Alternative Shaker transcripts express either rapidly inactivating or noninactivating K+ channels

Diversification of Potassium Currents in Excitable Cells via Kvβ Proteins

Excitable cells of the nervous and cardiovascular systems depend on an assortment of plasmalemmal potassium channels to control diverse cellular functions. Voltage-gated potassium (Kv) channels are central to the feedback control of membrane excitability in these processes due to their activation by depolarized membrane potentials permitting K⁺ efflux. Accordingly, Kv currents are differentially controlled not only by numerous cellular signaling paradigms that influence channel abundance and shape voltage sensitivity, but also by heteromeric configurations of channel complexes. In this context, we discuss the current knowledge related to how intracellular Kvβ proteins interacting with pore complexes of Shaker-related Kv1 channels may establish a modifiable link between excitability and metabolic state. Past studies in heterologous systems have indicated roles for Kvβ proteins in regulating channel stability, trafficking, subcellular targeting, and gating. More recent works identifying potential in vivo physiologic roles are considered in light of these earlier studies and key gaps in knowledge to be addressed by future research are described.

Cold acclimation increases depolarization resistance and tolerance in muscle fibers from a chill-susceptible insect, Locusta migratoria

Cold exposure depolarizes cells in insects due to a reduced electrogenic ion transport and a gradual increase in extracellular K⁺ concentration ([K⁺]). Cold-induced depolarization is linked to cold injury in chill-susceptible insects, and the locust, Locusta migratoria, has been shown to improve cold tolerance following cold acclimation through depolarization resistance. Here we investigate how cold acclimation influences depolarization resistance and how this resistance relates to improved cold tolerance. To address this question, we investigated if cold acclimation affects the electrogenic transport capacity and/or the relative K⁺ permeability during cold exposure by measuring membrane potentials of warm- and cold-acclimated locusts in the presence and absence of ouabain (Na⁺-K⁺ pump blocker) or 4-aminopyridine (4-AP; voltage-gated K⁺ channel blocker). In addition, we compared the membrane lipid composition of muscle tissue from warm- and cold-acclimated locust and the abundance of a range transcripts related to ion transport and cell injury accumulation. We found that cold-acclimated locusts are depolarization resistant due to an elevated K⁺ permeability, facilitated by opening of 4-AP-sensitive K⁺ channels. In accordance, cold acclimation was associated with an increased abundance of Shaker transcripts (gene encoding 4-AP-sensitive voltage-gated K⁺ channels). Furthermore, we found that cold acclimation improved muscle cell viability following exposure to cold and hyperkalemia even when muscles were depolarized substantially. Thus cold acclimation confers resistance to depolarization by altering the relative ion permeability, but cold-acclimated locusts are also more tolerant to depolarization.

Lethal effects of an insecticidal spider venom peptide involve positive allosteric modulation of insect nicotinic acetylcholine receptors

κ-Hexatoxins (κ-HXTXs) are a family of excitotoxic insect-selective neurotoxins from Australian funnel-web spiders that are lethal to a wide range of insects, but display no toxicity towards vertebrates. The prototypic κ-HXTX-Hv1c selectively blocks native and expressed cockroach large-conductance calcium-activated potassium (BK_Ca or K_Ca1.1) channels, but not their mammalian orthologs. Despite this potent and selective action on insect K_Ca1.1 channels, we found that the classical K_Ca1.1 blockers paxilline, charybdotoxin and iberiotoxin, which all block insect K_Ca1.1 channels, are not lethal in crickets. We therefore used whole-cell patch-clamp analysis of cockroach dorsal unpaired median (DUM) neurons to study the effects of κ-HXTX-Hv1c on sodium-activated (K_Na), delayed-rectifier (K_DR) and 'A-type' transient (K_A) K⁺ channels. 1 μM κ-HXTX-Hv1c failed to significantly inhibit cockroach K_Na and K_DR channels, but did cause a 30 ± 7% saturating inhibition of K_A channel currents, possibly via a Kv4 (Shal-like) action. However, this modest action at such a high concentration of κ-HXTX-Hv1c would indicate a different lethal target. Accordingly, we assessed the actions of κ-HXTX-Hv1c on neurotransmitter-gated ion channels in cockroach DUM neurons. We found that κ-HXTX-Hv1c failed to produce any major effects on GABA_A or glutamate-Cl receptors but dramatically slowed nicotine-evoked ACh receptor (nAChR) current decay and reversed nAChR desensitization. These actions occurred without any alterations to nAChR current amplitude or the nicotine concentration-response curve, and are consistent with a positive allosteric modulation of nAChRs. κ-HXTX-Hv1c therefore represents the first venom peptide that selectively modulates insect nAChRs with a mode of action similar to the excitotoxic insecticide spinosyn A. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Neuronal ClC-3 Splice Variants Differ in Subcellular Localizations, but Mediate Identical Transport Functions

