Quantification of isozyme-specific activation of phospholipase C-beta2 by Rac GTPases and phospholipase C-epsilon by Rho GTPases in an intact cell assay system

Mammalian phospholipase D: Function, and therapeutics

Despite being discovered over 60 years ago, the precise role of phospholipase D (PLD) is still being elucidated. PLD enzymes catalyze the hydrolysis of the phosphodiester bond of glycerophospholipids producing phosphatidic acid and the free headgroup. PLD family members are found in organisms ranging from viruses, and bacteria to plants, and mammals. They display a range of substrate specificities, are regulated by a diverse range of molecules, and have been implicated in a broad range of cellular processes including receptor signaling, cytoskeletal regulation and membrane trafficking. Recent technological advances including: the development of PLD knockout mice, isoform-specific antibodies, and specific inhibitors are finally permitting a thorough analysis of the in vivo role of mammalian PLDs. These studies are facilitating increased recognition of PLD's role in disease states including cancers and Alzheimer's disease, offering potential as a target for therapeutic intervention.

VEGAS as a Platform for Facile Directed Evolution in Mammalian Cells

Directed evolution, artificial selection toward designed objectives, is routinely used to develop new molecular tools and therapeutics. Successful directed molecular evolution campaigns repeatedly test diverse sequences with a designed selective pressure. Unicellular organisms and their viral pathogens are exceptional for this purpose and have been used for decades. However, many desirable targets of directed evolution perform poorly or unnaturally in unicellular backgrounds. Here, we present a system for facile directed evolution in mammalian cells. Using the RNA alphavirus Sindbis as a vector for heredity and diversity, we achieved 24-h selection cycles surpassing 10^-3 mutations per base. Selection is achieved through genetically actuated sequences internal to the host cell, thus the system's name: viral evolution of genetically actuating sequences, or "VEGAS." Using VEGAS, we evolve transcription factors, GPCRs, and allosteric nanobodies toward functional signaling endpoints each in less than 1 weeks' time.

Structural determinants of 5-HT2B receptor activation and biased agonism

Serotonin (5-hydroxytryptamine; 5-HT) receptors modulate a variety of physiological processes ranging from perception, cognition and emotion to vascular and smooth muscle contraction, platelet aggregation, gastrointestinal function and reproduction. Drugs that interact with 5-HT receptors effectively treat diseases as diverse as migraine headaches, depression and obesity. Here we present four structures of a prototypical serotonin receptor-the human 5-HT_2B receptor-in complex with chemically and pharmacologically diverse drugs, including methysergide, methylergonovine, lisuride and LY266097. A detailed analysis of these structures complemented by comprehensive interrogation of signaling illuminated key structural determinants essential for activation. Additional structure-guided mutagenesis experiments revealed binding pocket residues that were essential for agonist-mediated biased signaling and β-arrestin2 translocation. Given the importance of 5-HT receptors for a large number of therapeutic indications, insights derived from these studies should accelerate the design of safer and more effective medications.

5-HT2C Receptor Structures Reveal the Structural Basis of GPCR Polypharmacology

Drugs frequently require interactions with multiple targets-via a process known as polypharmacology-to achieve their therapeutic actions. Currently, drugs targeting several serotonin receptors, including the 5-HT_2C receptor, are useful for treating obesity, drug abuse, and schizophrenia. The competing challenges of developing selective 5-HT_2C receptor ligands or creating drugs with a defined polypharmacological profile, especially aimed at G protein-coupled receptors (GPCRs), remain extremely difficult. Here, we solved two structures of the 5-HT_2C receptor in complex with the highly promiscuous agonist ergotamine and the 5-HT_2A-C receptor-selective inverse agonist ritanserin at resolutions of 3.0 Å and 2.7 Å, respectively. We analyzed their respective binding poses to provide mechanistic insights into their receptor recognition and opposing pharmacological actions. This study investigates the structural basis of polypharmacology at canonical GPCRs and illustrates how understanding characteristic patterns of ligand-receptor interaction and activation may ultimately facilitate drug design at multiple GPCRs.

