Stimulation of Toll-Like Receptor 9 by Chronic Intraventricular Unmethylated Cytosine-Guanine DNA Infusion Causes Neuroinflammation and Impaired Spatial Memory

The Emerging Role of Toll-Like Receptor-Mediated Neuroinflammatory Signals in Psychiatric Disorders and Acquired Epilepsy

The new and evolving paradigms of psychiatric disorders pathogenesis are deeply inclined toward chronic inflammation that leads to disturbances in the neuronal networks of patients. A strong association has been established between the inflammation and neurobiology of depression which is mediated by different toll-like receptors (TLRs). TLRs and associated signalling pathways are identified as key immune regulators to stress and infections in neurobiology. They are a special class of transmembrane proteins, which are one of the broadly studied members of the Pattern Recognition Patterns family. This review focuses on summarizing the important findings on the role of TLRs associated with psychotic disorders and acquired epilepsy. This review also shows the promising potential of TLRs in immune response mediated through antidepressant therapies and TLRs polymorphism associated with various psychotic disorders. Moreover, this also sheds light on future directions to further target TLRs as a therapeutic approach for psychiatric disorders.

Biological agents and the aging brain: glial inflammation and neurotoxic signaling

Neuroinflammation is a universal characteristic of brain aging and neurological disorders, irrespective of the disease state. Glial inflammation mediates this signaling, through astrocyte and microglial polarization from neuroprotective to neurotoxic phenotypes. Glial reactivity results in the loss of homeostasis, as these cells no longer provide support to neurons, in addition to the production of chronically toxic pro-inflammatory mediators. These glial changes initiate an inflammatory brain state that injures the central nervous system (CNS) over time. As the brain ages, glia are altered, including increased glial cell numbers, morphological changes, and either a pre-disposition or inability to become reactive. These alterations induce age-related neuropathologies, ultimately leading to neuronal degradation and irreversible damage associated with disorders of the aged brain, including Alzheimer's Disease (AD) and other related diseases. While the complex interactions of these glial cells and the brain are well studied, the role additional stressors, such as infectious agents, play on age-related neuropathology has not been fully elucidated. Both biological agents in the periphery, such as bacterial infections, or in the CNS, including viral infections like SARS-CoV-2, push glia into neuroinflammatory phenotypes that can exacerbate pathology within the aging brain. These biological agents release pattern associated molecular patterns (PAMPs) that bind to pattern recognition receptors (PRRs) on glial cells, beginning an inflammatory cascade. In this review, we will summarize the evidence that biological agents induce reactive glia, which worsens age-related neuropathology.

Phospholipase D3 degrades mitochondrial DNA to regulate nucleotide signaling and APP metabolism

Phospholipase D3 (PLD3) polymorphisms are linked to late-onset Alzheimer's disease (LOAD). Being a lysosomal 5'-3' exonuclease, its neuronal substrates remained unknown as well as how a defective lysosomal nucleotide catabolism connects to AD-proteinopathy. We identified mitochondrial DNA (mtDNA) as a major physiological substrate and show its manifest build-up in lysosomes of PLD3-defective cells. mtDNA accretion creates a degradative (proteolytic) bottleneck that presents at the ultrastructural level as a marked abundance of multilamellar bodies, often containing mitochondrial remnants, which correlates with increased PINK1-dependent mitophagy. Lysosomal leakage of mtDNA to the cytosol activates cGAS-STING signaling that upregulates autophagy and induces amyloid precursor C-terminal fragment (APP-CTF) and cholesterol accumulation. STING inhibition largely normalizes APP-CTF levels, whereas an APP knockout in PLD3-deficient backgrounds lowers STING activation and normalizes cholesterol biosynthesis. Collectively, we demonstrate molecular cross-talks through feedforward loops between lysosomal nucleotide turnover, cGAS-STING and APP metabolism that, when dysregulated, result in neuronal endolysosomal demise as observed in LOAD.

