Publications by authors named "Christopher L-H Huang"

194 Publications

A Comparative Study of Systolic and Diastolic Mechanical Synchrony in Canine, Primate, and Healthy and Failing Human Hearts.

Front Cardiovasc Med 2021 28;8:750067. Epub 2021 Oct 28.

Institute of Molecular Medicine, College of Future Technology, Peking University, Beijing, China.

Mechanical dyssynchrony (MD) is associated with heart failure (HF) and may be prognostically important in cardiac resynchronization therapy (CRT). Yet, little is known about its patterns in healthy or diseased hearts. We here investigate and compare systolic and diastolic MD in both right (RV) and left ventricles (LV) of canine, primate and healthy and failing human hearts. RV and LV mechanical function were examined by pulse-wave Doppler in 15 beagle dogs, 59 rhesus monkeys, 100 healthy human subjects and 39 heart failure (HF) patients. This measured RV and LV pre-ejection periods (RVPEP and LVPEP) and diastolic opening times (Q-TVE and Q-MVE). The occurrence of right (RVMDs) and left ventricular systolic mechanical delay (LVMDs) was assessed by comparing RVPEP and LVPEP values. That of right (RVMDd) and left ventricular diastolic mechanical delay (LVMDd) was assessed from the corresponding diastolic opening times (Q-TVE and Q-MVE). These situations were quantified by values of interventricular systolic (IVMDs) and diastolic mechanical delays (IVMDd), represented as positive if the relevant RV mechanical events preceded those in the LV. Healthy hearts in all species examined showed greater LV than RV delay times and therefore positive IVMDs and IVMDd. In contrast a greater proportion of the HF patients showed both markedly increased IVMDs and negative IVMDd, with diastolic mechanical asynchrony negatively correlated with LVEF. The present IVMDs and IVMDd findings have potential clinical implications particularly for personalized setting of parameter values in CRT in individual patients to achieve effective treatment of HF.
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http://dx.doi.org/10.3389/fcvm.2021.750067DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8581184PMC
October 2021

Ventricular SK2 upregulation following angiotensin II challenge: Modulation by p21-activated kinase-1.

J Mol Cell Cardiol 2021 Nov 10. Epub 2021 Nov 10.

Key Laboratory of Medical Electrophysiology of the Ministry of Education, Medical Electrophysiological Key Laboratory of Sichuan Province, Institute of Cardiovascular Research, Southwest Medical University, Luzhou, Sichuan 646000, China; Department of Cardiovascular Medicine, the Affiliated Hospital of Southwest Medical University, Luzhou, Sichuan 646000, China. Electronic address:

Effects of hypertrophic challenge on small-conductance, Ca-activated K(SK2) channel expression were explored in intact murine hearts, isolated ventricular myocytes and neonatal rat cardiomyocytes (NRCMs). An established experimental platform applied angiotensin II (Ang II) challenge in the presence and absence of reduced p21-activated kinase (PAK1) (PAK1 vs. PAK1, or shRNA-PAK1 interference) expression. SK2 current contributions were detected through their sensitivity to apamin block. Ang II treatment increased such SK2 contributions to optically mapped action potential durations (APD) and their heterogeneity, and to patch-clamp currents. Such changes were accentuated in PAK1 compared to PAK1, intact hearts and isolated cardiomyocytes. They paralleled increased histological and echocardiographic hypertrophic indices, reduced cardiac contractility, and increased SK2 protein expression, changes similarly greater with PAK1 than PAK1. In NRCMs, Ang II challenge replicated such increases in apamin-sensitive SK patch clamp currents as well as in real-time PCR and western blot measures of SK2 mRNA and protein expression and cell hypertrophy. Furthermore, the latter were enhanced by shRNA-PAK1 interference and mitigated by the PAK1 agonist FTY720. Increased CaMKII and CREB phosphorylation accompanied these effects. These were rescued by both FTY720 as well as the CaMKII inhibitor KN93, but not its inactive analogue KN92. Such CREB then specifically bound to the KCNN2 promoter sequence in luciferase assays. These findings associate Ang II induced hypertrophy with increased SK2 expression brought about by a CaMKII/CREB signaling convergent with the PAK1 pathway thence upregulating the KCNN2 promoter activity. SK2 may then influence cardiac electrophysiology under conditions of cardiac hypertrophy and failure.
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http://dx.doi.org/10.1016/j.yjmcc.2021.11.001DOI Listing
November 2021

Gating of RYR2 channels from the arrhythmic RYR2-P2328S mouse heart and some unexpected actions of flecainide.

J Gen Physiol 2022 Sep 12;154(9). Epub 2021 Nov 12.

Physiological Laboratory, University of Cambridge, Cambridge, UK.

The P2328S mutation in mice is associated with arrhythmia and spontaneous diastolic calcium release in atrial and ventricular myocytes and there is a corresponding leftward shift in the Ca2+-activation curve for mutant RYR2 channels from homozygous mouse hearts (Salvage et al. 2019. J Cell Sci. https://doi.org/10.1242/jcs.229039). P2328 is located in helical domain 1 (HD1) of RYR2. Local structural changes likely result when structurally active proline residues are replaced by structurally inert serine residues. We speculate that local structural changes in HD1 lead to sequential intradomain and interdomain stearic changes through the protein to the distant channel gate, which favor the open pore conformation. The drug flecainide prevents arrhythmia in humans and mouse models of CPVT by blocking NaV1.5 and RYR2 channels. Conventionally, flecainide blocks RYR2 channels in a voltage-dependent manner. We did not observe voltage-dependent pore block. This was possibly because, in contrast to previous studies, the only channel modulators that we used to produce end-diastolic control channel activity were 1 µM cytoplasmic Ca2+ and 1 mM luminal Ca2+. We observed previously unreported, voltage-independent increases in WT and P2328S channel activity at low flecainide concentrations, followed by a decline in activity at higher concentrations. The increase in activity dominated the effect of flecainide on P2328S channels. These effects suggested high-affinity flecainide binding to an activation site and lower-affinity binding to an inhibition site, both distant from the channel pore (Salvage et al. 2021. Cells. https://doi.org/10.3390/cells10082101). Unlike channel block by flecainide, the drug under our conditions stabilized intrinsic sub-conductance activity at +40 mV and -40 mV. Since flecainide effectively reduces CPVT arrythmia clinically and in animal models, we conclude that voltage-independent inhibition and voltage-dependent channel block prevail under cellular conditions. However, channel activation is important to note as it may be unmasked in other circumstances such as acquired cardiac disorders, mutations, or additional drug applications.
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http://dx.doi.org/10.1085/jgp.2021ecc42DOI Listing
September 2022

Electrophysiological and Proarrhythmic Effects of Hydroxychloroquine Challenge in Guinea-Pig Hearts.

