Chloride movements in human neutrophils during phagocytosis: characterization and relationship to granule release

Decreased intracellular chloride enhances cell migration and invasion via activation of the ERK1/2 signaling pathway in DU145 human prostate carcinoma cells

Our previous study revealed that changes of the intracellular Cl- concentration ([Cl-]i) affected cell proliferation in cancer cells. However, the role of Cl- on cell migration and invasion in cancer cells remains unanalyzed. Therefore, the aim of the present study is to investigate whether changes of [Cl-]i affects cell migration and invasion of cancer cells. In human prostate cancer DU145 cells, cell migration and invasion were enhanced by culturing in the low Cl- medium (replacement of Cl- by NO3-). We also found that DU145 cells in the low Cl- condition caused significant transient ERK1/2 activation followed by an increase of MMP-1 mRNA levels. Inhibition of ERK1/2 activation in the low Cl- condition reduced enhancement of MMP-1 mRNA levels and decreased cell migration and invasion. These observations indicate that [Cl-]i plays important roles in metastatic function by regulating the ERK1/2 signaling pathway in human prostate cancer cells, and intracellular Cl- would be one of the key targets for anti-cancer therapy.

The role of Cl- and K+ efflux in NLRP3 inflammasome and innate immune response activation

Inflammation is part of innate immunity and is a natural response of the body to bacteria, virus, any other pathogen infections, or to damaged tissues. However, too much inflammation or chronic inflammation contributes to a wide variety of diseases such as inflammatory bowel disease, cancer, type 2 diabetes, heart disease, or autoimmune disease such as rheumatoid arthritis. Recent studies underscored the critical role of K⁺ and Cl^- efflux in the activation of the inflammasome. The NLRP3 inflammasome is a multiprotein complex that mediates the production of the proinflammatory cytokines IL-1β and IL-18 and initiates the inflammatory cell death or pyroptosis. The NLRP3 inflammasome can be activated by multiple stimuli such as extracellular ATP, microbial toxins, ROS, mitochondria DNA or particulate matter. Although the precise mechanisms of NLRP3 activation and regulation by these diverse agonists remain unclear, multiple reports indicate that all NLRP3 agonists ultimately lead to a drop in intracellular concentration of potassium (K⁺ efflux) and chloride (Cl^- efflux). The WNK-SPAK/OSR1-[N]KCC pathway plays a critical role maintaining K⁺ and Cl^- ions concentration in the cell. Recent advances indicate that the WNK-SPAK-[N]KCC pathway play a role in the activation of the innate immune response. This review highlights recent discoveries detailing how ion transport regulates innate immune cell response to inflammatory stimuli.

Abundant Monovalent Ions as Environmental Signposts for Pathogens during Host Colonization

Host colonization by a pathogen requires proper sensing and response to local environmental cues, to ensure adaptation and continued survival within the host. The ionic milieu represents a critical potential source of environmental cues, and indeed, there has been extensive study of the interplay between host and pathogen in the context of metals such as iron, zinc, and manganese, vital ions that are actively sequestered by the host. The inherent non-uniformity of the ionic milieu also extends, however, to "abundant" ions such as chloride and potassium, whose concentrations vary greatly between tissue and cellular locations, and with the immune response. Despite this, the concept of abundant ions as environmental cues and key players in host-pathogen interactions is only just emerging. Focusing on chloride and potassium, this review brings together studies across multiple bacterial and parasitic species that have begun to define both how these abundant ions are exploited as cues during host infection, and how they can be actively manipulated by pathogens during host colonization. The close links between ion homeostasis and sensing/response to different ionic signals, and the importance of studying pathogen response to cues in combination, are also discussed, while considering the fundamental insight still to be uncovered from further studies in this nascent area of inquiry.

