Exposure to metals induces morphological and functional alteration of AFD neurons in nematode Caenorhabditis elegans

Graphene oxide-induced neurotoxicity on neurotransmitters, AFD neurons and locomotive behavior in Caenorhabditis elegans

Graphene oxide (GO) and graphene-based nanomaterials have been widely applied in recent years, but their potential health risk and neurotoxic potentials remain poorly understood. In this study, neurotoxic potential of GO and its underlying molecular and cellular mechanism were investigated using the nematode, Caenorhabditis elegans. Deposition of GO in the head region and increased reactive oxygen species (ROS) was observed in C. elegans after exposure to GO. The neurotoxic potential of GO was then investigated, focusing on neurotransmitters contents and neuronal activity using AFD sensory neurons. The contents of all neurotransmitters, such as, tyrosine, tryptophan, dopamine, tyramine, and GABA, decreased significantly by GO exposure. Decreased fluorescence of Pgcy-8:GFP, a marker of AFD sensory neuron, by GO exposure suggested GO could cause neuronal damage on AFD neuron. GO exposure led decreased expression of ttx-1 and ceh-14, genes required for the function of AFD neurons also confirmed possible detrimental effect of GO to AFD neuron. To understand physiological meaning of AFD neuronal damage by GO exposure, locomotive behavior was then investigated in wild-type as well as in loss-of-function mutants of ttx-1 and ceh-14. GO exposure significantly altered locomotor behavior markers, such as, speed, acceleration, stop time, etc., in wild-type C. elegans, which were mostly rescued in AFD neuron mutants. The present study suggested the GO possesses neurotoxic potential, especially on neurotransmitters and AFD neuron in C. elegans. These findings provide useful information to understand the neurotoxic potential of GO and other graphene-based nanomaterials, which will guide their safe application.

Caenorhabditis elegans, a Biological Model for Research in Toxicology

Caenorhabditis elegans is a nematode of microscopic size which, due to its biological characteristics, has been used since the 1970s as a model for research in molecular biology, medicine, pharmacology, and toxicology. It was the first animal whose genome was completely sequenced and has played a key role in the understanding of apoptosis and RNA interference. The transparency of its body, short lifespan, ability to self-fertilize and ease of culture are advantages that make it ideal as a model in toxicology. Due to the fact that some of its biochemical pathways are similar to those of humans, it has been employed in research in several fields. C. elegans' use as a biological model in environmental toxicological assessments allows the determination of multiple endpoints. Some of these utilize the effects on the biological functions of the nematode and others use molecular markers. Endpoints such as lethality, growth, reproduction, and locomotion are the most studied, and usually employ the wild type Bristol N2 strain. Other endpoints use reporter genes, such as green fluorescence protein, driven by regulatory sequences from other genes related to different mechanisms of toxicity, such as heat shock, oxidative stress, CYP system, and metallothioneins among others, allowing the study of gene expression in a manner both rapid and easy. These transgenic strains of C. elegans represent a powerful tool to assess toxicity pathways for mixtures and environmental samples, and their numbers are growing in diversity and selectivity. However, other molecular biology techniques, including DNA microarrays and MicroRNAs have been explored to assess the effects of different toxicants and samples. C. elegans has allowed the assessment of neurotoxic effects for heavy metals and pesticides, among those more frequently studied, as the nematode has a very well defined nervous system. More recently, nanoparticles are emergent pollutants whose toxicity can be explored using this nematode. Overall, almost every type of known toxicant has been tested with this animal model. In the near future, the available knowledge on the life cycle of C. elegans should allow more studies on reproduction and transgenerational toxicity for newly developed chemicals and materials, facilitating their introduction in the market. The great diversity of endpoints and possibilities of this animal makes it an easy first-choice for rapid toxicity screening or to detail signaling pathways involved in mechanisms of toxicity.

Vitamin E ameliorates neurodegeneration related phenotypes caused by neurotoxicity of Al2O3-nanoparticles in C. elegans

Vitamin E has the potential to ameliorate the neurotoxicity of Al₂O₃-nanoparticles that induce neurodegeneration related phenotypes inC. elegans.

Quantum dots exposure alters both development and function of D-type GABAergic motor neurons in nematode Caenorhabditis elegans

Both translocation into targeted neurons and developmental and functional alterations in targeted neurons contribute to CdTe QDs neurotoxicity.

