In Vivo and in Vitro Analysis of Cardiac Troponin I Phosphorylation

Focus on cardiac troponin complex: From gene expression to cardiomyopathy

The cardiac troponin complex (cTn) is a regulatory component of sarcomere. cTn consists of three subunits: cardiac troponin C (cTnC), which confers Ca2+ sensitivity to muscle; cTnI, which inhibits the interaction of cross-bridge of myosin with thin filament during diastole; and cTnT, which has multiple roles in sarcomere, such as promoting the link between the cTnI-cTnC complex and tropomyosin within the thin filament and influencing Ca2+ sensitivity of cTn and force development during contraction. Conditions that interfere with interactions within cTn and/or other thin filament proteins can be key factors in the regulation of cardiac contraction. These conditions include alterations in myofilament Ca2+ sensitivity, direct changes in cTn function, and triggering downstream events that lead to adverse cardiac remodeling and impairment of heart function. This review describes gene expression and post-translational modifications of cTn as well as the conditions that can adversely affect the delicate balance among the components of cTn, thereby promoting contractile dysfunction.

The HCM - Linked Mutation Arg92Leu in TNNT2 Allosterically Alters the cTnC - cTnI Interface and Disrupts the PKA-mediated Regulation of Myofilament Relaxation

Background

Impaired left ventricular relaxation, high filling pressures, and dysregulation of Ca 2+ homeostasis are common findings contributing to diastolic dysfunction in hypertrophic cardiomyopathy (HCM). Studies have shown that impaired relaxation is an early observation in the sarcomere-gene-positive preclinical HCM cohort which suggests potential involvement of myofilament regulators of relaxation. Yet, a molecular level understanding of mechanism(s) at the level of the myofilament is lacking. We hypothesized that mutation-specific, allosterically mediated, changes to the cardiac troponin C-cardiac troponin I (cTnC-cTnI) interface can account for the development of early-onset diastolic dysfunction via decreased PKA accessibility to cTnI.

Methods

HCM mutations R92L-cTnT (Arg92Leu) and Δ160E-cTnT (Glu160 deletion) were studied in vivo , in vitro, and in silico via 2D echocardiography, western blotting, ex vivo hemodynamics, stopped-flow kinetics, time resolved fluorescence resonance energy transfer (TR-FRET), and molecular dynamics simulations.

Results

The HCM-causative mutations R92L-cTnT and Δ160E-cTnT result in different time-of-onset of diastolic dysfunction. R92L-cTnT demonstrated early-onset diastolic dysfunction accompanied by a localized decrease in phosphorylation of cTnI. Constitutive phosphorylation of cTnI (cTnI-D 23 D 24 ) was sufficient to recover diastolic function to Non-Tg levels only for R92L-cTnT. Mutation-specific changes in Ca 2+ dissociation rates associated with R92L-cTnT reconstituted with cTnI-D 23 D 24 led us to investigate potential involvement of structural changes in the cTnC-cTnI interface as an explanation for these observations. We probed the interface via TR-FRET revealing a repositioning of the N-terminus of cTnI, closer to cTnC, and concomitant decreases in distance distributions at sites flanking the PKA consensus sequence. Implementing TR-FRET distances as constraints into our atomistic model identified additional electrostatic interactions at the consensus sequence.

Conclusion

These data indicate that the early diastolic dysfunction observed in a subset of HCM is likely attributable to structural changes at the cTnC-cTnI interface that impair accessibility of PKA thereby blunting β-adrenergic responsiveness and identifying a potential molecular target for therapeutic intervention.

The lack of Troponin I Ser-23/24 phosphorylation is detrimental to in vivo cardiac function and exacerbates cardiac disease

Troponin I (TnI) is a key regulator of cardiac contraction and relaxation with TnI Ser-23/24 phosphorylation serving as a myofilament mechanism to modulate cardiac function. Basal cardiac TnI Ser-23/24 phosphorylation is high such that both increased and decreased TnI phosphorylation may modulate cardiac function. While the effects of increasing TnI Ser-23/24 phosphorylation on heart function are well established, the effects of decreasing TnI Ser-23/24 phosphorylation are not clear. To understand the in vivo role of decreased TnI Ser-23/24 phosphorylation, mice expressing TnI with Ser-23/24 mutated to alanine (TnI S23/24A) that lack the ability to be phosphorylated at these residues were subjected to echocardiography and pressure-volume hemodynamic measurements in the absence or presence of physiological (pacing increasing heart rate or adrenergic stimulation) or pathological (transverse aortic constriction (TAC)) stress. In the absence of pathological stress, the lack of TnI Ser-23/24 phosphorylation impaired systolic and diastolic function. TnI S23/24A mice also had an impaired systolic and diastolic response upon stimulation increased heart rate and an impaired adrenergic response upon dobutamine infusion. Following pathological cardiac stress induced by TAC, TnI S23/24A mice had a greater increase in ventricular mass, worse diastolic function, and impaired systolic and diastolic function upon increasing heart rate. These findings demonstrate that mice lacking the ability to phosphorylate TnI at Ser-23/24 have impaired in vivo systolic and diastolic cardiac function, a blunted cardiac reserve and a worse response to pathological stress supporting decreased TnI Ser23/24 phosphorylation is a modulator of these processes in vivo.

Suppression of lusitropy as a disease mechanism in cardiomyopathies

In cardiac muscle the action of adrenaline on β1 receptors of heart muscle cells is essential to adjust cardiac output to the body's needs. Adrenergic activation leads to enhanced contractility (inotropy), faster heart rate (chronotropy) and faster relaxation (lusitropy), mainly through activation of protein kinase A (PKA). Efficient enhancement of heart output under stress requires all of these responses to work together. Lusitropy is essential for shortening the heartbeat when heart rate increases. It therefore follows that, if the lusitropic response is not present, heart function under stress will be compromised. Current literature suggests that lusitropy is primarily achieved due to PKA phosphorylation of troponin I (TnI) and phospholamban (PLB). It has been well documented that PKA-induced phosphorylation of TnI releases Ca2+ from troponin C faster and increases the rate of cardiac muscle relaxation, while phosphorylation of PLB increases SERCA activity, speeding up Ca2+ removal from the cytoplasm. In this review we consider the current scientific evidences for the connection between suppression of lusitropy and cardiac dysfunction in the context of mutations in phospholamban and thin filament proteins that are associated with cardiomyopathies. We will discuss what advances have been made into understanding the physiological mechanism of lusitropy due to TnI and PLB phosphorylation and its suppression by mutations and we will evaluate the evidence whether lack of lusitropy is sufficient to cause cardiomyopathy, and under what circumstances, and consider the range of pathologies associated with loss of lusitropy. Finally, we will discuss whether suppressed lusitropy due to mutations in thin filament proteins can be therapeutically restored.

Mechanosignaling pathways alter muscle structure and function by post-translational modification of existing sarcomeric proteins to optimize energy usage

A transduced mechanical signal arriving at its destination in muscle alters sarcomeric structure and function. A major question addressed is how muscle mass and tension generation are optimized to match actual performance demands so that little energy is wasted. Three cases for improved energy efficiency are examined: the troponin complex for tuning force production, control of the myosin heads in a resting state, and the Z-disc proteins for sarcomere assembly. On arrival, the regulation of protein complexes is often controlled by post-translational modification (PTM), of which the most common are phosphorylation by kinases, deacetylation by histone deacetylases and ubiquitination by E3 ligases. Another branch of signals acts not through peptide covalent bonding but via ligand interactions (e.g. Ca²⁺ and phosphoinositide binding). The myosin head and the regulation of its binding to actin by the troponin complex is the best and earliest example of signal destinations that modify myofibrillar contractility. PTMs in the troponin complex regulate both the efficiency of the contractile function to match physiologic demand for work, and muscle mass via protein degradation. The regulation of sarcomere assembly by integration of incoming signaling pathways causing the same PTMs or ligand binding are discussed in response to mechanical loading and unloading by the Z-disc proteins CapZ, α-actinin, telethonin, titin N-termini, and others. Many human mutations that lead to cardiomyopathy and heart disease occur in the proteins discussed above, which often occur at their PTM or ligand binding sites.

