Activation of microglial group III metabotropic glutamate receptors protects neurons against microglial neurotoxicity

Activation of metabotropic glutamate receptor 5 improves recovery after spinal cord injury in rodents

OBJECTIVE

Activation of metabotropic glutamate receptor 5 (mGluR5) has neuroprotective properties in vitro and has been reported to limit postischemic lesion volume in vivo. Previously, mGluR5 has been identified on microglia in vitro, but the effects of mGluR5 activation on inflammation in vivo or on recovery after spinal cord injury is unknown.

METHODS

Rats received intrathecal infusion of the selective mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) for 7 days after moderate impact spinal cord injury at T9. Complementary studies examined CHPG effects on activated spinal microglia cultures.

RESULTS

Functional motor recovery was significantly increased by CHPG treatment up to 28 days after injury, with improvements in weight bearing, step taking, and coordination of stepping behavior. CHPG treatment significantly reduced lesion volume and increased white matter sparing at 28 days after injury. Administration of CHPG attenuated microglial-associated inflammatory responses in a dose-dependent fashion, including expression of ED1, Iba-1, Galectin-3, NADPH oxidase components, tumor necrosis factor-alpha, and inducible nitric oxide synthase. Because mGluR5 is expressed by microglial cells in the rat spinal cord, such effects may be mediated by direct action on microglial cells. mGluR5 stimulation also reduced microglial activation and decreased microglial-induced neurotoxicity in spinal cord microglia cultures; the latter effects were blocked by the selective mGluR5 antagonist MTEP.

INTERPRETATION

These data demonstrate that mGluR5 activation can reduce microglial-associated inflammation, suggesting that the protective effects of mGluR5 agonists may reflect this action. Ann Neurol 2009;66:63-74.

Microglial physiology: unique stimuli, specialized responses

Microglia, the macrophages of the central nervous system parenchyma, have in the normal healthy brain a distinct phenotype induced by molecules expressed on or secreted by adjacent neurons and astrocytes, and this phenotype is maintained in part by virtue of the blood-brain barrier's exclusion of serum components. Microglia are continually active, their processes palpating and surveying their local microenvironment. The microglia rapidly change their phenotype in response to any disturbance of nervous system homeostasis and are commonly referred to as activated on the basis of the changes in their morphology or expression of cell surface antigens. A wealth of data now demonstrate that the microglia have very diverse effector functions, in line with macrophage populations in other organs. The term activated microglia needs to be qualified to reflect the distinct and very different states of activation-associated effector functions in different disease states. Manipulating the effector functions of microglia has the potential to modify the outcome of diverse neurological diseases.

Metabotropic glutamate receptor 5 activation inhibits microglial associated inflammation and neurotoxicity

The Group I metabotropic glutamate receptor 5 (mGluR5) can modulate addiction, pain, and neuronal cell death. Expression of some mGluRs, such as Group II and III mGluRs, has been reported in microglia and may affect their activation. However, the expression and role of mGluR5 in microglia is unclear. Using immunocytochemistry and Western blot, we demonstrate that mGluR5 protein is expressed in primary microglial cultures. Activation of mGluR5 using the selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) significantly reduces microglial activation in response to lipopolysaccharide, as indicated by a reduction in nitric oxide, reactive oxygen species, and TNFalpha production. Microglial induced neurotoxicity is also markedly reduced by CHPG treatment. The anti-inflammatory effects of CHPG are not observed in microglial cultures from mGluR5 knockout mice and are blocked by selective mGluR5 antagonists, suggesting that these actions are mediated by the mGluR5 receptor. Anti-inflammatory actions of mGluR5 activation are attenuated by phospholipase C and protein kinase C inhibitors, as well as by calcium chelators, suggesting that the mGluR5 activation in microglia involves the G(alphaq)-protein signal transduction pathway. These data indicate that microglial mGluR5 may represent a novel target for modulating neuroinflammation, an important component of both acute and chronic neurodegenerative disorders.

The organotin compounds trimethyltin (TMT) and triethyltin (TET) but not tributyltin (TBT) induce activation of microglia co-cultivated with astrocytes

The organotin compounds trimethyltin (TMT), triethyltin (TET) and tributyltin (TBT) show different organotoxicities in vivo. While TMT and TET induce a strong neurotoxicity accompanied by microglial and astroglial activation, TBT rather effects the immune system. Previously, we have shown in an in vitro co-culture model that microglial cells can be activated by TMT in the presence of astrocytes. In this study, we wanted to investigate (a) if the neurotoxic organotin compound TET can also activate microglial cells in vitro similar to TMT and (b) if differences between the neurotoxicants TMT and TET on the one side and TBT on the other exist concerning microglial activation. Therefore, purified microglial and astroglial cell cultures from neonatal rat brains were treated either alone or in co-cultures for 24h with different concentrations of TMT, TET or TBT and the basal cytotoxicity and nitric oxide formation was determined. Furthermore, morphological changes of astrocytes were examined. Our results show that microglial activation can be increased in subcytolethal concentrations, but only in the presence of astrocytes and not in microglial cell cultures alone. This increase was induced by the neurotoxicants TMT and TET but not by TBT. Taken together, the differing microglia activating effect of the organotin compounds may contribute to the differing neurotoxic potential of this group of chemicals in vivo. In addition, our results emphasize the need for co-culture systems when studying interactions between different cell types for toxicity assessment.

Activation of metabotropic glutamate receptor 5 modulates microglial reactivity and neurotoxicity by inhibiting NADPH oxidase

Microglial-related factors have been implicated in the signaling cascades that contribute to neuronal cell death in various neurodegenerative disorders. Thus, strategies that reduce microglial activation and associated neurotoxicity may have therapeutic benefit. Group II and III metabotropic glutamate receptors (mGluRs) are expressed in microglia and can modulate microglial activity in primary cell cultures. We demonstrate that the group I receptor member mGluR5 is highly expressed in primary microglial cultures and the BV2 microglial cell line. Activation of mGluR5 using the selective agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) significantly attenuates microglial activation in response to lipopolysaccharide and interferon-gamma, as indicated by a reduction in the expression of inducible nitric-oxide synthase, production of nitric oxide and tumor necrosis factor-alpha, and intracellular generation of reactive oxygen species. In addition, microglial-induced neurotoxicity is also markedly reduced by CHPG treatment. The anti-inflammatory effects of CHPG are mediated by the mGluR5 receptor, because either a selective mGluR5 antagonist or small interference RNA knockdown attenuated the actions of this drug. CHPG blocked the lipopolysaccharide-induced increase in expression and enzymatic activity of NADPH oxidase. Moreover, the protective effects of CHPG were significantly reduced when the NADPH oxidase subunits p22(phox) or gp91(phox) were knocked down by small interference RNA. These data suggest that mGluR5 represents a novel target for modulating microglial-dependent neuroinflammation, and may have therapeutic relevance for neurological disorders that exhibit microglial-mediated neurodegeneration.

