Inhibitory effect of modified bafilomycins and concanamycins on P- and V-type adenosinetriphosphatases

A fast Ca2+-induced Ca2+-release mechanism in Dictyostelium discoideum

In vertebrate cells calcium-induced calcium release (CICR) is thought to be responsible for rapid cytosolic Ca(2+) elevations despite the occurrence of strong Ca(2+) buffering within the cytosol. In Dictyostelium, a CICR mechanism has not been reported. While analyzing Ca(2+) regulation in a vesicular fraction of Dictyostelium rich in Ca(2+)-flux activity, containing contractile vacuoles (CV) as the main component of acidic Ca(2+) stores and ER, we detected a rapid Ca(2+) change upon addition of Ca(2+) (CIC). CIC was three times larger in active stores accumulating Ca(2+) than before Ca(2+) uptake and in inactivated stores. Ca(2+) release was demonstrated with the calmodulin antagonist W7 that inhibits the V-type H(+)ATPase activity and Ca(2+) uptake of acidic Ca(2+) stores. W7 caused a rapid and large increase of extravesicular Ca(2+) ([Ca(2+)](e)), much faster and larger than thapsigargin (Tg), a Ca(2+)-uptake inhibitor of the ER. W7 treatment blocked CIC indicating that a large part of CIC is due to Ca(2+) release. The height of CIC depended on the filling state of the Ca(2+) stores. CIC was virtually unchanged in the iplA(-) strain that lacks a putative IP(3) or ryanodine receptor thought to be located at the endoplasmic reticulum. By contrast, CIC was reduced in two mutants, HGR8 and lvsA(-), that are impaired in acidic Ca(2+)-store function. Purified Ca(2+) stores enriched in CV still displayed CIC, indicating that CV are a source of Ca(2+)-induced Ca(2+) release. CIC-defective mutants were altered in their oscillatory properties. The irregularity of the HGR8 oscillation suggests that the principal oscillator is affected in this mutant.

A vacuolar-type H+-ATPase and a Na+/H+exchanger contribute to intracellular pH regulation in cockroach salivary ducts

Cells of the dopaminergically innervated salivary ducts in the cockroach Periplaneta americana have a vacuolar-type H+-ATPase(V-ATPase) of unknown function in their apical membrane. We have studied whether dopamine affects intracellular pH (pHi) in duct cells and whether and to what extent the apical V-ATPase contributes to pHiregulation. pHi measurements with double-barrelled pH-sensitive microelectrodes and the fluorescent dye BCECF have revealed: (1) the steady-state pHi is 7.3±0.1; (2) dopamine induces a dose-dependent acidification up to pH 6.9±0.1 at 1 μmol l–1 dopamine, EC50 at 30 nmol l–1dopamine; (3) V-ATPase inhibition with concanamycin A or Na+-free physiological saline (PS) does not affect the steady-state pHi; (4)concanamycin A, Na+ -free PS and Na+/H+exchange inhibition with 5-(N-ethyl-N-isopropyl)-amiloride(EIPA) each reduce the rate of pHi recovery from a dopamine-induced acidification or an acidification induced by an NH4Cl pulse; (5)pHi recovery after NH4Cl-induced acidification is almost completely blocked by concanamycin A in Na+-free PS or by concanamycin A applied together with EIPA; (6) pHi recovery after dopamine-induced acidification is also completely blocked by concanamycin A in Na+-free PS but only partially blocked by concanamycin A applied together with EIPA. We therefore conclude that the apical V-ATPase and a basolateral Na+/H+ exchange play a minor role in steady-state pHi regulation but contribute both to H+extrusion after an acute dopamine- or NH4Cl-induced acid load.

Characterisation of exogenous folate transport in Plasmodium falciparum

Folate salvage by Plasmodium falciparum is an important source of key cofactors, but little is known about the underlying mechanism. Using synchronised parasite cultures, we observed that uptake of this dianionic species against the negative-inward electrochemical gradient is highly dependent upon cell-cycle stage, temperature and pH, but not on mono- or divalent metal ions. Energy dependence was tested with different sugars; glucose was necessary for folate import, although fructose was also able to function in this role, unlike sugars that cannot be processed through the glycolytic pathway. Import into both infected erythrocytes and free parasites was strongly inhibited by the anion-channel blockers probenecid and furosemide, which are likely to be acting predominantly on specific folate transporters in both cases. Import was not affected by high concentrations of the antifolate drugs pyrimethamine and sulfadoxine, but was inhibited by the close folate analogue methotrexate. The pH optimum for folate uptake into infected erythrocytes was 6.5–7.0. Dinitrophenol and nigericin, which strongly facilitate the equilibration of H⁺ ions across biological membranes and thus abolish or substantially reduce the proton gradient, inhibited folate uptake profoundly. The ATPase inhibitor concanamycin A also greatly reduced folate uptake, further demonstrating a link to ATP-powered proton transport. These data strongly suggest that the principal folate uptake pathway in P. falciparum is specific, highly regulated, dependent upon the proton gradient across the parasite plasma membrane, and is likely to be mediated by one or more proton symporters.

VAMP4 cycles from the cell surface to the trans-Golgi network via sorting and recycling endosomes

VAMP4 is enriched in the trans-Golgi network (TGN) and functions in traffic from the early and recycling endosomes to the TGN, but its trafficking itinerary is unknown. Cells stably expressing TGN-enriched VAMP4 C-terminally-tagged with EGFP (VAMP4-EGFP) are able to internalize and transport EGFP antibody efficiently to the TGN, suggesting that VAMP4-EGFP cycles between the cell surface and the TGN. The N-terminal extension of VAMP4 endows a chimeric VAMP5 with the ability to cycle from the surface to the TGN. Detailed time-course analysis of EGFP antibody transport to the TGN as well as pharmacological and thermal perturbation experiments suggest that VAMP4-EGFP is endocytosed by clathrin-dependent pathways and is delivered to the sorting and then recycling endosomes. This is followed by a direct transport to the TGN, without going through the late endosome. The di-Leu motif of the TGN-targeting signal is important for internalization, whereas the acidic cluster is crucial for efficient delivery of internalized antibody from the endosome to the TGN. These results suggest that the TGN-targeting signal of VAMP4 mediates the efficient recycling of VAMP4 from the cell surface to the TGN via the sorting and recycling endosomes, thus conferring steady-state enrichment of VAMP4 at the TGN.

Colony-stimulating factor-1 increases osteoclast intracellular pH and promotes survival via the electroneutral Na/HCO3 cotransporter NBCn1

Colony-stimulating factor-1 (CSF-1) promotes the survival of osteoclasts, short-lived cells that resorb bone. Although a rise in intracellular pH (pH(i)) has been linked to inhibition of apoptosis, the effect of CSF-1 on pH(i) in osteoclasts has not been reported. The present study shows that, in the absence of CO(2)/HCO(3)(-), CSF-1 causes little change in osteoclast pH(i). In contrast, exposing these cells to CSF-1 in the presence of CO(2)/HCO(3)(-) causes a rapid and sustained cellular alkalinization. The CSF-1-induced rise in pH(i) is not blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, an inhibitor of HCO(3)(-) transporters but is abolished by removing extracellular sodium. This inhibition profile is similar to that of the electroneutral Na/HCO(3) cotransporter NBCn1. By RT-PCR, NBCn1 transcripts are present in both osteoclasts and osteoclast-like cells (OCLs), and by immunoblotting, the protein is present in OCLs. Moreover, CSF-1 promotes osteoclast survival in the presence of CO(2)/HCO(3)(-) buffer but not in its absence. Preventing the activation of NBCn1 markedly attenuates the ability of CSF-1 to 1) block activation of caspase-8 and 2) prolong osteoclast survival. Inhibiting caspase-3 or caspase-8 in OCLs prolongs osteoclast survival to the same extent as does CSF-1. This study provides the first evidence that osteoclasts express a CSF-1-regulated Na/HCO(3) cotransporter, which may play a role in cell survival.

