The PP2A inhibitor LB-100 mitigates lupus nephritis by suppressing tertiary lymphoid structure formation.

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by multi-organ involvement and autoantibody production. Patients with SLE face a substantial risk of developing lupus nephritis (LN), which imposes a substantial burden on both patients and their families. Protein phosphatase 2A (PP2A) is a widely distributed serine/threonine phosphatase that participates in regulating multiple signaling pathways. Inhibition of PP2A has been implicated in the treatment of various diseases. LB-100, a small molecule inhibitor of PP2A, has demonstrated anti-tumor therapeutic effects and high safety profile in preclinical experiments. However, the role of PP2A and its inhibitor has been insufficiently studied in LN. In this study, we assessed the potential effects of LB-100 in both MRL/lpr mice and R848-induced BALB/c mice. Our findings indicated that LB-100 administration led to reduced spleen enlargement, decreased deposition of immune complexes, ameliorated renal damage, and improved kidney function in both spontaneous and R848-induced lupus mouse models. Importantly, we observed the formation of tertiary lymphoid structures (TLSs) in the kidneys of two distinct lupus mouse models. The levels of signature genes of TLS were elevated in the kidneys of lupus mice, whereas LB-100 mitigated chemokine production and inhibited TLS formation. In addition, we confirmed that inhibition or knockdown of PP2A reduced the production of T cell-related chemokines by renal tubular epithelial cells (RTEC). In summary, our study highlighted the renal protective potential of the PP2A inhibitor LB-100 in two distinct lupus mouse models, suggesting its potential as a novel strategy for treating LN and other autoimmune diseases.

PP2A inhibitor SET promotes mTORC1 and Bmi1 signaling through Akt activation and maintains the colony-formation ability of cancer cells.

Protein phosphatase 2A (PP2A) is an essential tumor suppressor, with its activity often hindered in cancer cells by endogenous PP2A inhibitory proteins like SE translocation (SET). SET/PP2A axis plays a pivotal role in the colony-formation ability of cancer cells and the stabilization of c-Myc and E2F1 proteins implicated in this process. However, in osteosarcoma cell line HOS, SET knock-down (KD) suppresses the colony-formation ability without affecting c-Myc and E2F1. This study aimed to unravel the molecular mechanism through which SET enhances the colony-formation ability of HOS cells and determine if it is generalized to other cancer cells. Transcriptome analysis unveiled that SET KD suppressed mTORC1 signaling. SET KD inhibited Akt phosphorylation, an upstream kinase for mTORC1. PP2A inhibitor blocked SET KD-mediated decrease in phosphorylation of Akt and a mTORC1 substrate p70S6K. A constitutively active Akt restored decreased colony-formation ability by SET KD, indicating the SET/PP2A/Akt/mTORC1 axis. Additionally, enrichment analysis highlighted that Bmi-1, a polycomb group protein, is affected by SET KD. SET KD decreased Bmi-1 protein by Akt inhibition but not by mTORC1 inhibition, and exogenous Bmi-1 expression rescued the reduced colony formation by SET KD. Four out of eight cancer cell lines exhibited decreased Bmi-1 by SET KD. Further analysis of these cell lines revealed that Myc activity plays a role in SET KD-mediated Bmi-1 degradation. These findings provide new insights into the molecular mechanism of SET-regulated colony-formation ability, which involved Akt-mediated activation of mTORC1/p70S6K and Bmi-1 signaling.

Endogenous PP2A inhibitor CIP2A degradation by chaperone-mediated autophagy contributes to the antitumor effect of mitochondrial complex I inhibition.

