Tautomycetin is a novel and specific inhibitor of serine/threonine protein phosphatase type 1, PP1

Nuclear lamin A/C phosphorylation by loss of androgen receptor leads to cancer-associated fibroblast activation

Alterations in nuclear structure and function are hallmarks of cancer cells. Little is known about these changes in Cancer-Associated Fibroblasts (CAFs), crucial components of the tumor microenvironment. Loss of the androgen receptor (AR) in human dermal fibroblasts (HDFs), which triggers early steps of CAF activation, leads to nuclear membrane changes and micronuclei formation, independent of cellular senescence. Similar changes occur in established CAFs and are reversed by restoring AR activity. AR associates with nuclear lamin A/C, and its loss causes lamin A/C nucleoplasmic redistribution. AR serves as a bridge between lamin A/C and the protein phosphatase PPP1. Loss of AR decreases lamin-PPP1 association and increases lamin A/C phosphorylation at Ser 301, a characteristic of CAFs. Phosphorylated lamin A/C at Ser 301 binds to the regulatory region of CAF effector genes of the myofibroblast subtype. Expression of a lamin A/C Ser301 phosphomimetic mutant alone can transform normal fibroblasts into tumor-promoting CAFs.

Pharmacological control of angiogenesis by regulating phosphorylation of myosin light chain 2

BACKGROUND

Control of angiogenesis is widely considered a therapeutic strategy, but reliable control methods are still under development. Phosphorylation of myosin light chain 2 (MLC2), which regulates actin-myosin interaction, is critical to the behavior of vascular endothelial cells (ECs) during angiogenesis. MLC2 is phosphorylated by MLC kinase (MLCK) and dephosphorylated by MLC phosphatase (MLCP) containing a catalytic subunit PP1. We investigated the potential role of MLC2 in the pharmacological control of angiogenesis.

METHODS AND RESULTS

We exposed transgenic zebrafish Tg(fli1a:Myr-mCherry)ncv1 embryos to chemical inhibitors and observed vascular development. PP1 inhibition by tautomycetin increased length of intersegmental vessels (ISVs), whereas MLCK inhibition by ML7 decreased it; these effects were not accompanied by structural dysplasia. ROCK inhibition by Y-27632 also decreased vessel length. An in vitro angiogenesis model of human umbilical vein endothelial cells (HUVECs) showed that tautomycetin increased vascular cord formation, whereas ML7 and Y-27632 decreased it. These effects appear to be influenced by regulation of cell morphology rather than cell viability or motility. Actin co-localized with phosphorylated MLC2 (pMLC2) was abundant in vascular-like elongated-shaped ECs, but poor in non-elongated ECs. pMLC2 was associated with tightly arranged actin, but not with loosely arranged actin. Moreover, knockdown of MYL9 gene encoding MLC2 reduced total MLC2 and pMLC2 protein and inhibited angiogenesis in HUVECs.

CONCLUSION

The present study found that MLC2 is a pivotal regulator of angiogenesis. MLC2 phosphorylation may be involved in the regulation of of cell morphogenesis and cell elongation. The functionally opposite inhibitors positively or negatively control angiogenesis, probably through the regulating EC morphology. These findings may provide a unique therapeutic target for angiogenesis.

Neuronal Excitation Induces Tau Protein Dephosphorylation via Protein Phosphatase 1 Activation to Promote Its Binding with Stable Microtubules

The tau protein is a microtubule-associated protein that promotes microtubule stabilization. The phosphorylation of the tau protein has been linked to its dissociation from microtubules. Here, we examined the relationship between neuronal depolarization activity and tau protein phosphorylation by employing model systems in culture as well as in vivo. The KCl-evoked depolarization of cultured neurons has often been used to investigate the effects of neuronal activity. We found dephosphorylation at AT8 sites (S202, T205), T212, AT180 sites (T231, S235), and S396 in KCl-simulated cultured neurons. We also found that the KCl-induced tau protein dephosphorylation increases the level of the tau protein fractionated with stable microtubules. In an in vivo experiment, we demonstrated that the exposure of mice to a new environment activates protein phosphatase 1 in the mouse hippocampus and induces tau protein dephosphorylation. We also found an increased amount of the tau protein in a stable microtubule fraction, suggesting that the dephosphorylation of the tau protein may lead to its increased microtubule association in vivo. These results suggest that the association of microtubules with tau proteins may be regulated by the tau protein phosphorylation status affected by neuronal electrical activity.

Aging-regulated PNUTS maintains endothelial barrier function via SEMA3B suppression

Age-related diseases pose great challenges to health care systems worldwide. During aging, endothelial senescence increases the risk for cardiovascular disease. Recently, it was described that Phosphatase 1 Nuclear Targeting Subunit (PNUTS) has a central role in cardiomyocyte aging and homeostasis. Here, we determine the role of PNUTS in endothelial cell aging. We confirm that PNUTS is repressed in senescent endothelial cells (ECs). Moreover, PNUTS silencing elicits several of the hallmarks of endothelial aging: senescence, reduced angiogenesis and loss of barrier function. Findings are validate in vivo using endothelial-specific inducible PNUTS-deficient mice (Cdh5-CreERT2;PNUTSfl/fl), termed PNUTSEC-KO. Two weeks after PNUTS deletion, PNUTSEC-KO mice present severe multiorgan failure and vascular leakage. Transcriptomic analysis of PNUTS-silenced HUVECs and lungs of PNUTSEC-KO mice reveal that the PNUTS-PP1 axis tightly regulates the expression of semaphorin 3B (SEMA3B). Indeed, silencing of SEMA3B completely restores barrier function after PNUTS loss-of-function. These results reveal a pivotal role for PNUTS in endothelial homeostasis through a SEMA3B downstream pathway that provides a potential target against the effects of aging in ECs.

Dietary potassium stimulates Ppp1Ca-Ppp1r1a dephosphorylation of kidney NaCl cotransporter and reduces blood pressure

Consumption of low dietary potassium, common with ultraprocessed foods, activates the thiazide-sensitive sodium chloride cotransporter (NCC) via the with no (K) lysine kinase/STE20/SPS1-related proline-alanine-rich protein kinase (WNK/SPAK) pathway to induce salt retention and elevate blood pressure (BP). However, it remains unclear how high-potassium "DASH-like" diets (dietary approaches to stop hypertension) inactivate the cotransporter and whether this decreases BP. A transcriptomics screen identified Ppp1Ca, encoding PP1A, as a potassium-upregulated gene, and its negative regulator Ppp1r1a, as a potassium-suppressed gene in the kidney. PP1A directly binds to and dephosphorylates NCC when extracellular potassium is elevated. Using mice genetically engineered to constitutively activate the NCC-regulatory kinase SPAK and thereby eliminate the effects of the WNK/SPAK kinase cascade, we confirmed that PP1A dephosphorylated NCC directly in a potassium-regulated manner. Prior adaptation to a high-potassium diet was required to maximally dephosphorylate NCC and lower BP in constitutively active SPAK mice, and this was associated with potassium-dependent suppression of Ppp1r1a and dephosphorylation of its cognate protein, inhibitory subunit 1 (I1). In conclusion, potassium-dependent activation of PP1A and inhibition of I1 drove NCC dephosphorylation, providing a mechanism to explain how high dietary K+ lowers BP. Shifting signaling of PP1A in favor of activation of WNK/SPAK may provide an improved therapeutic approach for treating salt-sensitive hypertension.

