Vinculin organises apical Transcellular Actin Fibres to coordinate collective migration

Collective cell migration is coordinated by adherens junctions (AJs) which serve both as structural links between the actin cytoskeleton of adjacent cells, as well as mechano-transductory structures allowing cells to transmit mechanical signals. Vinculin contributes to AJ maturation in multiple ways, binding to actin and the cadherin-catenin complex, bundling actin filaments, antagonising branched actin polymerisation and recruiting late AJ proteins. Here we have analysed the effect of vinculin on junctional actin organisation and its role in collective migration during unjamming in the human epithelial cell line MCF10A. At the apical surface of MCF10A monolayers, we found transcellular actin fibres (TAFs) that are directionally coordinated across long ranges, up to 10 cells. These TAFs are contractile and "cross" cell-cell contacts at AJs. Analysis of a vinculin knockout cell line revealed that this protein is essential for the coordination of TAFs across multiple cells. Arp2/3 activity must be tightly regulated to establish a long-range network of TAFs, since its downregulation by CK666 treatment, as well as its upregulation by the expression of an activated Rac1 mutant or a mutation that prevents the vinculin-Arp2/3 interaction, all impair TAF formation. During hypotonic unjamming of monolayers, we found that MCF10A cells with long-range TAFs migrate more collectively than vinculin KO cells in which TAFs only connect adjacent cells. Similarly, space-induced unjamming of MCF10A and vinculin knockout monolayers showed that cells connected by the long-range TAFs can collectively coordinate the direction in which they extend their lamellipodia. Thus, we show that vinculin plays a novel role in organising long-range actin networks across multiple cells and coordinating collective migration within cell monolayers.