14-3-3 Proteins regulate K2P 5.1 surface expression on T lymphocytes

Proteomic Landscape and Deduced Functions of the Cardiac 14-3-3 Protein Interactome

Rationale: The 14-3-3 protein family is known to interact with many proteins in non-cardiac cell types to regulate multiple signaling pathways, particularly those relating to energy and protein homeostasis; and the 14-3-3 network is a therapeutic target of critical metabolic and proteostatic signaling in cancer and neurological diseases. Although the heart is critically sensitive to nutrient and energy alterations, and multiple signaling pathways coordinate to maintain the cardiac cell homeostasis, neither the structure of cardiac 14-3-3 protein interactome, nor potential functional roles of 14-3-3 protein–protein interactions (PPIs) in heart has been explored. Objective: To establish the comprehensive landscape and characterize the functional role of cardiac 14-3-3 PPIs. Methods and Results: We evaluated both RNA expression and protein abundance of 14-3-3 isoforms in mouse heart, followed by co-immunoprecipitation of 14-3-3 proteins and mass spectrometry in left ventricle. We identified 52 proteins comprising the cardiac 14-3-3 interactome. Multiple bioinformatic analyses indicated that more than half of the proteins bound to 14-3-3 are related to mitochondria; and the deduced functions of the mitochondrial 14-3-3 network are to regulate cardiac ATP production via interactions with mitochondrial inner membrane proteins, especially those in mitochondrial complex I. Binding to ribosomal proteins, 14-3-3 proteins likely coordinate protein synthesis and protein quality control. Localizations of 14-3-3 proteins to mitochondria and ribosome were validated via immunofluorescence assays. The deduced function of cardiac 14-3-3 PPIs is to regulate cardiac metabolic homeostasis and proteostasis. Conclusions: Thus, the cardiac 14-3-3 interactome may be a potential therapeutic target in cardiovascular metabolic and proteostatic disease states, as it already is in cancer therapy.

Alkaline-sensitive two-pore domain potassium channels form functional heteromers in pancreatic β-cells

Two-pore domain K⁺ channels (K_2P channels), active as dimers, produce inhibitory currents regulated by a variety of stimuli. Among them, TWIK1-related alkalinization-activated K⁺ channel 1 (TALK1), TWIK1-related alkalinization-activated K⁺ channel 2 (TALK2), and TWIK1-related acid-sensitive K⁺ channel 2 (TASK2) form a subfamily of structurally related K_2P channels stimulated by extracellular alkalosis. The human genes encoding these proteins are clustered at chromosomal region 6p21 and coexpressed in multiple tissues, including the pancreas. The question whether these channels form functional heteromers remained open. By analyzing single-cell transcriptomic data, we show that these channels are coexpressed in insulin-secreting pancreatic β-cells. Using in situ proximity ligation assay and electrophysiology, we show that they form functional heterodimers both upon heterologous expression and under native conditions in human pancreatic β-cells. We demonstrate that heteromerization of TALK2 with TALK1 or with TASK2 endows TALK2 with sensitivity to extracellular alkalosis in the physiological range. We further show that the association of TASK2 with TALK1 and TALK2 increases their unitary conductance. These results provide a new example of heteromerization in the K_2P channel family expanding the range of the potential physiological and pathophysiological roles of TALK1/TALK2/TASK2 channels, not only in insulin-secreting cells but also in the many other tissues in which they are coexpressed.

