Neural alterations from lead exposure in zebrafish

1H NMR-metabolomics studies on acute toxicity effect of lead in adult zebrafish (Danio rerio) model

Zebrafish (Danio rerio) is ideal for studying the effects of toxins like lead or plumbum (Pb) which persist in the environment and harm body systems when absorbed. Increasing Pb concentration could result in a higher mortality rate and alteration of behavior and metabolism. The present study evaluates the acute toxicity effect of Pb on metabolome and behavior in adult zebrafish. The zebrafish were exposed to various Pb concentrations ranging from 0 to 30 mg/L for different periods (24, 48, and 72 h) before the fish samples were subjected to Nuclear Magnetic Resonance (NMR)-multivariate data analysis (MVDA) with additional support from behavioral assessment. The behavior of zebrafish was significantly altered after Pb inducement and the differential metabolites increased in low (5 mg/L) while decreased in high (10 mg/L) Pb concentrations. An ideal Pb induction could be achieved by 5 mg/L concentration in 24 h, which induced significant metabolite changes without irreversible damage. Continuing research on the effects of lead toxicity is crucial to develop effective prevention and treatment strategies.

Neurotoxicity of Combined Exposure to the Heavy Metals (Pb and As) in Zebrafish (Danio rerio)

Lead (Pb) and arsenic (As) are commonly occurring heavy metals in the environment and produce detrimental impacts on the central nervous system. Although they have both been indicated to exhibit neurotoxic properties, it is not known if they have joint effects, and their mechanisms of action are likewise unknown. In this study, zebrafish were exposed to different concentrations of Pb (40 μg/L, 4 mg/L), As (32 μg/L, 3.2 mg/L) and their combinations (40 μg/L + 32 μg/L, 4 mg/L + 3.2 mg/L) for 30 days. The histopathological analyses showed significant brain damage characterized by glial scar formation and ventricular enlargement in all exposed groups. In addition, either Pb or As staining inhibited the swimming speed of zebrafish, which was enhanced by their high concentrations in a mixture. To elucidate the underlying mechanisms, we examined changes in acetylcholinesterase (AChE) activity, neurotransmitter (dopamine, 5-hydroxytryptamine) levels, HPI axis-related hormone (cortisol and epinephrine) contents and neurodevelopment-related gene expression in zebrafish brain. The observations suggest that combined exposure to Pb and As can cause abnormalities in swimming behavior and ultimately exacerbate neurotoxicity in zebrafish by interfering with the cholinergic system, dopamine and 5-hydroxytryptamine signaling, HPI axis function as well as neuronal development. This study provides an important theoretical basis for the mixed exposure of heavy metals and their toxicity to aquatic organisms.

Effects of continuous and pulse lead exposure on the swimming behavior of tadpoles revealed by brain-gut axis analysis

Lead (Pb) is present in aquatic environments with a continuous or pulse form due to the regular or irregular discharge of wastewater. These two modes of exposure result in different toxicological effects on aquatic animals. To compare the effects of Pb exposure mode on the swimming behavior of amphibian larvae, this study proposed a combination method to examine the brain-gut axis (gut bacteria, histopathology, metabolomics, and ethology) in order to evaluate the ecotoxic differences in Pelophylax nigromaculatus tadpoles (Gs 21-28) when exposed to continuous (CE100) versus pulse exposure (PE100) of environmental concentrations of Pb (100 μg/L). The results showed that: 1) CE100 significantly decreased the movement distance and swimming activity of the tadpoles compared to PE100 and the control, while there were no significant differences between the control group and PE100. 2) At the phyla level, compared to PE100, CE100 treatment significantly decreased the abundance of Actinobacteria, Firmicutes, Proteobacteria, and Bacteroidetes and increased the abundance of Fusobacteria in the gut. At the genus level, compared to PE100, CE100 significantly increased the abundance of U114 and decreased the abundance of Anaerorhabdus, Exiguobacterium and Microbacterium. 3) Compared to PE100, CE100 changed the metabolites of the brain-gut axis pathway, such as quinolinic acid, L-valine, L-dopa, L-histidine, urocanic acid, L-threonine, γ-aminobutyric acid (GABA), L-glutamate (Glu), acetylcholine (Ach), L-tyrosine (Tyr), L-tryptophan (Trp), and levodopa (DOPA). 4) CE100 and PE100 played a repressive role in the histidine metabolism and tyrosine metabolism pathways and played a promoting role in the purine metabolism and pyrimidine metabolism pathways. This study provides a method for evaluating the toxic effects of heavy metal exposure via two different exposure modes (pulse versus continuous) which tadpoles may encounter in the natural environment from a combined study examining the brain-gut axis.

