Effects of metal exposure on associative learning behavior in nematode Caenorhabditis elegans

Caenorhabditis elegans as a Prediction Platform for Nanotechnology-Based Strategies: Insights on Analytical Challenges

Nanotechnology-based strategies have played a pivotal role in innovative products in different technological fields, including medicine, agriculture, and engineering. The redesign of the nanometric scale has improved drug targeting and delivery, diagnosis, water treatment, and analytical methods. Although efficiency brings benefits, toxicity in organisms and the environment is a concern, particularly in light of global climate change and plastic disposal in the environment. Therefore, to measure such effects, alternative models enable the assessment of impacts on both functional properties and toxicity. Caenorhabditis elegans is a nematode model that poses valuable advantages such as transparency, sensibility in responding to exogenous compounds, fast response to perturbations besides the possibility to replicate human disease through transgenics. Herein, we discuss the applications of C. elegans to nanomaterial safety and efficacy evaluations from one health perspective. We also highlight the directions for developing appropriate techniques to safely adopt magnetic and organic nanoparticles, and carbon nanosystems. A description was given of the specifics of targeting and treatment, especially for health purposes. Finally, we discuss C. elegans potential for studying the impacts caused by nanopesticides and nanoplastics as emerging contaminants, pointing out gaps in environmental studies related to toxicity, analytical methods, and future directions.

The intertwining between lead and ethanol in the model organism Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) is a model organism widely used to evaluate the mechanistic aspects of toxicants with the potential to predict responses comparable to those of mammals. We report here the consequences of developmental lead (Pb) exposure on behavioral responses to ethanol (EtOH) in C. elegans. In addition, we present data on morphological alterations in the dopamine (DA) synapse and DA-dependent behaviors aimed to dissect the neurobiological mechanisms that underlie the relationship between these neurotoxicants. Finally, the escalation to superior animals that parallels the observed effects in both experimental models with references to EtOH metabolism and oxidative stress is also discussed. Overall, the literature revised here underpins the usefulness of C. elegans to evidence behavioral responses to a combination of neurotoxicants in mechanistic-orientated studies.

High-throughput assessment of toxic effects of metal mixtures of cadmium(Cd), lead(Pb), and manganese(Mn) in nematode Caenorhabditis elegans

Heavy metals, a class of persistent environmental toxicants, are harmful to human health. Cd and Pb are two of the most common toxic heavy metals that have been linked with cancers and malfunction of the nervous system. Notably, contamination of Mn usually coexisted with Cd and Pb in environmental and occupational settings. Studies regularly examined the toxic effects on individual metals; however, potential health and toxic effects of mixtures containing two or more heavy metals are unknown. Here, we investigated toxic effects of Cd, Pb, Mn, and their binary and ternary mixtures in the nematode Caenorhabdities elegans. The toxic outcomes, including effects on growth, reproduction, and feeding, were measured via high-throughput platform analysis. The transgenic strain BY250 with GFP in dopaminergic neurons was used to explore the neurodegenerative effects induced by single metals or their mixtures. The combination index(CI) for mixtures effect was calculated using isobolograms methods. Following the exposure, we found significant toxic effects in C. elegans. For single metals, the toxicity order for growth, reproduction, and feeding were Pb > Cd > Mn. For mixtures, the mixture of Cd + Mn induced a less than addictive effect in C. elegans, whereas the mixtures of Cd + Pb, Pb + Mn, and Cd + Pb + Mn induced greater-than-additive effects. Both single metals and their mixtures induced abnormality in dopaminergic neurons. These results showed combinative toxic and neurodegenerative effects of heavy metal mixtures, and future studies will focus on characterization of concentration-response patterns and identification of potential molecular mechanisms in C. elegans model.

C. elegans as a model in developmental neurotoxicology

Due to many advantages Caenorhabditis elegans (C. elegans) has become a preferred model of choice in many fields, including neurodevelopmental toxicity studies. This review discusses the benefits of using C. elegans as an alternative to mammalian systems and gives examples of the uses of the nematode in evaluating the effects of major known neurodevelopmental toxins, including manganese, mercury, lead, fluoride, arsenic and organophosphorus pesticides. Reviewed data indicates numerous similarities with mammals in response to these toxins. Thus, C. elegans studies have the potential to predict possible effects of developmental neurotoxicants in higher animals, and may be used to identify new molecular pathways behind neurodevelopmental disruptions, as well as new toxicants.

