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

A novel G protein-biased agonist at the μ opioid receptor induces substantial receptor desensitisation through G protein-coupled receptor kinase

BACKGROUND AND PURPOSE

G protein-biased μ opioid receptor agonists have the potential to induce less receptor desensitisation and tolerance than balanced opioids. Here, we investigated if the cyclic endomorphin analogue Tyr-c[D-Lys-Phe-Tyr-Gly] (Compound 1) is a G protein-biased μ agonist and characterised its ability to induce rapid receptor desensitisation in mammalian neurones.

EXPERIMENTAL APPROACH

The signalling and trafficking properties of opioids were characterised using bioluminescence resonance energy transfer assays, enzyme-linked immunosorbent assay and phosphosite-specific immunoblotting in human embryonic kidney 293 cells. Desensitisation of opioid-induced currents were studied in rat locus coeruleus neurones using whole-cell patch-clamp electrophysiology. The mechanism of Compound 1-induced μ receptor desensitisation was probed using kinase inhibitors.

KEY RESULTS

Compound 1 has similar intrinsic activity for G protein signalling as morphine. As predicted for a G protein-biased μ agonist, Compound 1 induced minimal agonist-induced internalisation and phosphorylation at intracellular μ receptor serine/threonine residues known to be involved in G protein-coupled receptor kinase (GRK)-mediated desensitisation. However, Compound 1 induced robust rapid μ receptor desensitisation in locus coeruleus neurons, to a greater degree than morphine. The extent of Compound 1-induced desensitisation was unaffected by activation or inhibition of protein kinase C (PKC) but was significantly reduced by inhibition of GRK.

CONCLUSION AND IMPLICATIONS

Compound 1 is a novel G protein-biased μ agonist that induces substantial rapid receptor desensitisation in mammalian neurons. Surprisingly, Compound 1-induced desensitisation was demonstrated to be GRK dependent despite its G protein bias. Our findings refute the assumption that G protein-biased agonists will evade receptor desensitisation and tolerance.

LINKED ARTICLES

This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Control of Gαq signaling dynamics and GPCR cross-talk by GRKs

Numerous processes contribute to the regulation of G protein-coupled receptors (GPCRs), but relatively little is known about rapid mechanisms that control signaling on the seconds time scale or regulate cross-talk between receptors. Here, we reveal that the ability of some GPCR kinases (GRKs) to bind Gαq both drives acute signaling desensitization and regulates functional interactions between GPCRs. GRK2/3-mediated acute desensitization occurs within seconds, is rapidly reversible, and can occur upon local, subcellular activation. This rapid desensitization is kinase independent, insensitive to pharmacological inhibition, and generalizable across receptor families and effectors. We also find that the ability of GRK2 to bind G proteins also enables it to regulate the extent and timing of Gαq-dependent signaling cross-talk between GPCRs. Last, we find that G protein/GRK2 interactions enable a novel form of GPCR trafficking cross-talk. Together, this work reveals potent forms of Gαq-dependent GPCR regulation with wide-ranging pharmacological and physiological implications.

G protein-coupled receptor signaling: transducers and effectors

G protein-coupled receptors (GPCRs) are of considerable interest due to their importance in a wide range of physiological functions and in a large number of Food and Drug Administration (FDA)-approved drugs as therapeutic entities. With continued study of their function and mechanism of action, there is a greater understanding of how effector molecules interact with a receptor to initiate downstream effector signaling. This review aims to explore the signaling pathways, dynamic structures, and physiological relevance in the cardiovascular system of the three most important GPCR signaling effectors: heterotrimeric G proteins, GPCR kinases (GRKs), and β-arrestins. We will first summarize their prominent roles in GPCR pharmacology before transitioning into less well-explored areas. As new technologies are developed and applied to studying GPCR structure and their downstream effectors, there is increasing appreciation for the elegance of the regulatory mechanisms that mediate intracellular signaling and function.

Allosteric Modulators of Metabotropic Glutamate Receptors as Novel Therapeutics for Neuropsychiatric Disease

Metabotropic glutamate (mGlu) receptors, a family of G-protein-coupled receptors, have been identified as novel therapeutic targets based on extensive research supporting their diverse contributions to cell signaling and physiology throughout the nervous system and important roles in regulating complex behaviors, such as cognition, reward, and movement. Thus, targeting mGlu receptors may be a promising strategy for the treatment of several brain disorders. Ongoing advances in the discovery of subtype-selective allosteric modulators for mGlu receptors has provided an unprecedented opportunity for highly specific modulation of signaling by individual mGlu receptor subtypes in the brain by targeting sites distinct from orthosteric or endogenous ligand binding sites on mGlu receptors. These pharmacological agents provide the unparalleled opportunity to selectively regulate neuronal excitability, synaptic transmission, and subsequent behavioral output pertinent to many brain disorders. Here, we review preclinical and clinical evidence supporting the utility of mGlu receptor allosteric modulators as novel therapeutic approaches to treat neuropsychiatric diseases, such as schizophrenia, substance use disorders, and stress-related disorders.

Targeting G protein-coupled receptors in cancer therapy

As basic research into GPCR signaling and its association with disease has come into fruition, greater clarity has emerged with regards to how these receptors may be amenable to therapeutic intervention. As a diverse group of receptor proteins, which regulate a variety of intracellular signaling pathways, research in this area has been slow to yield tangible therapeutic agents for the treatment of a number of diseases including cancer. However, recently such research has gained momentum based on a series of studies that have sought to define GPCR proteins dynamics through the elucidation of their crystal structures. In this chapter, we define the approaches that have been adopted in developing better therapeutics directed against the specific parts of the receptor proteins, such as the extracellular and the intracellular domains, including the ligands and auxiliary proteins that bind them. Finally, we also briefly outline how GPCR-derived signaling transduction pathways hold great potential as additional targets.

