Direct Correlation of Structures and Nanomechanical Properties of Multicomponent Lipid Bilayers

Abstract

Exploring the fine structures and physicochemical properties of physiologically relevant membranes is crucial to understanding biological membrane functions including membrane mechanical stability. We report a direct correlation of the self-organized structures exhibited in phase-segregated supported lipid bilayers consisting of dioleoylphosphatidylcholine/egg sphingomyelin/cholesterol (DEC) in the absence and presence of ceramide (DEC-Ceramide) with their nanomechanical properties using AFM imaging and high-resolution force mapping. Direct incorporation of ceramide into phase-segregated supported lipid bilayers formed ceramide-enriched domains, where the height topography was found to be imaging setpoint dependent. In contrast, liquid ordered domains in both DEC and DEC-Ceramide presented similar heights regardless of AFM imaging settings. Owing to its capability for simultaneous determination of the topology and interaction forces, AFM-based force mapping was used in our study to directly correlate the structures and mechanical responses of different coexisting phases. The intrinsic breakthrough forces, regarded as fingerprints of bilayer stability, along with elastic moduli, adhesion forces, and indentation of the different phases in the bilayers were systematically determined on the nanometer scale, and the results were presented as two-dimensional visual maps using a self-developed code for force curves batch analysis. The mechanical stability and compactness were increased in both liquid ordered domains and fluid disordered phases of DEC-Ceramide, attributed to the influence of ceramide in the organization of the bilayer, as well as to the displacement of cholesterol as a result of the generation of ceramide-enriched domains. The use of AFM force mapping in studying phase segregation of multicomponent lipid membrane systems is a valuable complement to other biophysical techniques such as imaging and spectroscopy, as it provides unprecedented insight into lipid membrane mechanical properties and functions.

Nanoscale mechanical properties of lipid bilayers and their relevance in biomembrane organization and function

“…As we already pointed out, the force spectroscopy technique appears as a valuable method to distinguish different regions of the lipid bilayer according to their different mechanical properties (Sullan et al, 2009). This is true both in the case of gel and liquidcrystalline domains and in the case of liquid disordered and liquid ordered domains.…”

Structure and Nanomechanics of Model Membranes by Atomic Force Microscopy and Spectroscopy: Insights into the Role of Cholesterol and Sphingolipids

“…These Cer-rich domains have an extremely high mechanical stability [91,92], confirming their tight lipid packing, most probably due to the strong affinity for hydrogen bonding with SM. In general, it has been determined that long-chain Cer incorporation leads to a lipid ordering and the whole mechanical stability of the membrane increases.…”

“…It has been observed that Cer molecules could efficiently displace Chol from Chol:SM rich domains, increasing the presence of Chol in the DOPC-rich phase, reflected also in an increase of the F b [89,91,92,93]. While for SLBs of DOPC:SM:Chol (40:40:20 molar ratio) the mean F b values are around 1.4 nN for the l d and 3.2 nN for the l o phase (Figure 5E), when Cer (20 mol %) is incorporated (Figure 5A–D), these values raise to 4.1 and 5 nN, respectively, while the new Cer-rich domains were not able to be indented for the maximum forces applied in the reported experiments (Figure 5C,F) [91,92]. Still, short-chain Cer have been reported to modify the lipid packing decreasing the mechanical stability of lipid bilayers [6].…”

“…Solid bars correspond to the l o domains, while hollow bars correspond to the l d phase. Adapted with permission from [91,92]. Copyright 2009 American Chemical Society.…”

Lipid-encapsulation of surface enhanced Raman scattering (SERS) nanoparticles and targeting to chronic lymphocytic leukemia (CLL) cells

“…The lipid coating consisted of a ternary mixture of dioleoylphosphatidylcholine (DOPC), egg sphingomyelin (ESM), and cholesterol (chol) in a 2:2:1 molar ratio (DEC221), which has been used in literature as a cell membrane mimic in model studies [21][22][23][24] . These coatings were found to form a bilayer around gold nanoparticles and greatly improve stability in acidic conditions and in solutions of biologically relevant ionic strength.…”