Voltage-gated proton channels and other proton transfer pathways

Counteracting effects of NADPH oxidase and the Na+/Ca2+ exchanger on membrane repolarisation and store-operated uptake of Ca2+ by chemoattractant-activated human neutrophils

This study was designed to investigate the possible involvement of NADPH oxidase and the Na(+)/Ca(2+) exchanger in regulating membrane repolarisation and store-operated uptake of Ca(2+) by FMLP (1 microM)-activated human neutrophils. Diphenyleneiodonium chloride (DPI, 5-10 microM) and KB-R7943 (2.5-10 microM), inhibitors of NADPH oxidase and the reverse mode of the Na(+)/Ca(2+) exchanger respectively, were used as pharmacological probes. Transmembrane fluxes of Ca(2+), K(+) and Na(+) were determined radiometrically, while alterations in membrane potential and cytosolic Ca(2+) were evaluated using spectrofluorimetric procedures. DPI, added to the cells at the time of maximum FMLP-activated membrane depolarisation, accelerated the rates of both membrane repolarisation and influx of Ca(2+), while KB-R7943 effectively antagonised these processes. SKF 96365 (10 microM), an antagonist of store-operated Ca(2+) channels, abolished the influx of Ca(2+) into FMLP-activated neutrophils, but had no effects on membrane repolarisation, suggesting that the Na(+)/Ca(2+) exchanger is primarily involved in mediating membrane repolarisation, thereby facilitating uptake of Ca(2+) via store-operated channels. These observations are compatible with prominent negative and positive regulatory roles for NADPH oxidase and the Na(+)/Ca(2+) exchanger respectively in regulating the rates of membrane repolarisation and store-operated uptake of Ca(2+) by chemoattractant-activated neutrophils.

NADPH oxidase-dependent acid production in airway epithelial cells

The purpose of this study was to determine the role of NADPH oxidase in H(+) secretion by airway epithelia. In whole cell patch clamp recordings primary human tracheal epithelial cells (hTE) and the human serous gland cell line Calu-3 expressed a functionally similar zinc-blockable plasma membrane H(+) conductance. However, the rate of H(+) secretion of confluent epithelial monolayers measured in Ussing chambers was 9-fold larger in hTE compared with Calu-3. In hTE H(+) secretion was blocked by mucosal ZnCl(2) and the NADPH oxidase blockers acetovanillone and 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), whereas these same blockers had no effect in Calu-3. We determined levels of transcripts for the NADPH oxidase transmembrane isoforms (Nox1 through -5, Duox1 and -2, and p22(phox)) and found Duox1, -2, and p22(phox) to be highly expressed in hTE, as well as the intracellular subunits p40(phox), p47(phox), and p67(phox). In contrast, Calu-3 lacked transcripts for Duox1, p40(phox), and p47(phox). Anti-Duox antibody staining resulted in prominent apical staining in hTE but no significant staining in Calu-3. When treated with amiloride to block the Na(+)/H(+) exchanger, intracellular pH in hTE acidified at significantly higher rates than in Calu-3, and treatment with AEBSF blocked acidification. These data suggest a role for an apically located Duox-based NADPH oxidase during intracellular H(+) production and H(+) secretion, but not in H(+) conduction.

On the origin of the electrostatic barrier for proton transport in aquaporin

The nature of the electrostatic barrier for proton transport in aquaporins is analyzed by semimacroscopic and microscopic models. It is found that the barrier is associated with the loss of the generalized solvation energy upon moving from the bulk solvent to the center of the channel. It is clarified that our solvation concept includes the effect of the protein polar groups and ionized residues. The nature of the contributions to the solvation barrier is examined by using the linear response approximation. It is found that the residues in the NPA region contribute much less than what would be deduced from calculations that do not consider the protein reorganization. It is clarified that the contributions of different structural or electrostatic elements to the solvation barrier can be established by removing these elements and examining the corresponding effect on the barrier height. Using this definition and "mutating" the NPA residues to their non-polar analogues establishes that these residues do not provide the major contribution to the solvation barrier.

