Role of the toll-like receptor 4 in neuroinflammation in Alzheimer's disease

Targeting the TLR4 signaling pathway by polyphenols: A novel therapeutic strategy for neuroinflammation

A wide array of cell signaling mediators and their interactions play vital roles in neuroinflammation associated with ischemia, brain trauma, developmental disorders and age-related neurodegeneration. Along with neurons, microglia and astrocytes are also affected by the inflammatory cascade by releasing pro-inflammatory cytokines, chemokines and reactive oxygen species. The release of pro-inflammatory mediators in response to neural dysfunction may be helpful, neutral or even deleterious to normal cellular survival. Moreover, the important role of NF-κB factors in the central nervous system (CNS) through toll-like receptor (TLR) activation has been well established. This review demonstrates recent findings regarding therapeutic aspects of polyphenolic compounds for the treatment of neuroinflammation, with the aim of regulating TLR4. Polyphenols including flavonoids, phenolic acids, phenolic alcohols, stilbenes and lignans, can target TLR4 signaling pathways in multiple ways. Toll interacting protein expression could be modulated by epigallocatechin-3-gallate. Resveratrol may also exert neuroprotective effects via the TLR4/NF-κB/STAT signaling cascade. Its role in activation of cascade via interfering with TLR4 oligomerization upon receptor stimulation has also been reported. Curcumin, another polyphenol, can suppress overexpression of inflammatory mediators via inhibiting the TLR4-MAPK/NF-κB pathway. It can also reduce neuronal apoptosis via a mechanism concerning the TLR4/MyD88/NF-κB signaling pathway in microglia/macrophages. Despite a symphony of in vivo and in vitro studies, many molecular and pharmacological aspects of neuroinflammation remain unclear. It is proposed that natural compounds targeting TLR4 may serve as important pharmacophores for the development of potent drugs for the treatment of neurological disorders.

Intramembrane attenuation of the TLR4-TLR6 dimer impairs receptor assembly and reduces microglia-mediated neurodegeneration

Recently, a single study revealed a new complex composed of Toll-like receptor 4 (TLR4), TLR6, and CD36 induced by fibrillary Aβ peptides, the hallmark of Alzheimer's disease. Unlike TLRs located on the plasma membrane that dimerize on the membrane after ligand binding to their extracellular domain, the TLR4-TLR6-CD36 complex assembly has been suggested to be induced by intracellular signals from CD36, similar to integrin inside-out signaling. However, the assembly site of TLR4-TLR6-CD36 and the domains participating in Aβ-induced signaling is still unknown. By interfering with TLR4-TLR6 dimerization using a TLR4-derived peptide, we show that receptor assembly is abrogated within the plasma membrane. Furthermore, we reveal that the transmembrane domains of TLR4 and TLR6 have an essential role in receptor dimerization and activation. Inhibition of TLR4-TLR6 assembly was associated with reduced secretion of proinflammatory mediators from microglia cells, ultimately rescuing neurons from death. Our findings support TLR4-TLR6 dimerization induced by Aβ. Moreover, we shed new light on TLR4-TLR6 assembly and localization and show the potential of inhibiting TLR4-TLR6 dimerization as a treatment of Alzheimer's disease.

Neuroinflammation and Infection: Molecular Mechanisms Associated with Dysfunction of Neurovascular Unit

Neuroinflammation is a complex inflammatory process in the central nervous system, which is sought to play an important defensive role against various pathogens, toxins or factors that induce neurodegeneration. The onset of neurodegenerative diseases and various microbial infections are counted as stimuli that can challenge the host immune system and trigger the development of neuroinflammation. The homeostatic nature of neuroinflammation is essential to maintain the neuroplasticity. Neuroinflammation is regulated by the activity of neuronal, glial, and endothelial cells within the neurovascular unit, which serves as a “platform” for the coordinated action of pro- and anti-inflammatory mechanisms. Production of inflammatory mediators (cytokines, chemokines, reactive oxygen species) by brain resident cells or cells migrating from the peripheral blood, results in the impairment of blood-brain barrier integrity, thereby further affecting the course of local inflammation. In this review, we analyzed the most recent data on the central nervous system inflammation and focused on major mechanisms of neurovascular unit dysfunction caused by neuroinflammation and infections.

Treadmill exercise produces neuroprotective effects in a murine model of Parkinson's disease by regulating the TLR2/MyD88/NF-κB signaling pathway

Parkinson's disease (PD) is characterized by progressive dopamine depletion and a loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Treadmill exercise is a promising non-pharmacological approach for reducing the risk of PD and other neuroinflammatory disorders, such as Alzheimer's disease. The goal of this study was to investigate the effects of treadmill exercise on α-synuclein-induced neuroinflammation and neuronal cell death in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD. Eight weeks of treadmill exercise improved motor deficits and reduced α-synuclein expression, a major causative factor of PD-like symptoms, in MPTP mice. Treadmill exercise also down-regulated the expression of toll-like receptor 2 and its associated downstream signaling molecules, including myeloid differentiation factor-88, tumor necrosis factor receptor-associated factor 6, and transforming growth factor-β-activated protein kinase 1. These effects were associated with reduced ionized calcium-binding adapter molecule 1 expression, decreased IκBα and nuclear transcription factor-κB phosphorylation, decreased tumor necrosis factor α and interleukin-1β expression, and decreased NADPH oxidase subunit expression in the SNpc and striatum. Additionally, it promoted the expression of tyrosine hydroxylase and the dopamine transporter, as well as plasma dopamine levels, in MPTP mice; these effects were associated with decreased caspase-3 expression and cleavage, as well as increased Bcl-2 expression in the SNpc. Taken together, our data suggest that treadmill exercise improves MPTP-associated motor deficits by exerting neuroprotective effects in the SNpc and striatum, supporting the notion that treadmill exercise is useful as a non-pharmacological tool for the management of PD.

Toll-like receptor 4 in glial inflammatory responses to air pollution in vitro and in vivo

Background

Exposure to traffic-related air pollution (TRAP) is associated with accelerated cognitive aging and higher dementia risk in human populations. Rodent brains respond to TRAP with activation of astrocytes and microglia, increased inflammatory cytokines, and neurite atrophy. A role for Toll-like receptor 4 (TLR4) was suggested in mouse TLR4-knockouts, which had attenuated lung macrophage responses to air pollution.

Methods

To further analyze these mechanisms, we examined mixed glial cultures (astrocytes and microglia) for RNA responses to nanoscale particulate matter (nPM; diameter <0.2 μm), a well-characterized nanoscale particulate matter subfraction of TRAP collected from a local freeway (Morgan et al. Environ Health Perspect 2011; 119,1003–1009, 2011). The nPM was compared with responses to the endotoxin lipopolysaccharide (LPS), a classic TLR4 ligand, using Affymetrix whole genome microarray in rats. Expression patterns were analyzed by significance analysis of microarrays (SAM) for fold change and by weighted gene co-expression network analysis (WGCNA) to identify modules of shared responses between nPM and LPS. Finally, we examined TLR4 activation in hippocampal tissue from mice chronically exposed to nPM.

Results

SAM and WGCNA analyses showed strong activation of TLR4 and NF-κB by both nPM and LPS. TLR4 siRNA attenuated TNFα and other inflammatory responses to nPM in vitro, via the MyD88-dependent pathway. In vivo, mice chronically exposed to nPM showed increased TLR4, MyD88, TNFα, and TNFR2 RNA, and decreased NF-κB and TRAF6 RNA TLR4 and NF-κB responses in the hippocampus.