G protein-coupled receptor kinase 2 (GRK2) as a multifunctional signaling hub

Accumulating evidence indicates that G protein-coupled receptor kinase 2 (GRK2) is a versatile protein that acts as a signaling hub by modulating G protein-coupled receptor (GPCR) signaling and also via phosphorylation or scaffolding interactions with an extensive number of non-GPCR cellular partners. GRK2 multifunctionality arises from its multidomain structure and from complex mechanisms of regulation of its expression levels, activity, and localization within the cell, what allows the precise spatio-temporal shaping of GRK2 targets. A better understanding of the GRK2 interactome and its modulation mechanisms is helping to identify the GRK2-interacting proteins and its substrates involved in the participation of this kinase in different cellular processes and pathophysiological contexts.

Nedd4 E3 ligase and beta-arrestins regulate ubiquitination, trafficking, and stability of the mGlu7 receptor

The metabotropic glutamate receptor 7 (mGlu7) is a class C G protein-coupled receptor that modulates excitatory neurotransmitter release at the presynaptic active zone. Although post-translational modification of cellular proteins with ubiquitin is a key molecular mechanism governing protein degradation and function, mGlu7 ubiquitination and its functional consequences have not been elucidated yet. Here, we report that Nedd4 ubiquitin E3 ligase and β-arrestins regulate ubiquitination of mGlu7 in heterologous cells and rat neurons. Upon agonist stimulation, β-arrestins recruit Nedd4 to mGlu7 and facilitate Nedd4-mediated ubiquitination of mGlu7. Nedd4 and β-arrestins regulate constitutive and agonist-induced endocytosis of mGlu7 and are required for mGlu7-dependent MAPK signaling in neurons. In addition, Nedd4-mediated ubiquitination results in the degradation of mGlu7 by both the ubiquitin-proteasome system and the lysosomal degradation pathway. These findings provide a model in which Nedd4 and β-arrestin act together as a complex to regulate mGlu7 surface expression and function at presynaptic terminals.

The Role of G Protein-coupled Receptor Kinases in Cancer

G protein-coupled receptors (GPCRs) are the largest family of plasma membrane receptors. Emerging evidence demonstrates that signaling through GPCRs affects numerous aspects of cancer biology such as vascular remolding, invasion, and migration. Therefore, development of GPCR-targeted drugs could provide a new therapeutic strategy to treating a variety of cancers. G protein-coupled receptor kinases (GRKs) modulate GPCR signaling by interacting with the ligand-activated GPCR and phosphorylating its intracellular domain. This phosphorylation initiates receptor desensitization and internalization, which inhibits downstream signaling pathways related to cancer progression. GRKs can also regulate non-GPCR substrates, resulting in the modulation of a different set of pathophysiological pathways. In this review, we will discuss the role of GRKs in modulating cell signaling and cancer progression, as well as the therapeutic potential of targeting GRKs.

G protein-coupled receptor kinases: Past, present and future

This review is provided in recognition of the extensive contributions of Dr. Robert J. Lefkowitz to the G protein-coupled receptor (GPCR) field and to celebrate his 75th birthday. Since one of the authors trained with Bob in the 80s, we provide a history of work done in the Lefkowitz lab during the 80s that focused on dissecting the mechanisms that regulate GPCR signaling, with a particular emphasis on the GPCR kinases (GRKs). In addition, we highlight structure/function characteristics of GRK interaction with GPCRs as well as a review of two recent reports that provide a molecular model for GRK-GPCR interaction. Finally, we offer our perspective on some future studies that we believe will drive this field.

Role of Spinophilin in Group I Metabotropic Glutamate Receptor Endocytosis, Signaling, and Synaptic Plasticity

Activation of Group I metabotropic glutamate receptors (mGluRs) activates signaling cascades, resulting in calcium release from intracellular stores, ERK1/2 activation, and long term changes in synaptic activity that are implicated in learning, memory, and neurodegenerative diseases. As such, elucidating the molecular mechanisms underlying Group I mGluR signaling is important for understanding physiological responses initiated by the activation of these receptors. In the current study, we identify the multifunctional scaffolding protein spinophilin as a novel Group I mGluR-interacting protein. We demonstrate that spinophilin interacts with the C-terminal tail and second intracellular loop of Group I mGluRs. Furthermore, we show that interaction of spinophilin with Group I mGluRs attenuates receptor endocytosis and phosphorylation of ERK1/2, an effect that is dependent upon the interaction of spinophilin with the C-terminal PDZ binding motif encoded by Group I mGluRs. Spinophilin knock-out results in enhanced mGluR5 endocytosis as well as increased ERK1/2, AKT, and Ca(2+) signaling in primary cortical neurons. In addition, the loss of spinophilin expression results in impaired mGluR5-stimulated LTD. Our results indicate that spinophilin plays an important role in regulating the activity of Group I mGluRs as well as their influence on synaptic activity.

Shifting towards a model of mGluR5 dysregulation in schizophrenia: Consequences for future schizophrenia treatment

Metabotropic glutamate receptor subtype 5 (mGluR5), encoded by the GRM5 gene, represents a compelling novel drug target for the treatment of schizophrenia. mGluR5 is a postsynaptic G-protein coupled glutamate receptor strongly linked with several critical cellular processes that are reported to be disrupted in schizophrenia. Accordingly, mGluR5 positive allosteric modulators show encouraging therapeutic potential in preclinical schizophrenia models, particularly for the treatment of cognitive dysfunctions against which currently available therapeutics are largely ineffective. More work is required to support the progression of mGluR5-targeting drugs into the clinic for schizophrenia treatment, although some obstacles may be overcome by comprehensively understanding how mGluR5 itself is involved in the neurobiology of the disorder. Several processes that are necessary for the regulation of mGluR5 activity have been identified, but not examined, in the context of schizophrenia. These processes include protein-protein interactions, dimerisation, subcellular trafficking, the impact of genetic variability or mutations on protein function, as well as epigenetic, post-transcriptional and post-translational processes. It is essential to understand these aspects of mGluR5 to determine whether they are affected in schizophrenia pathology, and to assess the consequences of mGluR5 dysfunction for the future use of mGluR5-based drugs. Here, we summarise the known processes that regulate mGluR5 and those that have already been studied in schizophrenia, and discuss the consequences of this dysregulation for current mGluR5 pharmacological strategies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Ca(2+)/calmodulin-dependent protein kinase II interacts with group I metabotropic glutamate and facilitates receptor endocytosis and ERK1/2 signaling: role of β-amyloid

