Voltage-dependent Potassium Channel Kv1 Research Articles

Characterization of endogenous Kv1.3 channel isoforms in T cells.

Voltage-dependent potassium channel Kv1.3 plays a key role on T-cell activation; however, lack of reliable antibodies has prevented its accurate detection under endogenous circumstances. To overcome this limitation, we created a Jurkat T-cell line with endogenous Kv1.3 channel tagged, to determine the expression, location, and changes upon activation of the native Kv1.3 channels. CRISPR-Cas9 technique was used to insert a Flag-Myc peptide at the C terminus of the KCNA3 gene. Basal or activated channel expression was studied using western blot analysis and imaging techniques. We identified two isoforms of Kv1.3 other than the canonical channel (54 KDa) differing on their N terminus: a longer isoform (70 KDa) and a truncated isoform (43 KDa). All three isoforms were upregulated after T-cell activation. We focused on the functional characterization of the truncated isoform (short form, SF), because it has not been previously described and could be present in the available Kv1.3-/- mice models. Overexpression of SF in HEK cells elicited small amplitude Kv1.3-like currents, which, contrary to canonical Kv1.3, did not induce HEK proliferation. To explore the role of endogenous SF isoform in a native system, we generated both a knockout Jurkat clone and a clone expressing only the SF isoform. Although the canonical isoform (long form) localizes mainly at the plasma membrane, SF remains intracellular, accumulating perinuclearly. Accordingly, SF Jurkat cells did not show Kv1.3 currents and exhibited depolarized resting membrane potential (VM ), decreased Ca2+ influx, and a reduction in the [Ca2+ ]i increase upon stimulation. Functional characterization of these Kv1.3 channel isoforms showed their differential contribution to signaling pathways involved in formation of the immunological synapse. We conclude that alternative translation initiation generates at least three endogenous Kv1.3 channel isoforms in T cells that exhibit different functional roles. For some of these functions, Kv1.3 proteins do not need to form functional plasma membrane channels.

Characterization of endogenous Kv1.3 channel isoforms in T cells

Voltage-dependent potassium channel Kv1.3 plays a key role on T cell activation; however, lack of reliable antibodies has prevented its accurate detection under endogenous circumstances. To overcome this limitation, we created a T cell-derived Jurkat cell line with endogenous Kv1.3 channel tagged in order to determine expression, location and changes upon activation of the native Kv1.3 channels. CRISPR-Cas9 technique was used to generate a double strand break on the gene locus of KCNA3 and a DNA-donor to insert a Flag-Myc peptide at the C-terminus directed by homologous recombination.

KCNE4-dependent functional consequences of Kv1.3-related leukocyte physiology

The voltage-dependent potassium channel Kv1.3 plays essential roles in the immune system, participating in leukocyte activation, proliferation and apoptosis. The regulatory subunit KCNE4 acts as an ancillary peptide of Kv1.3, modulates K+ currents and controls channel abundance at the cell surface. KCNE4-dependent regulation of the oligomeric complex fine-tunes the physiological role of Kv1.3. Thus, KCNE4 is crucial for Ca2+-dependent Kv1.3-related leukocyte functions. To better understand the role of KCNE4 in the regulation of the immune system, we manipulated its expression in various leukocyte cell lines. Jurkat T lymphocytes exhibit low KCNE4 levels, whereas CY15 dendritic cells, a model of professional antigen-presenting cells, robustly express KCNE4. When the cellular KCNE4 abundance was increased in T cells, the interaction between KCNE4 and Kv1.3 affected important T cell physiological features, such as channel rearrangement in the immunological synapse, cell growth, apoptosis and activation, as indicated by decreased IL-2 production. Conversely, ablation of KCNE4 in dendritic cells augmented proliferation. Furthermore, the LPS-dependent activation of CY15 cells, which induced Kv1.3 but not KCNE4, increased the Kv1.3-KCNE4 ratio and increased the expression of free Kv1.3 without KCNE4 interaction. Our results demonstrate that KCNE4 is a pivotal regulator of the Kv1.3 channelosome, which fine-tunes immune system physiology by modulating Kv1.3-associated leukocyte functions.