Phosphoprotein phosphatase activity positively regulates oligomeric pyrin to trigger inflammasome assembly in phagocytes

IMPORTANCE

Pyrin, a unique cytosolic receptor, initiates inflammatory responses against RhoA-inactivating bacterial toxins and effectors like Yersinia's YopE and YopT. Understanding pyrin regulation is crucial due to its association with dysregulated inflammatory responses, including Familial Mediterranean Fever (FMF), linked to pyrin gene mutations. FMF mutations historically acted as a defense mechanism against plague. Negative regulation of pyrin through PKN phosphorylation is well established, with Yersinia using the YopM effector to promote pyrin phosphorylation and counteract its activity. This study highlights the importance of phosphoprotein phosphatase activity in positively regulating pyrin inflammasome assembly in phagocytic cells of humans and mice. Oligomeric murine pyrin has S205 phosphorylated before inflammasome assembly, and this study implicates the dephosphorylation of murine pyrin S205 by two catalytic subunits of PP2A in macrophages. These findings offer insights for investigating the regulation of oligomeric pyrin and the balance of kinase and phosphatase activity in pyrin-associated infectious and autoinflammatory diseases.

Phosphatases maintain low catalytic activity of SGK1: DNA damage resets the balance in favor of phosphorylation

The serum- and glucocorticoid-induced kinase 1 (SGK1) promotes cell survival under stress conditions and facilitates the emergence of drug resistance in cancer. The underlying mechanisms of these observations are not fully understood. In this study, we found that SGK1 activity is suppressed by the action of the S/T phosphatases PP5 and PP2A, which constantly dephosphorylate SGK1. Using newly developed anti-phospho SGK1 antibodies and inhibitors of phosphatases, we determined that the high degree of dephosphorylation is caused by two factors: the tendency of SGK1 to unfold, which makes it dependent on Hsp90 chaperone complexes composed of four proteins, Hsp90/CDC37/PP5/SGK1, and where the phosphatase PP5 persistently dephosphorylates SGK1 within the complex. SGK1 binding to PP2A regulatory subunits B55γ and B55δ brings PP2A catalytic subunit close to exposed SGK1 phosphoresidues. A further association of phosphorylated pS37-FAM122A-an endogenous inhibitor of PP2A-to the holoenzyme diminishes dephosphorylation of SGK1 mediated by PP2A. Our study also reveals that genotoxic stress can reverse the dominant impact of phosphatases over kinases by activating the DNA-dependent protein kinase, which enhances mTORC2 activity directed to SGK1. Thus, our results provide insight into a molecular pathway that enables SGK1 to gain phosphorylation and catalytic activity and promote cell survival, potentially diminishing the efficacy of cancer treatments. As the DNA damage response operates in many cancer cells and is further induced by chemotherapies, the findings of this study could have significant implications for the development of novel cancer therapies targeting SGK1.

Oscillations in PP1 activity are essential for accurate progression through mammalian oocyte meiosis

Tightly controlled fluctuations in kinase and phosphatase activity play important roles in regulating M-phase transitions. Protein Phosphatase 1 (PP1) is one of these phosphatases, with oscillations in PP1 activity driving mitotic M-phase. Evidence from a variety of experimental systems also points to roles in meiosis. Here, we report that PP1 is important for M-phase transitions through mouse oocyte meiosis. We employed a unique small-molecule approach to inhibit or activate PP1 at distinct phases of mouse oocyte meiosis. These studies show that temporal control of PP1 activity is essential for the G2/M transition, metaphase I/anaphase I transition, and the formation of a normal metaphase II oocyte. Our data also reveal that inappropriate activation of PP1 is more deleterious at the G2/M transition than at prometaphase I-to-metaphase I, and that an active pool of PP1 during prometaphase is vital for metaphase I/anaphase I transition and metaphase II chromosome alignment. Taken together, these results establish that loss of oscillations in PP1 activity causes a range of severe meiotic defects, pointing to essential roles for PP1 in female fertility, and more broadly, M-phase regulation.

Nuclear lamin A/C phosphorylation by loss of Androgen Receptor is a global determinant of cancer-associated fibroblast activation

Alterations of nuclear structure and function, and associated impact on gene transcription, are a hallmark of cancer cells. Little is known of these alterations in Cancer-Associated Fibroblasts (CAFs), a key component of the tumor stroma. Here we show that loss of androgen receptor (AR), which triggers early steps of CAF activation in human dermal fibroblasts (HDFs), leads to nuclear membrane alterations and increased micronuclei formation, which are unlinked from induction of cellular senescence. Similar alterations occur in fully established CAFs, which are overcome by restored AR function. AR associates with nuclear lamin A/C and loss of AR results in a substantially increased lamin A/C nucleoplasmic redistribution. Mechanistically, AR functions as a bridge between lamin A/C with the protein phosphatase PPP1. In parallel with a decreased lamin-PPP1 association, AR loss results in a marked increase of lamin A/C phosphorylation at Ser 301, which is also a feature of CAFs. Phosphorylated lamin A/C at Ser 301 binds to the transcription promoter regulatory region of several CAF effector genes, which are upregulated due to the loss of AR. More directly, expression of a lamin A/C Ser301 phosphomimetic mutant alone is sufficient to convert normal fibroblasts into tumor-promoting CAFs of the myofibroblast subtype, without an impact on senescence. These findings highlight the pivotal role of the AR-lamin A/C-PPP1 axis and lamin A/C phosphorylation at Ser 301 in driving CAF activation.

Serine/Threonine Protein Phosphatases 1 and 2A in Lung Endothelial Barrier Regulation

Vascular barrier dysfunction is characterized by increased permeability and inflammation of endothelial cells (ECs), which are prominent features of acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and sepsis, and a major complication of the SARS-CoV-2 infection and COVID-19. Functional impairment of the EC barrier and accompanying inflammation arises due to microbial toxins and from white blood cells of the lung as part of a defensive action against pathogens, ischemia-reperfusion or blood product transfusions, and aspiration syndromes-based injury. A loss of barrier function results in the excessive movement of fluid and macromolecules from the vasculature into the interstitium and alveolae resulting in pulmonary edema and collapse of the architecture and function of the lungs, and eventually culminates in respiratory failure. Therefore, EC barrier integrity, which is heavily dependent on cytoskeletal elements (mainly actin filaments, microtubules (MTs), cell-matrix focal adhesions, and intercellular junctions) to maintain cellular contacts, is a critical requirement for the preservation of lung function. EC cytoskeletal remodeling is regulated, at least in part, by Ser/Thr phosphorylation/dephosphorylation of key cytoskeletal proteins. While a large body of literature describes the role of phosphorylation of cytoskeletal proteins on Ser/Thr residues in the context of EC barrier regulation, the role of Ser/Thr dephosphorylation catalyzed by Ser/Thr protein phosphatases (PPases) in EC barrier regulation is less documented. Ser/Thr PPases have been proposed to act as a counter-regulatory mechanism that preserves the EC barrier and opposes EC contraction. Despite the importance of PPases, our knowledge of the catalytic and regulatory subunits involved, as well as their cellular targets, is limited and under-appreciated. Therefore, the goal of this review is to discuss the role of Ser/Thr PPases in the regulation of lung EC cytoskeleton and permeability with special emphasis on the role of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A) as major mammalian Ser/Thr PPases. Importantly, we integrate the role of PPases with the structural dynamics of the cytoskeleton and signaling cascades that regulate endothelial cell permeability and inflammation.

