A small domain in the N terminus of the regulatory alpha-subunit Kv2. 3 modulates Kv2.1 potassium channel gating

A novel autism-associated KCNB1 mutation dramatically slows Kv2.1 potassium channel activation, deactivation and inactivation

KCNB1, on human chromosome 20q13.3, encodes the alpha subunit of the Kv2.1 voltage gated potassium channel. Kv2.1 is ubiquitously expressed throughout the brain and is critical in controlling neuronal excitability, including in the hippocampus and pyramidal neurons. Human KCNB1 mutations are known to cause global development delay or plateauing, epilepsy, and behavioral disorders. Here, we report a sibling pair with developmental delay, absence seizures, autism spectrum disorder, hypotonia, and dysmorphic features. Whole exome sequencing revealed a heterozygous variant of uncertain significance (c. 342 C>A), p. (S114R) in KCNB1, encoding a serine to arginine substitution (S114R) in the N-terminal cytoplasmic region of Kv2.1. The siblings' father demonstrated autistic features and was determined to be an obligate KCNB1 c. 342 C>A carrier based on familial genetic testing results. Functional investigation of Kv2.1-S114R using cellular electrophysiology revealed slowing of channel activation, deactivation, and inactivation, resulting in increased net current after longer membrane depolarizations. To our knowledge, this is the first study of its kind that compares the presentation of siblings each with a KCNB1 disorder. Our study demonstrates that Kv2.1-S114R has profound cellular and phenotypic consequences. Understanding the mechanisms underlying KCNB1-linked disorders aids clinicians in diagnosis and treatment and provides potential therapeutic avenues to pursue.

Kv2 channels contribute to neuronal activity within the vagal afferent-nTS reflex arc

Diversity in the functional expression of ion channels contributes to the unique patterns of activity generated in visceral sensory A-type myelinated neurons versus C-type unmyelinated neurons in response to their natural stimuli. In the present study, Kv2 channels were identified as underlying a previously uncharacterized delayed rectifying potassium current expressed in both A- and C-type nodose ganglion neurons. Kv2.1 and 2.2 appear confined to the soma and initial segment of these sensory neurons; however, neither was identified in their central presynaptic terminals projecting onto relay neurons in the nucleus of the solitary tract (nTS). Kv2.1 and Kv2.2 were also not detected in the peripheral axons and sensory terminals in the aortic arch. Functionally, in nodose neuron somas, Kv2 currents exhibited frequency-dependent current inactivation and contributed to action potential repolarization in C-type neurons but not A-type neurons. Within the nTS, the block of Kv2 currents does not influence afferent presynaptic calcium influx or glutamate release in response to afferent activation, supporting our immunohistochemical observations. On the other hand, Kv2 channels contribute to membrane hyperpolarization and limit action potential discharge rate in second-order neurons. Together, these data demonstrate that Kv2 channels influence neuronal discharge within the vagal afferent-nTS circuit and indicate they may play a significant role in viscerosensory reflex function.NEW & NOTEWORTHY We demonstrate the expression and function of the voltage-gated delayed rectifier potassium channel Kv2 in vagal nodose neurons. Within sensory neurons, Kv2 channels limit the width of the broader C-type but not narrow A-type action potential. Within the nucleus of the solitary tract (nTS), the location of the vagal terminal field, Kv2 does not influence glutamate release. However, Kv2 limits the action potential discharge of nTS relay neurons. These data suggest a critical role for Kv2 in the vagal-nTS reflex arc.

Adam, amigo, brain, and K channel

Voltage-dependent K+ (Kv) channels are diverse, comprising the classical Shab - Kv2, Shaker - Kv1, Shal - Kv4, and Shaw - Kv3 families. The Shaker family alone consists of Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5, Kv1.6, and Kv1.7. Moreover, the Shab family comprises two functional (Kv2.1 and Kv2.2) and several "silent" alpha subunits (Kv2.3, Kv5, Kv6, Kv8, and Kv9), which do not generate K current. However, e.g., Kv8.1, via heteromerization, inhibits outward currents of the same family or even that of Shaw. This property of Kv8.1 is similar to those of designated beta subunits or non-selective auxiliary elements, including ADAM or AMIGO proteins. Kv channels and, in turn, ADAM may modulate the synaptic long-term potentiation (LTP). Prevailingly, Kv1.1 and Kv1.5 are attributed to respective brain and heart pathologies, some of which may occur simultaneously. The aforementioned channel proteins are apparently involved in several brain pathologies, including schizophrenia and seizures.

