Cell volume regulation in response to hypotonicity is impaired in HeLa cells expressing a protein kinase Calpha mutant lacking kinase activity

Interactomic exploration of LRRC8A in volume-regulated anion channels

Ion channels are critical in enabling ion movement into and within cells and are important targets for pharmacological interventions in different human diseases. In addition to their ion transport abilities, ion channels interact with signalling and scaffolding proteins, which affects their function, cellular positioning, and links to intracellular signalling pathways. The study of "channelosomes" within cells has the potential to uncover their involvement in human diseases, although this field of research is still emerging. LRRC8A is the gene that encodes a crucial protein involved in the formation of volume-regulated anion channels (VRACs). Some studies suggest that LRRC8A could be a valuable prognostic tool in different types of cancer, serving as a biomarker for predicting patients' outcomes. LRRC8A expression levels might be linked to tumour progression, metastasis, and treatment response, although its implications in different cancer types can be varied. Here, publicly accessible databases of cancer patients were systematically analysed to determine if a correlation between VRAC channel expression and survival rate exists across distinct cancer types. Moreover, we re-evaluated the impact of LRRC8A on cellular proliferation and migration in colon cancer via HCT116 LRRC8A-KO cells, which is a current topic of debate in the literature. In addition, to investigate the role of LRRC8A in cellular signalling, we conducted biotin proximity-dependent identification (BioID) analysis, revealing a correlation between VRAC channels and cell-cell junctions, mechanisms that govern cellular calcium homeostasis, kinases, and GTPase signalling. Overall, this dataset improves our understanding of LRRC8A/VRAC and explores new research avenues while identifying promising therapeutic targets and promoting inventive methods for disease treatment.

Physiological Functions of the Volume-Regulated Anion Channel VRAC/LRRC8 and the Proton-Activated Chloride Channel ASOR/TMEM206

Volume-regulated anion channels (VRACs) and the acid-sensitive outwardly rectifying anion channel (ASOR) mediate flux of chloride and small organic anions. Although known for a long time, they were only recently identified at the molecular level. VRACs are heteromers consisting of LRRC8 proteins A to E. Combining the essential LRRC8A with different LRRC8 paralogues changes key properties of VRAC such as conductance or substrate selectivity, which is how VRACs are involved in multiple physiological functions including regulatory volume decrease, cell proliferation and migration, cell death, purinergic signalling, fat and glucose metabolism, insulin signalling, and spermiogenesis. VRACs are also involved in pathological conditions, such as the neurotoxic release of glutamate and aspartate. Certain VRACs are also permeable to larger, organic anions, including antibiotics and anti-cancer drugs, making them an interesting therapeutic target. ASOR, also named proton-activated chloride channel (PAC), is formed by TMEM206 homotrimers on the plasma membrane and on endosomal compartments where it mediates chloride flux in response to extracytosolic acidification and plays a role in the shrinking and maturation of macropinosomes. ASOR has been shown to underlie neuronal swelling which causes cell death after stroke as well as promoting the metastasis of certain cancers, making them intriguing therapeutic targets as well.

Stability of Intracellular Protein Concentration under Extreme Osmotic Challenge

Cell volume (CV) regulation is typically studied in short-term experiments to avoid complications resulting from cell growth and division. By combining quantitative phase imaging (by transport-of-intensity equation) with CV measurements (by the exclusion of an external absorbing dye), we were able to monitor the intracellular protein concentration (PC) in HeLa and 3T3 cells for up to 48 h. Long-term PC remained stable in solutions with osmolarities ranging from one-third to almost twice the normal. When cells were subjected to extreme hypoosmolarity (one-quarter of normal), their PC did not decrease as one might expect, but increased; a similar dehydration response was observed at high concentrations of ionophore gramicidin. Highly dilute media, or even moderately dilute in the presence of cytochalasin, caused segregation of water into large protein-free vacuoles, while the surrounding cytoplasm remained at normal density. These results suggest that: (1) dehydration is a standard cellular response to severe stress; (2) the cytoplasm resists prolonged dilution. In an attempt to investigate the mechanism behind the homeostasis of PC, we tested the inhibitors of the protein kinase complex mTOR and the volume-regulated anion channels (VRAC). The initial results did not fully elucidate whether these elements are directly involved in PC maintenance.