Diploids heterozygous for a vma13Delta mutation in Saccharomyces cerevisiae highlight the importance of V-ATPase subunit balance in supporting vacuolar acidification and silencing cytosolic V1-ATPase activity

The V-ATPase H subunit (encoded by the VMA13 gene) activates ATP-driven proton pumping in intact V-ATPase complexes and inhibits MgATPase activity in cytosolic V1 sectors (Parra, K. J., Keenan, K. L., and Kane, P. M. (2000) J. Biol. Chem. 275, 21761-21767). Yeast diploids heterozygous for a vma13Delta mutation show the pH- and calcium-dependent conditional lethality characteristic of mutants lacking V-ATPase activity, although they still contain one wild-type copy of VMA13. Vacuolar vesicles from this strain have approximately 50% of the ATPase activity of those from a wild-type diploid but do not support formation of a proton gradient. Compound heterozygotes with a second heterozygous deletion in another V1 subunit gene exhibit improved growth, vacuolar acidification, and ATP-driven proton transport in vacuolar vesicles. In contrast, compound heterozygotes with a second deletion in a Vo subunit grow even more poorly than the vma13Delta heterozygote, have very little vacuolar acidification, and have very low levels of V-ATPase subunits in isolated vacuoles. In addition, cytosolic V1 sectors from this strain and from the strain containing only the heterozygous vma13Delta mutation have elevated MgATPase activity. The results suggest that balancing levels of subunit H with those of other V-ATPase subunits is critical, both for allowing organelle acidification and for preventing unproductive hydrolysis of cytosolic ATP.

Biosynthetic Origin of the Methoxyl Extender Unit in Bafilomycin and Concanamycin using Stereospecifically Labeled Precursors

The microbial macrolides bafilomycin A1, B, and concanamycin A from Streptomyces spp. are potent and specific inhibitors of V-ATPases. The question of the biosynthetic origin of the two uncommon "glycolate units" of each of the macrolide structures was addressed by feeding experiments with stereospecifically 13C-labeled precursors. Our studies clearly indicate that glycerol is a source for the methoxylated C2-units and determines the orientation of the incorporation. Products from the carboxylic acid pool or TCA cycle are ruled out as key precursors. The data suggest the action of a glycerol kinase and point to phosphoglycerate as an intermediate in their biosynthesis. However, glycerate itself is not accepted as a precursor. We present the likely biosynthetic pathway and show the value of stereospecifically labeled presursors as an important tool for biosynthetic investigations.

Intracellular survival of Burkholderia cenocepacia in macrophages is associated with a delay in the maturation of bacteria-containing vacuoles

Strains of the Burkholderia cepacia complex (Bcc) are opportunistic bacteria that can cause life-threatening infections in patients with cystic fibrosis and chronic granulomatous disease. Previous work has shown that Bcc isolates can persist in membrane-bound vacuoles within amoeba and macrophages without bacterial replication, but the detailed mechanism of bacterial persistence is unknown. In this study, we have investigated the survival of the Burkholderia cenocepacia strain J2315 within RAW264.7 murine macrophages. Strain J2315 is a prototypic isolate of the widespread and transmissible ET12 clone. Unlike heat-inactivated bacteria, which reach lysosomes shortly after internalization, vacuoles containing live B. cenocepacia J2315 accumulate the late endosome/lysosome marker LAMP-1 and start fusing with lysosomal compartments only after 6 h post internalization. Using fluorescent fluid-phase probes, we also demonstrated that B. cenocepacia-containing vacuoles continued to interact with newly formed endosomes, and maintained a luminal pH of 6.4 +/- 0.12. In contrast, vacuoles containing heat-inactivated bacteria had an average pH of 4.8 +/- 0.03 and rapidly merged with lysosomes. Additional experiments using concanamycin A, a specific inhibitor of the vacuolar H+-ATPase, revealed that vacuoles containing live bacteria did not exclude the H+-ATPase. This mode of bacterial survival did not require type III secretion, as no differences were found between wild type and a type III secretion mutant strain. Collectively, our results suggest that intracellular B. cenocepacia cause a delay in the maturation of the phagosome, which may contribute to facilitate bacterial escape from the microbicidal activities of the host cell.

Degradation of oxidized proteins by autophagy during oxidative stress in Arabidopsis

Upon encountering oxidative stress, proteins are oxidized extensively by highly reactive and toxic reactive oxidative species, and these damaged, oxidized proteins need to be degraded rapidly and effectively. There are two major proteolytic systems for bulk degradation in eukaryotes, the proteasome and vacuolar autophagy. In mammalian cells, the 20S proteasome and a specific type of vacuolar autophagy, chaperone-mediated autophagy, are involved in the degradation of oxidized proteins in mild oxidative stress. However, little is known about how cells remove oxidized proteins when under severe oxidative stress. Using two macroautophagy markers, monodansylcadaverine and green fluorescent protein-AtATG8e, we here show that application of hydrogen peroxide or the reactive oxidative species inducer methyl viologen can induce macroautophagy in Arabidopsis (Arabidopsis thaliana) plants. Macroautophagy-defective RNAi-AtATG18a transgenic plants are more sensitive to methyl viologen treatment than wild-type plants and accumulate a higher level of oxidized proteins due to a lower degradation rate. In the presence of a vacuolar H(+)-ATPase inhibitor, concanamycin A, oxidized proteins were detected in the vacuole of wild-type root cells but not RNAi-AtATG18a root cells. Together, our results indicate that autophagy is involved in degrading oxidized proteins under oxidative stress conditions in Arabidopsis.

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.

Identification of a Domain in the Vo Subunit d That Is Critical for Coupling of the Yeast Vacuolar Proton-translocating ATPase

Vacuolar proton-translocating ATPase pumps consist of two domains, V(1) and V(o). Subunit d is a component of V(o) located in a central stalk that rotates during catalysis. By generating mutations, we showed that subunit d couples ATP hydrolysis and proton transport. The mutation F94A strongly uncoupled the enzyme, preventing proton transport but not ATPase activity. C-terminal mutations changed coupling as well; ATPase activity was decreased by 59-72%, whereas proton transport was not measurable (E328A) or was moderately reduced (E317A and C329A). Except for W325A, which had low levels of V(1)V(o), mutations allowed wild-type assembly regardless of the fact that subunits E and d were reduced at the membrane. N- and C-terminal deletions of various lengths were inhibitory and gradually destabilized subunit d, limiting V(1)V(o) formation. Both N and C terminus were required for V(o) assembly. The N-terminal truncation 2-19Delta prevented V(1)V(o) formation, although subunit d was available. The C terminus was required for retention of subunits E and d at the membrane. In addition, the C terminus of its bacterial homolog (subunit C from T. thermophilus) stabilized the yeast subunit d mutant 310-345Delta and allowed assembly of the rotor structure with subunits A and B. Structural features conserved between bacterial and eukaryotic subunit d and the significance of domain 3 for vacuolar proton-translocating ATPase function are discussed.

Oxidized ATP protection against anthrax lethal toxin

Bacillus anthracis lethal toxin (LT) induces rapid lysis (<90 min) of murine macrophages from certain inbred strains. The mechanism for LT-induced cytolysis is currently unknown. We hypothesized that the ATP-activated macrophage P2X7 receptors implicated in nucleotide-mediated macrophage lysis could play a role in LT-mediated cytolysis and discovered that a potent P2X7 antagonist, oxidized ATP (o-ATP), protects macrophages against LT. Other P2X7 receptor antagonists, however, had no effect on LT function, while oxidized nucleotides, o-ADP, o-GTP, and o-ITP, which did not act as receptor ligands, provided protection. Cleavage of the LT substrates, the mitogen-activated protein kinases, was inhibited by o-ATP in RAW274.6 macrophages and CHO cells. We investigated the various steps in the intoxication pathway and found that binding of the protective-antigen (PA) component of LT to cells and the enzymatic proteolytic ability of the lethal factor (LF) component of LT were unaffected by o-ATP. Instead, the drug inhibited formation of the sodium dodecyl sulfate-resistant PA oligomer, which occurs in acidified endosomes, but did not prevent cell surface PA oligomerization, as evidenced by binding and translocation of LF to a protease-resistant intracellular location. We found that o-ATP also protected cells from anthrax edema toxin and diphtheria toxin, which also require an acidic environment for escape from endosomes. Confocal microscopy using pH-sensitive fluorescent dyes showed that o-ATP increased endosomal pH. Finally, BALB/cJ mice injected with o-ATP and LT were completely protected against lethality.

A model for the proteolipid ring and bafilomycin/concanamycin-binding site in the vacuolar ATPase of Neurospora crassa

The vacuolar ATPase has been implicated in a variety of physiological processes in eukaryotic cells. Bafilomycin and concanamycin, highly potent and specific inhibitors of the vacuolar ATPase, have been widely used to investigate the enzyme. Derivatives have been developed as possible therapeutic drugs. We have used random mutagenesis and site-directed mutagenesis to identify 23 residues in the c subunit involved in binding these drugs. We generated a model for the structure of the ring of c subunits in Neurospora crassa by using data from the crystal structure of the homologous subunits of the bacterium Enterococcus hirae (Murata, T., Yamato, I., Kakinuma, Y., Leslie, A. G., and Walker, J. E. (2005) Science 308, 654-659). In the model 10 of the 11 mutation sites that confer the highest degree of resistance are closely clustered. They form a putative drug-binding pocket at the interface between helices 1 and 2 on one c subunit and helix 4 of the adjacent c subunit. The excellent fit of the N. crassa sequence to the E. hirae structure and the degree to which the structural model predicts the clustering of these residues suggest that the folding of the bacterial and eukaryotic polypeptides is very similar.

