Innate Immunity Stimulation via Toll-Like Receptor 9 Ameliorates Vascular Amyloid Pathology in Tg-SwDI Mice with Associated Cognitive Benefits

Microglial modulation as a therapeutic strategy in Alzheimer's disease: Focus on microglial preconditioning approaches

Alzheimer's disease (AD) is a progressive disease that causes an impairment of learning and memory. Despite the highly complex pathogenesis of AD, amyloid beta (Aβ) deposition and neurofibrillary tangles (NFTs) formation are the main hallmarks of AD. Neuroinflammation also has a crucial role in the development of AD. As the central nervous system's innate immune cells, microglial cells are activated in AD and induce inflammation by producing pro-inflammatory mediators. However, microglial activation is not always deleterious. M2-activated microglial cells are considered anti-inflammatory cells, which develop neuroprotection. Various approaches are proposed for managing AD, yet no effective therapy is available for this disorder. Considering the potential protective role of M2 microglia in neurodegenerative disorders and the improvement of these disorders by preconditioning approaches, it can be suggested that preconditioning of microglial cells may be beneficial for managing AD progression. Therefore, this study review microglial preconditioning approaches for preventing and improving AD.

From Immunity to Neurogenesis: Toll-like Receptors as Versatile Regulators in the Nervous System

Toll-like receptors (TLRs) are among the main components of the innate immune system. They can detect conserved structures in microorganisms and molecules associated with stress and cellular damage. TLRs are expressed in resident immune cells and both neurons and glial cells of the nervous system. Increasing evidence is emerging on the participation of TLRs not only in the immune response but also in processes of the nervous system, such as neurogenesis and cognition. Below, we present a review of the literature that evaluates the expression and role of TLRs in processes such as neurodevelopment, behavior, cognition, infection, neuroinflammation, and neurodegeneration.

Innate immune activation in neurodegenerative diseases

Activation of the innate immune system following pattern recognition receptor binding has emerged as one of the major pathogenic mechanisms in neurodegenerative disease. Experimental, epidemiological, pathological, and genetic evidence underscores the meaning of innate immune activation during the prodromal as well as clinical phases of several neurodegenerative disorders including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and frontotemporal dementia. Importantly, innate immune activation and the subsequent release of inflammatory mediators contribute mechanistically to other hallmarks of neurodegenerative diseases such as aberrant proteostatis, pathological protein aggregation, cytoskeleton abnormalities, altered energy homeostasis, RNA and DNA defects, and synaptic and network disbalance and ultimately to the induction of neuronal cell death. In this review, we discuss common mechanisms of innate immune activation in neurodegeneration, with particular emphasis on the pattern recognition receptors (PRRs) and other receptors involved in the detection of damage-associated molecular patterns (DAMPs).

Clearance of interstitial fluid (ISF) and CSF (CLIC) group-part of Vascular Professional Interest Area (PIA), updates in 2022-2023. Cerebrovascular disease and the failure of elimination of Amyloid-β from the brain and retina with age and Alzheimer's disease: Opportunities for therapy

This editorial summarizes advances from the Clearance of Interstitial Fluid and Cerebrospinal Fluid (CLIC) group, within the Vascular Professional Interest Area (PIA) of the Alzheimer's Association International Society to Advance Alzheimer's Research and Treatment (ISTAART). The overarching objectives of the CLIC group are to: (1) understand the age-related physiology changes that underlie impaired clearance of interstitial fluid (ISF) and cerebrospinal fluid (CSF) (CLIC); (2) understand the cellular and molecular mechanisms underlying intramural periarterial drainage (IPAD) in the brain; (3) establish novel diagnostic tests for Alzheimer's disease (AD), cerebral amyloid angiopathy (CAA), retinal amyloid vasculopathy, amyloid-related imaging abnormalities (ARIA) of spontaneous and iatrogenic CAA-related inflammation (CAA-ri), and vasomotion; and (4) establish novel therapies that facilitate IPAD to eliminate amyloid β (Aβ) from the aging brain and retina, to prevent or reduce AD and CAA pathology and ARIA side events associated with AD immunotherapy.

