Rat cardiac troponin T mutation (F72L)-mediated impact on thin filament cooperativity is divergently modulated by α- and β-myosin heavy chain isoforms

Characterization of NEB pathogenic variants in patients reveals novel nemaline myopathy disease mechanisms and omecamtiv mecarbil force effects

Nebulin, a critical protein of the skeletal muscle thin filament, plays important roles in physiological processes such as regulating thin filament length (TFL), cross-bridge cycling, and myofibril alignment. Pathogenic variants in the nebulin gene (NEB) cause NEB-based nemaline myopathy (NEM2), a genetically heterogeneous disorder characterized by hypotonia and muscle weakness, currently lacking curative therapies. In this study, we examined a cohort of ten NEM2 patients, each with unique pathogenic variants, aiming to understand their impact on mRNA, protein, and functional levels. Results show that pathogenic truncation variants affect NEB mRNA stability and lead to nonsense-mediated decay of the mutated transcript. Moreover, a high incidence of cryptic splice site activation was found in patients with pathogenic splicing variants that are expected to disrupt the actin-binding sites of nebulin. Determination of protein levels revealed patients with either relatively normal or markedly reduced nebulin. We observed a positive relation between the reduction in nebulin and a reduction in TFL, or reduction in tension (both maximal and submaximal tension). Interestingly, our study revealed a pathogenic duplication variant in nebulin that resulted in a four-copy gain in the triplicate region of NEB and a much larger nebulin protein and longer TFL. Additionally, we investigated the effect of Omecamtiv mecarbil (OM), a small-molecule activator of cardiac myosin, on force production of type 1 muscle fibers of NEM2 patients. OM treatment substantially increased submaximal tension across all NEM2 patients ranging from 87 to 318%, with the largest effects in patients with the lowest level of nebulin. In summary, this study indicates that post-transcriptional or post-translational mechanisms regulate nebulin expression. Moreover, we propose that the pathomechanism of NEM2 involves not only shortened but also elongated thin filaments, along with the disruption of actin-binding sites resulting from pathogenic splicing variants. Significantly, our findings highlight the potential of OM treatment to improve skeletal muscle function in NEM2 patients, especially those with large reductions in nebulin levels.

Characterization of NEB mutations in patients reveals novel nemaline myopathy disease mechanisms and omecamtiv mecarbil force effects

Nebulin, a critical protein of the skeletal muscle thin filament, plays important roles in physiological processes such as regulating thin filament length (TFL), cross-bridge cycling, and myofibril alignment. Mutations in the nebulin gene ( NEB ) cause NEB-based nemaline myopathy (NEM2), a genetically heterogeneous disorder characterized by hypotonia and muscle weakness, currently lacking therapies targeting the underlying pathological mechanisms. In this study, we examined a cohort of ten NEM2 patients, each with unique mutations, aiming to understand their impact on mRNA, protein, and functional levels. Results show that truncation mutations affect NEB mRNA stability and lead to nonsense-mediated decay of the mutated transcript. Moreover, a high incidence of cryptic splice site activation was found in patients with splicing mutations which is expected to disrupt the actin-binding sites of nebulin. Determination of protein levels revealed patients with relatively normal nebulin levels and others with markedly reduced nebulin. We observed a positive relation between the reduction in nebulin and a reduction in TFL, and a positive relation between the reduction in nebulin level and the reduction in tension (both maximal and submaximal tension). Interestingly, our study revealed a duplication mutation in nebulin that resulted in a larger nebulin protein and longer TFL. Additionally, we investigated the effect of Omecamtiv mecarbil (OM), a small-molecule activator of cardiac myosin, on force production of type I muscle fibers of NEM2 patients. OM treatment substantially increased submaximal tension across all NEM2 patients ranging from 87-318%, with the largest effects in patients with the lowest level of nebulin. In summary, this study indicates that post-transcriptional or post-translational mechanisms regulate nebulin expression. Moreover, we propose that the pathomechanism of NEM2 involves not only shortened but also elongated thin filaments, along with the disruption of actin-binding sites resulting from splicing mutations. Significantly, our findings highlight the potential of OM treatment to improve skeletal muscle function in NEM2 patients, especially those with large reductions in nebulin levels.

