The role of PSD-95 in the rearrangement of Kv1.3 channels to the immunological synapse

Kv1.3 in the spotlight for treating immune diseases

INTRODUCTION

Kv1.3 is the main voltage-gated potassium channel of leukocytes from both the innate and adaptive immune systems. Channel function is required for common processes such as Ca2+ signaling but also for cell-specific events. In this context, alterations in Kv1.3 are associated with multiple immune disorders. Excessive channel activity correlates with numerous autoimmune diseases, while reduced currents result in increased cancer prevalence and immunodeficiencies.

AREAS COVERED

This review offers a general view of the role of Kv1.3 in every type of leukocyte. Moreover, diseases stemming from dysregulations of the channel are detailed, as well as current advances in their therapeutic research.

EXPERT OPINION

Kv1.3 arises as a potential immune target in a variety of diseases. Several lines of research focused on channel modulation have yielded positive results. However, among the great variety of specific channel blockers, only one has reached clinical trials. Future investigations should focus on developing simpler administration routes for channel inhibitors to facilitate their entrance into clinical trials. Prospective Kv1.3-based treatments will ensure powerful therapies while minimizing undesired side effects.

Dynamics and spatial organization of Kv1.3 at the immunological synapse of human CD4+ T cells

Formation of the immunological synapse (IS) is a key event during initiation of an adaptive immune response to a specific antigen. During this process, a T cell and an antigen presenting cell form a stable contact that allows the T cell to integrate both internal and external stimuli in order to decide whether to activate. The threshold for T cell activation depends on the strength and frequency of the calcium (Ca2+) signaling induced by antigen recognition, and it must be tightly regulated to avoid undesired harm to healthy cells. Potassium (K+) channels are recruited to the IS to maintain the negative membrane potential required to sustain Ca2+ entry. However, the precise localization of K+ channels within the IS remains unknown. Here, we visualized the dynamic subsynaptic distribution of Kv1.3, the main voltage-gated potassium channel in human T cells. Upon T cell receptor engagement, Kv1.3 polarized toward the synaptic cleft and diffused throughout the F-actin rich distal compartment of the synaptic interface-an effect enhanced by CD2-CD58 corolla formation. As the synapse matured, Kv1.3 clusters were internalized at the center of the IS and released in extracellular vesicles. We propose a model in which specific distribution of Kv1.3 within the synapse indirectly regulates the channel function and that this process is limited through Kv1.3 internalization and release in extracellular vesicles.

A Meta-Analysis Study to Infer Voltage-Gated K+ Channels Prognostic Value in Different Cancer Types

Potassium channels are often highly expressed in cancer cells with respect to healthy ones, as they provide proliferative advantages through modulating membrane potential, calcium homeostasis, and various signaling pathways. Among potassium channels, Shaker type voltage-gated Kv channels are emerging as promising pharmacological targets in oncology. Here, we queried publicly available cancer patient databases to highlight if a correlation exists between Kv channel expression and survival rate in five different cancer types. By multiple gene comparison analysis, we found a predominant expression of KCNA2, KCNA3, and KCNA5 with respect to the other KCNA genes in skin cutaneous melanoma (SKCM), uterine corpus endometrial carcinoma (UCEC), stomach adenocarcinoma (STAD), lung adenocarcinoma (LUAD), and lung squamous cell carcinoma (LUSC). This analysis highlighted a prognostic role of KCNA3 and KCNA5 in SKCM, LUAD, LUSC, and STAD, respectively. Interestingly, KCNA3 was associated with a positive prognosis in SKCM and LUAD but not in LUSC. Results obtained by the analysis of KCNA3-related differentially expressed genes (DEGs); tumor immune cell infiltration highlighted differences that may account for such differential prognosis. A meta-analysis study was conducted to investigate the role of KCNA channels in cancer using cancer patients’ datasets. Our study underlines a promising correlation between Kv channel expression in tumor cells, in infiltrating immune cells, and survival rate.

