Troponin and tropomyosin: proteins that switch on and tune in the activity of cardiac myofilaments

Cooperative behavior of molecular motors

Both experimental evidence and theoretical models for collective effects in the working mechanism of molecular motors are reviewed at three different levels, namely: (i) interaction between the two heads of double-headed motors, particularly in processive motors like kinesin, myosin V and myosin VI, (ii) cooperative regulation of muscle thin filaments by accessory proteins and the Ca2+ level, and (iii) collective dynamic effects stemming from the mechanical coupling of molecular motors within macroscopic structures such as muscle thick filaments or axonemes. We aim to bridge the gap between structural information at the molecular level and physiological data with accompanying specific models on the one hand, and general stochastic physical models for the action of molecular motors on the other hand. An underlying assumption is that while, ultimately, the function of molecular motors will be explainable by a quantitative description of specific intramolecular dynamics and intermolecular interactions, for some coarse grained larger scale dynamic features it will be sufficient and illuminating to construct physical models that are simplified to the bare essentials.

Troponin I in the murine myocardium: influence on length-dependent activation and interfilament spacing

Cyclic AMP-dependent protein kinase (PKA) targets contractile proteins, troponin-I (TnI) and myosin binding protein C (MyBP-C) in the heart and induces a decrease in myofilament Ca2+ sensitivity. Yet, the effect of sarcomere length (SL) change on Ca2+ sensitivity (length-dependent activation: LDA) following PKA-dependent phosphorylation is not clear. To clarify the role of PKA-dependent phosphorylation of TnI and MyBP-C on LDA in the heart, we examined LDA in skinned myocytes from a non-transgenic (NTG) and a transgenic murine model in which the native cardiac isoform (cTnI) was completely replaced by the slow skeletal isoform of TnI (ssTnI-TG) lacking the phosphorylation sites for PKA, while retaining PKA sites on MyBP-C. In NTG myocytes, PKA treatment decreased Ca2+ sensitivity at each SL, but enhanced the impact of SL change on Ca2+ sensitivity. Despite a greater sensitivity to Ca2+ and a reduction in LDA, neither Ca2+ responsiveness nor LDA was affected by PKA treatment in ssTnI-TG myocytes. To determine whether the above observations could be explained by the lateral separation between thick and thin filaments, as suggested by others, we measured interfilament spacing by X-ray diffraction as a function of SL in skinned cardiac trabeculae in the passive state from both NTG and ssTnI-TG models before and following treatment with PKA. Phosphorylation by PKA increased lattice spacing at every SL in NTG trabeculae. However, the relationship between SL and myofilament lattice spacing in ssTnI-TG was markedly shifted downward to an overall decreased myofilament lattice spacing following PKA treatment. We conclude: (1) PKA-dependent phosphorylation enhances length-dependent activation in NTG hearts; (2) replacement of native TnI with ssTnI increases Ca2+ sensitivity of tension but reduces length-dependent activation; (3) MyBP-C phosphorylation by PKA does not alter calcium responsiveness and induces a decrease in myofilament lattice spacing at all sarcomere lengths and (4) length-dependent activation in the heart cannot be entirely explained by alterations in myofilament lattice spacing.

Ca2+-induced conformational transition in the inhibitory and regulatory regions of cardiac troponin I

Cardiac muscle activation is initiated by the binding of Ca(2+) to the single N-domain regulatory site of cardiac muscle troponin C (cTnC). Ca(2+) binding causes structural changes between cTnC and two critical regions of cardiac muscle troponin I (cTnI): the regulatory region (cTnI-R, residues 150-165) and the inhibitory region (cTnI-I, residues130-149). These changes are associated with a decreased cTnI affinity for actin and a heightened affinity for cTnC. Using Förster resonance energy transfer, we have measured three intra-cTnI distances in the deactivated (Mg(2+)-saturated) and Ca(2+)-activated (Ca(2+)-saturated) states in reconstituted binary (cTnC-cTnI) and ternary (cTnC-cTnI-cTnT) troponin complexes. Distance A (spanning cTnI-R) was unaltered by Ca(2+). Distances B (spanning both cTnI-R and cTnI-I) and C (from a residue flanking cTnI-I to a residue in the center of cTnI-R) exhibited Ca(2+)-induced increases of >8 A. These results compliment our previous determination of the distance between residues flanking cTnI-I alone. Together, the data suggest that Ca(2+) activation causes residues within cTnI-I to switch from a beta-turn/coil to an extended quasi-alpha-helical conformation as the actin-contacts are broken, whereas cTnI-R remains alpha-helical in both Mg(2+)- and Ca(2+)-saturated states. We have used the data to construct a structural model of the cTnI inhibitory and regulatory regions in the Mg(2+)- and Ca(2+)-saturated states.

Polycystin-2 associates with tropomyosin-1, an actin microfilament component

Polycystin-2 (PC2) is the product of the second cloned gene (PKD2) responsible for autosomal dominant polycystic kidney disease and has recently been shown to be a calcium-permeable cation channel. PC2 has been shown to connect indirectly with the actin microfilament. Here, we report a direct association between PC2 and the actin microfilament. Using a yeast two-hybrid screen, we identified a specific interaction between the PC2 cytoplasmic C-terminal domain and tropomyosin-1 (TM-1), a component of the actin microfilament complex. Tropomyosins constitute a protein family of more than 20 isoforms arising mainly from alternative splicing and are present in muscle as well as non-muscle cells. We identified a new TM-1 splicing isoform in kidney and heart (TM-1a) that differs from TM-1 in the C terminus and interacted with PC2. In vitro biochemical methods, including GST pull-down, blot overlay and microtiter binding assays, confirmed the interaction between PC2 and the two TM-1 isoforms. Further experiments targeted the interacting domains to G821-R878 of PC2 and A152-E196, a common segment of TM-1 and TM-1a. Indirect double immunofluorescence experiments showed partial co-localization of PC2 and TM-1 in transfected mouse fibroblast NIH 3T3 cells. Co-immunoprecipitation (co-IP) studies using 3T3 cells and Xenopus oocytes co-expressing PC2 and TM-1 (or TM-1a) revealed in vivo association between the protein pairs. Furthermore, the in vivo interaction between the endogenous PC2 and TM-1 was demonstrated also by reciprocal co-IP using native human embryonic kidney cells and human adult kidney. Considering previous reports that TM-1 acts as a suppressor of neoplastic growth of transformed cells, it is possible that TM-1 contributes to cyst formation/growth when the anchorage of PC2 to the actin microfilament via TM-1 is altered.

Does cross-bridge activation determine the time course of myofibrillar relaxation?

