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

Phospho-selective mechanisms of arrestin conformations and functions revealed by unnatural amino acid incorporation and (19)F-NMR

Specific arrestin conformations are coupled to distinct downstream effectors, which underlie the functions of many G-protein-coupled receptors (GPCRs). Here, using unnatural amino acid incorporation and fluorine-19 nuclear magnetic resonance (¹⁹F-NMR) spectroscopy, we demonstrate that distinct receptor phospho-barcodes are translated to specific β-arrestin-1 conformations and direct selective signalling. With its phosphate-binding concave surface, β-arrestin-1 ‘reads' the message in the receptor phospho-C-tails and distinct phospho-interaction patterns are revealed by ¹⁹F-NMR. Whereas all functional phosphopeptides interact with a common phosphate binding site and induce the movements of finger and middle loops, different phospho-interaction patterns induce distinct structural states of β-arrestin-1 that are coupled to distinct arrestin functions. Only clathrin recognizes and stabilizes GRK2-specific β-arrestin-1 conformations. The identified receptor-phospho-selective mechanism for arrestin conformation and the spacing of the multiple phosphate-binding sites in the arrestin enable arrestin to recognize plethora phosphorylation states of numerous GPCRs, contributing to the functional diversity of receptors.

G-protein-coupled receptors (GPCRs) signal via G proteins or arrestin-mediated pathways; the plasticity of arrestin proteins is thought to underlie their function. Here, the authors use NMR to examine how β-arrestin-1 recognizes different GPCR phospho-barcodes, and how this triggers structural rearrangements to fulfill selective functions.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

G Protein-Coupled Receptor Kinase 2 (GRK2) and 5 (GRK5) Exhibit Selective Phosphorylation of the Neurotensin Receptor in Vitro

G protein-coupled receptor kinases (GRKs) play an important role in the desensitization of G protein-mediated signaling of G protein-coupled receptors (GPCRs). The level of interest in mapping their phosphorylation sites has increased because recent studies suggest that the differential pattern of receptor phosphorylation has distinct biological consequences. In vitro phosphorylation experiments using well-controlled systems are useful for deciphering the complexity of these physiological reactions and understanding the targeted event. Here, we report on the phosphorylation of the class A GPCR neurotensin receptor 1 (NTSR1) by GRKs under defined experimental conditions afforded by nanodisc technology. Phosphorylation of NTSR1 by GRK2 was agonist-dependent, whereas phosphorylation by GRK5 occurred in an activation-independent manner. In addition, the negatively charged lipids in the immediate vicinity of NTSR1 directly affect phosphorylation by GRKs. Identification of phosphorylation sites in agonist-activated NTSR1 revealed that GRK2 and GRK5 target different residues located on the intracellular receptor elements. GRK2 phosphorylates only the C-terminal Ser residues, whereas GRK5 phosphorylates Ser and Thr residues located in intracellular loop 3 and the C-terminus. Interestingly, phosphorylation assays using a series of NTSR1 mutants show that GRK2 does not require acidic residues upstream of the phospho-acceptors for site-specific phosphorylation, in contrast to the β₂-adrenergic and μ-opioid receptors. Differential phosphorylation of GPCRs by GRKs is thought to encode a particular signaling outcome, and our in vitro study revealed NTSR1 differential phosphorylation by GRK2 and GRK5.

GRK3 suppresses L-DOPA-induced dyskinesia in the rat model of Parkinson's disease via its RGS homology domain

Degeneration of dopaminergic neurons causes Parkinson’s disease. Dopamine replacement therapy with L-DOPA is the best available treatment. However, patients develop L-DOPA-induced dyskinesia (LID). In the hemiparkinsonian rat, chronic L-DOPA increases rotations and abnormal involuntary movements modeling LID, via supersensitive dopamine receptors. Dopamine receptors are controlled by G protein-coupled receptor kinases (GRKs). Here we demonstrate that LID is attenuated by overexpression of GRK3 in the striatum, whereas knockdown of GRK3 by microRNA exacerbated it. Kinase-dead GRK3 and its separated RGS homology domain (RH) suppressed sensitization to L-DOPA, whereas GRK3 with disabled RH did not. RH alleviated LID without compromising anti-akinetic effect of L-DOPA. RH binds striatal Gq. GRK3, kinase-dead GRK3, and RH inhibited accumulation of ∆FosB, a marker of LID. RH-dead mutant was ineffective, whereas GRK3 knockdown exacerbated ∆FosB accumulation. Our findings reveal a novel mechanism of GRK3 control of the dopamine receptor signaling and the role of Gq in LID.

Determination of GPCR Phosphorylation Status: Establishing a Phosphorylation Barcode

G protein-coupled receptors (GPCRs) are rapidly phosphorylated following agonist occupation in a process that mediates receptor uncoupling from its cognate G protein, a process referred to as desensitization. In addition, this process provides a mechanism by which receptors can engage with arrestin adaptor molecules and couple to downstream signaling pathways. The importance of this regulatory process has been highlighted recently by the understanding that ligands can direct receptor signaling along one pathway in preference to another, the phenomenon of signaling bias that is partly mediated by the phosphorylation status or phosphorylation barcode of the receptor. Methods to determine the phosphorylation status of a GPCR in vitro and in vivo are necessary to understand not only the physiological mechanisms involved in GPCR signaling, but also to fully examine the signaling properties of GPCR ligands. This unit describes detailed methods for determining the overall phosphorylation pattern on a receptor (the phosphorylation barcode), as well as mass spectrometry approaches that can define the precise sites that become phosphorylated. These techniques, coupled with the generation and characterization of receptor phosphorylation-specific antibodies, provide a full palate of techniques necessary to determine the phosphorylation status of any given GPCR subtype.

Targeting β-arrestin2 in the treatment of L-DOPA-induced dyskinesia in Parkinson's disease

Parkinson's disease (PD) is characterized by severe locomotor deficits and is commonly treated with the dopamine (DA) precursor l-3,4-dihydroxyphenylalanine (L-DOPA), but its prolonged use causes dyskinesias referred to as L-DOPA-induced dyskinesias (LIDs). Recent studies in animal models of PD have suggested that dyskinesias are associated with the overactivation of G protein-mediated signaling through DA receptors. β-Arrestins desensitize G protein signaling at DA receptors (D1R and D2R) in addition to activating their own G protein-independent signaling events, which have been shown to mediate locomotion. Therefore, targeting β-arrestins in PD L-DOPA therapy might prove to be a desirable approach. Here we show in a bilateral DA-depletion mouse model of Parkinson's symptoms that genetic deletion of β-arrestin2 significantly limits the beneficial locomotor effects while markedly enhancing the dyskinesia-like effects of acute or chronic L-DOPA treatment. Viral rescue or overexpression of β-arrestin2 in knockout or control mice either reverses or protects against LIDs and its key biochemical markers. In other more conventional animal models of DA neuron loss and PD, such as 6-hydroxydopamine-treated mice or rats and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated nonhuman primates, β-arrestin2 overexpression significantly reduced dyskinesias while maintaining the therapeutic effect of L-DOPA. Considerable efforts are being spent in the pharmaceutical industry to identify therapeutic approaches to block LIDs in patients with PD. Our results point to a potential therapeutic approach, whereby development of either a genetic or pharmacological intervention to enhance β-arrestin2- or limit G protein-dependent D1/D2R signaling could represent a more mechanistically informed strategy.

C-terminal threonines and serines play distinct roles in the desensitization of rhodopsin, a G protein-coupled receptor

Rod photoreceptors generate measurable responses to single-photon activation of individual molecules of the G protein-coupled receptor (GPCR), rhodopsin. Timely rhodopsin desensitization depends on phosphorylation and arrestin binding, which quenches G protein activation. Rhodopsin phosphorylation has been measured biochemically at C-terminal serine residues, suggesting that these residues are critical for producing fast, low-noise responses. The role of native threonine residues is unclear. We compared single-photon responses from rhodopsin lacking native serine or threonine phosphorylation sites. Contrary to expectation, serine-only rhodopsin generated prolonged step-like single-photon responses that terminated abruptly and randomly, whereas threonine-only rhodopsin generated responses that were only modestly slower than normal. We show that the step-like responses of serine-only rhodopsin reflect slow and stochastic arrestin binding. Thus, threonine sites play a privileged role in promoting timely arrestin binding and rhodopsin desensitization. Similar coordination of phosphorylation and arrestin binding may more generally permit tight control of the duration of GPCR activity.

DOI:http://dx.doi.org/10.7554/eLife.05981.001

‘Rod’ cells in the eye enable us to see in starlight. Inside these cells, a protein called rhodopsin is activated by light, which leads to an electrical signal being produced that travels to the brain. The duration of the electrical signal depends on the time it takes for the rhodopsin to be deactivated. Rhodopsin is a member of a large class of receptor proteins known as G protein-coupled receptors that regulate many processes throughout the body.

