Prostaglandin signaling suppresses beneficial microglial function in Alzheimer's disease models

Prostaglandin A1 Inhibits the Cognitive Decline of APP/PS1 Transgenic Mice via PPARγ/ABCA1-dependent Cholesterol Efflux Mechanisms

Prostaglandins (PGs) are early and key contributors to chronic neurodegenerative diseases. As one important member of classical PGs, PGA1 has been reported to exert potential neuroprotective effects. However, the mechanisms remain unknown. To this end, we are prompted to investigate whether PGA1 is a useful neurological treatment for Alzheimer's disease (AD) or not. Using high-throughput sequencing, we found that PGA1 potentially regulates cholesterol metabolism and lipid transport. Interestingly, we further found that short-term administration of PGA1 decreased the levels of the monomeric and oligomeric β-amyloid protein (oAβ) in a cholesterol-dependent manner. In detail, PGA1 activated the peroxisome proliferator-activated receptor-gamma (PPARγ) and ATP-binding cassette subfamily A member 1 (ABCA1) signalling pathways, promoting the efflux of cholesterol and decreasing the intracellular cholesterol levels. Through PPARγ/ABCA1/cholesterol-dependent pathway, PGA1 decreased the expression of presenilin enhancer protein 2 (PEN-2), which is responsible for the production of Aβ. More importantly, long-term administration of PGA1 remarkably decreased the formation of Aβ monomers, oligomers, and fibrils. The actions of PGA1 on the production and deposition of Aβ ultimately improved the cognitive decline of the amyloid precursor protein/presenilin1 (APP/PS1) transgenic mice.

Repurposing the KCa3.1 inhibitor senicapoc for Alzheimer's disease

Objective

Microglia play a pivotal role in the initiation and progression of Alzheimer's disease (AD). We here tested the therapeutic hypothesis that the Ca²⁺‐activated potassium channel KCa3.1 constitutes a potential target for treating AD by reducing neuroinflammation.

Methods

To determine if KCa3.1 is relevant to AD, we tested if treating cultured microglia or hippocampal slices with Aβ oligomer (AβO) activated KCa3.1 in microglia, and if microglial KCa3.1 was upregulated in 5xFAD mice and in human AD brains. The expression/activity of KCa3.1 was examined by qPCR, Western blotting, immunohistochemistry, and whole‐cell patch‐clamp. To investigate the role of KCa3.1 in AD pathology, we resynthesized senicapoc, a clinically tested KCa3.1 blocker, and determined its pharmacokinetic properties and its effect on microglial activation, Aβ deposition and hippocampal long‐term potentiation (hLTP) in 5xFAD mice.

Results

We found markedly enhanced microglial KCa3.1 expression/activity in brains of both 5xFAD mice and AD patients. In hippocampal slices, microglial KCa3.1 expression/activity was increased by AβO treatment, and its inhibition diminished the proinflammatory and hLTP‐impairing activities of AβO. Senicapoc exhibited excellent brain penetrance and oral availability, and in 5xFAD mice, reduced neuroinflammation, decreased cerebral amyloid load, and enhanced hippocampal neuronal plasticity.

Interpretation

Our results prompt us to propose repurposing senicapoc for AD clinical trials, as senicapoc has excellent pharmacological properties and was safe and well‐tolerated in a prior phase‐3 clinical trial for sickle cell anemia. Such repurposing has the potential to expedite the urgently needed new drug discovery for AD.

Suppressing pro-inflammatory prostaglandin signaling attenuates excitotoxicity-associated neuronal inflammation and injury

Glutamate receptor-mediated excitotoxicity is a common pathogenic process in many neurological conditions including epilepsy. Prolonged seizures induce elevations in extracellular glutamate that contribute to excitotoxic damage, which in turn can trigger chronic neuroinflammatory reactions, leading to secondary damage to the brain. Blocking key inflammatory pathways could prevent such secondary brain injury following the initial excitotoxic insults. Prostaglandin E2 (PGE₂) has emerged as an important mediator of neuroinflammation-associated injury, in large part via activating its EP2 receptor subtype. Herein, we investigated the effects of EP2 receptor inhibition on excitotoxicity-associated neuronal inflammation and injury in vivo. Utilizing a bioavailable and brain-permeant compound, TG6-10-1, we found that pharmacological inhibition of EP2 receptor after a one-hour episode of kainate-induced status epilepticus (SE) in mice reduced seizure-promoted functional deficits, cytokine induction, reactive gliosis, blood-brain barrier impairment, and hippocampal damage. Our preclinical findings endorse the feasibility of blocking PGE₂/EP2 signaling as an adjunctive strategy to treat prolonged seizures. The promising benefits from EP2 receptor inhibition should also be relevant to other neurological conditions in which excitotoxicity-associated secondary damage to the brain represents a pathogenic event.

Alzheimer's Disease: Recent Concepts on the Relation of Mitochondrial Disturbances, Excitotoxicity, Neuroinflammation, and Kynurenines

The pathomechanism of Alzheimer's disease (AD) certainly involves mitochondrial disturbances, glutamate excitotoxicity, and neuroinflammation. The three main aspects of mitochondrial dysfunction in AD, i.e., the defects in dynamics, altered bioenergetics, and the deficient transport, act synergistically. In addition, glutamatergic neurotransmission is affected in several ways. The balance between synaptic and extrasynaptic glutamatergic transmission is shifted toward the extrasynaptic site contributing to glutamate excitotoxicity, a phenomenon augmented by increased glutamate release and decreased glutamate uptake. Neuroinflammation in AD is predominantly linked to central players of the innate immune system, with central nervous system (CNS)-resident microglia, astroglia, and perivascular macrophages having been implicated at the cellular level. Several abnormalities have been described regarding the activation of certain steps of the kynurenine (KYN) pathway of tryptophan metabolism in AD. First of all, the activation of indolamine 2,3-dioxygenase, the first and rate-limiting step of the pathway, is well-demonstrated. 3-Hydroxy-L-KYN and its metabolite, 3-hydroxy-anthranilic acid have pro-oxidant, antioxidant, and potent immunomodulatory features, giving relevance to their alterations in AD. Another metabolite, quinolinic acid, has been demonstrated to be neurotoxic, promoting glutamate excitotoxicity, reactive oxygen species production, lipid peroxidation, and microglial neuroinflammation, and its abundant presence in AD pathologies has been demonstrated. Finally, the neuroprotective metabolite, kynurenic acid, has been associated with antagonistic effects at glutamate receptors, free radical scavenging, and immunomodulation, giving rise to potential therapeutic implications. This review presents the multiple connections of KYN pathway-related alterations to three main domains of AD pathomechanism, such as mitochondrial dysfunction, excitotoxicity, and neuroinflammation, implicating possible therapeutic options.

Chronic neurodegeneration induces type I interferon synthesis via STING, shaping microglial phenotype and accelerating disease progression

Type I interferons (IFN‐I) are the principal antiviral molecules of the innate immune system and can be made by most cell types, including central nervous system cells. IFN‐I has been implicated in neuroinflammation during neurodegeneration, but its mechanism of induction and its consequences remain unclear. In the current study, we assessed expression of IFN‐I in murine prion disease (ME7) and examined the contribution of the IFN‐I receptor IFNAR1 to disease progression. The data indicate a robust IFNβ response, specifically in microglia, with evidence of IFN‐dependent genes in both microglia and astrocytes. This IFN‐I response was absent in stimulator of interferon genes (STING^−/−) mice. Microglia showed increased numbers and activated morphology independent of genotype, but transcriptional signatures indicated an IFNAR1‐dependent neuroinflammatory phenotype. Isolation of microglia and astrocytes demonstrated disease‐associated microglial induction of Tnfα, Tgfb1, and of phagolysosomal system transcripts including those for cathepsins, Cd68, C1qa, C3, and Trem2, which were diminished in IFNAR1 and STING deficient mice. Microglial increases in activated cathepsin D, and CD68 were significantly reduced in IFNAR1^−/− mice, particularly in white matter, and increases in COX‐1 expression, and prostaglandin synthesis were significantly mitigated. Disease progressed more slowly in IFNAR1^−/− mice, with diminished synaptic and neuronal loss and delayed onset of neurological signs and death but without effect on proteinase K‐resistant PrP levels. Therefore, STING‐dependent IFN‐I influences microglial phenotype and influences neurodegenerative progression despite occurring secondary to initial degenerative changes. These data expand our mechanistic understanding of IFN‐I induction and its impact on microglial function during chronic neurodegeneration.

