenizer and permeabilized by Triton X-100 (0.5%; v/v) for 5 min. They were subsequently incubated overnight at 4°C in exchange solution containing 1 mg/ml of recombinant human cTn complex with the addition of 4 mM CaCl₂, 4 mM DTT, 5 μl/ml protease inhibitor cocktail (P8340; Sigma), and 10 μl/ml phosphatase inhibitor cocktail 2 and 3 (P5726 and P0044; Sigma) (pH 6.9). The next day, the cardiomyocytes were washed twice in rigor solution and finally in relaxing solution (5.95 mM Na₂ATP, 6.04 mM MgCl₂, 2 mM EGTA, 139.6 mM KCl, and 10 mM imidazole pH 7.0). This method results in a homogenous distribution of recombinant cTn complex within the exchanged cardiomyocytes (22).

Determination of the degree of troponin exchange.

The degree of troponin exchange was determined as described previously (39). Briefly, to determine the degree of cTn exchange and to assess protein phosphorylation status, part of the suspension of cells was treated with 2D-clean-up kit (GE Healthcare) as described by the manufacturer protocol after overnight cTn exchange. Subsequently, tissue pellets were homogenized in sample buffer containing 15% glycerol, 62.5 mM Tris (pH 6.8), 1% (wt/vol) SDS, and 2% (wt/vol) DTT. Protein concentration measured with RCDC Protein Assay Kit II (Bio-Rad) ranged between 2 to 4 mg/ml.

Immunoblotting was used to determine the degree of exchange of endogenous cTn by recombinant cTn complex. Therefore, recombinant cTnT was labeled with a Myc-tag to allow discrimination between endogenous and recombinant cTn complex. In a previous study we demonstrated that Myc-tag labeling does not interfere with myofilament function (39). Proteins were separated on a one-dimensional 13% SDS-polyacrylamide gel and blotted onto a nitrocellulose membrane (Hybond) using the protocol supplied by the manufacturer in 1 h at 75 V. A specific monoclonal antibody against cTnT (Clone JLT-12; Sigma; dilution 1:1,250) was used to detect endogenous and recombinant cTnT by chemiluminescence (ECL; Amersham Biosciences). We have previously demonstrated that the affinity of the cTnT antibody was the same for cTnT compared with cTnT-Myc and that cTnT loading was within the linear range (39).

Myofilament protein phosphorylation.

The phosphorylation levels of sarcomeric proteins was determined before and after cTn exchange using ProQ-Diamond-stained 1D-gels, as described previously (39). The phosphorylation signals were normalized to the intensities of the SYPRO Ruby stained myosin light chain 2 bands to correct for small differences in protein loading. The PeppermintStick Phosphoprotein marker (Molecular Probes) was used to correct for differences in staining between gels (41). The ratio of the intensities of ProQ-Diamond and SYPRO Ruby stained ovalbumin band was used to correct for inter-gel differences.

The distribution of endogenous phosphorylated species of cTnI was analyzed using Phos-tag acrylamide gels (FMS Laboratory; Hiroshima University, Japan) as described before (10, 18).

Analysis of titin isoform composition.

Tissue samples were weighed and pulverized in liquid nitrogen using a mortar and a pestle. Tissue powder was solubilized in 8 M urea buffer with DTT and 50% glycerol solution with protease inhibitors (0.16 mmol/l leupeptin, 0.04 mmol/l E-64, and 0.2 mmol/l PMSF). Samples were loaded in triplicate on 1% agarose gels stained with SYPRO Ruby to determine titin isoform composition as described previously (27).

Isometric force measurements.

Force measurements in cardiomyocytes exchanged with recombinant cTn were performed as described previously (17, 37). Isometric force was measured at 15°C and LDA was determined by measuring force at different Ca²⁺ concentrations, first at a sarcomere length of 1.8 μm and subsequently at 2.2 μm. The following parameters were determined: passive force at pCa 9.0 (F_pas), maximal force at pCa 4.5 (F_max: total force minus F_pas), Ca²⁺ sensitivity of force development (pCa₅₀), and steepness of the sigmoidal force-pCa relation [Hill coefficient (n_Hill)]. Only a minor decrease in maximal force development was observed during force measurements at a sarcomere length of 1.8 (2.1 ± 2.5%) and 2.2 (7.8 ± 1.9%) μm. Force decline during the entire experiment did not differ between cells exchanged with the different troponin complexes. Troponin-exchanged donor cardiomyocytes were incubated with exogenous PKA for 40 min at 20°C in relaxing solution containing the catalytic subunit of PKA (100 U/incubation; Sigma) after which isometric force measurements were performed at 15°C.

Data analysis.

Data analysis was performed as previously described using the Hill equation to fit force-Ca²⁺ relations:

$$\text{F}({\text{Ca}}^{2+})/{\text{F}}_0={[{\text{Ca}}^{2+}]}^{n\text{Hill}}/({{\text{Ca}}_{50}}^{n\text{Hill}}+{[{\text{Ca}}^{2+}]}^{n\text{Hill}})$$

where F is steady-state force, F₀ the steady-state force at the saturating Ca²⁺ concentration, n_Hill the steepness of the relationship, and Ca₅₀ (or pCa₅₀) represents the midpoint of the relation. One-way ANOVA followed by a Bonferroni post hoc test was used to compare the amount of exchange of the different cTn-complexes. Two-way ANOVA repeated-measures followed by a Bonferroni post hoc test was used to compare groups exchanged with the different cTn complexes (#P < 0.05, significant difference compared with control; for WT, see Table 2–4; for 23A/24A, see Table 5). When two-way ANOVA revealed a significant effect for sarcomere length (P < 0.05), paired t-tests were performed to compare cell measurements at two different sarcomere lengths in each cTn-exchange group (*P < 0.05, 1.8 vs. 2.2 μm). Data values represent an average of all cells of the different IDCM hearts. No significant differences were found between the three IDCM hearts after cTn exchange with the same complex for all force parameters measured. Values are given as means ± SE of n myocytes.

Table 2.

