- Email: [email protected]
Mechanisms of metabotropic glutamate receptor desensitization: role in the patterning of effector enzyme activation
Neurochemistry International 41 (2002) 319–326
Mechanisms of metabotropic glutamate receptor desensitization: role in the patterning of effector enzyme activation Lianne B. Dale a , Andy V. Babwah a , Stephen S.G. Ferguson a,b,c,d,∗ a
John P. Robarts Research Institute, 100 Perth Drive, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6A 5K8 b Department of Medicine, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6G 5K8 c Department of Physiology, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6G 5K8 d Department of Pharmacology and Toxicology, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6G 5K8 Received 1 December 2001; accepted 28 February 2002
Abstract Metabotropic glutamate receptors (mGluRs) constitute an unique subclass of G protein-coupled receptors (GPCRs). These receptors are activated by the excitatory amino acid glutamate and play an essential role in regulating neural development and plasticity. In the present review, we overview the current understanding regarding the molecular mechanisms involved in the desensitization and endocytosis of Group 1 mGluRs as well as the relative contribution of desensitization to the spatial-temporal patterning of glutamate receptor signaling. Similar to what has been reported previously for prototypic GPCRs, mGluRs desensitization is mediated by second messenger-dependent protein kinases and GPCR kinases (GRKs). However, it remains to be determined whether mGluRs phosphorylation by GRKs and -arrestin binding are absolutely required for desensitization. Group 1 mGluRs endocytosis is both agonist-dependent and -independent. Agonist-dependent mGluRs internalization is mediated by a -arrestin- and dynamin-dependent clathrin-coated vesicle dependent endocytic pathway. The activation of Group 1 mGluRs also results in oscillatory Gq protein-coupling leading to the cyclical activation of phospholipase C thereby stimulating oscillations in both inositol 1,4,5-triphosphate formation and Ca2+ release from intracellular stores. These glutamate receptor-stimulated Ca2+ oscillations are translated into the synchronous activation of protein kinase C (PKC), which has led to the hypothesis that oscillatory mGluRs signaling involves the repetitive phosphorylation of mGluRs by PKC. However, recent experimental evidence suggests that oscillatory signaling is an intrinsic glutamate receptor property that is independent of feedback receptor phosphorylation by PKC. The challenge in the future will be to determine the structural determinants underlying mGluRs-mediated spatial-temporal signaling as well as to understand how complex signaling patterns can be interpreted by cells in both the developing and adult nervous systems. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Metabotropic glutamate receptor; G protein-coupled receptor; Desensitization
1. Introduction Metabotropic glutamate receptors (mGluRs) are members of the G protein-coupled receptor (GPCR) superfamily, the largest family of integral membrane receptor proteins. GPCRs are found in organisms ranging from slime mold and yeast to mammals and transduce the information provided by a wide variety of extracellular signals, including light, odor, taste, pheromones, hormones and neurotransmitters, to the interior of cells. GPCR signal transduction is achieved by the coupling of these receptors to a wide variety of effector systems through heterotrimeric guanine nucleotide binding proteins (G proteins; reviewed by Ferguson, 2001). Agonist-activated GPCRs function as guanine nucleotide ∗
Corresponding author. Tel.: +1-519-663-3825; fax: +1-519-663-3789. E-mail address: [email protected] (S.S.G. Ferguson).
exchange factors facilitating the exchange of GDP for GTP on the G protein ␣ subunit, thus allowing the functional dissociation of the G␣ and G␥ subunits. The freely dissociated heterotrimeric G protein, G␣ and G␥ subunits then either positively or negatively regulate a variety of effector systems resulting in changes in intracellular second messenger levels and/or ion conductance.
2. The metabotropic glutamate receptor family Glutamate is the major excitatory neurotransmitter in the central nervous system and is essential in the regulation of brain functions and neural cell development (Nakanishi, 1994; Conn and Pin, 1997). Glutamate mediates its actions through two distinct types of receptors, ionotropic glutamate receptors (iGluRs) and mGluRs. The iGluRs are
0197-0186/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 7 - 0 1 8 6 ( 0 2 ) 0 0 0 7 3 - 6
320
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
Fig. 1. Schematic representation of Group 1 mGluR signal transduction and desensitization mechanisms. Group 1 mGluR agonist activation results in receptor coupling to both Gq and Gs heterotrimeric G proteins. Gs receptor coupling stimulates adenylyl cyclase (AC) and the catalysis of cAMP from ATP. Gq receptor coupling stimulates the activity of phospholipase C (PLC), resulting in intracellular InsP3 and DAG formation. InsP3 activates the InsP3 receptor (InsP3 R) resulting in the release of intracellular Ca2+ stores from the endoplasmic reticulum (ER) required for the Ca2+ and DAG-dependent activation of PKC. Activated PKC may subsequently phosphorylate and desensitize the receptor. Receptor desensitization also occurs in response to GRKs, the plasma membrane targeting of GRK2 is mediated in part by the ␥ subunit of the heterotrimeric G protein.
cation-specific ion channels that mediate fast excitatory glutamate responses and are subdivided into AMPA/kainate and NMDA receptors (Nakanishi, 1994; Conn and Pin, 1997). In contrast, mGluRs mediate slower glutamate responses by coupling to a variety of second messenger cascades via heterotrimeric G proteins (Nakanishi, 1994; Conn and Pin, 1997). This property allows mGluRs to translate relatively short neuronal activation into long-lasting changes in synaptic activity. As a consequence, mGluR signaling plays an important role in the processes underlying synaptic plasticity (e.g. memory and learning; Riedel, 1996). The mGluR family of receptors constitutes an unique subclass of GPCRs that bear no sequence or structural homology to prototypic Class 1 (rhodopsin, 2 -adrenergic family) and Class 2 (secretin family) GPCRs other than the retention of the seven transmembrane spanning domain topology that is characteristic of a GPCR (4). Other members of the mGluR subfamily of receptors bear sequence homology to GABAB receptors, Ca2+ -sensing receptors and many pheromone receptors. Unlike prototypic GPCR family members, mGluRs couple to heterotrimeric G proteins via their second intracellular loop domain rather than the third intracellular loop domain utilized by Class 1 and 2 GPCRs (Gomeza et al., 1996). The mGluR family consists of eight receptor subtypes many of which exist as multiple splice variants (Conn and Pin, 1997). The mGluR subtypes can be subdivided into three groups on the basis of sequence homology, pharmacology and G protein-coupling specificity. Group 1 mGluRs (mGluR1 and mGluR5) are coupled via Gq to the stimulation of phospholipase C (PLC) leading to increases in intracellular inositol 1,4,5-triphosphate (InsP3 ) concentration, the release of intracellular Ca2+ stores, and the activation of protein kinase C (PKC) as well as through Gs to the activation of
adenylyl cyclase resulting in cAMP formation (Fig. 1). Group 2 (mGluR2 and mGluR3) and Group 3 mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8) are negatively coupled to adenylyl cyclase (Nakanishi, 1994; Conn and Pin, 1997). These receptors are localized at both pre- and post-synaptic sites where they regulate both glutamate release and the excitability of the post-synaptic membrane.
3. Global paradigms of GPCR desensitization Receptor desensitization fulfills an important physiological role by acting as the “thermostatic” or “feedback” mechanism limiting both acute and chronic over stimulation of GPCR signal transduction cascades. Receptor desensitization may be particularly relevant to the regulation of Group I mGluR activity, since the over stimulation of these receptors involves excitotoxicity leading to neuronal cell death associated with acute brain ischemia and neurotrauma (Nicoletti et al., 1996; Alessandri and Bullock, 1998; Bordi and Ugolini, 1999; Calabresi et al., 1999). GPCR desensitization involves the culmination of several distinct events: the uncoupling of receptors from their heterotrimeric G proteins, the internalization (endocytosis) of receptors to endosomes and down regulation. The time-frames over which these processes occur ranges from seconds (phosphorylation) to minutes (endocytosis) and even hours (down regulation). The extent of receptor desensitization varies from complete termination of signaling, as observed in the visual and olfactory systems, to the attenuation of agonist potency and maximal responsiveness, such as is observed for the 2 -adrenergic receptor (2 AR; Pippig et al., 1995; Sakmar, 1998). Receptor internalization not only contributes to the GPCR desensitization, but is also required for
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
the dephosphorylation and resensitization of many GPCRs (Pippig et al., 1995; Zhang et al., 1997). Down regulation, a decrease in the total cellular complement of GPCRs, occurs in response to long-term exposure to agonist (minutes to hours). The decrease in the total cellular complement of GPCRs results from two processes: increased lysosomal degradation of pre-existing receptors and decreased mRNA and protein synthesis (Bouvier et al., 1989).
4. Receptor phosphorylation At least four families of protein kinases exhibit the capacity to phosphorylate GPCRs: (1) second messengerdependent protein kinases (e.g. cAMP-dependent protein kinase, PKA and PKC); (2) casein kinase 1␣; (3) tyrosine kinases; (4) the G protein-coupled receptor kinases (GRKs; Valiquette et al., 1990; Premont et al., 1995; Budd et al., 2000). GRKs specifically phosphorylate the agonist-activated form of GPCRs and thereby promote the binding of arresting proteins (arrestins) that further uncouple the receptors by interdicting receptor/G protein interactions (Benovic et al., 1987; Lohse et al., 1990). While both second messenger-dependent protein kinases and GRKs contribute to the agonist-dependent desensitization of GPCRs, second messenger-dependent protein kinases exhibit the capacity to indiscriminately phosphorylate and desensitize receptors that have not been exposed to agonist (reviewed by Premont et al., 1995). The relative contribution of second messenger-dependent protein kinases and GRKs to the overall GPCR desensitization process still remains to be determined. Second messenger-dependent protein kinases are activated in response to GPCR-stimulated increases in intracellular second messengers such as cAMP, Ca2+ and diacylglycerol (DAG). These kinases phosphorylate GPCRs at relatively well-defined phosphorylation consensus sites within their intracellular loops and carboxyl-terminal tail domains. In contrast, the consensus sites for GRK-mediated phosphorylation remain ill-defined. There are seven members of the GRK family, that can be subdivided into three groups based on sequence and functional homology: (1) GRK1 (rhodopsin kinase) and GRK7 (a new candidate cone opsin kinase); (2) GRK2 (-adrenergic receptor kinase1, ARK1) and GRK3 (-adrenergic receptor kinase2, ARK2); (3) GRK4, GRK5 and GRK6. The targeting of these kinases to their GPCR targets is mediated by multiple mechanisms that all involve the GRK carboxyl-terminal domains. These mechanisms include: post-translational farnesylation (GRK1 and GRK7) and palmitoylation (GRK4 and GRK6), association with heterotrimeric G protein ␥ subunits and phospholipids (GRK2 and GRK3), and electrostatic interaction between a highly basic amino acid domain with plasma membrane phospholipids (GRK5). GRKs phosphorylate both serine and threonine residues localized within either the third intracellular loop or carboxyl-terminal
321
tail domains of GPCRs (reviewed by Premont et al., 1995; Ferguson, 2001). 4.1. Arrestins GRK-mediated phosphorylation is not sufficient to mediate full inactivation of GPCRs and requires the binding of cytosolic co-factor proteins called arrestins (Lohse et al., 1990). To date, four arrestin family members have been identified. The members of the family can be divided into two groups based on sequence homology, function, and tissue distribution: (1) visual arrestins, S antigen and X-arrestin or C-arrestin; (2) -arrestins, -arrestin1 and -arrestin 2 (reviewed by Ferguson, 2001). Arrestins are cytoplasmic proteins, which following agonist stimulation, translocate rapidly to the plasma membrane to bind their GPCR targets in a GRK-dependent manner (Barak et al., 1997). Arrestins preferentially bind to agonist activated and GRK-phosphorylated GPCRs as opposed to second messenger-dependent protein kinase-phosphorylated or non-phosphorylated receptors (Lohse et al., 1990). However, arrestins will bind to GPCRs with low affinity in the absence of phosphorylation and/or agonist activation (Gurevich et al., 1995). The mechanism(s) by which arrestins contribute to GPCR desensitization involves both the physical uncoupling of GPCRs from heterotrimeric G proteins and the targeting of GPCRs for endocytosis (reviewed by Ferguson, 2001).
