Role of neurexin2a in lead-induced locomotor defect in developing zebrafish

Lead specifically declines tyrosine hydroxylase activity to induce the onset of Parkinson's disease through disrupting dopamine biosynthesis in fly models

Parkinson's disease (PD) is one of the fastest-growing neurodegenerative diseases and has been linked to the exposure to numerous environmental neurotoxins. Although lead (Pb) exposure has been related to the development of PD, the molecular target of Pb to cause the onset of PD is insufficiently investigated. Herein, we explored the effects of Pb exposure on behavior, pathophysiology, and gene expression of wild-type (WT) fly (Drosophila melanogaster) by comparison with its PD model. After exposure to Pb, the WT flies showed PD-like locomotor impairments and selective loss of dopaminergic (DAergic) neurons, displaying similar phenotypes to fly PD model (PINK1). Transcriptomic analysis showed the similarity in gene expression profiles between Pb treatment WT flies and PINK1 mutant flies. Moreover, Pb exposure resulted in endogenous dopamine deficits in WT flies. Analyses of gene expression and enzyme activity confirmed that Pb exposure reduced tyrosine hydroxylase (TH) activity and led to failure of dopamine synthesis. Furthermore, molecular dynamics simulation confirmed that Pb was adsorbed by TH and subsequently inhibited the enzymatic activity. Exogenous injection of L-dopa and melatonin could partially rescue the pathological phenotypes of Pb-exposed flies and PD fly model. Antagonist injection of microRNA-133, which negatively regulated the expression of TH gene, ultimately rescued in the manifestation of PD phenotypes in flies. Involvement of TH overexpression mutants of fly strongly promoted the resistance to Pb exposure and rescued both behavior and the number of DAergic neurons. Therefore, our study elucidates the Pb molecular target in dopamine pathway and mechanism underlying the risks of Pb exposure on the occurrence of PD at environmentally-relevant concentrations.

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.

Dynamic alterations of locomotor activity and the microbiota in zebrafish larvae with low concentrations of lead exposure

Lead (Pb) is a ubiquitous heavy metal associated with developmental and behavioral disorders. The establishment of pioneer microbiota overlaps with the development of the brain during early life, and Pb-induced developmental neurotoxicity may be partially caused by early-life microbiota dysbiosis. This study investigated the locomotor activity and the microbiota in developing zebrafish at multiple developmental time points (five days post fertilization [5 dpf], 6 dpf, and 7 dpf) under exposure to low concentrations of lead (0.05 mg/L). Time-dependent reductions in the number of activities and the average movement distance of larvae compared to the control were observed following Pb exposure. Furthermore, Pb exposure significantly altered the composition of the gut microbiota of zebrafish larvae. At the phylum level, the abundance of Proteobacteria decreased from 5 to 7 dpf, while that of Actinobacteria increased in the control groups. At the class level, the proportion of Alphaproteobacteria decreased, while that of Actinobacteria increased in the control groups. Notably, all showed the opposite trend in Pb groups. A correlation analysis between indices of locomotor activity and microbial communities revealed genus-level features that were clearly linked to the neurobehavioral performance of zebrafish. Seven genera were significantly correlated with the two performance indicators of the locomotion analysis, namely Rhodococcus, Deinococcus, Bacillus, Bosea, Bradyrhizobium, Staphylococcus, and Rhizobium. Rhizobium was dominant in zebrafish and increased in the Pb groups in a time-dependent manner. In addition, the expression levels of bdnf, trkb1, trkb2, and p75ntr changed in zebrafish from 5 to 7 dpf under Pb exposure. Collectively, these results suggest that Pb-induced neurotoxicity could potentially be treated by targeting the gut microbiota.