Iron-enhanced paraquat-mediated dopaminergic cell death due to increased oxidative stress as a consequence of microglial activation

Environmental paraquat and neonatal iron exposure have both separately been suggested as potential risk factors for sporadic forms of Parkinson's disease (PD). In this study, we demonstrate that combined environmental exposure to these two agents results in modulations in microglial activation state. Apocynin, an NADPH oxidase inhibitor, was found to attenuate the release of superoxide from microglia stimulated by combined paraquat and iron and blocked paraquat-induced dopaminergic neuronal death. Furthermore, pretreatment with the synthetic superoxide dismutase/catalase mimetic, EUK-189, significantly decreased microglial activation mediated by combined paraquat and iron treatment. These findings support the notion that environmental PD risk factors may act synergetically to produce neurodegeneration associated with the disorder and that iron and paraquat may act via common oxidative stress-mediated mechanism involving microglial activation.

Metabotropic glutamate receptors as targets for multipotential treatment of neurological disorders

Glutamate is a major excitatory neurotransmitter in the CNS that is involved in numerous cellular functions, including cell death and survival. Metabotropic glutamate receptors (mGluR) are G-protein coupled receptors that have been classified into three groups on the basis of signal transduction pathways and pharmacological profiles. Group I, II, and III mGluRs are found on cell types within and peripheral to the CNS, including neurons, microglia, astrocytes, oligodendrocytes, T- and B-cell lymphocytes, osteoblasts, hepatocytes, and endothelial cells, among others. These receptors have a number of effects on cells that can influence outcome after trauma, including reducing neuronal and oligodendroglial cell death, inflammation, and endothelial permeability. Thus, mGluRs are a promising multipotential therapeutic approach. Because the pathology of CNS trauma and neurodegeneration is multifactorial (including, for example, oxidative stress, mitochondrial breakdown, and inflammation), therapies that serve to modulate multiple pathophysiological pathways may prove more effective than those directed at a single target. This review examines the multipotential therapeutic utility of mGluR modulation in acute and chronic injury and neurodegeneration.

Temporal correlation of the memory deficit with Alzheimer-like lesions induced by activation of glycogen synthase kinase-3

We have reported that activation of glycogen synthase kinase-3 (GSK-3) by ventricle injection of wortmannin (WT) and GF-109203X (GFX) induces Alzheimer-like memory deficit in rats [Liu et al., J. Neurochem. 87 (2003), 1333]. To further explore the factors responsible for the memory loss, we studied here the temporal alterations of GSK-3, tau phosphorylation, beta-amyloid (Abeta), and acetylcholine (ACh) after injection of WT/GFX, and analyzed their correlation with the memory loss. We observed that the severe memory deficit occurred at 24 and 48 h, and simultaneously, GSK-3 activation, tau hyperphosphorylation at Thr231, Ser396, and Ser404 and decline of ACh in hippocampus were detected, and these changes were mostly recovered at 72 and 96 h after the injection of WT/GFX. Remarkable increase of Abeta and intracellular accumulation of argentophilic substances were detected at 72 h. Pearson analysis showed that the memory deficit was correlated with GSK-3 activation, tau hyperphosphorylation, and decline of ACh but not with Abeta overproduction. Our data provide direct evidence demonstrating that activation of GSK-3 by WT/GFX may cause memory deficit through tau hyperphosphorylation and suppression of ACh in hippocampus.

Myelin-induced microglial neurotoxicity can be controlled by microglial metabotropic glutamate receptors

Microglia are present in an activated state in multiple sclerosis lesions. Incubation of primary cultured rat microglia with rat-brain derived myelin (0.1-1 microg/mL) for 24 h induced microglial activation; cells displayed enhanced ED1 staining, expression of inducible nitric oxide synthase, production and release of the cytokine tumour necrosis factor-alpha and glutamate release. Exposure of microglia to myelin induced the expression of neuronal caspases and ultimately neuronal death in cultured cerebellar granule cell neurons; neurotoxicity was directly because of microglial-derived soluble toxins. Co-incubation of microglia with agonists or antagonists of different metabotropic glutamate receptor (mGluR) subtypes ameliorated microglial neurotoxicity by inhibiting soluble neurotoxin production. Activation of microglial mGluR2 exacerbated myelin-evoked neurotoxicity whilst activation of mGluR3 was protective as was activation of group III mGluRs. These data show that myelin-induced microglial neurotoxicity can be prevented by regulation of mGluRs and suggest these receptors on microglia may be promising targets for therapeutic intervention in multiple sclerosis.

Low doses of cannabinoids enhance the antinociceptive effects of intracisternally administered mGluRs groups II and III agonists in formalin-induced TMJ nociception in rats

This study provides the first demonstration that central cannabinoids modulate the antinociceptive actions of metabotropic glutamate receptors (mGluRs) on formalin-induced temporomandibular joint (TMJ) nociception. Noxious scratching behavior induced by formalin injection in the TMJ was used as a model of pain. Intracisternal injection of 30mug of WIN 55,212-2, a non-subtype selective cannabinoid receptor agonist, attenuated the number of scratches by 75% as compared with the vehicle-treated group, whereas vehicle alone or 3 or 10 microg of WIN 55,212-2 had no effect. To explore the postulated interaction between central cannabinoid receptors and mGluRs, effects of combined administration of sub-analgesic doses of WIN 55,212-2 and group II or III mGluR agonists were tested. Group II or III mGluRs agonists were administered intracisternally 10 min after intracisternal administration of WIN 55,212-2. Neither 100 nmol APDC, a group II mGluRs agonist, nor L-AP4, a group III mGluR agonist, altered nociceptive behavior when given alone but significantly inhibited the formalin-induced nociceptive behavior in the presence of a sub-threshold dose ( 3microg) of WIN 55,212-2. The ED50 value of APDC or L-AP4 was significantly reduced upon co-treatment with WIN 55,212-2 than in the vehicle-treated group, highlighting the important therapeutic potential of the combined administration of group II or III mGluR agonists with cannabinoids to effectively treat inflammatory pain associated with the TMJ. Potentiating effects of group II or III mGluRs agonists will likely permit the administration of cannabinoids at doses that do not achieve significant accumulation to produce undesirable motor dysfunction.

