A signaling mechanism from G alpha q-protein-coupled metabotropic glutamate receptors to gene expression: role of the c-Jun N-terminal kinase pathway

Noncanonical Metabotropic Glutamate Receptor 5 Signaling in Alzheimer's Disease

Metabotropic glutamate receptor 5 (mGluR5) is ubiquitously expressed in brain regions responsible for memory and learning. It plays a key role in modulating rapid changes in synaptic transmission and plasticity. mGluR5 supports long-term changes in synaptic strength by regulating the transcription and translation of essential synaptic proteins. β-Amyloid 42 (Aβ42) oligomers interact with a mGluR5/cellular prion protein (PrP^C) complex to disrupt physiological mGluR5 signal transduction. Aberrant mGluR5 signaling and associated synaptic failure are considered an emerging pathophysiological mechanism of Alzheimer's disease (AD). Therefore, mGluR5 represents an attractive therapeutic target for AD, and recent studies continue to validate the efficacy of various mGluR5 allosteric modulators in improving memory deficits and mitigating disease pathology. However, sex-specific differences in the pharmacology of mGluR5 and activation of noncanonical signaling downstream of the receptor suggest that its utility as a therapeutic target in female AD patients needs to be reconsidered. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The elongation factor eEF1A2 controls translation and actin dynamics in dendritic spines

Synaptic plasticity involves structural modifications in dendritic spines that are modulated by local protein synthesis and actin remodeling. Here, we investigated the molecular mechanisms that connect synaptic stimulation to these processes. We found that the phosphorylation of isoform-specific sites in eEF1A2-an essential translation elongation factor in neurons-is a key modulator of structural plasticity in dendritic spines. Expression of a nonphosphorylatable eEF1A2 mutant stimulated mRNA translation but reduced actin dynamics and spine density. By contrast, a phosphomimetic eEF1A2 mutant exhibited decreased association with F-actin and was inactive as a translation elongation factor. Activation of metabotropic glutamate receptor signaling triggered transient dissociation of eEF1A2 from its regulatory guanine exchange factor (GEF) protein in dendritic spines in a phosphorylation-dependent manner. We propose that eEF1A2 establishes a cross-talk mechanism that coordinates translation and actin dynamics during spine remodeling.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Role of fragile X mental retardation protein in chronic pain

Chronic pain has detrimental effects on one's quality of life. However, its treatment options are very limited, and its underlying pathogenesis remains unclear. Recent research has suggested that fragile X mental retardation protein is involved in the development of chronic pain, making it a potential target for prevention and treatment. The current review of literature will examine the function of fragile X mental retardation protein and its associated pathways, through which we hope to gain insight into how fragile X mental retardation protein may contribute to nociceptive sensitization and chronic pain.

Aloe emodin induces hepatotoxicity by activating NF-κB inflammatory pathway and P53 apoptosis pathway in zebrafish

The aim of this study was to investigate the hepatotoxic effect and its underlying mechanism of aloe emodin (AE). AE was docked with the targets of NF-κB inflammatory pathway and P53 apoptosis pathway respectively by using molecular docking technique. To verify the results of molecular docking and further investigate the hepatotoxicity mechanism of AE, the zebrafish Tg (fabp10: EGFP) was used as an animal model in vivo. The pathological sections of zebrafish liver were analyzed to observe the histopathological changes and Sudan black B was used to study whether there were inflammatory reactions in zebrafish liver or not. Then TdT-mediated dUTP Nick-End Labeling (TUNEL) was used to detect the apoptotic signal of zebrafish liver cells, finally the mRNA expression levels as well as the protein expression levels of the targets in NF-κB and P53 pathways in zebrafish were measured by quantitative Real-Time PCR (qRT-PCR) and western blot. Molecular docking results showed that AE could successfully dock with all the targets of NF-κB and P53 pathways, and the docking scores of most of the targets were equal to or higher than that of the corresponding ligands. Pathological sections showed AE could cause zebrafish liver lesions and the result of Sudan black B staining revealed that AE blackened the liver of zebrafish with Sudan black B. Then TUNEL assay showed that a large number of dense apoptotic signals were observed in AE group, mainly distributed in the liver and yolk sac of zebrafish. The results of qRT-PCR and western blot showed that AE increased the mRNA and protein expression levels of pro-inflammatory and pro-apoptotic targets in NF-κB and P53 pathways. AE could activate the NF-κB inflammatory pathway and the P53 apoptosis pathway, and its hepatotoxic mechanism was related to activation of NF-κB-P53 inflammation-apoptosis pathways.

The involvement of the mGluR5-mediated JNK signaling pathway in rats with diabetic retinopathy

OBJECTIVE

To understand the involvement of the mGluR5-mediated JNK signaling pathway in rats with diabetic retinopathy (DR).

METHODS

This study established rat models of diabetes mellitus (DM), which were divided into Normal, DM, DM + CHPG (mGluR5 agonist CHPG), and DM + MTEP (mGluR5 antagonist MTEP) groups. The blood glucose and weight of rats were recorded. EB staining was used for observation of blood-retinal barrier (BRB) damage. Neural retina function was measured by pattern electroretinogram (ERG). PAS and NG2 immunohistochemistry were conducted to evaluate the retinal vascular morphology. The TUNEL assay and active caspase-3 immunohistochemistry were performed to detect retinal cell apoptosis. Additionally, the expression levels of superoxide dismutase (SOD) and methylenedioxyamphetamine (MDA) were measured. Moreover, expression levels of mGluR5 and JNK pathway-related proteins were detected by western blot.

RESULTS

When compared with control rats, rats in the DM group showed decreased amplitude and latency of the peak times in the ERG test; further, DM group rats presented increases in blood glucose, BRB permeability, a retinal capillary area density, retinal cell apoptosis with an increased number of active caspase-3-positive cells, MDA level, mGluR5 levels, and the ratio of p-JNK/JNK, and they showed reductions in body weight and SOD activity, as well as in the number of pericytes and in the pericyte coverage (all P < 0.05). However, rats in DM + CHPG group had stronger negative effects than those in DM group (all P < 0.05). Rats from DM + MTEP group showed an opposite trend compared with the DM rats (all P < 0.05).

CONCLUSION

The level of mGluR5 in DR rats was upregulated, whereas inhibition of mGluR5 alleviated retinal pathological damage and decreased cell apoptosis to improve DR via suppression of the JNK signaling pathway, which provided a scientific theoretical basis for the clinical treatment of DR.

