Protein phosphatase type-1, not type-2A, modulates actin microfilament integrity and myosin light chain phosphorylation in living nonmuscle cells

Dysregulated Signaling at Postsynaptic Density: A Systematic Review and Translational Appraisal for the Pathophysiology, Clinics, and Antipsychotics' Treatment of Schizophrenia

Emerging evidence from genomics, post-mortem, and preclinical studies point to a potential dysregulation of molecular signaling at postsynaptic density (PSD) in schizophrenia pathophysiology. The PSD that identifies the archetypal asymmetric synapse is a structure of approximately 300 nm in diameter, localized behind the neuronal membrane in the glutamatergic synapse, and constituted by more than 1000 proteins, including receptors, adaptors, kinases, and scaffold proteins. Furthermore, using FASS (fluorescence-activated synaptosome sorting) techniques, glutamatergic synaptosomes were isolated at around 70 nm, where the receptors anchored to the PSD proteins can diffuse laterally along the PSD and were stabilized by scaffold proteins in nanodomains of 50-80 nm at a distance of 20-40 nm creating "nanocolumns" within the synaptic button. In this context, PSD was envisioned as a multimodal hub integrating multiple signaling-related intracellular functions. Dysfunctions of glutamate signaling have been postulated in schizophrenia, starting from the glutamate receptor's interaction with scaffolding proteins involved in the N-methyl-D-aspartate receptor (NMDAR). Despite the emerging role of PSD proteins in behavioral disorders, there is currently no systematic review that integrates preclinical and clinical findings addressing dysregulated PSD signaling and translational implications for antipsychotic treatment in the aberrant postsynaptic function context. Here we reviewed a critical appraisal of the role of dysregulated PSD proteins signaling in the pathophysiology of schizophrenia, discussing how antipsychotics may affect PSD structures and synaptic plasticity in brain regions relevant to psychosis.

Spinophilin modulates pain through suppressing dendritic spine morphogenesis via negative control of Rac1-ERK signaling in rat spinal dorsal horn

Both spinophilin (SPN, also known as neurabin 2) and Rac1 (a member of Rho GTPase family) are believed to play key roles in dendritic spine (DS) remodeling and spinal nociception. However, how SPN interacts with Rac1 in the above process is unknown. Here, we first demonstrated natural existence of SPN-protein phosphatase 1-Rac1 complex in the spinal dorsal horn (DH) neurons by both double immunofluorescent labeling and co-immunoprecipitation, then the effects of SPN over-expression and down-regulation on mechanical and thermal pain sensitivity, GTP-bound Rac1-ERK signaling activity, and spinal DS density were studied. Over-expression of SPN in spinal neurons by intra-DH pAAV-CMV-SPN-3FLAG could block both mechanical and thermal pain hypersensitivity induced by intraplantar bee venom injection, however it had no effect on the basal pain sensitivity. Over-expression of SPN also resulted in a significant decrease in GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-MCS). In sharp contrast, knockdown of SPN in spinal neurons by intra-DH pAAV-CAG-eGFP-U6-shRNA[SPN] produced both pain hypersensitivity and dramatic elevation of GTP-Rac1-ERK activities, relative to naive and irrelevant control (pAAV-shRNA [NC]). Moreover, knockdown of SPN resulted in increase in DS density while over-expression of it had no such effect. Collectively, SPN is likely to serve as a regulator of Rac1 signaling to suppress DS morphogenesis via negative control of GTP-bound Rac1-ERK activities at postsynaptic component in rat DH neurons wherein both mechanical and thermal pain sensitivity are controlled.

Protein Serine/Threonine Phosphatases: Keys to Unlocking Regulators and Substrates

Protein serine/threonine phosphatases (PPPs) are ancient enzymes, with distinct types conserved across eukaryotic evolution. PPPs are segregated into types primarily on the basis of the unique interactions of PPP catalytic subunits with regulatory proteins. The resulting holoenzymes dock substrates distal to the active site to enhance specificity. This review focuses on the subunit and substrate interactions for PPP that depend on short linear motifs. Insights about these motifs from structures of holoenzymes open new opportunities for computational biology approaches to elucidate PPP networks. There is an expanding knowledge base of posttranslational modifications of PPP catalytic and regulatory subunits, as well as of their substrates, including phosphorylation, acetylation, and ubiquitination. Cross talk between these posttranslational modifications creates PPP-based signaling. Knowledge of PPP complexes, signaling clusters, as well as how PPPs communicate with each other in response to cellular signals should unlock the doors to PPP networks and signaling "clouds" that orchestrate and coordinate different aspects of cell physiology.

Dendritic spine alterations in schizophrenia

Schizophrenia is a chronic illness affecting approximately 0.5-1% of the world's population. The etiology of schizophrenia is complex, including multiple genes, and contributing environmental effects that adversely impact neurodevelopment. Nevertheless, a final common result, present in many subjects with schizophrenia, is impairment of pyramidal neuron dendritic morphology in multiple regions of the cerebral cortex. In this review, we summarize the evidence of reduced dendritic spine density and other dendritic abnormalities in schizophrenia, evaluate current data that informs the neurodevelopment timing of these impairments, and discuss what is known about possible upstream sources of dendritic spine loss in this illness.

Ethanol disrupts intestinal epithelial tight junction integrity through intracellular calcium-mediated Rho/ROCK activation

Evidence indicates that ethanol-induced intestinal barrier dysfunction and subsequent endotoxemia plays a key role in the pathogenesis of alcoholic liver disease. Recently, it has been demonstrated that ethanol induces RhoA kinase activation in intestinal epithelium, thereby disrupting barrier integrity. In this study, the role of a rise in intracellular calcium concentration ([Ca(2+)]i) in ethanol-induced Rho-associated coiled coil-forming kinase (Rho/ROCK) activation and barrier disruption was investigated in Caco-2 cell monolayers. Treatment of Caco-2 monolayers with 40 mmol/l ethanol induced [Ca(2+)]i release as indicated by increased relative fluorescent units of Fluo-3 from 0.06 ± 0.02 to 2.27 ± 1.96 (P < 0.0001). Pretreatment with 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) completely inhibited the release, whereas the inositol 1,4,5-triphosphate receptor (IP3R)-antagonist, Xestospongin C, partially inhibited the ethanol-induced [Ca(2+)]i release (from 2.27 ± 1.96 to 0.03 ± 0.01; P < 0.0001 and from 2.27 ± 1.96 to 1.19 ± 1.80; P < 0.001, respectively). The rise in [Ca(2+)]i was paralleled with increased intestinal permeability, which could be attenuated by either BAPTA-AM or Xestospongin C. Furthermore, ethanol induced Rho/ROCK activation, as indicated by increased phosphorylation of myosin-binding subunit, which could be prevented either by BAPTA, Xestospongin C, or the specific Rho/ROCK inhibitor Y27632. Finally, inhibition of Rho/ROCK kinase by Y27632 ameliorated the ethanol-induced redistribution of zonula occluden-1, adherens junction proteins including E-cadherin and β-catenin, and also disorganization of F-actin. These findings suggest that ethanol-induced [Ca(2+)]i release, mediated by stimulating IP3R-gated Ca(2+) channel, activates Rho/ROCK in Caco-2 cells, thereby contributing to ethanol-induced intestinal barrier dysfunction.

