A scaffolding for regulation of volume-sensitive Cl- channels

Volume-regulated chloride channel regulates cell proliferation and is involved in the possible interaction between TMEM16A and LRRC8A in human metastatic oral squamous cell carcinoma cells

OBJECTIVES

Volume-regulated anion channels (VRACs), expressed in various cells, play an important role in cell volume regulation. Despite being physiologically defined almost half a century ago, only the molecular candidates of VRAC, TMEM16A, LRRC8A, and bestrophin-1 (BEST1), are known. Here, we aimed to explore the functional significance of VRAC in, HST-1, an oral squamous cell carcinoma (OSCC) cell line.

METHODS

Cell proliferation assays, RT-PCR, Western blot, and flow cytometry were used to estimate changes in gene expression and cell proliferation. Ion channel activity was recorded using the patch-clamp technique. Specific genes were knocked-down by siRNA assays.

RESULTS

VRAC, identified as a hypotonicity-induced current, was highly functional and associated with the proliferation of HST-1 cells but not of HaCaT (a normal keratinocyte) cells. The pharmacological profile of VRAC in HST-1 was similar to that reported previously. DCPIB, a specific VRAC inhibitor, completely inhibited VRAC and proliferation of HST-1 cells, eventually leading to apoptosis. VRAC in HST-1 was attenuated by the knockdown of TMEM16A and LRRC8A, while knockdown of BEST1 affected cell proliferation. In situ proximity ligation assay showed that TMEM16A and LRRC8A co-localized under isotonic conditions (300 mOsM) but were separated under hypotonic conditions (250 mOsM) on the plasma membrane.

CONCLUSIONS

We have found that VRAC acts to regulate the proliferation of human metastatic OSCC cells and the composition of VRAC may involve in the interactions between TMEM16A and LRRC8A in HST-1 cells.

The Next Generation Non-competitive Active Polyester Nanosystems for Transferrin Receptor-mediated Peroral Transport Utilizing Gambogic Acid as a Ligand

The current methods for targeted drug delivery utilize ligands that must out-compete endogenous ligands in order to bind to the active site facilitating the transport. To address this limitation, we present a non-competitive active transport strategy to overcome intestinal barriers in the form of tunable nanosystems (NS) for transferrin receptor (TfR) utilizing gambogic acid (GA), a xanthanoid, as its ligand. The NS made using GA conjugated poly(lactide-co-glycolide) (PLGA) have shown non-competitive affinity to TfR evaluated in cell/cell-free systems. The fluorescent PLGA-GA NS exhibited significant intestinal transport and altered distribution profile compared to PLGA NS in vivo. The PLGA-GA NS loaded with cyclosporine A (CsA), a model peptide, upon peroral dosing to rodents led to maximum plasma concentration of CsA at 6 h as opposed to 24 h with PLGA-NS with at least 2-fold higher levels in brain at 72 h. The proposed approach offers new prospects for peroral drug delivery and beyond.

Caveolae act as membrane reserves which limit mechanosensitive I(Cl,swell) channel activation during swelling in the rat ventricular myocyte

Background

Many ion channels are preferentially located in caveolae where compartmentalisation/scaffolding with signal transduction components regulates their activity. Channels that are mechanosensitive may be additionally dependent on caveolar control of the mechanical state of the membrane. Here we test which mechanism underlies caveolar-regulation of the mechanosensitive I_Cl,swell channel in the adult cardiac myocyte.

Methodology/Principal Findings

Rat ventricular myocytes were exposed to solution of 0.02 tonicity (T; until lysis), 0.64T for 10–15 min (swelling), and/or methyl-β-cyclodextrin (MBCD; to disrupt caveolae). MBCD and 0.64T swelling reduced the number of caveolae visualised by electron microscopy by 75 and 50% respectively. MBCD stimulated translocation of caveolin 3 from caveolae-enriched buoyant membrane fractions, but both caveolin 1 and 3 remained in buoyant fractions after swelling. I_Cl,swell inhibition in control cells decreased time to half-maximal volume (t_0.5,vol; 0.64T), consistent with a role for I_Cl,swell in volume regulation. MBCD-treated cells showed reduced time to lysis (0.02T) and t_0.5,vol (0.64T) compared with controls. The negative inotropic response to swelling (an index of I_Cl,swell activation) was enhanced by MBCD.

