Inhibition of late sodium current via PI3K/Akt signaling prevents cellular remodeling in tachypacing-induced HL-1 atrial myocytes

An aberrant late sodium current (I_Na,Late) caused by a mutation in the cardiac sodium channel (Na_v1.5) has emerged as a contributor to electrical remodeling that causes susceptibility to atrial fibrillation (AF). Although downregulation of phosphoinositide 3-kinase (PI3K)/Akt signaling is associated with AF, the molecular mechanisms underlying the negative regulation of I_Na,Late in AF remain unclear, and potential therapeutic approaches are needed. In this work, we constructed a tachypacing-induced cellular model of AF by exposing HL-1 myocytes to rapid electrical stimulation (1.5 V/cm, 4 ms, 10 Hz) for 6 h. Then, we gathered data using confocal Ca²⁺ imaging, immunofluorescence, patch-clamp recordings, and immunoblots. The tachypacing cells displayed irregular Ca²⁺ release, delayed afterdepolarization, prolonged action potential duration, and reduced PI3K/Akt signaling compared with controls. Those detrimental effects were related to increased I_Na,Late and were significantly mediated by treatment with the I_Na,Late blocker ranolazine. Furthermore, decreased PI3K/Akt signaling via PI3K inhibition increased I_Na,Late and subsequent aberrant myocyte excitability, which were abolished by I_Na,Late inhibition, suggesting that PI3K/Akt signaling is responsible for regulating pathogenic I_Na,Late. These results indicate that PI3K/Akt signaling is critical for regulating I_Na,Late and electrical remodeling, supporting the use of PI3K/Akt-mediated I_Na,Late as a therapeutic target for AF.

Identification of two preclinical canine models of atrial fibrillation to facilitate drug discovery

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia occurring in humans, and new treatment strategies are critically needed. The lack of reliable preclinical animal models of AF is a major limitation to drug development of novel antiarrhythmic compounds.

OBJECTIVE

The purpose of this study was to provide a comprehensive head-to-head assessment of 5 canine AF models.

METHODS

Five canine models were evaluated for the efficacy of AF induction and AF duration. We tested 2 acute models: short-term atrial tachypacing (AT) for 6 hours with analysis of AF at hourly increments, and carbachol injection into a cardiac fat pad followed by short-term AT. We also tested 3 chronic models: pacemaker implantation followed by either 4 weeks of AT and subsequent atrial burst pacing or intermittent long-term AT for up to 4-5 months to generate AF ≥4.5 hours, and finally ventricular tachypacing to induce heart failure followed by atrial burst pacing to induce AF.

RESULTS

Careful evaluation showed that acute AT, AT for 4 weeks, and the heart failure model all were unsuccessful in generating reproducible AF episodes of sufficient duration to study antiarrhythmic drugs. In contrast, intermittent long-term AT generated AF lasting ≥4.5 hours in ∼30% of animals. The acute model using carbachol and short-term AT resulted in AF induction of ≥15 minutes in ≥75% of animals, thus enabling testing of antiarrhythmic drugs.

CONCLUSION

Intermittent long-term AT and the combination of local carbachol injection with successive short-term AT may contribute to future drug development efforts for AF treatment.

Chronic intermittent tachypacing by an optogenetic approach induces arrhythmia vulnerability in human engineered heart tissue

AIMS

Chronic tachypacing is commonly used in animals to induce cardiac dysfunction and to study mechanisms of heart failure and arrhythmogenesis. Human induced pluripotent stem cells (hiPSC) may replace animal models to overcome species differences and ethical problems. Here, 3D engineered heart tissue (EHT) was used to investigate the effect of chronic tachypacing on hiPSC-cardiomyocytes (hiPSC-CMs).

METHODS AND RESULTS

To avoid cell toxicity by electrical pacing, we developed an optogenetic approach. EHTs were transduced with lentivirus expressing channelrhodopsin-2 (H134R) and stimulated by 15 s bursts of blue light pulses (0.3 mW/mm2, 30 ms, 3 Hz) separated by 15 s without pacing for 3 weeks. Chronic optical tachypacing did not affect contractile peak force, but induced faster contraction kinetics, shorter action potentials, and shorter effective refractory periods. This electrical remodelling increased vulnerability to tachycardia episodes upon electrical burst pacing. Lower calsequestrin 2 protein levels, faster diastolic depolarization (DD) and efficacy of JTV-519 (46% at 1 µmol/L) to terminate tachycardia indicate alterations of Ca2+ handling being part of the underlying mechanism. However, other antiarrhythmic compounds like flecainide (69% at 1 µmol/L) and E-4031 (100% at 1 µmol/L) were also effective, but not ivabradine (1 µmol/L) or SEA0400 (10 µmol/L).

