Ca ²⁺ release from the sarcoplasmic reticulum mediated by the cardiac ryanodine receptor (RyR2) is a fundamental event in cardiac muscle contraction. RyR2 mutations suggested to cause defective Ca ²⁺ channel function have recently been identified in catecholaminergic polymorphic ventricular tachycardia (CPVT) and arrhythmogenic right ventricular dysplasia (ARVD) affected individuals. We report expression of three CPVT-linked human RyR2 (hRyR2) mutations (S ²²⁴⁶ L, N ⁴¹⁰⁴ K, and R ⁴⁴⁹⁷ C) in HL-1 cardiomyocytes displaying correct targeting to the endoplasmic reticulum. N ⁴¹⁰⁴ K also localized to the Golgi apparatus. Phenotypic characteristics including intracellular Ca ²⁺ handling, proliferation, viability, RyR2:FKBP12.6 interaction, and beat rate in resting HL-1 cells expressing mutant hRyR2 were indistinguishable from wild-type (WT) hRyR2. However, Ca ²⁺ release was augmented in cells expressing mutant hRyR2 after RyR activation (caffeine and 4-chloro-m-cresol) or β-adrenergic stimulation (isoproterenol). RyR2:FKBP12.6 interaction remained intact after caffeine or 4-CMC activation, but was dramatically disrupted by isoproterenol or forskolin, an activator of adenylate cyclase. Isoproterenol and forskolin elevated cyclic-AMP to similar magnitudes in all cells and were associated with equivalent hyperphosphorylation of mutant and WT hRyR2. CPVT-linked mutations in hRyR2 did not alter resting cardiomyocyte phenotype but mediated augmented Ca ²⁺ release on RyR-agonist or β-AR stimulation. Furthermore, equivalent interaction between mutant and WT hRyR2 and FKBP12.6 was demonstrated.

Trafficking pathways underlying the assembly of connexins into gap junctions were examined using living COS-7 cells expressing a range of connexin-aequorin (Cx-Aeq) chimeras. By measuring the chemiluminescence of the aequorin fusion partner, the translocation of oligomerized connexins from intracellular stores to the plasma membrane was shown to occur at different rates that depended on the connexin isoform. Treatment of COS-7 cells expressing Cx32-Aeq and Cx43-Aeq with brefeldin A inhibited the movement of these chimera to the plasma membrane by 84 ± 4 and 88 ± 4%, respectively. Nocodazole treatment of the cells expressing Cx32-Aeq and Cx43-Aeq produced 29 ± 16 and 4 ± 7% inhibition, respectively. In contrast, the transport of Cx26 to the plasma membrane, studied using a construct (Cx26/43T-Aeq) in which the short cytoplasmic carboxyl-terminal tail of Cx26 was replaced with the extended carboxyl terminus of Cx43, was inhibited 89 ± 5% by nocodazole and was minimally affected by exposure of cells to brefeldin A (17 ±11%). The transfer of Lucifer yellow across gap junctions between cells expressing wild-type Cx32, Cx43, and the corresponding Cx32-Aeq and Cx43-Aeq chimeras was reduced by nocodazole treatment and abolished by brefeldin A treatment. However, the extent of dye coupling between cells expressing wild-type Cx26 or the Cx26/43T-Aeq chimeras was not significantly affected by brefeldin A treatment, but after nocodazole treatment, transfer of dye to neighboring cells was greatly reduced. These contrasting effects of brefeldin A and nocodazole on the trafficking properties and intercellular dye transfer are interpreted to suggest that two pathways contribute to the routing of connexins to the gap junction.

