Surfing the DNA databases for K+ channels nets yet more diversity

Much more than a leak: structure and function of K₂p-channels

Over the last decade, we have seen an enormous increase in the number of experimental studies on two-pore-domain potassium channels (K2P-channels). The collection of reviews and original articles compiled for this special issue of Pflügers Archiv aims to give an up-to-date summary of what is known about the physiology and pathophysiology of K2P-channels. This introductory overview briefly describes the structure of K2P-channels and their function in different organs. Its main aim is to provide some background information for the 19 reviews and original articles of this special issue of Pflügers Archiv. It is not intended to be a comprehensive review; instead, this introductory overview focuses on some unresolved questions and controversial issues, such as: Do K2P-channels display voltage-dependent gating? Do K2P-channels contribute to the generation of action potentials? What is the functional role of alternative translation initiation? Do K2P-channels have one or two or more gates? We come to the conclusion that we are just beginning to understand the extremely complex regulation of these fascinating channels, which are often inadequately described as 'leak channels'.

Identification and analysis of cation channel homologues in human pathogenic fungi

Fungi are major causes of human, animal and plant disease. Human fungal infections can be fatal, but there are limited options for therapy, and resistance to commonly used anti-fungal drugs is widespread. The genomes of many fungi have recently been sequenced, allowing identification of proteins that may become targets for novel therapies. We examined the genomes of human fungal pathogens for genes encoding homologues of cation channels, which are prominent drug targets. Many of the fungal genomes examined contain genes encoding homologues of potassium (K⁺), calcium (Ca²⁺) and transient receptor potential (Trp) channels, but not sodium (Na⁺) channels or ligand-gated channels. Some fungal genomes contain multiple genes encoding homologues of K⁺ and Trp channel subunits, and genes encoding novel homologues of voltage-gated K_v channel subunits are found in Cryptococcus spp. Only a single gene encoding a homologue of a plasma membrane Ca²⁺ channel was identified in the genome of each pathogenic fungus examined. These homologues are similar to the Cch1 Ca²⁺ channel of Saccharomyces cerevisiae. The genomes of Aspergillus spp. and Cryptococcus spp., but not those of S. cerevisiae or the other pathogenic fungi examined, also encode homologues of the mitochondrial Ca²⁺ uniporter (MCU). In contrast to humans, which express many K⁺, Ca²⁺ and Trp channels, the genomes of pathogenic fungi encode only very small numbers of K⁺, Ca²⁺ and Trp channel homologues. Furthermore, the sequences of fungal K⁺, Ca²⁺, Trp and MCU channels differ from those of human channels in regions that suggest differences in regulation and susceptibility to drugs.

Characterization of TWIK-2, a two-pore domain K+ channel, cloned from the rat middle cerebral artery

TWIK-2, a member of the Two-Pore Domain K channel family, is expressed in a number of mammalian tissues including the vascular system. The function of TWIK-2 is not known. The purpose of this study was to clone the TWIK-2 channel from the rat middle cerebral artery, express it in CHO cells, and characterize the channel's electrical properties. In light of the fact that there are no specific TWIK-2 inhibitors or activators, a better characterization of the channel should enhance our understanding of its role in the vascular system. TWIK-2 was cloned from the rat middle cerebral artery and expressed with an N-terminal green fluorescence protein (GFP) in CHO cells. We report that rTWIK-2-GFP currents were relatively linear at physiological K(+) concentrations but become slightly inwardly rectifying in symmetrical K(+). rTWIK-2-GFP was insensitive to 10 mM TEA, 3 mM 4-aminopyridine, and 10 microM glibenclamide. However, rTWIK-2-GFP was inhibited by Ba(2+) with 50% of the current being blocked at 80 microM. rTWIK-2-GFP activity was enhanced 60% by 100 microM arachidonic acid. The electrophysiological characteristics of TWIK-2 indicate that it could serve an important role in ion homeostasis and regulation of the membrane potential in arteries and arterioles.

Direct observation of individual KCNQ1 potassium channels reveals their distinctive diffusive behavior

