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The blood-brain barrier is built up by endothelial cells lining the cerebral capillaries whereby the physical diffusion barrier is formed by tight junctions sealing the intercellular clefts. Chemical factors being released endogenously to the blood stream may regulate the barrier tightness. However, since tight junctions of the cerebral capillaries are more complex compared to those of other vessels, it becomes evident that the cells of the neurovascular unit play an important role in the induction and the maintenance of the barrier properties. Astrocytes and pericytes interact with the endothelial cells whereby the contact zone is built up by the extracellular matrix. Thus, in addition to chemical mediators released from either cells of the cerebrovascular unit leading to a crosstalk between those cells, the presence of given molecules of the extracellular matrix and also their assembly have to be considered in the transfer of signals able to induce or modulate the barrier. Here we report and summarize recent evidence that external factors like glucocorticoids act in concert with astroyctes in a co-culture system of primary porcine endothelial cells with astrocytes, but only if astrocytes are able to contact the endothelial cells. Moreover, evidence will be given to show that astrocytic and also the pericytic extracellular matrix produced by those cells are able to induce the barrier by an upregulation of the tight junction proteins occludin, claudin-5 and ZO-1, both on mRNA and at the protein level.

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The blood-brain barrier (BBB) comprises the microvascular endothelial cells, pericytes, and astrocytes, which are connected by the extracellular matrix (ECM). Current BBB models focus solely on the microvascular endothelial cells which constitute a physical barrier by formation of tight junctions (TJs), while the impact of pericytes on barrier regulation is poorly understood. We established a coculture model from primary porcine brain capillary endothelial cells (PBCECs) and pericytes (PBCPs) to approach the in vivo situation. This model allows the examination of pericyte impact on pharmacological, transport, migration, and metabolic activity of the BBB. In vivo the interaction between pericytes and endothelial cells is partly controlled by the ECM which is remodeled by matrix metalloproteinases (MMPs). Both endothelial cells and pericytes secrete MMPs which are important not only for ECM remodeling but also for TJ cleavage. In this chapter, current methods to study the interactions of these cell types by ECM signaling as well as MMP secretion are described.