Biochemistry of the renal V-ATPase

The vacuolar-type H-ATPase in ovine rumen epithelium is regulated by metabolic signals

In this study, the effect of metabolic inhibition (MI) by glucose substitution with 2-deoxyglucose (2-DOG) and/or application of antimycin A on ovine rumen epithelial cells (REC) vacuolar-type H⁺-ATPase (vH⁺-ATPase) activity was investigated. Using fluorescent spectroscopy, basal pH_i of REC was measured to be 7.3 ± 0.1 in HCO₃⁻-free, glucose-containing NaCl medium. MI induced a strong pH_i reduction (−0.44 ± 0.04 pH units) with a more pronounced effect of 2-DOG compared to antimycin A (−0.30 ± 0.03 versus −0.21 ± 0.03 pH units). Treatment with foliomycin, a specific vH⁺-ATPase inhibitor, decreased REC pH_i by 0.21 ± 0.05 pH units. After MI induction, this effect was nearly abolished (−0.03 ± 0.02 pH units). In addition, membrane-associated localization of vH⁺-ATPase B subunit disappeared. Metabolic control of vH⁺-ATPase involving regulation of its assembly state by elements of the glycolytic pathway could provide a means to adapt REC ATP consumption according to energy availability.

Tumour hypoxia induces a metabolic shift causing acidosis: a common feature in cancer

Maintenance of cellular pH homeostasis is fundamental to life. A number of key intracellular pH (pHi) regulating systems including the Na⁺/H⁺ exchangers, the proton pump, the monocarboxylate transporters, the HCO₃⁻ transporters and exchangers and the membrane-associated and cytosolic carbonic anhydrases cooperate in maintaining a pHi that is permissive for cell survival. A common feature of tumours is acidosis caused by hypoxia (low oxygen tension). In addition to oncogene activation and transformation, hypoxia is responsible for inducing acidosis through a shift in cellular metabolism that generates a high acid load in the tumour microenvironment. However, hypoxia and oncogene activation also allow cells to adapt to the potentially toxic effects of an excess in acidosis. Hypoxia does so by inducing the activity of a transcription factor the hypoxia-inducible factor (HIF), and particularly HIF-1, that in turn enhances the expression of a number of pHi-regulating systems that cope with acidosis. In this review, we will focus on the characterization and function of some of the hypoxia-inducible pH-regulating systems and their induction by hypoxic stress. It is essential to understand the fundamentals of pH regulation to meet the challenge consisting in targeting tumour metabolism and acidosis as an anti-tumour approach. We will summarize strategies that take advantage of intracellular and extracellular pH regulation to target the primary tumour and metastatic growth, and to turn around resistance to chemotherapy and radiotherapy.

Differential regulation of H+-ATPases in MDCK-C11 cells by aldosterone and vasopressin

The objective of the present work was to characterize the biochemical activity of the proton pumps present in the C11 clone of Madin–Darby canine kidney (MDCK) cells, akin to intercalated cells of the collecting duct, as well as to study their regulation by hormones like aldosterone and vasopressin. MDCK-C11 cells from passages 78 to 86 were utilized. The reaction to determine H+-ATPase activity was started by addition of cell homogenates to tubes contained the assay medium. The inorganic phosphate (Pi) released was determined by a colorimetric method modified from that described by Fiske and Subbarow. Changes in intracellular calcium concentration in the cells was determined using the Ca2+-sensing dye fluo-4 AM. Homogenates of MDCK-C11 cells present a bafilomycin-sensitive activity (vacuolar H+-ATPase), and a vanadate-sensitive activity (H+/K+-ATPase). The bafilomycin-sensitive activity showed a pH optimum of 6.12. ATPase activity was also stimulated in a dose-dependent fashion as K+ concentration was increased between 0 and 50 mmol·L–1, with an apparent Km for the release of Pi of 0.13 mmol·L–1 and Vmax of 22.01 nmol·mg–1·min–1. Incubation of cell monolayers with 10−8 mol·L–1 aldosterone for 24 h significantly increased vacuolar H+-ATPase activity, an effect prevented by 10−5 mol·L–1 spironolactone. Vacuolar H+-ATPase activity was also stimulated by 10−11 mol·L–1 vasopressin, an effect prevented by a V1 receptor-specific antagonist. This dose of vasopressin determined a sustained rise of cytosolic ionized calcium. We conclude that (i) homogenates of MDCK-C11 cells present a bafilomycin-sensitive (H+-ATPase) activity and a vanadate-sensitive (H+/K+-ATPase) activity, and (ii) vacuolar H+-ATPase activity is activated by aldosterone through a genomic pathway and by vasopressin through V1 receptors.