Anatomy and function of group III metabotropic glutamate receptors in gastric vagal pathways

Metabotropic glutamate receptors (mGluR) are classified into groups I (excitatory), II and III (inhibitory) mGluR. Activation of peripheral group III mGluR (mGluR4, mGluR6, mGluR7, mGluR8), particularly mGluR8, inhibits vagal afferent mechanosensitivity in vitro which translates into reduced triggering of transient lower oesophageal sphincter relaxations and gastroesophageal reflux in vivo. However, the expression and function of group III mGluR in central gastrointestinal vagal reflex pathways is not known. Here we assessed the expression of group III mGluR in identified gastric vagal afferents in the nodose ganglion (NG) and in the dorsal medulla. We also determined the central action of the mGluR8a agonist S-3,4-DCPG (DCPG) on nucleus tractus solitarius (NTS) neurons with gastric mechanosensory input in vivo. Labelling for mGluR4 and mGluR8 was abundant in gastric vagal afferents in the NG, at their termination site in the NTS (subnucleus gelatinosus) and in gastric vagal motorneurons, while labelling for mGluR6 and mGluR7 was weaker in these regions. DCPG (0.1 nmol or 0.001-10 nmol i.c.v.) inhibited or markedly attenuated responses of 8/10 NTS neurons excited by isobaric gastric distension with no effect on blood pressure or respiration; 2 NTS neurons were unaffected. The effects of DCPG were significantly reversed by the group III mGluR antagonist MAP4 (10 nmol, i.c.v.). In contrast, 4/4 NTS neurons inhibited by gastric distension were unaffected by DCPG. We conclude that group III mGluR are expressed in peripheral and central vagal pathways, and that mGluR8 within the NTS selectively reduce excitatory transmission along gastric vagal pathways.

Primary Neuron/Astrocyte Co-Culture on Polyelectrolyte Multilayer Films: A Template for Studying Astrocyte-Mediated Oxidative Stress in Neurons

We engineered patterned co-cultures of primary neurons and astrocytes on polyelectrolyte multilayer (PEM) films without the aid of adhesive proteins/ligands to study the oxidative stress mediated by astrocytes on neuronal cells. A number of studies have explored engineering co-culture of neurons and astrocytes predominantly using cell lines rather than primary cells owing to the difficulties involved in attaching primary cells onto synthetic surfaces. To our knowledge this is the first demonstration of patterned co-culture of primary neurons and astrocytes for studying neuronal metabolism. In our study, we used synthetic polymers, namely poly(diallyldimethylammoniumchloride) (PDAC) and sulfonated poly(styrene) (SPS) as the polycation and polyanion, respectively, to build the multilayers. Primary neurons attached and spread preferentially on SPS surfaces, while primary astrocytes attached to both SPS and PDAC surfaces. SPS patterns were formed on PEM surfaces, either by microcontact printing SPS onto PDAC surfaces or vice-versa, to obtain patterns of primary neurons and patterned co-cultures of primary neurons and astrocytes. We further used the patterned co-culture system to study the neuronal response to elevated levels of free fatty acids as compared to the response in separated monoculture by measuring the level of reactive oxygen species (ROS; a widely accepted marker of oxidative stress). The elevation in the ROS levels was observed to occur earlier in the patterned co-culture system than in the separated monoculture system. The results suggest that this technique may provide a useful tool for engineering neuronal co-culture systems, that may more accurately capture neuronal function and metabolism, and thus could be used to obtain valuable insights into neuronal cell function and perhaps even the pathogenesis of neurodegenerative diseases.

Neuronal 'On' and 'Off' signals control microglia

Recent findings indicate that neurons are not merely passive targets of microglia but rather control microglial activity. The variety of different signals that neurons use to control microglia can be divided into two categories: 'Off' signals constitutively keep microglia in their resting state and antagonize proinflammatory activity. 'On' signals are inducible and include purines, chemokines, glutamate. They instruct microglia activation under pathological conditions towards a beneficial or detrimental phenotype. Various neuronal signaling molecules thus actively control microglia function, thereby contribute to the inflammatory milieu of the central nervous system. Thus, neurons should be envisaged as key immune modulators in the brain.

Neurotransmitter receptors on microglia

Microglia are the intrinsic immune cells of the brain and express chemokine and cytokine receptors that interact with the peripheral immune cells. Recent studies have indicated that microglia also respond to the brain's classical signalling substances, the neurotransmitters. Here, we review the evidence for the expression of neurotransmitter receptors on microglia and the consequences of this receptor activation for microglial behaviour. It is evident that neurotransmitters instruct microglia to perform distinct types of responses, such as triggering an inflammatory cascade or acquiring a neuroprotective phenotype. Understanding how microglia respond to different neurotransmitters will thus have important implications for controlling the reactivity of these cells in acute injury, as well as for treating chronic neurodegenerative diseases.

Expression of insulin-like growth factor-II and leukemia inhibitory factor antibody immunostaining on the ionized calcium-binding adaptor molecule 1-positive microglias in the spinal cord of amyotrophic lateral sclerosis patients

Amyotrophic lateral sclerosis (ALS) is a progressive degenerative disease involving the upper and lower motor neuron systems. Activated microglia are reported to enhance motor neuron death by secreting neurotoxic cytokines in SOD1-transgenic mice. Recent studies have provided evidence that chronic stimulation leads microglia to acquire an anti-inflammatory phenotype, characterized by activated morphology and induction of neuroprotective and immunoregulatory molecules. However, little information is available on the protective functions of microglia in the ALS spinal cord. To investigate the roles of microglia in ALS, we examined the appearance of ionized calcium-binding adaptor molecule 1-positive (Iba1-positive) microglia as correlated to the disease duration and immunohistochemical expression of neurogrowth factors in the ALS spinal cord. In this study, the number of Ibal-positive rod-like microglia significantly increased in the ALS spinal cord compared to controls. The number of ramified microglia was positively correlated with the number of normal-looking neurons and clinical duration of ALS patients; however, the number of rod-like microglia was not correlated with that of abnormal neurons, nor with the clinical duration of the disease. Some rod-like microglia were positive for anti-insulin-like growth factor-II (IGF II) and anti-leukemia inhibitory factor (LIF) immunostaining. Motor neurons in the ALS spinal cords also showed immunore-activity for IGF-II, LIF and the receptors of IGF-II and LIE Taken together, these findings suggest that at least some microglia might have a protective effect on motor neurons in the ALS spinal cord. Neuroprotective and/or neurotoxic effects of microglia on motor neurons should be further studied.