Degradable Poly(amino acid) Vesicles Modulate DNA-Induced Inflammation after Traumatic Brain Injury

Following brain trauma, secondary injury from molecular and cellular changes causes progressive cerebral tissue damage. Acute/chronic neuroinflammation following traumatic brain injury (TBI) is a key player in the development of secondary injury. Rapidly elevated cell-free DNAs (cfDNAs) due to cell death could lead to production of inflammatory cytokines that aggravate TBI. Herein, we designed poly(amino acid)-based cationic nanoparticles (cNPs) and applied them intravenously in a TBI mice model with the purpose of scavenging cfDNA in the brain and suppressing the acute inflammation. In turn, these cNPs could effectively eliminate endogenous cfDNA, inhibit excessive activation of inflammation, and promote neural functional recovery.

Bacterial DNAemia in Alzheimer's Disease and Mild Cognitive Impairment: Association with Cognitive Decline, Plasma BDNF Levels, and Inflammatory Response

Microbial dysbiosis (MD) provokes gut barrier alterations and bacterial translocation in the bloodstream. The increased blood bacterial DNA (BB-DNA) may promote peripheral- and neuro-inflammation, contributing to cognitive impairment. MD also influences brain-derived neurotrophic factor (BDNF) production, whose alterations contribute to the etiopathogenesis of Alzheimer's disease (AD). The purpose of this study is to measure BB-DNA in healthy elderly controls (EC), and in patients with mild cognitive impairment (MCI) and AD to explore the effect on plasma BDNF levels (pBDNF), the inflammatory response, and the association with cognitive decline during a two-year follow-up. Baseline BB-DNA and pBDNF were significantly higher in MCI and AD than in EC. BB-DNA was positively correlated with pBDNF in AD, plasma Tumor necrosis factor-alpha (TNF-α), and Interleukin-10 (IL-10) levels in MCI. AD patients with BB-DNA values above the 50th percentile had lower baseline Mini-Mental State Examination (MMSE). After a two-year follow-up, AD patients with the highest BB-DNA tertile had a worse cognitive decline, while higher BB-DNA levels were associated with higher TNF-α and lower IL-10 in MCI. Our study demonstrates that, in early AD, the higher the BB-DNA levels, the higher the pBDNF levels, suggesting a defensive attempt; BB-DNA seems to play a role in the AD severity/progression; in MCI, higher BB-DNA may trigger an increased inflammatory response.

Host immune responses in the central nervous system during fungal infections

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

TLR-Mediated Signal Transduction and Neurodegenerative Disorders

A special class of proteins called Toll-like receptors (TLRs) are an essential part of the innate immune system, connecting it to the adaptive immune system. There are 10 different Toll-Like Receptors that have been identified in human beings. TLRs are part of the central nervous system (CNS), showing that the CNS is capable of the immune response, breaking the long-held belief of the brain's "immune privilege" owing to the blood-brain barrier (BBB). These Toll-Like Receptors are present not just on the resident macrophages of the central nervous system but are also expressed by the neurons to allow them for the production of proinflammatory agents such as interferons, cytokines, and chemokines; the activation and recruitment of glial cells; and their participation in neuronal cell death by apoptosis. This study is focused on the potential roles of various TLRs in various neurodegenerative diseases such as Parkinson's disease (PD) and Alzheimer's disease (AD), namely TLR2, TLR3, TLR4, TLR7, and TLR9 in AD and PD in human beings and a mouse model.

Oligodeoxynucleotides containing unmethylated cytosine-guanine motifs are effective immunostimulants against pneumococcal meningitis in the immunocompetent and neutropenic host

BACKGROUND

Bacterial meningitis is a fatal disease with a mortality up to 30% and neurological sequelae in one fourth of survivors. Available vaccines do not fully protect against this lethal disease. Here, we report the protective effect of synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG ODN) against the most frequent form of bacterial meningitis caused by Streptococcus pneumoniae.

METHODS

Three days prior to the induction of meningitis by intracerebral injection of S. pneumoniae D39, wild-type and Toll-like receptor (TLR9)^-/- mice received an intraperitoneal injection of 100 μg CpG ODN or vehicle. To render mice neutropenic, anti-Ly-6G monoclonal antibody was daily administrated starting 4 days before infection with a total of 7 injections. Kaplan-Meier survival analyses and bacteriological studies, in which mice were sacrificed 24 h and 36 h after infection, were performed.

