Coexpression of the KCNA3B gene product with Kv1.5 leads to a novel A-type potassium channel

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.

H258R mutation in KCNAB3 gene in a family with genetic epilepsy and febrile seizures plus

PURPOSE

The aim of this was to discover disease-causing gene mutations linked to genetic epilepsy with febrile seizures plus (GEFS+) in a family in the Southern Chinese Han population. Of a three-generation pedigree of 18 members in this family, 4 were affected with GEFS+.

METHOD

Blood samples of 7 family members-3 affected and 4 unaffected individuals-were collected. Whole-exome sequencing was performed to assess for genetic mutations in two of the affected individuals and two of the unaffected individuals.

RESULTS

Fourteen potentially consequential mutations were found in the two affected individuals and were validated with the Sanger sequencing method. Blood DNA tested in polymerase chain reaction with KCNAB3 primers revealed that one novel missense mutation, c.773A>G (p.H258R) in the KCNAB3 gene, which encoded the potassium voltage-gated channel subfamily A regulatory β subunit 3 (KCNAB3), was shared by all three affected and one unaffected family member. However, this mutation did not appear in 300 unrelated control subjects. According to the bioinformatics tools SIFT and PROVEAN, p.H258R was thought to affect protein function. Functional verification showed that the KCNAB3 mutation could accelerate the inactivation of potassium channels, thus inhibiting potassium current, increasing neuronal excitability, and promoting epileptic convulsion.

CONCLUSIONS

These results reveal that mutations in the KCNAB3 gene may be associated with GEFS+.

The Biosynthesis of Enzymatically Oxidized Lipids

Enzymatically oxidized lipids are a specific group of biomolecules that function as key signaling mediators and hormones, regulating various cellular and physiological processes from metabolism and cell death to inflammation and the immune response. They are broadly categorized as either polyunsaturated fatty acid (PUFA) containing (free acid oxygenated PUFA "oxylipins", endocannabinoids, oxidized phospholipids) or cholesterol derivatives (oxysterols, steroid hormones, and bile acids). Their biosynthesis is accomplished by families of enzymes that include lipoxygenases (LOX), cyclooxygenases (COX), cytochrome P450s (CYP), and aldo-keto reductases (AKR). In contrast, non-enzymatically oxidized lipids are produced by uncontrolled oxidation and are broadly considered to be harmful. Here, we provide an overview of the biochemistry and enzymology of LOXs, COXs, CYPs, and AKRs in humans. Next, we present biosynthetic pathways for oxylipins, oxidized phospholipids, oxysterols, bile acids and steroid hormones. Last, we address gaps in knowledge and suggest directions for future work.

Further studies of ion channels in the electroreceptor of the skate through deep sequencing, cloning and cross species comparisons

Our comparative studies seek to understand the structure and function of ion channels in cartilaginous fish that can detect very low voltage gradients in seawater. The principal channels of the electroreceptor include a calcium activated K channel whose α subunit is Kcnma1, and a voltage-dependent calcium channel, Cacna1d. It has also been suggested based on physiological and pharmacological evidence that a voltage-gated K channel is present in the basal membranes of the receptor cells which modulates synaptic transmitter release. Large conductance calcium-activated K channels (BK) are comprised of four α subunits, encoded by Kcnma1 and modulatory β subunits of the Kcnmb class. We recently cloned and published the skate Kcnma1 gene and most of Kcnmb4 using purified mRNA of homogenized electroreceptors. Bellono et al. have recently performed RNA sequencing (RNA-seq) on purified mRNA from skate electroreceptors and found several ion channels including Kcnma1. We searched the Bellono et al. RNA-seq repository for additional channels and subunits. Our most significant findings are the presence of two Shaker type voltage dependent K channel sequences which are grouped together as isoforms in the data repository. The larger of these is a skate ortholog of the voltage dependent fast potassium channel Kv1.1, which is expressed at appreciable levels. The second ortholog is similar to Kv1.5 but has fewer N-terminal amino acids than other species. The sequence for Kv1.5 in the skate is very strongly aligned with the recently reported sequence for potassium channels in the electroreceptors of the cat shark, S. retifer, which also modulate synaptic transmission. The latter channel was designated as Kv1.3 in the initial report, but we suggest that these channels are actually orthologs of each other, and that Kv1.5 is the prevailing designation. We also found a beta subunit sequence (Kcnab2) which may co-assemble with one or both of the voltage gated channels. The new channels and subunits were verified by RT-PCR and the Kv1.1 sequence was confirmed by cloning. We also searched the RNA-seq repository for accessory subunits of Kcnma1, and found a computer-generated assembly that contained a complete sequence of its β subunit, Kcnmb2. Skate Kcnmb2 has a total of 279 amino acids, with 51 novel amino acids at the N-terminus which may play a specific physiological role. This sequence was confirmed by PCR and cloning. However, skate Kcnmb2 is expressed at low levels in the electroreceptor compared to Kcnma1 and skate Kcnmb1 is absent. The evolutionary origin of the newly described K channels and their subunits was studied by alignments with mammalian sequences, including human, and also those in related fish: the whale shark (R. typus), the ghost shark (C.milii), and (S. retifer). There are also orthologous K channels of the lamprey, which has electroreceptors. Tree building and bootstrap programs were used to confirm phylogenetic inferences. Further research should focus on the subcellular locations of these channels, their gating behavior, and the effects of accessory subunits on gating.

Metabolic regulation of Kv channels and cardiac repolarization by Kvβ2 subunits

Voltage-gated potassium (Kv) channels control myocardial repolarization. Pore-forming Kvα proteins associate with intracellular Kvβ subunits, which bind pyridine nucleotides with high affinity and differentially regulate channel trafficking, plasmalemmal localization and gating properties. Nevertheless, it is unclear how Kvβ subunits regulate myocardial K⁺ currents and repolarization. Here, we tested the hypothesis that Kvβ2 subunits regulate the expression of myocardial Kv channels and confer redox sensitivity to Kv current and cardiac repolarization. Co-immunoprecipitation and in situ proximity ligation showed that in cardiac myocytes, Kvβ2 interacts with Kv1.4, Kv1.5, Kv4.2, and Kv4.3. Cardiac myocytes from mice lacking Kcnab2 (Kvβ2^-/-) had smaller cross sectional areas, reduced sarcolemmal abundance of Kvα binding partners, reduced I_to, I_K,slow1, and I_K,slow2 densities, and prolonged action potential duration compared with myocytes from wild type mice. These differences in Kvβ2^-/- mice were associated with greater P wave duration and QT interval in electrocardiograms, and lower ejection fraction, fractional shortening, and left ventricular mass in echocardiographic and morphological assessments. Direct intracellular dialysis with a high NAD(P)H:NAD(P)⁺ accelerated Kv inactivation in wild type, but not Kvβ2^-/- myocytes. Furthermore, elevated extracellular levels of lactate increased [NADH]_i and prolonged action potential duration in wild type cardiac myocytes and perfused wild type, but not Kvβ2^-/-, hearts. Taken together, these results suggest that Kvβ2 regulates myocardial electrical activity by supporting the functional expression of proteins that generate I_to and I_K,slow, and imparting redox and metabolic sensitivity to Kv channels, thereby coupling cardiac repolarization to myocyte metabolism.

Transient Potassium Channels: Therapeutic Targets for Brain Disorders

Transient potassium current channels (I_A channels), which are expressed in most brain areas, have a central role in modulating feedforward and feedback inhibition along the dendroaxonic axis. Loss of the modulatory channels is tightly associated with a number of brain diseases such as Alzheimer’s disease, epilepsy, fragile X syndrome (FXS), Parkinson’s disease, chronic pain, tinnitus, and ataxia. However, the functional significance of I_A channels in these diseases has so far been underestimated. In this review, we discuss the distribution and function of I_A channels. Particularly, we posit that downregulation of I_A channels results in neuronal (mostly dendritic) hyperexcitability accompanied by the imbalanced excitation and inhibition ratio in the brain’s networks, eventually causing the brain diseases. Finally, we propose a potential therapeutic target: the enhanced action of I_A channels to counteract Ca²⁺-permeable channels including NMDA receptors could be harnessed to restore dendritic excitability, leading to a balanced neuronal state.

