Enhanced troponin I binding explains the functional changes produced by the hypertrophic cardiomyopathy mutation A8V of cardiac troponin C

Troponin C gene mutations on cardiac muscle cell and skeletal Regulation: A comprehensive review

BACKGROUND

The troponin complex plays a crucial role in regulating skeletal and cardiac contraction. Congenital myopathies can occur due to several mutations in genes that encode skeletal troponin. Moreover, there is limited information regarding the composition of skeletal troponin. This review specifically examines a comprehensive review of the TNNC gene mutations on cardiac and skeletal regulations.

MAIN BODY

Troponin C (TNNC) has been linked to a newly discovered inherited muscle disorder. Genetic variations in genes that encode skeletal troponin can impair the function of sarcomeres. Various treatment approaches have been employed to mitigate the impact of variations, including the use of troponin activators, the injection of wild-type protein via AAV gene therapy, and myosin modification to enhance muscle contraction. The processes responsible for the pathophysiological implications of the variations in genes that encode skeletal troponin are not fully understood.

CONCLUSION

This comprehensive review will contribute to the understanding of the relationship between human cardiomyopathy and TNNC mutations and will guide the development of therapy approaches.

Mouse Models of Cardiomyopathies Caused by Mutations in Troponin C

Cardiac muscle contraction is regulated via Ca2+ exchange with the hetero-trimeric troponin complex located on the thin filament. Binding of Ca2+ to cardiac troponin C, a Ca2+ sensing subunit within the troponin complex, results in a series of conformational re-arrangements among the thin filament components, leading to an increase in the formation of actomyosin cross-bridges and muscle contraction. Ultimately, a decline in intracellular Ca2+ leads to the dissociation of Ca2+ from troponin C, inhibiting cross-bridge cycling and initiating muscle relaxation. Therefore, troponin C plays a crucial role in the regulation of cardiac muscle contraction and relaxation. Naturally occurring and engineered mutations in troponin C can lead to altered interactions among components of the thin filament and to aberrant Ca2+ binding and exchange with the thin filament. Mutations in troponin C have been associated with various forms of cardiac disease, including hypertrophic, restrictive, dilated, and left ventricular noncompaction cardiomyopathies. Despite progress made to date, more information from human studies, biophysical characterizations, and animal models is required for a clearer understanding of disease drivers that lead to cardiomyopathies. The unique use of engineered cardiac troponin C with the L48Q mutation that had been thoroughly characterized and genetically introduced into mouse myocardium clearly demonstrates that Ca2+ sensitization in and of itself should not necessarily be considered a disease driver. This opens the door for small molecule and protein engineering strategies to help boost impaired systolic function. On the other hand, the engineered troponin C mutants (I61Q and D73N), genetically introduced into mouse myocardium, demonstrate that Ca2+ desensitization under basal conditions may be a driving factor for dilated cardiomyopathy. In addition to enhancing our knowledge of molecular mechanisms that trigger hypertrophy, dilation, morbidity, and mortality, these cardiomyopathy mouse models could be used to test novel treatment strategies for cardiovascular diseases. In this review, we will discuss (1) the various ways mutations in cardiac troponin C might lead to disease; (2) relevant data on mutations in cardiac troponin C linked to human disease, and (3) all currently existing mouse models containing cardiac troponin C mutations (disease-associated and engineered).

The effect of Mg2+ on Ca2+ binding to cardiac troponin C in hypertrophic cardiomyopathy associated TNNC1 variants