Functional analysis of missense mutations in Kv8.2 causing cone dystrophy with supernormal rod electroretinogram

Mutations in KCNV2 have been proposed as the molecular basis for cone dystrophy with supernormal rod electroretinogram. KCNV2 codes for the modulatory voltage-gated potassium channel α-subunit, Kv8.2, which is incapable of forming functional channels on its own. Functional heteromeric channels are however formed with Kv2.1 in heterologous expression systems, with both α-subunit genes expressed in rod and cone photoreceptors. Of the 30 mutations identified in the KCNV2 gene, we have selected three missense mutations localized in the potassium channel pore and two missense mutations localized in the tetramerization domain for analysis. We characterized the differences between homomeric Kv2.1 and heteromeric Kv2.1/Kv8.2 channels and investigated the influence of the selected mutations on the function of heteromeric channels. We found that two pore mutations (W467G and G478R) led to the formation of nonconducting heteromeric Kv2.1/Kv8.2 channels, whereas the mutations localized in the tetramerization domain prevented heteromer generation and resulted in the formation of homomeric Kv2.1 channels only. Consequently, our study suggests the existence of two distinct molecular mechanisms involved in the disease pathology.

Electrically Silent Kv Subunits: Their Molecular and Functional Characteristics

Electrically silent voltage-gated potassium (KvS) α-subunits do not form homotetramers but heterotetramerize with Kv2 subunits, generating functional Kv2/KvS channel complexes in which the KvS subunits modulate the Kv2 current. This poses intriguing questions into the molecular mechanisms by which these KvS subunits cannot form functional homotetramers, why they only interact with Kv2 subunits, and how they modulate the Kv2 current.

Mutation of histidine 105 in the T1 domain of the potassium channel Kv2.1 disrupts heteromerization with Kv6.3 and Kv6.4

The voltage-activated K(+) channel subunit Kv2.1 can form heterotetramers with members of the Kv6 subfamily, generating channels with biophysical properties different from homomeric Kv2.1 channels. The N-terminal tetramerization domain (T1) has been shown previously to play a role in Kv channel assembly, but the mechanisms controlling specific heteromeric assembly are still unclear. In Kv6.x channels the histidine residue of the zinc ion-coordinating C3H1 motif of Kv2.1 is replaced by arginine or valine. Using a yeast two-hybrid assay, we found that substitution of the corresponding histidine 105 in Kv2.1 by valine (H105V) or arginine (H105R) disrupted the interaction of the T1 domain of Kv2.1 with the T1 domains of both Kv6.3 and Kv6.4, whereas interaction of the T1 domain of Kv2.1 with itself was unaffected by this mutation. Using fluorescence resonance energy transfer (FRET), interaction could be detected between the subunits Kv2.1/Kv2.1, Kv2.1/Kv6.3, and Kv2.1/Kv6.4. Reduced FRET signals were obtained after co-expression of Kv2.1(H105V) or Kv2.1(H105R) with Kv6.3 or Kv6.4. Wild-type Kv2.1 but not Kv2.1(H105V) could be co-immunoprecipitated with Kv6.4. Co-expression of dominant-negative mutants of Kv6.3 reduced the current produced Kv2.1, but not of Kv2.1(H105R) mutants. Co-expression of Kv6.3 or Kv6.4 with wt Kv2.1 but not with Kv2.1(H105V) or Kv2.1(H105R) changed the voltage dependence of activation of the channels. Our results suggest that His-105 in the T1 domain of Kv2.1 is required for functional heteromerization with members of the Kv6 subfamily. We conclude from our findings that Kv2.1 and Kv6.x subunits have complementary T1 domains that control selective heteromerization.