The ability of force-generating cross-bridges to activate the thin filament in cardiac muscle was tested by studying the effects of initial force and [MgADP] on force relaxation kinetics in subcellular myofibrillar bundles prepared from left ventricles of the guinea pig. Relaxation was initiated by rapidly reducing the [Ca(2+)] from pCa 4.5 to 7.5. Initiating relaxation from lower force levels during pre-steady-state force development did not significantly accelerate the kinetics of the force decay compared to relaxations initiated from steady-state force development. This suggests that the force-generating cross-bridges which become formed during maximally Ca(2+)-activated steady-state contractions do not maintain thin filament activation for significant enough times after Ca(2+)-removal to exert a rate-limiting influence on force relaxation kinetics. Adding 2 mM MgADP to solutions slowed down relaxation kinetics approximately 4-fold. To differentiate whether these slower kinetics result from either (1) MgADP favoring accumulation of cross-bridges during the preceding contraction in a state of activating capability or (2) slow-down of cross-bridge turnover by the presence of the product MgADP during relaxation, the [MgADP] was either increased or removed at the time of Ca(2+)-removal. The addition of 2 mM MgADP to activating solutions (subsequent relaxation in the absence of MgADP) slowed-down the kinetics of the initial, slow, linear force decay following Ca(2+)-removal approximately 1.5-fold, suggesting that the high [MgADP] during contraction favors formation of cross-bridges which contribute in rate-limiting early relaxation kinetics by transiently sustaining thin filament activation. On the other hand, the addition of 2 mM MgADP to the relaxing solution (preceding Ca(2+)-activation in absence of MgADP) slowed-down the kinetics of the initial force decay approximately 3-fold, more similar to the kinetics observed in the continuous presence of 2 mM MgADP both before and after Ca(2+)-removal. This suggest that, despite some influence of cross-bridge activation, the main effect of MgADP on relaxation kinetics results from product inhibition of cross-bridge turnover. In summary, whereas under certain conditions (high [MgADP]) cross-bridge activation of the thin filament can weakly take part in rate-limiting relaxation kinetics induced by complete Ca(2+)-removal, cross-bridge activation does not influence relaxation kinetics under more physiologically normal conditions.

The Special Structure and Function of Troponin I in Regulation of Cardiac Contraction and Relaxation

In this chapter I review evidence for a pivotal role of the sarcomeric thin filament protein, troponin I, in cardiac muscle activation and its modulation by covalent modifications, sarcomere length, and intracellular pH. This evidence demonstrates that the cardiac variant of troponin I (cTnI), which is the only isoform expressed in the adult myocardium, has unique structure and function that are specialized for extrinsic and intrinsic control of cardiac contraction and relaxation.

Activation of striated muscle: nearest-neighbor regulatory-unit and cross-bridge influence on myofilament kinetics

We have formulated a three-compartment model of muscle activation that includes both strong cross-bridge (XB) and Ca(2+)-activated regulatory-unit (RU) mediated nearest-neighbor cooperative influences. The model is based on the tight coupling premise--that XB retain activating Ca(2+) on the thin filament. Using global non-linear least-squares, the model produced excellent fits to experimental steady-state force-pCa and ATPase-pCa data from skinned rat soleus fibers. In terms of the model, nearest-neighbor influences over the range of Ca(2+) required for activation cause the Ca(2+) dissociation rate from regulatory-units (k(off)) to decrease and the cross-bridge association rate (f) to increase each more than ten-fold. Moreover, the rate variations occur in separate Ca(2+) regimes. The energy of activation governing f is strongly influenced by both neighboring RU and XB. In contrast, the energy of activation governing k(off) is less affected by neighboring XB than by neighboring RU. Nearest-neighbor cooperative influences provide both an overall sensitization to Ca(2+) and the well-known steep response of force to free Ca(2+). The apparent sensitivity for Ca(2+)-activation of force and ATPase is a function of cross-bridge kinetic rates. The model and derived parameter set produce simulated behavior in qualitative agreement with steady-state experiments reported in the literature for partial TnC replacement, increased [P(i)], increased [ADP], and MalNEt-S1 addition. The model is an initial attempt to construct a general theory of striated muscle activation-one that can be consistently used to interpret data from various types of muscle manipulation experiments.

Length-dependent activation in three striated muscle types of the rat

The process whereby sarcomere length modulates the sensitivity of the myofilaments to Ca(2+) is termed length-dependent activation. Length-dependent activation is a property of all striated muscles, yet the relative extent of length-dependent activation between skeletal muscle and cardiac muscle is unclear. Although length-dependent activation may be greater in fast skeletal muscle (FSM) than in slow skeletal muscle (SSM), there has not been a well controlled comparison of length-dependent activation between skeletal muscle and cardiac muscle (CM). Accordingly, we measured sarcomere length-dependent properties in skinned soleus (SSM), psoas (FSM) and ventricular trabeculae (CM) of the rat under carefully controlled conditions. The free Ca(2+)-force relationship was determined at sarcomere lengths (SL) of 1.95 microm, 2.10 microm and 2.25 microm and fitted to a modified Hill equation. FSM and SSM were more sensitive to Ca(2+) than CM. Length-dependent activation was ordered as CM > FSM > SSM. Cooperativity as measured by the Hill coefficient of the Ca(2+)-force relationship was not significantly different between CM and FSM, both of which exhibited greater cooperativity than SSM. SL did not significantly alter this parameter in each muscle type. To establish whether the observed differences can be explained by alterations in interfilament spacing, we measured myofilament lattice spacing (LS) by synchrotron X-ray diffraction in relaxed, skinned muscle preparations. LS was inversely proportional to SL for each muscle type. The slope of the SL-LS relationship, however, was not significantly different between striated muscle types. We conclude that (1) length-dependent activation differs among the three types of striated muscle and (2) these differences in the length-dependent properties among the striated muscle types may not solely be explained by the differences in the response of interfilament spacing to changes in muscle length in relaxed, skinned isolated muscle preparations.

Mechanisms of action of novel cardiotonic agents

Regulation of myocardial contractility by cardiotonic agents is achieved by an increase in intracellular Ca2+ mobilization (upstream mechanism), an increase in Ca2+ binding affinity to troponin C (central mechanism), or facilitation of the process subsequent to Ca2+ binding to troponin C (downstream mechanism). cAMP mediates the regulation induced by Ca2+ mobilizers such as beta-adrenoceptor agonists and selective phosphodiesterase III inhibitors acting through the upstream mechanism. These agents act likewise on the central mechanism to decrease Ca2+ sensitivity of troponin C in association with the cAMP-mediated phosphorylation of troponin I. In addition to such a well-known action of cAMP, recent experimental findings have revealed that Ca2+ sensitizers, such as levosimendan, OR-1896, and UD-CG 212 Cl, require the cAMP-mediated signaling for induction of Ca2+ sensitizing effect. These agents shift the [Ca2+] -force relationship to the left, but their positive inotropic effect (PIE) is inhibited by carbachol, which suppresses selectively the cAMP-mediated PIE. These findings imply that cAMP may play a crucial role in increasing the myofilament Ca2+ sensitivity by cross-talk with the action of individual cardiotonic agents. No clinically available cardiotonic agents act primarily via Ca2+ sensitization, but the PIE of pimobendan and levosimendan is partly mediated by an increase in myofilament Ca2+ sensitivity. Evidence is accumulating that cardiotonic agents with Ca2+ sensitizing action are more effective than agents that act purely via the upstream mechanism in clinical settings. Further clinical trials are required to establish the effectiveness of Ca2+ sensitizers in long-term therapy for congestive heart failure patients.

