Biochemical and ultrastructural changes of the liver and kidney of the phytoplanktivorous silver carp feeding naturally on toxic Microcystis blooms in Taihu Lake, China

Novel insights into DEHP-induced zebrafish spleen damage: Cellular apoptosis, mitochondrial dysfunction, and innate immunity

DEHP (Di(2-ethylhexyl) phthalate) is the most abundant phthalate component detected in environmental samples as it is widely used in the manufacturing of children's toys, medical devices and furniture. Due to its wide prevalence and propensity to accumulate in the food chain, significant concerns have risen about the safety profile of DEHP. Here, we used a zebrafish model to investigate the toxicity mechanisms of DEHP. Our results indicated that exposure to DEHP altered the ROS content in zebrafish spleen and inhibited the activities of antioxidant enzymes SOD and CAT, detoxification enzyme GSH-Px and induced histopathological damage. In addition, elucidated the mechanism of DEHP significantly promoted apoptosis and caused damage in spleen cells through the bax/bcl-2 pathway. Further genetic testing demonstrated significant alterations in mitochondrial biogenesis, fission, and fusion-related genes and suggested potential mechanistic pathways, including GM10532/m6A/FIS1 axis, the STAT3/POA1 axis, and the NFR1/TFAM axis. Serological and genomic analysis indicated that DEHP exposure activated the C3 complement cascade immune pathway and interfered with innate immune function. IBRv2 analysis proposes that innate immunity may serve as a signal indicator of early toxic responses to DEHP pollutants. This study provided comprehensive cellular and genetic data for DEHP toxicity studies and emphasized the need for future management and remediation of DEHP contamination. It also provides data to specifically support the health risk assessments of DEHP, as well as contributing to broader health and environmental research.

TBBPA and its alternative TCBPA induced ROS-dependent mitochondria-mediated apoptosis in the liver of Rana nigromaculata

Tetrabromobisphenol A (TBBPA), which is the most widely employed brominated flame retardant, and its alternative tetrachlorobisphenol A (TCBPA) are widely distributed in aquatic environments. In the present study, the hepatotoxicity induced by TBBPA and TCBPA was investigated in Rana nigromaculata, and the potential mechanisms were investigated with a particular focus on ROS (reactive oxygen species) -dependent mitochondria-mediated apoptosis. Healthy adult frogs were exposed to 0, 0.001, 0.01, 0.1, and 1 mg/L waterborne TBBPA and TCBPA for 14 days. The results showed that liver weight was significantly increased by 51.52%-98.99% in the 0.01, 0.1, and 1 mg/L TBBPA and TCBPA groups relative to the control. Histological examination revealed that the structure of the liver, to some extent, was influenced by TBBPA and TCBPA with nuclear shrinkage and mitochondrial swelling. Meanwhile, TBBPA and TCBPA have significantly increased the alanine transaminase level in serum and the content of ROS, while inhibiting the activity of superoxide dismutase in the liver. In addition, DNA fragments were observed in the TBBPA and TCBPA groups relative to the control. Expression of Cytochrome C was significantly increased by 1.13-, 1.38-, 1.60-, and 2.46-fold in 0.001, 0.01, 0.1, and 1 mg/L TBBPA, and by 1.26-, 1.51-, 2.14-, and 2.98- fold in 0.001, 0.01, 0.1, and 1 mg/L TCBPA, respectively, which indicated that TCBPA may be more toxic than TBBPA. Similarly, the ratio of Bax/Bcl-2 was increased in a dose-dependent manner. These results indicated that apoptosis in the ROS-dependent mitochondrial pathway mediates hepatotoxicity caused by TBBPA and TCBPA. The present study will facilitate an understanding of the toxicity mechanism of flame retardants.

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.

Cyanobacteria, cyanotoxins, and their histopathological effects on fish tissues in Fehérvárcsurgó reservoir, Hungary

Cyanobacteria are important members of lake plankton, but they have the ability to form blooms and produce cyanotoxins and thus cause a number of adverse effects. Freshwater ecosystems around the world have been investigated for the distribution of cyanobacteria and their toxins and the effects they have on the ecosystems. Similar research was performed on the Fehérvárcsurgó reservoir in Hungary during 2018. Cyanobacteria were present and blooming, and the highest abundance was recorded in July (2,822,000 cells/mL). The species present were Aphanizomenon flos-aquae, Microcystis flos-aquae, Microcystis wesenbergii, Cuspidothrix issatschenkoi, Dolichospermum flos-aquae, and Snowella litoralis. In July and September, the microcystin encoding gene mcyE and the saxitoxin encoding gene sxtG were amplified in the biomass samples. While a low concentration of microcystin-RR was found in one water sample from July, analyses of Abramis brama and Carassius gibelio caught from the reservoir did not show the presence of the investigated microcystins in the fish tissue. However, several histopathological changes, predominantly in gills and kidneys, were observed in the fish, and the damage was more severe during May and especially July, which coincides with the increase in cyanobacterial biomass during the summer months. Cyanobacteria may thus have adverse effects in this ecosystem.

Challenges of using blooms of Microcystis spp. in animal feeds: A comprehensive review of nutritional, toxicological and microbial health evaluation

