Inhibiting TLR2 activation attenuates amyloid accumulation and glial activation in a mouse model of Alzheimer's disease

Identification of cytochrome P450 2E1 as a novel target in neuroinflammation and development of its inhibitor Q11 as a treatment strategy

Neuroinflammation is implicated in nearly all pathological processes of central nervous system (CNS) diseases. However, no medications specifically targeting neuroinflammation are clinically available, and conventional anti-inflammatory drugs exhibit limited efficacy. Here, we identified cytochrome P450 2E1 (CYP2E1) as a novel therapeutic target in neuroinflammation. Elevated CYP2E1 levels were observed in hippocampal tissues of mouse and rat neuroinflammation models, as well as in LPS-stimulated primary microglia. Genetic ablation of Cyp2e1 improved spatial learning and memory in neuroinflammatory rats and reduced pro-inflammatory cytokine levels in Cyp2e1-deficient microglia. Furthermore, Q11 (1-(4-methyl-5-thiazolyl) ethanone), a novel CYP2E1 inhibitor developed and synthesized in our laboratory, effectively ameliorated Alzheimer's disease-related spatial learning and memory functions and depression-related anxiety-like behaviors in mice/rats. Mechanistically, Q11 attenuated microglial activation, neuronal damage, oxidative stress, and neuroinflammation by suppressing the PI3K/Akt, STAT1/3, and NF-κB signaling pathways. These findings establish CYP2E1 as a druggable target for neuroinflammation and propose Q11 as a promising candidate for treating neuroinflammation-related diseases.

Pro-Oxidative and Inflammatory Actions of Extracellular Hemoglobin and Heme: Molecular Events and Implications for Alzheimer's and Parkinson Disease

Hemoglobin (Hb) and heme, which are typically confined within red blood cells (RBCs), are essential for intravascular transport of gases and nutrients. However, these molecules acquire secondary functions upon exposure to the extracellular environment. Hb and heme generate reactive oxygen species (ROS), which are potent pro-inflammatory agents that contribute to oxidative stress and cellular damage. These events are relevant to neurodegenerative processes, where oxidative stress, irregular deposition of protein aggregates, and chronic inflammation are key pathological features. Extracellular Hb, heme, and oxidative stress derived from hemorrhagic events or RBC lysis may contribute to increased blood-brain barrier (BBB) permeability. These events allow Hb and heme to interact with neuroimmune cells and pathological protein aggregates, further amplifying pro-inflammatory signaling and the progression of Alzheimer's disease (AD) and Parkinson disease (PD). Chronic neuroinflammation, oxidative stress, and mitochondrial dysfunction lead to neuronal degeneration. Here, we sought to elucidate the pro-oxidative and inflammatory actions of extracellular Hb and heme, emphasizing their potential impact on AD and PD development.

Reducing microglial lipid load enhances β amyloid phagocytosis in an Alzheimer's disease mouse model

Macrophages accumulate lipid droplets (LDs) under stress and inflammatory conditions. Despite the presence of LD-loaded macrophages in many tissues, including the brain, their contribution to neurodegenerative disorders remains elusive. This study investigated the role of lipid metabolism in Alzheimer's disease (AD) by assessing the contribution of LD-loaded brain macrophages, including microglia and border-associated macrophages (BAMs), in an AD mouse model. Particularly, BAMs and activated CD11c+ microglia localized near β amyloid (Aβ) plaques exhibited a pronounced lipid-associated gene signature and a high LD load. Having observed that elevated intracellular LD content correlated inversely with microglial phagocytic activities, we subsequently inhibited LD formation specifically in CX3CR1+ brain macrophages using an inducible APP-KI/Fit2iΔMφ transgenic mouse model. We demonstrated that reducing LD content in microglia and CX3CR1+ BAMs remarkably improved their phagocytic ability. Furthermore, lowering microglial LDs consistently enhanced their efferocytosis capacities and notably reduced Aβ deposition in the brain parenchyma. Therefore, mitigating LD accumulation in brain macrophages provides perspectives for AD treatment.

Microglial activation states and their implications for Alzheimer's Disease

Alzheimer's Disease (AD) is a chronic neurodegenerative disorder characterized by the accumulation of toxic amyloid-beta (Aβ) plaques and neurofibrillary tangles (NFTs) of tau protein in the brain. Microglia, key immune cells of the central nervous system, play an important role in AD development and progression, primarily through their responses to Aβ and NFTs. Initially, microglia can clear Aβ, but in AD, chronic activation overwhelms protective mechanisms, leading to sustained neuroinflammation that enhances plaque toxicity, setting off a damaging cycle that affects neurons, astrocytes, cerebral vasculature, and other microglia. Current AD treatments have been largely ineffective, though emerging immunotherapies focusing on plaque removal show promise, but often overlook the role of neuroinflammation. Activated microglia display a complex range of phenotypes that can be broadly broken into pro- or anti-inflammatory states, although this dichotomy does not describe the significant overlap between states. Aβ can strongly induce inflammatory activity, triggering the production of reactive oxygen species, inflammatory cytokines (e.g., TNF-α, IL-1β, IL-6), synapse engulfment, blood-brain barrier compromise, and impaired Aβ clearance. These processes contribute to neural tissue loss, manifesting as cognitive decline such as impaired executive function and memory. Conversely, anti-inflammatory activation exerts neuroprotective effects by suppressing inflammatory pathways and releasing neurotrophic factors that aid neuron repair and protection. Induction of anti-inflammatory states may offer a dual therapeutic approach to address both neuroinflammation and plaque accumulation in AD. This approach suggests potential strategies to modulate microglial phenotypes, aiming to restore neuroprotective functions and mitigate disease progression by simultaneously targeting inflammation and plaque pathology.

Investigation of Novel Aronia Bioactive Fraction-Alginic Acid Nanocomplex on the Enhanced Modulation of Neuroinflammation and Inhibition of Aβ Aggregation

BACKGROUND/OBJECTIVES

Aronia extract or its active compounds, especially anthocyanin, have shown potential for Alzheimer's disease (AD)-related pathologies, including neuroinflammation, fibrillogenesis of amyloid beta (Aβ), and cognitive impairment. However, there was still concern about their structural instability in vivo and in vitro. To solve the instability of anthocyanins, we combined aronia bioactive factions (ABFs) and alginic acid via electrostatic molecular interactions and created an ABF-alginic acid nanocomplex (AANCP). We evaluated whether it is more stable and effective in cognitive disorder mice and neuroinflammation cell models.

METHODS

The physicochemical properties of the AANCP, such as nanoparticle size, structural stability, and release rate, were characterized. The AANCP was administered to scopolamine-injected Balb/c mice, and to BV2 microglia treated with lipopolysaccharide (LPS) and amyloid beta (Aβ). Inflammation responses were measured via qPCR and ELISA in vitro, and cognitive functions were measured via behavior tests in vivo.

RESULTS

The AANCP readily formed nanoparticles, 209.6 nm in size, with a negatively charged zeta potential. The AANCP exhibited better stability in four plasma samples (human, dog, rat, and mouse) and was slowly released in different pH conditions (pH 2.0, 7.4, and 8.0) compared with non-complexedABF. In vitro studies on microglial cells treated with AANCPs revealed a suppression of inflammatory cytokines (tumor necrosis factor-alpha and interleukin-6) induced by LPS. The AANCP increased microglial Aβ phagocytosis through the activation of triggering receptor expressed on myeloid cell 2 (TREM2)-related microglial polarization. The AANCP inhibited aggregation of Aβ in vitro and alleviated cognitive impairment in a scopolamine-induced in vivo dementia mouse model.

CONCLUSIONS

Our data indicate that AANCPs are more stable than ABFs and effective for cognitive disorders and neuroinflammation via modulation of M2 microglial polarization.