Structure of the regulatory N-domain of human cardiac troponin C in complex with human cardiac troponin I147-163 and bepridil

Cardiac troponin C (cTnC) is the Ca(2+)-dependent switch for contraction in heart muscle and a potential target for drugs in the therapy of heart failure. Ca(2+) binding to the regulatory domain of cTnC (cNTnC) induces little structural change but sets the stage for cTnI binding. A large "closed" to "open" conformational transition occurs in the regulatory domain upon binding cTnI(147-163) or bepridil. This raises the question of whether cTnI(147-163) and bepridil compete for cNTnC.Ca(2+). In this work, we used two-dimensional (1)H,(15)N-heteronuclear single quantum coherence (HSQC) NMR spectroscopy to examine the binding of bepridil to cNTnC.Ca(2+) in the absence and presence of cTnI(147-163) and of cTnI(147-163) to cNTnC.Ca(2+) in the absence and presence of bepridil. The results show that bepridil and cTnI(147-163) bind cNTnC.Ca(2+) simultaneously but with negative cooperativity. The affinity of cTnI(147-163) for cNTnC.Ca(2+) is reduced approximately 3.5-fold by bepridil and vice versa. Using multinuclear and multidimensional NMR spectroscopy, we have determined the structure of the cNTnC.Ca(2+).cTnI(147-163).bepridil ternary complex. The structure reveals a binding site for cTnI(147-163) primarily located on the A/B interhelical interface and a binding site for bepridil in the hydrophobic pocket of cNTnC.Ca(2+). In the structure, the N terminus of the peptide clashes with part of the bepridil molecule, which explains the negative cooperativity between cTnI(147-163) and bepridil for cNTnC.Ca(2+). This structure provides insights into the features that are important for the design of cTnC-specific cardiotonic drugs, which may be used to modulate the Ca(2+) sensitivity of the myofilaments in heart muscle contraction.

Interfilament spacing, Ca2+ sensitivity, and Ca2+ binding in skinned bovine cardiac muscle

The length-dependence of myofilament Ca2+ sensitivity in cardiac muscle appears to be a function of length-dependent variation in the lateral separation of actin and myosin filaments. The goal of this study was to determine how force, Ca2+ sensitivity, and Ca2+ binding to troponin C are correlated in skinned bovine ventricular muscle bundles set at sarcomere length 1.9 microm and subjected to varying degrees of osmotic compression with Dextran T-500. With 5, 10, and 15% Dextran T-500 the muscle diameter was reduced by 13, 21, and 25%, respectively. Addition of 5% Dextran T-500 caused increases in developed force, Ca2+ sensitivity, and in the affinity of Ca2+ for the regulatory binding site on troponin C. All of these parameters were reversed back toward control levels with 10% Dextran T-500. With 15% Dextran T-500 all parameters were decreased to below control levels. These data indicate that (1) there is an optimal filament separation at which both Ca2+ sensitivity and Ca2+ binding are maximized, and (2) Ca2+-troponin C affinity is linked to changes in Ca2+ sensitivity rather than to changes in interfilament spacing.

Length dependence of cardiac myofilament Ca(2+) sensitivity in the presence of substitute nucleoside triphosphates

Although ATP is the immediate source of energy for muscle contraction other nucleoside triphosphates (NTP) can substitute for ATP as substrates for myosin and as sources of energy for contraction of skinned muscle fibers. However, experiments with skinned skeletal muscle fibers in the presence of substitute NTP indicate significant differences with respect to cross-bridge kinetics, force generation, and Ca(2+) regulation. In this study the length dependence of Ca(2+) sensitivity of skinned bovine cardiac muscle was analyzed in the presence of MgATP, MgCTP, MgUTP, and MgITP. Ca(2+) regulation in the presence of MgCTP and MgUTP was essentially the same as in the presence of MgATP, although the maximum force generated (at sarcomere length 2.4 microm) was about 25% less. However, the length dependence of Ca(2+) sensitivity was eliminated in the presence of MgUTP. With MgITP the maximum force generated (at sarcomere length 2.4 microm) was about the same as in the presence of MgATP, but there was an impairment of relaxation such that at pCa 8 the force developed was about 50-60% of that developed at pCa 5. Moreover, the Ca(2+)-dependent component showed no length-dependent sensitivity. Thus length modulation of Ca(2+) sensitivity is a function of the myosin substrate. Taken in conjunction with other data, the results are consistent with the hypothesis that length-dependence of Ca(2+) sensitivity is modulated at a step upstream from the force-generating reaction.

Quantitative dynamics of site-specific protein phosphorylation determined using liquid chromatography electrospray ionization mass spectrometry

We have developed and validated a method that uses liquid chromatography/electrospray ionization-mass spectrometry to quantify site-specific protein phosphorylation. The method uses selected ion monitoring to determine the chromatographic peak areas of specific tryptic peptides from the protein of interest. The extent of phosphorylation is determined from the ratio of the phosphopeptide peak area to the peak area of an unmodified reference peptide that acts as internal standard, correcting for variations in protein amounts and peptide recovery in the digest preparation procedure. As a result, we refer to this protocol as the native reference peptide method. Mole of phosphate at the selected site per mole of protein is obtained from this ratio, using calibration curves of synthetic peptides to determine relative responses. Our method begins with protein separation by SDS-PAGE and is carried out on amounts of peptide produced by an in-gel digestion of single Coomassie blue-stained bands. To illustrate the utility of the method and provide validation, we used cardiac troponin I as analyte and monitored the time course of a protein kinase C betaII reaction. Those analyses appropriately demonstrate the time-dependent increase of phosphorylation at a PKC-preferred site, Ser44 in the peptide 41ISASPR45 and the concomitant consumption of the nonphosphorylated peptide. We believe that this method provides a novel tool to directly measure specific phosphorylation sites in proteins in different physiological states and expect that the method will be adaptable not only to a variety of samples types (i.e., culture cells, tissues, etc.) but to a variety of posttranslation modifications as well.

[Molecular genetics of cardiomyopathies]

In the last decade our understanding of cardiac pathophysiology has experienced significant advances linked to major advances in molecular genetics. Although many genes are associated today with cardiac diseases, the genetics of both hypertrophic cardiomyopathy and dilated cardiomyopathy have generated great interest. The familial nature of the disease in some patients has been very useful in this regard. In addition, there are also excellent experimental models to study the implications of the genetic abnormalities. Altogether the study of the molecular genetics of the cardiomyopathies should provide not only prognostic information but also new therapeutic alternatives.

Cardiac excitation-contraction coupling

Of the ions involved in the intricate workings of the heart, calcium is considered perhaps the most important. It is crucial to the very process that enables the chambers of the heart to contract and relax, a process called excitation-contraction coupling. It is important to understand in quantitative detail exactly how calcium is moved around the various organelles of the myocyte in order to bring about excitation-contraction coupling if we are to understand the basic physiology of heart function. Furthermore, spatial microdomains within the cell are important in localizing the molecular players that orchestrate cardiac function.