Cell- and virus-mediated regulation of the barrier-to-autointegration factor's phosphorylation state controls its DNA binding, dimerization, subcellular localization, and antipoxviral activity

Barrier-to-autointegration factor (BAF) is a DNA binding protein with multiple cellular functions, including the ability to act as a potent defense against vaccinia virus infection. This antiviral function involves BAF's ability to condense double-stranded DNA and subsequently prevent viral DNA replication. In recent years, it has become increasingly evident that dynamic phosphorylation involving the vaccinia virus B1 kinase and cellular enzymes is likely a key regulator of multiple BAF functions; however, the precise mechanisms are poorly understood. Here we analyzed how phosphorylation impacts BAF's DNA binding, subcellular localization, dimerization, and antipoxviral activity through the characterization of BAF phosphomimetic and unphosphorylatable mutants. Our studies demonstrate that increased phosphorylation enhances BAF's mobilization from the nucleus to the cytosol, while dephosphorylation restricts BAF to the nucleus. Phosphorylation also impairs both BAF's dimerization and its DNA binding activity. Furthermore, our studies of BAF's antiviral activity revealed that hyperphosphorylated BAF is unable to suppress viral DNA replication or virus production. Interestingly, the unphosphorylatable BAF mutant, which is capable of binding DNA but localizes predominantly to the nucleus, was also incapable of suppressing viral replication. Thus, both DNA binding and localization are important determinants of BAF's antiviral function. Finally, our examination of how phosphatases are involved in regulating BAF revealed that PP2A dephosphorylates BAF during vaccinia infection, thus counterbalancing the activity of the B1 kinase. Altogether, these data demonstrate that phosphoregulation of BAF by viral and cellular enzymes modulates this protein at multiple molecular levels, thus determining its effectiveness as an antiviral factor and likely other functions as well.

IMPORTANCE

The barrier-to-autointegration factor (BAF) contributes to cellular genomic integrity in multiple ways, the best characterized of which are as a host defense against cytoplasmic DNA and as a regulator of mitotic nuclear reassembly. Although dynamic phosphorylation involving both viral and cellular enzymes is likely a key regulator of multiple BAF functions, the precise mechanisms involved are poorly understood. Here we demonstrate that phosphorylation coordinately regulates BAF's DNA binding, subcellular localization, dimerization, and antipoxviral activity. Overall, our findings provide new insights into how phosphoregulation of BAF modulates this protein at multiple levels and governs its effectiveness as an antiviral factor against foreign DNA.

Phactr4: a new integrin modulator required for directional migration of enteric neural crest cells

The enteric nervous system (ENS) is critically important for many intestinal functions such as peristalsis and secretion. Defects in the embryonic formation of the ENS cause Hirschsprung disease (HSCR) or megacolon, a severe birth defect that affects approximately 1 in 5,000 newborns. One of the least understood aspects of ENS development are the cellular and molecular mechanisms that control chain migration of the ENS cells during their migration into and along the embryonic gut. We recently reported a mouse model of HSCR in which mutant embryos carrying a hypomorphic allele of the Phactr4 gene show an embryonic gastrointestinal defect due to loss of enteric neurons in the colon. We found that Phactr4 modulates integrin signaling and cofilin activity to coordinate the forces that drive enteric neural crest cell (ENCC) migration in the mammalian embryo. In this extra view, we briefly summarize the current knowledge on integrin signaling in ENCC migration and introduce the Phactr protein family. Employing the ENS as a model, we shed some light on the mechanisms by which Phactr4 regulates integrin signaling and controls the cell polarity required for directional ENCC migration in the mouse developing gut.

Mechanisms regulating cilia growth and cilia function in endothelial cells

The primary cilium is an important sensory organelle present in most mammalian cells. Our current studies aim at examining intracellular molecules that regulate cilia length and/or cilia function in vitro and ex vivo. For the first time, we show that intracellular cAMP and cAMP-dependent protein kinase (PKA) regulate both cilia length and function in vascular endothelial cells. Although calcium-dependent protein kinase modulates cilia length, it does not play a significant role in cilia function. Cilia length regulation also involves mitogen-activated protein kinase (MAPK), protein phosphatase-1 (PP-1), and cofilin. Furthermore, cofilin regulates cilia length through actin rearrangement. Overall, our study suggests that the molecular interactions between cilia function and length can be independent of one another. Although PKA regulates both cilia length and function, changes in cilia length by MAPK, PP-1, or cofilin do not have a direct correlation to changes in cilia function. We propose that cilia length and function are regulated by distinct, yet complex intertwined signaling pathways.

Actin cytoskeleton in myofibroblast differentiation: ultrastructure defining form and driving function

Myofibroblasts are modified fibroblasts characterized by the presence of a well-developed contractile apparatus and the formation of robust actin stress fibers. These mechanically active cells are thought to orchestrate extracellular matrix remodeling during normal wound healing in response to tissue injury; these cells are found also in aberrant tissue remodeling in fibrosing disorders. This review surveys the understanding of the role of actin stress fibers in myofibroblast biology. Actin stress fibers are discussed as a defining ultrastructural and morphologic feature and well-accepted observations demonstrating its participation in contraction, focal adhesion maturation, and extracellular matrix reorganization are presented. Finally, more recent observations are reviewed, demonstrating its role in transducing mechanical force into biochemical signals, transcriptional control of genes involved in locomotion, contraction, and matrix reorganization, as well as the localized regulation of messenger RNA (mRNA) translation. This breadth of functionality of the actin stress fiber serves to reinforce and amplify its mechanical function, via induced expression of proteins that themselves augment contraction, focal adhesion formation, and matrix remodeling. In composite, the functions of the actin cytoskeleton are most often aligned, allowing for the integration and amplification of signals promoting both myofibroblast differentiation and matrix remodeling during fibrogenesis.

Spinophilin is required for normal morphology, Ca(2+) homeostasis and contraction but dispensable for β-adrenergic stimulation of adult cardiomyocytes

Spinophilin (SPN) is a ubiquitously expressed scaffolding protein that interacts through several binding modules with a variety of target proteins. Thus, SPN bundles F-actin, targets protein phosphatase 1 to the ryanodine receptor, and targets regulators of G-protein signaling to G-protein coupled receptors in cardiomyocytes. In this work we studied the role of SPN on cardiomyocyte morphology, function, and β-adrenergic responsiveness using a homozygous SPN knock-out mouse model (SPN-/-). We show that spinophilin deficiency significantly (1) reduced cardiomyocyte length, (2) increases both Ca(2+) amplitude and maximal rate of Ca(2+) rise during systole, and (3) decreased shortening amplitude and maximal rate of shortening, while (4) β-adrenergic stimulation remained intact. Our data suggest that spinophilin is an upstream regulator required for normal growth and excitation-contraction coupling, but is dispensable for β-adrenergic stimulation of adult cardiomyocytes.

