Differential Expression of Branched-Chain Aminotransferase in the Rat Ocular Tissues

Branched-chain amino acids (BCAAs) play vital roles in metabolic and physiological processes, with their catabolism initiated by two branched-chain aminotransferase isozymes: cytosolic (BCATc) and mitochondrial (BCATm). These enzymes have tissue and cell-specific compartmentalization and are believed to shuttle metabolites between cells and tissues. Although their expression and localization have been established in most tissues, ocular tissues remain unknown. In this study, we used immunohistochemical analyses to investigate the expression and localization of BCAT enzymes in the normal eye tissues. As expected, BCATc was highly expressed in the neuronal cells of the retina, particularly in the ganglion cell layers, inner nuclear layer, and plexiform layer, with little to no expression in Müller cells. BCATc was also present in the cornea, retinal pigment epithelium (RPE), choroid, ciliary body, and iris but not in the lens. In contrast, BCATm was expressed across all ocular tissues, with strong expression in the Muller cells of the retina, the endothelial and epithelial layers of the cornea, the choroid and iris, and the epithelial cells at the lens's front. The extensive expression and distribution of BCAT isozymes in the ocular tissue, suggests that BCAA transamination is widespread in the eye, potentially aiding in metabolite transport between ocular tissues. The findings provide new insights into the physiological role of BCATs in the eye, particularly within the neuronal retina.