Metallome evolution in ageing C. elegans and a copper stable isotope perspective

Ageing is accompanied by important chemical deregulations that could serve as biomarkers of premature ageing conditions.

The time-dependent synergism of the six-component mixtures of substituted phenols, pesticides and ionic liquids to Caenorhabditis elegans

Traditional environmental risk assessment rarely focused on exposures to multi-component mixtures which may cause toxicological interactions and usually ignored that toxicity is a process in time, which may underestimate the environment risk of mixtures. In this paper, six chemicals belonging to three categories, two substituted phenols, two pesticides and two Ionic liquids, were picked to construct a six-component mixture system. To systematically examine the effects of various concentration compositions, the uniform design ray method was employed to design nine mixture rays with nine mixture ratios and for every mixture ray 12 concentration levels were specified by the fixed ratio ray design. The improved combination index was used to evaluate the combined toxicities of the mixtures to Caenorhabditis elegans (C. elegans) in the exposure times of 6, 12 and 24h. It was shown that the mixture rays display time-dependent synergism, i.e. the range of synergistic effect narrows and the strength of synergism runs down with exposure time, which illustrates that the mixture toxicity of some chemicals is not a sum of individual toxicities at some exposure times and it is necessary to consider the toxicological interaction in mixtures.

Neurodegeneration Induced by Metals in Caenorhabditis elegans

Metals are a component of a variety of ecosystems and organisms. They can generally be divided into essential and nonessential metals. The essential metals are involved in physiological processes once the deficiency of these metals has been associated with diseases. Although iron, manganese, copper, and zinc are important for life, it has been evidenced that they are also involved in neuronal damage in many neurodegenerative disorders. Nonessential metals, which are metals without physiological functions, are present in trace or higher levels in living organisms. Occupational, environmental, or deliberate exposures to lead, mercury, aluminum, and cadmium are clearly correlated with the increase of toxicity and varied kinds of pathological situations. Actually, the field of neurotoxicology needs to satisfy two opposing demands: the testing of a growing list of chemicals and resource limitations and ethical concerns associated with testing using traditional mammalian species. Toxicological assays using alternative animal models may relieve some of this pressure by allowing testing of more compounds while reducing expenses and using fewer mammals. The nervous system is by far the more complex system in C. elegans. Almost a third of their cells are neurons (302 neurons versus 959 cells in adult hermaphrodite). It initially underwent extensive development as a model organism in order to study the nervous system, and its neuronal lineage and the complete wiring diagram of its nervous system are stereotyped and fully described. The neurotransmission systems are phylogenetically conserved from nematodes to vertebrates, which allows for findings from C. elegans to be extrapolated and further confirmed in vertebrate systems. Different strains of C. elegans offer a new perspective on neurodegenerative processes. Some genes have been found to be related to neurodegeneration induced by metals. Studying these interactions may be an effective tool to slow neuronal loss and deterioration.

Integrative assessment of benzene exposure to Caenorhabditis elegans using computational behavior and toxicogenomic analyses

In this study, we investigated the toxic effects of benzene to the nematode Caenorhabditis elegans in an integrative manner, using computational behavior and toxicogenomics analyses, along with survival and reproduction. Benzene exposure led to changes in locomotive behavior and reproduction decline in C. elegans. Microarray followed by pathway analysis revealed that 228 genes were differentially expressed by benzene exposure, and cyp-35a2, pmk-1, and cep-1 were selected for further reproduction and multiparametric behavior analysis. Mutant analysis showed that benzene induced reproduction decline was rescued in cyp-35a2(gk317) mutant, whereas it was significantly exacerbated in pmk-1(km25) mutant, compared with the wildtype. The multiparametric behavior analysis on the mutants of selected genes revealed that each strain exhibits different response patterns, particularly, enhanced linear movement in the cyp-35a2(gk317) mutant, whereas the changes in partial body movement were observed in the pmk-1(km25) mutant by benzene exposure. A self-organizing map revealed that the pmk-1(km25) mutant group was the most densely clustered and located on the opposite side of the map of the cyp-35a2(gk317) mutant, each crossing that of the wildtype. Overall results suggest distinct roles of cyp-35a2 and pmk-1 genes in benzene-induced alterations in behavior and reproduction in C. elegans. This study also suggests computational behavior analysis is a suitable tool for addressing the integrative impact of chemical stress alongside with toxicogenomic approach.