Gill membrane remodeling with soft-water acclimation in zebrafish (Danio rerio)

Little is known regarding the ionoregulatory abilities of zebrafish exposed to soft water despite the popularity of this model organism for physiology and aquatic toxicology. We examined genomic and nongenomic changes to gills of zebrafish as they were progressively acclimated from moderately hard freshwater to typical soft water over 7 days and held in soft water for another 7 days. Gills were sampled daily and mRNA expression levels of gill Na(+)-K(+)-ATPase (NKA) alpha1a subunit, epithelium calcium channel (ECaC), carbonic anhydrase-1 and 2 (CA-1, CA-2), Na(+)/H(+) exchanger (NHE-2), V-type proton (H(+))-ATPase, and copper transport protein (CTR-1) were quantified by real-time PCR. Changes in enzyme activities of gill NKA were determined and protein levels of NKA and ECaC were quantified by Western blotting. Levels of mRNA for ECaC increased fourfold after day 6, with an associated increase in ECaC protein levels after 1 wk in soft water. CA-1 and CA-2 exhibited a 1.5- and 6-fold increase in gene expression on days 6 and 5, respectively. Likewise, there was a fivefold increase in NHE-2 expression after day 6. Surprisingly, CTR-1 mRNA showed a large transient increase (over threefold) on day 6, while H(+)-ATPase mRNA did not change. These data demonstrate a high degree of phenotypic plasticity in zebrafish gills exposed to an ion-poor environment. This not only enhances our understanding of ionoregulatory processes in fish but also highlights the need for proper experimental design for studies involving preacclimation to soft water (e.g., metal toxicity).

A vH+-ATPase is present in cultured sheep ruminal epithelial cells

In this study, the existence and functional activity of a vacuolar-type H(+)-ATPase (vH(+)-ATPase) was explored in primary cultures of sheep ruminal epithelial cells (REC). The mRNA transcripts of the E and B subunits of vH(+)-ATPase were detectable in RNA from REC samples by RT-PCR. Immunoblotting of REC protein extractions with antibodies directed against the B subunit of yeast vH(+)-ATPase revealed a protein band of the expected size (60 kDa). Using the fluorescent indicator BCECF and selective inhibitors (foliomycin, HOE 694, S3226), the contribution of vH(+)-ATPase and Na(+)/H(+) exchanger (NHE) subtype 1 and 3 activity to the regulation of intracellular pH (pH(i)) was determined in nominally HCO(3)(-)-free, HEPES-buffered NaCl medium containing 20 mM of the short-chain fatty acid butyrate as well as after reduction of the extracellular Cl(-) concentration ([Cl(-)](e)) from 136 to 36 mM. The initial pH(i) of REC was 7.4 +/- 0.1 in nominally HCO(3)(-)-free, HEPES-buffered NaCl medium and 7.0 +/- 0.1 after acid loading with butyrate. Selective inhibition of the vH(+)-ATPase with foliomycin decreased pH(i) by 0.19 +/- 0.03 pH units. On the basis of the observed decreases in pH(i) resulting from inhibition of vH(+)-ATPase as well as of subtypes 1 and 3 of NHE, vH(+)-ATPase activity appears to account for approximately 30% of H(+) extrusion, whereas the activities of NHE subtypes 3 and 1 account for 20 and 50% of H(+) extrusion, respectively. Lowering of [Cl(-)](e) induced a pH(i) decrease (-0.51 +/- 0.03 pH units) and impaired pH(i) recovery from butyrate-induced acid load. Moreover, reduction of [Cl(-)](e) abolished the inhibitory effect of foliomycin and markedly reduced the HOE 694- and S3226-sensitive components of pH(i), indicating a role of Cl(-) in the function of these H(+) extrusion mechanisms. We conclude that a vH(+)-ATPase is expressed in ovine REC and plays a considerable role in the pH(i) regulation of these cells.

New insights into the regulation of V-ATPase-dependent proton secretion

The vacuolar H(+)-ATPase (V-ATPase) is a key player in several aspects of cellular function, including acidification of intracellular organelles and regulation of extracellular pH. In specialized cells of the kidney, male reproductive tract and osteoclasts, proton secretion via the V-ATPase represents a major process for the regulation of systemic acid/base status, sperm maturation and bone resorption, respectively. These processes are regulated via modulation of the plasma membrane expression and activity of the V-ATPase. The present review describes selected aspects of V-ATPase regulation, including recycling of V-ATPase-containing vesicles to and from the plasma membrane, assembly/disassembly of the two domains (V(0) and V(1)) of the holoenzyme, and the coupling ratio between ATP hydrolysis and proton pumping. Modulation of the V-ATPase-rich cell phenotype and the pathophysiology of the V-ATPase in humans and experimental animals are also discussed.