Compensatory changes in Ca(2+) and myocardial O(2) consumption in beta-tropomyosin transgenic hearts

Transgenic mice overexpressing beta-tropomyosin have increased myofilament Ca(2+) sensitivity that we hypothesized would result in altered relationships among pressure and heart rates, intracellular Ca(2+), and myocardial O(2) consumption. In perfused hearts from transgenic mice there was a marked negative force-frequency response between 6 and 10 Hz with a 30 +/- 3% reduction in peak-positive first derivative of pressure development over time (dP/dt) compared with 14 +/- 2% in wild-type mice (P < 0.001). At 8 Hz systolic pressures were normal, though peak systolic intracellular Ca(2+) was significantly reduced in transgenic mice versus wild type (726 +/- 61 vs. 936 +/- 67 nM, P < 0.05) indicating an alteration in the pressure-Ca(2+) relationship. Over a wide range of positive and negative inotropic interventions there were normal developed pressures, though marked elevations in myocardial O(2) consumption (15-54%). Because pressures are normal and intracellular Ca(2+) decreased and myocardial O(2) consumption increased, this suggests that these abnormalities are at least in part compensatory mechanisms to the altered myofilament function.

Ischemic and anesthetic preconditioning reduces cytosolic [Ca2+] and improves Ca(2+) responses in intact hearts

Ca(+) loading during reperfusion after myocardial ischemia is linked to reduced cardiac function. Like ischemic preconditioning (IPC), a volatile anesthetic given briefly before ischemia can reduce reperfusion injury. We determined whether IPC and sevoflurane preconditioning (SPC) before ischemia equivalently improve mechanical and metabolic function, reduce cytosolic Ca(2+) loading, and improve myocardial Ca(2+) responsiveness. Four groups of guinea pig isolated hearts were perfused: no ischemia, no treatment before 30-min global ischemia and 60-min reperfusion (control), IPC (two 2-min occlusions) before ischemia, and SPC (3.5 vol%, two 2-min exposures) before ischemia. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured at the left ventricular (LV) free wall with the fluorescent probe indo 1. Ca(2+) responsiveness was assessed by changing extracellular [Ca(2+)]. In control hearts, initial reperfusion increased diastolic [Ca(2+)] and diastolic LV pressure (LVP), and the maximal and minimal derivatives of LVP (dLVP/dt(max) and dLVP/dt(min), respectively), O(2) consumption, and cardiac efficiency (CE). Throughout reperfusion, IPC and SPC similarly reduced ischemic contracture, ventricular fibrillation, and enzyme release, attenuated rises in systolic and diastolic [Ca(2+)], improved contractile and relaxation indexes, O(2) consumption, and CE, and reduced infarct size. Diastolic [Ca(2+)] at 50% dLVP/dt(min) was right shifted by 32-53 +/- 8 nM after 30-min reperfusion for all groups. Phasic [Ca(2+)] at 50% dLVP/dt(max) was not altered in control but was left shifted by -235 +/- 40 nM [Ca(2+)] after IPC and by -135 +/- 20 nM [Ca(2+)] after SPC. Both SPC and IPC similarly reduce Ca(2+) loading, while augmenting contractile responsiveness to Ca(2+), improving postischemia cardiac function and attenuating permanent damage.

The role of troponin abnormalities as a cause for stunned myocardium

Myocardial stunning is a form of ischemic injury, which occurs with transient ischemia followed by re-establishment of flow, and which results in reversible cardiac dysfunction. There is evidence that the molecular defect in stunning is at the level of the contractile apparatus. Selective proteolysis of the myofilament protein, troponin I, appears to underlie the phenotype of stunning in some models, but other myofilament protein modifications may also have a role.

Structure of the C-domain of human cardiac troponin C in complex with the Ca2+ sensitizing drug EMD 57033

Ca(2+) binding to cardiac troponin C (cTnC) triggers contraction in heart muscle. In heart failure, myofilaments response to Ca(2+) are often altered and compounds that sensitize the myofilaments to Ca(2+) possess therapeutic value in this syndrome. One of the most potent and selective Ca(2+) sensitizers is the thiadiazinone derivative EMD 57033, which increases myocardial contractile function both in vivo and in vitro and interacts with cTnC in vitro. We have determined the NMR structure of the 1:1 complex between Ca(2+)-saturated C-domain of human cTnC (cCTnC) and EMD 57033. Favorable hydrophobic interactions between the drug and the protein position EMD 57033 in the hydrophobic cleft of the protein. The drug molecule is orientated such that the chiral group of EMD 57033 fits deep in the hydrophobic pocket and makes several key contacts with the protein. This stereospecific interaction explains why the (-)-enantiomer of EMD 57033 is inactive. Titrations of the cCTnC.EMD 57033 complex with two regions of cardiac troponin I (cTnI(34-71) and cTnI(128-147)) reveal that the drug does not share a common binding epitope with cTnI(128-147) but is completely displaced by cTnI(34-71). These results have important implications for elucidating the mechanism of the Ca(2+) sensitizing effect of EMD 57033 in cardiac muscle contraction.

Localization of regions of troponin I important in deactivation of cardiac myofilaments by acidic pH

Ca2+-activation of cardiac muscle myofilaments is more sensitive to depression by acidic pH than is the case with skeletal myofilaments. We tested the hypothesis that this difference is related to specific regions of the TnI (troponin I) isoforms in these muscles. We exchanged native Tn complex in detergent-extracted fiber bundles from mouse ventricles with Tn containing various combinations of fast (fsTnI) or slow skeletal (ssTnI) complexed with either cardiac TnC (cTnC) or fsTnC, and with cTnC complexed with the following chimeras: (1) fsTnI N-terminal region (fN) plus cTnI inhibitory peptide (cIp) and cTnI C-terminal region (cC); and (2) cTnI N-terminal region (cN)-cIp-fsTnI C-terminal region (fC). We determined the change in half maximal Ca2+(DeltaEC50) for tension activation at pH 7.0 and pH 6.5. Similar DeltaEC50 values were obtained for unextracted controls (5.53+/-0.30 microm), for preparations containing cTnI-cTnC (5.74+/-0.40 microm), and preparations exchanged with cTnI-fsTnC (5.63+/-0.40 microm). However, replacement of cTnI with fsTnI significantly decreased DeltaEC50 to 3.95+/-0.17 microm. Replacement of cTnI with ssTnI also significantly depressed DeltaEC50 to 2.07+/-0.15 microm. Results of studies using the chimeras demonstrated that the C-terminal domains of cTnI and fsTnI are responsible for these differences. This conclusion also fits with data from experiments in which we measured Ca2+-binding to the regulatory site of cTnC in binary complexes containing cTnC with cTnI, fsTnI, or the chimeras. Our results localize a region of TnI important in effects of acidosis on cardiac myofilaments and extend our earlier data indicating that C-terminal regions of cTnI outside the Ip are critical for activation by Ca2+.

