Kir2.3 knock-down decreases IK1 current in neonatal rat cardiomyocytes

Ionic current changes underlying action potential repolarization responses to physiological pacing and adrenergic stimulation in adult rat ventricular myocytes

This study aimed to simulate ventricular responses to elevations in myocyte pacing and adrenergic stimulation using a novel electrophysiological rat model and investigate ion channel responses underlying action potential (AP) modulations. Peak ion currents and AP repolarization to 50% and 90% of full repolarization (APD_50-90 ) were recorded during simulations at 1-10 Hz pacing under control and adrenergic stimulation conditions. Further simulations were performed with incremental ion current block (L-type calcium current, I_Ca ; transient outward current, I_to ; slow delayed rectifier potassium current, I_Ks ; rapid delayed rectifier potassium current, I_Kr ; inward rectifier potassium current, I_K1 ) to identify current influence on AP response to exercise. Simulated APD_50-90 closely resembled experimental findings. Rate-dependent increases in I_Ks (6%-101%), I_Kr (141%-1339%), and I_Ca (0%-15%) and reductions in I_to (11%-57%) and I_K1 (1%-9%) were observed. Meanwhile, adrenergic stimulation triggered moderate increases in all currents (23%-67%) except I_K1 . Further analyses suggest AP plateau is most sensitive to modulations in I_to and I_Ca while late repolarization is most sensitive to I_K1 , I_Ca , and I_Ks , with alterations in I_Ks predominantly stimulating the greatest magnitude of influence on late repolarization (35%-846% APD₉₀ prolongation). The modified Leeds rat model (mLR) is capable of accurately modeling APs during physiological stress. This study highlights the importance of I_Ca , I_to , I_K1, and I_Ks in controlling electrophysiological responses to exercise. This work will benefit the study of cardiac dysfunction, arrythmia, and disease, though future physiologically relevant experimental studies and model development are required.

MicroRNA-547-5p-mediated interleukin-33/suppressor of tumorigenicity 2 signaling underlies the genesis and maintenance of neuropathic pain and is targeted by the therapy with bone marrow stromal cells

Interleukin-33 (IL-33)/suppressor of tumorigenicity 2 (ST2) signaling is known to promote inflammation and the genesis and maintenance of neuropathic pain. However, it remained mostly unknown how IL-33/ST2 signaling can be enhanced by neuropathic stimulations. Here, we report that the chronic constriction nerve injury (CCI)-induced increases in the expression of IL-33 and ST2 and a decrease in microRNA (miRNA)-547-5p not only in the dorsal root ganglia (DRG) but also in spinal dorsal horn (SDH) ipsilateral to the CCI. We found that increasing endogenous miRNA-547-5p by the intrathecal (i.t.) infusion of agomir-miR-547-5p did not produce any effect in naive rats but blocked the CCI-induced increases in the IL-33 and ST2, and pain sensitivity. The reducing endogenous miRNA-547-5p by the i.t. delivering antagomir-miR-547-5p into naive rats caused significant changes in IL-33 and ST2 expressions in both the DRG and SDH, and pain sensitivity, which were similar to those induced by the CCI. Since increasing IL-33 by the i.t. infusion of recombinant IL-33 produced no change in the expression of miR-547-5p, and the CCI still reduced miR-547-5p expression in rats with the IL-33 knockdown, we conclude that the reduction of miR-547-5p can be an upstream event leading to the enhancement of IL-33/ST2 signaling induced by the CCI. The intravenous application of bone marrow stromal cells (BMSCs) reduced the depression of miR-547-5p in both the DRG and SDH, and pain hypersensitivity produced by the CCI or antagomir-miR547-5p application. However, the BMSC effect was significantly occluded by the pretreatment with miR-547-5p agomir or the IL-33 knockdown, demonstrating a novel mechanism underlying the BMSC therapy.

The expression of the rare caveolin-3 variant T78M alters cardiac ion channels function and membrane excitability

Aims

Caveolinopathies are a family of genetic disorders arising from alterations of the caveolin-3 (cav-3) gene. The T78M cav-3 variant has been associated with both skeletal and cardiac muscle pathologies but its functional contribution, especially to cardiac diseases, is still controversial. Here, we evaluated the effect of the T78M cav-3 variant on cardiac ion channel function and membrane excitability.

Methods and results

We transfected either the wild type (WT) or T78M cav-3 in caveolin-1 knock-out mouse embryonic fibroblasts and found by immunofluorescence and electron microscopy that both are expressed at the plasma membrane and form caveolae. Two ion channels known to interact and co-immunoprecipitate with the cav-3, hKv1.5 and hHCN4, interact also with T78M cav-3 and reside in lipid rafts. Electrophysiological analysis showed that the T78M cav-3 causes hKv1.5 channels to activate and inactivate at more hyperpolarized potentials and the hHCN4 channels to activate at more depolarized potentials, in a dominant way. In spontaneously beating neonatal cardiomyocytes, the expression of the T78M cav-3 significantly increased action potential peak-to-peak variability without altering neither the mean rate nor the maximum diastolic potential. We also found that in a small cohort of patients with supraventricular arrhythmias, the T78M cav-3 variant is more frequent than in the general population. Finally, in silico analysis of both sinoatrial and atrial cell models confirmed that the T78M-dependent changes are compatible with a pro-arrhythmic effect.

