Mutations involving cardiac ion channels result in abnormal action potential formation or propagation, leading to cardiac arrhythmias. Despite the large impact on society of sudden cardiac death resulting from such arrhythmias, understanding of the underlying cellular mechanism is poor and clinical risk stratification and treatment consequently limited. Basic research using molecular techniques, as well as animal models, has proved extremely useful in improving our knowledge of inherited arrhythmogenic syndromes. This offers the practitioner tools to accurately diagnose rare disorders and provides novel markers for risk assessment and a basis for new strategies of treatment.

• Mice which are haploinsufficient in the Scn5a^+/− gene have reduced cardiac sodium channel (Na_v1.5) density and are used to model the Brugada syndrome. • Conduction velocity restitution showed lower initial values and earlier points of failure during incremental pacing in the murine Scn5a^+/− right ventricle (RV) epicardium particularly when treated with flecainide. • The broadness of the conduction velocity restitution function was a poor indicator of arrhythmia or alternans. Conduction velocity alternans occurred abruptly and was more marked in the flecainide-treated Scn5a^+/− RV epicardium. • Introduction of wavelength restitution yielded functions that converged to a common instability condition in contrast to action potential duration (APD) or conduction velocity restitution. • This occurred at significantly lower heart rates in Scn5a^+/− RV epicardium following flecainide or quinidine challenge, corresponding to a smaller total wavelength (basic cycle distance) resulting from a reduction in conduction velocity. • Wavelength restitution was superior at predicting alternans than either APD or conduction velocity restitution. Abstract Reductions in cardiac action potential wavelength, and the consequent wavebreak, have been implicated in arrhythmogenesis. Tachyarrhythmias are more common in the Brugada syndrome, particularly following pharmacological challenge, previously modelled using Scn5a^+/− murine hearts. Propagation latencies and action potential durations (APDs) from monophasic action potential recordings were used to assess wavelength changes with heart rate in Langendorff-perfused wild-type (WT) and Scn5a^+/− hearts. Recordings were obtained from right (RV) and left (LV) ventricular, epicardial and endocardial surfaces during incremental pacing, before and following flecainide or quinidine challenge. Conduction velocities (θ′), action potential wavelengths (λ′= APD ×θ′), and their corresponding alternans depended non-linearly upon diastolic interval (DI). Maximum θ′ was lower in Scn5a^+/− RV epicardium than endocardium. Flecainide further reduced θ′, accentuating this RV conduction block. Quinidine reduced maximum θ′ in WT and caused earlier conduction failure in the RV of both Scn5a^+/− and WT. Use of recovery wavelengths (λ′₀= DI ×θ′) rather than DI, provided novel λ restitution plots of λ′ against λ′₀, which sum to a basic cycle distance permitting feedback analysis. λ′ restitution gradient better correlated with alternans magnitude than either APD or θ restitution gradient. The large differences in θ′ and APD restitution contrasted with minor differences in maximum λ′ between epi- and endocardia of untreated hearts, and quinidine-treated WT hearts. Strikingly, all regions and conditions converged to a common instability point, implying a conserved relationship. Flecainide or quinidine decreased the pacing rates at which this occurred, through reducing basic cycle distance, in the Scn5a^+/− RV epicardium, directly predictive of its arrhythmic phenotype.

The identity and function of host factors required for efficient phagocytosis and intracellular maintenance of the protozoan parasite Leishmania donovani are poorly understood. Utilising the phagocytic capability of Drosophila S2 cells, together with available tools for modulating gene expression by RNAi, we have developed an experimental system in which to identify host proteins of this type on a genome-wide scale. We have shown that L. donovani amastigotes can be phagocytosed by S2 cells, in which they replicate and are maintained in a compartment with features characteristic of mammalian phagolysosomes. Screening with dsRNAs from 1920 conserved metazoan genes has identified transcripts that, when reduced in expression, cause either increased or decreased phagocytosis. Focussing on genes in the latter class, RNAi-mediated knockdown of the small GTPase Rab5, the prenylated SNARE protein YKT6, one sub-unit of serine palmitoyltransferase (spt2/lace), the Rac1-associated protein Sra1 and the actin cytoskeleton regulatory protein, SCAR, all lead to a significant reduction in parasite phagocytosis. A role for the lace mammalian homologue in amastigote uptake by mammalian macrophages has been verified using the serine palmitoyltransferase inhibitor, myriocin. These observations suggest that this experimental approach has the potential to identify a large number of host effectors required for efficient parasite uptake and maintenance.

Ryanodine receptor type 2 (RyR2) mutations are implicated in catecholaminergic polymorphic ventricular tachycardia (CPVT) thought to result from altered myocyte Ca²⁺ homeostasis reflecting inappropriate “leakiness” of RyR2-Ca²⁺ release channels arising from increases in their basal activity, alterations in their phosphorylation, or defective interactions with other molecules or ions. The latter include calstabin, calsequestrin-2, Mg²⁺, and extraluminal or intraluminal Ca²⁺. Recent clinical studies additionally associate RyR2 abnormalities with atrial arrhythmias including atrial tachycardia (AT), fibrillation (AF), and standstill, and sinus node dysfunction (SND). Some RyR2 mutations associated with CPVT in mouse models also show such arrhythmias that similarly correlate with altered Ca²⁺ homeostasis. Some examples show evidence for increased Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of RyR2. A homozygotic RyR2-P2328S variant demonstrates potential arrhythmic substrate resulting from reduced conduction velocity (CV) in addition to delayed afterdepolarizations (DADs) and ectopic action potential (AP) firing. Finally, one model with an increased RyR2 activity in the sino-atrial node (SAN) shows decreased automaticity in the presence of Ca²⁺-dependent decreases in I_{Ca, L} and diastolic sarcoplasmic reticular (SR) Ca²⁺ depletion.

Aim: To test a hypothesis attributing arrhythmia in Brugada Syndrome to right ventricular (RV) outflow tract (RVOT) conduction abnormalities arising from Na_v1.5 insufficiency and fibrotic change.Methods: Arrhythmic properties of Langendorff‐perfused Scn5a+/− and wild‐type mouse hearts were correlated with ventricular effective refractory periods (VERPs), multi‐electrode array (MEA) measurements of action potential (AP) conduction velocities and dispersions in conduction direction (CD), Na_v1.5 expression levels, and fibrotic change, as measured at the RVOT and RV. Two‐way anova was used to test for both independent and interacting effects of anatomical region and genotype on these parameters.Results: Scn5a+/− hearts showed greater arrhythmic frequencies during programmed electrical stimulation at the RVOT but not the RV. The Scn5a+/− genotype caused an independent increase of VERP regardless of whether the recording site was the RVOT or RV. Effective AP conduction velocities (CV†s), derived from fitting regression planes to arrays of observed local activation times were reduced in Scn5a+/− hearts and at the RVOT independently. AP conduction velocity magnitudes derived by averaging MEA results from local vector analyses, CV*, were reduced by the Scn5a+/− genotype alone. In contrast, dispersions in conduction direction, were greater in the RVOT than the RV, when the atrioventricular node was used as the pacing site. The observed reductions in Na_v1.5 expression were attributable to Scn5a+/−, whereas increased levels of fibrosis were associated with the RVOT.Conclusions: The Scn5a+/− RVOT recapitulates clinical findings of increased arrhythmogenicity through reduced CV† reflecting reduced CV* attributable to reduced Na_v1.5 expression and increased CD attributable to fibrosis.