Organic anion transporting polypeptides expressed in liver and brain mediate uptake of microcystin

Nodding syndrome: A role for environmental biotoxins that dysregulate MECP2 expression?

Nodding syndrome is an epileptic encephalopathy associated with neuroinflammation and tauopathy. This initially pediatric brain disease, which has some clinical overlap with Methyl-CpG-binding protein 2 (MECP2) Duplication Syndrome, has impacted certain impoverished East African communities coincident with local civil conflict and internal displacement, conditions that forced dependence on contaminated food and water. A potential role in Nodding syndrome for certain biotoxins (freshwater cyanotoxins plus/minus mycotoxins) with neuroinflammatory, excitotoxic, tauopathic, and MECP2-dysregulating properties, is considered here for the first time.

Toxicity of three microcystin variants on the histology, physiological and metabolism of hepatopancreas and intestinal microbiota of Litopenaeus vannamei

Microcystins (MCs) are prevalent harmful contaminants within shrimp aquaculture systems, exhibiting a diverse array of variants. Gut microbiota can engage in mutual interactions with the host through the gut-liver axis. In this study, the shrimp Litopenaeus vannamei were subjected to three different variants of MCs (LR, YR, RR) at a concentration of 1 μg/L each, and elucidated the alterations in both intestinal microbiota and hepatopancreas physiological homeostasis. The results showed that all three variants of MCs prompted histological alterations in the hepatopancreas, induced elevated levels of oxidative stress biomarkers (H2O2, T-SOD, and CAT), disturbed the transcription levels of immune-related genes (Crus, ALF, and Lys), along with an increase in apoptotic genes (Casp-3 and P53). Furthermore, the metabolic profiles of the hepatopancreas were perturbed, particularly in amino acid metabolism such as "lysine degradation" and "β-alanine metabolism"; the mTOR and FoxO signaling were also influenced, encompassing alterations in the transcription levels of related genes. Additionally, the alterations were observed in the intestinal microbiota's diversity and composition, particularly potential beneficial bacteria (Alloprevotella, Bacteroides, Collinsella, Faecalibacterium, and Prevotellaceae UCG-001), which exhibited a positive correlation with the metabolite berberine. These findings reveal that the three MCs variants can impact the health of the shrimp by interfering with the homeostasis of intestinal microbial and hepatopancreas physiology.

Alleviation of microcystin-leucine arginine -induced hepatotoxicity: An updated overview

OBJECTIVES

Contamination of surface waters is a major health threat for all living creatures. Some types of blue-green algae that naturally occur in fresh water, are able to produce various toxins, like Microcystins (MCs). Microcystin-leucine arginine (MC-LR) produced by Microcystis aeruginosa is the most toxic and abundant isoforms of MCs, and it causes hepatotoxicity. The present article reviews preclinical experiments examined different treatments, including herbal derivatives, dietary supplements and drugs against MC-LR hepatotoxicity.

METHODS

We searched scientific databases Web of Science, Embase, Medline (PubMed), Scopus, and Google Scholar using relevant keywords to find suitable studies until November 2023.

RESULTS

MC-LR through Organic anion transporting polypeptide superfamily transporters (OATPs) penetrates and accumulates in hepatocytes, and it inhibits protein phosphatases (PP1 and PP2A). Consequently, MC-LR disturbs many signaling pathways and induces oxidative stress thus damages cellular macromolecules. Some protective agents, especially plants rich in flavonoids, and natural supplements, as well as chemoprotectants were shown to diminish MC-LR hepatotoxicity.

CONCLUSION

The reviewed agents through blocking the OATP transporters (nontoxic nostocyclopeptide-M1, captopril, and naringin), then inhibition of MC-LR uptake (naringin, rifampin, cyclosporin-A, silymarin and captopril), and finally at restoration of PPAse activity (silybin, quercetin, morin, naringin, rifampin, captopril, azo dyes) exert hepatoprotective effect against MC-LR.

Microcystin-leucine arginine induces the proliferation of cholangiocytes and cholangiocarcinoma cells through the activation of the Wnt/β-catenin signaling pathway

Background

Microcystin-leucine arginine (MC-LR) is a cyanobacterial hepatotoxic toxin found in water sources worldwide, including in northeastern Thailand, where opisthorchiasis-associated cholangiocarcinoma (CCA) is most prevalent. MC-LR is a potential carcinogen; however, its involvement in liver fluke-associated CCA remains ambiguous. Here, we aimed to evaluate the effect of MC-LR on the progression of CCA via the Wnt/β-catenin pathway in vitro.

Methods

Cell division, migration, cell cycle transition, and MC-LR transporter expression were evaluated in vitro through MTT assay, wound healing assay, flow cytometry, and immunofluorescence staining, respectively. Following a 24-h treatment of cultured cells with 1, 10, 100, and 1,000 nM of MC-LR, the proliferative effect of MC-LR on the Wnt/β-catenin signaling pathway was investigated using immunoblotting and qRT-PCR analysis. Immunohistochemistry was used to determine β-catenin expression in CCA tissue compared to adjacent tissue.

Results

Human immortalized cholangiocyte cells (MMNK-1) and a human cell line established from opisthorchiasis-associated CCA (KKU-213B) expressed the MC-LR transporter and internalized MC-LR. Exposure to 10 nM and 100 nM of MC-LR notably enhanced cells division and migration in both cell lines (P < 0.05) and markedly elevated the percentage of S phase cells (P < 0.05). MC-LR elevated PP2A expression by activating the Wnt/β-catenin signaling pathway and suppressing phosphatase activity. Inhibition of the β-catenin destruction complex genes (Axin1 and APC) led to the upregulation of β-catenin and its downstream target genes (Cyclin D1 and c-Jun). Inhibition of Wnt/β-catenin signaling by MSAB confirmed these results. Additionally, β-catenin was significantly expressed in cancerous tissue compared to adjacent areas (P < 0.001).

Conclusions

Our findings suggest that MC-LR promotes cell proliferation and progression of CCA through Wnt/β-catenin pathway. Further evaluation using invivo experiments is needed to confirm this observation. This finding could promote health awareness regarding MC-LR intake and risk of CCA.

Urolithin a Partially Protects against Oxidative Damage Induced for Microcistyn-lr in C6 Cells

Microcystin-LR (MIC-LR) is a toxin which the mechanism of intoxication involves oxidative stress. Urolithin A (URO-A) is a metabolic product from the colonic fermentation of ellagic acid with antioxidant potential. This study aimed to evaluate the putative protective effect of URO-A against MIC-LR toxicity in C6 cells. C6 cells were incubated with MIC-LR (1 and 10 μM) and/or URO-A (3, 30, 60 and 100 μM) for 24 h. MIC-LR induced reactive species (RS) generation, depletion of total thiol (SH) groups, and survival loss when compared with the control group. Also, at 10 μM, MIC-LR induced CAT activity inhibition. URO-A caused CAT activity inhibition and showed a trend to increase RS generation (60 and 100 μM) per se. URO-A at 3 μM completely attenuated the RS generation and the impairment in SH groups caused by MIC-LR. Our results demonstrated that URO-A might offer a protective effect against toxicity caused by MIC-LR in glial cells by restoring the levels of RS and thiol groups.

Microcystin Contamination in Irrigation Water and Health Risk

Microcystins (MCs), natural hepatotoxic compounds produced by cyanobacteria, pose significant risks to water quality, ecosystem stability, and the well-being of animals, plants, and humans when present in elevated concentrations. The escalating contamination of irrigation water with MCs presents a growing threat to terrestrial plants. The customary practice of irrigating crops from local water sources, including lakes and ponds hosting cyanobacterial blooms, serves as a primary conduit for transferring these toxins. Due to their high chemical stability and low molecular weight, MCs have the potential to accumulate in various parts of plants, thereby increasing health hazards for consumers of agricultural products, which serve as the foundation of the Earth's food chain. MCs can bioaccumulate, migrate, potentially biodegrade, and pose health hazards to humans within terrestrial food systems. This study highlights that MCs from irrigation water reservoirs can bioaccumulate and come into contact with plants, transferring into the food chain. Additionally, it investigates the natural mechanisms that organisms employ for conjugation and the microbial processes involved in MC degradation. To gain a comprehensive understanding of the role of MCs in the terrestrial food chain and to elucidate the specific health risks associated with consuming crops irrigated with water contaminated with these toxins, further research is necessary.

Endogenous Production and Vibrational Analysis of Heavy-Isotope-Labeled Peptides from Cyanobacteria

Stable isotope labeling is an extremely useful tool for characterizing the structure, tracing the metabolism, and imaging the distribution of natural products in living organisms using mass-sensitive measurement techniques. In this study, a cyanobacterium was cultured in 15 N/13 C-enriched media to endogenously produce labeled, bioactive oligopeptides. The extent of heavy isotope incorporation in these peptides was determined with LC-MS, while the overall extent of heavy isotope incorporation in whole cells was studied with nanoSIMS and AFM-IR. Up to 98 % heavy isotope incorporation was observed in labeled cells. Three of the most abundant peptides, microcystin-LR (MCLR), cyanopeptolin-A (CYPA), and aerucyclamide-A (ACAA), were isolated and further studied with Raman and FTIR spectroscopies and DFT calculations. This revealed several IR and Raman active vibrations associated with functional groups not common in ribosomal peptides, like diene, ester, thiazole, thiazoline, and oxazoline groups, which could be suitable for future vibrational imaging studies. More broadly, this study outlines a simple and relatively inexpensive method for producing heavy-labeled natural products. Manipulating the bacterial culture conditions by the addition of specific types and amounts of heavy-labeled nutrients provides an efficient means of producing heavy-labeled natural products for mass-sensitive imaging studies.

