Materials and toxicological approaches to study metal and metal-oxide nanoparticles in the model organism Caenorhabditis elegans

Caenorhabditis elegans as a Model for Evaluating the Toxicology of Inorganic Nanoparticles

Inorganic nanoparticles are nanomaterials with a central core composed of inorganic specimens, especially metals, which give them interesting applications but can impact the environment and human health. Their short- and long-term effects are not completely known and to investigate that, alternative models have been successfully used. Among these, the nematode Caenorhabditis elegans has been increasingly applied in nanotoxicology in recent years because of its many features and advantages for toxicological screening. This non-parasitic nematode may inhabit any environment where organic matter is available; therefore, it is interesting for ecotoxicological assessments. Moreover, this worm has a high genetic homology to humans, making the findings translatable. A notable number of published studies unraveled the level of toxicity of different nanoparticles, including the mechanisms by which their toxicity occurs. This narrative review collects and describes the most relevant toxicological data for inorganic nanoparticles obtained using C. elegans and also supports its application in safety assessments for regulatory purposes.

GABA attenuates neurotoxicity of zinc oxide nanoparticles due to oxidative stress via DAF-16/FoxO and SKN-1/Nrf2 pathways

Zinc oxide nanoparticles (ZnO-NPs) are one of the most widely used metal oxide nanomaterials. The increased use of ZnO-NPs has exacerbated environmental pollution and raised the risk of neurological disorders in organisms through food chains, and it is urgent to look for detoxification strategies. γ-Aminobutyric acid (GABA) is an inhibitory neurotransmitter that has been shown to have anxiolytic, anti-aging and inhibitory effects on nervous system excitability. However, there are few reports on the prevention and control of the toxicity of nano-metal ions by GABA. In zebrafish, ZnO-NPs exposure led to increased mortality and behavioral abnormalities of larva, which could be moderated by GABA intervention. Similar results were investigated in Caenorhabditis elegans, showing lifespan extension, abnormal locomotor frequency and behavior recovery when worms fed with GABA under ZnO-NPs exposure. Moreover, GABA enhanced antioxidant enzyme activities by upregulating the expression of antioxidant-related genes and thus scavenged excessive O2-. In the case of ZnO-NPs exposure, inhibition of nuclear translocation of DAF-16 and SKN-1 was restored by GABA. Meanwhile, the protective effect of GABA was blocked in daf-16 (-) and skn-1 (-) mutant, suggesting that DAF-16/FoxO and SKN-1/Nrf2 pathways is the key targets of GABA. This study provides a new solution for the application of GABA and mitigation of metal nanoparticle neurotoxicity.

Size, polyglycerol grafting, and net surface charge of iron oxide nanoparticles determine their interaction and toxicity in Caenorhabditis elegans

The widespread application of engineered nanoparticles (NPs) in environmental remediation has raised public concerns about their toxicity to aquatic organisms. Although appropriate surface modification can mitigate the ecotoxicity of NPs, the lack of polymer coating to inhibit toxicity completely and the insufficient knowledge about charge effect hinder the development of safe nanomaterials. Herein, we explored the potential of polyglycerol (PG) functionalization in alleviating the environmental risks of NPs. Iron oxide NPs (ION) of 20, 100, and 200 nm sizes (IONS, IONM and IONL, respectively) were grafted with PG to afford ION-PG. We examined the interaction of ION and ION-PG with Caenorhabditis elegans (C. elegans) and found that PG suppressed non-specific interaction of ION with C. elegans to reduce their accumulation and to inhibit their translocation. Particularly, IONS-PG was completely excluded from worms of all developmental stages. By covalently introducing sulfate, carboxyl and amino groups onto IONS-PG, we further demonstrated that positively charged IONS-PG-NH3+ induced high intestinal accumulation, cuticle adhesion and distal translocation, whereas the negatively charged IONS-PG-OSO3- and IONS-PG-COO- were excreted out. Consequently, no apparent deleterious effects on brood size and life span were observed in worms treated by IONS-PG and IONS-PG bearing negatively charged groups. This study presents new surface functionalization approaches for developing ecofriendly nanomaterials.

