A novel H(+) conductance in eosinophils: unique characteristics and absence in chronic granulomatous disease

Inhibition of the neutrophil NADPH oxidase and associated H+ channel by diethyl pyrocarbonate (DEPC), a histidine-modifying agent: evidence for at least two target sites

Diethyl pyrocarbonate (DEPC), a histidine-modifying reagent, has been utilized to demonstrate the importance of histidine residues in the functioning of proteins. In previous studies of the NADPH oxidase, histidine residues have been determined to be important in the ability of gp91(phox) to function as an H(+) pathway and in the binding of haem and FAD. We have investigated the ability of DEPC to inhibit H(+) flux and superoxide generation by human neutrophils. Proton flux through the NADPH oxidase-associated H(+) channel was inhibited by DEPC only if applied simultaneously with an activator of the channel. This suggested that the site modified by DEPC is not accessible in the closed channel. Superoxide generation by the NADPH oxidase was also inhibited by DEPC when applied after or simultaneously with the activator. Translocation of the NADPH oxidase cytosolic components, p67(phox) and p47(phox), to the membrane was unaffected by DEPC. In a cell-free system, DEPC-treated membranes failed to support superoxide generation or the reduction of Iodonitrotetrazolium Violet and showed a loss of the characteristic cytochrome b(558) spectrum. Superoxide generation by DEPC-treated cytosol was inhibited slightly. Therefore it can be concluded that there are two sites within the NADPH oxidase that interact with DEPC, one in the H(+) pathway, only accessible in the activated oxidase, and a second accessible prior to activation of the NADPH oxidase. The latter non-proton pathway DEPC site is located within the membrane components of the NADPH oxidase and is associated with the binding of haem in the enzyme complex.

Heme histidine ligands within gp91(phox) modulate proton conduction by the phagocyte NADPH oxidase

The membrane subunit of the phagocyte NADPH oxidase, gp91(phox), possesses a H(+) channel motif formed by membrane-spanning histidines postulated to coordinate the two heme groups forming the redox center of the flavocytochrome. To study the role of heme-binding histidines on proton conduction, we stably expressed the gp91(phox) cytochrome in human embryonic kidney 293 cells and measured proton currents with the patch clamp technique. Similar to its shorter homologue, NADPH oxidase homologue 1, which is predicted not to bind heme, gp91(phox) generated voltage-activated, pH-dependent, H(+)-selective currents that were reversibly blocked by Zn(2+). The gp91(phox) currents, however, activated faster, deactivated more slowly, and were markedly affected by the inhibition of heme synthesis. Upon heme removal, the currents had larger amplitude, activated faster and at lower voltages, and became sensitive to the histidine reagent diethylpyrocarbonate. Mutation of the His-115 residue to leucine abolished both the gp91(phox) characteristic 558-nm absorbance peak and voltage-activated currents, indicating that His-115 is involved in both heme ligation and proton conduction. These results indicate that the gp91(phox) proton channel is activated upon release of heme from its His-115 ligand. During activation of the oxidase complex, changes in heme coordination within the cytochrome might increase the mobility of histidine ligands, thereby coupling electron and proton transport.

Voltage-gated proton channels in microglia

Microglia, macrophages that reside in the brain, can express at least 12 different ion channels, including voltage-gated proton channels. The properties of H+ currents in microglia are similar to those in other phagocytes. Proton currents are elicited by depolarizing the membrane potential, but activation also depends strongly on both intracellular pH (pH(i)) and extracellular pH (pH(o)). Increasing pH(o) or lowering pH(i) promotes H+ channel opening by shifting the activation threshold to more negative potentials. H+ channels in microglia open only when the pH gradient is outward, so they carry only outward current in the steady state. Time-dependent activation of H+ currents is slow, with a time constant roughly 1 s at room temperature. Microglial H+ currents are inhibited by inorganic polyvalent cations, which reduce H+ current amplitude and shift the voltage dependence of activation to more positive potentials. Cytoskeletal disruptive agents modulate H+ currents in microglia. Cytochalasin D and colchicine decrease the current density and slow the activation of H+ currents. Similar changes of H+ currents, possibly due to cytoskeletal reorganization, occur in microglia during the transformation from ameboid to ramified morphology. Phagocytes, including microglia, undergo a respiratory burst, in which NADPH oxidase releases bactericidal superoxide anions into the phagosome and stoichiometrically releases protons into the cell, tending to depolarize and acidify the cell. H+ currents may help regulate both the membrane potential and pH(i) during the respiratory burst. By compensating for the efflux of electrons and counteracting intracellular acidification, H+ channels help maintain superoxide anion production.

