Structure, function and regulation of the vacuolar (H+)-ATPases

The ATPases: a new family for a family-based drug design approach

The rapid discovery of new drugs is greatly facilitated when a family of related proteins is targeted with a similar approach in chemistry. Few protein families have so far been investigated using this kind of 'family-based' approach. Therefore, to increase the size of our Pharmacopeia and to cure human diseases more efficiently, new druggable protein families must be identified. It is shown in this review that ATPases are very good candidates for a family-based approach. The human proteome contains many ATPases, which are involved in several diseases. All the ATPases contain a nucleotide-binding site, and it is therefore possible to target all of them with a single strategy in chemistry; the design of competitive ATP inhibitors. Moreover, because a similar approach has been conducted with the protein kinases, the compound libraries and the knowledge developed in the kinase field can be directly applied to the ATPases.

Different roles of proteolipids and 70-kDa subunits of V-ATPase in growth and death of cultured human cells

BACKGROUND

The vacuolar-type proton-translocating adenosine triphosphatase (V-ATPase) plays important roles in cell growth and tumour progression. V-ATPase is composed of two distinct structures, a hydrophilic catalytic cytosolic sector (V(1)) and a hydrophobic transmembrane sector (V(0)). The V(1) sector is composed of 5-8 different subunits with the structure A(3)B(3)C(1)D(1)E(1)F(1)G(1)H(1). The V0 sector is composed of 5 different subunits with the structure 1161381191166. The over-expression of 16-kDa proteolipid subunit of V-ATPase in the perinuclear region of the human adventitial fibroblasts promotes phenotypic modulation that contributes to neointimal formation and medial thickening. A relationship between oncogenicity and the expression of the 16-kDa proteolipid has also been suggested in human pancreatic carcinoma tissue.

RESULTS

We found that the mRNA levels of the 16-kDa proteolipid but not of the 70-kDa subunit of V-ATPase in human myofibroblasts were more abundant in serum-containing medium (MF(+) cells) than serum-free medium (MF(-) cells). In HeLa cells, the levels of mRNA and protein of the 16-kDa, 21-kDa or 70-kDa were clearly suppressed when the corresponding anti-sense oligonucleotides were administered to the culture medium. The growth rate and viability (mostly due to necrosis) of HeLa cells were reduced markedly by the 16-kDa and 21-kDa anti-sense, but little by the 70-kDa anti-sense, and not at all by any sense oligonucleotides. The localization of 16-kDa/21-kDa proteolipid subunits was different from that of the 70-kDa subunit in HeLa cells.

CONCLUSION

These results suggest that the 16-kDa and 21-kDa proteolipid subunits of the V0 sector play crucial roles in growth and death of cultured human cells. Our results may provide new insights into the mechanism and therapeutic implications for vessel wall hyperplasia and tumorigenesis.

Increased tumour extracellular pH induced by Bafilomycin A1 inhibits tumour growth and mitosis in vivo and alters 5-fluorouracil pharmacokinetics

The aim was to determine if a specific inhibitor of vacuolar H(+)-ATPases (V-ATPases), Bafilomycin A1 (BFM), could increase the low extracellular pH (pHe) typical of solid tumours and thus inhibit their growth in vivo. BFM inhibited the proliferation of various human cells and rat pituitary GH3 tumour cells in vitro (IC50: 2.5-19.2 nM), and flow cytometry on GH3 cells showed a marked increase in S and G2M phases after 16-48 h, but no evidence of increased apoptosis. BFM caused significant inhibition of GH3 xenograft growth, and histomorphometry showed a 30% decrease in mitosis but no change in apoptosis. 31P-magnetic resonance spectroscopy (MRS) in vivo of GH3 xenografts showed that BFM increased pHe, but did not affect pHi, resulting in a decrease in the negative pH gradient (-delta pH). BFM decreased lactate formation suggesting a reduction in glycolysis. We suggest that BFM reduces extracellular H(+)-transport by inhibition of V-ATPases leading to an increase in pHe and decreased glycolysis, and thus reduced tumour cell proliferation. 19F-MRS in vivo showed that a smaller -delta pH was associated with decreased retention of 5-fluorouracil (5FU) which was consistent with our previous data in vivo implying the -delta pH controls tumour retention of 5 FU.

Localization of the vacuolar-type ATPase in swimbladder gas gland cells of the European eel (Anguilla anguilla)

The vacuolar ATPase is a multifunctional enzyme that consists of several subunits. Subunit B is part of the catalytic domain of the enzyme and is present in two isoforms in fish as well as in mammals. Possibly, these two isoforms - vatB1 (kidney isoform) and vatB2 (brain isoform) - serve different functions. A localization of the two isoforms was attempted in swimbladder gas gland cells of the European eel Anguilla anguilla by immunohistochemistry. Two antibodies were produced by immunization of rabbits with synthetic peptides. Specificity of the antibodies, on the one hand, an isoform-specific antibody for vatB1 and, on the other hand, an antibody that recognizes both isoforms (vatB1 and vatB2), was confirmed by western blot analysis using recombinant proteins produced in a bacterial expression system. The immunohistochemical localization with the antibody directed against both isoforms of the B subunit revealed a positive staining in apical membranes of swimbladder gas gland cells as well as in the basolateral membranes. Significant staining was observed in vesicles located near the apical membrane. Staining with the vatB1-specific antibody resulted in a similar picture in the apical region of the cells. In contrast to the staining with the first antibody, only a poor signal was observed in the basal region. The nature of the vesicles in the apical region of the gas gland cells was determined by using an antibody directed against surfactant protein D.

