CLIC4 (p64H1) and its putative transmembrane domain form poorly selective, redox-regulated ion channels

Chloride intracellular channel 1 functions in endothelial cell growth and migration

Background

Little is known about the role of CLIC1 in endothelium. These studies investigate CLIC1 as a regulator of angiogenesis by in vitro techniques that mimic individual steps in the angiogenic process.

Methods

Using shRNA against clic1, we determined the role of CLIC1 in primary human endothelial cell behavior.

Results

Here, we report that reduced CLIC1 expression caused a reduction in endothelial migration, cell growth, branching morphogenesis, capillary-like network formation, and capillary-like sprouting. FACS analysis showed that CLIC1 plays a role in regulating the cell surface expression of various integrins that function in angiogenesis including β1 and α3 subunits, as well as αVβ3 and αVβ5.

Conclusions

Together, these results indicate that CLIC1 is required for multiple steps of in vitro angiogenesis and plays a role in regulating integrin cell surface expression.

Metamorphic response of the CLIC1 chloride intracellular ion channel protein upon membrane interaction

A striking feature of the CLIC (chloride intracellular channel) protein family is the ability of its members to convert between a soluble state and an integral membrane channel form. Direct evidence of the structural transition required for the CLIC protein to autonomously insert into the membrane is lacking, largely because of the challenge of probing the conformation of the membrane-bound protein. However, insights into the CLIC transmembrane form can be gained by biophysical methods such as fluorescence resonance energy transfer (FRET) spectroscopy. This approach was used to measure distances from tryptophan 35, located within the CLIC1 putative N-domain transmembrane region, to three native cysteine residues within the C-terminal domain. These distances were computed both in aqueous solution and upon the addition of membrane vesicles. The FRET distances were used as constraints for modeling of a structure for the CLIC1 integral membrane form. The data are suggestive of a large conformational unfolding occurring between the N- and C-domains of CLIC1 upon interaction with the membrane. Consistent with previous findings, the N-terminal domain of CLIC1 is likely to insert into the lipid bilayer, while the C-domain remains in solution on the extravesicular side of the membrane.

Chloride intracellular channel 4 is critical for the epithelial morphogenesis of RPE cells and retinal attachment

A plasmid-based transfection method was used to cell-autonomously silence chloride intracellular channel 4 (CLIC4) in RPE in situ. These results show CLIC4 is critical for epithelial morphogenesis and retinal attachment. Novel candidate targets for retinal detachment therapy have also been identified.

Retinal detachment is a sight-threatening condition. The molecular mechanism underlying the adhesion between the RPE and photoreceptors is poorly understood because the intimate interactions between these two cell types are impossible to model and study in vitro. In this article, we show that chloride intracellular channel 4 (CLIC4) is enriched at apical RPE microvilli, which are interdigitated with the photoreceptor outer segment. We used a novel plasmid-based transfection method to cell-autonomously suppress CLIC4 in RPE in situ. CLIC4 silenced RPE cells exhibited a significant loss of apical microvilli and basal infoldings, reduced retinal adhesion, and epithelial-mesenchymal transition. Ectopically expressing ezrin failed to rescue the morphological changes exerted by CLIC4 silencing. Neural retinas adjacent to the CLIC4-suppressed RPE cells display severe dysplasia. Finally, a high level of aquaporin 1 unexpectedly appeared at the apical surfaces of CLIC4-suppressed RPE cells, together with a concomitant loss of basal surface expression of monocarboxylate transporter MCT3. Our results suggested that CLIC4 plays an important role in RPE-photoreceptor adhesion, perhaps by modulating the activity of cell surface channels/transporters. We propose that these changes may be attributable to subretinal fluid accumulation in our novel retinal detachment animal model.

