Cellular Binding of Charged Nanoparticle-Protein Complexes

Abstract

Nanoparticles used for biomedical applications are exposed to a complex mixture of extracellular serum proteins that non-specifically adsorb to the nanoparticle surface. The resulting “protein corona” dominates interactions of the nanoparticle with the cellular environment. As nanoparticle binding to the cell surface is the first step in the course of most biomedical applications, we have focused on the role of the initial nanoparticle surface charge in the cellular binding of nanoparticles. Cationic, amine-modified and anionic, carboxylate-modified polystyrene nanoparticles were studied as a model system. Although both cationic and anionic nanoparticles form a protein corona, cellular binding follows opposite trends as determined from fluorescence microscopy experiments. The cellular binding of cationic nanoparticles is enhanced in the presence of serum proteins, while anionic nanoparticle binding is inhibited by the presence of serum proteins. Competition assays performed with flow cytometry enabled us to identify the cellular receptors used by the nanoparticle-protein complexes. We have determined that complexes formed with anionic nanoparticles bind to native protein receptors, while those formed with cationic nanoparticles bind to scavenger receptors. Similar trends were observed for anionic nanoparticles with biomedical applications including quantum dots, Au nanospheres, and low-density lipoprotein. To probe the underlying cause for the charge-dependent differences in cellular binding, we are currently characterizing the structure of the adsorbed proteins using circular dichroism, fluorescence spectroscopy, and isothermal titration calorimetry. Preliminary results suggest that protein structure is lost upon binding to cationic nanoparticles, and protein structure is retained on anionic nanoparticle surfaces. Our results indicate that, in the presence of blood serum proteins, initial nanoparticle surface charge mediates nanoparticle-cell interactions.