Functional and molecular analysis of D-serine transport in retinal Muller cells

D-serine, an endogenous co-agonist of NMDA receptors in vertebrate retina, may modulate glutamate sensitivity of retinal neurons. This study determined at the functional and molecular level the transport process responsible for D-serine in retinal Muller cells. RT-PCR and immunoblotting showed that serine racemase (SR), the synthesizing enzyme for D-serine, is expressed in the rMC-1 Muller cell line and primary cultures of mouse Muller cells (1 degrees MCs). The relative contributions of different amino acid transport systems to D-serine uptake were determined based on differential substrate specificities and ion dependencies. D-serine uptake was obligatorily dependent on Na+, eliminating Na+-independent transporters (asc-1 and system L) for D-serine in Muller cells. The Na+:substrate stoichiometry for the transport process was 1:1. D-serine transport was inhibited by alanine, serine, cysteine, glutamine, and asparagine, but not anionic amino acids or cationic amino acids, suggesting that D-serine transport in Muller cells occurs via ASCT2 rather than ASCT1 or ATB(0,+). The expression of mRNAs specific for ASCT1, ASCT2, and ATB(0,+) was analyzed by RT-PCR confirming the expression of ASCT2 (and ASCT1) mRNA, but not ATB(0,+), in Muller cells. Immunoblotting detected ASCT2 in neural retina and in 1 degrees MCs; immunohistochemistry confirmed these data in retinal sections and in cultures of I'MCs. The efflux of D-serine via ASCT2 by ASCT2 substrates was demonstrable using the Xenopus laevis oocyte heterologous expression system. These data provide the first molecular evidence for SR and ASCT2 expression in a Muller cell line and in 1 degrees MCs and suggest that D-serine, synthesized in Wiler cells by SR, is effluxed via ASCT2 to regulate NMDA receptors in adjacent neurons. (c) 2006 Published by Elsevier Ltd.