Colocalization of metabotropic glutamate receptors in rat dorsal root ganglion cells

Glutamate is the main excitatory transmitter in both central and peripheral nervous systems. Discovery of metabotropic glutamate receptors (mGluRs) made it clear that glutamate can have excitatory or inhibitory effects on neuronal function, with group I mGluRs enhancing cell excitability but group II and III mGluRs decreasing excitability. The present study investigated the colocalization of mGluR subtypes representing groups I, II, or III in rat L5 dorsal root ganglion (DRG) cells. The analyses show that group III has the highest expression, with 75.0% of DRG cells expressing mGluR8, followed by group II, with 51.6% expressing mGluR2/3, followed by group I, with only 6.8% expressing mGluR1alpha. mGluR8 is expressed by small, medium, and large diameter cells. In contrast, mGluR1alpha and mGluR2/3 are expressed by mainly small and medium cells. Approximately half of cells expressing group I mGluR1alpha also express either group II mGluR2/3 or group III mGluR8. These mGluR1alpha double-labeled populations are not likely to overlap since >1.0% of mGluR1alpha are triple-labeled. As expected from the high percentage of single-labeled mGluR2/3 and mGluR8 cells, there is a considerable population of double-labeled cells with approximately 30% of each population expressing both receptors. Due to the fact that the number of mGluR1alpha-expressing cells in the DRG is low, the percentage of triple-labeled cells is also low ( approximately 1-2%). The prevalence of groups II and III indicate that glutamate could have a substantial inhibitory effect of primary afferent function, reducing and/or fine-tuning sensory input before transmission to the spinal cord. These anatomical data highlight the potential inhibitory role glutamate may play in peripheral sensory transmission.

Chromogranin A activates diverse pathways mediating inducible nitric oxide expression and apoptosis in primary microglia

Chromogranin A (CgA) is associated with microglial activation cascades implicated in neurodegeneration in Alzheimer's, Pick's and Parkinson's diseases. In primary rat microglia, CgA-mediated inducible nitric oxide (iNOS) expression, nitric oxide (NO) production, mitochondrial depolarisation and apoptosis were inhibited by PP2 (Src kinase inhibitor). CgA-mediated iNOS expression and NO production were also inhibited by U0126 (MEK inhibitor), but mitochondrial depolarisation and apoptosis were not. PP2 inhibited ERK phosphorylation; therefore, Src mediates CgA-induced ERK phosphorylation leading to iNOS expression and NO production. Glutamate release induced by CgA was independent of both pathways. These findings provide insights into the way microglia are activated by CgA and the microglial signalling mechanisms associated with neurological disorders such as Alzheimer's disease.

Central metabotropic glutamate receptors differentially participate in interleukin-1beta-induced mechanical allodynia in the orofacial area of conscious rats

The present study investigated the role of central metabotropic glutamate receptors (mGluRs) in interleukin-1beta (IL-1beta)-induced mechanical allodynia and mirror-image mechanical allodynia in the orofacial area. Experiments were carried out on male Sprague-Dawley rats weighing 230 to 280 g. After administration of 0.01, 0.1, 1, or 10 pg of IL-1beta into a subcutaneous area of the vibrissa pad, we examined the withdrawal behavioral responses produced by 10 successive trials of an air-puff ramp pressure applied ipsilaterally or contralaterally to the IL-1beta injection site. Subcutaneous injection of IL-1beta produced mechanical allodynia and mirror-image mechanical allodynia in the orofacial area. Intracisternal administration of CPCCOEt, a mGluR1 antagonist, or MPEP, a mGluR5 antagonist, reduced IL-1beta-induced mechanical allodynia and mirror-image mechanical allodynia. Intracisternal administration of APDC, a group II mGluR agonist, or L-AP4, a group III mGluR agonist, reduced both IL-1beta-induced mechanical allodynia and mirror-image mechanical allodynia. The antiallodynic effect, induced by APDC or L-AP4, was blocked by intracisternal pretreatment with LY341495, a group II mGluR antagonist, or CPPG, a group III mGluR antagonist. These results suggest that groups I, II, and III mGluRs differentially modulated IL-1beta-induced mechanical allodynia, as well as mirror-image mechanical allodynia, in the orofacial area.

PERSPECTIVE

Central group I mGluR antagonists and groups II and III mGluR agonists modulate IL-1beta-induced mechanical allodynia and mirror-image mechanical allodynia in the orofacial area. Therefore, the central application of group I mGluR antagonists or groups II and III mGluR agonists might be of therapeutic value in treating pain disorder.

Selective Activation of Group III Metabotropic Glutamate Receptors by l-(+)-2-Amino-4-phosphonobutryic Acid Protects the Nigrostriatal System against 6-Hydroxydopamine Toxicity in Vivo

Evidence from several studies suggests that the progressive degeneration of dopaminergic (DA) neurones of the substantia nigra pars compacta (SNc) in Parkinson's disease (PD) may in part be due to excessive release of glutamate from subthalamic projections onto nigral DA neurones. Previous in vitro studies have demonstrated that selective activation of Group III metabotropic glutamate receptors (mGluR) negatively modulates excitatory transmission in the SNc and is neuroprotective against glutamate-mediated toxicity. Consistent with this, we have reported preliminary data indicating that the selective group III mGluR agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) can also protect the nigrostriatal system against 6-hydroxydopamine (6-OHDA) toxicity in vivo. We have now extended these preliminary studies in this model and report here that both acute and subchronic intranigral injections of l-AP4 provide significant protection of the nigrostriatal system against 6-OHDA toxicity. This neuroprotection displays a bell-shaped profile with a clear concentration-dependent relationship. In contrast, when administered to animals 7 days post-6-OHDA lesioning, l-AP4 significantly protects the functionality but not the integrity of the nigrostriatal system. We further demonstrate that neuroprotection by l-AP4 in vivo is reversed by coadministration of the selective Group III mGluR antagonist (RS)-alpha-methylserine-O-phosphate, confirming a receptor-mediated mechanism of action. These data provide further compelling evidence that selective activation of Group III mGluR is neuroprotective in an in vivo experimental model of PD, a finding that may have important implications for the future treatment of this disease.

Kainate induces rapid redistribution of the actin cytoskeleton in ameboid microglia

Microglia are key mediators of the immune response in the central nervous system (CNS). They are closely related to macrophages and undergo dramatic morphological and functional changes after CNS trauma or excitotoxic lesions. Microglia can be directly stimulated by excitatory neurotransmitters and are known to express many neurotransmitter receptors. The role of these receptors, however, is not clear. This study describes the microglial response to the glutamate receptor agonist kainate (KA) and shows via immunochemistry that the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate receptor subunit GluR1 is present on cultured microglia. In the presence of 100 microM or 1 mM KA, cultured microglia underwent dramatic morphological and cytoskeletal changes as observed by time-lapse photography and quantitative confocal analysis of phalloidin labeling. KA-stimulated microglia showed condensation of cytoplasmic actin filaments, rapid de- and repolymerization, and cytoplasmic redistribution of condensed actin bundles. Rearrangement of actin filaments-thought to be involved in locomotion and phagocytosis and to indicate an increased level of activation (for reviews see Greenberg [ 1995] Trends Cell Biol. 5:93-99; Imai and Kohsaka [ 2002] Glia 40:164-174)-was significantly increased in treated vs. control cultures. Morphological plasticity and membrane ruffling were also seen. These findings suggest direct microglial excitation via glutamate receptor pathways. Thus, neurotransmitter release after brain or spinal cord injury might directly modulate the inflammatory response.

Metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlus) are a family of G-protein-coupled receptors activated by the neurotransmitter glutamate. Molecular cloning has revealed eight different subtypes (mGlu1-8) with distinct molecular and pharmacological properties. Multiplicity in this receptor family is further generated through alternative splicing. mGlus activate a multitude of signalling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of neurotransmitter release. In this review, we summarize anatomical findings (from our work and that of other laboratories) describing their distribution in the central nervous system. Recent evidence regarding the localization of these receptors in peripheral tissues will also be examined. The distinct regional, cellular and subcellular distribution of mGlus in the brain will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.

Spatially controlled co-culture of neurons and glial cells

The ability to create and maintain neuron and glial cell co-cultures is important for neuronal regeneration as well as for fundamental studies on neuron and glial cell interactions. We demonstrate here a method for spatially controlling the arrangement of neurons and glial cells. Line patterns of cell resistant, poly(oligoethyleneglycol methacrylate-co-methacrylic acid), was microcontact printed on various substrates to spatially control the attachment of neurons. Neuron-like cells, PC12 and SH-SY5Y cells, were confined within the unprinted line patterns and extended neurites along the line patterns. Subsequent attachment of glial cells was accomplished by converting the originally cell-resistant line patterns of poly(oligoethyleneglycol methacrylate-co-methacrylic acid) to cell adhesive by electrostatic adsorption of cationic poly-lysine, chitosan, or poly(ethyleneimine). This method for creating patterned co-cultures of neuron and glial cells provides a useful tool for investigating neuron-glial cell interactions and has potential applications in the repair or regeneration of nervous systems.

Activation of Group II/III metabotropic glutamate receptors attenuates LPS-induced astroglial neurotoxicity via promoting glutamate uptake

Altered glial function that leads to oxidative stress and excitotoxicity may contribute to the initiation or progression of neuronal death in neurodegenerative diseases. We report the pivotal role of astroglial Group II and III metabotropic glutamate receptors (mGluR) against neurotoxicity. Activation of Group II or III mGluR on astrocytes with selective agonists DCG-IV or L-AP4 respectively inhibited astroglial lipopolysaccharide (LPS)-conditioned medium induced apoptosis of primary cultured mesencephalic neurons. Specific Group II or III mGluR antagonists APICA or MSOP completely abolished the neuroprotective effects of DCG-IV and L-AP4. Morphologic analysis showed that DCG-IV or L-AP4 could also attenuate the astroglial neurotoxicity to dopaminergic neurons. Measurement of extracellular glutamate concentration and [(3)H]-glutamate uptake showed that the restoration of glutamate uptake capability in LPS-treated astrocytes might be involved in the neuroprotective effects of activating astroglial Group II or III mGluR. Furthermore, we found that the repression of astroglial uptake function could be revived by GSH, and both Group II and III mGluR agonists could recover the endogenous reduced glutathione (GSH) level in LPS-treated astrocytes. These results suggested that the possible mechanisms of neuroprotection by either Type II or Type III mGluR activation may involve restoration of endogenous GSH, in turn affording recovery of astroglial capability to take up glutamate.

Microglial control of neuronal death and synaptic properties

Microglia have long been characterized by their immune function in the nervous system and are still mainly considered in a beneficial versus detrimental dialectic. However a review of literature enables to shed novel lights on microglial function under physiological conditions. It is now relevant to position these cells as full time partners of neuronal function and more specifically of synaptogenesis and developmental apoptosis. Indeed, microglia can actively control neuronal death. It has actually been shown in retina that microglial nerve growth factor (NGF) is necessary for the developmental apoptosis to occur. Similarly, in cerebellum, microglia induces developmental Purkinje cells death through respiratory burst. Furthermore, in spinal cord, microglial TNFalpha commits motoneurons to a neurotrophic dependent developmental apoptosis. Microglia can also control synaptogenesis. This is suggested by the fact that a mutation in KARAP/DAP12, a key protein of microglial activation impacts synaptic functions in hippocampus, and synapses protein content. In addition it has been now demonstrated that microglial brain-derived neurotrophin factor (BDNF) directly regulates synaptic properties in spinal cord. In conclusion, microglia can control neuronal function under physiological conditions and it is known that neuronal activity reciprocally controls microglial activation. We will discuss the importance of this cross-talk which allows microglia to orchestrate the balance between synaptogenesis and neuronal death occurring during development or injuries.

Effect of metabotropic glutamate receptor agonists and signal transduction modulators on feeding by a caterpillar

Feeding in codling moth caterpillars was induced by the general glutamate receptor activator monosodium glutamate (MSG) and by three different mGluR agonists known to specifically stimulate different classes of vertebrate metabotropic glutamate receptors, including: (1S,3R)-ACPD, which stimulates group I mGluRs (2R,4R)-APDC, which stimulates group II mGluRs and L-AP4, which stimulates some group III mGluRs. Experiments exposing larvae to combinations of specific mGluR agonists and specific signal transduction modulators suggest that each tested mGluR uses a different signaling pathway. First, feeding stimulatory effects of (1S,3R)-ACPD were abolished by phospholipase C inhibitor, U 73122, but remained unaffected by adenylate cyclase activator, NKH 477, or phosphodiesterase inhibitor, Rolipram. Second, (2R,4R)-APDC induced feeding in presence of U 73122 or Rolipram, but lost its feeding stimulatory effects in presence of NKH 477. Finally, L-AP4 did not induce feeding in presence of Rolipram, but maintained its feeding stimulatory effects in presence of U 73122 or NKH 477. The activity of the general glutamate receptor activator MSG was abolished by NKH 477, and Rolipram. U 73122 did not affect MSG-stimulated feeding. These results suggest that transduction of MSG taste in the codling moth caterpillar relies mostly on cAMP-dependent signaling pathways.

Chromogranin-mediated secretion of mutant superoxide dismutase proteins linked to amyotrophic lateral sclerosis

Here we report that chromogranins, components of neurosecretory vesicles, interact with mutant forms of superoxide dismutase (SOD1) that are linked to amyotrophic lateral sclerosis (ALS), but not with wild-type SOD1. This interaction was confirmed by yeast two-hybrid screen and by co-immunoprecipitation assays using either lysates from Neuro2a cells coexpressing chromogranins and SOD1 mutants or lysates from spinal cord of ALS mice. Confocal and immunoelectron microscopy revealed a partial colocalization of mutant SOD1 with chromogranins in spinal cord of ALS mice. Mutant SOD1 was also found in immuno-isolated trans-Golgi network and in microsome preparations, suggesting that it can be secreted. Indeed we report evidence that chromogranins may act as chaperone-like proteins to promote secretion of SOD1 mutants. From these results, and our finding that extracellular mutant SOD1 can trigger microgliosis and neuronal death, we propose a new ALS pathogenic model based on the toxicity of secreted SOD1 mutants.

