Signaling in time and space: G protein-coupled receptors and mitogen-activated protein kinases

GPCR Intracellular Loop Regulation of Beta-Arrestin-Mediated Endosomal Signaling Dynamics

G protein-coupled receptors (GPCRs) are currently appreciated to be routed to diverse cellular platforms to generate both G protein-dependent and -independent signals. The latter has been best studied with respect to β-arrestin-associated receptor internalization and trafficking to signaling endosomes for extracellular signal-regulated kinase (ERK) activation. However, how GPCR structural and conformational variants regulate endosomal ERK signaling dynamics, which can be central in neural development, plasticity, and disease processes, is not well understood. Among class B GPCRs, the PACAP-selective PAC1 receptor is unique in the expression of variants that can contain intracellular loop 3 (ICL3) cassette inserts. The nervous system expresses preferentially the PAC1Null (no insert) and PAC1Hop (28-amino acid Hop insert) receptor variants. Our molecular modeling and signaling studies revealed that the PAC1Null and PAC1Hop receptor variants can associate with β-arrestin differentially, resulting in enhanced receptor internalization and ERK activation for the PAC1Hop variant. The study amplifies our understandings of GPCR intracellular loop structure/function relationships with the first example of how the duration of endosomal ERK activation can be guided by ICL3. The results provide a framework for how changes in GPCR variant expression can impact developmental and homeostatic processes and may be contributory to maladaptive neuroplasticity underlying chronic pain and stress-related disorders.

Adenoviral transduction of EGFR into pregnancy-adapted uterine artery endothelial cells remaps growth factor induction of endothelial dysfunction

During pregnancy, uterine vascular vasodilation is enhanced through adapted Ca2+ signaling, facilitated through increased endothelial connexin 43 (Cx43) gap junctional communication (GJC). In preeclampsia (PE), this adaptive response is missing. Of note, the angiogenic factor VEGF can also act via Src and ERK to close Cx43 gap junctions. While VEGFR2 is necessary for such closure, a role VEGFR1 is less clear. We reasoned if VEGFR2 is acting alone, then substituting another growth factor receptor with VEGFR2-like signaling should have the same effect. In uterine artery endothelial cells derived from pregnant sheep (P-UAEC), endogenous EGFR expression is very low. When we used adenovirus to raise EGFR, we also dose-dependently induced EGF-sensitive Cx43 phosphorylation mainly via ERK, and corresponding loss of Ca2+ bursts, but eliminated VEGF effects on phosphorylation of Cx43 or loss of Ca2+ bursting. This surprising observation suggests that while activated EGFR may indeed substitute for VEGFR2, it also sequesters a limited pool of effector molecules needed for VEGFR2 to phosphorylate Cx43. Thus, low endogenous EGFR expression in P-UAEC may be a necessary strategy to allow VEGFR-2 control of GJC, a first step in initiating angiogenesis in healthy pregnancy. Of further note, trophoblasts are rich in EGFR, and we have demonstrated shed PLAP+/EGFR + extracellular vesicles in maternal circulation in first trimester plasma samples using nanoscale high resolution flow cytometry. Collectively our data suggest that placenta derived exosomes positive for EGFR should be further considered as a possible cause of endothelial dysfunction in women with PE.

5-HT1B Receptor-Mediated Activation of ERK1/2 Requires Both Gαi/o and β-Arrestin Proteins

5-HT_1B receptors modulate synaptic serotonin (5-HT) levels and play a significant role in the regulation of emotional behaviors. These receptors are Gα_i/o-coupled and inhibit adenylyl cyclase but have also been reported to activate MAP kinases; however, the details of signaling cascades downstream of 5-HT_1B receptor activation remain unclear, particularly in neuronal cells. We generated a stable 5-HT_1B receptor-expressing Neuro2A (N2A-1B) neuronal cell line and demonstrate that activation of these receptors by the selective 5-HT_1B agonist CP-94253 results in activation of ERK1/2 but not of other closely related MAP kinases. Phosphoproteomics revealed four novel phosphorylation sites on the third intracellular loop of the 5-HT_1B receptor, and mutations of serine-256 and serine-291 to alanine led to reduced levels of ERK1/2 phosphorylation following receptor activation. Inhibition of Gα_i/o signaling with pertussis toxin, as well as MEK1/2 inhibition with U0126, also reduced 5-HT_1B-mediated ERK1/2 phosphorylation. Finally, we found that knockout of either β-arrestin 1 or β-arrestin 2 prevented 5-HT_1B-mediated phosphorylation of ERK1/2. Taken together, these results show that 5-HT_1B receptor activation selectively induces ERK1/2 activation through both the Gα_i subunit and β-arrestin proteins. This work elucidates the signal transduction pathway of 5-HT_1B receptors, as well as key phosphorylation sites within the receptor that modulate ERK1/2 activation, and further characterizes the intracellular mechanisms that underlie 5-HT_1B receptor function.

Model-free and kinetic modelling approaches for characterising non-equilibrium pharmacological pathway activity: Internalisation of cannabinoid CB1 receptors

BACKGROUND AND PURPOSE

Receptor internalisation is by nature kinetic. Application of a standard equilibrium dose response model to describe the properties of a ligand inducing internalisation, while commonly used, are therefore problematic. Here, we propose two quantitative approaches to address this issue-(a) a model-free method and (b) a kinetic modelling approach-and systematically evaluate the performance of these methods against traditional equilibrium methods to characterise the internalisation profiles of cannabinoid CB1 receptor agonists.

EXPERIMENTAL APPROACH

Kinetic internalisation assays were conducted using a concentration series of six CB1 receptor ligands. Internalisation rate analysis and snapshot equilibrium analysis were performed. A model-free method was developed based on the mean residence time of internalisation. A kinetic internalisation model was developed under the quasi-steady state assumption.

KEY RESULTS

Rates of receptor internalisation depended on both agonist and concentration. Agonist potencies from snapshot equilibrium analysis increased with stimulation time, and there was no single time point at which internalisation profiles could infer agonist properties in a comparative manner. The model-free method yielded a time-invariant measure of potency/efficacy for internalisation. The kinetic model adequately described the internalisation of CB1 receptors over time and provided robust estimates of both potency and efficacy.

CONCLUSION AND IMPLICATIONS

Applying equilibrium analysis to a non-equilibrium pathway cannot provide a reliable estimate of agonist potency. Both the model-free and kinetic modelling approaches characterised the internalisation profiles of CB1 receptor agonists. The kinetic model provides additional advantages as a method to capture changes in receptor number during other functional assays.

Muscarinic receptor antagonists activate ERK-CREB signaling to augment neurite outgrowth of adult sensory neurons

A major cellular effector activated by G protein coupled receptors is extracellular signal-regulated kinase (ERK). The ERK signaling cascade regulates a variety of cellular processes including growth and proliferation. Both G protein and β-arrestin-mediated signaling lead to ERK activation by phosphorylation through different kinases. Recently, we have shown muscarinic acetylcholine type 1 receptor (M₁R) antagonists, muscarinic toxin 7 (MT7) and pirenzepine, elevated neurite outgrowth and protected from small and large fiber neuropathy in adult sensory neurons in various animal models. Thus, we tested the novel hypothesis that muscarinic antagonists could drive neurite outgrowth through altered M₁R-ERK signaling. We have used two dimensional isoelectric focusing/SDS-PAGE combined with analysis using multiple phospho-epitope specific antibodies to study ERK1/2 phosphorylation and activation of its downstream nuclear effector cyclic response element binding protein (CREB). Activated CREB is known to exhibit neuroprotective and growth promoting effects. One hour of treatment with MT7 and pirenzepine activated ERK through M₁R and induced a significant increase in levels of pCREB(S133) in cultured sensory neurons. Further, pharmacological blockade or siRNA based knockdown of ERK abolished the MT7 and pirenzepine mediated neuritogenic effect. In addition, we have shown drug-induced alterations of charged protein fractions that may possess additional post-translationally modified forms of ERK and CREB. For the first time we show that long-term treatment, e.g. 1 h, with muscarinic antagonists selective or specific for M₁R can activate a biased β-arrestin dependent ERK-CREB signal cascade. Our study gives novel insight into muscarinic antagonist-mediated modulation of M₁R-ERK-CREB signaling which could be exploited for therapy in neuropathic diseases.