RESULTS

Pre-treatment with 100 μg CpG ODN prolonged survival of immunocompetent and neutropenic wild-type mice but not of TLR9^-/- mice. There was a trend towards lower mortality in CpG ODN-treated immunocompetent and neutropenic wild-type mice. CpG ODN caused an increase of IL-12/IL-23p40 levels in the spleen and serum in uninfected animals. The effects of CpG ODN on bacterial concentrations and development of clinical symptoms were associated with an increased number of microglia in the CNS during the early phase of infection. Elevated concentrations of IL-12/IL-23p40 and MIP-1α correlated with lower bacterial concentrations in the blood and spleen during infection.

CONCLUSIONS

Pre-conditioning with CpG ODN strengthened the resistance of neutropenic and immunocompetent mice against S. pneumoniae meningitis in the presence of TLR9. Administration of CpG ODN decreased bacterial burden in the cerebellum and reduced the degree of bacteremia. Systemic administration of CpG ODN may help to prevent or slow the progression to sepsis of bacterial CNS infections in healthy and immunocompromised individuals even after direct inoculation of bacteria into the intracranial compartments, which can occur after sinusitis, mastoiditis, open head trauma, and surgery, including placement of an external ventricular drain.

Danger-Sensing/Patten Recognition Receptors and Neuroinflammation in Alzheimer's Disease

Fibrillar aggregates and soluble oligomers of both Amyloid-β peptides (Aβs) and hyperphosphorylated Tau proteins (p-Tau-es), as well as a chronic neuroinflammation are the main drivers causing progressive neuronal losses and dementia in Alzheimer’s disease (AD). However, the underlying pathogenetic mechanisms are still much disputed. Several endogenous neurotoxic ligands, including Aβs, and/or p-Tau-es activate innate immunity-related danger-sensing/pattern recognition receptors (PPRs) thereby advancing AD’s neuroinflammation and progression. The major PRR families involved include scavenger, Toll-like, NOD-like, AIM2-like, RIG-like, and CLEC-2 receptors, plus the calcium-sensing receptor (CaSR). This quite intricate picture stresses the need to identify the pathogenetically topmost Aβ-activated PRR, whose signaling would trigger AD’s three main drivers and their intra-brain spread. In theory, the candidate might belong to any PRR family. However, results of preclinical studies using in vitro nontumorigenic human cortical neurons and astrocytes and in vivo AD-model animals have started converging on the CaSR as the pathogenetically upmost PRR candidate. In fact, the CaSR binds both Ca²⁺ and Aβs and promotes the spread of both Ca²⁺ dyshomeostasis and AD’s three main drivers, causing a progressive neurons’ death. Since CaSR’s negative allosteric modulators block all these effects, CaSR’s candidacy for topmost pathogenetic PRR has assumed a growing therapeutic potential worth clinical testing.

Microglia: Agents of the CNS Pro-Inflammatory Response

The pro-inflammatory immune response driven by microglia is a key contributor to the pathogenesis of several neurodegenerative diseases. Though the research of microglia spans over a century, the last two decades have increased our understanding exponentially. Here, we discuss the phenotypic transformation from homeostatic microglia towards reactive microglia, initiated by specific ligand binding to pattern recognition receptors including toll-like receptor-4 (TLR4) or triggering receptors expressed on myeloid cells-2 (TREM2), as well as pro-inflammatory signaling pathways triggered such as the caspase-mediated immune response. Additionally, new research disciplines such as epigenetics and immunometabolism have provided us with a more holistic view of how changes in DNA methylation, microRNAs, and the metabolome may influence the pro-inflammatory response. This review aimed to discuss our current knowledge of pro-inflammatory microglia from different angles, including recent research highlights such as the role of exosomes in spreading neuroinflammation and emerging techniques in microglia research including positron emission tomography (PET) scanning and the use of human microglia generated from induced pluripotent stem cells (iPSCs). Finally, we also discuss current thoughts on the impact of pro-inflammatory microglia in neurodegenerative diseases.