Combined effects of lead and manganese on locomotor activity and microbiota in zebrafish

Exposure to lead (Pb) and manganese (Mn) during early life influences neurodevelopment and increases the risk of neurodegenerative disorders. However, the level of developmental neurotoxicity due to combined exposure to the two metals remains unclear. Although the microbiota plays an essential part in the development of the nervous system via the gut-brain axis, there is a paucity of information regarding the interactions between exposure to Pb and Mn, the destruction of the microbiome, and neurodevelopmental impacts. To fill in this knowledge gap, we investigated the developmental neurotoxicity and effects on the microbiota of Pb (0.05 mg·L-1) alone and in combination with Mn (0.3 mg·L-1) in zebrafish larvae. Our results revealed that combined exposure precipitated higher malformation rates and lower locomotor activity levels than exposure to either Pb or Mn alone. Additionally, when we separated the combined exposure group from the other groups by applying unsupervised principal coordinates analysis (PCoA) and linear discriminant analysis (LEfSe) of microflora sequencing results, we observed extensive alterations in microbial abundances under combined-exposure conditions. Functional prediction analysis showed that combined exposure contributed to altered amino acid and lipid metabolism, and also that combined exposure to Pb and Mn reflected the greatest number of differentially activated biological pathways compared to the other three groups. ATP-binding cassette G (ABCG) genes and genes related to serotonin signaling and metabolism were altered following combined Pb and Mn exposure and exhibited disparate trends vis-à-vis Pb or Mn exposure alone. According to the results, the combined exposure to Pb and Mn led to more severe effects on both zebrafish locomotor activity and gut microbial composition. We suggest that the microbiota contributes to the combined neurotoxicity by increasing ABCG5 and ABCG8 gene expression.

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.

Joint toxicity of lead and cadmium on the behavior of zebrafish larvae: An antagonism

Although the individual toxicity of lead (Pb) and cadmium (Cd) was intensively studied, little is known about their joint toxicity on the development of circadian behavioral rhythm. Therefore, we co-exposed zebrafish to Pb and Cd to investigate the alterations of behavioral rhythm and the potential mechanism. Inductively coupled plasma mass spectrometry analysis was used to detect the internal exposure level of heavy metals. The behavioral rhythm was monitored by a video-track tracking system. The changes of gene expression regarding melatonin-related molecules and clock genes were analyzed by quantitative polymerase chain reaction and JTK-Cycle analysis. The results showed that the level of Pb2+ and Cd2+ accumulated in the co-exposure group were significantly lower than that in the Pb or Cd group. Exposed to Pb reduced the locomotor activity; the behavioral rhythms were disrupted by Cd, while the pattern in the co-exposure group showed an antagonistic effect on locomotor activity and behavioral rhythm. The expression rhythm of aanat1 was disturbed and the expression levels of mtnr1aa and mtnr1bb were decreased by co-exposure treatment, but mtnr1c was increased in Pb and Cd group, respectively. Exposure to Cd caused the disruption of expression rhythm in clock genes, like clock1b, clock2, and cry1b, while only the rhythm of clock2 was disrupted in the co-exposure group. The results suggest that the behavioral rhythm disruption caused by Cd exposure is associated with the disturbance of certain circadian genes, whereas Pb exposure only abates the locomotor activity; an antagonistic effect on the behavioral pattern when co-exposed zebrafish larvae to Pb and Cd.

Wild Zebrafish Sentinels: Biological Monitoring of Site Differences Using Behavior and Morphology

Environmental change poses a devastating risk to human and environmental health. Rapid assessment of water conditions is necessary for monitoring, evaluating, and addressing this global health danger. Sentinels or biological monitors can be deployed in the field using minimal resources to detect water quality changes in real time, quickly and cheaply. Zebrafish (Danio rerio) are ideal sentinels for detecting environmental changes due to their biomedical tool kit, widespread geographic distribution, and well-characterized phenotypic responses to environmental disturbances. Here, we demonstrate the utility of zebrafish sentinels by characterizing phenotypic differences in wild zebrafish between two field sites in India. Site 1 was a rural environment with flowing water, low-hypoxic conditions, minimal human-made debris, and high iron and lead concentrations. Site 2 was an urban environment with still water, hypoxic conditions, plastic pollution, and high arsenic, iron, and chromium concentrations. We found that zebrafish from Site 2 were smaller, more cohesive, and less active than Site 1 fish. We also found sexually dimorphic body shapes within the Site 2, but not the Site 1, population. Advancing zebrafish sentinel research and development will enable rapid detection, evaluation, and response to emerging global health threats.

