A new K+ channel beta subunit to specifically enhance Kv2.2 (CDRK) expression

The cellular pathways that maintain the quality control and transport of diverse potassium channels

Potassium channels are multi-subunit transmembrane proteins that permit the selective passage of potassium and play fundamental roles in physiological processes, such as action potentials in the nervous system and organismal salt and water homeostasis, which is mediated by the kidney. Like all ion channels, newly translated potassium channels enter the endoplasmic reticulum (ER) and undergo the error-prone process of acquiring post-translational modifications, folding into their native conformations, assembling with other subunits, and trafficking through the secretory pathway to reach their final destinations, most commonly the plasma membrane. Disruptions in these processes can result in detrimental consequences, including various human diseases. Thus, multiple quality control checkpoints evolved to guide potassium channels through the secretory pathway and clear potentially toxic, aggregation-prone misfolded species. We will summarize current knowledge on the mechanisms underlying potassium channel quality control in the secretory pathway, highlight diseases associated with channel misfolding, and suggest potential therapeutic routes.

Kv Channel Ancillary Subunits: Where Do We Go from Here?

Voltage-gated potassium (Kv) channels each comprise four pore-forming α-subunits that orchestrate essential duties such as voltage sensing and K+ selectivity and conductance. In vivo, however, Kv channels also incorporate regulatory subunits-some Kv channel specific, others more general modifiers of protein folding, trafficking, and function. Understanding all the above is essential for a complete picture of the role of Kv channels in physiology and disease.

High spatial density is associated with non-conducting Kv channels from two families

Ion channels are well known for their ability to regulate the cell membrane potential. However, many ion channels also have functions that do not involve ion conductance. Kv2 channels are one family of ion channels whose non-conducting functions are central to mammalian cell physiology. Kv2.1 and Kv2.2 channels form stable contact sites between the endoplasmic reticulum and plasma membrane via an interaction with endoplasmic reticulum resident proteins. To perform this structural role, Kv2 channels are expressed at extremely high densities on the plasma membranes of many cell types, including central pyramidal neurons, α-motoneurons, and smooth muscle cells. Research from our lab and others has shown that the majority of these plasma membrane Kv2.1 channels do not conduct potassium in response to depolarization. The mechanism of this channel silencing is unknown but is thought to be dependent on channel density in the membrane. Furthermore, the prevalence of a non-conducting population of Kv2.2 channels has not been directly tested. In this work we make improved measurements of the numbers of conducting and non-conducting Kv2.1 channels expressed in HEK293 cells and expand the investigation of non-conducting channels to three additional Kv α-subunits: Kv2.2, Kv1.4, and Kv1.5. By comparing the numbers of gating and conducting channels in individual HEK293 cells, we found that on average, only 50% of both Kv2.1 and Kv2.2 channels conducted potassium and, as previously suggested, that fraction decreased with increased channel density in the plasma membrane. At the highest spatial densities tested, which are comparable with those found at Kv2 clusters in situ, only 20% of Kv2.1 and Kv2.2 channels conducted potassium. We also show for the first time that Kv1.4 and Kv1.5 exhibit density-dependent silencing, suggesting that this phenomenon has an underlying mechanism that is shared by Kv channels from multiple families.

Potassium channels and the development of arousal-relevant action potential trains in primary hindbrain neurons

Neurons in nucleus gigantocellularis (NGC) have been shown by many lines of evidence to be important for regulating generalized CNS arousal. Our previous study on mouse pups suggested that the development of NGC neurons' capability to fire action potential (AP) trains may both lead to the development of behavioral arousal and may itself depend on an increase in delayed rectifier currents. Here with whole-cell patch clamp we studied delayed rectifier currents in two stages. First, primary cultured neurons isolated from E12.5 embryonic hindbrain (HB), a dissection which contains all of NGC, were used to take advantage of studying neurons in vitro over using neurons in situ or in brain slices. HB neurons were tested with Guangxitoxin-1E and Resveratrol, two inhibitors of Kv2 channels which mediate the main bulk of delayed rectifier currents. Both inhibitors depressed delayed rectifier currents, but differentially: Resveratrol, but not Guangxitoxin-1E, reduced or abolished action potentials in AP trains. Since Resveratrol affects the Kv2.2 subtype, the development of the delayed rectifier mediated through Kv2.2 channels may lead to the development of HB neurons' capability to generate AP trains. Stage Two in this work found that electrophysiological properties of the primary HB neurons recorded are essentially the same as those of NGC neurons. Thus, from the two stages combined, we propose that currents mediated through Kv2.2 are crucial for generating AP trains which, in turn, lead to the development of mouse pup behavioral arousal.

Tale of tail current

The largest biomass of channel proteins is located in unicellular organisms and bacteria that have no organs. However, orchestrated bidirectional ionic currents across the cell membrane via the channels are important for the functioning of organs of organisms, and equally concern both fauna or flora. Several ion channels are activated in the course of action potentials. One of the hallmarks of voltage-dependent channels is a 'tail current' - deactivation as observed after prior and sufficient activation predominantly at more depolarized potentials e.g. for Kv while upon hyperpolarization for HCN α subunits. Tail current also reflects the timing of channel closure that is initiated upon termination of stimuli. Finally, deactivation of currents during repolarization could be a selective estimate for given channel as in case of HERG, if dedicated long and more depolarized 'tail pulse' is used. Since from a holding potential of e.g. -70 mV are often a family of outward K⁺ currents comprising I_A and I_K are simultaneously activated in native cells.