Scapinin, the protein phosphatase 1 binding protein, enhances cell spreading and motility by interacting with the actin cytoskeleton

Scapinin, also named phactr3, is an actin and protein phosphatase 1 (PP1) binding protein, which is expressed in the adult brain and some tumor cells. At present, the role(s) of scapinin in the brain and tumors are poorly understood. We show that the RPEL-repeat domain of scapinin, which is responsible for its direct interaction with actin, inhibits actin polymerization in vitro. Next, we established a Hela cell line, where scapinin expression was induced by tetracycline. In these cells, expression of scapinin stimulated cell spreading and motility. Scapinin was colocalized with actin at the edge of spreading cells. To explore the roles of the RPEL-repeat and PP1-binding domains, we expressed wild-type and mutant scapinins as fusion proteins with green fluorescence protein (GFP) in Cos7 cells. Expression of GFP-scapinin (wild type) also stimulated cell spreading, but mutation in the RPEL-repeat domain abolished both the actin binding and the cell spreading activity. PP1-binding deficient mutants strongly induced cell retraction. Long and branched cytoplasmic processes were developed during the cell retraction. These results suggest that scapinin enhances cell spreading and motility through direct interaction with actin and that PP1 plays a regulatory role in scapinin-induced morphological changes.

Reduced dendritic spine density in auditory cortex of subjects with schizophrenia

We have previously identified reductions in mean pyramidal cell somal volume in deep layer 3 of BA 41 and 42 and reduced axon terminal density in deep layer 3 of BA 41. In other brain regions demonstrating similar deficits, reduced dendritic spine density has also been identified, leading us to hypothesize that dendritic spine density would also be reduced in BA 41 and 42. Because dendritic spines and their excitatory inputs are regulated in tandem, we further hypothesized that spine density would be correlated with axon terminal density. We used stereologic methods to quantify a marker of dendritic spines, spinophilin-immunoreactive (SP-IR) puncta, in deep layer 3 of BA 41 and 42 of 15 subjects with schizophrenia, each matched to a normal comparison subject. The effect of long-term haloperidol exposure on SP-IR puncta density was evaluated in nonhuman primates. SP-IR puncta density was significantly lower by 27.2% in deep layer 3 of BA 41 in the schizophrenia subjects, and by 22.2% in deep layer 3 of BA 42. In both BA 41 and 42, SP-IR puncta density was correlated with a marker of axon terminal density, but not with pyramidal cell somal volume. SP-IR puncta density did not differ between haloperidol-exposed and control monkeys. Lower SP-IR puncta density in deep layer 3 of BA 41 and 42 of subjects with schizophrenia may reflect concurrent reductions in excitatory afferent input. This may contribute to impairments in auditory sensory processing that are present in subjects with schizophrenia.

Regulation of protein phosphatase type 1 and cell cycle progression by PfLRR1, a novel leucine-rich repeat protein of the human malaria parasite Plasmodium falciparum

The protein called 'suppressor of the dis2 mutant (sds22+)' is an essential regulator of cell division in fission and budding yeasts, where its deletion causes mitotic arrest. Its role in cell cycle control appears to be mediated through the activation of protein phosphatase type 1 (PP1) in Schizosaccharomyces pombe. We have identified the Plasmodium falciparum Sds22 orthologue, which we designated PfLRR1 as it belongs to the leucine-rich repeat protein family. We showed by glutathione-S-transferase pull-down assay that the PfLRR1 gene product interacts with PfPP1, that the PfLRR1-PfPP1 complex is present in parasite extracts and that PfLRR1 inhibits PfPP1 activity. Functional studies in Xenopus oocytes revealed that PfLRR1 interacted with endogenous PP1 and overcame the G2/M cell cycle checkpoint by promoting progression to germinal vesicle breakdown (GVBD). Confirmatory results showing the appearance of GVBD were observed when oocytes were treated with anti-PP1 antibodies or okadaic acid. Taken together, these observations suggest that PfLRR1 can regulate the cell cycle by binding to PP1 and regulating its activity.

Effects of S1P on myoblastic cell contraction: possible involvement of Ca-independent mechanisms

Sphingosine-1-phosphate (S1P) is a lipid mediator, which affects many essential processes such as cell proliferation, differentiation and contraction in many cell types. We have previously demonstrated that the lipid mediator elicits Ca(2+) transients in a myoblastic cell line (C2C12) by interacting with its specific receptors (S1PR(s)). In the present study, we wanted to correlate the Ca(2+) response with activation of myoblastic cell contractility. C2C12 cells were first investigated for the expression and cellular organization of cytoskeletal proteins by immunoconfocal microscopy. We found that myoblasts exhibited a quite immature cytoskeleton, with filamentous actin dispersed as a web-like structure within the cytoplasm. To evaluate intracellular Ca(2+) mobilization, the cells were loaded with a fluorescent Ca(2+) indicator (Fluo-3), stimulated with S1P and simultaneously observed with differential interference contrast and fluorescence optics. Exogenous S1P-induced myoblastic cell contraction was temporally unrelated to S1P-induced intracellular Ca(2+) increase; cell contraction occurred within 5-8 s from stimulation, whereas intracellular Ca(2+) increase was evident only after 15-25 s. To support the Ca(2+) independence of myoblastic cell contraction, the cells were pretreated with a Ca(2+) chelator, BAPTA/AM, prior to stimulation with S1P. In these experimental conditions, the myoblasts were still able to contract, whereas the S1P-induced Ca(2+) transients were completely abolished. On the contrary, when C2C12 cells were induced to differentiate into skeletal myotubes, they responded to S1P with a rapid cell contraction concurrent with an increase in the intracellular Ca(2+). These data suggest that Ca(2+)-independent mechanism of cell contraction may be replaced by Ca(2+)-dependent ones during skeletal muscle differentiation.

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.

Neurabin/protein phosphatase-1 complex regulates dendritic spine morphogenesis and maturation

The majority of excitatory synapses in the mammalian brain form on filopodia and spines, actin-rich membrane protrusions present on neuronal dendrites. The biochemical events that induce filopodia and remodel these structures into dendritic spines remain poorly understood. Here, we show that the neuronal actin- and protein phosphatase-1-binding protein, neurabin-I, promotes filopodia in neurons and nonneuronal cells. Neurabin-I actin-binding domain bundled F-actin, promoted filopodia, and delayed the maturation of dendritic spines in cultured hippocampal neurons. In contrast, dimerization of neurabin-I via C-terminal coiled-coil domains and association of protein phosphatase-1 (PP1) with neurabin-I through a canonical KIXF motif inhibited filopodia. Furthermore, the expression of a neurabin-I polypeptide unable to bind PP1 delayed the maturation of neuronal filopodia into spines, reduced the synaptic targeting of AMPA-type glutamate (GluR1) receptors, and decreased AMPA receptor-mediated synaptic transmission. Reduction of endogenous neurabin levels by interference RNA (RNAi)-mediated knockdown also inhibited the surface expression of GluR1 receptors. Together, our studies suggested that disrupting the functions of a cytoskeletal neurabin/PP1 complex enhanced filopodia and impaired surface GluR1 expression in hippocampal neurons, thereby hindering the morphological and functional maturation of dendritic spines.