Palmitoylation of Voltage-Gated Ion Channels

Protein lipidation is one of the most common forms of posttranslational modification. This alteration couples different lipids, such as fatty acids, phospho- and glycolipids and sterols, to cellular proteins. Lipidation regulates different aspects of the protein’s physiology, including structure, stability and affinity for cellular membranes and protein–protein interactions. In this scenario, palmitoylation is the addition of long saturated fatty acid chains to amino acid residues of the proteins. The enzymes responsible for this modification are acyltransferases and thioesterases, which control the protein’s behavior by performing a series of acylation and deacylation cycles. These enzymes target a broad repertoire of substrates, including ion channels. Thus, protein palmitoylation exhibits a pleiotropic role by differential modulation of the trafficking, spatial organization and electrophysiological properties of ion channels. Considering voltage-gated ion channels (VGICs), dysregulation of lipidation of both the channels and the associated ancillary subunits correlates with the development of various diseases, such as cancer or mental disorders. Therefore, a major role for protein palmitoylation is currently emerging, affecting not only the dynamism and differential regulation of a moiety of cellular proteins but also linking to human health. Therefore, palmitoylation of VGIC, as well as related enzymes, constitutes a novel pharmacological tool for drug development to target related pathologies.

Role of C-Terminal Domain and Membrane Potential in the Mobility of Kv1.3 Channels in Immune Synapse Forming T Cells

Voltage-gated Kv1.3 potassium channels are essential for maintaining negative membrane potential during T-cell activation. They interact with membrane-associated guanylate kinases (MAGUK-s) via their C-terminus and with TCR/CD3, leading to enrichment at the immunological synapse (IS). Molecular interactions and mobility may impact each other and the function of these proteins. We aimed to identify molecular determinants of Kv1.3 mobility, applying fluorescence correlation spectroscopy on human Jurkat T-cells expressing WT, C-terminally truncated (ΔC), and non-conducting mutants of mGFP-Kv1.3. ΔC cannot interact with MAGUK-s and is not enriched at the IS, whereas cells expressing the non-conducting mutant are depolarized. Here, we found that in standalone cells, mobility of ΔC increased relative to the WT, likely due to abrogation of interactions, whereas mobility of the non-conducting mutant decreased, similar to our previous observations on other membrane proteins in depolarized cells. At the IS formed with Raji B-cells, mobility of WT and non-conducting channels, unlike ΔC, was lower than outside the IS. The Kv1.3 variants possessing an intact C-terminus had lower mobility in standalone cells than in IS-engaged cells. This may be related to the observed segregation of F-actin into a ring-like structure at the periphery of the IS, leaving much of the cell almost void of F-actin. Upon depolarizing treatment, mobility of WT and ΔC channels decreased both in standalone and IS-engaged cells, contrary to non-conducting channels, which themselves caused depolarization. Our results support that Kv1.3 is enriched at the IS via its C-terminal region regardless of conductivity, and that depolarization decreases channel mobility.

IL-15 Trans-Presentation Is an Autonomous, Antigen-Independent Process

IL-15 plays a pivotal role in the long-term survival of T cells and immunological memory. Its receptor consists of three subunits (IL-15Rα, IL-2/15Rβ, and γ_c). IL-15 functions mainly via trans-presentation (TP), during which an APC expressing IL-15 bound to IL-15Rα presents the ligand to the βγ_c receptor-heterodimer on a neighboring T/NK cell. To date, no direct biophysical evidence for the intercellular assembly of the IL-15R heterotrimer exists. Ag presentation (AP), the initial step of T cell activation, is also based on APC-T cell interaction. We were compelled to ask whether AP has any effect on IL-15 TP or whether they are independent processes. In our human Raji B cell-Jurkat T cell model system, we monitored inter-/intracellular protein interactions upon formation of IL-15 TP and AP receptor complexes by Förster resonance energy transfer measurements. We detected enrichment of IL-15Rα and IL-2/15Rβ at the synapse and positive Förster resonance energy transfer efficiency if Raji cells were pretreated with IL-15, giving direct biophysical evidence for IL-15 TP. IL-15Rα and MHC class II interacted and translocated jointly to the immunological synapse when either ligand was present, whereas IL-2/15Rβ and CD3 moved independently of each other. IL-15 TP initiated STAT5 phosphorylation in Jurkat cells, which was not further enhanced by AP. Conversely, IL-15 treatment slightly attenuated Ag-induced phosphorylation of the CD3ζ chain. Our studies prove that in our model system, IL-15 TP and AP can occur independently, and although AP enhances IL-15R assembly, it has no significant effect on IL-15 signaling during TP. Thus, IL-15 TP can be considered an autonomous, Ag-independent process.

