Potassium channels and their evolving gates

Macromolecular rate theory explains the temperature dependence of membrane conductance kinetics

The factor Q10 is used in neuroscience to adjust reaction rates of voltage-activated membrane conductances to different temperatures and is widely assumed to be constant. By performing an analysis of published data of the reaction rates of sodium, potassium, and calcium membrane conductances, we demonstrate that 1) Q10 is temperature dependent, 2) this relationship is similar across conductances, and 3) there is a strong effect at low temperatures (<15°C). We show that macromolecular rate theory (MMRT) explains this temperature dependency. MMRT predicts the existence of optimal temperatures at which reaction rates decrease as temperature increases, a phenomenon that we also found in the published data sets. We tested the consequences of using MMRT-adjusted reaction rates in the Hodgkin-Huxley model of the squid's giant axon. The MMRT-adjusted model reproduces the temperature dependence of the rising and falling times of the action potential. Furthermore, the model also reproduces these properties for different squid species that live in different climates. In a second example, we compare spiking patterns of biophysical models based on human pyramidal neurons from the Allen Cell Types database at room and physiological temperatures. The original models, calibrated at 34°C, failed to generate realistic spikes at room temperature in more than half of the tested models, while the MMRT produces realistic spiking in all conditions. In another example, we show that using the MMRT correction in hippocampal pyramidal cell models results in 100% differences in voltage responses. Finally, we show that the shape of the Q10 function results in systematic errors in predicting reaction rates. We propose that the optimal temperature could be a thermodynamical barrier to avoid over excitation in neurons. While this study is centered on membrane conductances, our results have important consequences for all biochemical reactions involved in cell signaling.

Germline deletion of chromosome 2p16-21 associated with Lynch syndrome

We identified a Japanese patient with Lynch syndrome with a novel large germline deletion of chromosome 2p16-21, including the EPCAM, MSH2, and KCNK12 genes. The proband was a 46-year-old man with ascending colon cancer. The clinical significance of germline KCNK12 gene deletion, which encodes one of the subfamilies of two-pore-domain potassium channels, is still unknown.

Identification and functional characterization of natural resistance-associated macrophage protein 2 from sea cucumber Apostichopus japonicus

As a member of natural resistance-associated macrophage protein (Nramp) family, Nramp2 conservatively exists in the cell membrane across species and is essential for normal iron homeostasis in an H⁺-dependent manner. Withholding available iron represents an important host defense strategy. However, the function of Nramp2 in response to invading pathogens is largely unknown in invertebrates. In this study, a unique echinoderm Nramp2 was identified from sea cucumber Apostichopus japonicus (designated as AjNramp2). The cDNA sequence of AjNramp2 was 2360 bp, with a putative open reading frame of 1713 bp, encoding a typical Nramp domain containing protein with 570 amino acid residues. Structural analysis revealed that AjNramp2 consisted of highly conserved helix regions similar with the human Nramp2. Spatial expression analysis revealed that AjNramp2 was ubiquitously expressed in all examined tissues, with the highest level found in the intestine. Immunohistochemistry assay showed that AjNramp2 was mainly located in the cellular membrane in coelomocytes. Vibrio splendidus challenge and lipopolysaccharide (LPS) stimulation could significantly promote the expression of AjNramp2, which was consistent with the cellular iron level in coelomocytes. Moreover, when the expression of AjNramp2 was knocked down by siRNA-AjNramp2, the cellular iron level was coordinately decreased in coelomocytes under LPS stimulation. Taken together, results indicated that AjNramp2 serves as an iron transport receptor to withhold available iron and may contribute to the nutritional immunity defense system of sea cucumber.

The KdpFABC complex - K+ transport against all odds

In bacteria, K⁺ is used to maintain cell volume and osmotic potential. Homeostasis normally involves a network of constitutively expressed transport systems, but in K⁺ deficient environments, the KdpFABC complex uses ATP to pump K⁺ into the cell. This complex appears to be a hybrid of two types of transporters, with KdpA descending from the superfamily of K⁺ transporters and KdpB belonging to the superfamily of P-type ATPases. Studies of enzymatic activity documented a catalytic cycle with hallmarks of classical P-type ATPases and studies of ion transport indicated that K⁺ import into the cytosol occurred in the second half of this cycle in conjunction with hydrolysis of an aspartyl phosphate intermediate. Atomic structures of the KdpFABC complex from X-ray crystallography and cryo-EM have recently revealed conformations before and after formation of this aspartyl phosphate that appear to contradict the functional studies. Specifically, structural comparisons with the archetypal P-type ATPase, SERCA, suggest that K⁺ transport occurs in the first half of the cycle, accompanying formation of the aspartyl phosphate. Further controversy has arisen regarding the path by which K⁺ crosses the membrane. The X-ray structure supports the conventional view that KdpA provides the conduit, whereas cryo-EM structures suggest that K⁺ moves from KdpA through a long, intramembrane tunnel to reach canonical ion binding sites in KdpB from which they are released to the cytosol. This review discusses evidence supporting these contradictory models and identifies key experiments needed to resolve discrepancies and produce a unified model for this fascinating mechanistic hybrid.

"Electrifying dysmorphology": Potassium channelopathies causing dysmorphic syndromes

Potassium channels are a heterogeneous group of membrane-bound proteins, whose functions support a diverse range of biological processes. Genetic disorders arising from mutations in potassium channels are classically recognized by symptoms arising from acute channel dysfunction, such as periodic paralysis, ataxia, seizures, or cardiac conduction abnormalities, often in a patient with otherwise normal examination findings. In this chapter, we review a distinct subgroup of rare potassium channelopathies whose presentations are instead suggestive of a developmental disorder, with features including intellectual disability, craniofacial dysmorphism or other physical anomalies. Known conditions within this subgroup are: Andersen-Tawil syndrome, Birk-Barel syndrome, Cantú syndrome, Keppen-Lubinsky syndrome, Temple-Baraitser syndrome, Zimmerman-Laband syndrome and a very similar disorder called Bauer-Tartaglia or FHEIG syndrome. Ion channelopathies are unlikely to be routinely considered in the differential diagnosis of children presenting with developmental concerns, and so detailed description and photographs of the clinical phenotype are provided to aid recognition. For several of these disorders, functional characterization of the genetic mutations responsible has led to identification of candidate therapies, including drugs already commonly used for other indications, which adds further impetus to their prompt recognition. Together, these cases illustrate the potential for mechanistic insights gained from genetic diagnosis to drive translational work toward targeted, disease-modifying therapies for rare disorders.