Roles in Innate Immunity

Microglia are best known as the resident phagocytes of the central nervous system (CNS). As a resident brain immune cell population, microglia play key roles during the initiation, propagation, and resolution of inflammation. The discovery of resident adaptive immune cells in the CNS has unveiled a relationship between microglia and adaptive immune cells for CNS immune-surveillance during health and disease. The interaction of microglia with elements of the peripheral immune system and other CNS resident cells mediates a fine balance between neuroprotection and tissue damage. In this chapter, we highlight the innate immune properties of microglia, with a focus on how pattern recognition receptors, inflammatory signaling cascades, phagocytosis, and the interaction between microglia and adaptive immune cells regulate events that initiate an inflammatory or neuroprotective response within the CNS that modulates immune-mediated disease exacerbation or resolution.

Vaccines and Dementia: Part II. Efficacy of BCG and Other Vaccines Against Dementia

 There is growing awareness that infections may contribute to the development of senile dementia including Alzheimer's disease (AD), and that immunopotentiation is therefore a legitimate target in the management of diseases of the elderly including AD. In Part I of this work, we provided a historical and molecular background to how vaccines, adjuvants, and their component molecules can elicit broad-spectrum protective effects against diverse agents, culminating in the development of the tuberculosis vaccine strain Bacille Calmette-Guérin (BCG) as a treatment for some types of cancer as well as a prophylactic against infections of the elderly such as pneumonia. In Part II, we critically review studies that BCG and other vaccines may offer a measure of protection against dementia development. Five studies to date have determined that intravesicular BCG administration, the standard of care for bladder cancer, is followed by a mean ∼45% reduction in subsequent AD development in these patients. Although this could potentially be ascribed to confounding factors, the finding that other routine vaccines such as against shingles (herpes zoster virus) and influenza (influenza A virus), among others, also offer a degree of protection against AD (mean 29% over multiple studies) underlines the plausibility that the protective effects are real. We highlight clinical trials that are planned or underway and discuss whether BCG could be replaced by key components of the mycobacterial cell wall such as muramyl dipeptide. We conclude that BCG and similar agents merit far wider consideration as prophylactic agents against dementia.

Behavioral and Neuronal Characterizations, across Ages, of the TgSwDI Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder that currently affects as many as 50 million people worldwide. It is neurochemically characterized by an aggregation of β-amyloid plaques and tau neurofibrillary tangles that result in neuronal dysfunction, cognitive decline, and a progressive loss of brain function. TgSwDI is a well-studied transgenic mouse model of AD, but no longitudinal studies have been performed to characterize cognitive deficits or β-amyloid plaque accumulation for use as a baseline reference in future research. Thus, we use behavioral tests (T-Maze, Novel Object Recognition (NOR), Novel Object Location (NOL)) to study long-term and working memory, and immunostaining to study β-amyloid plaque deposits, as well as brain size, in hippocampal, cerebellum, and cortical slices in TgSwDI and wild-type (WT) mice at 3, 5, 8, and 12 months old. The behavioral results show that TgSwDI mice exhibit deficits in their long-term spatial memory starting at 8 months old and in long-term recognition memory at all ages, but no deficits in their working memory. Immunohistochemistry showed an exponential increase in β-amyloid plaque in the hippocampus and cortex of TgSwDI mice over time, whereas there was no significant accumulation of plaque in WT mice at any age. Staining showed a smaller hippocampus and cerebellum starting at 8 months old for the TgSwDI compared to WT mice. Our data show how TgSwDI mice differ from WT mice in their baseline levels of cognitive function and β-amyloid plaque load throughout their lives.