Immune Synapse Residency of Orai1 Alters Ca2+ Response of T Cells

CRAC, which plays important role in Ca²⁺-dependent T-lymphocyte activation, is composed of the ER-resident STIM1 and the plasma membrane Orai1 pore-forming subunit. Both accumulate at the immunological synapse (IS) between a T cell and an antigen-presenting cell (APC). We hypothesized that adapter/interacting proteins regulate Orai1 residence in the IS. We could show that mGFP-tagged Orai1-Full channels expressed in Jurkat cells had a biphasic IS-accumulation kinetics peaked at 15 min. To understand the background of Orai1 IS-redistribution we knocked down STIM1 and SAP97 (adaptor protein with a short IS-residency (15 min) and ability to bind Orai1 N-terminus): the mGFP-Orai1-Full channels kept on accumulating in the IS up to the 60th minute in the STIM1- and SAP97-lacking Jurkat cells. Deletion of Orai1 N terminus (mGFP-Orai1-Δ72) resulted in the same time course as described for STIM1/SAP97 knock-down cells. Ca²⁺-imaging of IS-engaged T-cells revealed that of Orai1 residency modifies the Ca²⁺-response: cells expressing mGFP-Orai1-Δ72 construct or mGFP-Orai1-Full in SAP-97 knock-down cells showed higher number of Ca²⁺-oscillation up to the 90th minute after IS formation. Overall, these data suggest that SAP97 may contribute to the short-lived IS-residency of Orai1 and binding of STIM1 to Orai1 N-terminus is necessary for SAP97-Orai1 interaction.

Dlg1 Knockout Inhibits Microglial Activation and Alleviates Lipopolysaccharide-Induced Depression-Like Behavior in Mice

Microglia-mediated neuroinflammation is widely perceived as a contributor to numerous neurological diseases and mental disorders including depression. Discs large homolog 1 (Dlg1), an adaptor protein, regulates cell polarization and the function of K⁺ channels, which are reported to regulate the activation of microglia. However, little is known about the role of Dlg1 in microglia and the maintenance of central nervous system homeostasis. In this study, we found that Dlg1 knockdown suppressed lipopolysaccharide (LPS)-induced inflammation by down-regulating the activation of nuclear factor-κB signaling and the mitogen-activated protein kinase pathway in microglia. Moreover, using an inducible Dlg1 microglia-specific knockout (Dlg1^flox/flox; CX3CR1^CreER) mouse line, we found that microglial Dlg1 knockout reduced the activation of microglia and alleviated the LPS-induced depression-like behavior. In summary, our results demonstrated that Dlg1 plays a critical role in microglial activation and thus provides a potential therapeutic target for the clinical treatment of depression.

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.

Functional Consequences of the Variable Stoichiometry of the Kv1.3-KCNE4 Complex

The voltage-gated potassium channel Kv1.3 plays a crucial role during the immune response. The channel forms oligomeric complexes by associating with several modulatory subunits. KCNE4, one of the five members of the KCNE family, binds to Kv1.3, altering channel activity and membrane expression. The association of KCNEs with Kv channels is the subject of numerous studies, and the stoichiometry of such associations has led to an ongoing debate. The number of KCNE4 subunits that can interact and modulate Kv1.3 is unknown. KCNE4 transfers important elements to the Kv1.3 channelosome that negatively regulate channel function, thereby fine-tuning leukocyte physiology. The aim of this study was to determine the stoichiometry of the functional Kv1.3-KCNE4 complex. We demonstrate that as many as four KCNE4 subunits can bind to the same Kv1.3 channel, indicating a variable Kv1.3-KCNE4 stoichiometry. While increasing the number of KCNE4 subunits steadily slowed the activation of the channel and decreased the abundance of Kv1.3 at the cell surface, the presence of a single KCNE4 peptide was sufficient for the cooperative enhancement of the inactivating function of the channel. This variable architecture, which depends on KCNE4 availability, differentially affects Kv1.3 function. Therefore, our data indicate that the physiological remodeling of KCNE4 triggers functional consequences for Kv1.3, thus affecting cell physiology.