Molecular Signaling and Transcriptional Regulation of Histamine H1 Receptor Gene

Histamine-activated histamine H₁ receptor (H1R) signaling regulates many gene expressions, mainly through the protein kinase C (PKC)/extracellular signal-regulated kinases (ERK) signaling. Involvement of other signaling, including NF-κB, Wnt, RUNX-2, and Rho A signaling was also demonstrated. In addition, cAMP production through the activation of H1R signaling was reported. H1R gene itself is also up-regulated by the activation of H1R signaling with histamine. Here, we review our recent findings in the molecular signaling and transcriptional regulation of the H1R gene. Stimulation with histamine up-regulates H1R gene expression through the activation of H1R in HeLa cells. The PKCδ/ERK/poly(ADP)ribosyl transferase-1 (PARP-1) signaling was involved in this up-regulation. Heat shock protein 90 also plays an important role in regulating PKCδ translocation. Promoter analyses revealed the existence of two promoters in the human H1R gene in HeLa cells. H1R-activated H1R gene up-regulation in response to histamine was also observed in U373 astroglioma cells. However, this up-regulation was mediated not through the PKCδ signaling but possibly through the PKCα signaling. In addition, the promoter region responsible for histamine-induced H1R gene transcription in U373 cells was different from that of HeLa cells. These findings suggest that the molecular signaling and transcriptional regulation of the H1R gene are different between neuronal cells and non-neuronal cells.

TRPV4 channels mediate the mechanoresponse in retinal microglia

The physiological and neurological correlates of plummeting brain osmolality during edema, traumatic CNS injury, and severe ischemia are compounded by neuroinflammation. Using multiple approaches, we investigated how retinal microglia respond to challenges mediated by increases in strain, osmotic gradients, and agonists of the stretch-activated cation channel TRPV4. Dissociated and intact microglia were TRPV4-immunoreactive and responded to the selective agonist GSK1016790A and substrate stretch with altered motility and elevations in intracellular calcium ([Ca²⁺ ]_i ). Agonist- and hypotonicity-induced swelling was associated with a nonselective outwardly rectifying cation current, increased [Ca²⁺ ]_i , and retraction of higher-order processes. The antagonist HC067047 reduced the extent of hypotonicity-induced microglial swelling and inhibited the suppressive effects of GSK1016790A and hypotonicity on microglial branching. Microglial TRPV4 signaling required intermediary activation of phospholipase A2 (PLA2), cytochrome P450, and epoxyeicosatrienoic acid production (EETs). The expression pattern of vanilloid thermoTrp genes in retinal microglia was markedly different from retinal neurons, astrocytes, and cortical microglia. These results suggest that TRPV4 represents a primary retinal microglial sensor of osmochallenges under physiological and pathological conditions. Its activation, associated with PLA2, modulates calcium signaling and cell architecture. TRPV4 inhibition might be a useful strategy to suppress microglial overactivation in the swollen and edematous CNS.

SWELL1 promotes cell growth and metastasis of hepatocellular carcinoma in vitro and in vivo

Background

SWELL1 was recently demonstrated to be an indispensable part of the volume-regulated anion channel (VRAC). VRAC is reported to participate in cell proliferation, survival, and migration. However, the correlation between SWELL1 and hepatocellular carcinoma (HCC) remains poorly-understood. In this study, we tried to explore the role of SWELL1 in HCC.

