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

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Dun Y, Mysona B, Itagaki S, Martin-Studdard A, Ganapathy V, Smith SB

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

Exp Eye Res. 2007 Jan;84(1):191-9. Epub 2006 Nov 13.

PubMed ID

17094966 [ View in PubMed

]

Abstract

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 ATB0,+. The expression of mRNAs specific for ASCT1, ASCT2, and ATB0,+ was analyzed by RT-PCR confirming the expression of ASCT2 (and ASCT1) mRNA, but not ATB0,+, 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 1 degrees 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 Muller cells by SR, is effluxed via ASCT2 to regulate NMDA receptors in adjacent neurons.

DrugBank Data that Cites this Article

Drug Targets

Drug	Target	Kind	Organism	Pharmacological Action	Actions
Asparagine	Neutral amino acid transporter B(0)	Protein	Humans	Unknown	Not Available	Details

Drug Transporters

Drug	Transporter	Kind	Organism	Pharmacological Action	Actions
L-Glutamine	Neutral amino acid transporter B(0)	Protein	Humans	Unknown	Substrate	Details