DARPP-32/protein phosphatase 1 regulates Rasgrp2 as a novel component of dopamine D1 receptor signaling in striatum

Dopamine regulates psychomotor function by D1 receptor/PKA-dependent phosphorylation of DARPP-32. DARPP-32, phosphorylated at Thr34 by PKA, inhibits protein phosphatase 1 (PP1), and amplifies the phosphorylation of other PKA/PP1 substrates following D1 receptor activation. In addition to the D1 receptor/PKA/DARPP-32 signaling pathway, D1 receptor stimulation is known to activate Rap1/ERK signaling. Rap1 activation is mediated through the phosphorylation of Rasgrp2 (guanine nucleotide exchange factor; activation) and Rap1gap (GTPase-activating protein; inhibition) by PKA. In this study, we investigated the role of PP1 inhibition by phospho-Thr34 DARPP-32 in the D1 receptor-induced phosphorylation of Rasgrp2 and Rap1gap at PKA sites. The analyses in striatal and NAc slices from wild-type and DARPP-32 knockout mice revealed that the phosphorylation of Rasgrp2 at Ser116/Ser117 and Ser586, but not of Rasgrp2 at Ser554 or Rap1gap at Ser441 or Ser499 induced by a D1 receptor agonist, is under the control of the DARPP-32/PP1. The results were supported by pharmacological analyses using a selective PP1 inhibitor, tautomycetin. In addition, analyses using a PP1 and PP2A inhibitor, okadaic acid, revealed that all sites of Rasgrp2 and Rap1gap were regulated by PP2A. Thus, the interactive machinery of DARPP-32/PP1 may contribute to efficient D1 receptor signaling via Rasgrp2/Rap1 in the striatum.

Midbody Proteins Display Distinct Dynamics during Cytokinesis

The midbody is an organelle that forms between the two daughter cells during cytokinesis. It co-ordinates the abscission of the nascent daughter cells and is composed of a multitude of proteins that are meticulously arranged into distinct temporal and spatial localization patterns. However, very little is known about the mechanisms that regulate the localization and function of midbody proteins. Here, we analyzed the temporal and spatial profiles of key midbody proteins during mitotic exit under normal conditions and after treatment with drugs that affect phosphorylation and proteasome-mediated degradation to decipher the impacts of post-translational modifications on midbody protein dynamics. Our results highlighted that midbody proteins show distinct spatio-temporal dynamics during mitotic exit and cytokinesis that depend on both ubiquitin-mediated proteasome degradation and phosphorylation/de-phosphorylation. They also identified two discrete classes of midbody proteins: ‘transient’ midbody proteins—including Anillin, Aurora B and PRC1—which rapidly accumulate at the midbody after anaphase onset and then slowly disappear, and ‘stable’ midbody proteins—including CIT-K, KIF14 and KIF23—which instead persist at the midbody throughout cytokinesis and also post abscission. These two classes of midbody proteins display distinct interaction networks with ubiquitylation factors, which could potentially explain their different dynamics and stability during cytokinesis.

A bidirectional switch in the Shank3 phosphorylation state biases synapses toward up- or downscaling

Homeostatic synaptic plasticity requires widespread remodeling of synaptic signaling and scaffolding networks, but the role of post-translational modifications in this process has not been systematically studied. Using deep-scale quantitative analysis of the phosphoproteome in mouse neocortical neurons, we found widespread and temporally complex changes during synaptic scaling up and down. We observed 424 bidirectionally modulated phosphosites that were strongly enriched for synapse-associated proteins, including S1539 in the autism spectrum disorder-associated synaptic scaffold protein Shank3. Using a parallel proteomic analysis performed on Shank3 isolated from rat neocortical neurons by immunoaffinity, we identified two sites that were persistently hypophosphorylated during scaling up and transiently hyperphosphorylated during scaling down: one (rat S1615) that corresponded to S1539 in mouse, and a second highly conserved site, rat S1586. The phosphorylation status of these sites modified the synaptic localization of Shank3 during scaling protocols, and dephosphorylation of these sites via PP2A activity was essential for the maintenance of synaptic scaling up. Finally, phosphomimetic mutations at these sites prevented scaling up but not down, while phosphodeficient mutations prevented scaling down but not up. These mutations did not impact baseline synaptic strength, indicating that they gate, rather than drive, the induction of synaptic scaling. Thus, an activity-dependent switch between hypo- and hyperphosphorylation at S1586 and S1615 of Shank3 enables scaling up or down, respectively. Collectively, our data show that activity-dependent phosphoproteome dynamics are important for the functional reconfiguration of synaptic scaffolds and can bias synapses toward upward or downward homeostatic plasticity.

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.

Adaptations in Hippo-Yap signaling and myofibroblast fate underlie scar-free ear appendage wound healing in spiny mice

Spiny mice (Acomys cahirinus) are terrestrial mammals that evolved unique scar-free regenerative wound-healing properties. Myofibroblasts (MFs) are the major scar-forming cell type in skin. We found that following traumatic injury to ear pinnae, MFs appeared rapidly in both Acomys and mouse yet persisted only in mouse. The timing of MF loss in Acomys correlated with wound closure, blastema differentiation, and nuclear localization of the Hippo pathway target protein Yap. Experiments in vitro revealed an accelerated PP2A-dependent dephosphorylation activity that maintained nuclear Yap in Acomys dermal fibroblasts (DFs) and was not detected in mouse or human DFs. Treatment of Acomys in vivo with the nuclear Yap-TEAD inhibitor verteporfin prolonged MF persistence and converted tissue regeneration to fibrosis. Forced Yap activity prevented and rescued TGF-β1-induced human MF formation in vitro. These results suggest that Acomys evolved modifications of Yap activity and MF fate important for scar-free regenerative wound healing in vivo.

High dietary potassium causes ubiquitin-dependent degradation of the kidney sodium-chloride cotransporter

The thiazide-sensitive sodium-chloride cotransporter (NCC) in the renal distal convoluted tubule (DCT) plays a critical role in regulating blood pressure (BP) and K⁺ homeostasis. During hyperkalemia, reduced NCC phosphorylation and total NCC abundance facilitate downstream electrogenic K⁺ secretion and BP reduction. However, the mechanism for the K⁺-dependent reduction in total NCC levels is unknown. Here, we show that NCC levels were reduced in ex vivo renal tubules incubated in a high-K⁺ medium for 24–48 h. This reduction was independent of NCC transcription, but was prevented using inhibitors of the proteasome (MG132) or lysosome (chloroquine). Ex vivo, high K⁺ increased NCC ubiquitylation, but inhibition of the ubiquitin conjugation pathway prevented the high K⁺-mediated reduction in NCC protein. In tubules incubated in high K⁺ media ex vivo or in the renal cortex of mice fed a high K⁺ diet for 4 days, the abundance and phosphorylation of heat shock protein 70 (Hsp70), a key regulator of ubiquitin-dependent protein degradation and protein folding, were decreased. Conversely, in similar samples the expression of PP1α, known to dephosphorylate Hsp70, was also increased. NCC coimmunoprecipitated with Hsp70 and PP1α, and inhibiting their actions prevented the high K⁺-mediated reduction in total NCC levels. In conclusion, we show that hyperkalemia drives NCC ubiquitylation and degradation via a PP1α-dependent process facilitated by Hsp70. This mechanism facilitates K⁺-dependent reductions in NCC to protect plasma K⁺ homeostasis and potentially reduces BP.

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.