The novel cannabinoid receptor GPR55 mediates anxiolytic-like effects in the medial orbital cortex of mice with acute stress

The G protein-coupled receptor 55 (GPR55) is a novel cannabinoid receptor, whose exact role in anxiety remains unknown. The present study was conducted to explore the possible mechanisms by which GPR55 regulates anxiety and to evaluate the effectiveness of O-1602 in the treatment of anxiety-like symptoms. Mice were exposed to two types of acute stressors: restraint and forced swimming. Anxiety behavior was evaluated using the elevated plus maze and the open field test. We found that O-1602 alleviated anxiety-like behavior in acutely stressed mice. We used lentiviral shRNA to selective ly knockdown GPR55 in the medial orbital cortex and found that knockdown of GPR55 abolished the anxiolytic effect of O-1602. We also used Y-27632, a specific inhibitor of ROCK, and U73122, an inhibitor of PLC, and found that both inhibitors attenuated the effectiveness of O-1602. Western blot analysis revealed that O-1602 downregulated the expression of GluA1 and GluN2A in mice. Taken together, these results suggest that GPR55 plays an important role in anxiety and O-1602 may have therapeutic potential in treating anxiety-like symptoms.

Regulation of G Protein-Coupled Receptors by Ubiquitination

G protein-coupled receptors (GPCRs) comprise the largest family of membrane receptors that control many cellular processes and consequently often serve as drug targets. These receptors undergo a strict regulation by mechanisms such as internalization and desensitization, which are strongly influenced by posttranslational modifications. Ubiquitination is a posttranslational modification with a broad range of functions that is currently gaining increased appreciation as a regulator of GPCR activity. The role of ubiquitination in directing GPCRs for lysosomal degradation has already been well-established. Furthermore, this modification can also play a role in targeting membrane and endoplasmic reticulum-associated receptors to the proteasome. Most recently, ubiquitination was also shown to be involved in GPCR signaling. In this review, we present current knowledge on the molecular basis of GPCR regulation by ubiquitination, and highlight the importance of E3 ubiquitin ligases, deubiquitinating enzymes and β-arrestins. Finally, we discuss classical and newly-discovered functions of ubiquitination in controlling GPCR activity.

Crosstalk between FSH and relaxin at the end of the proliferative stage of rat Sertoli cells

Follicle-stimulating hormone (FSH) stimulates the proliferation of immature Sertoli cells through the activation of PI3K/AKT/mTORC1 and MEK/ERK1/2 pathways. Mature Sertoli cells stop proliferating and respond to FSH by stimulating cAMP production. To gain insight into possible mechanisms involved in this switch as well as the impact of paracrine factors that stimulate cell proliferation, we analyzed the effects of FSH and relaxin on intracellular signaling pathways involved with proliferation and differentiation in Sertoli cells from 15-day-old rats, which are close to the transition between the two stages. FSH stimulated ³H-thymidine incorporation and cyclin D1 expression, changes associated with proliferation. In contrast, FSH inhibited AKT and ERK1/2 phosphorylation, activated cAMP production and induced changes in several cell cycle genes that were compatible with differentiation. Relaxin also stimulated ³H-thymidine incorporation but increased phosphorylation of ERK1/2 and AKT. When both hormones were added simultaneously, relaxin attenuated FSH-mediated inhibition of ERK1/2 and AKT phosphorylation and FSH-mediated activation of cAMP production. FSH but not relaxin increased CREB phosphorylation, and relaxin but not FSH shifted NF-κB expression from the cytoplasm to the nucleus. Relaxin did not inhibit the effects of FSH on inhibin α and Bcl2 expression. We propose that at this time of Sertoli cell development, FSH starts to direct cells to differentiation through activation of cAMP/CREB and inhibition of ERK1/2 and AKT pathways. Relaxin counteracts FSH signaling through the inhibition of cAMP and activation of ERK1/2, AKT and NF-κB, but does not block the differentiation process triggered by FSH.

Regulation of N-Formyl Peptide Receptor Signaling and Trafficking by Arrestin-Src Kinase Interaction

Arrestins were originally described as proteins recruited to ligand-activated, phosphorylated G protein-coupled receptors (GPCRs) to attenuate G protein-mediated signaling. It was later revealed that arrestins also mediate GPCR internalization and recruit a number of signaling proteins including, but not limited to, Src family kinases, ERK1/2, and JNK3. GPCR-arrestin binding and trafficking control the spatial and temporal activity of these multi-protein complexes. In previous reports, we concluded that N-formyl peptide receptor (FPR)-mediated apoptosis, which occurs upon receptor stimulation in the absence of arrestins, is associated with FPR accumulation in perinuclear recycling endosomes. Under these conditions, inhibition of Src kinase and ERK1/2 prevented FPR-mediated apoptosis. To better understand the role of Src kinase in this process, in the current study we employed a previously described arrestin-2 (arr2) mutant deficient in Src kinase binding (arr2-P91G/P121E). Unlike wild type arrestin, arr2-P91G/P121E did not inhibit FPR-mediated apoptosis, suggesting that Src binding to arrestin-2 prevents apoptotic signaling. However, in cells expressing this mutant, FPR-mediated apoptosis was still blocked by inhibition of Src kinase activity, suggesting that activation of Src independent of arrestin-2 binding is involved in FPR-mediated apoptosis. Finally, while Src kinase inhibition prevented FPR-mediated-apoptosis in the presence of arr2-P91G/P121E, it did not prevent FPR-arr2-P91G/P121E accumulation in the perinuclear recycling endosome. On the contrary, inhibition of Src kinase activity mediated the accumulation of activated FPR-wild type arrestin-2 in recycling endosomes without initiating FPR-mediated apoptosis. Based on these observations, we conclude that Src kinase has two independent roles following FPR activation that regulate both FPR-arrestin-2 signaling and trafficking.

Azelastine desensitization of transient receptor potential vanilloid 1: a potential mechanism explaining its therapeutic effect in nonallergic rhinitis

BACKGROUND

Capsaicin, a prototypic transient receptor potential vanilloid 1 (TRPV1) agonist, has been shown to be more clinically effective in the treatment of nonallergic rhinitis (NAR) compared with other rhinitis subtypes. Azelastine has also been found to be clinically effective in the treatment of NAR but its mechanism(s) of action is still poorly elucidated. This study was designed to determine, using in vitro cell lines, whether topical therapies including azelastine have activity on TRPV1 ion channels similar to capsaicin.

METHODS

The effects of capsaicin (1 μM), azelastine (30 μM), bepotastine (10 μM), olopatadine (10 μM), and fluticasone (200 μM) on TRPV1 channels using mice neuronal cells (Cath.a), as surrogates for submucosal sensory neurons, and human nasal epithelial cells (hNEC) were determined and compared. For azelastine, bepotastine, and capsaicin, which elicited an agonist effect on TRPV1, live cell [Ca(2+)] signaling in Cath.a cells and hNECs expressing TRPV1 were performed in the absence and presence of capsazepine at 10 μM (a TRPV1 antagonist) or using wild-type mouse embryonic fibroblasts (wtMEFs) that express TRPV1 ion channels and TRPV1 homozygous null mutant (TRPV1-/-) knockout MEF cells as controls to establish TRPV1 channel selectivity. As azelastine has previously been found clinically effective in NAR, additional experiments were performed to determine its ability to desensitize TRPV1 ion channels and its effect on regulating intracellular calcium homeostasis.

RESULTS

Cath.a cells treated with azelastine, bepotastine, or capsaicin showed a significant increase in TRPV1-dependant (Ca(2+)) specific cytosolic fluorescence. Continuous treatment with azelastine or capsaicin resulted in desensitization of TRPV1 channels. In hNECs, azelastine stimulation resulted in Ca(2+) shifts from the cytosol to mitochondria and overexpression of hematopoietic cell-specific Lyn substrate 1-associated protein X1, which may thus be effective in cytosolic Ca(2+) homeostasis.

CONCLUSION

Azelastine, similar to capsaicin, exhibits direct activity on TRPV1 ion channels that may represent a novel mechanistic pathway explaining its clinical efficacy in NAR.

