Structure of protein phosphatase 2A core enzyme bound to tumor-inducing toxins

STRIPAK, a fundamental signaling hub of eukaryotic development

SUMMARYThe striatin-interacting phosphatase and kinase (STRIPAK) complex is involved in the regulation of many developmental processes in eukaryotic microorganisms and all animals, including humans. STRIPAK is a component of protein phosphatase 2A (PP2A), a highly conserved serine-threonine phosphatase composed of catalytic subunits (PP2Ac), a scaffolding subunit (PP2AA) and various substrate-directing B regulatory subunits. In particular, the B''' regulatory subunit called striatin has evoked major interest over the last 20 years. Studies in fungal systems have contributed substantially to our current knowledge about STRIPAK composition, assembly, and cellular localization, as well as its regulatory role in autophagy and the morphology of fungal development. STRIPAK represents a signaling hub with many kinases and thus integrates upstream and downstream information from many conserved eukaryotic signaling pathways. A profound understanding of STRIPAK's regulatory role in fungi opens the gateway to understanding the multifarious functions carried out by STRIPAK in higher eukaryotes, including its contribution to malignant cell growth.

Dephosphorylation of bZIP59 by PP2A ensures appropriate shade avoidance response in Arabidopsis

Changes in light quality and quantity experienced by many shade-intolerant plants grown in close proximity lead to transcriptional reprogramming and shade avoidance syndrome (SAS). Despite the importance of phosphorylation-dependent signaling in cellular physiology, phosphorylation events during SAS are largely unknown. Here, we examined shade-regulated phosphorylation events in Arabidopsis using quantitative phosphoproteomics. We confirmed shade-induced dephosphorylation of bZIP59, a basic region/leucine zipper motif (bZIP) transcription factor. Shade treatment promotes the nuclear localization of bZIP59, which can be mimicked by mutation of the phosphorylation sites on bZIP59. Phenotypic analysis identified that bZIP59 negatively regulated shade-induced hypocotyl elongation. bZIP59 repressed the shade-induced activation of certain growth-related genes, while shade increased the DNA binding of bZIP59. Furthermore, the protein phosphatase 2A (PP2A) mediated dephosphorylation of bZIP59. Our study characterized a previously unidentified mechanism by which the phytochrome B (phyB)-PP2A-bZIP59 regulatory module integrates shade signals and transcriptomes, broadening our knowledge of phosphorylation strategies for rapid adaptation to shade.

Derivatization of Microcystins Can Increase Target Inhibition while Reducing Cellular Uptake

Microcystins, a large family of nonribosomal cyclic heptapeptides known for their hepatotoxicity, are among the best-studied cyanobacterial toxins. Recently, they have been discussed as leads for the development of anticancer drug substances. Their main mode-of-action is inhibition of the eukaryotic serine/threonine protein phosphatases 1 and 2A. Unlike many cytotoxins that can cross cell membranes by passive diffusion, microcystins depend on active uptake via organic anion transporting polypeptides 1B1 or 1B3. Both phosphatase inhibition and transportability strongly depend on the structure of the individual microcystin. Here, we present how chemical modification of positions 2 and 4 of the microcystin core structure can alter these two properties. Aiming to reduce transportability and increase phosphatase inhibition, we used pharmacophore modeling to investigate the phosphatase inhibition potential of microcystins derivatized with small molecules containing a variety of functional groups. The respective derivatives were synthesized using click chemistry. We discovered that some derivatized microcystins can address a yet undescribed subpocket of the protein phosphatase 1. The derivatized microcystins were tested for phosphatase 1 inhibition and cytotoxicity on transporter-expressing cell lines, revealing that target inhibition and transportability of microcystins can independently be influenced by the physicochemical properties, especially of the residue located in position 2 of the microcystin. Derivatization with small acids or amino acids resulted in microcystins with a favorable ratio of inhibition to transportability, making these derivatives potentially suitable for drug development.

Protein phosphatase 2A regulates blood cell proliferation and differentiation in Drosophila larval lymph glands

Protein phosphatase 2A (PP2A), one of the most abundant protein phosphatases, has divergent functions in multiple types of cells. Its inactivation has been closely associated with leukemia diseases. However, the physiological function of PP2A for hematopoiesis has been poorly understood in organisms. Drosophila hematopoiesis parallels the vertebrate counterpart in developmental and functional features but involves a much simpler hematopoietic system. Here, utilizing the Drosophila major larval hematopoietic organ lymph gland, we studied the function of PP2A for hematopoiesis in vivo. By knocking down the expression of Pp2A-29B that encodes the scaffold subunit of the PP2A holoenzyme complex, we found that PP2A silencing in the differentiating hemocytes resulted in their excessive proliferation. Furthermore, this PP2A inhibition downregulated the expression of Smoothened (Smo), a crucial component in the Hedgehog pathway, and smo overexpression was able to rescue the phenotypes of PP2A depletion, indicating that Smo functions as a downstream effector of PP2A to restrict the hemocyte proliferation. PDGF/VEGF-receptor (Pvr) overexpression also restored the Smo expression and lymph gland morphology of PP2A silencing, suggesting a PP2A-Pvr-Smo axis to regulate lymph gland growth and hemocyte proliferation. Moreover, inhibiting PP2A activity in the blood progenitor cells promoted their differentiation, but which was independent with Smo. Together, our data suggested that PP2A plays a dual role in the Drosophila lymph gland by preserving the progenitor population and restraining the hemocyte proliferation, to properly regulate the hematopoietic process.

Integrated stress response plasticity governs normal cell adaptation to chronic stress via the PP2A-TFE3-ATF4 pathway

The integrated stress response (ISR) regulates cell fate during conditions of stress by leveraging the cell's capacity to endure sustainable and efficient adaptive stress responses. Protein phosphatase 2A (PP2A) activity modulation has been shown to be successful in achieving both therapeutic efficacy and safety across various cancer models. However, the molecular mechanisms driving its selective antitumor effects remain unclear. Here, we show for the first time that ISR plasticity relies on PP2A activation to regulate drug response and dictate cellular survival under conditions of chronic stress. We demonstrate that genetic and chemical modulation of the PP2A leads to chronic proteolytic stress and triggers an ISR to dictate whether the cell lives or dies. More specifically, we uncovered that the PP2A-TFE3-ATF4 pathway governs ISR cell plasticity during endoplasmic reticular and cellular stress independent of the unfolded protein response. We further show that normal cells reprogram their genetic signatures to undergo ISR-mediated adaptation and homeostatic recovery thereby avoiding toxicity following PP2A-mediated stress. Conversely, oncogenic specific cytotoxicity induced by chemical modulation of PP2A is achieved by activating chronic and irreversible ISR in cancer cells. Our findings propose that a differential response to chemical modulation of PP2A is determined by intrinsic ISR plasticity, providing a novel biological vulnerability to selectively induce cancer cell death and improve targeted therapeutic efficacy.

PP2Ac Regulates Autophagy via Mediating mTORC1 and ULK1 During Osteoclastogenesis in the Subchondral Bone of Osteoarthritis

The molecular mechanism underlying abnormal osteoclastogenesis triggering subchondral bone remodeling in osteoarthritis (OA) is still unclear. Here, single-cell and bulk transcriptomics sequencing analyses are performed on GEO datasets to identify key molecules and validate them using knee joint tissues from OA patients and rat OA models. It is found that the catalytic subunit of protein phosphatase 2A (PP2Ac) is highly expressed during osteoclastogenesis in the early stage of OA and is correlated with autophagy. Knockdown or inhibition of PP2Ac weakened autophagy during osteoclastogenesis. Furthermore, the ULK1 expression of the downstream genes is significantly increased when PP2Ac is knocked down. PP2Ac-mediated autophagy is dependent on ULK1 phosphorylation activity during osteoclastogenesis, which is associated with enhanced dephosphorylation of ULK1 Ser637 residue regulating at the post-translational level. Additionally, mTORC1 inhibition facilitated the expression level of PP2Ac during osteoclastogenesis. In animal OA models, decreasing the expression of PP2Ac ameliorated early OA progression. The findings suggest that PP2Ac is also a promising therapeutic target in early OA.

