Blood-to-retina transport of biotin via Na+-dependent multivitamin transporter (SMVT) at the inner blood-retinal barrier

The purpose of this study was to elucidate the mechanisms of biotin transport across the inner blood-retinal barrier (inner BRB). [3H]Biotin transport in the retina across the inner BRB was examined using an in vivo integration plot and retinal uptake index analyses in rats. The transport mechanism was characterized using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells) as an in vitro inner BRB model. The apparent influx permeability clearance (Kin) per gram retina of [3H]biotin was found to be 5.55 μL/(min g retina). The Kin of [3H]biotin was 8.9-fold greater than that of [3H]D-mannitol, a non-permeable paracellular marker. [3H]Biotin uptake by the retina was found to be significantly inhibited by biotin and pantothenic acid, supporting carrier-mediated influx transport of biotin at the inner BRB. [3H]Biotin uptake by TR-iBRB2 cells was Na+-, temperature-, and concentration-dependent with a Km of 146 μM. These forms of transport were significantly inhibited by Na+-dependent multivitamin transporter (SMVT) substrates such as biotin, pantothenic acid, lipoic acid, and desthiobiotin. These transport properties are consistent with those of biotin transport by SMVT. SMVT mRNA was expressed in TR-iBRB2 cells and isolated rat retinal vascular endothelial cells. Our findings suggest that SMVT is involved in the transport of biotin from the circulating blood to the retina across the inner BRB.