Oxidative stress and inflammation in Parkinson's disease: is there a causal link?

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a dramatic loss of dopaminergic neurons in the substantia nigra (SN). Among the many pathogenic mechanisms thought to contribute to the demise of these cells, dopamine-dependent oxidative stress has classically taken center stage due to extensive experimental evidence showing that dopamine-derived reactive oxygen species and oxidized dopamine metabolites are toxic to nigral neurons. In recent years, however, the involvement of neuro-inflammatory processes in nigral degeneration has gained increasing attention. Not only have activated microglia and increased levels of inflammatory mediators been detected in the striatum of deceased PD patients, but a large body of animal studies points to a contributory role of inflammation in dopaminergic cell loss. Recently, postmortem examination of human subjects exposed to the parkinsonism-inducing toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), revealed the presence of activated microglia decades after drug exposure, suggesting that even a brief pathogenic insult can induce an ongoing inflammatory response. Perhaps not surprisingly, non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to reduce the risk of developing PD. In the past few years, various pathways have come to light that could link dopamine-dependent oxidative stress and microglial activation, finally ascribing a pathogenic trigger to the chronic inflammatory response characteristic of PD.

Dopamine and noradrenaline control distinct functions in rodent microglial cells

Microglial cells are the immune-competent elements of the brain. They not only express receptors for chemokines and cytokines but also for neurotransmitters such as GABA [Charles et al., Mol. Cell Neurosci. 24 (2003) 214], glutamate [Noda et al., J. Neurosci. 20 (2000) 251], and adrenaline [Mori et al., Neuropharmacology 43 (2002) 1026]. Here we report the functional expression of dopamine receptors in mouse and rat microglia, in culture and brain slices. Using the patch clamp technique as the functional assay we identified D1- and D2-like dopamine receptors using subtype-specific ligands. They triggered the inhibition of the constitutive potassium inward rectifier and activated potassium outward currents in a subpopulation of microglia. Chronic dopamine receptor stimulation enhanced migratory activity and attenuated the lipopolysaccharide (LPS)-induced nitric oxide (NO) release similar as by stimulation of adrenergic receptors. While, however, noradrenaline attenuated the LPS-induced release of TNF-alpha and IL-6, dopamine was ineffective in modulating this response. We conclude that microglia express dopamine receptors which are distinct in function from adrenergic receptors.

Fibrin-modifying serine proteases thrombin, tPA, and plasmin in ischemic stroke: a review

Ischemic stroke is a sudden loss of circulation to a portion of the brain that results in a loss of neurologic function. Many ischemic strokes are embolic. They result from a thrombus traveling into the central circulation and occluding a blood vessel. Treatment of ischemic stroke with recombinant tissue plasminogen activator (tPA) can improve patient outcomes. However, tPA must be used during a specific time window after the stroke onset to be effective and it risks converting an ischemic stroke into a hemorrhagic one. We explore the basic effects of fibrin-modifying proteases on neurons, astrocytes, and microglia during ischemia. tPA, thrombin, and plasmin can initiate microglial activation and change both neuronal and astrocytic survival. As a result of these functions and of their role in blood homeostasis, all three of these proteases have profound effects on neurons and glial cells in the brain and are capable of altering the development and severity of ischemic stroke.

Microglia is activated by astrocytes in trimethyltin intoxication

Microglia participates in most acute and chronic neuropathologies and its activation appears to involve interactions with neurons and other glial cells. Trimethyltin (TMT)-induced brain damage is a well-characterized model of neurodegeneration, in which microglial activation occurs before neuronal degeneration. The aim of this in vitro study was to investigate the role of astroglia in TMT-induced microgliosis by using nitric oxide (NO), inducible NO synthase (iNOS), and morphological changes as parameters for microglial activation. Our investigation discusses (a) whether microglial cells can be activated directly by TMT; (b) if astroglial cells are capable of triggering or modulating microglial activation; (c) how the morphology and survival of microglia and astrocytes are affected by TMT treatment; and (d) whether microglial-astroglial interactions depend on direct cell contact or on soluble factors. Our results show that microglia are more vulnerable to TMT than astrocytes are and cannot be activated directly by TMT with regard to the examined parameters. In bilayer coculture with viable astroglial cells, microglia produce NO in significant amounts at subcytotoxic concentrations of TMT (20 micromol/l). At these TMT concentrations, microglial cells in coculture convert into small round cells without cell processes, whereas flat, fibroblast-like astrocytes convert into thin process bearing stellate cells with a dense and compact cell body. We conclude that astrocytes trigger microglial activation after treatment with TMT, although the mechanisms of this interaction remain unknown.

Oxidative stress in the brain: novel cellular targets that govern survival during neurodegenerative disease

Despite our present knowledge of some of the cellular pathways that modulate central nervous system injury, complete therapeutic prevention or reversal of acute or chronic neuronal injury has not been achieved. The cellular mechanisms that precipitate these diseases are more involved than initially believed. As a result, identification of novel therapeutic targets for the treatment of cellular injury would be extremely beneficial to reduce or eliminate disability from nervous system disorders. Current studies have begun to focus on pathways of oxidative stress that involve a variety of cellular pathways. Here we discuss novel pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Current work has identified exciting pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and their downstream substrates that include Forkhead transcription factors, glycogen synthase kinase-3beta, mitochondrial dysfunction, Bad, and Bcl-x(L). Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation. New therapeutic avenues that are just open to exploration, such as with brain temperature regulation, nicotinamide adenine dinucleotide modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat a variety of disorders that include stroke, Alzheimer's disease, and traumatic brain injury.

Pure albumin is a potent trigger of calcium signalling and proliferation in microglia but not macrophages or astrocytes

Microglial activation is implicated in the neurotoxicity of neurodegenerative diseases. Raised intracerebral levels of albumin are associated with the pathology of Alzheimer's disease, multiple sclerosis, and stroke where blood-brain barrier damage is evident. We report here that treatment of primary cultured microglia and the N9 microglial cell line with pure albumin, or albumin in which fatty acids and immunoglobulins remain attached (fraction V), induced a rise in intracellular calcium. This rise in intracellular calcium was mediated via Src tyrosine kinase and phospholipase C. The albumin-induced calcium response was coupled to microglial proliferation, which was prevented by BAPTA, U73122 or PP2 but not mimicked by thapsigargin. In contrast, peritoneal macrophages were resistant to albumin- or fraction V-induced calcium responses and proliferation, whilst primary cultured astrocytes or the TSA-3 astrocyte cell line were responsive to fraction V albumin but not pure albumin. Furthermore, cerebellar granule neurones did not respond to albumin. These data suggest that albumin may play a role in microglial activation in pathological situations involving blood-brain barrier impairment, and that the specific responses of microglia to albumin allow a distinction to be made between the signalling responses of microglia, blood-borne macrophages, astrocytes and neurones.

