A major challenge in computational models of cardiac electromechanics is the reconstruction of myocardial fiber architecture, as direct in vivo measurements of fiber orientation are not feasible. Consequently, rule-based methods are commonly adopted as surrogates. This study investigates the respective roles of macroscopic fiber architecture and microscopic fiber disarray in cardiac electromechanical simulations. A high-fidelity biventricular electromechanical model of a murine heart was developed using a high-resolution myocardial fiber field obtained via mesoscopic optical imaging, which serves as a reference ground truth. A spatial smoothing strategy is introduced to decouple macroscopic fiber organization from local disarray, and the resulting responses are also compared with those obtained using a rule-based fiber field. The results show that passive mechanics and electrophysiological activation are only weakly affected by fiber disarray, with global chamber compliance and activation times remaining largely unchanged across different fiber descriptions. In contrast, active mechanics is highly sensitive to fiber architecture. Moderate regularization of the experimentally measured fiber field enhances the ventricular pumping efficiency of the computational model by reducing microscopic disarray while preserving the macroscopic helical organization, whereas excessive smoothing or rule-based fiber reconstructions lead to unphysiologically strong or inefficient contraction. Within this framework, two commonly adopted surrogate strategies to account for fiber disarray are investigated: a reduction of the effective cross-bridge stiffness in the active tension model, and the introduction of controlled misalignment between active tension and the local fiber direction. Overall, the results reveal important limitations of commonly adopted surrogate approaches for modeling fiber disarray.

Background Mavacamten, a first-in-class allosteric myosin inhibitor, has demonstrated efficacy and safety in obstructive hypertrophic cardiomyopathy (oHCM), notably reducing symptoms, left ventricular outflow obstruction, and wall thickness over 30 weeks. We recently reported that the MYBPC3 :c.772G>A variant causes HCM through cMyBP-C haploinsufficiency, leading to accelerated sarcomere kinetics and higher energy consumption in patient myocardium and hiPSC- derived cardiomyocytes (hiPSC-CMs). These effects are counterbalanced by prolonged action potentials and slower Ca²⁺ transients, which preserve twitch duration but may increase arrhythmic risk. Mavacamten may reduce myocardial energetic defects in HCM. Objectives To investigate the long-term effects of Mavacamten on sarcomere structure, contractility, and transcriptional remodeling using patient-specific and CRISPR-corrected isogenic hiPSC-derived cardiomyocyte models of HCM. Methods HiPSC-CMs and engineered heart tissues (EHTs) derived from a MYBPC3:c.772G>A patient and its CRISPR-corrected line were first exposed to increasing concentrations of Mavacamten to assess acute dose–response relationships and determine IC50 values. Based on these data, chronic treatments (0.3– 0.75 μM for 20 days) were performed mechanical, structural, electrophysiological, and transcriptomic adaptations. Results Acute exposure produced a rapid and fully reversible reduction in active force, while chronic treatment for 20 days induced a sustained decrease in contractility with incomplete recovery after 4 days of washout, indicating a two-phase mechanism of action. Long-term force reduction was paralleled by decreased cell area and sarcomere density, indicating that structural disassembly contributes to sustained functional depression and re-assembly after washout. Electrophysiological analysis confirmed the specific alterations of the MYBPC3 :c.772G>A mutation previously observed, with no detectable effects following treatment with Mavacamten. In addition, transcriptome analysis was used to study the molecular mechanisms underlying the long-term effect. Conclusions Mavacamten induces a biphasic, persistent-to-reversible, reduction of sarcomeric force associated with structural remodeling, providing mechanistic insight into its capacity to promote favorable cardiac remodeling in oHCM. ### Competing Interest Statement Iacopo Olivotto has received advisory board fees/research grants from Bristol Myers Squibb, Cytokinetics, Sanofi Chiesi, Genzyme, Amicus, Bayer, Tenaya, Rocket Pharma and Edgewise and Lexeo. Sonette Steczina was a former graduate student at the University of Washington at the time of this work and is currently an employee at Cytokinetics HORIZON-HLTH-2023-TOOL-05, 101137115 European Union's Horizon 2020 research and innovation programme, 777204