Phosphorylation of Troponin I finely controls the positioning of Troponin for the optimal regulation of cardiac muscle contraction

Troponin is the Ca²⁺ molecular switch that regulates striated muscle contraction. In the heart, troponin Ca²⁺ sensitivity is also modulated by the PKA-dependent phosphorylation of a unique 31-residue N-terminal extension region of the Troponin I subunit (NH₂-TnI). However, the detailed mechanism for the propagation of the phosphorylation signal through Tn, which results in the enhancement of the myocardial relaxation rate, is difficult to examine within whole Tn. Several models exist for how phosphorylation modulates the troponin response in cardiac cells but these are mostly built from peptide-NMR studies and molecular dynamics simulations. Here we used a paramagnetic spin labeling approach to position and track the movement of the NH₂-TnI region within whole Tn. Through paramagnetic relaxation enhancement (PRE)-NMR experiments, we show that the NH₂-TnI region interacts with a broad surface area on the N-domain of the Troponin C subunit. This region includes the Ca²⁺ regulatory Site II and the TnI switch-binding site. Phosphorylation of the NH₂-TnI both weakens and shifts this region to an adjacent site on TnC. Interspin EPR distances between NH₂-TnI and TnC further reveal a phosphorylation induced re-orientation of the TnC N-domain under saturating Ca²⁺ conditions. We propose an allosteric model where phosphorylation triggered cooperative changes in both the interaction of the NH₂-TnI region with TnC, and the re-orientation of the TnC interdomain orientation, together promote the release of the TnI switch-peptide. Enhancement of the myocardial relaxation rate then occurs. Knowledge of this unique role of phosphorylation in whole Tn is important for understanding pathological processes affecting the heart.

Cardiac contractile dysfunction and protein kinase C-mediated myofilament phosphorylation in disease and aging

Increases in protein kinase C (PKC) are associated with diminished cardiac function, but the contribution of downstream myofilament phosphorylation is debated in human and animal models of heart failure. The current experiments evaluated PKC isoform expression, downstream cardiac troponin I (cTnI) S44 phosphorylation (p-S44), and contractile function in failing (F) human myocardium, and in rat models of cardiac dysfunction caused by pressure overload and aging. In F human myocardium, elevated PKCα expression and cTnI p-S44 developed before ventricular assist device implantation. Circulatory support partially reduced PKCα expression and cTnI p-S44 levels and improved cellular contractile function. Gene transfer of dominant negative PKCα (PKCαDN) into F human myocytes also improved contractile function and reduced cTnI p-S44. Heightened cTnI phosphorylation of the analogous residue accompanied reduced myocyte contractile function in a rat model of pressure overload and in aged Fischer 344 × Brown Norway F1 rats (≥26 mo). Together, these results indicate PKC-targeted cTnI p-S44 accompanies cardiac cellular dysfunction in human and animal models. Interfering with PKCα activity reduces downstream cTnI p-S44 levels and partially restores function, suggesting cTnI p-S44 may be a useful target to improve contractile function in the future.

Troponin I modulation of cardiac performance: Plasticity in the survival switch

Signaling complexes targeting the myofilament are essential in modulating cardiac performance. A central target of this signaling is cardiac troponin I (cTnI) phosphorylation. This review focuses on cTnI phosphorylation as a model for myofilament signaling, discussing key gaps and future directions towards understanding complex myofilament modulation of cardiac performance. Human heart cTnI is phosphorylated at 14 sites, giving rise to a complex modulatory network of varied functional responses. For example, while classical Ser23/24 phosphorylation mediates accelerated relaxation, protein kinase C phosphorylation of cTnI serves as a brake on contractile function. Additionally, the functional response of cTnI multi-site phosphorylation cannot necessarily be predicted from the response of individual sites alone. These complexities underscore the need for systematically evaluating single and multi-site phosphorylation on myofilament cellular and in vivo contractile function. Ultimately, a complete understanding of these multi-site responses requires work to establish site occupancy and dominance, kinase/phosphatase signaling balance, and the function of adaptive secondary phosphorylation. As cTnI phosphorylation is essential for modulating cardiac performance, future insight into the complex role of cTnI phosphorylation is important to establish sarcomere signaling in the healthy heart as well as identification of novel myofilament targets in the treatment of disease.

The continuing evolution of cardiac troponin I biomarker analysis: from protein to proteoform

INTRODUCTION

The troponin complex consists of three proteins that fundamentally couple excitation with contraction. Circulating cardiac-specific Troponin I (cTnI) serves as diagnostic biomarker tools for risk stratification of acute coronary syndromes and acute myocardial infarction (MI). Within the heart, cTnI oscillates between inactive and active conformations to either block or disinhibit actinomyosin formation. This molecular mechanism is fine-tuned through extensive protein modifications whose profiles are maladaptively altered with co-morbidities including hypertrophic cardiomyopathy, diabetes, and heart failure. Technological advances in analytical platforms over the last decade enable routine baseline cTnI analysis in patients without cardiovascular complications, and hold potential to expand cTnI readouts that include modified cTnI proteoforms. Areas covered: This review covers the current state, advances, and prospects of analytical platforms that now enable routine baseline cTnI analysis in patients. In parallel, improved mass spectrometry instrumentation and workflows already reveal an array of modified cTnI proteoforms with promising diagnostic implications. Expert commentary: New analytical capabilities provide clinicians and researchers with an opportunity to address important questions surrounding circulating cTnI in the improved diagnosis of specific patient cohorts. These techniques also hold considerable promise for new predictive and prescriptive applications for individualized profiling and improve patient care.

Different Sequences of Fractionated Low-Dose Proton and Single Iron-Radiation-Induced Divergent Biological Responses in the Heart

Deep-space travel presents risks of exposure to ionizing radiation composed of a spectrum of low-fluence protons (¹H) and high-charge and energy (HZE) iron nuclei (e.g., ⁵⁶Fe). When exposed to galactic cosmic rays, each cell in the body may be traversed by ¹H every 3-4 days and HZE nuclei every 3-4 months. The effects of low-dose sequential fractionated ¹H or HZE on the heart are unknown. In this animal model of simulated ionizing radiation, middle-aged (8-9 months old) male C57BL/6NT mice were exposed to radiation as follows: group 1, nonirradiated controls; group 2, three fractionated doses of 17 cGy ¹H every other day (¹H × 3); group 3, three fractionated doses of 17 cGy ¹H every other day followed by a single low dose of 15 cGy ⁵⁶Fe two days after the final ¹H dose (¹H × 3 + ⁵⁶Fe); and group 4, a single low dose of 15 cGy ⁵⁶Fe followed (after 2 days) by three fractionated doses of 17 cGy ¹H every other day (⁵⁶Fe + ¹H × 3). A subgroup of mice from each group underwent myocardial infarction (MI) surgery at 28 days postirradiation. Cardiac structure and function were assessed in all animals at days 7, 14 and 28 after MI surgery was performed. Compared to the control animals, the treatments that groups 2 and 3 received did not induce negative effects on cardiac function or structure. However, compared to all other groups, the animals in group 4, showed depressed left ventricular (LV) functions at 1 month with concomitant enhancement in cardiac fibrosis and induction of cardiac hypertrophy signaling at 3 months. In the irradiated and MI surgery groups compared to the control group, the treatments received by groups 2 and 4 did not induce negative effects at 1 month postirradiation and MI surgery. However, in group 3 after MI surgery, there was a 24% increase in mortality, significant decreases in LV function and a 35% increase in post-infarction size. These changes were associated with significant decreases in the angiogenic and cell survival signaling pathways. These data suggest that fractionated doses of radiation induces cellular and molecular changes that result in depressed heart functions both under basal conditions and particularly after myocardial infarction.

Functional communication between PKC-targeted cardiac troponin I phosphorylation sites

Increased protein kinase C (PKC) activity is associated with heart failure, and can target multiple cardiac troponin I (cTnI) residues in myocytes, including S23/24, S43/45 and T144. In earlier studies, cTnI-S43D and/or -S45D augmented S23/24 and T144 phosphorylation, which suggested there is communication between clusters. This communication is now explored by evaluating the impact of phospho-mimetic cTnI S43/45D combined with S23/24D (cTnIS4D) or T144D (cTnISDTD). Gene transfer of epitope-tagged cTnIS4D and cTnISDTD into adult cardiac myocytes progressively replaced endogenous cTnI. Partial replacement with cTnISDTD or cTnIS4D accelerated the time to peak (TTP) shortening and time to 50% re-lengthening (TTR_50%) on day 2, but peak shortening was only diminished by cTnIS4D. Extensive cTnIS4D replacement continued to accelerate TTP, and decrease shortening amplitude, while TTR_50% returned to baseline levels on day 4. In contrast, cTnISDTD modestly reduced shortening amplitude and continued to accelerate myocyte TTP and TTR_50%. These results indicate cTnIS43/45 communicates with S23/24 and T144, with S23/24 exacerbating and T144 attenuating the S43/45D-dependent functional deficit. In addition, more severe functional alterations in cTnIS4D myocytes were accompanied by higher levels of secondary phosphorylation compared to cTnISDTD. These results suggest that secondary phosphorylation helps to maintain steady-state contractile function during chronic cTnI phosphorylation at PKC sites.

