Arrestin serves as a molecular switch, linking endogenous alpha2-adrenergic receptor to SRC-dependent, but not SRC-independent, ERK activation

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

Sympathetic activity regulates epithelial proliferation and wound healing via adrenergic receptor α2A

Innervation of the intestinal mucosa by the sympathetic nervous system is well described but the effects of adrenergic receptor stimulation on the intestinal epithelium remain equivocal. We therefore investigated the effect of sympathetic neuronal activation on intestinal cells in mouse models and organoid cultures, to identify the molecular routes involved. Using publicly available single-cell RNA sequencing datasets we show that the α2A isoform is the most abundant adrenergic receptor in small intestinal epithelial cells. Stimulation of this receptor with norepinephrine or a synthetic specific α2A receptor agonist promotes epithelial proliferation and stem cell function, while reducing differentiation in vivo and in intestinal organoids. In an anastomotic healing mouse model, adrenergic receptor α2A stimulation resulted in improved anastomotic healing, while surgical sympathectomy augmented anastomotic leak. Furthermore, stimulation of this receptor led to profound changes in the microbial composition, likely because of altered epithelial antimicrobial peptide secretion. Thus, we established that adrenergic receptor α2A is the molecular delegate of intestinal epithelial sympathetic activity controlling epithelial proliferation, differentiation, and host defense. Therefore, this receptor could serve as a newly identified molecular target to improve mucosal healing in intestinal inflammation and wounding.

Quantitation of Plasma Membrane (G Protein-Coupled) Receptor Trafficking in Cultured Cells

Measuring the functional behavior of G protein-coupled receptors (GPCRs) has been a major focus of academic and pharmaceutical research for many decades. These efforts have led to the development of many assays to measure the downstream effects of ligand binding on receptor activity. In this chapter, we describe an internalization/recycling assay that can be used to track changes in receptor number at the plasma membrane. Used in concert with other assays, this antibody-based technique can provide dynamic information on GPCR activation by receptor-specific ligands.

Scaffolding of Mitogen-Activated Protein Kinase Signaling by β-Arrestins

β-arrestins were initially identified to desensitize and internalize G-protein-coupled receptors (GPCRs). Receptor-bound β-arrestins also initiate a second wave of signaling by scaffolding mitogen-activated protein kinase (MAPK) signaling components, MAPK kinase kinase, MAPK kinase, and MAPK. In particular, β-arrestins facilitate ERK1/2 or JNK3 activation by scaffolding signal cascade components such as ERK1/2-MEK1-cRaf or JNK3-MKK4/7-ASK1. Understanding the precise molecular and structural mechanisms of β-arrestin-mediated MAPK scaffolding assembly would deepen our understanding of GPCR-mediated MAPK activation and provide clues for the selective regulation of the MAPK signaling cascade for therapeutic purposes. Over the last decade, numerous research groups have attempted to understand the molecular and structural mechanisms of β-arrestin-mediated MAPK scaffolding assembly. Although not providing the complete mechanism, these efforts suggest potential binding interfaces between β-arrestins and MAPK signaling components and the mechanism for MAPK signal amplification by β-arrestin-mediated scaffolding. This review summarizes recent developments of cellular and molecular works on the scaffolding mechanism of β-arrestin for MAPK signaling cascade.

New Structural Perspectives in G Protein-Coupled Receptor-Mediated Src Family Kinase Activation

Src family kinases (SFKs) are key regulators of cell proliferation, differentiation, and survival. The expression of these non-receptor tyrosine kinases is strongly correlated with cancer development and tumor progression. Thus, this family of proteins serves as an attractive drug target. The activation of SFKs can occur via multiple signaling pathways, yet many of them are poorly understood. Here, we summarize the current knowledge on G protein-coupled receptor (GPCR)-mediated regulation of SFKs, which is of considerable interest because GPCRs are among the most widely used pharmaceutical targets. This type of activation can occur through a direct interaction between the two proteins or be allosterically regulated by arrestins and G proteins. We postulate that a rearrangement of binding motifs within the active conformation of arrestin-3 mediates Src regulation by comparison of available crystal structures. Therefore, we hypothesize a potentially different activation mechanism compared to arrestin-2. Furthermore, we discuss the probable direct regulation of SFK by GPCRs and investigate the intracellular domains of exemplary GPCRs with conserved polyproline binding motifs that might serve as scaffolding domains to allow such a direct interaction. Large intracellular domains in GPCRs are often understudied and, in general, not much is known of their contribution to different signaling pathways. The suggested direct interaction between a GPCR and a SFK could allow for a potential immediate allosteric regulation of SFKs by GPCRs and thereby unravel a novel mechanism of SFK signaling. This overview will help to identify new GPCR-SFK interactions, which could serve to explain biological functions or be used to modulate downstream effectors.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Dexmedetomidine attenuates sevoflurane‑induced neurocognitive impairment through α2‑adrenoceptors

It has been reported that sevoflurane induces neurotoxicity in the developing brain. Dexmedetomidine is an α2 adrenoceptor agonist used for the prevention of sevoflurane‑induced agitation in children in clinical practice. The aim of the present study was to determine whether dexmedetomidine could prevent sevoflurane‑induced neuroapoptosis, neuroinflammation, oxidative stress and neurocognitive impairment. Additionally, the involvement of α2 adrenoceptors in the neuroprotective effect of dexmedetomidine was assessed. Postnatal day (P)6 C57BL/6 male mice were randomly divided into four groups (n=6 in each group). Mice were pretreated with dexmedetomidine, either alone or together with yohimbine, an α2 adrenoceptor inhibitor, then exposed to 3% sevoflurane in 25% oxygen. Control mice either received normal saline alone or with sevoflurane exposure. Following sevoflurane exposure, the expression of cleaved caspase‑3 was detected by immunohistochemistry in hippocampal tissue sections. In addition, the levels of tumor necrosis factor‑α (TNF‑α), interleukin (IL)‑1β, IL‑6 and malondialdehyde, as well as superoxide dismutase (SOD) activity in the hippocampus were measured. At P35, the learning and memory abilities were assessed in each mouse using a Morris water maze test. Dexmedetomidine significantly decreased the expression of activated caspase‑3 following sevoflurane exposure. Moreover, dexmedetomidine significantly decreased the levels of TNF‑α, IL‑1β and IL‑6 in the hippocampus. SOD activity also increased in a dose‑dependent manner in dexmedetomidine‑treated mice. MDA decreased in a dose‑dependent manner in dexmedetomidine‑treated mice. Lastly, sevoflurane‑induced learning and memory impairment was reversed by dexmedetomidine treatment. By contrast, co‑administration of yohimbine significantly attenuated the neuroprotective effects of dexmedetomidine. These findings suggested that dexmedetomidine exerted a neuroprotective effect against sevoflurane‑induced apoptosis, inflammation, oxidative stress and neurocognitive impairment, which was mediated, at least in part, by α2 adrenoceptors.

