A molecular basis for different interactions of marine toxins with protein phosphatase-1. Molecular models for bound motuporin, microcystins, okadaic acid, and calyculin A

Structure, Toxicity, Prevalence, and Degradation of Six Understudied Freshwater Cyanopeptides

Anthropogenic influences have increased global warming and eutrophication, escalating the frequency and severity of harmful cyanobacterial blooms (cHABs) in freshwater ecosystems. These blooms release cyanopeptides, a diverse class of bioactive compounds with varying acute and chronic toxicities upon ingestion. To date, research has prioritized acutely toxic cyanopeptides like microcystins. As a result, significantly less is known about other freshwater cyanopeptides. This review highlights six understudied cyanopeptide classes, anabaenopeptins, cyanopeptolins, aeruginosamides, aeruginosins, microginins, and cyclamides, and provides a comprehensive overview of their molecular structures, toxicological profiles, environmental concentrations, and known degradation pathways. Given the potential toxicity, increased environmental abundance, and environmental stability of many cyanopeptides in freshwater sources, further research is needed to understand if degraded cyanopeptides are still biologically active prior to entering drinking water to ensure public health.

Health and Environmental Impacts of Cyanobacteria and Cyanotoxins from Freshwater to Seawater

Cyanobacterial harmful algal blooms (cyanoHABs) are a natural phenomenon produced mainly by the interaction between natural and anthropogenic events. CyanoHABs are characterized by the production of cyanotoxins that can have harmful effects on different species within the food web and even affect human health. Among the most prevalent toxin groups worldwide are microcystins (MCs), anatoxins (ATXs), cylindrospermopsins (CYNs) and nodularins (NODs), which are characterized as toxins with hepatotoxic, neurotoxic, and cytotoxic effects. This review summarizes and analyzes research on the influence of cyanoHABs, the main toxin-producing cyanobacteria and the most prevalent cyanotoxins in freshwater and marine bodies, highlighting their global occurrence, toxicology, and bioaccumulation dynamics in vectors of the food web, and the main cases of acute and chronic intoxications in humans. This review is useful for understanding the dynamics of cyanoHABs' interaction with the ecosystem and their impact on human health, and how the implementation of a surveillance and management framework for cyanobacteria and cyanotoxins could generate vital information for stakeholders to establish health guidelines on the risks and hazards of cyanoHABs for the ecosystem and humans.

Conformation and binding of 12 Microcystin (MC) congeners to PPP1 using molecular dynamics simulations: A potential approach in support of an improved MC risk assessment

Microcystins (MCs) occur frequently during cyanobacterial blooms worldwide, representing a group of currently about 300 known MC congeners, which are structurally highly similar. Human exposure to MCs via contaminated water, food or dietary supplements can lead to severe intoxications with ensuing high morbidity and in some cases mortality. Currently, one MC congener (MC-LR) is almost exclusively considered for risk assessment (RA) by the WHO. Many MC congeners co-occur during bloom events, of which MC-LR is not the most toxic. Indeed, MC congeners differ dramatically in their inherent toxicity, consequently raising question about the reliability of the WHO RA and the derived guidance values. Molecular dynamics (MD) simulation can aid in understanding differences in toxicity, as experimental validation for all known MC congeners is not feasible. Therefore, we present MD simulations of a total of twelve MC congeners, of which eight MC congeners were simulated for the first time. We show that depending on their structure and toxicity class, MCs adapt to different backbone conformations. These backbone conformations are specific to certain MC congeners and can change or shift to other conformations upon binding to PPP1, affecting the stability of the binding. Analysis of the interactions with PPP1 demonstrated that there are frequently occurring patterns for individual MC congeners, and that published PPP interactions could be reproduced. In addition, common but also unique patterns were found for individual MC congeners, suggesting differences in binding behaviour. The MD simulations presented here therefore enhance our understanding of MC congener-specific differences and demonstrated that congener-specific investigations are prerequisite for allowing characterisation of yet untested or even unknown MC congeners, thereby allowing for a novel potential approach in support of an improved RA of microcystins in humans.

Co-Occurrence of Taste and Odor Compounds and Cyanotoxins in Cyanobacterial Blooms: Emerging Risks to Human Health?

Cyanobacteria commonly form large blooms in waterbodies; they can produce cyanotoxins, with toxic effects on humans and animals, and volatile compounds, causing bad tastes and odors (T&O) at naturally occurring low concentrations. Notwithstanding the large amount of literature on either cyanotoxins or T&O, no review has focused on them at the same time. The present review critically evaluates the recent literature on cyanotoxins and T&O compounds (geosmin, 2-methylisoborneol, β-ionone and β-cyclocitral) to identify research gaps on harmful exposure of humans and animals to both metabolite classes. T&O and cyanotoxins production can be due to the same or common to different cyanobacterial species/strains, with the additional possibility of T&O production by non-cyanobacterial species. The few environmental studies on the co-occurrence of these two groups of metabolites are not sufficient to understand if and how they can co-vary, or influence each other, perhaps stimulating cyanotoxin production. Therefore, T&Os cannot reliably serve as early warning surrogates for cyanotoxins. The scarce data on T&O toxicity seem to indicate a low health risk (but the inhalation of β-cyclocitral deserves more study). However, no data are available on the effects of combined exposure to mixtures of cyanotoxins and T&O compounds and to combinations of T&O compounds; therefore, whether the co-occurrence of cyanotoxins and T&O compounds is a health issue remains an open question.