The effects of the tropomyosin cardiomyopathy mutations on the calcium regulation of actin-myosin interaction in the atrium and ventricle differ

The molecular mechanisms of pathogenesis of atrial myopathy associated with hypertrophic (HCM) and dilated (DCM) mutations of sarcomeric proteins are still poorly understood. For this, one needs to investigate the effects of the mutations on actin-myosin interaction in the atria separately from ventricles. We compared the impact of the HCM and DCM mutations of tropomyosin (Tpm) on the calcium regulation of the thin filament interaction with atrial and ventricular myosin using an in vitro motility assay. We found that the mutations differently affect the calcium regulation of actin-myosin interaction in the atria and ventricles. The DCM E40K Tpm mutation significantly reduced the maximum sliding velocity of thin filaments with ventricular myosin and its Ca²⁺-sensitivity. With atrial myosin, its effects were less pronounced. The HCM I172T mutation reduced the Ca²⁺-sensitivity of the sliding velocity of filaments with ventricular myosin but increased it with the atrial one. The HCM L185R mutation did not affect actin-myosin interaction in the atria. The results indicate that the difference in the effects of Tpm mutations on the actin-myosin interaction in the atria and ventricles may be responsible for the difference in pathological changes in the atrial and ventricular myocardium.

Developmental increase in β-MHC enhances sarcomere length-dependent activation in the myocardium

The expression of β-myosin heavy chain (β-MHC) in the guinea pig heart increases during postnatal development. Reda et al. show that this increase in β-MHC enhances length-mediated increases in myofilament Ca²⁺ sensitivity and sarcomere length–dependent changes in contractile function.

Shifts in myosin heavy chain (MHC) isoforms in cardiac myocytes have been shown to alter cardiac muscle function not only in healthy developing hearts but also in diseased hearts. In guinea pig hearts, there is a large age-dependent shift in MHC isoforms from 80% α-MHC/20% β-MHC at 3 wk to 14% α-MHC/86% β-MHC at 11 wk. Because kinetic differences in α- and β-MHC cross-bridges (XBs) are known to impart different cooperative effects on thin filaments, we hypothesize here that differences in α- and β-MHC expression in guinea pig cardiac muscle impact sarcomere length (SL)–dependent contractile function. We therefore measure steady state and dynamic contractile parameters in detergent-skinned cardiac muscle preparations isolated from the left ventricles of young (3 wk old) or adult (11 wk old) guinea pigs at two different SLs: short (1.9 µm) and long (2.3 µm). Our data show that SL-dependent effects on contractile parameters are augmented in adult guinea pig cardiac muscle preparations. Notably, the SL-mediated increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) is twofold greater in adult guinea pig muscle preparations (ΔpCa₅₀ being 0.11 units in adult preparations but only 0.05 units in young preparations). Furthermore, adult guinea pig cardiac muscle preparations display greater SL-dependent changes than young muscle preparations in (1) the magnitude of length-mediated increase in the recruitment of new force-bearing XBs, (2) XB detachment rate, (3) XB strain-mediated effects on other force-bearing XBs, and (4) the rate constant of force redevelopment. Our findings suggest that increased β-MHC expression enhances length-dependent activation in the adult guinea pig cardiac myocardium.

Regulation of myofilament force and loaded shortening by skeletal myosin binding protein C

Myosin binding protein C (MyBP-C) is thought to regulate the contraction of skeletal muscle. Robinett et al. show that phosphorylation of slow skeletal MyBP-C modulates contraction by recruiting cross-bridges, modifying cross-bridge kinetics, and altering internal drag forces in the C-zone.

