Evidence from gene knockout studies implicates Asc-1 as the primary transporter mediating d-serine reuptake in the mouse CNS

• Published in: The European journal of neuroscience 2007-03, Vol.25 (6), p.1757-1766
• Main Authors: Rutter, A. Richard, Fradley, Rosa L., Garrett, Elizabeth M., Chapman, Kerry L., Lawrence, Jason M., Rosahl, Thomas W., Patel, Smita
• Format: Article
• Language: English
• Subjects: amino acid; Amino Acid Transport System y+ - deficiency; Amino Acid Transport System y+ - physiology; Amino Acid Transport Systems - deficiency; Amino Acid Transport Systems - metabolism; Animals; Biological Transport - drug effects; Biological Transport - genetics; Cells, Cultured; Central Nervous System - cytology; Central Nervous System - metabolism; Dose-Response Relationship, Drug; Embryo, Mammalian; glycine; Mice; Mice, Inbred C57BL; Mice, Knockout; NMDA receptor; Serine - metabolism; Serine - pharmacokinetics; Sodium - metabolism; synaptosome; Synaptosomes - metabolism; Synaptosomes - ultrastructure
• ISSN: 0953-816X; 1460-9568
• DOI: 10.1111/j.1460-9568.2007.05446.x

In the mammalian central nervous system, transporter‐mediated reuptake may be critical for terminating the neurotransmitter action of d‐serine at the strychnine insensitive glycine site of the NMDA receptor. The Na+ independent amino acid transporter alanine–serine–cysteine transporter 1 (Asc‐1) has been proposed to account for synaptosomal d‐serine uptake by virtue of its high affinity for d‐serine and widespread neuronal expression throughout the brain. Here, we sought to validate the contribution of Asc‐1 to d‐serine uptake in mouse brain synaptosomes using Asc‐1 gene knockout (KO) mice. Total [3H]d‐serine uptake in forebrain and cerebellar synaptosomes from Asc‐1 knockout mice was reduced to 34 ± 5% and 22 ± 3% of that observed in wildtype (WT) mice, respectively. When the Na+ dependent transport components were removed by omission of Na+ ions in the assay buffer, d‐serine uptake in knockout mice was reduced to 8 ± 1% and 3 ± 1% of that measured in wildtype mice in forebrain and cerebellum, respectively, suggesting Asc‐1 plays a major role in the Na+ independent transport of d‐serine. Potency determination of d‐serine uptake showed that Asc‐1 mediated rapid high affinity Na+ independent uptake with an IC50 of 19 ± 1 µm. The remaining uptake was mediated predominantly via a low affinity Na+ dependent transporter with an IC50 of 670 ± 300 µm that we propose is the glial alanine–serine–cysteine transporter 2 (ASCT2) transporter. The results presented reveal that Asc‐1 is the only high affinity d‐serine transporter in the mouse CNS and is the predominant mechanism for d‐serine reuptake.