PACAP-Induced PAC1 Receptor Internalization and Recruitment of Endosomal Signaling Regulate Cardiac Neuron Excitability

Pituitary adenylate cyclase-activating polypeptide (PACAP, Adcyap1) activation of PAC1 receptors (Adcyap1r1) significantly increases excitability of guinea pig cardiac neurons. This modulation of excitability is mediated in part by plasma membrane G protein-dependent activation of adenylyl cyclase and downstream signaling cascades, as well as by endosomal signaling mechanisms. PACAP/PAC1 receptor-mediated activation of plasma membrane adenylyl cyclase (AC) and the resulting increase in cellular cAMP enhances a hyperpolarization-induced nonselective cationic current Ih, which contributes to the PACAP-induced increase in cardiac neuron excitability. Further, PACAP-mediated AC/cAMP/PKA downstream signaling also appears to enhance cardiac neuron IT to facilitate the excitatory responses. PACAP activation of PAC1 receptors rapidly stimulates receptor internalization, and reducing ambient temperature or treatments with the clathrin inhibitor Pitstop2 or the dynamin I/II inhibitor dynasore to block endocytic events can suppress PACAP-enhanced neuronal excitability. Thus, endocytosis inhibitors essentially eliminate PACAP-enhanced excitability suggesting that endosomal platforms represent a primary signaling mechanism. Endosomal signaling is associated canonically with ERK activation and in accord, PACAP-enhanced cardiac neuron excitability is reduced by MEK inhibitor pretreatments. PACAP activation of MEK/ERK signaling can enhance currents through voltage-dependent Nav1.7 channels. Hence, PACAP-induced PAC1 receptor internalization/endosomal signaling, recruitment of MEK/ERK signaling, and modulation of Nav1.7 are implicated as key mechanisms contributing to the PACAP-enhanced neuronal excitability. PACAP/PAC1 receptor-mediated endosomal ERK signaling in central circuits can play key roles in development of chronic pain and anxiety-related responses; thus, PAC1 endosomal signaling likely participates in a variety of homeostatic responses within neuronal circuits in the CNS.

Src family kinase inhibitors blunt PACAP-induced PAC1 receptor endocytosis, phosphorylation of ERK, and the increase in cardiac neuron excitability

Pituitary adenylate cyclase activating polypeptide (PACAP, Adcyap1) activation of PAC1 receptors ( Adcyap1r1) significantly increases excitability of guinea pig cardiac neurons. This modulation of excitability is mediated in part by plasma membrane G protein-dependent activation of adenylyl cyclase and downstream signaling cascades. However, additional mechanisms responsible for the enhanced excitability are activated following internalization of the PAC1 receptor and endosomal signaling. Src family kinases play critical roles mediating endocytosis of many trophic factor and G protein-coupled receptors. The present study investigated whether Src family kinases also support the PACAP-induced PAC1 receptor internalization, phosphorylation of ERK, and enhanced neuronal excitability. Using human embryonic kidney cells stably expressing a green fluorescent protein-tagged PAC1 receptor, treatment with the Src family kinase inhibitor PP2 (10 µM) markedly reduced the PACAP-induced PAC1 receptor internalization, and in parallel, both PP2 and Src inhibitor 1 (Src-1, 2 µM) reduced ERK activation determined by Western blot analysis. In contrast, Src family kinase inhibitors did not eliminate a PACAP-induced rise in global calcium generated by inositol (1,4,5)-trisphosphate-induced release of calcium from endoplasmic reticulum stores. From confocal analysis of phosphorylated ERK immunostaining, PP2 treatment significantly attenuated PACAP activation of ERK in neurons within cardiac ganglia whole mount preparations. Intracellular recordings demonstrated that PP2 also significantly blunted a PACAP-induced increase in cardiac neuron excitability. These studies demonstrate Src-related kinase activity in PAC1 receptor internalization, activation of MEK/ERK signaling, and regulation of neuronal excitability. The present results provide further support for the importance of PAC1 receptor endosomal signaling as a key mechanism regulating cellular function.

The amyloid precursor protein modulates α2A-adrenergic receptor endocytosis and signaling through disrupting arrestin 3 recruitment

The amyloid precursor protein (APP) has long been appreciated for its role in Alzheimer's disease (AD) pathology. However, less is known about the physiologic function of APP outside of AD. Particularly, whether and how APP may regulate functions of cell surface receptors, including GPCRs, remains largely unclear. In this study, we identified a novel direct interaction between APP and the α_2A-adrenergic receptor (α_2AAR) that occurs at the intracellular domains of both proteins. The APP interaction with α_2AAR is promoted by agonist stimulation and competes with arrestin 3 binding to the receptor. Consequently, the presence of APP attenuates α_2AAR internalization and desensitization, which are arrestin-dependent processes. Furthermore, in neuroblastoma neuro-2A cells and primary superior cervical ganglion neurons, where APP is highly expressed, the lack of APP leads to a dramatic increase in plasma membrane recruitment of endogenous arrestin 3 following α_2AAR activation. Concomitantly, agonist-induced internalization of α_2AAR is significantly enhanced in these neuronal cells. Our study provided the first evidence that APP fine tunes GPCR signaling and trafficking. Given the important role of α_2AAR in controlling norepinephrine release and response, this novel regulation of α_2AAR by APP may have an impact on modulation of noradrenergic activity and sympathetic tone.-Zhang, F., Gannon, M., Chen, Y., Zhou, L., Jiao, K., Wang, Q. The amyloid precursor protein modulates α_2A-adrenergic receptor endocytosis and signaling through disrupting arrestin 3 recruitment.

Diverse arrestin-recruiting and endocytic profiles of tricyclic antipsychotics acting as direct α2A adrenergic receptor ligands

The therapeutic mechanism of action underlying many psychopharmacological agents remains poorly understood, due largely to the extreme molecular promiscuity exhibited by these agents with respect to potential central nervous system targets. Agents of the tricyclic chemical class, including both antidepressants and antipsychotics, exhibit a particularly high degree of molecular promiscuity; therefore, any clarification of how these agents interact with specific central nervous system targets is of great potential significance to the field. Here, we present evidence demonstrating that tricyclic antipsychotics appear to segregate into three distinct groups based upon their molecular interactions with the centrally-important α_2A adrenergic receptor (AR). Specifically, while the α_2AAR binds all antipsychotics tested with similar affinities, and none of the agents are able to induce classical heterotrimeric G protein-mediated α_2AAR signaling, significant differences are observed with respect to arrestin3 recruitment and receptor endocytosis. All antipsychotics tested induce arrestin3 recruitment to the α_2AAR, but with differing strengths. Both chlorpromazine and clozapine drive significant α_2AAR endocytosis, but via differing clathrin-dependent and lipid raft-dependent pathways, while fluphenazine does not drive a robust response. Intriguingly, in silico molecular modeling suggests that each of the three exhibits unique characteristics in interacting with the α_2AAR ligand-binding pocket. In addition to establishing these three antipsychotics as novel arrestin-biased ligands at the α_2AAR, our findings provide key insights into the molecular actions of these clinically-important agents.