Crystal Structure of an LSD-Bound Human Serotonin Receptor

The prototypical hallucinogen LSD acts via serotonin receptors, and here we describe the crystal structure of LSD in complex with the human serotonin receptor 5-HT2B. The complex reveals conformational rearrangements to accommodate LSD, providing a structural explanation for the conformational selectivity of LSD's key diethylamide moiety. LSD dissociates exceptionally slow from both 5-HT2BR and 5-HT2AR-a major target for its psychoactivity. Molecular dynamics (MD) simulations suggest that LSD's slow binding kinetics may be due to a "lid" formed by extracellular loop 2 (EL2) at the entrance to the binding pocket. A mutation predicted to increase the mobility of this lid greatly accelerates LSD's binding kinetics and selectively dampens LSD-mediated β-arrestin2 recruitment. This study thus reveals an unexpected binding mode of LSD; illuminates key features of its kinetics, stereochemistry, and signaling; and provides a molecular explanation for LSD's actions at human serotonin receptors. PAPERCLIP.

Cyclin A2: At the crossroads of cell cycle and cell invasion

Cyclin A2 is an essential regulator of the cell division cycle through the activation of kinases that participate to the regulation of S phase as well as the mitotic entry. However, whereas its degradation by the proteasome in mid mitosis was thought to be essential for mitosis to proceed, recent observations show that a small fraction of cyclin A2 persists beyond metaphase and is degraded by autophagy. Its implication in the control of cytoskeletal dynamics and cell movement has unveiled its role in the modulation of RhoA activity. Since this GTPase is involved in both cell rounding early in mitosis and later, in the formation of the cleavage furrow, this suggests that cyclin A2 is a novel actor in cytokinesis. Taken together, these data point to this cyclin as a potential mediator of cell-niche interactions whose dysregulation could be taken as a hallmark of metastasis.

Cyclin A2 modulates EMT via β-catenin and phospholipase C pathways

We have previously demonstrated that Cyclin A2 is involved in cytoskeletal dynamics, epithelial-mesenchymal transition (EMT) and metastasis. This phenotype was potentiated by activated oncogenic H-Ras. However, the mechanisms governing EMT in these cells have not yet been elucidated. Here, we dissected the pathways that are responsible for EMT in cells deficient for Cyclin A2. In Cyclin A2-depleted normal murine mammary gland (NMuMG) cells expressing RasV12, we found that β-catenin was liberated from the cell membrane and cell-cell junctions and underwent nuclear translocation and activation. Components of the canonical wingless (WNT) pathway, including WNT8b, WNT10a, WNT10b, frizzled 1 and 2 and TCF4 were upregulated at the messenger RNA and protein levels following Cyclin A2 depletion. However, suppression of the WNT pathway using the acetyltransferase porcupine inhibitor C59 did not reverse EMT whereas a dominant negative form of TCF4 as well as inhibition of phospholipase C using U73122 were able to do so. This suggests that a WNT-independent mechanism of β-catenin activation via phospholipase C is involved in the EMT induced by Cyclin A2 depletion. Our findings will broaden our knowledge on how Cyclin A2 contributes to EMT and metastasis.

Membrane-induced allosteric control of phospholipase C-β isozymes

All peripheral membrane proteins must negotiate unique constraints intrinsic to the biological interface of lipid bilayers and the cytosol. Phospholipase C-β (PLC-β) isozymes hydrolyze the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2) to propagate diverse intracellular responses that underlie the physiological action of many hormones, neurotransmitters, and growth factors. PLC-β isozymes are autoinhibited, and several proteins, including Gαq, Gβγ, and Rac1, directly engage distinct regions of these phospholipases to release autoinhibition. To understand this process, we used a novel, soluble analog of PIP2 that increases in fluorescence upon cleavage to monitor phospholipase activity in real time in the absence of membranes or detergents. High concentrations of Gαq or Gβ1γ2 did not activate purified PLC-β3 under these conditions despite their robust capacity to activate PLC-β3 at membranes. In addition, mutants of PLC-β3 with crippled autoinhibition dramatically accelerated the hydrolysis of PIP2 in membranes without an equivalent acceleration in the hydrolysis of the soluble analog. Our results illustrate that membranes are integral for the activation of PLC-β isozymes by diverse modulators, and we propose a model describing membrane-mediated allosterism within PLC-β isozymes.