Combined inhibition of oxidative phosphorylation (OXPHOS) and glycolysis has been shown to activate a PP2A-dependent signaling pathway, leading to tumor cell death. Here, we analyze highly selective mitochondrial complex I or III inhibitors in vitro and in vivo to elucidate the molecular mechanisms leading to cell death following OXPHOS inhibition. We show that IACS-010759 treatment (complex I inhibitor) induces a reactive oxygen species (ROS)-dependent dissociation of CIP2A from PP2A, leading to its destabilization and degradation through chaperone-mediated autophagy. Mitochondrial complex III inhibition has analogous effects. We establish that activation of the PP2A holoenzyme containing B56δ regulatory subunit selectively mediates tumor cell death, while the arrest in proliferation that is observed upon IACS-010759 treatment does not depend on the PP2A-B56δ complex. These studies provide a molecular characterization of the events subsequent to the alteration of critical bioenergetic pathways and help to refine clinical studies aimed to exploit metabolic vulnerabilities of tumor cells.

Inhibition of PP2A by LB100 sensitizes bladder cancer cells to chemotherapy by inducing p21 degradation.

PURPOSE: Bladder carcinoma (BLCA) is the most common urinary tract malignancy and exhibits a poor response to chemotherapy. Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase involved in a wide variety of regulatory cellular processes, including apoptosis and the DNA-damage response (DDR). LB100, a small molecule inhibitor of PP2A, has been shown to act as a chemo-sensitizer in multiple types of cancer. However, the anti-tumor effect and mode of action of LB100 in BLCA have yet to be identified. METHODS: In vitro and in vivo experiments were performed to assess the anti-tumor effect of LB100 alone or in combination with gemcitabine. Mass spectrometry (MS)-based phosphoproteomics analysis was used to identify the downstream substrates of PP2A and to explore the mechanism underlying LB100-induced DNA damage and apoptosis. In addition, we established a chemo-resistant BLCA cell line (RT-112-R) by prolonged drug exposure and determined the effect of LB100 in enhancing genotoxicity in BLCA cell lines and xenograft mouse models. RESULTS: We found that LB100 is sufficient to induce an anti-tumor response in BLCA cells by inducing DNA damage and apoptosis both in vitro and in vivo. Furthermore, we found that PP2A potentially dephosphorylates p-p21-ser130 to stabilize p21. Inhibition of PP2A by LB100 increased the level of p-p21-ser130, subsequently leading to a reduction in p21 level in a dose-dependent manner. In addition, we found that treatment of LB100 abrogated the G1/S cell cycle checkpoint, resulting in increased phosphorylation of γH2AX in BLCA cells. Moreover, LB100 enhanced genotoxicity in chemo-resistant BLCA cells by inducing DNA damage and apoptosis in vitro and in vivo. CONCLUSION: Our findings indicate that PP2A may serve as a potential therapeutic target in BLCA through regulating p21 stability.

Combination of dasatinib and okadaic acid induces apoptosis and cell cycle arrest by targeting protein phosphatase PP2A in chronic myeloid leukemia cells.

Chronic myeloid leukemia (CML) is a cancer type of the white blood cells and because of BCR-ABL translocation it results in increased tyrosine kinase activity. For this purpose, dasatinib is the second-generation tyrosine kinase inhibitor that is used for inhibition of BCR-ABL. Effectively and safetly, dasatinib has been used for imatinib-intolerant/resistant CML patients. Protein phosphatase 2A (PP2A) is the major serine/threonine phosphatase ensuring cellular homeostasis in cells and is associated with many cancer types including leukemias. In this study, we aimed to investigate the effects of dasatinib and okadaic acid (OA), either alone or in combination, on apoptosis and cell cycle arrest and dasatinib effect on enzyme activity and protein-level changes of PP2A in K562 cell line. The cytotoxic effects of dasatinib were evaluated by WST-1 analysis. Apoptosis was determined by Annexin V and Apo-Direct assays by flow cytometry. Cell cycle arrest analysis was performed for the investigation of the cytostatic effect. We also used OA as a PP2A inhibitor to assess apoptosis and cell cycle arrest changes in case of reducing the level of PP2A. PP2A enyzme activity and protein levels of PP2A were examined by serine/threonine phosphatase assay and Western blot analysis, respectively. Apoptosis was increased with dasatinib and OA combination. Cell cycle arrest was determined especially after OA treatment. The enzyme activity was decreased depending on time after dasatinib application. PP2A regulatory and catalytic subunit protein levels were decreased compared to control. Targeting the PP2A by dasatinib and OA has potential for CML treatment.

