Exosomes and Exosomal Non-coding RNAs Are Novel Promises for the Mechanism-Based Diagnosis and Treatments of Atrial Fibrillation

Atrial fibrillation (AF) is the most common arrhythmia worldwide and has a significant impact on human health and substantial costs. Currently, there is a lack of accurate biomarkers for the diagnosis and prognosis of AF. Moreover, the long-term efficacy of the catheter ablation in the AF is unsatisfactory. Therefore, it is necessary to explore new biomarkers and treatment strategies for the mechanism-based AF. Exosomes are nano-sized biovesicles released by nearly all types of cells. Since the AF would be linked to the changes of the atrial cells and their microenvironment, and the AF would strictly influence the exosomal non-coding RNAs (exo-ncRNAs) expression, which makes them as attractive diagnostic and prognostic biomarkers for the AF. Simultaneously, the exo-ncRNAs have been found to play an important role in the mechanisms of the AF and have potential therapeutic prospects. Although the role of the exo-ncRNAs in the AF is being actively investigated, the evidence is still limited. Furthermore, there is a lack of consensus regarding the most appropriate approach for exosome isolation and characterization. In this article, we reviewed the new methodologies available for exosomes biogenesis, isolation, and characterization, and then discussed the mechanism of the AF and various levels and types of exosomes relevant to the AF, with the special emphasis on the exo-ncRNAs in the diagnosis, prognosis, and treatment of the mechanism-based AF.

GLYCOGEN AND ATRIAL FIBRILLATION ATRIAL MYOCYTES CONTRACTILITY AS MECHANICAL FACTOR SHIFTING INTRACELLULAR GLYCOGEN AGAINST INTERCALATED DISCS

The purpose of this communication is to introduce in the medical literature an additional factor until now hypothesized action of atrial cells depolarization as factor in intracellular flow of glycogen molecules coalescing against the gap junctions. The demonstrated effect of gap junction blockers on paired cells contractility combined with gap junction’s selectivity towards glycogen molecules; and the visualization of contrasting intracellular glycogen images during atrial fibrillation are shown. Published data supporting atrial myocytes contraction as a mechanism in intracellular glycogen molecules migration and its deleterious effects leading into atrial fibrillation (AF) is proposed.

In Silico Assessment of Class I Antiarrhythmic Drugs Effects on Pitx2-induced Atrial Fibrillation: Insights from Populations of Electrophysiological Models of Human Atrial Cells and Tissues

Electrical remodelling as a result of the homeodomain transcription factor 2 (Pitx2)‐dependent gene regulation was linked to atrial fibrillation (AF) and AF patients with single nucleotide polymorphisms at chromosome 4q25 responded favorably to Class I antiarrhythmic drugs (AADs). The possible reasons behind this remain elusive. The purpose of this study was to assess the efficacy of AADs disopyramide, quinidine, and propafenone on human atrial arrhythmias mediated by Pitx2-induced remodelling, from a single cell to the tissue level, using drug binding models with multi-channel pharmacology. Experimentally calibrated populations of human atrial action potential (AP) models in both sinus rhythm (SR) and Pitx2-induced AF conditions were constructed by using two distinct models to represent morphological subtypes of AP. Multi-channel pharmacological effects of disopyramide, quinidine, and propafenone on ionic currents were considered. Simulated results showed that Pitx2-induced remodelling increased maximum upstroke velocity (dVdtmax) and conduction velocity (CV), and decreased AP duration (APD) and wavelength (WL). At the concentrations tested in this study, these AADs decreased dVdtmax and CV and prolonged APD in the setting of Pitx2-induced AF. Our findings of alterations in WL indicated that quinidine and disopyramide may be more effective against Pitx2-induced AF than propafenone by prolonging WL.

Warmer, faster, stronger: Ca2+ cycling in avian myocardium

ABSTRACTBirds occupy a unique position in the evolution of cardiac design. Their hearts are capable of cardiac performance on par with, or exceeding that of mammals, and yet the structure of their cardiomyocytes resembles those of reptiles. It has been suggested that birds use intracellular Ca2+ stored within the sarcoplasmic reticulum (SR) to power contractile function, but neither SR Ca2+ content nor the cross-talk between channels underlying Ca2+-induced Ca2+ release (CICR) have been studied in adult birds. Here we used voltage clamp to investigate the Ca2+ storage and refilling capacities of the SR and the degree of trans-sarcolemmal and intracellular Ca2+ channel interplay in freshly isolated atrial and ventricular myocytes from the heart of the Japanese quail (Coturnix japonica). A trans-sarcolemmal Ca2+ current (ICa) was detectable in both quail atrial and ventricular myocytes, and was mediated only by L-type Ca2+ channels. The peak density of ICa was larger in ventricular cells than in atrial cells, and exceeded that reported for mammalian myocardium recorded under similar conditions. Steady-state SR Ca2+ content of quail myocardium was also larger than that reported for mammals, and reached 750.6±128.2 μmol l−1 in atrial cells and 423.3±47.2 μmol l−1 in ventricular cells at 24°C. We observed SR Ca2+-dependent inactivation of ICa in ventricular myocytes, indicating cross-talk between sarcolemmal Ca2+ channels and ryanodine receptors in the SR. However, this phenomenon was not observed in atrial myocytes. Taken together, these findings help to explain the high-efficiency avian myocyte excitation–contraction coupling with regard to their reptilian-like cellular ultrastructure.