Force measurements in failing cardiomyocytes after exchange with recombinant troponin

	WT	143D	143A
Fmax, kN/m2
1.8 μm	14.5 ± 0.8	13.8 ± 1.3	12.7 ± 1.8
2.2 μm	18.0 ± 1.0*	17.2 ± 1.5*	16.8 ± 1.5*
Fpas, kN/m2
1.8 μm	2.2 ± 0.1	2.2 ± 0.3	1.9 ± 0.3
2.2 μm	3.3 ± 0.3*	3.4 ± 0.4*	3.0 ± 0.3*
pCa50
1.8 μm	5.64 ± 0.02	5.72 ± 0.03†	5.62 ± 0.01
2.2 μm	5.67 ± 0.02*	5.76 ± 0.02*†	5.65 ± 0.01*
nHill
1.8 μm	2.9 ± 0.1	3.0 ± 0.3	3.1 ± 0.1
2.2 μm	2.7 ± 0.1	2.7 ± 0.2	2.8 ± 0.1*

Values are means ± SE. Myofilament force was measured at 1.8- and 2.2-μm sarcomere length and at different Ca²⁺ concentrations in failing cardiomyocytes (8 myocytes per complex) exchanged with troponin complex mutated at cTnI-Thr143 into alanine (143A; dephosphorylation) or aspartic acid (143D; phosphorylation) and wild-type cTn (WT). Maximum force (F_max) at saturating Ca²⁺ concentrations, passive force (F_pas) and Ca²⁺ sensitivity derived from the midpoint of the force-pCa relationship (pCa₅₀) were significantly increased at 2.2 μm compared with 1.8 μm for all complexes. The Hill coefficient (n_Hill; steepness of the force-pCa curves) was slightly lower at 2.2 μm compared with 1.8 μm in all groups. cTn exchange significantly increased Ca²⁺ sensitivity in 143D compared with WT and 143A at both sarcomere lengths. No significant differences among the 3 cTn complexes for F_max, F_pas, and n_Hill were found at both sarcomere lengths. A two-way ANOVA repeated measures followed by a Bonferroni post hoc test was used to compare groups exchanged with the different cTn complexes (

^†P < 0.05, significant difference compared with WT). When the two-way ANOVA revealed a significant effect for sarcomere length (P < 0.05), paired t-tests were performed to compare cell measurements at 2 different sarcomere lengths in each cTn-exchange group (

^*P < 0.05, 1.8 vs. 2.2 μm).

Table 4.

Force measurements in donor cardiomyocytes after exchange with recombinant troponin and subsequent PKA treatment

With PKA	WT	143D	23D/24D
Fmax, kN/m2
1.8 μm	12.8 ± 1.1	11.7 ± 1.1	11.1 ± 1.9
2.2 μm	14.9 ± 1.6*	14.5 ± 1.6	16.2 ± 2.2*
Fpas, kN/m2
1.8 μm	2.0 ± 0.2	2.0 ± 0.2	2.6 ± 0.1
2.2 μm	2.9 ± 0.2*	3.4 ± 0.5*	3.6 ± 0.3*
pCa50
1.8 μm	5.34 ± 0.06	5.47 ± 0.01†	5.37 ± 0.01
2.2 μm	5.44 ± 0.08*	5.56 ± 0.02*	5.48 ± 0.01*
nHill
1.8 μm	2.1 ± 0.1	2.3 ± 0.2	1.7 ± 0.1
2.2 μm	2.1 ± 0.1	2.4 ± 0.2	1.7 ± 0.2

Values are means ± SE. One donor sample exchanged with 143D, 23D/24D and WT troponin complex with PKA treatment (4–5 myocytes per complex). Myofilament force was measured at sarcomere lengths of 1.8 and 2.2 μm at different Ca²⁺ concentrations. Ca²⁺ sensitivity derived from the midpoint of the force-pCa relationship (pCa₅₀) significantly increased at 2.2 μm compared with 1.8 μm for all complexes. Stretching of cardiomyocytes from 1.8 to 2.2 μm increased maximal (F_max) and passive force (F_pas) in all groups, without affecting n_Hill (steepness of the force-pCa curves). Compared with WT, 143D increased Ca²⁺ sensitivity, reaching significance at sarcomere length 1.8 μm (†). pCa₅₀ of WT did not significantly differ from 23D/24D after PKA treatment. No significant differences among the 3 cTn complexes for F_max, F_pas, and n_Hill were found. A two-way ANOVA repeated measures followed by a Bonferroni post hoc test was used to compare groups exchanged with the different cTn complexes (

^†P < 0.05, significant difference compared with WT). When the two-way ANOVA revealed a significant effect for sarcomere length (P < 0.05), paired t-tests were performed to compare cell measurements at 2 different sarcomere lengths in each cTn-exchange group (

^*P < 0.05, 1.8 vs. 2.2 μm).

Table 5.

Force measurements in failing cardiomyocytes after exchange with recombinant troponin mutated at Ser23 and Ser24

	23A/24A	23A/24D	23D/24A	23D/24D
Fmax, kN/m2
1.8 μm	12.4 ± 0.6	11.9 ± 0.5	13.2 ± 0.5	11.5 ± 0.7
2.2 μm	15.4 ± 0.7*	17.3 ± 1.0*	17.7 ± 0.8*	14.5 ± 0.7*
Fpas, kN/m2
1.8 μm	2.2 ± 0.4	1.7 ± 0.2	1.5 ± 0.3	1.9 ± 0.2
2.2 μm	2.8 ± 0.6*	2.2 ± 0.3*	2.1 ± 0.3*	2.2 ± 0.2*
pCa50
1.8 μm	5.57 ± 0.02	5.60 ± 0.01	5.59 ± 0.02	5.40 ± 0.01†
2.2 μm	5.59 ± 0.02	5.62 ± 0.02	5.61 ± 0.02	5.49 ± 0.01*†
nHill
1.8 μm	2.4 ± 0.2	2.1 ± 0.1	2.2 ± 0.1	2.0 ± 0.1
2.2 μm	2.2 ± 0.3	1.9 ± 0.1	2.2 ± 0.1	2.1 ± 0.2