BACKGROUND

Agonist stimulation of Group I metabotropic glutamate receptors (mGluRs) initiates their coupling to the heterotrimeric G protein, Gαq/11, resulting in the activation of phospholipase C, the release of Ca(2+) from intracellular stores and the subsequent activation of protein kinase C. However, it is now recognized that mGluR5a also functions as a receptor for cellular prion protein (PrP(C)) and β-amyloid peptide (Aβ42) oligomers to facilitate intracellular signaling via the resulting protein complex. Intracellular mGluR5a signaling is also regulated by its association with a wide variety of intracellular regulation proteins.

RESULTS

In the present study, we utilized mass spectroscopy to identify calmodulin kinase IIα (CaMKIIα) as a protein that interacts with the second intracellular loop domain of mGluR5. We show that CaMKIIα interacts with both mGluR1a and mGluR5a in an agonist-independent manner and is co-immunoprecipitated with mGluR5a from hippocampal mouse brain. CaMKIIα positively regulates both mGluR1a and mGluR5a endocytosis, but selectively attenuates mGluR5a but not mGluR1a-stimulated ERK1/2 phosphorylation in a kinase activity-dependent manner. We also find that Aβ42 oligomers stimulate the association of CaMKIIα with mGluR5a and activate ERK1/2 in an mGluR5a-dependent manner. However, Aβ42 oligomer-stimulated ERK1/2 phosphorylation is not regulated by mGluR5a/CaMKIIα interactions suggesting that agonist and Aβ42 oligomers stabilize distinct mGluR5a activation states that are differentially regulated by CaMKIIα. The expression of both mGluR5a and PrP(C) together, but not alone resulted in the agonist-stimulated subcellular distribution of CaMKIIα into cytoplasmic puncta.

CONCLUSIONS

Taken together these results indicate that CaMKIIα selectively regulates mGluR1a and mGluR5a ERK1/2 signaling. As mGluR5 and CaMKIIα are involved in learning and memory and Aβ and mGluR5 are implicated in Alzheimer's disease, results of these studies could provide insight into potential pharmacological targets for treatment of Alzheimer's disease.

KISS1R: Hallmarks of an Effective Regulator of the Neuroendocrine Axis

Kisspeptin (KP) is now well recognized as a potent stimulator of gonadotropin-releasing hormone (GnRH) secretion and thereby a major regulator of the neuroendocrine-reproductive axis. KP signals via KISS1R, a G protein-coupled receptor (GPCR) that activates the G proteins Gαq/11. Modulation of the interaction of KP with KISS1R is therefore a potential new therapeutic target for stimulating (in infertility) or inhibiting (in hormone-dependent diseases) the reproductive hormone cascade. Major efforts are underway to target KISS1R in the treatment of sex steroid hormone-dependent disorders and to stimulate endogenous hormonal responses along the neuroendocrine axis as part of in vitro fertilization protocols. The development of analogs modulating KISS1R signaling will be aided by an understanding of the intracellular pathways and dynamics of KISS1R signaling under normal and pathological conditions. This review focuses on KISS1R recruitment of intracellular signaling (Gαq/11- and β-arrestin-dependent) pathways that mediate GnRH secretion and the respective roles of rapid desensitization, internalization, and recycling of resensitized receptors in maintaining an active population of KISS1R at the cell surface to facilitate prolonged KP signaling. Additionally, this review summarizes and discusses the major findings of an array of studies examining the desensitization of KP signaling in man, domestic and laboratory animals. This discussion highlights the major effects of ligand efficacy and concentration and the physiological, developmental, and metabolic status of the organism on KP signaling. Finally, the potential for the utilization of KP and analogs in stimulating and inhibiting the reproductive hormone cascade as an alternative to targeting the downstream GnRH receptor is discussed.

Metabotropic glutamate receptor 5 as a potential therapeutic target in Huntington's disease

INTRODUCTION

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein, which underlies the loss of striatal and cortical neurons. Glutamate has been implicated in a number of neurodegenerative diseases, and several studies suggest that the metabotropic glutamate receptor 5 (mGluR5) may represent a target for the treatment of HD.

AREAS COVERED

The main goal of this review is to discuss the current data in the literature regarding the role of mGluR5 in HD and evaluate the potential of mGluR5 as a therapeutic target for the treatment of HD. mGluR5 is highly expressed in the brain regions affected in HD and is involved in movement control. Moreover, mGluR5 interacts with htt and mutated htt profoundly affects mGluR5 signaling. However, mGluR5 stimulation can activate both neuroprotective and neurotoxic signaling pathways, depending on the context of activation.

EXPERT OPINION

Although the data published so far strongly indicate that mGluR5 plays a major role in HD-associated neurodegeneration, htt aggregation and motor symptoms, it is not clear whether mGluR5 stimulation can diminish or intensify neuronal cell loss and HD progression. Thus, future experiments will be necessary to further investigate the outcome of drugs acting on mGluR5 for the treatment of neurodegenerative diseases.

