Characterisation of the effects of anthranilic and (indanyloxy) acetic acid derivatives on chloride transport in membrane vesicles

Pannexin 1 sustains the electrophysiological responsiveness of retinal ganglion cells

Pannexin 1 (Panx1) forms ATP-permeable membrane channels that play a key role in purinergic signaling in the nervous system in both normal and pathological conditions. In the retina, particularly high levels of Panx1 are found in retinal ganglion cells (RGCs), but the normal physiological function in these cells remains unclear. In this study, we used patch clamp recordings in the intact inner retina to show that evoked currents characteristic of Panx1 channel activity were detected only in RGCs, particularly in the OFF-type cells. The analysis of pattern electroretinogram (PERG) recordings indicated that Panx1 contributes to the electrical output of the retina. Consistently, PERG amplitudes were significantly impaired in the eyes with targeted ablation of the Panx1 gene in RGCs. Under ocular hypertension and ischemic conditions, however, high Panx1 activity permeated cell membranes and facilitated the selective loss of RGCs or stably transfected Neuro2A cells. Our results show that high expression of the Panx1 channel in RGCs is essential for visual function in the inner retina but makes these cells highly sensitive to mechanical and ischemic stresses. These findings are relevant to the pathophysiology of retinal disorders induced by increased intraocular pressure, such as glaucoma.

CLIC1 regulates dendritic cell antigen processing and presentation by modulating phagosome acidification and proteolysis

Intracellular chloride channel protein 1 (CLIC1) participates in inflammatory processes by regulating macrophage phagosomal functions such as pH and proteolysis. Here, we sought to determine if CLIC1 can regulate adaptive immunity by actions on dendritic cells (DCs), the key professional antigen presenting cells. To do this, we first generated bone marrow-derived DCs (BMDCs) from germline CLIC1 gene-deleted (CLIC1^−/−) and wild-type (CLIC1^+/+) mice, then studied them in vitro and in vivo. We found phagocytosis triggered cytoplasmic CLIC1 translocation to the phagosomal membrane where it regulated phagosomal pH and proteolysis. Phagosomes from CLIC1^−/− BMDCs displayed impaired acidification and proteolysis, which could be reproduced if CLIC1^+/+, but not CLIC1^−/− cells, were treated with IAA94, a CLIC family ion channel blocker. CLIC1^−/− BMDC displayed reduced in vitro antigen processing and presentation of full-length myelin oligodendrocyte glycoprotein (MOG) and reduced MOG-induced experimental autoimmune encephalomyelitis. These data suggest that CLIC1 regulates DC phagosomal pH to ensure optimal processing of antigen for presentation to antigen-specific T-cells. Further, they indicate that CLIC1 is a novel therapeutic target to help reduce the adaptive immune response in autoimmune diseases.

Summary: DC phagosomes from CLIC1^−/− mice display impaired acidification and in vivo and in vitro antigen processing and presentation, revealing CLIC1^−/− as a potential therapeutic target in reducing the adaptive immune response in autoimmune diseases.

Transmembrane voltage potential of somatic cells controls oncogene-mediated tumorigenesis at long-range

The microenvironment is increasingly recognized as a crucial aspect of cancer. In contrast and complement to the field's focus on biochemical factors and extracellular matrix, we characterize a novel aspect of host:tumor interaction - endogenous bioelectric signals among non-excitable somatic cells. Extending prior work focused on the bioelectric state of cancer cells themselves, we show for the first time that the resting potentials of distant cells are critical for oncogene-dependent tumorigenesis. In the Xenopus laevis tadpole model, we used human oncogenes such as mutant KRAS to drive formation of tumor-like structures that exhibited overproliferation, increased nuclear size, hypoxia, acidity, and leukocyte attraction. Remarkably, misexpression of hyperpolarizing ion channels at distant sites within the tadpole significantly reduced the incidence of these tumors. The suppression of tumorigenesis could also be achieved by hyperpolarization using native CLIC1 chloride channels, suggesting a treatment modality not requiring gene therapy. Using a dominant negative approach, we implicate HDAC1 as the mechanism by which resting potential changes affect downstream cell behaviors. Based on published data on the voltage-mediated changes of butyrate flux through the SLC5A8 transporter, we present a model linking resting potentials of host cells to the ability of oncogenes to initiate tumorigenesis. Antibiotic data suggest that the relevant butyrate is generated by a native bacterial species, identifying a novel link between the microbiome and cancer that is mediated by alterations in bioelectric signaling.

Preferential regulation of rabbit cardiac L-type Ca2+ current by glycolytic derived ATP via a direct allosteric pathway