Microcystis spp., are Gram-negative, oxygenic, photosynthetic prokaryotes which use solar energy to convert carbon dioxide (CO₂) and minerals into organic compounds and biomass. Eutrophication, rising CO₂ concentrations and global warming are increasing Microcystis blooms globally. Due to its high availability and protein content, Microcystis biomass has been suggested as a protein source for animal feeds. This would reduce dependency on soybean and other agricultural crops and could make use of "waste" biomass when Microcystis scums and blooms are harvested. Besides proteins, Microcystis contain further nutrients including lipids, carbohydrates, vitamins and minerals. However, Microcystis produce cyanobacterial toxins, including microcystins (MCs) and other bioactive metabolites, which present health hazards. In this review, challenges of using Microcystis blooms in feeds are identified. First, nutritional and toxicological (nutri-toxicogical) data, including toxicity of Microcystis to mollusks, crustaceans, fish, amphibians, mammals and birds, is reviewed. Inclusion of Microcystis in diets caused greater mortality, lesser growth, cachexia, histopathological changes and oxidative stress in liver, kidney, gill, intestine and spleen of several fish species. Estimated daily intake (EDI) of MCs in muscle of fish fed Microcystis might exceed the provisional tolerable daily intake (TDI) for humans, 0.04 μg/kg body mass (bm)/day, as established by the World Health Organization (WHO), and is thus not safe. Muscle of fish fed M. aeruginosa is of low nutritional value and exhibits poor palatability/taste. Microcystis also causes hepatotoxicity, reproductive toxicity, cardiotoxicity, neurotoxicity and immunotoxicity to mollusks, crustaceans, amphibians, mammals and birds. Microbial pathogens can also occur in blooms of Microcystis. Thus, cyanotoxins/xenobiotics/pathogens in Microcystis biomass should be removed/degraded/inactivated sufficiently to assure safety for use of the biomass as a primary/main/supplemental ingredient in animal feed. As an ameliorative measure, antidotes/detoxicants can be used to avoid/reduce the toxic effects. Before using Microcystis in feed ingredients/supplements, further screening for health protection and cost control is required.

A Review of Nephrotoxicity of Microcystins

Cyanobacterial blooms triggered by eutrophication and climate change have become a global public health issue. The toxic metabolites microcystins (MCs) generated by cyanobacteria can accumulate in food chain and contaminate water, thus posing a potential threat to human and animals health. Studies have suggested that aside liver, the kidney may be another target organ of MCs intoxication. Therefore, this review provides various evidences on the nephrotoxicity of MCs. The review concludes that nephrotoxicity of MCs may be related to inhibition of protein phosphatases and excessive production of reactive oxygen species, cytoskeleton disruption, endoplasmic reticulum stress, DNA damage and cell apoptosis. To protect human from MCs toxic consequences, this paper also puts forward some directions for further research.

Seasonal variation of microcystins and their accumulation in fish in two large shallow lakes of China

Bioaccumulation of microcystins (MCs) has been widely observed in aquatic vertebrates and invertebrates, but its seasonal and specific variations remain unclear. In the present study, dissolved MCs in water, algal cell-bound MCs and muscle tissue MCs of nine fish species were investigated monthly in two of the largest shallow lakes in China: Lake Taihu and Lake Chaohu. The fish species were grouped as carnivorous, planktivorous, and omnivorous fish. Seasonal variations in dissolved and algal cell-bound MCs in water and MCs contents of fish hepatopancreas and muscle were investigated in the two lakes from 2009 to 2010. Dissolved MCs in water ranged from 0.35 to 2.56 µg l^-1 in Lake Taihu and 0.16 to 2.45 µg l^-1 in Lake Chaohu, and showed seasonally a unimodal distribution. Algal cell-bound MCs also showed a similar seasonal variation in both lakes, but dissolved MCs in water peaked about one month later than algal cell-bound MCs. The MCs content in the Fish muscle was higher MCs from October to December than in the other months. For most of the fish species, it exceeded the tolerable daily intake value established by the WHO. The averaged MCs content in the muscle of carnivorous, planktivorous, omnivorous fish was 48.2, 28.7 and 37.8 μg kg^-1 in Lake Taihu, respectively, and 27.8, 18.6 and 20.4 μg kg^-1 in Lake Chaohu. It was significantly higher in carnivorous fish than in planktivorous and omnivorous fish, indicating that carnivorous fish has a higher exposure risk to the local people when consuming the harvested fish. The average ratio of hepatopancreas to muscle MCs contents was 13.0, 25.2, 13.8 for carnivorous, planktivorous, omnivorous fishes in Lake Taihu, respectively, and 18.0, 24.9, 14.8 in Lake Chaohu. These ratio for planktivorous fish almost doubled that for carnivorous and omnivorous fish. High correlation of MC content in carnivorous, omnivorous and planktivorous fish indicates that MCs can be delivered along trophic levels in the food chains.

Chronic Microcystin-LR Exposure Induces Abnormal Lipid Metabolism via Endoplasmic Reticulum Stress in Male Zebrafish

In order to explore effects of low levels of continuous microcystin-LR (MC-LR) (a cyanotoxin) exposure on hepatic lipid metabolism on the basis of the endoplasmic reticulum stress (ERS) pathway, we exposed adult male zebrafish to MC-LR (0, 1, 5, and 25 μg/L) for 60 days, and hepatic histopathology as well as lipid metabolic parameters were determined with mRNA levels of ERS signal molecules and downstream factors, along with genes associated with lipid metabolism in zebrafish liver. The results revealed that prolonged exposure to MC-LR remarkably altered the levels of hepatic total cholesterol and triglyceride and led to hepatic steatosis, which was also confirmed by hepatic cytoplasmic vacuolization in Hematoxylin/eosin (H&E) stain and lipid droplet accumulation in Oil Red O stain. The severity of hepatic damage and lipidation was increased in a dose-related manner. MC-LR exposure significantly upregulated transcriptional levels of ERS markers including hspa5, mapk8, and chop, indicating the occurrence of ERS in the liver of zebrafish. Concurrently, MC-LR significantly improved mRNA expression of unfolded protein response (UPR) pathway-related genes including atf6, eif2ak3, ern1, and xbp1s, suggesting that all of the three UPR branches were activated by MC-LR. MC-LR also induced significant upregulation of downstream lipid metabolism-related factors and genes including srebf1, srebf2, fatty acid synthase (fasn), acetyl-CoA carboxylase (acaca), stearoyl-CoA desaturase (scd), HMG CoA reductase (hmgcra), and HMG CoA synthase (hmgcs1), and downregulation of genes associated with lipolysis such as triglyceride hydrolase gene (atgl), hormone-sensitive enzyme gene (hsla), and carnitine palmitoyltransferase gene (cpt1aa). Our present results indicated that the cause of hepatic lipid accumulation by MC-LR was mainly by upregulating lipogenic and cholesterol genes but downregulating the expression of lipolytic genes through the induction of srebf1 and srebf2, which were involved in the activation of ERS signal pathways.

