P‑Glycoprotein, Breast Cancer Resistance Protein, Organic Anion Transporter 3, and Transporting Peptide 1a4 during Blood–Brain Barrier Maturation: Involvement of Wnt/β-Catenin and Endothelin‑1 Signaling

Our current knowledge about drug transporters in the maturational brain is very limited. In this study, we provide a comprehensive overview of the expression and activity profile of P-glycoprotein (P-gp), Breast Cancer Resistance Protein (bcrp), Organic Anion Transporter 3 (oat3), and Transporting Peptide 1a4 (oatp1a4) transporters during blood–brain barrier (BBB) maturation. Gene and protein expressions of the analyzed transporters increase as the brain matures, with no variation in their activity for P-gp and bcrp, while the transport activity of oat3 and oatp1a4 increases during brain maturation from preterm up to adulthood. For the first time, we illustrate a downregulation of nuclear β-catenin expression in brain capillaries when bcrp, P-gp, oat3, and oatp1a4 transporters are at their highest expression levels. <i>In vivo</i> activation of β-catenin in rat brains, by intracerebroventricular (ICV) injection of a GSK-3 inhibitor, enhances the activity of P-gp, bcrp, oat3, and oatp1a4. Interestingly, in an <i>in vitro</i> BBB model consisting of a coculture of primary endothelial brain cells with astrocytes or <i>in vivo</i>, activation of β-catenin enhances the mRNA expression of ET-1. Interestingly, blocking the ETA receptor for endothelin-1 <i>in vivo</i> by ICV injection of a ETA antagonist decreases transporter activity mediated by the activation of β-catenin. These findings shed light on the role of an interaction between β-catenin and endothelin-1 signaling in the regulation of these transporters at the BBB.