A conserved cationic motif enhances membrane binding and insertion of the chloride intracellular channel protein 1 transmembrane domain

Chloride Intracellular Channel Proteins (CLICs) and Malignant Tumor Progression: A Focus on the Preventive Role of CLIC2 in Invasion and Metastasis

Simple Summary

Although chloride intracellular channel proteins (CLICs) have been identified as ion channel proteins, their true functions are still elusive. Recent in silico analyses show that CLICs may be prognostic markers in cancer. This review focuses on CLIC2 that plays preventive roles in malignant cell invasion and metastasis. CLIC2 is secreted extracellularly and binds to matrix metalloproteinase 14 (MMP14), while inhibiting its activity. As a result, CLIC2 may contribute to the development/maintenance of junctions between blood vessel endothelial cells and the inhibition of invasion and metastasis of tumor cells. CLIC2 may be a novel therapeutic target for malignancies.

Abstract

CLICs are the dimorphic protein present in both soluble and membrane fractions. As an integral membrane protein, CLICs potentially possess ion channel activity. However, it is not fully clarified what kinds of roles CLICs play in physiological and pathological conditions. In vertebrates, CLICs are classified into six classes: CLIC1, 2, 3, 4, 5, and 6. Recently, in silico analyses have revealed that the expression level of CLICs may have prognostic significance in cancer. In this review, we focus on CLIC2, which has received less attention than other CLICs, and discuss its role in the metastasis and invasion of malignant tumor cells. CLIC2 is expressed at higher levels in benign tumors than in malignant ones, most likely preventing tumor cell invasion into surrounding tissues. CLIC2 is also expressed in the vascular endothelial cells of normal tissues and maintains their intercellular adhesive junctions, presumably suppressing the hematogenous metastasis of malignant tumor cells. Surprisingly, CLIC2 is localized in secretory granules and secreted into the extracellular milieu. Secreted CLIC2 binds to MMP14 and inhibits its activity, leading to suppressed MMP2 activity. CLIC4, on the other hand, promotes MMP14 activity. These findings challenge the assumption that CLICs are ion channels, implying that they could be potential new targets for the treatment of malignant tumors.

CLIC1 recruits PIP5K1A/C to induce cell-matrix adhesions for tumor metastasis

Membrane protrusion and adhesion to the extracellular matrix, which involves the extension of actin filaments and formation of adhesion complexes, are the fundamental processes for cell migration, tumor invasion, and metastasis. How cancer cells efficiently coordinate these processes remains unclear. Here, we showed that membrane-targeted chloride intracellular channel 1 (CLIC1) spatiotemporally regulates the formation of cell-matrix adhesions and membrane protrusions through the recruitment of PIP5Ks to the plasma membrane. Comparative proteomics identified CLIC1 upregulated in human hepatocellular carcinoma (HCC) and associated with tumor invasiveness, metastasis, and poor prognosis. In response to migration-related stimuli, CLIC1 recruited PIP5K1A and PIP5K1C from the cytoplasm to the leading edge of the plasma membrane, where PIP5Ks generate a phosphatidylinositol 4,5-bisphosphate-rich (PIP2-rich) microdomain to induce the formation of integrin-mediated cell-matrix adhesions and the signaling for cytoskeleon extension. CLIC1 silencing inhibited the attachment of tumor cells to culture plates and the adherence and extravasation in the lung alveoli, resulting in suppressed lung metastasis in mice. This study reveals what we believe is an unrecognized mechanism that spatiotemporally coordinates the formation of both lamellipodium/invadopodia and nascent cell-matrix adhesions for directional migration and tumor invasion/metastasis. The unique traits of upregulation and membrane targeting of CLIC1 in cancer cells make it an excellent therapeutic target for tumor metastasis.

A chloride channel blocker prevents the suppression by inorganic phosphate of the cytosolic calcium signals that control muscle contraction

KEY POINTS

Accumulation of inorganic phosphate (P_i ) may contribute to muscle fatigue by precipitating calcium salts inside the sarcoplasmic reticulum (SR). Neither direct demonstration of this process nor definition of the entry pathway of P_i into SR are fully established.  We showed that P_i promoted Ca²⁺ release at concentrations below 10 mm and decreased it at higher concentrations. This decrease correlated well with that of [Ca²⁺ ]_SR .  Pre-treatment of permeabilized myofibres with 2 mm Cl^- channel blocker 9-anthracenecarboxylic acid (9AC) inhibited both effects of P_i .  The biphasic dependence of Ca²⁺ release on [P_i ] is explained by a direct effect of P_i acting on the SR Ca²⁺ release channel, combined with the intra-SR precipitation of Ca²⁺ salts. The effects of 9AC demonstrate that P_i enters the SR via a Cl^- pathway of an as-yet-undefined molecular nature.