VAMP2 interacts directly with the N terminus of Kv2.1 to enhance channel inactivation

Recently, we demonstrated that the Kv2.1 channel plays a role in regulated exocytosis of dense-core vesicles (DCVs) through direct interaction of its C terminus with syntaxin 1A, a plasma membrane soluble NSF attachment receptor (SNARE) component. We report here that Kv2.1 interacts with VAMP2, the vesicular SNARE partner that is also present at high concentration in neuronal plasma membrane. This is the first report of VAMP2 interaction with an ion channel. The interaction was demonstrated in brain membranes and characterized using electrophysiological and biochemical analyses in Xenopus oocytes combined with an in vitro binding analysis and protein modeling. Comparative study performed with wild-type and mutant Kv2.1, wild-type Kv1.5, and chimeric Kv1.5N/Kv2.1 channels revealed that VAMP2 enhanced the inactivation of Kv2.1, but not of Kv1.5, via direct interaction with the T1 domain of the N terminus of Kv2.1. Given the proposed role for surface VAMP2 in the regulation of the vesicle cycle and the important role for the sustained Kv2.1 current in the regulation of dendritic calcium entry during high-frequency stimulation, the interaction of VAMP2 with Kv2.1 N terminus may contribute, alongside with the interaction of syntaxin with Kv2.1 C terminus, to the activity dependence of DCV release.

An activation gating switch in Kv1.2 is localized to a threonine residue in the S2-S3 linker

The activation properties of Kv1.2 channels are highly variable, with reported half-activation (V((1/2))) values ranging from approximately -40 mV to approximately +30 mV. Here we show that this arises because Kv1.2 channels occupy two distinct gating modes ("fast" and "slow"). "Slow" gating (tau(act) = 90 +/- 6 ms at +35 mV) was associated with a V((1/2)) of activation of +16.6 +/- 1.1 mV, whereas "fast" gating (tau(act) = 4.5 +/- 1.7 ms at +35 mV) was associated with a V((1/2)) of activation of -18.8 +/- 2.3 mV. It was possible to switch between gating modes by applying a prepulse, which suggested that channels activate to a single open state along separate "fast" and "slow" activation pathways. Using chimeras and point mutants between Kv1.2 and Kv1.5 channels, we determined that introduction of a positive charge at or around threonine 252 in the S2-S3 linker of Kv1.2 abolished "slow" activation gating. Furthermore, dialysis of the cytoplasm or excision of cell-attached patches from cells expressing Kv1.2 channels switched gating from "slow" to "fast", suggesting involvement of cytoplasmic regulators. Collectively, these results demonstrate two modes of activation gating in Kv1.2 and specific residues in the S2-S3 linker that act as a switch between these modes.

Potassium channels

Potassium channels are integral membrane proteins that selectively transport K+ across the cell membrane. They are present in all mammalian cells and have a wide variety of roles in both excitable and nonexcitable cells. The phenotypic diversity required to accomplish their various roles is created by differences in conductance, the timecourse and mechanisms of different gating events, and the interaction of channels with a variety of accessory proteins. Through the integration of biophysical, molecular, structural, and theoretical studies, significant progress has been made toward understanding the structural basis of K+ channel function, and diseases associated with K+ channel dysfunction.

Fluorescence measurements reveal stoichiometry of K+ channels formed by modulatory and delayed rectifier alpha-subunits

Modulatory alpha-subunits, which comprise one-fourth of all voltagegated K(+) channel (Kv) alpha-subunits, do not assemble into homomeric channels, but selectively associate with delayed rectifier Kv2 subunits to form heteromeric channels of unknown stoichiometry. Their distinct expression patterns and unique functional properties have made these channels candidate molecular correlates for a broad set of native K(+) currents. Here, we combine FRET and electrophysiological measurements to determine the stoichiometry and geometry of heteromeric channels composed of the delayed rectifier Kv2.1 subunit and the modulatory Kv9.3 alpha-subunit. Kv channel alpha-subunits were fused with GFP variants, and heteromerization of different combinations of tagged and untagged alpha-subunits was studied. FRET, evaluated by acceptor photobleaching, was only observed upon formation of functional channels. Our results, obtained from two independent experimental paradigms, suggest the formation of heteromeric Kv2.1/Kv9.3 channels of fixed stoichiometry consisting of three Kv2.1 subunits and one Kv9.3 subunit. Strikingly, despite this uneven stoichiometry, we find that heteromeric Kv2.1/Kv9.3 channels maintain a pseudosymmetric arrangement of subunits around the central pore.

