BBB permeability is formed by slow rate of movement of vesicles

Andreone BJ, Chow BW, Tata A, Lacoste B, Ben-Zvi A, Bullock K, Deik AA, Ginty DD, Clish CB, Gu C. Blood-Brain Barrier Permeability Is Regulated by Lipid Transport-Dependent Suppression of Caveolae-Mediated Transcytosis.Neuron. 2017; 94:581-594.e5.

The blood-brain barrier (BBB) provides a constant homeostatic brain environment that is essential for proper neural function. An unusually low rate of vesicular ( vesicle = blob of cell membrane) transport (transcytosis) has been identified as one of the two unique properties of CNS endothelial cells, relative to peripheral endothelial cells (the other is tight  junctions which creates impermeable joins between cell), that maintain the restrictive quality of the BBB. However, it is not known how this low rate of transcytosis is achieved. Here we provide a mechanism whereby the regulation of CNS endothelial cell lipid composition specifically inhibits the caveolae-mediated transcytotic route readily used in the periphery. An 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. Furthermore, lipids transported by Mfsd2a establish a unique lipid environment that inhibits caveolae vesicle formation in CNS endothelial cells to suppress transcytosis and ensure BBB integrity.

Caveolae (Latin for “little caves”), which are a special type of lipid raft, are small (50–100 nanometerinvaginations of the plasma membrane in many cell types, especially in endothelial cells.These flask-shaped structures are rich in proteins as well as lipids such as cholesterol and sphingolipids and have several functions in signal transduction.They are also believed to play a role in endocytosis.

In this study they looked at the differences in the lipids between brain blood vessel cells = endothelial cells and they found  differences and this was related to the action of  Major facilitator superfamily domain-containing protein 2 (MFSD2 or MFSD2A), also known as sodium-dependent lysophosphatidylcholine symporter 1. This is a membrane transport protein that is expressed in the endothelium of the blood–brain barrier (BBB) and is involved in BBB formation. Genetic ablation of MFSD2A results in leaky BBB and increases central nervous system endothelial cell vesicular transcytosis without otherwise affecting tight junctions. In addition to transport of other lysophosphatidylcholines across the BBB, MSFD2A is the primary mechanism for docosahexaenoic acid (DHA, an omega-3 fatty acid) uptake and transport into the brain. There was a lot more  DHA lipid in the brain blood vessels cells. By incorporating DHA into the membrane, Mfsd2a is fundamentally changing the composition of the membrane and making it unfavorable for the formation of the caveolae. 

They may be able to manipulate things to get treatments into the brain

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