A dominant negative mutant of the G protein-coupled receptor kinase 2 selectively attenuates adenosine A2 receptor desensitization

A2B adenosine receptor signaling and regulation

The A2B adenosine receptor (A2BR) is one of the four adenosine-activated G protein-coupled receptors. In addition to adenosine, protein kinase C (PKC) was recently found to activate the A2BR. The A2BR is coupled to both Gs and Gi, as well as Gq proteins in some cell types. Many primary cells and cell lines, such as bladder and breast cancer, bronchial smooth muscle, skeletal muscle, and fat cells, express the A2BR endogenously at high levels, suggesting its potentially important role in asthma, cancer, diabetes, and other conditions. The A2BR has been characterized as both pro- and anti-inflammatory, inducing cell type-dependent secretion of IL-6, IL-8, and IL-10. Theophylline and enprofylline have long been used for asthma treatment, although it is still not entirely clear if their A2BR antagonism contributes to their therapeutic effects or side effects. The A2BR is required in ischemic cardiac preconditioning by adenosine. Both A2BR and protein kinase C (PKC) contribute to cardioprotection, and both modes of A2BR signaling can be blocked by A2BR antagonists. Inhibitors of PKC and A2BR are in clinical cancer trials. Sulforaphane and other isothiocyanates from cruciferous vegetables such as broccoli and cauliflower have been reported to inhibit A2BR signaling via reaction with an intracellular A2BR cysteine residue (C210). A full, A2BR-selective agonist, critical to elucidate many controversial roles of the A2BR, is still not available, although agonist-bound A2BR structures have recently been reported.

Characterisation of endogenous A2A and A2B receptor-mediated cyclic AMP responses in HEK 293 cells using the GloSensor™ biosensor: Evidence for an allosteric mechanism of action for the A2B-selective antagonist PSB 603

Endogenous adenosine A_2B receptors (A_2BAR) mediate cAMP accumulation in HEK 293 cells. Here we have used a biosensor to investigate the mechanism of action of the A_2BAR antagonist PSB 603 in HEK 293 cells. The A_2A agonist CGS 21680 elicited a small response in these cells (circa 20% of that obtained with NECA), suggesting that they also contain a small population of A_2A receptors. The responses to NECA and adenosine were antagonised by PSB 603, but not by the selective A_2AAR antagonist SCH 58261. In contrast, CGS 21680 responses were not antagonised by high concentrations of PSB 603, but were sensitive to inhibition by SCH 58261. Analysis of the effect of increasing concentrations of PSB 603 on the response to NECA indicated a non-competitive mode of action yielding a marked reduction in the NECA E_MAX with no significant effect on EC₅₀ values. Kinetics analysis of the effect of PSB 603 on the A_2BAR-mediated NECA responses confirmed a saturable effect that was consistent with an allosteric mode of antagonism. The possibility that PSB 603 acts as a negative allosteric modulator of A_2BAR suggests new approaches to the development of therapeutic agents to treat conditions where adenosine levels are high.

Agonist-induced internalization and desensitization of the apelin receptor

Apelin acts via the G protein-coupled apelin receptor (APJ) to mediate effects on cardiovascular and fluid homeostasis. G protein-coupled receptor (GPCR) trafficking has an important role in the regulation of receptor signalling pathways and cellular functions, however in the case of APJ the mechanisms and proteins involved in apelin-induced trafficking are not well understood. We generated a stable HEK-293 cell line expressing N-terminus HA-tagged mouse (m) APJ, and used a semi-automated imaging protocol to quantitate APJ trafficking and ERK1/2 activation following stimulation with [Pyr¹]apelin-13. The mechanisms of [Pyr¹]apelin-13-induced internalization and desensitization were explored using dominant-negative mutant (DNM) cDNA constructs of G protein-coupled receptor kinase 2 (GRK2), β-arrestin1, EPS15 and dynamin. The di-phosphorylated ERK1/2 (ppERK1/2) response to [Pyr¹]apelin-13 desensitized during sustained stimulation, due to upstream APJ-specific adaptive changes. Furthermore, [Pyr¹]apelin-13 stimulation caused internalization of mAPJ via clathrin coated vesicles (CCVs) and also caused a rapid reduction in cell surface and whole cell HA-mAPJ. Our data suggest that upon continuous agonist exposure GRK2-mediated phosphorylation targets APJ to CCVs that are internalized from the cell surface in a β-arrestin1-independent, EPS15- and dynamin-dependent manner. Internalization does not appear to contribute to the desensitization of APJ-mediated ppERK1/2 activation in these cells.

How does adenosine control neuronal dysfunction and neurodegeneration?

The adenosine modulation system mostly operates through inhibitory A₁ (A₁ R) and facilitatory A_2A receptors (A_2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A₁ R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: high-frequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A_2A R; A_2A R switch off A₁ R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A₁ R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A₁ R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A_2A R, probably to bolster adaptive changes, but this heightens brain damage since A_2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A_2A R, whereas astrocytic and microglia A_2A R might control the spread of damage. The A_2A R signaling mechanisms are largely unknown since A_2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A₁ R preconditioning and preventing excessive A_2A R function might afford maximal neuroprotection. The main physiological role of the adenosine modulation system is to sharp the salience of information encoding through a combined action of adenosine A_2A receptors (A_2A R) in the synapse undergoing an alteration of synaptic efficiency with an increased inhibitory action of A₁ R in all surrounding synapses. Brain insults trigger an up-regulation of A_2A R in an attempt to bolster adaptive plasticity together with adenosine release and A₁ R desensitization; this favors synaptotocity (increased A_2A R) and decreases the hurdle to undergo degeneration (decreased A₁ R). Maximal neuroprotection is expected to result from a combined A_2A R blockade and increased A₁ R activation. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Adenosine receptors: expression, function and regulation

Adenosine receptors (ARs) comprise a group of G protein-coupled receptors (GPCR) which mediate the physiological actions of adenosine. To date, four AR subtypes have been cloned and identified in different tissues. These receptors have distinct localization, signal transduction pathways and different means of regulation upon exposure to agonists. This review will describe the biochemical characteristics and signaling cascade associated with each receptor and provide insight into how these receptors are regulated in response to agonists. A key property of some of these receptors is their ability to serve as sensors of cellular oxidative stress, which is transmitted by transcription factors, such as nuclear factor (NF)-κB, to regulate the expression of ARs. Recent observations of oligomerization of these receptors into homo- and heterodimers will be discussed. In addition, the importance of these receptors in the regulation of normal and pathological processes such as sleep, the development of cancers and in protection against hearing loss will be examined.

Adenosine receptor desensitization and trafficking

As with the majority of G-protein-coupled receptors, all four of the adenosine receptor subtypes are known to undergo agonist-induced regulation in the form of desensitization and trafficking. These processes can limit the ability of adenosine receptors to couple to intracellular signalling pathways and thus reduce the ability of adenosine receptor agonists as well as endogenous adenosine to produce cellular responses. In addition, since adenosine receptors couple to multiple signalling pathways, these pathways may desensitize differentially, while the desensitization of one pathway could even trigger signalling via another. Thus, the overall picture of adenosine receptor regulation can be complex. For all adenosine receptor subtypes, there is evidence to implicate arrestins in agonist-induced desensitization and trafficking, but there is also evidence for other possible forms of regulation, including second messenger-dependent kinase regulation, heterologous effects involving G proteins, and the involvement of non-clathrin trafficking pathways such as caveolae. In this review, the evidence implicating these mechanisms is summarized for each adenosine receptor subtype, and we also discuss those issues of adenosine receptor regulation that remain to be resolved as well as likely directions for future research in this field.

