Purification of the synaptic vesicle-binding protein physophilin. Identification as 39-kDa subunit of the vacuolar H(+)-ATPase

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

A novel putative M9.2 isoform of V-ATPase expressed in the nervous system

We have identified a cDNA encoding a novel putative neuron-specific isoform of vacuolar proton-translocating ATPase (V-ATPase), NM9.2, from rat and mouse. Sequence analysis revealed that NM9.2 conserved similar characteristic amino acid sequences with 60-70% identity to M9.2 previously isolated from V-ATPase in chromaffin granules. Using Northern blot analysis, NM9.2 mRNA was specifically detected in the brain, whereas M9.2 mRNA was widely expressed in various tissues. In situ hybridization showed that NM9.2 gene expression was restricted mainly to neuronal cells and consistent with that of the a1/Ac116 subunit of V-ATPase. NM9.2 is a putative neuronal isoform of the 9.2 kDa subunit in V-ATPase.

Tetraspan vesicle membrane proteins: synthesis, subcellular localization, and functional properties

Tetraspan vesicle membrane proteins (TVPs) are characterized by four transmembrane regions and cytoplasmically located end domains. They are ubiquitous and abundant components of vesicles in most, if not all, cells of multicellular organisms. TVP-containing vesicles shuttle between various membranous compartments and are localized in biosynthetic and endocytotic pathways. Based on gene organization and amino acid sequence similarities TVPs can be grouped into three distinct families that are referred to as physins, gyrins, and secretory carrier-associated membrane proteins (SCAMPs). In mammals synaptophysin, synaptoporin, pantophysin, and mitsugumin29 constitute the physins, synaptogyrin 1-4 the gyrins, and SCAMP1-5 the SCAMPs. Members of each family are cell-type-specifically synthesized resulting in unique patterns of TVP coexpression and subcellular colocalization. TVP orthologs have been identified in most multicellular organisms, including diverse animal and plant species, but have not been detected in unicellular organisms. They are subject to protein modification, most notably to phosphorylation, and are part of multimeric complexes. Experimental evidence is reviewed showing that TVPs contribute to vesicle trafficking and membrane morphogenesis.

Tetanus toxin-mediated cleavage of cellubrevin inhibits proton secretion in the male reproductive tract

Our laboratory has previously shown that the vacuolar H(+)-ATPase, located in a subpopulation of specialized cells establishes a luminal acidic environment in the epididymis and proximal part of the vas deferens (Breton S, Smith PJS, Lui B, and Brown D. Nat Med 2: 470-472, 1996). Low luminal pH is critical for sperm maturation and maintenance of sperm in a quiescent state during storage in these organs. In the present study we examined the regulation of proton secretion in the epididymis and vas deferens. In vivo microtubule disruption by colchicine induced an almost complete loss of H(+)-ATPase apical polarity. Endocytotic vesicles, visualized by Texas red-dextran internalization, contain H(+)-ATPase, indicating active endocytosis of the pump. Cellubrevin, an analog of the vesicle soluble N-ethyl malemide-sensitive factor attachment protein (SNAP) receptor (v-SNARE) synaptobrevin, is highly enriched in H(+)-ATPase-rich cells of the epididymis and vas deferens, and tetanus toxin treatment markedly inhibited bafilomycin-sensitive proton secretion by 64.3+/-9.0% in the proximal vas deferens. Western blotting showed effective cleavage of cellubrevin by tetanus toxin in intact vas deferens, demonstrating that the toxin gained access to cellubrevin. These results suggest that H(+)-ATPase is actively endocytosed and exocytosed in proton-secreting cells of the epididymis and vas deferens and that net proton secretion requires the participation of the v-SNARE cellubrevin.

Neurotransmitter release at rapid synapses

The classical concept of the vesicular hypothesis for acetylcholine (ACh) release, one quantum resulting from exocytosis of one vesicle, is becoming more complicated than initially thought. 1) synaptic vesicles do contain ACh, but the cytoplasmic pool of ACh is the first to be used and renewed on stimulation. 2) The vesicles store not only ACh, but also ATP and Ca(2+) and they are critically involved in determining the local Ca(2+) microdomains which trigger and control release. 3) The number of exocytosis pits does increase in the membrane upon nerve stimulation, but in most cases exocytosis happens after the precise time of release, while it is a change affecting intramembrane particles which reflects more faithfully the release kinetics. 4) The SNARE proteins, which dock vesicles close to Ca(2+) channels, are essential for the excitation-release coupling, but quantal release persists when the SNAREs are inactivated or absent. 5) The quantum size is identical at the neuromuscular and nerve-electroplaque junctions, but the volume of a synaptic vesicle is eight times larger in electric organ; at this synapse there is enough ACh in a single vesicle to generate 15-25 large quanta, or 150-200 subquanta. These contradictions may be only apparent and can be resolved if one takes into account that an integral plasmalemmal protein can support the formation of ACh quanta. Such a protein has been isolated, characterised and called mediatophore. Mediatophore has been localised at the active zones of presynaptic nerve terminals. It is able to release ACh with the expected Ca(2+)-dependency and quantal character, as demonstrated using mediatophore-transfected cells and other reconstituted systems. Mediatophore is believed to work like a pore protein, the regulation of which is in turn likely to depend on the SNARE-vesicle docking apparatus.

