Preparation of Fluorescent Microcystin Derivatives by Direct Arginine Labelling and Their Biological Evaluation

Lethal and behavioral effects of semi-purified microcystins, Micropeptin and apolar compounds from cyanobacteria on freshwater microcrustacean Thamnocephalus platyurus

The mass proliferation of cyanobacteria, episodes known as blooms, is a concern worldwide. One of the most critical aspects during these blooms is the production of toxic secondary metabolites that are not limited to the four cyanotoxins recognized by the World Health Organization. These metabolites comprise a wide range of structurally diverse compounds that possess bioactive functions. Potential human and ecosystem health risks posed by these metabolites and co-produced mixtures remain largely unknown. We studied acute lethal and sublethal effects measured as impaired mobility on the freshwater microcrustaceans Thamnocephalus platyurus for metabolite mixtures from two cyanobacterial strains, a microcystin (MC) producer and a non-MC producer. Both cyanobacterial extracts, from the MC-producer and non-MC-producer, caused acute toxicity with LC50 (24 h) values of 0.50 and 2.55 mgdw_biomass/mL, respectively, and decreased locomotor activity. Evaluating the contribution of different cyanopeptides revealed that the Micropeptin-K139-dominated fraction from the MC-producer extract contributed significantly to mortality and locomotor impairment of the microcrustaceans, with potential mixture effect with other cyanopeptolins present in this fraction. In the non-MC-producer extract, compounds present in the apolar fraction contributed mainly to mortality, locomotor impairment, and morphological changes in the antennae of the microcrustacean. No lethal or sublethal effects were observed in the fractions dominated by other cyanopetides (Cyanopeptolin 959, Nostoginin BN741). Our findings contribute to the growing body of research indicating that cyanobacterial metabolites beyond traditional cyanotoxins cause detrimental effects. This underscores the importance of toxicological assessments of such compounds, also at sublethal levels.

Introducing fluorescent probe technology for detecting microcystin-LR in the water and cells

BACKGROUND

For a long time, the environment hazards caused by cyanobacteria bloom and associated microcystins have attracted attention worldwide. Microcystin-LR (MC-LR) is the most widely distributed and most toxic toxin. At present, numerous MC-LR detection methods exist many drawbacks. Therefore, a quick and accurate method for identifying and detecting MC-LR is crucial and necessary. In this work, we strived to introduce a novel fluorescence assay to detect MC-LR in the water and cells.

RESULTS

According to the special spatial configuration and physicochemical properties of MC-LR, we designed and constructed six fluorescent probes. The design concepts of the probes were exhaustively elaborated. MC-YdTPA, MC-YdTPE, MC-RdTPA, and MC-RdTPE could show significant fluorescence enhancement in MC-LR solution. Significantly, MC-YdTPA, MC-YdTPE, and MC-RdTPA could also response well in the cells treated with MC-LR, demonstrating these fluorescent probes' values. The recognition mechanism between probes and MC-LR were also deeply explored: (1) The polyphenylene ring structure of probes may have nested or hydrogen bond weak interaction with the ring structure of MC-LR. (2) The probes can generate a reaction to the hydrogen ions ionized by MC-LR.

SIGNIFICANCE

We proposed the novel ideas for designing MC-LR probes. This research can provide valuable experiences and important assistance in synthesizing MC-LR fluorescent probes. We expect that this work may bring new ideas to develop fluorescent probes for researching MC-LR in vivo and in vitro.