Surface-mediated proton-transfer reactions in membrane-bound proteins

As outlined by Peter Mitchell in the chemiosmotic theory, an intermediate in energy conversion in biological systems is a proton electrochemical potential difference ("proton gradient") across a membrane, generated by membrane-bound protein complexes. These protein complexes accommodate proton-transfer pathways through which protons are conducted. In this review, we focus specifically on the role of the protein-membrane surface and the surface-bulk water interface in the dynamics of proton delivery to these proton-transfer pathways. The general mechanisms are illustrated by experimental results from studies of bacterial photosynthetic reaction centres (RCs) and cytochrome c oxidase (CcO).

TRPV1 acts as proton channel to induce acidification in nociceptive neurons

The low extracellular pH of inflamed or ischemic tissues enhances painful sensations by sensitizing and activating the vanilloid receptor 1 (TRPV1). We report here that activation of TRPV1 results in a marked intracellular acidification in nociceptive dorsal root ganglion neurons and in a heterologous expression system. A characterization of the underlying mechanisms revealed a Ca(2+)-dependent intracellular acidification operating at neutral pH and an additional as yet unrecognized direct proton conductance through the poorly selective TRPV1 pore operating in acidic extracellular media. Large organic cations permeate through the activated TRPV1 pore even in the presence of physiological concentrations of Na(+), Mg(2+), and Ca(2+). The wide pore and the unexpectedly high proton permeability of TRPV1 point to a proton hopping permeation mechanism along the water-filled channel pore. In acidic media, the high relative proton permeability through TRPV1 defines a novel proton entry mechanism in nociceptive neurons.

During the respiratory burst, do phagocytes need proton channels or potassium channels, or both?

The NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidase enzyme complex, a crucial component of innate immunity, produces superoxide anion (O2-), which is a precursor to many reactive oxygen species. NADPH oxidase produces O2- by transferring electrons from intracellular NADPH across the membrane to extracellular (or phagosomal) oxygen and is thus electrogenic. It is widely believed that electroneutrality is preserved by proton flux through voltage-gated proton channels. A series of recent papers have challenged several key aspects of this view of the "respiratory burst." The most recent study solidifies the proposal that O2- and other reactive oxygen species produced by phagocytes are not toxic to microbes under physiological conditions. Further, an essential role for high-conductance, Ca2+-activated K+ (maxi-K+) channels in microbe killing is proposed. Finally, the results cast doubt on the widely held view that H+ efflux through voltage-gated proton channels (i) is the main mechanism of charge compensation, and (ii) is essential to continuous O2- production by the NADPH oxidase. My analysis of the new data and of a large body of data in the literature indicates that the proposed role of maxi-K+ channels in the respiratory burst is not yet credibly established. H+ efflux through proton channels thus remains the most viable mechanism for charge compensation and continuous O2- production. The important question of the toxicity of reactive oxygen species in phagocytes and in other cells, which has long been simply taken for granted, is a widespread assumption that deserves critical study.

X-Ray Structure Determination of Three Mutants of the Bacterial Photosynthetic Reaction Centers from Rb. sphaeroides

In the photosynthetic reaction center (RC) from Rhodobacter sphaeroides, the reduction of a bound quinone molecule Q(B) is coupled with proton uptake. When Asp-L213 is replaced by Asn, proton transfer is inhibited. Proton transfer was restored by two second-site revertant mutations, Arg-M233-->Cys and Arg-H177-->His. Kinetic effects of Cd(2+) on proton transfer showed that the entry point in revertant RCs to be the same as in the native RC. The structures of the parental and two revertant RCs were determined at resolutions of 2.10, 1.80, and 2.75 A. From the structures, we were able to delineate alternate proton transfer pathways in the revertants. The main changes occur near Glu-H173, which allow it to substitute for the missing Asp-L213. The electrostatic changes near Glu-H173 cause it to be a good proton donor and acceptor, and the structural changes create a cavity which accommodates water molecules that connect Glu-H173 to other proton transfer components.