Searching for novel Cdk5 substrates in brain by comparative phosphoproteomics of wild type and Cdk5-/- mice

Protein phosphorylation is the most common post-translational modification that regulates several pivotal functions in cells. Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase which is mostly active in the nervous system. It regulates several biological processes such as neuronal migration, cytoskeletal dynamics, axonal guidance and synaptic plasticity among others. In search for novel substrates of Cdk5 in the brain we performed quantitative phosphoproteomics analysis, isolating phosphoproteins from whole brain derived from E18.5 Cdk5^+/+ and Cdk5^−/− embryos, using an Immobilized Metal-Ion Affinity Chromatography (IMAC), which specifically binds to phosphorylated proteins. The isolated phosphoproteins were eluted and isotopically labeled for relative and absolute quantitation (iTRAQ) and mass spectrometry identification. We found 40 proteins that showed decreased phosphorylation at Cdk5^−/− brains. In addition, out of these 40 hypophosphorylated proteins we characterized two proteins, :MARCKS (Myristoylated Alanine-Rich protein Kinase C substrate) and Grin1 (G protein regulated inducer of neurite outgrowth 1). MARCKS is known to be phosphorylated by Cdk5 in chick neural cells while Grin1 has not been reported to be phosphorylated by Cdk5. When these proteins were overexpressed in N2A neuroblastoma cell line along with p35, serine phosphorylation in their Cdk5 motifs was found to be increased. In contrast, treatments with roscovitine, the Cdk5 inhibitor, resulted in an opposite effect on serine phosphorylation in N2A cells and primary hippocampal neurons transfected with MARCKS. In summary, the results presented here identify Grin 1 as novel Cdk5 substrate and confirm previously identified MARCKS as a a bona fide Cdk5 substrate.

Development of functionally selective, small molecule agonists at kappa opioid receptors

The kappa opioid receptor (KOR) is widely expressed in the CNS and can serve as a means to modulate pain perception, stress responses, and affective reward states. Therefore, the KOR has become a prominent drug discovery target toward treating pain, depression, and drug addiction. Agonists at KOR can promote G protein coupling and βarrestin2 recruitment as well as multiple downstream signaling pathways, including ERK1/2 MAPK activation. It has been suggested that the physiological effects of KOR activation result from different signaling cascades, with analgesia being G protein-mediated and dysphoria being mediated through βarrestin2 recruitment. Dysphoria associated with KOR activation limits the therapeutic potential in the use of KOR agonists as analgesics; therefore, it may be beneficial to develop KOR agonists that are biased toward G protein coupling and away from βarrestin2 recruitment. Here, we describe two classes of biased KOR agonists that potently activate G protein coupling but weakly recruit βarrestin2. These potent and functionally selective small molecule compounds may prove to be useful tools for refining the therapeutic potential of KOR-directed signaling in vivo.

Minireview: Targeting GPCR Activated ERK Pathways for Drug Discovery

It has become clear in recent years that multiple signal transduction pathways are employed upon GPCR activation. One of the major cellular effectors activated by GPCRs is extracellular signal-regulated kinase (ERK). Both G-protein and β-arrestin mediated signaling pathways can lead to ERK activation. However, depending on activation pathway, the subcellular destination of activated ERK1/2 may be different. G-protein -dependent ERK activation results in the translocation of active ERK to the nucleus, whereas ERK activated via an arrestin-dependent mechanism remains largely in the cytoplasm. The subcellular location of activated ERK1/2 determines the downstream signaling cascade. Many substrates of ERK1/2 are found in the nucleus: nuclear transcription factors that participate in gene transcription, cell proliferation and differentiation. ERK1/2 substrates are also found in cytosol and other cellular organelles: they may play roles in translation, mitosis, apoptosis and cross-talk with other signaling pathways. Therefore, determining specific subcellular locations of activated ERK1/2 mediated by GPCR ligands would be important in correlating signaling pathways with cellular physiological functions. While GPCR-stimulated selective ERK pathway activation has been studied in several receptor systems, exploitation of these different signaling cascades for therapeutics has not yet been seriously pursued. Many old drug candidates were identified from screens based on G-protein signaling assays, and their activity on β-arrestin signaling pathways being mostly unknown, especially regarding their subcellular ERK pathways. With today’s knowledge of complicated GPCR signaling pathways, drug discovery can no longer rely on single-pathway approaches. Since ERK activation is an important signaling pathway and associated with many physiological functions, targeting the ERK pathway, especially specific subcellular activation pathways should provide new avenues for GPCR drug discovery.

The A, B, Cs of G-protein-coupled receptor pharmacology in assay development for HTS

G-protein-coupled receptors represent one of the most important areas of research in the pharmaceutical industry, being one of the largest druggable gene families. Recognising this fact, manufacturers have developed a huge variety of homogeneous assay technologies that facilitate the quantification of receptor ligand binding events and their downstream signalling cascades. However, while early emphasis was placed on the most sensitive, high-throughput and cost-effective screening technologies to enable identification of the most lead matter for further development, in recent years emphasis has shifted to a focus on maximising the identification of compounds that are new and developing assays that are more biologically/pharmacologically relevant. Therefore, this review provides an overview of the binding and functional techniques available for high-throughput screening, with particular attention on how assay application and configuration can be maximised to ensure their successful identification of relevant chemical matter and thereby optimising project success.

Cardiac intercellular communication: are myocytes and fibroblasts fair-weather friends?

The cardiac fibroblast (CF) has historically been thought of as a quiescent cell of the heart, passively maintaining the extracellular environment for the cardiomyocytes (CM), the functional cardiac cell type. The increasingly appreciated role of the CF, however, extends well beyond matrix production, governing many aspects of cardiac function including cardiac electrophysiology and contractility. Importantly, its contributions to cardiac pathophysiology and pathologic remodeling have created a shift in the field's focus from the CM to the CF as a therapeutic target in the treatment of cardiac diseases. In response to cardiac injury, the CF undergoes a pathologic phenotypic transition into a myofibroblast, characterized by contractile smooth muscle proteins and upregulation of collagens, matrix proteins, and adhesion molecules. Further, the myofibroblast upregulates expression and secretion of a variety of pro-inflammatory, profibrotic mediators, including cytokines, chemokines, and growth factors. These mediators act in both an autocrine fashion to further activate CFs, as well as in a paracrine manner on both CMs and circulating inflammatory cells to induce myocyte dysfunction and chronic inflammation, respectively. Together, cell-specific cytokine-induced effects exacerbate pathologic remodeling and progression to HF. A better understanding of this dynamic intercellular communication will lead to novel targets for the attenuation of cardiac remodeling. Current strategies aimed at targeting cytokines have been largely unsuccessful in clinical trials, lending insights into ways that such intercellular cross talk can be more effectively attenuated. This review will summarize the current knowledge regarding CF functions in the heart and will discuss the regulation and signaling behind CF-mediated cytokine production and function. We will then highlight clinical trials that have exploited cytokine cross talk in the treatment of heart failure and provide novel strategies currently under investigation that may more effectively target pathologic CF-CM communication for the treatment of cardiac disease. This review explores novel mechanisms to directly attenuate heart failure progression through inhibition of signaling downstream of pro-inflammatory cytokines that are elevated after cardiac injury.

Signaling to cardiac hypertrophy: insights from human and mouse RASopathies

Cardiac hypertrophy is the heart's response to a variety of extrinsic and intrinsic stimuli, some of which might finally lead up to a maladaptive state. An integral part of the pathogenesis of the hypertrophic cardiomyopathy disease (HCM) is the activation of the rat sarcoma (RAS)/RAF/MEK (mitogen-activated protein kinase kinase)/MAPK (mitogen-activated protein kinase) cascade. Therefore, the molecular signaling involving RAS has been the subject of intense research efforts, particularly after the identification of the RASopathies. These constitute a class of developmental disorders caused by germline mutations affecting proteins contributing to the RAS pathway. Among other phenotypic features, a subset of these syndromes is characterized by HCM, prompting researchers and clinicians to delve into the chief signaling constituents of cardiac hypertrophy. In this review, we summarize current advances in the knowledge of the molecular signaling events involved in the pathogenesis of cardiac hypertrophy through work completed on patients and on genetically manipulated animals with HCM and RASopathies. Important insights are drawn from the recognition of parallels between cardiac hypertrophy and cancer. Future research promises to further elucidate the complex molecular interactions responsible for cardiac hypertrophy, possibly pointing the way for the identification of new specific targets for the treatment of HCM.

The functional size of GPCRs - monomers, dimers or tetramers?

In almost 16 years since the word "dimer" was used in a publication to describe the organization of G protein-coupled receptors (GPCRs), a large number of studies have since weighed in on this notion. Are native, functional GPCRs monomers, dimers or as some would suggest even higher order structures? Here, we review some of the latest evidence regarding the organization of these receptors in both homo- and hetero-oligomeric formats, with a particular focus on β-adrenergic receptors. This is particularly important for understanding the allosteric nature of receptor/receptor interactions. It is likely that, over the course of evolution, mechanisms have come into play using all of the possible variations in receptor/receptor stoichiometry, depending on the cell and the physiological context in question. Finally, we provide some data that suggests that higher order structures of GPCRs, as with dimers themselves are probably assembled in the ER.