Efficacy and Acceptability of Different Auxiliary Drugs in Pediatric Sevoflurane Anesthesia: A Network Meta-analysis of Mixed Treatment Comparisons

Emergence agitation preventive medicine should be combined with pediatric anesthesia because of the high frequency of emergence agitation. However, it is challenging to determine the most appropriate medication that can be introduced into pediatric anesthesia for the sake of emergence agitation prevention. We reviewed and retrieved the data from PubMed and Embase. Various medications were assessed based on several endpoints including Emergence agitation outcomes (EA), postoperative nausea and vomiting (PONV), the number of patients who required analgesic (RA), pediatric anesthesia emergence delirium (PAED), the extubation time, the emergency time and the duration of post-anesthesia care unit (PACU) stay. Both traditional and network meta-analysis were carried in this study. A total of 45 articles were complied with the selection criteria and the corresponding articles were reviewed. Fentanyl demonstrated the highest cumulative ranking probability which was followed by those of ketamine and dexmedetomidine with respect to EA and PAED. When PONV and RA were concerned together, clonidine exhibited the highest cumulative ranking probability compared to other medications. Our study suggested that dexmedetomidine perhaps is the most appropriate prophylactic treatment which can be introduced into anesthesia for preventing emergence agitation.

Activation of α2A-adrenergic signal transduction in chondrocytes promotes degenerative remodelling of temporomandibular joint

This study tested whether activation of adrenoreceptors in chondrocytes has roles in degenerative remodelling of temporomandibular joint (TMJ) and to determine associated mechanisms. Unilateral anterior crossbite (UAC) was established to induce TMJ degeneration in rats. Saline vehicle, α2- and β-adrenoreceptor antagonists or agonists were injected locally into the TMJ area of UAC rats. Cartilage degeneration, subchondral bone microarchitecture and the expression of adrenoreceptors, aggrecans, matrix metalloproteinases (MMPs) and RANKL by chondrocytes were evaluated. Chondrocytes were stimulated by norepinephrine to investigate signal transduction of adrenoreceptors. Increased α2A-adrenoreceptor expression was observed in condylar cartilage of UAC rats, together with cartilage degeneration and subchondral bone loss. Norepinephrine depresses aggrecans expression but stimulates MMP-3, MMP-13 and RANKL production by chondrocytes through ERK1/2 and PKA pathway; these effects were abolished by an α2A-adrenoreceptor antagonist. Furthermore, inhibition of α2A-adrenoreceptor attenuated degenerative remodelling in the condylar cartilage and subchondral bone, as revealed by increased cartilage thickness, proteoglycans and aggrecan expression, and decreased MMP-3, MMP-13 and RANKL expressions in cartilage, increased BMD, BV/TV, and decreased Tb.Sp in subchondral bone. Conversely, activation of α2A-adrenoreceptor intensified aforementioned degenerative changes in UAC rats. It is concluded that activation of α2A-adrenergic signal in chondrocytes promotes TMJ degenerative remodelling by chondrocyte-mediated pro-catabolic activities.

Quantitation of Plasma Membrane (G Protein-Coupled) Receptor Trafficking in Cultured Cells

Measuring the functional behavior of G protein-coupled receptors (GPCRs) has been a major focus of academic and pharmaceutical research for many decades. These efforts have led to the development of many assays to measure the downstream effects of ligand binding on receptor activity. In this chapter, we describe an internalization/recycling assay that can be used to track changes in receptor number at the plasma membrane. Used in concert with other assays, this antibody-based technique can provide important information on GPCR activation by receptor-specific ligands.

Unraveling the molecular architecture of a G protein-coupled receptor/β-arrestin/Erk module complex

β-arrestins serve as signaling scaffolds downstream of G protein-coupled receptors, and thus play a crucial role in a plethora of cellular processes. Although it is largely accepted that the ability of β-arrestins to interact simultaneously with many protein partners is key in G protein-independent signaling of GPCRs, only the precise knowledge of these multimeric arrangements will allow a full understanding of the dynamics of these interactions and their functional consequences. However, current experimental procedures for the determination of the three-dimensional structures of protein-protein complexes are not well adapted to analyze these short-lived, multi-component assemblies. We propose a model of the receptor/β-arrestin/Erk1 signaling module, which is consistent with most of the available experimental data. Moreover, for the β-arrestin/Raf1 and the β-arrestin/ERK interactions, we have used the model to design interfering peptides and shown that they compete with both partners, hereby demonstrating the validity of the predicted interaction regions.

Neurotoxic effects of dexmedetomidine in fetal cynomolgus monkey brains

The neuroprotective effects of dexmedetomidine have been reported by many investigators; however its underlying mechanism to reduce neuronal injury during a prolonged anesthesia remains unclear. In this study, we investigated the neurotoxic effects of dexmedetomidine in fetal monkey brains. In the present study, we compare the neurotoxic effects of dexmedetomidine and ketamine, a general anesthetic with a different mechanism of action, in fetal cynomolgus monkeys. Twenty pregnant monkeys at approximate gestation day 120 were divided into 4 groups: non-treatment controls (Group 1); ketamine at 20 mg/kg intramuscularly followed by a 12-hr infusion at 20-50 mg/kg/hr (Group 2); dexmedetomidine at 3 µg/kg intravenously (i.v.) over 10 min followed by a 12-hr infusion at the human equivalent dose (HED) of 3 µg/kg/hr (Group 3); and dexmedetomidine at 30 µg/kg i.v. over 10 min followed by a 12-hr infusion at 30 µg/kg/hr, 10 times HED (Group 4). Blood samples from both dams and fetuses were measured for concentration of dexmedetomidine. Each fetus was perfusion-fixed, serial sections were cut through the frontal cortex, and stained to detect for apoptosis (activated caspase 3 and TUNEL) and neurodegeneration (silver stain). In utero treatment with ketamine resulted in marked apoptosis and degeneration primarily in layers I and II of the frontal cortex. In contrast, fetal brains from animals treated with dexmedetomidine showed none to minimal neuroapoptotic or neurodegenerative lesions at both low- and high-dose treatments. Plasma levels confirmed systemic exposure of dexmedetomidine in both dams and fetuses. In conclusion, these results demonstrate that dexmedetomidine at both low-dose (HED) and high-dose (10 times HED) does not induce apoptosis in the frontal cortex (layers I, II, and III) of developing brain of cynomolgus monkeys.