Assessing the role of toll-like receptor in isolated, standard and enriched housing conditions

Depression is a common psychiatric disorder that has been poorly understood. Consequently, current antidepressant agents have clinical limitations. Until today, most have exhibited the slow onset of therapeutic action and, more importantly, their effect on remission has been minimal. Thus, the need to find new forms of therapeutic intervention is urgent. The inflammation hypothesis of depression is widely acknowledged and is one that theories the relationship between the function of the immune system and its contribution to the neurobiology of depression. In this research, we utilized an environmental isolation (EI) approach as a valid animal model of depression, employing biochemical, molecular, and behavioral studies. The aim was to investigate the anti-inflammatory effect of etanercept, a tumor necrosis factor-α inhibitor on a toll-like receptor 7 (TLR 7) signaling pathway in a depressive rat model, and compare these actions to fluoxetine, a standard antidepressant agent. The behavioral analysis indicates that depression-related symptoms are reduced after acute administration of fluoxetine and, to a lesser extent, etanercept, and are prevented by enriched environment (EE) housing conditions. Experimental studies were conducted by evaluating immobility time in the force swim test and pleasant feeling in the sucrose preference test. The mRNA expression of the TLR 7 pathway in the hippocampus showed that TLR 7, MYD88, and TRAF6 were elevated in isolated rats compared to the standard group, and that acute treatment with an antidepressant and anti-inflammatory drugs reversed these effects. This research indicates that stressful events have an impact on behavioral well-being, TLR7 gene expression, and the TLR7 pathway. We also found that peripheral administration of etanercept reduces depressive-like behaviour in isolated rats: this could be due to the indirect modulation of the TLR7 pathway and other TLRs in the brain. Furthermore, fluoxetine treatment reversed depressive-like behaviour and molecularly modulated the expression of TLR7, suggesting that fluoxetine exerts antidepressant effects partially by modulating the TLR7 signaling pathway.

Cell Type Specific Expression of Toll-Like Receptors in Human Brains and Implications in Alzheimer's Disease

Toll-like receptors mediate important cellular immune responses upon activation via various pathogenic stimuli such as bacterial or viral components. The activation and subsequent secretion of cytokines and proinflammatory factors occurs in the whole body including the brain. The subsequent inflammatory response is crucial for the immune system to clear the pathogen(s) from the body via the innate and adaptive immune response. Within the brain, astrocytes, neurons, microglia, and oligodendrocytes all bear unique compositions of Toll-like receptors. Besides pathogens, cellular damage and abnormally folded protein aggregates, such as tau and Amyloid beta peptides, have been shown to activate Toll-like receptors in neurodegenerative diseases such as Alzheimer's disease. This review provides an overview of the different cell type-specific Toll-like receptors of the human brain, their activation mode, and subsequent cellular response, as well as their activation in Alzheimer's disease. Finally, we critically evaluate the therapeutic potential of targeting Toll-like receptors for treatment of Alzheimer's disease as well as discussing the limitation of mouse models in understanding Toll-like receptor function in general and in Alzheimer's disease.

Role of Microglia TLRs in Neurodegeneration

Toll-like receptors (TLRs) are a group of receptors widely distributed in the organism. In the central nervous system, they are expressed in neurons, astrocytes and microglia. Although their involvement in immunity is notorious, different articles have demonstrated their roles in physiological and pathological conditions, including neurodegeneration. There is increasing evidence of an involvement of TLRs, especially TLR2, 4 and 9 in neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS). In this sense, their expression in microglia might modulate the activity of these cells, which in turn, lead to protective or deleterious effects over neurons and other cells. Therefore, TLRs might mediate the link between inflammation and neurodegenerative diseases. However, further studies have to be performed to elucidate the role of the other TLRs in these diseases and to further prove and confirm the pathophysiological role of all TLRs in neurodegeneration. In this article, we revise and summarize the current knowledge regarding the role of TLRs in neurodegeneration with the focus on the possible functions of these receptors in microglia.