Microglia-Associated Neuroinflammation in Alzheimer’s Disease and Its Therapeutic Potential

Background: Neuroinflammation has long been implicated in the progression of amyloid beta (Aβ) accumulation and the decline of cognitive function in Alzheimer’s disease (AD). The phenotype balance between A1 (toxic) and A2 (safe) microglial phenotypes to toxic illness in AD has become a hot research topic at present. Currently, many transcription factors, downstream signaling pathways, and molecular mechanisms that regulate the polarization of microglia are being explored. Furthermore, microglia may also exert a complex role in AD through the transformation of Aβ plaques or debris clearance, reflected in Aβ phagocytosis. One of the mediators of neuroinflammation in AD is the activated microglia. Therefore, the regulation of microglial function may be the key to successfully treating AD. Methods: This paper is a review article. PubMed, Embase, Scopus, and research meeting abstracts were searched up to 2024 for studies of microglia and neuroinflammation in Alzheimer’s Disease. Systematic information retrieval was performed, and appropriate studies were isolated based on important information available in the studies. The information from each of the articles was understood and extracted to form a database. Results: The similar neuropathological results between several animals and AD cases show the possibility of implementing microglia-related changes as an earlier diagnostic marker for AD in humans. The gene sets identified in various transcriptomic studies further foster this avenue of research by offering potential targets for therapeutic development. Substantial evidence, both in vitro and in vivo, has suggested that the loss of the normal A2 phenotype and the activation of toxic A1 microglia contribute to neurodegeneration in AD. Conclusions: Promoting or restoring the polarization of microglia towards the A2 phenotype may thus represent an effective therapeutic strategy for ameliorating neuroinflammation and progressive neurocognitive impairments. Multiple studies suggest that microglia-associated neuroinflammation at a special stage could also be protective, and, therefore, intervention should be delicate so that a beneficial response is retained.

Exploring the hub Genes and Potential Mechanisms of Complement system-related Genes in Parkinson Disease: Based on Transcriptome Sequencing and Mendelian Randomization

An accurate diagnosis of Parkinson's disease (PD) remains challenging and the exact cause of the disease is unclean. The aims are to identify hub genes associated with the complement system in PD and to explore their underlying molecular mechanisms. Initially, differentially expressed genes (DEGs) and key module genes related to PD were mined through differential expression analysis and WGCNA. Then, differentially expressed CSRGs (DE-CSRGs) were obtained by intersecting the DEGs, key module genes and CSRGs. Subsequently, MR analysis was executed to identify genes causally associated with PD. Based on genes with significant MR results, the expression level and diagnostic performance verification were achieved to yield hub genes. Functional enrichment and immune infiltration analyses were accomplished to insight into the pathogenesis of PD. qRT-PCR was employed to evaluate the expression levels of hub genes. After MR analysis and related verification, CD93, CTSS, PRKCD and TLR2 were finally identified as hub genes. Enrichment analysis indicated that the main enriched pathways for hub genes. Immune infiltration analysis found that the hub genes showed significant correlation with a variety of immune cells (such as myeloid-derived suppressor cell and macrophage). In the qRT-PCR results, the expression levels of CTSS, PRKCD and TLR2 were consistent with those we obtained from public databases. Hence, we mined four hub genes associated with complement system in PD which provided novel perspectives for the diagnosis and treatment of PD.

TLR2 immunotherapy suppresses neuroinflammation, tau spread, and memory loss in rTg4510 mice

In Alzheimer's disease, chronic neuroinflammation is accompanied by amyloid and tau pathologies. Especially, aberrant microglial activation is known to precede the regional tau pathology development, but the mechanisms how microglia affect tau spread remain largely unknown. Here, we found that toll-like receptor 2 (TLR2) in microglia recognizes oligomeric tau as a pathogenic ligand and induces inflammatory responses. Knockout of TLR2 reduced tau pathology and microglial activation in rTg4510 tau transgenic mice. Treatment of oligomeric tau induced TLR2 activation and increased inflammatory responses in microglial cells. TLR2 further mediated the tau-induced microglial activation and promoted tau uptake into neurons in neuron-microglia co-culture system and in mouse hippocampus after intracranial tau injection. Importantly, treatment with anti-TLR2 monoclonal antibody Tomaralimab blocked TLR2 activation and inflammatory responses in a dose-dependent manner, and significantly reduced tau spread and memory loss in rTg4510 mice. These results suggest that TLR2 plays a crucial role in tau spread by causing aberrant microglial activation in response to pathological tau, and blocking TLR2 with immunotherapy may ameliorate tau pathogenesis in Alzheimer's disease.

Ultrahigh field diffusion magnetic resonance imaging uncovers intriguing microstructural changes in the adult zebrafish brain caused by Toll-like receptor 2 genomic deletion

Toll-like receptor 2 (TLR2) belongs to the TLR protein family that plays an important role in the immune and inflammation response system. While TLR2 is predominantly expressed in immune cells, its expression has also been detected in the brain, specifically in microglia and astrocytes. Recent studies indicate that genomic deletion of TLR2 can result in impaired neurobehavioural function. It is currently not clear if the genomic deletion of TLR2 leads to any alterations in the microstructural features of the brain. In the current study, we noninvasively assess microstructural changes in the brain of TLR2-deficient (tlr2-/-) zebrafish using state-of-the art magnetic resonance imaging (MRI) methods at ultrahigh magnetic field strength (17.6 T). A significant increase in cortical thickness and an overall trend towards increased brain volumes were observed in young tlr2-/- zebrafish. An elevated T2 relaxation time and significantly reduced apparent diffusion coefficient (ADC) unveil brain-wide microstructural alterations, potentially indicative of cytotoxic oedema and astrogliosis in the tlr2-/- zebrafish. Multicomponent analysis of the ADC diffusivity signal by the phasor approach shows an increase in the slow ADC component associated with restricted diffusion. Diffusion tensor imaging and diffusion kurtosis imaging analysis revealed diminished diffusivity and enhanced kurtosis in various white matter tracks in tlr2-/- compared with control zebrafish, identifying the microstructural underpinnings associated with compromised white matter integrity and axonal degeneration. Taken together, our findings demonstrate that the genomic deletion of TLR2 results in severe alterations to the microstructural features of the zebrafish brain. This study also highlights the potential of ultrahigh field diffusion MRI techniques in discerning exceptionally fine microstructural details within the small zebrafish brain, offering potential for investigating microstructural changes in zebrafish models of various brain diseases.

Targeting Microglia in Alzheimer's Disease: Pathogenesis and Potential Therapeutic Strategies

Microglia, as resident macrophages in the central nervous system, play a multifunctional role in the pathogenesis of Alzheimer's disease (AD). Their clustering around amyloid-β (Aβ) deposits is a core pathological feature of AD. Recent advances in single-cell RNA sequencing (scRNA-seq) and single-nucleus RNA sequencing (snRNA-seq) have revealed dynamic changes in microglial phenotypes over time and across different brain regions during aging and AD progression. As AD advances, microglia primarily exhibit impaired phagocytosis of Aβ and tau, along with the release of pro-inflammatory cytokines that damage synapses and neurons. Targeting microglia has emerged as a potential therapeutic approach for AD. Treatment strategies involving microglia can be broadly categorized into two aspects: (1) enhancing microglial function: This involves augmenting their phagocytic ability against Aβ and cellular debris and (2) mitigating neuroinflammation: Strategies include inhibiting TNF-α signaling to reduce the neuroinflammatory response triggered by microglia. Clinical trials exploring microglia-related approaches for AD treatment have garnered attention. Additionally, natural products show promise in enhancing beneficial effects and suppressing inflammatory responses. Clarifying microglial dynamics, understanding their roles, and exploring novel therapeutic approaches will advance our fight against AD.

Comparison of Neuroinflammation Induced by Hyperphosphorylated Tau Protein Versus Ab42 in Alzheimer's Disease

Both neurofibrillary tangles and senile plaques are associated with inflammation in Alzheimer's disease (AD). Their relative degree of induced neuroinflammation, however, is not well established. Mouse models of AD that expressed either human Aβ42 (n = 7) or human hyperphosphorylated tau protein alone (n = 3), wild type (n = 10), and human AD samples (n = 29 with 18 controls) were studied. The benefit of using mouse models that possess only human tau or amyloid-b is that it allows for the individual evaluation of how each protein affects neuroinflammation, something not possible in human tissue. Three indicators of neuroinflammation were examined: TLRs/RIG1 expression, the density of astrocytes and microglial cells, and well-established mediators of neuroinflammation (IL6, TNFα, IL1β, and CXCL10). There was a statistically significant increase in neuroinflammation with all three variables in the mouse models with human tau only as compared to human Aβ42 only or wild-type mice (each at p < 0.0001). Only the Aβ42 5xFAD mice (n = 4) showed statistically higher neuroinflammation versus wild type (p = 0.0030). The human AD tissues were segregated into Aβ42 only or hyperphosphorylated tau protein with Aβ42. The latter areas showed increased neuroinflammation with each of the three variables compared to the areas with only Aβ42. Of the TLRs and RIG-1, TLR8 was significantly elevated in both the mouse model and human AD and only in areas with the abnormal tau protein. It is concluded that although Aβ42 and hyperphosphorylated tau protein can each induce inflammation, the latter protein is associated with a much stronger neuroinflammatory response vis-a-vis a significantly greater activated microglial response.

