Interaction of rabbit skeletal muscle troponin T and F-actin at physiological ionic strength

Structures of the troponin core domain containing the cardiomyopathy-causing mutants studied by small-angle X-ray scattering

Troponin (Tn), consisting of three subunits, TnC, TnI, and TnT, is a protein in the thin filaments in muscle, and, together with another thin-filament protein tropomyosin (Tm), plays a major role in regulation of muscle contraction. Various mutations of Tn cause familial hypertrophic cardiomyopathy. These mutations are directly related to aberrations in this regulatory mechanism. Here we focus on the mutations E244D and K247R of TnT, which reside in the middle of the pathway of the Ca(2+)-binding signal from TnC to Tm. These mutations induce an increase in the maximum tension of cardiac muscle without changes in Ca(2+)-sensitivity. As a first step toward elucidating the molecular mechanism underlying this functional aberration, we carried out small-angle X-ray scattering experiments on the Tn core domain containing the wild type subunits and those containing the mutant TnT in the absence and presence of Ca(2+). Changes in the overall shape induced by the mutations were detected for the first time by the changes in the radius of gyration and the maximum dimension between the wild type and the mutants. Analysis of the scattering curves by model calculations shows that TnC adopts a dumbbell structure regardless of the mutations, and that the mutations change the distributions of the conformational ensembles so that the flexible N- and C-terminal regions of TnT become close to the center of the whole moelcule. This suggests, since these regions are related to the Tn-Tm interactions, that alteration of the Tn-Tm interactions induced by the mutations causes the functional aberration.

Adaptation by alternative RNA splicing of slow troponin T isoforms in type 1 but not type 2 Charcot-Marie-Tooth disease

Slow troponin T (TnT) plays an indispensable role in skeletal muscle function. Alternative RNA splicing in the NH(2)-terminal region produces high-molecular-weight (HMW) and low-molecular-weight (LMW) isoforms of slow TnT. Normal adult slow muscle fibers express mainly HMW slow TnT. Charcot-Marie-Tooth disease (CMT) is a group of inherited peripheral polyneuropathies caused by various neuronal defects. We found in the present study that LMW slow TnT was significantly upregulated in demyelination form type 1 CMT (CMT1) but not axonal form type 2 CMT (CMT2) muscles. Contractility analysis showed an increased specific force in single fibers isolated from CMT1 but not CMT2 muscles compared with control muscles. However, an in vitro motility assay showed normal velocity of the myosin motor isolated from CMT1 and CMT2 muscle biopsies, consistent with their unchanged myosin isoform contents. Supporting a role of slow TnT isoform regulation in contractility change, LMW and HMW slow TnT isoforms showed differences in the molecular conformation in conserved central and COOH-terminal regions with changed binding affinity for troponin I and tropomyosin. In addition to providing a biochemical marker for the differential diagnosis of CMT, the upregulation of LMW slow TnT isoforms under the distinct pathophysiology of CMT1 demonstrates an adaptation of muscle function to neurological disorders by alternative splicing modification of myofilament proteins.

Troponin T core structure and the regulatory NH2-terminal variable region

The conserved central and COOH-terminal regions of troponin T (TnT) interact with troponin C, troponin I, and tropomyosin to regulate striated muscle contraction. Phylogenic data show that the NH2-terminal region has evolved as an addition to the conserved core structure of TnT. This NH2-terminal region does not bind other thin filament proteins, and its sequence is hypervariable between fiber type and developmental isoforms. Previous studies have demonstrated that NH2-terminal modifications alter the COOH-terminal conformation of TnT and thin filament Ca2+-activation, yet the functional core structure of TnT and the mechanism of NH2-terminal modulation are not well understood. To define the TnT core structure and investigate the regulatory role of the NH2-terminal variable region, we investigated two classes of model TnT molecules: (1) NH2-terminal truncated cardiac TnT and (2) chimera proteins consisting of an acidic or basic skeletal muscle TnT NH2-terminus spliced to the cardiac TnT core. Deletion of the TnT hypervariable NH2-terminus preserved binding to troponin I and tropomyosin and sustained cardiac muscle contraction in the heart of transgenic mice. Further deletion of the conserved central region diminished binding to tropomyosin. The reintroduction of differently charged NH2-terminal domains in the chimeric molecules produced long-range conformational changes in the central and COOH-terminal regions to alter troponin I and tropomyosin binding. Similar NH2-terminal charge effects are demonstrated in naturally occurring cardiac TnT isoforms, indicating a physiological significance. These results suggest that the hypervariable NH2-terminal region modulates the conformation and function of the TnT core structure to fine-tune muscle contractility.