Contrasting roles of endosomal pH and the cytoskeleton in infection of human glial cells by JC virus and simian virus 40

Infection of eukaryotic cells by pathogens requires the efficient use of host cell endocytic and cytoplasmic transport mechanisms. Understanding how these cellular functions are exploited by microorganisms allows us to better define the basic biology of pathogenesis while providing better insight into normal cellular functions. In this report we compare and contrast intracellular transport and trafficking of the human polyomavirus JC virus (JCV) with that of simian virus 40 (SV40). We have previously shown that infection of human glial cells by JCV requires clathrin-dependent endocytosis. In contrast, infection of cells by SV40 proceeds by caveola-dependent endocytosis. We now examine the roles of endosomal pH and the cellular cytoskeleton during infection of glial cells by both viruses. Our results demonstrate that JCV infection is sensitive to disruption of endosomal pH, whereas SV40 infection is pH independent. Infection by JCV is inhibited by treatment of glial cells with cytochalasin D, nocodazole, and acrylamide, whereas SV40 infection is affected only by nocodazole. These data point to critical differences between JCV and SV40 in terms of endocytosis and intracellular trafficking of their DNA genomes to the nucleus. These data also suggest a unique sequential involvement of cytoskeletal elements during infection of glial cells by JCV.

Enhanced expression of the human vacuolar H+-ATPase c subunit gene (ATP6L) in response to anticancer agents

We have isolated two overlapping genomic clones that contain the 5'-terminal portion of the human vacuolar H(+)-ATPase c subunit (ATP6L) gene. The sequence preceding the transcription initiation site, which is GC-rich, contains four GC boxes and one Oct1-binding site, but there is no TATA box or CCAAT box. In vivo footprint analysis in human cancer cells shows that two GC boxes and the Oct1-binding site are occupied by Sp1 and Oct1, respectively. We show here that treatment with anticancer agents enhances ATP6L expression. Although cisplatin did not induce ATP6L promoter activity, it altered ATP6L mRNA stability. On the other hand, the DNA topoisomerase II inhibitor, TAS-103, strongly induced promoter activity, and this effect was completely eradicated when a mutation was introduced into the Oct1-binding site. Treatment with TAS-103 increased the levels of both Sp1/Sp3 and Oct1 in nuclear extracts. Cooperative binding of Sp1 and Oct1 to the promoter is required for promoter activation by TAS-103. Incubation of a labeled oligonucleotide probe encompassing the -73/-68 GC box and -64/-57 Oct1-binding site with a nuclear extract from drug-treated KB cells yielded higher levels of the specific DNA-protein complex than an extract of untreated cells. Thus, the two transcription factors, Sp1 and Oct1 interact, in an adaptive response to DNA damage, by up-regulating expression of the vacuolar H(+)-ATPase genes. Furthermore, combination of the vacuolar H(+)-ATPase (V-ATPase) inhibitor, bafilomycin A1, with TAS-103 enhanced apoptosis of KB cells with an associated increase in caspase-3 activity. Our data suggest that the induction of V-ATPase expression is an anti-apoptotic defense, and V-ATPase inhibitors in combination with low-dose anticancer agents may provide a new therapeutic approach.

Plasmalemmal H+ extruders in mammalian alveolar macrophages

The distribution of plasmalemmal V-type H+-pumps (V-ATPase) among mammalian macrophages (mvarphi) is uncertain and, hence, the functional significance of mvarphi plasmalemmal V-ATPase is unclear. This study investigated the role of V-ATPase in the regulation of intracellular pH (pH(i)) by resident alveolar mvarphi from sheep, pigs, dogs and rabbits. The fluorescent probe 2',7'-biscarboxyethyl-5,6-carboxyfluorescein was used to monitor baseline pH(i) and the rate of pH(i) recovery (dpH(i)/dt) from intracellular acid-loads (NH(4)Cl prepulse). Baseline pH(i) was 7.1-7.2. In sheep, pig and dog studies, 10 microM bafilomycin A(1) (a selective V-ATPase inhibitor) caused a rapid fall in baseline pH(i) (0.15-0.20 units); baseline values were unaffected by 0.1 mM amiloride (a Na+ transport inhibitor). V-ATPase activity (bafilomycin-sensitive component of dpH(i)/dt) was solely responsible for pH(i) recovery from intracellular acid-loads at acid-loaded pH(i) values >6.8-6.9. Na+/H+ exchange (amiloride-sensitive component of dpH(i)/dt) was detected only at acid-loaded pH(i) values <6.8. The activity of both H+ extruders increased at lower pH(i) values, albeit the Na+/H+ exchanger was more pH-sensitive than was V-ATPase. In rabbit studies, 10 microM bafilomycin A(1) and 1 mM N-ethylmaleimide (a non-specific H+-pump inhibitor) produced similar falls in baseline mvarphi pH(i), but had significantly larger effects than did the selective V-ATPase inhibitor concanamycin A (<or=15 microM). The findings suggest that plasmalemmal V-ATPase activity plays a major role in pH(i) regulation by alveolar mvarphi of sheep, pigs, dogs and rabbits.

