Functional vacuolar ATPase (V-ATPase) proton pumps traffic to the enterocyte brush border membrane and require CFTR

Vacuolar ATPases (V-ATPases) are highly conserved proton pumps that regulate organelle pH. Epithelial luminal pH is also regulated by cAMP-dependent traffic of specific subunits of the V-ATPase complex from endosomes into the apical membrane. In the intestine, cAMP-dependent traffic of cystic fibrosis transmembrane conductance regulator (CFTR) channels and the sodium hydrogen exchanger (NHE3) in the brush border regulate luminal pH. V-ATPase was found to colocalize with CFTR in intestinal CFTR high expresser (CHE) cells recently. Moreover, apical traffic of V-ATPase and CFTR in rat Brunner's glands was shown to be dependent on cAMP/PKA. These observations support a functional relationship between V-ATPase and CFTR in the intestine. The current study examined V-ATPase and CFTR distribution in intestines from wild-type, CFTR(-/-) mice and polarized intestinal CaCo-2BBe cells following cAMP stimulation and inhibition of CFTR/V-ATPase function. Coimmunoprecipitation studies examined V-ATPase interaction with CFTR. The pH-sensitive dye BCECF determined proton efflux and its dependence on V-ATPase/CFTR in intestinal cells. cAMP increased V-ATPase/CFTR colocalization in the apical domain of intestinal cells and redistributed the V-ATPase Voa1 and Voa2 trafficking subunits from the basolateral membrane to the brush border membrane. Voa1 and Voa2 subunits were localized to endosomes beneath the terminal web in untreated CFTR(-/-) intestine but redistributed to the subapical cytoplasm following cAMP treatment. Inhibition of CFTR or V-ATPase significantly decreased pHi in cells, confirming their functional interdependence. These data establish that V-ATPase traffics into the brush border membrane to regulate proton efflux and this activity is dependent on CFTR in the intestine.

Bis-enoxacin inhibits bone resorption and orthodontic tooth movement

Enoxacin inhibits binding between the B-subunit of vacuolar H(+)-ATPase (V-ATPase) and microfilaments, and also between osteoclast formation and bone resorption in vitro. We hypothesized that a bisphosphonate derivative of enoxacin, bis-enoxacin (BE), which was previously studied as a bone-directed antibiotic, might have similar activities. BE shared a number of characteristics with enoxacin: It blocked binding between the recombinant B-subunit and microfilaments and inhibited osteoclastogenesis in cell culture with IC50s of about 10 µM in each case. BE did not alter the relative expression levels of various osteoclast-specific proteins. Even though tartrate-resistant acid phosphatase 5b was expressed, proteolytic activation of the latent pro-enzyme was inhibited. However, unlike enoxacin, BE stimulated caspase-3 activity. BE bound to bone slices and inhibited bone resorption by osteoclasts on BE-coated bone slices in cell culture. BE reduced the amount of orthodontic tooth movement achieved in rats after 28 days. Analysis of these data suggests that BE is a novel anti-resorptive molecule that is active both in vitro and in vivo and may have clinical uses.

ABBREVIATIONS

BE, bis-enoxacin; V-ATPase, vacuolar H(+)-ATPase; TRAP, tartrate-resistant acid phosphatase; αMEM D10, minimal essential media, alpha modification with 10% fetal bovine serum; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis; RANKL, receptor activator of nuclear factor kappa B-ligand; NFATc1, nuclear factor of activated T-cells; ADAM, a disintegrin and metalloprotease domain; OTM, orthodontic tooth movement.

Essential role for vacuolar acidification in Candida albicans virulence

Fungal infections are on the rise, with mortality above 30% in patients with septic Candida infections. Mutants lacking V-ATPase activity are avirulent and fail to acidify endomembrane compartments, exhibiting pleiotropic defects in secretory, endosomal, and vacuolar pathways. However, the individual contribution of organellar acidification to virulence and its associated traits is not known. To dissect their separate roles in Candida albicans pathogenicity we generated knock-out strains for the V0 subunit a genes VPH1 and STV1, which target the vacuole and secretory pathway, respectively. While the two subunits were redundant in many vma phenotypes, such as alkaline pH sensitivity, calcium homeostasis, respiratory defects, and cell wall integrity, we observed a unique contribution of VPH1. Specifically, vph1Δ was defective in acidification of the vacuole and its dependent functions, such as metal ion sequestration as evidenced by hypersensitivity to Zn(2+) toxicity, whereas stv1Δ resembled wild type. In growth conditions that elicit morphogenic switching, vph1Δ was defective in forming hyphae whereas stv1Δ was normal or only modestly impaired. Host cell interactions were evaluated in vitro using the Caco-2 model of intestinal epithelial cells, and murine macrophages. Like wild type, stv1Δ was able to inflict cellular damage in Caco-2 and macrophage cells, as assayed by LDH release, and escape by filamentation. In contrast, vph1Δ resembled a vma7Δ mutant, with significant attenuation in host cell damage. Finally, we show that VPH1 is required for fungal virulence in a murine model of systemic infection. Our results suggest that vacuolar acidification has an essential function in the ability of C. albicans to form hyphae and establish infection.

