Alveolar Epithelial Type I Cells Express β2-Adrenergic Receptors and G-protein Receptor Kinase 2

β2-Adrenergic agonists stimulate alveolar epithelial sodium (Na+) transport and lung fluid clearance. Alveolar type II (AT2) cells have been reported to express β2-adrenergic receptors (β2AR). Given the large surface area covered by alveolar type I (AT1) cells and their potential role in alveolar fluid removal, we were interested in learning if AT1 cells express β2AR as well. Because β2AR is potentially susceptible to desensitization by G-protein-coupled receptor kinase 2 (GRK2), we also undertook localization of GRK2. β2AR and GRK2 expression was evaluated in whole lung, isolated alveolar epithelial cells (AECs), and AECs in primary culture, and was localized to specific AEC phenotypes by immunofluorescence techniques. β2AR is highly expressed in AT1 cells. β2AR mRNA increases with time in culture as AT2 cells transdifferentiate towards the AT1 cell phenotype. Immunoreactive GRK2 is seen in both AT1 and AT2 cells in similar amounts. These data suggest that both AT1 and AT2 cells may contribute to the increased alveolar Na+ and water clearance observed after exposure to β2 adrenergic agents. Both cell types also express GRK2, suggesting that both may undergo desensitization of β2AR with subsequent decline in the stimulatory effects of β2-adrenergic agonists over time.

Acute ischemic cardiac dysfunction is attenuated via gene transfer of a peptide inhibitor of the beta-adrenergic receptor kinase (betaARK1)

Acute myocardial ischemia is a critical adverse effect potentially occurring during cardiac procedures. A peptide inhibitor of the beta-adrenergic receptor kinase (betaARK1), betaARKct, has been successful in rescuing chronic myocardial ischemia. The present study focused on the effects of adenoviral-mediated betaARKct (Adv-betaARKct) delivery on left ventricle (LV) dysfunction induced by acute coronary occlusion. Rabbits received intracoronary delivery of phosphate-buffered saline (PBS) (n=9) or 5x10(11) viral particles of betaARKct (n=8). A loose prolene 5-0 Potz-loop suture was placed around the circumflex coronary artery (LCx) with both ends buried under the skin. Four days later, the suture was retrieved and pulled to occlude the LCx. Ischemia was confirmed by immediate ECG changes. LV function was continuously recorded for 45 min. Contractility (LVdP/dtmax), relaxation (LVdP/dtmin) and end diastolic pressure (EDP) were less impaired in the betaARKct group as compared to PBS (P<0.05, two-way ANOVA). betaAR density was higher in the ischemic area of the LV in the betaARKct group (betaARKct: 71.9+/-4.6 fmol/mg protein, PBS: 54.5+/-4.0 fmol/mg protein, P<0.05). Adenylyl cyclase activity was also improved basally and in response to betaAR stimulation. betaARK1 activation was less in the betaARKct group (P<0.05). Therefore, inhibition of myocardial betaARK1 may represent a new strategy to prevent LV dysfunction induced by acute coronary ischemia.

Chronic beta-adrenergic receptor stimulation enhances the expression of G-Protein coupled receptor kinases, GRK2 and GRK5, in both the heart and peripheral lymphocytes

BACKGROUND

Enhanced expression of G protein-coupled receptor kinase (GRK) has been reported in failing hearts and in the present study the stability of enhanced GRK mRNA expression, and the correlation between the expression level of GRK mRNA in peripheral lymphocytes and in the heart were both evaluated.

METHODS AND RESULTS

Isoproterenol was injected into rats for 2 weeks, and then GRK5 mRNA was assessed by quantitative reverse transcriptase-palymerase chain reaction. An enhanced expression of cardiac GRK5 mRNA was observed even after 4 weeks of recovery. The isoproterenol-induced increased expression of GRK2 and GRK5 mRNA was equally observed in the heart and lymphocytes, and there was a close correlation between the heart and lymphocytes in the level of expression of each GRK mRNA.

CONCLUSIONS

The GRK mRNA level is maintained at a high level for a long period without continuous beta-adrenergic receptor stimulation. The level in circulating lymphocytes could be used as a surrogate marker to estimate the level of cardiac GRK expression and, presumably, the beta-adrenergic receptor function of cardiomyocytes.

The G protein-coupled receptor kinase-2 is a TGFbeta-inducible antagonist of TGFbeta signal transduction

Signaling from the activin/transforming growth factor beta (TGFbeta) family of cytokines is a tightly regulated process. Disregulation of TGFbeta signaling is often the underlying basis for various cancers, tumor metastasis, inflammatory and autoimmune diseases. In this study, we identify the protein G-coupled receptor kinase 2 (GRK2), a kinase involved in the desensitization of G protein-coupled receptors (GPCR), as a downstream target and regulator of the TGFbeta-signaling cascade. TGFbeta-induced expression of GRK2 acts in a negative feedback loop to control TGFbeta biological responses. Upon TGFbeta stimulation, GRK2 associates with the receptor-regulated Smads (R-Smads) through their MH1 and MH2 domains and phosphorylates their linker region. GRK2 phosphorylation of the R-Smads inhibits their carboxyl-terminal, activating phosphorylation by the type I receptor kinase, thus preventing nuclear translocation of the Smad complex, leading to the inhibition of TGFbeta-mediated target gene expression, cell growth inhibition and apoptosis. Furthermore, we demonstrate that GRK2 antagonizes TGFbeta-induced target gene expression and apoptosis ex vivo in primary hepatocytes, establishing a new role for GRK2 in modulating single-transmembrane serine/threonine kinase receptor-mediated signal transduction.

Protein kinase activity of phosphoinositide 3-kinase regulates beta-adrenergic receptor endocytosis

Phosphoinositide 3-kinase (PI(3)K) is a unique enzyme characterized by both lipid and protein kinase activities. Here, we demonstrate a requirement for the protein kinase activity of PI(3)K in agonist-dependent beta-adrenergic receptor (betaAR) internalization. Using PI(3)K mutants with either protein or lipid phosphorylation activity, we identify the cytoskeletal protein non-muscle tropomyosin as a substrate of PI(3)K, which is phosphorylated in a wortmannin-sensitive manner on residue Ser 61. A constitutively dephosphorylated (S61A) tropomyosin mutant blocks agonist-dependent betaAR internalization, whereas a tropomyosin mutant that mimics constitutive phosphorylation (S61D) complements the PI(3)K mutant, with only lipid phosphorylation activity reversing the defective betaAR internalization. Notably, knocking down endogenous tropomyosin expression using siRNAs that target different regions if tropomyosin resulted in complete inhibition of betaAR endocytosis, showing that non-muscle tropomyosin is essential for agonist-mediated receptor internalization. These studies demonstrate a previously unknown role for the protein phosphorylation activity of PI(3)K in betaAR internalization and identify non-muscle tropomyosin as a cellular substrate for protein kinase activity of PI(3)K.

A crucial role for GRK2 in regulation of endothelial cell nitric oxide synthase function in portal hypertension

Nitric oxide (NO) production by endothelial cell nitric oxide synthase (eNOS) in sinusoidal endothelial cells is reduced in the injured liver and leads to intrahepatic portal hypertension. We sought to understand the mechanism underlying defective eNOS function. Phosphorylation of the serine-threonine kinase Akt, which activates eNOS, was substantially reduced in sinusoidal endothelial cells from injured livers. Overexpression of Akt in vivo restored phosphorylation of Akt and production of NO and reduced portal pressure in portal hypertensive rats. We found that Akt physically interacts with G-protein-coupled receptor kinase-2 (GRK2), and that this interaction inhibits Akt activity. Furthermore, GRK2 expression increased in sinusoidal endothelial cells from portal hypertensive rats and knockdown of GRK2 restored Akt phosphorylation and NO production, and normalized portal pressure. Finally, after liver injury, GRK2-deficient mice developed less severe portal hypertension than control mice. Thus, an important mechanism underlying impaired activity of eNOS in injured sinusoidal endothelial cells is defective phosphorylation of Akt caused by overexpression of GRK2 after injury.

Inhibition of signal termination-related kinases by membrane-permeant bitter and sweet tastants: potential role in taste signal termination

Sweet and bitter taste sensations are believed to be initiated by the tastant-stimulated T1R and T2R G protein-coupled receptor (GPCR) subfamilies, respectively, which occur in taste cells. Although such tastants, with their significantly diverse chemical structures (e.g., sugar and nonsugar sweeteners), may share the same or similar T1Rs, some nonsugar sweeteners and many bitter tastants are amphipathic and produce a significant delay in taste termination (lingering aftertaste). We report that such tastants may permeate rat taste bud cells rapidly in vivo and inhibit known signal termination-related kinases in vitro, such as GPCR kinase (GRK)2, GRK5, and PKA. GRK5 and perhaps GRK2 and GRK6 are present in taste cells. A new hypothesis is proposed in which membrane-permeant tastants not only interact with taste GPCRs but also interact intracellularly with the receptors' downstream shutoff components to inhibit signal termination.