The mechanism of proton exclusion in aquaporin channels

The mechanism of proton exclusion in aquaporin channels is elucidated through free energy calculations of the pathway of proton transport. The second generation multistate empirical valence bond (MS-EVB2) model was applied to simulate the interaction of an excess proton with the channel environment. Jarzynski's equality was employed for rapid convergence of the free energy profile. A barrier sufficiently high to block proton transport is located near the channel center at the NPA motif-a site involved in bi-orientational ordering of the embedded water-wire in absence of the excess proton. A second and lower barrier is observed at the selectivity filter near the periplasmic outlet where the channel is narrowest. This secondary barrier may be essential in filtering other large solutes and cations.

A proton pore in a potassium channel voltage sensor reveals a focused electric field

Voltage-dependent potassium channels are essential for the generation of nerve impulses. Voltage sensitivity is conferred by charged residues located mainly in the fourth transmembrane segment (S4) of each of the four identical subunits that make up the channel. These charged segments relocate when the potential difference across the membrane changes, controlling the ability of the pore to conduct ions. In the crystal structure of the Aeropyrum pernix potassium channel KvAP, the S4 and part of the third (S3B) transmembrane alpha-helices are connected by a hairpin turn in an arrangement termed the 'voltage-sensor paddle'. This structure was proposed to move through the lipid bilayer during channel activation, transporting positive charges across a large fraction of the membrane. Here we show that replacing the first S4 arginine by histidine in the Shaker potassium channel creates a proton pore when the cell is hyperpolarized. Formation of this pore does not support the paddle model, as protons would not have access to a lipid-buried histidine. We conclude that, at hyperpolarized potentials, water and protons from the internal and external solutions must be separated by a narrow barrier in the channel protein that focuses the electric field to a small voltage-sensitive region.

Interactions between NADPH oxidase and voltage-gated proton channels: why electron transport depends on proton transport

Leukocytes kill microbes by producing reactive oxygen species, using a multi-component enzyme complex, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Electrons pass from intracellular NADPH through a redox chain within the enzyme, to reduce extracellular O2 to O2-. Electron flux is electrogenic, and rapidly depolarizes the membrane potential. Excessive depolarization can turn off electron transport by self-inhibition, but this is prevented by proton flux that balances the electron flux. Although the membrane potential depolarizes by approximately 100 mV during the respiratory burst (NADPH oxidase activity), NADPH oxidase activity is independent of voltage in this range, which permits optimal function and prevents self-inhibition.

The ABCs of solute carriers: physiological, pathological and therapeutic implications of human membrane transport proteinsIntroduction

The Human Genome Organisation (HUGO) Nomenclature Committee Database provides a list of transporter families of the solute carrier (SLC) gene series (see http://www.gene.ucl.ac.uk/nomenclature/). Currently, it includes 43 families and 298 transporter genes. This special issue features mini-reviews on each of these SLC families written by the experts in each field. A WEB site has been established (http://www.pharmaconference.org/slctable.asp) that gives the latest updates for the SLC families and their members as well as relevant links to gene databases and reviews in the literature. A list of all currently known SLC families, a discussion of additional SLC families and family members as well as a brief summary of non-SLC transporter genes is included in this introduction.

Phagocytosis by neutrophils

Phagocytosis is central to the microbicidal function of neutrophils. Pathogens are initially engulfed into a plasma membrane-derived vacuole, the phagosome, which proceeds to acquire degradative properties by a complex process termed maturation. In this chapter, we discuss the current knowledge of the molecular mechanisms underlying phagosome formation and maturation in neutrophils.

