KCNE4 suppresses Kv1.3 currents by modulating trafficking, surface expression and channel gating

Targeting of Kv7.5 (KCNQ5)/KCNE channels to surface microdomains of cell membranes

BACKGROUND

Kv7.5 (KCNQ5) channels conduct M-type potassium currents in the brain, are expressed in skeletal muscle, and contribute to vascular muscle tone.

METHODS

We coexpressed Kv7.5 and KCNE1-3 peptides in HEK293 cells and then analyzed their association using electrophysiology and co-immunoprecipitation, assessed localization using confocal microscopy, examined targeting of the oligomeric channels to cholesterol-rich membrane surface microdomains using lipid raft isolation, and evaluated their membrane dynamics using fluorescence recovery after photobleaching (FRAP).

RESULTS

Kv7.5 forms oligomeric channels specifically with KCNE1 and KCNE3. The expression of Kv7.5 targeted to cholesterol-rich membrane surface microdomains was very low. Oligomeric Kv7.5/KCNE1 and Kv7.5/KCNE3 channels did not localize to lipid rafts. However, Kv7.5 association impaired KCNE3 expression in lipid raft microdomains.

CONCLUSIONS

Our results indicate that Kv7.5 contributes to the spatial regulation of KCNE3. This new scenario could greatly assist in determining the physiological relevance of putative KCNE3 interactions in nerve and muscle.

Disruption of kv1.3 channel forward vesicular trafficking by hypoxia in human T lymphocytes

Hypoxia in solid tumors contributes to decreased immunosurveillance via down-regulation of Kv1.3 channels in T lymphocytes and associated T cell function inhibition. However, the mechanisms responsible for Kv1.3 down-regulation are not understood. We hypothesized that chronic hypoxia reduces Kv1.3 surface expression via alterations in membrane trafficking. Chronic hypoxia decreased Kv1.3 surface expression and current density in Jurkat T cells. Inhibition of either protein synthesis or degradation and endocytosis did not prevent this effect. Instead, blockade of clathrin-coated vesicle formation and forward trafficking prevented the Kv1.3 surface expression decrease in hypoxia. Confocal microscopy revealed an increased retention of Kv1.3 in the trans-Golgi during hypoxia. Expression of adaptor protein-1 (AP1), responsible for clathrin-coated vesicle formation at the trans-Golgi, was selectively down-regulated by hypoxia. Furthermore, AP1 down-regulation increased Kv1.3 retention in the trans-Golgi and reduced Kv1.3 currents. Our results indicate that hypoxia disrupts AP1/clathrin-mediated forward trafficking of Kv1.3 from the trans-Golgi to the plasma membrane thus contributing to decreased Kv1.3 surface expression in T lymphocytes.

KCNE2 forms potassium channels with KCNA3 and KCNQ1 in the choroid plexus epithelium

Cerebrospinal fluid (CSF) is crucial for normal function and mechanical protection of the CNS. The choroid plexus epithelium (CPe) is primarily responsible for secreting CSF and regulating its composition by mechanisms currently not fully understood. Previously, the heteromeric KCNQ1-KCNE2 K(+) channel was functionally linked to epithelial processes including gastric acid secretion and thyroid hormone biosynthesis. Here, using Kcne2(-/-) tissue as a negative control, we found cerebral expression of KCNE2 to be markedly enriched in the CPe apical membrane, where we also discovered expression of KCNQ1. Targeted Kcne2 gene deletion in C57B6 mice increased CPe outward K(+) current 2-fold. The Kcne2 deletion-enhanced portion of the current was inhibited by XE991 (10 μM) and margatoxin (10 μM) but not by dendrotoxin (100 nM), indicating that it arose from augmentation of KCNQ subfamily and KCNA3 but not KCNA1 K(+) channel activity. Kcne2 deletion in C57B6 mice also altered the polarity of CPe KCNQ1 and KCNA3 trafficking, hyperpolarized the CPe membrane by 9 ± 2 mV, and increased CSF [Cl(-)] by 14% compared with wild-type mice. These findings constitute the first report of CPe dysfunction caused by cation channel gene disruption and suggest that KCNE2 influences blood-CSF anion flux by regulating KCNQ1 and KCNA3 in the CPe.

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 K(V)1.3. The association of KCNE4, which alters the biophysical properties, trafficking and membrane localization of K(V)1.3, functions as an endogenous dominantnegative mechanism. Since both proteins are expressed in the immune system, K(V)1.3/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.