Processing and presentation of a mycobacterial antigen 85B epitope by murine macrophages is dependent on the phagosomal acquisition of vacuolar proton ATPase and in situ activation of cathepsin D

Mycobacterium tuberculosis (strain H37Rv) and bacillus Calmette-Guérin (BCG) vaccine inhibit phagosome maturation in macrophages and their effect on processing, and presentation of a secreted Ag85 complex B protein, Ag85B, by mouse macrophages was analyzed. Macrophages were infected with GFP-expressing mycobacterial strains and analyzed for in situ localization of vacuolar proton ATPase (v-ATPase) and cathepsin D (Cat D) using Western blot analysis and immunofluorescence. H37Rv and BCG phagosomes excluded the v-ATPase and maintained neutral pH while the attenuated H37Ra strain acquired v-ATPase and acidified. Mycobacterial phagosomes acquired Cat D, although strains BCG and H37Rv phagosomes contained the inactive 46-kDa form, whereas H37Ra phagosomes had the active 30-kDa form. Infected macrophages were overlaid with a T cell hybridoma specific for an Ag85B epitope complexed with MHC class II. Coincident with active Cat D, H37Ra-infected macrophages presented the epitope to T cells inducing IL-2, whereas H37Rv- and BCG-infected macrophages were less efficient in IL-2 induction. Bafilomycin inhibited the induction of macrophage-induced IL-2 from T cells indicating that v-ATPase was essential for macrophage processing of Ag85B. Furthermore, the small interfering RNA interference of Cat D synthesis resulted in a marked decrease in the levels of macrophage-induced IL-2. Thus, a v-ATPase-dependent phagosomal activation of Cat D was required for the generation of an Ag85B epitope by macrophages. Reduced processing of Ag85B by H37Rv- and BCG-infected macrophages suggests that phagosome maturation arrest interferes with the efficient processing of Ags in macrophages. Because Ag85B is immunodominant, this state may lead to a decreased ability of the wild-type as well as the BCG vaccine to induce protective immunity.

Regulation of branchial V-H(+)-ATPase, Na(+)/K(+)-ATPase and NHE2 in response to acid and base infusions in the Pacific spiny dogfish (Squalus acanthias)

To study the mechanisms of branchial acid-base regulation, Pacific spiny dogfish were infused intravenously for 24 h with either HCl (495+/- 79 micromol kg(-1) h(-1)) or NaHCO(3) (981+/-235 micromol kg(-1) h(-1)). Infusion of HCl produced a transient reduction in blood pH. Despite continued infusion of acid, pH returned to normal by 12 h. Infusion of NaHCO(3) resulted in a new steady-state acid-base status at approximately 0.3 pH units higher than the controls. Immunostained serial sections of gill revealed the presence of separate vacuolar proton ATPase (V-H(+)-ATPase)-rich or sodium-potassium ATPase (Na(+)/K(+)-ATPase)-rich cells in all fish examined. A minority of the cells also labeled positive for both transporters. Gill cell membranes prepared from NaHCO(3)-infused fish showed significant increases in both V-H(+)-ATPase abundance (300+/-81%) and activity. In addition, we found that V-H(+)-ATPase subcellular localization was mainly cytoplasmic in control and HCl-infused fish, while NaHCO(3)-infused fish demonstrated a distinctly basolateral staining pattern. Western analysis in gill membranes from HCl-infused fish also revealed increased abundance of Na(+)/H(+) exchanger 2 (213+/-5%) and Na(+)/K(+)-ATPase (315+/-88%) compared to the control.

Isoforms vatB1 and vatB2 of the vacuolar type ATPase subunit B are differentially expressed in embryos of the zebrafish (Danio rerio)

The v-type ATPase is a membrane anchored, multi-subunit proton pump, which in freshwater fish appears to play a major role in ionoregulative processes in the apical membrane of specialized gill cells. Very little is known about free-living fish embryos and larvae that are exposed to hypo-osmotic conditions with spawning but do not have their gills fully developed. By using reverse transcriptase-polymerase chain reaction and immunological methods, we could demonstrate the presence of two isoforms of the subunit B of this v-type ATPase in the early development of the zebrafish. Immunohistochemical analysis revealed the presence of one isoform (vatB1) in the apical membrane of embryonic skin cells, while vatB2 has been found ubiquitously. This differential localization of the two isoforms supports the hypothesis that vatB1 is preferentially involved in ionoregulative functions, while vatB2 may be preferentially responsible for acidification of intracellular vesicles.

Sodium and chloride transport in soft water and hard water acclimated zebrafish (Danio rerio)

While the zebrafish is commonly used for studies of developmental biology and toxicology, very little is known about their osmoregulatory physiology. The present investigation of Na(+) and Cl(-) transport revealed that the zebrafish is able to tolerate extremely low ambient ion concentrations and that this is achieved at least in part by a greatly enhanced apparent uptake capacity and affinity for both ions. Zebrafish maintain plasma and whole body electrolyte concentrations similar to most other freshwater teleosts even in deionized water containing only 35 microM NaCl, i.e soft water. We recorded an extremely low transport affinity constant (K(m)) of 8+/-1 microM for the active uptake of Cl(-) in soft water acclimated fish, while other transport kinetic parameters were in agreement with reports for other freshwater organisms. While both Na(+) and Cl(-) uptake in soft water clearly depends on apical proton pump activity, changes in abundance and possibly localization of this protein did not appear to contribute to soft water acclimation. Active Cl(-) uptake was strongly dependent on branchial carbonic anhydrase (CA) activity regardless of water type, while the response of Na(+) transport to a CA inhibitor was more variable. Differential response of Na(+) uptake to amiloride depending on acclimation medium suggests that different Na(+) transport mechanisms are employed by zebrafish acclimated to soft and hard water.