Myofilament Calcium Sensitivity: Mechanistic Insight into TnI Ser-23/24 and Ser-150 Phosphorylation Integration

Troponin I (TnI) is a major regulator of cardiac muscle contraction and relaxation. During physiological and pathological stress, TnI is differentially phosphorylated at multiple residues through different signaling pathways to match cardiac function to demand. The combination of these TnI phosphorylations can exhibit an expected or unexpected functional integration, whereby the function of two phosphorylations are different than that predicted from the combined function of each individual phosphorylation alone. We have shown that TnI Ser-23/24 and Ser-150 phosphorylation exhibit functional integration and are simultaneously increased in response to cardiac stress. In the current study, we investigated the functional integration of TnI Ser-23/24 and Ser-150 to alter cardiac contraction. We hypothesized that Ser-23/24 and Ser-150 phosphorylation each utilize distinct molecular mechanisms to alter the TnI binding affinity within the thin filament. Mathematical modeling predicts that Ser-23/24 and Ser-150 phosphorylation affect different TnI affinities within the thin filament to distinctly alter the Ca2+-binding properties of troponin. Protein binding experiments validate this assertion by demonstrating pseudo-phosphorylated Ser-150 decreases the affinity of isolated TnI for actin, whereas Ser-23/24 pseudo-phosphorylation is not different from unphosphorylated. Thus, our data supports that TnI Ser-23/24 affects TnI-TnC binding, while Ser-150 phosphorylation alters TnI-actin binding. By measuring force development in troponin-exchanged skinned myocytes, we demonstrate that the Ca2+ sensitivity of force is directly related to the amount of phosphate present on TnI. Furthermore, we demonstrate that Ser-150 pseudo-phosphorylation blunts Ser-23/24-mediated decreased Ca2+-sensitive force development whether on the same or different TnI molecule. Therefore, TnI phosphorylations can integrate across troponins along the myofilament. These data demonstrate that TnI Ser-23/24 and Ser-150 phosphorylation regulates muscle contraction in part by modulating different TnI interactions in the thin filament and it is the combination of these differential mechanisms that provides understanding of their functional integration.

The contributions of cardiac myosin binding protein C and troponin I phosphorylation to β-adrenergic enhancement of in vivo cardiac function

KEY POINTS

β-adrenergic stimulation increases cardiac myosin binding protein C (MyBP-C) and troponin I phosphorylation to accelerate pressure development and relaxation in vivo, although their relative contributions remain unknown. Using a novel mouse model lacking protein kinase A-phosphorylatable troponin I (TnI) and MyBP-C, we examined in vivo haemodynamic function before and after infusion of the β-agonist dobutamine. Mice expressing phospho-ablated MyBP-C displayed cardiac hypertrophy and prevented full acceleration of pressure development and relaxation in response to dobutamine, whereas expression of phosphor-ablated TnI alone had little effect on the acceleration of contractile function in response to dobutamine. Our data demonstrate that MyBP-C phosphorylation is the principal mediator of the contractile response to increased β-agonist stimulation in vivo. These results help us understand why MyBP-C dephosphorylation in the failing heart contributes to contractile dysfunction and decreased adrenergic reserve in response to acute stress. β-adrenergic stimulation plays a critical role in accelerating ventricular contraction and speeding relaxation to match cardiac output to changing circulatory demands. Two key myofilaments proteins, troponin I (TnI) and myosin binding protein-C (MyBP-C), are phosphorylated following β-adrenergic stimulation; however, their relative contributions to the enhancement of in vivo cardiac contractility are unknown. To examine the roles of TnI and MyBP-C phosphorylation in β-adrenergic-mediated enhancement of cardiac function, transgenic (TG) mice expressing non-phosphorylatable TnI protein kinase A (PKA) residues (i.e. serine to alanine substitution at Ser23/24; TnI(PKA-)) were bred with mice expressing non-phosphorylatable MyBP-C PKA residues (i.e. serine to alanine substitution at Ser273, Ser282 and Ser302; MyBPC(PKA-)) to generate a novel mouse model expressing non-phosphorylatable PKA residues in TnI and MyBP-C (DBL(PKA-)). MyBP-C dephosphorylation produced cardiac hypertrophy and increased wall thickness in MyBPC(PKA-) and DBL(PKA-) mice, and in vivo echocardiography and pressure-volume catheterization studies revealed impaired systolic function and prolonged diastolic relaxation compared to wild-type and TnI(PKA-) mice. Infusion of the β-agonist dobutamine resulted in accelerated rates of pressure development and relaxation in all mice; however, MyBPC(PKA-) and DBL(PKA-) mice displayed a blunted contractile response compared to wild-type and TnI(PKA-) mice. Furthermore, unanaesthesized MyBPC(PKA-) and DBL(PKA-) mice displayed depressed maximum systolic pressure in response to dobutamine as measured using implantable telemetry devices. Taken together, our data show that MyBP-C phosphorylation is a critical modulator of the in vivo acceleration of pressure development and relaxation as a result of enhanced β-adrenergic stimulation, and reduced MyBP-C phosphorylation may underlie depressed adrenergic reserve in heart failure.

Contribution of Post-translational Phosphorylation to Sarcomere-Linked Cardiomyopathy Phenotypes

Secondary shifts develop in post-translational phosphorylation of sarcomeric proteins in multiple animal models of inherited cardiomyopathy. These signaling alterations together with the primary mutation are predicted to contribute to the overall cardiac phenotype. As a result, identification and integration of post-translational myofilament signaling responses are identified as priorities for gaining insights into sarcomeric cardiomyopathies. However, significant questions remain about the nature and contribution of post-translational phosphorylation to structural remodeling and cardiac dysfunction in animal models and human patients. This perspective essay discusses specific goals for filling critical gaps about post-translational signaling in response to these inherited mutations, especially within sarcomeric proteins. The discussion focuses primarily on pre-clinical analysis of animal models and defines challenges and future directions in this field.

Effects of Cardiac Troponin I Mutation P83S on Contractile Properties and the Modulation by PKA-Mediated Phosphorylation

cTnI(P82S) (cTnI(P83S) in rodents) resides at the I-T arm of cardiac troponin I (cTnI) and was initially identified as a disease-causing mutation of hypertrophic cardiomyopathy (HCM). However, later studies suggested this may not be true. We recently reported that introduction of an HCM-associated mutation in either inhibitory-peptide (cTnI(R146G)) or cardiac-specific N-terminus (cTnI(R21C)) of cTnI blunts the PKA-mediated modulation on myofibril activation/relaxation kinetics by prohibiting formation of intrasubunit contacts between these regions. Here, we tested whether this also occurs for cTnI(P83S). cTnI(P83S) increased both Ca(2+) binding affinity to cTn (KCa) and affinity of cTnC for cTnI (KC-I), and eliminated the reduction of KCa and KC-I observed for phosphorylated-cTnI(WT). In isolated myofibrils, cTnI(P83S) maintained maximal tension (TMAX) and Ca(2+) sensitivity of tension (pCa50). For cTnI(WT) myofibrils, PKA-mediated phosphorylation decreased pCa50 and sped up the slow-phase relaxation (especially for those Ca(2+) conditions that heart performs in vivo). Those effects were blunted for cTnI(P83S) myofibrils. Molecular-dynamics simulations suggested cTnI(P83S) moderately inhibited an intrasubunit interaction formation between inhibitory-peptide and N-terminus, but this "blunting" effect was weaker than that with cTnI(R146G) or cTnI(R21C). In summary, cTnI(P83S) has similar effects as other HCM-associated cTnI mutations on troponin and myofibril function even though it is in the I-T arm of cTnI.

Functionally conservative substitutions at cardiac troponin I S43/45

A phospho-null Ala substitution at protein kinase C (PKC)-targeted cardiac troponin I (cTnI) S43/45 reduces myocyte and cardiac contractile function. The goal of the current study was to test whether cTnIS43/45N is an alternative, functionally conservative substitution in cardiac myocytes. Partial and more extensive endogenous cTnI replacement was similar at 2 and 4 days after gene transfer, respectively, for epitope-tagged cTnI and cTnIS43/45N. This replacement did not significantly change thin filament stoichiometry. In functional studies, there were no significant changes in the amplitude and/or rates of contractile shortening and re-lengthening after this partial (2 days) and extensive (4 days) replacement with cTnIS43/45N. The cTnIS43/45N substitution also was not associated with adaptive changes in the myocyte Ca(2+) transient or in phosphorylation of the protein kinase A and C-targeted cTnIS23/24 site. These results provide evidence that cTnIS43/45N is a functionally conservative substitution, and may be appropriate for use as a phospho-null in rodent models designed for studies on PKC modulation of cardiac performance.