Emerging roles for multifunctional ion channel auxiliary subunits in cancer

Several superfamilies of plasma membrane channels which regulate transmembrane ion flux have also been shown to regulate a multitude of cellular processes, including proliferation and migration. Ion channels are typically multimeric complexes consisting of conducting subunits and auxiliary, non-conducting subunits. Auxiliary subunits modulate the function of conducting subunits and have putative non-conducting roles, further expanding the repertoire of cellular processes governed by ion channel complexes to processes such as transcellular adhesion and gene transcription. Given this expansive influence of ion channels on cellular behaviour it is perhaps no surprise that aberrant ion channel expression is a common occurrence in cancer. This review will focus on the conducting and non-conducting roles of the auxiliary subunits of various Ca2+, K+, Na+ and Cl- channels and the burgeoning evidence linking such auxiliary subunits to cancer. Several subunits are upregulated (e.g. Cavβ, Cavγ) and downregulated (e.g. Kvβ) in cancer, while other subunits have been functionally implicated as oncogenes (e.g. Navβ1, Cavα2δ1) and tumour suppressor genes (e.g. CLCA2, KCNE2, BKγ1) based on in vivo studies. The strengthening link between ion channel auxiliary subunits and cancer has exposed these subunits as potential biomarkers and therapeutic targets. However further mechanistic understanding is required into how these subunits contribute to tumour progression before their therapeutic potential can be fully realised.

Biochemical and physiological properties of K+ channel-associated AKR6A (Kvβ) proteins

Voltage-gated potassium (Kv) channels play an essential role in the regulation of membrane excitability and thereby control physiological processes such as cardiac excitability, neural communication, muscle contraction, and hormone secretion. Members of the Kv1 and Kv4 families are known to associate with auxiliary intracellular Kvβ subunits, which belong to the aldo-keto reductase superfamily. Electrophysiological studies have shown that these proteins regulate the gating properties of Kv channels. Although the three gene products encoding Kvβ proteins are functional enzymes in that they catalyze the nicotinamide adenine dinucleotide phosphate (NAD[P]H)-dependent reduction of a wide range of aldehyde and ketone substrates, the physiological role for these proteins and how each subtype may perform unique roles in coupling membrane excitability with cellular metabolic processes remains unclear. Here, we discuss current knowledge of the enzymatic properties of Kvβ proteins from biochemical studies with their described and purported physiological and pathophysiological influences.

Insights into electrosensory organ development, physiology and evolution from a lateral line-enriched transcriptome

The anamniote lateral line system, comprising mechanosensory neuromasts and electrosensory ampullary organs, is a useful model for investigating the developmental and evolutionary diversification of different organs and cell types. Zebrafish neuromast development is increasingly well understood, but neither zebrafish nor Xenopus is electroreceptive and our molecular understanding of ampullary organ development is rudimentary. We have used RNA-seq to generate a lateral line-enriched gene-set from late-larval paddlefish (Polyodon spathula). Validation of a subset reveals expression in developing ampullary organs of transcription factor genes critical for hair cell development, and genes essential for glutamate release at hair cell ribbon synapses, suggesting close developmental, physiological and evolutionary links between non-teleost electroreceptors and hair cells. We identify an ampullary organ-specific proneural transcription factor, and candidates for the voltage-sensing L-type Ca_v channel and rectifying K_v channel predicted from skate (cartilaginous fish) ampullary organ electrophysiology. Overall, our results illuminate ampullary organ development, physiology and evolution.

Pubertal development in healthy children is mirrored by DNA methylation patterns in peripheral blood

Puberty marks numerous physiological processes which are initiated by central activation of the hypothalamic–pituitary–gonadal axis, followed by development of secondary sexual characteristics. To a large extent, pubertal timing is heritable, but current knowledge of genetic polymorphisms only explains few months in the large inter-individual variation in the timing of puberty. We have analysed longitudinal genome-wide changes in DNA methylation in peripheral blood samples (n = 102) obtained from 51 healthy children before and after pubertal onset. We show that changes in single methylation sites are tightly associated with physiological pubertal transition and altered reproductive hormone levels. These methylation sites cluster in and around genes enriched for biological functions related to pubertal development. Importantly, we identified that methylation of the genomic region containing the promoter of TRIP6 was co-ordinately regulated as a function of pubertal development. In accordance, immunohistochemistry identified TRIP6 in adult, but not pre-pubertal, testicular Leydig cells and circulating TRIP6 levels doubled during puberty. Using elastic net prediction models, methylation patterns predicted pubertal development more accurately than chronological age. We demonstrate for the first time that pubertal attainment of secondary sexual characteristics is mirrored by changes in DNA methylation patterns in peripheral blood. Thus, modulations of the epigenome seem involved in regulation of the individual pubertal timing.

Gene Mutation Analysis in 253 Chinese Children with Unexplained Epilepsy and Intellectual/Developmental Disabilities

Objective

Epilepsy and intellectual/developmental disabilities (ID/DD) have a high rate of co-occurrence. Here, we investigated gene mutations in Chinese children with unexplained epilepsy and ID/DD.

Methods

We used targeted next-generation sequencing to detect mutations within 300 genes related to epilepsy and ID/DD in 253 Chinese children with unexplained epilepsy and ID/DD. A series of filtering criteria was used to find the possible pathogenic variations. Validation and parental origin analyses were performed by Sanger sequencing. We reviewed the phenotypes of patients with each mutated gene.

Results

We identified 32 novel and 16 reported mutations within 24 genes in 46 patients. The detection rate was 18% (46/253) in the whole group and 26% (17/65) in the early-onset (before three months after birth) epilepsy group. To our knowledge, we are the first to report KCNAB1 is a disease-causing gene of epilepsy by identifying a novel de novo mutation (c.1062dupCA p.Leu355HisfsTer5) within this gene in one patient with early infantile epileptic encephalopathy (EIEE). Patients with an SCN1A mutation accounted for the largest proportion, 17% (8/46). A total of 38% (9/24) of the mutated genes re-occurred at least 2 times and 63% (15/24) occurred only one time. Ion channel genes are the most common (8/24) and genes related to synapse are the next most common to occur (5/24).

Significance

We have established genetic diagnosis for 46 patients of our cohort. Early-onset epilepsy had the highest detection rate. KCNAB1 mutation was first identified in EIEE patient. We expanded the phenotype and mutation spectrum of the genes we identified. The mutated genes in this cohort are mostly isolated. This suggests that epilepsy and ID/DD phenotypes occur as a consequence of brain dysfunction caused by a highly diverse population of mutated genes. Ion channel genes and genes related to synapse were more common mutated in this patient cohort.

17p13.1 microdeletion: genetic and clinical findings in a new patient with epilepsy and comparison with literature

Array comparative genomic hybridization is now a powerful tool to investigate patients with multiple congenital abnormalities and intellectual/motor impairment, and genomic imbalances are identified in a growing number of children with intellectual disability. Deletions in the 17p13.1 region have been reported in patients with dysmorphic features and developmental delay but a consistent phenotype has yet to emerge. Here, we report on the diagnosis of a 17p13.1 microdeletion of 829 kb in an 8-year-old girl presenting with profound cognitive disability, psychomotor delay, facial dysmorphisms, and refractory epilepsy. This deletion comprises 44 genes, including 8 OMIM morbid genes. We discuss genetic, clinical, and epileptic features comparing our patient with those previously reported in the literature.