Exercise-Induced Modulation of Neuroinflammation in Models of Alzheimer's Disease

 Alzheimer's disease (AD), a progressive, neurodegenerative condition characterised by accumulation of toxic βeta-amyloid (Aβ) plaques, is one of the leading causes of dementia globally. The cognitive impairment that is a hallmark of AD may be caused by inflammation in the brain triggered and maintained by the presence of Aβ protein, ultimately leading to neuronal dysfunction and loss. Since there is a significant inflammatory component to AD, it is postulated that anti-inflammatory strategies may be of prophylactic or therapeutic benefit in AD. One such strategy is that of regular physical activity, which has been shown in epidemiological studies to be protective against various forms of dementia including AD. Exercise induces an anti-inflammatory environment in peripheral organs and also increases expression of anti-inflammatory molecules within the brain. Here we review the evidence, mainly from animal models of AD, supporting the hypothesis that exercise can reduce or slow the cellular and cognitive impairments associated with AD by modulating neuroinflammation.

Macrophage de novo NAD+ synthesis specifies immune function in aging and inflammation

Recent advances highlight a pivotal role for cellular metabolism in programming immune responses. Here, we demonstrate that cell-autonomous generation of nicotinamide adenine dinucleotide (NAD⁺) via the kynurenine pathway (KP) regulates macrophage immune function in aging and inflammation. Isotope tracer studies revealed that macrophage NAD⁺ derives substantially from KP metabolism of tryptophan. Genetic or pharmacological blockade of de novo NAD⁺ synthesis depleted NAD⁺, suppressed mitochondrial NAD⁺-dependent signaling and respiration, and impaired phagocytosis and resolution of inflammation. Innate immune challenge triggered upstream KP activation but paradoxically suppressed cell-autonomous NAD⁺ synthesis by limiting the conversion of downstream quinolinate to NAD⁺, a profile recapitulated in aging macrophages. Increasing de novo NAD⁺ generation in immune-challenged or aged macrophages restored oxidative phosphorylation and homeostatic immune responses. Thus, KP-derived NAD⁺ operates as a metabolic switch to specify macrophage effector responses. Breakdown of de novo NAD⁺ synthesis may underlie declining NAD⁺ levels and rising innate immune dysfunction in aging and age-associated diseases.

Early Life Stress and Epigenetics in Late-onset Alzheimer's Dementia: A Systematic Review

Involvement of life stress in Late-Onset Alzheimer's Disease (LOAD) has been evinced in longitudinal cohort epidemiological studies, and endocrinologic evidence suggests involvements of catecholamine and corticosteroid systems in LOAD. Early Life Stress (ELS) rodent models have successfully demonstrated sequelae of maternal separation resulting in LOAD-analogous pathology, thereby supporting a role of insulin receptor signalling pertaining to GSK-3beta facilitated tau hyper-phosphorylation and amyloidogenic processing. Discussed are relevant ELS studies, and findings from three mitogen-activated protein kinase pathways (JNK/SAPK pathway, ERK pathway, p38/MAPK pathway) relevant for mediating environmental stresses. Further considered were the roles of autophagy impairment, neuroinflammation, and brain insulin resistance. For the meta-analytic evaluation, 224 candidate gene loci were extracted from reviews of animal studies of LOAD pathophysiological mechanisms, of which 60 had no positive results in human LOAD association studies. These loci were combined with 89 gene loci confirmed as LOAD risk genes in previous GWAS and WES. Of the 313 risk gene loci evaluated, there were 35 human reports on epigenomic modifications in terms of methylation or histone acetylation. 64 microRNA gene regulation mechanisms were published for the compiled loci. Genomic association studies support close relations of both noradrenergic and glucocorticoid systems with LOAD. For HPA involvement, a CRHR1 haplotype with MAPT was described, but further association of only HSD11B1 with LOAD found; however, association of FKBP1 and NC3R1 polymorphisms was documented in support of stress influence to LOAD. In the brain insulin system, IGF2R, INSR, INSRR, and plasticity regulator ARC, were associated with LOAD. Pertaining to compromised myelin stability in LOAD, relevant associations were found for BIN1, RELN, SORL1, SORCS1, CNP, MAG, and MOG. Regarding epigenetic modifications, both methylation variability and de-acetylation were reported for LOAD. The majority of up-to-date epigenomic findings include reported modifications in the well-known LOAD core pathology loci MAPT, BACE1, APP (with FOS, EGR1), PSEN1, PSEN2, and highlight a central role of BDNF. Pertaining to ELS, relevant loci are FKBP5, EGR1, GSK3B; critical roles of inflammation are indicated by CRP, TNFA, NFKB1 modifications; for cholesterol biosynthesis, DHCR24; for myelin stability BIN1, SORL1, CNP; pertaining to (epi)genetic mechanisms, hTERT, MBD2, DNMT1, MTHFR2. Findings on gene regulation were accumulated for BACE1, MAPK signalling, TLR4, BDNF, insulin signalling, with most reports for miR-132 and miR-27. Unclear in epigenomic studies remains the role of noradrenergic signalling, previously demonstrated by neuropathological findings of childhood nucleus caeruleus degeneration for LOAD tauopathy.

Impact of Dietary Fats on Brain Functions

BACKGROUND

Adequate dietary intake and nutritional status have important effects on brain functions and on brain health. Energy intake and specific nutrients excess or deficiency from diet differently affect cognitive processes, emotions, behaviour, neuroendocrine functions and synaptic plasticity with possible protective or detrimental effects on neuronal physiology. Lipids, in particular, play structural and functional roles in neurons. Here the importance of dietary fats and the need to understand the brain mechanisms activated by peripheral and central metabolic sensors. Thus, the manipulation of lifestyle factors such as dietary interventions may represent a successful therapeutic approach to maintain and preserve brain health along lifespan.

METHODS

This review aims at summarizing the impact of dietary fats on brain functions.

RESULTS

Starting from fat consumption, nutrient sensing and food-related reward, the impact of gut-brain communications will be discussed in brain health and disease. A specific focus will be on the impact of fats on the molecular pathways within the hypothalamus involved in the control of reproduction via the expression and the release of Gonadotropin-Releasing Hormone. Lastly, the effects of specific lipid classes such as polyunsaturated fatty acids and of the "fattest" of all diets, commonly known as "ketogenic diets", on brain functions will also be discussed.

CONCLUSION

Despite the knowledge of the molecular mechanisms is still a work in progress, the clinical relevance of the manipulation of dietary fats is well acknowledged and such manipulations are in fact currently in use for the treatment of brain diseases.

Sleep and circadian rhythm disruption and stress intersect in Alzheimer's disease

Alzheimer's disease (AD) was discovered and the pathological hallmarks were revealed more than a century ago. Subsequently, many remarkable discoveries and breakthroughs provided us with mechanistic insights into the pathogenesis of AD. The identification of the molecular underpinning of the disease not only provided the framework of AD pathogenesis but also targets for therapeutic inventions. Despite all the initial successes, no effective treatment for AD has emerged yet as all the late stages of clinical trials have failed. Many factors ranging from genetic to environmental factors have been critically appraised as the potential causes of AD. In particular, the role of stress on AD has been intensively studied while the relationship between sleep and circadian rhythm disruption (SCRD) and AD have recently emerged. SCRD has always been thought to be a corollary of AD pathologies until recently, multiple lines of evidence converge on the notion that SCRD might be a contributing factor in AD pathogenesis. More importantly, how stress and SCRD intersect and make their concerted contributions to AD phenotypes has not been reviewed. The goal of this literature review is to examine at multiple levels – molecular, cellular (e.g. microglia, gut microbiota) and holistic – how the interaction between stress and SCRD bi-directionally and synergistically exacerbate AD pathologies and cognitive impairment. AD, in turn, worsens stress and SCRD and forms the vicious cycle that perpetuates and amplifies AD.

The potential importance of myeloid-derived suppressor cells (MDSCs) in the pathogenesis of Alzheimer's disease

The exact cause of Alzheimer's disease (AD) is still unknown, but the deposition of amyloid-β (Aβ) plaques and chronic inflammation indicates that immune disturbances are involved in AD pathogenesis. Recent genetic studies have revealed that many candidate genes are expressed in both microglia and myeloid cells which infiltrate into the AD brains. Invading myeloid cells controls the functions of resident microglia in pathological conditions, such as AD pathology. AD is a neurologic disease with inflammatory component where the immune system is not able to eliminate the perpetrator, while, concurrently, it should prevent neuronal injuries induced by inflammation. Recent studies have indicated that AD brains are an immunosuppressive microenvironment, e.g., microglial cells are hyporesponsive to Aβ deposits and anti-inflammatory cytokines enhance Aβ deposition. Immunosuppression is a common element in pathological disorders involving chronic inflammation. Studies on cancer-associated inflammation have demonstrated that myeloid-derived suppressor cells (MDSCs) have a crucial role in the immune escape of tumor cells. Immunosuppression is not limited to tumors, since MDSCs can be recruited into chronically inflamed tissues where inflammatory mediators enhance the proliferation and activation of MDSCs. AD brains express a range of chemokines and cytokines which could recruit and expand MDSCs in inflamed AD brains and thus generate an immunosuppressive microenvironment. Several neuroinflammatory disorders, e.g., the early phase of AD pathology, have been associated with an increase in the level of circulating MDSCs. We will elucidate the immunosuppressive armament of MDSCs and present evidences in support of the crucial role of MDSCs in the pathogenesis of AD.