A review and assessment of cyanobacterial toxins as cardiovascular health hazards

Eutrophicated waters frequently support bloom-forming cyanobacteria, many of which produce potent cyanobacterial toxins (cyanotoxins). Cyanotoxins can cause adverse health effects in a wide range of organisms where the toxins may target the liver, other internal organs, mucous surfaces and the skin and nervous system. This review surveyed more than 100 studies concerning the cardiovascular toxicity of cyanotoxins and related topics. Over 60 studies have described various negative effects on the cardiovascular system by seven major types of cyanotoxins, i.e. the microcystin (MC), nodularin (NOD), cylindrospermopsin (CYN), anatoxin (ATX), guanitoxin (GNTX), saxitoxin (STX) and lyngbyatoxin (LTX) groups. Much of the research was done on rodents and fish using high, acutely toxin concentrations and unnatural exposure routes (such as intraperitoneal injection), and it is thus concluded that the emphasis in future studies should be on oral, chronic exposure of mammalian species at environmentally relevant concentrations. It is also suggested that future in vivo studies are conducted in parallel with studies on cells and tissues. In the light of the presented evidence, it is likely that cyanotoxins do not constitute a major risk to cardiovascular health under ordinary conditions met in everyday life. The risk of illnesses in other organs, in particular the liver, is higher under the same exposure conditions. However, adverse cardiovascular effects can be expected due to indirect effects arising from damage in other organs. In addition to risks related to extraordinary concentrations of the cyanotoxins and atypical exposure routes, chronic exposure together with co-existing diseases could make some of the cyanotoxins more dangerous to cardiovascular health.

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.

DNA nanotechnology: A recent advancement in the monitoring of microcystin-LR

The Microcystin-Leucine-Arginine (MC-LR) is the most toxic and widely distributed microcystin, which originates from cyanobacteria produced by water eutrophication. The MC-LR has deleterious effects on the aquatic lives and agriculture, and this highly toxic chemical could severely endanger human health when the polluted food was intaken. Therefore, the monitoring of MC-LR is of vital importance in the fields including environment, food, and public health. Utilizing the complementary base pairing between DNA molecules, DNA nanotechnology can realize the programmable and predictable regulation of DNA molecules. In analytical applications, DNA nanotechnology can be used to detect targets via target-induced conformation change and the nano-assemblies of nucleic acids. Compared with the conventional analytical technologies, DNA nanotechnology has the advantages of sensitive, versatile, and high potential in real-time and on-site applications. According to the molecular basis for recognizing MC-LR, the strategies of applying DNA nanotechnology in the MC-LR monitoring are divided into two categories in this review: DNA as a recognition element and DNA-assisted signal processing. This paper introduces state-of-the-art analytical methods for the detection of MC-LR based on DNA nanotechnology and provides critical perspectives on the challenges and development in this field.

Comprehensive insights into the occurrence and toxicological issues of nodularins

The occurrence of cyanobacterial toxins is being increasingly reported. Nodularins (NODs) are one of the cyanotoxins group mainly produced by Nodularia spumigena throughout the world. NODs may exert adverse effects on animal and human health, and NOD-R variant is the most widely investigated. However, research focused on them is still limited. In order to understand the realistic risk well, the aim of this review is to compile the available information in the scientific literature regarding NODs, including their sources, distribution, structural characteristics, physicochemical properties, biosynthesis and degradation, adverse effects in vitro and vivo, and toxicokinetics. More data is urgently needed to integrate the cumulative or synergistic effects of NODs on different species and various cells to better understand, anticipate and aggressively manage their potential toxicity after both short- and long-term exposure in ecosystem, and to minimize or prevent the adverse effects on human health, environment and the economy.

A new method for the simultaneous determination of cyanotoxins (Microcystins and Cylindrospermopsin) in mussels using SPE-UPLC-MS/MS

The aim of this study was to optimize the extraction conditions of Microcystin-LR (MC-LR), Microcystin-RR (MC-RR), Microcystin-YR (MC-YR) and Cylindrospermopsin (CYN) simultaneously from mussels by using response surface methodology (RSM) and to validate the method by a dual solid phase extraction (SPE) system combined with ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The optimal parameters were: 90% MeOH (% v/v) for the extraction, a solvent/sample ratio of 75 and 15% MeOH in the extract before loading onto SPE. Mussels were spiked at 10; 37.5 and 75 ng g^-1 fresh weight (f.w) of the 4 toxins, showing linear ranges of 0.5-75 ng g^-1 f.w; low values for the limits of detection (0.01-0.39 ng g^-1 f.w.) and quantification (0.23-0.40 ng g^-1 f.w.); acceptable recoveries (70.37-114.03%) and relative standard deviation (%RSD_IP) values (2.61-13.73%). The method was successfully applied to edible mussels exposed to cyanobacterial extracts under laboratory conditions, and it could allow the monitoring of these cyanotoxins in environmental mussel samples.

Structural Diversity, Characterization and Toxicology of Microcystins

Hepatotoxic microcystins (MCs) are the most widespread class of cyanotoxins and the one that has most often been implicated in cyanobacterial toxicosis. One of the main challenges in studying and monitoring MCs is the great structural diversity within the class. The full chemical structure of the first MC was elucidated in the early 1980s and since then, the number of reported structural analogues has grown steadily and continues to do so, thanks largely to advances in analytical methodology. The structures of some of these analogues have been definitively elucidated after chemical isolation using a combination of techniques including nuclear magnetic resonance, amino acid analysis, and tandem mass spectrometry (MS/MS). Others have only been tentatively identified using liquid chromatography-MS/MS without chemical isolation. An understanding of the structural diversity of MCs, the genetic and environmental controls for this diversity and the impact of structure on toxicity are all essential to the ongoing study of MCs across several scientific disciplines. However, because of the diversity of MCs and the range of approaches that have been taken for characterizing them, comprehensive information on the state of knowledge in each of these areas can be challenging to gather. We have conducted an in-depth review of the literature surrounding the identification and toxicity of known MCs and present here a concise review of these topics. At present, at least 279 MCs have been reported and are tabulated here. Among these, about 20% (55 of 279) appear to be the result of chemical or biochemical transformations of MCs that can occur in the environment or during sample handling and extraction of cyanobacteria, including oxidation products, methyl esters, or post-biosynthetic metabolites. The toxicity of many MCs has also been studied using a range of different approaches and a great deal of variability can be observed between reported toxicities, even for the same congener. This review will help clarify the current state of knowledge on the structural diversity of MCs as a class and the impacts of structure on toxicity, as well as to identify gaps in knowledge that should be addressed in future research.