Myosin binding protein C (MyBP-C) is a 125–140-kD protein located in the C-zone of each half-thick filament. It is thought to be an important regulator of contraction, but its precise role is unclear. Here we investigate mechanisms by which skeletal MyBP-C regulates myofilament function using rat permeabilized skeletal muscle fibers. We mount either slow-twitch or fast-twitch skeletal muscle fibers between a force transducer and motor, use Ca²⁺ to activate a range of forces, and measure contractile properties including transient force overshoot, rate of force development, and loaded sarcomere shortening. The transient force overshoot is greater in slow-twitch than fast-twitch fibers at all Ca²⁺ activation levels. In slow-twitch fibers, protein kinase A (PKA) treatment (a) augments phosphorylation of slow skeletal MyBP-C (sMyBP-C), (b) doubles the magnitude of the relative transient force overshoot at low Ca²⁺ activation levels, and (c) increases force development rates at all Ca²⁺ activation levels. We also investigate the role that phosphorylated and dephosphorylated sMyBP-C plays in loaded sarcomere shortening. We test the hypothesis that MyBP-C acts as a brake to filament sliding within the myofilament lattice by measuring sarcomere shortening as thin filaments traverse into the C-zone during lightly loaded slow-twitch fiber contractions. Before PKA treatment, shortening velocity decelerates as sarcomeres traverse from ∼3.10 to ∼3.00 µm. After PKA treatment, sarcomeres shorten a greater distance and exhibit less deceleration during similar force clamps. After sMyBP-C dephosphorylation, sarcomere length traces display a brief recoil (i.e., “bump”) that initiates at ∼3.06 µm during loaded shortening. Interestingly, the timing of the bump shifts with changes in load but manifests at the same sarcomere length. Our results suggest that sMyBP-C and its phosphorylation state regulate sarcomere contraction by a combination of cross-bridge recruitment, modification of cross-bridge cycling kinetics, and alteration of drag forces that originate in the C-zone.

Cardiomyopathy mutation (F88L) in troponin T abolishes length dependency of myofilament Ca2+ sensitivity

The F88L mutation in cardiac troponin T (TnT_F88L) is associated with hypertrophic cardiomyopathy. Reda and Chandra reveal that it abolishes length-mediated increase in myofilament Ca²⁺ sensitivity and attenuates cooperative mechanisms governing length-dependent activation.

Recent clinical studies have revealed a new hypertrophic cardiomyopathy–associated mutation (F87L) in the central region of human cardiac troponin T (TnT). However, despite its implication in several incidences of sudden cardiac death in young and old adults, whether F87L is associated with cardiac contractile dysfunction is unknown. Because the central region of TnT is important for modulating the muscle length–mediated recruitment of new force-bearing cross-bridges (XBs), we hypothesize that the F87L mutation causes molecular changes that are linked to the length-dependent activation of cardiac myofilaments. Length-dependent activation is important because it contributes significantly to the Frank–Starling mechanism, which enables the heart to vary stroke volume as a function of changes in venous return. We measured steady-state and dynamic contractile parameters in detergent-skinned guinea pig cardiac muscle fibers reconstituted with recombinant guinea pig wild-type TnT (TnT_WT) or the guinea pig analogue (TnT_F88L) of the human mutation at two different sarcomere lengths (SLs): short (1.9 µm) and long (2.3 µm). TnT_F88L increases pCa₅₀ (−log [Ca²⁺]_free required for half-maximal activation) to a greater extent at short SL than at long SL; for example, pCa₅₀ increases by 0.25 pCa units at short SL and 0.17 pCa units at long SL. The greater increase in pCa₅₀ at short SL leads to the abolishment of the SL-dependent increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) in TnT_F88L fibers, ΔpCa₅₀ being 0.10 units in TnT_WT fibers but only 0.02 units in TnT_F88L fibers. Furthermore, at short SL, TnT_F88L attenuates the negative impact of strained XBs on force-bearing XBs and augments the magnitude of muscle length–mediated recruitment of new force-bearing XBs. Our findings suggest that the TnT_F88L-mediated effects on cardiac thin filaments may lead to a negative impact on the Frank–Starling mechanism.

Omecamtiv Mecarbil Abolishes Length-Mediated Increase in Guinea Pig Cardiac Myofiber Ca2+ Sensitivity