ACS Pharmacol Transl Sci 2021 Oct 30;4(5):1639-1653. Epub 2021 Aug 30.

Henan SCOPE Research Institute of Electrophysiology Co. Ltd., Kaifeng 475000, China.

Hydroxychloroquine (HCQ), clinically established in antimalarial and autoimmune therapy, recently raised cardiac arrhythmogenic concerns when used alone or with azithromycin (HCQ+AZM) in Covid-19. We report complementary, experimental, studies of its electrophysiological effects. In patch clamped HEK293 cells expressing human cardiac ion channels, HCQ inhibited I and I at a therapeutic concentrations (ICs: 10 ± 0.6 and 34 ± 5.0 μM). I and I showed higher ICs; I and I were unaffected. AZM slightly inhibited I, I I and I, sparing I and I. (HCQ+AZM) inhibited I and I (ICs: 7.7 ± 0.8 and 30.4 ± 3.0 μM), sparing I, I, and I. Molecular induced-fit docking modeling confirmed potential HCQ-hERG but weak AZM-hERG binding. Effects of μM-HCQ were studied in isolated perfused guinea-pig hearts by multielectrode, optical RH237 voltage, and Rhod-2 mapping. These revealed reversibly reduced left atrial and ventricular action potential (AP) conduction velocities increasing their heterogeneities, increased AP durations (APDs), and increased durations and dispersions of intracellular [Ca] transients, respectively. Hearts also became bradycardic with increased electrocardiographic PR and QRS durations. The (HCQ+AZM) combination accentuated these effects. Contrastingly, (HCQ+AZM) and not HCQ alone disrupted AP propagation, inducing alternans and torsadogenic-like episodes on voltage mapping during forced pacing. O'Hara-Rudy modeling showed that the observed I and I effects explained the APD alterations and the consequently prolonged Ca transients. The latter might then downregulate I, reducing AP conduction velocity through recently reported I downregulation by cytosolic [Ca] in a novel scheme for drug action. The findings may thus prompt future investigations of HCQ's cardiac safety under particular, chronic and acute, clinical situations.
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http://dx.doi.org/10.1021/acsptsci.1c00166DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8506600PMC
October 2021

Ca2+-dependent modulation of voltage-gated myocyte sodium channels.

Biochem Soc Trans 2021 11;49(5):1941-1961

Department of Biochemistry, University of Cambridge, Cambridge, U.K.

Voltage-dependent Na+ channel activation underlies action potential generation fundamental to cellular excitability. In skeletal and cardiac muscle this triggers contraction via ryanodine-receptor (RyR)-mediated sarcoplasmic reticular (SR) Ca2+ release. We here review potential feedback actions of intracellular [Ca2+] ([Ca2+]i) on Na+ channel activity, surveying their structural, genetic and cellular and functional implications, translating these to their possible clinical importance. In addition to phosphorylation sites, both Nav1.4 and Nav1.5 possess potentially regulatory binding sites for Ca2+ and/or the Ca2+-sensor calmodulin in their inactivating III-IV linker and C-terminal domains (CTD), where mutations are associated with a range of skeletal and cardiac muscle diseases. We summarize in vitro cell-attached patch clamp studies reporting correspondingly diverse, direct and indirect, Ca2+ effects upon maximal Nav1.4 and Nav1.5 currents (Imax) and their half-maximal voltages (V1/2) characterizing channel gating, in cellular expression systems and isolated myocytes. Interventions increasing cytoplasmic [Ca2+]i down-regulated Imax leaving V1/2 constant in native loose patch clamped, wild-type murine skeletal and cardiac myocytes. They correspondingly reduced action potential upstroke rates and conduction velocities, causing pro-arrhythmic effects in intact perfused hearts. Genetically modified murine RyR2-P2328S hearts modelling catecholaminergic polymorphic ventricular tachycardia (CPVT), recapitulated clinical ventricular and atrial pro-arrhythmic phenotypes following catecholaminergic challenge. These accompanied reductions in action potential conduction velocities. The latter were reversed by flecainide at RyR-blocking concentrations specifically in RyR2-P2328S as opposed to wild-type hearts, suggesting a basis for its recent therapeutic application in CPVT. We finally explore the relevance of these mechanisms in further genetic paradigms for commoner metabolic and structural cardiac disease.
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http://dx.doi.org/10.1042/BST20200604DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8589445PMC
November 2021

Transcriptional profiles of genes related to electrophysiological function in Scn5a murine hearts.

Physiol Rep 2021 Oct;9(19):e15043

Faculty of Health and Medical Science, University of Surrey, Guildford, UK.

The Scn5a gene encodes the major pore-forming Na 1.5 (α) subunit, of the voltage-gated Na channel in cardiomyocytes. The key role of Na 1.5 in action potential initiation and propagation in both atria and ventricles predisposes organisms lacking Scn5a or carrying Scn5a mutations to cardiac arrhythmogenesis. Loss-of-function Na 1.5 genetic abnormalities account for many cases of the human arrhythmic disorder Brugada syndrome (BrS) and related conduction disorders. A murine model with a heterozygous Scn5a deletion recapitulates many electrophysiological phenotypes of BrS. This study examines the relationships between its Scn5a genotype, resulting transcriptional changes, and the consequent phenotypic presentations of BrS. Of 62 selected protein-coding genes related to cardiomyocyte electrophysiological or homeostatic function, concentrations of mRNA transcribed from 15 differed significantly from wild type (WT). Despite halving apparent ventricular Scn5a transcription heterozygous deletion did not significantly downregulate its atrial expression, raising possibilities of atria-specific feedback mechanisms. Most of the remaining 14 genes whose expression differed significantly between WT and Scn5a animals involved Ca homeostasis specifically in atrial tissue, with no overlap with any ventricular changes. All statistically significant changes in expression were upregulations in the atria and downregulations in the ventricles. This investigation demonstrates the value of future experiments exploring for and clarifying links between transcriptional control of Scn5a and of genes whose protein products coordinate Ca regulation and examining their possible roles in BrS.
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http://dx.doi.org/10.14814/phy2.15043DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8495800PMC
October 2021

Flecainide Paradoxically Activates Cardiac Ryanodine Receptor Channels under Low Activity Conditions: A Potential Pro-Arrhythmic Action.

Cells 2021 08 16;10(8). Epub 2021 Aug 16.

Eccles Institute of Neuroscience, John Curtin School of Medical Research, The Australian National University, 131 Garran Road, Acton 2601, Australia.

