Our previous studies of DARPP-32 in striatal slices have shown that activation of D1 receptors leads to cAMP-dependent dephosphorylation of Thr-75, the Cdk5 site in DARPP-32. In the current study, we have elucidated a mechanism whereby protein phosphatase 2A (PP2A) is activated by a cAMP/PKA-dependent pathway, leading to dephosphorylation of Thr-75. PP2A consists of a catalytic C subunit that associates with the scaffolding A subunit and a variety of B subunits. We have found that the A/C subunits of PP2A, in association with the B56␦ (or PPP2R5D) regulatory subunit, is an active DARPP-32 phosphatase. The B56␦ subunit expressed in HEK293 cells forms a heterotrimeric assembly that catalyzes PKA-mediated dephosphorylation at Thr-75 in DARPP-32 (also cotransfected into HEK293 cells). The B56␦ subunit is phosphorylated by PKA, and this increases the overall activity of PP2A in vitro and in vivo. Among four PKA-phosphorylation sites identified in B56␦ in vitro, Ser-566 was found to be critical for the regulation of PP2A activity. Moreover, Ser-566 was phosphorylated by PKA in response to activation of D1 receptors in striatal slices. Based on these studies, we propose that the B56␦/A/C PP2A complex regulates the dephosphorylation of DARPP-32 at Thr-75, thereby helping coordinate the efficacy of dopaminergic neurotransmission in striatal neurons. Moreover, stimulation of protein phosphatase activity by this mechanism may represent an important signaling pathway regulated by cAMP in neurons and other types of cell.cAMP ͉ DARPP-32 ͉ protein phosphorylation D ARPP-32 is a phosphoprotein that is highly enriched in dopaminoceptive medium-sized spiny neurons in the striatum and nucleus accumbens (1, 2). A variety of biochemical studies as well as targeted deletion and mutation of DARPP-32 in mice have shown that DARPP-32 plays a critical role in the actions of dopamine as well as in the actions of antipsychotic drugs, drugs of abuse, and other agents that modulate dopamine levels in the brain (2-5). Through activation of the D1 subclass of receptors, dopamine increases cAMP, activates protein kinase A (PKA), and phosphorylates Thr-34 of DARPP-32. Phosphorylation at Thr-34 converts DARPP-32 into a potent, high-affinity inhibitor of the broad specificity serine/threonine protein phosphatase, PP-1, leading to increased phosphorylation of many physiologically important substrates in medium spiny neurons, including neurotransmitter receptors, voltage-gated ion channels, ion pumps, protein kinases, and transcription factors (1, 2).In addition to Thr-34, DARPP-32 is phosphorylated at multiple sites by several protein kinases, including CK1, CK2 and Cdk5 (6-9). In particular, phosphorylation of Thr-75 by Cdk5 blocks PKA-mediated phosphorylation of Thr-34 of DARPP-32, thereby modulating the efficacy of the dopamine/D1/cAMP/ PKA/DARPP-32/PP1 signaling cascade (8). Our previous studies have found that there is a reciprocal relationship between the phosphorylation status of . Under basal conditions in striatal neurons in vivo or in vit...

We show that the PR72 (or B؆ or PPP2R3A) regulatory subunit of PP2A is highly expressed in striatum. Through the use of overexpression and down-regulation by using RNAi, we show that PP2A, in a heterotrimeric complex with the PR72 subunit, mediates Ca 2؉ -dependent dephosphorylation at Thr-75 of DARPP-32. The PR72 subunit contains two Ca 2؉ binding sites formed by E and F helices (EF-hands 1 and 2), and we show that the former is necessary for the ability of PP2A activity to be regulated by Ca 2؉ , both in vitro and in vivo. Our studies also indicate that the PR72-containing form of PP2A is necessary for the ability of glutamate acting at ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and NMDA receptors to regulate Thr-75 dephosphorylation. These studies further our understanding of the complex signal transduction pathways that regulate DARPP-32. In addition, our studies reveal an alternative intracellular mechanism whereby Ca 2؉ can activate serine/threonine phosphatase activity.calcium ͉ protein phosphorylation ͉ dopamine ͉ glutamate D opamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) is a phosphoprotein that is selectively enriched in medium spiny neurons in the neostriatum (1, 2). When phosphorylated at Thr-34 by protein kinase A (PKA), DARPP-32 is converted into a potent, high-affinity inhibitor of the broad specificity serine/threonine protein phosphatase, protein phosphatase 1 (PP1), leading to increased phosphorylation of many physiologically important substrates in medium spiny neurons, including neurotransmitter receptors, voltage-gated ion channels, ion pumps, protein kinases, and transcription factors (1, 2). These biochemical studies, as well as targeted deletion and mutation of DARPP-32 in mice, have shown that the protein plays a critical role in the actions of dopamine, as well as in the actions of antipsychotic drugs, drugs of abuse, and other agents that modulate dopamine levels in the brain (2-5). DARPP-32 is also phosphorylated at other sites by the protein kinases CK1, CK2, and Cdk5 (cyclin-dependent kinase 5), which serve to modulate the phosphorylation and dephosphorylation of . For example, phosphorylation at Thr-75 by Cdk5 inhibits PKA and blocks phosphorylation at Thr-34, thereby attenuating the dopamine/D1/cAMP/PKA/DARPP-32/PP1 signaling cascade (8).The mechanisms involved in the dephosphorylation of the various sites of DARPP-32 are also complex, involving the actions of various serine/threonine protein phosphatases. Phospho-Thr-34 is largely under the control of PP2B (or calcineurin), a Ca 2ϩ /calmodulin-dependent enzyme that is regulated by glutamate acting at both NMDA and ␣-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors. Activation of AMPA or NMDA receptors also results in Ca 2ϩ -dependent dephosphorylation of Thr-75, but our previous studies indicated that this appeared to involve a distinct PP2A-dependent pathway (10, 11). Phospho-Thr-75 is also dephosphorylated by a cAMP/ PKA-dependent pathway that regulates PP2A activity (12, 13). The ab...

