Nucleotide uptake by isolated cholinergic synaptic vesicles: evidence for a carrier of adenosine 5'-triphosphate

Physiopathological Role of the Vesicular Nucleotide Transporter (VNUT) in the Central Nervous System: Relevance of the Vesicular Nucleotide Release as a Potential Therapeutic Target

Vesicular storage of neurotransmitters, which allows their subsequent exocytotic release, is essential for chemical transmission in the central nervous system. Neurotransmitter uptake into secretory vesicles is carried out by vesicular transporters, which use the electrochemical proton gradient generated by a vacuolar H⁺-ATPase to drive neurotransmitter vesicular accumulation. ATP and other nucleotides are relevant extracellular signaling molecules that participate in a variety of biological processes. Although the active transport of nucleotides into secretory vesicles has been characterized from the pharmacological and biochemical point of view, the protein responsible for such vesicular accumulation remained unidentified for some time. In 2008, the human SLC17A9 gene, the last identified member of the SLC17 transporters, was found to encode the vesicular nucleotide transporter (VNUT). VNUT is expressed in various ATP-secreting cells and is able to transport a wide variety of nucleotides in a vesicular membrane potential-dependent manner. VNUT knockout mice lack vesicular storage and release of ATP, resulting in blockage of the purinergic transmission. This review summarizes the current studies on VNUT and analyzes the physiological relevance of the vesicular nucleotide transport in the central nervous system. The possible role of VNUT in the development of some pathological processes, such as chronic neuropathic pain or glaucoma is also discussed. The putative involvement of VNUT in these pathologies raises the possibility of the use of VNUT inhibitors for therapeutic purposes.

Vesicular nucleotide transporter (VNUT): appearance of an actress on the stage of purinergic signaling

Vesicular storage of ATP is one of the processes initiating purinergic chemical transmission. Although an active transport mechanism was postulated to be involved in the processes, a transporter(s) responsible for the vesicular storage of ATP remained unidentified for some time. In 2008, SLC17A9, the last identified member of the solute carrier 17 type I inorganic phosphate transporter family, was found to encode the vesicular nucleotide transporter (VNUT) that is responsible for the vesicular storage of ATP. VNUT transports various nucleotides in a membrane potential-dependent fashion and is expressed in the various ATP-secreting cells. Mice with knockout of the VNUT gene lose vesicular storage and release of ATP from neurons and neuroendocrine cells, resulting in blockage of the initiation of purinergic chemical transmission. Thus, VNUT plays an essential role in the vesicular storage and release of ATP. The VNUT knockout mice exhibit resistance for neuropathic pain and a therapeutic effect against diabetes by way of increased insulin sensitivity. Thus, VNUT inhibitors and suppression of VNUT gene expression may be used for therapeutic purposes through suppression of purinergic chemical transmission. This review summarizes the studies to date on VNUT and discusses what we have learned about the relevance of vesicular ATP release as a potential drug target.

ATP: The crucial component of secretory vesicles

The colligative properties of ATP and catecholamines demonstrated in vitro are thought to be responsible for the extraordinary accumulation of solutes inside chromaffin cell secretory vesicles, although this has yet to be demonstrated in living cells. Because functional cells cannot be deprived of ATP, we have knocked down the expression of the vesicular nucleotide carrier, the VNUT, to show that a reduction in vesicular ATP is accompanied by a drastic fall in the quantal release of catecholamines. This phenomenon is particularly evident in newly synthesized vesicles, which we show are the first to be released. Surprisingly, we find that inhibiting VNUT expression also reduces the frequency of exocytosis, whereas the overexpression of VNUT drastically increases the quantal size of exocytotic events. To our knowledge, our data provide the first demonstration that ATP, in addition to serving as an energy source and purinergic transmitter, is an essential element in the concentration of catecholamines in secretory vesicles. In this way, cells can use ATP to accumulate neurotransmitters and other secreted substances at high concentrations, supporting quantal transmission.

Neuronal involvement in muscular atrophy

The innervation of skeletal myofibers exerts a crucial influence on the maintenance of muscle tone and normal operation. Consequently, denervated myofibers manifest atrophy, which is preceded by an increase in sarcolemma permeability. Recently, de novo expression of hemichannels (HCs) formed by connexins (Cxs) and other none selective channels, including P2X7 receptors (P2X7Rs), and transient receptor potential, sub-family V, member 2 (TRPV2) channels was demonstrated in denervated fast skeletal muscles. The denervation-induced atrophy was drastically reduced in denervated muscles deficient in Cxs 43 and 45. Nonetheless, the transduction mechanism by which the nerve represses the expression of the above mentioned non-selective channels remains unknown. The paracrine action of extracellular signaling molecules including ATP, neurotrophic factors (i.e., brain-derived neurotrophic factor (BDNF)), agrin/LDL receptor-related protein 4 (Lrp4)/muscle-specific receptor kinase (MuSK) and acetylcholine (Ach) are among the possible signals for repression for connexin expression. This review discusses the possible role of relevant factors in maintaining the normal functioning of fast skeletal muscles and suppression of connexin hemichannel expression.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Vesicular neurotransmitter transporters: an approach for studying transporters with purified proteins