Activation of ATP-sensitive K-channel promotes the anticonvulsant properties of cannabinoid receptor agonist through mitochondrial ATP level reduction

Cannabinoid receptor (CBR) agonist could act as a protective agent against seizure susceptibility in animal models of epilepsy. Studies have shown that potassium channels could play a key role in ameliorating neuronal excitability. In this study, we attempted to evaluate how CBRs and Adenosine Tri-Phosphate (ATP)-sensitive potassium channels collaborate to affect seizure susceptibility by changing the clonic seizure threshold (CST). We used male Naval Medical Research Institute (NMRI) mice and treated them with the following drugs: cromakalim (a potassium channel opener, 10 μg/kg), glibenclamide (a potassium channel blocker, 0.03 and 1 mg/kg), 0.5 mg/kg of AM-251 (a selective CB1 antagonist), AM-630 (a selective CB2 antagonist), and 0.5, 3, and 10 mg/kg of WIN 55,212-2 (a nonselective agonist of CBRs); and CST was appraised after each type of administration. Also, we evaluated the ATP level of the hippocampus in each treatment to clarify the interaction between the cannabinoid system and potassium channel. Our results showed that administration of WIN 55,212-2 at 10 mg/kg significantly increased CST (P < 0.001). This change could be reversed by using AM-251(P < 0.001) but not AM-630. Also, either cromakalim (10 μg/kg) or glibenclamide (0.03 and 1 mg/kg) could not significantly affect the CST. In addition, glibenclamide (1 mg/kg) could reverse the anticonvulsant effect of WIN 55,212-2 (10 mg/kg) on CST (P < 0.001). However, the anticonvulsant effect was observed when cromakalim (10 μg/kg) was added to WIN 55,212-2 at its subeffective dose (3 mg/kg) in comparison to single-treated animals. Interestingly, we observed that CB1 agonist could significantly decrease ATP level. In conclusion, CB1 agonist accomplishes at least a part of its anticonvulsant actions through ATP-sensitive potassium channels, probably by decreasing the mitochondrial ATP level to open the potassium channel to induce its anticonvulsant effect.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

High-Performance Respiration-Based Biocell Using Artificial Nanochannel Regulation

Based on electron and proton transfer events occurring in biological respiration, a mitochondria-based biocell is constructed by combining with artificial nanochannels. In this biocell, mitochondria transfer electrons to the working electrode and pump protons into the electrolyte through the tricarboxylic acid cycle. The nanochannels provide passages for protons to transport along the transmembrane concentration gradient to consume electrons on the counter electrode, forming a continuous and stable current. Furthermore, the proton transmembrane transport behavior could be modulated by regulating the permeability area and surface charge of nanochannels. A high-performance biocell is obtained when equipped with the optimized nanochannels, which produces a current of ≈3.1 mA cm^-2 , a maximum power of ≈0.91 mW cm^-2 , and a lifetime over 60 h. This respiratory-based biocell shows great potential for the efficient utilization of bioelectricity.

Mitochondrial Ultrastructure and Glucose Signaling Pathways Attributed to the Kv1.3 Ion Channel

Gene-targeted deletion of the potassium channel Kv1.3 (Kv1.3^−∕−) results in “Super-smeller” mice with a sensory phenotype that includes an increased olfactory ability linked to changes in olfactory circuitry, increased abundance of olfactory cilia, and increased expression of odorant receptors and the G-protein, G_olf. Kv1.3^−∕− mice also have a metabolic phenotype including lower body weight and decreased adiposity, increased total energy expenditure (TEE), increased locomotor activity, and resistance to both diet- and genetic-induced obesity. We explored two cellular aspects to elucidate the mechanism by which loss of Kv1.3 channel in the olfactory bulb (OB) may enhance glucose utilization and metabolic rate. First, using in situ hybridization we find that Kv1.3 and the insulin-dependent glucose transporter type 4 (GLUT4) are co-localized to the mitral cell layer of the OB. Disruption of Kv1.3 conduction via construction of a pore mutation (W386F Kv1.3) was sufficient to independently translocate GLUT4 to the plasma membrane in HEK 293 cells. Because olfactory sensory perception and the maintenance of action potential (AP) firing frequency by mitral cells of the OB is highly energy demanding and Kv1.3 is also expressed in mitochondria, we next explored the structure of this organelle in mitral cells. We challenged wildtype (WT) and Kv1.3^−∕− male mice with a moderately high-fat diet (MHF, 31.8 % kcal fat) for 4 months and then examined OB ultrastructure using transmission electron microscopy. In WT mice, mitochondria were significantly enlarged following diet-induced obesity (DIO) and there were fewer mitochondria, likely due to mitophagy. Interestingly, mitochondria were significantly smaller in Kv1.3^−∕− mice compared with that of WT mice. Similar to their metabolic resistance to DIO, the Kv1.3^−∕− mice had unchanged mitochondria in terms of cross sectional area and abundance following a challenge with modified diet. We are very interested to understand how targeted disruption of the Kv1.3 channel in the OB can modify TEE. Our study demonstrates that Kv1.3 regulates mitochondrial structure and alters glucose utilization; two important metabolic changes that could drive whole system changes in metabolism initiated at the OB.

1.2 Å X-ray structure of the renal potassium channel Kv1.3 T1 domain

Here we present the structure of the T1 domain derived from the voltage-dependent potassium channel K(v)1.3 of Homo sapiens sapiens at 1.2 Å resolution crystallized under near-physiological conditions. The crystals were grown without precipitant in 150 mM KP(i), pH 6.25. The crystals show I4 symmetry typical of the natural occurring tetrameric assembly of the single subunits. The obtained structural model is based on the highest resolution currently achieved for tetramerization domains of voltage-gated potassium channels. We identified an identical fold of the monomer but inside the tetramer the single monomers show a significant rotation which leads to a different orientation of the tetramer compared to other known structures. Such a rotational movement inside the tetrameric assembly might influence the gating properties of the channel. In addition we see two distinct side chain configurations for amino acids located in the top layer proximal to the membrane (Tyr109, Arg116, Ser129, Glu140, Met142, Arg146), and amino acids in the bottom layer of the T1-domain distal from the membrane (Val55, Ile56, Leu77, Arg86). The relative populations of these two states are ranging from 50:50 for Val55, Tyr109, Arg116, Ser129, Glu140, 60:40 for Met142, 65:35 for Arg86, 70:30 for Arg146, and 80:20 for Ile56 and Leu77. The data suggest that in solution these amino acids are involved in an equilibrium of conformational states that may be coupled to the functional states of the whole potassium channel.

Conduits of life's spark: a perspective on ion channel research since the birth of neuron

Heartbeats, muscle twitches, and lightning-fast thoughts are all manifestations of bioelectricity and rely on the activity of a class of membrane proteins known as ion channels. The basic function of an ion channel can be distilled into, "The hole opens. Ions go through. The hole closes." Studies of the fundamental mechanisms by which this process happens and the consequences of such activity in the setting of excitable cells remains the central focus of much of the field. One might wonder after so many years of detailed poking at such a seemingly simple process, is there anything left to learn?