Oxidative and reductive metabolism of lipid-peroxidation derived carbonyls

Extensive research has shown that increased production of reactive oxygen species (ROS) results in tissue injury under a variety of pathological conditions and chronic degenerative diseases. While ROS are highly reactive and can incite significant injury, polyunsaturated lipids in membranes and lipoproteins are their main targets. ROS-triggered lipid-peroxidation reactions generate a range of reactive carbonyl species (RCS), and these RCS spread and amplify ROS-related injury. Several RCS generated in oxidizing lipids, such as 4-hydroxy trans-2-nonenal (HNE), 4-oxo-2-(E)-nonenal (ONE), acrolein, malondialdehyde (MDA) and phospholipid aldehydes have been shown to be produced under conditions of oxidative stress and contribute to tissue injury and dysfunction by depleting glutathione and other reductants leading to the modification of proteins, lipids, and DNA. To prevent tissue injury, these RCS are metabolized by several oxidoreductases, including members of the aldo-keto reductase (AKR) superfamily, aldehyde dehydrogenases (ALDHs), and alcohol dehydrogenases (ADHs). Metabolism via these enzymes results in RCS inactivation and detoxification, although under some conditions, it can also lead to the generation of signaling molecules that trigger adaptive responses. Metabolic transformation and detoxification of RCS by oxidoreductases prevent indiscriminate ROS toxicity, while at the same time, preserving ROS signaling. A better understanding of RCS metabolism by oxidoreductases could lead to the development of novel therapeutic interventions to decrease oxidative injury in several disease states and to enhance resistance to ROS-induced toxicity.

S-glutathionylation of an auxiliary subunit confers redox sensitivity to Kv4 channel inactivation

Reactive oxygen species (ROS) regulate ion channels, modulate neuronal excitability, and contribute to the etiology of neurodegenerative disorders. ROS differentially suppress fast "ball-and-chain" N-type inactivation of cloned Kv1 and Kv3 potassium channels but not of Kv4 channels, likely due to a lack of reactive cysteines in Kv4 N-termini. Recently, we discovered that N-type inactivation of Kv4 channel complexes can be independently conferred by certain N-terminal variants of Kv4 auxiliary subunits (DPP6a, DPP10a). Here, we report that both DPP6a and DPP10a, like Kv subunits with redox-sensitive N-type inactivation, contain a highly conserved cysteine in their N-termini (Cys-13). To test if N-type inactivation mediated by DPP6a or DPP10a is redox sensitive, Xenopus oocyte recordings were performed to examine the effects of two common oxidants, tert-butyl hydroperoxide (tBHP) and diamide. Both oxidants markedly modulate DPP6a- or DPP10a-conferred N-type inactivation of Kv4 channels, slowing the overall inactivation and increasing the peak current. These functional effects are fully reversed by the reducing agent dithiothreitol (DTT) and appear to be due to a selective modulation of the N-type inactivation mediated by these auxiliary subunits. Mutation of DPP6a Cys-13 to serine eliminated the tBHP or diamide effects, confirming the importance of Cys-13 to the oxidative regulation. Biochemical studies designed to elucidate the underlying molecular mechanism show no evidence of protein-protein disulfide linkage formation following cysteine oxidation. Instead, using a biotinylated glutathione (BioGEE) reagent, we discovered that oxidation by tBHP or diamide leads to S-glutathionylation of Cys-13, suggesting that S-glutathionylation underlies the regulation of fast N-type inactivation by redox. In conclusion, our studies suggest that Kv4-based A-type current in neurons may show differential redox sensitivity depending on whether DPP6a or DPP10a is highly expressed, and that the S-glutathionylation mechanism may play a previously unappreciated role in mediating excitability changes and neuropathologies associated with ROS.

The emerging Pannexin 1 signalome: a new nexus revealed?

Pannexins (Panxs) are a family of single-membrane, large-pore ion, and metabolite permeable channels. Of the three Panx proteins, Panx1 has been most extensively studied, and has recently emerged as an exciting, clinically relevant target in many physiological and pathophysiological settings. This channel is widely expressed across various cell and tissue types; however its links to precise signaling pathways are largely unknown. Here we review the current literature surrounding presently identified Panx1–protein interactions, a critical first step to unraveling the Panx1 signalome. First we elucidate the reported associations of Panx1 with other ion channels, receptors, and channel signaling complexes. Further, we highlight recently identified Panx1–cytoskeleton interactions. Finally, we discuss the implications of these protein–protein interactions for Panx1 function in various cell and tissue types, and identify key outstanding questions arising from this work.

Regulation of ion channels by pyridine nucleotides

Recent research suggests that in addition to their role as soluble electron carriers, pyridine nucleotides [NAD(P)(H)] also regulate ion transport mechanisms. This mode of regulation seems to have been conserved through evolution. Several bacterial ion-transporting proteins or their auxiliary subunits possess nucleotide-binding domains. In eukaryotes, the Kv1 and Kv4 channels interact with pyridine nucleotide-binding β-subunits that belong to the aldo-keto reductase superfamily. Binding of NADP(+) to Kvβ removes N-type inactivation of Kv currents, whereas NADPH stabilizes channel inactivation. Pyridine nucleotides also regulate Slo channels by interacting with their cytosolic regulator of potassium conductance domains that show high sequence homology to the bacterial TrkA family of K(+) transporters. These nucleotides also have been shown to modify the activity of the plasma membrane K(ATP) channels, the cystic fibrosis transmembrane conductance regulator, the transient receptor potential M2 channel, and the intracellular ryanodine receptor calcium release channels. In addition, pyridine nucleotides also modulate the voltage-gated sodium channel by supporting the activity of its ancillary subunit-the glycerol-3-phosphate dehydrogenase-like protein. Moreover, the NADP(+) metabolite, NAADP(+), regulates intracellular calcium homeostasis via the 2-pore channel, ryanodine receptor, or transient receptor potential M2 channels. Regulation of ion channels by pyridine nucleotides may be required for integrating cell ion transport to energetics and for sensing oxygen levels or metabolite availability. This mechanism also may be an important component of hypoxic pulmonary vasoconstriction, memory, and circadian rhythms, and disruption of this regulatory axis may be linked to dysregulation of calcium homeostasis and cardiac arrhythmias.

Alternative splicing in the aldo-keto reductase superfamily: implications for protein nomenclature

The aldo-keto reductase superfamily contains 173 proteins which are present in all phyla. Examination of the human and mouse genomes has identified that in some instances a single AKR gene can give rise to alternatively spliced mRNA variants which in some cases can give rise to more than one protein isoform. This is currently well documented in the AKR6A subfamily which contains the β-subunits of the voltage-gated potassium ion channels. With the emergence of second generation sequencing it is likely that the occurrence of transcript variants and protein isoforms from a single AKR gene may become common place. To deal with this issue we recommend that the Ensembl data-base nomenclature be used to annotate the transcript variants from a single AKR gene. However, since multiple transcript variants could give rise to either the same or multiple protein isoforms from the same AKR gene we also propose to expand the nomenclature of the AKR protein superfamily, so that when a protein isoform is shown to be expressed and is functional it would be assigned the standard AKR name followed by a "period or full-stop" and a number for that unique isoform. Numbers will be assigned chronologically and linked to the respective transcripts annotated in Ensembl e.g. AKR6A5.1 (Kvβ2.1) (AKR6A5-001, -006 and -201), followed by AKR6A5.2 (Kvβ2.2) (AKR6A5-002,-202). This nomenclature is expandable and it enables multiple protein isoforms to be assigned to their respective transcripts when they arise from the same AKR gene or for a single protein isoform to be assigned to multiple transcripts when the transcripts encode the same AKR protein.

AMPAR-independent effect of striatal αCaMKII promotes the sensitization of cocaine reward

Changes in CaMKII-regulated synaptic excitability are a means through which experience may modify neuronal function and shape behavior. While behavior in rodent addiction models is linked with CaMKII activity in the nucleus accumbens (NAc) shell, the key cellular adaptations that forge this link are unclear. Using a mouse strain with striatal-specific expression of autonomously active CaMKII (T286D), we demonstrate that while persistent CaMKII activity induces behaviors comparable to those in mice repeatedly exposed to psychostimulants, it is insufficient to increase AMPAR-mediated synaptic strength in NAc shell. However, autonomous CaMKII upregulates A-type K(+) current (IA) and decreases firing in shell neurons. Importantly, inactivating the transgene with doxycycline eliminates both the IA-mediated firing decrease and the elevated behavioral response to cocaine. This study identifies CaMKII regulation of IA in NAc shell neurons as a novel cellular contributor to the sensitization of cocaine reward.