Methods

Immunohistochemistry and quantitative real-time-PCR (qRT-PCR) was used to measure SWELL1 expression in HCC samples obtained from patients with HCC. The effects of SWELL1 on HCC cell proliferation, apoptosis, and metastasis were analysed by corresponding cytological experiments including Cell Counting Kit-8 (CCK8), colony-forming, 5-ethynyl-2′-deoxyuridine (EdU), cell cycle analysis, TUNEL, Annexin V and PI staining, wound healing, transwell, and so on. BALB/c nude mice were used for the in vivo assays. qRT-PCR and western blotting was performed for molecular mechanisms.

Findings

SWELL1 was highly expressed in HCC tissues, and related to the poor prognosis. In vitro, the over-expression of SWELL1 significantly induced cell proliferation and migration, and inhibited apoptosis, whereas suppressing SWELL1 had the opposite effects. Moreover, knockdown of SWELL1 suppressed the growth and metastasis of HCC in vivo. Further experiments revealed that SWELL1 induced cell growth by activating the cyclinD1/CDK2 pathway via the connection with PKCa at the signalling level, and regulated cell migration through the JNK pathway in HCC.

Interpretation

SWELL1 acts as a promoter in the growth and metastasis of HCC cells and may be a potential intervention target for HCC.

Fund

This work is supported by the National Natural Science Foundation of China (No. 81572422, 81700515).

More than just a pressure relief valve: physiological roles of volume-regulated LRRC8 anion channels

The volume-regulated anion channel (VRAC) is a key player in the volume regulation of vertebrate cells. This ubiquitously expressed channel opens upon osmotic cell swelling and potentially other cues and releases chloride and organic osmolytes, which contributes to regulatory volume decrease (RVD). A plethora of studies have proposed a wide range of physiological roles for VRAC beyond volume regulation including cell proliferation, differentiation and migration, apoptosis, intercellular communication by direct release of signaling molecules and by supporting the exocytosis of insulin. VRAC was additionally implicated in pathological states such as cancer therapy resistance and excitotoxicity under ischemic conditions. Following extensive investigations, 5 years ago leucine-rich repeat-containing family 8 (LRRC8) heteromers containing LRRC8A were identified as the pore-forming components of VRAC. Since then, molecular biological approaches have allowed further insight into the biophysical properties and structure of VRAC. Heterologous expression, siRNA-mediated downregulation and genome editing in cells, as well as the use of animal models have enabled the assessment of the proposed physiological roles, together with the identification of new functions including spermatogenesis and the uptake of antibiotics and platinum-based cancer drugs. This review discusses the recent molecular biological insights into the physiology of VRAC in relation to its previously proposed roles.

A FRET sensor of C-terminal movement reveals VRAC activation by plasma membrane DAG signaling rather than ionic strength

Volume-regulated anion channels (VRACs) are central to cell volume regulation. Recently identified as hetero-hexamers formed by LRRC8 proteins, their activation mechanism remains elusive. Here, we measured Förster resonance energy transfer (FRET) between fluorescent proteins fused to the C-termini of LRRC8 subunits. Inter-subunit FRET from LRRC8 complexes tracked VRAC activation. With patch-clamp fluorometry, we confirmed that the cytoplasmic domains rearrange during VRAC opening. With these FRET reporters, we determined VRAC activation, non-invasively, in live cells and their subcompartments. Reduced intracellular ionic strength did not directly activate VRACs, and VRACs were not activated on endomembranes. Instead, pharmacological manipulation of diacylglycerol (DAG), and protein kinase D (PKD) activity, activated or inhibited plasma membrane-localized VRACs. Finally, we resolved previous contradictory reports concerning VRAC activation, using FRET to detect robust activation by PMA that was absent during whole-cell patch clamp. Overall, non-invasive VRAC measurement by FRET is an essential tool for unraveling its activation mechanism.