Voltage-gated sodium channel (NaV) protein dissection creates a set of functional pore-only proteins

Many voltage-gated ion channel (VGIC) superfamily members contain six-transmembrane segments in which the first four form a voltage-sensing domain (VSD) and the last two form the pore domain (PD). Studies of potassium channels from the VGIC superfamily together with identification of voltage-sensor only proteins have suggested that the VSD and the PD can fold independently. Whether such transmembrane modularity is common to other VGIC superfamily members has remained untested. Here we show, using protein dissection, that the Silicibacter pomeroyi voltage-gated sodium channel (Na(V)Sp1) PD forms a stand-alone, ion selective pore (Na(V)Sp1p) that is tetrameric, α-helical, and that forms functional, sodium-selective channels when reconstituted into lipid bilayers. Mutation of the Na(V)Sp1p selectivity filter from LESWSM to LDDWSD, a change similar to that previously shown to alter ion selectivity of the bacterial sodium channel Na(V)Bh1 (NaChBac), creates a calcium-selective pore-only channel, Ca(V)Sp1p. We further show that production of PDs can be generalized by making pore-only proteins from two other extremophile Na(V)s: one from the hydrocarbon degrader Alcanivorax borkumensis (Na(V)Ab1p), and one from the arsenite oxidizer Alkalilimnicola ehrlichei (Na(V)Ae1p). Together, our data establish a family of active pore-only ion channels that should be excellent model systems for study of the factors that govern both sodium and calcium selectivity and permeability. Further, our findings suggest that similar dissection approaches may be applicable to a wide range of VGICs and, thus, serve as a means to simplify and accelerate biophysical, structural, and drug development efforts.

A synthetic S6 segment derived from KvAP channel self-assembles, permeabilizes lipid vesicles, and exhibits ion channel activity in bilayer lipid membrane

KvAP is a voltage-gated tetrameric K(+) channel with six transmembrane (S1-S6) segments in each monomer from the archaeon Aeropyrum pernix. The objective of the present investigation was to understand the plausible role of the S6 segment, which has been proposed to form the inner lining of the pore, in the membrane assembly and functional properties of KvAP channel. For this purpose, a 22-residue peptide, corresponding to the S6 transmembrane segment of KvAP (amino acids 218-239), and a scrambled peptide (S6-SCR) with rearrangement of only hydrophobic amino acids but without changing its composition were synthesized and characterized structurally and functionally. Although both peptides bound to the negatively charged phosphatidylcholine/phosphatidylglycerol model membrane with comparable affinity, significant differences were observed between these peptides in their localization, self-assembly, and aggregation properties onto this membrane. S6-SCR also exhibited reduced helical structures in SDS micelles and phosphatidylcholine/phosphatidylglycerol lipid vesicles as compared with the S6 peptide. Furthermore, the S6 peptide showed significant membrane-permeabilizing capability as evidenced by the release of calcein from the calcein-entrapped lipid vesicles, whereas S6-SCR showed much weaker efficacy. Interestingly, although the S6 peptide showed ion channel activity in the bilayer lipid membrane, despite having the same amino acid composition, S6-SCR was significantly inactive. The results demonstrated sequence-specific structural and functional properties of the S6 wild type peptide. The selected S6 segment is probably an important structural element that could play an important role in the membrane interaction, membrane assembly, and functional property of the KvAP channel.

Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family

Bacterial Trk and Ktr, fungal Trk and plant HKT form a family of membrane transporters permeable to K(+) and/or Na(+) and characterized by a common structure probably derived from an ancestral K(+) channel subunit. This transporter family, specific of non-animal cells, displays a large diversity in terms of ionic permeability, affinity and energetic coupling (H(+)-K(+) or Na(+)-K(+) symport, K(+) or Na(+) uniport), which might reflect a high need for adaptation in organisms living in fluctuating or dilute environments. Trk/Ktr/HKT transporters are involved in diverse functions, from K(+) or Na(+) uptake to membrane potential control, adaptation to osmotic or salt stress, or Na(+) recirculation from shoots to roots in plants. Structural analyses of bacterial Ktr point to multimeric structures physically interacting with regulatory subunits. Elucidation of Trk/Ktr/HKT protein structures along with characterization of mutated transporters could highlight functional and evolutionary relationships between ion channels and transporters displaying channel-like features.

Genetic linkage study of an autosomal recessive form of juvenile myoclonic epilepsy in a consanguineous Tunisian family

Juvenile myoclonic epilepsy (JME) is the most common idiopathic generalized epilepsies (IGEs), affecting 12-30% of all epilepsies in medical centers. To date genetic linkage studies have revealed putative loci on different chromosomes, but these findings are still inconclusive about which gene precisely is responsible for the disease. Here, we report the genetic and clinical analysis of a (JME) consanguineous Tunisian family with four affected children out of eight. A genome-wide search was carried out by using the Affymetrix GeneChip Mapping 500K NspI chip. Pairewise logarithm of the odds (LOD) scores were calculated with MERLIN (1.1) assuming an autosomal recessive model, and a complementary homozygous mapping analysis was performed with AutoSNPa software. The genome-wide parametric linkage analysis showed suggestive linkage to chromosome 2q. Interactive visual analysis of SNP data using AutoSNPa revealed two large regions of shared homozygosity by descent on 2q23.3 and on 2q24.1. We decided to sequence the exons of the two genes coding for such proteins located in 2q23.3, CACNB4 and 2q24.1, KCNJ3. No nucleotide variation--comprising the previously reported mutations--was detected.

Separation of heteromeric potassium channel Kcv towards probing subunit composition-regulated ion permeation and gating

The chlorella virus-encoded Kcv can form a homo-tetrameric potassium channel in lipid membranes. This miniature peptide can be synthesized in vitro, and the tetramer purified from the SDS-polyacrylamide gel retains the K(+) channel functionality. Combining this capability with the mass-tagging method, we propose a simple, straightforward approach that can generically manipulate individual subunits in the tetramer, thereby enabling the detection of contribution from individual subunits to the channel functions. Using this approach, we showed that the structural change in the selectivity filter from only one subunit is sufficient to cause permanent channel inactivation ("all-or-none" mechanism), whereas the mutation near the extracellular entrance additively modifies the ion permeation with the number of mutant subunits in the tetramer ("additive" mechanism).

The Olfactory Bulb: A Metabolic Sensor of Brain Insulin and Glucose Concentrations via a Voltage-Gated Potassium Channel

The voltage-gated potassium channel, Kv1.3, contributes a large proportion of the current in mitral cell neurons of the olfactory bulb where it assists to time the firing patterns of action potentials as spike clusters that are important for odorant detection. Gene-targeted deletion of the Kv1.3 channel, produces a "super-smeller" phenotype, whereby mice are additionally resistant to diet- and genetically-induced obesity. As assessed via an electrophysiological slice preparation of the olfactory bulb, Kv1.3 is modulated via energetically important molecules - such as insulin and glucose - contributing to the body's metabolic response to fat intake. We discuss a biophysical characterization of modulated synaptic communication in the slice following acute glucose and insulin stimulation, chronic elevation of insulin in mice that are in a conscious state, and induction of diet-induced obesity. We have discovered that Kv1.3 contributes an unusual nonconducting role - the detection of metabolic state.