Calmodulin-dependent KCNE4 dimerization controls membrane targeting

The voltage-dependent potassium channel Kv1.3 participates in the immune response. Kv1.3 is essential in different cellular functions, such as proliferation, activation and apoptosis. Because aberrant expression of Kv1.3 is linked to autoimmune diseases, fine-tuning its function is crucial for leukocyte physiology. Regulatory KCNE subunits are expressed in the immune system, and KCNE4 specifically tightly regulates Kv1.3. KCNE4 modulates Kv1.3 currents slowing activation, accelerating inactivation and retaining the channel at the endoplasmic reticulum (ER), thereby altering its membrane localization. In addition, KCNE4 genomic variants are associated with immune pathologies. Therefore, an in-depth knowledge of KCNE4 function is extremely relevant for understanding immune system physiology. We demonstrate that KCNE4 dimerizes, which is unique among KCNE regulatory peptide family members. Furthermore, the juxtamembrane tetraleucine carboxyl-terminal domain of KCNE4 is a structural platform in which Kv1.3, Ca2+/calmodulin (CaM) and dimerizing KCNE4 compete for multiple interaction partners. CaM-dependent KCNE4 dimerization controls KCNE4 membrane targeting and modulates its interaction with Kv1.3. KCNE4, which is highly retained at the ER, contains an important ER retention motif near the tetraleucine motif. Upon escaping the ER in a CaM-dependent pattern, KCNE4 follows a COP-II-dependent forward trafficking mechanism. Therefore, CaM, an essential signaling molecule that controls the dimerization and membrane targeting of KCNE4, modulates the KCNE4-dependent regulation of Kv1.3, which in turn fine-tunes leukocyte physiology.

Voltage-dependent conformational changes of Kv1.3 channels activate cell proliferation.

The voltage-dependent potassium channel Kv1.3 has been implicated in proliferation in many cell types, based on the observation that Kv1.3 blockers inhibited proliferation. By modulating membrane potential, cell volume, and/or Ca2+ influx, K+ channels caninfluence cell cycle progression. Also, noncanonical channel functions could contribute to modulate cell proliferation independentof K+ efflux. The specificity of the requirement of Kv1.3 channels for proliferation suggests the involvement of molecule-specific interactions, but the underlying mechanisms are poorly identified. Heterologous expression of Kv1.3 channels in HEK cells has been shown to increase proliferation independently ofK+ fluxes. Likewise, some of the molecular determinants of Kv1.3-induced proliferation have been located in the C-terminus region, where individual point mutations of putative phosphorylation sites (Y447A and S459A) abolished Kv1.3-induced proliferation. Here,we investigated the mechanisms linking Kv1.3 channels to proliferation exploring the correlation between Kv1.3 voltage-dependent molecular dynamics and cell cycle progression. Using transfected HEK cells, we analyzed both the effect of changes in resting membrane potential on Kv1.3-induced proliferation and the effect of mutated Kv1.3 channels with altered voltage dependence of gating. We conclude that voltage-dependent transitions of Kv1.3 channels enable the activation of proliferative pathways. We also found that Kv1.3 associated with IQGAP3, a scaffold protein involved in proliferation, and that membrane depolarization facilitates their interaction. The functional contribution of Kv1.3-IQGAP3 interplay to cell proliferation was demonstrated both in HEK cells and in vascular smooth muscle cells. Our data indicate that voltage-dependent conformational changes of Kv1.3 are an essential element in Kv1.3-induced proliferation.

