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

Extracellular molecular signals shaping dendrite architecture during brain development

Proper growth and branching of dendrites are crucial for adequate central nervous system (CNS) functioning. The neuronal dendritic geometry determines the mode and quality of information processing. Any defects in dendrite development will disrupt neuronal circuit formation, affecting brain function. Besides cell-intrinsic programmes, extrinsic factors regulate various aspects of dendritic development. Among these extrinsic factors are extracellular molecular signals which can shape the dendrite architecture during early development. This review will focus on extrinsic factors regulating dendritic growth during early neuronal development, including neurotransmitters, neurotrophins, extracellular matrix proteins, contact-mediated ligands, and secreted and diffusible cues. How these extracellular molecular signals contribute to dendritic growth has been investigated in developing nervous systems using different species, different areas within the CNS, and different neuronal types. The response of the dendritic tree to these extracellular molecular signals can result in growth-promoting or growth-limiting effects, and it depends on the receptor subtype, receptor quantity, receptor efficiency, the animal model used, the developmental time windows, and finally, the targeted signal cascade. This article reviews our current understanding of the role of various extracellular signals in the establishment of the architecture of the dendrites.

circ‑Grm1 promotes pulmonary artery smooth muscle cell proliferation and migration via suppression of GRM1 expression by FUS

Pulmonary arterial hypertension is a progressive and fatal disease. Recent studies suggest that circular RNA (circRNAs/circs) can regulate various biological processes, including cell proliferation. Therefore, it is possible that circRNA may have important roles in pulmonary artery smooth muscle cell proliferation in hypoxic pulmonary hypertension (HPH). The aim of the present study was to determine the role and mechanism of circRNA‑glutamate metabotropic receptor 1 (circ‑Grm1; mmu_circ_0001907) in pulmonary artery smooth muscle cell (PASMC) proliferation and migration in HPH. High‑throughput transcriptome sequencing was used to screen circRNAs and targeted genes involved in HPH. Cell Counting Kit‑8 (CCK‑8), 5‑ethynyl‑2‑deoxyuridine and wound healing assays were employed to assess cell viability and migration. Reverse transcription‑quantitative PCR and western blotting were used to detect target gene expression in different groups. Bioinformatical approaches were used to predict the interaction probabilities of circ‑Grm1 and Grm1 with FUS RNA binding protein (FUS). The interactions of circ‑Grm1, Grm1 and FUS were evaluated using RNA silencing and RNA immunoprecipitation assays. The results demonstrated that circ‑Grm1 was upregulated in hypoxic PASMCs. Further experiments revealed that the knockdown of circ‑Grm1 could suppress the proliferation and migration of hypoxic PASMCs. Transcriptome sequencing revealed that Grm1 could be the target gene of circ‑Grm1. It was found that circ‑Grm1 could competitively bind to FUS and consequently downregulate Grm1. Moreover, Grm1 could inhibit the function of circ‑Grm1 by promoting the proliferative and migratory abilities of hypoxic PASMCs. The results also demonstrated that circ‑Grm1 influenced the biological functions of PASMCs via the Rap1/ERK pathway by regulating Grm1. Overall, the current results suggested that circ‑Grm1 was associated with HPH and promoted the proliferation and migration of PASMCs via suppression of Grm1 expression through FUS.

Dissociable roles of the nucleus accumbens core and shell subregions in the expression and extinction of conditioned fear

The nucleus accumbens (NAc), consisting of core (NAcC) and shell (NAcS) sub-regions, has primarily been studied as a locus mediating the effects of drug reward and addiction. However, there is ample evidence that this region is also involved in regulating aversive responses, but the exact role of the NAc and its subregions in regulating associative fear processing remains unclear. Here, we investigated the specific contribution of the NAcC and NAcS in regulating both fear expression and fear extinction in C57BL/6J mice. Using Arc expression as an indicator of neuronal activity, we first show that the NAcC is specifically active only in response to an associative fear cue during an expression test. In contrast, the NAcS is specifically active during fear extinction. We next inactivated each subregion using lidocaine and demonstrated that the NAcC is necessary for fear expression, but not for extinction learning or consolidation of extinction. In contrast, we demonstrate that the NAcS is necessary for the consolidation of extinction, but not fear expression or extinction learning. Further, inactivation of mGluR1 or ERK signaling specifically in the NAcS disrupted the consolidation of extinction but had no effect on fear expression or extinction learning itself. Our data provide the first evidence for the importance of the ERK/MAPK pathway as the underlying neural mechanism facilitating extinction consolidation within the NAcS. These findings suggest that the NAc subregions play dissociable roles in regulating fear recall and the consolidation of fear extinction, and potentially implicate them as critical regions within the canonical fear circuit.

Mutations in Metabotropic Glutamate Receptor 1 Contribute to Natural Short Sleep Trait

Sufficient and efficient sleep is crucial for our health. Natural short sleepers can sleep significantly shorter than the average population without a desire for more sleep and without any obvious negative health consequences. In searching for genetic variants underlying the short sleep trait, we found two different mutations in the same gene (metabotropic glutamate receptor 1) from two independent natural short sleep families. In vitro, both of the mutations exhibited loss of function in receptor-mediated signaling. In vivo, the mice carrying the individual mutations both demonstrated short sleep behavior. In brain slices, both of the mutations changed the electrical properties and increased excitatory synaptic transmission. These results highlight the important role of metabotropic glutamate receptor 1 in modulating sleep duration.

MGluR5 activity is required for the induction of ethanol behavioral sensitization and associated changes in ERK MAP kinase phosphorylation in the nucleus accumbens shell and lateral habenula

Metabotropic glutamate receptor subtype-5 (mGluR5) activity regulates a variety of behavioral pathologies associated with alcohol addiction. The main goal of this study was to determine if mGluR5 regulates the induction of ethanol-induced locomotor sensitization, which is a model of experience-dependent plasticity following initial exposure to drugs of abuse. The extracellular signal-regulated kinase (ERK_1/2) pathway is downstream of mGluR5 and implicated in alcohol addiction; however, its role in sensitization remains unexplored. We sought to determine if mGluR5-mediated changes in ethanol-induced sensitization are associated with changes in ERK_1/2 phosphorylation (pERK_1/2) in specific brain regions. Adult male DBA/2 J mice were tested for acute locomotor response to ethanol (0 or 2 g/kg, IP) followed by a 9-day induction period in which the mGluR5 antagonist MPEP (0 or 30 mg/kg, IP) was administered prior to ethanol (0 or 2.5 g/kg, IP). One day later, ethanol (2 g/kg) produced a robust within- and between-group increase in locomotor activity, indicating sensitization in mice that received MPEP (0 mg/kg) during induction. MPEP (30 mg/kg) treatment during induction resulted in locomotor response to ethanol (2 g/kg) challenge that was equivalent to an acute response, indicating full blockade of sensitization. Sensitization was associated with increased pERK_1/2 immunoreactivity (IR) in nucleus accumbens shell (AcbSh) and a reduction in lateral habenula (LHb), both of which were blocked by MPEP treatment during induction. Sensitization was also associated with mGluR5-independent increases in pERK_1/2 IR in the nucleus accumbens core and decreases in the dentate gyrus and lateral septum. These data indicate that mGluR5 activity is required for the induction of ethanol locomotor sensitization and associated changes in ERK_1/2 phosphorylation in the AcbSh and LHb, which raises the hypothesis that mGluR5-mediated cell signaling in these brain regions may mediate the induction of sensitization. Elucidating mechanisms of sensitization may increase understanding of how ethanol hijacks behavioral functions during the development of addiction.

