Diversity of calcium signaling by metabotropic glutamate receptors

Involvement of Metabotropic Glutamate Receptors in regulation of immune response in the Pacific Oyster Crassostrea gigas

Metabotropic glutamate receptors (mGluRs) play a pivotal role in the neuroendocrine-immune regulation. In this study, eight mGluRs were identified in the Pacific Oyster Crassostrea gigas, which were classified into three subfamilies based on genetic similarity. All CgmGluRs harbor variable numbers of PBP1 domains at the N-terminus. The sequence and structural features of CgmGluRs are highly similar to mGluRs in other species. A uniformly upregulated expression of CgmGluRs was observed during D-shaped larval stage compared to early D-shaped larval stage. The transcripts of CgmGluRs were detectable in various tissues of oyster. Different CgmGluR exhibited diverse expression patterns response against different PAMP stimulations, among which CgmGluR5 was significantly downregulated under these stimulations, reflecting its sensitivity and broad-spectrum responsiveness to microbes. Following LPS stimulation, the mRNA expression of CgmGluR5 and CgCALM1 in haemocytes was suppressed within 6 h and returned to normal levels by 12 h. Inhibition of CgmGluR5 activity resulted in a significant reduction in CgCALM1 expression after 12 h. Further KEGG enrichment analysis suggested that CgmGluR5 might modulate calcium ion homeostasis and metabolic pathways by regulating CgCALM1. This research delivers the systematic analysis of mGluR in the Pacific Oyster, offering insights into evolutionary characteristics and immunoregulatory function of mGluR in mollusks.

Temperature sensing by the calcium-sensing receptor

Whether GPCRs support the sensing of temperature as well as other chemical and physical modalities is not well understood. Introduction: Extracellular Ca²⁺ concentration (Ca²⁺ _o) modulates core body temperature and the firing rates of temperature-sensitive CNS neurons, and hypocalcemia provokes childhood seizures. However, it is not known whether these phenomena are mediated by Ca²⁺ _o-sensing GPCRs, including the calcium-sensing receptor (CaSR). In favor of the hypothesis, CaSRs are expressed in hypothalamic regions that support core temperature regulation, and autosomal dominant hypocalcemia, due to CaSR activating mutations, is associated with childhood seizures. Methods: Herein, we tested whether CaSR-dependent signaling is temperature sensitive using an established model system, CaSR-expressing HEK-293 cells. Results: We found that the frequency of Ca²⁺ _o-induced Ca²⁺ _i oscillations but not the integrated response was linearly dependent on temperature in a pathophysiologically relevant range. Chimeric receptor analysis showed that the receptor's C-terminus is required for temperature-dependent modulation and experiments with the PKC inhibitor GF109203X and CaSR mutants T888A and T888M, which eliminate a key phosphorylation site, demonstrated the importance of repetitive phosphorylation and dephosphorylation. Discussion and Conclusion: CaSRs mediate temperature-sensing and the mechanism, dependent upon repetitive phosphorylation and dephosphorylation, suggests that GPCRs more generally contribute to temperature-sensing.

Redox and Calcium Alterations of a Müller Cell Line Exposed to Diabetic Retinopathy-Like Environment

Diabetic retinopathy (DR) is a common complication of diabetes mellitus and is the major cause of vision loss in the working-age population. Although DR is traditionally considered a microvascular disease, an increasing body of evidence suggests that neurodegeneration is an early event that occurs even before the manifestation of vasculopathy. Accordingly, attention should be devoted to the complex neurodegenerative process occurring in the diabetic retina, also considering possible functional alterations in non-neuronal cells, such as glial cells. In this work, we investigate functional changes in Müller cells, the most abundant glial population present within the retina, under experimental conditions that mimic those observed in DR patients. More specifically, we investigated on the Müller cell line rMC-1 the effect of high glucose, alone or associated with activation processes and oxidative stress. By fluorescence microscopy and cellular assays approaches, we studied the alteration of functional properties, such as reactive oxygen species production, antioxidant response, calcium homeostasis, and mitochondrial membrane potential. Our results demonstrate that hyperglycaemic-like condition per se is well-tolerated by rMC-1 cells but makes them more susceptible to a pro-inflammatory environment, exacerbating the effects of this stressful condition. More specifically, rMC-1 cells exposed to high glucose decrease their ability to counteract oxidative stress, with consequent toxic effects. In conclusion, our study offers new insights into Müller cell pathophysiology in DR and proposes a novel in vitro model which may prove useful to further investigate potential antioxidant and anti-inflammatory molecules for the prevention and/or treatment of DR.

Metabotropic Glutamate Receptors at the Aged Mossy Fiber - CA3 Synapse of the Hippocampus

Metabotropic glutamate receptors (mGluRs) are a group of G-protein-coupled receptors that exert a broad array of modulatory actions at excitatory synapses of the central nervous system. In the hippocampus, the selective activation of the different mGluRs modulates the intrinsic excitability, the strength of synaptic transmission, and induces multiple forms of long-term plasticity. Despite the relevance of mGluRs in the normal function of the hippocampus, we know very little about the changes that mGluRs functionality undergoes during the non-pathological aging. Here, we review data concerning the physiological actions of mGluRs, with particular emphasis on hippocampal area CA3. Later, we examine changes in the expression and functionality of mGluRs during the aging process. We complement this review with original data showing an array of electrophysiological modifications observed in the synaptic transmission and intrinsic excitability of aged CA3 pyramidal cells in response to the pharmacological stimulation of the different mGluRs.

Branched Photoswitchable Tethered Ligands Enable Ultra-efficient Optical Control and Detection of G Protein-Coupled Receptors In Vivo

The limitations of classical drugs have spurred the development of covalently tethered photoswitchable ligands to control neuromodulatory receptors. However, a major shortcoming of tethered photopharmacology is the inability to obtain optical control with an efficacy comparable with that of the native ligand. To overcome this, we developed a family of branched photoswitchable compounds to target metabotropic glutamate receptors (mGluRs). These compounds permit photo-agonism of G_i/o-coupled group II mGluRs with near-complete efficiency relative to glutamate when attached to receptors via a range of orthogonal, multiplexable modalities. Through a chimeric approach, branched ligands also allow efficient optical control of G_q-coupled mGluR5, which we use to probe the spatiotemporal properties of receptor-induced calcium oscillations. In addition, we report branched, photoswitch-fluorophore compounds for simultaneous receptor imaging and manipulation. Finally, we demonstrate this approach in vivo in mice, where photoactivation of SNAP-mGluR2 in the medial prefrontal cortex reversibly modulates working memory in normal and disease-associated states.

Regulation and Function of Activity-Dependent Homer in Synaptic Plasticity

Alterations in synaptic signaling and plasticity occur during the refinement of neural circuits over the course of development and the adult processes of learning and memory. Synaptic plasticity requires the rearrangement of protein complexes in the postsynaptic density (PSD), trafficking of receptors and ion channels and the synthesis of new proteins. Activity-induced short Homer proteins, Homer1a and Ania-3, are recruited to active excitatory synapses, where they act as dominant negative regulators of constitutively expressed, longer Homer isoforms. The expression of Homer1a and Ania-3 initiates critical processes of PSD remodeling, the modulation of glutamate receptor-mediated functions, and the regulation of calcium signaling. Together, available data support the view that Homer1a and Ania-3 are responsible for the selective, transient destabilization of postsynaptic signaling complexes to facilitate plasticity of the excitatory synapse. The interruption of activity-dependent Homer proteins disrupts disease-relevant processes and leads to memory impairments, reflecting their likely contribution to neurological disorders.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Molecular Mechanism: ERK Signaling, Drug Addiction, and Behavioral Effects

Addiction to psychostimulants has been considered as a chronic psychiatric disorder characterized by craving and compulsive drug seeking and use. Over the past two decades, accumulating evidence has demonstrated that repeated drug exposure causes long-lasting neurochemical and cellular changes that result in enduring neuroadaptation in brain circuitry and underlie compulsive drug consumption and relapse. Through intercellular signaling cascades, drugs of abuse induce remodeling in the rewarding circuitry that contributes to the neuroplasticity of learning and memory associated with addiction. Here, we review the role of the extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase, and its related intracellular signaling pathways in drug-induced neuroadaptive changes that are associated with drug-mediated psychomotor activity, rewarding properties and relapse of drug seeking behaviors. We also discuss the neurobiological and behavioral effects of pharmacological and genetic interferences with ERK-associated molecular cascades in response to abused substances. Understanding the dynamic modulation of ERK signaling in response to drugs may provide novel molecular targets for therapeutic strategies to drug addiction.

