Spatially and temporally resolved investigation of the mechanics of phagocytosis

• Applicant: Professor Dr. Holger Kreß
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics ; Biophysics
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 313448779

Cells are able to internalize extracellular objects by phagocytosis. This process is evolutionary highly conserved and plays a fundamental role for the mammalian immune system. Bacteria that bind to Fc gamma-receptors in the membrane of macrophages can activate intracellular signaling cascades that lead among others to an increased polymerization of actin filaments and an increased activity of myosin II motors. As a result of these processes, cell protrusions called phagocytic cups grow and wrap around the bacteria which leads to a pulling of the bacteria into the cell and finally to an internalization and in general to an intracellular digestion of the bacteria. A large number of molecules that are involved in phagocytosis are identified by now, however the mechanics of this process is largely unknown. So far, there have been no measurements that investigate the mechanical properties of cells during phagocytosis in a spatially resolved manner. Therefore it is not known, how strong mechanical cell parameters vary in space during phagocytosis. Based on in vitro experiments and theoretical models of cytoskeletal networks, we make the following hypotheses: We expect that during phagocytosis, the stiffness of the cell increases transiently in a spatially limited region around the phagocytic cup and that the cell stiffness decreases with increasing distance to the cup. Furthermore, we expect that actin polymerization and myosin activity are responsible for these spatial and temporal variations and that the actin and myosin concentrations are spatially and temporally correlated with the measured mechanical properties of the cell. We will test these hypotheses by trapping single and multiple functionalized microparticles with holographic optical tweezers and by binding them individually or simultaneously with well-defined distances to the membrane of single macrophages. As artificial bacteria, we will use particles that are coated with immunoglobulin G antibodies. Other coatings will allow the fabrication of inert probe particles that will not be phagocytosed by macrophages. By using blinking optical tweezers we will measure subsequently the mechanical properties of the cells in a spatially and temporally resolved manner. By using biochemical inhibitors, we will modulate the activity of essential molecules such as actin of myosin II to investigate their influence on the measured mechanical cell properties. Furthermore, we will determine the distributions of these molecules in a spatially and temporally resolved manner by means of fluorescence microscopy. By testing the above mentioned hypotheses, characteristic length- and time-scales for the variation of cell mechanical properties during phagocytosis can be measured. These measurements allow the testing of predictions that are based on in vitro experiments and theoretical models in an evolutionary strongly conserved and medically highly relevant cellular process.

• DFG Programme: Research Grants