Regulation of metabotropic glutamate receptor signaling, desensitization and endocytosis

Abstract

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 Gα_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 β-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, β-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 β₂-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.

Introduction

Glutamate is a major excitatory neurotransmitter in the central nervous system and it mediates its action through 2 distinct types of receptors: ionotropic and metabotropic (Pin & Duvosin, 1995). Ionotropic receptors are ion channels permeable to cations and are subdivided into N-methyl-d-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors based on agonist preference (Nakanishi, 1994). Metabotropic glutamate receptors (mGluRs) are members of the G protein-coupled receptor (GPCR) superfamily and they have been shown to play an important role in processes requiring synaptic plasticity, such as learning and memory, neuronal development, and neurodegeneration (Nakanishi, 1994, Pin & Bockaert, 1995, Pin & Duvosin, 1995, Conn & Pin, 1997, Dale et al., 2003).

Eight distinct mammalian mGluRs have been reported that share 30–60% identity and have been divided into 3 subgroups based on sequence similarities, preferred signal transduction pathways and pharmacology (Nakanishi, 1994, Pin & Bockaert, 1995, Pin & Duvosin, 1995, Conn & Pin, 1997, Dale et al., 2003). They are Group I (mGluR1 and mGluR5), Group II (mGluR2 and mGluR3) and Group III (mGluR4, mGluR6, mGluR7, and mGluR8). This review focuses on Group I mGluRs. Group I mGluRs are positively coupled to phospholipase C (PLC) via Gα_q/11 and inositol 1,4,5 trisphosphate (InsP3) formation (Abdul-Ghani et al., 1996). Their activation increases intracellular calcium levels by inducing calcium release from intracellular stores and stimulates protein kinase C (PKC) activation, potentiates L-type voltage-dependent calcium channels, and inhibits K⁺ conductances. These receptors also activate other G proteins, including Gα_s and Gα_i/o (Aramori & Nakanishi, 1992, Francesconi & Duvoisin, 2000). For example, mGluR1a, when transfected into CHO cells, stimulates both cAMP formation and arachidonic acid release (Aramori & Nakanishi, 1992), whereas mGluR5a does not couple with the stimulatory cAMP pathway (Abe et al., 1992). Group I mGluRs can also activate intracellular signals independent of G proteins, probably via Src (Heuss et al., 1999). Thus, these receptors can trigger a variety of signaling cascades and modulate the activity of ion and ligand-gated channels through functional coupling with transduction pathways such as PLC, adenylyl cyclase, phospholipase A2, phospholipase D (PLD), tyrosine kinases, extracellular signal-regulated protein kinases and mitogen-activated protein kinase kinase (Hermans & Challiss, 2001, Valenti et al., 2002).

Group I mGluRs are differentially distributed throughout the central nervous system. mGluR1 is highly expressed within CA3 pyramidal cells of the hippocampus, Purkinje cells of the cerebellum, the olfactory bulb, and thalamus. In contrast, mGluR5 is expressed in low levels in the cerebellum, but in high levels in the cortex, the striatum and nucleus accumbens of the basal ganglia, CA1 and CA3 pyramidal cells of the hippocampus and granule cells of the olfactory bulb (Bordi & Ugolini, 1999). There are also differences in the cortical distribution of Group I mGluRs among species, in particular between primates and rodents. The localization of mGluR1a to pyramidal cells in primate cortex contrasts with reports that mGluR1a is found almost exclusively in interneurons in rodent cortex (Muly et al., 2003, Kuwajima et al., 2004). Group I mGluRs are also found outside the brain in unmyelinated sensory afferent terminals in the skin, where they play an important role in pain sensation (Bhave et al., 2001). They are also found in melanocytes (Frati et al., 2000), osteoblasts (Gu & Publicover, 2000), heart cells (Gill et al., 1999), and hepatocytes (Storto et al., 2000). Although Group I mGluRs can be located at both presynaptic and postsynaptic sites in the brain, they primarily display a peri-synaptic localization at the postsynaptic membrane of glutamatergic neurons, where they often regulate neuronal excitability by modulating currents through ionotropic glutamate receptor channels (Shigermoto et al., 1993, Baude et al., 1997, Shigermoto et al., 1997).