From cradle to twilight: the carboxyl terminus directs the fate of the A(2A)-adenosine receptor

The extended carboxyl terminus of the A(2A)-adenosine receptor is known to engage several proteins other than those canonically involved in signalling by GPCRs (i.e., G proteins, G protein-coupled receptor kinases/GRKs, arrestins). The list includes the deubiquinating enzyme USP4, α-actinin, the guanine nucleotide exchange factor for ARF6 ARNO, translin-X-associated protein, calmodulin, the neuronal calcium binding protein NECAB2 and the synapse associated protein SAP102. However, if the fate of the A(2A)-receptor is taken into account - from its birthplace in the endoplasmic reticulum to its presumed site of disposal in the lysosome, it is evident that many more proteins must interact with the A(2A)-adenosine receptor. There are several arguments that support the conjecture that these interactions will preferentially occur with the carboxyl terminus of the A(2A)-adeonsine receptor: (i) the extended carboxyl terminus (of 122 residues=) offers the required space to accommodate companions; (ii) analogies can be drawn with other receptors, which engage several of these binding partners with their C-termini. This approach allows for defining the nature of the unknown territory. As an example, we posit a chaperone/coat protein complex-II (COPII) exchange model that must occur on the carboxyl terminus of the receptor. This model accounts for the observation that a minimum size of the C-terminus is required for correct folding of the receptor. It also precludes premature recruitment of the COPII-coat to a partially folded receptor.

A novel thromboxane A2 receptor D304N variant that abrogates ligand binding in a patient with a bleeding diathesis

We investigated the cause of mild mucocutaneous bleeding in a 14-year-old male patient (P1). Platelet aggregation and ATP secretion induced by arachidonic acid and the thromboxane A(2) receptor (TxA(2)R) agonist U46619 were reduced in P1 compared with controls, whereas the responses to other platelet agonists were retained. P1 was heterozygous for a transversion within the TBXA2R gene predictive of a D304N substitution in the TxA(2)R. In Chinese hamster ovary-K1 cells expressing the variant D304N TxA(2)R, U46619 did not increase cytosolic free Ca(2+) concentration, indicating loss of receptor function. The TxA(2)R antagonist [(3)H]-SQ29548 showed an approximate 50% decrease in binding to platelets from P1 but absent binding to Chinese hamster ovary-K1 cells expressing variant D304N TxA(2)R. This is the second naturally occurring TxA(2)R variant to be associated with platelet dysfunction and the first in which loss of receptor function is associated with reduced ligand binding. D304 lies within a conserved NPXXY motif in transmembrane domain 7 of the TxA(2)R that is a key structural element in family A G protein-coupled receptors. Our demonstration that the D304N substitution causes clinically significant platelet dysfunction by reducing ligand binding establishes the importance of the NPXXY motif for TxA(2)R function in vivo.

Allosteric modulators of g protein-coupled receptors: future therapeutics for complex physiological disorders

G protein-coupled receptors (GPCRs) are one of the most important classes of proteins in the genome, not only because of their tremendous molecular diversity but because they are the targets of nearly 50% of current pharmacotherapeutics. The majority of these drugs affect GPCR activity by binding to a similar molecular site as the endogenous cognate ligand for the receptor. These "orthosterically" targeted drugs currently dominate the existing pharmacopeia. Over the past two decades, novel opportunities for drug discovery have risen from a greater understanding of the complexity of GPCR signaling. A striking example of this is the appreciation that many GPCRs possess functional allosteric binding sites. Allosteric modulator ligands bind receptor domains topographically distinct from the orthosteric site, altering the biological activity of the orthosteric ligand by changing its binding affinity, functional efficacy, or both. This additional receptor signaling complexity can be embraced and exploited for the next generation of GPCR-targeted therapies. Despite the challenges associated with detecting and quantifying the myriad of possible allosteric effects on GPCR activity, allosteric ligands offer the prospect of engendering a facile stimulus-bias in orthosteric ligand signaling, paving the way for not only receptor-selective but also signaling pathway-selective therapies. Allosteric modulators possess specific advantages when considering the treatment of multifactorial syndromes, such as metabolic diseases or age-related cognitive impairment, because they may not greatly affect neurotransmitter or hormone release patterns, thus maintaining the integrity of complex signaling networks that underlie perception, memory patterns, or neuroendocrinological axes while introducing therapeutically beneficial signal bias.

Involvement of PKC alpha and G-protein-coupled receptor kinase 2 in agonist-selective desensitization of mu-opioid receptors in mature brain neurons

BACKGROUND AND PURPOSE

The ability of an agonist to induce desensitization of the mu-opioid receptor (MOR) depends upon the agonist used. Furthermore, previous data suggest that the intracellular mechanisms underlying desensitization may be agonist-specific. We investigated the mechanisms underlying MOR desensitization, in adult mammalian neurons, caused by morphine (a partial agonist in this system) and DAMGO (a high-efficacy agonist).

EXPERIMENTAL APPROACH

MOR function was measured in locus coeruleus neurons, by using whole-cell patch-clamp electrophysiology, in rat and mouse brain slices (both wild-type and protein kinase C (PKC)alpha knockout mice). Specific isoforms of PKC were inhibited by using inhibitors of the receptors for activated C-kinase (RACK), and in vivo viral-mediated gene-transfer was used to transfect neurons with dominant negative mutants (DNMs) of specific G-protein-coupled receptor kinases (GRKs).

KEY RESULTS

Morphine-induced desensitization was attenuated by using RACK inhibitors that inhibit PKCalpha, but not by other isoform-specific inhibitors. Further, the PKC component of morphine-induced desensitization was absent in locus coeruleus neurons from PKCalpha knockout mice. The PKC-enhanced morphine-induced desensitization was not affected by over-expression of a GRK2 dominant negative mutant (GRK2 DNM). In contrast, DAMGO-induced MOR desensitization was independent of PKC activity but was reduced by over-expression of the GRK2 DNM but not by that of a GRK6 DNM.

CONCLUSIONS AND IMPLICATIONS

In mature mammalian neurons, different MOR agonists can induce MOR desensitization by different mechanisms, morphine by a PKCalpha-mediated, heterologous mechanism and DAMGO by a GRK-mediated, homologous mechanism. These data represent functional selectivity at the level of receptor desensitization.

Identification and characterization of a novel P2Y 12 variant in a patient diagnosed with type 1 von Willebrand disease in the European MCMDM-1VWD study

We investigated whether defects in the P2Y(12) ADP receptor gene (P2RY12) contribute to the bleeding tendency in 92 index cases enrolled in the European MCMDM-1VWD study. A heterozygous mutation, predicting a lysine to glutamate (K174E) substitution in P2Y(12), was identified in one case with mild type 1 von Willebrand disease (VWD) and a VWF defect. Platelets from the index case and relatives carrying the K174E defect changed shape in response to ADP, but showed reduced and reversible aggregation in response to 10 muM ADP, unlike the maximal, sustained aggregation observed in controls. The reduced response was associated with an approximate 50% reduction in binding of [(3)H]2MeS-ADP to P2Y(12), whereas binding to the P2Y(1) receptor was normal. A hemagglutinin-tagged K174E P2Y(12) variant showed surface expression in CHO cells, markedly reduced binding to [(3)H]2MeS-ADP, and minimal ADP-mediated inhibition of forskolin-induced adenylyl cyclase activity. Our results provide further evidence for locus heterogeneity in type 1 VWD.