Metabotropic Glutamate Receptors in Glial Cells: A New Potential Target for Neuroprotection?

Neurodegenerative disorders are characterized by excitotoxicity and neuroinflammation that finally lead to slow neuronal degeneration and death. Although neurons are the principal target, glial cells are important players as they contribute by either exacerbating or dampening the events that lead to neuroinflammation and neuronal damage. A dysfunction of the glutamatergic system is a common event in the pathophysiology of these diseases. Metabotropic glutamate (mGlu) receptors belong to a large family of G protein-coupled receptors largely expressed in neurons as well as in glial cells. They often appear overexpressed in areas involved in neurodegeneration, where they can modulate glutamatergic transmission. Of note, mGlu receptor upregulation may involve microglia or, even more frequently, astrocytes, where their activation causes release of factors potentially able to influence neuronal death. The expression of mGlu receptors has been also reported on oligodendrocytes, a glial cell type specifically involved in the development of multiple sclerosis. Here we will provide a general overview on the possible involvement of mGlu receptors expressed on glial cells in the pathogenesis of different neurodegenerative disorders and the potential use of subtype-selective mGlu receptor ligands as candidate drugs for the treatment of neurodegenerative disorders. Negative allosteric modulators (NAM) of mGlu5 receptors might represent a relevant pharmacological tool to develop new neuroprotective strategies in these diseases. Recent evidence suggests that targeting astrocytes and microglia with positive allosteric modulators (PAM) of mGlu3 receptor or oligodendrocytes with mGlu4 PAMS might represent novel pharmacological approaches for the treatment of neurodegenerative disorders.

Role of DNA Methylation in Hypobaric Hypoxia-Induced Neurodegeneration and Spatial Memory Impairment

Hypobaric hypoxia (HH) is a major stress factor that is associated with physiological, biochemical, molecular and genomic alterations. Brain is the organ that reacts sensitively to oxygen deprivation, which leads to oxidative stress and cognitive function impairment. Our previous studies have reported that downregulation of brain derived neurotrophic factor (BDNF) leads to neurodegeneration and memory impairment. The aim of the present study was to investigate the effect of HH exposure on DNA methylation and its regulation in BDNF expression, neurodegeneration and spatial memory impairment. For this purpose, Sprague Dawley rats were exposed to HH at a simulated altitude of 25,000 feet for 14 days. Real-time polymerase chain reaction was used for transcriptional expression of DNA Methyltransferases (DNMTs) including DNMT1, DNMT3a and -DNMT3b, and immunoblotting was used for the translational expression of DNMT1, DNMT3a, DNMT3b, Methyl CpG binding protein 2 (MeCP2), pMeCP2 and BDNF in rat hippocampus. Additionally, neuronal morphology alteration and neurodegeneration in CA1 region of hippocampus were investigated though Cresyl violet (CV) staining and Fluoro-Jade C staining respectively. Results obtained suggested that HH exposure increased the expression of DNMT1 DNMT3b at the mRNA as well as protein level, whereas no significant change was observed in the level of DNMT3a. Furthermore, the level of pMeCP2 and BDNF were significantly decreased; however, the expression level of MeCP2 was significantly increased. The CV and Fluoro-Jade C-positive cells were significantly enhanced in the CA1 region of hippocampus in the HH exposed group as compared to unexposed rats. Thus, the present study concluded that HH decreases neuronal activation by the upregulation of DNA methylation and MeCP2 and decreased the expression of pMeCP2, which result in the downregulation of BDNF. The decreased BDNF expression is associated with neuronal loss and spatial memory impairment. This study highlights that DNMT inhibition could be an important therapeutic target for neurodegenerative diseases.

Gaq proteins: molecular pharmacology and therapeutic potential

Seven transmembrane G protein-coupled receptors (GPCRs) have gained much interest in recent years as it is the largest class among cell surface receptors. G proteins lie in the heart of GPCRs signalling and therefore can be therapeutically targeted to overcome complexities in GPCR responses and signalling. G proteins are classified into four families (Gi, Gs, G12/13 and Gq); Gq is further subdivided into four classes. Among them Gαq and Gαq/11 isoforms are most crucial and ubiquitously expressed; these isoforms are almost 88% similar at their amino acid sequence but may exhibit functional divergences. However, uncertainties often arise about Gαq and Gαq/11 inhibitors, these G proteins might also have suitability to the invention of novel-specific inhibitors for each isoforms. YM-254890 and UBO-QIC are discovered as potent inhibitors of Gαq functions and also investigated in thrombin protease-activated receptor (PAR)-1 inhibitors and platelet aggregation inhibition. The most likely G protein involved in PAR-1 stimulates responses is one of the Gαq family isoforms. In this review, we highlight the molecular structures and pharmacological responses of Gαq family which may reflect the biochemical and molecular role of Gαq and Gαq/11. The advanced understanding of Gαq and Gαq/11 role in GPCR signalling may shed light on our understanding on cell biology, cellular physiology and pathophysiology and also lead to the development of novel therapeutic agents for a number of diseases.

Osteosarcoma cell proliferation and survival requires mGluR5 receptor activity and is blocked by Riluzole

Osteosarcomas are malignant tumors of bone, most commonly seen in children and adolescents. Despite advances in modern medicine, the poor survival rate of metastatic osteosarcoma has not improved in two decades. In the present study we have investigated the effect of Riluzole on a human and mouse metastatic osteosarcoma cells. We show that LM7 cells secrete glutamate in the media and that mGluR5 receptors are required for the proliferation of LM7 cells. Riluzole, which is known to inhibit glutamate release, inhibits proliferation, induces apoptosis and prevents migration of LM7 cells. This is also seen with Fenobam, a specific blocker of mGluR5. We also show that Riluzole alters the phosphorylation status of AKT/P70 S6 kinase, ERK1/2 and JNK1/2. Thus Riluzole is an effective drug to inhibit proliferation and survival of osteosarcoma cells and has therapeutic potential for the treatment of osteosarcoma exhibiting autocrine glutamate signaling.

