Paradoxes of causal loops in spacetime

There is, among some scientists and philosophers, the idea that any theory that would allow the time travel would introduce causal issues. These types of temporal paradoxes can be avoided by the Novikov self-consistency principle or by a variation in the interpretation of many worlds with interacting worlds. The world in which we live has, according to David Lewis, a Parmenidean ontology: "a manifold of events in four dimensions," and the occupants of the world are the 4-dimensional aggregates of the stages - "temporal lines". The causal loops in backwards time travel involve events that appear to "come from nowhere," paradoxical "self-existent" objects or information, resulting in a bootstrap paradox. Many believe that causality loops are not impossible or unacceptable, but only inexplicable. DOI: 10.13140/RG.2.2.28792.7040

Identification through action potential clamp of proarrhythmic consequences of the short QT syndrome T618I hERG 'hotspot' mutation

The T618I KCNH2-encoded hERG mutation is the most frequently observed mutation in genotyped cases of the congenital short QT syndrome (SQTS), a cardiac condition associated with ventricular fibrillation and sudden death. Most T618I hERG carriers exhibit a pronounced U wave on the electrocardiogram and appear vulnerable to ventricular, but not atrial fibrillation (AF). The basis for these effects is unclear. This study used the action potential (AP) voltage clamp technique to determine effects of the T618I mutation on hERG current (I(hERG)) elicited by APs from different cardiac regions. Whole-cell patch-clamp recordings were made at 37 °C of I(hERG) from hERG-transfected HEK-293 cells. Maximal I(hERG) during a ventricular AP command was increased ∼4-fold for T618I I(hERG) and occurred much earlier during AP repolarization. The mutation also increased peak repolarizing currents elicited by Purkinje fibre (PF) APs. Maximal wild-type (WT) I(hERG) current during the PF waveform was 87.2 ± 4.5% of maximal ventricular repolarizing current whilst for the T618I mutant, the comparable value was 47.7 ± 2.7%. Thus, the T618I mutation exacerbated differences in repolarizing I(hERG) between PF and ventricular APs; this could contribute to heterogeneity of ventricular-PF repolarization and consequently to the U waves seen in T618I carriers. The comparatively shorter duration and lack of pronounced plateau of the atrial AP led to a smaller effect of the T618I mutation during the atrial AP, which may help account for the lack of reported AF in T618I carriers. Use of a paired ventricular AP protocol revealed an alteration to protective I(hERG) transients that affect susceptibility to premature excitation late in AP repolarization/early in diastole. These observations may help explain altered arrhythmia susceptibility in this form of the SQTS

Ranolazine inhibition of hERG potassium channels: Drug–pore interactions and reduced potency against inactivation mutants

AbstractThe antianginal drug ranolazine, which combines inhibitory actions on rapid and sustained sodium currents with inhibition of the hERG/IKr potassium channel, shows promise as an antiarrhythmic agent. This study investigated the structural basis of hERG block by ranolazine, with lidocaine used as a low potency, structurally similar comparator. Recordings of hERG current (IhERG) were made from cell lines expressing wild-type (WT) or mutant hERG channels. Docking simulations were performed using homology models built on MthK and KvAP templates. In conventional voltage clamp, ranolazine inhibited IhERG with an IC50 of 8.03μM; peak IhERG during ventricular action potential clamp was inhibited ~62% at 10μM. The IC50 values for ranolazine inhibition of the S620T inactivation deficient and N588K attenuated inactivation mutants were respectively ~73-fold and ~15-fold that for WT IhERG. Mutations near the bottom of the selectivity filter (V625A, S624A, T623A) exhibited IC50s between ~8 and 19-fold that for WT IhERG, whilst the Y652A and F656A S6 mutations had IC50s ~22-fold and 53-fold WT controls. Low potency lidocaine was comparatively insensitive to both pore helix and S6 mutations, but was sensitive to direction of K+ flux and particularly to loss of inactivation, with an IC50 for S620T-hERG ~49-fold that for WT IhERG. Docking simulations indicated that the larger size of ranolazine gives it potential for a greater range of interactions with hERG pore side chains compared to lidocaine, in particular enabling interaction of its two aromatic groups with side chains of both Y652 and F656. The N588K mutation is responsible for the SQT1 variant of short QT syndrome and our data suggest that ranolazine is unlikely to be effective against IKr/hERG in SQT1 patients

