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T.B. Redens, C. Liang, T.C. Welbourne, M.P. Langford; Distribution of –Glutamyltranspeptidase, Excitatory Amino Acid Transporters (GLAST–1, GLT–1, EAAC–1) and Intracellular Glutamate in Human Ciliary Body Epithelial Cells . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3790.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the distribution of γ–glutamyltranspeptidase (GGT; an extrinsic phosphate independent glutaminase enzyme that generates Glu from glutamine and glutathione), glutamate (Glu) transporters (i.e., excitatory amino acid transporters; EAAT1–3), and intracellular Glu in ciliary body epithelial cells and assess their relationships to Glu modulation of human blood–aqueous barrier (BAB) function. Methods: The indirect immunofluorescent antibody method was used to detect immuno–reactive antibodies to purified GGT from rat milk, to synthetic peptides of the rat glial excitatory amino acid transporter (EAAT1–3) proteins [i.e., GLAST–1, GLT–1 and EAAC–1, respectively], and to Glu in epithelial cells of human ciliary body tissue sections. Results: Immuno–reactive GGT was localized to the basolateral surfaces of human ciliary non–pigmented epithelial (NPE) and pigmented epithelial (PE) cells. EAAT–1 (GLAST–1) was localized to the NPE cells, while EAAT–2 (GLT–1) was detected on both ciliary PE and NPE cells. More interestingly, EAAT–3 (EAAC–1) was predominantly localized to the basolateral surface of the PE cells. Intracellular Glu was detected in NPE and PE cells. Notably, Glu was highly concentrated in vesicules in the apical regions of ciliary NPE cells in apposition to the apical melanosomes in PE cells. Conclusions: The results are consistent with the concept that the high concentrations of vesicular Glu are maintained at the apical surface of the NPE cells due in part to aqueous Glu generated from Gln by GGT and uptake by high affinity EAAT–1 and 2 transporters on NPE and PE cells. A similar association was detected for GGT and EAAT–3 in PE cells. The high concentration of Glu in NPE suggests the importance of Glu generation, transport, release and resorption in modulating the BAB function by ciliary epithelial cells. That is, increased BAB permeability would deliver Gln to the basolateral Glu generator contributing to the restoration of barrier function. Further, the results offers a plausible explanation to the lower than plasma levels of Glu in human aqueous and the susceptibility of the BAB epithelium to endogenous and exogenous agents that affect GGT and Glu transport activity.
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