Differentiation-associated Na⁺-dependent Inorganic Phosphate Cotransporter (DNPI) Is a Vesicular Glutamate Transporter in Endocrine Glutamatergic Systems

Vesicular glutamate transporter is present in neuronal synaptic vesicles and endocrine synaptic-like microvesicles and is responsible for vesicular storage of L-glutamate. A brain-specific Na⁺-dependent inorganic phosphate transporter (BNPI) functions as a vesicular glutamate transporter in synaptic vesicles, and the expression of this BNPI defines the glutamatergic phenotype in the central nervous system (Bellocchio, E. E., Reimer, R. J., Fremeau, R. T., Jr., and Edwards, R. H. (2000) Science 289, 957-960; Takamori, S., Rhee, J. S., Rosenmund, C., and Jahn, R. (2000) Nature 407, 189-194). However, since not all glutamatergic neurons contain BNPI, an additional transporter(s) responsible for vesicular glutamate uptake has been postulated. Here we report that differentiation-associated Na⁺-dependent inorganic phosphate cotransporter (DNPI), an isoform of BNPI (Aihara, Y., Mashima, H., Onda, H., Hisano, S., Kasuya, H., Hori, T., Yamada, S., Tomura, H., Yamada, Y., Inoue, I., Kojima, I., and Takeda, J. (2000) J. Neurochem. 74, 2622-2625), also transports L-glutamate at the expense of an electrochemical gradient of protons established by the vacuolar proton pump when expressed in COS7 cells. Molecular, biological, and immunohistochemical studies have indicated that besides its presence in neuronal cells DNPI is preferentially expressed in mammalian pinealocytes, αTC6 cells, clonal pancreatic α cells, and α cells of Langerhans islets, these cells being proven to secrete L-glutamate through Ca²⁺-dependent regulated exocytosis followed by its vesicular storage. Pancreatic polypeptide-secreting F cells of Langerhans islets also expressed DNPI. These results constitute evidence that DNPI functions as another vesicular transporter in glutamatergic endocrine cells as well as in neurons.