Single and joint effects of chronic exposure to chlorpyrifos and glyphosate based pesticides on structural biomarkers in Cnesterodon decemmaculatus

Worldwide freshwater bodies that cross agricultural or urban areas are exposed to mixtures of xenobiotics. In particular, pesticides are usually part of these mixtures and could come into direct or indirect contact with biota and therefore, organisms have to cope with this altered scenario and the detrimental effects of these substances. Commercial formulations of chlorpyrifos and glyphosate, and their mixtures were evaluated using a set of biomarkers in the native fish C. decemmaculatus exposed to relevant environmentally pesticides concentrations. The biomarkers measured were: histopathological indices and tissue ultrastructure in liver and nuclear abnormalities and micronuclei in erythrocytes. During 42 days adult females were exposed to the following concentrations of Clorfox and Roundup Max (chlorpyrifos and glyphosate, respectively): 0.84 nl/l and 8.4 nl/l of Clorfox (CF), 0.2 and 2 mg/l of Roundup Max (RM) and all the combinations of these concentrations. Being the low concentrations of both pesticides environmentally relevant. Nuclear abnormalities of erythrocytes were registered under CF, RM and only one mixture. Histological inflammatory alterations increased in individuals exposed to CF and two mixtures. Finally, some pesticide combinations increased the circulatory alterations in liver. Ultrastructural changes in hepatocytes were registered at all the pesticide treatments. The different biomarker responses showed in the mixtures treatments reflected complex interactions, showing the mixture of the low concentrations of both pesticides (the environmentally relevant), potentiated effects. According to our results the presence of these substances in freshwaters could impose important risks for natural populations by causing deleterious effects on the native fish species C. decemmaculatus.

Nitrite Enhances MC-LR-Induced Changes on Splenic Oxidation Resistance and Innate Immunity in Male Zebrafish

Hazardous contaminants, such as nitrite and microcystin-leucine arginine (MC-LR), are released into water bodies during cyanobacterial blooms and may adversely influence the normal physiological function of hydrobiontes. The combined effects of nitrite and MC-LR on the antioxidant defense and innate immunity were evaluated through an orthogonal experimental design (nitrite: 0, 29, 290 μM; MC-LR: 0, 3, 30 nM). Remarkable increases in malondialdehyde (MDA) levels have suggested that nitrite and/or MC-LR exposures induce oxidative stress in fish spleen, which were indirectly confirmed by significant downregulations of total antioxidant capacity (T-AOC), glutathione (GSH) contents, as well as transcriptional levels of antioxidant enzyme genes cat1, sod1 and gpx1a. Simultaneously, nitrite and MC-LR significantly decreased serum complement C3 levels as well as the transcriptional levels of splenic c3b, lyz, il1β, ifnγ and tnfα, and indicated that they could jointly impact the innate immunity of fish. The severity and extent of splenic lesions were aggravated by increased concentration of nitrite or MC-LR and became more serious in combined groups. The damages of mitochondria and pseudopodia in splenic macrophages suggest that oxidative stress exerted by nitrite and MC-LR aimed at the membrane structure of immune cells and ultimately disrupted immune function. Our results clearly demonstrate that nitrite and MC-LR exert synergistic suppressive effects on fish innate immunity via interfering antioxidant responses, and their joint toxicity should not be underestimated in eutrophic lakes.

Single and combined exposure of microcystin-LR and nitrite results in reproductive endocrine disruption via hypothalamic-pituitary-gonadal-liver axis

Microcystin-LR (MC-LR) released by Microcystis blooms degradation usually co-exists with a chemical called nitrite, posing a serious harm to aquatic organisms. To assess the single and combined effects of MC-LR and nitrite on the reproductive endocrine system, a fully factorial experiment was designed and adult male zebrafish (Danio rerio) were exposed to 9 treatment combinations of MC-LR (0, 3, 30 μg/L) and nitrite (0, 2, 20 mg/L) for 30 d. The results showed that both MC-LR and nitrite caused concentration-dependent effects including the growth inhibition, decreased gonad index as well as testicular injuries with widen intercellular spaces and seminiferous epithelium deteriorations. And testicular pathological changes in the co-exposure groups of MC-LR and nitrite were similar but more serious than those in single-factor exposure groups. Concurrently, exposure to MC-LR or nitrite alone could significantly decrease T levels by downregulating gene expressions (gnrh2, lhβ, ar, lhr) in the hypothalamic-pituitary-gonadal-liver-axis (HPGL-axis), and there were significant interactions between MC-LR and nitrite on them. In contrast, E2 levels as well as transcriptional levels of cyp19a1b, cyp19a1a and vtg1 showed significant inductions with increasing MC-LR concentrations, indicating an estrogen-like effect of MC-LR. Our findings illustrated that co-exposure of MC-LR and nitrite synergistically cause reproductive dysfunction by interfering with the HPGL axis in male fish, which prompt us to focus more on the potential risks in fish reproduction and even population dynamics due to the wide occurrence of toxic cyanobacterial blooms.

The synergistic effects of waterborne microcystin-LR and nitrite on hepatic pathological damage, lipid peroxidation and antioxidant responses of male zebrafish

Hazardous materials from decaying cyanobacterial blooms, such as microcystin-LR (MC-LR) and nitrite pose serious challenges to aquatic organisms. To assess combined toxic effects of MC-LR and nitrite on hepatic pathology, lipid peroxidation and antioxidant responses of fish, adult male zebrafish (Danio rerio) were exposed to solutions with different combined concentrations of MC-LR (0, 3, 30 μg/L) and nitrite (0, 2, 20 mg/L) for 30 d. The results showed that hepatic pathological lesions progressed in severity and extent with increasing concentration of single factor MC-LR or nitrite and became more severe in co-exposure groups. Concurrently, significant increases in malondialdehyde (MDA) revealed the occurrence of oxidative stress caused by MC-LR, nitrite and both of them, which was indirectly verified by remarkable decreases in the total antioxidant capacity (T-AOC) as well as the transcription and activity of antioxidant enzymes (CAT and GPx). Hepatic mitochondria were damaged as the common action site of MC-LR and nitrite, suggesting that oxidative stress played a significant role in the mechanisms of the hepatotoxicity of MC-LR and nitrite. The depletion of hepatic glutathione (GSH) indicated the importance of GSH/glutathione-S-transferases (GST) system in these two chemicals detoxification. These results clearly illustrated that MC-LR and nitrite have synergistic effects on the histostructure, antioxidant capacity and detoxification capability in the liver of zebrafish. Therefore, the combined pollution of MC-LR and nitrite in eutrophic lakes can reduce the defense mechanism of the fish and accelerate the consumption of GSH, which compromise the survival of the fish during prolonged cyanobacterial blooms episodes.

