Atypical antiinflammatory activation of microglia induced by apoptotic neurons: possible role of phosphatidylserine-phosphatidylserine receptor interaction

Role of COX-2 in inflammatory and degenerative brain diseases

In the last decade, the potential role of cyclooxygenase-2 (COX-2) and prostaglandins (PGs) in brain diseases has been extensively studied. COX-2 over-expression has been associated with neurotoxiticy in acute conditions, such as hypoxia/ischemia and seizures, as well as in inflammatory chronic diseases, including Creutzfeldt-Jakob disease (CJD) and Alzheimer's disease (AD). However, the role played by COX-2 in neurodegenerative diseases is still controversial and further clinical and experimental studies are warranted. In addition, the emerging role of COX-2 in behavioural and cognitive functions strongly indicates that studies aimed at improving our knowledge of the physiological role of COX-2 in the central nervous system are crucial to fully understand the pros and cons of its manipulation in disabling neurological diseases.

Cyclooxygenase-2, prostaglandin E2, and microglial activation in prion diseases

Cyclooxygenase (COX) catalyzes the first committed step in the synthesis of prostaglandins (PGs) and is the main target of nonsteroidal anti-inflammatory drugs (NSAIDs). The enzyme exists as constitutive (COX-1) and inducible (COX-2) isoforms, being the latter a major player in inflammation. In the brain, COX-2 expression has been associated with inflammatory and neurodegenerative processes of several human neurological diseases. Prion diseases, or transmissible spongiform encephalopathies, are a heterogeneous group of fatal neurodegenerative disorders, characterized by deposition of the protease-resistant prion protein, astrocytosis, and spongiform degeneration. In addition, an extensive microglial activation supports the occurrence of local chronic inflammatory response. In experimental prion diseases, COX-2 immunoreactivity was found specifically localized to microglial cells and increased with the progression of disease, along with the number of activated microglia. The induction of COX-2 was paralleled by a substantial raise in the brain homogenate PGE(2) levels. In these models, only few scattered COX-1-positive microglia-like cells were detected, suggesting that COX-2 is the major form in prion diseases. In line with the animal models, elevated levels of PGE(2) were found in the cerebrospinal fluid of subjects affected by sporadic, genetic, or variant CJD. In sporadic CJD patients, the most numerous group of patients examined, higher CSF levels of PGE(2) were associated with shorter survival. Although the mechanisms leading to microglial COX-2 expression as well as its potential implication in prion disease pathogenesis remain to be established, PGE(2) levels in the cerebrospinal fluid might represent an important index to predict survival and disease severity.

Innate immunity and protective neuroinflammation: new emphasis on the role of neuroimmune regulatory proteins

Brain inflammation due to infection, hemorrhage, and aging is associated with activation of the local innate immune system as expressed by infiltrating cells, resident glial cells, and neurons. The innate immune response relies on the detection of "nonself" and "danger-self" ligands behaving as "eat me signals" by a plethora of pattern recognition receptors (PRRs) expressed by professional and amateur phagocytes to promote the clearance of pathogens, toxic cell debris (amyloid fibrils, aggregated synucleins, prions), and apoptotic cells accumulating within the brain parenchyma and the cerebrospinal fluid (CSF). These PRRs (e.g., complement, TLR, CD14, scavenger receptors) are highly conserved between vertebrates and invertebrates and may represent the most ancestral innate scavenging system involved in tissue homeostasis. However, in some diseases, these protective mechanisms lead to neurodegeneration on the ground that several innate immune molecules have neurocytotoxic activities. The response is a "double-edged sword" representing a fine balance between protective and detrimental effects. Several key regulatory mechanisms have now been evidenced in the control of CNS innate immunity, and these could be harnessed to explore novel therapeutic avenues. We will herein provide new emphasis on the role of neuroimmune regulatory proteins (NIRegs), such as CD95L, TNF, CD200, CD47, sialic acids, CD55, CD46, fH, C3a, HMGB1, which are involved in silencing innate immunity at the cellular and molecular levels and suppression of inflammation. For instance, NIRegs may play an important role in controlling lymphocyte/macrophage/microglia hyperinflammatory responses, while sparing host defense and repair mechanisms. Moreover, NIRegs have direct beneficial effects on neurogenesis and contributing to brain tissue remodeling.