The signaling role for chloride in the bidirectional communication between neurons and astrocytes

It is well known that the electrical signaling in neuronal networks is modulated by chloride (Cl^-) fluxes via the inhibitory GABA_A and glycine receptors. Here, we discuss the putative contribution of Cl^- fluxes and intracellular Cl^- to other forms of information transfer in the CNS, namely the bidirectional communication between neurons and astrocytes. The manuscript (i) summarizes the generic functions of Cl^- in cellular physiology, (ii) recaps molecular identities and properties of Cl^- transporters and channels in neurons and astrocytes, and (iii) analyzes emerging studies implicating Cl^- in the modulation of neuroglial communication. The existing literature suggests that neurons can alter astrocytic Cl^- levels in a number of ways; via (a) the release of neurotransmitters and activation of glial transporters that have intrinsic Cl^- conductance, (b) the metabotropic receptor-driven changes in activity of the electroneutral cation-Cl^- cotransporter NKCC1, and (c) the transient, activity-dependent changes in glial cell volume which open the volume-regulated Cl^-/anion channel VRAC. Reciprocally, astrocytes are thought to alter neuronal [Cl^-]_i through either (a) VRAC-mediated release of the inhibitory gliotransmitters, GABA and taurine, which open neuronal GABA_A and glycine receptor/Cl^- channels, or (b) the gliotransmitter-driven stimulation of NKCC1. The most important recent developments in this area are the identification of the molecular composition and functional heterogeneity of brain VRAC channels, and the discovery of a new cytosolic [Cl^-] sensor - the Wnk family protein kinases. With new work in the field, our understanding of the role of Cl^- in information processing within the CNS is expected to be significantly updated.

Calcium is not required for triggering volume restoration in hypotonically challenged A549 epithelial cells

Maintenance of cell volume is a fundamental housekeeping function in eukaryotic cells. Acute cell swelling activates a regulatory volume decrease (RVD) process with poorly defined volume sensing and intermediate signaling mechanisms. Here, we analyzed the putative role of Ca²⁺ signaling in RVD in single substrate-adherent human lung epithelial A549 cells. Acute cell swelling was induced by perfusion of the flow-through imaging chamber with 50 % hypotonic solution at a defined fluid turnover rate. Changes in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and cell volume were monitored simultaneously with ratiometric Fura-2 fluorescence and 3D reconstruction of stereoscopic single-cell images, respectively. Hypotonic challenge caused a progressive swelling peaking at ∼20 min and followed, during the next 20 min, by RVD of 60 ± 7 % of the peak volume increase. However, at the rate of swelling used in our experiments, these processes were not accompanied by a measurable increment of [Ca²⁺]_i. Loading with intracellular Ca²⁺ chelator BAPTA slightly delayed peak of swelling but did not prevent RVD in 82 % of cells. Further, electrophysiology whole-cell patch-clamp experiments showed that BAPTA did not block activation of volume-regulated anion channel (VRAC) measured as swelling-induced outwardly rectifying 5-nitro-2-(3-phenylpropyl-amino) benzoic acid sensitive current. Together, our data suggest that intracellular Ca²⁺-mediated signaling is not essential for VRAC activation and subsequent volume restoration in A549 cells.

Phosphorylation of PACSIN2 by protein kinase C triggers the removal of caveolae from the plasma membrane

PACSIN2, a membrane-sculpting BAR domain protein, localizes to caveolae. Here, we found that protein kinase C (PKC) phosphorylates PACSIN2 at serine 313, thereby decreasing its membrane binding and tubulation capacities. Concomitantly, phosphorylation decreased the time span for which caveolae could be tracked at the plasma membrane (the 'tracking duration'). Analyses of the phospho-mimetic S313E mutant suggested that PACSIN2 phosphorylation was sufficient to reduce caveolar-tracking durations. Both hypotonic treatment and isotonic drug-induced PKC activation increased PACSIN2 phosphorylation at serine 313 and shortened caveolar-tracking durations. Caveolar-tracking durations were also reduced upon the expression of other membrane-binding-deficient PACSIN2 mutants or upon RNA interference (RNAi)-mediated PACSIN2 depletion, pointing to a role for PACSIN2 levels in modulating the lifetime of caveolae. Interestingly, the decrease in membrane-bound PACSIN2 was inversely correlated with the recruitment and activity of dynamin 2, a GTPase that mediates membrane scission. Furthermore, expression of EHD2, which stabilizes caveolae and binds to PACSIN2, restored the tracking durations of cells with reduced PACSIN2 levels. These findings suggest that the PACSIN2 phosphorylation decreases its membrane-binding activity, thereby decreasing its stabilizing effect on caveolae and triggering dynamin-mediated removal of caveolae.