PKA accelerates rate of force development in murine skinned myocardium expressing alpha- or beta-tropomyosin

In myocardium, protein kinase A (PKA) is known to phosphorylate troponin I (TnI) and myosin-binding protein-C (MyBP-C). Here, we used skinned myocardial preparations from nontransgenic (NTG) mouse hearts expressing 100% alpha-tropomyosin (alpha-Tm) to examine the effects of phosphorylated TnI and MyBP-C on Ca2+ sensitivity of force and the rate constant of force redevelopment (k(tr)). Experiments were also done using transgenic (TG) myocardium expressing approximately 60% beta-Tm to test the idea that the alpha-Tm isoform is required to observe the mechanical effects of PKA phosphorylation. Compared with NTG myocardium, TG myocardium exhibited greater Ca2+ sensitivity of force and developed submaximal forces at faster rates. Treatment with PKA reduced Ca2+ sensitivity of force in NTG and TG myocardium, had no effect on maximum k(tr) in either NTG or TG myocardium, and increased the rates of submaximal force development in both kinds of myocardium. These results show that PKA-mediated phosphorylation of myofibrillar proteins significantly alters the static and dynamic mechanical properties of myocardium, and these effects occur regardless of the type of Tm expressed.

Effects of sarcomere length and Ca(2+) binding on h reactivity of myofilament bound troponin C in porcine skinned cardiac muscle fibers

Length dependence of cardiac Ca(2+) activation is an essential component of the Frank-Starling relation. The aim of this study is to examine the length effects on the Ca(2+)-induced conformational changes of filament-bound cTnC in skinned cardiac muscle fibers. The two cysteine residues (Cys-35 and Cys-84) in the regulatory domain of cTnC allow for the attachment of conformational probes to this region. Their incorporation with the fluorescent probe, 7-diethylamino-3-[4'-maleimidylphenyl]-4-methylcoumarin (CPM), was used to determine the varying cTnC conformations in cardiac fibers. The data obtained show that the length-dependent Ca(2+)-mediated conformational changes require strong-binding cross-bridges for cardiac activation.

Comparative studies on the expression patterns of three troponin T genes during mouse development

In vertebrates, three troponin T (TnT) genes, cardiac TnT (cTnT), skeletal muscle fast-twitch TnT (fTnT), and slow-twitch TnT (sTnT), have evolved for the regulation of striated muscle contraction. To understand the mechanism for muscle fiber-specific expression of the TnT genes, we compared their expression patterns during mouse development. Our data revealed that the TnT expression in the developing embryo was not as restricted as that in the adult. In addition to a strong expression in the developing heart beginning at day 7.5 p.c (postcoitum), the cTnT transcript was detected at later stages in some skeletal muscles, where beginning at day 11.75 p.c. the fTnT and sTnT genes were also expressed. Only sTnT but not fTnT was found transiently in the developing heart. At day 13.5 p.c., expressions of all three genes were detected in the developing tongue and this co-expression continued to day 16.5 p.c. with the fTnT isoform being predominant. At this stage, overlapping and distinct expression patterns of both sTnT and fTnT genes were also evident in many developing skeletal muscles. These data suggest that different muscles during development undergo a complex change in TnT isoforms resulting in different contractile properties. Unexpectedly, the cTnT transcript was persistently found in the developing bladder, where presumably smooth muscle is present. In transgenic mice, expression of a LacZ gene driven by a rat cTnT promoter (-497 to +192 bp) was very similar to that of the endogenous cTnT gene, suggesting that this promoter contained regulatory elements sufficient for the control of tissue-specific cTnT expression during development.

Increased contractility and altered Ca(2+) transients of mouse heart myocytes conditionally expressing PKCbeta

Activation of protein kinase C (PKC) in heart muscle signals hypertrophy and may also directly affect contractile function. We tested this idea using a transgenic (TG) mouse model in which conditionally expressed PKCbeta was turned on at 10 wk of age and remained on for either 6 or 10 mo. Compared with controls, TG cardiac myocytes demonstrated an increase in the peak amplitude of the Ca(2+) transient, an increase in the extent and rate of shortening, and an increase in the rate of relengthening at both 6 and 10 mo of age. Phospholamban phosphorylation and Ca(2+)-uptake rates of sarcoplasmic reticulum vesicles were the same in TG and control heart preparations. At 10 mo, TG skinned fiber bundles demonstrated the same sensitivity to Ca(2+) as controls, but maximum tension was depressed and there was increased myofilament protein phosphorylation. Our results differ from studies in which PKCbeta was constitutively overexpressed in the heart and in studies that reported a depression of myocyte contraction with no change in the Ca(2+) transient.

[Troponine T as possible myocardial injury marker. Its application in myocardial stunning and silent ischemia]

INTRODUCTION AND OBJECTIVES

The need for more specific, more sensitive and earlier biochemical markers of acute myocardial infarction, has led to the development of alternative methods to CK-MB). The aim of this work is to assess the usefulness of TnT measurement, in comparison with other markers for detecting transitory ischemic processes without necrosis in some experimental models.

METHODS

The plasma levels of Troponine T, CK, CK-MB and adenosine were assessed as markers of ischemic myocardial injury. Two protocols were used: in Series I and II very brief (2 min ischemia with 3-min reperfusion) repeated (20 episodes) ischemias were induced, while Series III involved a single 15-min ischemia with a 60-min reperfusion. In Series I the coronary occlusor was placed close to the anterior descending coronary artery (AD); in Series II and III it was placed distally in the AD. Blood samples were taken from the peripheral vein (PVB) and corresponding coronary segment vein; in a basal situation, during ischemia, upon reperfusion, after 24 hours, and after 5 and 10 days. The plasma levels of adenosine, troponine T, CK and CK-MB as well as general and regional function parameters were measured.

RESULTS

In Series I we observed hypokinesis that lasted 10 days, reaching its maximum on days 4-5. In Series II and III regional function was restored by 24 hours. CK and CK-MB showed similar behaviour; they rose significantly when the chest was opened (p < 0.05) reaching the highest value at 24 hours in all the series. Adenosine rose significantly only during reperfusion (p < 0.05). Troponine T increased after ischemia but not before, remained high for 5 days in all series (PVB).

CONCLUSIONS

Troponine T rises in absence of necrosis, preferably when the ischemia is longer.

Skeletal and Cardiac Muscle Contractile Activation: Tropomyosin “Rocks and Rolls”

Changes in thin filament structure induced by Ca2+ binding to troponin and subsequent strong cross-bridge binding regulate additional strong cross-bridge attachment, force development, and dependence of force on sarcomere length in skeletal and cardiac muscle. Variations in activation properties account for functional differences between these muscle types.