4-Alkyloxyimino derivatives of uridine-5'-triphosphate: distal modification of potent agonists as a strategy for molecular probes of P2Y2, P2Y4, and P2Y6 receptors

Extended N(4)-(3-arylpropyl)oxy derivatives of uridine-5'-triphosphate were synthesized and potently stimulated phospholipase C stimulation in astrocytoma cells expressing G protein-coupled human (h) P2Y receptors (P2YRs) activated by UTP (P2Y2/4R) or UDP (P2Y6R). The potent P2Y4R-selective N(4)-(3-phenylpropyl)oxy agonist was phenyl ring-substituted or replaced with terminal heterocyclic or naphthyl rings with retention of P2YR potency. This broad tolerance for steric bulk in a distal region was not observed for dinucleoside tetraphosphate agonists with both nucleobases substituted. The potent N(4)-(3-(4-methoxyphenyl)-propyl)oxy analogue 19 (EC50: P2Y2R, 47 nM; P2Y4R, 23 nM) was functionalized for chain extension using click tethering of fluorophores as prosthetic groups. The BODIPY 630/650 conjugate 28 (MRS4162) exhibited EC50 values of 70, 66, and 23 nM at the hP2Y2/4/6Rs, respectively, and specifically labeled cells expressing the P2Y6R. Thus, an extended N(4)-(3-arylpropyl)oxy group accessed a structurally permissive region on three Gq-coupled P2YRs, and potency and selectivity were modulated by distal structural changes. This freedom of substitution was utilized to design of a pan-agonist fluorescent probe of a subset of uracil nucleotide-activated hP2YRs.

Small molecule inhibitors of phospholipase C from a novel high-throughput screen

Phospholipase C (PLC) isozymes are important signaling molecules, but few small molecule modulators are available to pharmacologically regulate their function. With the goal of developing a general approach for identification of novel PLC inhibitors, we developed a high-throughput assay based on the fluorogenic substrate reporter WH-15. The assay is highly sensitive and reproducible: screening a chemical library of 6280 compounds identified three novel PLC inhibitors that exhibited potent activities in two separate assay formats with purified PLC isozymes in vitro. Two of the three inhibitors also inhibited G protein-coupled receptor-stimulated PLC activity in intact cell systems. These results demonstrate the power of the high-throughput assay for screening large collections of small molecules to identify novel PLC modulators. Potent and selective modulators of PLCs will ultimately be useful for dissecting the roles of PLCs in cellular processes, as well as provide lead compounds for the development of drugs to treat diseases arising from aberrant phospholipase activity.

4-Alkyloxyimino-cytosine nucleotides: tethering approaches to molecular probes for the P2Y6 receptor

4-Alkyloxyimino derivatives of pyrimidine nucleotides display high potency as agonists of certain G protein-coupled P2Y receptors (P2YRs). In an effort to functionalize a P2Y₆R agonist for fluorescent labeling, we probed two positions (N⁴ and γ-phosphate of cytidine derivatives) with various functional groups, including alkynes for click chemistry. Functionalization of extended imino substituents at the 4 position of the pyrimidine nucleobase of CDP preserved P2Y₆R potency generally better than γ-phosphoester formation in CTP derivatives. Fluorescent Alexa Fluor 488 conjugate 16 activated the human P2Y₆R expressed in 1321N1 human astrocytoma cells with an EC₅₀ of 9 nM, and exhibited high selectivity for this receptor over other uridine nucleotide-activated P2Y receptors. Flow cytometry detected specific labeling with 16 to P2Y₆R-expressing but not to wild-type 1321N1 cells. Additionally, confocal microscopy indicated both internalized 16 (t_1/2 of 18 min) and surface-bound fluorescence. Known P2Y₆R ligands inhibited labeling. Theoretical docking of 16 to a homology model of the P2Y₆R predicted electrostatic interactions between the fluorophore and extracellular portion of TM3. Thus, we have identified the N⁴-benzyloxy group as a structurally permissive site for synthesis of functionalized congeners leading to high affinity molecular probes for studying the P2Y₆R.