Interaction mechanism of endogenous PP2A inhibitor protein ENSA with PP2A.

The vast diversity of protein phosphatase 2A (PP2A) holoenzyme composition ensures its multifaceted role in the regulation of cellular growth and signal transduction. In several pathological conditions, such as cancer, PP2A is inhibited by endogenous inhibitor proteins. Several PP2A inhibitor proteins have been identified, one of which is α-endosulfine (ENSA). ENSA inhibits PP2A activity when it is phosphorylated at Ser67 by Greatwall (Gwl) kinase. The role of ENSA in PP2A inhibition is rather well characterized, but knowledge of the mechanism of inhibition is scarce. In this study, we have performed comprehensive structural characterization of ENSA, and its interaction with PP2A A- and various B56-subunit isoforms by combining NMR spectroscopy, small-angle X-ray scattering (SAXS) and interaction assays. The results clearly indicate that ENSA is an intrinsically disordered protein containing three transient α-helical structures. ENSA was observed to interact PP2A mainly via A-subunit, as the affinity with the A-subunit is significantly stronger than with any of the B56 subunits. Based on our results, it seems that ENSA follows the dock-and-coalesce mechanism in associating with PP2A A-subunit. Taken together, our results provide an essential structural and molecular framework to understanding molecular bases of ENSA-mediated PP2A inhibition, which is crucial for the development of new therapies for diseases linked to PP2A inhibition.

Protein phosphatase 2A restrains DLK signaling to promote proper Drosophila synaptic development and mammalian cortical neuron survival.

Protein phosphatase 2A (PP2A) is a major cellular phosphatase with many protein substrates. As expected for a signaling molecule with many targets, inhibition of PP2A disrupts fundamental aspects of cellular physiology including cell division and survival. In post-mitotic neurons, the microtubule associated protein Tau is a particularly well-studied PP2A substrate as hyperphosphorylation of Tau is a hallmark of Alzheimer's disease. Although many cellular targets are likely altered by loss of PP2A, here we find that activation of a single pathway can explain important aspects of the PP2A loss-of-function phenotype in neurons. We demonstrate that PP2A inhibits activation of the neuronal stress kinase DLK and its Drosophila ortholog Wallenda. In the fly, PP2A inhibition activates a DLK/Wallenda-regulated transcriptional program that induces synaptic terminal overgrowth at the neuromuscular junction. In cultured mammalian neurons, PP2A inhibition activates a DLK-dependent apoptotic program that induces cell death. Since hyperphosphorylated Tau is toxic, we wished to test the hypothesis that dephosphorylation of Tau by PP2A is required for neuronal survival. Contrary to expectations, in the absence of Tau PP2A inhibition still activates DLK and induces neuronal cell death, demonstrating that hyperphosphorylated Tau is not required for cell death in this model. Moreover, hyperphosphorylation of Tau following PP2A inhibition does not require DLK. Hence, loss of PP2A function in cortical neurons triggers two independent neuropathologies: 1) Tau hyperphosphorylation and 2) DLK activation and subsequent neuronal cell death. These findings demonstrate that inhibition of the DLK pathway is an essential function of PP2A required for normal Drosophila synaptic terminal development and mammalian cortical neuron survival.

PP2A phosphatase inhibition is anti-fibrotic through Ser77 phosphorylation-mediated ARNT/ARNT homodimer formation.