Identification of HCN1 as a 14-3-3 client

Hyperpolarization activated cyclic nucleotide-gated channel 1 (HCN1) is expressed throughout the nervous system and is critical for regulating neuronal excitability, with mutations being associated with multiple forms of epilepsy. Adaptive modulation of HCN1 has been observed, as has pathogenic dysregulation. While the mechanisms underlying this modulation remain incompletely understood, regulation of HCN1 has been shown to include phosphorylation. A candidate phosphorylation-dependent regulator of HCN1 channels is 14-3-3. We used bioinformatics to identify three potential 14-3-3 binding sites in HCN1. We confirmed that 14-3-3 could pull down HCN1 from multiple tissue sources and used HEK293 cells to detail the interaction. Two sites in the intrinsically disordered C-terminus of HCN1 were necessary and sufficient for a phosphorylation-dependent interaction with 14-3-3. The same region of HCN1 containing the 14-3-3 binding peptides is required for phosphorylation-independent protein degradation. We propose a model in which phosphorylation of mouse S810 and S867 (human S789 and S846) recruits 14-3-3 to inhibit a yet unidentified factor signaling for protein degradation, thus increasing the half-life of HCN1.

A role for TASK2 channels in the human immunological synapse

The immunological synapse is a transient junction that occurs when the plasma membrane of a T cell comes in close contact with an APC after recognizing a peptide from the antigen-MHC. The interaction starts when CRAC channels embedded in the T cell membrane open, flowing calcium ions into the cell. To counterbalance the ion influx and subsequent depolarization, K_v 1.3 and KCa3.1 channels are recruited to the immunological synapse, increasing the extracellular K⁺ concentration. These processes are crucial as they initiate gene expression that drives T cell activation and proliferation. The T cell-specific function of the K_2P channel family member TASK2 channels and their role in autoimmune processes remains unclear. Using mass spectrometry analysis together with epifluorescence and super-resolution single-molecule localization microscopy, we identified TASK2 channels as novel players recruited to the immunological synapse upon stimulation. TASK2 localizes at the immunological synapse, upon stimulation with CD3 antibodies, likely interacting with these molecules. Our findings suggest that, together with K_v 1.3 and KCa3.1 channels, TASK2 channels contribute to the proper functioning of the immunological synapse, and represent an interesting treatment target for T cell-mediated autoimmune disorders.

14-3-3 modulation of the inflammatory response

Regulation of inflammation is a central part of the maintenance of homeostasis by the immune system. One important class of regulatory protein that has been shown to have effects on the inflammatory process are the 14-3-3 proteins. Herein we describe the roles that have been identified for 14-3-3 in regulation of the inflammatory response. These roles encompass regulation of the response that affect inflammation at the genetic, molecular and cellular levels. At a genetic level 14-3-3 is involved in the regulation of multiple transcription factors and affects the transcription of key effectors of the immune response. At a molecular level many of the constituent parts of the inflammatory process, such as pattern recognition receptors, protease activated receptors and cytokines are regulated through phosphorylation and recognition by 14-3-3 whilst disruption of the recognition processes has been observed to result in clinical syndromes. 14-3-3 is also involved in the regulation of cell proliferation and differentiation, this has been shown to affect the immune system, particularly T- and B-cells. Finally, we discuss how abnormal levels of 14-3-3 contribute to undesirable immune responses and chronic inflammatory conditions.

Intracellular fluoride influences TASK mediated currents in human T cells

The expression of Kv1.3 and K_Ca channels in human T cells is essential for maintaining cell activation, proliferation and migration during an inflammatory response. Recently, an additional residual current, sensitive to anandamide and A293, compounds specifically inhibiting currents mediated by TASK channels, was observed after complete pharmacological blockade of Kv1.3 and K_Ca channels. This finding was not consistently observed throughout different studies and, an in-depth review of the different recording conditions used for the electrophysiological analysis of K⁺ currents in T cells revealed fluoride as major anionic component of the pipette intracellular solutions in the initial studies. While fluoride is frequently used to stabilize electrophysiological recordings, it is known as G-protein activator and to influence the intracellular Ca²⁺ concentration, which are mechanisms known to modulate TASK channel functioning. Therefore, we systemically addressed different fluoride- and chloride-based pipette solutions in whole-cell patch-clamp experiments in human T cells and used specific blockers to identify membrane currents carried by TASK and Kv1.3 channels. We found that fluoride increased the decay time constant of K⁺ outward currents, reduced the degree of the sustained current component and diminished the effect of the specific TASK channels blocker A293. These findings indicate that the use of fluoride-based pipette solutions may hinder the identification of a functional TASK channel component in electrophysiological experiments.

