Activation of group II metabotropic glutamate receptors underlies microglial reactivity and neurotoxicity following stimulation with chromogranin A, a peptide up-regulated in Alzheimer's disease

Receptors on Microglia

Microglial cells are the most receptive cells in the central nervous system (CNS), expressing several classes of receptors reflecting their immune heritage and newly acquired neural specialisation. Microglia possess, depending on the particular context, receptors to neurotransmitters and neuromodulators as well as immunocompetent receptors. This rich complement allows microglial cells to monitor the functional status of the nervous system, contribute actively to the regulation of neural activity and plasticity and homeostasis, and guard against pathogens as well as other challenges to the CNS's integrity and function.

Proteomic analysis discovers potential biomarkers of early traumatic axonal injury in the brainstem

OBJECTIVE

Application of Tandem Mass Tags (TMT)-based LC-MS/MS analysis to screen for differentially expressed proteins (DEPs) in traumatic axonal injury (TAI) of the brainstem and to predict potential biomarkers and key molecular mechanisms of brainstem TAI.

METHODS

A modified impact acceleration injury model was used to establish a brainstem TAI model in Sprague-Dawley rats, and the model was evaluated in terms of both functional changes (vital sign measurements) andstructural changes (HE staining, silver-plating staining and β-APP immunohistochemical staining). TMT combined with LC-MS/MS was used to analyse the DEPs in brainstem tissues from TAI and Sham groups. The biological functions of DEPs and potential molecular mechanisms in the hyperacute phase of TAI were analysed by bioinformatics techniques, and candidate biomarkers were validated using western blotting and immunohistochemistry on brainstem tissues from animal models and humans.

RESULTS

Based on the successful establishment of the brainstem TAI model in rats, TMT-based proteomics identified 65 DEPs, and bioinformatics analysis indicated that the hyperacute phase of TAI involves multiple stages of biological processes including inflammation, oxidative stress, energy metabolism, neuronal excitotoxicity and apoptosis. Three DEPs, CBR1, EPHX2 and CYP2U1, were selected as candidate biomarkers and all three proteins were found to be significantly expressed in brainstem tissue 30 min-7 days after TAI in both animal models and humans.

CONCLUSION

Using TMT combined with LC-MS/MS analysis for proteomic study of early TAI in rat brainstem, we report for the first time that CBR1, EPHX2 and CYP2U1 can be used as biomarkers of early TAI in brainstem by means of western blotting and immunohistochemical staining, compensating for the limitations of silver-plating staining and β-APP immunohistochemical staining, especially in the case of very short survival time after TAI (shorter than 30 min). A number of other proteins that also have a potential marker role are also presented, providing new insights into the molecular mechanisms, therapeutic targets and forensic identification of early TAI in brainstem.

Gap Junctions and Connexins in Microglia-Related Oxidative Stress and Neuroinflammation: Perspectives for Drug Discovery

Microglia represent the immune system of the brain. Their role is central in two phenomena, neuroinflammation and oxidative stress, which are at the roots of different pathologies related to the central nervous system (CNS). In order to maintain the homeostasis of the brain and re-establish the equilibrium after a threatening imbalance, microglia communicate with each other and other cells within the CNS by receiving specific signals through membrane-bound receptors and then releasing neurotrophic factors into either the extracellular milieu or directly into the cytoplasm of nearby cells, such as astrocytes and neurons. These last two mechanisms rely on the activity of protein structures that enable the formation of channels in the membrane, namely, connexins and pannexins, that group and form gap junctions, hemichannels, and pannexons. These channels allow the release of gliotransmitters, such as adenosine triphosphate (ATP) and glutamate, together with calcium ion (Ca2+), that seem to play a pivotal role in inter-cellular communication. The aim of the present review is focused on the physiology of channel protein complexes and their contribution to neuroinflammatory and oxidative stress-related phenomena, which play a central role in neurodegenerative disorders. We will then discuss how pharmacological modulation of these channels can impact neuroinflammatory phenomena and hypothesize that currently available nutraceuticals, such as carnosine and N-acetylcysteine, can modulate the activity of connexins and pannexins in microglial cells and reduce oxidative stress in neurodegenerative disorders.

Understanding microglial responses in large animal models of traumatic brain injury: an underutilized resource for preclinical and translational research

Traumatic brain injury (TBI) often results in prolonged or permanent brain dysfunction with over 2.8 million affected annually in the U.S., including over 56,000 deaths, with over 5 million total survivors exhibiting chronic deficits. Mild TBI (also known as concussion) accounts for over 75% of all TBIs every year. Mild TBI is a heterogeneous disorder, and long-term outcomes are dependent on the type and severity of the initial physical event and compounded by secondary pathophysiological consequences, such as reactive astrocytosis, edema, hypoxia, excitotoxicity, and neuroinflammation. Neuroinflammation has gained increasing attention for its role in secondary injury as inflammatory pathways can have both detrimental and beneficial roles. For example, microglia-resident immune cells of the central nervous system (CNS)-influence cell death pathways and may contribute to progressive neurodegeneration but also aid in debris clearance and neuroplasticity. In this review, we will discuss the acute and chronic role of microglia after mild TBI, including critical protective responses, deleterious effects, and how these processes vary over time. These descriptions are contextualized based on interspecies variation, sex differences, and prospects for therapy. We also highlight recent work from our lab that was the first to describe microglial responses out to chronic timepoints after diffuse mild TBI in a clinically relevant large animal model. The scaled head rotational acceleration of our large animal model, paired with the gyrencephalic architecture and appropriate white:gray matter ratio, allows us to produce pathology with the same anatomical patterns and distribution of human TBI, and serves as an exemplary model to examine complex neuroimmune response post-TBI. An improved understanding of microglial influences in TBI could aid in the development of targeted therapeutics to accentuate positive effects while attenuating detrimental post-injury responses over time.