See the Unseen: Red-Emissive Carbon Dots for Visualizing the Nucleolar Structures in Two Model Animals and In Vivo Drug Toxicity

Nucleolus, which participates in many crucial cellular activities, is an ideal target for evaluating the state of a cell or an organism. Here, bright red-emissive carbon dots (termed CPCDs) with excitation-independent/polarity-dependent fluorescence emission are synthesized by a one-step hydrothermal reaction between congo red and p-phenylenediamine. The CPCDs can achieve wash-free, real-time, long-term, and high-quality nucleolus imaging in live cells, as well as in vivo imaging of two common model animals-zebrafish and Caenorhabditis elegans (C. elegans). Strikingly, CPCDs realize the nucleolus imaging of organs/flowing blood cells in zebrafish at a cellular level for the first time, and the superb nucleolus imaging of C. elegans suggests that the germ cells in the spermatheca probably have no intact nuclei. These previously unachieved imaging results of the cells/tissues/organs may guide the zebrafish-related studies and benefit the research of C. elegans development. More importantly, a novel strategy based on CPCDs for in vivo toxicity evaluation of materials/drugs (e.g., Ag⁺ ), which can visualize the otherwise unseen injuries in zebrafish, is developed. In conclusion, the CPCDs represent a robust tool for visualizing the structures and dynamic behaviors of live zebrafish and C. elegans, and may find important applications in cell biology and toxicology.

Expanding the Synthetic Toolbox through Metal-Enzyme Cascade Reactions

The combination of metal-, photo-, enzyme-, and/or organocatalysis provides multiple synthetic solutions, especially when the creation of chiral centers is involved. Historically, enzymes and transition metal species have been exploited simultaneously through dynamic kinetic resolutions of racemates. However, more recently, linear cascades have appeared as elegant solutions for the preparation of valuable organic molecules combining multiple bioprocesses and metal-catalyzed transformations. Many advantages are derived from this symbiosis, although there are still bottlenecks to be addressed including the successful coexistence of both catalyst types, the need for compatible reaction media and mild conditions, or the minimization of cross-reactivities. Therefore, solutions are here also provided by means of catalyst coimmobilization, compartmentalization strategies, flow chemistry, etc. A comprehensive review is presented focusing on the period 2015 to early 2022, which has been divided into two main sections that comprise first the use of metals and enzymes as independent catalysts but working in an orchestral or sequential manner, and later their application as bionanohybrid materials through their coimmobilization in adequate supports. Each part has been classified into different subheadings, the first part based on the reaction catalyzed by the metal catalyst, while the development of nonasymmetric or stereoselective processes was considered for the bionanohybrid section.

Coated Cu-doped ZnO and Cu nanoparticles as control agents against plant pathogenic fungi and nematodes

In the current study, coated copper nanoparticles with polyethylene glycol 8000 (Cu@PEG NPs) and copper-doped zinc oxide nanoparticles with diethylene glycol (Cu-doped ZnO@DEG NPs) have been synthesized via solvothermal and microwave-assisted process, physicochemical characterized, and studied as nano-fungicides and nano-nematicides. Spheroidal Cu-doped ZnO@DEG NPs and urchin-like Cu@PEG NPs have been isolated with average crystallite sizes of 12 and 21 nm, respectively. The Cu doping (11.3 wt%) in ZnO lattice (88.7 wt%) was investigated by Rietveld refinement analysis and confirmed by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). The Cu-doped ZnO@DEG and Cu@PEG NPs revealed a growth inhibition of fungi Botrytis cinerea (B. cinerea) and Sclerotinia sclerotiorum (S. sclerotiorum) and nematode paralysis of Meloidogyne javanica in a dose-dependent manner. Cu-doped ZnO@DEG NPs were more effective against M. javanica (EC₅₀ = 2.60 μg/mL) than the Cu@PEG NPs (EC₅₀ = 25 μg/mL). In contrast, the antifungal activity was approximately similar for both NPs, with EC₅₀ values at 310 and 327 μg/mL against B. cinerea, respectively, and 260 and 278 μg/mL against S. sclerotiorum, respectively. Lettuce (Lactuca sativa) plants were inoculated with S. sclerotiorum or M. javanica and sprayed with either Cu-doped ZnO@DEG NPs or Cu@PEG NPs. The antifungal effect was evaluated based on a disease index (DI), and nematicidal activity was assessed based on the total number of galls and nematode females per root gram. NPs successfully inhibited the growth of both pathogens without causing phytotoxicity on lettuce. The DI were significantly decreased as compared to the positive control (DI = 5.2), estimated equal to 1.7, 2.9 and 2.5 for Cu@PEG NPs, Cu-doped ZnO@DEG NPs and the chemical control (KOCIDE 2000), respectively. The reduction in galling and population of M. javanica ranged from 39.32% to 32.29%, statistically like chemical control. The treatment of lettuce plants with Cu-doped ZnO@DEG NPs increased the leaf net photosynthetic value at 4.60 and 6.66 μmol CO₂^-2 s^-1 in plants inoculated with S. sclerotiorum and M. javanica, respectively, as compared to the control (3.00 μmol CO₂^-2 s^-1). The antioxidant capacity of NPs treated lettuce plants was evaluated as 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity in leaf extracts. Plants inoculated with S. sclerotiorum and sprayed with Cu-doped ZnO@DEG and Cu@PEG NPs, exhibited a 34.22% and 32.70% increase in antioxidant capacity, respectively, higher than the control. Similarly, an increase in antioxidant capacity was measured (39.49 and 37.36%) in lettuce inoculated with M. javanica and treated with Cu-doped ZnO@DEG and Cu@PEG NPs, respectively. Moreover, an increase of phenolic compounds in lettuce leaf tissue treated with NPs was measured as compared to the control. Overall, foliar applied Cu and Cu-doped ZnO NPs could be a promising tool to control phytopathogenic fungi and nematodes contributing to sustainability of agri-food sector.