This study explores the use of polarized second-harmonic generation (pSHG) to investigate myosin conformation in the relaxed state, differentiating between the actin-available, disordered (ON) state and the energy-conserving, ordered (OFF) state. By shifting the ON/OFF equilibrium using both physical and chemical manipulations, we demonstrate the sensitivity of pSHG in quantifying the ON/OFF ratio in skeletal and cardiac tissues. Comparisons with X-ray diffraction measurements further validate our findings. Applying this approach to a sarcomeric mutation associated with hypertrophic cardiomyopathy, we show that R403Q/MYH7-mutated minipig ventricle tissue exhibits a higher ON fraction compared to controls. This difference is abolished under high concentrations of a myosin activator (2-deoxyATP) and an inhibitor (Mavacamten), indicating structural similarity between R403Q and controls in these two states. ATPase assays reveal increased resting ATPase activity in R403Q samples, which persists even in the presence of 2-deoxyATP, suggesting that the elevated energy consumption in the R403Q mutation is driven by both a population shift toward the ON state and enhanced myosin ATPase activity per motor head.

The heart relies on finely tuned spatial heterogeneity in wall structure, fibre orientation, blood distribution and excitation-contraction coupling to maintain mechanical homeostasis. Even minor deviations may interact with anisotropy to promote maladaptation and disease. While cardiomyopathies have classically been attributed to primary cellular defects, emerging evidence suggests that abnormal regional stress may act as a primary driver of pathology even in the absence of intrinsic sarcomeric dysfunction. This review proposes the concept of 'cardiac mechanopathies', conditions in which misdirected or disproportionate mechanical forces are hypothesised to act as primary determinants of maladaptive remodelling, culminating in a cardiomyopathic phenotype. Clinical paradigms include arrhythmogenic mitral valve prolapse and apical hypertrophic cardiomyopathy with papillary muscle displacement. While a direct causality link still needs to be established, this concept is conceived as a working hypothesis for future studies. Advanced imaging may help identify subtle structural abnormalities early, while insights into the molecular basis of maladaptation may reveal new therapeutic targets. Clinically, studies are needed to understand whether reducing mechanical stress through interventions and lifestyle modifications can mitigate disease expression and progression.

Among smart materials, Liquid Crystalline Elastomers (LCEs) combine programmable well-defined deformations with wireless control. To date, the successful fabrication of LCEs through 3D printing techniques, such as direct ink writing (DIW), requires precise control over the ink formulation, mesogen alignment, and curing processes, to get devices with uniform molecular orientations, and with optimized actuation performance. Here, we present a simple synthetic approach leading to a ten-of-gram-scale ink production suitable for low-cost DIW 3D printing of LCEs. The novel ink, containing a push-pull azobenzene directly linked to the polymer backbone, enabled 4D printing of fast responsive photo-mechanical actuators with programmable and reversible deformation. Our centimeter-scale LCE structures present active tensions twitches comparable to those of cardiac muscles, both in terms of magnitude (kPa range) and timescale (tens to hundreds of milliseconds). An all-round actuation characterization is also developed and reported. As a proof-of-concept demonstrator, an optical beam steerer was developed demonstrating a high control of the beam diffraction angle as a function of the control beam light power.

Duchenne muscular dystrophy (DMD) is a X-linked disease affecting skeletal and cardiac muscle and is caused by mutations in the dystrophin gene (DMD). Patient-derived induced pluripotent stem cells (iPSCs) serve as reliable in vitro disease models. Their genetic correction by CRISPR/Cas9 allows the generation of isogenic controls and holds promises for gene therapy. However, restoring full-length dystrophin, especially when deletions involve multiple exons, constitutes a technological challenge. This study aimed to fully repair the dystrophin gene from a DMD iPSC line carrying the deletion of exons 49–50 and to characterize the rescue of the cardiac phenotype. We developed an innovative CRISPR/Cas9-based approach involving the insertion of coding sequences of the deleted region, at the 3’ of exon 48, thereby generating a single continuous coding sequence encompassing exons 48-49-50. Subsequently, iPSCs were differentiated into cardiomyocytes and cardiac fibroblasts. Cardiac phenotypes were analysed by western blot, immunofluorescence, ELISA, FACS, Ionoptix, 3D engineered heart tissue (EHT) and single-nuclei RNA-seq. The correction of a two-exons DMD gene deletion in Duchenne iPSCs, using CRISPR/Cas9, enabled the re-expression of a stable and functional full-length dystrophin in cardiomyocytes resulting in the rescue of cardiac pathological phenotypes. Edited cardiomyocytes showed improved morphology, reduced release of the cardiomyocytes damage marker troponin I, and decreased ROS production. Moreover, dystrophin restoration enhanced contractility and ameliorated the Ca2+ kinetics. Notably, edited iPSC derived fibroblasts showed reduced pro-fibrotic stimuli response. In parallel, we also observed enhanced functioning of a 3D engineered heart tissue and profound change in the transcriptomic profile in both cardiomyocytes and fibroblasts after the re-expression of full-length dystrophin. We developed an innovative approach that enabled the re-expression of full-length dystrophin in a DMD iPSC line with consequent complete rescue of in vitro DMD cardiac phenotypes. On the long term, these results could lay a foundation for future applications of cell therapy or in vivo CRISPR/Cas9-based intervention to treat DMD.