Cardiac troponin C (cTnC) is the critical Ca2+ -sensing component of the troponin complex. Binding of Ca2+ to cTnC triggers a cascade of conformational changes within the myofilament that culminate in force production. Hypertrophic cardiomyopathy (HCM)-associated TNNC1 variants generally induce a greater degree and duration of Ca2+ binding, which may underly the hypertrophic phenotype. Regulation of contraction has long been thought to occur exclusively through Ca2+ binding to site II of cTnC. However, work by several groups including ours suggest that Mg2+ , which is several orders of magnitude more abundant in the cell than Ca2+ , may compete for binding to the same cTnC regulatory site. We previously used isothermal titration calorimetry (ITC) to demonstrate that physiological concentrations of Mg2+ may decrease site II Ca2+ -binding in both N-terminal and full-length cTnC. Here, we explore the binding of Ca2+ and Mg2+ to cTnC harbouring a series of TNNC1 variants thought to be causal in HCM. ITC and thermodynamic integration (TI) simulations show that A8V, L29Q and A31S elevate the affinity for both Ca2+ and Mg2+ . Further, L48Q, Q50R and C84Y that are adjacent to the EF hand binding motif of site II have a more significant effect on affinity and the thermodynamics of the binding interaction. To the best of our knowledge, this work is the first to explore the role of Mg2+ in modifying the Ca2+ affinity of cTnC mutations linked to HCM. Our results indicate a physiologically significant role for cellular Mg2+ both at baseline and when elevated on modifying the Ca2+ binding properties of cTnC and the subsequent conformational changes which precede cardiac contraction.

Cardiomyocyte Dysfunction in Inherited Cardiomyopathies

Inherited cardiomyopathies form a heterogenous group of disorders that affect the structure and function of the heart. Defects in the genes encoding sarcomeric proteins are associated with various perturbations that induce contractile dysfunction and promote disease development. In this review we aimed to outline the functional consequences of the major inherited cardiomyopathies in terms of myocardial contraction and kinetics, and to highlight the structural and functional alterations in some sarcomeric variants that have been demonstrated to be involved in the pathogenesis of the inherited cardiomyopathies. A particular focus was made on mutation-induced alterations in cardiomyocyte mechanics. Since no disease-specific treatments for familial cardiomyopathies exist, several novel agents have been developed to modulate sarcomere contractility. Understanding the molecular basis of the disease opens new avenues for the development of new therapies. Furthermore, the earlier the awareness of the genetic defect, the better the clinical prognostication would be for patients and the better the prevention of development of the disease.

De Novo Missense Mutations in TNNC1 and TNNI3 Causing Severe Infantile Cardiomyopathy Affect Myofilament Structure and Function and Are Modulated by Troponin Targeting Agents

Rare pediatric non-compaction and restrictive cardiomyopathy are usually associated with a rapid and severe disease progression. While the non-compaction phenotype is characterized by structural defects and is correlated with systolic dysfunction, the restrictive phenotype exhibits diastolic dysfunction. The molecular mechanisms are poorly understood. Target genes encode among others, the cardiac troponin subunits forming the main regulatory protein complex of the thin filament for muscle contraction. Here, we compare the molecular effects of two infantile de novo point mutations in TNNC1 (p.cTnC-G34S) and TNNI3 (p.cTnI-D127Y) leading to severe non-compaction and restrictive phenotypes, respectively. We used skinned cardiomyocytes, skinned fibers, and reconstituted thin filaments to measure the impact of the mutations on contractile function. We investigated the interaction of these troponin variants with actin and their inter-subunit interactions, as well as the structural integrity of reconstituted thin filaments. Both mutations exhibited similar functional and structural impairments, though the patients developed different phenotypes. Furthermore, the protein quality control system was affected, as shown for TnC-G34S using patient's myocardial tissue samples. The two troponin targeting agents levosimendan and green tea extract (-)-epigallocatechin-3-gallate (EGCg) stabilized the structural integrity of reconstituted thin filaments and ameliorated contractile function in vitro in some, but not all, aspects to a similar degree for both mutations.

Potential impacts of the cardiac troponin I mobile domain on myofilament activation and relaxation

