KCNQ2 and KCNQ3 potassium channel subunits: molecular correlates of the M-channel

M-channel gating and simulation

Single potassium M-channels in rat sympathetic neurons have multiple voltage-dependent kinetic components in their activity: short, medium, and long closed times (tau(CS), tau(CM), and tau(CL)) and short and long open times (tau(OS) and tau(OL)). All five components can be detected in cell-attached patches, but only four of them (tau(CS), tau(CM), tau(OS), and tau(OL)) in excised patches (, J. Physiol. (Lond.). 472:711-724; 1996, Neuron. 16:151-162; 1996, Neuropharmacology. 35:933-947). Analysis of the burst structure of activity recorded from cell-attached and excised inside-out patches showed it to be consistent with the sequential kinetic scheme C(L) left arrow over right arrow O(S) left arrow over right arrow C(M) left arrow over right arrow O(L) left arrow over right arrow C(S). Using this scheme and experimentally determined kinetic parameters, we successfully simulated the activity of M-channels both under steady-state conditions and during depolarizing voltage steps. Consistent with the characteristic behavior of macroscopic M-current, ensemble currents constructed from simulated M-channels had exponential activation and deactivation, with no delays, when tested in the range between -50 and -20 mV.

Separation of M-like current and ERG current in NG108-15 cells

Differentiated NG108-15 neuroblastoma x glioma hybrid cells were whole-cell voltage-clamped. Hyperpolarizing pulses, superimposed on a depolarized holding potential (-30 or -20 mV), elicited deactivation currents which consisted of two components, distinguishable by fitting with two exponential functions. Linopirdine [DuP 996, 3,3-bis(4-pyridinylmethyl)-1-phenylindolin-2-one), a neurotransmitter-release enhancer known as potent and selective blocker of the M-current of rat sympathetic neurons, in concentrations of 5 or 10 microM selectively inhibited the fast component (IC50 = 14.7 microM). The slow component was less sensitive to linopirdine (IC50>20 microM). The class III antiarrhythmics [(4-methylsulphonyl)amido]benzenesulphonamide (WAY-123.398) and 1-[2-(6-methyl-2-pyrydinil)ethyl]-4-(4-methylsulphonylaminobenz oyl) piperidine (E-4031), selective inhibitors of the inwardly rectifying ERG (ether-à-go-go-related gene) potassium channel, inhibited predominantly the slow component (IC50 = 38 nM for E-4031). The time constant of the WAY-123.398-sensitive current resembled the time constant of the slow component in size and voltage dependence. Inwardly rectifying ERG currents, recorded in K+ -rich bath at strongly negative pulse potentials, resembled the slow component of the deactivation current in their low sensitivity to linopirdine (28% inhibition at 50 microM). The size of the slow component varied greatly between cells. Accordingly, varied the effect of WAY-123.398 on deactivation current and holding current. RNA transcripts for the following members of the ether-à-go-go gene (EAG) K+ channel family were found in differentiated NG108-15 cells: ERG1, ERG2, EAGI, EAG-like (ELK)1, ELK2; ERG3 was only present in non-differentiated cells. In addition, RNA transcripts for KCNQ2 and KCNQ3 were found in differentiated and non-differentiated cells. We conclude that the fast component of the deactivation current is M-like current and the slow component is deactivating ERG current. The molecular correlates are probably KCNQ2/KCNQ3 and ERG1/ERG2, respectively.

Voltage-gated potassium channels: from hyperexcitability to excitement

The superfamily of voltage-activated potassium channels may express structurally and functionally diverse voltage-activated potassium channels in the nervous system. The roles of some voltage-activated potassium channel types, e.g. rapidly inactivating (transiently active type) channels and muscarine sensitive muscarine sensitive channels, are beginning to be understood. They may significantly influence dendritic action-potential back-propagation, signal to noise ratios in presynaptic excitability or the responsiveness of a neuron to synaptic input. Inherited disorders related to changes in excitability (episodic ataxia, epilepsy, heart arrhythmia) or to defects in sensory perception (hearing loss) have been associated with mutations in a few voltage-activated potassium channel genes. Most likely, more voltage-activated potassium channel genes will be linked to related disorders in the near future.

Ion channel genes and human neurological disease: recent progress, prospects, and challenges

What do epilepsy, migraine headache, deafness, episodic ataxia, periodic paralysis, malignant hyperthermia, and generalized myotonia have in common? These human neurological disorders can be caused by mutations in genes for ion channels. Many of the channel diseases are "paroxysmal disorders" whose principal symptoms occur intermittently in individuals who otherwise may be healthy and active. Some of the ion channels that cause human neurological disease are old acquaintances previously cloned and extensively studied by channel specialists. In other cases, however, disease-gene hunts have led the way to the identification of new channel genes. Progress in the study of ion channels has made it possible to analyze the effects of human neurological disease-causing channel mutations at the level of the single channel, the subcellular domain, the neuronal network, and the behaving organism.

Expression of RGS2 alters the coupling of metabotropic glutamate receptor 1a to M-type K+ and N-type Ca2+ channels

Group I mGluRs heterologously expressed in sympathetic neurons inhibited calcium (I(Ca)) and M-type potassium (I(M)) currents. Treatment with pertussis toxin (PTX) revealed a voltage-dependent (VD), PTX-sensitive component of I(Ca) inhibition and a voltage-independent (VI), PTX-insensitive component. Coexpression of RGS2 occluded mGluR1a inhibition of I(M) and made I(Ca) inhibition VD in PTX-treated cells, presumably by blocking the effects of G alpha(q/11)-GTP. These data indicate that mGluR1a can couple to G(i/o) as well as G(q/11). In addition, VI I(Ca) inhibition proceeds through a G alpha(q/11)-GTP-mediated pathway, which can be occluded by expressing RGS2, leaving the VD, G betagamma-mediated inhibition active. These data may reveal a functional role for the upregulation of RGS2 expression in in vivo systems.

Benign familial neonatal epilepsy with mutations in two potassium channel genes

The significant progress made over the past year in understanding the basis for a form of neonatal seizures can be attributed to the successful positional cloning of two new voltage-gated potassium channel genes. Expression studies have increased our understanding of the biology of these channels and their role in epilepsy.

Genetics of the epilepsies

Epilepsies, like other common diseases, have complex inheritance, and molecular genetic studies in such conditions are difficult. There has been recent success in identifying the molecular basis of certain epilepsies, particularly in those syndromes with autosomal dominant inheritance. All four genes discovered to date for idiopathic epilepsies code for ion channel subunits, either ligand-gated or voltage-gated. The idiopathic epilepsies thus appear, at least in part, to be a family of channelopathies.

KCNQ4, a novel potassium channel expressed in sensory outer hair cells, is mutated in dominant deafness

Potassium channels regulate electrical signaling and the ionic composition of biological fluids. Mutations in the three known genes of the KCNQ branch of the K+ channel gene family underlie inherited cardiac arrhythmias (in some cases associated with deafness) and neonatal epilepsy. We have now cloned KCNQ4, a novel member of this branch. It maps to the DFNA2 locus for a form of nonsyndromic dominant deafness. In the cochlea, it is expressed in sensory outer hair cells. A mutation in this gene in a DFNA2 pedigree changes a residue in the KCNQ4 pore region. It abolishes the potassium currents of wild-type KCNQ4 on which it exerts a strong dominant-negative effect. Whereas mutations in KCNQ1 cause deafness by affecting endolymph secretion, the mechanism leading to KCNQ4-related hearing loss is intrinsic to outer hair cells.