Synthesis and P2Y₂ receptor agonist activities of uridine 5'-phosphonate analogues

We explored the influence of modifications of uridine 5'-methylenephosphonate on biological activity at the human P2Y(2) receptor. Key steps in the synthesis of a series of 5-substituted uridine 5'-methylenephosphonates were the reaction of a suitably protected uridine 5'-aldehyde with [(diethoxyphosphinyl)methylidene]triphenylphosphorane, C-5 bromination and a Suzuki-Miyaura coupling. These analogues behaved as selective agonists at the P2Y(2) receptor, with three analogues exhibiting potencies in the submicromolar range. Although maximal activities observed with the phosphonate analogues were much less than observed with UTP, high concentrations of the phosphonates had no effect on the stimulatory effect of UTP. These results suggest that these phosphonates bind to an allosteric site of the P2Y(2) receptor.

The mechanism of cell membrane ruffling relies on a phospholipase D2 (PLD2), Grb2 and Rac2 association

Membrane ruffling is the formation of actin rich membrane protrusions, essential for cell motility. The exact mechanism of ruffling is not fully known. Using YFP and CFP fluorescent chimeras, we show for the first time a co-localization of Phospholipase D2 (PLD2) and Growth factor Receptor Bound protein-2 (Grb2) with actin-rich membrane protrusions of macrophages. Grb2 cooperates with PLD2 in enhancing membrane ruffling, whether in resting cells or in cells stimulated with the growth factor M-CSF, although in the latter an increase in dorsal ruffles was observed, consistent with receptor-ligand internalization. Cells transfected with PLD2 mutated in the PH domain (Y169F) or with Grb2 mutated in the SH2 site (R86K) negate this effect, indicating an association PLD2(Y169)-SH2-Grb2 that was confirmed by immunoprecipitation and Western blotting. The association results in enhanced PLD activity, but the lipase activity can only partially explain the formation of membrane ruffles in vivo. A third component involves the Rho-GTPase Rac2 and it is only when Rac2 is overexpressed along with PLD2 and Rac2 that a full biological effect, including actin polymerization in vivo, is obtained. The mechanism involved is, then, as follows: PLD enzymatic action, after having been increased due to the binding to Grb2-SH2 via Y169, cooperates with Rac2, and the three molecules stimulate actin polymerization and consequently, membrane ruffle formation. Since membrane ruffling precedes cell migration, the results herein provide a novel mechanism for control of membrane dynamics, crucial for the physiology of leukocytes.

Pyrimidine nucleotides with 4-alkyloxyimino and terminal tetraphosphate δ-ester modifications as selective agonists of the P2Y(4) receptor

P2Y(2) and P2Y(4) receptors are G protein-coupled receptors, activated by UTP and dinucleoside tetraphosphates, which are difficult to distinguish pharmacologically for lack of potent and selective ligands. We structurally varied phosphate and uracil moieties in analogues of pyrimidine nucleoside 5'-triphosphates and 5'-tetraphosphate esters. P2Y(4) receptor potency in phospholipase C stimulation in transfected 1321N1 human astrocytoma cells was enhanced in N(4)-alkyloxycytidine derivatives. OH groups on a terminal δ-glucose phosphoester of uridine 5'-tetraphosphate were inverted or substituted with H or F to probe H-bonding effects. N(4)-(Phenylpropoxy)-CTP 16 (MRS4062), Up(4)-[1]3'-deoxy-3'-fluoroglucose 34 (MRS2927), and N(4)-(phenylethoxy)-CTP 15 exhibit ≥10-fold selectivity for human P2Y(4) over P2Y(2) and P2Y(6) receptors (EC(50) values 23, 62, and 73 nM, respectively). δ-3-Chlorophenyl phosphoester 21 of Up(4) activated P2Y(2) but not P2Y(4) receptor. Selected nucleotides tested for chemical and enzymatic stability were much more stable than UTP. Agonist docking at CXCR4-based P2Y(2) and P2Y(4) receptor models indicated greater steric tolerance of N(4)-phenylpropoxy group at P2Y(4). Thus, distal structural changes modulate potency, selectivity, and stability of extended uridine tetraphosphate derivatives, and we report the first P2Y(4) receptor-selective agonists.