Rab8 modulates metabotropic glutamate receptor subtype 1 intracellular trafficking and signaling in a protein kinase C-dependent manner

Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors (GPCRs) that are activated by glutamate, the primary excitatory neurotransmitter in the CNS. Alterations in glutamate receptor signaling are implicated in neuropathologies such as Alzheimer's disease, ischemia, and Huntington's disease among others. Group 1 mGluRs (mGluR1 and mGluR5) are primarily coupled to Gα(q/11) leading to the activation of phospholipase C and the formation of diacylglycerol and inositol 1,4,5-trisphosphate, which results in the release of intracellular calcium stores and protein kinase C (PKC) activation. Desensitization, endocytosis, and recycling are major mechanisms of GPCR regulation, and the intracellular trafficking of GPCRs is linked to the Rab family of small G proteins. Rab8 is a small GTPase that is specifically involved in the regulation of secretory/recycling vesicles, modulation of the actin cytoskeleton, and cell polarity. Rab8 has been shown to regulate the synaptic delivery of AMPA receptors during long-term potentiation and during constitutive receptor recycling. We show here that Rab8 interacts with the C-terminal tail of mGluR1a in an agonist-dependent manner and plays a role in regulating of mGluR1a signaling and intracellular trafficking in human embryonic kidney 293 cells. Specifically, Rab8 expression attenuates mGluR1a-mediated inositol phosphate formation and calcium release from mouse neurons in a PKC-dependent manner, while increasing cell surface mGluR1a expression via decreased receptor endocytosis. These experiments provide us with an understanding of the role Rabs play in coordinated regulation of mGluR1a and how this impacts mGluR1a signaling.

Somatic mutations in GRM1 in cancer alter metabotropic glutamate receptor 1 intracellular localization and signaling

The activity of metabotropic glutamate receptors (mGluRs) is known to be altered as the consequence of neurodegenerative diseases such as Alzheimer, Parkinson, and Huntington disease. However, little attention has been paid to this receptor family's potential link with cancer. Recent reports indicate altered mGluR signaling in various tumor types, and several somatic mutations in mGluR1a in lung cancer were recently described. Group 1 mGluRs (mGluR1a and mGluR5) are coupled primarily to Gαq, leading to the activation of phospholipase C and to the formation of diacylglycerol and inositol 1,4,5-trisphosphate, leading to the release of Ca(2+) from intracellular stores and protein kinase C (PKC) activation. In the present study, we investigated the intracellular localization and G protein-dependent and -independent signaling of eight GRM1 (mGluR1a) somatic mutations. Two mutants found in close proximity to the glutamate binding domain and cysteine-rich region (R375G and G396V) show both decreased cell surface expression and basal inositol phosphate (IP) formation. However, R375G shows increased ERK1/2 activation in response to quisqualate stimulation. A mutant located directly in the glutamate binding site (A168V) shows increased quisqualate-induced IP formation and, similar to R375G, increased ERK1/2 activation. Additionally, a mutation in the G protein-coupled receptor kinase 2/PKC regulatory region (R696W) shows decreased ERK1/2 activation, whereas a mutation within the Homer binding region in the carboxyl-terminal tail (P1148L) does not alter the intracellular localization of the receptor, but it induces changes in cellular morphology and exhibits reduced ERK1/2 activation. Taken together, these results suggest that mGluR1a signaling in cancer is disrupted by somatic mutations with multiple downstream consequences.

Raf kinase inhibitory protein is required for cerebellar long-term synaptic depression by mediating PKC-dependent MAPK activation

It was demonstrated previously that a positive feedback loop, including protein kinase C (PKC) and mitogen-activated protein kinase (MAPK), is required for the gradual expression of cerebellar long-term depression (LTD). PKC and MAPK are mutually activated in this loop. MAPK-dependent PKC activation is likely to be mediated by phospholipase A2. On the other hand, it is not clear how PKC activates MAPK. Therefore, the entire picture of this loop was not fully understood. We here test the hypothesis that this loop is completed by the PKC substrate, Raf kinase inhibitory protein (RKIP). To test this hypothesis, we used a mutant form of RKIP that is not phosphorylated by PKC and thus constitutively inhibits Raf-1 and MEK, upstream kinases of MAPK. When this RKIP mutant was introduced into Purkinje cells of mouse cerebellar slices through patch-clamp electrodes, LTD was blocked, while wild-type (WT) RKIP had no effect on LTD. Physiological epistasis experiments demonstrated that RKIP works downstream of PKC and upstream of MAPK during LTD induction. Furthermore, biochemical analyses demonstrated that endogenous RKIP dissociates from Raf-1 and MEK during LTD induction in a PKC-dependent manner, suggesting that RKIP binding-dependent inhibition of Raf-1 and MEK is removed upon LTD induction. We therefore conclude that PKC-dependent regulation of RKIP leads to MAPK activation, with RKIP completing the positive feedback loop that is required for LTD.

Structure of metabotropic glutamate receptor C-terminal domains in contact with interacting proteins

Metabotropic glutamate receptors (mGluRs) regulate intracellular signal pathways that control several physiological tasks, including neuronal excitability, learning, and memory. This is achieved by the formation of synaptic signal complexes, in which mGluRs assemble with functionally related proteins such as enzymes, scaffolds, and cytoskeletal anchor proteins. Thus, mGluR associated proteins actively participate in the regulation of glutamatergic neurotransmission. Importantly, dysfunction of mGluRs and interacting proteins may lead to impaired signal transduction and finally result in neurological disorders, e.g., night blindness, addiction, epilepsy, schizophrenia, autism spectrum disorders and Parkinson's disease. In contrast to solved crystal structures of extracellular N-terminal domains of some mGluR types, only a few studies analyzed the conformation of intracellular receptor domains. Intracellular C-termini of most mGluR types are subject to alternative splicing and can be further modified by phosphorylation and SUMOylation. In this way, diverse interaction sites for intracellular proteins that bind to and regulate the glutamate receptors are generated. Indeed, most of the known mGluR binding partners interact with the receptors' C-terminal domains. Within the last years, different laboratories analyzed the structure of these domains and described the geometry of the contact surface between mGluR C-termini and interacting proteins. Here, I will review recent progress in the structure characterization of mGluR C-termini and provide an up-to-date summary of the geometry of these domains in contact with binding partners.