Effects of HCM cTnI mutation R145G on troponin structure and modulation by PKA phosphorylation elucidated by molecular dynamics simulations

Cardiac troponin (cTn) is a key molecule in the regulation of human cardiac muscle contraction. The N-terminal cardiac-specific peptide of the inhibitory subunit of troponin, cTnI (cTnI(1-39)), is a target for phosphorylation by protein kinase A (PKA) during β-adrenergic stimulation. We recently presented evidence indicating that this peptide interacts with the inhibitory peptide (cTnl(137-147)) when S23 and S24 are phosphorylated. The inhibitory peptide is also the target of the point mutation cTnI-R145G, which is associated with hypertrophic cardiomyopathy (HCM), a disease associated with sudden death in apparently healthy young adults. It has been shown that both phosphorylation and this mutation alter the cTnC-cTnI (C-I) interaction, which plays a crucial role in modulating contractile activation. However, little is known about the molecular-level events underlying this modulation. Here, we computationally investigated the effects of the cTnI-R145G mutation on the dynamics of cTn, cTnC Ca(2+) handling, and the C-I interaction. Comparisons were made with the cTnI-R145G/S23D/S24D phosphomimic mutation, which has been used both experimentally and computationally to study the cTnI N-terminal specific effects of PKA phosphorylation. Additional comparisons between the phosphomimic mutations and the real phosphorylations were made. For this purpose, we ran triplicate 150 ns molecular dynamics simulations of cTnI-R145G Ca(2+)-bound cTnC(1-161)-cTnI(1-172)-cTnT(236-285), cTnI-R145G/S23D/S24D Ca(2+)-bound cTnC(1-161)-cTnI(1-172)-cTnT(236-285), and cTnI-R145G/PS23/PS24 Ca(2+)-bound cTnC(1-161)-cTnI(1-172)-cTnT(236-285), respectively. We found that the cTnI-R145G mutation did not impact the overall dynamics of cTn, but stabilized crucial Ca(2+)-coordinating interactions. However, the phosphomimic mutations increased overall cTn fluctuations and destabilized Ca(2+) coordination. Interestingly, cTnI-R145G blunted the intrasubunit interactions between the cTnI N-terminal extension and the cTnI inhibitory peptide, which have been suggested to play a crucial role in modulating troponin function during β-adrenergic stimulation. These findings offer a molecular-level explanation for how the HCM mutation cTnI-R145G reduces the modulation of cTn by phosphorylation of S23/S24 during β-adrenergic stimulation.

Troponin I Mutations R146G and R21C Alter Cardiac Troponin Function, Contractile Properties, and Modulation by Protein Kinase A (PKA)-mediated Phosphorylation

Two hypertrophic cardiomyopathy-associated cardiac troponin I (cTnI) mutations, R146G and R21C, are located in different regions of cTnI, the inhibitory peptide and the cardiac-specific N terminus. We recently reported that these regions may interact when Ser-23/Ser-24 are phosphorylated, weakening the interaction of cTnI with cardiac TnC. Little is known about how these mutations influence the affinity of cardiac TnC for cTnI (KC-I) or contractile kinetics during β-adrenergic stimulation. Here, we tested how cTnI(R146G) or cTnI(R21C) influences contractile activation and relaxation and their response to protein kinase A (PKA). Both mutations significantly increased Ca(2+) binding affinity to cTn (KCa) and KC-I. PKA phosphorylation resulted in a similar reduction of KCa for all complexes, but KC-I was reduced only with cTnI(WT). cTnI(WT), cTnI(R146G), and cTnI(R21C) were complexed into cardiac troponin and exchanged into rat ventricular myofibrils, and contraction/relaxation kinetics were measured ± PKA phosphorylation. Maximal tension (Tmax) was maintained for cTnI(R146G)- and cTnI(R21C)-exchanged myofibrils, and Ca(2+) sensitivity of tension (pCa50) was increased. PKA phosphorylation decreased pCa50 for cTnI(WT)-exchanged myofibrils but not for either mutation. PKA phosphorylation accelerated the early slow phase relaxation for cTnI(WT) myofibrils, especially at Ca(2+) levels that the heart operates in vivo. Importantly, this effect was blunted for cTnI(R146G)- and cTnI(R21C)-exchanged myofibrils. Molecular dynamics simulations suggest both mutations inhibit formation of intra-subunit contacts between the N terminus and the inhibitory peptide of cTnI that is normally seen with WT-cTn upon PKA phosphorylation. Together, our results suggest that cTnI(R146G) and cTnI(R21C) blunt PKA modulation of activation and relaxation kinetics by prohibiting cardiac-specific N-terminal interaction with the cTnI inhibitory peptide.

PKA phosphorylation of cardiac troponin I modulates activation and relaxation kinetics of ventricular myofibrils

Protein kinase A (PKA) phosphorylation of myofibril proteins constitutes an important pathway for β-adrenergic modulation of cardiac contractility and relaxation. PKA targets the N-terminus (Ser-23/24) of cardiac troponin I (cTnI), cardiac myosin-binding protein C (cMyBP-C) and titin. The effect of PKA-mediated phosphorylation on the magnitude of contraction has been studied in some detail, but little is known about how it modulates the kinetics of thin filament activation and myofibril relaxation as Ca(2+) levels vary. Troponin C (cTnC) interaction with cTnI (C-I interaction) is a critical step in contractile activation that can be modulated by cTnI phosphorylation. We tested the hypothesis that altering C-I interactions by PKA, or by cTnI phosphomimetic mutations (S23D/S24D-cTnI), directly affects thin filament activation and myofilament relaxation kinetics. Rat ventricular myofibrils were isolated and endogenous cTn was exchanged with either wild-type cTnI, or S23D/S24D-cTnI recombinant cTn. Contractile mechanics were monitored at maximum and submaximal Ca(2+) concentrations. PKA treatment of wild-type cTn or exchange of cTn containing S23D/S24D-cTnI resulted in an increase in the rate of early, slow phase of relaxation (kREL,slow) and a decrease in its duration (tREL,slow). These effects were greater for submaximal Ca(2+) activated contractions. PKA treatment also reduced the rate of contractile activation (kACT) at maximal, but not submaximal Ca(2+), and reduced the Ca(2+) sensitivity of contraction. Using a fluorescent probe coupled to cTnC (C35S-IANBD), the Ca(2+)-cTn binding affinity and C-I interaction were monitored. Ca(2+) binding to cTn (pCa50) was significantly decreased when cTnI was phosphorylated by PKA (ΔpCa50 = 0.31). PKA phosphorylation of cTnI also weakened C-I interaction in the presence of Ca(2+). These data suggest that weakened C-I interaction, via PKA phosphorylation of cTnI, may slow thin filament activation and result in increased myofilament relaxation kinetics, the latter of which could enhance early phase diastolic relaxation during β-adrenergic stimulation.

Independent modulation of contractile performance by cardiac troponin I Ser43 and Ser45 in the dynamic sarcomere

Protein kinase C (PKC) targets cardiac troponin I (cTnI) S43/45 for phosphorylation in addition to other residues. During heart failure, cTnI S43/45 phosphorylation is elevated, and yet there is ongoing debate about its functional role due, in part, to the emergence of complex phenotypes in animal models. The individual functional influences of phosphorylated S43 and S45 also are not yet known. The present study utilizes viral gene transfer of cTnI with phosphomimetic S43D and/or S45D substitutions to evaluate their individual and combined influences on function in intact adult cardiac myocytes. Partial replacement (≤40%) with either cTnIS43D or cTnIS45D reduced the amplitude of contraction, and cTnIS45D slowed contraction and relaxation rates, while there were no significant changes in function with cTnIS43/45D. More extensive replacement (≥70%) with cTnIS43D, cTnIS45D, and cTnIS43/45D each reduced the amplitude of contraction. Additional experiments also showed cTnIS45D reduced myofilament Ca(2+) sensitivity of tension. At the same time, shortening rates returned toward control values with cTnIS45D and the later stages of relaxation also became accelerated in myocytes expressing cTnIS43D and/or S45D. Further studies demonstrated this behavior coincided with adaptive changes in myofilament protein phosphorylation. Taken together, the results observed in myocytes expressing cTnIS43D and/or S45D suggest these 2 residues reduce function via independent mechanism(s). The changes in function associated with the onset of adaptive myofilament signaling suggest the sarcomere is capable of fine tuning PKC-mediated cTnIS43/45 phosphorylation and contractile performance. This modulatory behavior also provides insight into divergent phenotypes reported in animal models with cTnI S43/45 phosphomimetic substitutions.