Endogenous Gs-coupled receptors in smooth muscle exhibit differential susceptibility to GRK2/3-mediated desensitization

Although G protein-coupled receptor (GPCR) kinases (GRKs) have been shown to mediate desensitization of numerous GPCRs in studies using cellular expression systems, their function under physiological conditions is less well understood. In the current study, we employed various strategies to assess the effect of inhibiting endogenous GRK2/3 on signaling and function of endogenously expressed G s-coupled receptors in human airway smooth muscle (ASM) cells. GRK2/3 inhibition by expression of a Gbetagamma sequestrant, a GRK2/3 dominant-negative mutant, or siRNA-mediated knockdown increased intracellular cAMP accumulation mediated via beta-agonist stimulation of the beta-2-adrenergic receptor (beta 2AR). Conversely, neither 5'-( N-ethylcarboxamido)-adenosine (NECA; activating the A2b adenosine receptor) nor prostaglandin E2 (PGE 2; activating EP2 or EP4 receptors)-stimulated cAMP was significantly increased by GRK2/3 inhibition. Selective knockdown using siRNA suggested the majority of PGE 2-stimulated cAMP in ASM was mediated by the EP2 receptor. Although a minor role for EP3 receptors in influencing PGE 2-mediated cAMP was determined, the GRK2/3-resistant nature of EP2 receptor signaling in ASM was confirmed using the EP2-selective agonist butaprost. Somewhat surprisingly, GRK2/3 inhibition did not augment the inhibitory effect of the beta-agonist on mitogen-stimulated increases in ASM growth. These findings demonstrate that with respect to G s-coupled receptors in ASM, GRK2/3 selectively attenuates beta 2AR signaling, yet relief of GRK2/3-dependent beta 2AR desensitization does not influence at least one important physiological function of the receptor.

Rapid resensitization of purinergic receptor function in human platelets

BACKGROUND

Adenosine diphosphate (ADP) is a critical regulator of platelet activation, mediating its actions through two G protein-coupled receptors (GPCRs), the P2Y(1) and P2Y(12) purinergic receptors. Recently, we demonstrated that both receptors desensitize and internalize in human platelets by differential kinase-dependent mechanisms.

OBJECTIVES

To demonstrate whether responses to P2Y(1) and P2Y(12) purinergic receptors resensitize in human platelets and determine the role of receptor traffic in this process.

METHODS

These studies were undertaken either in human platelets or in cells stably expressing epitope-tagged P2Y(1) and P2Y(12) purinergic receptor constructs.

RESULTS

In this study we show for the first time that responses to both of these receptors can rapidly resensitize following agonist-dependent desensitization in human platelets. Further, we show that in human platelets or in 1321N1 cells stably expressing receptor constructs, the disruption of receptor internalization, dephosphorylation or subsequent receptor recycling is sufficient to block resensitization of purinergic receptor responses. We also show that, in platelets, internalization of both these receptors is dependent upon dynamin, and that this process is required for resensitization of responses.

CONCLUSIONS

This study is therefore the first to show that both P2Y(1) and P2Y(12) receptor activities are rapidly and reversibly modulated in human platelets, and it reveals that the underlying mechanism requires receptor trafficking as an essential part of this process.

Adenosine A(2A) receptor dynamics studied with the novel fluorescent agonist Alexa488-APEC

G protein-coupled receptors, such as the adenosine A(2A) receptor, are dynamic proteins, which undergo agonist-dependent redistribution from the cell surface to intracellular membranous compartments, such as endosomes. In order to study the kinetics of adenosine A(2A) receptor redistribution in living cells, we synthesized a novel fluorescent agonist, Alexa488-APEC. Alexa488-APEC binds to adenosine A(2A) (K(i)=149+/-27 nM) as well as A(3) receptors (K(i)=240+/-160 nM) but not to adenosine A(1) receptors. Further, we characterized the dose-dependent increase in Alexa488-APEC-induced cAMP production as well as cAMP response element binding (CREB) protein phosphorylation, verifying the ligand's functionality at adenosine A(2A) but not A(2B) receptors. In live-cell imaging studies, Alexa488-APEC-induced adenosine A(2A) receptor internalization, which was blocked by the competitive reversible antagonist ZM 241385 and hyperosmolaric sucrose. Further, internalized adenosine A(2A) receptors co-localized with clathrin and Rab5, indicating that agonist stimulation promotes adenosine A(2A) receptor uptake through a clathrin-dependent mechanism to Rab5-positive endosomes. The basic characterization of Alexa488-APEC described here showed that it provides a useful tool for tracing adenosine A(2A) receptors in vitro.

The A(2A)-adenosine receptor: a GPCR with unique features?

The A(2A)-adenosine receptor is a prototypical G(s)-coupled receptor. However, the A(2A)-receptor has several structural and functional characteristics that make it unique. In contrast to the classical model of collision coupling described for the beta-adrenergic receptors, the A(2A)-receptor couples to adenylyl cyclase by restricted collision coupling and forms a tight complex with G(s). The mechanistic basis for this is not clear; restricted collision coupling may arise from the interaction of the receptor with additional proteins or due to the fact that G protein-coupling is confined to specialized membrane microdomains. The A(2A)-receptor has a long C-terminus (of >120 residues), which is for the most part dispensable for coupling to G(s). It was originally viewed as the docking site for kinases and the beta-arrestin family to initiate receptor desensitization and endocytosis. The A(2A)-receptor is, however, fairly resistant to agonist-induced internalization. Recently, the C-terminus has also been appreciated as a binding site for several additional 'accessory' proteins. Established interaction partners include alpha-actinin, ARNO, USP4 and translin-associated protein-X. In addition, the A(2A)-receptor has also been reported to form a heteromeric complex with the D(2)-dopamine receptor and the metabotropic glutamate receptor-5. It is clear that (i) this list cannot be exhaustive and (ii) that all these proteins cannot bind simultaneously to the receptor. There must be rules of engagement, which allow the receptor to elicit different biological responses, which depend on the cellular context and the nature of the concomitant signal(s). Thus, the receptor may function as a coincidence detector and a signal integrator.

Internalization and desensitization of adenosine receptors

Until now, more than 800 distinct G protein-coupled receptors (GPCRs) have been identified in the human genome. The four subtypes of the adenosine receptor (A₁, A_2A, A_2B and A₃ receptor) belong to this large family of GPCRs that represent the most widely targeted pharmacological protein class. Since adenosine receptors are widespread throughout the body and involved in a variety of physiological processes and diseases, there is great interest in understanding how the different subtypes are regulated, as a basis for designing therapeutic drugs that either avoid or make use of this regulation. The major GPCR regulatory pathway involves phosphorylation of activated receptors by G protein-coupled receptor kinases (GRKs), a process that is followed by binding of arrestin proteins. This prevents receptors from activating downstream heterotrimeric G protein pathways, but at the same time allows activation of arrestin-dependent signalling pathways. Upon agonist treatment, adenosine receptor subtypes are differently regulated. For instance, the A₁Rs are not (readily) phosphorylated and internalize slowly, showing a typical half-life of several hours, whereas the A_2AR and A_2BR undergo much faster downregulation, usually shorter than 1 h. The A₃R is subject to even faster downregulation, often a matter of minutes. The fast desensitization of the A₃R after agonist exposure may be therapeutically equivalent to antagonist occupancy of the receptor. This review describes the process of desensitization and internalization of the different adenosine subtypes in cell systems, tissues and in vivo studies. In addition, molecular mechanisms involved in adenosine receptor desensitization are discussed.

Developing oligodendrocytes express functional GABA(B) receptors that stimulate cell proliferation and migration

GABA(B) receptors (GABA(B)Rs) are involved in early events during neuronal development. The presence of GABA(B)Rs in developing oligodendrocytes has not been established. Using immunofluorescent co-localization, we have identified GABA(B)R proteins in O4 marker-positive oligodendrocyte precursor cells (OPCs) in 4-day-old mouse brain periventricular white matter. In culture, OPCs, differentiated oligodendrocytes (DOs) and type 2 astrocytes (ASTs) express both the GABA(B1abcdf) and GABA(B2) subunits of the GABA(B)R. Using semiquantitative PCR analysis with GABA(B)R isoform-selective primers we found that the expression level of GABA(B1abd) was substantially higher in OPCs or ASTs than in DOs. In contrast, the GABA(B2) isoform showed a similar level of expression in OPCs and DOs, and a significantly higher level in ASTs. This indicates that the expression of GABA(B1) and GABA(B2) subunits are under independent control during oligodendroglial development. Activation of GABA(B)Rs using the selective agonist baclofen demonstrated that these receptors are functionally active and negatively coupled to adenylyl cyclase. Manipulation of GABA(B)R activity had no effect on OPC migration in a conventional agarose drop assay, whereas baclofen significantly increased OPC migration in a more sensitive transwell microchamber-based assay. Exposure of cultured OPCs to baclofen increased their proliferation, providing evidence for a functional role of GABA(B)Rs in oligodendrocyte development. The presence of GABA(B)Rs in developing oligodendrocytes provides a new mechanism for neuronal-glial interactions during development and may offer a novel target for promoting remyelination following white matter injury.