Interactions between the C-terminus of Kv1.5 and Kvβ regulate pyridine nucleotide-dependent changes in channel gating

Voltage-gated potassium (Kv) channels are tetrameric assemblies of transmembrane Kv proteins with cytosolic N- and C-termini. The N-terminal domain of Kv1 proteins binds to β-subunits, but the role of the C-terminus is less clear. Therefore, we studied the role of the C-terminus in regulating Kv1.5 channel and its interactions with Kvβ-subunits. When expressed in COS-7 cells, deletion of the C-terminal domain of Kv1.5 did not affect channel gating or kinetics. Coexpression of Kv1.5 with Kvβ3 increased current inactivation, whereas Kvβ2 caused a hyperpolarizing shift in the voltage dependence of current activation. Inclusion of NADPH in the patch pipette solution accelerated the inactivation of Kv1.5-Kvβ3 currents. In contrast, NADP(+) decreased the rate and the extent of Kvβ3-induced inactivation and reversed the hyperpolarizing shift in the voltage dependence of activation induced by Kvβ2. Currents generated by Kv1.5ΔC+Kvβ3 or Kv1.5ΔC+Kvβ2 complexes did not respond to changes in intracellular pyridine nucleotide concentration, indicating that the C-terminus is required for pyridine nucleotide-dependent interactions between Kvβ and Kv1.5. A glutathione-S-transferase (GST) fusion protein containing the C-terminal peptide of Kv1.5 did not bind to apoKvβ2, but displayed higher affinity for Kvβ2:NADPH than Kvβ2:NADP(+). The GST fusion protein also precipitated Kvβ proteins from mouse brain lysates. Pull-down experiments, structural analysis and electrophysiological data indicated that a specific region of the C-terminus (Arg543-Val583) is required for Kvβ binding. These results suggest that the C-terminal domain of Kv1.5 interacts with β-subunits and that this interaction is essential for the differential regulation of Kv currents by oxidized and reduced nucleotides.

Electrophysiological, pharmacological and molecular profile of the transient outward rectifying conductance in rat sympathetic preganglionic neurons in vitro

Transient outward rectifying conductances or A-like conductances in sympathetic preganglionic neurons (SPN) are prolonged, lasting for hundreds of milliseconds to seconds and are thought to play a key role in the regulation of SPN firing frequency. Here, a multidisciplinary electrophysiological, pharmacological and molecular single-cell rt-PCR approach was used to investigate the kinetics, pharmacological profile and putative K+ channel subunits underlying the transient outward rectifying conductance expressed in SPN. SPN expressed a 4-aminopyridine (4-AP) sensitive transient outward rectification with significantly longer decay kinetics than reported for many other central neurons. The conductance and corresponding current in voltage-clamp conditions was also sensitive to the Kv4.2 and Kv4.3 blocker phrixotoxin-2 (1-10 μM) and the blocker of rapidly inactivating Kv channels, pandinotoxin-Kα (50 nM). The conductance and corresponding current was only weakly sensitive to the Kv1 channel blocker tityustoxin-Kα and insensitive to dendrotoxin I (200 nM) and the Kv3.4 channel blocker BDS-II (1 μM). Single-cell RT-PCR revealed mRNA expression for the α-subunits Kv4.1 and Kv4.3 in the majority and Kv1.5 in less than half of SPN. mRNA for accessory β-subunits was detected for Kvβ2 in all SPN with differential expression of mRNA for KChIP1, Kvβ1 and Kvβ3 and the peptidase homologue DPP6. These data together suggest that the transient outwardly rectifying conductance in SPN is mediated by members of the Kv4 subfamily (Kv4.1 and Kv4.3) in association with the β-subunit Kvβ2. Differential expression of the accessory β subunits, which may act to modulate channel density and kinetics in SPN, may underlie the prolonged and variable time-course of this conductance in these neurons.

Neuronal voltage-gated K+ (Kv) channels function in macromolecular complexes

Considerable evidence indicates that native neuronal voltage-gated K+ (Kv) currents reflect the functioning of macromolecular Kv channel complexes, composed of pore-forming (α)-subunits, cytosolic and transmembrane accessory subunits, together with regulatory and scaffolding proteins. The individual components of these macromolecular complexes appear to influence the stability, the trafficking, the localization and/or the biophysical properties of the channels. Recent studies suggest that Kv channel accessory subunits subserve multiple roles in the generation of native neuronal Kv channels. Additional recent findings suggest that Kv channel accessory subunits can respond to changes in intracellular Ca(2+) or metabolism and thereby integrate signaling pathways to regulate Kv channel expression and properties. Although studies in heterologous cells have provided important insights into the effects of accessory subunits on Kv channel expression/properties, it has become increasingly clear that experiments in neurons are required to define the physiological roles of Kv channel accessory and associated proteins. A number of technological and experimental hurdles remain that must be overcome in the design, execution and interpretation of experiments aimed at detailing the functional roles of accessory subunits and associated proteins in the generation of native neuronal Kv channels. With the increasing association of altered Kv channel functioning with neurological disorders, the potential impact of these efforts is clear.

Ancillary subunits associated with voltage-dependent K+ channels

Since the first discovery of Kvbeta-subunits more than 15 years ago, many more ancillary Kv channel subunits were characterized, for example, KChIPs, KCNEs, and BKbeta-subunits. The ancillary subunits are often integral parts of native Kv channels, which, therefore, are mostly multiprotein complexes composed of voltage-sensing and pore-forming Kvalpha-subunits and of ancillary or beta-subunits. Apparently, Kv channels need the ancillary subunits to fulfill their many different cell physiological roles. This is reflected by the large structural diversity observed with ancillary subunit structures. They range from proteins with transmembrane segments and extracellular domains to purely cytoplasmic proteins. Ancillary subunits modulate Kv channel gating but can also have a great impact on channel assembly, on channel trafficking to and from the cellular surface, and on targeting Kv channels to different cellular compartments. The importance of the role of accessory subunits is further emphasized by the number of mutations that are associated in both humans and animals with diseases like hypertension, epilepsy, arrhythmogenesis, periodic paralysis, and hypothyroidism. Interestingly, several ancillary subunits have in vitro enzymatic activity; for example, Kvbeta-subunits are oxidoreductases, or modulate enzymatic activity, i.e., KChIP3 modulates presenilin activity. Thus different modes of beta-subunit association and of functional impact on Kv channels can be delineated, making it difficult to extract common principles underlying Kvalpha- and beta-subunit interactions. We critically review present knowledge on the physiological role of ancillary Kv channel subunits and their effects on Kv channel properties.

Molecular dissection of I(A) in cortical pyramidal neurons reveals three distinct components encoded by Kv4.2, Kv4.3, and Kv1.4 alpha-subunits

The rapidly activating and inactivating voltage-gated K(+) (Kv) current, I(A), is broadly expressed in neurons and is a key regulator of action potential repolarization, repetitive firing, backpropagation (into dendrites) of action potentials, and responses to synaptic inputs. Interestingly, results from previous studies on a number of neuronal cell types, including hippocampal, cortical, and spinal neurons, suggest that macroscopic I(A) is composed of multiple components and that each component is likely encoded by distinct Kv channel alpha-subunits. The goals of the experiments presented here were to test this hypothesis and to determine the molecular identities of the Kv channel alpha-subunits that generate I(A) in cortical pyramidal neurons. Combining genetic disruption of individual Kv alpha-subunit genes with pharmacological approaches to block Kv currents selectively, the experiments here revealed that Kv1.4, Kv4.2, and Kv4.3 alpha-subunits encode distinct components of I(A) that together underlie the macroscopic I(A) in mouse (male and female) cortical pyramidal neurons. Recordings from neurons lacking both Kv4.2 and Kv4.3 (Kv4.2(-/-)/Kv4.3(-/-)) revealed that, although Kv1.4 encodes a minor component of I(A), the Kv1.4-encoded current was found in all the Kv4.2(-/-)/Kv4.3(-/-) cortical pyramidal neurons examined. Of the cortical pyramidal neurons lacking both Kv4.2 and Kv1.4, 90% expressed a Kv4.3-encoded I(A) larger in amplitude than the Kv1.4-encoded component. The experimental findings also demonstrate that the targeted deletion of the individual Kv alpha-subunits encoding components of I(A) results in electrical remodeling that is Kv alpha-subunit specific.