Values are means ± SE. Failing cardiomyocytes exchanged with 23A/24A, 23A/24D, 23D/24A, and 23D/24D (6–9 cells per group). Myofilament force was measured at sarcomere lengths of 1.8 and 2.2 μm at different Ca²⁺ concentrations. Ca²⁺ sensitivity derived from the midpoint of the force-pCa relationship (pCa₅₀) increased at 2.2 μm compared with 1.8 μm for all complexes, however, the increase was only significant in 23D/24D. Stretching of cardiomyocytes from 1.8 to 2.2 μm significantly increased maximal (F_max) and passive force (F_pas) in all groups. There was no significant effect of sarcomere length on n_Hill (steepness of the force-pCa curves). Ca²⁺ sensitivity was significantly lower in 23D/24D compared with 23A/24A at both sarcomere lengths (

^†P < 0.05). No significant differences among the 4 cTn complexes for F_max, F_pas, and n_Hill were found at both sarcomere lengths. A two-way ANOVA repeated measures followed by a Bonferroni post hoc test was used to compare groups exchanged with the different cTn complexes (†P < 0.05, significant difference compared with 23A/24A). When the two-way ANOVA revealed a significant effect for sarcomere length (P < 0.05), paired t-tests were performed to compare cell measurements at 2 different sarcomere lengths in each cTn-exchange group (

^*P < 0.05, 1.8 vs. 2.2 μm).

RESULTS

Pseudo-phosphorylation of Thr143 increases Ca²⁺ sensitivity without affecting length-dependent activation in IDCM cardiomyocytes.

As the effect of pseudo-phosphorylated Thr143 may be modulated by cTnI phosphorylation at Ser23/24, we first analyzed baseline cTnI phosphorylation in the IDCM samples used in the exchange experiments. Phostag analysis showed a relatively low cTnI phosphorylation in IDCM hearts (n = 3): 7.5 ± 0.9% bisphosphorylated, 27.2 ± 4.7% monophosphorylated and 65.3 ± 5.1% unphosphorylated cTnI (Fig. 1A).

Fig. 1.

Baseline cardiac troponin I phosphorylation and quantification of troponin exchange in cardiomyocytes. A: distribution of cardiac troponin I (cTnI) phosphospecies of the 3 idiopathic dilated cardiomyopathy (IDCM) samples and the donor sample used in the exchange experiments determined via a Phos tag-blot. B: immunoblot stained with an antibody against troponin T (cTnT) that recognizes both endogenous cTnT (lower band) and recombinant Myc-tag-labeled cTnT (cTnT-myc). An example is shown of a suspension of IDCM cardiomyocytes exchanged with recombinant cTn (143A, 143D, or WT). The bar graphs indicate average percentages of cTn exchange in cardiomyocytes after overnight incubation in exchange solution containing the different cTnI species (average values from 3 IDCM hearts). No significant differences were found in exchange percentage between the 3 complexes. Error bars are visible when larger than symbol size. 143A, pseudo-dephosphorylated cTnI at Thr143; 143D, pseudo-phosphorylated cTnI at Thr143; WT, wild-type cTnI.

Troponin exchange was determined using immunoblot detection of the myc-tag labeled cTnT. Figure 1B depicts a representative immunoblot loaded with IDCM cardiomyocytes incubated overnight with recombinant cTn containing pseudo-dephosphorylated Thr143 (143A), pseudo-phosphorylated Thr143 (143D), and WT cTnI. Myc-tagged cTnT migrates more slowly through the gel compared with endogenous cTnT and therefore two cTnT bands are found. The percentage of cTn exchange was calculated from the ratio of myc-tagged recombinant cTnT and the total amount of cTnT. The average exchange percentage of the three cTn complexes in IDCM amounted to 68.4 ± 3% and did not significantly differ (P = 0.58) between the three cTn complexes (Fig. 1B), indicating similar incorporation in the myofilaments.

Previous studies indicated that sarcomeric proteins, other than cTnI, may modify length-dependent activation (9, 19). To assess if cTn exchange affected phosphorylation of myofilament proteins other than troponin, ProQ-Diamond staining was performed before and after cTn exchange. In agreement with our previous study (39), no significant differences in protein phosphorylation were induced in the other contractile proteins studied (cardiac myosin-binding protein-C, myosin light chain 2, and desmin) upon cTn exchange (Table 1). Since endogenous cTnT phosphorylation level was low, no significant reduction in cTnT phosphorylation could be detected upon exchange with unphosphorylated whole troponin complex via ProQ-diamond staining. The level of cTnT phosphorylation did not differ between cells exchanged with the different cTn complexes (Table 1).

Table 1.

Myofilament protein phosphorylation before and after troponin exchange

	cMyBPc	Desmin	MLC2	cTnT
Before exchange	0.39 ± 0.02	0.47 ± 0.13	0.35 ± 0.10	0.20 ± 0.04
143A	0.37 ± 0.02	0.48 ± 0.14	0.36 ± 0.10	0.15 ± 0.01
143D	0.37 ± 0.03	0.46 ± 0.17	0.33 ± 0.09	0.15 ± 0.02
WT	0.35 ± 0.03	0.45 ± 0.08	0.29 ± 0.03	0.17 ± 0.02

Values are means ± SE of the ProQ/SYPRO intensity ratio. Shown is myofilament protein phosphorylation determined with ProQ-diamond staining before and after exchange in 3 idiopathic dilated cardiomyopathy samples. Troponin exchange did not affect myofilament protein phosphorylation (by one-way ANOVA comparing before exchange to the exchanged groups).

cMyBP-C, cardiac myosin-binding protein-C; MLC2, myosin light chain 2; cTnT, cardiac troponin T; A, alanine substitution; D, aspartic acid substitution; WT, wild type.

To determine the effects of Thr143 phosphorylation on myofilament force development, force measurements were performed in failing cardiomyocytes exchanged with 143D. As controls, 143A was used, where alanine was used to mimic dephosphorylation at Thr143, and unphosphorylated WT cTnI. Ca²⁺ sensitivity of force development was measured at 2.2 μm sarcomere length in the cTn-exchanged IDCM cardiomyocytes (1 mg/ml recombinant cTn; 3 IDCM hearts, 11–13 cells in total). Myofilament Ca²⁺ sensitivity was significantly increased in 143D, evident from the leftward shift of the force-pCa curve for 143D compared with WT (Fig. 2A), while 143A did not change Ca²⁺ sensitivity compared with WT. The midpoint of the force-pCa relation (pCa₅₀) was significantly higher in 143D-exchanged cells compared with both controls (WT and 143A; Fig. 2B).