Genome-Wide Analyses of Working-Memory Ability: A Review

Working memory, a theoretical construct from the field of cognitive psychology, is crucial to everyday life. It refers to the ability to temporarily store and manipulate task-relevant information. The identification of genes for working memory might shed light on the molecular mechanisms of this important cognitive ability and-given the genetic overlap between, for example, schizophrenia risk and working-memory ability-might also reveal important candidate genes for psychiatric illness. A number of genome-wide searches for genes that influence working memory have been conducted in recent years. Interestingly, the results of those searches converge on the mediating role of neuronal excitability in working-memory performance, such that the role of each gene highlighted by genome-wide methods plays a part in ion channel formation and/or dopaminergic signaling in the brain, with either direct or indirect influence on dopamine levels in the prefrontal cortex. This result dovetails with animal models of working memory that highlight the role of dynamic network connectivity, as mediated by dopaminergic signaling, in the dorsolateral prefrontal cortex. Future work, which aims to characterize functional variants influencing working-memory ability, might choose to focus on those genes highlighted in the present review and also those networks in which the genes fall. Confirming gene associations and highlighting functional characterization of those associations might have implications for the understanding of normal variation in working-memory ability and also for the development of drugs for mental illness.

Kv2 dysfunction after peripheral axotomy enhances sensory neuron responsiveness to sustained input

Peripheral nerve injuries caused by trauma are associated with increased sensory neuron excitability and debilitating chronic pain symptoms. Axotomy-induced alterations in the function of ion channels are thought to largely underlie the pathophysiology of these phenotypes. Here, we characterise the mRNA distribution of Kv2 family members in rat dorsal root ganglia (DRG) and describe a link between Kv2 function and modulation of sensory neuron excitability. Kv2.1 and Kv2.2 were amply expressed in cells of all sizes, being particularly abundant in medium-large neurons also immunoreactive for neurofilament-200. Peripheral axotomy led to a rapid, robust and long-lasting transcriptional Kv2 downregulation in the DRG, correlated with the onset of mechanical and thermal hypersensitivity. The consequences of Kv2 loss-of-function were subsequently investigated in myelinated neurons using intracellular recordings on ex vivo DRG preparations. In naïve neurons, pharmacological Kv2.1/Kv2.2 inhibition by stromatoxin-1 (ScTx) resulted in shortening of action potential (AP) after-hyperpolarization (AHP). In contrast, ScTx application on axotomized neurons did not alter AHP duration, consistent with the injury-induced Kv2 downregulation. In accordance with a shortened AHP, ScTx treatment also reduced the refractory period and improved AP conduction to the cell soma during high frequency stimulation. These results suggest that Kv2 downregulation following traumatic nerve lesion facilitates greater fidelity of repetitive firing during prolonged input and thus normal Kv2 function is postulated to limit neuronal excitability. In summary, we have profiled Kv2 expression in sensory neurons and provide evidence for the contribution of Kv2 dysfunction in the generation of hyperexcitable phenotypes encountered in chronic pain states.

•

Kv2.1 and Kv2.2 are expressed in rat dorsal root ganglion neurons.

•

Kv2 subunits are most abundant in myelinated sensory neurons.

•

Kv2.1 and Kv.2 subunits are downregulated in a traumatic nerve injury pain model.

•

Kv2 inhibition ex vivo allows higher firing rates during sustained stimulation.

•

We conclude that Kv2 channels contribute to limiting peripheral neuron excitability.

Control of voltage-gated potassium channel Kv2.2 expression by pyruvate-isocitrate cycling regulates glucose-stimulated insulin secretion

Recent studies have shown that the pyruvate-isocitrate cycling pathway, involving the mitochondrial citrate/isocitrate carrier and the cytosolic NADP-dependent isocitrate dehydrogenase (ICDc), is involved in control of glucose-stimulated insulin secretion (GSIS). Here we demonstrate that pyruvate-isocitrate cycling regulates expression of the voltage-gated potassium channel family member Kv2.2 in islet β-cells. siRNA-mediated suppression of ICDc, citrate/isocitrate carrier, or Kv2.2 expression impaired GSIS, and the effect of ICDc knockdown was rescued by re-expression of Kv2.2. Moreover, chronic exposure of β-cells to elevated fatty acids, which impairs GSIS, resulted in decreased expression of Kv2.2. Surprisingly, knockdown of ICDc or Kv2.2 increased rather than decreased outward K(+) current in the 832/13 β-cell line. Immunoprecipitation studies demonstrated interaction of Kv2.1 and Kv2.2, and co-overexpression of the two channels reduced outward K(+) current compared with overexpression of Kv2.1 alone. Also, siRNA-mediated knockdown of ICDc enhanced the suppressive effect of the Kv2.1-selective inhibitor stromatoxin1 on K(+) currents. Our data support a model in which a key function of the pyruvate-isocitrate cycle is to maintain levels of Kv2.2 expression sufficient to allow it to serve as a negative regulator of Kv channel activity.

