Molecular identification of the dominant-negative, splicing isoform of the two-pore domain K(+) channel K(2P)5.1 in lymphoid cells and enhancement of its expression by splicing inhibition

Exploring the therapeutic opportunities of potassium channels for the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease that affects the synovial joint, which leads to inflammation, loss of function, joint destruction, and disability. The disease biology of RA involves complex interactions between genetic and environmental factors and is strongly associated with various immune cells, and each of the cell types contributes differently to disease pathogenesis. Several immunomodulatory molecules, such as cytokines, are secreted from the immune cells and intervene in the pathogenesis of RA. In immune cells, membrane proteins such as ion channels and transporters mediate the transport of charged ions to regulate intracellular signaling pathways. Ion channels control the membrane potential and effector functions such as cytotoxic activity. Moreover, clinical studies investigating patients with mutations and alterations in ion channels and transporters revealed their importance in effective immune responses. Recent studies have shown that voltage-gated potassium channels and calcium-activated potassium channels and their subtypes are involved in the regulation of immune cells and RA. Due to the role of these channels in the pathogenesis of RA and from multiple pieces of clinical evidence, they can be considered therapeutic targets for the treatment of RA. Here, we describe the role of voltage-gated and calcium-activated potassium channels and their subtypes in RA and their pharmacological application as drug targets.

Role of K+ and Ca2+-Permeable Channels in Osteoblast Functions

Bone-forming cells or osteoblasts play an important role in bone modeling and remodeling processes. Osteoblast differentiation or osteoblastogenesis is orchestrated by multiple intracellular signaling pathways (e.g., bone morphogenetic proteins (BMP) and Wnt signaling pathways) and is modulated by the extracellular environment (e.g., parathyroid hormone (PTH), vitamin D, transforming growth factor β (TGF-β), and integrins). The regulation of bone homeostasis depends on the proper differentiation and function of osteoblast lineage cells from osteogenic precursors to osteocytes. Intracellular Ca²⁺ signaling relies on the control of numerous processes in osteoblast lineage cells, including cell growth, differentiation, migration, and gene expression. In addition, hyperpolarization via the activation of K⁺ channels indirectly promotes Ca²⁺ signaling in osteoblast lineage cells. An improved understanding of the fundamental physiological and pathophysiological processes in bone homeostasis requires detailed investigations of osteoblast lineage cells. This review summarizes the current knowledge on the functional impacts of K⁺ channels and Ca²⁺-permeable channels, which critically regulate Ca²⁺ signaling in osteoblast lineage cells to maintain bone homeostasis.

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.

Transcriptional repression of human epidermal growth factor receptor 2 by ClC-3 Cl- /H+ transporter inhibition in human breast cancer cells

Recent studies have indicated that the intracellular concentration of chloride ions (Cl⁻) regulates gene expression in several types of cells and that Cl⁻ modulators positively or negatively regulate the PI3K/AKT/mammalian target of rapamycin (mTOR) and signal transducer and activator of transcription (STAT)3 signaling pathways. We previously reported that the Ca²⁺‐activated Cl⁻ channel anoctamine (ANO)1 regulated human epidermal growth factor receptor 2 (HER2) transcription in breast cancer YMB‐1 cells. However, the mechanisms underlying ANO1‐regulated HER2 gene expression have not yet been elucidated. In the present study, we showed the involvement of intracellular organelle ClC‐3 Cl⁻/H⁺ transporter in HER2 transcription in breast cancer MDA‐MB‐453 cells. The siRNA‐mediated inhibition of ClC‐3, but not ANO1, markedly repressed HER2 transcription in MDA‐MB‐453 cells. Subsequently, treatments with the AKT inhibitor AZD 5363 and mTOR inhibitor everolimus significantly enhanced HER2 transcription in MDA‐MB‐453 cells, whereas that with the STAT3 inhibitor 5,15‐diphenylporphyrin (5,15‐DPP) inhibited it. AKT and mTOR inhibitors also significantly enhanced HER2 transcription in YMB‐1 cells. The siRNA‐mediated inhibition of ClC‐3 and ANO1 resulted in increased AKT phosphorylation and decreased STAT3 phosphorylation in MDA‐MB‐453 and YMB‐1 cells, respectively. The intracellular Cl⁻ channel protein CLIC1 was expressed in both cells; however, its siRNA‐mediated inhibition did not elicit the transcriptional repression of HER2. Collectively, our results demonstrate that intracellular Cl⁻ regulation by ANO1/ClC‐3 participates in HER2 transcription, mediating the PI3K/AKT/mTOR and/or STAT3 signaling pathway(s) in HER2‐positive breast cancer cells, and support the potential of ANO1/ClC‐3 blockers as therapeutic options for patients with resistance to anti‐HER2 therapies.