Fig. 2.

Myofilament Ca²⁺ sensitivity is increased upon Thr143 pseudo-phosphorylation without affecting length-dependent activation in cardiomyocytes from failing human hearts. A: Myofilament force development measured at 2.2 μm sarcomere length at various Ca²⁺ concentration Ca²⁺ in permeabilized cardiomyocytes from IDCM patients in which endogenous troponin complex was partially exchanged (68.4 ± 3%) with recombinant myc-tag-labeled troponin complexes (11–13 cardiomyocytes in total from 3 IDCM hearts per group). B: compared with unphosphorylated wild-type cTnI (WT), Ca²⁺ sensitivity derived from the midpoint of the force-pCa relationship (pCa₅₀) was significantly increased after exchange with pseudo-phosphorylated cTnI at Thr143 (143D). Pseudo-dephosphorylation at Thr143 (143A) did not alter Ca²⁺ sensitivity compared with WT. C: length-dependent increase in Ca²⁺ sensitivity was studied by measuring force at sarcomere lengths of 1.8 and 2.2 μm (8 cells for each group, 71.0 ± 4.4% exchange). D: exchange with 143D did not significantly alter the length-dependent increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀: 1.8 vs. 2.2 μm) compared with 143A and WT. *P < 0.05, 143D vs. 143A and WT in posttest Bonferroni analyses of one-way ANOVA.

To study the effect of Thr143 phosphorylation on length-dependent myofilament activation, force measurements were also performed at 1.8 μm (Fig. 2C). The effect of sarcomere lengthening on myofilament Ca²⁺ sensitivity is displayed as the ΔpCa₅₀, which is the difference in pCa₅₀ at a sarcomere length of 1.8 μm (Fig. 2C) and 2.2 μm (Fig. 2A). For all cTn complexes, Ca²⁺ sensitivity significantly increased upon cardiomyocyte lengthening. The increase in pCa₅₀ did not differ among groups (ΔpCa₅₀: WT: 0.04 ± 0.01; 143D: 0.05 ± 0.01; and 143A: 0.03 ± 0.01; Fig. 2D). Sarcomere lengthening from 1.8 to 2.2 μm significantly increased maximal (F_max) and passive (F_pas) force in all three cTn-exchanged groups (Table 2), which is in accordance with the well-known length-force relationship (1). The n_Hill slightly decreased upon sarcomere lengthening, only reaching a significant difference for 143A (Table 2). No significant differences in F_max, F_pas, and n_Hill were found between cells exchanged with the three cTn complexes at both sarcomere lengths. Exchange in IDCM cardiomyocytes with cTn complex containing unphosphorylated cTnI (WT and 143A) did not affect contractile parameters compared with control (no cTn exchange) IDCM cardiomyocytes (2 IDCM samples; 5 cells) at short and long sarcomere lengths, respectively: F_max: 12.2 ± 1.5 and 16.5 ± 2.3 kN/m²; F_pas: 1.5 ± 0.3 and 2.7 ± 0.5 kN/m²; pCa₅₀: 5.64 ± 0.02 and 5.68 ± 0.02; n_Hill: 3.2 ± 0.3 and 3.1 ± 0.2.

Overall, these data show that phosphorylation at Thr143 increases myofilament Ca²⁺ sensitivity but does not alter LDA of the sarcomeres in failing cardiomyocytes.

Pseudo-phosphorylation of Thr143 and Ser23/24 in donor cardiomyocytes.

The absence of an effect of Thr143 phosphorylation in failing cells may be due to a blunted LDA of failing myocardium, as a reduced length-dependent increase in myofilament Ca²⁺ sensitivity has been reported in failing compared with donor cardiac tissue (30). In our recent study we found a ΔpCa₅₀ of 0.10 ± 0.01 in six donor samples (31), which is significantly larger than the value found in the IDCM samples used in this study without cTn exchange (ΔpCa₅₀ = 0.04 ± 0.01). Such a blunted length-dependent increase in Ca²⁺ sensitivity in failing cardiomyocytes may be due to the low baseline cTnI phosphorylation but could also be related to changes in myofilament proteins other than troponin. Therefore, we studied the effects of Thr143 pseudo-phosphorylation in combination with PKA-mediated effects on LDA in nonfailing donor cardiomyocytes. The effects of pseudo-phosphorylation at Thr143 (143D) and at Ser23/24 (23D/24D) were studied without and with treatment with exogenous PKA. The average cTn exchange percentage in donor cardiomyocytes amounted to 82.6 ± 3.1% and did not differ among the three complexes (WT, 143D, and 23D/24D). Phosphorylation of other myofilament proteins than cTnI in exchanged donor samples corresponded to cMyBP-C: 0.57 ± 0.04; desmin: 0.47 ± 0.06; MLC2: 0.28 ± 0.01; and cTnT: 0.24 ± 0.02.

Similar to IDCM, 143D significantly increased Ca²⁺ sensitivity compared with WT in one donor sample at both sarcomere lengths, while Ca²⁺ sensitivity was significantly lower compared with WT in cells exchanged with 23D/24D (Fig. 3, A–C). No differences were found in ΔpCa₅₀ between WT and 143D, while the length-dependent change in pCa₅₀ was higher in cells exchanged with 23D/24D (Fig. 3D and Table 3).

Fig. 3.