Diversity of metabotropic glutamate receptor-interacting proteins and pathophysiological functions

In the mammalian brain, the large majority of excitatory synapses express pre- and postsynaptic glutamate receptors. These are ion channels and G protein-coupled membrane proteins that are organized into functional signaling complexes. Here, we will review the nature and pathophysiological functions of the scaffolding proteins associated to these receptors, focusing on the G protein-coupled subtypes.

The intra-molecular activation mechanisms of the dimeric metabotropic glutamate receptor 1 differ depending on the type of G proteins

Metabotropic glutamate receptor 1 (mGlu1) functions as a homodimer and activates not only the Gq but also the Gi/o and Gs pathways. Because of the dimeric configuration, different pathways could be activated either through the glutamate-bound subunit (cis-activation) and/or the other one (trans-activation). We here examined whether the intra-molecular activation mechanisms in the mGlu1 differ depending on the type of coupling G proteins, using various combinations of mGlu1 constructs that lack glutamate binding and/or G-protein coupling. The cis- or trans-activation alone was confirmed to trigger the Gq-coupled intracellular Ca(2+) transient. In contrast, the Gi/o-coupled G protein-dependent inward rectifying potassium (GIRK) channels were not activated either through the cis- or trans-activation alone. When one subunit of dimeric mGlu1 lacked the G-protein coupling, a significant decrease in the glutamate-induced GIRK current density was also observed. As the G protein-coupling-deficient subunit did not decrease the cell surface expression of mGlu1 and the Gq-coupled Ca(2+) transient, it was suggested that the coupling deficiency in one subunit of mGlu1 attenuates the Gi/o but not Gq coupling. In conclusion, multiple G-protein signaling was differentially activated by different intra-molecular activation mechanisms of the dimeric mGlu1.

Regulation of the epithelial Na+ channel by the RH domain of G protein-coupled receptor kinase, GRK2, and Galphaq/11

The G protein-coupled receptor kinase (GRK2) belongs to a family of protein kinases that phosphorylates agonist-activated G protein-coupled receptors, leading to G protein-receptor uncoupling and termination of G protein signaling. GRK2 also contains a regulator of G protein signaling homology (RH) domain, which selectively interacts with α-subunits of the Gq/11 family that are released during G protein-coupled receptor activation. We have previously reported that kinase activity of GRK2 up-regulates activity of the epithelial sodium channel (ENaC) in a Na(+) absorptive epithelium by blocking Nedd4-2-dependent inhibition of ENaC. In the present study, we report that GRK2 also regulates ENaC by a mechanism that does not depend on its kinase activity. We show that a wild-type GRK2 (wtGRK2) and a kinase-dead GRK2 mutant ((K220R)GRK2), but not a GRK2 mutant that lacks the C-terminal RH domain (ΔRH-GRK2) or a GRK2 mutant that cannot interact with Gαq/11/14 ((D110A)GRK2), increase activity of ENaC. GRK2 up-regulates the basal activity of the channel as a consequence of its RH domain binding the α-subunits of Gq/11. We further found that expression of constitutively active Gαq/11 mutants significantly inhibits activity of ENaC. Conversely, co-expression of siRNA against Gαq/11 increases ENaC activity. The effect of Gαq on ENaC activity is not due to change in ENaC membrane expression and is independent of Nedd4-2. These findings reveal a novel mechanism by which GRK2 and Gq/11 α-subunits regulate the activity ENaC.

GPCR Conformations: Implications for Rational Drug Design

G protein-coupled receptors (GPCRs) comprise a large class of transmembrane proteins that play critical roles in both normal physiology and pathophysiology. These critical roles offer targets for therapeutic intervention, as exemplified by the substantial fraction of current pharmaceutical agents that target members of this family. Tremendous contributions to our understanding of GPCR structure and dynamics have come from both indirect and direct structural characterization techniques. Key features of GPCR conformations derived from both types of characterization techniques are reviewed.

Metabotropic glutamate receptors: physiology, pharmacology, and disease

The metabotropic glutamate receptors (mGluRs) are family C G-protein-coupled receptors that participate in the modulation of synaptic transmission and neuronal excitability throughout the central nervous system. The mGluRs bind glutamate within a large extracellular domain and transmit signals through the receptor protein to intracellular signaling partners. A great deal of progress has been made in determining the mechanisms by which mGluRs are activated, proteins with which they interact, and orthosteric and allosteric ligands that can modulate receptor activity. The widespread expression of mGluRs makes these receptors particularly attractive drug targets, and recent studies continue to validate the therapeutic utility of mGluR ligands in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, anxiety, depression, and schizophrenia.