Regulatory mechanisms and physiological relevance of a voltage-gated H+ channel in murine osteoclasts: phorbol myristate acetate induces cell acidosis and the channel activation

The voltage-gated H+ channel is a powerful H+ extruding mechanism of osteoclasts, but its functional roles and regulatory mechanisms remain unclear. Electrophysiological recordings revealed that the H+ channel operated on activation of protein kinase C together with cell acidosis.

INTRODUCTION

H+ is a key signaling ion in bone resorption. In addition to H+ pumps and exchangers, osteoclasts are equipped with H+ conductive pathways to compensate rapidly for pH imbalance. The H+ channel is distinct in its strong H+ extrusion ability and voltage-dependent gatings.

METHODS

To investigate how and when the H+ channel is available in functional osteoclasts, the effects of phorbol 12-myristate 13-acetate (PMA), an activator for protein kinase C, on the H+ channel were examined in murine osteoclasts generated in the presence of soluble RANKL (sRANKL) and macrophage-colony stimulating factor (M-CSF).

RESULTS AND CONCLUSIONS

Whole cell recordings clearly showed that the H+ current was enhanced by increasing the pH gradient across the plasma membrane (delta(pH)), indicating that the H+ channel changed its activity by sensing delta(pH). The reversal potential (V(rev)) was a valuable tool for the real-time monitoring of delta(pH) in clamped cells. In the permeabilized patch, PMA (10 nM-1.6 microM) increased the current density and the activation rate, slowed decay of tail currents, and shifted the threshold toward more negative voltages. In addition, PMA caused a negative shift of V(rev), suggesting that intracellular acidification occurred. The PMA-induced cell acidosis was confirmed using a fluorescent pH indicator (BCECF), which recovered quickly in a K(+)-rich alkaline solution, probably through the activated H+ channel. Both cell acidosis and activation of the H+ channel by PMA were inhibited by staurosporine. In approximately 80% of cells, the PMA-induced augmentation in the current activity remained after compensating for the delta(pH) changes, implying that both delta(pH)-dependent and -independent mechanisms mediated the channel activation. Activation of the H+ channel shifted the membrane potential toward V(rev). These data suggest that the H+ channel may contribute to regulation of the pH environments and the membrane potential in osteoclasts activated by protein kinase C.

Transport kinetics of uncoupling proteins. Analysis of UCP1 reconstituted in planar lipid bilayers

According to alternative hypotheses, mitochondrial uncoupling protein 1 (UCP1) is either a proton channel ("buffering model") or a fatty acid anion carrier ("fatty acid cycling"). Transport across the proton channel along a chain of hydrogen bonds (Grotthus mechanism) may include fatty acid carboxyl groups or occur in the absence of fatty acids. In this work, we demonstrate that planar bilayers reconstituted with UCP1 exhibit an increase in membrane conductivity exclusively in the presence of fatty acids. Hence, we can exclude the hypothesis considering a preexisting H+ channel in UCP1, which does not require fatty acid for function. The augmented conductivity is nearly completely blocked by ATP. Direct application of transmembrane voltage and precise current measurements allowed determination of ATP-sensitive conductances at 0 and 150 mV as 11.5 and 54.3 pS, respectively, by reconstituting nearly 3 x 10(5) copies of UCP1. The proton conductivity measurements carried out in presence of a pH gradient (0.4 units) allowed estimation of proton turnover numbers per UCP1 molecule. The observed transport rate of 14 s-1 is compatible both with carrier and channel nature of UCP1.