Conclusions

These results show TLR4 activation is integral in brain inflammatory responses to air pollution, and warrant further study of TLR4 in accelerated cognitive aging by air pollution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0858-x) contains supplementary material, which is available to authorized users.

Hepcidin attenuates amyloid beta-induced inflammatory and pro-oxidant responses in astrocytes and microglia

Alzheimer's disease (AD) is characterized by extracellular senile plaques, intracellular neurofibrillary tangles, and neuronal death. Aggregated amyloid-β (Aβ) induces inflammation and oxidative stress, which have pivotal roles in the pathogenesis of AD. Hepcidin is a key regulator of systemic iron homeostasis. Recently, an anti-inflammatory response to hepcidin was reported in macrophages. Under the hypothesis that hepcidin mediates anti-inflammatory response in the brain, in this study, we evaluated the putative anti-inflammatory role of hepcidin on Aβ-activated astrocytes and microglia. Primary culture of astrocytes and microglia were treated with Aβ, with or without hepcidin, and cytokine levels were then evaluated. In addition, the toxicity of Aβ-treated astrocyte- or microglia-conditioned media was tested on neurons, evaluating cellular death and oxidative stress generation. Finally, mice were injected in the right lateral ventricle with Aβ, with or without hepcidin, and hippocampus glial activation and oxidative stress were evaluated. Pre-treatment with hepcidin reduced the expression and secretion of TNF-α and IL-6 in astrocytes and microglia treated with Aβ. Hepcidin also reduced neurotoxicity and oxidative damage triggered by conditioned media obtained from astrocytes and microglia treated with Aβ. Stereotaxic intracerebral injection of hepcidin reduced glial activation and oxidative damage triggered by Aβ injection in mice. Overall, these results are consistent with the hypothesis that in astrocytes and microglia hepcidin down-regulates the inflammatory and pro-oxidant processes induced by Aβ, thus protecting neighboring neurons. This is a newly described property of hepcidin in the central nervous system, which may be relevant for the development of strategies to prevent the neurodegenerative process associated with AD.

Small Molecule Inhibition of Interleukin-1 Receptor-Associated Kinase 4 (IRAK4)

In recent years, interleukin-1 receptor-associated kinase 4, IRAK4, has become an attractive target for many medicinal chemistry programmes. Target inhibition is of potential therapeutic value in areas including autoimmune disorders, cancer, inflammatory diseases, and possibly neurodegenerative diseases. Results from high-throughput screening efforts have led, in conjunction with structure-based drug design, to the identification of highly potent and selective small molecule IRAK4 inhibitors from many diverse chemical series. In vitro and in vivo studies with entities from distinct structural classes have helped elucidate the downstream pharmacological responses associated with IRAK4 inhibition as a proof of concept in disease models, leading to the recent initiation of human clinical trials. Within this review, we will highlight the considerable effort by numerous groups dedicated to the development of small molecule IRAK4 inhibitors for the treatment of human disease.

Association of adipocyte enhancer-binding protein 1 with Alzheimer's disease pathology in human hippocampi

Adipocyte enhancer binding protein 1 (AEBP1) activates inflammatory responses via the NF-κB pathway in macrophages and regulates adipogenesis in preadipocytes. Up-regulation of AEBP1 in the hippocampi of patients with Alzheimer's disease (AD) has been revealed by microarray analyses of autopsied brains from the Japanese general population (the Hisayama study). In this study, we compared the expression patterns of AEBP1 in normal and AD brains, including in the hippocampus, using immunohistochemistry. The subjects were 24 AD cases and 52 non-AD cases. Brain specimens were immunostained with antibodies against AEBP1, tau protein, amyloid β protein, NF-κB, GFAP and Iba-1. In normal brains, AEBP1 immunoreactivity mainly localized to the perikarya of hippocampal pyramidal neurons, and its expression was elevated in the pyramidal neurons and some astrocytes in AD hippocampi. Although AEBP1 immunoreactivity was almost absent in neurons containing neurofibrillary tangles, AEBP1 was highly expressed in neurons with pretangles and in the tau-immunopositive, dystrophic neurites of senile plaques. Nuclear localization of NF-κB was also observed in certain AEBP1-positive neurons in AD cases. Comparison of AD and non-AD cases suggested a positive correlation between the expression level of AEBP1 and the degree of amyloid β pathology. These findings imply that AEBP1 protein has a role in the progression of AD pathology.

Aging and amyloid β oligomers enhance TLR4 expression, LPS-induced Ca2+ responses, and neuron cell death in cultured rat hippocampal neurons

Background

Toll-like receptors (TLRs) are transmembrane pattern-recognition receptors of the innate immune system recognizing diverse pathogen-derived and tissue damage-related ligands. It has been suggested that TLR signaling contributes to the pathogenesis of age-related, neurodegenerative diseases, including Alzheimer’s disease (AD). AD is associated to oligomers of the amyloid β peptide (Aβo) that cause intracellular Ca²⁺ dishomeostasis and neuron cell death in rat hippocampal neurons. Here we assessed the interplay between inflammation and Aβo in long-term cultures of rat hippocampal neurons, an in vitro model of neuron aging and/or senescence.

Methods

Ca²⁺ imaging and immunofluorescence against annexin V and TLR4 were applied in short- and long-term cultures of rat hippocampal neurons to test the effects of TLR4-agonist LPS and Aβo on cytosolic [Ca²⁺] and on apoptosis as well as on expression of TLR4.

Results

LPS increases cytosolic [Ca²⁺] and promotes apoptosis in rat hippocampal neurons in long-term culture considered aged and/or senescent neurons, but not in short-term cultured neurons considered young neurons. TLR4 antagonist CAY10614 prevents both effects. TLR4 expression in rat hippocampal neurons is significantly larger in aged hippocampal cultures. Treatment of aged hippocampal cultures with Aβo increases TLR4 expression and enhances LPS-induced Ca²⁺ responses and neuron cell death.

Conclusions

Aging and amyloid β oligomers, the neurotoxin involved in Alzheimer’s disease, enhance TLR4 expression as well as LPS-induced Ca²⁺ responses and neuron cell death in rat hippocampal neurons aged in vitro.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0802-0) contains supplementary material, which is available to authorized users.

Fibrillar Amyloid-β Accumulation Triggers an Inflammatory Mechanism Leading to Hyperphosphorylation of the Carboxyl-Terminal End of Tau Polypeptide in the Hippocampal Formation of the 3×Tg-AD Transgenic Mouse

Alzheimer's disease (AD) is a degenerative and irreversible disorder whose progressiveness is dependent on age. It is histopathologically characterized by the massive accumulation of insoluble forms of tau and amyloid-β (Aβ) asneurofibrillary tangles and neuritic plaques, respectively. Many studies have documented that these two polypeptides suffer several posttranslational modifications employing postmortem tissue sections from brains of patients with AD. In order to elucidate the molecular mechanisms underlying the posttranslational modifications of key players in this disease, including Aβ and tau, several transgenic mouse models have been developed. One of these models is the 3×Tg-AD transgenic mouse, carrying three transgenes encoding APPSWE, S1M146V, and TauP301L proteins. To further characterize this transgenicmouse, we determined the accumulation of fibrillar Aβ as a function of age in relation to the hyperphosphorylation patterns of TauP301L at both its N- and C-terminus in the hippocampal formation by immunofluorescence and confocal microscopy. Moreover, we searched for the expression of activated protein kinases and mediators of inflammation by western blot of wholeprotein extracts from hippocampal tissue sections since 3 to 28 months as well. Our results indicate that the presence of fibrillar Aβ deposits correlates with a significant activation of astrocytes and microglia in subiculum and CA1 regions of hippocampus. Accordingly, we also observed a significant increase in the expression of TNF-α associated to neuritic plaques and glial cells. Importantly, there is an overexpression of the stress activated protein kinases SAPK/JNK and Cdk-5 in pyramidal neurons, which might phosphorylate several residues at the C-terminus of TauP301L. Therefore, the accumulation of Aβ oligomers results in an inflammatory environment that upregulates kinases involved in hyperphosphorylation of TauP301L polypeptide.