5. GPCR endocytosis An important aspect of GPCR activity and regulation is the internalization or endocytosis of agonist-activated receptors into the intracellular membrane compartments of the cell. GPCR internalization appears to be mediated by multiple distinct mechanisms, but the most common mechanism is the -arrestin-dependent targeting of GPCRs for internalization via a clathrin-vesicle mediated pathway (Ferguson et al., 1996; Goodman et al., 1996; Zhang et al., 1996). The targeting of -arrestin-bound receptors for endocytosis appears to involve the interaction of both the clathrin heavy chain and the 2-adaptin subunit of the heterotetrameric AP-2 adaptor complex with the carboxyl-terminal domain of -arrestin 1 and 2 (Goodman et al., 1996; Laporte et al., 1999; Laporte et al., 2000). The functional consequence of GPCR endocytosis appears to be multifaceted. The -arrestin-dependent endocytosis not only contributes to the prolonged desensitization of some GPCRs as the consequence of the internalization of -arrestin/GPCR complexes (Oakley et al., 1999; Anborgh et al., 2000), but it is also required for the resensitization of many GPCRs (reviewed by Ferguson, 2001). Moreover, -arrestin-dependent GPCR endocytosis both couples desensitized GPCRs to G protein-independent signaling pathways and serves to compartmentalize signaling complexes (Luttrell et al., 1999; DeFea et al., 2000).
322
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
6. Metabotropic glutamate receptor desensitization Recently, the mechanisms underlying Group 1 mGluR desensitization has been the subject of intensive investigation by several groups (Fig. 1). These studies reveal that mGluR desensitization may be mediated by both second messenger-dependent protein kinases and GRKs (Herrero et al., 1994; Desai et al., 1996; Gereau and Heinemann, 1998; Ciruela et al., 1999; Dale et al., 2000; Francesconi and Duvoisin, 2000; Sallese et al., 2000). However, these studies have often resulted in discordant results, which may either be the consequence of the cellular system employed or agonist ligand used to activate the receptor. The following section overviews our current understanding of the contribution of second messenger-dependent protein kinases and GRKs in the regulation of Group 1 mGluR signaling. 7. Role of second messenger-dependent protein kinases The desensitization of Group 1 mGluRs is characterized as a decrease in phosphoinositide hydrolysis in response to repeated exposure to agonist. Group 1 mGluR desensitization has been described in neuronal cultures, hippocampal slices and heterologous cell culture systems (e.g. Schoepp and Johnson, 1988; Herrero et al., 1994; Desai et al., 1996). The first indication that second messenger-dependent protein kinases mediate the desensitization of Group 1 mGluRs came from studies utilizing both activators and inhibitors of PKC (Schoepp and Johnson, 1988; Catania et al., 1991; Ciruela et al., 1999). PKC-dependent desensitization is also observed for mGluR1 splice variants (Ciruela et al., 1999). Molecular analysis has revealed that multiple PKC phosphorylation consensus sites are required for mGluR5 desensitization (Gereau and Heinemann, 1998). In particular, threonine residue 606 and serine residue 613 in the first intracellular loop, threonine residue 665 in the second intracellular loop and serine residues 881 and 890 in the carboxyl-terminal tail domains of mGluR5 all appear to contribute to mGluR5 desensitization (Gereau and Heinemann, 1998). The combination of these mutations completely abolishes mGluR5 desensitization (Gereau and Heinemann, 1998). Unfortunately, the potential for PKC-mediated phosphorylation of each of these residues has not been investigated. At the molecular level, the role of PKC phosphorylation in the regulation of mGluR1a signaling has been less intensively investigated. However, threonine residue 695 has been implicated in PKC-mediated mGluR1a desensitization and phosphorylation (Francesconi and Duvoisin, 2000). The replacement of threonine residue 695 with an alanine retards mGluR1a desensitization, whereas replacement with a glutamic acid residue to mimic phosphorylation tends to uncouple the receptor from Gq (Francesconi and Duvoisin, 2000). Interestingly, the desensitization of Group 1 mGluR-mediated Gq signaling processes at
hippocampal synapses is associated with a switch from facilitation to inhibition of excitatory synaptic transmission (Rodriquez-Moreno et al., 1998). This observation is particularly intriguing because mutation of threonine residue 695 to a glutamic acid residue disrupts mGluR1a coupling to Gq, but leaves Gs-signaling through this receptor subtype intact (Francesconi and Duvoisin, 2000). Consequently, the switch in mGluR signal transduction at hippocampal synapse may well be associated with PKC-dependent receptor desensitization. PKC-mediated phosphorylation of mGluRs may also contribute to other aspects of mGluR signaling. In particular, PKC-mediated phosphorylation of mGluR5 appears to antagonize the Ca2+ -dependent association of calmodulin with the receptor (Minakami et al., 1997). Calmodulin binds to mGluR5 at two sites and PKC appears to phosphorylate both regions to antagonize binding. It is hypothesized that this might provide a mechanism by which mGluR activation may regulate ion channel activity (Minakami et al., 1997).
8. Role of GRKs and arrestins Recently, GRKs were also implicated in the desensitization of Group 1 mGluR activity (Dale et al., 2000; Sallese et al., 2000). Dale et al. (2000) found that GRK2 and GRK5 contributed to both the phosphorylation and desensitization of mGluR1a in HEK 293 cells. Although the expression of GRK4 and GRK6 resulted in mGluR1a phosphorylation (Dale and Ferguson, unpublished data), these kinases did not contribute to the desensitization of mGluR1a signaling (Dale et al., 2000). Moreover, the expression of a kinase dead GRK2-K220R mutant blocked agonist-stimulated mGluR1a phosphorylation and resulted in significant impairment of mGluR1a desensitization. This observation is similar to what was reported for the parathyroid hormone receptor, where receptor desensitization was observed to be phosphorylation independent (Dicker et al., 1999). In addition, although -arrestins also contributed to the internalization of mGluR1a (see below), -arrestin binding did not appear to be required for mGluR1a desensitization (Dale et al., 2001a). Consistent with the observation that only GRK2 and GRK5 contributed to the desensitization of agonist-stimulated mGluR1a responses, only the expression of these kinases reduced basal mGluR1a activity (Dale et al., 2000). This reduction in basal mGluR activity was associated with increased cell survival that was mediated in part by reduced apoptotic cell death. In contrast to our own studies, Sallese et al. (2000) reported that GRK2, GRK4, GRK5 and GRK6 expression resulted in mGluR1a desensitization in HEK 293 cells, but that only GRK4 was required for mGluR1a desensitization in cerebellar Purkinje cells. Moreover, the overexpression of a kinase dead dominant-negative GRK4 mutant both blocked mGluR1a phosphorylation and desensitization (Sallese et al., 2000). These studies were originally
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
323
motivated by the interesting observation that GRK4 was expressed in cerebellar Purkinje cells. Consistent with this observation, GRK4 knock down using anti-sense in Purkinje cells resulted in increased mGluR1 signaling.
9. Metabotropic glutamate receptor endocytosis The internalization of mGluR1a appears to be mediated by both agonist-dependent and -independent mechanisms (Ciruela and McIlhinney, 1997; Doherty et al., 1999; Sallese et al., 2000; Dale et al., 2001a; Mundell et al., 2001). Unlike what is observed for many other GPCRs, substantial mGluR1a internalization is observed in the absence of agonist (Doherty et al., 1999; Dale et al., 2001a). This has been demonstrated both by the loss of cell surface receptors and the substantial localization of green fluorescent protein (GFP)-tagged mGluR to endocytic vesicles in the absence of agonist stimulation (Doherty et al., 1999; Dale et al., 2001a). Although this internalization may be mediated by the release of glutamate from cells in culture, treatment with antagonists has no effect on the extent of cell surface receptor loss (Dale et al., 2001a). The agonist-independent internalization of mGluR1a is insensitive to both -arrestin and dynamin dominant-negative mutants but still appears to be mediated by clathrin-coated vesicle mediated endocytic pathway (Dale et al., 2001a). The agonist-independent internalization of mGluR1a may be impeded in neurons through the association of Homer1c, potentially due to Homer1c-dependent clustering of receptors (Tadokoro et al., 1999; Ciruela et al., 2000). Agonist stimulation also promotes mGluR1a endocytosis in a GRK and -arrestin-dependent manner (Sallese et al., 2000; Dale et al., 2001b; Mundell et al., 2001; Fig. 2). Agonist-stimulated mGluR1a internalization can be blocked by the expression of both -arrestin and dynamin dominant-negative mutants (Mundell et al., 2001). mGluR1a activation also promotes the membrane translocation and internalization of GFP-tagged -arrestins (Dale et al., 2001a; Mundell et al., 2001). The overexpression of both -arrestin 1 and -arrestin 2 also increases the extent of mGluR1a internalization in response to glutamate (Mundell et al., 2001), but only -arrestin 1 appears to contribute to mGluR1a internalization in response to quisqualate (Dale et al., 2001a). These observations suggest that different mGluR agonists might stabilize different receptor conformations required for the association of arrestin proteins with mGluR1a. This is similar to what has previously been reported for the mu-opioid receptor (Zhang et al., 1998). Although it appears mGluRs internalized in the absence of agonist are efficiently recycled back to the cell surface, it is unknown whether mGluRs internalized in response to agonist can be recycled (Fig. 2). Consequently, a great deal remains to be learned about the mechanisms regulating both the agonist-dependent and -independent trafficking of mGluRs.
Fig. 2. Schematic representation of the mechanism underlying the agonist-stimulated internalization of Group 1 mGluRs. In response to agonist-stimulated GRK phosphorylation (P, phosphate), -arrestin1 (Arr1) translocates from the cytosol to the membrane and binds to the GRK-phosphorylated receptor. The association of -arrestin with both clathrin and the 2-adaptin subunit of the heterotetrameric AP2 adaptor complex targets mGluRs to clathrin-coated pits for internalization in clathrin-coated vesicles. The -arrestin 1 is internalized with the receptor, but it is unknown whether -arrestin 1 is released in the endosomal compartment to allow receptor dephosphorylation by a G protein-coupled receptor phosphatase (GRP). It is also unknown whether mGluRs internalized in response to agonist stimulation are recycled back to the cell surface.