In vivo toxicity assessment of eugenol and vanillin-functionalised silica particles using Caenorhabditis elegans

The toxicological properties of different silica particles functionalised with essential oil components (EOCs) were herein assessed using the in vivo model C. elegans. In particular, the effects of the acute and long-term exposure to three silica particle types (SAS, MCM-41 micro, MCM-41 nano), either bare or functionalised with eugenol or vanillin, were evaluated on different biological parameters of nematodes. Acute exposure to the different particles did not reduce nematodes survival, brood growth or locomotion, but reproduction was impaired by all the materials, except for vanillin-functionalised MCM-41 nano. Moreover, long-term exposure to particles led to strongly inhibited nematodes growth and reproduction. The eugenol-functionalised particles exhibited higher functionalisation yields and had the strongest effects during acute and long-term exposures. Overall, the vanillin-functionalised particles displayed milder acute toxic effects on reproduction than pristine materials, but severer toxicological responses for the 96-hour exposure assays. Our findings suggest that the EOC type anchored to silica surfaces and functionalisation yield are crucial for determining the toxicological effects of particles on C. elegans. The results obtained with this alternative in vivo model can help to anticipate potential toxic responses to these new materials for human health and the environment.

Mitigation of graphene oxide toxicity in C. elegans after chemical degradation with sodium hypochlorite

Graphene oxide (GO) is a promising and strategic carbon-based nanomaterial for innovative and disruptive technologies. It is therefore essential to address its environmental health and safety aspects. In this work, we evaluated the chemical degradation of graphene oxide by sodium hypochlorite (NaClO, bleach water) and its consequences over toxicity, on the nematode Caenorhabditis elegans. The morphological, chemical, and structural properties of GO and its degraded product, termed NaClO-GO, were characterized, exploring an integrated approach. After the chemical degradation of GO at room temperature, its flake size was reduced from 156 to 29 nm, while NaClO-GO showed changes in UV-vis absorption, and an increase in the amount of oxygenated surface groups, which dramatically improved its colloidal stability in moderately hard reconstituted water (EPA medium). Acute and chronic exposure endpoints (survival, growth, fertility, and reproduction) were monitored to evaluate material toxicities. NaClO-GO presented lower toxicity at all endpoints. For example, an increase of over 100% in nematode survival was verified for the degraded material when compared to GO at 10 mg L^-1. Additionally, enhanced dark-field hyperspectral microscopy confirmed the oral uptake of both materials by C. elegans. Finally, this work represents a new contribution toward a better understanding of the links between the transformation of graphene-based materials and nanotoxicity effects (mitigation), which is mandatory for the safety improvements that are required to maximize nanotechnological benefits to society.

Research Advances on the Adverse Effects of Nanomaterials in a Model Organism, Caenorhabditis elegans

Along with the rapid development of nanotechnology, the biosafety assessment of nanotechnology products, including nanomaterials (NMs), has become more and more important. The nematode Caenorhabditis elegans is a valuable model organism that has been widely used in the field of biology because of its excellent advantages, including low cost, small size, short life span, and highly conservative genomes with vertebral animals. In recent years, the number of nanotoxicological researchers using C. elegans has been growing. According to these available studies, the present review classified the adverse effects of NMs in C. elegans into systematic, cellular, and molecular toxicity, and focused on summarizing and analyzing the underlying mechanisms of metal, metal oxide, and nonmetallic NMs causing toxic effects in C. elegans. Our findings provide insights into what further studies are needed to assess the biosafety of NMs in the ecosystem using C. elegans. Environ Toxicol Chem 2021;40:2406-2424. © 2021 SETAC.

