Tightening of endothelial cell contacts: a physiologic response to cocultures with smooth-muscle-like 10T1/2 cells.

Tightening of endothelial cell-to-cell contacts is an important event at the end of angiogenesis in order to achieve controlled transfer of solutes between the blood stream and solid tissues. We found that tightening of endothelial cell-to-cell contacts and the formation of a permeability barrier can be induced in vitro by dibutyryl cAMP and hydrocortisone. This process is accompanied by increased junctional localization and cytoskeletal association of the adherens junctional plakoglobin and the tight junction associated proteins ZO-1, ZO-2, and occludin. Based on these findings, we proceeded to investigate whether smooth-muscle-like mesenchymal cells would influence endothelial junctional differentiation. For this purpose, human umbilical chord vein endothelial cells and murine smooth-muscle-like 10T1/2 cells were cocultivated and compared with their respective monocultures. Immunofluorescence on cells and Western blot analyses were performed for marker proteins of adherens and tight junctions. Functional permeability assays were performed for the tracer molecule biotin-dextran. The results indicated that 10T1/2 cells induced the tightening of endothelial cell-to-cell contacts. Plakoglobin, ZO-1, ZO-2, and occludin showed increased junctional localization when 10T1/2 cells were present. Cocultures also displayed a significantly higher permeability barrier for the tracer molecule biotin-dextran. In conclusion, mural cells such as smooth muscle cells and pericytes may be important for stabilizing endothelial cell-to-cell contacts and may influence vessel-type specific differences of the endothelial phenotype.