The left ventricular end-systolic pressure-volume relationship (ESPVr) is a key indicator of cardiac contractility. Despite its established importance, several studies suggested that the mechanical mode of contraction, such as isovolumetric or ejecting contractions, may affect the ESPVr, challenging the traditional notion of a single, consistent relationship. Furthermore, it remains unclear whether the observed effects of ejection on force generation are inherent to the ventricular chamber itself or are a fundamental property of the myocardial tissue, with the underlying mechanisms remaining poorly understood. We investigated these aspects by using a multiscale in silico model that allowed us to elucidate the links between subcellular mechanisms and organ-level function. Simulations of ejecting and isovolumetric beats with different preload and afterload resistance were performed by modulating calcium and cross-bridge kinetics. The results suggest that the ESPVr is not a fixed curve but depends on the mechanical history of the contraction, with potentially both positive and negative effects of ejection. Isolated tissue simulations suggest that these phenomena are intrinsic to the myocardial tissue, rather than properties of the ventricular chamber. Our results suggest that the ESPVr results from the balance of positive and negative effects of ejection, respectively related to a memory effect of the increased apparent calcium sensitivity at high sarcomere length, and to the inverse relationship between force and velocity. Numerical simulations allowed us to reconcile conflicting results in the literature and suggest translational implications for clinical conditions such as hypertrophic cardiomyopathy, where altered calcium dynamics and cross-bridge kinetics may impact the ESPVr.

Obscurin is a large muscle protein whose multiple functions include providing mechanical strength to the M-band and linking the sarcomere to the sarcoplasmic reticulum. Mutations in obscurin are linked to various forms of muscle diseases. This study compares cardiac function in a murine model of obscurin deletion (KO) with wild-type (WT) in vivo and ex vivo. Echocardiography showed that KO hearts had larger (+20%) end-diastolic and end-systolic volumes, reduced fractional shortening, and impaired ejection fraction, consistent with dilated cardiomyopathy. However, stroke volume and cardiac output were preserved due to increased end-diastolic volume. Morphological analyses revealed reduced sarcoplasmic reticulum volume, with preserved T-tubule network. While myofilament function was preserved in isolated myofibrils and skinned trabeculae, experiments in intact trabeculae revealed that Obscn KO hearts compared with WT displayed (1) reduced active tension at high frequencies and during resting-state contractions, (2) impaired positive inotropic and lusitropic response to β-adrenergic stimulation (isoproterenol 0.1 μM), and (3) faster mechanical restitution, suggesting reduced sarcoplasmic reticulum refractoriness. Intracellular [Ca2+]i measurements showed reduced peak systolic and increased diastolic levels in KO versus WT cardiomyocytes. Western blot experiments revealed lower SERCA and phospholamban (PLB) expression and reduced PLB phosphorylation in KO mice. While action potential parameters and conduction velocity were unchanged, β-adrenergic stimulation induced more frequent spontaneous Ca2+ waves and increased arrhythmia susceptibility in KO compared with WT. Taken together, these findings suggest that obscurin deletion, in adult mice, is linked to compensated dilated cardiomyopathy, altered E-C coupling, impaired response to inotropic agents, and increased propensity to arrhythmias.

The conventional relaxed (DRX) and the super-relaxed (SRX) states are widely assumed to correspond to a structure where myosin heads are in an open (free to interact with actin) or closed (binding to actin inhibited) configuration. Based on biochemical measurements, an increased DRX to SRX ratio has previously been linked to human hypertrophic cardiomyopathy (HCM) associated with sarcomeric mutations. In this condition, during the diastolic period, more heads can enter the force-generating phase leading to residual interactions with detrimental consequences for heart relaxation and overall energetical balance.