Meloxicam Improves Cognitive Impairment of Diabetic Rats through COX2-PGE2-EPs-cAMP/pPKA Pathway

Diabetics often face greater risk of cognitive impairment than nondiabetics. However, how to prevent this disease is still unconfirmed. In this study, we investigated the potential protection and mechanism of meloxicam on cognitive impairment in diabetic rats. The diabetic rat model was established with a high-fat diet and a small dose of streptozotocin (40 mg/kg). The changes of spatial learning and memory, histopathology, and the protein expressions of amyloid protein precursor (APP) and β-amyloid (Aβ) indicated that diabetic rats had neuronal injury and cognitive impairment. Tumor necrosis factor α (TNFα), interleukin 6 (IL-6), C reactive protein (CRP) and prostaglandin E2 (PGE2) levels, and microglial cell number were significantly increased in the diabetic rat brain. Meanwhile, the protein expressions of APP, Aβ, cyclooxygenases2 (COX2), E-type prostanoid recptors 1 (EP1) and EP2, and the level of cyclic adenosine monophosphate (cAMP) were significantly increased, while the protein expressions of EP3 and phosphorylated protein kinase A (pPKA) were significantly decreased in the diabetic rat hippocampus and cortex. However, the EP4 protein expression had no significant changes. Meloxicam significantly improved neuronal injury and cognitive impairment, and significantly decreased inflammatory cytokines levels. Meloxicam also significantly decreased the protein expressions of APP, Aβ, COX2, EP1 and EP2, and the level of cAMP and significantly increased the EP3 and pPKA protein expressions in rat hippocampus and cortex. However, meloxicam did not significantly influence the levels of blood glucose, lipids, and insulin of rats. Our results suggest that meloxicam could significantly protect diabetic rats from cognitive impairment via a mechanism that may be associated with rebalancing the COX2-PGE2-EPs-cAMP/PKA pathway.

Differentiation of Glial Cells From hiPSCs: Potential Applications in Neurological Diseases and Cell Replacement Therapy

Glial cells are the most abundant cell type in the central nervous system (CNS) and play essential roles in maintaining brain homeostasis, forming myelin, and providing support and protection for neurons, etc. Over the past decade, significant progress has been made in the reprogramming field. Given the limited accessibility of human glial cells, in vitro differentiation of human induced pluripotent stem cells (hiPSCs) into glia may provide not only a valuable research tool for a better understanding of the functions of glia in the CNS but also a potential cellular source for clinical therapeutic purposes. In this review, we will summarize up-to-date novel strategies for the committed differentiation into the three major glial cell types, i.e., astrocyte, oligodendrocyte, and microglia, from hiPSCs, focusing on the non-neuronal cell effects on the pathology of some representative neurological diseases. Furthermore, the application of hiPSC-derived glial cells in neurological disease modeling will be discussed, so as to gain further insights into the development of new therapeutic targets for treatment of neurological disorders.

[Quercetin ameliorates inflammation in CA1 hippocampal region in aged triple transgenic Alzheimer´s disease mice model.]

INTRODUCTION

Alzheimer's disease is the most common form of dementia. It is characterized by histopathological hallmarks such as senile plaques and neurofibrillary tangles, as well as a concomitant activation of microglial cells and astrocytes that release pro-inflammatory mediators such as IL-1β, iNOS, and COX-2, leading to neuronal dysfunction and death.

OBJECTIVE

To evaluate the effect of quercetin on the inflammatory response in the CA1 area of the hippocampus in a 3xTg-AD male and female mice model.

MATERIALS AND METHODS

Animals were injected intraperitoneally with quercetin every 48 hours during three months, and we conducted histological and biochemical studies.

RESULTS

We found that in quercetin-treated 3xTg-AD mice, reactive microglia and fluorescence intensity of Aβ aggregates significantly decreased. GFAP, iNOS, and COX-2 immunoreactivity also decreased and we observed a clear tendency in the reduction of IL-1β in hippocampal lysates.

CONCLUSION

Our work suggests an anti-inflammatory effect of quercetin in the CA1 hippocampal region of aged triple transgenic Alzheimer's disease mice.

Discovery of 2-Piperidinyl Phenyl Benzamides and Trisubstituted Pyrimidines as Positive Allosteric Modulators of the Prostaglandin Receptor EP2

Prostaglandin E2 (PGE2) via its Gαs-coupled EP2 receptor protects cerebral cortical neurons from excitotoxic and anoxic injury, though EP2 receptor activation can also cause secondary neurotoxicity in chronic inflammation. We performed a high-throughput screen of a library of 292 000 small molecules and identified several compounds that have a 2-piperidinyl phenyl benzamide or trisubstituted pyrimidine core as positive modulators for human EP2 receptor. The most active compounds increased the potency of PGE2 on EP2 receptor 4-5-fold at 20 μM without altering efficacy, indicative of an allosteric mechanism. These compounds did not augment the activity of the other Gαs-coupled PGE2 receptor subtype EP4 and showed neuroprotection against N-methyl-d-aspartate (NMDA)-induced excitotoxicity. These newly developed compounds represent second-generation allosteric potentiators for EP2 receptor and shed light on a promising neuroprotective strategy. They should prove valuable as molecular tools to achieve a better understanding of the dichotomous action of brain EP2 receptor activation.

Monoacylglycerol lipase inhibitor JZL184 prevents HIV-1 gp120-induced synapse loss by altering endocannabinoid signaling

Monoacylglycerol lipase (MGL) hydrolyzes 2-arachidonoylglycerol to arachidonic acid and glycerol. Inhibition of MGL may attenuate neuroinflammation by enhancing endocannabinoid signaling and decreasing prostaglandin (PG) production. Almost half of HIV infected individuals are afflicted with HIV-associated neurocognitive disorder (HAND), a neuroinflammatory disease in which cognitive decline correlates with synapse loss. HIV infected cells shed the envelope protein gp120 which is a potent neurotoxin that induces synapse loss. Here, we tested whether inhibition of MGL, using the selective inhibitor JZL184, would prevent synapse loss induced by gp120. The number of synapses between rat hippocampal neurons in culture was quantified by imaging clusters of a GFP-tagged antibody-like protein that selectively binds to the postsynaptic scaffolding protein, PSD95. JZL184 completely blocked gp120-induced synapse loss. Inhibition of MGL decreased gp120-induced interleukin-1β (IL-1β) production and subsequent potentiation of NMDA receptor-mediated calcium influx. JZL184-mediated protection of synapses was reversed by a selective cannabinoid type 2 receptor (CB2R) inverse agonist/antagonist. JZL184 also reduced gp120-induced prostaglandin E2 (PGE2) production; PG signaling was required for gp120-induced IL-1β expression and synapse loss. Inhibition of MGL prevented gp120-induced synapse loss by activating CB2R resulting in decreased production of the inflammatory cytokine IL-1β. Because PG signaling was required for gp120-induced synapse loss, JZL184-induced decreases in PGE2 levels may also protect synapses. MGL presents a promising target for preventing synapse loss in neuroinflammatory conditions such as HAND.

Cre Driver Mice Targeting Macrophages

The Cre/loxP system is a widely applied technology for site-specific genetic manipulation in mice. This system allows for deletion of the genes of interest in specific cells, tissues, and whole organism to generate a diversity of conditional knockout mouse strains. Additionally, the Cre/loxP system is useful for development of cell- and tissue-specific reporter mice for lineage tracing, and cell-specific conditional depletion models in mice. Recently, the Cre/loxP technique was extensively adopted to characterize the monocyte/macrophage biology in mouse models. Compared to other relatively homogenous immune cell types such as neutrophils, mast cells, and basophils, monocytes/macrophages represent a highly heterogeneous population which lack specific markers or transcriptional factors. Though great efforts have been made toward establishing macrophage-specific Cre driver mice in the past decade, all of the current available strains are not perfect with regard to their depletion efficiency and targeting specificity for endogenous macrophages. Here we overview the commonly used Cre driver mouse strains targeting macrophages and discuss their major applications and limitations.