Pathogenesis associated with a restrictive cardiomyopathy mutant in cardiac troponin T is due to reduced protein stability and greatly increased myofilament Ca2+ sensitivity

BACKGROUND

Dilated and hypertrophic cardiomyopathy mutations in troponin can blunt effects of protein kinase A (PKA) phosphorylation of cardiac troponin I (cTnI), decreasing myofilament Ca2+-sensitivity; however this effect has never been tested for restrictive cardiomyopathy (RCM) mutants. This study explores whether an RCM cardiac troponin T mutant (cTnT-ΔE96) interferes with convergent PKA regulation and if TnT instability contributes to greatly enhanced Ca2+-sensitivity in skinned fibers.

METHODS

Force of contraction in skinned cardiac porcine fiber and spectroscopic studies were performed.

RESULTS

A decrease of -0.26 and -0.25 pCa units in Ca2+-sensitivity of contraction after PKA incubation was observed for skinned fibers incorporated with WT or cTnT-ΔE96, respectively. To further assess whether cTnT-ΔE96 interferes solely with transmission of cTnI phosphorylation effects, skinned fibers were reconstituted with PKA pseudo-phosphorylated cTnI (cTnI-SS/DD.cTnC). Fibers displaced with cTnT-WT, reconstituted with cTnI-SS/DD.cTnC decreased Ca2+-sensitivity of force (pCa50=5.61) compared to control cTnI-WT.cTnC (pCa50=5.75), similarly affecting cTnT-ΔE96 (pCa50=6.03) compared to control \cTnI-WT.cTnC (pCa50=6.14). Fluorescence studies measuring cTnC(IAANS) Ca2+-affinity changes due to cTnT-ΔE96 indicated that higher complexity (thin filament) better recapitulates skinned fiber Ca2+ sensitive changes. Circular dichroism revealed reduced α-helicity and earlier thermal unfolding for cTnT-ΔE96 compared to WT.

CONCLUSIONS

Although ineffective in decreasing myofilament Ca2+-sensitivity to normal levels, cTnT-ΔE96 does not interfere with PKA cTnI phosphorylation mediated effects; 2) cTnT-ΔE96 requires actin to increase cTnC Ca2+-affinity; and 3) deletion of E96 reduces cTnT stability, likely disrupting crucial thin filament interactions.

GENERAL SIGNIFICANCE

The pathological effect of cTnT-ΔE96 is largely manifested by dramatic myofilament Ca2+-sensitization which still persists even after PKA phosphorylation mediated Ca2+-desensitization.

Computational studies of the effect of the S23D/S24D troponin I mutation on cardiac troponin structural dynamics

During β-adrenergic stimulation, cardiac troponin I (cTnI) is phosphorylated by protein kinase A (PKA) at sites S23/S24, located at the N-terminus of cTnI. This phosphorylation has been shown to decrease KCa and pCa50, and weaken the cTnC-cTnI (C-I) interaction. We recently reported that phosphorylation results in an increase in the rate of early, slow phase of relaxation (kREL,slow) and a decrease in its duration (tREL,slow), which speeds up the overall relaxation. However, as the N-terminus of cTnI (residues 1-40) has not been resolved in the whole cardiac troponin (cTn) structure, little is known about the molecular-level behavior within the whole cTn complex upon phosphorylation of the S23/S24 residues of cTnI that results in these changes in function. In this study, we built up the cTn complex structure (including residues cTnC 1-161, cTnI 1-172, and cTnT 236-285) with the N-terminus of cTnI. We performed molecular-dynamics (MD) simulations to elucidate the structural basis of PKA phosphorylation-induced changes in cTn structure and Ca(2+) binding. We found that introducing two phosphomimic mutations into sites S23/S24 had no significant effect on the coordinating residues of Ca(2+) binding site II. However, the overall fluctuation of cTn was increased and the C-I interaction was altered relative to the wild-type model. The most significant changes involved interactions with the N-terminus of cTnI. Interestingly, the phosphomimic mutations led to the formation of intrasubunit interactions between the N-terminus and the inhibitory peptide of cTnI. This may result in altered interactions with cTnC and could explain the increased rate and decreased duration of slow-phase relaxation seen in myofibrils.

Long term ablation of protein kinase A (PKA)-mediated cardiac troponin I phosphorylation leads to excitation-contraction uncoupling and diastolic dysfunction in a knock-in mouse model of hypertrophic cardiomyopathy

The cardiac troponin I (cTnI) R21C (cTnI-R21C) mutation has been linked to hypertrophic cardiomyopathy and renders cTnI incapable of phosphorylation by PKA in vivo. Echocardiographic imaging of homozygous knock-in mice expressing the cTnI-R21C mutation shows that they develop hypertrophy after 12 months of age and have abnormal diastolic function that is characterized by longer filling times and impaired relaxation. Electrocardiographic analyses show that older R21C mice have elevated heart rates and reduced cardiovagal tone. Cardiac myocytes isolated from older R21C mice demonstrate that in the presence of isoproterenol, significant delays in Ca(2+) decay and sarcomere relaxation occur that are not present at 6 months of age. Although isoproterenol and stepwise increases in stimulation frequency accelerate Ca(2+)-transient and sarcomere shortening kinetics in R21C myocytes from older mice, they are unable to attain the corresponding WT values. When R21C myocytes from older mice are treated with isoproterenol, evidence of excitation-contraction uncoupling is indicated by an elevation in diastolic calcium that is frequency-dissociated and not coupled to shorter diastolic sarcomere lengths. Myocytes from older mice have smaller Ca(2+) transient amplitudes (2.3-fold) that are associated with reductions (2.9-fold) in sarcoplasmic reticulum Ca(2+) content. This abnormal Ca(2+) handling within the cell may be attributed to a reduction (2.4-fold) in calsequestrin expression in conjunction with an up-regulation (1.5-fold) of Na(+)-Ca(2+) exchanger. Incubation of permeabilized cardiac fibers from R21C mice with PKA confirmed that the mutation prevents facilitation of mechanical relaxation. Altogether, these results indicate that the inability to enhance myofilament relaxation through cTnI phosphorylation predisposes the heart to abnormal diastolic function, reduced accessibility of cardiac reserves, dysautonomia, and hypertrophy.

Gene regulation, alternative splicing, and posttranslational modification of troponin subunits in cardiac development and adaptation: a focused review

Troponin plays a central role in regulating the contraction and relaxation of vertebrate striated muscles. This review focuses on the isoform gene regulation, alternative RNA splicing, and posttranslational modifications of troponin subunits in cardiac development and adaptation. Transcriptional and posttranscriptional regulations such as phosphorylation and proteolysis modifications, and structure-function relationships of troponin subunit proteins are summarized. The physiological and pathophysiological significances are discussed for impacts on cardiac muscle contractility, heart function, and adaptations in health and diseases.

Desensitization of myofilaments to Ca2+ as a therapeutic target for hypertrophic cardiomyopathy with mutations in thin filament proteins

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is a common genetic disorder caused mainly by mutations in sarcomeric proteins and is characterized by maladaptive myocardial hypertrophy, diastolic heart failure, increased myofilament Ca(2+) sensitivity, and high susceptibility to sudden death. We tested the following hypothesis: correction of the increased myofilament sensitivity can delay or prevent the development of the HCM phenotype.

METHODS AND RESULTS

We used an HCM mouse model with an E180G mutation in α-tropomyosin (Tm180) that demonstrates increased myofilament Ca(2+) sensitivity, severe hypertrophy, and diastolic dysfunction. To test our hypothesis, we reduced myofilament Ca(2+) sensitivity in Tm180 mice by generating a double transgenic mouse line. We crossed Tm180 mice with mice expressing a pseudophosphorylated cardiac troponin I (S23D and S24D; TnI-PP). TnI-PP mice demonstrated a reduced myofilament Ca(2+) sensitivity compared with wild-type mice. The development of pathological hypertrophy did not occur in mice expressing both Tm180 and TnI-PP. Left ventricle performance was improved in double transgenic compared with their Tm180 littermates, which express wild-type cardiac troponin I. Hearts of double transgenic mice demonstrated no changes in expression of phospholamban and sarcoplasmic reticulum Ca(2+) ATPase, increased levels of phospholamban and troponin T phosphorylation, and reduced phosphorylation of TnI compared with Tm180 mice. Moreover, expression of TnI-PP in Tm180 hearts inhibited modifications in the activity of extracellular signal-regulated kinase and zinc finger-containing transcription factor GATA in Tm180 hearts.

CONCLUSIONS

Our data strongly indicate that reduction of myofilament sensitivity to Ca(2+) and associated correction of abnormal relaxation can delay or prevent development of HCM and should be considered as a therapeutic target for HCM.