p116Rip Decreases Myosin II Phosphorylation by Activating Myosin Light Chain Phosphatase and by Inactivating RhoA

p116Rip was originally found to be a RhoA-binding protein, but its function has been unknown. Here, we clarify the function of p116Rip. Two critical findings were made. First, we found that p116Rip activated the GTPase activity of RhoA in vitro and that p116Rip overexpression in cells consistently diminished the epidermal growth factor-induced increase in GTP-bound RhoA. Second, p116Rip activated the myosin light chain phosphatase (MLCP) activity of the holoenzyme. p116Rip did not activate the catalytic subunit alone, indicating that the activation is due to the binding of p116Rip to the myosin phosphatase targeting subunit MYPT1. Interestingly, the activation of phosphatase was specific to myosin as substrate, and p116Rip directly bound to myosin, thus facilitating myosin/MLCP interaction. The gene silencing of p116Rip consistently and significantly increased myosin phosphorylation as well as stress fiber formation in cells. Based upon these findings, we propose that p116Rip is an important regulatory component that controls the RhoA signaling pathway, thus regulating MLCP activity and myosin phosphorylation in cells.

Modulation of acto-myosin contractility in skeletal muscle myoblasts uncouples growth arrest from differentiation

Cell-substratum interactions trigger key signaling pathways that modulate growth control and tissue-specific gene expression. We have previously shown that abolishing adhesive interactions by suspension culture results in G0 arrest of myoblasts. We report that blocking intracellular transmission of adhesion-dependent signals in adherent cells mimics the absence of adhesive contacts. We investigated the effects of pharmacological inhibitors of acto-myosin contractility on growth and differentiation of C2C12 myogenic cells. ML7 (5-iodonaphthalene-1-sulfonyl homopiperazine) and BDM (2,3, butanedione monoxime) are specific inhibitors of myosin light chain kinase, and myosin heavy chain ATPase, respectively. ML7 and BDM affected cell shape by reducing focal adhesions and stress fibers. Both inhibitors rapidly blocked DNA synthesis in a dose-dependent, reversible fashion. Furthermore, both ML7 and BDM suppressed expression of MyoD and myogenin, induced p27kip1 but not p21cip1, and inhibited differentiation. Thus, as with suspension-arrest, inhibition of acto-myosin contractility in adherent cells led to arrest uncoupled from differentiation. Over-expression of inhibitors of the small GTPase RhoA (dominant negative RhoA and C3 transferase) mimicked the effects of myosin inhibitors. By contrast, wild-type RhoA induced arrest, maintained MyoD and activated myogenin and p21 expression. The Rho effector kinase ROCK did not appear to mediate Rho's effects on MyoD. Thus, ROCK and MLCK play different roles in the myogenic program. Signals regulated by MLCK are critical, since inhibition of MLCK suppressed MyoD expression but inhibition of ROCK did not. Inhibition of contractility suppressed MyoD but did not reduce actin polymer levels. However, actin depolymerization with latrunculin B inhibited MyoD expression. Taken together, our observations indicate that actin polymer status and contractility regulate MyoD expression. We suggest that in myoblasts, the Rho pathway and regulation of acto-myosin contractility may define a control point for conditional uncoupling of differentiation and the cell cycle.

Functional diversity of protein phosphatase-1, a cellular economizer and reset button

The protein serine/threonine phosphatase protein phosphatase-1 (PP1) is a ubiquitous eukaryotic enzyme that regulates a variety of cellular processes through the dephosphorylation of dozens of substrates. This multifunctionality of PP1 relies on its association with a host of function-specific targetting and substrate-specifying proteins. In this review we discuss how PP1 affects the biochemistry and physiology of eukaryotic cells. The picture of PP1 that emerges from this analysis is that of a "green" enzyme that promotes the rational use of energy, the recycling of protein factors, and a reversal of the cell to a basal and/or energy-conserving state. Thus PP1 promotes a shift to the more energy-efficient fuels when nutrients are abundant and stimulates the storage of energy in the form of glycogen. PP1 also enables the relaxation of actomyosin fibers, the return to basal patterns of protein synthesis, and the recycling of transcription and splicing factors. In addition, PP1 plays a key role in the recovery from stress but promotes apoptosis when cells are damaged beyond repair. Furthermore, PP1 downregulates ion pumps and transporters in various tissues and ion channels that are involved in the excitation of neurons. Finally, PP1 promotes the exit from mitosis and maintains cells in the G1 or G2 phases of the cell cycle.

Phosphorylation of spinophilin modulates its interaction with actin filaments

Spinophilin is a protein phosphatase 1 (PP1)- and actin-binding protein that modulates excitatory synaptic transmission and dendritic spine morphology. We report that spinophilin is phosphorylated in vitro by protein kinase A (PKA). Phosphorylation of spinophilin was stimulated by treatment of neostriatal neurons with a dopamine D1 receptor agonist or with forskolin, consistent with spinophilin being a substrate for PKA in intact cells. Using tryptic phosphopeptide mapping, site-directed mutagenesis, and microsequencing analysis, we identified two major sites of phosphorylation, Ser-94 and Ser-177, that are located within the actin-binding domain of spinophilin. Phosphorylation of spinophilin by PKA modulated the association between spinophilin and the actin cytoskeleton. Following subcellular fractionation, unphosphorylated spinophilin was enriched in the postsynaptic density, whereas a pool of phosphorylated spinophilin was found in the cytosol. F-actin co-sedimentation and overlay analysis revealed that phosphorylation of spinophilin reduced the stoichiometry of the spinophilin-actin interaction. In contrast, the ability of spinophilin to bind to PP1 remained unchanged. Taken together, our studies suggest that phosphorylation of spinophilin by PKA modulates the anchoring of the spinophilin-PP1 complex within dendritic spines, thereby likely contributing to the efficacy and plasticity of synaptic transmission.

Experimental Parkinsonism Modulates Multiple Genes Involved in the Transduction of Dopaminergic Signals in the Striatum

The irreversible loss of the dopamine-mediated control of striatal function is considered the functional substrate of the motor symptoms of Parkinson's disease. This pathological event causes a complex rearrangement of neuronal activity which involves specific dopamine-regulated cellular functions and, secondarily, several other cellular properties and transmitter systems. In the present study, we applied recently developed cDNA microarray technology to investigate the genetic correlates of the alterations produced by 6-hydroxydopamine-induced dopamine denervation in the nucleus striatum. We found that chronic dopamine denervation caused the modulation of 50 different genes involved in several cellular functions. In particular, products of the genes modulated by this experimental manipulation are involved both in the intracellular transduction of dopamine signal and in the regulation of glutamate transmission in striatal neurons, providing some information on the possible neuronal events which lead to the reorganization of glutamate transmission in the striatum of parkinsonian rats.