We have directly observed the trafficking and fusion of ion channel containing vesicles and monitored the release of individual ion channels at the plasma membrane of live mammalian cells using total internal reflection fluorescence microscopy. Proteins were fused in-frame with green or red fluorescent proteins and expressed at low level in HL-1 and HEK293 cells. Dual color imaging revealed that vesicle trafficking involved motorized movement along microtubules followed by stalling, fusion, and subsequent release of individual ion channels at the plasma membrane. We found that KCNQ1-KCNE1 complexes were released in batches of about 5 molecules per vesicle. To elucidate the properties of ion channel complexes at the cell membrane we tracked the movement of individual molecules and compared the diffusive behavior of two types of potassium channel complex (KCNQ1-KCNE1 and Kir6.2-SUR2A) to that of a G-protein coupled receptor, the A1 adenosine receptor. Plots of mean squared displacement against time intervals showed that mobility depended on channel type, cell type, and temperature. Analysis of the mobility of wild type KCNQ1-KCNE1 complexes showed the existence of a significant immobile subpopulation and also a significant number of molecules that demonstrated periodic stalling of diffusive movements. This behavior was enhanced in cells treated with jasplakinolide and was abrogated in a C-terminal truncated form (KCNQ1(R518X)-KCNE1) of the protein. This mutant has been identified in patients with the long QT syndrome. We propose that KCNQ1-KCNE1 complexes interact intermittently with the actin cytoskeleton via the C-terminal region and this interaction may have a functional role.

cGMP does not activate two-pore domain K+ channels in cerebrovascular smooth muscle

Two-pore domain K(+) (K(2P)) channels are a new channel family. The goal of this study was to determine if K(2P) channels are activated by the nitric oxide (NO)/cGMP/PKG pathway in vascular smooth muscle. Relative levels of message for K(2P) channels were assessed in rat middle cerebral arteries (MCAs) using quantitative RT-PCR, and K(+) currents were measured in freshly dispersed vascular smooth muscle cells of the MCA. The rat MCA expresses a number of K(2P) channels. Message for TREK-1 was the most abundant K(2P) channel, followed by TASK-1 and TWIK-2, which were expressed at approximately 10% of the level of TREK-1. Message for other K(2P) channels was 1% or less than that of TREK-1. A number of K(2P) channels, including TREK-1, TWIK-2, and TASK-1, have putative PKG phosphorylation sites in the intracellular domains. The NO donor sodium nitroprusside (100 muM) or the membrane permeable analog of cGMP 8-bromo-cGMP (10 muM) elicited transient increases in whole cell current of vascular smooth muscle from the rat MCA. However, after large-conductance Ca(2+)-activated K(+) channels had been blocked with 10 mM tetraethylammonium (TEA), no increase in whole cell current was observed. Since K(2P) channels are resistant to the blocking effects of TEA, we conclude that K(2P) channels in vascular smooth muscle were not activated by the NO/cGMP/PKG pathway. Although K(2P) channels are highly expressed, K(2P) currents are not activated via the NO/cGMP pathway in rat MCA smooth muscle, despite the presence of numerous putative PKG phosphorylation sites.

Phylogeny as a guide to structure and function of membrane transport proteins

Protein phylogeny, based on primary amino acid sequence relatedness, reflects the evolutionary process and therefore provides a guide to structure, mechanism and function. Any two proteins that are related by common descent are expected to exhibit similar structures and functions to a degree proportional to the degree of their sequence similarity; but two independently evolving proteins should not. This principle provides the impetus to define protein phylogenetic relationships and interrelate families when possible. In this mini-review, we summarize the computational approaches and criteria we use to establish common evolutionary origin. We apply these tools to define distant superfamily relationships between several previously recognized transport protein families. In some cases, available structural and functional data are evaluated in order to substantiate our claim that molecular phylogeny provides a reliable guide to protein structure and function.

O2 sensing in the human ductus arteriosus: redox-sensitive K+ channels are regulated by mitochondria-derived hydrogen peroxide

The ductus arteriosus (DA) is a fetal artery that allows blood ejected from the right ventricle to bypass the pulmonary circulation in utero. At birth, functional closure of the DA is initiated by an O2-induced, vasoconstrictor mechanism which, though modulated by endothelial-derived endothelin and prostaglandins, is intrinsic to the smooth muscle cell (DASMC) [Michelakis et al., Circ. Res. 91 (2002); pp. 478-486]. As pO2 increases, a mitochondrial O2-sensor (electron transport chain complexes I or III) is activated, which generates a diffusible redox mediator (H2O2). H2O2 inhibits voltage-gated K+ channels (Kv) in DASMC. The resulting membrane depolarization activates L-type Ca2+ channels, thereby promoting vasoconstriction. Conversely, inhibiting mitochondrial ETC complexes I or III mimics hypoxia, depolarizing mitochondria, and decreasing H2O2 levels. The resulting increase in K+ current hyperpolarizes the DASMC and relaxes the DA. We have developed two models for study of the DA's O2-sensor pathway, both characterized by decreased O2-constriction and Kv expression: (i) preterm rabbit DA, (ii) ionically-remodeled, human term DA. The O2-sensitive channels Kv1.5 and Kv2.1 are important to DA O2-constriction and overexpression of either channel enhances DA constriction in these models. Understanding this O2-sensing pathway offers therapeutic targets to modulate the tone and patency of human DA in vivo, thereby addressing a common form of congenital heart disease in preterm infants.

