Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

SLC26 Anion Transporters

Solute carrier family 26 (SLC26) is a family of functionally diverse anion transporters found in all kingdoms of life. Anions transported by SLC26 proteins include chloride, bicarbonate, and sulfate, but also small organic dicarboxylates such as fumarate and oxalate. The human genome encodes ten functional homologs, several of which are causally associated with severe human diseases, highlighting their physiological importance. Here, we review novel insights into the structure and function of SLC26 proteins and summarize the physiological relevance of human members.

Halide-selective, proton-coupled anion transport by phenylthiosemicarbazones

Phenylthiosemicarbazones (PTSCs) are proton-coupled anion transporters with pH-switchable behaviour known to be regulated by an imine protonation equilibrium. Previously, chloride/nitrate exchange by PTSCs was found to be inactive at pH 7.2 due to locking of the thiourea anion binding site by an intramolecular hydrogen bond, and switched ON upon imine protonation at pH 4.5. The rate-determining process of the pH switch, however, was not examined. We here develop a new series of PTSCs and demonstrate their conformational behaviour by X-ray crystallographic analysis and pH-switchable anion transport properties by liposomal assays. We report the surprising finding that the protonated PTSCs are extremely selective for halides over oxyanions in membrane transport. Owing to the high chloride over nitrate selectivity, the pH-dependent chloride/nitrate exchange of PTSCs originates from the rate-limiting nitrate transport process being inhibited at neutral pH.

Imidazolinium-based Multiblock Amphiphile as Transmembrane Anion Transporter

Transmembrane anion transport is an important biological process in maintaining cellular functions. Thus, synthetic anion transporters are widely developed for their biological applications. Imidazolinium was introduced as anion recognition site to a multiblock amphiphilic structure that consists of octa(ethylene glycol) and aromatic units. Ion transport assay using halide-sensitive lucigenin and pH-sensitive 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) revealed that imidazolinium-based multiblock amphiphile (IMA) transports anions and showed high selectivity for nitrate, which plays crucial roles in many biological events. Temperature-dependent ion transport assay using 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) indicated that IMA works as a mobile carrier. ¹ H NMR titration experiments indicated that the C2 proton of the imidazolinium ring recognizes anions via a (C-H)⁺ ⋅⋅⋅X^- hydrogen bond. Furthermore, all-atom molecular dynamics simulations revealed a dynamic feature of IMA within the membranes during ion transportation.

Ligand-dependent modulation of hOCT1 transport reveals discrete ligand binding sites within the substrate translocation channel

The human hepatic organic cation transporter 1 (hOCT1) is a well-known transporter of both xenobiotic and endogenous cations. The substrates and inhibitors of hOCT1 are structurally and physiochemically diverse and include some widely prescribed drugs (metformin and imatinib), vitamins (thiamine), and neurotransmitters (serotonin). It has been demonstrated that the closely related renal isoform, hOCT2, is subject to ligand-dependent modulation, wherein one ligand may enhance or inhibit transport of a second, chemically unrelated, ligand. This phenomenon has important implications for drug-drug interactions due to the ubiquity of polypharmacy and the large number of drugs that are present as cations under physiological conditions. Therefore, the objective of this study was to determine if hOCT1 is subject to the same ligand-dependent modulation as hOCT2, and to identify unique putative ligand binding sites in the translocation channel for a sub-set of ligands using computational modeling. The competitive counter flow (CCF) assay was employed to examine ligand-dependent effects by utilizing four different radiolabeled probe substrates: MPP+, serotonin, metformin, and TEA. We identified 20 ligands that modulated the transport of the four test substrates examined. One of the putative ligands identified, BSP, is an anion at physiological pH. Direct uptake studies of radiolabeled BSP suggested that it is a hOCT1 substrate with a Km of 13.6 ± 2.6 µM and Vmax of 55.1 ± 4.1 pmol/mg protein/min. Each ligand identified was computationally docked into a homology model of hOCT1 using the UCSF DOCK software package. The docking study revealed three separate ligand binding pockets within the hOCT1 translocation pathway, defined by their interactions with three prototypical substrates: MPP+, TEA, and acyclovir. Our results suggest that hOCT1 is not only subject to ligand-dependent modulation, but also that individual ligand binding occurs at discrete sites within the hOCT1 translocation pathway which may influence ligand binding at the other sites.