Tetramerization domain mutations in KCNA5 affect channel kinetics and cause abnormal trafficking patterns

The activity of voltage-gated K(+) (K(V)) channels plays an important role in regulating pulmonary artery smooth muscle cell (PASMC) contraction, proliferation, and apoptosis. The highly conserved NH(2)-terminal tetramerization domain (T1) of K(V) channels is important for proper channel assembly, association with regulatory K(V) beta-subunits, and localization of the channel to the plasma membrane. We recently reported two nonsynonymous mutations (G182R and E211D) in the KCNA5 gene of patients with idiopathic pulmonary arterial hypertension, which localize to the T1 domain of KCNA5. To study the electrophysiological properties and expression patterns of the mutants compared with the wild-type (WT) channel in vitro, we transfected HEK-293 cells with WT KCNA5, G182R, E211D, or the double mutant G182R/E211D channel. The mutants form functional channels; however, whole cell current kinetic differences between WT and mutant channels exist. Steady-state inactivation curves of the G182R and G182R/E211D channels reveal accelerated inactivation; the mutant channels inactivated at more hyperpolarized potentials compared with the WT channel. Channel protein expression was also decreased by the mutations. Compared with the WT channel, which was present in its mature glycosylated form, the mutant channels are present in greater proportion in their immature form in HEK-293 cells. Furthermore, G182R protein level is greatly reduced in COS-1 cells compared with WT. Immunostaining data support the hypothesis that, while WT protein localizes to the plasma membrane, mutant protein is mainly retained in intracellular packets. Overall, these data support a role for the T1 domain in channel kinetics as well as in KCNA5 channel subcellular localization.

The potassium channel subunit Kvbeta3 interacts with pannexin 1 and attenuates its sensitivity to changes in redox potentials

Pannexin 1 (Panx1), a member of the second gap junction protein family identified in vertebrates, appears to preferentially form non-junctional membrane channels. A candidate regulatory protein of Panx1 is the potassium channel subunit Kvbeta3, previously identified by bacterial two-hybrid strategies. Here, we report on the physical association of Panx1 with Kvbeta3 by immunoprecipitation when co-expressed in a neuroblastoma cell line (Neuro2A). Furthermore, in vivo co-expression of Panx1 and Kvbeta3 was shown to occur in murine hippocampus and cerebellum. Kvbeta3 is known to accelerate inactivation of otherwise slowly inactivating potassium channels under reducing conditions. We subsequently found that Panx1 channel currents exhibit a significant reduction when exposed to reducing agents, and that this effect is attenuated in the presence of Kvbeta3. Apparently, Kvbeta3 is involved in regulating the susceptibility of Panx1 channels to redox potential. Furthermore, the Panx1 channel blockers carbenoxolone and Probenecid were less effective in inhibiting Panx1 currents when Kvbeta3 was co-expressed. The influence of Kvbeta3 on Panx1 is the first example of modulation of Panx1 channel function(s) by interacting proteins, and suggests the physiological importance of sensing changes in redox potentials.

Structural determinants of Kvbeta1.3-induced channel inactivation: a hairpin modulated by PIP2

Inactivation of voltage-gated Kv1 channels can be altered by Kvbeta subunits, which block the ion-conducting pore to induce a rapid ('N-type') inactivation. Here, we investigate the mechanisms and structural basis of Kvbeta1.3 interaction with the pore domain of Kv1.5 channels. Inactivation induced by Kvbeta1.3 was antagonized by intracellular PIP(2). Mutations of R5 or T6 in Kvbeta1.3 enhanced Kv1.5 inactivation and markedly reduced the effects of PIP(2). R5C or T6C Kvbeta1.3 also exhibited diminished binding of PIP(2) compared with wild-type channels in an in vitro lipid-binding assay. Further, scanning mutagenesis of the N terminus of Kvbeta1.3 revealed that mutations of L2 and A3 eliminated N-type inactivation. Double-mutant cycle analysis indicates that R5 interacts with A501 and T480 of Kv1.5, residues located deep within the pore of the channel. These interactions indicate that Kvbeta1.3, in contrast to Kvbeta1.1, assumes a hairpin structure to inactivate Kv1 channels. Taken together, our findings indicate that inactivation of Kv1.5 is mediated by an equilibrium binding of the N terminus of Kvbeta1.3 between phosphoinositides (PIPs) and the inner pore region of the channel.

Kinetics of nucleotide binding to the beta-subunit (AKR6A2) of the voltage-gated potassium (Kv) channel

The beta-subunits of the voltage-gated potassium (Kv) channels modulate the kinetics and the gating of Kv channels and assists in channel trafficking and membrane localization. These proteins are members of the AKR6 family. They share a common (alpha/beta)(8) barrel structural fold and avidly bind pyridine nucleotides. Low catalytic activity has been reported for these proteins. Kinetic studies with rat Kvbeta2 revealed that the chemical step is largely responsible for the rate-limitation but nucleotide exchange could also contribute to the overall rate. Herein we report our investigations on the kinetics of cofactor exchange using nucleotide-free preparations of Kvbeta2. Kinetic traces measuring quenching of Kvbeta2 fluorescence by NADP(+) were consistent with a two-step binding mechanism which includes rapid formation of a loose enzyme:cofactor complex followed by a slow conformational rearrangement to form a tight final complex. Closing of the nucleotide enfolding loop, which in the crystal structure folds over the bound cofactor, provides the structural basis for this rearrangement. The rate of the loop opening required to release the cofactor is similar for NADPH and NADP(+) (0.9 min(-1)) and is of the same order of magnitude as the rate of the chemical step estimated previously from kinetic studies with 4-nitrobenzaldehyde (0.3-0.8 min(-1), [S.M. Tipparaju, O.A. Barski, S. Srivastava, A. Bhatnagar, Catalytic mechanism and substrate specificity of the beta-subunit of the voltage-gated potassium channel, Biochemistry 47 (2008) 8840-8854]). Binding of NADPH is accompanied by a second conformational change that might be responsible for a 4-fold higher affinity observed with the reduced cofactor and the resulting difficulty in removing bound NADPH from the protein. These data provide evidence that nucleotide exchange occurs on a seconds-to-minutes time scale and set the upper limit for the maximal possible rate of catalysis by Kvbeta2. Slow cofactor exchange is consistent with the role of the beta-subunit as a metabolic sensor implicated in tonic regulation of potassium currents.

Cumulative activation of voltage-dependent KVS-1 potassium channels

In this study, we reveal the existence of a novel use-dependent phenomenon in potassium channels, which we refer to as cumulative activation (CA). CA consists of an increase in current amplitude in response to repetitive depolarizing step pulses to the same potential. CA persists for up to 20 s and is similar to a phenomenon called "voltage-dependent facilitation" observed in some calcium channels. The KVS-1 K+ channel, which exhibits CA, is a rapidly activating and inactivating voltage-dependent potassium channel expressed in chemosensory and other neurons of Caenorhabditis elegans. It is unusual in being most closely related to the Shab (Kv2) family of potassium channels, which typically behave like delayed rectifier K+ channels in other species. The magnitude of CA depends on the frequency, voltage, and duration of the depolarizing step pulse. CA also radically changes the activation and inactivation kinetics of the channel, suggesting that the channel may undergo a physical modification in a use-dependent manner; thus, a model that closely simulates the behavior of the channel postulates the existence of two populations of channels, unmodified and modified. Use-dependent changes in the behavior of potassium channels, such as CA observed in KVS-1, could be involved in functional mechanisms of cellular plasticity such as synaptic depression that represent the cellular basis of learning and memory.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

Kv4.3 is not required for the generation of functional Ito,f channels in adult mouse ventricles

Accumulated evidence suggests that the heteromeric assembly of Kv4.2 and Kv4.3 alpha-subunits underlies the fast transient Kv current (I(to,f)) in rodent ventricles. Recent studies, however, demonstrated that the targeted deletion of Kv4.2 results in the complete elimination of I(to,f) in adult mouse ventricles, revealing an essential role for the Kv4.2 alpha-subunit in the generation of mouse ventricular I(to,f) channels. The present study was undertaken to investigate directly the functional role of Kv4.3 by examining the effects of the targeted disruption of the KCND3 (Kv4.3) locus. Mice lacking Kv4.3 (Kv4.3-/-) appear indistinguishable from wild-type control animals, and no structural or functional abnormalities were evident in Kv4.3-/- hearts. Voltage-clamp recordings revealed that functional I(to,f) channels are expressed in Kv4.3-/- ventricular myocytes, and that mean I(to,f) densities are similar to those recorded from wild-type cells. In addition, I(to,f) properties (inactivation rates, voltage dependences of inactivation and rates of recovery from inactivation) in Kv4.3-/- and wild-type mouse ventricular myocytes were indistinguishable. Quantitative RT-PCR and Western blot analyses did not reveal any measurable changes in the expression of Kv4.2 or the Kv channel interacting protein (KChIP2) in Kv4.3-/- ventricles. Taken together, the results presented here suggest that, in contrast with Kv4.2, Kv4.3 is not required for the generation of functional mouse ventricular I(to,f) channels.

