Involvement of G protein-coupled receptor kinase-6 in desensitization of CGRP receptors

The Role of G Protein-Coupled Receptor Kinase 6 Regulation in Inflammation and Pain

The G protein-coupled receptor kinase 6 is associated with inflammation and pathological pain. Impairment of GRK6 expression was described in chronic inflammatory diseases such as rheumatoid arthritis and this was shown to be accompanied by an imbalance of downstream signaling pathways. Here, we discuss novel aspects of GRK6 interaction and its impact upon hyperalgesia and inflammatory processes. In this review, we compile important findings concerning GRK6 regulation for a better pathophysiological understanding of the intracellular interaction in the context of inflammation and show clinical implications-for example, the identification of possible therapy goals in the treatment of chronic inflammatory hyperalgesia.

Determining the Effects of Differential Expression of GRKs and β-arrestins on CLR-RAMP Agonist Bias

Signalling of the calcitonin-like receptor (CLR) is multifaceted, due to its interaction with receptor activity modifying proteins (RAMPs), and three endogenous peptide agonists. Previous studies have focused on the bias of G protein signalling mediated by the receptor and receptor internalisation of the CLR-RAMP complex has been assumed to follow the same pattern as other Class B1 G Protein-Coupled Receptors (GPCRs). Here we sought to measure desensitisation of the three CLR-RAMP complexes in response to the three peptide agonists, through the measurement of β-arrestin recruitment and internalisation. We then delved further into the mechanism of desensitisation through modulation of β-arrestin activity and the expression of GPCR kinases (GRKs), a key component of homologous GPCR desensitisation. First, we have shown that CLR-RAMP1 is capable of potently recruiting β-arrestin1 and 2, subsequently undergoing rapid endocytosis, and that CLR-RAMP2 and -RAMP3 also utilise these pathways, although to a lesser extent. Following this we have shown that agonist-dependent internalisation of CLR is β-arrestin dependent, but not required for full agonism. Overexpression of GRK2-6 was then found to decrease receptor signalling, due to an agonist-independent reduction in surface expression of the CLR-RAMP complex. These results represent the first systematic analysis of the importance of β-arrestins and GRKs in CLR-RAMP signal transduction and pave the way for further investigation regarding other Class B1 GPCRs.

mTORC1 and mTORC2 Converge on the Arp2/3 Complex to Promote KrasG12D-Induced Acinar-to-Ductal Metaplasia and Early Pancreatic Carcinogenesis

BACKGROUND & AIMS

Oncogenic KrasG12D induces neoplastic transformation of pancreatic acinar cells through acinar-to-ductal metaplasia (ADM), an actin-based morphogenetic process, and drives pancreatic ductal adenocarcinoma (PDAC). mTOR (mechanistic target of rapamycin kinase) complex 1 (mTORC1) and 2 (mTORC2) contain Rptor and Rictor, respectively, and are activated downstream of KrasG12D, thereby contributing to PDAC. Yet, whether and how mTORC1 and mTORC2 impact on ADM and the identity of the actin nucleator(s) mediating such actin rearrangements remain unknown.

METHODS

A mouse model of inflammation-accelerated KrasG12D-driven early pancreatic carcinogenesis was used. Rptor, Rictor, and Arpc4 (actin-related protein 2/3 complex subunit 4) were conditionally ablated in acinar cells to deactivate the function of mTORC1, mTORC2 and the actin-related protein (Arp) 2/3 complex, respectively.

RESULTS

We found that mTORC1 and mTORC2 are markedly activated in human and mouse ADM lesions, and cooperate to promote KrasG12D-driven ADM in mice and in vitro. They use the Arp2/3 complex as a common downstream effector to induce the remodeling the actin cytoskeleton leading to ADM. In particular, mTORC1 regulates the translation of Rac1 (Rac family small GTPase 1) and the Arp2/3-complex subunit Arp3, whereas mTORC2 activates the Arp2/3 complex by promoting Akt/Rac1 signaling. Consistently, genetic ablation of the Arp2/3 complex prevents KrasG12D-driven ADM in vivo. In acinar cells, the Arp2/3 complex and its actin-nucleation activity mediated the formation of a basolateral actin cortex, which is indispensable for ADM and pre-neoplastic transformation.

CONCLUSIONS

Here, we show that mTORC1 and mTORC2 attain a dual, yet nonredundant regulatory role in ADM and early pancreatic carcinogenesis by promoting Arp2/3 complex function. The role of Arp2/3 complex as a common effector of mTORC1 and mTORC2 fills the gap between oncogenic signals and actin dynamics underlying PDAC initiation.

Role of GRK6 in the Regulation of Platelet Activation through Selective G Protein-Coupled Receptor (GPCR) Desensitization

Platelet G protein-coupled receptors (GPCRs) regulate platelet function by mediating the response to various agonists, including adenosine diphosphate (ADP), thromboxane A₂, and thrombin. Although GPCR kinases (GRKs) are considered to have the crucial roles in most GPCR functions, little is known regarding the regulation of GPCR signaling and mechanisms of GPCR desensitization by GRKs in platelets. In this study, we investigated the functional role of GRK6 and the molecular basis for regulation of specific GPCR desensitization by GRK6 in platelets. We used GRK6 knockout mice to evaluate the functional role of GRK6 in platelet activation. Platelet aggregation, dense- and α-granule secretion, and fibrinogen receptor activation induced by 2-MeSADP, U46619, thrombin, and AYPGKF were significantly potentiated in GRK6^−/− platelets compared to the wild-type (WT) platelets. However, collagen-related peptide (CRP)-induced platelet aggregation and secretion were not affected in GRK6^−/− platelets. Interestingly, platelet aggregation induced by co-stimulation of serotonin and epinephrine which activate G_q-coupled 5HT_2A and G_z-coupled α_2A adrenergic receptors, respectively, was not affected in GRK6^−/− platelets, suggesting that GRK6 was involved in specific GPCR regulation. In addition, platelet aggregation in response to the second challenge of ADP and AYPGKF was restored in GRK6^−/− platelets whereas re-stimulation of the agonist failed to induce aggregation in WT platelets, indicating that GRK6 contributed to P2Y₁, P2Y₁₂, and PAR4 receptor desensitization. Furthermore, 2-MeSADP-induced Akt phosphorylation and AYPGKF-induced Akt, extracellular signal-related kinase (ERK), and protein kinase Cδ (PKCδ) phosphorylation were significantly potentiated in GRK6^−/− platelets. Finally, GRK6^−/− mice exhibited an enhanced and stable thrombus formation after FeCl₃ injury to the carotid artery and shorter tail bleeding times, indicating that GRK6^−/− mice were more susceptible to thrombosis and hemostasis. We conclude that GRK6 plays an important role in regulating platelet functional responses and thrombus formation through selective GPCR desensitization.

Extracellular tau induces microglial phagocytosis of living neurons in cell cultures

Tau is a microtubule-associated protein, found at high levels in neurons, and its aggregation is associated with neurodegeneration. Recently, it was found that tau can be actively secreted from neurons, but the effects of extracellular tau on neuronal viability are unclear. In this study, we investigated whether extracellular tau^2N4R can cause neurotoxicity in primary cultures of rat brain neurons and glial cells. Cell cultures were examined for neuronal loss, death, and phosphatidylserine exposure, as well as for microglial phagocytosis by fluorescence microscopy. Aggregation of tau^2N4R was assessed by atomic force microscopy. We found that extracellular addition of tau induced a gradual loss of neurons over 1-2 days, without neuronal necrosis or apoptosis, but accompanied by proliferation of microglia in the neuronal-glial co-cultures. Tau addition caused exposure of the 'eat-me' signal phosphatidylserine on the surface of living neurons, and this was prevented by elimination of the microglia or by inhibition of neutral sphingomyelinase. Tau also increased the phagocytic activity of pure microglia, and this was blocked by inhibitors of neutral sphingomyelinase or protein kinase C. The neuronal loss induced by tau was prevented by inhibitors of neutral sphingomyelinase, protein kinase C or the phagocytic receptor MerTK, or by eliminating microglia from the cultures. The data suggest that extracellular tau induces primary phagocytosis of stressed neurons by activated microglia, and identifies multiple ways in which the neuronal loss induced by tau can be prevented.

