Modulation of cardiac Na+ current phenotype by beta1-subunit expression

Biophysical comparison of sodium currents in native cardiac myocytes and human induced pluripotent stem cell-derived cardiomyocytes

INTRODUCTION

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are used for safety pharmacology and to investigate genetic diseases affecting cardiac ion channels. It is unclear whether adult myocytes or hiPSC-CMs are the better platform for cardiac safety pharmacology. We examined the biophysical and molecular properties of I_Na in adult myocytes and hiPSC-CMs.

METHODS

hiPSC-CMs were plated at low density. Atrial and ventricular cells were obtained from dog hearts. Whole cell patch clamp was used to record I_Na.

RESULTS

Voltage clamp recordings showed a large I_Na in all three cell types but different densities. Small differences in steady-state inactivation and recovery from inactivation were noted in the three cell types. Application of lidocaine to the three cell types showed a similar pattern of block of I_Na under voltage clamp; however, lidocaine produced different effects on AP waveform under current clamp. AP clamp experiments showed that application of ventricular or atrial cell waveforms to the same hiPSC-CM elicited a large I_Na while application of a sinoatrial node waveform elicited no I_Na. Molecular analysis of Na⁺ channel subunits showed SCN5A and SCN1B-4B were expressed in adult cells and iPSC-CMs. However, iPSC-CMs express both fetal (exon 6A) and adult (exon 6) isoforms of SCN5A.

DISCUSSION

There are major differences in I_Na density and smaller differences in other biophysical properties of I_Na in adult atrial, ventricular, and hiPSC-CMs. The depolarized maximum diastolic potential coupled with the presence of phase 4 depolarization limits the contribution of I_Na in hiPSC-CM action potentials. Our results suggest that hiPSC-CMs may be useful for drug screening of Na⁺ channel inhibitors under voltage clamp but not current clamp.

A novel transgenic rabbit model with reduced repolarization reserve: long QT syndrome caused by a dominant-negative mutation of the KCNE1 gene

BACKGROUND AND PURPOSE

The reliable assessment of proarrhythmic risk of compounds under development remains an elusive goal. Current safety guidelines focus on the effects of blocking the KCNH2/HERG ion channel-in tissues and animals with intact repolarization. Novel models with better predictive value are needed that more closely reflect the conditions in patients with cardiac remodelling and reduced repolarization reserve.

EXPERIMENTAL APPROACH

We have developed a model for the long QT syndrome type-5 in rabbits (LQT5 ) with cardiac-specific overexpression of a mutant (G52R) KCNE1 β-subunit of the channel that carries the slow delayed-rectifier K(+) -current (IKs ). ECG parameters, including short-term variability of the QT interval (STVQT ), a biomarker for proarrhythmic risk, and arrhythmia development were recorded. In vivo, arrhythmia susceptibility was evaluated by i.v. administration of the IKr blocker dofetilide. K(+) currents were measured with the patch-clamp technique.

KEY RESULTS

Patch-clamp studies in ventricular myocytes isolated from LQT5 rabbits revealed accelerated IKs and IKr deactivation kinetics. At baseline, LQT5 animals exhibited slightly but significantly prolonged heart-rate corrected QT index (QTi) and increased STVQT . Dofetilide provoked Torsade-de-Pointes arrhythmia in a greater proportion of LQT5 rabbits, paralleled by a further increase in STVQT .

CONCLUSION AND IMPLICATIONS

We have created a novel transgenic LQT5 rabbit model with increased susceptibility to drug-induced arrhythmias that may represent a useful model for testing proarrhythmic potential and for investigations of the mechanisms underlying arrhythmias and sudden cardiac death due to repolarization disturbances.

Antisense-mediated post-transcriptional silencing of SCN1B gene modulates sodium channel functional expression

BACKGROUND INFORMATION

Voltage-dependent sodium channels are membrane proteins essential for cell excitability. They are composed by a pore-forming α-subunit and one or more β subunits. Nine α subunit and five β subunit isoforms have been identified in mammals: β1, its splice variant β1B, β2, β3 and β4. Although they do not form the ion channel pore, β subunits modulate both function as well as expression of sodium channels on cell membrane.

RESULTS

To investigate the role of β1 subunit on the modulation of sodium channel expression, we silenced this auxiliary subunit with specific antisense oligonucleotides (ASONs) in two rat cell lines, the GH3 and the H9C2, from neuro-ectoderm and cardiac myocyte origin, respectively. Treatment of cells with ASONs determined a reduction of about 50% of β1 subunit mRNA and protein expression in both cell lines. We found that this level of β1 subunit silencing resulted in an overall decrease of α subunit mRNA, protein expression and a decrease of sodium current density, without altering significantly the voltage-dependent and kinetic properties of the currents. In GH3 cells, the β1 subunit silencing reduced the expression of Nav1.1, Nav1.3 and Nav1.6 isoforms, whereas the Nav 1.2 isoform expression remained unaltered. The expression of the only α subunit present in H9C2 cells, the Nav1.5, was also reduced by β1 subunit silencing.