A Systematic Investigation into the Environmental Fate of Microcystins and The Potential Risk: Study in Lake Taihu

A systematic investigation was conducted in Lake Taihu in autumn of 2013 and 2014, in order to understand the environmental fate of microcystins (MCs) and evaluate the health risk from MCs. Samples of water, algal cells, macrophytes, shrimps and fish were taken to detect MCs by HPLC-MS/MS after solid phase extraction. Widespread MC contamination in water, algal cells, macrophytes, shrimps and fish was found in Lake Taihu. The ubiquitous presence of MCs in water, algal cells and biota was found in 100% of samples. MC accumulation was in the order of primary producer > tertiary consumer > secondary consumer > primary consumer. The highest levels of MCs in macrophytes, shrimps and fish tissue were found in Potamogeton maackianus, Exopalaemon modestus, and Hyporhamphus intermedius, respectively. The MCs level in shrimps and the tissues of three fish species, Neosalanx tangkahkeii taihuensis, Coilia ectenes and silver carp, was closely linked to their dietary exposure. Ceratophyllum demersum L. was an ideal plant for introduction into lakes to protect against Microcystis blooms and MCs, due to its ability to absorb nutrients, accumulate large amounts of MCs and tolerate these toxins compared to other macrophytes. The average daily intakes (ADIs) of MCs for Exopalaemon modestus and three fish species, Coilia ectenes, Hyporhamphus intermedius and Carassius carassius, were all above the tolerable daily intakes (TDI) set by the World Health Organization (WHO), implying there existed potential threats to human health.

Life-cycle exposure to microcystin-LR interferes with the reproductive endocrine system of male zebrafish

Recently, MC-LR reproductive toxicity drew great attention. Limited information was available on endocrine-disrupting effects of MC-LR on the reproduction system in fish. In the present study, zebrafish hatchlings (5 d post-fertilization) were exposed to 0, 0.3, 3 and 30μg/L MC-LR for 90 d until they reached sexual maturity. Male zebrafish were selected, and changes in growth and developmental parameters, testicular histological structure as well as the levels of gonadal steroid hormones were studied along with the related-gene transcriptional responses in the hypothalamic-pituitary-gonadal axis (HPG-axis). The results, for the first time, show a life cycle exposure to MC-LR causes growth inhibition, testicular damage and delayed sperm maturation. A significant decrease in T/E2 ratio indicated that MC-LR disrupted sex steroid hormones balance. The changes in transcriptional responses of HPG-axis related genes revealed that MC-LR promoted the conversion of T to E2 in circulating blood. It was also noted that vtg1 mRNA expression in the liver was up-regulated, which implied that MC-LR could induce estrogenic-like effects at environmentally relevant concentrations and long-term exposure. Our findings indicated that a life cycle exposure to MC-LR causes endocrine disruption with organic and functional damage of the testis, which might compromise the quality of life for the survivors and pose a potent threat on fish reproduction and thus population dynamics in MCs-contaminated aquatic environments.

Quantitatively evaluating detoxification of the hepatotoxic microcystin-LR through the glutathione (GSH) pathway in SD rats

Glutathione (GSH) plays crucial roles in antioxidant defense and detoxification metabolism of microcystin-LR (MC-LR). However, the detoxification process of MC-LR in mammals remains largely unknown. This paper, for the first time, quantitatively analyzes MC-LR and its GSH pathway metabolites (MC-LR-GSH and MC-LR-Cys) in the liver of Sprague-Dawley (SD) rat after MC-LR exposure. Rats received intraperitoneal (i.p.) injection of 0.25 and 0.5 lethal dose 50 (LD50) of MC-LR with or without pretreatment of buthionine-(S,R)-sulfoximine (BSO), an inhibitor of GSH synthesis. The contents of MC-LR-GSH were relatively low during the experiment; however, the ratio of MC-LR-Cys to MC-LR reached as high as 6.65 in 0.5 LD50 group. These results demonstrated that MC-LR-GSH could be converted to MC-LR-Cys efficiently, and this metabolic rule was in agreement with the data of aquatic animals previously reported. MC-LR contents were much higher in BSO + MC-LR-treated groups than in the single MC-LR-treated groups. Moreover, the ratio of MC-LR-Cys to MC-LR decreased significantly after BSO pretreatment, suggesting that the depletion of GSH induced by BSO reduced the detoxification of MCs. Moreover, MC-LR remarkably induced liver damage, and the effects were more pronounced in BSO pretreatment groups. In conclusion, this study verifies the role of GSH in the detoxification of MC-LR and furthers our understanding of the biochemical mechanism for SD rats to counteract toxic cyanobacteria.

