The Role of MAPRE2 and Microtubules in Maintaining Normal Ventricular Conduction

BACKGROUND

Brugada syndrome is associated with loss-of-function SCN5A variants, yet these account for only ≈20% of cases. A recent genome-wide association study identified a novel locus within MAPRE2, which encodes EB2 (microtubule end-binding protein 2), implicating microtubule involvement in Brugada syndrome.

METHODS

A mapre2 knockout zebrafish model was generated using CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeat-associated protein 9) and validated by Western blot. Larval hearts at 5 days post-fertilization were isolated for voltage mapping and immunocytochemistry. Adult fish hearts were used for ECG, patch clamping, and immunocytochemistry. Morpholinos were injected into embryos at 1-cell stage for knockdown experiments. A transgenic zebrafish line with cdh2 tandem fluorescent timer was used to study adherens junctions. Microtubule plus-end tracking and patch clamping were performed in human induced pluripotent stem cell derived cardiomyocytes (iPSC-CMs) with MAPRE2 knockdown and knockout, respectively.

RESULTS

Voltage mapping of mapre2 knockout hearts showed a decrease in ventricular maximum upstroke velocity of the action potential and conduction velocity, suggesting loss of cardiac voltage-gated sodium channel function. ECG showed QRS prolongation in adult knockout fish, and patch clamping showed decreased sodium current density in knockout ventricular myocytes and arrhythmias in knockout iPSC-CMs. Confocal imaging showed disorganized adherens junctions and mislocalization of mature Ncad (N-cadherin) with mapre2 loss of function, associated with a decrease of detyrosinated tubulin. MAPRE2 knockdown in iPSC-CMs led to an increase in microtubule growth velocity and distance, indicating changes in microtubule dynamics. Finally, knockdown of ttl encoding tubulin tyrosine ligase in mapre2 knockout larvae rescued tubulin detyrosination and ventricular maximum upstroke velocity of the action potential.

CONCLUSIONS

Genetic ablation of mapre2 led to a decrease in voltage-gated sodium channel function, a hallmark of Brugada syndrome, associated with disruption of adherens junctions, decrease of detyrosinated tubulin as a marker of microtubule stability, and changes in microtubule dynamics. Restoration of the detyrosinated tubulin fraction with ttl knockdown led to rescue of voltage-gated sodium channel-related functional parameters in mapre2 knockout hearts. Taken together, our study implicates microtubule dynamics in the modulation of ventricular conduction.

The Brugada Syndrome Susceptibility Gene HEY2 Modulates Cardiac Transmural Ion Channel Patterning and Electrical Heterogeneity

RATIONALE

Genome-wide association studies previously identified an association of rs9388451 at chromosome 6q22.3 (near HEY2) with Brugada syndrome. The causal gene and underlying mechanism remain unresolved.

OBJECTIVE

We used an integrative approach entailing transcriptomic studies in human hearts and electrophysiological studies in Hey2^+/- (Hey2 heterozygous knockout) mice to dissect the underpinnings of the 6q22.31 association with Brugada syndrome.

METHODS AND RESULTS

We queried expression quantitative trait locus data acquired in 190 human left ventricular samples from the genotype-tissue expression consortium for cis-expression quantitative trait locus effects of rs9388451, which revealed an association between Brugada syndrome risk allele dosage and HEY2 expression (β=+0.159; P=0.0036). In the same transcriptomic data, we conducted genome-wide coexpression analysis for HEY2, which uncovered KCNIP2, encoding the β-subunit of the channel underlying the transient outward current (I_to), as the transcript most robustly correlating with HEY2 expression (β=+1.47; P=2×10^-34). Transcript abundance of Hey2 and the I_to subunits Kcnip2 and Kcnd2, assessed by quantitative reverse transcription-polymerase chain reaction, was higher in subepicardium versus subendocardium in both left and right ventricles, with lower levels in Hey2^+/- mice compared with wild type. Surface ECG measurements showed less prominent J waves in Hey2^+/- mice compared with wild-type. In wild-type mice, patch-clamp electrophysiological studies on cardiomyocytes from right ventricle demonstrated a shorter action potential duration and a lower V_max in subepicardium compared with subendocardium cardiomyocytes, which was paralleled by a higher I_to and a lower sodium current (I_Na) density in subepicardium versus subendocardium. These transmural differences were diminished in Hey2^+/- mice because of changes in subepicardial cardiomyocytes.

CONCLUSIONS

This study uncovers a role of HEY2 in the normal transmural electrophysiological gradient in the ventricle and provides compelling evidence that genetic variation at 6q22.31 (rs9388451) is associated with Brugada syndrome through a HEY2-dependent alteration of ion channel expression across the cardiac ventricular wall.

