Vesicular ATP transport is a hard (V)NUT to crack

Suo Quan Wan Protects Mouse From Early Diabetic Bladder Dysfunction by Mediating Motor Protein Myosin Va and Transporter Protein SLC17A9

Objective: To investigate the effects of Suo Quan Wan (SQW), a traditional Chinese herbal formula, on the overactive bladder (OAB) of type 2 diabetes mellitus (T2DM) mouse models, particularly on its function of mediating the gene and protein expression levels of myosin Va and SLC17A9.

Materials and Methods: After 4 weeks high-fat diet (HFD) feeding, C57BL/6J mice were injected with streptozotocin (100 mg/kg) for four times. After 3 weeks, the diabetic mice were treated with SQW for another 3 weeks. Voided stain on paper assay, fasting blood glucose (FBG) test, and oral glucose tolerance test (OGTT) were conducted. Urodynamic test, tension test [α,β-methylene ATP, electrical-field stimulation (EFS), KCl, and carbachol] and histomorphometry were also performed. Western blot analysis and qPCR assays were used to quantify the expression levels of myosin Va and SLC17A9.

Results: The diabetic mice exhibited decreased weight but increased water intake, urine production, FBG, and OGTT. No significant changes were observed after 3 weeks SQW treatment. Urodynamic test indicated that the non-voiding contraction (NVC) frequency, maximum bladder capacity (MBC), residual volume (RV), and bladder compliance (BC) were remarkably increased in the diabetic mice, whereas the voided efficiency (VE) was decreased as a feature of overactivity. Compared with the model mice, SQW treatment significantly improved urodynamic urination with decreased NVC, MBC, RV, and BC, and increased VE. Histomorphometry results showed that the bladder wall of the diabetic mice thickened, and SQW effectively attenuated the pathological alterations. The contract responses of bladder strips to all stimulators were higher in the DSM strips of diabetic mice, whereas SQW treatment markedly decreased the contraction response for all stimuli. Moreover, the protein and gene expression levels of myosin Va and SLC17A9 were up-regulated in the bladders of diabetic mice, but SQW treatment restored such alterations.

Conclusion: T2DM mice exhibited the early phase of diabetic bladder dysfunction (DBD) characterized by OAB and bladder dysfunction. SQW can improve the bladder storage and micturition of DBD mice by mediating the protein and gene expression levels of myosin Va and SLC17A9 in the bladder, instead of improving the blood glucose level.

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.

Osteoblastic Lrp4 promotes osteoclastogenesis by regulating ATP release and adenosine-A2AR signaling

Lrp4 is mutated in patients with high-bone-mass diseases. Loss of Lrp4 in osteoblasts (OBs) increases bone formation by OBs and decreases bone resorption by osteoclasts through an unclear mechanism. Xiong et al. show that overproduction of extracellular adenosine in Lrp4-deficient OBs, which are derived from ATP hydrolysis and signals through A_2AR and RANK, may underlie Lrp4 regulation of osteoclastogenesis.

Bone homeostasis depends on the functional balance of osteoblasts (OBs) and osteoclasts (OCs). Lrp4 is a transmembrane protein that is mutated in patients with high bone mass. Loss of Lrp4 in OB-lineage cells increases bone mass by elevating bone formation by OBs and reducing bone resorption by OCs. However, it is unclear how Lrp4 deficiency in OBs impairs osteoclastogenesis. Here, we provide evidence that loss of Lrp4 in the OB lineage stabilizes the prorenin receptor (PRR) and increases PRR/V-ATPase–driven ATP release, thereby enhancing the production of the ATP derivative adenosine. Both pharmacological and genetic inhibition of adenosine-_2A receptor (A_2AR) in culture and Lrp4 mutant mice diminishes the osteoclastogenic deficit and reduces trabecular bone mass. Furthermore, elevated adenosine-A_2AR signaling reduces receptor activator of nuclear factor κB (RANK)–mediated osteoclastogenesis. Collectively, these results identify a mechanism by which osteoblastic Lrp4 controls osteoclastogenesis, reveal a cross talk between A_2AR and RANK signaling in osteoclastogenesis, and uncover an unrecognized pathophysiological mechanism of high-bone-mass disorders.

