Deletion of v7-3 (SLC6A15) transporter allows assessment of its roles in synaptosomal proline uptake, leucine uptake and behaviors

v7-3 (SLC6A15) is the prototype for a gene subfamily whose members have sequence homologies to classical Na⁺- and Cl^--dependent neurotransmitter transporters but display unusual features that include characteristic large fourth extracellular loops. Interest in v7-3 has been increased by the elucidation of its expression in neurons located in cerebral cortex, hippocampus, cerebellum, midbrain and olfactory bulb. To help clarify the role of v7-3 in brain functions, we have created and characterized v7-3 knockout mice. These mice lack functional v7-3 protein but are viable and fertile. While our studies were in progress, v7-3 expression was reported to confer transport of proline and branched-chain amino acids in in vitro expression systems [Takanaga, H., Mackenzie, B., Peng, J.B., Hediger, M.A., 2005b. Characterization of a branched-chain amino-acid transporter SBAT1 (SLC6A15) that is expressed in human brain. Biochem. Biophys. Res. Commun. 337, 892-900; Broer, A., Tietze, N., Kowalczuk, S., Chubb, S., Munzinger, M., Bak, L.K., Broer, S., 2006. The orphan transporter v7-3 (slc6a15) is a Na⁺-dependent neutral amino acid transporter (B0AT2). Biochem. J. 393, 421-430]. Assessment of amino acid uptake into cortical synaptosomes of v7-3 knockouts identified 15% and 40% reductions in sodium-dependent proline and leucine transport, respectively, compared to wild type controls. Despite these biochemical changes, v7-3 knockout mice demonstrate only modest alterations in rotarod performance with aging and lack reproducible alterations in other motor, memory, anxiety or olfactory tests. Compensation for the lack of v7-3 via other amino acid carriers is likely to leave v7-3 knockouts without gross behavioral manifestations. The current results place v7-3 in the context of other brain transporters that accumulate proline and branched-chain amino acids.