Involvement of protein kinase Cdelta/extracellular signal-regulated kinase/poly(ADP-ribose) polymerase-1 (PARP-1) signaling pathway in histamine-induced up-regulation of histamine H1 receptor gene expression in HeLa cells

The histamine H(1) receptor (H1R) gene is up-regulated in patients with allergic rhinitis. However, the mechanism and reason underlying this up-regulation are still unknown. Recently, we reported that the H1R expression level is strongly correlated with the severity of allergic symptoms. Therefore, understanding the mechanism of this up-regulation will help to develop new anti-allergic drugs targeted for H1R gene expression. Here we studied the molecular mechanism of H1R up-regulation in HeLa cells that express H1R endogenously in response to histamine and phorbol 12-myristate 13-acetate (PMA). In HeLa cells, histamine stimulation caused up-regulation of H1R gene expression. Rottlerin, a PKCδ-selective inhibitor, inhibited up-regulation of H1R gene expression, but Go6976, an inhibitor of Ca(2+)-dependent PKCs, did not. Histamine or PMA stimulation resulted in PKCδ phosphorylation at Tyr(311) and Thr(505). Activation of PKCδ by H(2)O(2) resulted in H1R mRNA up-regulation. Overexpression of PKCδ enhanced up-regulation of H1R gene expression, and knockdown of the PKCδ gene suppressed this up-regulation. Histamine or PMA caused translocation PKCδ from the cytosol to the Golgi. U0126, an MEK inhibitor, and DPQ, a poly(ADP-ribose) polymerase-1 inhibitor, suppressed PMA-induced up-regulation of H1R gene expression. These results were confirmed by a luciferase assay using the H1R promoter. Phosphorylation of ERK and Raf-1 in response to PMA was also observed. However, real-time PCR analysis showed no inhibition of H1R mRNA up-regulation by a Raf-1 inhibitor. These results suggest the involvement of the PKCδ/ERK/poly(ADP-ribose) polymerase-1 signaling pathway in histamine- or PMA-induced up-regulation of H1R gene expression in HeLa cells.

Two conventional protein kinase C isoforms, alpha and beta I, are involved in the ATP-induced activation of volume-regulated anion channel and glutamate release in cultured astrocytes

Volume-regulated anion channels (VRACs) are activated by cell swelling and are permeable to inorganic and small organic anions, including the excitatory amino acids glutamate and aspartate. In astrocytes, ATP potently enhances VRAC activity and glutamate release via a P2Y receptor-dependent mechanism. Our previous pharmacological study identified protein kinase C (PKC) as a major signaling enzyme in VRAC regulation by ATP. However, conflicting results obtained with potent PKC blockers prompted us to re-evaluate the involvement of PKC in regulation of astrocytic VRACs by using small interfering RNA (siRNA) and pharmacological inhibitors that selectively target individual PKC isoforms. In primary rat astrocyte cultures, application of hypoosmotic medium (30% reduction in osmolarity) and 20 microM ATP synergistically increased the release of excitatory amino acids, measured with a non-metabolized analog of L-glutamate, D-[(3)H]aspartate. Both Go6976, the selective inhibitor of Ca(2+)-sensitive PKCalpha, betaI/II, and gamma, and MP-20-28, a cell permeable pseudosubstrate inhibitory peptide of PKCalpha and betaI/II, reduced the effects of ATP on D-[(3)H]aspartate release by approximately 45-55%. Similar results were obtained with a mixture of siRNAs targeting rat PKCalpha and betaI. Surprisingly, down-regulation of individual alpha and betaI PKC isozymes by siRNA was completely ineffective. These data suggest that ATP regulates VRAC activity and volume-sensitive excitatory amino acid release via cooperative activation of PKCalpha and betaI.