Metal and complementary molecular bioimaging in Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia in the elderly, affecting over 27 million people worldwide. AD represents a complex neurological disorder which is best understood as the consequence of a number of interconnected genetic and lifestyle variables, which culminate in multiple changes to brain structure and function. These can be observed on a gross anatomical level in brain atrophy, microscopically in extracellular amyloid plaque and neurofibrillary tangle formation, and at a functional level as alterations of metabolic activity. At a molecular level, metal dyshomeostasis is frequently observed in AD due to anomalous binding of metals such as Iron (Fe), Copper (Cu), and Zinc (Zn), or impaired regulation of redox-active metals which can induce the formation of cytotoxic reactive oxygen species and neuronal damage. Metal chelators have been administered therapeutically in transgenic mice models for AD and in clinical human AD studies, with positive outcomes. As a result, neuroimaging of metals in a variety of intact brain cells and tissues is emerging as an important tool for increasing our understanding of the role of metal dysregulation in AD. Several imaging techniques have been used to study the cerebral metallo-architecture in biological specimens to obtain spatially resolved data on chemical elements present in a sample. Hyperspectral techniques, such as particle-induced X-ray emission (PIXE), energy dispersive X-ray spectroscopy (EDS), X-ray fluorescence microscopy (XFM), synchrotron X-ray fluorescence (SXRF), secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled mass spectrometry (LA-ICPMS) can reveal relative intensities and even semi-quantitative concentrations of a large set of elements with differing spatial resolution and detection sensitivities. Other mass spectrometric and spectroscopy imaging techniques such as laser ablation electrospray ionization mass spectrometry (LA ESI-MS), MALDI imaging mass spectrometry (MALDI-IMS), and Fourier transform infrared spectroscopy (FTIR) can be used to correlate changes in elemental distribution with the underlying pathology in AD brain specimens. Taken together, these techniques provide new techniques to probe the pathobiology of AD and pave the way for identifying new therapeutic targets. The current review aims to discuss the advantages and challenges of using these emerging elemental and molecular imaging techniques, and highlight clinical achievements in AD research using bioimaging techniques.

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.

Phthalates induce neurotoxicity affecting locomotor and thermotactic behaviors and AFD neurons through oxidative stress in Caenorhabditis elegans

Background

Phthalate esters are ubiquitous environmental contaminants and numerous organisms are thus exposed to various levels of phthalates in their natural habitat. Considering the critical, but limited, research on human neurobehavioral outcomes in association with phthalates exposure, we used the nematode Caenorhabditis elegans as an in vivo model to evaluate phthalates-induced neurotoxicity and the possible associated mechanisms.

Principal Findings

Exposure to phthalates (DEHP, DBP, and DIBP) at the examined concentrations induced behavioral defects, including changes in body bending, head thrashing, reversal frequency, and thermotaxis in C. elegans. Moreover, phthalates (DEHP, DBP, and DIBP) exposure caused toxicity, affecting the relative sizes of cell body fluorescent puncta, and relative intensities of cell bodies in AFD neurons. The mRNA levels of the majority of the genes (TTX-1, TAX-2, TAX-4, and CEH-14) that are required for the differentiation and function of AFD neurons were decreased upon DEHP exposure. Furthermore, phthalates (DEHP, DBP, and DIBP) exposure at the examined concentrations produced elevated intracellular reactive oxygen species (ROS) in C. elegans. Finally, pretreatment with the antioxidant ascorbic acid significantly lowered the intracellular ROS level, ameliorated the locomotor and thermotactic behavior defects, and protected the damage of AFD neurons by DEHP exposure.

Conclusions

Our study suggests that oxidative stress plays a critical role in the phthalate esters-induced neurotoxic effects in C. elegans.

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.

Contributions of altered permeability of intestinal barrier and defecation behavior to toxicity formation from graphene oxide in nematode Caenorhabditis elegans

Graphene oxide (GO) has been extensively studied for potential biomedical applications. Meanwhile, potential GO toxicity arises in both biomedical applications and non-biomedical products where environmental exposures may occur. In the present study, we examined the potential adverse effects of GO and the underlying mechanism using nematode Caenorhabditis elegans as the assay system. We compared the in vivo effects of GO between acute exposure and prolonged exposure, and found that prolonged exposure to 0.5-100 mg L(-1) of GO caused damage on functions of both primary (intestine) and secondary (neuron and reproductive organ) targeted organs. In the intestine, ROS production was significantly correlated with the formation of adverse effects on functions of both primary and secondary targeted organs. GO could be translocated into intestinal cells with loss of microvilli, and distributed to be adjacent to or surrounding mitochondria. Prolonged exposure to GO resulted in a hyper-permeable state of the intestinal barrier, an increase in mean defecation cycle length, and alteration of genes required for intestinal development and defecation behavior. Thus, our data suggest that prolonged exposure to GO may cause potential risk to environmental organisms after release into the environment. GO toxicity may be due to the combinational effects of oxidative stress in the intestinal barrier, enhanced permeability of the biological barrier, and suppressed defecation behavior in C. elegans.