Development of a rapid zebrafish model for lead poisoning research and drugs screening

Lead (Pb) contamination is a worldwide public health threaten. Besides close restraint of lead exposure, it's emergency to discover compounds that could help to cue toxicities caused by lead. Zebrafish embryos and early larvae can serve as valuable screening tools in early pre-clinical phase of drug screening and research. In order to establish a zebrafish lead poisoning model that could be used for drug screening and research, zebrafish embryos at 6 h post-fertilization (hpf) were treated with lead at different concentrations by soaking intermittently, raised in lead work solution at night while in fish water during the day. After treated for 90 h, death and severe trunk curvature were observed on zebrafish in 640 μM group, obvious dysplasia, blood toxicity, excessive reactive oxygen species (ROS), severe neurotoxicity, such as shorter length of peripheral motor neurons, neuronal apoptosis, and axonal injury, and neurobehavior impairment were induced by lead at 80, 160 and 320 μM, similar to phenotypes reported in rodent. Moreover, the mRNA level of genes related to neurodevelopment, memory, and antioxidation were significantly down regulated, and apoptosis-related genes were up regulated, consistent to zebrafish phenotypic change. Finally, zebrafish were intermittently exposed to 80 μM lead solution to establish the lead poisoning model, and the efficacy of a safe chelating agent Meso-2,3-Dimercaptosuccinic acid (DMSA) was tested at a series of concentrations to validate the zebrafish model. The result showed concentration-dependent decrease of lead content in zebrafish in DMSA treated groups compared with model group. The above data fully demonstrated a zebrafish model of lead poisoning suitable for drug screening was successfully developed, which was expected to provide a rapid and economic tools for discovering antidotes of lead and drugs of neuroprotection.

The role of exposure window and dose in determining lead toxicity in developing Zebrafish

Heavy metal contamination is recognized worldwide as a serious threat to human health and wildlife, and reducing their emissions is a priority of international and EU actions. Due to its persistence, high bioaccumulation tendency, and toxicity properties, lead (Pb) is one of the heavy metals of greatest concern. Even at low concentrations, lead induces various clinical and subclinical conditions in both humans and animals, and it has been included in the priority list of hazardous substances. In the present study, we used zebrafish's early stages as a model, given their well-acknowledged predictive value in the risk assessment of chemicals. This study was designed to investigate the morphological and morphometric alterations induced by Pb during zebrafish's early development and disclose the putative effects stage- and/or dose-dependent. We examined injuries induced by two environmentally relevant and extremely low concentrations of Pb (2.5 μg/L and 5 μg/L) during two exposure windows: early (between 1 and 7 dpf) and late (between 2 and 8 dpf). We clearly demonstrated that the incidence and severity of morphological abnormalities increased with increasing Pb dose and exposure time in both early and late-exposed groups. Furthermore, we revealed that malformation severity was significantly higher in the early exposed group than in the late exposure group at all exposure times and for both tested doses, thus highlighting the high sensitivity of zebrafish during the initial stages of development. The information presented in this paper emphasizes the effectiveness of morphological biomarkers in unveiling threatening situations and supports the role of zebrafish embryos and larvae in risk assessment and environmental monitoring.

Hindbrain defects induced by Di-butyl phthalate (DBP) in developing zebrafish embryos

Di-butyl phthalate (DBP) is a globally used plasticizer found in alarmingly high concentrations in soil and water ecosystems. As phthalates are non-covalently bound to plastic polymers, phthalates easily leach into the aquatic environment. The effects of DBP on aquatic organisms is concerning, most notably, studies have focused on the endocrine-disrupting effects. However, reports on the developmental neurotoxicity of DBP are rare. Using the zebrafish vertebrate model system, we treated pre-gastrulation staged embryos with 2.5 μM DBP, a concentration environmentally noted. We find that general hindbrain structure and rhombomere patterning is disrupted at 72 h post fertilization (hpf). We investigated hindbrain specific neural patterning of cranial motor neurons and find defects in branchiomotor neuron patterning and migration. Furthermore, defects in r4 specific Mauthner neuron development were also noted. Thus, we conclude that DBP exposure during embryonic development induces defects to the hindbrain and concomitantly the neurons that are born and differentiate there.

