In mammals, toxic electrolytes of endogenous and exogenous origin are excreted through the urine and bile. Before excretion, these compounds cross numerous cellular membranes in a transporter-mediated manner. However, the protein transporters involved in the final excretion step are poorly understood. Here, we show that MATE1, a human and mouse orthologue of the multidrug and toxin extrusion family conferring multidrug resistance on bacteria, is primarily expressed in the kidney and liver, where it is localized to the luminal membranes of the urinary tubules and bile canaliculi. When expressed in HEK293 cells, MATE1 mediates H ؉ -coupled electroneutral exchange of tetraethylammonium and 1-methyl-4-phenylpyridinium. Its substrate specificity is similar to those of renal and hepatic H ؉ -coupled organic cations (OCs) export. Thus, MATE1 appears to be the long searched for polyspecific OC exporter that directly transports toxic OCs into urine and bile. multidrug and toxin extrusion ͉ multidrug export ͉ urinary tubule ͉ bile canaliculi

L-Glutamate is the major excitatory neurotransmitter in the central nervous system and plays important roles in many neuronal processes such as fast synaptic transmission and neuronal plasticity (1, 2). To use L-glutamate as an intercellular signaling molecule, neuronal cells develop the glutamatergic system. Thus, L-glutamate is accumulated in synaptic vesicles through vesicular glutamate transporters (VGLUTs), 1 and is secreted through exocytosis. The released L-glutamate binds to the receptor so as to transmit signals intercellularly. The excess amount of L-glutamate in synaptic cleft is sequestrated through plasma membrane-type glutamate transporter. Recent evidence has indicated that peripheral non-neuronal tissues also possess the glutamatergic system and use L-glutamate as an intercellular transmitter (3). The islet of Langerhans, a pancreatic miniature organ for the hormones regulating the blood glucose level, is composed of four major types of endocrine cells, i.e. insulin-secreting ␤ cells, glucagon-secreting ␣ cells, somatostatin-secreting ␦ cells, and pancreatic polypeptide-secreting F cells. These islet cells expresses functional glutamate receptors and plasma membrane-type glutamate transporter (4 -11), suggesting that L-glutamate functions as an intercellular transmitter in islet. In fact, L-glutamate has been shown to affect secretion of insulin or glucagon from islet cells, isolated islets, or perfused pancreas (4 -11). However, the role of L-glutamate as an intercellular chemical transmitter in the islets has been long controversial, mainly because critical issues, i.e. where, when, and how L-glutamate appears in the islets and what happens upon stimulation of glutamate receptors in the islets, remain unresolved.Recent findings indicate that brain-specific Na ϩ -dependent inorganic phosphate cotransporter (12) and its isoform, differentiation-associated Na ϩ -dependent inorganic phosphate cotransporter (13), function as VGLUTs and are thus abbreviated as VGLUT1 and VGLUT2, respectively (14 -21). These VGLUTs seem to be potential probes for the site of L-glutamate release in peripheral tissues as well as the central nervous system since these transporters are essential for L-glutamate signal output. We have shown that VGLUT2 is expressed in ␣TC6 cells, clonal ␣ cells, and islet ␣ cells, but not in ␤ or ␦ cells (18). These results are consistent with the occurrence of Ca 2ϩ -dependent exocytosis of L-glutamate from ␣TC6 cells (22) and suggest that ␣ cells are the sites of L-glutamate signal appearance.During course of the study, we noticed that the expression and subcellular localization of VGLUTs are of extraordinary

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In islets of Langerhans, L-glutamate is stored in glucagon-containing secretory granules of ␣-cells and cosecreted with glucagon under low-glucose conditions. The L-glutamate triggers secretion of ␥-aminobutyric acid (GABA) from ␤-cells, which in turn inhibits glucagon secretion from ␣-cells through the GABAA receptor. In the present study, we tested the working hypothesis that L-glutamate functions as an autocrine/paracrine modulator and inhibits glucagon secretion through a glutamate receptor (