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.

Disentangling the Role of Astrocytes in Alcohol Use Disorder

Several laboratories recently identified that astrocytes are critical regulators of addiction machinery. It is now known that astrocyte pathology is a common feature of ethanol (EtOH) exposure in both humans and animal models, as even brief EtOH exposure is sufficient to elicit long-lasting perturbations in astrocyte gene expression, activity, and proliferation. Astrocytes were also recently shown to modulate the motivational properties of EtOH and other strongly reinforcing stimuli. Given the role of astrocytes in regulating glutamate homeostasis, a crucial component of alcohol use disorder (AUD), astrocytes might be an important target for the development of next-generation alcoholism treatments. This review will outline some of the more prominent features displayed by astrocytes, how these properties are influenced by acute and long-term EtOH exposure, and future directions that may help to disentangle astrocytic from neuronal functions in the etiology of AUD.

Assessing the Effects of Acute Amyloid β Oligomer Exposure in the Rat

Alzheimer’s disease (AD) is the most common form of dementia, yet there are no therapeutic treatments that can either cure or delay its onset. Currently, the pathogenesis of AD is still uncertain, especially with respect to how the disease develops from a normal healthy brain. Amyloid β oligomers (AβO) are highly neurotoxic proteins and are considered potential initiators to the pathogenesis of AD. Rat brains were exposed to AβO via bilateral intracerebroventricular injections. Rats were then euthanized at either 1, 3, 7 or 21-days post surgery. Rat behavioural testing was performed using the Morris water maze and open field tests. Post-mortem brain tissue was immunolabelled for Aβ, microglia, and cholinergic neurons. Rats exposed to AβO showed deficits in spatial learning and anxiety-like behaviour. Acute positive staining for Aβ was only observed in the corpus callosum surrounding the lateral ventricles. AβO exposed rat brains also showed a delayed increase in activated microglia within the corpus callosum and a decreased number of cholinergic neurons within the basal forebrain. Acute exposure to AβO resulted in mild learning and memory impairments with co-concomitant white matter pathology within the corpus callosum and cholinergic cell loss within the basal forebrain. Results suggest that acute exposure to AβO in the rat may be a useful tool in assessing the early phases for the pathogenesis of AD.

Toll-like receptors: promising therapeutic targets for inflammatory diseases

The health of living organisms is constantly challenged by bacterial and viral threats. The recognition of pathogenic microorganisms by diverse receptors triggers a variety of host defense mechanisms, leading to their eradication. Toll-like receptors (TLRs), which are type I transmembrane proteins, recognize specific signatures of the invading microbes and activate a cascade of downstream signals inducing the secretion of inflammatory cytokines, chemokines, and type I interferons. The TLR response not only counteracts the pathogens but also initiates and shapes the adaptive immune response. Under normal conditions, inflammation is downregulated after the removal of the pathogen and cellular debris. However, a dysfunctional TLR-mediated response maintains a chronic inflammatory state and leads to local and systemic deleterious effects in host cells and tissues. Such inappropriate TLR response has been attributed to the development and progression of multiple diseases such as cancer, autoimmune, and inflammatory diseases. In this review, we discuss the emerging role of TLRs in the pathogenesis of inflammatory diseases and how targeting of TLRs offers a promising therapeutic strategy for the prevention and treatment of various inflammatory diseases. Additionally, we highlight a number of TLR-targeting agents that are in the developmental stage or in clinical trials.

Prothrombin Kringle-2: A Potential Inflammatory Pathogen in the Parkinsonian Dopaminergic System

Although accumulating evidence suggests that microglia-mediated neuroinflammation may be crucial for the initiation and progression of Parkinson's disease (PD), and that the control of neuroinflammation may be a useful strategy for preventing the degeneration of nigrostriatal dopaminergic (DA) projections in the adult brain, it is still unclear what kinds of endogenous biomolecules initiate microglial activation, consequently resulting in neurodegeneration. Recently, we reported that the increase in the levels of prothrombin kringle-2 (pKr-2), which is a domain of prothrombin that is generated by active thrombin, can lead to disruption of the nigrostriatal DA projection. This disruption is mediated by neurotoxic inflammatory events via the induction of microglial Toll-like receptor 4 (TLR4) in vivo , thereby resulting in less neurotoxicity in TLR4-deficient mice. Moreover, inhibition of microglial activation following minocycline treatment, which has anti-inflammatory activity, protects DA neurons from pKr-2-induced neurotoxicity in the substantia nigra (SN) in vivo. We also found that the levels of pKr-2 and microglial TLR4 were significantly increased in the SN of PD patients compared to those of age-matched controls. These observations suggest that there may be a correlation between pKr-2 and microglial TLR4 in the initiation and progression of PD, and that inhibition of pKr-2-induced microglial activation may be protective against the degeneration of the nigrostriatal DA system in vivo. To describe the significance of pKr-2 overexpression, which may have a role in the pathogenesis of PD, we have reviewed the mechanisms of pKr-2-induced microglial activation, which results in neurodegeneration in the SN of the adult brain.

Aged complement factor H knockout mice kept in a clean barriered environment have reduced retinal pathology

Age-related macular degeneration (AMD) is the largest cause of visual loss in those over 60 years in the West and is a condition increasing in prevalence. Many diseases result from genetic/environmental interactions and 50% of AMD cases have an association with polymorphisms of the complement system including complement factor H. Here we explore interactions between genetic predisposition and environmental conditions in triggering retinal pathology in two groups of aged complement factor H knock out (Cfh(-/-)) mice. Mice were maintained over 9 months in either a conventional open environment or a barriered pathogen free environment. Open environment Cfh(-/-) mice had significant increases in subretinal macrophage numbers, inflammatory and stress responses and reduced photoreceptor numbers over mice kept in a pathogen free environment. Hence, environmental factors can drive retinal disease in these mice when linked to complement deficits impairing immune function. Both groups of mice had similar levels of retinal amyloid beta accumulation. Consequently there is no direct link between this and inflammation in Cfh(-/-) mice.

Small-Molecule Carbohydrate-Based Immunostimulants

In this review, we discuss small-molecule, carbohydrate-based immunostimulants that target Toll-like receptor 4 (TLR-4) and cluster of differentiation 1D (CD1d) receptors. The design and use of these molecules in immunotherapy as well as results from their use in clinical trials are described. How these molecules work and their utilization as vaccine adjuvants are also discussed. Future applications and extensions for the use of these analogues as therapeutic agents will be outlined.