JNK pathway signaling: a novel and smarter therapeutic targets for various biological diseases

JNK pathway regulates various physiological processes including inflammatory responses, cell differentiation, cell proliferation, cell death, cell survival and expression of proteins. Deregulation of JNK is linked with various diseases including neurodegenerative disease, autoimmune disease, diabetes, cancer, cardiac hypertrophy and asthma. Three distinct genes JNK1, JNK2 and JNK3 have been identified as regulator of JNK pathway. JNK1 and JNK2 have broad tissue distribution and play a potential role in insulin resistance, inflammation and cell signaling. JNK3 is predominantly found in the CNS neurons, making it an attractive target for neurodegenerative disorders. In this review, we summarize the evidence supporting JNK as a potent therapeutic target, and small molecules from various chemical classes as JNK inhibitors.

Regulation of synaptic MAPK/ERK phosphorylation in the rat striatum and medial prefrontal cortex by dopamine and muscarinic acetylcholine receptors

Dopamine and acetylcholine are two principal transmitters in the striatum and are usually balanced to modulate local neural activity and to maintain striatal homeostasis. This study investigates the role of dopamine and muscarinic acetylcholine receptors in the regulation of a central signaling protein, i.e., the mitogen-activated protein kinase (MAPK). We focus on the synaptic pool of MAPKs because of the fact that these kinases reside in peripheral synaptic structures in addition to their somatic locations. We show that a systemic injection of dopamine D1 receptor (D1R) agonist SKF81297 enhances phosphorylation of extracellular signal-regulated kinases (ERKs), a prototypic subclass of MAPKs, in the adult rat striatum. Similar results were observed in another dopamine-responsive region, the medial prefrontal cortex (mPFC). The dopamine D2 receptor agonist quinpirole had no such effects. Pretreatment with a positive allosteric modulator (PAM) of muscarinic acetylcholine M4 receptors (M4Rs), VU0152100, attenuated the D1R agonist-stimulated ERK phosphorylation in the two regions, whereas the PAM itself did not alter basal ERK phosphorylation. All drug treatments had no effect on phosphorylation of c-Jun N-terminal kinases (JNKs), another MAPK subclass, in the striatum and mPFC. These results demonstrate that dopamine and acetylcholine are integrated to control synaptic ERK but not JNK activation in striatal and mPFC neurons in vivo. Activation of M4Rs exerts an inhibitory effect on the D1R-mediated upregulation of synaptic ERK phosphorylation.

c-Jun N-terminal kinase regulates mGluR-dependent expression of post-synaptic FMRP target proteins

Fragile X syndrome (FXS) is caused by the loss of functional fragile X mental retardation protein (FMRP). Loss of FMRP results in an elevated basal protein expression profile of FMRP targeted mRNAs, a loss of local metabotropic glutamate receptor (mGluR)-regulated protein synthesis, exaggerated long-term depression and corresponding learning and behavioral deficits. Evidence shows that blocking mGluR signaling in FXS models ameliorates these deficits. Therefore, understanding the signaling mechanisms downstream of mGluR stimulation may provide additional therapeutic targets for FXS. Kinase cascades are an integral mechanism regulating mGluR-dependent protein translation. The c-Jun N-terminal kinase (JNK) pathway has been shown to regulate mGluR-dependent nuclear transcription; however, the involvement of JNK in local, synaptic signaling has not been explored. Here, we show that JNK is both necessary and sufficient for mGluR-dependent expression of a subset of FMRP target proteins. In addition, JNK activity is basally elevated in fmr1 knockout mouse synapses, and blocking JNK activity reduces the over-expression of post-synaptic proteins in these mice. Together, these data suggest that JNK may be an important signaling mechanism downstream of mGluR stimulation, regulating FMRP-dependent protein synthesis. Furthermore, local, post-synaptic dysregulation of JNK activity may provide a viable target to ameliorate the deficits involved in FXS. Expression of many FMRP target proteins is enhanced in FXS. Here, we evaluated the role of JNKs in FXS. We found that JNK signaling is activated upon mGluR stimulation in wild-type neurons. Conversely, JNK activity is basally elevated in fmr1 knockout. Inhibiting JNK reduced the expression of FMRP target proteins and driving JNK activity increased the expression of these proteins.

The GluK4 kainate receptor subunit regulates memory, mood, and excitotoxic neurodegeneration

Though the GluK4 kainate receptor subunit shows limited homology and a restricted expression pattern relative to other kainate receptor subunits, its ablation results in distinct behavioral and molecular phenotypes. GluK4 knockout mice demonstrated impairments in memory acquisition and recall in a Morris water maze test, suggesting a previously unreported role for kainate receptors in spatial memory. GluK4 knockout mice also showed marked hyperactivity and impaired pre-pulse inhibition, thereby mirroring two of the hallmark endophenotypes of patients with schizophrenia and bipolar disorder. Furthermore, we found that GluK4 is a key mediator of excitotoxic neurodegeneration: GluK4 knockout mice showed robust neuroprotection in the CA3 region of the hippocampus following intrahippocampal injection of kainate and widespread neuroprotection throughout the hippocampus following hypoxia-ischemia. Biochemical analysis of kainate- or sham-treated wild-type and GluK4 knockout hippocampal tissue suggests that GluK4 may act through the JNK pathway to regulate the molecular cascades that lead to excitotoxicity. Together, our findings suggest that GluK4 may be relevant to the understanding and treatment of human neuropsychiatric and neurodegenerative disorders.

Amphetamine increases phosphorylation of MAPK/ERK at synaptic sites in the rat striatum and medial prefrontal cortex

Mitogen-activated protein kinases (MAPKs) play a central role in cell signaling. Extracellular signal-regulated kinase (ERK) is a prototypic subclass of MAPKs and is densely expressed in postmitotic neurons of adult mammalian brains. Active ERK translocates into the nucleus to regulate gene expression. Additionally, ERK is visualized in neuronal peripheries, such as distal synaptic structures. While nuclear ERK is a known sensitive target of psychostimulants, little is known about the responsiveness of synaptic ERK to stimulants. In this study, we focused on ERK at synaptic versus extrasynaptic sites and investigated its responses to the psychostimulant amphetamine in the adult rat striatum and medial prefrontal cortex (mPFC) in vivo. We used a pre-validated biochemical fractionation procedure to isolate synapse- and extrasynapse-enriched membranes. We found that two common ERK isoforms (ERK1 and ERK2) were concentrated more in extrasynaptic fractions than in synaptic fractions in striatal and cortical neurons under normal conditions. At synaptic sites, ERK2 was noticeably more abundant than ERK1. Acute injection of amphetamine induced an increase in ERK2 phosphorylation in the synaptic fraction of striatal neurons, while the drug did not alter extrasynaptic ERK2 phosphorylation. Similar results were observed in the mPFC. In both synaptic and extrasynaptic compartments, total ERK1/2 proteins remained stable in response to amphetamine. Our data establish the subsynaptic distribution pattern of MAPK/ERK in striatal and cortical neurons. Moreover, the synaptic pool of ERK2 in these neurons can be selectively activated by amphetamine.