Protein kinase A directly phosphorylates metabotropic glutamate receptor 5 to modulate its function

Metabotropic glutamate receptor 5 (mGluR5) regulates excitatory post-synaptic signaling in the central nervous system (CNS) and is implicated in various CNS disorders. Protein kinase A (PKA) signaling is known to play a critical role in neuropsychiatric disorders such as Parkinson's disease, schizophrenia, and addiction. Dopamine signaling is known to modulate the properties of mGluR5 in a cAMP- and PKA-dependent manner, suggesting that mGluR5 may be a direct target for PKA. Our study identifies mGluR5 at Ser870 as a direct substrate for PKA phosphorylation and demonstrates that this phosphorylation plays a critical role in the PKA-mediated modulation of mGluR5 functions such as extracellular signal-regulated kinase phosphorylation and intracellular Ca(2+) oscillations. The identification of the molecular mechanism by which PKA signaling modulates mGluR5-mediated cellular responses contributes to the understanding of the interaction between dopaminergic and glutamatergic neuronal signaling. We identified serine residue 870 (S870) in metabotropic glutamate receptor 5 (mGluR5) as a direct substrate for protein kinase A (PKA). The phosphorylation of this site regulates the ability of mGluR5 to induce extracellular signal-regulated kinase (ERK) phosphorylation and intracellular Ca(2+) oscillations. This study provides a direct molecular mechanism by which PKA signaling interacts with glutamate neurotransmission.

Guidepost neurons for the lateral olfactory tract: expression of metabotropic glutamate receptor 1 and innervation by glutamatergic olfactory bulb axons

The guidepost neurons for the lateral olfactory tract, which are called lot cells, are the earliest-generated neurons in the neocortex. They migrate tangentially and ventrally further down this tract, and provide scaffolding for the olfactory bulb axons projecting into this pathway. The molecular profiles of the lot cells are largely uncharacterized. We found that lot cells specifically express metabotropic glutamate receptor subtype-1 at a very early stage of development. This receptor is functionally competent and responds to a metabotropic glutamate receptor agonist with a transient increase in the intracellular calcium ion concentration. When the glutamatergic olfactory bulb axons were electrically stimulated, lot cells responded to the stimulation with a calcium increase mainly via ionotropic glutamate receptors, suggesting potential neurotransmission between the axons and lot cells during early development. Together with the finding that lot cells themselves are glutamatergic excitatory neurons, our results provide another notable example of precocious interactions between the projecting axons and their intermediate targets.

Compensatory molecular and functional mechanisms in nervous system of the Grm1(crv4) mouse lacking the mGlu1 receptor: a model for motor coordination deficits

The metabotropic glutamate type 1 (mGlu1) and type 5 (mGlu5) receptors, the only members of group I mGlu receptors, are implicated in synaptic plasticity and mechanisms of feedback control of glutamate release. They exhibit nearly complementary distributions throughout the central nervous system, well evident in the cerebellum, where mGlu1 receptor is most intensely expressed while mGlu5 receptor is not. Despite their different distribution, they show a similar subcellular localization and use common transducing pathways. We recently described the Grm1(crv4) mouse with motor coordination deficits and renal anomalies caused by a spontaneous mutation inactivating the mGlu1 receptor. To define the neuropathological mechanisms in these mice, we evaluated expression and function of the mGlu5 receptor in cerebral and cerebellar cortices. Western blot and immunofluorescence analyses showed mGlu5 receptor overexpression. Quantitative reverse transcriptase-polymerase chain reaction results indicated that the up-regulation is already evident at RNA level. Functional studies confirmed an enhanced glutamate release from cortical cerebral and cerebellar synaptosomes when compared with wild-type that is abolished by the mGlu5 receptor-specific inhibitor, 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP). Finally, acute MPEP treatment of Grm1(crv4/crv4) mice induced an evident although incomplete improvement of motor coordination, suggesting that mGlu5 receptors enhanced activity worsens, instead of improving, the motor-coordination defects in the Grm1(crv4/crv4) mice.

Defining protein kinase/phosphatase isoenzymic regulation of mGlu₅ receptor-stimulated phospholipase C and Ca²⁺ responses in astrocytes

BACKGROUND AND PURPOSE

Cyclical phosphorylation and dephosphorylation of a key residue within the C-terminal domain of the activated type 5 metabotropic glutamate (mGlu₅) receptor is believed to cause the synchronous, oscillatory changes in inositol 1,4,5-trisphosphate and Ca²⁺ levels observed in a variety of cell types. Here, we have attempted to better define the kinase and phosphatase enzymes involved in this modulation.

EXPERIMENTAL APPROACH

Ca²⁺ and [³H]inositol phosphate ([³H]IP(x) ) measurements in astrocyte preparations have been used to evaluate the effects of pharmacological inhibition of protein kinase C (PKC) and protein phosphatase activities and small interfering RNA-mediated specific PKC isoenzymic knock-down on mGlu₅ receptor signalling.

KEY RESULTS

Ca²⁺ oscillation frequency or [³H]IP(x) accumulation in astrocytes stimulated by mGlu₅ receptors, was concentration-dependently decreased by protein phosphatase-1/2A inhibition or by PKC activation. PKC inhibition also increased [³H]IP(x) accumulation two- to threefold and changed the Ca²⁺ response into a peak-plateau response. However, selective inhibition of conventional PKC isoenzymes or preventing changes in [Ca²⁺](i) concentration by BAPTA-AM loading was without effect on mGlu₅ receptor-stimulated [³H]IP(x) accumulation. Selective knock-down of PKCδ was without effect on glutamate-stimulated Ca²⁺ responses; however, selective PKCε knock-down in astrocytes changed Ca²⁺ responses from oscillatory into peak-plateau type.

CONCLUSION AND IMPLICATIONS

These data confirm the acute regulation of mGlu₅ receptor signalling by protein kinases and protein phosphatases and provide novel data pinpointing the isoenzymic dependence of this regulation in the native mGlu₅ receptor-expressing rat cortical astrocyte. These data also highlight a potential alternative mechanism by which mGlu₅ receptor signalling might be therapeutically manipulated.

Parasagittally aligned, mGluR1-dependent patches are evoked at long latencies by parallel fiber stimulation in the mouse cerebellar cortex in vivo

The parallel fibers (PFs) in the cerebellar cortex extend several millimeters along a folium in the mediolateral direction. The PFs are orthogonal to and cross several parasagittal zones defined by the olivocerebellar and corticonuclear pathways and the expression of molecular markers on Purkinje cells (PCs). The functions of these two organizations remain unclear, including whether the bands respond similarly or differentially to PF input. By using flavoprotein imaging in the anesthetized mouse in vivo, this study demonstrates that high-frequency PF stimulation, which activates a beamlike response at short latency, also evokes patches of activation at long latencies. These patches consist of increased fluorescence along the beam at latencies of 20-25 s with peak activation at 35 s. The long-latency patches are completely blocked by the type 1 metabotropic glutamate receptor (mGluR(1)) antagonist LY367385. Conversely, the AMPA and NMDA glutamate receptor antagonists DNQX and APV have little effect. Organized in parasagittal bands, the long-latency patches align with zebrin II-positive PC stripes. Additional Ca(2+) imaging demonstrates that the patches reflect increases in intracellular Ca(2+). Both the PLCβ inhibitor U73122 and the ryanodine receptor inhibitor ryanodine completely block the long-latency patches, indicating that the patches are due to Ca(2+) release from intracellular stores. Robust, mGluR(1)-dependent long-term potentiation (LTP) of the patches is induced using a high-frequency PF stimulation conditioning paradigm that generates LTP of PF-PC synapses. Therefore, the parasagittal bands, as defined by the molecular compartmentalization of PCs, respond differentially to PF inputs via mGluR(1)-mediated release of internal Ca(2+).

