Regulation of Transcytosis Underlies Blood-Brain Barrier Permeability

Abstract

The blood-brain barrier (BBB) provides a constant homeostatic brain environment that is essential for proper neural function. A single layer of continuous endothelial cells lining the walls of central nervous system (CNS) blood vessels forms the BBB, which seals the brain and controls solute flux. The low permeability of the BBB has largely been attributed to the restrictive tight junctions between endothelial cells. In recent years, however, an unusually low level of transcellular vesicular transport (transcytosis) has been identified as another unique property of central nervous system (CNS) endothelial cells, relative to peripheral endothelial cells, that maintains the restrictiveness of the BBB. While manipulation of transcytosis has been proposed as a strategy for CNS drug delivery, it is not known how the low rate of transcytosis is achieved at the BBB. The goal of this dissertation is to understand how transcytosis is suppressed in CNS endothelial cells to ensure proper BBB function. Here we provide a mechanism whereby the regulation of CNS endothelial cell lipid composition inhibits specifically the caveolae-mediated transcytotic route readily used in the periphery. Using a combination of mouse genetics, lipidomic mass spectrometry, and electron microscopy, we show that lipids transported by Mfsd2a establish a unique lipid environment that inhibits caveolae vesicle formation in CNS endothelial cells to suppress transcytosis. Moreover, unbiased lipidomic analysis reveals significant differences in endothelial cell lipid signatures from the CNS and periphery, which underlie a suppression of caveolae vesicle formation and trafficking in brain endothelial cells. Finally, we have identified an antibody that binds endogenous Mfsd2a protein, and we are currently examining its putative functional blocking capabilities, potentially opening the door to manipulation of caveolae-mediated transcytosis for therapeutic purposes. This study represents the first mechanistic investigation into the suppression of transcytosis at the BBB, and therefore adds a new level of understanding to how this unique cellular property regulates BBB permeability. Additionally, it establishes, for the first time, lipid composition of CNS endothelial cells as a key regulator of BBB function. Ultimately, the experiments described here open up avenues to develop new strategies to deliver drugs to the diseased CNS.