Bending and puncturing the influenza lipid envelope

Abstract

Lysosomes, enveloped viruses, as well as synaptic and secretory vesicles are all examples of natural nanocontainers (diameter ≈ 100 nm) which specifically rely on their lipid bilayer to protect and exchange their contents with the cell. We have applied methods primarily based on atomic force microscopy and finite element modeling that allow precise investigation of the mechanical properties of the influenza virus lipid envelope. The mechanical properties of small, spherical vesicles made from PR8 influenza lipids were probed by an atomic force microscopy tip applying forces up to 0.2 nN, which led to an elastic deformation up to 20%, on average. The liposome deformation was modeled using finite element methods to extract the lipid bilayer elastic properties. We found that influenza liposomes were softer than what would be expected for a gel phase bilayer and highly deformable: Consistent with previous suggestion that influenza lipids do not undergo a major phase transition, we observe that the stiffness of influenza liposomes increases gradually and weakly (within one order of magnitude) with temperature. Surprisingly, influenza liposomes were, in most cases, able to withstand wall-to-wall deformation, and forces >1 nN were generally required to puncture the influenza envelope, which is similar to viral protein shells. Hence, the choice of a highly flexible lipid envelope may provide as efficient a protection for a viral genome as a stiff protein shell.

Figure 1

AFM imaging and stiffness measurements on small unilamellar vesicles. (A) A tapping mode height image of a liposome in buffer (320 × 320 nm scan size, 256 × 256 pixels). (Inset below) Cross-section height profile of the liposome. (B) A reconstructed height image from a force map of the same liposome (320 × 320 nm scan size, 24 × 24 pixels). Each pixel contains one FZ curve from which the height and stiffness at that point can be measured. (Inset below) Cross-sectional height profile from the force map shows the same height as in tapping mode.

Figure 2

Dependency of the influenza liposome stiffness on the probed region. (A) The liposome was divided in concentric areas for which the average stiffness was measured (e.g., the pixels enclosed by the circles of 30-and 40-nm radius). The plot shows the normalized stiffness for 15 liposomes, the stiffness decreases when the liposome is probed further away from its center. (B) Averaged force versus indentation curve of a 71-nm-high influenza liposome, obtained by averaging four curves obtained within 20 nm from the center. The stiffness was obtained by performing a linear fit between 0.1 and 0.2 nN. (Shaded dots) Response calculated with FEM (d = 70 nm, E = 30 MPa). (Inset) Deformation of the thin-shell model by a hyperbolic tip and a flat surface.

Figure 3

Stiffness of influenza and DMPC liposomes. (A) Stiffness versus diameter plot of influenza liposomes. The stiffness of liposomes increased with decreasing liposome diameter. (B) 1/stiffness versus diameter plot of influenza liposomes compared with DMPC and DMPC/cholesterol 1:1 (mol/mol) liposomes. (Green crosses) Reciprocal of the data presented in Fig. 3A. (Black crosses) DMPC liposomes. (Red crosses) DMPC/cholesterol (1:1, mol/mol) liposomes. Each scatter plot was overlaid with a finite element model of the liposome indentation by AFM. After fitting the stiffness distributions with Eq. 5, the average stiffness for a 100-nm-diameter vesicle was calculated. From the ratio of the average stiffness values, we found that the DMPC/cholesterol liposomes were on average 100% stiffer, and influenza liposomes 110% stiffer than DMPC liposomes.

Figure 4

Temperature influence on the stiffness of influenza liposomes. Influenza liposomes at 13 ± 2°C (blue dots), 26 ± 2°C (green dots), and 37 ± 1°C (orange dots). After fitting the stiffness distributions for each case with Eq. 5 (solid line), the average stiffness for a 100-nm-diameter vesicle was calculated. Influenza liposomes at 13°C and 26°C were on average, respectively, 40% and 10% stiffer, than at 37°C.

Figure 5

Influenza liposome puncture at high forces. (A) Jumps in the indentation curves occur at high forces: Those are indicated by the black arrows (and identified by a change of sign of the slope). (Inset) Histogram of the distance to the surface for each puncture event (172 events on 19 particles). Most of the events occur at high indentation, e.g., when the tip is only separated from surface by the two apposed bilayers of the liposome. (B) The average puncturing force is 0.88 nN (172 events, 19 particles). More than 95% puncture events occurred above 0.4 nN. (C) Pushing at high forces led to only minor morphology and height changes. Images and height profile show a liposome before (left) and after (right) four pushes at 2 nN force. (D) Multiple successive pushes at forces between 0.4 and 2 nN did not lead to a change in height (shown is the average of 12 experiments). On the abscissa, the average number of pushes between each height measurement is indicated. The puncture experiments were performed at 28 ± 2°C.