Microcystins with Modified Adda5-Residues from a Heterologous Microcystin Expression System

Microcystins are hepatotoxic cyclic heptapeptides produced by some cyanobacterial species and usually contain the unusual β-amino acid 3S-amino-9S-methoxy-2S,6,8S-trimethyl-10-phenyl-4E,6E-decadienoic acid (Adda) at position-5. The full microcystin gene cluster from Microcystis aeruginosa PCC 7806 has been expressed in Escherichia coli. In an earlier study, the engineered strain was shown to produce MC-LR and [d-Asp3]MC-LR, the main microcystins reported in cultures of M. aeruginosa PCC 7806. However, analysis of the engineered strain of E. coli using semitargeted liquid chromatography with high-resolution tandem mass spectrometry (LC-HRMS/MS) and thiol derivatization revealed the presence of 15 additional microcystin analogues, including four linear peptide variants and, in total, 12 variants with modifications to the Adda moiety. Four of the Adda-variants lacked the phenyl group at the Adda-terminus, a modification that has not previously been reported in cyanobacteria. Their HRMS/MS spectra contained the product-ion from Adda at m/z 135.1168, but the commonly observed product-ion at m/z 135.0804 from Adda-containing microcystins was almost completely absent. In contrast, three of the variants were missing a methyl group between C-2 and C-8 of the Adda moiety, and their LC-HRMS/MS spectra displayed the product-ion from Adda at m/z 135.0804. However, instead of the product-ion at m/z 135.1168, these three variants gave product-ions at m/z 121.1011. These observations, together with spectra from microcystin standards using in-source fragmentation, showed that the product-ion at m/z 135.1168 found in the HRMS/MS spectra of most microcystins originated from the C-2 to C-8 region of the Adda moiety. Identification of the fragmentation pathways for the Adda side chain will facilitate the detection of microcystins containing modifications in their Adda moieties that could otherwise easily be overlooked with standard LC-MS screening methods. Microcystin variants containing Abu at position-1 were also prominent components of the microcystin profile of the engineered bacterium. Microcystin variants with Abu1 or without the phenyl group on the Adda side chain were not detected in the original host cyanobacterium. This suggests not only that the microcystin synthase complex may be affected by substrate availability within its host organism but also that it possesses an unexpected degree of biosynthetic flexibility.

Structural basis of Integrator-dependent RNA polymerase II termination

The Integrator complex can terminate RNA polymerase II (Pol II) in the promoter-proximal region of genes. Previous work has shed light on how Integrator binds to the paused elongation complex consisting of Pol II, the DRB sensitivity-inducing factor (DSIF) and the negative elongation factor (NELF) and how it cleaves the nascent RNA transcript1, but has not explained how Integrator removes Pol II from the DNA template. Here we present three cryo-electron microscopy structures of the complete Integrator-PP2A complex in different functional states. The structure of the pre-termination complex reveals a previously unresolved, scorpion-tail-shaped INTS10-INTS13-INTS14-INTS15 module that may use its 'sting' to open the DSIF DNA clamp and facilitate termination. The structure of the post-termination complex shows that the previously unresolved subunit INTS3 and associated sensor of single-stranded DNA complex (SOSS) factors prevent Pol II rebinding to Integrator after termination. The structure of the free Integrator-PP2A complex in an inactive closed conformation2 reveals that INTS6 blocks the PP2A phosphatase active site. These results lead to a model for how Integrator terminates Pol II transcription in three steps that involve major rearrangements.

Mapping the substrate landscape of protein phosphatase 2A catalytic subunit PPP2CA

Protein phosphatase 2A (PP2A) is an essential Ser/Thr phosphatase. The PP2A holoenzyme complex comprises a scaffolding (A), regulatory (B), and catalytic (C) subunit, with PPP2CA being the principal catalytic subunit. The full scope of PP2A substrates in cells remains to be defined. To address this, we employed dTAG proteolysis-targeting chimeras to efficiently and selectively degrade dTAG-PPP2CA in homozygous knock-in HEK293 cells. Unbiased global phospho-proteomics identified 2,204 proteins with significantly increased phosphorylation upon dTAG-PPP2CA degradation, implicating them as potential PPP2CA substrates. A vast majority of these are novel. Bioinformatic analyses revealed involvement of the potential PPP2CA substrates in spliceosome function, cell cycle, RNA transport, and ubiquitin-mediated proteolysis. We identify a pSP/pTP motif as a predominant target for PPP2CA and confirm some of our phospho-proteomic data with immunoblotting. We provide an in-depth atlas of potential PPP2CA substrates and establish targeted degradation as a robust tool to unveil phosphatase substrates in cells.

Mutations at Two Key Sites in PP2A Safeguard Caenorhabditis elegans Neurons from Microcystin-LR Toxicity

Microcystin-LR (MC-LR) is a secondary metabolite produced by cyanobacteria, globally renowned for its potent hepatotoxicity. However, an increasing body of research suggests that it also exhibits pronounced neurotoxicity. PP2A is a fundamental intracellular phosphatase that plays a pivotal role in cell development and survival. Although extensive research has focused on the binding of MC-LR to the C subunit of PP2A, few studies have explored the key amino acid sites that can prevent the binding of MC-LR to PP2A-C. Due to the advantages of Caenorhabditis elegans (C. elegans), such as ease of genetic editing and a short lifespan, we exposed nematodes to MC-LR in a manner that simulated natural exposure conditions based on MC-LR concentrations in natural water bodies (immersion exposure). Our findings demonstrate that MC-LR exerts comprehensive toxicity on nematodes, including reducing lifespan, impairing reproductive capabilities, and diminishing sensory functions. Notably, and for the first time, we observed that MC-LR neurotoxic effects can persist up to the F3 generation, highlighting the significant threat that MC-LR poses to biological populations in natural environments. Furthermore, we identified two amino acid sites (L252 and C278) in PP2A-C through mutations that prevented MC-LR binding without affecting PP2A activity. This discovery was robustly validated through behavioral studies and neuronal calcium imaging using nematodes. In conclusion, we identified two crucial amino acid sites that could prevent MC-LR from binding to PP2A-C, which holds great significance for the future development of MC-LR detoxification drugs.

Targeting phosphatases: From molecule design to clinical trials

Phosphatase is a kind of enzyme that can dephosphorylate target proteins, which can be divided into serine/threonine phosphatase and tyrosine phosphatase according to its mode of action. Current evidence showed multiple phosphatases were highly correlated with diseases including various cancers, demonstrating them as potential targets. However, currently, targeting phosphatases with small molecules faces many challenges, resulting in no drug approved. In this case, phosphatases are even regarded as "undruggable" targets for a long time. Recently, a variety of strategies have been adopted in the design of small molecule inhibitors targeting phosphatases, leading many of them to enter into the clinical trials. In this review, we classified these inhibitors into 4 types, including (1) molecular glues, (2) small molecules targeting catalytic sites, (3) allosteric inhibition, and (4) bifunctional molecules (proteolysis targeting chimeras, PROTACs). These molecules with diverse strategies prove the feasibility of phosphatases as drug targets. In addition, the combination therapy of phosphatase inhibitors with other drugs has also entered clinical trials, which suggests a broad prospect. Thus, targeting phosphatases with small molecules by different strategies is emerging as a promising way in the modulation of pathogenetic phosphorylation.

Altering phosphorylation in cancer through PP2A modifiers

Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase integral to the regulation of many cellular processes. Due to the deregulation of PP2A in cancer, many of these processes are turned toward promoting tumor progression. Considerable research has been undertaken to discover molecules capable of modulating PP2A activity in cancer. Because PP2A is capable of immense substrate specificity across many cellular processes, the therapeutic targeting of PP2A in cancer can be completed through either enzyme inhibitors or activators. PP2A modulators likewise tend to be effective in drug-resistant cancers and work synergistically with other known cancer therapeutics. In this review, we will discuss the patterns of PP2A deregulation in cancer, and its known downstream signaling pathways important for cancer regulation, along with many activators and inhibitors of PP2A known to inhibit cancer progression.