NKCC1: Newly Found as a Human Disease-Causing Ion Transporter

Among the electroneutral Na⁺-dependent chloride transporters, NKCC1 had until now evaded identification as a protein causing human diseases. The closely related SLC12A transporters, NKCC2 and NCC have been identified some 25 years ago as responsible for Bartter and Gitelman syndromes: two renal-dependent salt wasting disorders. Absence of disease was most surprising since the NKCC1 knockout mouse was shown in 1999 to be viable, albeit with a wide range of deleterious phenotypes. Here we summarize the work of the past 5 years that introduced us to clinical cases involving NKCC1. The most striking cases are of 3 children with inherited mutations, who have complete absence of NKCC1 expression. These cases establish that lack of NKCC1 causes deafness; CFTR-like secretory defects with mucus accumulation in lung and intestine; severe xerostomia, hypotonia, dysmorphic facial features, and severe neurodevelopmental disorder. Another intriguing case is of a patient with a dominant deleterious SLC12A2 allele. This de novo mutation introduced a premature stop codon leading to a truncated protein. This mutant transporter seems to exert dominant-negative effect on wild-type transporter only in epithelial cells. The patient who suffers from lung, bladder, intestine, pancreas, and multiple endocrine abnormalities has, however, normal hearing and cognition. Finally, new reports substantiate the haploinsufficiency prediction of the SLC12A2 gene. Cases with single allele mutations in SLC12A2 have been linked to hearing loss and neurodevelopmental disorders.

CLIC2α Chloride Channel Orchestrates Immunomodulation of Hemocyte Phagocytosis and Bactericidal Activity in Crassostrea gigas

Chloride ion plays critical roles in modulating immunological interactions. Herein, we demonstrated that the anion channel CLIC2α mediates Cl⁻ flux to regulate hemocytes functions in the Pacific oyster (Crassostrea gigas). Specifically, during infection by Vibrio parahemolyticus, chloride influx was activated following onset of phagocytosis. Phosphorylation of Akt was stimulated by Cl⁻ ions entering host cells, further contributing to signal transduction regulating internalization of bacteria through the PI3K/Akt signaling pathway. Concomitantly, Cl⁻ entered phagosomes, promoted the acidification and maturation of phagosomes, and contributed to production of HOCl to eradicate engulfed bacteria. Finally, genomic screening reveals CLIC2α as a major Cl⁻ channel gene responsible for regulating Cl⁻ influx in oysters. Knockdown of CLIC2α predictably impeded phagosome acidification and restricted bacterial killing in oysters. In conclusion, our work has established CLIC2α as a prominent regulator of Cl⁻ influx and thus Cl⁻ function in C. gigas in bacterial infection contexts.

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Influx of chloride ions is switched on during phagocytosis in oyster hemocytes

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PI3K/Akt signaling pathway mediates chloride-dependent activation of phagocytosis

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Cl⁻ promotes phagosomal acidification and HOCl production

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CLIC2α is the principal chloride channel encoding gene within oyster genome

Interpreting an apoptotic corpse as anti-inflammatory involves a chloride sensing pathway

Apoptotic cell clearance (efferocytosis) elicits an anti-inflammatory response by phagocytes, but the mechanisms that underlie this response are still being defined. Here, we uncover a chloride-sensing signalling pathway that controls both the phagocyte 'appetite' and its anti-inflammatory response. Efferocytosis transcriptionally altered the genes that encode the solute carrier (SLC) proteins SLC12A2 and SLC12A4. Interfering with SLC12A2 expression or function resulted in a significant increase in apoptotic corpse uptake per phagocyte, whereas the loss of SLC12A4 inhibited corpse uptake. In SLC12A2-deficient phagocytes, the canonical anti-inflammatory program was replaced by pro-inflammatory and oxidative-stress-associated gene programs. This 'switch' to pro-inflammatory sensing of apoptotic cells resulted from the disruption of the chloride-sensing pathway (and not due to corpse overload or poor degradation), including the chloride-sensing kinases WNK1, OSR1 and SPAK-which function upstream of SLC12A2-had a similar effect on efferocytosis. Collectively, the WNK1-OSR1-SPAK-SLC12A2/SLC12A4 chloride-sensing pathway and chloride flux in phagocytes are key modifiers of the manner in which phagocytes interpret the engulfed apoptotic corpse.

Dysregulated Calcium Homeostasis in Cystic Fibrosis Neutrophils Leads to Deficient Antimicrobial Responses