Structural characterisation of neuronal voltage-sensitive K+ channels heterologously expressed in Pichia pastoris

Neuronal voltage-dependent K(+) channels of the delayed rectifier type consist of multiple Kv alpha subunit variants, which assemble as hetero- or homotetramers, together with four Kv beta auxiliary subunits. Direct structural information on these proteins has not been forthcoming due to the difficulty in isolating the native K(+) channels. We have overexpressed the subunit genes in the yeast Pichia pastoris. The Kv1.2 subunit expressed alone is shown to fold into a native conformation as determined by high-affinity binding of 125I-labelled alpha-dendrotoxin, while co-expressed Kv1.2 and Kv beta 2 subunits co-assembled to form native-like oligomers. Sites of post-translational modifications causing apparent heterogeneity on SDS-PAGE were identified by site-directed mutagenesis. Engineering to include affinity tags and scale-up of production by fermentation allowed routine purification of milligram quantities of homo- and heteroligomeric channels. Single-particle electron microscopy of the purified channels was used to generate a 3D volume to 2.1 nm resolution. Protein domains were assigned by fitting crystal structures of related bacterial proteins. Addition of exogenous lipid followed by detergent dialysis produced well-ordered 2D crystals that exhibited mostly p12(1) symmetry. Projection maps of negatively stained crystals show the constituent molecules to be 4-fold symmetric, as expected for the octameric K(+) channel complex.

Family ties of gated pores: evolution of the sensor module

The six-transmembrane channels are thought to be composed of two modules: pore and sensor. Whereas the modular design of the pore has been established, the modularity of the sensor remains hypothetical. As a first step toward establishing the modularity of this region, we searched for genes where the sensor is found independent of the pore and have identified new members of the sensor superfamily. Analysis of these sensors reveals a motif shared among not only these newly discovered members and voltage-gated, transient receptor potential, and polycystin channel sensors, but also MscL, a bacterial mechanosensitive channel. Mutational analyses presented here and in previous studies demonstrate that highly conserved residues within this motif are required for normal channel activity; mutations of residues within this motif in different subfamilies lead to consistent channel phenotypes. Previous studies have demonstrated that peptides containing this motif and the adjacent conserved transmembrane domain elicit channel activities when reconstituted into lipid membranes. These data provide evidence for the modularity of the sensor, imply a model for its evolution, suggest a common origin for mechano- and voltage-sensing, and may offer a glimpse of the properties of the first sensor/channel.

Mutants of a temperature-sensitive two-P domain potassium channel

Within the Caenorhabditis elegans genome there exist at least 42 genes encoding TWK (two-P domain K(+)) channels, potassium channel subunits that contain two pore regions and four transmembrane domains. We now report the first functional characterization of a TWK channel from C. elegans. Although potassium channels have been reported to be activated by a variety of factors, TWK-18 currents increase dramatically with increases in temperature. Two mutant alleles of the twk-18 gene confer uncoordinated movement and paralysis in C. elegans. Expression of wild-type and mutant TWK-18 channels in Xenopus oocytes showed that mutant channels express much larger potassium currents than wild-type channels. Promoter-green fluorescent protein fusion experiments indicate that TWK-18 is expressed in body wall muscle. Our genetic and physiological data suggest that the movement defects observed in mutant twk-18 animals may be explained by an increased activity of the mutant TWK-18 channels.

Phylogeny of ion channels: clues to structure and function

Voltage-gated ion channels are responsible for the electrical activity in a variety of cell types in modern-day animals. However, they represent the result of many millions of years of evolution of a family of ion channel proteins that are also found in prokaryotes and diverse eukaryotes, and probably exist in all life forms. This review traces the evolution of ion channels, with particular emphasis on the factors and evolutionary pathways that may have given rise to voltage-gated potassium (K+), calcium (Ca2+), and sodium (Na+) channels. The review also highlights the utility of comparing phylogenetically distinct versions of the same protein as a means to better understand the structure and function of proteins.

KCNQ potassium channels: physiology, pathophysiology, and pharmacology

KCNQ genes encode a growing family of six transmembrane domains, single pore-loop, K(+) channel alpha-subunits that have a wide range of physiological correlates. KCNQ1 (KvLTQ1) is co-assembled with the product of the KCNE1 (minimal K(+)-channel protein) gene in the heart to form a cardiac-delayed rectifier-like K(+) current. Mutations in this channel can cause one form of inherited long QT syndrome (LQT1), as well as being associated with a form of deafness. KCNQ1 can also co-assemble with KCNE3, and may be the molecular correlate of the cyclic AMP-regulated K(+) current present in colonic crypt cells. KCNQ2 and KCNQ3 heteromultimers are thought to underlie the M-current; mutations in these genes may cause an inherited form of juvenile epilepsy. The KCNQ4 gene is thought to encode the molecular correlate of the I(K,n) in outer hair cells of the cochlea and I(K,L) in Type I hair cells of the vestibular apparatus, mutations in which lead to a form of inherited deafness. The recently identified KCNQ5 gene is expressed in brain and skeletal muscle, and can co-assemble with KCNQ3, suggesting it may also play a role in the M-current heterogeneity. This review will set this family of K(+) channels amongst the other known families. It will highlight the genes, physiology, pharmacology, and pathophysiology of this recently discovered, but important, family of K(+) channels.

