Mechanical Responses of Membranes probed with Force Spectroscopy

Abstract

Mechanical properties of membranes such as surface tension and elastic constants play an important role in cellular processes such as vesicular transport and cell migration. Conventionally, these parameters have been calculated by deforming the membrane into cylindrical nanotubes (‘membrane tethers’) using shear flow and micropipette aspiration. In recent times, more sensitive approaches involving optical traps and force spectroscopy have been used to measure forces involved in and determine energetic parameters of the tether pulling process. Here, we report our observations from constant speed tether-pulling experiments using force spectroscopy on mammalian cells in culture. Tethers pulled at a constant speed appear to relax in a discrete, step-wise manner. Strikingly, such experiments carried out with model membrane systems also display similar force profiles, suggesting that such a mechanical response may be an inherent property of the lipid bilayer. Importantly, we observe a pulling rate dependence of the characteristic parameters of force profiles obtained on both cell and model membranes. We interpret these results to reflect a transition of the lipid bilayer from a viscous to a viscoelastic regime, caused by an increase in pulling rate. Further, we focus on the rate dependence of a particular parameter of the force curve called the ‘tether force’ that provides a measure of the resistance offered by the membrane to deformation. Subtle differences in this rate dependence seen in cell and model membranes are currently under investigation. Our results suggest that physical parameters defining membrane behaviour need to be interpreted in the context of the timescale of membrane deformation.