Activation of the kidney sodium chloride cotransporter by the β2-adrenergic receptor agonist salbutamol increases blood pressure

The thiazide-sensitive sodium-chloride-cotransporter (NCC) in the kidney distal convoluted tubule (DCT) plays an essential role in sodium and potassium homeostasis. Here, we demonstrate that NCC activity is increased by the β2-adrenoceptor agonist salbutamol, a drug prevalently used to treat asthma. Relative to β1-adrenergic receptors, the β2-adrenergic receptors were greatly enriched in mouse DCT cells. In mice, administration of salbutamol increased NCC phosphorylation (indicating increased activity) within 30 minutes but also caused hypokalemia, which also increases NCC phosphorylation. In ex vivo kidney slices and isolated tubules, salbutamol increased NCC phosphorylation in the pharmacologically relevant range of 0.01-10 μM, an effect observed after 15 minutes and maintained at 60 minutes. Inhibition of the inwardly rectifying potassium channel (Kir) 4.1 or the downstream with-no-lysine kinases (WNKs) and STE20/SPS1-related proline alanine-rich kinase (SPAK) pathway greatly attenuated, but did not prevent, salbutamol-induced NCC phosphorylation. Salbutamol increased cAMP in tubules, kidney slices and mpkDCT cells (model of DCT). Phosphoproteomics indicated that protein phosphatase 1 (PP1) was a key upstream regulator of salbutamol effects. A role for PP1 and the PP1 inhibitor 1 (I1) was confirmed in tubules using inhibitors of PP1 or kidney slices from I1 knockout mice. On normal and high salt diets, salbutamol infusion increased systolic blood pressure, but this increase was normalized by thiazide suggesting a role for NCC. Thus, β2-adrenergic receptor signaling modulates NCC activity via I1/PP1 and WNK-dependent pathways, and chronic salbutamol administration may be a risk factor for hypertension.

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.

Phosphorylation and dephosphorylation of Ser852 and Ser889 control the clustering, localization and function of PAR3

Cell polarity is essential for various asymmetric cellular events, and the partitioning defective (PAR) protein PAR3 (encoded by PARD3 in mammals) plays a unique role as a cellular landmark to establish polarity. In epithelial cells, PAR3 localizes at the subapical border, such as the tight junction in vertebrates, and functions as an apical determinant. Although we know a great deal about the regulators of PAR3 localization, how PAR3 is concentrated and localized to a specific membrane domain remains an important question to be clarified. In this study, we demonstrate that ASPP2 (also known as TP53BP2), which controls PAR3 localization, links PAR3 and protein phosphatase 1 (PP1). The ASPP2-PP1 complex dephosphorylates a novel phosphorylation site, Ser852, of PAR3. Furthermore, Ser852- or Ser889-unphosphorylatable PAR3 mutants form protein clusters, and ectopically localize to the lateral membrane. Concomitance of clustering and ectopic localization suggests that PAR3 localization is a consequence of local clustering. We also demonstrate that unphosphorylatable forms of PAR3 exhibited a low molecular turnover and failed to coordinate rapid reconstruction of the tight junction, supporting that both the phosphorylated and dephosphorylated states are essential for the functional integrity of PAR3.

The Cell Cycle Checkpoint System MAST(L)-ENSA/ARPP19-PP2A is Targeted by cAMP/PKA and cGMP/PKG in Anucleate Human Platelets

The cell cycle is controlled by microtubule-associated serine/threonine kinase-like (MASTL), which phosphorylates the cAMP-regulated phosphoproteins 19 (ARPP19) at S62 and 19e/α-endosulfine (ENSA) at S67and converts them into protein phosphatase 2A (PP2A) inhibitors. Based on initial proteomic data, we hypothesized that the MASTL-ENSA/ARPP19-PP2A pathway, unknown until now in platelets, is regulated and functional in these anucleate cells. We detected ENSA, ARPP19 and various PP2A subunits (including seven different PP2A B-subunits) in proteomic studies of human platelets. ENSA-S109/ARPP19–S104 were efficiently phosphorylated in platelets treated with cAMP- (iloprost) and cGMP-elevating (NO donors/riociguat) agents. ENSA-S67/ARPP19-S62 phosphorylations increased following PP2A inhibition by okadaic acid (OA) in intact and lysed platelets indicating the presence of MASTL or a related protein kinase in human platelets. These data were validated with recombinant ENSA/ARPP19 and phospho-mutants using recombinant MASTL, protein kinase A and G. Both ARPP19 phosphorylation sites S62/S104 were dephosphorylated by platelet PP2A, but only S62-phosphorylated ARPP19 acted as PP2A inhibitor. Low-dose OA treatment of platelets caused PP2A inhibition, diminished thrombin-stimulated platelet aggregation and increased phosphorylation of distinct sites of VASP, Akt, p38 and ERK1/2 MAP kinases. In summary, our data establish the entire MASTL(like)–ENSA/ARPP19–PP2A pathway in human platelets and important interactions with the PKA, MAPK and PI3K/Akt systems.

Dynamic Phosphorylation and Dephosphorylation of Cyclase-Associated Protein 1 by Antagonistic Signaling through Cyclin-Dependent Kinase 5 and cAMP Are Critical for the Protein Functions in Actin Filament Disassembly and Cell Adhesion

Cyclase-associated protein 1 (CAP1) is a conserved actin-regulating protein that enhances actin filament dynamics and also regulates adhesion in mammalian cells. We previously found that phosphorylation at the Ser307/Ser309 tandem site controls its association with cofilin and actin and is important for CAP1 to regulate the actin cytoskeleton. Here, we report that transient Ser307/Ser309 phosphorylation is required for CAP1 function in both actin filament disassembly and cell adhesion. Both the phosphomimetic and the nonphosphorylatable CAP1 mutant, which resist transition between phosphorylated and dephosphorylated forms, had defects in rescuing the reduced rate of actin filament disassembly in the CAP1 knockdown HeLa cells. The phosphorylation mutants also had defects in alleviating the elevated focal adhesion kinase (FAK) activity and the enhanced focal adhesions in the knockdown cells. In dissecting further phosphoregulatory cell signals for CAP1, we found that cyclin-dependent kinase 5 (CDK5) phosphorylates both Ser307 and Ser309 residues, whereas cAMP signaling induces dephosphorylation at the tandem site, through its effectors protein kinase A (PKA) and exchange proteins directly activated by cAMP (Epac). No evidence supports an involvement of activated protein phosphatase in executing the dephosphorylation downstream from cAMP, whereas preventing CAP1 from accessing its kinase CDK5 appears to underlie CAP1 dephosphorylation induced by cAMP. Therefore, this study provides direct cellular evidence that transient phosphorylation is required for CAP1 functions in both actin filament turnover and adhesion, and the novel mechanistic insights substantially extend our knowledge of the cell signals that function in concert to regulate CAP1 by facilitating its transient phosphorylation.

Structural Mechanism of Regioselectivity in an Unusual Bacterial Acyl-CoA Dehydrogenase

Terminal alkenes are easily derivatized, making them desirable functional group targets for polyketide synthase (PKS) engineering. However, they are rarely encountered in natural PKS systems. One mechanism for terminal alkene formation in PKSs is through the activity of an acyl-CoA dehydrogenase (ACAD). Herein, we use biochemical and structural analysis to understand the mechanism of terminal alkene formation catalyzed by an γ,δ-ACAD from the biosynthesis of the polyketide natural product FK506, TcsD. While TcsD is homologous to canonical α,β-ACADs, it acts regioselectively at the γ,δ-position and only on α,β-unsaturated substrates. Furthermore, this regioselectivity is controlled by a combination of bulky residues in the active site and a lateral shift in the positioning of the FAD cofactor within the enzyme. Substrate modeling suggests that TcsD utilizes a novel set of hydrogen bond donors for substrate activation and positioning, preventing dehydrogenation at the α,β position of substrates. From the structural and biochemical characterization of TcsD, key residues that contribute to regioselectivity and are unique to the protein family were determined and used to identify other putative γ,δ-ACADs that belong to diverse natural product biosynthetic gene clusters. These predictions are supported by the demonstration that a phylogenetically distant homologue of TcsD also regioselectively oxidizes α,β-unsaturated substrates. This work exemplifies a powerful approach to understand unique enzymatic reactions and will facilitate future enzyme discovery, inform enzyme engineering, and aid natural product characterization efforts.