Cardiac ryanodine receptor (RyR2) mutations are implicated in the potentially fatal catecholaminergic polymorphic ventricular tachycardia (CPVT) and in atrial fibrillation. CPVT has been successfully treated with flecainide monotherapy, with occasional notable exceptions. Reported actions of flecainide on cardiac sodium currents from mice carrying the pro-arrhythmic homozygotic RyR2-P2328S mutation prompted our explorations of the effects of flecainide on their RyR2 channels. Lipid bilayer electrophysiology techniques demonstrated a novel, paradoxical increase in RyR2 activity. Preceding flecainide exposure, channels were mildly activated by 1 mM luminal Ca and 1 µM cytoplasmic Ca, with open probabilities () of 0.03 ± 0.01 (wild type, WT) or 0.096 ± 0.024 (P2328S). Open probability () increased within 0.5 to 3 min of exposure to 0.5 to 5.0 µM cytoplasmic flecainide, then declined with higher concentrations of flecainide. There were no such increases in a subset of high channels with ≥ 0.08, although then declined with ≥5 µM (WT) or ≥50 µM flecainide (P2328S). On average, channels with < 0.08 were significantly activated by 0.5 to 10 µM of flecainide (WT) or 0.5 to 50 µM of flecainide (P2328S). These results suggest that flecainide can bind to separate activation and inhibition sites on RyR2, with activation dominating in lower activity channels and inhibition dominating in more active channels.
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http://dx.doi.org/10.3390/cells10082101DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8394964PMC
August 2021

Finite element analysis predicts Ca microdomains within tubular-sarcoplasmic reticular junctions of amphibian skeletal muscle.

Sci Rep 2021 07 13;11(1):14376. Epub 2021 Jul 13.

Physiological Laboratory, University of Cambridge, Downing Street, Cambridge, CB2 3EG, UK.

A finite element analysis modelled diffusional generation of steady-state Ca microdomains within skeletal muscle transverse (T)-tubular-sarcoplasmic reticular (SR) junctions, sites of ryanodine receptor (RyR)-mediated SR Ca release. It used established quantifications of sarcomere and T-SR anatomy (radial diameter [Formula: see text]; axial distance [Formula: see text]). Its boundary SR Ca influx densities,[Formula: see text], reflected step impositions of influxes, [Formula: see text] deduced from previously measured Ca signals following muscle fibre depolarization. Predicted steady-state T-SR junctional edge [Ca], [Ca] matched reported corresponding experimental cytosolic [Ca] elevations given diffusional boundary efflux [Formula: see text] established cytosolic Ca diffusion coefficients [Formula: see text] and exit length [Formula: see text]. Dependences of predicted [Ca] upon [Formula: see text] then matched those of experimental [Ca] upon Ca release through their entire test voltage range. The resulting model consistently predicted elevated steady-state T-SR junctional ~ µM-[Ca] elevations radially declining from maxima at the T-SR junction centre along the entire axial T-SR distance. These [Ca] heterogeneities persisted through 10- and fivefold, variations in D and w around, and fivefold reductions in d below, control values, and through reported resting muscle cytosolic [Ca] values, whilst preserving the flux conservation ([Formula: see text] condition, [Formula: see text]. Skeletal muscle thus potentially forms physiologically significant ~ µM-[Ca] T-SR microdomains that could regulate cytosolic and membrane signalling molecules including calmodulin and RyR, These findings directly fulfil recent experimental predictions invoking such Ca microdomains in observed regulatory effects upon Na channel function, in a mechanism potentially occurring in similar restricted intracellular spaces in other cell types.
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http://dx.doi.org/10.1038/s41598-021-93083-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8277803PMC
July 2021

Molecular basis of ventricular arrhythmogenicity in a Pgc-1α deficient murine model.

Mol Genet Metab Rep 2021 Jun 9;27:100753. Epub 2021 Apr 9.

Faculty of Health and Medical Sciences, University of Surrey, GU2 7AL Guildford, United Kingdom.

Mitochondrial dysfunction underlying metabolic disorders such as obesity and diabetes mellitus is strongly associated with cardiac arrhythmias. Murine Pgc-1α hearts replicate disrupted mitochondrial function and model the associated pro-arrhythmic electrophysiological abnormalities. Quantitative PCR, western blotting and histological analysis were used to investigate the molecular basis of the electrophysiological changes associated with mitochondrial dysfunction. qPCR analysis implicated downregulation of genes related to Na-K ATPase activity (Atp1b1), surface Ca entry (Cacna1c), action potential repolarisation (Kcnn1), autonomic function (Adra1d, Adcy4, Pde4d, Prkar2a), and morphological properties (Myh6, Tbx3) in murine Pgc-1α ventricles. Western blotting revealed reduced Na1.5 but normal Cx43 expression. Histological analysis revealed increased tissue fibrosis in the Pgc-1α ventricles. These present findings identify altered transcription amongst a strategically selected set of genes established as encoding proteins involved in cardiac electrophysiological activation and therefore potentially involved in alterations in ventricular activation and Ca homeostasis in arrhythmic substrate associated with Pgc-1α deficiency. They complement and complete previous studies examining such expression characteristics in the atria and ventricles of Pgc-1 deficient murine hearts.
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http://dx.doi.org/10.1016/j.ymgmr.2021.100753DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8059080PMC
June 2021

Symmetric Projection Attractor Reconstruction analysis of murine electrocardiograms: Retrospective prediction of Scn5a genetic mutation attributable to Brugada syndrome.

Heart Rhythm O2 2020 Dec;1(5):368-375

Department of Mathematics, University of Surrey, Guildford, United Kingdom.

Background: Life-threatening arrhythmias resulting from genetic mutations are often missed in current electrocardiogram (ECG) analysis. We combined a new method for ECG analysis that uses all the waveform data with machine learning to improve detection of such mutations from short ECG signals in a mouse model.

Objective: We sought to detect consequences of Na channel deficiencies known to compromise action potential conduction in comparisons of Scn5a mutant and wild-type mice using short ECG signals, examining novel and standard features derived from lead I and II ECG recordings by machine learning algorithms.

Methods: Lead I and II ECG signals from anesthetized wild-type and Scn5a mutant mice of length 130 seconds were analyzed by extracting various groups of features, which were used by machine learning to classify the mice as wild-type or mutant. The features used were standard ECG intervals and amplitudes, as well as features derived from attractors generated using the novel Symmetric Projection Attractor Reconstruction method, which reformulates the whole signal as a bounded, symmetric 2-dimensional attractor. All the features were also combined as a single feature group.

Results: Classification of genotype using the attractor features gave higher accuracy than using either the ECG intervals or the intervals and amplitudes. However, the highest accuracy (96%) was obtained using all the features. Accuracies for different subgroups of the data were obtained and compared.

Conclusion: Detection of the Scn5a mutation from short mouse ECG signals with high accuracy is possible using our Symmetric Projection Attractor Reconstruction method.
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http://dx.doi.org/10.1016/j.hroo.2020.08.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7962089PMC
December 2020

Sarcoplasmic reticular Ca-ATPase inhibition paradoxically upregulates murine skeletal muscle Na1.4 function.