Acid-base regulation in fishes: cellular and molecular mechanisms

The mechanisms underlying acid-base transfers across the branchial epithelium of fishes have been studied for more than 70 years. These animals are able to compensate for changes to internal pH following a wide range of acid-base challenges, and the gill epithelium is the primary site of acid-base transfers to the water. This paper reviews recent molecular, immunohistochemical, and functional studies that have begun to define the protein transporters involved in the acid-base relevant ion transfers. Both Na(+)/H(+) exchange (NHE) and vacuolar-type H(+)-ATPase transport H(+) from the fish to the environment. While NHEs have been thought to carry out this function mainly in seawater-adapted animals, these proteins have now been localized to mitochondrial-rich cells in the gill epithelium of both fresh and saltwater-adapted fishes. NHEs have been found in the gill epithelium of elasmobranchs, teleosts, and an agnathan. In several species, apical isoforms (NHE2 and NHE3) appear to be up-regulated following acidosis. In freshwater teleosts, H(+)-ATPase drives H(+) excretion and is indirectly coupled to Na(+) uptake (via Na(+) channels). It has been localized to respiratory pavement cells and chloride cells of the gill epithelium. In the marine elasmobranch, both branchial NHE and H(+)-ATPase have been identified, suggesting that a combination of these mechanisms may be utilized by marine elasmobranchs for acid-base regulation. An apically located Cl(-)/HCO(3)(-) anion exchanger in chloride cells may be responsible for base excretion in fresh and seawater-adapted fishes. While only a few species have been examined to date, new molecular approaches applied to a wider range of fishes will continue to improve our understanding of the roles of the various gill membrane transport processes in acid-base balance.

Mechanisms through which ammonia regulates cortical collecting duct net proton secretion

Ammonia stimulates cortical collecting duct (CCD) net bicarbonate reabsorption by activating an apical H(+)-K(+)-ATPase through mechanisms that are independent of ammonia's known effects on intracellular pH and active sodium transport. The present studies examined whether this stimulation occurs through soluble N-ethylmaleimide-sensitive fusion attachment receptor (SNARE) protein-mediated vesicle fusion. Rabbit CCD segments were studied using in vitro microperfusion, and transepithelial bicarbonate transport was measured using microcalorimetry. Ammonia's stimulation of bicarbonate reabsorption was blocked by either chelating intracellular calcium with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester or by inhibiting microtubule polymerization with colchicine compared with parallel studies performed in the absence of these inhibitors. An inactive structural analog of colchicine, lumicolchicine, did not alter ammonia's stimulation of bicarbonate reabsorption. Tetanus toxin, a zinc endopeptidase specific for vesicle-associated SNARE (v-SNARE) proteins, prevented ammonia from stimulating net bicarbonate reabsorption. Consistent with the functional evidence for v-SNARE involvement, antibodies directed against a conserved region of isoforms 1-3 of the tetanus toxin-sensitive, vesicle-associated membrane protein (VAMP) members of v-SNARE proteins labeled the apical and subapical region of collecting duct intercalated cells. Similarly, antibodies to NSF protein, a protein involved in activation of SNARE proteins for subsequent vesicle fusion, localized to the apical and subapical region of collecting duct intercalated cells. These results indicate that ammonia stimulates CCD bicarbonate reabsorption through an intracellular calcium-dependent, microtubule-dependent, and v-SNARE-dependent mechanism that appears to involve insertion of cytoplasmic vesicles into the apical plasma membrane of CCD intercalated cells.

Overexpression of vacuolar ATPase 16-kDa subunit in 10T1/2 fibroblasts enhances invasion with concomitant induction of matrix metalloproteinase-2

Recent studies show that the vacuolar-type H(+)-ATPase (V-ATPase) 16 kDa subunit is expressed on plasma membrane of cancer cells. We hypothesized that V-ATPase 16 kDa subunit is directly involved in cell invasion. In the present study we established transfectants overexpressing V-ATPase 16 kDa subunit at the mRNA level, and found that these transfectants showed an enhanced invasiveness through matrigel with a concomitant increases in secretion of matrix metalloproteinase-2. Moreover, antisense oligonucleotides of the V-ATPase 16 kDa subunit suppressed invasive human A549 cell invasion with concomitant decreases in secretion of matrix metalloproteinase-2. The results suggest that the V-ATPase 16 kDa subunit is directly involved in cell invasion and that matrix metalloproteinase-2 is responsible for promoting the invasion by the V-ATPase 16 kDa subunit.