10. Role of Group 1 mGluR desensitization in the patterning of effector enzyme activity The activation of Group 1 mGluRs gives rise to repetitive base-line separated Ca2+ -transients (oscillations; Kawabata et al., 1996; Nakahara et al., 1997; Kawabata et al., 1998; Codazzi et al., 2001; Dale et al., 2001b). These Ca2+ oscillations in response to mGluR activation are observed in immature neuronal cultures, developing neocortex, astrocytes, and heterologous cell cultures (Kawabata et al., 1996; Nakahara et al., 1997; Kawabata et al., 1998; Flint et al., 1999; Codazzi et al., 2001; Dale et al., 2001b). Low frequency Ca2+ oscillations are observed in response to mGluR1a activation (1 per 100–300 s), whereas mGluR5a stimulates oscillations at higher frequencies (1 per 25–40 s; Kawabata et al., 1998; Dale et al., 2001b). The Ca2+ oscillations at different frequencies are likely important for the selective activation of different transcription factors and/or the activation of protein kinases. This may be important for modulating long-term changes in synaptic activity that either occur during neuronal development or are associated with learning and memory. Recently, we demonstrated that PK repetitively redistributes between the cytosol and plasma membrane in response to mGluR1a- and mGluR5a-stimulated oscillations in intracellular free Ca2+ (Dale et al., 2001b). The Ca2+ and
324
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
PK oscillations in response to both mGluR1a and mGluR5a activation are dependent upon the release of Ca2+ from intracellular stores, but only mGluR1a-stimulated oscillations are dependent upon extracellular Ca2+ (Kawabata et al., 1998; Dale et al., 2000b). Oscillatory mGluR signaling appears to involve the repetitive receptor coupling to Gq resulting in the cyclical activation of phospholipase C thereby driving oscillations in both InsP3 formation and Ca2+ release from intracellular stores (Dale et al., 2001b). The differences in the rate at which mGluR1a and mGluR5a stimulate InsP3 , Ca2+ , and PKC oscillations appears to be a receptor-specific phenomenon that is regulated by the identity of a single amino acid residue localized within the G protein coupling domain of the receptors (see below, Kawabata et al., 1996; Dale et al., 2001b). The oscillatory activation of conventional PKC isoforms is not only driven by the oscillatory release of Ca2+ from intracellular stores but also by oscillations in intracellular DAG concentrations (Codazzi et al., 2001). Oscillations in conventional PKC isoforms between the plasma membrane and cytosol have been observed in both astrocytes and heterologous cell cultures (Codazzi et al., 2001; Dale et al., 2001b). In astrocytes, PKC oscillations occur in response to the activation of endogenously expressed mGluR5 (Codazzi et al., 2001). Ca2+ oscillations observed in response to Group 1 mGluR activation may occur as the consequence of two distinct mechanisms: (1) repetitive G protein-coupling in response to repetitive cycles of receptor phosphorylation and dephosphorylation or (2) Ca2+ -induced Ca2+ release from intracellular stores in response to low levels of InsP3 formation followed by the inhibition of InsP3 receptors at the higher concentrations of intracellular Ca2+ attained at the peak of the Ca2+ release. Generally, Ca2+ oscillations in response to mGluR activation have been proposed to occur as the consequence of the repetitive feedback phosphorylation by PKC (Kawabata et al., 1996; Codazzi et al., 2001). However, the mutation of each of the PKC phosphorylation sites in mGluR1a either individually or in clusters has no effect on quisqualate-stimulated Ca2+ and PK oscillations (Dale et al., 2001b). Moreover, the treatment of cells with PKC inhibitors results in the inhibition of PKC oscillations without effecting Ca2+ oscillations in response to either mGluR1a or mGluR5a activation (Dale et al., 2001b). These observations indicate that repetitive cycles of mGluR phosphorylation and dephosphorylation are not sufficient to explain oscillatory mGluR signaling patterns. It is possible that mGluR-stimulated oscillatory signaling in response to agonist activation is related to the structure of the mGluR ligand-binding domain. The mGluR ligand-binding domain consists of two globular lobes that exist in an equilibrium between open and closed conformations in the absence of ligand (Kunishima et al., 2000). Agonist stimulation may shift the probability that the ligand-binding domain exists in a closed (active) conformation. However, it is possible that the ligand-binding domain may spontaneous shift to an open conformation (inactive)
even in the presence of agonist and that this shift may be influenced by the structural determinants of intracellular domains required for receptor G protein coupling. Consistent with this idea, mutagenesis of the aspartic acid residue at position 854 of mGluR1a to an alanine residue results in a mGluR1a mutant that signals Ca2+ and PKC oscillations in the absence of agonist stimulation (Dale et al., 2001b). However, this theoretical model still requires to be substantiated with empirical data. As indicated above, differences in the rate of Ca2+ and PKC oscillations in response to mGluR1a and mGluR5a activation are regulated by the identity of a single amino acid residue localized to the G protein-coupling domains of the receptors (Kawabata et al., 1996; Dale et al., 2001a). Aspartic acid residue at position 854 in mGluR1a and the corresponding threonine residue at position 840 in mGluR5a are the only amino acid residues that differ between the G protein-coupling domains of mGluR1a and mGluR5a. The threonine residue in mGluR5a falls within the context of a putative PKC phosphorylation consensus sequence and it was previously proposed that phosphorylation of this site might account for the observed differences in the frequency of Ca2+ oscillations stimulated by mGluR1a and mGluR5a (Kawabata et al., 1996). Indeed, substitution of aspartic acid residue at position 854 in mGluR1a with a threonine residue to reconstitute a PKC phosphorylation consensus sequence accelerated the rate of Ca2+ and PKC oscillations induced by mGluR1a activation (Kawabata et al., 1996; Dale et al., 2001b). However, the acceleration of mGluR1a-stimulated Ca2+ and PKC oscillation rates does not appear to require PKC phosphorylation, because the substitution of aspartic acid residue 854 with either alanine or glycine residues garners the same result (Dale et al., 2001b). Thus, it appears that receptor-specific oscillatory G protein-coupling rates represent intrinsic receptor properties that are regulated by charge density of the residue localized at position 854 in mGluR1a or position 840 in mGluR5a. It is likely that negatively charged residues antagonize and/or slow mGluR coupling to G proteins.
11. Conclusions In conclusion, the temporal regulation of Ca2+ signaling represents a universal mechanism exploited by both excitable and non-excitable cells to regulate specific cellular responses. The Ca2+ oscillations in response to mGluR activation are observed in immature neuronal cultures and developing neocortex. Several studies have now determined that mGluR desensitization is regulated by the same mechanisms involved in the desensitization of more prototypic GPCRs. The activation of this important class of GPCR also results in the receptor subtype-specific spatial-temporal patterning of effector enzyme activation. The patterning of these signals appears to be independent of receptor desensitization, but rather appears to occur as the consequence of
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
the unique structure activity of Group 1 mGluRs. Among the challenges awaiting investigators in the field will be to determine how information encoded by receptor-specific spatial-temporal signaling patterns are interpreted by cells to regulate diverse neuronal functions in both the developing and adult nervous systems.
Acknowledgements S.S.G.F. is the recipient of a McDonald Salary Award and Great West/London Life Salary Award from the Heart and Stroke Foundation of Canada and a Premier’s Research Excellence Award from the Province of Ontario. A.V.B. is the recipient of a Canadian Hypertension Society/Canadian Institutes of Health Research (CIHR) fellowship. This work was supported by CIHR Grant MA-15506 to S.S.G.F. References Anborgh, P.H., Dale, L.B., Seachrist, J., Ferguson, S.S.G., 2000. Differential regulation of 2 -adrenergic and angiotensin II type 1A receptor trafficking and resensitization: role of carboxyl-terminal domains. Mol. Endocrinol. 14, 2040–2053. Alessandri, B., Bullock, R., 1998. Glutamate and its receptors in the pathophysiology of brain and spinal cord injuries. Prog. Brain Res. 116, 303–330. Barak, L.S., Ferguson, S.S.G., Zhang, J., Caron, M.G., 1997. A -arrestin/ green fluorescent protein biosensor for detecting G protein-coupled receptor activation. J. Biol. Chem. 272, 27497–27500. Benovic, J.L., Kühn, H., Weyland, I., Codina, J., Caron, M.G., Lefkowitz, R.J., 1987. Functional desensitization of the isolated -adrenergic receptor by the -adrenergic receptor kinase: potential role of an analog of the retinal protein arrestin (48 kDa protein). Proc. Natl. Acad. Sci. U.S.A. 84, 8879–8882. Bordi, F., Ugolini, A., 1999. Group I metabotropic glutamate receptors: implications for brain diseases. Progr. Neurobiol. 59, 55–79. Bouvier, M., Collins, S., O’Dowd, B.F., Campbell, P.T., de Blasi, A., Kobilka, B.K., MacGregor, C., Irons, G.P., Caron, M.G., Lefkowitz, R.J., 1989. Two distinct pathways for cAMP-mediated down regulation of the 2 -adrenergic receptor: phosphorylation of the receptor and regulation of its mRNA. J. Biol. Chem. 264, 16786–16792. Budd, D.C., McDonald, J.E., Tobin, A.B., 2000. Phosphorylation and regulation of a Gq/11-coupled receptor by casein kinase 1␣. J. Biol. Chem. 275, 19667–19675. Calabresi, P., Centonze, D., Pisani, A., Bernardi, G., 1999. Metabotropic glutamate receptors and cell-type-specific vulnerability in the striatum: implications for ischemia and Huntington’s disease. Exp. Neurol. 158, 97–108. Catania, M.V., Aronica, E., Sortino, M.A., Canonico, P.L., Nicoletti, F., 1991. Desensitization of metabotropic glutamate receptors in neuronal cultures. J. Neurochem. 56, 1329–1335. Ciruela, F., McIlhinney, R.A., 1997. Differential internalization of mGluR1 splice variants in response to agonist and phorbol esters in permanently transfected BHK cells. FEBS Lett. 418, 83–86. Ciruela, F., Giacometti, A., McIlhinney, R.A., 1999. Functional regulation of metabotropic glutamate receptor type 1c: a role for phosphorylation in the desensitization of the receptor. FEBS Lett. 462, 278–282. Ciruela, F., Soloviev, M.M., Chan, W.Y., McIlhinney, R.A., 2000. Homer-1c/Vesl-1L modulates the cell surface targeting of metabotropic glutamate receptor type 1alpha: evidence for an anchoring function. Mol. Cell. Neurosci. 15, 36–50.
