Nuclear softening is essential for protease-independent migration.

During amoeboidal migration, cancer cells migrate in a protease-independent manner by squeezing through pre-existing gaps in the extracellular matrix (ECM). However, the extent to which cells alter their physical properties in order to sustain this mode of migration remains unclear. Here, we address this question by documenting biophysical changes in the properties of highly invasive MDA-MB-231 and HT-1080 cells upon inhibition of pericellular proteolysis. Remarkably, treatment with the broad spectrum MMP inhibitor GM6001 not only induces cell rounding and loss of actomyosin contractility, but also induces nuclear softening via increased phosphorylation of the nuclear membrane protein lamin A/C. Though nuclear softening is necessary for sustaining migration through sub-nuclear sized transwell pores, it is not sufficient. In addition, baseline levels of contractility mediating pore entry and peri-nuclear actin inside the pores mediating pore migration are also required. Taken together, our results suggest that protease-independent migration through sub-nuclear sized pre-existing tracks is enabled by deformation of a softened nucleus by contractility and the peri-nuclear actin network.