Protective effect of Huanglianjiedu Decoction on microcystin-LR induced nerve injury

Microcystin-LR (MC-LR) presented in eutrophic water has been identified as having the capacity to induce damage to the mammalian nervous system by crossing the blood-brain barrier through organic anion transporting polypeptides. However, the lack of effective preventive and protective strategies remains a concern. Huanglianjiedu Decoction (HLJD), a classical Chinese traditional formula originating from the Tang Dynasty and comprising Rhizoma Coptidis, Radix Scutellariae, Cortex Phellodendri, and Fructus Gardeniae, has exhibited neuroprotective effects attributed to its antioxidant properties. In this study, we investigated the potential of HLJD in counteracting the neurotoxic effects induced by MC-LR. Our findings revealed that MC-LR dose-dependently inhibited the activity of the PP2A enzyme in PC 12 cells and significantly elevated the phosphorylation levels of JNK, ERK1/2, and p38. Moreover, MC-LR administration resulted in synaptic damage in mouse neurons, hyperphosphorylation of the microtubule-related protein Tau, cognitive impairment, and deficits in learning and memory in C57BL/6J mice. Notably, HLJD effectively reversed the cytotoxicity caused by MC-LR in PC 12 cells, and attenuated MC-LR-induced neuronal damage while improving learning ability in mice. These results highlight the potential of HLJD as a promising protective strategy against MC-LR-induced neurological injury.

Protein Phosphatase 2A as a Therapeutic Target in Pulmonary Diseases

New disease targets and medicinal chemistry approaches are urgently needed to develop novel therapeutic strategies for treating pulmonary diseases. Emerging evidence suggests that reduced activity of protein phosphatase 2A (PP2A), a complex heterotrimeric enzyme that regulates dephosphorylation of serine and threonine residues from many proteins, is observed in multiple pulmonary diseases, including lung cancer, smoke-induced chronic obstructive pulmonary disease, alpha-1 antitrypsin deficiency, asthma, and idiopathic pulmonary fibrosis. Loss of PP2A responses is linked to many mechanisms associated with disease progressions, such as senescence, proliferation, inflammation, corticosteroid resistance, enhanced protease responses, and mRNA stability. Therefore, chemical restoration of PP2A may represent a novel treatment for these diseases. This review outlines the potential impact of reduced PP2A activity in pulmonary diseases, endogenous and exogenous inhibitors of PP2A, details the possible PP2A-dependent mechanisms observed in these conditions, and outlines potential therapeutic strategies for treatment. Substantial medicinal chemistry efforts are underway to develop therapeutics targeting PP2A activity. The development of specific activators of PP2A that selectively target PP2A holoenzymes could improve our understanding of the function of PP2A in pulmonary diseases. This may lead to the development of therapeutics for restoring normal PP2A responses within the lung.

The diverging role of CDC14B: from mitotic exit in yeast to cell fate control in humans

CDC14, originally identified as crucial mediator of mitotic exit in budding yeast, belongs to the family of dual-specificity phosphatases (DUSPs) that are present in most eukaryotes. Contradicting data have sparked a contentious discussion whether a cell cycle role is conserved in the human paralogs CDC14A and CDC14B but possibly masked due to redundancy. Subsequent studies on CDC14A and CDC14B double knockouts in human and mouse demonstrated that CDC14 activity is dispensable for mitotic progression in higher eukaryotes and instead suggested functional specialization. In this review, we provide a comprehensive overview of our current understanding of how faithful cell division is linked to phosphorylation and dephosphorylation and compare functional similarities and divergences between the mitotic phosphatases CDC14, PP2A, and PP1 from yeast and higher eukaryotes. Furthermore, we review the latest discoveries on CDC14B, which identify this nuclear phosphatase as a key regulator of gene expression and reveal its role in neuronal development. Finally, we discuss CDC14B functions in meiosis and possible implications in other developmental processes.

Substrate and phosphorylation site selection by phosphoprotein phosphatases

Dynamic protein phosphorylation and dephosphorylation are essential regulatory mechanisms that ensure proper cellular signaling and biological functions. Deregulation of either reaction has been implicated in several human diseases. Here, we focus on the mechanisms that govern the specificity of the dephosphorylation reaction. Most cellular serine/threonine dephosphorylation is catalyzed by 13 highly conserved phosphoprotein phosphatase (PPP) catalytic subunits, which form hundreds of holoenzymes by binding to regulatory and scaffolding subunits. PPP holoenzymes recognize phosphorylation site consensus motifs and interact with short linear motifs (SLiMs) or structural elements distal to the phosphorylation site. We review recent advances in understanding the mechanisms of PPP site-specific dephosphorylation preference and substrate recruitment and highlight examples of their interplay in the regulation of cell division.

PP2Acα-B'/PR61 Holoenzyme of Toxoplasma gondii Is Required for the Amylopectin Metabolism and Proliferation of Tachyzoites

Here, we report that the inhibition of the PP2A subfamily by okadaic acid results in an accumulation of polysaccharides in the acute infection stage (tachyzoites) of Toxoplasma gondii, which is a protozoan of global zoonotic importance and a model for the apicomplexan parasites. The loss of the catalytic subunit α of PP2A (ΔPP2Acα) in RHΔku80 leads to the polysaccharide accumulation phenotype in the base of tachyzoites as well as residual bodies and significantly compromises the intracellular growth in vitro and the virulence in vivo. A metabolomic analysis revealed that the accumulated polysaccharides in ΔPP2Acα are derived from interrupted glucose metabolism, which affects the production of ATP and energy homeostasis in the T. gondii knockout. The assembly of the PP2Acα holoenzyme complex involved in the amylopectin metabolism in tachyzoites is possibly not regulated by LCMT1 or PME1, and this finding contributes to the identification of the regulatory B subunit (B'/PR61). The loss of B'/PR61 results in the accumulation of polysaccharide granules in the tachyzoites as well as reduced plaque formation ability, exactly the same as ΔPP2Acα. Taken together, we have identified a PP2Acα-B'/PR61 holoenzyme complex that plays a crucial role in the carbohydrate metabolism and viability in T. gondii, and its deficiency in function remarkably suppresses the growth and virulence of this important zoonotic parasite both in vitro and in vivo. Hence, rendering the PP2Acα-B'/PR61 holoenzyme functionless should be a promising strategy for the intervention of Toxoplasma acute infection and toxoplasmosis. IMPORTANCE Toxoplasma gondii switches back and forth between acute and chronic infections, mainly in response to host immunologic status, which is characterized by flexible but specific energy metabolism. Polysaccharide granules are accumulated in the acute infection stage of T. gondii that have been exposed to a chemical inhibitor of the PP2A subfamily. The genetic depletion of the catalytic subunit α of PP2A leads to this phenotype and significantly affects the cell metabolism, energy production, and viability. Further, a regulatory B subunit PR61 is necessary for the PP2A holoenzyme to function in glucose metabolism and in the intracellular growth of T. gondii tachyzoites. A deficiency of this PP2A holoenzyme complex (PP2Acα-B'/PR61) in T. gondii knockouts results in the abnormal accumulation of polysaccharides and the disruption of energy metabolism, suppressing their growth and virulence. These findings provide novel insights into cell metabolism and identify a potential target for an intervention against a T. gondii acute infection.

Effects of Cyanobacterial Harmful Algal Bloom Toxin Microcystin-LR on Gonadotropin-Dependent Ovarian Follicle Maturation and Ovulation in Mice

BACKGROUND

Cyanobacterial harmful algal blooms (CyanoHABs) originate from the excessive growth or bloom of cyanobacteria often referred to as blue-green algae. They have been on the rise globally in both marine and freshwaters in recently years with increasing frequency and severity owing to the rising temperature associated with climate change and increasing anthropogenic eutrophication from agricultural runoff and urbanization. Humans are at a great risk of exposure to toxins released from CyanoHABs through drinking water, food, and recreational activities, making CyanoHAB toxins a new class of contaminants of emerging concern.

OBJECTIVES

We investigated the toxic effects and mechanisms of microcystin-LR (MC-LR), the most prevalent CyanoHAB toxin, on the ovary and associated reproductive functions.