Selective downregulation of the BKbeta1 subunit in diabetic arteriolar myocytes

Diabetic retinopathy is an important cause of visual loss. Functional abnormalities including vasoconstriction precede structural changes. Using the streptozotocin-model of diabetes in rats, we have identified downregulation of the beta1 subunit of the BK channel in arteriole myocytes as a possible molecular mechanism underlying these early changes. BKbeta1 mRNA levels were reduced as early as one month after induction of diabetes, and BK Ca(2+)-sensitivity and caffeine-evoked BK currents were reduced at three months. This effect appears to be selective for the regulatory subunit, as BKalpha subunit expression was not altered at the mRNA level, and voltage-activated BK currents were unaltered. No changes were seen in voltage activated Ca(2+)-current, Ca(2+)-activated Cl(-)current, or A-type voltage activated K(+)-currents. Reduced Ca(2+)-activated BK activity may promote depolarization, Ca(2+)-channel activation and increased contraction under resting conditions or in response to Ca(2+)-mobilizing agonists.

A new mode of regulation of N-type inactivation in a Caenorhabditis elegans voltage-gated potassium channel

N-type inactivation in voltage-gated K+ (Kv) channels is a widespread means to modulate neuronal excitability and signaling. Here we have shown a novel mechanism of N-type inactivation in a Caenorhabditis elegans Kv channel. The N-terminal sequence of KVS-1 contains a domain of 22 amino acids that resembles the inactivation ball in A-type channels, which is preceded by a domain of eighteen amino acids. Wild type KVS-1 currents can be described as A-type; however, their kinetics are significantly (approximately 5-fold) slower. When the putative inactivation ball is deleted, the current becomes non-inactivating. Inactivation is restored in non-inactivating channels by diffusion of the missing inactivation domain in the cytoplasm. Deletion of the domain in front of the ball speeds inactivation kinetics approximately 5-fold. We conclude that KVS-1 is the first example of a novel type of Kv channel simultaneously possessing an N-inactivating ball preceded by an N inactivation regulatory domain (NIRD) that acts to slow down inactivation through steric mechanisms.

SCAM analysis reveals a discrete region of the pore turret that modulates slow inactivation in Kv1.5

In Kv1.5, protonation of histidine 463 in the S5-P linker (turret) increases the rate of depolarization-induced inactivation and decreases the peak current amplitude. In this study, we examined how amino acid substitutions that altered the physico-chemical properties of the side chain at position 463 affected slow inactivation and then used the substituted cysteine accessibility method (SCAM) to probe the turret region (E456-P468) to determine whether residue 463 was unique in its ability to modulate the macroscopic current. Substitutions at position 463 of small, neutral (H463G and H463A) or large, charged (H463R, H463K, and H463E) side groups accelerated inactivation and induced a dependency of the current amplitude on the external potassium concentration. When cysteine substitutions were made in the distal turret (T462C-P468C), modification with either the positively charged [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET) or negatively charged sodium (2-sulfonatoethyl) methanethiosulfonate reagent irreversibly inhibited current. This inhibition could be antagonized either by the R487V mutation (homologous to T449V in Shaker) or by raising the external potassium concentration, suggesting that current inhibition by MTS reagents resulted from an enhancement of inactivation. These results imply that protonation of residue 463 does not modulate inactivation solely by an electrostatic interaction with residues near the pore mouth, as proposed by others, and that residue 463 is part of a group of residues within the Kv1.5 turret that can modulate P/C-type inactivation.

Kv1.5 is a major component underlying the A-type potassium current in retinal arteriolar smooth muscle

Little is known about the molecular characteristics of the voltage-activated K⁺ (K_v) channels that underlie the A-type K⁺ current in vascular smooth muscle cells of the systemic circulation. We investigated the molecular identity of the A-type K⁺ current in retinal arteriolar myocytes using patch-clamp techniques, RT-PCR, immunohistochemistry, and neutralizing antibody studies. The A-type K⁺ current was resistant to the actions of specific inhibitors for K_v3 and K_v4 channels but was blocked by the K_v1 antagonist correolide. No effects were observed with pharmacological agents against K_v1.1/2/3/6 and 7 channels, but the current was partially blocked by riluzole, a K_v1.4 and K_v1.5 inhibitor. The current was not altered by the removal of extracellular K⁺ but was abolished by flecainide, indicative of K_v1.5 rather than K_v1.4 channels. Transcripts encoding K_v1.5 and not K_v1.4 were identified in freshly isolated retinal arterioles. Immunofluorescence labeling confirmed a lack of K_v1.4 expression and revealed K_v1.5 to be localized to the plasma membrane of the arteriolar smooth muscle cells. Anti-K_v1.5 antibody applied intracellularly inhibited the A-type K⁺ current, whereas anti-K_v1.4 antibody had no effect. Co-expression of K_v1.5 with K_vβ1 or K_vβ3 accessory subunits is known to transform K_v1.5 currents from delayed rectifers into A-type currents. K_vβ1 mRNA expression was detected in retinal arterioles, but K_vβ3 was not observed. K_vβ1 immunofluorescence was detected on the plasma membrane of retinal arteriolar myocytes. The findings of this study suggest that K_v1.5, most likely co-assembled with K_vβ1 subunits, comprises a major component underlying the A-type K⁺ current in retinal arteriolar smooth muscle cells.

Voltage-gated potassium channels: regulation by accessory subunits

Voltage-gated potassium channels regulate cell membrane potential and excitability in neurons and other cell types. A precise control of neuronal action potential patterns underlies the basic functioning of the central and peripheral nervous system. This control relies on the adaptability of potassium channel activities. The functional diversity of potassium currents, however, far exceeds the considerable molecular diversity of this class of genes. Potassium current diversity contributes to the specificity of neuronal firing patterns and may be achieved by regulated transcription, RNA splicing, and posttranslational modifications. Another mechanism for regulation of potassium channel activity is through association with interacting proteins and accessory subunits. Here the authors highlight recent work that addresses this growing area of exploration and discuss areas of future investigation.

Transient outward K+ currents in rat dissociated subfornical organ neurones and angiotensin II effects

Although angiotensin II inhibits transient outward K+ currents (I(A)s) in subfornical organ neurones, there is no evidence concerning which Kv channels are involved. We investigated I(A)-generating Kv channels in dissociated rat subfornical organ neurones, using molecular, electrophysiological and pharmacological techniques, and studied the effects of angiotensin II. Conventional RT-PCR showed the presence of mRNAs for channels of the Kv3.4, Kv1.4 and Kv4 families, which are capable of generating I(A)s. Tetraethylammonium at 1 mm, which blocks Kv3 channel-derived currents, and blood-depressing substance-I, a Kv3.4-specific blocker, at 2 microm suppressed the I(A)-like component of whole-cell outward currents in some neurones. 4-Aminopyridine at 5 mm inhibited I(A)s in the presence of tetraethylammonium at 1 mm. Cd2+ at 300 microm shifted the activation and inactivation curves of the 4-aminopyridine-sensitive and tetraethylammonium-resistant I(A)s positively. The tetraethylammonium-resistant I(A)s showed fast and slow components during the process of recovery from inactivation, but the slow component was not seen in all neurones. The time constant of the fast recovery component was less than 200 ms, while that of the slow recovery component was around 1 s. Using single-cell RT-PCR, mRNAs for Kv4.2 and Kv4.3L were detected frequently, but those for Kv1.4 and Kv3.4 were seen only rarely. Angiotensin II at 30 nm inhibited the fast recovery component of tetraethylammonium-resistant I(A)s in many neurones. These results suggest that the fast recovery component of the tetraethylammonium-resistant I(A) in subfornical organ neurones depends upon Kv4, and that it can be modulated by angiotensin II.