Effects of TLR3 and TLR9 Signaling Pathway on Brain Protection in Rats Undergoing Sevoflurane Pretreatment during Cardiopulmonary Bypass

OBJECTIVE

To investigate the effects of TLR3 and TLR9 signaling pathway on brain injury during CPB in rats pretreated with sevoflurane and its possible molecular mechanism.

METHODS

SD rats were randomly assigned to sham group, CPB group, and Sev group. Brain tissue was obtained at before CPB (T0), at CPB for 30 minutes (T1), 1 hour after CPB (T3), and 3 hours after CPB (T5). ELISA was used to measure S100-β and IL-6. Western blot was utilized to determine TLR3 and TLR9 expression. TUNEL was applied to detect neuronal apoptosis.

RESULTS

Compared with CPB group, at T1, at termination after 1 hour of CPB (T2), T3, 2 hours after CPB (T4) and T5, S100-β and IL-6 decreased in Sev group. Compared with CPB group, IFN-β were increased in Sev group, except T0. Compared with CPB group, TLR3 expression increased, and TLR9 and NF-κB decreased in Sev group. The apoptotic neurons were less in Sev group than in CPB group (P < 0.05).

CONCLUSION

Sevoflurane intervention can activate TLR3 and TLR9 signaling pathway, upregulate TLR3 expression and downstream TRIF expression, decrease TLR9 expression, and downregulate downstream NF-κB expression in CPB rat models, thereby mitigating brain injury induced by inflammatory response during CPB.

Pharmacological Tools to Activate Microglia and their Possible use to Study Neural Network Patho-physiology

BACKGROUND

Microglia are the resident immunocompetent cells of the CNS and also constitute a unique cell type that contributes to neural network homeostasis and function. Understanding microglia cell-signaling not only will reveal their diverse functions but also will help to identify pharmacological and non-pharmacological tools to modulate the activity of these cells.

METHODS

We undertook a search of bibliographic databases for peer-reviewed research literature to identify microglial activators and their cell-specificity. We also looked for their effects on neural network function and dysfunction.

RESULTS

We identified several pharmacological targets to modulate microglial function, which are more or less specific (with the proper control experiments). We also identified pharmacological targets that would require the development of new potent and specific modulators. We identified a wealth of evidence about the participation of microglia in neural network function and their alterations in pathological conditions.

CONCLUSION

The identification of specific microglia-activating signals provides experimental tools to modulate the activity of this heterogeneous cell type in order to evaluate its impact on other components of the nervous system, and it also helps to identify therapeutic approaches to ease some pathological conditions related to microglial dysfunction.

Decreased Toll-Like Receptor 2 and Toll-Like Receptor 7/8-Induced Cytokines in Parkinson's Disease Patients

OBJECTIVES

Toll-like receptors (TLRs) are expressed in several immune cells including blood monocytes and resident macrophages, such as microglia in the central nervous system. TLRs recognize pathogen- or damage-associated molecular patterns, leading to the release of inflammatory and toxic molecules, which can contribute to neuroinflammation associated with Parkinson's disease (PD). The aim of this study was to compare the potential of peripheral blood cells from PD patients or healthy subjects to produce cytokines after exposure to TLR agonists, and to investigate TLR2 and TLR4 expression on monocyte subsets.

METHODS

Twenty-one patients and 21 healthy controls were recruited. Patients were evaluated according to the Unified Parkinson's Disease Rating Scale, and Hoehn and Yahr stage. Cytokines were measured in supernatants of whole blood cultures after incubation with TLR2, TLR4, or TLR7/8 agonists, by cytometric bead array. Expression of CD14, CD16, TLR2, and TLR4 was analyzed by cytometry.

RESULTS

Patient blood cells produced lower levels of cytokines in response to TLR2 and also after TLR7/8/R848 activation than controls. Percentages of CD14+CD16+ or CD14+CD16- monocytes and TLR2 and TLR4 expression were similar between patients and controls.