Activation of EP2 receptor suppresses poly(I: C) and LPS-mediated inflammation in primary microglia and organotypic hippocampal slice cultures: Contributing role for MAPKs

Brain inflammation is a critical factor involved in neurodegeneration. Recently, the prostaglandin E₂ (PGE₂ ) downstream members were suggested to modulate neuroinflammatory responses accompanying neurodegenerative diseases. In this study, we investigated the protective effects of prostaglandin E₂ receptor 2 (EP₂ ) during TLR3 and TLR4-driven inflammatory response using in vitro primary microglia and ex vivo organotypic hippocampal slice cultures (OHSCs). Depletion of microglia from OHSCs differentially affected TLR3 and TLR4 receptor expression. Poly(I:C) induced the production of prostaglandin E₂ in OHSCs by increasing cyclooxygenase (COX-2) and microsomal prostaglandin E synthase (mPGES)-1. Besides, stimulation of OHSCs and microglia with Poly(I:C) upregulated EP₂ receptor expression. Co-stimulation of OHSCs and microglia with the EP₂ agonist butaprost reduced inflammatory mediators induced by LPS and Poly(I:C). In Poly(I:C) challenged OHSCs, butaprost almost restored microglia ramified morphology and reduced Iba1 immunoreactivity. Importantly, microglia depletion prevented the induction of inflammatory mediators following Poly(I:C) or LPS challenge in OHSCs. Activation of EP₂ receptor reversed the Poly(I:C)/LPS-induced phosphorylation of the mitogen activated protein kinases (MAPKs) ERK, p38 MAPK and c-Jun N-terminal kinase (JNK) in microglia. Collectively, these data identify an anti-inflammatory function for EP₂ signaling in diverse innate immune responses, through a mechanism that involves the mitogen-activated protein kinases pathway.

Lipoprotein-associated Phospholipase A2 Is Associated with Risk of Mild Cognitive Impairment in Chinese Patients with Type 2 Diabetes

Type 2 diabetes mellitus (T2DM) is a low-grade chronic inflammatory diseases, which have been implicated in the pathogenesis of cognitive decline. We aim to evaluate associations between inflammatory markers and the risk of mild cognitive impairment (MCI) in T2DM. This study of 140 diabetic patients involved 71 with MCI and 69 controls. Clinical parameters, neuropsychological tests, high sensitivity C reactive protein (hsCRP), interleukin-6 (IL-6), lipoprotein-associated Phospholipase A2 (Lp-PLA2) mass and activity were measured. The results showed significantly higher plasma hsCRP, IL-6, Lp-PLA2 mass and activity in MCI group compared to controls. In T2DM with MCI, the Montreal Cognitive Assessment (MoCA) score was positively correlated with education level and high-density lipoprotein cholesterol (HDL-c), but inversely correlated with age, glycosylated hemoglobin, intima-media thickness (IMT), hsCRP, IL-6, and Lp-PLA2 mass and activity. Correlation analysis showed that both plasma Lp-PLA2 mass and activity were positively correlated with total cholesterol, low-density lipoprotein cholesterol, and IMT but negatively associated with MoCA score. Multivariable logistic regression analysis indicated higher hsCRP, Lp-PLA2 mass, Lp-PLA2 activity, and lower HDL-c to be independent risk factors increasing the possibility of MCI in T2DM. In conclusion, plasma Lp-PLA2 and hsCRP were found to be associated with the risk of MCI among T2DM patients.

Cyclooxygenase-2 contributes to oxidopamine-mediated neuronal inflammation and injury via the prostaglandin E2 receptor EP2 subtype

Cyclooxygenase-2 (COX-2) triggers pro-inflammatory processes that can aggravate neuronal degeneration and functional impairments in many neurological conditions, mainly via producing prostaglandin E2 (PGE₂) that activates four membrane receptors, EP1-EP4. However, which EP receptor is the culprit of COX-2/PGE₂-mediated neuronal inflammation and degeneration remains largely unclear and presumably depends on the insult types and responding components. Herein, we demonstrated that COX-2 was induced and showed nuclear translocation in two neuronal cell lines – mouse Neuro-2a and human SH-SY5Y – after treatment with neurotoxin 6-hydroxydopamine (6-OHDA), leading to the biosynthesis of PGE₂ and upregulation of pro-inflammatory cytokine interleukin-1β. Inhibiting COX-2 or microsomal prostaglandin E synthase-1 suppressed the 6-OHDA-triggered PGE₂ production in these cells. Treatment with PGE₂ or EP2 selective agonist butaprost, but not EP4 agonist CAY10598, increased cAMP response in both cell lines. PGE₂-initiated cAMP production in these cells was blocked by our recently developed novel selective EP2 antagonists – TG4-155 and TG6-10-1, but not by EP4 selective antagonist GW627368X. The 6-OHDA-promoted cytotoxicity was largely blocked by TG4-155, TG6-10-1 or COX-2 selective inhibitor celecoxib, but not by GW627368X. Our results suggest that PGE₂ receptor EP2 is a key mediator of COX-2 activity-initiated cAMP signaling in Neuro-2a and SH-SY5Y cells following 6-OHDA treatment, and contributes to oxidopamine-mediated neurotoxicity.

Alleviation of Microglial Activation Induced by p38 MAPK/MK2/PGE2 Axis by Capsaicin: Potential Involvement of other than TRPV1 Mechanism/s

Exaggerated inflammatory responses in microglia represent one of the major risk factors for various central nervous system’s (CNS) associated pathologies. Release of excessive inflammatory mediators such as prostaglandins and cytokines are the hallmark of hyper-activated microglia. Here we have investigated the hitherto unknown effects of capsaicin (cap) - a transient receptor potential vanilloid 1 (TRPV1) agonist- in murine primary microglia, organotypic hippocampal slice cultures (OHSCs) and human primary monocytes. Results demonstrate that cap (0.1–25 µM) significantly (p < 0.05) inhibited the release of prostaglandin E₂ (PGE₂)_, 8-iso-PGF_2α, and differentially regulated the levels of cytokines (TNF-α, IL-6 & IL-1β). Pharmacological blockade (via capsazepine & SB366791) and genetic deficiency of TRPV1 (TRPV1^−/−) did not prevent cap-mediated suppression of PGE₂ in activated microglia and OHSCs. Inhibition of PGE₂ was partially dependent on the reduced levels of PGE₂ synthesising enzymes, COX-2 and mPGES-1. To evaluate potential molecular targets, we discovered that cap significantly suppressed the activation of p38 MAPK and MAPKAPK2 (MK2). Altogether, we demonstrate that cap alleviates excessive inflammatory events by targeting the PGE₂ pathway in in vitro and ex vivo immune cell models. These findings have broad relevance in understanding and paving new avenues for ongoing TRPV1 based drug therapies in neuroinflammatory-associated diseases.

Inflammation in Alzheimer's disease: Lessons learned from microglia-depletion models

Microglia are the primary immune cell of the brain and function to protect the central nervous system (CNS) from injury and invading pathogens. In the homeostatic brain, microglia serve to support neuronal health through synaptic pruning, promoting normal brain connectivity and development, and through release of neurotrophic factors, providing support for CNS integrity. However, recent evidence indicates that the homeostatic functioning of these cells is lost in neurodegenerative disease, including Alzheimer's disease (AD), ultimately contributing to a chronic neuroinflammatory environment in the brain. Importantly, the development of compounds and genetic models to ablate the microglial compartment has emerged as effective tools to further our understanding of microglial function in AD. Use of these models has identified roles of microglia in several pathological facets of AD, including tau propagation, synaptic stripping, neuronal loss, and cognitive decline. Although culminating evidence utilizing these microglial ablation models reports an absence of CNS-endogenous and peripheral myeloid cell involvement in Aβ phagocytosis, recent data indicates that targeting microglia-evoked neuroinflammation in AD may be essential for potential therapeutics. Therefore, identifying altered signaling pathways in the microglia-devoid brain may assist with the development of effective inflammation-based therapies in AD.

Cerebroprotection by the neuronal PGE2 receptor EP2 after intracerebral hemorrhage in middle-aged mice

Inflammatory responses mediated by prostaglandins such as PGE₂ may contribute to secondary brain injury after intracerebral hemorrhage (ICH). However, the cell-specific signaling by PGE₂ receptor EP2 differs depending on whether the neuropathic insult is acute or chronic. Using genetic and pharmacologic approaches, we investigated the role of EP2 receptor in two mouse models of ICH induced by intrastriatal injection of collagenase or autologous arterial whole blood. We used middle-aged male mice to enhance the clinical relevance of the study. EP2 receptor was expressed in neurons but not in astrocytes or microglia after collagenase-induced ICH. Brain injury after collagenase-induced ICH was associated with enhanced cellular and molecular inflammatory responses, oxidative stress, and matrix metalloproteinase (MMP)-2/9 activity. EP2 receptor deletion exacerbated brain injury, brain swelling/edema, neuronal death, and neurobehavioral deficits, whereas EP2 receptor activation by the highly selective agonist AE1-259-01 reversed these outcomes. EP2 receptor deletion also exacerbated brain edema and neurologic deficits in the blood ICH model. These findings support the premise that neuronal EP2 receptor activation by PGE₂ protects brain against ICH injury in middle-aged mice through its anti-inflammatory and anti-oxidant effects and anti-MMP-2/9 activity. PGE₂/EP2 signaling warrants further investigation for potential use in ICH treatment.