Changes to cuticle surface ultrastructure and some biological functions in the nematode Caenorhabditis elegans exposed to excessive copper

Copper is an essential metal, but its toxic effects are pronounced when organisms are exposed to it in excessive amounts. However, information about the effects of chronic copper exposure on the cuticle ultrastructure of organisms is insufficient. Studies of the model organism, Caenorhabditis elegans, could further our understanding of the effect of chronic excessive copper exposure on human health. In this study, the cuticle surface ultrastructure of C. elegans was observed using scanning electron microscopy after excessive copper exposure. In addition to this, some biological functions, such as chemotaxis, reproduction, and development, were also analyzed. After chronic excessive copper exposure, the worms' body surface from vulva to tail was extensively wrinkled and folded along with the annulus. The worm's vulva size was significantly decreased, and the middle ridge of the alae was disrupted. Furthermore, some of the biological functions of nematodes were also affected: the chemotaxis index was partially changed, bags-of-worms were induced, development was delayed, and egg-laying number was decreased by copper treatment. The results of the present study shed new light on the effects of copper on C. elegans cuticle as well as some biological functions.

Adverse effects from clenbuterol and ractopamine on nematode Caenorhabditis elegans and the underlying mechanism

In the present study, we used Caenorhabditis elegans assay system to investigate in vivo toxicity from clentuberol and ractopamine and the possible underlying mechanism. Both acute and prolonged exposures to clentuberol or ractopamine decreased brood size and locomotion behavior, and induced intestinal autofluorescence and reactive oxygen species (ROS) production. Although acute exposure to the examined concentrations of clentuberol or ractopamine did not induce lethality, prolonged exposure to 10 µg/L of clentuberol and ractopamine reduced lifespan. At relatively high concentrations, ractopamine exhibited more severe toxicity than clentuberol on nematodes. Overexpression of sod-2 gene encoding a Mn-SOD to prevent induction of oxidative stress effectively inhibited toxicity from clentuberol or ractopamine. Besides oxidative stress, we found that clentuberol might reduce lifespan through influencing insulin/IGF signaling pathway; however, ractopamine might reduce lifespan through affecting both insulin/IGF signaling pathway and TOR signaling pathway. Ractopamine more severely decreased expression levels of daf-16, sgk-1, skn-1, and aak-2 genes than clentuberol, and increased expression levels of daf-2 and age-1 genes at the examined concentration. Therefore, the C. elegans assay system may be useful for assessing the possible toxicity from weight loss agents, and clentuberol and ractopamine may induce toxicity through different molecular mechanisms.

Biosafety assessment of titanium dioxide nanoparticles in acutely exposed nematode Caenorhabditis elegans with mutations of genes required for oxidative stress or stress response

We used Caenorhabditis elegans to investigate whether acute exposure to TiO2-NPs at the concentration of 20 μg L(-1) reflecting predicted environmental relevant concentration and 25 mg L(-1) reflecting concentration in food can cause toxicity on nematodes with mutations of susceptible genes. Among examined mutants associated with oxidative stress and stress response, we found that genes of sod-2, sod-3, mtl-2, and hsp-16.48 might be susceptible for TiO2-NPs toxicity. Mutations of these genes altered functions of both possible primary and secondary targeted organs in nematodes exposed to 25 mg L(-1) of TiO2-NPs for 24-h. Mutations of these genes caused similar expression patterns of genes required for oxidative stress in TiO2-NPs exposed mutant nematodes, implying their similar mechanisms to form the susceptible property. Nevertheless, acute exposure to 20 μg L(-1) of TiO2-NPs for 24-h and 25 mg L(-1) of TiO2-NPs for 0.48-h or 5.71-h did not influence functions of both possible primary and secondary targeted organs in sod-2, sod-3, mtl-2, and hsp-16.48 mutants. Therefore, our results suggest the relatively safe property of acute exposure to TiO2-NPs with certain durations at predicted environmental relevant concentrations or concentrations comparable to those in food in nematodes with mutations of some susceptible genes.

