ENDOTHELIAL-CELL TUBE FORMATION DEPENDS ON CADHERIN-5 AND CD31 INTERACTIONS WITH FILAMENTOUS ACTIN

Capillary tube formation represents a specialized endothelial cell fun ction and is a prerequisite for the establishment of a continuous vess el lumen. Tube formation is thought to depend on cell-cell adhesion mo lecules, but whereas cadherins, Ig superfamily proteins, and integrins have been considered as potential candidates, the specific proteins s ubserving this function are unknown. We identified cadherin 5 and CD31 as two molecules critical for endothelial cell tube formation. Using human dermal microvascular endothelial cells or human umbilical vein e ndothelial cells for an in vitro capillary tube formation assay or, fo r comparison, an in vivo wound healing model in SCID mice, we show tha t tube formation in vivo and in vitro were prevented by adhesion-block ing mAbs to both proteins when used in conjunction but not by either m Ab alone. In addition, both mAbs but not each individual mAb blocked c ytoplasmic filamentous actin (F-actin) reorganization that resulted in cytoplasmic F-actin clumps similar to those induced by cytochalasin D , suggesting that cadherin 5 and CD31 tune tube formation in concert w ith F-actin. Indeed, cytochalasin D blocked capillary tube formation b ut colchicine did not, the latter inhibiting microtubule but not F-act in assembly. Using immunoprecipitation methods, we show for the first time that not only cadherin 5 but also a portion of CD31 connects to b eta-catenin, which is part of the adherens junction complex known to b e linked to F-actin. In conclusion, we provide evidence that cadherin 5, CD31, beta-catenin, and F-actin shape a functional complex that con trols endothelial cell tube formation.