Elisabeth Fischer-Friedrich Group
Active rheology of the cytoskeleton
Many physiological processes, such as cell division or cell migration, include cellular shape changes. Like all solid matter, cells resist shape changes through their material stiffness. Understanding this material stiffness is therefore pivotal for our understanding of cell shape changes. As molecular force generators produce active contractile stress in cellular material, physical concepts for inanimate matter need to be extended to capture material properties of cells. Our group works on the development of theoretical and experimental tools for the quantification of active material properties of cells.
The rheology of the cellular cortex
The cell cortex is a cytoskeletal meshwork of polymerized actin proteins that mechanically supports the inner side of the cellular plasma membrane. The cortex is constantly renewing itself through cycles of polymerization and depolymerization. Due to this turnover, it has long been hypothesized that the cortex behaves fluid-like on timescales larger than its turnover timescale. However, it was difficult to test this hypothesis experimentally as the cortex is connected to other cellular structures that generally contribute to the cellular force response in cell-mechanical probing. We have developed a cell confinement assay using atomic force microscopy (AFM) that dominantly probes the cellular cortex. In this assay, we dynamically compress (non-adherent) cells between parallel plates thereby stretching the overall cell surface and thus the cortical layer of the cell. We could extract a frequency-dependent complex elastic modulus that characterizes the mechanical resistance of the cortex with respect to area dilation. Indeed, we find that the cell cortex starts to behave dominantly fluid-like for frequencies lower than ~0.01 Hz.
Cortical rheology is captured by a simple rheological model
We find that cellular rheology exhibits a characteristic timescale, which marks the onset of fluidity of the cellular cortex. Still, cortical rheology is not captured by a single relaxation timescale as in the case of a simple Maxwell model. Also, a power law as commonly found in the rheology of adherent cells does not capture cortical rheology as it is devoid of a characteristic timescale. We have established a new rheological model that captures cortical rheology. This model is defined through a constant relaxation spectrum up to a cut-off timescale. This cut-off timescale corresponds to a slowest relaxation mode in the material. Our new rheological model reconciles key features of the commonly used Maxwell and power law models, because it exhibits a (slowest) characteristic timescale but also a continuum of smaller relaxation timescales. Interestingly, we find that that the slowest relaxation timescale of the cortex is similar to turnover times of cortical cross-linkers. This finding strengthens the idea that cortex fluidization stems from cortical turnover. In fact, we could show that the abundance of actin cross-linker proteins tunes cortical rheology.
Future Projects and Goals
Our group establishes a new approach to cell mechanics characterizing cells as an actively prestressed material. Unlike inanimate matter, cells contain molecular force generators that produce active contractile stresses in the cellular material. In cell mechanical probing, the contribution of these active stresses has been mainly disregarded. A central aim of our group is to reveal active and passive contributions to the effective cellular shear modulus conventionally used in cell mechanics. Our insight will substantially enhance our understanding of dynamic cell shape regulation for instance in the context of cell migration as in cancer metastasis or during morphogenesis.
Methodological and Technical Expertise
- Mathematical modeling
- Numerical calculations (Mathematica, Matlab, C)
- Cell Culture
- Atomic Force Microscopy
- Confocal Imaging and FRAP