METHODS

Mouse models with either chronic daily oral or acute intraperitoneal exposure, an engineered three-dimensional ovarian follicle culture system, and human primary ovarian granulosa cells were tested with MC-LR of various dose levels. Single-follicle RNA sequencing, reverse transcription-quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, western blotting, immunohistochemistry (IHC), and benchmark dose modeling were used to examine the effects of MC-LR on follicle maturation, hormone secretion, ovulation, and luteinization.

RESULTS

Mice exposed long term to low-dose MC-LR did not exhibit any differences in the kinetics of folliculogenesis, but they had significantly fewer corpora lutea compared with control mice. Superovulation models further showed that mice exposed to MC-LR during the follicle maturation window had significantly fewer ovulated oocytes. IHC results revealed ovarian distribution of MC-LR, and mice exposed to MC-LR had significantly lower expression of key follicle maturation mediators. Mechanistically, in both murine and human granulosa cells exposed to MC-LR, there was reduced protein phosphatase 1 (PP1) activity, disrupted PP1-mediated PI3K/AKT/FOXO1 signaling, and less expression of follicle maturation-related genes.

DISCUSSION

Using both in vivo and in vitro murine and human model systems, we provide data suggesting that environmentally relevant exposure to the CyanoHAB toxin MC-LR interfered with gonadotropin-dependent follicle maturation and ovulation. We conclude that MC-LR may pose a nonnegligible risk to women's reproductive health by heightening the probability of irregular menstrual cycles and infertility related to ovulatory disorders. https://doi.org/10.1289/EHP12034.

Cooperative mechanics of PR65 scaffold underlies the allosteric regulation of the phosphatase PP2A

PR65, a horseshoe-shaped scaffold composed of 15 HEAT (observed in Huntingtin, elongation factor 3, protein phosphatase 2A, and the yeast kinase TOR1) repeats, forms, together with catalytic and regulatory subunits, the heterotrimeric protein phosphatase PP2A. We examined the role of PR65 in enabling PP2A enzymatic activity with computations at various levels of complexity, including hybrid approaches that combine full-atomic and elastic network models. Our study points to the high flexibility of this scaffold allowing for end-to-end distance fluctuations of 40-50 Å between compact and extended conformations. Notably, the intrinsic dynamics of PR65 facilitates complexation with the catalytic subunit and is retained in the PP2A complex enabling PR65 to engage the two domains of the catalytic subunit and provide the mechanical framework for enzymatic activity, with support from the regulatory subunit. In particular, the intra-repeat coils at the C-terminal arm play an important role in allosterically mediating the collective dynamics of PP2A, pointing to target sites for modulating PR65 function.

Antibody-free and selective detection of okadaic acid using an affinity peptide-based indirect assay

Okadaic acid (OA) is a type of marine biotoxin produced by some species of dinoflagellates in marine environments. Consumption of shellfish contaminated with OA can cause diarrhetic shellfish poisoning (DSP) in humans with symptoms that typically include abdominal pain, diarrhea and vomiting. In this study, we developed an affinity peptide-based direct competition enzyme-linked immunosorbent assay (dc-ELISA) for the detection of OA in real samples. The OA-specific peptide was successfully identified via M13 biopanning and a series of peptides were chemically synthesized and characterized their recognition activities. The dc-ELISA system showed good sensitivity and selectivity with a half-maximal inhibitory concentration (IC₅₀) of 148.7 ng/mL and a limit of detection (LOD) of 5.41 ng/mL (equivalent, 21.52 ng/g). Moreover, the effectiveness of the developed dc-ELISA was validated using OA-spiked shellfish samples, and the developed dc-ELISA showed a high recovery rate. These results suggest that the affinity peptide-based dc-ELISA can be a promising tool for detecting OA in shellfish samples.

C-glycosides analogues of the okadaic acid central fragment exert neuroprotection via restoration of PP2A-phosphatase activity: A rational design of potential drugs for Alzheimer's disease targeting tauopathies

Protein phosphatase 2A (PP2A) is an important Ser/Thr phosphatase that participates in the regulation of multiple cellular processes. This implies that any deficient activity of PP2A is the responsible of severe pathologies. For instance, one of the main histopathological features of Alzheimer's disease is neurofibrillary tangles, which are mainly comprised by hyperphosphorylated forms of tau protein. This altered rate of tau phosphorylation has been correlated with PP2A depression AD patients. With the goal of preventing PP2A inactivation in neurodegeneration scenarios, we have aimed to design, synthesize and evaluate new ligands of PP2A capable of preventing its inhibition. To achieve this goal, the new PP2A ligands present structural similarities with the central fragment C19-C27 of the well-established PP2A inhibitor okadaic acid (OA). Indeed, this central moiety of OA does not exert inhibitory actions. Hence, these compounds lack PP2A-inhibiting structural motifs but, in contrast, compete with PP2A inhibitors, thus recovering phosphatase activity. Proving this hypothesis, most compounds showed a good neuroprotective profile in neurodegeneration models related to PP2A impairment, highlighting derivative 10, named ITH12711, as the most promising one. This compound (1) restored in vitro and cellular PP2A catalytic activity, measured on a phospho-peptide substrate and by western-blot analyses, (2) proved good brain penetration measured by PAMPA, and (3) prevented LPS-induced memory impairment of mice in the object recognition test. Thus, the promising outcomes of the compound 10 validate our rational approach to design new PP2A-activating drugs based on OA central fragment.

PP1, PP2A and PP2B Interplay in the Regulation of Sperm Motility: Lessons from Protein Phosphatase Inhibitors

Male fertility relies on the ability of spermatozoa to fertilize the egg in the female reproductive tract (FRT). Spermatozoa acquire activated motility during epididymal maturation; however, to be capable of fertilization, they must achieve hyperactivated motility in the FRT. Extensive research found that three protein phosphatases (PPs) are crucial to sperm motility regulation, the sperm-specific protein phosphatase type 1 (PP1) isoform gamma 2 (PP1γ2), protein phosphatase type 2A (PP2A) and protein phosphatase type 2B (PP2B). Studies have reported that PP activity decreases during epididymal maturation, whereas protein kinase activity increases, which appears to be a requirement for motility acquisition. An interplay between these PPs has been extensively investigated; however, many specific interactions and some inconsistencies remain to be elucidated. The study of PPs significantly advanced following the identification of naturally occurring toxins, including calyculin A, okadaic acid, cyclosporin, endothall and deltamethrin, which are powerful and specific PP inhibitors. This review aims to overview the protein phosphorylation-dependent biochemical pathways underlying sperm motility acquisition and hyperactivation, followed by a discussion of the PP inhibitors that allowed advances in the current knowledge of these pathways. Since male infertility cases still attain alarming numbers, additional research on the topic is required, particularly using other PP inhibitors.

Measurement of Microcystin Activity in Human Plasma Using Immunocapture and Protein Phosphatase Inhibition Assay

Microcystins are toxic chemicals generated by certain freshwater cyanobacteria. These chemicals can accumulate to dangerous levels during harmful algal blooms. When exposed to microcystins, humans are at risk of hepatic injury, including liver failure. Here, we describe a method to detect microcystins in human plasma by using immunocapture followed by a protein phosphatase inhibition assay. At least 279 microcystins have been identified, and most of these compounds share a common amino acid, the Adda side chain. We targeted this Adda side chain using a commercial antibody and extracted microcystins from human samples for screening and analysis. To quantitate the extracted microcystins, we fortified plasma with microcystin-LR, one of the most well-studied, commonly detected, and toxic microcystin congeners. The quantitation range for the detection of microcystin in human plasma using this method is 0.030-0.50 ng/mL microcystin-LR equivalents. This method detects unconjugated and conjugated forms (cysteine and glutathione) of microcystins. Quality control sample accuracies varied between 98.9% and 114%, with a precision of 7.18-15.8%. Finally, we evaluated plasma samples from a community health surveillance project of Florida residents living or working near harmful algae blooms.