Lysophosphatidylcholine enhances the suppressive function of human naturally occurring regulatory T cells through TGF-β production

Naturally occurring CD4(+)CD25(+) regulatory T cells (nTregs) play a pivotal role in the maintenance of self-tolerance and immune homeostasis. To gain insight into the mechanism of action of nTregs in pathological and physiological immune responses, it is important to analyze bioactive molecules that modulate the maintenance and function of nTregs. From a library of bioactive lipids, we obtained lysophosphatidylcholine (LPC) as a molecule that enhanced the Foxp3 expression and suppressive function of human nTregs significantly in comparison with those of DMSO-treated nTregs (control). The expression levels of TGF-β1 mRNA and protein in LPC-treated nTregs were significantly higher than those in control nTregs. After treatment with anti-TGF-β1 antibody, the increases in Foxp3 expression and the suppressive properties of LPC-treated nTregs returned to the levels observed in control nTregs. These findings indicate that LPC enhances Foxp3 expression and the suppressive function of nTregs through TGF-β1 produced by nTregs themselves. Experimental knockdown of G2A and GPR4 showed that this LPC-induced TGF-β1 expression in nTregs was due to G2A signaling, and did not involve GPR4. Moreover, JNK was a major contributor to LPC-induced TGF-β1 expression in nTregs, although LPC activated MAPKs including ERK1/2, p38 MAPK, and JNK via G2A. LPC is a bioactive lysolipid highly abundant in the circulation. Therefore, LPC may contribute to the maintenance and function of human nTregs in vivo.

Involvement of P2X receptors in the regulation of insulin secretion, proliferation and survival in mouse pancreatic β-cells

In order to clarify the functional role of ionotropic purinergic (P2X) receptors in pancreatic β-cells, we examined the effect of several P2 receptor agonists and antagonists on insulin secretion by mouse pancreatic islets, mouse Beta-TC6 cell proliferation and survival of dispersed islet cells in culture. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed the expression of mRNAs of P2X(4) receptor in mouse islets and P2X(1), P2X(2), P2X(3), P2X(4), P2X(5) and P2X(7) receptors in Beta-TC6 cells. The presence of P2X(4) receptor proteins in islets and Beta-TC6 cells was confirmed by immunofluorescent staining and Western blot analysis. We have previously found that the functional P2Y(1) receptor but not P2Y(2) and P2Y(4) receptors was present in islets. In this study we found that a nonspecific P2 receptor agonist, ATP (1 μM) stimulated insulin secretion by islets in the presence of high glucose (20 mM) in culture. The effect of ATP was partially inhibited by a P2 receptor antagonist PPADS as well as a P2Y(1) receptor antagonist MRS2179. In addition, a P2X(4) receptor potentiator ivermectin per se augmented glucose-induced insulin secretion and slightly potentiated the effect of ATP. These results suggested the involvement of P2Y(1)and P2X receptors. We also found that ATP inhibited proliferation of Beta-TC6 cells in a concentration-dependent manner during 72 h culture. The inhibitory effect of ATP was completely reversed by PPADS and partially by treating cells with small interfering RNA targeted for P2X(4) receptor mRNA. Furthermore, we found that the phosphorylation of the extracellular signal-regulated kinase 1 and 2 (ERK1/2) was suppressed by treatment with ATP in Beta-TC6 cells. In addition, we found that ATP reduced the cell viability and DNA synthesis of islet cells in culture. The effect of ATP on the cell viability was blocked by PPADS or MRS2179. These results suggested that P2X receptors as well as the P2Y(1) receptor played a role in the modulation of insulin secretion, proliferation and cell viability in mouse pancreatic β-cells.

GPR54 regulates ERK1/2 activity and hypothalamic gene expression in a Gα(q/11) and β-arrestin-dependent manner

G protein-coupled receptor 54 (GPR54) is a G_q/11-coupled 7 transmembrane-spanning receptor (7TMR). Activation of GPR54 by kisspeptin (Kp) stimulates PIP₂ hydrolysis, Ca²⁺ mobilization and ERK1/2 MAPK phosphorylation. Kp and GPR54 are established regulators of the hypothalamic-pituitary-gonadal (HPG) axis and loss-of-function mutations in GPR54 are associated with an absence of puberty and hypogonadotropic hypogonadism, thus defining an important role of the Kp/GPR54 signaling system in reproductive function. Given the tremendous physiological and clinical importance of the Kp/GPR54 signaling system, we explored the contributions of the GPR54-coupled G_q/11 and β-arrestin pathways on the activation of a major downstream signaling molecule, ERK, using G_q/11 and β-arrestin knockout mouse embryonic fibroblasts. Our study revealed that GPR54 employs the G_q/11 and β-arrestin-2 pathways in a co-dependent and temporally overlapping manner to positively regulate ERK activity and pERK nuclear localization. We also show that while β-arrestin-2 potentiates GPR54 signaling to ERK, β-arrestin-1 inhibits it. Our data also revealed that diminished β-arrestin-1 and -2 expression in the GT1-7 GnRH hypothalamic neuronal cell line triggered distinct patterns of gene expression following Kp-10 treatment. Thus, β-arrestin-1 and -2 also regulate distinct downstream responses in gene expression. Finally, we showed that GPR54, when uncoupled from the G_q/11 pathway, as is the case for several naturally occurring GPR54 mutants associated with hypogonadotropic hypogonadism, continues to regulate gene expression in a G protein-independent manner. These new and exciting findings add significantly to our mechanistic understanding of how this important receptor signals intracellularly in response to kisspeptin stimulation.

Pharmacological characterization of GPR55, a putative cannabinoid receptor

GPR55 has recently attracted much attention as another member of the cannabinoid family, potentially explaining physiological effects that are non-CB1/CB2 mediated. However, the data gathered so far are conflicting with respect to its pharmacology. We review the primary literature to date on GPR55, describing its discovery, structure, pharmacology and potential physiological functions. The CB1 receptor antagonist/inverse agonist AM251 has been shown to be a GPR55 agonist in all reports in which it was evaluated, as has the lysophospholipid, lysophosphatidylinositol (LPI). Whether GPR55 responds to the endocannabinoid ligands anandamide and 2-arachidonylglycerol and the phytocannabinoids, delta-9-tetrahydrocannabidiol and cannabidiol, is cell type and tissue-dependent. GPR55 has been shown to utilize G(q), G(12), or G(13) for signal transduction; RhoA and phospholipase C are activated. Experiments with mice in which GPR55 has been inactivated reveal a role for this receptor in neuropathic and inflammatory pain as well as in bone physiology. Thus delineating the pharmacology of this receptor and the discovery of selective agonists and antagonists merits further study and could lead to new therapeutics.

Arrestin development: emerging roles for beta-arrestins in developmental signaling pathways

Arrestins were identified as mediators of G protein-coupled receptor (GPCR) desensitization and endocytosis. However, it is now clear that they scaffold many intracellular signaling networks to modulate the strength and duration of signaling by diverse types of receptors--including those relevant to the Hedgehog, Wnt, Notch, and TGFbeta pathways--and downstream kinases such as the MAPK and Akt/PI3K cascades. The involvement of arrestins in many discrete developmental signaling events suggests an indispensable role for these multifaceted molecular scaffolds.

