Universal behavior of membranes with sterols

Abstract

Lanosterol is the biosynthetic precursor of cholesterol and ergosterol, sterols that predominate in the membranes of mammals and lower eukaryotes, respectively. These three sterols are structurally quite similar, yet their relative effects on membranes have been shown to differ. Here we study the effects of cholesterol, lanosterol, and ergosterol on 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine lipid bilayers at room temperature. Micropipette aspiration is used to determine membrane material properties (area compressibility modulus), and information about lipid chain order (first moments) is obtained from deuterium nuclear magnetic resonance. We compare these results, along with data for membrane-bending rigidity, to explore the relationship between membrane hydrophobic thickness and elastic properties. Together, such diverse approaches demonstrate that membrane properties are affected to different degrees by these structurally distinct sterols, yet nonetheless exhibit universal behavior.

FIGURE 1

Structures of cholesterol, lanosterol, and ergosterol. In the biosynthetic pathway, the methyl groups on lanosterol's α-face are shed, giving rise to cholesterol. Ergosterol differs structurally from cholesterol in that it has two additional double bonds as well as a methyl group on the side chain.

FIGURE 2

Images showing an aspirated vesicle (diameter 40 μm) in the high-pressure regime: (a) the raw image and (b) the analyzed form. From the analysis (described in (37)), the radius of the outer sphere, R₂, the length of the cylinder, L, and the pipette radius R₁ are determined.

FIGURE 3

Plot of the data obtained from micropipette aspiration of a POPC lipid vesicle in the high-tension regime at 25°C. The data is presented in the form of the frame tension, τ (Eq. 3), as a function of the optically resolvable area, A_p (Eq. 2). The fitting range is limited to τ ≥ 2 mN/m, which yields the apparent area expansion modulus . The error bars are estimated by (δτ/τ) ∼ (δR₁/R₁) ∼ 1/25 and (δA_p/A_p) ∼(δR₂/R₂) ∼ 1.5 × 10⁻³. The zero frame tension area, A_p,0, is the intercept of the fitted linear curve and the x axis.

FIGURE 4

Plot of (a) the apparent area expansion modulus, and (b) the first moment of the ²H-NMR spectrum, M₁, as a function of sterol content. is determined by micropipette aspiration and M₁ by ²H-NMR. The extent to which these sterols increase and M₁ follows the sequence cholesterol > lanosterol > ergosterol for all measured sterol concentrations. Numerical values and error are reported in Table 1.

FIGURE 5

Spectra obtained by ²H-NMR for POPC-d₃₁ membranes containing (a) 10 mol%; (b) 20 mol%; and (c) 30 mol% lanosterol at 25°C. Acquisition parameters are documented in the text.

FIGURE 6

Plot of (a) the area expansion modulus, , as a function of the bending rigidity, κ, and (b) the ratio as a function of M₁.

FIGURE 7

Plot of (a) the area expansion modulus, , and (b) the bending rigidity, κ, as a function of the acyl chain order measured by M₁. Both mechanical moduli exhibit a unique functional dependence on M₁ independently of sterol structure and concentration. Numerical values and error are reported in Table 1.