S-nitrosylation regulates nuclear translocation of chloride intracellular channel protein CLIC4

Nuclear translocation of chloride intracellular channel protein CLIC4 is essential for its role in Ca(2+)-induced differentiation, stress-induced apoptosis, and modulating TGF-beta signaling in mouse epidermal keratinocytes. However, post-translational modifications on CLIC4 that govern nuclear translocation and thus these activities remain to be elucidated. The structure of CLIC4 is dependent on the redox environment, in vitro, and translocation may depend on reactive oxygen and nitrogen species in the cell. Here we show that NO directly induces nuclear translocation of CLIC4 that is independent of the NO-cGMP pathway. Indeed, CLIC4 is directly modified by NO through S-nitrosylation of a cysteine residue, as measured by the biotin switch assay. NO enhances association of CLIC4 with the nuclear import proteins importin alpha and Ran. This is likely a result of the conformational change induced by S-nitrosylated CLIC4 that leads to unfolding of the protein, as exhibited by CD spectra analysis and trypsinolysis of the modified protein. Cysteine mutants of CLIC4 exhibit altered nitrosylation, nuclear residence, and stability, compared with the wild type protein likely as a consequence of altered tertiary structure. Moreover, tumor necrosis factor alpha-induced nuclear translocation of CLIC4 is dependent on nitric-oxide synthase activity. Inhibition of nitric-oxide synthase activity inhibits tumor necrosis factor alpha-induced nitrosylation and association with importin alpha and Ran and ablates CLIC4 nuclear translocation. These results suggest that S-nitrosylation governs CLIC4 structure, its association with protein partners, and thus its intracellular distribution.

Modulation of intracellular chloride channels by ATP and Mg2+

We report the effects of ATP and Mg2+ on the activity of intracellular chloride channels. Mitochondrial and lysosomal membrane vesicles isolated from rat hearts were incorporated into bilayer lipid membranes, and single chloride channel currents were measured. The observed chloride channels (n=112) possessed a wide variation in single channel parameters and sensitivities to ATP. ATP (0.5-2 mmol/l) modulated and/or inhibited the chloride channel activities (n=38/112) in a concentration-dependent manner. The inhibition effect was irreversible (n=5/93) or reversible (n=15/93). The non-hydrolysable ATP analogue AMP-PNP had a similar inhibition effect as ATP, indicating that phosphorylation did not play a role in the ATP inhibition effect. ATP modulated the gating properties of the channels (n=6/93), decreased the channels' open dwell times and increased the gating transition rates. ATP (0.5-2 mmol/l) without the presence of Mg2+ decreased the chloride channel current (n=12/14), whereas Mg2+ significantly reversed the effect (n=4/4). We suggest that ATP-intracellular chloride channel interactions and Mg2+ modulation of these interactions may regulate different physiological and pathological processes.

Chloride channels of intracellular membranes

Proteins implicated as intracellular chloride channels include the intracellular ClC proteins, the bestrophins, the cystic fibrosis transmembrane conductance regulator, the CLICs, and the recently described Golgi pH regulator. This paper examines current hypotheses regarding roles of intracellular chloride channels and reviews the evidence supporting a role in intracellular chloride transport for each of these proteins.

The enigma of the CLIC proteins: Ion channels, redox proteins, enzymes, scaffolding proteins?

Chloride intracellular channel proteins (CLICs) are distinct from most ion channels in that they have both soluble and integral membrane forms. CLICs are highly conserved in chordates, with six vertebrate paralogues. CLIC-like proteins are found in other metazoans. CLICs form channels in artificial bilayers in a process favoured by oxidising conditions and low pH. They are structurally plastic, with CLIC1 adopting two distinct soluble conformations. Phylogenetic and structural data indicate that CLICs are likely to have enzymatic function. The physiological role of CLICs appears to be maintenance of intracellular membranes, which is associated with tubulogenesis but may involve other substructures.

