Functional characterization and comparison of the outer blood-retina barrier and the blood-brain barrier.
OBJECTIVE
To determine efflux systems of the outer blood-retina barrier (oBRB) and compare the oBRB with the blood-brain barrier (BBB).


METHODS
Porcine oBRB structure and transport characteristics of freshly dissected intact tissue sheets were investigated with scanning electron microscopy, immunocytochemistry, vital dye labeling, and pharmacological agents, using HPLC/mass spectrometry. To compare drug permeation across the oBRB and the BBB, three different systems were used: (1) oBRB tissue sheets in a two-chamber device in vitro; (2) an in vitro BBB model composed of purified astrocytes and brain capillary endothelial cells on transfilter membranes; and (3) an in vivo model based on the brain-plasma ratio of drugs in mice.


RESULTS
Efflux pumps (multidrug resistance protein [P-gp] and multidrug resistance-associated protein [MRP]) were demonstrated by antibody staining. Side-specific application of three P-gp and MRP substrates and selective transport inhibition suggested that both membrane proteins were preferentially located on the choroidal side of the oBRB. Therefore, the efflux was directed toward the blood, as in the BBB. To relate the transport characteristics of the oBRB to the BBB, up to nine different test compounds were used. The ranking of the permeability coefficients (P(e)) and the brain-plasma ratios of test compounds indicated that the oBRB has barrier and carrier features similar to those of the BBB in vitro and in vivo.


CONCLUSIONS
Despite the fact that epithelial oBRB and endothelial BBB have developed as separate entities with many site-specific functions, their transport and permeation characteristics display surprising similarities, that include the polarized expression of the two major efflux pumps P-gp and MRP.
Steuer
Heiko
H
NMI Natural and Medical Sciences Institute, University of Tübingen, Markwiesenstr. 55, D-72770 Rütlingen, Germany.
Jaworski
Andreas
A
Elger
Bernd
B
Kaussmann
Martin
M
Keldenich
Jörg
J
Schneider
Herbert
H
Stoll
Dieter
D
Schlosshauer
Burkhard
B
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D001812:Blood-Brain Barrier / Q000502:physiology*; D001813:Blood-Retinal Barrier / Q000502:physiology*; D001921:Brain / Q000098:blood supply*; D002199:Capillary Permeability; D002417:Cattle; D002560:Cerebrovascular Circulation / Q000502:physiology; D002851:Chromatography, High Pressure Liquid; D018920:Coculture Techniques; D004730:Endothelium, Vascular / Q000378:metabolism; D007150:Immunohistochemistry; D008565:Membrane Proteins / Q000378:metabolism; D008855:Microscopy, Electron, Scanning; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010857:Pigment Epithelium of Eye / Q000502:physiology* / Q000648:ultrastructure; D012171:Retinal Vessels / Q000502:physiology*; D013552:Swine; D062826:Zonula Occludens-1 Protein
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D008565:Membrane Proteins; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D010750:Phosphoproteins; D062826:Zonula Occludens-1 Protein; C569043:Tjp1 protein, mouse