Nitric Oxide Production by the Vacuolar-Type (H+)-ATPase Inhibitors Bafilomycin A1 and Concanamycin A and Its Possible Role in Apoptosis in RAW 264.7 Cells

In the mouse leukemic monocyte cell line RAW 264.7, the vacuolar-type (H(+))-ATPase (V-ATPase) inhibitors bafilomycin A1 and concanamycin A induced nitric oxide (NO) production through the expression of inducible nitric-oxide synthase mRNA and its protein and decreased cell growth and survival as determined by 3-(4,5-dimethyl(thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Bafilomycin A1 and concanamycin A activated nuclear factor (NF)-kappaB and activator protein-1 and decreased the level of IkappaB-alpha and increased that of phosphorylated c-Jun N-terminal kinase (JNK). NO production induced by these V-ATPase inhibitors was suppressed by the NF-kappaB inhibitor Bay 11-7082 [(E)3-[(4-methylphenyl)sulfonyl])-2-propenenitrile] and the JNK inhibitor SP600125 [anthra[1,9-cd]pyrazol-6(2H)-one] in parallel with the partial alleviation of the V-ATPase inhibitor-induced decrease in MTT response. The Na(+),K(+)-ATPase inhibitor dibucaine and the F-ATPase inhibitor oligomycin did not induce NO production at which concentrations the MTT response was decreased. The NO donor S-nitroso-N-acetyl-dl-penicillamine further lowered the V-ATPase inhibitor-induced decrease in the MTT response, and the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, sodium salt (carboxy-PTIO) alleviated it partially. Mitochondrial depolarization, an index of apoptosis, was induced by bafilomycin A1 and concanamycin A. On treatment with the nitric-oxide synthase inhibitor N(G)-monomethyl-l-arginine acetate, the disruption of mitochondrial membrane potential induced by bafilomycin A1 and concanamycin A was alleviated partially in parallel with the decrease in NO production. Carboxy-PTIO also alleviated it partially. Our findings suggest that the V-ATPase inhibitors bafilomycin A1 and concanamycin A similarly induce NO production and the newly produced NO participates partially in the V-ATPase inhibitor-induced apoptosis in RAW 264.7 cells.

Fluorescence measurements of serotonin-induced V-ATPase-dependent pH changes at the luminal surface in salivary glands of the blowfly Calliphora vicina

Secretion in blowfly salivary glands is induced by the neurohormone serotonin and powered by a vacuolar-type H(+)-ATPase (V-ATPase) located in the apical membrane of the secretory cells. We have established a microfluorometric method for analysing pH changes at the luminal surface of the secretory epithelial cells by using the fluorescent dye 5-N-hexadecanoyl-aminofluorescein (HAF). After injection of HAF into the lumen of the tubular salivary gland, the fatty acyl chain of the dye molecule partitions into the outer leaflet of the plasma membrane and its pH-sensitive fluorescent moiety is exposed at the cell surface. Confocal imaging has confirmed that HAF distributes over the entire apical membrane of the secretory cells and remains restricted to this membrane domain. Ratiometric analysis of HAF fluorescence demonstrates that serotonin leads to a reversible dose-dependent acidification at the luminal surface. Inhibition by concanamycin A confirms that the serotonin-induced acidification at the luminal surface is due to H(+) transport across the apical membrane via V-ATPase. Measurements with pH-sensitive microelectrodes corroborate a serotonin-induced luminal acidification and demonstrate that luminal pH decreases by about 0.4 pH units at saturating serotonin concentrations. We conclude that ratiometric measurements of HAF fluorescence provide an elegant method for monitoring V-ATPase-dependent H(+) transport in the blowfly salivary gland in vivo and for analysing the spatiotemporal pattern of pH changes at the luminal surface.

Enantioselective total synthesis of FD-891

The enantioselective synthesis of FD-891 has been achieved with a longest linear sequence of 21 steps. The synthetic strategy involves the use of aldol additions of a chlorotitanium enolate of N-acylthiazolidinethiones as the key reaction to establish 6 of the 10 stereogenic centers. A key cross-metathesis and a late-stage Julia olefination serve to assemble three key subunits.

Preferential energy- and potential-dependent accumulation of cisplatin-gutathione complexes in human cancer cell lines (GLC4 and K562): A likely role of mitochondria

cis-Diamminedichloroplatinum(II) (CDDP) is an important chemotherapeutic agent used in the treatment of a wide variety of solid tumors. We have recently shown that aquated forms of cisplatin (aqua-Pt) rapidly accumulate in K562 and GLC4 cultured cells, in comparison to CDDP. Thus, when cells are incubated with aquated forms of cisplatin a gradient of concentration is observed after a short time, approximately 40 min, with an intracellular concentration of aqua-Pt of 20-30 times higher than that of extracellular aqua-Pt. The same gradient of concentration is observed when cells are incubated with CDDP but it takes a longer time, i.e., about 24 h. Therefore, the question arises as to the identity of the intracellular sites of accumulation of aqua-Pt. Using several agents to modulate membrane potential, acidic compartment pH and/or ATP level, we obtained evidence that aqua-Pt may accumulate rapidly inside mitochondria as this accumulation is energy- and membrane-potential-dependent. However, aqua-Pt complexes are not characterized by a delocalized charge and a lipophilic character that would permit their movement through the inner membrane. Therefore, it is suggested that intracellular aqua-Pt reacts rapidly with glutathione with the resultant complex being transported inside the mitochondria via one of the known glutathione transporters, i.e., dicarboxylate and/or 2-oxoglutarate transporters present in the inner membrane.

Early, H+-V-ATPase-dependent proton flux is necessary for consistent left-right patterning of non-mammalian vertebrates

Biased left-right asymmetry is a fascinating and medically important phenomenon. We provide molecular genetic and physiological characterization of a novel, conserved, early, biophysical event that is crucial for correct asymmetry: H+ flux. A pharmacological screen implicated the H+-pump H+-V-ATPase in Xenopus asymmetry, where it acts upstream of early asymmetric markers. Immunohistochemistry revealed an actin-dependent asymmetry of H+-V-ATPase subunits during the first three cleavages. H+-flux across plasma membranes is also asymmetric at the four- and eight-cell stages, and this asymmetry requires H+-V-ATPase activity. Abolishing the asymmetry in H+ flux, using a dominant-negative subunit of the H+-V-ATPase or an ectopic H+ pump, randomized embryonic situs without causing any other defects. To understand the mechanism of action of H+-V-ATPase, we isolated its two physiological functions, cytoplasmic pH and membrane voltage (Vmem) regulation. Varying either pH or Vmem, independently of direct manipulation of H+-V-ATPase, caused disruptions of normal asymmetry, suggesting roles for both functions. V-ATPase inhibition also abolished the normal early localization of serotonin, functionally linking these two early asymmetry pathways. The involvement of H+-V-ATPase in asymmetry is conserved to chick and zebrafish. Inhibition of the H+-V-ATPase induces heterotaxia in both species; in chick, H+-V-ATPase activity is upstream of Shh; in fish, it is upstream of Kupffer's vesicle and Spaw expression. Our data implicate H+-V-ATPase activity in patterning the LR axis of vertebrates and reveal mechanisms upstream and downstream of its activity. We propose a pH- and Vmem-dependent model of the early physiology of LR patterning.

Inhibition of vacuolar-type (H+)-ATPase by the cytostatic macrolide apicularen A and its role in apicularen A-induced apoptosis in RAW 264.7 cells

Apicularen A and the known vacuolar-type (H(+))-ATPase (V-ATPase) inhibitor bafilomycin A(1) induced apoptosis of RAW 264.7 cells, while apicularen B, an N-acetyl-glucosamine glycoside of apicularen A, was far less effective. Apicularen A inhibited vital staining with acridine orange of the intracellular organelles of RAW 264.7 cells, inhibited the ATP-dependent proton transport into inside-out microsome vesicles, and inhibited the bafilomycin A(1)-sensitive ATP hydrolysis. The IC(50) values of the proton transport were 0.58 nM for apicularen A, 13 nM for apicularen B, and 0.95 nM for bafilomycin A(1). Furthermore, apicularen A inhibited the bafilomycin A(1)-sensitive ATP hydrolysis more potently than apicularen B. F-ATPase and P-ATPase were not inhibited by apicularen A. We concluded that apicularen A inhibits V-ATPase, and thus induces apoptosis in RAW 264.7 cells.