Ectopic expression of the K+ channel β subunits from Puccinellia tenuiflora (KPutB1) and rice (KOB1) alters K+ homeostasis of yeast and Arabidopsis

In this study, we cloned a cDNA for the K+ channel β subunit from the halophyte Puccinellia tenuiflora and named it KPutB1. KPutB1 was preferentially expressed in the roots and was transiently induced by K+-starvation, salt stress, or the combination of both stresses. By yeast two-hybrid assay, we demonstrated that KPutB1 interacts with PutAKT1, α subunit of an AKT1-type K+ channel of P. tenuiflora. The functional relevance of this interaction on K+-nutrition was investigated by co-expression experiments in yeast under various ionic conditions, and K+ channel α and β subunit homologues from rice (OsAKT1 and KOB1, respectively) were included for comparison. Yeast co-expressing PutAKT1 and the β subunits (KPutB1 and KOB1) had better growth and higher K+-uptake ability than yeast expressing PutAKT1 alone. In contrast, yeast co-expressing the β subunits (KPutB1 and KOB1) with OsAKT1 had slower growth and lower K+ uptake than yeast expressing OsAKT1 alone. Arabidopsis plants over-expressing the K+ channel β subunit of P. tenuiflora or rice showed increased shoot K+ content and decreased root Na+ content under control, 75 mM NaCl, and K+-starvation stress conditions. These results suggest that ectopic expression of the K+ channel β subunit could alter K+ and Na+ homeostasis in plants.

Trafficking of neuronal two pore domain potassium channels

The activity of two pore domain potassium (K2P) channels regulates neuronal excitability and cell firing. Post-translational regulation of K2P channel trafficking to the membrane controls the number of functional channels at the neuronal membrane affecting the functional properties of neurons. In this review, we describe the general features of K channel trafficking from the endoplasmic reticulum (ER) to the plasma membrane via the Golgi apparatus then focus on established regulatory mechanisms for K2P channel trafficking. We describe the regulation of trafficking of TASK channels from the ER or their retention within the ER and consider the competing hypotheses for the roles of the chaperone proteins 14-3-3, COP1 and p11 in these processes and where these proteins bind to TASK channels. We also describe the localisation of TREK channels to particular regions of the neuronal membrane and the involvement of the TREK channel binding partners AKAP150 and Mtap2 in this localisation. We describe the roles of other K2P channel binding partners including Arf6, EFA6 and SUMO for TWIK1 channels and Vpu for TASK1 channels. Finally, we consider the potential importance of K2P channel trafficking in a number of disease states such as neuropathic pain and cancer and the protection of neurons from ischemic damage. We suggest that a better understanding of the mechanisms and regulations that underpin the trafficking of K2P channels to the plasma membrane and to localised regions therein may considerably enhance the probability of future therapeutic advances in these areas.

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.

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.

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.

Increased functional diversity of plant K+ channels by preferential heteromerization of the shaker-like subunits AKT2 and KAT2

Assembly of plant Shaker subunits as heterotetramers, increasing channel functional diversity, has been reported. Here we focus on a new interaction, between AKT2 and KAT2 subunits. The assembly as AKT2/KAT2 heterotetramers is demonstrated by (i) a strong signal in two-hybrid tests with intracytoplasmic C-terminal regions, (ii) the effect of KAT2 on AKT2 subunit targeting in tobacco cells, (iii) the complete inhibition of AKT2 currents by co-expression with a dominant-negative KAT2 subunit in Xenopus oocytes, and reciprocally, and (iv) the appearance, upon co-expression of wild-type AKT2 and KAT2 subunits, of new channel functional properties that cannot be explained by the co-existence of two kinds of homotetrameric channels. In particular, the instantaneous current, characteristic of AKT2, displayed new functional features when compared with those of AKT2 homotetramers: activation by external acidification (instead of inhibition) and weak inhibition by calcium. Single channel current measurements in oocytes co-expressing AKT2 and KAT2 revealed a strong preference for incorporation of subunits into heteromultimers and a diversity of individual channels. In planta, these new channels, which may undergo specific regulations, are likely to be formed in guard cells and in the phloem, where they could participate in the control of membrane potential and potassium fluxes.

Analysis of voltage-gated potassium channel beta1 subunits in the porcine neonatal ductus arteriosus

The voltage-gated potassium channels (Kv) are partially responsible for the contraction/relaxation of blood vessels in response to changes in the Po(2) level. The present study determined the expression of Kvbeta1 and four oxygen-sensitive Kvalpha subunits (Kv1.2, Kv1.5, Kv2.1, and Kv9.3) in the ductus arteriosus (DA), the aorta (Ao), and the pulmonary artery (PA) in porcine neonates immediately after birth. We cloned three Kvbeta1 transcript variants (Kvbeta1.2, Kvbeta1.3, and Kvbeta1.4), Kv1.2, Kv1.5, and Kv9.3 from piglets. Three Kvbeta1 transcripts, Kv1.2, Kv1.5, and Kv9.3, encode predicted proteins of 401, 408, 202, 499, 600, and 491 residues. These Kv showed a high degree of sequence conservation with the corresponding Kv in human. Northern and quantitative real-time PCR (qr-PCR) analyses showed that Kvbeta1.2 expression was high in the DA and Ao but low in the PA. Kv1.5 expression was high in the Ao and PA but low in the DA. Expression of Kvbeta1.3, Kvbeta1.4, Kv1.2, Kv2.1, and Kv9.3 was low in these blood vessels. The inactivation property of Kvbeta1.2 against Kv1.5 was confirmed using Xenopus laevis oocytes. Our findings suggest that the molecular basis for the differential electrophysiological characteristics including opposing response to oxygen in the DA and the PA are partially due to diversity in expression of Kv1.5 and Kvbeta1.2 subunits. The high expression of Kvbeta1.2 and relatively low expression of Kv1.5 in the DA might be partially responsible for the ductal closure after birth.