Mechanism of proton transfer inhibition by Cd(2+) binding to bacterial reaction centers: determination of the pK(A) of functionally important histidine residues

The bacterial photosynthetic reaction center (RC) uses light energy to catalyze the reduction of a bound quinone molecule Q(B) to quinol Q(B)H(2). In RCs from Rhodobacter sphaeroides the protons involved in this process come from the cytoplasm and travel through pathways that involve His-H126 and His-H128 located near the proton entry point. In this study, we measured the pH dependence from 4.5 to 8.5 of the binding of the proton transfer inhibitor Cd(2+), which ligates to these surface His in the RC and inhibits proton-coupled electron transfer. At pH <6, the negative slope of the logarithm of the dissociation constant, K(D), versus pH approaches 2, indicating that, upon binding of Cd(2+), two protons are displaced; i.e., the binding is electrostatically compensated. At pH >7, K(D) becomes essentially independent of pH. A theoretical fit to the data over the entire pH range required two protons with pK(A) values of 6.8 and 6.3 (+/-0.5). To assess the contribution of His-H126 and His-H128 to the observed pH dependence, K(D) was measured in mutant RCs that lack the imidazole group of His-H126 or His-H128 (His --> Ala). In both mutant RCs, K(D) was approximately pH independent, showing that Cd(2+) does not displace protons upon binding in the mutant RCs, in contrast to the native RC in which His-H126 and His-H128 are the predominant contributors to the observed pH dependence of K(D). Thus, Cd(2+) inhibits RC function by binding to functionally important histidines.

The speed of the flagellar rotary motor of Escherichia coli varies linearly with protonmotive force

A protonmotive force (pmf) across the cell's inner membrane powers the flagellar rotary motor of Escherichia coli. Speed is known to be proportional to pmf when viscous loads are heavy. Here we show that speed also is proportional to pmf when viscous loads are light. Two motors on the same bacterium were monitored as the cell was slowly deenergized. The first motor rotated the entire cell body (a heavy load), while the second motor rotated a small latex bead (a light load). The first motor rotated slowly and provided a measure of the cell's pmf. The second motor rotated rapidly and was compared with the first, to give the speed-pmf relation for light loads. Experiments were done at 24.0 degrees C and 16.2 degrees C, with initial speeds indicating operation well into the high-speed, low-torque regime. Speed was found to be proportional to pmf over the entire (accessible) dynamic range (0-270 Hz). If the passage of a fixed number of protons carries the motor through each revolution, i.e., if the motor is tightly coupled, a linear speed-pmf relation is expected close to stall, where the work done against the viscous load matches the energy dissipated in proton flow. A linear relation is expected at high speeds if proton translocation is rate-limiting and involves multiple steps, a model that also applies to simple proton channels. The present work shows that a linear relation is true more generally, providing an additional constraint on possible motor mechanisms.

Properties of single voltage-gated proton channels in human eosinophils estimated by noise analysis and by direct measurement

Voltage-gated proton channels were studied under voltage clamp in excised, inside-out patches of human eosinophils, at various pHi with pHo 7.5 or 6.5 pipette solutions. H+ current fluctuations were observed consistently when the membrane was depolarized to voltages that activated H+ current. At pHi < or = 5.5 the variance increased nonmonotonically with depolarization to a maximum near the midpoint of the H+ conductance-voltage relationship, gH-V, and then decreased, supporting the idea that the noise is generated by H+ channel gating. Power spectral analysis indicated Lorentzian and 1/f components, both related to H+ currents. Unitary H+ current amplitude was estimated from stationary or quasi-stationary variance, sigmaH2. We analyze sigmaH2 data obtained at various voltages on a linearized plot that provides estimates of both unitary conductance and the number of channels in the patch, without requiring knowledge of open probability. The unitary conductance averaged 38 fS at pHi 6.5, and increased nearly fourfold to 140 fS at pHi 5.5, but was independent of pHo. In contrast, the macroscopic gH was only 1.8-fold larger at pHi 5.5 than at pHi 6.5. The maximum H+ channel open probability during large depolarizations was 0.75 at pHi 6.5 and 0.95 at pHi 5.5. Because the unitary conductance increases at lower pHi more than the macroscopic gH, the number of functional channels must decrease. Single H+ channel currents were too small to record directly at physiological pH, but at pHi < or = 5.5 near Vthreshold (the voltage at which gH turns on), single channel-like current events were observed with amplitudes 7-16 fA.