Brain endothelial hemostasis regulation by pericytes.

Pericytes are known to regulate brain capillary endothelial functions. The purpose of this study was to define the hemostatic regulatory role of human brain pericytes. We used blood-brain barrier models consisting of human pericytes grown on transwell membrane inserts and cocultured with human brain microvascular endothelial cells (HBEC), or pericytes grown in direct contact with HBEC. When grown in cocultures in which pericytes were physically separated from endothelial cells, pericytes induced significant changes in endothelial tissue plasminogen activator (tPA) messenger ribonucleic acid (mRNA) and protein: tPA mRNA level was decreased in pericyte cocultures (52%+/-25% of monocultures, P < 0.05) and tPA protein level was decreased (66%+/-23% of monocultures, P < 0.05). Pericyte effects on endothelial fibrinolysis were enhanced when the two cell types were cocultured in direct contact, with tPA protein reduced in cocultures compared with monocultures (25%+/-15% of monocultures, P < 0.05). Endotoxin (lipopolysaccharide (LPS)), used as a standardized stimulus to define brain-specific inflammation-induced change, amplified pericyte-induced enhanced release of the tPA inhibitor plasminogen activator inhibitor-1 (PAI-1); the latter was released by endothelial cells first cocultured with pericytes and then incubated with LPS in the absence of pericytes. Pericytes (in contrast to endothelial cells and astrocytes) were found to be the principal in vitro source of the serpin protease nexin-1 (PN-1), known to have primarily antithrombin effects. These in vitro findings suggest that pericytes negatively regulate brain endothelial cell fibrinolysis, while pericyte expression of PN-1 may provide endogenous anticoagulant activity.