Structure, Function, and Drug Interactions of Neurotransmitter Transporters in the Postgenomic Era

Vesicular neurotransmitter transporters are responsible for the accumulation of neurotransmitters in secretory vesicles and play essential roles in chemical transmission. The SLC17 family contributes to sequestration of anionic neurotransmitters such as glutamate, aspartate, and nucleotides. Identification and subsequent cellular and molecular biological studies of SLC17 transporters unveiled the principles underlying the actions of these transporters. Recent progress in reconstitution methods in combination with postgenomic approaches has advanced studies on neurotransmitter transporters. This review summarizes the molecular properties of SLC17-type transporters and recent findings regarding the novel SLC18 transporter.

Fine-tuned ATP signals are acute mediators in osteocyte mechanotransduction

Osteocytes are considered the primary mechanosensors of bone, but the signaling pathways they apply in mechanotransduction are still incompletely investigated and characterized. A growing body of data strongly indicates that P2 receptor signaling among osteoblasts and osteoclasts has regulatory effects on bone remodeling. Therefore, we hypothesized that ATP signaling is also applied by osteocytes in mechanotransduction. We applied a short fluid pulse on MLO-Y4 osteocyte-like cells during real-time detection of ATP and demonstrated that mechanical stimulation activates the acute release of ATP and that these acute ATP signals are fine-tuned according to the magnitude of loading. ATP release was then challenged by pharmacological inhibitors, which indicated a vesicular release pathway for acute ATP signals. Finally, we showed that osteocytes express functional P2X2 and P2X7 receptors and respond to even low concentrations of nucleotides by increasing intracellular calcium concentration. These results indicate that in osteocytes, vesicular ATP release is an acute mediator of mechanical signals and the magnitude of loading. These and previous results, therefore, implicate purinergic signaling as an early signaling pathway in osteocyte mechanotransduction.

Apyrase suppression raises extracellular ATP levels and induces gene expression and cell wall changes characteristic of stress responses

Plant cells release ATP into their extracellular matrix as they grow, and extracellular ATP (eATP) can modulate the rate of cell growth in diverse tissues. Two closely related apyrases (APYs) in Arabidopsis (Arabidopsis thaliana), APY1 and APY2, function, in part, to control the concentration of eATP. The expression of APY1/APY2 can be inhibited by RNA interference, and this suppression leads to an increase in the concentration of eATP in the extracellular medium and severely reduces growth. To clarify how the suppression of APY1 and APY2 is linked to growth inhibition, the gene expression changes that occur in seedlings when apyrase expression is suppressed were assayed by microarray and quantitative real-time-PCR analyses. The most significant gene expression changes induced by APY suppression were in genes involved in biotic stress responses, which include those genes regulating wall composition and extensibility. These expression changes predicted specific chemical changes in the walls of mutant seedlings, and two of these changes, wall lignification and decreased methyl ester bonds, were verified by direct analyses. Taken together, the results are consistent with the hypothesis that APY1, APY2, and eATP play important roles in the signaling steps that link biotic stresses to plant defense responses and growth changes.

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.

Inflammation promotes airway epithelial ATP release via calcium-dependent vesicular pathways

ATP in airway surface liquid (ASL) controls mucociliary clearance functions via the activation of airway epithelial purinergic receptors. However, abnormally elevated ATP levels have been reported in inflamed airways, suggesting that excessive ATP in ASL contributes to airway inflammation. Despite these observations, little is known about the mechanisms of ATP accumulation in the ASL covering inflamed airways. In this study, links between cystic fibrosis (CF)-associated airway inflammation and airway epithelial ATP release were investigated. Primary human bronchial epithelial (HBE) cells isolated from CF lungs exhibited enhanced IL-8 secretion after 6 to 11 days, but not 28 to 35 days, in culture, compared with normal HBE cells. Hypotonic cell swelling-promoted ATP release was increased in 6- to 11-day-old CF HBE cells compared with non-CF HBE cells, but returned to normal values after 28 to 35 days in culture. The exposure of non-CF HBE cells to airway secretions isolated from CF lungs, namely, sterile supernatants of mucopurulent material (SMM), also caused enhanced IL-8 secretion and increased ATP release. The SMM-induced increase in ATP release was sensitive to Ca(2+) chelation and vesicle trafficking/exocytosis inhibitors, but not to pannexin inhibition. Transcript levels of the vesicular nucleotide transporter, but not pannexin 1, were up-regulated after SMM exposure. SMM-treated cultures displayed increased basal mucin secretion, but mucin secretion was not enhanced in response to hypotonic challenge after the exposure of cells to either vehicle or SMM. We propose that CF airway inflammation up-regulates the capacity of airway epithelia to release ATP via Ca(2+)-dependent vesicular mechanisms not associated with mucin granule secretion.