Analysis of the pH requirement for membrane fusion of different isolates of the paramyxovirus parainfluenza virus 5

Paramyxoviruses enter cells by fusing their envelopes with the plasma membrane, a process that occurs at neutral pH. Recently, it has been found that there is an exception to this dogma in that a porcine isolate of the paramyxovirus parainfluenza virus 5 (PIV5), known as SER, requires a low-pH step for fusion (S. Seth, A. Vincent, and R. W. Compans, J. Virol. 77: 6520-6527, 2003). As a low-pH activation mechanism for fusion would greatly facilitate biophysical studies of paramyxovirus-mediated membrane fusion, we have reexamined the triggering of the PIV5 SER fusion protein. Using multiple assays, we could not find a requirement for low-pH triggering of PIV5 SER fusion. The challenge of discovering how the paramyxovirus receptor binding protein (HN, H, or G) activates the metastable fusion protein to cause membrane fusion at neutral pH remains.

Significant Inhibition of Human CD8+ Cytotoxic T Lymphocyte-Mediated Xenocytotoxicity by Overexpression of the Human Decoy Fas Antigen

BACKGROUND

Human CD8(+) CTL-mediated killing may be important for xenograft rejection. The purpose of this study was to explore the preventing methods for CTL-mediated xenocytotoxicity by overexpression of human decoy Fas, which lacks a death domain in its cytoplasmic region, by binding competition with endogenous pig Fas. Moreover, the cytoprotective effect of this CTL-killing of membrane-bound human FasL, which is resistant to metalloproteolytic cleavage, was also assessed.

METHODS

Human CTL were generated by the stimulation of human PBMC with swine endothelial cells (SEC) and human IL-2, subsequently a CD8(+) population were selected by magnetic beads and employed as the effector cells. Stable SEC transfectants expressing either decoy Fas or membrane-bound FasL were established. Double-transfectants were also created. The amelioration of cytotoxicity to these transfectants was examined with Cr release assay. RESULTS.: Human CD8(+) CTL were highly detrimental against parental SEC. This CTL-killing was strongly inhibited by anti-FasL mAb treatment, however partial suppression was observed by Concanamycin A treatment. The overexpression of either decoy Fas or membrane-bound FasL in SEC markedly inhibited CTL-xenocytotoxicity. The double expressions of these molecules also significantly reduced this xenocytotoxicity despite the low levels of expression of either decoy Fas or membrane-bound FasL.

CONCLUSION

These findings indicate that the strong xenocytotoxicity of human CD8(+) CTL is mediated mainly by the Fas/FasL pathway. The overexpression of either decoy Fas or membrane-bound FasL were quite effective in preventing CTL-killing. Furthermore, the combined expression of both molecules in pig cells may create a window of opportunity for prolonging xenograft survival.

Vacuolar H+-ATPase activity is required for endocytic and secretory trafficking in Arabidopsis

In eukaryotic cells, compartments of the highly dynamic endomembrane system are acidified to varying degrees by the activity of vacuolar H(+)-ATPases (V-ATPases). In the Arabidopsis thaliana genome, most V-ATPase subunits are encoded by small gene families, thus offering potential for a multitude of enzyme complexes with different kinetic properties and localizations. We have determined the subcellular localization of the three Arabidopsis isoforms of the membrane-integral V-ATPase subunit VHA-a. Colocalization experiments as well as immunogold labeling showed that VHA-a1 is preferentially found in the trans-Golgi network (TGN), the main sorting compartment of the secretory pathway. Uptake experiments with the endocytic tracer FM4-64 revealed rapid colocalization with VHA-a1, indicating that the TGN may act as an early endosomal compartment. Concanamycin A, a specific V-ATPase inhibitor, blocks the endocytic transport of FM4-64 to the tonoplast, causes the accumulation of FM4-64 together with newly synthesized plasma membrane proteins, and interferes with the formation of brefeldin A compartments. Furthermore, nascent cell plates are rapidly stained by FM4-64, indicating that endocytosed material is redirected into the secretory flow after reaching the TGN. Together, our results suggest the convergence of the early endocytic and secretory trafficking pathways in the TGN.

Receptor-mediated entry by equine infectious anemia virus utilizes a pH-dependent endocytic pathway

Previous studies of human and nonhuman primate lentiviral entry mechanisms indicate a predominant use of pH-independent pathways, although more recent studies of human immunodeficiency virus type 1 entry appear to reveal the use of a low-pH-dependent entry pathway in certain target cells. To expand the characterization of the specificity of lentiviral entry mechanisms, we have in the current study examined the entry pathway of equine infectious anemia virus (EIAV) during infection of its natural target, equine macrophages, permissive equine fibroblastic cell lines, and an engineered mouse cell line expressing the recently defined equine lentivirus receptor-1. The specificity of EIAV entry into these various cells was determined by assaying the effects of specific drug treatments on the level of virus entry as measured by quantitative real-time PCR assay of early reverse transcripts or by measurements of virion production. The results of these studies demonstrated that EIAV entry into all cell types was substantially inhibited in a dose-dependent manner by treatment with the vacuolar H+-ATPase inhibitors concanamycin A and bafilomycin A1 or the lysosomotropic weak base ammonium chloride. In contrast, treatments with sucrose to block clathrin-mediated endocytosis or with chloroquine to block organelle acidification failed to inhibit EIAV entry into the same target cells. The observed inhibition of EIAV entry was shown not to be related to cytotoxicity. Taken together, these experiments reveal for the first time that EIAV receptor-mediated entry into target cells is via a low-pH-dependent endocytic pathway.

Bafilomycin A1 inhibits chloroquine-induced death of cerebellar granule neurons

Treatment of cells with the macrolide antibiotic bafilomycin A1, an inhibitor of vacuolar (V)-ATPase, or with the lysosomotropic agent chloroquine, has been shown to pharmacologically inhibit autophagy as evidenced by an accumulation of autophagosomes, which in turn causes Bax-dependent apoptosis. However, bafilomycin A1 has also been reported to inhibit chloroquine-induced apoptosis, suggesting a complex interrelationship between these two inhibitors of autophagy. To determine whether the cytoprotective effect of bafilomycin A1 on chloroquine-treated cells was dependent on inhibition of V-ATPase, we examined the single and combined effects of bafilomycin and chloroquine on cultured cerebellar granule neurons. When added separately, chloroquine or high concentrations of bafilomycin A1 (> or =10 nM) induced a dose-dependent inhibition of autophagy (as measured by an increase in LC3-II, a marker specific for autophagosomes), followed by caspase-3 activation and cell death. When added in combination, bafilomycin A1 potently inhibited chloroquine-induced caspase-3 activity and cell death at concentrations (< or =1 nM) that neither altered vacuolar acidification nor inhibited autophagy. The neuroprotective effects of bafilomycin A1 against chloroquine were substantially greater than those produced by Bax deficiency. Bafilomycin A1-induced neuroprotection seemed to be stimulus-specific, in that staurosporine-induced death was not attenuated by coaddition of bafilomycin A1. Together, these data suggest that in addition to promoting death via inhibition of V-ATPase and autophagy, bafilomycin A1 possesses novel, neuroprotective properties that inhibit Bax-dependent activation of the intrinsic apoptotic pathway resulting from the pharmacological inhibition of autophagy.

V-ATPases as Drug Targets

V-ATPases are large, complex enzymes responsible for acidification of many internal compartments in eukaryotic cells. They also occur on plasma membranes of specialized cells, where they acidify the surrounding milieu. Numerous physiological processes depend on the activity of V-ATPases, and V-ATPases are implicated as a contributing factor in multiple diseases, including osteoporosis, deafness, and cancer. Three classes of natural products have been identified as potent inhibitors of V-ATPases. The bafilomycins and concanamycins, which inhibit all known eukaryotic V-ATPases, are the most extensively studied inhibitors. They bind the Vo subunit c and may inhibit the enzyme by preventing rotation of the c subunit ring. The salicylihalamides and lobatamides show remarkable specificity for animal V-ATPases. The chondropsins preferentially inhibit the fungal V-ATPase. Because of the variety of processes and diseases associated with V-ATPases and the possibility of designing selective inhibitors, the V-ATPases are attractive targets for development of therapeutic agents.