TIMAP inhibits endothelial myosin light chain phosphatase by competing with MYPT1 for the catalytic protein phosphatase 1 subunit PP1cβ

Transforming growth factor-β membrane associated protein (TIMAP) is an endothelial cell (EC)-predominant PP1 regulatory subunit and a member of the myosin phosphatase target (MYPT) protein family. The MYPTs preferentially bind the catalytic protein phosphatase 1 subunit PP1cβ, forming myosin phosphatase holoenzymes. We investigated whether TIMAP/PP1cβ could also function as a myosin phosphatase. Endogenous PP1cβ, myosin light chain 2 (MLC2), and myosin IIA heavy chain coimmunoprecipitated from EC lysates with endogenous TIMAP, and endogenous MLC2 colocalized with TIMAP in EC projections. Purified recombinant GST-TIMAP interacted directly with purified recombinant His-MLC2. However, TIMAP overexpression in EC enhanced MLC2 phosphorylation, an effect not observed with a TIMAP mutant that does not bind PP1cβ. Conversely, MLC2 phosphorylation was reduced in lung lysates from TIMAP-deficient mice and upon silencing of endogenous TIMAP expression in ECs. Ectopically expressed TIMAP slowed the rate of MLC2 dephosphorylation, an effect requiring TIMAP-PP1cβ interaction. The association of MYPT1 with PP1cβ was profoundly reduced in the presence of excess TIMAP, leading to proteasomal MYPT1 degradation. In the absence of TIMAP, MYPT1-associated PP1cβ readily bound immobilized microcystin-LR, an active-site inhibitor of PP1c. By contrast, TIMAP-associated PP1cβ did not interact with microcystin-LR, indicating that the active site of PP1cβ is blocked when it is bound to TIMAP. Thus, TIMAP inhibits myosin phosphatase activity in ECs by competing with MYPT1 for PP1cβ and blocking the PP1cβ active site.

An examination of microcystin-LR accumulation and toxicity using tethered bilayer lipid membranes (tBLMs)

Microcystin-LR (MC-LR) is a potent cyanobacterial toxin responsible for animal and human poisonings worldwide. MC-LR is found in organisms throughout the foodweb, however there is conjecture regarding whether it biomagnifies. Few studies have investigated how MC-LR interacts with lipid membranes, a determinant of biomagnification potential. We tested whether 1 μM MC-LR irreversibly associates with lipid bilayers or causes the creation of pore defects upon short and long-term exposure. Using tethered bilayer lipid membranes (tBLMs), we observed an increase in membrane conduction in tBLMs, representing an interaction of microcystin-LR with the lipid bilayer and a change in membrane packing properties. However, there were minimal changes in membrane capacitance upon short and long-term exposure, and MC-LR exhibited a rapid off-rate. Upon 24 h exposure to the toxin, no lipophilic multimeric complexes were detected capable of altering the toxin's off-rate. There was no evidence of the creation of new pores. This study demonstrates that MC-LR does not irreversibly imbed itself into lipids membranes after short or long-term exposure and suggests MC-LR does not biomagnify through the food web via lipid storage.

Effect of crude extracts from Nodularia spumigena on round goby (Neogobius melanostomus)

Nodularia spumigena is a nitrogen-fixing filamentous cyanobacteria in the Baltic Sea. Nodularin (NOD), the hepatotoxic peptide produced by this cyanobacterium, accumulates in the organisms from different trophic levels. In this paper, the effects of N. spumigena cell extract on the round goby (Neogobius melanostomus) was investigated under laboratory conditions. This benthic fish species feed on mussels in which nodularin accumulation was well documented. In current study a sharp increase in the NOD concentration in analyzed organs was observed after 24 h (PPIA) after 72 h of exposure (LC/MS). To determine the direction and strength of the changes induced in the fish by the toxin, several biochemical markers of exposure such as concentration of glutathione and activities of catalase, guaiacol peroxidase and glutathione S-transferase were used. In analyzed organs (liver, gills and muscle) of the round goby, the activity of these enzymes were suppressed. Higher GSH/protein amount and CAT and POD activity in gills than in liver reflects the importance of gills in NOD entering into analyzed fish body when exposed to toxin. The results indicate that the round goby (Neogobius melanostomus) exposed to extracts from N. spumigena cells triggered a defense system in a time-dependent manner. The obtained results contribute to a better understanding of fish response to the presence of compounds produced by N. spumigena.