Cardiac myosin binding protein-C: a structurally dynamic regulator of myocardial contractility

Cardiac myosin binding protein-C (cMyBP-C) is a modular protein anchored to the thick filament through interactions mediated by its C-terminal region. The N-terminal region of cMyBPC-C regulates myocardial contractility by modifying actin-myosin association. Phosphorylation of the N-terminal region diminishes cMyBP-C's capacity to regulate actin-myosin function. Despite a substantial body of literature, many issues remain unclear regarding the structural and functional roles of cMyBP-C. While no high-resolution structures of the intact protein exist, crystallographic and nuclear magnetic resonance (NMR) structures of isolated N-terminal domains provide important molecular details regarding cMyBP-C's role in controlling contractility. In this review, we summarize the emerging structural understanding of cMyBP-C with a particular emphasis placed on describing how its dynamic molecular interactions with both thin and thick filament proteins likely contribute to contractile regulation. Furthermore, we discuss the future directions and strategies by which we may improve the mechanistic understanding of its role in modulating cardiac muscle contraction.

Length dependence of striated muscle force generation is controlled by phosphorylation of cTnI at serines 23/24

  According to the Frank-Starling relationship, greater end-diastolic volume increases ventricular output. The Frank-Starling relationship is based, in part, on the length-tension relationship in cardiac myocytes. Recently, we identified a dichotomy in the steepness of length-tension relationships in mammalian cardiac myocytes that was dependent upon protein kinase A (PKA)-induced myofibrillar phosphorylation. Because PKA has multiple myofibrillar substrates including titin, myosin-binding protein-C and cardiac troponin I (cTnI), we sought to define if phosphorylation of one of these molecules could control length-tension relationships. We focused on cTnI as troponin can be exchanged in permeabilized striated muscle cell preparations, and tested the hypothesis that phosphorylation of cTnI modulates length dependence of force generation. For these experiments, we exchanged unphosphorylated recombinant cTn into either a rat cardiac myocyte preparation or a skinned slow-twitch skeletal muscle fibre. In all cases unphosphorylated cTn yielded a shallow length-tension relationship, which was shifted to a steep relationship after PKA treatment. Furthermore, exchange with cTn having cTnI serines 23/24 mutated to aspartic acids to mimic phosphorylation always shifted a shallow length-tension relationship to a steep relationship. Overall, these results indicate that phosphorylation of cTnI serines 23/24 is a key regulator of length dependence of force generation in striated muscle.

The role of nodose ganglia in the regulation of cardiovascular function following pulmonary exposure to ultrafine titanium dioxide

The inhalation of nanosized air pollutant particles is a recognised risk factor for cardiovascular disease; however, the link between occupational exposure to engineered nanoparticles and adverse cardiovascular events remains unclear. In the present study, the authors demonstrated that pulmonary exposure of rats to ultrafine titanium dioxide (UFTiO2) significantly increased heart rate and depressed diastolic function of the heart in response to isoproterenol. Moreover, pulmonary inhalation of UFTiO2 elevated mean and diastolic blood pressure in response to norepinephrine. Pretreatment of the rats ip with the transient receptor potential (TRP) channel blocker ruthenium red inhibited substance P synthesis in nodose ganglia and associated functional and biological changes in the cardiovascular system. In conclusion, the effects of pulmonary inhalation of UFTiO2 on cardiovascular function are most likely triggered by a lung-nodose ganglia-regulated pathway via the activation of TRP channels in the lung.

Integration of troponin I phosphorylation with cardiac regulatory networks

We focus here on the modulation of thin filament activity by cardiac troponin I phosphorylation as an integral and adaptive mechanism in cardiac homeostasis and as a mechanism vulnerable to maladaptive response to stress. We discuss a current concept of cardiac troponin I function in the A-band region of the sarcomere and potential signaling to cardiac troponin I in a network involving the ends of the thin filaments at the Z-disk and the M-band regions. The cardiac sarcomere represents a remarkable set of interacting proteins that functions not only as a molecular machine generating the heartbeat but also as a hub of signaling. We review how phosphorylation signaling to cardiac troponin I is integrated, with parallel signals controlling excitation-contraction coupling, hypertrophy, and metabolism.

TNNI3K is a novel mediator of myofilament function and phosphorylates cardiac troponin I

The phosphorylation of cardiac troponin I (cTnI) plays an important role in the contractile dysfunction associated with heart failure. Human cardiac troponin I-interacting kinase (TNNI3K) is a novel cardiac-specific functional kinase that can bind to cTnI in a yeast two-hybrid screen. The purpose of this study was to investigate whether TNNI3K can phosphorylate cTnI at specific sites and to examine whether the phosphorylation of cTnI caused by TNNI3K can regulate cardiac myofilament contractile function. Co-immunoprecipitation was performed to confirm that TNNI3K could interact with cTnI. Kinase assays further indicated that TNNI3K did not phosphorylate cTnI at Ser23/24 and Ser44, but directly phosphorylated Ser43 and Thr143 in vitro. The results obtained for adult rat cardiomyocytes also indicated that enhanced phosphorylation of cTnI at Ser43 and Thr143 correlated with rTNNI3K (rat TNNI3K) overexpression, and phosphorylation was reduced when rTNNI3K was knocked down. To determine the contractile function modulated by TNNI3K-mediated phosphorylation of cTnI, cardiomyocyte contraction was studied in adult rat ventricular myocytes. The contraction of cardiomyocytes increased with rTNNI3K overexpression and decreased with rTNNI3K knockdown. We conclude that TNNI3K may be a novel mediator of cTnI phosphorylation and contribute to the regulation of cardiac myofilament contraction function.

Myofilament incorporation and contractile function after gene transfer of cardiac troponin I Ser43/45Ala

Phosphorylation of cardiac troponin I serines 43/45 (cTnISer43/45) by protein kinase C (PKC) is associated with cardiac dysfunction and yet there is disagreement about the role this cluster plays in modulating contractile performance. The present study evaluates the impact of phospho-null Ala substitutions at Ser43/45 (cTnISer43/45Ala) on contractile performance in intact myocytes. Viral-based gene transfer of cardiac troponin I (cTnI) or cTnISer43/45Ala resulted in time-dependent increases in expression, with 70-80% of endogenous cTnI replaced within 4days. Western analysis of intact and permeabilized myocytes along with immunohistochemistry showed each exogenous cTnI was incorporated into the sarcomere of myocytes. In contractile function studies, there were no differences in shortening and re-lengthening for cTnI and cTnISer43/45Ala-expressing myocytes 2days after gene transfer. However, more extensive replacement with cTnISer43/45Ala after 4days diminished peak shortening amplitude and accelerated re-lengthening measured as the time to 50% re-lengthening (TTR50%). A decrease in myofilament Ca(2+) sensitivity of tension also was observed in permeabilized myocytes expressing cTnISer43/45Ala and is consistent with accelerated re-lengthening observed in intact myocytes under basal conditions. Phosphorylation of cTnI Ser23/24 and the Ca(2+) transient were not changed in these myocytes. These results demonstrate extensive sarcomere expression of cTnISer43/45Ala directly modulates myofilament function under basal conditions. In further work, the accelerated re-lengthening observed in control or cTnI-expressing myocytes treated with the PKC agonist, endothelin-1 (ET, 10nM) was slowed in myocytes expressing cTnISer43/45Ala. This outcome may indicate Ser43/45 is targeted for phosphorylation by ET-activated PKC and/or influences transduction of this agonist-activated response.

Cardiac myofilaments: from proteome to pathophysiology

This review addresses the functional consequences of altered post-translational modifications of cardiac myofilament proteins in cardiac diseases such as heart failure and ischemia. The modifications of thick and thin filament proteins as well as titin are addressed. Understanding the functional consequences of altered protein modifications is an essential step in the development of targeted therapies for common cardiac diseases.

Effect of cold acclimation on troponin I isoform expression in striated muscle of rainbow trout

In vertebrates each of the three striated muscle types (fast skeletal, slow skeletal, and cardiac) contain distinct isoforms of a number of different contractile proteins including troponin I (TnI). The functional characteristics of these proteins have a significant influence on muscle function and contractility. The purpose of this study was to characterize which TnI gene and protein isoforms are expressed in the different muscle types of rainbow trout ( Oncorhynchus mykiss) and to determine whether isoform expression changes in response to cold acclimation (4°C). Semiquantitative real-time PCR was used to characterize the expression of seven different TnI genes. The sequence of these genes, cloned from Atlantic salmon ( Salmo salar) and rainbow trout, were obtained from the National Center for Biotechnology Information databases. One-dimensional gel electrophoresis and tandem mass spectrometry were used to identify the TnI protein isoforms expressed in each muscle type. Interestingly, the results indicate that each muscle type expresses the gene transcripts of up to seven TnI isoforms. There are significant differences, however, in the expression pattern of these genes between muscle types. In addition, cold acclimation was found to increase the expression of specific gene transcripts in each muscle type. The proteomics analysis demonstrates that fast skeletal and cardiac muscle contain three TnI isoforms, whereas slow skeletal muscle contains four. No other vertebrate muscle to date has been found to express as many TnI protein isoforms. Overall this study underscores the complex molecular composition of teleost striated muscle and suggests there is an adaptive value to the unique TnI profiles of each muscle type.