Reprint of: Microglial toll-like receptors and Alzheimer's disease

Microglial activation represents an important pathological hallmark of Alzheimer's disease (AD), and emerging data highlight the involvement of microglial toll-like receptors (TLRs) in the course of AD. TLRs have been observed to exert both beneficial and detrimental effects on AD-related pathologies, and transgenic animal models have provided direct and credible evidence for an association between TLRs and AD. Moreover, analyses of genetic polymorphisms have suggested interactions between genetic polymorphisms in TLRs and AD risk, further supporting the hypothesis that TLRs are involved in AD. In this review, we summarize the key evidence in this field. Future studies should focus on exploring the mechanisms underlying the potential roles of TLRs in AD.

Complementing the inflammasome

The innate immune system is an ancient surveillance system able to sense microbial invaders as well as aberrations in normal cell function. No longer viewed as a static and non-specific part of immunity, the innate immune system employs a plethora of specialized pattern recognition sensors to monitor and achieve homeostasis; these include the Toll-like receptors, the retinoic acid-inducible gene-like receptors, the nucleotide-binding oligomerization domain receptors (NLRs), the C-type lectins and the complement system. In order to increase specificity and diversity, innate immunity uses homotypic and heterotypic associations among these different components. Multi-molecular assemblies are formed both on the cell surface and in the cytosol to respond to pathogen and danger signals. Diverse, but tailored, responses to a changing environment are orchestrated depending on the the nature of the challenge and the repertoire of interacting receptors and components available in the sensing cell. It is now emerging that innate immunity operates a system of 'checks and balances' where interaction among the sensors is key in maintaining normal cell function. Complement sits at the heart of this alarm system and it is becoming apparent that it is capable of interacting with all the other pathways to effect a tailored immune response. In this review, we will focus on complement interactions with NLRs, the so-called 'inflammasomes', describing the molecular mechanisms that have been revealed so far and discussing the circumstantial evidence that exists for these interactions in disease states.

The Role of Toll-Like Receptor 4 in Infectious and Noninfectious Inflammation

Toll-like receptor 4 (TLR4) belongs to the family of pattern recognition receptors (PRRs). They are highly conserved receptors that recognize conserved pathogen-associated molecular patterns (PAMPs), thus representing the first line of defense against infections. TLR4 has been long recognized as the sensing receptor for gram-negative lipopolysaccharide (LPS). In addition, it also binds endogenous molecules produced as a result of tissue injury. Hence, TLR4 represents a key receptor on which both infectious and noninfectious stimuli converge to induce a proinflammatory response. TLR4-mediated inflammation, triggered by exogenous or endogenous ligands, is also involved in several acute and chronic diseases, having a pivotal role as amplifier of the inflammatory response. This review focuses on the research progress about the role of TLR4 activation in infectious and noninfectious (e.g., sterile) inflammation and the effects of TLR4 signaling in some pathological conditions.

Neuroinflammation: the devil is in the details

There is significant interest in understanding inflammatory responses within the brain and spinal cord. Inflammatory responses that are centralized within the brain and spinal cord are generally referred to as 'neuroinflammatory'. Aspects of neuroinflammation vary within the context of disease, injury, infection, or stress. The context, course, and duration of these inflammatory responses are all critical aspects in the understanding of these processes and their corresponding physiological, biochemical, and behavioral consequences. Microglia, innate immune cells of the CNS, play key roles in mediating these neuroinflammatory responses. Because the connotation of neuroinflammation is inherently negative and maladaptive, the majority of research focus is on the pathological aspects of neuroinflammation. There are, however, several degrees of neuroinflammatory responses, some of which are positive. In many circumstances including CNS injury, there is a balance of inflammatory and intrinsic repair processes that influences functional recovery. In addition, there are several other examples where communication between the brain and immune system involves neuroinflammatory processes that are beneficial and adaptive. The purpose of this review is to distinguish different variations of neuroinflammation in a context-specific manner and detail both positive and negative aspects of neuroinflammatory processes. In this review, we will use brain and spinal cord injury, stress, aging, and other inflammatory events to illustrate the potential harm and benefits inherent to neuroinflammation. Context, course, and duration of the inflammation are highly important to the interpretation of these events, and we aim to provide insight into this by detailing several commonly studied insults. This article is part of the 60th anniversary supplemental issue.

High-throughput proteomics reveal alarmins as amplifiers of tissue pathology and inflammation after spinal cord injury

Spinal cord injury is characterized by acute cellular and axonal damage followed by aggressive inflammation and pathological tissue remodelling. The biological mediators underlying these processes are still largely unknown. Here we apply an innovative proteomics approach targeting the enriched extracellular proteome after spinal cord injury for the first time. Proteomics revealed multiple matrix proteins not previously associated with injured spinal tissue, including small proteoglycans involved in cell-matrix adhesion and collagen fibrillogenesis. Network analysis of transcriptomics and proteomics datasets uncovered persistent overexpression of extracellular alarmins that can trigger inflammation via pattern recognition receptors. In mechanistic experiments, inhibition of toll-like receptor-4 (TLR4) and the receptor for advanced glycation end-products (RAGE) revealed the involvement of alarmins in inflammatory gene expression, which was found to be dominated by IL1 and NFκΒ signalling. Extracellular high-mobility group box-1 (HMGB1) was identified as the likely endogenous regulator of IL1 expression after injury. These data reveal a novel tissue remodelling signature and identify endogenous alarmins as amplifiers of the inflammatory response that promotes tissue pathology and impedes neuronal repair after spinal cord injury.

GLP-2 Attenuates LPS-Induced Inflammation in BV-2 Cells by Inhibiting ERK1/2, JNK1/2 and NF-κB Signaling Pathways

The pathogenesis of Parkinson’s disease (PD) often involves the over-activation of microglia. Over-activated microglia could produce several inflammatory mediators, which trigger excessive inflammation and ultimately cause dopaminergic neuron damage. Anti-inflammatory effects of glucagon-like peptide-2 (GLP-2) in the periphery have been shown. Nonetheless, it has not been illustrated in the brain. Thus, in this study, we aimed to understand the role of GLP-2 in microglia activation and to elucidate the underlying mechanisms. BV-2 cells were pretreated with GLP-2 and then stimulated by lipopolysaccharide (LPS). Cells were assessed for the responses of pro-inflammatory enzymes (iNOS and COX-2) and pro-inflammatory cytokines (IL-1β, IL-6 and TNF-α); the related signaling pathways were evaluated by Western blotting. The rescue effect of GLP-2 on microglia-mediated neurotoxicity was also examined. The results showed that GLP-2 significantly reduced LPS-induced production of inducible nitric oxide synthase (iNOS), cyclooxygenase-s (COX-2), IL-1β, IL-6 and TNF-α. Blocking of Gα_s by NF449 resulted in a loss of this anti-inflammatory effect in BV-2 cells. Analyses in signaling pathways demonstrated that GLP-2 reduced LPS-induced phosphorylation of ERK1/2, JNK1/2 and p65, while no effect was observed on p38 phosphorylation. In addition, GLP-2 could suppress microglia-mediated neurotoxicity. All results imply that GLP-2 inhibits LPS-induced microglia activation by collectively regulating ERK1/2, JNK1/2 and p65.

Microglial toll-like receptors and Alzheimer's disease

Microglial activation represents an important pathological hallmark of Alzheimer's disease (AD), and emerging data highlight the involvement of microglial toll-like receptors (TLRs) in the course of AD. TLRs have been observed to exert both beneficial and detrimental effects on AD-related pathologies, and transgenic animal models have provided direct and credible evidence for an association between TLRs and AD. Moreover, analyses of genetic polymorphisms have suggested interactions between genetic polymorphisms in TLRs and AD risk, further supporting the hypothesis that TLRs are involved in AD. In this review, we summarize the key evidence in this field. Future studies should focus on exploring the mechanisms underlying the potential roles of TLRs in AD.

