Cochlear Nucleus Transcriptome of a Fragile X Mouse Model Reveals Candidate Genes for Hyperacusis

OBJECTIVE

Fragile X Syndrome (FXS) is a hereditary form of autism spectrum disorder. It is caused by a trinucleotide repeat expansion in the Fmr1 gene, leading to a loss of Fragile X Protein (FMRP) expression. The loss of FMRP causes auditory hypersensitivity: FXS patients display hyperacusis and the Fmr1- knock-out (KO) mouse model for FXS exhibits auditory seizures. FMRP is strongly expressed in the cochlear nucleus and other auditory brainstem nuclei. We hypothesize that the Fmr1-KO mouse has altered gene expression in the cochlear nucleus that may contribute to auditory hypersensitivity.

METHODS

RNA was isolated from cochlear nuclei of Fmr1-KO and WT mice. Using next-generation sequencing (RNA-seq), the transcriptomes of Fmr1-KO mice and WT mice (n = 3 each) were compared and analyzed using gene ontology programs.

RESULTS

We identified 270 unique, differentially expressed genes between Fmr1-KO and WT cochlear nuclei. Upregulated genes (67%) are enriched in those encoding secreted molecules. Downregulated genes (33%) are enriched in neuronal function, including synaptic pathways, some of which are ideal candidate genes that may contribute to hyperacusis.

CONCLUSION

The loss of FMRP can affect the expression of genes in the cochlear nucleus that are important for neuronal signaling. One of these, Kcnab2, which encodes a subunit of the Shaker voltage-gated potassium channel, is expressed at an abnormally low level in the Fmr1-KO cochlear nucleus. Kcnab2 and other differentially expressed genes may represent pathways for the development of hyperacusis. Future studies will be aimed at investigating the effects of these altered genes on hyperacusis.

LEVEL OF EVIDENCE

N/A Laryngoscope, 134:1363-1371, 2024.

Molecular rearrangements in S6 during slow inactivation in Shaker-IR potassium channels

Voltage-gated K+ channels have distinct gates that regulate ion flux: the activation gate (A-gate) formed by the bundle crossing of the S6 transmembrane helices and the slow inactivation gate in the selectivity filter. These two gates are bidirectionally coupled. If coupling involves the rearrangement of the S6 transmembrane segment, then we predict state-dependent changes in the accessibility of S6 residues from the water-filled cavity of the channel with gating. To test this, we engineered cysteines, one at a time, at S6 positions A471, L472, and P473 in a T449A Shaker-IR background and determined the accessibility of these cysteines to cysteine-modifying reagents MTSET and MTSEA applied to the cytosolic surface of inside-out patches. We found that neither reagent modified either of the cysteines in the closed or the open state of the channels. On the contrary, A471C and P473C, but not L472C, were modified by MTSEA, but not by MTSET, if applied to inactivated channels with open A-gate (OI state). Our results, combined with earlier studies reporting reduced accessibility of residues I470C and V474C in the inactivated state, strongly suggest that the coupling between the A-gate and the slow inactivation gate is mediated by rearrangements in the S6 segment. The S6 rearrangements are consistent with a rigid rod-like rotation of S6 around its longitudinal axis upon inactivation. S6 rotation and changes in its environment are concomitant events in slow inactivation of Shaker KV channels.

The Ion Channel Gene KCNAB2 Is Associated with Poor Prognosis and Loss of Immune Infiltration in Lung Adenocarcinoma

The malignancy with the greatest global mortality rate is lung cancer. Lung adenocarcinoma (LUAD) is the most common subtype. The evidence demonstrated that voltage-gated potassium channel subunit beta-2 (KCNAB2) significantly participated in the initiation of colorectal cancer and its progression. However, the biological function of KCNAB2 in LUAD and its effect on the tumor immune microenvironment are still unknown. In this study, we found that the expression of KCNAB2 in tissues of patients with LUAD was markedly downregulated, and its downregulation was linked to accelerated cancer growth and poor clinical outcomes. In addition, low KCNAB2 expression was correlated with a deficiency in immune infiltration. The mechanism behind this issue might be that KCNAB2 influenced the immunological process such that the directed migration of immune cells was affected. Furthermore, overexpression of KCNAB2 in cell lines promoted the expression of CCL2, CCL3, CCL4, CCL18, CXCL9, CXCL10, and CXCL12, which are necessary for the recruitment of immune cells. In conclusion, KCNAB2 may play a key function in immune infiltration and can be exploited as a predictive biomarker for evaluating prognosis and a possible immunotherapeutic target.

Age-related changes in Kv4/Shal and Kv1/Shaker expression in Drosophila and a role for reactive oxygen species

Age-related changes in ion channel expression are likely to affect neuronal signaling. Here, we examine how age affects Kv4/Shal and Kv1/Shaker K+ channel protein levels in Drosophila. We show that Kv4/Shal protein levels decline sharply from 3 days to 10 days, then more gradually from 10 to 40 days after eclosion. In contrast, Kv1/Shaker protein exhibits a transient increase at 10 days that then stabilizes and eventually declines at 40 days. We present data that begin to show a relationship between reactive oxygen species (ROS), Kv4/Shal, and locomotor performance. We show that Kv4/Shal levels are negatively affected by ROS, and that over-expression of Catalase or RNAi knock-down of the ROS-generating enzyme, Nicotinamide Adenine Dinucleotide Phosphate (NADPH) Oxidase (NOX), can attenuate the loss of Kv4/Shal protein. Finally, we compare levels of Kv4.2 and Kv4.3 in the hippocampus, olfactory bulb, cerebellum, and motor cortex of mice aged 6 weeks and 1 year. While there was no global decline in Kv4.2/4.3 that parallels what we report in Drosophila, we did find that Kv4.2/4.3 are differentially affected in various brain regions; this survey of changes may help inform mammalian studies that examine neuronal function with age.

Genome-wide identification of soybean Shaker K+ channel gene family and functional characterization of GmAKT1 in transgenic Arabidopsis thaliana under salt and drought stress

Potassium is a major cationic nutrient involved in numerous physiological processes in plants. The uptake of K+ is mediated by K+ channels and transporters, and the Shaker K+ channel gene family plays an essential role in K+ uptake and stress resistance in plants. However, little is known regarding this family in soybean. In this study, 14 members of the Shaker K+ channel gene family were identified in soybean and were classified into five groups. Protein domain analysis revealed that Shaker K+ channel gene members have an ion transport domain (ion trans), a cyclic nucleotide-binding domain, ankyrin repeat domains, and a dimerization domain in the potassium ion channel. Quantitative real-time polymerase chain reaction analysis indicated that the expression of eight genes (notably GmAKT1) in soybean leaves and roots was significantly increased in response to salt and drought stress. Furthermore, the overexpression of GmAKT1 in Arabidopsis enhanced root length, K+ concentration, and fresh/dry weight ratio compared with wild-type plants subjected to salt and drought stress; this suggests that GmAKT1 improves the tolerance of soybean to abiotic stress. Our results provide important insight into the characterization of Shaker K+ channel gene family members in soybean and highlight the function of GmAKT1 in soybean plants under salt and drought stress.

Interrelated In Vitro Mechanisms of Sibutramine-Induced Cardiotoxicity

Consumption of illicit pharmaceutical products containing sibutramine has been reported to cause cardiovascular toxicity problems. This study aimed to demonstrate the toxicity profile of sibutramine, and thereby provide important implications for the development of more effective strategies in both clinical approaches and drug design studies. Action potentials (APs) were determined from freshly isolated ventricular cardiomyocytes with whole-cell configuration of current clamp as online. The maximum amplitude of APs (MAPs), the resting membrane potential (RMP), and AP duration from the repolarization phases were calculated from original records. The voltage-dependent K+-channel currents (IK) were recorded in the presence of external Cd2+ and both inward and outward parts of the current were calculated, while their expression levels were determined with qPCR. The levels of intracellular free Ca2+ and H+ (pHi) as well as reactive oxygen species (ROS) were measured using either a ratiometric micro-spectrofluorometer or confocal microscope. The mechanical activity of isolated hearts was observed with Langendorff-perfusion system. Acute sibutramine applications (10-8-10-5 M) induced significant alterations in both MAPs and RMP as well as the repolarization phases of APs and IK in a concentration-dependent manner. Sibutramine (10 μM) induced Ca2+-release from the sarcoplasmic reticulum under either electrical or caffeine stimulation, whereas it depressed left ventricular developed pressure with a marked decrease in the end-diastolic pressure. pHi inhibition by sibutramine supports the observed negative alterations in contractility. Changes in mRNA levels of different IK subunits are consistent with the acute inhibition of the repolarizing IK, affecting AP parameters, and provoke the cardiotoxicity.