Modulation of Kv1.3 by the Gut Peptide Glucagon-Like Peptide 1

The voltage-dependent potassium channel Kv1.3 is a mammalian homolog of the Shaker subfamily that is implicated in processes related to immunity, glucose metabolism, energy homeostasis, and olfaction, in addition to action potential (AP) excitability. It has been previously demonstrated that Kv1.3 is modulated by important metabolic factors and hormones including insulin, glucose, and glucagon-like peptide 1 (GLP-1). Although GLP-1 is an incretin hormone secreted by L-cells of the intestine following a meal, we discovered GLP-1 signaling in the olfactory bulb wherein the peptide enhances AP firing of the major output neurons in a Kv1.3-dependent manner. Herein, we co-expressed Kv1.3 and the receptor for GLP-1 (GLP-1-R) in a heterologous expression system (HEK 293) to biophysically characterize the molecular mechanism of modulation. Recording macrocurrents from cell-attached patches in voltage-clamped cells, we found that Kv1.3+GLP-1-R co-expression did not alter basal biophysical properties of the channel, but bath application of the peptide decreased the peak outward current by 50% over a 25-minute recording interval and elicited a 6 mV hyperpolarizing shift in voltage at half-activation (V1/2). Using Quantity One Software, predicted Ser phosphorylation sites were mapped to S105, S470, S472, and S502 in the N- and C-terminal aspects of the channel. A channel was constructed with all four phosphorylation sites removed (S to A point mutations), named delta4SerKv1.3. Cells co-expressed with delta4SerKv1.3+GLP-1-R were resistant to current suppression and shift in V1/2 in response to GLP-1 peptide. We are using a channel immunoprecipitation strategy followed by Western Blot to determine if delta4SerKv1.3+GLP-1R transfected conditions fail to exhibit Kv1.3 phosphorylation when stimulated with GLP-1. These results highlight a phosphorylation-dependent modulation of Kv1.3 by GLP-1 signaling that could regulate metabolism in the olfactory bulb. Funded by the FSU LAC Scholarship and NIH grant DCR01013080 from the NIDCD.

Evaluation of neuroprotective effects of insulin on immuno-inflammatory and systemic disorders induced by kaliotoxin, a Kv1.3 channel blocker.

Kaliotoxin2 (KTX2) is a highly selective blocker of voltage-dependent potassium channels Kv1.3 containing 37 amino acid residues. It is purified from Androctonus australis scorpion venom. The binding of KTX2 to its targets is able to alter the neuronal excitability leading to neurological disorders, accompanied by an inflammatory response. In brain, activation of insulin receptor signaling pathway by insulin induces current suppression and concomitant tyrosine phosphorylation of Kv1.3 channel. The aim of this study is to evaluate the effect of insulin injected by i.c.v. route on the neuro-pathophysiological and systemic disorders induced by KTX2. Tissue damage, inflammatory response and oxidative stress biomarkers were assessed in NMRI mice at 24h after co-injection of KTX2 and insulin by intracerebroventricular route. Obtained results revealed that the central administration of insulin prevents cerebral cortex injury, brain edema and blood-brain barrier alteration induced by KTX2, these are accompanied by significant decrease of systemic disorders including serum cytokines, inflammatory and oxidative stress markers and tissue damage. These results indicate that insulin is able to reduce neuro-immunological effects and systemic disorders induced by KTX2. The neuroprotective effect of insulin may be due to its crucial role in the regulation of inflammation response and its properties to modulate the activity of Kv1.3 channels in brain.

Caveolar targeting links Kv1.3 with the insulin-dependent adipocyte physiology.

The voltage-dependent potassium channel Kv1.3 participates in peripheral insulin sensitivity. Genetic ablation of Kv1.3 triggers resistance to diet-induced weight gain, thereby pointing to this protein as a pharmacological target for obesity and associated type II diabetes. However, this role is under intense debate because Kv1.3 expression in adipose tissue raises controversy. We demonstrated that Kv1.3 is expressed in white adipose tissue from humans and rodents. Moreover, other channels, such as Kv1.1, Kv1.2, Kv1.4 and especially Kv1.5, from the same Shaker family are also present. Although elevated insulin levels and adipogenesis remodel the Kv phenotype, which could lead to multiple heteromeric complexes, Kv1.3 markedly participates in the insulin-dependent regulation of glucose uptake in mature adipocytes. Adipocyte differentiation increased the expression of Kv1.3, which is targeted to caveolae by molecular interactions with caveolin 1. Using a caveolin 1-deficient 3T3-L1 adipocyte cell line, we demonstrated that the localization of Kv1.3 in caveolar raft structures is important for proper insulin signaling. Insulin-dependent phosphorylation of the channel occurs at the onset of insulin-mediated signaling. However, when Kv1.3 was spatially outside of these lipid microdomains, impaired phosphorylation was exhibited. Our data shed light on the putative role of Kv1.3 in weight gain and insulin-dependent responses contributing to knowledge about adipocyte physiology.

