Quantitatively evaluating detoxification of the hepatotoxic microcystins through the glutathione and cysteine pathway in the cyanobacteria-eating bighead carp

Recent advances and challenges in the analysis of natural toxins

Natural toxins (NTs) are poisonous secondary metabolites produced by living organisms developed to ward off predators. Especially low molecular weight NTs (MW<∼1 kDa), such as mycotoxins, phycotoxins, and plant toxins, are considered an important and growing food safety concern. Therefore, accurate risk assessment of food and feed for the presence of NTs is crucial. Currently, the analysis of NTs is predominantly performed with targeted high pressure liquid chromatography tandem mass spectrometry (HPLC-MS/MS) methods. Although these methods are highly sensitive and accurate, they are relatively expensive and time-consuming, while unknown or unexpected NTs will be missed. To overcome this, novel on-site screening methods and non-targeted HPLC high resolution mass spectrometry (HRMS) methods have been developed. On-site screening methods can give non-specialists the possibility for broad "scanning" of potential geographical regions of interest, while also providing sensitive and specific analysis at the point-of-need. Non-targeted chromatography-HRMS methods can detect unexpected as well as unknown NTs and their metabolites in a lab-based approach. The aim of this chapter is to provide an insight in the recent advances, challenges, and perspectives in the field of NTs analysis both from the on-site and the laboratory perspective.

Congener-specific fate and impact of microcystins in the soil-earthworm system

Microcystins (MCs) have a significant influence on aquatic ecosystems, but little is known about their terrestrial fate and impact. Here, we investigated the fate of two MCs (MC-LR and MC-RR) in the soil-earthworm system, with consideration of their congener-specific impact on earthworm health, soil bacteria, and soil metabolome. Although MCs had little acute lethal effect on earthworms, they caused obvious growth inhibition and setae rupture. Relative to MC-RR, MC-LR exhibited higher bioaccumulation and the resulting dermal lesions and deformation of longitudinal muscles. While the incorporation of both MCs into soils stimulated pathogenic bacteria and depressed oxidative stress tolerant bacteria, the response among soil nitrification and glutathione metabolism differed between the two congeners. The dissipation kinetics of MCs obeyed the first-order model. Earthworms stimulated soil N-cycling enzyme activities, increased the abundance of MC-degrading bacteria, and promoted bacterial metabolic functions related to glutathione metabolism, xenobiotics biodegradation, and metabolism of amino acids that comprise MCs, which accelerated the dissipation of MC-LR and MC-RR by 227% and 82%, respectively. These results provide evidence of significant congener differences in the terrestrial fate and impact of MCs, which will enable a better understanding of their role in mediating soil functions and ecosystem services.

Health Risks of Chronic Exposure to Small Doses of Microcystins: An Integrative Metabolomic and Biochemical Study of Human Serum

Health risks of chronic exposure to microcystins (MCs), a family of aquatic contaminants produced mainly by cyanobacteria, are critical yet unsolved problems. Despite a few epidemiological studies, the metabolic profiles of humans exposed to MCs remain unknown, hindering the deep understanding of the molecular toxicity mechanisms. Here, sensitive nuclear magnetic resonance (NMR)- and liquid chromatography-mass spectrometry (LC-MS)-based metabolomics were applied to investigate the serum metabolic profiles of humans living near Lake Chao, where toxic cyanobacterial blooms occur annually. MCs were positively detected in 92 of 144 sera by ultra-high-pressure liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) with a median concentration of 0.016 μg/L. The estimated daily intake (0.15-0.27 μg MC-LReq/day) was less than the tolerable daily intake (TDI, 2.4 μg MC-LR for 60 kg adults) recommended by the World Health Organization (WHO). Obvious disruptions of the amino acid metabolism were confirmed and played important roles in renal impairments associated with serum MC burdens. Chronic oral exposure of mice to 30 μg MC-LR/kg body mass, which is less than the no observed adverse effect level, also led to obvious renal lesions and metabolic dysfunction. These observations provide the first evidence of metabolic disturbance of humans exposed to MCs and indicate that the WHO's TDI value determined traditionally should be lessened to protect human health effectively.