CONCLUSIONS

Blood leukocyte TLR2 and TLR7/8 responses are impaired in PD. This was neither associated with imbalance in monocyte subsets nor with TLR2/TLR4 expression on these cells. The association between a decreased TLR response in periphery and damage of brain in PD must be further investigated.

The impact of single and pairwise Toll-like receptor activation on neuroinflammation and neurodegeneration

Background

Toll-like receptors (TLRs) enable innate immune cells to respond to pathogen- and host-derived molecules. The central nervous system (CNS) exhibits most of the TLRs identified with predominant expression in microglia, the major immune cells of the brain. Although individual TLRs have been shown to contribute to CNS disorders, the consequences of multiple activated TLRs on the brain are unclear. We therefore systematically investigated and compared the impact of sole and pairwise TLR activation on CNS inflammation and injury.

Methods

Selected TLRs expressed in microglia and neurons were stimulated with their specific TLR ligands in varying combinations. Cell cultures were then analyzed by immunocytochemistry, FlowCytomix, and ELISA. To determine neuronal injury and neuroinflammation in vivo, C57BL/6J mice were injected intrathecally with TLR agonists. Subsequently, brain sections were analyzed by quantitative real-time PCR and immunohistochemistry.

Results

Simultaneous stimulation of TLR4 plus TLR2, TLR4 plus TLR9, and TLR2 plus TLR9 in microglia by their respective specific ligands results in an increased inflammatory response compared to activation of the respective single TLR in vitro. In contrast, additional activation of TLR7 suppresses the inflammatory response mediated by the respective ligands for TLR2, TLR4, or TLR9 up to 24 h, indicating that specific combinations of activated TLRs individually modulate the inflammatory response. Accordingly, the composition of the inflammatory response pattern generated by microglia varies depending on the identity and combination of the activated TLRs engaged. Likewise, neuronal injury occurs in response to activation of only selected TLRs and TLR combinations in vitro. Activation of TLR2, TLR4, TLR7, and TLR9 in the brain by intrathecal injection of the respective TLR ligand into C57BL/6J mice leads to specific expression patterns of distinct TLR mRNAs in the brain and causes influx of leukocytes and inflammatory mediators into the cerebrospinal fluid to a variable extent. Also, the intensity of the inflammatory response and neurodegenerative effects differs according to the respective activated TLR and TLR combinations used in vivo.

Conclusions

Sole and pairwise activation of TLRs modifies the pattern and extent of inflammation and neurodegeneration in the CNS, thereby enabling innate immunity to take account of the CNS diseases’ diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0166-7) contains supplementary material, which is available to authorized users.

Treatment implications of the altered cytokine-insulin axis in neurodegenerative disease

The disappointments of a series of large anti-amyloid trials have brought home the point that until the driving force behind Alzheimer's disease, and the way it causes harm, are firmly established and accepted, researchers will remain ill-equipped to find a way to treat patients successfully. The origin of inflammation in neurodegenerative diseases is still an open question. We champion and expand the argument that a shift in intracellular location of α-synuclein, thereby moving a key methylation enzyme from the nucleus, provides global hypomethylation of patients' cerebral DNA that, through being sensed by TLR9, initiates production of the cytokines that drive these cerebral inflammatory states. After providing a background on the relevant inflammatory cytokines, this commentary then discusses many of the known alternatives to the primary amyloid argument of the pathogenesis of Alzheimer's disease, and the treatment approaches they provide. A key point to appreciate is the weight of evidence that inflammatory cytokines, largely through increasing insulin resistance and thereby reducing the strength of the ubiquitously important signaling mediated by insulin, bring together most of these treatments under development for neurodegenerative disease under the one roof. Moreover, the principles involved apply to a wide range of inflammatory diseases on both sides of the blood brain barrier.

