Glutamate, Excitatory Amino Acid Transporters, Xc− Antiporter, Glutamine Synthetase, and γ-Glutamyltranspeptidase in Human Corneal Epithelium

Abstract

Purpose: The viability and functions of the corneal epithelium are dependent in large measure on the active uptake of nutrients, growth factors, and amino acids from stroma and tear. The present study presents the cellular distribution(s) of glutamate, the Na+-dependent glutamate/aspartate transporters (excitatory amino acid transporters; EAAT1-5), Na+-independent glutamate/cystine exchanger (Xc− antiporter) subunits (xCT light chain and 4F2hc heavy chain), glutamine synthetase (GS), and γ-glutamyltranspeptidase (GGT) in human corneal epithelium.Methods: Glutamate, EAAT1-5, xCT/4F2hc, GS, and GGT immunoreactive proteins were detected by immunofluorescence microscopy. Human corneal GGT activity was quantified using a standard colorimetric assay.Results: Glutamate, EAAT3>2>1, xCT/4F2hc, and GGT proteins were detected in the columnar and wing cells. Glutamate was reduced or absent in the EAAT negative, Xc− antiporter, and GS positive outer wing cell and flat superficial epithelial cell layers. All EAATs (EAAT3>4/5>1/2), xCT/4F2hc, GS, and GGT were detected in flat superficial epithelial cell layer.Conclusions: The localization of glutamate, multiple EAATs, Xc− antiporter proteins, and GGT to columnar and superficial epithelial cell layers suggests uptake of glutamate and cystine from the stroma and tear and supports their importance in regulation of glutamate/cystine and glutathione (GSH; a tripeptide of glutamate, cystine, and glycine) in the human cornea epithelium. In addition, the low glutamate levels in outer wing and flat superficial epithelial cells positive for Xc− antiporter and GS are consistent with exchange of glutamate by Xc− antiporter for extracellular cystine utilized in GSH synthesis and support coupling of ammonia detoxification with glutamate degradation by GS.