Group I metabotropic glutamate receptor-mediated gene transcription and implications for synaptic plasticity and diseases

Stimulation of group I metabotropic glutamate receptors (mGluRs) initiates a wide variety of signaling pathways. Group I mGluR activation can regulate gene expression at both translational and transcriptional levels, and induces translation or transcription-dependent synaptic plastic changes in neurons. The group I mGluR-mediated translation-dependent neural plasticity has been well reviewed. In this review, we will highlight group I mGluR-induced gene transcription and its role in synaptic plasticity. The signaling pathways (PKA, CaMKs, and MAPKs) which have been shown to link group I mGluRs to gene transcription, the relevant transcription factors (CREB and NF-κB), and target proteins (FMRP and ARC) will be documented. The significance and future direction for characterizing group I mGluR-mediated gene transcription in fragile X syndrome, schizophrenia, drug addiction, and other neurological disorders will also be discussed.

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.

Cocaine facilitates PKC maturation by upregulating its phosphorylation at the activation loop in rat striatal neurons in vivo

Newly synthesized protein kinase C (PKC) undergoes a series of phosphorylation to render a mature form of the enzyme. It is this mature PKC that possesses the catalytic competence to respond to second messengers for activation and downstream signaling. The first and rate-limiting phosphorylation occurs at a threonine residue in the activation loop (AL), which triggers the rest maturation processing of PKC and regulates PKC activity in response to cellular stimulation. Given the fact that PKC is enriched in striatal neurons, we investigated the regulation of PKC phosphorylation at the AL site in the rat striatum by the psychostimulant cocaine in vivo. We found that PKC was phosphorylated at the AL site at a moderate level in the normal rat brain. Acute systemic injection of cocaine increased the PKC-AL phosphorylation in the two striatal structures (caudate putamen and nucleus accumbens). Cocaine also elevated the PKC-AL phosphorylation in the medial prefrontal cortex. The cocaine-stimulated PKC phosphorylation in the striatum is rapid and transient. A reliable increase in PKC phosphorylation was seen 7 min after drug injection, which declined to the normal level by 1h. This kinetics corresponds to that seen for another striatum-enriched protein kinase, mitogen-activated protein kinase/extracellular signal-regulated kinase, in response to cocaine. This study suggests a new model for exploring the impact of cocaine on protein kinases in striatal neurons. By modifying PKC phosphorylation at the AL site, cocaine is believed to possess the ability to alter the maturation processing of the kinase in striatal neurons in vivo.

WITHDRAWN: Upregulation of conventional protein kinase C phosphorylation and translocation in the rat nucleus accumbens following cocaine administration

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mGluR5 is involved in proliferation of rat neural progenitor cells exposed to hypoxia with activation of mitogen-activated protein kinase signaling pathway

Hypoxia/ischemia induces proliferation of neural progenitor cells (NPCs) in rodent and human brain; however, the mechanisms remain unknown. We investigated the effects of metabotropic glutamate receptor 5 (mGluR5) on NPC proliferation under hypoxia, the expression of cyclin D1, and the activation of the mitogen-activated protein kinases (MAPKs) signaling pathway in cell culture. The results showed that hypoxia induced mGluR5 expression on NPCs in vitro. Under hypoxia, the mGluR5 agonists DHPG and CHPG significantly increased NPC proliferation in cell activity, diameter of neurospheres, bromodeoxyuridine (BrdU) incorporation and cell division, and expression of cyclin D1, with decreasing cell death. The mGluR5 siRNA and antagonist MPEP decreased the NPC proliferation and expression of cyclin D1, with increasing cell death. Phosphorylated JNK and ERK increased with the proliferation of NPCs after DHPG and CHPG treatment under hypoxia, while p-p38 level decreased. These results demonstrate that the expression of mGluR5 was upregulated during the proliferation of rat NPCs stimulated by hypoxia in vitro. The activation of the ERK and JNK signaling pathway and the expression of cyclin D1 were increased in this process. These finding suggest the involvement of mGluR5 in rat NPC proliferation and provide a target molecule in neural repair after ischemia/hypoxia injury of CNS.

Metabotropic glutamate receptor 5 promotes proliferation of human neural stem/progenitor cells with activation of mitogen-activated protein kinases signaling pathway in vitro

Metabotropic glutamate receptors (mGluRs) regulate neurogenesis in brain, but the mechanisms remain unknown. In this study, we investigated the effect of mGluR5 on the proliferation of human embryonic neural stem/progenitor cells (NPCs), the expression of cyclin D1 and the activation of signaling pathways of mitogen-activated protein kinases (MAPKs). Results showed that mGluR5 agonist (S)-3,5-dihydroxyphenylglycine hydrate (DHPG) increased the proliferation of NPCs by increasing cell activity, diameter of neurospheres and cell division, while mGluR5 siRNA and antagonist 6-methyl-2-(phenylethynyl) pyridine hydrochloride (MPEP) decreased the NPC proliferation. The mRNA and protein expressions of cyclin D1 increased with DHPG treatment and decreased after siRNA or MPEP treatment. It was also found that activation of extracellular signal-regulated protein kinase (ERK) and c-Jun N-terminal protein kinase (JNK) signaling pathways were involved in the proliferation of NPCs. After DHPG treatment, p-ERK1/2 and p-JNK2 levels increased, and meanwhile p-p38 level decreased; but p-ERK1/2 and p-JNK2 levels decreased after siRNA or MPEP treatment, and p-p38 level increased. Our findings demonstrated that mGluR5 promoted the proliferation of human embryonic cortical NPCs and increased cyclin D1 expression with the changes in phosphorylation of MAPKs signaling pathways in vitro, suggesting a novel mechanism for pharmacological study of treatment for ischemic brain injury and neurodegenerative disorders.