Vesicular storage and subsequent release of neurotransmitters are the key processes of chemical signal transmission. In this process, vesicular neurotransmitter transporters are responsible for loading the signaling molecules. The use of a "clean biochemical" approach with purified, recombinant transporters has helped in the identification of novel vesicular neurotransmitter transporters and in the analysis of the control of signal transmission.

SLC17: a functionally diverse family of organic anion transporters

Molecular studies have determined that the SLC17 transporters, a family of nine proteins initially implicated in phosphate transport, mediate the transport of organic anions. While their role in phosphate transport remains uncertain, it is now clear that the transport of organic anions facilitated by this family of proteins is involved in diverse processes ranging from the vesicular storage of the neurotransmitters, to urate metabolism, to the degradation and metabolism of glycoproteins.

ATP synthesis and storage

Since 1929, when it was discovered that ATP is a substrate for muscle contraction, the knowledge about this purine nucleotide has been greatly expanded. Many aspects of cell metabolism revolve around ATP production and consumption. It is important to understand the concepts of glucose and oxygen consumption in aerobic and anaerobic life and to link bioenergetics with the vast amount of reactions occurring within cells. ATP is universally seen as the energy exchange factor that connects anabolism and catabolism but also fuels processes such as motile contraction, phosphorylations, and active transport. It is also a signalling molecule in the purinergic signalling mechanisms. In this review, we will discuss all the main mechanisms of ATP production linked to ADP phosphorylation as well the regulation of these mechanisms during stress conditions and in connection with calcium signalling events. Recent advances regarding ATP storage and its special significance for purinergic signalling will also be reviewed.

Impaired purinergic neurotransmission to mesenteric arteries in deoxycorticosterone acetate-salt hypertensive rats

Sympathetic nerves release norepinephrine and ATP onto mesenteric arteries. In deoxycorticosterone acetate (DOCA)-salt hypertensive rats, there is increased arterial sympathetic neurotransmission attributable, in part, to impaired prejunctional regulation of norepinephrine release. Prejunctional regulation purinergic transmission in hypertension is less well understood. We hypothesized that alpha(2)-adrenergic receptor dysfunction alters purinergic neurotransmission to arteries in DOCA-salt hypertensive rats. Mesenteric artery preparations were maintained in vitro, and intracellular electrophysiological methods were used to record excitatory junction potentials (EJPs) from smooth muscle cells. EJP amplitude was reduced in smooth muscle cells from DOCA-salt (4+/-1 mV) compared with control arteries (9+/-1 mV; P<0.05). When using short trains of stimulation (0.5 Hz; 5 pulses), the alpha(2)adrenergic receptor antagonist yohimbine (1 micromol/L) potentiated EJPs in control more than in DOCA-salt arteries (180+/-35% versus 86+/-7%; P<0.05). Norepinephrine (0.1 to 3.0 micromol/L), the alpha(2)adrenergic receptor agonist UK 14304 (0.001 to 0.100 micromol/L), the A(1) adenosine receptor agonist cyclopentyladensosine (0.3 to 100.0 micromol/L), and the N-type calcium channel blocker omega-conotoxin GVIA (0.0003 to 0.1000 micromol/L) decreased EJP amplitude equally well in control and DOCA-salt arteries. Trains of stimuli (10 Hz) depleted ATP stores more completely, and the latency to EJP recovery was longer in DOCA-salt compared with control arteries. These data indicate that there is reduced purinergic input to mesenteric arteries of DOCA-salt rats because of decreased ATP bioavailability in sympathetic nerves. These data highlight the potential importance of impaired purinergic regulation of arterial tone as a target for drug treatment of hypertension.

Identification of a vesicular nucleotide transporter

ATP is a major chemical transmitter in purinergic signal transmission. Before secretion, ATP is stored in secretory vesicles found in purinergic cells. Although the presence of active transport mechanisms for ATP has been postulated for a long time, the proteins responsible for its vesicular accumulation remains unknown. The transporter encoded by the human and mouse SLC17A9 gene, a novel member of an anion transporter family, was predominantly expressed in the brain and adrenal gland. The mouse and bovine counterparts were associated with adrenal chromaffin granules. Proteoliposomes containing purified transporter actively took up ATP, ADP, and GTP by using membrane potential as the driving force. The uptake properties of the reconstituted transporter were similar to that of the ATP uptake by synaptic vesicles and chromaffin granules. Suppression of endogenous SLC17A9 expression in PC12 cells decreased exocytosis of ATP. These findings strongly suggest that SLC17A9 protein is a vesicular nucleotide transporter and should lead to the elucidation of the molecular mechanism of ATP secretion in purinergic signal transmission.

