The Role of LAT1 at the Inner Blood–Retinal Barrier in Supplying Large Neutral Amino Acids

Abstract

Abstract: : Purpose: L–Type amino acid transporters (LAT) prefer branched–chain and aromatic amino acids including neurotransmitter precursors. The purpose of this study was to clarify the expression and function of LAT at the inner blood–retinal barrier (BRB). Methods: [³H]L–Leucine transport across the inner BRB was characterized using in vivo integration plot analysis and in vitro uptake by a conditionally immortalized rat retinal capillary endothelial cell line (TR–iBRB cells). The expression and localization of the LAT1 were demonstrated by quantitative real–time RT–PCR, immunoblot, and immunohistochemical analyses. Results: The apparent influx permeability clearance of [³H]L–leucine in the rat retina was found to be 203 µL/min/g retina, supporting that carrier–mediated transport system is involved in [³H]L–leucine transport from the blood to the retina. [³H]L–Leucine uptake by TR–iBRB cells was an Na⁺–independent and concentration–dependent process with a K_m of 14.1 µM. This process was inhibited greater by substrates of LAT1, D–leucine, D–phenylalanine, and D–methionine, than those of LAT2, L–alanine and L–glutamine. The expression of LAT1 mRNA was 100– and 15–fold greater than that of LAT2 in TR–iBRB and magnetically isolated rat retinal vascular endothelial cells, respectively. The expression of LAT1 protein was observed in TR–iBRB and primary cultured human retinal endothelial cells. The immunostaining of LAT1 was observed along the rat retinal capillaries. Conclusions: LAT1 is expressed at the inner BRB and mediates the blood–to–retina L–leucine transport. This transport system would play a key role in maintaining large neutral amino acids as well as neurotransmitters in the neural retina.

Keywords: retina: neurochemistry • neurotransmitters/neurotransmitter systems • retina