Tempo-phosphate as an ESR tool to study phosphate transport

TEMPO-phosphate has been introduced as a phosphate analogue to study phosphate transport in erythrocytes. The nitroxide is reduced intracellularly upon entering the cells, the membrane transport being the rate-limiting step of the loss of ESR signal. The use of TEMPO-phosphate is convenient and avoids the hazard of radioactivity. We studied the inhibition of TEMPO-phosphate transport to human erythrocytes by various compounds. DIDS and SITS, inhibitors of Band 3, inhibited the TEMPO-phosphate transport. 1-cyano-4-hydroxycinnamic acid, inhibitor of monocarboxylate transporters, did not affect the permeation of TEMPO-phosphate. The transport of TEMPO-phosphate was inhibited by various polyphenols, especially curcumin, naringin, quercetin, luteolin and kaempferol. Interestingly, 3-bromopyruvic acid, an alkylating agent and potential anticancer agent, induced an apparent enhancement of TEMPO-phosphate transport into erythrocytes.

Anion-selective Formate/nitrite transporters: taxonomic distribution, phylogenetic analysis and subfamily-specific conservation pattern in prokaryotes

Background

The monovalent anions formate, nitrite and hydrosulphide are main metabolites of bacterial respiration during anaerobic mixed-acid fermentation. When accumulated in the cytoplasm, these anions become cytotoxic. Membrane proteins that selectively transport these monovalent anions across the membrane have been identified and they belong to the family of Formate/Nitrite Transporters (FNTs). Individual members that selectively transport formate, nitrite and hydrosulphide have been investigated. Experimentally determined structures of FNTs indicate that they share the same hourglass helical fold with aquaporins and aquaglyceroporins and have two constriction regions, namely, cytoplasmic slit and central constriction. Members of FNTs are found in bacteria, archaea, fungi and protists. However, no FNT homolog has been identified in mammals. With FNTs as potential drug targets for many bacterial diseases, it is important to understand the mechanism of selectivity and transport across these transporters.

Results

We have systematically searched the sequence databases and identified 2206 FNT sequences from bacteria, archaea and eukaryotes. Although FNT sequences are very diverse, homology modeling followed by structure-based sequence alignment revealed that nearly one third of all the positions within the transmembrane region exhibit high conservation either as a group or at the level of individual residues across all three kingdoms. Phylogenetic analysis of prokaryotic FNT sequences revealed eight different subgroups. Formate, nitrite and hydrosulphide transporters respectively are clustered into two (FocA and FdhC), three (NirC-α, NirC-β and NirC-γ) and one (HSC) subfamilies. We have also recognized two FNT subgroups (YfdC-α and YfdC-β) with unassigned function. Analysis of taxonomic distribution indicates that each subfamily prefers specific taxonomic groups. Structure-based sequence alignment of individual subfamily members revealed that certain positions in the two constriction regions and some residues facing the interior show subfamily-specific conservation. We have also identified examples of FNTs with the two constriction regions formed by residues that are less frequently observed. We have developed dbFNT, a database of FNT models and associated details. dbFNT is freely available to scientific community.

Conclusions

Taxonomic distribution and sequence conservation of FNTs exhibit subfamily-specific features. The conservation pattern in the central constriction and cytoplasmic slit in the open and closed states are distinct for YfdC and NirC subfamilies. The same is true for some residues facing the interior of the transporters. The specific residues in these positions can exert influence on the type of solutes that are transported by these proteins. With FNTs found in many disease-causing bacteria, the knowledge gained in this study can be used in the development and design of anti-bacterial drugs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3947-4) contains supplementary material, which is available to authorized users.

CK2 is a key regulator of SLC4A2-mediated Cl-/HCO3- exchange in human airway epithelia

Transepithelial bicarbonate secretion by human airway submucosal glands and surface epithelial cells is crucial to maintain the pH-sensitive innate defence mechanisms of the lung. cAMP agonists stimulate HCO₃^- secretion via coordinated increases in basolateral HCO₃^- influx and accumulation, as well as CFTR-dependent HCO₃^- efflux at the luminal membrane of airway epithelial cells. Here, we investigated the regulation of a basolateral located, DIDS-sensitive, Cl^-/HCO₃^- exchanger, anion exchanger 2 (AE2; SLC4A2) which is postulated to act as an acid loader, and therefore potential regulator of HCO₃^- secretion, in human airway epithelial cells. Using intracellular pH measurements performed on Calu-3 cells, we demonstrate that the activity of the basolateral Cl^-/HCO₃^- exchanger was significantly downregulated by cAMP agonists, via a PKA-independent mechanism and also required Ca²⁺ and calmodulin under resting conditions. AE2 contains potential phosphorylation sites by a calmodulin substrate, protein kinase CK2, and we demonstrated that AE2 activity was reduced in the presence of CK2 inhibition. Moreover, CK2 inhibition abolished the activity of AE2 in primary human nasal epithelia. Studies performed on mouse AE2 transfected into HEK-293T cells confirmed almost identical Ca²⁺/calmodulin and CK2 regulation to that observed in Calu-3 and primary human nasal cells. Furthermore, mouse AE2 activity was reduced by genetic knockout of CK2, an effect which was rescued by exogenous CK2 expression. Together, these findings are the first to demonstrate that CK2 is a key regulator of Cl^--dependent HCO₃^- export at the serosal membrane of human airway epithelial cells.

