Periodic Reporting for period 5 - MECHANICS (Mechanics of cells: the role of intermediate filaments)
Periodo di rendicontazione: 2023-05-01 al 2023-10-31
In this project our objective was to investigate how the remarkable mechanical properties of IFs are encoded in the molecular interactions of the protein monomers and how they are translated into the mechanical behavior of a whole cell. Thus, our research established a structure-mechanics-function relationship for this important component of the cytoskeleton. The genetic complexity of the IF protein family with 70 members that are expressed in a tissue-specific manner required a strategic approach with well-defined model systems and the combination of in vitro and cellular work. Direct mechanical testing by stress application was performed in a quantitative manner by optical tweezers.
Our work covers different length scales, from molecular interactions, which are investigated by numerical simulations, via single filament mechanics, interactions between filaments and within networks, to cellular mechanics.
We compared keratin and vimentin IFs concerning the mechanical properties and could - with the help of Monte Carlo simulations - reveal that differences in intra-filament coupling lead to intriguing differences between the two IF types. When stretched and relaxed in cycles, vimentin filaments become softer, but retains their length (like double-network gels). Keratin filaments, by contrast, become longer, but retains their stiffness (like metals).
We found that cytoskeletal filaments (vimentin-vimentin; vimentin-microtubules) interact directly with each other, due to electrostatic and hydrophobic interactions and that these forces are comparatively strong.
Our results have implications on biomedical research because the mechanical properties of cells play a major role in certain diseases, such as cancer, and in wound healing and embryogenesis. Furthermore the tunabilityof the properties is highly interesting for materials research and may serve a a "blueprint" for designing novel, sustainable matererials as functional as biomaterials.
1. A thorough understanding of the physical principles underlying the processes in a healthy cell is the necessary prerequisite for investigating situations in disease. Long alpha-helices are abundant in mechanically relevant proteins and our findings may be generalized in this respect.
2. Apart from the importance in biomedicine, I also expect this work to open up new opportunities for materials research. Apparently, the special hierarchical architecture of IFs leads to astonishing viscoelastic properties. We are now able to understand how these structural elements work individually and in concert (e.g. the high stretchability), mimicking the mechanical properties and development of equally remarkable materials might be possible.
3. As much as physics can help biology (see aspect 1), the other way around, highly complex biological systems, like IFs, provide a wealth of intriguing soft condensed matter physics problems, which can be studied on accessible time, length and force scales.
During the this project, we have thoroughly characterized the force-strain behavior of IFs, underlined with both analytical and numerical modeling. As a consequence, we suggested an alternative (three-state) model to the previous "alpha-to-beta transitions" within the protein monomers, which can now explain all experimental findings by others and us. We were furthermore able to explain the differences in mechanical behavior between vimentin and keratin IFs based on the amino acid sequences of the two proteins. We were able to measure the interactions between two vimentin IFs as well as between one vimentin IF and one microtubule and could show that the presence of vimentin stabilizes microtubules from rapid depolymerization (catastrophe). We also established methods to investigate cell mechanics, such as traction force microscopy, atomic force microscopy and cell stretching.