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J Biol Chem, Vol. 275, Issue 9, 6620-6627, March 3, 2000

Inherited Defects of Sodium-dependent Glutamate Transport Mediated by Glutamate/Aspartate Transporter in Canine Red Cells Due to a Decreased Level of Transporter Protein Expression^*

Kota Sato, Mutsumi Inaba^§¶, Yuki Suwa, Aya Matsuu, Yoshiaki Hikasa, Ken-ichiro Ono^§, and Katsumoto Kagota

From the  Department of Veterinary Internal Medicine, Faculty of Agriculture, Tottori University, Tottori 680-8553, Japan and the ^§ Laboratory of Veterinary Clinical Pathobiology, Department of Veterinary Medical Sciences, Graduate School of Agricultural and Life Sciences, University of Tokyo, Tokyo 113-8657, Japan

Canine red cells have a high affinity Na⁺/K⁺-dependent glutamate transporter. We herein demonstrate that this transport is mediated by the canine homologue of glutamate/aspartate transporter (GLAST), one of the glutamate transporter subtypes abundant in the central nervous system. We also demonstrate that GLAST is the most ubiquitous glutamate transporter among the transporter subtypes that have been cloned to date. The GLAST protein content was extremely reduced in variant red cells, low glutamate transport (LGlut) red cells characterized by an inherited remarkable decrease in glutamate transport activity. All LGluT dogs carried a missense mutation of Gly⁴⁹² to Ser (G492S) in either the heterozygous or homozygous state. The GLAST protein with G492S mutation was fully functional in glutamate transport in Xenopus oocytes. However, G492S GLAST exhibited a marked decrease in activity after the addition of cycloheximide, while the wild type showed no significant change, indicating that G492S GLAST was unstable compared with the wild-type transporter. Moreover, LGluT dogs, but not normal dogs, heterozygous for the G492S mutation showed a selective decrease in the accumulation of GLAST mRNA from the normal allele. Based on these findings, we conclude that a complicated heterologous combination of G492S mutation and some transcriptional defect contributes to the pathogenesis of the LGluT red cell phenotype.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EMBL Data Bank with accession number(s) AF067847 (canine GLAST cDNA), AF167076 (canine GLT-1 partial cDNA sequence), AF167075 (canine EAAC1 partial cDNA sequence), and AF167077 (canine EAAT4 partial cDNA sequence).

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