Altered DNA methylation profiles in blood from patients with sporadic Creutzfeldt-Jakob disease

Prion diseases are fatal and transmissible neurodegenerative disorders caused by the misfolding and aggregation of prion protein. Although recent studies have implicated epigenetic variation in common neurodegenerative disorders, no study has yet explored their role in human prion diseases. Here we profiled genome-wide blood DNA methylation in the most common human prion disease, sporadic Creutzfeldt–Jakob disease (sCJD). Our case–control study (n = 219), when accounting for differences in cell type composition between individuals, identified 38 probes at genome-wide significance (p < 1.24 × 10^–7). Nine of these sites were taken forward in a replication study, performed in an independent case–control (n = 186) cohort using pyrosequencing. Sites in or close to FKBP5, AIM2 (2 probes), UHRF1, KCNAB2 successfully replicated. The blood-based DNA methylation signal was tissue- and disease-specific, in that the replicated probe signals were unchanged in case–control studies using sCJD frontal-cortex (n = 84), blood samples from patients with Alzheimer’s disease, and from inherited and acquired prion diseases. Machine learning algorithms using blood DNA methylation array profiles accurately distinguished sCJD patients and controls. Finally, we identified sites whose methylation levels associated with prolonged survival in sCJD patients. Altogether, this study has identified a peripheral DNA methylation signature of sCJD with a variety of potential biomarker applications.

Characterization of a dual-action adulticidal and larvicidal interfering RNA pesticide targeting the Shaker gene of multiple disease vector mosquitoes

The existing mosquito pesticide repertoire faces great challenges to sustainability, and new classes of pesticides are vitally needed to address established and emerging mosquito-borne infectious diseases. RNA interference- (RNAi-) based pesticides are emerging as a promising new biorational mosquito control strategy. In this investigation, we describe characterization of an interfering RNA pesticide (IRP) corresponding to the mosquito Shaker (Sh) gene, which encodes an evolutionarily conserved voltage-gated potassium channel subunit. Delivery of the IRP to Aedes aegypti adult mosquitoes in the form of siRNA that was injected or provided as an attractive toxic sugar bait (ATSB) led to Sh gene silencing that resulted in severe neural and behavioral defects and high levels of adult mortality. Likewise, when provided to A. aegypti larvae in the form of short hairpin RNA (shRNA) expressed in Saccharomyces cerevisiae (baker’s yeast) that had been formulated into a dried inactivated yeast tablet, the yeast IRP induced neural defects and larval death. Although the Sh IRP lacks a known target site in humans or other non-target organisms, conservation of the target site in the Sh genes of multiple mosquito species suggested that it may function as a biorational broad-range mosquito insecticide. In support of this, the Sh IRP induced both adult and larval mortality in treated Aedes albopictus, Anopheles gambiae, and Culex quinquefasciatus mosquitoes, but was not toxic to non-target arthropods. These studies indicated that IRPs targeting Sh could one day be used in integrated biorational mosquito control programs for the prevention of multiple mosquito-borne illnesses. The results of this investigation also suggest that the species-specificity of ATSB technology, a new paradigm for vector control, could be enhanced through the use of RNAi-based pesticides.

New classes of environmentally-safe pesticides are vitally needed to address established and emerging mosquito-borne infectious diseases. In this investigation, we describe characterization of an interfering RNA pesticide corresponding to the mosquito Shaker gene. Although the pesticide recognizes a conserved target site in the Shaker genes of multiple species of disease vector mosquitoes, it lacks a known target site in humans or other non-target organisms. The pesticide killed adult mosquitoes when it was microinjected or provided to adults as an attractive toxic sugar bait. The pesticide also induced high mortality rates when fed to larvae using a yeast-based expression and delivery system. These studies demonstrated that interfering RNA pesticides targeting the mosquito Shaker gene could one day be used for the biorational control of mosquitoes and the prevention of multiple mosquito-borne illnesses.

Functional characterization and physiological roles of the single Shaker outward K+ channel in Medicago truncatula

The model legume Medicago truncatula possesses a single outward Shaker K⁺ channel, whereas Arabidopsis thaliana possesses two channels of this type, named AtSKOR and AtGORK, with AtSKOR having been shown to play a major role in K⁺ secretion into the xylem sap in the root vasculature and with AtGORK being shown to mediate the efflux of K⁺ across the guard cell membrane, leading to stomatal closure. Here we show that the expression pattern of the single M. truncatula outward Shaker channel, which has been named MtGORK, includes the root vasculature, guard cells and root hairs. As shown by patch-clamp experiments on root hair protoplasts, besides the Shaker-type slowly activating outwardly rectifying K⁺ conductance encoded by MtGORK, a second K⁺ -permeable conductance, displaying fast activation and weak rectification, can be expressed by M. truncatula. A knock-out (KO) mutation resulting in an absence of MtGORK activity is shown to weakly reduce K⁺ translocation to shoots, and only in plants engaged in rhizobial symbiosis, but to strongly affect the control of stomatal aperture and transpirational water loss. In legumes, the early electrical signaling pathway triggered by Nod-factor perception is known to comprise a short transient depolarization of the root hair plasma membrane. In the absence of the functional expression of MtGORK, the rate of the membrane repolarization is found to be decreased by a factor of approximately two. This defect was without any consequence on infection thread development and nodule production in plants grown in vitro, but a decrease in nodule production was observed in plants grown in soil.

Distinct expression of potassium channels regulates visual response properties of lamina neurons in Drosophila melanogaster

The computational organization of sensory systems depends on the diversification of individual cell types with distinct signal-processing capabilities. The Drosophila visual system, for instance, splits information into channels with different temporal properties directly downstream of photoreceptors in the first-order interneurons of the OFF pathway, L2 and L3. However, the biophysical mechanisms that determine this specialization are largely unknown. Here, we show that the voltage-gated K_a channels Shaker and Shal contribute to the response properties of the major OFF pathway input L2. L3 calcium response kinetics postsynaptic to photoreceptors resemble the sustained calcium signals of photoreceptors, whereas L2 neurons decay transiently. Based on a cell-type-specific RNA-seq data set and endogenous protein tagging, we identified Shaker and Shal as the primary candidates to shape L2 responses. Using in vivo two-photon imaging of L2 calcium signals in combination with pharmacological and genetic perturbations of these K_a channels, we show that the wild-type Shaker and Shal function is to enhance L2 responses and cell-autonomously sharpen L2 kinetics. Our results reveal a role for K_a channels in determining the signal-processing characteristics of a specific cell type in the visual system.

The Effect of AtHKT1;1 or AtSOS1 Mutation on the Expressions of Na⁺ or K⁺ Transporter Genes and Ion Homeostasis in Arabidopsis thaliana under Salt Stress

HKT1 and SOS1 are two key Na⁺ transporters that modulate salt tolerance in plants. Although much is known about the respective functions of HKT1 and SOS1 under salt conditions, few studies have examined the effects of HKT1 and SOS1 mutations on the expression of other important Na⁺ and K⁺ transporter genes. This study investigated the physiological parameters and expression profiles of AtHKT1;1, AtSOS1, AtHAK5, AtAKT1, AtSKOR, AtNHX1, and AtAVP1 in wild-type (WT) and athkt1;1 and atsos1 mutants of Arabidopsis thaliana under 25 mM NaCl. We found that AtSOS1 mutation induced a significant decrease in transcripts of AtHKT1;1 (by 56⁻62% at 6⁻24 h), AtSKOR (by 36⁻78% at 6⁻24 h), and AtAKT1 (by 31⁻53% at 6⁻24 h) in the roots compared with WT. This led to an increase in Na⁺ accumulation in the roots, a decrease in K⁺ uptake and transportation, and finally resulted in suppression of plant growth. AtHKT1;1 loss induced a 39⁻76% (6⁻24 h) decrease and a 27⁻32% (6⁻24 h) increase in transcripts of AtSKOR and AtHAK5, respectively, in the roots compared with WT. At the same time, 25 mM NaCl decreased the net selective transport capacity for K⁺ over Na⁺ by 92% in the athkt1;1 roots compared with the WT roots. Consequently, Na⁺ was loaded into the xylem and delivered to the shoots, whereas K⁺ transport was restricted. The results indicate that AtHKT1;1 and AtSOS1 not only mediate Na⁺ transport but also control ion uptake and the spatial distribution of Na⁺ and K⁺ by cooperatively regulating the expression levels of relevant Na⁺ and K⁺ transporter genes, ultimately regulating plant growth under salt stress.

