New in vitro model for the brain drug delivery research: Conditionally immortalized cell lines as novel models of the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB)

Elucidating the molecular mechanism of the brain barrier transport is a key step toward successful drug delivery to the brain. In this review, we focused on the usefulness and the limitations of the newly developed in vitro model. To overcome several limitations of classical in vitro model, conditionally immortalized mouse (TM-BBB) and rat (TR-BBB) brain capillary endothelial cell lines and a choroid plexus epithelial cell line (TR-CSFB) were established from the temperature sensitive SV 40 large T antigen gene transgenic animals. mRNA and protein expression of GLUT1 in TR-BBB was 100 folds greater than that of primary culture, Several known transporter genes were identified for TR-BBB or TM-BBB, including organic anion transporter 3 (OAT 3) and organic anion transporting polypeptide 2 (oatp 2). Employing TM-BBB, new brain barrier functions have been clarified such as creatine transporter (CRT) and GABA transporter (GAT 2/BGT-1). TR-BBB has been applied for the study of expressional regulation of transporters at the BBB, e. g., L-proline and glycine transporter (ATA 2) and taurine transporter (TAUT). Moreover, one of the most important subjects for the in vitro BBB model is to clarify the expression and regulation of the tight junction proteins at the BBB. Co-culture systems among TR-BBB, conditionally immortalized astrocyte (TR-AST) and pericyte (TR-PCT) cell lines were developed and revealed that the expression of occludin, a tight-junction component, was induced by soluble factors secreted from TR-AST and TR-PCT. For the brain drug delivery research, the new in vitro model will provide rational and efficient strategies.