Distinct roles for protein kinase C isoforms in regulating platelet purinergic receptor function

ADP is a critical regulator of platelet activation, mediating its actions through two G protein-coupled receptors (GPCRs), P2Y1 and P2Y12. We have shown previously that the receptors are functionally desensitized, in a homologous manner, by distinct kinase-dependent mechanisms in which P2Y1 is regulated by protein kinase C (PKC) and P2Y12 by G protein-coupled receptor kinases. In this study, we addressed whether different PKC isoforms play different roles in regulating the trafficking and activity of these two GPCRs. Expression of PKCalpha and PKCdelta dominant-negative mutants in 1321N1 cells revealed that both isoforms regulated P2Y1 receptor signaling and trafficking, although only PKCdelta was capable of regulating P2Y12, in experiments in which PKC was directly activated by the phorbol ester phorbol 12-myristate 13-acetate (PMA). These results were paralleled in human platelets, in which PMA reduced subsequent ADP-induced P2Y1 and P2Y12 receptor signaling. PKC isoform-selective inhibitors revealed that novel, but not conventional, isoforms of PKC regulate P2Y12 function, whereas both novel and classic isoforms regulate P2Y1 activity. It is also noteworthy that we studied receptor internalization in platelets by a radioligand binding approach showing that both receptors internalize rapidly in these cells. ADP-induced P2Y1 receptor internalization is attenuated by PKC inhibitors, whereas that of the P2Y12 receptor is unaffected. Both P2Y1 and P2Y12 receptors can also undergo PMA-stimulated internalization, and here again, novel but not classic PKCs regulate P2Y12, whereas both novel and classic isoforms regulate P2Y1 internalization. This study therefore is the first to reveal distinct roles for PKC isoforms in the regulation of platelet P2Y receptor function and trafficking.

Tumor Necrosis Factor-α Prevents Desensitization of Gαs-Coupled Receptors by Regulating GRK2 Association with the Plasma Membrane

We have reported previously that interleukin-1 and tumor necrosis factor (TNF)-alpha increase expression and function of adenosine A2A receptors (A2ARs), although the increased function is disproportionate to the increment in expression. We therefore studied the effect of TNF-alpha on A2A R function and desensitization in human monocytoid THP-1 cells. We observed that TNF-alpha regulates activity of A2A Rs and other G protein-coupled receptors (GPCRs) by altering their ligand-mediated desensitization. Pretreatment of resting cells with the A2AR agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680) or the pan-adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine quickly desensitized cAMP responses to CGS 21680 restimulation, but TNF-alpha treatment prevented A2AR desensitization. As expected, A2A R occupancy induced translocation of GPCR kinase-2 (GRK2) to the plasma membrane (PM). We were surprised to find that after TNF-alpha treatment, A2AR occupancy not only failed to induce GRK2 translocation to PM but also decreased GRK2 association with PM. TNF-alpha altered GRK2 translocation in response to the beta-adrenergic receptor agonist isoproterenol in a similar manner. Similar to GRK2, beta-arrestin associated with PM after A2A R stimulation in control cells but not in TNF-alpha-treated cells. C2-ceramide, a downstream mediator in the sphingomyelinase (SMase)-dependent pathway, mimicked the effect of TNF-alpha on GRK2 translocation. Moreover, inhibitors of the SMases and an inhibitor of c-Jun NH2-terminal kinase, also a downstream effector in the SMase pathway, reversed TNF-alpha-mediated effects on GRK2 translocation and A2A R desensitization. These results suggest a novel form of cross-talk between TNF-alpha receptors and GPCRs; TNF-alpha enhances GPCR function by preventing agonist-induced desensitization of GPCRs by diminishing agonist-dependent recruitment of GRK2 and beta-arrestin to PM by a SMase pathway-mediated mechanism.

Distinctive and synergistic signaling of human adenosine A2a and dopamine D2L receptors in CHO cells

Adenosine A(2a) receptor (A(2a)R) colocalizes with dopamine D(2) receptor (D(2)R) in the basal ganglia and modulates D(2)R-mediated dopaminergic activities. A(2a)R and D(2)R couple to stimulatory and inhibitory G proteins, respectively. Their opposing roles in regulating neuronal activities, such as locomotion and alcohol consumption, are mediated by their opposite actions on adenylate cyclase, which often serves as "co-incidence detector" of various activators. On the other hand, the neural actions of A(2a)R and D(2)R are also, at least partially, independent of each other, as indicated by studies using D(2)R and A(2a)R knock-out mice. Here we co-expressed human A(2a)R and human D(2L)R in CHO cells and examined their signaling characteristics. Human A(2a)R desensitized rapidly upon agonist stimulation. A(2a)R activity (80%) was diminished after 2 hr of pretreatment with its agonist CGS21680. In contrast, human D(2L)R activity was sustained even after 2 hr and 18 hr pretreatment with its agonist quinpirole. Long-term (18 hr) stimulation of human D(2L)R also increased basal cAMP levels in CHO cells, whereas long-term (18 hr) activation of human A(2a)R did not affect basal cAMP levels. Furthermore, long-term (18 hr) activation of D(2L)R dramatically sensitized A(2a)R-induced stimulation of adenylate cyclase in a pertussis toxin-sensitive way. Forskolin-induced cAMP accumulation was significantly increased after short-term (2 hr) human D(2L)R stimulation and further elevated after long-term (18 hr) D(2L)R activation. However, neither short-term (2 hr) nor long-term (18 hr) stimulation of A(2a)R affected the inhibitory effects of D(2L)R on adenylate cyclase. Co-stimulation of A(2a)R and D(2L)R could not induce desensitization or sensitization of D(2L)R either. In summary, signaling through A(2a)R and D(2L)R is distinctive and synergistic, supporting their unique and yet integrative roles in regulating neuronal functions when both receptors are present.

Phosphorylation of GRK1 and GRK7 by cAMP-dependent protein kinase attenuates their enzymatic activities

Phosphorylation of G protein-coupled receptors is a critical step in the rapid termination of G protein signaling. In rod cells of the vertebrate retina, phosphorylation of rhodopsin is mediated by GRK1. In cone cells, either GRK1, GRK7, or both, depending on the species, are speculated to initiate signal termination by phosphorylating the cone opsins. To compare the biochemical properties of GRK1 and GRK7, we measured the K(m) and V(max) of these kinases for ATP and rhodopsin, a model substrate. The results demonstrated that these kinases share similar kinetic properties. We also determined that cAMP-dependent protein kinase (PKA) phosphorylates GRK1 at Ser(21) and GRK7 at Ser(23) and Ser(36) in vitro. These sites are also phosphorylated when FLAG-tagged GRK1 and GRK7 are expressed in HEK-293 cells treated with forskolin to stimulate the endogenous production of cAMP and activation of PKA. Rod outer segments isolated from bovine retina phosphorylated the FLAG-tagged GRKs in the presence of dibutyryl-cAMP, suggesting that GRK1 and GRK7 are physiologically relevant substrates. Although both GRKs also contain putative phosphorylation sites for PKC and Ca(2+)/calmodulin-dependent protein kinase II, neither kinase phosphorylated GRK1 or GRK7. Phosphorylation of GRK1 and GRK7 by PKA reduces the ability of GRK1 and GRK7 to phosphorylate rhodopsin in vitro. Since exposure to light causes a decrease in cAMP levels in rod cells, we propose that phosphorylation of GRK1 and GRK7 by PKA occurs in the dark, when cAMP levels in photoreceptor cells are elevated, and represents a novel mechanism for regulating the activities of these kinases.