Aryl hydrocarbon receptor nuclear translocator (ARNT) mediates anti-fibrotic activity in kidney and liver through induction of ALK3-receptor expression and subsequently increased Smad1/5/8 signaling. While expression of ARNT can be pharmacologically induced by sub-immunosuppressive doses of FK506 or by GPI1046, its anti-fibrotic activity is only realized when ARNT-ARNT homodimers form, as opposed to formation of ARNT-AHR or ARNT-HIF1α heterodimers. Mechanisms underlying ARNTs dimerization decision to specifically form ARNT-ARNT homodimers and possible cues to specifically induce ARNT homodimerization have been previously unknown. Here, we demonstrate that phosphorylation of the Ser77 residue is critical for ARNT-ARNT homodimer formation and stabilization. We further demonstrate that inhibition of PP2A phosphatase activity by LB100 enhances ARNT-ARNT homodimers both in vivo and in vitro (mouse tubular epithelial cells and human embryonic kidney cells). In murine models of kidney fibrosis, and also of liver fibrosis, combinations of FK506 or GPI1046 (to induce ARNT expression) with LB100 (to enhance ARNT homodimerization) elicit additive anti-fibrotic activities. Our study provides additional evidence for the anti-fibrotic activity of ARNT-ARNT homodimers and reveals Ser77 phosphorylation as a novel pharmacological target to realize the therapeutic potential of increased ARNT transactivation activity.

Inhibiting PP2Acα Promotes the Malignant Phenotype of Gastric Cancer Cells through the ATM/METTL3 Axis.

This article is aimed at exploring the relationship between the phosphatase 2A catalytic subunit Cα (PP2Acα, encoded by PPP2CA) and methyltransferase-like 3 (METTL3) in the malignant progression of gastric cancer (GC). Through analyzing the bioinformatics database and clinical tissue immunohistochemistry results, we found that abnormal PP2Acα and METTL3 levels were closely related to the malignant progression of GC. To explore the internal connection between PP2Acα and METTL3 in the progression of GC, we carried out cellular and molecular experiments and finally proved that PP2Acα inhibition can upregulate METTL3 levels by activating ATM activity, thereby promoting the malignant progression of GC.

A Comparative Analysis of Methods (LC-MS/MS, LC-MS and Rapid Test Kits) for the Determination of Diarrhetic Shellfish Toxins in Oysters, Mussels and Pipis.

Rapid methods for the detection of biotoxins in shellfish can assist the seafood industry and safeguard public health. Diarrhetic Shellfish Toxins (DSTs) are produced by species of the dinoflagellate genus Dinophysis, yet the comparative efficacy of their detection methods has not been systematically determined. Here, we examined DSTs in spiked and naturally contaminated shellfish-Sydney Rock Oysters (Saccostrea glomerata), Pacific Oysters (Magallana gigas/Crassostrea gigas), Blue Mussels (Mytilus galloprovincialis) and Pipis (Plebidonax deltoides/Donax deltoides), using LC-MS/MS and LC-MS in 4 laboratories, and 5 rapid test kits (quantitative Enzyme-Linked Immunosorbent Assay (ELISA) and Protein Phosphatase Inhibition Assay (PP2A), and qualitative Lateral Flow Assay (LFA)). We found all toxins in all species could be recovered by all laboratories using LC-MS/MS (Liquid Chromatography-tandem Mass Spectrometry) and LC-MS (Liquid Chromatography-Mass Spectrometry); however, DST recovery at low and mid-level concentrations (<0.1 mg/kg) was variable (0-150%), while recovery at high-level concentrations (>0.86 mg/kg) was higher (60-262%). While no clear differences were observed between shellfish, all kits delivered an unacceptably high level (25-100%) of falsely compliant results for spiked samples. The LFA and the PP2A kits performed satisfactorily for naturally contaminated pipis (0%, 5% falsely compliant, respectively). There were correlations between spiked DSTs and quantitative methods was highest for LC-MS (r2 = 0.86) and the PP2A kit (r2 = 0.72). Overall, our results do not support the use of any DST rapid test kit as a stand-alone quality assurance measure at this time.

A novel dephosphorylation targeting chimera selectively promoting tau removal in tauopathies.