Endolysosomal proteolysis and its regulation

The endolysosomal system comprises a unique environment for proteolysis, which is regulated in a manner that apparently does not involve protease inhibitors. The system comprises a series of membrane-bound intracellular compartments, within which endocytosed material and redundant cellular components are hydrolysed. Endocytosed material tends to flow vectorially through the system, proceeding through the early endosome, the endosome carrier vesicle, the late endosome and the lysosome. Phagocytosis and autophagy provide alternative entry points into the system. Late endosomes, lysosome/late endosome hybrid organelles, phagosomes and autophagosomes are the principal sites for proteolysis. In each case, hydrolytic competence is due to components of the endolysosomal system, i.e. proteases, lysosome-associated membrane proteins, H(+)-ATPases and possibly cysteine transporters. The view is emerging that lysosomes are organelles for the storage of hydrolases, perhaps in an inactivated form. Once a substrate has entered a proteolytically competent environment, the rate-limiting proteolytic steps are probably effected by cysteine endoproteinases. As these are affected by pH and possibly redox potential, they may be regulated by the organelle luminal environment. Regulation is probably also affected, among other factors, by organelle fusion reactions, whereby the meeting of enzyme and substrate may be controlled. Such systems would permit simultaneous regulation of a number of unrelated hydrolases.

Vacuolar H(+)-ATPase: functional mechanisms and potential as a target for cancer chemotherapy

Tumor cells in vivo often exist in a hypoxic microenvironment with a lower extracellular pH than that surrounding normal cells. Ability to upregulate proton extrusion may be important for tumor cell survival. Such microenvironmental factors may be involved in the development of resistant subpopulations of tumor cells. In solid tumors, both intracellular and extracellular pH differ between drug-sensitive and -resistant cells, and pH appears critical to the therapeutic effectiveness of anticancer agents. Four major types of pH regulators have been identified in tumor cells: the sodium-proton antiporter, the bicarbonate transporter, the proton-lactate symporter and proton pumps. Understanding mechanisms regulating tumor acidity opens up novel opportunities for cancer chemotherapy. In this minireview, we describe the structure and function of certain proton pumps overexpressed in many tumors--vacuolar H(+)-ATPases--and consider their potential as targets for cancer chemotherapy.

Elevated expression of vacuolar proton pump genes and cellular PH in cisplatin resistance

V-ATPases are proton-translocating enzymes, which are found not only in numerous intracellular organelles but also in the plasma membranes of many eukaryotic cells. Using differential display, we have identified one of the proton pump subunit genes, ATP6C, as a cisplatin-inducible gene. Northern blot analysis demonstrated that expression of other members of the subunit is inducible by cisplatin treatment. Proton pump gene expression is also upregulated in 3 independent cisplatin-resistant cell lines but not in vincristine- or etoposide-resistant cell lines. Cellular pH was significantly higher in cisplatin-resistant cells than in sensitive parental cells. In vitro DNA-binding activity of cisplatin was markedly increased in acidic conditions, suggesting that the cytotoxicity of cisplatin is modulated by cellular pH. Furthermore, the proton pump inhibitor bafilomycin can synergistically potentiate the cytotoxicity of cisplatin but not of etoposide or camptothecin. These results indicate that cellular pH is one of the critical parameters for effective cancer chemotherapy with cisplatin.

Presentation of antigens by MHC class II molecules: getting the most out of them

The function of MHC class II molecules is to bind peptides derived from antigens that access the endocytic route of antigen presenting cells and display them on the plasma membrane for recognition by CD4(+) T cells. Formation of the MHC II-peptide complexes entails the confluence of the antigens and the MHC II molecules in the same compartments of the endocytic route. There, both the antigens and the MHC II molecules undergo a series of orchestrated changes that involve proteases, other hydrolases and chaperones, culminating in the generation of a wide repertoire of MHC II-peptide combinations. All the events that lead to formation of MHC II-peptide complexes show a considerable degree of flexibility; this lack of strict rules is advantageous in that it provides T cells with the maximum amount of information, ensuring that pathogens do not go undetected.

Microtubules are involved in glucose-dependent dissociation of the yeast vacuolar [H+]-ATPase in vivo

The vacuolar [H(+)]-ATPases (V-ATPases) are composed of a peripheral V(1) domain and a membrane-embedded V(0) domain. Reversible dissociation of the V(1) and V(0) domains has been observed in both yeast and insects and has been suggested to represent a general regulatory mechanism for controlling V-ATPase activity in vivo. In yeast, dissociation of the V-ATPase is triggered by glucose depletion, but the signaling pathways that connect V-ATPase dissociation and glucose metabolism have not been identified. We have found that nocodazole, an agent that disrupts microtubules, partially blocked dissociation of the V-ATPase in response to glucose depletion in yeast. By contrast, latrunculin, an agent that disrupts actin filaments, had no effect on glucose-dependent dissociation of the V-ATPase complex. Neither nocodazole nor latrunculin blocked reassembly of the V-ATPase upon re-addition of glucose to the medium. The effect of nocodazole appears to be specifically through disruption of microtubules since glucose-dependent dissociation of the V-ATPase was not blocked by nocodazole in yeast strains bearing a mutation in tubulin that renders it resistant to nocodazole. Because nocodazole has been shown to arrest cells in the G(2) phase of the cell cycle, it was of interest to determine whether nocodazole exerted its effect on dissociation of the V-ATPase through cell cycle arrest. Glucose-dependent dissociation of the V-ATPase was examined in four yeast strains bearing temperature-sensitive mutations that arrest cells in different stages of the cell cycle. Because dissociation of the V-ATPase occurred normally at both the permissive and restrictive temperatures in these mutants, the results suggest that in vivo dissociation is not dependent upon cell cycle phase.