Cystic fibrosis (CF), one of the most common human genetic diseases worldwide, is caused by a defect in the CF transmembrane conductance regulator (CFTR). Patients with CF are highly susceptible to infections caused by opportunistic pathogens (including Burkholderia cenocepacia), which induce excessive lung inflammation and lead to the eventual loss of pulmonary function. Abundant neutrophil recruitment into the lung is a key characteristic of bacterial infections in CF patients. In response to infection, inflammatory neutrophils release reactive oxygen species and toxic proteins, leading to aggravated lung tissue damage in patients with CF. The present study shows a defect in reactive oxygen species production by mouse Cftr^-/- , human F508del-CFTR, and CF neutrophils; this results in reduced antimicrobial activity against B. cenocepacia Furthermore, dysregulated Ca²⁺ homeostasis led to increased intracellular concentrations of Ca²⁺ that correlated with significantly diminished NADPH oxidase response and impaired secretion of neutrophil extracellular traps in human CF neutrophils. Functionally deficient human CF neutrophils recovered their antimicrobial killing capacity following treatment with pharmacological inhibitors of Ca²⁺ channels and CFTR channel potentiators. Our findings suggest that regulation of neutrophil Ca²⁺ homeostasis (via CFTR potentiation or by the regulation of Ca²⁺ channels) can be used as a new therapeutic approach for reestablishing immune function in patients with CF.

Hypertonic Saline Suppresses NADPH Oxidase-Dependent Neutrophil Extracellular Trap Formation and Promotes Apoptosis

Tonicity of saline (NaCl) is important in regulating cellular functions and homeostasis. Hypertonic saline is administered to treat many inflammatory diseases, including cystic fibrosis. Excess neutrophil extracellular trap (NET) formation, or NETosis, is associated with many pathological conditions including chronic inflammation. Despite the known therapeutic benefits of hypertonic saline, its underlying mechanisms are not clearly understood. Therefore, we aimed to elucidate the effects of hypertonic saline in modulating NETosis. For this purpose, we purified human neutrophils and induced NETosis using agonists such as diacylglycerol mimetic phorbol myristate acetate (PMA), Gram-negative bacterial cell wall component lipopolysaccharide (LPS), calcium ionophores (A23187 and ionomycin from Streptomyces conglobatus), and bacteria (Pseudomonas aeruginosa and Staphylococcus aureus). We then analyzed neutrophils and NETs using Sytox green assay, immunostaining of NET components and apoptosis markers, confocal microscopy, and pH sensing reagents. This study found that hypertonic NaCl suppresses nicotinamide adenine dinucleotide phosphate oxidase (NADPH2 or NOX2)-dependent NETosis induced by agonists PMA, Escherichia coli LPS (0111:B4 and O128:B12), and P. aeruginosa. Hypertonic saline also suppresses LPS- and PMA- induced reactive oxygen species production. It was determined that supplementing H₂O₂ reverses the suppressive effect of hypertonic saline on NOX2-dependent NETosis. Many of the aforementioned suppressive effects were observed in the presence of equimolar concentrations of choline chloride and osmolytes (d-mannitol and d-sorbitol). This suggests that the mechanism by which hypertonic saline suppresses NOX2-dependent NETosis is via neutrophil dehydration. Hypertonic NaCl does not significantly alter the intracellular pH of neutrophils. We found that hypertonic NaCl induces apoptosis while suppressing NOX2-dependent NETosis. In contrast, hypertonic solutions do not suppress NOX2-independent NETosis. Although hypertonic saline partially suppresses ionomycin-induced NETosis, it enhances A23187-induced NETosis, and it does not alter S. aureus-induced NETosis. Overall, this study determined that hypertonic saline suppresses NOX2-dependent NETosis induced by several agonists; in contrast, it has variable effects on neutrophil death induced by NOX2-independent NETosis agonists. These findings are important in understanding the regulation of NETosis and apoptosis in neutrophils.

Chloride flux in phagocytes

Phagocytes, such as neutrophils and macrophages, engulf microbes into phagosomes and launch chemical attacks to kill and degrade them. Such a critical innate immune function necessitates ion participation. Chloride, the most abundant anion in the human body, is an indispensable constituent of the myeloperoxidase (MPO)-H2 O2 -halide system that produces the potent microbicide hypochlorous acid (HOCl). It also serves as a balancing ion to set membrane potentials, optimize cytosolic and phagosomal pH, and regulate phagosomal enzymatic activities. Deficient supply of this anion to or defective attainment of this anion by phagocytes is linked to innate immune defects. However, how phagocytes acquire chloride from their residing environment especially when they are deployed to epithelium-lined lumens, and how chloride is intracellularly transported to phagosomes remain largely unknown. This review article will provide an overview of chloride protein carriers, potential mechanisms for phagocytic chloride preservation and acquisition, intracellular chloride supply to phagosomes for oxidant production, and methods to measure chloride levels in phagocytes and their phagosomes.