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

K_2P 5.1 channels (also called TASK-2 or Kcnk5) have already been shown to be relevant in the pathophysiology of autoimmune disease because they are known to be upregulated on peripheral and central T lymphocytes of multiple sclerosis (MS) patients. Moreover, overexpression of K_2P 5.1 channels in vitro provokes enhanced T-cell effector functions. However, the molecular mechanisms regulating intracellular K_2P 5.1 channel trafficking are unknown so far. Thus, the aim of the study is to elucidate the trafficking of K_2P 5.1 channels on T lymphocytes. Using mass spectrometry analysis, we have identified 14-3-3 proteins as novel binding partners of K_2P 5.1 channels. We show that a non-classical 14-3-3 consensus motif (R-X-X-pT/S-x) at the channel's C-terminus allows the binding between K_2P 5.1 and 14-3-3. The mutant K_2P 5.1/S266A diminishes the protein-protein interaction and reduces the amplitude of membrane currents. Application of a non-peptidic 14-3-3 inhibitor (BV02) significantly reduces the number of wild-type channels in the plasma membrane, whereas the drug has no effect on the trafficking of the mutated channel. Furthermore, blocker application reduces T-cell effector functions. Taken together, we demonstrate that 14-3-3 interacts with K_2P 5.1 and plays an important role in channel trafficking.

[Physiological Role of K(+) Channels in the Regulation of T Cell Function]

Potassium ion (K(+)) channels play an important role in the modulation of calcium ion (Ca(2+)) signaling via control of the membrane potential. In T-lymphocytes, the voltage-gated K(+) channel, KV1.3, and the intermediate-conductance Ca(2+)-activated K(+) channel, KCa3.1, predominantly contribute to K(+) conductance, and are responsible for cell proliferation, differentiation, apoptosis and infiltration. Inflammatory bowel disease (IBD), including ulcerative colitis and Crohn's disease, afflicts more than 0.1% of the population worldwide. In the chemically-induced IBD model mouse, an increase in KCa3.1 activity was observed in mesenteric lymph node CD4(+) T-lymphocytes, concomitant with an upregulation of KCa3.1 and a positive KCa3.1 regulator, NDPK-B. Pharmacological blockade of the KCa3.1 K(+) channel by TRAM-34 and/or ICA17043 elicited 1) a significant decrease in IBD severity, as assessed by diarrhea, visible fecal blood, inflammation and crypt damage of the colon; and 2) restoration of the expression levels of KCa3.1 and Th1 cytokines in CD4(+) T-lymphocytes in the IBD model. Recent studies have indicated the impact of K2P5.1 upregulation in T lymphocytes on the pathogenesis of autoimmune diseases such as rheumatoid arthritis and multiple sclerosis. The K2P5.1 K(+) channel is therefore highlighted as a potent therapeutic target in managing the pathogenesis of autoimmune diseases. Alternatively, pre-mRNA splicing of ion channels is associated with the development and progression of various diseases, including autoimmune diseases. Therefore, mRNA-splicing mechanisms underlying the transcriptional regulation of K2P5.1 K(+) channels may be a new strategic therapeutic target for autoimmune and inflammatory diseases.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.