Kinetic and system bias as drivers of metabotropic glutamate receptor 5 allosteric modulator pharmacology

Allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGlu₅) have been proposed as potential therapies for various CNS disorders. These ligands bind to sites distinct from the orthosteric (or endogenous) ligand, often with improved subtype selectivity and spatio-temporal control over receptor responses. We recently revealed that mGlu₅ allosteric agonists and positive allosteric modulators exhibit biased agonism and/or modulation. To establish whether negative allosteric modulators (NAMs) engender similar bias, we rigorously characterized the pharmacology of eight diverse mGlu₅ NAMs. Radioligand inhibition binding studies revealed novel modes of interaction with mGlu₅ for select NAMs, with biphasic or incomplete inhibition of the radiolabeled NAM, [³H]methoxy-PEPy. We assessed mGlu₅-mediated intracellular Ca²⁺ (iCa²⁺) mobilization and inositol phosphate (IP₁) accumulation in HEK293A cells stably expressing low levels of mGlu₅ (HEK293A-rat mGlu₅-low) and mouse embryonic cortical neurons. The apparent affinity of acetylenic NAMs, MPEP, MTEP and dipraglurant, was dependent on the signaling pathway measured, agonist used, and cell type (HEK293A-rat mGlu₅-low versus mouse cortical neurons). In contrast, the acetylenic partial NAM, M-5MPEP, and structurally distinct NAMs (VU0366248, VU0366058, fenobam), had similar affinity estimates irrespective of the assay or cellular background. Biased modulation was evident for VU0366248 in mouse cortical neurons where it was a NAM for DHPG-mediated iCa²⁺ mobilization, but neutral with DHPG in IP₁ accumulation assays. Overall, this study highlights the inherent complexity in mGlu₅ NAM pharmacology that we hypothesize may influence interpretation when translating into preclinical models and beyond in the design and development of novel therapeutics for neuropsychiatric and neurological disorders.

Permethrin, a pyrethroid insecticide, regulates ERK1/2 activation through membrane depolarization-mediated pathway in HepG2 hepatocytes

Permethrin is a pyrethroid insecticide that acts thru membrane depolarization and is known to disrupt calcium levels in neurons. Disrupted calcium homeostasis is linked to oxidative stress as well as many other cellular mis-functions and permethrin has been reported to disrupt lipid and glucose metabolism in animals and mammalian cell models. It is not known, however, if permethrin influences calcium levels and its associated cellular mechanisms in liver cells. Thus, the goal of the current study was to investigate the mechanisms of permethrin on calcium-mediated cellular signaling pathway, particularly on activation of extracellular signal-related kinase (ERK1/2 or p42/p44) using human hepatocytes, HepG2. The current results showed that permethrin treatment induced oxidative stress and phosphorylation of ERK1/2, which were dependent upon voltage-sensitive sodium channels (VSSC). It was further determined that permethrin-induced ERK1/2 activation was mediated by the metabotropic glutamate receptors (mGluRs)-phosphoinositide phospholipase C (PLC)-protein kinase C (PKC) pathway, but not by changes of intracellular calcium or ER stress-mediated mechanisms.

Interactions between estrogen receptors and metabotropic glutamate receptors and their impact on drug addiction in females

Contribution to Special Issue on Fast effects of steroids. Estrogen receptors α and β (ERα and ERβ) have a unique relationship with metabotropic glutamate receptors (mGluRs) in the female rodent brain such that estradiol is able to recruit intracellular G-protein signaling cascades to influence neuronal physiology, structure, and ultimately behavior. While this association between ERs and mGluRs exists in many cell types and brain regions, its effects are perhaps most striking in the nucleus accumbens (NAc). This review will discuss the original characterization of ER/mGluR signaling and how estradiol activity in the NAc confers increased sensitivity to drugs of abuse in females through this mechanism.

Exosomes released by metabotropic glutamate receptor 1 (GRM1) expressing melanoma cells increase cell migration and invasiveness

Exosomes are naturally occurring membrane-bound nanovesicles generated constitutively and released by various cell types, and often in higher quantities by tumor cells. Exosomes may facilitate communication between the primary tumor and its local microenvironment, supporting cell invasion and other early events in metastasis. A neuronal receptor, metabotropic glutamate receptor 1 (GRM1), when ectopically expressed in melanocytes, induces in vitro melanocytic transformation and spontaneous malignant melanoma development in vivo in a transgenic mouse model. Our earlier studies showed that genetic modulation in GRM1 expression by siRNA or disruption of GRM1-mediated glutamate signaling interfere with downstream effectors resulting in a decrease in both cell proliferation in vitro and tumor progression in vivo. In this study, we sought to determine whether exosome formation might play a role in GRM1 mediated melanoma development and progression. To test this, we utilized in vitro cultured cells in which GRM1 expression and function could be modulated by pharmacological and genetic means and determined effects on exosome production. We also tested the effects of exosomes from GRM1 expressing melanoma cells on growth, migration and invasion of GRM1 negative cells. Our results show that although GRM1 expression has no influence on exosome quantity, exosomes produced by GRM1-positive cells modulate the ability of the recipient cell to migrate, invade and exhibit anchorage-independent cell growth.

Homer1a drives homeostatic scaling-down of excitatory synapses during sleep

Sleep is an essential process that supports learning and memory by acting on synapses through poorly understood molecular mechanisms. Using biochemistry, proteomics, and imaging in mice, we find that during sleep, synapses undergo widespread alterations in composition and signaling, including weakening of synapses through removal and dephosphorylation of synaptic AMPA-type glutamate receptors. These changes are driven by the immediate early gene Homer1a and signaling from group I metabotropic glutamate receptors mGluR1/5. Homer1a serves as a molecular integrator of arousal and sleep need via the wake- and sleep-promoting neuromodulators, noradrenaline and adenosine, respectively. Our data suggest that homeostatic scaling-down, a global form of synaptic plasticity, is active during sleep to remodel synapses and participates in the consolidation of contextual memory.

Opposite Effects of mGluR1a and mGluR5 Activation on Nucleus Accumbens Medium Spiny Neuron Dendritic Spine Density

The group I metabotropic glutamate receptors (mGluR1a and mGluR5) are important modulators of neuronal structure and function. Although these receptors share common signaling pathways, they are capable of having distinct effects on cellular plasticity. We investigated the individual effects of mGluR1a or mGluR5 activation on dendritic spine density in medium spiny neurons in the nucleus accumbens (NAc), which has become relevant with the potential use of group I mGluR based therapeutics in the treatment of drug addiction. We found that systemic administration of mGluR subtype-specific positive allosteric modulators had opposite effects on dendritic spine densities. Specifically, mGluR5 positive modulation decreased dendritic spine densities in the NAc shell and core, but was without effect in the dorsal striatum, whereas increased spine densities in the NAc were observed with mGluR1a positive modulation. Additionally, direct activation of mGluR5 via CHPG administration into the NAc also decreased the density of dendritic spines. These data provide insight on the ability of group I mGluRs to induce structural plasticity in the NAc and demonstrate that the group I mGluRs are capable of producing not just distinct, but opposing, effects on dendritic spine density.