Structural basis for competition between drug binding and Kvbeta 1.3 accessory subunit-induced N-type inactivation of Kv1.5 channels

Kvbeta subunits are accessory proteins that modify gating of Kv1 channels. Kvbeta1.3 subunits bind to the N termini of Kv1.5 alpha-subunits and induce fast N-type inactivation, slow the rate of deactivation, and alter the voltage dependence and kinetics of channel activation. The N terminus of a Kvbeta subunit and quaternary ammonium compounds bind to the inner pore of Kv1 channels; however, it is unknown to what extent the pore binding sites for drugs and Kvbeta subunits overlap. Here, we used site-directed Ala mutagenesis to scan residues of the Kv1.5 pore to define the binding site for Kvbeta1.3 subunits. Individual mutations of five residues in the S6 domain (Val505, Ile508, Leu510, Val512, and Val516) greatly retarded or prevented Kvbeta1.3 induced inactivation, and reduced effects on Kv1.5 deactivation. Mutation of Thr479 and Thr480 enhanced Kvbeta1.3-induced N-type inactivation. None of the Ala mutations prevented the Kvbeta1.3-induced negative shifts in the voltage dependence of activation or slow C-type inactivation, suggesting that these gating effects are mediated by an interaction other than the one for N-type inactivation. Thr479, Thr480, Val505, Ile508, and Val512, of Kv1.5 channels are also important interaction sites for the anthranilic acid S0100176 (N-benzyl-N-pyridin-3-ylmethyl-2-(toluene-4-sulfonylamino)-benzamide hydrochloride). Leu510 and V516A prevented Kvbeta1.3-induced inactivation but did not alter drug block. Block of Kv1.5 by S0100176 was reduced and voltage-dependent in the presence of Kvbeta1.3 but not in the presence of an N-truncated form of the Kvbeta subunit. Thus, residues in the pore of Kv1.5 required for N-type inactivation overlap with but are not identical to the drug binding site.

Pattern of Kv beta subunit expression in macrophages depends upon proliferation and the mode of activation

Voltage-dependent potassium channels (Kv) in leukocytes are involved in the immune response. In bone marrow-derived macrophages (BMDM), proliferation and activation induce delayed rectifier K+ currents, generated by Kv1.3, via transcriptional, translational, and posttranslational controls. Furthermore, modulatory Kv beta subunits coassociate with Kv alpha subunits, increasing channel diversity and function. In this study we have identified Kv beta subunits in mouse BMDM, studied their regulation during proliferation and activation, and analyzed K+ current parameters influenced by these proteins. BMDM express all isoforms of Kv beta1 (Kv beta1.1, Kv beta1.2, and Kv beta1.3) and Kv beta2 (Kv beta2.1), but not Kv beta4, the alternatively spliced murine Kv beta3 variant. M-CSF-dependent proliferation induced all Kv beta isoforms. However, LPS- and TNF-alpha-induced activation differentially regulated these subunits. Although LPS increased Kv beta1.3, reduced Kv beta1.2, and maintained Kv beta1.1 mRNA levels constant, TNF-alpha up-regulated Kv beta1.1, down-regulated Kv beta1.2, and left Kv beta1.3 expression unchanged. Moreover, in contrast to TNF-alpha, M-CSF- and LPS- up-regulated Kv beta2.1. K+ currents from M-CSF- and LPS-stimulated BMDM exhibited faster inactivation, whereas TNF-alpha increased tau values. Although in M-CSF-stimulated cells the half-inactivation voltage shifted to more positive potentials, the incubation with LPS and TNF-alpha resulted in a hyperpolarizing displacement similar to that in resting BMDM. Furthermore, activation time constants of K+ currents and the kinetics of the tail currents were different depending upon the mode of activation. Our results indicate that differential Kv beta expression modifies the electrical properties of Kv in BMDM, dependent upon proliferation and the mode of activation. This could determine physiologically appropriate surface channel complexes, allowing for greater flexibility in the precise regulation of the immune response.

Genetic modifiers of the Kv beta2-null phenotype in mice

Shaker-type potassium (K+) channels are composed of pore-forming alpha subunits associated with cytoplasmic beta subunits. Kv beta2 is the predominant Kv beta subunit in the mammalian nervous system, but its functions in vivo are not clear. Kv beta2-null mice have been previously characterized in our laboratory as having reduced lifespans, cold swim-induced tremors and occasional seizures, but no apparent defect in Kv alpha-subunit trafficking. To test whether strain differences might influence the severity of this phenotype, we analyzed Kv beta2-null mice in different strain backgrounds: 129/SvEv (129), C57BL/6J (B6) and two mixed B6/129 backgrounds. We found that strain differences significantly affected survival, body weight and thermoregulation in Kv beta2-null mice. B6 nulls had a more severe phenotype than 129 nulls in these measures; this dramatic difference did not reflect alterations in seizure thresholds but may relate to strain differences we observed in cerebellar Kv1.2 expression. To specifically test whether Kv beta1 is a genetic modifier of the Kv beta2-null phenotype, we generated Kv beta1.1-deficient mice by gene targeting and bred them to Kv beta2-null mice. Kv beta1.1/Kv beta2 double knockouts had significantly increased mortality compared with either single knockout but still maintained surface expression of Kv1.2, indicating that trafficking of this alpha subunit does not require either Kv beta subunit. Our results suggest that genetic differences between 129/SvEv and C57Bl/6J are key determinants of the severity of defects seen in Kv beta2-null mice and that Kv beta1.1 is a specific although not strain-dependent modifier.

Differential modulation of Kv1 channel-mediated currents by co-expression of Kvbeta3 subunit in a mammalian cell-line

The effect of Kvbeta3 subunit co-expression on currents mediated by the Shaker-related channels Kv1.1 to Kv1.6 in Chinese hamster ovary (CHO) cells was studied with patch-clamp techniques. In the presence of Kvbeta3, differences in the voltage dependence of activation for Kv1.1, Kv1.3 and Kv1.6 were detected, but not for Kv1.2- and Kv1.4-mediated currents. Co-expression of Kvbeta3 did not cause a significant increase in current density for any of the tested channels. In contrast to previous studies in Xenopus oocyte expression system, Kvbeta3 confered a rapid inactivation to all except Kv1.3 channels. Also, Kv1.6 channels that possess an N-type inactivation prevention (NIP) domain for Kvbeta1.1, inactivated rapidly when co-expressed with Kvbeta3. Onset and recovery kinetics of channel inactivation distinctly differed for the various Kv1alpha/Kvbeta3 subunit combinations investigated in this study. The results indicate that the choice of expression system may critically determine Kvbeta3 inactivating activity. This suggests that the presence of an inactivating domain and a receptor in a channel pore, although necessary, may not be sufficient for an effective rapid N-type inactivation of Kv1 channels in heterologous expression systems.

Structure and function of Kv4-family transient potassium channels

Shal-type (Kv4.x) K(+) channels are expressed in a variety of tissue, with particularly high levels in the brain and heart. These channels are the primary subunits that contribute to transient, voltage-dependent K(+) currents in the nervous system (A currents) and the heart (transient outward current). Recent studies have revealed an enormous degree of complexity in the regulation of these channels. In this review, we describe the surprisingly large number of ancillary subunits and scaffolding proteins that can interact with the primary subunits, resulting in alterations in channel trafficking and kinetic properties. Furthermore, we discuss posttranslational modification of Kv4.x channel function with an emphasis on the role of kinase modulation of these channels in regulating membrane properties. This concept is especially intriguing as Kv4.2 channels may integrate a variety of intracellular signaling cascades into a coordinated output that dynamically modulates membrane excitability. Finally, the pathophysiology that may arise from dysregulation of these channels is also reviewed.