CGRP Receptor Signalling Pathways

Calcitonin gene-related peptide (CGRP) is a promiscuous peptide, similar to many other members of the calcitonin family of peptides. The potential of CGRP to act on many different receptors with differing affinities and efficacies makes deciphering the signalling from the CGRP receptor a challenging task for researchers.Although it is not a typical G protein-coupled receptor (GPCR), in that it is composed not just of a GPCR, the CGRP receptor activates many of the same signalling pathways common for other GPCRs. This includes the family of G proteins and a variety of protein kinases and transcription factors. It is now also clear that in addition to the initiation of cell-surface signalling, GPCRs, including the CGRP receptor, also activate distinct signalling pathways as the receptor is trafficking along the endocytic conduit.Given CGRP's characteristic of activating multiple GPCRs, we will first consider the complex of calcitonin receptor-like receptor (CLR) and receptor activity-modifying protein 1 (RAMP1) as the CGRP receptor. We will discuss the discovery of the CGRP receptor components, the molecular mechanisms controlling its internalization and post-endocytic trafficking (recycling and degradation) and the diverse signalling cascades that are elicited by this receptor in model cell lines. We will then discuss CGRP-mediated signalling pathways in primary cells pertinent to migraine including neurons, glial cells and vascular smooth muscle cells.Investigation of all the CGRP- and CGRP receptor-mediated signalling cascades is vital if we are to fully understand CGRP's role in migraine and will no doubt unearth new targets for the treatment of migraine and other CGRP-driven diseases.

Overexpression of GRK6 attenuates neuropathic pain via suppression of CXCR2 in rat dorsal root ganglion

G protein-coupled kinase (GRK) 6 is a member of the GRK family that mediates agonist-induced desensitization and signaling of G protein-coupled receptors (GPCRs), thus involving in a wide variety of processes including inflammation and nociception. Recent studies have indicated that chemokines play an important role in chronic pain via increased expression of respective GPCRs. This study was designed to investigate the role of GRK6 and its interaction with substrate chemokine receptors in dorsal root ganglion (DRG) in a rat model of neuropathic pain induced by chronic constriction injury (CCI). Following induction of CCI, GRK6 expression was significantly downregulated in rat DRGs at L4-L6 segments. Overexpression of GRK6 using lentiviral-mediated production strategy via sciatic nerve injection markedly attenuated mechanical allodynia and thermal hyperalgesia in CCI rats. Overexpression of GRK6 also drastically reversed the hyperexcitability of DRG neurons innervating the hind paw and suppressed the enhanced expression of CXCR2 in DRGs of CCI rats. In addition, co-immunoprecipitation, immunofluorescence, and correlation analysis supported the interaction between GRK6 and CXCR2. These results suggest that GRK6 might be a key molecular involved in peripheral mechanism of neuropathic pain and that overexpression of GRK6 might be a potential strategy for treatment for neuropathic pain through inhibition of CXCR2 signal pathway.

G-Protein Coupled Receptor Kinase 2 Minimally Regulates Melanopsin Activity in Intrinsically Photosensitive Retinal Ganglion Cells

Phosphorylation is a primary modulator of mammalian G-protein coupled receptor (GPCR) activity. The GPCR melanopsin is the photopigment of intrinsically photosensitive retinal ganglion cells (ipRGCs) in the mammalian retina. Recent evidence from in vitro experiments suggests that the G-protein coupled receptor kinase 2 (GRK2) phosphorylates melanopsin and reduces its activity following light exposure. Using an ipRGC-specific GRK2 loss-of-function mouse, we show that GRK2 loss alters melanopsin response dynamics and termination time in postnatal day 8 (P8) ipRGCs but not in older animals. However, the alterations are small in comparison to the changes reported for other opsins with loss of their cognate GRK. These results suggest GRK2 contributes to melanopsin deactivation, but that other mechanisms account for most of modulation of melanopsin activity in ipRGCs.

Functionally selective signaling for morphine and fentanyl antinociception and tolerance mediated by the rat periaqueductal gray

Functionally selective signaling appears to contribute to the variability in mechanisms that underlie tolerance to the antinociceptive effects of opioids. The present study tested this hypothesis by examining the contribution of G protein-coupled receptor kinase (GRK)/Protein kinase C (PKC) and C-Jun N-terminal kinase (JNK) activation on both the expression and development of tolerance to morphine and fentanyl microinjected into the ventrolateral periaqueductal gray of the rat. Microinjection of morphine or fentanyl into the periaqueductal gray produced a dose-dependent increase in hot plate latency. Microinjection of the non-specific GRK/PKC inhibitor Ro 32-0432 into the periaqueductal gray to block mu-opioid receptor phosphorylation enhanced the antinociceptive effect of morphine but had no effect on fentanyl antinociception. Microinjection of the JNK inhibitor SP600125 had no effect on morphine or fentanyl antinociception, but blocked the expression of tolerance to repeated morphine microinjections. In contrast, a microinjection of Ro 32-0432 blocked the expression of fentanyl, but not morphine tolerance. Repeated microinjections of Ro 32-0432 blocked the development of morphine tolerance and inhibited fentanyl antinociception whether rats were tolerant or not. Repeated microinjections of SP600125 into the periaqueductal gray blocked the development of tolerance to both morphine and fentanyl microinjections. These data demonstrate that the signaling molecules that contribute to tolerance vary depending on the opioid and methodology used to assess tolerance (expression vs. development of tolerance). This signaling difference is especially clear for the expression of tolerance in which JNK contributes to morphine tolerance and GRK/PKC contributes to fentanyl tolerance.

Caveolae-dependent internalization and homologous desensitization of VIP/PACAP receptor, VPAC₂, in gastrointestinal smooth muscle

The main membrane proteins of caveolae (caveolin-1, -2 and -3) oligomerize within lipid rich domains to form regular invaginations of smooth muscle plasma membrane and participate in receptor internalization and desensitization independent of clathrin-coated vesicle endocytosis. We have previously shown that Gs-coupled VIP/PACAP receptors, VPAC2, predominantly expressed in smooth muscle cells of the gut, are exclusively phosphorylated by GRK2 leading to receptor internalization and desensitization. Herein, we characterized the role of caveolin-1 in VPAC2 receptor internalization and desensitization in gastric smooth muscle using three approaches: (i) methyl β-cyclodextrin (MβCD) to deplete cholesterol and disrupt caveolae in dispersed muscle cells, (ii) caveolin-1 siRNA to suppress caveolin-1 expression in cultured muscle cells, and (iii) caveolin-1 knockout mice (caveolin-1(-/-)). Pretreatment of gastric muscle cells with VIP stimulated tyrosine phosphorylation of caveolin-1, and induced VPAC2 receptor internalization (measured as decrease in (125)I-VIP binding after pretreatment) and desensitization (measured as decrease in VIP-induced cAMP formation after pretreatment). Caveolin-1 phosphorylation, and VPAC2 receptor internalization and desensitization were blocked by disruption of caveolae with MβCD, suppression of caveolin-1 with caveolin-1 siRNA or inhibition of Src kinase activity by PP2. Pretreatment with VIP significantly inhibited adenylyl cyclase activity and muscle relaxation in response to subsequent addition of VIP in freshly dispersed muscle cells and in muscle strips isolated from wild type and caveolin-1(-/-) mice; however, the inhibition was significantly attenuated in caveolin-1(-/-) mice. These results suggest that caveolin-1 plays an important role in VPAC2 receptor internalization and desensitization.

Arrestins: role in the desensitization, sequestration, and vesicular trafficking of G protein-coupled receptors

Over the years, β-arrestins have emerged as multifunctional molecular scaffolding proteins regulating almost every imaginable G protein-coupled receptor (GPCR) function. Originally discovered as GPCR-desensitizing molecules, they have been shown to also serve as important regulators of GPCR signaling, sequestration, and vesicular trafficking. This broad functional role implicates β-arrestins as key regulatory proteins for cellular function. Hence, this chapter summarizes the current understanding of the β-arrestin family's unique ability to control the kinetics as well as the extent of GPCR activity at the level of desensitization, sequestration, and subsequent intracellular trafficking.