High concentration of vitamin E decreases thermosensation and thermotaxis learning and the underlying mechanisms in the nematode Caenorhabditis elegans

α-tocopherol is a powerful liposoluble antioxidant and the most abundant isoform of vitamin E in the body. Under normal physiological conditions, adverse effects of relatively high concentration of vitamin E on organisms and the underlying mechanisms are still largely unclear. In the present study, we used the nematode Caenorhabditis elegans as an in vivo assay system to investigate the possible adverse effects of high concentration of vitamin E on thermosensation and thermotaxis learning and the underlying mechanisms. Our data show that treatment with 100-200 µg/mL of vitamin E did not noticeably influence both thermosensation and thermotaxis learning; however, treatment with 400 µg/mL of vitamin E altered both thermosensation and thermotaxis learning. The observed decrease in thermotaxis learning in 400 µg/mL of vitamin E treated nematodes might be partially due to the moderate but significant deficits in thermosensation, but not due to deficits in locomotion behavior or perception to food and starvation. Treatment with 400 µg/mL of vitamin E did not noticeably influence the morphology of GABAergic neurons, but significantly decreased fluorescent intensities of the cell bodies in AFD sensory neurons and AIY interneurons, required for thermosensation and thermotaxis learning control. Treatment with 400 µg/mL of vitamin E affected presynaptic function of neurons, but had no remarkable effects on postsynaptic function. Moreover, promotion of synaptic transmission by activating PKC-1 effectively retrieved deficits in both thermosensation and thermotaxis learning induced by 400 µg/mL of vitamin E. Therefore, relatively high concentrations of vitamin E administration may cause adverse effects on thermosensation and thermotaxis learning by inducing damage on the development of specific neurons and presynaptic function under normal physiological conditions in C. elegans.

Carboxylic acid functionalization prevents the translocation of multi-walled carbon nanotubes at predicted environmentally relevant concentrations into targeted organs of nematode Caenorhabditis elegans

Carboxyl (-COOH) surface modified multi-walled carbon nanotubes (MWCNTs-COOH) can be used for targeted delivery of drugs and imaging. However, whether MWCNTs-COOH at environmentally relevant concentrations exert certain toxic effects on multicellular organisms and the underlying mechanisms are still largely unclear. In the present study, we applied the nematode Caenorhabditis elegans to evaluate the properties of MWCNTs-COOH at environmentally relevant concentrations by comparing the effects of MWCNTs and MWCNTs-COOH exposure on C. elegans from L1-larvae to adult at concentrations of 0.001-1000 μg L(-1). Exposure to MWCNTs could potentially damage the intestine (primary targeted organ) at concentrations greater than 0.1 μg L(-1) and functions of neurons and reproductive organ (secondary targeted organs) at concentrations greater than 0.001 μg L(-1). Carboxyl modification prevented the toxicity of MWCNTs on the primary and the secondary targeted organs at concentrations less than 100 μg L(-1), suggesting that carboxyl modification can effectively prevent the adverse effects of MWCNTs at environmentally relevant concentrations. After exposure, MWCNTs-COOH (1 mg L(-1)) were translocated into the spermatheca and embryos in the body through the primary targeted organs. However, MWCNTs-COOH (10 μg L(-1)) were not observed in spermatheca and embryos in the body of nematodes. Moreover, relatively high concentrations of MWCNTs-COOH exposed nematodes might have a hyper-permeable intestinal barrier, whereas MWCNTs-COOH at environmentally relevant concentrations effectively sustained the normally permeable state for the intestinal barrier. Therefore, we elucidated the cellular basis of carboxyl modification to prevent toxicity of MWCNTs at environmentally relevant concentrations. Our data highlights the key role of biological barriers in the primary targeted organs to block toxicity formation from MWCNTs, which will be useful for the design of effective prevention strategies against MWCNTs toxicity.

