Functional and anatomical identification of a vesicular transporter mediating neuronal ATP release

Role of vesicular nucleotide transporter VNUT (SLC17A9) in release of ATP from AR42J cells and mouse pancreatic acinar cells

ATP is released from cells in response to various stimuli. Our previous studies on pancreas indicated that pancreatic acini could be major stores of secreted ATP. In the present study, our aim was to establish the role of the vesicular nucleotide transporter (VNUT), SLC17A9, in storage and release of ATP. Freshly prepared acini from mice and AR42J rat acinar cells were used in this study. We illustrate that in AR42J cells, quinacrine (an ATP store marker) and Bodipy ATP (a fluorescent ATP analog) co-localized with VNUT-mCherry to vesicles/granules. Furthermore, in acini and AR42J cells, a marker of the zymogen granule membranes, Rab3D, and VNUT co-localized. Dexamethasone treatment of AR42J cells promoted formation of acinar structures, paralleled by increased amylase and VNUT expression, and increased ATP release in response to cholinergic stimulation. Mechanical stimulus (pressure) and cell swelling also induced ATP release, but this was not influenced by dexamethasone, most likely indicating different non-zymogen-related release mechanism. In conclusion, we propose that VNUT-dependent ATP release pathway is associated with agonist-induced secretion process and downstream purinergic signalling in pancreatic ducts.

Neuromodulation: purinergic signaling in respiratory control

The main functions of the respiratory neural network are to produce a coordinated, efficient, rhythmic motor behavior and maintain homeostatic control over blood oxygen and CO2/pH levels. Purinergic (ATP) signaling features prominently in these homeostatic reflexes. The signaling actions of ATP are produced through its binding to a diversity of ionotropic P2X and metabotropic P2Y receptors. However, its net effect on neuronal and network excitability is determined by the interaction between the three limbs of a complex system comprising the signaling actions of ATP at P2Rs, the distribution of multiple ectonucleotidases that differentially metabolize ATP into ADP, AMP, and adenosine (ADO), and the signaling actions of ATP metabolites, especially ADP at P2YRs and ADO at P1Rs. Understanding the significance of purinergic signaling is further complicated by the fact that neurons, glia, and the vasculature differentially express P2 and P1Rs, and that both neurons and glia release ATP. This article reviews at cellular, synaptic, and network levels, current understanding and emerging concepts about the diverse roles played by this three-part signaling system in: mediating the chemosensitivity of respiratory networks to hypoxia and CO2/pH; modulating the activity of rhythm generating networks and inspiratory motoneurons, and; controlling blood flow through the cerebral vasculature.

Neurovascular signaling in the brain and the pathological consequences of hypertension

The execution and maintenance of all brain functions are dependent on a continuous flow of blood to meet the metabolic needs of the tissue. To ensure the delivery of resources required for neural processing and the maintenance of neural homeostasis, the cerebral vasculature is elaborately and extensively regulated by signaling from neurons, glia, interneurons, and perivascular nerves. Hypertension is associated with impaired neurovascular regulation of the cerebral circulation and culminates in neurodegeneration and cognitive dysfunction. Here, we review the physiological processes of neurovascular signaling in the brain and discuss mechanisms of hypertensive neurovascular dysfunction.

Ecto-5'-nucleotidase (CD73)-mediated formation of adenosine is critical for the striatal adenosine A2A receptor functions