Gx-50 reduces β-amyloid-induced TNF-α, IL-1β, NO, and PGE2 expression and inhibits NF-κB signaling in a mouse model of Alzheimer's disease

Chronic inflammation, which is regulated by overactivated microglia in the brain, accelerates the occurrence and development of Alzheimer's disease (AD). Gx-50 has been investigated as a novel drug for the treatment of AD in our previous studies. Here, we investigated whether gx-50 possesses anti-inflammatory effects in primary rat microglia and a mouse model of AD, amyloid precursor protein (APP) Tg mice. The expression of TNF-α, IL-1β, NO, prostaglandin E2, and the expression of iNOS and COX2 were inhibited by gx-50 in amyloid β (Aβ) treated rat microglia; additionally, microglial activation and the expression of IL-1β, iNOS, and COX2 were also significantly suppressed by gx-50 in APP(+) transgenic mice. Furthermore, gx-50 inhibited the activation of NF-κB and MAPK cascades in vitro and in vivo in APP-Tg mice. Moreover, the expression of TLR4 and its downstream signaling proteins MyD88 and tumor necrosis factor receptor associated factor 6 (TRAF6) was reduced by gx-50 in vitro and in vivo. Interestingly, silencing of TLR4 reduced Aβ-induced upregulation of IL-1β and TRAF6 to levels similar to gx-50 inhibition; moreover, overexpression of TLR4 increased the expression of MyD88 and TRAF6, which was significantly reduced by gx-50. These findings provide strong evidence that gx-50 has anti-inflammatory effects against Aβ-triggered microglial overactivation via a mechanism that involves the TLR4-mediated NF-κBB/MAPK signaling cascade.

Microglia Function in the Normal Brain

The activation of microglia has been recognized for over a century by their morphological changes. Long slender microglia acquire a short sturdy ramified shape when activated. During the past 20 years, microglia have been accepted as an essential cellular component for understanding the pathogenic mechanism of many brain diseases, including neurodegenerative diseases. More recently, functional studies and imaging in mouse models indicate that microglia are active in the healthy central nervous system. It has become evident that microglia release several signal molecules that play key roles in the crosstalk among brain cells, i.e., astrocytes and oligodendrocytes with neurons, as well as with regulatory immune cells. Recent studies also reveal the heterogeneous nature of microglia diverse functions depending on development, previous exposure to stimulation events, brain region of residence, or pathological state. Subjects to approach by future research are still the unresolved questions regarding the conditions and mechanisms that render microglia protective, capable of preventing or reducing damage, or deleterious, capable of inducing or facilitating the progression of neuropathological diseases. This novel knowledge will certainly change our view on microglia as therapeutic target, shifting our goal from their general silencing to the generation of treatments able to change their activation pattern.

Neural stem cell transplantation decreases neuroinflammation in a transgenic mouse model of Alzheimer's disease

Inflammatory processes are considered to play an important role in the progression of neurodegenerative changes in Alzheimer's disease (AD). A number of studies have reported that inflammatory processes are highly correlated with cognitive deficits in AD-like mice. Transplantation of neural stem cells (NSCs) has been considered as a potential new therapy for the treatment of AD because of its effects in improving cognitive ability. However, NSCs have not been evaluated for their protective effects against inflammatory changes in AD. Here, we injected NSCs into amyloid precursor protein (APP)/PS1 transgenic mice to analyse cognitive function and to measure glial fibrillary acidic protein (GFAP), ionized calcium-binding adaptor molecule-1 (Iba-1) and toll-like receptors 4(TLR4) activation. We also quantified TLR-4 pathway-related agents, Aβ concentration and the levels of proinflammatory mediators. Our results showed that in NSC-injected APP/PS1 mice, activation of GFAP, Iba-1, TLR4 and TLR4 pathway-related agents (MyD88, TRIF, P38 MAPK and NF-κB P65) were significantly decreased with decreased expression of proinflammatory mediators (IL-1, IL-6, TNF-α and PGE2). These changes were associated with the amelioration of cognitive deficits, but no difference was found in Aβ concentration. Our results provide novel evidence that NSC transplantation in APP/PS1 mice significantly improved cognitive deficits and was accompanied by the attenuation of inflammatory injury via suppression of glial and TLR4-mediated inflammatory pathway activation. Our data indicate that these pathways may potentially be important therapeutic targets to prevent or delay AD. This study investigated the neuroprotective effect of neural stem cell (NSC) transplantation against Alzheimer's disease (AD) inflammation. We found that NSC treatment in APP/PS1 mice significantly improved cognitive deficits and was accompanied by the attenuation of inflammatory injury via suppression of glial and toll-like receptor 4 (TLR4) activation and its downstream signalling pathways. Our findings indicate that these pathways may be potentially important therapeutic targets to prevent or delay AD.

Cross talk between AT1 receptors and Toll-like receptor 4 in microglia contributes to angiotensin II-derived ROS production in the hypothalamic paraventricular nucleus

ANG II is thought to increase sympathetic outflow by increasing oxidative stress and promoting local inflammation in the paraventricular nucleus (PVN) of the hypothalamus. However, the relative contributions of inflammation and oxidative stress to sympathetic drive remain poorly understood, and the underlying cellular and molecular targets have yet to be examined. ANG II has been shown to enhance Toll-like receptor (TLR)4-mediated signaling on microglia. Thus, in the present study, we aimed to determine whether ANG II-mediated activation of microglial TLR4 signaling is a key molecular target initiating local oxidative stress in the PVN. We found TLR4 and ANG II type 1 (AT1) receptor mRNA expression in hypothalamic microglia, providing molecular evidence for the potential interaction between these two receptors. In hypothalamic slices, ANG II induced microglial activation within the PVN (∼65% increase, P < 0.001), an effect that was blunted in the absence of functional TLR4. ANG II increased ROS production, as indicated by dihydroethidium fluorescence, within the PVN of rats and mice (P < 0.0001 in both cases), effects that were also dependent on the presence of functional TLR4. The microglial inhibitor minocycline attenuated ANG II-mediated ROS production, yet ANG II effects persisted in PVN single-minded 1-AT1a knockout mice, supporting the contribution of a non-neuronal source (likely microglia) to ANG II-driven ROS production in the PVN. Taken together, these results support functional interactions between AT1 receptors and TLR4 in mediating ANG II-dependent microglial activation and oxidative stress within the PVN. More broadly, our results support a functional interaction between the central renin-angiotensin system and innate immunity in the regulation of neurohumoral outflows from the PVN.

Shock Wave Treatment Protects From Neuronal Degeneration via a Toll-Like Receptor 3 Dependent Mechanism: Implications of a First-Ever Causal Treatment for Ischemic Spinal Cord Injury

Background

Paraplegia following spinal cord ischemia represents a devastating complication of both aortic surgery and endovascular aortic repair. Shock wave treatment was shown to induce angiogenesis and regeneration in ischemic tissue by modulation of early inflammatory response via Toll‐like receptor (TLR) 3 signaling. In preclinical and clinical studies, shock wave treatment had a favorable effect on ischemic myocardium. We hypothesized that shock wave treatment also may have a beneficial effect on spinal cord ischemia.