Protein kinase Cɛ activity regulates mGluR5 surface expression in the rat nucleus accumbens

Type 5 metabotropic glutamate receptors (mGluR5) activate protein kinase C (PKC) via coupling to G_αq/11 protein signaling. We have previously demonstrated that the epsilon isoform of PKC (PKCɛ) is a critical downstream target of mGluR5 in regulating behavioral and biochemical responses to alcohol. Recent evidence suggests that PKC-mediated phosphorylation of mGluR5 can lead to receptor desensitization and internalization. We therefore sought to examine the specific involvement of PKCɛ in the regulation of mGluR5 surface expression in the nucleus accumbens (NAc), a key regulator of alcohol-associated behaviors. Coronal brain sections from male Wistar rats were analyzed for either colocalization of mGluR5 and PKCɛ via immunohistochemistry or changes in mGluR5 surface expression and PKCɛ phosphorylation following local application of PKCɛ translocation activator or inhibitor peptides and/or an orthosteric mGluR5 agonist. We observed colocalization of mGluR5 and PKCɛ in the NAc. We also showed that intra-NAc infusion of the PKCɛ translocation inhibitor ɛV1-2 increased mGluR5 surface expression under baseline conditions. Stimulation of mGluR5 with an orthosteric agonist DHPG, dose dependently increased ERK1/2 and PKCɛ phosphorylation as well as mGluR5 internalization in acute NAc slices. Finally, we observed that activation of PKCɛ translocation with Tat-ΨɛRACK peptide mediates agonist-independent mGluR5 internalization, whereas PKCɛ translocation inhibitor ɛV1-2 prevents agonist-dependent internalization of mGluR5 in NAc slice preparations. These findings suggest that the subcellular localization of mGluR5 in the NAc is regulated by PKCɛ under basal and stimulation conditions, which may influence the role of mGluR5-PKCɛ signaling in alcohol-related behaviors. © 2016 Wiley Periodicals, Inc.

Hippocampal metabotropic glutamate receptor long-term depression in health and disease: focus on mitogen-activated protein kinase pathways

Group I metabotropic glutamate receptor (mGluR) dependent long-term depression (LTD) is a major form of synaptic plasticity underlying learning and memory. The molecular mechanisms involved in mGluR-LTD have been investigated intensively for the last two decades. In this 60th anniversary special issue article, we review the recent advances in determining the mechanisms that regulate the induction, transduction and expression of mGluR-LTD in the hippocampus, with a focus on the mitogen-activated protein kinase (MAPK) pathways. In particular we discuss the requirement of p38 MAPK and extracellular signal-regulated kinase 1/2 (ERK 1/2) activation. The recent advances in understanding the signaling cascades regulating mGluR-LTD are then related to the cognitive impairments observed in neurological disorders, such as fragile X syndrome and Alzheimer's disease. mGluR-LTD is a form of synaptic plasticity that impacts on memory formation. In the hippocampus mitogen-activated protein kinases (MAPKs) have been found to be important in mGluR-LTD. In this 60th anniversary special issue article, we review the independent and complementary roles of two classes of MAPK, p38 and ERK1/2 and link this to the aberrant mGluR-LTD that has an important role in diseases. This article is part of the 60th Anniversary special issue.

Metabotropic glutamate receptors in cancer

Metabotropic glutamate receptors (mGluRs) are widely known for their roles in synaptic signaling. However, accumulating evidence suggests roles of mGluRs in human malignancies in addition to synaptic transmission. Somatic cell homeostasis presents intriguing possibilities of mGluRs and glutamate signaling as novel targets for human cancers. More recently, aberrant glutamate signaling has been shown to participate in the transformation and maintenance of various cancer types, including glioma, melanoma skin cancer, breast cancer, and prostate cancer, indicating that genes encoding mGluRs, GRMs, can function as oncogenes. Here, we provide a review on the interactions of mGluRs and their ligand, glutamate, in processes that promote the growth of tumors of neuronal and non-neuronal origins. Further, we discuss the evolution of riluzole, a glutamate release inhibitor approved for amyotrophic lateral sclerosis (ALS), but now fashioned as an mGluR1 inhibitor for melanoma therapy and as a radio-sensitizer for tumors that have metastasized to the brain. With the success of riluzole, it is not far-fetched to believe that other drugs that may act directly or indirectly on other mGluRs can be beneficial for multiple applications. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Metabotropic glutamate receptor subtype 5: molecular pharmacology, allosteric modulation and stimulus bias

The metabotropic glutamate receptor subtype 5 (mGlu5 ) is a family C GPCR that has been implicated in various neuronal processes and, consequently, in several CNS disorders. Over the past few decades, GPCR-based drug discovery, including that for mGlu5 receptors, has turned considerable attention to targeting allosteric binding sites. Modulation of endogenous agonists by allosteric ligands offers the advantages of spatial and temporal fine-tuning of receptor activity, increased selectivity and reduced adverse effects with the potential to elicit improved clinical outcomes. Further, with greater appreciation of the multifaceted nature of the transduction of mGlu5 receptor signalling, it is increasingly apparent that drug discovery must take into consideration unique receptor conformations and the potential for stimulus-bias. This novel paradigm proposes that different ligands may differentially modulate distinct signalling pathways arising from the same receptor. We review our current understanding of the complexities of mGlu5 receptor signalling and regulation, and how these relate to allosteric ligands. Ultimately, a deeper appreciation of these relationships will provide the foundation for targeted drug design of compounds with increased selectivity, not only for the desired receptor but also for the desired signalling outcome from the receptor. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

The current management of brain metastasis in melanoma: a focus on riluzole

Brain metastasis is a common endpoint in human malignant melanoma, and the prognosis for patients remains poor despite advancements in therapy. Current treatment for melanoma metastatic to the brain is grouped into those providing symptomatic relief such as corticosteroids and antiepileptic agents, to those that are disease modifying. Related to the latter group, recent studies have demonstrated that aberrant glutamate signaling plays a role in the transformation and maintenance of various cancer types, including melanoma. Glutamate secretion from these and surrounding cells have been found to stimulate regulatory pathways that control tumor growth, proliferation and survival in vitro and in vivo. The antiglutamatergic actions of an inhibitor of glutamate release, riluzole, have been detected by its ability to clear glutamate from the synapse, and it has been shown to inhibit glutamate release rather than directly inhibiting glutamate receptors. Preclinical studies have demonstrated the ability of riluzole to act as a radiosensitizing agent in melanoma. The effect of riluzole on downstream glutamatergic signaling has pointed to cross talk between the metabotropic G-protein-coupled glutamate receptors implicated in a subset of human melanomas with other signaling pathways, including apoptotic, angiogenic, ROS and cell invasion mechanisms, thus establishing its potential to be further explored in combination therapy regimens for both primary human melanoma and melanoma metastatic to the brain.

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.