Bradykinin promotes immune responses in differentiated embryonic neurospheres carrying APPswe and PS1dE9 mutations

BACKGROUND

Neural progenitor cells (NPCs) can be cultivated from developing brains, reproducing many of the processes that occur during neural development. They can be isolated from a variety of animal models, such as transgenic mice carrying mutations in amyloid precursor protein (APP) and presenilin 1 and 2 (PSEN 1 and 2), characteristic of familial Alzheimer's disease (fAD). Modulating the development of these cells with inflammation-related peptides, such as bradykinin (BK) and its antagonist HOE-140, enables the understanding of the impact of such molecules in a relevant AD model.

RESULTS

We performed a global gene expression analysis on transgenic neurospheres treated with BK and HOE-140. To validate the microarray data, quantitative real-time reverse-transcription polymerase chain reaction (RT-PCR) was performed on 8 important genes related to the immune response in AD such as CCL12, CCL5, CCL3, C3, CX3CR1, TLR2 and TNF alpha and Iba-1. Furthermore, comparative analysis of the transcriptional profiles was performed between treatments, including gene ontology and reactome enrichment, construction and analysis of protein-protein interaction networks and, finally, comparison of our data with human dataset from AD patients. The treatments affected the expression levels of genes mainly related to microglia-mediated neuroinflammatory responses, with BK promoting an increase in the expression of genes that enrich processes, biological pathways, and cellular components related to immune dysfunction, neurodegeneration and cell cycle. B2 receptor inhibition by HOE-140 resulted in the reduction of AD-related anomalies caused in this system.

CONCLUSIONS

BK is an important immunomodulatory agent and enhances the immunological changes identified in transgenic neurospheres carrying the genetic load of AD. Bradykinin treatments modulate the expression rates of genes related to microglia-mediated neuroinflammation. Inhibiting bradykinin activity in Alzheimer's disease may slow disease progression.

Potential Alzheimer's disease drug targets identified through microglial biology research

INTRODUCTION

Microglia, the primary immune cells in the brain, play multifaceted roles in Alzheimer's disease (AD). Microglia can potentially mitigate the pathological progression of AD by clearing amyloid beta (Aβ) deposits in the brain and through neurotrophic support. In contrast, disproportionate activation of microglial pro-inflammatory pathways, as well as excessive elimination of healthy synapses, can exacerbate neurodegeneration in AD. The challenge, therefore, lies in discerning the precise regulation of the contrasting microglial properties to harness their therapeutic potential in AD.

AREAS COVERED

This review examines the evidence relevant to the disease-modifying effects of microglial manipulators in AD preclinical models. The deleterious pro-inflammatory effects of microglia in AD can be ameliorated via direct suppression or indirectly through metabolic manipulation, epigenetic targeting, and modulation of the gut-brain axis. Furthermore, microglial clearance of Aβ deposits in AD can be enhanced via strategically targeting microglial membrane receptors, lysosomal functions, and metabolism.

EXPERT OPINION

Given the intricate and diverse nature of microglial responses throughout the course of AD, therapeutic interventions directed at microglia warrant a tactical approach. This could entail employing therapeutic regimens, which concomitantly suppress pro-inflammatory microglial responses while selectively enhancing Aβ phagocytosis.

Lung endothelium, tau, and amyloids in health and disease

Lung endothelia in the arteries, capillaries, and veins are heterogeneous in structure and function. Lung capillaries in particular represent a unique vascular niche, with a thin yet highly restrictive alveolar-capillary barrier that optimizes gas exchange. Capillary endothelium surveys the blood while simultaneously interpreting cues initiated within the alveolus and communicated via immediately adjacent type I and type II epithelial cells, fibroblasts, and pericytes. This cell-cell communication is necessary to coordinate the immune response to lower respiratory tract infection. Recent discoveries identify an important role for the microtubule-associated protein tau that is expressed in lung capillary endothelia in the host-pathogen interaction. This endothelial tau stabilizes microtubules necessary for barrier integrity, yet infection drives production of cytotoxic tau variants that are released into the airways and circulation, where they contribute to end-organ dysfunction. Similarly, beta-amyloid is produced during infection. Beta-amyloid has antimicrobial activity, but during infection it can acquire cytotoxic activity that is deleterious to the host. The production and function of these cytotoxic tau and amyloid variants are the subject of this review. Lung-derived cytotoxic tau and amyloid variants are a recently discovered mechanism of end-organ dysfunction, including neurocognitive dysfunction, during and in the aftermath of infection.

Metabolic regulation of microglial phagocytosis: Implications for Alzheimer's disease therapeutics

Microglia, the resident immune cells of the brain, are increasingly implicated in the regulation of brain health and disease. Microglia perform multiple functions in the central nervous system, including surveillance, phagocytosis and release of a variety of soluble factors. Importantly, a majority of their functions are closely related to changes in their metabolism. This natural inter-dependency between core microglial properties and metabolism offers a unique opportunity to modulate microglial activities via nutritional or metabolic interventions. In this review, we examine the existing scientific literature to synthesize the hypothesis that microglial phagocytosis of amyloid beta (Aβ) aggregates in Alzheimer's disease (AD) can be selectively enhanced via metabolic interventions. We first review the basics of microglial metabolism and the effects of common metabolites, such as glucose, lipids, ketone bodies, glutamine, pyruvate and lactate, on microglial inflammatory and phagocytic properties. Next, we examine the evidence for dysregulation of microglial metabolism in AD. This is followed by a review of in vivo studies on metabolic manipulation of microglial functions to ascertain their therapeutic potential in AD. Finally, we discuss the effects of metabolic factors on microglial phagocytosis of healthy synapses, a pathological process that also contributes to the progression of AD. We conclude by enlisting the current challenges that need to be addressed before strategies to harness microglial phagocytosis to clear pathological protein deposits in AD and other neurodegenerative disorders can be widely adopted.

Signaling pathways in macrophages: molecular mechanisms and therapeutic targets

Macrophages play diverse roles in development, homeostasis, and immunity. Accordingly, the dysfunction of macrophages is involved in the occurrence and progression of various diseases, such as coronavirus disease 2019 and atherosclerosis. The protective or pathogenic effect that macrophages exert in different conditions largely depends on their functional plasticity, which is regulated via signal transduction such as Janus kinase-signal transducer and activator of transcription, Wnt and Notch pathways, stimulated by environmental cues. Over the past few decades, the molecular mechanisms of signaling pathways in macrophages have been gradually elucidated, providing more alternative therapeutic targets for diseases treatment. Here, we provide an overview of the basic physiology of macrophages and expound the regulatory pathways within them. We also address the crucial role macrophages play in the pathogenesis of diseases, including autoimmune, neurodegenerative, metabolic, infectious diseases, and cancer, with a focus on advances in macrophage-targeted strategies exploring modulation of components and regulators of signaling pathways. Last, we discuss the challenges and possible solutions of macrophage-targeted therapy in clinical applications. We hope that this comprehensive review will provide directions for further research on therapeutic strategies targeting macrophage signaling pathways, which are promising to improve the efficacy of disease treatment.

Mechanistic insights into the role of herpes simplex virus 1 in Alzheimer's disease

Alzheimer's Disease (AD) is an aging-associated neurodegenerative disorder, threatening millions of people worldwide. The onset and progression of AD can be accelerated by environmental risk factors, such as bacterial and viral infections. Human herpesviruses are ubiquitous infectious agents that underpin numerous inflammatory disorders including neurodegenerative diseases. Published studies concerning human herpesviruses in AD imply an active role HSV-1 in the pathogenesis of AD. This review will summarize the current understanding of HSV-1 infection in AD and highlight some barriers to advance this emerging field.