Elucidation of a novel extracellular calcium-binding site on metabotropic glutamate receptor 1{alpha} (mGluR1{alpha}) that controls receptor activation

Metabotropic glutamate receptor 1α (mGluR1α) exerts important effects on numerous neurological processes. Although mGluR1α is known to respond to extracellular Ca(2+) ([Ca(2+)](o)) and the crystal structures of the extracellular domains (ECDs) of several mGluRs have been determined, the calcium-binding site(s) and structural determinants of Ca(2+)-modulated signaling in the Glu receptor family remain elusive. Here, we identify a novel Ca(2+)-binding site in the mGluR1α ECD using a recently developed computational algorithm. This predicted site (comprising Asp-318, Glu-325, and Asp-322 and the carboxylate side chain of the receptor agonist, Glu) is situated in the hinge region in the ECD of mGluR1α adjacent to the reported Glu-binding site, with Asp-318 involved in both Glu and calcium binding. Mutagenesis studies indicated that binding of Glu and Ca(2+) to their distinct but partially overlapping binding sites synergistically modulated mGluR1α activation of intracellular Ca(2+) ([Ca(2+)](i)) signaling. Mutating the Glu-binding site completely abolished Glu signaling while leaving its Ca(2+)-sensing capability largely intact. Mutating the predicted Ca(2+)-binding residues abolished or significantly reduced the sensitivity of mGluR1α not only to [Ca(2+)](o) and [Gd(3+)](o) but also, in some cases, to Glu. The dual activation of mGluR1α by [Ca(2+)](o) and Glu has important implications for the activation of other mGluR subtypes and related receptors. It also opens up new avenues for developing allosteric modulators of mGluR function that target specific human diseases.

Activation of metabotropic glutamate receptors induces periodic burst firing and concomitant cytosolic Ca2+ oscillations in cerebellar interneurons

Little is known about the generation of slow rhythms in brain neuronal circuits. Nevertheless, a few studies, both from reconstituted systems and from hippocampal slices, indicate that activation of metabotropic glutamate receptors (mGluRs) could generate such rhythms. Here we show in rat cerebellar slices that after either release of glutamate by repetitive stimulation, or direct stimulation of type 1 mGluRs, molecular layer interneurons exhibit repetitive slow Ca(2+) transients. By combining cell-attached patch-clamp recording with Ca(2+) imaging, we show that the regular Ca(2+) transients (mean frequency, 35 mHz induced by 2 microm quisqualate in the presence of ionotropic glutamate receptor blockers) are locked with bursts of action potentials. Nevertheless, the Ca(2+) transients are not blocked by tetrodotoxin, indicating that firing is not necessary to entrain oscillations. The first Ca(2+) transient within a train is different in several ways from subsequent transients. It is broader than the subsequent transients, displays a different phase relationship to associated spike bursts, and exhibits a distinct sensitivity to ionic and pharmacological manipulations. Whereas the first transient appears to involve entry of Ca(2+) ions through transient receptor potential channel-like channels and secondarily activated L-type Ca(2+) channels, subsequent transients rely mostly on an exchange of Ca(2+) ions between the cytosol and D-myo-inositol-1,4,5-triphosphate-sensitive intracellular Ca(2+) stores. The slow, highly regular oscillations observed in the present work are likely to drive pauses in postsynaptic Purkinje cells, and could play a role in coordinating slow oscillations involving the cerebello-olivar circuit loop.

Cellular localization of metabotropic glutamate receptors in cortical tubers and subependymal giant cell tumors of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder associated with cortical malformations (cortical tubers) and the development of glial tumors (subependymal giant-cell tumors, SGCTs). Expression of metabotropic glutamate receptor (mGluR) subtypes is developmentally regulated and several studies suggest an involvement of mGluR-mediated glutamate signaling in the regulation of proliferation and survival of neural stem-progenitor cells, as well as in the control of tumor growth. In the present study, we have investigated the expression and cell-specific distribution of group I (mGluR1, mGluR5), group II (mGluR2/3) and group III (mGluR4 and mGluR8) mGluR subtypes in human TSC specimens of both cortical tubers and SGCTs, using immunocytochemistry. Strong group I mGluR immunoreactivity (IR) was observed in the large majority of TSC specimens in dysplastic neurons and in giant cells within cortical tubers, as well as in tumor cells within SGCTs. In particular mGluR5 appeared to be most frequently expressed, whereas mGluR1alpha was detected in a subpopulation of neurons and giant cells. Cells expressing mGluR1alpha and mGluR5, demonstrate IR for phospho-S6 ribosomal protein (PS6), which is a marker of the mammalian target of rapamycin (mTOR) pathway activation. Group II and particularly group III mGluR IR was less frequently observed than group I mGluRs in dysplastic neurons and giant cells of tubers and tumor cells of SGCTs. Reactive astrocytes were mainly stained with mGluR5 and mGluR2/3. These findings expand our knowledge concerning the cellular phenotype in cortical tubers and in SGCTs and highlight the role of group I mGluRs as important mediators of glutamate signaling in TSC brain lesions. Individual mGluR subtypes may represent potential pharmacological targets for the treatment of the neurological manifestations associated with TSC brain lesions.

Inhibition of mGluR1 and IP3Rs impairs long-term memory formation in young chicks

Calcium (Ca(2+)) is involved in a myriad of cellular functions in the brain including synaptic plasticity. However, the role of intracellular Ca(2+) stores in memory processing remains poorly defined. The current study explored a role for glutamate-dependent intracellular Ca(2+) release in memory processing via blockade of metabotropic glutamate receptor subtype 1 (mGluR1) and inositol (1,4,5)-trisphosphate receptors (IP(3)Rs). Using a single-trial discrimination avoidance task developed for the young chick, administration of the specific and potent mGluR1 antagonist JNJ16259685 (500nM, immediately post-training, ic), or the IP(3)R antagonist Xestospongin C (5microM, immediately post-training, ic), impaired retention from 90min post-training. These findings are consistent with mGluR1 activating IP(3)Rs to release intracellular Ca(2+) required for long-term memory formation and have been interpreted within an LTP2 model. The consequences of different patterns of retention loss following ryanodine receptor (RyR) and IP(3)R inhibition are discussed.

Neuroprotective effects of the selective type 1 metabotropic glutamate receptor antagonist YM-202074 in rat stroke models

We describe in vitro properties and in vivo neuroprotective effects of a newly synthesized, high-affinity, selective allosteric metabotropic glutamate receptor type 1 (mGluR(1)) antagonist, N-cyclohexyl-6-{[(2-methoxyethyl)(methyl)amino]methyl}-N-methylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-202074). YM-202074 bound an allosteric site of rat mGluR(1) with a K(i) value of 4.8+/-0.37 nM. YM-202074 also inhibited the mGluR(1)-mediated inositol phosphates production in rat cerebellar granule cells with an IC(50) value of 8.6+/-0.9 nM, while showing selectivity over mGluR(2-7). When YM-202074 was infused intravenously at an initial dose of 20 mg/kg/h for 0.5 h followed by a dose of 5 mg/kg/h for 7.5 h, the free concentration of YM-202074 in the brain rapidly (<12 min) reached approximately 0.3 microM, reaching a steady-state phase within 1.5 h. We first treated rats such that they developed transient middle cerebral artery (MCA) occlusion. Results clearly demonstrate a dose-dependent improvement of neurological deficit and reduction of the infarct volume in both the hemisphere and cortex when YM-202074 was infused intravenously immediately after occlusion at a dose of 10 or 20 mg/kg/h for 0.5 h followed by a dose of 2.5 or 5 mg/kg/h for 23.5 h, respectively. Significant neuroprotection was maintained even when the administration of drugs was delayed by up to 2 h following the onset of ischemia. Furthermore, the improvement of neurological deficit and the reduction of infarct volume were sustained for 1 week following the onset of ischemia. These results suggest that YM-202074 exhibits great potential as a novel neuroprotective agent for the treatment of stroke.