Mechanisms of dopamine D1 receptor-mediated ERK1/2 activation in the parkinsonian striatum and their modulation by metabotropic glutamate receptor type 5

In animal models of Parkinson's disease, striatal overactivation of ERK1/2 via dopamine (DA) D1 receptors is the hallmark of a supersensitive molecular response associated with dyskinetic behaviors. Here we investigate the pathways involved in D1 receptor-dependent ERK1/2 activation using acute striatal slices from rodents with unilateral 6-hydroxydopamine (6-OHDA) lesions. Application of the dopamine D1-like receptor agonist SKF38393 induced ERK1/2 phosphorylation and downstream signaling in the DA-denervated but not the intact striatum. This response was mediated through a canonical D1R/PKA/MEK1/2 pathway and independent of ionotropic glutamate receptors but blocked by antagonists of L-type calcium channels. Coapplication of an antagonist of metabotropic glutamate receptor type 5 (mGluR5) or its downstream signaling molecules (PLC, PKC, IP3 receptors) markedly attenuated SKF38393-induced ERK1/2 activation. The role of striatal mGluR5 in D1-dependent ERK1/2 activation was confirmed in vivo in 6-OHDA-lesioned animals treated systemically with SKF38393. In one experiment, local infusion of the mGluR5 antagonist MTEP in the DA-denervated rat striatum attenuated the activation of ERK1/2 signaling by SKF38393. In another experiment, 6-OHDA lesions were applied to transgenic mice with a cell-specific knockdown of mGluR5 in D1 receptor-expressing neurons. These mice showed a blunted striatal ERK1/2 activation in response to SFK38393 treatment. Our results reveal that D1-dependent ERK1/2 activation in the DA-denervated striatum depends on a complex interaction between PKA- and Ca(2+)-dependent signaling pathways that is critically modulated by striatal mGluR5.

Disruption of GRM1-mediated signalling using riluzole results in DNA damage in melanoma cells

Gain of function of the neuronal receptor, metabotropic glutamate receptor 1 (Grm1), was sufficient to induce melanocytic transformation in vitro and spontaneous melanoma development in vivo when ectopically expressed in melanocytes. The human form of this receptor, GRM1, has been shown to be ectopically expressed in a subset of human melanomas but not benign nevi or normal melanocytes, suggesting that misregulation of GRM1 is involved in the pathogenesis of certain human melanomas. Sustained stimulation of Grm1 by the ligand, glutamate, is required for the maintenance of transformed phenotypes in vitro and tumorigenicity in vivo. In this study, we investigate the mechanism of an inhibitor of glutamate release, riluzole, on human melanoma cells that express metabotropic glutamate receptor 1 (GRM1). Various in vitro assays conducted show that inhibition of glutamate release in several human melanoma cell lines resulted in an increase of oxidative stress and DNA damage response markers.

High expression of metabotropic glutamate receptor 4: correlation with clinicopathologic characteristics and prognosis of osteosarcoma

PURPOSE

Analyze protein and gene expression of mGluRs (mGluR1, mGluR5, mGluR4) in osteosarcoma tissues and discuss the relation between expression level and clinical characteristics of osteosarcoma, and study the clinical significance.

METHODS

Detect protein and mRNA expression level of mGluRs (mGluR1, mGluR5, mGluR4) in 40 osteosarcoma tissues and the corresponding adjacent normal tissues by Western blot and RT-PCR accordingly. Immunohistochemistry was adopted to detect the expression of mGluRs (mGluR1, mGluR5, mGluR4) in 118 paraffin embedded osteosarcoma tissues and eight normal bone tissues. Then, the correlation between the expression and clinical characteristics of patients was analyzed. Furthermore, survival analysis of osteosarcoma was performed to study the relation between expression level of mGluRs and patient prognosis.

RESULTS

No correlation of mGluR1 and mGluR5 with clinicopathologic characteristics of osteosarcoma was found. Statistical analysis demonstrated that the expression level of mGluR4 shared no significant correlation with gender, age, histologic type and tumor location of patient, but was related to Enneking stage and tumor metastasis (P < 0.05). High mGluR4 expression is more frequently noted in the osteosarcoma tissues with higher Enneking stage and metastasis. The results of Western blot and RT-PCR indicated a significantly increased expression level of mGluR4 gene and protein in osteosarcoma tissues compared with normal tissues. Though higher gene and protein expression of mGluR5 and mGluR1 were also indicated in osteosarcoma tissues compared with normal tissues, no statistical significance was noted for the difference (P > 0.05). According to the survival analysis of 118 osteosarcoma patients, cases in the mGluR4 high-expression group showed inferior disease-free survival rate and poorer overall survival rate.

CONCLUSION

High expression of mGluR4 in osteosarcoma tissues is related to poor prognosis, thus holding certain reference value for estimating prognosis of osteosarcoma patients.

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

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

Dual effect of 17β-estradiol on NMDA-induced neuronal death: involvement of metabotropic glutamate receptor 1

Pretreatment with 10 nm 17β-estradiol (17βE2) or 100 μm of the metabotropic glutamate 1 receptor (mGlu1R) agonist, dihydroxyphenylglycine (DHPG), protected neurons against N-methyl-d-aspartate (NMDA) toxicity. This effect was sensitive to blockade of both estrogen receptors and mGlu1R by their respective antagonists. In contrast, 17βE2 and/or DHPG, added after a low-concentration NMDA pulse (45 μm), produced an opposite effect, i.e. an exacerbation of NMDA toxicity. Again this effect was prevented by both receptor antagonists. In support of an interaction of estrogen receptors and mGlu1R in mediating a neurotoxic response, exacerbation of NMDA toxicity by 17βE2 disappeared when cultures were treated with DHPG prior to NMDA challenge, and conversely, potentiation of NMDA-induced cell death by DHPG was prevented by pretreatment with 17βE2. Addition of calpain III inhibitor (10 μm), 2 h before NMDA, prevented the increased damage induced by the two agonists, an affect that can be secondary to cleavage of mGlu1R by calpain. Accordingly, NMDA stimulation reduced expression of the full-length (140 kDa) mGluR1, an effect partially reversed by calpain inhibitor. Finally, in the presence of NMDA, the ability of 17βE2 to stimulate phosphorylation of AKT and ERK was impaired. Pretreatment with calpain inhibitor prevented the reduction of phosphorylated ERK but had no significant effect on phosphorylated AKT. Accordingly, the inhibition of ERK signaling by U0126 (1 μm) counteracted the effect of calpain inhibition on 17βE2-induced exacerbation of NMDA toxicity. The present data confirm the dual role of estrogens in neurotoxicity/neuroprotection and highlight the role of the timing of exposure to estrogens.

Characterisation of AmphiAmR4, an amphioxus (Branchiostoma floridae) α₂-adrenergic-like G-protein-coupled receptor

Little is known about the evolutionary relationship between vertebrate adrenergic receptors and invertebrate octopamine and tyramine receptors. The complexity of the adrenergic signalling system is believed to be an innovation of the vertebrate lineage but the presence of noradrenaline has been reported in some invertebrate species. The cephalochordate, amphioxus (Branchiostoma floridae), is an ideal model organism for studying the evolution of vertebrate GPCRs, given its unique position at the base of the chordate lineage. Here, we describe the pharmacological characterisation and second messenger coupling abilities of AmphiAmR4, which clusters with α₂-adrenergic receptors in a phylogenetic tree but also shares a high sequence similarity to invertebrate octopamine/tyramine receptors in both BLAST and Hidden Markov Model analyses. Thus, it was of particular interest to determine if AmphiAmR4 displayed similar functional properties to the vertebrate α₂-adrenergic receptors or to invertebrate octopamine or tyramine receptors. When stably expressed in Chinese hamster ovary (CHO) cells, noradrenaline couples the receptor to both the activation of adenylyl cyclase and to the activation of the MAPKinase pathway. Pharmacological studies with a wide range of agonists and antagonists suggest that AmphiAmR4 functions as an α₂-adrenergic-like receptor when expressed in CHO cells.