Dlg1 Maintains Dendritic Cell Function by Securing Voltage-Gated K+ Channel Integrity

Dendritic cells (DCs) play key roles in Ab responses by presenting Ags to lymphocytes and by producing proinflammatory cytokines. In this study, we reported that DC-specific knockout of discs large homologue 1 (Dlg1) resulted in a significantly reduced capacity to mediate Ab responses to both thymus-independent and thymus-dependent Ags in Dlg1 ^fl/flCd11c-Cre-GFP mice. Mechanistically, Dlg1-deficient DCs showed severely impaired endocytosis and phagocytosis capacities upon Ag exposure. In parallel, loss of Dlg1 significantly jeopardized the proinflammatory cytokine production by DCs upon TLR stimulation. Thus, Dlg1-deficient DCs lost their functions to support innate and adaptive immunities. At a cellular level, Dlg1 exhibited an indispensable function to maintain membrane potential changes by securing potassium ion (K⁺) efflux and subsequent calcium ion (Ca²⁺) influx events in DCs upon stimulation, both of which are known to be required for proper function of DCs. At a molecular level, Dlg1 did so by retaining the integrity of voltage-gated K⁺ channels (including Kv1.3) in DCs. The loss of Dlg1 led to a decreased expression of K⁺ channels, resulting in impaired membrane potential changes and, as a consequence, reduced proinflammatory cytokine production, compromised Ag endocytosis, and phagocytosis. In conclusion, this study provided, to our knowledge, a novel insight into Dlg1 and the voltage-gated K⁺ channels axis in DC functions.

The C-terminal HRET sequence of Kv1.3 regulates gating rather than targeting of Kv1.3 to the plasma membrane

Kv1.3 channels are expressed in several cell types including immune cells, such as T lymphocytes. The targeting of Kv1.3 to the plasma membrane is essential for T cell clonal expansion and assumed to be guided by the C-terminus of the channel. Using two point mutants of Kv1.3 with remarkably different features compared to the wild-type Kv1.3 (A413V and H399K having fast inactivation kinetics and tetraethylammonium-insensitivity, respectively) we showed that both Kv1.3 channel variants target to the membrane when the C-terminus was truncated right after the conserved HRET sequence and produce currents identical to those with a full-length C-terminus. The truncation before the HRET sequence (NOHRET channels) resulted in reduced membrane-targeting but non-functional phenotypes. NOHRET channels did not display gating currents, and coexpression with wild-type Kv1.3 did not rescue the NOHRET-A413V phenotype, no heteromeric current was observed. Interestingly, mutants of wild-type Kv1.3 lacking HRET(E) (deletion) or substituted with five alanines for the HRET(E) motif expressed current indistinguishable from the wild-type. These results demonstrate that the C-terminal region of Kv1.3 immediately proximal to the S6 helix is required for the activation gating and conduction, whereas the presence of the distal region of the C-terminus is not exclusively required for trafficking of Kv1.3 to the plasma membrane.