Omecamtiv mecarbil (OM) is a pharmacological agent that augments cardiac contractile function by enhancing myofilament Ca²⁺ sensitivity. Given that interventions that increase myofilament Ca²⁺ sensitivity have the potential to alter length-dependent activation (LDA) of cardiac myofilaments, we tested the influence of OM on this fundamental property of the heart. This is significant not only because LDA is prominent in cardiac muscle but also because it contributes to the Frank-Starling law, a mechanism by which the heart increases stroke volume in response to an increase in venous return. We measured steady-state and dynamic contractile indices in detergent-skinned guinea pig (Cavia porcellus) cardiac muscle fibers in the absence and presence of 0.3 and 3.0 μM OM at two different sarcomere lengths (SLs), short SL (1.9 μm) and long SL (2.3 μm). Myofilament Ca²⁺ sensitivity, as measured by pCa₅₀ (-log of [Ca²⁺]_free concentration required for half-maximal activation), increased significantly at both short and long SLs in OM-treated fibers when compared to untreated fibers; however, the magnitude of increase in pCa₅₀ was twofold greater at short SL than at long SL. A consequence of this greater increase in pCa₅₀ at short SL was that pCa₅₀ did not increase any further at long SL, suggesting that OM abolished the SL dependency of pCa₅₀. Furthermore, the SL dependency of rate constants of cross-bridge distortion dynamics (c) and force redevelopment (k_tr) was abolished in 0.3-μM-OM-treated fibers. The negative impact of OM on the SL dependency of pCa₅₀, c, and k_tr was also observed in 3.0-μM-OM-treated fibers, indicating that cooperative mechanisms linked to LDA were altered by the OM-mediated effects on cardiac myofilaments.

Hypertrophic cardiomyopathy mutation in cardiac troponin T (R95H) attenuates length-dependent activation in guinea pig cardiac muscle fibers

The central region of cardiac troponin T (TnT) is important for modulating the dynamics of muscle length-mediated cross-bridge recruitment. Therefore, hypertrophic cardiomyopathy mutations in the central region may affect cross-bridge recruitment dynamics to alter myofilament Ca²⁺ sensitivity and length-dependent activation of cardiac myofilaments. Given the importance of the central region of TnT for cardiac contractile dynamics, we studied if hypertrophic cardiomyopathy-linked mutation (TnT_R94H)-induced effects on contractile function would be differently modulated by sarcomere length (SL). Recombinant wild-type TnT (TnT_WT) and the guinea pig analog of the human R94H mutation (TnT_R95H) were reconstituted into detergent-skinned cardiac muscle fibers from guinea pigs. Steady-state and dynamic contractile measurements were made at short and long SLs (1.9 and 2.3 µm, respectively). Our results demonstrated that TnT_R95H increased pCa₅₀ (-log of free Ca²⁺ concentration) to a greater extent at short SL; TnT_R95H increased pCa₅₀ by 0.11 pCa units at short SL and 0.07 pCa units at long SL. The increase in pCa₅₀ associated with an increase in SL from 1.9 to 2.3 µm (ΔpCa₅₀) was attenuated nearly twofold in TnT_R95H fibers; ΔpCa₅₀ was 0.09 pCa units for TnT_WT fibers but only 0.05 pCa units for TnT_R95H fibers. The SL dependency of rate constants of cross-bridge distortion dynamics and tension redevelopment was also blunted by TnT_R95H Collectively, our observations on the SL dependency of pCa₅₀ and rate constants of cross-bridge distortion dynamics and tension redevelopment suggest that mechanisms underlying the length-dependent activation cardiac myofilaments are attenuated by TnT_R95HNEW & NOTEWORTHY Mutant cardiac troponin T (TnT_R95H) differently affects myofilament Ca²⁺ sensitivity at short and long sarcomere length, indicating that mechanisms underlying length-dependent activation are altered by TnT_R95H TnT_R95H enhances myofilament Ca²⁺ sensitivity to a greater extent at short sarcomere length, thus attenuating the length-dependent increase in myofilament Ca²⁺ sensitivity.

Cardiomyopathy-related mutation (A30V) in mouse cardiac troponin T divergently alters the magnitude of stretch activation in α- and β-myosin heavy chain fibers