ATP and acetylcholine, equal brethren

Acetylcholine was the first neurotransmitter identified and ATP is the hitherto final compound added to the list of small molecule neurotransmitters. Despite the wealth of evidence assigning a signaling role to extracellular ATP and other nucleotides in neural and non-neural tissues, the significance of this signaling pathway was accepted very reluctantly. In view of this, this short commentary contrasts the principal molecular and functional components of the cholinergic signaling pathway with those of ATP and other nucleotides. It highlights pathways of their discovery and analyses tissue distribution, synthesis, uptake, vesicular storage, receptors, release, extracellular hydrolysis as well as pathophysiological significance. There are differences but also striking similarities. Comparable to ACh, ATP is taken up and stored in synaptic vesicles, released in a Ca(2+)-dependent manner, acts on nearby ligand-gated or metabotropic receptors and is hydrolyzed extracellularly. ATP and acetylcholine are also costored and coreleased. In addition, ATP is coreleased from biogenic amine storing nerve terminals as well as from at least subpopulations of glutamatergic and GABAergic terminals. Both ACh and ATP fulfill the criteria postulated for neurotransmitters. More recent evidence reveals that the two messengers are not confined to neural functions, exerting a considerable variety of non-neural functions in non-innervated tissues. While it has long been known that a substantial number of pathologies originate from malfunctions of the cholinergic system there is now ample evidence that numerous pathological conditions have a purinergic component.

Identification and characterization of SV31, a novel synaptic vesicle membrane protein and potential transporter

Synaptic vesicle proteins govern all relevant functions of the synaptic vesicle life cycle, including vesicle biogenesis, vesicle transport, uptake and storage of neurotransmitters, and regulated endocytosis and exocytosis. In spite of impressive progress made in the past years, not all known vesicular functions can be assigned to defined protein components, suggesting that the repertoire of synaptic vesicle proteins is still incomplete. We have identified and characterized a novel synaptic vesicle membrane protein of 31 kDa with six putative transmembrane helices that, according to its membrane topology and phylogenetic relation, may function as a vesicular transporter. The vesicular allocation is demonstrated by subcellular fractionation, heterologous expression, immunocytochemical analysis of brain sections and immunoelectron microscopy. The protein is expressed in select brain regions and contained in subpopulations of nerve terminals that immunostain for the vesicular glutamate transporter 1 and the vesicular GABA transporter VGaT (vesicular amino acid transporter) and may attribute specific and as yet undiscovered functions to subsets of glutamatergic and GABAergic synapses.

Characterization of DIDS-sensitive ATP accumulation in brain synaptic vesicles

ATP is an excitatory neurotransmitter in the central and peripheral nervous system. We investigated ATP accumulation in highly purified brain synaptic vesicles (SVs). Based on the amount of ATP accumulated in SVs under the conditions used, ATP is not transported against a concentration gradient but rather appears to have a Delta mu H(+)-independent mechanism. ATP transport was inhibited by DIDS and NEM, but was not affected by Mg(2+) or by pre-incubation with nucleotides. These results suggest a unique transport mechanism that does not involve exchange with other nucleotides or protons, unlike other known neurotransmitter transport systems.

Vesicular release of ATP at central synapses

Adenosine triphosphate (ATP) acts as a fast excitatory transmitter in several regions of the central nervous system (CNS) including the medial habenula, dorsal horn, locus coeruleus, hippocampus, and somatosensory cortex. Postsynaptic actions of ATP are mediated through an extended family of P2X receptors, widely expressed throughout the CNS. ATP is released via several pathways, including exocytosis from presynaptic terminals and diffusion through large transmembrane pores (e.g., hemichannels, P2X(7) receptors, or volume-sensitive chloride channels) expressed in astroglial membranes. In presynaptic terminals, ATP is accumulated and stored in the synaptic vesicles. In different presynaptic terminals, these vesicles may contain ATP only or ATP and another neurotransmitter [e.g., gamma-amino-butyric acid (GABA) or glutamate]; in the latter case, two transmitters can be coreleased. Here, we discuss the mechanisms of vesicular release of ATP in the CNS and present our own data, which indicate that in central neuronal terminals, ATP is primarily stored and released from distinct pool of vesicles; the release of ATP is not synchronized either with GABA or with glutamate.

