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

Identification and functional characterization of cardiac troponin I as a novel disease gene in autosomal dominant dilated cardiomyopathy

RATIONALE

Idiopathic dilated cardiomyopathy (DCM) is inherited in approximately one third of cases, usually as an autosomal dominant trait. More than 30 loci have been identified, several of which encode sarcomeric proteins which can also be mutated to cause hypertrophic cardiomyopathy. One contractile protein gene well known as a hypertrophic cardiomyopathy disease gene, but with no reported mutation in autosomal dominant DCM, is TNNI3 which encodes cardiac troponin I.

OBJECTIVE

To test TNNI3 as a candidate gene, a panel of 96 probands with DCM was analyzed.

METHODS AND RESULTS

Genomic DNA was isolated and TNNI3 exons screened by heteroduplex analysis. Exons with aberrant profiles were sequenced and variants evaluated by segregation analysis and study of normal controls. We report 2 novel TNNI3 missense mutations, Lys36Gln and Asn185Lys, each associated with severe and early onset familial DCM. Of the 5 mutation carriers, cardiac transplantation was required in 3, at ages 6, 15, and 24 years. Analysis of Ca(2+) regulation of actin-tropomyosin-activated myosin ATPase by troponin revealed that troponin reconstituted with either mutant troponin I gave lower maximum ATPase rates and lower Ca(2+) sensitivity than wild type. Furthermore, mutant thin filaments had reduced Ca(2+) affinity compared with normal.

CONCLUSIONS

The functional alterations mirror closely a consistent phenotype found in proven DCM mutations in other thin filament proteins, thus supporting the interpretation that these mutations are disease-causing. These are the first reported autosomal dominant DCM-causing mutations in TNNI3, and so the findings expand the spectrum of disease-causing genes that lead to either hypertrophic cardiomyopathy or DCM depending on the specific mutation.

Ala scanning of the inhibitory region of cardiac troponin I

In skeletal and cardiac muscles, troponin (Tn), which resides on the thin filament, senses a change in intracellular Ca(2+) concentration. Tn is composed of TnC, TnI, and TnT. Ca(2+) binding to the regulatory domain of TnC removes the inhibitory effect by TnI on the contraction. The inhibitory region of cardiac TnI spans from residue 138 to 149. Upon Ca(2+) activation, the inhibitory region is believed to be released from actin, thus triggering actin-activation of myosin ATPase. In this study, we created a series of Ala-substitution mutants of cTnI to delineate the functional contribution of each amino acid in the inhibitory region to myofilament regulation. We found that most of the point mutations in the inhibitory region reduced the ATPase activity in the presence of Ca(2+), which suggests the same region also acts as an activator of the ATPase. The thin filaments can also be activated by strong myosin head (S1)-actin interactions. The binding of N-ethylmaleimide-treated myosin subfragment 1 (NEM-S1) to actin filaments mimics such strong interactions. Interestingly, in the absence of Ca(2+) NEM-S1-induced activation of S1 ATPase was significantly less with the thin filaments containing TnI(T144A) than that with the wild-type TnI. However, in the presence of Ca(2+), there was little difference in the activation of ATPase activity between these preparations.

An internal domain of beta-tropomyosin increases myofilament Ca(2+) sensitivity

Tropomyosin (TM) is involved in Ca(2+)-mediated muscle contraction and relaxation in the heart. Striated muscle alpha-TM is the major isoform expressed in the heart. The expression of striated muscle beta-TM in the murine myocardium results in a decreased rate of relaxation and increased myofilament Ca(2+) sensitivity. Replacing the carboxyl terminus (amino acids 258-284) of alpha-TM with beta-TM (a troponin T-binding region) results in decreased rates of contraction and relaxation in the heart and decreased myofilament Ca(2+) sensitivity. We hypothesized that the putative internal troponin T-binding domain (amino acids 175-190) of beta-TM may be responsible for the increased myofilament Ca(2+) sensitivity observed when the entire beta-TM is expressed in the heart. To test this hypothesis, we generated transgenic mice that expressed chimeric TM containing beta-TM amino acids 175-190 in the backbone of alpha-TM (amino acids 1-174 and 191-284). These mice expressed 16-57% chimeric TM and did not develop cardiac hypertrophy or any other morphological changes. Physiological analysis showed that these hearts exhibited decreased rates of contraction and relaxation and a positive response to isoproterenol. Skinned fiber bundle analyses showed a significant increase in myofilament Ca(2+) sensitivity. Biophysical experiments demonstrated that the exchanged amino acids did not influence the flexibility of the TM. This is the first study to demonstrate that a specific domain within TM can increase the Ca(2+) sensitivity of the thin filament and affect sarcomeric performance. Furthermore, these results enhance the understanding of why TM mutations associated with familial hypertrophic cardiomyopathy demonstrate increased myofilament sensitivity to Ca(2+).

Biomarkers of pathophysiology in hypertrophic cardiomyopathy: implications for clinical management and prognosis

The study of biomarkers and their signalling pathways has allowed the development of new therapeutic strategies in a range of disorders. The aim of the present systematic review is to provide an overview of different biomarkers in patients with hypertrophic cardiomyopathy that could give some insight into the pathophysiologic mechanism(s) underlying the typical clinical and histological manifestations of the disease. Several pathophysiological models are presented and discussed, including studies that have investigated these biomarkers for diagnostic and prognostic reasons, in relation to disease progression and/or mortality.

Modulation of the effects of tropomyosin on actin and myosin conformational changes by troponin and Ca2+

The molecular mechanisms by which troponin (TN)-tropomyosin (TM) regulates the myosin ATPase cycle were investigated using fluorescent probes specifically bound to Cys36 of TM, Cys707 of myosin subfragment-1, and Cys374 of actin incorporated into ghost muscle fibers. Intermediate states of actomyosin were simulated by using nucleotides and non-hydrolysable ATP analogs. Multistep changes in mobility and spatial arrangement of SH1 helix of myosin motor domain and actin subdomain-1 during the ATPase cycle were observed. Each intermediate state of actomyosin induced a definite conformational state and specific position of TM strands on the surface of thin filament. TM increased the amplitude of myosin SH1 helix and actin subdomain-1 movements at transition from weak- to strong-binding states shifting to the center of thin filament at strong-binding and to the periphery of thin filament at weak-binding states. TN modulated those movements in a capital ES, Cyrillicsmall a, Cyrillic(2+)-dependent manner. At high-Ca(2+), TN enhanced the effect of TM on SH1 helix and subdomain-1 movements by transferring TM further to the center of thin filament at strong-binding states. In contrast, at low-Ca(2+), TN inhibited the effect of TM movements, "freezing" actin structure in "OFF" state and TM in the position typical for weak-binding states, resulting in disturbing the interplay of actin and myosin.

Molecular etiology and pathogenesis of hereditary cardiomyopathy

Cardiomyopathy is defined as a cardiac disease caused by functional abnormality of cardiac muscle, and the etiology of the functional abnormality includes both extrinsic and intrinsic factors. Cardiomyopathy caused by the intrinsic factors is defined as idiopathic or primary cardiomyopathy, and there are several clinical phenotypes, including hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). The major intrinsic factor is gene mutations, and linkage studies, as well as candidate gene approaches, have deciphered multiple disease genes for hereditary primary cardiomyopathy. Of note is that mutations in the same disease gene can be found in different clinical phenotypes of cardiomyopathy. Functional analyses of disease-related mutations have revealed that characteristic functional alterations are associated with the clinical phenotypes, such that increased and decreased Ca(2+) sensitivity because of sarcomere mutations are associated with HCM and DCM, respectively. In addition, recent data have suggested that mutations in the Z-disc components found in HCM and DCM may result in increased and decreased stiffness of the sarcomere (ie, stiff sarcomere and loose sarcomere, respectively). More recently, mutations in the components of the I region can be found in hereditary cardiomyopathy, further complicating the etiology of primary cardiomyopathy.