Effects of the phosphatase inhibitors, okadaic acid, ATPgammaS, and calyculin A on the dividing sand dollar egg

The effects of the phosphatase inhibitors, okadaic acid (OA), adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), and calyculin A (CL-A) on anaphase chromosome movement, cytokinesis, and cytoskeletal structures at cell division were examined by being microinjected into mitotic sand dollar eggs. When OA was injected, chromosome movement was inhibited and, moreover, chromosomes were ejected from the polar regions of the mitotic apparatus. By immunofluorescence, microtubules were observed to be severed in the OA-injected eggs, causing the smooth cell surface to be changed to an irregular surface. When ATPgammaS and CL-A were injected, the effect on cell shape was remarkable: In dividing eggs, furrowing stopped within several seconds after injection, small blebs appeared on the cell surface and became large, spherical or dumbbell cell shapes then changed to irregular forms, and subsequently cytoplasmic flow occurred. Microfilament detection revealed that actin accumulation in the cortex, which was not limited to the furrow cortex, occurred shortly after injection. Cortical accumulation of actin is thought to induce force generation and random cortical contraction, and accordingly to result in bleb extrusion from the cortex. Consequently, the phosphatase inhibitors inhibited the transition from mitosis to interphase by mediating cortical accumulation of actin filaments and/or fragmentation of microtubules.

Protein phosphatases 1 and 2A transiently associate with myosin during the peak rate of secretion from mast cells

Mast cells undergo cytoskeletal restructuring to allow secretory granules passage through the cortical actomyosin barrier to fuse with the plasma membrane and release inflammatory mediators. Protein phosphorylation is believed to regulate these rearrangements. Although some of the protein kinases implicated in this phosphorylation are known, the relevant protein phosphatases are not. At the peak rate of antigen-induced granule mediator release (2.5 min), protein phosphatases PP1 and PP2A, along with actin and myosin II, are transiently relocated to ruffles on the apical surface and a band at the peripheral edge of the cell. This leaves an area between the nucleus and the peripheral edge significantly depleted (3-5-fold) in these proteins. Phorbol 12-myristate 13-acetate (PMA) plus A23187 induces the same changes, at a time coincident with its slower rate of secretion. Coimmunoprecipitation experiments demonstrated a significantly increased association of myosin with PP1 and PP2A at the time of peak mediator release, with levels of association decreasing by 5 min. Jasplakinolide, an inhibitor of actin assembly, inhibits secretion and the cytoskeletal rearrangements. Surprisingly, jasplakinolide also affects myosin, inducing the formation of short rods throughout the cytoplasm. Inhibition of PP2A inhibited secretion, the cytoskeletal rearrangements, and led to increased phosphorylation of the myosin heavy and light chains at protein kinase C-specific sites. These findings indicate that a dynamic actomyosin cytoskeleton, partially regulated by both PP1 and PP2A, is required for mast cell secretion.

Actin filament assembly and actin-myosin contractility are necessary for anchorage- and EGF-dependent activation of phospholipase Cgamma

Formation of actin stress fibers and the focal adhesion complex between cell and the substratum are crucial for nonmalignant cells to achieve anchorage-dependent growth. We show here that the adhesion complex formed in normal human mammary epithelial (HME) cells which adhered to type IV collagen, involved the EGF receptor (EGFR) and phospholipase Cgamma (PLCgamma) as signaling molecules, in addition to integrin beta1, alpha-actinin, and actin even before stimulation of the cells with EGF. Stimulation of cells with EGF induced tyrosine phosphorylation of EGFR and activation of PLCgamma, as assessed by the production of a second messenger diacylglycerol (DAG), without any significant increase in the amount of EGFR-bound PLCgamma. Disruption of either actin filaments by cytochalasin D (CD) or actin-myosin contractility by ML-7, an inhibitor of myosin light chain kinase (MLCK), altered the flattened morphology of quiescent cells to a retracted one, without affecting the association between EGFR and PLCgamma. Stimulation of CD- or ML-7-treated cells with EGF failed to inhibit tyrosine phosphorylation of EGFR and its association and colocalization with PLCgamma, but inhibited the PLCgamma activation. Phosphatidylinositol 4,5-bisphosphate (PtdInsP2), substrate of PLCgamma, was tightly associated with alpha-actinin and the content of alpha-actinin-bound PtdInsP2 was reduced by treatment of cells with ML-7 but not with CD. The amount of PtdInsP2 bound to alpha-actinin was increased by the addition of EGF and this EGF-induced increase was blocked by either CD or ML-7. The present results suggest that anchorage-dependent EGF signaling in HME cells may require both actin filament assembly and actin-myosin contractility for the PLCgamma activation.

Protein phosphatase 2B inhibitor potentiates endothelial PKC activity and barrier dysfunction

Serine/threonine (Ser/Thr) protein phosphatases (PPs) are implicated in the recovery from endothelial barrier dysfunction caused by inflammatory mediators. We hypothesized that Ser/Thr PPs may regulate protein kinase C (PKC), a critical signaling molecule in barrier dysfunction, in the promotion of barrier recovery. Western analysis indicated that bovine pulmonary microvascular endothelial cells (BPMECs) expressed the three major Ser/Thr PPs, PP1, PP2A, and PP2B. Pretreatment with 100 ng/ml of FK506 (a PP2B inhibitor) but not with the PP1 and PP2A inhibitors calyculin A or okadaic acid potentiated the thrombin-induced increase in PKC phosphotransferase activity. FK506 also potentiated thrombin-induced PKC-alpha but not PKC-beta phosphorylation. FK506 but not calyculin A or okadaic acid inhibited recovery from the thrombin-induced decrease in transendothelial resistance. Neither FK506 nor okadaic acid altered the thrombin-induced resistance decrease, whereas calyculin A potentiated the decrease. Downregulation of PKC with phorbol 12-myristate 13-acetate rescued the FK506-mediated inhibition of recovery, which was consistent with the finding that the thrombin-induced phosphorylation of PKC-alpha was reduced during the recovery phase. These results indicated that PP2B may play a physiologically important role in returning endothelial barrier dysfunction to normal through the regulation of PKC.

Regulation of dendritic spine stability

Dendritic spines undergo several types of transformations, ranging from growth to collapse, and from elongation to shortening, and they experience dynamic morphological activity on a rapid time scale. Changes in spine number and morphology occur under pathological conditions like excitotoxicity, but also during normal central nervous system development, during hormonal fluctuations, and in response to neural activity under physiological circumstances. We briefly review evidence for various types of alterations in spines, and discuss the possible molecular basis for changes in spine stability. Filamentous actin appears to be the most important cytoskeletal component of spines, and a growing list of actin-associated and actin-regulatory proteins has been reported to reside within spines. We conclude that spines contain two distinct pools of actin filaments (one stable, the other unstable) that provide the spine with both a stable core structure and a dynamic, complex shape. Finally, we review the current state of knowledge of actin filament regulation, based on studies in nonneuronal cells.

Spinophilin regulates the formation and function of dendritic spines

Spinophilin, a protein that interacts with actin and protein phosphatase-1, is highly enriched in dendritic spines. Here, through the use of spinophilin knockout mice, we provide evidence that spinophilin modulates both glutamatergic synaptic transmission and dendritic morphology. The ability of protein phosphatase-1 to regulate the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors was reduced in spinophilin knockout mice. Consistent with altered glutamatergic transmission, spinophilin-deficient mice showed reduced long-term depression and exhibited resistance to kainate-induced seizures and neuronal apoptosis. In addition, deletion of the spinophilin gene caused a marked increase in spine density during development in vivo as well as altered filopodial formation in cultured neurons. In conclusion, spinophilin appears to be required for the regulation of the properties of dendritic spines.

