MECHANISMS OF EPITHELIAL CELL-CELL ADHESION AND CELL COMPACTION REVEALED BY HIGH-RESOLUTION TRACKING OF E-CADHERIN-GREEN FLUORESCENT PROTEIN

Cadherin-mediated adhesion initiates cell reorganization into tissues, but the mechanisms and dynamics of such adhesion are poorly understoo d. Using time-lapse imaging and photobleach recovery analyses of a ful ly functional E-cadherin/GFP fusion protein, we define three sequentia l stages in cell-cell adhesion and provide evidence for mechanisms inv olving E-cadherin and the actin cytoskeleton in transitions between th ese stages. In the first stage, membrane contacts between two cells in itiate coalescence of a highly mobile, diffuse pool of cell surface E- cadherin into immobile punctate aggregates along contacting membranes. These. E-cadherin aggregates are spatially coincident with membrane a ttachment sites for actin filaments branching off from circumferential actin cables that circumscribe each cell. In the second stage, circum ferential actin cables near cell-cell contact sites separate, and the resulting two ends of the cable swing outwards to the perimeter of the contact. Concomitantly, subsets of E-cadherin puncta are also swept t o the margins of the contact where they coalesce into large E-cadherin plaques. This reorganization results in the formation of a circumfere ntial actin cable that circumscribes both cells, and is embedded into each E-cadherin plaque at the contact margin. At this stage, the two c ells achieve maximum contact, a process referred to as compaction. The se changes in E-cadherin and actin distributions are repeated when add itional single cells adhere to large groups of cells. The third stage of adhesion occurs as additional cells are added to groups of >3 cells ; circumferential actin cables linked to E-cadherin plaques on adjacen t cells appear to constrict in a purse-string action, resulting in the further coalescence of individual plaques Into the vertices of multic ell contacts. The reorganization of E-cadherin and actin results in th e condensation of cells into colonies. We propose a model to explain h ow, through strengthening and compaction, E-cadherin and actin cables coordinate to remodel initial cell-cell contacts to the final condensa tion of cells into colonies.