The platelet-derived growth factor receptor-beta phosphorylates and activates G protein-coupled receptor kinase-2. A mechanism for feedback inhibition

G protein-coupled receptor kinase-2 (GRK2) serine-phosphorylates the platelet-derived growth factor receptor-beta (PDGFRbeta), and thereby diminishes signaling by the receptor. Because activation of GRK2 may involve phosphorylation of its N-terminal tyrosines by c-Src, we tested whether the PDGFRbeta itself could tyrosine-phosphorylate and activate GRK2. To do so, we used wild type (WT) and Y857F mutant PDGFRbetas in HEK cells, which lack endogenous PDGFRs. The Y857F PDGFRbeta autophosphorylates normally but does not phosphorylate exogenous substrates. Although PDGF-stimulated Y857F and WT PDGFRbetas activated c-Src equivalently, the WT PDGFRbeta tyrosine-phosphorylated GKR2 60-fold more than the Y857F PDGFRbeta in intact cells. With purified GRK2 and either WT or Y857F PDGFRbetas immunoprecipitated from HEK cells, GRK2 tyrosyl phosphorylation was PDGF-dependent and required the WT PDGFRbeta, even though the WT and Y857F PDGFRbetas autophosphorylated equivalently. This PDGFRbeta-mediated GRK2 tyrosyl phosphorylation enhanced GRK2 activity: GRK2-mediated seryl phosphorylation of the PDGFRbeta was 9-fold greater for the WT than for the Y857F in response to PDGF, but equivalent when GRK2 was activated by sequential stimulation of beta2-adrenergic and PDGF-beta receptors. Furthermore, both PDGFRbeta-mediated GRK2 tyrosyl phosphorylation and GRK2-mediated PDGFRbeta seryl phosphorylation were reduced approximately 50% in intact cells by mutation to phenylalanine of three tyrosines in the N-terminal domain of GRK2. We conclude that the activated PDGFRbeta itself phosphorylates GRK2 tyrosyl residues and thereby activates GRK2, which then serine-phosphorylates and desensitizes the PDGFRbeta.

G protein-coupled receptor kinase 2 in multiple sclerosis and experimental autoimmune encephalomyelitis

Many modulators of inflammation, including chemokines, neuropeptides, and neurotransmitters signal via G protein-coupled receptors (GPCR). GPCR kinases (GRK) can phosphorylate agonist-activated GPCR thereby promoting receptor desensitization. Here we describe that in leukocytes from patients with active relapsing-remitting multiple sclerosis (MS) or with secondary progressive MS, GRK2 levels are significantly reduced. Unexpectedly, cells from patients during remission express even lower levels of GRK2. The level of GRK2 in leukocytes of patients after stroke, a neurological disorder with paralysis but without an autoimmune component, was similar to GRK2 levels in cells from healthy individuals. In addition, we demonstrate that the course of recombinant myelin oligodendrocyte glycoprotein (1-125)-induced experimental autoimmune encephalomyelitis (EAE), an animal model for MS, is markedly different in GRK2(+/-) mice that express 50% of the GRK2 protein in comparison with wild-type mice. Onset of EAE was significantly advanced by 5 days in GRK2(+/-) mice. The earlier onset of EAE was associated with increased early infiltration of the CNS by T cells and macrophages. Although disease scores in the first phase of EAE were similar in both groups, GRK2(+/-) animals did not develop relapses, whereas wild-type animals did. The absence of relapses in GRK2(+/-) mice was associated with a marked reduction in inflammatory infiltrates in the CNS. Recombinant myelin oligodendrocyte glycoprotein-induced T cell proliferation and cytokine production were normal in GRK2(+/-) animals. We conclude that down-regulation of GRK2 expression may have important consequences for the onset and progression of MS.

Targeted Inhibition of β-Adrenergic Receptor Kinase-1–Associated Phosphoinositide-3 Kinase Activity Preserves β-Adrenergic Receptor Signaling and Prolongs Survival in Heart Failure Induced by Calsequestrin Overexpression

OBJECTIVES

Desensitization and down-regulation of beta-adrenergic receptors (betaARs) are prominent features of heart failure largely mediated by increased levels of betaAR kinase-1 (betaARK1).

BACKGROUND

beta-adrenergic receptor kinase 1 interacts with phosphoinositide-3 kinase (PI3K), and upon agonist stimulation, the betaARK1/PI3K complex is recruited to agonist-stimulated betaARs. Here we tested the hypothesis that in vivo selective inhibition of betaARK1-associated PI3K activity would preserve betaAR signaling and, therefore, improve cardiac function and survival in experimental heart failure.

METHODS

We used a murine model of heart failure induced by calsequestrin (CSQ) cardiac-specific overexpression; CSQ mice were crossed with mice overexpressing in the heart a catalytically inactive PI3Kgamma (PI3Kgamma(inact)) to competitively displace endogenous PI3K from betaARK1.

RESULTS

Catalytically inactive PI3KgammaPI3K overexpression in CSQ mice inhibited betaARK1-associated PI3K activity, normalized betaAR levels, and preserved betaAR responsiveness to isoproterenol (ISO). Restoration of betaAR signaling via PI3Kgamma(inact) overexpression resulted in marked improvement of cardiac function and a significant prolongation of survival. Importantly, the effects of PI3Kgamma(inact) overexpression were restricted to betaAR signaling, because cellular PI3K signaling was unaltered, as shown by the similar activation of multiple downstream signaling pathways in both CSQ and CSQ/PI3Kgamma(inact) mice.

CONCLUSIONS

These data in the CSQ model of cardiac dysfunction indicate that membrane-targeted PI3K activity plays a detrimental role in heart failure, and its inhibition represents a novel therapeutic approach to ameliorate cardiac dysfunction and improve survival.

Uncoupling and endocytosis of 5-hydroxytryptamine 4 receptors. Distinct molecular events with different GRK2 requirements

The 5-hydroxytryptamine type 4 receptors (5-HT4Rs) are involved in memory, cognition, feeding, respiratory control, and gastrointestinal motility through activation of a G(s)/cAMP pathway. We have shown that 5-HT4R undergoes rapid and profound homologous uncoupling in neurons. However, no significant uncoupling was observed in COS-7 or HEK293 cells, which expressed either no or a weak concentration of GRK2, respectively. High expression of GRK2 in neurons is likely to be the reason for this difference because overexpression of GRK2 in COS-7 and HEK293 cells reproduced rapid and profound uncoupling of 5-HT4R. We have also shown, for the first time, that GRK2 requirements for uncoupling and endocytosis were very different. Indeed, beta-arrestin/dynamin-dependent endocytosis was observed in HEK293 cells without any need of GRK2 overexpression. In addition to this difference, uncoupling and beta-arrestin/dynamin-dependent endocytosis were mediated through distinct mechanisms. Neither uncoupling nor beta-arrestin/dynamin-dependent endocytosis required the serine and threonine residues localized within the specific C-terminal domains of the 5-HT4R splice variants. In contrast, a cluster of serines and threonines, common to all variants, was an absolute requirement for beta-arrestin/dynamin-dependent receptor endocytosis, but not for receptor uncoupling. Furthermore, beta-arrestin/dynamin-dependent endocytosis and uncoupling were dependent on and independent of GRK2 kinase activity, respectively. These results clearly demonstrate that the uncoupling and endocytosis of 5-HT4R require different GRK2 concentrations and involve distinct molecular events.

L-DOPA reverses the MPTP-induced elevation of the arrestin2 and GRK6 expression and enhanced ERK activation in monkey brain

Dysregulation of dopamine receptors (DARs) is believed to contribute to Parkinson disease (PD) pathology. G protein-coupled receptors (GPCR) undergo desensitization via activation-dependent phosphorylation by G protein-coupled receptor kinases (GRKs) followed by arrestin binding. Using quantitative Western blotting, we detected profound differences in the expression of arrestin2 and GRKs among four experimental groups of nonhuman primates: (1) normal, (2) parkinsonian, (3) parkinsonian treated with levodopa without or (4) with dyskinesia. Arrestin2 and GRK6 expression was significantly elevated in the MPTP-lesioned group in most brain regions; GRK2 was increased in caudal caudate and internal globus pallidus. Neither levodopa-treated group differed significantly from control. The only dyskinesia-specific change was an elevation of GRK3 in the ventral striatum of the dyskinetic group. Changes in arrestin and GRK expression in the MPTP group were accompanied by enhanced ERK activation and elevated total ERK expression, which were also reversed by L-DOPA. The data suggest the involvement of arrestins and GRKs in Parkinson disease pathology and the effects of levodopa treatment.

Phosphorylation-independent regulation of metabotropic glutamate receptor 1 signaling requires g protein-coupled receptor kinase 2 binding to the second intracellular loop

Metabotropic glutamate receptors (mGluRs) are members of a unique class of G protein-coupled receptors (class III) that include the calcium-sensing and gamma-aminobutyric acid type B receptors. The activity of mGluRs is regulated by second messenger-dependent protein kinases and G protein-coupled receptor kinases (GRKs). The attenuation of both mGluR1a and mGluR1b signaling by GRK2 is phosphorylation- and beta-arrestin-independent and requires the concomitant association of GRK2 with both the receptor and Galpha(q/11). G protein interactions are mediated, in part, by the mGluR1 intracellular second loop, but the domains required for GRK2 binding are unknown. In the present study, we showed that GRK2 binds to the second intracellular loop of mGluR1a and mGluR1b and also to the mGluR1a carboxyl-terminal tail. Alanine scanning mutagenesis revealed a discrete domain within loop 2 that contributes to GRK2 binding, and the mutation of either lysine 691 or 692 to an alanine within this domain resulted in a loss of GRK2 binding to both mGluR1a and mGluR1b. Mutation of either Lys(691) or Lys(692) prevented GRK2-mediated attenuation of mGluR1b signaling, whereas the mutation of only Lys(692) prevented GRK2-mediated inhibition of mGluR1a signaling. Thus, the mGluR1a carboxyl-terminal tail may also be involved in regulating the signaling of the mGluR1a splice variant. Taken together, our findings indicated that kinase binding to an mGluR1 domain involved in G protein-coupling is essential for the phosphorylation-independent attenuation of signaling by GRK2.