Ubiquitination mediates Kv1.3 endocytosis as a mechanism for protein kinase C-dependent modulation

The voltage-dependent potassium channel Kv1.3 plays essential physiological functions in the immune system. Kv1.3, regulating the membrane potential, facilitates downstream Ca2+ -dependent pathways and becomes concentrated in specific membrane microdomains that serve as signaling platforms. Increased and/or delocalized expression of the channel is observed at the onset of several autoimmune diseases. In this work, we show that adenosine (ADO), which is a potent endogenous modulator, stimulates PKC, thereby causing immunosuppression. PKC activation triggers down-regulation of Kv1.3 by inducing a clathrin-mediated endocytic event that targets the channel to lysosomal-degradative compartments. Therefore, the abundance of Kv1.3 at the cell surface decreases, which is clearly compatible with an effective anti-inflammatory response. This mechanism requires ubiquitination of Kv1.3, catalyzed by the E3 ubiquitin-ligase Nedd4-2. Postsynaptic density protein 95 (PSD-95), a member of the MAGUK family, recruits Kv1.3 into lipid-raft microdomains and protects the channel against ubiquitination and endocytosis. Therefore, the Kv1.3/PSD-95 association fine-tunes the anti-inflammatory response in leukocytes. Because Kv1.3 is a promising multi-therapeutic target against human pathologies, our results have physiological relevance. In addition, this work elucidates the ADO-dependent PKC-mediated molecular mechanism that triggers immunomodulation by targeting Kv1.3 in leukocytes.

Deciphering the Kv1.3/Caveolin Interaction

Lipid rafts are specialized membrane microdomains rich in sphingolipids and cholesterol where signaling molecules and their targets converge. These domains are more ordered and tightly packed than the surrounding bilayer. Rafts contain caveolins, which are implicated in many physiological processes such as cholesterol transport to the membrane surface. The targeting of ion channels to these microdomains has emerged as a crucial mechanism of ion channel localization and function. The voltage-dependent potassium channel Kv1.3, which participates in the immune response, targets to the immunological synapse between T lymphocytes and the antigen presenting cell.

The C-terminal domain of Kv1.3 regulates functional interactions with the KCNE4 subunit.

The voltage-dependent K+ channel Kv1.3 (also known as KCNA3), which plays crucial roles in leukocytes, physically interacts with KCNE4. This interaction inhibits the K+ currents because the channel is retained within intracellular compartments. Thus, KCNE subunits are regulators of K+ channels in the immune system. Although the canonical interactions of KCNE subunits with Kv7 channels are under intensive investigation, the molecular determinants governing the important Kv1.3- KCNE4 association in the immune system are unknown. Our results suggest that the tertiary structure of the C-terminal domain of Kv1.3 is necessary and sufficient for such an interaction. However, this element is apparently not involved in modulating Kv1.3 gating. Furthermore, the KCNE4-dependent intracellular retention of the channel, which negatively affects the activity of Kv1.3, is mediated by two independent and additive mechanisms. First, KCNE4 masks the YMVIEE signature at the C-terminus of Kv1.3, which is crucial for the surface targeting of the channel. Second, we identify a potent endoplasmic reticulum retention motif in KCNE4 that further limits cell surface expression. Our results define specific molecular determinants that play crucial roles in the physiological function of Kv1.3 in leukocytes.