Protein Kinase Cδ Is Activated by Shiga Toxin and Regulates Its Transport

Protein kinase C (PKC) isozymes regulate different vesicular trafficking steps in the recycling or degradative pathways. However, a possible role of these kinases in the retrograde pathway from endosomes to the Golgi complex has previously not been investigated. We report here the involvement of a specific PKC isozyme, PKCdelta, in the intracellular transport of the glycolipid-binding Shiga toxin (Stx), which utilizes the retrograde pathway to intoxicate cells. Upon binding to cells, Stx was shown to specifically activate PKCdelta and not PKCalpha. The involvement of PKCdelta and PKCalpha in the retrograde transport of Stx was then monitored biochemically and by immunofluorescence after inhibition or depletion of the isozymes. PKCdelta, but not PKCalpha, was shown to selectively regulate the endosome-to-Golgi transport of StxB. Upon inhibition or knockdown of PKCdelta, StxB molecules colocalized less with giantin and more with EEA1, indicating that the molecules were accumulated in endosomes, unable to reach the Golgi complex. The inhibition of Golgi transport of Stx was reflected by a strong reduction in the toxic effect, demonstrating that transport of Stx to the cytosol is dependent on PKCdelta activity. These results are in agreement with our previous data, which show that Stx is able to stimulate its own transport.

Hypotonicity causes actin reorganization and recruitment of the actin-binding ERM protein moesin in membrane protrusions in collecting duct principal cells

Hypotonicity-induced cell swelling is characterized by a modification in cell architecture associated with actin cytoskeleton remodeling. The ezrin/radixin/moesin (ERM) family proteins are important signal transducers during actin reorganization regulated by the monomeric G proteins of the Rho family. We report here that in collecting duct CD8 cells hypotonicity-induced cell swelling resulted in deep actin reorganization, consisting of loss of stress fibers and formation of F-actin patches in membrane protrusions where the ERM protein moesin was recruited. Cell swelling increased the interaction between actin and moesin and induced the transition of moesin from an oligomeric to a monomeric functional conformation, characterized by both the COOH- and NH2-terminal domains being exposed. In this conformation, which is stabilized by phosphorylation of a conserved threonine in the COOH-terminal domain by PKC or Rho kinase, moesin can bind interacting proteins. Interestingly, hypotonic stress increased the amount of threonine-phosphorylated moesin, which was prevented by the PKC-α inhibitor Gö-6976 (50 nM). In contrast, the Rho kinase inhibitor Y-27632 (1 μM) did not affect the hypotonicity-induced increase in phosphorylated moesin. The present data represent the first evidence that hypotonicity-induced actin remodeling is associated with phosphorylated moesin recruitment at the cell border and interaction with actin.

Stimulation of histamine H1 receptor up-regulates histamine H1 receptor itself through activation of receptor gene transcription

Histamine is a major mediator in allergy acting mainly through the histamine H(1) receptor (H1R). Although H1R up-regulation has been suggested as an important step for induction of allergic symptoms, little is known about the regulation of H1R level. Here we report that the activation of H1R up-regulates H1R through augmentation of H1R mRNA expression in HeLa cells. Histamine stimulation significantly increased both H1R promoter activity and mRNA level without alteration in mRNA stability. H1R protein was also up-regulated by histamine. An H1R antagonist but not histamine H(2) receptor antagonist blocked histamine-induced up-regulation of both promoter activity and mRNA expression. A protein kinase C (PKC) activator, phorbol-12-myristate-13-acetate, increased H1R mRNA expression, whereas an activator of PKA or PKG (8-Br-cAMP or 8-Br-cGMP, respectively) did not. Furthermore, histamine-induced up-regulation of both promoter activity and mRNA level were completely suppressed by the PKC inhibitor Ro-31-8220. H1R antagonists have long been thought to block H1R and inhibit immediate allergy symptoms. In addition to this short-term effect, our data propose their long-term inhibitory effect against allergic diseases by suppressing PKC-mediated H1R gene transcription. This finding provides new insights into the therapeutic target of H1R antagonist in allergic diseases.