Hypoxia inducible NOD2 interacts with 3-O-sulfogalactoceramide and regulates vesicular homeostasis

BACKGROUND

Oxygen sensing in mammalian cells is a conserved signaling pathway regulated by hypoxia inducible factor type 1 (HIF-1). Inadequate oxygen supply (hypoxia) is common to many pathological disorders where autophagy plays an import role. The aim of this study was the identification and characterization of novel HIF-1 target genes that promote autophagy during hypoxia.

METHODS

Whole genome Chromatin Immune Precipitation from hypoxic HeLa cells was used to identify novel HIF-1 target genes. Hypoxia induced expression and transcription regulation was studied in wild type and HIF-deficient cells. siRNA silencing of candidate genes was used to establish their role during autophagy. Recombinant protein was used for screening immobilized glycosylated lipids to identify potential ligands.

RESULTS

We identified the Nucleotide Oligomerization Domain 2 (NOD2/CARD15) as a novel HIF-1 target and 3-O-sulfo-galactoceramide (sulfatide) and Mycobacterium sp. specific sulfolipid-1 as the first NOD2 ligands that both compete for binding to NOD2. Loss of NOD2 function impaired autophagy upstream of the autophagy inhibitor chloroquine by reducing the number of acidic vesicles. Inhibition of sulfatide synthesis elicited defects in autophagy similar to the NOD2 loss of function but did not influence NOD2-mediated NF-kB signaling.

CONCLUSIONS

Our findings suggest that the interaction of NOD2 with sulfatide may mediate the balance between autophagy and inflammation in hypoxic cells.

GENERAL SIGNIFICANCE

These findings may lead to a better understanding of complex inflammatory pathologies like Crohn's disease and tuberculosis where both NOD2 and hypoxia are implicated.

Glycolytic control of vacuolar-type ATPase activity: a mechanism to regulate influenza viral infection

As new influenza virus strains emerge, finding new mechanisms to control infection is imperative. In this study, we found that we could control influenza infection of mammalian cells by altering the level of glucose given to cells. Higher glucose concentrations induced a dose-specific increase in influenza infection. Linking influenza virus infection with glycolysis, we found that viral replication was significantly reduced after cells were treated with glycolytic inhibitors. Addition of extracellular ATP after glycolytic inhibition restored influenza infection. We also determined that higher levels of glucose promoted the assembly of the vacuolar-type ATPase within cells, and increased vacuolar-type ATPase proton-transport activity. The increase of viral infection via high glucose levels could be reversed by inhibition of the proton pump, linking glucose metabolism, vacuolar-type ATPase activity, and influenza viral infection. Taken together, we propose that altering glucose metabolism may be a potential new approach to inhibit influenza viral infection.

AMP-activated protein kinase regulates the vacuolar H+-ATPase via direct phosphorylation of the A subunit (ATP6V1A) in the kidney

The vacuolar H(+)-ATPase (V-ATPase) in intercalated cells contributes to luminal acidification in the kidney collecting duct and nonvolatile acid excretion. We previously showed that the A subunit in the cytoplasmic V1 sector of the V-ATPase (ATP6V1A) is phosphorylated by the metabolic sensor AMP-activated protein kinase (AMPK) in vitro and in kidney cells. Here, we demonstrate that treatment of rabbit isolated, perfused collecting ducts with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) inhibited V-ATPase-dependent H(+) secretion from intercalated cells after an acid load. We have identified by mass spectrometry that Ser-384 is a major AMPK phosphorylation site in the V-ATPase A subunit, a result confirmed by comparing AMPK-dependent phosphate labeling of wild-type A-subunit (WT-A) with that of a Ser-384-to-Ala A subunit mutant (S384A-A) in vitro and in intact HEK-293 cells. Compared with WT-A-expressing HEK-293 cells, S384A-A-expressing cells exhibited greater steady-state acidification of HCO3(-)-containing media. Moreover, AICAR treatment of clone C rabbit intercalated cells expressing the WT-A subunit reduced V-ATPase-dependent extracellular acidification, an effect that was blocked in cells expressing the phosphorylation-deficient S384A-A mutant. Finally, expression of the S384A-A mutant prevented cytoplasmic redistribution of the V-ATPase by AICAR in clone C cells. In summary, direct phosphorylation of the A subunit at Ser-384 by AMPK represents a novel regulatory mechanism of the V-ATPase in kidney intercalated cells. Regulation of the V-ATPase by AMPK may couple V-ATPase activity to cellular metabolic status with potential relevance to ischemic injury in the kidney and other tissues.