P2Y1 and P2Y12 receptors for ADP desensitize by distinct kinase-dependent mechanisms

Adenosine 5'-diphosphate (ADP) plays a central role in regulating platelet function by the activation of the G protein-coupled receptors P2Y(1) and P2Y(12). Although it is well established that aggregation responses of platelets to ADP desensitize, the underlying mechanisms involved remain unclear. In this study we demonstrate that P2Y(1)- and P2Y(12)-mediated platelet responses desensitize rapidly. Furthermore, we have established that these receptors desensitize by different kinase-dependent mechanisms. G protein-coupled receptor kinase (GRK) 2 and GRK6 are both endogenously expressed in platelets. Transient overexpression of dominant-negative mutants of these kinases or reductions in endogenous GRK expression by the use of specific siRNAs in 1321N1 cells showed that P2Y(12), but not P2Y(1), desensitization is mediated by GRKs. In contrast, desensitization of P2Y(1), but not P2Y(12), is largely dependent on protein kinase C activity. This study is the first to show that both P2Y(1) and P2Y(12) desensitize in human platelets, and it reveals ways in which their sensitivity to ADP may be differentially and independently altered.

Reciprocal cross-talk between P2Y1 and P2Y12 receptors at the level of calcium signaling in human platelets

Adenosine diphosphate (ADP), an important platelet agonist, acts through 2 G-protein-coupled receptors (GPCRs), P2Y(1) and P2Y(12), which signal through Gq and Gi, respectively. There is increasing evidence for cross-talk between signaling pathways downstream of GPCRs and here we demonstrate cross-talk between these 2 ADP receptors in human platelets. We show that P2Y(12) contributes to platelet signaling by potentiating the P2Y(1)-induced calcium response. This potentiation is mediated by 2 mechanisms: inhibition of adenylate cyclase and activation of phosphatidylinositol 3 (PI 3)-kinase. Furthermore, the Src family kinase inhibitor PP1 selectively potentiates the contribution to the calcium response by P2Y(12), although inhibition of adenylate cyclase by P2Y(12) is unaffected. Using PP1 in combination with the inhibitor of PI 3-kinase LY294002, we show that Src negatively regulates the PI 3-kinase-mediated component of the P2Y(12) calcium response. Finally, we were able to show that Src kinase is activated through P2Y(1) but not P2Y(12). Taken together, we present evidence for a complex signaling interplay between P2Y(1) and P2Y(12), where P2Y(12) is able to positively regulate P2Y(1) action and P2Y(1) negatively regulates this action of P2Y(12). It is likely that this interplay between receptors plays an important role in maintaining the delicate balance between platelet activation and inhibition during normal hemostasis.

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.

Gene and protein domain-specific patterns of genetic variability within the G-protein coupled receptor superfamily

INTRODUCTION

Guanine nucleotide binding proteins (G-proteins) represent the targets for >50% of all therapeutics. There is substantial interindividual variation in response to agonists and antagonists directed to these receptors, which may, in part, be due to genetic polymorphisms. As a class, the sequence variability of G-protein-coupled receptor (GPCR) genes has not been characterized.

STUDY DESIGN

This variability was investigated by sequencing promoter, 5'- and 3'-UTR, coding blocks, and intron-exon boundaries, of 64 GPCR genes in an ethnically diverse group of 82 individuals.

RESULTS

Of the 675 single-nucleotide variations found, 61% occurred in > or =1% of the population sample and the nature of these 412 single nucleotide polymorphisms (SNPs) was assessed. 5'-UTR (p = 0.002) and coding (p = 0.006) SNPs were observed more often in GPCR genes, compared with 309 non-GPCR genes similarly interrogated. The prevalence of non-synonymous coding SNPs was unexpectedly high, with 65% of GPCR genes having at least one. Intron-containing genes had half as many non-synonymous coding SNPs compared with intronless genes (p = 0.0009), suggesting that when introns are not available coding regions provide sites for variation. A distinct relationship between the prevalence of non-synonymous SNPs and receptor structural domains was evident (p = 0.0006 by ANOVA), with variability being most prominent in the transmembrane spanning domains (38%) and the intracellular loops (24%). Phosphoregulatory domains, particularly the carboxy terminus, often the site for agonist-promoted phosphorylation by G-protein coupled receptor kinases, were the least polymorphic (8%).

CONCLUSIONS

There is substantial genetic variability in potentially pharmacologically relevant coding and noncoding regions of GPCRs. Such variability should be considered in the development of new agents, or optimization of existing agents, targeted to these receptors.

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.

Adenosine receptors: G protein-mediated signalling and the role of accessory proteins

Ever since the discovery of the effects of adenosine in the circulation, adenosine receptors continue to represent a promising drug target. Firstly, this is due to the fact that the receptors are expressed in a large variety of cells; in particular, the actions of adenosine (or, respectively, of the antagonistic methylxanthines) in the central nervous system, in the circulation, on immune cells and on other tissues can be beneficial in certain disorders. Secondly, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes have been identified by molecular cloning; they belong to the family of G protein-coupled receptors, which transfer signals by activating heterotrimeric G proteins. It has been appreciated recently that accessory proteins impinge on the receptor/G protein interaction and thus modulate the signalling reaction. These accessory components may be thought as adaptors that redirect the signalling pathway to elicit a cell-specific response. Here, we review the recent literature on adenosine receptors and place a focus on the role of accessory proteins in the organisation of adenosine receptor signalling. These components have been involved in receptor sorting, in the control of signal amplification and in the temporal regulation of receptor activity, while the existence of others is postulated on the basis of atypical cellular reactions elicited by receptor activation.

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.

Rapid agonist-induced desensitization and internalization of the A(2B) adenosine receptor is mediated by a serine residue close to the COOH terminus

The G(s)-coupled rat A(2B) adenosine receptor (A(2B)-AR) was epitope-tagged at the NH(2) terminus with hemagglutinin (HA) and subjected to progressive deletions or point mutations of the COOH terminus in order to determine regions of the receptor that contribute to agonist-induced desensitization and internalization. When expressed stably in Chinese hamster ovary cells, a mutant receptor in which the final 2 amino acids were deleted, the Leu(330)-stop mutant, underwent rapid agonist-induced desensitization and internalization as did the wild type (WT) receptor. However, the Phe(328) and the Gln(325)-stop mutants were resistant to rapid agonist-induced desensitization and internalization. Co-expression of arrestin-2-green fluorescent protein (arrestin-2-GFP) with WT receptor or Leu(330)-stop mutant resulted in rapid translocation of arrestin-2-GFP from cytosol to membrane upon agonist addition. On the other hand, agonist activation of the Phe(328)-stop or Gln(325)-stop mutant did not result in translocation of arrestin-2-GFP from cytosol. A COOH terminus point mutant, S329G, was also unable to undergo rapid agonist-induced desensitization and internalization, indicating that Ser(329) is a critical residue for these processes. A further deletion mutant (Ser(326)-stop) unexpectedly underwent rapid agonist-induced desensitization and internalization. However, activation of this mutant did not promote translocation of arrestin-2-GFP from cytosol to membrane. In addition, whereas WT receptor internalization was markedly inhibited by co-expression of dominant negative mutants of arrestin-2 (arrestin-2-(319-418)), dynamin (dynamin K44A), or Eps-15 (EDelta95-295), Ser(326)-stop receptor internalization was only inhibited by dominant negative mutant dynamin. Taken together these results indicate that Ser(329), close to the COOH terminus of the rat A(2B)-AR, is critical for the rapid agonist-induced desensitization and internalization of the receptor. However, deletion of the COOH terminus also uncovers a motif that is able to redirect internalization of the receptor to an arrestin- and clathrin-independent pathway.