Excluding interference and detecting Microcystin-LR in the natural lakes and cells based a unique fluorescence method

Cyanobacteria blooms that cause the death of aquatic and terrestrial organisms have attracted considerable attention since the 19th century. The most typical toxin in cyanobacteria blooms is cyanobacteria toxin, particularly microcystin-LR (MC-LR). Therefore, a simple and highly efficient method for detecting MC-LR plays a role in studying the ecological toxicology of MC-LR. However, as MC-LR itself is located in a complex environment, traditional techniques present complex and false-positive defects. To address the above issues, novel technologies should be explored and discovered. Herein, we describe the development of MC-BDKZ as the first paradigm of probes that can concurrently report MC-LR in natural lakes and cells. This novel material shows large Stokes Shift and possesses good photostability and high sensitivity. Considering the properties mentioned above, MC-BDKZ not only achieves the detection of MC-LR in the lake water samples, but also completes the imaging of exogenous MC-LR in cells. Moreover, the interference of many factors in the lake and cells is excluded completely in the process of MC-LR detection. We comprehensively analyzed the response principle and potential application of MC-BDKZ in the process of MC-LR detection. Compared with the conventional MC-LR detection technologies, fluorescence probe technology shows better convenience and greatly reduces distance from the practical application in vitro and in vivo. We envisioned that the development of this visual research tool could provide crucial clues for exploring the pathogenesis of MC-LR in body.

Precursor-Directed Biosynthesis and Fluorescence Labeling of Clickable Microcystins

Microcystins, cyclic nonribosomal heptapeptides, are the most well-known cyanobacterial toxins. They are exceptionally well studied, but open questions remain concerning their physiological role for the producing microorganism or their suitability as lead compounds for anticancer drug development. One means to study specialized metabolites in more detail is the introduction of functional groups that make a compound amenable for bioorthogonal, so-called click reactions. Although it was reported that microcystins cannot be derivatized by precursor-directed biosynthesis, we successfully used this approach to prepare clickable microcystins. Supplementing different azide- or terminal alkyne containing amino acid analogues into the cultivation medium of microcystin-producing cyanobacteria strains, we found that these strains differ strongly in their substrate acceptance. Exploiting this flexibility, we generated more than 40 different clickable microcystins. We conjugated one of these derivatives with a fluorogenic dye and showed that neither incorporation of the unnatural amino acid analogue nor attachment of the fluorescent label significantly affects the cytotoxicity against cell lines expressing the human organic anion transporting polypeptides 1B1 or 1B3. Using time-lapse microscopy, we observed that the fluorescent microcystin is rapidly taken up into eukaryotic cells expressing these transporters.

Mimicking of Arginine by Functionalized N(ω)-Carbamoylated Arginine As a New Broadly Applicable Approach to Labeled Bioactive Peptides: High Affinity Angiotensin, Neuropeptide Y, Neuropeptide FF, and Neurotensin Receptor Ligands As Examples

Derivatization of biologically active peptides by conjugation with fluorophores or radionuclide-bearing moieties is an effective and commonly used approach to prepare molecular tools and diagnostic agents. Whereas lysine, cysteine, and N-terminal amino acids have been mostly used for peptide conjugation, we describe a new, widely applicable approach to peptide conjugation based on the nonclassical bioisosteric replacement of the guanidine group in arginine by a functionalized carbamoylguanidine moiety. Four arginine-containing peptide receptor ligands (angiotensin II, neurotensin(8-13), an analogue of the C-terminal pentapeptide of neuropeptide Y, and a neuropeptide FF analogue) were subject of this proof-of-concept study. The N(ω)-carbamoylated arginines, bearing spacers with a terminal amino group, were incorporated into the peptides by standard Fmoc solid phase peptide synthesis. The synthesized chemically stable peptide derivatives showed high receptor affinities with Ki values in the low nanomolar range, even when bulky fluorophores had been attached. Two new tritiated tracers for angiotensin and neurotensin receptors are described.