Inhibition of vacuolar ATPase attenuates the TRAIL-induced activation of caspase-8 and modulates the trafficking of TRAIL receptosomes

Tumour necrosis factor (TNF) related apoptosis inducing ligand (TRAIL), a membrane-bound ligand from the TNF family, has attracted significant attention due to its rather specific and effective ability to induce apoptotic death in various types of cancer cells via binding to and activating its pro-apoptotic death receptors. However, a significant number of primary cancer cells often develop resistance to TRAIL treatment, and the signalling platform behind this phenomenon is not fully understood. Upon blocking endosomal acidification by the vacuolar ATPase (V-ATPase) inhibitors bafilomycin A1 (BafA1) or concanamycin A, we observed a significantly reduced initial sensitivity of several, mainly colorectal, tumour cell lines to TRAIL-induced apoptosis. In cells pretreated with these inhibitors, the TRAIL-induced processing of caspase-8 and the aggregation and trafficking of the TRAIL receptor complexes were temporarily attenuated. Nuclear factor κB or mitogen activated protein/stress kinase signalling from the activated TRAIL receptors remained unchanged, and neither possible lysosomal permeabilization nor acid sphingomyelinase was involved in this process. The cell surface expression of TRAIL receptors and their TRAIL-induced internalization were not affected by V-ATPase inhibitors. The inhibitory effect of BafA1, however, was blunted by knockdown of the caspase-8 inhibitor cFLIP. Altogether, the data obtained provide the first evidence that endosomal acidification could represent an important regulatory node in the proximal part of TRAIL-induced pro-apoptotic signalling.

Origin of asymmetry at the intersubunit interfaces of V1-ATPase from Thermus thermophilus

V-type ATPase (V-ATPase) is one of the rotary ATPase complexes that mediate energy conversion between the chemical energy of ATP and the ion gradient across the membrane through a rotary catalytic mechanism. Because V-ATPase has structural features similar to those of well-studied F-type ATPase, the structure is expected to highlight the common essence of the torque generation of rotary ATPases. Here, we report a complete model of the extra-membrane domain of the V-ATPase (V1-ATPase) of a thermophilic bacterium, Thermus thermophilus, consisting of three A subunits, three B subunits, one D subunit, and one F subunit. The X-ray structure at 3.9Å resolution provides detailed information about the interactions between A3B3 and DF subcomplexes as well as interactions among the respective subunits, which are defined by the properties of side chains. Asymmetry at the intersubunit interfaces was detected from the structural differences among the three AB pairs in the different reaction states, while the large interdomain motion in the catalytic A subunits was not observed unlike F1 from various species and V1 from Enterococcus hirae. Asymmetry is mainly realized by rigid-body rearrangements of the relative position between A and B subunits. This is consistent with the previous observations by the high-resolution electron microscopy for the whole V-ATPase complexes. Therefore, our result plausibly implies that the essential motion for the torque generation is not the large interdomain movement of the catalytic subunits but the rigid-body rearrangement of subunits.

Close Association of Carbonic Anhydrase (CA2a and CA15a), Na(+)/H(+) Exchanger (Nhe3b), and Ammonia Transporter Rhcg1 in Zebrafish Ionocytes Responsible for Na(+) Uptake

Freshwater (FW) fishes actively absorb salt from their environment to tolerate low salinities. We previously reported that vacuolar-type H⁺-ATPase/mitochondrion-rich cells (H-MRCs) on the skin epithelium of zebrafish larvae (Danio rerio) are primary sites for Na⁺ uptake. In this study, in an attempt to clarify the mechanism for the Na⁺ uptake, we performed a systematic analysis of gene expression patterns of zebrafish carbonic anhydrase (CA) isoforms and found that, of 12 CA isoforms, CA2a and CA15a are highly expressed in H-MRCs at larval stages. The ca2a and ca15a mRNA expression were salinity-dependent; they were upregulated in 0.03 mM Na⁺ water whereas ca15a but not ca2a was down-regulated in 70 mM Na⁺ water. Immunohistochemistry demonstrated cytoplasmic distribution of CA2a and apical membrane localization of CA15a. Furthermore, cell surface immunofluorescence staining revealed external surface localization of CA15a. Depletion of either CA2a or CA15a expression by Morpholino antisense oligonucleotides resulted in a significant decrease in Na⁺ accumulation in H-MRCs. An in situ proximity ligation assay demonstrated a very close association of CA2a, CA15a, Na⁺/H⁺ exchanger 3b (Nhe3b), and Rhcg1 ammonia transporter in H-MRC. Our findings suggest that CA2a, CA15a, and Rhcg1 play a key role in Na⁺uptake under FW conditions by forming a transport metabolon with Nhe3b.