Involvement of reactive oxygen species (ROS) in the hepatopancreatic cytotoxicity, oxidative stress, and apoptosis induced by microcystin-LR in Eriocheir sinensis

There is limited knowledge about the toxicity of Microcystin-LR (MC-LR) in crustaceans, despite its high toxicity to aquatic organisms. This research aimed to explore the effects of MC-LR on cytotoxicity, oxidative stress, and apoptosis in the hepatopancreas of Eriocheir sinensis, as well as elucidate the involvement of reactive oxygen species (ROS) and potential mechanisms of toxicity. In vivo and in vitro exposures of crabs to MC-LR and N-acetylcysteine (NAC) were performed, followed by assessments of cell morphology, viability, tissue pathology, biochemical indicators, gene expression, and hepatopancreatic transcriptome. Results revealed that MC-LR facilitated the entry of the MC-LR transporter oatp3a into hepatopancreatic cells, leading to upregulated expression of phase I detoxification enzyme genes (cyp4c, cyp2e1, and cyp3) and downregulated the phase II enzyme genes (gst1, gpx, gsr2, gclc, and nqo1), resulting in increased ROS levels and cytotoxic effects. MC-LR exhibited cytotoxicity, reducing cell viability and inducing abnormal nuclear morphology with a 48 h-IC50 value of approximately 120 μm. MC-LR exposure caused biochemical changes indicative of oxidative stress damage and evident hepatopancreatic lesions. Additionally, MC-LR exposure regulated the levels of bax and bcl-2 expression, activating caspase 3 and 6 to induce cell apoptosis. Intervention with NAC attenuated MC-LR-induced ROS production and associated toxic effects. Transcriptome analysis revealed enrichment of differentially expressed genes in pathways related to cytochrome P450-mediated xenobiotic metabolism and the FoxO signaling pathway. These findings shed light on the potential mechanisms underlying MC-LR toxicity and provide valuable references for further research and conservation efforts regarding the health of aquatic animals.

Microcystin-LR-induced epithelial-mesenchymal transition-like cells acquire resistance to multi-toxins

The protein phosphatase inhibitor microcystin-LR (MC-LR), a hepatocyte-selective cyanotoxin, induces phenotypic changes in HEK293 OATP1B3-expressing (HEK293-OATP1B3) cells, which include cytoskeletal reorganization (HEK293-OATP1B3-AD) and anoikis resistance (HEK293-OATP1B3-FL) transformed cells, respectively. These cells acquire resistance to MC-LR and partial epithelial-mesenchymal transition (EMT) characteristics. In cancer cells, EMT is generally involved in multi-drug resistance. Here, we focused on the multi-drug resistance of HEK293-OATP1B3-AD and HEK293-OATP1B3-FL cells. The MTT assay and immunoblotting were conducted to examine the responses of HEK293-OATP1B3, HEK293-OATP1B3-AD, and HEK293-OATP1B3-FL cells to multiple toxins and drugs that function as substrates for OATP1B3, including MC-LR, nodularin (Nod), okadaic acid (OA), and cisplatin (CDDP). HEK293-OATP1B3-AD and HEK293-OATP1B3-FL cells were more resistant to MC-LR, Nod, and OA than HEK293-OATP1B3 cells. Conversely, the three cell types were equivalently sensitive to CDDP. By using protein phosphatase assay, the reduction of the inhibitory effect of MC-LR and Nod on phosphatase activity might be one reason for the resistance to MC-LR and Nod in HEK293-OATP1B3-AD and HEK293-OATP1B3-FL cells. Furthermore, the parental HEK293-OATP1B3 cells showed enhanced p53 phosphorylation and stabilization after MC-LR exposure, while p53 phosphorylation was attenuated in HEK293-OATP1B3-AD and HEK293-OATP1B3-FL cells. Moreover, in HEK293-OATP1B3-AD and HEK293-OATP1B3-FL cells, AKT phosphorylation was higher than that of the parental HEK293-OATP1B3 cell line. These results suggest that the multi-toxin resistance observed in HEK293-OATP1B3-AD and HEK293-OATP1B3-FL cells is associated with AKT activation and p53 inactivation.

Hepatotoxicity of cyanotoxin microcystin-LR in human: Insights into mechanisms of action in the 3D culture model Hepoid-HepaRG

Microcystin-LR (MC-LR) is a potent hepatotoxin produced by harmful cyanobacterial blooms (CyanoHABs). MC-LR targets highly differentiated hepatocytes expressing organic anion transporting polypeptides OATP1B1 and OATP1B3 that are responsible for hepatocellular uptake of the toxin. The present study utilized an advanced 3D in vitro human liver model Hepoid-HepaRG based on the cultivation of collagen-matrix embedded multicellular spheroids composed of highly differentiated and polarized hepatocyte-like cells. 14-d-old Hepoid-HepaRG cultures showed increased expression of OATP1B1/1B3 and sensitivity to MC-LR cytotoxicity at concentrations >10 nM (48 h exposure, EC20 = 26 nM). MC-LR induced neither caspase 3/7 activity nor expression of the endoplasmic reticulum stress marker gene BiP/GRP78, but increased release of pro-inflammatory cytokine IL-8, indicating a necrotic type of cell death. Subcytotoxic (10 nM) and cytotoxic (≥100 nM) MC-LR concentrations disrupted hepatocyte functions, such as xenobiotic metabolism phase-I enzyme activities (cytochrome P450 1A/1B) and albumin secretion, along with reduced expression of CYP1A2 and ALB genes. MC-LR also decreased expression of HNF4A gene, a critical regulator of hepatocyte differentiation and function. Genes encoding hepatobiliary membrane transporters (OATP1B1, BSEP, NTCP), hepatocyte gap junctional gene connexin 32 and the epithelial cell marker E-cadherin were also downregulated. Simultaneous upregulation of connexin 43 gene, primarily expressed by liver progenitor and non-parenchymal cells, indicated a disruption of tissue homeostasis. This was associated with a shift in the expression ratio of E-cadherin to N-cadherin towards the mesenchymal cell marker, a process linked to epithelial-mesenchymal transition (EMT) and hepatocarcinogenesis. The effects observed in the human liver cell in vitro model revealed mechanisms that can potentially contribute to the MC-LR-induced promotion and progression of hepatocellular carcinoma (HCC). Hepoid-HepaRG cultures provide a robust, accessible and versatile in vitro model, capable of sensitively detecting hepatotoxic effects at toxicologically relevant concentrations, allowing for assessing hepatotoxicity mechanisms, human health hazards and impacts of environmental hepatotoxins, such as MC-LR.

Toxic effects of three variants of microcystins on the intestinal histology, physiological and metabolic response of Litopenaeus vannamei

Microcystins (MCs) are harmful substances to the health of cultured shrimp, and there are many variants of MCs. Intestinal is the immune and metabolic center of the shrimp, and is also the target organ for MCs toxicity. In this study, the shrimp Litopenaeus vannamei juvenile were separately exposed to 1 μg/L of three MCs variants (LR, YR, RR) for 72 h respectively, and the changes of intestinal morphology, physiological response and metabolic function were analyzed. The results showed the three MCs variants stress caused intestinal mucosal damage and disordered the homeostasis of antimicrobial genes (ALF and Lys) expression. The mRNA expression levels of antioxidant genes (Nrf2 and GPx) and apoptosis factors (CytC and Casp-3) were increased, but that of detoxification gene (CYP450) was decreased. Furthermore, the intestinal metabolic pattern was also influenced by stresses, among which MC-LR induced more differential metabolites than that of MC-YR and MC-RR. The function of purine metabolism was highly influenced by the stress of three MCs variants, followed by amino acid metabolism, but there were differences in the types of amino acids. The metabolites citric acid, L-glutamine, L-tryptophan, spermine, UMP, and indole contributed to the metabolic pathway network. Nineteen stress-related metabolites were identified as candidates for subsequent screening of potential biomarkers. These results revealed the toxic effects of three MCs variants on the intestinal physiological and metabolic homeostasis of the shrimp.

Probiotic Lactobacillus rhamnosus alleviates the neurotoxicity of microcystin-LR in zebrafish (Danio rerio) through the gut-brain axis

Microcystin-LR (MCLR) is one of the most toxic cyanobacterial toxins and is harmful to the central nervous system of fish. Probiotic additives can improve neuroendocrine function in fish. Although both MCLR and probiotics aim at the nervous system, whether they interact with each other and the mechanisms remain unexplored. In the present study, 4-month-old zebrafish were exposed to 0, 2.2, and 22 μg/L of MCLR for 28 days with or without the probiotic L. rhamnosus. We found that MCLR exposure could inhibit the swimming speed of zebrafish, while the presence of L. rhamnosus mitigated this abnormality. To elucidate the mechanism of how L. rhamnosus alleviates MCLR-induced neurotoxicity, we examined the bioaccumulation of MCLR, changes in neurotransmitters, immune biochemical indicators, and hormone content of the hypothalamic-pituitary-interrenal (HPI) axis in zebrafish along the gut-brain axis. Our results showed L. rhamnosus could reverse the abnormal swimming behavior and eventually alleviate neurotoxicity in zebrafish by modulating intestinal and brain neural signaling, neuroinflammation, and HPI axis responses. This study provides implications for the application of probiotics in the aquaculture industry.