Molecular mechanisms underlying membrane-potential-mediated regulation of neuronal K2P2.1 channels

The activity of background K(2P) channels adjusts the resting membrane potential to enable plasticity of excitable cells. Here we have studied the regulation of neuronal K(2P)2.1 (KCNK2, TREK-1) channel activity by resting membrane potential. When heterologously expressed, K(2P)2.1 currents gradually increased at hyperpolarizing potentials and declined at depolarizing potentials, with a midpoint potential of -60 mV. As K(2P) channels are not equipped with an integral voltage sensor, we sought extrinsic cellular components that could convert changes in the membrane electrical field to cellular activity that would indirectly modify K(2P)2.1 currents. We propose that membrane depolarization activated the Gq protein-coupled receptor pathway, in the apparent absence of ligand, resulting in phosphatidylinositol-4,5-bisphosphate (PIP(2)) depletion through the action of phospholipase C. Our results suggest a novel mechanism in which an indirect pathway confers membrane potential regulation onto channels that are not intrinsically voltage sensitive to enhance regulation of neuronal excitability levels.

Conformational dynamics of the inner pore helix of voltage-gated potassium channels

Voltage-gated potassium (Kv) channels control the electrical excitability of neurons and muscles. Despite this key role, how these channels open and close or gate is not fully understood. Gating is usually attributed to the bending and straightening of pore-lining helices at glycine and proline residues. In this work we focused on the role of proline in the Pro-Val-Pro (PVP) motif of the inner S6 helix in the Kv1.2 channel. We started by developing a simple hinged-rod model to fully explore the configurational space of bent helices and we related these configurations to the degree of pore opening. We then carried out fully atomistic simulations of the S6 helices and compared these simulations to the hinged-rod model. Both methods suggest that Kv1 channels are not tightly closed when the inner helices are straight, unlike what is seen in the non-PVP containing channels KcsA and KirBac. These results invite the possibility that the S6 helices may be kinked when Kv1 channels are closed. Our simulations indicate that the wild-type helix adopts multiple spatially distinct configurations, which is consistent with its role in adopting a closed state and an open state. The two most dominant configurational basins correspond to a 6 A movement of the helix tail accompanied by the PVP region undergoing a local alpha-helix to 3(10)-helix transition. We explored how single point mutations affect the propensity of the S6 helix to adopt particular configurations. Interestingly, mutating the first proline, P405 (P473 in Shaker), to alanine completely removed the bistable nature of the S6 helix possibly explaining why this mutation compromises the channel. Next, we considered four other mutations in the area known to affect channel gating and we saw similarly dramatic changes to the helix's dynamics and range of motion. Our results suggest a possible mechanism of helix pore closure and they suggest differences in the closed state of glycine-only channels, like KcsA, and PVP containing channels.

Functional properties of native and cloned P2X receptors

Electrophysiological experiments on dissociated smooth muscle and neurons have revealed three distinct phenotypes of P2X receptor: (1) a rapidly desensitizing, beta-methylene ATP-sensitive response typical of most smooth muscle; (2) a non-desensitizing, alpha,beta-methylene ATP-insensitive response characteristic of PC12 phaeochromocytoma cells and rat superior cervical ganglion neurons; and (3) a non-desensitizing, alpha, beta-methylene ATP-sensitive response observed in sensory neurons. All of these purinoceptors share a similar cationic and high Ca2+ permeability and sensitivity to blockade by suramin, Cibacron blue, oxidized ATP, pyridoxal-5-phosphate and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid. Heterologous expression of two forms of cloned P2X receptors (from rat vas deferens and PC12 cells) reveals that each cloned receptor can reconstitute native responses with remarkable fidelity. Such results suggest that homo-oligomeric channels may be formed from single subunits of the P2X receptor in smooth muscle, PC12 cells and some neurons. The third phenotype observed in native cells might result from co-assembly of subunits of the cloned receptors. However, co-expression studies show that these two forms of the P2X receptor do not heteropolymerize. Therefore, the non-desensitizing, alpha, beta-methylene ATP-sensitive response observed in sensory neurons may result from a distinct P2X receptor or from heteropolymerization of more than one distinct P2X purinoceptor.

P2X receptors: a third major class of ligand-gated ion channels

Three classes of ligand-gated ion channels are defined by their molecular architecture. The first embraces nicotinic, 5-HT3, glycine and GABA receptors. The second class contains the glutamate receptors-AMPA, kainate and NMDA types. The third class is the P2X receptors for ATP. Current knowledge of the structure of these channels is reviewed, and set beside what is known of their basic functional properties. The aim of this paper is to consider how our more complete understanding of the first two classes of channels might be helpful in forming a molecular picture of P2X receptor function.

Molecular cloning, characterization and expression analysis of natural resistance associated macrophage protein (Nramp) cDNA from turbot (Scophthalmus maximus)

Nramp (natural resistance associated macrophage protein) has been identified as one of the major candidate genes for controlling natural resistance and/or susceptibility to intracellular pathogens in vertebrates. However, few reports are available about the structure and function of Nramp in teleost. We have recently isolated the cDNA encoding Nramp from turbot (Scophthalmus maximus). The full-length cDNA of the Nramp is 2584 bp in length, including 69 bp 5' terminal UTR, 850 bp 3' terminal UTR and 1665 bp open reading frame for a protein with 554 amino acid residues (Genbank accession number: DQ263240). Comparison of amino acid sequence indicated that turbot Nramp consists of 12 transmembrane regions (TM) domains. A consensus transport motif (CTM) containing 20 residues was observed between transmembrane domains 8 and 9. The deduced amino acid sequence of turbot Nramp exhibited between 60 and 92% homology with 13 other vertebrate Nramp sequences. Nramp transcripts were found to be highly abundant in head kidney, kidney and spleen, abundant in intestine and gill, less abundant in liver, brain, heart and gonad, least in muscle and skin. The level of Nramp mRNA in embryos gradually increases during embryogenesis from blastula stage to fry stage. Challenge of turbot with pathogenic bacteria, Vibrio anguillarum, elevated Nramp mRNA levels in liver and spleen. The Nramp transcripts were detected in turbot embryonic cell line (TEC). Challenge of the TEC cell cultures with pathogenic bacteria, V. anguillarum, significantly elevated Nramp mRNA levels in TEC cell cultures.