Constitutive expression of CmSKOR, an outward K+ channel gene from melon, in Arabidopsis thaliana involved in saline tolerance

Shaker-like K⁺ outward rectifying channel (SKOR) is involved in mediating long-distance K⁺ transport from roots to shoots. In this study, a Shaker-like outward K⁺ channel gene CmSKOR (GenBank accession number MF447462) was isolated from melon (Cucumis melo L.). Phylogenetic analysis showed that CmSKOR belongs to the SKOR-subfamily in the Shaker-like K⁺ channel family. Electrophysiological experiments indicated that CmSKOR was a K⁺-permeable channel with low affinity. Expressed in Xenopus oocytes, CmSKOR displayed classical Shaker-like outwardly rectifying K⁺ currents. Confocal imaging of a CmSKOR - yellow fluorescent fusion protein (YFP) in transgenic Nicotiana tabacum leaves indicated that CmSKOR was located in the plasma membrane. Transcript analysis showed CmSKOR predominantly expressed in melon roots and with lower abundance in stem and leaves. In addition, both external K⁺ and NaCl treatment could up-regulate the expression of CmSKOR in melon and enhance the K⁺ content in shoot. Constitutive overexpressed CmSKOR in Arabidopsis thaliana, the transgenic plants showed changes in root length in MS plates, displayed higher maximum photochemical efficiency of PSII (Fv/Fm), higher fresh and dry weight, and accumulation of K⁺ in shoot, together with the changes of transcript amount of CmSKOR with NaCl treatments in mixture substrate. In conclusion, it was proposed that CmSKOR may play the role on distributing K⁺ to the shoot in melon and its constitutive expression in Arabidopsis improved saline tolerance by maintaining K⁺ homeostasis in the plant.

Paternal age effects on sperm FOXK1 and KCNA7 methylation and transmission into the next generation

Children of older fathers carry an increased risk for developing autism and other disorders. To elucidate the underlying mechanisms, we investigated the correlation of sperm DNA methylation with paternal age and its impact on the epigenome of the offspring. Methylation levels of nine candidate genes and LINE-1 repeats were quantified by bisulfite pyrosequencing in sperm DNA of 162 donors and 191 cord blood samples of resulting children (conceived by IVF/ICSI with the same sperm samples). Four genes showed a significant negative correlation between sperm methylation and paternal age. For FOXK1 and KCNA7, the age effect on the sperm epigenome was replicated in an independent cohort of 188 sperm samples. For FOXK1, paternal age also significantly correlated with foetal cord blood (FCB) methylation. Deep bisulfite sequencing and allele-specific pyrosequencing allowed us to distinguish between maternal and paternal alleles in FCB samples with an informative SNP. FCB methylation of the paternal FOXK1 allele was negatively correlated with paternal age, whereas maternal allele was unaffected by maternal age. Since FOXK1 duplication has been associated with autism, we studied blood FOXK1 methylation in 74 children with autism and 41 age-matched controls. The FOXK1 promoter showed a trend for accelerated demethylation in the autism group. Dual luciferase reporter assay revealed that FOXK1 methylation influences gene expression. Collectively, our study demonstrates that age-related DNA methylation changes in sperm can be transmitted to the next generation and may contribute to the increased disease risk in offspring of older fathers.

Nitrate-Dependent Control of Shoot K Homeostasis by the Nitrate Transporter1/Peptide Transporter Family Member NPF7.3/NRT1.5 and the Stelar K+ Outward Rectifier SKOR in Arabidopsis

Root-to-shoot translocation and shoot homeostasis of potassium (K) determine nutrient balance, growth, and stress tolerance of vascular plants. To maintain the cation-anion balance, xylem loading of K(+) in the roots relies on the concomitant loading of counteranions, like nitrate (NO3 (-)). However, the coregulation of these loading steps is unclear. Here, we show that the bidirectional, low-affinity Nitrate Transporter1 (NRT1)/Peptide Transporter (PTR) family member NPF7.3/NRT1.5 is important for the NO3 (-)-dependent K(+) translocation in Arabidopsis (Arabidopsis thaliana). Lack of NPF7.3/NRT1.5 resulted in K deficiency in shoots under low NO3 (-) nutrition, whereas the root elemental composition was unchanged. Gene expression data corroborated K deficiency in the nrt1.5-5 shoot, whereas the root responded with a differential expression of genes involved in cation-anion balance. A grafting experiment confirmed that the presence of NPF7.3/NRT1.5 in the root is a prerequisite for proper root-to-shoot translocation of K(+) under low NO3 (-) supply. Because the depolarization-activated Stelar K(+) Outward Rectifier (SKOR) has previously been described as a major contributor for root-to-shoot translocation of K(+) in Arabidopsis, we addressed the hypothesis that NPF7.3/NRT1.5-mediated NO3 (-) translocation might affect xylem loading and root-to-shoot K(+) translocation through SKOR. Indeed, growth of nrt1.5-5 and skor-2 single and double mutants under different K/NO3 (-) regimes revealed that both proteins contribute to K(+) translocation from root to shoot. SKOR activity dominates under high NO3 (-) and low K(+) supply, whereas NPF7.3/NRT1.5 is required under low NO3 (-) availability. This study unravels nutritional conditions as a critical factor for the joint activity of SKOR and NPF7.3/NRT1.5 for shoot K homeostasis.

Deep conservation of genes required for both Drosphila melanogaster and Caenorhabditis elegans sleep includes a role for dopaminergic signaling

OBJECTIVES

Cross-species conservation of sleep-like behaviors predicts the presence of conserved molecular mechanisms underlying sleep. However, limited experimental evidence of conservation exists. Here, this prediction is tested directly.

MEASUREMENTS AND RESULTS

During lethargus, Caenorhabditis elegans spontaneously sleep in short bouts that are interspersed with bouts of spontaneous locomotion. We identified 26 genes required for Drosophila melanogaster sleep. Twenty orthologous C. elegans genes were selected based on similarity. Their effect on C. elegans sleep and arousal during the last larval lethargus was assessed. The 20 most similar genes altered both the quantity of sleep and arousal thresholds. In 18 cases, the direction of change was concordant with Drosophila studies published previously. Additionally, we delineated a conserved genetic pathway by which dopamine regulates sleep and arousal. In C. elegans neurons, G-alpha S, adenylyl cyclase, and protein kinase A act downstream of D1 dopamine receptors to regulate these behaviors. Finally, a quantitative analysis of genes examined herein revealed that C. elegans arousal thresholds were directly correlated with amount of sleep during lethargus. However, bout duration varies little and was not correlated with arousal thresholds.

CONCLUSIONS

The comprehensive analysis presented here suggests that conserved genes and pathways are required for sleep in invertebrates and, likely, across the entire animal kingdom. The genetic pathway delineated in this study implicates G-alpha S and previously known genes downstream of dopamine signaling in sleep. Quantitative analysis of various components of quiescence suggests that interdependent or identical cellular and molecular mechanisms are likely to regulate both arousal and sleep entry.

Differential expression of the Kv1 voltage-gated potassium channel family in the rat nephron

Several potassium (K(+)) channels contribute to maintaining the resting membrane potential of renal epithelial cells. Apart from buffering the cell membrane potential and cell volume, K(+) channels allow sodium reabsorption in the proximal tubule (PT), K(+) recycling and K(+) reabsorption in the thick ascending limb (TAL) and K(+) secretion and K(+) reabsorption in the distal convoluted tubule (DCT), connecting tubule (CNT) and collecting duct. Previously, we identified Kv.1.1, Kv1.3 and Kv1.6 channels in collecting ducts of the rat inner medulla. We also detected intracellular Kv1.3 channel in the acid secretory intercalated cells, which is trafficked to the apical membrane in response to dietary K(+) to function as a secretory K(+) channel. In this work we sought to characterize the expression of all members of the Kv1 family in the rat nephron. mRNA and protein expression were detected for all Kv1 channels. Immunoblots identified differential expression of each Kv1 in the cortex, outer and inner medulla. Immunofluorescence labeling detected Kv1.5 in Bowman´s capsule and endothelial cells and Kv1.7 in podocytes, endothelial cells and macula densa in glomeruli; Kv1.4, Kv1.5 and Kv1.7 in PT; Kv1.2, Kv1.4 and Kv1.6 in TAL; Kv1.1, Kv1.4 and Kv1.6 in DCT and CNT and Kv1.3 in DCT, and all the Kv1 family in the cortical and medullary collecting ducts. Recently, some hereditary renal syndromes have been attributed to mutations in K(+) channels. Our results expand the repertoire of K(+) channels that contribute to K(+) homeostasis to include the Kv1 family.

