Transcriptional and Behavioral Responses of Zebrafish Larvae to Microcystin-LR Exposure

Microcystin-LR in drinking water: An emerging role of mitochondrial-induced epigenetic modifications and possible mitigation strategies

Algal blooms are a serious menace to freshwater bodies all over the world. These blooms typically comprise cyanobacterial outgrowths that produce a heptapeptide toxin, Microcystin-LR (MC-LR). Chronic MC-LR exposure impairs mitochondrial-nuclear crosstalk, ROS generation, activation of DNA damage repair pathways, apoptosis, and calcium homeostasis by interfering with PC/MAPK/RTK/PI3K signaling. The discovery of the toxin's biosynthesis pathways paved the way for the development of molecular techniques for the early detection of microcystin. Phosphatase inhibition-based bioassays, high-performance liquid chromatography, and enzyme-linked immunosorbent tests have recently been employed to identify MC-LR in aquatic ecosystems. Biosensors are an exciting alternative for effective on-site analysis and field-based characterization. Here, we present a synthesis of evidence supporting MC-LR as a mitotoxicant, examine various detection methods, and propose a novel theory for the relevance of MC-LR-induced breakdown of mitochondrial machinery and its myriad biological ramifications in human health and disease.

Visual toxicity in zebrafish larvae following exposure to 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), triphenyl phosphate (TPhP), and isopropyl phenyl diphenyl phosphate (IPPP)

TPhP and IPPP, alternatives to PBDEs as flame retardants, have been studied for their developmental toxicity, but their visual toxicities are less understood. In this study, zebrafish larvae were exploited to evaluate the potential ocular impairments following exposure to BDE-47, TPhP, and IPPP. The results revealed a range of ocular abnormalities, including malformation, vascular issues within the eyes, and histopathological changes in the retina. Notably, the visually mediated behavioral changes were primarily observed in IPPP and TPhP, indicating that they caused more severe eye malformations and vision impairment than BDE-47. Molecular docking and MD simulations showed stronger binding affinity of TPhP and IPPP to RAR and RBP receptors. Elevated ROS and T3 levels induced by these compounds led to apoptosis in larvae eyes, and increased GABA levels induced by TPhP and IPPP hindered retinal repair. In summary, our results indicate TPhP and IPPP exhibit severer visual toxicity than BDE-47, affecting eye development and visually guided behaviors. The underlying mechanism involves disruptions in RA signaling, retinal neurotransmitters imbalance, thyroid hormones up-regulation, and apoptosis in larvae eyes. This work highlights novel insights into the need for cautious use of these flame retardants due to their potential biological hazards, thereby offering valuable guidance for their safer applications.

Cyanobacterial Blooms in City Parks: A Case Study Using Zebrafish Embryos for Toxicity Characterization

Cyanobacteria are photosynthetic prokaryotes that play an important role in the ecology of aquatic ecosystems. However, they can also produce toxins with negative effects on aquatic organisms, wildlife, livestock, domestic animals, and humans. With the increasing global temperatures, urban parks, renowned for their multifaceted contributions to society, have been largely affected by blooms of toxic cyanobacteria. In this work, the toxicity of two different stages of development of a cyanobacterial bloom from a city park was assessed, evaluating mortality, hatching, development, locomotion (total distance, slow and rapid movements, and path angles) and biochemical parameters (oxidative stress, neurological damage, and tissue damage indicators) in zebrafish embryos/larvae (Danio rerio). Results showed significant effects for the samples with more time of evolution at the developmental level (early hatching for low concentrations (144.90 mg/L), delayed hatching for high concentrations (significant values above 325.90 mg/L), and delayed development at all concentrations), behavioral level (hypoactivity), and biochemical level (cholinesterase (ChE)) activity reduction and interference with the oxidative stress system for both stages of evolution). This work highlights the toxic potential of cyanobacterial blooms in urban environments. In a climate change context where a higher frequency of cyanobacterial proliferation is expected, this topic should be properly addressed by competent entities to avoid deleterious effects on the biodiversity of urban parks and poisoning events of wildlife, pets and people.