Metabotropic glutamate receptor-mediated cell signaling pathways are altered in a mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a polyglutamine expansion in the huntingtin protein (Htt). Group I metabotropic glutamate receptors (mGluRs) are coupled to G(alphaq) and play an important role in neuronal survival. We have previously demonstrated that mGluRs interact with Htt. Here we used striatal neuronal primary cultures and acute striatal slices to demonstrate that mGluR-mediated signaling pathways are altered in a presymptomatic mouse model of HD (Hdh(Q111/Q111)), as compared to those of control mice (Hdh(Q20/Q20)). mGluR1/5-mediated inositol phosphate (InsP) formation is desensitized in striatal slices from Hdh(Q111/Q111) mice and this desensitization is PKC-mediated. Despite of decreased InsP formation, (S)-3,5-dihydroxylphenylglycine (DHPG)-mediated Ca(2+) release is higher in Hdh(Q111/Q111) than in Hdh(Q20/Q20) neurons. Furthermore, mGluR1/5-stimulated AKT and extracellular signal-regulated kinase (ERK) activation is altered in Hdh(Q111/Q111) mice. Basal AKT activation is higher in Hdh(Q111/Q111) neurons and this increase is mGluR5 dependent. Moreover, mGluR5 activation leads to higher levels of ERK activation in Hdh(Q111/Q111) than in Hdh(Q20/Q20) striatum. PKC inhibition not only brings Hdh(Q111/Q111) DHPG-stimulated InsP formation to Hdh(Q20/Q20) levels, but also causes an increase in neuronal cell death in Hdh(Q111/Q111) neurons. However, PKC inhibition does not modify neuronal cell death in Hdh(Q20/Q20) neurons, suggesting that PKC-mediated desensitization of mGluR1/5 in Hdh(Q111/Q111) mice might be protective in HD. Together, these data indicate that group I mGluR-mediated signaling pathways are altered in HD and that these cell signaling adaptations could be important for striatal neurons survival.

Role of the amino terminus of G protein-coupled receptor kinase 2 in receptor phosphorylation

G protein-coupled receptor kinases (GRKs) specifically phosphorylate activated G protein-coupled receptors. While the X-ray crystal structures of several GRKs have been determined, the mechanism of interaction of GRK with GPCRs is currently unknown. To further characterize the role of the GRK2 amino terminus in receptor interaction and phosphorylation, we generated a series of point mutations within the first 10 amino acids of GRK2 and tested their ability to phosphorylate receptor and nonreceptor substrates. Although all mutants exhibited some impairment in receptor phosphorylation, three of the mutants, D3K, L4A, and D10A, were the most severely affected. Using the beta2-adrenergic receptor and rhodopsin as receptor substrates and tubulin as a nonreceptor substrate, we demonstrated that the kinase activity toward the receptors was severely decreased in the mutants, while they fully retained their ability to phosphorylate tubulin. Moreover, the amino-terminal mutants were able to bind to the receptor but, in contrast to wild-type GRK2, were not activated by receptor binding. A synthetic peptide containing residues 1-14 of GRK2 served as a noncompetitive inhibitor of receptor phosphorylation by GRK2, while a comparable peptide from GRK5 had no effect on GRK2 activity. Secondary structure prediction and circular dichroism suggest that the GRK2 amino-terminal peptide forms an amphipathic alpha-helix. Taken together, we propose a mechanism whereby the extreme amino terminus of GRK2 forms an intramolecular interaction that selectively enhances the catalytic activity of the kinase toward receptor substrates.

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

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

Calcineurin inhibitor protein (CAIN) attenuates Group I metabotropic glutamate receptor endocytosis and signaling

Group I metabotropic glutamate receptors (mGluRs) are coupled via phospholipase Cbeta to the hydrolysis of phosphoinositides and function to modulate neuronal excitability and synaptic transmission at glutamatergic synapses. The desensitization of Group I mGluR signaling is thought to be mediated primarily via second messenger-dependent protein kinases and G protein-coupled receptor kinases. We show here that both mGluR1 and mGluR5 interact with the calcineurin inhibitor protein (CAIN). CAIN is co-immunoprecipitated in a complex with Group I mGluRs from both HEK 293 cells and mouse cortical brain lysates. Purified CAIN and its C-terminal domain specifically interact with glutathione S-transferase fusion proteins corresponding to the second intracellular loop and the distal C-terminal tail domains of mGluR1. The interaction of CAIN with mGluR1 could also be blocked using a Tat-tagged peptide corresponding to the mGluR1 second intracellular loop domain. Overexpression of full-length CAIN attenuates the agonist-stimulated endocytosis of both mGluR1a and mGluR5a in HEK 293 cells, but expression of the CAIN C-terminal domain does not alter mGluR5a internalization. In contrast, overexpression of either full-length CAIN or the CAIN C-terminal domain impairs agonist-stimulated inositol phosphate formation in HEK 293 cells expressing mGluR1a. This CAIN-mediated antagonism of mGluR1a signaling appears to involve the disruption of receptor-Galpha(q/11) complexes. Taken together, these observations suggest that the association of CAIN with intracellular domains involved in mGluR/G protein coupling provides an additional mechanism by which Group I mGluR endocytosis and signaling are regulated.

Phosphorylation-independent regulation of metabotropic glutamate receptor 5 desensitization and internalization by G protein-coupled receptor kinase 2 in neurons

The uncoupling of metabotropic glutamate receptors (mGluRs) from heterotrimeric G proteins represents an essential feedback mechanism that protects neurons against receptor overstimulation that may ultimately result in damage. The desensitization of mGluR signaling is mediated by both second messenger-dependent protein kinases and G protein-coupled receptor kinases (GRKs). Unlike mGluR1, the attenuation of mGluR5 signaling in HEK 293 cells is reported to be mediated by a phosphorylation-dependent mechanism. However, the mechanisms regulating mGluR5 signaling and endocytosis in neurons have not been investigated. Here we show that a 2-fold overexpression of GRK2 leads to the attenuation of endogenous mGluR5-mediated inositol phosphate (InsP) formation in striatal neurons and siRNA knockdown of GRK2 expression leads to enhanced mGluR5-mediated InsP formation. Expression of a catalytically inactive GRK2-K220R mutant also effectively attenuates mGluR5 signaling, but the expression of a GRK2-D110A mutant devoid in Galpha(q/11) binding increases mGluR5 signaling in response to agonist stimulation. Taken together, these results indicate that the attenuation of mGluR5 responses in striatal neurons is phosphorylation-independent. In addition, we find that mGluR5 does not internalize in response to agonist treatment in striatal neuron, but is efficiently internalized in cortical neurons that have higher levels of endogenous GRK2 protein expression. When overexpressed in striatal neurons, GRK2 promotes agonist-stimulated mGluR5 internalization. Moreover, GRK2-mediated promotion of mGluR5 endocytosis does not require GRK2 catalytic activity. Thus, we provide evidence that GRK2 mediates phosphorylation-independent mGluR5 desensitization and internalization in neurons.