Inhibition of myosin/moesin phosphatase by expression of the phosphoinhibitor protein CPI-17 alters microfilament organization and retards cell spreading

Cell migration and cytokinesis require reorganization of the cytoskeleton, involving phosphorylation and dephosphorylation of proteins such as myosin II and moesin. Myosin and moesin bind directly to a regulatory subunit of myosin/moesin phosphatase (MMP) that contains a protein type-1 phosphatase (PP1) catalytic subunit. Here we examined the role of MMP in cytoskeletal dynamics using a phosphorylation-dependent inhibitor protein specific for MMP, called CPI-17. Fibroblasts do not express CPI-17, making them a null background to study effects of expression. Wild type CPI-17 in rat embryo fibroblasts caused (1) abnormal accumulation of cortical F-actin fibers, distinct from the stress fibers induced by expression of active RhoA; (2) progressive contraction of cell area, leaving behind filamentous extensions that stained for F-actin and moesin, but not myosin; and (3) significantly retarded spreading of fibroblasts on fibronectin with elevated myosin II light chain phosphorylation. A phosphorylation site mutant CPI-17(T38A) and inhibitor-2 (Inh2), another PP1-specific inhibitor protein, served as controls and did not elicit these same responses when expressed at the same level as CPI-17. Inhibition of myosin light chain kinase by ML-9 prevented the abnormal accumulation of cortical microfilaments by CPI-17, but did not reverse shrinkage in area, whereas kinase inhibitors HA1077 and H7 prevented CPI-17-induced changes in microfilament distribution and cell contraction. These results highlight the physiological importance of myosin/moesin phosphatase regulation to dynamic remodeling of the cytoskeleton.

Effect of calyculin A on the surface structure of unfertilized sea urchin eggs

Calyculin A, a potent inhibitor of type 1 and type 2A protein phosphatases, induces contractile ring formation when applied to unfertilized sea urchin eggs [Tosuji et al., 1992: Proc. Natl. Acad. Sci. USA 89:10613-10617]. We report here the elongation of microvilli in the unfertilized eggs exposed to calyculin A. The elongated microvilli and associated sperm-egg binding sites (egg receptor for sperm) then became concentrated into a constriction site corresponding to the cleavage furrow. The egg receptor for sperm was also in close connection to the microfilaments. Okadaic acid is another known inhibitor of protein phosphatase type-1 and type-2A. Its effect, however, is about a hundredfold feebler for type-1 phosphatase than type-2A. Even after treatment with okadaic acid, no change was observed, suggesting that these morphological changes were induced by calyculin A solely though its inhibitory effect on the type-1 protein phosphatase.

Regulation of LPA-promoted myofibroblast contraction: role of Rho, myosin light chain kinase, and myosin light chain phosphatase

Myofibroblasts generate the contractile force responsible for wound healing and pathological tissue contracture. In this paper the stress-relaxed collagen lattice model was used to study lysophosphatidic acid (LPA)-promoted myofibroblast contraction and the role of the small GTPase Rho and its downstream targets Rho kinase and myosin light chain phosphatase (MLCPPase) in regulating myofibroblast contraction. In addition, the regulation of myofibroblast contraction was compared with that of smooth muscle cells. LPA-promoted myofibroblast contraction was inhibited by the myosin light chain kinase (MLCK) inhibitors KT5926 and ML-7; however, in contrast to that observed in smooth muscle cells, elevation of intracellular calcium alone was not sufficient to promote myofibroblast contraction. These results suggest that Ca(2+)-mediated activation of MLCK, while necessary, is not sufficient to promote myofibroblast contraction. The specific Rho inactivator C3-transferase and the Rho kinase inhibitor Y-27632 inhibited LPA-promoted myofibroblast contraction, suggesting that contraction depends on activation of the Rho/Rho kinase pathway. Calyculin, a type 1 phosphatase inhibitor known to inhibit MLCPPase, could promote myofibroblast contraction in the absence of LPA, as well as restore contraction in the presence of C3-transferase or Y-27632. Together these results support a model whereby Rho/Rho kinase-mediated inhibition of MLCPPase is necessary for LPA-promoted myofibroblast contraction, in contrast to smooth muscle cells in which Ca(2+) activation of MLCK alone is sufficient to promote contraction.

The apical Na(+)/H(+) exchanger isoform NHE3 is regulated by the actin cytoskeleton

The epithelial isoform of the Na(+)/H(+) exchanger, NHE3, associates with at least two related regulatory factors called NHERF1/EBP50 and NHERF2/TKA-1/E3KARP. These factors in addition interact with the cytoskeletal protein ezrin, which in turn binds to actin. The possible linkage of NHE3 with the cytoskeleton prompted us to test the effect of actin-modifying agents on NHE3 activity. Cytochalasins B and D and latrunculin B, which interfere with actin polymerization, induced a profound inhibition of NHE3 activity. The effect was isoform-specific inasmuch as the "housekeeping" exchanger NHE1 was virtually unaffected. Cytoskeletal disorganization was associated with a subcellular redistribution of NHE3, which accumulated at sites where actin aggregated, suggesting a physical interaction of exchangers with the cytoskeleton. An interaction was further suggested by the co-sedimentation of a detergent-insoluble fraction of NHE3 with the actin cytoskeleton. Inhibition of transport was not due to diminution in the number of transporters at the plasmalemma. Functional analyses of NHE1/NHE3 chimeras revealed that the cytoplasmic domain of NHE3 conferred sensitivity to cytochalasin B. Progressive carboxyl-terminal and internal deletions of the cytoplasmic region of NHE3 indicated that the region between residues 650 and 684 is critical for this response. This region overlaps with the domain reported to interact with NHERF and also contains a putative ezrin-binding site; hence, it likely plays a role in interactions with the cytoskeleton.

Vimentin dephosphorylation by protein phosphatase 2A is modulated by the targeting subunit B55

The intermediate filament protein vimentin is a major phosphoprotein in mammalian fibroblasts, and reversible phosphorylation plays a key role in its dynamic rearrangement. Selective inhibition of type 2A but not type 1 protein phosphatases led to hyperphosphorylation and concomitant disassembly of vimentin, characterized by a collapse into bundles around the nucleus. We have analyzed the potential role of one of the major protein phosphatase 2A (PP2A) regulatory subunits, B55, in vimentin dephosphorylation. In mammalian fibroblasts, B55 protein was distributed ubiquitously throughout the cytoplasm with a fraction associated to vimentin. Specific depletion of B55 in living cells by antisense B55 RNA was accompanied by disassembly and increased phosphorylation of vimentin, as when type 2A phosphatases were inhibited using okadaic acid. The presence of B55 was a prerequisite for PP2A to efficiently dephosphorylate vimentin in vitro or to induce filament reassembly in situ. Both biochemical fractionation and immunofluorescence analysis of detergent-extracted cells revealed that fractions of PP2Ac, PR65, and B55 were tightly associated with vimentin. Furthermore, vimentin-associated PP2A catalytic subunit was displaced in B55-depleted cells. Taken together these data show that, in mammalian fibroblasts, the intermediate filament protein vimentin is dephosphorylated by PP2A, an event targeted by B55.

