A Cell Culture Model of the Blood-Brain Barrier

Endothelial cells that make up brain capillaries and constitute the blood-brain barrier become different from peripheral endothelial cells in response to inductive factors found in the nervous system . We have established a cell culture model of the bloodbrain barrier by treating brain endothelial cells with a combination of astrocyte-conditioned medium and agents that elevate intracellular cAMP These cells form THE blood-brain barrier (BBB),' interposed between the circulatory system and the central nervous system (CNS), is relatively impermeable to ions, many amino acids, small peptides, and proteins . In vertebrates, the BBB exists at the level ofthe endothelial cells (ECs) that make up brain capillaries (Brightman, 1989) . These ECs differ from thoseinmost peripheralcapillaries in two important respects . First, tight junctions with extremely high electrical resistance are present between brain capillary ECs (Crone and Olesen, 1982 ; Butt et al ., 1990) . This limits the amount of paracellular flux . Second, brain ECs undergo a relatively slow rate offluid-phase endocytosis, as assayed by the uptake of tracer molecules, such as HRP (Reese and Karnovsky, 1967) . This limits the amount of transcellular flux . In short, it is the combination ofthe limited paracellular and transcellular movement that accounts for the existence of the BBB. In recent years, the manner in which ECs in the brain become different from those in the periphery has been examined . Stewart and Wiley (1981) found that ECs derived from brain capillaries that are then permitted to vascularize peripheral tissue become relatively leaky. Conversely, ECs originally derived from peripheral capillaries become less leaky when they vascularize brain tissue. They concluded that tissue environment influences EC phenotype . Janzer and Raff (1987) showed further that type 1 astrocytes, which normally project to brain capillary ECs in vivo, are able to provide some of the inductive influences of brain tissue, again L . L . Rubin's current address is Eisai London Laboratories and Department ofAnatomy and Developmental Biology, Darwin Building, University College London, London WOE 6BT, England . 1 . Abbreviations used in this paper: BBB, blood-brain barrier ; CNS, central nervous system; CPTCAMP, 8-(4-chlorophenylthio)-adenosine-3':5' monophosphate, cyclic ; CNTF, ciliary neurotrophic factor ; EC, endothelial cell . © The Rockefeller University Press, 0021-9525/91/12/1725/11 $2 .00 The Journal of Cell Biology, Volume 115, Number 6, December 1991 1725-1735 1725 high resistance tight junctions and exhibit low rates of paracellular leakage and fluid-phase endocytosis . They also undergo a dramatic structural reorganization as they form tight junctions . Results from these studies suggest modes of manipulating the permeability of the blood-brain barrier, potentially providing the basis for increasing the penetration of drugs into the central