Differential roles of mGlu8 receptors in the regulation of glutamate and gamma-aminobutyric acid release at periaqueductal grey level

We investigated the role of group III metabotropic glutamate (mGlu) receptors on glutamate and GABA releases at the periaqueductal grey (PAG) level by using in vivo microdialysis in rats. Intra-PAG perfusion of either L-(+)-2-amino-4-phosphonobutyric acid (L-AP4, 100-300 microM), (RS)-4-phosphonophenylglycine ((RS)-PPG, 100-300 microM) selective agonists of group III mGlu receptors, or (S)-3,4-dicarboxyphenylglycine ((S)-3,4-DCPG, 50-100 microM), a selective agonist of mGlu8 receptor, increased glutamate and decreased GABA extracellular concentrations. (RS)-alpha-methylserine-O-phosphate (MSOP, 0.5 mM), a selective group III receptor antagonist, perfused in combination with (S)-3,4-DCPG, L-AP4 or (RS)-PPG, antagonised the effects induced by these agonists on both extracellular glutamate and GABA values. alpha-Methyl-3-methyl-4-phosphonophenylglycine (UBP1112, 300 microM), a group III mGlu receptor antagonist, perfused in combination with (RS)-PPG or (S)-3,4-DCPG, antagonised the effects induced by these agonists. Intra-PAG perfusion with forskolin (100 microM), an activator of adenylate cyclase, increased dialysate glutamate and GABA levels. Moreover, intra-PAG perfusion with N-[2-(p-bromocinnamyl-amino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89) (100 microM), a protein kinase (PKA) inhibitor, abolished the effect of (S)-3,4-DCPG on both glutamate and GABA releases. H-89, per se, did not modify glutamate release but reduced extracellular GABA value at the higher dosage used (200 microM). These data suggest that group III mGlu receptors in the PAG modulate the releases of glutamate and GABA conversely. In particular, both the facilitation of glutamate and the inhibition of GABA releases require the participation of coupling to adenylate cyclase and the subsequent activation of the PKA pathway.

Enhancement of glutamate uptake mediates the neuroprotection exerted by activating group II or III metabotropic glutamate receptors on astrocytes

We investigated whether the activation of astroglial group II and III metabotropic glutamate receptors (mGluRs) could exert neuroprotective effects and whether the neuroprotection was related to glutamate uptake. Our results showed that the activation of astroglial group II or III mGluRs exerted neuroprotection against 1-methyl-4-phenylpyridinium (MPP+) astroglial conditioned medium-induced neurotoxicity in midbrain neuron cultures. Furthermore, MPP+ decreased glutamate uptake of primary astrocytes and C6 glioma cells, which was recovered by activating group II or III mGluRs. Specific group II or III mGluRs antagonists completely abolished the neuroprotective effects and the enhancement of glutamate uptake of their respective agonists. Our results showed that the primary cultured rat astrocytes and C6 glioma cells expressed receptor proteins for group II mGluR2/3, group III mGluR4, mGluR6 and mGluR7. C6 glioma cells expressed mRNA for group II mGluR3, group III mGluR4, mGluR6, mGluR7 and mGluR8. In conclusion, we confirmed that the activation of astroglial mGluRs exerted neuroprotection, and demonstrated that the mechanism underlying this protective role was at least partially related to the enhancement of glutamate uptake.

Future directions in the treatment of Alzheimer’s disease

Alzheimer's disease (AD) remains the most common of the neurodegenerative disorders. In the elderly, it represents the most frequently occurring form of dementia, especially if considered alongside concomitant cerebrovascular disease. Current treatment involves the use of acetylcholinesterase inhibitors, which have shown symptomatic benefits in the recognised domains of cognition, function and behaviour. While they may have intrinsic disease-modifying activity, this is yet to be proven, and strategies to alter the fundamental neuropathological changes in AD continue to be sought. Much of the evidence suggests that the accumulation of amyloid-beta may play a pivotal role, therefore the bulk of current research is focused on possible intervention along the amyloid pathways. However, the abnormal phosphorylation of tau is also a reasonable target and as the molecular basis of AD is better delineated, more targeted treatment approaches are being proposed. This paper reports on the current data that is setting the future directions for research into AD.

A new PCR-based approach for the preparation of RNA probe

A number of PCR-based in situ hybridization (ISH) techniques have been reported to facilitate the procedure. However, those techniques require additional gene specific primers with RNA polymerase binding site. We developed a new PCR-based ISH technique without extra gene-specific primers. We amplified gene specific PCR products with regular gene-specific primer pairs. Special linker, including T7 RNA polymerase binding site, was adapted to amplified PCR products. Secondary PCR was performed with T7 primer, and forward or reverse primer, used for the first PCR to prepare template DNA for RNA transcription. We were able to generate sense and anti-sense probes for ISH in a day. Recently, real-time PCR and ISH are required to validate microarray results quantitatively and qualitatively. This technique can be expected to facilitate the high-throughput validation of transcripts detected by microarrays.

Stress in the brain: novel cellular mechanisms of injury linked to Alzheimer's disease

More than a century has elapsed since the description of Alois Alzheimer's patient Auguste D. Yet, the well-documented generation of beta-amyloid aggregates and neurofibrillary tangles that define Alzheimer's disease is believed to represent only a portion of the cellular processes that can determine the course of Alzheimer's disease. Understanding of the complex nature of this disorder has evolved with an increased appreciation for pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Recent work has identified novel pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and the formation of neurofibrillary tangles through their downstream substrates that include glycogen synthase kinase-3beta, Bad, and Bcl-xL. Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation for the processing of amyloid precursor protein, tau protein cleavage, and presenilin disposal. New therapeutic avenues that are just open to exploration, such as with nicotinamide adenine dinucleotide modulation, cell cycle modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat Alzheimer's disease.

Metabotropic glutamate receptor 4-mediated 5-Fluorouracil resistance in a human colon cancer cell line

PURPOSE

5-Fluorouracil (5-FU) has been the mainstay treatment for colorectal cancer for the past few decades. However, as with other cancers, development of 5-FU resistance has been a major obstacle in colorectal cancer chemotherapy. The purpose of this study was to gain further understanding of the mechanisms underlying 5-FU resistance in colorectal cancer cells.

EXPERIMENTAL DESIGN

A 5-FU-resistant cell line was established from the human colon cancer cell line SNU-769A. Protein extracts from these two cell lines (parent and resistant) were analyzed using comparative proteomics to identify differentially expressed proteins.