Checkpoint signaling and error correction require regulation of the MPS1 T-loop by PP2A-B56

During mitosis, the formation of microtubule-kinetochore attachments is monitored by the serine/threonine kinase monopolar spindle 1 (MPS1). MPS1 is recruited to unattached kinetochores where it phosphorylates KNL1, BUB1, and MAD1 to initiate the spindle assembly checkpoint. This arrests the cell cycle until all kinetochores have been stably captured by microtubules. MPS1 also contributes to the error correction process rectifying incorrect kinetochore attachments. MPS1 activity at kinetochores requires autophosphorylation at multiple sites including threonine 676 in the activation segment or "T-loop." We now demonstrate that the BUBR1-bound pool of PP2A-B56 regulates MPS1 T-loop autophosphorylation and hence activation status in mammalian cells. Overriding this regulation using phosphomimetic mutations in the MPS1 T-loop to generate a constitutively active kinase results in a prolonged mitotic arrest with continuous turnover of microtubule-kinetochore attachments. Dynamic regulation of MPS1 catalytic activity by kinetochore-localized PP2A-B56 is thus critical for controlled MPS1 activity and timely cell cycle progression.

Synthetic Studies toward the Total Synthesis of Tautomycetin

The studies culminating in the synthesis of two large subunits of tautomycetin are described. The first one, fragment C1-C12 that has an anti-1,3-dimethyl system and a terminal diene unit, was accomplished in 10 linear steps in 7.4% overall yield. The second one, fragment C13-C25 which bears the sensitive anhydride framework and the majority of the stereogenic centers, was prepared in 13 linear steps (longest sequence) in 8% overall yield. Among the key transformations used, a regioselective epoxide opening, a Pd-catalyzed addition of terminal alkyne to acceptor alkyne, a Mukaiyama aldol reaction, a Yamaguchi esterification, and a homemade mild di-esterification can be cited. The chosen strategies allowed good yields, stereoselectivity, reproducibility, and scalability for several important intermediates.

Dok3-protein phosphatase 1 interaction attenuates Card9 signaling and neutrophil-dependent antifungal immunity

Invasive fungal infection is a serious health threat with high morbidity and mortality. Current antifungal drugs only demonstrate partial success in improving prognosis. Furthermore, mechanisms regulating host defense against fungal pathogens remain elusive. Here, we report that the downstream of kinase 3 (Dok3) adaptor negatively regulates antifungal immunity in neutrophils. Our data revealed that Dok3 deficiency increased phagocytosis, proinflammatory cytokine production, and netosis in neutrophils, thereby enhancing mutant mouse survival against systemic infection with a lethal dose of the pathogenic fungus Candida albicans. Biochemically, Dok3 recruited protein phosphatase 1 (PP1) to dephosphorylate Card9, an essential player in innate antifungal defense, to dampen downstream NF-κB and JNK activation and immune responses. Thus, Dok3 suppresses Card9 signaling, and disrupting Dok3-Card9 interaction or inhibiting PP1 activity represents therapeutic opportunities to develop drugs to combat candidaemia.

Regulation of Neuronal Na+/K+-ATPase by Specific Protein Kinases and Protein Phosphatases

The Na⁺/K⁺-ATPase (NKA) is a ubiquitous membrane-bound enzyme responsible for generating and maintaining the Na⁺ and K⁺ electrochemical gradients across the plasmalemma of living cells. Numerous studies in non-neuronal tissues have shown that this transport mechanism is reversibly regulated by phosphorylation/dephosphorylation of the catalytic α subunit and/or associated proteins. In neurons, Na⁺/K⁺ transport by NKA is essential for almost all neuronal operations, consuming up to two-thirds of the neuron's energy expenditure. However, little is known about its cellular regulatory mechanisms. Here we have used an electrophysiological approach to monitor NKA transport activity in male rat hippocampal neurons in situ We report that this activity is regulated by a balance between serine/threonine phosphorylation and dephosphorylation. Phosphorylation by the protein kinases PKG and PKC inhibits NKA activity, whereas dephosphorylation by the protein phosphatases PP-1 and PP-2B (calcineurin) reverses this effect. Given that these kinases and phosphatases serve as downstream effectors in key neuronal signaling pathways, they may mediate the coupling of primary messengers, such as neurotransmitters, hormones, and growth factors, to the NKAs, through which multiple brain functions can be regulated or dysregulated.SIGNIFICANCE STATEMENT The Na⁺/K⁺-ATPase (NKA), known as the "Na⁺ pump," is a ubiquitous membrane-bound enzyme responsible for generating and maintaining the Na⁺ and K⁺ electrochemical gradients across the plasma membrane of living cells. In neurons, as in most types of cells, the NKA generates the negative resting membrane potential, which is the basis for almost all aspects of cellular function. Here we used an electrophysiological approach to monitor physiological NKA transport activity in single hippocampal pyramidal cells in situ We have found that neuronal NKA activity is oppositely regulated by phosphorylation and dephosphorylation, and we have identified the main protein kinases and phosphatases mediating this regulation. This fundamental form of NKA regulation likely plays a role in multiple brain functions.

Dopamine receptor antagonists as potential therapeutic agents for ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is caused mostly by mutations in polycystin-1 or polycystin-2. Fluid flow leads to polycystin-dependent calcium influx and nuclear export of histone deacetylase 5 (HDAC5), which facilitates the maintenance of renal epithelial architecture by de-repression of MEF2C target genes. Here, we screened a small-molecule library to find drugs that promotes nuclear export of HDAC5. We found that dopamine receptor antagonists, domperidone and loxapine succinate, stimulate export of HDAC5, even in Pkd1^–/–cells. Domperidone targets Drd3 receptor to modulate the phosphorylation of HDAC5. Domperidone treatment increases HDAC5 phosphorylation likely by reducing protein phosphatase 2A (PP2A) activity, thus shifting the equilibrium towards HDAC5-P and export from the nucleus. Treating Pkd1^–/–mice with domperidone showed significantly reduced cystic growth and cell proliferation. Further, treated mice displayed a reduction in glomerular cyst and increased body weight and activity. These results suggest that HDAC5 nucleocytoplasmic shuttling may be modulated to impede disease progression in ADPKD and uncovers an unexpected role for a class of dopamine receptors in renal epithelial morphogenesis.

Phosphatase inhibitor PPP1R11 modulates resistance of human T cells toward Treg-mediated suppression of cytokine expression

Regulatory T cells (Tregs) act as indispensable unit for maintaining peripheral immune tolerance mainly by regulating effector T cells. T cells resistant to suppression by Tregs pose therapeutic challenges in the treatment of autoimmune diseases, while augmenting susceptibility to suppression may be desirable for cancer therapy. To understand the cell intrinsic signals in T cells during suppression by Tregs, we have previously performed a global phosphoproteomic characterization. We revealed altered phosphorylation of protein phosphatase 1 regulatory subunit 11 (PPP1R11; Inhibitor‐3) in conventional T cells upon suppression by Tregs. Here, we show that silencing of PPP1R11 renders T cells resistant toward Treg‐mediated suppression of TCR‐induced cytokine expression. Furthermore, whole‐transcriptome sequencing revealed that PPP1R11 differentially regulates not only the expression of specific T cell stimulation‐induced cytokines but also other molecules and pathways in T cells. We further confirmed the target of PPP1R11, PP1, to augment TCR‐induced cytokine expression. In conclusion, we present PPP1R11 as a novel negative regulator of T cell activation‐induced cytokine expression. Targeting PPP1R11 may have therapeutic potential to regulate the T cell activation status including modulating the susceptibility of T cells toward Treg‐mediated suppression, specifically altering the stimulation‐induced T cell cytokine milieu.

