(349d) Lipid Chemical Structure Regulates the Interactions of Nanoparticles with Lipid Membranes

The mechanisms by which nanomaterials disrupt the cell membrane, thereby causing cytotoxicity, has been a matter of debate. It has been reported that that electrostatic attraction between oppositely-charged particles and lipids in the membrane is the primary mechanism through which nanoparticles bind to and disrupt lipid membranes. However, our results, using negatively-charged nanoparticles with different surface moieties have challenged this idea and suggest that specific chemical moieties on nanoparticle surface and in the lipid structure regulates nanoparticle-membrane interactions. The present study used unmodified silica, carboxyl-modified silica and unmodified polystyrene nanoparticles (nominal 50 nm) and phospholipids with various headgroups, acyl chain saturation, and backbone to examine how the lipid chemical structure regulates nanoparticle-induced membrane disruption.

Phosphatidylcholines, terminated by phosphate and choline, with unsaturated, 1,2-dioleoyl-sn-glycero-3-phosphocholine (18:1 (9Z) PC, DOPC), or saturated, 1,2-distearoyl-sn-glycero-3-phosphocholine (18:0 PC, DSPC), acyl chains were used to examine the role of chain saturation in nanoparticle-lipid interactions. In addition, 2-((2,3-bis(oleoyloxy)propyl) dimethylammonio) ethyl hydrogen phosphate (18:1 (9Z) inverse PC (iPC), DOCP) was also used. This synthetic lipid contains an inverted headgroup, in which the phosphate moiety is exposed to the surface instead of the trimethylammonium group in normal PCs. Two sphingomyelins (SMs) were also used in this study. N-stearoyl-D-erythro-sphingosylphosphorylcholine (18:0 SM, d18:1/18:0) and N-oleoyl-D-erythro-sphingosylphosphorylcholine (18:1 SM, d18:1/18:1(9Z)) were employed as SM models. These lipids differ in their saturation levels, which allows for studies on the role of acyl chain saturation in SMs, as well as PCs. Vesicle integrity was studied by encapsulating the self-quenching fluorescent probe, carboxyfluorescein, in vesicles and studying its leakage before and after exposure to nanoparticles at 25 °C. Nanoparticles binding were also studied using Forster resonance energy transfer (FRET). Effects of nanoparticle on lipid packing were studied using (1,6-diphenyl-1,3,5-hexatriene) DPH anisotropy.

It was found that the lipid headgroup, backbone, and acyl chain saturation all play an important role in nanoparticle-induced membrane disruption. Nanoparticles bound to and disrupted unsaturated, but not saturated, vesicles. A slight change in the chemical moiety exposed at the vesicle surface significantly affected nanoparticle-induced membrane damage. Carboxyl-modified silica nanoparticles were found to only disrupt vesicles containing a sphingosine backbone, indicating the importance of the backbone structure in the outcome of the interactions. Nanoparticles also changed lipid packing, likely resulting in the formation of pores in the process. Taken together, our observations reveal that the lipid chemical structure and specific interactions between chemical moieties on nanoparticles and in lipids regulate nanoparticle-induced membrane damage.