RESULTS

5-FU-resistant human colon cancer cells were found to overexpress metabotropic glutamate receptor 4 (mGluR4). Other experiments showed cellular resistance to 5-FU (i.e., cell survival) was altered by the mGluR4 agonist l-2-amino-4-phosphonobutyric acid (L-AP 4), and by the mGluR4 antagonist (S)-amino-2-methyl-4-phosphonobutanoic acid (MAP 4), in that L-AP 4 increased 5-FU resistance in SNU-769A cells, whereas MAP 4 ablated 5-FU resistance in 5-FU-resistant cells. However, there was no significant effect of L-AP 4 or MAP 4 on basal cAMP and thymidylate synthase levels. Interestingly, 5-FU down-regulated mGluR4 expression, and MAP 4 suppressed proliferation in both cell lines.

CONCLUSIONS

We here report mGluR4 expression in human colon cancer cell line, which provides further evidence for extra-central nervous system expression of glutamate receptors. Overexpression of mGluR4 may tentatively be responsible for 5-FU resistance and, although activation by agonist promotes cell survival in the presence of 5-FU, decreased mGluR4 expression or inactivation by antagonist contributes to cell death.

Chymotrypsin-like proteases contribute to human monocytic THP-1 cell as well as human microglial neurotoxicity

Activated microglia have been observed in various neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis, and multiple sclerosis. They may exacerbate neuronal damage by secreting various toxic molecules. The list of candidate toxins includes proteases. Since it is currently not known which, if any, proteases are involved in human microglia neurotoxicity, we studied the effects of a panel of protease inhibitors on the toxicity of cell-free supernatants of stimulated human microglia and THP-1 monocytic cells to human SH-SY5Y cells. Five structurally distinct inhibitors that are known to inhibit chymotrypsin-like proteases were partially protective. They included chymostatin, AEBSF (Pefabloc SC), alpha1-antichymotrypsin, bromoenol lactone, and 3,4-dichloroisocoumarin. The data suggest that certain protease inhibitors could inhibit microglial-mediated toxicity. They might represent a novel class of drugs with benefit in diseases where overactivity of microglia contributes to the pathogenesis.

Glutamate-mediated glial injury: Mechanisms and clinical importance

Primary and/or secondary glial cell death can cause and/or aggravate human diseases of the central nervous system (CNS). Like neurons, glial cells are vulnerable to glutamate insults. Astrocytes, microglia, and oligodendrocytes express a wide variety of glutamate receptors and transporters that mediate many of the deleterious effects of glutamate. Astrocytes are responsible for most glutamate uptake in synaptic and nonsynaptic areas and consequently, are the major regulators of glutamate homeostasis. Microglia in turn may secrete cytokines, which can impair glutamate uptake and reduce the expression of glutamate transporters. Finally, oligodendrocytes, the myelinating cells of the CNS, are very sensitive to excessive glutamate signaling, which can lead to the apoptosis or necrosis of these cells. This review aims at summarizing the mechanisms leading to glial cell death as a consequence of alterations in glutamate signaling, and their clinical relevance. A thorough understanding of these events will undoubtedly lead to better therapeutic strategies to treat CNS diseases affecting glia and in particular, those that involve damage to white matter tracts.

Expression patterns of Group III metabotropic glutamate receptors mGluR4 and mGluR8 in multiple sclerosis lesions

Recent evidence supports a role for metabotropic glutamate receptors (mGluRs) in neuroinflammatory diseases. In the present study, we have investigated whether the group III mGluR subtypes mGluR4 and mGluR8 are expressed in MS lesions at various stages of evolution. In control patient tissue and in normal-appearing MS white matter (NAWM), no microglial or astrocyte staining was detected. In contrast, in active lesions, mGluR8 immunoreactivity (IR) was detected in cells of the microglia/macrophage lineage. Fewer macrophage-like cells were positive for mGluR8 in chronic active and inactive lesions. No mGluR4 IR was detected in cells of the microglia/macrophage lineage in the MS lesions studied. In chronic active lesions, however, a population of reactive astrocytes localized in the rim of the lesions expressed both mGluR4 and mGluR8. Our results suggest a role for these receptor subtypes in the inflammatory response in MS that involves both astrocytes and cells of the microglia/macrophage lineage.

Microglia

Microglia--the macrophage equivalent of the CNS--safeguards and supports neuronal functions. Threats to the CNS homeostasis can trigger a rapid transformation of these cells from a normally "resting" into alerted and "activated" states. Microglia primarily serves the tissue defence and protection when participating in mechanisms of innate and adaptive immunity. On the contrary, excessive acute or chronic microglial activation can provoke severe neuronal and glial damage by carrying or fuelling destructive cascades. Several factors and conditions have already been identified that maintain the resting phenotype or organize and control the activation process. Cells are thereby able to recognize a dangerous signal as well as to sense functional disturbance. Microglial activation is also proving a much more variable and adaptive process than previously noticed. Aiming at microglia as a therapeutic target, research may focus on intracellular pathways that are probably common to activation scenarios as triggered by various receptor systems. Certain signalling elements may have key roles in the cytosolic integration of sensory inputs and a conversion into programs of executive performance. As the integrative aspect of microglial activation becomes illuminated hope builds up also on strategies for selective interference with harmful outcomes in favour of the--phylogenetically approved--beneficial potential of these fascinating cells.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

Heterogeneity and potentiation of AMPA type of glutamate receptors in rat cultured microglia

alpha-amino-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor in rat cultured microglia were analyzed precisely using flop- and flip-preferring allosteric modulators of AMPA receptors, 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyacetamide (PEPA) and cyclothiazide (CTZ), respectively. Glutamate (Glu)- or kainite (KA)-induced currents were completely inhibited by a specific blocker of AMPA receptor, LY300164, indicating that functional Glu-receptors in cultured microglia are mostly AMPA receptor but not KA receptor in many cells. Glu- and KA-induced currents were potentiated by PEPA and CTZ in a concentration-dependent manner. The ratio of the potentiation by PEPA to the potentiation by cyclothiazide varied with cells between 0.1 and 0.9, suggesting cell-to-cell heterogeneity of AMPA receptor subunits expressed in microglia. Quantitative RT-PCR revealed that GluR1-3 mainly occurred in the flip forms, which agreed with the stronger potentiation of receptor currents by CTZ vs. PEPA. Finally, the potentiation of microglial AMPA receptors by PEPA and CTZ inhibited the Glu-induced release of tumor necrosis factor-alpha (TNF-alpha) unpredictably. The increase in TNF-alpha release by Glu or KA required extracellular Na+ and Ca2+ ions but not mitogen-activated protein kinase (MAPK), suggesting the effects of PEPA and CTZ were not due to the inhibition of MAPK. These results suggest that potentiation of microglial AMPA receptors serves as a negative feedback mechanism for the regulation of TNF-alpha release and may contribute to the ameliorating effects of allosteric modulators of AMPA receptors.