ABSTRACTT-tubules (TT) form a complex network of sarcolemmal membrane invaginations, essential for well-coordinated excitation-contraction coupling (ECC) and, thus, homogeneous mechanical activation of cardiomyocytes. ECC is initiated by rapid depolarization of the sarcolemmal membrane. Whether TT membrane depolarisation is active (local generation of action potentials; AP) or passive (following depolarisation of the outer cell surface sarcolemma; SS) has not been experimentally assessed in cardiomyocytes. Based on the assessment of ion flux pathways needed for AP generation, we hypothesise that TT are excitable. We therefore explored TT excitability experimentally, using an all-optical approach to stimulate and record trans-membrane potential changes in TT that were electrically insulated from the SS membrane by transient osmotic shock. Our results establish that cardiomyocyte TT can generate AP. These AP show electrical features that differ substantially from those observed in SS, consistent with differences in the density of ion channels and transporters in the two different membrane domains. We propose that TT-generated AP represent a safety mechanism for TT AP propagation and ECC, which may be particularly relevant in pathophysiological settings where morpho-functional changes reduce the electrical connectivity between SS and TT membranes.KEY POINTSCardiomyocytes are characterized by a complex network of membrane invaginations (the T-tubular system) that propagate action potentials to the core of the cell, ensuring synchronous and uniform cell contraction.In this study, we investigate whether the T-tubular system is able to generate action potentials autonomously, rather than following depolarization of the outer cell surface sarcolemma.For this purpose, we developed a fully optical platform to probe and manipulate the electrical dynamics of sub-cellular membrane domains.Our findings demonstrate that T-tubules are intrinsically excitable, revealing distinct characteristics of self-generated T-tubular action potentials.This active electrical capability may serve as a protective mechanism against voltage drops occurring within the T-tubular network.

Left-ventricular hypertrophy (LVH) is an adaptive condition to hemodynamic stress often involving the left ventricle (LV). This pathological condition is associated with clinical complications such as ventricular arrhythmias and diastolic dysfunction, the molecular and cellular mechanisms of which have been poorly investigated in the human heart. We collected myocardial samples from the upper interventricular septum of 132 patients with hypertrophic cardiomyopathy (HCM), 42 patients with aortic stenosis and severe LVH (AoS-LVH) and 12 non-failing non-hypertrophic patients with valve disease (NF-NH), who underwent myectomy operations at our cardiac surgery center. Samples were used to isolate single viable cardiomyocytes from the left ventricle to perform patch-clamp electrophysiological studies and intracellular calcium measurements with fluorescent dyes. Intact trabeculae were also dissected to perform isometric force measurements of electrically-stimulated twitches. Single-cell patch-clamp studies revealed a marked prolongation of action potential duration (APD) in HCM (N=82, mean APD at 90% repolarization=763±252ms at 0.5Hz) and AoS-LVH (N=22, APD90%=579±147ms) samples with respect to NF-NH (N=12, APD90%=447±88ms). In both HCM and AoS-LVH cardiomyocytes, APD prolongation was associated with increased late-Na + current with respect to NF-NH, while L-type Ca 2+ current was enlarged only in HCM samples. Ca 2+ -fluorescence studies revealed markedly slower Ca-transient (CaT) kinetics in myocardial samples from HCM (N=38, mean CaT 50% decay time at 0.5 Hz = 658±179ms) and AoS-LVH patients (N=9, CaT50%= 568±187ms), when compared with NF-NH samples (N=8, CaT50%=283±59ms), paralleled by elevated diastolic [Ca 2+ ]. Twitch force measurements in intact trabeculae revealed prolonged isometric contractions in HCM (N=78, overall twitch duration at 0.5Hz= 730±147ms) and in AoS-LVH patient-samples (N=24, TwD=669±116ms), as compared with NF-NH (N=7, TwD=511±73ms). Force-frequency relationship was flat in pathological samples, while NF-NH trabeculae showed a clear increase in twitch amplitude while increasing pacing rate to 2Hz. Our results suggest that the main features of functional cardiomyocyte remodeling (that is, changes in the expression and/or function of ion-channel and EC-coupling proteins) are qualitatively similar in primary vs. secondary LVH, albeit abnormalities are quantitatively more extensive in HCM samples.