Functional consequences of the human cardiac troponin I hypertrophic cardiomyopathy mutation R145G in transgenic mice

In this study, we addressed the functional consequences of the human cardiac troponin I (hcTnI) hypertrophic cardiomyopathy R145G mutation in transgenic mice. Simultaneous measurements of ATPase activity and force in skinned papillary fibers from hcTnI R145G transgenic mice (Tg-R145G) versus hcTnI wild type transgenic mice (Tg-WT) showed a significant decrease in the maximal Ca(2+)-activated force without changes in the maximal ATPase activity and an increase in the Ca(2+) sensitivity of both ATPase and force development. No difference in the cross-bridge turnover rate was observed at the same level of cross-bridge attachment (activation state), showing that changes in Ca(2+) sensitivity were not due to changes in cross-bridge kinetics. Energy cost calculations demonstrated higher energy consumption in Tg-R145G fibers compared with Tg-WT fibers. The addition of 3 mm 2,3-butanedione monoxime at pCa 9.0 showed that there was approximately 2-4% of force generating cross-bridges attached in Tg-R145G fibers compared with less than 1.0% in Tg-WT fibers, suggesting that the mutation impairs the ability of the cardiac troponin complex to fully inhibit cross-bridge attachment under relaxing conditions. Prolonged force and intracellular [Ca(2+)] transients in electrically stimulated intact papillary muscles were observed in Tg-R145G compared with Tg-WT. These results suggest that the phenotype of hypertrophic cardiomyopathy is most likely caused by the compensatory mechanisms in the cardiovascular system that are activated by 1) higher energy cost in the heart resulting from a significant decrease in average force per cross-bridge, 2) slowed relaxation (diastolic dysfunction) caused by prolonged [Ca(2+)] and force transients, and 3) an inability of the cardiac TnI to completely inhibit activation in the absence of Ca(2+) in Tg-R145G mice.

The unique functions of cardiac troponin I in the control of cardiac muscle contraction and relaxation

We review development of evidence and current perceptions of the multiple and significant functions of cardiac troponin I in regulation and modulation of cardiac function. Our emphasis is on the unique structure function relations of the cardiac isoform of troponin I, especially regions containing sites of phosphorylation. The data indicate that modifications of specific regions cardiac troponin I by phosphorylations either promote or reduce cardiac contractility. Thus, a homeostatic balance in these phosphorylations is an important aspect of control of cardiac function. A new concept is the idea that the homeostatic mechanisms may involve modifications of intra-molecular interactions in cardiac troponin I.

Dilated and hypertrophic cardiomyopathy mutations in troponin and alpha-tropomyosin have opposing effects on the calcium affinity of cardiac thin filaments

Dilated cardiomyopathy and hypertrophic cardiomyopathy (HCM) can be caused by mutations in thin filament regulatory proteins of the contractile apparatus. In vitro functional assays show that, in general, the presence of dilated cardiomyopathy mutations decreases the Ca(2+) sensitivity of contractility, whereas HCM mutations increase it. To assess whether this functional phenomenon was a direct result of altered Ca(2+) affinity or was caused by altered troponin-tropomyosin switching, we assessed Ca(2+) binding of the regulatory site of cardiac troponin C in wild-type or mutant troponin complex and thin filaments using a fluorescent probe (2-[4'-{iodoacetamido}aniline]-naphthalene-6-sulfonate) attached to Cys35 of cardiac troponin C. The Ca(2+)-binding affinity (pCa(50)=6.57+/-0.03) of reconstituted troponin complex was unaffected by all of the HCM and dilated cardiomyopathy troponin mutants tested, with the exception of the troponin I Arg145Gly HCM mutation, which caused an increase (DeltapCa(50)=+0.31+/-0.05). However, when incorporated into regulated thin filaments, all but 1 of the 10 troponin and alpha-tropomyosin mutants altered Ca(2+)-binding affinity. Both HCM mutations increased Ca(2+) affinity (DeltapCa(50)=+0.41+/-0.02 and +0.51+/-0.01), whereas the dilated cardiomyopathy mutations decreased affinity (DeltapCa(50)=-0.12+/-0.04 to -0.54+/-0.04), which correlates with our previous functional in vitro assays. The exception was the troponin T Asp270Asn mutant, which caused a significant decrease in cooperativity. Because troponin is the major Ca(2+) buffer in the cardiomyocyte sarcoplasm, we suggest that Ca(2+) affinity changes caused by cardiomyopathy mutant proteins may directly affect the Ca(2+) transient and hence Ca(2+)-sensitive disease state remodeling pathways in vivo. This represents a novel mechanism for this class of mutation.

Phosphorylation-dependent conformational transition of the cardiac specific N-extension of troponin I in cardiac troponin

We present here the solution structure for the bisphosphorylated form of the cardiac N-extension of troponin I (cTnI(1-32)), a region for which there are no previous high-resolution data. Using this structure, the X-ray crystal structure of the cardiac troponin core, and uniform density models of the troponin components derived from neutron contrast variation data, we built atomic models for troponin that show the conformational transition in cardiac troponin induced by bisphosphorylation. In the absence of phosphorylation, our NMR data and sequence analyses indicate a less structured cardiac N-extension with a propensity for a helical region surrounding the phosphorylation motif, followed by a helical C-terminal region (residues 25-30). In this conformation, TnI(1-32) interacts with the N-lobe of cardiac troponin C (cTnC) and thus is positioned to modulate myofilament Ca2+-sensitivity. Bisphosphorylation at Ser23/24 extends the C-terminal helix (residues 21-30) which results in weakening interactions with the N-lobe of cTnC and a re-positioning of the acidic amino terminus of cTnI(1-32) for favorable interactions with basic regions, likely the inhibitory region of cTnI. An extended poly(L-proline)II helix between residues 11 and 19 serves as the rigid linker that aids in re-positioning the amino terminus of cTnI(1-32) upon bisphosphorylation at Ser23/24. We propose that it is these electrostatic interactions between the acidic amino terminus of cTnI(1-32) and the basic inhibitory region of troponin I that induces a bending of cTnI at the end that interacts with cTnC. This model provides a molecular mechanism for the observed changes in cross-bridge kinetics upon TnI phosphorylation.

DCM troponin C mutant Gly159Asp blunts the response to troponin phosphorylation

Dilated cardiomyopathy (DCM) can be caused by a Gly159Asp mutation in cardiac troponin C (cTnC). Our previous work found that partial replacement of endogenous troponin in skinned muscle fibres with human cardiac troponin containing Gly159Asp cTnC had no significant effect on maximum force generation, Ca(2+)-sensitivity or cooperativity, but halved the activation rate. In order to examine whether the mutant affected contractility when troponin was phosphorylated, Gly159Asp cTnC was introduced in the presence of a phosphomimic of protein kinase A phosphorylation of troponin I (Ser23Asp,Ser24Asp). The increased force production of the muscle fibres caused by this phosphomimic was significantly depressed. Furthermore, in the presence of the protein kinase C phosphomimic of troponin T (Thr203Glu), Gly159Asp mutant significantly reversed the decrease in Ca(2+)-sensitivity. We conclude that this DCM mutant significantly blunts the contractile response to phosphorylation and this novel mechanism may contribute to its pathogenic effect.

Prevalence, clinical significance, and genetic basis of hypertrophic cardiomyopathy with restrictive phenotype

OBJECTIVES

The purpose of this study was to determine the prevalence, clinical significance, and genetic basis of hypertrophic cardiomyopathy (HCM) with "restrictive phenotype" characterized by restrictive filling and minimal or no left ventricular hypertrophy.

BACKGROUND

Hypertrophic cardiomyopathy is a heterogeneous myocardial disorder with a broad spectrum of clinical presentation and morphologic features. Recent reports indicated that some patients with restrictive cardiomyopathy, which is an uncommon condition defined by restrictive filling and reduced diastolic volumes with normal or near normal left ventricular wall thickness and contractile function, have features suggestive of HCM with mutations in cardiac troponin I, myocyte disarray at explant/autopsy, and relatives with HCM. Systematic evaluation of the restrictive phenotype in HCM patients has not been performed.

METHODS

We evaluated 1,226 patients from 688 consecutive HCM families to identify individuals who fulfilled diagnostic criteria for "restrictive phenotype."

RESULTS

Nineteen of 1,226 affected individuals (1.5%) from 16 families (2.3%) had the "restrictive phenotype." During follow up (53.7 +/- 49.2 months), 17 patients (89%) experienced dyspnea (New York Heart Association functional class > or =2). The 5-year survival rate from all-cause mortality, cardiac transplantation, or implantable cardioverter-defibrillator discharge was 56.4%. Mutation analysis for 5 sarcomere genes was feasible in 15 of 16 probands. Mutations were found in 8: 4 in beta-myosin heavy chain, and 4 in cardiac troponin I.

