Two-Point Microrheology of Phase-Separated Domains in Lipid Bilayers

Abstract

Though the importance of membrane fluidity for cellular function has been well established for decades, methods for measuring lipid bilayer viscosity remain challenging to devise and implement. Recently, approaches based on characterizing the Brownian dynamics of individual tracers such as colloidal particles or lipid domains have provided insights into bilayer viscosity. For fluids in general, however, methods based on single-particle trajectories provide a limited view of hydrodynamic response. The technique of two-point microrheology, in which correlations between the Brownian dynamics of pairs of tracers report on the properties of the intervening medium, characterizes viscosity at length-scales that are larger than that of individual tracers and has less sensitivity to tracer-induced distortions, but has never been applied to lipid membranes. We present, to our knowledge, the first two-point microrheological study of lipid bilayers, examining the correlated motion of domains in phase-separated lipid vesicles and comparing one- and two-point results. We measure two-point correlation functions in excellent agreement with the forms predicted by two-dimensional hydrodynamic models, analysis of which reveals a viscosity intermediate between those of the two lipid phases, indicative of global fluid properties rather than the viscosity of the local neighborhood of the tracer.

Hydrodynamic shear dissipation and transmission in lipid bilayers

“…The viscosity measured for pure DPPC membranes further agrees with measurements on DPPC liposomes obtained from the fluorescence lifetime imaging of membrane-bound molecular rotors, although our value is slightly higher ( 26 , 59 ). Other studies of different mixtures have reported viscosities ranging from Pa s m ( 53 , 54 , 56 – 58 , 60 ), in agreement with the viscosity we report for DOPC/DPPC mixtures. Finally, for pure DOPC bilayers, the range we deduce for the viscosity Pa s m is consistent with the lower viscosities previously reported for DOPC membranes Pa s m ( 14 , 55 ).…”

“…Finally, for pure DOPC bilayers, the range we deduce for the viscosity Pa s m is consistent with the lower viscosities previously reported for DOPC membranes Pa s m ( 14 , 55 ). It is noteworthy that for the mixture, we measure an intermediate viscosity between the values measured for the two homogeneous membranes, as has been found with other two-point microrheology approaches ( 60 ).…”

Imaging non-classical mechanical responses of lipid membranes using molecular rotors

“…Common ways to quantitatively evaluate membrane fluidity and microviscosity, such as fluorescence correlation spectroscopy, 41 fluorescence recovery after photobleaching 42 or single particle tracking, 43 are not easily compatible with mapping large sample areas. Furthermore, their time resolution is limited, which prevents them from monitoring dynamic processes.…”

Lipid bilayer hydrodynamic drag

“…Interfaces between lipid bilayers and aqueous solutions are present in countless environments both natural, such as at cell and organelle membranes, and artificial, such as in suspensions of liposome-encapsulated drugs. The hydrodynamics of bilayers and bilayer-bound objects are therefore of considerable interest [1][2][3][4][5][6][7][8]. In particular, the rheology of red blood cells in vivo [9] and suspensions of cells and liposomes in vitro [10,11] depends directly on the nature of the flow boundary condition of the bilayer / water interface.…”

“…It has been widely assumed throughout work spanning many decades that this interface is described by the no-slip condition characteristic of solidliquid interfaces [3,[12][13][14][15][16]. However, lipid bilayers are Newtonian fluids [5,7,17], and it has been speculated that their aqueous interfaces may therefore behave more like no-shear-stress boundaries, or intermediate between solid-like and liquid-like extremes [18]. Remarkably, despite its fundamental importance and widespread applicability, we are aware of almost no measurements of the flow boundary conditions at lipid bilayer surfaces.…”