Reconstitution of mediatophore-supported quantal acetylcholine release

Synaptic transmission of a nerve impulse is an extremely rapid event relying on transfer of brief chemical impulses from one cell to another. This transmission is dependent upon Ca2+ and known to be quantal, which led to the widely accepted vesicular hypothesis of neurotransmitter release. However, at least in the case of rapid synaptic transmission the hypothesis has been found difficult to reconcile with a number of observations. In this article, we shall review data from experiments dealing with reconstitution of quantal and Ca2+-dependent acetylcholine release in: i) proteoliposomes, ii) Xenopus oocytes, and iii) release-deficient cell lines. In these three experimental models, release is dependent on the expression of the mediatophore, a protein isolated from the plasma membrane of cholinergic nerve terminals of the Torpedo electric organ. We shall discuss the role of mediatophore in quantal acetylcholine release, its possible involvement in morphological changes affecting presynaptic membrane during the release, and its interactions with others proteins of the cholinergic nerve terminal.

Synaptic vesicle biogenesis

Synaptic vesicles, which have been a paradigm for the fusion of a vesicle with its target membrane, also serve as a model for understanding the formation of a vesicle from its donor membrane. Synaptic vesicles, which are formed and recycled at the periphery of the neuron, contain a highly restricted set of neuronal proteins. Insight into the trafficking of synaptic vesicle proteins has come from studying not only neurons but also neuroendocrine cells, which form synaptic-like microvesicles (SLMVs). Formation and recycling of synaptic vesicles/SLMVs takes place from the early endosome and the plasma membrane. The cytoplasmic machinery of synaptic vesicle/SLMV formation and recycling has been studied by a variety of experimental approaches, in particular using cell-free systems. This has revealed distinct machineries for membrane budding and fission. Budding is mediated by clathrin and clathrin adaptors, whereas fission is mediated by dynamin and its interacting protein SH3p4, a lysophosphatidic acid acyl transferase.

Acetylcholine release. Reconstitution of the elementary quantal mechanism

Choline acetyltransferase and vesicular acetylcholine transporter genes are the products of two adjacent genes defining a cholinergic locus. The release mechanism is expressed independently of this locus in some cell lines. A cholinergic neuron will therefore have to coordinate the expression of release with that of the cholinergic locus. Transfection of a plasmid encoding Torpedo mediatophore in cells that are unable to release this transmitter endows them with a Ca2(+)-dependent and quantal release mechanism. The synchronization of mediatophore activation results from a control of calcium microdomains by the synaptic vesicles. It is therefore dependent on the proteins that dock vesicles close to calcium channels.

Brain Ac39/physophilin: cloning, coexpression and colocalization with synaptophysin

Physophilin is an oligomeric protein that binds the synaptic vesicle protein synaptophysin constituting a complex that has been hypothesized to form the exocytotic fusion pore. Microsequencing of several physophilin peptides putatively identified this protein as the Ac39 subunit of the V-ATPase. Ac39 has recently been shown to be present in a synaptosomal complex which, in addition to synaptophysin, includes the bulk of synaptobrevin II, and subunits c and Ac115 of the V0 sector of the V-ATPase. We have cloned physophilin from mouse brain and found a differential region of 12 amino acids when compared with the previously reported sequence of Ac39 from bovine adrenal medulla. RT-PCR cloning from the bovine adrenal medulla demonstrates that sequencing errors occurred in the previous cloning study, and shows that the amino acid sequences of physophilin and Ac39 are completely identical. In situ hybridization in rat brain reveals a largely neuronal distribution of Ac39/physophilin mRNA which spatio-temporally correlates with those of subunit c and synaptophysin. Immunohistochemical analysis shows that Ac39/physophilin is mostly concentrated in the neuropil with a pattern identical to subunit A and very similar to synaptophysin. Double-labelling immunofluorescence shows a complete colocalization of Ac39/physophilin with subunit A and a partial colocalization with synaptophysin in the neuropil. Our findings bring anatomical support for the in vivo occurrence of the synaptophysin-Ac39/physophilin interaction and further suggest a coordinated transcription of V-ATPase and synaptophysin genes. A putative role of Ac39/physophilin in the inactivation of the V-ATPase by disassembly of its V1 sector is also discussed.

