Cardiac arrhythmias are a significant cause of morbidity and mortality worldwide, accounting for 10% to 15% of all deaths. Although most arrhythmias are due to acquired heart disease, inherited channelopathies and cardiomyopathies disproportionately affect children and young adults. Arrhythmogenesis is complex, involving anatomic structure, ion channels and regulatory proteins, and the interplay between cells in the conduction system, cardiomyocytes, fibroblasts, and the immune system. Animal models of arrhythmia are powerful tools for studying not only molecular and cellular mechanism of arrhythmogenesis but also more complex mechanisms at the whole heart level, and for testing therapeutic interventions. This review summarizes basic and clinical arrhythmia mechanisms followed by an in-depth review of published animal models of genetic and acquired arrhythmia disorders.

The ACTN2 gene encodes α-actinin 2, located in the Z-disc of the sarcomeres in striated muscle. In this study, we sought to investigate the effects of an ACTN2 missense variant of unknown significance (p.A868T) on cardiac muscle structure and function. Left ventricular free wall samples were obtained at the time of cardiac transplantation from a heart failure patient with the ACTN2 A868T heterozygous variant. This variant is in the EF 3–4 domain known to interact with titin and α-actinin. At the ultrastructural level, ACTN2 A868T cardiac samples presented small structural changes in cardiomyocytes when compared to healthy donor samples. However, contractile mechanics of permeabilized ACTN2 A868T variant cardiac tissue displayed higher myofilament Ca²⁺ sensitivity of isometric force, reduced sinusoidal stiffness, and faster rates of tension redevelopment at all Ca²⁺ levels. Small-angle X-ray diffraction indicated increased separation between thick and thin filaments, possibly contributing to changes in muscle kinetics. Molecular dynamics simulations indicated that while the mutation does not significantly impact the structure of α-actinin on its own, it likely alters the conformation associated with titin binding. Our results can be explained by two Z-disc mediated communication pathways: one pathway that involves α-actinin’s interaction with actin, affecting thin filament regulation, and the other pathway that involves α-actinin’s interaction with titin, affecting thick filament activation. This work establishes the role of α-actinin 2 in modulating cross-bridge kinetics and force development in the human myocardium as well as how it can be involved in the development of cardiac disease.

Loss-of-function (LOF) variants in the K_V11.1 potassium channel cause long QT syndrome (LQTS). Most variants disrupt intracellular channel transport (trafficking) to the cell membrane. Since some channel inhibitors improve trafficking of K_V11.1 variants, a high-throughput screening (HTS) assay to detect trafficking enhancement would be valuable to the identification of drug candidates. The thallium (Tl⁺) flux assay technique, widely used for drug screening, was optimized using human embryonic kidney (HEK-293) cells expressing a trafficking-deficient K_V11.1 variant in 384-well plates. Assay quality was assessed using Z prime (Z’) scores comparing vehicle to E-4031, a drug that increases K_V11.1 membrane trafficking. The optimized assay was validated by immunoblot, electrophysiology experiments, and a pilot drug screen. The combination of: 1) truncating the trafficking-deficient variant K_V11.1-G601S (K_V11.1-G601S-G965*X) with the addition of 2) K_V11.1 channel activator (VU0405601) and 3) cesium (Cs⁺) to the Tl⁺ flux assay buffer resulted in an outstanding Z’ of 0.83. To validate the optimized trafficking assay, we carried out a pilot screen that identified three drugs (ibutilide, azaperone, and azelastine) that increase K_V11.1 trafficking. The new assay exhibited 100% sensitivity and specificity. Immunoblot and voltage-clamp experiments confirmed that all three drugs identified by the new assay improved membrane trafficking of two additional LQTS K_V11.1 variants. We report two new ways to increase target-specific activity in trafficking assays—genetic modification and channel activation—that yielded a novel HTS assay for identifying drugs that improve membrane expression of pathogenic K_V11.1 variants. SIGNIFICANCE STATEMENT This manuscript reports the development of a high-throughput assay (thallium flux) to identify drugs that can increase function in K_V11.1 variants that are trafficking-deficient. Two key aspects that improved the resolving power of the assay and could be transferable to other ion channel trafficking–related assays include genetic modification and channel activation.