Bafilomycin A1inhibits rhinovirus infection in human airway epithelium: effects on endosome and ICAM-1

To examine the effects of bafilomycin A1, a blocker of vacuolar H+-ATPase, on rhinovirus (RV) infection in the airway epithelium, primary cultures of human tracheal epithelial cells were infected with RV14. Viral infection was confirmed by showing that viral RNA in the infected cells and the viral titers in the supernatants of infected cells increased with time. RV14 infection upregulated the production of cytokines and mRNA of intercellular adhesion molecule (ICAM)-1 in epithelial cells. Bafilomycin A1reduced the viral titers of RV14 and inhibited the production of cytokines and ICAM-1 before and after RV14 infection. Bafilomycin A1reduced susceptibility of epithelial cells to RV14 infection. RV14 increased activated nuclear factor-κB in the cells, and bafilomycin A1reduced the activated nuclear factor-κB. Bafilomycin A1decreased the number of acidic endosomes in the epithelial cells. These results suggest that bafilomycin A1may inhibit infection by RV14 by not only blocking RV RNA entry into the endosomes but also reducing ICAM-1 expression in the epithelial cells. Bafilomycin A1may therefore modulate airway inflammation after RV infection.

Expression of multiple Na+/H+ exchanger isoforms in cultured epithelial cells from rat efferent duct and cauda epididymidis

Although earlier work has pointed to the presence of Na+/H+ exchangers (NHEs) in the rat epididymis, little is known about the regional distribution of various NHE isoforms and their functions. In the present work, expression of different isoforms of NHE in cultured epithelia of the efferent duct and cauda epdidymidis were studied. Reverse transcription-polymerase chain reaction revealed the presence of NHE1, NHE2, and NHE3, but not NHE4, message in both cultured epithelia. Western blot analysis detected the presence of NHE1 and NHE2 proteins in both cultured epithelia, but NHE3 protein was only detected in the cultured epithelial cells from the efferent duct. Immunohistochemical studies demonstrated that NHE2 was localized in the cytoplasm of the ciliated cells, whereas NHE3 was localized at the apical membrane of the principal cells of the efferent duct. The NHE activities in both cultured epithelia were inhibited by 10 microM HOE-694 (3-methylsulphonyl-4-piperidinobenzoyl guanidine methanesulphonate), a NHE1 inhibitor, by approximately 76%. The HOE-694-resistant NHE activities in the cultured epithelia of efferent duct and cauda epididymidis were completely inhibited by 20 microM S3226 (3-[2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methyl-phenyl]-N:-isopropylidene-2-methyl-acrylamide dihydrochloride), a NHE3 inhibitor, and 300 microM HOE-694 (a dose that can completely block NHE2), respectively. These results indicated that NHE1, NHE2, and NHE3 were expressed in the cultured epithelial cells of the efferent duct, whereas only NHE1 and NHE2 were expressed in the cultured epithelial cells of the cauda epididymidis. It is suggested that NHE1 may provide "housekeeping" functions in both epithelia, whereas NHE2 in the cauda epididymidis and NHE3 in the efferent duct may be involved in Na+ reabsorption and regulation of pH of the luminal fluid.

Cellular pathology and pathogenic aspects of neuronal ceroid lipofuscinoses

Lysosomal accumulation of autofluorescent, ceroid lipopigment material in various tissues and organs is a common feature of the neuronal ceroid lipofuscinoses (NCLs). However, recent clinicopathologic and genetic studies have evidenced that NCLs encompass a group of highly heterogeneous disorders. In five of the eight NCL variants distinguished at present, genes associated with the disease process have been isolated and characterized (CLN1, CLN2, CLN3, CLN5, CLN8). Only products of two of these genes, CLN 1 and CLN2, have structural and functional properties of lysosomal enzymes. Nevertheless, according to the nature of the material accumulated in the lysosomes, NCLs in humans as well as natural animal models of these disorders can be divided into two major groups: those characterized by the prominent storage of saposins A and D, and those showing the predominance of subunit c of mitochondrial ATP synthase accumulation. Thus, taking into account the chemical character of the major component of the storage material, NCLs can be classified currently as proteinoses. Of importance, although lysosomal storage material accumulates in NCL subjects in various organs, only brain tissue shows severe dysfunction and cell death, another common feature of the NCL disease process. However, the relation between the genetic defects associated with the NCL forms, the accumulation of storage material, and tissue damage is still unknown. This chapter introduces the reader to the complex pathogenesis of NCLs and summarizes our current knowledge of the potential consequences of the genetic defects of NCL-associated proteins on the biology of the cell.

Characterization of constitutive and putative differentially expressed mRNAs by means of expressed sequence tags, differential display reverse transcriptase-PCR and randomly amplified polymorphic DNA-PCR from the sand fly vector Lutzomyia longipalpis

Molecular studies of insect disease vectors are of paramount importance for understanding parasite-vector relationship. Advances in this area have led to important findings regarding changes in vectors' physiology upon blood feeding and parasite infection. Mechanisms for interfering with the vectorial capacity of insects responsible for the transmission of diseases such as malaria, Chagas disease and dengue fever are being devised with the ultimate goal of developing transgenic insects. A primary necessity for this goal is information on gene expression and control in the target insect. Our group is investigating molecular aspects of the interaction between Leishmania parasites and Lutzomyia sand flies. As an initial step in our studies we have used random sequencing of cDNA clones from two expression libraries made from head/thorax and abdomen of sugar fed L. longipalpis for the identification of expressed sequence tags (EST). We applied differential display reverse transcriptase-PCR and randomly amplified polymorphic DNA-PCR to characterize differentially expressed mRNA from sugar and blood fed insects, and, in one case, from a L. (V.) braziliensis-infected L. longipalpis. We identified 37 cDNAs that have shown homology to known sequences from GeneBank. Of these, 32 cDNAs code for constitutive proteins such as zinc finger protein, glutamine synthetase, G binding protein, ubiquitin conjugating enzyme. Three are putative differentially expressed cDNAs from blood fed and Leishmania-infected midgut, a chitinase, a V-ATPase and a MAP kinase. Finally, two sequences are homologous to Drosophila melanogaster gene products recently discovered through the Drosophila genome initiative.