The mechanisms of microcystin-LR-induced genotoxicity and neurotoxicity in fish and mammals: Bibliometric analysis and meta-analysis

Microcystin-leucine arginine (MC-LR) is a typical cyanobacterial toxin, and the threat of this toxin is increasing among organisms. Despite extensive toxicological studies on MC-LR, there is no comprehensive analysis based on previously published data. Therefore, we conducted bibliometric analysis and meta-analysis to identify research hotspots and to elucidate the key mechanism of the relationship between MC-LR and genotoxicity and neurotoxicity among fish and mammals. One of the hotspots is toxic mechanisms (indicated by the frequent appearance of oxidative stress, DNA damage, apoptosis, neurotoxicity, genotoxicity, ROS, comet assay, signalling pathway, and gene expression indicate as keywords). The density visualization shows a high frequency of "microcystin-LR" and "toxicology," and the overlay visualization emphasizes the prominence of "neurotoxicity" in recent years. These findings confirm the importance of studying MC-LR toxicity. Meta-analysis indicated that in both fish and mammals, MC-LR exposure increased ROS levels by 294 % and increased DNA damage biomarkers by 174 % but decreased neurotoxicity biomarkers by 9 %. Intergroup comparisons revealed that the exposure concentration of MC-LR was significantly correlated with genotoxicity and neurotoxicity levels in both fish and mammals (p < 0.05). Furthermore, the random forest (RF) model revealed that exposure concentration was the primary determinant associated with the induction of ROS, genotoxicity, and neurotoxicity induced by MC-LR. This is likely the dominant mechanism by which excessive ROS production induced by MC-LR causes oxidative stress, ultimately leading to genotoxicity and neurotoxicity in both fish and mammals.

Combined effects of microcystin-LR and rice straw-derived biochar on the hepatic antioxidant capacity of zebrafish: Insights from LC-MS/MS-based metabolomics analysis

Microcystin-LR (MC-LR) produced by cyanobacteria blooms poses a serious risk to aquatic organisms. Rice straw-derived biochar (BC) is gradually being utilized as an effective adsorbent to remove water pollutants. In the present study, the combined toxicity of MC-LR and BC on hepatic antioxidant capacity and metabolic phenotype of zebrafish (Danio rerio) were conducted due to the increasing concern of eutrophication in aquatic environments. Female zebrafish were exposed to solutions of MC-LR (10 μg/L) and BC (100 μg/L) individually and in combination for 30 days. The results indicated that sub-chronic MC-LR exposure induced oxidative stress and metabolic disorders, with a significant elevation of several amino acids, glucose as well as unsaturated fatty acids. Metabolic pathway analysis showed that the ascorbate and aldarate metabolism and biosynthesis of unsaturated fatty acids were affected under MC-LR stress. Significantly increased MDA levels along with significantly decreased CAT and GPx activities were observed in the MC-LR group. Nevertheless, MDA levels, antioxidant enzyme activities, and the relevant gene expressions (cat1, nrf2a, HO-1, keap1a) returned to baseline in the co-exposure group. These findings revealed that MC-LR resulted in metabolic disorders of protein, sugar, and lipid related to energy production, and BC could relieve MC-LR-induced metabolic disorder and oxidative stress in the liver of zebrafish. However, the potential risk of BC-induced metabolic disorder should not be neglected. Our present results highlight the potential of BC as a tool for mitigating the negative impacts of MC-LR on aquatic organisms in blooms-contaminated water.

Harmful and beneficial properties of cyanotoxins: Two sides of the same coin

Cyanotoxins are by definition "harmful agents" produced by cyanobacteria. Their toxicity has been extensively studied and reviewed over the years. Cyanotoxins have been commonly classified, based on their poisonous effects on mammals, into three main classes, neurotoxins, hepatotoxins and dermatotoxins, and, considering their chemical features, mainly identified as peptides, alkaloids and lipopolysaccharides. Here we propose a broader subdivision of cyanotoxins into eight distinct classes, taking into account their molecular structures, biosynthesis and modes of action: alkaloids, non-ribosomal peptides, polyketides, non-protein amino acids, indole alkaloids, organophosphates, lipopeptides and lipoglycans. For each class, the structures and primary mechanisms of toxicity of the main representative cyanotoxins are reported. Despite their powerful biological activities, only recently scientists have considered the biotechnological potential of cyanotoxins, and their applications both in medical and in industrial settings, even if only a few of these have reached the biotech market. In this perspective, we discuss the potential uses of cyanotoxins as anticancer, antimicrobial, and biocidal agents, as common applications for cytotoxic compounds. Furthermore, taking into account their mechanisms of action, we describe peculiar potential bioactivities for several cyanotoxin classes, such as local anaesthetics, antithrombotics, neuroplasticity promoters, immunomodulating and antifouling agents. In this review, we aim to stimulate research on the potential beneficial roles of cyanotoxins, which require interdisciplinary cooperation to facilitate the discovery of innovative biotechnologies.

Protective effect of Huanglianjiedu Decoction on microcystin-LR induced nerve injury

Microcystin-LR (MC-LR) presented in eutrophic water has been identified as having the capacity to induce damage to the mammalian nervous system by crossing the blood-brain barrier through organic anion transporting polypeptides. However, the lack of effective preventive and protective strategies remains a concern. Huanglianjiedu Decoction (HLJD), a classical Chinese traditional formula originating from the Tang Dynasty and comprising Rhizoma Coptidis, Radix Scutellariae, Cortex Phellodendri, and Fructus Gardeniae, has exhibited neuroprotective effects attributed to its antioxidant properties. In this study, we investigated the potential of HLJD in counteracting the neurotoxic effects induced by MC-LR. Our findings revealed that MC-LR dose-dependently inhibited the activity of the PP2A enzyme in PC 12 cells and significantly elevated the phosphorylation levels of JNK, ERK1/2, and p38. Moreover, MC-LR administration resulted in synaptic damage in mouse neurons, hyperphosphorylation of the microtubule-related protein Tau, cognitive impairment, and deficits in learning and memory in C57BL/6J mice. Notably, HLJD effectively reversed the cytotoxicity caused by MC-LR in PC 12 cells, and attenuated MC-LR-induced neuronal damage while improving learning ability in mice. These results highlight the potential of HLJD as a promising protective strategy against MC-LR-induced neurological injury.

Cyanotoxins Increase Cytotoxicity and Promote Nonalcoholic Fatty Liver Disease Progression by Enhancing Cell Steatosis

Freshwater prokaryotic cyanobacteria within harmful algal blooms produce cyanotoxins which are considered major pollutants in the aquatic system. Direct exposure to cyanotoxins through inhalation, skin contact, or ingestion of contaminated drinking water can target the liver and may cause hepatotoxicity. In the current study, we investigated the effect of low concentrations of cyanotoxins on cytotoxicity, inflammation, modulation of unfolded protein response (UPR), steatosis, and fibrosis signaling in human hepatocytes and liver cell models. Exposure to low concentrations of microcystin-LR (MC-LR), microcystin-RR (MC-RR), nodularin (NOD), and cylindrospermopsin (CYN) in human bipotent progenitor cell line HepaRG and hepatocellular carcinoma (HCC) cell lines HepG2 and SK-Hep1 resulted in increased cell toxicity. MC-LR, NOD, and CYN differentially regulated inflammatory signaling, activated UPR signaling and lipogenic gene expression, and induced cellular steatosis and fibrotic signaling in HCC cells. MC-LR, NOD, and CYN also regulated AKT/mTOR signaling and inhibited autophagy. Chronic exposure to MC-LR, NOD, and CYN upregulated the expression of lipogenic and fibrosis biomarkers. Moreover, RNA sequencing (RNA seq) data suggested that exposure of human hepatocytes, HepaRG, and HCC HepG2 cells to MC-LR and CYN modulated expression levels of several genes that regulate non-alcoholic fatty liver disease (NAFLD). Our data suggest that low concentrations of cyanotoxins can cause hepatotoxicity and cell steatosis and promote NAFLD progression.

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.

Cyanotoxin exposure and hepatocellular carcinoma

Cyanobacteria are ubiquitous in aquatic and terrestrial environments worldwide and include a number of species producing tumor-promoting hepatotoxins. Human exposure to cyanobacteria and cyanotoxins primarily occurs though ingestion of contaminated drinking water and food sources. In a Northeast U.S. population, we recently reported an independent association of oral cyanobacteria with risk of hepatocellular carcinoma (HCC). In a cross-sectional study of 55 HCC patients in Hawaii, U.S.A., serum microcystin/nodularin (MC/NOD), cylindrospermopsin (CYN), and anabaenopeptin (AB) were measured by ELISA. In a subset of 16 patients, cyanotoxin levels were compared by tumor expression of over 700 genes analyzed via the Nanostring nCounter Fibrosis panel. MC/NOD, CYN, and AB were detected in all HCC patients. MC/NOD and CYN levels significantly varied by etiology with the highest levels in cases attributed to metabolic risk factors, specifically, hyperlipidemia, type 2 diabetes, and non-alcoholic fatty liver disease/non-alcoholic steatohepatitis. Cyanotoxin levels were significantly positively correlated with tumor expression of genes functioning in PPAR signaling and lipid metabolism. Our study provides novel albeit limited evidence that cyanotoxins may a role in the pathogenesis of HCC through the dysregulation of lipid metabolism and progression of hepatic steatosis.