Investigation of the Effects of the Short QT Syndrome D172N Kir2.1 Mutation on Ventricular Action Potential Profile Using Dynamic Clamp

The congenital short QT syndrome (SQTS) is a cardiac condition that leads to abbreviated ventricular repolarization and an increased susceptibility to arrhythmia and sudden death. The SQT3 form of the syndrome is due to mutations to the KCNJ2 gene that encodes Kir2.1, a critical component of channels underlying cardiac inwardly rectifying K(+) current, I(K1). The first reported SQT3 KCNJ2 mutation gives rise to the D172N Kir2.1 mutation, the consequences of which have been studied on recombinant channels in vitro and in ventricular cell and tissue simulations. The aim of this study was to establish the effects of the D172N mutation on ventricular repolarization through real-time replacement of I(K1) using the dynamic clamp technique. Whole-cell patch-clamp recordings were made from adult guinea-pig left ventricular myocytes at physiological temperature. Action potentials (APs) were elicited at 1 Hz. Intrinsic I(K1) was inhibited with a low concentration (50 µM) of Ba(2+) ions, which led to AP prolongation and triangulation, accompanied by a ∼6 mV depolarization of resting membrane potential. Application of synthetic I(K1) through dynamic clamp restored AP duration, shape and resting potential. Replacement of wild-type (WT) I(K1) with heterozygotic (WT-D172N) or homozygotic (D172N) mutant formulations under dynamic clamp significantly abbreviated AP duration (APD(90)) and accelerated maximal AP repolarization velocity, with no significant hyperpolarization of resting potential. Across stimulation frequencies from 0.5 to 3 Hz, the relationship between APD(90) and cycle length was downward shifted, reflecting AP abbreviation at all stimulation frequencies tested. In further AP measurements at 1 Hz from hiPSC cardiomyocytes, the D172N mutation produced similar effects on APD and repolarization velocity; however, resting potential was moderately hyperpolarized by application of mutant I(K1) to these cells. Overall, the results of this study support the major changes in ventricular cell AP repolarization with the D172N predicted from prior AP modelling and highlight the potential utility of using adult ventricular cardiomyocytes for dynamic clamp exploration of functional consequences of Kir2.1 mutations

Inhibition of the hERG potassium channel by phenanthrene:a polycyclic aromatic hydrocarbon pollutant

The lipophilic polycyclic aromatic hydrocarbon (PAH) phenanthrene is relatively abundant in polluted air and water and can access and accumulate in human tissue. Phenanthrene has been reported to interact with cardiac ion channels in several fish species. This study was undertaken to investigate the ability of phenanthrene to interact with hERG (human Ether-à-go-go-Related Gene) encoded Kv11.1 K(+) channels, which play a central role in human ventricular repolarization. Pharmacological inhibition of hERG can be proarrhythmic. Whole-cell patch clamp recordings of hERG current (I(hERG)) were made from HEK293 cells expressing wild-type (WT) and mutant hERG channels. WT I(hERG1a) was inhibited by phenanthrene with an IC(50) of 17.6 ± 1.7 µM, whilst I(hERG1a/1b) exhibited an IC(50) of 1.8 ± 0.3 µM. WT I(hERG) block showed marked voltage and time dependence, indicative of dependence of inhibition on channel gating. The inhibitory effect of phenanthrene was markedly impaired by the attenuated inactivation N588K mutation. Remarkably, mutations of S6 domain aromatic amino acids (Y652, F656) in the canonical drug binding site did not impair the inhibitory action of phenanthrene; the Y652A mutation augmented I(hERG) block. In contrast, the F557L (S5) and M651A (S6) mutations impaired the ability of phenanthrene to inhibit I(hERG), as did the S624A mutation below the selectivity filter region. Computational docking using a cryo-EM derived hERG structure supported the mutagenesis data. Thus, phenanthrene acts as an inhibitor of the hERG K(+) channel by directly interacting with the channel, binding to a distinct site in the channel pore domain. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00018-021-03967-8