14-3-3γ, a novel regulator of the large-conductance Ca2+-activated K+ channel

14-3-3γ is a small protein regulating its target proteins through binding to phosphorylated serine/threonine residues. Sequence analysis of large-conductance Ca²⁺-activated K⁺ (BK) channels revealed a putative 14-3-3 binding site in the COOH-terminal region. Our previous data showed that 14-3-3γ is widely expressed in the mouse kidney. Therefore, we hypothesized that 14-3-3γ has a novel role in the regulation of BK channel activity and protein expression. We used electrophysiology, Western blot analysis, and coimmunoprecipitation to examine the effects of 14-3-3γ on BK channels both in vitro and in vivo. We demonstrated the interaction of 14-3-3γ with BK α-subunits (BKα) by coimmunoprecipitation. In human embryonic kidney-293 cells stably expressing BKα, overexpression of 14-3-3γ significantly decreased BK channel activity and channel open probability. 14-3-3γ inhibited both total and cell surface BKα protein expression while enhancing ERK1/2 phosphorylation in Cos-7 cells cotransfected with flag-14-3-3γ and myc-BK. Knockdown of 14-3-3γ by siRNA transfection markedly increased BKα expression. Blockade of the ERK1/2 pathway by incubation with the MEK-specific inhibitor U0126 partially abolished 14-3-3γ-mediated inhibition of BK protein expression. Similarly, pretreatment of the lysosomal inhibitor bafilomycin A1 reversed the inhibitory effects of 14-3-3γ on BK protein expression. Furthermore, overexpression of 14-3-3γ significantly increased BK protein ubiquitination in embryonic kidney-293 cells stably expressing BKα. Additionally, 3 days of dietary K⁺ challenge reduced 14-3-3γ expression and ERK1/2 phosphorylation while enhancing renal BK protein expression and K⁺ excretion. These data suggest that 14-3-3γ modulates BK channel activity and protein expression through an ERK1/2-mediated ubiquitin-lysosomal pathway.

Possible Contribution of Inflammation-Associated Hypoxia to Increased K2P5.1 K+ Channel Expression in CD4+ T cells of the Mouse Model for Inflammatory Bowel Disease

Previous studies have reported the up-regulation of the two-pore domain K⁺ channel K_2P5.1 in the CD4⁺ T cells of patients with multiple sclerosis (MS) and rheumatoid arthritis (RA), as well as in a mouse model of inflammatory bowel disease (IBD). However, the mechanisms underlying this up-regulation remain unclear. Inflammation-associated hypoxia is involved in the pathogenesis of autoimmune diseases, such as IBD, MS, and RA, and T cells are exposed to a hypoxic environment during their recruitment from inflamed tissues to secondary lymphoid tissues. We herein investigated whether inflammation-associated hypoxia is attributable to the increased expression and activity of K_2P5.1 in the splenic CD4⁺ T cells of chemically-induced IBD model mice. Significant increases in hypoxia-inducible factor (HIF)-1α transcripts and proteins were found in the splenic CD4⁺ T cells of the IBD model. In the activated splenic CD4⁺ T cells, hypoxia (1.5% O₂) increased K_2P5.1 expression and activity, whereas a treatment with the HIF inhibitor FM19G11 but not the selective HIF-2 inhibitor exerted the opposite effect. Hypoxia-exposed K_2P5.1 up-regulation was also detected in stimulated thymocytes and the mouse T-cell line. The class III histone deacetylase sirtuin-1 (SIRT1) is a downstream molecule of HIF-1α signaling. We examined the effects of the SIRT1 inhibitor NCO-01 on K_2P5.1 transcription in activated CD4⁺ T cells, and we found no significant effects on the K_2P5.1 transcription. No acute compensatory responses of K_2P3.1–K_2P5.1 up-regulation were found in the CD4⁺ T cells of the IBD model and the hypoxia-exposed T cells. Collectively, these results suggest a mechanism for K_2P5.1 up-regulation via HIF-1 in the CD4⁺ T cells of the IBD model.