Phosphorylation of Ser23 and Ser24 regulates the length-dependent increase in Ca²⁺ sensitivity in donor cardiomyocytes. Myofilament force development was measured at a sarcomere length of 1.8 (A) and 2.2 μm (B) at various Ca²⁺ concnetrations in donor cardiomyocytes in which endogenous troponin complex was partially exchanged (82.6 ± 3.1%) with recombinant myc-tag-labeled troponin complexes (4–5 cardiomyocytes per group). C: compared with unphosphorylated wild-type cTnI (WT), Ca²⁺ sensitivity derived from the midpoint of the force-pCa relationship (pCa₅₀) was significantly increased after exchange with pseudo-phosphorylated cTnI at Thr143 (143D) and significantly decreased upon pseudo-bisphosphorylation of Ser23/Ser24 (23D/24D) at 2.2 μm. D: length-dependent increase in Ca²⁺ sensitivity (ΔpCa₅₀) did not differ between 143D and WT. However, exchange with 23D/24D significantly increased the length-dependent Ca²⁺ sensitivity increase compared with WT. E and F: troponin-exchanged donor cardiomyocytes were incubated with exogenous PKA. E: PKA treatment decreased Ca²⁺ sensitivity in cells exchanged with WT and 143D compared with untreated cardiomyoctes after exchange but had no effect in 23D/24D cells (Fig. 3, E vs. C). F: PKA treatment significantly increased the length-dependent increase in Ca²⁺ sensitivity for WT and 143D compared with untreated cells after exchange (Fig. 3, D vs. F). No significant differences in ΔpCa₅₀ were found after PKA treatment of cells exchanged with the 3 different cTn complexes. *P < 0.05, WT vs. 143D and 23D/24D complexes in posttest Bonferroni analyses of one-way ANOVA. PKA treated cardiomyocytes were compared with untreated cardiomyocytes after exchange with the same complex via a Student's t-test. G: myofilament proteins of cardiomyocytes after cTn exchanged without (lane 1) and with (lane 2) PKA treatment were separated by 1D-gel electrophoresis. Staining differences between gels were corrected with phosphorylated ovalbumin of the peppermint marker (PM). Phosphorylation signal on ProQ Diamond-stained gels was divided by the SYPRO signal of the same protein to correct for minor differences in loading. Phosphorylation levels of cTnI and cMyBP-C increased after PKA treatment.

Table 3.

Force measurements in donor cardiomyocytes after exchange with recombinant troponin without PKA treatment

Without PKA	WT	143D	23D/24D
Fmax, kN/m2
1.8 μm	12.4 ± 0.5	13.3 ± 1.4	11.7 ± 1.1
2.2 μm	16.2 ± 0.5*	16.7 ± 1.2*	16.3 ± 0.9*
Fpas, kN/m2
1.8 μm	2.2 ± 0.3	2.5 ± 0.6	2.9 ± 0.3
2.2 μm	3.9 ± 0.6*	4.1 ± 0.8*	4.0 ± 0.3*
pCa50
1.8 μm	5.55 ± 0.05	5.69 ± 0.02†	5.37 ± 0.01†
2.2 μm	5.57 ± 0.05	5.71 ± 0.02†	5.46 ± 0.01*†
nHill
1.8 μm	1.9 ± 0.1	2.7 ± 0.4†	2.0 ± 0.1
2.2 μm	1.9 ± 0.1	2.4 ± 0.3	1.7 ± 0.1

Values are means ± SE. One donor sample exchanged with 143D, 23D/24D, and WT troponin complex (4–5 myocytes per complex). Myofilament force was measured at sarcomere lengths of 1.8 and 2.2 μm at different Ca²⁺ concentrations. Ca²⁺ sensitivity derived from the midpoint of the force–pCa relationship (pCa₅₀) increased at 2.2 μm compared with 1.8 μm for all complexes; however, the difference was only significant in 23D/24D. Stretching of cardiomyocytes from 1.8 to 2.2 μm increased maximal (F_max) and passive force (F_pas) in all groups. There was no significant effect of sarcomere length on n_Hill (steepness of the force-pCa curves). Compared with WT, 143D significantly increased Ca²⁺ sensitivity and 23D/24D decreased Ca²⁺ sensitivity at both sarcomere lengths. No significant differences among the 3 cTn complexes for F_max and F_pas were found. n_Hill was significantly higher in 143D compared with WT only at sarcomere length 1.8 μm. A two-way ANOVA repeated measures followed by a Bonferroni post hoc test was used to compare groups exchanged with the different cTn complexes (

^†P < 0.05, significant difference compared with WT). When the two-way ANOVA revealed a significant effect for sarcomere length (P < 0.05), paired t-tests were performed to compare cell measurements at 2 different sarcomere lengths in each cTn-exchange group (

^*P < 0.05, 1.8 vs. 2.2 μm).

The relatively low ΔpCa₅₀ in WT and 143D compared with donor cardiomyocytes without cTn exchange (4 cells; ΔpCa₅₀ = 0.10 ± 0.03) and 23D/24D-exchanged cells (Fig. 3D) may be explained by removal of phosphorylated endogenous cTnI upon exchange with recombinant unphosphorylated cTn complex. It is known that donor cardiomyocytes are relatively highly phosphorylated at Ser23/24 (10, 39). Phostag analysis showed 52.7% bisphosphorylated, 35.9% monophosphorylated, and 11.4% unphosphorylated cTnI at baseline in the donor tissue used in this study (Fig. 1A). Treatment of cTn-exchanged donor cardiomyocytes with exogenous PKA reduced Ca²⁺ sensitivity in WT and 143D (Fig. 3E) compared with cTn-exchanged cells without PKA treatment (Fig. 3C). Moreover, PKA treatment increased ΔpCa₅₀ in WT and 143D (Fig. 3, F vs. D). ProQ-Diamond staining illustrated that cTnI and cMyBP-C phosphorylation was 5.5 ± 0.4 and 1.8 ± 0.1 times higher, respectively, in cTn-exchanged donor cells treated with PKA compared with untreated cTn-exchanged donor cells (Fig. 3G). This is in line with our previous study (31), where selective PKA-mediated phosphorylation of cMyBP-C and cTnI was found. After PKA treatment, WT and 143D displayed a similar length-dependent increase in myofilament Ca²⁺ sensitivity as 23D/24D-exchanged cells (Fig. 3F). No effect of PKA-mediated phosphorylation on the length-dependent increase in Ca²⁺ sensitivity was found in donor cardiomyocytes after exchange with 23D/24D compared with 23D/24D without PKA treatment (Fig. 3, D and F). This most likely demonstrates that PKA-mediated cTnI phosphorylation at Ser23/24 was responsible for the observed enhancement in the length-dependent increase in Ca²⁺ sensitivity in human donor cardiomyocytes after exchange with WT and 143D. However, this does not exclude that phosphorylation of other PKA-mediated phosphorylation targets like titin and cMyBP-C may affect the length-dependent increase in Ca²⁺ sensitivity, since it is possible that phosphorylation of cMyBP-C and titin exerted opposite effects on the length-dependent change in Ca²⁺ sensitivity (6, 9).