Comparative effects of nodularin and microcystin-LR in zebrafish: 2. Uptake and molecular effects in eleuthero-embryos and adult liver with focus on endoplasmic reticulum stress

Microcystin (MC) and nodularin are structurally similar cyanobacterial toxins that inhibit protein phosphatases. Additional modes of action are poorly known, in particular for nodularin. In our associated work, we showed that active cellular uptake is mediated by the organic anion transporting polypeptide drOatp1d1 in zebrafish (Faltermann et al., 2016). Here, we assessed the transcriptional expression of three genes encoding three uptake transporters during embryonic development from 24h post fertilization (hpf) to 168 hpf. Transcripts of drOatp1d1 and drOatp2b1 are present at 24 hpf. The abundance increased after hatching and remained about constant up to 168 hpf. Transcripts of drOatp2b1 were most abundant, while drOapt1f transcripts showed very low relative abundance compared to drOatp1d1 and drOatp2b1. We further demonstrated the uptake of fluorescent labeled MC-LR in eleuthero-embryos and its accumulation in the glomerulus of the pronephros. An important molecular effect of MC-LR in human liver cells is the induction of endoplasmic reticulum (ER)-stress. Here, we investigated, whether MC-LR and nodularin similarly lead to induction of ER-stress in zebrafish by analyzing changes of mRNA levels of genes indicative of ER-stress. In zebrafish liver organ cultures short- and long-term exposures to 0.15 and 0.3 μmol L(-1) MC-LR, and 0.5 and 1 μM L(-1) nodularin led to significant transcriptional induction of several ER-stress marker genes, including the chaperone glucose regulated protein 78 (bip), the spliced form of x-box binding protein (xbp-1s), the CCAAT-enhancer-binding protein homologous protein (chop) and activating transcription factor 4 (atf4). Furthermore, strong transcriptional changes occurred for tumor necrosis factor alpha (tnfa) and dual specificity phosphatase 5 (dusp5), associated with mitogen activated protein kinase (MAPK) pathway. However, no alterations in transcript levels of pro-apoptotic genes Bcl-2 like protein 4 (bax) and p53 occurred. In contrast to adult liver, MC-LR and nodularin did not result in detectable changes of mRNA levels of selected target genes involved in ER-stress in zebrafish eleuthero-embryos, nor was the abundance of transcripts belonging to the MAPK and pro-apoptosis pathways altered. In conclusion, our data indicate that MC-LR and nodularin have similar transcriptional effects. They lead to changes in mRNA levels of genes that suggest induction of ER-stress, and furthermore, lead to increased level of tnfα mRNA in the adult liver, which suggests a novel (transcriptional) mode of action in fish. However, although taken up by eleuthero-embryos, no transcriptional changes induced by these cyanobacterial toxins were detected. This is probably due to action to specific organs such as liver and kidneys that could not be identified by whole-embryo sampling.

Comparative effects of nodularin and microcystin-LR in zebrafish: 1. Uptake by organic anion transporting polypeptide Oatp1d1 (Slco1d1)

Microcystin-LR (MC-LR) and nodularin are hepatotoxins produced by several cyanobacterial species. Their toxicity is based on active cellular uptake and subsequent inhibition of protein phosphatases PP1/2A, leading to hyperphosphorylation and cell death. To date, uptake of MC-LR and nodularin in fish is poorly understood. Here, we investigated the role of the organic anion transporting polypeptide Oatp1d1 in zebrafish (drOatp1d1, Slco1d1) in cellular uptake in zebrafish. We stably transfected CHO and HEK293 cell lines expressing drOatp1d1. In both transfectants, uptake of MC-LR and nodularin was demonstrated by competitive inhibition of uptake with fluorescent substrate lucifer yellow. Direct uptake of MC-LR was demonstrated by immunostaining, and indirectly by the high cytotoxicity in stable transfectants. By means of a synthesized fluorescent labeled MC-LR derivative, direct uptake was further confirmed in HEK293 cells expressing drOatp1d1. Additionally, uptake and toxicity was investigated in the permanent zebrafish liver cell line ZFL. These cells had only a low relative abundance of drOatp1d1, drOatp2b1 and drOatp1f transcripts, which correlated with the lack of MC-LR induced cytotoxicity and transcriptional changes of genes indicative of endoplasmic reticulum stress, a known effect of this toxin. Our study demonstrates that drOatp1d1 functions as an uptake transporter for both MC-LR and nodularin in zebrafish.