MAP kinase protects G protein-coupled receptor kinase 2 from proteasomal degradation

The G protein-coupled receptor kinase 2 (GRK2) phosphorylates and shuts down signaling from 7-transmembrane receptors (7TMs). Although, receptor activity controls GRK2 expression levels, the underlying molecular mechanisms are poorly understood. We have previously shown that extracellular signal-regulated kinase (ERK1/2) activation increases GRK2 expression [J. Theilade, J. Lerche Hansen, S. Haunso, S.P. Sheikh, Extracellular signal-regulated kinases control expression of G protein-coupled receptor kinase 2 (GRK2), FEBS Lett. 518 (2002) 195-199]. In the present study, we found that ERK1/2 regulates GRK2 degradation rather than synthesis. ERK1/2 blockade using PD98059 decreased GRK2 cellular levels to 0.25-fold of control in Cos7 cells. This effect was due to enhanced degradation of the GRK2 protein, since proteasome blockade prevented down-regulation of GRK2 protein levels in the presence of PD98059. Further, ERK blockade had no effect on GRK2 synthesis as probed using a reporter construct carrying the GRK2 promoter upstream of the luciferase gene. We predict ERK1/2 mediated GRK2 protection could be a general phenomenon as proteasome inhibition increased GRK2 expression in two other cell lines, HEK293 and NIH3T3.

The Role of Gβγ and Domain Interfaces in the Activation of G Protein-Coupled Receptor Kinase 2†

In response to extracellular signals, G protein-coupled receptors (GPCRs) catalyze guanine nucleotide exchange on Galpha subunits, enabling both activated Galpha and Gbetagamma subunits to target downstream effector enzymes. One target of Gbetagamma is G protein-coupled receptor kinase 2 (GRK2), an enzyme that initiates homologous desensitization by phosphorylating activated GPCRs. GRK2 consists of three distinct domains: an RGS homology (RH) domain, a protein kinase domain, and a pleckstrin homology (PH) domain, through which it binds Gbetagamma. The crystal structure of the GRK2-Gbetagamma complex revealed that the domains of GRK2 are intimately associated and left open the possibility for allosteric regulation by Gbetagamma. In this paper, we report the 4.5 A structure of GRK2, which shows that the binding of Gbetagamma does not induce large domain rearrangements in GRK2, although small rotations of the RH and PH domains relative to the kinase domain are evident. Mutation of residues within the larger domain interfaces of GRK2 generally leads to diminished expression and activity, suggesting that these interfaces are important for stability and remain intact upon activation of GRK2. Geranylgeranylated Gbetagamma, but not a soluble mutant of Gbetagamma, protects GRK2 from clostripain digestion at a site within its kinase domain that is 80 A away from the Gbetagamma binding site. Equilibrium ultracentrifugation experiments indicate that neither abnormally large detergent micelles nor protein oligomerization can account for the observed protection. The Gbetagamma-mediated binding of GRK2 to CHAPS micelles or lipid bilayers therefore appears to rigidify the kinase domain, perhaps by encouraging stable contacts between the RH and kinase domains.

c-Src is involved in regulating signal transmission from PDGFbeta receptor-GPCR(s) complexes in mammalian cells

We have reported that the platelet-derived growth factor receptor-beta (PDGFbeta) forms a novel signaling complex with G protein-coupled receptors (GPCR) (e.g. S1P(1) receptor) that enables more efficient activation of p42/p44 mitogen-activated protein kinase (MAPK) in response to PDGF and sphingosine 1-phosphate (S1P). We now demonstrate that c-Src participates in regulating the endocytosis of PDGFbeta receptor-GPCR complexes in response to PDGF. This leads to association of cytoplasmic p42/p44 MAPK with the receptor complex in endocytic vesicles. c-Src is regulated by G protein betagamma subunits and can interact with beta-arrestin. Indeed, the PDGF-dependent activation of p42/p44 MAPK was reduced by over-expression of the C-terminal domain of GRK2 (sequesters Gbetagamma subunits), the clathrin-binding domain of beta-arrestin and by inhibitors of c-Src and clathrin-mediated endocytosis. Moreover, PDGF and S1P induce the recruitment of c-Src to the PDGFbeta receptor-S1P(1) receptor complex. This leads to a G protein/c-Src-dependent tyrosine phosphorylation of Gab1 and accumulation of dynamin II at the plasma membrane, a step required for endocytosis of the PDGFbeta receptor-GPCR complex. These findings provide important information concerning the molecular organisation of novel receptor tyrosine kinase (RTK)-GPCR signal relays in mammalian cells.

Alterations of beta-adrenoceptor responsiveness in postischemic myocardium after 72 h of reperfusion

To determine the effect of a completely developed reperfused myocardial infarction model on beta-adrenoceptor responsiveness, we induced a 90-min regional ischemia followed by 72 h of reperfusion in dog hearts. Regional myocardial blood flow was determined after 60 min of ischemia using radioactive microspheres. beta-adrenoceptor density was reduced in the ischemic endocardium (95+/-16 fmol/mg) and epicardium (160+/-13 fmol/mg) compared to the nonischemic region (304+/-21 fmol/mg). beta-adrenoceptor density in the ischemic endocardium varied with the degree of collateral blood flow measured (r2=0.79, P<0.05); this relation was the opposite of that in the ischemic epicardium (r2=0.77, P<0.05). Higher levels of tissue catecholamines and G protein-coupled receptor kinase 2 (GRK2) were observed in the ischemic epicardium as compared to nonischemic tissue. Forskolin-induced adenylyl cyclase activities were depressed in both ischemic regions as compared to nonischemic region, correlating with a reduction in regional myocardial blood flow. Using forskolin stimulation as covariate, no difference in isoproterenol-induced adenylyl cyclase activity was identified in the different regions. It is concluded that cAMP production induced by beta-adrenoceptor activation is dependent upon adenylyl cyclase enzyme activity rather than beta-adrenoceptor density in the ischemic myocardium. However, the density of the beta-adrenoceptor in the viable ischemic regions can be modified by the presence of GRK2 and tissue catecholamines, an index of regional sympathetic efferent postganglionic nerve terminal activity.

α-Synuclein phosphorylation controls neurotoxicity and inclusion formation in a Drosophila model of Parkinson disease

Alpha-synuclein is phosphorylated at serine 129 (Ser129) in intracellular protein aggregates called Lewy bodies. These inclusion bodies are the characteristic pathologic lesions of Parkinson disease. Here we define the role of phosphorylation of Ser129 in alpha-synuclein toxicity and inclusion formation using a Drosophila model of Parkinson disease. Mutation of Ser129 to alanine to prevent phosphorylation completely suppresses dopaminergic neuronal loss produced by expression of human alpha-synuclein. In contrast, altering Ser129 to the negatively charged residue aspartate, to mimic phosphorylation, significantly enhances alpha-synuclein toxicity. The G protein-coupled receptor kinase 2 (Gprk2) phosphorylates Ser129 in vivo and enhances alpha-synuclein toxicity. Blocking phosphorylation at Ser129 substantially increases aggregate formation. Thus Ser129 phosphorylation status is crucial in mediating alpha-synuclein neurotoxicity and inclusion formation. Because increased number of inclusion bodies correlates with reduced toxicity, inclusion bodies may protect neurons from alpha-synuclein toxicity.

Targeted overexpression of G protein-coupled receptor kinase-2 in osteoblasts promotes bone loss

To investigate the role of G protein-coupled receptor kinases (GRKs) in regulating bone formation in vivo, we overexpressed the potent G protein-coupled receptor (GPCR) regulator GRK2 in osteoblasts, using the osteocalcin gene-2 promoter to target expression to osteoblastic cells. Using the parathyroid hormone (PTH) receptor as a model system, we found that overexpression of GRK2 in osteoblasts attenuated PTH-induced cAMP generation by mouse calvaria ex vivo. This decrease in GPCR responsiveness was associated with a reduction in bone mineral density (BMD) in transgenic (TG) mice compared with non-TG littermate controls. The decrease in BMD was most prominent in trabecular-rich lumbar spine and was not observed in cortical bone of the femoral shaft. Quantitative computed tomography indicated that the loss of trabecular bone was due to a decrease in trabecular thickness, with little change in trabecular number. Histomorphometric analyses confirmed the decrease in trabecular bone volume and demonstrated reduced bone remodeling, as evidenced by a decrease in osteoblast numbers and osteoblast-mediated bone formation. Osteoclastic activity also appeared to be reduced because urinary excretion of the osteoclastic activity marker deoxypyridinoline was decreased in TG mice compared with control animals. Consistent with reduced coupling of osteoblast-mediated bone formation to osteoclastic bone resorption, mRNA levels of both osteoprotegrin and receptor activator of NF-kappaB ligand were altered in calvaria of TG mice in a pattern that would promote a low rate of bone remodeling. Taken together, these data suggest that enhancing GRK2 activity and consequently reducing GPCR activity in osteoblasts produces a low bone-turnover state that reduces bone mass.