Eukaryotic transmembrane solute transport systems

A comprehensive classification system for transmembrane molecular transporters has been proposed. This system is based on (i) mode of transport and energy-coupling mechanism, (ii) protein phylogenetic family, (iii) phylogenetic cluster, and (iv) substrate specificity. The proposed "Transport Commission" (TC) system is superficially similar to that implemented decades ago by the Enzyme Commission for enzymes, but it differs from the latter system in that it uses phylogenetic and functional data for classification purposes. Very few families of transporters include members that do not function exclusively in transport. Analyses reported reveal that channels, primary carriers, secondary carriers (uni-, sym-, and antiporters), and group translocators comprise distinct categories of transporters, and that transport mode and energy coupling are relatively immutable characteristics. By contrast, substrate specificity and polarity of transport are often readily mutable. Thus, with very few exceptions, a unified family of transporters includes members that function by a single transport mode and energy-coupling mechanism although a variety of substrates may be transported with either inwardly or outwardly directed polarity. The TC system allows cross-referencing according to substrates transported and protein sequence database accession numbers. Thus, familial assignments of newly sequenced transport proteins are facilitated. In this article I examine families of transporters that are eukaryotic specific. These families include (i) channel proteins, mostly from animals; (ii) facilitators and secondary active transport carriers; (iii) a few ATP-dependent primary active transporters; and (iv) transporters of unknown mode of action or energy-coupling mechanism. None of the several ATP-independent primary active transport energy-coupling mechanisms found in prokaryotes is represented within the eukaryotic-specific families. The analyses reported provide insight into transporter families that may have arisen in eukaryotes after the separation of eukaryotes from archaea and bacteria. On the basis of the reported analyses, it is suggested that the horizontal transfer of genes encoding transport proteins between eukaryotes and members of the other two domains of life occurred very infrequently during evolutionary history.

Characterization of TASK-4, a novel member of the pH-sensitive, two-pore domain potassium channel family

We report the primary sequence of TASK-4, a novel member of the acid-sensitive subfamily of tandem pore K(+) channels. TASK-4 transcripts are widely expressed in humans, with highest levels in liver, lung, pancreas, placenta, aorta and heart. In Xenopus oocytes TASK-4 generated K(+) currents displaying a marked outward rectification which was lost by elevation of extracellular K(+). TASK-4 currents were efficiently blocked by barium (83% inhibition at 2 mM), only weakly inhibited by 1 mM concentrations of quinine, bupivacaine and lidocaine, but not blocked by tetraethylammonium, 4-aminopyridine and Cs(+). TASK-4 was sensitive to extracellular pH, but in contrast to other TASK channels, pH sensitivity was shifted to more alkaline pH. Thus, TASK-4 in concert with other TASK channels might regulate cellular membrane potential over a wide range of extracellular pH.

GYGD pore motifs in neighbouring potassium channel subunits interact to determine ion selectivity

Cells maintain a negative resting membrane potential through the constitutive activity of background K+ channels. A novel multigene family of such K+ channels has recently been identified. A unique characteristic of these K+ channels is the presence of two homologous, subunit-like domains, each containing a pore-forming region. Sequence co-variations in the GYGD signature motifs of the two pore regions suggested an interaction between neighbouring pore domains. Mutations of the GYGD motif in the rat drk1 (Kv2.1) K+ channel showed that the tyrosine (Y) position was important for K+ selectivity and single channel conductance, whereas the aspartate (D) position was a critical determinant of open state stability. Tandem constructs engineered to mimic the GYGx-GxGD pattern seen in two-domain K+ channels delineated a co-operative intersubunit interaction between the Y and D positions, which determined ion selectivity, conductance and gating. In the bacterial KcsA K+ channel crystal structure, the equivalent aspartate residue (D80) does not directly interact with permeating K+ ions. However, the data presented here show that the D position is able to fine-tune ion selectivity through a functional interaction with the Y position in the neighbouring subunit. These data indicate a physiological basis for the extensive sequence variation seen in the GYGD motifs of two-domain K+ channels. It is suggested that a cell can precisely regulate its resting membrane potential by selectively expressing a complement of two-domain K+ channels.