Dark-field/hyperspectral microscopy for detecting nanoscale particles in environmental nanotoxicology research

Nanoscale contaminants (including engineered nanoparticles and nanoplastics) pose a significant threat to organisms and environment. Rapid and non-destructive detection and identification of nanosized materials in cells, tissues and organisms is still challenging, although a number of conventional methods exist. These approaches for nanoparticles imaging and characterisation both inside the cytoplasm and on the cell or tissue outer surfaces, such as electron or scanning probe microscopies, are unquestionably potent tools, having excellent resolution and supplemented with chemical analysis capabilities. However, imaging and detection of nanomaterials in situ, in wet unfixed and even live samples, such as living isolated cells, microorganisms, protozoans and miniature invertebrates using electron microscopy is practically impossible, because of the elaborate sample preparation requiring chemical fixation, contrast staining, matrix embedding and exposure into vacuum. Atomic force microscopy, in several cases, can be used for imaging and mechanical analysis of live cells and organisms under ambient conditions, however this technique allows for investigation of surfaces. Therefore, a different approach allowing for imaging and differentiation of nanoscale particles in wet samples is required. Dark-field microscopy as an optical microscopy technique has been popular among researchers, mostly for imaging relatively large specimens. In recent years, the so-called "enhanced dark field" microscopy based on using higher numerical aperture light condensers and variable numerical aperture objectives has emegred, which allows for imaging of nanoscale particles (starting from 5 nm nanospheres) using almost conventional optical microscopy methodology. Hyperspectral imaging can turn a dark-field optical microscope into a powerful chemical characterisation tool. As a result, this technique is becoming popular in environmental nanotoxicology studies. In this Review Article we introduce the reader into the methodology of enhanced dark-field and dark-field-based hyperspectral microscopy, covering the most important advances in this rapidly-expanding area of environmental nanotoxicology.

Xenobiotic metabolism and transport in Caenorhabditis elegans

Caenorhabditis elegans has emerged as a major model in biomedical and environmental toxicology. Numerous papers on toxicology and pharmacology in C. elegans have been published, and this species has now been adopted by investigators in academic toxicology, pharmacology, and drug discovery labs. C. elegans has also attracted the interest of governmental regulatory agencies charged with evaluating the safety of chemicals. However, a major, fundamental aspect of toxicological science remains underdeveloped in C. elegans: xenobiotic metabolism and transport processes that are critical to understanding toxicokinetics and toxicodynamics, and extrapolation to other species. The aim of this review was to initially briefly describe the history and trajectory of the use of C. elegans in toxicological and pharmacological studies. Subsequently, physical barriers to chemical uptake and the role of the worm microbiome in xenobiotic transformation were described. Then a review of what is and is not known regarding the classic Phase I, Phase II, and Phase III processes was performed. In addition, the following were discussed (1) regulation of xenobiotic metabolism; (2) review of published toxicokinetics for specific chemicals; and (3) genetic diversity of these processes in C. elegans. Finally, worm xenobiotic transport and metabolism was placed in an evolutionary context; key areas for future research highlighted; and implications for extrapolating C. elegans toxicity results to other species discussed.

Engineering the surface of graphene oxide with bovine serum albumin for improved biocompatibility in Caenorhabditis elegans

Graphene oxide (GO) has been extensively studied for its potential biomedical applications. However, its potential risk associated with the interactions of GO in a biological system hampers its biomedical applications. Therefore, there is an urgent need to enhance the biocompatibility of GO. In the present study, we decorated the surface of GO with bovine serum albumin (GO-BSA) to mitigate the in vivo toxic properties of GO. An in vivo model Caenorhabditis elegans has been used to study the potential protective effect of BSA decoration in mitigating GO induced toxicity. The BSA decoration on the surface of GO prevents the acute and prolonged toxicity induced by GO in primary and secondary organs by maintaining normal intestinal permeability, defecation behavior, development, and reproduction. Notably, GO-BSA treatment at 0.5-100 mg L^-1 does not affect the intracellular redox status and lifespan of C. elegans. Reporter gene expression analysis revealed that exposure to GO-BSA (100 mg L^-1) did not significantly influence the nuclear accumulation and expression patterns of DAF-16/FOXO and SKN-1/Nrf2 transcription factors and their downstream target genes sod-3, hsp-16.2, ctl-1,2,3, gcs-1, and gst-4 when compared to exposure to pristine GO. Also, quantitative real-time PCR results showed that GO-BSA did not alter the expression of genes involved in regulating DNA damage checkpoints (cep-1, hus-1 and egl-1) and core signaling pathways of apoptosis (ced-4, ced-3 and ced-9), in contrast to GO treatment. All these findings will have an impact on the future development of safer nanomaterial formulations of graphene and graphene-based materials for environmental and biomedical applications.