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Osteoclasts are involved in the catabolism of the bone matrix and eliminate the resulting degradation products through transcytosis, but the molecular mechanism and regulation of transcytosis remain poorly understood. Upon differentiation, osteoclasts express vesicular glutamate transporter 1 (VGLUT1), which is essential for vesicular storage and subsequent exocytosis of glutamate in neurons. VGLUT1 is localized in transcytotic vesicles and accumulates L‐glutamate. Osteoclasts secrete L‐glutamate and the bone degradation products upon stimulation with KCl or ATP in a Ca2+‐dependent manner. KCl‐ and ATP‐dependent secretion of L‐glutamate was absent in osteoclasts prepared from VGLUT1−/− knockout mice. Osteoclasts express mGluR8, a class III metabotropic glutamate receptor. Its stimulation by a specific agonist inhibits secretion of L‐glutamate and bone degradation products, whereas its suppression by a specific antagonist stimulates bone resorption. Finally, it was found that VGLUT1−/− mice develop osteoporosis. Thus, in bone‐resorbing osteoclasts, L‐glutamate and bone degradation products are secreted through transcytosis and the released L‐glutamate is involved in autoregulation of transcytosis. Glutamate signaling may play an important role in the bone homeostasis.

S U M M A R YThe wide-ranging expression of glutamate receptors in peripheral tissues suggests an unexpectedly wider role(s) of L -glutamate as an intercellular signaling molecule. However, the peripheral glutamatergic system is poorly understood, partly because the sites of L -glutamate signal appearance are less well characterized. Vesicular glutamate transporters (VGLUTs) are potential probes for the sites of vesicular storage and subsequent secretion of L -glutamate. In this study we raised specific polyclonal antibodies against two VGLUT isoforms, VGLUT1 and VGLUT2, and investigated their localization in peripheral tissues of rat. We detected the expression of either VGLUT1 or VGLUT2, or both, in pancreas, stomach, intestine, and testis. In pancreas, VGLUT1 and VGLUT2 are present in pancreatic polypeptide-containing secretory granules in F-cells in the islets of Langerhans. In stomach, VGLUT2 is abundant in the antrum and pylorus and is present in a subset of pancreatic polypeptide-containing cells. In intestine, VGLUT2 is abundant in the ileum and is co-localized with glucagon-like immunoreactive peptide and polypeptide YY (PYY). In testis, VGLUT2 is expressed and localized in the outer acrosomal membrane of spermatids, where KA1 and GluR5, kainate receptor subunits, are almost always localized. Taken together, these results strongly suggest the occurrence of a peripheral glutamatergic system in the gastroenteropancreatic system and testis.

Many metabolic factors affect the secretion of insulin from ␤-cells and glucagon from ␣-cells of the islets of Langerhans to regulate blood glucose. Somatostatin from ␦-cells, considered a local inhibitor of islet function, reduces insulin and glucagon secretion by activating somatostatin receptors in islet cells. Somatostatin secretion from ␦-cells is increased by high glucose via glucose metabolism in a similar way to insulin secretion from ␤-cells. However, it is unknown how low glucose triggers somatostatin secretion. Because L-glutamate is cosecreted with glucagon from ␣-cells under low-glucose conditions and acts as a primary intercellular messenger, we hypothesized that glutamate signaling triggers the secretion of somatostatin. In this study, we showed that ␦-cells express GluR4c-flip, a newly identified splicing variant of GluR4, an (RS)-␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type ionotropic glutamate receptor of rat. After treatment with L-glutamate, AMPA, or kainate, secretion of somatostatin from isolated islets was significantly stimulated under low-glucose conditions. The glutamatedependent somatostatin secretion was Ca 2؉ dependent and blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Somatostatin in turn inhibited the secretion of L-glutamate and glucagon from ␣-cells. These results indicate that L-glutamate triggers somatostatin secretion from ␦-cells by way of the GluR4c-flip receptor under lowglucose conditions. The released somatostatin may complete the feedback inhibition of ␣-cells. Thus, ␣-and ␦-cells may communicate with each other through Lglutamate and somatostatin signaling.