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.

Both the stimulation and inhibition of root hair growth induced by extracellular nucleotides in Arabidopsis are mediated by nitric oxide and reactive oxygen species

Root hairs secrete ATP as they grow, and extracellular ATP and ADP can trigger signaling pathways that regulate plant cell growth. In several plant tissues the level of extracellular nucleotides is limited in part by ectoapyrases (ecto-NTPDases), and the growth of these tissues is strongly influenced by their level of ectoapyrase expression. Both chemical inhibition of ectoapyrase activity and suppression of the expression of two ectoapyrase enzymes by RNAi in Arabidopsis resulted in inhibition of root hair growth. As assayed by a dose-response curve, different concentrations of the poorly hydrolysable nucleotides, ATPγS and ADPβS, could either stimulate (at 7.5-25 μM) or inhibit (at ≥ 150 μM) the growth rate of root hairs in less than an hour. Equal amounts of AMPS, used as a control, had no effect on root hair growth. Root hairs of nia1nia2 mutants, which are suppressed in nitric oxide (NO) production, and of atrbohD/F mutants, which are suppressed in the production of H(2)O(2), did not show growth responses to applied nucleotides, indicating that the growth changes induced by these nucleotides in wild-type plants were likely transduced via NO and H(2)O(2) signals. Consistent with this interpretation, treatment of root hairs with different concentrations of ATPγS induced different accumulations of NO and H(2)O(2) in root hair tips. Two mammalian purinoceptor antagonists also blocked the growth responses induced by extracellular nucleotides, suggesting that they were initiated by a receptor-based mechanism.

ATP storage and uptake by isolated pancreatic zymogen granules

ATP is released from pancreatic acini in response to cholinergic and hormonal stimulation. The same stimuli cause exocytosis of ZG (zymogen granules) and release of digestive enzymes. The aim of the present study was to determine whether ZG stored ATP and to characterize the uptake mechanism for ATP transport into the ZG. ZG were isolated and the ATP content was measured using luciferin/luciferase assays and was related to protein in the sample. The estimate of ATP concentration in freshly isolated granules was 40-120 microM. The ATP uptake had an apparent Km value of 4.9+/-2.1 mM when granules were incubated without Mg2+ and a Km value of 0.47+/-0.05 mM in the presence of Mg2+, both in pH 6.0 buffers. The uptake of ATP was significantly higher at pH 7.2 compared with pH 6.0 solutions. The anion transport blockers DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulfonate) and Evans Blue inhibited ATP transport. Western blot analysis on the ZG showed the presence of VNUT (vesicular nucleotide transporter). Together, these findings indicate that VNUT may be responsible for the ATP uptake into ZG. Furthermore, the present study shows the presence of ATP together with digestive enzymes in ZG. This indicates that co-released ATP would regulate P2 receptors in pancreatic ducts and, thus, ductal secretion, and this would aid delivery of enzymes to the duodenum.

Combinatorial method for overexpression of membrane proteins in Escherichia coli

Membrane proteins constitute 20-30% of all proteins encoded by the genome of various organisms. Large amounts of purified proteins are required for activity and crystallization attempts. Thus, there is an unmet need for a heterologous membrane protein overexpression system for purification, crystallization, and activity determination. We developed a combinatorial method for overexpressing and purifying membrane proteins using Escherichia coli. This method utilizes short hydrophilic bacterial proteins, YaiN and YbeL, fused to the ends of the membrane proteins to serve as facilitating factors for expression and purification. Fourteen prokaryotic and mammalian membrane proteins were expressed using this system. Moderate to high expression was obtained for most proteins, and detergent solubilization combined with a short purification process produced stable, monodispersed membrane proteins. Five of the mammalian membrane proteins, overexpressed using our system, were reconstituted into liposomes and exhibited transport activity comparable with the native transporters.