Fluctuations of cytoskeleton-bound microbeads—the effect of bead–receptor bindingdynamics

Abstract

The cytoskeleton (CSK) of living cells is a crosslinked fiber network, subject to ongoingbiochemical remodeling processes that can be visualized by tracking the spontaneousmotion of CSK-bound microbeads. The bead motion is characterized by anomalousdiffusion with a power-law time evolution of the mean square displacement (MSD),and can be described as a stochastic transport process with apparent diffusivityD and power-lawexponent β: MSD ∼ D (t/t0)β. Here we studiedwhether D and β change with the time that has passed after the initial bead–cell contact, and whether they aresensitive to bead coating (fibronectin, integrin antibodies, poly-L-lysine, albumin) and bead size (0.5–4.5 µm). The measurements are interpreted in the framework of a simple model that describes thebead as an overdamped particle coupled to the fluctuating CSK network by an elasticspring. The viscous damping coefficient characterizes the degree of bead internalization intothe cell, and the spring constant characterizes the strength of the binding of the bead tothe CSK. The model predicts distinctive signatures of the MSD that change withtime as the bead couples more tightly to the CSK and becomes internalized.Experimental data show that the transition from the unbound to the tightly boundstate occurs in an all-or-nothing manner. The time point of this transition showsconsiderable variability between individual cells (2–30 min) and depends on the beadsize and bead coating. On average, this transition occurs later for smaller beadsand beads coated with ligands that trigger the formation of adhesion complexes(fibronectin, integrin antibodies). Once the bead is linked to the CSK, however, theligand type and bead size have little effect on the MSD. On longer timescales ofseveral hours after bead addition, smaller beads are internalized into the cellmore readily, leading to characteristic changes in the MSD that are consistentwith increased viscous damping by the cytoplasm and reduced binding strength.