Pyrimidine ribonucleotides with enhanced selectivity as P2Y(6) receptor agonists: novel 4-alkyloxyimino, (S)-methanocarba, and 5'-triphosphate gamma-ester modifications

The P2Y(6) receptor is a cytoprotective G-protein-coupled receptor (GPCR) activated by UDP (EC(50) = 0.30 microM). We compared and combined modifications to enhance P2Y(6) receptor agonist selectivity, including ribose ring constraint, 5-iodo and 4-alkyloxyimino modifications, and phosphate modifications such as alpha,beta-methylene and extension of the terminal phosphate group into gamma-esters of UTP analogues. The conformationally constrained (S)-methanocarba-UDP is a full agonist (EC(50) = 0.042 microM). 4-Methoxyimino modification of pyrimidine enhanced P2Y(6), preserved P2Y(2) and P2Y(4), and abolished P2Y(14) receptor potency, in the appropriate nucleotide. N(4)-Benzyloxy-CDP (15, MRS2964) and N(4)-methoxy-Cp(3)U (23, MRS2957) were potent, selective P2Y(6) receptor agonists (EC(50) of 0.026 and 0.012 microM, respectively). A hydrophobic binding region near the nucleobase was explored with receptor modeling and docking. UTP-gamma-aryl and cycloalkyl phosphoesters displayed only intermediate P2Y(6) receptor potency but had enhanced stability in acid and cell membranes. UTP-glucose was inactive, but its (S)-methanocarba analogue and N(4)-methoxycytidine 5'-triphospho-gamma-[1]glucose were active (EC(50) of 2.47 and 0.18 microM, respectively). Thus, the potency, selectivity, and stability of pyrimidine nucleotides as P2Y(6) receptor agonists may be enhanced by modest structural changes.

Genetic deletion of p90 ribosomal S6 kinase 2 alters patterns of 5-hydroxytryptamine 2A serotonin receptor functional selectivity

The concept of functional selectivity has now thoroughly supplanted the previously entrenched notion of intrinsic efficacy by explaining how agonists and antagonists exhibit a range of efficacies for distinct receptor-mediated responses. It is noteworthy that functional selectivity accommodates significant changes in efficacy resulting from differential expression of G protein-coupled receptor modifying proteins (i.e., "conditional efficacy")-a phenomenon with profound implications for drug discovery. We have uncovered a novel regulatory mechanism whereby p90 ribosomal S6 kinase 2 (RSK2) interacts with 5-hydroxytryptamine(2A) (5-HT(2A)) serotonin receptors and attenuates receptor signaling via direct receptor phosphorylation (Proc Natl Acad Sci U S A 103:4717-4722, 2006; J Biol Chem 284:5557-5573, 2009). This discovery, together with the mounting evidence for conditional efficacy, suggested to us that 5-HT(2A) agonist signaling might be disproportionately affected by alterations in RSK2 expression. To test this hypothesis, we evaluated a chemically diverse set of 5-HT(2A) agonists at three readouts of 5-HT(2A) receptor activation in both wild-type (WT) and RSK2 knock-out (KO) mouse embryonic fibroblasts (MEFs). Here we report that 5-HT(2A) receptor agonist efficacies were significantly and variably augmented in RSK2 KO MEFs compared with WT MEFs. As a result, relative agonist efficacies were significantly altered, and even reversed, between WT and RSK2 KO MEFs for a single effector readout. This study provides the first evidence that deletion of a single kinase can elicit profound changes in patterns of agonist functional selectivity.