The present study investigated the functional consequences of the human hypertrophic cardiomyopathy (HCM) mutation A28V in cardiac troponin T (TnT). The A28V mutation is located within the NH2 terminus of TnT, a region known to be important for full activation of cardiac thin filaments. The functional consequences of the A28V mutation in TnT remain unknown. Given how α- and β-myosin heavy chain (MHC) isoforms differently alter the functional effect of the NH2 terminus of TnT, we hypothesized that the A28V-induced effects would be differently modulated by α- and β-MHC isoforms. Recombinant wild-type mouse TnT (TnTWT) and the mouse equivalent of the human A28V mutation (TnTA30V) were reconstituted into detergent-skinned cardiac muscle fibers extracted from normal (α-MHC) and transgenic (β-MHC) mice. Dynamic and steady-state contractile parameters were measured in reconstituted muscle fibers. Step-like length perturbation experiments demonstrated that TnTA30V decreased the magnitude of the muscle length-mediated recruitment of new force-bearing cross bridges (ER) by 30% in α-MHC fibers. In sharp contrast, TnTA30V increased ER by 55% in β-MHC fibers. Inferences drawn from other dynamic contractile parameters suggest that directional changes in ER in TnTA30V + α-MHC and TnTA30V + β-MHC fibers result from a divergent impact on cross bridge-regulatory unit (troponin-tropomyosin complex) cooperativity. TnTA30V-mediated effects on Ca2+-activated maximal tension and instantaneous muscle fiber stiffness (ED) were also divergently affected by α- and β-MHC. Our study demonstrates that TnTA30V + α-MHC and TnTA30V + β-MHC fibers show contrasting contractile phenotypes; however, only the observations from β-MHC fibers are consistent with the clinical data for A28V in humans.

NEW & NOTEWORTHY

The differential impact of α- and β-myosin heavy chain (MHC) on contractile dynamics causes a mutant cardiac troponin T (TnTA30V) to differently modulate cardiac contractile function. TnTA30V attenuated Ca2+-activated maximal tension and length-mediated cross-bridge recruitment against α-MHC but augmented these parameters against β-MHC, suggesting divergent contractile phenotypes.

L71F mutation in rat cardiac troponin T augments crossbridge recruitment and detachment dynamics against α-myosin heavy chain, but not against β-myosin heavy chain

The N-terminal extension of human cardiac troponin T (TnT), which modulates myofilament Ca²⁺ sensitivity, contains several hypertrophic cardiomyopathy (HCM)-causing mutations including S69F. However, the functional consequence of S69F mutation is unknown. The human analog of S69F in rat TnT is L71F (TnT_L71F). Because the functional consequences due to structural changes in the N-terminal extension are influenced by the type of myosin heavy chain (MHC) isoform, we hypothesized that the TnT_L71F-mediated effect would be differently modulated by α- and β-MHC isoforms. TnT_L71F and wild-type rat TnT were reconstituted into de-membranated muscle fibers from normal (α-MHC) and propylthiouracil-treated rat hearts (β-MHC) to measure steady-state and dynamic contractile parameters. The magnitude of the TnT_L71F-mediated attenuation of Ca²⁺-activated maximal tension was greater in α- than in β-MHC fibers. For example, TnT_L71F attenuated maximal tension by 31% in α-MHC fibers but only by 10% in β-MHC fibers. Furthermore, TnT_L71F reduced myofilament Ca²⁺ sensitivity by 0.11 pCa units in α-MHC fibers but only by 0.05 pCa units in β-MHC fibers. TnT_L71F augmented rate constants of crossbridge recruitment and crossbridge detachment dynamics in α-MHC fibers but not in β-MHC fibers. Collectively, our data demonstrate that TnT_L71F induces greater contractile deficits against α-MHC than against β-MHC background.

Dilated Cardiomyopathy Mutation (R134W) in Mouse Cardiac Troponin T Induces Greater Contractile Deficits against α-Myosin Heavy Chain than against β-Myosin Heavy Chain

Many studies have demonstrated that depressed myofilament Ca²⁺ sensitivity is common to dilated cardiomyopathy (DCM) in humans. However, it remains unclear whether a single determinant-such as myofilament Ca²⁺ sensitivity-is sufficient to characterize all cases of DCM because the severity of disease varies widely with a given mutation. Because dynamic features dominate in the heart muscle, alterations in dynamic contractile parameters may offer better insight on the molecular mechanisms that underlie disparate effects of DCM mutations on cardiac phenotypes. Dynamic features are dominated by myofilament cooperativity that stem from different sources. One such source is the strong tropomyosin binding region in troponin T (TnT), which is known to modulate crossbridge (XB) recruitment dynamics in a myosin heavy chain (MHC)-dependent manner. Therefore, we hypothesized that the effects of DCM-linked mutations in TnT on contractile dynamics would be differently modulated by α- and β-MHC. After reconstitution with the mouse TnT equivalent (TnT_R134W) of the human DCM mutation (R131W), we measured dynamic contractile parameters in detergent-skinned cardiac muscle fiber bundles from normal (α-MHC) and transgenic mice (β-MHC). TnT_R134W significantly attenuated the rate constants of tension redevelopment, XB recruitment dynamics, XB distortion dynamics, and the magnitude of length-mediated XB recruitment only in α-MHC fiber bundles. TnT_R134W decreased myofilament Ca²⁺ sensitivity to a greater extent in α-MHC (0.14 pCa units) than in β-MHC fiber bundles (0.08 pCa units). Thus, our data demonstrate that TnT_R134W induces a more severe DCM-like contractile phenotype against α-MHC than against β-MHC background.