Adenosine is a neuromodulator acting through inhibitory A1 receptors (A1Rs) and facilitatory A2ARs, which have similar affinities for adenosine. It has been shown that the activity of intracellular adenosine kinase preferentially controls the activation of A1Rs, but the source of the adenosine activating A2ARs is unknown. We now show that ecto-5'-nucleotidase (CD73), the major enzyme able to convert extracellular AMP into adenosine, colocalizes with A2ARs in the basal ganglia. In addition to astrocytes, striatal CD73 is prominently localized to postsynaptic sites. Notably, CD73 coimmunoprecipitated with A2ARs and proximity ligation assays confirmed the close proximity of CD73 and A2ARs in the striatum. Accordingly, the cAMP formation in synaptosomes as well as the hypolocomotion induced by a novel A2AR prodrug that requires CD73 metabolization to activate A2ARs were observed in wild-type mice, but not in CD73 knock-out (KO) mice or A2AR KO mice. Moreover, CD73 KO mice displayed increased working memory performance and a blunted amphetamine-induced sensitization, mimicking the phenotype of global or forebrain-A2AR KO mice, as well as upon pharmacological A2AR blockade. These results show that CD73-mediated formation of extracellular adenosine is responsible for the activation of striatal A2AR function. This study points to CD73 as a new target that can fine-tune A2AR activity, and a novel therapeutic target to manipulate A2AR-mediated control of striatal function and neurodegeneration.

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.

Are vesicular neurotransmitter transporters potential treatment targets for temporal lobe epilepsy?

The vesicular neurotransmitter transporters (VNTs) are small proteins responsible for packing synaptic vesicles with neurotransmitters thereby determining the amount of neurotransmitter released per vesicle through fusion in both neurons and glial cells. Each transporter subtype was classically seen as a specific neuronal marker of the respective nerve cells containing that particular neurotransmitter or structurally related neurotransmitters. More recently, however, it has become apparent that common neurotransmitters can also act as co-transmitters, adding complexity to neurotransmitter release and suggesting intriguing roles for VNTs therein. We will first describe the current knowledge on vesicular glutamate transporters (VGLUT1/2/3), the vesicular excitatory amino acid transporter (VEAT), the vesicular nucleotide transporter (VNUT), vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT) and the vesicular γ-aminobutyric acid (GABA) transporter (VGAT) in the brain. We will focus on evidence regarding transgenic mice with disruptions in VNTs in different models of seizures and epilepsy. We will also describe the known alterations and reorganizations in the expression levels of these VNTs in rodent models for temporal lobe epilepsy (TLE) and in human tissue resected for epilepsy surgery. Finally, we will discuss perspectives on opportunities and challenges for VNTs as targets for possible future epilepsy therapies.

Microglia release ATP by exocytosis

Microglia survey the brain environment by sensing several types of diffusible molecules, among which extracellular nucleotides released/leaked from damaged cells have central roles. Microglia sense ATP or other nucleotides by multiple P2 receptors, after which they change into several different phenotypes. However, so far, it is largely unknown whether microglia themselves release ATP and, if so, by what mechanism. Here we show that exocytosis is the mechanism by which microglia release ATP. When we stimulated microglia with ionomycin, they released ATP and the release was dependent on Ca²⁺, vesicular H⁺-ATPase, or SNAREs but independent of connexin/pannexin hemichannels. VNUT was found to be expressed in microglia and exhibited no colocalization with lysosome. We also visualized the exocytosis of ATP by a quinacrine-based fluorescent time-lapse imaging. Moreover, we found that lipopolysaccharide increased the ionomycin-induced release of ATP, which was dependent on the increase in VNUT. Taken together, our data suggested that exocytosis is the mechanism of ATP release from microglia. When activated, they would release ATP by increasing VNUT-dependent exocytotic mechanisms.

P2Y1 receptors expressed by C1 neurons determine peripheral chemoreceptor modulation of breathing, sympathetic activity, and blood pressure