Toll-like receptor 9 deficiency impacts sensory and motor behaviors

Toll-like receptors (TLRs) mediate the induction of the innate immune system in response to pathogens, injury and disease. However, they also play non-immune roles and are expressed in the central nervous system (CNS) during prenatal and postnatal stages including adulthood. Little is known about their roles in the CNS in the absence of pathology. Several members of the TLR family have been implicated in the development of neural and cognitive function although the contribution of TLR9 to these processes has not been well defined. The current studies were undertaken to determine whether developmental TLR9 deficiency affects motor, sensory or cognitive functions. We report that TLR9 deficient (TLR9(-/-)) mice show a hyper-responsive sensory and motor phenotype compared to wild type (TLR9(+/+)) controls. This is indicated by hypersensitivity to thermal stimuli in the hot plate paw withdrawal test, enhanced motor-responsivity under anxious conditions in the open field test and greater sensorimotor reactivity in the acoustic startle response. Prepulse inhibition (PPI) of the acoustic startle response was also enhanced, which indicates abnormal sensorimotor gating. In addition, subtle, but significant, gait abnormalities were noted in the TLR9(-/-) mice on the horizontal balance beam test with higher foot slip numbers than TLR9(+/+) controls. In contrast, spatial learning and memory, assessed by the Morris water maze, was similar in the TLR9(-/-) and TLR9(+/+) mice. These findings support the notion that TLR9 is important for the appropriate development of sensory and motor behaviors.

CpG oligodeoxynucleotides induce the expression of the antimicrobial peptide cathelicidin in glial cells

During bacterial infections, antimicrobial peptides are synthesised as an important part of the innate immune system. However, expression and function in the central nervous system (CNS) need further investigations. The aim of this study was to examine the involvement of the pattern-recognition-receptor toll-like receptor 9 (TLR9) in the expression of the cathelin-related antimicrobial peptide (CRAMP) and to characterise the participating signal transduction pathways. In primary TLR9 deficient and wildtype mice astrocytes as well as microglia cells, the expression of CRAMP after treatment with the TLR9 agonist unmethylated cytosine-guanine oligodeoxynucleotide motifs (CpG-DNA) was examined in vitro. In vivo CRAMP expression after intraventricular infusion of CpG-DNA in TLR9 deficient and wildtype mice as well as in mice with pneumococcal meningitis localised in glial cells was determined. Furthermore, the regulation of different signal transduction pathways involved in CpG-DNA-induced CRAMP expression in glial cells was analysed. An in vitro and in vivo CpG-DNA-induced increase of CRAMP expression in astrocytes and microglia cells using real time RT-PCR and immunofluorescence was demonstrated. Different signal transduction pathways such as mitogen-activated protein kinases and inflammatory mediated pathways are involved in the expression of CRAMP in primary glial cells. Interestingly, TLR9-deficient glial cells showed a reduced but not completely abolished CRAMP mRNA expression and ERK1/2 phosphorylation in response to CpG-DNA treatment. On the other side in vivo, TLR9 deletion did not change CRAMP expression after bacterial infection. In conclusion, our results show that TLR9 can induce the expression of antimicrobial peptides such as CRAMP in response to bacterial DNA motifs in primary glial cells. Additional findings suggest also that CpG-DNA-induced effects are not only mediated by TLR9, but also mediated by other pattern recognition receptors.

TLR7 and TLR9 trigger distinct neuroinflammatory responses in the CNS

Toll-like receptors (TLRs) 7 and 9 recognize nucleic acid determinants from viruses and bacteria and elicit the production of type I interferons and proinflammatory cytokines. TLR7 and TLR9 are similar regarding localization and signal transduction mechanisms. However, stimulation of these receptors has differing effects in modulating viral pathogenesis and in direct toxicity in the central nervous system (CNS). In the present study, we examined the potential of the TLR7 agonist imiquimod and the TLR9 agonist cytosine-phosphate-guanosine oligodeoxynucleotide (CpG-ODN) to induce neuroinflammation after intracerebroventricular inoculation. CpG-ODN induced a more robust inflammatory response than did imiquimod after inoculation into the CNS, with higher levels of several proinflammatory cytokines and chemokines. The increase in cytokines and chemokines correlated with breakdown of the blood-cerebrospinal fluid barrier and recruitment of peripheral cells to the CNS in CpG-ODN-inoculated mice. In contrast, TLR7 agonists induced a strong interferon β response in the CNS but only low levels of other cytokines. The difference in response to these agonists was not due to differences in distribution or longevity of the agonists but rather was correlated with cytokine production by choroid plexus cells. These results indicate that despite the high similarity of TLR7 and TLR9 in binding nucleic acids and inducing similar downstream signaling, the neuroinflammation response induced by these receptors differs dramatically due, at least in part, to activation of cells in the choroid plexus.