Elderly patients have an altered gut-brain axis regardless of the presence of cirrhosis

Cognitive difficulties manifested by the growing elderly population with cirrhosis could be amnestic (memory-related) or non-amnestic (memory-unrelated). The underlying neuro-biological and gut-brain changes are unclear in this population. We aimed to define gut-brain axis alterations in elderly cirrhotics compared to non-cirrhotic individuals based on presence of cirrhosis and on neuropsychological performance. Age-matched outpatients with/without cirrhosis underwent cognitive testing (amnestic/non-amnestic domains), quality of life (HRQOL), multi-modal MRI (fMRI go/no-go task, volumetry and MR spectroscopy), blood (inflammatory cytokines) and stool collection (for microbiota). Groups were studied based on cirrhosis/not and also based on neuropsychological performance (amnestic-type, amnestic/non-amnestic-type and unimpaired). Cirrhotics were impaired on non-amnestic and selected amnestic tests, HRQOL and systemic inflammation compared to non-cirrhotics. Cirrhotics demonstrated significant changes on MR spectroscopy but not on fMRI or volumetry. Correlation networks showed that Lactobacillales members were positively while Enterobacteriaceae and Porphyromonadaceae were negatively linked with cognition. Using the neuropsychological classification amnestic/non-amnestic-type individuals were majority cirrhosis and had worse HRQOL, higher inflammation and decreased autochthonous taxa relative abundance compared to the rest. This classification also predicted fMRI, MR spectroscopy and volumetry changes between groups. We conclude that gut-brain axis alterations may be associated with the type of neurobehavioral decline or inflamm-aging in elderly cirrhotic subjects.

The endocytic pathway in microglia during health, aging and Alzheimer's disease

Microglia, the main phagocytes of the central nervous system (CNS), are involved in the surveillance and maintenance of nervous tissue. During normal tissue homeostasis, microglia migrates within the CNS, phagocytose dead cells and tissue debris, and modulate synapse pruning and spine formation via controlled phagocytosis. In the event of an invasion by a foreign body, microglia are able to phagocytose the invading pathogen and process it proteolytically for antigen presentation. Internalized substrates are incorporated and sorted within the endocytic pathway and thereafter transported via complex vesicular routes. When targeted for degradation, substrates are delivered to acidic late endosomes and lysosomes. In these, the enzymatic degradation relies on pH and enzyme content. Endocytosis, sorting, transport, compartment acidification and degradation are regulated by complex signaling mechanisms, and these may be altered during aging and pathology. In this review, we discuss the endocytic pathway in microglia, with insight into the mechanisms controlling lysosomal biogenesis and pH regulation. We also discuss microglial lysosome function associated with Alzheimer's disease (AD) and the mechanisms of amyloid-beta (Aβ) internalization and degradation. Finally, we explore some therapies currently being investigated to treat AD and their effects on microglial response to Aβ, with insight in those involving enhancement of lysosomal function.

Inhibition of STAT3- and MAPK-dependent PGE2 synthesis ameliorates phagocytosis of fibrillar β-amyloid peptide (1-42) via EP2 receptor in EMF-stimulated N9 microglial cells

Background

Prostaglandin E₂ (PGE₂)-involved neuroinflammatory processes are prevalent in several neurological conditions and diseases. Amyloid burden is correlated with the activation of E-prostanoid (EP) 2 receptors by PGE₂ in Alzheimer’s disease. We previously demonstrated that electromagnetic field (EMF) exposure can induce pro-inflammatory responses and the depression of phagocytosis in microglial cells, but the signaling pathways involved in phagocytosis of fibrillar β-amyloid (fAβ) in microglial cells exposed to EMF are poorly understood. Given the important role of PGE₂ in neural physiopathological processes, we investigated the PGE₂-related signaling mechanism in the immunomodulatory phagocytosis of EMF-stimulated N9 microglial cells (N9 cells).

Methods

N9 cells were exposed to EMF with or without pretreatment with the selective inhibitors of cyclooxygenase-2 (COX-2), Janus kinase 2 (JAK2), signal transducer and activator of transcription 3 (STAT3), and mitogen-activated protein kinases (MAPKs) and antagonists of PG receptors EP1-4. The production of endogenous PGE₂ was quantified by enzyme immunoassays. The phagocytic ability of N9 cells was evaluated based on the fluorescence intensity of the engulfed fluorescent-labeled fibrillar β-amyloid peptide (1-42) (fAβ₄₂) measured using a flow cytometer and a fluorescence microscope. The effects of pharmacological agents on EMF-activated microglia were investigated based on the expressions of JAK2, STAT3, p38/ERK/JNK MAPKs, COX-2, microsomal prostaglandin E synthase-1 (mPGES-1), and EP2 using real-time PCR and/or western blotting.

Results

EMF exposure significantly increased the production of PGE₂ and decreased the phagocytosis of fluorescent-labeled fAβ₄₂ by N9 cells. The selective inhibitors of COX-2, JAK2, STAT3, and MAPKs clearly depressed PGE₂ release and ameliorated microglial phagocytosis after EMF exposure. Pharmacological agents suppressed the phosphorylation of JAK2-STAT3 and MAPKs, leading to the amelioration of the phagocytic ability of EMF-stimulated N9 cells. Antagonist studies of EP1-4 receptors showed that EMF depressed the phagocytosis of fAβ₄₂ through the PGE₂ system, which is linked to EP2 receptors.

Conclusions

This study indicates that EMF exposure could induce phagocytic depression via JAK2-STAT3- and MAPK-dependent PGE₂-EP2 receptor signaling pathways in microglia. Therefore, pharmacological inhibition of PGE₂ synthesis and EP2 receptors may be a potential therapeutic strategy to combat the neurobiological deterioration that follows EMF exposure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0762-9) contains supplementary material, which is available to authorized users.

Targeting innate immunity for neurodegenerative disorders of the central nervous system

Neuroinflammation is critically involved in numerous neurodegenerative diseases, and key signaling steps of innate immune activation hence represent promising therapeutic targets. This mini review series originated from the 4th Venusberg Meeting on Neuroinflammation held in Bonn, Germany, 7-9th May 2015, presenting updates on innate immunity in acute brain injury and chronic neurodegenerative disorders, such as traumatic brain injury and Alzheimer disease, on the role of astrocytes and microglia, as well as technical developments that may help elucidate neuroinflammatory mechanisms and establish clinical relevance. In this meeting report, a brief overview of physiological and pathological microglia morphology is followed by a synopsis on PGE2 receptors, insights into the role of arginine metabolism and further relevant aspects of neuroinflammation in various clinical settings, and concluded by a presentation of technical challenges and solutions when working with microglia and astrocyte cultures. Microglial ontogeny and induced pluripotent stem cell-derived microglia, advances of TREM2 signaling, and the cytokine paradox in Alzheimer's disease are further contributions to this article. Neuroinflammation is critically involved in numerous neurodegenerative diseases, and key signaling steps of innate immune activation hence represent promising therapeutic targets. This mini review series originated from the 4th Venusberg Meeting on Neuroinflammation held in Bonn, Germany, 7-9th May 2015, presenting updates on innate immunity in acute brain injury and chronic neurodegenerative disorders, such as traumatic brain injury and Alzheimer's disease, on the role of astrocytes and microglia, as well as technical developments that may help elucidate neuroinflammatory mechanisms and establish clinical relevance. In this meeting report, a brief overview on physiological and pathological microglia morphology is followed by a synopsis on PGE2 receptors, insights into the role of arginine metabolism and further relevant aspects of neuroinflammation in various clinical settings, and concluded by a presentation of technical challenges and solutions when working with microglia cultures. Microglial ontogeny and induced pluripotent stem cell-derived microglia, advances of TREM2 signaling, and the cytokine paradox in Alzheimer's disease are further contributions to this article.

Dietary arachidonic acid as a risk factor for age-associated neurodegenerative diseases: Potential mechanisms

Alzheimer's disease and associated diseases constitute a major public health concern worldwide. Nutrition-based, preventive strategies could possibly be effective in delaying the occurrence of these diseases and lower their prevalence. Arachidonic acid is the second major polyunsaturated fatty acid (PUFA) and several studies support its involvement in Alzheimer's disease. The objective of this review is to examine how dietary arachidonic acid contributes to Alzheimer's disease mechanisms and therefore to its prevention. First, we explore the sources of neuronal arachidonic acid that could potentially originate from either the conversion of linoleic acid, or from dietary sources and transfer across the blood-brain-barrier. In a second part, a brief overview of the role of the two main agents of Alzheimer's disease, tau protein and Aβ peptide is given, followed by the examination of the relationship between arachidonic acid and the disease. Third, the putative mechanisms by which arachidonic acid could influence Alzheimer's disease occurrence and evolution are presented. The conclusion is devoted to what remains to be determined before integrating arachidonic acid in the design of preventive strategies against Alzheimer's disease and other neurodegenerative diseases.