CONCLUSIONS

These results indicate that the β1 subunit may exert an isoform-specific fine-tuned modulation of sodium channel expression.

TGF-β1, released by myofibroblasts, differentially regulates transcription and function of sodium and potassium channels in adult rat ventricular myocytes

Cardiac injury promotes fibroblasts activation and differentiation into myofibroblasts, which are hypersecretory of multiple cytokines. It is unknown whether any of such cytokines are involved in the electrophysiological remodeling of adult cardiomyocytes. We cultured adult cardiomyocytes for 3 days in cardiac fibroblast conditioned medium (FCM) from adult rats. In whole-cell voltage-clamp experiments, FCM-treated myocytes had 41% more peak inward sodium current (I_Na) density at −40 mV than myocytes in control medium (p<0.01). In contrast, peak transient outward current (I_to) was decreased by ∼55% at 60 mV (p<0.001). Protein analysis of FCM demonstrated that the concentration of TGF-β1 was >3 fold greater in FCM than control, which suggested that FCM effects could be mediated by TGF-β1. This was confirmed by pre-treatment with TGF-β1 neutralizing antibody, which abolished the FCM-induced changes in both I_Na and I_to. In current-clamp experiments TGF-β1 (10 ng/ml) prolonged the action potential duration at 30, 50, and 90 repolarization (p<0.05); at 50 ng/ml it gave rise to early afterdepolarizations. In voltage-clamp experiments, TGF-β1 increased I_Na density in a dose-dependent manner without affecting voltage dependence of activation or inactivation. I_Na density was −36.25±2.8 pA/pF in control, −59.17±6.2 pA/pF at 0.1 ng/ml (p<0.01), and −58.22±6.6 pA/pF at 1 ng/ml (p<0.01). In sharp contrast, I_to density decreased from 22.2±1.2 pA/pF to 12.7±0.98 pA/pF (p<0.001) at 10 ng/ml. At 1 ng/ml TGF-β1 significantly increased SCN5A (Na_V1.5) (+73%; p<0.01), while reducing KCNIP2 (Kchip2; −77%; p<0.01) and KCND2 (K_V4.2; −50% p<0.05) mRNA levels. Further, the TGF-β1-induced increase in I_Na was mediated through activation of the PI3K-AKT pathway via phosphorylation of FOXO1 (a negative regulator of SCN5A). TGF-β1 released by myofibroblasts differentially regulates transcription and function of the main cardiac sodium channel and of the channel responsible for the transient outward current. The results provide new mechanistic insight into the electrical remodeling associated with myocardial injury.

The intracellular domain of the beta 2 subunit modulates the gating of cardiac Na v 1.5 channels

We have previously shown that the transmembrane segment plus either the extracellular or intracellular domain of the beta1 subunit are required to modify cardiac Na(v)1.5 channels. In this study, we coexpressed the intracellular domain of the beta2 subunit in a beta1/beta2 chimera with Na(v)1.5 channels in Xenopus oocytes and obtained an atypical recovery behavior of Na(v)1.5 channels not reported before for other Na(+) channels: Recovery times of up to 20 ms at -120 mV produced a similar fast recovery as observed for Na(v)1.5/beta1 channels, but the current amplitude decreased again at longer recovery times and reached a steady-state level after 1-2 s with current amplitudes of only 43 +/- 2% of the value at 20 ms. Current reduction was accompanied by slowed inactivation and by a shift of steady-state activation toward depolarized potentials by 9 mV. All effects were reversible and they were not seen when deleting the beta2 intracellular domain. These results describe the first functional effects of a beta2 subunit region on Na(v)1.5 channels and suggest a novel closed state in cardiac Na(+) channels accessible at hyperpolarized potentials.

Modulation of cardiac Na(+) and Ca(2+) currents by CaM and CaMKII

Sarcolemmal sodium (Na) and calcium (Ca) currents are fundamentally involved in shaping the cardiac action potential. Alterations in Na or Ca currents can change action potential characteristics and therefore might result in cardiac arrhythmias. Also, these ions contribute to excitation-contraction coupling and therefore are important in myocyte shortening and contractility of the heart. This review article summarizes how sarcolemmal Na and Ca channels are regulated with an emphasis on the novel role of Ca-dependent proteins Calmodulin (CaM) and especially Ca/CaM-dependent protein kinase II (CaMKII) to modulate sarcolemmal Na and Ca channels in the heart.