Modulation of hepatocellular swelling-activated K+currents by phosphoinositide pathway-dependent protein kinase C

K+channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K+current ( IKVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K+flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to IKVolactivation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP2) increased IKVol, whereas conditions that decrease cellular PIP2decreased IKVol. GTP and AlF4−stimulated IKVoldevelopment, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased IKVoland inhibited the stimulatory response to PIP2or GTP. Protein kinase C (PKC) is involved, because K+current was enhanced by 1-oleoyl-2-acetyl- sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both IKVoland the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated IKVolactivation involves the cytoskeleton. Under isotonic conditions, a slowly developing K+current similar to IKVolwas activated by PIP2, lipid phosphatase inhibitors to counter PIP2depletion, a PLC-coupled α1-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate IKVolthrough a PIP2/PKC-dependent pathway. The results indicate that PIP2indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.

Physiological regulation of ATP release at the apical surface of human airway epithelia

Extracellular ATP and its metabolite adenosine regulate mucociliary clearance in airway epithelia. Little has been known, however, regarding the actual ATP and adenosine concentrations in the thin ( approximately 7 microm) liquid layer lining native airway surfaces and the link between ATP release/metabolism and autocrine/paracrine regulation of epithelial function. In this study, chimeric Staphylococcus aureus protein A-luciferase (SPA-luc) was bound to endogenous antigens on primary human bronchial epithelial (HBE) cell surface and ATP concentrations assessed in real-time in the thin airway surface liquid (ASL). ATP concentrations on resting cells were 1-10 nm. Inhibition of ecto-nucleotidases resulted in ATP accumulation at a rate of approximately 250 fmol/min/cm2, reflecting the basal ATP release rate. Following hypotonic challenge to promote cell swelling, cell-surface ATP concentration measured by SPA-luc transiently reached approximately 1 microm independent of ASL volume, reflecting a transient 3-log increase in ATP release rates. In contrast, peak ATP concentrations measured in bulk ASL by soluble luciferase inversely correlated with volume. ATP release rates were intracellular calcium-independent, suggesting that non-exocytotic ATP release from ciliated cells, which dominate our cultures, mediated hypotonicity-induced nucleotide release. However, the cystic fibrosis transmembrane conductance regulator (CFTR) did not participate in this function. Following the acute swelling phase, HBE cells exhibited regulatory volume decrease which was impaired by apyrase and facilitated by ATP or UTP. Our data provide the first evidence that ATP concentrations at the airway epithelial surface reach the range for P2Y2 receptor activation by physiological stimuli and identify a role for mucosal ATP release in airway epithelial cell volume regulation.

Effects of polyol pathway hyperactivity on protein kinase C activity, nociceptive peptide expression, and neuronal structure in dorsal root ganglia in diabetic mice

We explored the specific impact of polyol pathway hyperactivity on dorsal root ganglia (DRG) using transgenic mice that overexpress human aldose reductase because DRG changes are crucial for the development of diabetic sensory neuropathy. Littermate mice served as controls. Half of the animals were made diabetic by streptozotocin injection and followed for 12 weeks. After diabetes onset, diabetic transgenic mice showed a significant elevation of pain sensation threshold after transient decrease and marked slowing of motor and sensory nerve conduction at the end of the study, while these changes were modest in diabetic littermate mice. Protein kinase C (PKC) activities were markedly reduced in diabetic transgenic mice, and the changes were associated with reduced expression of membrane PKC-alpha isoform that was translocated to cytosol. Membrane PKC-betaII isoform expression was contrariwise increased. Calcitonin gene-related peptide-and substance P-positive neurons were reduced in diabetic transgenic mice and less severely so in diabetic littermate mice. Morphometric analysis disclosed neuronal atrophy only in diabetic transgenic mice. Treatment with an aldose reductase inhibitor (fidarestat 4 mg x kg(-1) x day(-1), orally) corrected all of the changes detected in diabetic transgenic mice. These findings underscore the pathogenic role of aldose reductase in diabetic sensory neuropathy through the altered cellular signaling and peptide expressions in DRG neurons.

