LongQt: A cardiac electrophysiology simulation platform

ActionPytential: An open source tool for analyzing and visualizing cardiac action potential data

The action potential forms the basis of cardiac pacemaking, conduction, and contraction. Action potentials can be recorded from numerous preparation types, including ventricular or atrial trabecules, Purkinje fibers, isolated cardiac myocytes. Numerous techniques are also available as well, such as the conventional microelectrode and the single-cell current clamp techniques, optical mapping, or in silico modeling. With such a vast array of electrophysiological methods comes an array of available hardware and software solutions. In this work, we present a software with an intuitive graphical user interface, ActionPytential, that enables the analysis of any type of cardiac action potential, regardless of acquisition method or tissue type. In most available software tools, the analysis of continuous (gap-free) recordings often requires manual user interaction to segment the individual action potentials. We provide an automated solution for this, both for slow-response and for externally paced action potentials. As of now, ActionPytential calculates 34 parameters from each action potential. The most often utilized ones, including amplitude, maximal rate of depolarization, and action potential duration values, were validated on 1200 action potentials from human, dog, rabbit, guinea pig, and rat cardiac preparations. We also provide new parameters that were previously only measurable manually, including the position and the depth of the notch in potentials showing a spike-and-dome morphology. Further notable features include a Butterworth-type low-pass filter, the averaging of multiple potentials, automated corrections for baseline drifting, aided manual analysis, high-quality plots, and batch processing for any number of potentials. ActionPytential is available for all major platforms (Windows, MacOS, GNU + Linux, BSD).

Statistical Approach to Incorporating Experimental Variability into a Mathematical Model of the Voltage-Gated Na+ Channel and Human Atrial Action Potential

The voltage-gated Na⁺ channel Na_v1.5 is critical for normal cardiac myocyte excitability. Mathematical models have been widely used to study Na_v1.5 function and link to a range of cardiac arrhythmias. There is growing appreciation for the importance of incorporating physiological heterogeneity observed even in a healthy population into mathematical models of the cardiac action potential. Here, we apply methods from Bayesian statistics to capture the variability in experimental measurements on human atrial Na_v1.5 across experimental protocols and labs. This variability was used to define a physiological distribution for model parameters in a novel model formulation of Na_v1.5, which was then incorporated into an existing human atrial action potential model. Model validation was performed by comparing the simulated distribution of action potential upstroke velocity measurements to experimental measurements from several different sources. Going forward, we hope to apply this approach to other major atrial ion channels to create a comprehensive model of the human atrial AP. We anticipate that such a model will be useful for understanding excitability at the population level, including variable drug response and penetrance of variants linked to inherited cardiac arrhythmia syndromes.

Giant ankyrin-G regulates cardiac function

Cardiovascular disease (CVD) remains the most common cause of adult morbidity and mortality in developed nations. As a result, predisposition for CVD is increasingly important to understand. Ankyrins are intracellular proteins required for the maintenance of membrane domains. Canonical ankyrin-G (AnkG) has been shown to be vital for normal cardiac function, specifically cardiac excitability, via targeting and regulation of the cardiac voltage-gated sodium channel. Noncanonical (giant) AnkG isoforms play a key role in neuronal membrane biogenesis and excitability, with evidence for human neurologic disease when aberrant. However, the role of giant AnkG in cardiovascular tissue has yet to be explored. Here, we identify giant AnkG in the myocardium and identify that it is enriched in 1-week-old mice. Using a new mouse model lacking giant AnkG expression in myocytes, we identify that young mice displayed a dilated cardiomyopathy phenotype with aberrant electrical conduction and enhanced arrhythmogenicity. Structural and electrical dysfunction occurred at 1 week of age, when giant AnkG was highly expressed and did not appreciably change in adulthood until advanced age. At a cellular level, loss of giant AnkG results in delayed and early afterdepolarizations. However, surprisingly, giant AnkG cKO myocytes display normal I_Na, but abnormal myocyte contractility, suggesting unique roles of the large isoform in the heart. Finally, transcript analysis provided evidence for unique pathways that may contribute to the structural and electrical findings shown in giant AnkG cKO animals. In summary, we identify a critical role for giant AnkG that adds to the diversity of ankyrin function in the heart.

Protein Phosphatase 2A Regulates Cardiac Na+ Channels

RATIONALE

Voltage-gated Na⁺ channel ( I_Na) function is critical for normal cardiac excitability. However, the Na⁺ channel late component ( I_Na,L) is directly associated with potentially fatal forms of congenital and acquired human arrhythmia. CaMKII (Ca²⁺/calmodulin-dependent kinase II) enhances I_Na,L in response to increased adrenergic tone. However, the pathways that negatively regulate the CaMKII/Na_v1.5 axis are unknown and essential for the design of new therapies to regulate the pathogenic I_Na,L.

OBJECTIVE

To define phosphatase pathways that regulate I_Na,L in vivo.