State-of-the-art review on the ecotoxicology, health hazards, and economic loss of the impact of microcystins and their ultrastructural cellular changes

Cyanobacteria are ubiquitously globally present in both freshwater and marine environments. Ample reports have been documented by researchers worldwide for pros and cons of cyanobacterial toxins. The implications of cyanobacterial toxin on health have received much attention in recent decades. Microcystins (MCs) represent the unique class of toxic metabolites produced by cyanobacteria. Although the beneficial aspects of cyanobacterial are numerous, the deleterious effect of MCs overlooked. Several studies on MCs evidently reported that MCs exhibit a plethora of harmful effect on animals, plants, and cell lines. Accordingly, numerous histopathological studies have also found that MCs cause detrimental effects to cells by damaging cellular organelles, including nuclear envelope, Golgi apparatus, endoplasmic reticulum, mitochondria, plastids, flagellum, pilus membrane structures and integrity, vesicle structures, and autolysosomes and autophagosomes. Such ultrastructural cellular damages holistically influence the morphological, biochemical, physiological, and genetic status of the host. Indeed, MCs have also been found to cause the deleterious effect to different animals and plants. Such deleterious effects of MCs have greater impact on agriculture, public health which in turn influences ecotoxicology and economic consequences. The impairments correspond to oxidative stress, organ failure, carcinogenesis, aquaculture loss, with an emphasis for blooms and respective bioaccumulation prospects. The preservation of mortality among life forms is addressed in a critical cellular perspective for multitude benefits. The comprehensive cellular assessment could provide opportunity to develop strategy for therapeutic implications.

Dual characteristics of Bellamya aeruginosa encountering Microcystis aeruginosa: Algal control and toxin depuration

The benthic gastropods Bellamya aeruginosa (B. aeruginosa) is ubiquitous in freshwater in China and neighboring countries with great edible value. It has been recognized as a potential manipulator to control harmful algal blooms due to its filtration on algal cells. In this study, the control effect of B. aeruginosa on toxic and non-toxic Microcystis aeruginosa (M. aeruginosa), and the accumulation and depuration of microcystins (MCs) in the snail were systematically explored. Results indicated that although toxic M. aeruginosa could protect itself via producing MCs, the introduction of B. aeruginosa could still effectively inhibit the algae with cell density below 1 × 10⁶ cells/mL. Hepatopancreas was the primary target of MCs in all tissues of B. aeruginosa, presenting a maximum of 3089.60 ng/g DW when exposed to toxic M. aeruginosa of 1.0 × 10⁷ cells/mL. The enrichment of MCs in other tissues following the order of digestive tract > gonad > mantle > muscle. Interestingly, snail could again excrete previously enriched MCs when transferred to non-toxic M. aeruginosa, giving rise to over 80% reduction of MCs in the body. After depuration, the estimated daily intake (EDI) of free MCs in intact individuals and the edible parts of B. aeruginosa were both lower than the tolerable daily intake (TDI). These results implicated that B. aeruginosa could control low density of M. aeruginosa in spring. Particularly, the snail could be perfectly safe to consume by purifying for a while after using as manipulator.

A Review of Common Cyanotoxins and Their Effects on Fish

Global warming and human-induced eutrophication drive the occurrence of various cyanotoxins in aquatic environments. These metabolites reveal diversified mechanisms of action, encompassing cyto-, neuro-, hepato-, nephro-, and neurotoxicity, and pose a threat to aquatic biota and human health. In the present paper, we review data on the occurrence of the most studied cyanotoxins, microcystins, nodularins, cylindrospermopsin, anatoxins, and saxitoxins, in the aquatic environment, as well as their potential bioaccumulation and toxicity in fish. Microcystins are the most studied among all known cyanotoxins, although other toxic cyanobacterial metabolites are also commonly identified in aquatic environments and can reveal high toxicity in fish. Except for primary toxicity signs, cyanotoxins adversely affect the antioxidant system and anti-/pro-oxidant balance. Cyanotoxins also negatively impact the mitochondrial and endoplasmic reticulum by increasing intracellular reactive oxygen species. Furthermore, fish exposed to microcystins and cylindrospermopsin exhibit various immunomodulatory, inflammatory, and endocrine responses. Even though cyanotoxins exert a complex pressure on fish, numerous aspects are yet to be the subject of in-depth investigation. Metabolites other than microcystins should be studied more thoroughly to understand the long-term effects in fish and provide a robust background for monitoring and management actions.

miR-146a-5p Promotes the Inflammatory Response in PBMCs Induced by Microcystin-Leucine-Arginine

Background

Microcystin-leucine-arginine (MC-LR) is the most abundant and most toxic variant of microcystin isomers. Various experiments have clearly shown that MC-LR has hepatotoxicity and carcinogenicity, but there are relatively few studies on its immune damage effect. In addition, numerous studies have shown that microRNAs (miRNAs) are involved in a wide range of biological processes. Do miRNAs also play a role in inflammatory response caused by microcystin exposure? This is the question to be answered in this study. Moreover, this study can also provides experimental evidence for the significance of miRNA applications.

Objective

To investigate the effect of MC-LR on the expressions of miR-146a and pro/anti-inflammatory cytokines in human peripheral blood mononuclear cells (PBMCs) and to further explore the role of miR-146a in the inflammatory responses caused by MC-LR.

Methods

Serum samples from 1789 medical examiners were collected and detect the concentrations of MCs, and 30 serum samples with concentrations of MCs around P₂₅, P₅₀, and p₇₅ were randomly selected for the detection of inflammatory factors. PBMCs from fresh peripheral blood extracted from these 90 medical examiners were subsequently tested for relative miR-146a expression. In vitro, the MC-LR were exposed to the PBMCs to detect the levels of inflammatory factors as well as the relative expression of miR-146a-5p. Then, a miRNA transfection assay was performed to verify the regulation of inflammatory factors by miR-146a-5p.

Results

In population samples, the expression of inflammatory factors and miR-146a-5p increased with increasing MCs concentration. In vitro experiments showed that the expression of inflammatory factors and miR-146a-5p in PBMCs increased with MC-LR exposure time or exposure dose too. In addition, inhibiting the expression of miR-146a-5p in PBMCs reduced inflammatory factor levels.

Conclusion

miR-146a-5p exerts a promoting effect on the MC-LR-induced inflammatory response by positively regulating inflammatory factor levels.

Microcystin-LR modulates multixenobiotic resistance proteins in the middle intestine of rainbow trout, Oncorhynchus mykiss

Global climate change favors explosive population growth events (blooms) of phytoplanktonic species, often producing toxic products, e.g., several genera of cyanobacteria synthesize a family of cyanotoxins called microcystins (MCs). Freshwater fish such as the rainbow trout Oncorhynchus mykiss can uptake MCs accumulated in the food chain. We studied the toxic effects and modulation of the activity and expression of multixenobiotic resistance proteins (ABCC transporters and the enzyme glutathione S-transferase (GST) in the O. mykiss middle intestine by microcystin-LR (MCLR). Juvenile fish were fed with MCLR incorporated in the food every 12 h and euthanized at 12, 24, or 48 h. We estimated the ABCC-mediated transport in ex vivo intestinal strips to estimate ABCC-mediated transport activity. We measured total and reduced (GSH) glutathione contents and GST and glutathione reductase (GR) activities. We studied MCLR cytotoxicity by measuring protein phosphatase 1 (PP1) activity and lysosomal membrane stability. Finally, we examined the relationship between ROS production and lysosomal membrane stability through in vitro experiments. Dietary MCLR had a time-dependent effect on ABCC-mediated transport, from inhibition at 12 h to a significant increase after 48 h. GST activity decreased only at 12 h, and GR activity only increased at 48 h. There were no effects on GSH or total glutathione contents. MCLR inhibited PP1 activity and diminished the lysosomal membrane stability at the three experimental times. In the in vitro study, the lysosomal membrane stability decreased in a concentration-dependent fashion from 0 to 5 µmol L ^- 1 MCLR, while ROS production increased only at 5 µmol L ^- 1 MCLR. MCLR did not affect mRNA expression of abcc2 or gst-π. We conclude that MCLR modulates ABCC-mediated transport activity in O. mykiss's middle intestine in a time-dependent manner. The transport rate increase does not impair MCLR cytotoxic effects.