Previous studies have shown that rhodopsin is deactivated by the attachment of phosphate groups to the protein. This allows another protein called arrestin to bind to rhodopsin. The phosphates can be attached to particular amino acids—the building blocks of proteins—at one end of rhodopsin. Three of these are a type of amino acid called serine. Previous work has shown that light increases the speed at which phosphate groups are added to these serines, suggesting that they are important for producing rapid electrical signals. The other three amino acids are of a different type—called threonine—but it is less clear what role they play in deactivating rhodopsin.

Here, Azevedo et al. studied mutant forms of rhodopsin that were missing the serines or threonines in mice. The experiments show that loss of the serines only slightly slowed the electrical signals. However, loss of the threonines resulted in much slower electrical signals that ended at random times. This was due to rhodopsin being less able to bind to arrestin.

Azevedo et al. propose a new model for how rhodopsin is deactivated. Once light activates the protein, phosphate groups are rapidly added to the serines, which begins to lower the activity of rhodopsin. However, it is the slower addition of phosphates to the threonines that is essential to promote arrestin binding and fully deactivate the protein. Other proteins belonging to the G protein-coupled receptor family also have these serines and threonines, and thus, may be regulated in a similar way.

DOI:http://dx.doi.org/10.7554/eLife.05981.002

The evolving impact of g protein-coupled receptor kinases in cardiac health and disease

G protein-coupled receptors (GPCRs) are important regulators of various cellular functions via activation of intracellular signaling events. Active GPCR signaling is shut down by GPCR kinases (GRKs) and subsequent β-arrestin-mediated mechanisms including phosphorylation, internalization, and either receptor degradation or resensitization. The seven-member GRK family varies in their structural composition, cellular localization, function, and mechanism of action (see sect. II). Here, we focus our attention on GRKs in particular canonical and novel roles of the GRKs found in the cardiovascular system (see sects. III and IV). Paramount to overall cardiac function is GPCR-mediated signaling provided by the adrenergic system. Overstimulation of the adrenergic system has been highly implicated in various etiologies of cardiovascular disease including hypertension and heart failure. GRKs acting downstream of heightened adrenergic signaling appear to be key players in cardiac homeostasis and disease progression, and herein we review the current data on GRKs related to cardiac disease and discuss their potential in the development of novel therapeutic strategies in cardiac diseases including heart failure.

G Protein-coupled Receptor Kinases of the GRK4 Protein Subfamily Phosphorylate Inactive G Protein-coupled Receptors (GPCRs)

G protein-coupled receptor (GPCR) kinases (GRKs) play a key role in homologous desensitization of GPCRs. It is widely assumed that most GRKs selectively phosphorylate only active GPCRs. Here, we show that although this seems to be the case for the GRK2/3 subfamily, GRK5/6 effectively phosphorylate inactive forms of several GPCRs, including β2-adrenergic and M2 muscarinic receptors, which are commonly used as representative models for GPCRs. Agonist-independent GPCR phosphorylation cannot be explained by constitutive activity of the receptor or membrane association of the GRK, suggesting that it is an inherent ability of GRK5/6. Importantly, phosphorylation of the inactive β2-adrenergic receptor enhanced its interactions with arrestins. Arrestin-3 was able to discriminate between phosphorylation of the same receptor by GRK2 and GRK5, demonstrating preference for the latter. Arrestin recruitment to inactive phosphorylated GPCRs suggests that not only agonist activation but also the complement of GRKs in the cell regulate formation of the arrestin-receptor complex and thereby G protein-independent signaling.

Advances in receptor conformation research: the quest for functionally selective conformations focusing on the β2-adrenoceptor

Seven-transmembrane receptors, also called GPCRs, represent the largest class of drug targets. Upon ligand binding, a GPCR undergoes conformational rearrangement and thereby changes its interaction with effector proteins including the cognate G-proteins and the multifunctional adaptor proteins, β-arrestins. These proteins, by initiating distinct signal transduction mechanisms, mediate one or several functional responses. Recently, the concept of ligand-directed GPCR signalling, also called functional selectivity or biased agonism, has been proposed to explain the phenomenon that chemically diverse ligands exhibit different efficacies towards the different signalling pathways of a single GPCR, and thereby act as functionally selective or 'biased' ligands. Current concepts support the notion that ligand-specific GPCR conformations are the basis of ligand-directed signalling. Multiple studies using fluorescence spectroscopy, X-ray crystallography, mass spectroscopy, nuclear magnetic resonance spectroscopy, single-molecule force spectroscopy and other techniques have provided the evidence to support this notion. It is anticipated that these techniques will ultimately help elucidate the structural basis of ligand-directed GPCR signalling at a precision meaningful for structure-based drug design and how a specific ligand molecular structure induces a unique receptor conformation leading to biased signalling. In this review, we will summarize recent advances in experimental techniques applied in the study of functionally selective GPCR conformations and breakthrough data obtained in these studies particularly those of the β2-adrenoceptor.

Molecular mechanism of phosphorylation-dependent arrestin activation

The past years have seen tremendous progress towards understanding how arrestins recognize phosphorylated G protein-coupled receptors (GPCRs). Two arrestin crystal structures, one of a pre-activated splice variant and one bound to a GPCR phosphopeptide, provided insights into the conformational changes upon phosphate recognition. Scanning mutagenesis and spectroscopic studies complete the picture of arrestin activation and receptor binding. Most perspicuous is the C-tail exchange mechanism, by which the C-tail of arrestin is released from its basal conformation and replaced by the phosphorylated GPCR C-terminus. Three positively charged clusters could act as conserved arrestin phosphosensors. Variations in the pattern of phosphorylation in a GPCR and variations within the C-terminus of different GPCRs may encode specificity to arrestin subtypes and particular physiological responses.

Engineered hyperphosphorylation of the β2-adrenoceptor prolongs arrestin-3 binding and induces arrestin internalization

G protein-coupled receptor phosphorylation plays a major role in receptor desensitization and arrestin binding. It is, however, unclear how distinct receptor phosphorylation patterns may influence arrestin binding and subsequent trafficking. Here we engineer phosphorylation sites into the C-terminal tail of the β2-adrenoceptor (β2AR) and demonstrate that this mutant, termed β2AR(SSS), showed increased isoprenaline-stimulated phosphorylation and differences in arrestin-3 affinity and trafficking. By measuring arrestin-3 recruitment and the stability of arrestin-3 receptor complexes in real time using fluorescence resonance energy transfer and fluorescence recovery after photobleaching, we demonstrate that arrestin-3 dissociated quickly and almost completely from the β2AR, whereas the interaction with β2AR(SSS) was 2- to 4-fold prolonged. In contrast, arrestin-3 interaction with a β2-adrenoceptor fused to the carboxyl-terminal tail of the vasopressin type 2 receptor was nearly irreversible. Further analysis of arrestin-3 localization revealed that by engineering phosphorylation sites into the β2-adrenoceptor the receptor showed prolonged interaction with arrestin-3 and colocalization with arrestin in endosomes after internalization. This is in contrast to the wild-type receptor that interacts transiently with arrestin-3 at the plasma membrane. Furthermore, β2AR(SSS) internalized more efficiently than the wild-type receptor, whereas recycling was very similar for both receptors. Thus, we show how the interaction between arrestins and receptors can be increased with minimal receptor modification and that relatively modest increases in receptor-arrestin affinity are sufficient to alter arrestin trafficking.

Loss of morphine reward and dependence in mice lacking G protein-coupled receptor kinase 5

BACKGROUND

The clinical benefits of opioid drugs are counteracted by the development of tolerance and addiction. We provide in vivo evidence for the involvement of G protein-coupled receptor kinases (GRKs) in opioid dependence in addition to their roles in agonist-selective mu-opioid receptor (MOR) phosphorylation.

METHODS

In vivo MOR phosphorylation was examined by immunoprecipitation and nanoflow liquid chromatography-tandem mass spectrometry analysis. Using the hot-plate and conditioned place preference test, we investigated opioid-related antinociception and reward effects in mice lacking GRK3 or GRK5.

RESULTS

Etonitazene and fentanyl stimulated the in vivo phosphorylation of multiple carboxyl-terminal phosphate acceptor sites, including threonine 370, serine 375, and threonine 379, which was predominantly mediated by GRK3. By contrast, morphine promoted a selective phosphorylation of serine 375 that was predominantly mediated by GRK5. In contrast to GRK3 knockout mice, GRK5 knockout mice exhibited reduced antinociceptive responses after morphine administration and developed morphine tolerance similar to wild-type mice but fewer signs of physical dependence. Also, morphine was ineffective in inducing conditioned place preference in GRK5 knockout mice, whereas cocaine conditioned place preference was retained. However, the reward properties of morphine were evident in knock-in mice expressing a phosphorylation-deficient S375A mutation of the MOR.

CONCLUSIONS

These findings show for the first time that MOR phosphorylation is regulated by agonist-selective recruitment of distinct GRK isoforms that influence different opioid-related behaviors. Modulation of GRK5 function could serve as a new approach for preventing addiction to opioids, while maintaining the analgesic properties of opioid drugs at an effective level.

