Function and expression of a splicing variant of vesicular glutamate transporter 1

Vesicular glutamate transporter (VGLUT) is an active transporter responsible for vesicular storage of glutamate in synaptic vesicles and plays an essential role in glutamatergic neurotransmission. VGLUT consists of three isoforms, VGLUT1, VGLUT2, and VGLUT3. The VGLUT1 variant, VGLUTlv, with an additional 75-base pair sequence derived from a second intron between exons 2 and 3, which corresponds to 25 amino acid residues in the 1st loop of VGLUT1, is the only splicing variant among VGLUTs, although whether VGLUTlv protein is actually translated at the protein level remains unknown. In the present study, VGLUTly was expressed in insect cells, solubilized, purified to near homogeneity, and its transport activity was examined. Proteoliposomes containing purified VGLUTlv were shown to accumulate glutamate upon imposition of an inside-positive membrane potential (Delta psi). The Delta psi-driven glutamate uptake activity requires Cl- and its pharmacological profile and kinetics are comparable to those of other VGLUTs. The retinal membrane contained two VGLUT1 moieties with apparent molecular masses of 65 and 57 kDa. VGLUT1v-specific antibodies against an inserted 25-amino acid residue sequence identified a 65-kDa immunoreactive polypeptide. Immunohistochemical analysis indicated that VGLUTly immunoreactivity is present in photoreceptor cells and is associated with synaptic vesicles. VGLUT1v immunoreactivity is also present in pinealocytes, but not in other areas, including the brain. These results indicated that VGLUTly exists in a functional state in rat photosensitive cells and is involved in glutamatergic chemical transmission. (C) 2017 Elsevier B.V. All rights reserved.