Histopathological Evaluation of the Exposure by Cyanobacteria Cultive Containing [d-Leu¹]Microcystin-LR on Lithobates catesbeianus Tadpoles

This study evaluated the effects of [d-Leu¹]Microcystin-LR variant by the exposure of Lithobates catesbeianus tadpole to unialgal culture Microcystis aeruginosa NPLJ-4 strain. The Tadpole was placed in aquariums and exposed to Microcystis aeruginosa culture or disrupted cells. For 16 days, 5 individuals were removed every 2 days, and tissue samples of liver, skeletal muscle, and intestinal tract were collected for histopathology and bioaccumulation analyses. After exposure, those surviving tadpoles were placed in clean water for 15 days to evaluate their recovery. A control without algae and toxins was maintained in the same conditions and exhibited normal histology and no tissue damage. In exposed tadpoles, samples were characterized by serious damages that similarly affected the different organs, such as loss of adhesion between cells, nucleus fragmentation, necrosis, and hemorrhage. Samples showed signs of recovery but severe damages were still observed. Neither HPLC-PDA nor mass spectrometry analysis showed any evidence of free Microcystins bioaccumulation.

Determination of the Role of Microcystis aeruginosa in Toxin Generation Based on Phosphoproteomic Profiles

Microcystis aeruginosa is the most common species responsible for toxic cyanobacterial blooms and is considered a significant contributor to the production of cyanotoxins, particularly the potent liver toxins called microcystins. Numerous studies investigating Microcystis spp. blooms have revealed their deleterious effects in freshwater environments. However, the available knowledge regarding the global phosphoproteomics of M. aeruginosa and their regulatory roles in toxin generation is limited. In this study, we conducted comparative phosphoproteomic profiling of non-toxic and toxin-producing strains of M. aeruginosa. We identified 59 phosphorylation sites in 37 proteins in a non-toxic strain and 26 phosphorylation sites in 18 proteins in a toxin-producing strain. The analysis of protein phosphorylation abundances and functions in redox homeostasis, energy metabolism, light absorption and photosynthesis showed marked differences between the non-toxic and toxin-producing strains of M. aeruginosa, indicating that these processes are strongly related to toxin generation. Moreover, the protein-protein interaction results indicated that BJ0JVG8 can directly interact with the PemK-like toxin protein B0JQN8. Thus, the phosphorylation of B0JQN8 appears to be associated with the regulatory roles of toxins in physiological activity.

Effects of toxic Microcystis aeruginosa on the silver carp Hypophthalmichtys molitrix revealed by hepatic RNA-seq and miRNA-seq

High-throughput sequencing was applied to analyze the effects of toxic Microcystis aeruginosa on the silver carp Hypophthalmichthys molitrix. Silver carps were exposed to two cyanobacteria species (toxic and non-toxic) for RNA-seq and miRNA-seq analysis. RNA-seq revealed that the liver tissue contained 105,379 unigenes. Of these genes, 143 were significantly differentiated, 82 were markedly up-regulated, and 61 were remarkably down-regulated. GO term enrichment analysis indicated that 35 of the 154 enriched GO terms were significantly enriched. KEGG pathway enrichment analysis demonstrated that 17 of the 118 enriched KEGG pathways were significantly enriched. A considerable number of disease/immune-associated GO terms and significantly enriched KEGG pathways were also observed. The sequence length determined by miRNA-seq was mainly distributed in 20-23 bp and composed of 882,620 unique small RNAs, and 53% of these RNAs were annotated to miRNAs. As confirmed, 272 known miRNAs were differentially expressed, 453 novel miRNAs were predicted, 112 miRNAs were well matched with 7,623 target genes, and 203 novel miRNAs were matched with 15,453 target genes. qPCR also indicated that Steap4, Cyp7a1, CABZ01088134.1, and PPP1R3G were significantly differentially expressed and might play major roles in the toxic, detoxifying, and antitoxic mechanisms of microcystin in fish.

Calyculin: Nature's way of making the sponge-derived cytotoxin

Covering: up to 2015.Calyculin A is a major cytotoxic compound isolated from the Japanese marine sponge Discodermia calyx. Its potent cytotoxicity is attributable to the specific inhibition of protein phosphatases 1 and 2A, as in the case of okadaic acid and the microcystins. Its chemical structure is well-designed not only for enzyme inhibition but also for higher membrane permeability in order to impart its potent cytotoxicity. The biosynthetic gene cluster of this densely functionalized polyketide and nonribosomal peptide hybrid molecule was recently identified from the sponge-microbe association. The producer organism and the dynamic bioconversion process were also revealed. In this highlight, we focus on the recent studies addressing nature's design and biogenesis of the sponge-derived cytotoxin, calyculin A.

Uncovering Potential Applications of Cyanobacteria and Algal Metabolites in Biology, Agriculture and Medicine: Current Status and Future Prospects

Cyanobacteria and algae having complex photosynthetic systems can channelize absorbed solar energy into other forms of energy for production of food and metabolites. In addition, they are promising biocatalysts and can be used in the field of "white biotechnology" for enhancing the sustainable production of food, metabolites, and green energy sources such as biodiesel. In this review, an endeavor has been made to uncover the significance of various metabolites like phenolics, phytoene/terpenoids, phytols, sterols, free fatty acids, photoprotective compounds (MAAs, scytonemin, carotenoids, polysaccharides, halogenated compounds, etc.), phytohormones, cyanotoxins, biocides (algaecides, herbicides, and insecticides) etc. Apart from this, the importance of these metabolites as antibiotics, immunosuppressant, anticancer, antiviral, anti-inflammatory agent has also been discussed. Metabolites obtained from cyanobacteria and algae have several biotechnological, industrial, pharmaceutical, and cosmetic uses which have also been discussed in this review along with the emerging technology of their harvesting for enhancing the production of compounds like bioethanol, biofuel etc. at commercial level. In later sections, we have discussed genetically modified organisms and metabolite production from them. We have also briefly discussed the concept of bioprocessing highlighting the functioning of companies engaged in metabolites production as well as their cost effectiveness and challenges that are being addressed by these companies.