Different patterns of Ca²⁺ signals are induced by low compared to high concentrations of P2Y agonists in microglia

Brain-resident macrophages (microglia) are key cellular elements in the preservation of tissue integrity. On the other hand, they can also contribute to the development of pathological events by causing an extensive and inappropriate inflammatory response. A growing number of reports indicate the involvement of nucleotides in the control of microglial functions. With this study on P2Y receptors in rat microglia, we want to contribute to the definition of their expression profile and to the characterisation of their signalling mechanisms leading to Ca²⁺ movements. Endogenous nucleotides, when applied at a concentration of 100 μM, elicited robust Ca²⁺ transients, thanks to a panel of metabotropic receptors comprising mainly P2Y₂, P2Y₆ and P2Y₁₂ subtypes. The involvement of P2Y₁₂ receptors in Ca²⁺ responses induced by adenine nucleotides was confirmed by the pharmacological and pertussis toxin sensitivity of the response induced by adenosine diphosphate (ADP). Beside the G protein involved, Gi and Gq respectively, adenine and uracil nucleotides differed also for induction by the latter of a capacitative Ca²⁺ plateau. Moreover, when applied at low (sub-micromolar) concentrations with a long-lasting challenge, uracil nucleotides elicited oscillatory Ca²⁺ changes with low frequency of occurrence (≤ 1 min⁻), sometimes superimposed to an extracellular Ca²⁺-dependent sustained Ca²⁺ rise. We conclude that different patterns of Ca²⁺ transients are induced by low (i.e., oscillatory Ca²⁺ activity) compared to high (i.e., fast release followed by sustained raise) concentrations of nucleotides, which can suggest different roles played by receptor stimulation depending not only on the type but also on the concentration of nucleotides.

The glutamate-glutamine cycle as an inducible, protective face of macrophage activation

Neuronal damage in HIV infection results mainly from chronic activation of brain tissue and involves inflammation, oxidative stress, and glutamate-related neurotoxicity. Glutamate toxicity acts via two distinct pathways: an excitotoxic one, in which glutamate receptors are hyperactivated, and an oxidative one, in which cystine uptake is inhibited, resulting in glutathione depletion, oxidative stress, and cell degeneration. A number of studies have shown that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. They, in turn, provide the trophic amino acid glutamine via their expression of glutamine synthetase. These protective and trophic actions are inhibited in HIV infection, probably as a result of the effects of inflammatory mediators and viral proteins. In vitro and in vivo studies have demonstrated that activated microglia and brain macrophages (AMM) express the transporters and enzymes of the glutamate cycle. This suggests that in addition to their recognized neurotoxic properties in HIV infection, these cells exhibit some neuroprotective properties, which may partly compensate for the inhibited astrocytic function. This hypothesis might explain the discrepancy between microglial activation, which occurs early in the disease, and neuronal apoptosis and neuronal loss, which are late events. In this review, we discuss the possible neuroprotective and neurotrophic roles of AMM and their relationships with inflammation and oxidative stress.

Prostaglandin E2 and BDNF levels in rat hippocampus are negatively correlated with status epilepticus severity: No impact on survival of seizure-generated neurons

Partial and generalized status epilepticus (pSE and gSE) trigger the same level of progenitor cell proliferation in adult dentate gyrus, but survival of new neurons is poor after gSE. Here, we show markedly elevated levels of prostaglandin E2 (PGE2) and brain-derived neurotrophic factor (BDNF) in rat hippocampal formation at 7 days following pSE but not gSE. Administration of the cyclooxygenase (COX) inhibitor flurbiprofen for 1 week, starting at day 8 post-SE, abated PGE2 and decreased BDNF levels, but did not affect survival of new neurons 4 weeks later. Thus, high PGE2 and BDNF levels induced by pSE are probably not of major importance for survival of new neurons during the first days after formation. We propose that they modulate other aspects of synaptic and cellular plasticity, and thereby may influence epileptogenesis.

