Heterogeneity of Endothelial Sheet Migration: Role in Angiogenic Plasticity.

The vascular system is a complex network of conduit and microvascular vessels exposed to different microenvironments that imprints unique phenotypic traits on individual endothelial cell populations. Endothelial cells in fully differentiated blood vessels in adult tissues have a quiescent phenotype characterized by an increased resistance to proliferate, migrate or undergo apoptosis. The intrinsic capacity of endothelial cells to switch from a quiescent to angiogenic phenotype however plays an important role in wound healing, several vascular proliferative disorders and tumor angiogenesis. Endothelial cells revert to an angiogenic phenotype as isolated cells in culture nonetheless they are contact inhibited at confluence reflecting their in vivo phenotype in the endothelium. In this study, we used time-lapse video microscopy to study early events in endothelial sheet migration in confluent monolayers of primary endothelial cells derived from conduit (PAECs) and microvascular (PMVECs) blood vessels. Recordings were made in a live cell chamber and were restricted to six hours to minimize the effect of proliferation on sheet migration. PMVECs at the wound edge were significantly highly spread and squamous in appearance compared to PAECs, which had a distinctly more cuboidal morphology. Majority (>95%) of PMVECs at the wound edge produced extensive lamellipodia based on morphology and dynamics that measured 25 μm ± 4 μm. By contrast, PAECs formed significantly smaller lamelipodia which extended by 8 μm ± 4 μm. On average PMVEC sheets migrated at a speed of 12.5 μm per hour covering a total distance of 75 μm ± 15 μm (n=6). Sheet migration rate in PMVECs was 3-fold faster than in PAECs (3.8 μm per hour), which covered a total distance of 23 μm ± 10 μm in the same time period. To unravel the molecular basis for this functional diversity, gene micro array analysis was performed. We identified unique transcriptional profiles for cell-cell adhesion molecules, integrins and disintegrin-metalloproteases each with a distinct role in collective cell migration. In particular, integrin alpha 7, which is a major regulator of lamellipodia formation was found to be 20-fold more abundant in PMVECs than in PAECs. This study provides molecular and functional evidence for heterogeneity of endothelial sheet migration. This central finding highlights variability in angiogenic plasticity in fully differentiated endothelial cells, which may have important ramifications for anti-angiogenesis therapy.