Catecholaminergic C1 cells of the rostral ventrolateral medulla (RVLM) are key determinants of the sympathoexcitatory response to peripheral chemoreceptor activation. Overactivation of this reflex is thought to contribute to increased sympathetic activity and hypertension; however, molecular mechanisms linking peripheral chemoreceptor drive to hypertension remain poorly understood. We have recently determined that activation of P2Y1 receptors in the RVLM mimicked effects of peripheral chemoreceptor activation. Therefore, we hypothesize that P2Y1 receptors regulate peripheral chemoreceptor drive in this region. Here, we determine whether P2Y1 receptors are expressed by C1 neurons in the RVLM and contribute to peripheral chemoreceptor control of breathing, sympathetic activity, and blood pressure. We found that injection of a specific P2Y1 receptor agonist (MRS2365) into the RVLM of anesthetized adult rats increased phrenic nerve activity (≈55%), sympathetic nerve activity (38 ± 6%), and blood pressure (23 ± 1 mm Hg), whereas application of a specific P2Y1 receptor antagonist (MRS2179) decreased peripheral chemoreceptor-mediated activation of phrenic nerve activity, sympathetic nerve activity, and blood pressure. To establish that P2Y1 receptors are expressed by C1 cells, we determine in the brain slice preparation using cell-attached recording techniques that cells responsive to MRS2365 are immunoreactive for tyrosine hydroxylase (a marker of C1 cells), and we determine in vivo that C1-lesioned animals do not respond to RVLM injection of MRS2365. These data identify P2Y1 receptors as key determinants of peripheral chemoreceptor regulation of breathing, sympathetic nerve activity, and blood pressure.

Vesicular nucleotide transporter regulates the nucleotide content in airway epithelial mucin granules

Nucleotides within the airway surface liquid promote fluid secretion via activation of airway epithelial purinergic receptors. ATP is stored within and released from mucin granules as co-cargo with mucins, but the mechanism by which ATP, and potentially other nucleotides, enter the lumen of mucin granules is not known. We assessed the contribution of the recently identified SLC17A9 vesicle nucleotide transporter (VNUT) to the nucleotide availability within isolated mucin granules and further examined the involvement of VNUT in mucin granule secretion-associated nucleotide release. RT-PCR and Western blot analyses indicated that VNUT is abundantly expressed in airway epithelial goblet-like Calu-3 cells, migrating as a duplex with apparent mobility of 55 and 60 kDa. Subcellular fractionation studies indicated that VNUT55 was associated with high-density mucin granules, whereas VNUT60 was associated with low-density organelles. Immunofluorescence studies showed that recombinant VNUT localized to mucin granules and other organelles. Mucin granules isolated from VNUT short hairpin RNA-expressing cells exhibited a marked reduction of ATP, ADP, AMP, and UTP levels within granules. Ca(2+)-regulated vesicular ATP release was markedly reduced in these cells, but mucin secretion was not affected. These results suggest that VNUT is the relevant nucleotide transporter responsible for the uptake of cytosolic nucleotides into mucin granules. By controlling the entry of nucleotides into mucin granules, VNUT contributes to the release of purinergic signaling molecules necessary for the proper hydration of co-released mucins.

Differential sensitivity of brainstem versus cortical astrocytes to changes in pH reveals functional regional specialization of astroglia

Astrocytes might function as brain interoceptors capable of detecting different (chemo)sensory modalities and transmitting sensory information to the relevant neural networks controlling vital functions. For example, astrocytes that reside near the ventral surface of the brainstem (central respiratory chemosensitive area) respond to physiological decreases in pH with vigorous elevations in intracellular Ca(2+) and release of ATP. ATP transmits astroglial excitation to the brainstem respiratory network and contributes to adaptive changes in lung ventilation. Here we show that in terms of pH-sensitivity, ventral brainstem astrocytes are clearly distinct from astrocytes residing in the cerebral cortex. We monitored vesicular fusion in cultured rat brainstem astrocytes using total internal reflection fluorescence microscopy and found that ∼35% of them respond to acidification with an increased rate of exocytosis of ATP-containing vesicular compartments. These fusion events require intracellular Ca(2+) signaling and are independent of autocrine ATP actions. In contrast, the rate of vesicular fusion in cultured cortical astrocytes is not affected by changes in pH. Compared to cortical astrocytes, ventral brainstem astrocytes display higher levels of expression of genes encoding proteins associated with ATP vesicular transport and fusion, including vesicle-associated membrane protein-3 and vesicular nucleotide transporter. These results suggest that astrocytes residing in different parts of the rat brain are functionally specialized. In contrast to cortical astrocytes, astrocytes of the brainstem chemosensitive area(s) possess signaling properties that are functionally relevant-they are able to sense changes in pH and respond to acidification with enhanced vesicular release of ATP.