The molecular genetic basis for hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM), a relatively common disease, is diagnosed clinically by unexplained cardiac hypertrophy and pathologically by myocyte hypertrophy, disarray, and interstitial fibrosis. HCM is the most common cause of sudden cardiac death (SCD) in the young and a major cause of morbidity and mortality in elderly. Hypertrophy and fibrosis are the major determinants of morbidity and SCD. More than 100 mutations in nine genes, all encoding sarcomeric proteins have been identified in patients with HCM, which had led to the notion that HCM is a disease of contractile sarcomeric proteins. The beta -myosin heavy chain (MyHC), cardiac troponin T (cTnT) and myosin binding protein-C (MyBP-C) are the most common genes accounting for approximately 2/3 of all HCM cases. Genotype-phenotype correlation studies suggest that mutations in the beta -MyHC gene are associated with more extensive hypertrophy and a higher risk of SCD as compared to mutations in genes coding for other sarcomeric proteins, such as MyBP-C and cTnT. The prognostic significance of mutations is related to their hypertrophic expressivity and penetrance, with the exception of those in the cTnT, which are associated with mild hypertrophic response and a high incidence of SCD. However, there is a significant variability and factors, such as modifier genes and probably the environmental factors affect the phenotypic expression of HCM. The molecular pathogenesis of HCM is not completely understood. In vitro and in vivo studies suggest that mutations impart a diverse array of functional defects including reduced ATPase activity of myosin, acto-myosin interaction, cross-bridging kinetics, myocyte contractility, and altered Ca2+ sensitivity. Hypertrophy and other clinical and pathological phenotypes are considered compensatory phenotypes secondary to functional defects. In summary, the molecular genetic basis of HCM has been identified, which affords the opportunity to delineate its pathogenesis. Understanding the pathogenesis of HCM could provide for genetic based diagnosis, risk stratification, treatment and prevention of cardiac phenotypes.

Cell biology of cardiac development

Building a vertebrate heart is a complex task and involves several tissues, including the myocardium, endocardium, neural crest, and epicardium. Interactions between these tissues result in the changes in function and morphology (and also in the extracellular matrix, which serves as a substrate for morphological change) that are requisite for development of the heart. Some of the signaling pathways that mediate these changes have now been identified and several investigators are now filling in the missing pieces in these pathways in hopes of ultimately understanding the molecular mechanisms that govern healthy heart development. In addition, transcription factors that regulate various aspects of heart development have been identified. Transcription factors of the GATA and Nkx2 families are of particular importance for early specification of the heart field and for regulating expression of genes that encode proteins of the contractile apparatus. This chapter highlights some of the most significant discoveries made in the rapidly expanding field of heart development.

Transgenic incorporation of skeletal TnT into cardiac myofilaments blunts PKC-mediated depression of force

Protein kinase C (PKC)-mediated phosphorylation of cardiac troponin I (cTnI) and troponin T (cTnT) has been shown to diminish maximum activation of myofilaments. The functional role of cTnI phosphorylation has been investigated. However, the impact of cTnT phosphorylation on myofilament force is not well studied. We tested the effect of endogenous PKC activation on steady-state tension development and Ca(2+) sensitivity in skinned fiber bundles from transgenic (TG) mouse hearts expressing fast skeletal TnT (fsTnT), which naturally lacks the PKC sites present in cTnT. The 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment induced a 29% (46.1 +/- 2.5 vs. 33.4 +/- 2.6 mN/mm(2)) reduction in maximum tension in the nontransgenic (NTG) preparations (n = 7) and was inhibited with chelerythrine. However, TPA did not induce a change in the maximum tension in the TG preparations (n = 11). TPA induced a small but significant (P < 0.02) increase in Ca(2+) sensitivity (untreated pCa(50) = 5.63 +/- 0.01 vs. treated pCa(50) = 5.72 +/- 0.01) only in TG preparations. In TG preparations, (32)P incorporation was not evident in TnT and was also significantly diminished in cTnI, compared with NTG. Our data indicate that incorporation of fsTnT into the cardiac myofilament lattice blunts PKC-mediated depression of maximum tension. These data also suggest that cTnT may play an important role in amplifying the myofilament depression induced by PKC-mediated phosphorylation of cTnI.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

Cardiac contractile impairment associated with increased phosphorylation of troponin I in endotoxemic rats

The subcellular mechanisms underlying intrinsic myocardial depression during sepsis remain poorly defined, in particular the relative roles of altered intracellular Ca2+ transients versus changes in myofilament properties. We studied contractile function of cardiac myocytes isolated 12 h after induction of endotoxemia (5 mg/kg intravenous E. coli lipopolysaccharide [LPS]) in conscious rats. Cardiomyocytes from LPS-injected rats had depressed twitch shortening compared with control cells (4.10.2% versus 7.80.3%; P2+ transients (peak indo-1 ratio 1.130.02 versus 1.120.02; P = NS). Contractile depression was unaffected by inhibitors of nitric oxide synthase. Steady-state myofilament response to Ca2+, assessed by tetanization of intact cells over a range of [Ca2+], was reduced significantly in the LPS group (P2+ was unaffected by isoproterenol (3 nmol/L) in endotoxemic cells, whereas there was a rightward shift in control cells. A reduction in myofilament response to Ca2+ is the major determinant of intrinsic cardiac depression in systemic endotoxemia. This condition appears to be related to an increase in myocardial troponin I phosphorylation.

Ca(2+) activation of myofilaments from transgenic mouse hearts expressing R92Q mutant cardiac troponin T

The functional consequences of the R92Q mutation in cardiac troponin T (cTnT), linked to familial hypertrophic cardiomyopathy in humans, are not well understood. We have studied steady- and pre-steady-state mechanical activity of detergent-skinned fiber bundles from a transgenic (TG) mouse model in which 67% of the total cTnT in the heart was replaced by the R92Q mutant cTnT. TG fibers were more sensitive to Ca(2+) than nontransgenic (NTG) fibers [negative logarithm of half maximally activating molar Ca(2+) (pCa(50)) = 5.84 +/- 0.01 and 6.12 +/- 0.01 for NTG and TG fibers, respectively]. The shift in pCa(50) caused by increasing the sarcomere length from 1.9 to 2.3 microm was significantly higher for TG than for NTG fibers (DeltapCa(50) = 0.13 +/- 0.01 and 0.29 +/- 0.02 for NTG and TG fibers, respectively). The relationships between rate of ATP consumption and steady-state isometric tension were linear, and the slopes were the same in NTG and TG fibers. Rate of tension redevelopment was more sensitive to Ca(2+) in TG than in NTG fibers (pCa(50) = 5.71 +/- 0.02 and 6.07 +/- 0.02 for NTG and TG fibers, respectively). We concluded that overall cross-bridge cycling kinetics are not altered by the R92Q mutation but that altered troponin-tropomyosin interactions could be responsible for the increase in myofilament Ca(2+) sensitivity in TG myofilaments.

Calcium, cross-bridges, and the Frank-Starling relationship

The steep relationship between active force and length in cardiac muscle is based on a length dependence of myofilament Ca(2+) sensitivity. However, it is not muscle length but the lateral spacing between actin and myosin filaments that sets the level of Ca(2+) sensitivity, mainly through modulation of myosin-mediated activation of the thin filament.