The G protein beta subunit is a determinant in the coupling of Gs to the beta 1-adrenergic and A2a adenosine receptors

The signaling specificity of five purified G protein betagamma dimers, beta(1)gamma(2), beta(2)gamma(2), beta(3)gamma(2), beta(4)gamma(2), and beta(5)gamma(2), was explored by reconstituting them with G(s) alpha and receptors or effectors in the adenylyl cyclase cascade. The ability of the five betagamma dimers to support receptor-alpha-betagamma interactions was examined using membranes expressing the beta(1)-adrenergic or A2a adenosine receptors. These receptors discriminated among the defined heterotrimers based solely on the beta isoform. The beta(4)gamma(2) dimer demonstrated the highest coupling efficiency to either receptor. The beta(5)gamma(2) dimer coupled poorly to each receptor, with EC(50) values 40-200-fold higher than those observed with beta(4)gamma(2). Strikingly, whereas the EC(50) of the beta(1)gamma(2) dimer at the beta(1)-adrenergic receptor was similar to beta(4)gamma(2), its EC(50) was 20-fold higher at the A2a adenosine receptor. Inhibition of adenylyl cyclase type I (AC1) and stimulation of type II (AC2) by the betagamma dimers were measured. betagamma dimers containing Gbeta(1-4) were able to stimulate AC2 similarly, and beta(5)gamma(2) was much less potent. beta(1)gamma(2), beta(2)gamma(2), and beta(4)gamma(2) inhibited AC1 equally; beta(3)gamma(2) was 10-fold less effective, and beta(5)gamma(2) had no effect. These data argue that the beta isoform in the betagamma dimer can determine the specificity of signaling at both receptors and effectors.

GRK3 mediates desensitization of CRF1 receptors: a potential mechanism regulating stress adaptation

Potential G protein-coupled receptor kinase (GRK) and protein kinase A (PKA) mediation of homologous desensitization of corticotropin-releasing factor type 1 (CRF1) receptors was investigated in human retinoblastoma Y-79 cells. Inhibition of PKA activity by PKI(5-22) or H-89 failed to attenuate homologous desensitization of CRF1 receptors, and direct activation of PKA by forskolin or dibutyryl cAMP failed to desensitize CRF-induced cAMP accumulation. However, treatment of permeabilized Y-79 cells with heparin, a nonselective GRK inhibitor, reduced homologous desensitization of CRF1 receptors by approximately 35%. Furthermore, Y-79 cell uptake of a GRK3 antisense oligonucleotide (ODN), but not of a random or mismatched ODN, reduced GRK3 mRNA expression by approximately 50% without altering GRK2 mRNA expression and inhibited homologous desensitization of CRF1 receptors by approximately 55%. Finally, Y-79 cells transfected with a GRK3 antisense cDNA construct exhibited an approximately 50% reduction in GRK3 protein expression and an ~65% reduction in homologous desensitization of CRF1 receptors. We conclude that GRK3 contributes importantly to the homologous desensitization of CRF1 receptors in Y-79 cells, a brain-derived cell line.

Functional consequences of altering myocardial adrenergic receptor signaling

From the ability to successfully manipulate the mouse genome has come important transgenic and gene-targeted knockout models that impact many areas of biomedical research. Genetically engineered mouse models geared toward the study of cardiovascular regulation have recently been described and provide powerful tools to study normal and compromised cardiac physiology. The genetic manipulation of the adrenergic receptor (AR) signaling system in the heart, including its regulation by desensitizing kinases, has shed light on the role of this signaling pathway in the regulation of cardiac contractility. One major finding, supported by several mouse models, is that in vivo contractility can be enhanced via alteration of myocardial AR signaling. Thus genetic manipulation of this critical receptor system in the heart represents a novel therapeutic approach for improving function of the failing heart.

Retrovirally mediated transfer of a G protein-coupled receptor kinase (GRK) dominant-negative mutant enhances endogenous calcitonin receptor signaling in Chinese hamster ovary cells. GRK inhibition enhances expression of receptors and receptor mRNA

G protein-coupled receptor kinases (GRKs) initiate pathways leading to agonist-dependent phosphorylation and desensitization of G protein-coupled receptors. However, the role of GRKs in modulation of signaling properties of native receptors has not been clearly defined. Here we addressed this question by generating Chinese hamster ovary (CHO) cells stably expressing a dominant-negative mutant of GRK2 (DN-GRK2), K220R, using retrovirally mediated gene transfer, and we assessed function of the endogenously expressed calcitonin (CT) receptors. We found that CT-mediated responses were prominently enhanced in CHO cells expressing DN-GRK2 compared with mock-infected control CHO cells with approximately 3-fold increases in CT-promoted cAMP production in whole cells and adenylyl cyclase activity in membrane fractions. CT-promoted phosphoinositide hydrolysis was also enhanced in DN-GRK2 cells. The number of CT receptors was increased approximately 3-fold in DN-GRK2 cells, as assessed by (125)I-salmon CT-specific binding, and this was associated with increased CT receptor mRNA levels. These results indicate that DN-GRK2 has multiple consequences for CT receptor signaling, but a primary effect is an increase in CT receptor mRNA and receptor number and, in turn, enhanced CT receptor signaling. As such, our findings provide a mechanistic basis for previous observations regarding agonist-promoted down-regulation of CT receptors and for resistance and escape from response to CT in vitro and in vivo. Moreover, the data suggest that blunting of receptor desensitization by DN-GRK2 blocks a GRK-mediated tonic inhibition of CT receptor expression and response. We speculate that GRKs play a similar role for other G protein-coupled receptors as well.

Polarity of A2b adenosine receptor expression determines characteristics of receptor desensitization

It is not known if, in polarized cells, desensitization events can be influenced by the domain on which the receptor resides. Desensitization was induced by 5'-(N-ethylcarboxamido)adenosine (NECA) and was quantitated by measurement of short-circuit current (I(sc)) in response to adenosine. NECA added to either the apical or basolateral compartments rapidly desensitized receptors on these respective domains. Although apical NECA had no effect on the basolateral receptor stimulation, basolateral NECA induced a complete desensitization of the apical receptor. We hypothesized that desensitization of apical receptor by basolateral desensitization could relate to a trafficking step in which A2b receptor is first targeted basolaterally upon synthesis and transported to the apical surface via vesicular transport/microtubules. Because desensitization is associated with downregulation of receptors, apical adenosine receptor can thus be affected by basolateral desensitization. Both low temperature and nocodazole inhibited I(sc) induced by apical and not basolateral adenosine.

IN CONCLUSION

1) a single receptor subtype, here modeled by the A2b receptor, differentially desensitizes based on the membrane domain on which it is expressed, 2) agonist exposure on one domain can result in desensitization of receptors on the opposite domain, 3) cross-domain desensitization can display strict polarity, and 4) receptor trafficking may play a role in the cross-desensitization process.

Selective regulation of endogenous G protein-coupled receptors by arrestins in HEK293 cells

Arrestins play an important role in regulating desensitization and trafficking of G protein-coupled receptors (GPCRs). However, limited insight into the specificity of arrestin-mediated regulation of GPCRs is currently available. Recently, we used an antisense strategy to reduce arrestin levels in HEK293 cells and characterize the role of arrestins on endogenous G(s)-coupled receptors (Mundell, S. J., Loudon, R. B., and Benovic, J. L. (1999) Biochemistry 38, 8723-8732). Here, we characterized GPCRs coupled to either G(q) (M(1) muscarinic acetylcholine receptor (M(1)AchR) and P2y(1) and P2y(2) purinergic receptors) or G(i) (somatostatin and AT1 angiotensin receptors) in wild type and arrestin antisense HEK293 cells. The agonist-specific desensitization of the M(1)Ach and somatostatin receptors was significantly attenuated in antisense-expressing cells, whereas desensitization of P2y(1) and P2y(2) purinergic and AT1 angiotensin receptors was unaffected by reduced arrestin levels. To further examine arrestin/GPCR specificity, we studied the effects of endogenous GPCR activation on the redistribution of arrestin-2 epitope tagged with the green fluorescent protein (arrestin-2-GFP). These studies revealed a receptor-specific movement of arrestin-2-GFP that mirrored the arrestin-receptor specificity observed in the antisense cells. Thus, agonist-induced activation of endogenous beta(2)-adrenergic, prostaglandin E(2), M(1)Ach, and somatostatin receptors induced arrestin-2-GFP redistribution to early endosomes, whereas P2y(1) and P2y(2) purinergic and AT1 angiotensin receptor activation did not. Thus, endogenous arrestins mediate the regulation of selective G(q)- and G(i)-coupled receptors in HEK293 cells.