Supramolecular signalling complexes in the nervous system

It is now apparent that multiprotein signalling complexes or "signalling machines" are responsible for orchestrating many complex signalling pathways in the cell. The synapse is a sub-cellular specialisation which transmits and converts patterns of electrical activity into cellular memory. This processing of electrical information is mediated by the protein components of the synapse. The organisation of synaptic proteins has been investigated over the last number of years using proteomic methods and with the application ofbioinformatics; a landscape of modular protein complexes at the synapse is emerging. Many share a common organisation centred on a receptor/channel, a protein scaffold, (in which the signalling molecules are localised) and membrane to cytoskeleton interactions. The use of PDZ-domain based protein scaffolds is a particularly common feature in the construction of neuronal protein complexes and the differential presence of these proteins in complexes can have functional consequences. Here we overview current proteomic methodologies for the analysis of multiprotein complexes. In addition, we describe the characterisation of a number of multiprotein complexes associated with ion channels (NMDAR, P2X7 and Kir2) and GPCRs (5-HT2A/5-HT2C, D2 and mGluR5) and discuss common their common components and organisation.

Role of spinal metabotropic glutamate receptor subtype 5 in the development of tolerance to morphine-induced antinociception in rat

Prolonged intrathecal (i.t.) administration of morphine results in tolerance to morphine-induced antinociception. We found that co-administration of selective metabotropic glutamate receptor subtype 5 antagonist MPEP with morphine could suppress the loss of morphine-induced antinociception and inhibit the development of tolerance to morphine-induced antinociceptive effect. Whereas, the specific metabotropic glutamate receptor subtype 5 agonist CHPG does the opposite. As the activation of NMDA receptor after chronic morphine administration has been verified, we suppose there is an enhanced activation of mGluR5 during the development of tolerance to morphine-induced antinociception. Activation of mGluR5 may mobilize the release of intracellular Ca(2+) and activate PKC, leading to morphine-induced antinociception suppression. We conclude that mGluR5 contributes to the development of tolerance to morphine-induced antinociception after chronic morphine exposure.

mGlu5 receptor and protein kinase C implicated in the development and induction of neuropathic pain following chronic ethanol consumption

The central mechanisms of neuropathic pain following chronic ethanol consumption are poorly understood. We previously reported that the levels of metabotropic glutamate 5 (mGlu5) receptor and phosphorylated-protein kinase C (PKC) were significantly increased in the spinal cord following chronic ethanol consumption. The aim of this study was to investigate whether mGlu5 receptor and PKC inhibitors directly attenuate the neuropathic pain-like state induced by chronic ethanol treatment in rats. A significant decrease in the mechanical nociceptive threshold was observed 5 weeks of chronic ethanol consumption. This hyperalgesia was significantly attenuated by repeated i.p. injection of (S)-2,6-diamino-N-[[1-(oxotridecyl)-2-piperidinyl]methyl] hexanamide dihydrochloride (NPC15437), a selective PKC inhibitor, once a day for a week after 4 weeks of ethanol treatment. Furthermore, this hyperalgesia was also significantly attenuated by repeated i.p. injection of 6-methyl-2-[phenylethynyl]-pyridine (MPEP), a selective mGlu5 receptor inhibitor, once a day for a week after 4 weeks of ethanol treatment. Furthermore, the hyperalgesia that developed after 5 weeks of ethanol treatment was significantly suppressed by a single i.p. post-injection with either NPC15437 or MPEP. These findings constitute direct evidence that spinal mGlu5 receptor and PKC play substantial roles in the development and maintenance of an ethanol-dependent neuropathic pain-like state in rats.

Involvement of mGluR5 in the ethanol-induced neuropathic pain-like state in the rat

Alcohol neuropathy has been thought to involve decreased nerve function following chronic ethanol consumption. However, there is no reliably successful therapy, largely due to a lack of understanding of the central underlying mechanisms. The aim of this study was to investigate the mechanisms that contribute to the neuropathic pain-like state induced by chronic ethanol treatment in rats. Rats were chronically treated with ethanol diet (1.25-5% of ethanol) for over 70 days. Mechanical hyperalgesia was observed during ethanol consumption and even after ethanol withdrawal. Under these conditions, an immunohistochemical study showed an increase in metabotropic glutamate receptor 5 (mGluR5) immunoreactivity in the superficial spinal dorsal horn of chronic ethanol-fed rats. Furthermore, immunoblot analysis revealed that the protein level of mGluR5 was clearly increased following chronic ethanol consumption. These findings support the idea that the increased levels of mGluR5 in the spinal cord may be, at least in part, involved in the induction of ethanol-dependent neuropathic pain-like state.

Metabotropic glutamate receptor subtype 5 antagonists MPEP and MTEP

Glutamate regulates the function of central nervous system (CNS), in part, through the cAMP and/or IP3/DAG second messenger-associated metabotropic glutamate receptors (mGluRs). The mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) has been extensively used to elucidate potential physiological and pathophysiological functions of mGluR5. Unfortunately, recent evidence indicates significant non-specific actions of MPEP, including inhibition of NMDA receptors. In contrast, in vivo and in vitro characterization of the newer mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) indicates that it is more highly selective for mGluR5 over mGluR1, has no effect on other mGluR subtypes, and has fewer off-target effects than MPEP. This article reviews literature on both of these mGluR5 antagonists, which suggests their possible utility in neurodegeneration, addiction, anxiety and pain management.

Involvement of spinal metabotropic glutamate receptor 5 in the development of tolerance to morphine-induced antinociception

It is well known that prolonged exposure to morphine results in tolerance to morphine-induced antinociception. In the present study, we found that either intrathecal (i.t.) or subcutaneous (s.c.) injection of the selective metabotropic glutamate receptor 5 (mGluR5) antagonist, methyl-6-(phenylethynyl)-pyridine hydrochloride (MPEP), attenuated the development of tolerance to morphine-induced antinociception. Using the receptor binding assay, we found here that the number of mGluR5 in the mouse spinal cord was significantly increased by repeated treatment with morphine. Furthermore, repeated treatment with morphine produced a significant increase in the level of mGluR5 immunoreactivity in the dorsal horn of the mouse spinal cord. Double-labeling experiments showed that the increased mGluR5 was predominantly expressed in the neurons and sparsely expressed in the processes of astrocytes following repeated treatment with morphine. Consistent with these results, the response of Ca2+ to the selective group I mGluR agonist, 3,5-dihydroxyphenylglycine (DHPG), in cultured spinal cord neurons was potently enhanced by 3 days of in vitro treatment with morphine. These findings support the idea that the increased mGluR5 following repeated treatment with morphine leads to enhanced neuronal excitability and synaptic transmission in the dorsal horn of the spinal cord and, in turn, suppresses the morphine-induced antinociception in mice.