Cyanobacteria Microcystis aeruginosa Contributes to the Severity of Fish Diseases: A Study on Spring Viraemia of Carp

Fish are exposed to numerous stressors in the environment including pollution, bacterial and viral agents, and toxic substances. Our study with common carps leveraged an integrated approach (i.e., histology, biochemical and hematological measurements, and analytical chemistry) to understand how cyanobacteria interfere with the impact of a model viral agent, Carp sprivivirus (SVCV), on fish. In addition to the specific effects of a single stressor (SVCV or cyanobacteria), the combination of both stressors worsens markers related to the immune system and liver health. Solely combined exposure resulted in the rise in the production of immunoglobulins, changes in glucose and cholesterol levels, and an elevated marker of impaired liver, alanine aminotransferase (ALT). Analytical determination of the cyanobacterial toxin microcystin-LR (MC-LR) and its structurally similar congener MC-RR and their conjugates showed that SVCV affects neither the levels of MC in the liver nor the detoxification capacity of the liver. MC-LR and MC-RR were depurated from liver mostly in the form of cysteine conjugates (MC-LR-Cys, MC-RR-Cys) in comparison to glutathione conjugates (LR-GSH, RR-GSH). Our study brought new evidence that cyanobacteria worsen the effect of viral agents. Such inclusion of multiple stressor concept helps us to understand how and to what extent the relevant environmental stressors co-influence the health of the fish population.

Acute exposure to microcystin-LR induces hepatopancreas toxicity in the Chinese mitten crab (Eriocheir sinensis)

The Chinese mitten crab is an important economic species in the Chinese aquaculture industry due to its rich nutritional value and distinct flavor. The hepatopancreas is a popular edible part of the Chinese mitten crab, and therefore, hepatopancreatic health directly determines its quality. However, a large-scale outbreak of hepatopancreatic necrosis syndrome ("Shuibiezi" disease in Chinese), which is caused by abiotic agents correlated with cyanobacteria bloom outbreaks, adversely affects the Chinese mitten crab breeding industry. Cyanobacterial blooms that occur in high-density farming ponds can produce microcystin-LR (MC-LR), which is hepatotoxic in fish and mammals. Hepatopancreas toxicity of MC-LR (0, 25, 50 and 75 μg/kg) was investigated after 48 h of exposure. The MC-LR can cause hepatopancreatic injury by inducing hepatopancreatic structural damage, subcellular structural changes, and cell apoptosis, followed by enhanced lipid peroxidase, reactive oxygen species, and apoptosis-related enzyme (Caspase 3, 8, and 9) activities. These in turn promote gene and protein expression of apoptosis-associated proteases (Caspase 3, 7, and 8, Bcl-2, and Bax), and alter antioxidant system responses (superoxide dismutase, glutathione S-transferase, glutathione peroxidase, glutathione reductase activities, and glutathione content). The present study is the first report on MC-LR hepatotoxicity in the Chinese mitten crab and confirms hepatopancreas toxicity, providing a theoretical basis for enhancing MCs resistance and developing preventive and curative measures against hepatopancreatic disease in the Chinese mitten crab breeding industry.

Sub-chronic microcystin-LR renal toxicity in rats fed a high fat/high cholesterol diet

Microcystin-LR (MCLR) is a liver and kidney toxin produced by cyanobacteria. Recently, it was demonstrated that MCLR exposure drives the progression of high fat/high cholesterol (HFHC) induced nonalcoholic fatty liver disease (NAFLD) to a more severe state. NAFLD is also a risk factor for chronic kidney disease (CKD), and the current study investigated MCLR renal toxicity in the context of an HFHC diet. Sprague Dawley rats were fed either a control diet or an HFHC diet for 10 weeks. After 6 weeks of diet, animals were administered either vehicle, 10 μg/kg, or 30 μg/kg MCLR via intraperitoneal injection every other day for 4 weeks. HFHC diet alone increased the renal glomerular change histopathology score, and 30 μg/kg MCLR exposure increased this score in both the control group and the HFHC group. In contrast, 30 μg/kg MCLR caused greater proteinuria and cast formation and decreased protein phosphatase 1 and 2A protein expression in the HFHC group. Urinary excretion of KIM-1 increased, but albumin and tamm-horsfall protein did not change after MCLR exposure. The general concordance between KIM-1, polyuria, proteinuria, and renal casts after MCLR exposure suggests that proximal tubule cell damage contributed to these connected pathologies. The control group adapted to repeated MCLR exposure by increasing the urinary elimination of MCLR and its metabolites, whereas this adaptation was blunted in the HFHC group. These data suggest an HFHC diet may increase the severity of certain MCLR-elicited renal toxicities.