Vitamin C Mitigates Oxidative Stress and Behavioral Impairments Induced by Deltamethrin and Lead Toxicity in Zebrafish

Environmental contamination from toxic metals and pesticides is an issue of great concern due to their harmful effects to human health and the ecosystems. In this framework, we assessed the adverse effects when aquatic organisms are exposed to toxicants such as deltamethrin (DM) and lead (Pb), alone or in combination, using zebrafish as a model. Moreover, we likewise evaluated the possible protective effect of vitamin C (VC) supplementation against the combined acute toxic effects of the two toxicants. Juvenile zebrafish were exposed to DM (2 μg L^-1) and Pb (60 μg L^-1) alone and in combination with VC (100 μg L^-1) and responses were assessed by quantifying acetylcholinesterase (AChE) activity, lipid peroxidation (MDA), some antioxidant enzyme activities (SOD and GPx), three-dimension locomotion responses and changes of elements concentrations in the zebrafish body. Our results show that VC has mitigative effects against behavioral and biochemical alterations induced by a mixture of contaminants, demonstrating that it can be used as an effective antioxidant. Moreover, the observations in the study demonstrate zebrafish as a promising in vivo model for assessing the neuroprotective actions of bioactive compounds.

In vivo assessment of respiratory burst inhibition by xenobiotic exposure using larval zebrafish

Currently, assessment of the potential immunotoxicity of a given agent involves a tiered approach for hazard identification and mechanistic studies, including observational studies, evaluation of immune function, and measurement of susceptibility to infectious and neoplastic diseases. These studies generally use costly low-throughput mammalian models. Zebrafish, however, offer an excellent alternative due to their rapid development, ease of maintenance, and homology to mammalian immune system function and development. Larval zebrafish also are a convenient model to study the innate immune system with no interference from the adaptive immune system. In this study, a respiratory burst assay (RBA) was utilized to measure reactive oxygen species (ROS) production after developmental xenobiotic exposure. Embryos were exposed to non-teratogenic doses of chemicals and at 96 h post-fertilization, the ability to produce ROS was measured. Using the RBA, 12 compounds with varying immune-suppressive properties were screened. Seven compounds neither suppressed nor enhanced the respiratory burst; five reproducibly suppressed global ROS production, but with varying potencies: benzo[a]pyrene, 17β-estradiol, lead acetate, methoxychlor, and phenanthrene. These five compounds have all previously been reported as immunosuppressive in mammalian innate immunity assays. To evaluate whether the suppression of ROS by these compounds was a result of decreased immune cell numbers, flow cytometry with transgenic zebrafish larvae was used to count the numbers of neutrophils and macrophages after chemical exposure. With this assay, benzo[a]pyrene was found to be the only chemical that induced a change in the number of immune cells by increasing macrophage but not neutrophil numbers. Taken together, this work demonstrates the utility of zebrafish larvae as a vertebrate model for identifying compounds that impact innate immune function at non-teratogenic levels and validates measuring ROS production and phagocyte numbers as metrics for monitoring how xenobiotic exposure alters the innate immune system.

Effects of thermal stress-induced lead (Pb) toxicity on apoptotic cell death, inflammatory response, oxidative defense, and DNA methylation in zebrafish (Danio rerio) embryos

Lead (Pb) is a toxic environmental pollutant that is frequently present in effluents from urban, mining, and industrial sources. The combinatorial effects of heavy metal exposure and temperature in aquatic organisms have received considerable attention as heat stress occurs simultaneously in conjunction with several contaminants in a natural environment. In this study, we examined the potential effects of Pb exposure in conditions of thermal stress (34 °C) in zebrafish (Danio rerio) embryos. Thermal stress at 34 °C induced a dramatic decrease in the survival rate, although exposure to Pb at 26 °C decreased the survival rate of the embryos. Malformations, such as the curved body shape, were increased in response to exposure to a combination of Pb and heat stress. The combination of Pb and heat stress also caused a decrease in the heart rate. Moreover, Pb and high-temperature exposure induced the upregulation of SOD, CAT, TNF-α, IL-1β, p53, and BAX transcripts, and downregulation of Dnmt1 and Dnmt3b transcripts. Thermal stress enhanced transcriptional responses of eight indicator genes following Pb toxicity. The induction of cell death in response to combined exposures was also confirmed in the body of zebrafish by fluorescence intensity image analysis. These data indicated that thermal stress enhanced the poisonous effects of Pb exposure on antioxidant defense, inflammation, and apoptotic mechanisms. Transcriptional inhibition of DNA methylation-related genes might serve as a crucial factor contributing to the possibility of epigenetic adaptation by altering combined stress. We suggest that a careful evaluation of the potential effects of climate change (especially temperature) should be considered when investigating the toxic levels of metal pollution, such as Pb, in an aquatic environment.