Effects of Kv1.2 intracellular regions on activation of Kv2.1 channels

Depolarizing voltage steps activate voltage-dependent K(+) (Kv) channels by moving the voltage sensor, which triggers a coupling reaction leading to the opening of the pore. We constructed chimeric channels in which intracellular regions of slowly activating Kv2.1 channels were replaced by respective regions of rapidly activating Kv1.2 channels. Substitution of either the N-terminus, S4-S5 linker, or C-terminus generated chimeric Kv2.1/1.2 channels with a paradoxically slow and approximately exponential activation time course consisting of a fast and a slow component. Using combined chimeras, each of these Kv1.2 regions further slowed activation at the voltage of 0 mV, irrespective of the nature of the other two regions, whereas at the voltage of 40 mV both slowing and accelerating effects were observed. These results suggest voltage-dependent interactions of the three intracellular regions. This observation was quantified by double-mutant cycle analysis. It is concluded that interactions between N-terminus, S4-S5 linker, and/or C-terminus modulate the activation time course of Kv2.1 channels and that part of these interactions is voltage dependent.

Modification of Kv2.1 K+ currents by the silent Kv10 subunits

Human and rat Kv10.1a and b cDNAs encode silent K+ channel pore-forming subunits that modify the electrophysiological properties of Kv2.1. These alternatively spliced variants arise by the usage of an alternative site of splicing in exon 1 producing an 11-amino acid insertion in the linker between the first and second transmembrane domains in Kv10.1b. In human, the Kv10s mRNA were detected by Northern blot in brain kidney lung and pancreas. In brain, they were expressed in cortex, hippocampus, caudate, putamen, amygdala and weakly in substantia nigra. In rat, Kv10.1 products were detected in brain and weakly in testes. In situ hybridization in rat brain shows that Kv10.1 mRNAs are expressed in cortex, olfactory cortical structures, basal ganglia/striatal structures, hippocampus and in many nuclei of the amygdala complex. The CA3 and dentate gyrus of the hippocampus present a gradient that show a progression from high level of expression in the caudo-ventro-medial area to a weak level in the dorso-rostral area. The CA1 and CA2 areas had low levels throughout the hippocampus. Several small nuclei were also labeled in the thalamus, hypothalamus, pons, midbrain, and medulla oblongata. Co-injection of Kv2.1 and Kv10.1a or b mRNAs in Xenopus oocytes produced smaller currents that in the Kv2.1 injected oocytes and a moderate reduction of the inactivation rate without any appreciable change in recovery from inactivation or voltage dependence of activation or inactivation. At higher concentration, Kv10.1a also reduces the activation rate and a more important reduction in the inactivation rate. The gene that encodes for Kv10.1 mRNAs maps to chromosome 2p22.1 in human, 6q12 in rat and 17E4 in mouse, locations consistent with the known systeny for human, rat and mouse chromosomes.

Molecular cloning and partial functional characterization of Tsha3--a novel modulatory potassium channel alpha-subunit of trout CNS

A novel Shaker-related potassium channel subunit termed Tsha3 that is widely expressed in the CNS of trout was PCR-cloned and sequenced: its deduced amino acid sequence showed an extended N-terminal domain with a high proportion of negatively charged residues and possessed highest similarity with KCNA10, a human epithelial potassium channel. Upon heterologous expression in Sf21 cells, homomeric Tsha3 did not yield voltage-activated potassium channels but produced only ohmic currents that reversed at -15 mV. After co-expression with Tsha1, a novel outward rectifier current was generated that differed from homomeric Tsha1 by its slower kinetics of activation, its partial current inactivation, and its partial blockade by 5 mM TEA as well as 1 microM DTX. Co-immunoprecipitation studies using anti-Tsha3 antibodies confirmed that Tsha3 tightly bound with Tsha1 in co-infected Sf21 cells. As revealed from GFP- and DsRed-labeling studies, the pattern of distribution of Tsha1 was profoundly altered after co-infection with Tsha3 subunits.

Structural determinants of the regulation of the voltage-gated potassium channel Kv2.1 by the modulatory α-subunit Kv9.3