GTP uptake into rat brain synaptic vesicles

Uptake of neurotransmitters into synaptic vesicles is driven by an electrochemical gradient generated by a vacuolar-type proton pump ATPase. This uptake implies a key role for synaptic vesicles in the regulation of neurotransmitter systems. Guanine nucleoside and nucleotides are involved in the inhibition of glutamate-induced cellular responses via an extracellular action and diverse trophic, proliferative, and modulatory effects of guanine nucleotides on neural cells have been shown. Here, we characterized the uptake of GTP into synaptic vesicles isolated from whole rat brain, by using a tritiated poorly-hydrolyzable GTP analog, 5'-guanylylimidodiphosphate ([3H]GppNHp). Uptake of GTP into synaptic vesicles is saturable, time- and temperature-dependent, and relies on a proton-eletrochemical gradient. However, [3H]GMP and [3H]GDP radioactive labeling in synaptic vesicles is not dependent on temperature and vesicular ATPase activity, which indicates that these nucleotides only bind to and are not taken up into synaptic vesicles. GTP is taken up by the same eletrochemical gradient-dependent transport system, as are neurotransmitters storage, which indicates that this guanine nucleotide may also function as a neurotransmitter.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Characterization of nucleotide transport into rat brain synaptic vesicles

ATP transport to synaptic vesicles from rat brain has been studied using the fluorescent substrate analogue 1,N6-ethenoadenosine 5'-triphosphate (epsilon-ATP). The increase in intravesicular concentration was time dependent for the first 30 min, epsilon-ATP being the most abundant nucleotide. The complexity of the saturation curve indicates the existence of kinetic and allosteric cooperativity in the nucleotide transport, which exhibits various affinity states with K0.5 values of 0.39 +/- 0.06 and 3.8 +/- 0.1 mM with epsilon-ATP as substrate. The Vmax values obtained were 13.5 +/- 1.4 pmol x min(-1) x mg of protein(-1) for the first curve and 28.3 +/- 1.6 pmol x min(-1) x mg of protein(-1) considering both components. This kinetic behavior can be explained on the basis of a mnemonic model. The nonhydrolyzable adenine nucleotide analogues adenosine 5'-O-3-(thiotriphosphate), adenosine 5'-O-2-(thiodiphosphate), and adenosine 5'-(beta,gamma-imino)triphosphate and the diadenosine polyphosphates P1,P3-di(adenosine)triphosphate, P1,P4-di(adenosine)tetraphosphate, and P1,P5-di(adenosine)pentaphosphate inhibited the nucleotide transport. The mitochondrial ATP/ADP exchange inhibitor atractyloside, N-ethylmaleimide, and polysulfonic aromatic compounds such as Evans blue and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid also inhibit epsilon-ATP vesicular transport.

Biochemistry and toxicology of the diterpenoid glycoside atractyloside

Atractyloside (Atr) is a diterpenoid glycoside that occurs naturally in plants (many of which are used in ethnomedicines) found in Europe, Africa, South America, Asia and the far East. It is also present in animal grazing forage. Atr (and its analogues) may be present at levels as high as 600 mg/kg dried plant material. Consumption of the plants containing Atr or carboxyatractyloside (carboxyAtr) has caused fatal renal proximal tubule necrosis and/or centrilobular hepatic necrosis in man and farm animals. Although pure Atr and crude plant extracts disrupt carbohydrate homeostasis and induce similar pathophysiological lesions in the kidney and liver, it is also possible that the toxicity of Atr may be confounded by the presence of other natural constituents in plants. Atr competitively inhibits the adenine nucleoside carrier in isolated mitochondria and thus blocks oxidative phosphorylation. This has been assumed to explain changes in carbohydrate metabolism and the toxic effects in liver and kidney. Although the acute toxicity of Atr is well described, many aspects of Atr toxicity (subchronic and chronic toxicity, reproductive toxicity, mutagenicity and carcinogenicity) have not been investigated and pharmacokinetic and metabolism data are limited. In vitro proximal tubular cells are selectively sensitive to Atr, whereas other renal cell types are quite resistant. There are also differences in the response of liver and renal tissue to Atr. Thus, not all of the clinical, biochemical and morphological changes caused by Atr can simply be explained on the basis of inhibition of mitochondrial phosphorylation. The relevance to a wider human risk is shown by the presence of Atr analogues in dried roasted Coffea arabica beans (17.5 32 mg/kg). There are no data to help identify the risk of low dose chronic exposure in human coffee consumers, nor is there information on the levels of Atr or its analogues in other commonly consumed human foodstuffs.