325
Codazzi, F., Teruel, M.N., Meyer, T., 2001. Control of astrocyte Ca2+ oscillations and waves by oscillating translocation and activation of protein kinase. Curr. Biol. 11, 1089–1097. Conn, P.J., Pin, J.-P., 1997. Pharmacology and functions of metabotropic glutamate receptors. Ann. Rev. Pharmacol. Toxicol. 37, 205–237. Dale, L.B., Bhattacharya, M., Anborgh, P.H., Murdoch, B., Bhatia, M., Nakanishi, S., Ferguson, S.S.G., 2000. G protein-coupled receptor kinase mediated desensitization of metabotropic glutamate receptor 1␣ protects against cell death. J. Biol. Chem. 275, 38213–38220. Dale, L.B., Bhattacharya, M., Seachrist, J.L., Anborgh, P.H., Ferguson, S.S.G., 2001a. Agonist-dependent and -independent internalization of metabotropic glutamate receptor 1a: -arrestin isoform specific endocytosis. Mol. Pharmacol. 60, 1243–1253. Dale, L.B., Babwah, A.V., Bhattacharya, M., Kelvin, D.J., Ferguson, S.S.G., 2001b. Spatial-temporal dynamics of metabotropic glutamate receptor-mediated inositol 1,4,5-triphosphate, Ca2+ , and protein kinase C oscillations. J. Biol. Chem. 276, 35900–35908. DeFea, K.A., Zalevsky, J., Thoma, M.S., Dery, O., Mullins, R.D., Bunnett, N.W., 2000. -Arrestin-dependent endocytosis of proteinase-activated receptor 2 is required for intracellular targeting of activated ERK1/2. J. Cell Biol. 148, 1267–1281. Dicker, F., Quitterer, U., Winstel, R., Honold, K., Lohse, M.J., 1999. Phosphorylation-independent inhibition of parathyroid hormone receptor signaling by G protein-coupled receptor kinases. Proc. Natl. Acad. Sci. U.S.A. 96, 5476–5481. Desai, M.A., Burnett, J.P., Mayne, N.G., Schoepp, D.D., 1996. Pharmacological characterization of desensitization in a human mGlu1 alpha-expressing non-neuronal cell line co-transfected with a glutamate transporter. Br. J. Pharmacol. 118, 1558–15564. Doherty, A.J., Coutinho, V., Collingridge, G.L., Henley, J.M., 1999. Rapid internalization and surface expression of a functional, fluorescently tagged G-protein-coupled glutamate receptor. Biochem. J. 341, 415–422. Ferguson, S.S.G., Downey Jr, W.E., Colapietro, A.M., Barak, L.S., Menard, L., Caron, M.G., 1996. Role of -arrestin in mediating agonist-promoted G protein-coupled receptor internalization. Science 271, 363–366. Ferguson, S.S.G., 2001. Evolving concepts in G protein-coupled receptor endocytosis: the role in receptor desensitization and signaling. Pharmacol. Rev. 53, 1–24. Flint, A.C., Dammerman, R.S., Kriegstein, A.R., 1999. Endogenous activation of metabotropic glutamate receptors in neocortical development causes calcium oscillations. Proc. Natl. Acad. Sci. U.S.A. 96, 12144–12149. Francesconi, A., Duvoisin, R.M., 2000. Opposing effects of protein kinase C and protein kinase A on metabotropic glutamate receptor signaling: selective desensitization of the inositol triphosphate/Ca2+ pathway by phosphorylation of the receptor-G protein-coupling domain. Proc. Natl. Acad. Sci. U.S.A. 97, 6185–6190. Gereau, R.W., Heinemann, S.F., 1998. Role of protein kinase C phosphorylation in rapid desensitization of metabotropic glutamate receptor 5. Neuron 20, 143–151. Goodman, O.B., Krupnick Jr., J.G., Santini, F., Gurevich, V.V., Penn, R.B., Gagnon, A.W., Keen, J.H., Benovic, J.L., 1996. -Arrestin acts as a clathrin adaptor in endocytosis of the 2 -adrenergic receptor. Nature 383, 447–450. Gomeza, J., Joly, C., Kuhn, R., Knopfel, T., Bockaert, J., Pin, J.P., 1996. The second intracellular loop of metabotropic glutamate receptor 1 cooperates with the other intracellular domains to control coupling to G-proteins. J. Biol. Chem. 271, 2199–2205. Gurevich, V.V., Dion, S.B., Onorato, J.J., Ptasienski, J., Kim, C.M., Sterne-Marr, R., Hosey, M.M., Benovic, J.L., 1995. Arrestin interactions with G protein-coupled receptors: direct binding studies of wild type and mutant arrestins with rhodopsin, 2 -adrenergic, and m2 muscarinic cholinergic receptors. J. Biol. Chem. 270, 720–731. Herrero, I., Miras-Portugal, T., Sanchez-Prieto, J., 1994. Rapid desensitization of the metabotropic glutamate receptor that facilitates glutamate
326
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
release in rat cerebrocortical nerve terminals. Eur. J. Neurosci. 6, 115–120. Kawabata, S., Tsutsumi, R., Kohara, A., Yamaguchi, T., Nakanishi, S., Okada, M., 1996. Control of calcium oscillations by phosphorylation of metabotropic glutamate receptors. Nature 383, 89–92. Kawabata, S., Kohara, A., Tsutsumi, R., Itahana, H., Hatashibe, S., Yamaguchi, T., Okada, M., 1998. Diversity of calcium signaling by metabotropic glutamate receptors. J. Biol. Chem. 273, 17381–17385. Kunishima, N., Shimada, Y., Tsuji, Y., Sato, T., Yamamoto, M., Kumasaka, T., Nakanishi, S., Jingami, H., Morikawa, K., 2000. Structural basis of glutamate recognition by a dimeric metabotropic glutamate receptor. Nature 407, 971–977. Laporte, S.A., Oakley, R.H., Zhang, J., Holt, J.A., Ferguson, S.S.G., Caron, M.G., Barak, L.S., 1999. The 2 -adrenergic receptor/-arrestin complex recruits the clathrin adaptor AP-2 during endocytosis. Proc. Natl. Acad. Sci. U.S.A. 96, 3712–3717. Laporte, S.A., Oakley, R.H., Holt, J.A., Barak, L.S., Caron, M.G., 2000. The interaction of -arrestin with the AP-2 adaptor, rather than clathrin, is required for the clustering of 2 -adrenergic receptor into clathrin-coated pits. J. Biol. Chem. 275, 23120–32126. Lohse, M.J., Benovic, J.L., Codina, J., Caron, M.G., Lefkowitz, R.J., 1990. -Arrestin: a protein that regulates -adrenergic receptor function. Science 248, 1547–1550. Luttrell, L.M., Ferguson, S.S.G., Daaka, Y., Miller, W.E., Maudsley, S., Della Rocca, G.J., Lin, F., Kawakatsu, H., Owada, K., Luttrell, D.K., Caron, M.G., Lefkowitz, R.J., 1999. -Arrestin-dependent formation of 2 -adrenergic receptor-Src protein kinase complexes. Science 283, 655–661. Minakami, R., Jinnai, N., Sugiyama, H., 1997. Phosphorylation and calmodulin binding of the metabotropic glutamate receptor subtype 5 (mGluR5) are antagonistic in vitro. J. Biol. Chem. 272, 20291–20298. Mundell, S.J., Matharu, A.L., Pula, G., Roberts, P.J., Kelly, E., 2001. Agonist-induced internalization of the metabotropic glutamate receptor 1␣ is arrestin- and dynamin-dependent. J. Neurochem. 78, 546–551. Nakahara, K., Okada, M., Nakanishi, S., 1997. The metabotropic glutamate receptor mGluR5 induces calcium oscillations in cultured astrocytes via protein kinase C phosphorylation. J. Neurochem. 69, 1467–1475. Nicoletti, F., Brunom, V., Copanim, A., Casabona, G., Knopfel, T., 1996. Metabotropic glutamate receptors: a new target for the therapy of neurodegenerative disorders? Trends Neurosci. 19, 267–271. Nakanishi, S., 1994. Metabotropic glutamate receptors: synaptic transmission, modulation, and plasticity. Neuron 13, 1031–1037. Oakley, R.H., Laporte, S.A., Holt, J.A., Barak, L.S., Caron, M.G., 1999. Association of -arrestin with G protein-coupled receptors
during clathrin-mediated endocytosis dictates the profile of receptor resensitization. J. Biol. Chem. 274, 32248–32257. Premont, R.T., Inglese, J., Lefkowitz, R.J., 1995. Protein kinases that phosphorylate activated G protein-coupled receptors. FASEB J. 9, 175–182. Pippig, S., Andexinger, S., Lohse, M.J., 1995. Sequestration and recycling of 2 -adrenergic receptors permit resensitization. Mol. Pharmacol. 47, 666–676. Riedel, G., 1996. Function of metabotropic glutamate receptors in learning and memory. Trends Neurosci. 19, 219–224. Rodriquez-Moreno, A., Sistiaga, A., Lerma, J., Sanchez-Prieto, J., 1998. Switch from facilitation to inhibition in the control of glutamate release by metabotropic glutamate receptor. Neuron 21, 1477–1486. Sallese, M., Salvatore, L., D’Urbano, E., Sala, G., Storto, M., Launey, T., Nicoletti, F., Knopfel, T., De Blasi, A., 2000. The G-protein-coupled receptor kinase GRK4 mediates homologous desensitization of metabotropic glutamate receptor 1. FASEB J. 14, 2569–2580. Sakmar, T.P., 1998. Rhodopsin: a prototypical G protein-coupled receptor. Prog. Nucl. Acid Res. Mol. Biol. 59, 1–34. Schoepp, D.D., Johnson, B.G., 1988. Selective inhibition of excitatory amino acid-stimulated phosphoinositide hydrolysis in the rat hippocampus by activation of protein kinase C. Biochem. Pharmacol. 37, 4299–4305. Tadokoro, S., Tachibana, T., Imanaka, T., Nishida, W., Sobue, K., 1999. Involvement of unique leucine-zipper motif of PSD-Zip45 (Homer 1c/Vesl-1L) in Group 1 metabotropic glutamate receptor clustering. Proc. Natl. Acad. Sci. U.S.A. 96, 13801–13806. Valiquette, M., Bonin, H., Hnatowich, M., Caron, M.G., Lefkowitz, R.J., Bouvier, M., 1990. Involvement of tyrosine residues located in the carboxyl tail of the human 2 -adrenergic receptor in agonist-induced down regulation of the receptor. Proc. Natl. Acad. Sci. U.S.A. 87, 5089–5093. Zhang, J., Ferguson, S.S.G., Barak, L.S., Menard, L., Caron, M.G., 1996. Dynamin and -arrestin reveal distinct mechanisms for G protein-coupled receptor internalization. J. Biol. Chem. 271, 18302– 18305. Zhang, J., Barak, L.S., Winkler, K.E., Caron, M.G., Ferguson, S.S.G., 1997. A central role for -arrestins and clathrin-coated vesicle-mediated endocytosis in 2 -adrenergic receptor resensitization. J. Biol. Chem. 272, 27005–27014. Zhang, J., Ferguson, S.S.G., Barak, L.S., Bodduluri, S., Laporte, S.A., Law, P.-Y., Caron, M.G., 1998. Role for G protein-coupled receptor kinase in agonist-specific regulation of -opioid receptor responsiveness. Proc. Natl. Acad. Sci. U.S.A. 95, 7157–7162.