Nanobiohybrids: a new concept for metal nanoparticles synthesis

In recent years, nanoscience and nanotechnology have brought a great revolution in different areas. In particular, the synthesis of transition metal nanoparticles has been of great relevance for their use in areas such as biomedicine, antimicrobial properties or catalytic applications for chemical synthesis. Recently, an innovative straightforward and very efficient synthesis of these nanoparticles by simply using enzymes as inductors in aqueous media has been described. This represents a very green alternative to the different methodologies described in the literature for metal nanoparticles preparation where harsh conditions are necessary. In this review the most recent advances in the synthesis of metal nanoparticles by this green technology, explaining the synthetic mechanism, the role of the enzyme in the formation of the nanoparticles and the effect on the final properties of these nanoparticles, are summarised. The application of these novel metal nanoparticles-enzyme hybrids in synthetic chemistry as heterogeneous catalysts with metal or dual (enzymatic and metallic) activity and their capacity as environmental and antimicrobial agents have also been discussed.

Comparative toxicity of a food additive TiO2, a bulk TiO2, and a nano-sized P25 to a model organism the nematode C. elegans

To help fill the knowledge gap regarding the potential human health impacts of food pigment TiO₂, a comparative toxicity study was performed on a food-grade TiO₂ (f-TiO₂), a bulk TiO₂ (b-TiO₂), and a nano-sized TiO₂ (Degussa P25), and in the nematode Caenorhabditis elegans. Acute phototoxicity and chronic toxicity effects including reproduction, lifespan, and vulval integrity were evaluated. The f-TiO₂, b-TiO₂, and P25 had a primary particle size (size range) of 149 (53-308) nm, 129 (64-259) nm, and 26 (11-52) nm, respectively. P25 showed the greatest phototoxicity with a 24-h LC50 of 6.0 mg/L (95% CI 5.95, 6.3), followed by the f-TiO₂ (LC50 = 6.55 mg/L (95% CI 6.35, 6.75)), and b-TiO₂ was the least toxic. All three TiO₂ (1-10 mg/L) induced concentration-dependent effects on the worm's reproduction, with a reduction in brood size by 8.5 to 34%. They all caused a reduction of worm lifespan, accompanied by an increased frequency of age-associated vulval integrity defects (Avid). The impact on lifespan and Avid phenotype was more notable for P25 than the f-TiO₂ or b-TiO₂. Ingestion and accumulation of TiO₂ particles in the worm intestine was observed for all three materials by light microscopy. These findings demonstrate that the food pigment TiO₂ induces toxicity effects in the worm and further studies are needed to elucidate the human health implication of such toxicities.

Carbon Nanotubes and Other Engineered Nanoparticles Induced Pathophysiology on Mesothelial Cells and Mesothelial Membranes

Nanoparticles have great potential for numerous applications due to their unique physicochemical properties. However, concerns have been raised that they may induce deleterious effects on biological systems. There is accumulating evidence that, like asbestos, inhaled nanomaterials of >5 μm and high aspect ratio (3:1), particularly rod-like carbon nanotubes, may inflict pleural disease including mesothelioma. Additionally, a recent set of case reports suggests that inhalation of polyacrylate/nanosilica could in part be associated with inflammation and fibrosis of the pleura of factory workers. However, the adverse outcomes of nanoparticle exposure to mesothelial tissues are still largely unexplored. In that context, the present review aims to provide an overview of the relevant pathophysiological implications involving toxicological studies describing effects of engineered nanoparticles on mesothelial cells and membranes. In vitro studies primarily emphasize on simulating cellular uptake and toxicity of nanotubes on benign or malignant cell lines. On the other hand, in vivo studies focus on illustrating endpoints of serosal pathology in rodent animal models. From a molecular aspect, some nanoparticle categories are shown to be cytotoxic and genotoxic after acute treatment, whereas chronic incubation may lead to malignant-like transformation. At an organism level, a number of fibrous shaped nanotubes are related with features of chronic inflammation and MWCNT-7 is the only type to consistently inflict mesothelioma.