1. The activity of Ca2+ channels is regulated by a number of mechanisms including direct allosteric modulation by intracellular ATP. Since ATP derived from glycolysis is preferentially used for membrane function, we hypothesized that glycolytic ATP also preferentially regulates cardiac L-type Ca2+ channels. 2. To test this hypothesis, peak L-type Ca2+ currents (ICa) were measured in voltage-clamped rabbit cardiomyocytes during glycolytic inhibition (2-deoxyglucose + pyruvate), oxidative inhibition (cyanide + glucose) or both (full metabolic inhibition; FMI). 3. A 10 min period of FMI resulted in a 40.0 % decrease in peak ICa at +10 mV (-5.1 +/- 0.6 versus -3.1 +/- 0.4 pA pF-1; n = 5, P < 0.01). Similar decreases in peak ICa were observed during glycolytic inhibition using 2-deoxyglucose (-6.2 +/- 0.2 versus -3.7 +/- 0.2 pA pF-1; n = 5, P < 0.01) or iodoacetamide (-6.7 +/- 0.3 versus -3.7 +/- 0.2 pA pF-1; n = 7, P < 0.01), but not following oxidative inhibition (-6.2 +/- 0.4 versus -6.4 +/- 0.3 pA pF-1; n = 5, n.s.). The reduction in ICa following glycolytic inhibition was not mediated by phosphate sequestration by 2-deoxyglucose or changes in intracellular pH. 4. Reductions in ICa were still observed when inorganic phosphate and creatine were included in the pipette, confirming a critical role for glycolysis in ICa regulation. 5. With 5 mM MgATP in the pipette during FMI, peak ICa decreased by only 18.4 % (-6.8 +/- 0.6 versus -5.5 +/- 0.3 pA pF-1; n = 4, P < 0.05), while inclusion of 5 mM MgAMP-PCP (beta,gamma-methyleneadenosine 5'-triphosphate, Mg2+ salt) completely prevented the decrease in peak ICa (-6.9 +/- 0.3 versus -6.5 +/- 0.3 pA pF-1; n = 5, n.s.). 6. Together, these results suggest that ICa is regulated by intracellular ATP derived from glycolysis and does not require hydrolysis of ATP. This regulation is expected to be energy conserving during periods of metabolic stress and myocardial ischaemia.

Cochlear effects of indacrinone are not altered by penicillin

Indacrinone is a loop diuretic structurally related to ethacrynic acid. Indacrinone is a racemic mixture. Previous studies have shown that the (-) enantiomer caused reduction of endocochlear potential (EP) and elevation of compound action potential (CAP) threshold (Rybak and Whitworth, 1987a). It has been demonstrated that organic acids such as penicillin, probenecid and sodium salicylate prevent the reduction of EP normally observed after furosemide administration (Rybak et al., 1992a). The present study was designed to determine whether penicillin pretreatment could prevent changes in EP and CAP threshold in (-)-indacrinone treated chinchillas. Adult chinchillas were anesthetized with ketamine and pentobarbital. A microelectrode was advanced into the scala media using the round window approach, and CAP responses to clicks were measured. One group was treated with (-)-indacrinone 100 mg/kg via the jugular vein. A second group of animals received penicillin 50 mg/kg i.v. thirty minutes before (-)-indacrinone. The mean EP change in the indacrinone-treated animals was 38.38 +/- 19.32 millivolts (mv). The reduction of EP in the group receiving penicillin was 24.43 +/- 20.74 mv (P > 0.09). The mean CAP threshold changes in animals receiving indacrinone was 20 +/- 14.14 dB whereas those pretreated with penicillin showed a threshold shift of 21.43 +/- 20.35 dB (P > 0.05). These findings are consistent with previous studies which showed that the effect of ethacrynic acid on the EP and CAP was not changed by the pretreatment with penicillin (Rybak et al., 1990).

Chloride channels and anion fluxes in a human colonic epithelium (HCA-7)

1. Colonic epithelial cells, derived from a human adenocarcinoma (HCA-7), were examined by the patch clamp technique. 2. Outwardly rectifying anion (Cl-) channels were identified in the apical membrane. The conductance was g(in) approximately 26 pS, g(out) approximately 40 pS. The open state probability of the channels increased with depolarization and the selectivity for Cl- over K+ (PCl/PK) was approximately 7.5. 3. The channels were sensitive to intracellular adenosine 3':5'-cyclic monophosphate (cyclic AMP, 0.1 mM), but not to Ca2+ (at concentrations up to 1 mM). At depolarized potentials the channels were blocked by pirentanide (1-5 microM) applied intracellularly. 4. HCA-7 monolayers loaded with 125I- (as a marker for Cl-) were used to measure I- efflux and converted to instantaneous rate constants. 5. The rate constant for I- efflux was increased by forskolin and lysylbradykinin (LBK). The effects of forskolin were not effected by BAPTA (an intracellular calcium chelator). The effects of LBK were inhibited by BAPTA and by Ba2+, indicating that LBK raised intracellular Ca2+ (Cai) which activates Ca(2+)-sensitive K-channels, the latter being blocked by Ba2+. 6. Although it cannot be conclusively proved that the outwardly rectifying chloride channels described here are solely or partially responsible for the increased anion efflux or transepithelial chloride secretion, the channels are likely to be more relevant for cyclic AMP-requiring rather than Ca(2+)-requiring secretagogues.

Chemical probes for anion transporters of mammalian cell membranes

Mammalian cell membranes harbor several types of chloride channels, chloride-cation symporters/cotransporters, and several classes of anion exchangers/antiporters. These transport systems subserve different cellular or organismic functions, depending on the nature of the cell, the spatial organization of transporters, and their functional interplay. Chemical probing has played a central role in the structural and functional delineation of the various anion transport systems. The design of specific probes or their selection from existing sources coupled with their judicious application to the most appropriate biological system had led to the identification of specific anion transporters and to the elucidation of the underlying molecular transport mechanism. In many instances, chemical probing has remained the major or exclusive analytical tool for the functional definition or identification of a given transport system, particularly for discerning among the various anion transporters which operate in highly heterogeneous cell membrane systems. This work critically reviews the present state of the chemical armamentarium available for the most common anion transporters found in mammalian cell membranes. It encompasses the description of the most useful or commonly used probes in terms of their chemical, biochemical, physiological, and pharmacological properties. The review deals primarily with what chemical probes tell about anion transporters and, most importantly, with the limitations inherent in the use of probes in transport studies.