Role of nucleotides and phosphoinositides in the stability of electron and proton currents associated with the phagocytic NADPH oxidase

VSOP/Hv1 proton channels sustain calcium entry, neutrophil migration, and superoxide production by limiting cell depolarization and acidification

Neutrophils kill microbes with reactive oxygen species generated by the NADPH oxidase, an enzyme which moves electrons across membranes. Voltage-gated proton channels (voltage-sensing domain only protein [VSOP]/Hv1) are required for high-level superoxide production by phagocytes, but the mechanism of this effect is not established. We show that neutrophils from VSOP/Hv1^−/− mice lack proton currents but have normal electron currents, indicating that these cells have a fully functional oxidase that cannot conduct protons. VSOP/Hv1^−/− neutrophils had a more acidic cytosol, were more depolarized, and produced less superoxide and hydrogen peroxide than neutrophils from wild-type mice. Hydrogen peroxide production was rescued by providing an artificial conductance with gramicidin. Loss of VSOP/Hv1 also aborted calcium responses to chemoattractants, increased neutrophil spreading, and decreased neutrophil migration. The migration defect was restored by the addition of a calcium ionophore. Our findings indicate that proton channels extrude the acid and compensate the charge generated by the oxidase, thereby sustaining calcium entry signals that control the adhesion and motility of neutrophils. Loss of proton channels thus aborts superoxide production and causes a severe signaling defect in neutrophils.

The intimate and mysterious relationship between proton channels and NADPH oxidase

Voltage gated proton channels and NADPH oxidase function cooperatively in phagocytes during the respiratory burst, when reactive oxygen species are produced to kill microbial invaders. Although these molecules are distinct entities, with no proven physical interaction, their presence and activity in many cells appears to be coordinated. We describe these interactions and discuss several types of mechanisms that might explain them.

Voltage-gated proton channels: what's next?

This review is an attempt to identify and place in context some of the many questions about voltage-gated proton channels that remain unsolved. As the gene was identified only 2 years ago, the situation is very different than in fields where the gene has been known for decades. For the proton channel, most of the obvious and less obvious structure-function questions are still wide open. Remarkably, the proton channel protein strongly resembles the voltage-sensing domain of many voltage-gated ion channels, and thus offers a novel approach to study gating mechanisms. Another surprise is that the proton channel appears to function as a dimer, with two separate conduction pathways. A number of significant biological questions remain in dispute, unanswered, or in some cases, not yet asked. This latter deficit is ascribable to the intrinsic difficulty in evaluating the importance of one component in a complex system, and in addition, to the lack, until recently, of a means of performing an unambiguous lesion experiment, that is, of selectively eliminating the molecule in question. We still lack a potent, selective pharmacological inhibitor, but the identification of the gene has allowed the development of powerful new tools including proton channel antibodies, siRNA and knockout mice.

Voltage-gated proton channels

The history of research on voltage-gated proton channels is recounted, from their proposed existence in dinoflagellates by Hastings in 1972 and their demonstration in snail neurons by Thomas and Meech in 1982 to the discovery in 2006 (after a decade of controversy) of genes that unequivocally code for proton channels. Voltage-gated proton channels are perfectly selective for protons, conduct deuterons half as well, and the conductance is strongly temperature dependent. These properties are consistent with a conduction mechanism involving hydrogen-bonded-chain transfer, in which the selectivity filter is a titratable amino acid residue. Channel opening is regulated stringently by pH such that only outward current is normally activated. Main functions of proton channels include acid extrusion from cells and charge compensation for the electrogenic activity of the phagocyte NADPH oxidase. Genetic approaches hold the promise of rapid progress in the near future.

Sustained activation of proton channels and NADPH oxidase in human eosinophils and murine granulocytes requires PKC but not cPLA2 alpha activity

The prevailing hypothesis that a signalling pathway involving cPLA(2)alpha is required to enhance the gating of the voltage-gated proton channel associated with NADPH oxidase was tested in human eosinophils and murine granulocytes. This hypothesis invokes arachidonic acid (AA) liberated by cPLA(2)alpha as a final activator of proton channels. In human eosinophils studied in the perforated-patch configuration, phorbol myristate acetate (PMA) stimulation elicited NADPH oxidase-generated electron current (I(e)) and enhanced proton channel gating identically in the presence or absence of three specific cPLA(2)alpha inhibitors, Wyeth-1, pyrrolidine-2 and AACOCF(3) (arachidonyl trifluoromethyl ketone). In contrast, PKC inhibitors GFX (GF109203X) or staurosporine prevented the activation of either proton channels or NADPH oxidase. PKC inhibition during the respiratory burst reversed the activation of both molecules, suggesting that ongoing phosphorylation is required. This effect of GFX was inhibited by okadaic acid, implicating phosphatases in proton channel deactivation. Proton channel activation by AA was partially reversed by GFX or staurosporine, indicating that AA effects are due in part to activation of PKC. In granulocytes from mice with the cPLA(2)alpha gene disrupted (knockout mice), PMA or fMetLeuPhe activated NADPH oxidase and proton channels in a manner indistinguishable from the responses of control cells. Thus, cPLA(2)alpha is not essential to activate the proton conductance or for a normal respiratory burst. Instead, phosphorylation of the proton channel or an activating molecule converts the channel to its activated gating mode. The existing paradigm for regulation of the concerted activity of proton channels and NADPH oxidase must be revised.