Conclusions/Significance

These data show that disrupting caveolae removes essential membrane reserves, which speeds swelling in hyposmotic conditions, and thereby promotes activation of I_Cl,swell. They illustrate a general principle whereby caveolae as a membrane reserve limit increases in membrane tension during stretch/swelling thereby restricting mechanosensitive channel activation.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Hypotonic swelling stimulates L-type Ca2+ channel activity in vascular smooth muscle cells through PKC

It has been suggested that L-type Ca(2+) channels play an important role in cell swelling-induced vasoconstriction. However, there is no direct evidence that Ca(2+) channels in vascular smooth muscle are modulated by cell swelling. We tested the hypothesis that L-type Ca(2+) channels in rabbit portal vein myocytes are modulated by hypotonic cell swelling via protein kinase activation. Ba(2+) currents (I(Ba)) through L-type Ca(2+) channels were recorded in smooth muscle cells freshly isolated from rabbit portal vein with the conventional whole cell patch-clamp technique. Superfusion of cells with hypotonic solution reversibly enhanced Ca(2+) channel activity but did not alter the voltage-dependent characteristics of Ca(2+) channels. Bath application of selective inhibitors of protein kinase C (PKC), Ro-31-8425 or Go-6983, prevented I(Ba) enhancement by hypotonic swelling, whereas the specific protein kinase A (PKA) inhibitor KT-5720 had no effect. Bath application of phorbol 12,13-dibutyrate (PDBu) significantly increased I(Ba) under isotonic conditions and prevented current stimulation by hypotonic swelling. However, PDBu did not have any effect on I(Ba) when cells were first exposed to hypotonic solution. Furthermore, downregulation of endogenous PKC by overnight treatment of cells with PDBu prevented current enhancement by hypotonic swelling. These data suggest that hypotonic cell swelling can enhance Ca(2+) channel activity in rabbit portal vein smooth muscle cells through activation of PKC.

Targeting of ion channels to membrane microdomains: localization of KV channels to lipid rafts

Voltage-gated K(+) channels are an important determinant of cellular excitability and key components of multiple signal transduction pathways. However, relatively little is known about the mechanisms of K(V) channel localization or their membrane partitioning. Lipid rafts are specialized membrane microdomains that are rich in sphingolipids and cholesterol. These rafts have been implicated in the organization of many membrane-associated signaling pathways and are currently the focus of intense interest in the scientific community. Biochemical and functional evidence indicate that K(V) channels, in addition to other ion channels, localize to lipid raft microdomains on the cell surface. Although several important questions regarding specific mechanisms of channel localization remain, emerging data indicate that protein-lipid interactions should be considered as a new mechanism of ion channel localization and compartmentation that might permit the therapeutic modulation of channel properties via alteration in membrane lipids.

Regulatory volume increase is associated with p38 kinase-dependent actin cytoskeleton remodeling in rat kidney MTAL

The kidney medulla is physiologically exposed to variations in extracellular osmolality. In response to hypertonic cell shrinkage, cells of the rat kidney medullary thick ascending limb of Henle's loop undergo p38 kinase-dependent regulatory volume increase (RVI). In the present study, we investigated the role of actin cytoskeleton reorganization in this process. Addition of hyperosmotic NaCl or sucrose, which activates MAP kinases and reduces cellular volume, induced a sustained actin polymerization occurring after 10 min and concurrently with RVI. In contrast, hyperosmotic urea, which does not modify MAP kinase activity and cellular volume, did not induce sustained actin polymerization. Fluorescence microscopy revealed that hyperosmotic NaCl and sucrose, but not urea, induced the redistribution of F-actin from a dense cortical ring to a diffuse network of actin bundles. Stabilization of actin filaments by jasplakinolide and inhibition of the generation of new actin filaments by swinholide A prevented RVI, whereas depolymerization of actin filaments by latrunculin B attenuated cell shrinkage and enhanced RVI. These actin-interfering drugs did not alter extracellular regulated kinase and p38 kinase activation under hypertonic conditions. Similar to swinholide A, inhibiting p38 kinase with SB-203580 abolished sustained actin polymerization, actin redistribution, and decreased RVI efficacy. We therefore propose that in rat kidney the medullary thick ascending limb of Henle's loop exposed to extracellular hypertonicity, p38 kinase activation induces depolymerization of the F-actin cortical ring and polymerization of a dense diffuse F-actin network that both contribute to increase RVI efficacy.

