Blood-brain barrier genomics and cloning of a novel organic anion transporter.

A novel organic anion transporter selectively expressed at the blood-brain barrier (BBB), originally designated BBB-specific anion transporter type 1 (BSAT1), and now classified as Slco1c1, has been cloned from a BBB genomics program as a partial cDNA; this study describes the cloning and expression of the full-length cDNA from a rat brain capillary cDNA library. Northern analysis revealed the selective expression of the transporter at the BBB, and the transporter was expressed after permanent transfection of human 293 cells with cDNA encoding either the full length or open reading frame mRNA. The full-length transporter cDNA was 2.6 kb, and the mRNA was highly expressed at the rat brain microvasculature, but not in kidney, liver, heart, or lung, or in glial cells or brain glial tumors. Blood-brain barrier-specific anion transporter type 1 expression in 293 cells was poor after the transfection of the full-length cDNA, whereas transporter expression in 293 cells was high after transfection of the open reading frame. The transporter showed asymmetric kinetic properties in comparison of the influx and efflux of model substrates, thyroxine (T4), triiodothyronine (T3), and estradiol-glucuronide (E2G). Thyroxine and T3 inhibited the influx of E2G, but E2G did not inhibit thyroxine influx, and T3 only weakly inhibited the influx of T4. Extracellular E2G stimulated the transefflux of intracellular T4. Blood-brain barrier-specific anion transporter type 1 is a novel organic anion transporter that is a sodium-independent exchanger that may participate in the active efflux of iodothyronines and steroid conjugates at the BBB.