Progressive familial heart block type I (PFHBI) is a progressive cardiac bundle branch disease in the His-Purkinje system that exhibits autosomal-dominant inheritance. In 3 branches of a large South African Afrikaner pedigree with an autosomal-dominant form of PFHBI, we identified the mutation c.19G→A in the transient receptor potential cation channel, subfamily M, member 4 gene (TRPM4) at chromosomal locus 19q13.3. This mutation predicted the amino acid substitution p.E7K in the TRPM4 amino terminus. TRPM4 encodes a Ca 2+ -activated nonselective cation (CAN) channel that belongs to the transient receptor potential melastatin ion channel family. Quantitative analysis of TRPM4 mRNA content in human cardiac tissue showed the highest expression level in Purkinje fibers. Cellular expression studies showed that the c.19G→A missense mutation attenuated deSUMOylation of the TRPM4 channel. The resulting constitutive SUMOylation of the mutant TRPM4 channel impaired endocytosis and led to elevated TRPM4 channel density at the cell surface. Our data therefore revealed a gain-of-function mechanism underlying this type of familial heart block.

Background-Isolated cardiac conduction block is a relatively common condition in young and elderly populations. Genetic predisposing factors have long been suspected because of numerous familial case reports. Deciphering genetic predisposing factors of conduction blocks may give a hint at stratifying conduction block carriers in a more efficient way. Methods and Results-One Lebanese family and 2 French families with autosomal dominant isolated cardiac conduction blocks were used for linkage analysis. A maximum combined multipoint lod score of 10.5 was obtained on a genomic interval including more than 300 genes. After screening 12 genes of this interval for mutation, we found a heterozygous missense mutation of the TRPM4 gene in each family (p.Arg164Trp, p.Ala432Thr, and p.Gly844Asp). This gene encodes the TRPM4 channel, a calcium-activated nonselective cation channel of the transient receptor potential melastatin (TRPM) ion channel family. All 3 mutations result in an increased current density. This gain of function is due to an elevated TRPM4 channel density at the cell surface secondary to impaired endocytosis and deregulation of Small Ubiquitin MOdifier conjugation (SUMOylation). Furthermore, we showed by immunohistochemistry that TRPM4 channel signal level is higher in atrial cardiomyocytes than in common ventricular cells, but is highest in Purkinje fibers. Small bundles of highly TRPM4-positive cells were found in the subendocardium and in rare intramural bundles. Conclusions-the TRPM4 gene is a causative gene in isolated cardiac conduction disease with mutations resulting in a gain of function and TRPM4 channel being highly expressed in cardiac Purkinje fibers. (Circ Cardiovasc Genet. 2010;3:374-385.)Key Words: bundle-branch block Ⅲ heart block Ⅲ ion channel Ⅲ sudden death Ⅲ genetics C onduction block is a condition in which the depolarization wave initiated in the sinus node of the heart is slowed down or even blocked on the way to ventricular cardiomyocytes. The position of the block, its completeness (partial or complete), and the number of blocks are largely variable from one individual to the other. Conduction blocks are not rare in the general population either among young people or late in life. Right bundle-branch block (RBBB) has a prevalence of about 0.1% in normal children, with a male predominance. 1 Complete RBBB with normal left ejection fraction and no diagnoses of cardiac disease was observed in 0.3% of individuals in a population of adults with a median age of 64 years in a community-based study lead by Miller et al. 2 The natural evolution of isolated conduction blocks is unpredictable, and deciphering the genetic predisposing factors might help in unmasking those that are dangerous. Rare cases of familial conduction blocks are secondary to mutations in SCN5A 3 or NKX2.5. 4 In the latter gene, the conduction block is often associated with a congenital heart defect (typically an atrial septal defect). Recently, a South African family with cardiac conduction block was reported to carry a ...

Gating the ion-permeation pathway in K(+) channels requires conformational changes in activation and inactivation gates. Here we have investigated the structural alterations associated with pH-dependent inactivation gating of the KcsA-Kv1.3 K(+) channel using solid-state NMR spectroscopy in direct reference to electrophysiological and pharmacological experiments. Transition of the KcsA-Kv1.3 K(+) channel from a closed state at pH 7.5 to an inactivated state at pH 4.0 revealed distinct structural changes within the pore, correlated with activation-gate opening and inactivation-gate closing. In the inactivated K(+) channel, the selectivity filter adopts a nonconductive structure that was also induced by binding of a pore-blocking tetraphenylporphyrin derivative. The results establish a structural link between inactivation and block of a K(+) channel in a membrane setting.

Potassium (K þ )-channel gating is choreographed by a complex interplay between external stimuli, K þ concentration and lipidic environment. We combined solid-state NMR and electrophysiological experiments on a chimeric KcsA-Kv1.3 channel to delineate K þ , pH and blocker effects on channel structure and function in a membrane setting. Our data show that pH-induced activation is correlated with protonation of glutamate residues at or near the activation gate. Moreover, K þ and channel blockers distinctly affect the open probability of both the inactivation gate comprising the selectivity filter of the channel and the activation gate. The results indicate that the two gates are coupled and that effects of the permeant K þ ion on the inactivation gate modulate activation-gate opening. Our data suggest a mechanism for controlling coordinated and sequential opening and closing of activation and inactivation gates in the K þ -channel pore.

Controlled opening and closing of an ion-selective pathway in response to changes of membrane potential is a fundamental feature of voltage-gated ion channels. In recent decades, various details of this process have been revealed with unprecedented precision based on studies of prototypic potassium channels. Though current scientific efforts are focused more on a thorough description of voltage-sensor movement, much less is known about the similarities and differences of the gating mechanisms among potassium channels. Here, we describe the peculiarities of the KCNQ1 gating process in parallel comparison to Shaker. We applied alanine scanning mutagenesis to the S4-S5 linker and pore region and followed the regularities of gating perturbations in KCNQ1. We found a fractional constitutive conductance for wild-type KCNQ1. This component increased significantly in mutants with considerably leftward-shifted steady-state activation curves. In contrast to Shaker, no correlation between V(1/2) and Z parameters was observed for the voltage-dependent fraction of KCNQ1. Our experimental findings are explained by a simple allosteric gating scheme with voltage-driven and voltage-independent transitions. Allosteric features are discussed in the context of extreme gating adaptability of KCNQ1 upon interaction with KCNE β-subunits.

This protocol describes the preparation of hippocampal slice cultures from rat or mouse pups using sterile conditions that do not require the use of antibiotics or antimycotics. Combining very good optical and electrophysiological accessibility with a lifetime approaching that of the intact animal, many fundamental questions about synaptic plasticity and long-term dynamics of network connectivity can be addressed with this preparation.

K+ channels control transmembrane action potentials by gating open or closed in response to external stimuli. Inactivation gating, involving a conformational change at the K+ selectivity filter, has recently been recognized as a major K+ channel regulatory mechanism. In the K+ channel hERG, inactivation controls the length of the human cardiac action potential. Mutations impairing hERG inactivation cause life-threatening cardiac arrhythmia, which also occur as undesired side effects of drugs. In this paper, we report atomistic molecular dynamics simulations, complemented by mutational and electrophysiological studies, which suggest that the selectivity filter adopts a collapsed conformation in the inactivated state of hERG. The selectivity filter is gated by an intricate hydrogen bond network around residues S620 and N629. Mutations of this hydrogen bond network are shown to cause inactivation deficiency in electrophysiological measurements. In addition, drug-related conformational changes around the central cavity and pore helix provide a functional mechanism for newly discovered hERG activators.

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