Microcystin-LR aerosol induces inflammatory responses in healthy human primary airway epithelium

Harmful algal blooms plague bodies of freshwater globally. These blooms are often composed of outgrowths of cyanobacteria capable of producing the heptapeptide Microcystin-LR (MC-LR) which is a well-known hepatotoxin. Recently, MC-LR has been detected in aerosols generated from lake water. However, the risk for human health effects due to MC-LR inhalation exposure have not been extensively investigated. In this study, we exposed a fully differentiated 3D human airway epithelium derived from 14 healthy donors to MC-LR-containing aerosol once a day for 3 days. Concentrations of MC-LR ranged from 100 pM to 1 µM. Although there were little to no detrimental alterations in measures of the airway epithelial function (i.e. cell survival, tissue integrity, mucociliary clearance, or cilia beating frequency), a distinct shift in the transcriptional activity was found. Genes related to inflammation were found to be upregulated such as C-C motif chemokine 5 (CCL5; log2FC = 0.57, p = 0.03) and C-C chemokine receptor type 7 (CCR7; log2FC = 0.84, p = 0.03). Functionally, conditioned media from MC-LR exposed airway epithelium was also found to have significant chemo-attractive properties for primary human neutrophils. Additionally, increases were found in the concentration of secreted chemokine proteins in the conditioned media such as CCL1 (log2FC = 5.07, p = 0.0001) and CCL5 (log2FC = 1.02, p = 0.046). These results suggest that MC-LR exposure to the human airway epithelium is capable of inducing an inflammatory response that may potentiate acute or chronic disease.

Microcystins in Water: Detection, Microbial Degradation Strategies, and Mechanisms

Microcystins are secondary metabolites produced by some cyanobacteria, a class of cyclic heptapeptide toxins that are stable in the environment. Microcystins can create a variety of adverse health effects in humans, animals, and plants through contaminated water. Effective methods to degrade them are required. Microorganisms are considered to be a promising method to degrade microcystins due to their high efficiency, low cost, and environmental friendliness. This review focuses on perspectives on the frontiers of microcystin biodegradation. It has been reported that bacteria and fungi play an important contribution to degradation. Analysis of the biodegradation mechanism and pathway is an important part of the research. Microcystin biodegradation has been extensively studied in the existing research. This review provides an overview of (1) pollution assessment strategies and hazards of microcystins in water bodies and (2) the important contributions of various bacteria and fungi in the biodegradation of microcystins and their degradation mechanisms, including mlr gene-induced (gene cluster expressing microcystinase) degradation. The application of biodegradable technology still needs development. Further, a robust regulatory oversight is required to monitor and minimize MC contamination. This review aims to provide more references regarding the detection and removal of microcystins in aqueous environments and to promote the application of biodegradation techniques for the purification of microcystin-contaminated water.

Structural, enzymatic and spatiotemporal regulation of PP2A-B55 phosphatase in the control of mitosis

Cells require major physical changes to induce a proper repartition of the DNA. Nuclear envelope breakdown, DNA condensation and spindle formation are promoted at mitotic entry by massive protein phosphorylation and reversed at mitotic exit by the timely and ordered dephosphorylation of mitotic substrates. This phosphorylation results from the balance between the activity of kinases and phosphatases. The role of kinases in the control of mitosis has been largely studied, however, the impact of phosphatases has long been underestimated. Recent data have now established that the regulation of phosphatases is crucial to confer timely and ordered cellular events required for cell division. One major phosphatase involved in this process is the phosphatase holoenzyme PP2A-B55. This review will be focused in the latest structural, biochemical and enzymatic insights provided for PP2A-B55 phosphatase as well as its regulators and mechanisms of action.

Coupling to short linear motifs creates versatile PME-1 activities in PP2A holoenzyme demethylation and inhibition

Protein phosphatase 2A (PP2A) holoenzymes target broad substrates by recognizing short motifs via regulatory subunits. PP2A methylesterase 1 (PME-1) is a cancer-promoting enzyme and undergoes methylesterase activation upon binding to the PP2A core enzyme. Here, we showed that PME-1 readily demethylates different families of PP2A holoenzymes and blocks substrate recognition in vitro. The high-resolution cryoelectron microscopy structure of a PP2A-B56 holoenzyme–PME-1 complex reveals that PME-1 disordered regions, including a substrate-mimicking motif, tether to the B56 regulatory subunit at remote sites. They occupy the holoenzyme substrate-binding groove and allow large structural shifts in both holoenzyme and PME-1 to enable multipartite contacts at structured cores to activate the methylesterase. B56 interface mutations selectively block PME-1 activity toward PP2A-B56 holoenzymes and affect the methylation of a fraction of total cellular PP2A. The B56 interface mutations allow us to uncover B56-specific PME-1 functions in p53 signaling. Our studies reveal multiple mechanisms of PME-1 in suppressing holoenzyme functions and versatile PME-1 activities derived from coupling substrate-mimicking motifs to dynamic structured cores.

De novo design of DNA aptamers that target okadaic acid (OA) by docking-then-assembling of single nucleotides

Okadaic acid (OA) is a diarrhetic shellfish poison widespread in ocean, so its detection is of great significance to seafood safety. Because of good sensitivity and low cost, biosensors using nucleic-acid aptamers as the recognition molecules are emerging as an important detection tool. However, the traditional SELEX screening method for acquiring OA high-affinity aptamers is time- and resource-intensive. Alternatively, here we developed a de novo design method based on the 3D structure of a target molecule, such as OA. Without experimental screening, this method designs OA aptamers by a computational approach of docking-then-assembling (DTA) of single nucleotides (A, C, G and T) as: (1) determining the high-affinity nucleotide binding sites of the target molecule via saturated molecular docking; (2) assembling the bound nucleotides into binding units to the target molecule; (3) constructing full-length aptamers by introducing stabilizing units to connect these binding units. In this way, five OA aptamers were designed, and microscale thermophoresis (MST) experiments verified that their K_d values are in the range of 100-600 nM; and one of them (named 9CGAT_4_a) could specifically bind to OA with low affinities for the other three marine biotoxins. Therefore, this study provides high-affinity and specific aptamers for the development of OA biosensors, and presents a promising de novo design method applicable to other target molecules.

Prior exposure to microcystin alters host gut resistome and is associated with dysregulated immune homeostasis in translatable mouse models

A strong association between exposure to the common harmful algal bloom toxin microcystin and the altered host gut microbiome has been shown. We tested the hypothesis that prior exposure to the cyanotoxin microcystin-LR may alter the host resistome. We show that the mice exposed to microcystin-LR had an altered microbiome signature that harbored antibiotic resistance genes. Host resistome genotypes such as mefA, msrD, mel, ant6, and tet40 increased in diversity and relative abundance following microcystin-LR exposure. Interestingly, the increased abundance of these genes was traced to resistance to common antibiotics such as tetracycline, macrolides, glycopeptide, and aminoglycosides, crucial for modern-day treatment of several diseases. Increased abundance of these genes was positively associated with increased expression of PD1, a T-cell homeostasis marker, and pleiotropic inflammatory cytokine IL-6 with a concomitant negative association with immunosurveillance markers IL-7 and TLR2. Microcystin-LR exposure also caused decreased TLR2, TLR4, and REG3G expressions, increased immunosenescence, and higher systemic levels of IL-6 in both wild-type and humanized mice. In conclusion, the results show a first-ever characterization of the host resistome following microcystin-LR exposure and its connection to host immune status and antimicrobial resistance that can be crucial to understand treatment options with antibiotics in microcystin-exposed subjects in clinical settings.

Bioinformatic Characterization of Whole Blood Neutrophils in Pelvic Inflammatory Disease: A Potential Prognostic Indicator for Transumbilical Single-Port Laparoscopic Pelvic Abscess Surgery

The purpose of this research is to determine the prognosis of patients treated with transumbilical single-port laparoscopic surgery for acute pelvic inflammatory illness. Postoperative data on 129 patients treated with laparoscopic surgery for acute pelvic inflammatory illness were obtained retrospectively. It was observed that the shorter the time required for postoperative leukocyte recovery to normal, the shorter the time required for postoperative pain and diet recovery, as well as hospital stay, in such individuals. CIBERSORT was used to examine patient data from GEO. The most significant difference between the normal and pelvic inflammatory groups was in neutrophil content. Association study found a substantial positive correlation between the quantity of neutrophils infiltrating the immune system and the abundance of monocyte M0 infiltrating the immune system. Neutrophil immune infiltration was strongly inversely linked with plasma cells, activated CD8+ Tm cells, and active CD4+ Tm cells. Four mRNAs linked with pelvic inflammatory illness were revealed to be strongly associated with neutrophil immune infiltration, notably CALML4, COQ10B, DCPS, and PPP2R1A. The ROC revealed that CALML4 (area under the curve (AUC): 0.769, 95% confidence interval (CI): 0.638-0.881), COQ10B (AUC: 0.742, 95% CI: 0.587-0.881), PPP2R1A (AUC: 0.733 95% CI: 0.593-0.857), and DCPS (AUC: 0.745, 95% CI: 0.571-0.900) were potential markers for predicting pelvic inflammatory disease. CALML4, COQ10B, PPP2R1A, and DCPS may be critical determinants determining the amount of preoperative neutrophil infiltration and the time required for leukocyte recovery after single-port laparoscopy in acute pelvic inflammatory illness.

