Potassium channels in the Cx43 gap junction perinexus modulate ephaptic coupling: an experimental and modeling study

Abstract

Background It was recently demonstrated that cardiac sodium channels (Nav1.5) localized at the perinexus, an intercalated disc nanodomain associated with gap junctions (GJ), may contribute to electrical coupling between cardiac myocytes via an ephaptic mechanism. Impairment of ephaptic coupling by acute interstitial edema (AIE)-induced swelling of the perinexus was associated with arrhythmogenic, anisotropic conduction slowing. Given that Kir2.1 has also recently been reported to localize at intercalated discs (ID), we hypothesized that Kir2.1 channels may reside within the perinexus and that inhibiting them may mitigate arrhythmogenic conduction slowing observed during AIE. Methods and Results Using gSTED and STORM super-resolution microscopy, we indeed find that a significant proportion of Kir2.1 channels reside within the perinexus. Moreover, whereas Nav1.5 inhibition during AIE exacerbated arrhythmogenic conduction slowing, inhibiting Kir2.1 channels during AIE preferentially increased transverse conduction velocity - decreasing anisotropy and ameliorating arrhythmia risk compared to AIE alone. Comparison of our results with a nanodomain computer model identified enrichment of both Nav1.5 and Kir2.1 at intercalated discs as key factors underlying the experimental observations. Conclusions We demonstrate that Kir2.1 channels are localized within the perinexus alongside Nav1.5 channels. Further, targeting Kir2.1 modulates intercellular coupling between cardiac myocytes, anisotropy of conduction and arrhythmia propensity in a manner consistent with a role for ephaptic coupling in cardiac conduction.

Reciprocal Modulation of IK1–INa Extends Excitability in Cardiac Ventricular Cells

“…Similar results were also obtained by Shaw and Rudy (1997, Figures 4, 6) and Veeraraghavan et al (2016, Figure 7) using different non-linear ionic models and our results are qualitatively consistent with these earlier studies. While Veeraraghavan et al (2016) also showed that a potassium conductance had a qualitatively similar effect on transverse conduction velocity to that seen here, this conductance had biphasic effect on longitudinal conduction velocity; however, their potassium channel was of the delayed rectifier type and may explain the differences between their results and those in this paper.…”

“…However, their explanation was that Kir2.1 upregulation hyperpolarizes the cell resting membrane potential, thereby increasing sodium channel availability. Figure 2F shows that increasing G K1 alone actually decreases conduction velocity and this is in agreement with the experimental results of (Veeraraghavan and Poelzing, 2008) and Veeraraghavan et al (2016) and with data on reduction of conduction velocity by pinacidil activation of I K−ATP . With the discovery of reciprocal modulation (Milstein et al, 2012), we can see that I K1 upregulation induces a parallel upregulation of I Na which is the root cause of the increase in conduction velocity.…”

“…Simulations of ephaptic coupling by Mori et al (2008) have also generalized the cable model and shown how this mechanism may play a role under conditions of reduced gap junction coupling. The experimental and modeling results of Veeraraghavan et al (2016) also suggest a role for Nav1.5 and Kir2.1 in intercellular communication through an ephaptic coupling mechanism in addition to gap junctions, myocyte geometry, and tissue architecture; by comparing a “ uniform ” model where ion channels are uniformly distributed with a “ polarized ” model where ion channels are spatially localized, it was shown that the polarized model was able to better explain anisotropic conduction patterns. Wei et al (2016) also used an ephaptic coupling model to study anisotropic phenomena under conditions of reduced gap junction coupling.…”

Intramolecular signaling in a cardiac connexin: Role of cytoplasmic domain dimerization

“…However, Cx45 is upregulated in failing human hearts and is thought to be responsible for cell-to-cell coupling in Cx43-null ventricular myocytes [12–14]. Of note, recent evidence suggest ephaptic mechanisms might also account for residual conduction in Cx43 knockout mice [15, 16]. The resulting altered coupling and slowed conductance enhances susceptibility to ventricular tachy-arrhythmias [17].…”

STORM and TEM Identify the Cardiac Ephapse: An Intercalated Disk Nanodomain with Previously Unanticipated Functions in Cardiac Conduction

“…In recent studies, we have used transmission electron microscopy (TEM) and gSTED super-resolution imaging that the perinexus, an intercalated disk (ID) nanodomain surrounding connexin43 (Cx43) gap junctions (GJ), possesses both characteristics. [4,5] Further, experimental disruption of close apposition between perinexal membranes slowed conduction and precipitated arrhythmias. Therefore, we investigated the localization and functions of Na V 1.5 and Na V β1 (a sodium channel auxiliary subunit and cell adhesion molecule [6]) within different ID nanodomains.…”