Cellular uptake of nanoparticles: journey inside the cell

Abstract

Nanoscale materials are increasingly found in consumer goods, electronics, and pharmaceuticals. While these particles interact with the body in myriad ways, their beneficial and/or deleterious effects ultimately arise from interactions at the cellular and subcellular level. Nanoparticles (NPs) can modulate cell fate, induce or prevent mutations, initiate cell-cell communication, and modulate cell structure in a manner dictated largely by phenomena at the nano-bio interface. Recent advances in chemical synthesis have yielded new nanoscale materials with precisely defined biochemical features, and emerging analytical techniques have shed light on nuanced and context-dependent nano-bio interactions within cells. In this review, we provide an objective and comprehensive account of our current understanding of the cellular uptake of NPs and the underlying parameters controlling the nano-cellular interactions, along with the available analytical techniques to follow and track these processes.

Fig. 1

Schematic illustration of the opsonization process, initiated by the adsorption of immunoglobulins or other complement proteins (opsonins) to the nanoparticle’s surface. Opsonized particles are subsequently identified through receptors on phagocytic cells and internalized.

Fig. 2

Effect of surface properties on opsonization and subsequent internalization of nanoparticles into the cell. The Fig. compares PEG-coated nanoparticles to uncoated ones. The PEG shell repels complement proteins, minimizing protein adsorption and hence, cellular uptake. Accordingly, the uncoated nanoparticles undergo greater cellular uptake.

Fig. 3

Schematic of clathrin-mediated endocytosis. The process is initiated by ligand recognition and then the formation of clathrin-coated pits. With the aid of clathrin triskelions, a hexagonal lattice is formed, inducing the invagination of the plasma membrane. Afterwards, dynamin (a scission protein) releases the vesicle into the cytoplasm, where decoating takes place.

Fig. 4

Schematic of caveolae-mediated endocytosis. Caveolin proteins play the main role in curvature formation. As in CME, dynamin is the scission protein that allows for budding of the vesicle and release into the cell.

Fig. 5

Schematic illustration for the process of macropinocytosis. Upon recognition, intracellular signaling pathways are activated to promote the formation of large membrane extensions. These ruffles then fuse back to form a large vesicle entrapping the content of the extracellular fluid by engulfment with membrane extensions/processes.

Fig. 6

Schematic illustration for some other non-endocytic entry mechanisms.

Fig. 7

Schematic illustration of the internalization of NPs into the liposome through active process.

Fig. 8

NPs at temperatures below the thermal transition temperature of their outer shell exhibit hydrophilic chain-extended polymers (left-hand side) and enter cells less readily than the same NPs above their polymer thermal transition temperature (right-hand side). Reproduced with permission from Ref.

Fig. 9

Schematic of endocytosis (gray arrows), intracellular trafficking (blue arrows), and cellular exocytosis (red arrows) of NPs. After cellular uptake, NPs are usually delivered to early endosomes, which are the main sorting stations in endocytosis; even vesicles related to non-receptor mediated entry mechanisms fuse with early endosomes. In the early endosome, some NPs are transported along with receptors to recycling endosomes and subsequently excreted; others that remain in early endosomes move slowly along microtubules toward the cell interior and fuse with late endosomes. Finally, late endosomes fuse with lysosomes, which are not necessarily the end of the pathway; some undergo exocytosis and release their undigested content by fusion with plasma membranes. On the pathway to multivesicular bodies (MVB) or even in lysosomes, a portion of NPs may escape from vesicular compartments to the cytoplasm; in addition, some NPs may begin by entering the cytoplasm via unspecific mechanisms. NPs in the cytoplasm or trapped in vesicles can enter the nucleus, mitochondria, endoplasmic reticulum (ER), and Golgi apparatus via unknown mechanisms. In fact, vesicles containing NPs can fuse with ER, Golgi, and other organelles. NPs that enter the ER or Golgi may leave the cell via vesicles related to the conventional secretion system. NPs that are localized in the cytoplasm can leave the cells via re-entering the vesicular system or directly via unspecific mechanisms. The “question marks” in the schematic denote unknown mechanisms.

Fig. 10. Probing Cellular Interactions of Nanoparticles

a) Super-resolution fluorescence microscopy (STORM) of the trafficking of 80-nm polystyrene nanoparticles (red) in HeLa cervical carcinoma cells (plasma membrane, green) compared to conventional wide-field microscopic techniques. b) Transmission electron microscopy (TEM) images of carbon nanotubes (CNTs, left) interacting with lung alveolar cells (right) following intratracheal administration in C57BL/6 mice. c) Atomic force microscopy (AFM) imaging of tattoo ink nanoparticles in cryosectioned human skin. Large (black arrows) and small (white arrows) agglomerates, as well as the underlying collagen fibril network, are visible in AFM height (left) and amplitude (right) images. d) Focused ion beam scanning electron microscopy (FIB-SEM) of HIV-mimetic 80-nm Au NPs infecting cells expressing the glycosphingolipid receptor CD169. Cell surface-bound particles are diffusely spread, while intracellular nanoparticles appear sequestered through as-yet-undetermined mechanisms. e) Real-time, live-cell Raman scattering images of murine macrophages with internalized 50-nm gold nanoparticle probes. Surface-enhance Raman scattering (SERS) from individual nanoparticles is detected (left) and spectral features from nanoparticle region-of-interest (right) are reported. f) Flow cytometry of PPC-1 prostate cancer cells treated with fluorescent, peptide-targeted, and etchable silver nanoparticles. Both cell fluorescence and side-scattering increase with peptide targeting (R-) of silver nanoparticles, while both decrease in response to the etching of cell surface-bound particulates. g) Real-time, dark-field scattering microscopy of respiratory syncytial virus (RSV) trafficking and infection in larynx epidermal cells as imaged using gold nanoparticle surface labels. h) Photoacoustic (PA) microscopy of human leukocytes. Composite images (532 nm, green; 600 nm, red) illustrating chromatin, nuclear, and cytoplasmic morphology. Although not used here, nanoparticles commonly serve as strong PA contrast agents. i) Laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) imaging (heatmap) of mouse fibroblasts (grayscale) incubated with Au (left) and Ag (right) nanoparticles. Reproduced with permission from (a) , (b) , (c) , (d) , (e) , (f) , (g) , (h) , and (i) . Copyright (a) 2016 American Chemical Society, (b) 2014 Købler et al., (c) 2015 Grant et al., (d) 2014 Nature Publishing Group, (e) 2013 Nature Publishing Group, (f) 2014 Nature Publishing Group, (g) 2014 Nature Publishing Group, (h) 2016 Strohm et al., and (i) 2012 American Chemical Society.

Fig. 11

Scheme showing the use of most important techniques in the correlative microscopy. The image is reproduced with permission from American Chemical Society.

Fig. 12. Trends in nanomaterial cellular trafficking research over the past decade

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