Parallel functional activity profiling reveals valvulopathogens are potent 5-hydroxytryptamine(2B) receptor agonists: implications for drug safety assessment

Drug-induced valvular heart disease (VHD) is a serious side effect of a few medications, including some that are on the market. Pharmacological studies of VHD-associated medications (e.g., fenfluramine, pergolide, methysergide, and cabergoline) have revealed that they and/or their metabolites are potent 5-hydroxytryptamine(2B) (5-HT(2B)) receptor agonists. We have shown that activation of 5-HT(2B) receptors on human heart valve interstitial cells in vitro induces a proliferative response reminiscent of the fibrosis that typifies VHD. To identify current or future drugs that might induce VHD, we screened approximately 2200 U.S. Food and Drug Administration (FDA)-approved or investigational medications to identify 5-HT(2B) receptor agonists, using calcium-based high-throughput screening. Of these 2200 compounds, 27 were 5-HT(2B) receptor agonists (hits); 14 of these had previously been identified as 5-HT(2B) receptor agonists, including seven bona fide valvulopathogens. Six of the hits (guanfacine, quinidine, xylometazoline, oxymetazoline, fenoldopam, and ropinirole) are approved medications. Twenty-three of the hits were then "functionally profiled" (i.e., assayed in parallel for 5-HT(2B) receptor agonism using multiple readouts to test for functional selectivity). In these assays, the known valvulopathogens were efficacious at concentrations as low as 30 nM, whereas the other compounds were less so. Hierarchical clustering analysis of the pEC(50) data revealed that ropinirole (which is not associated with valvulopathy) was clearly segregated from known valvulopathogens. Taken together, our data demonstrate that patterns of 5-HT(2B) receptor functional selectivity might be useful for identifying compounds likely to induce valvular heart disease.

Synthesis and P2Y receptor activity of nucleoside 5'-phosphonate derivatives

Ribose-based nucleoside 5'-diphosphates and triphosphates and related nucleotides were compared in their potency at the P2Y receptors with the corresponding nucleoside 5'-phosphonate derivatives. Phosphonate derivatives of UTP and ATP activated the P2Y(2) receptor but were inactive or weakly active at P2Y(4) receptor. Uridine 5'-(diphospho)phosphonate was approximately as potent at the P2Y(2) receptor as at the UDP-activated P2Y(6) receptor. These results suggest that removal of the 5'-oxygen atom from nucleotide agonist derivatives reduces but does not prevent interaction with the P2Y(2) receptor. Uridine 5'-(phospho)phosphonate as well as the 5'-methylenephosphonate equivalent of UMP were inactive at the P2Y(4) receptor and exhibited maximal effects at the P2Y(2) receptor that were 50% of that of UTP suggesting novel action of these analogues.

Development of selective high affinity antagonists, agonists, and radioligands for the P2Y1 receptor

The P2Y(1) receptor is a member of the P2Y family of nucleotide-activated G protein-coupled receptors, and it is an important therapeutic target based on its broad tissue distribution and essential role in platelet aggregation. We have designed a set of highly selective and diverse pharmacological tools for studying the P2Y(1) receptor using a rational approach to ligand design. Based on the discovery that bisphosphate analogues of the P2Y(1) receptor agonist, ADP, are partial agonists/competitive antagonists of this receptor, an iterative approach was used to develop competitive antagonists with enhanced affinity and selectivity. Halogen substitutions of the 2-position of the adenine ring provided increased affinity while an N(6) methyl substitution eliminated partial agonist activity. Furthermore, various replacements of the ribose ring with symmetrically branched, phosphorylated acyclic structures revealed that the ribose is not necessary for recognition at the P2Y(1) receptor. Finally, replacement of the ribose ring with a five member methanocarba ring constrained in the Northern conformation conferred dramatic increases in affinity to both P2Y(1) receptor antagonists as well as agonists. These combined structural modifications have resulted in a series of selective high affinity antagonists of the P2Y(1) receptor, two broadly applicable radioligands, and a high affinity agonist capable of selectively activating the P2Y(1) receptor in human platelets. Complementary receptor modeling and computational ligand docking have provided a putative structural framework for the drug-receptor interactions. A similar rational approach is being applied to develop selective ligands for other subtypes of P2Y receptors.