Genomic organization and chromosomal localization of the murine 2 P domain potassium channel gene Kcnk8: conservation of gene structure in 2 P domain potassium channels

A 2 P domain potassium channel expressed in eye, lung, and stomach, Kcnk8, has recently been identified. To initiate further biochemical and genetic studies of this channel, we assembled the murine Kcnk8 cDNA sequence, characterized the genomic structure of the Kcnk8 gene, determined its chromosomal localization, and analyzed its activity in a Xenopus laevis oocyte expression system. The composite cDNA has an open reading frame of 1029 bp and encodes a protein of 343 amino acids with a predicted molecular mass of 36 kDa. Structure analyses predict 2 P domains and four potential transmembrane helices with a potential single EF-hand motif and four potential SH3-binding motifs in the COOH-terminus. Cloning of the Kcnk8 chromosomal gene revealed that it is composed of three exons distributed over 4 kb of genomic DNA. Genome database searching revealed that one of the intron/exon boundaries identified in Kcnk8 is present in other mammalian 2 P domain potassium channels genes and many C. elegans 2P domain potassium channel genes, revealing evolutionary conservation of gene structure. Using fluorescence in situ hybridization, the murine Kcnk8 gene was mapped to chromosome 19, 2B, the locus of the murine dancer phenotype, and syntenic to 11q11-11q13, the location of the human homologue. No significant currents were generated in a Xenopus laevis oocyte expression system using the composite Kcnk8 cDNA sequence, suggesting, like many potassium channels, additional channel subunits, modulator substances, or cellular chaperones are required for channel function.

Molecular identification of O2 sensors and O2-sensitive potassium channels in the pulmonary circulation

Small, muscular pulmonary arteries (PAs) constrict within seconds of the onset of alveolar hypoxia, diverting blood flow to better-ventilated lobes, thereby matching ventilation to perfusion and optimizing systemic PO2. This hypoxic pulmonary vasoconstriction (HPV) is enhanced by endothelial derived vasoconstrictors, such as endothelin, and inhibited by endothelial derived nitric oxide. However, the essence of the response is intrinsic to PA smooth muscle cells in resistance arteries (PASMCs). HPV is initiated by inhibition of the Kv channels in PASMCs which set the membrane potential (EM). It is currently uncertain whether this reflects an initial inhibitory effect of hypoxia on the K+ channels or an initial release of intracellular Ca2+, which then inhibits K+ channels. In either scenario, the resulting depolarization activates L-type, voltage gated Ca2+ channels, which raises cytosolic calcium levels [Ca2+]i and causes vasoconstriction. Nine families of Kv channels are recognized from cloning studies (Kv1-Kv9), each with subtypes (i.e. Kv1.1-1.6). The contribution of an individual Kv channel to the whole-cell current (IK) is difficult to determine pharmacologically because Kv channel inhibitors are nonspecific. Furthermore, the PASMC's IK is an ensemble, reflecting activity of several channels. The K+ channels which set EM, and inhibition of which initiates HPV, conduct an outward current which is slowly inactivating, and which is blocked by the Kv inhibitor 4-aminopyridine (4-AP) but not by inhibitors of Ca(2+)- or ATP-sensitive K+ channels. Using anti-Kv antibodies to immunolocalize and inhibit Kv channels, we showed that the PASMC contains numerous types of Kv channels from the Kv1 and Kv2 families., Furthermore Kv1.5 and Kv2.1 may be important in determining the EM and play a role as effectors of HPV in PASMCs. While the Kv channels in PASMCs are the "effectors" of HPV, it is uncertain whether they are intrinsically O2-sensitive or are under the control of an "O2 sensor". Certain Kv channels are rich in cysteine, and respond to the local redox environment, tending to open when oxidized and close when reduced. Candidate sensors vary the PASMC redox potential in proportion to O2. These include Nicotinamide Adenine Dinucleotide Phosphate Oxidase, (NADPH oxidase) and the cytosolic ratio of reduced/oxidized redox couples (i.e. glutathione GSH/GSSG), as controlled by electron flux in the mitochondrial electron transport chain (ETC). Using a mouse that lacks the gp91phox component of NADPH oxidase, we have recently shown that loss of the gp91phox-containing NADPH oxidase as a source of activated oxygen species does not impair HPV. However, inhibition of complex 1 of the mitochondrial electron transport chain mimics hypoxia in that it inhibits IK, reduces the production of activated O2 species and causes vasoconstriction. We hypothesize that a redox O2 sensor, perhaps in the mitochondrion, senses O2 through changes in the accumulation of freely diffusible electron donors. Changes in the ratio of reduced/oxidized redox couples, such as NADH/NAD+ and glutathione (GSH/GSSG) can reduce or oxidize the K+ channels, resulting in alterations of PA tone.