Studies on the release and extracellular metabolism of endogenous ATP in rat superior cervical ganglion: support for neurotransmitter role of ATP

The release of endogenous ATP, measured by the luciferin-luciferase assay, and the release of [3H]acetylcholine from the isolated superior cervical ganglion of the rat loaded with [3H]choline were studied simultaneously. Electrical field stimulation enhanced the release of endogenous ATP and acetylcholine in a [Ca2+]o-dependent manner. The Na+ channel blocker, tetrodotoxin (1 microM) inhibited the stimulation-evoked release of endogenous ATP and of [3H]acetylcholine, but did not change the resting release. The release of ATP was dependent on the frequency of stimulation between 2 and 10 Hz. when the number of shocks was kept constant (360 shocks), while acetylcholine was not released in a frequency-dependent fashion. Ten days after cutting of the preganglionic nerve of the superior cervical ganglion the stimulation-evoked release of acetylcholine and ATP was abolished and the uptake of [3H]choline was significantly reduced but not inhibited. Hexamethonium, (100 microM) a nicotinic acetylcholine receptor antagonist, significantly reduced the release of both acetylcholine and ATP, indicating a positive feedback modulation of ACh and ATP release. 8-Cyclopentyl-1,3-dipropylxanthine (10 nM), the selective A1-adenosine receptor antagonist exhibited similar effect on the release of ATP and acetylcholine: both of them were augmented, showing that the stimulation-evoked release of ATP and acetylcholine are under the inhibitory control of A1-adenosine receptors. When the temperature was reduced to 7 degrees C to inhibit carrier-mediated processes, the resting and stimulated release of acetylcholine was not changed. Conversely, the release of ATP in response to stimulation was reduced by 79.9 +/- 5.6%, and the basal release was also almost completely blocked. Carbamylcholine by itself was able to release ATP, but not acetylcholine, in a hexamethonium-inhibitable manner, even from ganglia whose preganglionic nerve had been cut 10 days prior to experiments, suggesting that ATP release can occur in response to nicotinic receptor stimulation of postsynaptic cells. The breakdown of ATP or AMP by superior cervical ganglion was measured by high performance liquid chromatography combined with UV detection. ATP and AMP, added to the tissues, were readily decomposed: the Km (apparent Michaelis constant) and Vmax (apparent maximal velocity) were 475 +/- 24 microM and 3.50 +/- 0.18 nmol/min per mg for ectoATPase and 1550 +/- 120 microM and 14.5 +/- 0.9 nmol/min per mg tissue for 5'-nucleotidase. In addition, by using electron microscopic enzyme histochemistry, the presence of ectoATPase was also shown in the superior cervical ganglion. It is concluded that endogenous ATP and acetylcholine are released simultaneously in response to stimulation of preganglionic nerve terminals in the superior cervical ganglion in a [Ca2+]o-dependent, tetrodotoxin-sensitive manner and is metabolized by ectoenzymes present in the tissue. The dissociation of the release of ATP and acetylcholine at different stimulation frequencies and temperatures shows that the release-ratio of acetylcholine and ATP can vary upon the condition of stimulation: this can reflect either the different composition of synaptic vesicles in the preganglionic nerve terminals or a significant contribution of non-exocytotic, carrier-mediated type of release of ATP to the bulk release.

Presence of epsilon-adenosine tetraphosphate in chromaffin granules after transport of epsilon-ATP

Adenosine 5'-tetraphosphate (Ap4) is a natural constituent of chromaffin granules with concentration values of 2.2 +/- 0.1 nmol/mg of protein and a ratio 245 +/- 40 times lower with respect to ATP (n = 4). The granular transport of epsilon-ATP resulted in a time- and concentration-dependent production of epsilon-adenosine tetraphosphate (epsilon-Ap4) at the intragranular level. The epsilon-Ap4 formation followed a hyperbolic saturation kinetic at low epsilon-ATP concentrations with K(m) value of 0.4 microM epsilon-ATP intragranular (1.15 pmol/mg of granular protein). Intragranular concentrations of epsilon-ATP higher than 500 pmol/mg of protein (approximately to 175 microM intragranular) resulted in a non-saturable production of epsilon-Ap4.