Neurabin I is a brain-specific actin-binding protein. Here we show that neurabin I binds protein phosphatase 1 (PP1) and inhibits PP1 activity. Neurabin I interacted with PP1alpha in an overlay assay, in yeast two-hybrid interaction analysis, and in coprecipitation and co-immunoprecipitation experiments. Neurabin I also copurified with both the alpha and gamma isoforms of PP1. A glutathione S-transferase (GST)-neurabin I fusion protein (residues 318-661) containing the putative PP1 binding domain (residues 456-460) inhibited PP1 activity (K(i) = 2.7 +/- 1.2 nM). This fusion protein was also rapidly phosphorylated in vitro by PKA (K(m) = 6 microM) to a stoichiomtry of 1 mol/mol. The phosphorylated residue was identified as serine 461 by HPLC-MS analysis of a tryptic digest. Phosphorylation of GST-neurabin I (residues 318-661) by PKA significantly reduced its binding to PP1 by overlay and by glutathione-Sepharose coprecipitation assays. A 35-fold decrease in inhibitory potency was also observed using a S461E mutant, which mimics phosphorylation of S461. These findings identify a signaling mechanism involving the regulation of PP1 activity and localization mediated by the cAMP pathway.

The concentration of the second messenger cAMP is tightly controlled in cells by the activity of phosphodiesterases. We have previously described how the protein kinase A-anchoring protein mAKAP serves as a scaffold for the cAMP-dependent protein kinase PKA and the cAMP-specific phosphodiesterase PDE4D3 in cardiac myocytes. PKA and PDE4D3 constitute a negative feedback loop whereby PKA-catalyzed phosphorylation and activation of PDE4D3 attenuate local cAMP levels. We now show that protein phosphatase 2A (PP2A) associated with mAKAP complexes is responsible for reversing the activation of PDE4D3 by catalyzing the dephosphorylation of PDE4D3 serine residue 54. Mapping studies reveal that a C-terminal mAKAP domain (residues 2085-2319) binds PP2A. Binding to mAKAP is required for PP2A function, such that deletion of the C-terminal domain enhances both base-line and forskolin-stimulated PDE4D3 activity. Interestingly, PP2A holoenzyme associated with mAKAP complexes in the heart contains the PP2A targeting subunit B56␦. Like PDE4D3, B56␦ is a PKA substrate, and PKA phosphorylation of mAKAP-bound B56␦ enhances phosphatase activity 2-fold in the complex. Accordingly, expression of a B56␦ mutant that cannot be phosphorylated by PKA results in increased PDE4D3 phosphorylation. Taken together, our findings demonstrate that PP2A associated with mAKAP complexes promotes PDE4D3 dephosphorylation, serving both to inhibit PDE4D3 in unstimulated cells and also to mediate a cAMP-induced positive feedback loop following adenylyl cyclase activation and B56␦ phosphorylation. In general, PKA⅐PP2A⅐mAKAP complexes exemplify how protein kinases and phosphatases may participate in molecular signaling complexes to dynamically regulate localized intracellular signaling.