Salmonella pathogenicity island 2-encoded type III secretion system mediates exclusion of NADPH oxidase assembly from the phagosomal membrane

Salmonella typhimurium requires a type III secretion system encoded by pathogenicity island (SPI)-2 to survive and proliferate within macrophages. This survival implies that S. typhimurium avoids or withstands bactericidal events targeted to the microbe-containing vacuole, which include intraphagosomal production of reactive oxygen species (ROS), phagosomal acidification, and delivery of hydrolytic enzymes to the phagosome via fusion with lysosomes. Recent evidence suggests that S. typhimurium alters ROS production by murine macrophages in an SPI-2-dependent manner. To gain insights into the mechanism by which S. typhimurium inhibits intraphagosomal ROS production, we analyzed the subcellular distribution of NADPH oxidase components during infection of human monocyte-derived macrophages by wild-type (WT) or several SPI-2 mutant strains of S. typhimurium. We found that the membrane component of the NADPH oxidase, flavocytochrome b(558), was actively excluded or rapidly removed from the phagosomal membrane of WT-infected monocyte-derived macrophages, thereby preventing assembly of the NADPH oxidase complex and intraphagosomal production of superoxide anion. In contrast, the NADPH oxidase assembled on and generated ROS in phagosomes containing SPI-2 mutant S. typhimurium. Subversion of NADPH oxidase assembly by S. typhimurium was accompanied by increased bacterial replication relative to that of SPI-2 mutant strains, suggesting that the ability of WT S. typhimurium to prevent NADPH oxidase assembly at the phagosomal membrane represents an important virulence factor influencing its intracellular survival.

Accelerated calcium influx and hyperactivation of neutrophils in chronic granulomatous disease

The relationship between activation of NADPH-oxidase, alterations in membrane potential and triggering of Ca2+ fluxes in human phagocytes has been investigated using neutrophils from four subjects with chronic granulomatous disease (CGD). Cytosolic Ca2+ and membrane potential were measured by spectrofluorimetry, and net efflux and influx of Ca2+ by radiometric procedures. Exposure of normal neutrophils to the chemotactic tripeptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP; 1 microM) was accompanied by an abrupt increase in cytosolic Ca2+ coincident with membrane depolarization and efflux of the cation. These events terminated at around 30 s after the addition of FMLP and were followed by membrane repolarization and store-operated influx of Ca2+, both of which were superimposable and complete after about 5 min. Activation of CGD neutrophils was also accompanied by an increase in cytosolic Ca2+, which, in spite of an efficient efflux response, was prolonged in relation to that observed in normal cells. This prolonged increase in cytosolic Ca2+ in activated CGD neutrophils occurred in the setting of trivial membrane depolarization and accelerated influx of Ca2+, and was associated with hyperactivity of the cells according to excessive release of elastase and increased activity of phospholipase A2. Treatment of CGD neutrophils with the type 4 phosphodiesterase inhibitor, rolipram (1 microM) restored Ca2+ homeostasis and attenuated the increase in elastase release. These findings support the involvement of NADPH-oxidase in regulating membrane potential and Ca2+ influx in activated neutrophils, and may explain the disordered inflammatory responses and granuloma formation which are characteristic of CGD.

Involvement of ion channels in human eosinophil respiratory burst

BACKGROUND

Human eosinophils possess a variety of ion channels that play a crucial role in the regulation of cellular activity. During eosinophil respiratory burst, efflux of H(+) ions through H(+) channels provides an efficient mechanism of H(+) extrusion and charge compensation. Interestingly, recent studies suggest that other ion channels may also be involved in this process.

OBJECTIVE

We sought to investigate the role of ion channels in phorbol 12-myristate 13-acetate-induced superoxide (O(2)(*-)) generation by human eosinophils.

METHODS

O(2)(*-) production was measured by using the superoxide dismutase-inhibitable reduction of cytochrome c. Ion channel expression and function were studied by using RT-PCR and the patch clamp technique, respectively.

RESULTS

O(2)(*-) generation was affected by several ion channel blockers, especially 4,4-diisothio-cyanostilbene-2,2'-disulfonic acid. The involvement of Cl(-) channels in this process was confirmed by replacement of Cl(-) with gluconate or other anions. The halide dependence of O(2)(*-) production could be described by the sequence Cl(-)> or =Br(-)>I(-), which is similar to the selectivity sequence of several members of the chloride channel (ClC) family. RT-PCR studies performed with primers for ClC-2, ClC-3, ClC-4, ClC-5, ClC-6, and the cystic fibrosis transmembrane conductance regulator showed only the expression of ClC-3. The presence of phorbol 12-myristate 13-acetate-sensitive Cl(-) channels in human eosinophils with biophysical properties similar to the ClC-3 channel has been studied.