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.

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.

Selectivity and potency of microcystin congeners against OATP1B1 and OATP1B3 expressing cancer cells

Microcystins are potent phosphatase inhibitors and cellular toxins. They require active transport by OATP1B1 and OATP1B3 transporters for uptake into human cells, and the high expression of these transporters in the liver accounts for their selective hepatic toxicity. Several human tumors have been shown to have high levels of expression of OATP1B3 but not OATP1B1, the main transporter in liver cells. We hypothesized that microcystin variants could be isolated that are transported preferentially by OATP1B3 relative to OATP1B1 to advance as anticancer agents with clinically tolerable hepatic toxicity. Microcystin variants have been isolated and tested for cytotoxicity in cancer cells stably transfected with OATP1B1 and OATP1B3 transporters. Microcystin variants with cytotoxic OATP1B1/OATP1B3 IC₅₀ ratios that ranged between 0.2 and 32 were found, representing a 150-fold range in transporter selectivity. As microcystin structure has a significant impact on transporter selectivity, it is potentially possible to develop analogs with even more pronounced OATP1B3 selectivity and thus enable their development as anticancer drugs.

Isolation of Microcystins from the Cyanobacterium Planktothrix rubescens Strain No80

Three minor microcystins have been isolated from a Planktothrix rubescens strain. Their structures have been elucidated by one- and two-dimensional NMR spectroscopy and high-resolution tandem mass spectrometry as the compounds [Asp(3),(E)-Dhb(7)]MC-LY (1), [Asp(3),(E)-Dhb(7)]MC-HtyW (2), and [Asp(3),(E)-Dhb(7)]MC-LW (3). The amino acids found at the variable positions 2 and 4 of the microcystin core structure are in accordance with the predicted amino acid substrate activation selectivities of the non-ribosomal peptide synthetases McyA and McyB described earlier for this strain. All structural microcystin variants produced by this strain were shown to inhibit protein phosphatase 1 in the nanomolar range.

Multi-directional function of the protein phosphatase 1 regulatory subunit TIMAP

TIMAP is an endothelial-cell predominant member of the MYPT family of PP1c regulatory subunits. This study explored the TIMAP-PP1c interaction and substrate specificity in vitro. TIMAP associated with all three PP1c isoforms, but endogenous endothelial cell TIMAP preferentially co-immunoprecipitated with PP1cβ. Structural modeling of the TIMAP/PP1c complex predicts that the PP1c C-terminus is buried in the TIMAP ankyrin cluster, and that the PP1c active site remains accessible. Consistent with this model, C-terminal PP1c phosphorylation by cdk2-cyclinA was masked by TIMAP, and PP1c bound TIMAP when the active site was occupied by the inhibitor microcystin. TIMAP inhibited PP1c activity toward phosphorylase a in a concentration-dependent manner, with half-maximal inhibition in the 0.4-1.2 nM range, an effect modulated by the length, and by Ser333/Ser337 phosphomimic mutations of the TIMAP C-terminus. TIMAP-bound PP1cβ effectively dephosphorylated MLC2 and TIMAP itself. By contrast, TIMAP inhibited the PP1cβ activity toward the putative substrate LAMR1, and instead masked LAMR1 PKA- and PKC-phosphorylation sites. This is direct evidence that MLC2 is a TIMAP/PP1c substrate. The data also indicate that TIMAP can modify protein phosphorylation independent of its function as a PP1c regulatory subunit, namely by masking phosphorylation sites of binding partners like PP1c and LAMR1.

Apoptotic responses of Carassius auratus lymphocytes to nodularin exposure in vitro

Nodularin, a metabolite of Nodularin spumigena, is widely detected in water blooms worldwide and causes serious negative effects on fish. The apoptosis-related cytotoxic effects and mechanisms of nodularin on Carassius auratus lymphocytes were investigated. Transmission electron microscopy results showed that nodularin-treated lymphocytes display a series of morphological changes, including condensed cytoplasm, nuclear chromatin agglutination and marginalization. DNA fragmentation was verified by the DNA-ladder and formation of sub-G1 DNA peaks. These cell characteristics confirmed the occurrence of apoptosis in lymphocytes. Flow cytometric results showed that the percentages of apoptotic cells incubated with 1, 5, 10, and 100 μg/L nodularin for 12 h reached 15.76%, 17.36%, 20.34% and 44.21%, respectively; controls showed low rates of apoptosis (2.4%). The mechanism of apoptosis induced by nodularin was determined, and results showed that nodularin exposure caused a significant increase in intracellular reactive oxygen species (ROS), loss of mitochondrial transmembrane potential in a dose-dependent manner, upregulation of intracellular Ca²⁺, downregulation of Bcl-2 and upregulation of Bax expression at the mRNA and protein levels, and activation of caspase-3 and caspase-9 without caspase-8. In summary, all the results suggest that nodularin induces lymphocyte apoptosis via the mitochondrial apoptotic pathway and destroys the immune response of fish.