ABSTRACT

Fatiguing exercise causes hydrolysis of phosphocreatine, increasing the intracellular concentration of inorganic phosphate (P_i ). P_i diffuses into the sarcoplasmic reticulum (SR) where it is believed to form insoluble Ca²⁺ salts, thus contributing to the impairment of Ca²⁺ release. Information on the P_i entrance pathway is still lacking. In amphibian muscles endowed with isoform 3 of the RyR channel, Ca²⁺ spark frequency is correlated with the Ca²⁺ load of the SR and can be used to monitor this variable. We studied the effects of P_i on Ca²⁺ sparks in permeabilized fibres of the frog. Relative event frequency (f/f_ref ) rose with increasing [P_i ], reaching 2.54 ± 1.6 at 5 mm, and then decreased monotonically, reaching 0.09 ± 0.03 at [P_i ] = 80 mm. Measurement of [Ca²⁺ ]_SR confirmed a decrease correlated with spark frequency at high [P_i ]. A large [Ca²⁺ ]_SR surge was observed upon P_i removal. Anion channels are a putative path for P_i into the SR. We tested the effect of the chloride channel blocker 9-anthracenecarboxylic acid (9AC) on P_i entrance. 9AC (400 µm) applied to the cytoplasm produced a non-significant increase in spark frequency and reduced the P_i effects on this parameter. Fibre treatment with 2 mm 9AC in the presence of high cytoplasmic Mg²⁺ suppressed the effects of P_i on [Ca²⁺ ]_SR and spark frequency up to 55 mm [P_i ]. These results suggest that chloride channels (or transporters) provide the main pathway of inorganic phosphate into the SR and confirm that P_i impairs Ca²⁺ release by accumulating and precipitating with Ca²⁺ inside the SR, thus contributing to myogenic fatigue.

Intracellular Chloride Channels: Novel Biomarkers in Diseases

Ion channels are integral membrane proteins present on the plasma membrane as well as intracellular membranes. In the human genome, there are more than 400 known genes encoding ion channel proteins. Ion channels are known to regulate several cellular, organellar, and physiological processes. Any mutation or disruption in their function can result in pathological disorders, both common or rare. Ion channels present on the plasma membrane are widely acknowledged for their role in various biological processes, but in recent years, several studies have pointed out the importance of ion channels located in intracellular organelles. However, ion channels located in intracellular organelles are not well-understood in the context of physiological conditions, such as the generation of cellular excitability and ionic homeostasis. Due to the lack of information regarding their molecular identity and technical limitations of studying them, intracellular organelle ion channels have thus far been overlooked as potential therapeutic targets. In this review, we focus on a novel class of intracellular organelle ion channels, Chloride Intracellular Ion Channels (CLICs), mainly documented for their role in cardiovascular, neurophysiology, and tumor biology. CLICs have a single transmembrane domain, and in cells, they exist in cytosolic as well as membranous forms. They are predominantly present in intracellular organelles and have recently been shown to be localized to cardiomyocyte mitochondria as well as exosomes. In fact, a member of this family, CLIC5, is the first mitochondrial chloride channel to be identified on the molecular level in the inner mitochondrial membrane, while another member, CLIC4, is located predominantly in the outer mitochondrial membrane. In this review, we discuss this unique class of intracellular chloride channels, their role in pathologies, such as cardiovascular, cancer, and neurodegenerative diseases, and the recent developments concerning their usage as theraputic targets.

Three Decades of Chloride Intracellular Channel Proteins: From Organelle to Organ Physiology

Intracellular organelles are membranous structures central for maintaining cellular physiology and the overall health of the cell. To maintain cellular function, intracellular organelles are required to tightly regulate their ionic homeostasis. Any imbalance in ionic concentrations can disrupt energy production (mitochondria), protein degradation (lysosomes), DNA replication (nucleus), or cellular signaling (endoplasmic reticulum). Ionic homeostasis is also important for volume regulation of intracellular organelles and is maintained by cation and anion channels as well as transporters. One of the major classes of ion channels predominantly localized to intracellular membranes is chloride intracellular channel proteins (CLICs). They are non-canonical ion channels with six homologs in mammals, existing as either soluble or integral membrane protein forms, with dual functions as enzymes and channels. Provided in this overview is a brief introduction to CLICs, and a summary of recent information on their localization, biophysical properties, and physiological roles. © 2018 by John Wiley & Sons, Inc.

Interaction of Human Chloride Intracellular Channel Protein 1 (CLIC1) with Lipid Bilayers: A Fluorescence Study

Chloride intracellular channel protein 1 (CLIC1) is very unusual as it adopts a soluble glutathione S-transferase-like canonical fold but can also autoinsert into lipid bilayers to form an ion channel. The conversion between these forms involves a large, but reversible, structural rearrangement of the CLIC1 module. The only identified environmental triggers controlling the metamorphic transition of CLIC1 are pH and oxidation. Until now, there have been no high-resolution structural data available for the CLIC1 integral membrane state, and consequently, a limited understanding of how CLIC1 unfolds and refolds across the bilayer to form a membrane protein with ion channel activity exists. Here we show that fluorescence spectroscopy can be used to establish the interaction and position of CLIC1 in a lipid bilayer. Our method employs a fluorescence energy transfer (FRET) approach between CLIC1 and a dansyl-labeled lipid analogue to probe the CLIC1-lipid interface. Under oxidizing conditions, a strong FRET signal between the single tryptophan residue of CLIC1 (Trp35) and the dansyl-lipid analogue was detected. When considering the proportion of CLIC1 interacting with the lipid bilayer, as estimated by fluorescence quenching experiments, the FRET distance between Trp35 and the dansyl moiety on the membrane surface was determined to be ∼15 Å. This FRET-detected interaction provides direct structural evidence that CLIC1 associates with membranes. The results presented support the current model of an oxidation-driven interaction of CLIC1 with lipid bilayers and also propose a membrane anchoring role for Trp35.