Single nucleotide polymorphisms in TLR9 are highly associated with susceptibility to bacterial meningitis in children

BACKGROUND

Bacterial meningitis (BM) is a severe infection mainly caused by Streptococcus pneumoniae and Neisseria meningitidis (NM). However, genetically determined susceptibility to develop severe infections by these microorganisms is variable between individuals. Toll-like receptor 9 (TLR9) recognizes bacterial DNA leading to intracellular inflammatory signaling. Single nucleotide polymorphisms (SNPs) within the TLR9 gene are associated with susceptibility to several diseases, no such association with meningitis has been described.

METHODS

We studied the role of TLR9 SNPs in host defense against BM. Two TLR9 SNPs and 4 TLR9 haplotypes were determined in 472 survivors of BM and compared to 392 healthy controls.

RESULTS

Carriage of the TLR9+2848-A mutant was significantly decreased in meningococcal meningitis (MM) patients compared with controls (p: .0098, odds ratio [OR]: .6, 95% confidence interval [CI]: .4-.9). TLR9 haplotype I was associated with an increased susceptibility to MM (p: .0237, OR 1.3, 95% CI: 1.0-1.5). In silico analysis shows a very strong immunoinhibitory potential for DNA of NM upon recognition by TLR9 (CpG index of -106.8).

CONCLUSIONS

We report an association of TLR9 SNPs with susceptibility to BM, specifically MM indicating a protective effect for the TLR9+2848-A allele. We hypothesize that the TLR9+2848-A mutant results in an up-regulation of TLR9 induced immune response compensating the strong inhibitory potential of NM CpG DNA.

Toll-like receptor signaling in neural plasticity and disease

Toll-like receptors (TLRs) are a family of innate immune system receptors that respond to pathogen-derived and tissue damage-related ligands. TLR signaling in immune cells, glia and neurons can play roles in the pathogenesis of stroke, Alzheimer's disease (AD) and multiple sclerosis (MS). Recent findings suggest that TLR signaling also influences multiple dynamic processes in the developing and adult central nervous system including neurogenesis, axonal growth and structural plasticity. In addition, TLRs are implicated in the regulation of behaviors including learning, memory and anxiety. This review describes recently discovered and unexpected roles for TLRs in neuroplasticity, and the implications of these findings for future basic and translational research studies.

Mechanisms of injury in bacterial meningitis

PURPOSE OF REVIEW

This review describes the pathophysiology of cellular and axonal injury in bacterial meningitis.

RECENT FINDINGS

Toll-like receptors have been recognized as important mediators for the initiation of the immune response within the central nervous system. Activation of microglial cells by bacterial products through these receptors increases their ability to phagocytose bacteria, but can also lead to destruction of neurons. The cholesterol-binding hemolysin pneumolysin has a direct toxic effect on neuronal cells. Adjuvant therapy with corticosteroids and glycerol improved the outcome of bacterial meningitis in clinical studies.

SUMMARY

Brain damage in bacterial meningitis leading to long-term neurologic sequelae and death is caused by several mechanisms. Bacterial invasion and the release of bacterial compounds promote inflammation, invasion of leukocytes and stimulation of microglia. Leukocytes, macrophages and microglia release free radicals, proteases, cytokines and excitatory amino acids, finally leading to energy failure and cell death. Vasculitis, focal ischemia and brain edema subsequent to an increase in cerebrospinal fluid outflow resistance, breakdown of the blood-brain barrier and swelling of necrotic cells cause secondary brain damage.