Monocultures of primary porcine brain capillary endothelial cells: Still a functional in vitro model for the blood-brain-barrier.

The main obstacle for the treatment of brain diseases is the restriction of the passage of pharmaceuticals across the blood-brain barrier. Endothelial cells line up the cerebral micro vessels and prevent the uncontrolled transfer of polar substances by intercellular tight junctions. In addition to this physical barrier, active transporters of the multi-drug-resistance prevent the passage of hydrophobic substances by refluxing them back to the blood stream. This paper reviews the development and selected applications of an in vitro porcine brain derived primary cell culture system established in the authors lab that closely resembles the BBB in vivo and could thus be used to study beyond other applications drug delivery to the brain. An essential technique to control the intactness or destruction of the barrier, the impedance spectroscopy, will be introduced. It will be shown that nanoparticles can cross the blood brain barrier by two mechanisms: opening the tight junctions and thus allowing parallel import of substances into the brain as well as receptor mediated endocytosis using brain specific target molecules. However cytotoxic effects have to be considered as well which beside standard cytotoxicity assays could be also determined by impedance technology. Moreover it will be shown that enzymes e.g. for enzyme replacement therapy could be transferred across the barrier by proper tuning or chemical modification of the enzyme. Since this review is based on a conference presentation it will mainly focus on applications of the monoculture system developed in the authors lab which under given culture conditions is useful due to its easy availability, robustness, good reproducibility and also due to its simplicity. Improvements of this model made by other groups will be acknowledged but not discussed here in detail.