Diversity in genomic organisation, developmental regulation and distribution of the murine PR72/B" subunits of protein phosphatase 2A

Hypercontractile cardiac phenotype in mice overexpressing the regulatory subunit PR72 of protein phosphatase 2A

Background

The activity, localization, and substrate specificity of the protein phosphatase 2A (PP2A) heterotrimer are controlled by various regulatory B subunits. PR72 belongs to the B'' gene family and has been shown to be upregulated in human heart failure. However, little is known about the functions of PR72 in the myocardium.

Methods

To address this issue, we generated a transgenic mouse model with heart-specific overexpression of PP2A-PR72. Biochemical and physiological methods were used to determine contractility, Ca2+ cycling parameters, and protein phosphorylation.

Results

A 2.5-fold increase in PR72 expression resulted in moderate cardiac hypertrophy. Maximal ventricular pressure was increased in catheterized transgenic mice (TG) compared to wild-type (WT) littermates. This was accompanied by an increased shortening of sarcomere length and faster relaxation at the single-cell level in TG. In parallel with these findings, the peak amplitude of Ca2+ transients was increased, and the decay in intracellular Ca2+ levels was shortened in TG compared to WT. The changes in Ca2+ cycling in TG were also evident from an increase in the full duration and width at half maximum of Ca2+ sparks. Consistent with the contractile data, phosphorylation of phospholamban at threonine-17 was higher in TG hearts. The lower expression of the Na+/Ca2+ exchanger may also contribute to the hypercontractile state in transgenic myocardium.

Conclusion

Our results suggest that PP2A-PR72 plays an important role in regulating cardiac contractile function and Ca2+ cycling, indicating that the upregulation of PR72 in heart failure is an attempt to compensate functionally.

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.

Clustering analysis and prognostic model based on PI3K/AKT-related genes in pancreatic cancer

Background

Pancreatic cancer is one of most aggressive malignancies with a dismal prognosis. Activation of PI3K/AKT signaling is instrumental in pancreatic cancer tumorigenesis. The aims of this study were to identify the molecular clustering, prognostic value, relationship with tumor immunity and targeting of PI3K/AKT-related genes (PARGs) in pancreatic cancer using bioinformatics.

Methods

The GSEA website was searched for PARGs, and pancreatic cancer-related mRNA data and clinical profiles were obtained through TCGA downloads. Prognosis-related genes were identified by univariate Cox regression analysis, and samples were further clustered by unsupervised methods to identify significant differences in survival, clinical information and immune infiltration between categories. Next, a prognostic model was constructed using Lasso regression analysis. The model was well validated by univariate and multivariate Cox regression analyses, Kaplan-Meier survival analysis and ROC curves, and correlations between risk scores and patient pathological characteristics were identified. Finally, GSEA, drug prediction and immune checkpoint protein analyses were performed.

Results

Pancreatic cancers were divided into Cluster 1 (C1) and Cluster 2 (C1) according to PARG mRNA expression. C1 exhibited longer overall survival (OS) and higher immune scores and CTLA4 expression, whereas C2 exhibited more abundant PD-L1. A 6-PARG-based prognostic model was constructed to divide pancreatic cancer patients into a high-risk score (HRS) group and a low-risk score (LRS) group, where the HRS group exhibited worse OS. The risk score was defined as an independent predictor of OS. The HRS group was significantly associated with pancreatic cancer metastasis, aggregation and immune score. Furthermore, the HRS group exhibited immunosuppression and was sensitive to radiotherapy and guitarbine chemotherapy. Multidrug sensitivity prediction analysis indicated that the HRS group may be sensitive to PI3K/AKT signaling inhibitors (PIK-93, GSK2126458, CAL-101 and rapamycin) and ATP concentration regulators (Thapsigargin). In addition, we confirmed the oncogenic effect of protein phosphatase 2 regulatory subunit B'' subunit alpha (PPP2R3A) in pancreatic cancer in vitro and in vivo.

Conclusions

PARGs predict prognosis, tumor immune profile, radiotherapy and chemotherapy drug sensitivity and are potential predictive markers for pancreatic cancer treatment that can help clinicians make decisions and personalize treatment.