Electrophysiological characterization of three non-synonymous single nucleotide polymorphisms (R87Q, A251T, and P307S) found in hKv1.5

Non-synonymous single nucleotide polymorphisms (SNPs) in the KCNA5/hKv1.5 gene, which encodes for a voltage-gated K+ channel responsible for the I (Kur) current in the human atria, have been recently reported. To gain further knowledge on potential influence of hKv1.5 SNPs, we searched for their presence in a specific population of 96 French-Canadians and characterized electrophysiological properties of the variants in two cell lines. The presumed promoter (-83 bp) and coding regions were sequenced. We found three heterozygous SNPs: R87Q, A251T, and P307S. Functional analysis of SNPs transfected in Chinese hamster ovary (CHO) cells showed that both R87Q and P307S diminished the inactivation amplitude (e.g., at +60 mV, amplitudes were 89+/-26, 23+/-4, and 22+/-7 pA/pF for the wild type, R87Q and P307S, respectively; n=8, 6, and 8, respectively). Inactivation was slowed with these variants (e.g., tau (fast) at +50 mV were 270+/-48, 490+/-66, and 340+/-45 ms for the wild type, R87Q, and P307S, respectively) while R87Q additionally accelerated the rate of hKv1.5 channel opening. A dominant-negative effect was observed for R87Q but not for P307S. SNPs properties were not reproduced when expressed in the HEK293 cell line, suggesting that the regulatory beta-subunit present in CHO cells (and the human heart) is essential for the SNPs effects that we have observed.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

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.

Auxiliary subunits of Shaker-type potassium channels

Voltage-gated potassium channels are important determinants of membrane excitability. This family of ion channels is composed of several classes of proteins, including the pore-forming Kvalpha subunits and the recently identified auxiliary Kvbeta subunits. A combination of a large number of genes that encode various alpha subunits and beta subunits and the selective formation of alpha-alpha and alpha-beta heteromultimeric channels provides rich molecular diversity that allows for regulated functional heterogeneity in both excitable tissues and nonexcitable tissues. Because the Kvbeta subunits can either upregulate or downregulate potassium currents, depending on the specific subunit combination, it is essential to understand their function at the molecular level. Furthermore, targeted changes of the Kvbeta expression or disruption of certain alpha-beta interactions could serve as a molecular basis for designing drugs and therapy to regulate excitability clinically.

Protein-protein interactions of a Kv? subunit in the cochlea

Accessory subunits associated with voltage-gated potassium (Kv) channels can influence the biophysical properties and promote the surface expression of channel-forming alpha-subunits. Previously, we cloned several alpha-subunits and a beta-subunit from a cDNA library of the chicken cochlea. In the present study, we raised an antibody against the N-terminus of chicken Kvbeta1.1 (cKvbeta1.1) and characterized the Kvbeta-related polypeptide in cochlear tissues and heterologous cells. The anti-cKvbeta1.1 antibody recognizes a 45-kDa polypeptide in chick cochlear extracts as well as in Chinese hamster ovary (CHO) cells transfected with cKvbeta1.1. The accessory subunit was localized to the ganglion cells of the chick cochlea using immunohistochemistry and in situ hybridization. Coimmunoprecipitation studies show that Kvbeta1.1 interacts with Shaker channel members Kvalpha1.2 and 1.3, both of which colocalize with beta to the cochlear ganglion cells. Additionally, coimmunoprecipitation studies show that Kvalpha1.2 and 1.3 interact with each other, suggesting that these ion channels are formed by heteromultimers. In comparison, Kvbeta did not coprecipitate with a member of the Shal subfamily. The presence of Kvbeta in the cochlea suggests that this subunit contributes to the modulation of auditory signals in the ganglion cells, presumably by regulating properties of inactivation as well as surface expression of Kvalpha channels.

Carboxyl Tail Region of the Kv2.2 Subunit Mediates Novel Developmental Regulation of Channel Density

Molecular mechanisms underlying the acquisition of stable electrical phenotypes in developing neurons remain poorly defined. As Xenopus embryonic spinal neurons mature, they initially exhibit dramatic changes in excitability due to a threefold increase in voltage-gated potassium current ( IKv) density. Later when mature neurons begin synapse formation, IKvdensity remains stable. Elevation of Kv1.1 and Kv2.1 RNA levels indicates that excess transcript levels of these Kv genes can increase current density in both young and mature neurons. In contrast, Kv2.2 overexpression increases IKvdensity in young but not mature neurons despite the presence of protein translated from injected RNA at this stage. Because protein domains can determine biophysical as well as subcellular localization properties of channel subunits, we tested whether a region of the Kv2.2 subunit regulated functional expression in mature neurons. We focused on the large cytoplasmic carboxy tail, a region that differs most between Kv2.2 and the structurally related Kv2.1 subunit. Chimeric Kv2 subunits were generated in which different regions of the large cytoplasmic carboxyl tail were exchanged between Kv2.1 and Kv2.2 subunits. All chimeric Kv2 subunits induced voltage-gated potassium currents when expressed heterologously in oocytes. In vivo chimeric subunits increased IKvdensity in young neurons on overexpression in the developing embryo. In contrast, in mature neurons, only those chimeras lacking a domain in the proximal carboxy terminus, proxC, increased IKvdensity when overexpressed. Thus the proxC domain mediates developmental and subunit-specific regulation of IKvand identifies a novel function for protein domains.