Cocaine increases phosphorylation of MeCP2 in the rat striatum in vivo: a differential role of NMDA receptors

Methyl CpG-binding protein-2 (MeCP2) is a transcriptional regulator that binds to methylated DNA at CpG sites and functions to silence DNA transcription. MeCP2 is subject to the phosphorylation modification at serine 421 (S421), which releases MeCP2 from DNA and thus facilitates gene expression. As a transcriptional repressor densely expressed in limbic reward circuits of adult mammalian brains, MeCP2 is recently emerging as a critical epigenetic factor in experience-dependent neural plasticity and psychostimulant addiction. In this study, we investigated the regulation of MeCP2 phosphorylation in the rat striatum by the psychostimulant cocaine in vivo. We found that acute systemic injection of cocaine increased MeCP2 phosphorylation at S421 in the rat striatum, including both the caudate putamen and the nucleus accumbens, while cocaine did not affect MeCP2 phosphorylation in the medial prefrontal cortex. The cocaine-stimulated MeCP2 phosphorylation in the nucleus accumbens was a rapid and transient event, as it was evident at 20 min and returned to normal levels 3h after drug injection. The cocaine effect in the caudate putamen was however relatively delayed. Reliable induction of MeCP2 phosphorylation in this region was detected at 60 min. Pretreatment with an N-methyl-d-aspartate (NMDA) glutamate receptor antagonist significantly reduced the cocaine-stimulated MeCP2 phosphorylation in the caudate putamen, although not in the nucleus accumbens. Our data support that MeCP2 is a sensitive target of psychostimulants. Its phosphorylation status is regulated by psychostimulant exposure. NMDA receptors play a region-specific role in linking cocaine to MeCP2 phosphorylation in striatal neurons in vivo.

Linking cocaine to endoplasmic reticulum in striatal neurons: role of glutamate receptors

The endoplasmic reticulum (ER) controls protein folding. Accumulation of unfolded and misfolded proteins in the ER triggers an ER stress response to accelerate normal protein folding or if failed to cause apoptosis. The ER stress response is a conserved cellular response in mammalian cells and is sensitive to various physiological or pathophysiological stimuli. Recent studies unravel that this response in striatal neurons is subject to the tight modulation by psychostimulants. Cocaine and amphetamines markedly increased expression of multiple ER stress reporter proteins in the dorsal striatum (caudate putamen) and other basal ganglia sites. This evoked ER stress response is mediated by activation of group I metabotropic glutamate receptors and N-methyl-D-aspartate receptors. Converging Ca(2+) signals derived from activation of these receptors activate the c-Jun N-terminal kinase pathway to evoke ER stress responses. The discovery of robust ER stress responses to stimulant exposure establishes a previously unrecognized stimulant-ER coupling. This inducible coupling seems to contribute to neurotoxicity of stimulants related to various neuropsychiatric and neurodegenerative illnesses. Elucidating cellular mechanisms linking cocaine and other stimulants to ER is therefore important for the development of therapeutic agents for treating neurological disorders resulted from stimulant toxicity.

Effects of mutant huntingtin on mGluR5-mediated dual signaling pathways: implications for therapeutic interventions

Glutamate excitotoxicity is thought to play an important role in Huntington's disease (HD), which is caused by a polyglutamine expansion in the HD protein huntingtin (htt). Overactivation of group I metabotropic glutamate receptors (mGluRs), which include mGluR1 as well as mGluR5 and are coupled via phospholipase C to the inositol phosphate pathway, is found to be involved in mutant htt-mediated neurotoxicity. However, activation of mGluR5 also leads to neuronal protection. Here, we report that mutant htt can activate both mGluR5-mediated ERK and JNK signaling pathways. While increased JNK signaling causes cell death, activation of ERK signaling pathway is protective against cell death. Expression of mutant htt in cultured cells causes greater activation of JNK than ERK. These findings suggest that selective inhibition of the JNK signaling pathway may offer an effective therapeutic approach for reducing htt-mediated excitotoxicity.

AMN082 promotes the proliferation and differentiation of neural progenitor cells with influence on phosphorylation of MAPK signaling pathways

Metabotropic glutamate receptors (mGluRs) are expressed in neural progenitor cells (NPCs) and may play important roles in the neurogenesis during embryonic development and adult brain repair following injuries. In the present study, we investigated the expression of metabotropic glutamate receptor 7 (mGluR7) and the possible roles of this receptor in the proliferation and differentiation of NPCs isolated from embryonic Sprague-Dawley (SD) rats. The results showed that under the normal culture and the hypoxic condition, both mRNA and protein of mGluR 7 are expressed in NPCs. Administration of AMN082, a selective agonist ofmGluR7, promoted the proliferation and differentiation of NPCs. We also demonstrated that activation of JNK and ERK signaling pathways are involved in the differentiation of NPCs into neurons following AMN082 treatment. AMN082 stimulated p-ERK and p-JNK2 expression in both normal and hypoxic conditions at different time points. But p-p38 decreased in normoxia and increased in hypoxia condition at 6h following treated with AMN082 activation. In conclusion, mGluR7 possesses the potential in promoting rat NPCs proliferation and differentiation in vitro with changes in phosphorylation of mitogen-activated protein kinases (MAPK) signaling pathways, suggesting that mGluR7 may exert an important role in brain development and repair of the central nervous system after injury.

Activation of c-Jun N-terminal kinase is required for the regulation of endoplasmic reticulum stress response in the rat dorsal striatum following repeated cocaine administration

Repeated exposure to cocaine upregulates endoplasmic reticulum (ER) stress response and c-Jun N-terminal kinase (JNK) phosphorylation is associated with the ER stress response in neurons. In this study, we investigated the involvement of JNK in the regulation of the ER stress response following repeated cocaine administration in the dorsal striatum in vivo. The results showed that systemic injections of cocaine (20 mg/kg) for seven consecutive days increased the induction of p46 JNK (JNK) phosphorylation, immunoglobulin heavy chain binding protein (BiP), the ER stress-associated protein caspase-12, and behavioral locomotor activity. This enhancement of BiP and caspase-12 expression and locomotor response was reduced by inhibiting JNK. Similar reduction of elevated JNK phosphorylation was induced by blocking dopamine D1 receptors, N-methyl-D-aspartate (NMDA) receptors, and group I metabotropic glutamate receptors (mGluRs). These data suggest that JNK activation following repeated cocaine administration is required for the regulation of the ER stress protein expression and behavioral alteration in the dorsal striatum. Stimulation of dopamine D1 receptors, NMDA receptors or group I mGluRs participates in the regulation of JNK activation.