Membrane topology structure of human high-affinity, sodium-dependent dicarboxylate transporter

High-affinity, sodium-dependent dicarboxylate transporter (NaDC3) is responsible for transport of Krebs cycle intermediates and may involve in regulation of aging and life span. Hydropathy analysis predicts that NaDC3 contains 11 or 12 hydrophobic transmembrane (TM) domains. However, the actual membrane topological structure of NaDC3 remains unknown. In this study, confocal immunofluorescence microscopy and membrane biotinylation of epitope-tagged N and C termini of NaDC3 provide evidence of an extracellular C terminus and an intracellular N terminus, indicating an odd number of transmembrane regions. The position of hydrophilic loops within NaDC3 was identified with antibodies against the loops domains combined with cysteine accessibility methods. A confocal image of membrane localization and transport activity assay of the cysteine insertion mutants show behavior similar to that of wild-type NaDC3 in transfected HEK293 cells, suggesting that these mutants retain a native protein configuration. We find that NaDC3 contains 11 transmembrane helices. The loops 1, 3, 5, 7, and 9 face the extracellular side, and loops 2, 4, 6, and 10 face the cytoplasmic side. A re-entrant loop-like structure between TM8 and TM9 may protrude into the membrane. Our results support the topography of 11 transmembrane domains with an extracellular C terminus and an intracellular N terminus of NaDC3, and for the first time provide experimental evidence for a novel topological model for NaDC3.

Increased plasma manganese, partially reduced ascorbate, 1 and absence of mitochondrial oxidative stress in type 2 diabetes mellitus: implications for the superoxide uncoupling protein 2 (UCP-2) pathway

Oxidative stress is an important component of diabetes and its complications. Manganese (Mn), the key component of the Mitochondrial antioxidant (MnSOD), plays a key role in the superoxide uncoupling protein 2 (UCP-2) pathway in inhibiting of glucose-stimulated insulin secretion (GSIS). The interactions of Mn with ascorbate and other components of this pathway have not been defined in type-2 diabetes. Fifty established type 2 diabetics (30 males, 20 females) and 30 non-diabetics (controls; 18 males, 12 females) matched for age and sex were investigated. Dietary intake, particularly of micronutrients as assessed by 24-h dietary recall was similar between diabetics and controls. Weight and height of all subjects were determined and body mass index (BMI) computed after clinical assessment. Fasting plasma glucose, manganese, ascorbic acid, creatinine and K+ levels were determined; K+ was to assess the K+ channels, whereas creatinine was to assess probability of oxidative stress nephropathy. Body mass index was greater in DM than in controls (p < 0.001). Fasting plasma glucose and Mn levels (p < 0.00 and p < 0.01, respectively) were higher in diabetes than in the controls. Manganese level was greater than twice the levels in controls. Ascorbic acid was not significantly different (p > 0.05), but was 50% lower than the level in non-diabetics. Potassium like Mn and glucose was significantly higher in diabetes mellitus (DM) than in controls (p < 0.001). Creatinine was not significantly different between diabetics and controls (p > 0.05). Correlations among all parameters were not significantly different. These findings suggest absence of significant oxidative stress in the mitochondria, probably excluding a role for UCP-2-superoxide pathway in the inhibition of glucose-stimulated insulin secretion (GSIS), calling for caution in the precocious conclusion that interruption of UCP-2 activity may provide a viable strategy to improve beta-cell dysfunction in type 2 diabetes mellitus.

Identification and characterization of a novel member of the ATP-sensitive K+ channel subunit family, Kir6.3, in zebrafish

ATP-sensitive K+ (KATP) channels play a crucial role in coupling cellular metabolism to membrane potential. In addition to the orthologs corresponding to Kir6.1 and Kir6.2 of mammals, we have identified a novel member, designated Kir6.3 (zKir6.3), of the inward rectifier K+ channel subfamily Kir6.x in zebrafish. zKir6.3 is a protein of 432 amino acids that shares 66% identity with mammalian Kir6.2 but differs considerably from mammalian Kir6.1 and Kir6.2 in the COOH terminus, which contain an Arg-Lys-Arg (RKR) motif, an endoplasmic reticulum (ER) retention signal. Single-channel recordings of reconstituted channels show that zKir6.3 requires the sulfonylurea receptor 1 (SUR1) subunit to produce KATP channel currents with single-channel conductance of 57.5 pS. Confocal microscopic analysis shows that zebrafish Kir6.3 requires the SUR1 subunit for its trafficking to the plasma membrane. Analyses of chimeric protein between human Kir6.2 and zKir6.3 and a COOH-terminal deletion of zKir6.3 indicate that interaction between the COOH terminus of zKir6.3 and SUR1 is critical for both channel activity and trafficking to the plasma membrane. We also identified zebrafish orthologs corresponding to mammalian SUR1 (zSUR1) and SUR2 (zSUR2) by the genomic database. Both Kir6.3 and SUR1 are expressed in embryonic brain of zebrafish, as assessed by whole mount in situ hybridization. These data indicate that Kir6.3 and SUR1 form functional KATP channels at the plasma membrane in zebrafish through a mechanism independent from ER retention by the RKR motif.

All four putative selectivity filter glycine residues in KtrB are essential for high affinity and selective K+ uptake by the KtrAB system from Vibrio alginolyticus

The subunit KtrB of bacterial Na+-dependent K+-translocating KtrAB systems belongs to a superfamily of K+ transporters. These proteins contain four repeated domains, each composed of two transmembrane helices connected by a putative pore loop (p-loop). The four p-loops harbor a conserved glycine residue at a position equivalent to a glycine selectivity filter residue in K+ channels. We investigated whether these glycines also form a selectivity filter in KtrB. The single residues Gly70, Gly185, Gly290, and Gly402 from p-loops P(A) to P(D) of Vibrio alginolyticus KtrB were replaced with alanine, serine, or aspartate. The three alanine variants KtrB(A70), KtrB(A185), and KtrB(A290) maintained a substantial activity in KtrAB-mediated K+ uptake in Escherichia coli. This activity was associated with a decrease in the affinity for K+ by 2 orders of magnitude, with little effect on Vmax. Minor activities were also observed for three other variants: KtrB(A402), KtrB(S70), and KtrB(D185). With all of these variants, the property of Na+ dependence of K+ transport was preserved. Only the four serine variants mediated Na+ uptake, and these variants differed considerably in their K+/Na+ selectivity. Experiments on cloned ktrB in the pBAD18 vector showed that V. alginolyticus KtrB alone was still active in E. coli. It mediated Na+-independent, slow, high affinity, and mutation-specific K+ uptake as well as K+-independent Na+ uptake. These data demonstrate that KtrB contains a selectivity filter for K+ ions and that all four conserved p-loop glycine residues are part of this filter. They also indicate that the role of KtrA lies in conferring velocity and ion coupling to the Ktr complex.

