Amino acid transporter SNAT5 localizes to glial cells in the rat brain

The SNAT5 transporter is a neutral amino acid carrier whose function remains unclear. Structural and mechanistically, SNAT5 is closely related to the SNAT3 transporter that mediates the efflux of glutamine from glial cells and that participates in the glutamate-glutamine cycle in the brain. In this study, we have analyzed the distribution of SNAT5 in the rat central nervous system using specific antibodies. Through immunoblotting we observed that SNAT5 is ubiquitously but unevenly distributed in the CNS. It accumulates most intensely in the neocortex, the hippocampus, the striatum, and the spinal cord, whereas moderate levels were found in the thalamus, hypothalamus, and brainstem. Light microscopy revealed that the distribution of SNAT5 paralleled that of the vesicular glutamate transporter vGLUT1 in the forebrain regions, whereas in the diencephalon and brainstem, SNAT5 staining was better correlated with that of vGLUT1 and vGLUT2. However, the cellular localization differed from that of the glutamatergic markers, since SNAT5 was expressed exclusively in astrocyte cell bodies and their processes, ensheathing glutamatergic GABAergic and glycinergic terminals. The presence of SNAT5 in astrocyte processes was confirmed by electron microscopy. They were seen not only to surround different neuronal structures, but they were also found in astrocyte endfeet. Taking into consideration the higher levels of SNAT5 in the neighborhood of glutamatergic terminals and the ability of this transporter family to promote the efflux of amino acids from intracellular stores (including glutamine and perhaps glycine), this transporter is likely to be involved in glutamatergic pathways in the brain.