CLN3 disease process: missense point mutations and protein depletion in vitro

Although the CLN3 gene associated with the disease process in subjects with the juvenile form of neuronal ceroid lipofuscinosis was discovered in 1995, our knowledge of the physiological function of its gene product, CLN3 protein, is still incomplete. To gain more insight into the structural properties and function of CLN3 protein we studied at present: i) how the naturally occurring point mutations Arg334Cys and Leu101Pro affect the biological properties of CLN3 protein, and ii) whether depletion of CLN3 protein synthesis by using an antisense approach induces a distinct phenotype in cells of neuronal origin in vitro. Here we report that although both CLN3 mutant proteins are targeted to lysosomes, thus similar to wild-type CLN3 protein, they are devoid of the biological activity of wild-type CLN3 protein such as its effect on lysosomal pH or intracellular processing of amyloid-beta protein precursor and cathepsin D in vitro. The Leu101Pro mutation affected significantly the maturation and stability of CLN3 protein. The Arg334Cys mutation influenced mildly the maturation and turnover of CLN3 protein, but at the same time abolished the function of CLN3 protein in vitro, which suggests that the Arg334 may constitute a part of the active site of CLN3 protein. In addition, we show that depletion of CLN3 protein synthesis in human neuroblastoma cells in vitro induces outgrowth of long cellular processes and formation of cellular aggregates and affects the viability of these cells. This finding suggests that CLN3 protein is implicated in biological processes associated with the differentiation of cells of neuronal origin.

Altered distribution of the yeast plasma membrane H+-ATPase as a feature of vacuolar H+-ATPase null mutants

The effect of vacuolar H(+)-ATPase (V-ATPase) null mutations on the targeting of the plasma membrane H(+)-ATPase (Pma1p) through the secretory pathway was analyzed. Gas1p, which is another plasma membrane component, was used as a control for the experiments with Pma1p. Contrary to Gas1p, which is not affected by the deletion of the V-ATPase complex in the V-ATPase null mutants, the amount of Pma1p in the plasma membrane is markedly reduced, and there is a large accumulation of the protein in the endoplasmic reticulum. Kex2p and Gef1p, which are considered to reside in the post-Golgi vesicles, were suggested as required for the V-ATPase function; hence, their null mutant phenotype should have been similar to the V-ATPase null mutants. We show that, in addition to the known differences between those yeast phenotypes, deletions of KEX2 or GEF1 in yeast do not affect the distribution of Pma1p as the V-ATPase null mutant does. The possible location of the vital site of acidification by V-ATPase along the secretory pathway is discussed.

Acidic endomembrane organelles are required for mouse postimplantation development

Vacuolar-type H(+)-ATPase (V-ATPase) plays a major role in endomembrane and plasma membrane proton transport in eukaryotes. We found that the acidic compartments generated by V-ATPase are present from the one-cell stage of mouse preimplantation embryos. Upon differentiation of trophoblasts and the inner cell mass at the blastocyst stage, these compartments exhibited a polarized perinuclear distribution. PL16(-/-) embryos, lacking the V-ATPase 16-kDa proteolipid (c subunit), developed to the blastocyst stage and were implanted in the uterine epithelium, but died shortly thereafter. This mutant showed severe defects in development of the embryonic and extraembryonic tissues at a stage that coincided with rapid cell proliferation. When cultured in vitro, PL16(-/-) blastocysts could hatch and become attached to the surface of a culture dish, but the inner cell mass grew significantly slower and most cells failed to survive for more than 4 days. PL16(-/-) cells showed impaired endocytosis as well as organellar acidification. The Golgi complex became swollen and vacuolated, possibly due to the absence of the luminal acidic pH. These results clearly indicate that acidic compartments are essential for development after implantation.

Regulation of yeast ectoapyrase ynd1p activity by activator subunit Vma13p of vacuolar H+-ATPase

CD39-like ectoapyrases are involved in protein and lipid glycosylation in the Golgi lumen of Saccharomyces cerevisiae. By using a two-hybrid screen, we found that an activator subunit (Vma13p) of yeast vacuolar H(+)-ATPase (V-ATPase) binds to the cytoplasmic domain of Ynd1p, a yeast ectoapyrase. Interaction of Ynd1p with Vma13p was demonstrated by direct binding and co-immunoprecipitation. Surprisingly, the membrane-bound ADPase activity of Ynd1p in a vma13Delta mutant was drastically increased compared with that of Ynd1p in VMA13 cells. A similar increase in the apyrase activity of Ynd1p was found in a vma1Delta mutant, in which the catalytic subunit A of V-ATPase is missing, and the membrane peripheral subunits including Vma13p are dissociated from the membranes. However, the E286Q mutant of VMA1, which assembles inactive V-ATPase complex including Vma13p in the membrane, retained wild type levels of Ynd1p activity, demonstrating that the presence of Vma13p rather than the function of V-ATPase in the membrane represses Ynd1p activity. These results suggest that association of Vma13p with the cytoplasmic domain of Ynd1p regulates its apyrase activity in the Golgi lumen.