Current Trends and New Challenges in Marine Phycotoxins

Marine phycotoxins are a multiplicity of bioactive compounds which are produced by microalgae and bioaccumulate in the marine food web. Phycotoxins affect the ecosystem, pose a threat to human health, and have important economic effects on aquaculture and tourism worldwide. However, human health and food safety have been the primary concerns when considering the impacts of phycotoxins. Phycotoxins toxicity information, often used to set regulatory limits for these toxins in shellfish, lacks traceability of toxicity values highlighting the need for predefined toxicological criteria. Toxicity data together with adequate detection methods for monitoring procedures are crucial to protect human health. However, despite technological advances, there are still methodological uncertainties and high demand for universal phycotoxin detectors. This review focuses on these topics, including uncertainties of climate change, providing an overview of the current information as well as future perspectives.

Exercise decreases PP2A-specific reversible thiol oxidation in human erythrocytes: Implications for redox biomarkers

New readily accessible systemic redox biomarkers are needed to understand the biological roles reactive oxygen species (ROS) play in humans because overtly flawed, technically fraught, and unspecific assays severely hamper translational progress. The antibody-linked oxi-state assay (ALISA) makes it possible to develop valid ROS-sensitive target-specific protein thiol redox state biomarkers in a readily accessible microplate format. Here, we used a maximal exercise bout to disrupt redox homeostasis in a physiologically meaningful way to determine whether the catalytic core of the serine/threonine protein phosphatase PP2A is a candidate systemic redox biomarker in human erythrocytes. We reasoned that: constitutive oxidative stress (e.g., haemoglobin autoxidation) would sensitise erythrocytes to disrupted ion homeostasis as manifested by increased oxidation of the ion regulatory phosphatase PP2A. Unexpectedly, an acute bout of maximal exercise lasting ~16 min decreased PP2A-specific reversible thiol oxidation (redox ratio, rest: 0.46; exercise: 0.33) without changing PP2A content (rest: 193 pg/ml; exercise: 191 pg/ml). The need for only 3-4 μl of sample to perform ALISA means PP2A-specific reversible thiol oxidation is a capillary-fingertip blood-compatible candidate redox biomarker. Consistent with biologically meaningful redox regulation, thiol reductant-inducible PP2A activity was significantly greater (+10%) at rest compared to exercise. We establish a route to developing new readily measurable protein thiol redox biomarkers for understanding the biological roles ROS play in humans.

Microcystins can be extracted from Microcystis aeruginosa using amino acid-derived biosurfactants

Microcystin, a cyanotoxin produced by Microcystis aeruginosa growing in eutrophic waters, can promote liver tumors in people ingesting contaminated water. To date, water treatment systems have not been effective in removing or degrading these cyanotoxins. In this work, we investigated the inhibitory activity of surfactants on the growth of M. aeruginosa and their application to extract the intracellular produced cyanotoxins. The experiments involving growth inhibition and extraction of cyanotoxins were carried out using the non-biodegradable surfactant cetyl trimethyl ammonium bromide (CTAB) in addition to other biodegradable surfactants. These were Tween 80 and surfactants derived from amino acids and peptides, respectively, from arginine, SDA, and hydrolyzed peptone, SDP. We demonstrated that the tested surfactants could be used to inhibit the growth of M. aeruginosa. At this point, CTAB and SDA proved to be the most competent surfactants in reducing cyanobacterial growth. Moreover, microcystins have been successfully removed from the water employing a cloud point extraction protocol based on the use of these surfactants and ammonium sulfate.

PP2A-B55: substrates and regulators in the control of cellular functions

PP2A is a major serine/threonine phosphatase class involved in the regulation of cell signaling through the removal of protein phosphorylation. This class of phosphatases is comprised of different heterotrimeric complexes displaying distinct substrate specificities. The present review will focus on one specific heterocomplex, the phosphatase PP2A-B55. Herein, we will report the direct substrates of this phosphatase identified to date, and its impact on different cell signaling cascades. We will additionally describe its negative regulation by its inhibitors Arpp19 and ENSA and their upstream kinase Greatwall. Finally, we will describe the essential molecular features defining PP2A-B55 substrate specificity that confer the correct temporal pattern of substrate dephosphorylation. The main objective of this review is to provide the reader with a unique source compiling all the knowledge of this particular holoenzyme that has evolved as a key enzyme for cell homeostasis and cancer development.

MCLR-elicited hepatic fibrosis and carcinogenic gene expression changes persist in rats with diet-induced nonalcoholic steatohepatitis through a 4-week recovery period

Nonalcoholic steatohepatitis (NASH) causes liver extracellular matrix (ECM) remodeling and is a risk factor for fibrosis and hepatocellular carcinoma (HCC). Microcystin-LR (MCLR) is a hepatotoxin produced by fresh-water cyanobacteria that causes a NASH-like phenotype, liver fibrosis, and is also a risk factor for HCC. The focus of the current study was to investigate and compare hepatic recovery after cessation of MCLR exposure in healthy versus NASH animals. Male Sprague-Dawley rats were fed either a control or a high fat/high cholesterol (HFHC) diet for eight weeks. Animals received either vehicle or 30 μg/kg MCLR (i.p: 2 weeks, alternate days). Animals were euthanized at one of three time points: at the completion of the MCLR exposure period and after 2 and 4 weeks of recovery. Histological staining suggested that after four weeks of recovery the MCLR-exposed HFHC group had less steatosis and more fibrosis compared to the vehicle-exposed HFHC group and MCLR-exposed control group. RNA-Seq analysis revealed dysregulation of ECM genes after MCLR exposure in both control and HFHC groups that persisted only in the HFHC groups during recovery. After 4 weeks of recovery, MCLR hepatotoxicity in pre-existing NASH persistently dysregulated genes related to cellular differentiation and HCC. These data demonstrate impaired hepatic recovery and persistent carcinogenic changes after MCLR toxicity in pre-existing NASH.

Structural Insights into Protein Regulation by Phosphorylation and Substrate Recognition of Protein Kinases/Phosphatases

Protein phosphorylation is one of the most widely observed and important post-translational modification (PTM) processes. Protein phosphorylation is regulated by protein kinases, each of which covalently attaches a phosphate group to an amino acid side chain on a serine (Ser), threonine (Thr), or tyrosine (Tyr) residue of a protein, and by protein phosphatases, each of which, conversely, removes a phosphate group from a phosphoprotein. These reversible enzyme activities provide a regulatory mechanism by activating or deactivating many diverse functions of proteins in various cellular processes. In this review, their structures and substrate recognition are described and summarized, focusing on Ser/Thr protein kinases and protein Ser/Thr phosphatases, and the regulation of protein structures by phosphorylation. The studies reviewed here and the resulting information could contribute to further structural, biochemical, and combined studies on the mechanisms of protein phosphorylation and to drug discovery approaches targeting protein kinases or protein phosphatases.