Rosiglitazone restores renal D1A receptor-Gs protein coupling by reducing receptor hyperphosphorylation in obese rats

Dopamine D(1A) receptor function is impaired in obesity-induced insulin resistance, contributing to sodium retention. We showed previously that uncoupling of D(1A) receptors from G proteins is responsible for diminished natriuretic response to dopamine in obese Zucker rats (OZRs). We hypothesized that overexpression of G protein-coupled receptor kinases (GRKs) leads to increased phosphorylation of D(1A) receptors, which in turn causes uncoupling of the receptors from G(s) proteins in proximal tubules of OZRs. We also examined effects of an insulin sensitizer, rosiglitazone, in correcting these defects. We found that basal and agonist (fenoldopam)-induced coupling of D(1A) receptors to G(s) proteins was impaired in proximal tubules of OZRs compared with lean Zucker rats (LZRs). Moreover, basal serine phosphorylation of D(1A) receptors was elevated two- to threefold in proximal tubules of OZRs compared with LZRs. Fenoldopam increased D(1A) receptor phosphorylation in proximal tubules of LZRs but not OZRs. Compared with that in LZRs, GRK4 expression in OZRs was elevated 200-300% in proximal tubule cell lysates and GRK2 expression was approximately 30% higher in plasma membranes isolated from proximal tubules of OZRs. Rosiglitazone treatment restored basal and agonist-induced coupling of D(1A) receptors to G(s) proteins and reduced basal serine phosphorylation of D(1A) receptors, GRK4 expression, and translocation of GRK2 to the plasma membrane in proximal tubules of OZRs. Furthermore, rosiglitazone significantly reduced fasting blood glucose and plasma insulin in OZRs. Collectively, these results suggest that insulin resistance is responsible for GRK4 overexpression and GRK2 translocation leading to hyperphosphorylation of D(1A) receptors and their uncoupling from G(s) proteins as rosiglitazone treatment corrects these defects in OZRs.

Exposure to TARC alters beta2-adrenergic receptor signaling in human peripheral blood T lymphocytes

The beta(2)-adrenergic receptor (beta(2)-AR) negatively regulates T cell activity through the activation of the G(s)/adenylyl cyclase/cAMP pathway. beta(2)-AR desensitization, which can be induced by its phosphorylation, may have important consequences for the regulation of T cell function in asthma. In the present study we demonstrate that the C-C chemokine thymus and activation-regulated chemokine (TARC) impairs the ability of beta(2)-agonist fenoterol to activate the cAMP downstream effector cAMP-responsive element binding protein (CREB) in freshly isolated human T cells. The TARC-induced activation of Src kinases resulted in membrane translocation of both G protein-coupled receptor kinase (GRK) 2 and beta-arrestin. Moreover, TARC was able to induce Src-dependent serine phosphorylation of the beta(2)-AR as well as its association with GRK2 and beta-arrestin. Finally, in contrast to CREB, phosphorylation of Src and extracellular signal-regulated kinase was enhanced by fenoterol upon TARC pretreatment. In summary, we show for the first time that TARC exposure impairs beta(2)-AR function in T cells. Our data suggest that this is mediated by Src-dependent activation of GRK2, resulting in receptor phosphorylation, binding to beta-arrestin, and a switch from cAMP-dependent signaling to activation of the MAPK pathway. We propose that aberrant T cell control in the presence of endogenous beta-agonists promotes T cell-mediated inflammation in asthma.

Roles of phosphorylation-dependent and -independent mechanisms in the regulation of M1 muscarinic acetylcholine receptors by G protein-coupled receptor kinase 2 in hippocampal neurons

When co-expressed with the inositol 1,4,5-trisphosphate biosensor eGFP-PH(PLC delta), G protein-coupled receptor kinase 2 (GRK2) can suppress M1 muscarinic acetylcholine (mACh) receptor-mediated phospholipase C signaling in hippocampal neurons through a phosphorylation-independent mechanism, most likely involving the direct binding of the RGS homology domain of GRK2 to G alpha(q/11). To define the importance of this mechanism in comparison with classical, phosphorylation-dependent receptor regulation by GRKs, we have examined M1 mACh receptor signaling in hippocampal neurons following depletion of GRK2 and also in the presence of non-G alpha(q/11)-binding GRK2 mutants. Depletion of neuronal GRK2 using an antisense strategy almost completely inhibited M1 mACh receptor desensitization without enhancing acute agonist-stimulated phospholipase C activity. By stimulating neurons with a submaximal agonist concentration before (R1) and after (R2) a period of exposure to a maximal agonist concentration, an index (R2/R1) of agonist-induced desensitization of signaling could be obtained. Co-transfection of neurons with either a non-G alpha(q/11)-binding (D110A) GRK2 mutant or the catalytically inactive (D110A,K220R)GRK2 did not suppress acute M1 mACh receptor-stimulated inositol 1,4,5-trisphosphate production. However, using the desensitization (R2/R1) protocol, it could be shown that expression of (D110A)GRK2 enhanced, whereas (D110A,K220R)GRK2 inhibited, agonist-induced M1 mACh receptor desensitization. In Chinese hamster ovary cells, the loss of G alpha(q/11) binding did not affect the ability of the (D110A)GRK2 mutant to phosphorylate M1 mACh receptors, whereas expression of (D110A,K220R)GRK2 had no effect on receptor phosphorylation. These data indicate that in hippocampal neurons endogenous GRK2 is a key regulator of M1 mACh receptor signaling and that the regulatory process involves both phosphorylation-dependent and -independent mechanisms.

Catheter-mediated delivery of adenoviral vectors expressing beta-adrenergic receptor kinase C-terminus inhibits intimal hyperplasia and luminal stenosis in rabbit iliac arteries

BACKGROUND

Previous studies have shown that incubation of balloon-injured rat carotid arteries with adenoviral vectors encoding the carboxyl terminus of the beta-adrenergic receptor kinase (Ad2/betaARKct) for 30 min reduces neointima formation. However, it is unclear whether this beneficial effect of betaARKct could be achieved using a catheter-based vector delivery system and whether the observed inhibition of neointima formation translated into a reduction of vessel stenosis.

METHODS

In this study, Ad2/betaARKct was infused into the balloon-injured site of rabbit iliac arteries using a porous infusion catheter over 2 min. Twenty-eight days after gene transfer, angiographic and histological assessments were performed.

RESULTS

Angiographic and histological assessments indicate significant (p < 0.05) inhibition of iliac artery neointima formation and lumen stenosis by Ad2/betaARKct. Our studies demonstrate that an inhibitory effect of Ad2/betaARKct on neointima formation is achievable using a catheter-based vector delivery system and that the inhibition of neointima formation translates into a gain in the vessel minimal luminal diameter. The extent of inhibition (35%) was comparable to that observed with adenoviral-mediated expression of thymidine kinase plus ganciclovir treatment, a cytotoxic gene therapy approach for restenosis.

CONCLUSIONS

These results suggest that adenoviral-mediated gene transfer of betaARKct is a clinically viable cytostatic gene therapy strategy for the treatment of restenosis.

Morphine-induced changes of gene expression in the brain

Repeated opiate administration alters gene expression in different brain regions of rodents, an effect which may contribute to plastic changes associated with addictive behaviour. There is increasing evidence that multiple transcription factors are induced in morphine tolerance, sensitization and during morphine withdrawal. Whereas morphine treatment does not lead to major alterations in the expression of mu-opioid receptors (MOR), there is transcriptional regulation of proteins involved in MOR trafficking such as GRK2 or beta arrestin 2 as well as altered expression of other receptors such as dopamine receptors, NMDA receptors, GABA(A) receptor and alpha(2A) adrenoceptor. Recent gene expression profiling studies reveal additional clusters of morphine-responsive genes: whereas single dose administration has been shown to predominantly reduce expression of genes involved in metabolic function, ascending morphine doses leading to morphine tolerance revealed induction of genes which alter patterns of synaptic connectivity such as arc or ania-3. These genes remained elevated after precipitated withdrawal, which also triggered the expression of several transcriptional activators and repressors. In addition, morphine has been shown to be a strong inducer of heat shock protein 70, a cell protective protein which might counter-regulate opiate-induced neurotoxicity. Temporal expression profiles during a chronic morphine application schedule revealed discrete and fluctuating expression of gene clusters such as transcription factors, G-protein-coupled receptors and neuropeptides. Prolonged abstinence seems to be characterized by up-regulation of several transcription factors and persistent down-regulation of ligand gated ion channels such as glutamatergic and GABA-ergic receptor subunits. These long-term changes in receptor expression suggest a persistent alteration of synaptic signalling after morphine treatment.