Metabotropic glutamate receptor 1 (Grm1) is an oncogene in epithelial cells

Non-neuronal expression of components of the glutamatergic system has been increasingly observed, and our laboratory previously had demonstrated the etiological role of ectopically expressed metabotropic glutamate receptor 1 (Grm1/mGluR1) in mouse models of melanoma. We hypothesize that inappropriate glutamatergic signaling in other cell types can dysregulate growth leading to transformation and tumorigenesis. As most cancers are carcinomas, we selected an immortalized primary baby mouse kidney (iBMK) cell model to assess whether Grm1 can transform epithelial cells. These iBMK cells, engineered to be immortal yet non-tumorigenic and retaining normal epithelial characteristics, were used as recipients for exogenous Grm1 cDNA. Several stable Grm1 expressing clones were isolated and the Grm1-receptors were shown to be functional, as evidenced by the accumulation of second messengers in response to Grm1 agonist. Additionally activated by agonist were MAPK and AKT signaling cascades, major intracellular pathways shown by many investigators to be critical in melanomagenesis and other neoplasms. These Grm1-iBMK cells exhibited enhanced cell proliferation in in vitro MTT assays and significant tumorigenicity in in vivo allografts. Persistent Grm1 expression was required for the maintenance of the in vivo tumorigenic phenotype as demonstrated by an inducible Grm1-silencing RNA. These are the first results that indicate Grm1 can be an oncogene in epithelial cells. Additionally, relevance to human disease in the corresponding tumor type of renal cell carcinoma (RCC) may be suggested by observed expression of GRM1/mGluR1 in a number of RCC tumor biopsy samples and cell lines, and the effects of GRM1 modulation on tumorigenicity therein. Moreover RCC cell lines exhibited elevated levels of extracellular glutamate, and some lines responded to drugs which modulate the glutamatergic system. These findings imply a possible role for glutamate signaling apparatus in RCC cell growth, and that the glutamatergic system may be a therapeutic target in renal cell carcinoma.

Investigating metabotropic glutamate receptor 5 allosteric modulator cooperativity, affinity, and agonism: enriching structure-function studies and structure-activity relationships

Drug discovery programs increasingly are focusing on allosteric modulators as a means to modify the activity of G protein-coupled receptor (GPCR) targets. Allosteric binding sites are topographically distinct from the endogenous ligand (orthosteric) binding site, which allows for co-occupation of a single receptor with the endogenous ligand and an allosteric modulator that can alter receptor pharmacological characteristics. Negative allosteric modulators (NAMs) inhibit and positive allosteric modulators (PAMs) enhance the affinity and/or efficacy of orthosteric agonists. Established approaches for estimation of affinity and efficacy values for orthosteric ligands are not appropriate for allosteric modulators, and this presents challenges for fully understanding the actions of novel modulators of GPCRs. Metabotropic glutamate receptor 5 (mGlu(5)) is a family C GPCR for which a large array of allosteric modulators have been identified. We took advantage of the many tools for probing allosteric sites on mGlu(5) to validate an operational model of allosterism that allows quantitative estimation of modulator affinity and cooperativity values. Affinity estimates derived from functional assays fit well with affinities measured in radioligand binding experiments for both PAMs and NAMs with diverse chemical scaffolds and varying degrees of cooperativity. We observed modulation bias for PAMs when we compared mGlu(5)-mediated Ca(2+) mobilization and extracellular signal-regulated kinase 1/2 phosphorylation data. Furthermore, we used this model to quantify the effects of mutations that reduce binding or potentiation by PAMs. This model can be applied to PAM and NAM potency curves in combination with maximal fold-shift data to derive reliable estimates of modulator affinities.

Functional effects of GRM1 suppression in human melanoma cells

Ectopic expression of a neuronal receptor, metabotropic glutamate receptor 1 (Grm1), in melanocytes has been implicated in melanoma development in mouse models. The human relevance of this receptor's involvement in melanoma pathogenesis was shown by detecting GRM1 expression in subsets of human melanomas, an observation lacking in benign nevi or normal melanocytes. Grm1-transformed mouse melanocytes and a conditional Grm1 transgenic mouse model confirmed a requirement for sustained expression of Grm1 for the maintenance of transformed phenotypes in vitro and tumorigenicity in vivo. Here, we investigate if continued GRM1 expression is also required in human melanoma cell lines by using two inducible, silencing RNA systems: the ecdysone/Ponasterone A and tetracycline on/off approaches to regulate GRM1 expression in the presence of each inducer. Various in vitro assays were conducted to assess the consequences of a reduction in GRM1 expression on cell proliferation, apoptosis, downstream targeted signaling pathways, and in vivo tumorigenesis. We showed that suppression of GRM1 expression in several human melanoma cell lines resulted in a reduction in the number of viable cells and a decrease in stimulated mitogen-activated protein kinase (MAPK) and PI3K/AKT and suppressed tumor progression in vivo. These results reinforce earlier observations where a reduction in cell growth in vitro and tumorigenesis in vivo were correlated with decreased GRM1 activities by pharmacologic inhibitors of the receptor, supporting the notion that GRM1 plays a role in the maintenance of transformed phenotypes in human melanoma cells in vitro and in vivo and could be a potential therapeutic target for the treatment of melanoma.

Regulation of the ERK pathway in the dentate gyrus by in vivo dopamine D1 receptor stimulation requires glutamatergic transmission

Acute systemic administration of the dopamine D1/D5 receptors (D1Rs) agonist, SKF81297, activates the extracellular signal-regulated protein kinases (ERK) pathway selectively in the granule cells of the dentate gyrus. In this study, we examined the mechanisms involved in this regulation and investigated the molecular components that could promote ERK-dependent transcription and translation. SKF81297 induced phosphorylation of ERK and histone H3 required intact glutamatergic transmission. Blockade of glutamate release achieved by the mGluR2/3 agonist, LY354740 or the selective adenosine A1R agonist, CCPA as well as neurotoxic lesions of lateral entorhinal cortex reduced the ability of SKF81297 to induce ERK activation in the dentate gyrus. This activation required the combined stimulation of NR2B-containing NMDARs, mGluR1 and mGluR5. SKF81297 evoked phosphorylation of the ribosomal protein S6 (rpS6) selectively at the Ser235/236 site while the Ser240/244 site remains unchanged. The SKF81297 induced increased phosphorylation of rpS6 was dependent on PKC and ERK/p90RSK activation. Surprisingly, administration of D1Rs agonist suppressed mTORC1/p70S6K pathway suggesting an mTOR-independent regulation of rpS6 phosphorylation. Taken together, our results show that intact glutamatergic transmission plays a major role in the regulation of ERK-dependent phosphorylation of histone H3 and rpS6 observed in the mouse dentate gyrus after systemic administration of SKF81297.

