Mechanisms of the penetration of blood-borne substances into the brain

Abstract

The blood-brain barrier (BBB) impedes the influx of intravascular compounds from the blood to the brain. Few blood-borne macromolecules are transferred into the brain because vesicular transcytosis in the endothelial cells is considerably limited and the tight junction is located between the endothelial cells. At the first line of the BBB, the endothelial glycocalyx which is a negatively charged, surface coat of proteoglycans, and adsorbed plasma proteins, contributes to the vasculoprotective effects of the vessels wall and are involved in maintaining vascular permeability. In the endothelial cytoplasm of cerebral capillaries, there is an asymmetrical array of metabolic enzymes such as alkaline phosphatase, acid phosphatase, 5'-nucleotidase, adenosine triphosphatase, and nucleoside diphosphatase and these enzymes contribute to inactivation of substrates. In addition, there are several types of influx or efflux transporters at the BBB, such as P-glycoprotein (P-gp), multidrug resistance associated protein, breast cancer resistance protein, organic anion transporters, organic cation transporters, organic cation transporter novel type transporters, and monocarboxylic acid transporters. P-gp, energy-dependent efflux transporter protein, is instrumental to the barrier function. Several findings recently reported indicate that endothelial P-gp contributes to efflux of undesirable substances such as beta-amyloid protein from the brain or periarterial interstitial fluid, while P-gp likely plays a crucial role in the genesis of multiple vascular abnormalities that accompany hypertension. In this review, influx and efflux mechanisms of drugs at the BBB are also reviewed and how medicines pass the BBB to reach the brain parenchyma is discussed.

Keywords: Blood-brain barrier; P-glycoprotein; tight junction..

Fig. (1)

Blood-brain barrier and the tight junction. (A) Schematic outline of a capillary of the blood-brain barrier in a transverse section showing the endothelium with a tight junction, basement membrane, a pericyte, and astrocytes. The localization of a gap junction between end-feet of the astrocytes is conceptually shown. (B) An electron micrograph showing a typical mammalian endothelial cell combined with a tight junction (arrows) and cationized ferritin (arrowheads) bound to the luminal endothelial surface representing the endothelial glycocalyx. There are scarce vesicular structures in the typical ECs of the brain.

Fig. (2)

(A) Hypothetical transcytotic routes through ECs of the BBB and membrane events associated with endocytic processes are represented. Direct transendothelial vesicular transport or an indirect route through the endosomal compartment (E) may apply to the receptor-mediated transcytosis of blood-borne ligands. Some endocytic vesicles derived from the luminal surface membranes are likely directed to endosomes that give rise to exocytic vesicles. Macromolecules entering the endothelium by adsorptive endocytosis are channeled to endosomes and either direcly or indirectly to the inner saccule of the Golgi complex (G); this Golgi saccule is responsible for packaging macromolecules for exocytosis at the abluminal and luminal faces of the endothelium. Fluid-phase macromolecules and some macromolecules taken into the endothelium by receptor-mediated and adsorptive endocytic processes are directed to endosomes (E) and lysosomes (L). The macromolecules deposited in lysosomes are enzymatically degraded rather than transcytosed. The multidrug resistance efflux transporter P-gp is shown at the luminal and abluminal membranes of the endothelial cytoplasm. (B) A rat brain was removed after perfusion with physiological saline and immersion-fixed with 4% paraformaldehyde in 0.1M phosphate buffer (PB). The brain tissue was embedded in LR White resin after additional fixation in 1% glutaraldehyde in 0.1M PB for 1 hour. Ultrathin sections were stained with anti-P-gp antibody, followed by incubation in a solution of anti-mouse IgG antibody conjugated with colloidal gold particles of 10 nm diameter (EY Laboratories, CA), diluted with phosphate buffered saline (1:25), for 1 h at RT. The ultrathin secions were stained with uranyl acetate and Reynold’s lead citrate, and were examined in a JEM-1200EX electron microcope (JEM, Tokyo, Japan). Labeling by 10-nm gold particles conjugated with the antibody against P-gp is found in the cytoplasm including the luminal (long arrows) and abluminal (short arrows) membranes of the ECs, and the basal lamina. The scale bar indicates 500 nm.