Alanine or aspartic acid substitutions at serine23/24 of cardiac troponin I decrease thin filament activation, with no effect on crossbridge detachment kinetics

Ala/Asp substitutions at Ser23/24 have been employed to investigate the functional impact of cardiac troponin I (cTnI) phosphorylation by protein kinase A (PKA). Some limitations of previous studies include the use of heterologous proteins and confounding effects arising from phosphorylation of cardiac myosin binding protein-C. Our goal was to probe the effects of cTnI phosphorylation using a homologous assay, so that altered function could be solely attributed to changes in cTnI. We reconstituted detergent-skinned rat cardiac papillary fibers with homologous rat cardiac troponin subunits to study the impact of Ala and Asp substitutions at Ser23/24 of rat cTnI (RcTnI S23A/24A and RcTnI S23D/24D). Both RcTnI S23A/24A and RcTnI S23D/24D showed a ~36% decrease in Ca(2+)-activated maximal tension. Both RcTnI S23A/24A and RcTnI S23D/24D showed a ~18% decrease in ATPase activity. Muscle fiber stiffness measurements suggested that the decrease in thin filament activation observed in RcTnI S23A/24A and RcTnI S23D/24D was due to a decrease in the number of strongly-bound crossbridges. Another major finding was that Ala and Asp substitutions in cTnI did not affect crossbridge detachment kinetics.

Protein kinase C depresses cardiac myocyte power output and attenuates myofilament responses induced by protein kinase A

Following activation by G-protein-coupled receptor agonists, protein kinase C (PKC) modulates cardiac myocyte function by phosphorylation of intracellular targets including myofilament proteins cardiac troponin I (cTnI) and cardiac myosin binding protein C (cMyBP-C). Since PKC phosphorylation has been shown to decrease myofibril ATPase activity, we hypothesized that PKC phosphorylation of cTnI and cMyBP-C will lower myocyte power output and, in addition, attenuate the elevation in power in response to protein kinase A (PKA)-mediated phosphorylation. We compared isometric force and power generating capacity of rat skinned cardiac myocytes before and after treatment with the catalytic subunit of PKC. PKC increased phosphorylation levels of cMyBP-C and cTnI and decreased both maximal Ca(2+) activated force and Ca(2+) sensitivity of force. Moreover, during submaximal Ca(2+) activations PKC decreased power output by 62 %, which arose from both the fall in force and slower loaded shortening velocities since depressed power persisted even when force levels were matched before and after PKC. In addition, PKC blunted the phosphorylation of cTnI by PKA, reduced PKA-induced spontaneous oscillatory contractions, and diminished PKA-mediated elevations in myocyte power. To test whether altered thin filament function plays an essential role in these contractile changes we investigated the effects of chronic cTnI pseudo-phosphorylation on myofilament function using myocyte preparations from transgenic animals in which either only PKA phosphorylation sites (Ser-23/Ser-24) (PP) or both PKA and PKC phosphorylation sites (Ser-23/Ser-24/Ser-43/Ser-45/T-144) (All-P) were replaced with aspartic acid. Cardiac myocytes from All-P transgenic mice exhibited reductions in maximal force, Ca(2+) sensitivity of force, and power. Similarly diminished power generating capacity was observed in hearts from All-P mice as determined by in situ pressure-volume measurements. These results imply that PKC-mediated phosphorylation of cTnI plays a dominant role in depressing contractility, and, thus, increased PKC isozyme activity may contribute to maladaptive behavior exhibited during the progression to heart failure.

Effect of hypertrophic cardiomyopathy-linked troponin C mutations on the response of reconstituted thin filaments to calcium upon troponin I phosphorylation

The objective of this work was to investigate the effect of hypertrophic cardiomyopathy-linked A8V and E134D mutations in cardiac troponin C (cTnC) on the response of reconstituted thin filaments to calcium upon phosphorylation of cardiac troponin I (cTnI) by protein kinase A. The phosphorylation of cTnI at protein kinase A sites was mimicked by the S22D/S23D double mutation in cTnI. Our results demonstrate that the A8V and E134D mutations had no effect on the extent of calcium desensitization of reconstituted thin filaments induced by cTnI pseudophosphorylation. However, the A8V mutation enhanced the effect of cTnI pseudophosphorylation on the rate of dissociation of calcium from reconstituted thin filaments and on the calcium dependence of actomyosin ATPase. Consequently, while the A8V mutation still led to a slower rate of dissociation of calcium from reconstituted thin filaments upon pseudophosphorylation of cTnI, the ability of the A8V mutation to decrease the rate of calcium dissociation was weakened. In addition, the ability of the A8V mutation to sensitize actomyosin ATPase to calcium was weakened after cTnI was replaced by the phosphorylation mimetic of cTnI. Consistent with the hypothesis that the E134D mutation is benign, it exerted a minor to no effect on the rate of dissociation of calcium from reconstituted thin filaments or on the calcium sensitivity of actomyosin ATPase, regardless of the cTnI phosphorylation status. In conclusion, our study enhances our understanding of how cardiomyopathy-linked cTnC mutations affect the response of reconstituted thin filaments to calcium upon cTnI phosphorylation.

Synergistic effects between phosphorylation of phospholamban and troponin I promote relaxation at higher heart rate

We hypothesized that the extent of frequency-dependent acceleration of relaxation (FDAR) would be less than that of isoproterenol-(ISO-)dependent acceleration of relaxation (IDAR) at the same increment of heart rates, and ISO may improve FDAR. Cardiac function and phosphorylation of PLB and cTnI were compared in pacing, ISO treatment, and combined pacing and ISO treatment in isolated working heart. The increase in cardiac output and the degree of relaxation was less in pacing than in ISO treatment at the same increment of heart rates. The increasing stimulation frequency induced more significant relaxant effect in ISO perfusion than that in physiological salt perfusion. The pacing only phosphorylated PLB at Thr17, but ISO induced phosphorylation of cTnI and PLB at Ser16 and Thr17. Those results suggest that the synergistic effects of PLB and cTnI induce higher degree of relaxation which makes a sufficient diastolic filling of the ventricle at higher heart rate.

The role of protein kinase C-mediated phosphorylation of sarcomeric proteins in the heart-detrimental or beneficial?

Protein kinase C (PKC) is a family of serine/threonine protein kinases, and alterations have been found in PKC isoform expression and localization in the failing heart. These alterations in PKC activation levels influence the PKC-mediated phosphorylation status of cellular target proteins involved in Ca²⁺-handling and sarcomeric contraction. The differences observed in the effects due to PKC-mediated phosphorylation may underlie part of the contractile dysfunction observed in the failing heart. It is therefore important to establish the beneficial and detrimental effects of this kinase in the healthy and failing heart. The function of PKC has been studied intensively; however, the complexity of the regulation of this kinase makes the interpretation of the different effects difficult. The main focus of this review is the (patho)physiological impact of phosphorylation of sarcomeric proteins, myosin light chain-2, troponin I and T, desmin, myosin binding protein-C, and titin by PKC.

Molecular mechanisms of sarcomere dysfunction in dilated and hypertrophic cardiomyopathy

The sarcomeres form the molecular motor of the cardiomyocyte and consist of a complex multi-protein of thick and thin filaments which are anchored to the cytoskeleton. The thick filament, composed of myosin and associated proteins, and the thin filament composed of actin, tropomyosin and the troponins develop actinmyosin crossbridges which cycle in response to calcium resulting in sliding of the filaments and contraction. The thin filament in fixed to the cardiomyocyte cytoskeleton at the Z-disc, a complex of structural and regulatory proteins. A giant protein, titin, provides an external scaffold and regulates passive force in diastole. Both genetic disorders and acquired conditions may affect proteins of the sarcomere. Genetic disorders of the thick and thin filament proteins are the predominant cause of hypertrophic cardiomyopathy. These mutations lead to abnormal sarcomere function, often an enhanced sensitivity to calcium, and impaired relaxation. This may result in secondary changes in calcium cycling and amplification of hypertrophic signaling cascades. Correcting the abnormal function of the sarcomere as well as intervening in later stages of the pathophysiologic cascades may ameliorate disease. In dilated cardiomyopathy genetic abnormalities in the sarcomere, Z-disc, calcium regulatory and cytoskeletal proteins as well as the dystrophin complex may be causal for disease. In dilated cardiomyopathy, disturbances in post-translational modifications of the sarcomere my also play a prominent role. Experimental models indicate that altered phosphorylation of sarcomeric proteins may impair systolic and diastolic function as well as the response to heart rate and afterload. Thus correcting these post-translational changes are legitimate targets for future therapeutic strategies for dilated cardiomyopathy.