Reduction of gut microbial diversity and short chain fatty acids in BALB/c mice exposure to microcystin-LR

Gut microbiota has been shown to play critical roles in host health. The present study was to determine the toxicological effects of microcystin-LR (MCLR) on gut microbial community and metabolites using 16S rDNA sequencing and gas chromatography-mass spectrometry (GC-MS). MCLR was administered to BALB/c mice by gavage for eight weeks. Results of the microbial alpha-diversity (Sobs, Chao1, ACE and Shannon indexes) decreased in MCLR-treated group versus controls. Phylum Candidatus Saccharibacteria decreased significantly in MCLR-treated group versus controls. Correspondingly, more than thirties genera in relative abundance decreased, especially short chain fatty acid (SCFA)-producing bacteria (e.g., Alistipes and Ruminococcus). These results indicated that the gut microbial community structure was remarkably changed by MCLR. Furthermore, concentrations of SCFAs were significantly decreased after MCLR exposure (P < 0.01), where butyrate decreased as high as 4.9-fold. Consequently, sub-chronic exposure to MCLR could not only alter the microbial composition but metabolites. This study offered novel insights into the toxic mechanism of MCs from gut microbiota, and facilitated further clarification of risks to human health from MCs exposure.

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.

In vitro detoxication of microcystins in human samples: variability among variants with different hydrophilicity and structure

Cyanotoxins, among which >200 variants of Microcystins (MC), constitute an emerging issue in food safety. Microcystins (MC) toxicity is congener-specific; however, the in vitro inhibition of PP1/PP2A (the key molecular event of MC toxicity) by single MC variants is comparable and MC toxicokinetics seems to be the critical point. Here, the variability in GSH conjugation catalysed by human recombinant enzymes and human hepatic cytosol has been compared between hydrophilic (MC-LR and MC-RR) and hydrophobic (MC-LW, MC-YR and MC-LF) variants, according to measured logPow. In vitro detoxication reaction (spontaneous plus enzymatic) is favored by the variant hydrophilicity, with MC-LF very poorly detoxified. With MC-YR and -LW the spontaneous reaction always gave the major contribution, whereas with MC-LR and -RR the enzymatic reaction became by far predominant when GSH was depleted. Consequently, the well-known GST polymorphisms seems not to be the major driver for potential human variability in susceptibility towards the MC-toxicity, except for MC-RR and -LR when GSH is depleted. Looking at these results and literature data, MC-RR (the least cytotoxic and acutely toxic in rodents) is the more hydrophilic, has the lowest OATP-mediated hepatic uptake and the highest detoxication efficiency. The opposite is true for the most lipophilic MC-LF: once entered in the cells with the highest uptake, it is very poorly detoxified, and resulted as the most toxic in various cell types. MC-dependent TK should be considered in order to estimate the variability in toxicity and to support the use of quantitative in vitro-in vivo extrapolation models of single toxins and their mixtures co-occurring in the environment.

GST transcriptional changes induced by a toxic Microcystis aeruginosa strain in two bivalve species during exposure and recovery phases