Regulation of group I metabotropic glutamate receptor expression in the rat striatum and prefrontal cortex in response to amphetamine in vivo

G protein-coupled metabotropic glutamate receptors (mGluRs) are expressed in widespread regions of the mammalian brain and are involved in the regulation of a variety of neuronal and synaptic activities. Group I mGluRs (mGluR1 and mGluR5 subtypes) are expressed in striatal medium spiny output neurons and are believed to play an important role in the modulation of cellular responses to dopamine stimulation with psychostimulants. In this study, we investigated the effect of a single dose of the psychostimulant amphetamine on mGluR1/5 protein expression in the rat forebrain in vivo. We found that acute systemic injection of amphetamine at a behaviorally active dose (5 mg/kg) was able to reduce mGluR5 protein levels in a confined biochemical fraction of synaptosomal plasma membranes enriched from the striatum. In contrast to the striatum, amphetamine increased mGluR5 protein levels in the medial prefrontal cortex. These changes in mGluR5 expression in both the striatum and the medial prefrontal cortex were transient and reversible. In addition, protein levels of mGluR1 in the enriched synaptosomal fraction from both the striatum and the medial prefrontal cortex remained stable in response to acute amphetamine. Similarly, Homer1b/c proteins, which are prominent anchoring proteins of mGluR1/5 and are highly expressed in the striatum and the medial prefrontal cortex, showed no change in their protein abundance in striatal and cortical synaptosomes after amphetamine administration. These data demonstrate differential sensitivity of mGluR1 and mGluR5 expression to amphetamine. Acute amphetamine injection is able to alter mGluR5 protein levels at synaptic sites in a subtype- and region-specific manner.

Metabotropic glutamate receptor-mediated long-term depression: molecular mechanisms

The ability to modify synaptic transmission between neurons is a fundamental process of the nervous system that is involved in development, learning, and disease. Thus, synaptic plasticity is the ability to bidirectionally modify transmission, where long-term potentiation and long-term depression (LTD) represent the best characterized forms of plasticity. In the hippocampus, two main forms of LTD coexist that are mediated by activation of either N-methyl-d-aspartic acid receptors (NMDARs) or metabotropic glutamate receptors (mGluRs). Compared with NMDAR-LTD, mGluR-LTD is less well understood, but recent advances have started to delineate the underlying mechanisms. mGluR-LTD at CA3:CA1 synapses in the hippocampus can be induced either by synaptic stimulation or by bath application of the group I selective agonist (R,S)-3,5-dihydroxyphenylglycine. Multiple signaling mechanisms have been implicated in mGluR-LTD, illustrating the complexity of this form of plasticity. This review provides an overview of recent studies investigating the molecular mechanisms underlying hippocampal mGluR-LTD. It highlights the role of key molecular components and signaling pathways that are involved in the induction and expression of mGluR-LTD and considers how the different signaling pathways may work together to elicit a persistent reduction in synaptic transmission.

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.

Amphetamine alters Ras-guanine nucleotide-releasing factor expression in the rat striatum in vivo

Ras-guanine nucleotide-releasing factors (Ras-GRFs) are densely expressed in neurons of the mammalian brain. As a Ras-specific activator predominantly concentrated at synaptic sites, Ras-GRFs activate the Ras-mitogen-activated protein kinase (Ras-MAPK) cascade in response to changing synaptic inputs, thereby modifying a variety of cellular and synaptic activities. While the Ras-MAPK cascade in the limbic reward circuit is well-known to be sensitive to dopamine inputs, the sensitivity of its upstream activator (Ras-GRFs) to dopamine remains to be investigated. In this study, the response of Ras-GRFs in their protein expression to dopamine stimulation was evaluated in the rat striatum in vivo. A single systemic injection of the psychostimulant amphetamine produced an increase in Ras-GRF1 protein levels in both the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum. The increase in Ras-GRF1 proteins was dose-dependent. The reliable increase was seen 2.5h after drug injection and returned to normal levels by 6h. In contrast to Ras-GRF1, protein levels of Ras-GRF2 in the striatum were not altered by amphetamine. In addition to the striatum, the medial prefrontal cortex is another forebrain site where amphetamine induced a parallel increase in Ras-GRF1 but not Ras-GRF2. No significant change in Ras-GRF1/2 proteins was observed in the hippocampus. These data demonstrate that Ras-GRF1 is a susceptible and selective target of amphetamine in striatal and cortical neurons. Its protein expression is subject to the modulation by acute exposure of amphetamine.

Increased operant responding for ethanol in male C57BL/6J mice: specific regulation by the ERK1/2, but not JNK, MAP kinase pathway

RATIONALE

Extracellular signal-regulated protein kinase (ERK(1/2)) is a member of the mitogen-activated protein kinase (MAPK) signaling pathway and a key molecular target for ethanol (EtOH) and other drugs of abuse.

OBJECTIVE

The aim of the study was to assess the role of two MAPK pathways, ERK(1/2) and c-Jun N-terminal kinase (JNK), on the modulation of EtOH and sucrose self-administration.

MATERIALS AND METHODS

C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule with 9% EtOH/2% sucrose, or 2% sucrose, as the reinforcer. In experiments 1 and 2, mice were injected with the MEK(1/2) inhibitor SL 327 (0-100 mg/kg) and the JNK inhibitor AS 6012452 (0-56 mg/kg) prior to self-administration. In experiment 3, SL 327 (0-100 mg/kg) was administered prior to performance on a progressive ratio (PR) schedule of EtOH reinforcement. In experiment 4, SL 327 and AS 601245 were injected 2 h before a locomotor test.

RESULTS

SL 327 (30 mg/kg) significantly increased EtOH self-administration without affecting locomotion. Higher doses of SL 327 and AS 601245 reduced EtOH-reinforced responding and locomotor activity. Reductions of both ligands on sucrose self-administration were due to decreases in motor activity. SL 327 pretreatment had no effect on PR responding.

CONCLUSIONS

ERK(1/2) activity is more directly involved in modulating the reinforcing properties of EtOH than JNK activity due to its selective potentiation of EtOH-reinforced responding. The specificity of this effect to EtOH self-administration, rather than sucrose self-administration, suggests that the mechanism by which ERK(1/2) increases EtOH-reinforced responding does not generalize to all reinforcing solutions and is not due to increased motivation to consume EtOH.