Increased G protein-coupled receptor kinase (GRK) expression in the anterior cingulate cortex in schizophrenia

BACKGROUND

Current pharmacological treatments for schizophrenia target G protein-coupled receptors (GPCRs), including dopamine receptors. Ligand-bound GPCRs are regulated by a family of G protein-coupled receptor kinases (GRKs), members of which uncouple the receptor from heterotrimeric G proteins, desensitize the receptor, and induce receptor internalization via the arrestin family of scaffolding and signaling molecules. GRKs initiate the activation of downstream signaling pathways, can regulate receptors and signaling molecules independent of GPCR phosphorylation, and modulate epigenetic regulators like histone deacetylases (HDACs). We hypothesize that the expression of GRK proteins is altered in schizophrenia, consistent with previous findings of alterations upstream and downstream from this family of molecules that facilitate intracellular signaling processes.

METHODS

In this study, we measured protein expression via Western blot analysis for GRKs 2, 3, 5, and 6 in the anterior cingulate cortex of patients with schizophrenia (n=36) and a comparison group (n=33). To control for antipsychotic treatment, we measured these same targets in haloperidol-treated vs. untreated rats (n=10 for both).

RESULTS

We found increased levels of GRK5 in schizophrenia. No changes were detected in GRK protein expression in rats treated with haloperidol decanoate for 9 months.

CONCLUSION

These data suggest that increased GRK5 expression may contribute to the pathophysiology of schizophrenia via abnormal regulation of the cytoskeleton, endocytosis, signaling, GPCRs, and histone modification.

Overlapping and opposing functions of G protein-coupled receptor kinase 2 (GRK2) and GRK5 during heart development

G protein-coupled receptor kinases 2 (GRK2) and 5 (GRK5) are fundamental regulators of cardiac performance in adults but are less well characterized for their function in the hearts of embryos. GRK2 and -5 belong to different subfamilies and function as competitors in the control of certain receptors and signaling pathways. In this study, we used zebrafish to investigate whether the fish homologs of GRK2 and -5, Grk2/3 and Grk5, also have unique, complementary, or competitive roles during heart development. We found that they differentially regulate the heart rate of early embryos and equally facilitate heart function in older embryos and that both are required to develop proper cardiac morphology. A loss of Grk2/3 results in dilated atria and hypoplastic ventricles, and the hearts of embryos depleted in Grk5 present with a generalized atrophy. This Grk5 morphant phenotype was associated with an overall decrease of early cardiac progenitors as well as a reduction in the area occupied by myocardial progenitor cells. In the case of Grk2/3, the progenitor decrease was confined to a subset of precursor cells with a committed ventricular fate. We attempted to rescue the GRK loss-of-function heart phenotypes by downstream activation of Hedgehog signaling. The Grk2/3 loss-of-function embryos were rescued by this approach, but Grk5 embryos failed to respond. In summary, we found that GRK2 and GRK5 control cardiac function as well as morphogenesis during development although with different morphological outcomes.

Developmental expression of GPR3 in rodent cerebellar granule neurons is associated with cell survival and protects neurons from various apoptotic stimuli

G-protein coupled receptor 3 (GPR3), GPR6, and GPR12 belong to a family of constitutively active Gs-coupled receptors that activate 3'-5'-cyclic adenosine monophosphate (cAMP) and are highly expressed in the brain. Among these receptors, the endogenous expression of GPR3 in cerebellar granule neurons (CGNs) is increased following development. GPR3 is important for neurite outgrowth and neural maturation; however, the physiological functions of GPR3 remain to be fully elucidated. Here, we investigated the survival and antiapoptotic functions of GPR3 under normal and apoptosis-inducing culture conditions. Under normal culture conditions, CGNs from GPR3-knockout mice demonstrated lower survival than did CGNs from wild-type or GPR3-heterozygous mice. Cerebellar sections from GPR3-/- mice at P7, P14, and P21 revealed more caspase-3-positive neurons in the internal granular layer than in cerebellar sections from wild-type mice. Conversely, in a potassium-deprivation model of apoptosis, increased expression of these three receptors promoted neuronal survival. The antiapoptotic effect of GPR3 was also observed under hypoxic (1% O2/5% CO2) and reactive oxygen species (ROS)-induced apoptotic conditions. We further investigated the signaling pathways involved in the GPR3-mediated antiapoptotic effect. The addition of the PKA inhibitor KT5720, the MAP kinase inhibitor U0126, and the PI3 kinase inhibitor LY294002 abrogated the GPR3-mediated antiapoptotic effect in a potassium-deprivation model of apoptosis, whereas the PKC inhibitor Gö6976 did not affect the antiapoptotic function of GPR3. Furthermore, downregulation of endogenous GPR3 expression in CGNs resulted in a marked reduction in the basal levels of ERK and Akt phosphorylation under normal culture conditions. Finally, we used a transient middle cerebral artery occlusion (tMCAO) model in wild-type and GPR3-knockout mice to determine whether GPR3 expression modulates neuronal survival after brain ischemia. After tMCAO, GPR3-knockout mice exhibited a significantly larger infarct area than did wild-type mice. Collectively, these in vitro and in vivo results suggest that the developmental expression of constitutively active Gs-coupled GPR3 activates the ERK and Akt signaling pathways at the basal level, thereby protecting neurons from apoptosis that is induced by various stimuli.

β-Arrestin-1 mediates thyrotropin-enhanced osteoblast differentiation

Thyrotropin (TSH) activation of the TSH receptor (TSHR), a 7-transmembrane-spanning receptor (7TMR), may have osteoprotective properties by direct effects on bone. TSHR activation by TSH phosphorylates protein kinases AKT1, p38α, and ERK1/2 in some cells. We found TSH-induced phosphorylation of these kinases in 2 cell lines engineered to express TSHRs, human embryonic kidney HEK-TSHR cells and human osteoblastic U2OS-TSHR cells. In U2OS-TSHR cells, TSH up-regulated pAKT1 (7.1±0.5-fold), p38α (2.9±0.4-fold), and pERK1/2 (3.1±0.2-fold), whereas small molecule TSHR agonist C2 had no or little effect on pAKT1 (1.8±0.08-fold), p38α (1.2±0.09-fold), and pERK1/2 (1.6±0.19-fold). Furthermore, TSH increased expression of osteoblast marker genes ALPL (8.2±4.6-fold), RANKL (21±5.9-fold), and osteopontin (OPN; 17±5.3-fold), whereas C2 had little effect (ALPL, 1.7±0.5-fold; RANKL, 1.3±0.6-fold; and OPN, 2.2±0.7-fold). β-Arrestin-1 and -2 can mediate activatory signals by 7TMRs. TSH stimulated translocation of β-arrestin-1 and -2 to TSHR, whereas C2 failed to translocate either β-arrestin. Down-regulation of β-arrestin-1 by siRNA inhibited TSH-stimulated phosphorylation of ERK1/2, p38α, and AKT1, whereas down-regulation of β-arrestin-2 increased phosphorylation of AKT1 in both cell types and of ERK1/2 in HEK-TSHR cells. Knockdown of β-arrestin-1 inhibited TSH-stimulated up-regulation of mRNAs for OPN by 87 ± 1.7% and RANKL by 73 ± 2.4%, and OPN secretion by 74 ± 10%. We conclude that TSH enhances osteoblast differentiation in U2OS cells that is, in part, caused by activatory signals mediated by β-arrestin-1.

Recent developments in biased agonism

The classic paradigm of G protein-coupled receptor (GPCR) activation was based on the understanding that agonist binding to a receptor induces or stabilizes a conformational change to an 'active' conformation. In the past decade, however, it has been appreciated that ligands can induce distinct 'active' receptor conformations with unique downstream functional signaling profiles. Building on the initial recognition of the existence of such 'biased ligands', recent years have witnessed significant developments in several areas of GPCR biology. These include increased understanding of structural and biophysical mechanisms underlying biased agonism, improvements in characterization and quantification of ligand efficacy, as well as clinical development of these novel ligands. Here we review recent major developments in these areas over the past several years.

Distinct pathways of ERK1/2 activation by hydroxy-carboxylic acid receptor-1

Mechanistic investigations have shown that, upon agonist activation, hydroxy-carboxylic acid receptor-1(HCA1) couples to a Gi protein and inhibits adenylate cyclase activity, leading to inhibition of liberation of free fatty acid. However, the underlying molecular mechanisms for HCA1 signaling remain largely unknown. Using CHO-K1 cells stably expressing HCA1, and L6 cells, which endogenously express rat HCA1 receptors, we found that activation of ERK1/2 by HCA1 was rapid, peaking at 5 min, and was significantly blocked by pertussis toxin. Furthermore, time course experiments with different kinase inhibitors demonstrated that HCA1 induced ERK1/2 activation via the extracellular Ca2+, PKC and IGF-I receptor transactivation-dependent pathways. In addition, we observed that pretreated the cells with M119K, an inhibitor of Gβγ subunit-dependent signaling, effectively attenuated the ERK1/2 activation triggered by HCA1, suggesting a critical role for βγ-subunits in HCA1-activated ERK1/2 phosphorylation. Furthermore, the present results also indicated that the arrestin2/3 were not required for ERK1/2 activation. In conclusion, our findings demonstrate that upon binding to agonist, HCA1 receptors initially activate Gi, leading to dissociation of the Gβγ subunit from activated Gi, and subsequently induce ERK1/2 activation via two distinct pathways: one PKC-dependent pathway and the other IGF-IR transactivation-dependent pathway. Our results provide the first in-depth evidence that defines the molecular mechanism of HCA1-mediated ERK1/2 activation.

