Gap junctions are comprised of connexins that form cell-to-cell channels which couple neighboring cells to accommodate the exchange of information. The need for communication does, however, change over time and therefore must be tightly controlled. Although the regulation of connexin protein expression by transcription and translation is of great importance, the trafficking, channel activity and degradation are also under tight control. The function of connexins can be regulated by several post translational modifications, which affect numerous parameters; including number of channels, open probability, single channel conductance or selectivity. The most extensively investigated post translational modifications are phosphorylations, which have been documented in all mammalian connexins. Besides phosphorylations, some connexins are known to be ubiquitinated, SUMOylated, nitrosylated, hydroxylated, acetylated, methylated and γ-carboxyglutamated. The aim of the present review is to summarize our current knowledge of post translational regulation of the connexin family of proteins.

Large-conductance Ca²⁺- and voltage-activated K⁺ channels (Kca1.1/BK/MaxiK) are widely expressed ion channels. They provide a Ca²⁺-dependent feedback mechanism for the regulation of various body functions such as blood flow, neurotransmitter release, uresis, and immunity. In addition, a mitochondrial K⁺ channel with KCa1.1-resembling properties has been found in the heart, where it may be involved in regulation of energy consumption. In the present study, the effect of a novel NeuroSearch compound, 1-(3,5-bis-trifluoromethyl-phenyl)-3-[4-bromo-2-(1H-tetrazol-5-yl)-phenyl]-thiourea (NS11021), was investigated on cloned KCa1.1 expressed in Xenopus laevis oocytes and mammalian cells using electrophysiological methods. NS11021 at concentrations above 0.3 μM activated KCa1.1 in a concentration-dependent manner by parallel-shifting the channel activation curves to more negative potentials. Single-channel analysis revealed that NS11021 increased the open probability of the channel by altering gating kinetics without affecting the single-channel conductance. NS11021 (10 μM) influenced neither a number of cloned Kv channels nor endogenous Na⁺ and Ca²⁺ channels (L- and T-type) in guinea pig cardiac myocytes. In conclusion, NS11021 is a novel KCa1.1 channel activator with better specificity and a 10 times higher potency compared with the most broadly applied KCa1.1 opener, NS1619. Thus, NS11021 might be a valuable tool compound when addressing the physiological and pathophysiological roles of KCa1.1 channels.

Atrial fibrillation (AF) is the most common cardiac arrhythmia, and early-onset lone AF has been linked to mutations in genes encoding ion channels. Mutations in the pore forming subunit K_V4.3 leading to an increase in the transient outward potassium current (I_to) have previously been associated with the Brugada Syndrome. Here we aim to determine if mutations in K_V4.3 or in the auxiliary subunit K⁺ Channel-Interacting Protein (KChIP) 2 are associated with early-onset lone AF. Two hundred and nine unrelated early-onset lone AF patients (<40 years) were recruited. The entire coding sequence of KCND3 and KCNIP2 was bidirectionally sequenced. One novel non-synonymous mutation A545P was found in KCND3 and was neither present in the control group (n = 432 alleles) nor in any publicly available database. The proband had onset of persistent AF at the age of 22, and no mutations in genes previously associated with AF were found. Electrophysiological analysis of K_V4.3-A545P expressed in CHO-K1 cells, revealed that peak-current density was increased and the onset of inactivation was slower compared with WT, resulting in a significant gain-of-function both in the absence and the presence of KChIP2. Gain-of-function mutations in K_V4.3 have previously been described in Brugada Syndrome, however, this is the first report of a K_V4.3 gain-of-function mutation in early-onset lone AF. This association of K_V4.3 gain-of-function and early-onset lone AF further supports the hypothesis that increased potassium current enhances AF susceptibility.

Transient outward potassium current (I_to) is thought to be central to the pathogenesis of the Brugada syndrome (BrS). However, an I_to activator has not been available with which to validate this hypothesis. Here, we provide a direct test of the hypothesis using a novel I_to activator, NS5806. Isolated canine ventricular myocytes and coronary-perfused wedge preparations were used. Whole-cell patch-clamp studies showed that NS5806 (10 µM) increased peak I_to at +40 mV by 79 ± 4% (24.5 ± 2.2 to 43.6 ± 3.4 pA/pF, n = 7) and slowed the time constant of inactivation from 12.6 ± 3.2 to 20.3 ± 2.9 ms (n = 7). The total charge carried by I_to increased by 186% (from 363.9 ± 40.0 to 1042.0 ± 103.5 pA·ms/pF, n = 7). In ventricular wedge preparations, NS5806 increased phase 1 and notch amplitude of the action potential in the epicardium, but not in the endocardium, and accentuated the ECG J-wave, leading to the development of phase 2 re-entry and polymorphic ventricular tachycardia (n = 9). Although sodium and calcium channel blockers are capable of inducing BrS only in right ventricular (RV) wedge preparations, the I_to activator was able to induce the phenotype in wedges from both ventricles. NS5806 induced BrS in 4/6 right and 2/10 left ventricular wedge preparations. The I_to activator NS5806 recapitulates the electrographic and arrhythmic manifestation of BrS, providing evidence in support of its pivotal role in the genesis of the disease. Our findings also suggest that a genetic defect leading to a gain of function of I_to could explain variants of BrS, in which ST-segment elevation or J-waves are evident in both right and left ECG leads.

ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue (“fast gate”) known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-β-synthase (CBS) domains and the intracellular vestibule (“slow gating”). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1–related diseases. Chloride transporting CLC proteins are expressed in a wide range of organisms, and the family encompasses several members with numerous roles in human health and disease by allowing movement of chloride ions across the membranes that encapsulate cells and cellular organelles. Structurally, CLCs form dimers possessing a separate ion translocation pathway in each monomer, and they can operate as either channels or transporters that exchange chloride for protons. The CLC channel ClC-1 is critical to skeletal muscle excitability and has been proposed as a target to alleviate neuromuscular disorders. Here, we have analyzed the structure of human ClC-1 and revealed the high similarity of its ion conducting pathway to those observed in other CLC members, including prokaryotic and algal transporters. Our data suggest how ClC-1 is regulated by environmental cues to allow opening and closure, thereby permitting attenuation of muscle function. Our results help with understanding the principal determinants that govern CLC proteins and may guide downstream translational applications to combat muscle pathologies.