Glutamate metabolism in HIV-infected macrophages: implications for the CNS

Central nervous system disorders are still a common complication of human immunodeficiency virus (HIV) infection and can lead to dementia and death. They are mostly the consequences of an inflammatory macrophagic activation and relate to glutamate-mediated excitotoxicity. However, recent studies also suggest neuroprotective aspects of macrophage activation through the expression of glutamate transporters and glutamine synthetase. We thus aimed to study whether HIV infection or activation of macrophages could modulate glutamate metabolism in these cells. We assessed the effect of HIV infection on glutamate transporter expression as well as on glutamate uptake by macrophages and showed that glutamate transport was partially decreased in the course of virus replication, whereas excitatory amino acid transporter-2 (EAAT-2) gene expression was dramatically increased. The consequences of HIV infection on glutamine synthetase were also measured and for the first time we show the functional expression of this key enzyme in macrophages. This expression was repressed during virus production. We then quantified EAAT-1 and EAAT-2 gene expression as well as glutamate uptake in differentially activated macrophages and show that the effects of HIV are not directly related to pro- or anti-inflammatory mediators. Finally, this study shows that glutamate transport by macrophages is less affected than what has been described in astrocytes. Macrophages may thus play a role in neuroprotection against glutamate in the infected brain, through their expression of both EAATs and glutamine synthetase. Because glutamate metabolism by activated macrophages is sensitive to both HIV infection and inflammation, it may thus be of potential interest as a therapeutic target in HIV encephalitis.

Macrophage activation and human immunodeficiency virus infection: HIV replication directs macrophages towards a pro-inflammatory phenotype while previous activation modulates macrophage susceptibility to infection and viral production

Macrophages are pivotal for the regulation of immune and inflammatory responses, but whether their role in HIV infection is protective or deleterious remains unclear. In this study, we investigated the effect of pro- and anti-inflammatory stimuli on macrophage sensitivity to two different aspects of HIV infection: their susceptibility to infection stricto sensu, which we measured by endpoint titration method, and their ability to support virus spread, which we measured by using an RT activity assay in infection kinetics. We show a partially protective role for pro-inflammatory agents as well as for IL-4. We also illustrate that various different stimuli display differential effects on macrophage susceptibility to HIV and on virus replication that occurs thereafter. On the other hand, HIV replication strongly repressed CD206 and CD163 expression, thus clearly orientating macrophages towards a pro-inflammatory phenotype, but independently of TNF. Taken together, our results emphasize that HIV infection of macrophages sets up inflammation at the cell level but through unexpected mechanisms. This may limit target susceptibility and participate in virus clearance but may also result in tissue damage.

Inflammation and neurodegeneration

Many CNS diseases of primarily noninflammatory origin, such as chronic neurodegenerative diseases, stroke and trauma, display inflammatory features. Conversely, damage to nerve cells and axons has emerged as a clinically important parameter of autoimmune neuroinflammatory conditions such as multiple sclerosis. Experimental data are conflicting as to whether neuroinflammatory reactions should be regarded as detrimental, or as an apt response serving to minimize nervous tissue damage. Despite this, modulation of inflammation is one of the most dynamic areas in the search for new therapeutic targets for a spectrum of CNS diseases. Recent developments in the field have unravelled an intricate regulation of neuroinflammation and disclosed several avenues that, with further exploration, may result in new ways of treating common and disabling CNS diseases.

Inflammation, the complement system and the diseases of aging

Inflammation is characteristic of neurodegenerative diseases of aging. Neuropathological evidence of activated microglia and activated astrocytes in lesioned areas, combined with epidemiological evidence of sparing of Alzheimer's disease (AD), Parkinson's disease (PD) and age-related macular degeneration (AMD) in long-term users of anti-inflammatory agents, indicates that inflammation is autodestructive of neurons. Locally produced autodestructive molecules include the membrane attack complex (MAC) of complement and oxygen-free radicals. Stimulation is provided by a variety of inflammatory cytokines. Agents which reduce the intensity of inflammation should have broad spectrum application in degenerative diseases of aging.

Blockade of chloride conductance antagonizes PMA-induced ramification in the murine microglial cell line, BV-2

In microglial cells, activation of ion channels and ion transporters is associated with the transformation from an amoeboid to a ramified phenotype and vice versa. In the present study, we evaluated the contributions of protein kinase C (PKC) activity and ion conductance to the phorbol 12-myristate 13-acetate (PMA)-dependent ramification in the murine microglial cell line, BV-2. In a first set of experiments, we showed that PMA, a commonly used activator of PKC, but not the bioinactive analog 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), induces ramification in BV-2 cells. Surprisingly, the PKC inhibitors calphostin C, chelerythrine, or bisindolylmaleimide II did not antagonize PMA-induced ramification. In a further set of experiments, we found that 4,4'-diisocyanatostilbene-2,2' disulfonic acid (DIDS), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), which block chloride channels and K-Cl cotransporters, and SKF 96365, a non-selective ion channel blocker, consistently suppressed PMA-induced ramification in BV-2 cells. Additional ion channel blockers, including lanthanides, amiloride, Ba2+, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), and flufenamic acid did not affect PMA-induced ramification in BV-2 cells. Cs+ accentuated the PMA-dependent ramification in BV-2 cells. Thus, our results indicate (1) that a PMA-binding protein, excluding PKC isoforms, is critical in structural remodeling of microglial cells and (2) that chloride conductance plays a pivotal role in induction of ramification in microglial cells.