Multiple functions of G protein-coupled receptor kinases

Desensitization is a physiological feedback mechanism that blocks detrimental effects of persistent stimulation. G protein-coupled receptor kinase 2 (GRK2) was originally identified as the kinase that mediates G protein-coupled receptor (GPCR) desensitization. Subsequent studies revealed that GRK is a family composed of seven isoforms (GRK1–GRK7). Each GRK shows a differential expression pattern. GRK1, GRK4, and GRK7 are expressed in limited tissues. In contrast, GRK2, GRK3, GRK5, and GRK6 are ubiquitously expressed throughout the body. The roles of GRKs in GPCR desensitization are well established. When GPCRs are activated by their agonists, GRKs phosphorylate serine/threonine residues in the intracellular loops and the carboxyl-termini of GPCRs. Phosphorylation promotes translocation of β-arrestins to the receptors and inhibits further G protein activation by interrupting receptor-G protein coupling. The binding of β-arrestins to the receptors also helps to promote receptor internalization by clathrin-coated pits. Thus, the GRK-catalyzed phosphorylation and subsequent binding of β-arrestin to GPCRs are believed to be the common mechanism of GPCR desensitization and internalization. Recent studies have revealed that GRKs are also involved in the β-arrestin-mediated signaling pathway. The GRK-mediated phosphorylation of the receptors plays opposite roles in conventional G protein- and β-arrestin-mediated signaling. The GRK-catalyzed phosphorylation of the receptors results in decreased G protein-mediated signaling, but it is necessary for β-arrestin-mediated signaling. Agonists that selectively activate GRK/β-arrestin-dependent signaling without affecting G protein signaling are known as β-arrestin-biased agonists. Biased agonists are expected to have potential therapeutic benefits for various diseases due to their selective activation of favorable physiological responses or avoidance of the side effects of drugs. Furthermore, GRKs are recognized as signaling mediators that are independent of either G protein- or β-arrestin-mediated pathways. GRKs can phosphorylate non-GPCR substrates, and this is found to be involved in various physiological responses, such as cell motility, development, and inflammation. In addition to these effects, our group revealed that GRK6 expressed in macrophages mediates the removal of apoptotic cells (engulfment) in a kinase activity-dependent manner. These studies revealed that GRKs block excess stimulus and also induce cellular responses. Here, we summarized the involvement of GRKs in β-arrestin-mediated and G protein-independent signaling pathways.

G protein-coupled receptor kinase 2 moderates recruitment of THP-1 cells to the endothelium by limiting histamine-invoked Weibel-Palade body exocytosis

BACKGROUND

G protein-coupled receptors (GPCRs) are a major family of signaling molecules, central to the regulation of inflammatory responses. Their activation upon agonist binding is attenuated by GPCR kinases (GRKs), which desensitize the receptors through phosphorylation. G protein-coupled receptor kinase 2(GRK2) down-regulation in leukocytes has been closely linked to the progression of chronic inflammatory disorders such as rheumatoid arthritis and multiple sclerosis. Because leukocytes must interact with the endothelium to infiltrate inflamed tissues, we hypothesized that GRK2 down-regulation in endothelial cells would also be pro-inflammatory.

OBJECTIVES

To determine whether GRK2 down-regulation in endothelial cells is pro-inflammatory.

METHODS

siRNA-mediated ablation of GRK2 in human umbilical vein endothelial cells (HUVECs) was used in analyses of the role of this kinase. Microscopic and biochemical analyses of Weibel-Palade body (WPB) formation and functioning, live cell imaging of calcium concentrations and video analyses of adhesion of monocyte-like THP-1 cells provide clear evidence of GRK2 function in histamine activation of endothelial cells.

RESULTS

G protein-coupled receptor kinase 2 depletion in HUVECs increases WPB exocytosis and P-selectin-dependent adhesion of THP-1 cells to the endothelial surface upon histamine stimulation, relative to controls. Further, live imaging of intracellular calcium concentrations reveals amplified histamine receptor signaling in GRK2-depleted cells, suggesting GRK2 moderates WPB exocytosis through receptor desensitization.

CONCLUSIONS

G protein-coupled receptor kinase 2 deficiency in endothelial cells results in increased pro-inflammatory signaling and enhanced leukocyte recruitment to activated endothelial cells. The ability of GRK2 to modulate initiation of inflammatory responses in endothelial cells as well as leukocytes now places GRK2 at the apex of control of this finely balanced process.

Biased signalling in follicle stimulating hormone action

Follicle-stimulating hormone (FSH) plays a crucial role in the control of reproduction by specifically binding to and activating a membrane receptor (FSHR) that belongs to the G protein-coupled receptor (GPCR) family. Similar to all GPCRs, FSHR activation mechanisms have generally been viewed as a two-state process connecting a unique FSH-bound active receptor to the Gs/cAMP pathway. Over the last decade, paralleling the breakthroughs that were made in the GPCR field, our understanding of FSH actions at the molecular level has dramatically changed. There are numerous facts indicating that the active FSHR is connected to a complex signalling network rather than the sole Gs/cAMP pathway. Consistently, the FSHR probably exists in equilibrium between multiple conformers, a subset of them being stabilized upon ligand binding. Importantly, the nature of the stabilized conformers of the receptor directly depends on the chemical structure of the ligand bound. This implies that it is possible to selectively control the intracellular signalling pathways activated by using biased ligands. Such biased ligands can be of different nature: small chemical molecules, glycosylation variants of the hormone or antibody/hormone complexes. Likewise, mutations or polymorphisms affecting the FSHR can also lead to stabilization of preferential conformers, hence to selective modulation of signalling pathways. These emerging notions offer a new conceptual framework that could potentially lead to the development of more specific drugs while also improving the way FSHR mutants/variants are functionally characterized.

Osmoregulation, vasopressin, and cAMP signaling in autosomal dominant polycystic kidney disease

PURPOSE OF REVIEW

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited nephropathy. This review will focus on the vasopressin and 3'-5'-cyclic adenosine monophosphate (cAMP) signaling pathways in ADPKD and will discuss how these insights offer new possibilities for the follow-up and treatment of the disease.

RECENT FINDINGS

Defective osmoregulation is an early manifestation of ADPKD and originates from both peripheral (renal effect of vasopressin) and central (release of vasopressin) components. Copeptin, which is released from the vasopressin precursor, may identify ADPKD patients at risk for rapid disease progression. Increased levels of cAMP in tubular cells, reflecting modifications in intracellular calcium homeostasis and abnormal stimulation of the vasopressin V2 receptor (V2R), play a central role in cystogenesis. Blocking the V2R lowers cAMP in cystic tissues, slows renal cystic progression and improves renal function in preclinical models. A phase III clinical trial investigating the effect of the V2R antagonist tolvaptan in ADPKD patients has shown that this treatment blunts kidney growth, reduces associated symptoms and slows kidney function decline when given over 3 years.

SUMMARY

These advances open perspectives for the understanding of cystogenesis in ADPKD, the mechanisms of osmoregulation, the role of polycystins in the brain, and the pleiotropic action of vasopressin.

Mechanisms of homologous and heterologous phosphorylation of FFA receptor 4 (GPR120): GRK6 and PKC mediate phosphorylation of Thr³⁴⁷, Ser³⁵⁰, and Ser³⁵⁷ in the C-terminal tail

Free fatty acid receptor 4 (FFA4), previously known as GPR120, is a G protein-coupled receptor that promotes numerous anti-inflammatory and antidiabetic effects upon its agonism by long chained unsaturated fatty acids. We have previously demonstrated that agonism of FFA4 with docosahexaenoic acid (DHA) and alpha-linoleic acid (ALA) facilitates rapid and transient phosphorylation of FFA4 expressed ectopically on the surface of HEK293 cells. However, the precise mechanisms that promote FFA4 phosphorylation remain elusive. In the current study, we examined the mechanisms behind both heterologous and homologous phosphorylation of FFA4 and set out to identify the foci of FFA4 phosphorylation. Our results demonstrate that basal and heterologous phosphorylation of FFA4 are mediated by protein kinase C (PKC), while G protein-coupled receptor kinase 6 (GRK6) plays the predominant role in DHA-mediated phosphorylation of FFA4. Furthermore, we identify Thr(347), Ser(350), and Ser(357) in the C-terminal tail as major sites of FFA4 phosphorylation. Concurrent mutation of these three sites leads to a FFA4 receptor that seemingly affects Gαq/11 signaling in a positive manner as demonstrated by heightened intracellular Ca(2+) responses following agonism with DHA. Importantly, this phosphodefective FFA4 mutant lacked the ability to promote β-arrestin-2 recruitment to the cell membrane. Since many of the functionally beneficial physiological effects of FFA4 are noted to be β-arrestin mediated, these findings could provide insight into the structural requirements for FFA4 function.