The R130S mutation significantly affects the function of prestin, the outer hair cell motor protein

A missense mutation, R130S, was recently found in the prestin gene, SLC26A5, of patients with moderate to severe hearing loss (DFNB61). In order to define the pathology of hearing loss associated with this missense mutation, a recombinant prestin construct harboring the R130S mutation (R130S-prestin) was generated, and its functional consequences examined in a heterologous expression system. We found that R130S-prestin targets the plasma membrane but less efficiently compared to wild-type. The voltage operating point and voltage sensitivity of the motor function of R130S-prestin were similar to wild-type prestin. However, the motor activity of R130S-prestin is greatly reduced at higher voltage stimulus frequencies, indicating a reduction in motor kinetics. Our study thus provides experimental evidence that supports a causal relationship between the R130S mutation in the prestin gene and hearing loss found in patients with this missense mutation.

KEY MESSAGE

Membrane targeting of prestin is impaired by the R130S missense mutation. The fast motor kinetics of prestin is impaired by the R130S missense mutation. Our study strongly suggests that the prestin R130S missense mutation is pathogenic.

Loss-of-function mutation of rice SLAC7 decreases chloroplast stability and induces a photoprotection mechanism in rice

Plants absorb sunlight to power the photochemical reactions of photosynthesis, which can potentially damage the photosynthetic machinery. However, the mechanism that protects chloroplasts from the damage remains unclear. In this work, we demonstrated that rice (Oryza sativa L.) SLAC7 is a generally expressed membrane protein. Loss-of-function of SLAC7 caused continuous damage to the chloroplasts of mutant leaves under normal light conditions. Ion leakage indicators related to leaf damage such as H2 O2 and abscisic acid levels were significantly higher in slac7-1 than in the wild type. Consistently, the photosynthesis efficiency and Fv/Fm ratio of slac7-1 were significantly decreased (similar to photoinhibition). In response to chloroplast damage, slac7-1 altered its leaf morphology (curled or fused leaf) by the synergy between plant hormones and transcriptional factors to decrease the absorption of light, suggesting that a photoprotection mechanism for chloroplast damage was activated in slac7-1. When grown in dark conditions, slac7-1 displayed a normal phenotype. SLAC7 under the control of the AtSLAC1 promoter could partially complement the phenotypes of Arabidopsis slac1 mutants, indicating a partial conservation of SLAC protein functions. These results suggest that SLAC7 is essential for maintaining the chloroplast stability in rice.

A new erythrocyte-based biochemical approach to predict the antiproliferative effects of heterocyclic scaffolds: The case of indolone

BACKGROUND

The indole core is a key structural feature of many natural products and biomolecules with broad spectrum chemotherapeutic properties. Some of us have recently synthesized a library of biologically promising indolone-based compounds. The present study focuses on the effects of one of them, namely DPIT, on human erythrocytes.

METHODS

We have examined the influence of DPIT on band 3 protein, intracellular ATP concentration and transport, caspase 3 activation, metabolic adaptation and membrane stability.

RESULTS

Our study elucidates that DPIT, intercalated into the phospholipid bilayer, decreases the anion transport, the intracellular ATP concentration and the cytosolic pH, inducing a direct activation of caspase 3.

CONCLUSIONS

Starting from the metabolic similarity between erythrocytes and cancer cells, we investigate how the metabolic derangements and membrane alterations induced by selected heterocycles could be related to the antiproliferative effects.

GENERAL SIGNIFICANCE

Our work aims to propose a new model of study to predict the antiproliferative effects of heterocyclic scaffolds, pointing out that only one of the listed conditions would be unfavorable to the life cycle of neoplastic cells.