Cell nuclei as cytoplasmic rheometers

Some researchers probe the mechanics of cells by perturbing them from the outside, such as using an atomic force microscope probe to record the amount of deformation of the cell in response to applying a prescribed force at a defined speed. Other researchers probe the mechanics of cells by perturbing them from the inside, an example of which is particle-tracking microrheology, in which the spontaneous motion of submicron, passive fluorescent beads ballistically injected earlier into the cell decodes the cell moduli. Both types of probes are typically composed of nonliving material. In this issue of Biophysical Journal, Moradi and Nazockdas cleverly propose to use the cell nucleus itself as a rheological probe for the mechanics of the cytoplasm (1). The cell nucleus is typically the largest and the stiffest organelle in eukaryotic cells. The surrounding cytoplasm contains other organelles and the cytoskeleton, which is comprised different kinds of semiflexible polymers, including actin, microtubules, and intermediate filaments. For cells that are confined by geometries on the scale of the size of the cell, the nucleus is minimally deformed and can therefore be approximated as a rigid sphere. It is in this limit that the authors ask the following questions. As a cell moves inside a microchannel, what does the motion of the cell nucleus, in response to deformations in the cell cortex, reveal about the rheology of the cytoplasm? Is it viscoelastic? Is it porous? Is it a poroelastic network? Is it something else? Answers to such questions will help us better understand cell function, such as how the cytoplasm reorganizes in response to changes in a cell physical environment