Role of neuroinflammation in neurodegeneration development

Studies in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and Amyotrophic lateral sclerosis, Huntington's disease, and so on, have suggested that inflammation is not only a result of neurodegeneration but also a crucial player in this process. Protein aggregates which are very common pathological phenomenon in neurodegeneration can induce neuroinflammation which further aggravates protein aggregation and neurodegeneration. Actually, inflammation even happens earlier than protein aggregation. Neuroinflammation induced by genetic variations in CNS cells or by peripheral immune cells may induce protein deposition in some susceptible population. Numerous signaling pathways and a range of CNS cells have been suggested to be involved in the pathogenesis of neurodegeneration, although they are still far from being completely understood. Due to the limited success of traditional treatment methods, blocking or enhancing inflammatory signaling pathways involved in neurodegeneration are considered to be promising strategies for the therapy of neurodegenerative diseases, and many of them have got exciting results in animal models or clinical trials. Some of them, although very few, have been approved by FDA for clinical usage. Here we comprehensively review the factors affecting neuroinflammation and the major inflammatory signaling pathways involved in the pathogenicity of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. We also summarize the current strategies, both in animal models and in the clinic, for the treatment of neurodegenerative diseases.

Lambda-cyhalothrin enhances inflammation in nigrostriatal region in rats: Regulatory role of NF-κβ and JAK-STAT signaling

The risk to develop neurobehavioural abnormalities in humans on exposure to lambda-cyhalothrin (LCT) - a type II synthetic pyrethroid has enhanced significantly due to its extensive uses in agriculture, homes, veterinary practices and public health programs. Earlier, we found that the brain dopaminergic system is vulnerable to LCT and affects motor functions in rats. In continuation to this, the present study is focused to unravel the role of neuroinflammation in LCT-induced neurotoxicity in substantia nigra and corpus striatum in rats. Increase in the mRNA expression of proinflammatory cytokines (TNF- α, IL-1β, IL-6) and iNOS whereas decrease in anti-inflammatory cytokine (IL-10) was distinct both in substantia nigra and corpus striatum of rats treated with LCT (0.5, 1.0, 3.0 mg/kg body weight, p.o, for 45 days) as compared to control rats. Further, LCT-treated rats exhibited increased levels of glial fibrillary acidic protein (GFAP) and ionized calcium-binding adapter molecule 1 (Iba-1), the glial marker proteins both in substantia nigra and corpus striatum as compared to controls. Exposure of rats to LCT also caused alterations in the levels of heat shock protein 60 (HSP60) and mRNA expression of toll-like receptors (TLR2 and TLR4) in the substantia nigra and corpus striatum. An increase in the phosphorylation of key proteins involved in NF-kβ (P65, Iκβ, IKKα, IKKβ) and JAK/STAT (STAT1, STAT3) signaling and alteration in the protein levels of JAK1 and JAK2 was prominent in LCT-treated rats. Histological studies revealed damage of dopaminergic neurons and reactive gliosis as evidenced by the presence of darkly stained pyknotic neurons and decrease in Nissl substance and an increase in infiltration of immune cells both in substantia nigra and corpus striatum of LCT-treated rats. Presence of reactive microglia and astrocytes in LCT-treated rats was also distinct in ultrastructural studies. The results exhibit that LCT may damage dopaminergic neurons in the substantia nigra and corpus striatum by inducing inflammation as a result of stimulation of neuroglial cells involving activation of NF-κβ and JAK/STAT signaling.

Lactobacillaceae improve cognitive dysfunction via regulating gut microbiota and suppressing Aβ deposits and neuroinflammation in APP/PS1 mice

Alzheimer's disease (AD), the most prevalent neurodegenerative disease, has a significant relationship with alteration of the gut microbiota (GM), and the GM-gut-brain axis has been explored to find novel therapeutic approaches for AD. The present study aimed to evaluate the effect of human Lactobacillaceae (HLL) on cognitive function in APP/PS1 mice. The results showed that HLL treatment significantly improved the cognitive function of mice via MWM and NOR tests. Furthermore, the expression of Aβ plaques, tau phosphorylation and neuroinflammation were markedly reduced in the hippocampus. Meanwhile, HLL treatment significantly increased the activity of GSH-PX and decreased the expression levels of IL-6 and MDA in the brain, and simultaneously increased the abundance of beneficial bacteria and restrained pathogenic bacteria in the intestine. Interestingly, significant correlations were observed between significant changes in abundance of GMs and AD-related markers. Collectively, these findings reveal that HLL is a promising therapeutic agent and potential probiotics, which might improve the cognitive function and AD pathologies.

A Polyaminobiaryl-Based β-secretase Modulator Alleviates Cognitive Impairments, Amyloid Load, Astrogliosis, and Neuroinflammation in APPSwe/PSEN1ΔE9 Mice Model of Amyloid Pathology

The progress in Alzheimer’s disease (AD) treatment suggests a combined therapeutic approach targeting the two lesional processes of AD, which include amyloid plaques made of toxic Aβ species and neurofibrillary tangles formed of aggregates of abnormally modified Tau proteins. A pharmacophoric design, novel drug synthesis, and structure-activity relationship enabled the selection of a polyamino biaryl PEL24-199 compound. The pharmacologic activity consists of a non-competitive β-secretase (BACE1) modulatory activity in cells. Curative treatment of the Thy-Tau22 model of Tau pathology restores short-term spatial memory, decreases neurofibrillary degeneration, and alleviates astrogliosis and neuroinflammatory reactions. Modulatory effects of PEL24-199 towards APP catalytic byproducts are described in vitro, but whether PEL24-199 can alleviate the Aβ plaque load and associated inflammatory counterparts in vivo remains to be elucidated. We investigated short- and long-term spatial memory, Aβ plaque load, and inflammatory processes in APPSwe/PSEN1ΔE9 PEL24-199 treated transgenic model of amyloid pathology to achieve this objective. PEL24-199 curative treatment induced the recovery of spatial memory and decreased the amyloid plaque load in association with decreased astrogliosis and neuroinflammation. The present results underline the synthesis and selection of a promising polyaminobiaryl-based drug that modulates both Tau and, in this case, APP pathology in vivo via a neuroinflammatory-dependent process.

Early activation of Toll-like receptor-3 reduces the pathological progression of Alzheimer's disease in APP/PS1 mouse

BACKGROUND

Toll-like receptor 3 (TLR3) plays an important role in the immune/inflammatory response in the nervous system and is a main pathological feature of Alzheimer's disease (AD). This study investigates the role of early activation of TLR3 in the pathophysiological process of AD.

METHODS

In the experiment, the agonist of TLR3, Poly(I:C), was intraperitoneally injected into the APP/PS1 mouse model of AD and wild-type control mice starting from the age of 4 to 9 months. At the age of 14 months, behavioral tests were conducted. Western blot and immunohistochemistry staining were used to evaluate the level of amyloid β-protein (Aβ), the activation of inflammatory cells, and neuron loss. In addition, the levels of inflammatory cytokines were measured using a quantitative polymerase chain reaction.

RESULTS

The results demonstrated that the early activation of TLR3 attenuated neuronal loss and neurobehavioral dysfunction. Moreover, the early activation of TLR3 reduced Aβ deposition, inhibited the activation of microglia and astrocytes, and decreased the transcription of pro-inflammatory factors in the hippocampus.

CONCLUSIONS

The results indicated that the activation of TLR3 by Poly (I:C) in the early stage of development of AD in a mouse model attenuated neuron loss and improved neurobehavioral functions. The underlying mechanisms could be attributed to its role in Aβ clearance, the inhibition of glial cells, and the regulation of neuroinflammation in the hippocampus.

Interlink between the gut microbiota and inflammation in the context of oxidative stress in Alzheimer's disease progression

The microbiota-gut-brain axis is an important pathway of communication and may dynamically contribute to Alzheimer's disease (AD) pathogenesis. Pathological commensal gut microbiota alterations, termed as dysbiosis, can influence intestinal permeability and break the blood-brain barrier which may trigger AD pathogenesis via redox signaling, neuronal, immune, and metabolic pathways. Dysbiosis increases the oxidative stress. Oxidants affect the innate immune system through recognizing microbial-derived pathogens by Toll-like receptors and initiating the inflammatory process. Most of the gut microbiome research work highlights the relationship between the gut microbiota and AD, but the contributory connection between precise bacteria and brain dysfunction in AD pathology cannot be fully demonstrated. Here, we summarize the current information of the fundamental connections between oxidative stress, inflammation, and gut dysbiosis in AD. This review emphasizes on the involvement of gut microbiota in the regulation of oxidative stress, inflammation, immune responses including central and peripheral cross-talk. It provides insights for novel preventative and therapeutic approaches in AD.