Glucose sensitivity of mouse olfactory bulb neurons is conveyed by a voltage‐gated potassium channel

The olfactory bulb has recently been proposed to serve as a metabolic sensor of internal chemistry, particularly that modified by metabolism. Because the voltage-dependent potassium channel Kv1.3 regulates a large proportion of the outward current in olfactory bulb neurons and gene-targeted deletion of the protein produces a phenotype of resistance to diet-induced obesity in mice, we hypothesized that this channel may play a role in translating energy availability into a metabolic signal. Here we explored the ability of extracellular glucose concentration to modify evoked excitability of the mitral neurons that principally regulate olfactory coding and processing of olfactory information. Using voltage-clamp electrophysiology of heterologously expressed Kv1.3 channels in HEK 293 cells, we found that Kv1.3 macroscopic currents responded to metabolically active (d-) rather than inactive (l-) glucose with a response profile that followed a bell-shaped curve. Olfactory bulb slices stimulated with varying glucose concentrations showed glucose-dependent mitral cell excitability as evaluated by current-clamp electrophysiology. While glucose could be either excitatory or inhibitory, the majority of the sampled neurons displayed a decreased firing frequency in response to elevated glucose concentration that was linked to increased latency to first spike and decreased action potential cluster length. Unlike modulation attributed to phosphorylation, glucose modulation of mitral cells was rapid, less than one minute, and was reversible within the time course of a patch recording. Moreover, we report that modulation targets properties of spike firing rather than action potential shape, involves synaptic activity of glutamate or GABA signalling circuits, and is dependent upon Kv1.3 expression. Given the rising incidence of metabolic disorders attributed to weight gain, changes in neuronal excitability in brain regions regulating sensory perception of food are of consequence.

Amiodarone inhibits tissue factor expression in monocytic THP-1 cells

There is a possibility thrombus formation is closely involved in sudden cardiac death. Amiodarone, a potassium channel inhibitor, is known to reduce mortality in patients with coronary artery disease or low ejection fraction, having antithrombotic actions. Using human monocytic THP-1 cells, we investigated the effects of amiodarone on tissue factor mRNA and protein expression. The involvement of the two main potassium channels existing in THP-1 cells was also investigated. Amiodarone (10μM) significantly and almost completely inhibited the increase of tissue factor mRNA and protein expression induced by tumor necrosis factor-α (100ng/ml). The inhibitory effects of amiodarone on tissue factor mRNA expression showed dose-dependency. Margatoxin (1nM), a selective blocker of voltage-dependent potassium channel Kv1.3, also inhibited tissue factor protein expression, but didn't significantly inhibit mRNA expression. Ba2+, a blocker of inwardly rectifying potassium channel Kir2.1, partly inhibited the increase of tissue factor mRNA and protein expression. This is the first study that shows amiodarone inhibits tissue factor expression in monocytic cells, by inhibiting mRNA transcription. The result may correlate with the facts amiodarone has antithrombotic actions in patients under extraordinary conditions where thrombus formation is enhanced. The inhibitory effects of amiodarone on tissue factor expression are drastic, different from those of margatoxin and Ba2+. The result suggests amiodarone has an underlying mechanism to intensely inhibit tissue factor expression other than blocking Kv1.3 and Kir2.1.

Voltage-Dependent Potassium Channels Kv1.3 and Kv1.5 in Human Cancer

Membrane ion channels participate in cancerous processes such as proliferation, migration and invasion, which contribute to metastasis. Increasing evidence indicates that voltage-dependent K(+) (Kv) channels are involved in the proliferation of many types of cells, including tumor cells. Kv channels have generated immense interest as a promising tool for developing new anti-tumor therapies. Therefore, the identification of potential biomarkers and therapeutic targets in specific cancers is an important prerequisite for the treatment. Since Kv1.3 and Kv1.5 are involved in the proliferation of many mammalian cells, we aimed to study the expression of Kv1.3 and Kv1.5 in a plethora of human cancers. Thus, tissues from breast, stomach, kidney, bladder, lung, skin, colon, ovary, pancreas, brain, lymph node, skeletal muscle and some of their malignant counterparts have been analyzed. Whereas Kv1.3 expression was either decreased or did not change in most tumors, Kv1.5 was overexpressed. However, the presence of Kv1.3 was mostly associated with inflammatory lymphoplasmocytic cells. Independent of the suitability of individual channels as therapeutic targets, the identification of a Kv phenotype from tumor specimens could have a diagnostic value of its own. Our results demonstrate that Kv1.5, and to some extent Kv1.3, are aberrantly expressed in a number of human cancers. These channels could serve both as novel markers of the metastatic phenotype and as potential new therapeutic targets. The concept of Kv channels as therapeutic targets or prognostic biomarkers attracts increasing interest and warrants further investigation.