The LRRC8A Mediated "Swell Activated" Chloride Conductance Is Dispensable for Vacuolar Homeostasis in Neutrophils

The dialysis of human and mouse neutrophils in patch clamp experiments in the conventional whole-cell mode induces the emergence of a chloride (Cl^-) current that appeared to be primarily regulated by cytoplasmic ionic strength. The characteristics of this current resembled that of the classical, and ubiquitous volume-sensitive outwardly rectifying Cl^- current: strong outward rectification, selectivity sequence of the Eisenman1 type, insensitivity to external pH and strong inhibition by tamoxifen, DCPIB and WW781. We show that this current is essentially supported by the leucine rich repeat containing 8 A (LRRC8A); the naturally occurring LRRC8A truncation mutant in ebo/ebo mice drastically reduced Cl^- conductance in neutrophils. Remarkably, the residual component presents a distinct pharmacology, but appears equally potentiated by reduced ionic strength. We have investigated the role of the LRRC8A-supported current in the ionic homeostasis of the phagosomal compartment. The vacuolar pH, measured using SNARF-1 labeled Candida albicans, normally rises because of NADPH oxidase activity, and this elevation is blocked by certain Cl^- channel inhibitors. However, the pH rise remains intact in neutrophils from the ebo/ebo mice which also demonstrate preserved phagocytic and respiratory burst capacities and normal-sized vacuoles. Thus, the LRRC8A-dependent conductance of neutrophils largely accounts for their “swell activated” Cl^- current, but is not required for homeostasis of the phagosomal killing compartment.

An Exploration of Charge Compensating Ion Channels across the Phagocytic Vacuole of Neutrophils

Neutrophils phagocytosing bacteria and fungi exhibit a burst of non-mitochondrial respiration that is required to kill and digest the engulfed microbes. This respiration is accomplished by the movement of electrons across the wall of the phagocytic vacuole by the neutrophil NADPH oxidase, NOX2. In this study, we have attempted to identify the non-proton ion channels or transporters involved in charge compensation by examining the effect of inhibitors on vacuolar pH and cross-sectional area, and on oxygen consumption. The chloride channel inhibitors 4-[(2-Butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]butanoic acid (DCPIB) and flufenamic acid (FFA) were the most effective inhibitors of alkalinisation in human neutrophil vacuoles, suggesting an efflux of chloride from the vacuole. The proton channel inhibitor, zinc (Zn²⁺), combined with DCPIB caused more vacuolar swelling than either compound alone, suggesting the conductance of osmotically active cations into the vacuole. Support for cation influx was provided by the broad-spectrum cation transport inhibitors anandamide and quinidine which inhibited vacuolar alkalinisation and swelling when applied with zinc. Oxygen consumption was generally unaffected by these anion or cation inhibitors alone, but when combined with Zn²⁺ it was dramatically reduced, suggesting that multiple channels in combination can compensate the charge. In an attempt to identify specific channels, we tested neutrophils from knock-out mouse models including CLIC1, ClC3, ClC4, ClC7, KCC3, KCNQ1, KCNE3, KCNJ15, TRPC1/3/5/6, TRPA1/TRPV1, TRPM2, and TRPV2, and double knockouts of CLIC1, ClC3, KCC3, TRPM2, and KCNQ1 with HVCN1, and humans with channelopathies involving BEST1, ClC7, CFTR, and MCOLN1. No gross abnormalities in vacuolar pH or area were found in any of these cells suggesting that we had not tested the correct channel, or that there is redundancy in the system. The respiratory burst was suppressed in the KCC3^-/- and enhanced in the CLIC1^-/- cells, but was normal in all others, including ClC3^-/-. These results suggest charge compensation by a chloride conductance out of the vacuole and by cation/s into it. The identity of these channels remains to be established.

Mechanotransduction in neutrophil activation and deactivation

Mechanotransduction refers to the processes through which cells sense mechanical stimuli by converting them to biochemical signals and, thus, eliciting specific cellular responses. Cells sense mechanical stimuli from their 3D environment, including the extracellular matrix, neighboring cells and other mechanical forces. Incidentally, the emerging concept of mechanical homeostasis,long term or chronic regulation of mechanical properties, seems to apply to neutrophils in a peculiar manner, owing to neutrophils' ability to dynamically switch between the activated/primed and deactivated/deprimed states. While neutrophil activation has been known for over a century, its deactivation is a relatively recent discovery. Even more intriguing is the reversibility of neutrophil activation and deactivation. We review and critically evaluate recent findings that suggest physiological roles for neutrophil activation and deactivation and discuss possible mechanisms by which mechanical stimuli can drive the oscillation of neutrophils between the activated and resting states. We highlight several molecules that have been identified in neutrophil mechanotransduction, including cell adhesion and transmembrane receptors, cytoskeletal and ion channel molecules. The physiological and pathophysiological implications of such mechanically induced signal transduction in neutrophils are highlighted as a basis for future work. This article is part of a Special Issue entitled: Mechanobiology.

