Characterization of Amino Acid Transporters in the Brain Molecular and Functional Studies of Members within the Solute Carrier Families SLC38 and SLC6

Abstract: Solute carriers (SLCs) comprise the largest group of transporters in humans and there are currently 52 SLC families. They are embedded in cellular membranes and transport numerous molecules; defects in many of the genes encoding SLCs have been connected to pathological conditions, and several SLCs are potential drug targets.The SLC38 family has in total eleven members in humans and they encode transporters called SNATs. In paper I and paper II, we reported molecular and functional characterization of Slc38a7 and Slc38a8, two of the previous orphan members in the family which we suggested to be named SNAT7 and SNAT8, respectively. Using in situ hybridization and immunohistochemistry, these transporters showed similar expression pattern and localized to neurons in the brain For functional characterization proteins were overexpressed in X. laevis oocytes and an Uptake Assay and electrophysiological recordings showed preferred transport of L-glutamine, L-histidine, L-alanine, L-asparagine, L-aspartate and L-arginine for SNAT7. A similar pattern was seen for SNAT8 in a slightly different order of affinities. We classified SNAT7 as a system N transporter and SNAT8 as belonging to system A, and suggests that SNAT7 and SNAT8 could play a role in the glutamine/glutamate(GABA) cycle (GGC) in the brain.Furthermore, we studied the vesicular B0AT3 (Slc6a17) transporter in paper III, and the sodium-coupled amino acid transporter B0AT2 (Slc6a15) in paper IV. Tissue expression studies showed similar localization of Slc6a17 and Slc6a15 mRNA using in situ hybridization and real-time PCR. In paper III, vesicular localization of B0AT2 was shown in both excitatory and inhibitory neurons. When challenging the monoaminergic system with drugs both Slc6a17 and Slc6a15 were upregulated. Suggested roles for the transporters are thereby in synaptic remodeling by regulating the availability of free amino acids used as precursors needed in neurotransmitter synthesis. Moreover, in paper IV, immunohistochemistry showed B0AT3 localization to neurons, astrocytes and epithelial cells of the choroid plexus. Leucine injections caused a smaller reduction of food intake as well as higher neuronal activation in the paraventricular hypothalamic nucleus in Slc6a15 KO mice, compared with wild type mice. This suggests B0AT2 involvement in the anorexigenic effects of leucine.