Freshwater Cyanobacterial Toxins, Cyanopeptides and Neurodegenerative Diseases

Cyanobacteria produce a wide range of structurally diverse cyanotoxins and bioactive cyanopeptides in freshwater, marine, and terrestrial ecosystems. The health significance of these metabolites, which include genotoxic- and neurotoxic agents, is confirmed by continued associations between the occurrence of animal and human acute toxic events and, in the long term, by associations between cyanobacteria and neurodegenerative diseases. Major mechanisms related to the neurotoxicity of cyanobacteria compounds include (1) blocking of key proteins and channels; (2) inhibition of essential enzymes in mammalian cells such as protein phosphatases and phosphoprotein phosphatases as well as new molecular targets such as toll-like receptors 4 and 8. One of the widely discussed implicated mechanisms includes a misincorporation of cyanobacterial non-proteogenic amino acids. Recent research provides evidence that non-proteinogenic amino acid BMAA produced by cyanobacteria have multiple effects on translation process and bypasses the proof-reading ability of the aminoacyl-tRNA-synthetase. Aberrant proteins generated by non-canonical translation may be a factor in neuronal death and neurodegeneration. We hypothesize that the production of cyanopeptides and non-canonical amino acids is a more general mechanism, leading to mistranslation, affecting protein homeostasis, and targeting mitochondria in eukaryotic cells. It can be evolutionarily ancient and initially developed to control phytoplankton communities during algal blooms. Outcompeting gut symbiotic microorganisms may lead to dysbiosis, increased gut permeability, a shift in blood-brain-barrier functionality, and eventually, mitochondrial dysfunction in high-energy demanding neurons. A better understanding of the interaction between cyanopeptides metabolism and the nervous system will be crucial to target or to prevent neurodegenerative diseases.

TRK-fused gene (TFG) regulates ULK1 stability via TRAF3-mediated ubiquitination and protects macrophages from LPS-induced pyroptosis

TRK-fused gene (TFG) is known to be involved in protein secretion and plays essential roles in an antiviral innate immune response. However, its function in LPS-induced inflammation and pyroptotic cell death is still unknown. Here, we reported that TFG promotes the stabilization of Unc-51 like autophagy activating kinase (ULK1) and participates in LPS plus nigericin (Ng) induced pyroptotic cell death. Our results showed that TFG-deficient THP-1 macrophages exhibit higher mitochondrial ROS production. LPS/Ng stimulation triggers a much higher level of ROS and induces pyroptotic cell death. ULK1 undergoes a rapid turnover in TFG-deficient THP-1 cells. TFG forms complex with an E3 ligase, tumor necrosis factor receptor-associated factor 3 (TRAF3), and stabilizes ULK1 via disturbing ULK1-TRAF3 interaction. Knockdown of TFG facilitates the interaction of ULK1 with TRAF3 and subsequent K48-linked ULK1 ubiquitination and proteasome degradation. Rescue of ULK1 expression blocks LPS/Ng-induced cell death in TFG-deficient THP-1 macrophages. Taken together, TFG plays an essential role in LPS/Ng-induced pyroptotic cell death via regulating K48-linked ULK1 ubiquitination in macrophages.

BMAA and MCLR Interact to Modulate Behavior and Exacerbate Molecular Changes Related to Neurodegeneration in Larval Zebrafish

Exposure to toxins produced by cyanobacteria (ie, cyanotoxins) is an emerging health concern due to their increasing prevalence and previous associations with neurodegenerative diseases including amyotrophic lateral sclerosis. The objective of this study was to evaluate the neurotoxic effects of a mixture of two co-occurring cyanotoxins, β-methylamino-l-alanine (BMAA) and microcystin leucine and arginine (MCLR), using the larval zebrafish model. We combined high-throughput behavior-based toxicity assays with discovery proteomic techniques to identify behavioral and molecular changes following 6 days of exposure. Although neither toxin caused mortality, morphological defects, nor altered general locomotor behavior in zebrafish larvae, both toxins increased acoustic startle sensitivity in a dose-dependent manner by at least 40% (p < .0001). Furthermore, startle sensitivity was enhanced by an additional 40% in larvae exposed to the BMAA/MCLR mixture relative to those exposed to the individual toxins. Supporting these behavioral results, our proteomic analysis revealed a 4-fold increase in the number of differentially expressed proteins in the mixture-exposed group. Additionally, prediction analysis reveals activation and/or inhibition of 8 enriched canonical pathways (enrichment p-value < .01; z-score≥|2|), including ILK, Rho Family GTPase, RhoGDI, and calcium signaling pathways, which have been implicated in neurodegeneration. We also found that expression of TDP-43, of which cytoplasmic aggregates are a hallmark of amyotrophic lateral sclerosis pathology, was significantly upregulated by 5.7-fold following BMAA/MCLR mixture exposure. Together, our results emphasize the importance of including mixtures of cyanotoxins when investigating the link between environmental cyanotoxins and neurodegeneration as we reveal that BMAA and MCLR interact in vivo to enhance neurotoxicity.

