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Force transmission during adhesion-independent migration
of several kilopascals (ref. 7). In contrast, force transmission during focal-adhesion-independent migration has not been investigated experimentally and the origin and magnitude of the forces involved remain unclear. Several theoretical models have been proposed. For example, in smooth 3D confinement, such as during migration through dense tissues in cancer invasion or in development810,
nonspecific substrate friction could account for force transmission11.
However, a friction-based mechanism has not been demonstrated experimentally, and it is unknown whether friction alone can generate sucient force to mediate cell body translocation.
To investigate forces exerted during focal-adhesion-independent migration, we used a non-adherent subline of Walker 256 carcinosarcoma (henceforth Walker) cells as a model. Walker cells polarize spontaneously in suspension and form blebs at their leading edge. They are unable to migrate on 2D substrates, but move eectively when confined in three dimensions12. We verified
that Walker cells migrate in a variety of confined environments, including in confinement between an agarose pad and serum-coated glass, within 3D collagen gels and in polydimethylsiloxane (PDMS) microfluidic channels passivated with bovine serum albumin (BSA) or beta-lactoglobulin to prevent cell adhesion13,14 (Fig. 1a
and Supplementary Fig. 1a,b). Walker cells remained motile when confined between agarose, which is non-adhesive15, and other
non-adhesive surfaces (Fig. 1b and Supplementary Fig. 1d). We therefore reasoned that Walker cells migrate in confinement without using specific substrate adhesions. To test this directly, we first attempted to localize focal adhesion components in these cells.We found that neither GFP-tagged focal adhesion kinase (FAK) nor speckle-GFPvinculin, a low-expression construct of vinculin allowing for detection of faint focal adhesion points16, formed foci at the basal surface of Walker cells migrating under agarose (Fig. 1c and Supplementary Fig. 1c). Furthermore, Walker cells with reduced
1MRC Laboratory for Molecular Cell Biology, UCL, London WC1E 6BT, UK. 2Institute for the Physics of Living Systems, UCL, London WC1E 6BT, UK. 3Max Planck Institute for the Physics of Complex Systems, Dresden 01187, Germany. 4MRC National Institute for Medical Research, London NW7 1AA, UK. 5Department of Cell and Developmental Biology, UCL, London WC1E 6BT, UK. 6London Centre for Nanotechnology and Department of Cell and Developmental Biology, UCL, London WC1H 0AH, UK. 7These authors contributed equally to this work. 8Present...