Protective Effect of Chlorogenic Acid and Its Analogues on Lead-Induced Developmental Neurotoxicity Through Modulating Oxidative Stress and Autophagy

Lead (Pb) is among the deleterious heavy metal and has caused global health concerns due to its tendency to cause a detrimental effect on the development of the central nervous system (CNS). Despite being a serious health concern, treatment of Pb poisoning is not yet available, reflecting the pressing need for compounds that can relieve Pb-induced toxicity, especially neurotoxicity. In the quest of exploring protective strategies against Pb-induced developmental neurotoxicity, compounds from natural resources have gained increased attention. Chlorogenic acid (CGA) and its analogues neochlorogenic acid (NCGA) and cryptochlorogenic acid (CCGA) are the important phenolic compounds widely distributed in plants. Herein, utilizing zebrafish as a model organism, we modeled Pb-induced developmental neurotoxicity and investigated the protective effect of CGA, NCGA, and CCGA co-treatment. In zebrafish, Pb exposure (1,000 μg/L) for 5 days causes developmental malformation, loss of dopaminergic (DA) neurons, and brain vasculature, as well as disrupted neuron differentiation in the CNS. Additionally, Pb-treated zebrafish exhibited abnormal locomotion. Notably, co-treatment with CGA (100 µM), NCGA (100 µM), and CCGA (50 µM) alleviated these developmental malformation and neurotoxicity induced by Pb. Further underlying mechanism investigation revealed that these dietary phenolic acid compounds may ameliorate Pb-induced oxidative stress and autophagy in zebrafish, therefore protecting against Pb-induced developmental neurotoxicity. In general, our study indicates that CGA, NCGA, and CCGA could be promising agents for treating neurotoxicity induced by Pb, and CCGA shows the strongest detoxifying activity.

Perinatal Lead Exposure Alters Calsyntenin-2 and Calsyntenin-3 Expression in the Hippocampus and Causes Learning Deficits in Mice Post-weaning

Calsyntenin-2 (Clstn2) and calsyntenin-3 (Clstn3) are the members of the cadherin superfamily and function to regulate the postsynaptic activity. Both proteins are known to play an important role in memory and learning. This study was designed to test the hypothesis that exposure of mothers to Pb in drinking water may alter the expression of Clstn2 and Clstn3 in offspring, which contributes to the Pb-induced learning deficiency. Pregnant mice were exposed to Pb in drinking water as Pb acetate from gestation to weaning. At the postnatal day 21, the learning and memory ability of pups was tested by Morris water maze, and the blood and brain tissues from pups were collected for metal and protein analyses. Data showed that perinatal Pb exposure resulted in a dose-dependent increase of Pb concentrations in blood (6-20-fold), hippocampus (2-7-fold), and cerebral cortex (2-8-fold) in offspring, as compared to controls (p < 0.05).The ability of learning and memory was decreased in lead exposure group, as compared to controls (p < 0.05). Both immunofluorescence and Western blot analyses revealed a striking difference in the expression of Clstn2 vs. Clstn3 following perinatal Pb exposure. In pregnant mice exposed to 0.1%, 0.2%, and 0.5% Pb, the expression of Clstn2 in offspring showed a Pb dose-related decrease by 39.2%, 76.5%, and 96.1% in hippocampus and by12.5%, 59.4%, and 78.1% in cerebral cortex, respectively (p < 0.05). In contrast, Clstn3 expression in these offspring brain regions was significantly increased (p < 0.05), after perinatal Pb exposure. The nature of Pb differential effect on Clstn2 and Clstn3 remains unknown. These observations suggest that Clstn2 and Clstn3 may have different roles in synaptic development and differentiation. Pb-induced learning defects may partly relate to the altered expression of calsyntenin proteins.