Mechanisms of metabotropic glutamate receptor desensitization: role in the patterning of effector enzyme activation Lianne B. Dale a , Andy V. Babwah a , Stephen S.G. Ferguson a,b,c,d,∗ a
John P. Robarts Research Institute, 100 Perth Drive, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6A 5K8 b Department of Medicine, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6G 5K8 c Department of Physiology, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6G 5K8 d Department of Pharmacology and Toxicology, University of Western Ontario, P.O. Box 5015, London, Ont., Canada N6G 5K8 Received 1 December 2001; accepted 28 February 2002
Abstract Metabotropic glutamate receptors (mGluRs) constitute an unique subclass of G protein-coupled receptors (GPCRs). These receptors are activated by the excitatory amino acid glutamate and play an essential role in regulating neural development and plasticity. In the present review, we overview the current understanding regarding the molecular mechanisms involved in the desensitization and endocytosis of Group 1 mGluRs as well as the relative contribution of desensitization to the spatial-temporal patterning of glutamate receptor signaling. Similar to what has been reported previously for prototypic GPCRs, mGluRs desensitization is mediated by second messenger-dependent protein kinases and GPCR kinases (GRKs). However, it remains to be determined whether mGluRs phosphorylation by GRKs and -arrestin binding are absolutely required for desensitization. Group 1 mGluRs endocytosis is both agonist-dependent and -independent. Agonist-dependent mGluRs internalization is mediated by a -arrestin- and dynamin-dependent clathrin-coated vesicle dependent endocytic pathway. The activation of Group 1 mGluRs also results in oscillatory Gq protein-coupling leading to the cyclical activation of phospholipase C thereby stimulating oscillations in both inositol 1,4,5-triphosphate formation and Ca2+ release from intracellular stores. These glutamate receptor-stimulated Ca2+ oscillations are translated into the synchronous activation of protein kinase C (PKC), which has led to the hypothesis that oscillatory mGluRs signaling involves the repetitive phosphorylation of mGluRs by PKC. However, recent experimental evidence suggests that oscillatory signaling is an intrinsic glutamate receptor property that is independent of feedback receptor phosphorylation by PKC. The challenge in the future will be to determine the structural determinants underlying mGluRs-mediated spatial-temporal signaling as well as to understand how complex signaling patterns can be interpreted by cells in both the developing and adult nervous systems. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Metabotropic glutamate receptor; G protein-coupled receptor; Desensitization
1. Introduction Metabotropic glutamate receptors (mGluRs) are members of the G protein-coupled receptor (GPCR) superfamily, the largest family of integral membrane receptor proteins. GPCRs are found in organisms ranging from slime mold and yeast to mammals and transduce the information provided by a wide variety of extracellular signals, including light, odor, taste, pheromones, hormones and neurotransmitters, to the interior of cells. GPCR signal transduction is achieved by the coupling of these receptors to a wide variety of effector systems through heterotrimeric guanine nucleotide binding proteins (G proteins; reviewed by Ferguson, 2001). Agonist-activated GPCRs function as guanine nucleotide ∗
Corresponding author. Tel.: +1-519-663-3825; fax: +1-519-663-3789. E-mail address: [email protected] (S.S.G. Ferguson).
exchange factors facilitating the exchange of GDP for GTP on the G protein ␣ subunit, thus allowing the functional dissociation of the G␣ and G␥ subunits. The freely dissociated heterotrimeric G protein, G␣ and G␥ subunits then either positively or negatively regulate a variety of effector systems resulting in changes in intracellular second messenger levels and/or ion conductance.
2. The metabotropic glutamate receptor family Glutamate is the major excitatory neurotransmitter in the central nervous system and is essential in the regulation of brain functions and neural cell development (Nakanishi, 1994; Conn and Pin, 1997). Glutamate mediates its actions through two distinct types of receptors, ionotropic glutamate receptors (iGluRs) and mGluRs. The iGluRs are
0197-0186/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 1 9 7 - 0 1 8 6 ( 0 2 ) 0 0 0 7 3 - 6
320
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
Fig. 1. Schematic representation of Group 1 mGluR signal transduction and desensitization mechanisms. Group 1 mGluR agonist activation results in receptor coupling to both Gq and Gs heterotrimeric G proteins. Gs receptor coupling stimulates adenylyl cyclase (AC) and the catalysis of cAMP from ATP. Gq receptor coupling stimulates the activity of phospholipase C (PLC), resulting in intracellular InsP3 and DAG formation. InsP3 activates the InsP3 receptor (InsP3 R) resulting in the release of intracellular Ca2+ stores from the endoplasmic reticulum (ER) required for the Ca2+ and DAG-dependent activation of PKC. Activated PKC may subsequently phosphorylate and desensitize the receptor. Receptor desensitization also occurs in response to GRKs, the plasma membrane targeting of GRK2 is mediated in part by the ␥ subunit of the heterotrimeric G protein.
cation-specific ion channels that mediate fast excitatory glutamate responses and are subdivided into AMPA/kainate and NMDA receptors (Nakanishi, 1994; Conn and Pin, 1997). In contrast, mGluRs mediate slower glutamate responses by coupling to a variety of second messenger cascades via heterotrimeric G proteins (Nakanishi, 1994; Conn and Pin, 1997). This property allows mGluRs to translate relatively short neuronal activation into long-lasting changes in synaptic activity. As a consequence, mGluR signaling plays an important role in the processes underlying synaptic plasticity (e.g. memory and learning; Riedel, 1996). The mGluR family of receptors constitutes an unique subclass of GPCRs that bear no sequence or structural homology to prototypic Class 1 (rhodopsin, 2 -adrenergic family) and Class 2 (secretin family) GPCRs other than the retention of the seven transmembrane spanning domain topology that is characteristic of a GPCR (4). Other members of the mGluR subfamily of receptors bear sequence homology to GABAB receptors, Ca2+ -sensing receptors and many pheromone receptors. Unlike prototypic GPCR family members, mGluRs couple to heterotrimeric G proteins via their second intracellular loop domain rather than the third intracellular loop domain utilized by Class 1 and 2 GPCRs (Gomeza et al., 1996). The mGluR family consists of eight receptor subtypes many of which exist as multiple splice variants (Conn and Pin, 1997). The mGluR subtypes can be subdivided into three groups on the basis of sequence homology, pharmacology and G protein-coupling specificity. Group 1 mGluRs (mGluR1 and mGluR5) are coupled via Gq to the stimulation of phospholipase C (PLC) leading to increases in intracellular inositol 1,4,5-triphosphate (InsP3 ) concentration, the release of intracellular Ca2+ stores, and the activation of protein kinase C (PKC) as well as through Gs to the activation of
adenylyl cyclase resulting in cAMP formation (Fig. 1). Group 2 (mGluR2 and mGluR3) and Group 3 mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8) are negatively coupled to adenylyl cyclase (Nakanishi, 1994; Conn and Pin, 1997). These receptors are localized at both pre- and post-synaptic sites where they regulate both glutamate release and the excitability of the post-synaptic membrane.
3. Global paradigms of GPCR desensitization Receptor desensitization fulfills an important physiological role by acting as the “thermostatic” or “feedback” mechanism limiting both acute and chronic over stimulation of GPCR signal transduction cascades. Receptor desensitization may be particularly relevant to the regulation of Group I mGluR activity, since the over stimulation of these receptors involves excitotoxicity leading to neuronal cell death associated with acute brain ischemia and neurotrauma (Nicoletti et al., 1996; Alessandri and Bullock, 1998; Bordi and Ugolini, 1999; Calabresi et al., 1999). GPCR desensitization involves the culmination of several distinct events: the uncoupling of receptors from their heterotrimeric G proteins, the internalization (endocytosis) of receptors to endosomes and down regulation. The time-frames over which these processes occur ranges from seconds (phosphorylation) to minutes (endocytosis) and even hours (down regulation). The extent of receptor desensitization varies from complete termination of signaling, as observed in the visual and olfactory systems, to the attenuation of agonist potency and maximal responsiveness, such as is observed for the 2 -adrenergic receptor (2 AR; Pippig et al., 1995; Sakmar, 1998). Receptor internalization not only contributes to the GPCR desensitization, but is also required for
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
the dephosphorylation and resensitization of many GPCRs (Pippig et al., 1995; Zhang et al., 1997). Down regulation, a decrease in the total cellular complement of GPCRs, occurs in response to long-term exposure to agonist (minutes to hours). The decrease in the total cellular complement of GPCRs results from two processes: increased lysosomal degradation of pre-existing receptors and decreased mRNA and protein synthesis (Bouvier et al., 1989).
4. Receptor phosphorylation At least four families of protein kinases exhibit the capacity to phosphorylate GPCRs: (1) second messengerdependent protein kinases (e.g. cAMP-dependent protein kinase, PKA and PKC); (2) casein kinase 1␣; (3) tyrosine kinases; (4) the G protein-coupled receptor kinases (GRKs; Valiquette et al., 1990; Premont et al., 1995; Budd et al., 2000). GRKs specifically phosphorylate the agonist-activated form of GPCRs and thereby promote the binding of arresting proteins (arrestins) that further uncouple the receptors by interdicting receptor/G protein interactions (Benovic et al., 1987; Lohse et al., 1990). While both second messenger-dependent protein kinases and GRKs contribute to the agonist-dependent desensitization of GPCRs, second messenger-dependent protein kinases exhibit the capacity to indiscriminately phosphorylate and desensitize receptors that have not been exposed to agonist (reviewed by Premont et al., 1995). The relative contribution of second messenger-dependent protein kinases and GRKs to the overall GPCR desensitization process still remains to be determined. Second messenger-dependent protein kinases are activated in response to GPCR-stimulated increases in intracellular second messengers such as cAMP, Ca2+ and diacylglycerol (DAG). These kinases phosphorylate GPCRs at relatively well-defined phosphorylation consensus sites within their intracellular loops and carboxyl-terminal tail domains. In contrast, the consensus sites for GRK-mediated phosphorylation remain ill-defined. There are seven members of the GRK family, that can be subdivided into three groups based on sequence and functional homology: (1) GRK1 (rhodopsin kinase) and GRK7 (a new candidate cone opsin kinase); (2) GRK2 (-adrenergic receptor kinase1, ARK1) and GRK3 (-adrenergic receptor kinase2, ARK2); (3) GRK4, GRK5 and GRK6. The targeting of these kinases to their GPCR targets is mediated by multiple mechanisms that all involve the GRK carboxyl-terminal domains. These mechanisms include: post-translational farnesylation (GRK1 and GRK7) and palmitoylation (GRK4 and GRK6), association with heterotrimeric G protein ␥ subunits and phospholipids (GRK2 and GRK3), and electrostatic interaction between a highly basic amino acid domain with plasma membrane phospholipids (GRK5). GRKs phosphorylate both serine and threonine residues localized within either the third intracellular loop or carboxyl-terminal
321
tail domains of GPCRs (reviewed by Premont et al., 1995; Ferguson, 2001). 4.1. Arrestins GRK-mediated phosphorylation is not sufficient to mediate full inactivation of GPCRs and requires the binding of cytosolic co-factor proteins called arrestins (Lohse et al., 1990). To date, four arrestin family members have been identified. The members of the family can be divided into two groups based on sequence homology, function, and tissue distribution: (1) visual arrestins, S antigen and X-arrestin or C-arrestin; (2) -arrestins, -arrestin1 and -arrestin 2 (reviewed by Ferguson, 2001). Arrestins are cytoplasmic proteins, which following agonist stimulation, translocate rapidly to the plasma membrane to bind their GPCR targets in a GRK-dependent manner (Barak et al., 1997). Arrestins preferentially bind to agonist activated and GRK-phosphorylated GPCRs as opposed to second messenger-dependent protein kinase-phosphorylated or non-phosphorylated receptors (Lohse et al., 1990). However, arrestins will bind to GPCRs with low affinity in the absence of phosphorylation and/or agonist activation (Gurevich et al., 1995). The mechanism(s) by which arrestins contribute to GPCR desensitization involves both the physical uncoupling of GPCRs from heterotrimeric G proteins and the targeting of GPCRs for endocytosis (reviewed by Ferguson, 2001).