A comparison of gene expression in mouse liver and kidney in obstructive cholestasis utilizing high-density oligonucleotide microarray technology

AIM: To assess the effects of obstructive cholestasis on a wider range of gene expression using microarray technology.

METHODS: Male C57BL/6J mice underwent common bile duct ligation (BDL) and were matched with pair-fed sham-operated controls. After 7 d, the animals were sacrificed and total RNA was isolated from livers and kidneys. Equal amounts of RNA from each tissue were pooled for each group and hybridized to Affymetrix GeneChip^®MG-U74Av2 containing a total of 12 488 probe sets. Data analysis was performed using GeneSpring^®6.0 software. Northern analysis and immunofluorescence were used for validation.

RESULTS: In sham-operated and BDL mice, 44 and 50% of 12 488 genes were expressed in livers, whereas 49 and 51% were expressed in kidneys, respectively. Seven days after BDL, 265 liver and 112 kidney genes with GeneOntology annotation were up-regulated and 113 liver and 36 kidney genes were down-regulated in comparison with sham-operated controls. Many genes were commonly regulated in both tissues and metabolism-related genes represented the largest functional group.

CONCLUSION: Following BDL, microarray analysis reveals a broad range of gene alterations in both liver and kidney.

Gene-specific therapy for inherited arrhythmogenic diseases

In the last few years, major advancement has been made in the understanding of the genetic basis of inherited arrhythmogenic diseases. Interestingly, the information obtained with the application of molecular genetics to these diseases is now influencing their clinical management, allowing gene-specific risk stratification and gene-specific management. The first attempt for a gene-specific therapy was made in 1995 with the use of mexiletine in long-QT syndrome (LQTS) patients with mutations in the SCN5A gene. Since then, several investigators have proposed novel therapeutic approaches based on the identification of the functional consequences of genetic mutations. In some instances, these novel therapies have already been introduced in clinical practice, and data are being collected to establish their long-term efficacy. In this review, we will summarize the current understanding of the molecular bases of inherited arrhythmias, with a specific focus toward discussing the most recent advancements toward the development of gene-specific therapies.

Cardiac sodium channel diseases

In the last few years, a very active line of research took place after the first identification of SCN5A mutations associated with an inherited form of cardiac arrhythmias and sudden death, the LQT3 variant of the long QT syndrome. Subsequently, two allelic diseases additional to LQT3 were shown to be due to mutations in the same gene, the Brugada syndrome (BrS) and the Lev-Lenegre syndrome (progressive cardiac conduction defect). Genotype-phenotype correlation and in vitro expression studies provide evidence that structure-function relationships of the SCN5A protein are much more complex than initially anticipated. The biophysical characterization of the sodium channel defects associated with different phenotypes and the genotype-phenotype correlation studies brought to the attention of the scientific community a plethora of mechanisms by which even a single amino acid substitution may remarkably affect cardiac excitability. Finally, the evidence of patients harboring an SCN5A mutation and overlapping clinical presentations creates a need for a revision of the traditional classification of the above mentioned diseases. It is now appropriate to consider the "sodium channel syndrome" as a unique clinical entity that may manifest itself with a spectrum of possible phenotypes.

Haploinsufficiency in combination with aging causes SCN5A-linked hereditary Lenègre disease

OBJECTIVES

The goal of this study was to investigate the genotype-to-phenotype relationship between SCN5A gene mutation and progressive cardiac conduction defect in order to gain insights into the pathophysiologic mechanisms of the disease.

BACKGROUND

Progressive cardiac conduction defect is a frequent disease commonly attributed to degeneration and fibrosis of the His bundle and its branches. In a French family, we have identified a splicing mutation in the SCN5A gene leading to hereditary progressive cardiac conduction defect.

METHODS

We have extended the size of the pedigree and phenotyped and genotyped all family members, and also investigated in vitro the functional consequences of the mutation.

RESULTS

Among 65 potentially affected members, 25 individuals were carriers of the IVS.22+2 T-->C SCN5A mutation. In relation to aging, gene carriers exhibit various types of conduction defects. P-wave, PR, and QRS duration increased progressively with age in gene carriers and in noncarriers. Whatever the age, conduction parameters were longer in gene carriers. The widening in the QRS complex with aging was more pronounced in gene carriers older than 40 years. Functional studies show that the IVS.22+2 T-->C SCN5A mutation lead to exon 22 skipping and to a complete loss of function of the affected allele, but to a normal trafficking of the mutated gene product.

CONCLUSIONS

Our findings demonstrate that hereditary Lenègre disease is caused by a haploinsufficiency mechanism, which in combination with aging leads to progressive alteration in conduction velocity.

