(353f) Interactions of Engineered Sub-Micron Silica Particles with Cell Membrane Models

The greater prevalence of engineered nanoparticles in industrial and biomedical applications has led to increased exposure of mammalian cells to such particles. The plasma membrane, which delineates the extracellular surface, is the first cellular component that meets exogenous particles. A number of fundamental studies have used simplified membrane models to elucidate the interactions of nanomaterials with the cell membrane, but the role of nanoparticle physicochemical properties on these interactions remains obscure. In addition, it is unclear how the physical characteristics of the membrane model might affect the complex interactions of nanomaterials with lipid surfaces. The current study focuses on understanding the interactions of engineered silica nanoparticles with different surface properties, with two common membrane models, lipid monolayers and lipid vesicles, in an effort to elucidate how the functional groups on the surface of nanoparticles affect their interactions with different membrane models.

Lipid monolayers and vesicles comprised of an equimolar mixture of sphingomyelin, cholesterol, and 1,2-dioleoyl-sn-glycero-3-phosphocholine (SM/Chol/DOPC) were used as model membranes mimicking the outer leaflet and both layers of the cell membrane, respectively. Five silica particles with the same diameter (104 ± 5 nm), but coated with different surface-functional groups: hydroxyl, amine, and polyethylene glycol (PEG), with different molecular weights (2k, 5k and 20k) were used as the particle model, and were applied to the membrane models at three different concentrations (0.0001 g/L to 0.01 g/L). Studies with lipid monolayers were performed by spreading the lipids on a purified water subphase in a Langmuir trough and obtaining surface pressure isotherms by recording the surface tension using a Wilhelmy plate balance while compressing the monolayer. Nanoparticle effects on the integrity of lipid vesicles was studied by encapsulating the self-quenching fluorescent probe, 5(6)-carboxyfluorescein (CF), in vesicles and monitoring the release of CF from liposomes (vesicle leakage) upon exposure to nanoparticles. Fluorescence anisotropy, using the lipophilic probe, diphenyl-1,3,5-hexatriene (DPH), was employed to monitor the changes in lipid order in the cell membrane.

Surface pressure isotherms indicated that PEG-coated silica particles, regardless of their molecular weight, increase the packing of lipids, as evidenced by a rise in surface pressure, while amine and hydroxyl-coated particles did not significantly change the surface pressure isotherm. In contrast, amine and hydroxyl-coated particles induced a time-dependent leakage in lipid vesicles – demonstrating a loss of vesicle integrity. Unlike the monolayer studies, the effect of PEGylated particles on lipid vesicles was dependent on PEG molecular weight. While particles coated with PEG 20k induced significant vesicle leakage, particles coated with PEG 2k and 5k did not disrupt the vesicles demonstrating that PEG molecular weight regulates the disruptive effects of engineered particles. In addition, DPH anisotropy experiments revealed that vesicle leakage was not caused by particle entrapment in the lipid bilayer as no significant changes were observed in fluorescence anisotropy after exposure to the particles. In summary, these observations indicate that surface-engineered silica particles show differential effects on lipid monolayers compared to bilayers and their interaction is altered with changes in particle surface-functional groups. Future studies are focused on evaluating the effects of particles on the phase segregation of lipids by imaging giant unilamellar vesicles as the membrane model.