Methods and Results

A spinal cord ischemia model in mice and spinal slice cultures ex vivo were performed. Treatment groups received immediate shock wave therapy, which resulted in decreased neuronal degeneration and improved motor function. In spinal slice cultures, the activation of TLR3 could be observed. Shock wave effects were abolished in spinal slice cultures from TLR3^−/− mice, whereas the effect was still present in TLR4^−/− mice. TLR4 protein was found to be downregulated parallel to TLR3 signaling. Shock wave–treated animals showed significantly better functional outcome and survival. The protective effect on neurons could be reproduced in human spinal slices.

Conclusions

Shock wave treatment protects from neuronal degeneration via TLR3 signaling and subsequent TLR4 downregulation. Consequently, it represents a promising treatment option for the devastating complication of spinal cord ischemia after aortic repair.

Critical role of the Mac1/NOX2 pathway in mediating reactive microgliosis-generated chronic neuroinflammation and progressive neurodegeneration

As average life expectancy rises throughout the world, neurodegenerative diseases have emerged as one of the greatest global public heath challenges in modern times. Substantial efforts have been made in researching neurodegenerative diseases over the last few decades, yet their predominantly sporadic nature has made uncovering their etiologies challenging. Mounting evidence has suggested that factors like damage-associated molecular patterns (DAMPs) released by stressed and dying neurons are likely involved in disease pathology and in stimulating chronic activation of microglia that contributes to neuronal oxidative stress and degeneration. This review focuses on how the microglial integrin receptor Mac1 and its downstream effector NADPH oxidase (NOX2) contribute to maintaining chronic neuroinflammation and are crucial in inflammation-driven neurotoxicity in neurodegenerative diseases. Our hope is to provide new insights on novel targets and therapies that could slow or even halt neurodegeneration.

Lipopolysaccharide from Rhodobacter sphaeroides Attenuates Microglia-Mediated Inflammation and Phagocytosis and Directs Regulatory T Cell Response

Microglia activation and neuroinflammation are key events during the progression of neurodegenerative disorders. Microglia exhibits toll-like receptors (TLRs), with predominant expression of TLR4, inducing aberrant neuroinflammation and exacerbated neurotoxicity. Studies suggest that microglia initiate infiltration of T cells into the brain that critically influence disease conditions. We report that LPS-Rs, through TLR4 antagonism, significantly inhibit TLR4 mediated inflammatory molecules like IL-1β, IL-6, TNF-α, COX-2, iNOS, and NO. LPS-Rs regulates JNK/p38 MAPKs and p65-NF-κB signaling pathways, which we report as indispensible for LPS induced neuroinflammation. LPS-Rs mitigates microglial phagocytic activity and we are first to report regulatory role of LPS-Rs which blocked microglia mediated inflammation and apoptotic cell death. LPS-Rs significantly inhibits expression of costimulatory molecules CD80, CD86, and CD40. Chemokine receptor, CCR5, and T cell recruitment chemokines, MIP-1α and CCL5, were negatively regulated by LPS-Rs. Furthermore, LPS-Rs significantly inhibited lymphocyte proliferation with skewed regulatory T (Treg) cell response as evidenced by increased FOXP3, IL-10, and TGF-β. Additionally, LPS-Rs serves to induce coordinated immunosuppressive response and confer tolerogenic potential to activated microglia extending neurosupportive microenvironment. TLR4 antagonism can be a strategy providing neuroprotection through regulation of microglia as well as the T cells.

TREM2 in CNS homeostasis and neurodegenerative disease

Myeloid-lineage cells accomplish a myriad of homeostatic tasks including the recognition of pathogens, regulation of the inflammatory milieu, and mediation of tissue repair and regeneration. The innate immune receptor and its adaptor protein—triggering receptor expressed on myeloid cells 2 (TREM2) and DNAX-activating protein of 12 kDa (DAP12)—possess the ability to modulate critical cellular functions via crosstalk with diverse signaling pathways. As such, mutations in TREM2 and DAP12 have been found to be associated with a range of disease phenotypes. In particular, mutations in TREM2 increase the risk for Alzheimer's disease and other neurodegenerative disorders. The leading hypothesis is that microglia, the resident immune cells of the central nervous system, are the major myeloid cells affected by dysregulated TREM2-DAP12 function. Here, we review how impaired signaling by the TREM2-DAP12 pathway leads to altered immune responses in phagocytosis, cytokine production, and microglial proliferation and survival, thus contributing to disease pathogenesis.

Inflammatory Cyclooxygenase Activity and PGE2 Signaling in Models of Alzheimer's Disease

The inflammatory response is a fundamental driving force in the pathogenesis of Alzheimer’s disease (AD). In the setting of accumulating immunogenic Aß peptide assemblies, microglia, the innate immune cells of the brain, generate a non-resolving immune response and fail to adequately clear accumulating Aß peptides, accelerating neuronal and synaptic injury. Pathological, biomarker, and imaging studies point to a prominent role of the innate immune response in AD development, and the molecular components of this response are beginning to be unraveled. The inflammatory cyclooxygenase-PGE₂ pathway is implicated in pre-clinical development of AD, both in epidemiology of normal aging populations and in transgenic mouse models of Familial AD. The cyclooxygenase-PGE₂ pathway modulates the inflammatory response to accumulating Aß peptides through actions of specific E-prostanoid G-protein coupled receptors.

Integrated genomic approaches identify major pathways and upstream regulators in late onset Alzheimer's disease

Previous studies have evaluated gene expression in Alzheimer’s disease (AD) brains to identify mechanistic processes, but have been limited by the size of the datasets studied. Here we have implemented a novel meta-analysis approach to identify differentially expressed genes (DEGs) in published datasets comprising 450 late onset AD (LOAD) brains and 212 controls. We found 3124 DEGs, many of which were highly correlated with Braak stage and cerebral atrophy. Pathway Analysis revealed the most perturbed pathways to be (a) nitric oxide and reactive oxygen species in macrophages (NOROS), (b) NFkB and (c) mitochondrial dysfunction. NOROS was also up-regulated, and mitochondrial dysfunction down-regulated, in healthy ageing subjects. Upstream regulator analysis predicted the TLR4 ligands, STAT3 and NFKBIA, for activated pathways and RICTOR for mitochondrial genes. Protein-protein interaction network analysis emphasised the role of NFKB; identified a key interaction of CLU with complement; and linked TYROBP, TREM2 and DOK3 to modulation of LPS signalling through TLR4 and to phosphatidylinositol metabolism. We suggest that NEUROD6, ZCCHC17, PPEF1 and MANBAL are potentially implicated in LOAD, with predicted links to calcium signalling and protein mannosylation. Our study demonstrates a highly injurious combination of TLR4-mediated NFKB signalling, NOROS inflammatory pathway activation, and mitochondrial dysfunction in LOAD.