CONCLUSIONS

The "restrictive phenotype" in isolation is an uncommon presentation of the clinical spectrum of HCM and is associated with severe limitation and poor prognosis. This phenotype may be associated with beta-myosin heavy chain and cardiac troponin I mutations.

Computational simulation of hypertrophic cardiomyopathy mutations in Troponin I: Influence of increased myofilament calcium sensitivity on isometric force, ATPase and [Ca2+]i

Familial hypertrophic cardiomyopathy (FHC) is an inherited disease that is characterized by ventricular hypertrophy, cardiac arrhythmias and increased risk of premature sudden death. FHC is caused by autosomal-dominant mutations in genes for a number of sarcomeric proteins; many mutations in Ca(2+)-regulatory proteins of the cardiac thin filament are associated with increased Ca(2+) sensitivity of myofilament function. Computational simulations were used to investigate the possibility that these mutations could affect the Ca(2+) transient and mechanical response of a myocyte during a single cardiac cycle. We used existing experimental data for specific mutations of cardiac troponin I that exhibit increased Ca(2+) sensitivity in physiological and biophysical assays. The simulated Ca(2+) transients were used as input for a three-dimensional half-sarcomere biomechanical model with filament compliance to predict the resulting force. Mutations with the highest Ca(2+) affinity (lowest K(m)) values, exhibit the largest decrease in peak Ca(2+) assuming a constant influx of Ca(2+) into the cytoplasm; they also prolong Ca(2+) removal but have little effect on diastolic Ca(2+). Biomechanical model results suggest that these cTnI mutants would increase peak force despite the decrease in peak [Ca(2+)](i). There is a corresponding increase in net ATP hydrolysis, with no change in tension cost (ATP hydrolyzed per unit of time-integrated tension). These simulations suggest that myofilament-initiated hypertrophic signaling could be associated with decreased [Ca(2+)](i), increased stress/strain, and/or increased ATP flux.

Mutations in fast skeletal troponin I, troponin T, and β‐tropomyosin that cause distal arthrogryposis all increase contractile function

Distal arthrogryposes (DAs) are a group of disorders characterized by congenital contractures of distal limbs without overt neurological or muscle disease. Unexpectedly, mutations in genes encoding the fast skeletal muscle regulatory proteins troponin T (TnT), troponin I (TnI), and beta-tropomyosin (beta-TM) have been shown to cause autosomal dominant DA. We tested how these mutations affect contractile function by comparing wild-type (WT) and mutant proteins in actomyosin ATPase assays and in troponin-replaced rabbit psoas fibers. We have analyzed all four reported mutants: Arg63His TnT, Arg91Gly beta-TM, Arg174Gln TnI, and a TnI truncation mutant (Arg156ter). Thin filaments, reconstituted using actin and WT troponin and beta-TM, activated myosin subfragment-1 ATPase in a calcium-dependent, cooperative manner. Thin filaments containing either a troponin or beta-TM DA mutant produced significantly enhanced ATPase rates at all calcium concentrations without alternating calcium-sensitivity or cooperativity. In troponin-exchanged skinned fibers, each mutant caused a significant increase in Ca2+ sensitivity, and Arg156ter TnI generated significantly higher maximum force. Arg91Gly beta-TM was found to have a lower actin affinity than WT and form a less stable coiled coil. We propose the mutations cause increased contractility of developing fast-twitch skeletal muscles, thus causing muscle contractures and the development of the observed limb deformities.

Myofibrillar remodeling in cardiac hypertrophy, heart failure and cardiomyopathies

BACKGROUND

A wide variety of pathological conditions have been shown to result in cardiac remodelling and myocardial dysfunction. However, the mechanisms of transition from adaptive to maladaptive alterations, as well as those for changes in cardiac performance leading to heart failure, are poorly understood.

OBSERVATIONS

Extensive studies have revealed a broad spectrum of progressive changes in subcellular structures and function, as well as in signal transduction and metabolism in the heart, among different cardiovascular disorders. The present review is focused on identifying the alterations in molecular and biochemical structure of myofibrils (myofibrillar remodelling) in hypertrophied and failing myocardium in different types of heart diseases. Numerous changes at the level of gene expression for both contractile and regulatory proteins have already been reported in failing hearts and heart diseases; these changes are potential precursors for heart failure such as cardiac hypertrophy and cardiomyopathies. Myofibrillar remodelling, as a consequence of proteolysis, oxidation, and phosphorylation of some functional groups in both contractile and regulatory proteins in hearts failing due to different etiologies, has also been described.

CONCLUSIONS

Although myofibrillar remodelling appears to be associated with cardiac dysfunction, alterations in both contractile and regulatory proteins are dependent on the type and stage of heart disease.

Increased Ca2+ affinity of cardiac thin filaments reconstituted with cardiomyopathy-related mutant cardiac troponin I

To understand the molecular mechanisms whereby cardiomyopathy-related cardiac troponin I (cTnI) mutations affect myofilament activity, we have investigated the Ca2+ binding properties of various assemblies of the regulatory components that contain one of the cardiomyopahty-related mutant cTnI. Acto-S1 ATPase activities in reconstituted systems were also determined. We investigated R145G and R145W mutations from the inhibitory region and D190H and R192H mutations from the second actin-tropomyosin-binding site. Each of the four mutations sensitized the acto-S1 ATPase to Ca2+. Whereas the mutations from the inhibitory region increased the basal level of ATPase activity, those from the second actin-tropomyosin-binding site did not. The effects on the Ca2+ binding properties of the troponin ternary complex and the troponin-tropomyosin complex with one of four mutations were either desensitization or no effect compared with those with wild-type cTnI. All of the mutations, however, affected the Ca2+ sensitivities of the reconstituted thin filaments in the same direction as the acto-S1 ATPase activity. Also the thin filaments with one of the mutant cTnIs bound Ca2+ with less cooperativity compared with those with wild-type cTnI. These data indicate that the mutations found in the inhibitory region and those from the second actin-tropomyosin site shift the equilibrium of the states of the thin filaments differently. Moreover, the increased Ca2+ bound to myofilaments containing the mutant cTnIs may be an important factor in triggered arrhythmias associated with the cardiomyopathy.

Ionic interventions that alter the association of troponin C C-domain with the thin filaments of vertebrate striated muscle

The regulatory complex of vertebrate skeletal muscle integrates information about cross-bridge binding, divalent cations and other intracellular ionic conditions to control activation of muscle contraction. Relatively little is known about the role of the troponin C (TnC) C-domain in the absence of Ca2+. Here, we use a standardized condition for measuring isometric tension in rabbit psoas skinned fibers to track TnC attachment and detachment in the absence of Ca2+ under different conditions of ionic strength, pH and MgATP. In the presence of MgATP and Mg2+, TnC detaches more readily and has a 1.5- to 2-fold lower affinity for the intact thin filament at pH 8 and 250 mM K+ than at pH 6 or in 30 mM K+; changes in affinity are fully reversible. The response to ionic strength is lost when Mg2+ and MgATP are absent, whereas the response to pH persists, suggesting that weaker electrostatic TnC-TnI-TnT interactions can be overridden by strongly bound cross-bridges. In solution, titration of a fluorescent C-domain mutant (F154W TnC) with Mg2+ reveals no significant changes in Mg2+ affinity with pH or ionic strength, suggesting that these parameters influence TnC binding by acting directly on electrostatic forces between TnC and TnI rather than by changing Mg2+ binding to C-domain sites III and IV.

The genetic basis for cardiac remodeling

Cardiomyopathies are primary disorders of cardiac muscle associated with abnormalities of cardiac wall thickness, chamber size, contraction, relaxation, conduction, and rhythm. They are a major cause of morbidity and mortality at all ages and, like acquired forms of cardiovascular disease, often result in heart failure. Over the past two decades, molecular genetic studies of humans and analyses of model organisms have made remarkable progress in defining the pathogenesis of cardiomyopathies. Hypertrophic cardiomyopathy can result from mutations in 11 genes that encode sarcomere proteins, and dilated cardiomyopathy is caused by mutations at 25 chromosome loci where genes encoding contractile, cytoskeletal, and calcium regulatory proteins have been identified. Causes of cardiomyopathies associated with clinically important cardiac arrhythmias have also been discovered: Mutations in cardiac metabolic genes cause hypertrophy in association with ventricular pre-excitation and mutations causing arrhythmogenic right ventricular dysplasia were recently discovered in protein constituents of desmosomes. This considerable genetic heterogeneity suggests that there are multiple pathways that lead to changes in heart structure and function. Defects in myocyte force generation, force transmission, and calcium homeostasis have emerged as particularly critical signals driving these pathologies. Delineation of the cell and molecular events triggered by cardiomyopathy gene mutations provide new fundamental knowledge about myocyte biology and organ physiology that accounts for cardiac remodeling and defines mechanistic pathways that lead to heart failure.