Metabotropic glutamate receptor function and regulation of sleep-wake cycles

Metabotropic glutamate (mGlu) receptors are the most abundant family of G-protein coupled receptors and are widely expressed throughout the central nervous system (CNS). Alterations in glutamate homeostasis, including dysregulations in mGlu receptor function, have been indicated as key contributors to multiple CNS disorders. Fluctuations in mGlu receptor expression and function also occur across diurnal sleep-wake cycles. Sleep disturbances including insomnia are frequently comorbid with neuropsychiatric, neurodevelopmental, and neurodegenerative conditions. These often precede behavioral symptoms and/or correlate with symptom severity and relapse. Chronic sleep disturbances may also be a consequence of primary symptom progression and can exacerbate neurodegeneration in disorders including Alzheimer's disease (AD). Thus, there is a bidirectional relationship between sleep disturbances and CNS disorders; disrupted sleep may serve as both a cause and a consequence of the disorder. Importantly, comorbid sleep disturbances are rarely a direct target of primary pharmacological treatments for neuropsychiatric disorders even though improving sleep can positively impact other symptom clusters. This chapter details known roles of mGlu receptor subtypes in both sleep-wake regulation and CNS disorders focusing on schizophrenia, major depressive disorder, post-traumatic stress disorder, AD, and substance use disorder (cocaine and opioid). In this chapter, preclinical electrophysiological, genetic, and pharmacological studies are described, and, when possible, human genetic, imaging, and post-mortem studies are also discussed. In addition to reviewing the important relationships between sleep, mGlu receptors, and CNS disorders, this chapter highlights the development of selective mGlu receptor ligands that hold promise for improving both primary symptoms and sleep disturbances.

Microglia-Astrocyte Communication in Alzheimer's Disease

Microglia and astrocytes are regarded as active participants in the central nervous system under various neuropathological conditions, including Alzheimer's disease (AD). Both microglia and astrocyte activation have been reported to occur with a spatially and temporarily distinct pattern. Acting as a double-edged sword, glia-mediated neuroinflammation may be both detrimental and beneficial to the brain. In a variety of neuropathologies, microglia are activated before astrocytes, which facilitates astrocyte activation. Yet reactive astrocytes can also prevent the activation of adjacent microglia in addition to helping them become activated. Studies describe changes in the genetic profile as well as cellular and molecular responses of these two types of glial cells that contribute to dysfunctional immune crosstalk in AD. In this paper, we construct current knowledge of microglia-astrocyte communication, highlighting the multifaceted functions of microglia and astrocytes and their role in AD. A thorough comprehension of microglia-astrocyte communication could hasten the creation of novel AD treatment approaches.

The Implication of Glial Metabotropic Glutamate Receptors in Alzheimer's Disease

Alzheimer's disease (AD) was first identified more than 100 years ago, yet aspects pertaining to its origin and the mechanisms underlying disease progression are not well known. To this date, there is no therapeutic approach or disease-modifying drug that could halt or at least delay disease progression. Until recently, glial cells were seen as secondary actors in brain homeostasis. Although this view was gradually refuted and the relevance of glial cells for the most diverse brain functions such as synaptic plasticity and neurotransmission was vastly proved, many aspects of its functioning, as well as its role in pathological conditions, remain poorly understood. Metabotropic glutamate receptors (mGluRs) in glial cells were shown to be involved in neuroinflammation and neurotoxicity. Besides its relevance for glial function, glutamatergic receptors are also central in the pathology of AD, and recent studies have shown that glial mGluRs play a role in the establishment and progression of AD. AD-related alterations in Ca2+ signalling, APP processing, and Aβ load, as well as AD-related neurodegeneration, are influenced by glial mGluRs. However, different types of mGluRs play different roles, depending on the cell type and brain region that is being analysed. Therefore, in this review, we focus on the current understanding of glial mGluRs and their implication in AD, providing an insight for future therapeutics and identifying existing research gaps worth investigating.

Sensory Experience as a Regulator of Structural Plasticity in the Developing Whisker-to-Barrel System

Cellular structures provide the physical foundation for the functionality of the nervous system, and their developmental trajectory can be influenced by the characteristics of the external environment that an organism interacts with. Historical and recent works have determined that sensory experiences, particularly during developmental critical periods, are crucial for information processing in the brain, which in turn profoundly influence neuronal and non-neuronal cortical structures that subsequently impact the animals' behavioral and cognitive outputs. In this review, we focus on how altering sensory experience influences normal/healthy development of the central nervous system, particularly focusing on the cerebral cortex using the rodent whisker-to-barrel system as an illustrative model. A better understanding of structural plasticity, encompassing multiple aspects such as neuronal, glial, and extra-cellular domains, provides a more integrative view allowing for a deeper appreciation of how all aspects of the brain work together as a whole.

Glia: A major player in glutamate-GABA dysregulation-mediated neurodegeneration

The imbalance between glutamate and γ-aminobutyric acid (GABA) results in the loss of synaptic strength leading to neurodegeneration. The dogma on the field considered neurons as the main players in this excitation-inhibition (E/I) balance. However, current strategies focusing only on neurons have failed to completely understand this condition, bringing up the importance of glia as an alternative modulator for neuroinflammation as glia alter the activity of neurons and is a source of both neurotrophic and neurotoxic factors. This review's primary goal is to illustrate the role of glia over E/I balance in the central nervous system and its interaction with neurons. Rather than focusing only on the neuronal targets, we take a deeper look at glial receptors and proteins that could also be explored as drug targets, as they are early responders to neurotoxic insults. This review summarizes the neuron-glia interaction concerning GABA and glutamate, possible targets, and its involvement in the E/I imbalance in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis.

Behavioural and biochemical responses to methamphetamine are differentially regulated by mGlu2 and mGlu3 metabotropic glutamate receptors in male mice

Group II metabotropic glutamate receptors (mGlu2 and mGlu3 receptors) shape mechanisms of methamphetamine addiction, but the individual role played by the two subtypes is unclear. We measured methamphetamine-induced conditioned place preference (CPP) and motor responses to single or repeated injections of methamphetamine in wild-type, mGlu2^-/-, and mGlu3^-/-mice. Only mGlu3^-/-mice showed methamphetamine preference in the CPP test. Motor response to the first methamphetamine injection was dramatically reduced in mGlu2^-/-mice, unless these mice were treated with the mGlu5 receptor antagonist, MTEP. In contrast, methamphetamine-induced sensitization was increased in mGlu3^-/-mice compared to wild-type mice. Only mGlu3^-/-mice sensitized to methamphetamine showed increases in phospho-ERK1/2 levels in the nucleus accumbens (NAc) and free radical formation in the NAc and medial prefrontal cortex. These changes were not detected in mGlu2^-/-mice. We also measured a series of biochemical parameters related to the mechanism of action of methamphetamine in naïve mice to disclose the nature of the differential behavioural responses of the three genotypes. We found a reduced expression and activity of dopamine transporter (DAT) and vesicular monoamine transporter-2 in the NAc and striatum of mGlu2^-/-and mGlu3^-/-mice, whereas expression of the DAT adaptor, syntaxin 1A, was selectively increased in the striatum of mGlu3^-/-mice. Methamphetamine-stimulated dopamine release in striatal slices was largely reduced in mGlu2^-/-, but not in mGlu3^-/-, mice. These findings suggest that drugs that selectively enhance mGlu3 receptor activity or negatively modulate mGlu2 receptors might be beneficial in the treatment of methamphetamine addiction and associated brain damage.