The secret life of ion channels: Kv1.3 potassium channels and proliferation

Kv1.3 channels are involved in the switch to proliferation of normally quiescent cells, being implicated in the control of cell cycle in many different cell types and in many different ways. They modulate membrane potential controlling K⁺ fluxes, sense changes in potential, and interact with many signaling molecules through their intracellular domains. From a mechanistic point of view, we can describe the role of Kv1.3 channels in proliferation with at least three different models. In the "membrane potential model," membrane hyperpolarization resulting from Kv1.3 activation provides the driving force for Ca²⁺ influx required to activate Ca²⁺-dependent transcription. This model explains most of the data obtained from several cells from the immune system. In the "voltage sensor model," Kv1.3 channels serve mainly as sensors that transduce electrical signals into biochemical cascades, independently of their effect on membrane potential. Kv1.3-dependent proliferation of vascular smooth muscle cells (VSMCs) could fit this model. Finally, in the "channelosome balance model," the master switch determining proliferation may be related to the control of the Kv1.3 to Kv1.5 ratio, as described in glial cells and also in VSMCs. Since the three mechanisms cannot function independently, these models are obviously not exclusive. Nevertheless, they could be exploited differentially in different cells and tissues. This large functional flexibility of Kv1.3 channels surely gives a new perspective on their functions beyond their elementary role as ion channels, although a conclusive picture of the mechanisms involved in Kv1.3 signaling to proliferation is yet to be reached.

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.

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.

The anti-proliferative effect of cation channel blockers in T lymphocytes depends on the strength of mitogenic stimulation

Ion channels are crucially important for the activation and proliferation of T lymphocytes, and thus, for the function of the immune system. Previous studies on the effects of channel blockers on T cell proliferation reported variable effectiveness due to differing experimental systems. Therefore our aim was to investigate how the strength of the mitogenic stimulation influences the efficiency of cation channel blockers in inhibiting activation, cytokine secretion and proliferation of T cells under standardized conditions. Human peripheral blood lymphocytes were activated via monoclonal antibodies targeting the TCR-CD3 complex and the co-stimulator CD28. We applied the blockers of Kv1.3 (Anuroctoxin), KCa3.1 (TRAM-34) and CRAC (2-Apb) channels of T cells either alone or in combination with rapamycin, the inhibitor of the mammalian target of rapamycin (mTOR). Five days after the stimulation ELISA and flow cytometric measurements were performed to determine IL-10 and IFN-γ secretion, cellular viability and proliferation. Our results showed that ion channel blockers and rapamycin inhibit IL-10 and IFN-γ secretion and cell division in a dose-dependent manner. Simultaneous application of the blockers for each channel along with rapamycin was the most effective, indicating synergy among the various activation pathways. Upon increasing the extent of mitogenic stimulation the anti-proliferative effect of the ion channel blockers diminished. This phenomenon may be important in understanding the fine-tuning of T cell activation.

Unconventional EGF-induced ERK1/2-mediated Kv1.3 endocytosis

The potassium channel Kv1.3 plays roles in immunity, neuronal development and sensory discrimination. Regulation of Kv1.3 by kinase signaling has been studied. In this context, EGF binds to specific receptors (EGFR) and triggers tyrosine kinase-dependent signaling, which down-regulates Kv1.3 currents. We show that Kv1.3 undergoes EGF-dependent endocytosis. This EGF-mediated mechanism is relevant because is involved in adult neural stem cell fate determination. We demonstrated that changes in Kv1.3 subcellular distribution upon EGFR activation were due to Kv1.3 clathrin-dependent endocytosis, which targets the Kv1.3 channels to the lysosomal degradative pathway. Interestingly, our results further revealed that relevant tyrosines and other interacting motifs, such as PDZ and SH3 domains, were not involved in the EGF-dependent Kv1.3 internalization. However, a new, and yet undescribed mechanism, of ERK1/2-mediated threonine phosphorylation is crucial for the EGF-mediated Kv1.3 endocytosis. Our results demonstrate that EGF triggers the down-regulation of Kv1.3 activity and its expression at the cell surface, which is important for the development and migration of adult neural progenitors.

The C-terminus SH3-binding domain of Kv1.3 is required for the actin-mediated immobilization of the channel via cortactin

Polarization of Kv1.3 channels is a necessary step in T-cell activation and motility. Nonetheless, the mechanisms regulating the Kv1.3 channel's membrane movement are not understood. This study provides evidence that cortactin, an actin-binding protein, controls Kv1.3 membrane motility via the channel's SH3 domain.