Expression of two vacuolar-type ATPase B subunit isoforms in swimbladder gas gland cells of the European eel: nucleotide sequences and deduced amino acid sequences

The poly(A)(+) RNA of swimbladder gas gland cells of the European eel Anguilla anguilla was isolated and used for cDNA synthesis. Using a pair of degenerate PCR primers directed towards the evolutionary highly conserved central part of the B subunit of vacuolar type H(+)-ATPase (V-ATPase) a fragment of 388 bp was amplified. By sequencing the cloned PCR products two different amplicons with a sequence identity of about 86% were obtained. BLASTN searches revealed a high degree of similarity of both to V-ATPase B subunits of other species. The sequences were completed by performing rapid amplification of cDNA ends PCR, subsequent cloning, and sequencing of the obtained products. The expression of two different isoforms of the V-ATPase B subunit is already demonstrated for Homo sapiens and Bos taurus. This is the first report that attributes the same phenomenon to a non-mammalian species, A. anguilla. The first isoform found in eel (vatB2) shows the highest degree of amino acid sequence homology with the human brain isoform (98.2%), the second one (vatB1) with the B subunit sequence of rainbow trout (Oncorhynchus mykiss) gill and kidney (98, 6%). The alignment of the deduced amino acid sequences of vatB1 and vatB2 shows that the highest sequence variation between these two isoforms is found at the amino-terminus, where vatB1 is nine amino acids shorter than vatB2, while at the carboxy-terminus it is two amino acids longer than vatB2. This has also been reported for the human and bovine kidney isoforms when compared with the brain isoforms. Northern blot analysis using specific hybridization probes revealed the expression of two mRNA's with lengths of about 2.9 kb and 3.5 kb for vatB1 and vatB2, respectively. For mammals, it is well known that V-ATPases containing the kidney isoforms of the B subunit are responsible for the extrusion of protons across the plasma membranes of several cell types. The fact that eel vatB1 seems to share structural features with the kidney isoforms in mammals supports the hypothesis that in gas gland cells a V-ATPase contributes to the acidification of the blood in the swimbladder.

Light and electron microscopic study of the hindgut of the ant (Formica nigricans, hymenoptera): II. Structure of the rectum

The rectum of the ant Formica nigricans is composed of six ovoid rectal papillae inserted into a rectal pouch. The wall of the rectal pouch is made up of a flat epithelium of simple rectal cells lined by cuticle, and surrounded by a circular muscle layer. Each rectal papilla is comprised by a simple columnar epithelium of principal cells facing the lumen, and a simple cuboid epithelium of secondary cells towards the hemolymph; a group of 20-25 slender junctional cells lies laterally between both epithelia enclosing an intrapapillar sinus. The muscle layer of the rectal wall also surrounds the base of the papillae. Principal cells do not exhibit extensive infoldings at the apical and basal plasma membranes. Lateral membranes, in contrast, develop highly folded mitochondria-scalariform junction complexes enclosing very narrow intercellular canaliculi between adjacent cells. These canaliculi open to wider intercellular sinuses that ultimately drain into the intrapapillar sinus at the sites of entry of tracheal cells. The lateral plasma membranes do not link to the apical or basal plasma membrane, thus originating a syncytium throughout the principal cells. The apical plasma membrane of secondary cells shows invaginations in relation with an apical tubulovacuolar system, bearing portasomes to the cytoplasmic side of the membrane. Secondary cells unite by convoluted septate junctions, and basolateral infoldings are also developed. These ultrastructural traits, some of them different from those found in other insects, are discussed and examined in relation to their role in water and solute absorption. A route for rectal transport in F. nigricans is proposed.

beta(1) integrin binds the 16-kDa subunit of vacuolar H(+)-ATPase at a site important for human papillomavirus E5 and platelet-derived growth factor signaling

Integrins mediate adhesive interactions between cells and the extracellular matrix, and play a role in cell migration, proliferation, differentiation, cytoskeletal organization, and signal transduction. We have identified an interaction between the beta(1) integrin and the 16-kDa subunit of vacuolar H(+)-ATPase (16K). This interaction was first isolated in a yeast two-hybrid screen and confirmed by coimmunoprecipitation and in in vitro binding assays using bacterially expressed proteins. Immunofluorescent studies performed in L6 myoblasts expressing both native and epitope-tagged 16K demonstrate co-localization with beta(1) integrin in focal adhesions. Deletion of the fourth of four transmembrane helices in 16K results in loss of interaction with beta(1) integrin in vitro and in the two-hybrid system, and less prominent staining in focal adhesions. This helix is also required for ligand-independent activation of platelet-derived growth factor-beta receptor signaling by the human papillomavirus E5 oncoprotein. Overexpression of 16K or expression of 16K lacking this helix alters the morphology of myoblasts and fibroblasts, suggesting that the interaction of 16K with integrins could be important for cell growth control. We also discuss the possible role 16K might play in integrin movement.