Clinical characteristics of heart disease patients with a good prognosis in spite of markedly increased plasma levels of type-B natriuretic peptide (BNP): anomalous behavior of plasma BNP in hypertrophic cardiomyopathy

BACKGROUND

Although it is not rare to encounter patients with plasma B-type natriuretic peptide (BNP) levels unequivalent to the severity of heart failure (HF), there has been little investigation to clarify the causative background of this phenomenon.

METHODS AND RESULTS

Among the 1,838 outpatients whose plasma BNP was measured, persistently increased levels of BNP above 500 pg/ml was observed for more than 6 months in 14 subjects with few HF symptoms. Among these, all of 4 patients without any following cardiac events (E-/high) for 12 months showed hypertrophic nonobstructive cardiomyopathy (HNCM). When we compared the clinical parameters of these patients with those of 22 HNCM patients without any following cardiac events whose plasma BNP levels were less than 200 pg/ml, there were only 2 clinical characteristics to be distinguished: (i) plasma renin activity (PRA) and norepinephrine (NE) levels were low in spite of markedly increased levels of plasma BNP in E-/high HNCM; and (ii) echocardiographic investigation revealed that only global left atrial fractional shortening was significantly lower in E-/high HNCM.

CONCLUSIONS

Plasma BNP levels do not always reflect the severity of HF in HNCM. It might be considered to utilize other clinical parameters such as NE and PRA to recognize HF severity in such patients.

Mutations in human cardiac troponin I that are associated with restrictive cardiomyopathy affect basal ATPase activity and the calcium sensitivity of force development

Human cardiac Troponin I (cTnI) is the first sarcomeric protein for which mutations have been associated with restrictive cardiomyopathy. To determine whether five mutations in cTnI (L144Q, R145W, A171T, K178E, and R192H) associated with restrictive cardiomyopathy were distinguishable from hypertrophic cardiomyopathy-causing mutations in cTnI, actomyosin ATPase activity and skinned fiber studies were carried out. All five mutations investigated showed an increase in the Ca2+ sensitivity of force development compared with wild-type cTnI. The two mutations with the worst clinical phenotype (K178E and R192H) both showed large increases in Ca2+ sensitivity (deltapCa50 = 0.47 and 0.36, respectively). Although at least one of these mutations is not in the known inhibitory regions of cTnI, all of the mutations investigated caused a decrease in the ability of cTnI to inhibit actomyosin ATPase activity. Mixtures of wild-type and mutant cTnI showed that cTnI mutants could be classified into three different groups: dominant (L144Q, A171T and R192H), equivalent (K178E), or weaker (R145W) than wild-type cTnI in actomyosin ATPase assays in the absence of Ca2+. Although most of the mutants were able to activate actomyosin ATPase similarly to wild-type cTnI, L144Q had significantly lower maximal ATPase activities than any of the other mutants or wild-type cTnI. Three mutants (L144Q, R145W, and K178E) were unable to fully relax contraction in the absence of Ca2+. The inability of the five cTnI mutations investigated to fully inhibit ATPase activity/force development and the generally larger increases in Ca2+ sensitivity than observed for most hypertrophic cardiomyopathy mutations would likely lead to severe diastolic dysfunction and may be the major physiological factors responsible for causing the restrictive cardiomyopathy phenotype in some of the genetically affected individuals.

Transgenic rabbit model for human troponin I-based hypertrophic cardiomyopathy

BACKGROUND

Transgenic and gene-targeted models have focused on the mouse. Fundamental differences between the mouse and human exist in Ca2+ handling during contraction/relaxation and in alterations in Ca2+ flux during heart failure, with the rabbit more accurately reflecting the human system.

METHODS AND RESULTS

Cardiac troponin I (cTnI) mutations can cause familial hypertrophic cardiomyopathy. An inhibitory domain mutation, arginine146-->glycine (cTnI(146Gly)), was modeled with the use of transgenic expression in the rabbit ventricle. cTnI(146Gly) levels >40% of total cTnI were perinatally lethal, whereas replacement levels of 15% to 25% were well tolerated. cTnI(146Gly) expression led to a leftward shift in the force-pCa2+ curves with cardiomyocyte disarray, fibrosis, and altered connexin43 organization. In isolated cTnI(146Gly) myocytes, twitch relaxation amplitudes were smaller than in normal cells, but [Ca]i transients and sarcoplasmic reticulum Ca2+ load were not different. Detrended fluctuation analysis of the QT(max) intervals was used to evaluate the cardiac repolarization phase and showed a significantly higher scaling exponent in the transgenic animals.

CONCLUSIONS

Expression of modest amounts of cTnI(146Gly) led to subtle defects without severely affecting cardiac function. Aberrant connexin organization, subtle morphological deficits, and an altered fractal pattern of the repolarization phase of transgenic rabbits, in the absence of entropy or other ECG abnormalities, may indicate an early developing pathology before the onset of more obvious repolarization abnormalities or major alterations in cardiac mechanics.

Effects of the mutation R145G in human cardiac troponin I on the kinetics of the contraction-relaxation cycle in isolated cardiac myofibrils

Familial hypertrophic cardiomyopathy (FHC) has been linked to mutations in sarcomeric proteins such as human cardiac troponin I (hcTnI). To elucidate the functional consequences of the mutation hcTnI(R145G) on crossbridge kinetics, force kinetics were analysed in murine cardiac myofibrils carrying either the mutant or the wild-type protein. The mutation was introduced into the myofibrils in two different ways: in the first approach, the endogenous Tn was replaced by incubation of the myofibrils with an excess of reconstituted recombinant hcTn containing either hcTnI(WT) or hcTnI(R145G). Alternatively, myofibrils were isolated either from non-transgenic or transgenic mice expressing the corresponding mcTnI(R146G) mutation. In myofibrils from both models, the mutation leads to a significant upward shift of the passive force-sarcomere length relation determined at pCa 7.5. Addition of 5 mm BDM (2,3-butandione-2-monoxime), an inhibitor of actomyosin ATPase partially reverses this shift, suggesting that the mutation impairs the normal function of cTnI to fully inhibit formation of force-generating crossbridges in the absence of Ca(2)(+). Maximum force development (F(max)) is significantly decreased by the mutation only in myofibrils exchanged with hcTnI(R145G) in vitro. Ca(2)(+) sensitivity of force development was reduced by the mutation in myofibrils from transgenic mice but not in exchanged myofibrils. In both models the rate constant of force development k(ACT) is reduced at maximal [Ca(2)(+)] but not at low [Ca(2)(+)] where it is rather increased. Force relaxation is significantly prolonged due to a reduction of the relaxation rate constant k(REL). We therefore assume that the impairment in the regulatory function of TnI by the mutation leads to modulations in crossbridge kinetics that significantly alter the dynamics of myofibrillar contraction and relaxation.

Structural based insights into the role of troponin in cardiac muscle pathophysiology

Troponin is a molecular switch, directly regulating the Ca2+-dependent activation of myofilament in striated muscle contraction. Cardiac troponin is subject to covalent and noncovalent modifications; phosphorylation modulates myofilament physiology, mutations are linked to familial hypertrophic cardiomyopathy, intracellular acidification causes myocardial infarction, and cardiotonic drugs modify myofilament response to Ca2+. The structure of troponin provides insights into the mechanism of this molecular switch and an understanding of the effects of protein modification under pathophysiological conditions. Although the structure of troponin C has been solved in various Ca2+-bound states for some time, structural information on troponin I and troponin T has only emerged recently. This review summarizes recent advances on the structure of complexes of troponin subunits with the aim of assessing how these proteins interact with each other to execute its role as a molecular switch and how covalent and noncovalent modifications affect the structure of troponin and the switch mechanism. We focus on pinpointing the specific amino acid residues involved in phosphorylation and mutation and the pH sensitive regions in the structure of troponin. We also present recent structural work that have identified the docking sites of several cardiotonic drugs on cardiac troponin C and discuss their relevance in the direction of troponin based drug design in the therapy of heart disease.