Intraneuronal accumulation of hyperphosphorylated tau is a hallmark pathology shown in over twenty neurodegenerative disorders, collectively termed as tauopathies, including the most common Alzheimer's disease (AD). Therefore, selectively removing or reducing hyperphosphorylated tau is promising for therapies of AD and other tauopathies. Here, we designed and synthesized a novel DEPhosphorylation TArgeting Chimera (DEPTAC) to specifically facilitate the binding of tau to Bα-subunit-containing protein phosphatase 2A (PP2A-Bα), the most active tau phosphatase in the brain. The DEPTAC exhibited high efficiency in dephosphorylating tau at multiple AD-associated sites and preventing tau accumulation both in vitro and in vivo. Further studies revealed that DEPTAC significantly improved microtubule assembly, neurite plasticity, and hippocampus-dependent learning and memory in transgenic mice with inducible overexpression of truncated and neurotoxic human tau N368. Our data provide a strategy for selective removal of the hyperphosphorylated tau, which sheds new light for the targeted therapy of AD and related-tauopathies.

The potent protein phosphatase 2A inhibitors aminocytostatins: new derivatives of cytostatin.

Specific inhibitors of protein phosphatase 2A (PP2A) mediate anticancer effects by augmenting the tumor-killing activity of natural killer (NK) cells. In this study, new PP2A inhibitors, aminocytostatins A-E, were isolated from Kitasatospora sp. MJ654-NF4 and structurally characterized. Aminocytostatins are derivatives of cytostatin, which is a specific PP2A inhibitor isolated from the same organism, and aminocytostatins have a characteristic amino group within the lactone moiety. Compared to cytostatin, aminocytostatin A showed a stronger inhibitory activity against PP2A in vitro and augmented the tumor-killing activity of NK cells in vivo. Furthermore, a docking model was generated to demonstrate the favorable activities of aminocytostatin A.

Protein Phosphatase 2A as a Therapeutic Target in Small Cell Lung Cancer.

Protein phosphatase 2A (PP2A), a serine/threonine phosphatase involved in the regulation of apoptosis, proliferation, and DNA-damage response, is overexpressed in many cancers, including small cell lung cancer (SCLC). Here we report that LB100, a small molecule inhibitor of PP2A, when combined with platinum-based chemotherapy, synergistically elicited an antitumor response both in vitro and in vivo with no apparent toxicity. Using inductively coupled plasma mass spectrometry, we determined quantitatively that sensitization via LB100 was mediated by increased uptake of carboplatin in SCLC cells. Treatment with LB100 alone or in combination resulted in inhibition of cell viability in two-dimensional culture and three-dimensional spheroid models of SCLC, reduced glucose uptake, and attenuated mitochondrial and glycolytic ATP production. Combining LB100 with atezolizumab increased the capacity of T cells to infiltrate and kill tumor spheroids, and combining LB100 with carboplatin caused hyperphosphorylation of the DNA repair marker γH2AX and enhanced apoptosis while attenuating MET signaling and invasion through an endothelial cell monolayer. Taken together, these data highlight the translational potential of inhibiting PP2A with LB100 in combination with platinum-based chemotherapy and immunotherapy in SCLC.

Deregulation of protein phosphatase 2A inhibitor SET is associated with malignant progression in breast cancer.

To understand the mechanism underlying metastasis, identification of a mechanism-based and common biomarker for circulating tumour cells (CTCs) in heterogenous breast cancer is needed. SET, an endogenous inhibitor of protein phosphatase 2A, was overexpressed in all subtypes of invasive breast carcinoma tissues. Treatment with SET-targeted siRNAs reduced the motility of MCF-7 and MDA-MB-231 cells in transwell assay. SET knockdown reduced the number of mammospheres by 60-70% in MCF-7 and MDA-MB-231 cells, which was associated with the downregulation of OCT4 and SLUG. Hence, we analysed the presence of SET-expressing CTCs (SET-CTCs) in 24 breast cancer patients. CTCs were enriched using a size-based method and then immunocytochemically analysed using an anti-SET antibody. SET-CTCs were detected in 6/6 (100%) patients with recurrent breast cancer with a median value of 12 (12 cells/3 mL blood), and in 13/18 (72.2%) patients with stage I-III breast cancer with a median value of 2.5, while the median value of healthy controls was 0. Importantly, high numbers of SET-CTCs were correlated with lymph node metastasis in patients with stage I-III disease. Our results indicate that SET contributes to breast cancer progression and can act as a potential biomarker of CTCs for the detection of metastasis.

