Mutations in the N- and D-helices of the N-domain of troponin C affect the C-domain and regulatory function

Anomalous structural dynamics of minimally frustrated residues in cardiac troponin C triggers hypertrophic cardiomyopathy

Cardiac TnC (cTnC) is highly conserved among mammals, and genetic variants can result in disease by perturbing Ca2+-regulation of myocardial contraction. Here, we report the molecular basis of a human mutation in cTnC's αD-helix (TNNC1-p.C84Y) that impacts conformational dynamics of the D/E central-linker and sampling of discrete states in the N-domain, favoring the "primed" state associated with Ca2+ binding. We demonstrate cTnC's αD-helix normally functions as a central hub that controls minimally frustrated interactions, maintaining evolutionarily conserved rigidity of the N-domain. αD-helix perturbation remotely alters conformational dynamics of the N-domain, compromising its structural rigidity. Transgenic mice carrying this cTnC mutation exhibit altered dynamics of sarcomere function and hypertrophic cardiomyopathy. Together, our data suggest that disruption of evolutionary conserved molecular frustration networks by a myofilament protein mutation may ultimately compromise contractile performance and trigger hypertrophic cardiomyopathy.

Meta-analysis of cardiomyopathy-associated variants in troponin genes identifies loci and intragenic hot spots that are associated with worse clinical outcomes

INTRODUCTION

Troponin (TNN)-encoded cardiac troponins (Tn) are critical for sensing calcium and triggering myofilament contraction. TNN variants are associated with development of cardiomyopathy; however, recent advances in genetic analysis have identified rare population variants. It is unclear how certain variants are associated with disease while others are tolerated.

OBJECTIVE

To compare probands with TNNT2, TNNI3, and TNNC1 variants and utilize high-resolution variant comparison mapping of pathologic and rare population variants to identify loci associated with disease pathogenesis.

METHODS

Cardiomyopathy-associated TNN variants were identified in the literature and topology mapping conducted. Clinical features were compiled and compared. Rare population variants were obtained from the gnomAD database. Signal-to-noise (S:N) normalized pathologic variant frequency against population variant frequency. Abstract review of clinical phenotypes was applied to "significant" hot spots.

RESULTS

Probands were compiled (N = 70 studies, 224 probands) as were rare variants (N = 125,748 exomes; 15,708 genomes, MAF <0.001). TNNC1-positive probands demonstrated the youngest age of presentation (20.0 years; P = .016 vs TNNT2; P = .004 vs TNNI3) and the highest death, transplant, or ventricular fibrillation events (P = .093 vs TNNT2; P = .024 vs TNNI3; Kaplan Meir: P = .025). S:N analysis yielded hot spots of diagnostic significance within the tropomyosin-binding domains, α-helix 1, and the N-Terminus in TNNT2 with increased sudden cardiac death and ventricular fibrillation (P = .004). The inhibitory region and C-terminal region in TNNI3 exhibited increased restrictive cardiomyopathy (P =.008). HCM and RCM models tended to have increased calcium sensitivity and DCM decreased sensitivity (P < .001). DCM and HCM studies typically showed no differences in Hill coefficient which was decreased in RCM models (P < .001). CM models typically demonstrated no changes to Fmax (P = .239).

CONCLUSION

TNNC1-positive probands had younger ages of diagnosis and poorer clinical outcomes. Mapping of TNN variants identified locations in TNNT2 and TNNI3 associated with heightened pathogenicity, RCM diagnosis, and increased risk of sudden death.

Myosin Rod Hypophosphorylation and CB Kinetics in Papillary Muscles from a TnC-A8V KI Mouse Model