The role of receptor structure in determining adenosine receptor activity

Adenosine produces a wide variety of physiological effects through the activation of cell surface adenosine receptors (ARs). ARs are members of the G-protein-coupled receptor family, and currently, four subtypes, the A1AR, A2AAR, A2BAR, and A3AR, are recognized. This review focuses on the role of receptor structure in governing various facets of AR activity. Ligand-binding properties of ARs are primarily dictated by amino acids in the transmembrane domains of the receptors, although a role for extracellular domains of certain ARs has been suggested. Studies have identified certain amino acids conserved amongst AR subtypes that are critical for ligand recognition, as well as additional residues that may differentiate between agonist and antagonist ligands. Receptor regions responsible for activation of Gs have been identified for the A2AAR. The location of these intracellular sites is consistent with findings described for other G-protein-coupled receptors. Site-directed mutagenesis has been employed to analyze the structural basis for the differences in the kinetics of the desensitization response displayed by various AR subtypes. For the A2AAR and A3AR, agonist-stimulated phosphorylation of the AR, presumably via a G-protein receptor kinase, has been shown to occur. For these AR subtypes, intracellular regions or individual amino acids that may be targets for this phosphorylation have been identified. Finally, the role of A1AR gene structure in regulating the expression of this AR subtype is reviewed.

A cluster of Ser/Thr residues at the C-terminus of mu-opioid receptor is required for G protein-coupled receptor kinase 2-mediated desensitization

To investigate the functional role of G protein-coupled receptor kinases (GRK) in homologous desensitization of the mu-opioid receptor, human embryonic kidney (HEK) 293 cells, which express a significant level of GRK2, were stably transfected with the cDNA encoding the rat mu-opioid receptor. Wild-type mu-opioid receptors developed homologous desensitization after 30 min pretreatment with DAMGO ([D-Ala2,N-methyl-Phe4,Gly-ol5]enkephalin), a specific mu-opioid receptor agonist. The ability of mu-opioid receptors to develop homologous desensitization was greatly impaired following the transfection of a cDNA fragment encoding the GRK2(495-689) polypeptide, which is believed to block Gbetagamma-mediated transduction events including the membrane translocation and activation of GRK2. The mu(Cdelta45) receptor, a deletion mutant that lacks 45 C-terminal amino acids, failed to exhibit homologous desensitization after 30 min pretreatment of DAMGO. The mu(Cdelta41) receptor, which differs from the mu(Cdelta45) receptor by having four more Ser/Thr residues (Thr354Ser355Ser356Thr357), developed GRK2-mediated desensitization. These results suggest that homologous desensitization of rat mu-opioid receptors results from the activation of GRK2 and that a cluster of Ser/Thr residues (Thr354Ser355Ser356Thr357) at the intracellular carboxyl tail plays an important role in GRK2-mediated mu-opioid receptor desensitization.

Alternative splicing produces transcripts encoding four variants of mouse G-protein-coupled receptor kinase 6

A family of protein kinases, termed G-protein-coupled receptor kinases (GRK1-6), is known to phosphorylate agonist-occupied G-protein-coupled receptors. We have identified mRNAs encoding four distinct mouse GRK6 isoforms (mGRK6), designated mGRK6-A through mGRK6-D. Mouse GRK6-B and mGRK6-C diverge from the known human GRK6 (577 residues) at residue 560 and are 13 residues longer and 16 residues shorter, respectively, than human GRK6, while mGRK6-A very likely represents the mouse equivalent of human GRK6. Mouse GRK6-D is identical to the other mGRK6 variants in the amino-terminal region, but comprises only 59 of the 263 amino acids of the putative catalytical domain. As mGRK6-D retains the region involved in interacting with activated receptors, but most likely lacks catalytic activity, this variant might represent a naturally occurring inhibitor of other GRKs. Analysis of the genomic organization of mGRK6 gene revealed that the four mRNAs are generated by alternative RNA splicing from a single approximately 14. 5-kb gene, made up of at least 17 exons and located on mouse chromosome 13. Similar to human GRK6, mGRK6-A contains three cysteine residues within its carboxyl-terminal region known to serve as substrates for palmitoylation. Mouse GRK6-B lacks these palmitoylation sites, but carries a basic carboxyl-terminus containing consensus sequences for phosphorylation by protein kinases C and cAMP/cGMP-dependent protein kinases. Mouse GRK6-C displays none of these motifs. Thus, mGRK6-A, mGRK6-B, and mGRK6-C are predicted to differ in terms of their regulation by carboxyl-terminal posttranslational modification. Analysis of mRNA expression revealed that the four mGRK6 mRNAs are differentially expressed in mouse tissues, suggesting that the four mGRK6 isoforms are involved in regulating tissue- or cell type-specific functions in vivo.

Effect of G protein-coupled receptor kinase 2 on the sensitivity of M4 muscarinic acetylcholine receptors to agonist-induced internalization and desensitization in NG108-15 cells

NG108-15 cells express predominantly the M4 subtype of the muscarinic receptor for acetylcholine. Stimulation of these receptors by the agonist carbachol causes an inhibition of cellular adenylyl cyclase and a consequent fall in the intracellular cyclic AMP concentration. Pretreatment of the cells with carbachol caused both internalization and desensitization of the M4 receptor. Overexpression of G protein-coupled receptor kinase (GRK) 2 caused an increase in the rate constant for receptor endocytosis (from 0.06 to 0.18 min(-1)) and a decrease in the EC50 for carbachol stimulation of internalization (from 15 to 3 microM). Overexpression of a dominant negative form of GRK2 had more modest effects, reducing the rate constant for endocytosis (from 0.11 to 0.07 min(-1)) and increasing the EC50 for carbachol stimulation of internalization (from 8 to 17 microM). Neither GRK2 nor dominant negative GRK2 overexpression had any effect on the rate constant for receptor recycling following agonist removal. The time course and extent of receptor desensitization in control cells were identical to the corresponding values for receptor internalization, and the rate and extent of desensitization were again increased by GRK2 overexpression. Exposure of the cells to hyperosmolar sucrose (0.6 M) almost completely blocked agonist-induced receptor internalization in both control and GRK2-overexpressing cells. Sucrose treatment also blocked agonist-induced desensitization. We conclude that the internalization and desensitization of the M4 muscarinic receptor in NG108-15 cells can be modulated in response to changes in GRK2 activity and also that internalization plays a key role in desensitization.

Analysis of domain responsible for desensitization of beta1-adrenergic receptor

When the wild type beta1-adrenergic receptor (WT-beta1AR) was expressed in Sf9 cells, the beta1AR-stimulated adenylyl cyclase activities were desensitized by prior treatment with isoproterenol. The extent of beta1AR desensitization was not modified, and the onset was not promoted by the overexpression of G protein-coupled receptor kinase 2 (GRK2), GRK5 or GRK6. However, overexpression of the dominant negative mutant of GRK2 appeared to inhibit desensitization of the beta1AR. The change of the potential protein kinase A phosphorylation site located at the intracellular third loop did not affect beta1AR desensitization. Desensitization of the truncated mutant, in which nearly all of the serine and threonine residues from the carboxyl terminus were eliminated, was the same as that of the WT-beta1AR. A deletion mutant that lacked serine and threonine residues of the intracellular third loop was also desensitized by isoproterenol stimulation. Furthermore, the deletion of serine and threonine residues from both the intracellular third loop and carboxyl terminus did not affect desensitization of the beta1AR. These results suggested that phosphorylation by endogenous GRKs in Sf9 cells contributed to desensitization of the beta1AR and that the regions other than third intracellular loop and carboxyl terminus may be responsible for beta1AR desensitization.