Considering the variety of signal transduction pathways that can be activated by Group I mGluRs, it is not surprising that these receptors are involved in a large variety of physiological events. Group I mGluRs have been shown to play an important role in neuronal development (Catania et al., 1991, Plenz & Kitai, 1998, Flint et al., 1999, Hannan et al., 2001), synaptic plasticity (Zhong et al., 2000, Wu et al., 2001, Gubellini et al., 2003), neurodegeneration (Kingston et al., 1999, Bruno et al., 2001), and the induction of reactive astrocytes (Aronica et al., 2000). Although Group I mGluRs (mGluR1 and mGluR5) are highly homologous among themselves and are coupled to Gα_q/11 proteins and phosphoinositide hydrolysis, they appear to fulfill different roles in the central nervous system (Valenti et al., 2002). Functional differences for mGluR1 and mGluR5 activity have been suggested by their distinct anatomical distribution, but have also been detected when they are coexpressed within the same cell. For example, activation of mGluR1 and mGluR5 in transfected cells generates oscillatory diacylglycerol, InsP3 and calcium responses at distinct frequencies that can be translated as oscillations in the activation of PKC (Dale et al., 2001a, Dale et al., 2001b, Babwah et al., 2003). These differences in oscillation frequency appear to be regulated by a single amino acid residue in the G protein-coupling domains of the mGluR1 (D854) and mGluR5 (T840) (Kawabata et al., 1996, Dale et al., 2001a, Dale et al., 2001b) (Fig. 1). The activation of mGluR5, but not of mGluR1 potentiates NMDA responses in striatal projection neurons (Pisani et al., 1997), whereas the presynaptic activation of mGluR1, but not of mGluR5, modulates GABAergic transmission and mediates neuroprotective effects against NMDA toxicity (Battaglia et al., 2001). Functional differences between mGluR1 and mGluR5 have also been reported in other cell types, including hippocampal CA1 pyramidal, globus pallidus, substantial-nigra pars reticulate, and midbrain dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area (Valenti et al., 2002).

The activation of Group I mGluRs has also been shown to play an important role in synaptic plasticity, which is defined as long- and short-term changes in the efficacy of synaptic transmission. Two classical forms of the synaptic plasticity long-term depression (LTD) and long-term potentiation (LTP) are observed in several brain regions and have been proposed to represent the cellular mechanisms for motor learning and behaviour. The expression of LTD has been shown to require the selective activation of mGluR1, as the amplitude of LTD is reduced in slices from mGluR1−/− mice (Conquet et al., 1994, Gubellini et al., 2001). The activation of Group I mGluRs also mediate the induction of corticostriatal LTP, as the blockade of both mGluR1 and mGluR5 can completely interfere with the induction of LTP (Gubellini et al., 2003). This is also supported by electrophysiological experiments utilizing mGluR1−/− and mGluR5−/− mice (Conquet et al., 1994, Chiamulera et al., 2001). Both groups of animals showed a reduced striatal LTP, but in different brain regions. In the hippocampus, mGluR1-deficient mice exhibit either normal or only slightly reduced CA1 and dentate LTP, but impaired CA3 mossy fiber LTP. These mice displayed a reduced performance in context-dependent fear conditioning tasks (Aiba et al., 1994). Conversely, mGluR5-deficient mice display a reduction in CA1 hippocampal LTP, but demonstrate normal CA3 mossy fiber LTP and have a reduced performance in spatial learning tasks (Lu et al., 1997). mGluR1-deficient mice display a reduced cerebellar LTD that is associated with a deficiency in eye blink reflex and motor coordination (Conquet et al., 1994, Gubellini et al., 2001). The role of Group I mGluRs on LTP has been proposed to be related to the enhancement of NMDA receptor-mediated responses through the activation of several calcium-dependent mechanisms (Calabresi et al., 1996, Dineley et al., 2001).

Altered glutamatergic transmission has been postulated as the pathophysiological basis of several neurological and neurodegenerative disorders, such as ischemia, brain trauma, chronic pain (Bhave et al., 2001, Karim et al., 2001, Zhou et al., 2001), epilepsy (Chapman et al., 2000), multiple sclerosis (Geurts et al., 2003), amyotropic lateral sclerosis (Laslo et al., 2001), diabetes (Berent-Spillson et al., 2003), Huntington's disease (Calabresi et al., 1999) and Parkinson's disease (Awad et al., 2000). In this context, the positive modulation of NMDA receptors by Group I mGluRs may lead to a massive influx of calcium ions and an enhanced glutamatergic calcium signal resulting in the selective loss of neurons involved in various diseases. However, it remains unknown whether the toxic effects of the glutamate receptors are the primary cause of the various pathological conditions or whether excessive glutamate levels are just consequences of the disorder leading to cell death. Nevertheless, the drugs targeted against the mGluRs (antagonists) are the focus of a number of therapeutic strategies for treating various neurological disorders.