Inter-subunit interactions across the upper voltage sensing-pore domain interface contribute to the concerted pore opening transition of Kv channels

The tight electro-mechanical coupling between the voltage-sensing and pore domains of Kv channels lies at the heart of their fundamental roles in electrical signaling. Structural data have identified two voltage sensor pore inter-domain interaction surfaces, thus providing a framework to explain the molecular basis for the tight coupling of these domains. While the contribution of the intra-subunit lower domain interface to the electro-mechanical coupling that underlies channel opening is relatively well understood, the contribution of the inter-subunit upper interface to channel gating is not yet clear. Relying on energy perturbation and thermodynamic coupling analyses of tandem-dimeric Shaker Kv channels, we show that mutation of upper interface residues from both sides of the voltage sensor-pore domain interface stabilizes the closed channel state. These mutations, however, do not affect slow inactivation gating. We, moreover, find that upper interface residues form a network of state-dependent interactions that stabilize the open channel state. Finally, we note that the observed residue interaction network does not change during slow inactivation gating. The upper voltage sensing-pore interaction surface thus only undergoes conformational rearrangements during channel activation gating. We suggest that inter-subunit interactions across the upper domain interface mediate allosteric communication between channel subunits that contributes to the concerted nature of the late pore opening transition of Kv channels.

Genetic and anatomical basis of the barrier separating wakefulness and anesthetic-induced unresponsiveness

A robust, bistable switch regulates the fluctuations between wakefulness and natural sleep as well as those between wakefulness and anesthetic-induced unresponsiveness. We previously provided experimental evidence for the existence of a behavioral barrier to transitions between these states of arousal, which we call neural inertia. Here we show that neural inertia is controlled by processes that contribute to sleep homeostasis and requires four genes involved in electrical excitability: Sh, sss, na and unc79. Although loss of function mutations in these genes can increase or decrease sensitivity to anesthesia induction, surprisingly, they all collapse neural inertia. These effects are genetically selective: neural inertia is not perturbed by loss-of-function mutations in all genes required for the sleep/wake cycle. These effects are also anatomically selective: sss acts in different neurons to influence arousal-promoting and arousal-suppressing processes underlying neural inertia. Supporting the idea that anesthesia and sleep share some, but not all, genetic and anatomical arousal-regulating pathways, we demonstrate that increasing homeostatic sleep drive widens the neural inertial barrier. We propose that processes selectively contributing to sleep homeostasis and neural inertia may be impaired in pathophysiological conditions such as coma and persistent vegetative states.

An annual 234 million surgical procedures are performed worldwide, making general anesthetics among the most common drugs administered to humans. Remarkably, however, we still do not understand the mechanisms by which general anesthetics render patients unconscious or the processes that re-establish consciousness upon emergence from anesthesia. We previously showed that the brain resists transitions between the wakeful and anesthesia states by generating a barrier to such transitions in both directions. We also showed that the existence of this barrier is conserved from invertebrates to mammals. In our present work, we use the genetic tractability and the simplified nervous system of the fruit fly Drosophila melanogaster to show that four genes are required to maintain this barrier. We also show that, as in mammals, there is overlap between pathways regulating natural sleep and general anesthesia. We propose that some of these shared pathways are impaired in conditions such as coma and persistent vegetative states, in which the barrier to transitioning to the waking state appears to be insurmountable.

Conserved N-terminal negative charges support optimally efficient N-type inactivation of Kv1 channels

N-type inactivation is produced by the binding of a potassium channel's N-terminus within the open pore, blocking conductance. Previous studies have found that introduction of negative charges into N-terminal inactivation domains disrupts inactivation; however, the Aplysia AKv1 N-type inactivation domain contains two negatively charged residues, E2 and E9. Rather than being unusual, sequence analysis shows that this N-terminal motif is highly conserved among Kv1 sequences across many phyla. Conservation analysis shows some tolerance at position 9 for other charged residues, like D9 and K9, whereas position 2 is highly conserved as E2. To examine the functional importance of these residues, site directed mutagenesis was performed and effects on inactivation were recorded by two electrode voltage clamp in Xenopus oocytes. We find that inclusion of charged residues at positions 2 and 9 prevents interactions with non-polar sites along the inactivation pathway increasing the efficiency of pore block. In addition, E2 appears to have additional specific electrostatic interactions that stabilize the inactivated state likely explaining its high level of conservation. One possible explanation for E2's unique importance, consistent with our data, is that E2 interacts electrostatically with a positive charge on the N-terminal amino group to stabilize the inactivation domain at the block site deep within the pore. Simple electrostatic modeling suggests that due to the non-polar environment in the pore in the blocked state, even a 1 Å larger separation between these charges, produced by the E2D substitution, would be sufficient to explain the 65× reduced affinity of the E2D N-terminus for the pore. Finally, our studies support a multi-step, multi-site N-type inactivation model where the N-terminus interacts deep within the pore in an extended like structure placing the most N-terminal residues 35% of the way across the electric field in the pore blocked state.

Ca2+-dependent protein kinase11 and 24 modulate the activity of the inward rectifying K+ channels in Arabidopsis pollen tubes

Potassium (K(+)) influx into pollen tubes via K(+) transporters is essential for pollen tube growth; however, the mechanism by which K(+) transporters are regulated in pollen tubes remains unknown. Here, we report that Arabidopsis thaliana Ca(2+)-dependent protein kinase11 (CPK11) and CPK24 are involved in Ca(2+)-dependent regulation of the inward K(+) (K(+)in) channels in pollen tubes. Using patch-clamp analysis, we demonstrated that K(+)in currents of pollen tube protoplasts were inhibited by elevated [Ca(2+)]cyt. However, disruption of CPK11 or CPK24 completely impaired the Ca(2+)-dependent inhibition of K(+)in currents and enhanced pollen tube growth. Moreover, the cpk11 cpk24 double mutant exhibited similar phenotypes as the corresponding single mutants, suggesting that these two CDPKs function in the same signaling pathway. Bimolecular fluorescence complementation and coimmunoprecipitation experiments showed that CPK11 could interact with CPK24 in vivo. Furthermore, CPK11 phosphorylated the N terminus of CPK24 in vitro, suggesting that these two CDPKs work together as part of a kinase cascade. Electrophysiological assays demonstrated that the Shaker pollen K(+)in channel is the main contributor to pollen tube K(+)in currents and acts as the downstream target of the CPK11-CPK24 pathway. We conclude that CPK11 and CPK24 together mediate the Ca(2+)-dependent inhibition of K(+)in channels and participate in the regulation of pollen tube growth in Arabidopsis.

RNA editing underlies temperature adaptation in K+ channels from polar octopuses

To operate in the extreme cold, ion channels from psychrophiles must have evolved structural changes to compensate for their thermal environment. A reasonable assumption would be that the underlying adaptations lie within the encoding genes. Here, we show that delayed rectifier K(+) channel genes from an Antarctic and a tropical octopus encode channels that differ at only four positions and display very similar behavior when expressed in Xenopus oocytes. However, the transcribed messenger RNAs are extensively edited, creating functional diversity. One editing site, which recodes an isoleucine to a valine in the channel's pore, greatly accelerates gating kinetics by destabilizing the open state. This site is extensively edited in both Antarctic and Arctic species, but mostly unedited in tropical species. Thus adenosine-to-inosine RNA editing can respond to the physical environment.

Transcript analysis and comparative evaluation of shaker and slowmo gene homologues from the European corn borer, Ostrinia nubilalis

The movement and dispersal of larval Lepidoptera impact their survival and distribution within the natural landscape. Homologues of the Drosophila behaviour-linked genes shaker (shkr) and slowmo (slmo) were identified from Ostrinia nubilalis (Lepidoptera: Crambidae). Onshkr was isolated as a 1610-nucleotide (nt) constitutively expressed transcript encoding a membrane-localized 469-amino-acid (aa) protein with a conserved tetramerization domain and the six-domain architecture necessary for the molecule to fold into an active K(+) channel. Three expressed splice variants of 682, 970 and 1604 nt were identified for the Onslmo gene, and encode predicted 141 and 228 aa proteins with a conserved protein of relevant evolutionary and lymphoid interest (PRELI) domain that may function in mitochondrial protein sorting and perinuclear protein localization. Onshkr and Onslmo protein sequences aligned within monophyletic lepidopteran groups.