Ribosomal S6 kinase 2 directly phosphorylates the 5-hydroxytryptamine 2A (5-HT2A) serotonin receptor, thereby modulating 5-HT2A signaling

The 5-hydroxytryptamine 2A (5-HT(2A)) receptor is a member of the G protein-coupled receptor superfamily (GPCR) and plays a key role in transducing a variety of cellular signals elicited by 5-hydroxytryptamine in both peripheral and central tissues. Despite its broad physiological importance, our current understanding of 5-HT(2A) receptor regulation is incomplete. We recently reported the novel finding that the multifunctional ERK effector ribosomal S6 kinase 2 (RSK2) physically interacts with the 5-HT(2A) receptor third intracellular (i3) loop and modulates receptor signaling (Sheffler, D. J., Kroeze, W. K., Garcia, B. G., Deutch, A. Y., Hufeisen, S. J., Leahy, P., Bruning, J. C., and Roth, B. L. (2006) Proc. Natl. Acad. Sci. U. S. A. 103, 4717-4722). We report here that RSK2 directly phosphorylates the 5-HT(2A) receptor i3 loop at the conserved residue Ser-314, thereby modulating 5-HT(2A) receptor signaling. Furthermore, these studies led to the discovery that RSK2 is required for epidermal growth factor-mediated heterologous desensitization of the 5-HT(2A) receptor. We arrived at these conclusions via multiple lines of evidence, including in vitro kinase experiments, tandem mass spectrometry, and site-directed mutagenesis. Our findings were further validated using phospho-specific Western blot analysis, metabolic labeling studies, and whole-cell signaling experiments. These results support a novel regulatory mechanism in which a downstream effector of the ERK/MAPK pathway directly interacts with, phosphorylates, and modulates signaling of the 5-HT(2A) serotonin receptor. To our knowledge, these findings are the first to demonstrate that a downstream member of the ERK/MAPK cascade phosphorylates a GPCR as well as mediates cross-talk between a growth factor and a GPCR.

Synthesis and potency of novel uracil nucleotides and derivatives as P2Y2 and P2Y6 receptor agonists

The phosphate, uracil, and ribose moieties of uracil nucleotides were varied structurally for evaluation of agonist activity at the human P2Y(2), P2Y(4), and P2Y(6) receptors. The 2-thio modification, found previously to enhance P2Y(2) receptor potency, could be combined with other favorable modifications to produce novel molecules that exhibit high potencies and receptor selectivities. Phosphonomethylene bridges introduced for stability in analogues of UDP, UTP, and uracil dinucleotides markedly reduced potency. Truncation of dinucleotide agonists of the P2Y(2) receptor, in the form of Up(4)-sugars, indicated that a terminal uracil ring is not essential for moderate potency at this receptor and that specific SAR patterns are observed at this distal end of the molecule. Key compounds reported in this study include 9, alpha,beta-methylene-UDP, a P2Y(6) receptor agonist; 30, Up(4)-phenyl ester and 34, Up(4)-[1]glucose, selective P2Y(2) receptor agonists; dihalomethylene phosphonate analogues 16 and 41, selective P2Y(2) receptor agonists; 43, the 2-thio analogue of INS37217 (P(1)-(uridine-5')-P(4)-(2'-deoxycytidine-5')tetraphosphate), a potent and selective P2Y(2) receptor agonist.

Crystal structure of Rac1 bound to its effector phospholipase C-β2

Although diverse signaling cascades require the coordinated regulation of heterotrimeric G proteins and small GTPases, these connections remain poorly understood. We present the crystal structure of the GTPase Rac1 bound to phospholipase C-beta2 (PLC-beta2), a classic effector of heterotrimeric G proteins. Rac1 engages the pleckstrin-homology (PH) domain of PLC-beta2 to optimize its orientation for substrate membranes. Gbetagamma also engages the PH domain to activate PLC-beta2, and these two activation events are compatible, leading to additive stimulation of phospholipase activity. In contrast to PLC-delta, the PH domain of PLC-beta2 cannot bind phosphoinositides, eliminating this mode of regulation. The structure of the Rac1-PLC-beta2 complex reveals determinants that dictate selectivity of PLC-beta isozymes for Rac GTPases over other Rho-family GTPases, and substitutions within PLC-beta2 abrogate its stimulation by Rac1 but not by Gbetagamma, allowing for functional dissection of this integral signaling node.