Mutated Toll-like receptor 9 increases Alzheimer's disease risk by compromising innate immunity protection

The development of Alzheimer's disease (AD) involves central and peripheral immune deregulation. Gene identification and studies of AD genetic variants of peripheral immune components may aid understanding of peripheral-central immune crosstalk and facilitate new opportunities for therapeutic intervention. In this study, we have identified in a Flanders-Belgian family a novel variant p.E317D in the Toll-like receptor 9 gene (TLR9), co-segregating with EOAD in an autosomal dominant manner. In human, TLR9 is an essential innate and adaptive immune component predominantly expressed in peripheral immune cells. The p.E317D variant caused 50% reduction in TLR9 activation in the NF-κB luciferase assay suggesting that p.E317D is a loss-of-function mutation. Cytokine profiling of human PBMCs upon TLR9 activation revealed a predominantly anti-inflammatory response in contrast to the inflammatory responses from TLR7/8 activation. The cytokines released upon TLR9 activation suppressed inflammation and promoted phagocytosis of Aβ42 oligomers in human iPSC-derived microglia. Transcriptome analysis identified upregulation of AXL, RUBICON and associated signaling pathways, which may underline the effects of TLR9 signaling-induced cytokines in regulating the inflammatory status and phagocytic property of microglia. Our data suggest a protective role of TLR9 signaling in AD pathogenesis, and we propose that TLR9 loss-of-function may disrupt a peripheral-central immune crosstalk that promotes dampening of inflammation and clearance of toxic protein species, leading to the build-up of neuroinflammation and pathogenic protein aggregates in AD development.

Loss of TREM2 exacerbates parenchymal amyloid pathology but diminishes CAA in Tg-SwDI mice

Alzheimer's disease (AD) is a progressive neurodegenerative disease, and it is the most common cause of dementia worldwide. Recent genome-wide association studies (GWAS) identified TREM2 (triggering receptor expressed on myeloid cells 2) as one of the major risk factors for AD. TREM2 is a surface receptor expressed on microglia and largely mediates microglial functions and immune homeostasis in the brain. The functions of TREM2 in AD pathogenesis, including in the formation of the key pathology parenchymal amyloid-β (Aβ) plaques, have been investigated by introducing Trem2 deficiency in AD mouse models. However, the role of TREM2 in cerebrovascular amyloidosis, in particular cerebral amyloid angiopathy (CAA) remains unexplored. CAA features Aβ deposition along the cerebral vessels, signifying an intersection between AD and vascular dysfunction. Using a well-characterized CAA-prone, transgenic mouse model of AD, Tg-SwDI (SwDI), we found that loss of TREM2 led to a marked increase in overall Aβ load in the brain, but a dramatic decrease in CAA in microvessel-rich regions, along with reduced microglial association with CAA. Transcriptomic analysis revealed that in the absence of Trem2 , microglia were activated but trapped in transition to the fully reactive state. Like microglia, perivascular macrophages were activated with upregulation of cell junction related pathways in Trem2 -deficient SwDI mice. In addition, vascular mural cells and astrocytes exhibited distinct responses to Trem2 deficiency, contributing to the pathological changes in the brain of Trem2 -null SwDI mice. Our study provides the first evidence that TREM2 differentially modulates parenchymal and vascular Aβ pathologies, which may have significant implications for both TREM2- and Aβ-targeting therapies for AD.

FDA-approved carbonic anhydrase inhibitors reduce amyloid β pathology and improve cognition, by ameliorating cerebrovascular health and glial fitness

INTRODUCTION

Cerebrovascular pathology is an early and causal hallmark of Alzheimer's disease (AD), in need of effective therapies.

METHODS

Based on the success of our previous in vitro studies, we tested for the first time in a model of AD and cerebral amyloid angiopathy (CAA), the carbonic anhydrase inhibitors (CAIs) methazolamide and acetazolamide, Food and Drug Administration-approved against glaucoma and high-altitude sickness.

RESULTS

Both CAIs reduced cerebral, vascular, and glial amyloid beta (Aβ) accumulation and caspase activation, diminished gliosis, and ameliorated cognition in TgSwDI mice. The CAIs also improved microvascular fitness and induced protective glial pro-clearance pathways, resulting in the reduction of Aβ deposition. Notably, we unveiled that the mitochondrial carbonic anhydrase-VB (CA-VB) is upregulated in TgSwDI brains, CAA and AD+CAA human subjects, and in endothelial cells upon Aβ treatment. Strikingly, CA-VB silencing specifically reduces Aβ-mediated endothelial apoptosis.

DISCUSSION

This work substantiates the potential application of CAIs in clinical trials for AD and CAA.