Salt, chloride, bleach, and innate host defense

Salt provides 2 life-essential elements: sodium and chlorine. Chloride, the ionic form of chlorine, derived exclusively from dietary absorption and constituting the most abundant anion in the human body, plays critical roles in many vital physiologic functions, from fluid retention and secretion to osmotic maintenance and pH balance. However, an often overlooked role of chloride is its function in innate host defense against infection. Chloride serves as a substrate for the generation of the potent microbicide chlorine bleach by stimulated neutrophils and also contributes to regulation of ionic homeostasis for optimal antimicrobial activity within phagosomes. An inadequate supply of chloride to phagocytes and their phagosomes, such as in CF disease and other chloride channel disorders, severely compromises host defense against infection. We provide an overview of the roles that chloride plays in normal innate immunity, highlighting specific links between defective chloride channel function and failures in host defense.

Protein chlorination in neutrophil phagosomes and correlation with bacterial killing

Neutrophils ingest and kill bacteria within phagocytic vacuoles. We investigated where they produce hypochlorous acid (HOCl) following phagocytosis by measuring conversion of protein tyrosine residues to 3-chlorotyrosine. We also examined how varying chloride availability affects the relationship between HOCl formation in the phagosome and bacterial killing. Phagosomal proteins, isolated following ingestion of opsonized magnetic beads, contained 11.4 Cl-Tyr per thousand tyrosine residues. This was 12 times higher than the level in proteins from the rest of the neutrophil and ~6 times higher than previously recorded for protein from ingested bacteria. These results indicate that HOCl production is largely localized to the phagosomes and a substantial proportion reacts with phagosomal protein before reaching the microbe. This will in part detoxify the oxidant but should also form chloramines which could contribute to the killing mechanism. Neutrophils were either suspended in chloride-free gluconate buffer or pretreated with formyl-Met-Leu-Phe, a procedure that has been reported to deplete intracellular chloride. These treatments, alone or in combination, decreased both chlorination in phagosomes and killing of Staphylococcus aureus by up to 50%. There was a strong positive correlation between the two effects. Killing was predominantly oxidant and myeloperoxidase dependent (88% inhibition by diphenylene iodonium and 78% by azide). These results imply that lowering the chloride concentration limits HOCl production and oxidative killing. They support a role for HOCl generation, rather than an alternative myeloperoxidase activity, in the killing process.

A regulatory role of K(+)-Cl(-) cotransporter in the cell cycle progression of breast cancer MDA-MB-231 cells

K(+)-Cl(-) cotransporter (KCC) has been shown to be involved in cell proliferation as well as cell volume regulation. A regulatory role of KCC in cell cycle progression of breast cancer MDA-MB-231 cells was explored by using synchronized MDA-MB-231 cells and dihydro-indenyloxy-alkanoic acid (DIOA), a potent inhibitor of KCC. MDA-MB-231 cells cultured in the presence of DIOA exhibited an increase in cell volume, a decrease in intracellular Cl(-) concentration, and reduction in cell proliferation with the G0/G1 phase arrest, which was accompanied with down-regulation of cyclin D1 and cyclin E2, and up-regulation of p21. Among these molecules, the expression of cyclin E2, a molecule essential for the transition from G1 to S phase, was markedly suppressed by DIOA treatment. DIOA-mediated up- or down-regulation of these molecules occurred at the transcriptional level. These findings suggest that KCC plays an important role in the early phase of cell cycle progression by regulating the expression of cyclin D1, cyclin E2, and p21, the molecules essential for the cell cycle progression.

Redox reactions and microbial killing in the neutrophil phagosome

SIGNIFICANCE

When neutrophils kill microorganisms, they ingest them into phagosomes and bombard them with a burst of reactive oxygen species.

