Measuring Cellular Reactions upon Particle Approach by Photonic Force Microscopy

Abstract

The locally changing mobility of a particle in the vicinity of the plasma membrane of a living cell is relevant to medicine and biology, since the viscous drag γ changes significantly with the distance to the interface, thereby triggering mechanosensitive proteins located at the cell surface and influencing the molecular processes of particle binding and endocytosis.In this study we use photonic force microscopy to investigate changes in the diffusive behavior of an optically trapped 1µm polystyrene bead, which approaches the membrane of different biological cells. The bead's temporal fluctuations are tracked interferometrically in 3D with nanometer precision and on a broad spectral bandwidth up to 2 MHz.The autocorrelation of the bead's motion reveals the friction coefficient γ(d) as a function of bead-membrane distance d. The viscous modulus G‘‘(ω,d) and the elastic modulus G’(ω,d) are obtained by analyzing the fluctuation data via the Kramers-Kronig-Relations and give additional access to the frequency-resolved viscous and elastic parameters γ(ω,d) and κ(ω,d), respectively.We investigated the hydrodynamic coupling lengths Λ(d) for different cell types (J774, HT29, MDCK) and a giant unilamellar vesicle (GUV), the latter consisting only of a lipid bilayer and lacking cytoskeleton and molecular motors. All coupling lengths perpendicular to living cell membranes are significantly longer than those of a GUV, leading to strongly enlarged and direction-dependent diffusion times for particle motion close to the membrane. Also, the maximum viscous drag close to cell membranes is significantly larger than at the GUV membrane, giving rise to the conclusion that the composition of the cell interior influences the hydrodynamic interaction. By analyzing the microrheological quantities G(ω,d), we found evidence of a viscoelastic interaction of the bead with the cell coat, which is unique to HT29 cells and additionally regulates the momentum transfer to the cell surface and its mechanosensitive receptors.