The effects of microglia-associated neuroinflammation on Alzheimer's disease

Alzheimer's disease (AD) is defined as a severe chronic degenerative neurological disease in human. The pathogenic mechanism of AD has been convincingly elucidated by the "amyloid cascade hypothesis" with the main focus of the pathological accretion of β-amyloid (Aβ) peptides outside the cell. However, increasing evidence suggests that this hypothesis is weak in explaining the pathogenesis of AD. Neuroinflammation is crucial in the development of AD, which is proven by the elevated levels of inflammatory markers and the identification of AD risk genes relevant to the innate immune function. Here, we summarize the effects of microglia-mediated neuroinflammation on AD, focusing on the temporal and spatial changes in microglial phenotype, the interactions among microglia, Aβ, tau, and neurons, and the prospects and recent advances in neuroinflammation as a diagnostic and therapeutic target of AD.

Inflammation context in Alzheimer's disease, a relationship intricate to define

Alzheimer's disease (AD), the most common form of dementia, is characterized by the accumulation of amyloid β (Aβ) and hyperphosphorylated tau protein aggregates. Importantly, Aβ and tau species are able to activate astrocytes and microglia, which release several proinflammatory cytokines, such as tumor necrosis factor α (TNF-α) and interleukin 1β (IL-1β), together with reactive oxygen (ROS) and nitrogen species (RNS), triggering neuroinflammation. However, this inflammatory response has a dual function: it can play a protective role by increasing Aβ degradation and clearance, but it can also contribute to Aβ and tau overproduction and induce neurodegeneration and synaptic loss. Due to the significant role of inflammation in the pathogenesis of AD, several inflammatory mediators have been proposed as AD markers, such as TNF-α, IL-1β, Iba-1, GFAP, NF-κB, TLR2, and MHCII. Importantly, the use of anti-inflammatory drugs such as NSAIDs has emerged as a potential treatment against AD. Moreover, diseases related to systemic or local inflammation, including infections, cerebrovascular accidents, and obesity, have been proposed as risk factors for the development of AD. In the following review, we focus on key inflammatory processes associated with AD pathogenesis.

Interferon γ: a master cytokine in microglia-mediated neural network dysfunction and neurodegeneration

Traditionally, lymphocytic interferon γ (IFN-γ) was considered to be a simple 'booster' of proinflammatory responses by microglia (brain-resident macrophages) during bacterial or viral infection. Recent slice culture (in situ) and in vivo studies suggest, however, that IFN-γ has a unique role in microglial activation. Priming by IFN-γ results in proliferation (microgliosis), enhanced synapse elimination, and moderate nitric oxide release sufficient to impair synaptic transmission, gamma rhythm activity, and cognitive functions. Moreover, IFN-γ is pivotal for driving Toll-like receptor (TLR)-activated microglia into neurotoxic phenotypes that induce energetic and oxidative stress, severe network dysfunction, and neuronal death. Pharmacological targeting of activated microglia could be beneficial during elevated IFN-γ levels, blood-brain barrier leakage, and parenchymal T lymphocyte infiltration associated with, for instance, encephalitis, multiple sclerosis, and Alzheimer's disease.

The Hidden Role of Non-Canonical Amyloid β Isoforms in Alzheimer's Disease

Recent advances have placed the pro-inflammatory activity of amyloid β (Aβ) on microglia cells as the focus of research on Alzheimer's Disease (AD). Researchers are confronted with an astonishing spectrum of over 100 different Aβ variants with variable length and chemical modifications. With the exception of Aβ1-42 and Aβ1-40, the biological significance of most peptides for AD is as yet insufficiently understood. We therefore aim to provide a comprehensive overview of the contributions of these neglected Aβ variants to microglia activation. First, the impact of Aβ receptors, signaling cascades, scavenger mechanisms, and genetic variations on the physiological responses towards various Aβ species is described. Furthermore, we discuss the importance of different types of amyloid precursor protein processing for the generation of these Aβ variants in microglia, astrocytes, oligodendrocytes, and neurons, and highlight how alterations in secondary structures and oligomerization affect Aβ neurotoxicity. In sum, the data indicate that gene polymorphisms in Aβ-driven signaling pathways in combination with the production and activity of different Aβ variants might be crucial factors for the initiation and progression of different forms of AD. A deeper assessment of their interplay with glial cells may pave the way towards novel therapeutic strategies for individualized medicine.

Altered serum amyloid beta and cerebral perfusion and their associations with cognitive function in patients with subcortical ischemic vascular disease

Subcortical ischemic vascular disease (SIVD) is one of the important causes of cognitive dysfunction, altered amyloid-beta (Aβ) and cerebral perfusion may be involved in the pathophysiological mechanism of SIVD and are closely related to cognitive function. We aimed to investigate altered serum Aβ and cerebral perfusion in patients with SIVD and their correlation with cognitive function. Seventy-four healthy controls (HCs) and 74 SIVD patients, including 38 SIVD patients with no cognitive impairment (SIVD-NCI) and 36 SIVD patients with mild cognitive impairment (SIVD-MCI) underwent the measurement of serum Aβ40 and Aβ42 levels, pseudo-continuous arterial spin labeling MRI scanning, and cognitive evaluation. Compared to the healthy controls (HCs), the level of serum Aβ40 and Aβ40/42 ratio increased and Aβ42 decreased in SIVD patients. The serum Aβ40 level and Aβ40/42 ratio in patients with SIVD-MCI were significantly higher than those in the HCs and SIVD-NCI, and the level of Aβ42 in the SIVD-MCI was lower than the HCs. In addition, the serum Aβ40/42 ratio provided high diagnostic accuracy for SIVD and SIVD-MCI, it was further identified as an independent risk factor for cognitive impairment. Patients with SIVD-NCI and SIVD-MCI exhibited both increased and decreased cerebral blood flow (CBF) in regional. The Aβ40/42 ratio was associated with global CBF, while altered global and regional CBF was associated with cognitive deficits. In addition, white matter hyperintensities volume (WMHV) correlated with Aβ40/42 ratio, CBF, and cognition. The relationship between Aβ40/42 ratio and cognition was partially mediated by altered CBF. Based on these results, we conclude that the serum Aβ40/42 ratio may be a potential biomarker that can complement current methods for the prediction and diagnosis of cognitive impairment in SIVD patients. In addition, serum Aβ may play a role in cognitive function by regulating CBF, which provides new insights into the intervention, treatment, and prevention of cognitive impairment in SIVD.

The Innate and Adaptive Immune Cells in Alzheimer’s and Parkinson’s Diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common neurodegenerative disorders of the central nervous system (CNS). Increasing evidence supports the view that dysfunction of innate immune cells initiated by accumulated and misfolded proteins plays essential roles in the pathogenesis and progression of these diseases. The TLR family was found to be involved in the regulation of microglial function in the pathogenesis and progression of AD or PD, making it as double-edged sword in these diseases. Altered function of peripheral innate immune cells was found in AD and PD and thus contributed to the development and progression of AD and PD. Alteration of different subsets of T cells was found in the peripheral blood and CNS in AD and PD. The CNS-infiltrating T cells can exert both neuroprotective and neurotoxic effects in the pathogenesis and progression. Here, we review recent evidences for the roles of innate and adaptive immune cells in the pathogenesis and progression of AD and PD.

Probiotics Treatment Can Improve Cognition in Patients with Mild Cognitive Impairment: A Systematic Review

Background: In recent years, the existence of the gut-brain axis and the impact of intestinal microbiota on brain function has received much attention. Accumulated evidence has prompted the postulation of the infectious hypothesis underlying or facilitating neurodegenerative diseases, such as Alzheimer’s disease. Under this hypothesis, intervention with probiotics could be useful at a preventive and therapeutic level. Objective: The objective of this systematic review is to reveal a benefit of improved cognitive function following the use of probiotics in individuals with mild cognitive impairment. Methods: We searched bibliographic databases and analyzed in detail the evidence and methodological quality of five recent randomized, double-blind, placebo-controlled clinical trials using the Cochrane Tool and the SIGN checklist. Results: Overall, and with satisfactory methodological quality, the studies evaluated support the use of probiotics as a weapon to slow the progression of cognitive decline in subjects with mild cognitive impairment. The literature review also indicates that maximum benefit of probiotics is found in subjects with incipient cognitive dysfunction and has no effect in those with advanced disease or absence of disease. Conclusion: These results support the intervention with probiotics, especially as a preventive approach. However, caution is required in the interpretation of the results as microbiota has not been evaluated in all studies, and further large-scale research with a prolonged study period is necessary to ensure the translatability of the results into real practice.