Functional Na V 1.8 Channels in Intracardiac Neurons

Rationale:

The SCN10A gene encodes the neuronal sodium channel isoform Na V 1.8. Several recent genome-wide association studies have linked SCN10A to PR interval and QRS duration, strongly suggesting an as-yet unknown role for Na V 1.8 in cardiac electrophysiology.

Objective:

To demonstrate the functional presence of SCN10A /Nav1.8 in intracardiac neurons of the mouse heart.

Methods and Results:

Immunohistochemistry on mouse tissue sections showed intense Na V 1.8 labeling in dorsal root ganglia and intracardiac ganglia and only modest Na V 1.8 expression within the myocardium. Immunocytochemistry further revealed substantial Na V 1.8 staining in isolated neurons from murine intracardiac ganglia but no Na V 1.8 expression in isolated ventricular myocytes. Patch-clamp studies demonstrated that the Na V 1.8 blocker A-803467 (0.5–2 μmol/L) had no effect on either mean sodium current (I Na ) density or I Na gating kinetics in isolated myocytes but significantly reduced I Na density in intracardiac neurons. Furthermore, A-803467 accelerated the slow component of current decay and shifted voltage dependence of inactivation toward more negative voltages, as expected for blockade of Na V 1.8-based I Na . In line with these findings, A-803467 did not affect cardiomyocyte action potential upstroke velocity but markedly reduced action potential firing frequency in intracardiac neurons, confirming a functional role for Na V 1.8 in cardiac neural activity.

Conclusions:

Our findings demonstrate the functional presence of SCN10A /Na V 1.8 in intracardiac neurons, indicating a novel role for this neuronal sodium channel in regulation of cardiac electric activity.

Re-evaluation of the action potential upstroke velocity as a measure of the Na+ current in cardiac myocytes at physiological conditions

BACKGROUND

The SCN5A encoded sodium current (I(Na)) generates the action potential (AP) upstroke and is a major determinant of AP characteristics and AP propagation in cardiac myocytes. Unfortunately, in cardiac myocytes, investigation of kinetic properties of I(Na) with near-physiological ion concentrations and temperature is technically challenging due to the large amplitude and rapidly activating nature of I(Na), which may seriously hamper the quality of voltage control over the membrane. We hypothesized that the alternating voltage clamp-current clamp (VC/CC) technique might provide an alternative to traditional voltage clamp (VC) technique for the determination of I(Na) properties under physiological conditions.

PRINCIPAL FINDINGS

We studied I(Na) under close-to-physiological conditions by VC technique in SCN5A cDNA-transfected HEK cells or by alternating VC/CC technique in both SCN5A cDNA-transfected HEK cells and rabbit left ventricular myocytes. In these experiments, peak I(Na) during a depolarizing VC step or maximal upstroke velocity, dV/dt(max), during VC/CC served as an indicator of available I(Na). In HEK cells, biophysical properties of I(Na), including current density, voltage dependent (in)activation, development of inactivation, and recovery from inactivation, were highly similar in VC and VC/CC experiments. As an application of the VC/CC technique we studied I(Na) in left ventricular myocytes isolated from control or failing rabbit hearts.

CONCLUSIONS

Our results demonstrate that the alternating VC/CC technique is a valuable experimental tool for I(Na) measurements under close-to-physiological conditions in cardiac myocytes.

HERG channel (dys)function revealed by dynamic action potential clamp technique

The human ether-a-go-go-related gene (HERG) encodes the rapid component of the cardiac delayed rectifier potassium current (I(Kr)). Per-Arnt-Sim domain mutations of the HERG channel are linked to type 2 long-QT syndrome. We studied wild-type and/or type 2 long-QT syndrome-associated mutant (R56Q) HERG current (I(HERG)) in HEK-293 cells, at both 23 and 36 degrees C. Conventional voltage-clamp analysis revealed mutation-induced changes in channel kinetics. To assess functional implication(s) of the mutation, we introduce the dynamic action potential clamp technique. In this study, we effectively replace the native I(Kr) of a ventricular cell (either a human model cell or an isolated rabbit myocyte) with I(HERG) generated in a HEK-293 cell that is voltage-clamped by the free-running action potential of the ventricular cell. Action potential characteristics of the ventricular cells were effectively reproduced with wild-type I(HERG), whereas the R56Q mutation caused a frequency-dependent increase of the action potential duration in accordance with the clinical phenotype. The dynamic action potential clamp approach also revealed a frequency-dependent transient wild-type I(HERG) component, which is absent with R56Q channels. This novel electrophysiological technique allows rapid and unambiguous determination of the effects of an ion channel mutation on the ventricular action potential and can serve as a new tool for investigating cardiac channelopathies.