Characterization of Bitter Taste Responses of Intestinal STC-1 Cells

Cellular responses of STC-1 cells to two bitter tastants (denatonium and caffeine) were investigated using a calcium-imaging technique and compared with the response to bombesin. Caffeine is known to stimulate taste receptor cells, but the properties of its signaling have not been well studied. STC-1 cells responded to all three molecules in a dose-dependent manner, and when a reverse transcriptase-polymerase chain reaction (RT-PCR) for denatonium receptor was performed, the product of predicted size was detected in STC-1 cells. Furthermore, all three signaling pathways were blocked by a phospholipase C (PLC) inhibitor, demonstrating the essential involvement of PLC in cellular responses. To study the regulatory system of G protein signaling in STC-1 cells, we searched G protein-coupled receptor kinases (GRKs) by the degenerate-primer PCR method and found that GRK2 is expressed. We also demonstrated that three GRKs (GRK2, GRK3 and GRK5) are differentially distributed in the circumvallate papilla while only GRK2 is present in taste bud cells. Finally, we overexpressed GRK2 in SCT-1 cells and found that bombesin-induced response was strongly inhibited by GRK2 but denatonium-activated signaling was not affected. In the case of caffeine, response was decreased by expression of GRK2 only when cells were activated by 1 mM caffeine. Thus, we showed that STC-1 cells emerge as a cell model for studying the molecular mechanism of bitter taste signaling, and could indicate properties of caffeine-induced signaling in comparison with other signaling.

Upregulation of beta-adrenergic receptors in heart failure due to volume overload

To examine the mechanisms of changes in beta-adrenergic signal transduction in heart failing due to volume overload, we studied the status of beta-adrenoceptors (beta-ARs), G protein-coupled receptor kinase (GRK), and beta-arrestin in heart failure due to aortocaval shunt (AVS). Heart failure in rats was induced by creating AVS for 16 wk, and beta-AR binding, GRK activity, as well as their protein content, and mRNA levels were determined in both left and right ventricles. The density and protein content for beta1-ARs, unlike those for beta2-ARs, were increased in the failing hearts. Furthermore, protein contents for GRK isoforms and beta-arrestin-1 were decreased in membranous fractions and increased in cytosolic fractions from the failing hearts. On the other hand, steady-state mRNA levels for beta1-ARs and GRK2, as well as protein content for Gbetagamma-subunits, did not change in the failing heart. Basal cardiac function was depressed; however, both in vivo and ex vivo positive inotropic responses of the failing hearts to isoproterenol were augmented. Treatment of AVS animals with imidapril (1 mg.kg(-1).day(-1)) or losartan (20 mg.kg(-1).day(-1)) retarded the progression of heart failure; partially prevented changes in beta1-ARs, GRKs, and beta-arrestin-1 in the failing myocardium; and attenuated the increase in positive inotropic effect of isoproterenol. These results indicate that upregulation of beta1-ARs is associated with subcellular redistribution of GRKs and beta-arrestin-1 in the failing heart due to volume overload. Furthermore, attenuation of alterations in beta-adrenergic system by imidapril or losartan may be due to blockade of the renin-angiotensin system in the AVS model of heart failure.

Beta-arrestin2 enhances beta2-adrenergic receptor-mediated nuclear translocation of ERK

Beta-arrestin mediates desensitization and internalization of beta-adrenergic receptors (betaARs), but also acts as a scaffold protein in extracellular signal-regulated kinase (ERK) cascade. Thus, we have examined the role of beta-arrestin2 in the betaAR-mediated ERK signaling pathways. Isoproterenol stimulation equally activated cytoplasmic and nuclear ERK in COS-7 cells expressing beta1AR or beta2AR. However, the activity of nuclear ERK was enhanced by co-expression of beta-arrestin2 in beta2AR-but not beta1AR-expressing cells. Pertussis toxin treatment and blockade of Gbetagamma action inhibited beta-arrestin2-enhanced nuclear activation of ERK, suggesting that beta-arrestin2 promotes nuclear ERK localization in a Gbetagamma dependent mechanism upon receptor stimulation. beta2AR containing the carboxyl terminal region of beta1AR lost the beta-arrestin2-promoted nuclear translocation. As the carboxyl terminal region is important for beta-arrestin binding, these results demonstrate that recruitment of beta-arrestin2 to carboxyl terminal region of beta2AR is important for ERK localization to the nucleus.

Altered beta2-adrenergic regulation of T cell activity after allergen challenge in asthma

BACKGROUND

Airway inflammation in asthma is orchestrated by recruitment of T helper (Th)2 lymphocytes to the lung and subsequent production of Th2-like cytokines upon allergen challenge.

OBJECTIVE

To examine whether allergen-induced dysfunction of the beta2-adrenergic receptor (beta2-AR) contributes to the enhanced T(h2) cell activity in asthma.

METHODS

Beta2-adrenergic regulation of cytokine mRNA expression was studied in alpha-CD3/alpha-CD28-activated peripheral blood lymphocytes from seven asthma patients before and 6 h after allergen challenge, in conjunction with the effects of beta2-agonist fenoterol on T cell chemotaxis and signalling pathways.

RESULTS

A complete loss of beta2-AR control over expression of the Th2 cytokines IL-4, IL-5 and IL-13, but not of the Th1 cytokine IFN-gamma, was observed after allergen challenge. Furthermore, we found impaired beta2-AR regulation of T cell migration as well as signal transduction pathways, i.e. the phosphorylation of cyclic adenosine monophosphate-responsive element binding protein and the inhibition of the mitogen-activated protein kinase pathway. The loss of beta2-AR control was associated with increased beta-adrenergic receptor kinase expression, which might be involved in beta2-AR desensitization. In addition, we demonstrate for the first time that T cells exposed to the chemokine thymus and activation-regulated chemokine show hyporesponsiveness to fenoterol.

CONCLUSION

Our results suggest that allergen-induced loss of beta2-AR control, possibly mediated by chemokine release, plays an important role in enhanced Th2-like activity in asthma.

Serial alterations of beta-adrenergic signaling in dilated cardiomyopathic hamsters: possible role of myocardial oxidative stress

BACKGROUND

The relationship between enhanced myocardial oxidative stress and impaired beta-adrenergic signaling remains to be characterized during the development of dilated cardiomyopathy.

METHODS AND RESULTS

Alterations in myocardial oxidative stress and beta-adrenergic signaling, as well as left ventricular (LV) functional and structural changes, were evaluated during the development of cardiomyopathy in TO-2 hamsters; F1B hamsters served as controls. LV dysfunction was first apparent at 8 weeks of age and deteriorated thereafter in the TO-2 hamsters. At 32 weeks, the animals exhibited heart failure with an increased plasma norepinephrine concentration. Cardiac myolysis, as demonstrated by elevated plasma concentration of cardiac troponin T, peaked at 8 weeks. The glutathione redox ratio revealed increased oxidative stress in the LV myocardium in TO-2 hamsters even at 4 weeks and became manifest after 8 weeks. The hearts of TO-2 hamsters had significantly reduced superoxide dismutase activity from 8 weeks onward compared with control hamsters. However, glutathione peroxidase activity was unchanged at any time point. The LV functional response to isoproterenol was markedly reduced at 8 weeks, without any apparent changes in the amount of beta-adrenergic signaling molecules, and it deteriorated thereafter. Adenylyl cyclase activity was significantly decreased, despite increased amounts of both G(s) alpha mRNA and protein, in the LV myocardium at 18 weeks.

CONCLUSIONS

Myocardial oxidative stress is actually enhanced in the initial development of LV dysfunction. Both activation of myocardial oxidative stress and impairment of beta-adrenergic signaling become prominent at the stage of severe LV dysfunction. Myocardial oxidative stress may be involved in the development of beta-adrenergic desensitization.

Tumor Necrosis Factor α, Rapid Ventricular Tachyarrhythmias, and Infarct Size in Canine Models of Myocardial Infarction

Etanercept (2 mg/kg), a TNFalpha sequestrant, was administered 24 hours and 1 hour before LAD coronary artery ligation to examine the role of TNFalpha on lethal ventricular tachyarrhythmias and myocardial necrosis. Dogs treated with etanercept had decreased very rapid (>360 bpm) ventricular triplets (6 +/- 1/h, n = 8) 2 to 24 hours following coronary artery ligation compared with saline (21 +/- 6/h, n = 10, P < 0.05). This was concordant with 8-fold salvage of beta-adrenergic receptor kinase 1 (betaARK) activity compared with control (33.8 +/- 7.2% versus 4.3 +/- 2.2% of unoperated control tissue, P < 0.01, n = 5). Salvage of betaARK occurred without change in the thickness of the epicardial tissue overlying the infarct. In dogs pretreated with etanercept before a 2-hour occlusion/4-hour reperfusion of the LAD coronary artery, infarct mass decreased by 61% (% area at risk) and 55% (% left ventricular mass) in the etanercept group (n = 8) compared with saline (n = 9, P < 0.05). This was concordant with an etanercept-mediated six-fold decrease in leukocyte accumulation within ischemically injured myocardium. TNFalpha antagonism decreases malignant ventricular tachyarrhythmias and may relate to partial protection of normal betaARK-mediated desensitization of beta-adrenergic receptors. TNFalpha sequestration also decreases infarct size in an occlusion/reperfusion model of myocardial ischemia.