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

Glutathione (GSH) and cysteine (Cys) conjugation have long been recognized to be important in the detoxification of microcystins (MCs) in animal organs, however, studies quantitatively estimating this process are rare, especially those simultaneously determining multiple toxins and their metabolites. This paper, for the first time, simultaneously quantified MC-LR (leucine arginine), MC-RR (arginine arginine), MCLR-GSH/Cys and MCRR-GSH/Cys in the liver, kidney, intestine and muscle of the cyanobacteria-eating bighead carp i.p. injected with two doses of MCs using liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). MCLR-Cys and MCRR-Cys content were much higher in kidney than in liver, intestine and muscle, suggesting the organotropism to kidney, while MCLR-GSH and MCRR-GSH were always below the detection limit. Bighead carp effectively metabolized MC-LR and MC-RR into the cysteine conjugates in kidney, as the ratios of MCLR-Cys to MC-LR and MCRR-Cys to MC-RR reached as high as 9.04 and 19.10, respectively. MC-LR and MC-RR were excreted mostly in the form of MCLR/RR-Cys rather than MCLR/RR-GSH, while MCs-GSH might act as mid-metabolites and changed to the more stable MCs-Cys rapidly. Cysteine conjugation of MCs appears to be an important biochemical mechanism for the cyanobacteria-eating fish to resist toxic cyanobacteria. A comparison of such detoxification mechanisms between fish and mammals would be interesting in the future studies.

The role of cysteine conjugation in the detoxification of microcystin-LR in liver of bighead carp (Aristichthys nobilis): a field and laboratory study

The role of glutathione (GSH) and cysteine (Cys) conjugates in the detoxification of microcystin-LR (MC-LR) in bighead carp (Aristichthys nobilis) was examined under laboratory and field conditions. Wild individuals of bighead carp were collected from 5 eutrophic lakes along the Yangtze River, while in laboratory experiment, bighead carp were injected intraperitoneally with 500 μg purified MC-LR/kg body weight (bw). Contents of MC-LR and its glutathione (MC-LR-GSH) and cysteine conjugates (MC-LR-Cys) in the liver of bighead carp were determined by liquid chromatography electrospray ionization mass spectrum (LC-ESI-MS). In laboratory experiment, low concentrations of MC-LR-GSH (mean: 0.042 μg/g dry weight (DW)) were always detectable, and the mean ratio of MC-LR-Cys to MC-LR-GSH was 6.55. While, in field study, relatively high MC-LR-Cys concentration (mean: 0.22 μg/g DW) was detected, whereas MC-LR-GSH was occasionally detectable, and the average ratio of MC-LR-Cys to MC-LR-GSH was as high as 71.49. A positive correlation was found between MC-LR-Cys concentration in the liver of bighead carp and MC-LR content in seston from the five lakes (r = 0.85). These results suggest that MC-LR-Cys might be much more important than MC-LR-GSH in the detoxification of MC-LR in fish liver, and that cysteine conjugation of MC-LR might be a physiological mechanism for the phytoplanktivorous bighead carp to counteract toxic cyanobacteria.

Natural products from the Lithistida: a review of the literature since 2000

Lithistid sponges are known to produce a diverse array of compounds ranging from polyketides, cyclic and linear peptides, alkaloids, pigments, lipids, and sterols. A majority of these structurally complex compounds have very potent and interesting biological activities. It has been a decade since a thorough review has been published that summarizes the literature on the natural products reported from this amazing sponge order. This review provides an update on the current taxonomic classification of the Lithistida, describes structures and biological activities of 131 new natural products, and discusses highlights from the total syntheses of 16 compounds from marine sponges of the Order Lithistida providing a compilation of the literature since the last review published in 2002.

Comparative protein expression in different strains of the bloom-forming cyanobacterium Microcystis aeruginosa

Toxin production in algal blooms presents a significant problem for the water industry. Of particular concern is microcystin, a potent hepatotoxin produced by the unicellular freshwater species Microcystis aeruginosa. In this study, the proteomes of six toxic and nontoxic strains of M. aeruginosa were analyzed to gain further knowledge in elucidating the role of microcystin production in this microorganism. This represents the first comparative proteomic study in a cyanobacterial species. A large diversity in the protein expression profiles of each strain was observed, with a significant proportion of the identified proteins appearing to be strain-specific. In total, 475 proteins were identified reproducibly and of these, 82 comprised the core proteome of M. aeruginosa. The expression of several hypothetical and unknown proteins, including four possible operons was confirmed. Surprisingly, no proteins were found to be produced only by toxic or nontoxic strains. Quantitative proteome analysis using the label-free normalized spectrum abundance factor approach revealed nine proteins that were differentially expressed between toxic and nontoxic strains. These proteins participate in carbon-nitrogen metabolism and redox balance maintenance and point to an involvement of the global nitrogen regulator NtcA in toxicity. In addition, the switching of a previously inactive toxin-producing strain to microcystin synthesis is reported.

Diversity and impact of prokaryotic toxins on aquatic environments: a review

Microorganisms are ubiquitous in all habitats and are recognized by their metabolic versatility and ability to produce many bioactive compounds, including toxins. Some of the most common toxins present in water are produced by several cyanobacterial species. As a result, their blooms create major threats to animal and human health, tourism, recreation and aquaculture. Quite a few cyanobacterial toxins have been described, including hepatotoxins, neurotoxins, cytotoxins and dermatotoxins. These toxins are secondary metabolites, presenting a vast diversity of structures and variants. Most of cyanobacterial secondary metabolites are peptides or have peptidic substructures and are assumed to be synthesized by non-ribosomal peptide synthesis (NRPS), involving peptide synthetases, or NRPS/PKS, involving peptide synthetases and polyketide synthases hybrid pathways. Besides cyanobacteria, other bacteria associated with aquatic environments are recognized as significant toxin producers, representing important issues in food safety, public health, and human and animal well being. Vibrio species are one of the most representative groups of aquatic toxin producers, commonly associated with seafood-born infections. Some enterotoxins and hemolysins have been identified as fundamental for V. cholerae and V. vulnificus pathogenesis, but there is evidence for the existence of other potential toxins. Campylobacter spp. and Escherichia coli are also water contaminants and are able to produce important toxins after infecting their hosts. Other bacteria associated with aquatic environments are emerging as toxin producers, namely Legionella pneumophila and Aeromonas hydrophila, described as responsible for the synthesis of several exotoxins, enterotoxins and cytotoxins. Furthermore, several Clostridium species can produce potent neurotoxins. Although not considered aquatic microorganisms, they are ubiquitous in the environment and can easily contaminate drinking and irrigation water. Clostridium members are also spore-forming bacteria and can persist in hostile environmental conditions for long periods of time, contributing to their hazard grade. Similarly, Pseudomonas species are widespread in the environment. Since P. aeruginosa is an emergent opportunistic pathogen, its toxins may represent new hazards for humans and animals. This review presents an overview of the diversity of toxins produced by prokaryotic microorganisms associated with aquatic habitats and their impact on environment, life and health of humans and other animals. Moreover, important issues like the availability of these toxins in the environment, contamination sources and pathways, genes involved in their biosynthesis and molecular mechanisms of some representative toxins are also discussed.