Progress in understanding of inhibitory purinergic neuromuscular transmission in the gut

Recent studies with genetic deletion of P2Y1 receptor (P2Y1-/-) have clinched its role in enteric purinergic inhibitory neurotransmission and suggested that β-NAD may be the purinergic inhibitory neurotransmitter in the colon. In this issue of the Journal, Gil and colleagues extend their earlier observations to the cecum and gastric antrum, showing that P2Y1 receptor mediated purinergic inhibition may be a general phenomenon in the gut. However, the authors made an unexpected observation in contrast with their earlier findings in the colon that neither the selective P2Y1 receptor antagonist MRS2500, nor P2Y1 receptor deletion, blocked the hyperpolarizing action of β-NAD in the cecum. These observations suggest that β-NAD may be the purinergic inhibitory neurotransmitter in the colon, but not in the cecum. This group had previously reported that the selective P2Y1 receptor antagonist MRS 2179 suppressed the hyperpolarizing action of ATP or ADP. Further studies are now needed to determine whether the hyperpolarizing actions of ATP and ADP are suppressed by the more potent P2Y1 antagonist MRS2500, and in P2Y1-/- mutants to test the intriguing possibility that different purines serve as purinergic inhibitory neurotransmitters in the colon and cecum and perhaps in different parts of the gut. Studies in P2Y1-/- mice will resolve other issues in purinergic neurotransmission including cellular localization of the β-NAD or ATP-activated P2Y1 receptors on either smooth muscle cells or PDGFRα+ fibroblast-like cells, relationship of purinergic to nitrergic neurotransmission and understanding the physiological and clinical importance of purinergic transmission in gastrointestinal motility and its disorders.

Vesicular nucleotide transporter-mediated ATP release regulates insulin secretion

Extracellular ATP plays a critical role in regulating insulin secretion in pancreatic β cells. The ATP released from insulin secretory vesicles has been proposed to be a major source of extracellular ATP. Currently, the mechanism by which ATP accumulates into insulin secretory granules remains elusive. In this study, the authors identified the expression of a vesicular nucleotide transporter (VNUT) in mouse pancreas, isolated mouse islets, and MIN6 cells, a mouse β cell line. Immunohistochemistry and immunofluorescence revealed that VNUT colocalized extensively with insulin secretory granules. Functional studies showed that suppressing endogenous VNUT expression in β cells by small hairpin RNA knockdown greatly reduced basal- and glucose-induced ATP release. Importantly, knocking down VNUT expression by VNUT small hairpin RNA in MIN6 cells and isolated mouse islets dramatically suppressed basal insulin release and glucose-stimulated insulin secretion (GSIS). Moreover, acute pharmacologic blockade of VNUT with Evans blue, a VNUT antagonist, greatly attenuated GSIS in a dose-dependent manner. Exogenous ATP treatment effectively reversed the insulin secretion defect induced by both VNUT knockdown and functional inhibition, indicating that VNUT-mediated ATP release is essential for maintaining normal insulin secretion. In contrast to VNUT knockdown, overexpression of VNUT in β cells resulted in excessive ATP release and enhanced basal insulin secretion and GSIS. Elevated insulin secretion induced by VNUT overexpression was reversed by pharmacologic inhibition of P2X but not P2Y purinergic receptors. This study reveals VNUT is expressed in pancreatic β cells and plays an essential and novel role in regulating insulin secretion through vesicular ATP release and extracellular purinergic signaling.