G protein-coupled receptor kinase 6 acts as a critical regulator of cytokine-induced hyperalgesia by promoting phosphatidylinositol 3-kinase and inhibiting p38 signaling

The molecular mechanisms determining magnitude and duration of inflammatory pain are still unclear. We assessed the contribution of G protein-coupled receptor kinase (GRK)-6 to inflammatory hyperalgesia in mice. We showed that GRK6 is a critical regulator of severity and duration of cytokine-induced hyperalgesia. In GRK6⁻/⁻ mice, a significantly lower dose (100 times lower) of intraplantar interleukin (IL)-1β was sufficient to induce hyperalgesia compared with wild-type (WT) mice. In addition, IL-1β hyperalgesia lasted much longer in GRK6⁻/⁻ mice than in WT mice (8 d in GRK6⁻/⁻ versus 6 h in WT mice). Tumor necrosis factor (TNF)-α-induced hyperalgesia was also enhanced and prolonged in GRK6⁻/⁻ mice. In vitro, IL-1β-induced p38 phosphorylation in GRK6⁻/⁻ dorsal root ganglion (DRG) neurons was increased compared with WT neurons. In contrast, IL-1β only induced activation of the phosphatidylinositol (PI) 3-kinase/Akt pathway in WT neurons, but not in GRK6⁻/⁻ neurons. In vivo, p38 inhibition attenuated IL-1β- and TNF-α-induced hyperalgesia in both genotypes. Notably, however, whereas PI 3-kinase inhibition enhanced and prolonged hyperalgesia in WT mice, it did not have any effect in GRK6-deficient mice. The capacity of GRK6 to regulate pain responses was also apparent in carrageenan-induced hyperalgesia, since thermal and mechanical hypersensitivity was significantly prolonged in GRK6⁻/⁻ mice. Finally, GRK6 expression was reduced in DRGs of mice with chronic neuropathic or inflammatory pain. Collectively, these findings underline the potential role of GRK6 in pathological pain. We propose the novel concept that GRK6 acts as a kinase that constrains neuronal responsiveness to IL-1β and TNF-α and cytokine-induced hyperalgesia via biased cytokine-induced p38 and PI 3-kinase/Akt activation.

Regulation of GPCR trafficking by RAMPs

AM and CGRP receptors undergo differential intracellular receptor trafficking upon ligand stimulation. Intracellular trafficking of CLR/RAMP receptor complexes is regulated by posttranslational modifications and protein-protein interactions that differ for each cell type. Recent evidence is accumulating to suggest that the RAMP isoform in complex with CLR may play a role in determining the intracellular trafficking and fate of ligand-stimulated receptor complexes. In this chapter, we will review the current literature on mechanisms of regulating receptor trafficking and roles that have been demonstrated for RAMPs in this regulation.

Characterization of kinases involved in the phosphorylation of aggregated α-synuclein

α-Synuclein (α-syn) is the major component of pathological inclusions characteristic of several neurodegenerative disorders, such as Parkinson's disease. The major posttranslational modification of α-syn is phosphorylation at S129, and previous studies estimate that approximately 90% of α-syn in proteinaceous, pathological inclusions is phosphorylated at this site. α-Syn can be phosphorylated by polo-like kinases (PLKs) 1-3 and casein kinases (CK) 1 and 2; however, the kinases associated with the hyperphosphorylation of aggregated α-syn are still under debate. Using a high-efficiency cellular model of α-syn aggregate formation, we found that selective inhibitors for CK2 and PLKs each partially inhibited S129 phosphorylation of soluble (nonaggregated) α-syn, but only PLK inhibitors modestly attenuated the phosphorylation of aggregated α-syn. In addition, none of the kinase inhibitors used had a substantial effect on the propensity of α-syn to aggregate. Overexpression of all PLKs promoted robust phosphorylation of soluble α-syn, but none altered the propensity of α-syn to aggregate. Overexpression of only PLK2 increased phosphorylation of aggregated α-syn at S129, which likely is due to increased phosphorylation of soluble α-syn, which then was incorporated into aggregates. Overexpression of PLK1 and treatment with BI2536 resulted in a significant reduction of phosphorylated, aggregated α-syn protein, beyond that of BI2536 treatment alone. These studies suggest that phosphorylation of α-syn is independent of α-syn aggregate formation, that PLK1 is involved in the phosphorylation of aggregated α-syn at S129 in this system, and that mechanisms resulting in hyperphosphorylation of aggregated α-syn appear to be independent of those responsible for the phosphorylation of soluble α-syn.

Combining resonance energy transfer methods reveals a complex between the alpha2A-adrenergic receptor, Galphai1beta1gamma2, and GRK2

Traditionally, G-protein-coupled receptor (GPCR) interactions with their G proteins and regulatory proteins, GPCR kinases (GRKs) and arrestins, are described as sequential events involving rapid assemblies/disassemblies. To directly monitor the dynamics of these interactions in living cells, we combined two spectrally resolved bioluminescence and one fluorescence resonance energy transfer (RET) methods. The RET combination analysis revealed that stimulation of the α(2A)-adrenergic receptor (α(2A)AR) leads to the recruitment of GRK2 at a receptor still associated with the Gα(i1)β(1)γ(2) complex. The interaction kinetics of GRKs with Gγ(2) (2.8 ± 0.4 s) and α(2A)AR (5.2 ± 0.5 s) were similar to that of the receptor-promoted change in RET between Gα(i1) and Gγ(2) (5.2 ± 1.2 s), and persisted until the translocation of βarrestin2 to the receptor, indicating that GRK2 remains associated to the receptor/G-protein complex for longer periods than anticipated. Moreover, GRK2 or a kinase-deficient GRK2 mutant, but not GRK5, potentiated the receptor-promoted changes in RET between Gα(i1) and Gγ(2) and abrogated the α(2A)AR-stimulated calcium response, suggesting that the recruitment of GRK2 to the complex contributes to the structural rearrangement and functional regulation of the signaling unit, independently of the kinase activity. RET combination analysis revealed unanticipated dynamics in GPCR signaling and will be applicable to many biological systems.

Stimuli of sensory-motor nerves terminate arterial contractile effects of endothelin-1 by CGRP and dissociation of ET-1/ET(A)-receptor complexes

Background

Endothelin-1 (ET-1), a long-acting paracrine mediator, is implicated in cardiovascular diseases but clinical trials with ET-receptor antagonists were not successful in some areas. We tested whether the quasi-irreversible receptor-binding of ET-1 (i) limits reversing effects of the antagonists and (ii) can be selectively dissociated by an endogenous counterbalancing mechanism.

Methodology/Principal findings

In isolated rat mesenteric resistance arteries, ET_A-antagonists, endothelium-derived relaxing factors and synthetic vasodilators transiently reduced contractile effects of ET-1 but did not prevent persistent effects of the peptide. Stimuli of peri-vascular vasodilator sensory-motor nerves such as capsaicin not only reduced but also terminated long-lasting effects of ET-1. This was prevented by CGRP-receptor antagonists and was mimicked by exogenous calcitonin gene-related peptide (CGRP). Using 2-photon laser scanning microscopy in vital intact arteries, capsaicin and CGRP, but not ET_A-antagonism, were observed to promote dissociation of pre-existing ET-1/ET_A-receptor complexes.

Conclusions

Irreversible binding and activation of ET_A-receptors by ET-1 (i) occur at an antagonist-insensitive site of the receptor and (ii) are selectively terminated by endogenously released CGRP. Hence, natural stimuli of sensory-motor nerves that stimulate release of endogenous CGRP can be considered for therapy of diseases involving ET-1.

The effect of protein kinase C and G protein-coupled receptor kinase inhibition on tolerance induced by mu-opioid agonists of different efficacy

Differences in the mechanisms underlying tolerance and mu-opioid receptor desensitization resulting from exposure to opioid agonists of different efficacy have been suggested previously. The objective of this study was to determine the effects of protein kinase C (PKC) and G protein-coupled receptor kinase (GRK) inhibition on antinociceptive tolerance in vivo to opioid agonists of different efficacy. A rapid (8-h) tolerance-induction model was used where each opioid was repeatedly administered to naive mice. Animals were then challenged with the opioid after injection of a kinase inhibitor to determine its effects on the level of tolerance. Tolerance to meperidine, morphine, or fentanyl was fully reversed by the PKC inhibitor 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)carbazole (Gö6976). However, in vivo tolerance to [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) was not reversed by PKC inhibition. The novel small-molecule GRK inhibitors beta-adrenergic receptor kinase 1 inhibitor and 2-(8-[(dimethylamino) methyl]-6,7,8,9-tetrahydropyridol[1,2-a]indol-3-yl)-3-(1-methylindol-3-yl)maleimide (Ro 32-0432) did not reverse the tolerance to meperidine, fentanyl, or morphine but did reverse the tolerance to DAMGO. To correlate GRK-dependent DAMGO-induced tolerance with mu-opioid receptor desensitization, we used in vitro whole-cell patch-clamp recording from mouse locus coeruleus neurons and observed that the GRK inhibitors reduced DAMGO-induced desensitization of mu-opioid receptors, whereas the PKC inhibitor had no effect. These results suggest that tolerance induced by low- and moderate-efficacy mu-opioid receptor agonists is dependent on PKC, whereas tolerance induced by the high-efficacy agonist DAMGO is dependent on GRK.