Basolateral distribution of caveolin-1 in the kidney. Absence from H+-atpase-coated endocytic vesicles in intercalated cells

In kidney epithelial cells, a variety of physiological processes are dependent on the active recycling of membrane proteins between intracellular vesicles and the cell surface. Although clathrin-mediated endocytosis occurs in several renal cell types, endocytosis can also occur by non-clathrin-coated vesicles, including pinocytotic structures known as caveolae that contain a novel coat protein, caveolin. Exo- and endocytosis of a vacuolar H+-ATPase in intercalated cells also occurs via specialized "coated" vesicles that do not contain clathrin. The aim of this study was to localize caveolin in the kidney and, in addition, to determine whether it could be a component of the H+-ATPase recycling process. Using an antibody against the alpha- and beta-isoforms of caveolin-1, our immunocytochemical data show a marked heterogeneity in the cellular expression of this isoform of caveolin in kidney. In contrast, caveolin-3 was not detectable in renal epithelial cells. Caveolin-1 was abundant in endothelial cells and smooth muscle cells and was present in the parietal cells of Bowman's capsule. Distal tubule cells, connecting tubule cells, and collecting duct principal cells exhibited marked punctate basolateral staining, corresponding to the presence of caveolae detected by electron microscopy, whereas all intercalated cells were negative in both cortex and medulla. These data indicate that although caveolin-1 may participate in basolateral events in some kidney epithelial cell types, it does not appear to be involved in the regulated recycling of H+-ATPase in intercalated cells. Therefore, these cells recycle H+-ATPase by a mechanism that involves neither clathrin nor caveolin-1.

Acetylcholine release and the cholinergic genomic locus

Choline acetyltransferase and vesicular acetylcholine-transporter genes are adjacent and coregulated. They define a cholinergic locus that can be turned on under the control of several factors, including the neurotrophins and the cytokines. Hirschprung's disease, or congenital megacolon, is characterized by agenesis of intramural cholinergic ganglia in the colorectal region. It results from mutations of the RET (GDNF-activated) and the endothelin-receptor genes, causing a disregulation in the cholinergic locus. Using cultured cells, it was shown that the cholinergic locus and the proteins involved in acetylcholine (ACh) release can be expressed separately ACh release could be demonstrated by means of biochemical and electrophysiological assays even in noncholinergic cells following preloading with the transmitter. Some noncholinergic or even nonneuronal cell types were found to be capable of releasing ACh quanta. In contrast, other cells were incompetent for ACh release. Among them, neuroblastoma N18TG-2 cells were rendered release-competent by transfection with the mediatophore gene. Mediatophore is an ACh-translocating protein that has been purified from plasma membranes of Torpedo nerve terminal; it confers a specificity for ACh to the release process. The mediatophores are activated by Ca2+; but with a slower time course, they can be desensitized by Ca2+. A strictly regulated calcium microdomain controls the synchronized release of ACh quanta at the active zone. In addition to ACh and ATP, synaptic vesicles have an ATP-dependent Ca2+ uptake system; they transiently accumulate Ca2+ after a brief period of stimulation. Those vesicles that are docked close to Ca2+ channels are therefore in the best position to control the profile and dynamics of the Ca2+ microdomains. Thus, vesicles and their whole set of associated proteins (SNAREs and others) are essential for the regulation of the release mechanism in which the mediatophore seems to play a key role.

Cell lines expressing an acetylcholine release mechanism; correction of a release-deficient cell by mediatophore transfection

Several neuronal and non-neuronal cell lines express a Ca(2+)-dependent mechanism of transmitter release that can be demonstrated after loading the cells with acetylcholine during culture. In contrast, a particular cell line, the neuroblastoma N18TG-2, was found to be deficient for release. We transfected N18TG-2 cells with a plasmid encoding Torpedo mediatophore, a protein able to translocate acetylcholine in response to calcium. The N18TG-2 cells expressed the Torpedo protein which reached their plasma membrane. At the same time, these cells acquired a Ca(2+)-dependent quantal release mechanism similar to the one naturally expressed by other cell lines. Hence, the presence of mediatophore in the plasma membrane seems essential for quantal release.