Molecular and cellular mechanisms of dopamine-mediated behavioral plasticity in the striatum

The striatum is the input structure of the basal ganglia system. By integrating glutamatergic signals from cortical and subcortical regions and dopaminergic signals from mesolimbic nuclei the striatum functions as an important neural substrate for procedural and motor learning as well as for reward-guided behaviors. In addition, striatal activity is significantly altered in pathological conditions in which either a loss of dopamine innervation (Parkinson's disease) or aberrant dopamine-mediated signaling (drug addiction and L-DOPA induced dyskinesia) occurs. Here we discuss cellular mechanisms of striatal synaptic plasticity and aspects of cell signaling underlying striatum-dependent behavior, with a major focus on the neuromodulatory action of the endocannabinoid system and on the role of the Ras-ERK cascade.

Posttranslation modification of G protein-coupled receptor in relationship to biased agonism

Biased signaling has been reported with a series of G protein-coupled receptors (GPCRs), including β(2)-adrenergic receptor and μ-opioid receptor (OPRM1). The concept of biased signaling suggests that the agonists of one particular receptor may activate the downstream signaling pathways with different efficacies. Thus in an extreme case, agonists might activate different sets of signaling pathways, which provide a new route to develop drugs with increased efficacies and decreased side effects. Among the many factors, posttranslation modifications of receptor proteins have major roles in influencing the biased signaling. Take OPRM1, for example, the phosphorylation and palmitoylation of receptor can regulate the biased signaling induced by agonists. Thus, by modulating these posttranslation modifications, the biased signaling of GPCRs can be regulated. In addition, although it is not considered as posttranslation modification normally, the distribution of GPCRs on cell membrane, especially the distribution between lipid-raft and non-raft microdomains, also contributes to the biased signaling. Thus in this chapter, we described the methods used in our laboratory to study receptor phosphorylation, receptor palmitoylation, and membrane distribution of receptor by using OPRM1 as a model. A functional model was also provided on these posttranslational modifications at the last section of this chapter.

α2 adrenergic receptor dysregulation in depressive disorders: implications for the neurobiology of depression and antidepressant therapy

Dysfunction in noradrenergic neurotransmission has long been theorized to occur in depressive disorders. The α2 adrenergic receptor (AR) family, as a group of key players in regulating the noradrenergic system, has been investigated for involvement in the neurobiology of depression and mechanisms of antidepressant therapies. However, a clear picture of the α2ARs in depressive disorders has not been established due to the existence of apparently conflicting findings in the literature. In this article, we report that a careful accounting of methodological differences within the literature can resolve the present lack of consensus on involvement of α2ARs in depression. In particular, the pharmacological properties of the radioligand (e.g. agonist versus antagonist) utilized for determining receptor density are crucial in determining study outcome. Upregulation of α2AR density detected by radiolabeled agonists but not by antagonists in patients with depressive disorders suggests a selective increase in the density of high-affinity conformational state α2ARs, which is indicative of enhanced G protein coupling to the receptor. Importantly, this high-affinity state α2AR upregulation can be normalized with antidepressant treatments. Thus, depressive disorders appear to be associated with increased α2AR sensitivity and responsiveness, which may represent a physiological basis for the putative noradrenergic dysfunction in depressive disorders. In addition, we review changes in some key α2AR accessory proteins in depressive disorders and discuss their potential contribution to α2AR dysfunction.

Desipramine selectively potentiates norepinephrine-elicited ERK1/2 activation through the α2A adrenergic receptor

The precise physiological effects of antidepressant drugs, and in particular their actions at non-monoamine transporter targets, are largely unknown. We have recently identified the tricyclic antidepressant drug desipramine (DMI) as a direct ligand at the α(2A) adrenergic receptor (AR) without itself driving heterotrimeric G protein/downstream effector activation [5]. In this study, we report our novel finding that DMI modulates α(2A)AR signaling in response to the endogenous agonist norepinephrine (NE). DMI acted as a signaling potentiator, selectively enhancing NE-induced α(2A)AR-mediated ERK1/2 MAPK signaling. This potentiation of ERK1/2 activation was observed as an increase in NE response sensitivity and a prolongation of the activation kinetics. DMI in a physiologically relevant ratio with NE effectively turned on ERK1/2 signaling that is lacking in response to physiological NE alone. Further, the DMI-induced ERK1/2 potentiation relied on heterotrimeric G(i/o) proteins and was arrestin-independent. This modulatory effect of DMI on NE signaling provides novel insight into the effects of this antidepressant drug on the noradrenergic system which it regulates, insight which enhances our understanding of the therapeutic mechanism for DMI.

S32212, a novel serotonin type 2C receptor inverse agonist/α2-adrenoceptor antagonist and potential antidepressant: I. A mechanistic characterization

Although most antidepressants suppress serotonin (5-HT) and/or noradrenaline reuptake, blockade of 5-HT(2C) receptors and α(2)-adrenoceptors likewise enhances monoaminergic transmission. These sites are targeted by the urea derivative N- [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-1,2-dihydro-3-H-benzo[e]indole-3-carboxamide (S32212). S32212 was devoid of affinity for monoamine reuptake sites, yet displayed pronounced affinity (pK(i), 8.2) for constitutively active human 5-HT(2CINI) (h5-HT(2CINI)) receptors, behaving as an inverse agonist in reducing basal Gα(q) activation, [(3)H]inositol-phosphate production, and the spontaneous association of h5-HT(2CINI)-Renilla luciferase receptors with β-arrestin2-yellow fluorescent protein. Furthermore, upon 18-h pretreatment, S32212 enhanced the plasma membrane expression of h5-HT(2CINI) receptors as visualized by confocal microscopy and quantified by enzyme-linked immunosorbent assay. Its actions were prevented by the neutral antagonist 6-chloro-5-methyl-N-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl]indoline-1-carboxamide (SB242,084), which also impeded the induction by long-term exposure to S32212 of otherwise absent Ca(2+) mobilization in mouse cortical neurones. In vivo, S32212 blunted the inhibitory influence of the 5-HT(2C) agonist 2-(3-chlorobenzyloxy)-6-(1-piperazinyl)pyrazine (CP809,101) on ventrotegmental dopaminergic neurones. S32212 also blocked 5-HT-induced Gα(q) and phospholipase C activation at the h5-HT(2A) and, less potently, h5-HT(2B) receptors and suppressed the discriminative stimulus properties of the 5-HT(2A) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane in rats. S32212 manifested marked affinity for human α(2A)- (pK(i) 7.2), α(2B)- (pK(i) 8.2), and α(2C)- (pK(i) 7.4) adrenoceptors, at which it abolished noradrenaline-induced recruitment of Gα(i3), Gα(o), adenylyl cyclase, and extracellular-regulated kinase1/2. Moreover, S32212 dose-dependently abolished the discriminative stimulus effects of the α(2)-adrenoceptor agonist (S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4,2'-(1',2',3',4'-tetrahydronaphthalene)] (S18616). Finally, S32212 displayed negligible affinity for α(1A)-adrenoceptors, histamine H(1) receptors, and muscarinic M(1) receptors. In conclusion, S32212 behaves as an inverse agonist at h5-HT(2C) receptors and as an antagonist at human α(2)-adrenoceptors (and h5-HT(2A) receptors). Its promising profile in preclinical models potentially relevant to the treatment of depression is described in J Pharmacol Exp Ther 340:765-780, 2012.