The role of serine/threonine phosphatases in human development: Evidence from congenital disorders

Reversible protein phosphorylation is a fundamental regulation mechanism in eukaryotic cell and organismal physiology, and in human health and disease. Until recently, and unlike protein kinases, mutations in serine/threonine protein phosphatases (PSP) had not been commonly associated with disorders of human development. Here, we have summarized the current knowledge on congenital diseases caused by mutations, inherited or de novo, in one of 38 human PSP genes, encoding a monomeric phosphatase or a catalytic subunit of a multimeric phosphatase. In addition, we highlight similar pathogenic mutations in genes encoding a specific regulatory subunit of a multimeric PSP. Overall, we describe 19 affected genes, and find that most pathogenic variants are loss-of-function, with just a few examples of gain-of-function alterations. Moreover, despite their widespread tissue expression, the large majority of congenital PSP disorders are characterised by brain-specific abnormalities, suggesting a generalized, major role for PSPs in brain development and function. However, even if the pathogenic mechanisms are relatively well understood for a small number of PSP disorders, this knowledge is still incomplete for most of them, and the further identification of downstream targets and effectors of the affected PSPs is eagerly awaited through studies in appropriate in vitro and in vivo disease models. Such lacking studies could elucidate the exact mechanisms through which these diseases act, and possibly open up new therapeutic avenues.

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.

Protein phosphatase 2A in the healthy and failing heart: New insights and therapeutic opportunities

Protein phosphatases have emerged as critical regulators of phosphoprotein homeostasis in settings of health and disease. Protein phosphatase 2A (PP2A) encompasses a large subfamily of enzymes that remove phosphate groups from serine/threonine residues within phosphoproteins. The heterogeneity in PP2A structure, which arises from the grouping of different catalytic, scaffolding and regulatory subunit isoforms, creates distinct populations of catalytically active enzymes (i.e. holoenzymes) that localise to different parts of the cell. This structural complexity, combined with other regulatory mechanisms, such as interaction of PP2A heterotrimers with accessory proteins and post-translational modification of the catalytic and/or regulatory subunits, enables PP2A holoenzymes to target phosphoprotein substrates in a highly specific manner. In this review, we summarise the roles of PP2A in cardiac physiology and disease. PP2A modulates numerous processes that are vital for heart function including calcium handling, contractility, β-adrenergic signalling, metabolism and transcription. Dysregulation of PP2A has been observed in human cardiac disease settings, including heart failure and atrial fibrillation. Efforts are underway, particularly in the cancer field, to develop therapeutics targeting PP2A activity. The development of small molecule activators of PP2A (SMAPs) and other compounds that selectively target specific PP2A holoenzymes (e.g. PP2A/B56α and PP2A/B56ε) will improve understanding of the function of different PP2A species in the heart, and may lead to the development of therapeutics for normalising aberrant protein phosphorylation in settings of cardiac remodelling and dysfunction.

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.

Epidermal growth factor receptor-dependent maintenance of cardiac contractility

AIMS

Epidermal growth factor receptor (EGFR) is essential to the development of multiple tissues and organs and is a target of cancer therapeutics. Due to the embryonic lethality of global EGFR deletion and conflicting reports of cardiac-overexpressed EGFR mutants, its specific impact on the adult heart, normally or in response to chronic stress, has not been established. Using complimentary genetic strategies to modulate cardiomyocyte-specific EGFR expression, we aim to define its role in the regulation of cardiac function and remodeling.

METHODS AND RESULTS

A floxed EGFR mouse model with α-myosin heavy chain-Cre-mediated cardiomyocyte-specific EGFR downregulation (CM-EGFR-KD mice) developed contractile dysfunction by 9 weeks of age, marked by impaired diastolic relaxation, as monitored via echocardiographic, hemodynamic and isolated cardiomyocyte contractility analyses. This contractile defect was maintained over time without overt cardiac remodeling until 10 months of age, after which the mice ultimately developed severe heart failure and reduced lifespan. Acute downregulation of EGFR in adult floxed EGFR mice with adeno-associated virus 9 (AAV9)-encoded Cre with a cardiac troponin T promoter (AAV9-cTnT-Cre) recapitulated the CM-EGFR-KD phenotype, while AAV9-cTnT-EGFR treatment of adult CM-EGFR-KD mice rescued the phenotype. Notably, chronic administration of the β-adrenergic receptor (βAR) agonist isoproterenol effectively and reversibly compensated for the contractile dysfunction in the absence of cardiomyocyte hypertrophy in CM-EGFR-KD mice. Mechanistically, EGFR downregulation reduced the expression of protein phosphatase 2 A (PP2A) regulatory subunit Ppp2r3a/PR72, which was associated with decreased phosphorylation of phospholamban (PLB) and Ca2+ clearance, and whose re-expression via AAV9-cTnT-PR72 rescued the CM-EGFR-KD phenotype.