Metal-induced neurodegeneration in C. elegans

The model species, Caenorhabditis elegans, has been used as a tool to probe for mechanisms underlying numerous neurodegenerative diseases. This use has been exploited to study neurodegeneration induced by metals. The allure of the nematode comes from the ease of genetic manipulation, the ability to fluorescently label neuronal subtypes, and the relative simplicity of the nervous system. Notably, C. elegans have approximately 60-80% of human genes and contain genes involved in metal homeostasis and transport, allowing for the study of metal-induced degeneration in the nematode. This review discusses methods to assess degeneration as well as outlines techniques for genetic manipulation and presents a comprehensive survey of the existing literature on metal-induced degeneration studies in the worm.

Metabolic profiling in Caenorhabditis elegans provides an unbiased approach to investigations of dosage dependent lead toxicity

The nematode, Caenorhabditis elegans (CE), serves as a model system in which to explore the impact of particularly low-levels of lead [250, 500, 1000 and 2000 parts per million (ppm) (1.4 × 10(-6) M to 1.1 × 10(-5) M/nematode)] on specific metabolic pathways and processes. Chromatographic profiles of redox active metabolites are captured through application of high performance liquid chromatography coupled to electrochemical detection (Coularray/HPLC). Principal Component Analysis (PCA: unbiased cluster analysis) and the application of a slicing program, located significant areas of difference occurring within the 2.8-4.58 min section of the chromatograms. It is within this region of the data profiles that known components of the purine pathway reside. Two analytes of unknown structure were detected at 3.5 and 4 min respectively. Alterations in levels of the purine, tryptophan and tyrosine pathway intermediates measured in response to differing concentrations of lead acetate indicate that the effect of lead on these pathways is not linear, yet the ratio of the pathway precursors, tryptophan and tyrosine remains relatively constant. The application of the above combined analytical approaches enhances the value of data generated. Exposure of CE to very low levels of lead produced significant alterations in profiles of electrochemically active compounds. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11306-012-0438-0) contains supplementary material, which is available to authorized users.

Small sizes of TiO2-NPs exhibit adverse effects at predicted environmental relevant concentrations on nematodes in a modified chronic toxicity assay system

In Caenorhabditis elegans, although acute toxicity of TiO(2) nanoparticles (TiO(2)-NPs) at high concentrations has been investigated, we still know little about chronic toxicity of TiO(2)-NPs. Our data here showed that acute TiO(2)-NPs exposure in the range of μg/L had no obviously adverse effects on nematodes, but the chronic toxicities of large sizes (60 nm and 90 nm) of TiO(2)-NPs in the range of μg/L were detected in nematodes in a modified chronic toxicity assay system. Moreover, chronic toxicities of small sizes (4 nm and 10nm) of TiO(2)-NPs in the range of ng/L were observed in nematodes with locomotion behavior and ROS production as endpoints. In nematodes chronically exposed to small sizes of TiO(2)-NPs at predicted environmental relevant concentrations, locomotion behavior was significantly (P<0.01) correlated with ROS production. Furthermore, treatment with antioxidants (ascorbate and N-acetyl-l-cysteine) inhibited both the induction of ROS production and the decrease of locomotion behaviors observed in nematodes chronically exposed to small sizes of TiO(2)-NPs at predicted environmental relevant concentrations. Therefore, chronic exposure to small sizes of TiO(2)-NPs at predicted environmental relevant concentrations can cause adverse effects on nematodes, and formation of such adverse effects may be largely due to the induction of oxidative stress.