MicroRNA target prediction in glaucoma

Glaucoma is a progressive optic neuropathy and is one of the leading causes of blindness in the industrialized countries. The aim of this study is to investigate microRNA (miRNA) regulation in glaucoma and other neurodegenerative diseases, that share similar pathways, by means of in silico approaches such as bibliographic search and access to bioinformatic resources. First of all, data mining was carried out on Human miRNA Disease Database (HMDD) and miR2Disease databases. Then, predictions of deregulated miRNAs were carried out accessing to microrna.org database. Finally, the potential combinatorial effect of miRNAs, on regulation of biochemical pathways, was studied by an enrichment analysis performed by DIANA-miRPath v.2.0. We found, from literature search, 8 deregulated miRNAs in glaucoma and 9 and 23 in age-related macular degeneration (AMD) and Alzheimer's disease (AD), respectively. One miRNA is commonly deregulated in glaucoma and AMD (miR-23a). Two miRNAs (miR-29a, miR-29b) are common to glaucoma and AD, and four miRNAs were identified to be commonly deregulated in AMD and AD (miR-9, miR-21, miR-34a, miR-146a). The match of the miRNA common to glaucoma and the other two neurodegenerative diseases (AMD and AD) did not generate any output. Enrichment of information has been reached through miRNAs prediction: 88 predicted miRNAs are common to glaucoma and AMD, 19 are common to glaucoma and AD, and 9 are common to AMD and AD. Indeed, predicted miRNAs common to the three neurodegenerative diseases are nine (miR-107, miR-137, miR-146a, miR-181c, miR-197, miR-21, miR-22, miR-590, miR-9). DIANA-miRPath predicted that those nine miRNAs might regulate pathways involved in inflammation. The findings hereby obtained provide a valuable hint to assess deregulation of specific miRNA, as potential biomarkers and therapeutic targets, in glaucoma and other neurodegenerative diseases by means of preclinical and clinical studies.

On the antioxidant, neuroprotective and anti-inflammatory properties of S-allyl cysteine: An update

Therapeutic approaches based on isolated compounds obtained from natural products to handle central and peripheral disorders involving oxidative stress and inflammation are more common nowadays. The validation of nutraceutics vs. pharmaceutics as tools to induce preventive and protective profiles in human health alterations is still far of complete acceptance, but the basis to start more solid experimental and clinical protocols with natural products has already begun. S-allyl cysteine (SAC) is a promising garlic-derived organosulfur compound exhibiting a considerable number of positive actions in cell models and living systems. An update, in the form of review, is needed from time to time to get access to the state-of-the-art on this topic. In this review we visited recent and refreshing evidence of new already proven and potential targets to explain the benefits of using SAC against toxic and pathological conditions. The broad spectrum of protective actions covered by this molecule comprises antioxidant, redox modulatory and anti-inflammatory activities, accompanied by anti-apoptotic, pro-energetic and signaling capacities. Herein, we detail the evidence on these aspects to provide the reader a more complete overview on the promising aspects of SAC in research.

Absence of toll-like receptor 4 (TLR4) extends survival in the hSOD1 G93A mouse model of amyotrophic lateral sclerosis

Background

Amyotrophic lateral sclerosis (ALS) is a devastating late onset neurodegenerative disorder that is characterised by the progressive loss of upper and lower motor neurons. The mechanisms underlying ALS pathogenesis are unclear; however, there is emerging evidence the innate immune system, including components of the toll-like receptor (TLR) system, may drive disease progression. For example, toll-like receptor 4 (TLR4) antagonism in a spontaneous ‘wobbler mouse’ model of ALS increased motor function, associated with a decrease in microglial activation. This study therefore aimed to extend from these findings and determine the expression and function of TLR4 signalling in hSOD1^G93A mice, the most widely established preclinical model of ALS.

Findings

TLR4 and one of its major endogenous ligands, high-mobility group box 1 (HMGB1), were increased during disease progression in hSOD1^G93A mice, with TLR4 and HMGB1 expressed by activated microglia and astrocytes. hSOD1^G93A mice lacking TLR4 showed transient improvements in hind-limb grip strength and significantly extended survival when compared to TLR4-sufficient hSOD1^G93A mice.

Conclusion

These results suggest that enhanced glial TLR4 signalling during disease progression contributes to end-stage ALS pathology in hSOD1^G93A mice.

Activation of MyD88-dependent TLR1/2 signaling by misfolded α-synuclein, a protein linked to neurodegenerative disorders

Synucleinopathies, such as Parkinson's disease and diffuse Lewy body disease, are progressive neurodegenerative disorders characterized by selective neuronal death, abnormal accumulation of misfolded α-synuclein, and sustained microglial activation. In addition to inducing neuronal toxicity, higher-ordered oligomeric α-synuclein causes proinflammatory responses in the brain parenchyma by triggering microglial activation, which may exacerbate pathogenic processes by establishing a chronic neuroinflammatory milieu. We found that higher-ordered oligomeric α-synuclein induced a proinflammatory microglial phenotype by directly engaging the heterodimer TLR1/2 (Toll-like receptor 1 and 2) at the cell membrane, leading to the nuclear translocation of NF-κB (nuclear factor κB) and the increased production of the proinflammatory cytokines TNF-α (tumor necrosis factor-α) and IL-1β (interleukin-1β) in a MyD88-dependent manner. Blocking signaling through the TLR1/2 heterodimer with the small-molecule inhibitor CU-CPT22 reduced the nuclear translocation of NF-κB and secretion of TNF-α from cultured primary mouse microglia. Candesartan cilexetil, a drug approved for treating hypertension and that inhibits the expression of TLR2, reversed the activated proinflammatory phenotype of primary microglia exposed to oligomeric α-synuclein, supporting the possibility of repurposing this drug for synucleinopathies.

APOE-modulated Aβ-induced neuroinflammation in Alzheimer's disease: current landscape, novel data, and future perspective

Chronic glial activation and neuroinflammation induced by the amyloid-β peptide (Aβ) contribute to Alzheimer's disease (AD) pathology. APOE4 is the greatest AD-genetic risk factor; increasing risk up to 12-fold compared to APOE3, with APOE4-specific neuroinflammation an important component of this risk. This editorial review discusses the role of APOE in inflammation and AD, via a literature review, presentation of novel data on Aβ-induced neuroinflammation, and discussion of future research directions. The complexity of chronic neuroinflammation, including multiple detrimental and beneficial effects occurring in a temporal and cell-specific manner, has resulted in conflicting functional data for virtually every inflammatory mediator. Defining a neuroinflammatory phenotype (NIP) is one way to address this issue, focusing on profiling the changes in inflammatory mediator expression during disease progression. Although many studies have shown that APOE4 induces a detrimental NIP in peripheral inflammation and Aβ-independent neuroinflammation, data for APOE-modulated Aβ-induced neuroinflammation are surprisingly limited. We present data supporting the hypothesis that impaired apoE4 function modulates Aβ-induced effects on inflammatory receptor signaling, including amplification of detrimental (toll-like receptor 4-p38α) and suppression of beneficial (IL-4R-nuclear receptor) pathways. To ultimately develop APOE genotype-specific therapeutics, it is critical that future studies define the dynamic NIP profile and pathways that underlie APOE-modulated chronic neuroinflammation. In this editorial review, we present data supporting the hypothesis that impaired apoE4 function modulates Aβ-induced effects on inflammatory receptor signaling, including amplification of detrimental (TLR4-p38α) and suppression of beneficial (IL-4R-nuclear receptor) pathways, resulting in an adverse NIP that causes neuronal dysfunction. NIP, Neuroinflammatory phenotype; P.I., pro-inflammatory; A.I., anti-inflammatory.