Previous studies have demonstrated the modulation of glutathione transferases (GSTs) induced by microcystin (MC) alone or in combination with other cyanobacterial secondary metabolites in bivalves. However, interspecies information about which and how GST isoforms are affected by these secondary metabolites is still scarce, especially considering the dynamic process involving their uptake and elimination routes. In this context, the role of GSTs gene expression changes in response to a toxic Microcystis aeruginosa extract were examined for Mytilus galloprovincialis and Ruditapes philippinarum during exposure and recovery phases. The expression levels of sigma 1, sigma 2, pi and mu-class GST genes were analyzed in the hepatopancreas of both bivalve species during cyanobacteria extract exposure (24 h) and post-exposure (24 and 72 h). Only a significant induction of sigma 1-class GST expression was observed for R. philippinarum upon 24-hour exposure of both bivalve species to Microcystis extract. During the recovery phase, GST transcriptional changes for M. galloprovincialis were characterized by an early induction (24 h) of sigma 1 and sigma 2 transcripts. On the other hand, GST transcriptional changes for R. philippinarum during post-exposure phase were characterized by an early induction (24 h) of sigma 1 and mu transcripts and a later induction (72 h) of the four analyzed GST transcripts. Such differences reflect variable GST response mechanisms to cope with MC-producing cyanobacterial blooms exposure between these two bivalve species, revealing a higher sensitivity of R. philippinarum to Microcystis-induced stress than M. galloprovincialis. The results also suggest a much higher level of activity of the GST detoxification system during the recovery phase compared to the period of the stress exposure for both bivalve species.

Gastrointestinal toxicity induced by microcystins

Microcystins (MCs) are produced by certain bloom-forming cyanobacteria that can induce toxicity in various organs, including renal toxicity, reproductive toxicity, cardiotoxicity, and immunosuppressive effects. It has been a significant global environmental issue due to its harm to the aquatic environment and human health. Numerous investigators have demonstrated that MC exposure can induce a widespread epidemic of enterogastritis with symptoms similar to food poisoning in areas close to lakes. Both in vivo and in vitro studies have provided evidence of positive associations between MC exposure and gastrointestinal toxicity. The toxicity of MCs on the gastrointestinal tract is multidimensional. MCs can affect gastrointestinal barrier function and shift the structure of gut microbiota in different gut regions. Furthermore, MCs can inhibit the secretion of gastrointestinal digestive enzymes and the release of inflammatory cytokines, which affects the expression of immune-related genes in the intestine. The damage of the intestine is closely correlated to MC exposure because the intestine is the main site for the digestion and absorption of nutrients. The damage to the gastrointestinal tract due to MCs was summarized from different aspects, which can be used as a foundation for further exploration of molecular damage mechanisms.

Uptake and accumulation of Microcystin-LR based on exposure through drinking water: An animal model assessing the human health risk

Harmful Algal Blooms (HABs) in freshwater systems and intensified aquaculture have increased the risk to human health through exposure to cyanotoxins such as microcystin-LR (MC-LR). To understand the uptake and processing of MC-LR in humans, the pig was chosen as an animal model. This was assessed by repeated exposure for 13 weeks of eight animals dosed daily with MC-LR at 0.04 µg/kg bw, repeated with six animals over five weeks at a dose 50 times higher at 2 µg/kg bw. An analytical method was developed for MC-LR in porcine serum and also to analyse levels of free MC-LR in harvested porcine tissues, with Lemieux Oxidation employed to determine bound MC-LR in these tissues. MC-LR was not detected in the serum of treated animals from either experiment but free MC-LR was observed in the large intestine and kidney from two animals from the higher dosed group at levels of 1.4 and 1.9 µg/kg dry weight (dw) respectively. The results indicated 50% of higher dosed animals accumulated bound MC-LR in liver tissue, averaging 26.4 µg, approximately 1.1% of the dose administered. These results point to the potential uptake and accumulation of MC-LR in human liver tissue exposed chronically to sub-acute doses.

Excretion pattern and dynamics of glutathione detoxification of microcystins in Sprague Dawley rat

The excretion route and dynamics of the glutathione (GSH) conjugate of microcystin-RR (MCRR), MCRR-GSH, were quantitatively studied in Sprague Dawley rat exposed with MCRR-GSH via liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). In the MCRR-GSH-treated rat, the average MCRR-Cysteine (MCRR-Cys)/MCRR-GSH ratio reached as high as 105.3, which indicated that the intermediate conjugate MCRR-GSH was rapidly converted to the product compound MCRR-Cys. Besides, MCRR was consistently detected in MCRR-GSH-treated rat, which suggested that MCRR can be dissociated from the MCRR-GSH conjugate and the reversibility of the MC-GSH conjugate. Results of total MC contents analysis in excrement showed that the total MC contents in urine were significantly higher than those in feces. The ratio of the total MC content in urine to feces was as high as 129.3, which demonstrates that the urine is the main route of excretion after MCRR-GSH-treatment. In urine, the MCRR-Cys concentration was 27.8-fold, 19.4-fold higher than MCRR-GSH and MCRR, respectively. Our results, for the first time, quantitatively found that MCRR-GSH was rapidly converted to MCRR-Cys after exposed to rat, and was excreted mainly through urine in the form of the MCRR-Cys conjugate. This study suggests that the GSH detoxification pathway of MCs could help to explain the greater sensitivity of mammals to MCs.