Sci Rep 2021 02 2;11(1):2846. Epub 2021 Feb 2.

Physiological Laboratory, University of Cambridge, Cambridge, CB2 3EG, UK.

Skeletal muscle Na channels possess Ca- and calmodulin-binding sites implicated in Nav1.4 current (I) downregulation following ryanodine receptor (RyR1) activation produced by exchange protein directly activated by cyclic AMP or caffeine challenge, effects abrogated by the RyR1-antagonist dantrolene which itself increased I. These findings were attributed to actions of consequently altered cytosolic Ca, [Ca], on Na1.4. We extend the latter hypothesis employing cyclopiazonic acid (CPA) challenge, which similarly increases [Ca], but through contrastingly inhibiting sarcoplasmic reticular (SR) Ca-ATPase. Loose patch clamping determined Na current (I) families in intact native murine gastrocnemius skeletal myocytes, minimising artefactual [Ca] perturbations. A bespoke flow system permitted continuous I comparisons through graded depolarizing steps in identical stable membrane patches before and following solution change. In contrast to the previous studies modifying RyR1 activity, and imposing control solution changes, CPA (0.1 and 1 µM) produced persistent increases in I within 1-4 min of introduction. CPA pre-treatment additionally abrogated previously reported reductions in I produced by 0.5 mM caffeine. Plots of peak current against voltage excursion demonstrated that 1 µM CPA increased maximum I by ~ 30%. It only slightly decreased half-maximal activating voltages (V) and steepness factors (k), by 2 mV and 0.7, in contrast to the V and k shifts reported with direct RyR1 modification. These paradoxical findings complement previously reported downregulatory effects on Nav1.4 of RyR1-agonist mediated increases in bulk cytosolic [Ca]. They implicate possible local tubule-sarcoplasmic triadic domains containing reduced [Ca] in the observed upregulation of Nav1.4 function following CPA-induced SR Ca depletion.
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http://dx.doi.org/10.1038/s41598-021-82493-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7854688PMC
February 2021

Protein expression profiles in murine ventricles modeling catecholaminergic polymorphic ventricular tachycardia: effects of genotype and sex.

Ann N Y Acad Sci 2020 10 26;1478(1):63-74. Epub 2020 Jul 26.

Faculty of Health and Medical Sciences, University of Surrey, Guildford, United Kingdom.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is associated with mutations in the cardiac ryanodine receptor (RyR2). These result in stress-induced ventricular arrhythmic episodes, with clinical symptoms and prognosis reported more severe in male than female patients. Murine homozygotic RyR2-P2328S (RyR2 ) hearts replicate the proarrhythmic CPVT phenotype of abnormal sarcoplasmic reticular Ca leak and disrupted Ca homeostasis. In addition, RyR2 hearts show decreased myocardial action potential conduction velocities (CV), all features implicated in arrhythmic trigger and substrate. The present studies explored for independent and interacting effects of RyR2 genotype and sex on expression levels of molecular determinants of Ca homeostasis (CASQ2, FKBP12, SERCA2a, NCX1, and Ca 1.2) and CV (Na 1.5, Connexin (Cx)-43, phosphorylated-Cx43, and TGF-β1) in mice. Expression levels of Ca homeostasis proteins were not altered, hence implicating abnormal RyR2 function alone in disrupted cytosolic Ca homeostasis. Furthermore, altered Na 1.5, phosphorylated Cx43, and TGF-β1 expression were not implicated in the development of slowed CV. By contrast, decreased Cx43 expression correlated with slowed CV, in female, but not male, RyR2 mice. The CV changes may reflect acute actions of the increased cytosolic Ca on Na 1.5 and Cx43 function.
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http://dx.doi.org/10.1111/nyas.14426DOI Listing
October 2020

Cell-Adhesion Properties of β-Subunits in the Regulation of Cardiomyocyte Sodium Channels.

Biomolecules 2020 07 1;10(7). Epub 2020 Jul 1.

Department of Biochemistry, University of Cambridge, Cambridge CB2 1QW, UK.

Voltage-gated sodium (Nav) channels drive the rising phase of the action potential, essential for electrical signalling in nerves and muscles. The Nav channel α-subunit contains the ion-selective pore. In the cardiomyocyte, Nav1.5 is the main Nav channel α-subunit isoform, with a smaller expression of neuronal Nav channels. Four distinct regulatory β-subunits (β1-4) bind to the Nav channel α-subunits. Previous work has emphasised the β-subunits as direct Nav channel gating modulators. However, there is now increasing appreciation of additional roles played by these subunits. In this review, we focus on β-subunits as homophilic and heterophilic cell-adhesion molecules and the implications for cardiomyocyte function. Based on recent cryogenic electron microscopy (cryo-EM) data, we suggest that the β-subunits interact with Nav1.5 in a different way from their binding to other Nav channel isoforms. We believe this feature may facilitate -cell-adhesion between β1-associated Nav1.5 subunits on the intercalated disc and promote ephaptic conduction between cardiomyocytes.
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http://dx.doi.org/10.3390/biom10070989DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7407995PMC
July 2020

Editorial: Optogenetics: An Emerging Approach in Cardiac Electrophysiology.

Front Physiol 2020 28;11:414. Epub 2020 Apr 28.

Key Laboratory of Medical Electrophysiology of the Ministry of Education and Institute of Cardiovascular Research, Southwest Medical University, Luzhou, China.

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http://dx.doi.org/10.3389/fphys.2020.00414DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7198899PMC
April 2020

The complexity of clinically-normal sinus-rhythm ECGs is decreased in equine athletes with a diagnosis of paroxysmal atrial fibrillation.

Sci Rep 2020 04 22;10(1):6822. Epub 2020 Apr 22.

Faculty of Health and Medical Sciences, University of Surrey, Guildford, GU2 7AL, United Kingdom.

Equine athletes have a pattern of exercise which is analogous to human athletes and the cardiovascular risks in both species are similar. Both species have a propensity for atrial fibrillation (AF), which is challenging to detect by ECG analysis when in paroxysmal form. We hypothesised that the proarrhythmic background present between fibrillation episodes in paroxysmal AF (PAF) might be detectable by complexity analysis of apparently normal sinus-rhythm ECGs. In this retrospective study ECG recordings were obtained during routine clinical work from 82 healthy horses and from 10 horses with a diagnosis of PAF. Artefact-free 60-second strips of normal sinus-rhythm ECGs were converted to binary strings using threshold crossing, beat detection and a novel feature detection parsing algorithm. Complexity of the resulting binary strings was calculated using Lempel-Ziv ('76 & '78) and Titchener complexity estimators. Dependence of Lempel-Ziv '76 and Titchener T-complexity on the heart rate in ECG strips obtained at low heart rates (25-60 bpm) and processed by the feature detection method was found to be significantly different in control animals and those diagnosed with PAF. This allows identification of horses with PAF from sinus-rhythm ECGs with high accuracy.
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http://dx.doi.org/10.1038/s41598-020-63343-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7176685PMC
April 2020

Bisphosphonates and atrial fibrillation: revisiting the controversy.