Atypical responsiveness of the orphan receptor GPR55 to cannabinoid ligands

The cannabinoid receptor 1 (CB(1)) and CB(2) cannabinoid receptors, associated with drugs of abuse, may provide a means to treat pain, mood, and addiction disorders affecting widespread segments of society. Whether the orphan G-protein coupled receptor GPR55 is also a cannabinoid receptor remains unclear as a result of conflicting pharmacological studies. GPR55 has been reported to be activated by exogenous and endogenous cannabinoid compounds but surprisingly also by the endogenous non-cannabinoid mediator lysophosphatidylinositol (LPI). We examined the effects of a representative panel of cannabinoid ligands and LPI on GPR55 using a beta-arrestin-green fluorescent protein biosensor as a direct readout of agonist-mediated receptor activation. Our data demonstrate that AM251 and SR141716A (rimonabant), which are cannabinoid antagonists, and the lipid LPI, which is not a cannabinoid receptor ligand, are GPR55 agonists. They possess comparable efficacy in inducing beta-arrestin trafficking and, moreover, activate the G-protein-dependent signaling of protein kinase CbetaII. Conversely, the potent synthetic cannabinoid agonist CP55,940 acts as a GPR55 antagonist/partial agonist. CP55,940 blocks GPR55 internalization, the formation of beta-arrestin GPR55 complexes, and the phosphorylation of ERK1/2; CP55,940 produces only a slight amount of protein kinase CbetaII membrane recruitment but does not stimulate membrane remodeling like LPI, AM251, or rimonabant. Our studies provide a paradigm for measuring the responsiveness of GPR55 to a variety of ligand scaffolds comprising cannabinoid and novel compounds and suggest that at best GPR55 is an atypical cannabinoid responder. The activation of GPR55 by rimonabant may be responsible for some of the off-target effects that led to its removal as a potential obesity therapy.

Cardiac hypertrophy: targeting Raf/MEK/ERK1/2-signaling

Over the past two decades, basic research has revealed a complex network of regulatory mechanisms that control the ERK1/2-signaling cascade. ERK1/2 mediate cardiac hypertrophy, a major risk factor for the development of arrhythmias, heart failure and sudden death, but also beneficial effects, e.g. protection of the heart from cell death and ischemic injury. Selective targeting of these ambiguous ERK functions could provide a powerful tool in the treatment of cardiac disease. This short review will discuss new mechanistic insights into ERK1/2-dependent development of cardiac hypertrophy and the prospect to translate this knowledge into future therapeutic strategies.

Ligusticum chuanxiong as a potential neuroprotectant for preventing serum deprivation-induced apoptosis in rat pheochromocytoma cells: functional roles of mitogen-activated protein kinases

ETHNOPHARMACOLOGICAL RELEVANCE

Ligusticum chuanxiong (LC) as a common component in many traditional Chinese medicinal formulas and decoctions has been used to treat different central nervous diseases, suggesting a neuroprotective function.

AIM OF THE STUDY

To investigate the functional roles of mitogen-activated protein kinases (MAPKs) in mediating the neuroprotection of LC.

MATERIALS AND METHODS

Different extractions of LC were applied with or without MAPK inhibitor to test their protection against serum deprivation-induced apoptosis in rat neuronal-like pheochromocytoma (PC12) cells as revealed by an MTT assay or Hoechst staining. Western blot was used to identify the activations of MAPKs.

RESULTS

The most effective butanol extraction (LC-BuOH) was used in the following experiments. LC-BuOH reversed serum deprivation-induced decreased phosphorylation of extracellular signal-regulated kinase (ERK) and increased phosphorylation of c-Jun NH(2)-terminal kinase (JNK) and p38, the family of MAPKs. A PKA inhibitor, blocked the protection of LC-BuOH and partially blocked LC-BuOH-induced alterations in MAPKs, suggesting that the LC-BuOH regulates MAPKs through both PKA-dependent and -independent pathways. Although PD 98059, an inhibitor of MEK which activates ERK, blocked LC-BuOH-induced ERK phosphorylation, it did not block the protection of LC-BuOH.

CONCLUSIONS

LC-BuOH mediates protection by suppressing JNK/p38 instead of activating ERK activity.

Somatostatin receptors 1 and 5 heterodimerize with epidermal growth factor receptor: agonist-dependent modulation of the downstream MAPK signalling pathway in breast cancer cells

The role of somatostatin (SST) and epidermal growth factor (EGF) in breast cancer is undisputed; however, the molecular mechanisms underlying their antiproliferative or proliferative effects are not well understood. We initially confirmed that breast tumour tissues express all five somatostatin receptors (SSTR1-5) and four epidermal growth factor receptors (ErbB1-4). Subsequently, to gain insight into the function of SSTRs and ErbBs in oestrogen receptor (ER)-positive (MCF-7) or ERalpha-negative (MDA-MB-231) breast cancer cells, we defined SSTR1, SSTR5 and ErbB1 mRNA and protein expression in these two tumour cell lines. Consistent with previous studies showing SSTR1/SSTR5 heterodimerization and having seen cell-specific and ligand-selective alterations in receptor expression, we next elucidated whether SSTR1 and SSTR5 functionally interact with ErbB1 using pbFRET analysis. We subsequently determined the effects of SST and EGF either alone, or in combination, on selected downstream signalling molecules such as erk1/2, p38 and JNK. Here, we showed that both SST and EGF influenced erk1/2 phosphorylation and that SST modulated the effects of EGF in a cell-specific manner. We also demonstrated agonist-, time and cell-dependent regulation of p38 phosphorylation. We further investigated modulation of Grb2, SOS, Shc, SH-PTP1 and SH-PTP2. ErbB1 adaptor proteins known to play a role in MAPK activation, Shc, Grb2 and SOS, changed in an agonist- and cell-specific manner whereas, SH-PTP1 and SH-PTP2, adaptor proteins reported to interact with SSTRs, translocated from the cytosol to membrane in a cell-specific manner following SST and/or EGF treatment. Although several previous studies have shown crosstalk between RTKs and GPCRs, there are no reports describing SSTR (GPCR) modulation of ErbBs (RTK) in breast cancer. To the best of our knowledge, this is the first report describing crosstalk/interactions between SSTRs and ErbBs.

Panax ginseng induces anterograde transport of pigment organelles in Xenopus melanophores

Melanophores from Xenopus laevis are pigmented cells, capable of quick colour changes through cyclic adenosine 3':5'-monophosphate (cAMP) coordinated transport of their intracellular pigment granules, melanosomes. In this study we use the melanophore cell line to evaluate the effects of Panax ginseng extract G115 on organelle transport. Absorbance readings of melanophore-coated microplates, Correlate-EIA direct cAMP enzyme immunoassay kit, and western blot were used to measure the melanosome movement and changes in intracellular signalling. We show that Panax ginseng induces a fast concentration-dependent anterograde transport of the melanosomes. No significant increase in the cAMP level was seen and pre-incubation of melanophores with the protein kinase C (PKC) inhibitor EGF-R Fragment 651-658 (M-EGF) only partly decreased the ginseng-induced dispersion. We also demonstrate that Panax ginseng, endothelin-3 (ET-3) and alpha-melanocyte stimulating hormone (MSH) stimulate an activation of mitogen activated protein kinase (MAPK). Pre-incubation with M-EGF decreased the MAPK activity induced by ET-3 and MSH, but again only marginally affected the response of Panax ginseng. Thus, in melanophores we suggest that Panax ginseng stimulates an anterograde transport of pigment organelles via a non-cAMP and mainly PKC-independent pathway.

Extracellular acidification elicits spatially and temporally distinct Ca2+ signals

Extracellular acidification accompanies neoplastic transformation of tissues and increases with tumor aggressiveness [1, 2]. The intracellular signaling cascade triggered by this process remains poorly understood and may be linked to recently discovered proton-activated G protein-coupled receptors such as OGR1 and G2A [3, 4]. Here, we report that OGR1 and G2A are expressed in human medulloblastoma tissue and its corresponding neuronal cell line. We show that extracellular acidification activates phospholipase C, IP(3) formation, and subsequent Ca2+ release from thapsigargin-sensitive stores in neurons. The number of responsive cells and the amount of Ca2+ released from stores correlated positively with the extent of extracellular acidification. Ca2+ release recruited the MEK/ERK pathway, providing a mechanistic explanation for how acidification stimulates cell growth. In addition, acidification activated Ca2+-permeable ion channels through a mechanism dependent on phospholipase C but independent of store depletion or a cytoplasmic Ca2+ rise. Hence, extracellular acidification, to levels seen in tumor tissue, activates temporally and spatially distinct pathways that elevate Ca2+ and may be directly relevant for tumor cell biology.