Interleukin-1 increases vacuolar-type H+-ATPase activity in murine peritoneal macrophages

Maintenance of cytoplasmic pH (pHi) within a narrow physiological range is crucial to normal cellular function. This is of particular relevance to phagocytic cells within the acidic inflammatory microenvironment where the pHi tends to be acid loaded. We have previously reported that a vacuolar-type H(+)-ATPase (V-ATPase) situated in the plasma membrane of macrophages and poised to extrude protons from the cytoplasmic to the extracellular space is an important pHi regulatory mechanism within the inflammatory milieu. Since this microenvironment is frequently characterized by the influx of cells known to release inflammatory cytokines, we performed studies to examine the effect of one such mediator molecule, interleukin-1 (IL-1), on pHi regulation in peritoneal macrophages. IL-1 caused a time- and dose-dependent increase in macrophage pHi recovery from an acute acid load. This effect was specific to IL-1 and was due to enhanced plasmalemmal V-ATPase activity. The increased V-ATPase activity by IL-1 occurred following a lag period of several hours and required de novo protein and mRNA synthesis. However, Northern blot analysis revealed that IL-1 did not exert its effect via alterations in the levels of mRNA transcripts for the A or B subunits of the V-ATPase complex. Finally, stimulation of both cAMP-dependent protein kinase and protein kinase C was required for the stimulatory effect of IL-1 on V-ATPase activity. Thus, cytokines present within the inflammatory milieu are able to modulate pHi regulatory mechanisms. These data may represent a novel mechanism whereby cytokines may improve cellular function at inflammatory sites.

The V0 sector of the V-ATPase, synaptobrevin, and synaptophysin are associated on synaptic vesicles in a Triton X-100-resistant, freeze-thawing sensitive, complex

Anti-synaptobrevin 2 immunoprecipitates obtained from freshly prepared Triton X-100 extracts of rat synaptosomes contained, in addition to synaptophysin, a 10-kDa band, which we identified by peptide sequencing and Western blotting as the c subunit of the vacuolar proton pump (V-ATPase) also called ductin or mediatophore. Ac39 and Ac116, two other transmembrane subunits of the V0 sector of the V-ATPase, were also found by Western blotting to be enriched in the immunoprecipitates. None of these V-ATPase subunits, or synaptophysin, was present in anti-synaptobrevin 2 immunoprecipitates obtained from frozen-thawed Triton X-100 extracts, which were greatly enriched, instead, in SNAP-25 and syntaxin 1. Accordingly, V-ATPase subunit c was found in anti-synaptophysin immunoprecipitates. Thus, the two complexes appear to be mutually exclusive. Subcellular fractionation of rat brain demonstrated that V-ATPase subunit c is localized with synaptobrevin 2 and synaptophysin in synaptic vesicles. The coprecipitation of V-ATPase subunit c with the synaptobrevin-synaptophysin complex suggests that this interaction may play a role in recruiting the proton pump into synaptic vesicles. Freeze-thawing, which involves a mild denaturing step, may produce a conformational change which dissociates the complex and mimics a change which occurs in vivo as a prerequisite to SNARE complex formation.

Mediatophore and other presynaptic proteins. A cybernetic linking at the active zone

In rapidly transmitting synapses, the mediatophore, a protein located in the presynaptic membrane, seems to play a key role in the last step of transmitter release. Reconstituted either in proteoliposomes or in Xenopus oocytes, or transfected in particular cell lines, the mediatophore is able to release acetylcholine with characteristics which meet several typical features of transmitter release in natural synapses. Good correspondence between the two conditions was found for: i) the dependency of release upon calcium concentration; ii) the desensitisation of release by persistence of internal calcium; iii) the effect of several drugs; iv) the fleeting formation of a population of large intramembrane particles during the precise time of release; and v) the pulsatile or quantal nature of transmitter release. All these features therefore could well be ascribed to intrinsic properties of the mediatophore molecule. How is the mediatophore integrated in the whole presynaptic apparatus? To what extent is its function regulated by the other proteins of the active zone? These questions are far from being solved. We want nevertheless to propose here a general view in which characteristic presynaptic functions such as transmitter release, calcium entry, sequestration and extrusion, regulation of short- and long-term changes in release efficiency, are supported by an ordered succession of molecular events involving the proteins of the active zone. It will be seen that some proteins compete for a common binding site. It is thus expected that they will occupy this site in a regulated succession, according to simple cybernetic rules.