Chromium exhibits adverse effects at environmental relevant concentrations in chronic toxicity assay system of nematode Caenorhabditis elegans

Here we investigated whether the assay system (10-d) in Caenorhabditis elegans can be used to evaluate chronic toxicity of chromium (Cr(VI)) at environmental relevant concentrations ranging from 5.2 μg L(-1) to 260 μg L(-1). The results indicated that lethality, locomotion behavior as revealed by head thrash, body bend, and forward turn, metabolism as revealed by pumping rate and mean defecation cycle length, intestinal autofluorescence, and ROS production were severely altered in Cr chronically exposed nematodes at environmental relevant concentrations. The most surprising observations were that head thrash, body bend, intestinal autofluorescence, and ROS production in 13 μg L(-1) Cr exposed nematodes were significantly influenced. The observed adverse effects of Cr on survival, locomotion behavior, and metabolism were largely due to forming severe intestinal autofluorescence and ROS production. Therefore, our findings demonstrate the usefulness of chronic toxicity assay system in C. elegans in evaluating the chronic toxicity of toxicants at environmental relevant concentrations.

Inhibition of ROS elevation and damage to mitochondrial function prevents lead-induced neurotoxic effects on structures and functions of AFD neurons in Caenorhabditis elegans

Here we investigated the possible roles of oxidative stress in the formation of decreased thermotaxis to cultivation temperature in lead (Pb)-exposed nematodes Caenorhabditis elagans. Exposure to Pb at the examined concentrations decreased thermotaxis behaviors, and induced severe deficits in the structural properties of AFD sensory neurons. Meanwhile, Pb exposure caused the induction of severe oxidative damage, reactive oxygen species (ROS) production, and mitochondrial dysfunction in young adults. Moreover, pre-treatment with the antioxidants dimethyl sulfoxide (DMSO), ascorbate and N-acetyl-L-cysteine (NAC), used to inhibit both the ROS elevation and the mitochondrial dysfunction caused by Pb exposure, at the L2-larval stage prevented the induction of oxidative damage and the formation of severe deficits in thermotaxis and structural properties of AFD sensory neurons in Pb-exposed young adults. Therefore, the formation of oxidative stress caused by Pb exposure may be due to both the induction of ROS elevation and damage to mitochondrial function, and oxidative stress may play a key role in inducing the neurotoxic effects on the structures and function of AFD sensory neurons in Pb-exposed nematodes.

Modulation of the assay system for the sensory integration of 2 sensory stimuli that inhibit each other in nematode Caenorhabditis elegans

OBJECTIVE

To perform the modulation of an assay system for the sensory integration of 2 sensory stimuli that inhibit each other.

METHODS

The assay system for assessing the integrative response to 2 reciprocally-inhibitory sensory stimuli was modulated by changing the metal ion barrier. Moreover, the hen-1, ttx-3 and casy-1 mutants having known defects in integrative response were used to evaluate the modulated assay systems. Based on the examined assay systems, new genes possibly involved in the sensory integration control were identified.

RESULTS

In the presence of different metal ion barriers and diacetyl, locomotion behaviors, basic movements, pan-neuronal, cholinergic and GABAergic neuronal GFP expressions, neuronal development, structures of sensory neurons and interneurons, and stress response of nematodes in different regions of examined assay systems were normal, and chemotaxis toward different concentrations of diacetyl and avoidance of different concentrations of metal ions were inhibited. In the first group, most of the nematodes moved to diacetyl by crossing the barrier of Fe(2+), Zn(2+), or Mn(2+). In the second group, almost half of the nematodes moved to diacetyl by crossing the barrier of Ag(+), Cu(2+), Cr(2+), or Cd(2+). In the third group, only a small number of nematodes moved to diacetyl by crossing the barrier of Pb(2+) or Hg(2+). Moreover, when nematodes encountered different metal ion barriers during migration toward diacetyl, the percentage of nematodes moving back and then turning and that of nematodes moving straight to diacetyl were very different. With the aid of examined assay systems, it was found that mutations of fsn-1 that encodes a F-box protein, and its target scd-2 that encodes a receptor tyrosine kinase, caused severe defects in integrative response, and the sensory integration defects of fsn-1 mutants were obviously inhibited by scd-2 mutation.

CONCLUSION

Based on the nematode behaviors in examined assay systems, 3 groups of assay systems were obtained. The first group may be helpful in evaluating or identifying the very subtle deficits in sensory integration, and the third group may be useful for the final confirmation of sensory integration defects of mutants identified in the first or the second group of assay systems. Furthermore, the important association of sensory integration regulation with stabilization or destabilization of synaptic differentiation may exist in C. elegans.