Tcap gene mutations in hypertrophic cardiomyopathy and dilated cardiomyopathy

OBJECTIVES

We sought to explore the relationship between a Tcap gene (TCAP) abnormality and cardiomyopathy.

BACKGROUND

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) cause severe heart failure and sudden death. Recent genetic investigations have revealed that mutations of genes encoding Z-disc components, including titin and muscle LIM protein (MLP), are the primary cause of both HCM and DCM. The Z-disc plays a role in establishing the mechanical coupling of sarcomeric contraction and stretching, with the titin/Tcap/MLP complex serving as a mechanical stretch sensor. Tcap interacts with the calsarcin, which tethers the calcineurin to the Z-disc.

METHODS

The TCAP was analyzed in 346 patients with HCM (236 familial and 110 sporadic cases) and 136 patients with DCM (34 familial and 102 sporadic cases). Two different in vitro qualitative assays-yeast two-hybrid and glutathion S-transferase pull-down competition-were performed in order to investigate functional changes in Tcap's interaction with MLP, titin, and calsarcin-1 caused by the identified mutations and a reported DCM-associated mutation, R87Q.

RESULTS

Two TCAP mutations, T137I and R153H, were found in patients with HCM, and another TCAP mutation, E132Q, was identified in a patient with DCM. It was demonstrated by the qualitative assays that the HCM-associated mutations augment the ability of Tcap to interact with titin and calsarcin-1, whereas the DCM-associated mutations impair the interaction of Tcap with MLP, titin, and calsarcin-1.

CONCLUSIONS

These observations suggest that the difference in clinical phenotype (HCM or DCM) may be correlated with the property of altered binding among the Z-disc components.

Biology of the troponin complex in cardiac myocytes

Troponin is the regulatory complex of the myofibrillar thin filament that plays a critical role in regulating excitation-contraction coupling in the heart. Troponin is composed of three distinct gene products: troponin C (cTnC), the 18-kD Ca(2+)-binding subunit; troponin I (cTnI), the approximately 23-kD inhibitory subunit that prevents contraction in the absence of Ca2+ binding to cTnC; and troponin T (cTnT), the approximately 35-kD subunit that attaches troponin to tropomyosin (Tm) and to the myofibrillar thin filament. Over the past 45 years, extensive biochemical, biophysical, and structural studies have helped to elucidate the molecular basis of troponin function and thin filament activation in the heart. At the onset of systole, Ca2+ binds to the N-terminal Ca2+ binding site of cTnC initiating a conformational change in cTnC, which catalyzes protein-protein associations activating the myofibrillar thin filament. Thin filament activation in turn facilitates crossbridge cycling, myofibrillar activation, and contraction of the heart. The intrinsic length-tension properties of cardiac myocytes as well as the Frank-Starling properties of the intact heart are mediated primarily through Ca(2+)-responsive thin filament activation. cTnC, cTnI, and cTnT are encoded by distinct single-copy genes in the human genome, each of which is expressed in a unique cardiac-restricted developmentally regulated fashion. Elucidation of the transcriptional programs that regulate troponin transcription and gene expression has provided insights into the molecular mechanisms that regulate and coordinate cardiac myocyte differentiation and provided unanticipated insights into the pathogenesis of cardiac hypertrophy. Autosomal dominant mutations in cTnI and cTnT have been identified and are associated with familial hypertrophic and restrictive cardiomyopathies.

Phosphorylation and mutation of human cardiac troponin I deferentially destabilize the interaction of the functional regions of troponin I with troponin C

We have utilized 2D [(1)H,(15)N]HSQC NMR spectroscopy to elucidate the binding of three segments of cTnI in native, phosphorylated, and mutated states to cTnC. The near N-terminal region (cRp; residues 34-71) contains the protein kinase C (PKC) phosphorylation sites S41 and S43, the inhibitory region (cIp; residues 128-147) contains another PKC site T142 and a familial hypertrophic cardiomyopathy (FHC) mutation R144G, and the switch region (cSp; residues 147-163) contains the novel p21-activated kinase (PAK) site S149 and another FHC mutation R161W. While S41/S43 phosphorylation of cRp had minimal disruption in the interaction of cRp and cTnC.3Ca(2+), T142 phosphorylation reduced the affinity of cIp for cCTnC.2Ca(2+) by approximately 14-fold and S149 phosphorylation reduced the affinity of cSp for cNTnC.Ca(2+) by approximately 10-fold. The mutation R144G caused an approximately 6-fold affinity decrease of cIp for cCTnC.2Ca(2+) and mutation R161W destabilized the interaction of cSp and cNTnC.Ca(2+) by approximately 1.4-fold. When cIp was both T142 phosphorylated and R144G mutated, its affinity for cCTnC.2Ca(2+) was reduced approximately 19-fold, and when cSp was both S149 phosphorylated and R161W mutated, its affinity for cNTnC.Ca(2+) was reduced approximately 4-fold. Thus, while the FHC mutation R144G enhances the effect of T142 phosphorylation on the interaction of cIp and cCTnC.2Ca(2+), the FHC mutation R161W suppresses the effect of S149 phosphorylation on the interaction of cSp and cNTnC.Ca(2+), demonstrating linkages between the FHC mutation and phosphorylation of cTnI. The observed alterations corroborate well with structural data. These results suggest that while the modifications in the cRp region have minimal influence, those in the key functional cIp-cSp region have a pronounced effect on the interaction of cTnI and cTnC, which may correlate with the altered myofilament function and cardiac muscle contraction under pathophysiological conditions.

C-Terminal Truncation of Cardiac Troponin I Causes Divergent Effects on ATPase and Force

Myocardial stunning is a form of reversible myocardial ischemia/reperfusion injury associated with systolic and diastolic contractile dysfunction. In the isolated rat heart model, myocardial stunning is characterized by specific C-terminal proteolysis of the myofilament protein, troponin I (cTnI) that yields cTnI 1-193 . To determine the effect of this particular C-terminal truncation of cTnI, without the confounding factor of other stunning-induced protein modifications, a series of solution biochemical assays has been undertaken using the human homologue of mouse/rat cTnI 1-193 , cTnI 1-192 . Affinity chromatography and actin sedimentation experiments detected little, or no, difference between the binding of cTnI (cTnI 1-209 ) and cTnI 1-192 to actin-tropomyosin, troponin T, or troponin C. Both cTnI and cTnI 1-192 inhibit the actin-tropomyosin–activated ATPase activity of myosin subfragment 1 (S1), and this inhibition is released by troponin C in the presence of Ca 2+ . However, cTnI 1-192 , when reconstituted as part of the troponin complex (cTn 1-192 ), caused a 54±11% increase in the maximum Ca 2+ -activated actin-tropomyosin-S1 ATPase activity, compared with troponin reconstituted with cTnI (cTn). Furthermore, cTn 1-192 increased Ca 2+ sensitivity of both the actin-tropomyosin-activated S1 ATPase activity and the Ca 2+ -dependent sliding velocity of reconstituted thin filaments, in an in vitro motility assay, compared with cTn. In an in vitro force assay, the actin-tropomyosin filaments bearing cTn 1-192 developed only 76±4% ( P <0.001) of the force obtained with filaments composed of reconstituted cTn. We suggest that cTnI proteolysis may contribute to the pathophysiology of myocardial stunning by altering the Ca 2+ -sensing and chemomechanical properties of the myofilaments.

Cardiomyopathic tropomyosin mutations that increase thin filament Ca2+ sensitivity and tropomyosin N-domain flexibility

The relationship between tropomyosin thermal stability and thin filament activation was explored using two N-domain mutants of alpha-striated muscle tropomyosin, A63V and K70T, each previously implicated in familial hypertrophic cardiomyopathy. Both mutations had prominent effects on tropomyosin thermal stability as monitored by circular dichroism. Wild type tropomyosin unfolded in two transitions, separated by 10 degrees C. The A63V and K70T mutations decreased the melting temperature of the more stable of these transitions by 4 and 10 degrees C, respectively, indicating destabilization of the N-domain in both cases. Global analysis of all three proteins indicated that the tropomyosin N-domain and C-domain fold with a cooperative free energy of 1.0-1.5 kcal/mol. The two mutations increased the apparent affinity of the regulatory Ca2+ binding sites of thin filament in two settings: Ca2+-dependent sliding speed of unloaded thin filaments in vitro (at both pH 7.4 and 6.3), and Ca2+ activation of the thin filament-myosin S1 ATPase rate. Neither mutation had more than small effects on the maximal ATPase rate in the presence of saturating Ca2+ or on the maximal sliding speed. Despite the increased tropomyosin flexibility implied by destabilization of the N-domain, neither the cooperativity of thin filament activation by Ca2+ nor the cooperative binding of myosin S1-ADP to the thin filament was altered by the mutations. The combined results suggest that a more dynamic tropomyosin N-domain influences interactions with actin and/or troponin that modulate Ca2+ sensitivity, but has an unexpectedly small effect on cooperative changes in tropomyosin position on actin.