Synthesis and Characterization of a Small, Membrane-Permeant, Caspase-Activatable Far-Red Fluorescent Peptide for Imaging Apoptosis

To image apoptosis in vivo with a small, membrane-permeant probe, TcapQ(647) was synthesized comprising a Tat-peptide-based permeation peptide sequence, an effector caspase recognition sequence, DEVD, and a flanking optically activatable pair comprising a far-red quencher, QSY 21, and a fluorophore, Alexa Fluor 647. Under baseline conditions, high quenching efficiencies were observed resulting in low background fluorescence. Upon exposure to executioner caspases, TcapQ(647) was specifically cleaved, thereby releasing the fluorophore from the quencher and enabling imaging of apoptosis.

Neuroprotection by adenosine in the brain: From A(1) receptor activation to A (2A) receptor blockade

Adenosine is a neuromodulator that operates via the most abundant inhibitory adenosine A(1) receptors (A(1)Rs) and the less abundant, but widespread, facilitatory A(2A)Rs. It is commonly assumed that A(1)Rs play a key role in neuroprotection since they decrease glutamate release and hyperpolarize neurons. In fact, A(1)R activation at the onset of neuronal injury attenuates brain damage, whereas its blockade exacerbates damage in adult animals. However, there is a down-regulation of central A(1)Rs in chronic noxious situations. In contrast, A(2A)Rs are up-regulated in noxious brain conditions and their blockade confers robust brain neuroprotection in adult animals. The brain neuroprotective effect of A(2A)R antagonists is maintained in chronic noxious brain conditions without observable peripheral effects, thus justifying the interest of A(2A)R antagonists as novel protective agents in neurodegenerative diseases such as Parkinson's and Alzheimer's disease, ischemic brain damage and epilepsy. The greater interest of A(2A)R blockade compared to A(1)R activation does not mean that A(1)R activation is irrelevant for a neuroprotective strategy. In fact, it is proposed that coupling A(2A)R antagonists with strategies aimed at bursting the levels of extracellular adenosine (by inhibiting adenosine kinase) to activate A(1)Rs might constitute the more robust brain neuroprotective strategy based on the adenosine neuromodulatory system. This strategy should be useful in adult animals and especially in the elderly (where brain pathologies are prevalent) but is not valid for fetus or newborns where the impact of adenosine receptors on brain damage is different.

Role of inflammation in neurodegenerative diseases

PURPOSE OF REVIEW

Inflammation is a self-defensive reaction aimed at eliminating or neutralizing injurious stimuli, and restoring tissue integrity. In neurodegenerative diseases inflammation occurs as a local response driven by microglia, in the absence of leucocyte infiltration. Like peripheral inflammation, neuroinflammation may become a harmful process, and it is now widely accepted that it may contribute to the pathogenesis of many central nervous system disorders, including chronic neurodegenerative diseases. This review addresses some of the most recent advances in our understanding of neuroinflammation.

RECENT FINDINGS

The presence of activated microglia surrounding amyloid plaques and increased levels of complement elements, cytokines, chemokines and free radicals support the existence of a self-propagating toxic cycle and provide a rationale for anti-inflammatory approaches to prevent or delay neurodegeneration. Nonetheless, 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. The causes and consequences of this atypical phenotype have just begun to be unravelled.

SUMMARY

Although significant advances have been made in our knowledge of degenerative diseases, there remains controversy regarding whether neuroinflammation and microglial activation are beneficial or detrimental. Strategies aimed at both preventing and boosting microglial activation are presently under investigation, and these studies might reveal new potentially effective treatments for these neurological disorders.

Activation of alpha7 nicotinic acetylcholine receptor by nicotine selectively up-regulates cyclooxygenase-2 and prostaglandin E2 in rat microglial cultures

Background

Nicotinic acetylcholine (Ach) receptors are ligand-gated pentameric ion channels whose main function is to transmit signals for the neurotransmitter Ach in peripheral and central nervous system. However, the α7 nicotinic receptor has been recently found in several non-neuronal cells and described as an important regulator of cellular function. Nicotine and ACh have been recently reported to inhibit tumor necrosis factor-α (TNF-α) production in human macrophages as well as in mouse microglial cultures. In the present study, we investigated whether the stimulation of α7 nicotinic receptor by the specific agonist nicotine could affect the functional state of activated microglia by promoting and/or inhibiting the release of other important pro-inflammatory and lipid mediator such as prostaglandin E₂.