AtKC1 is a general modulator of Arabidopsis inward Shaker channel activity

A functional Shaker potassium channel requires assembly of four α-subunits encoded by a single gene or various genes from the Shaker family. In Arabidopsis thaliana, AtKC1, a Shaker α-subunit that is silent when expressed alone, has been shown to regulate the activity of AKT1 by forming heteromeric AtKC1-AKT1 channels. Here, we investigated whether AtKC1 is a general regulator of channel activity. Co-expression in Xenopus oocytes of a dominant negative (pore-mutated) AtKC1 subunit with the inward Shaker channel subunits KAT1, KAT2 or AKT2, or the outward subunits SKOR or GORK, revealed that the three inward subunits functionally interact with AtKC1 while the outward ones cannot. Localization experiments in plant protoplasts showed that KAT2 was able to re-locate AtKC1 fused to GFP from endomembranes to the plasma membrane, indicating that heteromeric AtKC1-KAT2 channels are efficiently targeted to the plasma membrane. Functional properties of heteromeric channels involving AtKC1 and KAT1, KAT2 or AKT2 were analysed by voltage clamp after co-expression of the respective subunits in Xenopus oocytes. AtKC1 behaved as a regulatory subunit within the heterotetrameric channel, reducing the macroscopic conductance and negatively shifting the channel activation potential. Expression studies showed that AtKC1 and its identified Shaker partners have overlapping expression patterns, supporting the hypothesis of a general regulation of inward channel activity by AtKC1 in planta. Lastly, AtKC1 disruption appeared to reduce plant biomass production, showing that AtKC1-mediated channel activity regulation is required for normal plant growth.

Random insertion of split-cans of the fluorescent protein venus into Shaker channels yields voltage sensitive probes with improved membrane localization in mammalian cells

FlaSh-YFP, a fluorescent protein (FP) voltage sensor that is a fusion of the Shaker potassium channel with yellow fluorescent protein (YFP), is primarily expressed in the endoplasmic reticulum (ER) of mammalian cells, possibly due to misfolded monomers. In an effort to improve plasma membrane expression, the FP was split into two non-fluorescent halves. Each half was randomly inserted into Shaker monomers via a transposon reaction. Shaker subunits containing the 5' half were co-expressed with Shaker subunits containing the 3' half. Tetramerization of Shaker subunits is required for re-conjugation of the FP. The misfolded monomers trapped in ER are unlikely to tetramerize and reconstitute the beta-can structure, and thus intracellular fluorescence might be reduced. This split-can transposon approach yielded 56 fluorescent probes, 30 (54%) of which were expressed at the plasma membrane and were capable of optically reporting changes in membrane potential. The largest signal from these novel FP-sensors was a -1.4% in ΔF/F for a 100 mV depolarization, with on time constants of about 15 ms and off time constants of about 200 ms. This split-can transposon approach has the potential to improve other multimeric probes.

Shaker-like potassium channels in Populus, regulated by the CBL-CIPK signal transduction pathway, increase tolerance to low-K+ stress

Shaker-like potassium channels in plants play an important role in potassium absorption and transport. Here, we characterized 11 genes encoding shaker-like channels from Populus trichocarpa. Furthermore, two homologs from this family were isolated from Populus euphratica and named PeKC1 and PeKC2. Subcellular localization analysis of them in Nicotiana benthamiana revealed that they are located in the cell membrane. Yeast two-hybrid assays showed that they not only interacted strongly with PeCIPK24, a homolog of AtCIPK23, but also interacted with several other CIPK members, including PeCIPK10 and PeCIPK17. To further analyze their function, we over-expressed PeKC1 or PeKC2 in akt1 mutant, the results show that the transgenic plant can recover the mutant phonotype sensitive to low-K(+) stress. This means PeKC1 or PeKC2 can complement the function of AKT1 in akt1 mutant, involved in the CBL1-CIPK23 signal transduction pathway and play an important role under low-K(+) stress.

Characterization of ion channels involved in the proliferative response of femoral artery smooth muscle cells

OBJECTIVE

Vascular smooth muscle cells (VSMCs) contribute significantly to occlusive vascular diseases by virtue of their ability to switch to a noncontractile, migratory, and proliferating phenotype. Although the participation of ion channels in this phenotypic modulation (PM) has been described previously, changes in their expression are poorly defined because of their large molecular diversity. We obtained a global portrait of ion channel expression in contractile versus proliferating mouse femoral artery VSMCs, and explored the functional contribution to the PM of the most relevant changes that we observed.

METHODS AND RESULTS

High-throughput real-time polymerase chain reaction of 87 ion channel genes was performed in 2 experimental paradigms: an in vivo model of endoluminal lesion and an in vitro model of cultured VSMCs obtained from explants. mRNA expression changes showed a good correlation between the 2 proliferative models, with only 2 genes, Kv1.3 and Kvbeta2, increasing their expression on proliferation. The functional characterization demonstrates that Kv1.3 currents increased in proliferating VSMC and that their selective blockade inhibits migration and proliferation.

CONCLUSIONS

These findings establish the involvement of Kv1.3 channels in the PM of VSMCs, providing a new therapeutical target for the treatment of intimal hyperplasia.

Double gaps along Shaker S4 demonstrate omega currents at three different closed states

The aim of the present study was to investigate in detail how the voltage sensor in the Shaker potassium channel moves during the gating process. After the publication of the open channel structure from the crystallized K(V)AP channel in 2003, an alternative so-called "paddle" model was put forward in contrast to the existing helical screw model. The voltage sensor S4 contains 4 arginine residues relevant for gating, R1(362), R2(365), R3(368) and R4(371), each separated by 2 neutral residues. These charged residues coil as one of three threads on the S4-alpha-helix. Based on a previous finding that the mutation R1S leads to the so-called omega leak current through a "gating-pore" in the closed state, we introduced gaps systematically along the arginine thread substituting long arginines by short serines. Mutations R2S or R3S did neither create transient nor steady leaks. The fact that the native residue A359, which is located three amino acids in front of R1, is a short one, motivated us to check its role. Mutation of A359 to arginine blocked the omega current in the R1S mutant indicating that the omega pore is occupied by A359 and R1. Introducing further double gaps (RR to SS) at sequential positions (0 + 1, 1 + 2, 2 + 3), produced clear leak currents which were remarkably stable over a wide voltage range. These leaks contradict that S4 would swing together with S3 in lipid according to the paddle hypothesis. Rather, our results show that during gating the S4 segment moves in 3 helical steps through a fixed pore formed by the channel protein.

Distributed structures underlie gating differences between the kin channel KAT1 and the Kout channel SKOR

The family of voltage-gated (Shaker-like) potassium channels in plants includes both inward-rectifying (K(in)) channels that allow plant cells to accumulate K(+) and outward-rectifying (K(out)) channels that mediate K(+) efflux. Despite their close structural similarities, K(in) and K(out) channels differ in their gating sensitivity towards voltage and the extracellular K(+) concentration. We have carried out a systematic program of domain swapping between the K(out) channel SKOR and the K(in) channel KAT1 to examine the impacts on gating of the pore regions, the S4, S5, and the S6 helices. We found that, in particular, the N-terminal part of the S5 played a critical role in KAT1 and SKOR gating. Our findings were supported by molecular dynamics of KAT1 and SKOR homology models. In silico analysis revealed that during channel opening and closing, displacement of certain residues, especially in the S5 and S6 segments, is more pronounced in KAT1 than in SKOR. From our analysis of the S4-S6 region, we conclude that gating (and K(+)-sensing in SKOR) depend on a number of structural elements that are dispersed over this approximately 145-residue sequence and that these place additional constraints on configurational rearrangement of the channels during gating.

A grapevine Shaker inward K(+) channel activated by the calcineurin B-like calcium sensor 1-protein kinase CIPK23 network is expressed in grape berries under drought stress conditions

Grapevine (Vitis vinifera), the genome sequence of which has recently been reported, is considered as a model species to study fleshy fruit development and acid fruit physiology. Grape berry acidity is quantitatively and qualitatively affected upon increased K(+) accumulation, resulting in deleterious effects on fruit (and wine) quality. Aiming at identifying molecular determinants of K(+) transport in grapevine, we have identified a K(+) channel, named VvK1.1, from the Shaker family. In silico analyses indicated that VvK1.1 is the grapevine counterpart of the Arabidopsis AKT1 channel, known to dominate the plasma membrane inward conductance to K(+) in root periphery cells, and to play a major role in K(+) uptake from the soil solution. VvK1.1 shares common functional properties with AKT1, such as inward rectification (resulting from voltage sensitivity) or regulation by calcineurin B-like (CBL)-interacting protein kinase (CIPK) and Ca(2+)-sensing CBL partners (shown upon heterologous expression in Xenopus oocytes). It also displays distinctive features such as activation at much more negative membrane voltages or expression strongly sensitive to drought stress and ABA (upregulation in aerial parts, downregulation in roots). In roots, VvK1.1 is mainly expressed in cortical cells, like AKT1. In aerial parts, VvK1.1 transcripts were detected in most organs, with expression levels being the highest in the berries. VvK1.1 expression in the berry is localized in the phloem vasculature and pip teguments, and displays strong upregulation upon drought stress, by about 10-fold.VvK1.1 could thus play a major role in K(+) loading into berry tissues, especially upon drought stress.