Ann N Y Acad Sci 2020 08 24;1474(1):15-26. Epub 2020 Mar 24.

Faculty of Health and Medical Sciences, University of Surrey, Guildford, United Kingdom.

Bisphosphonates (BPs) are widely prescribed drugs used to treat osteoporosis, commonly arising in postmenopausal women and in chronic glucocorticoid use. Their mechanism of action is through inhibiting osteoclast-induced bone remodeling, and they also possess calcium sequestering properties. Common side effects involve the gastrointestinal system and rare but serious side effects, including osteonecrosis of the jaw. However, a link between BPs and atrial fibrillation (AF) has been proposed, with early clinical trials, such as the Fracture Intervention Trial and the HORIZON Pivotal Fracture Trial, reporting that BPs are associated with increased risk of AF. Nevertheless, subsequent studies have reported contrasting results, ranging from no effect of BPs to antiarrhythmic effects of BPs. Preclinical and electrophysiological studies on any proarrhythmic effect of BPs are limited in scope and number, but suggest possible mechanisms that include antiangionesis-related myocardial remodeling, calcium handling abnormalities, and inflammatory changes. Contrastingly, some studies indicate that BPs are antiarrhythmic by inhibiting fibrotic myocardial remodeling. In order to continue established clinical prescribing of BPs within absolute margins of safety, it will be necessary to systematically rule in/rule out these mechanisms. Thus, we discuss these studies and examine in detail the potential mechanistic links, with the aim of suggesting further avenues for research.
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http://dx.doi.org/10.1111/nyas.14332DOI Listing
August 2020

Factors affecting the electrocardiographic QT interval in malaria: A systematic review and meta-analysis of individual patient data.

PLoS Med 2020 03 5;17(3):e1003040. Epub 2020 Mar 5.

Cardiovascular Institute, Hospital Clinic, University of Barcelona, Barcelona, Spain.

Background: Electrocardiographic QT interval prolongation is the most widely used risk marker for ventricular arrhythmia potential and thus an important component of drug cardiotoxicity assessments. Several antimalarial medicines are associated with QT interval prolongation. However, interpretation of electrocardiographic changes is confounded by the coincidence of peak antimalarial drug concentrations with recovery from malaria. We therefore reviewed all available data to characterise the effects of malaria disease and demographic factors on the QT interval in order to improve assessment of electrocardiographic changes in the treatment and prevention of malaria.

Methods And Findings: We conducted a systematic review and meta-analysis of individual patient data. We searched clinical bibliographic databases (last on August 21, 2017) for studies of the quinoline and structurally related antimalarials for malaria-related indications in human participants in which electrocardiograms were systematically recorded. Unpublished studies were identified by the World Health Organization (WHO) Evidence Review Group (ERG) on the Cardiotoxicity of Antimalarials. Risk of bias was assessed using the Pharmacoepidemiological Research on Outcomes of Therapeutics by a European Consortium (PROTECT) checklist for adverse drug events. Bayesian hierarchical multivariable regression with generalised additive models was used to investigate the effects of malaria and demographic factors on the pretreatment QT interval. The meta-analysis included 10,452 individuals (9,778 malaria patients, including 343 with severe disease, and 674 healthy participants) from 43 studies. 7,170 (68.6%) had fever (body temperature ≥ 37.5°C), and none developed ventricular arrhythmia after antimalarial treatment. Compared to healthy participants, patients with uncomplicated falciparum malaria had shorter QT intervals (-61.77 milliseconds; 95% credible interval [CI]: -80.71 to -42.83) and increased sensitivity of the QT interval to heart rate changes. These effects were greater in severe malaria (-110.89 milliseconds; 95% CI: -140.38 to -81.25). Body temperature was associated independently with clinically significant QT shortening of 2.80 milliseconds (95% CI: -3.17 to -2.42) per 1°C increase. Study limitations include that it was not possible to assess the effect of other factors that may affect the QT interval but are not consistently collected in malaria clinical trials.

Conclusions: Adjustment for malaria and fever-recovery-related QT lengthening is necessary to avoid misattributing malaria-disease-related QT changes to antimalarial drug effects. This would improve risk assessments of antimalarial-related cardiotoxicity in clinical research and practice. Similar adjustments may be indicated for other febrile illnesses for which QT-interval-prolonging medications are important therapeutic options.
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http://dx.doi.org/10.1371/journal.pmed.1003040DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7058280PMC
March 2020

Ryanodine receptor modulation by caffeine challenge modifies Na current properties in intact murine skeletal muscle fibres.

Sci Rep 2020 02 10;10(1):2199. Epub 2020 Feb 10.

Physiological Laboratory, University of Cambridge, Downing Street, Cambridge, CB2 3EG, United Kingdom.

We investigated effects of the ryanodine receptor (RyR) modulator caffeine on Na current (I) activation and inactivation in intact loose-patch clamped murine skeletal muscle fibres subject to a double pulse procedure. I activation was examined using 10-ms depolarising, V, steps to varying voltages 0-80 mV positive to resting membrane potential. The dependence of the subsequent, I inactivation on V was examined by superimposed, V, steps to a fixed depolarising voltage. Current-voltage activation and inactivation curves indicated that adding 0.5 and 2 mM caffeine prior to establishing the patch seal respectively produced decreased (within 1 min) and increased (after ~2 min) peak I followed by its recovery to pretreatment levels (after ~40 and ~30 min respectively). These changes accompanied negative shifts in the voltage dependence of I inactivation (within 10 min) and subsequent superimposed positive activation and inactivation shifts, following 0.5 mM caffeine challenge. In contrast, 2 mM caffeine elicited delayed negative shifts in both activation and inactivation. These effects were abrogated if caffeine was added after establishing the patch seal or with RyR block by 10 μM dantrolene. These effects precisely paralleled previous reports of persistently (~10 min) increased cytosolic [Ca] with 0.5 mM, and an early peak rapidly succeeded by persistently reduced [Ca] likely reflecting gradual RyR inactivation with ≥1.0 mM caffeine. The latter findings suggested inhibitory effects of even resting cytosolic [Ca] on I. They suggest potentially physiologically significant negative feedback regulation of RyR activity on Na1.4 properties through increased or decreased local cytosolic [Ca], Ca-calmodulin and FKBP12.
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http://dx.doi.org/10.1038/s41598-020-59196-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7010675PMC
February 2020

The cardiac CaMKII-Na1.5 relationship: From physiology to pathology.