Hydrogen peroxide potentiates volume-sensitive excitatory amino acid release via a mechanism involving Ca2+/calmodulin-dependent protein kinase II

Excessive excitatory amino acid (EAA) release in cerebral ischemia is a major mechanism responsible for neuronal damage and death. A substantial fraction of ischemic EAA release occurs via volume-regulated anion channels (VRACs). Hydrogen peroxide (H2O2), which is abundantly produced during ischemia and reperfusion, activates a number of protein kinases critical for VRAC functioning and has recently been reported to activate VRACs. In the present study, we explored the effects of H2O2 on volume-dependent EAA release in cultured astrocytes, measured as the release of preloaded D-[3H]aspartate. 100-1,000 microm H2O2 enhanced swelling-induced EAA release by approximately 2.5-3-fold (EC50 approximately 10 microM). The VRAC blockers ATP, phloretin, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) potently inhibited both control swelling-induced and the H2O2-potentiated release, suggesting a role for VRACs. The H2O2-induced component of EAA release was attenuated by the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM) and completely eliminated by the calmodulin antagonists trifluoperazine and W-7 and the Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93. Inhibitors of tyrosine kinases, protein kinase C, and the myosin light chain kinase were ineffective in blocking the H2O2 response. H2O2 treatment of swollen astrocytes, but not swelling alone, resulted in CaMKII activation that was inhibited by KN-93, as determined by a phospho-Thr286 CaMKII antibody. These data demonstrate that H2O2 strongly up-regulates astrocytic volume-sensitive EAA release via a CaMKII-dependent mechanism and in this way may potently promote pathological EAA release and brain damage in ischemia.

Voltage-dependent anion channel-1 (VDAC-1) contributes to ATP release and cell volume regulation in murine cells

Extracellular ATP regulates several elements of the mucus clearance process important for pulmonary host defense. However, the mechanisms mediating ATP release onto airway surfaces remain unknown. Mitochondrial voltage-dependent anion channels (mt-VDACs) translocate a variety of metabolites, including ATP and ADP, across the mitochondrial outer membrane, and a plasmalemmal splice variant (pl-VDAC-1) has been proposed to mediate ATP translocation across the plasma membrane. We tested the involvement of VDAC-1 in ATP release in a series of studies in murine cells. First, the full-length coding sequence was cloned from a mouse airway epithelial cell line (MTE7b⁻) and transfected into NIH 3T3 cells, and pl-VDAC-1-transfected cells exhibited higher rates of ATP release in response to medium change compared with mock-transfected cells. Second, ATP release was compared in cells isolated from VDAC-1 knockout [VDAC-1 (−/−)] and wild-type (WT) mice. Fibroblasts from VDAC-1 (−/−) mice released less ATP than WT mice in response to a medium change. Well-differentiated cultures from nasal and tracheal epithelia of VDAC-1 (−/−) mice exhibited less ATP release in response to luminal hypotonic challenge than WT mice. Confocal microscopy studies revealed that cell volume acutely increased in airway epithelia from both VDAC-1 (−/−) and WT mice after luminal hypotonic challenge, but VDAC-1 (−/−) cells exhibited a slower regulatory volume decrease (RVD) than WT cells. Addition of ATP or apyrase to the luminal surface of VDAC-1 (−/−) or WT cultures with hypotonic challenge produced similar initial cell height responses and RVD kinetics in both cell types, suggesting that involvement of VDAC-1 in RVD is through ATP release. Taken together, these studies suggest that VDAC-1, directly or indirectly, contributes to ATP release from murine cells. However, the observation that VDAC-1 knockout cells released a significant amount of ATP suggests that other molecules also play a role in this function.