Afamin is synthesized by cerebrovascular endothelial cells and mediates α-tocopherol transport across an in vitro model of the blood-brain barrier

alpha-Tocopherol (alpha TocH), a member of the vitamin E family, is essential for normal neurological function. Despite the importance of alpha TocH transport into the CNS, transfer mechanisms across the blood-brain barrier (BBB) are not entirely clear. We here investigate whether afamin, a known alpha TocH-binding protein, contributes to alpha TocH transport across an in vitro model of the BBB consisting of primary porcine brain capillary endothelial cells (BCEC) and basolaterally cultured astrocytoma cells. Exogenously added afamin had no adverse effects on BCEC viability or barrier function and was transported across BCEC Transwell cultures. Furthermore, alpha TocH transport across polarized BCEC cultures to astrocytoma cells is facilitated by afamin, though to a lesser extent than by high-density lipoprotein-mediated transport, an essential and in vivo operating alpha TocH import pathway at the cerebrovasculature. We also demonstrate that porcine BCEC endogenously synthesize afamin. In line with these in vitro findings, afamin was detected by immunohistochemistry in porcine, human postmortem, and mouse brain, where prominent staining was observed almost exclusively in the cerebrovasculature. The demonstration of afamin mRNA expression in isolated brain capillaries suggests that afamin might be a new family member of binding/transport proteins contributing to alpha TocH homeostasis at the BBB in vivo.