Pericyte-endothelial cell interaction increases MMP-9 secretion at the blood-brain barrier in vitro.

The extracellular matrix (ECM) connecting brain capillary endothelial cells (BCEC) with the surrounding brain resident cells is an essential part of the blood-brain barrier (BBB). Represented by the basement membrane which joins BCEC with brain neuroglia (astrocytes and pericytes) it forms a neurovascular unit. Neuroglia-secreted matrix metalloproteinases (MMPs) control the ECM composition and are involved in the integrity and function of the BBB during cell diapedesis and BBB breakdown after ischemia and other CNS diseases. We examined the involvement of pericytes and astrocytes in endothelial MMP secretion and their effect on endothelial barrier properties in a primary cell culture model using porcine BCEC. We applied puromycin to eliminate pericyte growth and demonstrated a significant (to about 30%) reduction of endothelial MMP-9 production in pericyte-free cultures. In contrast, BCEC co-culture with pericytes resulted in an increased amount of endothelial MMP-9 and active MMPs measured by both zymography and fluorimetric assay. Astrocyte co-culture in a filter setup with BCEC allowing a cell-cell signaling via soluble factors revealed significantly reduced endothelial MMP activity. These data were directly correlated with improved BBB integrity under pericyte elimination and astrocyte co-culture conditions as indicated by transendothelial electrical resistance (TEER) values. Our data define pericyte interactions as a main inducer of endothelial MMP secretion and propose a new role for pericyte-endothelial cell crosstalk at the BBB in vitro.