J Mol Cell Cardiol 2020 02 18;139:190-200. Epub 2020 Jan 18.

Faculty of Health and Medical Sciences, University of Surrey, Guildford, GU2 7AL, United Kingdom; Physiological Laboratory, University of Cambridge, Downing Street, Cambridge, CB2 3EG, United Kingdom. Electronic address:

The SCN5A gene encodes Na1.5, which, as the cardiac voltage-gated Na channel's pore-forming α subunit, is crucial for the initiation and propagation of atrial and ventricular action potentials. The arrhythmogenic propensity of inherited SCN5A mutations implicates the Na channel in determining cardiomyocyte excitability under normal conditions. Cytosolic kinases have long been known to alter the kinetic profile of Na1.5 inactivation via phosphorylation of specific residues. Recent substantiation of both the role of calmodulin-dependent kinase II (CaMKII) in modulating the properties of the Na1.5 inactivation gate and the significant rise in oxidation-dependent autonomous CaMKII activity in structural heart disease has raised the possibility of a novel pathway for acquired arrhythmias - the CaMKII-Na1.5 relationship. The aim of this review is to: (1) outline the relationship's translation from physiological adaptation to pathological vicious circle; and (2) discuss the relative merits of each of its components as pharmacological targets.
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http://dx.doi.org/10.1016/j.yjmcc.2019.12.014DOI Listing
February 2020

Supramolecular clustering of the cardiac sodium channel Nav1.5 in HEK293F cells, with and without the auxiliary β3-subunit.

FASEB J 2020 03 16;34(3):3537-3553. Epub 2020 Jan 16.

Deparment of Biochemistry, University of Cambridge, Cambridge, UK.

Voltage-gated sodium channels comprise an ion-selective α-subunit and one or more associated β-subunits. The β3-subunit (encoded by the SCN3B gene) is an important physiological regulator of the heart-specific sodium channel, Nav1.5. We have previously shown that when expressed alone in HEK293F cells, the full-length β3-subunit forms trimers in the plasma membrane. We extend this result with biochemical assays and use the proximity ligation assay (PLA) to identify oligomeric β3-subunits, not just at the plasma membrane, but throughout the secretory pathway. We then investigate the corresponding clustering properties of the α-subunit and the effects upon these of the β3-subunits. The oligomeric status of the Nav1.5 α-subunit in vivo, with or without the β3-subunit, has not been previously investigated. Using super-resolution fluorescence imaging, we show that under conditions typically used in electrophysiological studies, the Nav1.5 α-subunit assembles on the plasma membrane of HEK293F cells into spatially localized clusters rather than individual and randomly dispersed molecules. Quantitative analysis indicates that the β3-subunit is not required for this clustering but β3 does significantly change the distribution of cluster sizes and nearest-neighbor distances between Nav1.5 α-subunits. However, when assayed by PLA, the β3-subunit increases the number of PLA-positive signals generated by anti-(Nav1.5 α-subunit) antibodies, mainly at the plasma membrane. Since PLA can be sensitive to the orientation of proteins within a cluster, we suggest that the β3-subunit introduces a significant change in the relative alignment of individual Nav1.5 α-subunits, but the clustering itself depends on other factors. We also show that these structural and higher-order changes induced by the β3-subunit do not alter the degree of electrophysiological gating cooperativity between Nav1.5 α-subunits. Our data provide new insights into the role of the β3-subunit and the supramolecular organization of sodium channels, in an important model cell system that is widely used to study Nav channel behavior.
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http://dx.doi.org/10.1096/fj.201701473RRDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7079131PMC
March 2020

Update on antiarrhythmic drug pharmacology.

J Cardiovasc Electrophysiol 2020 02 22;31(2):579-592. Epub 2020 Jan 22.

Key Laboratory of Medical Electrophysiology of Ministry of Education, Institute of Cardiovascular Research, Southwest Medical University, Luzhou, China.

Cardiac arrhythmias constitute a major public health problem. Pharmacological intervention remains mainstay to their clinical management. This, in turn, depends upon systematic drug classification schemes relating their molecular, cellular, and systems effects to clinical indications and therapeutic actions. This approach was first pioneered in the 1960s Vaughan-Williams classification. Subsequent progress in cardiac electrophysiological understanding led to a lag between the fundamental science and its clinical translation, partly addressed by The working group of the European Society of Cardiology (1991), which, however, did not emerge with formal classifications. We here utilize the recent Revised Oxford Classification Scheme to review antiarrhythmic drug pharmacology. We survey drugs and therapeutic targets offered by the more recently characterized ion channels, transporters, receptors, intracellular Ca handling, and cell signaling molecules. These are organized into their strategic roles in cardiac electrophysiological function. Following analysis of the arrhythmic process itself, we consider (a) pharmacological agents directly targeting membrane function, particularly the Na and K ion channels underlying depolarizing and repolarizing events in the cardiac action potential. (b) We also consider agents that modify autonomic activity that, in turn, affects both the membrane and (c) the Ca homeostatic and excitation-contraction coupling processes linking membrane excitation to contractile activation. Finally, we consider (d) drugs acting on more upstream energetic and structural remodeling processes currently the subject of clinical trials. Such systematic correlations of drug actions and arrhythmic mechanisms at different molecular to systems levels of cardiac function will facilitate current and future antiarrhythmic therapy.
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http://dx.doi.org/10.1111/jce.14347DOI Listing
February 2020

Is the sigma-1 receptor a potential pharmacological target for cardiac pathologies? A systematic review.

Int J Cardiol Heart Vasc 2020 Feb 28;26:100449. Epub 2019 Dec 28.

School of Veterinary Medicine, Faculty of Health and Medical Sciences, University of Surrey, Surrey GU2 7AL, UK.

Sigma-1 receptors are ligand-regulated chaperone proteins, involved in several cellular mechanisms. The aim of this systematic review was to examine the effects that the sigma-1 receptor has on the cardiovascular system. The interaction targets and proposed mechanisms of action of sigma-1 receptors were explored, with the aim of determining if the sigma-1 receptor is a potential pharmacological target for cardiac pathologies. This systematic review was conducted according to the PRISMA guidelines and these were used to critically appraise eligible studies. Pubmed and Scopus were systematically searched for articles investigating sigma-1 receptors in the cardiovascular system. Papers identified by the search terms were then subject to analysis against pre-determined inclusion criteria. 23 manuscripts met the inclusion criteria and were included in this review. The experimental platforms, experimental techniques utilised and the results of the studies were summarised. The sigma-1 receptor is found to be implicated in cardioprotection, via various mechanisms including stimulating the Akt-eNOS pathway, and reduction of Ca2 + leakage into the cytosol via modulating certain calcium channels. Sigma-1 receptors are also found to modulate other cardiac ion channels including different subtypes of potassium and sodium channels and have been shown to modulate intracardiac neuron excitability. The sigma-1 receptor is a potential therapeutic target for treatment of cardiac pathologies, particularly cardiac hypertrophy. We therefore suggest investigating the cardioprotective mechanisms of sigma-1 receptor function, alongside proposed potential ligands that can stimulate these functions.
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http://dx.doi.org/10.1016/j.ijcha.2019.100449DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6939113PMC
February 2020

Cardiac arrhythmogenesis: a tale of two clocks?