Sodium channel heterologous expression in mammalian cells and the role of the endogenous beta1-subunits

Sodium currents in cell lines transfected with the sole alpha-subunit, or constitutively expressing sodium channels, have an inactivation that is always prevalently mono-exponential. Differently, expression of alpha-subunit in Xenopus oocytes exerts slow inactivating currents with biphasic decay, while simultaneous co-transfection of alpha and beta1 restores a mono-exponential (normal) inactivation. A hypothesis for such differences is that an endogenous presence of beta1 or beta1-alternative splicing, beta1A, in cells could account for the normal inactivation. To test this hypothesis and to evaluate the role for the beta1A, we inhibited the expression of beta1/beta1A by antisense oligonucleotides on Nav1.4-transfected human embryonic cell line 293 (HEK) cells. Reduction of beta1/beta1A produces no significant functional effects in Nav1.4-HEK. This result invalidates the hypothesis that the lack of slow-mode in cell lines is simply due to a constitutive expression of beta1/beta1A.

Late Na currents affected by alpha subunit isoform and beta1 subunit co-expression in HEK293 cells

Peak Na current underlies excitability and conduction in the heart, and late non-inactivating or slowly inactivating Na current plays a role in action potential duration. We hypothesized that different alpha subunit isoforms or beta1 subunit co-expression might affect late Na current. The human Na channel alpha subunits hNa(v)1.5 (hH1a) and hNa(v)1.4 (hSkM1) were transfected with and without the hNa(V)beta1 (beta1) subunit in HEK293 cells and studied by whole cell patch clamp. The inactivation relationship for hH1a was 28mV negative to that for hSkM1, and beta1 shifted the midpoint positively by 22mV for hH1a and 8mV for hSkM1. When pre-pulse duration was varied from 10ms to 10s, "steady-state" was approached more slowly for hH1a. beta1 caused hH1a but not hSkM1 to reach "steady-state" earlier. Both isoforms showed two recovery components but hH1a showed a "cardiac phenotype" with a smaller slow component that was unaffected by beta1. The amplitude of a late current (at 750ms) was significantly greater for hH1a than hSkM1, but beta1 decreased late current for hH1a and eliminated the difference. Under the study conditions the alpha subunit isoforms have distinct functional phenotypes and co-expression with beta1 tends to diminish these distinctions. These properties may provide mechanisms for regional and transmural distribution of late Na current and late Na current amplitudes during development and in disease states.

Mouse heart Na+ channels: primary structure and function of two isoforms and alternatively spliced variants

We isolated two full-length cDNA clones from the adult murine heart that encode two different voltage-gated Na+ channels: mH1 and mH2. Sequence comparisons indicated that mH1 is highly homologous to rat SCN5A, whereas mH2 is highly homologous to SCN4A, expressed in rat skeletal muscle. Electrophysiological properties of mH1 channels strongly resembled the tetrodotoxin (TTX)-resistant Na+ current of mouse ventricular cells, whereas mH2 channels activated at more positive potentials and were highly sensitive to TTX [50% inhibitory constant (IC50) = 11 nM]. We found that mH2 is not expressed in cardiac cells of neonatal mice, but appears to be upregulated during the development. Besides these Na+ channel isoforms, we also detected two alternatively spliced mH1 variants that were characterized by deletions within the sequence coding for the intracellular loop between domains II and III. One of the shortened channels, mH1-2, developed Na+ currents indistinguishable from those of mH1. The other splice variant (mH1-3) did not form functional channels. Quantitative reverse transcriptase-polymerase chain reaction indicated that RNA preparations of the adult mouse heart contain 54% mH1, 25% mH1-2, 16% mH2, and 5% mH1-3. Conclusively, mH1 generates the main portion of the mouse cardiac TTX-resistant Na+ current and mH2 is a candidate for TTX-sensitive currents previously described in adult cardiomyocytes. Furthermore, the presence of mH1-2 and mH1-3 transcripts indicates that alternative splicing plays a role in the regulation of functional Na+ channels in cardiomyocytes.

Regulation of the cardiac voltage-gated Na+ channel (H1) by the ubiquitin-protein ligase Nedd4

The cardiac voltage-gated Na+ channel H1, involved in the generation of cardiac action potential, contains a C-terminal PY motif (xPPxY). Since PY motifs are known ligands to WW domains, we investigated their role for H1 regulation and the possible involvement of the WW domain containing ubiquitin-protein ligase Nedd4, taking advantage of the Xenopus oocyte system. Mutation of the PY motif leads to higher peak currents when compared to wild-type channel. Moreover, co-expression of Nedd4 reduced the peak currents, whereas an enzymatically inactive Nedd4 mutant increased them, likely by competing with endogenous Nedd4. The effect of Nedd4 was not observed in the PY motif mutated channel or in the skeletal muscle voltage-gated Na+ channel, which lacks a PY motif. We conclude that H1 may be regulated by Nedd4 depending on WW-PY interaction, and on an active ubiquitination site.