IFN-{gamma}-mediated inhibition of MAPK phosphatase expression results in prolonged MAPK activity in response to M-CSF and inhibition of proliferation

Macrophages have the capacity to proliferate in response to specific growth factors, such as macrophage-colony stimulating factor (M-CSF). In the presence of several cytokines and activating factors, macrophages undergo growth arrest, become activated, and participate in the development of an immune response. We have previously observed that activation of extracellularly regulated kinase 1/2 (ERK-1/2) is required for macrophage proliferation in response to growth factors. A short and early pattern of ERK activity correlated with the proliferative response. In contrast, slightly prolonged patterns of activity of these kinases were induced by signals that lead to macrophage activation and growth arrest. IFN-gamma is the main endogenous Th1-type macrophage activator. Here we report that stimulation with IFN-gamma prolongs the pattern of ERK activity induced by M-CSF in macrophages. These effects correlate with IFN-gamma-mediated inhibition of the expression of several members of the MAPK phosphatase family, namely MKP-1, -2, and -4. Moreover, inhibition of MKP-1 expression using siRNA technology or synthetic inhibitors also led to elongated ERK activity and significant blockage of M-CSF-dependent proliferation. These data suggest that subtle changes in the time course of activity of members of the MAPK family contribute to the antiproliferative effects of IFN-gamma in macrophages.

Biology of somatostatin in breast cancer

The biological effects of the neuropeptide somatostatin (SST) are mediated via a family of five somatostatin receptors (SSTRs) belonging to a family of G-protein-coupled receptors (GPCRs). SSTR regulate the secretion of hormones, growth factors, neurotransmission and cell growth in receptor-specific manner. In addition, SST plays an inhibitory role in several mammary cancer models. These effects are mediated both indirectly through inhibition of hormones and growth factors which promote tumor growth as well as directly via SSTRs present on tumor cells to inhibit mitogenic signaling of growth factor receptor kinases leading to growth arrest and induction of apoptosis. Here, we present an overview on the role of SST and its analogs in breast cancer.

Intracellular mechanisms regulating corticotropin-releasing hormone receptor-2beta endocytosis and interaction with extracellularly regulated kinase 1/2 and p38 mitogen-activated protein kinase signaling cascades

Many important physiological roles of the urocortin (UCN) family of peptides as well as CRH involve the type 2 CRH receptor (CRH-R2) and downstream activation of multiple pathways. To characterize molecular determinants of CRH-R2 functional activity, we used HEK293 cells overexpressing recombinant CRH-R2beta and investigated mechanisms involved in attenuation of CRH-R2 signaling activity and uncoupling from intracellular effectors. CRH-R2beta-mediated adenylyl cyclase activation was sensitive to homologous desensitization induced by pretreatment with either UCN-II or the weaker agonist CRH. CRH-R2beta activation induced transient beta-arrestin1 and beta-arrestin2, as well as clathrin, recruitment to the plasma membrane. Beta-arrestin2 appeared to be the main beta-arrestin subtype associated with the receptor. This was followed by CRH-R2beta endocytosis in a mechanism that exhibited distinct agonist-dependent temporal characteristics. CRH-R2beta also induced transient activation of the ERK1/2 and p38MAPK signaling cascades that peaked at 5 min and returned to basal within 20-30 min. Unlike p38MAPK, activated ERK1/2 was localized both in the cytoplasm and nucleus. Experiments employing inhibitors of receptor endocytosis showed that CRH-R2beta-MAPK interaction does not require beta-arrestin, clathrin, or receptor endocytosis. Site-directed mutagenesis studies on CRH-R2beta C terminus showed that the amino acid cassette TAAV at the end of the C terminus is important for CRH-R2beta signaling because loss of a potential phospho-acceptor site in mutant receptors containing deletion or Ala substitution of the cassette TAAV resulted in reduced ERK1/2 activation and accelerated receptor internalization. These findings provide new insights about the signaling mechanisms regulating CRH-R2beta functional activity and determining its biological responses.

Dehydroepiandrosterone induces a neuroendocrine phenotype in nerve growth factor-stimulated chromaffin pheochromocytoma PC12 cells

The adrenal androgen dehydroepiandrosterone (DHEA) is produced in the inner zone of the adrenal cortex, which is in direct contact to adrenal medullary cells. Due to their close anatomical proximity and tightly intermingled cell borders, a direct interaction of adrenal cortex and medulla has been postulated. In humans congenital adrenal hyperplasia due to 21-hydroxylase deficiency results in androgen excess accompanied by severe adrenomedullary dysplasia and chromaffin cell dysfunction. Therefore, to define the mechanisms of DHEA action on chromaffin cell function, we investigated its effect on cell survival and differentiation processes on a molecular level in the chromaffin cell line PC12. DHEA lessened the positive effect of NGF on cell survival and neuronal differentiation. Nerve growth factor (NGF)-mediated induction of a neuronal phenotype was inhibited by DHEA as indicated by reduced neurite outgrowth and decreased expression of neuronal marker proteins such as synaptosome-associated protein of 25 kDa and vesicle-associated membrane protein-2. We examined whether DHEA may stimulate the cells toward a neuroendocrine phenotype. DHEA significantly elevated catecholamine release from unstimulated PC12 cells in the presence but not absence of NGF. Accordingly, DHEA enhanced the expression of the neuroendocrine marker protein chromogranin A. Next, we explored the possible molecular mechanisms of DHEA and NGF interaction. We demonstrate that NGF-induced ERK1/2 phosphorylation was reduced by DHEA. In summary, our data show that DHEA influences cell survival and differentiation processes in PC12 cells, possibly by interacting with the ERK1/2 MAPK pathway. DHEA drives NGF-stimulated cells toward a neuroendocrine phenotype, suggesting that the interaction of intraadrenal steroids and growth factors is required for the maintenance of an intact adrenal medulla.

Selective regulation of somatostatin receptor subtype signaling: evidence for constitutive receptor activation

Anterior pituitary hormone secretion is under tonic suppression by hypothalamic somatostatin signaling through somatostatin receptor subtypes (SSTs). Because some hormonal axes are known to be abnormally regulated by ligand-independent constitutively active G protein-coupled receptors, we tested pituitary SSTs for selective constitutive signaling. We therefore differentially silenced endogenous SST2, SST3, and SST5 in somatostatin-sensitive ACTH-secreting mouse AtT-20 pituitary corticotroph cells using small inhibitory RNA (siRNA) and analyzed downstream SSTs-regulated pathways. Transfection with siRNA reduced specific receptor subtype mRNA expression up to 82%. Specificity of receptor silencing was validated against negative controls with different gene-selective siRNAs, concordance of mRNA and cAMP changes, reduced potency of receptor-selective agonists, and phenotype rescue by overexpression of the silenced receptor. Mouse SST3 > SST5 > SST2 knockdown increased basal cAMP accumulation (up to 200%) and ACTH secretion (up to 60%). SST2- and SST5-selective agonist potencies were reduced by SST3- and SST5-silencing, respectively. SST5 > SST2 = SST3 silencing also increased basal levels of ERK1/2 phosphorylation. SST3- and SST5-knockdown increased cAMP was only partially blocked by pertussis toxin. The results show that SST2, SST3, and SST5 exhibit constitutive activity in mouse pituitary corticotroph cells, restraining adenylate cyclase and MAPK activation and ACTH secretion. SST3 mainly inhibits cAMP accumulation and ACTH secretion, whereas SST5 predominantly suppresses MAPK pathway activation. Therefore, SST receptor subtypes control pituitary cell function not only through somatostatin binding to variably expressed cell membrane receptor subtypes, but also by differential ligand-independent receptor-selective constitutive action.

alpha(1)-Adrenergic receptor stimulation of cell motility requires phospholipase D-mediated extracellular signal-regulated kinase activation

Phospholipase D is suspected to play a role in tumorigenesis, and the inhibition of phospholipase D has been associated with changes in several cellular events including invasion and migration. We report here that the specific alpha(1)-adrenergic receptor agonist, phenylepherine, signals to a growth factor pathway in a manner that requires phospholipase D activity in CCL39 fibroblasts. Phenylepherine increased extracellular signal-regulated kinase phosphorylation eightfold and promoted stress fiber formation threefold. Stress fiber formation was blocked when extracellular signal-regulated kinase activation was inhibited. Stimulation of CCL39 fibroblasts by phenylepherine increased the rate of wound healing fourfold in a wounding assay, while treatment with the MEK inhibitor, PD98059 reduced the closure of phenylepherine-induced wound healing to control levels. Addition of 1-butanol but not 2-butanol inhibited extracellular signal-regulated kinase activation by phenylepherine, presumably by blocking the formation of phosphatidic acid. Exogenously added cell permeable phosphatidic acid increased extracellular signal-regulated kinase activation in a time- and dose-dependent manner as well as stimulated the formation of stress fibers. 1-butanol also significantly inhibited the ability of phenylepherine to stimulate stress fiber formation and wound healing. Taken together, these results indicate a novel role for phospholipase D in the activation of the extracellular signal-regulated kinase growth factor pathway to stimulate early cellular events induced by phenylepherine.