On the chemistry, toxicology and genetics of the cyanobacterial toxins, microcystin, nodularin, saxitoxin and cylindrospermopsin

The cyanobacteria or "blue-green algae", as they are commonly termed, comprise a diverse group of oxygenic photosynthetic bacteria that inhabit a wide range of aquatic and terrestrial environments, and display incredible morphological diversity. Many aquatic, bloom-forming species of cyanobacteria are capable of producing biologically active secondary metabolites, which are highly toxic to humans and other animals. From a toxicological viewpoint, the cyanotoxins span four major classes: the neurotoxins, hepatotoxins, cytotoxins, and dermatoxins (irritant toxins). However, structurally they are quite diverse. Over the past decade, the biosynthesis pathways of the four major cyanotoxins: microcystin, nodularin, saxitoxin and cylindrospermopsin, have been genetically and biochemically elucidated. This review provides an overview of these biosynthesis pathways and additionally summarizes the chemistry and toxicology of these remarkable secondary metabolites.

Cyanobacterial cyclopeptides as lead compounds to novel targeted cancer drugs

Cyanobacterial cyclopeptides, including microcystins and nodularins, are considered a health hazard to humans due to the possible toxic effects of high consumption. From a pharmacological standpoint, microcystins are stable hydrophilic cyclic heptapeptides with a potential to cause cellular damage following uptake via organic anion-transporting polypeptides (OATP). Their intracellular biological effects involve inhibition of catalytic subunits of protein phosphatase 1 (PP1) and PP2, glutathione depletion and generation of reactive oxygen species (ROS). Interestingly, certain OATPs are prominently expressed in cancers as compared to normal tissues, qualifying MC as potential candidates for cancer drug development. In the era of targeted cancer therapy, cyanotoxins comprise a rich source of natural cytotoxic compounds with a potential to target cancers expressing specific uptake transporters. Moreover, their structure offers opportunities for combinatorial engineering to enhance the therapeutic index and resolve organ-specific toxicity issues. In this article, we revisit cyanobacterial cyclopeptides as potential novel targets for anticancer drugs by summarizing existing biomedical evidence, presenting structure-activity data and discussing developmental perspectives.

Calyculins and related marine natural products as serine-threonine protein phosphatase PP1 and PP2A inhibitors and total syntheses of calyculin A, B, and C

Calyculins, highly cytotoxic polyketides, originally isolated from the marine sponge Discodermia calyx by Fusetani and co-workers, belong to the lithistid sponges group. These molecules have become interesting targets for cell biologists and synthetic organic chemists. The serine/threonine protein phosphatases play an essential role in the cellular signalling, metabolism, and cell cycle control. Calyculins express potent protein phosphatase 1 and 2A inhibitory activity, and have therefore become valuable tools for cellular biologists studying intracellular processes and their control by reversible phosphorylation. Calyculins might also play an important role in the development of several diseases such as cancer, neurodegenerative diseases, and type 2-diabetes mellitus. The fascinating structures of calyculins have inspired various groups of synthetic organic chemists to develop total syntheses of the most abundant calyculins A and C. However, with fifteen chiral centres, a cyano-capped tetraene unit, a phosphate-bearing spiroketal, an anti, anti, anti dipropionate segment, an alpha-chiral oxazole, and a trihydroxylated gamma-amino acid, calyculins reach versatility that only few natural products can surpass, and truly challenge modern chemists' asymmetric synthesis skills.

Selective reduction of C=C double bonds in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of microcystins

Cyanobacteria are photosynthetic bacteria encountered in various aquatic environments. Some of them are able to produce powerful toxins called cyanotoxins. Among cyanotoxins, microcystins (MCs) constitute a group of closely related cyclic heptapeptides. Their sequences are made up of classical amino acids as well as post- translational modified ones. Interestingly, in vivo metabolism of microcystins seems to be greatly dependent on various minor structural differences and particularly those of the seventh amino acid, which can be either dehydroalanine (or a derivative), dehydroaminobutyric acid (or a derivative), serine or alanine. As a consequence, microcystins have been classified on the basis of the nature of this singular amino acid. A major difficulty in the classification of such toxins is that some of them share the same molecular masses and the same molecular formulas. Consequently, a simple mass measurement is not sufficient to determine the structure and the class of a toxin of interest. Heavy and expensive techniques are used to classify them, such as multi-dimensional nuclear magnetic resonance and amino acid analysis. In this work, a new matrix-assisted laser desorption/ionization time-of-flight method leading to an easy classification of MCs is proposed. The methodology relies on the reductive properties of the matrix 1,5-diaminonaphtalene (1,5-DAN) which appears to be able to selectively reduce the double carbon-carbon bond belonging to the seventh amino acid. Moreover, the yield of reduction seems to be influenced by the degree of substitution of this double bond, allowing a discrimination between dehydroalanine and dehydroaminobutyric acid. This selective reduction was confirmed by the study of three synthetic peptides by mass spectrometry and tandem mass spectrometry. According to these results, the use of reductive matrices seems to be promising in the study of microcystins and in their classification. More generally, 1,5-DAN allows the selective reduction of double carbon-carbon bonds. This property could also be employed in the characterization of others types of compound displaying double bonds (petrochemistry, metabolomics....).

