Endothelial Cell Polarization During Lumen Formation, Tubulogenesis, and Vessel Maturation in 3D Extracellular Matrices

A fundamental event during vascular morphogenesis is the formation of endothelial cell (EC) lumens and tube networks in 3D extracellular matrices (ECM). This morphogenic step leads to rapid EC polarization in that apical and basal membrane domains are created, with fluid contacting the developing apical membrane surface, while ECM is in contact with the basal membrane surface. Interestingly, it has been difficult to identify specific apical vs. basal markers in ECs. This suggests that their ability to polarize is distinct from other cell types such as epithelial cells which have more extensive cell–cell junctional contacts and more specialized apical domains with exocrine abilities where substantial fluids can be generated and released apically. One key difference is that ECs in contact with blood must be flat in order to properly function and respond to fluid flow forces. Interestingly, our recent studies reveal that tubulins and, in particular, modified tubulins such as acetylated tubulin are distributed in a subapical domain that supports the developing apical membrane surface during EC lumen formation and tube maintenance. Disruption of the tubulin cytoskeleton can lead to failure of tube formation and rapid tube collapse suggesting that the polarized shape of EC-lined tubes in 3D extracellular matrices is strongly dependent on the tubulin cytoskeleton rather than the actin cytoskeleton (although both are clearly important). EC tube polarization is also strongly affected by the recruitment of mural cells (i.e., pericytes) which, in conjunction with ECs, leads to critical extracellular matrix remodeling including deposition of the vascular basement membrane matrix which is laid down on the basal surface in between the basally distributed pericytes which co-contribute this matrix along with ECs. In conclusion, the mechanisms that stimulate EC tubulogenesis establish apical–basal cytoskeletal and membrane polarity during morphogenic events in 3D matrices. EC polarization is further enhanced by the recruitment of mural cells such as pericytes along the abluminal EC surface which stimulates vascular basement membrane assembly along this basal surface to further contribute to EC polarity and maintenance of EC-lined tubes. At this point, flow forces and components of blood together will provide additional polarizing information to the EC apical surface and contribute to vascular tube stabilization and the development of EC quiescence in the mature vasculature.