Biomarkers involved in energy metabolism and oxidative stress response in the liver of Goodea gracilis Hubbs and Turner, 1939 exposed to the microcystin-producing Microcystis aeruginosa LB85 strain

Goodea gracilis is an endemic fish that only habitats in some water bodies of Central Mexico that are contaminated with cyanobacteria-producing microcystins (MC); however, a lack of information on this topic prevails. With the aim to generate the first approximation about the physiological changes elicited by cyanobacterium that produce MC congeners in this fish species, specimens born in the laboratory was exposed for 96 h to cell densities of 572.5, 1145, 2290, 4580, and 9160 × 10(6) cells of Microcystis aeruginosa strain LB85/L, and a set of novel endpoint related to hepatic gluconeogenesis (ADH/LDH) and pro-oxidant forces O2., H2 O2 ) in addition to biomarkers of oxidative damage and antioxidant response was evaluated in the liver. Results suggest that high inhibition of protein serine/threonine phosphatase (PP) may trigger many metabolic processes, such as those related to hepatic gluconeogenesis (ADH/LDH) and pro-oxidant O2⋅, H2 O2 , TBARS, ROOH, RC=O) as well as antioxidant (SOD, CAT, GPx) response to oxidative stress. Particularly, we observed that inhibition of LDH and PP, and H2 O2 increase and TBARS production were the key damages induced by high densities of M. aeruginosa. However, changes between aerobic and anaerobic metabolism related with ROS metabolism and ADH/LDH balance are apparently an acclimation of this fish species to exposure to cyanobacteria or their MCs. Fish species living in environments potentially contaminated with cyanobacteria or their MCs possess mechanisms of acclimation that allow them to offset the damage induced, even in the case of fish that have never been exposed to MCs.

Toxicopathology induced by microcystins and nodularin: a histopathological review

Cyanobacteria are present in all aquatic ecosystems throughout the world. They are able to produce toxic secondary metabolites, and microcystins are those most frequently found. Research has displayed a negative influence of microcystins and closely related nodularin on fish, and various histopathological alterations have been observed in many organs of the exposed fish. The aim of this article is to summarize the present knowledge of the impact of microcystins and nodularin on the histology of fish. The observed negative effects of cyanotoxins indicate that cyanobacteria and their toxins are a relevant medical (due to irritation, acute poisoning, tumor promotion, and carcinogenesis), ecotoxicological, and economic problem that may affect both fish and fish consumers including humans.

Damage and recovery of the ovary in female zebrafish i.p.-injected with MC-LR

Up to now, in vivo studies on toxic effects of microcystins (MCs) on the reproductive system are limited and the underlying molecular mechanisms of MCs-induced reproductive toxicity remain to be elucidated. In an acute toxic experiment, female zebrafish (Danio rerio) were injected intraperitoneally (i.p.) at doses of 50 and 200 μg MC-LR/kg body weight (BW) respectively, and histopathological lesions and antioxidant enzymatic activities and gene expression in the ovary were studied at 1, 3, 12, 24, 48 and 168 h post injection (hpi). Pathological lesions of zebrafish ovary progressed in severity and extent with the increasing exposure time and dose within 12 hpi. Concurrently, the increases in malondialdehyde (MDA) contents as well as the enzymatic activities and transcriptional levels of antioxidant enzymes catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) showed the occurrence of oxidative stress, indicating that MC-LR induced adverse effects on the structure and functional activity of zebrafish ovary. Oxidative stress plays a significant role in the reproductive toxicity of MC-LR. The significant decrease of glutathione (GSH) content in zebrafish ovary suggested the importance of MC-LR detoxification by glutathione S-transferases (GST) via GSH. The final recovery of histostructure and antioxidative indices indicated that ovarian efficient antioxidant defense system might be an important mechanism of zebrafish to counteract MC-LR. Although the negative effects of MC-LR can be overcome by ovarian antioxidant system in this study, the potential reproductive risks of MC-LR should not be neglected because of its wide occurrence.

Presence and bioaccumulation of microcystins and cylindrospermopsin in food and the effectiveness of some cooking techniques at decreasing their concentrations: a review

Microcystins (MCs) and cylindrospermopsin (CYN) are among the cyanotoxins which occur naturally, produced by different cyanobacteria species when they grow or proliferate under favorable environmental conditions. From a toxicological point of view, their relevance is due to the deleterious effects that they have been reported to induce in a wide range of organisms, including humans. Cyanotoxins intake from contaminated water and food is an important source of human exposure. Various edible aquatic organisms, plants, and food supplements based on algae, can bioaccumulate these toxins. A thorough review of the scientific data available on this topic is provided, the studies on MCs being much more numerous than those focused on CYN. The scientific literature suggests that these cyanotoxins can be accumulated at concentrations higher than their respective recommended tolerable daily intake (TDI). Finally, the influence of different cooking procedures on their levels in food has been considered. In this regard, again studies on the matter dealing with CYN have been not yet raised. MCs contents have been reported to be reduced in muscle of fish after boiling, or cooking in a microwave-oven, although the effect of other traditional cooking processes such as frying, roasting or grilling have not been demonstrated.

Short-term uptake of microcystin-LR by Coregonus lavaretus: GST activity and genotoxicity

In the present study, juvenile whitefish weighing 2 g were exposed by force-feeding to two ecologically relevant doses (0.05 and 0.5 μg per fish) of microcystin-LR (MC-LR). Then over 96 h the MC uptake in fish liver and muscle was measured, as the activity of the detoxification enzyme glutathione S-transferase (GST) in the liver, and the genotoxicity impact on red blood cells. Results show that (1) the MC-LR equivalent concentrations increased for both doses and in both organs of whitefish with approximately threefold lower concentrations for the low dose compared to the high dose in both organs and threefold lower concentrations in the muscle compared to the liver for each dose (2) the liver GST activity increased during the first 48 h of exposure with fivefold higher GST activity for the highest dose at 48 h compared to control and (3) MC-LR leads to deoxyribonucleic acid strand breaks that were detected by the comet assay and shown to be partially repaired. This work demonstrates that European whitefish could be impacted by cyanobacteria toxins due to rapid microcystin uptake, especially in the context of chronic contamination, which can occur during long bloom episodes.