The cardiac troponin C (TnC)-A8V mutation is associated with hypertrophic and restrictive cardiomyopathy (HCM and RCM) in human and mice. The residue affected lies in the N-helix, a region known to affect Ca²⁺-binding affinity to the N-terminal domain. Here we report on the functional effects of this mutation in skinned papillary muscle fibers from homozygous knock-in TnC-A8V mice. Muscle fibers from left ventricle were activated at 25°C under the ionic conditions of working cardiomyocytes. The pCa-tension relationship showed a 3× increase in Ca²⁺-sensitivity and a decrease (0.8×) in cooperativity (n_H) in mutant fibers. The elementary steps of the cross-bridge (CB) cycle were investigated by sinusoidal analysis. The ATP study revealed that there is no significant change in the affinity of ATP (K₁) for the myosin head. In TnC-A8V mutant fibers, the CB detachment rate (k₂) and its equilibrium constant (K₂) increased (1.5×). The phosphate study revealed that rate constant of the force-generation step (k₄) decreased (0.5×), reversal step (k_-4) increased (2×), and the phosphate-release step (1/K₅) increased (2×). Pro-Q Diamond staining of the skinned fibers samples revealed no significant changes in total phosphorylation of multiple sarcomeric proteins. Further investigation using liquid chromatography-tandem mass spectrometry revealed hypophosphorylation of the rod domain of myosin heavy chain in TnC-A8V mutant fibers compared to wild-type. Immunoblotting confirmed the results observed in the mass spectrometry analysis. The results suggest perturbed CB kinetics-possibly caused by changes in the α-myosin heavy chain phosphorylation profile-as a novel mechanism, to our knowledge, by which a mutation in TnC can have rippling effects in the myofilament and contribute to the pathogenesis of HCM/RCM.

Amide hydrogens reveal a temperature-dependent structural transition that enhances site-II Ca2+-binding affinity in a C-domain mutant of cardiac troponin C

The hypertrophic cardiomyopathy-associated mutant D145E, in cardiac troponin C (cTnC) C-domain, causes generalised instability at multiple sites in the isolated protein. As a result, structure and function of the mutant are more susceptible to higher temperatures. Above 25 °C there are large, progressive increases in N-domain Ca²⁺-binding affinity for D145E but only small changes for the wild-type protein. NMR-derived backbone amide temperature coefficients for many residues show a sharp transition above 30–40 °C, indicating a temperature-dependent conformational change that is most prominent around the mutated EF-hand IV, as well as throughout the C-domain. Smaller, isolated changes occur in the N-domain. Cardiac skinned fibres reconstituted with D145E are more sensitive to Ca²⁺ than fibres reconstituted with wild-type, and this defect is amplified near body-temperature. We speculate that the D145E mutation destabilises the native conformation of EF-hand IV, leading to a transient unfolding and dissociation of helix H that becomes more prominent at higher temperatures. This creates exposed hydrophobic surfaces that may be capable of binding unnaturally to a variety of targets, possibly including the N-domain of cTnC when it is in its open Ca²⁺-saturated state. This would constitute a potential route for propagating signals from one end of TnC to the other.

In Vivo Analysis of Troponin C Knock-In (A8V) Mice: Evidence that TNNC1 Is a Hypertrophic Cardiomyopathy Susceptibility Gene

BACKGROUND

Mutations in thin-filament proteins have been linked to hypertrophic cardiomyopathy, but it has never been demonstrated that variants identified in the TNNC1 (gene encoding troponin C) can evoke cardiac remodeling in vivo. The goal of this study was to determine whether TNNC1 can be categorized as an hypertrophic cardiomyopathy susceptibility gene, such that a mouse model can recapitulate the clinical presentation of the proband.

METHODS AND RESULTS

The TNNC1-A8V proband diagnosed with severe obstructive hypertrophic cardiomyopathy at 34 years of age exhibited mild-to-moderate thickening in left and right ventricular walls, decreased left ventricular dimensions, left atrial enlargement, and hyperdynamic left ventricular systolic function. Genetically engineered knock-in (KI) mice containing the A8V mutation (heterozygote=KI-TnC-A8V(+/-); homozygote=KI-TnC-A8V(+/+)) were characterized by echocardiography and pressure-volume studies. Three-month-old KI-TnC-A8V(+/+) mice displayed decreased ventricular dimensions, mild diastolic dysfunction, and enhanced systolic function, whereas KI-TnC-A8V(+/-) mice displayed cardiac restriction at 14 months of age. KI hearts exhibited atrial enlargement, papillary muscle hypertrophy, and fibrosis. Liquid chromatography-mass spectroscopy was used to determine incorporation of mutant cardiac troponin C (≈ 21%) into the KI-TnC-A8V(+/-) cardiac myofilament. Reduced diastolic sarcomeric length, increased shortening, and prolonged Ca(2+) and contractile transients were recorded in intact KI-TnC-A8V(+/-) and KI-TnC-A8V(+/+) cardiomyocytes. Ca(2+) sensitivity of contraction in skinned fibers increased with mutant gene dose: KI-TnC-A8V(+/+)>KI-TnC-A8V(+/-)>wild-type, whereas KI-TnC-A8V(+/+) relaxed more slowly on flash photolysis of diazo-2.