p38-MAPK induced dephosphorylation of alpha-tropomyosin is associated with depression of myocardial sarcomeric tension and ATPase activity

Our objective in work presented here was to understand the mechanisms by which activated p38alpha MAPK depresses myocardial contractility. To test the hypothesis that activation of p38 MAPK directly influences sarcomeric function, we used transgenic mouse models with hearts in which p38 MAPK was constitutively turned on by an upstream activator (MKK6bE). These hearts demonstrated a significant depression in ejection fraction after induction of the transgene. We also studied hearts of mice expressing a dominant negative p38alpha MAPK. Simultaneous determination of tension and ATPase activity of detergent-skinned fiber bundles from left ventricular papillary muscle demonstrated a significant inhibition of both maximum tension and ATPase activity in the transgenic-MKK6bE hearts. Fibers from hearts expressing dominant negative p38alpha MAPK demonstrated no significant change in tension or ATPase activity. There were no significant changes in phosphorylation level of troponin-T3 and troponin-T4, or myosin light chain 2. However, compared with controls, there was a significant depression in levels of phosphorylation of alpha-tropomyosin and troponin I in fiber bundles from transgenic-MKK6bE hearts, but not from dominant negative p38alpha MAPK hearts. Our experiments also showed that p38alpha MAPK colocalizes with alpha-actinin at the Z-disc and complexes with protein phosphatases (PP2alpha, PP2beta). These data are the first to indicate that chronic activation of p38alpha MAPK directly depresses sarcomeric function in association with decreased phosphorylation of alpha-tropomyosin.

Differences in myofilament calcium sensitivity in rat psoas fibers reconstituted with troponin T isoforms containing the alpha- and beta-exons

The carboxy terminus of fast skeletal muscle troponin T (fsTnT) is highly conserved. However, mutually exclusive splicing of exons 16 and 17 in the fsTnT gene results in the expression of either the alpha- or beta-fsTnT isoform. The alpha-isoform is expressed only in adult fast skeletal muscle, whereas the beta-isoform is expressed in varying quantities throughout muscle development. Reconstitution of detergent-skinned adult rat psoas muscle fibers with rat fast skeletal troponin complexes containing either fsTnT isoform demonstrated that reconstitution with alpha-fsTnT resulted in greater myofilament Ca(2+) sensitivity than reconstitution with beta-fsTnT, without changes to Ca(2+)-activated maximal tension, ATPase activity or tension cost. The observed isoform-specific differences in myofilament Ca(2+) sensitivity may be due to changes in the transition of the thin-filament regulatory unit from the off to the on state, possibly due to altered interactions of the C-terminus of fsTnT with troponins I and/or C.

Functional consequence of mutation in rat cardiac troponin T is affected differently by myosin heavy chain isoforms