Familial hypertrophic cardiomyopathy mutations in troponin I (K183D, G203S, K206Q) enhance filament sliding

A major cause of familial hypertrophic cardiomyopathy (FHC) is dominant mutations in cardiac sarcomeric genes. Linkage studies identified FHC-related mutations in the COOH terminus of cardiac troponin I (cTnI), a region with unknown function in Ca(2+) regulation of the heart. Using in vitro assays with recombinant rat troponin subunits, we tested the hypothesis that mutations K183Delta, G203S, and K206Q in cTnI affect Ca(2+) regulation. All three mutants enhanced Ca(2+) sensitivity and maximum speed (s(max)) of filament sliding of in vitro motility assays. Enhanced s(max) (pCa 5) was observed with rabbit skeletal and rat cardiac (alpha-MHC or beta-MHC) heavy meromyosin (HMM). We developed a passive exchange method for replacing endogenous cTn in permeabilized rat cardiac trabeculae. Ca(2+) sensitivity and maximum isometric force did not differ between preparations exchanged with cTn(cTnI,K206Q) or wild-type cTn. In both trabeculae and motility assays, there was no loss of inhibition at pCa 9. These results are consistent with COOH terminus of TnI modulating actomyosin kinetics during unloaded sliding, but not during isometric force generation, and implicate enhanced cross-bridge cycling in the cTnI-related pathway(s) to hypertrophy.

Hypertrophic cardiomyopathy:a paradigm for myocardial energy depletion

Genetic analysis of hypertrophic cardiomyopathy (HCM), a mendelian form of cardiac hypertrophy, indicates that the primary defect is in sarcomeric function. However, the initial proposal that depressed myocardial contraction leads to a 'compensatory' hypertrophy has proven inconsistent with laboratory and clinical evidence. Drawing on observations of mutant contractile protein function, together with mouse models and clinical studies, we propose that sarcomeric HCM mutations lead to inefficient ATP utilization. The suggestion that energy depletion underlies HCM is supported by the HCM-like phenotype found with mutations in a variety of metabolic genes. A central role for compromised energetics would also help explain the unresolved clinical observations of delayed onset and asymmetrical hypertrophy in HCM, and would have implications for therapy in HCM and, potentially, in more-common forms of cardiac hypertrophy and failure.

Actin binding proteins: regulation of cytoskeletal microfilaments

The actin cytoskeleton is a complex structure that performs a wide range of cellular functions. In 2001, significant advances were made to our understanding of the structure and function of actin monomers. Many of these are likely to help us understand and distinguish between the structural models of actin microfilaments. In particular, 1) the structure of actin was resolved from crystals in the absence of cocrystallized actin binding proteins (ABPs), 2) the prokaryotic ancestral gene of actin was crystallized and its function as a bacterial cytoskeleton was revealed, and 3) the structure of the Arp2/3 complex was described for the first time. In this review we selected several ABPs (ADF/cofilin, profilin, gelsolin, thymosin beta4, DNase I, CapZ, tropomodulin, and Arp2/3) that regulate actin-driven assembly, i.e., movement that is independent of motor proteins. They were chosen because 1) they represent a family of related proteins, 2) they are widely distributed in nature, 3) an atomic structure (or at least a plausible model) is available for each of them, and 4) each is expressed in significant quantities in cells. These ABPs perform the following cellular functions: 1) they maintain the population of unassembled but assembly-ready actin monomers (profilin), 2) they regulate the state of polymerization of filaments (ADF/cofilin, profilin), 3) they bind to and block the growing ends of actin filaments (gelsolin), 4) they nucleate actin assembly (gelsolin, Arp2/3, cofilin), 5) they sever actin filaments (gelsolin, ADF/cofilin), 6) they bind to the sides of actin filaments (gelsolin, Arp2/3), and 7) they cross-link actin filaments (Arp2/3). Some of these ABPs are essential, whereas others may form regulatory ternary complexes. Some play crucial roles in human disorders, and for all of them, there are good reasons why investigations into their structures and functions should continue.

Expression and purification of human cardiac troponin subunits and their functional incorporation into isolated cardiac mouse myofibrils

The three subunits of the human cardiac troponin complex (hcTnC, hcTnI, hcTnT) were overexpressed in E. coli, purified and reconstituted to form the hcTn complex. This complex was then incorporated into subcellular bundles of mouse cardiac myofibrils whereby the native mcTn complex was replaced. On thus exchanged myofibrils, isometric force kinetics following sudden changes in free Ca(2+) concentration were measured using atomic force cantilevers. Following the exchange, the myofibrillar force remained fully Ca(2+) regulated, i.e. myofibrils were completely relaxed at pCa 7.5 and developed the same maximum Ca(2+)-activated isometric force upon increasing the pCa to 4.5 as unexchanged myofibrils. The replacement of endogenous mcTn by wild-type hcTn neither altered the kinetics of Ca(2+)-induced force development of the mouse myofibrils nor the kinetics of force relaxation induced by the sudden, complete removal of Ca(2+). Preparations of functional Tn reconstituted myofibrils provide a promising model to study the role of Tn in kinetic mechanisms of cardiac myofibrillar contraction and relaxation.

Idiopathic restrictive cardiomyopathy is part of the clinical expression of cardiac troponin I mutations

Restrictive cardiomyopathy (RCM) is an uncommon heart muscle disorder characterized by impaired filling of the ventricles with reduced volume in the presence of normal or near normal wall thickness and systolic function. The disease may be associated with systemic disease but is most often idiopathic. We recognized a large family in which individuals were affected by either idiopathic RCM or hypertrophic cardiomyopathy (HCM). Linkage analysis to selected sarcomeric contractile protein genes identified cardiac troponin I (TNNI3) as the likely disease gene. Subsequent mutation analysis revealed a novel missense mutation, which cosegregated with the disease in the family (lod score: 4.8). To determine if idiopathic RCM is part of the clinical expression of TNNI3 mutations, genetic investigations of the gene were performed in an additional nine unrelated RCM patients with restrictive filling patterns, bi-atrial dilatation, normal systolic function, and normal wall thickness. TNNI3 mutations were identified in six of these nine RCM patients. Two of the mutations identified in young individuals were de novo mutations. All mutations appeared in conserved and functionally important domains of the gene. This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Folding and function of the troponin tail domain. Effects of cardiomyopathic troponin T mutations

Troponin contains a globular Ca(2+)-binding domain and an elongated tail domain composed of the N terminus of subunit troponin T (TnT). The tail domain anchors troponin to tropomyosin and actin, modulates myosin function, and is a site of cardiomyopathy-inducing mutations. Critical interactions between tropomyosin and troponin are proposed to depend on tail domain residues 112-136, which are highly conserved across phyla. Most cardiomyopathy mutations in TnT flank this region. Three such mutations were examined and had contrasting effects on peptide TnT-(1-156), promoting folding and thermal stability assessed by circular dichroism (F110I) or weakening folding and stability (T104V and to a small extent R92Q). Folding of both TnT-(1-156) and whole troponin was promoted by replacing bovine TnT Thr-104 with human TnT Ala-104, further indicating the importance of this cardiomyopathy site residue for protein folding. Mutation F110I markedly stabilized the troponin tail but weakened binding of holo-troponin to actin-tropomyosin 8-fold, suggesting that loss of flexibility impairs troponin tail function. The effect of the F110I mutation on troponin-tropomyosin binding to actin was much less, indicating this flexibility is particularly important for the interactions of troponin with tropomyosin. We suggest that most cardiomyopathic mutations in the troponin tail alter muscle function indirectly, by perturbing interactions between troponin and tropomyosin requisite for the complex effects of these proteins on myosin.