Rationale:: Flecainide, a class 1c antiarrhythmic, has emerged as an effective therapy in preventing arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) refractory to β-adrenergic receptor blockade. It has been proposed that the clinical efficacy of flecainide in CPVT is because of the combined actions of direct blockade of ryanodine receptors (RyR2) and Na ⁺ channel inhibition. However, there is presently no direct evidence to support the notion that flecainide blocks RyR2 Ca ²⁺ flux in the physiologically relevant (luminal-to-cytoplasmic) direction. The mechanism of flecainide action remains controversial. Objective:: To examine, in detail, the effect of flecainide on the human RyR2 channel and to establish whether the direct blockade of physiologically relevant RyR2 ion flow by the drug contributes to its therapeutic efficacy in the clinical management of CPVT. Methods and Results:: Using single-channel analysis, we show that, even at supraphysiological concentrations, flecainide did not inhibit the physiologically relevant, luminal-to-cytosolic flux of cations through the channel. Moreover, flecainide did not alter RyR2 channel gating and had negligible effect on the mechanisms responsible for the sarcoplasmic reticulum charge-compensating counter current. Using permeabilized cardiac myocytes to eliminate any contribution of plasmalemmal Na ⁺ channels to the observed actions of the drug at the cellular level, flecainide did not inhibit RyR2-dependent sarcoplasmic reticulum Ca ²⁺ release. Conclusions:: The principal action of flecainide in CPVT is not via a direct interaction with RyR2. Our data support a model of flecainide action in which Na ⁺ -dependent modulation of intracellular Ca ²⁺ handling attenuates RyR2 dysfunction in CPVT.

TLR overactivation may lead to end organ damage and serious acute and chronic inflammatory conditions. TLR responses must therefore be tightly regulated to control disease outcomes. We show in this study the ability of the soluble form of TLR2 (sTLR2) to regulate proinflammatory responses, and demonstrate the mechanisms underlying sTLR2 regulatory capacity. Cells overexpressing sTLR2, or stimulated in the presence of the sTLR2 protein, are hyporesponsive to TLR2 ligands. Regulation was TLR2 specific, and affected NF-κB activation, phagocytosis, and superoxide production. Natural sTLR2-depleted serum rendered leukocytes hypersensitive to TLR2-mediated stimulation. Mice administered sTLR2 together with Gram-positive bacteria-derived components showed lower peritoneal levels of the neutrophil (PMN) chemoattractant, keratinocyte-derived chemokine; lower PMN numbers; and a reduction in late apoptotic PMN. Mononuclear cell recruitment remained unaffected, and endogenous peritoneal sTLR2 levels increased. Notably, the capacity of sTLR2 to modulate acute inflammatory parameters did not compromise the ability of mice to clear live Gram-positive bacteria-induced infection. Mechanistically, sTLR2 interfered with TLR2 mobilization to lipid rafts for signaling, acted as a decoy microbial receptor, and disrupted the interaction of TLR2 with its coreceptor, CD14, by associating with CD14. These findings establish sTLR2 as a regulator of TLR2-mediated inflammatory responses, capable of blunting immune responses without abrogating microbial recognition and may inform the design of novel therapeutics against acute and chronic inflammatory conditions.

Heart failure (HF) is a chronic multi-factorial disease characterized by sarcoplasmic reticulum (SR) dysfunction that manifests as severely reduced contractility and increased risk of arrhythmia. Several lines of evidence have revealed the existence of defective ryanodine receptor (RyR2)-mediated Ca²⁺ leak in HF, although its relevance as a causative factor rather than a phenotypic consequence of the disease is questioned. This review will consider the relative contribution of RyR2-mediated Ca²⁺ leak to the profound cellular, transcriptional and electrical remodelling associated with HF. In particular, it will focus on our current understanding of the role of defective phosphorylation of RyR2 as a both a chronic mediator of excitation–contraction coupling (ECC) dysfunction and as a potent catalyst of RyR2-dependent arrhythmogenesis. A hypothetical concept that SR Ca²⁺ leak fundamentally underlies the increased arrhythmogenic susceptibility in HF, but that it may not directly contribute to contractile dysfunction, which may involve maladaptive perturbations in metabolism and energy utilization, is also discussed.