Microtubule and motor-dependent endocytic vesicle sorting in vitro

Endocytic vesicles undergo fission to sort ligand from receptor. Using quantitative immunofluorescence and video imaging, we provide the first in vitro reconstitution of receptor-ligand sorting in early endocytic vesicles derived from rat liver. We show that to undergo fission, presegregation vesicles must bind to microtubules (MTs) and move upon addition of ATP. Over 13% of motile vesicles elongate and are capable of fission. After fission, one vesicle continues to move, whereas the other remains stationary, resulting in their separation. On average, almost 90% receptor is found in one daughter vesicle, whereas ligand is enriched by approximately 300% with respect to receptor in the other daughter vesicle. Although studies performed on polarity marked MTs showed approximately equal plus and minus end-directed motility, immunofluorescence microscopy revealed that kinesins, but not dynein, were associated with these vesicles. Motility and fission were prevented by addition of 1 mM 5'-adenylylimido-diphosphate (AMP-PNP, an inhibitor of kinesins) or incubation with kinesin antibodies, but were unaffected by addition of 5 microM vanadate (a dynein inhibitor) or dynein antibodies. These studies indicate an essential role of kinesin-based MT motility in endocytic vesicle sorting, providing a system in which factors required for endocytic vesicle processing can be identified and characterized.

CLN3 protein regulates lysosomal pH and alters intracellular processing of Alzheimer's amyloid-beta protein precursor and cathepsin D in human cells

Maintenance of the appropriate pH in the intracellular vacuolar compartments is essential for normal cell function. Here, we report that CLN3 protein, which is associated with the juvenile form of neuronal ceroid lipofuscinosis (JNCL), participates in lysosomal pH homeostasis in human cells. We show that CLN3 protein increases lysosomal pH in cultured human embryonal kidney cells, whereas inhibition of CLN3 protein synthesis by antisense approach acidifies lysosomal compartments. These changes in lysosomal pH are sufficient to exert a significant biological effect and modify intracellular processing of amyloid-beta protein precursor and cathepsin D, model proteins whose metabolism is influenced by the pH of acidic organelles. Mutant CLN3 protein (R334C) that is associated with the classical JNCL phenotype was devoid of biological activities of wild-type CLN3 protein. These data suggest that the pathogenesis of juvenile neuronal ceroid lipofuscinosis is associated with altered acidification of lysosomal compartments. Furthermore, our study indicates that CLN3 protein affects metabolism of proteins essential for cell functions, such as amyloid-beta protein precursor, implicated in Alzheimer's disease pathogenesis.

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.

Rab7: a key to lysosome biogenesis

The molecular machinery behind lysosome biogenesis and the maintenance of the perinuclear aggregate of late endocytic structures is not well understood. A likely candidate for being part of this machinery is the small GTPase Rab7, but it is unclear whether this protein is associated with lysosomes or plays any role in the regulation of the perinuclear lysosome compartment. Previously, Rab7 has mainly been implicated in transport from early to late endosomes. We have now used a new approach to analyze the role of Rab7: transient expression of Enhanced Green Fluorescent Protein (EGFP)-tagged Rab7 wt and mutant proteins in HeLa cells. EGFP-Rab7 wt was associated with late endocytic structures, mainly lysosomes, which aggregated and fused in the perinuclear region. The size of the individual lysosomes as well as the degree of perinuclear aggregation increased with the expression levels of EGFP-Rab7 wt and, more dramatically, the active EGFP-Rab7Q67L mutant. In contrast, upon expression of the dominant-negative mutants EGFP-Rab7T22N and EGFP-Rab7N125I, which localized mainly to the cytosol, the perinuclear lysosome aggregate disappeared and lysosomes, identified by colocalization of cathepsin D and lysosome-associated membrane protein-1, became dispersed throughout the cytoplasm, they were inaccessible to endocytosed molecules such as low-density lipoprotein, and their acidity was strongly reduced, as determined by decreased accumulation of the acidotropic probe LysoTracker Red. In contrast, early endosomes associated with Rab5 and the transferrin receptor, late endosomes enriched in the cation-independent mannose 6-phosphate receptor, and the trans-Golgi network, identified by its enrichment in TGN-38, were unchanged. These data demonstrate for the first time that Rab7, controlling aggregation and fusion of late endocytic structures/lysosomes, is essential for maintenance of the perinuclear lysosome compartment.

Cloning and molecular characterisation of the trout (Oncorhynchus mykiss) vacuolar H(+)-ATPase B subunit