CONCLUSIONS

Altogether our study highlights a previously unrecognized role for EGFR in maintaining contractile homeostasis under physiologic conditions in the adult heart via regulation of PR72 expression.

TRANSLATIONAL PERSPECTIVE

Our study highlights a previously unrecognized role for EGFR in maintaining contractile homeostasis under physiologic conditions in the adult heart via regulation of PR72, a PP2A regulatory subunit with an unknown impact on cardiac function. Further, we have shown that cardiomyocyte-expressed EGFR is required for the promotion of cardiac hypertrophy under conditions of chronic catecholamine stress. Altogether, our study provides new insight into the dynamic nature of cardiomyocyte-specific EGFR.

The PP2A subunit PR130 is a key regulator of cell development and oncogenic transformation

Protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase. This enzyme is involved in a plethora of cellular processes, including apoptosis, autophagy, cell proliferation, and DNA repair. Remarkably, PP2A can act as a context-dependent tumor suppressor or promoter. Active PP2A complexes consist of structural (PP2A-A), regulatory (PP2A-B), and catalytic (PP2A-C) subunits. The regulatory subunits define the substrate specificity and the subcellular localization of the holoenzyme. Here we condense the increasing evidence that the PP2A B-type subunit PR130 is a critical regulator of cell identity and oncogenic transformation. We summarize knowledge on the biological functions of PR130 in normal and transformed cells, targets of the PP2A-PR130 complex, and how diverse extra- and intracellular stimuli control the expression and activity of PR130. We additionally review the impact of PP2A-PR130 on cardiac functions, neuronal processes, and anti-viral defense and how this might affect cancer development and therapy.

Protein phosphatase 2 regulatory subunit B''Alpha silencing inhibits tumor cell proliferation in liver cancer

Aim

To explore the effects of protein phosphatase 2 regulatory subunit B''Alpha (PPP2R3A) on the proliferation and migration of liver cancer cells.

Methods

Expression of PPP2R3A in tumor tissues of hepatocellular carcinoma (HCC) patients was detected by immunohistochemistry and western blotting. In two liver cancer cell lines (HepG2 and HuH7), PPP2R3A expression was silenced and then overexpression with PPP2R3A lentiviral vectors, and the effects of PPP2R3A knockdown or overexpression on the proliferation, cell cycle, migration, and invasion of HCC cells were determined in vitro. In a xenograft cancer model in nude mice, the in vivo effects of PPP2R3A knockdown on tumor growth and cancer cell proliferation were evaluated.

Results

PPP2R3A expression was found in tumor foci in six of eight HCC samples, at a level higher than that in the adjacent para‐tumor tissues. PPP2R3A expression was observed primarily in the cytoplasm of the cancer cells. Knockdown of PPP2R3A resulted in significant inhibition of hepatoma cell proliferation (P < .05), migration (P < .01), and invasion (P < .01) as well as a significant delay in the G1/S transition in both liver cancer lines (P < .05) and increased p53 expression. Conversely, overexpression of PPP2R3A promoted the proliferation (P < .05) and altered cell cycle progression (P < .05) of both liver cancer cell lines. In vivo, PPP2R3A knockdown in liver cancer cells led to significant reductions in the tumor volume (P < .001) and the expression of Ki‐67 in tumor tissues (P < .05).

Conclusion

PPP2R3A may play a role in liver cancer via the regulation of tumor cell proliferation and invasion.

Discovery of PPP2R3A and TMX3 pathogenic variants in a Zhuang family with coronary artery disease using whole-exome sequencing

Whole-exome sequencing (WES) was used to explore pathogenic variants present in a Zhuang family with coronary artery disease (CAD). The 11 Zhuang family members enrolled in this study include three generations of direct relatives with a history of CAD. All members currently live in Guangxi Zhuang Autonomous Region (China), in which all members were also born. Clinical and biochemical tests were performed on the members who were divided into the CAD group (four members), the high-risk CAD group (four members), or normal control group (three members). Whole-exome sequencing was performed on two randomly selected CAD members and one control member in order to screen for candidate genes. All members of the family were genetically verified using Sanger sequencing. Bioinformatics analysis performed on WES data led to the discovery of five candidate genes. Having been confirmed using Sanger sequencing, two pathogenic mutations, PPP2R3A (c.721G; p.Glu241Lys>A) and TMX3 (c.1035 + 38delA), were found in four members of the CAD group and two members of the high-risk group. These variants were not detected in the three control group members. PPP2R3A and TMX3 may be new pathogenic mutations associated with CAD in the Zhuang population of Guangxi province.