The MinK-related peptides

Voltage-gated potassium (Kv) channels mediate rapid, selective diffusion of K+ ions through the plasma membrane, controlling cell excitability, secretion and signal transduction. KCNE genes encode a family of single transmembrane domain proteins called MinK-related peptides (MiRPs) that function as ancillary or beta subunits of Kv channels. When co-expressed in heterologous systems, MiRPs confer changes in Kv channel conductance, gating kinetics and pharmacology, and are fundamental to recapitulation of the properties of some native currents. Inherited mutations in KCNE genes are associated with diseases of cardiac and skeletal muscle, and the inner ear. This article reviews our current understanding of MiRPs--their functional roles, the mechanisms underlying their association with Kv alpha subunits, their patterns of native expression and emerging evidence of the potential roles of MiRPs in the brain. The ubiquity of MiRP expression and their promiscuous association with Kv alpha subunits suggest a prominent role for MiRPs in channel dependent 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.

Functional properties of K+ currents in adult mouse ventricular myocytes

Although the K+ currents expressed in hearts of adult mice have been studied extensively, detailed information concerning their relative sizes and biophysical properties in ventricle and atrium is lacking. Here we describe and validate pharmacological and biophysical methods that can be used to isolate the three main time- and voltage-dependent outward K+ currents which modulate action potential repolarization. A Ca2+ -independent transient outward K+ current, Ito, can be separated from total outward current using an 'inactivating prepulse'. The rapidly activating, slowly inactivating delayed rectifier K+ current, IKur, can be isolated using submillimolar concentrations of 4-aminopyridine (4-AP). The remaining K+ current, Iss, can be obtained by combining these two procedures: (i) inactivating Ito and (ii) eliminating IKur by application of low concentration of 4-AP. Iss activates relatively slowly and shows very little inactivation, even during depolarizations lasting several seconds. Our findings also show that the rate of reactivation of Ito is more than 20-fold faster than that of IKur. These results demonstrate that the outward K+ currents in mouse ventricles can be separated based on their distinct time and voltage dependence, and different sensitivities to 4-AP. Data obtained at both 22 and 32 degrees C demonstrate that although the duration of the inactivating prepulse has to be adapted for the recording temperature, this approach for separation of K+ current components is also valid at more physiological temperatures. To demonstrate that these methods also allow separation of these K+ currents in other cell types, we have applied this same approach to myocytes from mouse atria. Molecular approaches have been used to compare the expression levels of different K+ channels in mouse atrium and ventricle. These findings provide new insights into the functional roles of IKur, Ito and Iss during action potential repolarization.

Modulation of delayed rectifier potassium channel by protein kinase C zeta-containing signaling complex in pheochromocytoma cells

Voltage-dependent delayed rectifier K(+) (Kv) channels are fundamental components in the regulation of neuronal excitability. We found that nerve growth factor (NGF) treatment of PC12 cells induced a hyperpolarizing shift of the Kv current activation curve by about 15 mV. This effect was similar to the effect of the modulatory subunit, Kv beta, on the cloned Kv channel, and required the activity of protein kinase C (PKC)zeta. Since NGF treatment of PC12 cells is known to increase the expression of p62 protein, which binds both to Kv beta and to PKC zeta, our results are consistent with the model in which p62 functions as a physical link in the assembly of signaling complex, PKC zeta-p62-Kv channel. In agreement with this model, the transient expression of p62 induced the same change in the Kv current activation curve as NGF, and the suppression of p62 expression inhibited the effect of NGF. The amount of bound Kv beta to p62 was increased by NGF treatment. These results suggest that the increased p62 protein induces the formation of the signaling complexes, enabling PKC zeta to modulate Kv channels. Thus, this may constitute a new way of modulating Kv channel activities.

An LQT mutant minK alters KvLQT1 trafficking

Cardiac I(Ks), the slowly activated delayed-rectifier K(+) current, is produced by the protein complex composed of alpha- and beta-subunits: KvLQT1 and minK. Mutations of genes encoding KvLQT1 and minK are responsible for the hereditary long QT syndrome (loci LQT1 and LQT5, respectively). MinK-L51H fails to traffic to the cell surface, thereby failing to produce effective I(Ks). We examined the effects that minK-L51H and an endoplasmic reticulum (ER)-targeted minK (minK-ER) exerted over the electrophysiology and biosynthesis of coexpressed KvLQT1. Both minK-L51H and minK-ER were sequestered primarily in the ER as confirmed by lack of plasma membrane expression. Glycosylation and immunofluorescence patterns of minK-L51H were qualitatively different for minK-ER, suggesting differences in trafficking. Cotransfection with the minK mutants resulted in reduced surface expression of KvLQT1 as assayed by whole cell voltage clamp and immunofluorescence. MinK-L51H reduced current amplitude by 91% compared with wild-type (WT) minK/KvLQT1, and the residual current was identical to KvLQT1 without minK. The phenotype of minK-L51H on I(Ks) was not dominant because coexpressed WT minK rescued the current and surface expression. Collectively, our data suggest that ER quality control prevents minK-L51H/KvLQT1 complexes from trafficking to the plasma membrane, resulting in decreased I(Ks). This is the first demonstration that a minK LQT mutation is capable of conferring trafficking defects onto its associated alpha-subunit.