The cardiac thin filament is regulated in a Ca²⁺-dependent manner through conformational changes of troponin and tropomyosin (Tm). It has been generally understood that under conditions of low Ca²⁺ the inhibitory peptide domain (IP) of troponin I (TnI) binds to actin and holds Tm over the myosin binding sites on actin to prevent crossbridge formation. More recently, evidence that the C-terminal mobile domain (MD) of TnI also binds actin has made for a more complex scenario. This study uses a computational model to investigate the consequences of assuming that TnI regulates Tm movement via two actin-binding domains rather than one. First, a 16-state model of the cardiac thin filament regulatory unit was created with TnI-IP as the sole regulatory domain. Expansion of this to include TnI-MD formed a 24-state model. Comparison of these models showed that assumption of a second actin-binding site allows the individual domains to have a lower affinity for actin than would be required for IP acting alone. Indeed, setting actin affinities of the IP and MD to 25% of that assumed for the IP in the single-site model was sufficient to achieve precisely the same degree of Ca²⁺ regulation. We also tested the 24-state model's ability to represent steady-state experimental data in the case of disruption of either the IP or MD. We were able to capture qualitative changes in several properties that matched what was seen in the experimental data. Lastly, simulations were run to examine the effect of disruption of the IP or MD on twitch dynamics. Our results suggest that both domains are required to keep diastolic cross-bridge activity to a minimum and accelerate myofilament relaxation. Overall, our analyses support a paradigm in which two domains of TnI bind with moderate affinity to actin, working in tandem to complete Ca²⁺-dependent regulation of the thin filament.

A comprehensive guide to genetic variants and post-translational modifications of cardiac troponin C

Familial cardiomyopathy is an inherited disease that affects the structure and function of heart muscle and has an extreme range of phenotypes. Among the millions of affected individuals, patients with hypertrophic (HCM), dilated (DCM), or left ventricular non-compaction (LVNC) cardiomyopathy can experience morphologic changes of the heart which lead to sudden death in the most detrimental cases. TNNC1, the gene that codes for cardiac troponin C (cTnC), is a sarcomere gene associated with cardiomyopathies in which probands exhibit young age of presentation and high death, transplant or ventricular fibrillation events relative to TNNT2 and TNNI3 probands. Using GnomAD, ClinVar, UniProt and PhosphoSitePlus databases and published literature, an extensive list to date of identified genetic variants in TNNC1 and post-translational modifications (PTMs) in cTnC was compiled. Additionally, a recent cryo-EM structure of the cardiac thin filament regulatory unit was used to localize each functionally studied amino acid variant and each PTM (acetylation, glycation, s-nitrosylation, phosphorylation) in the structure of cTnC. TNNC1 has a large number of variants (> 100) relative to other genes of the same transcript size. Surprisingly, the mapped variant amino acids and PTMs are distributed throughout the cTnC structure. While many cardiomyopathy-associated variants are localized in α-helical regions of cTnC, this was not statistically significant χ² (p = 0.72). Exploring the variants in TNNC1 and PTMs of cTnC in the contexts of cardiomyopathy association, physiological modulation and potential non-canonical roles provides insights into the normal function of cTnC along with the many facets of TNNC1 as a cardiomyopathic gene.

Eliminating the First Inactive State and Stabilizing the Active State of the Cardiac Regulatory System Alters Behavior in Solution and in Ordered Systems

Calcium binding to troponin C (TnC) is insufficient for full activation of myosin ATPase activity by actin-tropomyosin-troponin. Previous attempts to investigate full activation utilized ATP-free myosin or chemically modified myosin to stabilize the active state of regulated actin. We utilized the Δ14-TnT and the A8V-TnC mutants to stabilize the activated state at saturating Ca²⁺ and to eliminate one of the inactive states at low Ca²⁺. The observed effects differed in solution studies and in the more ordered in vitro motility assay and in skinned cardiac muscle preparations. At saturating Ca²⁺, full activation with Δ14-TnT·A8V-TnC decreased the apparent K_M for actin-activated ATPase activity compared to bare actin filaments. Rates of in vitro motility increased at both high and low Ca²⁺ with Δ14-TnT; the maximum shortening speed at high Ca²⁺ increased 1.8-fold. Cardiac muscle preparations exhibited increased Ca²⁺ sensitivity and large increases in resting force with either Δ14-TnT or Δ14-TnT·A8V-TnC. We also observed a significant increase in the maximal rate of tension redevelopment. The results of full activation with Ca²⁺ and Δ14-TnT·A8V-TnC confirmed and extended several earlier observations using other means of reaching full activation. Furthermore, at low Ca²⁺, elimination of the first inactive state led to partial activation. This work also confirms, in three distinct experimental systems, that troponin is able to stabilize the active state of actin-tropomyosin-troponin without the need for high-affinity myosin binding. The results are relevant to the reason for two inactive states and for the role of force producing myosin in regulation.