Phosphoinositide turnover in Toll-like receptor signaling and trafficking

Lipid components in biological membranes are essential for maintaining cellular function. Phosphoinositides, the phosphorylated derivatives of phosphatidylinositol (PI), regulate many critical cell processes involving membrane signaling, trafficking, and reorganization. Multiple metabolic pathways including phosphoinositide kinases and phosphatases and phospholipases tightly control spatio-temporal concentration of membrane phosphoinositides. Metabolizing enzymes responsible for PI 4,5-bisphosphate (PI(4,5)P2) production or degradation play a regulatory role in Toll-like receptor (TLR) signaling and trafficking. These enzymes include PI 4-phosphate 5-kinase, phosphatase and tensin homolog, PI 3-kinase, and phospholipase C. PI(4,5)P2 mediates the interaction with target cytosolic proteins to induce their membrane translocation, regulate vesicular trafficking, and serve as a precursor for other signaling lipids. TLR activation is important for the innate immune response and is implicated in diverse pathophysiological disorders. TLR signaling is controlled by specific interactions with distinct signaling and sorting adaptors. Importantly, TLR signaling machinery is differentially formed depending on a specific membrane compartment during signaling cascades. Although detailed mechanisms remain to be fully clarified, phosphoinositide metabolism is promising for a better understanding of such spatio-temporal regulation of TLR signaling and trafficking. [BMB Reports 2014; 47(7): 361-368]

Modulation of microglial function by the antidepressant drug venlafaxine

An increasing amount of data suggests that depression is an inflammatory disease. Depressed patients have higher peripheral blood levels of inflammatory markers which have been shown to access the brain and interact with the pathophysiological domain known to be involved in depression. Furthermore, microglia activation may play an important role in the inflammatory pathophysiology of depression.

In BV-2 microglia cell line, the present study investigated the potential anti-inflammatory effects of venlafaxine, along with its potential influence on injury of lipopolysaccharide (LPS)-stimulated cells. Although venlafaxine showed only marginal influence on the majority of the pro-inflammatory parameters assessed (in particular NO release, phagocytosis and proliferation), it significantly suppressed superoxide production by the stimulated cells. In addition, venlafaxine exerted also a protective effect on mitochondrial membrane potential and lysosomes of the stimulated microglia.

In conclusion, our results suggest that although VEN might have only a marginal effect on major pro-inflammatory parameters of microglia, its inhibitory effect on superoxide generation can contribute to the prevention of harmful effects of oxidative and nitrosative stress involved in the pathogenesis of depression. Moreover, the protective effect of VEN on viability of microglia can prevent a rapid reduction of these cells, thus avoiding limitations of several physiological processes in the brain and possibly also the progression of depression.

Genistein alleviates β-amyloid-induced inflammatory damage through regulating Toll-like receptor 4/nuclear factor κB

Genistein (GEN), a major soybean isoflavone (SIF), might possess neuroprotective properties through its anti-inflammatory activity. We hypothesized that GEN could prevent the inflammatory damage detected in C6 cells induced by β-amyloid peptides 25-35 (Aβ25-35). Accordingly, we evaluated the inflammatory damage induced by Aβ25-35 and the protective effect of GEN against Aβ25-35 in C6 cells. In our study, the C6 glial cells (rats glioma cell lines) were preincubated with or without GEN for 2 h following incubation with Aβ25-35 for another 24 h. Then, methylthiazolyl tetrazolium (MTT) assay was used to assess the cell viability. Immunofluorescence staining was used to identify the C6 cells. Inflammatory factors tumor necrosis factor (TNF)-α and interleukin (IL)-1β were analyzed by using enzyme-linked immunosorbent assay (ELISA). Western blot analysis and reverse transcription-polymerase chain reaction analysis were performed to assess the expression of Toll-like receptors 4 (TLR4), inhibitor of kappaB-alpha (IκB-α). The current results showed that GEN could alleviate Aβ25-35-induced cell apoptosis and prevent Aβ25-35-induced TNF-α and IL-1β release from C6 cells. In addition, GEN prevented Aβ25-35-induced upregulation of the gene and protein expression of TLR4, and GEN significantly upregulated the expression of IκB-α in C6 cells damaged by Aβ25-35. These results suggest that GEN can alleviate the inflammatory stress caused by Aβ25-35 treatment, which might be associated with the neuroprotective effect of GEN regulating the TLR4/NFκB signaling pathway.

MicroRNA-181c negatively regulates the inflammatory response in oxygen-glucose-deprived microglia by targeting Toll-like receptor 4

Cerebral hypoxia/ischemia rapidly induces inflammation in the brain, which is characterized by microglial activation and the release of inflammatory cytokines. We have previously demonstrated that miR-181c can directly regulate tumor necrosis factor (TNF)-α production post-transcriptionally. Here, we determined that hypoxia up-regulated TLR4 expression but down-regulated miR-181c expression in primary microglia. We also demonstrated that miR-181c suppresses TLR4 by directly binding its 3'-untranslated region. In addition, miR-181c inhibited NF-κB activation and the downstream production of proinflammatory mediators, such as TNF-α, IL-1β, and iNOS. Knocking down TLR4 in microglia significantly decreased TLR4 expression and inhibited NF-κB activation and the downstream production of proinflammatory mediators, whereas ectopic TLR4 expression significantly abrogated the suppressed inflammatory response induced by miR-181c. Therefore, our study identified an important role for the miR-181c-TLR4 pathway in hypoxic microglial activation and neuroinflammation. This pathway could represent a potential therapeutic target for cerebral hypoxic diseases associated with microglial activation and the inflammatory response. Cerebral hypoxia/ischemia induces microglial activation and the release of inflammatory cytokines. We found that hypoxia down-regulated miR-181c in primary microglia. In addition, miR-181c inhibited TLR4 expression through binding to its 3'UTR, thus inhibiting NF-kB activation and the production of downstream proinflammatory mediators. Therefore, the miR-181c-TLR4 pathway may be a potential therapeutic target for the treatment of cerebral hypoxic diseases.

Amyloid beta receptors responsible for neurotoxicity and cellular defects in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease. Although a major cause of AD is the accumulation of amyloid-β (Aβ) peptide that induces neuronal loss and cognitive impairments, our understanding of its neurotoxic mechanisms is limited. Recent studies have identified putative Aβ-binding receptors that mediate Aβ neurotoxicity in cells and models of AD. Once Aβ interacts with a receptor, a toxic signal is transduced into neurons, resulting in cellular defects including endoplasmic reticulum stress and mitochondrial dysfunction. In addition, Aβ can also be internalized into neurons through unidentified Aβ receptors and induces malfunction of subcellular organelles, which explains some part of Aβ neurotoxicity. Understanding the neurotoxic signaling initiated by Aβ-receptor binding and cellular defects provide insight into new therapeutic windows for AD. In the present review, we summarize the findings on Aβ-binding receptors and the neurotoxicity of oligomeric Aβ.

Bioenergetic dysfunction and inflammation in Alzheimer's disease: a possible connection

Inflammation is observed in Alzheimer’s disease (AD) subject brains. Inflammation-relevant genes are increasingly implicated in AD genetic studies, and inflammatory cytokines to some extent even function as peripheral biomarkers. What underlies AD inflammation is unclear, but no “foreign” agent has been implicated. This suggests that internally produced damage-associated molecular pattern (DAMPs) molecules may drive inflammation in AD. A more complete characterization and understanding of AD-relevant DAMPs could advance our understanding of AD and suggest novel therapeutic strategies. In this review, we consider the possibility that mitochondria, intracellular organelles that resemble bacteria in many ways, trigger and maintain chronic inflammation in AD subjects. Data supporting the possible nexus between AD-associated bioenergetic dysfunction are discussed.