Interplay Between the Effects of Dilated Cardiomyopathy Mutation (R206L) and the Protein Kinase C Phosphomimic (T204E) of Rat Cardiac Troponin T Are Differently Modulated by α- and β-Myosin Heavy Chain Isoforms

Background

We hypothesized that the functional effects of R206L—a rat analog of the dilated cardiomyopathy (DCM) mutation R205L in human cardiac troponin T (TnT)—were differently modulated by myosin heavy chain (MHC) isoforms and T204E, a protein kinase C (PKC) phosphomimic of TnT. Our hypothesis was based on two observations: (1) α‐ and β‐MHC differentially influence the functional effects of TnT; and (2) PKC isoforms capable of phosphorylating TnT are upregulated in failing human hearts.

Methods and Results

We generated 4 recombinant TnT variants: wild type; R206L; T204E; and R206L+T204E. Functional effects of the TnT variants were tested in cardiac muscle fibers (minimum 14 per group) from normal (α‐MHC) and propylthiouracil‐treated rats (β‐MHC) using steady‐state and dynamic contractile measurements. Notably, in α‐MHC fibers, Ca²⁺‐activated maximal tension was attenuated by R206L (≈32%), T204E (≈63%), and R206L+T204E (≈64%). In β‐MHC fibers, maximal tension was unaffected by R206L, but was attenuated by T204E (≈33%) and R206L+T204E (≈40%). Thus, β‐MHC differentially counteracted the attenuating effects of the TnT variants on tension. However, in β‐MHC fibers, R206L+T204E attenuated tension to a greater extent when compared to T204E alone. In β‐MHC fibers, R206L+T204E attenuated the magnitude of the length‐mediated recruitment of new cross‐bridges (≈28%), suggesting that the Frank‐Starling mechanism was impaired.

Conclusions

Our findings are the first (to our knowledge) to demonstrate that the functional effects of a DCM‐linked TnT mutation are not only modulated by MHC isoforms, but also by the pathology‐associated post‐translational modifications of TnT.

The effect of cardiomyopathy mutation (R97L) in mouse cardiac troponin T on the muscle length-mediated recruitment of crossbridges is modified divergently by α- and β-myosin heavy chain

Hypertrophic cardiomyopathy mutations in cardiac troponin T (TnT) lead to sudden cardiac death. Augmented myofilament Ca(2+) sensitivity is a common feature in TnT mutants, but such observations fail to provide a rational explanation for severe cardiac phenotypes. To better understand the mutation-induced effect on the cardiac phenotype, it is imperative to determine the effects on dynamic contractile features such as the muscle length (ML)-mediated activation against α- and β-myosin heavy chain (MHC) isoforms. α- and β-MHC are not only differentially expressed in rodent and human hearts, but they also modify ML-mediated activation differently. Mouse analog of human TnTR94L (TnTR97L) or wild-type TnT was reconstituted into de-membranated muscle fibers from normal (α-MHC) and transgenic (β-MHC) mouse hearts. TnTR97L augmented myofilament Ca(2+) sensitivity by a similar amount in α- and β-MHC fibers. However, TnTR97L augmented the negative impact of strained crossbridges on other crossbridges (γ) by 22% in α-MHC fibers, but attenuated γ by 21% in β-MHC fibers. TnTR97L decreased the magnitude of ML-mediated recruitment of crossbridges (ER) by 37% in α-MHC fibers, but increased ER by 35% in β-MHC fibers. We provide a mechanistic basis for the TnTR97L-induced effects in α- and β-MHC fibers and discuss the relevance to human hearts.