Does a physiological role for KCNE subunits exist in the immune system?

The study of channel modulation by regulatory subunits has attracted considerable attention. Evidence indicates a pivotal role for accessory proteins in the channelosome. For instance, these regulatory subunits are necessary to recapitulate in vivo ion currents and to further understand the physiological role of ion channels. KCNEs are a family of regulatory subunits that interact with a wide range of channels. We have described for the first time a molecular interaction between KCNE4 and the voltage-dependent potassium channel Kv1.3. The association of KCNE4, which alters the biophysical properties, trafficking and membrane localization of Kv1.3, functions as an endogenous dominant-negative mechanism. Since both proteins are expressed in the immune system, Kv1.3/KCNE4 channels may contribute to the fine-tuning of the immune response. Therefore, our results point to KCNE4 as a novel target for immunomodulation. KCNE4 is not the only KCNE which is expressed in leukocytes. All KCNEs (KCNE1-5) are present, and some members demonstrate modulation during proliferation and cancer. In summary, regulatory KCNE subunits are expressed in the immune system. In addition, several voltage-dependent K+ channels, which could interact with KCNEs, are also detected. Therefore, KCNE subunits may play a yet undiscovered role in the physiology of the immune system.

Voltage-dependent Potassium Channels Kv1.3 and Kv1.5 in Human Fetus

Voltage-dependent K⁺ channels (Kv) control repolarization and membrane potential in electrically excitable cells. In addition, Kv channels are involved in the maintenance of vascular smooth muscle tone, insulin release, epithelial K⁺ transport, cell proliferation and leukocyte activation. Kv1.3 and Kv1.5 are widely distributed throughout the body and are involved in a variety of physiological processes taking place in the immune system, brain and muscle. Since the developmental pattern of Kv channels has an essential role in the maturing human, we aimed to study Kv1.3 and Kv1.5 channels in 8-10 week human fetal tissues. We chose that gestational age because all organs are in place and the nervous system, although not fully developed. However, the human embryo is undergoing major changes, which will lead to a defined adult pattern. Our results indicated that numerous tissues expressed Kv1.3 and Kv1.5. While Kv1.3 overlapped with the central and peripheral nervous systems, Kv1.5 was mostly localized in the central nervous system. In addition, both channels were abundantly expressed in the hematopoietic fetal liver. Finally, Kv1.5 heavily stained skeletal muscle and heart, whereas Kv1.3 was slightly present. This is the first study to analyze Kv1.3 and Kv1.5 in human during the beginning of fetal development.

Voltage-Dependent Potassium Channels Kv1.3 and Kv1.5 in Human Cancer

Membrane ion channels participate in cancerous processes such as proliferation, migration and invasion, which contribute to metastasis. Increasing evidence indicates that voltage-dependent K(+) (Kv) channels are involved in the proliferation of many types of cells, including tumor cells. Kv channels have generated immense interest as a promising tool for developing new anti-tumor therapies. Therefore, the identification of potential biomarkers and therapeutic targets in specific cancers is an important prerequisite for the treatment. Since Kv1.3 and Kv1.5 are involved in the proliferation of many mammalian cells, we aimed to study the expression of Kv1.3 and Kv1.5 in a plethora of human cancers. Thus, tissues from breast, stomach, kidney, bladder, lung, skin, colon, ovary, pancreas, brain, lymph node, skeletal muscle and some of their malignant counterparts have been analyzed. Whereas Kv1.3 expression was either decreased or did not change in most tumors, Kv1.5 was overexpressed. However, the presence of Kv1.3 was mostly associated with inflammatory lymphoplasmocytic cells. Independent of the suitability of individual channels as therapeutic targets, the identification of a Kv phenotype from tumor specimens could have a diagnostic value of its own. Our results demonstrate that Kv1.5, and to some extent Kv1.3, are aberrantly expressed in a number of human cancers. These channels could serve both as novel markers of the metastatic phenotype and as potential new therapeutic targets. The concept of Kv channels as therapeutic targets or prognostic biomarkers attracts increasing interest and warrants further investigation.