The Potassium Channel Odyssey: Mechanisms of Traffic and Membrane Arrangement

Ion channels are transmembrane proteins that conduct specific ions across biological membranes. Ion channels are present at the onset of many cellular processes, and their malfunction triggers severe pathologies. Potassium channels (KChs) share a highly conserved signature that is necessary to conduct K⁺ through the pore region. To be functional, KChs require an exquisite regulation of their subcellular location and abundance. A wide repertoire of signatures facilitates the proper targeting of the channel, fine-tuning the balance that determines traffic and location. These signature motifs can be part of the secondary or tertiary structure of the protein and are spread throughout the entire sequence. Furthermore, the association of the pore-forming subunits with different ancillary proteins forms functional complexes. These partners can modulate traffic and activity by adding their own signatures as well as by exposing or masking the existing ones. Post-translational modifications (PTMs) add a further dimension to traffic regulation. Therefore, the fate of a KCh is not fully dependent on a gene sequence but on the balance of many other factors regulating traffic. In this review, we assemble recent evidence contributing to our understanding of the spatial expression of KChs in mammalian cells. We compile specific signatures, PTMs, and associations that govern the destination of a functional channel.

Plasma kallikrein modulates immune cell trafficking during neuroinflammation via PAR2 and bradykinin release

Blood-brain barrier (BBB) disruption and transendothelial trafficking of immune cells into the central nervous system (CNS) are pathophysiological hallmarks of neuroinflammatory disorders like multiple sclerosis (MS). Recent evidence suggests that the kallikrein-kinin and coagulation system might participate in this process. Here, we identify plasma kallikrein (KK) as a specific direct modulator of BBB integrity. Levels of plasma prekallikrein (PK), the precursor of KK, were markedly enhanced in active CNS lesions of MS patients. Deficiency or pharmacologic blockade of PK renders mice less susceptible to experimental autoimmune encephalomyelitis (a model of MS) and is accompanied by a remarkable reduction of BBB disruption and CNS inflammation. In vitro analysis revealed that KK modulates endothelial cell function in a protease-activated receptor-2-dependent manner, leading to an up-regulation of the cellular adhesion molecules Intercellular Adhesion Molecule 1 and Vascular Cell Adhesion Molecule 1, thereby amplifying leukocyte trafficking. Our study demonstrates that PK is an important direct regulator of BBB integrity as a result of its protease function. Therefore, KK inhibition can decrease BBB damage and cell invasion during neuroinflammation and may offer a strategy for the treatment of MS.

The kinase activity of the channel-kinase protein TRPM7 regulates stability and localization of the TRPM7 channel in polarized epithelial cells

The channel-kinase transient receptor potential melastatin 7 (TRPM7) is a bifunctional protein with ion channel and kinase domains. The kinase activity of TRPM7 has been linked to the regulation of a broad range of cellular activities, but little is understood as to how the channel itself is regulated by its own kinase activity. Here, using several mammalian cell lines expressing WT TRPM7 or kinase-inactive variants, we discovered that compared with the cells expressing WT TRPM7, cells in which TRPM7's kinase activity was inactivated had faster degradation, elevated ubiquitination, and increased intracellular retention of the channel. Mutational analysis of TRPM7 autophosphorylation sites further revealed a role for Ser-1360 of TRPM7 as a key residue mediating both TRPM7 stability and intracellular trafficking. Additional trafficking roles were uncovered for Ser-1403 and Ser-1567, whose phosphorylation by TRPM7's kinase activity mediated the interaction of the channel with the signaling protein 14-3-3θ. In summary, our results point to a critical role for TRPM7's kinase activity in regulating proteasome-mediated turnover of the TRPM7 channel and controlling its cellular localization in polarized epithelial cells. Overall, these findings improve our understanding of the significance of TRPM7's kinase activity for functional regulation of its channel activity.