In line with our experiments in IDCM cardiomyoctes, a sarcomere length increase from 1.8 to 2.2 μm in donor cardiomyocytes without (Table 3) or with (Table 4) PKA treatment increased F_max and F_pas for all cTn complexes, without changing n_Hill. Between groups (WT, 143D, and 23D/24D), no significant differences were found for F_max and F_pas (Tables 3 and 4). The n_Hill was significantly higher in 143D compared with WT at a sarcomere length of 1.8 μm without PKA treatment (Table 3).

Overall, our data from failing and nonfailing human cardiomyocytes show that cTnI pseudo-phosphorylation at the PKC site Thr143 does not modulate length-dependent myofilament activation.

Both Ser23 and Ser24 need to be phosphorylated to increase length-dependent activation.

Our data in donor cardiomyocytes confirm a previous study (16) that indirectly demonstrated that phosphorylation of cTnI-Ser23/24 enhances the increase in Ca²⁺ sensitivity upon an increase in sarcomere length. However, while monophosphorylation of Ser23 or Ser24 has been reported in the human heart (42, 43), no functional effect of monophosphorylation has been found (39, 44). Therefore, we investigated if monophosphorylation of Ser23 or Ser24 would affect LDA of the myofilaments. For these experiments IDCM cardiomyocytes were used, which showed <7% endogenous bisphosphorylation at Ser23/24 (Fig. 1A). Due to the low baseline cTnI phosphorylation, the remaining endogenous Ser23/24 bisphosphorylation after cTn exchange does not interfere with functional effects of mono-and bisphosphorylated cTn complexes. IDCM cardiomyocytes (6–9 cells per cTn complex) were exchanged with four different pseudo-phosphorylation variants of Ser23/24. The average cTn exchange percentage was 76.6 ± 1.9% and did not differ between the complexes. In accordance with our previous observations in nonfailing donor cardiomyocytes (39), only pseudo-bisphosphorylation decreased Ca²⁺ sensitivity compared with mono- and dephosphorylated Ser23/24 in IDCM cardiomyocytes (Fig. 4A). Compared with pseudo-dephosphorylated Ser23/24 (23A/24A), pseudo-monophosphorylation at Ser23 (23D/24A) or Ser24 (23A/24D) did not affect LDA (Fig. 4B and Table 5). Upon exchange with pseudo-bisphosphorylated Ser23/24 (23D/24D), the length-dependent increase in Ca²⁺ sensitivity was significantly increased compared with 23A/24A (Fig. 4B). The enhanced length-dependent increase in Ca²⁺ sensitivity in 23D/24D was significantly higher compared with control IDCM cardiomyocytes without exchange. The sarcomere length increase from 1.8 to 2.2 μm increased F_max and F_pas for all cTn complexes, without changing n_Hill (Table 5).

Fig. 4.

Phosphorylation of both Ser23 and Ser24 is required to increase length-dependent activation in failing cardiomyocytes. Myofilament force development was measured at sarcomere lengths of 1.8 and 2.2 μm at various Ca²⁺ concentrations in failing cardiomyocytes in which endogenous troponin complex was partially exchanged (76.6 ± 1.9%) with recombinant myc-tag-labeled troponin complexes (6–9 cells per group). A: compared with pseudo-unphosphorylated Ser23/24 (23A/24A), Ca²⁺ sensitivity derived from the midpoint of the force-pCa relationship (pCa₅₀) decreased upon pseudo-bisphosphorylation of Ser23 and Ser24 (23D/24D), while no effect was seen with mono-phosphorylated complexes (23A/24D and 23D/24A). B: length-dependent increase in Ca²⁺ sensitivity was not changed compared with pseudo-unphosphorylated Ser23/24 (23A/24A) upon pseudo-monophosphorylation of Ser23 or Ser24 (23D/24A, 23A/24D). Pseudo-bisphosphorylation of Ser23 and Ser24 enhanced the length-dependent Ca²⁺ sensitivity increase compared with 23A/24A. *P < 0.05, 23A/24A vs. 23A/24D, 23D/24A, and 23D/24D complexes in posttest Bonferroni analyses of one-way ANOVA.

Length-dependent myofilament activation and titin-based passive stiffness.

Titin-based passive stiffness has been shown to exert a profound effect on length-dependent activation. A high expression of the stiff N2B isoform in cardiac tissue has been associated with a higher passive force and larger length-dependent increase in myofilament Ca²⁺ sensitivity compared with tissue with a high expression of compliant N2BA isoform (9). Thus changes in titin isoform composition, as have been reported in human heart failure (21, 23), may alter length-dependent activation. In our exchange experiments, passive tension did not differ between the different experimental groups (Tables 2–5). Moreover, no differences were observed in F_pas and titin isoform composition (Fig. 5) between the failing and donor samples used in the present study.

Fig. 5.

Titin isoform composition in the IDCM and donor tissue. Titin isoform composition was analyzed in the IDCM and donor samples. A SYPRO-stained gel shows the compliant N2BA and stiff N2B titin isoform. On average, titin composition in the IDCM samples did not differ from the donor sample.

DISCUSSION

This study demonstrates that cTnI pseudo-phosphorylation at the PKC-site Thr143 increases myofilament Ca²⁺ sensitivity without affecting the length-dependent increase in maximal force or Ca²⁺ sensitivity in human failing and nonfailing donor cardiomyocytes. The length-dependent increase in Ca²⁺ sensitivity is enhanced by pseudo-phosphorylation of Ser23/24 only upon phosphorylation of both sites.

Phosphorylation of Thr143 increases Ca²⁺ sensitivity without affecting length-dependent activation.