The molecular biology of invertebrate voltage-gated Ca(2+) channels

The importance of voltage-gated Ca(2+) channels in cellular function is illustrated by the many distinct types of Ca(2+) currents found in vertebrate tissues, a variety that is generated in part by numerous genes encoding Ca(2+) channel subunits. The degree to which this genetic diversity is shared by invertebrates has only recently become apparent. Cloning of Ca(2+) channel subunits from various invertebrate species, combined with the wealth of information from the Caenorhabditis elegans genome, has clarified the organization and evolution of metazoan Ca(2+) channel genes. Functional studies have employed novel structural information gained from invertebrate Ca(2+) channels to complement ongoing research on mammalian Ca(2+) currents, while demonstrating that the strict correspondence between pharmacological and molecular classes of vertebrate Ca(2+) channels does not fully extend to invertebrate tissues. Molecular structures can now be combined with physiological data to develop a more cogent system of categorizing invertebrate channel subtypes. In this review, we examine recent progress in the characterization of invertebrate Ca(2+) channel genes and its relevance to the diversity of invertebrate Ca(2+) currents.

Identification and characterization of a putative C. elegans potassium channel gene (Ce-slo-2) distantly related to Ca(2+)-activated K(+) channels

Two putative homologues of large conductance Ca(2+)-activated K(+) channel alpha-subunit gene (slowpoke or slo) were revealed by C. elegans genome sequencing. One of the two genes, F08B12.3 (Ce-slo-2), shows a relatively low amino acid sequence similarity to other Slo sequences and lacks key functional motifs, which are important for calcium and voltage sensing. However, its overall structure and regions of homology, which are conserved in all Slo proteins, suggest that Ce-SLO-2 should belong to the Slo channel family. We have cloned a full-length cDNA of the Ce-slo-2, which encodes a protein containing six putative transmembrane segments with a K(+)-selective pore and a large C-terminal cytosolic domain. Green fluorescent protein (GFP) and whole-mount immunostaining analyses revealed that Ce-slo-2 is specifically expressed in neuronal cells at the nerve ring, at the ventral nerve cord of the mid-body, and at the tail region. We have also identified a putative human counterpart of Ce-slo-2 from a human brain EST database, which shows a stretch of highly conserved amino acid residues. Northern blot and mRNA dot blot analyses revealed a strong and specific expression in brain and skeletal muscle. Taken together, our data suggest that Ce-slo-2 may constitute an evolutionarily conserved gene encoding a potassium channel that has specific functions in neuronal cells.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

A Ca2+/calmodulin-dependent protein kinase modulates Drosophila photoreceptor K+ currents: a role in shaping the photoreceptor potential

Light activation of Drosophila photoreceptors leads to the generation of a depolarizing receptor potential via opening of transient receptor potential and transient receptor potential-like cationic channels. Counteracting the light-activated depolarizing current are two voltage-gated K+ conductances, IA and IK, that are expressed in these sensory neurons. Here we show that Drosophila photoreceptors IA and IK are regulated by calcium-calmodulin (Ca2+/calmodulin) via a Ca2+/calmodulin-dependent protein kinase (CaM kinase), with IK being far more sensitive than IA. Inhibition of Ca2+/calmodulin by N-(6 aminohexyl)-5-chloro-1-naphthalenesulfonamide or trifluoperazine markedly reduced the K+ current amplitudes. Likewise, inhibition of CaM kinases by KN-93 potently depressed IK and accelerated its C-type inactivation kinetics. The effect of KN-93 was specific because its structurally related but functionally inactive analog KN-92 was totally ineffective. In Drosophila photoreceptor mutant ShKS133, which allows isolation of IK, we demonstrate by current-clamp recording that inhibition of IK by quinidine or tetraethylammonium increased the amplitude of the photoreceptor potential, depressed light adaptation, and slowed down the termination of the light response. Similar results were obtained when CaM kinases were blocked by KN-93. These findings place photoreceptor K+ channels as an additional target for Ca2+/calmodulin and suggest that IK is well suited to act in concert with other components of the signaling machinery to sharpen light response termination and fine tune photoreceptor sensitivity during light adaptation.