Regulation of GPR54 signaling by GRK2 and {beta}-arrestin

Kisspeptin and its receptor, GPR54, are major regulators of the hypothalamic-pituitary-gonadal axis as well as regulators of human placentation and tumor metastases. GPR54 is a G(q/11)-coupled G protein-coupled receptor (GPCR), and activation by kisspeptin stimulates phosphatidy linositol 4, 5-biphosphate hydrolysis, Ca(2+) mobilization, arachidonic acid release, and ERK1/2 MAPK phosphorylation. Physiological evidence suggests that GPR54 undergoes agonist-dependent desensitization, but underlying molecular mechanisms are unknown. Furthermore, very little has been reported on the early events that regulate GPR54 signaling. The lack of information in these important areas led to this study. Here we report for the first time on the role of GPCR serine/threonine kinase (GRK)2 and beta-arrestin in regulating GPR54 signaling in human embryonic kidney (HEK) 293 cells, a model cell system for studying the molecular regulation of GPCRs, and genetically modified MDA MB-231 cells, an invasive breast cancer cell line expressing about 75% less beta-arrestin-2 than the control cell line. Our study reveals that in HEK 293 cells, GPR54 is expressed both at the plasma membrane and intracellularly and also that plasma membrane expression is regulated by cytoplasmic tail sequences. We also demonstrate that GPR54 exhibits constitutive activity, internalization, and association with GRK2 and beta- arrestins-1 and 2 through sequences in the second intracellular loop and cytoplasmic tail of the receptor. We also show that GRK2 stimulates the desensitization of GPR54 in HEK 293 cells and that beta-arrestin-2 mediates GPR54 activation of ERK1/2 in MDA-MB-231 cells. The significance of these findings in developing molecular-based therapies for treating certain endocrine-related disorders is discussed.

G protein-coupled receptor kinase 6 controls post-inflammatory visceral hyperalgesia

Post-inflammatory pain is a poorly understood phenomenon. G protein-coupled receptors are involved in regulating pain signaling in the context of inflammation. G protein-coupled receptor kinases (GRK) modulate signaling through these receptors. We investigated whether GRK6 contributes to post-inflammatory visceral hyperalgesia. Colitis was induced in female mice by 1% dextran sodium sulphate in drinking water for 7 days. Disease score, colon length, and colonic cytokines were determined. On day 49, when animals had recovered from colitis, we induced visceral pain by intracolonic capsaicin instillation. Behavioral responses to capsaicin were monitored for 20 min. Referred hyperalgesia was measured using von Frey hairs. Spinal cord c-Fos was visualized by immunohistochemistry. In contrast to our earlier observations in male GRK6-/- and wild type (WT) mice, we did not detect differences in the course of colitis or in expression of colonic cytokines between female GRK6-/- and WT mice. After recovery from colitis, capsaicin-induced behavioral pain responses and spinal cord c-Fos expression were more pronounced in female GRK6-/- than WT mice. Naive GRK6-/- and WT animals did not differ in pain and c-Fos responses to capsaicin. Capsaicin-induced referred hyperalgesia post-colitis was increased in GRK6-/- compared to WT mice. However, referred hyperalgesia post-colitis was not affected by ablation of GRK6. Furthermore, in vitro IL-1beta sensitized the capsaicin receptor TRPV1 and this process was inhibited by over-expression of GRK6. We describe the novel concept that GRK6 inhibits post-inflammatory visceral hyperalgesia but does not contribute to visceral pain in naive animals. We propose that GRK6 regulates inflammation-induced sensitization of TRPV1.

Protein kinase C is a common component of CGRP receptor desensitization induced by distinct agonists

The calcitonin gene-related peptide (CGRP) is a neuropeptide involved in vasodilation and other physiological functions throughout the body. The receptor for CGRP has been cloned and well studied, but the mechanism of CGRP receptor desensitization has not been fully elucidated. In the present study, we evaluated the kinetics for agonist-mediated desensitization of the adenylate cyclase response in human neuroblastoma SK-N-MC cells. Distinct CGRP receptor agonists were used, including alpha and beta isoforms of CGRP, the linearized derivative cys(Et)2,7 alphaCGRP, adrenomedullin, and adrenomedullin 2. betaCGRP was 4-600 times more potent at desensitizing the cAMP production as compared to the other receptor-activating ligands, and all of the desensitization effects were blocked by a CGRP receptor antagonist. Although the different agonists vary in their ability to induce functional desensitization, a pretreatment/washout sequence with each peptide was able to reduce the activation potency of the other members of the calcitonin/CGRP peptide family. Next we tested whether the desensitizing effects of the distinct peptides involve protein kinase C (PKC) or protein kinase A (PKA). A PKC inhibitor, Ro 31-8220, concentration-dependently reduced the desensitization induced by the 5 CGRP receptor agonists, while having little effect on their desensitization potencies. PKA inhibitors KT-5720 and H-89, on the other hand, showed little effect on the induced level of desensitization. The findings indicate that functional desensitization is produced by distinct peptides acting through the active site of CGRP receptors, and involves the activation of PKC as a common component necessary to achieve maximal desensitization of receptor signaling.

Desensitization and re-sensitization of CGRP receptor function in human neuroblastoma SK-N-MC cells

Calcitonin gene-related peptide (CGRP) is a highly potent vasodilator known to be involved in many physiological functions within the cardiovascular, gastrointestinal, immune, and nervous systems. This study assessed the desensitization of CGRP receptors by measuring agonist-mediated activation of adenylate cyclase in a model system employing human neuroblastoma-derived SK-N-MC cells. In these cells, we demonstrated that pre-incubation with CGRP (20 nM) induces a rapid desensitization of CGRP signaling (t(1/2)<or=3 min) by causing a decrease in potency and efficacy. CGRP's desensitization potency (DC(50)=0.29 nM) is similar to its activation potency on non-desensitized cells (EC(50)=0.20 nM). The desensitized receptors exhibited slow and incomplete re-sensitization upon removal of the pre-incubated ligand, resulting in 52-65% functional recovery after 3-5 h while CGRP binding sites were completely restored. Additional agonists within the calcitonin/CGRP family of peptides (calcitonin, amylin, adrenomedullin, and adrenomedullin 2) were compared to CGRP with regard to their ability to activate and desensitize CGRP receptors. Calcitonin and amylin did not cause receptor activation nor did they produce desensitization. Adrenomedullin and adrenomedullin 2 activated the receptors and produced desensitization, but at a slower rate and with a weaker desensitization potency than CGRP-induced desensitization. Adrenomedullin exhibited similar potency for receptor activation and desensitization, whereas adrenomedullin 2 has a 4-fold higher preference for receptor desensitization than for receptor activation. Activation and desensitization induced by CGRP, adrenomedullin and adrenomedullin 2 were blocked by the CGRP receptor antagonist CGRP8-37. These data indicate that CGRP receptors are desensitized by select peptides in the calcitonin/CGRP family. Slow recovery from the desensitized state may provide a strategy for timed modulation of the CGRP signaling pathway.

Expression of terminally glycosylated calcitonin receptor-like receptor in uterine leiomyoma: endothelial phenotype and association with microvascular density

PURPOSE

The role for the hypoxia-inducible angiogenic factor adrenomedullin (AM) in tumor growth and progression has been suggested. Calcitonin receptor-like receptor (CL) is a G protein-coupled receptor (GPCR) that mediates effects of AM, but little information is available on its expression and functional state in human tumors. The present study attempted to determine CL potential for antiangiogenic therapy of uterine leiomyoma.

EXPERIMENTAL DESIGN AND RESULTS

GPCR CL is transported to the cell surface and recognized by AM only when terminally/mature glycosylated. The presence and localization of this form of the receptor in tumor and surrounding myometrial tissues obtained from leiomyoma-bearing uteri were examined using deglycosylation, immunoblotting, and immunofluorescence analysis. The mature CL glycoprotein was expressed in both tissues and localized exclusively in normal and tumor endothelium within leiomyoma-bearing uteri. The functionality of the receptor expressed in myometrial microvascular endothelial cells (MMVEC) was examined in vitro using receptor internalization and angiogenic assays. The mature CL glycoprotein expressed by primary MMVECs was functional because AM interacted with this GPCR and induced its internalization as well as angiogenic effects (proliferation and migration) in MMVECs in vitro. Finally, the levels of tissue-expressed mature CL glycoprotein as a functional form of this GPCR were analyzed by immunoblotting. The expression of this functional form of the receptor in vivo was significantly decreased (P = 0.01) in leiomyoma tissue, and this was concurrent with the decrease in microvascular density (measured by Chalkley counting) in tumor compared with surrounding myometrium (P = 0.031).