Acute exposure to environmentally relevant lead levels induces oxidative stress and neurobehavioral alterations in larval zebrafish (Danio rerio)

The ubiquitous contamination of environmental lead (Pb) remains a worldwide threat. Improper Pb mine waste disposal from an abandoned lead-zinc mine has recently unearthed widespread Pb poisoning in children in Kabwe Zambia. Although the adverse effects of Pb on human health have begun to receive attention, the ecotoxicological effects on aquatic vertebrates still need further investigation. In addition, there is paucity in the knowledge on the behavioural and molecular subcellular responses in larval zebrafish exposed to Pb within the range of environmental relevant concentration (average 3 μg/L with maximum of 94 μg/L) on aquatic organisms such as zebrafish. The adverse effects of environmentally relevant levels of Pb on larval zebrafish was evaluated by measuring swimming behaviour under alternating dark and light conditions. Larval zebrafish acutely exposed to environmentally relevant Pb exhibited neuro-behavioural alteration including enhanced hyperactivity under light conditions evidenced by increased distanced covered and speed compared to the control. The alteration of entire behavioral profiles was further associated with the disturbed expression patterns of mRNA level of key genes associated with antioxidant (HO-1, Ucp-2 and CoxI), proapoptotic gene (TP53), and antiapoptotic gene (Bcl-2). To our knowledge, this is the first report on the effects of environmentally relevant Pb levels from Kabwe, Zambia and their adverse neurobehavioural effects and subcellular molecular oxidative responses in larval zebrafish acutely exposed within a 30 min period. The current results would be beneficial in our understanding of the effects of low Pb levels acutely discharged into an aquatic environment and the life of aquatic organisms.

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.

Comparative effects of mercury chloride and methylmercury exposure on early neurodevelopment in zebrafish larvae

Mercury (Hg) is a ubiquitous environmental toxicant with important public health implications. Hg causes neurotoxicity through astrocytes, Ca²⁺, neurotransmitters, mitochondrial damage, elevations of reactive oxygen species and post-translational modifications. However, the similarities and differences between the neurotoxic mechanisms caused by different chemical forms of Hg remain unclear. Zebrafish embryos were exposed to methylmercury (MeHgCl) or mercury chloride (HgCl₂) (0, 4, 40, 400 nM) up for 96 h. HgCl₂ exposure could significantly decrease survival rate, body length and eye size, delay the hatching period, induce tail bending and reduce the locomotor activity, and these effects were aggravated in the MeHgCl group. The compounds could increase the number of apoptotic cells in the brain and downregulate the expression of Shha, Ngn1 and Nrd, which contribute to early nervous development. The underlying mechanisms were investigated by metabolomics data. Galactose metabolism, tyrosine metabolism and starch and sucrose metabolism pathways were disturbed after HgCl₂ or MeHgCl exposure. In addition, the levels of three neurotransmitters including tyrosine, dopamine and tryptophan were reduced after HgCl₂ or MeHgCl exposure. Oxidative stress is related to metabolite changes, such as changes in the putrescine, niacinamide and uric acid contents in the HgCl₂ group, and squalene in the MeHgCl group. These data indicated that downregulation of these genes and abnormal metabolic profile and pathways contribute to the neurotoxicity of HgCl₂ and MeHgCl.

The metabolomics and neurodevelopmental endpoints were integrated to reveal that abnormal metabolic pathway and expression of Shha, Ngn1 and Nrd may contribute to neurotoxicity induced by MeHg, which was more toxic than HgCl₂ in zebrafish larvae.