Toll-like receptor-mediated neuroinflammation: relevance for cognitive dysfunctions

Toll-like receptors (TLRs) are pattern recognition receptors (PRRs) with a well-documented role in the innate and adaptive immune responses. Interestingly, TLR activation has also been linked to several brain functions including neurogenesis and synaptogenesis. Increasing evidence supports TLR involvement in peripheral and central inflammation underlying normal aging and the pathogenesis of clinical conditions characterized by cognitive decline. These include not only major neurodegenerative diseases but also traumatic brain injuries, surgeries, and alcohol consumption- and chemotherapy-induced cognitive impairment. We first summarize the physiological roles of TLRs in the nervous system, and then illustrate the emerging involvement of TLRs in cognitive functions, pointing to these receptors as novel enticing pharmacological targets to develop more efficient drugs for the treatment of cognitive impairment.

When the infectious environment meets the AD brain

Background

The Amyloid theory of Alzheimer’s disease (AD) suggests that the deposition of Amyloid β (Aβ) in the brain triggers a chain of events, involving the deposition of phosphorylated Tau and other misfolded proteins, leading to neurodegeneration via neuroinflammation, oxidative stress, and neurovascular factors. The infectious theory linked various infectious agents with the development of AD, raising the possibility that they serve as etiological causes of the disease. Are these theories mutually exclusive, or do they coincide?

Main body

In this review, we will discuss how the two theories converge. We present a model by which (1) the systemic infectious burden accelerates the development of AD brain pathology via bacterial Amyloids and other pathogen-associated molecular patterns (PAMPs), and (2) the developing AD brain pathology increases its susceptibility to the neurotoxicity of infectious agents -derived PAMPs, which drive neurodegeneration via activated microglia.

Conclusions

The reciprocal effects of amyloid deposition and systemic infectious burden may lead to a vicious cycle fueling Alzheimer’s disease pathogenesis.

Identification of TLR2 as a Key Target in Neuroinflammation in Vascular Dementia

Vascular dementia (VaD) is the second most common cause of dementia. At present, precise molecular processes of VaD are unclear. We attempted to discover the VaD relevant candidate genes, enrichment biological processes and pathways, key targets, and the underlying mechanism by microarray bioinformatic analysis. We selected GSE122063 related to the autopsy samples of VaD for analysis. We first took use of Weighted Gene Co-expression Network Analysis (WGCNA) to achieve modules related to VaD and hub genes. Second, we filtered out significant differentially expressed genes (DEGs). Third, significant DEGs then went through Geno Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Fourth, Gene Set Enrichment Analysis (GSEA) was performed. At last, we constructed the protein–protein interaction (PPI) network. The results showed that the yellow module had the strongest correlation with VaD, and we finally identified 21 hub genes. Toll-like receptor 2 (TLR2) was the top hub gene and was strongly correlated with other possible candidate genes. In total, 456 significant DEGs were filtered out and these genes were found to be enriched in the Toll receptor signaling pathway and several other immune-related pathways. In addition, Gene Set Enrichment Analysis results showed that similar pathways were significantly over-represented in TLR2-high samples. In the PPI network, TLR2 was still an important node with high weight and combined scores. We concluded that the TLR2 acts as a key target in neuroinflammation which may participate in the pathophysiological process of VaD.

Innate Immune Cell Death in Neuroinflammation and Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder molecularly characterized by the formation of amyloid β (Aβ) plaques and type 2 microtubule-associated protein (Tau) abnormalities. Multiple studies have shown that many of the brain's immunological cells, specifically microglia and astrocytes, are involved in AD pathogenesis. Cells of the innate immune system play an essential role in eliminating pathogens but also regulate brain homeostasis and AD. When activated, innate immune cells can cause programmed cell death through multiple pathways, including pyroptosis, apoptosis, necroptosis, and PANoptosis. The cell death often results in the release of proinflammatory cytokines that propagate the innate immune response and can eliminate Aβ plaques and aggregated Tau proteins. However, chronic neuroinflammation, which can result from cell death, has been linked to neurodegenerative diseases and can worsen AD. Therefore, the innate immune response must be tightly balanced to appropriately clear these AD-related structural abnormalities without inducing chronic neuroinflammation. In this review, we discuss neuroinflammation, innate immune responses, inflammatory cell death pathways, and cytokine secretion as they relate to AD. Therapeutic strategies targeting these innate immune cell death mechanisms will be critical to consider for future preventive or palliative treatments for AD.

Cellular Reprogramming and Its Potential Application in Alzheimer's Disease

Alzheimer's disease (AD) has become the most common age-related dementia in the world and is currently incurable. Although many efforts have been made, the underlying mechanisms of AD remain unclear. Extracellular amyloid-beta deposition, intracellular tau hyperphosphorylation, neuronal death, glial cell activation, white matter damage, blood-brain barrier disruption, and other mechanisms all take part in this complicated disease, making it difficult to find an effective therapy. In the study of therapeutic methods, how to restore functional neurons and integrate myelin becomes the main point. In recent years, with the improvement and maturity of induced pluripotent stem cell technology and direct cell reprogramming technology, it has become possible to induce non-neuronal cells, such as fibroblasts or glial cells, directly into neuronal cells in vitro and in vivo. Remarkably, the induced neurons are functional and capable of entering the local neural net. These encouraging results provide a potential new approach for AD therapy. In this review, we summarized the characteristics of AD, the reprogramming technique, and the current research on the application of cellular reprogramming in AD. The existing problems regarding cellular reprogramming and its therapeutic potential for AD were also reviewed.

Does SARS-CoV-2 affect neurodegenerative disorders? TLR2, a potential receptor for SARS-CoV-2 in the CNS

The coronavirus (COVID-19) pandemic, caused by severe acute respiratory system coronavirus 2 (SARS-CoV-2), has created significant challenges for scientists seeking to understand the pathogenic mechanisms of SARS-CoV-2 infection and to identify the best therapies for infected patients. Although ACE2 is a known receptor for the virus and has been shown to mediate viral entry into the lungs, accumulating reports highlight the presence of neurological symptoms resulting from infection. As ACE2 expression is low in the central nervous system (CNS), these neurological symptoms are unlikely to be caused by ACE2-virus binding. In this review, we will discuss a proposed interaction between SARS-CoV-2 and Toll-like receptor 2 (TLR2) in the CNS. TLR2 is an innate immune receptor that recognizes exogenous microbial components but has also been shown to interact with multiple viral components, including the envelope (E) protein of SARS-CoV-2. In addition, TLR2 plays an important role in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). Based on these observations, we hypothesize that TLR2 may play a critical role in the response to SARS-CoV-2 infiltration in the CNS, thereby resulting in the induction or acceleration of AD and PD pathologies in patients.

Microglia and Neuroinflammation: Crucial Pathological Mechanisms in Traumatic Brain Injury-Induced Neurodegeneration

Traumatic brain injury (TBI) is one of the most common diseases in the central nervous system (CNS) with high mortality and morbidity. Patients with TBI usually suffer many sequelae in the life time post injury, including neurodegenerative disorders such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). However, the pathological mechanisms connecting these two processes have not yet been fully elucidated. It is important to further investigate the pathophysiological mechanisms underlying TBI and TBI-induced neurodegeneration, which will promote the development of precise treatment target for these notorious neurodegenerative consequences after TBI. A growing body of evidence shows that neuroinflammation is a pivotal pathological process underlying chronic neurodegeneration following TBI. Microglia, as the immune cells in the CNS, play crucial roles in neuroinflammation and many other CNS diseases. Of interest, microglial activation and functional alteration has been proposed as key mediators in the evolution of chronic neurodegenerative pathology following TBI. Here, we review the updated studies involving phenotypical and functional alterations of microglia in neurodegeneration after injury, survey key molecules regulating the activities and functional responses of microglia in TBI pathology, and explore their potential implications to chronic neurodegeneration after injury. The work will give us a comprehensive understanding of mechanisms driving TBI-related neurodegeneration and offer novel ideas of developing corresponding prevention and treatment strategies for this disease.