Cardiac troponin T (cTnT) is an essential component of the thin filament regulatory unit (RU) that regulates Ca2+ activation of tension in the heart muscle. Because there is coupling between the RU and myosin crossbridges, the functional outcome of cardiomyopathy-related mutations in cTnT may be modified by the type of myosin heavy chain (MHC) isoform. Ca2+ activation of tension and ATPase activity were measured in muscle fibres from normal rat hearts containing alpha-MHC isoform and propylthiouracil (PTU)-treated rat hearts containing beta-MHC isoform. Muscle fibres from normal and PTU-treated rat hearts were reconstituted with two different mutations in rat cTnT; the deletion of Glu162 (cTnT(E162DEL)) and the deletion of Lys211 (cTnT(K211DEL)). Alpha-MHC and beta-MHC isoforms had contrasting impact on tension-dependent ATP consumption (tension cost) in cTnT(E162DEL) and cTnT(K211DEL) reconstituted muscle fibres. Significant increases in tension cost in alpha-MHC-containing muscle fibres corresponded to 17% (P < 0.01) and 23% (P < 0.001) when reconstituted with cTnT(E162DEL) and cTnT(K211DEL), respectively. In contrast, tension cost decreased when these two cTnT mutants were reconstituted in muscle fibres containing beta-MHC; by approximately 24% (P < 0.05) when reconstituted with cTnT(E162DEL) and by approximately 17% (P = 0.09) when reconstituted with cTnT(K211DEL). Such differences in tension cost were substantiated by the mechano-dynamic analysis of cTnT mutant reconstituted muscle fibres from normal and PTU-treated rat hearts. Our observation demonstrates that qualitative changes in MHC isoform alters the nature of cardiac myofilament dysfunction induced by mutations in cTnT.

Troponin T modulates sarcomere length-dependent recruitment of cross-bridges in cardiac muscle

The heterogenic nature of troponin T (TnT) isoforms in fast skeletal and cardiac muscle suggests important functional differences. Dynamic features of rat cardiac TnT (cTnT) and rat fast skeletal TnT (fsTnT) reconstituted cardiac muscle preparations were captured by fitting the force response of small amplitude (0.5%) muscle length changes to the recruitment-distortion model. The recruitment of force-bearing cross-bridges (XBs) by increases in muscle length was favored by cTnT. The recruitment magnitude was approximately 1.5 times greater for cTnT- than for fsTnT-reconstituted muscle fibers. The speed of length-mediated XB recruitment (b) in cTnT-reconstituted muscle fiber was 0.50-0.57 times as fast as fsTnT-reconstituted muscle fibers (3.05 vs. 5.32 s(-1) at sarcomere length, SL, of 1.9 microm and 4.16 vs. 8.36 s(-1) at SL of 2.2 microm). Due to slowing of b in cTnT-reconstituted muscle fibers, the frequency of minimum stiffness (f(min)) was shifted to lower frequencies of muscle length changes (at SL of 1.9 microm, 0.64 Hz, and 1.16 Hz for cTnT- and fsTnT-reconstituted muscle fibers, respectively; at SL of 2.2 microm, 0.79 Hz, and 1.11 Hz for cTnT- and fsTnT-reconstituted muscle fibers, respectively). Our model simulation of the data implicates TnT as a participant in the process by which SL- and XB-regulatory unit cooperative interactions activate thin filaments. Our data suggest that the amino-acid sequence differences in cTnT may confer a heart-specific regulatory role. cTnT may participate in tuning the heart muscle by decreasing the speed of XB recruitment so that the heart beats at a rate commensurate with f(min).

Increase in tension-dependent ATP consumption induced by cardiac troponin T mutation

