PITX2 Knockout Induces Key Findings of Electrical Remodeling as Seen in Persistent Atrial Fibrillation

Calcium Homeostatic Feedback Control Predicts Atrial Fibrillation Initiation, Remodeling, and Progression

BACKGROUND

Atrial fibrillation (AF) is a progressive disorder, with arrhythmia episodes becoming increasingly longer and ultimately permanent. The chaotic electrical activity by itself is well known to drive progression, a process classically summarized as "AF begets AF." However, the mechanisms underlying this progression are not yet well defined.

OBJECTIVES

We hypothesize that calcium homeostatic feedback regulating ion channel expression is a critical mechanistic component of this pathological process.

METHODS

We propose a modeling framework that tracks both short-term beat-to-beat electrical and calcium activity and long-term tissue substrate remodeling as a single coupled dynamical system. Importantly, the full AF progression from healthy to pathological remodeled tissue is reproduced, in contrast with prior studies that consider "snapshots" of various AF stages.

RESULTS

Simulations predict that single cells respond to fast pacing by maintaining intracellular calcium concentrations through dynamic ion channel expression and electrical phenotype changes. In 2-dimensional homogeneous tissue, spontaneous spiral waves stabilize into permanent re-entry. In 2-dimensional heterogeneous tissue, we observe the initiation of re-entrant activity in response to fast pacing, followed by increasingly longer intermittent, and then permanent, arrhythmic activity. Simulations predict critical properties of re-entrant wave locations, leading to a novel hypothesis: spiral wave activity itself drives underlying substrate remodeling and the emergence of remodeled tissue "niches" that support the stabilization of fast re-entrant activity.

CONCLUSIONS

Thus, the model joins multiple lines of inquiry (ie, long-term calcium regulation, ion channel coexpression and remodeling, and tissue-scale arrhythmia spatiotemporal organization) into a single coherent framework, and for the first time, captures the dynamics of the long-term natural history of AF.

p21-Activated Kinase 1 (Pak1) as an Element in Functional and Dysfunctional Interplay Among the Myocardium, Adipose Tissue, and Pancreatic Beta Cells

This review focuses on p21-activated kinase 1 (Pak1), a multifunctional, highly conserved enzyme that regulates multiple downstream effectors present in many tissues. Upstream signaling via Ras-related small G-proteins, Cdc42/Rac1 promotes the activity of Pak1. Our hypothesis is that this signaling cascade is an important element in communication among the myocardium, adipose tissue, and pancreatic β-cells. Evidence indicates that a shared property of these tissues is that structure/function stability requires homeostatic Pak1 activity. Increases or decreases in Pak1 activity may promote dysfunction or increase susceptibility to stressors. Evidence that increased levels of Pak1 activity may be protective provides support for efforts to develop therapeutic approaches activating Pak1 with potential use in prevalent disorders associated with obesity, diabetes, and myocardial dysfunction. On the other hand, since increased Pak1 activity is associated with cancer progression, there has been a significant effort to develop Pak1 inhibitors. These opposing therapeutic approaches highlight the need for a deep understanding of Pak1 signaling in relation to the development of effective and selective therapies with minimal or absent off-target effects.

PITX2 deficiency leads to atrial mitochondrial dysfunction

AIMS

Reduced left atrial PITX2 is associated with atrial cardiomyopathy and atrial fibrillation (AF). PITX2 is restricted to left atrial cardiomyocytes (aCMs) in the adult heart. The links between PITX2 deficiency, atrial cardiomyopathy, and AF are not fully understood.

METHODS AND RESULTS

To identify mechanisms linking PITX2 deficiency to AF, we generated and characterized PITX2-deficient human aCMs derived from human induced pluripotent stem cells (hiPSC) and their controls. PITX2-deficient hiPSC-derived atrial cardiomyocytes showed shorter and disorganized sarcomeres and increased mononucleation. Electron microscopy found an increased number of smaller mitochondria compared with isogenic controls. Mitochondrial protein expression was altered in PITX2-deficient hiPSC-derived atrial cardiomyocytes. Single-nuclear RNA-sequencing found differences in cellular respiration pathways and differentially expressed mitochondrial and ion channel genes in PITX2-deficient hiPSC-derived atrial cardiomyocytes. PITX2 repression in hiPSC-derived atrial cardiomyocytes replicated dysregulation of cellular respiration. Mitochondrial respiration was shifted to increased glycolysis in PITX2-deficient hiPSC-derived atrial cardiomyocytes. PITX2-deficient human hiPSC-derived atrial cardiomyocytes showed higher spontaneous beating rates. Action potential duration was more variable with an overall prolongation of early repolarization, consistent with metabolic defects. Gene expression analyses confirmed changes in mitochondrial genes in left atria from 42 patients with AF compared with 43 patients with sinus rhythm. Dysregulation of left atrial mitochondrial (COX7C) and metabolic (FOXO1) genes was associated with PITX2 expression in human left atria.

CONCLUSION

PITX2 deficiency causes atrial mitochondrial dysfunction and a metabolic shift to glycolysis in human aCMs. PITX2-dependent metabolic changes can contribute to the structural and functional defects found in PITX2-deficient atria.