Biochemistry, localization and functional roles of ecto-nucleotidases in the nervous system

Nucleotides such as ATP, ADP, UTP or the diadenosine polyphosphates and possibly even NAD+ are extracellular signaling substances in the brain and in other tissues. Enzymes located on the cell surface catalyze the hydrolysis of these compounds and thus limit their spatio-temporal activity. As a final hydrolysis product they generate the nucleoside and phosphate. The paper discusses the biochemical properties, cellular localization and functional properties of surface-located enzymes that hydrolyse nucleotides released from nervous tissue. This is preceded by a brief discussion of nucleotide receptors, cellular storage and mechanisms of nucleotide release. In nervous tissue nucleoside 5'-triphosphates are hydrolysed by ecto-ATP-diphosphohydrolase and possibly in addition also by ecto-nucleoside triphosphatase and ecto-nucleoside diphosphatase. The molecular identity of the ATP-diphosphohydrolase has now been revealed. The hydrolysis of nucleoside 5'-monophosphates is catalysed by 5'-nucleotidase whose biochemical properties and molecular structure have been studied in detail. Little is known about the molecular properties of the diadenosine polyphosphatases. Surface located enzymes for the extracellular hydrolysis of NAD+ and also ecto-protein kinases are discussed briefly. The cellular localization of the ecto-nucleotidases is only partly defined. Whereas in adult mammalian brain activity for hydrolysis of ATP and ADP may be associated with nerve cells or glial cells 5'-nucleotidase appears to have a preferential glial allocation in the adult mammal. The extracellular hydrolysis of the nucleotides is of functional importance not only during synaptic transmission where it functions in signal elimination. It plays a crucial role also for the survival and differentiation of neural cells in vitro and presumably during neuronal development in vivo.

Identification of a glycosylphosphatidylinositol-anchored glycoprotein with nucleoside phosphatase activity on the membrane of pig pancreatic zymogen granules

The molecular events between the second messenger-mediated triggering of regulated exocytosis and the subsequent fusion of the secretory granules with the apical plasma membrane are unclear. The glycoprotein GP-2, the most abundant of the very few proteins of the pancreatic zymogen granule membrane has been cloned and sequenced in dog and rat, but no (enzymatic) function has so far been ascribed to it. Nucleoside phosphatase activities associated with the pig zymogen granule membrane were recently assumed to be related to GP-2. To identify the protein(s) carrying these activities we have used a novel combination of native and denaturing one- and two-dimensional polyacrylamide gel electrophoresis in the detergents CHAPS, Triton X-100 or SDS. Histochemical examination on the gels and incubation with lectins and phosphatidylinositol phospholipase-C have allowed characterization of the protein with the nucleoside di- and tri-phosphatase activities. SDS-PAGE of the single protein spot with nucleoside phosphatase activity excised from Triton X-100 2-dimensional gels showed the presence of 92 kDa and 67 kDa glycoproteins. The isolated protein had an isoelectric point of 5.2, formed high molecular weight complexes, was shown to be glycosylphosphatidylinositol-anchored and contained complex carbohydrate structures. It hydrolyses di- and tri-phosphate nucleotides in dependence of the glycosylphosphatidylinositol anchor and is sensitive to non-mitochondrial diphosphohydrolase inhibitors. In summary, this paper identifies GP-2 as a nucleoside phosphatase within the zymogen granule membrane, suggesting it may be involved in energy-requiring processes on the cytosolic side of the granules.

Nucleotide vesicular transporter of bovine chromaffin granules. Evidence for a mnemonic regulation

The nucleotide vesicular transport has been studied with the fluorescent substrate analogues, the (1,N6-ethenoadenosine) nucleotides. The transport experiments were carried out with granular preparations from bovine adrenal medulla, and epsilon-ATP, epsilon-ADP, and epsilon-AMP were quantified after separation by high performance liquid chromatography. The granular concentration increase of all three nucleotides was time-dependent. The concentration dependence of epsilon-nucleotide transport to chromaffin granules did not follow the Michaelis-Menten kinetics and presented a similar three-step curve with cooperativity. This shape can be considered to be the result of the addition of three sigmoidal curves with their corresponding kinetic parameters. epsilon-ATP exhibited K values of 0.25, 1, and 3 mM and Vmax values of 0.02, 0.04 and 0.19 nmol.min-1.mg of protein-1, for the first, second, and third curves for each step, respectively. epsilon-ADP exhibited K values of 0.15, 0.9, and 3.6 mM and Vmax values of 0.025, 0.035, and 0.3 nmol.min-1.mg of protein-1, respectively for the first, second, and third curves. epsilon-AMP exhibited K values of 0.2, 1.2, and 3.2 mM, and Vmax values of 0.01, 0.04, and 0.055 nmol.min-1.mg of protein-1, also for the first to third steps. The Hill numbers for epsilon-ATP, epsilon-ADP, and epsilon-AMP were not constant but a function of the transport saturation. The nonhydrolyzable ATP analogues AMPPNP, ATP gamma S, and ADP beta S were activators of epsilon-nucleotide transport at concentrations under 1 mM and inhibitors at higher concentrations. Atractyloside and N-ethylmaleimide partially inhibited the nucleotide granular transport. High extragranular ATP concentrations specifically induced the exit of the previously transporter granular epsilon-ATP.