Arrestin-dependent activation of ERK and Src family kinases

The four members of the mammalian arrestin family, two visual and two nonvisual, share the property of stimulus-dependent docking to G protein-coupled receptors. This conformational selectivity permits them to function in receptor desensitization, as arrestin binding sterically inhibits G protein coupling. The two nonvisual arrestins further act as adapter proteins, linking receptors to the clathrin-dependent endocytic machinery and regulating receptor sequestration, intracellular trafficking, recycling, and degradation. Arrestins also function as ligand-regulated scaffolds, recruiting catalytically active proteins into receptor-based multiprotein "signalsome" complexes. Arrestin binding thus marks the transition from a transient G protein-coupled state on the plasma membrane to a persistent arrestin-coupled state that continues to signal as the receptor internalizes. Two of the earliest discovered and most studied arrestin-dependent signaling pathways involve regulation of Src family nonreceptor tyrosine kinases and the ERK1/2 mitogen-activated kinase cascade. In each case, arrestin scaffolding imposes constraints on kinase activity that dictate signal duration and substrate specificity. Evidence suggests that arrestin-bound ERK1/2 and Src not only play regulatory roles in receptor desensitization and trafficking but also mediate longer term effects on cell growth, migration, proliferation, and survival.

Lentivirus vector-mediated Rho guanine nucleotide dissociation inhibitor 2 overexpression induces beta-2 adrenergic receptor desensitization in airway smooth muscle cells

BACKGROUND

Beta-2 adrenergic receptor (β2AR) downregulation is critical to asthma rescue therapy; however, tolerance, also known as β2AR or bronchodilator desensitization, mechanisms potentially resulting in life-threatening rescue treatment failure remain poorly understood.

METHODS

Airway smooth muscle cells (ASMCs) from BALB/c mice were primarily cultured. The full-length Rho guanine nucleotide dissociation inhibitor 2 (RhoGDI2) gene from ASMCs was amplified by RT-PCR, and RhoGDI2 gene was subcloned into the digested PWPXL plasmid. The recombinant lentivirus PWPXL-RhoGDI2 expression plasmid was packaged into mature lentivirus by 293T cells and used to infect ASMCs. Fluorescent quantitation RT-PCR and Western Blot were used to detect the level of mRNA and protein expression of RhoGDI2, β2AR, guanine nucleotide exchange factor (GEF), GTPase-activating protein (GAP) and G protein-coupled receptor kinases (GRKs) in overexpression RhoGDI2-ASMCs group, negative GFP control ASMCs group and normal control ASMCs group. Membrane receptor numbers of β2AR was observed by radioligand receptor binding assay in overexpression RhoGDI2-ASMCs group, negative GFP control ASMCs group and normal control ASMCs group.

RESULTS

RhoGDI2 vector successfully transfected ASMCs, with infection efficiency (the percentage of GFP-positive cells) >80%. RhoGDI2, GEF and G-protein-coupled receptor kinase 2 (GRK2) expressions significantly increased in the RhoGDI2 overexpression group compared to control and negative control groups (all P<0.05). Conversely, β2AR and GAP expressions were significantly lower in the RhoGDI2 overexpression group (both P<0.05), exhibiting an inverse correlation with RhoGDI2 expression. Control and negative control groups exhibiting β2AR density more than 2-fold higher than that observed in the RhoGDI2 overexpression group.

CONCLUSIONS

RhoGDI2 reduces β2AR density, potentially by reducing β2AR and GAP expressions and increase GEF and GRK2 expressions. Thus, RhoGDI2 is central in cellular β2AR desensitization, though this full mechanism and intermediates merit further investigation.

β-Arrestin regulation of myosin light chain phosphorylation promotes AT1aR-mediated cell contraction and migration

Over the last decade, it has been established that G-protein-coupled receptors (GPCRs) signal not only through canonical G-protein-mediated mechanisms, but also through the ubiquitous cellular scaffolds β-arrestin-1 and β-arrestin-2. Previous studies have implicated β-arrestins as regulators of actin reorganization in response to GPCR stimulation while also being required for membrane protrusion events that accompany cellular motility. One of the most critical events in the active movement of cells is the cyclic phosphorylation and activation of myosin light chain (MLC), which is required for cellular contraction and movement. We have identified the myosin light chain phosphatase Targeting Subunit (MYPT-1) as a binding partner of the β-arrestins and found that β-arrestins play a role in regulating the turnover of phosphorylated myosin light chain. In response to stimulation of the angiotensin Type 1a Receptor (AT1aR), MLC phosphorylation is induced quickly and potently. We have found that β-arrestin-2 facilitates dephosphorylation of MLC, while, in a reciprocal fashion, β-arrestin 1 limits dephosphorylation of MLC. Intriguingly, loss of either β-arrestin-1 or 2 blocks phospho-MLC turnover and causes a decrease in the contraction of cells as monitored by atomic force microscopy (AFM). Furthermore, by employing the β-arrestin biased ligand [Sar¹,Ile⁴,Ile⁸]-Ang, we demonstrate that AT1aR-mediated cellular motility involves a β-arrestin dependent component. This suggests that the reciprocal regulation of MLC phosphorylation status by β-arrestins-1 and 2 causes turnover in the phosphorylation status of MLC that is required for cell contractility and subsequent chemotaxic motility.

Selective regulation of recombinantly expressed mGlu7 metabotropic glutamate receptors by G protein-coupled receptor kinases and arrestins

mGlu7 receptors are coupled to Gi/Go-proteins and activate multiple transduction pathways, including inhibition of adenylyl cyclase activity and stimulation of ERK1/2 and JNK pathways. mGlu7 receptors play an important role in cognition and emotion and are involved in stress-related disorders such as anxiety and depression and in susceptibility to convulsive seizures. In spite of these potential clinical implications, little is known on the mechanisms that regulate mGlu7-receptor signaling. Here we show that mGlu7 receptor-dependent signaling pathways were regulated in a complementary manner by different GRK subtypes, with GRK4 affecting the adenylyl cyclase and the JNK pathways, and GRK2 selectively affecting the ERK1/2 pathway. Additionally we found that the two isoforms of non-visual arrestins, i.e. β-arrestin1 and β-arrestin2, exerted opposite effects on mGlu7-receptor signaling, with β-arrestin1 positively modulating ERK1/2 and inhibiting JNK, and β-arrestin2 doing the opposite. This represents a remarkable example of "reciprocal regulation" of receptor signaling by the two isoforms of β-arrestin. Finally we found that β-arrestin1 amplified mGlu7 receptor-dependent ERK1/2 activation in response to L-AP4 (an orthosteric agonist), but not in response to AMN082 (an atypical mGlu7-receptor allosteric agonist). The different effect of β-arrestin1 on L-AP4- and AMN082-stimulated ERK1/2 phosphorylation is in line with the emerging concept of β-arrestin-biased agonists. The present study may open new perspectives in elucidating the physio-pathological roles of the mGlu7 receptor and may provide new insights for the possibility to develop specific (biased) agonists that can selectively activate different signaling pathways.

Mutation of light-dependent phosphorylation sites of the Drosophila transient receptor potential-like (TRPL) ion channel affects its subcellular localization and stability

The Drosophila phototransduction cascade terminates in the opening of the ion channel transient receptor potential (TRP) and TRP-like (TRPL). Contrary to TRP, TRPL undergoes light-dependent subcellular trafficking between rhabdomeric photoreceptor membranes and an intracellular storage compartment, resulting in long term light adaptation. Here, we identified in vivo phosphorylation sites of TRPL that affect TRPL stability and localization. Quantitative mass spectrometry revealed a light-dependent change in the TRPL phosphorylation pattern. Mutation of eight C-terminal phosphorylation sites neither affected multimerization of the channels nor the electrophysiological response of flies expressing the mutated channels. However, these mutations resulted in mislocalization and enhanced degradation of TRPL after prolonged dark-adaptation. Mutation of subsets of the eight C-terminal phosphorylation sites also led to a reduction of TRPL content and partial mislocalization in the dark. This suggests that a light-dependent switch in the phosphorylation pattern of the TRPL channel mediates stable expression of TRPL in the rhabdomeres upon prolonged dark-adaptation.

G-protein receptor kinase 5 regulates the cannabinoid receptor 2-induced up-regulation of serotonin 2A receptors

We have recently reported that cannabinoid agonists can up-regulate and enhance the activity of serotonin 2A (5-HT2A) receptors in the prefrontal cortex (PFCx). Increased expression and activity of cortical 5-HT2A receptors has been associated with neuropsychiatric disorders, such as anxiety and schizophrenia. Here we report that repeated CP55940 exposure selectively up-regulates GRK5 proteins in rat PFCx and in a neuronal cell culture model. We sought to examine the mechanism underlying the regulation of GRK5 and to identify the role of GRK5 in the cannabinoid agonist-induced up-regulation and enhanced activity of 5-HT2A receptors. Interestingly, we found that cannabinoid agonist-induced up-regulation of GRK5 involves CB2 receptors, β-arrestin 2, and ERK1/2 signaling because treatment with CB2 shRNA lentiviral particles, β-arrestin 2 shRNA lentiviral particles, or ERK1/2 inhibitor prevented the cannabinoid agonist-induced up-regulation of GRK5. Most importantly, we found that GRK5 shRNA lentiviral particle treatment prevented the cannabinoid agonist-induced up-regulation and enhanced 5-HT2A receptor-mediated calcium release. Repeated cannabinoid exposure was also associated with enhanced phosphorylation of CB2 receptors and increased interaction between β-arrestin 2 and ERK1/2. These latter phenomena were also significantly inhibited by GRK5 shRNA lentiviral treatment. Our results suggest that sustained activation of CB2 receptors, which up-regulates 5-HT2A receptor signaling, enhances GRK5 expression; the phosphorylation of CB2 receptors; and the β-arrestin 2/ERK interactions. These data could provide a rationale for some of the adverse effects associated with repeated cannabinoid agonist exposure.