D1R/PP2A/p-CaMKIIα signaling in the caudate putamen is involved in acute methamphetamine-induced hyperlocomotion.

Drug addiction is underscored by the transition from experimental use to dependent use of addictive drugs. Acute use of methamphetamine (METH) causes a range of clinical symptoms, including hyperlocomotion. Dopamine D1 receptor (D1R)-mediated negative regulation of phosphorylated calcium/calmodulin-dependent protein kinase IIα (p-CaMKIIα, threonine [Thr] 286) is involved in the acute effects induced by single METH administration. Protein phosphatase 2A (PP2A) is a potential bridge that links D1R and p-CaMKIIα (Thr 286) after acute METH administration. However, the mechanisms underlying hyperlocomotion induced by single METH administration remain unclear. In this study, SCH23390 (a D1R inhibitor) and LB100 (a PP2A inhibitor) were administered to examine the involvement of D1R and PP2A signaling in acute METH-induced hyperlocomotion in mice. The protein levels of methylated PP2A-C (m-PP2A-C, leucine [Leu] 309), phosphorylated PP2A-C (p-PP2A-C, tyrosine [Tyr] 307), PP2A-C, p-CaMKIIα (Thr 286), and CaMKIIα in the prefrontal cortex (PFc), nucleus accumbens (NAc), and caudate putamen (CPu) were measured. Administration of 0.5 mg/kg SCH23390 reversed the acute METH-induced increase in protein levels of m-PP2A-C (Leu 309) and the decrease in protein levels of p-PP2A-C (Tyr 307) in the CPu, but not in the PFC and NAc. Moreover, prior administration of 0.1 mg/kg LB100 attenuated hyperlocomotion induced by single METH administration and reversed the decrease in protein levels of p-CaMKII (Thr 286) in the PFC, NAc, and CPu. Collectively, these results indicate that the D1R/PP2A/p-CaMKIIα signaling cascade in the CPu may be involved in hyperlocomotion after a single administration of METH.

Deletion or inhibition of prolyl oligopeptidase blocks lithium-induced phosphorylation of GSK3b and Akt by activation of protein phosphatase 2A.

Alterations in prolyl oligopeptidase (PREP) activity have been connected, for example, with bipolar and major depressive disorder, and several studies have reported that lack or inhibition of PREP blocks the effects of lithium on inositol 1,4,5-triphosphate (IP3 ) levels. However, the impact of PREP modulation on other intracellular targets of lithium, such as glycogen synthase kinase 3 beta (GSK3b) or protein kinase B (Akt), has not been studied. We recently found that PREP regulates protein phosphatase 2A (PP2A), and because GSK3b and Akt are PP2A substrates, we studied if PREP-related lithium insensitivity is dependent on PP2A. To assess this, HEK-293 and SH-SY5Y cells with PREP deletion or PREP inhibition (KYP-2047) were exposed to lithium, and thereafter, the phosphorylation levels of GSK3b and Akt were measured by Western blot. As expected, PREP deletion and inhibition blocked the lithium-induced phosphorylation on GSK3b and Akt in both cell lines. When lithium exposure was combined with okadaic acid, a PP2A inhibitor, KYP-2047 did not have effect on lithium-induced GSK3b and Akt phosphorylation. Therefore, we conclude that PREP deletion or inhibition blocks the intracellular effects of lithium on GSK3b and Akt via PP2A activation.

MHC Phosphopeptides: Promising Targets for Immunotherapy of Cancer and Other Chronic Diseases.