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.

Modulation of AMPA receptor GluR1 subunit phosphorylation in neurons by the intravenous anaesthetic propofol

BACKGROUND

The ionotropic glutamate receptor is a potential molecular site in the central nervous system that general anaesthetics may interact with to produce some of their biological actions. Protein phosphorylation has been well documented to occur in the intracellular C-terminal domain of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptors, which represents a pivotal mechanism for the post-translational modulation of AMPA receptor functions. In this study, we investigated a possible influence of an i.v. anaesthetic agent propofol on the phosphorylation of AMPA receptor GluR1 subunits in cultured neurons.

METHODS

The effect of propofol on phosphorylation of GluR1 subunits at serine 831 and 845 was assayed in cultured rat striatal and cortical neurons by western blot with phospho- and site-specific antibodies.

RESULTS

Propofol consistently elevated phosphorylation of GluR1 subunits at the C-terminal serine 845 site in both striatal and cortical neurons. The elevation in phosphorylation was concentration-dependent and started at a low concentration (3 microM). This increase in serine 845 phosphorylation was rapid and sustained during the entire course of propofol exposure. In contrast to serine 845, phosphorylation of GluR1 at serine 831 was not altered by propofol in striatal and cortical neurons. Total GluR1 abundance remained unchanged in response to propofol incubation.

CONCLUSIONS

These data indicate that propofol possesses the ability to upregulate AMPA receptor GluR1 subunit phosphorylation at a specific serine 845 site in neurons and provide evidence supporting the AMPA receptor as a molecular target for general anaesthetics.

Transcriptional profiling of the rat frontal cortex following administration of the mGlu5 receptor antagonists MPEP and MTEP

The development of selective type 5 metabotropic glutamate receptor (mGlu5) antagonists, such as 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP), has revealed an important role for these receptors in various disorders of the nervous system including depression, anxiety, epilepsy, Parkinson's disease, drug addiction, and alcoholism. In this study, we used microarray technology to examine changes in gene expression induced by repeated administration of the mGlu5 antagonists MPEP and MTEP. Male Wistar rats (n=5 per treatment group) were administered MPEP (10 mg/kg), MTEP (10 mg/kg) or vehicle intraperitoneally twice daily for 5 days. Approximately 30 min following the final drug administration, rats were sacrificed and frontal cortices were then dissected and examined for changes in gene expression by cDNA microarray analysis. Changes in gene expression with p-values less than 0.01 were considered to be statistically significant. The expression of 63 genes was changed by both MPEP and MTEP, with 58 genes down-regulated and 5 genes up-regulated. Quantitative PCR verified the magnitude and direction of change in expression of 9 of these genes (r2=0.556, p=0.017). Pathway analysis revealed that many of the biological processes altered by repeated MPEP and MTEP treatment were related to ATP synthesis, hydrolase activity, and signaling pathways associated with mitogen-activated protein kinase (MAPK). Our results demonstrate diverse effects of MPEP and MTEP gene expression in the frontal cortex, and these results may help elucidate the mechanisms by which these compounds produce beneficial effects in animal models of various disorders of the central nervous system.

Long-lasting up-regulation of orexin receptor type 2 protein levels in the rat nucleus accumbens after chronic cocaine administration

Hypothalamic orexin (hypocretin) neurons project to the key structures of the limbic system and orexin receptors, both orexin receptor type 1 (OXR1) and type 2 (OXR2), are expressed in most limbic regions. Emerging evidence suggests that orexin is among important neurotransmitters that regulate addictive properties of drugs of abuse. In this study, we examined the effect of psychostimulant cocaine on orexin receptor protein abundance in the rat limbic system in vivo. Intermittent administration of cocaine (20 mg/kg, i.p., once daily for 5 days) caused a typical behavioral sensitization response to a challenge cocaine injection at a 14-day withdrawal period. Repeated cocaine administration at the same withdrawal time also increased OXR2 protein levels in the nucleus accumbens while repeated cocaine had no effect on OXR1 and orexin neuropeptide (both orexin-A and orexin-B) levels in this region. In contrast to the nucleus accumbens, OXR2 levels in the frontal cortex, the ventral tegmental area, the hippocampus, and the dorsal striatum (caudate putamen) were not altered by cocaine. Remarkably, the up-regulated OXR2 levels in the nucleus accumbens showed a long-lasting nature as it persisted up to 60 days after the discontinuation of repeated cocaine treatments. In contrast to chronic cocaine administration, an acute cocaine injection was insufficient to modify levels of any orexin receptor and peptide. Our data identify the up-regulation of OXR2 in the nucleus accumbens as an enduring molecular event that is correlated well with behavioral plasticity in response to chronic psychostimulant administration. This OXR2 up-regulation may reflect a key adaptation of limbic orexinergic transmission to chronic drug exposure and may thus be critical for the expression of motor plasticity.

Activator protein-1 responsive to the group II metabotropic glutamate receptor subtype in association with intracellular calcium in cultured rat cortical neurons

Activation of ionotropic glutamate (Glu) receptors, such as N-methyl-d-aspartate receptors, is shown to modulate the gene transcription mediated by the transcription factor activator protein-1 (AP1) composed of Fos and Jun family proteins in the brain, while little attention has been paid to the modulation of AP1 expression by metabotropic Glu receptors (mGluRs). In cultured rat cortical neurons, where constitutive expression was seen with all groups I, II and III mGluR subtypes, a significant and selective increase was seen in the DNA binding activity of AP1 120 min after the brief exposure to the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) for 5 min. In cultured rat cortical astrocytes, by contrast, a significant increase was induced by a group I mGluR agonist, but not by either a group II or III mGluR agonist. The increase by DCG-IV was significantly prevented by a group II mGluR antagonist as well as by either an intracellular Ca(2+) chelator or a voltage-sensitive Ca(2+) channel blocker, but not by an intracellular Ca(2+) store inhibitor. Moreover, DCG-IV significantly prevented the increase of cAMP formation by forskolin in cultured neurons. Western blot analysis revealed differential expression profiles of Fos family members in neurons briefly exposed to DCG-IV and NMDA. Prior or simultaneous exposure to DCG-IV led to significant protection against neuronal cell death by NMDA. These results suggest that activation of the group II mGluR subtype would modulate the gene expression mediated by AP1 through increased intracellular Ca(2+) levels in cultured rat cortical neurons.