Tachampa et al. (33) showed that substitution of a single amino acid at position 143 of cTnI (from Thr to Pro) greatly diminished the length-dependent increase in myofilament Ca²⁺ sensitivity. Since Thr143 is a well-known PKC-mediated phosphorylation site that has been implicated in heart failure (25, 32, 43), we investigated if phosphorylation of Thr143 may modify LDA of the myofilaments in human cardiomyocytes. We show that pseudo-phosphorylation of Thr143 does not affect the length-dependent increase in maximal force and Ca²⁺ sensitivity irrespective of the level of cTnI phosphorylation at PKA sites.

The finding that Thr143 phosphorylation does not affect LDA is not in conflict with the previous finding that a substitution of Thr143 by a proline, as present in ssTnI and fast skeletal TnI, severely blunts LDA in cardiac muscle. A proline is a neutral amino acid with very specific properties [reviewed by Williamson (40)]. Replacement of a Thr by a Pro alters the amino acid charge differently compared with threonine phosphorylation. Likewise, mutations in the inhibitory region of cTnI, which are associated with inherited forms of cardiomyopathies, may disrupt LDA. We recently reported blunted LDA in myocardium from patients with hypertrophic cardiomyopathy harboring a mutation (R145W) in cTnI (31). Overall, these results indicate that mutation-induced changes in amino acids in the inhibitory region of cTnI and phosphorylation-mediated changes of Thr143 exert different effects on sarcomere function. While the threonine at position 143 is key for LDA in cardiac muscle, phosphorylation at this site does not modulate LDA.

Pseudo-phosphorylation of Thr143 using aspartic acid at amino acid 143 increased Ca²⁺ sensitivity both in IDCM and donor cardiomyocytes. In contrast, exchange in mouse fibers with pseudo-phosphorylated Thr143 mimicked by a glutamate did not change (5) or nonsignificantly decreased Ca²⁺ sensitivity (32) compared with WT exchange. Our data are in line with a study of Wang et al. (38) where incorporation of phosphate instead of pseudo-phosphorylation was used. By using genetically modified mice harboring alanine mutations at the different phosphorylation sites of cTnI, they demonstrated that PKC-βII preferentially phosphorylates Thr143 and thereby increased Ca²⁺ sensitivity. This suggests that an aspartic acid pseudo-phosphorylation at Thr143 more closely resembles PKC-mediated phosphorylation of Thr143 compared with a glutamate pseudo-phosphorylation.

The absence of an effect of Thr143 pseudo-phosphorylation on LDA is in line with the preserved LDA at the myofilament level in a mouse model of heart failure induced by overexpression of PKCε (20), which has been demonstrated to phosphorylate Thr143 (15). Thr143 phosphorylation did not increase the relatively small length-dependent increase in Ca²⁺ sensitivity when Ser23/24 are largely unphosphorylated in failing cells. One possibility to explain this may be the elevated Ca²⁺ sensitivity, limiting further Ca²⁺ sensitivity enhancement upon lengthening. However, pseudo-phosphorylation of Thr143 in combination with phosphorylation of Ser23/24 also did not change the enhanced length-dependent increase in Ca²⁺ sensitivity. In donor cardiomyocytes, WT without PKA showed a pCa₅₀ of 5.57 ± 0.05 and a relatively low length-dependent increase in Ca²⁺ sensitivity (Fig. 3, C and D), while 143D after PKA displayed the same pCa₅₀ (5.56 ± 0.02) with an enhanced length-dependent Ca²⁺ sensitivity increase (Fig. 3, E and F). These observations support the conclusion that an enhanced length-dependent increase in Ca²⁺ sensitivity is not directly related to a lower myofilament Ca²⁺ sensitivity induced by phosphorylation of Ser23/24. This is in line with our previous study where a low Ca²⁺ sensitivity at a sarcomere length of 2.2 μm was found in a patient with a cTnI mutation (R145W) after treatment with exogenous PKA together with a blunted length-dependent increase in Ca²⁺ sensitivity (31). Titin isoform composition did not differ between the samples used for exchange experiments and therefore most likely did not contribute to the changes observed in length-dependent myofilament activation in our exchange experiments. In summary, our data show that PKC-mediated phosphorylation at Thr143 sensitizes the myofilament to Ca²⁺ without affecting length dependence of the sarcomeres independent of the initial value of Ca²⁺ sensitivity, which is set by the phosphorylation status of other troponin sites and sarcomeric proteins.

Phosphorylation of both Ser23 and Ser24 increases length-dependent activation.

Exchange with WT troponin complex did not alter the length-dependent increase in Ca²⁺ sensitivity in IDCM cardiomyocytes compared with (unexchanged) control cardiomyocytes, whereas it significantly reduced the length-dependent increase in Ca²⁺ sensitivity in donor cardiomyocytes compared with control cardiomyocytes. An important difference in cTn between donor and IDCM cardiomyocytes is that donor cardiomyocytes are highly bisphosphorylated at Ser23/24 (>50%), in contrast to end-stage IDCM (<10%) (10, 39). LDA has been reported to be regulated by PKA-mediated phosphorylation of cTnI at these sites, although a reduction (14) and enhancement (16) of the length-dependent change in Ca²⁺ sensitivity has been reported upon PKA incubation. In addition, contrasting results have been reported when pseudo-phosphorylated cTnI at Ser23/24 was exchanged, showing a blunted (28) or enhanced (24) change in length-dependent Ca²⁺ sensitivity compared with WT exchange. The reason for these differences is unclear; however, one possible explanation for the blunted length-dependent increase in Ca²⁺ sensitivity that was found in previous studies may be that a different protocol was used (14, 28). In the present study we first performed all force measurements at a low sarcomere length of 1.8 μm and thereafter performed measurements at 2.2 μm. We did not stretch cardiomyocytes to a sarcomere length of 2.3 μm to prevent damage to the sarcomeres. However, Rao et al. (28) and Kajiwara et al. (14) first measured at a sarcomere length of 2.3 μm and thereafter at a shorter sarcomere length of 2.0 μm.