Crystallization and preliminary X-ray diffraction studies of the tetramerization domain derived from the human potassium channel Kv1.3

The tetramerization domain (T1 domain) derived from the voltage-dependent potassium channel Kv1.3 of Homo sapiens was recombinantly expressed in Escherichia coli and purified. The crystals were first grown in an NMR tube in 150 mM potassium phosphate pH 6.5 in the absence of additional precipitants. The crystals showed I4 symmetry characteristic of the naturally occurring tetrameric assembly of the single subunits. A complete native data set was collected to 1.2 A resolution at 100 K using synchrotron radiation.

Soluble β-amyloid oligomers alter biophysical properties of Kv1.3 channels

The aggregation of amyloid peptides in brain tissue is a hallmark of Alzheimer's disease. Amyloid beta (Aβ) exists in several assembly states, which may play different physiological or pathophysiological roles. The effects of Aβ on voltage-dependent ion channels in neurons and microglia were implicated in early stages of neurodegeneration. We tested the effect of soluble oligomers (Aβ1-42) of amyloid precursor protein (APP) on voltage-dependent potassium channels Kv1.3. Potassium current was measured during whole-cell recording from L929 cells, stably expressing Kv1.3. Acute application of Aβ1-42 reversibly reduced peak current amplitudes and affected kinetics of current activation, inactivation and deactivation in a voltage- and a dose-dependent manner. The time constant of K+ current activation during depolarizing voltage pulses from the holding membrane potential of −80 mV was reduced by Aβ1-42 at low (<10 mV) voltages, but was not changed at higher voltages. Slow C-type inactivation of K+ current was significantly faster in the presence of Aβ1-42 with the effect being most prominent at −20 mV (lowest voltage measured) and diminishing with increasing voltage. The time constant of K+ current deactivation was significantly reduced by Aβ1-42, and the effect progressively increased with voltage increase. Under the same conditions, the voltage sensitivity of Kv1.3 conductance was not significantly changed by Aβ1-42. Our results reveal acute effects of biologically active soluble β-amyloid oligomers on voltage-dependent potassium channels Kv1.3. Faster inactivation and deactivation of K+ current in the presence of Aβ could impair regulation of the membrane potential, ultimately leading to pathophysiological changes in the cell.Supported by the Hillblom Foundation.

Posttranslational modification of voltage-dependent potassium channel Kv1.5: COOH-terminal palmitoylation modulates its biological properties

The physiological function of ion channels is affected by protein-protein and protein-membrane interactions that modulate their activity and/or localization. Palmitoylation modulates protein function by facilitating targeted membrane association, interaction with other proteins, and determining subcellular localization. In this study, we demonstrate that the voltage-dependent potassium (Kv) channel Kv1.5 is palmitoylated and that the mutation of COOH-terminal cysteines is sufficient to abolish the palmitoylation of the Kv1.5 polypeptide in Chinese hamster ovary (CHO) cells. The labeling represented the thioester linkage of the labeled palmitic acid to cysteine rather than amide and oxygen ester linkages as judged by the release of the palmitic acid upon the treatment of the gel with hydroxylamine at a neutral pH. Site-directed mutagenesis and radiolabeling studies revealed that C593 was the sole site of palmitoylation. The elucidation of the biological function of palmitoylation revealed that the expression of the FLAG-Kv1.5 palmitoylation-deficient mutant (FL-Kv1.5(Palm-)) in stable CHO cells increased membrane expression as determined by the biotinylation of surface proteins and quantitative immunofluorescence analyses of these cells, in turn enhancing the outward potassium current. This enhanced surface expression and the currents were consequential to the slower rate of internalization, causing an increased localization of FL-Kv1.5(Palm-) in the plasma membrane compared with the wild-type FL-Kv1.5 channels. We conclude that the Kv1.5 channel is palmitoylated and that its palmitoylation modulates its biological functions and, therefore, might provide a physiological link between the metabolic state and the expression of Kv1.5 on the plasma membrane.