Kinase activity-independent regulation of cyclin pathway by GRK2 is essential for zebrafish early development

G protein-coupled receptor (GPCR) kinases (GRKs) are known as a family of serine/threonine kinases that function as key regulators of GPCRs, as well as other types of receptors. Extensive studies of GRKs at the cellular and organismal levels have led to a consensus that GRK-catalyzed phosphorylation of receptors is the primary mechanism underlying their physiological functions. Here, we report that down-regulation of GRK2 in zebrafish embryos with GRK2 morpholino results in developmental early arrest and, interestingly, that this arrest can be rescued by exogenous expression of a GRK2 kinase-dead mutant, K220R. A physical interaction between GRK2 and cyclin B1 regulator patched homolog 1 (PTCH1), stimulated by Hedgehog (Hh), rather than GRK2-mediated phosphorylation of downstream targets, appears as the underlying mechanism. We identify residues 262-379 as the PTCH1-binding region (BP). Interaction of GRK2, K220R, and BP with PTCH1 reduces the association of PTCH1 with cyclin B1 and disrupts PTCH1-mediated inhibition of cyclin B1 nuclear translocation, whereas the PTCH1-binding deficient GRK2 mutant (Delta312-379) does not. Cell cycle and cell proliferation assays show that overexpressing PTCH1 remarkably inhibited cell growth and this effect could be attenuated by GRK2, K220R, or BP, but not Delta312-379. In vivo studies show that BP, as well as the nuclear-localizing cyclin B1 mutant, is effective in rescuing the early arrest phenotype in GRK2 knockdown embryos, but Delta312-379 is not. Our data thus reveal a novel kinase activity-independent function for GRK and establish a role for GRK2 as a cell-cycle regulator during early embryonic development.

Phosphorylation of group I metabotropic glutamate receptors (mGluR1/5) in vitro and in vivo

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are densely expressed in mammalian brain. They are actively involved in the regulation of normal cellular activity and synaptic plasticity, and are frequently linked to the pathogenesis of various mental illnesses. Like ionotropic glutamate receptors, group I mGluRs are subject to the regulation by protein phosphorylation. Accumulative data demonstrate sufficient phosphorylation of the intracellular mGluR1/5 domains at specific serine/threonine sites by protein kinase C in heterologous cells or neurons, which serves as an important mechanism for regulating the receptor signaling and desensitization. Emerging evidence also shows the significant involvements of G protein-coupled receptor kinases, Ca2+/calmodulin-dependent protein kinase II, tyrosine kinases, and protein phosphatases in controlling the phosphorylation status of group I mGluRs. This review analyzes the recent data concerning group I mGluR phosphorylation and the phosphorylation-dependent regulation of group I mGluR function. Future research directions in this area with newly available high throughput and proteomic approaches are also discussed in the end.

M3 muscarinic acetylcholine receptor-mediated signaling is regulated by distinct mechanisms

We have used RNA interference previously to demonstrate that G protein-coupled receptor kinase 2 (GRK2) regulates endogenously expressed H1 histamine receptor in human embryonic kidney 293 cells. In this report, we investigate the regulation of endogenously expressed M(3) muscarinic acetylcholine receptor (M(3) mAChR). We show that knockdown of GRK2, GRK3, or GRK6, but not GRK5, significantly increased carbachol-mediated calcium mobilization. Stable expression of wild-type GRK2 or a kinase-dead mutant (GRK2-K220R) reduced calcium mobilization after receptor activation, whereas GRK2 mutants defective in Galpha(q) binding (GRK2-D110A, GRK2-R106A, and GRK2-R106A/K220R) had no effect on calcium signaling, suggesting that GRK2 primarily regulates G(q) after M(3) mAChR activation. The knockdown of arrestin-2 or arrestin-3 also significantly increased carbachol-mediated calcium mobilization. Knockdown of GRK2 and the arrestins also significantly enhanced carbachol-mediated activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), whereas prolonged ERK1/2 activation was only observed with GRK2 or arrestin-3 knockdown. We also investigated the role of casein kinase-1alpha (CK1alpha) and found that knockdown of CK1alpha increased calcium mobilization but not ERK activation. In summary, our data suggest that multiple proteins dynamically regulate M(3) mAChR-mediated calcium signaling, whereas GRK2 and arrestin-3 play the primary role in regulating ERK activation.

Phosphorylation-independent attenuation of GPCR signalling

The uncoupling of G-protein-coupled receptors (GPCRs) from their cognate heterotrimeric G proteins provides an essential physiological 'feedback' mechanism that protects against both acute and chronic overstimulation of receptors. The primary mechanism by which GPCR activity is regulated is the feedback phosphorylation of activated GPCRs by kinases that are dependent on second messengers, GPCR kinases (GRKs) and arrestins. It has recently become apparent, however, that GRK2-mediated regulation of GPCR responsiveness also involves a phosphorylation-independent component that requires both heterotrimeric G-protein alpha-subunit interactions and GPCR binding. Moreover, in addition to GRK2, a growing number of GPCR-interacting proteins might contribute to the phosphorylation-independent G-protein uncoupling of GPCRs. Here, new information about the mechanisms underlying this phosphorylation-independent regulation of receptor and G-protein coupling is reviewed.