Major histocompatibility complex-associated peptides have been considered as potential immunotherapeutic targets for many years. MHC class I phosphopeptides result from dysregulated cell signaling pathways that are common across cancers and both viral and bacterial infections. These antigens are recognized by central memory T cells from healthy donors, indicating that they are considered antigenic by the immune system and that they are presented across different individuals and diseases. Based on these responses and the similar dysregulation, phosphorylated antigens are promising candidates for prevention or treatment of different cancers as well as a number of other chronic diseases.

Targeting protein phosphatase PP2A for cancer therapy: development of allosteric pharmaceutical agents.

Tumor initiation is driven by oncogenes that activate signaling networks for cell proliferation and survival involving protein phosphorylation. Protein kinases in these pathways have proven to be effective targets for pharmaceutical inhibitors that have progressed to the clinic to treat various cancers. Here, we offer a narrative about the development of small molecule modulators of the protein Ser/Thr phosphatase 2A (PP2A) to reduce the activation of cell proliferation and survival pathways. These novel drugs promote the assembly of select heterotrimeric forms of PP2A that act to limit cell proliferation. We discuss the potential for the near-term translation of this approach to the clinic for cancer and other human diseases.

PP1/PP2A phosphatase inhibition-induced metaplasticity in protein synthesis blocker-treated hippocampal slices: LTP and LTD, or There and Back again.

Long-term potentiation (LTP) and long-term depression (LTD) are key forms of synaptic plasticity in the hippocampus. LTP and LTD are believed to underlie the processes occurring during learning and memory. Search of mechanisms responsible for switching from LTP to LTD and vice versa is an important fundamental task. Protein synthesis blockers (PSB) are widely used in models of memory impairment and LTP suppression. Here, we found that blockade of serine/threonine phosphatases 1 (PP1) and 2A (PP2A) with the specific blockers, calyculin A (CalyA) or okadaic acid (OA), and simultaneous blockade of the protein translation by anisomycin or cycloheximide leads to a switch from PSB-impaired LTP to LTD. PP1/PP2A-dependent LTD was extremely sensitive to the intensity of the test stimuli, whose increase restored the field excitatory postsynaptic potentials (fEPSP) to the values corresponding to control LTP in the non-treated slices. PP1/PP2A blockade affected the basal synaptic transmission, increasing the paired-pulse facilitation (PPF) ratio, and restored the PSB-impaired PPF 3 h after tetanus. Prolonged exposure to anisomycin led to the NO synthesis increase (measured using fluorescent dye) both in the dendrites and somata of CA1, CA3, dentate gyrus (DG) hippocampal layers. OA partially prevented the NO production in the CA1 dendrites, as well in the CA3 and DG somas. Direct measurements of changes in serine/threonine phosphatase (STPP) activity revealed importance of the PP1/PP2A-dependent component in the late LTP phase (L-LTP) in anisomycin-treated slices. Thus, serine/threonine phosphatases PP1/PP2A influence both basal synaptic transmission and stimulation-induced synaptic plasticity.

The M-phase regulatory phosphatase PP2A-B55δ opposes protein kinase A on Arpp19 to initiate meiotic division.

Oocytes are held in meiotic prophase for prolonged periods until hormonal signals trigger meiotic divisions. Key players of M-phase entry are the opposing Cdk1 kinase and PP2A-B55δ phosphatase. In Xenopus, the protein Arpp19, phosphorylated at serine 67 by Greatwall, plays an essential role in inhibiting PP2A-B55δ, promoting Cdk1 activation. Furthermore, Arpp19 has an earlier role in maintaining the prophase arrest through a second serine (S109) phosphorylated by PKA. Prophase release, induced by progesterone, relies on Arpp19 dephosphorylation at S109, owing to an unknown phosphatase. Here, we identified this phosphatase as PP2A-B55δ. In prophase, PKA and PP2A-B55δ are simultaneously active, suggesting the presence of other important targets for both enzymes. The drop in PKA activity induced by progesterone enables PP2A-B55δ to dephosphorylate S109, unlocking the prophase block. Hence, PP2A-B55δ acts critically on Arpp19 on two distinct sites, opposing PKA and Greatwall to orchestrate the prophase release and M-phase entry.