Expression and Localization of Branched-Chain Ketoacid Dehydrogenase E1 Subunits and LAT1 Transporter in Rat Retinal and Ocular Tissues.

Branched-chain amino acids (BCAAs) are essential for various metabolic and physiological functions. The enzymes responsible for breaking down and transporting them are organized in a tissue-specific manner, playing a crucial role in moving metabolites between cells but often being overlooked. This is particularly true for the rate-limiting enzyme, branched-chain keto-acid dehydrogenase (BCKD). To address this gap, our study investigated the expression and distribution of the BCKDE1 subunits and the BCAA transporter LAT1 in normal rat eye tissues, with a focus on the retina, by applying immunohistochemistry (IHC) and immunofluorescence (IF) with specific antibodies. Our findings show that BCKDE1 subunits are highly expressed in retinal neurons, specifically in the ganglion cell layer (GCL), inner and outer nuclear layers (INL/ONL), and the plexiform layers (IPL/OPL), although they are notably absent in Müller cells. Expression was also strong in the epithelial cells of the lens, iris, and ciliary body. Similarly, we observed LAT1 localized in the GCL and INL of the retina, as well as in the iris, ciliary body, and lens epithelium. These results complement our previous work, which indicated that branched-chain aminotransferase (BCAT) isozymes are widely expressed across most ocular tissues. Overall, this evidence strongly indicates that a complete BCAA metabolic pathway exists in the eye. This provides a comprehensive understanding of BCAA metabolism in eye tissues, highlighting its crucial role in maintaining amino acid balance, neurotransmitter production, and energy generation. Furthermore, this study lays a foundation for future studies on how disruptions in these enzymatic pathways might affect neurodegeneration in diabetic retinopathy and contribute to other eye conditions.