Atrial hiPSC-CM as a Pharmacologic Model to Evaluate Anti-AF Drugs: Some Lessons From I Kur

ABSTRACT

Human induced pluripotent stem cells (hiPSC) and atrial hiPSC-derived cardiomyocytes (hiPSC-CM) have entered the arena of preclinical atrial fibrillation research. A central question is whether they reproduce the physiologic contribution of atrial selective potassium currents (such as the ultrarapid potassium current, I Kur ) to repolarization. Of note, 2 studies in single atrial hiPSC-CM reported prolongation of action potential duration by I Kur block indicating that I Kur might in fact represent a valuable target for the treatment of human atrial fibrillation. However, the results and interpretation are at odds with the literature on I Kur block in human atria and the results of clinical studies. We believe that the discrepancies indicate that experiments in single atrial CM (both adult atrial CM and atrial hiPSC-CM) might be misleading. Under particular experimental conditions, atrial hiPSC-CMs may not closely resemble the electrophysiology of the human atrium. Therefore, we recapitulate here methodological issues evaluating potential value of the I Kur as an antiarrhythmic target when investigated in animal models, in human atrial tissues, and finally in atrial hiPSC-CM.

Genetics and Pharmacogenetics of Atrial Fibrillation: A Mechanistic Perspective

The heritability of atrial fibrillation (AF) is well established. Over the last decade genetic architecture of AF has been unraveled by genome-wide association studies and family-based studies. However, the translation of these genetic discoveries has lagged owing to an incomplete understanding of the pathogenic mechanisms underlying the genetic variants, challenges in classifying variants of uncertain significance (VUS), and limitations of existing disease models. We review the mechanistic insight provided by basic science studies regarding AF mechanisms, recent developments in high-throughput classification of VUS, and advances in bioengineered cardiac models for developing personalized therapy for AF.

Human induced pluripotent stem cell-derived atrial cardiomyocytes recapitulate contribution of the slowly activating delayed rectifier currents IKs to repolarization in the human atrium

AIMS

Human induced pluripotent stem cell-derived atrial cardiomyocytes (hiPSC-aCM) could be a helpful tool to study the physiology and diseases of the human atrium. To fulfil this expectation, the electrophysiology of hiPSC-aCM should closely resemble the situation in the human atrium. Data on the contribution of the slowly activating delayed rectifier currents (IKs) to repolarization are lacking for both human atrium and hiPSC-aCM.

METHODS AND RESULTS

Human atrial tissues were obtained from patients with sinus rhythm (SR) or atrial fibrillation (AF). Currents were measured in human atrial cardiomyocytes (aCM) and compared with hiPSC-aCM and used to model IKs contribution to action potential (AP) shape. Action potential was recorded by sharp microelectrodes. HMR-1556 (1 µM) was used to identify IKs and to estimate IKs contribution to repolarization. Less than 50% of hiPSC-aCM and aCM possessed IKs. Frequency of occurrence, current densities, activation/deactivation kinetics, and voltage dependency of IKs did not differ significantly between hiPSC-aCM and aCM, neither in SR nor AF. β-Adrenoceptor stimulation with isoprenaline did not increase IKs neither in aCM nor in hiPSC-aCM. In tissue from SR, block of IKs with HMR-1556 did not lengthen the action potential duration, even when repolarization reserve was reduced by block of the ultra-rapid repolarizing current with 4-aminopyridine or the rapidly activating delayed rectifier potassium outward current with E-4031.

CONCLUSION

I Ks exists in hiPSC-aCM with biophysics not different from aCM. As in adult human atrium (SR and AF), IKs does not appear to relevantly contribute to repolarization in hiPSC-aCM.

Generation of a human iPSC-derived cardiomyocyte/fibroblast engineered heart tissue model

Animal models have proven integral to broadening our understanding of complex cardiac diseases but have been hampered by significant species-dependent differences in cellular physiology. Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have shown great promise in the modelling of cardiac diseases despite limitations in functional and structural maturity. 3D stem cell-derived cardiac models represent a step towards mimicking the intricate microenvironment present in the heart as an in vitro model. Incorporation of non-myocyte cell types, such as cardiac fibroblasts, into engineered heart tissue models (EHTs) can help better recapitulate the cell-to-cell and cell-to-matrix interactions present in the human myocardium. Integration of human-induced pluripotent stem cell-derived cardiac fibroblasts (hiPSC-CFs) and hiPSC-CM into EHT models enables the generation of a genetically homogeneous modelling system capable of exploring the abstruse structural and electrophysiological interplay present in cardiac pathophysiology. Furthermore, the construction of more physiologically relevant 3D cardiac models offers great potential in the replacement of animals in heart disease research. Here we describe efficient and reproducible protocols for the differentiation of hiPSC-CMs and hiPSC-CFs and their subsequent assimilation into EHTs. The resultant EHT consists of longitudinally arranged iPSC-CMs, incorporated alongside hiPSC-CFs. EHTs with both hiPSC-CMs and hiPSC-CFs exhibit slower beating frequencies and enhanced contractile force compared to those composed of hiPSC-CMs alone. The modified protocol may help better characterise the interplay between different cell types in the myocardium and their contribution to structural remodelling and cardiac fibrosis.