Computational quest for understanding the role of astrocyte signaling in synaptic transmission and plasticity

The complexity of the signaling network that underlies astrocyte-synapse interactions may seem discouraging when tackled from a theoretical perspective. Computational modeling is challenged by the fact that many details remain hitherto unknown and conventional approaches to describe synaptic function are unsuitable to explain experimental observations when astrocytic signaling is taken into account. Supported by experimental evidence is the possibility that astrocytes perform genuine information processing by means of their calcium signaling and are players in the physiological setting of the basal tone of synaptic transmission. Here we consider the plausibility of this scenario from a theoretical perspective, focusing on the modulation of synaptic release probability by the astrocyte and its implications on synaptic plasticity. The analysis of the signaling pathways underlying such modulation refines our notion of tripartite synapse and has profound implications on our understanding of brain function.

Vesicular uptake and exocytosis of L-aspartate is independent of sialin

The mechanism of release and the role of l-aspartate as a central neurotransmitter are controversial. A vesicular release mechanism for l-aspartate has been difficult to prove, as no vesicular l-aspartate transporter was identified until it was found that sialin could transport l-aspartate and l-glutamate when reconstituted into liposomes. We sought to clarify the release mechanism of l-aspartate and the role of sialin in this process by combining l-aspartate uptake studies in isolated synaptic vesicles with immunocyotchemical investigations of hippocampal slices. We found that radiolabeled l-aspartate was taken up into synaptic vesicles. The vesicular l-aspartate uptake, relative to the l-glutamate uptake, was twice as high in the hippocampus as in the whole brain, the striatum, and the entorhinal and frontal cortices and was not inhibited by l-glutamate. We further show that sialin is not essential for exocytosis of l-aspartate, as there was no difference in ATP-dependent l-aspartate uptake in synaptic vesicles from sialin-knockout and wild-type mice. In addition, expression of sialin in PC12 cells did not result in significant vesicle uptake of l-aspartate, and depolarization-induced depletion of l-aspartate from hippocampal nerve terminals was similar in hippocampal slices from sialin-knockout and wild-type mice. Further, there was no evidence for nonvesicular release of l-aspartate via volume-regulated anion channels or plasma membrane excitatory amino acid transporters. This suggests that l-aspartate is exocytotically released from nerve terminals after vesicular accumulation by a transporter other than sialin.

Neuronal and extraneuronal release of ATP and NAD(+) in smooth muscle

Adenosine 5'-triphosphate (ATP) and nicotinamide adenine dinucleotide (NAD(+) ) are key intracellular constituents involved in energy transfer and redox homeostasis in the cell. ATP is also released in the extracellular space and in the past half century it has been assumed to be the purinergic neurotransmitter in many systems including smooth muscle. In some smooth muscles (i.e., the human urinary bladder detrusor muscle), ATP does appear to be primarily released from nerves upon action potential firings, but in other smooth muscles (i.e., the human large intestine), ATP does not mimic the endogenous purine neurotransmitter. It was recently found that NAD(+) , another ubiquitous intracellular adenine nucleotide, also follows a regulated release in neurosecretory cells, vascular and visceral smooth muscles, and the brain. In some cases, NAD(+) fulfills presynaptic and postsynaptic criteria for a neurotransmitter better than ATP. Therefore, the purine hypothesis of neural regulation in smooth muscle is in need of reevaluation. This article will briefly review the current understanding of neuronal and extraneuronal release of purines in smooth muscle with emphasis on the roles of extracellular ATP and NAD(+) and, further, will discuss more recent information about the likely involvement of multiple purines in smooth muscle neurotransmission.

Exocytosis in astrocytes: transmitter release and membrane signal regulation

Astrocytes, a type of glial cells in the brain, are eukaryotic cells, and a hallmark of these are subcellular organelles, such as secretory vesicles. In neurons vesicles play a key role in signaling. Upon a stimulus-an increase in cytosolic concentration of free Ca(2+) ([Ca(2+)](i))-the membrane of vesicle fuses with the presynaptic plasma membrane, allowing the exit of neurotransmitters into the extracellular space and their diffusion to the postsynaptic receptors. For decades it was thought that such vesicle-based mechanisms of gliotransmitter release were not present in astrocytes. However, in the last 30 years experimental evidence showed that astrocytes are endowed with mechanisms for vesicle- and non-vesicle-based gliotransmitter release mechanisms. The aim of this review is to focus on exocytosis, which may play a role in gliotransmission and also in other forms of cell-to-cell communication, such as the delivery of transporters, ion channels and antigen presenting molecules to the cell surface.