Ligand bias at metabotropic glutamate 1a receptors: molecular determinants that distinguish β-arrestin-mediated from G protein-mediated signaling

The metabotropic glutamate 1a (mGlu1a) receptor is a G protein-coupled receptor linked with phosphoinositide (PI) hydrolysis and with β-arrestin-1-mediated sustained extracellular signal-regulated kinase (ERK) phosphorylation and cytoprotective signaling. Previously, we reported the existence of ligand bias at this receptor, inasmuch as glutamate induced both effects, whereas quisqualate induced only PI hydrolysis. In the current study, we showed that mGlu1 receptor agonists such as glutamate, aspartate, and l-cysteate were unbiased and activated both signaling pathways, whereas quisqualate and (S)-3,5-dihydroxyphenylglycine stimulated only PI hydrolysis. Competitive antagonists inhibited only PI hydrolysis and not the β-arrestin-dependent pathway, whereas a noncompetitive mGlu1 receptor antagonist blocked both pathways. Mutational analysis of the ligand binding domain of the mGlu1a receptor revealed that Thr188 residues were essential for PI hydrolysis but not for protective signaling, whereas Arg323 and Lys409 residues were required for β-arrestin-1-mediated sustained ERK phosphorylation and cytoprotective signaling but not for PI hydrolysis. Therefore, the mechanism of ligand bias appears to involve different modes of agonist interactions with the receptor ligand binding domain. Although some mGlu1a receptor agonists are biased toward PI hydrolysis, we identified two endogenous compounds, glutaric acid and succinic acid, as new mGlu1 receptor agonists that are fully biased toward β-arrestin-mediated protective signaling. Pharmacological studies indicated that, in producing the two effects, glutamate interacted in two distinct ways with mGlu1 receptors, inasmuch as competitive mGlu1 receptor antagonists that blocked PI hydrolysis did not inhibit cytoprotective signaling. Quisqualate, which is biased toward PI hydrolysis, failed to inhibit glutamate-induced protection, and glutaric acid, which is biased toward protection, did not interfere with glutamate-induced PI hydrolysis. Taken together, these data indicate that ligand bias at mGlu1 receptors is attributable to different modes of receptor-glutamate interactions, which are differentially coupled to PI hydrolysis and β-arrestin-mediated cytoprotective signaling, and they reveal the existence of new endogenous agonists acting at mGlu1 receptors.

Polymorphisms within the metabotropic glutamate receptor 1 gene are associated with depression phenotypes

OBJECTIVES

Glutamate has been implicated in the pathophysiology and treatment of mood disorders possibly by affecting the regulation of the hypothalamus-pituitary-adrenocortical (HPA) axis. Growing evidence suggests an important role of the metabotropic glutamate receptor 1 (mGlu1) in depression-related phenotypes. To test whether these findings can also be supported by human genetics data, we explored polymorphisms within the metabotropic glutamate receptor 1 gene (GRM1) for their association with unipolar depression (UPD) as well as with biological phenotypes of this disorder.

METHODS

We first tested the association of 43 tag-SNPs covering the GRM1 locus with UPD in 350 patients and 370 matched controls. We then investigated the effects of the associated SNPs on hippocampal glutamate levels estimated using ¹H-MR-spectroscopy (¹H-MRS) and on endocrine measures from the combined dexamethasone-suppression/CRH stimulation (dex/CRH) test.

RESULTS

Within the GRM1 locus, 22 SNPs showed nominally significant association with UPD, of which 6 withstood corrections for multiple testing (rs2268666 with best allelic p=7.0×10⁻⁵). Supportive evidence for an association with UPD was gained from a second independent sample with 904 patients and 1012 controls. Furthermore, patients homozygous for the non-risk genotypes showed reduced hippocampal glutamate levels as measured by ¹H-MRS, a more pronounced normalization of HPA-axis hyperactivity as well as a better antidepressant treatment outcome.

CONCLUSIONS

These results suggest that the combination of genetic and biological markers may allow to subgroup patients into etiopathogenetically more relevant subcategories which could guide clinicians in their antidepressant treatment choices.

N-acetylcysteine protects against apoptosis through modulation of group I metabotropic glutamate receptor activity

The activation of group I metabotropic glutamate receptor (group I mGlus) has been shown to produce neuroprotective or neurotoxic effects. In this study, we investigated the effects of N-acetylcysteine (NAC), a precursor of the antioxidant glutathione, on group I mGlus activation in apoptosis of glial C6 and MN9D cell lines, and a rat model of Parkinson's disease (PD). We demonstrated that NAC protected against apoptosis through modulation of group I mGlus activity. In glial C6 cells, NAC promoted phosphorylation of ERK induced by (s)-3,5- dihydroxy-phenylglycine (DHPG), an agonist of group I mGlus. NAC enhanced the group I mGlus-mediated protection from staurosporine (STS)-induced apoptosis following DHPG treatment. Moreover, in rotenone-treated MN9D cells and PD rat model, NAC protected against group I mGlus-induced toxicity by compromising the decrease in phosphorylation of ERK, phosphorylation or expression level of TH. Furthermore, the results showed that NAC prohibited the level of ROS and oxidation of cellular GSH/GSSG (E_h) accompanied by activated group I mGlus in the experimental models. Our results suggest that NAC might act as a regulator of group I mGlus-mediated activities in both neuroprotection and neurotoxicity via reducing the oxidative stress, eventually to protect cell survival. The study also suggests that NAC might be a potential therapeutics targeting for group I mGlus activation in the treatment of PD.

Involvement of src-kinase activation in ischemic preconditioning induced protection of mouse brain

AIMS

To investigate the role of src-kinase in ischemic preconditioning induced reversal of ischemia and reperfusion induced cerebral injury in mice.

MAIN METHODS

Bilateral carotid artery occlusion of 17min followed by reperfusion for 24h was employed to produce ischemia and reperfusion induced cerebral injury in mice. Cerebral infarct size was measured using triphenyltetrazolium chloride staining using both by volume and by weight methods differently. Memory was evaluated using elevated plus maze test. Rota rod test was employed to assess motor incoordination.

KEY FINDINGS

Bilateral carotid artery occlusion followed by reperfusion produced cerebral infarction and impaired memory and motor co-ordination. Three preceding episodes of bilateral carotid artery occlusion for 1min and reperfusion of 1min (ischemic preconditioning) prevented markedly ischemia-reperfusion-induced cerebral injury measured in terms of infarct size (38.5±1.3% and 38.5±2.9% mean infarct of control animals was reduced to 24.3±1.2% and 23.5±1.8% of the preconditioning groups respectively), loss of memory (72.2±3.6 mean transfer latency time of control animals was reduced to 25.6±5.2 of the preconditioning group respectively) and motor coordination (78.3±17.6s mean falling down latency time of control animals was increased to a mean value of 180.9±6.5s of the preconditioning groups respectively). SU6656 (2mg/kg, ip) and PP1 (0.1mg/kg, ip), highly selective src-kinase inhibitors, attenuated this neuroprotective effect of ischemic preconditioning.

SIGNIFICANCE

Therefore, neuroprotective effect of ischemic preconditioning may be due to src-kinase linked mechanism.