Memory and long-term potentiation (LTP) dissociated: normal spatial memory despite CA1 LTP elimination with Kv1.4 antisense

Long-term potentiation (LTP) in the hippocampal slice preparation has been proposed as an in vitro model for long-term memory. However, correlation of LTP with memory in living animals has been difficult to demonstrate. Furthermore, in the last few years evidence has accumulated that dissociate the two. Because potassium channels might determine the weight of synapses in networks, we studied the role of Kv1.4, a presynaptic A-type voltage-dependent K+ channel, in both memory and LTP. Reverse transcription-PCR and Western blot analysis with specific antibodies showed that antisense oligodeoxyribonucleotide to Kv1.4 microinjected intraventricularly into rat brains obstructed hippocampal Kv1.4 mRNA, "knocking down" the protein in the hippocampus. This antisense knockdown had no effect on rat spatial maze learning, memory, or exploratory behavior, but eliminated both early- and late-phase LTP and reduced paired-pulse facilitation (a presynaptic effect) in CA1 pyramidal neurons without affecting dentate gyrus LTP. This presynaptic Kv1.4 knockdown together with previous postsynaptic Kv1.1 knockdown demonstrates that CA1 LTP is neither necessary nor sufficient for rat spatial memory.

Detection of transcripts for delayed rectifier potassium channels in the Xenopus laevis inner ear

Reverse transcriptase polymerase chain reaction (RT-PCR) was used to amplify sequences for delayed rectifier potassium (drk) channel transcripts in Xenopus laevis inner ear and brain. We used degenerate primers that spanned a region between the N-terminal cytoplasmic portion and a region located between the S2 and S3 transmembrane domains of the potassium channel protein. When inner ear total RNA or brain mRNA was used as a template for RT-PCR, a unique product of the expected size (approximately 560 bp) was observed as a single band after electrophoresis on agarose gels. The PCR product from reactions using X. laevis genomic DNA as template was similarly sized, indicating a lack of introns in this region. The RT-PCR products from inner ear and brain were isolated, cloned, and sequenced. Sequence analysis showed that the X. laevis inner ear and brain clones were identical. Sequence alignments of the cloned RT-PCR products with posted GenBank sequences established that the drk sequences from X. laevis inner ear and brain share highest identity with larval X. laevis brain, mouse, rat, and human Kv2 sequences. Positive signals were obtained from inner ear and brain mRNA in Northern dot blots hybridized with digoxigenin labeled probes from the inner ear clone. Taken together, results provide evidence for the expression of Kv2 sequences in the X. laevis inner ear and brain.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.

Characterization of delayed rectifier Kv channels in oligodendrocytes and progenitor cells

We examined the molecular identity of K+ channel genes underlying the delayed rectifier (IK) in differentiated cultured oligodendrocytes (OLGs) and oligodendrocyte progenitor (OP) cells. Using reverse transcription-PCR cloning, we found that OP cells and OLGs expressed multiple Kv transcripts, namely Kv1.2, Kv1.4, Kv.1.5, and Kv1.6. Immunocytochemical and Western blot analyses revealed that Kv1.5 and Kv1.6 as well as Kv1.2 and Kv1.4 channel proteins could be detected in these cells, but definitive evidence for functional K+ channel expression was obtained only for the Kv1.5 channel. In addition, mRNA and immunoreactive protein levels of both Kv1.5 and Kv1.6 channels were significantly lower in differentiated OLGs when compared with levels in OP cells. Proliferation of OP cells was inhibited by K+ channel blockers, but not by incubation with either Kv1.5 or Kv1.6 antisense oligonucleotides. We conclude that (1) IK in OP cells and OLGs is encoded partly by Kv1.5 subunits, possibly forming heteromultimeric channels with Kv1.6 or other Kv subunits; and (2) inhibition of Kv1.5 or Kv1.6 channel expression alone does not prevent mitogenesis. Concomitant inhibition of other Kv channels underlying IK may be necessary for OP cells to exit from cell cycle.

Age-dependent expression of the apamin-sensitive calcium-activated K+ channel in fast and slow rat skeletal muscle

An altered expression of the apamin-sensitive K+ channel from skeletal muscle is apparently implicated in human myotonic dystrophy (MD). We found, in rats, that the expression of this channel depends on age and the type of muscle. This result may be one of the bases of the different susceptibilities of fast and slow muscles to drug-induced myotonia.

New structure and function in plant K+ channels: KCO1, an outward rectifier with a steep Ca2+ dependency

Potassium (K+) channels mediating important physiological functions are characterized by a common pore-forming (P) domain. We report the cloning and functional analysis of the first higher plant outward rectifying K+ channel (KCO1) from Arabidopsis thaliana. KCO1 belongs to a new class of 'two-pore' K+ channels recently described in human and yeast. KCO1 has four putative transmembrane segments and tandem calcium-binding EF-hand motifs. Heterologous expression of KCO1 in baculovirus-infected insect (Spodoptera frugiperda) cells resulted in outwardly rectifying, K+-selective currents elicited by depolarizing voltage pulses in whole-cell measurements. Activation of KCO1 was strongly dependent on the presence of nanomolar concentrations of cytosolic free Ca2+ [Ca2+]cyt. No K+ currents were detected when [Ca2+]cyt was adjusted to <150 nM. However, KCO1 strongly activated at increasing [Ca2+]cyt, with a saturating activity observed at approximately 300 nM [Ca2+]cyt. KCO1 single channel analysis on excised membrane patches, resulting in a single channel conductance of 64 pS, confirmed outward rectification as well as Ca2+-dependent activation. These data suggest a direct link between calcium-mediated signaling processes and K+ ion transport in higher plants. The identification of KCO1 as the first plant K+ outward channel opens a new field of structure-function studies in plant ion channels.