Mitochondrial DNA and Inflammation in Alzheimer's Disease

Mitochondrial dysfunction and neuroinflammation are implicated in the pathogenesis of most neurodegenerative diseases, such as Alzheimer's disease (AD). In fact, although a growing number of studies show crosstalk between these two processes, there remain numerous gaps in our knowledge of the mechanisms involved, which requires further clarification. On the one hand, mitochondrial dysfunction may lead to the release of mitochondrial damage-associated molecular patterns (mtDAMPs) which are recognized by microglial immune receptors and contribute to neuroinflammation progression. On the other hand, inflammatory molecules released by glial cells can influence and regulate mitochondrial function. A deeper understanding of these mechanisms may help identify biomarkers and molecular targets useful for the treatment of neurodegenerative diseases. This review of works published in recent years is focused on the description of the mitochondrial contribution to neuroinflammation and neurodegeneration, with particular attention to mitochondrial DNA (mtDNA) and AD.

Increased TSPO expression, pyroglutamate-modified amyloid beta (AβN3(pE)) accumulation and transient clustering of microglia in the thalamus of Tg-SwDI mice

Epidemiological studies showed that Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA) frequently co-occur; however, the precise mechanism is not well understood. A unique animal model (Tg-SwDI mice) was developed to investigate the early-onset and robust accumulation of both parenchymal and vascular Aβ in the brain. Tg-SwDI mice have been extensively used to study the mechanisms of cerebrovascular dysfunction, neuroinflammation, neurodegeneration, and cognitive decline observed in AD/CAA patients and to design biomarkers and therapeutic strategies. In the present study, we documented interesting new features in the thalamus of Tg-SwDI mice: 1) a sharp increase in the expression of ionized calcium-binding adapter molecule 1 (Iba-1) in microglia in 6-month-old animals; 2) microglia clustering at six months that disappeared in old animals; 3) N-truncated/modified AβN3(pE) peptide in 9-month-old female and 12-month-old male mice; 4) an age-dependent increase in translocator protein (TSPO) expression. These findings reinforce the versatility of this model for studying multiple pathological issues involved in AD and CAA.

Emerging concepts towards a translational framework in Alzheimer's disease

Over the past decades, significant efforts have been made to understand the precise mechanisms underlying the pathogenesis of Alzheimer's disease (AD), the most common cause of dementia. However, clinical trials targeting AD pathological hallmarks have consistently failed. Refinement of AD conceptualization, modeling, and assessment is key to developing successful therapies. Here, we review critical findings and discuss emerging ideas to integrate molecular mechanisms and clinical approaches in AD. We further propose a refined workflow for animal studies incorporating multimodal biomarkers used in clinical studies - delineating critical paths for drug discovery and translation. Addressing unresolved questions with the proposed conceptual and experimental framework may accelerate the development of effective disease-modifying strategies for AD.

Common Signaling Pathways Involved in Alzheimer's Disease and Stroke: Two Faces of the Same Coin

Alzheimer's disease (AD) and stroke are two interrelated neurodegenerative disorders which are the leading cause of death and affect the neurons in the brain and central nervous system. Although amyloid-β aggregation, tau hyperphosphorylation, and inflammation are the hallmarks of AD, the exact cause and origin of AD are still undefined. Recent enormous fundamental discoveries suggest that the amyloid hypothesis of AD has not been proven and anti-amyloid therapies that remove amyloid deposition have not yet slowed cognitive decline. However, stroke, mainly ischemic stroke (IS), is caused by an interruption in the cerebral blood flow. Significant features of both disorders are the disruption of neuronal circuitry at different levels of cellular signaling, leading to the death of neurons and glial cells in the brain. Therefore, it is necessary to find out the common molecular mechanisms of these two diseases to understand their etiological connections. Here, we summarized the most common signaling cascades including autotoxicity, ApoE4, insulin signaling, inflammation, mTOR-autophagy, notch signaling, and microbiota-gut-brain axis, present in both AD and IS. These targeted signaling pathways reveal a better understanding of AD and IS and could provide a distinguished platform to develop improved therapeutics for these diseases.