CONCLUSIONS

The TNNC1-A8V mutant increases the Ca(2+)-binding affinity of the thin filament and elicits changes in Ca(2+) homeostasis and cellular remodeling, which leads to diastolic dysfunction. These in vivo alterations further implicate the role of TNNC1 mutations in the development of cardiomyopathy.

Functional characterization of TNNC1 rare variants identified in dilated cardiomyopathy

TNNC1, which encodes cardiac troponin C (cTnC), remains elusive as a dilated cardiomyopathy (DCM) gene. Here, we report the clinical, genetic, and functional characterization of four TNNC1 rare variants (Y5H, M103I, D145E, and I148V), all previously reported by us in association with DCM (Hershberger, R. E., Norton, N., Morales, A., Li, D., Siegfried, J. D., and Gonzalez-Quintana, J. (2010) Circ. Cardiovasc. Genet. 3, 155-161); in the previous study, two variants (Y5H and D145E) were identified in subjects who also carried MYH7 and MYBPC3 rare variants, respectively. Functional studies using the recombinant human mutant cTnC proteins reconstituted into porcine papillary skinned fibers showed decreased Ca(2+) sensitivity of force development (Y5H and M103I). Furthermore, the cTnC mutants diminished (Y5H and I148V) or abolished (M103I) the effects of PKA phosphorylation on Ca(2+) sensitivity. Only M103I decreased the troponin activation properties of the actomyosin ATPase when Ca(2+) was present. CD spectroscopic studies of apo (absence of divalent cations)-, Mg(2+)-, and Ca(2+)/Mg(2+)-bound states indicated that all of the cTnC mutants (except I148V in the Ca(2+)/Mg(2+) condition) decreased the α-helical content. These results suggest that each mutation alters the function/ability of the myofilament to bind Ca(2+) as a result of modifications in cTnC structure. One variant (D145E) that was previously reported in association with hypertrophic cardiomyopathy and that produced results in vivo in this study consistent with prior hypertrophic cardiomyopathy functional studies was found associated with the MYBPC3 P910T rare variant, likely contributing to the observed DCM phenotype. We conclude that these rare variants alter the regulation of contraction in some way, and the combined clinical, molecular, genetic, and functional data reinforce the importance of TNNC1 rare variants in the pathogenesis of DCM.

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

A functional and structural study of troponin C mutations related to hypertrophic cardiomyopathy

Recently four new hypertrophic cardiomyopathy mutations in cardiac troponin C (cTnC) (A8V, C84Y, E134D, and D145E) were reported, and their effects on the Ca(2+) sensitivity of force development were evaluated (Landstrom, A. P., Parvatiyar, M. S., Pinto, J. R., Marquardt, M. L., Bos, J. M., Tester, D. J., Ommen, S. R., Potter, J. D., and Ackerman, M. J. (2008) J. Mol. Cell. Cardiol. 45, 281-288). We performed actomyosin ATPase and spectroscopic solution studies to investigate the molecular properties of these mutations. Actomyosin ATPase activity was measured as a function of [Ca(2+)] utilizing reconstituted thin filaments (TFs) with 50% mutant and 50% wild type (WT) and 100% mutant cardiac troponin (cTn) complexes: A8V, C84Y, and D145E increased the Ca(2+) sensitivity with only A8V demonstrating lowered Ca(2+) sensitization at the 50% ratio when compared with 100%; E134D was the same as WT at both ratios. Of these four mutants, only D145E showed increased ATPase activation in the presence of Ca(2+). None of the mutants affected ATPase inhibition or the binding of cTn to the TF measured by co-sedimentation. Only D145E increased the Ca(2+) affinity of site II measured by 2-(4'-(2''-iodoacetamido)phenyl)aminonaphthalene-6-sulfonic acid fluorescence in isolated cTnC or the cTn complex. In the presence of the TF, only A8V was further sensitized to Ca(2+). Circular dichroism measurements in different metal-bound states of the isolated cTnCs showed changes in the secondary structure of A8V, C84Y, and D145E, whereas E134D was the same as WT. PyMol modeling of each cTnC mutant within the cTn complex revealed potential for local changes in the tertiary structure of A8V, C84Y, and D145E. Our results indicate that 1) three of the hypertrophic cardiomyopathy cTnC mutants increased the Ca(2+) sensitivity of the myofilament; 2) the effects of the mutations on the Ca(2+) affinity of isolated cTnC, cTn, and TF are not sufficient to explain the large Ca(2+) sensitivity changes seen in reconstituted and fiber assays; and 3) changes in the secondary structure of the cTnC mutants may contribute to modified protein-protein interactions along the sarcomere lattice disrupting the coupling between the cross-bridge and Ca(2+) binding to cTnC.