Regulation of K+ channel activities in plants: from physiological to molecular aspects

Plant voltage-gated channels belonging to the Shaker family participate in sustained K+ transport processes at the cell and whole plant levels, such as K+ uptake from the soil solution, long-distance K+ transport in the xylem and phloem, and K+ fluxes in guard cells during stomatal movements. The attention here is focused on the regulation of these transport systems by protein-protein interactions. Clues to the identity of the regulatory mechanisms have been provided by electrophysiological approaches in planta or in heterologous systems, and through analogies with their animal counterparts. It has been shown that, like their animal homologues, plant voltage-gated channels can assemble as homo- or heterotetramers associating polypeptides encoded by different Shaker genes, and that they can bind auxiliary subunits homologous to those identified in mammals. Furthermore, several regulatory processes (involving, for example, protein kinases and phosphatases, G proteins, 14-3-3s, or syntaxins) might be common to plant and animal Shakers. However, the molecular identification of plant channel partners is still at its beginning. This paper reviews current knowledge on plant K+ channel regulation at the physiological and molecular levels, in the light of the corresponding knowledge in animal cells, and discusses perspectives for the deciphering of regulatory networks in the future.

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.

Voltage-dependent K+ channel beta subunits in muscle: differential regulation during postnatal development and myogenesis

Voltage-dependent potassium channels contribute to the electrical properties of nerve and muscle by affecting action potential shape and duration. The complexity of the currents generated is further enhanced by the presence of accessory beta subunits. Here we report that while all Kvbeta mRNA isoforms are present in rat brain, muscle tissues express only Kvbeta1 (Kvbeta1.1-Kvbeta1.3) and Kvbeta2, but not Kvbeta3. Kvbeta subunits were close regulated through post-natal development in brain and striated muscle, as well as during myogenesis in the rat skeletal muscle cell line L6E9. While the alternatively spliced Kvbeta mRNA products from Kvbeta1 gene were differentially expressed, Kvbeta2.1 was associated with myogenesis. These results show that Kvbeta genes are strongly regulated in muscle and suggest a physiological role for voltage-gated K(+) channels during development and myotube formation.

Early changes in gene expression in two models of Batten disease

Infantile and juvenile neuronal ceroid lipofuscinosis (NCLs) are progressive neurodegenerative disorders of childhood with distinct ages of clinical onset, but with a similar pathological outcome. Infantile and juvenile NCL are inherited in an autosomal recessive manner due to mutations in the CLN1 and CLN3 genes, respectively. Recently developed Cln1- and Cln3-knockout mouse models share similarities in pathology with the respective human disease. Using oligonucleotide arrays we identified reproducible changes in gene expression in the brains of both 10-week-old Cln1- and Cln3-knockout mice as compared to wild-type controls, and confirmed changes in levels of several of the cognate proteins by immunoblotting. Despite the similarities in pathology, the two mutations affect the expression of different, non-overlapping sets of genes. The possible significance of these changes and the pathological mechanisms underlying NCL diseases are discussed.

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.

KCNE4 is an inhibitory subunit to the KCNQ1 channel

KCNE4 is a membrane protein belonging to a family of single transmembrane domain proteins known to have dramatic effect on the gating of certain potassium channels. However, no functional role of KCNE4 has been suggested so far. In the present paper we demonstrate that KCNE4 is an inhibitory subunit to KCNQ1 channels. Co-expression of KCNQ1 and KCNE4 in Xenopus oocytes completely inhibited the KCNQ1 current. This was reproduced in mammalian CHO-K1 cells. Experiments with delayed expression of mRNA coding for KCNE4 in KCNQ1-expressing oocytes suggested that KCNE4 exerts its effect on KCNQ1 channels already expressed in the plasma membrane. This notion was supported by immunocytochemical studies and Western blotting, showing no significant difference in plasma membrane expression of KCNQ1 channels in the presence or absence of KCNE4. The impact of KCNE4 on KCNQ1 was specific since no effect of KCNE4 could be detected if co-expressed with KCNQ2-5 channels or hERG1 channels. RT-PCR studies revealed high KCNE4 expression in embryos and adult uterus, where significant expression of KCNQ1 channels has also been demonstrated.

Potassium channel ontogeny

Potassium channels are multi-subunit complexes, often composed of several polytopic membrane proteins and cytosolic proteins. The formation of these oligomeric structures, including both biogenesis and trafficking, is the subject of this review. The emphasis is on events in the endoplasmic reticulum (ER), particularly on how, where, and when K(+) channel polypeptides translocate and integrate into the bilayer, oligomerize and fold to form pore-forming units, and associate with auxiliary subunits to create the mature channel complex. Questions are raised with respect to the sequence of these events, when biogenic decisions are made, models for integration of K(+) channel transmembrane segments, crosstalk between the cell surface and ER, and recognition of compatible partner subunits. Also considered are determinants of subunit composition and stoichiometry, their consequence for trafficking, mechanisms for ER retention and export, and sequence motifs that direct channels to the cell surface. It is these mechanistic issues that govern the differential distributions of K(+) conductances at the cell surface, and hence the electrical activity of cells and tissues underlying both the physiology and pathophysiology of an organism.

Molecular basis of hypoxia-induced pulmonary vasoconstriction: role of voltage-gated K+ channels

The hypoxia-induced membrane depolarization and subsequent constriction of small resistance pulmonary arteries occurs, in part, via inhibition of vascular smooth muscle cell voltage-gated K+ (KV) channels open at the resting membrane potential. Pulmonary arterial smooth muscle cell KV channel expression, antibody-based dissection of the pulmonary arterial smooth muscle cell K+ current, and the O2 sensitivity of cloned KV channels expressed in heterologous expression systems have all been examined to identify the molecular components of the pulmonary arterial O2-sensitive KV current. Likely components include Kv2.1/Kv9.3 and Kv1.2/Kv1.5 heteromeric channels and the Kv3.1b alpha-subunit. Although the mechanism of KV channel inhibition by hypoxia is unknown, it appears that KV alpha-subunits do not sense O2 directly. Rather, they are most likely inhibited through interaction with an unidentified O2 sensor and/or beta-subunit. This review summarizes the role of KV channels in hypoxic pulmonary vasoconstriction, the recent progress toward the identification of KV channel subunits involved in this response, and the possible mechanisms of KV channel regulation by hypoxia.