ClC-3 is a member of the CLC family of anion channels and transporters, for which multiple functional properties and subcellular localizations have been reported. Since alternative splicing often results in proteins with diverse properties, we investigated to what extent alternative splicing might influence subcellular targeting and function of ClC-3. We identified three alternatively spliced ClC-3 isoforms, ClC-3a, ClC-3b, and ClC-3c, in mouse brain, with ClC-3c being the predominant splice variant. Whereas ClC-3a and ClC-3b are present in late endosomes/lysosomes, ClC-3c is targeted to recycling endosomes via a novel N-terminal isoleucine-proline (IP) motif. Surface membrane insertion of a fraction of ClC-3c transporters permitted electrophysiological characterization of this splice variant through whole-cell patch clamping on transfected mammalian cells. In contrast, neutralization of the N-terminal dileucine-like motifs was required for functional analysis of ClC-3a and ClC-3b. Heterologous expression of ClC-3a or ClC-3b carrying mutations in N-terminal dileucine motifs as well as WTClC-3c in HEK293T cells resulted in outwardly rectifying Cl(-) currents with significant capacitive current components. We conclude that alternative splicing of Clcn3 results in proteins with different subcellular localizations, but leaves the transport function of the proteins unaffected.

A novel Na+ channel splice form contributes to the regulation of an androgen-dependent social signal

Na(+) channels are often spliced but little is known about the functional consequences of splicing. We have been studying the regulation of Na(+) current inactivation in an electric fish model in which systematic variation in the rate of inactivation of the electric organ Na(+) current shapes the electric organ discharge (EOD), a sexually dimorphic, androgen-sensitive communication signal. Here, we examine the relationship between an Na(+) channel (Na(v)1.4b), which has two splice forms, and the waveform of the EOD. One splice form (Na(v)1.4bL) possesses a novel first exon that encodes a 51 aa N-terminal extension. This is the first report of an Na(+) channel with alternative splicing in the N terminal. This N terminal is present in zebrafish suggesting its general importance in regulating Na(+) currents in teleosts. The extended N terminal significantly speeds fast inactivation, shifts steady-state inactivation, and dramatically enhances recovery from inactivation, essentially fulfilling the functions of a beta subunit. Both splice forms are equally expressed in muscle in electric fish and zebrafish but Na(v)1.4bL is the dominant form in the electric organ implying electric organ-specific transcriptional regulation. Transcript abundance of Na(v)1.4bL in the electric organ is positively correlated with EOD frequency and lowered by androgens. Thus, shaping of the EOD waveform involves the androgenic regulation of a rapidly inactivating splice form of an Na(+) channel. Our results emphasize the role of splicing in the regulation of a vertebrate Na(+) channel and its contribution to a known behavior.

Bridging behavior and physiology: ion-channel perspective on mushroom body-dependent olfactory learning and memory in Drosophila

An important body of evidence documents the differential expression of ion channels in brains, suggesting they are essential to endow particular brain structures with specific physiological properties. Because of their role in correlating inputs and outputs in neurons, modulation of voltage-dependent ion channels (VDICs) can profoundly change neuronal network dynamics and performance, and may represent a fundamental mechanism for behavioral plasticity, one that has received less attention in learning and memory studies. Revisiting three paradigmatic mutations altering olfactory learning and memory in Drosophila (dunce, leonardo, amnesiac) a link was established between each mutation and the operation of VDICs in Kenyon cells, the intrinsic neurons of the mushroom bodies (MBs). In Drosophila, MBs are essential to the emergence of olfactory associative learning and retention. Abnormal ion channel operation might underlie failures in neuronal physiology, and be crucial to understand the abnormal associative learning and retention phenotypes the mutants display. We also discuss the only case in which a mutation in an ion channel gene (shaker) has been directly linked to olfactory learning deficits. We analyze such evidence in light of recent discoveries indicating an unusual ion current profile in shaker mutant MB intrinsic neurons. We anticipate that further studies of acquisition and retention mutants will further confirm a link between such mutations and malfunction of specific ion channel mechanisms in brain structures implicated in learning and memory.

