Disopyramide is effective and safe in patients with obstructive hypertrophic cardiomyopathy. However, its cellular and molecular mechanisms of action are unknown. We tested disopyramide in cardiomyocytes from the septum of surgical myectomy patients: disopyramide inhibits multiple ion channels, leading to lower Ca transients and force, and shortens action potentials, thus reducing cellular arrhythmias. The electrophysiological profile of disopyramide explains the efficient reduction of outflow gradients but also the limited prolongation of the QT interval and the absence of arrhythmic side effects observed in 39 disopyramide-treated patients. In conclusion, our results support the idea that disopyramide is safe for outpatient use in obstructive patients.

Rationale:: Despite major advances in cardiovascular medicine, heart disease remains a leading cause of death worldwide. However, the field of tissue engineering has been growing exponentially in the last decade and restoring heart functionality is now an affordable target; yet, new materials are still needed for effectively provide rapid and long-lasting interventions. Liquid crystalline elastomers (LCEs) are biocompatible polymers able to reversibly change shape in response to a given stimulus and generate movement. Once stimulated, LCEs can produce tension or movement like a muscle. However, so far their application in biology was limited by slow response times and a modest possibility to modulate tension levels during activation. Objective:: To develop suitable LCE-based materials to assist cardiac contraction. Methods and Results:: Thanks to a quick, simple, and versatile synthetic approach, a palette of biocompatible acrylate-based light-responsive LCEs with different molecular composition was prepared and mechanically characterized. Out of this, the more compliant one was selected. This material was able to contract for some weeks when activated with very low light intensity within a physiological environment. Its contraction was modulated in terms of light intensity, stimulation frequency, and t _on /t _off ratio to fit different contraction amplitude/time courses, including those of the human heart. Finally, LCE strips were mounted in parallel with cardiac trabeculae, and we demonstrated their ability to improve muscular systolic function, with no impact on diastolic properties. Conclusions:: Our results indicated LCEs are promising in assisting cardiac mechanical function and developing a new generation of contraction assist devices.

Cardiomyocytes from human induced pluripotent stem cells (hiPSC-CMs) are the most promising human source with preserved genetic background of healthy individuals or patients. This study aimed to establish a systematic procedure for exploring development of hiPSC-CM functional output to predict genetic cardiomyopathy outcomes and identify molecular targets for therapy. Biomimetic substrates with microtopography and physiological stiffness can overcome the immaturity of hiPSC-CM function. We have developed a custom-made apparatus for simultaneous optical measurements of hiPSC-CM action potential and calcium transients to correlate these parameters at specific time points (day 60, 75 and 90 post differentiation) and under inotropic interventions. In later-stages, single hiPSC-CMs revealed prolonged action potential duration, increased calcium transient amplitude and shorter duration that closely resembled those of human adult cardiomyocytes from fresh ventricular tissue of patients. Thus, the major contribution of sarcoplasmic reticulum and positive inotropic response to β-adrenergic stimulation are time-dependent events underlying excitation contraction coupling (ECC) maturation of hiPSC-CM; biomimetic substrates can promote calcium-handling regulation towards adult-like kinetics. Simultaneous optical recordings of long-term cultured hiPSC-CMs on biomimetic substrates favor high-throughput electrophysiological analysis aimed at testing (mechanistic hypothesis on) disease progression and pharmacological interventions in patient-derived hiPSC-CMs.

Cardiomyopathy is currently the leading cause of death for patients with Duchenne muscular dystrophy (DMD), a severe neuromuscular disorder affecting young boys. Animal models have provided insight into the mechanisms by which dystrophin protein deficiency causes cardiomyopathy, but there remains a need to develop human models of DMD to validate pathogenic mechanisms and identify therapeutic targets. Here, we have developed human engineered heart tissues (EHTs) from CRISPR-edited, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) expressing a truncated dystrophin protein lacking part of the actin-binding domain. The 3D EHT platform enables direct measurement of contractile force, simultaneous monitoring of Ca²⁺ transients, and assessment of myofibril structure. Dystrophin-mutant EHTs produced less contractile force as well as delayed kinetics of force generation and relaxation, as compared to isogenic controls. Contractile dysfunction was accompanied by reduced sarcomere length, increased resting cytosolic Ca²⁺ levels, delayed Ca²⁺ release and reuptake, and increased beat rate irregularity. Transcriptomic analysis revealed clear differences between dystrophin-deficient and control EHTs, including downregulation of genes related to Ca²⁺ homeostasis and extracellular matrix organization, and upregulation of genes related to regulation of membrane potential, cardiac muscle development, and heart contraction. These findings indicate that the EHT platform provides the cues necessary to expose the clinically-relevant, functional phenotype of force production as well as mechanistic insights into the role of Ca²⁺ handling and transcriptomic dysregulation in dystrophic cardiac function, ultimately providing a powerful platform for further studies in disease modeling and drug discovery.

Full muscle relaxation happens when [Ca²⁺] falls below the threshold for force activation. Several experimental models, from whole muscle organs and intact muscle down to skinned fibers, have been used to explore the cascade of kinetic events leading to mechanical relaxation. The use of single myofibrils together with fast solution switching techniques, has provided new information about the role of cross-bridge (CB) dissociation in the time course of isometric force decay. Myofibril’s relaxation is biphasic starting with a slow seemingly linear phase, with a rate constant, slow k_REL, followed by a fast mono-exponential phase. Sarcomeres remain isometric during the slow force decay that reflects CB detachment under isometric conditions while the final fast relaxation phase begins with a sudden give of few sarcomeres and is then dominated by intersarcomere dynamics. Based on a simple two-state model of the CB cycle, myofibril slow k_REL represents the apparent forward rate with which CBs leave force generating states (g_app) under isometric conditions and correlates with the energy cost of tension generation (ATPase/tension ratio); in short slow k_REL ~ g_app ~ tension cost. The validation of this relationship is obtained by simultaneously measuring maximal isometric force and ATP consumption in skinned myocardial strips that provide an unambiguous determination of the relation between contractile and energetic properties of the sarcomere. Thus, combining kinetic experiments in isolated myofibrils and mechanical and energetic measurements in multicellular cardiac strips, we are able to provide direct evidence for a positive linear correlation between myofibril isometric relaxation kinetics (slow k_REL) and the energy cost of force production both measured in preparations from the same cardiac sample. This correlation remains true among different types of muscles with different ATPase activities and also when CB kinetics are altered by cardiomyopathy-related mutations. Sarcomeric mutations associated to hypertrophic cardiomyopathy (HCM), a primary cardiac disorder caused by mutations in genes encoding sarcomeric proteins, have been often found to accelerate CB turnover rate and increase the energy cost of myocardial contraction. Here we review data showing that faster CB detachment results in a proportional increase in the energetic cost of tension generation in heart samples from both HCM patients and mouse models of the disease.