Induced toxicity in early-life stage zebrafish (Danio rerio) and its behavioral analysis after exposure to non-doped, nitrogen-doped and nitrogen, sulfur-co doped carbon quantum dots

In this study, the effects of doping of CQDs with alternative functional groups (dopants) were evaluated through embryonic development of zebrafish (Danio rerio). The CQDs were synthesized using simple and low-cost sources: Non-doped (citric acid was used as the carbon source), nitrogen-doped (N-doped) and nitrogen, sulfur-co-doped (N,S-doped). The CQDs induced significant toxicity to zebrafish (>150 μg/mL) and the toxic effects were dose-dependent. The N,S-doped CQDs were the most toxic (LD50 = 149.92 μg/mL), followed by the N-doped CQDs (LD50 = 399.95 μg/mL) while the non-doped CQDs were the least toxic (LD50 = 548.48 μg/mL) of the three. The growth rate (GR) was affected following the toxicity pattern (GR_NS-doped<GR_N-doped<GR_non-doped <GR_blanc), which, in turn, greatly depends on the type of dopant. Morphological malformations, such as pericardial edema, yolk sac edema, tail and spinal curvature were observed to zebrafish embryos as the toxicity, concentration and exposure time to the nanomaterial increased. Behavioral analysis showed that locomotor activity increases as the toxicity of the nanomaterial rises. The differences in toxicity, growth rate and malfunctions of CQDs were attributed to their doping with different heteroatoms. The N,S-doped CQDs, unequivocally, exhibited the most pronounced effects.

Synergistic toxicity of microcystin-LR and Cu to zebrafish (Danio rerio)

Toxic cyanobacterial blooms often coincide with metal pollution in a freshwater environment because of surface run-off enriched with nutrients and metals. However, the joint toxic effects of cyanobacterial toxins and metals on aquatic animals remain unknown. In this study, single and joint toxic effects and mechanisms of microcystin-LR (MCLR) and copper (Cu) were investigated in the early development of zebrafish (Danio rerio). The LC50_72-h values were 2.79 mg/L for MCLR and 3.23 mg/L for Cu. The sublethal concentrations of MCLR (≤600 μg/L) did not affect the normal development of zebrafish but increased its hatchability. Strong synergistic toxic effects were observed after co-exposure to MCLR and Cu at environmental concentrations (≤60 μg/L). The synergistic toxic effects of these two compounds could be attributed to the increased bioaccumulation of MCLR and Cu, which was mediated by MCLR transporters (e.g., oatp1d1 and oatp2b1) and Cu transporters (e.g., ctr1 and atp7a), in zebrafish. Such bioaccumulation caused oxidative stress, as suggested by the disrupted gene expression of anti-oxidative enzymes (e.g., Cu/Zn-SOD, Mn-SOD, and CAT). Our results revealed for the first time the synergistic toxic effects and potential toxic mechanism of MCLR-Cu in aquatic animals. These synergistic effects should be considered when assessing the ecological risk of toxic cyanobacterial blooms.

Toxic effects of fluridone on early developmental stages of Japanese Medaka (Oryzias latipes)

The herbicide fluridone is intensively applied to control invasive aquatic plants globally, including in the Sacramento and San Joaquin Delta (the Delta), California, USA. Our previous study revealed that the adult stage of Delta Smelt showed acute and sub-lethal adverse effects following 6 h of exposure to environmentally relevant concentrations of fluridone. To further investigate mechanisms of toxicity of fluridone and to assess its toxicity to early life stages of fish, we performed additional exposures using the fish model Japanese Medaka (Oryzias latipes). Male and female Medaka embryos were exposed to concentrations of fluridone for 14 d and showed reduced hatching success in a dose dependent manner. The half maximal effective concentration for the hatching success was 2.3 mg L^-1. In addition, male and female Medaka larvae were acute exposed to fluridone for 6 h to assess their swimming behavior and gene expression patterns. Fish exposed to fluridone at 4.2 mg L^-1 or higher became lethargic and showed abnormal swimming behavior. The response to the stimuli was significantly impaired by fluridone at 21 mg L^-1 and above in males, and at 104 mg L^-1 in females. Transcriptome analysis identified a total of 799 genes that were significantly differentially expressed, comprising 555 up-regulated and 244 down-regulated genes in males exposed to 21 mg L^-1 of fluridone. The gene set enrichment analysis indicated a number of biological processes altered by fluridone. Among the genes involved in those biological processes, the expression of the genes, acetylcholinesterase, retinoic acid receptor, insulin receptor substrate, glutathione reductase, and glutathione S transferase, exhibited dose- and sex-dependent responses to fluridone. The study indicated that fluridone exposure led to detrimental toxic effects at early developmental stages of fish, by disturbing the biological processes of growth and development, and the nervous system, inducing oxidative stress and endocrine disruption.