Distinct roles of β-arrestin 1 and β-arrestin 2 in ORG27569-induced biased signaling and internalization of the cannabinoid receptor 1 (CB1)

The cannabinoid receptor 1 (CB1) is a G protein-coupled receptor primarily expressed in brain tissue that has been implicated in several disease states. CB1 allosteric compounds, such as ORG27569, offer enormous potential as drugs over orthosteric ligands, but their mechanistic, structural, and downstream effects upon receptor binding have not been established. Previously, we showed that ORG27569 enhances agonist binding affinity to CB1 but inhibits G protein-dependent agonist signaling efficacy in HEK293 cells and rat brain expressing the CB1 receptor (Ahn, K. H., Mahmoud, M. M., and Kendall, D. A. (2012) J. Biol. Chem. 287, 12070-12082). Here, we identify the mediators of CB1 receptor internalization and ORG27569-induced G protein-independent signaling. Using siRNA technology, we elucidate an ORG27569-induced signaling mechanism for CB1 wherein β-arrestin 1 mediates short term signaling to ERK1/2 with a peak at 5 min and other upstream kinase components including MEK1/2 and c-Src. Consistent with these findings, we demonstrate co-localization of CB1-GFP with red fluorescent protein-β-arrestin 1 upon ORG27569 treatment using confocal microscopy. In contrast, we show the critical role of β-arrestin 2 in CB1 receptor internalization upon treatment with CP55940 (agonist) or treatment with ORG27569. These results demonstrate for the first time the involvement of β-arrestin in CB1-biased signaling by a CB1 allosteric modulator and also define the differential role of the two β-arrestin isoforms in CB1 signaling and internalization.

Arrestins as regulators of kinases and phosphatases

The discovery that, in addition to mediating G protein-coupled receptor (GPCR) desensitization and endocytosis, arrestins bind to diverse catalytically active nonreceptor proteins and act as ligand-regulated signaling scaffolds led to a paradigm shift in the study of GPCR signal transduction. Research over the past decade has solidified the concept that arrestins confer novel GPCR-signaling capacity by recruiting protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into receptor-based multiprotein "signalsome" complexes. Signalsomes regulate downstream pathways controlled by Src family nonreceptor tyrosine kinases, mitogen-activated protein kinases, protein kinase B (AKT), glycogen synthase kinase 3, protein phosphatase 2A, nuclear factor-κB, and several others, imposing spatial and temporal control on their function. While many arrestin-bound kinases and phosphatases are involved in the control of cytoskeletal rearrangement, vesicle endocytosis, exocytosis, and cell migration, other signals reach into the nucleus, affecting cell proliferation, apoptosis, and survival. Indeed, the kinase/phosphatase network regulated by arrestins may be fully as diverse as that regulated by heterotrimeric G proteins.

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

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

Involvement of distinct arrestin-1 elements in binding to different functional forms of rhodopsin

Solution NMR spectroscopy of labeled arrestin-1 was used to explore its interactions with dark-state phosphorylated rhodopsin (P-Rh), phosphorylated opsin (P-opsin), unphosphorylated light-activated rhodopsin (Rh*), and phosphorylated light-activated rhodopsin (P-Rh*). Distinct sets of arrestin-1 elements were seen to be engaged by Rh* and inactive P-Rh, which induced conformational changes that differed from those triggered by binding of P-Rh*. Although arrestin-1 affinity for Rh* was seen to be low (K(D) > 150 μM), its affinity for P-Rh (K(D) ~80 μM) was comparable to the concentration of active monomeric arrestin-1 in the outer segment, suggesting that P-Rh generated by high-gain phosphorylation is occupied by arrestin-1 under physiological conditions and will not signal upon photo-activation. Arrestin-1 was seen to bind P-Rh* and P-opsin with fairly high affinity (K(D) of~50 and 800 nM, respectively), implying that arrestin-1 dissociation is triggered only upon P-opsin regeneration with 11-cis-retinal, precluding noise generated by opsin activity. Based on their observed affinity for arrestin-1, P-opsin and inactive P-Rh very likely affect the physiological monomer-dimer-tetramer equilibrium of arrestin-1, and should therefore be taken into account when modeling photoreceptor function. The data also suggested that complex formation with either P-Rh* or P-opsin results in a global transition in the conformation of arrestin-1, possibly to a dynamic molten globule-like structure. We hypothesize that this transition contributes to the mechanism that triggers preferential interactions of several signaling proteins with receptor-activated arrestins.

Desensitization, trafficking, and resensitization of the pituitary thyrotropin-releasing hormone receptor

The pituitary receptor for thyrotropin-releasing hormone (TRH) is a calcium-mobilizing G protein-coupled receptor (GPCR) that signals through Gq/11, elevating calcium, and activating protein kinase C. TRH receptor signaling is quickly desensitized as a consequence of receptor phosphorylation, arrestin binding, and internalization. Following activation, TRH receptors are phosphorylated at multiple Ser/Thr residues in the cytoplasmic tail. Phosphorylation catalyzed by GPCR kinase 2 (GRK2) takes place rapidly, reaching a maximum within seconds. Arrestins bind to two phosphorylated regions, but only arrestin bound to the proximal region causes desensitization and internalization. Phosphorylation at Thr365 is critical for these responses. TRH receptors internalize in clathrin-coated vesicles with bound arrestin. Following endocytosis, vesicles containing phosphorylated TRH receptors soon merge with rab5-positive vesicles. Over approximately 20 min these form larger endosomes rich in rab4 and rab5, early sorting endosomes. After TRH is removed from the medium, dephosphorylated receptors start to accumulate in rab4-positive, rab5-negative recycling endosomes. The mechanisms responsible for sorting dephosphorylated receptors to recycling endosomes are unknown. TRH receptors from internal pools help repopulate the plasma membrane. Dephosphorylation of TRH receptors begins when TRH is removed from the medium regardless of receptor localization, although dephosphorylation is fastest when the receptor is on the plasma membrane. Protein phosphatase 1 is involved in dephosphorylation but the details of how the enzyme is targeted to the receptor remain obscure. It is likely that future studies will identify biased ligands for the TRH receptor, novel arrestin-dependent signaling pathways, mechanisms responsible for targeting kinases and phosphatases to the receptor, and principles governing receptor trafficking.

Activated human hydroxy-carboxylic acid receptor-3 signals to MAP kinase cascades via the PLC-dependent PKC and MMP-mediated EGFR pathways

BACKGROUND AND PURPOSE 3-Hydroxy-octanoate, recently identified as a ligand for, the orphan GPCR, HCA(3), is of particular interest given its ability to treat lipid disorders and atherosclerosis. Here we demonstrate the pathway of HCA(3)-mediated activation of ERK1/2. EXPERIMENTAL APPROACH Using CHO-K1 cells stably expressing HCA(3) receptors and A431 cells, a human epidermoid cell line with high levels of endogenous expression of functional HCA(3) receptors, HCA(3)-mediated activation of ERK1/2 was measured by Western blot. KEY RESULTS HCA(3)-mediated activation of ERK1/2 was rapid, peaking at 5 min, and was Pertussis toxin sensitive. Our data, obtained by time course analyses in combination with different kinase inhibitors, demonstrated that on agonist stimulation, HCA(3) receptors evoked ERK1/2 activation via two distinct pathways, the PLC/PKC pathway at early time points (≤ 2 min) and the MMP/ epidermal growth factor receptor (EGFR) transactivation pathway with a maximum response at 5 min. Furthermore, our present results also indicated that the βγ-subunits of the G(i) protein play a critical role in HCA(3)-activated ERK1/2 phosphorylation, whereas β-arrestins and Src were not required for ERK1/2 activation. CONCLUSIONS AND IMPLICATIONS We have described the molecular mechanisms underlying the coupling of human HCA(3) receptors to the ERK1/2 MAP kinase pathway in CHO-K1 and A431 cells, which implicate the G(i) protein-initiated, PLC/PKC -and platelet-derived growth factor receptor/EGFR transactivation-dependent pathways. These observations may provide new insights into the pharmacological effects and the physiological functions modulated by the HCA(3)-mediated activation of ERK1/2.