RECENT ADVANCES

This review focuses on what oxidants are produced and how they kill. The neutrophil NADPH oxidase is activated and shuttles electrons from NADPH in the cytoplasm to oxygen in the phagosomal lumen. Superoxide is generated in the narrow space between the ingested organism and the phagosomal membrane and kinetic modeling indicates that it reaches a concentration of around 20 μM. Degranulation leads to a very high protein concentration with up to millimolar myeloperoxidase (MPO). MPO has many substrates, but its main phagosomal reactions should be to dismutate superoxide and, provided adequate chloride, catalyze efficient conversion of hydrogen peroxide to hypochlorous acid (HOCl). Studies with specific probes have shown that HOCl is produced in the phagosome and reacts with ingested bacteria. The amount generated should be high enough to kill. However, much of the HOCl reacts with phagosomal proteins. Generation of chloramines may contribute to killing, but the full consequences of this are not yet clear.

CRITICAL ISSUES

Isolated neutrophils kill most of the ingested microorganisms rapidly by an MPO-dependent mechanism that is almost certainly due to HOCl. However, individuals with MPO deficiency rarely have problems with infection. A possible explanation is that HOCl provides a frontline response that kills most of the microorganisms, with survivors killed by nonoxidative processes. The latter may deal adequately with low-level infection but with high exposure, more efficient HOCl-dependent killing is required.

FUTURE DIRECTIONS

Better quantification of HOCl and other oxidants in the phagosome should clarify their roles in antimicrobial action.

Chloride transport in functionally active phagosomes isolated from Human neutrophils

Chloride anion is critical for hypochlorous acid (HOCl) production and microbial killing in neutrophil phagosomes. However, the molecular mechanism by which this anion is transported to the organelle is poorly understood. In this report, membrane-enclosed and functionally active phagosomes were isolated from human neutrophils by using opsonized paramagnetic latex microspheres and a rapid magnetic separation method. The phagosomes recovered were highly enriched for specific protein markers associated with this organelle such as lysosomal-associated membrane protein-1, myeloperoxidase (MPO), lactoferrin, and NADPH oxidase. When FITC-dextran was included in the phagocytosis medium, the majority of the isolated phagosomes retained the fluorescent label after isolation, indicative of intact membrane structure. Flow cytometric measurement of acridine orange, a fluorescent pH indicator, in the purified phagosomes demonstrated that the organelle in its isolated state was capable of transporting protons to the phagosomal lumen via the vacuolar-type ATPase proton pump (V-ATPase). When NADPH was supplied, the isolated phagosomes constitutively oxidized dihydrorhodamine 123, indicating their ability to produce hydrogen peroxide. The preparations also showed a robust production of HOCl within the phagosomal lumen when assayed with the HOCl-specific fluorescent probe R19-S by flow cytometry. MPO-mediated iodination of the proteins covalently conjugated to the phagocytosed beads was quantitatively measured. Phagosomal uptake of iodide and protein iodination were significantly blocked by chloride channel inhibitors, including CFTRinh-172 and NPPB. Further experiments determined that the V-ATPase-driving proton flux into the isolated phagosomes required chloride cotransport, and the cAMP-activated CFTR chloride channel was a major contributor to the chloride transport. Taken together, the data suggest that the phagosomal preparation described herein retains ion transport properties, and multiple chloride channels including CFTR are responsible for chloride supply to neutrophil phagosomes.

Enhancement of tubulin polymerization by Cl(-)-induced blockade of intrinsic GTPase

In growing neurite of neuronal cells, it is suggested that α/β-tubulin heterodimers assemble to form microtubule, and assembly of microtubule promotes neurite elongation. On the other hand, recent studies reveal importance of intracellular Cl(-) in regulation of various cellular functions such as cell cycle progression, differentiation, cell migration, and elongation of neurite in neuronal cells. In this study, we investigated effects of Cl(-) on in vitro tubulin polymerization. We found that efficiency of in vitro tubulin polymerization (the number of microtubule) was higher (3 to 5-fold) in Cl(-)-containing solutions than that in Cl(-)-free solutions containing Br(-) or NO(3)(-). On the other hand, GTPase activity of tubulin was lower (2/3-fold) in Cl(-)-containing solutions than that in Cl(-)-free solutions containing Br(-) or NO(3)(-). Efficiency of in vitro tubulin polymerization in solutions containing a non-hydrolyzable analogue of GTP (GpCpp) instead of GTP was much higher than that in the presence of GTP. Effects of replacement of GTP with GpCpp on in vitro tubulin polymerization was weaker in Cl(-) solutions (10-fold increases) than that in Br(-) or NO(3)(-) solutions (20-fold increases), although the efficiency of in vitro tubulin polymerization in Cl(-) solutions containing GpCpp was still higher than that in Br(-) or NO(3)(-) solutions containing GpCpp. Our results suggest that a part of stimulatory effects of Cl(-) on in vitro tubulin polymerization is mediated via an inhibitory effect on GTPase activity of tubulin, although Cl(-) would also regulate in vitro tubulin polymerization by factors other than an inhibitory effect on GTPase activity.