Molecular aspects of the histamine H3 receptor

The cloning of the histamine H(3) receptor (H(3)R) cDNA in 1999 by Lovenberg et al. [10] allowed detailed studies of its molecular aspects and indicated that the H(3)R can activate several signal transduction pathways including G(i/o)-dependent inhibition of adenylyl cyclase, activation of phospholipase A(2), Akt and the mitogen activated kinase as well as the inhibition of the Na(+)/H(+) exchanger and inhibition of K(+)-induced Ca(2+) mobilization. Moreover, cloning of the H(3)R has led to the discovery several H(3)R isoforms generated through alternative splicing of the H(3)R mRNA. The H(3)R has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders like obesity, myocardial ischemia, migraine, inflammatory diseases and several CNS disorders like Alzheimer's disease, attention-deficit hyperactivity disorder and schizophrenia. In this paper, we review various molecular aspects of the hH(3)R including its signal transduction, dimerization and the occurrence of different H(3)R isoforms.

Seven-transmembrane receptor signalling and ERK compartmentalization

Vast numbers of extracellular signalling molecules exert effects on their target cells by activation of a relatively limited number of mitogen-activated protein kinase (MAPK) cascades, raising the question of how specificity is achieved. To a large extent, this appears to be attributable to differences in kinetics and compartmentalization of MAPK protein activation that are dictated by MAPK-associated proteins serving as scaffolds, anchors, activators or effectors. Here, we review spatiotemporal aspects of signalling via the Ras-Raf-extracellular signal-regulated kinase pathway, emphasizing recent work on roles of arrestins as scaffolds and transducers for seven transmembrane receptor signalling.

The V2 vasopressin receptor stimulates ERK1/2 activity independently of heterotrimeric G protein signalling

The V2 vasopressin receptor (V2R) activates the mitogen activated protein kinases (MAPK) ERK1/2 through a mechanism involving the scaffolding protein beta arrestin. Here we report that this activating pathway is independent of G alpha s, G alpha i, G alpha q or G betagamma and that the V2R-mediated activation of G alpha s inhibits ERK1/2 activity in a cAMP/PKA-dependent manner. In the HEK293 cells studied, the beta arrestin-promoted activation was found to dominate over the PKA-mediated inhibition of the pathway, leading to a strong vasopressin-stimulated ERK1/2 activation. Despite the strong MAPK activation and in contrast with other GPCR, V2R did not induce any significant increase in DNA synthesis, consistent with the notion that the stable interaction between V2R and beta arrestin prevents signal propagation to the nucleus. Beta arrestin was found to be essential for the ERK1/2 activation, indicating that the recruitment of the scaffolding protein is necessary and sufficient to initiate the signal in the absence of any other stimulatory cues. Based on the use of selective pharmacological inhibitors, dominant negative mutants and siRNA, we conclude that the beta arrestin-dependent activation of ERK1/2 by the V2R involves c-Src and a metalloproteinase-dependent trans-activation event. These findings demonstrate that beta arrestin is a genuine signalling initiator that can, on its own, engage a MAPK activation machinery upon stimulation of a GPCR by its natural ligand.

Stimulation of mitogen-activated protein kinase kinases (MEK1/2) by mu-, delta- and kappa-opioid receptor agonists in the rat brain: regulation by chronic morphine and opioid withdrawal

Opioid addiction modulates the extracellular signal-regulated kinase (ERK) leading to synaptic plasticity in the brain. ERK1/2 are stimulated by mitogen-activated protein kinase kinases (MEK1/2), but little is known about the regulation of MEK activity by opioid drugs. This study was designed to assess the acute effects of selective mu-, delta-, and kappa-opioid receptor agonists, as well as those induced by chronic morphine and opioid withdrawal, on the content of phosphorylated MEK1/2 in the rat brain. Sufentanil (1-30 microg/kg, 30-120 min) induced dose- and time-dependent increases in MEK1/2 phosphorylation in the cerebral cortex and corpus striatum (30-177%) through a naloxone-sensitive mechanism. Morphine (100 mg/kg, 2 h) also augmented MEK1/2 phosphorylation in the both brain regions (50-70%). Similarly, the selective delta-opioid receptor agonist SNC-80 (10 mg/kg, 30 min) increased MEK1/2 activity in the cortex (60%) that was antagonized by naltrindole. In contrast, the selective kappa-opioid receptor agonist (-)-U50488H (10 mg/kg, 30-120 min) did not modify significantly MEK1/2 phosphorylation in the cortex. Chronic morphine (10-100 mg/kg, 5 days) was not associated with alterations in the content of phosphorylated MEK1/2 in the brain (induction of tachyphylaxis to the acute effects). In morphine-dependent rats, however, naloxone (2 mg/kg)-precipitated withdrawal (2-6 h) induced robust increases in MEK1/2 phosphorylation in cortex (27-49%) and striatum (83-123%). Spontaneous opioid withdrawal (24 h) in morphine-dependent rats did not alter MEK1/2 activity in the brain. The findings may be relevant in the context of the pivotal role played by the MEK/ERK pathway in various long-lasting forms of synaptic plasticity associated with opioid addiction.

Rapid, nongenomic responses to ecdysteroids and catecholamines mediated by a novel Drosophila G-protein-coupled receptor

Nongenomic response pathways mediate many of the rapid actions of steroid hormones, but the mechanisms underlying such responses remain controversial. In some cases, cell-surface expression of classical nuclear steroid receptors has been suggested to mediate these effects, but, in a few instances, specific G-protein-coupled receptors (GPCRs) have been reported to be responsible. Here, we describe the activation of a novel, neuronally expressed Drosophila GPCR by the insect ecdysteroids ecdysone (E) and 20-hydroxyecdysone (20E). This is the first report of an identified insect GPCR interacting with steroids. The Drosophila melanogaster dopamine/ecdysteroid receptor (DmDopEcR) shows sequence homology with vertebrate beta-adrenergic receptors and is activated by dopamine (DA) to increase cAMP levels and to activate the phosphoinositide 3-kinase pathway. Conversely, E and 20E show high affinity for the receptor in binding studies and can inhibit the effects of DA, as well as coupling the receptor to a rapid activation of the mitogen-activated protein kinase pathway. The receptor may thus represent the Drosophila homolog of the vertebrate "gamma-adrenergic receptors," which are responsible for the modulation of various activities in brain, blood vessels, and pancreas. Thus, DmDopEcR can function as a cell-surface GPCR that may be responsible for some of the rapid, nongenomic actions of ecdysteroids, during both development and signaling in the mature adult nervous system.

Arrestin-mediated ERK Activation by Gonadotropin-releasing Hormone Receptors

Activation of seven-transmembrane region receptors typically causes their phosphorylation with consequent arrestin binding and desensitization. Arrestins also act as scaffolds, mediating signaling to Raf and ERK and, for some receptors, inhibiting nuclear translocation of ERK. GnRH receptors (GnRHRs) act via Gq/11 to stimulate the phospholipase C/Ca2+/protein kinase C (PKC) cascade and the Raf/MEK/ERK cassette. Uniquely, type I mammalian GnRHRs lack the C-tails that are found in other seven-transmembrane region receptors (including nonmammalian GnRHRs) and are implicated in arrestin binding. Here we have compared ERK signaling by human GnRHRs (hGnRHRs) and Xenopus GnRHRs (XGnRHRs). In HeLa cells, XGnRHRs underwent rapid and arrestin-dependent internalization and caused arrestin/green fluorescent protein (GFP) translocation to the membrane and endosomes, whereas hGnRHRs did not. Internalized XGnRHRs were co-localized with arrestin-GFP, whereas hGnRHRs were not. Both receptors mediated transient ERK phosphorylation and nuclear translocation (revealed by immunohistochemistry or by imaging of co-transfected ERK2-GFP), and for both, ERK phosphorylation was reduced by PKC inhibition but not by inhibiting epidermal growth factor receptor autophosphorylation. In the presence of PKC inhibitor, Deltaarrestin-(319-418) blocked XGnRHR-mediated, but not hGnRHR-mediated, ERK phosphorylation. When receptor number was varied, hGnRHRs activated phospholipase C and ERK more efficiently than XGnRHRs but were less efficient at causing ERK2-GFP translocation. At high receptor number, XGnRHRs and hGnRHRs both caused ERK2-GFP translocation to the nucleus, but at low receptor number, XGnRHRs caused ERK2-GFP translocation, whereas hGnRHRs did not. Thus, experiments with XGnRHRs have revealed the first direct evidence of arrestin-mediated (probably G protein-independent) GnRHR signaling, whereas those with hGnRHRs imply that scaffolds other than arrestins can determine GnRHR effects on ERK compartmentalization.