METHODS AND RESULTS

A mouse model lacking a key regulatory subunit (B56α) of the PP (protein phosphatase) 2A holoenzyme displayed aberrant action potentials after adrenergic stimulation. Unbiased computational modeling of B56α KO (knockout) mouse myocyte action potentials revealed an unexpected role of PP2A in I_Na,L regulation that was confirmed by direct I_Na,L recordings from B56α KO myocytes. Further, B56α KO myocytes display decreased sensitivity to isoproterenol-induced induction of arrhythmogenic I_Na,L, and reduced CaMKII-dependent phosphorylation of Na_v1.5. At the molecular level, PP2A/B56α complex was found to localize and coimmunoprecipitate with the primary cardiac Na_v channel, Na_v1.5.

CONCLUSIONS

PP2A regulates Na_v1.5 activity in mouse cardiomyocytes. This regulation is critical for pathogenic Na_v1.5 late current and requires PP2A-B56α. Our study supports B56α as a novel target for the treatment of arrhythmia.

Defining new mechanistic roles for αII spectrin in cardiac function

Spectrins are cytoskeletal proteins essential for membrane biogenesis and regulation and serve critical roles in protein targeting and cellular signaling. αII spectrin (SPTAN1) is one of two α spectrin genes and αII spectrin dysfunction is linked to alterations in axon initial segment formation, cortical lamination, and neuronal excitability. Furthermore, human αII spectrin loss-of-function variants cause neurological disease. As global αII spectrin knockout mice are embryonic lethal, the in vivo roles of αII spectrin in adult heart are unknown and untested. Here, based on pronounced alterations in αII spectrin regulation in human heart failure we tested the in vivo roles of αII spectrin in the vertebrate heart. We created a mouse model of cardiomyocyte-selective αII spectrin-deficiency (cKO) and used this model to define the roles of αII spectrin in cardiac function. αII spectrin cKO mice displayed significant structural, cellular, and electrical phenotypes that resulted in accelerated structural remodeling, fibrosis, arrhythmia, and mortality in response to stress. At the molecular level, we demonstrate that αII spectrin plays a nodal role for global cardiac spectrin regulation, as αII spectrin cKO hearts exhibited remodeling of αI spectrin and altered β-spectrin expression and localization. At the cellular level, αII spectrin deficiency resulted in altered expression, targeting, and regulation of cardiac ion channels NaV1.5 and KV4.3. In summary, our findings define critical and unexpected roles for the multifunctional αII spectrin protein in the heart. Furthermore, our work provides a new in vivo animal model to study the roles of αII spectrin in the cardiomyocyte.

Ca2+/calmodulin-dependent kinase II-dependent regulation of atrial myocyte late Na+ current, Ca2+ cycling, and excitability: a mathematical modeling study

Atrial fibrillation (AF) affects more than three million people per year in the United States and is associated with high morbidity and mortality. Both electrical and structural remodeling contribute to AF, but the molecular pathways underlying AF pathogenesis are not well understood. Recently, a role for Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) in the regulation of persistent "late" Na⁺ current ( I_Na,L) has been identified. Although I_Na,L inhibition is emerging as a potential antiarrhythmic strategy in patients with AF, little is known about the mechanism linking I_Na,L to atrial arrhythmogenesis. A computational approach was used to test the hypothesis that increased CaMKII-activated I_Na,L in atrial myocytes disrupts Ca²⁺ homeostasis, promoting arrhythmogenic afterdepolarizations. Dynamic CaMKII activity and regulation of multiple downstream targets [ I_Na,L, L-type Ca²⁺ current, phospholamban, and the ryanodine receptor sarcoplasmic reticulum Ca²⁺-release channel (RyR2)] were incorporated into an existing well-validated computational model of the human atrial action potential. Model simulations showed that constitutive CaMKII-dependent phosphorylation of Na_v1.5 and the subsequent increase in I_Na,L effectively disrupt intracellular atrial myocyte ion homeostasis and CaMKII signaling. Specifically, increased I_Na,L promotes intracellular Ca²⁺ overload via forward-mode Na⁺/Ca²⁺ exchange activity, which greatly increases RyR2 open probability beyond that observed for CaMKII-dependent phosphorylation of RyR2 alone. Increased I_Na,L promotes atrial myocyte repolarization defects (afterdepolarizations and alternans) in the setting of acute β-adrenergic stimulation. We anticipate that our modeling efforts will help identify new mechanisms for atrial Na_V1.5 regulation with direct relevance for human AF. NEW & NOTEWORTHY Here, we present a novel computational model to study the effects of late Na⁺ current ( I_Na,L) in human atrial myocytes. Simulations predict that I_Na,L promotes intracellular accumulation of Ca²⁺, with subsequent dysregulation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) signaling and ryanodine receptor 2-mediated Ca²⁺ release. Although I_Na,L plays a small role in regulating atrial myocyte excitability at baseline, CaMKII-dependent enhancement of the current promoted arrhythmogenic dynamics. Listen to this article's corresponding podcast at http://ajpheart.podbean.com/e/camkii-dependent-regulation-of-atrial-late-sodium-current-and-excitability/ .