A gradient of force generation at rest differentiates cardiomyopathy outcomes with variants of actin located at the same residue

Distinct mechanisms drive divergent phenotypes in hypertrophic and dilated cardiomyopathy-associated TPM1 variants

Heritable forms of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) represent starkly diverging clinical phenotypes, yet may be caused by mutations to the same sarcomeric protein. The precise mechanisms by which point mutations within the same gene bring about phenotypic diversity remain unclear. Our objective was to develop a mechanistic explanation of diverging phenotypes in two TPM1 mutations, E62Q (HCM) and E54K (DCM). Drawing on data from the literature and experiments with stem cell-derived cardiomyocytes expressing the TPM1 mutations of interest, we constructed computational simulations that provide plausible explanations of the distinct muscle contractility caused by each variant. In E62Q, increased calcium sensitivity and hypercontractility was explained most accurately by a reduction in effective molecular stiffness of tropomyosin and alterations in its interactions with the actin thin filament that favor the "closed" regulatory state. By contrast, the E54K mutation appeared to act via long-range allosteric interactions to increase the association rate of the C-terminal troponin I mobile domain to tropomyosin/actin. These mutation-linked molecular events produced diverging alterations in gene expression that can be observed in human engineered heart tissues. Modulators of myosin activity confirmed our proposed mechanisms by rescuing normal contractile behavior in accordance with predictions.

Duality in disease: How two amino acid substitutions at actin residue 312 result in opposing forms of cardiomyopathy

Two common types of cardiovascular disease are hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) which occur from changes to sarcomere contractile mechanisms and activity. Actin amino acid substitutions R312C and R312H have been found in HCM and DCM patients, respectively. Previously, we observed that R312C/H variants display both hyperactivity and hypoactivity in vitro, contradicting traditional characterizations of HCM- and DCM-causing variants. Here, we further characterized R312C/H actin variants in vitro and conducted in silico modeling to better understand the mechanisms differentiating HCM and DCM. Our results suggest that R312C/H substitutions cause structural changes that differentially impact actomyosin activity. A gradient of altered interactions with regulatory proteins troponin, tropomyosin, and the C0C2 domains of myosin-binding protein C was also observed, influencing the accessibility of active and inhibitory conformations of these proteins. The results presented here support our previous suggestion of a gradient of factors that differentiate between HCM and DCM. Further characterization of HCM- and DCM-causing actin variants using in vitro and in silico methods is required for better understanding cardiomyopathy and improving clinical outcomes.

Glutamate 139 of tropomyosin is critical for cardiac thin filament blocked-state stabilization

The cardiac thin filament proteins troponin and tropomyosin control actomyosin formation and thus cardiac contractility. Calcium binding to troponin changes tropomyosin position along the thin filament, allowing myosin head binding to actin required for heart muscle contraction. The thin filament regulatory proteins are hot spots for genetic mutations causing heart muscle dysfunction. While much of the thin filament structure has been characterized, critical regions of troponin and tropomyosin involved in triggering conformational changes remain unresolved. A poorly resolved region, helix-4 (H4) of troponin I, is thought to stabilize tropomyosin in a position on actin that blocks actomyosin interactions at low calcium concentrations during muscle relaxation. We have proposed that contact between glutamate 139 on tropomyosin and positively charged residues on H4 leads to blocking-state stabilization. In this study, we attempted to disrupt these interactions by replacing E139 with lysine (E139K) to define the importance of this residue in thin filament regulation. Comparison of mutant and wild-type tropomyosin was carried out using in-vitro motility assays, actin co-sedimentation, and molecular dynamics simulations to determine perturbations in troponin-tropomyosin function caused by the tropomyosin mutation. Motility assays revealed that mutant thin filaments moved at higher velocity at low calcium with increased calcium sensitivity demonstrating that tropomyosin residue 139 is vital for proper tropomyosin-mediated inhibition during relaxation. Similarly, molecular dynamic simulations revealed a mutation-induced decrease in interaction energy between tropomyosin-E139K and troponin I (R170 and K174). These results suggest that salt-bridge stabilization of tropomyosin position by troponin IH4 is essential to prevent actomyosin interactions during cardiac muscle relaxation.