Why does tautomycetin thioesterase prefer hydrolysis to macrocyclization? Theoretical study on its catalytic mechanism

In this study, we attempted to uncover the reasons why Tautomycetin thioesterase (TMC TE) prefers hydrolysis rather than macrocyclization, and reveal the molecular basis of TE-catalyzed hydrolysis and macrocyclization.

Biochemical and Structural Characterization of TtnD, a Prenylated FMN-Dependent Decarboxylase from the Tautomycetin Biosynthetic Pathway

Tautomycetin (TTN) is a polyketide natural product featuring a terminal alkene. Functional characterization of the genes within the ttn gene cluster from Streptomyces griseochromogenes established the biosynthesis of the TTN polyketide backbone, its dialkylmaleic anhydride moiety, the coupling of the two moieties to form the nascent intermediate TTN F-1, and the tailoring steps converting TTN F-1 to TTN. Here, we report biochemical and structural characterization of TtnD, a prenylated FMN (prFMN)-dependent decarboxylase belonging to the UbiD family that catalyzes the penultimate step of TTN biosynthesis. TtnD catalyzes decarboxylation of TTN D-1 to TTN I-1, utilizing prFMN as a cofactor generated by the TtnC flavin prenyltransferase; both TtnD and TtnC are encoded within the ttn biosynthetic gene cluster. TtnD exhibits substrate promiscuity but accepts only TTN D-1 congeners that feature an α,β-unsaturated acid, supporting the [3+2] cycloaddition mechanism during catalysis that requires the double bond of an α,β-unsaturated acid substrate. TtnD shares a similar overall structure with other members of the UbiD family but forms a homotetramer in solution. Each protomer is composed of three domains with the active site located between the middle and C-terminal domains; R169-E272-E277, constituting the catalytic triad, and E228, involved in Mn(II)-mediated binding of prFMN, were confirmed by site-directed mutagenesis. TtnD represents the first example of a prFMN-dependent decarboxylase involved in polyketide biosynthesis, expanding the substrate scope of the UbiD family of decarboxylases beyond simple aromatic and cinnamic acids. TtnD and its homologues are widespread in nature and could be exploited as biocatalysts for organic synthesis.

The logic of biologically active small molecules: amazing ability of microorganisms*

In this review article, I will outline my way of thinking about biologically active small molecules. The structure of liposidomycin B from Streptomyces species resulted in my initial sense that a structure tells its function. A biologically active small molecule may save directly or indirectly a number of people. Even if the molecule has not been used as a therapeutic agent, it can be used as a useful chemical probe for dissecting a living cell into different biochemical pieces. Such biologically active small molecules derived from microorganisms have been primarily found in cultivable microorganisms that make up only 1% of total microbes in nature. Discovery of novel growth factors, zincmethylphyrin, zinc coproporphyrin, and coproporphyrin enabled laboratory cultivation of previously uncultured Leucobacter sp. These findings might expand the possibility for further discovery of novel therapeutic agents or chemical probes.

PP1:Tautomycetin Complex Reveals a Path toward the Development of PP1-Specific Inhibitors

Selective inhibitors for each serine/threonine phosphatase (PPP) are essential to investigate the biological actions of PPPs and to guide drug development. Biologically diverse organisms (e.g., cyanobacteria, dinoflagellates, beetles) produce structurally distinct toxins that are catalytic inhibitors of PPPs. However, most toxins exhibit little selectivity, typically inhibiting multiple family members with similar potencies. Thus, the use of these toxins as chemical tools to study the relationship between individual PPPs and their biological substrates, and how disruptions in these relationships contributes to human disease, is severely limited. Here, we show that tautomycetin (TTN) is highly selective for a single PPP, protein phosphatase 1 (PP1/PPP1C). Our structure of the PP1:TTN complex reveals that PP1 selectivity is defined by a covalent bond between TTN and a PP1-specific cysteine residue, Cys127. Together, these data provide key molecular insights needed for the development of novel probes targeting single PPPs, especially PP1.

Calyculin: Nature's way of making the sponge-derived cytotoxin

Covering: up to 2015.Calyculin A is a major cytotoxic compound isolated from the Japanese marine sponge Discodermia calyx. Its potent cytotoxicity is attributable to the specific inhibition of protein phosphatases 1 and 2A, as in the case of okadaic acid and the microcystins. Its chemical structure is well-designed not only for enzyme inhibition but also for higher membrane permeability in order to impart its potent cytotoxicity. The biosynthetic gene cluster of this densely functionalized polyketide and nonribosomal peptide hybrid molecule was recently identified from the sponge-microbe association. The producer organism and the dynamic bioconversion process were also revealed. In this highlight, we focus on the recent studies addressing nature's design and biogenesis of the sponge-derived cytotoxin, calyculin A.

The PP2A-B56 Phosphatase Opposes Cyclin E Autocatalytic Degradation via Site-Specific Dephosphorylation

Cyclin E, in conjunction with its catalytic partner cyclin-dependent kinase 2 (CDK2), regulates cell cycle progression as cells exit quiescence and enter S-phase. Multiple mechanisms control cyclin E periodicity during the cell cycle, including phosphorylation-dependent cyclin E ubiquitylation by the SCF^Fbw7 ubiquitin ligase. Serine 384 (S384) is the critical cyclin E phosphorylation site that stimulates Fbw7 binding and cyclin E ubiquitylation and degradation. Because S384 is autophosphorylated by bound CDK2, this presents a paradox as to how cyclin E can evade autocatalytically induced degradation in order to phosphorylate its other substrates. We found that S384 phosphorylation is dynamically regulated in cells and that cyclin E is specifically dephosphorylated at S384 by the PP2A-B56 phosphatase, thereby uncoupling cyclin E degradation from cyclin E-CDK2 activity. Furthermore, the rate of S384 dephosphorylation is high in interphase but low in mitosis. This provides a mechanism whereby interphase cells can oppose autocatalytic cyclin E degradation and maintain cyclin E-CDK2 activity while also enabling cyclin E destruction in mitosis, when inappropriate cyclin E expression is genotoxic.

Human RIF1 and protein phosphatase 1 stimulate DNA replication origin licensing but suppress origin activation

The human RIF1 protein controls DNA replication, but the molecular mechanism is largely unknown. Here, we demonstrate that human RIF1 negatively regulates DNA replication by forming a complex with protein phosphatase 1 (PP1) that limits phosphorylation-mediated activation of the MCM replicative helicase. We identify specific residues on four MCM helicase subunits that show hyperphosphorylation upon RIF1 depletion, with the regulatory N-terminal domain of MCM4 being particularly strongly affected. In addition to this role in limiting origin activation, we discover an unexpected new role for human RIF1-PP1 in mediating efficient origin licensing. Specifically, during the G1 phase of the cell cycle, RIF1-PP1 protects the origin-binding ORC1 protein from untimely phosphorylation and consequent degradation by the proteasome. Depletion of RIF1 or inhibition of PP1 destabilizes ORC1, thereby reducing origin licensing. Consistent with reduced origin licensing, RIF1-depleted cells exhibit increased spacing between active origins. Human RIF1 therefore acts as a PP1-targeting subunit that regulates DNA replication positively by stimulating the origin licensing step, and then negatively by counteracting replication origin activation.