Allograft rejection following solid-organ transplantation is a major cause of graft dysfunction and mortality. Current diagnostic approaches rely on histology, which exhibits wide diagnostic variability and lacks clinically relevant molecular phenotyping. Here we leverage image-based spatial transcriptomics at subcellular resolution in longitudinal human cardiac biopsies to characterize transcriptional heterogeneity in 62 adult and pediatric heart transplant recipients during and following histologically diagnosed rejection. Across 28 cell types, we identified significant differences in abundance in immune and parenchymal cells across different classes of rejection. We observed broad overlap in transcriptional states across rejection severity and significant heterogeneity within rejection grades. Responders and nonresponders to augmented therapies had distinct transcriptomic profiles, with nonresponders exhibiting baseline T cell hyperactivation and tissue remodeling genes. We also identified cell-specific genes linked to long-term outcomes after heart transplant. These results underscore the importance of subtyping cellular states during rejection to stratify immune–cardiac interactions relevant to short- and long-term outcomes.

Background: Ventricular tachycardia (VT) is responsible for sudden death in chronic ischemic heart disease (CIHD) patients. The cardiac ryanodine receptor (RyR2) releases Ca2+ from the sarcoplasmic reticulum (SR) and links electrical excitation to contraction. RyR2 hyperactivity has been widely documented in CIHD and may contribute to VT risk and progressive LV remodeling. Objective: To test the hypothesis that targeting RyR2 hyperactivity plays a mechanistic role in VT inducibility and progressive heart failure in CIHD that can be prevented by the RyR2 inhibitor dantrolene. Methods: CIHD was induced in C57BL/6J mice by left coronary artery ligation. Four weeks later, mice were randomized to either acute or chronic (6 weeks via osmotic mini-pump) treatment with dantrolene or vehicle. VT inducibility was assessed by programmed stimulation in vivo and in isolated hearts. Electrical substrate remodeling was assessed by optical mapping. Ca2+ sparks and spontaneous Ca2+ releases were measured in isolated cardiomyocytes. Cardiac remodeling was assessed by histology and qRT-PCR. Cardiac function and contractility were assessed by echocardiography. Results: Compared to vehicle, acute dantrolene treatment reduced VT inducibility and improved LV contractility in vivo . Optical mapping in isolated hearts demonstrated reentrant VT prevention by dantrolene, which normalized the shortened refractory period (VERP) and prolonged action potential duration (APD), preventing APD alternans. In single CIHD cardiomyocytes, dantrolene normalized RyR2 hyperactivity and prevented spontaneous SR Ca ²⁺ release. Chronic dantrolene treatment reduced peripheral muscle strength but had no adverse effects on body weight or mortality. Chronic dantrolene not only reduced VT inducibility but also reduced peri-infarct fibrosis and prevented the progression of LV dysfunction in CIHD mice. Conclusion: RyR2 hyperactivity plays a mechanistic role for VT risk, infarct remodeling, and contractile dysfunction in CIHD mice. Our data provide proof of concept for the anti-arrhythmic and anti-fibrotic efficacy of dantrolene in CIHD. Clinical Perspective: What is New?: The mouse CIHD model is a more clinically relevant model in which treatment is started late after infarction, when heart failure is already established. Acute and chronic dantrolene treatment suppresses VT inducibility by restoring myocyte APD, terminating APD alternans and normalizing VERP. Chronic dantrolene treatment prevents pathological remodeling and peri-infarct fibrosis, the substrate for reentry VT. Cardiac function is improved with chronic dantrolene therapy. Clinical Implications: Treatment with dantrolene, which is already approved for clinical use, is a promising therapy in patients with ischemic heart disease, in whom other antiarrhythmic drugs are contraindicated. Dantrolene inhibition of RyR2 not only suppresses VT but also improves cardiac function in chronic ischemic heart disease.