K⁺-Cl⁻ cotransport mediates the bactericidal activity of neutrophils by regulating NADPH oxidase activation

Neutrophilic phagocytosis is an essential component of innate immunity. During phagocytosis, the generation of bactericidal hypochlorous acid(HOCl) requires the substrates, Cl− and superoxide produced by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase to kill the internalized pathogens. Here we show that the neutrophilic K+–Cl− cotransporter (KCC) constitutes aCl− permeation pathway and mediates the bactericidal activity by regulating NADPH oxidase activation. Dihydroindenyloxy alkanoic acid (DIOA), a KCC inhibitor, suppressed the toxin- or chemical-induced efflux of 36Cl− or 86Rb+, and diminished the production of superoxide in human and murine neutrophils. Inhibition of KCC activity or knockdown of KCC expression, in particular KCC3, reduced the phosphorylation as well as the membrane recruitment of oxidase components. Activated neutrophils displayed a significant colocalization of KCC3 and early endosomal marker, indicating that KCC3 could be localized on the phagosomes once neutrophils are activated. The NADPH oxidase activity and the phosphorylation level of oxidase component were 50% lower in the neutrophils isolated from KCC3−/− mice than in the neutrophils isolated from KCC3+/+ mice.Mortality rate after intraperitoneal challenge with Staphylococcus aureus was higher in KCC3−/− mice, and the bacterial clearance was impaired in the survivors.We conclude that, in activated neutrophil, NADPH oxidase complexes are associated with KCC3 at the plasma membrane and are internalized to form phagosomes, where KCC activity and expression level affect the production of oxidants.

Quercetin stimulates NGF-induced neurite outgrowth in PC12 cells via activation of Na(+)/K(+)/2Cl(-) cotransporter

We have recently reported that Na(+)/K(+)/2Cl(-) cotransporter isoform 1 (NKCC1) plays an essential role in nerve growth factor (NGF)-induced neurite outgrowth in PC12D cells. On the other hand, it has been reported that dietary flavonoids, such as quercetin, apigenin, and luteolin, stimulate various ion transporters. In the present report, we investigated the effect of quercetin, a flavonoid, on NGF-induced neurite outgrowth in PC12 cells (the parental strain of PC12D cells). Quercetin stimulated the NGF-induced neurite outgrowth in a dose-dependent manner. Knockdown of NKCC1 by RNAi methods abolished the stimulatory effect of flavonoid. Quercetin stimulated NKCC1 activity (measured as bumetanide-sensitive (86)Rb influx) without any increase in the expression level of NKCC1 protein. The stimulatory effect of quercetin on neurite outgrowth was dependent upon extracellular Cl(-). These observations indicate that quercetin stimulates the NGF-induced neurite outgrowth via an increase in Cl(-) incorporation into the intracellular space by activating NKCC1 in PC12 cell.

Voltage-gated proton channels find their dream job managing the respiratory burst in phagocytes

The voltage-gated proton channel bears surprising resemblance to the voltage-sensing domain (S1-S4) of other voltage-gated ion channels but is a dimer with two conduction pathways. The proton channel seems designed for efficient proton extrusion from cells. In phagocytes, it facilitates the production of reactive oxygen species by NADPH oxidase.