CONCLUSIONS

Our findings suggest that GPCR CL mediates angiogenic effects of AM in myometrium and that further evaluation of the properties of the CL expressed in both normal and tumor endothelium in vivo may be essential before targeting this endothelial GPCR for antiangiogenic therapies.

Adrenomedullin and CGRP interact with endogenous calcitonin-receptor-like receptor in endothelial cells and induce its desensitisation by different mechanisms

Adrenomedullin (AM) and calcitonin gene-related peptide (CGRP) are related peptides with distinct pharmacological profiles. Calcitonin-receptor-like receptor (CRLR, now known as CL) can function as either an AM receptor or a CGRP receptor, when cotransfected with receptor-activity-modifying proteins (RAMPs) that define ligand-binding specificity. The aim of the present study was to determine the role of endogenously expressed CL (EndoCL) in generating endogenous AM and CGRP receptors. We raised anti-human CL antibody and identified microvascular endothelial cells (MVECs) as a major CL-expressing cell type in tissues by immunohistochemistry. Cultured MVECs continue to express EndoCL as well as fully active endogenous AM- and CGRP-sensitive receptors in vitro, as demonstrated by the ability of both peptides to induce migration and Akt phosphorylation. We therefore tested the hypothesis that endothelial EndoCL can interact with both AM and CGRP by examining receptor internalisation and desensitisation (loss of the ability to induce Akt phosphorylation). We found that agonist-mediated internalisation of EndoCL occurs in response to AM but not CGRP in MVECs. However, AM-induced EndoCL internalisation was blocked by antagonists of both AM and CGRP receptors: AM(22-52) and CGRP(8-37), respectively. Furthermore, AM-induced EndoCL internalisation resulted in desensitisation not only of AM but also of CGRP receptors. Finally, CGRP also induced desensitisation of both endogenous AM and CGRP receptors, but did not mediate EndoCL internalisation despite interaction with this receptor. Thus, EndoCL interacts with both AM and CGRP, and simultaneously acts as a receptor for both peptides (i.e acting as an endogenous AM/CGRP receptor) in endothelial cells. Interaction with either ligand is sufficient to induce EndoCL desensitisation to both AM and CGRP, but differential mechanisms are involved since only AM induces EndoCL internalisation. These novel findings regarding regulation of EndoCL function in endothelial cells are likely to be of importance in conditions where AM or CGRP levels are elevated, such as cardiovascular disease, diabetes and inflammation.

GRKs and beta-arrestins: roles in receptor silencing, trafficking and signaling

Stimulation of cell-surface seven-transmembrane receptors (7TMRs) elicits biological responses to a wide range of extracellular signals, including many hormones. Classically, heterotrimeric GTP-binding proteins (G proteins) are recruited to the activated conformation of 7TMRs. Only two other families of protein have this remarkable characteristic: G-protein-coupled receptor kinases and beta-arrestins. These two protein families have long been known to have a central and coordinated role in the "desensitization" of G protein activation by 7TMRs. In addition, G-protein-coupled receptor kinases and beta-arrestins are involved in an increasing number of interactions with non-receptor proteins, broadening the variety of their cellular functions. These newly appreciated attributes of these two families of protein highlight their unique ability to coordinate the various aspects of 7TMR functions.

Adrenomedullin: a new and promising target for drug discovery

Adrenomedullin (AM) is a 52 amino acid peptide that plays a critical role in several diseases such as hypertension, cancer, diabetes, cardiovascular and renal disorders, among others. Interestingly, AM behaves as a protective agent against some pathologies, yet is a stimulating factor for other disorders. Thus, AM can be considered as a new and promising target for the design of non-peptidic modulators that could be useful for the treatment of those pathologies, by regulating AM levels or the activity of AM. A full decade on from its discovery, much more is known about AM molecular biology and pharmacology, but this knowledge still needs to be applied to the development of clinically useful drugs.

Orphanin FQ/nociceptin potentiates [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin-Induced mu-opioid receptor phosphorylation

In this study, we investigate the molecular mechanisms by which acute orphanin FQ/nociceptin (OFQ/N), acting through the nociceptin opioid peptide (NOP) receptor, desensitizes the mu-opioid receptor. We described previously the involvement of protein kinase C and G-protein-coupled receptor kinases (GRK) 2 and 3 in OFQ/N-induced mu receptor desensitization. Because phosphorylation of the mu receptor triggers the successive regulatory mechanisms responsible for desensitization, such as receptor uncoupling, internalization, and down-regulation, we investigated the ability of OFQ/N to modulate [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO)-induced mu receptor phosphorylation in BE(2)-C human neuroblastoma cells transfected with epitope-tagged mu receptors. OFQ/N treatment (100 nM, 60 min) potentiated DAMGO-induced mu receptor phosphorylation; inhibition of GRK2 or protein kinase C concomitant with OFQ/N treatment blocked the OFQ/N-mediated increase in DAMGO-induced phosphorylation. Inclusion of the NOP antagonist peptide III-BTD during OFQ/N pretreatment blocked the potentiation of DAMGO-induced phosphorylation by OFQ/N, which is consistent with the potentiation being mediated via actions of the NOP receptor. In addition, in cells expressing mu receptors in which the GRK-mediated phosphorylation site Ser(375) was mutated to alanine, OFQ/N treatment failed to potentiate DAMGO-induced mu receptor phosphorylation and failed to desensitize the mu receptor. However, DAMGO-induced mu receptor phosphorylation and OFQ/N-induced mu receptor desensitization occurred in cells expressing mu receptors lacking non-GRK phosphorylation sites. These data suggest that OFQ/N binds to NOP receptors and activates protein kinase C, which then increases the ability of GRK2 to phosphorylate the agonist-occupied mu receptor, heterologously regulating homologous mu receptor desensitization.

BIBN4096BS and CGRP(8-37) antagonize the relaxant effects of alpha-CGRP more than those of beta-CGRP in human extracranial arteries

We hypothesize that dilatation of extracranial arteries during migraine could be caused by CGRP. We compared the relaxant effects of alpha-calcitonin gene-related peptide (alpha-CGRP) and beta-calcitonin gene-related peptide (beta-CGRP) and the antagonism by BIBN4096BS and CGRP(8-37) on rings of human temporal and occipital arteries precontracted with KCl. beta-CGRP relaxed temporal (-logEC50M = 8.1) and occipital arteries (-logEC50M = 7.6) with 19-fold and 29-fold lower potencies respectively than alpha-CGRP. Nearly maximal effective concentrations of alpha-CGRP (4 nM) and beta-CGRP (50 nM) caused stable relaxations of the temporal artery for 4 h without fading. BIBN4094BS antagonized the effects of alpha-CGRP (pK(B) = 10.1 and 9.9, respectively) more than beta-CGRP (pK(B) = 9.3 and 9.2 respectively) on both temporal and occipital arteries. CGRP(8-37) antagonized the effects of alpha-CGRP (pK(B) = 6.6 and 6.4 respectively) more than beta-CGRP (pK(B) = 5.7 and 5.5 respectively) on both temporal and occipital arteries. Antagonism of the relaxant effects of alpha-CGRP (4 nM) and beta-CGRP (50 nM) by BIBN4096BS (10 and 100 nM) was reversible for beta-CGRP, but irreversible for alpha-CGRP, 1 h after BIBN4096BS washout. We conclude that alpha-CGRP and beta-CGRP interact either at different binding sites of the same CGRP receptor system or all together with different receptor systems in human extracranial arteries. BIBN4096BS binds more firmly to the receptor activated by alpha-CGRP than to the receptor activated by beta-CGRP.

Pathophysiological roles of G-protein-coupled receptor kinases

G-protein-coupled receptor kinases (GRKs) interact with the agonist-activated form of G-protein-coupled receptors (GPCRs) to effect receptor phosphorylation and to initiate profound impairment of receptor signalling, or desensitization. GPCRs form the largest family of cell surface receptors known and defects in GRK function have the potential consequence to affect GPCR-stimulated biological responses in many pathological situations. This review focuses on the physiological role of GRKs revealed by genetically modified animals but also develops the involvement of GRKs in human diseases as, Oguchi disease, heart failure, hypertension or rhumatoid arthritis. Furthermore, the regulation of GRK levels in opiate addiction, cancers, psychiatric diseases, cystic fibrosis and cardiac diseases is discussed. Both transgenic mice and human pathologies have demonstrated the importance of GRKs in the signalling pathways of rhodopsin, beta-adrenergic and dopamine-1 receptors. The modulation of GRK activity in animal models of cardiac diseases can be effective to restore cardiac function in heart failure and opens a novel therapeutic strategy in diseases with GPCR dysregulation.