Conclusion

This study demonstrates that the T78M cav-3 induces complex modifications in ion channel function that ultimately alter membrane excitability. The presence of the T78M cav-3 can thus generate a susceptible substrate that, in concert with other structural alterations and/or genetic mutations, may become arrhythmogenic.

Extracellular pH regulates autophagy via the AMPK-ULK1 pathway in rat cardiomyocytes

Various pathological conditions contribute to pH fluctuations and affect the functions of vital organs such as the heart. In this study, we show that in rat cardiomyocytes, acidic extracellular pH (pHe) inhibits autophagy, whereas alkaline pHe stimulates it. We also find that adenosine monophosphate-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR) and Unc-51-like kinase 1 (ULK1) are very sensitive to pHe changes. Furthermore, by interfering with AMPK, mTOR or ULK1 activity, we demonstrate that the AMPK-ULK1 pathway, but not the mTOR pathway, plays a crucial role on pHe-regulated autophagy and cardiomyocyte viability. These data provide a potential therapeutic strategy against cardiomyocyte injury triggered by pH fluctuations.

Genetically engineered excitable cardiac myofibroblasts coupled to cardiomyocytes rescue normal propagation and reduce arrhythmia complexity in heterocellular monolayers

Rationale and Objective

The use of genetic engineering of unexcitable cells to enable expression of gap junctions and inward rectifier potassium channels has suggested that cell therapies aimed at establishing electrical coupling of unexcitable donor cells to host cardiomyocytes may be arrhythmogenic. Whether similar considerations apply when the donor cells are electrically excitable has not been investigated. Here we tested the hypothesis that adenoviral transfer of genes coding Kir2.1 (I_K1), Na_V1.5 (I_Na) and connexin-43 (Cx43) proteins into neonatal rat ventricular myofibroblasts (NRVF) will convert them into fully excitable cells, rescue rapid conduction velocity (CV) and reduce the incidence of complex reentry arrhythmias in an in vitro model.

Methods and Results

We used adenoviral (Ad-) constructs encoding Kir2.1, Na_V1.5 and Cx43 in NRVF. In single NRVF, Ad-Kir2.1 or Ad-Na_V1.5 infection enabled us to regulate the densities of I_K1 and I_Na, respectively. At varying MOI ratios of 10/10, 5/10 and 5/20, NRVF co-infected with Ad-Kir2.1+ Na_V1.5 were hyperpolarized and generated action potentials (APs) with upstroke velocities >100 V/s. However, when forming monolayers only the addition of Ad-Cx43 made the excitable NRVF capable of conducting electrical impulses (CV = 20.71±0.79 cm/s). When genetically engineered excitable NRVF overexpressing Kir2.1, Na_V1.5 and Cx43 were used to replace normal NRVF in heterocellular monolayers that included neonatal rat ventricular myocytes (NRVM), CV was significantly increased (27.59±0.76 cm/s vs. 21.18±0.65 cm/s, p<0.05), reaching values similar to those of pure myocytes monolayers (27.27±0.72 cm/s). Moreover, during reentry, propagation was faster and more organized, with a significantly lower number of wavebreaks in heterocellular monolayers formed by excitable compared with unexcitable NRVF.

Conclusion

Viral transfer of genes coding Kir2.1, Na_V1.5 and Cx43 to cardiac myofibroblasts endows them with the ability to generate and propagate APs. The results provide proof of concept that cell therapies with excitable donor cells increase safety and reduce arrhythmogenic potential.

Postnatal developmental decline in IK1 in mouse ventricular myocytes isolated by the Langendorff perfusion method: comparison with the chunk method

Expression and function of cardiac ion channels exhibit postnatal developmental changes, which, however, has not yet been proven in ventricular myocytes isolated using similar techniques. In this study, ventricular myocytes were enzymatically dissociated from mouse heart at different postnatal ages (including postnatal day 0) by similar techniques using Langendorff perfusion. Whole-cell patch-clamp experiments were performed to record action potentials, I (K1), I (Kr), I (Kur), I (ss), and I (Ca,L), in ventricular myocytes freshly isolated from postnatal days 0, 7, and 14 and adult mice. Viable ventricular myocytes of day-0 mouse heart exhibited spindle-shaped appearance having cell length of approximately 50 μm, which gradually developed to a rod-shaped one having clear cross striation with cell length of approximately 120 μm (adult). The action potential duration markedly shortened, while the resting membrane potential depolarized to a small but significant extent during postnatal development. I (K1) density was maximal in postnatal day-0 ventricular myocytes and gradually decreased during development, which was accompanied by postnatal depolarization of resting membrane potential. However, I (K1) density was markedly decreased by approximately 80% in postnatal day-0 ventricular myocytes, when isolated by the chunk method. Quantitative real-time polymerase chain reaction (PCR) and western blot analyses demonstrated higher Kir2.3 expression but lower expression levels of Kir2.1 and Kir2.2 in day-0 mouse ventricles, compared with those of day-14 and adult mouse ventricles. Whereas I (Kr) exhibited marked decrease during postnatal development, I (Kur), I (ss), and I (Ca,L) exhibited postnatal developmental increase. The present cell isolation method using the Langendorff perfusion thus found that, in mouse ventricles, I (K1) exhibited postnatal developmental decrease, associated with depolarization of resting potential.