Cross-bridge interaction kinetics in rat myocardium are accelerated by strong binding of myosin to the thin filament

To determine the ability of strong-binding myosin cross-bridges to activate the myocardial thin filament, we examined the Ca2+ dependence of force and cross-bridge interaction kinetics at 15 degrees C in the absence and presence of a strong-binding, non-force-generating derivative of myosin subfragment-1 (NEM-S1) in chemically skinned myocardium from adult rats. Relative to control conditions, application of 6 microM NEM-S1 significantly increased Ca2+-independent tension, measured at pCa 9.0, from 0.8 +/- 0.3 to 3.7 +/- 0.8 mN mm-2. Furthermore, NEM-S1 potentiated submaximal Ca2+-activated forces and thereby increased the Ca2+ sensitivity of force, i.e. the [Ca2+] required for half-maximal activation (pCa50) increased from pCa 5.85 +/- 0.05 to 5.95 +/- 0.04 (change in pCa50 (dpCa50) = 0.11 +/- 0.02). The augmentation of submaximal force by NEM-S1 was accompanied by a marked reduction in the steepness of the force-pCa relationship for forces less than 0.50 Po (maximum Ca2+-activated force), i.e. the Hill coefficient (n2) decreased from 4.72 +/- 0.38 to 1.54 +/- 0.07. In the absence of NEM-S1, the rate of force redevelopment (ktr) was found to increase from 1.11 +/- 0.21 s-1 at submaximal [Ca2+] (pCa 6.0) to 9.28 +/- 0.41 s-1 during maximal Ca2+ activation (pCa 4.5). Addition of NEM-S1 reduced the Ca2+ dependence of ktr by eliciting maximal values at low levels of Ca2+, i.e. ktr was 9.38 +/- 0.30 s-1 at pCa 6.6 compared to 9.23 +/- 0.27 s-1 at pCa 4. At intermediate levels of Ca2+, ktr was less than maximal but was still greater than values obtained at the same pCa in the absence of NEM-S1. NEM-S1 dramatically reduced both the extent and rate of relaxation from steady-state submaximal force following flash photolysis of the caged Ca2+ chelator diazo-2. These data demonstrate that strongly bound myosin cross-bridges increase the level of thin filament activation in myocardium, which is manifested by an increase in the rate of cross-bridge attachment, potentiation of force at low levels of free Ca2+, and slowed rates of relaxation.

Ca2+ - and cross-bridge-dependent changes in N- and C-terminal structure of troponin C in rat cardiac muscle

Linear dichroism of 5'-tetramethylrhodamine (5'ATR)-labeled cardiac troponin C (cTnC) was measured to monitor cTnC structure during Ca2+-activation of force in rat skinned myocardium. Mono-cysteine mutants allowed labeling at Cys-84 (cTnC(C84), near the D/E helix linker); Cys-35 (cTnC(C35), at nonfunctional site I); or near the C-terminus with a cysteine inserted at site 98 (cTnC-C35S,C84S,S98C, cTnC(C98)). With 5'ATR-labeled cTnC(C84) and cTnC(C98) dichroism increased with increasing [Ca2+], while rigor cross-bridges caused dichroism to increase more with 5'ATR-labeled cTnC(C84) than cTnC(C98). The pCa50 values and n(H) from Hill analysis of the Ca2+-dependence of force and dichroism were 6.4 (+/-0.02) and 1.08 (+/-0.04) for force and 6.3 (+/-0.04) and 1.02 (+/-0.09) (n = 5) for dichroism in cTnC(C84) reconstituted trabeculae. Corresponding data from cTnC(C98) reconstituted trabeculae were 5.53 (+/-0.03) and 3.1 (+/-0.17) for force, and 5.39 (+/-0.03) and 1.87 (+/-0.17) (n = 5) for dichroism. The contribution of active cycling cross-bridges to changes in cTnC structure was determined by inhibition of force to 6% of pCa 4.0 controls with 1.0 mM sodium vanadate (Vi). With 5'ATR-labeled cTnC(C84) Vi caused both the pCa50)of dichroism and the maximum value at pCa 4.0 to decrease, while with 5'ATR-labeled cTnC(C98) the pCa50 of dichroism decreased with no change of dichroism at pCa 4.0. The dichroism of 5'ATR-labeled cTnC(C35) was insensitive to either Ca2+ or strong cross-bridges. These data suggest that both Ca2+ and cycling cross-bridges perturb the N-terminal structure of cTnC at Cys-84, while C-terminal structure is altered by site II Ca2+-binding, but not cross-bridges.

Cellular and functional defects in a mouse model of heart failure

Heart failure and dilated cardiomyopathy develop in mice that lack the muscle LIM protein (MLP) gene (MLP(-/-)). The character and extent of the heart failure that occurs in MLP(-/-) mice were investigated using echocardiography and in vivo pressure-volume (P-V) loop measurements. P-V loop data were obtained with a new method for mice (sonomicrometry) using two pairs of orthogonal piezoelectric crystals implanted in the endocardial wall. Sonomicrometry revealed right-shifted P-V loops in MLP(-/-) mice, depressed systolic contractility, and additional evidence of heart failure. Cellular changes in MLP(-/-) mice were examined in isolated single cells using patch-clamp and confocal Ca(2+) concentration ([Ca(2+)]) imaging techniques. This cellular investigation revealed unchanged Ca(2+) currents and Ca(2+) spark characteristics but decreased intracellular [Ca(2+)] transients and contractile responses and a defect in excitation-contraction coupling. Normal cellular and whole heart function was restored in MLP(-/-) mice that express a cardiac-targeted transgene, which blocks the function of beta-adrenergic receptor (beta-AR) kinase-1 (betaARK1). These data suggest that, despite the persistent stimulus to develop heart failure in MLP(-/-) mice (i.e., loss of the structural protein MLP), downregulation and desensitization of the beta-ARs may play a pivotal role in the pathogenesis. Furthermore, this work suggests that the inhibition of betaARK1 action may prove an effective therapy for heart failure.

Biological function and site II Ca2+-induced opening of the regulatory domain of skeletal troponin C are impaired by invariant site I or II Glu mutations

To investigate the roles of site I and II invariant Glu residues 41 and 77 in the functional properties and calcium-induced structural opening of skeletal muscle troponin C (TnC) regulatory domain, we have replaced them by Ala in intact F29W TnC and in wild-type and F29W N domains (TnC residues 1-90). Reconstitution of intact E41A/F29W and E77A/F29W mutants into TnC-depleted muscle skinned fibers showed that Ca(2+)-induced tension is greatly reduced compared with the F29W control. Circular dichroism measurements of wild-type N domain as a function of pCa (= -log[Ca(2+)]) demonstrated that approximately 90% of the total change in molar ellipticity at 222 nm ([theta](222 nm)) could be assigned to site II Ca(2+) binding. With E41A, E77A, and cardiac TnC N domains this [theta](222 nm) change attributable to site II was reduced to < or =40% of that seen with wild type, consistent with their structures remaining closed in +Ca(2+). Furthermore, the Ca(2+)-induced changes in fluorescence, near UV CD, and UV difference spectra observed with intact F29W are largely abolished with E41A/F29W and E77A/F29W TnCs. Taken together, the data indicate that the major structural change in N domain, including the closed to open transition, is triggered by site II Ca(2+) binding, an interpretation relevant to the energetics of the skeletal muscle TnC and cardiac TnC systems.