Voltage-gated potassium (Kv) channels containing alpha-subunits of the Kv2 subfamily mediate delayed rectifier currents in excitable cells. Channels formed by Kv2.1 alpha-subunits inactivate from open- and closed states with both forms of inactivation serving different physiological functions. Here we show that open- and closed-state inactivation of Kv2.1 can be distinguished by the sensitivity to intracellular tetraethylammonium and extracellular potassium and lead to the same inactivated conformation. The functional properties of Kv2.1 are regulated by its association with modulatory alpha-subunits (Kv5, Kv6, Kv8, and Kv9). For instance, Kv9.3 changes the state preference of Kv2.1 inactivation by accelerating closed-state inactivation and inhibiting open-state inactivation. An N-terminal regulatory domain (NRD) has been suggested to determine the function of the modulatory alpha-subunit Kv8.1. However, when we tested the NRD of Kv9.3, we found that the functional properties of chimeric Kv2.1 channels containing the NRD of Kv9.3 (Kv2.1(NRD)) did not resemble those of Kv2.1/Kv9.3 heteromers, thus questioning the role of the NRD in Kv9 subunits. A further region of interest is a PXP motif in the sixth transmembrane segment. This motif is conserved among all alpha-subunits of the Kv1, Kv2, Kv3, and Kv4 subfamilies, whereas the second proline is not conserved in any modulatory alpha-subunit. Exchanging this proline in Kv2.1 for the corresponding residue of Kv9.3 resulted in channels (Kv2.1-P410T) that show all hallmarks of the regulation of Kv2.1 by Kv9.3. The effect prevailed in heteromeric channels following co-expression of Kv2.1-P410T with Kv2.1. These data suggest that the alteration of the PXP motif is an important determinant of the regulatory function of modulatory alpha-subunits.

Delayed rectifier K+ currents, IK, are encoded by Kv2 alpha-subunits and regulate tonic firing in mammalian sympathetic neurons

Previous studies have revealed the presence of four kinetically distinct voltage-gated K+ currents, I(Af), I(As), I(K), and I(SS), in rat superior cervical ganglion (SCG) neurons and demonstrated that I(K) and I(SS) are expressed in all cells, whereas I(Af) and I(As) are differentially distributed. Previous studies have also revealed the presence of distinct components of I(Af) encoded by alpha-subunits of the Kv1 and Kv4 subfamilies. In the experiments described here, pore mutants of Kv2.1 (Kv2.1W365C/Y380T) and Kv2.2 (Kv2.2W373C/Y388T) that function as Kv2 subfamily-specific dominant negatives (Kv2.1DN and Kv2.2DN) were generated to probe the functional role(s) of Kv2 alpha-subunits. Expression of Kv2.1DN or Kv2.2DN in human embryonic kidney-293 cells selectively attenuates Kv2.1- or Kv2.2-encoded K+ currents, respectively. Using the Biolistics Gene Gun, cDNA constructs encoding either Kv2.1DN or Kv2.2DN [and enhanced green fluorescent protein (EGFP)] were introduced into SCG neurons. Whole-cell recordings from EGFP-positive Kv2.1DN or Kv2.2DN-expressing cells revealed selective decreases in I(K). Coexpression of Kv2.1DN and Kv2.2DN eliminates I(K) in most (75%) SCG cells and, in the remaining (25%) cells, I(K) density is reduced. Together with biochemical data revealing that Kv2.1 and Kv2.2 alpha-subunits do not associate in rat SCGs, these results suggest that Kv2.1 and Kv2.2 form distinct populations of I(K) channels, and that Kv2 alpha-subunits underlie (most of) I(K) in SCG neurons. Similar to wild-type cells, phasic, adapting, and tonic firing patterns are evident in SCG cells expressing Kv2.1DN or Kv2.2DN, although action potential durations in tonic cells are prolonged. Expression of Kv2.2DN also results in membrane depolarization, suggesting that Kv2.1- and Kv2.2-encoded I(K) channels play distinct roles in regulating the excitability of SCG neurons.