Naringin Prevents Cognitive Dysfunction in Aging Rats by Inhibiting Toll-Like Receptor 4 (TLR4)/NF-κB Pathway and Endoplasmic Reticulum Stress

Objective

Naringin is a flavonoid derived from Chinese herbs. According to earlier studies, naringin may have the potential to alleviate aging-induced cognitive dysfunction. Therefore, this study attempted to explore the protective effect and underlying mechanism of naringin on aging rats with cognitive dysfunction.

Methods

After the construction of a model of aging rats with cognitive dysfunction through subcutaneous injection of D-galactose (D-gal; 150 mg/kg), intragastric administration of naringin (100 mg/kg) was performed for treatment. Behavioral tests, including Morris water maze test (MWM), novel object recognition test (NORT), and fear conditioning test, were used to measure the cognitive function; ELISA and biochemical tests were used to determine the levels of interleukin (IL)-1β, IL-6, monocyte chemoattractant protein-1 (MCP-1), brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), malondialdehyde (MDA), and glutathione peroxidase (GSH-Px) in the hippocampus of rats in each group, respectively; H&E staining was used to observe the pathological changes in the hippocampus; Western blot was used to examine the expression of toll-like receptor 4 (TLR4)/NF-κB pathway-related proteins and endoplasmic reticulum (ER) stress-related proteins in the hippocampus.

Results

The model was successfully constructed by subcutaneous injection of D-gal (150 mg/kg). The behavioral test results showed that naringin could ameliorate the cognitive dysfunction and alleviate the histopathological damage of hippocampus. Moreover, naringin significantly improve the inflammatory response (the levels of IL-1β, IL-6, and MCP-1 were decreased), oxidative stress response (MDA level was increased while GSH-Px activity was decreased), and ER stress (the expression of glucose-regulated protein 78 (GRP78), C/-EBP homologous protein (CHOP), and transcription factor 6 (ATF6) expression was downregulated), and increased the levels of neurotrophic factors BDNF and NGF in D-gal rats. Besides, further mechanistic studies revealed the downregulation of naringin on TLR4/NF-κB pathway activity.

Conclusion

Naringin may inhibit inflammatory response, oxidative stress, and ER stress by downregulating TLR4/NF-κB pathway activity, thereby improving cognitive dysfunction and alleviating histopathological damage of hippocampus in aging rats. Briefly, naringin is an effective drug for the treatment of cognitive dysfunction.

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.

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.

Transcriptomic Profile of Blood–Brain Barrier Remodeling in Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA) is a small vessel disease characterized by amyloid β (Aβ) peptide deposition within the walls of medium to small-caliber blood vessels, cerebral microhemorrhage, and blood–brain barrier (BBB) leakage. It is commonly associated with late-stage Alzheimer’s disease. BBB dysfunction is indicated as a pathological substrate for CAA progression with hyperpermeability, enhancing the extravasation of plasma components and inducing neuroinflammation, further worsening BBB injury and contributing to cognitive decline. Although significant effort has been made in defining the gene mutations and risk factors involved in microvascular alterations with vascular dementia and Alzheimer’s disease, the intra- and intercellular pathogenic mechanisms responsible for vascular hyperpermeability are still largely unknown. The present study aimed to elucidate the transcriptional profile of the cerebral microvessels (BBB) in a murine model with CAA vasculopathy to define potential causes and underlying mechanisms of BBB injury. A comprehensive RNA sequencing analysis was performed of CAA vasculopathy in Tg-SwDI mice at 6 and 18 months in comparison to age-matched wildtype controls to examine how age and amyloid accumulation impact the transcriptional signature of the BBB. Results indicate that Aβ has a critical role in triggering brain endothelial cell and BBB dysfunction in CAA vasculopathy, causing an intense proinflammatory response, impairing oxidative metabolism, altering the coagulation status of brain endothelial cells, and remodeling barrier properties. The proinflammatory response includes both adaptive and innate immunity, with pronounced induction of genes that regulate macrophage/microglial activation and chemokines/adhesion molecules that support T and B cell transmigration. Age has an important impact on the effects of Aβ, increasing the BBB injury in CAA vasculopathy. However, early inflammation, particularly microglia/macrophage activation and the mediators of B lymphocytes’ activities are underlying processes of BBB hyperpermeability and cerebral microbleeds in the early stage of CAA vasculopathy. These findings reveal a specific profile of the CAA-associated BBB injury that leads to a full progression of CAA.