The role of pH dynamics and the Na+/H+ antiporter in the etiopathogenesis and treatment of cancer. Two faces of the same coin--one single nature

Looked at from the genetic point-of-view cancer represents a daunting and, frankly, confusing multiplicity of diseases (at least 100) that require an equally large variety of therapeutic strategies and substances designed to treat the particular tumor. However, when analyzed phenotypically cancer is a relatively uniform disease of very conserved 'hallmark' behaviors across the entire spectrum of tissue and genetic differences [D. Hanahan, R.A. Weinberg, Hallmarks of cancer, Cell 100 (2000) 57-70]. This suggests that cancers do, indeed, share common biochemical and physiological characteristics that are independent of the varied genetic backgrounds, and that there may be a common mechanism underlying both the neoplastic transformation/progression side and the antineoplastic/therapy side of oncology. The challenge of modern oncology is to integrate all the diverse experimental data to create a physiological/metabolic/energetic paradigm that can unite our thinking in order to understand how both neoplastic progression and therapies function. This reductionist view gives the hope that, as in chemistry and physics, it will possible to identify common underlying driving forces that define a tumor and will permit, for the first time, the actual calculated manipulation of their state. That is, a rational therapeutic design. In the present review, we present evidence, obtained from a great number of studies, for a fundamental, underlying mechanism involved in the initiation and evolution of the neoplastic process. There is an ever growing body of evidence that all the important neoplastic phenotypes are driven by an alkalization of the transformed cell, a process which seems specific for transformed cells since the same alkalinization has no effect in cells that have not been transformed. Seen in that light, different fields of cancer research, from etiopathogenesis, cancer cell metabolism and neovascularization, to multiple drug resistance (MDR), selective apoptosis, modern cancer chemotherapy and the spontaneous regression of cancer (SRC) all appear to have in common a pivotal characteristic, the aberrant regulation of hydrogen ion dynamics [S. Harguindey, J.L. Pedraz, R. García Cañero, J. Pérez de Diego, E.J. Cragoe Jr., Hydrogen ion-dependent oncogenesis and parallel new avenues to cancer prevention and treatment using a H+-mediated unifying approach: pH-related and pH-unrelated mechanisms, Crit. Rev. Oncog. 6 (1) (1995) 1-33]. Cancer cells have an acid-base disturbance that is completely different than observed in normal tissues and that increases in correspondence with increasing neoplastic state: an interstitial acid microenvironment linked to an intracellular alkalosis.

Subunit a of the yeast V-ATPase participates in binding of bafilomycin

Bafilomycin and concanamycin are potent and highly specific inhibitors of the vacuolar (H(+))-ATPases (V-ATPases), typically inhibiting at nanomolar concentrations. Previous studies have shown that subunit c of the integral V(0) domain participates in bafilomycin binding, and that this site resembles the oligomycin binding site of the F-ATPase (Bowman, B. J., and Bowman, E. J. (2002) J. Biol. Chem. 277, 3965-3972). Because mutations in F-ATPase subunit a also confer resistance to oligomycin, we investigated whether the a subunit of the V-ATPase might participate in binding bafilomycin. Twenty-eight subunit a mutations were constructed just N-terminal to the critical Arg(735) residue in transmembrane 7 required for proton transport, a region similar to that shown to participate in oligomycin binding by the F-ATPase. The mutants appeared to assemble normally and all but two showed normal growth at pH 7.5, whereas all but three had at least 25% of wild-type levels of proton transport and ATPase activity. Of the functional mutants, three displayed K(i) values for bafilomycin significantly different from wild-type (0.22 +/- 0.03 nm). These included E721K (K(i) 0.38 +/- 0.03 nm), L724A (0.40 +/- 0.02 nm), and N725F (0.54 +/- 0.06 nm). Only the N725F mutation displayed a K(i) for concanamycin (0.84 +/- 0.04 nm) that was slightly higher than wild-type (0.60 +/- 0.07 nm). These results suggest that subunit a of V-ATPase participates along with subunit c in binding bafilomycin.

Selective induction of apoptosis with proton pump inhibitor in gastric cancer cells

PURPOSE

To survive in an ischemic microenvironment with a lower extracellular pH, ability to up-regulate proton extrusion is critical for cancer cell survival. Gastric H+/K(+)-ATPase exchanges luminal K+ for cytoplasmic H+ and is the enzyme primarily responsible for gastric acidification. On the basis of the fact that blocking the clearance of acidic metabolites are known to induce the cell death, we hypothesized that pantoprazole (PPZ), one of gastric H+/K(+)-ATPase inhibitors used frequently to treat acid-related diseases, could inhibit growth of tumor cells.

EXPERIMENTAL DESIGN

Genomic DNA fragmentation, terminal deoxynucleotidyl transferase (Tdt)-mediated nick end labeling assay, and annexin V staining were performed to detect PPZ-induced apoptosis. Mitogen-activated protein kinase activation and heat shock proteins expression were determined by immunoblot with specific antibodies. The antitumor effect of PPZ was evaluated in vivo by a xenograft model of nude mice.

RESULTS

After PPZ treatment, apoptotic cell death was seen selectively in cancer cells and was accompanied with extracellular signal-regulated kinase deactivation. By contrast, normal gastric mucosal cells showed the resistance to PPZ-induced apoptosis through the overexpression of antiapoptotic regulators including HSP70 and HSP27. In a xenograft model of nude mice, administration of PPZ significantly inhibited tumorigenesis and induced large-scale apoptosis of tumor cells.

CONCLUSIONS

PPZ selectively induced in vivo and in vitro apoptotic cell death in gastric cancer, suggesting that proton pump inhibitors could be used for selective anticancer effects.

The B1-subunit of the H(+) ATPase is required for maximal urinary acidification

The multisubunit vacuolar-type H(+)ATPases mediate acidification of various intracellular organelles and in some tissues mediate H(+) secretion across the plasma membrane. Mutations in the B1-subunit of the apical H(+)ATPase that secretes protons in the distal nephron cause distal renal tubular acidosis in humans, a condition characterized by metabolic acidosis with an inappropriately alkaline urine. To examine the detailed cellular and organismal physiology resulting from this mutation, we have generated mice deficient in the B1-subunit (Atp6v1b1(-/-) mice). Urine pH is more alkaline and metabolic acidosis is more severe in Atp6v1b1(-/-) mice after oral acid challenge, demonstrating a failure of normal urinary acidification. In Atp6v1b1(-/-) mice, the normal urinary acidification induced by a lumen-negative potential in response to furosemide infusion is abolished. After an acute intracellular acidification, Na(+)-independent pH recovery rates of individual Atp6v1b1(-/-) intercalated cells of the cortical collecting duct are markedly reduced and show no further decrease after treatment with the selective H(+)ATPase inhibitor concanamycin. Apical expression of the alternative B-subunit isoform, B2, is increased in Atp6v1b1(-/-) medulla and colocalizes with the H(+)ATPase E-subunit; however, the greater severity of metabolic acidosis in Atp6v1b1(-/-) mice after oral acid challenge indicates that the B2-subunit cannot fully functionally compensate for the loss of B1. Our results indicate that the B1 isoform is the major B-subunit isoform that incorporates into functional, plasma membrane H(+)ATPases in intercalated cells of the cortical collecting duct and is required for maximal urinary acidification.

Endocytosis and degradation of BOR1, a boron transporter of Arabidopsis thaliana, regulated by boron availability

Boron (B) is essential for plants but toxic when present in excess. Arabidopsis thaliana BOR1 is a B exporter for xylem loading and is essential for efficient B translocation from roots to shoots under B limitation. B translocation to shoots was enhanced under B limitation in WT but not in bor1-1 mutant plants. The enhanced translocation was suppressed upon resupply of high levels of B within several hours. Unlike a number of transporters for essential mineral nutrients, BOR1 mRNA accumulation was not strongly affected by B conditions. However, accumulation of a constitutively expressed BOR1-GFP fusion protein was elevated under conditions of limited B supply. Upon resupply of high levels of B, BOR1-GFP was degraded within several hours. These findings demonstrate that posttranscriptional mechanisms play a major role in regulation of BOR1 accumulation. Confocal laser scanning microscopy of root tip cells showed that BOR1-GFP is localized to the plasma membrane under B limitation. Shortly after B application, the protein was observed in dot-like structures in the cytoplasm before degradation. Colocalization studies of the fusion protein with an endocytic tracer FM4-64 and an endosomal Rab-GTPase Ara7 fused to monomeric red fluorescent protein suggested that BOR1 is transferred from the plasma membrane via the endosomes to the vacuole for degradation. These results establish that endocytosis and degradation of BOR1 are regulated by B availability, to avoid accumulation of toxic levels of B in shoots under high-B supply, while protecting the shoot from B deficiency under B limitation.