Environmental Microcystin exposure in underlying NAFLD-induced exacerbation of neuroinflammation, blood-brain barrier dysfunction, and neurodegeneration are NLRP3 and S100B dependent

Nonalcoholic fatty liver disease (NAFLD) has been shown to be associated with extrahepatic comorbidities including neuronal inflammation and Alzheimer's-like pathology. Environmental and genetic factors also act as a second hit to modulate severity and are expected to enhance the NAFLD-linked neuropathology. We hypothezied that environmental microcystin-LR (MC-LR), a toxin produced by harmful algal blooms of cyanobacteria, exacerbates the neuroinflammation and degeneration of neurons associated with NAFLD. Using a mouse model of NAFLD, exposed to MC-LR subsequent to the onset of fatty liver, we show that the cyanotoxin could significantly increase proinflammatory cytokine expression in the frontal cortex and cause increased expression of Lcn2 and HMGB1. The above effects were NLRP3 inflammasome activation-dependent since the use of NLRP3 knockout mice abrogated the increase in inflammation. NLRP3 was also responsible for decreased expression of the blood-brain barrier (BBB) tight junction proteins Occludin and Claudin 5 suggesting BBB dysfunction was parallel to neuroinflammation following microcystin exposure. An increased circulatory S100B release, a hallmark of astrocyte activation in MC-LR exposed NAFLD mice also confirmed BBB integrity loss, but the astrocyte activation observed in vivo was NLRP3 independent suggesting an important role of a secondary S100B mediated crosstalk. Mechanistically, conditioned medium from reactive astrocytes and parallel S100B incubation in neuronal cells caused increased inducible NOS, COX-2, and higher BAX/ Bcl2 protein expression suggesting oxidative stress-mediated neuronal cell apoptosis crucial for neurodegeneration. Taken together, MC-LR exacerbated neuronal NAFLD-linked comorbidities leading to cortical inflammation, BBB dysfunction, and neuronal apoptosis.

Microcystin Toxicokinetics, Molecular Toxicology, and Pathophysiology in Preclinical Rodent Models and Humans

Microcystins are ubiquitous toxins produced by photoautotrophic cyanobacteria. Human exposures to microcystins occur through the consumption of contaminated drinking water, fish and shellfish, vegetables, and algal dietary supplements and through recreational activities. Microcystin-leucine-arginine (MCLR) is the prototypical microcystin because it is reported to be the most common and toxic variant and is the only microcystin with an established tolerable daily intake of 0.04 µg/kg. Microcystin toxicokinetics is characterized by low intestinal absorption, rapid and specific distribution to the liver, moderate metabolism to glutathione and cysteinyl conjugates, and low urinary and fecal excretion. Molecular toxicology involves covalent binding to and inhibition of protein phosphatases, oxidative stress, cell death (autophagy, apoptosis, necrosis), and cytoskeleton disruption. These molecular and cellular effects are interconnected and are commonly observed together. The main target organs for microcystin toxicity are the intestine, liver, and kidney. Preclinical data indicate microcystins may also have nervous, pulmonary, cardiac, and reproductive system toxicities. Recent evidence suggests that exposure to other hepatotoxic insults could potentiate microcystin toxicity and increase the risk for chronic diseases. This review summarizes the current knowledge for microcystin toxicokinetics, molecular toxicology, and pathophysiology in preclinical rodent models and humans. More research is needed to better understand human toxicokinetics and how multifactorial exposures contribute to disease pathogenesis and progression.

Identification of 24-O-β-d-Glycosides and 7-Deoxy-Analogues of Okadaic Acid and Dinophysistoxin-1 and -2 in Extracts from Dinophysis Blooms, Dinophysis and Prorocentrum Cultures, and Shellfish in Europe, North America and Australasia

Two high-mass polar compounds were observed in aqueous side-fractions from the purification of okadaic acid (1) and dinophysistoxin-2 (2) from Dinophysis blooms in Spain and Norway. These were isolated and shown to be 24-O-β-d-glucosides of 1 and 2 (4 and 5, respectively) by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and enzymatic hydrolysis. These, together with standards of 1, 2, dinophysistoxin-1 (3), and a synthetic specimen of 7-deoxy-1 (7), combined with an understanding of their mass spectrometric fragmentation patterns, were then used to identify 1–5, the 24-O-β-d-glucoside of dinophysistoxin-1 (6), 7, 7-deoxy-2 (8), and 7-deoxy-3 (9) in a range of extracts from Dinophysis blooms, Dinophysis cultures, and contaminated shellfish from Spain, Norway, Ireland, Canada, and New Zealand. A range of Prorocentrum lima cultures was also examined by liquid chromatography–high resolution tandem mass spectrometry (LC–HRMS/MS) and was found to contain 1, 3, 7, and 9. However, although 4–6 were not detected in these cultures, low levels of putative glycosides with the same exact masses as 4 and 6 were present. The potential implications of these findings for the toxicology, metabolism, and biosynthesis of the okadaic acid group of marine biotoxins are briefly discussed.

Protein phosphatase 2A - structure, function and role in neurodevelopmental disorders

Neurodevelopmental disorders (NDDs), including intellectual disability (ID), autism and schizophrenia, have high socioeconomic impact, yet poorly understood etiologies. A recent surge of large-scale genome or exome sequencing studies has identified a multitude of mostly de novo mutations in subunits of the protein phosphatase 2A (PP2A) holoenzyme that are strongly associated with NDDs. PP2A is responsible for at least 50% of total Ser/Thr dephosphorylation in most cell types and is predominantly found as trimeric holoenzymes composed of catalytic (C), scaffolding (A) and variable regulatory (B) subunits. PP2A can exist in nearly 100 different subunit combinations in mammalian cells, dictating distinct localizations, substrates and regulatory mechanisms. PP2A is well established as a regulator of cell division, growth, and differentiation, and the roles of PP2A in cancer and various neurodegenerative disorders, such as Alzheimer's disease, have been reviewed in detail. This Review summarizes and discusses recent reports on NDDs associated with mutations of PP2A subunits and PP2A-associated proteins. We also discuss the potential impact of these mutations on the structure and function of the PP2A holoenzymes and the etiology of NDDs.

Roles of constitutive and signal-dependent protein phosphatase 2A docking motifs in burst attenuation of the cyclic AMP response element-binding protein

The cAMP response element-binding protein (CREB) is an important regulator of cell growth, metabolism, and synaptic plasticity. CREB is activated through phosphorylation of an evolutionarily conserved Ser residue (S133) within its intrinsically disordered kinase-inducible domain (KID). Phosphorylation of S133 in response to cAMP, Ca²⁺, and other stimuli triggers an association of the KID with the KID-interacting (KIX) domain of the CREB-binding protein (CBP), a histone acetyl transferase (HAT) that promotes transcriptional activation. Here we addressed the mechanisms of CREB attenuation following bursts in CREB phosphorylation. We show that phosphorylation of S133 is reversed by protein phosphatase 2A (PP2A), which is recruited to CREB through its B56 regulatory subunits. We found that a B56-binding site located at the carboxyl-terminal boundary of the KID (BS2) mediates high-affinity B56 binding, while a second binding site (BS1) located near the amino terminus of the KID mediates low affinity binding enhanced by phosphorylation of adjacent casein kinase (CK) phosphosites. Mutations that diminished B56 binding to BS2 elevated both basal and stimulus-induced phosphorylation of S133, increased CBP interaction with CREB, and potentiated the expression of CREB-dependent reporter genes. Cells from mice harboring a homozygous Creb^E153D mutation that disrupts BS2 exhibited increased S133 phosphorylation stoichiometry and elevated transcriptional bursts to cAMP. These findings provide insights into substrate targeting by PP2A holoenzymes and establish a new mechanism of CREB attenuation that has implications for understanding CREB signaling in cell growth, metabolism, synaptic plasticity, and other physiologic contexts.

Strategies for Targeting Serine/Threonine Protein Phosphatases with Small Molecules in Cancer

Among numerous posttranslational regulation patterns, phosphorylation is reversibly controlled by the balance of kinases and phosphatases. The major form of cellular signaling involves the reversible phosphorylation of proteins on tyrosine, serine, or threonine residues. However, altered phosphorylation levels are found in diverse diseases, including cancer, making kinases and phosphatases ideal drug targets. In contrast to the success of prosperous kinase inhibitors, design of small molecules targeting phosphatase is struggling due to past bias and difficulty. This is especially true for serine/threonine phosphatases, one of the largest phosphatase families. From this perspective, we aim to provide insights into serine/threonine phosphatases and the small molecules targeting these proteins for drug development, especially in cancer. Through highlighting the modulation strategies, we aim to provide basic principles for the design of small molecules and future perspectives for the application of drugs targeting serine/threonine phosphatases.

