Large deformations in the cytoplasm and cytoskeleton plasticity

Cells spread, proliferate, migrate, divide, and die. Large deformations of the cytoplasm are associated with these processes.  Considerable research has generated a mechanical description of the cytoplasm; however, most of this work has concentrated on the small deformation linear regime. In this talk, I will describe the physics of large cytoplasm deformations.

In this experiment work, we use an electrical magnetic tweezer to pull ferromagnetic particles through the cytoplasm. We induce large deformations and measure the plasticity of the cytoplasm.  Further, we visualize how fluorescently-tagged cytoskeleton filaments: microtubules and vimentin reorganize themselves during this process. We find that the strain rate of large deformations has a distinct limit, independent of the applied force. This strain rate limit is dictated by the cytoskeleton dynamics. It can be altered by chemical treatments that either change the cytoskeleton density or inhibit the cytoskeleton dynamics. 

We were also able to differentiate the role of each filament type. In the mouse embryonic fibroblast cell line, we find the microtubules and the intermediate filaments are important in the cytoplasm plastic deformation due to their abundance. In addition, they act synergistically to generate deformation resistance: removal of either filament completely changes the physics of the large deformation.

This research joins two longtime biophysics concepts together: the capacity of the cell to deform over time,e.g. cargo transport, cell migration, cell division, and the cytoskeleton dynamics. We hope this study can enrich the mechanical and physical understanding of the complicated and fascinating cellular biosystem.