Regulation on the toxicity of microcystin-LR target to protein phosphatase 1 by biotransformation pathway: effectiveness and mechanism

From in-silico screening to in-vitro evaluation: Enhancing the detection of Microcystins with engineered PP1 mutant variants

Cyanotoxins produced during harmful algal blooms (CyanoHABs) have become a worldwide issue of concern. Microcystins (MC) are the most ubiquitous group of cyanotoxins and have known carcinogenic and hepatotoxic effects. The protein phosphatase inhibition assays (PPIAs), based on the inhibition of Protein Phosphatase 1/2A (PP1/PP2A) by MC, are one of the most cost-effective options for detecting MC. In this work, we aimed to design in-silico and evaluate in-vitro mutant variants of the PP1 protein, in order to enhance their capabilities as a MC biosensor. To this end, we performed an in-silico active site-saturated mutagenesis screening, followed by stability and docking affinity calculation with the MCLR cyanotoxin. Candidates with improved both affinity and stability were further tested in a fully flexible active-site docking. The best-scored mutations (19) were individually analysed regarding their locations and interactions. Four of them (p.D197F; p.Q249Y; p.S129W; p.D220Q) were selected for in-vitro expression and evaluation. Mutant p.D197F, exhibited a significant increment in inhibition by MCLR with respect to the WT, while showing a non-significant difference in stability nor activity. This successful PP1 inhibition enhancement suggests the potential of the p.D197F variant for practical MC detection applications.

Regulation Effectiveness and Mechanism of Biotransformation Pathway on the Toxicity of Microcystin-LR Target to Protein Phosphatase 2A

Biotransformation is recognized as a potential pathway to regulate the environmental risk of microcystins (MCs). To explore the regulation effectiveness and mechanism of the biotransformation pathway, six typical MCLR-biotransformation products (MCLR-BTPs) were prepared, and their inhibition effects on protein phosphatase 2A (PP2A) were evaluated. The inhibition effects of the MCLR-BTPs generally decreased with the increase in biothiol molecular weights and polarity, indicating that biotransformation was an effective pathway through which to regulate MCLR toxicity. To further explore the regulation mechanism, the key interaction processes between the MCLR/MCLR-BTPs and the PP2A were explored by homology modeling and molecular docking. The introduced biothiols blocked the covalent binding of Mdha7 to Cys269 but strengthened the hydrogen bond "Mdha7"→Arg268. The changed "Mdha7" intervened the combination of MCLR-BTPs to PP2A by weakening the hydrogen bonds Arg4←Arg214, Arg4→Pro213, Adda5←His118, and Ala1←Arg268, and the ionic bond Glu6-Mn12+. The weakening combination of the MCLR-BTPs to PP2A further attenuated the interactions between the conserved domain and the Mn2+ ions (including the ionic bonds Asp57-Mn12+ and Asp85-Mn12+ and the metal bonds Asp57-Mn12+ and Asn117-Mn12+) and increased the exposure of the Mn2+ ions. Meanwhile, the weakened hydrogen bond Arg4←Arg214 facilitated the combination of the phosphate group to Arg214 (with increased exposure). In this way, the catalytic activity of the PP2A was restored.

Insight into the Molecular Mechanism for the Discrepant Inhibition of Microcystins (MCLR, LA, LF, LW, LY) on Protein Phosphatase 2A

Microcystins (MCs) exhibit diversified inhibition effects on protein phosphatases (PPs) due to their structural differences. To fully evaluate the potential mechanism for the discrepant inhibition effects, the five most frequent MCs with varying residues at position Z4 were selected as the tested toxins. Their inhibition sequence on PP2A was detected as follows: MCLR > MCLW > MCLA > MCLF > MCLY. Combined with homology modeling and molecular docking technology, the major interaction parameters between the MCs and PP2A were obtained. The correlation analysis for the major interaction parameters and inhibition effects showed that the hydrophobicity of Z4 had an important influence on the interaction of the MCs to PP2A. The introduction of hydrophobic Z4 directly weakened hydrogen bonds Z4→Pro213 and Z4←Arg214, indirectly weakened hydrogen bonds Adda5←Asn117, Glu6←Arg89, and MeAsp3←Arg89, but indirectly enhanced ionic bonds Glu6←Arg89, Glu6-Mn12+, and Glu6-Mn22+. In this way, the combination of the MCs with PP2A was blocked, and thus, the interactions between PP2A and the Mn2+ ions (in the catalytic center) were further affected; metal bonds Asp85-Mn12+ and Asp85-Mn22+ were weakened, while metal bond His241-Mn12+ was enhanced. As a result, the interactions in the catalytic center were inhibited to varying degrees, resulting in the reduced toxicity of MCs.

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

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

Recent developments in the methods of quantitative analysis of microcystins

Cyanotoxins are produced by the toxic cyanobacterial species present in algal blooms formed in water bodies due to nutrient over-enrichment by human influences and natural environmental conditions. Extensive studies are available on the most widely encountered cyanotoxins, microcystins (MCs) in fresh and brackish water bodies. MC contaminated water poses severe risks to human health, environmental sustainability, and aquatic life. Therefore, commonly occurring MCs should be monitored. Occasionally, detection and quantification of these toxins are difficult due to the unavailability of pure standards. Enzymatic, immunological assays, and analytical techniques like protein phosphatase inhibition assay, enzyme-linked immunosorbent assay, high-performance liquid chromatography, liquid chromatography-mass spectrometry, and biosensors are used for their detection and quantification. There is no single method for the detection of all the different types of MCs; therefore, various techniques are often combined to yield reliable results. Biosensor development offered a problem-solving approach in the detection of MCs due to their high accuracy, sensitivity, rapid response, and portability. In this review, an endeavor has been made to uncover emerging techniques used for the detection and quantification of the MCs.