The non-specific ion channel in Torpedo ocellata fused synaptic vesicles

1. Synaptic vesicles were isolated and fused into large structures with a diameter of more than 20 microns to characterize their ionic channels. The 'cell'-attached and inside-out configurations of the patch clamp technique were used. 2. Two types of ion channels were most frequently observed: a low conductance chloride channel and a high conductance non-specific channel. 3. The non-specific channel has a main conducting state and a substate. The main conducting state has a slope conductance of 246 +/- 15 pS (+/- S.E.M., n = 15), in the presence of different combinations of KCl and potassium glutamate. 4. From the reversal potentials of the current-voltage (I-V) relation, it was concluded that this channel conducts both Cl- and K+. 5. The non-specific channel is highly voltage dependent: under steady-state voltages it has a high open probability near 0 mV and does not inactivate; when the membrane is hyperpolarized (pipette side more positive), the open probability decreases dramatically. 6. Voltage pulses showed that upon hyperpolarization (from holding potentials between -20 and + 20 mV), the channels deactivated; when the membrane was stepped back to the holding potential, the channels reactivated rapidly. 7. In a number of experiments, when the pipette side was made more negative than the bath, the open probability also decreased. 8. Frequently, a substate with a conductance of about 44 +/- 4% (+/- S.E.M., n = 3) of the main state was detected. 9. We speculate that this non-specific ion channel may have different roles at the various stages of the life cycle of the synaptic vesicle. When the synaptic vesicle is an intracellular structure, it might help its transmitter-concentrating capacity by dissipating the polarization. After fusion with the surface membrane, it might constitute an additional conductance pathway, taking part in frequency modulation of synaptic transmission.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Endogenous adenosine modulates stimulation-induced depression at the frog neuromuscular junction

1. Endogenous adenosine, which is produced by enzymatic degradation of ATP released from synaptic vesicles, has been shown to be a potent inhibitor of acetylcholine release from motor nerve terminals. It has been proposed that this auto-inhibition mechanism might contribute significantly to tetanic stimulation-induced depression. 2. Levels of facilitation and depression during a 20 Hz stimulus train differ greatly in different terminals, but are strongly and non-linearly correlated with the terminal's release characteristics (the amount of transmitter released per unit terminal length, or 'release efficacy'). There is a weaker, approximately linear, correlation between depression and release efficacy at 2 Hz stimulation. 3. The effects of both endogenous and exogenously applied adenosine are also highly variable for different nerve terminals. We have shown that much of this variability can be attributed to the release efficacy of each terminal in the case of endogenous effects, and to the size of the nerve terminal in the case of exogenously applied adenosine receptor agonists. 4. When nerve terminals are pooled according to their individual release characteristics, endogenous adenosine can be shown to contribute significantly to stimulation-induced depression of release primarily in terminals that release enough transmitter to generate significant levels of adenosine, but do not release so much transmitter that depletion of releasable quanta is severe.

Effects of potassium depolarization on intracellular compartmentalization of ATP in cholinergic synaptosomes isolated from Torpedo electric organ

It is well known that acetylcholine (ACh) and ATP are co-stored and co-released in nerve terminals of the electric organ of Torpedo. Cholinergic synaptosomes were subjected to a cycle of freezing and thawing showing that ATP is distributed in two operational pools like those described for ACh. The bound pool is resistant to freezing and thawing, and it is presumably protected by membranes. When metabolically active ATP was prelabelled with [3H]adenosine, 76% of the radioactivity was associated with the free pool of ATP. When the preparation was depolarized in a calcium containing medium, there was a decrease in the specific radioactivity of ATP in the free pool and an increase in the bound pool. These results reflect that the patterns of distribution of ACh and ATP, in this synaptosomal preparation, are similar in resting conditions and during K+ depolarization.

The role of thiols in nucleotide uptake into synaptic vesicles from Torpedo marmorata

We have employed sulfhydryl group reagents in an attempt to determine the mechanism by which the transport of nucleotides into synaptic vesicles is controlled. Transport proved to be sensitive to N-ethylmaleimide; radiolabelled N-ethylmaleimide was used to locate the sulfhydryl group to the translocase-associated molecule previously identified as a polypeptide of Mr 34,000 [Lee and Witzemann (1983) Biochemistry 22, 6123-6130]. The nucleotide uptake was 75% inhibited by the mercurials rho-hydroxymercuribenzoate and rho-chloromercuriphenylsulfonate. Uptake was also sensitive to the reagents phenylarsine oxide and iodosobenzoic acid, which are specific for dithiols. These results indicate that a readily accessible dithiol is critical for nucleotide transport. Using the lipophilic oxidants iodosobenzoic acid and plumbagin, we demonstrated that nucleotide uptake was inhibited upon oxidation of the dithiol but that this did not involve an alteration in the affinity of the translocase for its substrate.