Calcium sensitivity, force frequency relationship and cardiac troponin I: critical role of PKA and PKC phosphorylation sites

Transgenic models with pseudo phosphorylation mutants of troponin I, PKA sites at Ser 22 and 23 (cTnIDD(22,23) mice) or PKC sites at Ser 42 and 44 (cTnIAD(22,23)DD(42,44)) displayed differential force-frequency relationships and afterload relaxation delay in vivo. We hypothesized that cTnI PKA and PKC phosphomimics impact cardiac muscle rate-related developed twitch force and relaxation kinetics in opposite directions. cTnIDD(22,23) transgenic mice produce a force frequency relationship (FFR) equivalent to control NTG albeit at lower peak [Ca(2+)](i), while cTnIAD(22,23)DD(42,44) TG mice had a flat FFR with normal peak systolic [Ca(2+)](i), thus suggestive of diminished responsiveness to [Ca(2+)](i) at higher frequencies. Force-[Ca(2+)](i) hysteresis analysis revealed that cTnIDD(22,23) mice have a combined enhanced myofilament calcium peak response with an enhanced slope of force development and decline per unit of [Ca(2+)](i), whereas cTnIAD(22,23)DD(42,44) transgenic mice showed the opposite. The computational ECME model predicts that the TG lines may be distinct from each other due to different rate constants for association/dissociation of Ca(2+) at the regulatory site of cTnC. Our data indicate that cTnI phosphorylation at PKA sites plays a critical role in the FFR by increasing relative myofilament responsiveness, and results in a distinctive transition between activation and relaxation, as displayed by force-[Ca(2+)](i) hysteresis loops. These findings may have important implications for understanding the specific contribution of cTnI to beta-adrenergic inotropy and lusitropy and to adverse contractile effects of PKC activation, which is relevant during heart failure development.

Why does troponin I have so many phosphorylation sites? Fact and fancy

We discuss a current controversy regarding the relative role of phosphorylation sites on cardiac troponin I (cTnI) (Fig. 1) in physiological and patho-physiological cardiac function. Studies with mouse models and in vitro studies indicate that multi-site phosphorylations are involved in both control of maximum tension and sarcomeric responsiveness to Ca(2+). Thus one hypothesis is that cardiac function reflects a balance of cTnI phosphorylations and a tilt in this balance may be maladaptive in acquired and genetic disorders of the heart. Studies on human heart samples taken mainly at end-stage heart failure, and in depth proteomic analysis of human and rat heart samples demonstrate that Ser23/Ser24 are the major and perhaps the only sites likely to be relevant to control cardiac function. Thus functional significance of Ser23/Ser24 phosphorylation is taken as fact, whereas the function of some other sites is treated as fancy. Maybe the extremes will meet: in any case we both agree that further work needs to be carried out with relatively large mammals and with determination of the time course of changes in phosphorylation to identify transient modifications that may be relevant at a beat-to-beat basis. Moreover, we agree that the changes and effects of cTnI phosphorylation need to be fully integrated into the effects of other phosphorylations in the cardiac myocyte.

Left ventricular and myocardial function in mice expressing constitutively pseudophosphorylated cardiac troponin I

RATIONALE

Protein kinase (PK)C-induced phosphorylation of cardiac troponin (cTn)I has been shown to regulate cardiac contraction.

OBJECTIVE

Characterize functional effects of increased PKC-induced cTnI phosphorylation and identify underlying mechanisms using a transgenic mouse model (cTnI(PKC-P)) expressing mutant cTnI (S43E, S45E, T144E).

METHODS AND RESULTS

Two-dimensional gel analysis showed 7.2+/-0.5% replacement of endogenous cTnI with the mutant form. Experiments included: mechanical measurements (perfused isolated hearts, isolated papillary muscles, and skinned fiber preparations), biochemical and molecular biological measurements, and a mathematical model-based analysis for integrative interpretation. Compared to wild-type mice, cTnI(PKC-P) mice exhibited negative inotropy in isolated hearts (14% decrease in peak developed pressure), papillary muscles (53% decrease in maximum developed force), and skinned fibers (14% decrease in maximally activated force, F(max)). Additionally, cTnI(PKC-P) mice exhibited slowed relaxation in both isolated hearts and intact papillary muscles. The cTnI(PKC-P) mice showed no differences in calcium sensitivity, cooperativity, steady-state force-MgATPase relationship, calcium transient (amplitude and relaxation), or baseline phosphorylation of other myofilamental proteins. The model-based analysis revealed that experimental observations in cTnI(PKC-P) mice could be reproduced by 2 simultaneous perturbations: a decrease in the rate of cross-bridge formation and an increase in calcium-independent persistence of the myofilament active state.

CONCLUSIONS

A modest increase in PKC-induced cTnI phosphorylation ( approximately 7%) can significantly alter cardiac muscle contraction: negative inotropy via decreased cross-bridge formation and negative lusitropy via persistence of myofilament active state. Based on our data and data from the literature we speculate that effects of PKC-mediated cTnI phosphorylation are site-specific (S43/S45 versus T144).

In vivo phosphorylation site mapping in mouse cardiac troponin I by high resolution top-down electron capture dissociation mass spectrometry: Ser22/23 are the only sites basally phosphorylated

Cardiac troponin I (cTnI) is the inhibitory subunit of cardiac troponin, a key myofilament regulatory protein complex located on the thin filaments of the contractile apparatus. cTnI is uniquely specific for the heart and is widely used in clinics as a serum biomarker for cardiac injury. Phosphorylation of cTnI plays a critical role in modulating cardiac function. cTnI is known to be regulated by protein kinase A and protein kinase C at five sites, Ser22/Ser23, Ser42/44, and Thr143, primarily based on results from in vitro phosphorylation assays by the specific kinase(s). However, a comprehensive characterization of phosphorylation of mouse cTnI occurring in vivo has been lacking. Herein, we have employed top-down mass spectrometry (MS) methodology with electron capture dissociation for precise mapping of in vivo phosphorylation sites of cTnI affinity purified from wild-type and transgenic mouse hearts. As demonstrated, top-down MS (analysis of intact proteins) is an extremely valuable technology for global characterization of labile phosphorylation occurring in vivo without a priori knowledge. Our top-down MS data unambiguously identified Ser22/23 as the only two sites basally phosphorylated in wild-type mouse cTnI with full sequence coverage, which was confirmed by the lack of phosphorylation in cTnI-Ala(2) transgenic mice where Ser22/23 in cTnI have been rendered nonphosphorylatable by mutation to alanine.

The cardiac troponin C mutation Leu29Gln found in a patient with hypertrophic cardiomyopathy does not alter contractile parameters in skinned murine myocardium

The present study investigates the effects of the first mutation of troponin C (hcTnC^L29Q) found in a patient with hypertrophic cardiomyopathy (HCM) on force–pCa relations and the interplay with phosphorylation of sarcomeric PKA substrates. In triton-skinned murine cardiac fibers, the endogenous mcTnC was extracted and the fibers were subsequently reconstituted with recombinant wild-type and mutant hcTnC. Force–pCa relations of preparations containing hcTnC^L29Q or hcTnC^WT were similar. Incubation of fibers reconstituted with the recombinant proteins with phosphatase to dephosphorylate sarcomeric PKA substrates induced an increase in Ca²⁺ sensitivity, slightly more pronounced (0.04 pCa units) in hcTnC^L29Q-containing fibers. Incubation of the dephosphorylated fibers with PKA induced significant rightward shifts of force–pCa relations of similar magnitude with both, hcTnC^L29Q and hcTnC^WT. No significant effects of hcTnC^L29Q on the velocity of unloaded shortening were observed. In conclusion, no major differences in contractile parameters of preparations containing hcTnC^L29Q compared to hcTnC^WT were observed. Therefore, it appears unlikely that hcTnC^L29Q induces the development of HCM by affecting the regulation of Ca²⁺-activated force and interference with PKA-mediated modulation of the Ca²⁺ sensitivity of contraction.