Cardiovasc Res 2020 12;116(14):e205-e209

Physiological Laboratory and Department of Biochemistry, University of Cambridge, Cambridge CB2 3EG, UK.

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http://dx.doi.org/10.1093/cvr/cvz283DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7695354PMC
December 2020

Molecular basis of arrhythmic substrate in ageing murine peroxisome proliferator-activated receptor γ co-activator deficient hearts modelling mitochondrial dysfunction.

Biosci Rep 2019 12;39(12)

Faculty of Health and Medical Sciences, University of Surrey, Guildford GU2 7AL, United Kingdom.

Introduction: Ageing and chronic metabolic disorders are associated with mitochondrial dysfunction and cardiac pro-arrhythmic phenotypes which were recently attributed to slowed atrial and ventricular action potential (AP) conduction in peroxisome proliferator-activated receptor γ co-activator deficient (Pgc-1β-/-) mice.

Methods: We compared expression levels of voltage-gated Na+ channel (NaV1.5) and gap junction channels, Connexins 40 and 43 (Cx40 and Cx43) in the hearts of young and old, and wild-type (WT) and Pgc-1β-/- mice. This employed Western blotting (WB) for NaV1.5, Cx40 and Cx43 in atrial/ventricular tissue lysates, and immunofluorescence (IF) from Cx43 was explored in tissue sections. Results were analysed using two-way analysis of variance (ANOVA) for independent/interacting effects of age and genotype.

Results: In atria, increased age and Pgc-1β-/- genotype each independently decreased both Cx40 and Cx43 expression without interacting effects. In IF experiments, both age and Pgc-1β deletion independently reduced Cx43 expression. In ventricles, age and genotype exerted interacting effects in WB studies of NaV1.5 expression. Young Pgc-1β-/- then showed greater NaV1.5 expression than young WT ventricles. However, neither age nor Pgc-1β deletion affected Cx43 expression, independently or through interacting effects in both WB and IF studies.

Conclusion: Similar pro-arrhythmic atrial/ventricular phenotypes arise in aged/Pgc-1β-/- from differing contributions of altered protein expression and functional effects that may arise from multiple acute mechanisms.
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http://dx.doi.org/10.1042/BSR20190403DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6911157PMC
December 2019

Gating control of the cardiac sodium channel Nav1.5 by its β3-subunit involves distinct roles for a transmembrane glutamic acid and the extracellular domain.

J Biol Chem 2019 12 28;294(51):19752-19763. Epub 2019 Oct 28.

Department of Biochemistry, University of Cambridge, Cambridge CB2 1QW, United Kingdom

The auxiliary β3-subunit is an important functional regulator of the cardiac sodium channel Nav1.5, and some β3 mutations predispose individuals to cardiac arrhythmias. The β3-subunit uses its transmembrane α-helix and extracellular domain to bind to Nav1.5. Here, we investigated the role of an unusually located and highly conserved glutamic acid (Glu-176) within the β3 transmembrane region and its potential for functionally synergizing with the β3 extracellular domain (ECD). We substituted Glu-176 with lysine (E176K) in the WT β3-subunit and in a β3-subunit lacking the ECD. Patch-clamp experiments indicated that the E176K substitution does not affect the previously observed β3-dependent depolarizing shift of of steady-state inactivation but does attenuate the accelerated recovery from inactivation conferred by the WT β3-subunit. Removal of the β3-ECD abrogated both the depolarizing shift of steady-state inactivation and the accelerated recovery, irrespective of the presence or absence of the Glu-176 residue. We found that steady-state inactivation and recovery from inactivation involve movements of the S4 helices within the DIII and DIV voltage sensors in response to membrane potential changes. Voltage-clamp fluorometry revealed that the E176K substitution alters DIII voltage sensor dynamics without affecting DIV. In contrast, removal of the ECD significantly altered the dynamics of both DIII and DIV. These results imply distinct roles for the β3-Glu-176 residue and the β3-ECD in regulating the conformational changes of the voltage sensors that determine channel inactivation and recovery from inactivation.
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http://dx.doi.org/10.1074/jbc.RA119.010283DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6926464PMC
December 2019

Principles of Optogenetic Methods and Their Application to Cardiac Experimental Systems.

Front Physiol 2019 11;10:1096. Epub 2019 Sep 11.

Physiological Laboratory, University of Cambridge, Cambridge, United Kingdom.

Optogenetic techniques permit studies of excitable tissue through genetically expressed light-gated microbial channels or pumps permitting transmembrane ion movement. Light activation of these proteins modulates cellular excitability with millisecond precision. This review summarizes optogenetic approaches, using examples from neurobiological applications, and then explores their application in cardiac electrophysiology. We review the , including depolarizing and hyperpolarizing variants, as well as modulators of G-protein coupled intracellular signaling. We discuss the biophysical properties that determine the ability of microbial opsins to evoke reliable, precise stimulation or silencing of electrophysiological activity. We also review variants offering possibilities for enhanced depth of tissue penetration, combinatorial stimulation for targeting different cell subpopulations, or all-optical read-in and read-out studies. in the cardiac cell of interest then requires, at the , introduction of opsin-encoding genes by viral transduction, or coupling "spark cells" to primary cardiomyocytes or a stem-cell derived counterpart. At the , this requires construction of transgenic mice expressing ChR2 in their cardiomyocytes, or injection (myocardial or systemic) of adenoviral expression systems. , by laser or LED, with widespread or multipoint illumination, although relatively straightforward may be technically challenged by cardiac motion and light-scattering in biological tissue. from cardiac optogenetic stimulation include single cell patch clamp recordings, multi-unit microarray recordings from cell monolayers or slices, and electrical recordings from isolated Langendorff perfused hearts. Optical readouts of specific cellular events, including ion transients, voltage changes or activity in biochemical signaling cascades, using small detecting molecules or genetically encoded sensors now offer powerful opportunities for all-optical control and monitoring of cellular activity. Use of optogenetics has expanded in cardiac physiology, mainly using optically controlled depolarizing ion channels to control heart rate and for optogenetic defibrillation. ChR2-expressing cardiomyocytes show normal baseline and active excitable membrane and Ca signaling properties and are sensitive even to ~1 ms light pulses. They have been employed in studies of the intrinsic cardiac adrenergic system and of cardiac arrhythmic properties.
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http://dx.doi.org/10.3389/fphys.2019.01096DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6749684PMC
September 2019

Generation of induced pluripotent stem cells (iPSCs) from an infant with catecholaminergic polymorphic ventricular tachycardia carrying the double heterozygous mutations A1855D in RyR2 and Q1362H in SCN10A.