Understanding microglial diversity and implications for neuronal function in health and disease

Genetic data implicate microglia as central players in brain health and disease, urging the need to better understand what microglia do in the brain. Microglia are critical partners in neuronal wiring and function during development and disease. Emerging literature suggests that microglia have diverse functional roles, raising the intriguing question of which functions of microglia become impaired in disease to undermine proper neuronal function. It is also becoming increasingly clear that microglia exist in heterogeneous cell states. Microglial cell states appear context-dependent, that is, age, sex, location, and health of their microenvironment; these are further influenced by external signaling factors including gut microbiota and lipid metabolites. These data altogether suggest that microglia exist in functional clusters that impact, and are impacted by, surrounding neuronal microenvironment. However, we still lack understanding of how we can translate microglia cell states into function. Here, we summarize the state-of-the-art on the diverse functions of microglia in relation to neuronal health. Then, we discuss heterogeneity during developing, healthy adult and diseased brains, and whether this may be predetermined by origin and/or regulated by local milieu. Finally, we propose that it is critical to gain high-resolution functional discernment into microglia-neuron interactions while preserving the spatial architecture of the tissue. Such insight will reveal specific targets for biomarker and therapeutic development toward microglia-neuron crosstalk in disease.

Genetic Deletion of mGlu3 Metabotropic Glutamate Receptors Amplifies Ischemic Brain Damage and Associated Neuroinflammation in Mice

Backgroud: Type-3 metabotropic glutamate (mGlu3) receptors are found in both neurons and glial cells and regulate synaptic transmission, astrocyte function, and microglial reactivity. Here we show that the genetic deletion of mGlu3 receptors amplifies ischemic brain damage and associated neuroinflammation in adult mice. An increased infarct size was observed in mGlu3^-/- mice of both CD1 and C57Black strains 24 h following a permanent occlusion of the middle cerebral artery (MCA) as compared to their respective wild-type (mGlu3^+/+ mice) counterparts. Increases in the expression of selected pro-inflammatory genes including those encoding interleukin-1β, type-2 cycloxygenase, tumor necrosis factor-α, CD86, and interleukin-6 were more prominent in the peri-infarct region of mGlu3^-/- mice. In contrast, the expression of two genes associated with the anti-inflammatory phenotype of microglia (those encoding the mannose-1-phosphate receptor and the α-subunit of interleukin-4 receptor) and the gene encoding the neuroprotective factor, glial cell line-derived neurotrophic factor, was enhanced in the peri-infarct region of wild-type mice, but not mGlu3^-/- mice, following MCA occlusion. In C57Black mice, the genetic deletion of mGlu3 receptors worsened the defect in the paw placement test as assessed in the contralateral forepaw at short times (4 h) following MCA occlusion. These findings suggest that mGlu3 receptors are protective against ischemic brain damage and support the way to the use of selective mGlu3 receptor agonists or positive allosteric modulators in experimental animal models of ischemic stroke.

Neuroinflammation in Sepsis: Molecular Pathways of Microglia Activation

Frequently underestimated, encephalopathy or delirium are common neurological manifestations associated with sepsis. Brain dysfunction occurs in up to 80% of cases and is directly associated with increased mortality and long-term neurocognitive consequences. Although the central nervous system (CNS) has been classically viewed as an immune-privileged system, neuroinflammation is emerging as a central mechanism of brain dysfunction in sepsis. Microglial cells are major players in this setting. Here, we aimed to discuss the current knowledge on how the brain is affected by peripheral immune activation in sepsis and the role of microglia in these processes. This review focused on the molecular pathways of microglial activity in sepsis, its regulatory mechanisms, and their interaction with other CNS cells, especially with neuronal cells and circuits.

Effects of Haloperidol, Risperidone, and Aripiprazole on the Immunometabolic Properties of BV-2 Microglial Cells

Microglial cells are resident macrophages in the brain that have been implicated in the pathophysiology of schizophrenia. There is a lack of studies covering the effects of antipsychotics on microglial cells. The current literature points to a possible anti-inflammatory action without clear mechanisms of action. The aim of this study is to characterize the effects of haloperidol, risperidone and aripiprazole on BV-2 microglial cells in in vitro conditions. We have used immunofluorescence and flow cytometry to analyze the classical pro and anti-inflammatory markers, while a real-time metabolic assay (Seahorse) was used to assess metabolic function. We analyzed the expression of p70S6K to evaluate the mTOR pathway activity with Western blot. In this study, we demonstrate the varying effects of haloperidol, risperidone and aripiprazole administration in BV-2 microglial cells. All three tested antipsychotics were successful in reducing the pro-inflammatory action of microglial cells, although only aripiprazole increased the expression of anti-inflammatory markers. Most significant differences in the possible mechanisms of action were seen in the real-time metabolic assays and in the mTORC1 signaling pathway activity, with aripiprazole being the only antipsychotic to reduce the mTORC1 activity. Our results shed some new light on the effects of haloperidol, risperidone and aripiprazole action in microglial cells, and reveal a novel possible mechanism of action for aripiprazole.

mGlu3 receptor regulates microglial cell reactivity in neonatal rats

BACKGROUND

Perinatal inflammation is a key factor of brain vulnerability in neonates born preterm or with intra-uterine growth restriction (IUGR), two leading conditions associated with brain injury and responsible for neurocognitive and behavioral disorders. Systemic inflammation is recognized to activate microglia, known to be the critical modulators of brain vulnerability. Although some evidence supports a role for metabotropic glutamate receptor 3 (mGlu3 receptor) in modulation of neuroinflammation, its functions are still unknown in the developing microglia.

METHODS

We used a double-hit rat model of perinatal brain injury induced by a gestational low-protein diet combined with interleukin-1β injections (LPD/IL-1β), mimicking both IUGR and prematurity-related inflammation. The effect of LPD/IL-1β on mGlu3 receptor expression and the effect of mGlu3 receptor modulation on microglial reactivity were investigated using a combination of pharmacological, histological, and molecular and genetic approaches.