Regulation of volume-sensitive outwardly rectifying anion channels in pulmonary arterial smooth muscle cells by PKC

We tested the possible role of endogenous protein kinase C (PKC) in the regulation of native volume-sensitive organic osmolyte and anion channels (VSOACs) in acutely dispersed canine pulmonary artery smooth muscle cells (PASMC). Hypotonic cell swelling activated native volume-regulated Cl(-) currents (I(Cl.vol)) which could be reversed by exposure to phorbol 12,13-dibutyrate (0.1 microM) or by hypertonic cell shrinkage. Under isotonic conditions, calphostin C (0.1 microM) or Ro-31-8425 (0.1 microM), inhibitors of both conventional and novel PKC isozymes, significantly activated I(Cl.vol) and prevented further modulation by subsequent hypotonic cell swelling. Bisindolylmaleimide (0.1 microM), a selective conventional PKC inhibitor, was without effect. Dialyzing acutely dispersed and cultured PASMC with epsilon V1-2 (10 microM), a translocation inhibitory peptide derived from the V1 region of epsilon PKC, activated I(Cl.vol) under isotonic conditions and prevented further modulation by cell volume changes. Dialyzing PASMC with beta C2-2 (10 microM), a translocation inhibitory peptide derived from the C2 region of beta PKC, had no detectable effect. Immunohistochemistry in cultured canine PASMC verified that hypotonic cell swelling is accompanied by translocation of epsilon PKC from the vicinity of the membrane to cytoplasmic and perinuclear locations. These data suggest that membrane-bound epsilon PKC controls the activation state of native VSOACs in canine PASMC under isotonic and anisotonic conditions.

Activation of volume-regulated Cl(-) channels by ACh and ATP in Xenopus follicles

Osmolarity-dependent ionic currents from follicle-enclosed Xenopus oocytes (follicles) were studied using electrophysiological techniques. Whole follicle currents were monitored using a two-electrode voltage clamp and single-channel activity was measured using the patch-clamp technique. In follicles held at -60 mV two chloride currents were activated in external hyposmotic solutions. One was the habitual volume-regulated current elicited by external hyposmolarity (ICl,swell), and the second was a slow and smooth current (Sin) generated by ACh or ATP application. In follicles, the permeability ratios for different anions with respect to Cl- were similar for both ICl,swell and Sin, with a sequence of: SCN- > I- > Br- >= NO3- >= Cl- > gluconate >= cyclamate > acetate > SO42-. Extracellular ATP blocked the outward component of Sin. Also, extracellular pH modulated the inactivation kinetics of Sin elicited by ACh; e.g. inactivation at +80 mV was approximately 100 % slower at pH 8.0 compared with that at pH 6.0. Lanthanides inhibited ICl, swell and Sin. La3+ completely inhibited ICl,swell with a half-maximal inhibitory concentration (IC50) of 17 +/- 1.9 microM, while Sin was blocked up to 55 % with an apparent IC50 of 36 +/- 2.6 microM. Patch-clamp recordings in follicular cells showed that hyposmotic challenge opened inward single-channel currents. The single channel conductance (4.7 +/- 0.4 pS) had a linear current-voltage relationship with a reversal membrane potential close to -20 mV. This single-channel activity was increased by application of ACh or ATP. The ICl,swell generation was not affected by pirenzepine or metoctramine, and did not affect the purinergic activation of the chloride current named Fin. Thus, ICl,swell was not generated via neurotransmitters released during cellular swelling. All together, equal discrimination for different anions, similar modulatory effects by extracellular pH, the blocking effects by ATP and La3+, and the same single-channel activity, strongly suggest that ICl,swell and Sin currents depend on the opening of the same type or a closely related class of volume-regulated chloride channels.