Abstract 281: Human Atrial Extracellular Matrix Drives Differentiation of Human Induced Pluripotent Stem Cell-derived Cardiomyocytes Toward an Atrial Phenotype

Extracellular matrix (ECM) can directly modulate cell proliferation, migration and differentiation by mediating diverse growth factors and signaling interactions. Protocols for cardiomyocyte differentiation of induced pluripotent stem cells (iPSCs) that recapitulate cardiac development frequently result in a mixed cardiac cell population dominated overwhelmingly by ventricular-like cells. Utilizing the inherent biological capabilities of decellularized ECM (dECM) from human myocardium, we developed a method for committing human iPSCs to an atrial-like cell phenotype. We employed a modified decellularization method to generate small particles (125-500 μm) of human atrial and ventricular dECM. The particles presented a fractal dimension (1.63 and 1.71) that suggested self-similarity across particle sizes of both atrial and ventricular dECM. Quantifications of DNA (3.37±0.50 and 2.77±0.62% of cadaveric), GAG (0.44±0.08 and 0.59±0.13 μg/mg), and SDS (2.46±1.20 and 2.91±2.53 μg/mg) validated the absence of difference of atrial and ventricular dECM. Proteomic profiling revealed dECM chamber-specific clustered populations. Ventricular and atrial dECM segregated into ventricular and atrial parts based on component 1 (19.5%) and component 2 (13.9%). A total of 14% of atrial proteins were matrisome atrial-related and 13% of ventricle proteins were matrisome ventricular-related. Myocytes differentiated in the presence of atrial dECM showed similar differentiation efficiency (66.6±10.2 vs 65.5±12.7% of cTNT) and, importantly, increased atrial markers, as confirmed by qPCR (SLP and COUPF-I) and flow cytometry (43.5%±12.7% vs 23.9%±10.8% of MLC2a) in comparison to control. We observed an increase in atrial cells (38.4% vs 14.8%) by action potential duration (APD), with statistical differences in cAPD10 (57.1±20.2 vs 104.4±48.7 ms) and cAPD20 (76.2±22 vs 126±47.4 ms). Altogether, we demonstrate that human atrial ECM retains cues to drive cardiac differentiation to an atrial fate, doubling the number of atrial cells with a functional atrial phenotype. These findings are a critical step toward generating sufficient quantities of atrial cells, which can be used for chamber-specific cardiac disease modeling and drug development.

Cell-to-cell Mathematical Modeling of Arrhythmia Phenomena in the Heart

With an aperiodic, self-similar distribution of two-dimensional arrangement of atrial cells, it is possible to simulate such phenomena as Fibrillation, Fluttering, and a sequence of Fibrillation-Fluttering. The topology of a network of cells may facilitate the initiation and development of arrhythmias such as Fluttering and Fibrillation. Using a GPU parallel architecture, two basic cell topologies were considered in this simulation, an aperiodic, fractal distribution of connections among 462 cells, and a chessboard-like geometry of 60×60 and 600×600 cells. With a complex set of initial conditions, it is possible to produce tissue behavior that may be identified with arrhythmias. Finally, we found several sets of initial conditions that show how a mesh of cells may exhibit Fibrillation that evolves into Fluttering.

Hypokalemia Promotes Arrhythmia by Distinct Mechanisms in Atrial and Ventricular Myocytes

Rationale: Hypokalemia occurs in up to 20% of hospitalized patients and is associated with increased incidence of ventricular and atrial fibrillation. It is unclear whether these differing types of arrhythmia result from direct and perhaps distinct effects of hypokalemia on cardiomyocytes. Objective: To investigate proarrhythmic mechanisms of hypokalemia in ventricular and atrial myocytes. Methods and Results: Experiments were performed in isolated rat myocytes exposed to simulated hypokalemia conditions (reduction of extracellular [K + ] from 5.0 to 2.7 mmol/L) and supported by mathematical modeling studies. Ventricular cells subjected to hypokalemia exhibited Ca 2+ overload and increased generation of both spontaneous Ca 2+ waves and delayed afterdepolarizations. However, similar Ca 2+ -dependent spontaneous activity during hypokalemia was only observed in a minority of atrial cells that were observed to contain t-tubules. This effect was attributed to close functional pairing of the Na + -K + ATPase and Na + -Ca 2+ exchanger proteins within these structures, as reduction in Na + pump activity locally inhibited Ca 2+ extrusion. Ventricular myocytes and tubulated atrial myocytes additionally exhibited early afterdepolarizations during hypokalemia, associated with Ca 2+ overload. However, early afterdepolarizations also occurred in untubulated atrial cells, despite Ca 2+ quiescence. These phase-3 early afterdepolarizations were rather linked to reactivation of nonequilibrium Na + current, as they were rapidly blocked by tetrodotoxin. Na + current-driven early afterdepolarizations in untubulated atrial cells were enabled by membrane hyperpolarization during hypokalemia and short action potential configurations. Brief action potentials were in turn maintained by ultra-rapid K + current (I Kur ); a current which was found to be absent in tubulated atrial myocytes and ventricular myocytes. Conclusions: Distinct mechanisms underlie hypokalemia-induced arrhythmia in the ventricle and atrium but also vary between atrial myocytes depending on subcellular structure and electrophysiology.