KCNQ-like Potassium Channels in Caenorhabditis elegans

The human KCNQ gene family encodes potassium channels linked to several genetic syndromes including neonatal epilepsy, cardiac arrhythmia, and progressive deafness. KCNQ channels form M-type potassium channels, which are critical regulators of neuronal excitability that mediate autonomic responses, pain, and higher brain function. Fundamental mechanisms of the normal and abnormal cellular roles for these channels may be gained from their study in simple model organisms. Here we report that a multigene family of KCNQ-like channels is present in the nematode, Caenorhabditis elegans. We show that many aspects of the functional properties, tissue expression pattern, and modulation of these C. elegans channels are conserved, including suppression by the M1 muscarinic receptor. We also describe a conserved mechanism of modulation by diacylglycerol for a subset of C. elegans and vertebrate KCNQ/KQT channels, which is dependent upon the carboxyl-terminal domains of channel subunits and activated protein kinase C.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

Coupled motions between pore and voltage-sensor domains: a model for Shaker B, a voltage-gated potassium channel

A high-resolution crystal structure of KvAP, an archeabacterial voltage-gated potassium (Kv) channel, complexed with a monoclonal Fab fragment has been recently determined. Based on this structure, a mechanism for the activation (opening) of Kv channels has been put forward. This mechanism has since been criticized, suggesting that the resolved structure is not representative of the family of voltage-gated potassium channels. Here, we propose a model of the transmembrane domain of Shaker B, a well-characterized Kv channel, built by homology modeling and docking calculations. In this model, the positively charged S4 helices are oriented perpendicular to the membrane and localized in the groove between segments S5 and S6 of adjacent subunits. The structure and the dynamics of the full atomistic model embedded in a hydrated lipid bilayer were investigated by means of two large-scale molecular dynamics simulations under transmembrane-voltage conditions known to induce, respectively, the resting state (closed) and the activation (opening) of voltage-gated channels. Upon activation, the model undergoes conformational changes that lead to an increase of the hydration of the charged S4 helices, correlated with an upward translation and a tilting of the latter, concurrently with movements of the S5 helices and the activation gate. Although small, these conformational changes ultimately result in an alteration of the ion-conduction pathway. Our findings support the transporter model devised by Bezanilla and collaborators, and further underline the crucial role played by internal hydration in the activation of the channel.

Role of G protein-gated inwardly rectifying potassium channels in P2Y12 receptor-mediated platelet functional responses

The role of the G(i)-coupled platelet P2Y(12) receptor in platelet function has been well established. However, the functional effector or effectors contributing directly to alphaIIbbeta3 activation in human platelets has not been delineated. As the P2Y(12) receptor has been shown to activate G protein-gated, inwardly rectifying potassium (GIRK) channels, we investigated whether GIRK channels mediate any of the functional responses of the platelet P2Y(12) receptor. Western blot analysis revealed that platelets express GIRK1, GIRK2, and GIRK4. In aspirin-treated and washed human platelets, 2 structurally distinct GIRK inhibitors, SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride) and U50488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(pyrrolidinyl)cyclohexyl] benzeneacetamide methanesulfonate), inhibited adenosine diphosphate (ADP)-, 2-methylthioADP (2-MeSADP)-, U46619-, and low-dose thrombin-mediated platelet aggregation. However, the GIRK channel inhibitors did not affect platelet aggregation induced by high concentrations of thrombin, AYPGKF, or convulxin. Furthermore, the GIRK channel inhibitors reversed SFLLRN-induced platelet aggregation, inhibited the P2Y(12)-mediated potentiation of dense granule secretion and Akt phosphorylation, and did not affect the agonist-induced G(q)-mediated platelet shape change and intracellular calcium mobilization. Unlike AR-C 69931MX, a P2Y(12) receptor-selective antagonist, the GIRK channel blockers did not affect the ADP-induced adenlylyl cyclase inhibition, indicating that they do not directly antagonize the P2Y(12) receptor. We conclude that GIRK channels are important functional effectors of the P2Y(12) receptor in human platelets.

Activation properties of Kv4.3 channels: time, voltage and [K+]o dependence

Rapidly inactivating, voltage-dependent K(+) currents play important roles in both neurones and cardiac myocytes. Kv4 channels form the basis of these currents in many neurones and cardiac myocytes and their mechanism of inactivation appears to differ significantly from that reported for Shaker and Kv1.4 channels. In most channel gating models, inactivation is coupled to the kinetics of activation. Hence, there is a need for a rigorous model based on comprehensive experimental data on Kv4.3 channel activation. To develop a gating model of Kv4.3 channel activation, we studied the properties of Kv4.3 channels in Xenopus oocytes, without endogenous KChIP2 ancillary subunits, using the perforated cut-open oocyte voltage clamp and two-electrode voltage clamp techniques. We obtained high-frequency resolution measurements of the activation and deactivation properties of Kv4.3 channels. Activation was sigmoid and well described by a fourth power exponential function. The voltage dependence of the activation time constants was best described by a biexponential function corresponding to at least two different equivalent charges for activation. Deactivation kinetics are voltage dependent and monoexponential. In contrast to other voltage-sensitive K(+) channels such as HERG and Shaker, we found that elevated extracellular [K(+)] modulated the activation process by slowing deactivation and stabilizing the open state. Using these data we developed a model with five closed states and voltage-dependent transitions between the first four closed states coupled to a voltage-insensitive transition between the final closed (partially activated) state and the open state. Our model closely simulates steady-state and kinetic activation and deactivation data.

Immunohistochemical localization of the voltage-gated potassium channel subunit Kv1.4 in the central nervous system of the adult rat

A large set of voltage-gated potassium channels is involved in regulating essential aspects of neuronal function in the central nervous system, thus contributing to the ability of neurons to respond to a given input. In the present study, we used immunocytochemical methods to elucidate the regional, cellular and subcellular distribution of the voltage-gated potassium channel subunit Kv1.4, a member of the Shaker subfamily, in the brain. At the light microscopic level, the Kv1.4 subunit showed a unique distribution pattern, being localized in specific neuronal populations of the rat brain. The neuronal regions expressing the highest levels of Kv1.4 protein included the cerebral cortex, the hippocampus, the posterolateral and posteromedial ventral thalamic nuclei, the dorsolateral and medial geniculate nuclei, the substantia nigra and the dorsal cochlear nucleus. The Kv1.4 subunit was also present in other neuronal populations, with different levels of Kv1.4 immunoreactivity. In all immunolabeled regions, the Kv1.4 subunit was mostly diffusely distributed and, to a lesser extent, it stained cell bodies and proximal dendrites. Furthermore, Kv1.4 immunoreactivity was also detected in nerve terminals and axonal terminal fields. At the electron microscopic level, Kv1.4 was located postsynaptically in dendritic spines and shafts at extrasynaptic sites, as well as presynaptically in axon and active zone of axon terminals, in the neocortex and hippocampus. The findings indicate that Kv1.4 channels are widely distributed in the rat brain and suggest that activation of this channel would have different modulatory effects on neuronal excitability.