Selective targeting of CREB-binding protein/β-catenin inhibits growth of and extracellular matrix remodelling by airway smooth muscle

BACKGROUND AND PURPOSE

Asthma is a heterogeneous chronic inflammatory disease, characterized by the development of structural changes (airway remodelling). β-catenin, a transcriptional co-activator, is fundamentally involved in airway smooth muscle growth and may be a potential target in the treatment of airway smooth muscle remodelling.

EXPERIMENTAL APPROACH

We assessed the ability of small-molecule compounds that selectively target β-catenin breakdown or its interactions with transcriptional co-activators to inhibit airway smooth muscle remodelling in vitro and in vivo.

KEY RESULTS

ICG-001, a small-molecule compound that inhibits the β-catenin/CREB-binding protein (CBP) interaction, strongly and dose-dependently inhibited serum-induced smooth muscle growth and TGFβ1-induced production of extracellular matrix components in vitro. Inhibition of β-catenin/p300 interactions using IQ-1 or inhibition of tankyrase 1/2 using XAV-939 had considerably less effect. In a mouse model of allergic asthma, β-catenin expression in the smooth muscle layer was found to be unaltered in control versus ovalbumin-treated animals, a pattern that was found to be similar in smooth muscle within biopsies taken from asthmatic and non-asthmatic donors. However, β-catenin target gene expression was highly increased in response to ovalbumin; this effect was prevented by topical treatment with ICG-001. Interestingly, ICG-001 dose-dependently reduced airway smooth thickness after repeated ovalbumin challenge, but had no effect on the deposition of collagen around the airways, mucus secretion or eosinophil infiltration.

CONCLUSIONS AND IMPLICATIONS

Together, our findings highlight the importance of β-catenin/CBP signalling in the airways and suggest ICG-001 may be a new therapeutic approach to treat airway smooth muscle remodelling in asthma.

Deletion of Pr130 Interrupts Cardiac Development in Zebrafish

Protein phosphatase 2 regulatory subunit B, alpha (PPP2R3A), a regulatory subunit of protein phosphatase 2A (PP2A), is a major serine/threonine phosphatase that regulates crucial function in development and growth. Previous research has implied that PPP2R3A was involved in heart failure, and PR130, the largest transcription of PPP2R3A, functioning in the calcium release of sarcoplasmic reticulum (SR), plays an important role in the excitation-contraction (EC) coupling. To obtain a better understanding of PR130 functions in myocardium and cardiac development, two pr130-deletion zebrafish lines were generated using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas) system. Pr130-knockout zebrafish exhibited cardiac looping defects and decreased cardiac function (decreased fractional area and fractional shortening). Hematoxylin and eosin (H&amp;E) staining demonstrated reduced cardiomyocytes. Subsequent transmission electron microscopy revealed that the bright and dark bands were narrowed and blurred, the Z- and M-lines were fogged, and the gaps between longitudinal myocardial fibers were increased. Additionally, increased apoptosis was observed in cardiomyocyte in pr130-knockout zebrafish compared to wild-type (WT). Taken together, our results suggest that pr130 is required for normal myocardium formation and efficient cardiac contractile function.

The broken "Off" switch in cancer signaling: PP2A as a regulator of tumorigenesis, drug resistance, and immune surveillance

Aberrant activation of signal transduction pathways can transform a normal cell to a malignant one and can impart survival properties that render cancer cells resistant to therapy. A diverse set of cascades have been implicated in various cancers including those mediated by serine/threonine kinases such RAS, PI3K/AKT, and PKC. Signal transduction is a dynamic process involving both "On" and "Off" switches. Activating mutations of RAS or PI3K can be viewed as the switch being stuck in the "On" position resulting in continued signaling by a survival and/or proliferation pathway. On the other hand, inactivation of protein phosphatases such as the PP2A family can be seen as the defective "Off" switch that similarly can activate these pathways. A problem for therapeutic targeting of PP2A is that the enzyme is a hetero-trimer and thus drug targeting involves complex structures. More importantly, since PP2A isoforms generally act as tumor suppressors one would want to activate these enzymes rather than suppress them. The elucidation of the role of cellular inhibitors like SET and CIP2A in cancer suggests that targeting these proteins can have therapeutic efficacy by mechanisms involving PP2A activation. Furthermore, drugs such as FTY-720 can activate PP2A isoforms directly. This review will cover the current state of knowledge of PP2A role as a tumor suppressor in cancer cells and as a mediator of processes that can impact drug resistance and immune surveillance.

