Mechanical and Functional Study of Cell Physiology at the Nanoscale

By live-cell imaging of biological samples dynamic cellular processes can be resolved. Fluorescence microscopy (FM) and atomic force microscopy (AFM) are both capable of imaging live cells. By combining these techniques structural as well as functional information of the same biological samples are measured. While the dynamics of specific proteins can be imaged by FM, the AFM provides the structural context. To study how intracellular processes affect the mechanics, structure and kinetics of cells at the nanoscale we combined a high-speed AFM (HS-AFM) and an inverted optical microscope (OM) into one instrument. With this system cell physiological processes at different time scales are imaged. Mammalian cell migration happening at minute time scales is studied by correlated super-resolution microscopy and HS-AFM. Bacterial cell separation happening within 10s of milliseconds is characterized as well as bacterial growth over multiple generations. Additionally to imaging, the capability of the AFM to quantitatively measure mechanical properties of tissues is deployed in combination with FM on mouse corneal tissues. Different to most other rod-shaped bacteria where division happens through constriction of the cell wall at midcell, mycobacteria keep the shape of their outer cell envelope unchanged during septation. The septal wall is formed by peptidoglycan branching off from the outer cell envelop eventually splitting the cell into two sister compartments. Nanomechanical measurements show that from the initiation of septation the stiffness at the septum increases significantly. High-temporal resolution imaging of the following bacterial separation process indicated that the sibling cells separate within tens of milliseconds. The connection of the outer cell envelop at the former septum is broken apart suggesting a mechanical break. The timing of the cell separation is regulated by the tensile stress at the septum and the ultimate tensile strength of the cell wall material. The positions where these separations occur are closely associated with morphological features on the undulating cell surface. These features are formed at the poles and migrate towards midcell as the cell grows. Interestingly, sibling cell separation is restricted to the center-most wave-trough making these features the first characteristic associated with division site placement. Experiments with mutant strains show that even asymmetric divisions occur within wave-troughs emphasizing their role in division site placement. Chronic inflammation induces an increase in the stiffness of the corneal tissue and is associated with dramatic changes in the composition of the tissue. Stem cells migrating along this tissue layer of increased stiffness experience increased activation of mechanotransduction pathways. This eventually leads to induction of a skin-like rather than corneal-like character of these regenerating stem cells. It demonstrates that the stiffness of the tissue and thus pathways involved in mechanotransduction decisively influence the stem cell fate.