Induction of cardiac fibrosis by β-blocker in G protein-independent and G protein-coupled receptor kinase 5/β-arrestin2-dependent Signaling pathways

G-protein coupled receptors (GPCRs) have long been known as receptors that activate G protein-dependent cellular signaling pathways. In addition to the G protein-dependent pathways, recent reports have revealed that several ligands called "biased ligands" elicit G protein-independent and β-arrestin-dependent signaling through GPCRs (biased agonism). Several β-blockers are known as biased ligands. All β-blockers inhibit the binding of agonists to the β-adrenergic receptors. In addition to β-blocking action, some β-blockers are reported to induce cellular responses through G protein-independent and β-arrestin-dependent signaling pathways. However, the physiological significance induced by the β-arrestin-dependent pathway remains much to be clarified in vivo. Here, we demonstrate that metoprolol, a β(1)-adrenergic receptor-selective blocker, could induce cardiac fibrosis through a G protein-independent and β-arrestin2-dependent pathway. Metoprolol, a β-blocker, increased the expression of fibrotic genes responsible for cardiac fibrosis in cardiomyocytes. Furthermore, metoprolol induced the interaction between β(1)-adrenergic receptor and β-arrestin2, but not β-arrestin1. The interaction between β(1)-adrenergic receptor and β-arrestin2 by metoprolol was impaired in the G protein-coupled receptor kinase 5 (GRK5)-knockdown cells. Metoprolol-induced cardiac fibrosis led to cardiac dysfunction. However, the metoprolol-induced fibrosis and cardiac dysfunction were not evoked in β-arrestin2- or GRK5-knock-out mice. Thus, metoprolol is a biased ligand that selectively activates a G protein-independent and GRK5/β-arrestin2-dependent pathway, and induces cardiac fibrosis. This study demonstrates the physiological importance of biased agonism, and suggests that G protein-independent and β-arrestin-dependent signaling is a reason for the diversity of the effectiveness of β-blockers.

The chemokine receptors CXCR1 and CXCR2 couple to distinct G protein-coupled receptor kinases to mediate and regulate leukocyte functions

The chemokine receptors, CXCR1 and CXCR2, couple to Gαi to induce leukocyte recruitment and activation at sites of inflammation. Upon activation by CXCL8, these receptors become phosphorylated, desensitized, and internalized. In this study, we investigated the role of different G protein-coupled receptor kinases (GRKs) in CXCR1- and CXCR2-mediated cellular functions. To that end, short hairpin RNA was used to inhibit GRK2, 3, 5, and 6 in RBL-2H3 cells stably expressing CXCR1 or CXCR2, and CXCL8-mediated receptor activation and regulation were assessed. Inhibition of GRK2 and GRK6 increased CXCR1 and CXCR2 resistance to phosphorylation, desensitization, and internalization, respectively, and enhanced CXCL8-induced phosphoinositide hydrolysis and exocytosis in vitro. GRK2 depletion diminished CXCR1-induced ERK1/2 phosphorylation but had no effect on CXCR2-induced ERK1/2 phosphorylation. GRK6 depletion had no significant effect on CXCR1 function. However, peritoneal neutrophils from mice deficient in GRK6 (GRK6(-/-)) displayed an increase in CXCR2-mediated G protein activation but in vitro exhibited a decrease in chemotaxis, receptor desensitization, and internalization relative to wild-type (GRK6(+/+)) cells. In contrast, neutrophil recruitment in vivo in GRK6(-/-) mice was increased in response to delivery of CXCL1 through the air pouch model. In a wound-closure assay, GRK6(-/-) mice showed enhanced myeloperoxidase activity, suggesting enhanced neutrophil recruitment, and faster wound closure compared with GRK6(+/+) animals. Taken together, the results indicate that CXCR1 and CXCR2 couple to distinct GRK isoforms to mediate and regulate inflammatory responses. CXCR1 predominantly couples to GRK2, whereas CXCR2 interacts with GRK6 to negatively regulate receptor sensitization and trafficking, thus affecting cell signaling and angiogenesis.

Competing G protein-coupled receptor kinases balance G protein and β-arrestin signaling

The molecular mechanisms and hidden dynamics governing ERK activation by the angiotensin II type 1A receptor are studied and deciphered, revealing a signal balancing mechanism that is found to be relevant to a range of other seven transmembrane receptors.

An ODE-based dynamical model of ERK activation by the prototypical angiotensin II type-1A seven transmembrane receptor has been built and validated.

In order to deal with a limited number of experimental read-outs, unknown parameters have been inferred by simultaneously fitting control and perturbed conditions.

In addition to its well-established function in G-protein uncoupling, G protein-coupled receptor kinase 2 has been shown to exert a strong negative effect on β-arrestin-dependent signaling and by doing so, to balance G-protein and β-arrestin signaling.

This novel function of G protein-coupled receptor kinase 2 has also been evidenced in primary vascular smooth muscle cells naturally expressing the AT_1AR and in HEK293 cells expressing other 7TMRs.

Seven-transmembrane receptors (7TMRs) are involved in nearly all aspects of chemical communications and represent major drug targets. 7TMRs transmit their signals not only via heterotrimeric G proteins but also through β-arrestins, whose recruitment to the activated receptor is regulated by G protein-coupled receptor kinases (GRKs). In this paper, we combined experimental approaches with computational modeling to decipher the molecular mechanisms as well as the hidden dynamics governing extracellular signal-regulated kinase (ERK) activation by the angiotensin II type 1A receptor (AT_1AR) in human embryonic kidney (HEK)293 cells. We built an abstracted ordinary differential equations (ODE)-based model that captured the available knowledge and experimental data. We inferred the unknown parameters by simultaneously fitting experimental data generated in both control and perturbed conditions. We demonstrate that, in addition to its well-established function in the desensitization of G-protein activation, GRK2 exerts a strong negative effect on β-arrestin-dependent signaling through its competition with GRK5 and 6 for receptor phosphorylation. Importantly, we experimentally confirmed the validity of this novel GRK2-dependent mechanism in both primary vascular smooth muscle cells naturally expressing the AT_1AR, and HEK293 cells expressing other 7TMRs.

The complexities of G-protein-coupled receptor kinase function in Hedgehog signaling

Hedgehog (Hh) signaling is essential for proper tissue patterning and maintenance and has a substantial impact on human disease. While many of the main components and mechanisms involved in transduction of the Hh signal have been identified, the details of how the pathway functions are continually being refined. One aspect that has attracted much attention recently is the involvement of G-protein-coupled receptor kinases (GRKs) in the pathway. These regulators of G-protein-coupled receptor (GPCR) signaling have an evolutionarily-conserved function in promoting high-threshold Hh target gene expression through regulation of Smoothened (Smo), a GPCR family member that activates intracellular Hh signaling. Several models of how GRKs impact on Smo to increase downstream signaling have been proposed. Recently, we demonstrated that these kinases have surprisingly complex and conflicting roles, acting to limit signaling through the pathway while also promoting Smo activity. In addition to the previously described direct effects of Gprk2 on Smo activation, Gprk2 also indirectly affects Hh signaling by controlling production of the second messenger cyclic AMP to influence Protein kinase A activity.

Selective recruitment of G protein-coupled receptor kinases (GRKs) controls signaling of the insulin-like growth factor 1 receptor

β-Arrestins are multifunctional proteins that play central roles in G protein-coupled receptor (GPCR) trafficking and signaling. β-Arrestin1 is also recruited to the insulin-like growth factor-1 receptor (IGF-1R), a receptor tyrosine kinase (RTK), mediating receptor degradation and signaling. Because GPCR phosphorylation by GPCR-kinases (GRKs) governs interactions of the receptors with β-arrestins, we investigated the regulatory roles of the four widely expressed GRKs on IGF-1R signaling/degradation. By suppressing GRK expression with siRNA, we demonstrated that lowering GRK5/6 abolishes IGF1-mediated ERK and AKT activation, whereas GRK2 inhibition increases ERK activation and partially inhibits AKT signaling. Conversely, β-arrestin-mediated ERK signaling is enhanced by overexpression of GRK6 and diminished by GRK2. Similarly, we demonstrated opposing effects of GRK2 and -6 on IGF-1R degradation: GRK2 decreases whereas GRK6 enhances ligand-induced degradation. GRK2 and GRK6 coimmunoprecipitate with IGF-1R and increase IGF-1R serine phosphorylation, promoting β-arrestin1 association. Using immunoprecipitation, confocal microscopy, and FRET analysis, we demonstrated β-arrestin/IGF-1R association to be transient for GRK2 and stable for GRK6. Using bioinformatic studies we identified serines 1248 and 1291 as the major serine phosphorylation sites of the IGF-1R, and subsequent mutation analysis demonstrated clear effects on IGF-1R signaling and degradation, mirroring alterations by GRKs. Targeted mutation of S1248 recapitulates GRK2 modulation, whereas S1291 mutation resembles GRK6 effects on IGF-1R signaling/degradation, consistent with GRK isoform-specific serine phosphorylation. This study demonstrates distinct roles for GRK isoforms in IGF-1R signaling through β-arrestin binding with divergent functional outcomes.