Characterization of the endocannabinoid system, CB(1) receptor signalling and desensitization in human myometrium

BACKGROUND AND PURPOSE

The endocannabinoid plays vital roles in several aspects of reproduction, including gametogenesis, fertilization and parturition. However, little is known regarding the presence or role of the endocannabinoid system in myometrial function. Here the presence of the endocannabinoid system and signalling properties of cannabinoid receptors were characterized.

EXPERIMENTAL APPROACH

Components of the endocannabinoid system were identified using qRT-PCR, immunohistochemical, immunoblotting and radioligand binding experiments. Cannabinoid receptor signalling pathways were characterized using standard MAPK and second messenger assays.

KEY RESULTS

Primary myometrium expresses the endocannabinoid synthesizing enzyme N-acyl-phosphatidyl ethanolamine-specific phospholipase D, endocannabinoid degrading enzyme fatty acid amide hydrolase and cannabinoid CB(1) , but not CB(2) receptors or transient receptor potential vanilloid-type-1 channels. The CB(1) receptor ligand anandamide caused a Gα(i/o) -dependent inhibition of adenylate cyclase reducing intracellular cAMP levels, and Gα(i/o) , phosphoinositide-3-kinase, Src-kinase-dependent ERK activation. CB(1) receptor-generated signals declined following continual anandamide stimulation, possibly due to ligand metabolism since free anandamide concentrations declined during the experiment from 2.5 µM initially, to 500 nM after >30 min. However, identical loss of CB(1) receptor responsiveness occurred in the presence of the metabolically stable derivative methanandamide. Moreover, RNAi-mediated depletion of arrestin3 (a negative regulator of receptor signalling) prevented loss of CB(1) receptor activity, enhancing and prolonging ERK signals.

CONCLUSIONS AND IMPLICATIONS

The myometrium has the capacity to synthesize, respond to and degrade endocannabinoids. Furthermore, reduced CB(1) receptor responsiveness occurs as a consequence of receptor desensitization, not agonist depletion and we identify a key role for arrestin3 in this process.

The antidepressant desipramine is an arrestin-biased ligand at the α(2A)-adrenergic receptor driving receptor down-regulation in vitro and in vivo

The neurobiological mechanisms of action underlying antidepressant drugs remain poorly understood. Desipramine (DMI) is an antidepressant classically characterized as an inhibitor of norepinephrine reuptake. Available evidence, however, suggests a mechanism more complex than simple reuptake inhibition. In the present study, we have characterized the direct interaction between DMI and the α(2A)-adrenergic receptor (α(2A)AR), a key regulator of noradrenergic neurotransmission with altered expression and function in depression. DMI alone was found to be sufficient to drive receptor internalization acutely and a robust down-regulation of α(2A)AR expression and signaling following prolonged stimulation in vitro. These effects are achieved through arrestin-biased regulation of the receptor, as DMI selectively induces recruitment of arrestin but not activation of heterotrimeric G proteins. Meanwhile, a physiologically relevant concentration of endogenous agonist (norepinephrine) was unable to sustain a down-regulation response. Prolonged in vivo administration of DMI resulted in significant down-regulation of synaptic α(2A)AR expression, a response that was lost in arrestin3-null animals. We contend that direct DMI-driven arrestin-mediated α(2A)AR down-regulation accounts for the therapeutically desirable but mechanistically unexplained adaptive alterations in receptor expression associated with this antidepressant. Our results provide novel insight into both the pharmacology of this antidepressant drug and the targeting of the α(2A)AR in depression.

Cyclic adenosine monophosphate-independent tyrosine phosphorylation of NR2B mediates cocaine-induced extracellular signal-regulated kinase activation

BACKGROUND

Activation of the extracellular signal-regulated kinase (ERK) in the striatum is crucial for long-term behavioral alterations induced by drugs of abuse. In response to cocaine, ERK phosphorylation (i.e., activation) is restricted to medium-sized spiny neurons expressing dopamine D1 receptor (D1R) and depends on a concomitant stimulation of D1R and glutamate N-methyl-D-aspartate receptor (NMDAR). However, the mechanisms responsible for this activation, especially the respective contribution of D1R and NMDAR, remain unknown.

METHODS

We studied striatal neurons in culture stimulated with D1R agonist and/or glutamate and wild-type or genetically modified mice treated with cocaine. Biochemical, immunohistochemical, and imaging studies were performed. Mice were also subjected to behavioral experiments.

RESULTS

Stimulation of D1R cannot activate ERK by itself but potentiates glutamate-mediated calcium influx through NMDAR that is responsible for ERK activation. Potentiation of NMDAR by D1R depends on a cyclic adenosine monophosphate-independent signaling pathway, which involves tyrosine phosphorylation of the NR2B subunit of NMDAR by Src family kinases. We also demonstrate that the D1R/Src family kinases/NR2B pathway is responsible for ERK activation by cocaine in vivo. Inhibition of this pathway abrogates cocaine-induced locomotor sensitization and conditioned place preference.

CONCLUSIONS

Our results show that potentiation of NR2B-containing NMDAR by D1R is necessary and sufficient to trigger cocaine-induced ERK activation. They highlight a new cyclic adenosine monophosphate-independent pathway responsible for the integration of dopamine and glutamate signals by the ERK cascade in the striatum and for long-term behavioral alterations induced by cocaine.