Molecular aspects of microcystin-induced hepatotoxicity and hepatocarcinogenesis

It is known that microcystin (MC) is a cyanotoxin that is a potent environmental inhibitor of eucariotic protein serine/threonine phosphatase 1 and 2A, both in vitro and in vivo. Consequently, these cyanobacterial toxins (MC-IARC group 2B carcinogen, MC extracts-group 3) are potent tumor promoters and there is an indication that they may also act as tumor initiators. The ability of microcystin-LR (MC-LR) to act as a tumor initiator is based on fact that it can induce DNA damage either by direct interaction with DNA or by indirect mechanisms through formation of reactive oxygen species (ROS). Both acute and chronic exposures, to either low or high doses of MC-LR, can activate apoptotic pathways. Chronic exposure to low concentrations of MC-LR contributes to increased risk for cancer development. Epidemiological studies, in certain areas of China, have suggested that MC is one of the risk factors for the high incidence of primary liver cancer (PLC). Recently, we have reported a correlation between PLC and cyanobacterial "blooms" in reservoirs used as a source for drinking water supply in central Serbia. It appears that the combination of acute and chronic exposures to both high and low doses of MC can lead to PLC initiation and promotion. Based on this, we propose that the requirement for the co-factors such as aflatoxin B1 and other mycotoxins, HBV, HCV, alcohol, etc. is not needed for initiation and promotion of PLC by MC-LR as was suggested earlier. The possible mechanisms of the genotoxicity of MC and its role as a hepatocarcinogen are outlined in this review. Furthermore, we show that the exposure of hepatocytes to MC can lead either to malignant proliferation or apoptosis.

Cyanobacterial toxins and liver diseaseThis article is one of a selection of papers published in a special issue celebrating the 125th anniversary of the Faculty of Medicine at the University of Manitoba

Blue-green algae, also known as cyanobacteria, produce a variety of toxins, some of which have been implicated in the pathogenesis of severe and potentially life-threatening diseases in humans. As the growth of cyanobacteria within freshwater lakes increases worldwide, it is important to review our present understanding of their toxicity and potential carcinogenicity to gain insight into how these organisms impact human health. This review addresses each of these topics, with special emphasis given to cyanobacterial hepatotoxins within freshwater environments.

Determination of microcystin-LR and its metabolites in snail (Bellamya aeruginosa), shrimp (Macrobrachium nipponensis) and silver carp (Hypophthalmichthys molitrix) from Lake Taihu, China

This paper describes seasonal changes of microcystin-LR (MC-LR) and its glutathione (MC-LR-GSH) and cysteine conjugates (MC-LR-Cys) in three aquatic animals--snail (Bellamya aeruginosa), shrimp (Macrobrachium nipponensis) and silver carp (Hypophthalmichthys molitrix) collected from Lake Taihu, China. MC-LR, MC-LR-GSH, and MC-LR-Cys were determined by liquid chromatography electrospray ionization mass spectrum (LC-ESI-MS). The mean MC-LR concentrations in the hepatopancreas of snail and shrimp and liver of silver carp were 6.61, 0.24, and 0.027 microg g(-1) dry weight (DW), respectively; while the average MC-LR-Cys concentrations were 0.50, 0.97, and 5.72 microg g(-1) DW, respectively. MC-LR-GSH was usually not detectable in these samples. The above results suggest that: (1) in aquatic animals, especially fish, the main excretion form of MC-LR could be MC-LR-Cys, but not MC-LR-GSH, whereas MC-LR-Cys might play an important role in detoxication of MC-LR and (2) that efficiency of MC-LR-Cys formation differs among species. The main detoxication pathway of MC-LR in aquatic animals is suggested as follows: when MC-LR enters into liver/hepatopancreas, it firstly conjugates with polypeptide or protein (including GSH, PP-1 and 2A) containing Cys residues, perhaps also some free cysteine; subsequently, MC-LR-Cys is degraded from these polypeptide or protein; and finally is excreted from animals by the compound of MC-LR-Cys.

Carcinogenic aspects of protein phosphatase 1 and 2A inhibitors

Okadaic acid is functionally a potent tumor promoter working through inhibition of protein phosphatases 1 and 2A (PP1 and PP2A), resulting in sustained phosphorylation of proteins in cells. The mechanism of tumor promotion with okadaic acid is thus completely different from that of the classic tumor promoter phorbol ester. Other potent inhibitors of PP1 and PP2A - such as dinophysistoxin-1, calyculins A-H, microcystin-LR and its derivatives, and nodularin - were isolated from marine organisms, and their structural features including the crystal structure of the PP1-inhibitor complex, tumor promoting activities, and biochemical and biological effects, are here reviewed. The compounds induced tumor promoting activity in three different organs, including mouse skin, rat glandular stomach and rat liver, initiated with three different carcinogens. The results indicate that inhibition of PP1 and PP2A is a general mechanism of tumor promotion applicable to various organs. This study supports the concept of endogenous tumor promoters in human cancer development.