G-protein coupled receptor kinases as modulators of G-protein signalling

G-protein coupled receptors (GPCRs) comprise one of the largest classes of signalling molecules. A wide diversity of activating ligands induce the active conformation of GPCRs and lead to signalling via heterotrimeric G-proteins and downstream effectors. In addition, a complex series of reactions participate in the 'turn-off' of GPCRs in both physiological and pharmacological settings. Some key players in the inactivation or 'desensitization' of GPCRs have been identified, whereas others remain the target of ongoing studies. G-protein coupled receptor kinases (GRKs) specifically phosphorylate activated GPCRs and initiate homologous desensitization. Uncoupling proteins, such as members of the arrestin family, bind to the phosphorylated and activated GPCRs and cause desensitization by precluding further interactions of the GPCRs and G-proteins. Adaptor proteins, including arrestins, and endocytic machinery participate in the internalization of GPCRs away from their normal signalling milieu. In this review we discuss the roles of these regulatory molecules as modulators of GPCR signalling.

Desensitization of G-protein-coupled receptors in the cardiovascular system

Multiple mechanisms exist to control the signaling and density of G-protein-coupled receptors (GPRs). Upon agonist binding and receptor activation, a series of reactions participate in the turn off or desensitization of GPRs. Many GPRs are phosphorylated by protein kinases and consequently uncoupled from G proteins. In addition, many GPRs are sequestered from the cell surface and become inaccessible to their activating ligands. Both receptor:G protein uncoupling and receptor sequestration may involve the participation of arrestins or other proteins. A model for receptor regulation has been developed from studies of the beta-adrenergic receptor. However, recent studies suggest that other GPRs important in the cardiovascular system, such as the muscarinic cholinergic receptors that regulate heart rate, might be regulated by mechanisms other than those that regulate the beta-adrenergic receptors. This review summarizes our current understanding of the processes involved in the desensitization of GPRs.

Somatostatin receptor desensitization in NG108-15 cells. A consequence of receptor sequestration

In NG108-15 cells inhibition of both N-type calcium channel current and adenylyl cyclase by somatostatin (SRIF) was not sustained but rapidly desensitized in the continued presence of the drug. The degree and rate of desensitization were concentration-dependent, and the desensitization was homologous with respect to the delta-opioid receptor. We have been unable to obtain evidence for the involvement of G protein-coupled receptor kinases (GRKs) in this desensitization. SRIF-induced desensitization of N-type calcium channel currents was not reduced in cells stably overexpressing a dominant negative mutant of GRK2 or following intracellular dialysis with GRK2- and GRK3-blocking peptides or with heparin. Inhibitors of protein kinase A, protein kinase C, and protein kinase G were also without effect. In contrast, both the rate and degree of SRIF-induced desensitization were reduced by pretreatment with phenylarsine oxide or concanavalin A, both inhibitors of receptor endocytosis. Furthermore, SRIF-induced desensitization was enhanced by monensin, which prevents receptor recycling back to the plasma membrane. Similarly, SRIF-induced desensitization of adenylyl cyclase inhibition was not reduced in cells stably overexpressing dominant negative mutant GRK2 but was reduced in cells pretreated with the receptor endocytosis inhibitor hyperosmotic sucrose or concanavalin A. These data are consistent with the view that SRIF-induced desensitization in NG108-15 cells results from receptor internalization.

The effect of inhibitors of receptor internalization on the desensitization and resensitization of three Gs-coupled receptor responses

Many G protein-coupled receptors (GPCRs) are known to internalize following agonist exposure, however the relative importance of this mechanism for the desensitization and resensitization of different GPCRs is unclear. In the present study, we have pretreated NG108-15 cells with hypertonic sucrose or concanavalin A (con A), to investigate the effects of these inhibitors of internalization on the agonist-induced desensitization and subsequent resensitization of three Gs-coupled receptor responses. Incubation of cells with sucrose or con A did not affect subsequent acute agonist stimulation of the A2A adenosine receptor or the agonist-induced desensitization of this receptor response. However, the resensitization of the A2A adenosine receptor response following agonist removal was abolished in the presence of sucrose or con A. Sucrose or con A treatment affected neither the desensitization nor resensitization of IP-prostanoid receptor responsiveness. On the other hand con A but not sucrose reduced the agonist-induced desensitization of secretin receptor responsiveness. However, secretin receptor responsiveness did not resensitize within the time period studied whether or not inhibitors of internalization were present. These results indicate that receptor internalization appears to subserve different functions for different GPCRs.

G protein-coupled receptor kinases

G protein-coupled receptor kinases (GRKs) constitute a family of six mammalian serine/threonine protein kinases that phosphorylate agonist-bound, or activated, G protein-coupled receptors (GPCRs) as their primary substrates. GRK-mediated receptor phosphorylation rapidly initiates profound impairment of receptor signaling, or desensitization. This review focuses on the regulation of GRK activity by a variety of allosteric and other factors: agonist-stimulated GPCRs, beta gamma subunits of heterotrimeric GTP-binding proteins, phospholipid cofactors, the calcium-binding proteins calmodulin and recoverin, posttranslational isoprenylation and palmitoylation, autophosphorylation, and protein kinase C-mediated GRK phosphorylation. Studies employing recombinant, purified proteins, cell culture, and transgenic animal models attest to the general importance of GRKs in regulating a vast array of GPCRs both in vitro and in vivo.

Enhanced expression of G protein-coupled receptor kinase 2 selectively increases the sensitivity of A2A adenosine receptors to agonist-induced desensitization

1. G protein-coupled receptor kinases (GRKs) are thought to be important in mediating the agonist-induced phosphorylation and consequent desensitization of G protein-coupled receptor (GPCR) responses. We have previously shown that stable expression of a dominant negative mutant G protein-coupled receptor kinase 2 (GRK2) construct in NG108-15 mouse neuroblastoma x rat glioma cells suppresses the agonist-induced desensitization of A2A and A2B adenosine receptor-stimulated adenylyl cyclase activity (Mundell et al., 1997). To further determine the role of GRK2 in agonist-induced desensitization of these adenosine receptors, we stably overexpressed wild type GRK2 in NG108-15 cells. 2. In homogenates prepared from cells overexpressing GRK2, the acute stimulation of adenylyl cyclase by activation of A2A and A2B adenosine receptors was markedly reduced, but could be reversed by pretreating the cells with AD (adenosine deaminase), to remove extracellular adenosine from the medium. On the other hand, acute stimulation of adenylyl cyclase by secretin, iloprost, NaF and forskolin was the same in GRK2 overexpressing cells and plasmid-transfected control cells. 3. Cells overexpressing GRK2 were more sensitive to adenosine receptor agonist-induced desensitization than plasmid-transfected control cells. This effect was selective since the agonist sensitivity of desensitization for secretin and IP-prostanoid receptor-stimulated adenylyl cyclase activity was not affected by GRK2 overexpression. 4. These results further implicate GRK2 as the likely mechanism by which A2 adenosine receptors undergo short-term desensitization in NG108-15 cells, and indicate that even when overexpressed, GRK2 retains its substrate specificity for native receptors in intact cells. Furthermore, the susceptibility of GPCRs to desensitization appears to depend on the level of GRK expression, such that in cells that express high levels of GRK2, low agonist concentrations may be sufficient to trigger GRK-mediated desensitization.

G-protein-coupled receptors: turn-ons and turn-offs

Advances in the study of G-protein-coupled receptor regulation have provided novel insights into the role of G-protein-coupled receptor kinases and arrestins in this process. Of particular interest are recent studies that have dramatically expanded the known cellular functions of these molecules to include roles in receptor endocytosis and activation of MAP kinase signalling pathways.