NPT1220-312, a TLR2/TLR9 Small Molecule Antagonist, Inhibits Pro-Inflammatory Signaling, Cytokine Release, and NLRP3 Inflammasome Activation

Toll-like receptors (TLRs) play a critical role in innate immune system responses to damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). A growing body of evidence suggests that excessive TLR-mediated innate immune system activation can lead to neuronal damage and precipitate or perpetuate neurodegenerative diseases. Among TLR subtypes, both TLR2 and TLR9 have been implicated in neurodegenerative disorders with increased expression of these receptors in the central nervous system being associated with pro-inflammatory signaling and increased burdens of pathologic aggregated proteins. In the current study, we characterized the actions of a combined TLR2/TLR9 antagonist, NPT1220-312, on pro-inflammatory signaling and cytokine release in monocyte/macrophage-derived heterologous cells, human microglia, and murine and human whole blood. NPT1220-312 potently blocked TLR2- and TLR9-mediated release of inflammatory cytokines in monocyte/macrophage cells and in human microglia. NPT1220-312 also blocked TLR2-mediated activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome including IL-1β, IL-18, and apoptosis-associated speck-like protein containing a CARD (ASC) release to the culture medium of human differentiated macrophages. The ability of NPT1220-312 to inhibit TLR2 mediated pro-inflammatory release of chemokines and cytokines in situ was demonstrated using murine and human whole blood. Together, these findings suggest that blockade of TLR2 and TLR9 may reduce inappropriate production of pro-inflammatory cytokines and chemokines from peripheral and central immune cells and thus potentially provide therapeutic benefit in neuroinflammatory/neurodegenerative disorders.

Palmitic Acid and Oleic Acid Differently Modulate TLR2-Mediated Inflammatory Responses in Microglia and Macrophages

The relationship between systemic immunity and neuroinflammation is widely recognised. Infiltration of peripheral immune cells to the CNS during certain chronic inflammatory states contributes significantly to neuropathology. Obesity and its co-morbidities are primary risk factors for neuroinflammatory and neurodegenerative conditions, including Alzheimer’s disease (AD). Dietary fats are among the most proinflammatory components of the obesogenic diet and play a prominent role in the low-grade systemic inflammation associated with the obese state. Saturated fatty acid (SFA) is largely implicated in the negative consequences of obesity, while the health benefits of monounsaturated fatty acid (MUFA) are widely acknowledged. The current study sought to explore whether SFA and MUFA differently modulate inflammatory responses in the brain, compared with peripheral immune cells. Moreover, we assessed the neuroinflammatory impact of high-fat-induced obesity and hypothesised that a MUFA-rich diet might mitigate inflammation despite obesogenic conditions. Toll-like receptor (TLR)2 mediates the inflammation associated with both obesity and AD. Using the TLR2 agonist lipoteichoic acid (LTA), we report that pre-exposure to either palmitic acid (PA) or oleic acid (OA) attenuated cytokine secretion from microglia, but heightened sensitivity to nitric oxide (NO) production. The reduction in cytokine secretion was mirrored in LTA-stimulated macrophages following exposure to PA only, while effects on NO were restricted to OA, highlighting important cell-specific differences. An obesogenic diet over 12 weeks did not induce prominent inflammatory changes in either cortex or hippocampus, irrespective of fat composition. However, we reveal a clear disparity in the effects of MUFA under obesogenic and non-obesogenic conditions.

Bacterial Amyloid Curli Associated Gut Epithelial Neuroendocrine Activation Predominantly Observed in Alzheimer's Disease Mice with Central Amyloid-β Pathology

BACKGROUND

Substantial evidence from recent research suggests an influential and underappreciated force in Alzheimer's disease (AD) pathogenesis: the pathological signals originate from outside the brain. Pathogenic bacteria produce amyloid-like proteins "curli" that form biofilms and show functional similarities to human amyloid-β (Aβ). These proteins may contribute to neurological disease progression via signaling cascade from the gut to the brain.

OBJECTIVE

We propose that curli causes neuroendocrine activation from the gut to brain that promotes central Aβ pathology.

METHODS

PGP9.5 and TLR2 levels in response to curli in the lumen of Tg2576 AD mice were analyzed by immunohistochemical and qRT-PCR analysis. Western blot and human 3D in vitro enteroids culture systems were also used. 16S rRNA gene sequencing was used to investigate bacterial dysbiosis.

RESULTS

We found significant increase in bacterial-amyloid curli with elevated TLR2 at the mRNA level in the pre- and symptomatic Tg-AD gut compared to littermate WT controls. This data associates with increased gram-positive bacterial colonization in the ileum of the symptomatic AD mice. We found fundamental evidence for vagus nerve activation in response to bacterial curli. Neuroendocrine marker PGP9.5 was significantly elevated in the gut epithelium of symptomatic AD mice, and this was colocalized with increased TLR2 expression. Enteroids, 3D-human ileal mini-gut monolayer in vitro model system also revealed increase levels of TLR2 upon stimulation with purified bacterial curli fibrils.

CONCLUSION

These findings reveal the importance of pathological changes within the gut-vagus-brain signaling in response to luminal bacterial amyloid that might play a vital role in central Aβ pathogenesis seen in the AD brain.

Innate Immunity and Cell Death in Alzheimer's Disease

The innate immune system plays key roles in controlling Alzheimer's disease (AD), while secreting cytokines to eliminate pathogens and regulating brain homeostasis. Recent research in the field of AD has shown that the innate immune-sensing ability of pattern recognition receptors on brain-resident macrophages, known as microglia, initiates neuroinflammation, Aβ accumulation, neuronal loss, and memory decline in patients with AD. Advancements in understanding the role of innate immunity in AD have laid a strong foundation to elucidate AD pathology and devise therapeutic strategies for AD in the future. In this review, we highlight the present understanding of innate immune responses, inflammasome activation, inflammatory cell death pathways, and cytokine secretion in AD. We also discuss how the AD pathology influences these biological processes.

Targeting innate immunity to protect and cure Alzheimer's disease: opportunities and pitfalls

Innate immunity has been the focus of many new directions to understand the mechanisms involved in the aetiology of brain diseases, especially Alzheimer's disease (AD). AD is a multifactorial disorder, with the innate immune response and neuroinflammation at the forefront of the pathology. Thus, microglial cells along with peripheral circulating monocytes and more generally the innate immune response have been the target of several pre-clinical and clinical studies. More than a decade ago, inhibiting innate immune cells was considered to be the critical angle for preventing and treating brain diseases. After the failing of numerous clinical trials and the discovery that it may actually be the opposite in various pre-clinical models, the field has changed considerably. Here, we present both sides of the story with a particular emphasis on the beneficial properties of innate immune cells and how they can be targeted to have neuroprotective properties.

Novel Therapeutic Approaches for Alzheimer's Disease: An Updated Review

Alzheimer's disease (AD) is a progressive neurodegenerative disease and accounts for most cases of dementia. The prevalence of AD has increased in the current rapidly aging society and contributes to a heavy burden on families and society. Despite the profound impact of AD, current treatments are unable to achieve satisfactory therapeutic effects or stop the progression of the disease. Finding novel treatments for AD has become urgent. In this paper, we reviewed novel therapeutic approaches in five categories: anti-amyloid therapy, anti-tau therapy, anti-neuroinflammatory therapy, neuroprotective agents including N-methyl-D-aspartate (NMDA) receptor modulators, and brain stimulation. The trend of therapeutic development is shifting from a single pathological target to a more complex mechanism, such as the neuroinflammatory and neurodegenerative processes. While drug repositioning may accelerate pharmacological development, non-pharmacological interventions, especially repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS), also have the potential for clinical application. In the future, it is possible for physicians to choose appropriate interventions individually on the basis of precision medicine.

Tlr2 Gene Deletion Delays Retinal Degeneration in Two Genetically Distinct Mouse Models of Retinitis Pigmentosa

Although considered a rare retinal dystrophy, retinitis pigmentosa (RP) is the primary cause of hereditary blindness. Given its diverse genetic etiology (>3000 mutations in >60 genes), there is an urgent need for novel treatments that target common features of the disease. TLR2 is a key activator of innate immune response. To examine its role in RP progression we characterized the expression profile of Tlr2 and its adaptor molecules and the consequences of Tlr2 deletion in two genetically distinct models of RP: Pde6brd10/rd10 (rd10) and RhoP23H/+ (P23H/+) mice. In both models, expression levels of Tlr2 and its adaptor molecules increased in parallel with those of the proinflammatory cytokine Il1b. In rd10 mice, deletion of a single Tlr2 allele had no effect on visual function, as evaluated by electroretinography. However, in both RP models, complete elimination of Tlr2 attenuated the loss of visual function and mitigated the loss of photoreceptor cell numbers. In Tlr2 null rd10 mice, we observed decreases in the total number of microglial cells, assessed by flow cytometry, and in the number of microglia infiltrating the photoreceptor layers. Together, these results point to TLR2 as a mutation-independent therapeutic target for RP.