Microcystin-LR aerosol induces inflammatory responses in healthy human primary airway epithelium

Harmful algal blooms plague bodies of freshwater globally. These blooms are often composed of outgrowths of cyanobacteria capable of producing the heptapeptide Microcystin-LR (MC-LR) which is a well-known hepatotoxin. Recently, MC-LR has been detected in aerosols generated from lake water. However, the risk for human health effects due to MC-LR inhalation exposure have not been extensively investigated. In this study, we exposed a fully differentiated 3D human airway epithelium derived from 14 healthy donors to MC-LR-containing aerosol once a day for 3 days. Concentrations of MC-LR ranged from 100 pM to 1 µM. Although there were little to no detrimental alterations in measures of the airway epithelial function (i.e. cell survival, tissue integrity, mucociliary clearance, or cilia beating frequency), a distinct shift in the transcriptional activity was found. Genes related to inflammation were found to be upregulated such as C-C motif chemokine 5 (CCL5; log2FC = 0.57, p = 0.03) and C-C chemokine receptor type 7 (CCR7; log2FC = 0.84, p = 0.03). Functionally, conditioned media from MC-LR exposed airway epithelium was also found to have significant chemo-attractive properties for primary human neutrophils. Additionally, increases were found in the concentration of secreted chemokine proteins in the conditioned media such as CCL1 (log2FC = 5.07, p = 0.0001) and CCL5 (log2FC = 1.02, p = 0.046). These results suggest that MC-LR exposure to the human airway epithelium is capable of inducing an inflammatory response that may potentiate acute or chronic disease.

Unraveling the intestinal epithelial barrier in cyanotoxin microcystin-treated Caco-2 cell monolayers

Microcystin is a widespread cyanobacterial toxin that affects the intestine to produce diarrheal symptoms after ingestion of freshwater blue-green algae. Our study aimed to characterize the mechanism by which the toxin leads to diarrhea via epithelial barrier dysfunction in a small intestine Caco-2 cell model. Microcystin-treated human Caco-2 epithelial monolayers were functionally and molecularly analyzed for barrier dysfunction. Tight junctions (TJs) and cell damage were analyzed in relation to transepithelial electrical resistance (TER) changes. TER of microcystin-treated Caco-2 cells was reduced by 65% of the initial value after 24 h; concomitantly, permeability for fluorescein increased 2.6-fold. Western blot analysis showed reduced claudin-1 expression, while expression of claudin-3 and -4 remained unchanged. Super-resolution stimulated emission depletion microscopy revealed that TJ integrity was compromised by fraying and splitting of the TJ domain of the epithelial cells. Epithelial apoptosis did not significantly contribute to epithelial barrier dysfunction, while cytoskeletal actomyosin constriction was associated with TJ disintegration and the barrier defect. Our results indicate that microcystin causes intestinal barrier leakiness, which helps to explain the leak flux type of diarrhea as the main pathomechanism after ingestion of cyanobacterial toxin.

The Impact of Cyanobacteria Blooms on the Aquatic Environment and Human Health

Cyanobacteria blooms are a global aquatic environment problem. In recent years, due to global warming and water eutrophication, the surface cyanobacteria accumulate in a certain area to form cyanobacteria blooms driven by wind. Cyanobacteria blooms change the physical and chemical properties of water and cause pollution. Moreover, cyanobacteria release organic matter, N (nitrogen) and P (phosphorus) into the water during their apoptosis, accelerating the eutrophication of the water, threatening aquatic flora and fauna, and affecting the community structure and abundance of microorganisms in the water. Simultaneously, toxins and carcinogens released from cyanobacteria can be enriched through the food chain/web, endangering human health. This study summarized and analyzed the research of the influence of cyanobacteria blooms on the aquatic environment and human health, which is helpful to understand further the harm of cyanobacteria blooms and provide some reference for a related research of cyanobacteria blooms.

Congeners-Specific Intestinal Absorption Of Microcystins In An In Vitro 3D Human Intestinal Epithelium: The Role Of Influx/Efflux Transporters

Microcystins constitute a group of over 200 variants and are increasingly considered as emerging toxins in food and feed safety, particularly with regards to sea-food and fish consumption. Toxicity of MCs is congener-specific, being characterised by different acute potencies, likely related to the differential activity of metabolic enzymes and transporters proteins involved in their cellular uptake. However, the active transport of MCs across intestinal membranes has not been fully elucidated. Our results, obtained using a fit for purpose 3D human reconstructed intestinal epithelium, provide new information on the complex mechanisms involved in the absorption of 5 MC variants’: it is indeed characterised by the equilibrium between uptake and extrusion, since the selected congeners are substrates of both influx and efflux proteins. In the range of tested nominal concentrations (10–40 µM) fully representative of relevant exposure scenarios, none of the active tested transporters were saturated. The comparison of permeability (Papp) values of MCs variants highlighted a dose independent relationship for MC-LR, -YR and -RR (Papp x 10^–7 ranged from 2.95 to 3.54 cm/s), whereas -LW and–LF showed a dose dependent increase in permeability reaching Papp values which were similar to the other congeners at 40 µM. MC-RR, -LR, -YR show absorption values around 5% of the administered dose. Due to their lipophilicity, MC-LW and -LF were also detected within the cellular compartment. The intestinal uptake was only partially attributable to OATPs, suggesting the involvement of additional transporters. Regarding the efflux proteins, MCs are not P-gp substrates whereas MRP2 and to a lesser extent Breast cancer resistance protein are active in their extrusion. Despite the presence of GST proteins, as an indication of metabolic competence, in the intestinal tissue, MC-conjugates were never detected in our experimental settings.

Impact of Cyanotoxin Ingestion on Liver Cancer Development Using an At-Risk Two-Staged Model of Mouse Hepatocarcinogenesis

Exposure to cyanobacterial hepatotoxins has been linked to the promotion and increased incidence of liver cancer in pre-clinical and epidemiologic studies. The family of hepatotoxins, microcystins (MCs), are produced by over 40 cyanobacterial species found in harmful algal blooms (HABs) worldwide, with MC-LR being the most common and potent MC congener. In the current study, we hypothesized that the low-dose chronic ingestion of Microcystis cyanotoxins via drinking water would promote liver carcinogenesis in pre-initiated mice. Four groups of C3H/HeJ mice received one intraperitoneal (i.p.) injection of diethylnitrosamine (DEN) at 4 weeks of age. Three weeks later, the mice were administered ad libitum drinking water containing one of the following: (1) reverse osmosis, deionized water; (2) water containing 500 mg/L phenobarbital (PB500); (3) water with purified MC-LR (10 µg/L) added; or (4) water containing lysed Microcystis aeruginosa (lysate; 10 µg/L total MCs). The exposure concentrations were based on environmentally relevant concentrations and previously established Ohio EPA recreational water MC guidelines. Throughout the 30-week exposure, mouse weights, food consumption, and water consumption were not significantly impacted by toxin ingestion. We found no significant differences in the number of gross and histopathologic liver lesion counts across the treatment groups, but we did note that the PB500 group developed lesion densities too numerous to count. Additionally, the proportion of lesions classified as hepatocellular carcinomas in the MC-LR group (44.5%; p < 0.05) and lysate group (55%; p < 0.01) was significantly higher compared to the control group (14.9%). Over the course of the study, the mice ingesting the lysate also had a significantly lower survival probability (64.4%; p < 0.001) compared to water (96.8%), PB500 (95.0%), and MC-LR (95.7%) exposures. Using cyanotoxin levels at common recreational water concentration levels, we demonstrate the cancer-promoting effects of a single cyanotoxin MC congener (MC-LR). Furthermore, we show enhanced hepatocarcinogenesis and significant mortality associated with combinatorial exposure to the multiple MCs and bioactive compounds present in lysed cyanobacterial cells—a scenario representative of the ingestion exposure route, such as HAB-contaminated water and food.

Prior exposure to microcystin alters host gut resistome and is associated with dysregulated immune homeostasis in translatable mouse models

A strong association between exposure to the common harmful algal bloom toxin microcystin and the altered host gut microbiome has been shown. We tested the hypothesis that prior exposure to the cyanotoxin microcystin-LR may alter the host resistome. We show that the mice exposed to microcystin-LR had an altered microbiome signature that harbored antibiotic resistance genes. Host resistome genotypes such as mefA, msrD, mel, ant6, and tet40 increased in diversity and relative abundance following microcystin-LR exposure. Interestingly, the increased abundance of these genes was traced to resistance to common antibiotics such as tetracycline, macrolides, glycopeptide, and aminoglycosides, crucial for modern-day treatment of several diseases. Increased abundance of these genes was positively associated with increased expression of PD1, a T-cell homeostasis marker, and pleiotropic inflammatory cytokine IL-6 with a concomitant negative association with immunosurveillance markers IL-7 and TLR2. Microcystin-LR exposure also caused decreased TLR2, TLR4, and REG3G expressions, increased immunosenescence, and higher systemic levels of IL-6 in both wild-type and humanized mice. In conclusion, the results show a first-ever characterization of the host resistome following microcystin-LR exposure and its connection to host immune status and antimicrobial resistance that can be crucial to understand treatment options with antibiotics in microcystin-exposed subjects in clinical settings.