Basic residues within the cardiac troponin T C terminus are required for full inhibition of muscle contraction and limit activation by calcium

Striated muscle is activated by myosin- and actin-linked processes, with the latter being regulated through changes in the position of tropomyosin relative to the actin surface. The C-terminal region of cardiac troponin T (TnT), a tropomyosin-associated protein, is required for full TnT inactivation at low Ca²⁺ and for limiting its activation at saturating Ca²⁺ Here, we investigated whether basic residues in this TnT region are involved in these activities, whether the TnT C terminus undergoes Ca²⁺-dependent conformational changes, and whether these residues affect cardiac muscle contraction. We generated a human cardiac TnT variant in which we replaced seven C-terminal Lys and Arg residues with Ala and added a Cys residue at either position 289 or 275 to affix a fluorescent probe. At pCa 3.7, actin filaments containing high-alanine TnT had an elevated ATPase rate like that obtained when the last TnT 14 residues were deleted. Acrylodan-tropomyosin fluorescence changes and S1-actin binding kinetics revealed that at pCa 8, the high-alanine TnT-containing filaments did not enter the first inactive state. FRET analyses indicated that the C-terminal TnT region approached Cys-190 of tropomyosin as actin filaments transitioned to the inactive B state; that transition was abolished with high-alanine TnT. High-alanine TnT-containing cardiac muscle preparations had increased Ca²⁺ sensitivity of both steady-state isometric force and sinusoidal stiffness as well as increased maximum steady-state isometric force and sinusoidal stiffness. We conclude that C-terminal basic residues in cardiac TnT are critical for the regulation of cardiac muscle contraction.

Structural and functional impact of troponin C-mediated Ca2+ sensitization on myofilament lattice spacing and cross-bridge mechanics in mouse cardiac muscle

Acto-myosin cross-bridge kinetics are important for beat-to-beat regulation of cardiac contractility; however, physiological and pathophysiological mechanisms for regulation of contractile kinetics are incompletely understood. Here we explored whether thin filament-mediated Ca²⁺ sensitization influences cross-bridge kinetics in permeabilized, osmotically compressed cardiac muscle preparations. We used a murine model of hypertrophic cardiomyopathy (HCM) harboring a cardiac troponin C (cTnC) Ca²⁺-sensitizing mutation, Ala8Val in the regulatory N-domain. We also treated wild-type murine muscle with bepridil, a cTnC-targeting Ca²⁺ sensitizer. Our findings suggest that both methods of increasing myofilament Ca²⁺ sensitivity increase cross-bridge cycling rate measured by the rate of tension redevelopment (k_TR); force per cross-bridge was also enhanced as measured by sinusoidal stiffness and I_1,1/I_1,0 ratio from X-ray diffraction. Computational modeling suggests that Ca²⁺ sensitization through this cTnC mutation or bepridil accelerates k_TR primarily by promoting faster cross-bridge detachment. To elucidate if myofilament structural rearrangements are associated with changes in k_TR, we used small angle X-ray diffraction to simultaneously measure myofilament lattice spacing and isometric force during steady-state Ca²⁺ activations. Within in vivo lattice dimensions, lattice spacing and steady-state isometric force increased significantly at submaximal activation. We conclude that the cTnC N-domain controls force by modulating both the number and rate of cycling cross-bridges, and that the both methods of Ca²⁺ sensitization may act through stabilization of cTnC's D-helix. Furthermore, we propose that the transient expansion of the myofilament lattice during Ca²⁺ activation may be an additional factor that could increase the rate of cross-bridge cycling in cardiac muscle. These findings may have implications for the pathophysiology of HCM.