5. GPCR endocytosis An important aspect of GPCR activity and regulation is the internalization or endocytosis of agonist-activated receptors into the intracellular membrane compartments of the cell. GPCR internalization appears to be mediated by multiple distinct mechanisms, but the most common mechanism is the -arrestin-dependent targeting of GPCRs for internalization via a clathrin-vesicle mediated pathway (Ferguson et al., 1996; Goodman et al., 1996; Zhang et al., 1996). The targeting of -arrestin-bound receptors for endocytosis appears to involve the interaction of both the clathrin heavy chain and the 2-adaptin subunit of the heterotetrameric AP-2 adaptor complex with the carboxyl-terminal domain of -arrestin 1 and 2 (Goodman et al., 1996; Laporte et al., 1999; Laporte et al., 2000). The functional consequence of GPCR endocytosis appears to be multifaceted. The -arrestin-dependent endocytosis not only contributes to the prolonged desensitization of some GPCRs as the consequence of the internalization of -arrestin/GPCR complexes (Oakley et al., 1999; Anborgh et al., 2000), but it is also required for the resensitization of many GPCRs (reviewed by Ferguson, 2001). Moreover, -arrestin-dependent GPCR endocytosis both couples desensitized GPCRs to G protein-independent signaling pathways and serves to compartmentalize signaling complexes (Luttrell et al., 1999; DeFea et al., 2000).
322
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
6. Metabotropic glutamate receptor desensitization Recently, the mechanisms underlying Group 1 mGluR desensitization has been the subject of intensive investigation by several groups (Fig. 1). These studies reveal that mGluR desensitization may be mediated by both second messenger-dependent protein kinases and GRKs (Herrero et al., 1994; Desai et al., 1996; Gereau and Heinemann, 1998; Ciruela et al., 1999; Dale et al., 2000; Francesconi and Duvoisin, 2000; Sallese et al., 2000). However, these studies have often resulted in discordant results, which may either be the consequence of the cellular system employed or agonist ligand used to activate the receptor. The following section overviews our current understanding of the contribution of second messenger-dependent protein kinases and GRKs in the regulation of Group 1 mGluR signaling. 7. Role of second messenger-dependent protein kinases The desensitization of Group 1 mGluRs is characterized as a decrease in phosphoinositide hydrolysis in response to repeated exposure to agonist. Group 1 mGluR desensitization has been described in neuronal cultures, hippocampal slices and heterologous cell culture systems (e.g. Schoepp and Johnson, 1988; Herrero et al., 1994; Desai et al., 1996). The first indication that second messenger-dependent protein kinases mediate the desensitization of Group 1 mGluRs came from studies utilizing both activators and inhibitors of PKC (Schoepp and Johnson, 1988; Catania et al., 1991; Ciruela et al., 1999). PKC-dependent desensitization is also observed for mGluR1 splice variants (Ciruela et al., 1999). Molecular analysis has revealed that multiple PKC phosphorylation consensus sites are required for mGluR5 desensitization (Gereau and Heinemann, 1998). In particular, threonine residue 606 and serine residue 613 in the first intracellular loop, threonine residue 665 in the second intracellular loop and serine residues 881 and 890 in the carboxyl-terminal tail domains of mGluR5 all appear to contribute to mGluR5 desensitization (Gereau and Heinemann, 1998). The combination of these mutations completely abolishes mGluR5 desensitization (Gereau and Heinemann, 1998). Unfortunately, the potential for PKC-mediated phosphorylation of each of these residues has not been investigated. At the molecular level, the role of PKC phosphorylation in the regulation of mGluR1a signaling has been less intensively investigated. However, threonine residue 695 has been implicated in PKC-mediated mGluR1a desensitization and phosphorylation (Francesconi and Duvoisin, 2000). The replacement of threonine residue 695 with an alanine retards mGluR1a desensitization, whereas replacement with a glutamic acid residue to mimic phosphorylation tends to uncouple the receptor from Gq (Francesconi and Duvoisin, 2000). Interestingly, the desensitization of Group 1 mGluR-mediated Gq signaling processes at
hippocampal synapses is associated with a switch from facilitation to inhibition of excitatory synaptic transmission (Rodriquez-Moreno et al., 1998). This observation is particularly intriguing because mutation of threonine residue 695 to a glutamic acid residue disrupts mGluR1a coupling to Gq, but leaves Gs-signaling through this receptor subtype intact (Francesconi and Duvoisin, 2000). Consequently, the switch in mGluR signal transduction at hippocampal synapse may well be associated with PKC-dependent receptor desensitization. PKC-mediated phosphorylation of mGluRs may also contribute to other aspects of mGluR signaling. In particular, PKC-mediated phosphorylation of mGluR5 appears to antagonize the Ca2+ -dependent association of calmodulin with the receptor (Minakami et al., 1997). Calmodulin binds to mGluR5 at two sites and PKC appears to phosphorylate both regions to antagonize binding. It is hypothesized that this might provide a mechanism by which mGluR activation may regulate ion channel activity (Minakami et al., 1997).
8. Role of GRKs and arrestins Recently, GRKs were also implicated in the desensitization of Group 1 mGluR activity (Dale et al., 2000; Sallese et al., 2000). Dale et al. (2000) found that GRK2 and GRK5 contributed to both the phosphorylation and desensitization of mGluR1a in HEK 293 cells. Although the expression of GRK4 and GRK6 resulted in mGluR1a phosphorylation (Dale and Ferguson, unpublished data), these kinases did not contribute to the desensitization of mGluR1a signaling (Dale et al., 2000). Moreover, the expression of a kinase dead GRK2-K220R mutant blocked agonist-stimulated mGluR1a phosphorylation and resulted in significant impairment of mGluR1a desensitization. This observation is similar to what was reported for the parathyroid hormone receptor, where receptor desensitization was observed to be phosphorylation independent (Dicker et al., 1999). In addition, although -arrestins also contributed to the internalization of mGluR1a (see below), -arrestin binding did not appear to be required for mGluR1a desensitization (Dale et al., 2001a). Consistent with the observation that only GRK2 and GRK5 contributed to the desensitization of agonist-stimulated mGluR1a responses, only the expression of these kinases reduced basal mGluR1a activity (Dale et al., 2000). This reduction in basal mGluR activity was associated with increased cell survival that was mediated in part by reduced apoptotic cell death. In contrast to our own studies, Sallese et al. (2000) reported that GRK2, GRK4, GRK5 and GRK6 expression resulted in mGluR1a desensitization in HEK 293 cells, but that only GRK4 was required for mGluR1a desensitization in cerebellar Purkinje cells. Moreover, the overexpression of a kinase dead dominant-negative GRK4 mutant both blocked mGluR1a phosphorylation and desensitization (Sallese et al., 2000). These studies were originally
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
323
motivated by the interesting observation that GRK4 was expressed in cerebellar Purkinje cells. Consistent with this observation, GRK4 knock down using anti-sense in Purkinje cells resulted in increased mGluR1 signaling.
9. Metabotropic glutamate receptor endocytosis The internalization of mGluR1a appears to be mediated by both agonist-dependent and -independent mechanisms (Ciruela and McIlhinney, 1997; Doherty et al., 1999; Sallese et al., 2000; Dale et al., 2001a; Mundell et al., 2001). Unlike what is observed for many other GPCRs, substantial mGluR1a internalization is observed in the absence of agonist (Doherty et al., 1999; Dale et al., 2001a). This has been demonstrated both by the loss of cell surface receptors and the substantial localization of green fluorescent protein (GFP)-tagged mGluR to endocytic vesicles in the absence of agonist stimulation (Doherty et al., 1999; Dale et al., 2001a). Although this internalization may be mediated by the release of glutamate from cells in culture, treatment with antagonists has no effect on the extent of cell surface receptor loss (Dale et al., 2001a). The agonist-independent internalization of mGluR1a is insensitive to both -arrestin and dynamin dominant-negative mutants but still appears to be mediated by clathrin-coated vesicle mediated endocytic pathway (Dale et al., 2001a). The agonist-independent internalization of mGluR1a may be impeded in neurons through the association of Homer1c, potentially due to Homer1c-dependent clustering of receptors (Tadokoro et al., 1999; Ciruela et al., 2000). Agonist stimulation also promotes mGluR1a endocytosis in a GRK and -arrestin-dependent manner (Sallese et al., 2000; Dale et al., 2001b; Mundell et al., 2001; Fig. 2). Agonist-stimulated mGluR1a internalization can be blocked by the expression of both -arrestin and dynamin dominant-negative mutants (Mundell et al., 2001). mGluR1a activation also promotes the membrane translocation and internalization of GFP-tagged -arrestins (Dale et al., 2001a; Mundell et al., 2001). The overexpression of both -arrestin 1 and -arrestin 2 also increases the extent of mGluR1a internalization in response to glutamate (Mundell et al., 2001), but only -arrestin 1 appears to contribute to mGluR1a internalization in response to quisqualate (Dale et al., 2001a). These observations suggest that different mGluR agonists might stabilize different receptor conformations required for the association of arrestin proteins with mGluR1a. This is similar to what has previously been reported for the mu-opioid receptor (Zhang et al., 1998). Although it appears mGluRs internalized in the absence of agonist are efficiently recycled back to the cell surface, it is unknown whether mGluRs internalized in response to agonist can be recycled (Fig. 2). Consequently, a great deal remains to be learned about the mechanisms regulating both the agonist-dependent and -independent trafficking of mGluRs.
Fig. 2. Schematic representation of the mechanism underlying the agonist-stimulated internalization of Group 1 mGluRs. In response to agonist-stimulated GRK phosphorylation (P, phosphate), -arrestin1 (Arr1) translocates from the cytosol to the membrane and binds to the GRK-phosphorylated receptor. The association of -arrestin with both clathrin and the 2-adaptin subunit of the heterotetrameric AP2 adaptor complex targets mGluRs to clathrin-coated pits for internalization in clathrin-coated vesicles. The -arrestin 1 is internalized with the receptor, but it is unknown whether -arrestin 1 is released in the endosomal compartment to allow receptor dephosphorylation by a G protein-coupled receptor phosphatase (GRP). It is also unknown whether mGluRs internalized in response to agonist stimulation are recycled back to the cell surface.
