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Abstract Liposomal carriers provide a flexible and effective strategy for delivering therapeutics across a broad spectrum of diseases. Cholesterol is frequently included in these systems to improve membrane rigidity and limit permeability. Despite its widespread use, the optimal cholesterol-to-lipid proportion for achieving stable and efficient liposome performance remains to be fully determined. In this work, we apply all-atom molecular dynamics simulations to explore how different cholesterol concentrations influence the structural and dynamic characteristics of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) bilayers, considering both planar and curved membrane geometries. Bilayers with cholesterol molar ratios of 0%, 10%, 20%, 30%, 40%, and 50% were simulated, and key biophysical parameters including area per lipid (APL), membrane thickness, leaflet interdigitation, and deuterium order parameters (SCD) were analyzed. In planar bilayers, increasing cholesterol concentration led to a progressive decrease in APL from approximately 60°A2to 40°A2, accompanied by increased membrane thickness and lipid ordering, consistent with cholesterol’s classical condensing effect. In contrast, curved bilayers exhibited a cholesterol-induced expansion effect, particularly in the inner leaflet, where APL increased from approximately 60°A2to 90°A2with rising cholesterol levels. SCD profiles showed that cholesterol enhanced tail ordering up to 40% concentration, beyond which the effect plateaued or slightly declined, suggesting structural saturation or packing frustration. Membrane thickness displayed a monotonic increase in planar bilayers but followed a nonlinear trend in curved systems due to curvature-induced stress. These findings highlight that cholesterol’s influence on membrane properties is highly dependent on bilayer geometry and asymmetry. While planar bilayers exhibit predictable responses, curved systems reveal nonclassical behaviors that challenge traditional models of cholesterol-lipid interactions. This work provides molecular-level insights and establishes a computational framework for the rational design of liposomal systems, emphasizing the need to account for curvature and asymmetry in membrane engineering.
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Phosphatidic Acid Accumulates at Areas of Curvature in Tubulated Lipid Bilayers and Liposomes
Phosphatidic acid (PA) is a signaling lipid that is produced enzymatically from phosphatidylcholine (PC), lysophosphatidic acid, or diacylglycerol. Compared to PC, PA lacks a choline moiety on the headgroup, making the headgroup smaller than that of PC and PA, and PA has a net negative charge. Unlike the cylindrical geometry of PC, PA, with its small headgroup relative to the two fatty acid tails, is proposed to support negatively curved membranes. Thus, PA is thought to play a role in a variety of biological processes that involve bending membranes, such as the formation of intraluminal vesicles in multivesicular bodies and membrane fusion. Using supported tubulated lipid bilayers (STuBs), the extent to which PA localizes to curved membranes was determined. STuBs were created via liposome deposition with varying concentrations of NaCl (500 mM to 1 M) on glass to form supported bilayers with connected tubules. The location of fluorescently labeled lipids relative to tubules was determined by imaging with total internal reflection or confocal fluorescence microscopy. The accumulation of various forms of PA (with acyl chains of 16:0-6:0, 16:0-12:0, 18:1-12:0) were compared to PC and the headgroup labeled phosphatidylethanolamine (PE), a lipid that has been shown to accumulate at regions of curvature. PA and PE accumulated more at tubules and led to the formation of more tubules than PC. Using large unilamellar liposomes in a dye-quenching assay, the location of the headgroup labeled PE was determined to be mostly on the outer, positively curved leaflet, whereas the tail labeled PA was located more on the inner, negatively curved leaflet. This study demonstrates that PA localizes to regions of negative curvature in liposomes and supports the formation of curved, tubulated membranes. This is one way that PA could be involved with curvature formation during a variety of cell processes.
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- Award ID(s):
- 1807455
- PAR ID:
- 10477969
- Publisher / Repository:
- MDPI
- Date Published:
- Journal Name:
- Biomolecules
- Volume:
- 12
- Issue:
- 11
- ISSN:
- 2218-273X
- Page Range / eLocation ID:
- 1707
- Subject(s) / Keyword(s):
- PA, PLD1
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/biom12111707