G protein-coupled receptor kinase regulates dopamine D3 receptor signaling by modulating the stability of a receptor-filamin-beta-arrestin complex. A case of autoreceptor regulation

In addition to its postsynaptic role, the dopamine D3 receptor (D3R) serves as a presynaptic autoreceptor, where it provides continuous feedback regulation of dopamine release at nerve terminals for processes as diverse as emotional tone and locomotion. D3R signaling ability is supported by an association with filamin (actin-binding protein 280), which localizes the receptor with G proteins in plasma membrane lipid rafts but is not appreciably antagonized in a classical sense by the ligand-mediated activation of G protein-coupled receptor kinases (GRKs) and beta-arrestins. In this study, we investigate GRK-mediated regulation of D3R.filamin complex stability and its effect on D3R.G protein signaling potential. Studies in HEK-293 cells show that in the absence of agonist the D3R immunoprecipitates in a complex containing both filamin A and beta-arrestin2. Moreover, the filamin directly interacts with beta-arrestin2 as assessed by immunoprecipitation and yeast two-hybrid studies. With reductions in basal GRK2/3 activity, an increase in the basal association of filamin A and beta-arrestin2 with D3R is observed. Conversely, increases in the basal GRK2/3 activity result in a reduction in the interaction between the D3R and filamin but a relative increase in the agonist-mediated interaction between beta-arrestin2 and the D3R. Our data suggest that the D3R, filamin A, and beta-arrestin form a signaling complex that is destabilized by agonist- or expression-mediated increases in GRK2/3 activity. These findings provide a novel GRK-based mechanism for regulating D3R signaling potential and provide insight for interpreting D3R autoreceptor behavior.

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.

Functional antagonism of different G protein-coupled receptor kinases for beta-arrestin-mediated angiotensin II receptor signaling

beta-arrestins bind to G protein-coupled receptor kinase (GRK)-phosphorylated seven transmembrane receptors, desensitizing their activation of G proteins, while concurrently mediating receptor endocytosis, and some aspects of receptor signaling. We have used RNA interference to assess the roles of the four widely expressed isoforms of GRKs (GRK 2, 3, 5, and 6) in regulating beta-arrestin-mediated signaling to the mitogen-activated protein kinase, extracellular signal-regulated kinase (ERK) 1/2 by the angiotensin II type 1A receptor. Angiotensin II-stimulated receptor phosphorylation, beta-arrestin recruitment, and receptor endocytosis are all mediated primarily by GRK2/3. In contrast, inhibiting GRK 5 or 6 expression abolishes beta-arrestin-mediated ERK activation, whereas lowering GRK 2 or 3 leads to an increase in this signaling. Consistent with these findings, beta-arrestin-mediated ERK activation is enhanced by overexpression of GRK 5 and 6, and reciprocally diminished by GRK 2 and 3. These findings indicate distinct functional capabilities of beta-arrestins bound to receptors phosphorylated by different classes of GRKs.

Level of beta-adrenergic receptor kinase 1 inhibition determines degree of cardiac dysfunction after chronic pressure overload-induced heart failure

BACKGROUND

Heart failure is characterized by abnormalities in beta-adrenergic receptor (betaAR) signaling, including increased level of myocardial betaAR kinase 1 (betaARK1). Our previous studies have shown that inhibition of betaARK1 with the use of the Gbetagamma sequestering peptide of betaARK1 (betaARKct) can prevent cardiac dysfunction in models of heart failure. Because inhibition of betaARK activity is pivotal for amelioration of cardiac dysfunction, we investigated whether the level of betaARK1 inhibition correlates with the degree of heart failure.

METHODS AND RESULTS

Transgenic (TG) mice with varying degrees of cardiac-specific expression of betaARKct peptide underwent transverse aortic constriction (TAC) for 12 weeks. Cardiac function was assessed by serial echocardiography in conscious mice, and the level of myocardial betaARKct protein was quantified at termination of the study. TG mice showed a positive linear relationship between the level of betaARKct protein expression and fractional shortening at 12 weeks after TAC. TG mice with low betaARKct expression developed severe heart failure, whereas mice with high betaARKct expression showed significantly less cardiac deterioration than wild-type (WT) mice. Importantly, mice with a high level of betaARKct expression had preserved isoproterenol-stimulated adenylyl cyclase activity and normal betaAR densities in the cardiac membranes. In contrast, mice with low expression of the transgene had marked abnormalities in betaAR function, similar to the WT mice.

CONCLUSIONS

These data show that the level of betaARK1 inhibition determines the degree to which cardiac function can be preserved in response to pressure overload and has important therapeutic implications when betaARK1 inhibition is considered as a molecular target.

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.

Activity-dependent internalization of smoothened mediated by beta-arrestin 2 and GRK2

Binding of Sonic Hedgehog (Shh) to Patched (Ptc) relieves the latter's tonic inhibition of Smoothened (Smo), a receptor that spans the cell membrane seven times. This initiates signaling which, by unknown mechanisms, regulates vertebrate developmental processes. We find that two molecules interact with mammalian Smo in an activation-dependent manner: G protein-coupled receptor kinase 2 (GRK2) leads to phosphorylation of Smo, and beta-arrestin 2 fused to green fluorescent protein interacts with Smo. These two processes promote endocytosis of Smo in clathrin-coated pits. Ptc inhibits association of beta-arrestin 2 with Smo, and this inhibition is relieved in cells treated with Shh. A Smo agonist stimulated and a Smo antagonist (cyclopamine) inhibited both phosphorylation of Smo by GRK2 and interaction of beta-arrestin 2 with Smo. beta-Arrestin 2 and GRK2 are thus potential mediators of signaling by activated Smo.

Structure/function analysis of alpha2A-adrenergic receptor interaction with G protein-coupled receptor kinase 2

G protein-coupled receptors (GPCRs) mediate the ability of a diverse array of extracellular stimuli to control intracellular signaling. Many GPCRs are phosphorylated by G protein-coupled receptor kinases (GRKs), a process that mediates agonist-specific desensitization in many cells. Although GRK binding to activated GPCRs results in kinase activation and receptor phosphorylation, relatively little is known about the mechanism of GRK/GPCR interaction or how this interaction results in kinase activation. Here, we used the alpha2A-adrenergic receptor (alpha(2A)AR) as a model to study GRK/receptor interaction because GRK2 phosphorylation of four adjacent serines within the large third intracellular loop of this receptor is known to mediate desensitization. Various domains of the alpha(2A)AR were expressed as glutathione S-transferase fusion proteins and tested for the ability to bind purified GRK2. The second and third intracellular loops of the alpha(2A)AR directly interacted with GRK2, whereas the first intracellular loop and C-terminal domain did not. Truncation mutagenesis identified three discrete regions within the third loop that contributed to GRK2 binding, the membrane proximal N- and C-terminal regions as well as a central region adjacent to the phosphorylation sites. Site-directed mutagenesis revealed a critical role for specific basic residues within these regions in mediating GRK2 interaction with the alpha(2A)AR. Mutation of these residues within the holo-alpha(2A)AR diminished GRK2-promoted phosphorylation of the receptor as well as the ability of the kinase to be activated by receptor binding. These studies provide new insight into the mechanism of interaction and activation of GRK2 by GPCRs and suggest that GRK2 binding is critical not only for receptor phosphorylation but also for full activity of the kinase.

Role of G protein-coupled receptor kinases in the homologous desensitization of the human and mouse melanocortin 1 receptors

The melanocortin 1 receptor, a G protein-coupled receptor positively coupled to adenylyl cyclase, is a key regulator of epidermal melanocyte proliferation and differentiation and a determinant of human skin phototype and skin cancer risk. Despite its potential importance for regulation of pigmentation, no information is available on homologous desensitization of this receptor. We found that the human melanocortin 1 receptor (MC1R) and its mouse ortholog (Mc1r) undergo homologous desensitization in melanoma cells. Desensitization is not dependent on protein kinase A, protein kinase C, calcium mobilization, or MAPKs, but is agonist dose-dependent. Both melanoma cells and normal melanocytes express two members of the G protein-coupled receptor kinase (GRK) family, GRK2 and GRK6. Cotransfection of the receptor and GRK2 or GRK6 genes in heterologous cells demonstrated that GRK2 and GRK6 impair agonist-dependent signaling by MC1R or Mc1r. However, GRK6, but not GRK2, was able to inhibit MC1R agonist-independent constitutive signaling. Expression of a dominant negative GRK2 mutant in melanoma cells increased their cAMP response to agonists. Agonist-stimulated cAMP production decreased in melanoma cells enriched with GRK6 after stable transfection. Therefore, GRK2 and GRK6 seem to be key regulators of melanocortin 1 receptor signaling and may be important determinants of skin pigmentation.