Cholinergic synaptic vesicles from Torpedo marmorata contain an atractyloside-binding protein related to the mitochondrial ADP/ATP carrier

Atractyloside is known to bind to the ADP/ATP translocase of the inner mitochondrial membrane, a complex formed by two basic protein subunits of relative molecular mass around 30 000. We found that synaptic vesicles from the electric organ of Torpedo marmorata, which store acetylcholine and ATP, bind atractyloside as well. Similarly to mitochondria, a protein-atractyloside complex could be solubilized from vesicle membranes with Triton X-100. Characterization of the complex by gel filtration, isoelectric focusing and gel electrophoresis revealed that atractyloside was bound to protein V11, earlier described as a major vesicle membrane component with a relative molecular mass around 34 000 and a basic isoelectric point. Since earlier experiments have already shown that uptake of ATP into isolated vesicles in vitro is inhibited by atractyloside, we can conclude now that V11 constitutes the nucleotide carrier of this secretory organelle. The structural and functional relationship of the mitochondrial and vesicular nucleotide translocases suggest a common evolutionary origin.

Adenine nucleotide and phosphoenolpyruvate transport by bovine chromaffin granule "ghosts"

Purified bovine adrenal chromaffin granule "ghosts" (resealed membranes) accumulate nucleotides and phosphoenolpyruvate in a temperature-dependent process. Equilibration takes several hours, the compounds being accumulated to not more than twice their concentration in the medium. This uptake occurs primarily by a non-saturable diffusion process, and rates and extents show little dependence on membrane energization. Uptake rates are inhibited less than 50% by compounds that discharge the "ghost" membrane potential, or by atractylate or low concentrations of Cibacron Blue. The data gave little support to the theory that chromaffin granules have a membrane potential-dependent adenine nucleotide transport process and that nucleotides are in equilibrium with the positive-inside potential across the granule membrane.

Specificity and properties of the nucleotide carrier in chromaffin granules from bovine adrenal medulla

1. The influence of various substances on the uptake of [3H]ATP and [14C]-noradrenaline into isolated bovine chromaffin granules was investigated. The carrier-mediated [3H]ATP uptake is specifically inhibited by SO42-, PO43- and phosphoenolpyruvate. Compounds with carboxylic acid or sulphonic acid groups had no significant inhibitory effects on either uptake. 2. 35SO42-, 32PO43- and phosphoenol[14C]pyruvate are taken up into chromaffin granules by a temperature-dependent process that is inhibited by atractyloside, uncouplers of oxidative phosphorylation and lipid-permeant anions. The apparent Km of 35SO42- uptake is 0.4 mM. 3. These results indicate that the nucleotide carrier in chromaffin granules has a broad specificity, transporting compounds with two strong negative charges. 4. Amino acid probes influence the uptake of ATP and catecholamines differently. Pyridoxal phosphate inhibits both uptake processes, 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid preferentially blocks ATP uptake, whereas phenylglyoxal blocks only ATP transport. It is suggested that the nucleotide carrier possesses arginine residues in a functionally important position. 5. The significance of these results obtained on isolated granules for the function of chromaffin granules within the cell is discussed.

Calmodulin stimulation of 45Ca2+ transport and protein phosphorylation in cholinergic synaptic vesicles

Cholinergic synaptic vesicles isolated from the electric organ of Torpedo californica exhibit ATP-dependent uptake of 45Ca2+ that is stimulated by exogenous calmodulin. ATP-independent uptake also occurs, but it is only weakly stimulated by calmodulin. Saturating calmodulin decreased the Michaelis constant for ATP-dependent 45Ca2+ uptake from 52 +/- 0.4 to 12 +/- 0.2 microM and increased the maximal velocity from 3.4 +/- 0.3 to 5.2 +/- 0.5 nmol/mg of protein per min. The dose-response curve for calmodulin-dependent stimulation showed a maximal increase of 3.5-fold in the uptake rate; 0.2 microM calmodulin gave half-maximal stimulation. The activity of the vesicle-associated ATPase was unaffected. Incubation of vesicles with [gamma-32P]ATP and Ca2+ resulted in phosphorylation of four polypeptides of molecular weights about 64,000, 58,000, 54,000, and 41,000 when calmodulin was added. Vesicles that were previously phosphorylated and purified exhibited 2-fold enhanced ATP-independent uptake of 45Ca2+. Cyclic AMP could not substitute for calmodulin. The calcium transport system of the cholinergic synaptic vesicle is regulated by a calcicalmodulin-dependent protein kinase that is vesicle-associated.