Pharmacogenomics of metabotropic glutamate receptor subtype 1 and in vivo malignant melanoma formation

We have previously shown that ectopic expression of metabotropic glutamate receptor subtype 1 in melanocytes is essential for both development and in vivo growth of melanoma using newly developed transgenic mice which conditionally express metabotropic glutamate receptor subtype 1 (mGluR1). In this study, we developed conditional transgenic mice, which harbor melanocytes not only in the dermis and hair follicles but also in the epidermis using stem cell factor transgenic mice. Pigmented plaques on the backs, tails, ears or groins of the transgenic mice began to appear 13 weeks after activation of the mGluR1 transgene, and the transgenic mice produced melanomas at a frequency of 100% 36 weeks after transgene activation. Although this transgenic mouse harbors melanocytes in the epidermis, proliferation of melanoma cells took place in the dermis. To elucidate the signals involved in development and growth of melanoma, inhibitors to phospholipase C, protein kinase C and mitogen-activated protein kinase kinase 1/2, and antagonists to Ca(2+) and calmodulin were administrated to transgenic mice. Each signal inhibitor to phospholipase, protein kinase C, Ca(2+) release, calmodulin and mitogen-activated protein kinase kinase 1/2 inhibited melanoma development. However, once melanoma was developed, the growth of melanoma was dramatically inhibited only by the inhibitor to mitogen-activated protein kinase kinase 1/2 with partial inhibition by inhibitors to protein kinase C and phospholipase C. This inhibition of melanoma growth was well correlated with the expression of phosphorylated extracellular signal-regulated kinase 1/2 and Ki-67. These results indicate that for development of melanoma, activation of every signaling pathway from mGluR1 is required. However, for growth of melanoma, the extracellular signal-regulated kinase pathway plays a key role.

Interneuron subtype specific activation of mGluR1/5 during epileptiform activity in hippocampus

PURPOSE

Specific inhibitory interneurons in area CA1 of the hippocampus, notably those located in stratum oriens-alveus (O/A-INs), are selectively vulnerable in patients and animal models of temporal lobe epilepsy (TLE). The excitotoxic mechanisms underlying the selective vulnerability of interneurons have not been identified but could involve group I metabotropic glutamate receptor subtypes (mGluR1/5), which have generally proconvulsive actions and activate prominent cationic currents and calcium responses specifically in O/A-INs.

METHODS

In this study, we examine the role of mGluR1/5 in interneurons during epileptiform activity using whole-cell recordings from CA1 O/A-INs and selective antagonists of mGluR1α (LY367385) and mGluR5 (MPEP) in a disinhibited rat hippocampal slice model of epileptiform activity.

RESULTS

Our data indicate more prominent epileptiform burst discharges and paroxysmal depolarizations (PDs) in O/A-INs than in interneurons located at the border of strata radiatum and lacunosum/moleculare (R/LM-INs). In addition, mGluR1 and mGluR5 significantly contributed to epileptiform responses in O/A-INs but not in R/LM-INs. Epileptiform burst discharges in O/A-INs were partly dependent on mGluR5. PDs and associated postsynaptic currents were dependent on both mGluR1α and mGluR5. These receptors contributed differently to postsynaptic currents underlying PDs, with mGluR5 contributing to the fast and slow components and mGluR1α to the slow component.

DISCUSSION

These findings support interneuron subtype-specific activation and differential contributions of mGluR1α and mGluR5 to epileptiform activity in O/A-INs, which could be important for their selective vulnerability in TLE.

The promiscuous mGlu5 receptor--a range of partners for therapeutic possibilities?

The issue of non-specific effects for potential therapeutics is particularly salient in neurological/psychiatric disorders, where adverse drug reactions could impair critical brain functions. The issue of specificity is not limited to candidate molecules, as receptor targets themselves often influence physiological as well as pathological outcomes. Metabotropic glutamate receptor 5 (mGlu5) is an example of a "promiscuous" receptor target that has been implicated in addiction, but also many other processes. However, if receptor modulation could be restricted to specific pathways/brain regions, mGlu5 may still prove to be a viable therapeutic target for various indications. Using this premise, a number of possible methods to refine drug development strategy are discussed, including exploiting specific interactions of mGlu5 with other receptors to narrow the influence of pharmacological agents, and also the use of RNA interference targeted to specific cells/regions of the brain.

AKT2 is a downstream target of metabotropic glutamate receptor 1 (Grm1)

We reported earlier on the oncogenic properties of Grm1 by demonstrating that stable Grm1-mouse-melanocytic clones proliferate in the absence of growth supplement and anchorage in vitro. In addition, these clones also exhibit aggressive tumorigenic phenotypes in vivo with short latency in tumor formation in both immunodeficient and syngeneic mice. We also detected strong activation of AKT in allograft tumors specifically AKT2 as the predominant isoform involved. In parallel, we assessed several human melanoma biopsy samples and found again that AKT2 was the predominantly activated AKT in these human melanoma biopsies. In cultured stable Grm1-mouse-melanocytic clones, as well as an metabotropic glutamate receptor 1 (Grm1) expressing human melanoma cell line, C8161, stimulation of Grm1 by its agonist led to the activation of AKT, while preincubation with Grm1-antagonist abolished Grm1-agonist-induced AKT activation. In addition, a reduction in tumor volume of Grm1-mouse-melanocytic-allografts was detected in the presence of small interfering AKT2 RNA (siAKT2). Taken together, these results showed that, in addition to the MAPK pathway previously reported being a downstream target of stimulated Grm1, AKT2 is another downstream target in Grm1 mediated melanocyte transformation.

Src kinase up-regulates the ERK cascade through inactivation of protein phosphatase 2A following cerebral ischemia

BACKGROUND

The regulation of protein phosphorylation requires a balance in the activity of protein kinases and protein phosphatases. Our previous data indicates that Src can increase ERK activity through Raf kinase in response to ischemic stimuli. This study examined the molecular mechanisms by which Src activates ERK cascade through protein phosphatases following cerebral ischemia.

RESULTS

Ischemia-induced Src activation is followed by phosphorylation of PP2A at Tyr307 leading to its inhibition in the rat hippocampus. SU6656, a Src inhibitor, up-regulates PP2A activity, resulting in a significant decreased activity in ERK and its targets, CREB and ERalpha. In addition, the PP2A inhibitor, cantharidin, led to an up-regulation of ERK activity and was able to counteract Src inhibition during ischemia.

CONCLUSION

Src induces up-regulation of ERK activity and its target transcription factors, CREB and ERalpha, through attenuation of PP2A activity. Therefore, activation of ERK is the result of a crosstalk between two pathways, Raf-dependent positive regulators and PP2A-dependent negative regulators.

Fragile x syndrome and autism: from disease model to therapeutic targets

Autism is an umbrella diagnosis with several different etiologies. Fragile X syndrome (FXS), one of the first identified and leading causes of autism, has been modeled in mice using molecular genetic manipulation. These Fmr1 knockout mice have recently been used to identify a new putative therapeutic target, the metabotropic glutamate receptor 5 (mGluR5), for the treatment of FXS. Moreover, mGluR5 signaling cascades interact with a number of synaptic proteins, many of which have been implicated in autism, raising the possibility that therapeutic targets identified for FXS may have efficacy in treating multiple other causes of autism.