In this study, we demonstrate using PKA-mediated phosphorylation and the exchange of pseudo-phosphorylated Ser23/24 that phosphorylation of Ser23/24 enhances the length-dependent increase in Ca²⁺ sensitivity in human cardiomyocytes because 1) exchange with WT complex removed endogenous phosphorylation at Ser23/24 in donor cardiomyocytes and blunted the length-dependent increase in Ca²⁺ sensitivity compared with baseline measurements in donor cells. 2) PKA treatment after WT exchange restored the length-dependent increase in Ca²⁺ sensitivity in donor cardiomyocytes. 3) Exchange with pseudo-phosphorylated Ser23/24 preserved the length-dependent increase in Ca²⁺ sensitivity in donor cardiomyocytes and enhanced the length-dependent increase in Ca²⁺ sensitivity in IDCM cardiomyocytes to levels observed in donor. In support, it has been demonstrated that PKA-mediated phosphorylation shifts a shallow length-tension relationship to a steeper length-tension relationship in rat cardiomyocytes (12). In accordance, exchange with pseudo-phosphorylated Ser23/24 in rat cardiomyocytes was associated with a steepened length-tension relationship providing evidence that phosphorylation of cTnI-Ser23/24 is essential to enhance length-dependent activation (11). In addition, we demonstrated using mono-and bisphosphorylated cTn complexes that phosphorylation at both Ser23 and Ser24 is required to enhance the length-dependent increase in myofilament Ca²⁺ sensitivity.

Implication of cTnI phosphorylation at Thr143 and Ser23/24 and in health and disease.

In the present study, we provide evidence that phosphorylation of the PKC site Thr143 does not affect LDA, while bisphosphorylation of the PKA-sites Ser23/24 enhances the length-dependent increase in Ca²⁺ sensitivity in human cardiomyocytes (Fig. 6). In the healthy heart, sympathetic activation during stress and exercise increases heart rate and stroke volume to meet the demands of the body. This is accomplished via activation of β₁-adrenergic receptors, which leads to activation of PKA. PKA phosphorylation of Ca²⁺ handling proteins and myofilament proteins enhances cardiomyocyte contraction and relaxation (for reviews: see Refs. 3, 35). The enhanced length-dependent increase in Ca²⁺ sensitivity induced upon PKA phosphorylation of cTnI-Ser23/24 will increase the range of myofilament Ca²⁺ sensitivities at which the heart can operate within a heart cycle (between short and long sarcomere lengths) as is illustrated in Fig. 6 (WT vs. WT + PKA, 23D/24D, and 23D/24D + PKA, which resembles activation of the heart upon β-adrenergic receptor stimulation).

Fig. 6.

Schematic representation of changes in length-dependent Ca²⁺ sensitivity upon phosphorylation of Thr143 and/or Ser23/24. Data obtained in troponin-exchanged donor cells without and with treatment with exogenous PKA (data from Fig. 3 and Tables 3 and 4) were combined to illustrate the range at which myofilament Ca²⁺ sensitivity (pCa₅₀) may vary in response to phosphorylation at the PKC site Thr143 and to bisphosphorylation at Ser23 and Ser24. Boxes represent the range of Ca²⁺ sensitivity measured at a sarcomere length of 1.8 (lower line) and 2.2 μm (upper line). This figure demonstrates that the sarcomere length-dependent shift in Ca²⁺ sensitivity is relatively small in WT and 143D without PKA (i.e., when cTnI phosphorylation at Ser23 and Ser24 is low). PKA treatment and pseudo-phosphorylation of Ser23/24 increase the range in Ca²⁺ sensitivity at which the sarcomere is operating upon changes in sarcomere length between 1.8 and 2.2 μm.

In end-stage heart failure, the Frank-Starling relationship has been reported to be depressed (13, 29). LDA is believed to be the cellular basis of the Frank-Starling mechanism (26, 34). Schwinger et al. (30) previously reported a reduced length-dependent increase in Ca²⁺ sensitivity in skinned fibers from terminally failing human myocardium. In our study, replacement of endogenous cTn with pseudo-bisphosphorylated cTnI at Ser23/24 enhanced the length-dependent increase in myofilament Ca²⁺ sensitivity in failing cardiomyocytes (Fig. 4B). This suggests that the blunted length-dependent increase in myofilament Ca²⁺ sensitivity, which has been reported previously (30) in end-stage heart failure, may be at least partly caused by reduced bisphosphorylation at Ser23/24 of cTnI relative to explanted donor tissue, which has been reported in several studies (4, 18, 36, 42).

Apart from reduced PKA-mediated cTnI phosphorylation, increased PKC-mediated phosphorylation of Thr143 may modify LDA of cardiac muscle in heart failure since this amino acid has been demonstrated to play a key role in the length-dependent increase in Ca²⁺ sensitivity (33). Pseudo-phosphorylation of the PKC site Thr143 did not affect LDA properties of human heart muscle but did increase Ca²⁺ sensitivity as illustrated in Fig. 6 (WT vs. 143D; WT + PKA vs. 143D + PKA). Based on our results, PKC-mediated phosphorylation of Thr143, which has been implicated in cardiac disease (25, 32, 43), does not restore the blunted length-dependent Ca²⁺ sensitivity increase in the failing heart, although it does increase Ca²⁺ sensitivity. The high Ca²⁺ sensitivity may aid to maintain cardiac output of the failing heart; however, it may also contribute to diastolic dysfunction.

GRANTS

This work was supported by National Heart, Lung, and Blood Institute Grants R01-HL-063038 and R01-HL-76038 and Netherlands Organization for Scientific Research (NWO; VIDI Grant).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

Author contributions: P.J.M.W., A.M.M., G.J.M.S., and J.v.d.V. conception and design of research; P.J.M.W., V.S., D.B.F., and Y.L. performed experiments; P.J.M.W., V.S., G.J.M.S., and J.v.d.V. analyzed data; P.J.M.W., V.S., D.B.F., Y.L., C.G.d.R., A.M.M., G.J.M.S., and J.v.d.V. interpreted results of experiments; P.J.M.W. prepared figures; P.J.M.W., V.S., D.B.F., Y.L., C.G.d.R., A.M.M., G.J.M.S., and J.v.d.V. drafted manuscript; P.J.M.W., G.J.M.S., and J.v.d.V. edited and revised manuscript; P.J.M.W., V.S., D.B.F., Y.L., C.G.d.R., A.M.M., G.J.M.S., and J.v.d.V. approved final version of manuscript.