How different mutations in cardiac troponin T (cTnT) lead to distinct secondary downstream cellular remodeling in familial hypertrophic cardiomyopathy (FHC) remains elusive. To explore the molecular basis for the distinct impact of different mutations in cTnT on cardiac myocytes, we studied mechanical activity of detergent-skinned muscle fiber bundles from different lines of transgenic (TG) mouse hearts that express wild-type cTnT (WTTG), R92W cTnT, R92L cTnT, and Delta-160 cTnT (deletion of amino acid 160). The amount of mutant cTnT is approximately 50% of the total myocellular cTnT in both R92W and R92L TG mouse hearts and approximately 35% in Delta-160 TG mouse hearts. Myofilament Ca2+ sensitivity was enhanced in all mutant cTnT TG cardiac muscle fibers. Compared with the WTTG fibers, Ca2+ sensitivity increased significantly at short sarcomere length (SL) of 1.9 microm (P < 0.001) in R92W TG fibers by 2.2-fold, in R92L by 2.0-fold, and in Delta-160 by 1.3-fold. At long SL of 2.3 microm, Ca2+ sensitivity increased significantly (P < 0.01) in a similar manner (R92W, 2.5-fold; R92L, 1.9-fold; Delta-160, 1.3-fold). Ca2+-activated maximal tension remained unaltered in all TG muscle fibers. However, tension-dependent ATP consumption increased significantly in Delta-160 TG muscle fibers at both short SL (23%, P < 0.005) and long SL (37%, P < 0.0001), suggesting a mutation-induced change in cross-bridge detachment rate constant. Chronic stresses on relative cellular ATP level in cardiac myocytes may cause a strain on energy-dependent Ca2+ homeostatic mechanisms. This may result in pathological remodeling that we observed in Delta-160 TG cardiac myocytes where the ratio of sarco(endo)plasmic reticulum Ca2+-ATPase 2/phospholamban decreased significantly. Our results suggest that different types of stresses imposed on cardiac myocytes would trigger distinct cellular signaling, which leads to remodeling that may be unique to some mutants.

Cellular Fate of Truncated Slow Skeletal Muscle Troponin T Produced by Glu180 Nonsense Mutation in Amish Nemaline Myopathy

A nonsense mutation at codon Glu180 in exon 11 of slow skeletal muscle troponin T (TnT) gene (TNNT1) causes an autosomal-recessive inherited nemaline myopathy. We previously reported the absence of intact or prematurely terminated slow TnT polypeptide in Amish nemaline myopathy (ANM) patient muscle. The present study further investigates the expression and fate of mutant slow TnT in muscle cells. Intact slow TnT mRNA was readily detected in patient muscle, indicating unaffected transcription and RNA splicing. Sequence of the cloned cDNAs revealed the single nucleotide mutation in two alternatively spliced isoforms of slow TnT mRNA. Mutant TNNT1 cDNA is translationally active in Escherichia coli and non-muscle eukaryotic cells, producing the expected truncated slow TnT protein. The mutant mRNA was expressed at significant levels in differentiated C2C12 myotubes, but unlike intact exogenous TnT, truncated slow TnT protein was not detected. Transfective expression in undifferentiated myoblasts produced slow TnT mRNA but not a detectable amount of truncated or intact slow TnT proteins, indicating a muscle cell-specific proteolysis of TnT when it is not integrated into myofilaments. The slow TnT-(1-179) fragment has substantially lower affinity for binding to tropomyosin, in keeping with the loss of one of two tropomyosin-binding sites. Our findings suggest that inefficient incorporation into myofilament is responsible for the instability of mutant slow TnT in ANM muscle. Rapid degradation of the truncated slow TnT protein, rather than instability of the nonsense mRNA, provides the protective mechanism against the potential dominant negative effect of the mutant TnT fragment.

Cooperative interaction between developmentally regulated troponin T and tropomyosin isoforms in the absence of F-actin

Troponin T (TnT) is the tropomyosin (Tm) binding subunit of the troponin complex that mediates the Ca(2+) regulation of actomyosin interaction in striated muscles. Troponin T isoform diversity is marked by a developmentally regulated acidic to basic switch that may modulate muscle contractility. We previously reported that transgenic expression of fast skeletal muscle TnT altered the cooperativity of cardiac muscle. In the present study, we have demonstrated that the binding of acidic TnT to troponin I is weaker than that of basic TnT. However, affinity chromatography experiments showed that Tm bound to acidic TnT with a greater affinity than to basic TnT, consistent with the significantly higher maximal binding of acidic TnT to Tm in solid phase binding assays. Competition and co-immunoprecipitation experiments demonstrated that the binding of TnT to Tm was cooperative in the absence of F-actin. The cooperativity between TnT molecules for Tm binding can be initiated by the conserved COOH-terminal T2 fragment of TnT. This indicates that the interaction of TnT with Tm induces a conformational change in Tm, promoting interaction of TnT with adjacent Tm dimers. This finding suggests a role for TnT and its acidic and basic isoforms in the cooperative release of the inhibition of striated muscle actomyosin interaction.

