Kinetics of cell attachment and spreading on hard and soft substrates
A very important aspect for the functioning of an organism is that cells adapt their behaviour to external stimuli. They continuously interact with their environment, and biochemical and physical cues can activate cellular signalling, which leads to changes in cell behaviour such as proliferation and shape. Understanding cells' interactions with their environment is also important for understanding diseases. For example mechanosensing, which is the sensing of the cell's mechanical environment, has been associated with cancer development. In order for a cell to be able to sense its mechanical environment, it needs to form attachments to the environment. In my thesis, I have worked on three different tasks: the development of a new measurement technique and the study of initial cell adhesion and of cell spreading. When a cell from suspension first comes into contact with a substrate, it forms initial attachment bonds with proteins on the substrate surface. These bonds are mediated through integrins, which are transmembrane heterodimers, binding to the cell's environment on one side and to the cell's cytoskeleton on the other side. I study this initial cell attachment by measuring the force needed to detach cells, called cell adhesion strength. For these experiments I built a detachment device, which allows the detachment of cells from a substrate by vibrating the substrate in liquid. The device combines cell incubation, detachment and imaging. I measured the dependence of initial integrin bond formation on external factors such as incubation temperature and substrate stiffness. Once initial integrin bonds are formed, many different proteins are recruited to the adhesion site in order to form stronger adhesions. Amongst these proteins are signalling proteins, which direct the behaviour of the cell as a whole. One of the first cellular reactions to a substrate after initial integrin binding is cell spreading. This can be seen by the cell changing its shape from spherical to dome-like on the substrate. Because cell spreading is a very early response of a cell to a substrate, the onset time of spreading can be used as a quantitative measure for the time it takes the cell to sense a substrate and signal shape change. In my work, I look at the distribution of the time of initial cell spreading in a population of cells. I measure this distribution under different growth conditions such as pH, change of incubation medium from DMEM to PBS, substrate stiffness and incubation temperature. In my detachment experiments, I observe that vibration accelerates cell spreading in those cells which remain on the substrate. This is a connection between the detachment experiments and the cell spreading experiments and it shows how cells react to external forces. By changing the medium temperature in the cell detachment and cell spreading experiments, I am able to analyse the kinetics of these two processes. I use a signalling network model to analyse the internal cellular signalling path that leads from a spherical to a spread cell.
Cell adhesion, Cell spreading, Adhesion strength assay
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