Membrane mechanical properties of synthetic asymmetric phospholipid vesicles

Abstract

Synthetic lipid vesicles have served as important model systems to study cellular membrane biology. Research has shown that the mechanical properties of bilayer membranes significantly affects their biological behavior. The properties of a lipid bilayer are governed by lipid acyl chain length, headgroup type, and the presence of membrane proteins. However, few studies have explored how membrane architecture, in particular trans-bilayer lipid asymmetry, influences membrane mechanical properties. In this study, we investigated the effects of lipid bilayer architecture (i.e. asymmetry) on the mechanical properties of biological membranes. This was achieved using a customized micropipette aspiration system and a novel microfluidic technique previously developed by our team for building asymmetric phospholipid vesicles with tailored bilayer architecture. We found that the bending modulus and area expansion modulus of the synthetic asymmetric bilayers were up to 50% larger than the values acquired for symmetric bilayers. This was caused by the dissimilar lipid distribution in each leaflet of the bilayer for the asymmetric membrane. To the best of our knowledge, this is the first report on the impact of trans-bilayer asymmetry on the area expansion modulus of synthetic bilayer membranes. Since the mechanical properties of bilayer membranes play an important role in numerous cellular processes, these results have significant implications for membrane biology studies.

Figure 1

Schematics of five types of giant unilamellar vesicles (GUVs) with different lipid compositions, including three symmetric GUVs made of DMPC, DOPC, and DMPC/DOPC (1:1 mixture), respectively, and two asymmetric GUVs made of DMPC (or DOPC) on the inner leaflet and DOPC (or DMPC) on the outer leaflet. We have created synthetic GUVs with trans-bilayer asymmetries as high as 95%.

Figure 2

Schematic representation of a customized hydrostatic pressure system: (a, b) water reservoir, (c, d) linear translation stage, (e, f, g) two-way stop valve, (h) low differential pressure transducer, (i) micropipette connection fitting, and (j) PDMS sample holder.

Figure 3

Images of an asymmetric GUV (Inner leaflet: DOPC – Outer leaflet: DMPC) aspirated into a micropipette with increasing suction pressure (i.e. membrane tension) from image (a) to (d). The aspiration length at each suction pressure was recorded and compared to that in image (a), which has the lowest suction pressure, to calculate the change in aspiration length (ΔL). The measurement data of this GUV is shown in Figure 4.

Figure 4

(a) Schematic of tension-strain measurement on a GUV including the bending regime and area expansion regime. (b) Tension-strain measurement for an asymmetric GUV (Inner leaflet: DOPC – Outer leaflet: DMPC) made at T = 22.5°C. The blue squares denote the natural log of the tension (τ) against apparent area strain (α_app). A linear fit is made within the low-tension regime (<0.5 mN/m). The red triangles denote τ against α_app, which is nearly linear in the high-tension regime (>0.5 mN/m). Subtracting out the contribution of subvisible thermal undulations from α_app in the high-tension regime gives the direct area strain (α_dir). Thus, the re-plotted points (black squares) shift the curve to the left. The bending (κ), apparent area expansion (K_app), and direct area expansion (K_dir) moduli of this asymmetric GUV were measured to be 17.5 × 10⁻²⁰ J, 217 mN/m, and 245 mN/m, respectively.