Hydrodynamic Propulsion of Liposomes Electrostatically Attracted to a Lipid Membrane Reveals Size-Dependent Conformational Changes

Abstract

The efficiency of lipid nanoparticle uptake across cellular membranes is strongly dependent on the very first interaction step. Detailed understanding of this step is in part hampered by the large heterogeneity in the physicochemical properties of lipid nanoparticles, such as liposomes, making conventional ensemble-averaging methods too blunt to address details of this complex process. Here, we contribute a means to explore whether individual liposomes become deformed upon binding to fluid cell-membrane mimics. This was accomplished by using hydrodynamic forces to control the propulsion of nanoscale liposomes electrostatically attracted to a supported lipid bilayer. In this way, the size of individual liposomes could be determined by simultaneously measuring both their individual drift velocity and diffusivity, revealing that for a radius of ∼45 nm, a close agreement with dynamic light scattering data was observed, while larger liposomes (radius ∼75 nm) displayed a significant deformation unless composed of a gel-phase lipid. The relevance of being able to extract this type of information is discussed in the context of membrane fusion and cellular uptake.

Effect of Glucose on the Mobility of Membrane-Adhering Liposomes

“…Similar conclusions were previously derived from diffusivity and drift velocity measurements of membrane-adhering liposomes in sheared glucose solutions. 27 Based on geometrical considerations (supporting section S10), the observation of a constant liposome height suggests, that the reduced liposome diffusivity (cf. Fig.…”

“…Measuring both the diffusivity and the drift velocity, allowed computing both the contact area and the height of the liposomes. 27 By adding 500 mM glucose, the contact area was observed to increase, without appreciable changes in the liposome height. 27 This, together with the analysis in supporting section S10, suggests that glucose induces adhesion forces, which enhance the contact area, without changing the liposome height.…”

“…To further confirm the Brownian character of the liposome diffusivity, the inset of It is re-emphasized, that we interpret the liposome diffusivity data, by assuming, that the Brownian motion of the liposome is electrostatically slaved to that of a disk-shaped cluster of lipids in the underlying membrane. [25][26][27] The size of the cluster is referred to as the contact area, which is defined as the membrane region, within one Debye length to the liposome; see Fig. 1b…”

“…12,33 In previous work, we furthermore found that adding glucose (nonionic osmolyte) reduces the diffusivity, and we elucidated the governing mechanism, by applying a shear flow over the membrane. 27 By measuring both the diffusivity as well as the shear-induced drift velocity of the liposomes, we computed the hydrodynamic height of the liposomes, showing negligible change after adding glucose. Assuming that the membrane viscosity is unaltered, this observation would suggest, that glucose extends the intermembrane contact area via adhesion forces, rather than via osmotic forces.…”

“…In Ref. [27] it was hypothesized, that the responsible adhesion force is a "depletion force". Owing to its large size (~1.5 nm), it is conceivable that glucose molecules are depleted from the intermembrane hydration layer (~1 nm), 34 causing an adhesive force, [35][36] as illustrated in Fig.…”