Early events in integrin alphavbeta6-mediated cell entry of foot-and-mouth disease virus

We have shown that foot-and-mouth disease virus (FMDV) infection mediated by the integrin alphavbeta6 takes place through clathrin-dependent endocytosis but not caveolae or other endocytic pathways that depend on lipid rafts. Inhibition of clathrin-dependent endocytosis by sucrose treatment or expression of a dominant-negative version of AP180 inhibited virus entry and infection. Similarly, inhibition of endosomal acidification inhibited an early step in infection. Blocking endosomal acidification did not interfere with surface expression of alphavbeta6, virus binding to the cells, uptake of the virus into endosomes, or cytoplasmic virus replication, suggesting that the low pH within endosomes is a prerequisite for delivery of viral RNA into the cytosol. Using immunofluorescence confocal microscopy, FMDV colocalized with alphavbeta6 at the cell surface but not with the B subunit of cholera toxin, a marker for lipid rafts. At 37 degrees C, virus was rapidly taken up into the cells and colocalized with markers for early and recycling endosomes but not with a marker for lysosomes, suggesting that infection occurs from within the early or recycling endosomal compartments. This conclusion was supported by the observation that FMDV infection is not inhibited by nocodazole, a reagent that inhibits vesicular trafficking between early and late endosomes (and hence trafficking to lysosomes). The integrin alphavbeta6 was also seen to accumulate in early and recycling endosomes on virus entry, suggesting that the integrin serves not only as an attachment receptor but also to deliver the virus to the acidic endosomes. These findings are all consistent with FMDV infection proceeding via clathrin-dependent endocytosis.

An envelope-determined, pH-independent endocytic route of viral entry determines the susceptibility of human immunodeficiency virus type 1 (HIV-1) and HIV-2 to Lv2 restriction

We identified a postentry restriction, termed Lv2, which determines the cellular tropism of two related human immunodeficiency virus type 2 (HIV-2) isolates and is dependent on the sequence of the capsid (CA) and envelope (Env) proteins. To explain the reliance on both CA and Env, we proposed that restrictive Envs deliver susceptible capsids to a compartment where Lv2 is active whereas nonrestrictive Envs deliver capsids into a compartment where Lv2 is either absent or less active. To test this model, we used compounds that affect endocytic pathways (ammonium chloride, bafilomycin A1, hypertonic sucrose) or lipid rafts (methyl-beta-cyclodextrin) to treat restrictive cells and show that restricted virus can be rescued from Lv2 if a lipid-raft-dependent, pH-independent endocytic pathway is inhibited. Furthermore, viral entry into HeLa/CD4 cells containing a tailless CD4 receptor, located outside lipid rafts, was fully permissive. Finally, we show that a variety of primary HIV-1 and HIV-2 viruses are susceptible to Lv2. Thus, we show that the route of entry, determined by the viral envelope, can influence cellular tropism by avoiding intracellular blocks to infection.

Archazolid and apicularen: novel specific V-ATPase inhibitors

Background

V-ATPases constitute a ubiquitous family of heteromultimeric, proton translocating proteins. According to their localization in a multitude of eukaryotic membranes, they energize many different transport processes. Since their malfunction is correlated with various diseases in humans, the elucidation of the properties of this enzyme for the development of selective inhibitors and drugs is one of the challenges in V-ATPase research.

Results

Archazolid A and B, two recently discovered cytotoxic macrolactones produced by the myxobacterium Archangium gephyra, and apicularen A and B, two novel benzolactone enamides produced by different species of the myxobacterium Chondromyces, exerted a similar inhibitory efficacy on a wide range of mammalian cell lines as the well established plecomacrolidic type V-ATPase inhibitors concanamycin and bafilomycin. Like the plecomacrolides both new macrolides also prevented the lysosomal acidification in cells and inhibited the V-ATPase purified from the midgut of the tobacco hornworm, Manduca sexta, with IC₅₀ values of 20–60 nM. However, they did not influence the activity of mitochondrial F-ATPase or that of the Na⁺/K⁺-ATPase. To define the binding sites of these new inhibitors we used a semi-synthetic radioactively labelled derivative of concanamycin which exclusively binds to the membrane V_o subunit c. Whereas archazolid A prevented, like the plecomacrolides concanamycin A, bafilomycin A₁ and B₁, labelling of subunit c by the radioactive I-concanolide A, the benzolactone enamide apicularen A did not compete with the plecomacrolide derivative.

Conclusion

The myxobacterial antibiotics archazolid and apicularen are highly efficient and specific novel inhibitors of V-ATPases. While archazolid at least partly shares a common binding site with the plecomacrolides bafilomycin and concanamycin, apicularen adheres to an independent binding site.

Autophagic nutrient recycling in Arabidopsis directed by the ATG8 and ATG12 conjugation pathways

Autophagy is an important mechanism for nonselective intracellular breakdown whereby cytosol and organelles are encapsulated in vesicles, which are then engulfed and digested by lytic vacuoles/lysosomes. In yeast, this encapsulation employs a set of autophagy (ATG) proteins that direct the conjugation of two ubiquitin-like protein tags, ATG8 and ATG12, to phosphatidylethanolamine and the ATG5 protein, respectively. Using an Arabidopsis (Arabidopsis thaliana) atg7 mutant unable to ligate either tag, we previously showed that the ATG8/12 conjugation system is important for survival under nitrogen-limiting growth conditions. By reverse-genetic analyses of the single Arabidopsis gene encoding ATG5, we show here that the subpathway that forms the ATG12-ATG5 conjugate also has an essential role in plant nutrient recycling. Similar to plants missing ATG7, those missing ATG5 display early senescence and are hypersensitive to either nitrogen or carbon starvation, which is accompanied by a more rapid loss of organellar and cytoplasmic proteins. Multiple ATG8 isoforms could be detected immunologically in seedling extracts. Their abundance was substantially elevated in both the atg5 and atg7 mutants, caused in part by an increase in abundance of several ATG8 mRNAs. Using a green fluorescent protein-ATG8a fusion in combination with concanamycin A, we also detected the accumulation of autophagic bodies inside the vacuole. This accumulation was substantially enhanced by starvation but blocked in the atg7 background. The use of this fusion in conjunction with atg mutants now provides an important marker to track autophagic vesicles in planta.

Roscovitine triggers excitotoxicity in cultured granule neurons by enhancing glutamate release

Cerebellar granule neurons are highly susceptible to injury in vivo and in vitro, and primary cultures are widely used to characterize relevant receptors and signaling pathways. However, there are problems associated with their use. In particular, cultures are typically grown in medium supplemented with elevated KCl levels because it improves survival, but accumulating evidence indicates that this causes profound neuroadaptations. For example, growth in elevated KCl levels renders neurons electrically silent. Thus, they cannot be used to examine excitotoxicity of synaptic origins. On the other hand, cultures grown in physiological medium are rarely studied because a proportion undergoes apoptosis. Herein, we provide evidence that mature neurons cultured in physiological KCl develop spontaneous action potentials that support survival through N-methyl-D-aspartate (NMDA) receptor-mediated mechanisms. Furthermore, the cdk inhibitor roscovitine enhances the coupling between tetrodotoxin-sensitive action potentials and P/Q-type voltage-dependent calcium channels (VDCCs), thereby converting this survival program to excitotoxicity of synaptic origin. Therefore, roscovitine-triggered necrosis requires spontaneous Na+-based action potentials (tetrodotoxin inhibits, (+/-)-2-amino-4-phosphonobutyric acid enhances), P/Q-type VDCC currents (omega-agatoxin-IVA and omega-conotoxin-MVIIC inhibit, but not omega-conotoxin-GVIA), intact vesicle fusion processes (tetanus toxin inhibits), and transmitter-filled vesicles (concanamycin and bafilomycin inhibit). From a postsynaptic standpoint, roscovitine-mediated excitotoxicity requires the functionally linked activation of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/KA) and NMDA receptors, which is consistent with evidence that activated AMPA/KA receptors relieve the voltage-dependent Mg2+ block of NMDA receptors, resulting in excitotoxic Ca2+ influx. In the end, NMDA receptor-linked pathways transduce excitotoxicity. On the other hand, L-type VDCC blockers are not protective. Further characterization of this new model is expected to provide important insights about excitotoxicity of synaptic origins and about roscovitine as a selective modulator of this process.