KCNE4 is an inhibitory subunit to Kv1.1 and Kv1.3 potassium channels

Kv1 potassium channels are widely distributed in mammalian tissues and are involved in a variety of functions from controlling the firing rate of neurons to maturation of T-lymphocytes. Here we show that the newly described KCNE4 beta-subunit has a drastic inhibitory effect on currents generated by Kv1.1 and Kv1.3 potassium channels. The inhibition is found on channels expressed heterologously in both Xenopus oocytes and mammalian HEK293 cells. mKCNE4 does not inhibit Kv1.2, Kv1.4, Kv1.5, or Kv4.3 homomeric complexes, but it does significantly reduce current through Kv1.1/Kv1.2 and Kv1.2/Kv1.3 heteromeric complexes. Confocal microscopy and Western blotting reveal that Kv1.1 is present at the cell surface together with KCNE4. Real-time RT-PCR shows a relatively high presence of mKCNE4 mRNA in several organs, including uterus, kidney, lung, intestine, and in embryo, whereas a much lower mRNA level is detected in the heart and in five different parts of the brain. Having the broad distribution of Kv1 channels in mind, the demonstrated inhibitory property of KCNE4-subunits could locally and/or transiently have a dramatic influence on cellular excitability and on setting resting membrane potentials.

What are the roles of the many different types of potassium channel expressed in cerebellar granule cells?

Potassium (K) channels have a key role in the regulation of neuronal excitability. Over a hundred different subunits encoding distinct K channel subtypes have been identified so far. A major challenge is to relate these many different channel subunits to the functional K currents observed in native neurons. In this review, we have concentrated on cerebellar granule neurons (CGNs). We have considered each of the three principal super families of K channels in turn, namely, the six transmembrane domain, voltage-gated super family, the two transmembrane domain, inward-rectifier super family and the four transmembrane domain, leak channel super family. For each super family, we have identified the subunits that are expressed in CGNs and related the properties of these expressed channel subunits to the functional currents seen in electrophysiological recordings from these neurons. In some cases, there are strong molecular candidates for proteins underlying observed currents. In other cases the correlation is less clear. We show that at least 26 potassium channel alpha subunits are moderately or strongly expressed in CGNs. Nevertheless, a good empirical model of CGN function has been obtained with just six distinct K conductances. The transient KA current in CGNs, seems due to expression of Kv4.2 channels or Kv4.2/4.3 heteromers, while the KCa current is due to expression of large-conductance slo channels. The G-protein activated KIR current is probably due to heteromeric expression of KIR3.1 and KIR3.2. Perhaps KIR2.2 subunits underlie the KIR current when it is constitutively active. The leak conductance can be attributed to TASK-1 and or TASK-3 channels. With less certainty, the IK-slow current may be due to expression of one or more members of the KCNQ or EAG family. Lastly, the delayed-rectifier Kv current has as many as six different potential contributors from the extensive Kv family of alpha subunits. Since many of these subunits are highly regulated by neurotransmitters, physiological regulators and, often, auxiliary subunits, the resulting electrical properties of CGNs may be highly dynamic and subject to constant fine-tuning.

PKA modulation of Kv4.2-encoded A-type potassium channels requires formation of a supramolecular complex

A-type channels, encoded by the pore-forming alpha-subunits of the Kv4.x family, are particularly important in regulating membrane excitability in the CNS and the heart. Given the key role of modulation of A currents by kinases, we sought to investigate the protein structure-function relationships underlying the regulation of these currents by PKA. We have previously shown the existence of two PKA phosphorylation sites in the Kv4.2 sequence; therefore, we focused this study on the Kv4.2 primary subunit. In the present studies we made the surprising finding that PKA phosphorylation of the Kv4.2 alpha-subunit is necessary but not sufficient for channel modulation; channel modulation by PKA required the presence of an ancillary subunit, the K+ channel interacting protein (KChIP3). Therefore, these findings indicate a surprising complexity to kinase regulation of A currents, in that an interaction of two separate molecular events, alpha-subunit phosphorylation and the association of an ancillary subunit (KChIP3), are necessary for phosphorylation-dependent regulation of Kv4.2-encoded A channels by PKA. Overall, our studies indicate that PKA must of necessity act on a supramolecular complex of pore-forming alpha-subunits plus ancillary subunits to alter channel properties.

Loss of the potassium channel beta-subunit gene, KCNAB2, is associated with epilepsy in patients with 1p36 deletion syndrome

PURPOSE

Clinical features associated with chromosome 1p36 deletion include characteristic craniofacial abnormalities, mental retardation, and epilepsy. The presence and severity of specific phenotypic features are likely to be correlated with loss of a distinct complement of genes in each patient. We hypothesize that hemizygous deletion of one, or a few, critical gene(s) controlling neuronal excitability is associated with the epilepsy phenotype. Because ion channels are important determinants of seizure susceptibility and the voltage-gated K(+) channel beta-subunit gene, KCNAB2, has been localized to 1p36, we propose that deletion of this gene may be associated with the epilepsy phenotype.

METHODS

Twenty-four patients were evaluated by fluorescence in situ hybridization with a probe containing KCNAB2. Clinical details were obtained by neurologic examination and EEG.

RESULTS

Nine patients are deleted for the KCNAB2 locus, and eight (89%) of these have epilepsy or epileptiform activity on EEG. The majority of patients have a severe seizure phenotype, including infantile spasms. In contrast, of those not deleted for KCNAB2, only 27% have chronic seizures, and none had infantile spasms.

CONCLUSIONS

Lack of the beta subunit would be predicted to reduce K(+) channel-mediated membrane repolarization and increase neuronal excitability, suggesting a possible relation between loss of this gene and the development of seizures. Because some patients with seizures were not deleted for KCNAB2, there may be additional genes within 1p36 that contribute to epilepsy in this syndrome. Hemizygosity of this gene in a majority of monosomy 1p36 syndrome patients with epilepsy suggests that haploinsufficiency for KCNAB2 is a significant risk factor for epilepsy.

Coupling of voltage-dependent potassium channel inactivation and oxidoreductase active site of Kvbeta subunits

The accessory beta subunits of voltage-dependent potassium (Kv) channels form tetramers arranged with 4-fold rotational symmetry like the membrane-integral and pore-forming alpha subunits (Gulbis, J. M., Mann, S., and MacKinnon, R. (1999) Cell. 90, 943-952). The crystal structure of the Kvbeta2 subunit shows that Kvbeta subunits are oxidoreductase enzymes containing an active site composed of conserved catalytic residues, a nicotinamide (NADPH)-cofactor, and a substrate binding site. Also, Kvbeta subunits with an N-terminal inactivating domain like Kvbeta1.1 (Rettig, J., Heinemann, S. H., Wunder, F., Lorra, C., Parcej, D. N., Dolly, O., and Pongs, O. (1994) Nature 369, 289-294) and Kvbeta3.1 (Heinemann, S. H., Rettig, J., Graack, H. R., and Pongs, O. (1996) J. Physiol. (Lond.) 493, 625-633) confer rapid N-type inactivation to otherwise non-inactivating channels. Here we show by a combination of structural modeling and electrophysiological characterization of structure-based mutations that changes in Kvbeta oxidoreductase activity may markedly influence the gating mode of Kv channels. Amino acid substitutions of the putative catalytic residues in the Kvbeta1.1 oxidoreductase active site attenuate the inactivating activity of Kvbeta1.1 in Xenopus oocytes. Conversely, mutating the substrate binding domain and/or the cofactor binding domain rescues the failure of Kvbeta3.1 to confer rapid inactivation to Kv1.5 channels in Xenopus oocytes. We propose that Kvbeta oxidoreductase activity couples Kv channel inactivation to cellular redox regulation.