Different G protein-coupled receptor kinases govern G protein and beta-arrestin-mediated signaling of V2 vasopressin receptor

Signaling through beta-arrestins is a recently appreciated mechanism used by seven-transmembrane receptors. Because G protein-coupled receptor kinase (GRK) phosphorylation of such receptors is generally a prerequisite for beta-arrestin binding, we studied the roles of different GRKs in promoting beta-arrestin-mediated extracellular signal-regulated kinase (ERK) activation by a typical seven-transmembrane receptor, the Gs-coupled V2 vasopressin receptor. Gs- and beta-arrestin-mediated pathways to ERK activation could be distinguished with H89, an inhibitor of protein kinase A, and beta-arrestin 2 small interfering RNA, respectively. The roles of GRK2, -3, -5, and -6 were assessed by suppressing their expression with specific small interfering RNA sequences. By using this approach, we demonstrated that GRK2 and -3 are responsible for most of the agonist-dependent receptor phosphorylation, desensitization, and recruitment of beta-arrestins. In contrast, GRK5 and -6 mediated much less receptor phosphorylation and beta-arrestin recruitment, but yet appeared exclusively to support beta-arrestin 2-mediated ERK activation. GRK2 suppression actually increased beta-arrestin-stimulated ERK activation. These results suggest that beta-arrestin recruited in response to receptor phosphorylation by different GRKs has distinct functional potentials.

Regulation of corticotropin-releasing hormone receptor type 1alpha signaling: structural determinants for G protein-coupled receptor kinase-mediated phosphorylation and agonist-mediated desensitization

Attenuation of CRH receptor type 1 (CRH-R1) signaling activity might involve desensitization and uncoupling of CRH-R1 from intracellular effectors. We investigated the desensitization of native CRH-R in human myometrial cells from pregnant women and recombinant CRH-R1alpha stably overexpressed in human embryonic kidney (HEK) 293 cells. In both cell types, CRH-R1-mediated adenylyl cyclase activation was susceptible to homologous desensitization induced by pretreatment with high concentrations of CRH. Time course studies showed half-maximal desensitization occurring after approximately 40 min of pretreatment and full recovery of CRH-R1alpha functional response within 2 h of removal of CRH pretreatment. In HEK 293 cells, desensitization of CRH-R1alpha was associated with receptor phosphorylation and subsequent endocytosis. To analyze the mechanism leading to CRH-R1alpha desensitization, we overexpressed a truncated beta-arrestin (319-418) and performed coimmunoprecipitation and G protein-coupled receptor kinase (GRK) translocation studies. We found that GRK3 and GRK6 are the main isoforms that interact with CRH-R1alpha, and that recruitment of GRK3 requires Gbetagamma-subunits as well as beta-arrestin. Site-directed mutagenesis of Ser and Thr residues in the CRH-R1alpha C terminus, identified Thr399 as important for GRK-induced receptor phosphorylation and desensitization.We conclude that homologous desensitization of CRH-R1alpha involves the coordinated action of multiple GRK isoforms, Gbeta gamma dimers and beta-arrestin. Based on our identification of key amino acid(s) for GRK-dependent phosphorylation, we demonstrate the importance of the CRH-R1alpha carboxyl tail for regulation of receptor activity.

Homologous desensitization of calcitonin gene-related peptide-induced relaxation in rat intramural coronary arteries

We investigated the type of desensitization of calcitonin gene-related peptide (CGRP)-induced responses in rat isolated intramural coronary arteries using isometric myograph and FURA-2 technique. In coronary arteries precontracted with 9,11-dideoxy-11alpha,9alpha-epoxymethanoprostaglandin F2alpha (U46619), development of tachyphylaxis to CGRP is characterized by significant attenuation of CGRP-induced maximal reduction in the tension and [Ca2+](i) during the second CGRP concentration-response curve; however, there was no further reduction in the CGRP-induced maximum relaxation during the third CGRP concentration-response curve. There was no sign of tachyphylaxis to CGRP when CGRP concentration-response curves were recorded in 36 mM K+-depolarized coronary arteries contrary to the results obtained in 300 nM U46619-precontracted coronary arteries. Preincubation with colchicine did not prevent the development of tachyphylaxis to CGRP in U46619-precontracted coronary arteries, indicating no role for endocytosis. Development of tachyphylaxis to CGRP was completely abolished by preincubating the coronary arteries with 1 microM RO 31-8220, indicating a role for protein kinases. Pre-exposure of the coronary arteries to isoprenaline or forskolin did not attenuate the CGRP-induced relaxation in these vessels, indicating that the cAMP-protein kinase A (PKA) pathway is not involved. Like CGRP, the coronary arteries developed tachyphylaxis toward isoprenaline during the second exposure. However, there was no sign of tachyphylaxis to either forskolin or dibutyryl cAMP (dbcAMP) during the second exposure. In conclusion, these results suggest that development of tachyphylaxis to CGRP in U46619-precontracted coronary is related to CGRP receptor-mediated activation of protein kinase.

Serine 447 in the carboxyl tail of human VPAC1 receptor is crucial for agonist-induced desensitization but not internalization of the receptor

The VPAC1 receptor for vasoactive intestinal peptide (VIP) belongs to the class II family of G protein-coupled receptors and is coupled to Gs protein/adenylyl cyclase. We assessed whether 10 different Ser/Thr residues in human VPAC1 receptor intracellular domains play a role in the process of VIP-induced desensitization/internalization by performing a site-directed mutagenesis study. The Ser/Thr residues mutated to Ala include potential G protein-coupled receptor kinase, protein kinase A and protein kinase C targets that are of particular interest for VPAC1 receptor desensitization. The data show that when Chinese hamster ovary cells expressing wild-type receptors were pretreated for 5 min with VIP (50 nM), receptor desensitization occurred with a 10-fold right shift of the ED50 for adenylyl cyclase activation. When the construct with the widest span of mutations was studied, there was no longer any short-term desensitization. By using constructs with fewer and fewer mutations, we identified Ser447 in the C-terminal tail to be crucial for rapid desensitization. We also showed that Ser447 plays an essential role for VIP-induced VPAC1 phosphorylation in Chinese hamster ovary cells. Furthermore, we demonstrated that none of the mutated Ser/Thr residues was involved in down-regulation after a 12-h treatment of cells with 50 nM VIP. Neither were they involved in VIP and VIP-induced receptor internalization as shown using a novel fluorescein-tagged VIP and VPAC1 receptor bearing a Flag epitope in the N-terminal domain and a green fluorescent protein at the C terminus. We conclude that Ser447, a likely G protein-coupled receptor kinase target, is crucial for VIP-induced phosphorylation and rapid desensitization of VPAC1 receptor.

Non-visual GRKs: are we seeing the whole picture?

G-protein-coupled receptor kinases (GRKs) comprise a family of seven mammalian serine/threonine protein kinases that phosphorylate and regulate agonist-occupied or constitutively active G-protein-coupled receptors (GPCRs). Studies of the details and consequences of these mechanisms have focused heavily on the original beta-adrenoceptor kinase (beta-ARK) family (GRK2 and GRK3) and, in particular, on phosphorylation-dependent recruitment of adaptor proteins such as the beta-arrestins. However, recent work has indicated roles for the other, non-visual GRKs (GRK4, GRK5 and GRK6) and has revealed potential phosphorylation-independent regulation of GPCRs by GRK2 and GRK3. In this article, we review this newer information and attempt to put it into context with GRKs as physiological regulators that could be appropriate targets for future pharmacological intervention.

