A hybrid total internal reflection fluorescence and optical tweezers microscope to study cell adhesion and membrane protein dynamics of single living cells

The dynamics of cell surface membrane proteins plays an important role in cell-cell interactions. The onset of the interaction is typically not precisely controlled by current techniques, making especially difficult the visualization of early-stage dynamics. We have developed a novel method where optical tweezers are used to trap cells and precisely control in space and time the initiation of interactions between a cell and a functionalized surface. This approach is combined with total internal reflection fluorescence microscopy to monitor dynamics of membrane bound proteins. We demonstrate an accuracy of approximately 2 s in determining the onset of the interaction. Furthermore, we developed a data analysis method to determine the dynamics of cell adhesion and the organization of membrane molecules at the contact area. We demonstrate and validate this approach by studying the dynamics of the green fluorescent protein tagged membrane protein activated leukocyte cell adhesion molecule expressed in K562 cells upon interaction with its ligand CD6 immobilized on a coated substrate. The measured cell spreading is in excellent agreement with existing theoretical models. Active redistribution of activated leukocyte cell adhesion molecule is observed from a clustered to a more homogenous distribution upon contact initiation. This redistribution follows exponential decay behaviour with a characteristic time of 35 s.