Deficiency in either COX-1 or COX-2 genes does not affect amyloid beta protein burden in amyloid precursor protein transgenic mice

Epidemiologic studies indicate that chronic use of non-steroidal anti-inflammatory drugs (NSAIDs) is associated with a lower risk for developing Alzheimer's disease (AD). Because the primary mode of action of NSAIDs is to inhibit cyclooxygenase (COX) activity, it has been proposed that perturbed activity of COX-1 or COX-2 contributes to AD pathogenesis. To test the role of COX-1 or COX-2 in amyloid deposition and amyloid-associated inflammatory changes, we examined amyloid precursor protein (APP) transgenic mice in the context of either COX-1 or COX-2 deficiency. Our studies showed that loss of either COX-1 or COX-2 gene did not alter amyloid burden in brains of the APP transgenic mice. However, one marker of microglial activation (CD45) was decreased in brains of COX-1 deficient/APP animals and showed a strong trend in reduction in COX-2 deficient/APP animals. These results suggest that COX activity and amyloid deposition in brain are likely independent processes. Further, if NSAIDs do causally reduce the risks of AD, then our findings indicate that the mechanisms are likely not due primarily to their inhibition on COX or γ-secretase modulation activity, the latter reported recently after acute dosing of ibuprofen in humans and nonhuman primates.

Microglial dysfunction connects depression and Alzheimer's disease

Alzheimer's disease (AD) and major depressive disorder (MDD) are highly prevalent neuropsychiatric conditions with intriguing epidemiological overlaps. Depressed patients are at increased risk of developing late-onset AD, and around one in four AD patients are co-diagnosed with MDD. Microglia are the main cellular effectors of innate immunity in the brain, and their activation is central to neuroinflammation - a ubiquitous process in brain pathology, thought to be a causal factor of both AD and MDD. Microglia serve several physiological functions, including roles in synaptic plasticity and neurogenesis, which may be disrupted in neuroinflammation. Following early work on the 'sickness behavior' of humans and other animals, microglia-derived inflammatory cytokines have been shown to produce depressive-like symptoms when administered exogenously or released in response to infection. MDD patients consistently show increased circulating levels of pro-inflammatory cytokines, and anti-inflammatory drugs show promise for treating depression. Activated microglia are abundant in the AD brain, and concentrate around senile plaques, hallmark lesions composed of aggregated amyloid-β peptide (Aβ). The Aβ burden in affected brains is regulated largely by microglial clearance, and the complex activation state of microglia may be crucial for AD progression. Intriguingly, recent reports have linked soluble Aβ oligomers, toxins that accumulate in AD brains and are thought to cause memory impairment, to increased brain cytokine production and depressive-like behavior in mice. Here, we review recent findings supporting the inflammatory hypotheses of AD and MDD, focusing on microglia as a common player and therapeutic target linking these devastating disorders.

Insights on the pathophysiology of Alzheimer's disease: The crosstalk between amyloid pathology, neuroinflammation and the peripheral immune system

Alzheimer's disease (AD) is the most common form of dementia, whose prevalence is growing along with the increased life expectancy. Although the accumulation and deposition of amyloid beta (Aβ) peptides in the brain is viewed as one of the pathological hallmarks of AD and underlies, at least in part, brain cell dysfunction and behavior alterations, the etiology of this neurodegenerative disease is still poorly understood. Noticeably, increased amyloid load is accompanied by marked inflammatory alterations, both at the level of the brain parenchyma and at the barriers of the brain. However, it is debatable whether the neuroinflammation observed in aging and in AD, together with alterations in the peripheral immune system, are responsible for increased amyloidogenesis, decreased clearance of Aβ out of the brain and/or the marked deficits in memory and cognition manifested by AD patients. Herein, we scrutinize some important traits of the pathophysiology of aging and AD, focusing on the interplay between the amyloidogenic pathway, neuroinflammation and the peripheral immune system.

An EP2 Agonist Facilitates NMDA-Induced Outward Currents and Inhibits Dendritic Beading through Activation of BK Channels in Mouse Cortical Neurons

Prostaglandin E₂ (PGE₂), a major metabolite of arachidonic acid produced by cyclooxygenase pathways, exerts its bioactive responses by activating four E-prostanoid receptor subtypes, EP1, EP2, EP3, and EP4. PGE₂ enables modulating N-methyl-D-aspartate (NMDA) receptor-mediated responses. However, the effect of E-prostanoid receptor agonists on large-conductance Ca²⁺-activated K⁺ (BK) channels, which are functionally coupled with NMDA receptors, remains unclear. Here, we showed that EP2 receptor-mediated signaling pathways increased NMDA-induced outward currents (I_NMDA-OUT), which are associated with the BK channel activation. Patch-clamp recordings from the acutely dissociated mouse cortical neurons revealed that an EP2 receptor agonist activated I_NMDA-OUT, whereas an EP3 receptor agonist reduced it. Agonists of EP1 or EP4 receptors showed no significant effects on I_NMDA-OUT. A direct perfusion of 3,5′-cyclic adenosine monophosphate (cAMP) through the patch pipette facilitated I_NMDA-OUT, which was abolished by the presence of protein kinase A (PKA) inhibitor. Furthermore, facilitation of I_NMDA-OUT caused by an EP2 receptor agonist was significantly suppressed by PKA inhibitor. Finally, the activation of BK channels through EP2 receptors facilitated the recovery phase of NMDA-induced dendritic beading in the primary cultured cortical neurons. These results suggest that a direct activation of BK channels by EP2 receptor-mediated signaling pathways plays neuroprotective roles in cortical neurons.

From Molecular Circuit Dysfunction to Disease: Case Studies in Epilepsy, Traumatic Brain Injury, and Alzheimer's Disease

Complex circuitry with feed-forward and feed-back systems regulate neuronal activity throughout the brain. Cell biological, electrical, and neurotransmitter systems enable neural networks to process and drive the entire spectrum of cognitive, behavioral, and motor functions. Simultaneous orchestration of distinct cells and interconnected neural circuits relies on hundreds, if not thousands, of unique molecular interactions. Even single molecule dysfunctions can be disrupting to neural circuit activity, leading to neurological pathology. Here, we sample our current understanding of how molecular aberrations lead to disruptions in networks using three neurological pathologies as exemplars: epilepsy, traumatic brain injury (TBI), and Alzheimer's disease (AD). Epilepsy provides a window into how total destabilization of network balance can occur. TBI is an abrupt physical disruption that manifests in both acute and chronic neurological deficits. Last, in AD progressive cell loss leads to devastating cognitive consequences. Interestingly, all three of these neurological diseases are interrelated. The goal of this review, therefore, is to identify molecular changes that may lead to network dysfunction, elaborate on how altered network activity and circuit structure can contribute to neurological disease, and suggest common threads that may lie at the heart of molecular circuit dysfunction.

Morin downregulates nitric oxide and prostaglandin E2 production in LPS-stimulated BV2 microglial cells by suppressing NF-κB activity and activating HO-1 induction

Morin possesses anti-inflammatory activity against septic shock and allergic responses, and prevents acute liver damage. However, the biological mechanism of action of morin in neuroinflammation remains largely unknown. Therefore, the present study investigated whether morin has the ability to attenuate expression of proinflammatory mediators such as nitric oxide (NO) and prostaglandin E2 (PGE2) in lipopolysaccharide (LPS)-stimulated BV2 microglial cells. Morin inhibited the expression of LPS-induced proinflammatory mediators such as NO and PGE2, without any cytotoxic effects. Furthermore, LPS-induced inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2) were inhibited both at the mRNA and protein levels in response to morin. Morin also attenuated LPS-induced DNA-binding activity of nuclear transcription factor-κB (NF-κB) and its promoter activity. Pyrrolidine dithiocarbamate (PDTC), a specific NF-κB inhibitor, downregulated the expression of LPS-induced iNOS and COX-2, which suggests that morin-mediated NF-κB inhibition is the main signaling pathway responsible for the inhibition of iNOS and COX-2 expression. Additionally, morin increased induction of heme oxygenase-1 (HO-1) activity, leading to the suppression of NO and PGE2 production. Our results indicate that morin downregulates the expression of proinflammatory genes, such as iNOS and COX-2, involved in the synthesis of NO and PGE2 in LPS-stimulated BV2 microglial cells by suppressing NF-κB activity and activation of HO-1. Taken together, the findings of the present study suggest that morin may have potential as a therapeutic for the prevention of neuroinflammation.

Evaluating cell reprogramming, differentiation and conversion technologies in neuroscience

The scarcity of live human brain cells for experimental access has for a long time limited our ability to study complex human neurological disorders and elucidate basic neuroscientific mechanisms. A decade ago, the development of methods to reprogramme somatic human cells into induced pluripotent stem cells enabled the in vitro generation of a wide range of neural cells from virtually any human individual. The growth of methods to generate more robust and defined neural cell types through reprogramming and direct conversion into induced neurons has led to the establishment of various human reprogramming-based neural disease models.

