Development and Maintenance of the Blood–Brain Barrier

Endothelial Cells

Endothelial cells comprise the core of the BBB, as this is the cell type in direct contact with the blood. Endothelial cells are held together via adherens junctions and seal tightly through TJs ( Fig. 9.1 ). The resulting tubular, single-cell-thick, “cobblestone” arrangement of endothelial cells comprises the arterioles, venules, and capillaries of the CNS. These cells house the proteins that maintain a physical and biochemical barrier between blood and brain. Paracellular transit of polar compounds between endothelial cells is prevented by TJs, which also serve to maintain endothelial cell polarity by limiting lateral diffusion of lipids and membrane proteins, and provide an intracellular scaffold for signaling complexes .

TJs between endothelial cells in the BBB primarily comprise occludin, claudins (particularly claudin-3 and -5), and (junctional adhesion molecules A, B, C) JAM-A, -B, and -C. Extracellular strands of TJ proteins from one endothelial cell adhere to the extracellular strands of TJ proteins on a neighboring endothelial cell. In electron microscopy images, the TJs bring two neighboring cells into such close association that the outer leaflets of the plasma membranes appear to fuse. Molecules such as transforming growth factor β (TGF-β) and glucocorticoids initiate signaling cascades that affect the expression and function of TJ proteins, and posttranslational modifications of the TJ proteins (e.g., phosphorylation, palmitoylation) further regulate TJ properties . Furthermore, the trafficking of TJ proteins from intracellular compartments and insertion of such complexes into the endothelial membrane is another means by which barrier properties can be modulated . The intracellular portion of TJ proteins form complexes with zonula occludens (ZO) proteins that, in turn, bind actin and thereby anchor the TJ complex to the cytoskeleton of the endothelial cell as well as provide a means to relay TJ status to intracellular compartments via ZO proteins or other signaling molecules .

Transporters located in the luminal and/or abluminal membranes of endothelial cells facilitate efflux of many potential neurotoxicants into the blood or influx of nutrients from the blood. ABC transporters are typically efflux transporters that couple the hydrolysis of ATP to the movement of substrates against their concentration gradients. P-glycoprotein, breast cancer resistance protein, and members of the multidrug resistance protein family are examples of ABC transporters. These transporters efflux a vast number of substrates. Some, like chlorpyrifos or ivermectin, would be toxic if they crossed the BBB; others are drugs that require access to the CNS (e.g., morphine, indinavir, taxol, prazosin) for efficacy. SLC transporters are also present at the BBB. These transporters have a wide variety of substrates that contain both toxicants (e.g., aflatoxinB1, ethidium bromide) and drugs (e.g., paclitacel, acyclovir, ketoprofen). The expression and trafficking of these transport proteins at the endothelial membrane are regulated by a multitude of signaling pathways and are capable of being altered in pain or disease states .

Finally, intracellular enzymes such as the cytochrome P450 family members, monoamine oxidases, glutathione- S -transferases, and catechol- O -methyltransferase act upon compounds that gain entry into the endothelial cell from the vessel lumen to inactivate or increase the clearance of these compounds . Drugs such as caffeine (CYP1B1), dextromethorphan (CYP46A1), and risperidone (CYP2D6) are examples of compounds that are metabolized by such enzymes. The capacity of these enzymes to affect CNS bioavailability of certain compounds is appreciated, but not yet exhaustively studied.