Role of nuclear ATPases in nuclear mechanics and cell migration through confined spaces: opposite effects of BRG1 and cohesin

Abstract

Deformation of the nucleus often presents a barrier to cell migration through tight spaces, such as those encountered as cells move through tissues or across extracellular matrix barriers. Reorganization of the nucleus to allow its passage through spaces much smaller than its resting diameter requires forces generated by the cytoskeleton, as well as active reorganization within the nucleus driven by ATPases that crosslink or move chromatin. Here, we show that two different nuclear ATPases, the BRG1/SMARCA4 motor of the BAF or SWI/SNF complex and the bifunctional crosslinking and loop extruding complex, cohesin, have opposite effects on the stiffness of isolated nuclei. Inhibition of BRG1 stiffens the nucleus, and cohesin softens it in karyoplasts derived from multiple cell types, including four different cancer cells, fibroblasts, and mesenchymal stem cells. The effects on isolated nuclear stiffness coincide with the effects of these ATPases on the ability of cells to migrate through tight spaces. Stiffening the nucleus inhibits single cell migration through micron-sized pores and the outward migration of tumor cell spheroids into a surrounding collagen matrix. Softening the nucleus by inhibiting cohesin has the opposite effect: it enhances single-cell migration through pores, at least for some cell types, and facilitates the outgrowth of cells from a tumor cell spheroid into the surrounding matrix. These results emphasize the importance of active motions generated within the nucleus for the global mechanics of the nucleus and the way that it deforms in response to externally generated stresses. Statement of significanceCells often move through tight spaces within three-dimensional materials. This constricted motion requires deformation of the nucleus, which is often stiffer and less dynamic than the rest of the cell. Nuclear deformation is achieved in part by forces generated outside the nucleus, primarily by cytoskeletal motors, but nuclear deformation is also affected by intranuclear motor proteins that move and reorganize chromatin. Here we show that the activity of DNA binding chromatin remodeling ATPases affects not only the stiffness of the isolated nucleus but also the ability of cells to deform as they move through tight spaces. Inhibition of motors such as BRG1 that fluidize the nucleus prevents cell movement through tight spaces, whereas inactivation of the crosslinking ATPase cohesin tends to enhance it.