Stimulation of group I metabotropic glutamate receptors evokes calcium signals and c-jun and c-fos gene expression in human T cells

To study if the activation of group I mGlu receptors in human T cells modifies intracellular Ca2+ concentration ([Ca2+](i)) and cell function, we measured [Ca2+](i) on cell suspensions (spectrofluorimetric method) or single cell (digital Ca2+ imaging system) using fura-2 as indicator. Early-inducible gene (c-jun and c-fos) expression was studied by reverse transcriptase-polymerase chain reaction assay as representative of Ca(2+)-sensitive gene expression. (1S,3R)-ACPD (100 microM), the selective mGlu receptor agonist, evoked a significant increase (34.1+/-4.9%) of [Ca2+](i), pharmacologically characterized as mediated by group I mGlu receptors, since both (S)-3,5-DHPG (100 microM), a selective group I mGlu receptor agonist and CHPG (1mM), the specific mGlu5 receptor agonist, reproduced the effects, that were abolished by AIDA (1mM), a selective group I mGlu receptor antagonist. (S)-3,5-DHPG-induced a rapid [Ca2+](i) rise (initial phase) followed by a slow decrease (second phase) to the baseline. Both extracellular Ca2+ and Ca2+ released from intracellular stores contribute to the [Ca2+](i) increase which depend on PLC activation. In a Ca(2+)-free buffer, the second phase rapidly return to the baseline; LaCl3 (1-10 microM), an inhibitor of extracellular Ca2+ influx, significantly reduced the second phase only; thapsigargin (1microM), by discharging intracellular Ca2+ stores, U 73122 (10 microM) and D609 (300 microM), by inhibiting PLC activity, prevented both phases. In our system, PTX pre-treatment increased (S)-3,5-DHPG effects, demonstrating that PXT-sensitive G(i/o) proteins are involved. Finally, specific stimulation of these receptors in Jurkat cells upregulates c-jun and c-fos gene expression, thus activating multiple downstream signalling regulating important T cell functions.

Protein Kinase C Phosphorylation of the Metabotropic Glutamate ReceptormGluR5 on Serine 839 Regulates Ca2+Oscillations

The activation of Group 1 metabotropic glutamate receptors, mGluR5 and mGluR1alpha, triggers intracellular calcium release; however, mGluR5 activation is unique in that it elicits Ca2+ oscillations. A short region of the mGluR5 C terminus is the critical determinant and differs from the analogous region of mGluR1alpha by a single amino acid residue, Thr-840, which is an aspartic acid (Asp-854) in mGluR1alpha. Previous studies show that mGluR5-elicited Ca2+ oscillations require protein kinase C (PKC)-dependent phosphorylation and identify Thr-840 as the phosphorylation site. However, direct phosphorylation of mGluR5 has not been studied in detail. We have used biochemical analyses to directly investigate the phosphorylation of the mGluR5 C terminus. We showed that Ser-839 on mGluR5 is directly phosphorylated by PKC, whereas Thr-840 plays a permissive role. Although Ser-839 is conserved in mGluR1alpha (Ser-853), it is not phosphorylated, as the adjacent residue (Asp-854) is not permissive; however, mutagenesis of Asp-854 to a permissive alanine residue allows phosphorylation of Ser-853 on mGluR1alpha. We investigated the physiological consequences of mGluR5 Ser-839 phosphorylation using Ca2+ imaging. Mutations that eliminate Ser-839 phosphorylation prevent the characteristic mGluR5-dependent Ca2+ oscillations. However, mutation of Thr-840 to alanine, which prevents potential Thr-840 phosphorylation but is still permissive for Ser-839 phosphorylation, has no effect on Ca2+ oscillations. Thus, we showed that it is phosphorylation of Ser-839, not Thr-840, that is absolutely required for the unique Ca2+ oscillations produced by mGluR5 activation. The Thr-840 residue is important only in that it is permissive for the PKC-dependent phosphorylation of Ser-839.

Acute up-regulation of glutamate uptake mediated by mGluR5a in reactive astrocytes

Excitatory transmission in the CNS necessitates the existence of dynamic controls of the glutamate uptake achieved by astrocytes, both in physiological conditions and under pathological circumstances characterized by gliosis. In this context, this study was aimed at evaluating the involvement of group I metabotropic glutamate receptors (mGluR) in the regulation of glutamate transport in a model of rat astrocytes undergoing in vitro activation using a cocktail of growth factors (G5 supplement). The vast majority of the cells were found to take up aspartate, mainly through the glutamate/aspartate transporter (GLAST), and at least 60% expressed functional mGluR5a. When exposed for 15 s to the selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine, reactive astrocytes showed a significant increase in their capacity to take up aspartate. This effect was confirmed at the single-cell level, since activation of mGluRs significantly increased the initial slope of aspartate-dependent Na+ entry associated with the activity of glutamate transporters. This up-regulation was inhibited by an antagonist of mGluR5 and, more importantly, was sensitive to a specific glutamate transporter 1 (GLT-1) blocker. The acute influence of mGluR5 on aspartate uptake was phospholipase C- and protein kinase C-dependent, and was mimicked by phorbol esters. We conclude that mGluR5a contributes to a dynamic control of GLT-1 function in activated astrocytes, acting as a glial sensor of the extracellular glutamate concentration in order to acutely regulate the excitatory transmission.

A family of octopamine [corrected] receptors that specifically induce cyclic AMP production or Ca2+ release in Drosophila melanogaster

In invertebrates, the biogenic-amine octopamine is an important physiological regulator. It controls and modulates neuronal development, circadian rhythm, locomotion, 'fight or flight' responses, as well as learning and memory. Octopamine mediates its effects by activation of different GTP-binding protein (G protein)-coupled receptor types, which induce either cAMP production or Ca(2+) release. Here we describe the functional characterization of two genes from Drosophila melanogaster that encode three octopamine receptors. The first gene (Dmoa1) codes for two polypeptides that are generated by alternative splicing. When heterologously expressed, both receptors cause oscillatory increases of the intracellular Ca(2+) concentration in response to applying nanomolar concentrations of octopamine. The second gene (Dmoa2) codes for a receptor that specifically activates adenylate cyclase and causes a rise of intracellular cAMP with an EC(50) of approximately 3 x 10(-8) m octopamine. Tyramine, the precursor of octopamine biosynthesis, activates all three receptors at > or = 100-fold higher concentrations, whereas dopamine and serotonin are non-effective. Developmental expression of Dmoa genes was assessed by RT-PCR. Overlapping but not identical expression patterns were observed for the individual transcripts. The genes characterized in this report encode unique receptors that display signature properties of native octopamine receptors.

Group I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to cyclic AMP response element binding protein (CREB) through a common Ca2+ - and protein kinase C-dependent pathway

Coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the cAMP response element binding protein (CREB) has been studied in Chinese hamster ovary cell lines where receptor expression is under the control of an inducible promoter. Both receptors stimulate CREB phosphorylation with similar time courses, and agonist potency was also comparable between the two receptors. Stimulation of cells in Ca(2+)-free medium containing EGTA (100 microm), with or without the additional depletion of intracellular stores, caused marked decreases in agonist-mediated responses in both cell lines. Down-regulation of protein kinase C (PKC) activity by phorbol ester treatment, or treatment with the broad spectrum PKC inhibitor Ro 31-8220, partially attenuated both mGlu1a and mGlu5a receptor-mediated responses. Furthermore, stimulation of cells in the absence of extracellular Ca(2+) following prior PKC down-regulation resulted in additive inhibitory effects. The involvement of extracellular signal-regulated kinases (ERK1/2), Ca(2+)/calmodulin or Ca(2+)/calmodulin-dependent protein kinases was assessed using pharmacological inhibitors. Results indicated that coupling of the group I mGlu receptors to CREB phosphorylation occurs independently of these pathways. Thus, although the [Ca(2+)](i) signatures activated by these mGlu receptors differ, they couple to CREB with comparable potency and recruit similar downstream components to execute CREB phosphorylation.

Regulation of Homer and group I metabotropic glutamate receptors by nicotine

The present study focuses on the nicotine-induced modulation of mRNA and protein expression of a number of genes involved in glutamatergic synaptic transmission in rat brain over different time periods of exposure. A subchronic (3 days) but not the chronic (7 or 14 days) administration of nicotine resulted in the up-regulation of Homer2a/b mRNA in the amygdala while in the ventral tegmental area (VTA) no change in expression of either Homer2a/b or Homer1b/c was observed. Although the increase in Homer2a/b mRNA was not translated into the protein level in the amygdala, a slight but significant up-regulation of Homer1b/c protein was observed in the same region at day 3. Both Homer forms were up-regulated at the protein level in the VTA at day 3. In the nucleus accumbens, 14 days of nicotine treatment up-regulated mRNA of Homer2b/c by 68.2% (P < 0.05), while the short form Homer1a gene was down-regulated by 65.0% at day 3 (P < 0.05). In regard to other components of the glutamatergic signalling, we identified an acute and intermittent increase in the mRNA and protein levels of mGluR1 and mGluR5 in the amygdala. In the VTA, however, the effects of nicotine on mGluR mRNA expression were long-lasting but rather specific to mGluR1. Nevertheless, mGluR1 protein levels in the VTA area were up-regulated only at day 3, as in the amygdala. These data provide further evidence for the involvement of nicotine in the glutamatergic neuronal synaptic activity in vivo, suggesting a role for the newly identified Homer proteins in this paradigm.