A genomic screen for yeast vacuolar membrane ATPase mutants

V-ATPases acidify multiple organelles, and yeast mutants lacking V-ATPase activity exhibit a distinctive set of growth defects. To better understand the requirements for organelle acidification and the basis of these growth phenotypes, approximately 4700 yeast deletion mutants were screened for growth defects at pH 7.5 in 60 mm CaCl(2). In addition to 13 of 16 mutants lacking known V-ATPase subunits or assembly factors, 50 additional mutants were identified. Sixteen of these also grew poorly in nonfermentable carbon sources, like the known V-ATPase mutants, and were analyzed further. The cwh36Delta mutant exhibited the strongest phenotype; this mutation proved to disrupt a previously uncharacterized V-ATPase subunit. A small subset of the mutations implicated in vacuolar protein sorting, vps34Delta, vps15Delta, vps45Delta, and vps16Delta, caused both Vma- growth phenotypes and lower V-ATPase activity in isolated vacuoles, as did the shp1Delta mutation, implicated in both protein sorting and regulation of the Glc7p protein phosphatase. These proteins may regulate V-ATPase targeting and/or activity. Eight mutants showed a Vma- growth phenotype but no apparent defect in vacuolar acidification. Like V-ATPase-deficient mutants, most of these mutants rely on calcineurin for growth, particularly at high pH. A requirement for constitutive calcineurin activation may be the predominant physiological basis of the Vma- growth phenotype.

The formation, vacuolar localization, and tonoplast transport of salicylic acid glucose conjugates in tobacco cell suspension cultures

The metabolism of salicylic acid (SA) in tobacco (Nicotiana tabacum L. cv. KY 14) cell suspension cultures was examined by adding [7-14C]SA to the cell cultures for 24 h and identifying the metabolites through high performance liquid chromatography analysis. The three major metabolites of SA were SA 2-O-beta-D: -glucose (SAG), methylsalicylate 2-O-beta-D: -glucose (MeSAG) and methylsalicylate. Studies on the intracellular localization of the metabolites revealed that all of the SAG associated with tobacco protoplasts was localized in the vacuole. However, the majority of the MeSAG was located outside the vacuole. The tobacco cells contained an SA inducible SA glucosyltransferase (SAGT) enzyme that formed SAG. The SAGT enzyme was not associated with the vacuole and appeared to be a cytoplasmic enzyme. The vacuolar transport of SAG was characterized by measuring the uptake of [14C]SAG into tonoplast vesicles isolated from tobacco cell cultures. SAG uptake was stimulated eightfold by the addition of MgATP. The ATP-dependent uptake of SAG was inhibited by bafilomycin A1 (a specific inhibitor of the vacuolar H(+)-ATPase) and dissipation of the transtonoplast H(+)-electrochemical gradient. Vanadate was not an inhibitor of SAG uptake. Several beta-glucose conjugates were strong inhibitors of SAG uptake, whereas glutathione and glucuronide conjugates were only marginally inhibitory. The SAG uptake exhibited Michaelis-Menten type saturation kinetics with a K(m) and V(max) value of 11 microM and 205 pmol min-1 mg-1, respectively, for SAG. Based on the transport characteristics it appears as if the vacuolar uptake of SAG in tobacco cells occurs through an H(+)-antiport-type mechanism.

Insertion of targeting domains into the envelope glycoprotein of Moloney murine leukemia virus (MoMLV)-based vectors modulates the route of mCAT-1-mediated viral entry

Several groups have inserted targeting domains into the envelope glycoprotein (Env) of Moloney murine leukemia virus (MoMLV) in an attempt to produce targeted retroviral vectors for human gene therapy. While binding of these modified Envs to the target molecule expressed on the surface of human cells was observed, specific high-titer infection of human cells expressing the target molecule was not achieved. Here we investigate the initial steps in the entry process of targeted MoMLV vectors both in murine and human cells expressing the MoMLV receptor, the mouse cationic amino acid transporter-1 (mCAT-1). We show that insertion of a small ligand targeted to E-selectin and of a single chain antibody (scFv) targeted to folate-binding protein (FBP) into the N-terminus of MoMLV Env results in the reduction of the infectivity and the kinetics of entry of the MoMLV vectors. The use of soluble receptor-binding domain (sRBD), bafilomycin A1 (BafA1) and methyl-beta-cyclodextrin (MbetaC) increase the infectivity of the MoMLV vectors targeted to FBP (MoMLV-FBP) suggesting that the scFv targeted to FBP increases the threshold for fusion and might re-route entry of the targeted MoMLV-FBP vector towards an endocytic, non-productive pathway.

Mutational analysis of the stator subunit E of the yeast V-ATPase

Subunit E is a component of the peripheral stalk(s) that couples membrane and peripheral subunits of the V-ATPase complex. In order to elucidate the function of subunit E, site-directed mutations were performed at the amino terminus and carboxyl terminus. Except for S78A and D233A/T202A, which exhibited V(1)V(o) assembly defects, the function of subunit E was resistant to mutations. Most mutations complemented the growth phenotype of vma4Delta mutants, including T6A and D233A, which only had 25% of the wild-type ATPase activity. Residues Ser-78 and Thr-202 were essential for V(1)V(o) assembly and function. The mutation S78A destabilized subunit E and prevented assembly of V(1) subunits at the membranes. Mutant T202A membranes exhibited 2-fold increased V(max) and about 2-fold less of V(1)V(o) assembly; the mutation increased the specific activity of V(1)V(o) by enhancing the k(cat) of the enzyme 4-fold. Reduced levels of V(1)V(o) and V(o) complexes at T202A membranes suggest that the balance between V(1)V(o) and V(o) was not perturbed; instead, cells adjusted the amount of assembled V-ATPase complexes in order to compensate for the enhanced activity. These results indicated communication between subunit E and the catalytic sites at the A(3)B(3) hexamer and suggest potential regulatory roles for the carboxyl end of subunit E. At the carboxyl end, alanine substitution of Asp-233 significantly reduced ATP hydrolysis, although the truncation 229-233Delta and the point mutation K230A did not affect assembly and activity. The implication of these results for the topology and functions of subunit E within the V-ATPase complex are discussed.

Chemical genetic screening identifies sulfonamides that raise organellar pH and interfere with membrane traffic

Chemical genetics seeks to identify small molecules that afford functional dissection of cell biological pathways. Previous screens for small molecule inhibitors of exocytic membrane traffic yielded the identification and characterization of several compounds that block traffic from the Golgi to the cell surface as well as transport from the endoplasmic reticulum to the Golgi network [Feng et al. Proc Natl Acad Sci USA 2003;100:6469-6474; Yarrow et al. Comb Chem High Throughput Screen 2003;6:279-286; Feng et al. EMBO Reports 2004: in press]. Here, we screened these inhibitors for potential effects on endocytic membrane traffic. Two structurally related sulfonamides were found to be potent and reversible inhibitors of transferrin-mediated iron uptake. These inhibitors do not block endoplasmic reticulum-to-Golgi transport, but do disrupt Golgi-to-cell surface traffic. The compounds are members of a novel class of sulfonamides that elevate endosomal and lysosomal pH, down-regulate cell surface receptors, and impair recycling of internalized transferrin receptors to the plasma membrane. In vitro experiments revealed that the sulfonamides directly inhibit adenosine triphosphate (ATP) hydrolysis by the V-ATPase and that they also possess a potent proton ionophore activity. While maintenance of organellar pH is known to be a critical factor in both endocytosis and exocytosis, the precise role of acidification, beyond the uncoupling of ligands from their receptors, remains largely unknown. Identification of this novel class of sulfonamide inhibitors provides new chemical tools to better understand the function of organelle pH in membrane traffic and the activity of V-ATPases in particular.

