Caenorhabditis elegans Nematode: A Versatile Model to Evaluate the Toxicity of Nanomaterials In Vivo

Abstract

Using in vivo models in assessing the toxicity of nanomaterials is a promising way to investigate the toxicity of nanomaterials on live organisms. Free-living nematodes Caenorhabditis elegans have been extensively used in toxicity assays because of their fully sequenced genome (closely homologous to human genome), short life span (ca. 3 weeks), and tiny transparent ~1-mm-long body built up from a fixed number of cells. In this chapter we overview our recent studies on nanotoxicity employing C. elegans nematodes. We have pioneered surface modification of C. elegans with polyelectrolyte multilayer shells (pure or doped with 20 nm Au nanoparticles) and the direct magnetic functionalization of nematodes with iron oxide nanoparticles. Magnetically functionalized nematodes were effectively separated and spatially moved using magnetic field. The cuticle-modified nematodes preserve their viability and reproduction. Next, we introduced a nanomaterial delivery method into C. elegans intestines based on using nanoparticle-coated bacteria as “nanobaits” ingested by nematodes as a sole food source. Nematodes feed on the nanoparticle-coated bacteria (Escherichia coli) and microalgae (Chlorella pyrenoidosa), resulting in ingestion of nanoparticles, which were detected exclusively inside the intestine. Using iron oxide nanoparticles to produce nanobaits, we were able to magnetically label live nematodes, rendering them magnetically responsive. Using this method, we analyzed the nanosafety of halloysite, a clay tubular nanomaterial having 50 nm diameter and 1.5 μm length. Halloysite nanotubes were found to be safe for C. elegans at relatively high concentrations (1 mg/mL) which is of about 1000× higher than any likely soil contamination concentration. We have used dark-field microscopy and physiological tests to confirm that halloysite is localized in the alimentary system only, without inducing severe toxic effects. We also introduced PeakForce Tapping non-resonance atomic force microscopy (AFM) to image and produce nanomechanical maps of C. elegans and Turbatrix aceti worms. The animals were imaged at nanoscale in gas and liquid environment. We have obtained high-resolution AFM images and nanomechanical maps of various cuticle features demonstrating the differences in topography and structure between animals of different age and different species. Nanomechanical mapping of surface deformation, modulus, and non-specific adhesion has confirmed the nonuniform mechanical properties of the nematode cuticle. Consequently, we suggest that AFM in PeakForce Tapping mode can be effective to investigate the surface coatings of relatively large live immobilized multicellular organisms.