Actin accumulates nesprin-2 at the front of the nucleus during confined cell migration

The mechanisms by which cells exert forces on their nuclei to migrate through openings smaller than the nuclear diameter remain unclear. In microfluidic devices, the hourglass shape of the nucleus and its strain patterns as it translocates through narrow constrictions suggest pulling forces. We use CRISPR/Cas9 to label nesprin-2 giant, a protein that links the cytoskeleton to the interior of the nucleus. We demonstrate that nesprin-2 giant accumulates at the front of the nucleus during nuclear deformation through narrow constrictions, independently of the nuclear lamina. We find that nesprins are more mobile than lamin A/C, and hypothesize that nesprin accumulation at the front is a consequence of forward pulling by the cytoskeleton. Using artificial constructs, we show indeed that the actin-binding domain of nesprin-2 is necessary and sufficient to generate this accumulation, and that microtubules are not involved. Actin filaments are organized in a barrel structure around the moving nucleus in the direction of movement. This and estimates of nesprin elongation suggest that actin pulling forces on nesprins are distributed around the deformed nucleus towards the front.