Combined Danio rerio embryo morbidity, mortality and photomotor response assay: A tool for developmental risk assessment from chronic cyanoHAB exposure

Freshwater harmful algal blooms produce a broad array of bioactive compounds, with variable polarity. Acute exposure to cyanotoxins can impact the liver, nervous system, gastrointestinal tract, skin, and immune function. Increasing evidence suggests chronic effects from low-level exposures of cyanotoxins and other associated bioactive metabolites of cyanobacterial origin. These sundry compounds persist in drinking and recreational waters and challenge resource managers in detection and removal. A systematic approach to assess the developmental toxicity of cyanobacterial metabolite standards was employed utilizing a robust and high throughput developmental Danio rerio embryo platform that incorporated a neurobehavioral endpoint, photomotor response. Subsequently, we applied the platform to cyanobacterial bloom surface water samples taken from temperate recreational beaches and tropical lake subsistence drinking water sources as a model approach. Dechorionated Danio rerio embryos were statically immersed beginning at four to six hours post fertilization at environmentally relevant concentrations, and then assessed at 24 h and 5 days for morbidity, morphological changes, and photomotor response. At least one assessed endpoint deviated significantly for exposed embryos for 22 out of 25 metabolites examined. Notably, the alkaloid lyngbyatoxin-a resulted in profound, dose-dependent morbidity and mortality beginning at 5 μg/L. In addition, hydrophobic components of extracts from beach monitoring resulted in potent morbidity and mortality despite only trace cyanotoxins detected. The hydrophilic extracts with several order of magnitude higher concentrations of microcystins resulted in no morbidity or mortality. Developmental photomotor response was consistently altered in environmental bloom samples, independent of the presence or concentration of toxins detected in extracts. While limited with respect to more polar compounds, this novel screening approach complements specific fingerprinting of acutely toxic metabolites with robust assessment of developmental toxicity, critical for chronic exposure scenarios.

Effects of the Essential Oil from Pistacia lentiscus Var. chia on the Lateral Line System and the Gene Expression Profile of Zebrafish (Danio rerio)

Mastic essential oil exhibits anti-bacterial, anti-inflammatory, and anti-oxidant properties. With the growing interest of the use of mastic oil in the food and pharmaceutical industry, systematic in vivo studies are needed to address controlled usage and safety issues. In the present work we evaluated the safety of mastic oil using as a model the zebrafish lateral line system. In addition, we studied the gene expression profile of zebrafish fed with mastic oil-supplemented diet using microarray analysis. Our results showed that the hair cells of lateral line neuromasts are functional upon exposure of zebrafish larvae up to 20 ppm of mastic essential oil, while treatment with higher concentrations, 100 and 200 ppm, resulted in increased larvae mortality. Dietary supplementation of zebrafish with mastic essential oil led to differential expression of interferon response-related genes as well as the immune responsive gene 1 (irg1) that links cellular metabolism with immune defense. Notably, mucin 5.2, a constituent of the mucus hydrogel that protects the host against invading pathogens, was up-regulated. Our in vivo work provides information concerning the safety of mastic essential oil use and suggests dietary effects on gene expression related with the physical and immunochemical properties of the gastrointestinal system.

Application of Benchmark Concentration (BMC) Analysis on Zebrafish Data: A New Perspective for Quantifying Toxicity in Alternative Animal Models

Over the past decade, the zebrafish is increasingly being used as a model to screen for chemical-mediated toxicities including developmental toxicity (DT) and neurotoxicity (NT). One of the major challenges is lack of harmonization in data analysis approaches, thereby posing difficulty in comparing findings across laboratories. To address this, we sought to establish a unified data analysis strategy for both DT and NT data, by adopting the benchmark concentration (BMC) analysis. There are two critical aspects in the BMC analysis: having a toxicity endpoint amenable for BMC and selecting a proper benchmark response (BMR) for the endpoint. For the former, in addition to the typical endpoints in NT assay (eg, hyper/hypo- response quantified by distance moved), we also used endpoints that assess the differences in movement patterns between chemical-treated embryos and control embryos. For the latter, we standardized the selection of BMR, which is analogous to minimum activity threshold, based on intrinsic response variations in the endpoint. When comparing our BMC results with a traditionally used LOAEL method (lowest-observed-adverse-effect level), we found high active compound concordance (100% for DT vs 74% for NT); generally, the BMC was more sensitive than LOAEL (no. of BMC more sensitive/no. of concordant active compounds, 43/50 for DT vs 16/26 for NT). Using the BMC with standardized toxicity endpoints and an appropriate BMR, we may now have a unified data-analysis approach to comparing results across different zebrafish datasets, for a better understanding of strengths and challenges when using the zebrafish as a screening tool.