Regulation of neuronal activation by Alpha2A adrenergic receptor agonist

Stress factors induce neuronal activation in brain areas that are related to anxiety and fear. High doses of caffeine induce neuronal activation with Ca2+ influx followed by expression of the immediate early gene c-fos. In the present study, we investigated c-Fos protein expression in stress-responsive brain areas induced by caffeine, as well as the role of alpha2A receptor in the regulation of neuronal activation. Immunohistochemical analysis showed that an acute effect of caffeine induced c-Fos protein expression in the hippocampus, the bed nucleus of stria terminalis (BNST), the lateral septum, the basolateral and central amygdala, the paraventricular hypothalamic nucleus (PVN), the locus coeruleus, and the lateral parabrachial nucleus (LPBN). However, c-Fos expression was attenuated after repeated treatment of caffeine, spaced 24 h apart, compared to a single acute effect. Alpha2A receptor activation with the agonist guanfacine attenuated the acute effect of caffeine in terms of c-Fos expression in neurons in the CA1-CA3 areas of hippocampus, the locus coeruleus and the LPBN as compared with effect of caffeine alone, whereas the number of c-Fos expressing neurons increased in the lateral septum, the dorsal BNST, the central amygdala, and the PVN, areas that are densely innervated by noradrenergic neurons. Guanfacine alone induced c-Fos protein expression in neurons in the central amygdala, the dorsal BNST, the PVN, the LPBN, and the caudal nucleus of the solitary tract. Guanfacine alone also induced phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in neurons expressing c-Fos in the dorsal BNST, the central amygdala, and the LPBN. These results suggest that alpha2A receptor activation modulates synaptic transmission in neuronal circuits that are correlated with stress in vivo.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Arrestin orchestrates crosstalk between G protein-coupled receptors to modulate the spatiotemporal activation of ERK MAPK

RATIONALE

G protein-coupled receptors (GPCRs) respond to diversified extracellular stimuli to modulate cellular function. Despite extensive studies investigating the regulation of single GPCR signaling cascades, the effects of concomitant GPCR activation on downstream signaling and cellular function remain unclear.

OBJECTIVE

We aimed to characterize the cellular mechanism by which GPCR crosstalk regulates mitogen-activated protein kinase (MAPK) activation.

METHODS AND RESULTS

Adrenergic receptors on cardiac fibroblasts were manipulated to examine the role of arrestin in the spatiotemporal regulation of extracellular signal-regulated kinase (ERK)1/2 MAPK signaling. We show a general mechanism in which arrestin activation by one GPCR is capable of regulating signaling originating from another GPCR. Activation of Gq coupled-receptor signaling leads to prolonged ERK1/2 MAPK phosphorylation, nuclear accumulation, and cellular proliferation. Interestingly, coactivation of these receptors with the beta-adrenergic receptors induced transient ERK signaling localized within the cytosol, which attenuated cell proliferation. Further studies revealed that recruitment of arrestin3 to the beta2-adrenergic receptor orchestrates the sequestration of Gq-coupled receptor-induced ERK to the cytosol through direct binding of ERK to arrestin.

CONCLUSIONS

This is the first evidence showing that arrestin3 acts as a coordinator to integrate signals from multiple GPCRs. Our studies not only provide a novel mechanism explaining the integration of mitogenic signaling elicited by different GPCRs, but also underscore the critical role of signaling crosstalk among GPCRs in vivo.

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.

Regulation of human mast cell and basophil function by anaphylatoxins C3a and C5a

Allergic diseases such as asthma result from inappropriate immunologic responses to common environmental allergens in genetically susceptible individuals. Following allergen exposure, interaction of dendritic cells (DC) with CD4(+) T cells leads to the production of Th2 cytokines, which induce B cells to synthesize IgE molecules (sensitization phase). These IgE molecules bind to their high affinity receptors (FcvarepsilonRI) on the surface of mast cells and basophils and their subsequent cross-linking by allergen results in the release of preformed and newly synthesized mediators, which cause bronchoconstriction, lung inflammation and airway hyperresponsiveness (AHR) in asthma (effector phase). The complement components C3a and C5a levels are increased in the lungs of patients with asthma and are likely generated via the actions of both allergen and mast cell proteases. In vivo studies with rodents have shown that while C3a facilitates allergen sensitization in some models C5a inhibits this response. Despite this difference, both anaphylatoxins promote lung inflammation and AHR in vivo indicating that cells other than DC and T cells likely mediate the functional effects of C3a and C5a in asthma. This review focuses on the contribution of C3a and C5a in the pathogenesis of asthma with a particular emphasis on mast cells and basophils. It discusses the mechanisms by which anaphylatoxins activate mast cells and basophils and the associated signaling pathways via which their receptors are regulated by priming and desensitization.

Minireview: physiological and pathological actions of RAS in the ovary

The small G proteins of the RAS superfamily act as molecular switches in the transduction of cellular signals critical for a wide range of normal developmental events as well as pathological processes. However, the functions of Ras genes in ovarian cells have only started to be unveiled. RAS, most likely KRAS that is highly expressed in granulosa cells of growing follicles, appears crucial for mediating the gonadotropin-induced events associated with the unique physiological process of ovulation. By contrast, conditional expression of a constitutively active Kras(G12D) mutant in granulosa cells results in ovulation defects due to the complete disruption of normal follicular growth, cessation of granulosa cell proliferation, and blockage of granulosa cell apoptosis and differentiation. When the tumor suppressor Pten is disrupted conditionally in the Kras(G12D)-expressing granulosa cells, granulosa cell tumors fail to develop. However, ovarian surface epithelial cells expressing the same Pten;Kras(G12D) mutations rapidly become ovarian surface epithelial serous cystadenocarcinomas. In this minireview, we summarize some of the physiological as well as pathological functions of RAS in the rodent ovary, discuss the implications of the Kras(G12D) mutant mouse models for understanding human diseases such as premature ovarian failure and ovarian cancers, and highlight new questions raised by the results of recent studies.

KG-135, enriched with selected ginsenosides, inhibits the proliferation of human prostate cancer cells in culture and inhibits xenograft growth in athymic mice

Sun ginseng (SG) was recently developed as a heat-processed form of ginseng. The Rg3, Rk1, and Rg5 ginsenosides are its main ginsenoside components. SG has been reported to have more potent pharmacological activities than red ginseng (RG), where these pharmacological activities include vasodilatory, anti-oxidant and anti-tumorigenic effects. In the present study, we investigated KG-135, the ginsenoside-rich fraction of SG and demonstrated that this fraction inhibits proliferation of human prostate cancer cells both in vitro and in vivo. KG-135 caused a significant growth inhibition of DU145 and PC-3 human prostate cancer cells. KG-135 induced cell cycle arrest in the G1 phase and caused an associated increase in the p21(Cip1) protein levels. When KG-135 was fed to mice that had been xenografted with DU145 tumors, a time-dependent inhibition of tumor growth was noted without any observed toxicity. Immunohistochemical analysis of the tumor tissues showed that KG-135 led to a decrease in the expression of proliferating cell nuclear antigen (PCNA). Microarray analysis of the tumors revealed that KG-135 inhibited tumor growth and also caused changes in the expression levels of multiple cancer-related genes. These data suggest that KG-135 effectively inhibits prostate cancer cell proliferation. Its mechanism of action likely involves cyclin inhibition and regulation of the expression of the TNFRSF25 and ADRA2A genes.

