Identification of the Differentiation-Associated Na⁺/P_ITransporter as a Novel Vesicular Glutamate Transporter Expressed in a Distinct Set of Glutamatergic Synapses

TLDR

Glutamate must be transported into synaptic vesicles to enable regulated neurosecretion. The study aims to functionally identify a second isoform of the vesicular glutamate transporter, previously known as the differentiation‑associated Na⁺/Pi transporter. The newly identified VGLUT2 selectively transports glutamate into vesicles in a H⁺‑ and Cl⁻‑dependent manner, driven by membrane potential and proton gradient, is enriched on specific synaptic vesicles, and together with the first isoform provides comprehensive glutamate uptake across all central glutamatergic neurons.

Abstract

Glutamate transport into synaptic vesicles is a prerequisite for its regulated neurosecretion. Here we functionally identify a second isoform of the vesicular glutamate transporter (VGLUT2) that was previously identified as a plasma membrane Na + -dependent inorganic phosphate transporter (differentiation-associated Na + /P I transporter). Studies using intracellular vesicles from transiently transfected PC12 cells indicate that uptake by VGLUT2 is highly selective for glutamate, is H + dependent, and requires Cl − ion. Both the vesicular membrane potential (Δψ) and the proton gradient (ΔpH) are important driving forces for vesicular glutamate accumulation under physiological Cl − concentrations. Using an antibody specific for VGLUT2, we also find that this protein is enriched on synaptic vesicles and selective for a distinct class of glutamatergic nerve terminals. The pathway-specific, complementary expression of two different vesicular glutamate transporters suggests functional diversity in the regulation of vesicular release at excitatory synapses. Together, the two isoforms may account for the uptake of glutamate by synaptic vesicles from all central glutamatergic neurons.