The trick of the tail: protein-protein interactions of metabotropic glutamate receptors

It was initially believed that G-protein-coupled receptors, such as metabotropic glutamate receptors, could simply be described as individual proteins that are associated with intracellular signal cascades via G-proteins. This view is no longer tenable. Today we know that metabotropic glutamate receptors (mGluRs) can dimerize and bind to a variety of proteins in addition to trimeric G-proteins. These newly identified protein interactions led to the discovery of new regulatory mechanisms that are independent of and sometimes synergistic with the classical G-protein-coupled second messenger pathways. Notably, several of these mechanisms connect mGluR-mediated signaling to other receptor classes, thereby creating a network of different receptor types and associated signal cascades. The intracellular C-termini of mGluRs play a key role in the regulation of these networks, and various new protein interactions of these domains were described recently. Because mGluRs are involved in a variety of physiological and pathophysiological processes, some of the proteins interacting with this receptor class have potential as valuable pharmaceutical targets. This review will give a comprehensive overview of proteins interacting with mGluR C-termini, highlight new evolving regulatory mechanisms for glutamatergic signal transduction and discuss possibilities for future drug development.

The G protein-coupled receptor kinase (GRK) interactome: role of GRKs in GPCR regulation and signaling

G protein-coupled receptor kinases (GRKs) and arrestins are key participants in the canonical pathways leading to phosphorylation-dependent GPCR desensitization, endocytosis, intracellular trafficking and resensitization as well as in the modulation of important intracellular signaling cascades by GPCR. Novel studies have revealed a phosphorylation-independent desensitization mechanism operating through their RGS-homology (RH) domain and the recent determination of the crystal structures of GRK2 and GRK6 has uncovered interesting details on the structure-function relationships of these kinases. Emerging evidence indicates that the activity of GRKs is tightly modulated by mechanisms including phosphorylation by different kinases and interaction with several cellular proteins such as calmodulin, caveolin or RKIP. In addition, GRKs are involved in multiple interactions with non-receptor proteins (PI3K, Akt, GIT or MEK) that point to novel GRK cellular roles. In this article, our purpose is to describe the ever increasing map of functional interactions for GRK proteins as a basis to better understand its contribution to cellular processes.

Regulation of metabotropic glutamate receptor signaling, desensitization and endocytosis

Metabotropic glutamate receptors (mGluRs) comprise a unique family of G protein-coupled receptors (GPCR) that can be classified into 3 groups based on G protein coupling specificity and sequence similarity. Group I mGluRs (mGluR1 and mGluR5) are coupled to the heterotrimeric G protein Galpha(q/11) and trigger the release of calcium from intracellular stores. In the present review, we discuss the molecular mechanisms involved in the desensitization and endocytosis of group I mGluRs. Group I mGluRs desensitize in response to both second-messenger-dependent protein kinases and G protein-coupled receptor kinases (GRK). However, GRK2-mediated mGluR1 desensitization appears to be both phosphorylation- and beta-arrestin-independent. In addition to GRK-mediated uncoupling of mGluRs from heterotrimeric G proteins, the huntingtin-interacting protein, optineurin, also contributes to mGluR1 and mGluR5 desensitization. The G protein-uncoupling activity of optineurin appears to be facilitated by the presence of polyglutamine-expanded mutant huntingtin but not wild-type huntingtin. Group I mGluRs also undergo both agonist-dependent and -independent endocytosis in both heterologous cell expression systems and primary neuronal cultures. The present review overviews the current understanding of the contribution of second messenger-dependent protein kinases, beta-arrestins and a novel Ral/phospholipase D2 (PLD2)-mediated endocytic pathway to the regulation of Group I mGluR endocytosis. Overall, the regulation of Group I mGluR desensitization and endocytosis appears to be mediated by the same molecular intermediates as have been described for more typical GPCR such as the beta(2)-adrenergic receptor. However, there appears to be subtle, but important, differences in the mechanisms by which these intermediates are employed to regulate Group I mGluR desensitization and endocytosis.

Novel features of G protein-coupled receptor kinase 4

The defining characteristic of G protein-coupled receptor homologous desensitization is that the receptor must be occupied by an agonist or in an activated conformation that mimics an agonist-induced state. In most instances, the mechanistic basis for this characteristic is the high selectivity of G protein-coupled receptor kinases for the activated receptor. In this issue, Rankin et al. (p. 759) demonstrate that under some conditions, at least, the G protein-coupled receptor kinase GRK4 does not display a preference for the agonist-occupied D1 dopamine receptor. Coexpression of GRK4 and the D1 receptor in a heterologous system induces phosphorylation of the receptor in the absence of agonist, causing constitutive desensitization and internalization of the receptor. Lacking the normal rapid feedback mechanisms associated with homologous desensitization, a system incorporating constitutively active GRK4 will be prone to dysregulation, perhaps explaining the generally low expression of GRK4. Indeed, considerable evidence suggests that just such dysregulation resulting from mutationally activated GRK4 contributes to the heritable component of human essential hypertension (Physiol Genomics 19:223-246, 2004).

Techniques: How to boost GPCR mutagenesis studies using yeast

G-protein-coupled receptors (GPCRs) are the major targets of today's medicines. To elucidate the mechanism of activation of GPCRs and the interaction of these receptors with their G proteins, mutagenesis studies have proven to be a powerful tool and have provided insight into the structure and function of GPCRs. Random mutagenesis is useful in this respect particularly when combined with a robust screening assay that is based on the functional properties of the mutants. In this article, the use of random mutagenesis combined with a functional screening assay in yeast is described and compared with alternative approaches such as site-directed mutagenesis per se, alanine/cysteine scanning and another screening assay, receptor selection and amplification technology (R-SAT). Screening in yeast of randomly mutated GPCRs has proven successful in the identification of ligands for orphan receptors and in high-throughput approaches. Moreover, it has provided substantial insight into G-protein coupling and receptor activation.