Renoprotection of Microcystin-RR in Unilateral Ureteral Obstruction-Induced Renal Fibrosis: Targeting the PKM2-HIF-1α Pathway

Renal fibrosis is a pathological characteristic of the endpoint of chronic kidney disease (CKD), which remains a major public health problem. None of the current therapies is effective in stopping kidney fibrosis progression. In light of our novel detection of a potential antifibrosis of microcystins (MCs), we investigate the renoprotection effect of MCs with UUO-induced renal fibrosis. The treatment of MCs was initiated in model animals in advance of UUO operation. After determining that the antifibrotic effect of MCs was independent of its toxicity, our study focused on the renoprotection of microcystin-RR (MC-RR), a lower toxic congener of MCs, in UUO mice and the cell models in vitro. The co-immunoprecipitation assay and recombination plasmid transfection were used in the investigation of the mechanism of antifibrosis of MC-RR. The data show that MC-RR substantially exerts an effect on renoprotection with suppression of the expression of TGF-β1/Smad signaling molecules and a blockage in epithelial dedifferentiation and myofibroblast activation in UUO model animals. MC-RR shows a binding directly to pyruvate kinase M2 (PKM2), downregulates PKM2-HIF-1α signaling, restores the inhibited expression of MMP-7 and MMP-13, and reduces the upregulated expression of MMP-9 in UUO renal tissues. The current study demonstrates a novel effect of MC-RR on renoprotection in kidney damage, which could be conducted in therapeutics for chronic kidney disease.

Movement Disorder and Neurotoxicity Induced by Chronic Exposure to Microcystin-LR in Mice

Microcystins are produced by some species of cyanobacteria, which are hazardous materials to the environment and human beings. It has been demonstrated that microcystin-LR (MC-LR) could disrupt the blood-brain barrier and cause learning and memory deficits, but the neurotoxicity of MC-LR on motor function remains unclear. In this study, the mice were exposed to MC-LR dissolved in drinking water at doses of 1, 7.5, or 15 μg/L for 15 months. We observed that 15 μg/L MC-LR could enter mouse brain tissues such as the cortex, hippocampus, and substantia nigra (SN). And 15 μg/L MC-LR also caused hypokinesia in mice and induced the loss and apoptosis of SN dopaminergic neurons (DA neurons). Meanwhile, MC-LR induced the accumulation of alpha synuclein (α-syn) in DA neurons and decreased the proteins of tyrosine hydroxylase (TH), dopa decarboxylase (DDC) and dopamine transporter (DAT), resulting in a reduction in dopamine (DA) content, which are pathological features of Parkinson's disease (PD). These results suggested that chronic MC-LR might induce PD-like lesions in mice. Moreover, chronic MC-LR exposure caused the inflammatory response in the SN, manifested by the increased numbers of glial cells and the release of inflammatory factors (TNF-α, MCP-1, and IL-6). In vitro, it was proved that MC-LR mediated SH-SY5Y cell apoptosis by activating oxidative stress and damaging mitochondria. Collectively, this study revealed a novel molecular mechanism for MC-LR neurotoxicity with significant implications for human health and the public environment.

Microcystin Contamination and Toxicity: Implications for Agriculture and Public Health

Microcystins are natural hepatotoxic metabolites secreted by cyanobacteria in aquatic ecosystems. When present at elevated concentrations, microcystins can affect water quality aesthetics; contaminate drinking water reservoirs and recreational waters; disrupt normal ecosystem functioning; and cause health hazards to animals, plants, and humans. Animal and human exposures to microcystins generally result from ingesting contaminated drinking water or physically contacting tainted water. Much research has identified a multitude of liver problems from oral exposure to microcystins, varying from hepatocellular damage to primary liver cancer. Provisional guidelines for microcystins in drinking and recreational water have been established to prevent toxic exposures and protect public health. With increasing occurrences of eutrophication in freshwater systems, microcystin contamination in groundwater and surface waters is growing, posing threats to aquatic and terrestrial plants and agricultural soils used for crop production. These microcystins are often transferred to crops via irrigation with local sources of water, such as bloom-forming lakes and ponds. Microcystins can survive in high quantities in various parts of plants (roots, stems, and leaves) due to their high chemical stability and low molecular weight, increasing health risks for consumers of agricultural products. Studies have indicated potential health risks associated with contaminated fruits and vegetables sourced from irrigated water containing microcystins. This review considers the exposure risk to humans, plants, and the environment due to the presence of microcystins in local water reservoirs used for drinking and irrigation. Additional studies are needed to understand the specific health impacts associated with the consumption of microcystin-contaminated agricultural plants.

Are little brown bats (Myotis lucifugus) impacted by dietary exposure to microcystin?✰

The cyanobacterium, Microcystis aeruginosa, can produce the hepatotoxin microcystin. When toxic M. aeruginosa overwinters in the sediments of lakes, it may be ingested by aquatic insects and bioaccumulate in nymphs of Hexagenia mayflies. When volant Hexagenia emerge from lakes to reproduce, they provide an abundant, albeit temporary, food source for many terrestrial organisms including bats. Little brown bats, Myotis lucifugus, feed opportunistically on aquatic insects including Hexagenia. To determine if microcystin moves from aquatic to terrestrial ecosystems via trophic transfer, we combined a dietary analysis with the quantification of microcystin in bat livers and feces. In June 2014, coincident with the local Hexagenia emergence, bat feces were collected from underneath a maternity roost near Little Traverse Lake (Leelanau County, Michigan, USA). Insects in the diet were identified via molecular analyses of fecal pellets from the roost and from individual bats. Livers and feces were collected from 19 female M. lucifugus, and the concentrations of microcystin in these liver tissues and feces were measured using an enzyme-linked immunosorbent assay (ELISA) and liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). We show that the majority of the bats' diets consisted of aquatic insects and that microcystin was detected in high concentrations (up to 129.9 μg/kg dw) in the bat feces by ELISA. Histopathological examination of three bat livers with the highest concentrations of microcystin showed no evidence of phycotoxicosis, indicating that M. lucifugus may not be immediately affected by the ingestion of microcystin. Future work could examine whether bats suffer delayed physiological effects from ingestion of microcystin.

As We Drink and Breathe: Adverse Health Effects of Microcystins and Other Harmful Algal Bloom Toxins in the Liver, Gut, Lungs and Beyond

Freshwater harmful algal blooms (HABs) are increasing in number and severity worldwide. These HABs are chiefly composed of one or more species of cyanobacteria, also known as blue-green algae, such as Microcystis and Anabaena. Numerous HAB cyanobacterial species produce toxins (e.g., microcystin and anatoxin—collectively referred to as HAB toxins) that disrupt ecosystems, impact water and air quality, and deter recreation because they are harmful to both human and animal health. Exposure to these toxins can occur through ingestion, inhalation, or skin contact. Acute health effects of HAB toxins have been well documented and include symptoms such as nausea, vomiting, abdominal pain and diarrhea, headache, fever, and skin rashes. While these adverse effects typically increase with amount, duration, and frequency of exposure, susceptibility to HAB toxins may also be increased by the presence of comorbidities. The emerging science on potential long-term or chronic effects of HAB toxins with a particular emphasis on microcystins, especially in vulnerable populations such as those with pre-existing liver or gastrointestinal disease, is summarized herein. This review suggests additional research is needed to define at-risk populations who may be helped by preventative measures. Furthermore, studies are required to develop a mechanistic understanding of chronic, low-dose exposure to HAB toxins so that appropriate preventative, diagnostic, and therapeutic strategies can be created in a targeted fashion.

Neurotoxicity induced by combined exposure of microcystin-LR and nitrite in male zebrafish (Danio rerio): Effects of oxidant-antioxidant system and neurotransmitter system

With the intensification of water eutrophication around the world, cyanobacterial blooms have been becoming a common environmental pollution problem. The levels of microcystin-LR (MC-LR) and nitrite rise sharply during the cyanobacterial bloom period, which may have potential joint toxicity on aquatic organisms. In this study, adult male zebrafish were immersed into different joint solutions of MC-LR (0, 3, 30 μg/L) and nitrite (0, 2, 20 mg/L) for 30 days to explore the neurotoxic effects and underlying mechanisms. The results showed that single factor MC-LR or nitrite caused a concentration-dependent damage in brain ultrastructure and the effects of their joint exposure were much more intense. Downregulated expression of mbp and bdnf associated with myelination of nerve fibers further confirmed that MC-LR and nitrite could damage the structure and function of neuron. The decreases in dopamine content, acetylcholinesterase activity and related gene mRNA levels indicated that MC-LR and nitrite adversely affected the normal function of the dopaminergic and cholinergic systems in zebrafish brain. In addition, the significant increase in malondialdehyde content suggested the occurrence of oxidative stress caused by MC-LR, nitrite and their joint-exposure, which paralleled a significant decrease in antioxidant enzyme‑manganese superoxide dismutase activity and its transcription level. In conclusion, MC-LR + Nitrite joint-exposure has synergistic neurotoxic effects on the structure and neurotransmitter systems of fish brain, and antioxidant capacity disruption caused by these two factors might be one of the underlying synergistic mechanisms. Therefore, there is a risk of being induced neurotoxicity in fish during sustained cyanobacterial bloom events.