The study of the determinants controlling Arpp19 phosphatase-inhibitory activity reveals an Arpp19/PP2A-B55 feedback loop

Arpp19 is a potent PP2A-B55 inhibitor that regulates this phosphatase to ensure the stable phosphorylation of mitotic/meiotic substrates. At G2-M, Arpp19 is phosphorylated by the Greatwall kinase on S67. This phosphorylated Arpp19 form displays a high affinity to PP2A-B55 and a slow dephosphorylation rate, acting as a competitor of PP2A-B55 substrates. The molecular determinants conferring slow dephosphorylation kinetics to S67 are unknown. PKA also phosphorylates Arpp19. This phosphorylation performed on S109 is essential to maintain prophase I-arrest in Xenopus oocytes although the underlying signalling mechanism is elusive. Here, we characterize the molecular determinants conferring high affinity and slow dephosphorylation to S67 and controlling PP2A-B55 inhibitory activity of Arpp19. Moreover, we show that phospho-S109 restricts S67 phosphorylation by increasing its catalysis by PP2A-B55. Finally, we discover a double feed-back loop between these two phospho-sites essential to coordinate the temporal pattern of Arpp19-dependent PP2A-B55 inhibition and Cyclin B/Cdk1 activation during cell division.

Progression of the cell division cycle requires feedback loops including those of phosphorylation and dephosphorylation; however the precise regulation of phosphorylation kinetics of Arpp19, an inhibitor of protein phosphatase 2A, is unclear. Here, the authors report that feedback between phosphorylation states of Ser67 and Ser109 of Arpp19 coordinates Arpp19-dependent inhibition of PP2A-B55 and Cyclin B activation during cell cycle progression.

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.

Tautomycetin Synthetic Analogues: Selective Inhibitors of Protein Phosphatase I

Ser/Thr protein phosphatases (PPs) regulate a substantial range of cellular processes with protein phosphatases 1 (PP1) and 2 A (PP2A) accounting for over 90 % of the activity within cells. Nevertheless, tools to study PPs are limited as PPs inhibitors, particularly those selective for PP1 inhibition, are relatively scarce. Two examples of PP1-selective inhibitors, which share structural similarities, are tautomycin (TTM) and tautomycetin (TTN). This work describes the development of PP1/PP2A inhibitors that incorporate key structural features of TTM and TTN and are designed to conserve regions known to bind the active site of PP1/PP2A but vary regions that differentially contact the hydrophobic groove of PP1/PP2A. In all 28 TTN analogues were synthetically generated that inhibit PP1/PP2A activity at <250 mM; seven possessed inhibition activity at 100 nM. The IC50 values were determined for the seven most active analogues, which ranged from 34 to 1500 nM (PP1) and 70 to 6800 nM (PP2A). Four of the seven analogues possessed PP1 selectivity, and one demonstrated eightfold selectivity in the nanomolar range (PP1 IC50 =34 nM, PP2A IC50 =270 nM). A rationale is given for the observed differences in selectivity.

Higher intestinal and circulatory lactate associated NOX2 activation leads to an ectopic fibrotic pathology following microcystin co-exposure in murine fatty liver disease

Clinical studies implicated an increased risk of intestinal fibrosis in patients with nonalcoholic fatty liver disease (NAFLD). Our previous studies have shown that microcystin-LR (MC-LR) exposure led to altered gut microbiome and increased abundance of lactate producing bacteria and intestinal inflammation in underlying NAFLD. This led us to further investigate the effects of the MC-LR, a PP2A inhibitor in activating the TGF-β fibrotic pathway in the intestines that might be mediated by increased lactate induced redox enzyme NOX2. Exposure to MC-LR led to higher lactate levels in circulation and in the intestinal content. The higher lactate levels were associated with NOX2 activation in vivo that led to increased Smad2/3-Smad4 co-localization and high alpha-smooth muscle actin (α-SMA) immunoreactivity in the intestines. Mechanistically, primary mouse intestinal epithelial cells treated with lactate and MC-LR separately led to higher NOX2 activation, phosphorylation of TGFβR1 receptor and subsequent Smad 2/3-Smad4 co-localization inhibitable by apocynin (NOX2 inhibitor), FBA (a peroxynitrite scavenger) and DMPO (a nitrone spin trap), catalase and superoxide dismutase. Inhibition of NOX2-induced redox signaling also showed a significant decrease in collagen protein thus suggesting a strong redox signaling induced activation of an ectopic fibrotic manifestation in the intestines. In conclusion, the present study provides mechanistic insight into the role of microcystin in dysbiosis-linked lactate production and subsequently advances our knowledge in lactate-induced NOX2 exacerbation of the cell differentiation and fibrosis in the NAFLD intestines.

Synthesis and Biological Evaluation of the Southern Hemisphere of Spirastrellolide A and Analogues

The synthesis and biological evaluation of truncated spirastrellolide A analogues comprised of the southern hemisphere against protein phosphatase 2A are described. A convergent synthesis was designed featuring two gold-catalyzed cyclization reactions, specifically, a dehydrative cyclization of monoallylic diols for the synthesis of the tetrahydropyran (A-ring) and a regioselective spiroketalization for the efficient generation of the [6,6]-spiroketal (B, C-ring system). The synthesis of the southern hemisphere of spirastrellolide A was achieved involving the longest linear sequence of 19 steps. A total of eight spirastrellolide A analogues were synthesized, and preliminary PP2A enzyme assay inhibition studies were performed for the first time on analogues of the southern hemisphere. Several analogues showed inhibition, which is a positive indication and perhaps suggests that the unsaturated spiroketal fragment might be crucial to induce PP2A inhibition.

Early microcystin-LR exposure-linked inflammasome activation in mice causes development of fatty liver disease and insulin resistance

Evidence from pediatric studies show that infants and children are at risk for early exposure to microcystin. The present report tests the hypothesis that early life exposure to microcystin (MC), a principal component of harmful algal blooms followed by a juvenile exposure to high-fat diet feeding potentiate the development of nonalcoholic fatty liver disease phenotype in adulthood. Results showed classical symptoms of early NAFLD linked inflammation. Cytokines and chemokines such as CD68, IL-1β, MCP-1, and TNF-α, as well as α-SMA were increased in the groups that were exposed to MC-LR with the high-fat diet compared to the vehicle group. Also, mechanistically, NLRP3 KO mice showed a significant decrease in the inflammation and NAFLD phenotype and resisted the metabolic changes such as insulin resistance and glucose metabolism in the liver. The data suggested that MC-LR exposure and subsequent NLRP3 inflammasome activation in childhood could impact liver health in juveniles.

The broad phenotypic spectrum of PPP2R1A-related neurodevelopmental disorders correlates with the degree of biochemical dysfunction

Purpose

Neurodevelopmental disorders (NDD) caused by protein phosphatase 2A (PP2A) dysfunction have mainly been associated with de novo variants in PPP2R5D and PPP2CA, and more rarely in PPP2R1A. Here, we aimed to better understand the latter by characterizing 30 individuals with de novo and often recurrent variants in this PP2A scaffolding Aα subunit.

Methods

Most cases were identified through routine clinical diagnostics. Variants were biochemically characterized for phosphatase activity and interaction with other PP2A subunits.

Results

We describe 30 individuals with 16 different variants in PPP2R1A, 21 of whom had variants not previously reported. The severity of developmental delay ranged from mild learning problems to severe intellectual disability (ID) with or without epilepsy. Common features were language delay, hypotonia, and hypermobile joints. Macrocephaly was only seen in individuals without B55α subunit-binding deficit, and these patients had less severe ID and no seizures. Biochemically more disruptive variants with impaired B55α but increased striatin binding were associated with profound ID, epilepsy, corpus callosum hypoplasia, and sometimes microcephaly.

Conclusion

We significantly expand the phenotypic spectrum of PPP2R1A-related NDD, revealing a broader clinical presentation of the patients and that the functional consequences of the variants are more diverse than previously reported.