Stem Cell Res 2019 08 24;39:101509. Epub 2019 Jul 24.

Xi'an Medicine University, Xi'an, 710021, China.

Induced pluripotent stem cells (iPSCs) were generated from peripheral blood mononuclear cells (PBMCs) isolated from the peripheral blood of a 4 month-old boy with catecholaminergic polymorphic ventricular tachycardia carrying the double heterozygous mutations RyR2-A1855D and SCN10A-Q1362H. PBMCs were reprogrammed using non-integrative Sendai viral vectors containing reprogramming factors OCT4, SOX2, KLF4 and C-MYC. The iPSCs were shown to express pluripotent markers, have trilineage differentiation potential, carry RyR2-A1855D and SCN10A-Q1362H mutations and have a normal karyotype. They will be useful for studying the pathogenesis of CPVT patients with ≥2 variants.
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http://dx.doi.org/10.1016/j.scr.2019.101509DOI Listing
August 2019

Autonomic modulation of the electrical substrate in mice haploinsufficient for cardiac sodium channels: a model of the Brugada syndrome.

Am J Physiol Cell Physiol 2019 09 10;317(3):C576-C583. Epub 2019 Jul 10.

William Harvey Research Institute, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, London, United Kingdom.

A murine line haploinsufficient in the cardiac sodium channel has been used to model human Brugada syndrome: a disease causing sudden cardiac death due to lethal ventricular arrhythmias. We explored the effects of cholinergic tone on electrophysiological parameters in wild-type and genetically modified, heterozygous, knockout mice. ventricular slices showed longer refractory periods than wild-type both at baseline and during isoprenaline challenge. hearts also showed lower conduction velocities and increased mean increase in delay than did littermate controls at baseline and blunted responses to isoprenaline challenge. Carbachol exerted limited effects but reversed the effects of isoprenaline with coapplication. mice showed a reduction in conduction reserve in that isoprenaline no longer increased conduction velocity, and this was not antagonized by muscarinic agonists.
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http://dx.doi.org/10.1152/ajpcell.00028.2019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6766618PMC
September 2019

Atrial Transcriptional Profiles of Molecular Targets Mediating Electrophysiological Function in Aging and β Deficient Murine Hearts.

Front Physiol 2019 24;10:497. Epub 2019 Apr 24.

Faculty of Health and Medical Sciences, University of Surrey, Guildford, United Kingdom.

Background: Deficiencies in the transcriptional co-activator, peroxisome proliferative activated receptor, gamma, coactivator-1β are implicated in deficient mitochondrial function. The latter accompanies clinical conditions including aging, physical inactivity, obesity, and diabetes. Recent electrophysiological studies reported that β mice recapitulate clinical age-dependent atrial pro-arrhythmic phenotypes. They implicated impaired chronotropic responses to adrenergic challenge, compromised action potential (AP) generation and conduction despite normal AP recovery timecourses and background resting potentials, altered intracellular Ca homeostasis, and fibrotic change in the observed arrhythmogenicity.

Objective: We explored the extent to which these age-dependent physiological changes correlated with alterations in gene transcription in murine β atria.

Methods And Results: RNA isolated from murine atrial tissue samples from young (12-16 weeks) and aged (>52 weeks of age), wild type (WT) and mice were studied by pre-probed quantitative PCR array cards. We examined genes encoding sixty ion channels and other strategic atrial electrophysiological proteins. genotype independently reduced gene transcription underlying Na-K-ATPase, sarcoplasmic reticular Ca-ATPase, background K channel and cholinergic receptor function. Age independently decreased Na-K-ATPase and fibrotic markers. Both factors interacted to alter channel activity underlying atrial automaticity. However, neither factor, whether independently or interactively, affected transcription of cardiac Na, voltage-dependent K channels, surface or intracellular Ca channels. Nor were gap junction channels, β-adrenergic receptors or transforming growth factor-β affected.

Conclusion: These findings limit the possible roles of gene transcriptional changes in previously reported age-dependent pro-arrhythmic electrophysiologial changes observed in β atria to an altered Ca-ATPase () expression. This directly parallels previously reported arrhythmic mechanism associated with p21-activated kinase type 1 deficiency. This could add to contributions from the direct physiological outcomes of mitochondrial dysfunction, whether through reactive oxygen species (ROS) production or altered Ca homeostasis.
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http://dx.doi.org/10.3389/fphys.2019.00497DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6491872PMC
April 2019

Ion channel gating in cardiac ryanodine receptors from the arrhythmic RyR2-P2328S mouse.

J Cell Sci 2019 05 21;132(10). Epub 2019 May 21.

Eccles Institute of Neuroscience, John Curtin School of Medical Research, The Australian National University, 131 Garran Road, Acton ACT 2601, Australia

Mutations in the cardiac ryanodine receptor Ca release channel (RyR2) can cause deadly ventricular arrhythmias and atrial fibrillation (AF). The RyR2-P2328S mutation produces catecholaminergic polymorphic ventricular tachycardia (CPVT) and AF in hearts from homozygous RyR2 (denoted RyR2) mice. We have now examined P2328S RyR2 channels from RyR2 hearts. The activity of wild-type (WT) and P2328S RyR2 channels was similar at a cytoplasmic [Ca] of 1 mM, but P2328S RyR2 was significantly more active than WT at a cytoplasmic [Ca] of 1 µM. This was associated with a >10-fold shift in the half maximal activation concentration (AC) for Ca activation, from ∼3.5 µM Ca in WT RyR2 to ∼320 nM in P2328S channels and an unexpected >1000-fold shift in the half maximal inhibitory concentration (IC) for inactivation from ∼50 mM in WT channels to ≤7 μM in P2328S channels, which is into systolic [Ca] levels. Unexpectedly, the shift in Ca activation was not associated with changes in sub-conductance activity, S2806 or S2814 phosphorylation or the level of FKBP12 (also known as FKBP1A) bound to the channels. The changes in channel activity seen with the P2328S mutation correlate with altered Ca homeostasis in myocytes from RyR2 mice and the CPVT and AF phenotypes.This article has an associated First Person interview with the first author of the paper.
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http://dx.doi.org/10.1242/jcs.229039DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6550012PMC
May 2019
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