Osteoblast calcium-sensing receptor has characteristics of ANF/7TM receptors

There is evidence for a functionally important extracellular calcium-sensing receptor in osteoblasts, but there is disagreement regarding its identity. Candidates are CASR and a putative novel calcium-sensing receptor, called Ob.CASR. To further characterize Ob.CASR and to distinguish it from CASR, we examined the extracellular cation-sensing response in MC3T3-E1 osteoblasts and in osteoblasts derived from CASR null mice. We found that extracellular cations activate ERK and serum response element (SRE)-luciferase reporter activity in osteoblasts lacking CASR. Amino acids, but not the calcimimetic NPS-R568, an allosteric modulator of CASR, also stimulate Ob.CASR-dependent SRE-luciferase activation in MC3T3-E1 osteoblasts. In addition, we found that the dominant negative Galphaq(305-359) construct inhibited cation-stimulated ERK activation, consistent with Ob.CASR coupling to Galphaq-dependent pathways. Ob.CASR is also a target for classical GPCR desensitization mechanisms, since beta-arrestins, which bind to and uncouple GRK phosphorylated GPCRs, attenuated cation-stimulated SRE-luciferase activity in CASR deficient osteoblasts. Finally, we found that Ob.CASR and CASR couple to SRE through distinct signaling pathways. Ob.CASR does not activate RhoA and C3 toxin fails to block Ob.CASR-induced SRE-luciferase activity. Mutational analysis of the serum response factor (SRF) and ternary complex factor (TCF) elements in SRE demonstrates that Ob.CASR predominantly activates TCF-dependent mechanisms, whereas CASR activates SRE-luciferase mainly through a RhoA and SRF-dependent mechanism. The ability of Ob.CASR to sense cations and amino acids and function like a G-protein coupled receptor suggests that it may belong to the family of receptors characterized by an evolutionarily conserved amino acid sensing motif (ANF) linked to an intramembranous 7 transmembrane loop region (7TM).

GPCR Screening via ERK 1/2: A Novel Platform for Screening G Protein–Coupled Receptors

Discovery of novel agonists and antagonists for G protein–coupled receptors (GPCRs) relies heavily on cell-based assays because determination of functional consequences of receptor engagement is often desirable. Currently, there are several key parameters measured to achieve this, including mobilization of intracellular Ca2+ and formation of cyclic adenosine monophosphate or inositol triphosphate. However, no single assay platform is suitable for all situations, and all of the assays have limitations. The authors have developed a new high-throughput homogeneous assay platform for GPCR discovery as an alternative to current assays, which employs detection of phosphorylation of the key signaling molecule p42/44 MAP kinase (ERK 1/2). The authors show that ERK 1/2 is consistently activated in cells stimulated by Gq-coupled GPCRs and provides a new high-throughput platform for screening GPCR drug candidates. The activation of ERK 1/2 in Gq-coupled GPCR systems generates comparable pharmacological data for receptor agonist and antagonist data obtained by other GPCR activation measurement techniques.

Delayed adenosine A1 receptor preconditioning in rat myocardium is MAPK dependent but iNOS independent

Adenosine A1 receptor delayed preconditioning (PC) against myocardial infarction has been well described; however, there have been limited investigations of the signaling mechanisms that mediate this phenomenon. In addition, there are multiple conflicting reports on the role of inducible nitric oxide synthase (iNOS) in mediating A1 late-phase PC. The purpose of this study was to determine the roles of the p38 and extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases (MAPKs) in in vivo delayed A1 receptor PC and whether this protection at the myocyte level is due to upregulation of iNOS. Myocardial infarct size was measured in open-chest anesthetized rats 24 h after treatment with vehicle or the adenosine A1 agonist 2-chloro-N6-cyclopentyladenosine (CCPA; 100 microg/kg ip). Additional rats receiving CCPA were pretreated with the p38 inhibitor SB-203580 (1 mg/kg ip) or the MAPK/ERK kinase (MEK) inhibitor PD-098059 (0.5 mg/kg ip). At 24 h after CCPA administration, a group of animals was given the iNOS inhibitor 1400 W 10 min before ischemia. Treatment with CCPA reduced infarct size from 48 +/- 2 to 28 +/- 2% of the area at risk, an effect that was blocked by both SB-203580 and PD-098059 but not 1400 W. Ventricular myocytes isolated 24 h after CCPA injection exhibited significantly reduced oxidative stress during H2O2 exposure compared with myocytes from vehicle-injected animals, and this effect was not blocked by the iNOS inhibitor 1400 W. Western blot analysis of whole heart and cardiac myocyte protein samples revealed no expression of iNOS 6 or 24 h after CCPA treatment. These results indicate that adenosine A1 receptor delayed PC in rats is mediated by MAPK-dependent mechanisms, but this phenomenon is not associated with the early or late expression of iNOS.

Cloning and characterization of the G protein betagamma subunits from Trichoplusia ni (High Five cells)

Baculoviral-mediated expression in insect cells has become a method of choice where high-level protein expression is desired and where expression in Escherichia coliform (E. coli.) is unsuitable. Genes of interest are inserted into the baculoviral genome of Autographa californica nuclear polyhedrosis virus (AcNPV) under the extremely strong, but very late polyhedron gene (PolH). The preferred host lines are derived from Spodoptera frugiperda (Sf9 or Sf21) or Tricoplusia ni (High Five, Invitrogen). Viral expression in insect cells is commonly used in the signal transduction field, due to the more than satisfactory capacity to express membrane proteins. However, co-association and/or co-purification of contaminating endogenous host G protein subunits, for example, may potentially threaten the functional and structural homogeneity of membrane preparations. The undefined G protein composition is complicated by the limited sequence data of either the S. frugiperda or Tricoplusia ni genomes. Here we report the isolation of cDNAs encoding two members of the heterotrimeric G protein family, Gbeta (Tn-Gbeta) and Ggamma (Tn-Ggamma), from Tricoplusia ni. Tn-Gbeta shares approximately 90% amino acid sequence identity with Gbeta from Drosophila melanogaster and 84% identity with mammalian Gbeta (human Gbeta1). Tn-Ggamma shares approximately 71% amino acid identity with D. melanogaster Ggamma1 and 42% identity with mammalian Ggamma (human Ggamma2). Tn-Gbetagamma is also functionally similar to mammalian Gbeta1gamma2 by virtue of their capacity to form a complex with mammalian Galpha subunits, support G-protein-dependent agonist binding to a mammalian G protein-coupled receptor (beta2-adrenergic receptor) and directly regulate effectors such as adenylyl cyclase.

ERK Is Regulated by Sodium-Proton Exchanger in Rat Aortic Vascular Smooth Muscle Cells

The purposes of this study were to test 1) the relationship between two widely studied mitogenic effector pathways, and 2) the hypothesis that sodium-proton exchanger type 1 (NHE-1) is a regulator of extracellular signal-regulated protein kinase (ERK) activation in rat aortic smooth muscle (RASM) cells. Angiotensin II (Ang II) and 5-hydroxytryptamine (5-HT) stimulated both ERK and NHE-1 activities, with activation of NHE-1 preceding that of ERK. The concentration-response curves for 5-HT and Ang II were superimposable for both processes. Inhibition of NHE-1 with pharmacological agents or by isotonic replacement of sodium in the perfusate with choline or tetramethylammonium greatly attenuated ERK activation by 5-HT or Ang II. Similar maneuvers significantly attenuated 5-HT- or Ang II-mediated activation of MEK and Ras but not transphosphorylation of the epidermal growth factor (EGF) receptor. EGF receptor blockade attenuated ERK activation, but not NHE-1 activation by 5-HT and Ang II, suggesting that the EGF receptor and NHE-1 work in parallel to stimulate ERK activity in RASM cells, converging distal to the EGF receptor but at or above the level of Ras in the Ras-MEK-ERK pathway. Receptor-independent activation of NHE-1 by acute acid loading of RASM cells resulted in the rapid phosphorylation of ERK, which could be blocked by pharmacological inhibitors of NHE-1 or by isotonic replacement of sodium, closely linking the proton transport function of NHE-1 to ERK activation. These studies identify NHE as a new regulator of ERK activity in RASM cells.