RESULTS

Exposure to LPD/IL-1β significantly downregulated Grm3 gene expression in the developing microglia. Both transcriptomic analyses and pharmacological modulation of mGlu3 receptor demonstrated its central role in the control of inflammation in resting and activated microglia. Microglia reactivity to inflammatory challenge induced by LPD/IL-1β exposure was reduced by an mGlu3 receptor agonist. Conversely, both specific pharmacological blockade, siRNA knock-down, and genetic knock-out of mGlu3 receptors mimicked the pro-inflammatory phenotype observed in microglial cells exposed to LPD/IL-1β.

CONCLUSIONS

Overall, these data show that Grm3 plays a central role in the regulation of microglial reactivity in the immature brain. Selective pharmacological activation of mGlu3 receptors may prevent inflammatory-induced perinatal brain injury.

The dialog between neurons and microglia in Alzheimer's disease: The neurotransmitters view

Microglia play a vital role in maintaining brain homeostasis. Their continuous sensing of surrounding micro-environments is crucial for their activity. Cross talk between specific neurons and microglia might occur through specific neurotransmitter receptors on microglia. Impairment with this interaction might result in pathological activity of microglia against potential insults. The reason for this activity in many neurodegenerative diseases such as Alzheimer's disease (AD) is not known. However, several papers report of the effects of different neurotransmitter agonists on microglial cells function that relate to their activity in AD. This review aims to summarize those works and to raise potential fundamental questions for future research.

The Dual Role of Glutamatergic Neurotransmission in Alzheimer's Disease: From Pathophysiology to Pharmacotherapy

Alzheimer's disease (AD) is an age-related dementia and neurodegenerative disorder, characterized by Aβ and tau protein deposition impairing learning, memory and suppressing synaptic plasticity of neurons. Increasing evidence suggests that there is a link between the glucose and glutamate alterations with age that down-regulates glucose utilization reducing glutamate levels in AD patients. Deviations in brain energy metabolism reinforce the development of AD by hampering glutamate levels in the brain. Glutamate is a nonessential amino acid and the major excitatory neurotransmitter synthesized from glucose. Alterations in cerebral glucose and glutamate levels precede the deposition of Aβ plaques. In the brain, over 40% of neuronal synapses are glutamatergic and disturbances in glutamatergic function have been implicated in pathophysiology of AD. Nevertheless, targeting the glutamatergic system seems to be a promising strategy to develop novel, improved therapeutics for AD. Here, we review data supporting the involvement of the glutamatergic system in AD pathophysiology as well as the efficacy of glutamatergic agents in this neurodegenerative disorder. We also discuss exciting new prospects for the development of improved therapeutics for this devastating disorder.

Myocardial infarction-induced anxiety-like behavior is associated with epigenetic alterations in the hippocampus of rat

Epidemiological and experimental animal studies indicate that there is a high risk for the incidence of neuropsychiatric disorders suffering from cardiovascular diseases such as myocardial infarction (MI). However, the potential mechanism of this association remains largely unknown. This study sought to evaluate whether epigenetic alterations in the hippocampus is associated with MI-induced anxiety-like behavior in rats. MI was induced by occlusion of the left anterior descending artery in adult female rats. Anxiety-like behavior was examined by elevated plus maze, light-dark box, and open field test. Relative gene and protein levels expression in the hippocampus were tested by qRT-PCR and western blotting, respectively. We found that MI rats exhibited anxiety-like behavior compared with those in controls, and there is a positive correlation between MI and anxiety-like behavior. We also found that MI decreased KDM6B while increased SIRT1 expression in the hippocampus of MI rats relative to those in controls. In addition, MI not only increased levels of IL-1β, bax, and cleaved-caspase 3, but also increased Iba-1 and GFAP expression in the hippocampus, as compared to those in controls, suggesting a promotion of neuro-inflammation and apoptosis in hippocampus. Co-immunoprecipitation assay illustrated that H3K27me3 functioned by counteracting with YAP activation in the hippocampus of MI rats relative to those in controls. Together, these results suggest a potential role of hippocampal epigenetic signaling in MI-induced anxiety-like behavior in rats, and pharmacological targeting KDM6B or SIRT1 could be a strategy to ameliorate anxiety-like behavior induced by MI.

Neuromodulation of Glial Function During Neurodegeneration

Glia, a non-excitable cell type once considered merely as the connective tissue between neurons, is nowadays acknowledged for its essential contribution to multiple physiological processes including learning, memory formation, excitability, synaptic plasticity, ion homeostasis, and energy metabolism. Moreover, as glia are key players in the brain immune system and provide structural and nutritional support for neurons, they are intimately involved in multiple neurological disorders. Recent advances have demonstrated that glial cells, specifically microglia and astroglia, are involved in several neurodegenerative diseases including Amyotrophic lateral sclerosis (ALS), Epilepsy, Parkinson's disease (PD), Alzheimer's disease (AD), and frontotemporal dementia (FTD). While there is compelling evidence for glial modulation of synaptic formation and regulation that affect neuronal signal processing and activity, in this manuscript we will review recent findings on neuronal activity that affect glial function, specifically during neurodegenerative disorders. We will discuss the nature of each glial malfunction, its specificity to each disorder, overall contribution to the disease progression and assess its potential as a future therapeutic target.

Glutamate-induced excitotoxicity in Parkinson's disease: The role of glial cells

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamate transmission efficiency depends on the correct functionality and expression of a plethora of receptors and transporters, located both on neurons and glial cells. Of note, glutamate reuptake by dedicated transporters prevents its accumulation at the synapse as well as non-physiological spillover. Indeed, extracellular glutamate increase causes aberrant synaptic signaling leading to neuronal excitotoxicity and death. Moreover, extrasynaptic glutamate diffusion is strongly associated with glia reaction and neuroinflammation. Glutamate-induced excitotoxicity is mainly linked to an impaired ability of glial cells to reuptake and respond to glutamate, then this is considered a common hallmark in many neurodegenerative diseases, including Parkinson's disease (PD). In this review, we discuss the function of astrocytes and microglia in glutamate homeostasis, focusing on how glial dysfunction causes glutamate-induced excitotoxicity leading to neurodegeneration in PD.

