Astrocyte-endothelial interactions and blood-brain barrier permeability

Abstract

The blood-brain barrier (BBB) is formed by brain endothelial cells lining the cerebral microvasculature, and is an important mechanism for protecting the brain from fluctuations in plasma composition, and from circulating agents such as neurotransmitters and xenobiotics capable of disturbing neural function. The barrier also plays an important role in the homeostatic regulation of the brain microenvironment necessary for the stable and co-ordinated activity of neurones. The BBB phenotype develops under the influence of associated brain cells, especially astrocytic glia, and consists of more complex tight junctions than in other capillary endothelia, and a number of specific transport and enzyme systems which regulate molecular traffic across the endothelial cells. Transporters characteristic of the BBB phenotype include both uptake mechanisms (e.g. GLUT-1 glucose carrier, L1 amino acid transporter) and efflux transporters (e.g. P-glycoprotein). In addition to a role in long-term barrier induction and maintenance, astrocytes and other cells can release chemical factors that modulate endothelial permeability over a time-scale of seconds to minutes. Cell culture models, both primary and cell lines, have been used to investigate aspects of barrier induction and modulation. Conditioned medium taken from growing glial cells can reproduce some of the inductive effects, evidence for involvement of diffusible factors. However, for some features of endothelial differentiation and induction, the extracellular matrix plays an important role. Several candidate molecules have been identified, capable of mimicking aspects of glial-mediated barrier induction of brain endothelium; these include TGFbeta, GDNF, bFGF, IL-6 and steroids. In addition, factors secreted by brain endothelial cells including leukaemia inhibitory factor (LIF) have been shown to induce astrocytic differentiation. Thus endothelium and astrocytes are involved in two-way induction. Short-term modulation of brain endothelial permeability has been shown for a number of small chemical mediators produced by astrocytes and other nearby cell types. It is clear that endothelial cells are involved in both long- and short-term chemical communication with neighbouring cells, with the perivascular end feet of astrocytes being of particular importance. The role of barrier induction and modulation in normal physiology and in pathology is discussed.

Fig. 1

Schematic drawing showing some features of the perivascular astrocytic end feet, which form ‘rosette’-like structures on the brain capillary surface. This arrangement would be expected to be optimal for two-way induction and communication between the astrocytes and endothelium, while not forming a physical barrier, so preserving free diffusion between the endothelium and the brain parenchyma. Based on Kacem et al. (1998).

Fig. 2

Development of tightness of the BBB model formed by culture of ECV304 cells, which show a robust endothelial phenotype. When grown without C6 glioma cells (–C6), the transendothelial electrical resistance (TEER, a measure of tight junctional restriction of the paracellular pathway) develops slowly over 15 days. TEER is enhanced when the cells are co-cultured, grown on filters with C6 glioma cells in the base of the wells (+C6). The model has been used to examine the mechanisms by which bradykinin modulates [Ca²⁺]_i and TEER. From Easton & Abbott (1997).

Fig. 3

Evidence for a role of free radicals in bradykinin-mediated increase in permeability of rat pial microvessels in situ, measured with a fluorescence technique. A, the free radical scavengers superoxide dismutase (SOD) and catalase (CAT) (each 100 U mL⁻¹) applied separately to the brain surface had little effect on the permeability response to 5 μM bradykinin, but completely blocked it when used in combination. B, the lipid peroxidation chain blocker butylated-hydroxytoluene (BHT; 1 mM) also inhibited the permeability response to 5 μM bradykinin. (n.s., not significant; **P < 0.01). From Sarker et al. (2000), by permission.

Fig. 4

Evidence for nucleotide receptors coupled to elevation of [Ca²⁺]_i on primary cultured rat brain endothelial cells grown on a biological matrix. (a) Calcium responses measured with fura2 (fluorescence ratio) were elicited by 100 μM ATP and several related agonists; evidence for presence of different receptors includes the observed response to UTP (P2Y₂) and to 2-MeSATP/ 2-MeSADP/ADP (P2Y₁). (b) The response to 2-MeSATP was similar in calcium-free and normal medium, evidence for mediation by a metabotropic rather than ionotropic receptor. From Sipos et al. (2000), by permission.