Altered hemodynamics in transgenic mice harboring mutant tropomyosin linked to hypertrophic cardiomyopathy

We used transgenic (TG) mice overexpressing mutant alpha-tropomyosin [alpha-Tm(Asp175Asn)], linked to familial hypertrophic cardiomyopathy (FHC), to test the hypothesis that this mutation impairs cardiac function by altering the sensitivity of myofilaments to Ca(2+). Left ventricular (LV) pressure was measured in anesthetized nontransgenic (NTG) and TG mice. In control conditions, LV relaxation was 6,970 +/- 297 mmHg/s in NTG and 5,624 +/- 392 mmHg/s in TG mice (P < 0.05). During beta-adrenergic stimulation, the rate of relaxation increased to 8,411 +/- 323 mmHg/s in NTG and to 6,080 +/- 413 mmHg/s in TG mice (P < 0.05). We measured the pCa-force relationship (pCa = -log [Ca(2+)]) in skinned fiber bundles from LV papillary muscles of NTG and TG hearts. In control conditions, the Ca(2+) concentration producing 50% maximal force (pCa(50)) was 5.77 +/- 0.02 in NTG and 5.84 +/- 0.01 in TG myofilament bundles (P < 0.05). After protein kinase A-dependent phosphorylation, the pCa(50) was 5.71 +/- 0.01 in NTG and 5.77 +/- 0. 02 in TG myofilament bundles (P < 0.05). Our results indicate that mutant alpha-Tm(Asp175Asn) increases myofilament Ca(2+)-sensitivity, which results in decreased relaxation rate and blunted response to beta-adrenergic stimulation.

Evolutionary and physiological variation in cardiac troponin C in relation to thermal strategies of fish

Striated muscle contraction is initiated when troponin C (TnC) binds Ca(2+), which activates actinomyosin ATPase. We investigated (i) the variation between cardiac TnC (cTnC) primary structure within teleost fish and (ii) the pattern of TnC expression in response to temperature acclimation. There were few differences between rainbow trout (Oncorhynchus mykiss), yellowfin tuna (Thunnus albacares), yellow perch (Perca flavescens), goldfish (Carassius auratus), white sucker (Catostomus commersoni), and icefish (Chaenocephalus aceratus) in cTnC amino acid sequence. No variation existed in the regulatory Ca(2+)-binding site (site 2). The site 3 and 4 substitutions were limited to residues not directly involved in Ca(2+) coordination. Fish cTnC primary structure was highly conserved between species (93%-98%) and collectively divergent from the highly conserved sequence seen in birds and mammals. Northern blots and polymerase chain reaction showed that thermal acclimation of trout (3 degrees, 18 degrees C) did not alter the TnC isoform pattern. While cardiac and white muscle had the expected isoforms-cTnC and fast troponin C (fTnC), respectively-red muscle unexpectedly expressed primarily ftnC. Cold acclimation did not alter myofibrillar ATPase Ca(2+) sensitivity, but maximal velocity increased by 60%. We found no evidence that TnC variants, arising between species or in response to thermal acclimation, play a major role in mitigating the effects of temperature on contractility of the adult fish heart.

Remodeling the cardiac sarcomere using transgenesis

An underpinning of basic physiology and clinical medicine is that specific protein complements underlie cell and organ function. In the heart, contractile protein changes correlating with functional alterations occur during both normal development and the development of numerous pathologies. What has been lacking for the majority of these observations is an extension of correlation to causative proof. More specifically, different congenital heart diseases are characterized by shifts in the motor proteins, and the genetic etiologies of a number of different dilated and hypertrophic cardiomyopathies have been established as residing at loci encoding the contractile proteins. To establish cause, or to understand development of the pathophysiology over an animal's life span, it is necessary to direct the heart to synthesize, in the absence of other pleiotropic changes, the candidate protein. Subsequently one can determine whether or how the protein's presence causes the effects either directly or indirectly. By affecting the heart's protein complement in a defined manner, the potential to establish the function of different proteins and protein isoforms exists. Transgenesis provides a means of stably modifying the mammalian genome. By directing expression of engineered proteins to the heart, cardiac contractile protein profiles can be effectively remodeled and the resultant animal used to study the consequences of a single, genetic manipulation at the molecular, biochemical, cytological, and physiological levels.

Attenuation of length dependence of calcium activation in myofilaments of transgenic mouse hearts expressing slow skeletal troponin I

We compared sarcomere length (SL) dependence of the Ca2+-force relation of detergent-extracted bundles of fibres dissected from the left ventricle of wild-type (WT) and transgenic mouse hearts expressing slow skeletal troponin I (ssTnI-TG). Fibre bundles from the hearts of the ssTnI-TG demonstrated a complete replacement of the cardiac troponin I (cTnI) by ssTnI. Compared to WT controls, ssTnI-TG fibre bundles were more sensitive to Ca2+ at both short SL (1.9 +/- 0.1 micrometer) and long SL (2.3 +/- 0.1 micrometer). However, compared to WT controls, the increase in Ca2+ sensitivity (change in half-maximally activating free Ca2+; DeltaEC50) associated with the increase in SL was significantly blunted in the ssTnI-TG myofilaments. Agents that sensitize the myofilaments to Ca2+ by promoting the actin-myosin reaction (EMD 57033 and CGP-48506) significantly reduced the length-dependent DeltaEC50 for Ca2+ activation, when SL in WT myofilaments was increased from 1.9 to 2.3 micrometer. Exposure of myofilaments to calmidazolium (CDZ), which binds to cTnC and increases its affinity for Ca2+, sensitized force developed by WT myofilaments to Ca2+ at SL 1.9 micrometer and desensitized the WT myofilaments at SL 2.3 micrometer. There were no significant effects of CDZ on ssTnI-TG myofilaments at either SL. Our results indicate that length-dependent Ca2+ activation is modified by specific changes in thin filament proteins and by agents that promote the actin-myosin interaction. Thus, these in vitro results provide a basis for using these models to test the relative significance of the length dependence of activation in situ.

Altered regulatory properties of human cardiac troponin I mutants that cause hypertrophic cardiomyopathy

Cardiac troponin I (cTnI) is the inhibitory component of the troponin complex and is involved in the calcium control of heart muscle contraction. Recently, specific missense mutations of the cTnI gene (TNNI3) have been shown to cause familial hypertrophic cardiomyopathy (HCM). We have analyzed the functional effects of two HCM mutations (R145G and R162W) using purified recombinant cTnI. Both mutations gave reduced inhibition of actin-tropomyosin-activated myosin ATPase, both in experiments using cTnI alone and in those using reconstituted human cardiac troponin under relaxing conditions. Both mutant troponin complexes also conferred increased calcium sensitivity of ATPase regulation. Studies on wild type/R145G mutant mixtures showed that the wild type phenotype was dominant in that the inhibition and the calcium sensitivity conferred by a 50:50 mixture was more similar to wild type than expected. Surface plasmon resonance-based assays showed that R162W mutant had an increased affinity for troponin C in the presence of calcium. This observation may contribute to the increased calcium sensitivity found with this mutant and also corroborates recent structural data. The observed decreased inhibition and increased calcium sensitivity suggest that these mutations may cause HCM via impaired relaxation rather than the impaired contraction seen with some other classes of HCM mutants.