10. Role of Group 1 mGluR desensitization in the patterning of effector enzyme activity The activation of Group 1 mGluRs gives rise to repetitive base-line separated Ca2+ -transients (oscillations; Kawabata et al., 1996; Nakahara et al., 1997; Kawabata et al., 1998; Codazzi et al., 2001; Dale et al., 2001b). These Ca2+ oscillations in response to mGluR activation are observed in immature neuronal cultures, developing neocortex, astrocytes, and heterologous cell cultures (Kawabata et al., 1996; Nakahara et al., 1997; Kawabata et al., 1998; Flint et al., 1999; Codazzi et al., 2001; Dale et al., 2001b). Low frequency Ca2+ oscillations are observed in response to mGluR1a activation (1 per 100–300 s), whereas mGluR5a stimulates oscillations at higher frequencies (1 per 25–40 s; Kawabata et al., 1998; Dale et al., 2001b). The Ca2+ oscillations at different frequencies are likely important for the selective activation of different transcription factors and/or the activation of protein kinases. This may be important for modulating long-term changes in synaptic activity that either occur during neuronal development or are associated with learning and memory. Recently, we demonstrated that PK repetitively redistributes between the cytosol and plasma membrane in response to mGluR1a- and mGluR5a-stimulated oscillations in intracellular free Ca2+ (Dale et al., 2001b). The Ca2+ and
324
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
PK oscillations in response to both mGluR1a and mGluR5a activation are dependent upon the release of Ca2+ from intracellular stores, but only mGluR1a-stimulated oscillations are dependent upon extracellular Ca2+ (Kawabata et al., 1998; Dale et al., 2000b). Oscillatory mGluR signaling appears to involve the repetitive receptor coupling to Gq resulting in the cyclical activation of phospholipase C thereby driving oscillations in both InsP3 formation and Ca2+ release from intracellular stores (Dale et al., 2001b). The differences in the rate at which mGluR1a and mGluR5a stimulate InsP3 , Ca2+ , and PKC oscillations appears to be a receptor-specific phenomenon that is regulated by the identity of a single amino acid residue localized within the G protein coupling domain of the receptors (see below, Kawabata et al., 1996; Dale et al., 2001b). The oscillatory activation of conventional PKC isoforms is not only driven by the oscillatory release of Ca2+ from intracellular stores but also by oscillations in intracellular DAG concentrations (Codazzi et al., 2001). Oscillations in conventional PKC isoforms between the plasma membrane and cytosol have been observed in both astrocytes and heterologous cell cultures (Codazzi et al., 2001; Dale et al., 2001b). In astrocytes, PKC oscillations occur in response to the activation of endogenously expressed mGluR5 (Codazzi et al., 2001). Ca2+ oscillations observed in response to Group 1 mGluR activation may occur as the consequence of two distinct mechanisms: (1) repetitive G protein-coupling in response to repetitive cycles of receptor phosphorylation and dephosphorylation or (2) Ca2+ -induced Ca2+ release from intracellular stores in response to low levels of InsP3 formation followed by the inhibition of InsP3 receptors at the higher concentrations of intracellular Ca2+ attained at the peak of the Ca2+ release. Generally, Ca2+ oscillations in response to mGluR activation have been proposed to occur as the consequence of the repetitive feedback phosphorylation by PKC (Kawabata et al., 1996; Codazzi et al., 2001). However, the mutation of each of the PKC phosphorylation sites in mGluR1a either individually or in clusters has no effect on quisqualate-stimulated Ca2+ and PK oscillations (Dale et al., 2001b). Moreover, the treatment of cells with PKC inhibitors results in the inhibition of PKC oscillations without effecting Ca2+ oscillations in response to either mGluR1a or mGluR5a activation (Dale et al., 2001b). These observations indicate that repetitive cycles of mGluR phosphorylation and dephosphorylation are not sufficient to explain oscillatory mGluR signaling patterns. It is possible that mGluR-stimulated oscillatory signaling in response to agonist activation is related to the structure of the mGluR ligand-binding domain. The mGluR ligand-binding domain consists of two globular lobes that exist in an equilibrium between open and closed conformations in the absence of ligand (Kunishima et al., 2000). Agonist stimulation may shift the probability that the ligand-binding domain exists in a closed (active) conformation. However, it is possible that the ligand-binding domain may spontaneous shift to an open conformation (inactive)
even in the presence of agonist and that this shift may be influenced by the structural determinants of intracellular domains required for receptor G protein coupling. Consistent with this idea, mutagenesis of the aspartic acid residue at position 854 of mGluR1a to an alanine residue results in a mGluR1a mutant that signals Ca2+ and PKC oscillations in the absence of agonist stimulation (Dale et al., 2001b). However, this theoretical model still requires to be substantiated with empirical data. As indicated above, differences in the rate of Ca2+ and PKC oscillations in response to mGluR1a and mGluR5a activation are regulated by the identity of a single amino acid residue localized to the G protein-coupling domains of the receptors (Kawabata et al., 1996; Dale et al., 2001a). Aspartic acid residue at position 854 in mGluR1a and the corresponding threonine residue at position 840 in mGluR5a are the only amino acid residues that differ between the G protein-coupling domains of mGluR1a and mGluR5a. The threonine residue in mGluR5a falls within the context of a putative PKC phosphorylation consensus sequence and it was previously proposed that phosphorylation of this site might account for the observed differences in the frequency of Ca2+ oscillations stimulated by mGluR1a and mGluR5a (Kawabata et al., 1996). Indeed, substitution of aspartic acid residue at position 854 in mGluR1a with a threonine residue to reconstitute a PKC phosphorylation consensus sequence accelerated the rate of Ca2+ and PKC oscillations induced by mGluR1a activation (Kawabata et al., 1996; Dale et al., 2001b). However, the acceleration of mGluR1a-stimulated Ca2+ and PKC oscillation rates does not appear to require PKC phosphorylation, because the substitution of aspartic acid residue 854 with either alanine or glycine residues garners the same result (Dale et al., 2001b). Thus, it appears that receptor-specific oscillatory G protein-coupling rates represent intrinsic receptor properties that are regulated by charge density of the residue localized at position 854 in mGluR1a or position 840 in mGluR5a. It is likely that negatively charged residues antagonize and/or slow mGluR coupling to G proteins.
11. Conclusions In conclusion, the temporal regulation of Ca2+ signaling represents a universal mechanism exploited by both excitable and non-excitable cells to regulate specific cellular responses. The Ca2+ oscillations in response to mGluR activation are observed in immature neuronal cultures and developing neocortex. Several studies have now determined that mGluR desensitization is regulated by the same mechanisms involved in the desensitization of more prototypic GPCRs. The activation of this important class of GPCR also results in the receptor subtype-specific spatial-temporal patterning of effector enzyme activation. The patterning of these signals appears to be independent of receptor desensitization, but rather appears to occur as the consequence of
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
the unique structure activity of Group 1 mGluRs. Among the challenges awaiting investigators in the field will be to determine how information encoded by receptor-specific spatial-temporal signaling patterns are interpreted by cells to regulate diverse neuronal functions in both the developing and adult nervous systems.
Acknowledgements S.S.G.F. is the recipient of a McDonald Salary Award and Great West/London Life Salary Award from the Heart and Stroke Foundation of Canada and a Premier’s Research Excellence Award from the Province of Ontario. A.V.B. is the recipient of a Canadian Hypertension Society/Canadian Institutes of Health Research (CIHR) fellowship. This work was supported by CIHR Grant MA-15506 to S.S.G.F. References Anborgh, P.H., Dale, L.B., Seachrist, J., Ferguson, S.S.G., 2000. Differential regulation of 2 -adrenergic and angiotensin II type 1A receptor trafficking and resensitization: role of carboxyl-terminal domains. Mol. Endocrinol. 14, 2040–2053. Alessandri, B., Bullock, R., 1998. Glutamate and its receptors in the pathophysiology of brain and spinal cord injuries. Prog. Brain Res. 116, 303–330. Barak, L.S., Ferguson, S.S.G., Zhang, J., Caron, M.G., 1997. A -arrestin/ green fluorescent protein biosensor for detecting G protein-coupled receptor activation. J. Biol. Chem. 272, 27497–27500. Benovic, J.L., Kühn, H., Weyland, I., Codina, J., Caron, M.G., Lefkowitz, R.J., 1987. Functional desensitization of the isolated -adrenergic receptor by the -adrenergic receptor kinase: potential role of an analog of the retinal protein arrestin (48 kDa protein). Proc. Natl. Acad. Sci. U.S.A. 84, 8879–8882. Bordi, F., Ugolini, A., 1999. Group I metabotropic glutamate receptors: implications for brain diseases. Progr. Neurobiol. 59, 55–79. Bouvier, M., Collins, S., O’Dowd, B.F., Campbell, P.T., de Blasi, A., Kobilka, B.K., MacGregor, C., Irons, G.P., Caron, M.G., Lefkowitz, R.J., 1989. Two distinct pathways for cAMP-mediated down regulation of the 2 -adrenergic receptor: phosphorylation of the receptor and regulation of its mRNA. J. Biol. Chem. 264, 16786–16792. Budd, D.C., McDonald, J.E., Tobin, A.B., 2000. Phosphorylation and regulation of a Gq/11-coupled receptor by casein kinase 1␣. J. Biol. Chem. 275, 19667–19675. Calabresi, P., Centonze, D., Pisani, A., Bernardi, G., 1999. Metabotropic glutamate receptors and cell-type-specific vulnerability in the striatum: implications for ischemia and Huntington’s disease. Exp. Neurol. 158, 97–108. Catania, M.V., Aronica, E., Sortino, M.A., Canonico, P.L., Nicoletti, F., 1991. Desensitization of metabotropic glutamate receptors in neuronal cultures. J. Neurochem. 56, 1329–1335. Ciruela, F., McIlhinney, R.A., 1997. Differential internalization of mGluR1 splice variants in response to agonist and phorbol esters in permanently transfected BHK cells. FEBS Lett. 418, 83–86. Ciruela, F., Giacometti, A., McIlhinney, R.A., 1999. Functional regulation of metabotropic glutamate receptor type 1c: a role for phosphorylation in the desensitization of the receptor. FEBS Lett. 462, 278–282. Ciruela, F., Soloviev, M.M., Chan, W.Y., McIlhinney, R.A., 2000. Homer-1c/Vesl-1L modulates the cell surface targeting of metabotropic glutamate receptor type 1alpha: evidence for an anchoring function. Mol. Cell. Neurosci. 15, 36–50.