Omecamtiv mecarbil (OM) is a small molecule that has been shown to improve the function of the slow human ventricular myosin (MyHC) motor through a complex perturbation of the thin/thick filament regulatory state of the sarcomere mediated by binding to myosin allosteric sites coupled to inorganic phosphate (Pi) release. Here, myofibrils from samples of human left ventricle (β-slow MyHC-7) and left atrium (α-fast MyHC-6) from healthy donors were used to study the differential effects of μmolar [OM] on isometric force in relaxing conditions (pCa 9.0) and at maximal (pCa 4.5) or half-maximal (pCa 5.75) calcium activation, both under control conditions (15 °C; equimolar DMSO; contaminant inorganic phosphate [Pi] ~170 μM) and in the presence of 5 mM [Pi]. The activation state and OM concentration within the contractile lattice were rapidly altered by fast solution switching, demonstrating that the effect of OM was rapid and fully reversible with dose-dependent and myosin isoform-dependent features. In MyHC-7 ventricular myofibrils, OM increased submaximal and maximal Ca2+-activated isometric force with a complex dose-dependent effect peaking (40% increase) at 0.5 μM, whereas in MyHC-6 atrial myofibrils, it had no effect or—at concentrations above 5 µM—decreased the maximum Ca2+-activated force. In both ventricular and atrial myofibrils, OM strongly depressed the kinetics of force development and relaxation up to 90% at 10 μM [OM] and reduced the inhibition of force by inorganic phosphate. Interestingly, in the ventricle, but not in the atrium, OM induced a large dose-dependent Ca2+-independent force development and an increase in basal ATPase that were abolished by the presence of millimolar inorganic phosphate, consistent with the hypothesis that the widely reported Ca2+-sensitising effect of OM may be coupled to a change in the state of the thick filaments that resembles the on–off regulation of thin filaments by Ca2+. The complexity of this scenario may help to understand the disappointing results of clinical trials testing OM as inotropic support in systolic heart failure compared with currently available inotropic drugs that alter the calcium signalling cascade.

Excel file containing all raw data, Origin Lab files for each independent panel, and raw images.Data are related to:Figure 1: Patch-clamp recordings on CTRL and DETUB cells.Figure 2: In silico prediction of TT membrane excitability.Figure 3: All-optical AP recording in TT und SS membrane sites of CTRL cardiomyocytes. Figure 4: All-optical AP recording in TT und SS membrane sites of DETUB cardiomyocytes.Figure 5: Loss of excitability of detached t-tubules.

BACKGROUND:The pathogenesis of MYBPC3-associated hypertrophic cardiomyopathy (HCM) is still unresolved. In our HCM patient cohort, a large and well-characterized population carrying the MYBPC3:c772G>A variant (p.Glu258Lys, E258K) provides the unique opportunity to study the basic mechanisms of MYBPC3-HCM with a comprehensive translational approach.METHODS:We collected clinical and genetic data from 93 HCM patients carrying the MYBPC3:c772G>A variant. Functional perturbations were investigated using different biophysical techniques in left ventricular samples from 4 patients who underwent myectomy for refractory outflow obstruction, compared with samples from non-failing non-hypertrophic surgical patients and healthy donors. Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and engineered heart tissues (EHTs) were also investigated.RESULTS:Haplotype analysis revealed MYBPC3:c772G>A as a founder mutation in Tuscany. In ventricular myocardium, the mutation leads to reduced cMyBP-C (cardiac myosin binding protein-C) expression, supporting haploinsufficiency as the main primary disease mechanism. Mechanical studies in single myofibrils and permeabilized muscle strips highlighted faster cross-bridge cycling, and higher energy cost of tension generation. A novel approach based on tissue clearing and advanced optical microscopy supported the idea that the sarcomere energetics dysfunction is intrinsically related with the reduction in cMyBP-C. Studies in single cardiomyocytes (native and hiPSC-derived), intact trabeculae and hiPSC-EHTs revealed prolonged action potentials, slower Ca2+ transients and preserved twitch duration, suggesting that the slower excitation-contraction coupling counterbalanced the faster sarcomere kinetics. This conclusion was strengthened by in silico simulations.CONCLUSIONS:HCM-related MYBPC3:c772G>A mutation invariably impairs sarcomere energetics and cross-bridge cycling. Compensatory electrophysiological changes (eg, reduced potassium channel expression) appear to preserve twitch contraction parameters, but may expose patients to greater arrhythmic propensity and disease progression. Therapeutic approaches correcting the primary sarcomeric defects may prevent secondary cardiomyocyte remodeling.