The current model of transepithelial ion movements in the gill of freshwater fish incorporates an apically oriented vacuolar H(+)-ATPase (H(+)V-ATPase; proton pump) that is believed to facilitate both acid excretion and Na(+) uptake. To substantiate this model, we have cloned and sequenced a cDNA encoding the B subunit of the rainbow trout (Oncorhynchus mykiss) H(+)V-ATPase. The cloning of the B subunit enabled an examination by northern analysis of its tissue distribution and expression during external hypercapnia. Degenerate oligonucleotide primers to the B subunit of the H(+)V-ATPase were designed and used in a semi-nested polymerase chain reaction (PCR) to amplify an 810 base pair (bp) product from a trout gill/kidney cDNA library. This PCR product was cloned and sequenced and then used to screen the same cDNA library. The assembled 2262 bp cDNA included an open reading frame coding for a deduced protein of 502 amino acid residues. A BLAST search of the GenBank nucleotide database revealed numerous matches to other vertebrate and invertebrate H(+)V-ATPase B subunits. Protein alignment demonstrated that the trout H(+)V-ATPase B subunit is more than 85 % identical and more than 90 % similar to those in other vertebrate species. An initial analysis of H(+)V-ATPase mRNA tissue distribution revealed significant expression in blood. Although a comparison of perfused tissues (blood removed) with non-perfused tissues demonstrated no obvious contribution of the blood to total tissue H(+)-ATPase mRNA levels, all subsequent experiments were performed using perfused tissues. Levels of H(+)V-ATPase mRNA expression were high in the gill, kidney (anterior or posterior), intestine, heart and spleen, but lower in liver and white muscle. Exposure of the fish to 12 h of external hypercapnia (water P(CO2)=7. 5 mmHg; 1 kPa) was associated with a transient increase (at 2 h) in the levels of H(+)V-ATPase B subunit mRNA in gill and kidney; liver mRNA levels were unaffected. These results are consistent with the hypothesis of an apically localised plasma membrane H(+)V-ATPase in the freshwater trout gill and that the expression of this proton pump is increased during periods of acidosis, at least in part because of an increased steady-state level of H(+)V-ATPase mRNA.

Regulation and reversibility of vacuolar H(+)-ATPase

Arabidopsis thaliana vacuolar H(+)-translocating pyrophosphatase (V-PPase) was expressed functionally in yeast vacuoles with endogenous vacuolar H(+)-ATPase (V-ATPase), and the regulation and reversibility of V-ATPase were studied using these vacuoles. Analysis of electrochemical proton gradient (DeltamuH) formation with ATP and pyrophosphate indicated that the proton transport by V-ATPase or V-PPase is not regulated strictly by the proton chemical gradient (DeltapH). On the other hand, vacuolar membranes may have a regulatory mechanism for maintaining a constant membrane potential (DeltaPsi). Chimeric vacuolar membranes showed ATP synthesis coupled with DeltamuH established by V-PPase. The ATP synthesis was sensitive to bafilomycin A(1) and exhibited two apparent K(m) values for ADP. These results indicate that V-ATPase is a reversible enzyme. The ATP synthesis was not observed in the presence of nigericin, which dissipates DeltapH but not DeltaPsi, suggesting that DeltapH is essential for ATP synthesis.

Ischemic preconditioning reduces Na(+) accumulation and cell killing in isolated rat hepatocytes exposed to hypoxia

Short periods of ischemia followed up by reperfusion are known to protect the heart against injury caused by a subsequent sustained ischemia. This phenomenon, known as ischemic preconditioning, has also been recently shown to reduce ischemic liver damage, but the mechanisms involved are still unknown. By using isolated hepatocytes as an in vitro model of liver preconditioning, we have investigated the possible effect of preconditioning on intracellular pH and Na(+) homeostasis. Freshly isolated rat hepatocytes were preconditioned by 10 minutes of incubation under hypoxic conditions followed up by 10 minutes of reoxygenation and subsequently exposed to 90 minutes of hypoxia. Although preconditioning did not ameliorate adenosine triphosphate (ATP) depletion, preconditioned hepatocytes exhibited an increased resistance to cell killing during hypoxic incubation. Intracellular acidosis and Na(+) accumulation developing during hypoxia were appreciably reduced in preconditioned cells. The effects of preconditioning on intracellular pH, Na(+) homeostasis, and cytotoxicity were mimicked by stimulating protein kinase C (PKC) with 4beta-phorbol-12-myristate-13-acetate (PMA) or 1,2 dioctanoyl-glycerol (1,2 DOG). Conversely, inhibiting PKC with chelerythrine or blocking vacuolar proton ATPase (V-ATPase) with bafilomycin A(1) abolished the protection given by preconditioning or by PMA treatment on hypoxic acidosis, Na(+) overload, and hepatocyte killing. Similarly, the addition of Na(+) ionophore monensin also reverted the cytoprotection exerted by preconditioning. This indicated that ischemic preconditioning of isolated hepatocytes decreased cell killing during hypoxia by preventing intracellular Na(+) accumulation. We propose that, after preconditioning, the stimulation of PKC might activate proton extrusion through V-ATPase, thus, limiting intracellular acidosis and Na(+) overload promoted by Na(+)-dependent acid buffering systems.