KChAP/Kvbeta1.2 interactions and their effects on cardiac Kv channel expression

KChAP and voltage-dependent K+ (Kv) beta-subunits are two different types of cytoplasmic proteins that interact with Kv channels. KChAP acts as a chaperone for Kv2.1 and Kv4.3 channels. It also binds to Kv1.x channels but, with the exception of Kv1.3, does not increase Kv1.x currents. Kvbeta-subunits are assembled with Kv1.x channels; they exhibit "chaperone-like" behavior and change gating properties. In addition, KChAP and Kvbeta-subunits interact with each other. Here we examine the consequences of this interaction on Kv currents in Xenopus oocytes injected with different combinations of cRNAs, including Kvbeta1.2, KChAP, and either Kv1.4, Kv1.5, Kv2.1, or Kv4.3. We found that KChAP attenuated the depression of Kv1.5 currents produced by Kvbeta1.2, and Kvbeta1.2 eliminated the increase of Kv2.1 and Kv4.3 currents produced by KChAP. Both KChAP and Kvbeta1.2 are expressed in cardiomyocytes, where Kv1.5 and Kv2.1 produce sustained outward currents and Kv4.3 and Kv1.4 generate transient outward currents. Because they interact, either KChAP or Kvbeta1.2 may alter both sustained and transient cardiac Kv currents. The interaction of these two different classes of modulatory proteins may constitute a novel mechanism for regulating cardiac K+ currents.

GUARD CELL SIGNAL TRANSDUCTION

Guard cells surround stomatal pores in the epidermis of plant leaves and stems. Stomatal pore opening is essential for CO2 influx into leaves for photosynthetic carbon fixation. In exchange, plants lose over 95% of their water via transpiration to the atmosphere. Signal transduction mechanisms in guard cells integrate hormonal stimuli, light signals, water status, CO2, temperature, and other environmental conditions to modulate stomatal apertures for regulation of gas exchange and plant survival under diverse conditions. Stomatal guard cells have become a highly developed model system for characterizing early signal transduction mechanisms in plants and for elucidating how individual signaling mechanisms can interact within a network in a single cell. In this review we focus on recent advances in understanding signal transduction mechanisms in guard cells.

Kvbeta subunits increase expression of Kv4.3 channels by interacting with their C termini

Auxiliary Kvbeta subunits form complexes with Kv1 family voltage-gated K(+) channels by binding to a part of the N terminus of channel polypeptide. This association influences expression and gating of these channels. Here we show that Kv4.3 proteins are associated with Kvbeta2 subunits in the brain. Expression of Kvbeta1 or Kvbeta2 subunits does not affect Kv4.3 channel gating but increases current density and protein expression. The increase in Kv4.3 protein is larger at longer times after transfection, suggesting that Kvbeta-associated channel proteins are more stable than those without the auxiliary subunits. This association between Kv4.3 and Kvbeta subunits requires the C terminus but not the N terminus of the channel polypeptide. Thus, Kvbeta subunits utilize diverse molecular interactions to stimulate the expression of Kv channels from different families.

BK-Type K(Ca) channels in two parasympathetic cell types: differences in kinetic properties and developmental expression

The intrinsic electrical properties of identified choroid and ciliary neurons of the chick ciliary ganglion were examined by patch-clamp recording methods. These neurons are derived from a common pool of mesencephalic neural crest precursor cells but innervate different target tissues and have markedly different action potential waveforms and intrinsic patterns of repetitive spike discharge. Therefore it is important to determine whether these cell types express different types of plasma membrane ionic channels, and to ascertain the developmental stages at which these cell types begin to diverge. This study has focused on large-conductance Ca(2+)-activated K(+) channels (K(Ca)), which are known to regulate spike waveform and repetitive firing in many cell types. Both ciliary ganglion cell types, identified on the basis of size and somatostatin immunoreactivity, express a robust macroscopic K(Ca) carried by a kinetically homogeneous population of large-conductance (BK-type) K(Ca) channels. However, the kinetic properties of these channels are different in the two cell types. Steady-state fluctuation analyses of macroscopic K(Ca) produced power spectra that could be fitted with a single Lorentzian curve in both cell types. However, the resulting corner frequency was significantly lower in choroid neurons than in ciliary neurons, suggesting that the underlying K(Ca) channels have a longer mean open-time in choroid neurons. Consistent with fluctuation analyses, significantly slower gating of K(Ca) channels in choroid neurons was also observed during macroscopic activation and deactivation at membrane potentials positive to -30 mV. Differences in the kinetic properties of K(Ca) channels could also be observed directly in single-channel recordings from identified embryonic day 13 choroid and ciliary neurons. The mean open-time of large-conductance K(Ca) channels was significantly greater in choroid neurons than in ciliary neurons in excised inside-out patches. The developmental expression of functional K(Ca) channels appears to be regulated differently in the two cell types. Although both cell types acquire functional K(Ca) at the same developmental stages (embryonic days 9-13), functional expression of these channels in ciliary neurons requires target-derived trophic factors. In contrast, expression of functional K(Ca) channels proceeds normally in choroid neurons developing in vitro in the absence of target-derived trophic factors. Consistent with this, extracts of ciliary neuron target tissues (striated muscle of the iris/ciliary body) contain K(Ca) stimulatory activity. However, K(Ca) stimulatory activity cannot be detected in extracts of the smooth muscle targets of choroid neurons.