DARPP-32 interaction with adducin may mediate rapid environmental effects on striatal neurons

Environmental enrichment has multiple effects on behaviour, including modification of responses to psychostimulant drugs mediated by striatal neurons. However, the underlying molecular and cellular mechanisms are not known. Here we show that DARPP-32, a hub signalling protein in striatal neurons, interacts with adducins, which are cytoskeletal proteins that cap actin filaments' fast-growing ends and regulate synaptic stability. DARPP-32 binds to adducin MARCKS domain and this interaction is modulated by DARPP-32 Ser97 phosphorylation. Phospho-Thr75-DARPP-32 facilitates β-adducin Ser713 phosphorylation through inhibition of a cAMP-dependent protein kinase/phosphatase-2A cascade. Caffeine or 24-h exposure to a novel enriched environment increases adducin phosphorylation in WT, but not T75A mutant mice. This cascade is implicated in the effects of brief exposure to novel enriched environment on dendritic spines in nucleus accumbens and cocaine locomotor response. Our results suggest a molecular pathway by which environmental changes may rapidly alter responsiveness of striatal neurons involved in the reward system.

Changes in environment are known to alter reward system responses, although the underlying mechanisms are unclear. Here, Engmann et al. show that DARPP-32 interacts directly with β-adducin in the mouse striatum to regulate structural and behavioural plasticity in response to novel environment and drug exposure.

Endogenous casein kinase-1 modulates NMDA receptor activity of hypothalamic presympathetic neurons and sympathetic outflow in hypertension

KEY POINTS

Increased NMDA receptor activity and excitability of presympathetic neurons in the hypothalamus can increase sympathetic nerve discharges leading to hypertension. In this study, we determined how protein kinases and phosphatases are involved in regulating NMDA receptor activity and firing activity of presympathetic neurons in the hypothalamus in normotensive and hypertensive rats. We show that casein kinase-1 inhibition increases NMDA receptor activity and excitability of presympathetic neurons in the hypothalamus and augments sympathetic nerve discharges in normotensive, but not in hypertensive, rats. Our data indicate that casein kinase-1 tonically regulates NMDA receptor activity by interacting with casein kinase-2 and protein phosphatases in the hypothalamus and that imbalance of NMDA receptor phosphorylation can augment the excitability of hypothalamic presympathetic neurons and sympathetic nerve discharges in hypertension. These findings help us understand the neuronal mechanism of hypertension, and reducing the NMDA receptor phosphorylation level may be effective for treating neurogenic hypertension.

ABSTRACT

Increased N-methyl-d-aspartate receptor (NMDAR) activity in the paraventricular nucleus (PVN) of the hypothalamus is involved in elevated sympathetic outflow in hypertension. However, the molecular mechanisms underlying augmented NMDAR activity in hypertension remain unclear. In this study, we determined the role of casein kinase-1 (CK1) in regulating NMDAR activity in the PVN. NMDAR-mediated excitatory postsynaptic currents (EPSCs) and puff NMDA-elicited currents were recorded in spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. The basal amplitudes of evoked NMDAR-EPSCs and puff NMDA currents were significantly higher in SHRs than in WKY rats. The CK1 inhibitor PF4800567 or PF670462 significantly increased the amplitude of NMDAR-EPSCs and puff NMDA currents in PVN neurons in WKY rats but not in SHRs. PF4800567 caused an NMDAR-dependent increase in the excitability of PVN neurons only in WKY rats. Also, the CK1ε protein level in the PVN was significantly lower in SHRs than in WKY rats. Furthermore, intracerebroventricular infusion of PF4800567 increased blood pressure and lumbar sympathetic nerve activity in WKY rats, and this effect was eliminated by microinjection of the NMDAR antagonist into the PVN. In addition, PF4800567 failed to increase NMDAR activity in brain slices of WKY rats pretreated with the protein phosphatase 1/2A, calcineurin, or casein kinase-2 inhibitor. Our findings suggest that CK1 tonically suppresses NMDAR activity in the PVN by reducing the NMDAR phosphorylation level. Diminished CK1 activity may contribute to potentiated glutamatergic synaptic input to PVN presympathetic neurons and elevated sympathetic vasomotor tone in neurogenic hypertension.

Tissue-specific distribution of serine/threonine protein phosphatase 1 of Toxocara canis

Serine/threonine protein phosphatase 1 (PP1) is expressed in developing and reproductively active male Toxocara canis. To investigate the tissue-specific expression of PP1 in T. canis, the PP1 protein was expressed in Escherichia coli, and the recombinant protein was used to generate a rabbit polyclonal antiserum. Indirect fluorescence immunohistochemical analysis of adult male T. canis showed that PP1 was expressed in the germ line tissues, primarily in the testis, seminal vesicle, vas deferens, and sperm cells, indicating the potential roles of PP1 in spermatogenesis. What's more, structural predictions of PP1 in T. canis were performed. The predictions of the structure indicated that PP1 may be a potential target for antihelmintic drugs. This is the first report of the tissue distributions and structural prediction of PP1 in T. canis, which might lead to the development of novel, innovative strategies for controlling T. canis infestations.

Targeting protein tyrosine phosphatase SHP2 for therapeutic intervention

Protein tyrosine phosphatases have been the focus of considerable research efforts aimed at developing novel therapeutics; however, these targets are often characterized as being ‘undruggable’ due to the challenge of achieving selectivity, potency and cell permeability. More recently, there has been renewed interest in developing inhibitors of the tyrosine phosphatase SHP2 (PTPN11) in the light of its broad role in cancer, specifically juvenile myelomonocytic leukemia, and recent studies that implicate SHP2 as a key factor in breast cancer progression. Recent significant advances in the field of SHP2 inhibitor development raise the question: are we on the verge of a new era of protein tyrosine phosphatase-directed therapeutics? This article critically appraises recent developments, assesses ongoing challenges and presents a perspective on possible future directions.

Distinct molecular components for thalamic- and cortical-dependent plasticity in the lateral amygdala

N-methyl-D-aspartate receptor (NMDAR)-dependent long-term depression (LTD) in the lateral nucleus of the amygdala (LA) is a form of synaptic plasticity thought to be a cellular substrate for the extinction of fear memory. The LA receives converging inputs from the sensory thalamus and neocortex that are weakened following fear extinction. Combining field and patch-clamp electrophysiological recordings in mice, we show that paired-pulse low-frequency stimulation can induce a robust LTD at thalamic and cortical inputs to LA, and we identify different underlying molecular components at these pathways. We show that while LTD depends on NMDARs and activation of the protein phosphatases PP2B and PP1 at both pathways, it requires NR2B-containing NMDARs at the thalamic pathway, but NR2C/D-containing NMDARs at the cortical pathway. LTD appears to be induced post-synaptically at the thalamic input but presynaptically at the cortical input, since post-synaptic calcium chelation and NMDAR blockade prevent thalamic but not cortical LTD. These results highlight distinct molecular features of LTD in LA that may be relevant for traumatic memory and its erasure, and for pathologies such as post-traumatic stress disorder (PTSD).