CFTR-mediated halide transport in phagosomes of human neutrophils

Chloride serves as a critical component of innate host defense against infection, providing the substrate for MPO-catalyzed production of HOCl in the phagosome of human neutrophils. Here, we used halide-specific fluorescent sensors covalently coupled to zymosan particles to investigate the kinetics of chloride and iodide transport in phagosomes of human neutrophils. Using the self-ratioable fluorescent probe specific for chloride anion, we measured chloride dynamics within phagosomes in response to extracellular chloride changes by quantitative fluorescence microscopy. Under the experimental conditions used, normal neutrophils showed rapid phagosomal chloride uptake with an initial influx rate of 0.31 +/- 0.04 mM/s (n=5). GlyH-101, a CFTR(inh), decreased the rate of uptake in a dose-dependent manner. Neutrophils isolated from CF patients showed a significantly slower rate of chloride uptake by phagosomes, having an initial influx rate of 0.043 +/- 0.012 mM/s (n=5). Interestingly, the steady-state level of chloride in CF phagosomes was approximately 26 mM, significantly lower than that of the control ( approximately 68 mM). As CFTR transports chloride as well as other halides, we conjugated an iodide-sensitive probe as an independent approach to confirm the results. The dynamics of iodide uptake by neutrophil phagosomes were monitored by flow cytometry. CFTR(inh)172 blocked 40-50% of the overall iodide uptake by phagosomes in normal neutrophils. In a parallel manner, the level of iodide uptake by CF phagosomes was only 20-30% of that of the control. Taken together, these results implicate CFTR in transporting halides into the phagosomal lumen.

Advances in neutrophil biology: clinical implications

Many lung diseases are characterized by neutrophil-dominated inflammation; therefore, an understanding of neutrophil function is of considerable importance to respiratory physicians. This review will focus on recent advances in our understanding of how neutrophils are produced, how these cells leave the circulation, the molecular events regulating neutrophil activation and, ultimately, how these cells die and are removed. The neutrophil is now recognized as a highly versatile and sophisticated cell with significant synthetic capacity and an important role in linking the innate and adaptive arms of the immune response. One of the key challenges in conditions such as COPD, bronchiectasis, cystic fibrosis, and certain forms of asthma is how to manipulate neutrophil function in a way that does not compromise antibacterial and antifungal capacity. The possession by neutrophils of a unique repertoire of surface receptors and signaling proteins may make such targeted therapy possible.

Role of Nox2 in elimination of microorganisms

NADPH oxidase of the phagocytic cells (Nox2) transfers electrons from cytosolic NADPH to molecular oxygen in the extracellular or intraphagosomal space. The produced superoxide anion (O*2) provides the source for formation of all toxic oxygen derivatives, but continuous O*2 generation depends on adequate charge compensation. The vital role of Nox2 in efficient elimination of microorganisms is clearly indicated by human pathology as insufficient activity of the enzyme results in severe, recurrent bacterial infections, the typical symptoms of chronic granulomatous disease. The goals of this contribution are to provide critical review of the Nox2-dependent cellular processes that potentially contribute to bacterial killing and degradation and to indicate possible targets of pharmacological interventions.

The role of chloride anion and CFTR in killing of Pseudomonas aeruginosa by normal and CF neutrophils

Chloride anion is essential for myeloperoxidase (MPO) to produce hypochlorous acid (HOCl) in polymorphonuclear neutrophils (PMNs). To define whether chloride availability to PMNs affects their HOCl production and microbicidal capacity, we examined how extracellular chloride concentration affects killing of Pseudomonas aeruginosa (PsA) by normal neutrophils. PMN-mediated bacterial killing was strongly dependent on extracellular chloride concentration. Neutrophils in a chloride-deficient medium killed PsA poorly. However, as the chloride level was raised, the killing efficiency increased in a dose-dependent manner. By using specific inhibitors to selectively block NADPH oxidase, MPO, and cystic fibrosis transmembrane conductance regulator (CFTR) functions, neutrophil-mediated killing of PsA could be attributed to three distinct mechanisms: CFTR-dependent and oxidant-dependent; chloride-dependent but not CFTR- and oxidant-dependent; and independent of any of the tested factors. Therefore, chloride anion is involved in oxidant- and nonoxidant-mediated bacterial killing. We previously reported that neutrophils from CF patients are defective in chlorination of ingested bacteria, suggesting that the chloride channel defect might impair the MPO-hydrogen peroxide-chloride microbicidal function. Here, we compared the competence of killing PsA by neutrophils from normal donors and CF patients. The data demonstrate that the killing rate by CF neutrophils was significantly lower than that by normal neutrophils. CF neutrophils in a chloride-deficient environment had only one-third of the bactericidal capacity of normal neutrophils in a physiological chloride environment. These results suggest that CFTR-dependent chloride anion transport contributes significantly to killing PsA by normal neutrophils and when defective as in CF, may compromise the ability to clear PsA.