Opening and closing of KCNKO potassium leak channels is tightly regulated

Potassium-selective leak channels control neuromuscular function through effects on membrane excitability. Nonetheless, their existence as independent molecular entities was established only recently with the cloning of KCNKO from Drosophila melanogaster. Here, the operating mechanism of these 2 P domain leak channels is delineated. Single KCNKO channels switch between two long-lived states (one open and one closed) in a tenaciously regulated fashion. Activation can increase the open probability to approximately 1, and inhibition can reduce it to approximately 0.05. Gating is dictated by a 700-residue carboxy-terminal tail that controls the closed state dwell time but does not form a channel gate; its deletion (to produce a 300-residue subunit with two P domains and four transmembrane segments) yields unregulated leak channels that enter, but do not maintain, the closed state. The tail integrates simultaneous input from multiple regulatory pathways acting via protein kinases C, A, and G.

Molecular basis of functional voltage-gated K+ channel diversity in the mammalian myocardium

In the mammalian heart, Ca2+-independent, depolarization-activated potassium (K+) currents contribute importantly to shaping the waveforms of action potentials, and several distinct types of voltage-gated K+ currents that subserve this role have been characterized. In most cardiac cells, transient outward currents, Ito,f and/or Ito,s, and several components of delayed reactivation, including IKr, IKs, IKur and IK,slow, are expressed. Nevertheless, there are species, as well as cell-type and regional, differences in the expression patterns of these currents, and these differences are manifested as variations in action potential waveforms. A large number of voltage-gated K+ channel pore-forming (alpha) and accessory (beta, minK, MiRP) subunits have been cloned from or shown to be expressed in heart, and a variety of experimental approaches are being exploited in vitro and in vivo to define the relationship(s) between these subunits and functional voltage-gated cardiac K+ channels. Considerable progress has been made in defining these relationships recently, and it is now clear that distinct molecular entities underlie the various electrophysiologically distinct repolarizing K+ currents (i.e. Ito,f, Ito,s, IKr, IKs, IKur, IK,slow, etc.) in myocyardial cells.

Developmental changes in the expression of Shaker- and Shab-related K(+) channels in neurons of the rat trigeminal ganglion

We have investigated properties of voltage-gated K(+) channels in neurons of the pre- and postnatal rat trigeminal ganglion (TG). To correlate functional data with information on gene expression of Shaker- and Shab-related channels in these pseudo-unipolar neurons, the patch-clamp technique was combined with the single-cell reverse transcription-polymerase chain reaction (RT-PCR). A majority (80%) of prenatal TG neurons possessed only sustained delayed rectifier currents with half-maximal current inactivation at -30 mV. In the postnatal cells, steady-state inactivation of sustained currents occurred at more negative voltages (half-maximal inactivation at -58 mV). About 65% of the postnatal cells displayed a transient outward component in addition to the sustained currents. With increasing age, the sensitivity of sustained currents to 4-aminopyridine (4-AP) decreased significantly. The Shaker channel toxins, alpha-dendrotoxin and agitoxin-2 (50 and 10 nM), were much less effective. Discrimination between both stages with tetraethylammonium chloride (5 mM) was not possible since the currents were reduced generally by about 50%. After recording, the cell content was harvested and single-cell RT-PCR was performed to compare K(+) current properties and mRNA expression within the same cell. Most cells simultaneously expressed several different Shaker- and Shab-like transcripts. At postnatal day 14, the frequency of cells carrying transcripts encoding Kv1.1 decreased. Detailed analysis revealed a higher 4-AP sensitivity of TG neurons expressing Kv1.1 transcripts.

Evaluation of functional interaction between K(+) channel alpha- and beta-subunits and putative inactivation gating by Co-expression in Xenopus laevis oocytes

Animal K(+) channel alpha- (pore-forming) subunits form native proteins by association with beta-subunits, which are thought to affect channel function by modifying electrophysiological parameters of currents (often by inducing fast inactivation) or by stabilizing the protein complex. We evaluated the functional association of KAT1, a plant K(+) channel alpha-subunit, and KAB1 (a putative homolog of animal K(+) channel beta-subunits) by co-expression in Xenopus laevis oocytes. Oocytes expressing KAT1 displayed inward-rectifying, non-inactivating K(+) currents that were similar in magnitude to those reported in prior studies. K(+) currents recorded from oocytes expressing both KAT1 and KAB1 had similar gating kinetics. However, co-expression resulted in greater total current, consistent with the possibility that KAB1 is a beta-subunit that stabilizes and therefore enhances surface expression of K(+) channel protein complexes formed by alpha-subunits such as KAT1. K(+) channel protein complexes formed by alpha-subunits such as KAT1 that undergo (voltage-dependent) inactivation do so by means of a "ball and chain" mechanism; the ball portion of the protein complex (which can be formed by the N terminus of either an alpha- or beta-subunit) occludes the channel pore. KAT1 was co-expressed in oocytes with an animal K(+) channel alpha-subunit (hKv1.4) known to contain the N-terminal ball and chain. Inward currents through heteromeric hKv1. 4:KAT1 channels did undergo typical voltage-dependent inactivation. These results suggest that inward currents through K(+) channel proteins formed at least in part by KAT1 polypeptides are capable of inactivation, but the structural component facilitating inactivation is not present when channel complexes are formed by either KAT1 or KAB1 in the absence of additional subunits.