Functional significance of cannabinoid-mediated, depolarization-induced suppression of inhibition (DSI) in the hippocampus

A number of recent studies have demonstrated that a well-known form of short-term plasticity at hippocampal GABAergic synapses, called depolarization-induced suppression of inhibition (DSI), is in fact mediated by the retrograde actions of endocannabinoids released in response to depolarization of the postsynaptic cells. These studies suggest that endogenous cannabinoids may play an important role in regulating inhibitory tone in the mammalian CNS. Despite the widespread interest and potential physiological importance of DSI, many questions regarding the physiological relevance of DSI remain. To that end, this study set out to define the specific limiting conditions that could elicit DSI at GABAergic synapses in CA1 hippocampal pyramidal neurons and to determine if DSI could be elicited with pulse trains that mimic hippocampal cell-firing patterns that occur in vivo. Whole cell recordings from hippocampal neurons under voltage-clamp configuration were made in rat hippocampal slices. Spontaneous and evoked gamma-aminobutyric acid-A (GABAA) receptor-mediated inhibitory postsynaptic currents (sIPSCs and eIPSCs, respectively) were recorded prior to and following depolarization of CA1 hippocampal pyramidal cells. Depolarizing voltage pulses were shaped to evoke currents in QX-314-treated cells similar to those accompanying single spontaneous voltage-clamped action potentials recorded from the soma. Attempts were made to elicit DSI with trains of these pulses that mimicked hippocampal cell firing patterns in vivo, for instance, when animals traverse place fields or are performing a short-term memory task. DSI could not be elicited by such pulse trains or by a number of other combinations of behaviorally specific firing parameters. The minimum duration of depolarization necessary to elicit DSI in hippocampal neurons determined by paired-pulse manipulation was 50 -75 ms at a critical interval of 20 -30 ms between pulse pairs. Under the conditions tested, the normal firing patterns of hippocampal neurons that occur in vivo do not appear to elicit DSI.

Physiological and pathophysiological roles of ATP-sensitive K+ channels

ATP-sensitive potassium (K(ATP)) channels are present in many tissues, including pancreatic islet cells, heart, skeletal muscle, vascular smooth muscle, and brain, in which they couple the cell metabolic state to its membrane potential, playing a crucial role in various cellular functions. The K(ATP) channel is a hetero-octamer comprising two subunits: the pore-forming subunit Kir6.x (Kir6.1 or Kir6.2) and the regulatory subunit sulfonylurea receptor SUR (SUR1 or SUR2). Kir6.x belongs to the inward rectifier K(+) channel family; SUR belongs to the ATP-binding cassette protein superfamily. Heterologous expression of differing combinations of Kir6.1 or Kir6.2 and SUR1 or SUR2 variant (SUR2A or SUR2B) reconstitute different types of K(ATP) channels with distinct electrophysiological properties and nucleotide and pharmacological sensitivities corresponding to the various K(ATP) channels in native tissues. The physiological and pathophysiological roles of K(ATP) channels have been studied primarily using K(ATP) channel blockers and K(+) channel openers, but there is no direct evidence on the role of the K(ATP) channels in many important cellular responses. In addition to the analyses of naturally occurring mutations of the genes in humans, determination of the phenotypes of mice generated by genetic manipulation has been successful in clarifying the function of various gene products. Recently, various genetically engineered mice, including mice lacking K(ATP) channels (knockout mice) and mice expressing various mutant K(ATP) channels (transgenic mice), have been generated. In this review, we focus on the physiological and pathophysiological roles of K(ATP) channels learned from genetic manipulation of mice and naturally occurring mutations in humans.

Characterization of amino acid substitutions in KdpA, the K+-binding and -translocating subunit of the KdpFABC complex of Escherichia coli

When grown under K+ limitation, Escherichia coli induces the K+-translocating KdpFABC complex. The stimulation of ATPase activity by NH4+ ions was shown for the first time. Substitutions in KdpA, which is responsible for K+ binding and translocation, revealed that enzyme complexes KdpA:G232A and KdpA:G232S have completely lost their cation selectivity.

Neurotrophin modulation of voltage-gated potassium channels in rat through TrkB receptors is time and sensory experience dependent

The whole-cell configuration of the patch-clamp technique, immunoprecipitation experiments and unilateral naris occlusions were used to investigate whether the voltage-gated potassium channel Kv1.3 was a substrate for neurotrophin-induced tyrosine phosphorylation and subsequent functional modulation of current properties in cultured rat olfactory bulb (OB) neurons. Membrane proteins of the OB included all three Trk receptor kinases, but the truncated form of the receptor, lacking an intact kinase domain, was the predominant form of the protein for TrkA and TrkC, while TrkB was predominantly found as the full-length receptor. Acute (15 min) stimulation of OB neurons with bath application of 50 ng ml(-1) brain-derived neurotrophic factor (BDNF), which is a selective ligand for TrkB, caused suppression of the whole-cell outward current and no changes in the kinetics of inactivation or deactivation. Acute stimulation with either nerve growth factor or neurotrophin-3 failed to evoke any changes in Kv1.3 function in the OB neurons. Chronic exposure to BDNF (days) caused an increase in the magnitude of Kv1.3 current and speeding of the inactivation and deactivation of the channel. Acute BDNF-induced activation of TrkB receptors significantly increased tyrosine phosphorylation of Kv1.3 in the OB, as shown using a combined immunoprecipitation and Western blot analysis. With unilateral naris occlusion, the acute BDNF-induced tyrosine phosphorylation of Kv1.3 was increased in neurons lacking odour sensory experience. In summary, the duration of neurotrophin exposure and the sensory-dependent state of a neuron can influence the degree of phosphorylation of a voltage-gated ion channel and its concomitant functional modulation by neurotrophins.

Inwardly rectifying K+ (Kir) channels in Drosophila. A crucial role of cellular milieu factors Kir channel function

Three cDNAs encoding inwardly rectifying potassium (Kir) channels were isolated from Drosophila melanogaster. The protein sequences of Drosophila KirI (dKirI) and dKirII are moderately (<44%) and dKirIII sequence is weakly (<27%) identical to human Kir channel subunits. During fly development, five dKir channel transcripts derived from three genes are differentially expressed. Whole mount in situ hybridizations revealed dKirI transcripts absent from embryos, but dKirII and dKirIII are expressed in the embryonic hind gut and in Malpighian tubules, respectively, thus covering the entire osmoregulatory system of the developing fly. In the head of adult flies, predominantly dKirII transcripts were detected. When expressed in Xenopus oocytes, dKir channel activity was only observed after amino acid substitutions in their cytosolic tails (e.g. exchange of a unique valine in the NH(2) terminus). In contrast, heterologous expression of wild type dKirI and dKirII in Drosophila S2 cells readily evoked typical inwardly rectifying K(+) currents, which were weakly sensitive to Ba(2+). Thus, the specific milieu of insect cells provides a crucial cellular environment for proper function of dKir channels.