Fe deficiency differentially affects the vacuolar proton pumps in cucumber and soybean roots

Iron uptake in dicots depends on their ability to induce a set of responses in root cells including rhizosphere acidification through H(+) extrusion and apoplastic Fe(III) reduction by Fe(III)-chelate reductase. These responses must be sustained by metabolic rearrangements aimed at providing the required NAD(P)H, ATP and H(+). Previous results in Fe-deficient cucumber roots showed that high H(+) extrusion is accompanied by increased phosphoenolpyruvate carboxylase (PEPC) activity, involved in the cytosol pH-stat; moreover (31)P-NMR analysis revealed increased vacuolar pH and decreased vacuolar [inorganic phosphate (Pi)]. The opposite was found in soybean: low rhizosphere acidification, decreased PEPC activity, vacuole acidification, and increased vacuolar [Pi]. These findings, highlighting a different impact of the Fe deficiency responses on cytosolic pH in the two species, lead to hypothesize different roles for H(+) and Pi movements across the tonoplast in pH homeostasis. The role of vacuole in cytosolic pH-stat involves the vacuolar H(+)-ATPase (V-ATPase) and vacuolar H(+)-pyrophosphatase (V-PPase) activities, which generating the ΔpH and ΔΨ, mediate the transport of solutes, among which Pi, across the tonoplast. Fluxes of Pi itself in its two ionic forms, H2PO4 (-) predominating in the vacuole and HPO4 (2-) in the cytosol, may be involved in pH homeostasis owing to its pH-dependent protonation/deprotonation reactions. Tonoplast enriched fractions were obtained from cucumber and soybean roots grown with or without Fe. Both V-ATPase and V-PPase activities were analyzed and the enrichment and localization of the corresponding proteins in root tissues were determined by Western blot and immunolocalization. V-ATPase did not change its activity and expression level in response to Fe starvation in both species. V-PPase showed a different behavior: in cucumber roots its activity and abundance were decreased, while in Fe-deficient soybean roots they were increased. The distinct role of the two H(+) pumps in Pi fluxes between cytoplasm and vacuole in Fe-deficient cucumber and soybean root cells is discussed.

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.

Multiple Transport Pathways for Mediating Intracellular pH Homeostasis: The Contribution of H(+)/ion Exchangers

Intracellular pH homeostasis is an essential process in all plant cells. The transport of H(+) into intracellular compartments is critical for providing pH regulation. The maintenance of correct luminal pH in the vacuole and in compartments of the secretory/endocytic pathway is important for a variety of cellular functions including protein modification, sorting, and trafficking. It is becoming increasingly evident that coordination between primary H(+) pumps, most notably the V-ATPase, and secondary ion/H(+) exchangers allows this endomembrane pH maintenance to occur. This article describes some of the recent insights from the studies of plant cation/H(+) exchangers and anion/H(+) exchangers that demonstrate the fundamental roles of these transporters in pH homeostasis within intracellular compartments.

Vma8p-GFP fusions can be functionally incorporated into V-ATPase, suggesting structural flexibility at the top of V1

The vacuolar ATPase (V-ATPase) complex of yeast (Saccharomyces cerevisiae) is comprised of two sectors, V(1) (catalytic) and V(O) (proton transfer). The hexameric (A(3)B(3)) cylinder of V(1) has a central cavity that must accommodate at least part of the rotary stalk of V-ATPase, a key component of which is subunit D (Vma8p). Recent electron microscopy (EM) data for the prokaryote V-ATPase complex (Thermus thermophilus) suggest that subunit D penetrates deeply into the central cavity. The functional counterpart of subunit D in mitochondrial F(1)F(O)-ATP synthase, subunit γ, occupies almost the entire length of the central cavity. To test whether the structure of yeast Vma8p mirrors that of subunit γ, we probed the location of the C-terminus of Vma8p by attachment of a large protein adduct, green fluorescent protein (GFP). We found that truncated Vma8p proteins lacking up to 40 C-terminal residues fused to GFP can be incorporated into functional V-ATPase complexes, and are able to support cell growth under alkaline conditions. We conclude that large protein adducts can be accommodated at the top of the central cavity of V(1) without compromising V-ATPase function, arguing for structural flexibility of the V(1) sector.