Shal and shaker differential contribution to the K+ currents in the Drosophila mushroom body neurons

Shaker, a voltage-dependent K+ channel, is enriched in the mushroom bodies (MBs), the locus of olfactory learning in Drosophila. Mutations in the shaker locus are known to alter excitability, neurotransmitter release, synaptic plasticity, and olfactory learning. However, a direct link of Shaker channels to MB intrinsic neuron (MBN) physiology has not been documented. We found that transcripts for shab, shaw, shaker, and shal, among which only Shaker and Shal have been reported to code for A-type currents, are present in the MBs. The electrophysiological data showed that the absence of functional Shaker channels modifies the distribution of half-inactivation voltages (V(i1/2)) in the MBNs, indicating a segregation of Shaker channels to only a subset (approximately 28%) of their somata. In harmony with this notion, we found that approximately one-fifth of MBNs lacking functional Shaker channels displayed dramatically slowed-down outward current inactivation times and reduced peak-current amplitudes. Furthermore, whereas all MBNs were sensitive to 4-aminopyridine, a nonspecific A-type current blocker, a subset of neurons (approximately 24%) displayed little sensitivity to a Shal-specific toxin. This subset of neurons displaying toxin-insensitive outward currents had more depolarized V(i1/2) values attributable to Shaker channels. Our findings provide the first direct evidence that altered Shaker channel function disrupts MBN physiology in Drosophila. To our surprise, the experimental data also indicate that Shaker channels segregate to a minor fraction of MB neuronal somata (20-30%), and that Shal channels contribute the somatic A-type current in the majority of MBNs.

A tale of two tails: cytosolic termini and K(+) channel function

The enormous variety of neuronal action potential waveforms can be ascribed, in large part, to the sculpting of their falling phases by currents through voltage-gated potassium channels. These proteins play several additional roles in other tissues such as the regulation of heartbeat and of insulin release from pancreatic cells as well as auditory signal processing in the cochlea. The functional channel is a tetramer with either six or two transmembrane segments per monomer. Selectivity filters, voltage sensors and gating elements have been mapped to residues within the transmembrane region. Cytoplasmic residues, which are accessible targets for signal transduction cascades and provide attractive means of regulation of channel activity, are now seen to be capable of modulating various aspects of channel function. Here we review structural studies on segments of the cytoplasmic tails of K(+) channels, as well as the range of modulatory activities of these tails.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

52-Week oral gavage chronic toxicity study with ubiquinone in rats with a 4-week recovery

Potential ubiquinone (CoQ10; a natural fermentation product) toxicity was assessed in rats administered CoQ(10) by oral gavage for 1 year at 100, 300, 600, and 1200 mg/(kg day). No adverse changes in mortality, clinical signs, body weight, food consumption, or clinical pathology results occurred. CoQ(10) had elimination half-lives ranging from 10.7 to 15.2 h. At 1200 mg/(kg day), a high incidence of orange, granular, lumenal exudate in nasal turbinates occurred; microscopically, findings similar to those in the turbinates were occasionally observed in small granulomas within lung alveoli. A dose-related increased incidence of vacuolated macrophages (mesenteric lymph nodes) and vacuolated hepatic periportal cells was noted. Neither were associated with tissue damage or organ dysfunction, so they were not considered to be adverse. The nasal turbinate and lung findings were probably secondary to incidental exposure to crystallized test material. Overall, CoQ(10) was well tolerated by male and female rats at dose levels up to 1200 mg/(kg day).

Genetic dissection of functional contributions of specific potassium channel subunits in habituation of an escape circuit in Drosophila

Potassium channels have been implicated in central roles in activity-dependent neural plasticity. The giant fiber escape pathway of Drosophila has been established as a model for analyzing habituation and its modification by memory mutations in an identified circuit. Several genes in Drosophila encoding K+ channel subunits have been characterized, permitting examination of the contributions of specific channel subunits to simple conditioning in an identified circuit that is amenable to genetic analysis. Our results show that mutations altering each of four K+ channel subunits (Sh, slo, eag, and Hk) have distinct effects on habituation at least as strong as those of dunce and rutabaga, memory mutants with defective cAMP metabolism (). Habituation, spontaneous recovery, and dishabituation of the electrically stimulated long-latency giant fiber pathway response were shown in each mutant type. Mutations of Sh (voltage-gated) and slo (Ca2+-gated) subunits enhanced and slowed habituation, respectively. However, mutations of eag and Hk subunits, which confer K+-current modulation, had even more extreme phenotypes, again enhancing and slowing habituation, respectively. In double mutants, Sh mutations moderated the strong phenotypes of eag and Hk, suggesting that their modulatory functions are best expressed in the presence of intact Sh subunits. Nonactivity-dependent responses (refractory period and latency) at two stages of the circuit were altered only in some mutants and do not account for modifications of habituation. Furthermore, failures of the long-latency response during habituation, which normally occur in labile connections in the brain, could be induced in the thoracic circuit stage in Hk mutants. Our work indicates that different K+ channel subunits play distinct roles in activity-dependent neural plasticity and thus can be incorporated along with second messenger "memory" loci to enrich the genetic analysis of learning and memory.