Chronic Exposure to Low Concentration Lead Chloride-Induced Anxiety and Loss of Aggression and Memory in Zebrafish

Lead and lead-derived compounds have been extensively utilized in industry, and their chronic toxicity towards aquatic animals has not been thoroughly addressed at a behavioral level. In this study, we assessed the risk of exposure to lead at a waterborne environmental concentration in adult zebrafish by behavioral and biochemical analyses. Nine tests, including three-dimension (3D) locomotion, novel tank exploration, mirror biting, predator avoidance, social interaction, shoaling, circadian rhythm locomotor activity, color preference, and a short-term memory test, were performed to assess the behavior of adult zebrafish after the exposure to 50 ppb PbCl₂ for one month. The brain tissues were dissected and subjected to biochemical assays to measure the relative expression of stress biomarkers and neurotransmitters to elucidate the underlying mechanisms for behavioral alterations. The results of the behavioral tests showed that chronic exposure to lead could elevate the stress and anxiety levels characterized by elevated freezing and reduced exploratory behaviors. The chronic exposure to PbCl₂ at a low concentration also induced a sharp reduction of aggressiveness and short-term memory. However, no significant change was found in predator avoidance, social interaction, shoaling, or color preference. The biochemical assays showed elevated cortisol and reduced serotonin and melatonin levels in the brain, thus, altering the behavior of the PbCl₂-exposed zebrafish. In general, this study determined the potential ecotoxicity of long-term lead exposure in adult zebrafish through multiple behavioral assessments. The significant findings were that even at a low concentration, long-term exposure to lead could impair the memory and cause a decrease in the aggressiveness and exploratory activities of zebrafish, which may reduce their survival fitness.

Zebrafish behavioral phenomics applied for phenotyping aquatic neurotoxicity induced by lead contaminants of environmentally relevant level

Environmental lead (Pb) exposure is a worldwide threat due to the ubiquitous contamination. Although the adverse effects of Pb on human health have previously been extensively explored, the eco-toxicological effects on aquatic vertebrates still need further investigation. In addition, there is a paucity in the knowledge of behavioral and physiological effects of Pb within the range of environmental relevant concentration (under 100 μg/L) on aquatic organisms such as zebrafish. Herein, we demonstrated that adult male zebrafish (Danio rerio) exposed to Pb at environmental concentration level (1 μg/L, 10 μg/L and 100 μg/L) for 14 days, exhibited obvious neuro-behavioral alteration including disturbed light dark preference, impaired exploratory behaviors and inhibited spatial working memory. The alteration of entire behavioral profiles was further associated with the disturbed expression patterns of mRNA level of key genes involved in neurodevelopment (gap43, syn2a, th, dat, and drd1b), neurotoxic effects (c-fos and gfap), and stress responses (tap, mt1, hsp70, and hsp90). To determine the comprehensively effect of aquatic contaminants on the entire behavioral profiles, behavioral phenomic data were obtained by hierarchical clustering analysis. Overall, we employed behavioral phenomics methods to find that Pb within standard chronic Pb toxic criteria, significantly altered behavioral phenotype and brain physiology, which would exert profound ecological consequences and offer the reference for adjustment of aquatic toxic criteria.

Synergistic effects of Pb and repeated heat pulse on developmental neurotoxicity in zebrafish

Pollutant discharges to the aquatic environment often contain multiple environmental stressors, affecting aquatic organisms. To mimic the discharges from nuclear and industry facilities, the combined effects of two independent types of stressors, heavy metal Pb and repeated heat pulse were addressed in this study. We investigated the developmental toxicity of combined treatment, especially its toxic effects on zebrafish neurodevelopment. The normal embryos at 4 hpf were exposed to 0.2 mM of Pb dissolved in the bathing medium with different temperatures (30, 32, and 34 °C) and then maintained in an incubator at 28 °C. After performing above treatment once every 24 h for 6 days, we found that combined treatment significantly affected neural development, including loss of dopaminergic (DA) neurons and brain vasculature, disruption of locomotor activity and neurodevelopmental genes expression in a temperature-dependent manner as compared to the Pb alone exposure group, indicating that repeated heat pulse enhances these negative impacts induced by Pb. In contrast, no apparent toxicity was observed in repeated heat pulse alone groups, suggesting that Pb treatment reduces thermal tolerance in zebrafish, which emphasized the importance to evaluate synergistic effects of Pb and repeated heat pulse. Moreover, repeated heat pulse aggravated Pb-induced apoptosis in the zebrafish brain. Further study of the underlying mechanism suggested that Caspase 3 regulated apoptosis was involved in this process. Taken together, our findings shed light on the full understanding of toxic effects of discharges from industrial applications on living organisms and its environmental impact.

