Fig. 1

Barrier and interface functions of the vascular BBB. Left panel—barrier functions of brain parenchymal capillaries are shown on the left and include a suppression of micropinocytosis via lipid transporters such as Msfd2a which prevents transcellular leakage, b expression of tight junction proteins that prevent paracellular leakage, c expression of efflux transporters which prevent the diffusion of hydrophobic substances and certain xenobiotics across the BBB, and d expression of metabolic enzymes which degrade substances taken up by the endothelium. Interface functions are shown on the right and include e transcellular diffusion of substances that are membrane permeant, f facilitated diffusion or active transport through solute carriers, g transport via receptor-mediated transcytosis, secreting (i, k) or responding to secretions (h, j) in the blood or brain compartments, and l inducing uptake and transport via adsorptive transcytosis. Right upper panel m depicts the positioning of the brain capillary (pink) with an associated pericyte (green) and astrocyte endfeet (blue). Note the relative absence of perivascular space, and red blood cell in the capillary lumen. Right lower panel n illustrates a post-capillary venule which is the predominant site of leukocyte trafficking across the vascular BBB. Diapedesis of a leukocyte (grey) into the perivascular space is depicted. This figure was created with BioRender