Functional Expression of Two KvLQT1-related Potassium Channels Responsible for an Inherited Idiopathic Epilepsy

Abstract

Benign familial neonatal convulsions (BFNC), a class of idiopathic generalized epilepsy, is an autosomal dominantly inherited disorder of newborns. BFNC has been linked to mutations in two putative K ؉ channel genes, KCNQ2 and KCNQ3. Amino acid sequence comparison reveals that both genes share strong homology to Kv-LQT1, the potassium channel encoded by KCNQ1, which is responsible for over 50% of inherited long QT syndrome. Here we describe the cloning, functional expression, and characterization of K ؉ channels encoded by KCNQ2 and KCNQ3 cDNAs. Individually, expression of KCNQ2 or KCNQ3 in Xenopus oocytes elicits voltagegated, rapidly activating K ؉ -selective currents similar to KCNQ1. However, unlike KCNQ1, KCNQ2 and KCNQ3 currents are not augmented by coexpression with the KCNQ1 ␤ subunit, KCNE1 (minK, IsK). Northern blot analyses reveal that KCNQ2 and KCNQ3 exhibit similar expression patterns in different regions within the brain. Interestingly, coexpression of KCNQ2 and KCNQ3 results in a substantial synergistic increase in current amplitude. Coexpression of KCNE1 with the two channels strongly suppressed current amplitude and slowed kinetics of activation. The pharmacological and biophysical properties of the K ؉ currents observed in the coinjected oocytes differ somewhat from those observed after injecting either KCNQ2 or KCNQ3 by itself. The functional interaction between KCNQ2 and KCNQ3 provides a framework for understanding how mutations in either channel can cause a form of idiopathic generalized epilepsy.Potassium channels are the largest and most diverse group of ion channels. They are primary regulators of resting membrane potential and action potential configuration and, therefore, modulate excitability of neurons, cardiac myocytes, and other electrically active cells. Recent identification of KCNQ1 (KvLQT1), the gene responsible for more than 50% of inherited cardiac long QT syndrome (LQTS), 1 established a new family of six-transmembrane domain K ϩ channels (1). KCNQ1, in combination with the KCNE1 subunit, encodes the slow component of the cardiac delayed rectifier K ϩ current (2-4), and mutations in KCNQ1, which occur in LQTS patients, partially or completely inhibit the channel in a dominant-negative fashion (5, 6). In an attempt to identify additional members of the KCNQ1 K ϩ channel gene family, the KCNQ1 sequence was used to search DNA and protein sequence data banks. Two additional KCNQ1-related genes, KCNQ2 and KCNQ3, were identified.Recent publications indicate that mutations in KCNQ2 or KCNQ3 are associated with BFNC, an autosomal dominantly inherited epilepsy in newborns (7-9). Preliminary functional characterization of KCNQ2 confirmed that this gene encodes a voltage-activated K ϩ channel (9). Here we describe the cloning, tissue distribution, and functional expression of both KCNQ2 and KCNQ3. More importantly, we demonstrate that these two channels interact functionally with each other and with KCNE1. EXPERIMENTAL PROCEDURES Molecular Cloning and Expression of KCNQ2 and KCNQ3-5ЈRap...

Reconstitution of Muscarinic Modulation of the KCNQ2/KCNQ3 K+Channels That Underlie the Neuronal M Current

“…As a marker for successfully transfected cells, all transfections included cDNA coding for green fluorescent protein, and cells emitting green fluorescence were chosen for study. Cotransfection of the plasmids for the two channel subunits yielded slowly activating and deactivating currents with M-current-like voltage dependence, kinetics, and pharmacology similar to those reported previously in oocytes (Wang et al, 1998;Yang et al, 1998).…”

“…The KCNQ2 channel has a tyrosine at position 284, analogous to position 449 in Shaker K ϩ channels, which has been shown to confer high sensitivity to external TEA (MacKinnon and Yellen, 1990;Heginbotham and MacKinnon, 1992), and KCNQ2 homomultimers are very TEA-sensitive (Wang et al, 1998;Yang et al, 1998). The KCNQ3 channel has a threonine at this position and is much less sensitive to TEA (Yang et al, 1998). We compared the TEA sensitivity of putative homo-and heteromeric channels.…”

“…The similarity to the heteromeric currents made it seem prudent to check that the currents were indeed attributable to homomeric expression. One pharmacological characteristic that should distinguish KCNQ2 from KCNQ3 is sensitivity to external tetraethylammonium ion (TEA) (Wang et al, 1998;Yang et al, 1998). The KCNQ2 channel has a tyrosine at position 284, analogous to position 449 in Shaker K ϩ channels, which has been shown to confer high sensitivity to external TEA (MacKinnon and Yellen, 1990;Heginbotham and MacKinnon, 1992), and KCNQ2 homomultimers are very TEA-sensitive (Wang et al, 1998;Yang et al, 1998).…”

Anxiolytic Effects of Maxipost (BMS-204352) and Retigabine via Activation of Neuronal Kv7 Channels

“…There are five known ␣-subunits (K v 7.1-K v 7.5), and the sequence alignment of the hydrophobic regions divides them into two groups, one comprising K v 7.1 and the other K v 7.2 to K v 7.5. K v 7.1 is found in the heart (Yang et al, 1997) and in epithelial tissues (Schroeder et al, 2000b), whereas K v 7.2 to K v 7.5 are expressed primarily in neurons and are therefore collectively referred to as neuronal (Biervert et al, 1998;Singh et al, 1998;Yang et al, 1998;Kharkovets et al, 2000). All K v 7 channels, with the exception of K v 7.5, have been linked with hereditary diseases: K v 7.1 with cardiac long QT syndrome, K v 7.2 and K v 7.3 with benign neonatal familial convulsions, and K v 7.4 with deafness (for reviews, see Robbins, 2001;Gribkoff, 2003).…”

“…The threshold for activation of K v 7 channels is around Ϫ60 mV (Yang et al, 1998), which is approximately the resting membrane potential of many neurons. A range of pharmacological tools elucidates the importance of K v 7 channels in controlling neuronal excitability.…”