Submicrometer and Nanoscale Inorganic Particles Exploit the Actin Machinery To Be Propelled along Microvilli-like Structures into Alveolar Cells

Abstract

The growing commerce in micro- and nanotechnology is expected to increase human exposure to submicrometer and nanoscale particles, including certain forms of amorphous silica. When inhaled, these particles are likely to reach the alveoli, where alveolar type II epithelial cells that are distinguished by apical microvilli are found. These cells play critical roles in the function of the alveoli and participate in the immune response to amorphous silica and other particles by releasing chemokines. The cellular interactions of the particles, which drive the cellular responses, are still unclear. Adverse effects of nanoparticles have been attributed, in part, to the unique properties of materials at the nanoscale. However, little is known about the cellular interactions of individual or small nanoparticle aggregates, mostly because of their tendency to agglomerate under experimental conditions. Here we investigate the interaction and internalization pathway of individual precipitated amorphous silica particles with specific surface properties and size, by following one particle at a time. We find that both 100 and 500 nm particles can take advantage of the actin turnover machinery within filopodia and microvilli-like structures to advance their way into alveolar type II epithelial cells. This pathway is strictly dependent on the positive surface charge of the particle and on the integrity of the actin filaments, unraveling the coupling of the particle with the intracellular environment across the cell membrane. The retrograde pathway brings a new mechanism by which positive surface charge supports particle recruitment, and potential subsequent toxicity, by polarized epithelial cells bearing microvilli.

The intensity of internalization and cytotoxicity of superparamagnetic iron oxide nanoparticles with different surface modifications in human tumor and diploid lung cells

“…In contrast, a preferential uptake of negatively charged particles into immortalized alveolar epithelial type 1 cells has been found [47]. Surface charge as an important parameter in biological activity of nanoparticles has been supported also by other authors [12,[48][49][50][51]. However, a mechanical hindrance of MNPs uptake cannot be excluded.…”

Shifts in oxidation states of cerium oxide nanoparticles detected inside intact hydrated cells and organelles

“…NPs enter the cellular environment mostly encased within endocytic vesicles [69–72] and in rare cases can also directly cross the cell membrane [73]. In both cases, the NPs enter an environment where the pH is around 7.4.…”

Cellular recognition and trafficking of amorphous silica nanoparticles by macrophage scavenger receptor A

“…The cellular uptake of ENPs is known to require active transport mechanisms (Orr et al 2007), yet there are few reports identifying specific biological molecules as ENP receptors. Using complementary cell models where levels of SR-A expression were selectively modulated combined with high resolution microscopy, we provide strong evidence that SR-A mediates the cellular uptake of anionic silica ENPs particles through a pathway characteristic of clathrin-mediated endocytosis.…”

“…The amorphous nature of the particles expected from solution precipitation methods was confirmed by X-ray and electron diffraction. For microscopy studies, the particles were labeled with either Alexa 488 or Alexa 546 carboxylic acid, succinimidyl ester fluorescent dyes (Molecular Probes/InVitrogen, Carlsbad, CA, USA) as previously described (Orr et al 2007). Briefly, dye labeling was done by reacting (3-aminopropyl) triethoxysilane (APTES) with the succinimidyl ester conjugated dye in dimethylformamide to form a dye-silane peptide linkage.…”

“…The dye amounts added to the suspension were controlled in order to produce relatively low dye coverages, 2.3% and 4.6% by surface area, for the Alexa 546 and Alexa 488 dyes, respectively. Low dye coverage labeling was developed in order to minimize the effect of the dye on the silica surface properties (Orr et al 2007).…”