Genetic dissection of functional contributions of specific potassium channel subunits in habituation of an escape circuit in Drosophila

Potassium channels have been implicated in central roles in activity-dependent neural plasticity. The giant fiber escape pathway of Drosophila has been established as a model for analyzing habituation and its modification by memory mutations in an identified circuit. Several genes in Drosophila encoding K+ channel subunits have been characterized, permitting examination of the contributions of specific channel subunits to simple conditioning in an identified circuit that is amenable to genetic analysis. Our results show that mutations altering each of four K+ channel subunits (Sh, slo, eag, and Hk) have distinct effects on habituation at least as strong as those of dunce and rutabaga, memory mutants with defective cAMP metabolism (). Habituation, spontaneous recovery, and dishabituation of the electrically stimulated long-latency giant fiber pathway response were shown in each mutant type. Mutations of Sh (voltage-gated) and slo (Ca2+-gated) subunits enhanced and slowed habituation, respectively. However, mutations of eag and Hk subunits, which confer K+-current modulation, had even more extreme phenotypes, again enhancing and slowing habituation, respectively. In double mutants, Sh mutations moderated the strong phenotypes of eag and Hk, suggesting that their modulatory functions are best expressed in the presence of intact Sh subunits. Nonactivity-dependent responses (refractory period and latency) at two stages of the circuit were altered only in some mutants and do not account for modifications of habituation. Furthermore, failures of the long-latency response during habituation, which normally occur in labile connections in the brain, could be induced in the thoracic circuit stage in Hk mutants. Our work indicates that different K+ channel subunits play distinct roles in activity-dependent neural plasticity and thus can be incorporated along with second messenger "memory" loci to enrich the genetic analysis of learning and memory.

Alternative splicing in the pore-forming region of shaker potassium channels

We have cloned cDNAs for the shaker potassium channel gene from the spiny lobster Panulirus interruptus. As previously found in Drosophila, there is alternative splicing at the 5' and 3' ends of the coding region. However, in Panulirus shaker, alternative splicing also occurs within the pore-forming region of the protein. Three different splice variants were found within the P region, two of which bestow unique electrophysiological characteristics to channel function. Pore I and pore II variants differ in voltage dependence for activation, kinetics of inactivation, current rectification, and drug resistance. The pore 0 variant lacks a P region exon and does not produce a functional channel. This is the first example of alternative splicing within the pore-forming region of a voltage-dependent ion channel. We used a recently identified potassium channel blocker, kappa-conotoxin PVIIA, to study the physiological role of the two pore forms. The toxin selectively blocked one pore form, whereas the other form, heteromers between the two pore forms, and Panulirus shal were not blocked. When it was tested in the Panulirus stomatogastric ganglion, the toxin produced no effects on transient K+ currents or synaptic transmission between neurons.

Alternative splicing in the pore-forming region of shaker potassium channels

We have cloned cDNAs for the shaker potassium channel gene from the spiny lobster Panulirus interruptus. As previously found in Drosophila, there is alternative splicing at the 5' and 3' ends of the coding region. However, in Panulirus shaker, alternative splicing also occurs within the pore-forming region of the protein. Three different splice variants were found within the P region, two of which bestow unique electrophysiological characteristics to channel function. Pore I and pore II variants differ in voltage dependence for activation, kinetics of inactivation, current rectification, and drug resistance. The pore 0 variant lacks a P region exon and does not produce a functional channel. This is the first example of alternative splicing within the pore-forming region of a voltage-dependent ion channel. We used a recently identified potassium channel blocker, kappa-conotoxin PVIIA, to study the physiological role of the two pore forms. The toxin selectively blocked one pore form, whereas the other form, heteromers between the two pore forms, and Panulirus shal were not blocked. When it was tested in the Panulirus stomatogastric ganglion, the toxin produced no effects on transient K+ currents or synaptic transmission between neurons.

Diverse expression and distribution of Shaker potassium channels during the development of the Drosophila nervous system

The spatio-temporal expression of Shaker (Sh) potassium channels (Kch) in the developing and adult nervous system of Drosophila has been studied at the molecular and histological level using specific antisera. Sh Kch are distributed in most regions of the nervous system, but their expression is restricted to only certain populations of cells. Sh Kch have been found in the following three locations: in synaptic areas of neuropile, in axonal fiber tracks, and in a small number of neuronal cell bodies. This wide subcellular localization, together with a diverse distribution, implicates Sh Kch in multiple neuronal functions. Experiments performed with Sh mutants that specifically eliminate a few of the Sh Kch splice variants clearly demonstrate an abundant differential expression and usage of the wide repertoire of Sh isoforms, but they do not support the idea of extensive segregation of these isoforms among different populations of neurons. Sh Kch are predominantly expressed at late stages of postembryonic development and adulthood. Strikingly, wide changes in the repertoire of Sh splice isoforms occur some time after the architecture of the nervous system is complete, indicating that the expression of Sh Kch contributes to the final refinements of neuronal differentiation. These late changes in the expression and distribution of Sh Kch seem to correlate with activity patterns suggesting that Sh Kch may be involved in adaptative mechanisms of excitability.