Amino-terminal determinants of U-type inactivation of voltage-gated K+ channels

The T1 domain is a cytosolic NH2-terminal domain present in all Kv (voltage-dependent potassium) channels, and is highly conserved between Kv channel subfamilies. Our characterization of a truncated form of Kv1.5 (Kv1.5deltaN209) expressed in myocardium demonstrated that deletion of the NH2 terminus of Kv1.5 imparts a U-shaped inactivation-voltage relationship to the channel, and prompted us to investigate the NH2 terminus as a regulatory site for slow inactivation of Kv channels. We examined the macroscopic inactivation properties of several NH2-terminal deletion mutants of Kv1.5 expressed in HEK 293 cells, demonstrating that deletion of residues up to the T1 boundary (Kv1.5deltaN19, Kv1.5deltaN91, and Kv1.5deltaN119) did not alter Kv1.5 inactivation, however, deletion mutants that disrupted the T1 structure consistently exhibited inactivation phenotypes resembling Kv1.5deltaN209. Chimeric constructs between Kv1.5 and the NH2 termini of Kv1.1 and Kv1.3 preserved the inactivation kinetics observed in full-length Kv1.5, again suggesting that the Kv1 T1 domain influences slow inactivation. Furthermore, disruption of intersubunit T1 contacts by mutation of residues Glu(131) and Thr(132) to alanines resulted in channels exhibiting a U-shaped inactivation-voltage relationship. Fusion of the NH2 terminus of Kv2.1 to the transmembrane segments of Kv1.5 imparted a U-shaped inactivation-voltage relationship to Kv1.5, whereas fusion of the NH2 terminus of Kv1.5 to the transmembrane core of Kv2.1 decelerated Kv2.1 inactivation and abolished the U-shaped voltage dependence of inactivation normally observed in Kv2.1. These data suggest that intersubunit T1 domain interactions influence U-type inactivation in Kv1 channels, and suggest a generalized influence of the T1 domain on U-type inactivation between Kv channel subfamilies.

Functional and molecular analysis of transient voltage-dependent K+ currents in rat hippocampal granule cells

1. We have investigated voltage-dependent outward K+ currents of dentate granule cells (DGCs) in acute brain slices from young and adult rats using nucleated and outside-out patch recordings. 2. In adult DGCs, the outward current pattern was dominated by a transient K+ current component. One portion of this current (approximately 60%) was blocked by micromolar concentrations of tetraethylammonium (TEA; IC50 42 microM) and BDS-I, a specific blocker of Kv3.4 subunits (2.5 microM). A second component was insensitive to tetraethylammonium (10 mM) and BDS-I. The transient outward current could be completely blocked by 4-aminopyridine (IC50 296 microM). 3. The TEA- and BDS-I-sensitive and the TEA-resistant current components were isolated pharmacologically. The current component that was blocked by BDS-I and TEA showed a depolarized threshold of activation (approximately -30 mV) reminiscent of Kv3.4 subunits, while the current component resistant to TEA activated at more hyperpolarized potentials (approximately -60 mV). 4. In nucleated patches obtained by placing the patch pipette adjacent to the apical dendrite, only small Na+ currents and small BDS-I-sensitive transient currents were detected. Nucleated patches obtained from either the cell soma (see above) or the axon hillock showed significantly larger amplitude Na+ currents as well as larger BDS-I-sensitive currents, indicating that this current was predominantly localized within the axosomatic compartment. This result was in good agreement with the distribution of Kv3.4 protein as determined by immunohistochemistry. 5. Current-clamp as well as mock action potential-clamp experiments revealed that the BDS-sensitive current component contributes to action potential repolarization. 6. A comparison of the two age groups (4-10 days and 60-100 days) revealed a marked developmental up-regulation of the BDS-I-sensitive component. These functional changes are paralleled by a developmental increase in Kv3.4 mRNA expression determined by quantitative real-time RT-PCR, as well as a pronounced up-regulation of Kv3.4 on the protein level determined by immunohistochemistry. 7. These functional and molecular results argue that Kv3.4 channels located predominantly in the axosomatic compartment underlie a transient K+ current in adult DGCs, and that these channels are functionally important for regulating spike repolarization. The marked developmental regulation suggests an important role of Kv3.4 in neuronal maturation.

The pore of plant K(+) channels is involved in voltage and pH sensing: domain-swapping between different K(+) channel alpha-subunits

Plant K(+) uptake channel types differ with respect to their voltage, Ca(2)+, and pH dependence. Here, we constructed recombinant chimeric channels between KST1, a member of the inward-rectifying, acid-activated KAT1 family, and AKT3, a member of the weakly voltage-dependent, proton-blocked AKT2/3 family. The homologous pore regions of AKT3 (amino acids 216 to 287) and KST1 (amino acids 217 to 289) have been exchanged to generate the two chimeric channels AKT3/(p)KST1 and KST1/(p)AKT3. In contrast to AKT3 wild-type channels, AKT3/(p)KST1 revealed a strong inward rectification reminiscent of that of KST1. Correspondingly, the substitution of the KST1 by the AKT3 pore led to less pronounced rectification properties of KST1/(p)AKT3 compared with wild-type KST1. Besides the voltage dependence, the interaction between the chimera and extracellular H(+) and Ca(2)+ resembled the properties of the inserted rather than the respective wild-type pore. Whereas AKT3/(p)KST1 was acid activated and Ca(2)+ insensitive, extracellular protons and Ca(2)+ inhibited KST1/(p)AKT3. The regulation of the chimeric channels by cytoplasmic protons followed the respective wild-type backbone of the chimeric channels, indicating that the intracellular pH sensor is located outside the P domain. We thus conclude that essential elements for external pH and Ca(2)+ regulation and for the rectification of voltage-dependent K(+) uptake channels are located within the channel pore.