Induction of Nitric Oxide Production by the Cytostatic Macrolide Apicularen A [2,4-Heptadienamide,N-[(1E)-3-[(3S,5R,7R,9S)-3,4,5,6,7,8,9,10-octahydro-7,14 Dihydroxy-1-oxo-5,9-epoxy-1H-2-benzoxacyclododecin-3-yl]-1 propenyl]-, (2Z,4Z)-(9CI)] and Possible Role of Nitric Oxide in Apicularen A-Induced Apoptosis in RAW 264.7 Cells

We previously reported that apicularen A [2,4-heptadienamide, N-[(1E)-3-[(3S,5R,7R,9S)-3,4,5,6,7,8,9,10-octahydro-7,14 dihydroxy-1-oxo-5,9-epoxy-1H-2-benzoxacyclododecin-3-yl]-1 propenyl]-, (2Z,4Z)-(9CI)], a highly cytostatic macrolide isolated from the myxobacterial genus Chondromyces, induces apoptosis in the mouse leukemic monocyte cell line RAW 264.7. To analyze the action mechanism of apicularen A for the induction of apoptosis, effects of apicularen A on nitric oxide (NO) production in RAW 264.7 cells were examined. It was demonstrated that apicularen A at 10 and 100 nM induced nitrite production, whereas apicularen B [2,4-heptadienamide, N-[(1E)-3-[(3S,5R,7R,9S)-7-[[2-(acetylamino)-2-deoxy-beta-d-glucopyranosyl]oxy]-3,4,5,6,7,8,9,10-octahydro-14-hydroxy-1-oxo-5,9-epoxy-1H-2-benzoxacyclododecin-3-yl]-1 propenyl]-, (2Z,4Z)-(9CI)], an N-acetyl-glucosamine glycoside of apicularen A, had no effect at 100 nM. The apicularen A-induced nitrite production was accompanied by an increase in the level of inducible nitric-oxide synthase (iNOS) and its mRNA and was suppressed by the NOS inhibitor N(G)-monomethyl-l-arginine acetate (l-NMMA). In addition, apicularen A activated nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) and decreased the level of IkappaB-alpha and increased that of phosphorylated c-Jun N-terminal kinase (JNK). Furthermore, the apicularen A-induced nitrite production was suppressed by the NF-kappaB inhibitor Bay 11-7082 [(E)-3-(4-methylphenylsulfonyl)-2-propenenitrile] and the JNK inhibitor SP600125 [anthra[1,9-cd]pyrazol-6(2H)-one]. These findings suggested that apicularen A activates NF-kappaB and AP-1, thus triggering the expression of iNOS mRNA and iNOS protein and induces NO production. Finally, apicularen A decreased cell growth and survival and cell viability and disrupted the mitochondrial membrane potential. The addition of l-NMMA partially recovered the apicularen A-induced decrease in cell growth and survival and cell viability and the disruption of mitochondrial membrane potential. These findings suggested that NO produced by apicularen A treatment participate partially in the apicularen A-induced apoptosis in RAW 264.7 cells.

Subcellular localization and calcium and pH requirements for proteolytic processing of the Hendra virus fusion protein

Proteolytic cleavage of the Hendra virus fusion (F) protein results in the formation of disulfide-linked F1 and F2 subunits, with cleavage occurring after residue K109 in the sequence GDVK/L. This unusual cleavage site and efficient propagation of Hendra virus in a furin-deficient cell line indicate that the Hendra F protein is not cleaved by furin, the protease responsible for proteolytic activation of many viral fusion proteins. To identify the subcellular site of Hendra F processing, Vero cells transfected with pCAGGS-Hendra F or pCAGGS-SV5 F were metabolically labeled and chased in the absence and presence of inhibitors of exocytosis. The addition of carbonyl-cyanide-3-chlorophenylhydrazone, monensin, brefeldin A, or NaF-AlCl3 or incubation of cells at 20 degrees C all inhibited processing of the Hendra F protein, suggesting that cleavage of Hendra F occurs either in secretory vesicles budding from the trans-Golgi network or at the cell surface. In contrast to proteolytic cleavage of the simian virus 5 (SV5) F protein by the Ca(2+)-dependent protease furin, proteolytic cleavage of the Hendra F protein was not significantly inhibited by decreases in Ca2+ levels following incubation with EGTA or A23187. However, in the presence of weak amines and H+ V-ATPase inhibitors, known to raise intracellular pH, cleavage of Hendra F protein was inhibited while processing of the SV5 F protein was not significantly affected. The subcellular location, sensitivity to pH changes, and decreased Ca2+ requirement suggest that the protease responsible for cleavage of Hendra F protein differs from proteases previously shown to be involved in the processing of other viral glycoproteins.

V-type ATPase is involved in biogenesis of GLUT4 vesicles

Proton pumps participate in several aspects of endocytic protein trafficking. However, their involvement specifically in the GLUT4 pathway has been a matter of great controversy. Here, we report that incubation of 3T3-L1 adipocytes with specific inhibitors of V-type ATPase, concanamycin A and bafilomycin A1, inhibits insulin-regulated glucose transport and results in accumulation of GLUT4 in heavy, rapidly sedimenting intracellular membranes. Correspondingly, the amount of small responsive GLUT4 vesicles in concanamycin A- and bafilomycin A1-treated cells is decreased. We conclude that these drugs block translocation of GLUT4 in adipose cells by inhibiting formation of small insulin-responsive vesicles on donor intracellular membranes. At the same time, proton pump inhibitors do not affect insulin-dependent translocation of preexisting vesicles or GLUT4 sorting in recycling endosomes. On the contrary, wortmannin acutely inhibits insulin-dependent translocation of the preexisting vesicles but has no effect on vesicle formation.

The putative elaiophylin biosynthetic gene cluster in Streptomyces sp. DSM4137 is adjacent to genes encoding adenosylcobalamin-dependent methylmalonyl CoA mutase and to genes for synthesis of cobalamin

A type I PKS gene probe obtained from RAPB of the rapamycin producer Streptomyces hygroscopicus, strongly hybridised to 92 out of 1120 cosmids from a genomic library of the elaiophylin-producing strain Streptomyces sp. DSM4137. Partial cosmid sequencing suggested the presence of 10 separate sequences encoding type I PKS genes. One entire DNA sequence was obtained and found exactly to match the gene organisation expected for the biosynthesis of the unusual macrodiolide polyketide elaiophylin. The putative elaiophylin gene cluster contains five large open-reading frames encoding typical modular polyketide synthases, which together catalyse the synthesis of the octaketide monomer of elaiophylin. Other genes were identified that would be required for provision of the ethylmalonate extender unit, for the synthesis and attachment of 2-deoxy-L-fucose and in regulation, or in export of the product. Immediately adjacent to the putative elaiophylin biosynthetic gene cluster is a 30-kbp region containing the gene for adenosylcobalamin-dependent methylmalonyl CoA mutase and also genes involved in the biosynthesis of the cobalamin cofactor. Analysis of the latter gene set confirms the view that cbiD of the anaerobic pathway and cobF in the aerobic pathway catalyse the same methylation of precorrin-5. The proximity of these genes to the putative elaiophylin gene cluster can best be rationalised if in this organism succinyl-CoA is a significant source of the methylmalonate units for complex polyketide biosynthesis.

The insect plasma membrane H+ V-ATPase: intra-, inter-, and supramolecular aspects

The plasma membrane H+ V-ATPase from the midgut of larval Manduca sexta, commonly called the tobacco hornworm, is the sole energizer of epithelial ion transport in this tissue, being responsible for the alkalinization of the gut lumen up to a pH of more than 11 and for any active ion movement across the epithelium. This minireview deals with those topics of our recent research on this enzyme that may contribute novel aspects to the biochemistry and physiology of V-ATPases. Our research approaches include intramolecular aspects such as subunit topology and the inhibition by macrolide antibiotics, intermolecular aspects such as the hormonal regulation of V-ATPase biosynthesis and the interaction of the V-ATPase with the actin cytoskeleton, and supramolecular aspects such as the interactions of V-ATPase, K+/H+ antiporter, and ion channels, which all function as an ensemble in the transepithelial movement of potassium ions.

TM2 but not TM4 of subunit c'' interacts with TM7 of subunit a of the yeast V-ATPase as defined by disulfide-mediated cross-linking

The vacuolar (H+)-ATPase (or V-ATPase) is an ATP-dependent proton pump which couples the energy released upon ATP hydrolysis to rotational movement of a ring of proteolipid subunits (c, c', and c'') relative to the integral subunit a. The proteolipid subunits each contain a single buried acidic residue that is essential for proton transport, with this residue located in TM4 of subunits c and c' and TM2 of subunit c''. Subunit c'' contains an additional buried acidic residue in TM4 that is not required for proton transport. The buried acidic residues of the proteolipid subunits are believed to interact with an essential arginine residue (Arg735) in TM7 of subunit a during proton translocation. We have previously shown that the helical face of TM7 of subunit a containing Arg735 interacts with the helical face of TM4 of subunit c' bordered by Glu145 and Leu147 (Kawasaki-Nishi et al. (2003) J. Biol. Chem. 278, 41908-41913). We have now analyzed interaction of subunits a and c'' using disulfide-mediated cross-linking. The results indicate that the helical face of TM7 of subunit a containing Arg735 interacts with the helical face of TM2 of subunit c'' centered on Ile105, with the essential glutamic acid residue (Glu108) located near the opposite border of this face compared with TM4 of subunit c'. By contrast, TM4 of subunit c'' does not form strong cross-links with TM7 of subunit a, suggesting that these transmembrane segments are not normally in close proximity. These results are discussed in terms of a model involving rotation of interacting helices in subunit a and the proteolipid subunits relative to each other.