Putative mGluR4 positive allosteric modulators activate Gi-independent anti-inflammatory mechanisms in microglia

Chronic dysregulated microglial activation may lead to persistent inflammation and progressive neurodegeneration. A previous study reported that ADX88178, a putative metabotropic glutamate receptor 4 (mGluR4) positive allosteric modulator (PAM), exerts anti-inflammatory effects in microglia by activating mGluR4. We employed in vitro models of immortalized microglia cell lines and primary microglia to elucidate the molecular mechanisms responsible for the regulation of inflammatory pathways by ADX88178 and other mGluR4 PAMs. ADX88178 downregulated lipopolysaccharide (LPS)-induced expression of pro-inflammatory mediators, including TNF-α, IL-1β, CCL-2, IL-6, NOS2, and miR-155, as well as NO levels, in BV2 cells and primary microglia. Other mGluR4 modulators had divergent activities; VU0361737 (PAM) showed anti-inflammatory effects, whereas the orthosteric group III agonist, L-AP4, and VU0155041 (PAM) displayed no anti-inflammatory actions. In contrast to the earlier report, ADX88178 anti-inflammatory effects appeared to be mGluR4-independent as mGluR4 expression in our in vitro models was very low and its actions were not altered by pharmacological or molecular inhibition of mGluR4. Moreover, we showed that ADX88178 activated G_i-independent, alternative signaling pathways as indicated by the absence of pertussis toxin-mediated inhibition and by increased phosphorylation of cAMP-response element binding protein (CREB), an inhibitor of the NFkB pro-inflammatory pathway. ADX88178 also attenuated NFkB activation by reducing the degradation of IkB and the associated translocation of NFkB-p65 to the nucleus. ADX88178 did not exert its anti-inflammatory effects through adenosine receptors, reported as mGluR4 heteromerization partners. Thus, our results indicate that in microglia, putative mGluR4 PAMs activate mGluR4/G_i-independent mechanisms to attenuate pro-inflammatory pathways.

Do Mast Cells Have a Role in Tendon Healing and Inflammation?

Understanding the links between the tendon healing process, inflammatory mechanisms, and tendon homeostasis/pain after tissue damage is crucial in developing novel therapeutics for human tendon disorders. The inflammatory mechanisms that are operative in response to tendon injury are not fully understood, but it has been suggested that inflammation occurring in response to nerve signaling, i.e., neurogenic inflammation, has a pathogenic role. The mechanisms driving such neurogenic inflammation are presently not clear. However, it has recently been demonstrated that mast cells present within the injured tendon can express glutamate receptors, raising the possibility that mast cells may be sensitive to glutamate signaling and thereby modulate neurogenic inflammation following tissue injury. In this review, we discuss the role of mast cells in the communication with peripheral nerves, and their emerging role in tendon healing and inflammation after injury.

Regulation of microglia by glutamate and its signal pathway in neurodegenerative diseases

Microglia are an essential component of the central nervous system (CNS) and are involved in the primary response to microorganisms, neuroinflammation, homeostasis, and tissue regeneration, as well as contributing to the pathogenesis of neurodegenerative diseases. Research has shown that microglial diversity, multifunctionality, and their relationship with glutamate are crucial to determining their roles in these diseases. In this review, we focus on recent progress in determining microglial characteristics and the role of glutamate and its receptors in microglia regulation, which could be a novel therapeutic strategy for neurodegenerative diseases.

Post-Ischaemic Immunological Response in the Brain: Targeting Microglia in Ischaemic Stroke Therapy

Microglia, the major endogenous immune cells of the central nervous system, mediate critical degenerative and regenerative responses in ischaemic stroke. Microglia become "activated", proliferating, and undergoing changes in morphology, gene and protein expression over days and weeks post-ischaemia, with deleterious and beneficial effects. Pro-inflammatory microglia (commonly referred to as M1) exacerbate secondary neuronal injury through the release of reactive oxygen species, cytokines and proteases. In contrast, microglia may facilitate neuronal recovery via tissue and vascular remodelling, through the secretion of anti-inflammatory cytokines and growth factors (a profile often termed M2). This M1/M2 nomenclature does not fully account for the microglial heterogeneity in the ischaemic brain, with some simultaneous expression of both M1 and M2 markers at the single-cell level. Understanding and regulating microglial activation status, reducing detrimental and promoting repair behaviours, present the potential for therapeutic intervention, and open a longer window of opportunity than offered by acute neuroprotective strategies. Pharmacological modulation of microglial activation status to promote anti-inflammatory gene expression can increase neurogenesis and improve functional recovery post-stroke, based on promising preclinical data. Cell-based therapies, using preconditioned microglia, are of interest as a method of therapeutic modulation of the post-ischaemic inflammatory response. Currently, there are no clinically-approved pharmacological options targeting post-ischaemic inflammation. A major developmental challenge for clinical translation will be the selective suppression of the deleterious effects of microglial activity after stroke whilst retaining (or enhancing) the neurovascular repair and remodelling responses of microglia.

Transcutaneous Electrical Nerve Stimulation Reduces Knee Osteoarthritic Pain by Inhibiting Spinal Glial Cells in Rats

BACKGROUND

Transcutaneous electrical nerve stimulation (TENS) is commonly used for pain control. However, the effects of TENS on osteoarthritis (OA) pain and potential underlying mechanisms remain unclear.

OBJECTIVE

The objective of this study was to investigate the effect of TENS on OA pain treatment and underlying mechanisms related to glial cell inhibition.

DESIGN

This was an experimental study.

METHODS

OA was induced by injection of monosodium iodoacetate into the synovial space of the right knee joint of rats. High-frequency (HF) TENS (100 Hz), low-frequency (LF) TENS (4 Hz), or sham TENS was applied to the ipsilateral knee joint for 20 minutes. Paw withdrawal threshold (PWT), weight bearing, and knee bend score (KBS) were measured. Immunohistochemistry for microglia and astrocytes was performed with L3 to L5 spinal segment samples. To investigate the effects of glial inhibition on OA pain, minocycline, l-α-aminoadipate, or artificial cerebrospinal fluid was injected intrathecally, and PWT and KBS were measured.

RESULTS

Compared with sham TENS, both HF TENS and LF TENS significantly increased PWT, decreased KBS, and inhibited activated microglia in the L3 to L5 segments but did not decrease the total number of microglia, except in the L4 segment (HF TENS). Astrocyte expression was significantly decreased in the L3 to L5 segments following LF TENS and in the L3 segment following HF TENS. Compared with artificial cerebrospinal fluid, both minocycline and l-α-aminoadipate increased PWT and decreased KBS.

LIMITATIONS

These results cannot be generalized to humans.

CONCLUSIONS

TENS alleviates OA pain in rats by inhibiting activated microglia and reducing astrocyte expression in the spinal cord. Although these results may not be generalizable to chronic pain in patients with OA, within the limitation of the experimental animal model used in the present study, they suggest a possible mechanism and preclinical evidence supporting further experimentation or clinical use of TENS in humans.

Evaluation of neurotoxicity and hepatotoxicity effects of acute and sub-acute oral administration of unripe ackee (Blighia sapida) fruit extract

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Acute oral dose of 2000 mg/kg of unripe B. sapida fruit extract (BSE) was toxic to mice.