Quantitative analysis of glutathione and cysteine S-conjugates of microcystin-LR in the liver, kidney and muscle of common carp (Cyprinus carpio) in Lake Taihu

Tissue distribution of microcystin (MC)-LR-GSH, MC-LR-Cys and MC-LR of omnivorous fish in Lake Taihu was investigated. MC-LR and MC-LR-Cys were detected in liver, kidney and muscle. The concentration of MC-LR in liver and kidney was 0.052 μg g^-1 DW and 0.067 μg g^-1 DW, respectively. MC-LR-Cys appeared to be an important metabolite with average contents of 1.104 μg g^-1 DW and 0.724 μg g^-1 DW in liver and kidney, and the MC-LR-Cys/MC-LR ratio in liver and kidney reaching as high as 21.4 and 10.8. High MC-LR-Cys/MC-LR ratio and a significant correlation between MC-LR-Cys and MC-LR concentration in liver, suggest that liver is more active in detoxification of MC-LR by formation of MC-LR-Cys for omnivorous fish. Furthermore, there might be a balance between the accumulation and depuration/metabolism of MC-LR-Cys in kidney. The MC-LR-Cys can be formed in kidney directly, or transported from liver or other tissues, while the MC-LR-Cys in kidney might be dissociated to MC-LR or excreted. Although MC-LR and its metabolites were scarcely detected in muscle, it is necessary to investigate the distribution of toxic metabolites in edible muscle.

Gut content microbiota of introduced bigheaded carps (Hypophthalmichthys spp.) inhabiting the largest shallow lake in Central Europe

Studying the microbiota in the alimentary tract of bigheaded carps (Hypophthalmichthys spp.) gained special interest recently, as these types of investigations on non-native fish species may lead to a better understanding of their ecological role and feeding habits in an invaded habitat. For microbiological examinations, bigheaded carp gut contents and water column samples from Lake Balaton (Hungary) were collected from spring to autumn in 2013. Denaturing Gradient Gel Electrophoresis (DGGE) and pyrosequencing of the 16S rRNA gene were performed to reveal the composition. According to the DGGE patterns, bacterial communities of water samples separated clearly from that of the intestines. Moreover, the bacterial communities in the foreguts and hindguts were also strikingly dissimilar. Based on pyrosequencing, both foregut and hindgut samples were predominated by the fermentative genus Cetobacterium (Fusobacteria). The presence of some phytoplankton taxa and the high relative abundance of cellulose-degrading bacteria in the guts suggest that intestinal microbes may have an important role in digesting algae and making them utilizable for bigheaded carps that lack cellulase enzyme. In turn, the complete absence of typical heterotrophic freshwater bacteria in all studied sections of the intestines indicated that bacterioplankton probably has a negligible role in the nutrition of bigheaded carps.

Gender-Specific Toxicological Effects of Chronic Exposure to Pure Microcystin-LR or Complex Microcystis aeruginosa Extracts on Adult Medaka Fish

Cyanobacterial blooms often occur in freshwater lakes and constitute a potential health risk to human populations, as well as to other organisms. However, their overall and specific implications for the health of aquatic organisms that are chronically and environmentally exposed to cyanobacteria producing hepatotoxins, such as microcystins (MCs), together with other bioactive compounds have still not been clearly established and remain difficult to assess. The medaka fish was chosen as the experimental aquatic model for studying the cellular and molecular toxicological effects on the liver after chronic exposures (28 days) to environmentally relevant concentrations of pure MC-LR, complex extracts of MC producing or nonproducing cyanobacterial biomasses, and of a Microcystis aeruginosa natural bloom. Our results showed a higher susceptibility of females to the different treatments compared to males at both the cellular and the molecular levels. Although hepatocyte lysis increased with MC-containing treatments, lysis always appeared more severe in the liver of females compare to males, and the glycogen cellular reserves also appeared to decrease more in the liver of females compared to those in the males. Proteomic investigations reveal divergent responses between males and females exposed to all treatments, especially for proteins involved in metabolic and homeostasis processes. Our observations also highlighted the dysregulation of proteins involved in oogenesis in female livers. These results suggest that fish populations exposed to cyanobacteria blooms may potentially face several ecotoxicological issues.