Troponin T: genetics, properties and function

Troponin T (TnT) is present in striated muscle of vertebrates and invertebrates as a group of homologous proteins with molecular weights usually in the 31-36 kDa range. It occupies a unique role in the regulatory protein system in that it interacts with TnC and TnI of the troponin complex and the proteins of the myofibrillar thin filament, tropomyosin and actin. In the myofibril the molecule is about 18 nm long and for much its length interacts with tropomyosin. The ability of TnT to form a complex with tropomyosin is responsible for locating the troponin complex with a periodicity of 38.5 nm along the thin filament of the myofibril. In addition to it structural role, TnT has the important function of transforming the TnI-TnC complex into a system, the inhibitory activity of which, on the tropomyosin-actomyosin MgATPase of the myofibril, becomes sensitive to calcium ions. Different genes control the expression of TnT in fast skeletal, slow skeletal and cardiac muscles. In all muscles, and particularly in fast skeletal, alternative splicing of mRNA produces a series of isoforms in a developmentally regulated manner. In consequence TnT exists in many more isoforms than any of the other thin filament proteins, the TnT superfamily. Despite the general homology of TnT isoforms, this alternative splicing leads to variable regions close to the N- and C-termini. As the isoforms have slightly different effects on the calcium sensitivity of the actomyosin MgATPase, modulation of the contractile response to calcium can occur during development and in different muscle types. TnT has recently aroused clinical interest in its potential for detecting myocardial damage and the association of mutations in the cardiac isoform with hypertrophic cardiomyopathy.

Distinct regions of troponin I regulate Ca2+-dependent activation and Ca2+ sensitivity of the acto-S1-TM ATPase activity of the thin filament

The regions of troponin I (TnI) responsible for Ca2+-dependent activation and Ca2+ sensitivity of the actin-myosin subfragment 1-tropomyosin ATPase (acto-S1-TM) activity have been determined. A colorimetric ATPase assay at pH 7.8 has been applied to reconstituted skeletal muscle thin filaments at actin:S1:TM ratios of 6:1:2. Several TnI fragments (TnI-(104-115), TnI-(1-116), and TnI-(96-148)) and TnI mutants with single amino acid substitutions within the inhibitory region (residues 104-115) were assayed to determine their roles on the regulatory function of TnI. TnI-(104-115) is sufficient for achieving maximum inhibition of the acto-S1-TM ATPase activity and its importance was clearly shown by the reduced potency of TnI mutants with single amino acid substitutions within this region. However, the function of the inhibitory region is modulated by other regions of TnI as observed by the poor inhibitory activity of TnI-(1-116) and the increased potency of the inhibitory region by TnI-(96-148). The regulatory complex composed of TnI-(96-148) plus troponin T-troponin C complex (TnT.C) displays the same Ca2+ sensitivity (pCa50) as intact troponin (Tn) or TnI plus TnT.C while those regulatory complexes composed of TnT.C plus either TnI-(104-115) or TnI-(1-116) had an increase in their pCa50 values. This indicates that the Ca2+ sensitivity or responsiveness of the thin filament is controlled by TnI residues 96-148. The ability of Tn to activate the acto-S1-TM ATPase activity in the presence of calcium to the level of the acto-S1 rate was mimicked by the regulatory complex composed of TnI-(1-116) plus TnT.C and was not seen with complexes composed with either TnI-(104-115) or TnI-(96-148). This indicates that the N terminus of TnI in conjunction with TnT controls the degree of activation of the ATPase activity. Although the TnI inhibitory region (104-115) is the Ca2+-sensitive switch which changes binding sites from actin-TM to TnC in the presence of calcium, its function is modulated by both the C-terminal and N-terminal regions of TnI. Thus, distinct regions of TnI control different aspects of Tn's biological function.