Palmitoylation regulates the magnitude of HCN4-mediated currents in mammalian cells

The sinoatrial node (SAN) and subsidiary pacemakers in the cardiac conduction system generate spontaneous electrical activity which is indispensable for electrical and therefore contractile function of the heart. The hyperpolarisation-activated cyclic nucleotide-gated channel HCN4 is responsible for genesis of the pacemaker “funny” current during diastolic depolarisation. S-palmitoylation, the reversible conjugation of the fatty acid palmitate to protein cysteine sulfhydryls, regulates the activity of key cardiac Na+ and Ca2+ handling proteins, influencing their membrane microdomain localisation and function. We investigated HCN4 palmitoylation and its functional consequences in engineered human embryonic kidney 293T cells as well as endogenous HCN4 in neonatal rat ventricular myocytes. HCN4 was palmitoylated in all experimental systems investigated. We mapped the HCN4 palmitoylation sites to a pair of cysteines in the HCN4 intracellular amino terminus. A double cysteine-to-alanine mutation CC93A/179AA of full length HCN4 caused a ∼67% reduction in palmitoylation in comparison to wild type HCN4. We used whole-cell patch clamp to evaluate HCN4 current (IHCN4) in stably transfected 293T cells. Removal of the two N-terminal palmitoylation sites did not significantly alter half maximal activation voltage of IHCN4 or the activation slope factor. IHCN4 was significantly larger in cells expressing wild type compared to non-palmitoylated HCN4 across a range of voltages. Phylogenetic analysis revealed that although cysteine 93 is widely conserved across all classes of HCN4 vertebrate orthologs, conservation of cysteine 179 is restricted to placental mammals. Collectively, we provide evidence for functional regulation of HCN4 via palmitoylation of its amino terminus in vertebrates. We suggest that by recruiting the amino terminus to the bilayer, palmitoylation enhances the magnitude of HCN4-mediated currents, but does not significantly affect the kinetics

The effects of acidosis on the hERG potassium channel

The human ether-a-go-go-related gene (hERG) encodes channels mediating the rapid delayed rectifier K+ current (IKr). IKr participates in cardiac action potential (AP) repolarisation and may also protect the ventricles against premature stimulation. The heart is exposed to acidosis (low pH) in a number of pathological conditions including myocardial ischemia. Extracellular acidosis is known to modulate hERG current (IhERG) function, although a number of aspects of the modulation remain incompletely understood. The aims of this investigation were to establish the effects of acidosis on: (i) IhERG amplitude, kinetics and the response to premature stimulation at mammalian physiological temperature; (ii) the hERG blocking potency of selected anti arrhythmic drugs. Whole-cell patch-clamp recordings of IhERG were made from mammalian cells (CHO or HEK 293) at 37 QC. Lowering external pH from 7.4 to 6.3 reduced the magnitude of IhERG by reducing macroscopic hERG conductance and modulating IhERG kinetics, with positively shifted activation and accelerated deactivation. Results from experiments using an acidic pipette solution showed that the actions of protons occurred from the external surface and not from secondary intracellular acidosis. Experimental and computer simulation work demonstrated that acidosis impairs the protective role of IhERG against premature stimulation. The effects of extracellular acidosis on IhERG kinetics were preserved when the shortened hERG I b isoform was studied, indicating that a full-length N-terminus is not necessary for acidic modulation of hERG channel function. Interestingly, the inhibitory effect of acidosis on IhERG was greater for hERG 1 band hERG lall b than for hERG la. Extracellular acidosis decreased the hERG blocking potency of flecainide, dofetilide and ranolazine, whilst the potency of amiodarone was unaffected. IhERG inactivation was found to be important for ranolazine's inhibitory action and a series of S6 and inner helix residues (Y652, F656, T623, S624 and V625) were identified as contributing to ranolazine binding. I.EThOS - Electronic Theses Online ServiceGBUnited Kingdo