CONCLUSION

We demonstrated a high vulnerability to tachycardia of optically tachypaced hiPSC-CMs in EHT and the effective termination by ryanodine receptor stabilization, sodium or hERG potassium channel inhibition. This new model might serve as a preclinical tool to test antiarrhythmic drugs increasing the insight in treating ventricular tachycardia.

Inhibition of Histone Deacetylases Induces K+ Channel Remodeling and Action Potential Prolongation in HL-1 Atrial Cardiomyocytes

BACKGROUND/AIMS

Cardiac arrhythmias are triggered by environmental stimuli that may modulate expression of cardiac ion channels. Underlying epigenetic regulation of cardiac electrophysiology remains incompletely understood. Histone deacetylases (HDACs) control gene expression and cardiac integrity. We hypothesized that class I/II HDACs transcriptionally regulate ion channel expression and determine action potential duration (APD) in cardiac myocytes.

METHODS

Global class I/II HDAC inhibition was achieved by administration of trichostatin A (TSA). HDAC-mediated effects on K+ channel expression and electrophysiological function were evaluated in murine atrial cardiomyocytes (HL-1 cells) using real-time PCR, Western blot, and patch clamp analyses. Electrical tachypacing was employed to recapitulate arrhythmia-related effects on ion channel remodeling in the absence and presence of HDAC inhibition.

RESULTS

Global HDAC inhibition increased histone acetylation and prolonged APD90 in atrial cardiomyocytes compared to untreated control cells. Transcript levels of voltage-gated or inwardly rectifying K+ channels Kcnq1, Kcnj3 and Kcnj5 were significantly reduced, whereas Kcnk2, Kcnj2 and Kcnd3 mRNAs were upregulated. Ion channel remodeling was similarly observed at protein level. Short-term tachypacing did not induce significant transcriptional K+ channel remodeling.

CONCLUSION

The present findings link class I/II HDAC activity to regulation of ion channel expression and action potential duration in atrial cardiomyocytes. Clinical implications for HDAC-based antiarrhythmic therapy and cardiac safety of HDAC inhibitors require further investigation.

Consequences of Atrial or Ventricular Tachypacing on the Heat Shock Proteins (HSP) level of Expression and Phosphorylation

BACKGROUND

Uncontrolled atrial fibrillation (AF) results in complex changes in the cardiomyocyte electrical and contractile functioning that promote atrial remodeling and the continuation of AF. Recently there has been a growing interest in understanding the role of heat shock proteins (HSPs), which are cytoprotective molecular chaperones, in the pathophysiology of AF. Several groups have examined HSP expression in patients with AF but have yielded mixed results. To allow for better consistency and reproducibility between subjects, we utilized canine models to reproduce AFpromoting conditions to better investigate the role of HSPs in the pathophysiology of AF.

METHODS

AF promoting conditions were simulated in canine models with fifteen adult mongrel dogs (20.6 to 36.0 kg) divided into three groups: (1) Control (n=5), (2) two week ventricular tachypacing (VTP) induced congestive heart failure (CHF) (n=5), and (3) one week atrial tachypacying (ATP) (n=5). Quick frozen right atrial free wall tissue samples were used for protein isolation and were analyzed via Western blotting with data was expressed as a relative ratio and were analyzed using a two-tailed, unpaired ttest and significance was set at p < 0.05. The expression levels of HSP 90, 70, and 25 were studied along with the phosphorylation status of HSP27 at serine-78.

RESULTS

We first examined the effects of the ATP and CHF heart models on the expression of a select group of HSPs via Western Blot. We found that there was no significant difference in levels of expression of HSP 90, 70, or 25 when either ATP or CHF models were compared to control canines. The phosphorylation status of HSP27 was significantly decreased in the CHF canine model when compared to control (p < 0.0111) and it tended towards a decrease in the ATP canine model when compared to control (p=0.0923).

CONCLUSION

This study showed that even though the expression levels of HSPs may remain constant, there are protein phosphorylation and dephosphorylation events that occur in AF that may have important consequences in its pathophysiology. It is therefore necessary to investigate the full scale of HSP modifications during AF and AF-promoting conditions.