Subunit interactions in the assembly of neuronal Kir3.0 inwardly rectifying K+ channels

Cardiac G protein-activated Kir (GIRK) channels may assemble as heterotetrameric polypeptides from two subunits, Kir3.1 and Kir3.4. For a functional comparison with native channels in the CNS we investigated all possible combinations of heteromeric channel formation from brain Kir3.1, Kir3.2, Kir3.3, and Kir3.4 subunits in mRNA-injected Xenopus oocytes. Analysis of macroscopic current amplitudes and channel gating kinetics indicated that individual subunits or combinations of Kir3.2, Kir3.3, and Kir3.4 formed functional channels ineffectively. Each of these subunits gave rise to prominent currents with distinct characteristics only in the presence of Kir3.1 subunits. Functional expression of concatemeric constructs between Kir3.1 and Kir3.2/3.4 subunits as well as coimmunoprecipitations with subunit-specific antibodies confirmed heteromeric channel formation. Mutational swapping between subunits of a single pore loop residue (Kir3.1F137S; Kir3.3S114F; a phenylalanine confers slow channel gating in Kir3.1 subunits) revealed that Kir3.1 subunits are an important constituent for native heteromeric channels and dominate their functional properties. However, homomeric channels from Kir3.1 subunits in vivo may not exist due to the spatial conflict of bulky phenylalanines in the pore structure.

ORK1, a potassium-selective leak channel with two pore domains cloned from Drosophila melanogaster by expression in Saccharomyces cerevisiae

A K+ channel gene has been cloned from Drosophila melanogaster by complementation in Saccharomyces cerevisiae cells defective for K+ uptake. Naturally expressed in the neuromuscular tissues of adult flies, this gene confers K+ transport capacity on yeast cells when heterologously expressed. In Xenopus laevis oocytes, expression yields an ungated K(+)-selective current whose attributes resemble the "leak" conductance thought to mediate the resting potential of vertebrate myelinated neurons but whose molecular nature has long remained elusive. The predicted protein has two pore (P) domains and four membrane-spanning helices and is a member of a newly recognized K+ channel family. Expression of the channel in flies and yeast cells makes feasible studies of structure and in vivo function using genetic approaches that are not possible in higher animals.

Do voltage-gated Kv1.1 and inward rectifier Kir2.1 potassium channels form heteromultimers?

Possible heteromultimer formation between Kv- and Kir-type K+ channels was investigated, in connection with the known functional diversity of K+ channels in vivo. Voltage-clamp experiments were performed on Xenopus oocytes, either injected with concatenated Kir2.1-Kv1.1 mRNA, or co-injected with Kv1.1 and Kir2.1 mRNA. K+ currents could be approximated by the algebraic sum of the 2 K+ current types alone. The tandem construct did not show functional expression, although it could be detected by Western blotting. We conclude that Kv1.1 and Kir2.1 alpha-subunit proteins fail to assemble and do not contribute functional diversity to K+ channels.

Presynaptic potassium channels

The past year has witnessed some significant improvements in our understanding of the molecular diversity, subunit composition, and functional properties of K+ channels in heterologous expression systems. Immunocytochemical studies have yielded important information on the localization of K+ channel proteins to synaptic terminals in mammalian brain. Although a coherent picture of native presynaptic K+ channels' function in the mammalian central nervous system is not yet available, it may emerge from improvements in patch-clamp techniques and new applications of targeted knock-out technologies.

Kinetic variability and modulation of dSlo, a cloned calcium-dependent potassium channel

We have examined, using patch recording, the modulation by ATP gamma S of the cloned Drosophila slopoke calcium-dependent potassium channel (dSlo) expressed in Xenopus oocytes. There is a large variation in the gating kinetics, open probability, and conductance level of the channel in this expression system, which complicates the analysis of modulatory events. Addition of ATP gamma S to the intracellular face of the patch does not consistently alter the overall open probability of dSlo, but it does increase the frequency of appearance of an exceptionally long-lived closed state of the channel. This modulation is not blocked by an inhibitor of several serine/threonine protein kinases, nor by mutation of a serine residue that is a target for phosphorylation by protein kinase A. Thus, ATP gamma S may alter dSlo kinetic properties by some mechanism other than serine/threonine phosphorylation.