Phosphatase holoenzyme PP1/GADD34 negatively regulates TLR response by inhibiting TAK1 serine 412 phosphorylation

The molecular mechanisms that fine tune TLRs responses need to be fully elucidated. Protein phosphatase-1 (PP1) has been shown to be important in cell death and differentiation. However, the roles of PP1 in TLR-triggered immune response remain unclear. In this study, we demonstrate that PP1 inhibits the activation of the MAPK and NF-κB pathway and the production of TNF-α, IL-6 in macrophages triggered by TLR3, TLR4, and TLR9 in a phosphatase-dependent manner. Conversely, PP1 knockdown increases TLRs-triggered signaling and proinflammatory cytokine production. Tautomycetin, a specific inhibitor of PP1, aggravates LPS-induced endotoxin shock in mice. We further demonstrate that PP1 negatively regulates TLR-triggered signaling by targeting TGF-β-activated kinase 1 (TAK1) serine 412 (Ser412) phosphorylation, which is required for activation of TAK1-mediated IL-1R and TLR signaling. Mutation of TAK1 Serine 412 to alanine (S412A) significantly inhibits TLR/IL-1R-triggered NF-κB and MAPK activation and induction of proinflammatory cytokines in macrophage and murine embryonic fibroblast cells. DNA damage-inducible protein 34 (GADD34) specifies PP1 to dephosphorylate TAK1 at Ser412. GADD34 depletion abolished the interaction between TAK1 and PP1, and it relieved PP1 overexpression-induced inhibition of TLRs signaling and proinflammatory cytokine production. In addition, knockdown of GADD34 significantly promotes TLR-induced TAK1 Ser412 phosphorylation, downstream NF-κB and MAPK activation, and proinflammatory cytokine production. Therefore, PP1, as a physiologic inhibitor, together with its regulatory subunit GADD34, tightly controls TLR-induced TAK1 Ser412 phosphorylation, preventing excessive activation of TLRs and protecting the host from overwhelmed inflammatory immune responses.

Greatwall-phosphorylated Endosulfine is both an inhibitor and a substrate of PP2A-B55 heterotrimers

During M phase, Endosulfine (Endos) family proteins are phosphorylated by Greatwall kinase (Gwl), and the resultant pEndos inhibits the phosphatase PP2A-B55, which would otherwise prematurely reverse many CDK-driven phosphorylations. We show here that PP2A-B55 is the enzyme responsible for dephosphorylating pEndos during M phase exit. The kinetic parameters for PP2A-B55's action on pEndos are orders of magnitude lower than those for CDK-phosphorylated substrates, suggesting a simple model for PP2A-B55 regulation that we call inhibition by unfair competition. As the name suggests, during M phase PP2A-B55's attention is diverted to pEndos, which binds much more avidly and is dephosphorylated more slowly than other substrates. When Gwl is inactivated during the M phase-to-interphase transition, the dynamic balance changes: pEndos dephosphorylated by PP2A-B55 cannot be replaced, so the phosphatase can refocus its attention on CDK-phosphorylated substrates. This mechanism explains simultaneously how PP2A-B55 and Gwl together regulate pEndos, and how pEndos controls PP2A-B55. DOI: http://dx.doi.org/10.7554/eLife.01695.001.

TNF-α downregulates inhibitory neurotransmission through protein phosphatase 1-dependent trafficking of GABA(A) receptors

Inflammation has been implicated in the progression of neurological disease, yet precisely how inflammation affects neuronal function remains unclear. Tumor necrosis factor-α (TNFα) is a proinflammatory cytokine that regulates synapse function by controlling neurotransmitter receptor trafficking and homeostatic synaptic plasticity. Here we characterize the mechanisms through which TNFα regulates inhibitory synapse function in mature rat and mouse hippocampal neurons. Acute application of TNFα induces a rapid and persistent decrease of inhibitory synaptic strength and downregulation of cell-surface levels of GABA(A)Rs containing α1, α2, β2/3, and γ2 subunits. We show that trafficking of GABA(A)Rs in response to TNFα is mediated by neuronally expressed TNF receptor 1 and requires activation of p38 MAPK, phosphatidylinositol 3-kinase, protein phosphatase 1 (PP1), and dynamin GTPase. Furthermore, TNFα enhances the association of PP1 with GABA(A)R β3 subunits and dephosphorylates a site on β3 known to regulate phospho-dependent interactions with the endocytic machinery. Conversely, we find that calcineurin and PP2A are not essential components of the signaling pathway and that clustering of the scaffolding protein gephyrin is only reduced after the initial receptor endocytosis. Together, these findings demonstrate a distinct mechanism of regulated GABA(A)R endocytosis that may contribute to the disruption of circuit homeostasis under neuroinflammatory conditions.

A novel effect of MARCKS phosphorylation by activated PKC: the dephosphorylation of its serine 25 in chick neuroblasts

MARCKS (Myristoylated Alanine-Rich C Kinase Substrate) is a peripheral membrane protein, especially abundant in the nervous system, and functionally related to actin organization and Ca-calmodulin regulation depending on its phosphorylation by PKC. However, MARCKS is susceptible to be phosphorylated by several different kinases and the possible interactions between these phosphorylations have not been fully studied in intact cells. In differentiating neuroblasts, as well as some neurons, there is at least one cell-type specific phosphorylation site: serine 25 (S25) in the chick. We demonstrate here that S25 is included in a highly conserved protein sequence which is a Cdk phosphorylatable region, located far away from the PKC phosphorylation domain. S25 phosphorylation was inhibited by olomoucine and roscovitine in neuroblasts undergoing various states of cell differentiation in vitro. These results, considered in the known context of Cdks activity in neuroblasts, suggest that Cdk5 is the enzyme responsible for this phosphorylation. We find that the phosphorylation by PKC at the effector domain does not occur in the same molecules that are phosphorylated at serine 25. The in situ analysis of the subcellular distribution of these two phosphorylated MARCKS variants revealed that they are also segregated in different protein clusters. In addition, we find that a sustained stimulation of PKC by phorbol-12-myristate-13-acetate (PMA) provokes the progressive disappearance of phosphorylation at serine 25. Cells treated with PMA, but in the presence of several Ser/Thr phosphatase (PP1, PP2A and PP2B) inhibitors indicated that this dephosphorylation is achieved via a phosphatase 2A (PP2A) form. These results provide new evidence regarding the existence of a novel consequence of PKC stimulation upon the phosphorylated state of MARCKS in neural cells, and propose a link between PKC and PP2A activity on MARCKS.

Hyperphosphorylation of polycystin-2 at a critical residue in disease reveals an essential role for polycystin-1-regulated dephosphorylation

Mutations in PKD1 (85%) or PKD2 (15%) account for almost all cases of autosomal dominant polycystic kidney disease (ADPKD). The ADPKD proteins, termed as polycystin-1 (PC1) and polycystin-2 (PC2), interact via their C-termini to form a receptor-ion channel complex whose function and regulation are not fully understood. Here, we report the first phosphorylated residue (Ser(829)) in PC2, whose dephosphorylation is mediated by PC1 binding through the recruitment of protein phosphatase-1 alpha (PP1α). Using a new phosphospecific antibody (pPC2) to this site, we demonstrate that Ser(829) is phosphorylated by Protein kinase A (PKA) but remains constitutively phosphorylated in cells and tissues lacking PC1. cAMP increased pSer(829) basolateral localization in MDCK cells in a time dependent manner and was essential for pronephric development in Xenopus embryos. When constitutively expressed, a complex phenotype associated with enhanced ATP-dependent ER Ca(2+) release and loss of growth suppression was observed in cycling cells. These results reveal a reciprocal functional link between PC1 and PC2 which is critically dependent on their interaction. Unopposed cAMP stimulated hyperphosphorylation of PC2 in the absence of functional PC1 could contribute to cyst initiation in PKD1 patients and represents a new molecular paradigm in understanding ADPKD pathogenesis.