Distinct roles of the last transmembrane domain in controlling Arabidopsis K+ channel activity

The family of voltage-gated potassium channels in plants presumably evolved from a common ancestor and includes both inward-rectifying (K(in)) channels that allow plant cells to accumulate K(+) and outward-rectifying (K(out)) channels that mediate K(+) efflux. Despite their close structural similarities, the activity of K(in) channels is largely independent of K(+) and depends only on the transmembrane voltage, whereas that of K(out) channels responds to the membrane voltage and the prevailing extracellular K(+) concentration. Gating of potassium channels is achieved by structural rearrangements within the last transmembrane domain (S6). Here we investigated the functional equivalence of the S6 helices of the K(in) channel KAT1 and the K(out) channel SKOR by domain-swapping and site-directed mutagenesis. Channel mutants and chimeras were analyzed after expression in Xenopus oocytes. We identified two discrete regions that influence gating differently in both channels, demonstrating a lack of functional complementarity between KAT1 and SKOR. Our findings are supported by molecular models of KAT1 and SKOR in the open and closed states. The role of the S6 segment in gating evolved differently during specialization of the two channel subclasses, posing an obstacle for the transfer of the K(+)-sensor from K(out) to K(in) channels.

Outward-rectifying K+ channel activities regulate cell elongation and cell division of tobacco BY-2 cells

Potassium ions (K+) are required for plant growth and development, including cell division and cell elongation/expansion, which are mediated by the K+ transport system. In this study, we investigated the role of K+ in cell division using tobacco BY-2 protoplast cultures. Gene expression analysis revealed induction of the Shaker-like outward K+ channel gene, NTORK1, under cell-division conditions, whereas the inward K+ channel genes NKT1 and NtKC1 were induced under both cell-elongation and cell-division conditions. Repression of NTORK1 gene expression by expression of its antisense construct repressed cell division but accelerated cell elongation even under conditions promoting cell division. A decrease in the K+ content of cells and cellular osmotic pressure in dividing cells suggested that an increase in cell osmotic pressure by K+ uptake is not required for cell division. In contrast, K+ depletion, which reduced cell-division activity, decreased cytoplasmic pH as monitored using a fluorescent pH indicator, SNARF-1. Application of K+ or the cytoplasmic alkalizing reagent (NH(4))(2)SO(4) increased cytoplasmic pH and suppressed the reduction in cell-division activity. These results suggest that the K+ taken up into cells is used to regulate cytoplasmic pH during cell division.

Phylogenomic analyses of KCNA gene clusters in vertebrates: why do gene clusters stay intact?

Background

Gene clusters are of interest for the understanding of genome evolution since they provide insight in large-scale duplications events as well as patterns of individual gene losses. Vertebrates tend to have multiple copies of gene clusters that typically are only single clusters or are not present at all in genomes of invertebrates. We investigated the genomic architecture and conserved non-coding sequences of vertebrate KCNA gene clusters. KCNA genes encode shaker-related voltage-gated potassium channels and are arranged in two three-gene clusters in tetrapods. Teleost fish are found to possess four clusters. The two tetrapod KNCA clusters are of approximately the same age as the Hox gene clusters that arose through duplications early in vertebrate evolution. For some genes, their conserved retention and arrangement in clusters are thought to be related to regulatory elements in the intergenic regions, which might prevent rearrangements and gene loss. Interestingly, this hypothesis does not appear to apply to the KCNA clusters, as too few conserved putative regulatory elements are retained.

Results

We obtained KCNA coding sequences from basal ray-finned fishes (sturgeon, gar, bowfin) and confirmed that the duplication of these genes is specific to teleosts and therefore consistent with the fish-specific genome duplication (FSGD). Phylogenetic analyses of the genes suggest a basal position of the only intron containing KCNA gene in vertebrates (KCNA7). Sistergroup relationships of KCNA1/2 and KCNA3/6 support that a large-scale duplication gave rise to the two clusters found in the genome of tetrapods. We analyzed the intergenic regions of KCNA clusters in vertebrates and found that there are only a few conserved sequences shared between tetrapods and teleosts or between paralogous clusters. The orthologous teleost clusters, however, show sequence conservation in these regions.

Conclusion

The lack of overall conserved sequences in intergenic regions suggests that there are either other processes than regulatory evolution leading to cluster conservation or that the ancestral regulatory relationships among genes in KCNA clusters have been changed together with their regulatory sites.

Rice shaker potassium channel OsKAT1 confers tolerance to salinity stress on yeast and rice cells

We screened a rice (Oryza sativa L. 'Nipponbare') full-length cDNA expression library through functional complementation in yeast (Saccharomyces cerevisiae) to find novel cation transporters involved in salt tolerance. We found that expression of a cDNA clone, encoding the rice homolog of Shaker family K(+) channel KAT1 (OsKAT1), suppressed the salt-sensitive phenotype of yeast strain G19 (Deltaena1-4), which lacks a major component of Na(+) efflux. It also suppressed a K(+)-transport-defective phenotype of yeast strain CY162 (Deltatrk1Deltatrk2), suggesting the enhancement of K(+) uptake by OsKAT1. By the expression of OsKAT1, the K(+) contents of salt-stressed G19 cells increased during the exponential growth phase. At the linear phase, however, OsKAT1-expressing G19 cells accumulated less Na(+) than nonexpressing cells, but almost the same K(+). The cellular Na(+) to K(+) ratio of OsKAT1-expressing G19 cells remained lower than nonexpressing cells under saline conditions. Rice cells overexpressing OsKAT1 also showed enhanced salt tolerance and increased cellular K(+) content. These functions of OsKAT1 are likely to be common among Shaker K(+) channels because OsAKT1 and Arabidopsis (Arabidopsis thaliana) KAT1 were able to complement the salt-sensitive phenotype of G19 as well as OsKAT1. The expression of OsKAT1 was restricted to internodes and rachides of wild-type rice, whereas other Shaker family genes were expressed in various organs. These results suggest that OsKAT1 is involved in salt tolerance of rice in cooperation with other K(+) channels by participating in maintenance of cytosolic cation homeostasis during salt stress and thus protects cells from Na(+).

A direct demonstration of closed-state inactivation of K+ channels at low pH

Lowering external pH reduces peak current and enhances current decay in Kv and Shaker-IR channels. Using voltage-clamp fluorimetry we directly determined the fate of Shaker-IR channels at low pH by measuring fluorescence emission from tetramethylrhodamine-5-maleimide attached to substituted cysteine residues in the voltage sensor domain (M356C to R362C) or S5-P linker (S424C). One aspect of the distal S3-S4 linker α-helix (A359C and R362C) reported a pH-induced acceleration of the slow phase of fluorescence quenching that represents P/C-type inactivation, but neither site reported a change in the total charge movement at low pH. Shaker S424C fluorescence demonstrated slow unquenching that also reflects channel inactivation and this too was accelerated at low pH. In addition, however, acidic pH caused a reversible loss of the fluorescence signal (pKa = 5.1) that paralleled the reduction of peak current amplitude (pKa = 5.2). Protons decreased single channel open probability, suggesting that the loss of fluorescence at low pH reflects a decreased channel availability that is responsible for the reduced macroscopic conductance. Inhibition of inactivation in Shaker S424C (by raising external K⁺ or the mutation T449V) prevented fluorescence loss at low pH, and the fluorescence report from closed Shaker ILT S424C channels implied that protons stabilized a W434F-like inactivated state. Furthermore, acidic pH changed the fluorescence amplitude (pKa = 5.9) in channels held continuously at −80 mV. This suggests that low pH stabilizes closed-inactivated states. Thus, fluorescence experiments suggest the major mechanism of pH-induced peak current reduction is inactivation of channels from closed states from which they can activate, but not open; this occurs in addition to acceleration of P/C-type inactivation from the open state.