Alternative mechanisms of vacuolar acidification in H(+)-ATPase-deficient yeast

Acidification of the endosomal/lysosomal pathway by the vacuolar-type proton translocating ATPase (V-ATPase) is necessary for a variety of essential eukaryotic cellular functions. Nevertheless, yeasts lacking V-ATPase activity (Deltavma) are viable when grown at low pH, suggesting alternative methods of organellar acidification. This was confirmed by directly measuring the vacuolar pH by ratio fluorescence imaging. When Deltavma yeasts were cultured and tested in the acidic conditions required for growth of V-ATPase-deficient mutants, the vacuolar pH was 5.9. Fluid-phase pinocytosis of acidic extracellular medium cannot account for these observations, because the V-ATPase-independent vacuolar acidification was unaffected in mutants deficient in endocytosis. Similarly, internalization of the plasmalemmal H(+)-ATPase (Pma1p) was ruled out, because overexpression of Pma1p failed to complement the Deltavma phenotype and did not potentiate the vacuolar acidification. To test whether weak electrolytes present in the culture medium could ferry acid equivalents to the vacuole, wild-type and the Deltavma yeasts were subjected to sudden changes in extracellular pH. In both cell types, the vacuoles rapidly alkalinized when external pH was raised from 5.5 (the approximate pH of the culture medium) to 7.5 and re-acidified when the yeasts were returned to a medium of pH 5.5. Importantly, these rapid pH changes were only observed when NH(4)(+), routinely added as a nitrogen source, was present. The NH(4)(+)-dependent acidification was not due to efflux of NH(3) from the vacuole, as cells equilibrated to pH 7.5 in the absence of weak electrolytes rapidly acidified when challenged with an acidic medium containing NH(4)(+). These findings suggest that although NH(3) can act as a cell-permeant proton scavenger, NH(4)(+) may function as a protonophore, facilitating equilibration of the pH across the plasma and vacuolar membranes of yeast. The high concentration of NH(4)(+) frequently added as a nitrogen source to yeast culture media together with effective NH(4)(+) transporters thereby facilitate vacuolar acidification when cells are suspended in acidic solutions.

Catalytic properties and conformation of hydrophobized alpha-chymotrypsin incorporated into a bilayer lipid membrane

A set of artificially hydrophobized alpha-chymotrypsin derivatives, carrying 2-11 stearoyl residues per enzyme molecule, were synthesized and their catalytic parameters and conformation in water solution and in the liposome-bound state were investigated. Hydrophobization of alpha-chymotrypsin and its further incorporation into phosphatidylcholine (PC) liposomes have no effect on the rate constant of the N-acetyl-L-tyrosine ethyl ester (ATEE) ester bond hydrolysis (k(cat)). At the same time, an increase in the number of stearoyl residues attached to the enzyme results in a drastic decrease of ATEE binding to the active center (K(M) increase). Incorporation of the hydrophobized enzyme into the PC liposome membrane results in K(M) recovery to nearly that of native alpha-chymotrypsin. The above changes are accompanied by partial unfolding of the enzyme molecules observed by fluorescence measurements. The obtained results are of interest to mimic the contribution of surface hydrophobic sites in the functioning of membrane proteins.

Characterization of a vacuolar pyrophosphatase in Trypanosoma brucei and its localization to acidocalcisomes

Inorganic pyrophosphate promoted the acidification of an intracellular compartment in permeabilized procyclic trypomastigotes of Trypanosoma brucei, as measured by acridine orange uptake. The proton gradient generated by pyrophosphate was collapsed by addition of nigericin or NH(4)Cl. Pyrophosphate-driven proton translocation was stimulated by potassium ions and inhibited by KF, by the pyrophosphate analogs imidodiphosphate and aminomethylenediphosphonate (AMDP), and by the thiol reagent p-hydroxymercuribenzoate at concentrations similar to those that inhibit the plant vacuolar H(+)-pyrophosphatase (PPase). The proton translocation activity had a pH optimum around 7.5 and was partially inhibited by 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (10 microM) and unaffected by bafilomycin A(1) (40 nM), concanamycin A (5 nM), sodium o-vanadate (500 microM), oligomycin (1 microM), N-ethylmaleimide (100 microM), and KNO(3). AMDP-sensitive pyrophosphate hydrolysis was detected in both procyclic and bloodstream trypomastigotes. Measurements of acridine orange uptake in permeabilized procyclic trypomastigotes in the presence of different substrates and inhibitors suggested the presence of H(+)-ATPase, H(+)-PPase, and (ADP-dependent) H(+)/Na(+) antiport activity in the same compartment. Separation of bloodstream and procyclic trypomastigote extracts on Percoll gradients yielded fractions that contained H(+)-PPase (both stages) and H(+)/Na(+) exchanger (procyclics) activities but lacked markers for mitochondria, glycosomes, and lysosomes. The organelles in these fractions were identified by electron microscopy and X-ray microanalysis as acidocalcisomes (electron-dense vacuoles). These results provide further evidence for the unique nature of acidocalcisomes in comparison with other, previously described, organelles.

Inhibition of intravacuolar acidification by antisense RNA decreases osteoclast differentiation and bone resorption in vitro

The role of proton transport and production in osteoclast differentiation was studied in vitro by inhibiting the transcription/translation of carbonic anhydrase II (CA II) and vacuolar H(+)-ATPase (V-ATPase) by antisense RNA molecules. Antisense RNAs targeted against CA II, or the 16 kDa or 60 kDa subunit of V-ATPase were used to block the expression of the specific proteins. A significant decrease in bone resorption rate and TRAP-positive osteoclast number was seen in rat bone marrow cultures and fetal mouse metacarpal cultures after antisense treatment. Intravacuolar acidification in rat bone marrow cells was also significantly decreased after antisense treatment. The CA II antisense RNA increased the number of TRAP-positive mononuclear cells, suggesting inhibition of osteoclast precursor fusion. Antisense molecules decreased the number of monocytes and macrophages, but increased the number of granulocytes in marrow cultures. GM-CSF, IL-3 and IL-6 were used to stimulate haematopoietic stem cell differentiation. The 16 kDa V-ATPase antisense RNA abolished the stimulatory effect of GM-CSF, IL-3 and IL-6 on TRAP-positive osteoclast formation, but did not affect the formation of monocytes and macrophages after IL-3 treatment, or the formation of granulocytes after IL-6 treatment. These results suggest that CA II and V-ATPase are needed, not only for the actual resorption, but also for osteoclast formation in vitro.