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.

Protective efficacy of selenite against lead-induced neurotoxicity in Caenorhabditis elegans

Background

Selenium is an essential micronutrient that has a narrow exposure window between its beneficial and toxic effects. This study investigated the protective potential of selenite (IV) against lead (Pb(II))-induced neurotoxicity in Caenorhabditis elegans.

Principal Findings

The results showed that Se(IV) (0.01 µM) pretreatment ameliorated the decline of locomotion behaviors (frequencies of body bends, head thrashes, and reversal ) of C. elegans that are damaged by Pb(II) (100 µM) exposure. The intracellular ROS level of C. elegans induced by Pb(II) exposure was significantly lowered by Se(IV) supplementation prior to Pb(II) exposure. Finally, Se(IV) protects AFD sensory neurons from Pb(II)-induced toxicity.

Conclusions

Our study suggests that Se(IV) has protective activities against Pb(II)-induced neurotoxicity through its antioxidant property.

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.

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.

Pre-treatment with mild UV irradiation increases the resistance of nematode Caenorhabditis elegans to toxicity on locomotion behaviors from metal exposure

UV irradiation at 10J/m(2)/min induced a mild toxicity on locomotion behaviors and stress response in Caenorhabditis elegans. Pre-treatment with UV irradiation at 10J/m(2)/min at L2-larva stage prevented the formation of locomotion behavioral defects, and activated a noticeable reduction of stress response and oxidative damage in 50 and 100μM metal (Hg, Pb, and Cr) exposed nematodes. Pre-treatment with UV irradiation at 20J/m(2)/min caused a significant decrease of locomotion behaviors in metal exposed nematodes, and pre-treatment with mild UV irradiation could not prevent the formation of locomotion behavioral defects in 200μM metal exposed nematodes. Moreover, the adaptive response to toxicity on locomotion behaviors induced by metal exposure was not formed in mev-1 mutants. Therefore, pre-treatment to mild UV irradiation activates the cross-adaptation response to toxicity on locomotion behaviors induced by metal exposure, and this kind of adaptive response may be under the control of MEV-1 function.

Pre-treatment with mild metal exposure suppresses the neurotoxicity on locomotion behavior induced by the subsequent severe metal exposure in Caenorhabditis elegans

Adaptive response to neurotoxicity on locomotion behavior by severe metal exposure was investigated in Caenorhabditis elegans. Exposure to 2.5μM of metals induced a moderate but significant reduction of locomotion behavior and induction of hsp-16.2::gfp expression. After pre-exposure to 2.5μM of metals, the reduced locomotion behavior induced by subsequent 50 and 100μM of metal exposure were significantly prevented, and the induction of hsp-16.2::gfp expression caused by subsequent 50 and 100μM of metal exposure were significantly suppressed. In contrast, after pre-exposure to 50μM examined metals, the reduced locomotion behavior induced by subsequent 50 and 100μM metal exposure were further decreased, and the noticeable induction of hsp-16.2::gfp expression caused by subsequent severe metal exposure were further enhanced. Therefore, pre-treatment with mild metal exposure can activate the adaptive response to neurotoxicity on locomotion behavior induced by subsequent severe metal exposure in nematodes.

Lethality toxicities induced by metal exposure during development in nematode Caenorhabditis elegans

Lethality changes were investigated during development in 4 h metal exposed Caenorhabditis elegans. Exposure to examined metals caused severe lethality toxicities in L1- and L2-larvae, in L3-larvae exposed to examined metals at concentrations of 50 and 100 microM and to Pb, Hg, and Cr at the concentration of 2.5 microM, in L4-larvae exposed to examined metals at concentrations of 50 and 100 microM, and in adults exposed to Pb, Hg, and Cr at the concentration of 100 microM. Moreover, the lethality toxicities induced by Pb and Hg in L1 larvae for 4 h could be largely comparable to those in young adults for 24 h.