Myofilament calcium sensitivity and cardiac disease: insights from troponin I isoforms and mutants

The heightened Ca2+ sensitivity of force found with hypertrophic cardiomyopathy (HCM)-associated mutant cardiac troponin I (cTnIR145G; R146G in rodents) has been postulated to be an underlying cause of hypertrophic growth and premature sudden death in humans and in animal models of the disease. Expression of slow skeletal TnI (ssTnI), a TnI isoform naturally expressed in developing heart, also increases myofilament Ca2+ sensitivity, yet its expression in transgenic mouse hearts is not associated with overt cardiac disease. Gene transfer of TnI isoforms or mutants into adult cardiac myocytes is used here to ascertain if expression levels or functional differences between HCM TnI and ssTnI could help explain these divergent organ-level effects. Results showed significantly reduced myofilament incorporation of cTnIR146G compared with ssTnI or wild-type cTnI. Despite differences in myofilament incorporation, ssTnI and cTnIR146G expression each resulted in enhanced myofilament tension in response to submaximal Ca2+ under physiological ionic conditions. Myofilament expression of an analogous HCM mutation in ssTnI (ssTnIR115G) did not further increase myofilament Ca2+ sensitivity of tension compared with ssTnI. In contrast, there was a divergent response under acidic pH conditions, a condition associated with the myocardial ischemia that often accompanies hypertrophic cardiomyopathy. The acidic pH-induced decrease in myofilament Ca2+ sensitivity was significantly greater in myocytes expressing cTnIR146G and ssTnIR115G compared with ssTnI. These results suggest that differences in pH sensitivities between wild-type ssTnI and mutant TnI proteins may be one factor in helping explain the divergent organ and organismal outcomes in TnI HCM- and ssTnI-expressing mice.

Alterations in thin filament regulation induced by a human cardiac troponin T mutant that causes dilated cardiomyopathy are distinct from those induced by troponin T mutants that cause hypertrophic cardiomyopathy

We have compared the in vitro regulatory properties of recombinant human cardiac troponin reconstituted using wild type troponin T with troponin containing the DeltaLys-210 troponin T mutant that causes dilated cardiomyopathy (DCM) and the R92Q troponin T known to cause hypertrophic cardiomyopathy (HCM). Troponin containing DeltaLys-210 troponin T inhibited actin-tropomyosin-activated myosin subfragment-1 ATPase activity to the same extent as wild type at pCa8.5 (>80%) but produced substantially less enhancement of ATPase at pCa4.5. The Ca(2+) sensitivity of ATPase activation was increased (DeltapCa(50) = +0.2 pCa units) and cooperativity of Ca(2+) activation was virtually abolished. Equimolar mixtures of wild type and DeltaLys-210 troponin T gave a lower Ca(2+) sensitivity than with wild type, while maintaining the diminished ATPase activation at pCa4.5 observed with 100% mutant. In contrast, R92Q troponin gave reduced inhibition at pCa8.5 but greater activation than wild type at pCa4.5; Ca(2+) sensitivity was increased but there was no change in cooperativity. In vitro motility assay of reconstituted thin filaments confirmed the ATPase results and moreover indicated that the predominant effect of the DeltaLys-210 mutation was a reduced sliding speed. The functional consequences of this DCM mutation are qualitatively different from the R92Q or any other studied HCM troponin T mutation, suggesting that DCM and HCM may be triggered by distinct primary stimuli.

Identification of novel interactions between domains of Myosin binding protein-C that are modulated by hypertrophic cardiomyopathy missense mutations

Cardiac myosin binding protein-C (cMyBPC) is a modular protein consisting of 11 domains whose precise function and sarcomeric arrangement are incompletely understood. Identification of hypertrophic cardiomyopathy (HCM)--causing missense mutations in cMyBPC has highlighted the significance of certain domains. Of particular interest is domain C5, an immunoglobulin-like domain with a cardiac-specific insert, which is of unknown function yet is the site of two HCM-causing missense mutations. To identify interactors with this region, a human cardiac cDNA library was screened in a yeast two-hybrid (Y2H) assay using the C5 sequence as bait. Screening >7x10(6) clones surprisingly revealed that domain C5 preferentially bound to clones encoding C-terminal fragments of cMyBPC; the interacting region was narrowed to domain C8 by deletion mapping. A surface plasmon resonance assay using purified recombinant cMyBPC domains was used to measure the affinity of C5 and C8 in vitro (K(a)=1x10(5) mol/L(-1)). This affinity was decreased about 2-fold by the HCM mutation R654H, and by at least 10-fold by the mutation N755K. Further Y2H assays also demonstrated specific binding between domains C7 and C10 of cMyBPC. Based on these novel interactions, and previous biochemical and structural data, we propose that cMyBPC molecules trimerize into a collar around the thick filament, with overlaps of domains C5-C7 of one cMyBPC with C8-C10 of another. We speculate that this interaction may be dynamically formed and released, thereby restricting or favoring cross-bridge formation, respectively. We suggest that the HCM mutations act by altering the cMyBPC collar, indicating its importance in thick filament structure and regulation.

Effects of T142 phosphorylation and mutation R145G on the interaction of the inhibitory region of human cardiac troponin I with the C-domain of human cardiac troponin C

Cardiac troponin I (cTnI) is the inhibitory component of the troponin complex, and its interaction with cardiac troponin C (cTnC) plays a critical role in transmitting the Ca(2+) signal to the other myofilament proteins in heart muscle contraction. The switch between contraction and relaxation involves a movement of the inhibitory region of cTnI (cIp) from cTnC to actin-tropomyosin. This region of cTnI is prone to missense mutations in heart disease, and a specific mutation, R145G, has been associated with familial hypertrophic cardiomyopathy. It also contains the unique cardiac PKC phosphorylation site at residue T142. To determine the structural consequences of the mutation R145G and the T142 phosphorylation on the interaction of cIp with cTnC, we have utilized 2D [(1)H, (15)N]-HSQC NMR spectroscopy to monitor the binding of native cIp, cIp-R (R145G), and cIp-P (phosphorylated T142), respectively, to the Ca(2+)-saturated C-domain of cTnC (cCTnC.2Ca(2+)). We also report a strategy for cloning, expression, and purification of cTnI peptide, and both synthetic and recombinant peptides are used in this study. NMR chemical shift mapping indicates that the binding epitope of cIp on cCTnC.2Ca(2+) is not greatly affected, but the affinity is reduced by approximately 14-fold by the T142 phosphorylation and approximately 4-fold by the mutation R145G, respectively. This suggests that these modifications of cIp have an adverse effect on the binding of cIp to cCTnC.2Ca(2+). These perturbations may correlate with the impairment or loss of cTnI function in heart muscle contraction.

Global gene expression profiling of end-stage dilated cardiomyopathy using a human cardiovascular-based cDNA microarray

To obtain a genomic portrait of heart failure derived from end-stage dilated cardiomyopathy (DCM), we explored expression analysis using the CardioChip, a nonredundant 10,848-element human cardiovascular-based expressed sequence tag glass slide cDNA microarray constructed in-house. RNA was extracted from the left ventricular free wall of seven patients undergoing transplantation, and five nonfailing heart samples. Cy3- and Cy5-labeled (and reverse dye-labeled) cDNA probes were synthesized from individual diseased or nonfailing adult heart RNA, and hybridized to the array. More than 100 transcripts were consistently differentially expressed in DCM >1.5-fold (versus pooled nonfailing heart, P < 0.05). Atrial natriuretic peptide was found to be up-regulated in DCM (19-fold compared to nonfailing, P < 0.05), as well as numerous sarcomeric and cytoskeletal proteins (eg, cardiac troponin, tropomyosin), stress response proteins (eg, HSP 40, HSP 70), and transcription/translation regulators (eg, CCAAT box binding factor, eIF-1AY). Down-regulation was most prominently observed with cell-signaling channels and mediators, particularly those involved in Ca(2+) pathways (Ca(2+)/calmodulin-dependent kinase, inositol 1,4,5-trisphosphate receptor, SERCA). Most intriguing was the co-expression of several novel, cardiac-enriched expressed sequence tags. Quantitative real-time reverse transcriptase-polymerase chain reaction of a selection of these clones verified expression. Our study provides a preliminary molecular profile of DCM using the largest human heart-specific cDNA microarray to date.