A preformed signaling complex mediates GnRH-activated ERK phosphorylation of paxillin and FAK at focal adhesions in L beta T2 gonadotrope cells

Most receptor tyrosine kinases and G protein-coupled receptors (GPCRs) operate via a limited number of MAPK cascades but still exert diverse functions, and therefore signal specificity remains an enigma. Also, most GPCR ligands utilize families of receptors for mediation of diverse biological actions; however, the mammalian type I GnRH receptor (GnRHR) seems to be the sole receptor mediating GnRH-induced gonadotropin synthesis and release. Signaling complexes associated with GPCRs may thus provide the means for signal specificity. Here we describe a signaling complex associated with the GnRHR, which is a unique GPCR lacking a C-terminal tail. Unlike other GPCRs, this signaling complex is preformed, and exposure of L beta T2 gonadotropes to GnRH induces its dynamic rearrangement. The signaling complex includes c-Src, protein kinase C delta, -epsilon, and -alpha, Ras, MAPK kinase 1/2, ERK1/2, tubulin, focal adhesion kinase (FAK), paxillin, vinculin, caveolin-1, kinase suppressor of Ras-1, and the GnRHR. Exposure to GnRH (5 min) causes MAPK kinase 1/2, ERK1/2, tubulin, vinculin, and the GnRHR to detach from c-Src, but they reassociate within 30 min. On the other hand, FAK, paxillin, the protein kinase Cs, and caveolin-1 stay bound to c-Src, whereas kinase suppressor of Ras-1 appears in the complex only 30 min after GnRH stimulation. GnRH was found to activate ERK1/2 in the complex in a c-Src-dependent manner, and the activated ERK1/2 subsequently phosphorylates FAK and paxillin. In parallel, caveolin-1, FAK, vinculin, and paxillin are phosphorylated on Tyr residues apparently by GnRH-activated c-Src. Receptor tyrosine kinases and GPCRs translocate ERK1/2 to the nucleus to phosphorylate and activate transcription factors. We therefore propose that the role of the multiprotein signaling complex is to sequester a cytosolic pool of activated ERK1/2 to phosphorylate FAK and paxillin at focal adhesions.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Altered expression and subcellular distribution of GRK subtypes in the dopamine-depleted rat basal ganglia is not normalized by l-DOPA treatment

Dysregulation of dopamine (DA) receptors is believed to underlie Parkinson's disease pathology and l-DOPA-induced motor complications. DA receptors are subject to regulation by G protein-coupled receptor kinases (GRKs) and arrestins. DA lesion with 6-hydroxydopamine caused multiple protein- and brain region-specific changes in the expression of GRKs. In the globus pallidus, all four GRK isoforms (GRK2, 3, 5, 6) were reduced in the lesioned hemisphere. In the caudal caudate-putamen (cCPu) three GRK isoforms (GRK2, 3, 6) were decreased by DA depletion. The decrease in GRK proteins in globus pallidus, but not cCPu, was mirrored by reduction in mRNA. GRK3 protein was reduced in the rostral caudate-putamen (rCPu), whereas other isoforms were either unchanged or up-regulated. GRK6 protein and mRNA were up-regulated in rCPu and nucleus accumbens. l-DOPA (25 mg/kg, twice daily for 10 days) failed to reverse changes caused by DA depletion, whereas D(2)/D(3) agonist pergolide (0.25 mg/kg daily for 10 days) restored normal levels of expression of GRK5 and 6. In rCPu, GRK2 protein was increased in most subcellular fractions by l-DOPA but not by DA depletion alone. Similarly, l-DOPA up-regulated arrestin3 in membrane fractions in both regions. GRK5 was down-regulated by l-DOPA in cCPu in the light membrane fraction, where this isoform is the most abundant. The data suggest that alterations in the expression and subcellular distribution of arrestins and GRKs contribute to pathophysiology of Parkinson's disease. Thus, these proteins may be targets for antiparkinsonian therapy.

Beta-arrestins and heterotrimeric G-proteins: collaborators and competitors in signal transduction

G-protein-coupled receptors (GPCRs), also known as seven transmembrane receptors (7-TMRs), are the largest protein receptor superfamily in the body. These receptors and their ligands direct a diverse array of physiological responses, and hence have broad relevance to numerous diseases. As a result, they have generated considerable interest in the pharmaceutical industry as drug targets. Recently, GPCRs have been demonstrated to elicit signals through interaction with the scaffolding proteins, beta-arrestins-1 and 2, independent of heterotrimeric G-protein coupling. This review discusses several known G-protein-independent, beta-arrestin-dependent pathways and their potential physiological and pharmacological significance. The emergence of G-protein-independent signalling changes the way in which GPCR signalling is evaluated, from a cell biological to a pharmaceutical perspective and raises the possibility for the development of pathway specific therapeutics.

Follicle-Stimulating Hormone Induces Multiple Signaling Cascades: Evidence that Activation of Rous Sarcoma Oncogene, RAS, and the Epidermal Growth Factor Receptor Are Critical for Granulosa Cell Differentiation

FSH regulates ovarian granulosa cell differentiation not only by activating adenylyl cyclase and protein kinase A (PKA) but also by other complex mechanisms. Using primary rat granulosa cell cultures, we provide novel evidence that FSH rapidly activates two small GTP-binding proteins RAP1 and RAS. FSH activation of RAP1 requires cAMP-mediated activation of exchange factor activated by cAMP/RAPGEF3 whereas FSH activation of RAS and downstream signaling cascades involves multiple factors. Specifically, FSH activation of RAS required Rous sarcoma oncogene (SRC) family tyrosine kinase (SFK) and epidermal growth factor receptor (EGFR) tyrosine kinase activities but not PKA. FSH-induced phosphorylation of ERK1/2 was blocked by dominant-negative RAS as well as by inhibitors of EGFR tyrosine kinase, metalloproteinases involved in growth factor shedding, and SFKs. In contrast, FSH-induced phosphorylation of protein kinase B (PKB/AKT) and the Forkhead transcription factor, FOXO1a occurred by SFK-dependent but RAS-independent mechanisms. The SFKs, c-SRC and FYN, and the SRC-related tyrosine kinase ABL were present and phosphorylated rapidly in response to FSH. Lastly, the EGF-like factor amphiregulin (AREG) activated RAS and ERK1/2 phosphorylation in granulosa cells by mechanisms that were selectively blocked by an EGFR antagonist but not by an SFK antagonist. However, AREG-mediated phosphorylation of PKB and FOXO1a required both EGFR and SFK activation. Moreover, we show that FSH induces AREG and that activation of the EGFR impacts granulosa cell differentiation and the expression of genes characteristic of the luteal cell phenotype. Thus, FSH orchestrates the coordinate activation of three diverse membrane-associated signaling cascades (adenylyl cyclase, RAS, and SFKs) that converge downstream to activate specific kinases (PKA, ERK1/2, and PKB/FOXO1a) that control granulosa cell function and differentiation.