Role of myosin Va in purinergic vesicular neurotransmission in the gut

We examined the hypothesis that myosin Va, by transporting purinergic vesicles to the varicosity membrane for exocytosis, plays a key role in purinergic vesicular neurotransmission. Studies were performed in wild-type (WT) and myosin Va-deficient dilute, brown, nonagouti (DBA) mice. Intracellular microelectrode recordings were made in mouse antral muscle strips. Purinergic inhibitory junction potential (pIJP) was recorded under nonadrenergic noncholinergic conditions after masking the nitrergic junction potentials. DBA mice showed reduced pIJP but normal hyperpolarizing response to P2Y1 receptor agonist MRS-2365. To investigate the mechanism of reduced purinergic transmission in DBA mice, studies were performed in isolated varicosities obtained from homogenates of whole gut tissues by ultracentrifugation and sucrose cushion purification. Purinergic varicosities were identified in tissue sections and in isolated varicosities by immunostaining for the vesicular ATP transporter, the solute carrier protein SLC17A9. The varicosities were similar in WT and DBA mice. Myosin Va was markedly reduced in DBA varicosities compared with the WT varicosities. Proximity ligation assay showed that myosin Va was closely associated with SLC17A9. Vesicular exoendocytosis was examined by FM1-43 staining of varicosities, which showed that exoendocytosis after KCl stimulation was impaired in DBA varicosities compared with WT varicosities. These studies show that SLC17A9 identifies ATP-containing purinergic varicosities. Myosin Va associates with SLC17A9-stained vesicles and possibly transports them to varicosity membrane for exocytosis. In myosin Va-deficient mice, purinergic inhibitory neurotransmission is impaired.

Chemical phenotypes of P2X2 purinoreceptor immunoreactive cell bodies in the area postrema

Purines such as adenosine 5'-triphosphate (ATP) act as extracellular messengers through specific purinergic receptors. Three different classes of purinergic receptors have been identified and termed P1, P2X, and P2Y. The purinergic receptor subunit P2X2 is a ligand-gated ion channel that is widely expressed by neurons in the CNS. In the brainstem medulla oblongata, the ionotropic P2X2 receptor (P2X2R) is enriched in the area postrema (AP). Two different antisera to P2X2R were used to determine the chemical nature of P2X2R immunoreactive cell bodies in the rat AP, an area lacking a blood-brain barrier. Subcellularly, P2X2R immunoreactivity was located to the periphery of individual cell bodies. The majority of P2X2R-immunoreactive cells were shown to contain tyrosine hydroxylase (TH) (63.5 ± 7.7%) and dopamine β-hydroxylase (61.5 ± 5.1%). Phenylethanolamine N-methyltransferase (PNMT)-containing cells were not detected in the AP, supporting a noradrenergic nature of P2X2R cells in the AP. There were no P2X2R-immunoreactive cells in the AP that contained the GABA-synthesizing enzyme glutamic acid decarboxylase 65. Only single vesicular glutamate transporter 2-immunoreactive cell bodies that were not P2X2R-positive were demonstrated in the AP. Some P2X2R-positive cells in the AP were immunoreactive for the neuropeptides substance P and pituitary adenylate cyclase-activating polypeptide, whereas dynorphin-, enkephalin-, or cholecystokinin-positive cells were not P2X2R-immunoreactive. Presence of P2X2R in a majority of noradrenergic cells of the AP implies that ATP may have a regulatory action on neuronal noradrenaline release from the AP, a circumventricular organ with a strategic position enabling interactions between circulating substances and the central nervous system.