Proteomic analysis of in vivo phosphorylated synaptic proteins

In the nervous system, protein phosphorylation is an essential feature of synaptic function. Although protein phosphorylation is known to be important for many synaptic processes and in disease, little is known about global phosphorylation of synaptic proteins. Heterogeneity and low abundance make protein phosphorylation analysis difficult, particularly for mammalian tissue samples. Using a new approach, combining both protein and peptide immobilized metal affinity chromatography and mass spectrometry data acquisition strategies, we have produced the first large scale map of the mouse synapse phosphoproteome. We report over 650 phosphorylation events corresponding to 331 sites (289 have been unambiguously assigned), 92% of which are novel. These represent 79 proteins, half of which are novel phosphoproteins, and include several highly phosphorylated proteins such as MAP1B (33 sites) and Bassoon (30 sites). An additional 149 candidate phosphoproteins were identified by profiling the composition of the protein immobilized metal affinity chromatography enrichment. All major synaptic protein classes were observed, including components of important pre- and postsynaptic complexes as well as low abundance signaling proteins. Bioinformatic and in vitro phosphorylation assays of peptide arrays suggest that a small number of kinases phosphorylate many proteins and that each substrate is phosphorylated by many kinases. These data substantially increase existing knowledge of synapse protein phosphorylation and support a model where the synapse phosphoproteome is functionally organized into a highly interconnected signaling network.

Altered agonist sensitivity and desensitization of neuronal mGluR1 responses in knock-in mice by a single amino acid substitution at the PKC phosphorylation site

mGluR1 and mGluR5 of the metabotropic glutamate receptor family are coupled to inositol trisphosphate-Ca2+ signal cascades and evoke distinct Ca2+ responses in neural cells and heterologously expressing cells. In heterologous cells, stimulation of recombinant mGluR1 evokes a single-peaked Ca2+ response whereas mGluR5 elicits an oscillatory Ca2+ response. The distinct Ca2+ responses are interchangeable by single amino substitution of aspartate for threonine at the corresponding position of the carboxy-terminal cytoplasmic regions of mGluR1 and mGluR5, respectively. In this investigation, we generated knock-in mice, termed mGluR1 D854T mice, in which aspartate of mGluR1 was replaced with threonine. We examined the effect of this D854T substitution on Ca2+ and current responses mediated by mGluR1 in cultured cerebellar Purkinje cells. Stimulation of mGluR1 D854T by a group 1 mGluR agonist, 3,5-dihydroxyphenylglycine (DHPG) evoked, as in wild-type mGluR1, only single-peaked Ca2+ responses as measured by Ca2+ fluorometric analysis. We then examined mGluR1-induced inward currents carried by nonselective cation channels during whole-cell recordings from cultured Purkinje cells. The mGluR1 D854T mutation abolished the responsiveness of mGluR1 to low concentrations of DHPG (0.5-500 nM) and reduced its desensitization during prolonged agonist application. mGluR1 D854T homozygous mutants showed no apparent behavioural abnormality as analysed by motor movement tests. These results indicate that, although additional modulatory mechanisms seem to be required to produce oscillatory Ca2+ responses of mGluR1, the single amino acid substitution at position 854 of mGluR1 is capable of influencing the kinetics of neuronal mGluR1 responses, most probably through PKC-mediated phosphorylation.

The mGlu5 metabotropic glutamate receptor is expressed in zones of active neurogenesis of the embryonic and postnatal brain

Metabotropic glutamate (mGlu) receptors have been implicated in the regulation of developmental plasticity. Here, we examined the expression of mGlu1a-b, -2, -3, -4a-b, and -5a receptor subtypes from embryonic day 12 (E12) to the early and late postnatal life. While all transcripts (with the exception of mGlu4 mRNA) were detected prenatally, only the mGlu5 receptor protein was found in detectable amounts in the embryonic brain. Immunohistochemical analysis showed that the mGlu5 receptor was mainly expressed by cells surrounding the ventricles at E15, whereas it was more diffusely expressed at E18. In the postnatal life, besides its classical expression sites, the mGlu5 receptor was found in zones of active neurogenesis such as the external granular layer (EGL) of the cerebellar cortex and the subventricular zone. In these regions, the presence of actively proliferating progenitor cells was detected by BrdU staining. No other subtype (among those we have examined) was found to be expressed in regions enriched of BrdU(+) cells. These data suggest a role for mGlu5 receptors in the early brain development and in basic cellular processes such as proliferation and/or differentiation.

The protein phosphatase 1/2A inhibitor okadaic acid increases CREB and Elk-1 phosphorylation and c-fos expression in the rat striatum in vivo

Activation of group I metabotropic glutamate receptors (mGluRs) up-regulates transcription factor cyclic AMP response element-binding protein (CREB) and Elk-1 phosphorylation via extracellular signal-regulated kinase 1/2 (ERK1/2) in the striatum in vivo. Protein phosphatase 1/2A further regulates immediate early gene expression by inactivating (dephosphorylating) CREB. In this study, using semi-quantitative immunohistochemical and western blot analyses and in situ hybridization histochemistry, we found that intrastriatal infusion of the protein phosphatase 1/2A inhibitor okadaic acid (0.005, 0.05 and 0.5 nmol) increased CREB and Elk-1 phosphorylation and c-Fos immunoreactivity in the injected dorsal striatum in a dose-dependent manner. In addition, okadaic acid (0.05 and 0.5 nM) increased c-fos mRNA expression in the dorsal striatum in a dose-dependent manner. Intrastriatal infusion of the group I agonist 3,5-dihydroxyphenylglycine (DHPG) at 100 and 250 nM also increased CREB and Elk-1 phosphorylation. Pre-treatment of okadaic acid (0.05 nm) did not alter DHPG-induced increases in the phosphorylation of the two transcription factors. These data suggest that protein phosphatase 1/2A in striatal neurons is tonically active in dephosphorylating CREB and Elk-1 and thus suppressing constitutive c-fos mRNA and protein expression. Inhibition of the phosphatase 1/2A may contribute to the group I mGluR-regulated phosphorylation of these transcription factors and c-fos expression.

Modulation of stretch-induced enhancement of neuronal NMDA receptor current by mGluR1 depends upon presence of glia

Stretching of cultured neurons has been used to model diffuse axonal injury associated with brain trauma. N-Methyl-D-aspartate receptor (NMDAR) activation and group I metabotropic glutamate receptors (mGluRs) are implicated in the pathophysiology of such injury. Here we detail the effects of culture condition and mGluR1 modulation on stretch-enhanced NMDA receptor activity, and show the presence of mGluR1 in addition to mGluR5 in glia. In cortical neurons grown in the absence (PN) or presence (NG) of a glial monolayer, stretch injury (5.7 mm) enhances NMDAR activity by increasing maximal NMDAR current, decreasing the voltage-dependent Mg(2+) block, and altering the kinetic behavior of these receptors. In PN cultures, activation of mGluR1 increases stretch-enhanced NMDAR activity, whereas in NG cultures, such activity is reduced. In contrast, inhibition of mGluR1 in PN cultures limits stretch-enhanced NMDAR activity, whereas in NG cultures activity is increased. MGluR1 modulate stretch-enhanced NMDAR activity through multiple mechanisms including: altering peak or steady state current, affecting Mg(2+) blockade of the NMDAR, or by changing NMDAR kinetics. The presence of glia significantly alters the nature of mGluR1-mediated modulation of NMDAR activity and stretch-induced injury. Together these data indicate a significant neuronal/glial interaction between glial mGluR1 and neuronal NMDA receptor activity.