Specificity of g protein-coupled receptor kinase 6-mediated phosphorylation and regulation of single-cell m3 muscarinic acetylcholine receptor signaling

Previously we have shown that G protein-coupled receptor kinase (GRK) 6 plays a major role in the regulation of the human M3 muscarinic acetylcholine receptor (M3 mAChR) in the human neuroblastoma SH-SY5Y. However, 30-fold overexpression of the catalytically inactive, dominant-negative K215RGRK6 produced only a 50% suppression of M3 mAChR phosphorylation and desensitization. Here, we have attempted to determine whether other endogenous kinases play a role in the regulation of M3 mAChR signaling. In contrast to the clear attenuating effect of K215RGRK6 expression on M3 mAChR regulation, dominant-negative forms of GRKs (K220RGRK2, K220RGRK3, K215RGRK5) and casein kinase 1alpha (K46RCK1alpha) were without effect. In addition, inhibition of a variety of second-messenger-regulated kinases and the tyrosine kinase Src also had no effect upon agonist-stimulated M3 mAChR regulation. To investigate further the desensitization process we have followed changes in inositol 1,4,5-trisphosphate in single SHSY5Y cells using the pleckstrin homology domain of PLCdelta1 tagged with green fluorescent protein (eGFP-PHPLCdelta1). Stimulation of cells with approximate EC50 concentrations of agonist before and after a desensitizing period of agonist exposure resulted in a marked attenuation of the latter response. Altered GRK6 activity, through overexpression of wild-type GRK6 or K215RGRK6, enhanced or reduced the degree of M3 mAChR desensitization, respectively. Taken together, our data indicate that M3 mAChR desensitization is mediated by GRK6 in human SH-SY5Y cells, and we show that receptor desensitization of phospholipase C signaling can be monitored in 'real-time' in single, living cells.

Mu-opioid-induced desensitization of opioid receptor-like 1 and mu-opioid receptors: differential intracellular signaling determines receptor sensitivity

Mu-Opioid receptors have been shown to contribute to orphanin FQ/nociceptin (OFQ/N)-mediated analgesia and hyperalgesia, indicating that both pro- and antinociceptive actions of OFQ/N are influenced by mu-opioid receptors. A 60-min activation of mu-or opioid receptor-like 1 (ORL1) opioid receptors natively expressed in BE(2)-C human neuroblastoma cells desensitized both mu- and ORL1 receptor-mediated inhibition of cAMP accumulation. The mechanism(s) of OFQ/N-mediated mu and ORL1 cross talk involves the conventional protein kinase C isozyme, PKC-alpha, and G protein-coupled receptor kinases (GRKs) 2 and 3. Unlike OFQ/N-mediated desensitization of ORL1 and mu-opioid receptors, [d-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO)-mediated ORL1 desensitization in BE(2)-C cells is PKC-independent. However, DAMGO (1 microM) pretreatment increased membrane levels of GRK2 and GRK3, indicating their translocation to the membrane upon activation. This suggests that DAMGO activation of mu-opioid receptors results in GRK2 and GRK3 inactivation of ORL1 upon challenge with OFQ/N. Antisense, but not sense, DNA selectively targeting GRK2 or GRK3 blocks DAMGO-mediated mu- and ORL1 desensitization, respectively. However, in SH-SY5Y neuroblastoma cells, DAMGO failed to desensitize ORL1 or alter membrane PKC-alpha or GRK levels. Instead, DAMGO stimulated PKC-epsilon translocation to the cell membrane and produced micro-receptor desensitization. These results indicate that acute exposure to mu-receptor agonists can regulate ORL1 function, but the ability to do so varies from cell type to cell type. These results also confirm the existence of multiple signaling mechanisms for mu-opioid receptors and the importance of these mechanisms for mu-receptor-mediated-heterologous effects.

Endogenous G protein-coupled receptor kinase 6 triggers homologous beta-adrenergic receptor desensitization in primary uterine smooth muscle cells

We previously reported that G protein-coupled receptor kinase (GRK) may contribute to beta-adrenergic receptor (beta-AR) uncoupling occurring just before parturition in rat uterine muscle (myometrium). To identify the GRK involved, we set up in this study a primary cell culture retaining the morphological and functional characteristics of myometrial tissue as well as the in vivo pattern of GRK expression (GRK2, GRK5, and GRK6). In this model, homologous beta-AR desensitization was assessed by an approximately 60% decrease in cAMP production to a subsequent challenge with the beta-agonist, isoproterenol. Desensitization was reduced by 36% with a GRK inhibitor, heparin, and by 31% with a protein kinase A in-hibitor, H89. Using antibodies known to specifically inhibit either GRK2/3 or GRK4-6 families, we demonstrated that only the GRK4-6 family mediated beta-AR desensitization. To discriminate between endogenous GRK5 and GRK6, we attempted to inhibit their action by introducing, into myometrial cells, kinase-dead dominant-negative mutants ((K215R)GRK5 and (K215R)GRK6). Expression of (K215R)GRK6 increased by approximately 70% the cAMP response to isoproterenol without effect on forskolin stimulation. Conversely, expression of (K215R)GRK5 or (K220R)GRK2 had no effect on beta-adrenergic signaling. These results strongly suggest that endogenous GRK6 mediate homologous beta-AR desensitization in myometrial cells.

Desensitisation of adrenomedullin and CGRP receptors

Adrenomedullin (AM), a potent vasoactive peptide, is elevated in certain disease states such as sepsis. Its role as a physiologically relevant peptide has been confirmed with the advent of the homozygous lethal AM peptide knockout mouse. So far, there have been few and conflicting studies which examine the regulatory role of AM at the receptor level. In this article, we discuss the few studies that have been presented on the desensitisation of AM receptors and also present novel data on the desensitisation of endogenous AM receptors in Rat-2 fibroblasts.

Regulation of intracellular cyclic AMP in skeletal muscle cells involves the efflux of cyclic nucleotide to the extracellular compartment

(1) This report analyses the intracellular and extracellular accumulation of cyclic AMP in primary rat skeletal muscle cultures, after direct and receptor-dependent stimulation of adenylyl cyclase (AC). (2) Isoprenaline, calcitonin gene-related peptide (CGRP) and forskolin induced a transient increase in the intracellular cyclic AMP that peaked 5 min after onset stimulation. (3) Under stimulation with isoprenaline or CGRP, the intracellular cyclic AMP initial rise was followed by an exponential decline, reaching 46 and 52% of peak levels in 10 min, respectively. (4) Conversely, the forskolin-dependent accumulation of intracellular cyclic AMP decreased slowly and linearly, reaching 49% of the peak level in 30 min. (5) The loss of intracellular cyclic AMP from peak levels, induced by direct or receptor-induced activation of AC, was followed by an increase in the extracellular cyclic AMP. (6) This effect was independent on PDEs, since it was obtained in the presence of 3-isobutyl-1-methylxanthine (IBMX). (7) Besides, in isoprenaline treated cells, the beta-adrenoceptor antagonist propranolol reduced both intra- and extracellular accumulation of cyclic AMP, whereas the organic anion transporter inhibitor probenecid reduced exclusively the extracellular accumulation. (8) Together our data show that direct or receptor-dependent activation of skeletal muscle AC results in a transient increase in the intracellular cyclic AMP, despite the continuous presence of the stimulus. The temporal declining of intracellular cyclic AMP was not dependent on the cyclic AMP breakdown but associated to the efflux of cyclic nucleotide to the extracellular compartment, by an active transport since it was prevented by probenecid.

G protein-coupled receptor kinase 6 (GRK6) selectively regulates endogenous secretin receptor responsiveness in NG108-15 cells

1. To determine the role of G protein-coupled receptor kinases (GRKs) in the regulation of endogenous secretin receptor responsiveness, we have transiently overexpressed both wild-type (WT) and dominant negative mutant (DNM) GRKs in NG108-15 mouse neuroblastoma x rat glioma hybrid cells and investigated the effects of this on agonist-stimulated adenylyl cyclase activity. 2. Overexpression of WT GRK6 selectively inhibited secretin-stimulated cyclic AMP accumulation (fold stimulation of cyclic AMP above basal following 15 min incubation with 100 nM secretin was 12.1+/-2.0 and 6.2+/- 0.8 in control and WT GRK overexpressing cells, respectively) without affecting cyclic AMP responses mediated by the adenosine A(2) receptor agonist 5'-(N-ethylcarboxamido) adenosine (NECA) or the prostanoid-IP receptor agonist iloprost, or the direct activator of adenylyl cyclase, forskolin. On the other hand DNM GRK6 (Lys(215)Arg) overexpression produced the opposite effect--a selective increase in the secretin-stimulated cyclic AMP response was observed in cells overexpressing DNM GRK6 compared to plasmid-transfected cells (fold stimulation of cyclic AMP above basal following 15 min incubation with 100 nM secretin was 12.6+/-2.7 and 29.6+/-5.6 for control and DNM GRK6-overexpressing cells, respectively). 3. Overexpression of WT GRK5 likewise inhibited the secretin-stimulated cyclic AMP response, however, this effect was not as selective as with GRK6, since adenosine A(2) receptor responsiveness was also suppressed by GRK5 overexpression. Unlike DNM GRK6, overexpression of DNM GRK5 failed to modulate secretin or A(2) adenosine receptor signalling suggesting that endogenous GRK5 is unlikely to regulate desensitization of these receptors in NG108-15 cells. 4. Overexpression of WT GRK2 did not affect secretin-stimulated cyclic AMP accumulation. Instead, GRK2 overexpression selectively inhibited A(2) adenosine receptor responsiveness, confirming our previous findings. 5. Together these results suggest a selective role of endogenous GRK6 in regulating secretin receptor responsiveness in NG108-15 cells. In addition, these data indicate that GRKs exert a surprising degree of selectivity in the regulation of natively expressed GPCR responses.

