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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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Cerebral endothelial cells accomplish the barrier functions between blood and brain interstitium. Structural features are the tight junctions between adjacent endothelial cells and the formation of marginal folds at the cell-cell contacts. The glucocorticoid hydrocortisone (HC) has been reported to enforce the blood-brain-barrier in vitro measurable by an increase of the transendothelial electrical resistance. This study shows the impact of HC on the mechanical and morphological properties of confluent cell layers of brain microvascular endothelial cells. HC induces an increase in height of these marginal folds and a reduction of the intercellular contact surface. These morphological changes are accompanied by changes in cell elasticity. Staining of fibrous actin indicates that HC induces a reorganization of the actin cortex. The quantitative determination of the local elastic properties of cells reveals for the first time an HC-induced increase of the representative Young's modulus according to cytoskeletal rearrangements. For this study, cells of two different species, porcine brain capillary endothelial cells and murine brain capillary endothelial cells, were used yielding similar results, which clearly demonstrates that the HC effect on the cell elasticity is species independent.

The influence of ectoine and sarcosine on the mechanical properties of surface bound fibronectin has been investigated by means of force microscopy. Single molecule stretching experiments of fibronectin molecules reveal that ectoine and sarcosine increase the tendency of the polypeptide to coil, thus decreasing its apparent persistence length. This behavior can be explained by means of the preferential exclusion model implying that the osmolytes are expelled from the protein surface due to the increase in chemical potential of the denatured, i.e. stretched, state forcing the protein into a more compact structure. Detailed analysis of the unfolding forces, which are extracted from the successive unfolding of the individual subunits of fibronectin, shows that neither ectoine nor sarcosine influence the binding strength in a significant way even if the concentration of osmolytes exceeds 1 M.