Prolyl hydroxylase 2: a novel regulator of β2 -adrenoceptor internalization

Adrenergic receptor (AR)-mediated signalling is modulated by oxygen levels. Prolyl hydroxylases (PHDs) are crucial for intracellular oxygen sensing and organism survival. However, it remains to be clarified whether or how PHDs are involved in the regulation of β₂-adrenoceptor (β₂-AR) signalling. Here we show that PHD2 can modulate the rate of β₂-AR internalization through interactions with β-arrestin 2. PHD2 hydroxylates β-arrestin 2 at the proline (Pro)¹⁷⁶, Pro¹⁷⁹ and Pro¹⁸¹ sites, which retards the recruitment of β-arrestin 2 to the plasma membrane and inhibits subsequent co-internalization with β₂-AR into the cytosol. β₂-AR internalization is critical to control the temporal and spatial aspects of β₂-AR signalling. Identifying novel regulators of β₂-AR internalization will enable us to develop new strategies to manipulate receptor signalling and provide potential targets for drug development in the prevention and treatment of diseases associated with β₂-AR signalling dysregulation.

Engagement of β-arrestin by transactivated insulin-like growth factor receptor is needed for V2 vasopressin receptor-stimulated ERK1/2 activation

G protein-coupled receptors (GPCRs) have been shown to activate the mitogen-activated protein kinases, ERK1/2, through both G protein-dependent and -independent mechanisms. Here, we describe a G protein-independent mechanism that unravels an unanticipated role for β-arrestins. Stimulation of the V2 vasopressin receptor (V2R) in cultured cells or in vivo in rat kidney medullar collecting ducts led to the activation of ERK1/2 through the metalloproteinase-mediated shedding of a factor activating the insulin-like growth factor receptor (IGFR). This process was found to be both Src- and β-arrestin-dependent. Whereas Src was found to act upstream of the metalloproteinase activation and be required for the release of the IGFR-activating factor, β-arrestins were found to act downstream of the IGFR transactivation. Unexpectedly, the engagement of β-arrestins by the IGFR but not by the V2R was needed to promote the vasopressin-stimulated ERK1/2 activation, indicating that a pool of β-arrestins distinct from those β-arrestins recruited to the V2R acts downstream of the receptor tyrosine kinase to activate ERK1/2. Such a dual site of action for β-arrestins helps explain the pleiotropic actions of this scaffolding protein. Given the role that V2R-stimulated ERK1/2 plays in kidney cell proliferation, this transactivation mechanism may have important implications for renal pathophysiology. Still, the role of β-arrestins downstream of a transactivation event is not limited to the V2R, because we observed a similar involvement for an unrelated GPCR (the platelet-activating factor receptor), indicating that it may be a general mechanism shared among GPCRs.

Fluorescence/bioluminescence resonance energy transfer techniques to study G-protein-coupled receptor activation and signaling

Fluorescence and bioluminescence resonance energy transfer (FRET and BRET) techniques allow the sensitive monitoring of distances between two labels at the nanometer scale. Depending on the placement of the labels, this permits the analysis of conformational changes within a single protein (for example of a receptor) or the monitoring of protein-protein interactions (for example, between receptors and G-protein subunits). Over the past decade, numerous such techniques have been developed to monitor the activation and signaling of G-protein-coupled receptors (GPCRs) in both the purified, reconstituted state and in intact cells. These techniques span the entire spectrum from ligand binding to the receptors down to intracellular second messengers. They allow the determination and the visualization of signaling processes with high temporal and spatial resolution. With these techniques, it has been demonstrated that GPCR signals may show spatial and temporal patterning. In particular, evidence has been provided for spatial compartmentalization of GPCRs and their signals in intact cells and for distinct physiological consequences of such spatial patterning. We review here the FRET and BRET technologies that have been developed for G-protein-coupled receptors and their signaling proteins (G-proteins, effectors) and the concepts that result from such experiments.

The role of GRK6 in animal models of Parkinson's disease and L-DOPA treatment

G protein-coupled Receptor Kinase 6 (GRK6) belongs to a family of kinases that phosphorylate GPCRs. GRK6 levels were found to be altered in Parkinson's Disease (PD) and D₂ dopamine receptors are supersensitive in mice lacking GRK6 (GRK6-KO mice). To understand how GRK6 modulates the behavioral manifestations of dopamine deficiency and responses to L-DOPA, we used three approaches to model PD in GRK6-KO mice: 1) the cataleptic response to haloperidol; 2) introducing GRK6 mutation to an acute model of absolute dopamine deficiency, DDD mice; 3) hemiparkinsonian 6-OHDA model. Furthermore, dopamine-related striatal signaling was analyzed by assessing the phosphorylation of AKT/GSK3β and ERK1/2. GRK6 deficiency reduced cataleptic behavior, potentiated the acute effect of L-DOPA in DDD mice, reduced rotational behavior in hemi-parkinsonian mice, and reduced abnormal involuntary movements induced by chronic L-DOPA. These data indicate that approaches to regulate GRK6 activity could be useful in modulating both therapeutic and side-effects of L-DOPA.

Regulation of basal resistance by a powdery mildew-induced cysteine-rich receptor-like protein kinase in barley

The receptor-like protein kinases (RLKs) constitute a large and diverse group of proteins controlling numerous plant physiological processes, including development, hormone perception and stress responses. The cysteine-rich RLKs (CRKs) represent a prominent subfamily of transmembrane-anchored RLKs. We have identified a putative barley (Hordeum vulgare) CRK gene family member, designated HvCRK1. The mature putative protein comprises 645 amino acids, and includes a putative receptor domain containing two characteristic 'domain 26 of unknown function' (duf26) domains in the N-terminal region, followed by a rather short 17-amino-acid transmembrane domain, which includes an AAA motif, two features characteristic of endoplasmic reticulum (ER)-targeted proteins and, finally, a characteristic putative protein kinase domain in the C-terminus. The HvCRK1 transcript was isolated from leaves inoculated with the biotrophic fungal pathogen Blumeria graminis f.sp. hordei (Bgh). HvCRK1 transcripts were observed to accumulate transiently following Bgh inoculation of susceptible barley. Transient silencing of HvCRK1 expression in bombarded epidermal cells led to enhanced resistance to Bgh, but did not affect R-gene-mediated resistance. Silencing of HvCRK1 phenocopied the effective penetration resistance found in mlo-resistant barley plants, and the possible link between HvCRK1 and MLO was substantiated by the fact that HvCRK1 induction on Bgh inoculation was dependent on Mlo. Finally, using both experimental and in silico approaches, we demonstrated that HvCRK1 localizes to the ER of barley cells. The negative effect on basal resistance against Bgh and the functional aspects of MLO- and ER-localized HvCRK1 signalling on Bgh inoculation are discussed.

Role of receptor-attached phosphates in binding of visual and non-visual arrestins to G protein-coupled receptors

Arrestins are a small family of proteins that regulate G protein-coupled receptors (GPCRs). Arrestins specifically bind to phosphorylated active receptors, terminating G protein coupling, targeting receptors to endocytic vesicles, and initiating G protein-independent signaling. The interaction of rhodopsin-attached phosphates with Lys-14 and Lys-15 in β-strand I was shown to disrupt the interaction of α-helix I, β-strand I, and the C-tail of visual arrestin-1, facilitating its transition into an active receptor-binding state. Here we tested the role of conserved lysines in homologous positions of non-visual arrestins by generating K2A mutants in which both lysines were replaced with alanines. K2A mutations in arrestin-1, -2, and -3 significantly reduced their binding to active phosphorhodopsin in vitro. The interaction of arrestins with several GPCRs in intact cells was monitored by a bioluminescence resonance energy transfer (BRET)-based assay. BRET data confirmed the role of Lys-14 and Lys-15 in arrestin-1 binding to non-cognate receptors. However, this was not the case for non-visual arrestins in which the K2A mutations had little effect on net BRET(max) values for the M2 muscarinic acetylcholine (M2R), β(2)-adrenergic (β(2)AR), or D2 dopamine receptors. Moreover, a phosphorylation-deficient mutant of M2R interacted with wild type non-visual arrestins normally, whereas phosphorylation-deficient β(2)AR mutants bound arrestins at 20-50% of the level of wild type β(2)AR. Thus, the contribution of receptor-attached phosphates to arrestin binding varies depending on the receptor-arrestin pair. Although arrestin-1 always depends on receptor phosphorylation, its role in the recruitment of arrestin-2 and -3 is much greater in the case of β(2)AR than M2R and D2 dopamine receptor.

GRK2: multiple roles beyond G protein-coupled receptor desensitization

G protein-coupled receptor kinases (GRKs) regulate numerous G protein-coupled receptors (GPCRs) by phosphorylating the intracellular domain of the active receptor, resulting in receptor desensitization and internalization. GRKs also regulate GPCR trafficking in a phosphorylation-independent manner via direct protein-protein interactions. Emerging evidence suggests that GRK2, the most widely studied member of this family of kinases, modulates multiple cellular responses in various physiological contexts by either phosphorylating non-receptor substrates or interacting directly with signaling molecules. In this review, we discuss traditional and newly discovered roles of GRK2 in receptor internalization and signaling as well as its impact on non-receptor substrates. We also discuss novel exciting roles of GRK2 in the regulation of dopamine receptor signaling and in the activation and trafficking of the atypical GPCR, Smoothened (Smo).