Reduction of G protein-coupled receptor kinase 2 expression in U-937 cells attenuates H2 histamine receptor desensitization and induces cell maturation

Histamine and H2 agonists transiently induce an important cAMP response in promonocytic U-937 cells but fail to induce monocytic differentiation because of a rapid receptor desensitization mediated by G protein-coupled receptor kinases (GRKs). The aims of the present study were to investigate the participation of GRK2 in the desensitization mechanism of the H2 receptor in U-937 cells by reducing GRK2 levels through antisense technology and to evaluate the differentiating capacity of cells expressing lower GRK2 level, stimulated by H2 agonists. By stable U-937 cell transfection with a GRK2-antisense cDNA, we obtained D5 and A2 cell clones exhibiting a reduction in GRK2 expression and an H3 clone with no significant difference in GRK2 expression from control cells. The cAMP response induced by the H2 agonist in D5 and A2 but not in H3 cells was higher than in U-937 and persisted for a longer period of time, although the number of H2 receptors in D5 and A2 cells was lower than in U-937. Furthermore, D5 and A2 cells treated with H2 agonist showed patterns of c-Fos and CD88 expression consistent with monocytic differentiated cells. Overall, these results indicate a direct correlation between the expression of GRK2 and the desensitization of natively expressed H2 receptors in U-937 cells, suggesting that GRK2 plays a major role in the regulation of these receptors' response. In turn, desensitization process is a key component of H2 receptor signaling, determining the differentiation capability of promonocytic cells.

Threonine 308 within a putative casein kinase 2 site of the cytoplasmic tail of leukotriene B(4) receptor (BLT1) is crucial for ligand-induced, G-protein-coupled receptor-specific kinase 6-mediated desensitization

Desensitization of G-protein-coupled receptors may involve phosphorylation of serine and threonine residues. The leukotriene B(4) (LTB(4)) receptor (BLT1) contains 14 intracellular serines and threonines, 8 of which are part of consensus target sequences for protein kinase C (PKC) or casein kinase 2. In this study, we investigated the importance of PKC and GPCR-specific kinase (GRK) phosphorylation in BLT1 desensitization. Pretreatment of BLT1-transfected COS-7 cells with PKC activators caused a decrease of LTB(4)-induced inositol phosphate (IP) accumulation. This reduction was prevented with the PKC inhibitor, staurosporine, and not observed in cells expressing a BLT1 deletion mutant (G291stop) lacking the cytoplasmic tail. Moreover LTB(4)-induced IP accumulation was significantly inhibited by overexpression of GRK2, GRK5, and especially GRK6, in cells expressing wild type BLT1 but not in those expressing G291stop. GRK6-mediated desensitization correlated with increased phosphorylation of BLT1. The G319stop truncated BLT1 mutant displayed functional characteristics comparable with wild type BLT1 in terms of desensitization by GRK6, but not by PKC. Substitution of Thr(308) within a putative casein kinase 2 site to proline or alanine in the full-length BLT1 receptor prevented most of GRK6-mediated inhibition of LTB(4)-induced IP production but only partially affected LTB(4)-induced BLT1 phosphorylation. Our findings thus suggest that Thr(308) is a major residue involved in GRK6-mediated desensitization of BLT1 signaling.

Endogenous G protein-coupled receptor kinase 6 Regulates M3 muscarinic acetylcholine receptor phosphorylation and desensitization in human SH-SY5Y neuroblastoma cells

We have previously shown that overexpression of G protein-coupled receptor kinase 6 (GRK6) enhanced the phosphorylation and desensitization of the endogenously expressed M(3) muscarinic acetylcholine (mACh) receptor in human SH-SY5Y neuroblastoma cells. In this study we have examined the potential role of endogenous GRK6 in the regulation of M(3) mACh receptor by blocking its action through the introduction of a kinase-dead, dominant-negative GRK6 ((K215R)GRK6). (K215R)GRK6 expression inhibited methacholine-stimulated M(3) mACh receptor phosphorylation by 50% compared with plasmid transfected control cells. Guanosine-5'-O-(3-[(35)S]thio)triphosphate binding and immunoprecipitation studies, conducted after agonist pretreatment (3 min), indicated that M(3) mACh receptor-G alpha(q/11) uncoupling was attenuated by 50% in cells expressing (K215R)GRK6 when compared with control cells. In contrast, expression of the related dominant-negative kinase (K215R)GRK5 had no effect on M(3) mACh receptor phosphorylation or uncoupling. Time course studies also showed that agonist-stimulated [(3)H]inositol phosphate accumulations were more sustained in cells expressing (K215R)GRK6 compared with control and (K215R)GRK5-expressing cells, whereas (K215R)GRK6 expression had no effect on the phospholipase C response to direct stimulation of G proteins with AlF(4)(-). The ability of (K215R)GRK6 to inhibit agonist-mediated M(3) mACh receptor phosphorylation and G protein uncoupling suggests that endogenous GRK6 mediates, at least in part, M(3) mACh receptor desensitization in the SH-SY5Y cell line.

Desensitization of endogenously expressed delta-opioid receptors: no evidence for involvement of G protein-coupled receptor kinase 2

The involvement of G protein-coupled receptor kinase 2 (GRK2) in the agonist-induced desensitization of delta-opioid receptor-mediated inhibition of cAMP formation in NG108-15 mouse neuroblastomaxrat glioma hybrid cells was investigated. Pretreatment of wild-type cells with the delta-opioid receptor agonist [D-Pen(2,5)]-enkephalin (DPDPE; 100 nM) for as little as 5 min produced marked desensitization of subsequent DPDPE-mediated inhibition of iloprost (300 nM)-stimulated cAMP formation. In NG108-15 cells stably overexpressing wild-type GRK2 or dominant negative mutant GRK2 (DNM GRK2), the DPDPE-induced desensitization of cAMP inhibition was the same as in plasmid-transfected control cells. Pretreatment of wild-type cells with the inhibitors of receptor internalization, concanavalin A (0.25 mg ml(-1)) or hypertonic sucrose (0.4 M), also failed to inhibit DPDPE-mediated desensitization. Finally, in NG108-15 cells stably overexpressing G protein-coupled receptor kinase 6 (GRK6), DPDPE-induced desensitization was significantly increased as compared to plasmid-transfected control cells. These results indicate that GRK2 is unlikely to mediate the desensitization of endogenous delta-opioid receptors in NG108-15 cells, but that other GRKs, such as GRK6, may be more important.

Agonist-promoted internalization of a ternary complex between calcitonin receptor-like receptor, receptor activity-modifying protein 1 (RAMP1), and beta-arrestin

The calcitonin receptor-like receptor (CRLR) is a seven-transmembrane domain (7TM) protein that requires the receptor activity-modifying protein 1 (RAMP1) to be expressed at the cell surface as a functional calcitonin gene-related peptide (CGRP) receptor. Although dimerization between the two molecules is well established, very little is known concerning the trafficking of this heterodimer upon receptor activation. Also, the subcellular localization and biochemical state of this ubiquitously expressed protein, in the absence of CRLR, remains poorly characterized. Here we report that when expressed alone RAMP1 is retained inside the cells where it is found in the endoplasmic reticulum and the Golgi predominantly as a disulfide-linked homodimer. In contrast, when expressed with CRLR, it is targeted to the cell surface as a 1:1 heterodimer with the 7TM protein. Although heterodimer formation does not involve intermolecular disulfide bonds, RAMP-CRLR association promotes the formation of intramolecular disulfide bonds within RAMP1. CGRP binding and receptor activation lead to the phosphorylation of CRLR and the internalization of the receptor as a stable complex. The internalization was found to be both dynamin- and beta-arrestin-dependent, indicating that the formation of a ternary complex between CRLR, RAMP1, and beta-arrestin leads to clathrin-coated pit-mediated endocytosis. These results therefore indicate that although atypical by its heterodimeric composition and its targeting to the plasma membrane, the CGRP receptor shares endocytotic mechanisms that are common to most classical 7TM receptors.