Novel channels of the inner mitochondrial membrane

Along with a large number of carriers, exchangers and "pumps", the inner mitochondrial membrane contains ion-conducting channels which endow it with controlled permeability to small ions. Some have been shown to be the mitochondrial counterpart of channels present also in other cellular membranes. The manuscript summarizes the current state of knowledge on the major inner mitochondrial membrane channels, properties, identity and proposed functions. Considerable attention is currently being devoted to two K(+)-selective channels, mtK(ATP) and mtBK(Ca). Their activation in "preconditioning" is considered by many to underlie the protection of myocytes and other cells against subsequent ischemic damage. We have recently shown that in apoptotic lymphocytes inner membrane mtK(V)1.3 interacts with the pro-apoptotic protein Bax after the latter has inserted into the outer mitochondrial membrane. Whether the just-discovered mtIK(Ca) has similar cellular role(s) remains to be seen. The Ca(2+) "uniporter" has been characterized electrophysiologically, but still awaits a molecular identity. Chloride-selective channels are represented by the 107 pS channel, the first mitochondrial channel to be observed by patch-clamp, and by a approximately 400 pS pore we have recently been able to fully characterize in the inner membrane of mitochondria isolated from a colon tumour cell line. This we propose to represent a component of the Permeability Transition Pore. The available data exclude the previous tentative identification with porin, and indicate that it coincides instead with the still molecularly unidentified "maxi" chloride channel.

Interactions of tight junctions with membrane channels and transporters

Tight junctions are unique organelles in epithelial cells. They are localized to the apico-lateral region and essential for the epithelial cell transport functions. The paracellular transport process that occurs via tight junctions is extensively studied and is intricately regulated by various extracellular and intracellular signals. Fine regulation of this transport pathway is crucial for normal epithelial cell functions. Among factors that control tight junction permeability are ions and their transporters. However, this area of research is still in its infancy and much more needs to be learned about how these molecules regulate tight junction structure and functions. In this review we have attempted to compile literature on ion transporters and channels involved in the regulation of tight junctions.

Functional reconstitution of mammalian 'chloride intracellular channels' CLIC1, CLIC4 and CLIC5 reveals differential regulation by cytoskeletal actin

Chloride intracellular channels (CLICs) are soluble, signal peptide-less proteins that are distantly related to Omega-type glutathione-S-transferases. Although some CLICs bypass the classical secretory pathway and autoinsert into cell membranes to form ion channels, their cellular roles remain unclear. Many CLICs are strongly associated with cytoskeletal proteins, but the role of these associations is not known. In this study, we incorporated purified, recombinant mammalian CLIC1, CLIC4 and (for the first time) CLIC5 into planar lipid bilayers, and tested the hypothesis that the channels are regulated by actin. CLIC5 formed multiconductance channels that were almost equally permeable to Na(+), K(+) and Cl(-), suggesting that the 'CLIC' nomenclature may need to be revised. CLIC1 and CLIC5, but not CLIC4, were strongly and reversibly inhibited (or inactivated) by 'cytosolic' F-actin in the absence of any other protein. This inhibition effect on channels could be reversed by using cytochalasin to disrupt the F-actin. We suggest that actin-regulated membrane CLICs could modify solute transport at key stages during cellular events such as apoptosis, cell and organelle division and fusion, cell-volume regulation, and cell movement.

Stilbene derivatives inhibit the activity of the inner mitochondrial membrane chloride channels

Ion channels selective for chloride ions are present in all biological membranes, where they regulate the cell volume or membrane potential. Various chloride channels from mitochondrial membranes have been described in recent years. The aim of our study was to characterize the effect of stilbene derivatives on single-chloride channel activity in the inner mitochondrial membrane. The measurements were performed after the reconstitution into a planar lipid bilayer of the inner mitochondrial membranes from rat skeletal muscle (SMM), rat brain (BM) and heart (HM) mitochondria. After incorporation in a symmetric 450/450 mM KCl solution (cis/trans), the chloride channels were recorded with a mean conductance of 155 ± 5 pS (rat skeletal muscle) and 120 ± 16 pS (rat brain). The conductances of the chloride channels from the rat heart mitochondria in 250/50 mM KCl (cis/trans) gradient solutions were within the 70–130 pS range. The chloride channels were inhibited by these two stilbene derivatives: 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) and 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS). The skeletal muscle mitochondrial chloride channel was blocked after the addition of 1 mM DIDS or SITS, whereas the brain mitochondrial channel was blocked by 300 μM DIDS or SITS. The chloride channel from the rat heart mitochondria was inhibited by 50–100 μM DIDS. The inhibitory effect of DIDS was irreversible. Our results confirm the presence of chloride channels sensitive to stilbene derivatives in the inner mitochondrial membrane from rat skeletal muscle, brain and heart cells.