Characterization of anandamide-stimulated cannabinoid receptor signaling in human ULTR myometrial smooth muscle cells

Accumulating evidence highlights the importance of the endocannabinoid anandamide (AEA) as a key mediator in reproductive physiology. Current data suggest potential roles for AEA in gametogenesis, fertilization, and parturition. AEA exerts its actions through two G protein-coupled receptors, termed cannabinoid receptor 1 (CB1), and 2 (CB2), and the ligand-gated transient receptor potential vanilloid receptor type 1 (TRPV1) ion channel. At present, the cellular mechanism(s) and consequences of AEA signaling in reproductive tissues, especially the myometrium, are poorly understood. Here, we examine the expression of CB1, CB2, and TRPV1 in the human myometrial smooth muscle cell-line (ULTR) and characterize intracellular signaling after stimulation with AEA. Radioligand binding analysis revealed a total CB receptor expression of 76 +/- 24 fmol/mg protein, with both quantitative PCR and competition binding studies indicating a negligible CB2 component. AEA caused Galpha(i/o)-dependent inhibition of adenylate cyclase to reduce intracellular cAMP levels. In addition, AEA caused a 2.5- to 3.5-fold increase in ERK activation, which was ablated by inhibition of Galpha(i/o), phosphoinositide-3-kinase and Src-kinase activities, but not by inhibition of Ca(2+)/calmodulin-dependent protein kinase or protein kinase C activities. TRPV1 channel activation with capsaicin failed to activate ERK. Consistent with these findings, the selective agonists, arachidonyl-2-chloroethylamide (CB1) and L759656 (CB2), and selective antagonists AM251 (CB1) and JTE907 (CB2), provided pharmacological evidence that the ERK signaling pathway is activated through endogenously expressed CB1. These findings provide an insight into myometrial AEA signaling, highlighting a potential role for endocannabinoids in the regulation of gene expression in myometrial smooth muscle cells.

Regulation of extracellular signal-regulated kinase phosphorylation in cultured rat striatal neurons

Recent studies demonstrate that activation of Ca(2+)-permeable N-methyl-D-aspartate (NMDA) receptors upregulates phosphorylation of mitogen-activated protein kinases (MAPKs) in heterologous cells and neurons. In cultured rat striatal neurons, the present work systematically evaluated the role of a number of protein kinases in forming a signaling cascade transducing NMDA receptor signals to MAPKs. It was found that a brief NMDA application consistently induced rapid and transient phosphorylation of the extracellular signal-regulated kinase 1/2 (ERK1/2), a best characterized subclass of MAPKs. This ERK1/2 phosphorylation was resistant to the inhibition of protein kinase C, p38 MAPK, cyclin-dependent kinase 5, receptor tyrosine kinase (epidermal growth factor receptors), or non-receptor tyrosine kinases (including Src) by their selective inhibitors. However, the increase in ERK1/2 phosphorylation was partially blocked by a protein kinase A (PKA) inhibitor. The inhibitors for Ca(2+)/calmodulin-dependent protein kinase (CaMK) or phosphatidylinositol 3-kinase (PI3-kinase) completely blocked the NMDA-stimulated ERK1/2 phosphorylation. In an attempt to characterize the sequential role of CaMK and PI3-kinase, we found that NMDA increased PI3-kinase phosphorylation on Tyr(508), which kinetically corresponded to the ERK1/2 phosphorylation and was blocked by the CaMK inhibitor. These results indicate that the protein kinases are differentially involved in linking NMDA receptors to ERK1/2 in striatal neurons.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Metabotropic glutamate receptor subtype-1 is essential for in vivo growth of melanoma

Ectopic expression of metabotropic glutamate receptor subtype 1 (mGluR1) in mouse melanocytes induces melanoma formation. Although requirement of mGluR1 for development of melanoma in the initial stage has been demonstrated, its role in melanoma growth in vivo remains unclear. In this study, we developed novel transgenic mice that conditionally express mGluR1 in melanocytes, using a tetracycline regulatory system. Pigmented lesions on the ears and tails of the transgenic mice began to appear 29 weeks after activation of the mGluR1 transgene, and the transgenic mice produced melanomas at a frequency of 100% 52 weeks after transgene activation. Subsequent inactivation of the mGluR1 transgene in melanoma-bearing mice inhibited melanoma growth with reduction of immunoreactivity to phosphorylated ERK1/2, whereas mice with persistent expression of mGluR1 developed larger melanoma burdens. mGluR1 expression is thus required not only for melanoma development but also for melanoma growth in vivo. These findings suggest that growth of melanoma can be inhibited in vivo by eliminating only one of the multiple genetic anomalies involved in tumorigenesis.

Allosteric potentiators of metabotropic glutamate receptor subtype 1a differentially modulate independent signaling pathways in baby hamster kidney cells

Recent studies suggest that subtype specific activators of metabotropic glutamate receptors (mGluRs) have exciting potential for the development of novel treatment strategies for numerous psychiatric and neurological disorders. A number of positive allosteric modulators (PAMs) have been identified that are highly selective for mGluR1, including the compounds Ro 01-6128, Ro 67-4853, and Ro 67-7476. These PAMs have been previously found to interact with a site distinct from that of negative allosteric modulators (NAMs), typified by R214127. These mGluR1 PAMs do not have an effect on baseline calcium levels but induce leftward shifts in the concentration-response of mGluR1 to agonists. However, their effects on a variety of signaling pathways and their mechanism of action have not been fully explored and are of critical importance for further development of mGluR1 allosteric modulators as novel drugs. In baby hamster kidney (BHK) cells, mGluR1 activates calcium mobilization, cAMP production, and extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation; signaling cascades which are distinct and differentially regulated. In contrast to their effects on calcium mobilization, these compounds were found to activate ERK1/2 phosphorylation in the absence of exogenously added agonist, an effect that was fully blocked by both orthosteric (LY341495) and allosteric (R214127) mGluR1 antagonists. The mGluR1 PAMs were also found to activate cAMP production in the absence of agonist. Thus, these mGluR1 PAMs have qualitatively different effects on a variety of mGluR1-mediated signal transduction cascades. Together, these data provide further evidence that allosteric compounds can differentially modulate the coupling of a single receptor to independent signaling pathways or act in a system-dependent manner.

G-protein-coupled receptors and melanoma

G-protein-coupled receptors (GPCR) are the largest family of receptors with over 500 members. Evaluation of GPCR gene expression in primary human tumors identified over-expression of GPCR in several tumor types. Analysis of cancer samples in different disease stages also suggests that some GPCR may be involved in early tumor progression and others may play a critical role in tumor invasion and metastasis. Currently, >50% of drug targets to various human diseases are based on GPCR. In this review, the relationships between several GPCR and melanoma development and/or progression will be discussed. Finally, the possibility of using one or more of these GPCR as therapeutic targets in melanoma will be summarized.

Metabotropic glutamate receptors: phosphorylation and receptor signaling

Metabotropic glutamate receptors (mGluRs) play important roles in neurotransmission, neuronal development, synaptic plasticity, and neurological disorders. Recent studies have revealed a sophisticated interplay between mGluRs and protein kinases: activation of mGluRs regulates the activity of a number of kinases, and direct phosphorylation of mGluRs affects receptor signaling, trafficking, and desensitization. Here we review the emerging literature on mGluR phosphorylation, signaling, and synaptic function.