Receptor-mediated endocytosis in kidney proximal tubules: recent advances and hypothesis

Preparation of kidney proximal tubules in suspension allows the study of receptor-mediated endocytosis, protein reabsorption, and traffic of endosomal vesicles. The study of tubular protein transport in vitro coupled with that of the function of endosomal preparation offers a unique opportunity to investigate a receptor-mediated endocytosis pathway under physiological and pathological conditions. We assume that receptor-mediated endocytosis of albumin in kidney proximal tubules in situ and in vitro can be regulated, on the one hand, by the components of the acidification machinery (V-type H+-ATPase, Cl(-)-channel and Na+/H+-exchanger), giving rise to formation and dissipation of a proton gradient in endosomal vesicles, and, on the other hand, by small GTPases of the ADP-ribosylation factor (Arf)-family. In this paper we thus analyze the recent advances of the studies of cellular and molecular mechanisms underlying the identification, localization, and function of the acidification machinery (V-type H+-ATPase, Cl(-)-channel) as well as Arf-family small GTPases and phospholipase D in the endocytotic pathway of kidney proximal tubules. Also, we explore the possible functional interaction between the acidification machinery and Arf-family small GTPases. Finally, we propose the hypothesis of the regulation of translocation of Arf-family small GTPases by an endosomal acidification process and its role during receptor-mediated endocytosis in kidney proximal tubules. The results of this study will not only enhance our understanding of the receptor-mediated endocytosis pathway in kidney proximal tubules under physiological conditions but will also have important implications with respect to the functional consequences under some pathological circumstances. Furthermore, it may suggest novel targets and approaches in the prevention and treatment of various diseases (cystic fibrosis, Dent's disease, diabetes and autosomal dominant polycystic kidney disease).

Characterization and cellular distribution of the osteoclast ruffled membrane vacuolar H+-ATPase B-subunit using isoform-specific antibodies

Acidification of the bone surface, leading to bone resorption, is accomplished by a vacuolar-type H+-ATPase present in a specialized domain of the plasma membrane of the osteoclast known as the ruffled membrane. Structure and function appears to be highly conserved within this class of multisubunit enzymes. However, cloning and sequencing of complementary DNA has shown that one of the subunits in the catalytic domain, the B-subunit, exists in at least two forms, B1 and B2. B1 messenger RNA has been found almost exclusively in the kidney, whereas messenger RNA for B2 has been found in all tissues studied, including the kidney. It has been speculated that the B1 isoform might be involved in targeting to the plasma membrane. In the present study, we have characterized the B-subunit of the chicken osteoclast H+-ATPase using antibodies directed against peptides with isoform-specific or conserved sequences of the B-subunit. Western analysis was performed on chicken osteoclast membrane vesicles and on partially purified chicken osteoclast H+-ATPase and was compared with similar analysis of H+-ATPase isolated from bovine kidney and brain. The B1-specific antibody reacted with a polypeptide of approximately 56 kD on immunoblots of the renal H+-ATPase, whereas no reaction could be detected against the osteoclast H+-ATPase or the osteoclast membrane vesicle preparation. In contrast, the antibody against a B2-specific sequence reacted with a peptide of approximately 56 kD on immunoblots of the osteoclast H+-ATPase, the renal H+-ATPase, and the clathrin-coated vesicle H+-ATPase. The antibody against a conserved region of the B-subunit did not generate any evidence for the presence of isoforms other than B2 in the osteoclast. Immunocytochemistry of rat osteoclasts on bovine bone slices using the B2 antibody showed intense polarized staining along the plasma membrane facing the bone surface in actively resorbing osteoclasts whereas nonresorbing osteoclasts were diffusely stained throughout the cytoplasm. By confocal microscopy, the B2 staining was located to the level of the ruffled membrane and appeared to be concentrated to the peripheral areas of the membrane adjacent to the sealing zone. We conclude that the osteoclast vacuolar H+-ATPase contains the B2 isoform and suggest that upon initiation of resorption the pump is translocated from the cell interior to a special domain of the ruffled membrane close to the sealing zone.

Proton gradient formation in early endosomes from proximal tubules

Heavy endosomes were isolated from proximal tubules using a combination of magnesium precipitation and wheat-germ agglutinin negative selection techniques. Two small GTPases (Rab4 and Rab5) known to be specifically present in early endosomes were identified in our preparations. Endosomal acidification was followed fluorimetrically using acridine orange. In presence of chloride ions and ATP, the formation of a proton gradient (delta pH) was observed. This process is due to the activity of an electrogenic V-type ATPase present in the endosomal membrane since specific inhibitors bafilomycin and folimycin effectively prevented or eliminated endosomal acidification. In presence of chloride ions (K(m) = 30 mM) the formation of the proton gradient was optimal. Inhibitors of chloride channel activity such as DIDS and NPPB reduced acidification. The presence of sodium ions stimulated the dissipation of the proton gradient. This effect of sodium was abolished by amiloride derivative (MIA) but only when loaded into endosomes, indicating the presence of a physiologically oriented Na+/H(+)-exchanger in the endosomal membrane. Monensin restored the gradient dissipation. Thus three proteins (V-type ATPase, Cl(-)-channel, Na+/H(+)-exchanger) present in early endosomes isolated from proximal tubules may regulate the formation, maintenance and dissipation of the proton gradient.