In vivo regulation of Homer1a expression in the striatum by cocaine

The glutamate receptor adaptor protein Homer is concentrated in the postsynaptic density of excitatory synapses and is critical for normal operation of synaptic transmission. In this study, we investigated the responsiveness of Homer family proteins to dopamine stimulation with the psychostimulant cocaine in rat striatal neurons both in vivo and in vitro. We found that a single dose of cocaine specifically induced a rapid and transient increase in protein levels of the Homer1a, but not Homer1b/c and Homer2a/b, isoforms in the striatum. This selective Homer1a induction was mediated primarily through activation of dopamine D1, but not D2, receptors. Both protein kinase A and Ca(2+)/calmodulin-dependent protein kinases are important for mediating the cocaine stimulation of Homer1a expression. At the transcriptional level, cAMP response element-binding protein serves as a prime transcription factor transmitting the signals derived from D1 receptors and associative pathways to the CaCRE sites within the Homer1a promoter. From a functional perspective, non-cross-linking Homer1a, once induced, competed with the cross-linking isoforms of Homer proteins (Homer1b/c and Homer2a/b) to uncouple the connection of group I metabotropic glutamate receptors (mGluRs) with inositol-1,4,5-triphosphate receptors. These results indicate that cocaine possesses the ability to stimulate Homer1a expression in striatal neurons through a specific synapse-to-nucleus pathway. Moreover, inducible Homer1a expression may represent a transcription-dependent mechanism underlying the dynamic regulation of submembranous macromolecular complex formation between group I mGluRs and their anchoring proteins.

Protein kinase C-regulated cAMP response element-binding protein phosphorylation in cultured rat striatal neurons

The transcription factor cAMP response element-binding protein (CREB) promotes target DNA transcription in response to cellular stimulation in brain neurons. Phosphorylation of CREB is regulated by a variety of extracellular and intracellular signals. In this study, protein kinase C (PKC)-regulated CREB phosphorylation was investigated in cultured rat striatal neurons. We found that PKC activation with phorbol 12-myristate 13-acetate (PMA) produced a rapid and transient phosphorylation of CREB. The increase in CREB phosphorylation was dose-dependent and prevented by the two PKC selective inhibitors (chelerythrine and Gö6983). Interestingly, the PMA-induced CREB phosphorylation was also blocked by a calcium/calmodulin-dependent protein kinase inhibitor KN93 and the two mitogen-activated protein kinase (MAPK) kinase inhibitors PD98059 and U0126, but not by a p38 MAPK inhibitor SB203580. PMA activation of PKC markedly increased phosphorylation of MAPK/extracellular signal-regulated kinase 1/2. The protein kinase A (PKA) inhibitor H89 at a dose that completely blocked the PKA activator (8-br-cAMP)-induced CREB phosphorylation partially blocked the PMA-stimulated CREB phosphorylation. Furthermore, blockade of NMDA and AMPA glutamate receptors and L-type voltage-operated Ca(2+) channels did not alter the ability of PMA to induce CREB phosphorylation. These results demonstrate that PKC is among the protein kinases that can positively modulate CREB phosphorylation in striatal neurons, and the PKC signals to CREB activation are mediated via signaling mechanisms involving multiple downstream protein kinases.

Molecular signaling mechanisms underlying epileptogenesis

Epilepsy, a disorder of recurrent seizures, is a common and frequently devastating neurological condition. Available therapy is only symptomatic and often ineffective. Understanding epileptogenesis, the process by which a normal brain becomes epileptic, may help identify molecular targets for drugs that could prevent epilepsy. A number of acquired and genetic causes of this disorder have been identified, and various in vivo and in vitro models of epileptogenesis have been established. Here, we review current insights into the molecular signaling mechanisms underlying epileptogenesis, focusing on limbic epileptogenesis. Study of different models reveals that activation of various receptors on the surface of neurons can promote epileptogenesis; these receptors include ionotropic and metabotropic glutamate receptors as well as the TrkB neurotrophin receptor. These receptors are all found in the membrane of a discrete signaling domain within a particular type of cortical neuron--the dendritic spine of principal neurons. Activation of any of these receptors results in an increase Ca2+ concentration within the spine. Various Ca2+-regulated enzymes found in spines have been implicated in epileptogenesis; these include the nonreceptor protein tyrosine kinases Src and Fyn and a serine-threonine kinase [Ca2+-calmodulin-dependent protein kinase II (CaMKII)] and phosphatase (calcineurin). Cross-talk between astrocytes and neurons promotes increased dendritic Ca2+ and synchronous firing of neurons, a hallmark of epileptiform activity. The hypothesis is proposed that limbic epilepsy is a maladaptive consequence of homeostatic responses to increases of Ca2+ concentration within dendritic spines induced by abnormal neuronal activity.

Regulation of mitogen-activated protein kinases by glutamate receptors

Glutamate receptors regulate gene expression in neurons by activating intracellular signaling cascades that phosphorylate transcription factors within the nucleus. The mitogen-activated protein kinase (MAPK) cascade is one of the best characterized cascades in this regulatory process. The Ca(2+)-permeable ionotropic glutamate receptor, mainly the NMDA receptor subtype, activates MAPKs through a biochemical route involving the Ca(2+)-sensitive Ras-guanine nucleotide releasing factor, Ca(2+)/calmodulin-dependent protein kinase II, and phosphoinositide 3-kinase. The metabotropic glutamate receptor (mGluR), however, activates MAPKs primarily through a Ca(2+)-insensitve pathway involving the transactivation of receptor tyrosine kinases. The adaptor protein Homer also plays a role in this process. As an information superhighway between surface glutamate receptors and transcription factors in the nucleus, active MAPKs phosphorylate specific transcription factors (Elk-1 and CREB), and thereby regulate distinct programs of gene expression. The regulated gene expression contributes to the development of multiple forms of synaptic plasticity related to long-lasting changes in memory function and addictive properties of drugs of abuse. This review, by focusing on new data from recent years, discusses the signaling mechanisms by which different types of glutamate receptors activate MAPKs, features of each MAPK cascade in regulating gene expression, and the importance of glutamate/MAPK-dependent synaptic plasticity in memory and addiction.