Neuronal trafficking of voltage-gated potassium channels

The computational ability of CNS neurons depends critically on the specific localization of ion channels in the somatodendritic and axonal membranes. Neuronal dendrites receive synaptic inputs at numerous spines and integrate them in time and space. The integration of synaptic potentials is regulated by voltage-gated potassium (Kv) channels, such as Kv4.2, which are specifically localized in the dendritic membrane. The synaptic potentials eventually depolarize the membrane of the axon initial segment, thereby activating voltage-gated sodium channels to generate action potentials. Specific Kv channels localized in the axon initial segment, such as Kv1 and Kv7 channels, determine the shape and the rate of action potentials. Kv1 and Kv7 channels present at or near nodes of Ranvier and in presynaptic terminals also influence the propagation of action potentials and neurotransmitter release. The physiological significance of proper Kv channel localization is emphasized by the fact that defects in the trafficking of Kv channels are observed in several neurological disorders including epilepsy. In this review, we will summarize the current understanding of the mechanisms of Kv channel trafficking and discuss how they contribute to the establishment and maintenance of the specific localization of Kv channels in neurons.

Building Excitable Membranes

Multiple plasma membrane proteins such as ion transporters and ion channels are involved in electrogenesis by setting resting membrane potentials and triggering/propagating action potentials. Recent findings strongly suggest that some of these membrane proteins are selectively transported into membrane microdomains termed lipid rafts. There appear to be multiple mechanisms for the specific protein translocation to lipid rafts, and many of these proteins exhibit distinct properties when inserted into the raft microdomains. Here the authors review the plasma membrane ion channels specifically localized at membrane lipid rafts in neurons. The mechanisms to selectively translocate these molecules to the lipid rafts and the consequences of the trafficking are also discussed.

NKS1, Na(+)- and K(+)-sensitive 1, regulates ion homeostasis in an SOS-independent pathway in Arabidopsis

An Arabidopsis thaliana mutant, nks1-1, exhibiting enhanced sensitivity to NaCl was identified in a screen of a T-DNA insertion population in the genetic background of Col-0 gl1sos3-1. Analysis of the genome sequence in the region flanking the T-DNA left border indicated two closely linked mutations in the gene encoded at locus At4g30996. A second allele, nks1-2, was obtained from the Arabidopsis Biological Resource Center. NKS1 mRNA was detected in all parts of wild-type plants but was not detected in plants of either mutant, indicating inactivation by the mutations. Both mutations in NKS1 were associated with increased sensitivity to NaCl and KCl, but not to LiCl or mannitol. NaCl sensitivity was associated with nks1 mutations in Arabidopsis lines expressing either wild type or null alleles of SOS1, SOS2 or SOS3. The NaCl-sensitive phenotype of the nks1-2 mutant was complemented by expression of a full-length NKS1 allele from the CaMV35S promoter. When grown in medium containing NaCl, nks1 mutants accumulated more Na(+) than wild type and K(+)/Na(+) homeostasis was perturbed. It is proposed NKS1, a plant-specific gene encoding a 19kDa endomembrane-localized protein of unknown function, is part of an ion homeostasis regulation pathway that is independent of the SOS pathway.

Kv1.5 in the immune system: the good, the bad, or the ugly?

For the last 20 years, knowledge of the physiological role of voltage-dependent potassium channels (Kv) in the immune system has grown exponentially. Leukocytes express a limited repertoire of Kv channels, which contribute to the membrane potential. These proteins are involved in the immune response and are therefore considered good pharmacological targets. Although there is a clear consensus about the physiological relevance of Kv1.3, the expression and the role of Kv1.5 are controversial. However, recent reports indicate that certain heteromeric Kv1.3/Kv1.5 associations may provide insight on Kv1.5. Here, we summarize what is known about this issue and highlight the role of Kv1.5 partnership interactions that could be responsible for this debate. The Kv1.3/Kv1.5 heterotetrameric composition of the channel and their possible differential associations with accessory regulatory proteins warrant further investigation.