Troponin C Mutations Partially Stabilize the Active State of Regulated Actin and Fully Stabilize the Active State When Paired with Δ14 TnT

Striated muscle contraction is regulated by the actin-associated proteins tropomyosin and troponin. The extent of activation of myosin ATPase activity is lowest in the absence of both Ca²⁺ and activating cross-bridges (i.e., S1-ADP or rigor S1). Binding of activating species of myosin to actin at a saturating Ca²⁺ concentration stabilizes the most active state (M state) of the actin-tropomyosin-troponin complex (regulated actin). Ca²⁺ binding alone produces partial stabilization of the active state. The extent of stabilization at a saturating Ca²⁺ concentration depends on the isoform of the troponin subunits, the phosphorylation state of troponin, and, in the case of cardiac muscle, the presence of hypertrophic cardiomyopathy-producing mutants of troponin T and troponin I. Cardiac dysfunction is also associated with mutations of troponin C (TnC). Troponin C mutants A8V, C84Y, and D145E increase the Ca²⁺ sensitivity of ATPase activity. We show that these mutants change the distribution of regulated actin states. The A8V and C84Y TnC mutants decreased the inactive B state distribution slightly at low Ca²⁺ concentrations, but the D145E mutants had no effect on that state. All TnC mutants increased the level of the active M state compared to that of the wild type, at a saturating Ca²⁺ concentration. Troponin complexes that contained two mutations that stabilize the active M state, A8V TnC and Δ14 TnT, appeared to be completely in the active state in the presence of only Ca²⁺. Because Ca²⁺ gives full activation, in this situation, troponin must be capable of positioning tropomyosin in the active M state without the need for rigor myosin binding.

Changes in the dynamics of the cardiac troponin C molecule explain the effects of Ca2+-sensitizing mutations

Cardiac troponin C (cTnC) is the regulatory protein that initiates cardiac contraction in response to Ca²⁺ TnC binding Ca²⁺ initiates a cascade of protein-protein interactions that begins with the opening of the N-terminal domain of cTnC, followed by cTnC binding the troponin I switch peptide (TnI_SW). We have evaluated, through isothermal titration calorimetry and molecular-dynamics simulation, the effect of several clinically relevant mutations (A8V, L29Q, A31S, L48Q, Q50R, and C84Y) on the Ca²⁺ affinity, structural dynamics, and calculated interaction strengths between cTnC and each of Ca²⁺ and TnI_SW Surprisingly the Ca²⁺ affinity measured by isothermal titration calorimetry was only significantly affected by half of these mutations including L48Q, which had a 10-fold higher affinity than WT, and the Q50R and C84Y mutants, each of which had affinities 3-fold higher than wild type. This suggests that Ca²⁺ affinity of the N-terminal domain of cTnC in isolation is insufficient to explain the pathogenicity of these mutations. Molecular-dynamics simulation was used to evaluate the effects of these mutations on Ca²⁺ binding, structural dynamics, and TnI interaction independently. Many of the mutations had a pronounced effect on the balance between the open and closed conformations of the TnC molecule, which provides an indirect mechanism for their pathogenic properties. Our data demonstrate that the structural dynamics of the cTnC molecule are key in determining myofilament Ca²⁺ sensitivity. Our data further suggest that modulation of the structural dynamics is the underlying molecular mechanism for many disease mutations that are far from the regulatory Ca²⁺-binding site of cTnC.

Hypertrophic Cardiomyopathy Cardiac Troponin C Mutations Differentially Affect Slow Skeletal and Cardiac Muscle Regulation

Mutations in TNNC1-the gene encoding cardiac troponin C (cTnC)-that have been associated with hypertrophic cardiomyopathy (HCM) and cardiac dysfunction may also affect Ca2+-regulation and function of slow skeletal muscle since the same gene is expressed in both cardiac and slow skeletal muscle. Therefore, we reconstituted rabbit soleus fibers and bovine masseter myofibrils with mutant cTnCs (A8V, C84Y, E134D, and D145E) associated with HCM to investigate their effects on contractile force and ATPase rates, respectively. Previously, we showed that these HCM cTnC mutants, except for E134D, increased the Ca2+ sensitivity of force development in cardiac preparations. In the current study, an increase in Ca2+ sensitivity of isometric force was only observed for the C84Y mutant when reconstituted in soleus fibers. Incorporation of cTnC C84Y in bovine masseter myofibrils reduced the ATPase activity at saturating [Ca2+], whereas, incorporation of cTnC D145E increased the ATPase activity at inhibiting and saturating [Ca2+]. We also tested whether reconstitution of cardiac fibers with troponin complexes containing the cTnC mutants and slow skeletal troponin I (ssTnI) could emulate the slow skeletal functional phenotype. Reconstitution of cardiac fibers with troponin complexes containing ssTnI attenuated the Ca2+ sensitization of isometric force when cTnC A8V and D145E were present; however, it was enhanced for C84Y. In summary, although the A8V and D145E mutants are present in both muscle types, their functional phenotype is more prominent in cardiac muscle than in slow skeletal muscle, which has implications for the protein-protein interactions within the troponin complex. The C84Y mutant warrants further investigation since it drastically alters the properties of both muscle types and may account for the earlier clinical onset in the proband.