5-hydroxytryptamine 4 receptor activation of the extracellular signal-regulated kinase pathway depends on Src activation but not on G protein or beta-arrestin signaling

The 5-hydroxytryptamine(4) (5-HT(4)) receptors have recently emerged as key modulators of learning, memory, and cognitive processes. In neurons, 5-hydroxytryptamine(4) receptors (5-HT(4)Rs) activate cAMP production and protein kinase A (PKA); however, nothing is known about their ability to activate another key signaling pathway involved in learning and memory: the extracellular signal-regulated kinase (ERK) pathway. Here, we show that 5-HT(4)R stimulation, in primary neurons, produced a potent but transient activation of the ERK pathway. Surprisingly, this activation was mostly PKA independent. Similarly, using pharmacological, genetic, and molecular tools, we observed that 5-HT(4)Rs in human embryonic kidney 293 cells, activated the ERK pathway in a G(s)/cAMP/PKA-independent manner. We also demonstrated that other classical G proteins (G(q)/G(i)/G(o)) and associated downstream messengers were not implicated in the 5-HT(4)R-activated ERK pathway. The 5-HT(4)R-mediated ERK activation seemed to be dependent on Src tyrosine kinase and yet totally independent of beta-arrestin. Immunocytofluorescence revealed that ERK activation by 5-HT(4)R was restrained to the plasma membrane, whereas p-Src colocalized with the receptor and carried on even after endocytosis. This phenomenon may result from a tight interaction between 5-HT(4)R and p-Src detected by coimmunoprecipitation. Finally, we confirmed that the main route by which 5-HT(4)Rs activate ERKs in neurons was Src dependent. Thus, in addition to classical cAMP/PKA signaling pathways, 5-HT(4)Rs may use ERK pathways to control memory process.

An opioid agonist that does not induce mu-opioid receptor--arrestin interactions or receptor internalization

G protein-coupled receptor desensitization and trafficking are important regulators of opioid receptor signaling that can dictate overall drug responsiveness in vivo. Furthermore, different mu-opioid receptor (muOR) ligands can lead to varying degrees of receptor regulation, presumably because of distinct structural conformations conferred by agonist binding. For example, morphine binding produces a muOR with low affinity for beta-arrestin proteins and limited receptor internalization, whereas enkephalin analogs promote robust trafficking of both beta-arrestins and the receptors. Here, we evaluate muOR trafficking in response to activation by a novel mu-selective agonist derived from the naturally occurring plant product, salvinorin A. It is interesting that this compound, termed herkinorin, does not promote the recruitment of beta-arrestin-2 to the muOR and does not lead to receptor internalization. Moreover, whereas G protein-coupled receptor kinase overexpression can promote morphine-induced beta-arrestin interactions and muOR internalization, such manipulations do not promote herkinorin-induced trafficking. Studies in mice have shown that beta-arrestin-2 plays an important role in the development of morphine-induced tolerance, constipation, and respiratory depression. Therefore, drugs that can activate the receptor without recruiting the arrestins may be a promising step in the development of opiate analgesics that distinguish between agonist activity and receptor regulation and may ultimately lead to therapeutics designed to provide pain relief without the adverse side effects normally associated with the opiate narcotics.

New roles for beta-arrestins in cell signaling: not just for seven-transmembrane receptors

beta-arrestins, originally discovered as molecules that bind to and desensitize the activated and phosphorylated form of the G protein-coupled beta2-adrenergic receptor (beta2-AR), have recently emerged as multifunctional adaptor/scaffold proteins that dynamically assemble a wide range of multiprotein complexes in response to stimulation of most seven-transmembrane receptors (7TMRs). These complexes mediate receptor signaling, trafficking, and degradation. Moreover, beta-arrestins are increasingly found to perform analogous functions for receptors from structurally diverse classes, including atypical 7TMRs such as frizzled and smoothened, the nicotinic cholinergic receptors, receptor tyrosine kinases, and cytokine receptors, thereby regulating a growing list of cellular processes such as chemotaxis, apoptosis, and metastasis.

Regulation of alpha2AR trafficking and signaling by interacting proteins

The continuing discovery of new G protein-coupled receptor (GPCR) interacting proteins and clarification of the functional consequences of these interactions has revealed multiple roles for these events. Some of these interactions serve to scaffold GPCRs to particular cellular micro-compartments or to tether them to defined signaling molecules, while other GPCR-protein interactions control GPCR trafficking and the kinetics of GPCR-mediated signaling transduction. This review provides a general overview of the variety of GPCR-protein interactions reported to date, and then focuses on one prototypical GPCR, the alpha(2)AR, and the in vitro and in vivo significance of its reciprocal interactions with arrestin and spinophilin. It seems appropriate to recognize the life and career of Arthur Hancock with a summary of studies that both affirm and surprise our preconceived notions of how nature is designed, as his career-long efforts similarly affirmed the complexity of human biology and attempted to surprise pathological changes in that biology with novel, discovery-based therapeutic interventions. Dr. Hancock's love of life, of family, and of commitment to making the world a better place are a model of the life well lived, and truly missed by those who were privileged to know, and thus love, him.

Dosage-dependent switch from G protein-coupled to G protein-independent signaling by a GPCR

G-protein-coupled receptors (GPCRs) mostly signal through heterotrimeric G proteins. Increasing evidence suggests that GPCRs could function in a G-protein-independent manner. Here, we show that at low concentrations of an agonist, beta(2)-adrenergic receptors (beta(2)-ARs) signal through Galpha(s) to activate the mitogen-activated protein kinase pathway in mouse embryonic fibroblast cells. At high agonist concentrations, signals are also transduced through beta(2)-ARs via an additional pathway that is G-protein-independent but tyrosine kinase Src-dependent. This new dosage-dependent switch of signaling modes of GPCRs has significant implications for GPCR intrinsic properties and desensitization.