PP2A binds to the LIM domains of lipoma-preferred partner through its PR130/B″ subunit to regulate cell adhesion and migration

Here, we identify the LIM protein lipoma-preferred partner (LPP) as a binding partner of a specific protein phosphatase 2A (PP2A) heterotrimer that is characterised by the regulatory PR130/B″α1 subunit (encoded by PPP2R3A). The PR130 subunit interacts with the LIM domains of LPP through a conserved Zn²⁺-finger-like motif in the differentially spliced N-terminus of PR130. Isolated LPP-associated PP2A complexes are catalytically active. PR130 colocalises with LPP at multiple locations within cells, including focal contacts, but is specifically excluded from mature focal adhesions, where LPP is still present. An LPP-PR130 fusion protein only localises to focal adhesions upon deletion of the domain of PR130 that binds to the PP2A catalytic subunit (PP2A/C), suggesting that PR130-LPP complex formation is dynamic and that permanent recruitment of PP2A activity might be unfavourable for focal adhesion maturation. Accordingly, siRNA-mediated knockdown of PR130 increases adhesion of HT1080 fibrosarcoma cells onto collagen I and decreases their migration in scratch wound and Transwell assays. Complex formation with LPP is mandatory for these PR130-PP2A functions, as neither phenotype can be rescued by re-expression of a PR130 mutant that no longer binds to LPP. Our data highlight the importance of specific, locally recruited PP2A complexes in cell adhesion and migration dynamics.

Cardiomyocyte specific deletion of PP2A causes cardiac hypertrophy

Cardiac hypertrophy is a common pathological alteration in heart disease, which has been reported to be connected with serine/threonine protein phosphatases that control the dephosphorylation of a variety of cardiac proteins. Herein, we generated protein phosphatase type 2A knockout expressing a tamoxifen-inducible Cre recombinase protein fused to two mutant estrogen-receptor ligand-binding domains (MerCreMer) under the control of the a-myosin heavy chain promoter. Cardiac function of mice was determined by echocardiography. Decrease in PP2A activity leads to increased cardiomyocyte hypertrophy and fibrosis. Loss of PP2ACα leads to the heart failure, including the changes of EF, FS, LV, ANP and BNP. On the molecular level, knockout mice shows increased expression of B55a and B56e at 60 days after tamoxifen injection. Additionally, the regulation of the Akt/GSK3β/β-catenin pathway is severely disturbed in knockout mice. In conclusion, cardiomyocyte specific deletion of PP2A gene causes the cardiac hypertrophy. We will use the knockout mice to generate a type of cardiomyocyte hypertrophy mouse model with myocardial fibrosis.

The Basic Biology of PP2A in Hematologic Cells and Malignancies

Reversible protein phosphorylation plays a crucial role in regulating cell signaling. In normal cells, phosphoregulation is tightly controlled by a network of protein kinases counterbalanced by several protein phosphatases. Deregulation of this delicate balance is widely recognized as a central mechanism by which cells escape external and internal self-limiting signals, eventually resulting in malignant transformation. A large fraction of hematologic malignancies is characterized by constitutive or unrestrained activation of oncogenic kinases. This is in part achieved by activating mutations, chromosomal rearrangements, or constitutive activation of upstream kinase regulators, in part by inactivation of their anti-oncogenic phosphatase counterparts. Protein phosphatase 2A (PP2A) represents a large family of cellular serine/threonine phosphatases with suspected tumor suppressive functions. In this review, we highlight our current knowledge about the complex structure and biology of these phosphatases in hematologic cells, thereby providing the rationale behind their diverse signaling functions. Eventually, this basic knowledge is a key to truly understand the tumor suppressive role of PP2A in leukemogenesis and to allow further rational development of therapeutic strategies targeting PP2A.