CONCLUSION

Cl(-) channels play an important role in the regulation of O(2)(*-) production by human eosinophils.

Simultaneous activation of NADPH oxidase-related proton and electron currents in human neutrophils

Generation of reactive oxygen species by the NADPH oxidase complex is an important bactericidal weapon of phagocytes. Phorbol myristate acetate (PMA) is a potent agonist for this "respiratory burst" in human neutrophils. Although stoichiometric H(+) efflux occurs during the respiratory burst, efforts to stimulate voltage-gated H(+) channels by PMA in whole-cell patch-clamped phagocytes have been unsuccessful. We have used a modification of the permeabilized-patch configuration that allows control of intracellular pH and preserves second-messenger pathways. Using this method, we show that PMA dramatically enhances and alters voltage-gated proton currents in human neutrophils. PMA produced four alterations in H(+) current properties, each of which increases the H(+) current at any given voltage: (i) a 40-mV negative shift in the H(+) conductance-voltage (g(H)-V) relationship; (ii) faster activation [smaller activation time constant (tau(act))] during depolarizing pulses; (iii) slower deactivation [larger deactivation time constant (tau(tail))] on repolarization; and (iv) a larger maximum H(+) conductance (g(H, max)). Inward current that directly reflects electron transport by NADPH oxidase was also activated by PMA stimulation. The identity of this electron current was confirmed by its sensitivity to diphenylene iodinium, an inhibitor of NADPH oxidase. Diphenylene iodinium also reversed the slowing of tau(tail) with a time course paralleling the inhibition of electron current. However, the amplitudes of H(+) and electron currents activated by PMA were not correlated. A complex interaction between NADPH oxidase and voltage-gated proton channels is indicated. The data suggest that PMA stimulation modulates preexisting H(+) channels rather than inducing a new H(+) channel.

A mammalian H+ channel generated through alternative splicing of the NADPH oxidase homolog NOH-1

Voltage-gated proton (H+) channels are found in many human and animal tissues and play an important role in cellular defense against acidic stress. However, a molecular identification of these unique ion conductances has so far not been achieved. A 191-amino acid protein is described that, upon heterologous expression, has properties indistinguishable from those of native H+ channels. This protein is generated through alternative splicing of messenger RNA derived from the gene NOH-1 (NADPH oxidase homolog 1, where NADPH is the reduced form of nicotinamide adenine dinucleotide phosphate).

pH-dependent inhibition of voltage-gated H(+) currents in rat alveolar epithelial cells by Zn(2+) and other divalent cations

Inhibition by polyvalent cations is a defining characteristic of voltage-gated proton channels. The mechanism of this inhibition was studied in rat alveolar epithelial cells using tight-seal voltage clamp techniques. Metal concentrations were corrected for measured binding to buffers. Externally applied ZnCl(2) reduced the H(+) current, shifted the voltage-activation curve toward positive potentials, and slowed the turn-on of H(+) current upon depolarization more than could be accounted for by a simple voltage shift, with minimal effects on the closing rate. The effects of Zn(2+) were inconsistent with classical voltage-dependent block in which Zn(2+) binds within the membrane voltage field. Instead, Zn(2+) binds to superficial sites on the channel and modulates gating. The effects of extracellular Zn(2+) were strongly pH(o) dependent but were insensitive to pH(i), suggesting that protons and Zn(2+) compete for external sites on H(+) channels. The apparent potency of Zn(2+) in slowing activation was approximately 10x greater at pH(o) 7 than at pH(o) 6, and approximately 100x greater at pH(o) 6 than at pH(o) 5. The pH(o) dependence suggests that Zn(2+), not ZnOH(+), is the active species. Evidently, the Zn(2+) receptor is formed by multiple groups, protonation of any of which inhibits Zn(2+) binding. The external receptor bound H(+) and Zn(2+) with pK(a) 6.2-6.6 and pK(M) 6.5, as described by several models. Zn(2+) effects on the proton chord conductance-voltage (g(H)-V) relationship indicated higher affinities, pK(a) 7 and pK(M) 8. CdCl(2) had similar effects as ZnCl(2) and competed with H(+), but had lower affinity. Zn(2+) applied internally via the pipette solution or to inside-out patches had comparatively small effects, but at high concentrations reduced H(+) currents and slowed channel closing. Thus, external and internal zinc-binding sites are different. The external Zn(2+) receptor may be the same modulatory protonation site(s) at which pH(o) regulates H(+) channel gating.