Spirulina platensis protects against microcystin-LR-induced toxicity in rats

Microcystis aeruginosa produces an abundant cyanotoxin (microcystins (MCs) in freshwater supplies. MCs have adverse health hazards to animals and humans. Microcystin-leucine-arginine (microcystin-LR or MC-LR) is the most studied among these MCs due to their high toxicity. So, this study was designed to evaluate the possible therapeutic role of the natural algal food supplement, Spirulina platensis (SP), against MC-LR-induced toxic effects in male Wistar rats. Forty rats were randomly divided into five groups. Control and SP groups orally administered distilled water and SP (1000 mg/kg/daily), respectively, for 21 days. MC-LR group was intraperitoneally injected with MC-LR (10 μg/kg/day) for 14 days. MC-LR-SP₅₀₀ and MC-LR-SP₁₀₀₀ groups were orally treated with SP (500 and 1000 mg/kg, respectively) for 7 days and concomitantly with MC-LR for 14 days. MC-LR induced oxidative hepatorenal damage, cardiotoxicity, and neurotoxicity greatly, which was represented by reduction of reduced glutathione content and the activities of glutathione peroxidase, catalase, and superoxide dismutase and elevation of concentrations of nitric oxide and malondialdehyde in renal, hepatic, brain, and heart tissues. In addition, it increased serum levels of urea, creatinine, tumor necrosis factor-alfa, interleukin-1beta and interleukin-6 and serum activities of alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, creatine kinase, and creatine kinase-MB. However, S. platensis restored normal levels of measured serum parameters, ameliorated MC-LR-induced oxidative damage, and normalized tissue antioxidant biomarkers. In conclusion, SP alleviated MC-induced organ toxicities by mitigating oxidative and nitrosative stress and lipid peroxidation.

Microcystin-leucine arginine exposure induced intestinal lipid accumulation and MC-LR efflux disorder in Lithobates catesbeianus tadpoles

Few studies exist on the toxic effects of chronic exposure to microcystins (MCs) on amphibian intestines, and the toxicity mechanisms are unclear. Here, we evaluated the impact of subchronic exposure (30 days) to environmentally realistic microcystin-leucine arginine (MC-LR) concentrations (0 μg/L, 0.5 μg/L and 2 μg/L) on tadpole (Lithobates catesbeianus) intestines by analyzing the histopathological and subcellular microstructural damage, the antioxidative and oxidative enzyme activities, and the transcriptome levels. Histopathological results showed severe damage accompanied by inflammation to the intestinal tissues as the MC-LR exposure concentration increased from 0.5 μg/L to 2 μg/L. RNA-sequencing analysis identified 634 and 1,147 differentially expressed genes (DEGs) after exposure to 0.5 μg/L and 2 μg/L MC-LR, respectively, compared with those of the control group (0 μg/L). Biosynthesis of unsaturated fatty acids and the peroxisome proliferator-activated receptor (PPAR) signaling pathway were upregulated in the intestinal tissues of the exposed groups, with many lipid droplets being observed on transmission electron microscopy, implying that MC-LR may induce lipid accumulation in frog intestines. Moreover, 2 μg/L of MC-LR exposure inhibited the xenobiotic and toxicant biodegradation related to detoxification, implying that the tadpoles' intestinal detoxification ability was weakened after exposure to 2 μg/L MC-LR, which may aggravate intestinal toxicity. Lipid accumulation and toxin efflux disorder may be caused by MC-LR-induced endoplasmic reticular stress. This study presents new evidence that MC-LR harms amphibians by impairing intestinal lipid metabolism and toxin efflux, providing a theoretical basis for evaluating the health risks of MC-LR to amphibians.

Investigation of microcystin conformation and binding towards PPP1 by molecular dynamics simulation

Microcystins (MC) are a group of structurally similar cyanotoxins with currently 279 described structural variants. Human exposure is frequent by consumption of contaminated water, food or food supplements. MC can result in serious intoxications, commensurate with ensuing pathology in various organs or in rare cases even mortality. The current WHO risk assessment primarily considers MC-LR, while all other structural variants are treated as equivalent to MC-LR, despite that current data strongly suggest that MC-LR is not the most toxic MC, and toxicity can be very different for MC congeners. To investigate and analyse binding and conformation of different MC congeners, we applied for the first time Molecular Dynamics (MD) simulation to four MC congeners (MC-LR, MC-LF, [Enantio-Adda5]MC-LF, [β-D-Asp3,Dhb7]MC-RR). We could show that ser/thr protein phosphatase 1 is stable in all MD simulations and that MC-LR backbone adopts to a second conformation in solvent MD simulation, which was previously unknown. We could also show that MC congeners can adopt to different backbone conformation when simulated in solvent or in complex with ser/thr protein phosphatase 1 and differ in their binding behaviour. Our findings suggest that MD Simulation of different MC congeners aid in understanding structural differences and binding of this group of structurally similar cyanotoxins.

Expression and Function of ABC Proteins in Fish Intestine

In fish, the intestine is fundamental for digestion, nutrient absorption, and other functions like osmoregulation, acid-base balance, and excretion of some metabolic products. These functions require a large exchange surface area, which, in turn, favors the absorption of natural and anthropogenic foreign substances (xenobiotics) either dissolved in water or contained in the food. According to their chemical nature, nutrients, ions, and water may cross the intestine epithelium cells' apical and basolateral membranes by passive diffusion or through a wide array of transport proteins and also through endocytosis and exocytosis. In the same way, xenobiotics can cross this barrier by passive diffusion or taking advantage of proteins that transport physiological substrates. The entry of toxic substances is counterbalanced by an active efflux transport mediated by diverse membrane proteins, including the ATP binding cassette (ABC) proteins. Recent advances in structure, molecular properties, and functional studies have shed light on the importance of these proteins in cellular and organismal homeostasis. There is abundant literature on mammalian ABC proteins, while the studies on ABC functions in fish have mainly focused on the liver and, to a minor degree, on the kidney and other organs. Despite their critical importance in normal physiology and as a barrier to prevent xenobiotics incorporation, fish intestine's ABC transporters have received much less attention. All the ABC subfamilies are present in the fish intestine, although their functionality is still scarcely studied. For example, there are few studies of ABC-mediated transport made with polarized intestinal preparations. Thus, only a few works discriminate apical from basolateral transport activity. We briefly describe the main functions of each ABC subfamily reported for mammals and other fish organs to help understand their roles in the fish intestine. Our study considers immunohistochemical, histological, biochemical, molecular, physiological, and toxicological aspects of fish intestinal ABC proteins. We focus on the most extensively studied fish ABC proteins (subfamilies ABCB, ABCC, and ABCG), considering their apical or basolateral location and distribution along the intestine. We also discuss the implication of fish intestinal ABC proteins in the transport of physiological substrates and aquatic pollutants, such as pesticides, cyanotoxins, metals, hydrocarbons, and pharmaceutical products.

New insights into toxicity of microcystins produced by cyanobacteria using in silico ADMET prediction

In silico methodologies can be used in the discovery of new drugs for measuring toxicity, predicting effects of substances not yet analyzed by in vivo methodologies. The ADMET Predictor® software (absorption, distribution, metabolism, elimination, and toxicity [ADMET]) was used in this work to predict toxic effects of microcystin variants MC-LR, MC-YR, MC-RR, and MC-HarHar. In the case of rodents, predictive results for all analyzed variants indicated carcinogenic potential. The predictive model of respiratory sensitivity in this group differentiated microcystins into 2 categories: sensitizer (MC-LR and -YR) and non-sensitizer (MC-HarHar and -RR). Predictive results for humans indicated that MC-LR and -RR are phospholipidosis inducers; on the other hand, MC-LR showed the highest predictive value of permeability in rabbit cornea and probability of crossing lipoprotein barriers (MC-LR>-YR>-HarHar>-RR). Considering bioavailable fractions, microcystins are more likely to cause biological effects in rats than humans, showing significant differences between models. The results of ADMET predictions add valuable information on microcystin toxicity, especially in the case of variants not yet studied experimentally.

MCLR-elicited hepatic fibrosis and carcinogenic gene expression changes persist in rats with diet-induced nonalcoholic steatohepatitis through a 4-week recovery period

Nonalcoholic steatohepatitis (NASH) causes liver extracellular matrix (ECM) remodeling and is a risk factor for fibrosis and hepatocellular carcinoma (HCC). Microcystin-LR (MCLR) is a hepatotoxin produced by fresh-water cyanobacteria that causes a NASH-like phenotype, liver fibrosis, and is also a risk factor for HCC. The focus of the current study was to investigate and compare hepatic recovery after cessation of MCLR exposure in healthy versus NASH animals. Male Sprague-Dawley rats were fed either a control or a high fat/high cholesterol (HFHC) diet for eight weeks. Animals received either vehicle or 30 μg/kg MCLR (i.p: 2 weeks, alternate days). Animals were euthanized at one of three time points: at the completion of the MCLR exposure period and after 2 and 4 weeks of recovery. Histological staining suggested that after four weeks of recovery the MCLR-exposed HFHC group had less steatosis and more fibrosis compared to the vehicle-exposed HFHC group and MCLR-exposed control group. RNA-Seq analysis revealed dysregulation of ECM genes after MCLR exposure in both control and HFHC groups that persisted only in the HFHC groups during recovery. After 4 weeks of recovery, MCLR hepatotoxicity in pre-existing NASH persistently dysregulated genes related to cellular differentiation and HCC. These data demonstrate impaired hepatic recovery and persistent carcinogenic changes after MCLR toxicity in pre-existing NASH.