Roles of phosphotase 2A in nociceptive signal processing

Multiple protein kinases affect the responses of dorsal horn neurons through phosphorylation of synaptic receptors and proteins involved in intracellular signal transduction pathways, and the consequences of this modulation may be spinal central sensitization. In contrast, the phosphatases catalyze an opposing reaction of de-phosphorylation, which may also modulate the functions of crucial proteins in signaling nociception. This is an important mechanism in the regulation of intracellular signal transduction pathways in nociceptive neurons. Accumulated evidence has shown that phosphatase 2A (PP2A), a serine/threonine specific phosphatase, is implicated in synaptic plasticity of the central nervous system and central sensitization of nociception. Therefore, targeting protein phosphotase 2A may provide an effective and novel strategy for the treatment of clinical pain. This review will characterize the structure and functional regulation of neuronal PP2A and bring together recent advances on the modulation of PP2A in targeted downstream substrates and relevant multiple nociceptive signaling molecules.

Circumventing embryonic lethality with Lcmt1 deficiency: generation of hypomorphic Lcmt1 mice with reduced protein phosphatase 2A methyltransferase expression and defects in insulin signaling

Protein phosphatase 2A (PP2A), the major serine/threonine phosphatase in eukaryotic cells, is a heterotrimeric protein composed of structural, catalytic, and targeting subunits. PP2A assembly is governed by a variety of mechanisms, one of which is carboxyl-terminal methylation of the catalytic subunit by the leucine carboxyl methyltransferase LCMT1. PP2A is nearly stoichiometrically methylated in the cytosol, and although some PP2A targeting subunits bind independently of methylation, this modification is required for the binding of others. To examine the role of this methylation reaction in mammalian tissues, we generated a mouse harboring a gene-trap cassette within intron 1 of Lcmt1. Due to splicing around the insertion, Lcmt1 transcript and LCMT1 protein levels were reduced but not eliminated. LCMT1 activity and methylation of PP2A were reduced in a coordinate fashion, suggesting that LCMT1 is the only PP2A methyltransferase. These mice exhibited an insulin-resistance phenotype, indicating a role for this methyltransferase in signaling in insulin-sensitive tissues. Tissues from these animals will be vital for the in vivo identification of methylation-sensitive substrates of PP2A and how they respond to differing physiological conditions.

Structure, regulation, and pharmacological modulation of PP2A phosphatases

Protein phosphatases of the type 2A family (PP2A) represent a major fraction of cellular Ser/Thr phosphatase activity in any given human tissue. In this review, we describe how the holoenzymic nature of PP2A and the existence of several distinct PP2A composing subunits allow for the generation of multiple structurally and functionally different PP2A complexes, explaining why PP2A is involved in the regulation of so many diverse cell biological and physiological processes. Moreover, in human disease, most notably in several cancers and Alzheimer's Disease, PP2A expression and/or activity have been found significantly decreased, underscoring its important functions as a major tumor suppressor and tau phosphatase. Hence, several recent preclinical studies have demonstrated that pharmacological restoration of PP2A activity, as well as pharmacological PP2A inhibition, under certain conditions, may be of significant future therapeutic value.

The biogenesis of active protein phosphatase 2A holoenzymes: a tightly regulated process creating phosphatase specificity

Protein phosphatase type 2A (PP2A) enzymes constitute a large family of Ser/Thr phosphatases with multiple functions in cellular signaling and physiology. The composition of heterotrimeric PP2A holoenzymes, resulting from the combinatorial assembly of a catalytic C subunit, a structural A subunit, and regulatory B-type subunit, provides the essential determinants for substrate specificity, subcellular targeting, and fine-tuning of phosphatase activity, largely explaining why PP2A is functionally involved in so many diverse physiological processes, sometimes in seemingly opposing ways. In this review, we highlight how PP2A holoenzyme biogenesis and enzymatic activity are controlled by a sophisticatedly coordinated network of five PP2A modulators, consisting of α4, phosphatase 2A phosphatase activator (PTPA), leucine carboxyl methyl transferase 1 (LCMT1), PP2A methyl esterase 1 (PME-1) and, potentially, target of rapamycin signaling pathway regulator-like 1 (TIPRL1), which serve to prevent promiscuous phosphatase activity until the holoenzyme is completely assembled. Likewise, these modulators may come into play when PP2A holoenzymes are disassembled following particular cellular stresses. Malfunctioning of these cellular control mechanisms contributes to human disease. The potential therapeutic benefits or pitfalls of interfering with these regulatory mechanisms will be briefly discussed.