Cyclooxygenase inhibition targets neurons to prevent early behavioural decline in Alzheimer's disease model mice

Identifying preventive targets for Alzheimer's disease is a central challenge of modern medicine. Non-steroidal anti-inflammatory drugs, which inhibit the cyclooxygenase enzymes COX-1 and COX-2, reduce the risk of developing Alzheimer's disease in normal ageing populations. This preventive effect coincides with an extended preclinical phase that spans years to decades before onset of cognitive decline. In the brain, COX-2 is induced in neurons in response to excitatory synaptic activity and in glial cells in response to inflammation. To identify mechanisms underlying prevention of cognitive decline by anti-inflammatory drugs, we first identified an early object memory deficit in APPSwe-PS1ΔE9 mice that preceded previously identified spatial memory deficits in this model. We modelled prevention of this memory deficit with ibuprofen, and found that ibuprofen prevented memory impairment without producing any measurable changes in amyloid-β accumulation or glial inflammation. Instead, ibuprofen modulated hippocampal gene expression in pathways involved in neuronal plasticity and increased levels of norepinephrine and dopamine. The gene most highly downregulated by ibuprofen was neuronal tryptophan 2,3-dioxygenase (Tdo2), which encodes an enzyme that metabolizes tryptophan to kynurenine. TDO2 expression was increased by neuronal COX-2 activity, and overexpression of hippocampal TDO2 produced behavioural deficits. Moreover, pharmacological TDO2 inhibition prevented behavioural deficits in APPSwe-PS1ΔE9 mice. Taken together, these data demonstrate broad effects of cyclooxygenase inhibition on multiple neuronal pathways that counteract the neurotoxic effects of early accumulating amyloid-β oligomers.

Microglia communication: Parallels between aging and Alzheimer's disease

Aging alters the functional integrity of the adult brain, driving cognitive impairments and susceptibility to neurodegenerative disorders in healthy individuals. In fact, aging remains the most dominant risk factor for Alzheimer's disease (AD). Recent findings have expanded our understanding of microglia function in the normal aging and AD brain, provoking an appreciation for microglia involvement in remodeling neuronal connections and maintaining brain integrity. This homeostatic function of microglia is achieved in part through the ability of microglia to interact extensively with and rapidly respond to changes in the brain microenvironment to enable adequate phenotypic transformations. Here, we discuss pro‐inflammatory drivers of microglia transformation in aging and AD by focusing on the immune‐modulatory functions of secreted factors, such as cytokines, complement factors and extracellular vesicles. We highlight the involvement of these secreted factors in aging and AD‐associated cellular changes in microglia immune activation, surveillance function, and phagocytosis. Finally, we discuss how pro‐inflammatory phenotypic changes associated with altered immune communication could both facilitate and exacerbate impairments in synaptic plasticity and cognitive function observed in the aged and AD brain.

Nrf2 activation in the treatment of neurodegenerative diseases: a focus on its role in mitochondrial bioenergetics and function

The nuclear factor erythroid-derived 2 (NF-E2)-related factor 2 (Nrf2) is a transcription factor well-known for its function in controlling the basal and inducible expression of a variety of antioxidant and detoxifying enzymes. As part of its cytoprotective activity, increasing evidence supports its role in metabolism and mitochondrial bioenergetics and function. Neurodegenerative diseases are excellent candidates for Nrf2-targeted treatments. Most neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, frontotemporal dementia and Friedreich's ataxia are characterized by oxidative stress, misfolded protein aggregates, and chronic inflammation, the common targets of Nrf2 therapeutic strategies. Together with them, mitochondrial dysfunction is implicated in the pathogenesis of most neurodegenerative disorders. The recently recognized ability of Nrf2 to regulate intermediary metabolism and mitochondrial function makes Nrf2 activation an attractive and comprehensive strategy for the treatment of neurodegenerative disorders. This review aims to focus on the potential therapeutic role of Nrf2 activation in neurodegeneration, with special emphasis on mitochondrial bioenergetics and function, metabolism and the role of transporters, all of which collectively contribute to the cytoprotective activity of this transcription factor.

Exercise as a pro-cognitive, pro-neurogenic and anti-inflammatory intervention in transgenic mouse models of Alzheimer's disease

It is now well established, at least in animal models, that exercise elicits potent pro-cognitive and pro-neurogenic effects. Alzheimer's disease (AD) is one of the leading causes of dementia and represents one of the greatest burdens on healthcare systems worldwide, with no effective treatment for the disease to date. Exercise presents a promising non-pharmacological option to potentially delay the onset of or slow down the progression of AD. Exercise interventions in mouse models of AD have been explored and have been found to reduce amyloid pathology and improve cognitive function. More recent studies have expanded the research question by investigating potential pro-neurogenic and anti-inflammatory effects of exercise. In this review we summarise studies that have examined exercise-mediated effects on AD pathology, cognitive function, hippocampal neurogenesis and neuroinflammation in transgenic mouse models of AD. Furthermore, we attempt to identify the optimum exercise conditions required to elicit the greatest benefits, taking into account age and pathology of the model, as well as type and duration of exercise.

Broad impact of deleting endogenous cannabinoid hydrolyzing enzymes and the CB1 cannabinoid receptor on the endogenous cannabinoid-related lipidome in eight regions of the mouse brain

BACKGROUND AND PURPOSE

The enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) hydrolyze endogenous cannabinoids (eCBs), N-arachidonoyl ethanolamine (AEA) and 2-arachidonoyl glycerol (2-AG), respectively. These enzymes also metabolize eCB analogs such as lipoamines and 2-acyl glycerols, most of which are not ligands at CB1. To test the hypothesis that deleting eCB hydrolyzing enzymes and CB1 shifts lipid metabolism more broadly and impacts more families of eCB structural analogs, targeted lipidomics analyses were performed on FAAH KO, MAGL KO, and CB1 KO mice and compared to WT controls in 8 brain regions.

EXPERIMENTAL APPROACH

Methanolic extracts of discrete brain regions (brainstem, cerebellum, cortex, hippocampus, hypothalamus, midbrain, striatum and thalamus) were partially purified on C-18 solid-phase extraction columns. Over 70 lipids per sample were then analyzed with HPLC/MS/MS.

KEY RESULTS

AEA and 2-AG were unaffected throughout the brain in CB1 KO mice; however, there was an increase in the arachidonic acid (AA) metabolite, PGE2 in the majority of brain areas. By contrast, PGE2 and AA levels were significantly reduced throughout the brain in the MAGL KO corresponding to significant increases in 2-AG. No changes in AA or PGE2 were seen throughout in the FAAH KO brain, despite significant increases in AEA, suggesting AA liberated by FAAH does not contribute to steady state levels of AA or PGE2. Changes in the lipidome were not confined to the AA derivatives and showed regional variation in each of the eCB KO models.

CONCLUSIONS AND IMPLICATIONS

AEA and 2-AG hydrolyzing enzymes and the CB1 receptor link the eCB system to broader lipid signaling networks in contrasting ways, potentially altering neurotransmission and behavior independently of cannabinoid receptor signaling.

Untangling the Web: Toxic and Protective Effects of Neuroinflammation and PGE2 Signaling in Alzheimer's Disease

The neuroinflammatory response has received increasing attention as a key factor in the pathogenesis of Alzheimer's disease (AD). Microglia, the innate immune cells and resident phagocytes of the brain, respond to accumulating Aβ peptides by generating a nonresolving inflammatory response. While this response can clear Aβ peptides from the nervous system in some settings, its failure to do so in AD accelerates synaptic injury, neuronal loss, and cognitive decline. The complex molecular components of this response are beginning to be unraveled, with identification of both damaging and protective roles for individual components of the neuroinflammatory response. Even within one molecular pathway, contrasting effects are often present. As one example, recent studies of the inflammatory cyclooxygenase-prostaglandin pathway have revealed both beneficial and detrimental effects dependent on the disease context, cell type, and downstream signaling pathway. Nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit cyclooxygenases, are associated with reduced AD risk when taken by cognitively normal populations, but additional clinical and mouse model studies have added complexities and caveats to this finding. Downstream of cyclooxygenase activity, prostaglandin E2 signaling exerts both damaging pro-inflammatory and protective anti-inflammatory effects through actions of specific E-prostanoid G-protein coupled receptors on specific cell types. These complexities underscore the need for careful study of individual components of the neuroinflammatory response to better understand their contribution to AD pathogenesis and progression.

Eliminating microglia in Alzheimer's mice prevents neuronal loss without modulating amyloid-β pathology

In addition to amyloid-β plaque and tau neurofibrillary tangle deposition, neuroinflammation is considered a key feature of Alzheimer's disease pathology. Inflammation in Alzheimer's disease is characterized by the presence of reactive astrocytes and activated microglia surrounding amyloid plaques, implicating their role in disease pathogenesis. Microglia in the healthy adult mouse depend on colony-stimulating factor 1 receptor (CSF1R) signalling for survival, and pharmacological inhibition of this receptor results in rapid elimination of nearly all of the microglia in the central nervous system. In this study, we set out to determine if chronically activated microglia in the Alzheimer's disease brain are also dependent on CSF1R signalling, and if so, how these cells contribute to disease pathogenesis. Ten-month-old 5xfAD mice were treated with a selective CSF1R inhibitor for 1 month, resulting in the elimination of ∼80% of microglia. Chronic microglial elimination does not alter amyloid-β levels or plaque load; however, it does rescue dendritic spine loss and prevent neuronal loss in 5xfAD mice, as well as reduce overall neuroinflammation. Importantly, behavioural testing revealed improvements in contextual memory. Collectively, these results demonstrate that microglia contribute to neuronal loss, as well as memory impairments in 5xfAD mice, but do not mediate or protect from amyloid pathology.