Diverse expression and distribution of Shaker potassium channels during the development of the Drosophila nervous system

The spatio-temporal expression of Shaker (Sh) potassium channels (Kch) in the developing and adult nervous system of Drosophila has been studied at the molecular and histological level using specific antisera. Sh Kch are distributed in most regions of the nervous system, but their expression is restricted to only certain populations of cells. Sh Kch have been found in the following three locations: in synaptic areas of neuropile, in axonal fiber tracks, and in a small number of neuronal cell bodies. This wide subcellular localization, together with a diverse distribution, implicates Sh Kch in multiple neuronal functions. Experiments performed with Sh mutants that specifically eliminate a few of the Sh Kch splice variants clearly demonstrate an abundant differential expression and usage of the wide repertoire of Sh isoforms, but they do not support the idea of extensive segregation of these isoforms among different populations of neurons. Sh Kch are predominantly expressed at late stages of postembryonic development and adulthood. Strikingly, wide changes in the repertoire of Sh splice isoforms occur some time after the architecture of the nervous system is complete, indicating that the expression of Sh Kch contributes to the final refinements of neuronal differentiation. These late changes in the expression and distribution of Sh Kch seem to correlate with activity patterns suggesting that Sh Kch may be involved in adaptative mechanisms of excitability.

Sodium channel functioning based on an octagonal structure model

The complete amino acid sequence of a sodium channel from squid Loligo bleekeri has been deduced by cloning and sequence analysis of the complementary DNA. A unique feature of the squid sodium channel is the 1,522 residue sequence, approximately three-fourths of those of the rat sodium channels I, II and III. On the basis of the sequence, and in comparison with those of vertebrate sodium channels, we have proposed a tertiary structure model of the sodium channel where the transmembrane segments are octagonally aligned and the four linkers of S5-6 between segments S5 and S6 play a crucial role in the activation gate, voltage sensor and ion selective pore, which can slide, depending on membrane potentials, along inner walls consisting of alternating segments S2 and S4. The proposed octagonal structure model is contrasted with that of Noda et al. (Nature 320; 188-192, 1986). The octagonal structure model can explain the gating of activation and inactivation, and ion selectivity, as well as the action mechanism of both tetrodotoxin (TTX) and alpha-scorpion toxin (ScTX), and can be applied not only to the sodium channel, but also to the calcium channel, potassium channel and cGMP-gated channel.

Cloning and functional expression of a Shaker-related voltage-gated potassium channel gene from Schistosoma mansoni (Trematoda: Digenea)

We have isolated a cDNA (SKv1.1) encoding a Shaker-related K+ channel from an adult cDNA library of the human parasitic trematode Schistosoma mansoni. The deduced amino acid sequence (512 aa, 56.5 kDa) contains 6 putative membrane-spanning domains (S1-S6) and a pore-forming domain (H5). SKv1.1 is grouped in the Shaker family, but forms a unique branch within this family, on the basis of dendrogram analysis. SKv1.1 shows significant sequence identity with most other Shaker channels, with 64-74% identity in the core region (S1-S6). It has the highest sequence identity with the K+ channel (Ak01a) from Aplysia. Northern blot analysis detected a single primary transcript of 2.8 kb. Southern blot analysis indicated that SKv1.1 is present as a single copy in the genomic DNA of S. mansoni. Expression of SKv1.1 in Xenopus oocytes produced a rapidly activating and inactivating outward K+ current which is highly sensitive to 4-aminopyridine, but is insensitive to tetraethylammonium, mast cell degranulating peptide, dendrotoxin and charybdotoxin. The presence of a Shaker homologue in Schistosoma suggests that Sh subfamilies may exist in other lower invertebrates as well as platyhelminths.

Mutational analysis of ion conduction and drug binding sites in the inner mouth of voltage-gated K+ channels

Pore properties that distinguish two cloned, voltage-gated K+ channels, Kv2.1 and Kv3.1, include single-channel conductance, block by external and internal tetraethylammonium, and block by 4-aminopyridine. To define the inner mouth of voltage-gated K+ channels, segmental exchanges and point mutations of nonconserved residues were used. Transplanting the cytoplasmic half of either transmembrane segments S5 or S6 from Kv3.1 into Kv2.1 reduced sensitivity to block by internal tetraethylammonium, increased sensitivity to 4-aminopyridine, and reduced single-channel conductance. In S6, changes in single-channel conductance and internal tetraethylammonium sensitivity were associated with point mutations V400T and L403 M, respectively. Although individual residues in both S5 and S6 were found to affect 4-aminopyridine blockade, the most effective change was L327F in S5. Thus, both S5 and S6 contribute to the inner mouth of the pore but different residues regulate ion conduction and blockade by internal tetraethylammonium and 4-aminopyridine.