Modeling Ca2+-Bound Troponin in Excitation Contraction Coupling

To explain disparate decay rates of cytosolic Ca²⁺ and structural changes in the thin filaments during a twitch, we model the time course of Ca²⁺-bound troponin (Tn) resulting from the free Ca²⁺ transient of fast skeletal muscle. In fibers stretched beyond overlap, the decay of Ca²⁺ as measured by a change in fluo-3 fluorescence is significantly slower than the intensity decay of the meridional 1/38.5 nm⁻¹ reflection of Tn; this is not simply explained by considering only the Ca²⁺ binding properties of Tn alone (Matsuo et al., 2010). We apply a comprehensive model that includes the known Ca²⁺ binding properties of Tn in the context of the thin filament with and without cycling crossbridges. Calculations based on the model predict that the transient of Ca²⁺-bound Tn correlates with either the fluo-3 time course in muscle with overlapping thin and thick filaments or the intensity of the meridional 1/38.5 nm⁻¹ reflection in overstretched muscle. Hence, cycling crossbridges delay the dissociation of Ca²⁺ from Tn. Correlation with the fluo-3 fluorescence change is not causal given that the transient of Ca²⁺-bound Tn depends on sarcomere length, whereas the fluo-3 fluorescence change does not. Transient positions of tropomyosin calculated from the time course of Ca²⁺-bound Tn are in reasonable agreement with the transient of measured perturbations of the Tn repeat in overlap and non-overlap muscle preparations.

Cardiac Troponin and Tropomyosin: Structural and Cellular Perspectives to Unveil the Hypertrophic Cardiomyopathy Phenotype

Inherited myopathies affect both skeletal and cardiac muscle and are commonly associated with genetic dysfunctions, leading to the production of anomalous proteins. In cardiomyopathies, mutations frequently occur in sarcomeric genes, but the cause-effect scenario between genetic alterations and pathological processes remains elusive. Hypertrophic cardiomyopathy (HCM) was the first cardiac disease associated with a genetic background. Since the discovery of the first mutation in the β-myosin heavy chain, more than 1400 new mutations in 11 sarcomeric genes have been reported, awarding HCM the title of the “disease of the sarcomere.” The most common macroscopic phenotypes are left ventricle and interventricular septal thickening, but because the clinical profile of this disease is quite heterogeneous, these phenotypes are not suitable for an accurate diagnosis. The development of genomic approaches for clinical investigation allows for diagnostic progress and understanding at the molecular level. Meanwhile, the lack of accurate in vivo models to better comprehend the cellular events triggered by this pathology has become a challenge. Notwithstanding, the imbalance of Ca²⁺ concentrations, altered signaling pathways, induction of apoptotic factors, and heart remodeling leading to abnormal anatomy have already been reported. Of note, a misbalance of signaling biomolecules, such as kinases and tumor suppressors (e.g., Akt and p53), seems to participate in apoptotic and fibrotic events. In HCM, structural and cellular information about defective sarcomeric proteins and their altered interactome is emerging but still represents a bottleneck for developing new concepts in basic research and for future therapeutic interventions. This review focuses on the structural and cellular alterations triggered by HCM-causing mutations in troponin and tropomyosin proteins and how structural biology can aid in the discovery of new platforms for therapeutics. We highlight the importance of a better understanding of allosteric communications within these thin-filament proteins to decipher the HCM pathological state.