Microcystin-LR induces cytotoxicity and affects carp immune cells by impairment of their phagocytosis and the organization of the cytoskeleton

Microcystin-LR (MC-LR) is the main isoform of hepatotoxin produced by cyanobacteria, which occur worldwide in the aquatic environment. The present study investigated the in vitro toxic MC-LR effects on immune cells isolated from the blood of carp. Cells were exposed to different MC-LR concentrations ranging from 0.01 to 1 µg ml(-1) for 2, 6 and 24 h. In addition, the effect of the toxin on the phagocytic activity of leukocytes and on actin and tubulin re-organization in phagocytic cells was studied. We observed that MC-LR induces apoptosis in lymphocytes 2 h after incubation, whereas high toxin concentrations induced necrosis in lymphocytes in a time- and concentration-dependent manner. Incubation of the cells for 2 h with 0.1 and 1 µg ml(-1) MC-LR inhibited phagocytosis without affecting apoptosis or glutathione (GSH) levels. Moreover, at this time point and with these concentrations, the toxin also induced a significant re-organization of the actin cytoskeleton in phagocytes, which subsequently collapsed around the nucleus leading to cell shrinkage and the disappearance of filopodia. These results suggest that both phagocytes and lymphocytes are targets for MC-LR and the disturbances of phagocytosis may impair the balance of the immune system.

Involvement of endoplasmic reticulum and autophagy in microcystin-LR toxicity in Vero-E6 and HepG2 cell lines

This work investigates the involvement of the endoplasmic reticulum (ER) and autophagy in microcystin-LR (MCLR) toxicity in Vero-E6 and HepG2 cell lines. Additionally, morphological alterations induced by MCLR in lysosomes and mitochondria were studied. Cytotoxicity evaluation showed that pure MCLR and MCLR from LMECYA110 extract induce concentration dependent viability decays after 24h exposure. HepG2 cells showed an increased sensitivity to MCLR than Vero cells, with lower cytotoxic thresholds and EC(50) values. Conversely, LC3B immunofluorescence showed that autophagy is triggered in both cell lines as a survival response to low MCLR concentrations. Furthermore, MCLR induced a MCLR concentration-dependent decrease of GRP94 expression in HepG2 cells while in Vero cells no alteration was observed. This suggests the involvement of the ER in HepG2 apoptosis elicited by MCLR, while in Vero cells ER destructuration could be a consequence of cytoskeleton inflicted damages. Additionally, in both cell lines, lysosomal destabilization preceded mitochondrial impairment which occurred at high toxin concentrations. Although not an early cellular target of MCLR, mitochondria appears to serve as central mediators of different signaling pathways elicited by the organelles involved in MCLR toxicity. As a result, kidney and hepatic cell lines exhibit cell type and dose-dependent mechanisms to overcome MCLR toxicity.

Oxidative stress and detoxification biomarker responses in aquatic freshwater vertebrates exposed to microcystins and cyanobacterial biomass

Cyanobacterial blooms represent a serious threat to the aquatic environment. Among other effects, biochemical markers have been studied in aquatic vertebrates after exposures to toxic cyanobacteria. Some parameters such as protein phosphatases may serve as selective markers of exposure to microcystins, but under natural conditions, fish are exposed to complex mixtures, which affect the overall biomarker response. This review aims to provide a critical summary of biomarker responses in aquatic vertebrates (mostly fish) to toxic cyanobacteria with a special focus on detoxification and oxidative stress. Detoxification biomarkers such as glutathione (GSH) and glutathione-S-transferase (GST) showed very high variability with poor general trends. Often, stimulations and/or inhibitions and/or no effects at GSH or GST have been reported, even within a single study, depending on many variables, including time, dose, tissue, species, etc. Most of the oxidative stress biomarkers (e.g., superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase) provided more consistent responses, but only lipid peroxidation (LPO) seemed to fulfill the criteria needed for biomarkers, i.e., a sufficiently long half-life and systematic response. Indeed, reviewed papers demonstrated that toxic cyanobacteria systematically elevate levels of LPO, which indicates the important role of oxidative damage in cyanobacterial toxicity. In summary, the measurement of biochemical changes under laboratory conditions may provide information on the mode of toxic action. However, comparison of different studies is very difficult, and the practical use of detoxification or oxidative stress biomarkers as diagnostic tools or early warnings of cyanobacterial toxicity is questionable.

Nephrotoxic effects from chronic toxic cyanobacterial blooms in fishes with different trophic levels in a large Chinese lake

Physiological and biochemical responses in kidneys of fishes with different trophic levels to toxic cyanobacterial blooms were studied. We sampled four fishes: the phytoplanktivorous Hypophthalmichthys molitrix and Aristichthys nobilis, the omnivorous Carassius auratus, and the carnivorous Culter ilishaeformis. Alterations of the antioxidant (GSH) and the major antioxidant enzymes (CAT, SOD, GPx, GST) were monitored monthly. Catalase and glutathione S-transferase were significantly higher during blooms than before and after blooms. All fishes showed ultrastructural alterations during blooms, which mainly are inosculation of foot processes in epithelial cell of glomeruli and mitochondria swelling in the proximal tubules. The results suggested that kidney impairment from chronic exposure of toxic cyanobacterial blooms might be the first step, and then followed by hepatic failure. Compared with livers in terms of physiological status, the weaker antioxidant ability of kidney made it more susceptible to chronic MCs exposure, besides its effective accumulation of MC metabolites.

Early biochemical effects of Microcystis aeruginosa extracts on juvenile rainbow trout (Oncorhynchus mykiss)

Microcystins (MC) are usually the predominant cyanotoxins associated with cyanobacterial blooms in natural surface waters. These toxins are well-known hepatotoxic agents that proceed by inhibiting protein phosphatase in aquatic biota; recent studies have also reported oxidative stress and disruption of ion regulation in aquatic organisms. In the present study, young trout (Oncorhynchus mykiss) were exposed to crude extracts of Microsystis aeruginosa for four days at 15 °C. The level of microcystins was calculated to confirm the presence of toxins in these crude extracts: 0, 0.75, 1.8 and 5 μg/L. Protein phosphatase measured in the liver increased by at least 3-fold and is significantly as a result of exposure to these sublethal concentrations of crude extract, his indicates an early defense response against protein phosphatase inhibition from cyanotoxins. This was corroborated by the decreased phosphate content in proteins found in the liver and brain. No increase in glutathione-S transferase (GST) activity was observed and lipid peroxidation was unaffected in both liver and brain tissue exposed to the cyanobacterial extracts. The data revealed that the proportion of the reduced (metal-binding) form of metallothionein (MT) decreased by two-fold relative to the control group (with a concomitant increase in the proportion of the oxidized form). The level of phosphate associated with MT increased by 1.5-fold at the highest concentration of crude extract. Acetylcholinesterase (AChE) activity in brain tissue was decreased after exposure to the highest concentration of crude extract, suggesting a slowdown in neural activity. However, no biotransformation processes or detoxification of GST was triggered. Our findings show early sign of biochemical effects of MC-LR in young trout.