325
Codazzi, F., Teruel, M.N., Meyer, T., 2001. Control of astrocyte Ca2+ oscillations and waves by oscillating translocation and activation of protein kinase. Curr. Biol. 11, 1089–1097. Conn, P.J., Pin, J.-P., 1997. Pharmacology and functions of metabotropic glutamate receptors. Ann. Rev. Pharmacol. Toxicol. 37, 205–237. Dale, L.B., Bhattacharya, M., Anborgh, P.H., Murdoch, B., Bhatia, M., Nakanishi, S., Ferguson, S.S.G., 2000. G protein-coupled receptor kinase mediated desensitization of metabotropic glutamate receptor 1␣ protects against cell death. J. Biol. Chem. 275, 38213–38220. Dale, L.B., Bhattacharya, M., Seachrist, J.L., Anborgh, P.H., Ferguson, S.S.G., 2001a. Agonist-dependent and -independent internalization of metabotropic glutamate receptor 1a: -arrestin isoform specific endocytosis. Mol. Pharmacol. 60, 1243–1253. Dale, L.B., Babwah, A.V., Bhattacharya, M., Kelvin, D.J., Ferguson, S.S.G., 2001b. Spatial-temporal dynamics of metabotropic glutamate receptor-mediated inositol 1,4,5-triphosphate, Ca2+ , and protein kinase C oscillations. J. Biol. Chem. 276, 35900–35908. DeFea, K.A., Zalevsky, J., Thoma, M.S., Dery, O., Mullins, R.D., Bunnett, N.W., 2000. -Arrestin-dependent endocytosis of proteinase-activated receptor 2 is required for intracellular targeting of activated ERK1/2. J. Cell Biol. 148, 1267–1281. Dicker, F., Quitterer, U., Winstel, R., Honold, K., Lohse, M.J., 1999. Phosphorylation-independent inhibition of parathyroid hormone receptor signaling by G protein-coupled receptor kinases. Proc. Natl. Acad. Sci. U.S.A. 96, 5476–5481. Desai, M.A., Burnett, J.P., Mayne, N.G., Schoepp, D.D., 1996. Pharmacological characterization of desensitization in a human mGlu1 alpha-expressing non-neuronal cell line co-transfected with a glutamate transporter. Br. J. Pharmacol. 118, 1558–15564. Doherty, A.J., Coutinho, V., Collingridge, G.L., Henley, J.M., 1999. Rapid internalization and surface expression of a functional, fluorescently tagged G-protein-coupled glutamate receptor. Biochem. J. 341, 415–422. Ferguson, S.S.G., Downey Jr, W.E., Colapietro, A.M., Barak, L.S., Menard, L., Caron, M.G., 1996. Role of -arrestin in mediating agonist-promoted G protein-coupled receptor internalization. Science 271, 363–366. Ferguson, S.S.G., 2001. Evolving concepts in G protein-coupled receptor endocytosis: the role in receptor desensitization and signaling. Pharmacol. Rev. 53, 1–24. Flint, A.C., Dammerman, R.S., Kriegstein, A.R., 1999. Endogenous activation of metabotropic glutamate receptors in neocortical development causes calcium oscillations. Proc. Natl. Acad. Sci. U.S.A. 96, 12144–12149. Francesconi, A., Duvoisin, R.M., 2000. Opposing effects of protein kinase C and protein kinase A on metabotropic glutamate receptor signaling: selective desensitization of the inositol triphosphate/Ca2+ pathway by phosphorylation of the receptor-G protein-coupling domain. Proc. Natl. Acad. Sci. U.S.A. 97, 6185–6190. Gereau, R.W., Heinemann, S.F., 1998. Role of protein kinase C phosphorylation in rapid desensitization of metabotropic glutamate receptor 5. Neuron 20, 143–151. Goodman, O.B., Krupnick Jr., J.G., Santini, F., Gurevich, V.V., Penn, R.B., Gagnon, A.W., Keen, J.H., Benovic, J.L., 1996. -Arrestin acts as a clathrin adaptor in endocytosis of the 2 -adrenergic receptor. Nature 383, 447–450. Gomeza, J., Joly, C., Kuhn, R., Knopfel, T., Bockaert, J., Pin, J.P., 1996. The second intracellular loop of metabotropic glutamate receptor 1 cooperates with the other intracellular domains to control coupling to G-proteins. J. Biol. Chem. 271, 2199–2205. Gurevich, V.V., Dion, S.B., Onorato, J.J., Ptasienski, J., Kim, C.M., Sterne-Marr, R., Hosey, M.M., Benovic, J.L., 1995. Arrestin interactions with G protein-coupled receptors: direct binding studies of wild type and mutant arrestins with rhodopsin, 2 -adrenergic, and m2 muscarinic cholinergic receptors. J. Biol. Chem. 270, 720–731. Herrero, I., Miras-Portugal, T., Sanchez-Prieto, J., 1994. Rapid desensitization of the metabotropic glutamate receptor that facilitates glutamate
326
L.B. Dale et al. / Neurochemistry International 41 (2002) 319–326
release in rat cerebrocortical nerve terminals. Eur. J. Neurosci. 6, 115–120. Kawabata, S., Tsutsumi, R., Kohara, A., Yamaguchi, T., Nakanishi, S., Okada, M., 1996. Control of calcium oscillations by phosphorylation of metabotropic glutamate receptors. Nature 383, 89–92. Kawabata, S., Kohara, A., Tsutsumi, R., Itahana, H., Hatashibe, S., Yamaguchi, T., Okada, M., 1998. Diversity of calcium signaling by metabotropic glutamate receptors. J. Biol. Chem. 273, 17381–17385. Kunishima, N., Shimada, Y., Tsuji, Y., Sato, T., Yamamoto, M., Kumasaka, T., Nakanishi, S., Jingami, H., Morikawa, K., 2000. Structural basis of glutamate recognition by a dimeric metabotropic glutamate receptor. Nature 407, 971–977. Laporte, S.A., Oakley, R.H., Zhang, J., Holt, J.A., Ferguson, S.S.G., Caron, M.G., Barak, L.S., 1999. The 2 -adrenergic receptor/-arrestin complex recruits the clathrin adaptor AP-2 during endocytosis. Proc. Natl. Acad. Sci. U.S.A. 96, 3712–3717. Laporte, S.A., Oakley, R.H., Holt, J.A., Barak, L.S., Caron, M.G., 2000. The interaction of -arrestin with the AP-2 adaptor, rather than clathrin, is required for the clustering of 2 -adrenergic receptor into clathrin-coated pits. J. Biol. Chem. 275, 23120–32126. Lohse, M.J., Benovic, J.L., Codina, J., Caron, M.G., Lefkowitz, R.J., 1990. -Arrestin: a protein that regulates -adrenergic receptor function. Science 248, 1547–1550. Luttrell, L.M., Ferguson, S.S.G., Daaka, Y., Miller, W.E., Maudsley, S., Della Rocca, G.J., Lin, F., Kawakatsu, H., Owada, K., Luttrell, D.K., Caron, M.G., Lefkowitz, R.J., 1999. -Arrestin-dependent formation of 2 -adrenergic receptor-Src protein kinase complexes. Science 283, 655–661. Minakami, R., Jinnai, N., Sugiyama, H., 1997. Phosphorylation and calmodulin binding of the metabotropic glutamate receptor subtype 5 (mGluR5) are antagonistic in vitro. J. Biol. Chem. 272, 20291–20298. Mundell, S.J., Matharu, A.L., Pula, G., Roberts, P.J., Kelly, E., 2001. Agonist-induced internalization of the metabotropic glutamate receptor 1␣ is arrestin- and dynamin-dependent. J. Neurochem. 78, 546–551. Nakahara, K., Okada, M., Nakanishi, S., 1997. The metabotropic glutamate receptor mGluR5 induces calcium oscillations in cultured astrocytes via protein kinase C phosphorylation. J. Neurochem. 69, 1467–1475. Nicoletti, F., Brunom, V., Copanim, A., Casabona, G., Knopfel, T., 1996. Metabotropic glutamate receptors: a new target for the therapy of neurodegenerative disorders? Trends Neurosci. 19, 267–271. Nakanishi, S., 1994. Metabotropic glutamate receptors: synaptic transmission, modulation, and plasticity. Neuron 13, 1031–1037. Oakley, R.H., Laporte, S.A., Holt, J.A., Barak, L.S., Caron, M.G., 1999. Association of -arrestin with G protein-coupled receptors
during clathrin-mediated endocytosis dictates the profile of receptor resensitization. J. Biol. Chem. 274, 32248–32257. Premont, R.T., Inglese, J., Lefkowitz, R.J., 1995. Protein kinases that phosphorylate activated G protein-coupled receptors. FASEB J. 9, 175–182. Pippig, S., Andexinger, S., Lohse, M.J., 1995. Sequestration and recycling of 2 -adrenergic receptors permit resensitization. Mol. Pharmacol. 47, 666–676. Riedel, G., 1996. Function of metabotropic glutamate receptors in learning and memory. Trends Neurosci. 19, 219–224. Rodriquez-Moreno, A., Sistiaga, A., Lerma, J., Sanchez-Prieto, J., 1998. Switch from facilitation to inhibition in the control of glutamate release by metabotropic glutamate receptor. Neuron 21, 1477–1486. Sallese, M., Salvatore, L., D’Urbano, E., Sala, G., Storto, M., Launey, T., Nicoletti, F., Knopfel, T., De Blasi, A., 2000. The G-protein-coupled receptor kinase GRK4 mediates homologous desensitization of metabotropic glutamate receptor 1. FASEB J. 14, 2569–2580. Sakmar, T.P., 1998. Rhodopsin: a prototypical G protein-coupled receptor. Prog. Nucl. Acid Res. Mol. Biol. 59, 1–34. Schoepp, D.D., Johnson, B.G., 1988. Selective inhibition of excitatory amino acid-stimulated phosphoinositide hydrolysis in the rat hippocampus by activation of protein kinase C. Biochem. Pharmacol. 37, 4299–4305. Tadokoro, S., Tachibana, T., Imanaka, T., Nishida, W., Sobue, K., 1999. Involvement of unique leucine-zipper motif of PSD-Zip45 (Homer 1c/Vesl-1L) in Group 1 metabotropic glutamate receptor clustering. Proc. Natl. Acad. Sci. U.S.A. 96, 13801–13806. Valiquette, M., Bonin, H., Hnatowich, M., Caron, M.G., Lefkowitz, R.J., Bouvier, M., 1990. Involvement of tyrosine residues located in the carboxyl tail of the human 2 -adrenergic receptor in agonist-induced down regulation of the receptor. Proc. Natl. Acad. Sci. U.S.A. 87, 5089–5093. Zhang, J., Ferguson, S.S.G., Barak, L.S., Menard, L., Caron, M.G., 1996. Dynamin and -arrestin reveal distinct mechanisms for G protein-coupled receptor internalization. J. Biol. Chem. 271, 18302– 18305. Zhang, J., Barak, L.S., Winkler, K.E., Caron, M.G., Ferguson, S.S.G., 1997. A central role for -arrestins and clathrin-coated vesicle-mediated endocytosis in 2 -adrenergic receptor resensitization. J. Biol. Chem. 272, 27005–27014. Zhang, J., Ferguson, S.S.G., Barak, L.S., Bodduluri, S., Laporte, S.A., Law, P.-Y., Caron, M.G., 1998. Role for G protein-coupled receptor kinase in agonist-specific regulation of -opioid receptor responsiveness. Proc. Natl. Acad. Sci. U.S.A. 95, 7157–7162.