Selective interaction of microcystin congeners with zebrafish (Danio rerio) Oatp1d1 transporter

Microcystins (MCs) are the most studied cyanotoxins. The uptake of MCs in cells and tissues of mammals and fish species is mostly mediated by organic anion-transporting polypeptides (OATPs in humans and rodents; Oatps in other species), and the Oatp1d1 appears to be a major transporter for MCs in fish. In this study, six MC congeners of varying physicochemical properties (MC-LR, -RR, -YR, -LW, -LF, -LA) were tested by measuring their effect on the uptake of model Oatp1d1 fluorescent substrate Lucifer yellow (LY) in HEK293T cells transiently or stably overexpressing zebrafish Oatp1d1. MC-LW and -LF showed the strongest interaction resulting in an almost complete inhibition of LY transport with IC₅₀ values of 0.21 and 0.26 μM, while congeners -LR, -YR and -LA showed lower inhibitory effects. To discern between Oatp1d1 substrates and inhibitors, results were complemented by Michaelis-Menten kinetics and chemical analytical determinations of MCs uptake, along with molecular docking studies performed using the developed zebrafish Oatp1d1 homology model. Our study showed that Oatp1d1-mediated transport of MCs could be largely dependent on their basic physicochemical properties, with log P_OW being the most obvious determinant. Finally, apart from determination of the chemical composition of cynobacterial blooms, a reliable risk assessment should take into account the interaction of identified MC congeners with Oatp1d1 as their primary transporter, and herewith we demonstrated that such a comprehensive approach could be based on the use of highly specific in vitro models, accompanied by chemical assessment and in silico molecular docking studies.

Toxic effects of cyanotoxins in teleost fish: A comprehensive review

The phenomenon of eutrophication leads to the global occurrence of algal blooms. Cyanotoxins as produced by many cyanobacterial species can lead to detrimental effects to the biome due to their stability and potential biomagnification along food webs. Therefore, understanding of the potential risks these toxins pose to the most susceptible organisms is an important prerequisite for ecological risks assessment of cyanobacteria blooms. Fishes are an important component of aquatic ecosystems that are prone to direct exposure to cyanotoxins. However, relatively few investigations have focused on measuring the toxic potentials of cyanotoxins in teleost fishes. This review comprehensively describes the major toxicological impacts (such as hepatotoxicity, neurotoxicity, immune toxicity, reproductive toxicity and cytogenotoxicity) of commonly occurring cyanotoxins in teleost fishes. The present work encompasses recent research progresses with special emphasis on the basic molecular mechanisms by which different cyanotoxins impose their toxicities in teleost fishes. The major research areas, which need to be focused on in future scientific investigations, have also been highlighted. Protein kinase inhibition, transcriptional dysregulation, disruption of redox homeostasis and the induction of apoptotic pathways appear to be the key drivers of the toxicological effects of cyanotoxins in fish. Analyses also showed that the impacts of cyanotoxins on specific reproductive processes are relatively less described in teleosts in comparison to mammalian systems. In fact, as compared to other toxicological effects of cyanotoxins, their reproductive toxicity (such as impacts on oocyte development, maturation and their hormonal regulation) is poorly understood in fish, and thus requires further studies. Furthermore, additonal studies characterizing the molecular mechanisms responsible for the cellular uptake of cyanotoxins need to be investigated.

Microcystin-Induced Immunotoxicity in Fishes: A Scoping Review

Cyanobacteria (blue-green algae) have been present on Earth for over 2 billion years, and can produce a variety of bioactive molecules, such as cyanotoxins. Microcystins (MCs), the most frequently detected cyanotoxins, pose a threat to the aquatic environment and to human health. The classic toxic mechanism of MCs is the inhibition of the protein phosphatases 1 and 2A (PP1 and PP2A). Immunity is known as one of the most important physiological functions in the neuroendocrine-immune network to prevent infections and maintain internal homoeostasis in fish. The present review aimed to summarize existing papers, elaborate on the MC-induced immunotoxicity in fish, and put forward some suggestions for future research. The immunomodulatory effects of MCs in fish depend on the exposure concentrations, doses, time, and routes of exposure. Previous field and laboratory studies provided strong evidence of the associations between MC-induced immunotoxicity and fish death. In our review, we summarized that the immunotoxicity of MCs is primarily characterized by the inhibition of PP1 and PP2A, oxidative stress, immune cell damage, and inflammation, as well as apoptosis. The advances in fish immunoreaction upon encountering MCs will benefit the monitoring and prediction of fish health, helping to achieve an ecotoxicological goal and to ensure the sustainability of species. Future studies concerning MC-induced immunotoxicity should focus on adaptive immunity, the hormesis phenomenon and the synergistic effects of aquatic microbial pathogens.

Cyanotoxins and the Nervous System

Cyanobacteria are capable of producing a wide range of bioactive compounds with many considered to be toxins. Although there are a number of toxicological outcomes with respect to cyanobacterial exposure, this review aims to examine those which affect the central nervous system (CNS) or have neurotoxicological properties. Such exposures can be acute or chronic, and we detail issues concerning CNS entry, detection and remediation. Exposure can occur through a variety of media but, increasingly, exposure through air via inhalation may have greater significance and requires further investigation. Even though cyanobacterial toxins have traditionally been classified based on their primary mode of toxicity, increasing evidence suggests that some also possess neurotoxic properties and include known cyanotoxins and unknown compounds. Furthermore, chronic long-term exposure to these compounds is increasingly being identified as adversely affecting human health.

Environmental Microcystin exposure in underlying NAFLD-induced exacerbation of neuroinflammation, blood-brain barrier dysfunction, and neurodegeneration are NLRP3 and S100B dependent

Nonalcoholic fatty liver disease (NAFLD) has been shown to be associated with extrahepatic comorbidities including neuronal inflammation and Alzheimer's-like pathology. Environmental and genetic factors also act as a second hit to modulate severity and are expected to enhance the NAFLD-linked neuropathology. We hypothezied that environmental microcystin-LR (MC-LR), a toxin produced by harmful algal blooms of cyanobacteria, exacerbates the neuroinflammation and degeneration of neurons associated with NAFLD. Using a mouse model of NAFLD, exposed to MC-LR subsequent to the onset of fatty liver, we show that the cyanotoxin could significantly increase proinflammatory cytokine expression in the frontal cortex and cause increased expression of Lcn2 and HMGB1. The above effects were NLRP3 inflammasome activation-dependent since the use of NLRP3 knockout mice abrogated the increase in inflammation. NLRP3 was also responsible for decreased expression of the blood-brain barrier (BBB) tight junction proteins Occludin and Claudin 5 suggesting BBB dysfunction was parallel to neuroinflammation following microcystin exposure. An increased circulatory S100B release, a hallmark of astrocyte activation in MC-LR exposed NAFLD mice also confirmed BBB integrity loss, but the astrocyte activation observed in vivo was NLRP3 independent suggesting an important role of a secondary S100B mediated crosstalk. Mechanistically, conditioned medium from reactive astrocytes and parallel S100B incubation in neuronal cells caused increased inducible NOS, COX-2, and higher BAX/ Bcl2 protein expression suggesting oxidative stress-mediated neuronal cell apoptosis crucial for neurodegeneration. Taken together, MC-LR exacerbated neuronal NAFLD-linked comorbidities leading to cortical inflammation, BBB dysfunction, and neuronal apoptosis.

A Brief Review of the Structure, Cytotoxicity, Synthesis, and Biodegradation of Microcystins

Harmful cyanobacterial blooms pose an environmental health hazard due to the release of water-soluble cyanotoxins. One of the most prevalent cyanotoxins in nature is microcystins (MCs), a class of cyclic heptapeptide hepatotoxins, and they are produced by several common cyanobacteria in aquatic environments. Once released from cyanobacterial cells, MCs are subjected to physical chemical and biological transformations in natural environments. MCs can also be taken up and accumulated in aquatic organisms and their grazers/predators and induce toxic effects in several organisms, including humans. This brief review aimed to summarize our current understanding on the chemical structure, exposure pathway, cytotoxicity, biosynthesis, and environmental transformation of microcystins.

Repeated exposure to microcystin-leucine-arginine potentiates excitotoxicity induced by a low dose of kainate

Microcystin-leucine-arginine (MLCR) is a cyanobacterial toxin, and has been demonstrated to cause neurotoxicity. In addition, MCLR has been identified as an inhibitor of protein phosphatase (PP)1 and PP2A, which are known to regulate the phosphorylation of various molecules related to synaptic excitability. Thus, in the present study, we examined whether MCLR exposure affects seizures induced by a low dose of kainic acid (KA; 0.05 μg, i.c.v.) administration. KA-induced seizure occurrence and seizure score significantly increased after repeated exposure to MCLR (2.5 or 5.0 μg/kg, i.p., once a day for 10 days), but not after acute MCLR exposure (2.5 or 5.0 μg/kg, i.p., 2 h and 30 min prior to KA administration), and hippocampal neuronal loss was consistently facilitated by repeated exposure to MCLR. In addition, repeated MCLR significantly elevated the membrane expression of kainate receptor GluK2 subunits, p-pan-protein kinase C (PKC), and p-extracellular signal-related kinase (ERK) at 1 h after KA. However, KA-induced membrane expression of Ca²⁺/calmodulin-dependent kinase II (CaMKII) was significantly reduced by repeated MCLR exposure. Consistent with the enhanced seizures and neurodegeneration, MCLR exposure significantly potentiated KA-induced oxidative stress and microglial activation, which was accompanied by increased expression of p-ERK and p-PKCδ in the hippocampus. The combined results suggest that repeated MCLR exposure potentiates KA-induced excitotoxicity in the hippocampus by increasing membrane GluK2 expression and enhancing oxidative stress and neuroinflammation through the modulation of p-CaMKII, p-PKC, and p-ERK.