Functional expression of TREK-2 K+ channel in cultured rat brain astrocytes

Background K+ channels whose subunit contains four transmembrane segments and two pore-forming domains (4TM/2P) have been cloned recently. We studied whether 4TM/2P K+ channels are functionally expressed in astrocytes that are known to have a large background (resting) K+ conductance and a large resting membrane potential. Reverse transcriptase-PCR analysis showed that, among five 4TM/2P K+ channels examined, TASK-1, TASK-3 and TREK-2 mRNAs were expressed in cultured astrocytes from rat cortex. In cell-attached patches, we mainly observed three K+ channels with single-channel conductances of 30, 117 and 176 pS (-40 mV) in symmetrical 140 mM KCl. The 30 pS channel was the inward rectifying K+ channel that has been previously described in astrocytes. The 117 pS K+ channel also showed inward rectification and was insensitive to 1 mM tetraethylammonium and 1 mM 4-aminopyridine. The 176 pS channel was the Ca2+-activated K+ channel. The 117 pS K+ channel was determined to be TREK-2, as judged by its electrophysiological properties and activation by membrane stretch, free fatty acids and intracellular acidosis. In approximately 50% of astrocytes in culture, whole-cell K+ current increased markedly following application of arachidonic acid. The number of TREK-2 channels in these cells was estimated to be approximately 500-1000/cell. Our results show that TREK-2 is functionally expressed in cortical astrocytes in culture, and suggest that TREK-2 may be involved in K+ homeostasis of astrocytes during pathological states.

Evolution of voltage-gated Na+ channels

Voltage-gated Na+ channels play important functional roles in the generation of electrical excitability in most vertebrate and invertebrate species. These channels are members of a superfamily that includes voltage-gated K+, voltage-gated Ca2+ and cyclic-nucleotide-gated channels. There are nine genes encoding voltage-gated Na+ channels in mammals, with a tenth homologous gene that has not been shown to encode a functional channel. Other vertebrate and invertebrate species have a smaller number of Na+ channel genes. The mammalian genes can be classified into five branches in a phylogenetic tree, and they are localized on four chromosomes. Four of the branches representing the four chromosomal locations probably resulted from the chromosomal duplications that led to the four Hox gene clusters. These duplications occurred close to the emergence of the first vertebrates. The fifth branch probably evolved from a separate ancestral Na+ channel gene. There are two branches in the invertebrate tree, although members of only one of those branches have been demonstrated to encode functional voltage-gated Na+ channels. It is possible that the other branch may have diverged, so that its members do not represent true voltage-gated Na+ channels. Vertebrate and invertebrate Na+ channels appear to be derived from a single primordial channel that subsequently evolved independently in the two lineages.

Pharmacology and biochemistry of spider venoms

Spider venoms represent an incredible source of biologically active substances which selectively target a variety of vital physiological functions in both insects and mammals. Many toxins isolated from spider venoms have been invaluable in helping to determine the role and diversity of neuronal ion channels and the process of exocytosis. In addition, there is enormous potential for the use of insect specific toxins from animal sources in agriculture. For these reasons, the past 15-20 years has seen a dramatic increase in studies on the venoms of many animals, particularly scorpions and spiders. This review covers the pharmacological and biochemical activities of spider venoms and the nature of the active components. In particular, it focuses on the wide variety of ion channel toxins, novel non-neurotoxic peptide toxins, enzymes and low molecular weight compounds that have been isolated. It also discusses the intraspecific sex differences in given species of spiders.

Vacuolar membrane localization of the Arabidopsis 'two-pore' K+ channel KCO1

Potassium (K+) channels play multiple roles in higher plants, and have been characterized electrophysiologically in various subcellular membranes. The K+ channel AtKCO1 from Arabidopsis thaliana is the prototype of a new family of plant K+ channels. In a previous study the protein has been functionally characterized after heterologous expression in Baculovirus-infected insect cells. In order to obtain further information on the physiological function of AtKCO1, the gene expression pattern and subcellular localization of the protein in plants were investigated. The regulatory function of the 5' region of the AtKCO1 gene was examined in transgenic A. thaliana plants carrying beta-glucuronidase (GUS) fusion constructs. Our analysis demonstrates that the AtKCO1 promoter is active in various tissues and cell types, and the highest GUS activity could be detected in mitotically active tissues of the plant. Promoter activity was strongly dependent on the presence of a 5' leader intron. The same overall structure was identified in two genes encoding AtKCO1-like K+ channels from Solanum tuberosum (StKCO1alpha and StKCO1beta). To investigate the subcellular localization of AtKCO1, the channel protein, as well as a fusion protein of AtKCO1 with green fluorescence protein (GFP), were expressed in transgenic tobacco BY2 cells. In sucrose density gradients, both proteins co-fractionate with tonoplast markers (Nt-TIPa, vATPase). In fluorescence images from transgenic AtKCO1-GFP BY2 cells fluorescence was exclusively detected in the tonoplast. Thus AtKCO1 is the first cloned K+ channel demonstrated to be a vacuolar K+ channel.

Bacterial ion channels and their eukaryotic homologues

Due to the relative ease of obtaining their crystal structures, bacterial ion channels provide a unique opportunity to analyse structure and function of their eukaryotic homologues. This review describes prokaryotic channels whose structures have been determined. These channels are KcsA, a bacterial homologue of eukaryotic potassium channels, MscL, a bacterial mechanosensitive ion channel and ClC0, a prokaryotic homologue of the eukaryotic ClC family of anion-selective channels. General features of their structure and function are described with a special emphasis on the advantages that these channels offer for understanding the properties of their eukaryotic homologues. We present amino-acid sequences of eukaryotic proteins related in their primary sequences to bacterial mechanosensitive channels. The usefulness of bacterial mechanosensitive channels for the studies on general principles of mechanosensation is discussed.

Escherichia coli as an expression system for K(+) transport systems from plants

The value of the Escherichia coli expression system has long been established because of its effectiveness in characterizing the structure and function of exogenously expressed proteins. When eukaryotic membrane proteins are functionally expressed in E. coli, this organism can serve as an alternative to eukaryotic host cells. A few examples have been reported of functional expression of animal and plant membrane proteins in E. coli. This mini-review describes the following findings: 1) homologous K(+) transporters exist in prokaryotic cells and in eukaryotic cells; 2) plant K(+) transporters can functionally complement mutant K(+) transporter genes in E. coli; and 3) membrane structures of plant K(+) transporters can be elucidated in an E. coli system. These experimental findings suggest the possibility of utilizing the E. coli bacterium as an expression system for other eukaryotic membrane transport proteins.