Nuclear deformation and dynamics of migrating cells in 3D confinement reveal adaptation of pulling and pushing forces

Eukaryotic cells show an astounding ability to migrate through pores and constrictions smaller than their nuclear diameter. However, the forces engaged in nuclear deformation and their effect on confined cell dynamics remain unclear. Here, we study the mechanics and dynamics of nuclei of mesenchymal cancer cells as they spontaneously and repeatedly transition through 3D compliant hydrogel channels. We find a biphasic dependence of migration speed and transition frequency on channel width, revealing maximal transition rates at widths comparable to the nuclear diameter. Using confocal imaging and hydrogel bead displacement, we determine the nuclear deformation and corresponding forces during spontaneous confined migration. We find the nucleus to reversibly deform with an elastic modulus not adapting to the confinement. Instead, with decreasing channel width, the nuclear shape during transmigration changes biphasically concomitant with the transitioning dynamics. The nucleus exhibits a prolate form along the migration direction in wide channels and a more compressed oblate shape in narrow channels. We propose a physical model for confined cell migration that explains the observed nuclear shapes and slowing down in terms of the cytoskeletal force-generation adapting from a pulling-to a pushing-dominated mechanism with increasing nuclear confinement.