Atrial fibrillation: pathophysiology, genetic and epigenetic mechanisms

Atrial fibrillation (AF) is the most common supraventricular arrhythmia affecting up to 1% of the general population. Its prevalence dramatically increases with age and could reach up to ∼10% in the elderly. The management of AF is a complex issue that is object of extensive ongoing basic and clinical research, it depends on its genetic and epigenetic causes, and it varies considerably geographically and also according to the ethnicity. Mechanistically, over the last decade, Genome Wide Association Studies have uncovered over 100 genetic loci associated with AF, and have shown that European ancestry is associated with elevated risk of AF. These AF-associated loci revolve around different types of disturbances, including inflammation, electrical abnormalities, and structural remodeling. Moreover, the discovery of epigenetic regulatory mechanisms, involving non-coding RNAs, DNA methylation and histone modification, has allowed unravelling what modifications reshape the processes leading to arrhythmias. Our review provides a current state of the field regarding the identification and functional characterization of AF-related genetic and epigenetic regulatory networks, including ethnic differences. We discuss clear and emerging connections between genetic regulation and pathophysiological mechanisms of AF.

Role of Genetic Variation in Transcriptional Regulatory Elements in Heart Rhythm

Genetic predisposition to cardiac arrhythmias has been a field of intense investigation. Research initially focused on rare hereditary arrhythmias, but over the last two decades, the role of genetic variation (single nucleotide polymorphisms) in heart rate, rhythm, and arrhythmias has been taken into consideration as well. In particular, genome-wide association studies have identified hundreds of genomic loci associated with quantitative electrocardiographic traits, atrial fibrillation, and less common arrhythmias such as Brugada syndrome. A significant number of associated variants have been found to systematically localize in non-coding regulatory elements that control the tissue-specific and temporal transcription of genes encoding transcription factors, ion channels, and other proteins. However, the identification of causal variants and the mechanism underlying their impact on phenotype has proven difficult due to the complex tissue-specific, time-resolved, condition-dependent, and combinatorial function of regulatory elements, as well as their modest conservation across different model species. In this review, we discuss research efforts aimed at identifying and characterizing-trait-associated variant regulatory elements and the molecular mechanisms underlying their impact on heart rate or rhythm.

A critical role of retinoic acid concentration for the induction of a fully human-like atrial action potential phenotype in hiPSC-CM

Retinoic acid (RA) induces an atrial phenotype in human induced pluripotent stem cells (hiPSCs), but expression of atrium-selective currents such as the ultrarapid (IKur) and acetylcholine-stimulated K+ current is variable and less than in the adult human atrium. We suspected methodological issues and systematically investigated the concentration dependency of RA. RA treatment increased IKur concentration dependently from 1.1 ± 0.54 pA/pF (0 RA) to 3.8 ± 1.1, 5.8 ± 2.5, and 12.2 ± 4.3 at 0.01, 0.1, and 1 μM, respectively. Only 1 μM RA induced enough IKur to fully reproduce human atrial action potential (AP) shape and a robust shortening of APs upon carbachol. We found that sterile filtration caused substantial loss of RA. We conclude that 1 μM RA seems to be necessary and sufficient to induce a full atrial AP shape in hiPSC-CM in EHT format. RA concentrations are prone to methodological issues and may profoundly impact the success of atrial differentiation.

PITX2 gain-of-function mutation associated with atrial fibrillation alters mitochondrial activity in human iPSC atrial-like cardiomyocytes

Atrial fibrillation (AF) is the most common cardiac arrhythmia worldwide; however, the underlying causes of AF initiation are still poorly understood, particularly because currently available models do not allow in distinguishing the initial causes from maladaptive remodeling that induces and perpetuates AF. Lately, the genetic background has been proven to be important in the AF onset. iPSC-derived cardiomyocytes, being patient- and mutation-specific, may help solve this diatribe by showing the initial cell-autonomous changes underlying the development of the disease. Transcription factor paired-like homeodomain 2 (PITX2) has been identified as a key regulator of atrial development/differentiation, and the PITX2 genomic locus has the highest association with paroxysmal AF. PITX2 influences mitochondrial activity, and alterations in either its expression or function have been widely associated with AF. In this work, we investigate the activity of mitochondria in iPSC-derived atrial cardiomyocytes (aCMs) obtained from a young patient (24 years old) with paroxysmal AF, carrying a gain-of-function mutation in PITX2 (rs138163892) and from its isogenic control (CTRL) in which the heterozygous point mutation has been reverted to WT. PITX2 aCMs show a higher mitochondrial content, increased mitochondrial activity, and superoxide production under basal conditions when compared to CTRL aCMs. However, increasing mitochondrial workload by FCCP or β-adrenergic stimulation allows us to unmask mitochondrial defects in PITX2 aCMs, which are incapable of responding efficiently to the higher energy demand, determining ATP deficiency.