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Repeated treatment with BSE impaired locomotor function, memory performance and shortened seizure latency in mice.

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Repeated treatment with BSE significantly up-regulate acetylcholinesterase enzyme activity in mice.

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Repeated treatment with BSE elevates oxidative stress in the brain and liver of mice.

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Repeated treatment with BSE showed histopathological evidences of toxicity in mice brain and liver.

Ackee (Blighia sapida) is a commonly eaten fruit that is indigenous to West Africa and Jamaica. Ackee poisoning in young children have been reported in parts of Nigeria due to consumption of the unripe fruits. This study was designed to identify potential mechanisms of acute and sub-acute toxicity of unripe B. sapida fruit extract (BSE).

Acute toxic effect was investigated in mice of either sex administered BSE 2000 mg/kg. The sub-acute toxicity effects were investigated in mice of either sex that received 28 days repeated administration of BSE (100 and 500 mg/kg, p.o.). Locomotor activity and memory performance were measured as well as seizure vulnerability in PTZ-induced model. Liver enzymes were assessed in the serum. Acetylcholinesterase, oxidative stress parameters and histopathological changes were assessed in the brain and liver tissues.

Signs and symptoms of toxicity such as urination, tremor, depressed locomotion and death were observed in acute toxicity test. Sub-acute dosing caused significant impairment in locomotor activity and memory performance in mice. Seizure threshold was shortened in BSE-treated compared to control mice. Brain acetylcholinesterase activity was significantly increased. Alkaline phosphatase (ALP) was significantly elevated in mice that received BSE (500 mg/kg). Furthermore, BSE caused significant increase in oxidative stress expressed in nitrite, malondialdehyde, reduced glutathione and catalase in the brain and liver tissues. Histological staining revealed neuronal damage of brain hippocampus and hepatocellular swelling and necrosis.

Blighia sapida unripe fruit extract increased susceptibility to seizure and impaired locomotor and memory function. The biochemical and histopathological findings revealed potential toxicity mechanisms related to neurotoxicity and hepatotoxicity.

Curcumin restores innate immune Alzheimer's disease risk gene expression to ameliorate Alzheimer pathogenesis

Alzheimer's disease (AD) genetics implies a causal role for innate immune genes, TREM2 and CD33, products that oppose each other in the downstream Syk tyrosine kinase pathway, activating microglial phagocytosis of amyloid (Aβ). We report effects of low (Curc-lo) and high (Curc-hi) doses of curcumin on neuroinflammation in APPsw transgenic mice. Results showed that Curc-lo decreased CD33 and increased TREM2 expression (predicted to decrease AD risk) and also increased TyroBP, which controls a neuroinflammatory gene network implicated in AD as well as phagocytosis markers CD68 and Arg1. Curc-lo coordinately restored tightly correlated relationships between these genes' expression levels, and decreased expression of genes characteristic of toxic pro-inflammatory M1 microglia (CD11b, iNOS, COX-2, IL1β). In contrast, very high dose curcumin did not show these effects, failed to clear amyloid plaques, and dysregulated gene expression relationships. Curc-lo stimulated microglial migration to and phagocytosis of amyloid plaques both in vivo and in ex vivo assays of sections of human AD brain and of mouse brain. Curcumin also reduced levels of miR-155, a micro-RNA reported to drive a neurodegenerative microglial phenotype. In conditions without amyloid (human microglial cells in vitro, aged wild-type mice), Curc-lo similarly decreased CD33 and increased TREM2. Like curcumin, anti-Aβ antibody (also reported to engage the Syk pathway, increase CD68, and decrease amyloid burden in human and mouse brain) increased TREM2 in APPsw mice and decreased amyloid in human AD sections ex vivo. We conclude that curcumin is an immunomodulatory treatment capable of emulating anti-Aβ vaccine in stimulating phagocytic clearance of amyloid by reducing CD33 and increasing TREM2 and TyroBP, while restoring neuroinflammatory networks implicated in neurodegenerative diseases.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Targeting mGlu Receptors for Optimization of Antipsychotic Activity and Disease-Modifying Effect in Schizophrenia

Metabotropic glutamate (mGlu) receptors are considered as candidate drug targets for the treatment of schizophrenia. These receptors form a family of eight subtypes (mGlu1 to -8), of which mGlu1 and -5 are coupled to G_q/11, and all other subtypes are coupled to G_i/o. Here, we discuss the possibility that selective ligands of individual mGlu receptor subtypes may be effective in controlling the core symptoms of schizophrenia, and, in some cases, may impact mechanisms underlying the progression of the disorder. Recent evidence indicates that activation of mGlu1 receptors inhibits dopamine release in the meso-striatal system. Hence, selective positive allosteric modulators (PAMs) of mGlu1 receptors hold promise for the treatment of positive symptoms of schizophrenia. mGlu5 receptors are widely expressed in the CNS and regulate the activity of cells that are involved in the pathophysiology of schizophrenia, such as cortical GABAergic interneurons and microglial cells. mGlu5 receptor PAMs are under development for the treatment of schizophrenia and cater the potential to act as disease modifiers by restraining neuroinflammation. mGlu2 receptors have attracted considerable interest because they negatively modulate 5-HT_2A serotonin receptor signaling in the cerebral cortex. Both mGlu2 receptor PAMs and orthosteric mGlu2/3 receptor agonists display antipsychotic-like activity in animal models, and the latter drugs are inactive in mice lacking mGlu2 receptors. So far, mGlu3 receptors have been left apart as drug targets for schizophrenia. However, activation of mGlu3 receptors boosts mGlu5 receptor signaling, supports neuronal survival, and drives microglial cells toward an antiinflammatory phenotype. This strongly encourages research of mGlu3 receptors in schizophrenia. Finally, preclical studies suggest that mGlu4 receptors might be targeted by novel antipsychotic drugs, whereas studies of mGlu7 and mGlu8 receptors in animal models of psychosis are still at their infancy.

Physiology of Microglia

Microglial cells derive from fetal macrophages which immigrate into and disseminate throughout the central nervous system (CNS) in early embryogenesis. After settling in the nerve tissue, microglial progenitors acquire an idiosyncratic morphological phenotype with small cell body and moving thin and highly ramified processes currently defined as "resting or surveillant microglia". Physiology of microglia is manifested by second messenger-mediated cellular excitability, low resting membrane conductance, and expression of receptors to pathogen- or damage-associated molecular patterns (PAMPs and DAMPs), as well as receptors to classical neurotransmitters and neurohormones. This specific physiological profile reflects adaptive changes of myeloid cells to the CNS environment.