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.

The fate of microcystins in the environment and challenges for monitoring

Microcystins are secondary metabolites produced by cyanobacteria that act as hepatotoxins in higher organisms. These toxins can be altered through abiotic processes, such as photodegradation and adsorption, as well as through biological processes via metabolism and bacterial degradation. Some species of bacteria can degrade microcystins, and many other organisms metabolize microcystins into a series of conjugated products. There are toxicokinetic models used to examine microcystin uptake and elimination, which can be difficult to compare due to differences in compartmentalization and speciation. Metabolites of microcystins are formed as a detoxification mechanism, and little is known about how quickly these metabolites are formed. In summary, microcystins can undergo abiotic and biotic processes that alter the toxicity and structure of the microcystin molecule. The environmental impact and toxicity of these alterations and the metabolism of microcystins remains uncertain, making it difficult to establish guidelines for human health. Here, we present the current state of knowledge regarding the alterations microcystins can undergo in the environment.

Dietary exposure of Daphnia to microcystins: no in vivo relevance of biotransformation

Anthropogenic nutrient input into lakes has contributed to the increased frequency of toxic cyanobacterial blooms. Daphnia populations have been shown to be locally adapted to toxic cyanobacteria and are able to suppress bloom formation; little is known about the physiology behind this phenomenon. Microcystin-LR (MCLR) is the most widespread cyanobacterial toxin, and, based on in vitro experiments, it is assumed that the enzyme glutathione-S-transferase (GST) might act as the first step of detoxification in Daphnia by conjugating MCLR with glutathione. In the present study Daphnia magna was fed a diet of 100% Microcystis aeruginosa PCC7806, a cyanobacterial strain that contains MCLR in high amounts (4.8-5.6 fg cell(-1)), in order to test for a possible conjugation of MCLR with GST in Daphnia in vivo. We used high-resolution LCMS to analyze incubation water, cyanobacterial cells and Daphnia tissue for the presence of MCLR conjugation products as well as unconjugated MCLR. Newly formed conjugation products were detected neither in Daphnia tissue nor in the incubation water. Moreover, the presence of Daphnia led to a decrease in unconjugated MCLR in the cyanobacterial cell fraction due to grazing, in comparison to a control without daphnids, which was well reflected by a similar increase of MCLR in the respective incubation water. As a consequence, the MCLR content did not change due to Daphnia presence within the entire experimental setup. In summary, MCLR ingestion by Daphnia led neither to the formation of conjugation products, nor to a decrease of unconjugated MCLR. GST-mediated conjugation thus seems to be of minor relevance for microcystin (MC) tolerance in Daphnia in vivo. This finding is supported by the fact that GST activity in Daphnia feeding on the MC-containing wildtype or a MC-free mutant of M. aeruginosa PCC7806 revealed an identical increase of specific activity in comparison to a cyanobacteria-free diet. Therefore, the frequently observed induction of GST activity upon exposure to toxic cyanobacteria is not a specific MC effect but a general cyanobacterial effect. This suggests that GST in Daphnia is involved in an oxidative stress response rather than in the specific detoxification of MCs. Furthermore, our results indicate the presence of an efficient transport mechanism which efficiently removes unconjugated MCLR from the Daphnia tissue. Further studies are needed to elucidate the nature of this transport mechanism.