Experimental model of microcystin accumulation in the liver of Oreochromis niloticus exposed subchronically to a toxic bloom of Microcystis sp

Although accumulation of the liver toxin microcystin in phytoplanktivorous fish has been demonstrated in captive fish and in natural ecosystems, the relation between microcystin in ingested algae and the pattern of buildup of microcystin in fish is poorly known. In this month-long study performed at a Brazilian fish farm, 45 mature Oreochromis niloticus were fed daily with fresh seston periodically dominated by toxic Microcystis sp. Microcystin was measured daily in the food and every 5 days in liver and muscle samples. Control fish received a diet of seston that was low in toxic cyanobacteria. Initially, in treatment ponds, microcystin available for fish increased from 6.5 to 66.9 ng microcystin fish(-1)day(-1), which was accompanied by an increase from 5.5 to 35.4 ng microcysting liver(-1). Microcystin in muscle was below our detection limit of 4 ng g tissue(-1) for the entire study. In the bloom phase, available microcystin reached its highest concentration (4450 ng MC fish(-1)day(-1)) then decreased to 910 ng microcystin fish(-1)day(-1) on day 31. During this period, microcystin reached its highest concentration of 81.6 ng MC g liver(-1) and stayed high until the end of the experiment. A model based on rapid uptake, saturation, and exponential loss was built with these experimental results, and verified with data from the literature. Our model showed that accumulation was up to 50% of ingestion at low doses, but at intermediate doses, the onset of elimination led to a decline of liver burden. Although the accumulation rate confirms the high contamination potential of microcystin, it was balanced by a high depuration rate and this efficient systemic elimination may explain the tolerance of these fish to toxic blooms in the wild.

Health risks associated with consumption of microcystin-contaminated fish and shellfish in three Chinese lakes: significance for freshwater aquacultures

The risks associated with consuming aquatic products were systematically evaluated by analyzing 26 economically important fish and shellfish species which were harvested monthly from three large lakes in China during the fisheries catch season. Results indicate most of the aquatic products from the three large lakes seem to be unsafe for human consumption due to microcystin accumulations, with the estimated daily intake (EDI) values 5-148 times, 2-50 times and 1.5-4 times higher than the tolerable daily intake (TDI) value in Taihu, Chaohu and Dianchi, respectively. In addition, the toxin accumulation in the harvested organisms varied intensity from month to month and by species which suggests that consumption risks may be reduced or avoided by either adjusting the legal fishing seasons or the species of fish and shellfish harvested. This study will provide new information about the risks associated with the consumption of aquatic products and suggests possible management strategies to reduce or avoid potential health risks.

Histopathology and microcystin distribution in Lymnaea stagnalis (Gastropoda) following toxic cyanobacterial or dissolved microcystin-LR exposure

The accumulation of hepatotoxic microcystins (MCs) in gastropods has been demonstrated to be higher following grazing of toxic cyanobacteria than from MCs dissolved in ambient water. Previous studies, however, did not adequately consider MCs covalently bound to protein phosphatases, which may represent a considerably part of the MC body burden. Thus, using an immunohistochemical method, we examined and compared the histopathology and organ distribution of covalently bound MCs in Lymnaea stagnalis following a 5-week exposure to (i) dmMC-LR, dmMC-RR, and MC-YR-producing Planktothrix agardhii (5 microg MC-LReqL(-1)) and (ii) dissolved MC-LR (33 and 100 microgL(-1)). A subsequent 3-week depuration investigated potential MC elimination and tissue regeneration. Following both exposures, bound MCs were primarily observed in the digestive gland and tract of L. stagnalis. Snails exposed to toxic cyanobacteria showed severe and widespread necrotic changes in the digestive gland co-occurring with a pronounced cytoplasmic presence of MCs in digestive cells and in the lumen of digestive lobules. Snails exposed to dissolved MC-LR showed moderate and negligible pathological changes of the digestive gland co-occurring with a restrained presence of MCs in the apical membrane of digestive cells and in the lumen of digestive lobules. These results confirm lower uptake of dissolved MC-LR and correspondingly lower cytotoxicity in the digestive gland of L. stagnalis. In contrast, after ingestion of MC-containing cyanobacterial filaments, the most likely longer residual time within the digestive gland and/or the MC variant involved (e.g., MC-YR) allowed for increased MC uptake, consequently a higher MC burden in situ and thus a more pronounced ensuing pathology. While no pathological changes were observed in kidney, foot and the genital gland, MCs were detected in spermatozoids and oocytes of all exposed snails, most likely involving a hemolymph transport from the digestive system to the genital gland. The latter results indicate the potential for adverse impact of MCs on gastropod health and reproduction as well as the possible transfer of MCs to higher trophic levels of the food web.

Plasma biochemical responses of the planktivorous filter-feeding silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) to prolonged toxic cyanobacterial blooms in natural waters

The planktivorous filter-feeding silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) are the attractive candidates for bio-control of plankton communities to eliminate odorous populations of cyanobacteria. However, few studies focused on the health of such fishes in natural water body with vigorous toxic blooms. Blood parameters are useful and sensitive for diagnosis of diseases and monitoring of the physiological status of fish exposed to toxicants. To evaluate the impact of toxic cyanobacterial blooms on the planktivorous fish, 12 serum chemistry variables were investigated in silver carp and bighead carp for 9 months, in a large net cage in Meiliang Bay, a hypereutrophic region of Lake Taihu. The results confirmed adverse effects of cyanobacterial blooms on two phytoplanktivorous fish, which mainly characterized with potential toxicogenomic effects and metabolism disorders in liver, and kidney dysfunction. In addition, cholestasis was intensively implied by distinct elevation of all four related biomarkers (ALP, GGT, DBIL, TBIL) in bighead carp. The combination of LDH, AST activities and DBIL, URIC contents for silver carp, and the combination of ALT, ALP activities and TBIL, DBIL, URIC concentrations for bighead carps were found to most strongly indicate toxic effects from cyanobacterial blooms in such fishes by a multivariate discriminant analysis.

