Abstract:

The blood-brain barrier (BBB) and drug delivery: In our modern societies, the prevalence of nervous system disorders is increasing in relation to the aging population. To overcome this problem, several interesting pharmacons and biomolecules are introduced and tested each day. The biggest obstacle to delivering drugs into the brain parenchyma is the presence of the BBB.The BBB is a regulatory interface, which limits free transport of large and small molecules between the systemic circulation and the central nervous system. This regulation enables proper neuronal function and protection from outside toxic insults as well as maintenance of a stable ionic homeostasis. The morphological basis of the BBB is the monolayer of brain endothelial cells (BECs) in the cerebral microvessels. The adult human brain contains more than 600 km of capillaries, which differ fundamentally from other endothelial interfaces in the body. The tight inter-endothelial junctions connecting the cells reduce paracellular permeation of ions and other small hydrophilic solutes as well as larger molecules. Consequently, molecules have to enter the brain via transcellular transport mechanisms, but this access is also subjected to restrictive control. The metabolic and enzymatic barrier, formed by a unique expression pattern of enzymes and efflux pumps at the luminal membrane of BEC, limits BBB penetration of lipophilic drugs and other xenobiotics. Thus, the supply of essential nutrients to brain cells is tightly controlled via the vesicular-mediated transcellular transport mechanism. Transcytosis of larger molecules such as peptides and proteins are initially endocytosed by absorptive- and receptor-mediated mechanisms and then transcytosed via subcellular vesicles. This regulated vesicular transport is also known as absorptive- and receptor-mediated transcytosis. In addition to essential supplements, cell surface receptors are considered a potential gate for targeted delivery of large drugs to the brain. In the last decade, several publications have focused on the transferrin receptor as a target for bispecific antibodies and nanoparticles with pharmaceutical effect. Additionally, the cross-talk among endothelial cells and neighboring cells such as astroglia, pericytes, microglia, and neurons should be mentioned, which induce a unique barrier phenotype in BEC. This interaction is important for drug delivery, as it is known to affect expression of tight junction molecules, receptors, and transporters, as well as influence the subcellular vesicular system. In this paper, we introduce high content screening microscopy as an approach to analyze the subcellular vesicular structure and the trafficking system of the BBB in vitro. The method is particular useful to describe and compare differences between different culture set-ups.