Protein phosphatase 2C (PP2C) is an archetype of the PPM Ser/Thr phosphatases, characterized by dependence on divalent magnesium or manganese cofactors, absence of known regulatory proteins, and resistance to all known Ser/Thr phosphatase inhibitors. We have used virtual ligand screening with the AutoDock method and the National Cancer Institute Diversity Set to identify small molecule inhibitors of PP2Cα activity at a protein substrate. These inhibitors are active in the micromolar range, and represent the first non-phosphate-based molecules found to inhibit a type 2C phosphatase. The compounds docked to three recurrent binding sites near the PP2Cα active site and displayed novel Ser/Thr phosphatase selectivity profiles. Common chemical features of these compounds may form the basis for development of a PP2C inhibitor pharmacophore and may facilitate investigation of PP2C control and cellular function.

Protein kinase A (PKA)-dependent signaling cascades play an important role in mediating the effects of dopamine and other neurotransmitters in striatal medium spiny neurons. We have identified a prominent striatal PKA substrate as Rap1-GTPase activating protein (Rap1GAP), a negative regulator of Rap1 signaling. Although present throughout the brain, Rap1GAP is enriched in striatal medium spiny neurons and is phosphorylated by PKA at Ser-441 and Ser-499 in response to activation of D1 dopamine receptors. Phosphorylation of Rap1GAP is associated with inhibition of GAP activity, as demonstrated by increased Rap1 activity in striatal neurons. Phosphorylation of Rap1GAP is also associated with increased dendritic spine head size in cultured neurons. These findings suggest that phosphorylation of Rap1GAP by PKA plays an important role in striatal neurons by modulating Rap1 actions.

The actions of LH to induce ovulation and luteinization of preovulatory follicles are mediated principally by activation of cAMP-dependent protein kinase (PKA) in granulosa cells. PKA activity is targeted to specific locations in many cells by A kinase-anchoring proteins (AKAPs). We previously showed that FSH induces expression of microtubule-associated protein (MAP) 2D, an 80-kDa AKAP, in rat granulosa cells, and that MAP2D coimmunoprecipitates with PKA-regulatory subunits in these cells. Here we report a rapid and targeted dephosphorylation of MAP2D at Thr256/Thr259 after treatment with human chorionic gonadotropin, an LH receptor agonist. This event is mimicked by treatment with forskolin or a cAMP analog and is blocked by the PKA inhibitor myristoylated-PKI, indicating a role for cAMP and PKA signaling in phosphoregulation of granulosa cell MAP2D. Furthermore, we show that Thr256/Thr259 dephosphorylation is blocked by the protein phosphatase 2A (PP2A) inhibitor, okadaic acid, and demonstrate interactions between MAP2D and PP2A by coimmunoprecipitation and microcystin-agarose pull-down. We also show that MAP2D interacts with glycogen synthase kinase (GSK) 3beta and is phosphorylated at Thr256/Thr259 by this kinase in the basal state. Increased phosphorylation of GSK3beta at Ser9 and the PP2A B56delta subunit at Ser566 is observed after treatment with human chorionic gonadotropin and appears to result in LH receptor-mediated inhibition of GSK3beta and activation of PP2A, respectively. Taken together, these results show that the phosphorylation status of the AKAP MAP2D is acutely regulated by LH receptor-mediated modulation of kinase and phosphatase activities via PKA.

BACKGROUND Cerebrospinal fluid (CSF) tau is a common biomarker for Alzheimer disease (AD). Measurements of tau have historically been performed using immunoassays. Given the molecular diversity of tau in CSF, the selectivity of these immunoassays has often been questioned. Therefore, we aimed to develop an analytically sensitive and selective immunoaffinity liquid chromatography–tandem mass spectrometry (LC-MS/MS) (IA-MS) assay. METHODS IA-MS sample analysis involved the addition of an internal standard, immunoaffinity purification of tau using a tau monoclonal antibody coupled to magnetic beads, trypsin digestion, and quantification of a surrogate tau peptide by LC-MS/MS using a Waters Trizaic nanoTile ultraperformance LC microfluidic device. Further characterization of tau peptides was performed by full-scan MS using a Thermo Orbitrap LC-MS. CSF samples from a cohort of age-matched controls and patients with AD were analyzed by the IA-MS method as well as a commercially available immunoassay. RESULTS The IA-MS assay had intra- and interassay imprecision values of 3.2% to 8.1% CV and 7.8% to 18.9% C, respectively, a mean recovery of 106%, and a limit of quantification of 0.25 pmol/L and was able to quantify tau concentrations in all human specimens tested. The IA-MS assay showed a correlation of R2 = 0.950 against a total-tau immunoassay. In patients with AD, tau was increased approximately 2-fold. CONCLUSIONS Combining immunoaffinity enrichment with microflow LC-MS/MS analysis is an effective approach for the development of a highly selective assay to measure total tau and, potentially, other posttranslationally modified forms of tau in CSF.