Elasticity and mechanical metamaterials
Does bending a membrane sort its cholesterol?
Open access · cc by · source: Europe PMC
Stretching cell-sized lipid bubbles with laser tweezers loosened lipid packing at the highly curved ends and moved cholesterol toward the flatter middle, measured without any fluorescent labels.
Study at a glance
- Design
- Other — Giant lipid vesicles trapped and stretched with holographic optical tweezers while Raman spectra are taken simultaneously at centre and edges; deuterated cholesterol tracks cholesterol, and data are fitted to a curvature-sorting model.
- N
- N=17 · 17 cholesterol-labelled vesicles in the pooled model fit; chain-packing trends came from one vesicle stretched cyclically plus seven more vesicles, and two vesicles were used for single-vesicle cyclic fits.
- Population
- Giant unilamellar vesicles of cholesterol, POPC and sphingomyelin (1:1:1)
- Outcome
- Raman band ratios for lipid chain packing and C-D band intensity (cholesterol content) at centre versus edges
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Key findings
Relaxed vesicles showed identical spectra at centre and edges, but stretching lowered the packing-sensitive band ratios at the edges, for example from 1.03 to 0.79 for one ratio as elongation rose, meaning lipid chains became less ordered where curvature was high. Cholesterol signal also dropped at the edges, with the model predicting and the data consistent with roughly 1.3-fold more cholesterol at the centre at maximum stretch. Fitting across 17 vesicles gave an asymmetry parameter of about 0.63, implying the outer leaflet holds more cholesterol than the inner one.
Methodology
The researchers made giant vesicles from cholesterol, an unsaturated phospholipid and sphingomyelin, trapped each one with three laser traps, and pulled them into ellipsoids up to about 1.3 times their original length. The same laser beams excited Raman scattering, so spectra were collected at the same time from the vesicle centre (low curvature) and edges (high curvature). Chain packing was read from ratios of C-H stretching bands, and cholesterol was tracked by replacing it with deuterium-labelled cholesterol, whose C-D bands sit in an otherwise empty part of the spectrum. The cholesterol data were fitted to a spontaneous-curvature model with two leaflets.
Limitations
Raman spectra measure total cholesterol across both leaflets, so the leaflet asymmetry comes from a model that ignores cholesterol flip-flop and assumes a uniform asymmetry parameter. Vesicles could only be stretched to about 1.3 before escaping the traps, trap positions were not always symmetric, and spectra were taken at only three spots, so the predicted detailed profile could not be checked. The vesicles are simple three-component models, not real cell membranes, and some outlier points were left unexplained.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
Stiffer components migrate away from strongly bent regions.
Curvature redistributes lipids: stretching giant vesicles with optical tweezers loosened chain packing at the highly curved ends and left roughly 1.3-fold more cholesterol at the flatter centre at maximum stretch, consistent with stiff, cholesterol-rich lipid avoiding high curvature.
Evidence for the claim as stated.
Stiffer components migrate away from strongly bent regions.
Curvature redistributes lipids: stretching giant vesicles with optical tweezers loosened chain packing at the highly curved ends and left roughly 1.3-fold more cholesterol at the flatter centre at maximum stretch, consistent with stiff, cholesterol-rich lipid avoiding high curvature.
Scope note — 17 vesicles, elongation limited to about 1.3; leaflet asymmetry is model-derived.
Limits the claim's scope: a different population, assay, or outcome.
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