Transcytosis at the blood-brain barrier

Abstract

The blood-brain barrier (BBB) is a functional interface separating the brain from the circulatory system and is essential for homeostasis of the central nervous system (CNS). The BBB regulates molecular flux to maintain an optimal environment for neuronal function and protects the brain from toxins and pathogens. Endothelial cells forming the walls of CNS blood vessels constitute the BBB. CNS endothelial cells exhibit two features that underlie the restrictive properties of the BBB: specialized tight junctions that prevent paracellular passage between the blood and the brain, and unusually low levels of vesicle trafficking that limit transcellular transport or transcytosis. While the prevailing view in the field was that specialized tight junctions contributed to CNS barrier properties, recent findings have revealed the importance of maintaining low rates of transcytosis at the BBB. It is now clear that suppression of transcytosis at the BBB is an active process and CNS-specific genetic programs inhibit this pathway to maintain a functional barrier.

Figure 1. Neurovascular unit and transcytosis at the BBB

(A) Cross-section of the neurovascular unit depicting endothelial cells surrounded by pericytes and astrocytes on their abluminal side. (B) Schematic illustrating the two kinds of transcytosis in CNS endothelial cells: molecules with specific receptors on CNS endothelial cells such as insulin and transferrin undergo receptor-mediated transcytosis while molecules such as albumin get endocytosed through charged interactions and undergo adsorptive transcytosis.

Figure 2. Existence of multiple endocytic pathways in mammalian cells

Macromolecules can be internalized into cells by clathrin-mediated, caveolae-mediated or by other vesicular and tubular endocytic pathways that are independent of both clathrin and caveolin-1.