Kv1.3 channels play a pivotal role in the activation and migration of T-lymphocytes. These functions are accompanied by the channels' polarization, which is essential for associated downstream events. However, the mechanisms that govern the membrane movement of Kv1.3 channels remain unclear. F-actin polymerization occurs concomitantly to channel polarization, implicating the actin cytoskeleton in this process. Here we show that cortactin, a factor initiating the actin network, controls the membrane mobilization of Kv1.3 channels. FRAP with EGFP-tagged Kv1.3 channels demonstrates that knocking down cortactin decreases the actin-based immobilization of the channels. Using various deletion and mutation constructs, we show that the SH3 motif of Kv1.3 mediates the channel immobilization. Proximity ligation assays indicate that deletion or mutation of the SH3 motif also disrupts interaction of the channel with cortactin. In T-lymphocytes, the interaction between HS1 (the cortactin homologue) and Kv1.3 occurs at the immune synapse and requires the channel's C-terminal domain. These results show that actin dynamics regulates the membrane motility of Kv1.3 channels. They also provide evidence that the SH3 motif of the channel and cortactin plays key roles in this process.

Protein kinase A-phosphorylated KV1 channels in PSD95 signaling complex contribute to the resting membrane potential and diameter of cerebral arteries

RATIONALE

Postsynaptic density-95 (PSD95) is a scaffolding protein that associates with voltage-gated, Shaker-type K(+) (KV1) channels and promotes the expression of KV1 channels in vascular smooth muscle cells of the cerebral (cVSMCs) circulation. However, the physiological role of PSD95 in mediating molecular signaling in cVSMCs is unknown.

OBJECTIVE

We explored whether a specific interaction between PSD95 and KV1 channels enables protein kinase A phosphorylation of KV1 channels in cVSMCs to promote vasodilation.

METHODS AND RESULTS

Rat cerebral arteries were used for analyses. A membrane-permeable peptide (KV1-C peptide) corresponding to the postsynaptic density-95, discs large, zonula occludens-1 binding motif in the C terminus of KV1.2α was designed as a dominant-negative peptide to disrupt the association of KV1 channels with PSD95. Application of KV1-C peptide to cannulated, pressurized cerebral arteries rapidly induced vasoconstriction and depolarized cVSMCs. These events corresponded to reduced coimmunoprecipitation of the PSD95 and KV1 proteins without altering surface expression. Middle cerebral arterioles imaged in situ through cranial window also constricted rapidly in response to local application of KV1-C peptide. Patch-clamp recordings confirmed that KV1-C peptide attenuates KV1 channel blocker (5-(4-phenylalkoxypsoralen))-sensitive current in cVSMCs. Western blots using a phospho-protein kinase A substrate antibody revealed that cerebral arteries exposed to KV1-C peptide showed markedly less phosphorylation of KV1.2α subunits. Finally, phosphatase inhibitors blunted both KV1-C peptide-mediated and protein kinase A inhibitor peptide-mediated vasoconstriction.

CONCLUSIONS

These findings provide initial evidence that protein kinase A phosphorylation of KV1 channels is enabled by a dynamic association with PSD95 in cerebral arteries and suggest that a disruption of such association may compromise cerebral vasodilation and blood flow.

Ion channels and anti-cancer immunity

The outcome of a malignant disease depends on the efficacy of the immune system to destroy cancer cells. Key steps in this process, for example the generation of a proper Ca(2+) signal induced by recognition of a specific antigen, are regulated by various ion channel including voltage-gated Kv1.3 and Ca(2+)-activated KCa3.1 K(+) channels, and the interplay between Orai and STIM to produce the Ca(2+)-release-activated Ca(2+) (CRAC) current required for T-cell proliferation and function. Understanding the immune cell subset-specific expression of ion channels along with their particular function in a given cell type, and the role of cancer tissue-dependent factors in the regulation of operation of these ion channels are emerging questions to be addressed in the fight against cancer disease. Answering these questions might lead to a better understanding of the immunosuppression phenomenon in cancer tissue and the development of drugs aimed at skewing the distribution of immune cell types towards killing of the tumour cells.