Bidirectional signaling between the cytoskeleton and integrins

Clustering of integrins into focal adhesions and focal complexes is regulated by the actin cytoskeleton. In turn, actin dynamics are governed by Rho family GTPases. Integrin-mediated adhesion activates these GTPases, triggering assembly of filopodia, lamellipodia and stress fibers. In the past few years, signaling pathways have begun to be identified that promote focal adhesion disassembly and integrin dispersal. Many of these pathways result in decreased myosin-mediated cell contractility.

Signaling of de-adhesion in cellular regulation and motility

Adhesion is a process that can be divided into three separate stages: (1) cell attachment, (2) cell spreading, and (3) the formation of focal adhesions and stress fibers. With each stage the adhesive strength of the cell increases. De-adhesion can be defined as the process involving the transition of the cell from a strongly adherent state, characterized by focal adhesions and stress fibers, to a state of intermediate adherence, represented by a cell that is spread, but that lacks stress fibers terminating at adhesion plaques. We propose that this modification of the structural link between the actin cytoskeleton and the extracellular matrix results in a more malleable cellular state conducive for dynamic processes such as cytokinesis, mitogenesis, and motility. Anti-adhesive proteins, including thrombospondin, tenascin, and SPARC, rapidly signal de-adhesion, potentially mediating proliferation and migration during development and wound healing. Intracellular signaling molecules involved in the regulation of de-adhesion are only beginning to be identified. Interestingly, many of the same signaling proteins recognized to play important roles during the process of adhesion have also been found to act during de-adhesion. Characterization of the precise mechanisms by which these signals modulate adhesive structures and the cytoskeleton will further our understanding of the regulation of adhesive strength and its function in cellular physiology.

Role of Ca2+/calmodulin-dependent phosphatase 2B in thrombin-induced endothelial cell contractile responses

Thrombin-induced Ca2+ mobilization, activation of Ca2+/calmodulin-dependent myosin light chain (MLC) kinase (MLCK), and increased phosphorylation of MLCs precede and are critical to endothelial cell (EC) barrier dysfunction. Net MLC dephosphorylation after thrombin is nearly complete by 60 min and involves type 1 phosphatase (PPase 1) activity. We now report that thrombin does not alter total PPase 1 activity in EC homogenates but rather decreases myosin-associated PPase 1 activity. The PPase 1 inhibitor calyculin fails to prevent thrombin-induced MLC dephosphorylation. However, thrombin significantly increased the activity of Ca2+-dependent PPase 2B in EC homogenates (approximately 1.5- to 2-fold), with PPase 2B activation correlating with phosphorylation of the PPase 2B catalytic subunit. Western immunoblotting revealed PPase 2B to be present in cytoskeletal EC fractions, with specific PPase 2B inhibitors such as cyclosporin (200 nM) and deltamethrin (100 nM to 1 microM) attenuating thrombin-induced cytoskeletal protein dephosphorylation, including EC MLC dephosphorylation. These results suggest a model whereby thrombin-inducible contraction is determined by the phosphorylation status of EC MLC regulated by the balance between EC MLCK, PPase 1 (constitutive), and PPase 2B (inducible) activities.

Thrombin inactivates myosin light chain phosphatase via Rho and its target Rho kinase in human endothelial cells

The role of Rho GTPase and its downstream targets Rho kinase and myosin light chain phosphatase in thrombin-induced endothelial cell contraction was investigated. The specific Rho inactivator C3-transferase from Clostridium botulinum as well as microinjection of the isolated Rho-binding domain of Rho kinase or active myosin light chain phosphatase abolished thrombin-stimulated endothelial cell contraction. Conversely, microinjection of constitutively active V14Rho, constitutively active catalytic domain of Rho kinase, or treatment with the phosphatase inhibitor tautomycin caused contraction. These data are consistent with the notion that thrombin activates Rho/Rho kinase to inactivate myosin light chain phosphatase in endothelial cells. In fact, we demonstrate that thrombin transiently inactivated myosin light chain phosphatase, and this correlated with a peak in myosin light chain phosphorylation. C3-transferase abolished the decrease in myosin light chain phosphatase activity as well as the subsequent increase in myosin light chain phosphorylation and cell contraction. These data suggest that thrombin activates the Rho/Rho kinase pathway to inactivate myosin light chain phosphatase as part of a signaling network that controls myosin light chain phosphorylation/contraction in human endothelial cells.

Protein phosphatase inhibitors and heat preconditioning prevent Hsp27 dephosphorylation, F-actin disruption and deterioration of morphology in ATP-depleted endothelial cells

The vascular endothelium response to ischemic depletion of ATP was studied in vitro. Endothelial cells (EC) cultured from human aorta or umbilical vein were incubated in a glucose-free medium containing CCCP or rotenone. Such blockade of energy metabolism caused a drop in ATP, destruction of actin filaments, morphological changes, and eventually disintegration of EC monolayer within 2-2.5 h. While ATP fell and F-actin collapsed, the 27-kDa heat shock protein (Hsp27) lost basal phosphorylation and became Triton X-100-insoluble forming granules inside the cell nuclei. Protein phosphatase (PP) inhibitors (okadaic acid, cantharidin, sodium orthovanadate) did not delay the ATP decrease in energy-deprived EC but arrested both the alterations in the Hsp27 status and the changes for the worse in F-actin and cell morphology. Similarly, the Hsp27 dephosphorylation/insolubilization/granulation and the cytoskeletal and morphological disturbances resulting from lack of ATP were suppressed in heat-preconditioned (thermotolerant) cultures, this effect being sensitive to quercetin, a blocker of Hsp induction. The longer preservation of the cytosolic pool of phosphorylated Hsp27 during ATP depletion in the PP inhibitor-treated or thermotolerant EC correlated with the acquired resistance of F-actin and morphology. These data suggest that PP inhibitors as well as heat-inducible Hsp(s) can protect ischemia-stressed cells by preventing the ATP loss-provoked protein dephosphorylation and breakdown of the actin cytoskeleton.

Autoregulation of protein phosphatase type 2A expression

Protein phosphatases are involved in many cellular processes. One of the most abundant of these enzymes, the serine/threonine-specific protein phosphatase type 2A (PP2A), is present in most eukaryotic cells and serves a variety of functions. However, the detailed study of its regulation and function has been hampered by the difficulty of manipulating its expression level in cell culture. By using a new mammalian expression vector to forcibly overexpress PP2A in the mouse fibroblast cell line NIH3T3, we now show that the catalytic subunit of PP2A is subject to a potent autoregulatory mechanism that adjusts PP2A protein to constant levels. This control is exerted at the translational level and does not involve regulation of transcription or RNA processing. Thus, our results demonstrate tight control of PP2A expression, and provide an explanation for the difficulty of increasing PP2A expression experimentally.