PP1 and PP2A phosphatases--cooperating partners in modulating retinoblastoma protein activation

The retinoblastoma/pocket protein family is one of the master regulators of the eukaryotic cell cycle. It includes the retinoblastoma protein (Rb) and the related p107 and p130 proteins. The importance of the Rb pathway for homeostasis and tumour suppression is evident from the fact that inactivating mutations in Rb are frequently associated with many cancers. Rbs regulate the cell cycle by controlling the activity of the E2F family of transcription factors. The activity of Rb proteins themselves is modulated by their phosphorylation status at several Ser/Thr residues: phosphorylation by cyclin-dependent kinases inactivates Rb proteins and positively influences the transcription of genes necessary for cell cycle progression. Although the mechanisms of cyclin-dependent kinase-mediated inactivation of Rb proteins are understood in great detail, our knowledge of the process that counteracts Rb phosphorylation is still quite limited. The present review focuses on the Ser/Thr phosphatases that are responsible for the dephosphorylation and thus activation of Rb proteins. Two major scenarios are considered: (a) when pocket proteins are dephosphorylated during regular cell cycle progression and (b) when rapid dephosphorylation is dictated by external stress or growth inhibitory conditions, such as oxidative stress, UV radiation or other DNA-damaging stimuli, and cell differentiation factors. It transpires that protein phosphatase 1 and protein phosphatase 2A can efficiently modulate pocket protein activity in a highly context-dependent manner and both are tightly regulated by the presence of different regulatory subunits or interacting proteins.

Protein phosphatase 2A PR130/B''alpha1 subunit binds to the SH2 domain-containing inositol polyphosphate 5-phosphatase 2 and prevents epidermal growth factor (EGF)-induced EGF receptor degradation sustaining EGF-mediated signaling

To elucidate novel cell biological functions of specific protein phosphatase 2A (PP2A) holoenzymes, we identified and biochemically characterized a complex between the Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase 2 (SHIP2) and a PP2A holoenzyme comprising PR130/B''alpha1 as a regulatory subunit (PP2A(T130)) in several mammalian cell lines. PR130/B''alpha1 and SHIP2 partially colocalize in untreated HeLa cells, and both translocate to the cell membrane on epidermal growth factor (EGF) stimulation. Concomitantly, a transient EGF-dependent interaction of PR130/B''alpha1 with the EGF receptor (EGFR) was observed, whereas the SHIP2-PR130 interaction remained constitutive. As previously reported for SHIP2, RNA interference-mediated knockdown of PR130 in COS-7 cells resulted in increased EGF-induced proteasome-dependent EGFR degradation, and an increased interaction of EGFR with the E3 ligase c-Cbl. In concordance with faster EGFR clearance or desensitization, intrinsic EGFR kinase activity (phospho-Tyr-1068) and downstream protein kinase B and extracellular signal-regulated kinase/mitogen-activated protein kinase pathways were more rapidly inactivated in PR130-knockdown cells. Notably, these effects could be rescued by reintroduction of RNA interference-resistant Myc-PR130, excluding any off-target effect. These data highlight a novel biological role of the PP2A(T130) holoenzyme in EGF signaling through interaction with EGFR and the phosphatidylinositol (3,4,5)-trisphosphate 5-phosphatase SHIP2. This interaction may be of clinical relevance as dysfunction of EGF-mediated signaling has been linked to various human cancers.

Specific regulation of protein phosphatase 2A PR72/B'' subunits by calpain

Protein phosphatase 2A (PP2A) represents a family of multimeric serine/threonine phosphatases with pleiotropic roles in signal transduction. We previously described the functional analysis of two Ca(2+)-binding EF-hands in the PR72/B'' class of regulatory PP2A subunits. Now we report partial degradation of PR72/B"alpha2 and PR130/B"alpha1 into a 45-48kDa proteolysis-resistant fragment ('PR45') by the Ca(2+)-dependent protease m-calpain. This limited proteolysis is dependent on EF-hand integrity, independent of two PEST-domains, and highly specific as PP2A(C), PR65/A and representatives of PR55/B and PR61/B' subunit families are calpain-resistant. PR45 was also generated in staurosporine-induced apoptotic MCF7 cells in a calpain-dependent way. Calpain treatment weakens the PR72-core enzyme interaction, activates basal PP2A(T72) phosphatase activity and dramatically increases its sensitivity for and activation by polycations. This unique property can be exploited in a specific biochemical assay for these holoenzymes. We propose local calpain action in vivo may constitute a novel regulatory mechanism of these holoenzymes.