Methods

Expression of α7 nicotinic receptor in rat microglial cell was examined by RT-PCR, immunofluorescence staining and Western blot. The functional effects of α7 receptor activation were analyzed in resting or lipopolysaccharide (LPS) stimulated microglial cells pre-treated with nicotine. Culture media were assayed for the levels of tumor necrosis factor, interleukin-1β, nitric oxide, interleukin-10 and prostaglandin E₂. Total RNA was assayed by RT-PCR for the expression of COX-2 mRNA.

Results

Rat microglial cells express α7 nicotinic receptor, and its activation by nicotine dose-dependently reduces the LPS-induced release of TNF-α, but has little or no effect on nitric oxide, interleukin-10 and interleukin-1β. By contrast, nicotine enhances the expression of cyclooxygenase-2 and the synthesis of one of its major products, prostaglandin E₂.

Conclusions

Since prostaglandin E₂ modulates several macrophage and lymphocyte functions, which are instrumental for inflammatory resolution, our study further supports the existence of a brain cholinergic anti-inflammatory pathway mediated by α7 nicotinic receptor that could be potentially exploited for novel treatments of several neuropathologies in which local inflammation, sustained by activated microglia, plays a crucial role.

Microglial activation in chronic neurodegenerative diseases: roles of apoptotic neurons and chronic stimulation

In chronic neurodegenerative diseases, microglial activation is an early sign that often precedes neuronal death. Increasing evidence indicates that in these chronic pathologies activated microglia sustain a local inflammatory response. Nonetheless, the potential detrimental or protective roles of such reaction remain to date not fully understood, mainly because of the lack of direct evidence of the functional properties acquired by microglia in the course of chronic diseases. Purified microglial cultures have been extensively used to investigate microglial functions associated with activation, but they are often criticized for some experimental constrains, including the abrupt addition of activators, the limited time of stimulation, and the absence of interactions with neurons or other elements of brain parenchyma. To limit these confounding factors, we developed in vitro models in which microglial cells were repeatedly challenged with lipopolysaccharide or co-cultured with healthy, apoptotic, or necrotic neuronal cells. We found that chronic stimulation and interaction with phosphatidylserine-expressing apoptotic cells induced microglial cells to release immunoregulatory and neuroprotective agents (prostaglandin E(2), transforming growth factor-beta, and nerve growth factor), whereas the synthesis of pro-inflammatory molecules (tumor necrosis factor-alpha and nitric oxide) was inhibited. These findings suggest that signals that are relevant to chronic diseases lead to a progressive down-regulation of pro-inflammatory microglial functions and may help in understanding the atypical microglial activation that begins to be recognized in some chronic neuropathologies.

Cyclooxygenase-2 (COX-2) in Inflammatory and Degenerative Brain Diseases

Cyclooxygenase (COX) catalyses the first committed step in the synthesis of prostanoids, a large family of arachidonic acid metabolites comprising prostaglandins, prostacyclin, and thromboxanes, and is a major target of non-steroidal anti-inflammatory drugs (NSAIDs). COX exists as constitutive and inducible isoforms. COX-2 is the inducible isoform, rapidly expressed in several cell types in response to growth factors, cytokines, and pro-inflammatory molecules. Since its discovery in the early 1990s, COX-2 has emerged as a major player in inflammatory reactions in peripheral tissues. By extension, COX-2 expression in brain has been associated with pro-inflammatory activities, thought to be instrumental in neurodegenerative processes of several acute and chronic diseases. However, 2 major aspects should be borne in mind. First, in the central nervous system, COX-2 is expressed under normal conditions and contributes to fundamental brain functions, such as synaptic activity, memory consolidation, and functional hyperemia. Second, "neuroinflammation" is a much more controlled reaction than inflammation in peripheral tissues, and in many cases is triggered and sustained by activation of resident cells, particularly microglia. In spite of the intense research of the last decade, the evidence of a direct role of COX-2 in neurodegenerative events is still controversial. This article will review new data in this area, focusing on some major human neurological diseases, such as multiple sclerosis, amyotrophic lateral sclerosis, Parkinson disease, Creutzfeldt-Jakob disease, and Alzheimer disease. Furthermore, the emerging role of COX-2 in behavioral and cognitive functions will be discussed.