Sex differences in and hormonal regulation of Kv1 potassium channel gene expression in the electric organ: molecular control of a social signal

Electric fish communicate with electric organ (EO) discharges (EODs) that are sexually dimorphic, hormone-sensitive, and often individually distinct. The cells of the EO (electrocytes) of the weakly electric fish Sternopygus possess delayed rectifying K+ currents that systematically vary in their activation and deactivation kinetics, and this precise variation in K+ current kinetics helps shape sex and individual differences in the EOD. Because members of the Kv1 subfamily produce delayed rectifier currents, we cloned a number of genes in the Kv1 subfamily from the EO of Sternopygus. Using our sequences and those from genome databases, we found that in teleost fish Kv1.1 and Kv1.2 exist as duplicate pairs (Kv1.1a&b, Kv1.2a&b) whereas Kv1.3 does not. Using real-time quantitative RT-PCR, we found that Kv1.1a and Kv1.2a, but not Kv1.2b, expression in the EO is higher in high EOD frequency females (which have fast EO K+ currents) than in low EOD frequency males (which have slow EO K+ currents). Systemic treatment with dihydrotestosterone decreased Kv1.1a and Kv1.2a, but not Kv1.2b, expression in the EO, whereas treatment with human chorionic gonadotropin (hCG) increased Kv1.2a but not Kv1.1a or Kv1.2b expression in the EO. Thus, systematic variation in the ratios of Kv1 channels expressed in the EO is correlated with individual differences in and sexual dimorphism of a communication signal.

Drosophila Hyperkinetic mutants have reduced sleep and impaired memory

In mammals, sleep is thought to be important for health, cognition, and memory. Fruit flies share most features of mammalian sleep, and a recent study found that Drosophila lines carrying loss-of-function mutations in Shaker (Sh) are short sleeping, suggesting that the Sh current plays a major role in regulating daily sleep amount. The Sh current is potentiated by a beta modulatory subunit coded by Hyperkinetic (Hk). Here, we demonstrate that severe loss-of-function mutations of Hk reduce sleep and do so primarily by affecting the Sh current. Moreover, we prove, using a transgenic approach, that a wild-type copy of Hk is sufficient to restore normal sleep. Furthermore, we show that short-sleeping Hk mutant lines have a memory deficit, whereas flies carrying a weaker hypomorphic Hk allele have normal sleep and normal memory. By comparing six short-sleeping Sh lines with two normal sleeping ones, we also found that only alleles that reduce sleep also impair memory. These data identify a gene, Hk, which is necessary to maintain normal sleep, and provide genetic evidence that short sleep and poor memory are linked.

Probing the cavity of the slow inactivated conformation of shaker potassium channels

Slow inactivation involves a local rearrangement of the outer mouth of voltage-gated potassium channels, but nothing is known regarding rearrangements in the cavity between the activation gate and the selectivity filter. We now report that the cavity undergoes a conformational change in the slow-inactivated state. This change is manifest as altered accessibility of residues facing the aqueous cavity and as a marked decrease in the affinity of tetraethylammonium for its internal binding site. These findings have implications for global alterations of the channel during slow inactivation and putative coupling between activation and slow-inactivation gates.

A hydrophobic element secures S4 voltage sensor in position in resting Shaker K+ channels

The S4 transmembrane alpha-helix in voltage-gated channels contains several regularly spaced basic amino acid residues that could be protonated and moved across the membrane electric field in response to membrane potential changes. The translocation of the charge-carrying S4 transduces membrane voltage to gating conformational changes of the channel, but how it is positioned and moved with respect to membrane lipid remains controversial. We found that hydrophilic and especially arginine and lysine substitution for L361 at the external end of S4 causes a large negative shift with shallowed slope of both activation and inactivation curves in Shaker K+ channels. Also, the macroscopic kinetics of activation and inactivation become much faster and barely voltage dependent, especially in the L361R mutant channel. These steady-state and kinetic data suggest that the replacement of one single hydrophobic residue, leucine, with arginine may profoundly destabilize the resting conformation of S4, which therefore takes a partially extruded position (partly activated position) at resting potentials (e.g. -120 mV). Consistently, the L361R point mutation gives rise to an extracellularly exposed R365C that is readily modified by external hydrophilic sulfhydryl-specific agents in the resting channel. Moreover, the extruded S4 in the L361R mutant channel could be retracted by strong hyperpolarizing potentials ( approximately -180 mV), from which the mutant channel is gated with slower kinetics but evidently stronger voltage dependence. We conclude that hydrophobic interaction involving a highly conserved residue at the top of S4 is crucial for properly securing the gating voltage sensor in the resting position and thus appropriate gating control of the voltage-gated channels.

Biodiversity of voltage sensor domain proteins

The six-transmembrane type voltage-gated ion channels play an essential role in neuronal excitability, muscle contraction, and secretion. The voltage sensor domain (VSD) is the key element of voltage-gated ion channels for sensing transmembrane potential, and has been studied at the levels of both biophysics and protein structure. Two recently identified proteins containing VSD without a pore domain showed unexpected biological roles: regulation of phosphatase activity and proton permeation. These proteins not only provide novel platforms to understand mechanisms of voltage sensing and ion permeation but also highlight previously unappreciated roles of membrane potential in non-neuronal cells.

Functional interactions at the interface between voltage-sensing and pore domains in the Shaker K(v) channel

Voltage-activated potassium (K(v)) channels contain a central pore domain that is partially surrounded by four voltage-sensing domains. Recent X-ray structures suggest that the two domains lack extensive protein-protein contacts within presumed transmembrane regions, but whether this is the case for functional channels embedded in lipid membranes remains to be tested. We investigated domain interactions in the Shaker K(v) channel by systematically mutating the pore domain and assessing tolerance by examining channel maturation, S4 gating charge movement, and channel opening. When mapped onto the X-ray structure of the K(v)1.2 channel the large number of permissive mutations support the notion of relatively independent domains, consistent with crystallographic studies. Inspection of the maps also identifies portions of the interface where residues are sensitive to mutation, an external cluster where mutations hinder voltage sensor activation, and an internal cluster where domain interactions between S4 and S5 helices from adjacent subunits appear crucial for the concerted opening transition.

The twisted ion-permeation pathway of a resting voltage-sensing domain

Proteins containing voltage-sensing domains (VSDs) translate changes in membrane potential into changes in ion permeability or enzymatic activity. In channels, voltage change triggers a switch in conformation of the VSD, which drives gating in a separate pore domain, or, in channels lacking a pore domain, directly gates an ion pathway within the VSD. Neither mechanism is well understood. In the Shaker potassium channel, mutation of the first arginine residue of the S4 helix to a smaller uncharged residue makes the VSD permeable to ions ('omega current') in the resting conformation ('S4 down'). Here we perform a structure-guided perturbation analysis of the omega conductance to map its VSD permeation pathway. We find that there are four omega pores per channel, which is consistent with one conduction path per VSD. Permeating ions from the extracellular medium enter the VSD at its peripheral junction with the pore domain, and then plunge into the core of the VSD in a curved conduction pathway. Our results provide a model of the resting conformation of the VSD.

Differential contributions of Shaker and Shab K+ currents to neuronal firing patterns in Drosophila

Different K(+) currents participate in generating neuronal firing patterns. The Drosophila embryonic "giant" neuron culture system has facilitated current- and voltage-clamp recordings to correlate distinct excitability patterns with the underlying K(+) currents and to delineate the mutational effects of identified K(+) channels. Mutations of Sh and Shab K(+) channels removed part of inactivating I(A) and sustained I(K), respectively, and the remaining I(A) and I(K) revealed the properties of their counterparts, e.g., Shal and Shaw channels. Neuronal subsets displaying the delayed, tonic, adaptive, and damping spike patterns were characterized by different profiles of K(+) current voltage dependence and kinetics and by differential mutational effects. Shab channels regulated membrane repolarization and repetitive firing over hundreds of milliseconds, and Shab neurons showed a gradual decline in repolarization during current injection and their spike activities became limited to high-frequency, damping firing. In contrast, Sh channels acted on events within tens of milliseconds, and Sh mutations broadened spikes and reduced firing rates without eliminating any categories of firing patterns. However, removing both Sh and Shal I(A) by 4-aminopyridine converted the delayed to damping firing pattern, demonstrating their actions in regulating spike initiation. Specific blockade of Shab I(K) by quinidine mimicked the Shab phenotypes and converted tonic firing to a damping pattern. These conversions suggest a hierarchy of complexity in K(+) current interactions underlying different firing patterns. Different lineage-defined neuronal subsets, identifiable by employing the GAL4-UAS system, displayed different profiles of spike properties and K(+) current compositions, providing opportunities for mutational analysis in functionally specialized neurons.