Determination of the folding of proteins as a function of denaturants, osmolytes or ligands using circular dichroism

Circular dichroism (CD) is an excellent tool for examining the interactions and stability of proteins. This protocol covers methods to obtain and analyze circular dichroism spectra to measure changes in the folding of proteins as a function of denaturants, osmolytes or ligands. Applications include determination of the free energy of folding of a protein, the effects of mutations on protein stability and the estimation of binding constants for the interactions of proteins with other proteins, DNA or ligands, such as substrates or inhibitors. The experiments require 2-5 h.

Converting troponin C into calmodulin: effects of mutations in the central helix and of changes in temperature

Calmodulin (CaM) and troponin C (TnC) are the most similar members of EF-hand family and show few differences in the primary structure. Here, we use mutants of troponin that mimic calmodulin and changes in temperature to investigate the factors that determine their specificity as regulatory proteins. Using a double mutant of troponin that resembles calmodulin in lacking both the N-terminal helix and KGK(91-93) we observe a small difference from troponin in binding to the erythrocyte Ca(2+)-ATPase, and an improvement in enzyme activation. A triple mutant, where in addition, the residues 88-90 are replaced with the corresponding sequence from calmodulin is equivalent to calmodulin in maximal activation, and it restores protein ability to increase Ca(2+) affinity for the enzyme. However, this mutant also binds less tightly (1/100) than calmodulin. Remarkably, a decrease in temperature has a more marked effect in protein binding than either mutation, reducing the difference in affinities to 18-fold, but without any improvement in their ability to increase Ca(2+) affinity for the enzyme. Spectroscopic analysis of hydrophobic domain exposure in EF-hand proteins was carried out using 8-anilino-1-naphthalenesulfonic acid (ANS). The probe shows a much higher fluorescence when bound to the complex Ca(4)-calmodulin than to Ca(4)-troponin. Decreasing the temperature exposes additional hydrophobic regions of troponin. Changing the Mg(2+) concentration does not affect their bindings to the enzyme. It is suggested that the requirements for troponin to mimic calmodulin in binding to the target enzyme, and those for activating it, are met by different regions of the protein.

Magnesium-calcium exchange in cardiac troponin C bound to cardiac troponin I

Understanding the process of Ca(2+)/Mg(2+)exchange during muscle excitation and relaxation is fundamental to elucidating the mechanism of Ca(2+)-regulated muscle contraction. During the resting phase, the C-domain of cardiac troponin C may be occupied by either Ca(2+)or Mg(2+). Here, complexes of recombinant cardiac troponin C(81-161) and the N terminus of cardiac troponin I, representing residues 33-80, were generated in the presence of saturating Mg(2+). Heteronuclear multi-dimensional nuclear magnetic resonance experiments were used to obtain backbone assignments of the Mg(2+)-loaded complex. In the presence of cardiac troponin I, the affinity of site IV for Mg(2+)is increased. Comparison of Mg(2+)and Ca(2+)-loaded complexes reveals that chemical shift differences are primarily localized to metal-binding sites III and IV, defining positions within these sites that have distinct Ca(2+)/Mg(2+)conformations. The observed transition from the Mg(2+)-loaded to Ca(2+)-loaded form demonstrates that sites III and IV fill simultaneously with Ca(2+)displacing Mg(2+). However, even in the absence of excess Ca(2+), Mg(2+)does not readily displace Ca(2+)in the isolated binary complex. Thus, the Mg(2+)-loaded conformer may only represent a small fraction of the total cardiac troponin C found in the sarcomere.