In vivo studies on the toxic effects of microcystins on mitochondrial electron transport chain and ion regulation in liver and heart of rabbit

This study examined the toxic effects of microcystins on mitochondria of liver and heart of rabbit in vivo. Rabbits were injected i.p. with extracted microcystins (mainly MC-RR and -LR) at two doses, 12.5 and 50 MC-LReq. microg/kg bw, and the changes in mitochondria of liver and heart were studied at 1, 3, 12, 24 and 48 h after injection. MCs induced damage of mitochondrial morphology and lipid peroxidation in both liver and heart. MCs influenced respiratory activity through inhibiting NADH dehydrogenase and enhancing succinate dehydrogenase (SDH). MCs altered Na(+)-K(+)-ATPase and Ca(2+)-Mg(2+)-ATPase activities of mitochondria and consequently disrupted ionic homeostasis, which might be partly responsible for the loss of mitochondrial membrane potential (MMP). MCs were highly toxic to mitochondria with more serious damage in liver than in heart. Damage of mitochondria showed reduction at 48 h in the low dose group, suggesting that the low dose of MCs might have stimulated a compensatory response in the rabbits.

Detoxification and oxidative stress responses along with microcystins accumulation in Japanese quail exposed to cyanobacterial biomass

The cyanobacterial exposure has been implicated in mass mortalities of wild birds, but information on the actual effects of cyanobacteria on birds in controlled studies is missing. Effects on detoxification and antioxidant parameters as well as bioaccumulation of microcystins (MCs) were studied in birds after sub-lethal exposure to natural cyanobacterial biomass. Four treatment groups of model species Japanese quail (Coturnix coturnix japonica) were exposed to controlled doses of cyanobacterial bloom during acute (10 days) and sub-chronic (30 days) experiment. The daily doses of cyanobacterial biomass corresponded to 0.2-224.6 ng MCs/g body weight. Significant accumulation of MCs was observed in the liver for both test durations and slight accumulation also in the muscles of the highest treatment group from acute test. The greatest accumulation was observed in the liver of the highest treatment group in the acute test reaching average concentration of 43.7 ng MCs/g fresh weight. The parameters of detoxification metabolism and oxidative stress were studied in the liver, heart and brain. The cyanobacterial exposure caused an increase of activity of cytochrome P-450-dependent 7-ethoxyresorufin O-deethylase representing the activation phase of detoxification metabolism. Also the conjugation phase of detoxification, namely the activity of glutathione-S-transferase, was altered. Cyanobacterial exposure also modulated oxidative stress responses including the level of glutathione and activities of glutathione-related enzymes and caused increase in lipid peroxidation. The overall pattern of detoxification parameters and oxidative stress responses clearly separated the control and the lowest exposure group from all the higher exposed groups. This is the first controlled study documenting the induction of oxidative stress along with MCs accumulation in birds exposed to natural cyanobacterial biomass. The data also suggest that increased activities of detoxification enzymes could lead to greater biotransformation and elimination of the MCs at the longer exposure time.

Microcystin-induced variations in transcription of GSTs in an omnivorous freshwater fish, goldfish

The glutathione S-transferases are important enzymes in the microcystin-induced detoxication processes. In this experiment, we cloned the full-length cDNA of alpha, pi and theta-class-like glutathione S-transferase genes from goldfish (Carassius auratus L). Their derived amino acid sequences were clustered with other vertebrate alpha, pi and theta-class GSTs in a phylogenetic tree and the goldfish GST sequences have the highest similarity with those from common carp and zebrafish. Goldfish were i.p. injected with microcystins extract at two doses (50 and 200microgkg(-1)BW MC-LReq) and the relative changes of the mRNA abundance in liver, kidney and intestine were analyzed by real-time PCR. The transcription of GST alpha was suppressed in both liver and intestine, but induced in the kidney. Decreased transcription of GST theta was detected in liver, kidney and intestine in the low-dose group. The transcription of GST pi was suppressed in liver and intestine post-injection in both dose groups. These results suggested that the transcription of GST isoforms varied in different ways within an organ and among organs of goldfish exposed to MCs.

Acute Effects of Microcystins MC-LR and MC-RR on Acid and Alkaline Phosphatase Activities and Pathological Changes in Intraperitoneally Exposed Tilapia Fish (Oreochromis sp.)

Microcystins (MC) are frequently present in cyanobacterial blooms in rivers and lakes, increasing the risk of toxicity to both animals and humans. There more than eighty reported microcystins, and the present study was undertaken to determine whether MC-LR and MC-RR can induce different enzyme alterations and histopathological changes in tilapia fish ( Oreochromis sp.) exposed to a single intraperitoneal (i.p.) injection of the pure standards (MC-LR and MC-RR) at a dose of 500 μg/kg; the tilapia fish were then observed for seven days. The two MC variants caused significant changes in the activities of acid and alkaline phosphatases (ACP and ALP) in vital organs, showing a different response pattern. The livers and kidneys of fish injected with MC-LR were particularly affected. MC-RR induced a very pronounced increase of ACP in the kidney and a significant increase of ALP in the liver. Both MC variants caused pathological lesions in hepatic tissues, such as megalocytosis, necrotic process, and microvesicular steatosis, particularly in fish treated with MC-LR, and degenerative renal changes, glomerulopathy, were more severe in tilapias exposed to MC-RR. In addition, both microcystins also caused significant myopathy in the heart. In contrast, the gills did not show any change in enzyme activity or histopathological injury.