Dynamics of integrin clustering at focal contacts of endothelial cells studied by multimode imaging microscopy

Human umbilical vein endothelial cells were stained with FITC-labeled anti-β₁ integrin antibody and plated on a glass cover slip to elucidate the mechanism of integrin clustering during focal contact formation. The process of integrin clustering was observed by time-lapse total-internal-reflection fluorescence microscopy, which can selectively visualize the labeled integrins at the basal surface of living cells. The clustering of integrins at focal contacts started at 1 hour after plating and individual clusters kept growing for ∼6 hours. Most integrin clusters (∼80%) elongated towards the cell center or along the cell margin at a rate of 0.29±0.24 μm minute⁻¹. Photobleaching and recovery experiments with evanescent illumination revealed that the integrins at the extending tip of the clusters were supplied from the intracellular space. Simultaneous time-lapse imaging of exocytosis of integrin-containing vesicles and elongating focal contacts showed that most exocytosis occurred at or near the focal contacts followed by their elongation. Double staining of F-actins and integrins demonstrated that stress fibers were located near the integrin clusters and that intracellular punctate integrins were associated with these stress fibers. These results suggest that the clustering of integrins is mediated by actin-fiber-dependent translocation of integrins to the extending tip of focal contacts.