Identification of a vesicular glutamate transporter that defines a glutamatergic phenotype in neurons

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Synaptic vesicles are loaded with neurotransmitter by mean s of specific vesicular transporters. Here we show that expression of BNPI, a vesicle-bound transporter associated with sodium-dependent phosphate tra nsport(1-3), results in glutamate uptake by intracellular vesicles. Substra te specificity and energy dependence are very similar to glutamate uptake b y synaptic vesicles. Stimulation of exocytosis-fusion of the vesicles with the cell membrane and release of their contents-resulted in quantal release of glutamate from BNPI-expressing cells. Furthermore, we expressed BNPI in neurons containing GABA (gamma-aminobutyric acid) and maintained them as c ultures of single, isolated neurons that form synapses to themselves. After stimulation of these neurons, a component of the postsynaptic current is m ediated by glutamate as it is blocked by a combination of the glutamate rec eptor antagonists, but is insensitive to a GABA(A) receptor antagonist. We conclude that BNPI functions as vesicular glutamate transporter and that ex pression of BNPI suffices to define a glutamatergic phenotype in neurons.