The polar T1 interface is linked to conformational changes that open the voltage-gated potassium channel

Kv voltage-gated potassium channels share a cytoplasmic assembly domain, T1. Recent mutagenesis of two T1 C-terminal loop residues implicates T1 in channel gating. However, structural alterations of these mutants leave open the question concerning direct involvement of T1 in gating. We find in mammalian Kv1.2 that gating depends critically on residues at complementary T1 surfaces in an unusually polar interface. An isosteric mutation in this interface causes surprisingly little structural alteration while stabilizing the closed channel and increasing the stability of T1 tetramers. Replacing T1 with a tetrameric coiled-coil destabilizes the closed channel. Together, these data suggest that structural changes involving the buried polar T1 surfaces play a key role in the conformational changes leading to channel opening.

Differential effects of amino-terminal distal and proximal domains in the regulation of human erg K(+) channel gating

The participation of amino-terminal domains in human ether-a-go-go (eag)-related gene (HERG) K(+) channel gating was studied using deleted channel variants expressed in Xenopus oocytes. Selective deletion of the HERG-specific sequence (HERG Delta138-373) located between the conserved initial amino terminus (the eag or PAS domain) and the first transmembrane helix accelerates channel activation and shifts its voltage dependence to hyperpolarized values. However, deactivation time constants from fully activated states and channel inactivation remain almost unaltered after the deletion. The deletion effects are equally manifested in channel variants lacking inactivation. The characteristics of constructs lacking only about half of the HERG-specific domain (Delta223-373) or a short stretch of 19 residues (Delta355-373) suggest that the role of this domain is not related exclusively to its length, but also to the presence of specific sequences near the channel core. Deletion-induced effects are partially reversed by the additional elimination of the eag domain. Thus the particular combination of HERG-specific and eag domains determines two important HERG features: the slow activation essential for neuronal spike-frequency adaptation and maintenance of the cardiac action potential plateau, and the slow deactivation contributing to HERG inward rectification.

Frequency-dependent regulation of rat hippocampal somato-dendritic excitability by the K+ channel subunit Kv2.1

The voltage-dependent potassium channel subunit Kv2.1 is widely expressed throughout the mammalian CNS and is clustered primarily on the somata and proximal dendrites, but not axons, of both principal neurones and inhibitory interneurones of the cortex and hippocampus. This expression pattern suggests that Kv2.1-containing channels may play a role in the regulation of pyramidal neurone excitability. To test this hypothesis and to determine the functional role of Kv2. 1-containing channels, cultured hippocampal slices were incubated with antisense oligonucleotides directed against Kv2.1 mRNA. Western blot analysis demonstrated that Kv2.1 protein content of cultured slices decreased > 90 % following 2 weeks of treatment with antisense oligonucleotides, when compared with either control missense-treated or untreated cultures. Similarly, Kv2.1 immunostaining was selectively decreased in antisense-treated cultures. Sustained outward potassium currents, recorded in both whole-cell and outside-out patch configurations, demonstrated a selective reduction of amplitude only in antisense-treated CA1 pyramidal neurones. Under current-clamp conditions, action potential durations were identical in antisense-treated, control missense-treated and untreated slices when initiated by low frequency stimulation (0.2 Hz). In contrast, spike repolarization was progressively prolonged during higher frequencies of stimulation (1 Hz) only in cells from antisense-treated slices. Similarly, action potentials recorded during electrographic interictal activity in the 'high [K+]o' model of epilepsy demonstrated pronounced broadening of their late phase only in cells from antisense-treated slices. Consistent with the frequency-dependent spike broadening, calcium imaging experiments from single CA1 pyramidal neurones revealed that high frequency Schaffer collateral stimulation resulted in a prolonged elevation of dendritic [Ca2+]i transients only in antisense-treated neurones. These studies demonstrate that channels containing Kv2.1 play a role in regulating pyramidal neurone somato-dendritic excitability primarily during episodes of high frequency synaptic transmission.