Cross talk between activation and slow inactivation gates of Shaker potassium channels

This study addresses the energetic coupling between the activation and slow inactivation gates of Shaker potassium channels. To track the status of the activation gate in inactivated channels that are nonconducting, we used two functional assays: the accessibility of a cysteine residue engineered into the protein lining the pore cavity (V474C) and the liberation by depolarization of a Cs⁺ ion trapped behind the closed activation gate. We determined that the rate of activation gate movement depends on the state of the inactivation gate. A closed inactivation gate favors faster opening and slower closing of the activation gate. We also show that hyperpolarization closes the activation gate long before a channel recovers from inactivation. Because activation and slow inactivation are ubiquitous gating processes in potassium channels, the cross talk between them is likely to be a fundamental factor in controlling ion flux across membranes.

Binding of a gating modifier toxin induces intersubunit cooperativity early in the Shaker K channel's activation pathway

Potassium currents from voltage-gated Shaker K channels activate with a sigmoid rise. The degree of sigmoidicity in channel opening kinetics confirms that each subunit of the homotetrameric Shaker channel undergoes more than one conformational change before the channel opens. We have examined effects of two externally applied gating modifiers that reduce the sigmoidicity of channel opening. A toxin from gastropod mucus, 6-bromo-2-mercaptotryptamine (BrMT), and divalent zinc are both found to slow the same conformational changes early in Shaker's activation pathway. Sigmoidicity measurements suggest that zinc slows a conformational change independently in each channel subunit. Analysis of activation in BrMT reveals cooperativity among subunits during these same early steps. A lack of competition with either agitoxin or tetraethylammonium indicates that BrMT binds channel subunits outside of the external pore region in an allosterically cooperative fashion. Simulations including negatively cooperative BrMT binding account for its ability to induce gating cooperativity during activation. We conclude that cooperativity among K channel subunits can be greatly altered by experimental conditions.

Molecular and Functional Differences between Heart mKv1.7 Channel Isoforms

Ion channels are membrane-spanning proteins that allow ions to permeate at high rates. The kinetic characteristics of the channels present in a cell determine the cell signaling profile and therefore cell function in many different physiological processes. We found that Kv1.7 channels from mouse heart muscle have two putative translation initiation start sites that generate two channel isoforms with different functional characteristics, mKv1.7L (489 aa) and a shorter mKv1.7S (457 aa). The electrophysiological analysis of mKv1.7L and mKv1.7S channels revealed that the two channel isoforms have different inactivation kinetics. The channel resulting from the longer protein (L) inactivates faster than the shorter channels (S). Our data supports the hypothesis that mKv1.7L channels inactivate predominantly due to an N-type related mechanism, which is impaired in the mKv1.7S form. Furthermore, only the longer version mKv1.7L is regulated by the cell redox state, whereas the shorter form mKv1.7S is not. Thus, expression starting at each translation initiation site results in significant functional divergence. Our data suggest that the redox modulation of mKv1.7L may occur through a site in the cytoplasmic N-terminal domain that seems to encompass a metal coordination motif resembling those found in many redox-sensitive proteins. The mRNA expression profile and redox modulation of mKv1.7 kinetics identify these channels as molecular entities of potential importance in cellular redox-stress states such as hypoxia.

State-dependent block of BK channels by synthesized shaker ball peptides

Crystal structures of potassium channels have strongly corroborated an earlier hypothetical picture based on functional studies, in which the channel gate was located on the cytoplasmic side of the pore. However, accessibility studies on several types of ligand-sensitive K⁺ channels have suggested that their activation gates may be located near or within the selectivity filter instead. It remains to be determined to what extent the physical location of the gate is conserved across the large K⁺ channel family. Direct evidence about the location of the gate in large conductance calcium-activated K⁺ (BK) channels, which are gated by both voltage and ligand (calcium), has been scarce. Our earlier kinetic measurements of the block of BK channels by internal quaternary ammonium ions have raised the possibility that they may lack a cytoplasmic gate. We show in this study that a synthesized Shaker ball peptide (ShBP) homologue acts as a state-dependent blocker for BK channels when applied internally, suggesting a widening at the intracellular end of the channel pore upon gating. This is consistent with a gating-related conformational change at the cytoplasmic end of the pore-lining helices, as suggested by previous functional and structural studies on other K⁺ channels. Furthermore, our results from two BK channel mutations demonstrate that similar types of interactions between ball peptides and channels are shared by BK and other K⁺ channel types.

Sub-cellular Ca2+ dynamics affected by voltage- and Ca2+-gated K+ channels: Regulation of the soma-growth cone disparity and the quiescent state in Drosophila neurons

Using Drosophila mutants and pharmacological blockers, we provide the first evidence that distinct types of K(+) channels differentially influence sub-cellular Ca(2+) regulation and growth cone morphology during neuronal development. Fura-2-based imaging revealed in cultured embryonic neurons that the loss of either voltage-gated, inactivating Shaker channels or Ca(2+)-gated Slowpoke BK channels led to robust spontaneous Ca(2+) transients that preferentially occurred within the growth cone. In contrast, loss of voltage-gated, non-inactivating Shab channels did not show such a disparity and sometimes produced soma-specific Ca(2+) transients. The fast spontaneous transients in both the soma and growth cone were suppressed by the Na(+) channel blocker tetrodotoxin, indicating that these Ca(2+) fluctuations stemmed from increases in membrane excitability. Similar differences in regional Ca(2+) regulation were observed upon membrane depolarization by high K(+)-containing saline. In particular, Shaker and slowpoke mutations enhanced the size and dynamics of the depolarization-induced Ca(2+) increase in the growth cone. In contrast, Shab mutations greatly prolonged the Ca(2+) increase in the soma. Differential effects of these excitability mutations on neuronal development were indicated by their distinct alterations in growth cone morphology. Loss of Shaker currents increased the size of lamellipodia and the number of filopodia, structures associated with the actin cytoskeleton. Interestingly, loss of Slowpoke currents strongly influenced tubulin regulation, enhancing the number of microtubule loop structures per growth cone. Together, our findings support the idea that individual K(+) channel subunits differentially regulate spontaneous sub-cellular Ca(2+) fluctuations in growing neurons that may influence activity-dependent growth cone formation.

Molecular template for a voltage sensor in a novel K+ channel. II. Conservation of a eukaryotic sensor fold in a prokaryotic K+ channel

KvLm, a novel bacterial depolarization-activated K⁺ (Kv) channel isolated from the genome of Listeria monocytogenes, contains a voltage sensor module whose sequence deviates considerably from the consensus sequence of a Kv channel sensor in that only three out of eight conserved charged positions are present. Surprisingly, KvLm exhibits the steep dependence of the open channel probability on membrane potential that is characteristic of eukaryotic Kv channels whose sensor sequence approximates the consensus. Here we asked if the KvLm sensor shared a similar fold to that of Shaker, the archetypal eukaryotic Kv channel, by examining if interactions between conserved residues in Shaker known to mediate sensor biogenesis and function were conserved in KvLm. To this end, each of the five non-conserved residues in the KvLm sensor were mutated to their Shaker-like charged residues, and the impact of these mutations on the voltage dependence of activation was assayed by current recordings from excised membrane patches of Escherichia coli spheroplasts expressing the KvLm mutants. Conservation of pairwise interactions was investigated by comparison of the effect of single mutations to the impact of double mutations presumed to restore wild-type fold and voltage sensitivity. We observed significant functional coupling between sites known to interact in Shaker Kv channels, supporting the notion that the KvLm sensor largely retains the fold of its eukaryotic homologue.

Light-induced depolarization of neurons using a modified Shaker K(+) channel and a molecular photoswitch

To trigger action potentials in neurons, most investigators use electrical or chemical stimulation. Here we describe an optical stimulation method based on semi-synthetic light-activated ion channels. These SPARK (synthetic photoisomerizable azobenzene-regulated K(+)) channels consist of a synthetic azobenzene-containing photoswitch and a genetically modified Shaker K(+) channel protein. SPARK channels with a wild-type selectivity filter elicit hyperpolarization and suppress action potential firing when activated by 390 nm light. A mutation in the pore converts the K(+)-selective Shaker channel into a nonselective cation channel. Activation of this modified channel with the same wavelength of light elicits depolarization of the membrane potential. Expression of these depolarizing SPARK channels in neurons allows light to rapidly and reversibly trigger action potential firing. Hence, hyper- and depolarizing SPARK channels provide a means for eliciting opposite effects on neurons in response to the same light stimulus.