Phosphorylation of ARHGAP19 by CDK1 and ROCK regulates its subcellular localization and function during mitosis

ARHGAP19 is a hematopoietic-specific Rho GTPase-activating protein (RhoGAP) that acts through the RhoA/ROCK pathway to critically regulate cell elongation and cytokinesis during lymphocyte mitosis. We report here that, during mitosis progression, ARHGAP19 is sequentially phosphorylated by the RhoA-activated kinases ROCK1 and ROCK2 (hereafter ROCK) on serine residue 422, and by CDK1 on threonine residues 404 and 476. The phosphorylation of ARHGAP19 by ROCK occurs before mitosis onset and generates a binding site for 14-3-3 family proteins. ARHGAP19 is then phosphorylated by CDK1 in prometaphase. The docking of 14-3-3 proteins to phosphorylated S422 protects ARHGAP19 from dephosphorylation of the threonine sites and prevents ARHGAP19 from relocating to the plasma membrane during prophase and metaphase, thus allowing RhoA to become activated. Disruption of these phosphorylation sites results in premature localization of ARHGAP19 at the cell membrane and in its enrichment to the equatorial cortex in anaphase leading to cytokinesis failure and cell multinucleation.

Acetylcholine-dependent upregulation of TASK-1 channels in thalamic interneurons by a smooth muscle-like signalling pathway

KEY POINTS

The ascending brainstem transmitter acetylcholine depolarizes thalamocortical relay neurons while it induces hyperpolarization in local circuit inhibitory interneurons. Sustained K⁺ currents are modulated in thalamic neurons to control their activity modes; for the interneurons the molecular nature of the underlying ion channels is as yet unknown. Activation of TASK-1 K⁺ channels results in hyperpolarization of interneurons and suppression of their action potential firing. The modulation cascade involves a non-receptor tyrosine kinase, c-Src. The present study identifies a novel pathway for the activation of TASK-1 channels in CNS neurons that resembles cholinergic signalling and TASK-1 current modulation during hypoxia in smooth muscle cells.

ABSTRACT

The dorsal part of the lateral geniculate nucleus (dLGN) is the main thalamic site for state-dependent transmission of visual information. Non-retinal inputs from the ascending arousal system and inhibition provided by γ-aminobutyric acid (GABA)ergic local circuit interneurons (INs) control neuronal activity within the dLGN. In particular, acetylcholine (ACh) depolarizes thalamocortical relay neurons by inhibiting two-pore domain potassium (K_2P ) channels. Conversely, ACh also hyperpolarizes INs via an as-yet-unknown mechanism. By using whole cell patch-clamp recordings in brain slices and appropriate pharmacological tools we here report that stimulation of type 2 muscarinic ACh receptors induces IN hyperpolarization by recruiting the G-protein βγ subunit (Gβγ), class-1A phosphatidylinositol-4,5-bisphosphate 3-kinase, and cellular and sarcoma (c-Src) tyrosine kinase, leading to activation of two-pore domain weakly inwardly rectifying K⁺ channel (TWIK)-related acid-sensitive K⁺ (TASK)-1 channels. The latter was confirmed by the use of TASK-1-deficient mice. Furthermore inhibition of phospholipase Cβ as well as an increase in the intracellular level of phosphatidylinositol-3,4,5-trisphosphate facilitated the muscarinic effect. Our results have uncovered a previously unknown role of c-Src tyrosine kinase in regulating IN function in the brain and identified a novel mechanism by which TASK-1 channels are activated in neurons.