Cell entry of one-dimensional nanomaterials occurs by tip recognition and rotation

Our plain-English summary of Cell entry of one-dimensional nanomaterials occurs by tip recognition and rotation, by X. Shi, A. von dem Bussche, R.H. Hurt, et al (Nature Nanotechnology, 2011).

The study

• Authors: X. Shi, A. von dem Bussche, R.H. Hurt, A.B. Kane, H. Gao
• Journal: Nature Nanotechnology
• Published: 2011;6(11):714-9
• Type of article: Letter
• Cited by: 296 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Materials with high aspect ratio, such as carbon nanotubes and asbestos fibres, have been shown to cause length-dependent toxicity in certain cells because these long materials prevent complete ingestion and this frustrates the cell. Biophysical models have been proposed to explain how spheres and elliptical nanostructures enter cells, but one-dimensional nanomaterials have not been examined. Here, we show experimentally and theoretically that cylindrical one-dimensional nanomaterials such as carbon nanotubes enter cells through the tip first. For nanotubes with end caps or carbon shells at their tips, uptake involves tip recognition through receptor binding, rotation that is driven by asymmetric elastic strain at the tube-bilayer interface, and near-vertical entry. The precise angle of entry is governed by the relative timescales for tube rotation and receptor diffusion. Nanotubes without caps or shells on their tips show a different mode of membrane interaction, posing an interesting question as to whether modifying the tips of tubes may help avoid frustrated uptake by cells.

In plain English

This laboratory and computer-modelling study looked at how long, thin ("one-dimensional") materials enter cells. The authors exposed two cell lines, mouse AML12 liver cells and human LP9 mesothelial cells (a peritoneal cell line), to multi-walled carbon nanotubes, gold nanowires and crocidolite asbestos fibres. They then imaged the cells with scanning electron microscopy. Nanotubes caught entering cells mostly stuck out at a steep angle, entering tip first. Tip entry was also the main mode seen for the gold nanowires and crocidolite fibres. In the liver cells, nanotube uptake fell significantly at 4 degrees C and in the presence of metabolic inhibitors. The authors took this as evidence of an energy-dependent process (endocytosis). Computer simulations and a mathematical model suggested that a nanotube with a capped tip is first bound at the tip and then rotates toward a near-vertical angle. In the simulations, open-ended tubes did not enter tip first. The authors concluded that tip entry offers a physical explanation for incomplete uptake ("frustrated phagocytosis") of long fibres. They stated that the simulation timescales can only be used qualitatively.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) cell experiments with electron and confocal microscopy, combined with coarse-grained molecular dynamics computer simulations and an analytical mathematical model
• Who was studied: Two immortalized, non-tumorigenic cell lines: mouse AML12 liver cells and human LP9 mesothelial cells (described as an hTERT-immortalized normal human peritoneal cell line); no human subjects
• Exposure: Three high-aspect-ratio materials: multi-walled carbon nanotubes made by chemical vapour deposition, crocidolite asbestos fibres (Duke Scientific; 30 to 3,000 nm in diameter), and amine-terminated gold nanowires (30 nm in diameter). The cells were exposed for 24 hours at 20 ppm for imaging. The energy-dependence tests used 80 ppm carbon nanotubes for 2 hours. The computer simulations modelled capped nanotubes entering a model lipid bilayer.
• Main results:
  • In electron micrographs, carbon nanotubes undergoing active entry into liver and mesothelial cells were mainly found protruding at a high angle. The membrane was pulled inward at the point of entry, which the authors describe as characteristic of endocytosis.
  • Tip entry was the dominant uptake mode observed for the nanotubes and also for the other materials tested: gold nanowires and crocidolite asbestos fibres.
  • In mouse liver cells, nanotube uptake was significantly reduced at 4 degrees C compared with 37 degrees C (P less than 0.0005). It was also reduced with a metabolic inhibitor mixture (P less than 0.005).
  • Nanotubes lying flat on the cell membrane showed no visible signs of active uptake.
  • In computer simulations, a capped nanotube placed at a 45-degree angle was pulled in by membrane receptors and rotated toward an entry angle close to 90 degrees. At a higher receptor density (0.33), it was fully wrapped before reaching 90 degrees. In the extreme non-specific case (density 1), it adopted a very small entry angle.
  • In simulations, open-ended nanotubes did not undergo tip entry, which the authors attribute to the lack of receptor-binding sites on a cap.
  • In the analytical model, tip rotation took about 4 seconds, compared with about 20 to 200 seconds for full uptake. The authors describe this as one or two orders of magnitude faster.
• The authors' conclusion: The authors concluded that tip entry gives a physical and mechanical explanation for incomplete endocytosis or frustrated phagocytosis. In their view, a fibre that enters vertically gives the cell no opportunity to sense its full length. They suggested that modifying fibre tips might in future help avoid frustrated uptake.
• Limitations the authors state:
  • The simulations could not reach the real cellular wrapping timescale, so their timescales can only be used qualitatively.
  • The simulation model does not correctly capture the viscosity of the cell cytosol, which can strongly affect how fast a nanotube rotates.
  • The analysis was limited to positive initial angles of approach and does not cover a nanotube landing completely flat.
  • A fully three-dimensional expression of the membrane's elastic energy was beyond the scope of the work. Instead, an empirical fit to the simulations was used.

Topics

Medical terms in this study, explained in our Lexicon: Aspect Ratio (Length-to-diameter ratio).

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