Cell-mediated compaction of the extracellular matrix (ECM) plays a critical role in tissue engineering, woundhealing, embryonic development, and many disease states. The ECM is compacted as a result of cellular traction forces. Wehypothesize that a cell mechanically remodels the nearby ECM until some target conditions are obtained, and then the cell stopscompacting. A key feature of this hypothesis is that ECM compaction primarily occurs in the pericellular region and the propertiesof the ECM in the pericellular region govern cellular force generation. We developed a mathematical model to describe theamount of macroscopic compaction of cell-populated collagen gels in terms of the initial cell and collagen densities, as wellas the final conditions of the pericellular environment (defined as the pericellular volume where the collagen is compacted(V) and the mass of collagen within this volume (m)). This model qualitatively predicts the effects of varying initial cell andcollagen concentrations on the extent of gel compaction, and by fitting V and m, provides reasonable quantitative agreementwith the extent of gel compaction observed in experiments with endothelial cells and fibroblasts. Microscopic analysis ofcompacted gels supports the assumption that collagen compaction occurs primarily in the pericellular environment.
Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP) proteins are key actin regulators that localize at regions of dynamic actin remodeling, including cellular protrusions and cell-cell and cell-matrix junctions. Several studies have suggested that Ena/VASP proteins are involved in the formation and function of cellular junctions. Here, we establish the importance of Ena/VASP in endothelial junctions in vivo by analysis of Ena/VASP-deficient animals. In the absence of Ena/VASP, the vasculature exhibits patterning defects and lacks structural integrity, leading to edema, hemorrhaging, and late stage embryonic lethality. In endothelial cells, we find that Ena/VASP activity is required for normal F-actin content, actomyosin contractility, and proper response to shear stress. These findings demonstrate that Ena/VASP is critical for actin cytoskeleton remodeling events involved in the maintenance of functional endothelia.