On the mechanism of 4-aminopyridine action on the cloned mouse brain potassium channel mKv1.1

1. This study used the whole-cell patch clamp technique to investigate the mechanism of action of the K+ channel blocker 4-aminopyridine (4-AP) on the cloned K+ channel mouse Kv1.1 (mKv1.1) expressed in Chinese hamster ovary cells. 2. Cells transfected with mKv1.1 expressed a non-inactivating, delayed rectifier-type K+ current. 4-AP induced a dose-, voltage- and use-dependent block of mKv1.1. 3. 4-AP blockade of mKv1.1 was similar whether 4-AP was administered extracellularly (IC50 = 147 microM) or intracellularly (IC50 = 117 microM). 4. Inclusion of the first twenty amino acids of the N-terminus sequence of the Shaker B K+ channel ('inactivation peptide') in the patch electrode transformed mKv1.1 into a rapidly inactivating current. The time constant of decay for the modified current was dependent on the concentration of inactivation peptide, and under these conditions extracellular 4-AP had a reduced potency (IC50 values of 471 and 537 microM for 0.5 and 2 mg ml-1 inactivation peptide, respectively). 5. A permanently charged analogue of 4-AP, 4-aminopyridine methiodide (4-APMI), was found to block mKv1.1 when applied inside the cell, but was without effect when administered externally. 6. Decreasing the intracellular pH (pHi) to 6.4 caused an increase in 4-AP potency (IC50 = 76 microM), whereas at pHi 9.0, the 4-AP potency fell (IC50 = 295 microM). Conversely, increasing extracellular pH (pHo) to 9.0 caused an increase in 4-AP potency (IC50 = 93 microM), whereas at pHo 6.4, 4-AP potency decreased (IC50 = 398 microM). 7. Taken together, these findings support the hypotheses that the uncharged form of 4-AP crosses the membrane, and that it is predominantly the cationic form which acts on mKv1.1 channels intracellularly, possibly at or near to the binding site for the inactivation peptide.

Divalent cations selectively alter the voltage dependence of inactivation of A-currents in chick autonomic neurons

A-currents were studied in acutely dissociated chick autonomic neurons. Switching from salines containing 4 mM Mg2+ to salines containing 4 mM Ca2+ caused a large positive shift in the voltage dependence of steady-state inactivation, but had no effect on the voltage dependence or kinetics of activation, deactivation, the rate at which channels became inactivated, or the rate at which channels recovered from inactivation. This effect saturated with increasing concentrations of Ca2+. Other group IIA divalent cations were less effective than Ca2+, in the order Ca2+ > Ba2+ > Mg2+ = Sr2+. Application of 4 mM Mg2+ partially antagonized the effects of 4 mM Ca2+. The results suggest that divalent cations selectively alter the voltage dependence of steady-state inactivation by acting at sites accessible from the outside of the cell.

Inactivation determined by a single site in K+ pores

An N-terminus peptide or a C-terminus mechanism involving a single residue in transmembrane segment 6 produces inactivation in voltage-dependent K+ channels. Here we show that a single position in the pore of K+ channels can produce inactivation having characteristics distinct from either N- or C-type inactivation. In a chimeric K+ channel (CHM), the point reversion CHM V369K produced fast inactivation and CHM V369S had the additional effect of halving K+ conductance consistent with a position in the pore. The result was not restricted to CHM; mutating position 369 in the naturally occurring channel Kv2.1 also produced fast inactivation. Like N- and C-types of inactivation, pore or P-type inactivation was characterized by short bursts terminated by rapid entry into the inactivated state. Unlike C-type inactivation, in which external tetraethylammonium (TEA) produced a simple blockade that slowed inactivation and reduced currents, in P-type inactivation external TEA increased currents. Unlike N-type inactivation, internal TEA produced a simple reduction in current and K+ occupancy of the pore had no effect. External TEA was not the only cation to increase current; external K+ enhanced channel availability and recovery from inactivation. Additional features of P-type inactivation were residue-specific effects on the extent of inactivation and removal of inactivation by a point reversion at position 374, which also regulates conductance. The demonstration of P-type inactivation indicates that pore residues in K+ channels may be part of the inactivation gating machinery.

Molecular basis of K+ channel inactivation gating

The gating of many K+, Na+ and Ca++ channels is driven by changes in membrane potential. Part of the gating mechanism, the voltage sensing S4, a proposed transmembrane segment, has been identified. Movement in the membrane electric field of the charged S4 is thought to precede the opening and closing of the activation gate. The physical basis of the conformational changes involved in gating has yet to be elucidated. Here, we discuss a domain that appears to lie at the cytoplasmic mouth of K+ channels and to form a receptor for the inactivation gate. We examine the possibility that a) the physical attachment of this receptor/mouth to the S4 allows inactivation to be coupled to the voltage dependent conformational changes that open the channel and b) explains the immobilization of gating charge by inactivation. We also address the physiological ramifications of such structural coupling.