Purification and properties of a cytosolic V1-ATPase

The native V1 complex of the tobacco hornworm vacuolar type ATPase (V-ATPase) was purified from cytosolic extracts of molting larval midgut. It consisted of the established V-ATPase subunits A, B, and E along with the 14-kDa subunit F and the novel 13-kDa subunit G. The final amount of purified V1 complex made up an unexpectedly high 2% of the total cytosolic protein, with a yield of approximately 0.4 mg/g of tissue. An equally high amount of cytosolic V1 complex was obtained from starving intermolt larvae. By contrast, the cytosolic V1 pool was reduced drastically in feeding intermolt larvae or in larvae that had been refed after starvation. The activity of the membrane-bound V-ATPase holoenzyme was inversely related to the size of the cytosolic V1 pool, suggesting that the insect plasma membrane V-ATPase is regulated by reversible disassembly of the V1 complex as a function of the feeding condition of the larvae. Like F1-ATPases, the purified V1 complex exhibited Ca2+-dependent ATPase activity and, in the presence of 25% methanol, exhibited Mg2+-dependent ATPase activity. Therefore, we designate the native V1 complex, V1-ATPase. Both enzyme activities were completely inhibited by micromolar N-ethylmaleimide. In contrast to the Ca2+-dependent V1-ATPase activity, the Mg2+/methanol-dependent V1-ATPase activity did not decrease with the incubation time and thus was not inhibited by ADP. Methanol appears to induce a conformational change of the V1 complex, leading to enzymatic properties of the V1-ATPase that are similar to those of the membrane-bound V-ATPase holoenzyme. This is the first time that a native and enzymatically active V1 complex has been purified from the cytosol.

Analysis of nucleotide binding by a vacuolar proton-translocating adenosine triphosphatase

The vacuolar-type proton-translocatine adenosine triphosphatase from bovine adrenal secretory granules (chromaffin granules) was purified and reconstituted into proteoliposomes. The binding of nucleotides to the enzyme was studied by quantifying their effects on the rate of inactivation by N-ethylmaleimide (MalNEt) of ATP-dependent proton translocation, and by direct measurement of the binding of [3H]MgADP. The results of these experiments are consistent with a model of the enzyme that had been developed as a result of kinetic experiments, the features of which are that the enzyme exists in two states, each containing three nucleotide-binding sites on catalytic subunits, and that nucleoside diphosphates regulate the enzyme by binding with high affinity to a single site in the inactive T state of the enzyme. Under the conditions of the experiments, MalNEt inactivated the ATPase in a pseudo-first order reaction. Rate constants of inactivation were reduced in the presence of MgADP, MgIDP and free ADP; the kinetics of protection suggested that the two conformational states of the enzyme were inactivated at different rates and also confirmed the existence of two different types of binding site for MgADP. Low nucleotide concentrations afforded partial protection from MalNEt; this was ascribed to binding of nucleotide to the regulatory site causing a shift in the conformational equilibrium towards the T state, which was more slowly inactivated than the unliganded R state of the enzyme. At higher nucleotide concentrations, binding at the catalytic site afforded complete protection from MalNEt. Protection by MgADP[S] and magnesium 2'- and 3'-O-[4-benzoylbenzoyl]adenosine 5'-triphosphate showed simpler kinetics but was also consistent with previously reported kinetic results. Analysis of subunit labelling with [3H]MalNEt showed that the three 72-kDa (catalytic) subunits were alkylated by MalNEt with similar rate constants, consistent with a symmetrical arrangement of the catalytic subunits, in contrast to the situation in F-type ATPases. Analysis of the binding of [3H]MgADP also confirmed the results of kinetic experiments. MgADP was shown to bind to the enzyme with an apparent dissociation constant of about 66 nM; assuming that the nucleotide binds only to the T-state, the true dissociation constant is < 1 nM. Using Blue Native polyacrylamide gel electrophoresis to separate the holo-ATPase from the membrane sector, the stoichiometry of binding was calculated to be 0.6 mol/mol enzyme, confirming the existence of a single regulatory site for MgADP. However, binding of MgADP to the enzyme was much slower than could be accounted for by the measured dissociation constants, suggesting that it is rate limited by a step such as a protein conformational change. Treatment designed to remove endogenous nucleotide had no effect on the rate or extent of binding of MgADP.

Biochemical characterization of a V-ATPase of tracheal smooth muscle plasma membrane fraction

A biochemical characterization of a Mg(2+)-ATPase activity associated with a plasma membrane fraction isolated from airway (tracheal) smooth muscle was performed. This enzyme is an integral part of the membrane remaining tightly bound after 0.6 M KCl extraction. This enzyme activity showed a cold inactivation in the presence of ATP and Mg2+. Also, this Mg(2+)-ATPase was stimulated by monovalent anions being Cl-, the best anion for such stimulation, even though Br- and I- were good substitutes and F- was ineffective. This Cl--stimulated activity showed a powerful nucleosidetriphosphatase activity having the following divalent cation specificity: Mg2+ > Mn2+ > Ca2+, where Zn2+ and Fe2+ were ineffective. This ATPase activity was not inhibited by ouabain oligomycin C and vanadate indicating that neither P- or F-ATPases were associated with this enzyme activity. However, the existence of a V-ATPase was shown by the significant inhibition causes by bafilomycin A1. Additionally, this V-ATPase seems to be coupled to Cl- conductor because duramycin inhibited this ATPase activity. The presence of a H+ pump associated to this V-ATPase was shown indirectly, through the stimulatory effect produced by uncouplers such as FCCP and 1799, which were able to produce significant stimulation of this V-ATPase indicating the existence of a H(+)-ATPase. Finally, the immunodetection of a 72 kDa polypeptide using a specific antibody against the A subunit (72 kDa) of V-ATPase from chromaffin granule demonstrated the presence of a V-ATPase in this plasma membrane fraction.