Impact of KCNE subunits on KCNQ1 (Kv7.1) channel membrane surface targeting

The KCNQ1 (Kv7.1) channel plays an important role in cardiovascular physiology. Cardiomyocytes co-express KCNQ1 with KCNE1-5 proteins. KCNQ1 may co-associate with multiple KCNE regulatory subunits to generate different biophysically and pharmacologically distinct channels. Increasing evidence indicates that the location and targeting of channels are important determinants of their function. In this context, the presence of K(+) channels in sphingolipid-cholesterol-enriched membrane microdomains (lipid rafts) is under investigation. Lipid rafts are important for cardiovascular functioning. We aimed to determine whether KCNE subunits modify the localization and targeting of KCNQ1 channels in lipid rafts microdomains. HEK-293 cells were transiently transfected with KCNQ1 and KCNE1-5, and their traffic and presence in lipid rafts were analyzed. Only KCNQ1 and KCNE3, when expressed alone, co-localized in raft fractions. In addition, while KCNE2 and KCNE5 notably stained the cell surface, KCNQ1 and the rest of the KCNEs showed strong intracellular retention. KCNQ1 targets multiple membrane surface microdomains upon association with KCNE peptides. Thus, while KCNQ1/KCNE1 and KCNQ1/KCNE2 channels target lipid rafts, KCNQ1 associated with KCNE3-5 did not. Channel membrane dynamics, analyzed by fluorescence recovery after photobleaching (FRAP) experiments, further supported these results. In conclusion, the trafficking and targeting pattern of KCNQ1 can be influenced by its association with KCNEs. Since KCNQ1 is crucial for cardiovascular physiology, the temporal and spatial regulations that different KCNE subunits may confer to the channels could have a dramatic impact on membrane electrical activity and putative endocrine regulation.

Ancillary subunits associated with voltage-dependent K+ channels

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

Potassium secretion by voltage-gated potassium channel Kv1.3 in the rat kidney

The fine regulation of Na(+) and K(+) transport takes place in the cortical distal nephron. It is well established that K(+) secretion occurs through apical K(+) channels: the ROMK and the Ca(2+)- and voltage-dependent maxi-K. Previously, we identified the voltage-gated Kv1.3 channel in the inner medulla of the rat kidney (Escobar LI, Martínez-Téllez JC, Salas M, Castilla SA, Carrisoza R, Tapia D, Vázquez M, Bargas J, Bolívar JJ. Am J Physiol Cell Physiol 286: C965-C974, 2004). To examine the role of Kv1.3 in the renal regulation of K(+) homeostasis, we characterized the effect of dietary K(+) on the molecular and functional expression of this channel. We performed real-time-PCR and immunoblot assays in kidneys from rats fed a control (CK; 1.2% wt/wt) or high-K(+) (HK; 10% wt/wt) diet for 5-15 days. Kv1.3 mRNA and protein expression did not change with HK in the whole kidney. However, dietary K(+) loading provoked a change in the cellular distribution of Kv1.3 from the cytoplasm to apical membranes. Immunolocalization of Kv1.3 detected the channel exclusively in the intercalated cells. We investigated whether Kv1.3 mediated K(+) transport in microperfused cortical collecting ducts (CCDs). The HK diet led to an increase in net K(+) transport from 7.4 +/- 1.1 (CK) to 11.4 +/- 1.0 (HK) pmol x min(-1.) mm(-1). Luminal margatoxin, a specific blocker of Kv1.3, decreased net K(+) secretion in HK CCDs to 6.0 +/- 1.6 pmol x min(-1.) mm(-1). Our data provide the first evidence that Kv1.3 channels participate in K(+) secretion and that apical membrane localization of Kv1.3 is enhanced in the intercalated cells by dietary K(+) loading.

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

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

Immunomodulation of voltage-dependent K+ channels in macrophages: molecular and biophysical consequences

Voltage-dependent potassium (K_v) channels play a pivotal role in the modulation of macrophage physiology. Macrophages are professional antigen-presenting cells and produce inflammatory and immunoactive substances that modulate the immune response. Blockage of K_v channels by specific antagonists decreases macrophage cytokine production and inhibits proliferation. Numerous pharmacological agents exert their effects on specific target cells by modifying the activity of their plasma membrane ion channels. Investigation of the mechanisms involved in the regulation of potassium ion conduction is, therefore, essential to the understanding of potassium channel functions in the immune response to infection and inflammation. Here, we demonstrate that the biophysical properties of voltage-dependent K⁺ currents are modified upon activation or immunosuppression in macrophages. This regulation is in accordance with changes in the molecular characteristics of the heterotetrameric K_v1.3/K_v1.5 channels, which generate the main K_v in macrophages. An increase in K⁺ current amplitude in lipopolysaccharide-activated macrophages is characterized by a faster C-type inactivation, a greater percentage of cumulative inactivation, and a more effective margatoxin (MgTx) inhibition than control cells. These biophysical parameters are related to an increase in K_v1.3 subunits in the K_v1.3/K_v1.5 hybrid channel. In contrast, dexamethasone decreased the C-type inactivation, the cumulative inactivation, and the sensitivity to MgTx concomitantly with a decrease in K_v1.3 expression. Neither of these treatments apparently altered the expression of K_v1.5. Our results demonstrate that the immunomodulation of macrophages triggers molecular and biophysical consequences in K_v1.3/K_v1.5 hybrid channels by altering the subunit stoichiometry.