Beta-arrestin binding to the beta2-adrenergic receptor requires both receptor phosphorylation and receptor activation

Homologous desensitization of beta2-adrenergic receptors has been shown to be mediated by phosphorylation of the agonist-stimulated receptor by G-protein-coupled receptor kinase 2 (GRK2) followed by binding of beta-arrestins to the phosphorylated receptor. Binding of beta-arrestin to the receptor is a prerequisite for subsequent receptor desensitization, internalization via clathrin-coated pits, and the initiation of alternative signaling pathways. In this study we have investigated the interactions between receptors and beta-arrestin2 in living cells using fluorescence resonance energy transfer. We show that (a) the initial kinetics of beta-arrestin2 binding to the receptor is limited by the kinetics of GRK2-mediated receptor phosphorylation; (b) repeated stimulation leads to the accumulation of GRK2-phosphorylated receptor, which can bind beta-arrestin2 very rapidly; and (c) the interaction of beta-arrestin2 with the receptor depends on the activation of the receptor by agonist because agonist withdrawal leads to swift dissociation of the receptor-beta-arrestin2 complex. This fast agonist-controlled association and dissociation of beta-arrestins from prephosphorylated receptors should permit rapid control of receptor sensitivity in repeatedly stimulated cells such as neurons.

Decline in caveolin-1 expression and scaffolding of G protein receptor kinase-2 with age in Fischer 344 aortic vascular smooth muscle

Beta-adrenergic receptor (beta-AR)-mediated vasorelaxation declines with age in humans and animal models. This is not caused by changes in expression of beta-AR, G alpha s, adenylyl cyclase, or protein kinase A but is associated with decreased cAMP production. Expression and activity of G protein receptor kinase-2 (GRK-2), which phosphorylates and desensitizes the beta-AR, increases with age in rat aortic tissue. Caveolin scaffolds the beta-AR, GRK, and other proteins within "signaling pockets" and inhibits GRK activity when bound. We questioned the effect of age on caveolin-1 expression and interaction between caveolin-1 and GRK-2 in vascular smooth muscle (VSM) isolated from 2-, 6-, 12-, and 24-mo-old male Fischer 344 rat aorta. Western blot analysis found expression of caveolin-1 declined with age (6-, 12- and 24-mo-old rat aortas express 92, 50, and 42% of 2-mo-old rat aortas, respectively). Results from density-buoyancy analysis showed a lower percentage of GRK in caveolin-1-specific fractions with age (6-, 12- and 24-mo-old rat aortas express 95, 56, and 12% of 2-mo-old rat aortas, respectively). Coimmunoprecipitation confirmed this finding; density of GRK in caveolin-1 immunoprecipitates was 97, 30, and 21% of 2-mo-old aortas compared with 6-, 12- and 24-mo-old animals, respectively. Immunohistocytochemistry and confocal microscopy confirmed that GRK-2 and caveolin-1 colocalize in VSM. These results suggest that in nonoverexpressed, intact tissue, the decline in beta-AR-mediated vasorelaxation may be caused by both a reduction in caveolin-1 expression and a reduction in binding of GRK-2 by caveolin-1. This could lead to an increase in the fraction of free GRK-2, which could phosphorylate and desensitize the beta-AR.

Hypertensive left ventricular hypertrophy: relation to beta-adrenergic receptor kinase-1 (betaARK1) in peripheral lymphocytes

BACKGROUND

Left ventricular hypertrophy (LVH) is associated with increased cardiovascular risk and altered sympathetic regulation in hypertension.

OBJECTIVES

To determine whether the level of beta-adrenergic receptor kinase-1 (betaARK1) in lymphocytes is related to LVH in patients with hypertension.

METHODS

Forty-nine patients with untreated essential hypertension were recruited to the study and classified into two groups: left ventricular hypertrophy (LVH: left ventricular mass index > or =134 g/m in men and > or =110 g/m in women; ages 52.4 +/- 12.8 years, n = 25) and non-LVH (NLVH: left ventricular mass index < 134 g/m in men and < 110 g/m in women; ages 50.8 +/- 13.1 years, n = 24). Lymphocytes were isolated from patients and quantitative-competitive reverse transcriptase-polymerase chain reaction (RT-PCR) and immunoblotting were used to estimate the expression of betaARK1 in the lymphocytes. G-protein-coupled receptor kinase activity was assessed by rhodopsin phosphorylation assay.

RESULTS

The expression of betaARK1 in lymphocytes was greater in the LVH group than in the NLVH group (0.0069 +/- 0.002 ng compared with 0.0048 +/- 0.0017 ng, P < 0.01) and correlated well with left ventricular mass index (r = 0.527, P < 0.001) and relative wall thickness (r = 0.627, P < 0.001). The concentration of betaARK1 protein in lymphocytes from individuals with LVH was increased two-fold compared with that in the NLVH group (both n = 7). Lymphocyte G-protein-coupled receptor kinase activity from LVH was enhanced 1.7-fold compared with NLVH (1.03 +/- 2.16 and 1.79 +/- 1.87 pmol phosphate/min per mg protein, respectively; P < 0.05, n = 7 for each group).

CONCLUSIONS

The concentration of betaARK1 in lymphocytes is greater in hypertensive individuals with LVH than in those without LVH and parallels the degree of hypertrophy. Generalized alterations in beta-adrenergic signalling, including betaARK1, could be a major contributory factor in the development of LVH in hypertension, and the concentration of betaARK1 in lymphocytes can reflect the development of LVH in a patient with hypertension.

Activation of tyrosine kinase of EGFR induces Gβγ-dependent GRK-EGFR complex formation

This study demonstrated that activation of tyrosine kinase of epidermal growth factor receptor (EGFR) induces its association with G protein-coupled receptor kinase 2 (GRK2). Immunoprecipitation experiments showed that EGF stimulation increased GRK2 binding to EGFR complex in HEK293 cells coexpressing EGFR and GRK2. The EGF-induced GRK2-EGFR complex formation was greatly reduced by perturbation of EGFR and Src tyrosine kinase activity. Furthermore, studies with GRK2 mutants showed that neither catalytic activity nor the N-terminal domain of GRK2 was required for EGF-induced GRK2-EGFR complex formation. However, overexpression of Gbetagamma scavengers blocked EGF-induced formation of GRK2-EGFR complex.

Bimodal regulation of the human H1 histamine receptor by G protein-coupled receptor kinase 2

The H1 histamine receptor (H1HR) is a member of the G protein-coupled receptor superfamily and regulates numerous cellular functions through its activation of the G(q/11) subfamily of heterotrimeric G proteins. Although the H1HR has been shown to undergo desensitization in multiple cell types, the mechanisms underlying the regulation of H1HR signaling are poorly defined. To address this issue, we examined the effects of wild type and mutant G protein-coupled receptor kinases (GRKs) on the phosphorylation and signaling of human H1HR in HEK293 cells. Overexpression of GRK2 promoted H1HR phosphorylation in intact HEK293 cells and completely inhibited inositol phosphate production stimulated by H1HR, whereas GRK5 and GRK6 had lesser effects on H1HR phosphorylation and signaling. Interestingly, catalytically inactive GRK2 (GRK2-K220R) also significantly attenuated H1HR-mediated inositol phosphate production, as did an N-terminal fragment of GRK2 previously characterized as a regulator of G protein signaling (RGS) protein for Galpha(q/11). Disruption of this RGS function in holo-GRK2 by mutation (GRK2-D110A) partially reversed the quenching effect of GRK2, whereas deletion of both the kinase activity and RGS function (GRK2-D110A/K220R) effectively relieved the inhibition of inositol phosphate generation. To evaluate the role of endogenous GRKs on H1HR regulation, we used small interfering RNAs to selectively target GRK2 and GRK5, two of the primary GRKs expressed in HEK293 cells. A GRK2-specific small interfering RNA effectively reduced GRK2 expression and resulted in a significant increase in histamine-promoted calcium flux. In contrast, knockdown of GRK5 expression was without effect on H1HR signaling. These findings demonstrate that GRK2 is the principal kinase mediating H1 histamine receptor desensitization in HEK293 cells and suggest that rapid termination of H1HR signaling is mediated by both the kinase activity and RGS function of GRK2.

Post-receptorial mechanisms underlie functional disregulation of beta2-adrenergic receptors in lymphocytes from Multiple Sclerosis patients

Increased density of beta2-adrenergic receptors has been demonstrated on peripheral blood mononuclear cells (PBMCs) from Multiple Sclerosis (MS) patients. In this study we found that isoproterenol reduces T-cell proliferation and IFNgamma secretion in PBMCs cultures from healthy controls and IFNbeta-treated but not untreated MS patients. Reduced expression levels of G protein coupled receptor kinase (GRK)2/3 (p < 0.05) and increased isoproterenol-induced cAMP accumulation (p < 0.0001) were found in PBMCs from all MS patients. Dibutyryl cAMP reduced the proliferation of PBMCs from all subgroups but in a slighter manner in untreated MS patients. We conclude that signalling through beta2-adrenergic receptors is chronically up-regulated but functionally uncoupled to immunoregulatory functions of lymphocytes from MS patients. Disregulation downstream the cAMP-associated signalling may underlie such a phenomenon.

Kinetics of G-protein-coupled receptor signalling and desensitization

The kinetics of G-protein-coupled receptor activation and deactivation has, so far, been measured only indirectly, most frequently by assessing the production of various second messengers. We have developed methods based on fluorescence resonance energy transfer to quantify the kinetics of receptor activation by agonist (measured as conformational change in the receptor), the kinetics of G-protein activation (measured as G-protein subunit rearrangement) and the kinetics of receptor inactivation by arrestins (measured as receptor–arrestin interaction). Using these methods, we show that receptor activation by agonists and signalling to G-proteins occur on the subsecond time scale, whereas receptor desensitization is limited by receptor phosphorylation and proceeds more slowly.