Receptor sites for open channel blockers of Shaker voltage-gated potassium channels--molecular approaches

The Shaker locus encodes a family of voltage-gated potassium (K) channels expressed in the central and peripheral nervous system as well as in muscle. Members of the Shaker K-family have variant amino- and carboxy-terminal sequences, which assemble into homo- and hetero-multimeric K-channels. The channels have distinct kinetics of activation and inactivation. Electrophysiological characterization of wild type and mutant K-channels allows to correlate particular domains and critical amino acid residues with receptor sites of open channel blockers such as tetraethylammonium, charybdotoxin and dendrotoxin.

Mechanism of asymmetric block of K channels by tetraalkylammonium ions in mouse neuroblastoma cells

Experiments were performed to compare the mechanism of block of voltage-dependent K channels by various short and long alkyl chain tetraalkylammonium (TAA) ions at internal and external sites. Current through single channels was recorded from excised membrane patches of cultured neuroblastoma cells using the patch-clamp technique. All of the TAA derivatives tested blocked the open channel when applied to either side of the membrane. Tetraethylammonium (TEA) reduced the amplitude of current through the open channel. Tetrabutylammonium (TBA) and tetrapentylammonium (TPeA) reduced the open time as a function of the concentration. An additional nonconducting state was observed when TBA or TPeA was applied internally or externally, due to the presence of a drug-bound and blocked state of the channel. The closing rate under control conditions was similar to that in the presence of external tetramethylammonium (TMA), suggesting that channel closing is independent of external drug binding. The concentration for half maximal block of the channel by external TEA was 80 microM. The channel was less sensitive to internal TEA, which half blocked the channel at 27 mM. The dissociation rate of long alkyl chain TAA ions from the channel was slower when applied to the inside, compared to external application, suggesting the presence of distinct internal and external receptors. Long alkyl chain TAA derivatives, such as TBA had a faster association rate with the open channel when applied to the inside of the membrane than when applied to the outside.

Differential effects of elevating [K]o on three transient outward potassium channels. Dependence on channel inactivation mechanisms

We carried out a systematic study on the effects of elevating [K]o on the properties of a transient outward potassium channel encoded by a cardiac cDNA (RHK1) and compared them with those on two Shaker potassium channels (H-4 and H-37). The amino acid sequences of all three channels are known, and their structure-function relations have been partially characterized. All three channels were expressed in Xenopus oocytes and studied under double-microelectrode voltage-clamp conditions. For all three channels, elevating [K]o caused an increase in the channels' chord conductances and a negative shift in the calculated activation curves. However, in other aspects of channel properties that are related to the channels' inactivation processes, there were differences in the changes induced by increasing [K]o: 1) Elevating [K]o caused a positive shift in the steady-state inactivation curves of RHK1 and H-4 but did not cause any shift in H-37. 2) Elevating [K]o slowed the time course of inactivation of H-37 but did not cause any significant changes in the time course of RHK1 or H-4. 3) Elevating [K]o accelerated the rate of recovery from inactivation of RHK1 and H-4 but slowed the recovery time course of H-37. Our experiments show that elevating [K]o can cause a wide range of effects on the transient outward potassium channels. Furthermore, raising [K]o induced similar changes in RHK1 and H-4 (inactivation mediated by an "N-type" mechanism) that were different from the changes in H-37 (inactivation mediated by a "C-type" mechanism). Therefore, our data suggest that part of the effects of elevating [K]o on channel properties may depend on the channel's inactivation mechanism. This hypothesis is supported by results from experiments studying the effects of elevating [K]o on a mutant RHK1 channel (RHK1 delta 3-25), which apparently lacks the N-type and C-type inactivation mechanisms.

Level of expression controls modes of gating of a K+ channel

Several distinct subfamilies of K+ channel genes have been discovered by molecular cloning, however, in some cases the structural differences among them do not account for the diversity of K+ current types, ranging from transient A-type to slowly inactivating delayed rectifier-type, as members within each subfamily have been shown to code for K+ channels of different inactivation kinetics and pharmacological properties. We show that a single K+ channel cDNA of the Shaker subfamily (ShH4) can express in Xenopus oocytes not only a transient A-type K+ current but also, upon increased level of expression, slowly inactivating K+ currents with markedly reduced sensitivity to tetraethylammonium. In correlation with the macroscopic currents there are single-channel gating modes ranging from the fast-inactivation mode which underlies the transient A-type current, to slow-inactivation modes characterized by bursts of longer openings, and corresponding to the slowly inactivating macroscopic currents.