Expression and functional properties of group I metabotropic glutamate receptors in bovine chromaffin cells

We demonstrate the presence and functional properties of Group I metabotropic glutamate receptors (mGluRs) expressed in chromaffin cells. Immunocytochemical techniques revealed that two mGluR subtypes (mGluR1alpha and mGluR5) are expressed in chromaffin cells, located in both the cytoplasmic membrane and the cytosol surrounding the nucleus. These mGluRs are functionally active on catecholamine (CA) secretion in chromaffin cells because both (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) and the specific agonist of Group I mGluRs, (S)-3,5-dihydroxyphenylglycine (DHPG), were able to stimulate the release of CAs (adrenaline and noradrenaline) in a dose-response manner. These effects were specifically reversed by L-(+)-2-amino-3-phosphonopropionic acid (L-AP3), a selective antagonist of the Group I metabotropic glutamate receptors. t-ACPD induced an increase in CA secretion in both the presence and absence of extracellular calcium, the former effect being accompanied by cell membrane depolarization. Noradrenaline (NA) release was higher in the presence of extracellular calcium than in its absence, whereas adrenaline release was of the same order under both conditions. These results indicate that different subtypes of Group I mGluRs are present in noradrenergic and adrenergic cells. Fluorescence imaging techniques in single cells showed different t-ACPD-induced increases in intracellular calcium in different chromaffin cells: in chromaffin cells, 67% expressed functional metabotropic glutamate receptors and with nicotinic receptors, whereas the remaining 33% expressed only nicotinic receptors. In the absence of external calcium, only about 25% of cells responded to t-ACPD-increased intracellular calcium by increasing inositol 1,4,5-trisphosphate (IP(3)) concentration and subsequent calcium mobilization from intracellular stores, whereas the remaining 75% increased intracellular calcium by promoting Ca(2+) influx from the extracellular medium through L- and N- but not P/Q voltage-dependent calcium channels.

Isoform-specific phosphorylation of metabotropic glutamate receptor 5 by protein kinase C (PKC) blocks Ca2+ oscillation and oscillatory translocation of Ca2+-dependent PKC

Prolonged activation of metabotropic glutamate receptor 5a (mGluR5a) causes synchronized oscillations in intracellular calcium, inositol 1,4,5-trisphosphate production, and protein kinase C (PKC) activation. Additionally, mGluR5 stimulation elicited cyclical translocations of myristoylated alanine-rich protein kinase C substrate, which were opposite to that of gammaPKC (i.e. from plasma membrane to cytosol) and dependent on PKC activity, indicating that myristoylated alanine-rich protein kinase C substrate is repetitively phosphorylated by oscillating gammaPKC on the plasma membrane. Mutation of mGluR5 Thr(840) to aspartate abolished the oscillation of gammaPKC, but the mutation to alanine (T840A) did not. Cotransfection of gammaPKC with betaIIPKC, another Ca2+-dependent PKC, resulted in synchronous oscillatory translocation of both classical PKCs. In contrast, cotransfection of deltaPKC, a Ca2+-independent PKC, abolished the oscillations of both gammaPKC and inositol 1,4,5-trisphosphate. Regulation of the oscillations was dependent on deltaPKC kinase activity but not on gammaPKC. Furthermore, the T840A-mGluR5-mediated oscillations were not blocked by the deltaPKC overexpression. These results revealed that activation of mGluR5 causes translocation of both gammaPKC and deltaPKC to the plasma membrane. deltaPKC, but not gammaPKC, phosphorylates mGluR5 Thr(840), leading to the blockade of both Ca2+ oscillations and gammaPKC cycling. This subtype-specific targeting proposes the molecular basis of the multiple functions of PKC.

[Analysis of PKC targeting mechanism using PKC fused with fluorescent proteins]

Protein kinase C (PKC) is a family, which consists of at least ten subtypes. To elucidate subtype-specific functions of PKC, we have developed the methods to observe PKC translocation in real time and in the living state using PKC fused with fluorescent proteins, including GFP and DsRed. Based on the live imaging of PKC translocation, we have demonstrated that PKC showed stimulus- and subtype-specific translocation, which can recognize its specific substrate and induce its specific cellular response (PKC targeting). These findings suggest that PKC targeting is the molecular basis underlying the diversity of PKC functions. Live imaging of PKC translocation has been proved to be a beneficial tool for understanding not only PKC functions, but also PKC-mediated signal transduction pathways. We have further analyzed PKC functions in the central nervous system using transgenic mice, which can express PKC-GFP in a brain-region-specific manner.

Protein kinase C isoform-specific differences in the spatial-temporal regulation and decoding of metabotropic glutamate receptor1a-stimulated second messenger responses

Metabotropic glutamate receptors (mGluRs) coupled via Gq to the hydrolysis of phosphoinositides stimulate Ca(2+) and PKCbetaII oscillations in both excitable and non-excitable cells. In the present study, we show that mGluR1a activation stimulates the repetitive plasma membrane translocation of each of the conventional and novel, but not atypical, PKC isozymes. However, despite similarities in sequence and cofactor regulation by diacyglycerol and Ca(2+), conventional PKCs exhibit isoform-specific oscillation patterns. PKCalpha and PKCbetaI display three distinct patterns of activity: (1) agonist-independent oscillations, (2) agonist-stimulated oscillations, and (3) persistent plasma membrane localization in response to mGluR1a activation. In contrast, only agonist-stimulated PKCbetaII translocation responses are observed in mGluR1a-expressing cells. PKCbetaI expression also promotes persistent increases in intracellular diacyglycerol concentrations in response to mGluR1a stimulation without affecting PKCbetaII oscillation patterns in the same cell. PKCbetaII isoform-specific translocation patterns are regulated by specific amino acid residues localized within the C-terminal PKC V5 domain. Specifically, Asn-625 and Lys-668 localized within the V5 domain of PKCbetaII cooperatively suppress PKCbetaI-like response patterns for PKCbetaII. Thus, redundancy in PKC isoform expression and differential decoding of second messenger response provides a novel mechanism for generating cell type-specific responses to the same signal.

Group I metabotropic glutamate receptor-mediated calcium signalling and immediate early gene expression in cultured rat striatal neurons

Group I metabotropic glutamate receptors (mGluRs) are positively coupled to phospholipase C (PLC) via Galphaq-proteins and are expressed in the medium-sized projection neurons of striatum. To characterize the group I mGluR/PLC-sensitive modulation of intracellular Ca2+ ([Ca2+]i) signalling, primary neuronal cultures were prepared from the striatum of E19 rat embryos or neonatal day-1 rat pups. Cytoplasmic Ca2+ signals were examined with fura-2/AM at a signal cell level. After 17-18 days in culture, a profound Ca2+ response consisting of two phases was induced in cultured striatal neurons following bath application of the selective group I agonist, 3,5-dihydroxyphenylglycine (DHPG). The [Ca2+]i elevation was concentration- and time-dependent, and was blocked by coexposure to the group I antagonist, N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC), or the PLC inhibitor, U-73122, but not to the group II/III antagonist (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE). A series of further pharmacological studies demonstrated that the initial spike-like transient was dependent on intracellular Ca2+ mobilization through 1,4,5-triphosphate-sensitive stores, and the second long-lasting rise was dependent on extracellular Ca2+ influx through N-methyl-d-aspartate (NMDA) receptors and especially L-type voltage-operated Ca2+ channels. Lastly, using an immediate early gene c-fos as a report of inducible gene expression, the resultant [Ca2+]i elevation contributes to DHPG-stimulated c-fos mRNA and Fos protein expression in striatal neurons as revealed by quantitative in situ hybridization and immunocytochemistry, respectively. These results demonstrate that group I mGluRs are able to affect Ca2+ homeostasis at multiple levels and trigger Ca2+-sensitive gene transcription in striatal neurons.