Functional analysis of a troponin I (R145G) mutation associated with familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy has been associated with several mutations in the gene encoding human cardiac troponin I (HCTnI). A missense mutation in the inhibitory region of TnI replaces an arginine residue at position 145 with a glycine and cosegregates with the disease. Results from several assays indicate that the inhibitory function of HCTnI(R145G) is significantly reduced. When HCTnI(R145G) was incorporated into whole troponin, Tn(R145G) (HCTnT small middle dotHCTnI(R145G) small middle dotHCTnC), only partial inhibition of the actin-tropomyosin-myosin ATPase activity was observed in the absence of Ca(2+) compared with wild type Tn (HCTnT small middle dotHCTnI small middle dotHCTnC). Maximal activation of actin-tropomyosin-myosin ATPase in the presence of Ca(2+) was also decreased in Tn(R145G) when compared with Tn. Using skinned cardiac muscle fibers, we determined that in comparison with the wild type complex 1) the complex containing HCTnI(R145G) only inhibited 84% of Ca(2+)-unregulated force, 2) the recovery of Ca(2+)-activated force was decreased, and 3) there was a significant increase in the Ca(2+) sensitivity of force development. Computer modeling of troponin C and I variables predicts that the primary defect in TnI caused by these mutations would lead to diastolic dysfunction. These results suggest that severe diastolic dysfunction and somewhat decreased contractility would be prominent clinical features and that hypertrophy could arise as a compensatory mechanism.

New molecular insights into heart failure and cardiomyopathy: potential strategies and therapies

BACKGROUND

In the most severely affected patients the mortality for congestive heart failure exceeds that of many cancers. While therapies are largely aimed at attenuating neurohumoral responses recent molecular insights reveal other potential targets for therapy.

AIMS

To summarise some of the recent developments in the management of heart failure and provide the clinician who treats heart failure with new insights into emerging approaches.

METHODS

A literature review was conducted of the recent literature together with personal research data.

RESULTS

Large randomised trials will provide a more comprehensive understanding of the interaction of beta-blockers and other heart failure therapies with gene polymorphisms. Cytokines are important in the progression of heart failure, yet therapy aimed at blocking cytokine effects has not been successful. More selective use of anti-cytokine therapy may have beneficial effects. Gene therapy to improve heart failure has not yet reached clinical trials. The molecular genetics of hypertrophic and dilated cardiomyopathy is rapidly improving our understanding so that genetic diagnostics and counselling may soon be performed for patients and families.

CONCLUSIONS

The emergence of a molecular based understanding of heart failure will hopefully improve therapy of this common condition.

Two mutations in troponin I that cause hypertrophic cardiomyopathy have contrasting effects on cardiac muscle contractility

We investigated the effects of two mutations in human cardiac troponin I, Arg(145)-->Gly and Gly(203)-->Ser, that are reported to cause familial hypertrophic cardiomyopathy. Mutant and wild-type troponin I, overexpressed in Escherichia coli, were used to reconstitute troponin complexes in vanadate-treated guinea pig cardiac trabeculae skinned fibres, and thin filaments were reconstituted with human cardiac troponin and tropomyosin along with rabbit skeletal muscle actin for in vitro motility and actomyosin ATPase assays. Troponin containing the Arg(145)-->Gly mutation inhibited force in skinned trabeculae less than did the wild-type, and had almost no inhibitory function in the in vitro motility assay. There was an enhanced inhibitory function with mixtures of 10-30% [Gly(145)]troponin I with the wild-type protein. Skinned trabeculae reconstituted with troponin I containing the Gly(203)-->Ser mutation and troponin C produced less Ca(2+)-activated force (64+/-8% of wild-type) and demonstrated lower Ca(2+) sensitivity [Delta(p)Ca(50) (log of the Ca(2+) concentration that gave 50% of maximal activation) 0.25 unit (P<0.05)] compared with wild-type troponin I, but thin filaments containing [Ser(203)]-troponin I were indistinguishable from those containing the wild-type protein in in vitro motility and ATPase assays. Thus these two mutations each result in hypertrophic cardiomyopathy, but have opposite effects on the overall contractility of the muscle in the systems we investigated, indicating either that we have not yet identified the relevant alteration in contractility for the Gly(203)->Ser mutation, or that the disease does not result directly from any particular alteration in contractility.

Ca(2+)-desensitizing effect of a deletion mutation Delta K210 in cardiac troponin T that causes familial dilated cardiomyopathy

A deletion mutation Delta K210 in cardiac troponin T (cTnT) was recently found to cause familial dilated cardiomyopathy (DCM). To explore the effect of this mutation on cardiac muscle contraction under physiological conditions, we determined the Ca(2+)-activated force generation in permeabilized rabbit cardiac muscle fibers into which the mutant and wild-type cTnTs were incorporated by using our TnT exchange technique. The free Ca(2+) concentrations required for the force generation were higher in the mutant cTnT-exchanged fibers than in the wild-type cTnT-exchanged ones, with no statistically significant differences in maximal force-generating capability and cooperativity. Exchanging the mutant cTnT into isolated cardiac myofibrils also increased the free Ca(2+) concentrations required for the activation of ATPase. In contrast, a deletion mutation Delta E160 in cTnT that causes familial hypertrophic cardiomyopathy (HCM) decreased the free Ca(2+) concentrations required for force generation, just as in the case of the other HCM-causing mutations in cTnT. The results indicate that cTnT mutations found in the two distinct forms of cardiomyopathy (i.e., HCM and DCM) change the Ca(2+) sensitivity of cardiac muscle contraction in opposite directions. The present study strongly suggests that Ca(2+) desensitization of force generation in sarcomere is a primary mechanism for the pathogenesis of DCM associated with the deletion mutation Delta K210 in cTnT.

Molecular Mechanisms of Inherited Cardiomyopathies

Cardiomyopathies are diseases of heart muscle that may result from a diverse array of conditions that damage the heart and other organs and impair myocardial function, including infection, ischemia, and toxins. However, they may also occur as primary diseases restricted to striated muscle. Over the past decade, the importance of inherited gene defects in the pathogenesis of primary cardiomyopathies has been recognized, with mutations in some 18 genes having been identified as causing hypertrophic cardiomyopathy (HCM) and/or dilated cardiomyopathy (DCM). Defining the role of these genes in cardiac function and the mechanisms by which mutations in these genes lead to hypertrophy, dilation, and contractile failure are major goals of ongoing research. Pathophysiological mechanisms that have been implicated in HCM and DCM include the following: defective force generation, due to mutations in sarcomeric protein genes; defective force transmission, due to mutations in cytoskeletal protein genes; myocardial energy deficits, due to mutations in ATP regulatory protein genes; and abnormal Ca2+ homeostasis, due to altered availability of Ca2+ and altered myofibrillar Ca2+ sensitivity. Improved understanding that will result from these studies should ultimately lead to new approaches for the diagnosis, prognostic stratification, and treatment of patients with heart failure.

Effects of phosphorylation and mutation R145G on human cardiac troponin I function

We have studied functional consequences of the mutations R145G, S22A, and S23A of human cardiac troponin I (cTnI) and of phosphorylation of two adjacent N-terminal serine residues in the wild-type cTnI and the mutated proteins. The mutation R145G has been linked to the development of familial hypertrophic cardiomyopathy. Cardiac troponin was reconstituted from recombinant human subunits including either wild-type or mutant cTnI and was used for reconstitution of thin filaments with skeletal muscle actin and tropomyosin. The Ca(2+)-dependent thin filament-activated myosin subfragment 1 ATPase (actoS1-ATPase) activity and the in vitro motility of these filaments driven by myosin were measured as a function of the cTnI phosphorylation state. Bisphosphorylation of wild-type cTnI decreases the Ca(2+) sensitivity of the actoS1-ATPase activity and the in vitro thin filament motility by about 0.15-0.21 pCa unit. The nonconservative replacement R145G in cTnI enhances the Ca(2+) sensitivity of the actoS1-ATPase activity by about 0.6 pCa unit independent of the phosphorylation state of cTnI. Furthermore, it mimics a strong suppressing effect on both the maximum actoS1-ATPase activity and the maximum in vitro filament sliding velocity which has been observed upon bisphosphorylation of wild-type cTnI. Bisphosphorylation of the mutant cTnI-R145G itself had no such suppressing effects anymore. Differential analysis of the effect of phosphorylation of each of the two serines, Ser23 in cTnI-S22A and Ser22 in cTnI-S23A, indicates that phosphorylation of Ser23 may already be sufficient for causing the reduction of maximum actoS1-ATPase activity and thin filament sliding velocity seen upon phosphorylation of both of these serines.