Does oxidatively damaged DNA drive amyloid-β generation in Alzheimer's disease? A hypothesis

In Alzheimer's disease (AD), amyloid-β (Aβ) generation and upstream β-secretase 1 (BACE1) expression appear to be driven by oxidative stress via c-Jun N-terminal kinase (JNK), p38, and Interferon-Induced, Double-Stranded RNA-Activated Protein Kinase (PKR). In addition, inflammatory molecules, including lipopolysaccharide (LPS), induce genes central to Aβ genesis, such as BACE1, via nuclear factor-κB (NFκB). However, additional triggers of Aβ generation remain poorly understood and might represent novel opportunities for therapeutic intervention. Based on mechanistic studies and elevated ectopic oxidatively damaged DNA (oxoDNA) levels in preclinical AD, mild cognitive impairment, and AD patients, we hypothesize oxoDNA contributes to β-amyloidosis starting from the earliest stages of AD through multiple pathways. OxoDNA induces mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4), thereby sensitizing the brain to oxidative stress-induced JNK activation and BACE1 transcription. It also induces myeloid differentiation primary response 88 (MyD88) and activates protein kinase CK2, thereby increasing NFκB activation and BACE1 induction. OxoDNA increases oxidative stress via nuclear factor erythroid 2-related factor 2 (Nrf2) ectopic localization, likely augmenting JNK-mediated BACE1 induction. OxoDNA likely also promotes β-amyloidosis via absent in melanoma 2 (AIM2) induction. Falsifiable predictions of this hypothesis include that deoxyribonuclease treatment should decrease Aβ and possibly slow cognitive decline in AD patients. While formal testing of this hypothesis remains to be performed, a case report has found deoxyribonuclease I treatment improved a severely demented AD patient's Mini-Mental Status Exam score from 3 to 18 at 2 months. There is preliminary preclinical and clinical evidence suggesting that ectopic oxidatively damaged DNA may act as an inflammatory damage-associated molecular pattern contributing to Aβ generation in AD, and deoxyribonuclease I should be formally evaluated to test whether it can decrease Aβ levels and slow cognitive decline in AD patients.

Modulating neuroinflammation in neurodegeneration-related dementia: can microglial toll-like receptors pull the plug?

Neurodegeneration-associated dementia disorders (NADDs), namely Alzheimer and Parkinson diseases, are developed by a significant portion of the elderly population globally. Extensive research has provided critical insights into the molecular basis of the pathological advancements of these diseases, but an efficient curative therapy seems elusive. A common attribute of NADDs is neuroinflammation due to a chronic inflammatory response within the central nervous system (CNS), which is primarily modulated by microglia. This response within the CNS is positively regulated by cytokines, chemokines, secondary messengers or cyclic nucleotides, and free radicals. Microglia mediated immune activation is regulated by a positive feedback loop in NADDs. The present review focuses on evaluating the crosstalk between inflammatory mediators and microglia, which aggravates both the clinical progression and extent of NADDs by forming a persistent chronic inflammatory milieu within the CNS. We also discuss the role of the human gut microbiota and its effect on NADDs as well as the suitability of targeting toll-like receptors for an immunotherapeutic intervention targeting the deflation of an inflamed milieu within the CNS.

The Role of Microglia in Perioperative Neuroinflammation and Neurocognitive Disorders

The aseptic trauma of peripheral surgery activates a systemic inflammatory response that results in neuro-inflammation; the microglia, the resident immunocompetent cells in the brain, are a key element of the neuroinflammatory response. In most settings microglia perform a surveillance role in the brain detecting and responding to “invaders” to maintain homeostasis. However, microglia have also been implicated in producing harm possibly by changing its phenotype from its beneficial, anti-inflammatory state (termed M2) into an injurious pro-inflammatory state (termed M1); it is likely that there are intermediates states between these polar phenotypes and some consider that a gradient exists with a number of intermediates, rather than a strict dichotomy between M1 and M2. In the pro-inflammatory phenotypes, microglia can disrupt synaptic plasticity such as long- term potentiation that can result in disorders of learning and memory of the type observed in Peri-operative Neurocognitive Disorders. Therefore, investigators have sought strategies to prevent microglia from provoking this adverse event in the perioperative period. In preclinical studies microglia can be depleted by removing trophic factors required for its maintenance; subsequent repopulation with a more beneficial microglial phenotype may result in memory enhancement, improved sensory motor function, as well as suppression of neuroinflammatory and oxidative stress pathways. Another approach consists of preventing microglial activation using the non-specific P38 MAP kinase blockers such as minocycline. Perhaps a more physiologic approach is the use of inhibitors of potassium (K⁺) channels that are required to convert the microglia into an active state. In this context the specific K⁺ channels that are implicated are termed Kv1.3 and KCa3.1 and high selective inhibitors for each have been developed. Data are accumulating demonstrating the utility of these K⁺ channel blockers in preventing Perioperative Neurocognitive Disorders.

Toll-like receptor-2 gene knockout results in neurobehavioral dysfunctions and multiple brain structural and functional abnormalities in mice

OBJECTIVE

Toll-like receptor-2 (TLR2), a member of TLR family, plays an important role in the induction and regulation of immune/inflammation. TLR2 gene knockout (TLR2KO) mice have been widely used for animal models of neurological diseases. Since there is close relationship between immune system and neurobehavioral functions, it is important to clarify the exact role of TLR2 defect itself in neurobehavioral functions. The present study aimed to investigate the effect of TLR2KO on neurobehavioral functions in mice and the mechanisms underlying the observed changes.

METHODS

Male TLR2KO and wild type (WT) mice aged 3, 7, and 12 months were used for neurobehavioral testing and detection of protein expression by Western blot. Brain magnetic resonance imaging (MRI), electrophysiological recording, and Evans blue (EB) assay were applied to evaluate regional cerebral blood flow (rCBF), synaptic function, and blood-brain barrier (BBB) integrity in 12-month-old TLR2KO and age-matched WT mice.

RESULTS

Compared to WT mice, TLR2KO mice showed decreased cognitive function and locomotor activity, as well as increased anxiety, which developed from middle age (before 7-month-old) to old age. In addition, significantly reduced regional cerebral blood flow (rCBF), inhibited long-term potentiation (LTP), and increased blood-brain barrier (BBB) permeability were observed in 12-month-old TLR2KO mice. Furthermore, compared with age-matched WT mice, significant reduction in protein levels of tight junction proteins (ZO-1, Occludin, and Claudin-5) and increased neurofilament protein (SMI32) were observed in 7 and 12-month-old TLR2KO mice, and that myelin basic protein (MBP) decreased in 12-month-old TLR2KO mice.

CONCLUSION

Our data demonstrated that TLR2 defect resulted in significantly observable neurobehavioral dysfunctions in mice starting from middle age, as well as multiple abnormalities in brain structure, function, and molecular metabolism.

Therapeutic Targeting Strategies for Early- to Late-Staged Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia, typically showing progressive neurodegeneration in aging brains. The key signatures of the AD progression are the deposition of amyloid-beta (Aβ) peptides, the formation of tau tangles, and the induction of detrimental neuroinflammation leading to neuronal loss. However, conventional pharmacotherapeutic options are merely relying on the alleviation of symptoms that are limited to mild to moderate AD patients. Moreover, some of these medicines discontinued to use due to either the insignificant effectiveness in improving the cognitive impairment or the adverse side effects worsening essential bodily functions. One of the reasons for the failure is the lack of knowledge on the underlying mechanisms that can accurately explain the major causes of the AD progression correlating to the severity of AD. Therefore, there is an urgent need for the better understanding of AD pathogenesis and the development of the disease-modifying treatments, particularly for severe and late-onset AD, which have not been covered thoroughly. Here, we review the underlying mechanisms of AD progression, which have been employed for the currently established therapeutic strategies. We believe this will further spur the discovery of a novel disease-modifying treatment for mild to severe, as well as early- to late-onset, AD.