Spinophilin, a novel protein phosphatase 1 binding protein localized to dendritic spines

Dendritic spines receive the vast majority of excitatory synaptic contacts in the mammalian brain and are presumed to contain machinery for the integration of various signal transduction pathways. Protein phosphatase 1 (PP1) is greatly enriched in dendritic spines and has been implicated in both the regulation of ionic conductances and long-term synaptic plasticity. The molecular mechanism whereby PP1 is localized to spines is unknown. We have now characterized a novel protein that forms a complex with the catalytic subunit of PP1 and is a potent modulator of PP1 enzymatic activity in vitro. Within the brain this protein displays a remarkably distinct localization to the heads of dendritic spines and has therefore been named spinophilin. Spinophilin has the properties expected of a scaffolding protein localized to the cell membrane and contains a single consensus sequence in PSD95/DLG/zo-1, which implies cross-linking of PP1 to transmembrane protein complexes. We propose that spinophilin represents a novel targeting subunit for PP1, which directs the enzyme to those substrates in the dendritic spine compartment, e.g., neurotransmitter receptors, which mediate the regulation of synaptic function by PP1.

Constitutively active protein phosphatase 1alpha causes Rb-dependent G1 arrest in human cancer cells

BACKGROUND

The retinoblastoma protein (Rb) needs to be phosphorylated by cyclin-dependent kinases (CDKs) before mammalian cells can enter the S phase of the cell cycle. As protein phosphatase 1 (PP1) activates Rb and is itself a target for inhibitory phosphorylation by CDKs in vitro, we asked whether any effects of PP1 on cell cycle progression depend on its phosphorylation and are mediated through Rb.

RESULTS

Using electrotransfer of recombinant protein into Rb-positive and Rb-negative cells, we have compared the effects of a wild-type PP1 catalytic subunit, PP1alpha, and a constitutively active mutant of this subunit (PP1alphaT320A) on G1 progression, proliferation rates, and cell viability. In treated cells, PP1alpha levels were elevated 6-16-fold and remained stable for at least 48 hours. In Rb-positive cells, PP1alphaT320A, but not PP1alpha, caused cell cycle arrest in late G1, which was associated with a lack of Rb phosphorylation. In Rb-negative cells, neither wild-type nor mutant phosphatase caused any change in cell cycle progression. Increased cell death was observed in both Rb-positive and Rb-negative cells, however, upon introduction of excess PP1alpha.

CONCLUSIONS

The difference between the effects of wild-type and mutant forms of PP1alpha suggests that PP1alpha has the potential to arrest cell growth in G1 unless it is inactivated by periodic phosphorylation at Thr320, presumably by CDKs that regulate passage through the G1-S cell cycle transition. Together, the effects in both cell types suggest that PP1alpha requires functional Rb to induce growth arrest, and that possibly another pool of PP1alpha induces cell death. This identifies PP1 as a potential target for therapeutic anti-proliferative strategies.

Differential localization of myosin and myosin phosphatase subunits in smooth muscle cells and migrating fibroblasts

Myosin II light chains (MLC20) are phosphorylated by a Ca2+/calmodulin-activated kinase and dephosphorylated by a phosphatase that has been purified as a trimer containing the delta isoform of type 1 catalytic subunit (PP1C delta), a myosin-binding 130-kDa subunit (M130) and a 20-kDa subunit. The distribution of M130 and PP1C as well as myosin II was examined in smooth muscle cells and fibroblasts by immunofluorescence microscopy and immunoblotting after differential extraction. Myosin and M130 colocalized with actin stress fibers in permeabilized cells. However, in nonpermeabilized cells the staining for myosin and M130 was different, with myosin mostly at the periphery of the cell and the M130 appearing diffusely throughout the cytoplasm. Accordingly, most M130 was recovered in a soluble fraction during permeabilization of cells, but the conditions used affected the solubility of both M130 and myosin. The PP1C alpha isoform colocalized with M130 and also was in the nucleus, whereas the PP1C delta isoform was localized prominently in the nucleus and in focal adhesions. In migrating cells, M130 concentrated in the tailing edge and was depleted from the leading half of the cell, where double staining showed myosin II was present. Because the tailing edge of migrating cells is known to contain phosphorylated myosin, inhibition of myosin LC20 phosphatase, probably by phosphorylation of the M130 subunit, may be required for cell migration.

Protein phosphatases maintain the organization and structural interactions of hepatic keratin intermediate filaments

The importance of protein phosphatases in the maintenance of cytoskeletal structure is supported by the serious liver injury caused by microcystin-LR, a hepatotoxic inhibitor of type-1 and type-2A serine/threonine protein phosphatases. We used the microcystin-LR-induced cell injury as a model to study the roles of protein dephosphorylation in maintaining cytoskeletal structure and cellular interactions in primary rat hepatocyte cultures. Confocal microscopy revealed that the first visible effect of microcystin-LR is disruption of desmoplakin organization at the cell surface, indicating dissociation of desmosomes. This effect is followed by a dramatic reorganization of both the intermediate filament (keratins 8 and 18) and microfilament networks, resulting in a merged structure in which the intermediate filaments are organized around a condensed actin core. Keratin 8, keratin 18 and desmoplakin I/II are the major cytoskeleton-associated targets for microcystin-LR-induced phosphorylation. Hyperphosphorylation of keratin 8 and 18 is accompanied by an increased keratin solubility, which correlates with the observed morphological effects. Phosphopeptide mapping shows that four specific tryptic phosphopeptides are highly phosphorylated predominantly in the soluble pool of keratin 18, whereas keratin 8 shows no indications of such assembly state-specific sites. Phosphopeptide maps of keratins phosphorylated in vivo and in vitro indicate that Ca2+/calmodulin-dependent kinase may be involved in regulating the serine-specific phosphorylation of both keratin 8 and keratin 18, while cAMP-dependent protein kinase does not seem to play a major role in this context. Taken together, our results show that the interactions between keratin intermediate filaments and desmosomes as well as the assembly states of their main constituent proteins, are directly regulated by serine/threonine kinase/phosphatase equilibria.

Shear stress-induced myosin association with cytoskeleton and phosphorylation in human platelets

Upon platelet activation by a high shear stress (108 dyne/cm2), actin and actin-binding protein increased rapidly into the Triton-insoluble cytoskeleton, whereas the association of myosin increased gradually. The amounts of cytoskeleton-associated myosin depended on the extent of aggregation. Preceding the maximal aggregation and ATP secretion, the 20 kDa light chain of myosin (MLC) is rapidly phosphorylated to approx. 45% of 20 kDa MLC and is then dephosphorylated. Cytoskeletal association of myosin and phosphorylation of 20 kDa MLC was inhibited by OP-41483, a prostaglandin I2 analog, which inhibited the full aggregation response to shear stress. Exposure to high shear stress resulted in an increased association of myosin light chain kinase and protein phosphatase types 1 and 2A with the cytoskeleton, while the cytoskeletal association of protein kinase C was not evident. These results indicate that 20 kDa MLC phosphorylation is involved in shear stress-induced platelet activation, and that cytoskeletal association of protein phosphatases may regulate the phosphorylation level of cytoskeletal elements such as myosin together with myosin light chain kinase.