Curcumin Ameliorates the Reduction Effect of PGE2 on Fibrillar β-Amyloid Peptide (1-42)-Induced Microglial Phagocytosis through the Inhibition of EP2-PKA Signaling in N9 Microglial Cells

Inflammatory activation of microglia and β amyloid (Aβ) deposition are considered to work both independently and synergistically to contribute to the increased risk of Alzheimer's disease (AD). Recent studies indicate that long-term use of phenolic compounds provides protection against AD, primarily due to their anti-inflammatory actions. We previously suggested that phenolic compound curcumin ameliorated phagocytosis possibly through its anti-inflammatory effects rather than direct regulation of phagocytic function in electromagnetic field-exposed N9 microglial cells (N9 cells). Here, we explored the prostaglandin-E2 (PGE2)-related signaling pathway that involved in curcumin-mediated phagocytosis in fibrillar β-amyloid peptide (1-42) (fAβ42)-stimulated N9 cells. Treatment with fAβ42 increased phagocytosis of fluorescent-labeled latex beads in N9 cells. This increase was attenuated in a dose-dependent manner by endogenous and exogenous PGE2, as well as a selective EP2 or protein kinase A (PKA) agonist, but not by an EP4 agonist. We also found that an antagonist of EP2, but not EP4, abolished the reduction effect of PGE2 on fAβ42-induced microglial phagocytosis. Additionally, the increased expression of endogenous PGE2, EP2, and cyclic adenosine monophosphate (AMP), and activation of vasodilator-stimulated phosphoprotein, cyclic AMP responsive element-binding protein, and PKA were depressed by curcumin administration. This reduction led to the amelioration of the phagocytic abilities of PGE2-stimulated N9 cells. Taken together, these data suggested that curcumin restored the attenuating effect of PGE2 on fAβ42-induced microglial phagocytosis via a signaling mechanism involving EP2 and PKA. Moreover, due to its immune modulatory effects, curcumin may be a promising pharmacological candidate for neurodegenerative diseases.

Prostaglandin J2: a potential target for halting inflammation-induced neurodegeneration

Prostaglandins (PGs) are produced via cyclooxygenases, which are enzymes that play a major role in neuroinflammation. Epidemiological studies show that chronic treatment with low levels of cyclooxygenase inhibitors (nonsteroidal anti-inflammatory drugs (NSAIDs)) lowers the risk for Alzheimer's disease (AD) and Parkinson's disease (PD) by as much as 50%. Unfortunately, inhibiting cyclooxygenases with NSAIDs blocks the synthesis of downstream neuroprotective and neurotoxic PGs, thus producing adverse side effects. We focus on prostaglandin J2 (PGJ2) because it is highly neurotoxic compared to PGA1, D2, and E2. Unlike other PGs, PGJ2 and its metabolites have a cyclopentenone ring with reactive α,β-unsaturated carbonyl groups that form covalent Michael adducts with key cysteines in proteins and GSH. Cysteine-binding electrophiles such as PGJ2 are considered to play an important role in determining whether neurons will live or die. We discuss in vitro and in vivo studies showing that PGJ2 induces pathological processes relevant to neurodegenerative disorders such as AD and PD. Further, we discuss our work showing that increasing intracellular cAMP with the lipophilic peptide PACAP27 counteracts some of the PGJ2-induced detrimental effects. New therapeutic strategies that neutralize the effects of specific neurotoxic PGs downstream from cyclooxygenases could have a significant impact on the treatment of chronic neurodegenerative disorders with fewer adverse side effects.

The Lipoxygenases: Their Regulation and Implication in Alzheimer's Disease

Inflammatory processes and alterations of lipid metabolism play a crucial role in Alzheimer’s disease (AD) and other neurodegenerative disorders. Polyunsaturated fatty acids (PUFA) metabolism impaired by cyclooxygenases (COX-1, COX-2), which are responsible for formation of several eicosanoids, and by lipoxygenases (LOXs) that catalyze the addition of oxygen to linolenic, arachidonic (AA), and docosahexaenoic acids (DHA) and other PUFA leading to formation of bioactive lipids, significantly affects the course of neurodegenerative diseases. Among several isoforms, 5-LOX and 12/15-LOX are especially important in neuroinflammation/neurodegeneration. These two LOXs are regulated by substrate concentration and availability, and by phosphorylation/dephosphorylation through protein kinases PKA, PKC and MAP-kinases, including ERK1/ERK2 and p38. The protein/protein interaction also is involved in the mechanism of 5-LOX regulation through FLAP protein and coactosin-like protein. Moreover, non-heme iron and calcium ions are potent regulators of LOXs. The enzyme activity significantly depends on the cell redox state and is differently regulated by various signaling pathways. 5-LOX and 12/15-LOX convert linolenic acid, AA, and DHA into several bioactive compounds e.g. hydroperoxyeicosatetraenoic acids (5-HPETE, 12S-HPETE, 15S-HPETE), which are reduced to corresponding HETE compounds. These enzymes synthesize several bioactive lipids, e.g. leucotrienes, lipoxins, hepoxilins and docosahexaenoids. 15-LOX is responsible for DHA metabolism into neuroprotectin D1 (NPD1) with significant antiapoptotic properties which is down-regulated in AD. In this review, the regulation and impact of 5-LOX and 12/15-LOX in the pathomechanism of AD is discussed. Moreover, we describe the role of several products of LOXs, which may have significant pro- or anti-inflammatory activity in AD, and the cytoprotective effects of LOX inhibitors.

Generation of Bioactive Oxylipins from Exogenously Added Arachidonic, Eicosapentaenoic and Docosahexaenoic Acid in Primary Human Brain Microvessel Endothelial Cells

The human blood-brain barrier (BBB) is the restrictive barrier between the brain parenchyma and the circulating blood and is formed in part by microvessel endothelial cells. The brain contains significant amounts of arachidonic acid (ARA), and docosahexaenoic acid (DHA), which potentially give rise to the generation of bioactive oxylipins. Oxylipins are oxygenated fatty acid metabolites that are involved in an assortment of biological functions regulating neurological health and disease. Since it is not known which oxylipins are generated by human brain microvessel endothelial cells (HBMECs), they were incubated for up to 30 min in the absence or presence of 0.1-mM ARA, eicosapentaenoic acid (EPA) or DHA bound to albumin (1:1 molar ratio), and the oxylipins generated were examined using high performance liquid chromatography-tandem mass spectrometry (HPLC/MS/MS). Of 135 oxylipins screened in the media, 63 were present at >0.1 ng/mL at baseline, and 95 were present after incubation with fatty acid. Oxylipins were rapidly generated and reached maximum levels by 2-5 min. While ARA, EPA and DHA each stimulated the production of oxylipins derived from these fatty acids themselves, ARA also stimulated the production of oxylipins from endogenous 18- and 20-carbon fatty acids, including α-linolenic acid. Oxylipins generated by the lipoxygenase pathway predominated both in resting and stimulated states. Oxylipins formed via the cytochrome P450 pathway were formed primarily from DHA and EPA, but not ARA. These data indicate that HBMECs are capable of generating a plethora of bioactive lipids that have the potential to modulate BBB endothelial cell function.

GWAS of longitudinal amyloid accumulation on 18F-florbetapir PET in Alzheimer's disease implicates microglial activation gene IL1RAP

Brain amyloid deposition is thought to be a seminal event in Alzheimer's disease. To identify genes influencing Alzheimer's disease pathogenesis, we performed a genome-wide association study of longitudinal change in brain amyloid burden measured by (18)F-florbetapir PET. A novel association with higher rates of amyloid accumulation independent from APOE (apolipoprotein E) ε4 status was identified in IL1RAP (interleukin-1 receptor accessory protein; rs12053868-G; P = 1.38 × 10(-9)) and was validated by deep sequencing. IL1RAP rs12053868-G carriers were more likely to progress from mild cognitive impairment to Alzheimer's disease and exhibited greater longitudinal temporal cortex atrophy on MRI. In independent cohorts rs12053868-G was associated with accelerated cognitive decline and lower cortical (11)C-PBR28 PET signal, a marker of microglial activation. These results suggest a crucial role of activated microglia in limiting amyloid accumulation and nominate the IL-1/IL1RAP pathway as a potential target for modulating this process.