Blood-Brain-Barrier

Nowhere in the body is there more need for homeostasis than in the brain. The mechanism for maintaining this barrier function lies in the capillary network supplying blood to the brain.

These endothelial cells of capillaries in the brain are different to those found in peripheral tissues in various ways:

1. Brain endothelial cells are joined by tight junctions of high electrical resistance providing an effective barrier against molecules.
2. In peripheral endothelial cells there is good transcellular movement of molecules. There is no such movement in brain endothelial cells.
3. Brain capillaries are in contact with foot processes of astrocytes which essentially separate the capillaries from the neurones.

The blood brain barrier is both a physical barrier and a system of cellular transport mechanisms.

Lipid soluble molecules are able to penetrate through the barrier relatively easily via the lipid membranes of the cells. In contrast, water soluble molecules (e.g. ions) are unable to transverse the barrier without the use of specialized carrier- mediated transport mechanisms. Transporter proteins play a significant role in the transport of compounds across the BBB. Substrate specificity, expression levels and activity of the uptake and efflux transporters that are expressed in the BBB differ. Two efflux transporters - MDR1 and BCRP are of main importance in preventing xenobiotics to cross the BBB. Both transporters are expressed on the apical membrane, pumping their substrates from the cell into the bloodstream.

Localization of efflux and uptake transporters in the brain capillary endothelial cell.
(Adopted from Loscher and Potschka, Nature Reviews Neuroscience, 2005 6(8):591-602)

Available assays within the BBB package

• MRP1 PREDEASY ATPase
• MDR1 PREDEASY ATPase
• MDR1 PREDIVEZ vesicular transport
• BCRP-HAM-PREDEASY-ATPase
• BCRP-M PREDIVEZ vesicular transport

• MDR1 Calcein Assay
• Hoechst Assay (BCRP)

• MDCKII BCRP AB/BA transport
• MDCKII mouseBcrp1 AB/BA transport
• LLC-PK1-MDR1 / LLC-PK1-rMdr1b (under development)
  
  
• Rat brain endothelial cell monolayer assay   NEW

Screening strategy

1. Specific transporter assay package: Brain endothelial cells in the BBB are packed together more tightly then elsewhere in the body, therefore the role of membrane transporters in the penetration of low permeability and even high permeability compounds is dominant. Transporter specific membrane based assays form the first step of this strategy. Carry out tests on the P-gp and BCRP with ATPase and Calcein or vesicular transport assays. Further assays can be carried out on MRP1, and MRP5 transporters.
   Once the possible interactions of the test compound with these transporters have been clarified, more complex and less cost-effective steps may follow: primary endothelial cell assays as well as microdialysis.