Distinct functional roles of the metabotropic glutamate receptors 1 and 5 in the rat globus pallidus

Group I metabotropic glutamate receptors (mGluRs) 1 and 5 frequently colocalize in the same neurons throughout the CNS. Because both receptors can couple to the same effector systems, the purpose of their cellular coexpression remains unclear. Here, we report that group I mGluR1 and mGluR5 have distinct functional roles in type II neurons of the rat globus pallidus (GP). Type II GP neurons form a large population of GABAergic projection neurons that are characterized by the presence of inwardly rectifying current I(h), low-threshold voltage-activated calcium current I(t), and activity at rest. Although immunocytochemical analysis reveals a high degree of neuronal colocalization of the two group I mGluRs in the GP, activation of mGluR1 only directly depolarizes type II GP neurons. Interestingly, blockade of mGluR5 by a highly selective antagonist, methylphenylethynylpyridine, leads to the potentiation of the mGluR1-mediated depolarization in this neuronal subpopulation. Metabotropic GluR1 desensitizes during repeated activation with the agonist in type II GP neurons, and blocking mGluR5 prevents the desensitization of the mGluR1-mediated depolarization. Elimination of the activity of protein kinase C (PKC) by an application of 1 microm bisendolylmaleimide or 1 microm chelerythrine, both protein kinase C inhibitors, potentiates the mGluR1-mediated response and prevents the desensitization of mGluR1 in type II GP neurons, suggesting that the effect of mGluR5 on mGluR1 signaling may involve PKC. Together, these data illustrate a novel mechanism by which mGluR1 and mGluR5, members of the same family of G-protein-coupled receptors, can interact to modulate neuronal activity in the rat GP.

Regulation of transcription factor phosphorylation by metabotropic glutamate receptor-associated signaling pathways in rat striatal neurons

The group I metabotropic glutamate receptors (mGluRs) are positively coupled to phospholipase C. Through phospholipase C, group I mGluR activation increases intracellular concentrations of diacylglycerol which is known as a strong activator of protein kinase C (PKC). This study investigated the putative role of PKC in the regulation of transcription factor phosphorylation induced by group I mGluR activation in the rat striatum in vivo. We found that the group I agonist 3,5-dihydroxyphenylglycine (DHPG) injected into the dorsal striatum (caudate-putamen) increased phosphorylation of the two transcription factors, cAMP response element-binding protein (CREB) and Elk-1, and extracellular signal-regulated kinase 1/2 (ERK1/2) in the injected striatum. Inhibition of PKC with GF109203X significantly attenuated DHPG-stimulated CREB, Elk-1, and ERK1/2 phosphorylation. Activation of PKC with intracaudate injection of 12-O-tetradecanoylphorbol-13-acetate (TPA) mimicked DHPG actions in facilitating the phosphorylation of CREB, Elk-1, and ERK1/2. Blockade of N-methyl-D-aspartate (NMDA) glutamate receptors with the non-competitive antagonist MK801 or the competitive antagonist AP5 attenuated TPA-induced CREB, Elk-1, and ERK1/2 phosphorylation. Similarly, inhibition of Ca(2+)/calmodulin-dependent protein kinases (CaMK) with KN62 also resulted in a significant attenuation of TPA induction of the three phosphoproteins. The data obtained from this study indicate that selective activation of PKC is needed for the group I agonist-induced CREB, Elk-1, and ERK1/2 phosphorylation in striatal neurons. Activated PKC may, at least in part, facilitate the phosphorylation of transcription factors via an NMDA/CaMK-sensitive pathway.

Group-I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to extracellular signal-regulated kinase (ERK) activation via distinct, but overlapping, signalling pathways

The coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the extracellular signal-regulated protein kinase (ERK) pathway has been studied in Chinese hamster ovary cell-lines where receptor expression is under inducible control. Both mGlu receptors stimulated comparable, robust and agonist concentration-dependent ERK activations in the CHO cell-lines. The mGlu1a receptor-mediated ERK response was almost completely attenuated by pertussis toxin (PTx) pretreatment, whereas the mGlu5a-ERK response, and the phosphoinositide response to activation of either receptor, was PTx-insensitive. mGlu1a and mGlu5a receptor coupling to ERK occurred via mechanisms independent of phosphoinositide 3-kinase activity and intracellular and/or extracellular Ca2+ concentration. While acute treatment with a protein kinase C (PKC) inhibitor did not attenuate agonist-stimulated ERK activation, down-regulation of PKCs by phorbol ester treatment for 24 h did attenuate both mGlu1a and mGlu5a receptor-mediated responses. Further, inhibition of Src non-receptor tyrosine kinase activity by PP1 attenuated the ERK response generated by both receptor subtypes, but only mGlu1a receptor-ERK activation was attenuated by PDGF receptor tyrosine kinase inhibitor AG1296. These findings demonstrate that, although expressed in a common cell background, these closely related mGlu receptors utilize different G proteins to cause ERK activation and may recruit different tyrosine kinases to facilitate this response.

Mechanisms of metabotropic glutamate receptor desensitization: role in the patterning of effector enzyme activation

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 beta-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 beta-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 Cbeta thereby stimulating oscillations in both inositol 1,4,5-triphosphate formation and Ca(2+) release from intracellular stores. These glutamate receptor-stimulated Ca(2+) 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.

Determinants of metabotropic glutamate receptor-5-mediated Ca2+ and inositol 1,4,5-trisphosphate oscillation frequency. Receptor density versus agonist concentration

Diverse patterns of Ca(2+)(i) release differentially regulate Ca(2+)-sensitive enzymes and gene transcription, and generally the extent of agonist activation of phospholipase C-linked G protein-coupled receptors determines the type of Ca(2+) signal. We have studied global Ca(2+) oscillations arising through activation of the metabotropic glutamate receptor mGluR5a expressed in Chinese hamster ovary cells and find that these oscillations are largely insensitive to agonist concentration. Using an inducible receptor expression system and a non-competitive antagonist, in conjunction with the translocation of eGFP-PH(PLCdelta) to monitor inositol 1,4,5-trisphosphate (InsP(3)) oscillations in single cells, we show that mGluR5a density determines the frequency of these oscillations. The predominant underlying mechanism resulted from a negative feedback loop whereby protein kinase C (PKC) inhibited InsP(3) generation. Down-regulation of PKC by prolonged exposure to phorbol ester revealed a second form of Ca(2+)(i) oscillation at low agonist concentrations. These Ca(2+)(i) signals showed features typical of classic repetitive Ca(2+)-induced Ca(2+) release and were sensitive to agonist concentration. Therefore, a single receptor can stimulate two types of InsP(3)-mediated Ca(2+) signal dependent upon feedback inhibition, producing two distinct means of controlling the final pattern of Ca(2+)(i) release. Our results have physiological implications for Ca(2+) signaling in general and emphasize the importance of mGluR5 surface expression for modulating synaptic plasticity.

Complex interactions between mGluR1 and mGluR5 shape neuronal network activity in the rat hippocampus

Group I metabotropic glutamate receptors (mGluRs) cause increased neuronal excitability that can lead to epileptogenesis and neurodegeneration. Here we have examined how individual members of this subgroup of mGluRs affect synchronised hippocampal synaptic activity under normal and disinhibited conditions similar to those that occur during certain epileptic states. We demonstrate that activation of both mGluR1 and mGluR5 are important in increasing neuronal synaptic excitability by increasing synchrony between cells and driving correlated network activity in circuits that contain, or are devoid of, GABA(A) receptor-mediated synaptic inputs. The precise patterning of activity that occurs is complex and depends upon: (1) the existing pattern of ongoing network activity prior to mGluR activation; and (2) the relative extent of activation of each mGluR subtype. However, mGluR5 appears to be the principal mGluR subtype that initiates bursting activity irrespective of the inhibitory synaptic tone within the neuronal network.