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.

Glia and central cardiorespiratory pathology

Respiration and blood pressure are primarily controlled by somatic and autonomic motor neurones, respectively. Central cardiorespiratory control is critical in moment-to-moment survival, but it also has a role in the development and maintenance of chronic pathological conditions such as hypertension. The glial cells of the brain are non-neuronal cells with metabolic, immune, and developmental functions. Recent evidence shows that glia play an active role in supporting and regulating the neuronal circuitry which drives the cardiorespiratory system. Here we will review the activities of two key types of glial cell, microglia and astrocytes, in assisting normal central cardiorespiratory control and in pathology.

Importance of GPCR-Mediated Microglial Activation in Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder associated with impairment of cognition, memory deficits and behavioral abnormalities. Accumulation of amyloid beta (Aβ) is a characteristic hallmark of AD. Microglia express several GPCRs, which, upon activation by modulators, mediate microglial activation and polarization phenotype. This GPCR-mediated microglial activation has both protective and detrimental effects. Microglial GPCRs are involved in amyloid precursor protein (APP) cleavage and Aβ generation. In addition, microglial GPCRs are featured in the regulation of Aβ degradation and clearance through microglial phagocytosis and chemotaxis. Moreover, in response to Aβ binding on microglial Aβ receptors, they can trigger multiple inflammatory pathways. However, there is still a lack of insight into the mechanistic link between GPCR-mediated microglial activation and its pathological consequences in AD. Currently, the available drugs for the treatment of AD are mostly symptomatic and dominated by acetylcholinesterase inhibitors (AchEI). The selection of a specific microglial GPCR that is highly expressed in the AD brain and capable of modulating AD progression through Aβ generation, degradation and clearance will be a potential source of therapeutic intervention. Here, we have highlighted the expression and distribution of various GPCRs connected to microglial activation in the AD brain and their potential to serve as therapeutic targets of AD.

Microglial immune checkpoint mechanisms

Microglia differentiate from progenitors that infiltrate the nascent CNS during early embryonic development. They then remain in this unique immune-privileged environment throughout life. Multiple immune mechanisms, which we collectively refer to as microglial checkpoints, ensure efficient and tightly regulated microglial responses to perturbations in the CNS milieu. Such mechanisms are essential for proper CNS development and optimal physiological function. However, in chronic disease or aging, when a robust immune response is required, such checkpoint mechanisms may limit the ability of microglia to protect the CNS. Here we survey microglial checkpoint mechanisms and their roles in controlling microglial function throughout life and in disease, and discuss how they may be targeted therapeutically.

Targeting metabotropic glutamate receptors for the treatment of neuroinflammation

A large body of evidence suggests that neuroinflammation lies at the core of nearly all CNS disorders, including psychiatric disorders. Invading and local immune cells orchestrate the series of events that lead to either tissue repair or damage in response to neuroinflammation. Both lymphocytes and microglia express metabotropic glutamate (mGlu) receptors, which respond to glutamate or other endogenous activators (e.g. some kynurenine metabolites of tryptophan metabolism) influencing immune phenotype and the balance between pro-inflammatory and anti-inflammatory cytokines. Here, we offer an up-to-date on the role of individual mGlu receptor subtypes in the regulation of innate and adaptive immune response, highlighting the relevance of this information in the development of subtype-selective mGlu receptor ligands for treatment of CNS disorders.

Metabotropic glutamate receptors: the potential for therapeutic applications in Alzheimer's disease

A dysfunction of glutamate signaling is implicated at several levels in the pathogenesis of Alzheimer's disease. Currently, metabotropic glutamate receptors, which have a wide distribution in the central nervous system and activate a multitude of cell signaling pathways, are pursued as targets for therapeutic intervention in Alzheimer's disease. Research is still limited, but results underscore the relevance of ongoing studies. Here we discuss the latest updates regarding metabotropic glutamate receptors and their role in Alzheimer's disease, as well as promising metabotropic glutamate receptor ligands that have been investigated in preclinical models of Alzheimer's disease.

Let's make microglia great again in neurodegenerative disorders

All of the common neurodegenerative disorders-Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and prion diseases-are characterized by accumulation of misfolded proteins that trigger activation of microglia; brain-resident mononuclear phagocytes. This chronic form of neuroinflammation is earmarked by increased release of myriad cytokines and chemokines in patient brains and biofluids. Microglial phagocytosis is compromised early in the disease process, obfuscating clearance of abnormal proteins. This review identifies immune pathologies shared by the major neurodegenerative disorders. The overarching concept is that aberrant innate immune pathways can be targeted for return to homeostasis in hopes of coaxing microglia into clearing neurotoxic misfolded proteins.

Neurotransmitter receptors on microglia

As the resident immune cells in the central nervous system, microglia have long been hypothesised to promote neuroinflammation and exacerbate neurotoxicity. However, this traditional view has undergone recent revision as evidence has accumulated that microglia exert beneficial and detrimental effects depending on activation status, polarisation phenotype and cellular context. A variety of neurotransmitter receptors are expressed on microglia and help mediate the bidirectional communication between neurons and microglia. Here we review data supporting the importance of neurotransmitter receptors on microglia, with a special emphasis on glutamate, γ-aminobutyric acid (GABA), norepinephrine, cannabinoid and acetylcholine receptors. We summarise evidence favouring a significant role for neurotransmitter receptors in modulating microglial activation, phagocytic clearance and phenotypic polarisation. Elucidating the effects of neurotransmitter receptors on microglia and dissecting the underlying mechanisms may help accelerate the discovery of novel drugs that tap the therapeutic potential of microglia.