Rapid conversion and reversible conjugation of glutathione detoxification of microcystins in bighead carp (Aristichthys nobilis)

The glutathione and cysteine conjugates of microcystin (MC-GSH and MC-Cys, respectively) are two important metabolites in the detoxification of microcystins (MCs). Although studies have quantitated both conjugates, the reason why the amounts of MC-GSH are much lower than those of MC-Cys in various animal organs remains unknown. In this study, MC-RR-GSH and MC-RR-Cys were respectively i.p. injected into the cyanobacteria-eating bighead carp (Aristichthys nobilis), to explore the biotransformation and detoxification mechanisms of the two conjugates. The contents of MC-RR, MC-RR-GSH, MC-RR-Cys and MC-RR-N-acetyl-cysteine (MC-RR-Nac, the acetylation product of MC-RR-Cys) in the liver, kidney, intestine and blood of bighead carp in both groups were quantified via liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). In the MC-RR-GSH-treated group, the MC-RR-Cys content in the kidney increased 96.7-fold from 0.25 to 0.5h post-injection, demonstrating that MC-RR-GSH acts as a highly reactive intermediate and is rapidly converted to MC-RR-Cys. The presence of MC-RR in both MC-RR-GSH- and MC-RR-Cys-treated groups indicates, for the first time, that MC conjugation with the thiol of GSH/Cys is a reversible process in vivo. Total MC-RR concentrations dissociated from MC-RR-Cys were lower than those from MC-RR-GSH, suggesting that MC-RR-Cys is more capable of detoxifying MC-RR. MC-RR-Cys was the most effectively excreted form in both the kidney and intestine, as the ratios of MC-RR-Cys to MC-RR reached as high as 15.2, 2.9 in the MC-RR-GSH-treated group and 63.4, 19.1 in the MC-RR-Cys-treated group. Whereas MC-RR-Nac could not be found in all of the samples of the present study. Our results indicate that MC-RR-GSH was rapidly converted to MC-RR-Cys and then excreted, and that both glutathione and cysteine conjugates could release MC-RR. This study quantitatively proves the importance of the GSH detoxification pathway and furthers our understanding of the biochemical mechanism by which bighead carp are resistant to toxic cyanobacteria.

Hepatotoxic seafood poisoning (HSP) due to microcystins: a threat from the ocean?

Cyanobacterial blooms are a major and growing problem for freshwater ecosystems worldwide that increasingly concerns public health, with an average of 60% of blooms known to be toxic. The most studied cyanobacterial toxins belong to a family of cyclic heptapeptide hepatotoxins, called microcystins. The microcystins are stable hydrophilic cyclic heptapeptides with a potential to cause cell damage following cellular uptake via organic anion-transporting proteins (OATP). Their intracellular biologic effects presumably involve inhibition of catalytic subunits of protein phosphatases (PP1 and PP2A) and glutathione depletion. The microcystins produced by cyanobacteria pose a serious problem to human health, if they contaminate drinking water or food. These toxins are collectively responsible for human fatalities, as well as continued and widespread poisoning of wild and domestic animals. Although intoxications of aquatic organisms by microcystins have been widely documented for freshwater ecosystems, such poisonings in marine environments have only occasionally been reported. Moreover, these poisonings have been attributed to freshwater cyanobacterial species invading seas of lower salinity (e.g., the Baltic) or to the discharge of freshwater microcystins into the ocean. However, recent data suggest that microcystins are also being produced in the oceans by a number of cosmopolitan marine species, so that Hepatotoxic Seafood Poisoning (HSP) is increasingly recognized as a major health risk that follows consumption of contaminated seafood.

Epidemiology of primary liver cancer in Serbia and possible connection with cyanobacterial blooms

Today, the occurrence of harmful cyanobacterial blooms is a common phenomenon and a potential global health problem. Cyanobacteria can produce metabolites highly toxic to humans. More than 80% of reservoirs used for water supply in Central Serbia have bloomed over the past 80 years. A 10-year epidemiological study showed a significant increase in the incidence of primary liver cancer (PLC) in the regions where water from the blooming reservoirs was used for human consumption. At the same time, no correlation was found between the incidence of PLC and other risk factors, such as cirrhosis and hepatitis viruses. Given the strong association with PLC induction and various known possible mechanisms of carcinogenic action, it is highly possible that, cyanotoxins--acting as initiator and promoter--may be the major risk factor that acts synergistically with other risk factors to cause increased incidence of PLC. However, at present, it is still not certain whether cyanotoxins alone were sufficient to induce PLC. Therefore, additional assessment of the health risks that may arise from human exposure to cyanotoxins is advisable.