Characterization of the GRK2 binding site of Galphaq

Heterotrimeric guanine nucleotide-binding proteins (G proteins) transmit signals from membrane bound G protein-coupled receptors (GPCRs) to intracellular effector proteins. The G(q) subfamily of Galpha subunits couples GPCR activation to the enzymatic activity of phospholipase C-beta (PLC-beta). Regulators of G protein signaling (RGS) proteins bind to activated Galpha subunits, including Galpha(q), and regulate Galpha signaling by acting as GTPase activating proteins (GAPs), increasing the rate of the intrinsic GTPase activity, or by acting as effector antagonists for Galpha subunits. GPCR kinases (GRKs) phosphorylate agonist-bound receptors in the first step of receptor desensitization. The amino termini of all GRKs contain an RGS homology (RH) domain, and binding of the GRK2 RH domain to Galpha(q) attenuates PLC-beta activity. The RH domain of GRK2 interacts with Galpha(q/11) through a novel Galpha binding surface termed the "C" site. Here, molecular modeling of the Galpha(q).GRK2 complex and site-directed mutagenesis of Galpha(q) were used to identify residues in Galpha(q) that interact with GRK2. The model identifies Pro(185) in Switch I of Galpha(q) as being at the crux of the interface, and mutation of this residue to lysine disrupts Galpha(q) binding to the GRK2-RH domain. Switch III also appears to play a role in GRK2 binding because the mutations Galpha(q)-V240A, Galpha(q)-D243A, both residues within Switch III, and Galpha(q)-Q152A, a residue that structurally supports Switch III, are defective in binding GRK2. Furthermore, GRK2-mediated inhibition of Galpha(q)-Q152A-R183C-stimulated inositol phosphate release is reduced in comparison to Galpha(q)-R183C. Interestingly, the model also predicts that residues in the helical domain of Galpha(q) interact with GRK2. In fact, the mutants Galpha(q)-K77A, Galpha(q)-L78D, Galpha(q)-Q81A, and Galpha(q)-R92A have reduced binding to the GRK2-RH domain. Finally, although the mutant Galpha(q)-T187K has greatly reduced binding to RGS2 and RGS4, it has little to no effect on binding to GRK2. Thus the RH domain A and C sites for Galpha(q) interaction rely on contacts with distinct regions and different Switch I residues in Galpha(q).

Unmasking the osteoinductive effects of a G-protein-coupled receptor (GPCR) kinase (GRK) inhibitor by treatment with PTH(1-34)

The effects of GPCR systems in bone are regulated by a family of enzymes termed GRKs. We found that (1) GRK inhibition in osteoblasts has age-dependent effects on bone mass, and (2) the anabolic actions of GRK inhibition are revealed by treatment with PTH(1-34).

INTRODUCTION

The effects of G-protein-coupled receptor (GPCR) systems in bone are modulated by a family of enzymes termed GPCR kinases (GRKs). These enzymes directly phosphorylate GPCR substrate and desensitize receptor signaling. We previously found that expression of a GRK inhibitor in osteoblasts using transgenic (TG) technologies enhanced bone remodeling, and in turn, increased BMD in 6-week-old TG mice compared with non-TG littermate controls, presumably because of enhanced GPCR function. The aim of this study was to determine the age-dependent effects of the transgene.

MATERIALS AND METHODS

BMD was monitored in TG mice and in controls at 6-week, 3-month, and 6-month time-points. To determine if the transgene enhanced responsiveness of bone to parathyroid hormone (PTH), we measured cyclic adenosine monophosphate (cAMP) generation by mouse calvaria ex vivo as well as the effects of treatment with PTH(1-34) on BMD, bone histomorphometry, and expression of the PTH-responsive gene RANKL in both TG mice and non-TG controls.

RESULTS

Consistent with our previous findings, we found that BMD was increased in TG mice compared with controls at 6 weeks of age. The increase in BMD was most prominent in trabecular-rich lumbar spine and was not observed in cortical bone of the femoral shaft. In contrast to younger animals, however, BMD in older TG mice was not statistically different compared with non-TG mice at 3 months of age and was similar to non-TG animals at 6 months of age. The GRK inhibitor seemed to promote GPCR activation in older mice, however, because (1) PTH-induced cAMP generation by mouse calvaria ex vivo was enhanced in TG mice compared with controls, (2) GRK inhibition increased responsiveness of lumbar spine to the osteoinductive actions of PTH(1-34), and (3) the enhanced anabolic effect of PTH(1-34) was associated with increased expression of the PTH-responsive gene RANKL in calvaria of the TG animals. Bone histomorphometry confirmed that PTH(1-34) increased trabecular bone volume in TG mice and found that this increase in bone mass was caused by enhanced bone formation, predominantly as a result of an increase in the mineral apposition rate (MAR).

CONCLUSIONS

These data suggest that the anabolic effects of GRK inhibition are age dependent. The osteoinductive actions of the GRK inhibitor are, however, unmasked by treatment with PTH(1-34).

Hope for a broken heart?

Heart failure affects 23 million people worldwide and results from cardiac dysfunction characterized by decreased responsiveness to beta-adrenergic stimulation. A recent publication by W.J. Koch and colleagues highlights evidence for targeted beta-adrenergic receptor kinase (betaARK1) inhibition by gene transfer to improve contractile function and beta-adrenergic responsiveness in failing human myocardium. This proof-of-concept study has great importance for future heart failure therapy because it provides evidence for the therapeutic effectiveness of betaARK1 inhibition in failing human myocardium.

Cellular G protein-coupled receptor kinase levels regulate sensitivity of the {alpha}2b-adrenergic receptor to undergo agonist-induced down-regulation

Chronic coactivation of alpha(2B)- and beta(2)-adrenoceptors (AR) was recently reported to down-regulate the alpha(2B)-AR at a lower threshold epinephrine (EPI) concentration compared with the activation of alpha(2B)-AR alone. This is the result of a modest beta(2)-AR-dependent up-regulation of G protein-coupled receptor kinase 3 (GRK3). In the present study, we determined that increasing GRK2 or GRK3 levels, independent of beta(2)-AR activation, decreases the EC(50) concentration for agonist-induced down-regulation of the alpha(2B)-AR using NG108 cells with or without overexpression (2- to 10-fold) of GRK2 or GRK3. In parental NG108 cells, the EC(50) concentration of EPI required for down-regulation of the alpha(2B)-AR is 30 microM. A 2- to 3-fold overexpression of GRK3 in NG108 cells, however, reduces the EC(50) to 0.2 microM (a 150-fold decrease), whereas a comparable overexpression of GRK2 reduces it to 1 microM (a 30-fold decrease). However, when GRK3 or GRK2 in NG108 cells are overexpressed 8- to 10-fold, the EC(50) concentration (0.02 microM EPI) for alpha(2B)-AR down-regulation is reduced 1000-fold. These data clearly suggest that a modest (2- to 3-fold) up-regulation of GRK3 is more effective at enhancing the sensitivity of alpha(2B)-AR to down-regulation after exposure to EPI than a modest up-regulation of GRK2, but that both GRK2 and GRK3 are equally effective at inducing alpha(2B)-AR down-regulation when up-regulated 8- to 10-fold. To our knowledge, this is the first report to systematically demonstrate that GRKs, particularly GRK3, play a pivotal role in modulating the agonist EC(50) concentration that down-regulates the alpha(2B)-AR and thus adds a new dimension to an already intricate signaling network.

Involvement of the C-terminal proline-rich motif of G protein-coupled receptor kinases in recognition of activated rhodopsin

G protein-coupled receptor kinases (GRKs) are a family of serine/threonine kinases that phosphorylate many activated G protein-coupled receptors (GPCRs) and play an important role in GPCR desensitization. Our previous work has demonstrated that the C-terminal conserved region (CC) of GRK-2 participates in interaction with rhodopsin and that this interaction is necessary for GRK-2-mediated receptor phosphorylation (Gan, X. Q., Wang, J. Y., Yang, Q. H., Li, Z., Liu, F., Pei, G., and Li, L. (2000) J. Biol. Chem. 275, 8469-8474). In this report, we further investigated whether the CC of other GRKs had the same functions and defined the specific sequences in CC that are required for the functions. The CC regions of GRK-1, GRK-2, and GRK-5, representatives of the three subfamilies of GRKs, could bind rhodopsin in vitro and inhibit GRK-2-mediated phosphorylation of rhodopsin, but not a peptide GRK substrate. Through a series of mutagenesis analyses, a proline-rich motif in the CC was identified as the key element involved in the interaction between the CC region and rhodopsin. Point mutations of this motif not only disrupted the interaction of GRK-2 with rhodopsin but also abolished the ability of GRK-2 to phosphorylate rhodopsin. The findings that the CC region of GRKs interact only with the light-activated but not the non-activated rhodopsin and that the N-terminal domain of GRK-2 interacts with rhodopsin in a light-independent manner suggest that the CC region is responsible for the recognition of activated GPCRs in the canonical model.