Molecular mechanisms regulating the myofilament response to Ca2+: implications of mutations causal for familial hypertrophic cardiomyopathy

In this chapter we consider a current perception of the molecular mechanisms controlling myofilament activation with emphasis on alterations that may occur in familial hypertrophic cardiomyopathy (FHC). FHC is a sarcomeric disease (100) with an autosomal dominant pattern of heritability (27, 51). There is a substantial body of evidence implicating missense mutations in the beta-MHC gene as causal for the development of this disease. Recently, mutations in genes of two thin filament regulatory proteins, cardiac troponin T(cTnT) and alpha-tropomyosin (alpha-Tm), have also been linked to FHC. The commonality among the functional consequences of these mutations remains an important question. This review discusses how these pathological mutations may impact the activation process by disrupting critical structure function relations in both the thick and thin filaments.

NH2-terminal truncation of skeletal muscle troponin T does not alter the Ca2+ sensitivity of thin filament assembly

To investigate how Ca2+ binding to troponin C regulates muscle contraction, the Ca(2+)-sensitive properties of thin filament assembly were studied as the tropomyosin binding, NH2-terminal region of troponin T was progressively shortened. Troponin complexes were prepared that contained skeletal muscle troponin C, troponin I, and either intact troponin T (TnT) (residues 1-259) or fragment TnT-(70-259), TnT-(151-259), or TnT-(159-259). In the absence of Ca2+ their respective affinities for pyrene-labeled tropomyosin were 2.3 x 10(7) M-1, 1.2 x 10(7) M-1, 1.9 x 10(5) M-1, and 1.9 x 10(5) M-1. Ca2+ had only a small effect on these affinities: 1.1 x 10(7) M-1 for whole troponin, 2 x 10(5) M-1 for troponin-(151-259), and 2.8 x 10(5) M-1 for troponin-(159-259). Forms of troponin that bound weakly to tropomyosin in the absence of actin increased the actin affinity of tropomyosin only 2-3-fold, even in the absence of Ca2+; weak binding of troponin to tropomyosin correlated with weak effects on tropomyosin-actin binding. In contrast, whole troponin had an approximately 500-fold effect on tropomyosin binding to actin, regardless of whether Ca2+ was present. The small effect of Ca2+ on the energetics of thin filament assembly is not attributable to the amino-terminal region of troponin T. The results suggest that Ca2+ causes the interaction between actin and the globular region of troponin to switch between two energetically similar states.

Troponin C - troponin I interactions and molecular signalling in cardiac myofilaments

This chapter describes a current perception of the molecular interactions regulating myofilament activity in heart cells. The focus is on the interaction between troponin-C (TnC), the Ca(2+)-receptor and troponin I (TnI), an inhibitory protein. It is this interaction that appears to form a molecular switch that turns on the thin filament. It will be seen that control of the actin-myosin reaction is not only through Ca(2+)-binding to TnC, but also through steric, cooperative and allosteric processes involving all of the main myofilament proteins-actin, myosin, tropomyosin (Tm), troponin T (TnT), TnC, and TnI. The process is modulated by covalent and non-covalent mechanisms. The process is altered in diverse myopathies and pathologies of the heart and is a target for pharmacological manipulation by a new class of inotropic agents, the "Ca(2+)-sensitizers".

Caldesmon150, caldesmon77 and skeletal muscle troponin T share a common antigenic determinant

Caldesmon and troponin are known to control actin-myosin interaction in smooth and striated muscle. We showed that polyclonal antibodies to smooth muscle caldesmon150 cross-react with non-muscle caldesmon77 and with skeletal muscle troponin T. The epitope recognized by caldesmon antibodies is located in the CB 2 peptide [residues 71-151] of troponin T. It is supposed that the observed cross-reactivity is due to the common structure of the tropomyosin-binding site of caldesmon and troponin T.