The peripheral complex of the tobacco hornworm V-ATPase contains a novel 13-kDa subunit G

A prominent 16-kDa protein copurifies with the V-ATPase isolated from both posterior midgut and Malpighian tubules of Manduca sexta larvae and thus was believed to represent a V-ATPase subunit. [14C]N,N'-dicyclohexylcarbodiimide labeling and its position on SDS-electrophoresis gels revealed that this protein was different from the 17-kDa proteolipid. A cDNA clone encoding a highly hydrophilic protein with a calculated molecular mass of 13,692 Da was obtained by immunoscreening. Monospecific antibodies, affinity-purified to the 13-kDa recombinant protein expressed in Escherichia coli, specifically recognized the 16-kDa protein of the purified V-ATPase, confirming that a cDNA encoding this protein had been cloned. In vitro translation of the cRNA showed that the cloned 13-kDa subunit behaved like a 16-kDa protein on SDS-electrophoresis gels. The cloned protein showed 37% amino acid sequence identity to the 13-kDa V-ATPase subunit Vma10p recently cloned from yeast and some similarity to subunit b of bacterial F-ATPases. In contrast to the Vma10p protein, which behaved like a V0 subunit, the M. sexta 13-kDa protein behaved like a V1 subunit, since it could be stripped from the membrane by treatment with the chaotropic salt KI and by cold inactivation. When KI dissociated V-ATPase subunits were reassociated by dialysis that removed the KI, a soluble, 450-kDa complex of the M. sexta V-ATPase could be purified by gel chromatography. This V1 complex consisted of subunits A, B, E, and the 13-kDa subunit, confirming that the cloned protein is a new V-ATPase subunit and a member of the peripheral V1 complex of the V-ATPase. We designate this new V1 component subunit G.

Bafilomycin A1 inhibits lysosomal, phagosomal, and plasma membrane H(+)-ATPase and induces lysosomal enzyme secretion in macrophages

Bafilomycin A1, a specific inhibitor of H(+)-ATPases of the vacuolar type, was in the present study shown, at similar concentrations, to induce secretion of lysosomal enzyme and to elevate lysosomal pH in mouse macrophages. These results lend support to the previous suggestion of a triggering role for an increase in lysosomal pH and a permissive role for cytosolic pH in the exocytosis of macrophage lysosomal enzyme. Vacuolar H(+)-ATPases are present in the macrophage plasma membrane as well as in intracellular membranes, for example, those of the lysosomal and phagosomal compartments. Phagosomal acidification was shown to be achieved in part by a mechanism with a similar sensitivity to bafilomycin A1 as lysosomal H+ transport and in part by an early, bafilomycin A1-insensitive mechanism. We found a lesser sensitivity towards bafilomycin A1 of the lysosomal and phagosomal H(+)-ATPase than that localized in the plasma membrane, indicating differences among H(+)-ATPases at the subcellular level. Also, by attempts to mobilize lysosomal H(+)-ATPase to the plasma membrane, support was obtained for the notion that subcellular H(+)-ATPase populations differ and thus possibly could be differentially regulated.

Regulation of plasma membrane V-ATPase activity by dissociation of peripheral subunits

The plasma membrane V-ATPase of Manduca sexta larval midgut is an electrogenic proton pump located in goblet cell apical membranes (GCAM); it energizes, by the voltage component of its proton motive force, an electrophoretic K+/nH+ antiport and thus K+ secretion (Wieczorek, H., Putzenlechner, M., Zeiske, W., and Klein, U. (1991) J. Biol Chem. 266, 15340-15347). Midgut transepithelial voltage, indicating net active K+ transport, was found to be more than 100 mV during intermoult stages but was abolished during moulting. Simultaneously, ATP hydrolysis and ATP-dependent proton transport in GCAM vesicles were found to be reduced to 10-15% of the intermoult level. Immunocytochemistry of midgut cryosections as well as SDS-polyacrylamide gel electrophoresis and immunoblots of GCAM demonstrated that loss of ATPase activity paralleled the disappearance of specific subunits. The subunits missing were those considered to compose the peripheral V1 sector, whereas the membrane integral V0 subunits remained in the GCAM of moulting larvae. The results provide, for the first time, evidence that a V-ATPase activity can be controlled in vivo by the loss of the peripheral V1 domain.