PROFILIN-1 IN CAPILLARY MORPHOGENESIS OF VASCULAR ENDOTHELIAL CELLS

Vascular endothelial cells (VEC) assemble into capillary-like structures during angiogenesis, and this neovascularization process plays an important role in a wide range of physiological and pathological scenarios. Based on significant upregulation of its expression in VEC during capillary morphogenesis, profilin-1 (Pfn1 - a ubiquitously expressed actin-binding protein) was previously implicated in capillary morphogenesis of VEC. The overall objective of the present study was to investigate whether and how loss of Pfn1 function affects a) the various cellular functions that are important for capillary morphogenesis such as VEC migration, invasion and proliferation, and b) the overall capillary forming ability of VEC. Loss of Pfn1 function in VEC was achieved either by suppressing the overall expression of Pfn1 by RNA interference method or selectively abrogating specific ligand-interactions (actin, proline-rich ligands) of Pfn1 by expressing various point-mutants of Pfn1 in a near-null endogenous Pfn1 background (knockdown and knock-in approach). Loss of Pfn1 expression causes a major change in actin cytoskeleton in VEC. Particularly, there is a significant depletion of actin filaments and focal adhesions in VEC when Pfn1 expression was silenced. Silencing of Pfn1 expression also significantly impairs the migratory ability of VEC. Analyses of leading edge dynamics revealed that Pfn1 depletion results in decreased velocity and frequency of lamellipodial protrusion. Further experiments with point-mutants of Pfn1 showed that both actin and polyproline interactions of Pfn1 are required for efficient lamellipodial protrusion and overall migration of VEC. Loss of Pfn1 expression is associated with reduced dynamics of VE-cadherin dependent cell-cell adhesion, which was also found to be correlated with increased nuclear accumulation of p27 Kip1 (a major cell-cycle inhibitor) and reduced VEC proliferation. Finally, we found that loss of overall expression of Pfn1 significantly impairs collagen gel invasion and three-dimensional (3-D) capillary morphogenesis of VEC. Abolishing either of actin or polyproline interactions of Pfn1 also leads to a dramatic inhibition of capillary mophogenesis of VEC. Taken together, these results demonstrate that Pfn1 plays a critical role in capillary morphogenesis of VEC through its interactions with both actin and polyproline ligands. This study may further imply that Pfn1 could be a novel angiogenesis target