The impact of metabotropic glutamate receptors into active neurodegenerative processes: A "dark side" in the development of new symptomatic treatments for neurologic and psychiatric disorders

Metabotropic glutamate (mGlu) receptor ligands are under clinical development for the treatment of CNS disorders with high social and economic burden, such as schizophrenia, major depressive disorder (MDD), and Parkinson's disease (PD), and are promising drug candidates for the treatment of Alzheimer's disease (AD). So far, clinical studies have shown symptomatic effects of mGlu receptor ligands, but it is unknown whether these drugs act as disease modifiers or, at the opposite end, they accelerate disease progression by enhancing neurodegeneration. This is a fundamental issue in the treatment of PD and AD, and is also an emerging theme in the treatment of schizophrenia and MDD, in which neurodegeneration is also present and contribute to disease progression. Moving from in vitro data and preclinical studies, we discuss the potential impact of drugs targeting mGlu2, mGlu3, mGlu4 and mGlu5 receptor ligands on active neurodegeneration associated with AD, PD, schizophrenia, and MDD. We wish to highlight that our final comments on the best drug candidates are not influenced by commercial interests or by previous or ongoing collaborations with drug companies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Targeting type-2 metabotropic glutamate receptors to protect vulnerable hippocampal neurons against ischemic damage

Background

To examine whether metabotropic glutamate (mGlu) receptors have any role in mechanisms that shape neuronal vulnerability to ischemic damage, we used the 4-vessel occlusion (4-VO) model of transient global ischemia in rats. 4-VO in rats causes a selective death of pyramidal neurons in the hippocampal CA1 region, leaving neurons of the CA3 region relatively spared. We wondered whether changes in the expression of individual mGlu receptor subtypes selectively occur in the vulnerable CA1 region during the development of ischemic damage, and whether post-ischemic treatment with drugs targeting the selected receptor(s) affords neuroprotection.

Results

We found that 4-VO caused significantly reduction in the transcript of mGlu2 receptors in the CA1 region at times that preceded the anatomical evidence of neuronal death. Down-regulation of mGlu2 receptors was associated with reduced H3 histone acetylation at the Grm2 promoter. The transcripts of other mGlu receptor subtypes were unchanged in the CA1 region of 4-VO rats. Ischemia did not cause changes in mGlu2 receptor mRNA levels in the resistant CA3 region, which, interestingly, were lower than in the CA1 region. Targeting the mGlu2 receptors with selective pharmacologic ligands had profound effects on ishemic neuronal damage. Post-ischemic oral treatment with the selective mGlu2 receptor NAM (negative allosteric modulator), ADX92639 (30 mg/kg), was highly protective against ischemic neuronal death. In contrast, s.c. administration of the mGlu2 receptor enhancer, LY487379 (30 mg/kg), amplified neuronal damage in the CA1 region and extended the damage to the CA3 region.

Conclusion

These findings suggest that the mGlu2 receptor is an important player in mechanisms regulating neuronal vulnerability to ischemic damage, and that mGlu2 receptor NAMs are potential candidates in the experimental treatments of disorders characterized by brain hypoperfusion, such as hypovolemic shock and cardiac arrest.

Neuroglial Transmission

Neuroglia, the "glue" that fills the space between neurons in the central nervous system, takes active part in nerve cell signaling. Neuroglial cells, astroglia, oligodendroglia, and microglia, are together about as numerous as neurons in the brain as a whole, and in the cerebral cortex grey matter, but the proportion varies widely among brain regions. Glial volume, however, is less than one-fifth of the tissue volume in grey matter. When stimulated by neurons or other cells, neuroglial cells release gliotransmitters by exocytosis, similar to neurotransmitter release from nerve endings, or by carrier-mediated transport or channel flux through the plasma membrane. Gliotransmitters include the common neurotransmitters glutamate and GABA, the nonstandard amino acid d-serine, the high-energy phosphate ATP, and l-lactate. The latter molecule is a "buffer" between glycolytic and oxidative metabolism as well as a signaling substance recently shown to act on specific lactate receptors in the brain. Complementing neurotransmission at a synapse, neuroglial transmission often implies diffusion of the transmitter over a longer distance and concurs with the concept of volume transmission. Transmission from glia modulates synaptic neurotransmission based on energetic and other local conditions in a volume of tissue surrounding the individual synapse. Neuroglial transmission appears to contribute significantly to brain functions such as memory, as well as to prevalent neuropathologies.

A Subset of Cerebrospinal Fluid Proteins from a Multi-Analyte Panel Associated with Brain Atrophy, Disease Classification and Prediction in Alzheimer's Disease

In this exploratory neuroimaging-proteomic study, we aimed to identify CSF proteins associated with AD and test their prognostic ability for disease classification and MCI to AD conversion prediction. Our study sample consisted of 295 subjects with CSF multi-analyte panel data and MRI at baseline downloaded from ADNI. Firstly, we tested the statistical effects of CSF proteins (n = 83) to measures of brain atrophy, CSF biomarkers, ApoE genotype and cognitive decline. We found that several proteins (primarily CgA and FABP) were related to either brain atrophy or CSF biomarkers. In relation to ApoE genotype, a unique biochemical profile characterised by low CSF levels of Apo E was evident in ε4 carriers compared to ε3 carriers. In an exploratory analysis, 3/83 proteins (SGOT, MCP-1, IL6r) were also found to be mildly associated with cognitive decline in MCI subjects over a 4-year period. Future studies are warranted to establish the validity of these proteins as prognostic factors for cognitive decline. For disease classification, a subset of proteins (n = 24) combined with MRI measurements and CSF biomarkers achieved an accuracy of 95.1% (Sensitivity 87.7%; Specificity 94.3%; AUC 0.95) and accurately detected 94.1% of MCI subjects progressing to AD at 12 months. The subset of proteins included FABP, CgA, MMP-2, and PPP as strong predictors in the model. Our findings suggest that the marker of panel of proteins identified here may be important candidates for improving the earlier detection of AD. Further targeted proteomic and longitudinal studies would be required to validate these findings with more generalisability.

The p53 Transcriptional Network Influences Microglia Behavior and Neuroinflammation

The tumor-suppressor protein p53 belongs to a family of proteins that play pivotal roles in multiple cellular functions including cell proliferation, cell death, genome stability, and regulation of inflammation. Neuroinflammation is a common feature of central nervous system (CNS) pathology, and microglia are the specialized resident population of CNS myeloid cells that initiate innate immune responses. Microglia maintain CNS homeostasis through pathogen containment, phagocytosis of debris, and initiation of tissue-repair cascades. However, an unregulated pro-inflammatory response can lead to tissue injury and dysfunction in both acute and chronic inflammatory states. Therefore, regulation of the molecular signals that control the induction, magnitude, and resolution of inflammation are necessary for optimal CNS health. We and others have described a novel mechanism by which p53 transcriptional activity modulates microglia behaviors in vitro and in vivo. Activation of p53 induces expression of microRNAs (miRNAs) that support microglia pro-inflammatory functions and suppress anti-inflammatory and tissue repair behaviors. In this review, we introduce the previously described roles of the p53 signaling network and discuss novel functions of p53 in the microglia-mediated inflammatory response in CNS health and disease. Ultimately, improved understanding of the molecular regulators modulated by p53 transcriptional activity in microglia will enhance the development of rational therapeutic strategies to harness the homeostatic and tissue repair functions of microglia.