Completely functional double-barreled chloride channel expressed from a single Torpedo cDNA

We have performed an electrophysiological analysis of the recently cloned Torpedo marmorata Cl- channel. Functional expression of Cl- channels in oocytes of Xenopus laevis previously injected with cRNA yielded an outward-rectifying current activated by hyperpolarization. Replacement of Cl- with other anions significantly reduced or inhibited the current. Single-channel recordings from cell-attached patches exhibited burst-like Cl- channel activity with rapid fluctuations between three equally spaced substates (0 pS, 9 pS, and 18 pS). The properties of the cloned Cl- channel were almost identical to those of the reconstituted native T. californica Cl- channel and were in full agreement with the predictions of the double-barreled channel model [Hanke, W. & Miller, C. (1983) J. Gen. Physiol. 82, 25-45]. Our results imply that the cloned cDNA codes for the completely functional Torpedo electroplax Cl- channel.

Developmentally regulated alternative RNA splicing of rat brain sodium channel mRNAs

Two rat brain Na channel alpha-subunit cDNAs, named RII and RIIA, have almost identical coding regions, with a divergence of only 36 nucleotides (0.6%) over a total length of 6015 residues. A cluster of 20 divergent residues occurs within a 90 nucleotide segment of cDNA sequence. We now demonstrate that this 90 nucleotide segment is encoded twice in the RII/RIIA genomic sequence. Furthermore, the mutually exclusive selection of these two exons is developmentally regulated. RII mRNAs are relatively abundant at birth but are gradually replaced by RIIA mRNAs as development proceeds. The two mRNAs also appear to have different regional distributions in the developing rat brain. Strikingly, although 30 amino acids are encoded by each alternative exon, only amino acid position 209 is altered between the two, specifying asparagine in RII and aspartate in RIIA. Alternative RNA splicing may modulate the RII/RIIA sodium channel properties during neuronal development.

Regulation of fast inactivation of cloned mammalian IK(A) channels by cysteine oxidation

Modulation of neuronal excitability by regulation of K+ channels potentially plays a part in short-term memory but has not yet been studied at the molecular level. Regulation of K+ channels by protein phosphorylation and oxygen has been described for various tissues and cell types; regulation of fast-inactivating K+ channels mediating IK(A) currents has not yet been described. Functional expression of cloned mammalian K+ channels has provided a tool for studying their regulation at the molecular level. We report here that fast-inactivating K+ currents mediated by cloned K+ channel subunits derived from mammalian brain expressed in Xenopus oocytes are regulated by the reducing agent glutathione. This type of regulation may have a role in vivo to link metabolism to excitability and to regulate excitability in specific membrane areas of mammalian neurons.

Swapping of functional domains in voltage-gated K+ channels

Functionally significant properties of domains in the amino acid sequence of potassium (K+) channel-forming proteins have been investigated by constructing chimeric K+ channels. The N-terminal domain of ShA2 channels was responsible for the fast inactivation (IKA) and also determined a shift in the threshold of activation whereas the membrane domain determined the timecourse of slow inactivation. The binding site for dendrotoxin (DTX), but not for mast cell degranulating peptide (MCDP), is completely located on the loop between the membrane spanning segments S5 and S6 in RCK1 channels. A certain part of this region which has recently been designated as a narrow part of the pore was found to be not responsible for the differences in the single-channel current amplitude between RCK4 and RCK2 K+ channels. Interchange of the C-terminal domain did not influence activation or inactivation of the channels.

Molecular basis for different rates of recovery from inactivation in the Shaker potassium channel family

The two alternative carboxyl-termini of Shaker K+ channels strongly influence the rates of inactivation and of recovery from channel inactivation. We show that this distinct inactivation behaviour is due to an alanine/valine amino acid replacement within the Shaker carboxyl-terminus at a site that occurs within the proposed membrane spanning segment S6.

Gating currents of inactivating and non-inactivating potassium channels expressed in Xenopus oocytes

The Xenopus oocyte expression system in combination with patch-clamp techniques allows the measurement of ionic currents from a single class of genetically engineered ion channels. Ionic currents in the nanoampere range from oocytes injected with cRNA, corresponding to potassium channels, can be recorded in the inside-out patch configuration. These recordings have a high time resolution at low background noise. Substitution of impermeant ions for potassium and blocking of the channel conductance with tetraethylammonium allows the recording of potassium gating currents, Ig, which is hampered in natural excitable cells by the simultaneous presence of sodium channels and a variety of different potassium channels. The "on" transients, Ig(on), are fast and can have amplitudes of up to several tens of pA. Upon repolarization to -100 mV after small depolarizations, "off" gating currents, Ig(off)g, which reverse most of the "on" charge displacement, Q(on), within 1 ms, are readily observed. However, this fast recovery of the gating charge is drastically reduced upon increasing the amplitude of the depolarizing pulse. In contrast to sodium channels, this temporary charge immobilization is complete within a few milliseconds at positive membrane potentials. Furthermore, there seems to be no direct correlation between charge immobilization and inactivation because the same phenomenon occurs for channels that do not inactivate.