Effects of exposure to multiple heavy metals on biochemical and histopathological alterations in common carp, Cyprinus carpio L

Heavy metals are frequently encountered as mixtures of essential and non-essential elements. Therefore, evaluation of their toxic effects individually does not offer a realistic estimate of their impact on biological processes. We studied effects of exposure to mixtures of essential and toxic metals (Cr, Cd and Pb) on biochemical, immunotoxicity level and morphological characteristics of the various tissues of a biomarker freshwater fish common carp using environmentally relevant concentrations. Fish were exposed to metal mixture through tank water for 7, 15 and 30 days, under controlled laboratory conditions. Tissue accumulation of the metals was measured using Atomic Absorption Spectrophotometric techniques. Chromium, cadmium and lead accumulation in muscle, gills, liver, kidney and intestine, tissue of common carp exposed to mixture metals for 30 days increased significant compared with control group (p < 0.001). However, the activity of antioxidant enzymes such as catalase (CAT) and superoxide dismutase (SOD) levels was significant altered in various tissues of exposed fish. Besides, the lipid peroxidation (LPO) was significant (p < 0.001) increased. Moreover, the tumor necrosis factor - α (TNF-α), interleukin (IL-6), and interferon-γ (IFN-γ) contents in tissues of muscle, gills, liver, kidney and intestine were increased significant compared with control fish (p < 0.001). In addition, microscopic examination of the main alterations in general morphology of fish gills included spiking and fusion of secondary lamellae, formation of club-shaped filaments epithelium in the interlamellar regions and hepatocytes showed damage of central vein and rupture of irregular hepatic plate with more number of vacuoles in the fish exposed to metal mixture for a longer duration (30 days). These results of this study clearly demonstrate that concentration individual and mixtures of metals in aquatic systems will greatly influence the cytokine alterations may result in an immune suppression or excessive activation in the treated common carp as well as may cause immune dysfunction or reduced immunity. In conclusion, toxicity of multiple metal mixtures of Cr, Cd and Pb has antioxidant and immunotoxic effects on C. carpio.

Altered RNA metabolism due to a homozygous RBM7 mutation in a patient with spinal motor neuropathy

The exosome complex is the most important RNA processing machinery within the cell. Mutations in its subunits EXOSC8 and EXOSC3 cause pontocerebellar hypoplasia, spinal muscular atrophy (SMA) and central nervous system demyelination. We present a patient with SMA-like phenotype carrying a homozygous mutation in RBM7-a subunit of the nuclear exosome targeting (NEXT) complex-which is known to bind and carry specific subtypes of coding and non-coding RNAs to the exosome. The NEXT complex with other protein complexes is responsible for the substrate specificity of the exosome. We performed RNA-sequencing (RNA-seq) analysis on primary fibroblasts of patients with mutations in EXOSC8 and RBM7 and gene knock-down experiments using zebrafish as a model system. RNA-seq analysis identified significantly altered expression of 62 transcripts shared by the two patient cell lines. Knock-down of rbm7, exosc8 and exosc3 in zebrafish showed a common pattern of defects in motor neurons and cerebellum. Our data indicate that impaired RNA metabolism may underlie the clinical phenotype by fine tuning gene expression which is essential for correct neuronal differentiation.

Evaluation of the developmental toxicity of lead in the Danio rerio body

Lead has been utilized throughout history and is widely distributed and mobilized globally. Although lead in the environment has been somewhat mitigated, the nature of lead and its extensive uses in the past prohibit it from being completely absent from our environment and exposure to lead is still a public health concern. Most studies regarding lead toxicity have focused on the brain. However, little is found in the literature on the effects of lead in other tissues. Here, we utilize the zebrafish model system to investigate effects of lead exposure during early developmental time windows at 24, 48 and 72 h post fertilization in the body. We analyze whole body, notochord and somatic muscle changes, vascular changes of the body, as well as motor neuron alterations. We find lead exposure induces a curved body phenotype with concomitant changes in somite length, decreased notochord staining and abnormal muscle staining using live and in situ approaches. Furthermore, altered vasculature within the somatic regions, loss and/or alternations of motor neuron extension both dorsally and ventrally from the spinal cord, loss of Rohon-Beard sensory neurons, and increased areas of apoptosis were found. We conclude that lead is developmentally toxic to other areas of the developing embryo, not just the brain.