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.

Complement System in Alzheimer's Disease

Alzheimer’s disease is a type of dementia characterized by problems with short-term memory, cognition, and difficulties with activities of daily living. It is a progressive, neurodegenerative disorder. The complement system is an ancient part of the innate immune system and comprises of more than thirty serum and membrane-bound proteins. This system has three different activating pathways and culminates into the formation of a membrane attack complex that ultimately causes target cell lysis (usually pathogens) The complement system is involved in several important functions in the central nervous system (CNS) that include neurogenesis, synaptic pruning, apoptosis, and neuronal plasticity. Here, we discuss how the complement system is involved in the effective functioning of CNS, while also contributing to chronic neuroinflammation leading to neurodegenerative disorders such as Alzheimer’s disease. We also discuss potential targets in the complement system for stopping its harmful effects via neuroinflammation and provide perspective for the direction of future research in this field.

The microglial lysosomal system in Alzheimer's disease: Guardian against proteinopathy

Microglia, the brain-resident immune cells, play an essential role in the upkeep of brain homeostasis. They actively adapt into specific activation states based on cues from the microenvironment. One of these encompasses the activated response microglia (ARMs) phenotype. It arises along a healthy aging process and in a range of neurodegenerative diseases, including Alzheimer's disease (AD). As the phenotype is characterized by an increased lipid metabolism, phagocytosis rate, lysosomal protease content and secretion of neuroprotective agents, it leaves to reason that the phenotype is adapted in an attempt to restore homeostasis. This is important to the conundrum of inflammatory processes. Inflammation per se may not be deleterious; it is only when microglial reactions become chronic or the microglial subtype is made dysfunctional by (multiple) risk proteins with single-nucleotide polymorphisms that microglial involvement becomes deleterious instead of beneficial. Interestingly, the ARMs up- and downregulate many late-onset AD-associated risk factor genes, the products of which are particularly active in the endolysosomal system. Hence, in this review, we focus on how the endolysosomal system is placed at the crossroad of inflammation and microglial capacity to keep pace with degradation.

Dissecting the Crosstalk between Endothelial Mitochondrial Damage, Vascular Inflammation, and Neurodegeneration in Cerebral Amyloid Angiopathy and Alzheimer's Disease

Alzheimer’s disease (AD) is the most prevalent cause of dementia and is pathologically characterized by the presence of parenchymal senile plaques composed of amyloid β (Aβ) and intraneuronal neurofibrillary tangles of hyperphosphorylated tau protein. The accumulation of Aβ also occurs within the cerebral vasculature in over 80% of AD patients and in non-demented individuals, a condition called cerebral amyloid angiopathy (CAA). The development of CAA is associated with neurovascular dysfunction, blood–brain barrier (BBB) leakage, and persistent vascular- and neuro-inflammation, eventually leading to neurodegeneration. Although pathologically AD and CAA are well characterized diseases, the chronology of molecular changes that lead to their development is still unclear. Substantial evidence demonstrates defects in mitochondrial function in various cells of the neurovascular unit as well as in the brain parenchyma during the early stages of AD and CAA. Dysfunctional mitochondria release danger-associated molecular patterns (DAMPs) that activate a wide range of inflammatory pathways. In this review, we gather evidence to postulate a crucial role of the mitochondria, specifically of cerebral endothelial cells, as sensors and initiators of Aβ-induced vascular inflammation. The activated vasculature recruits circulating immune cells into the brain parenchyma, leading to the development of neuroinflammation and neurodegeneration in AD and CAA.

The NLRP3 Inflammasome in the Pathogenesis and Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia. Although AD is one of the most socioeconomically devastating diseases confronting humanity, no "curative" disease modifying drug has been identified. Recent decades have witnessed repeated failures of drug trials and have called into question the utility of the amyloid hypothesis approach to AD therapeutics design. Accordingly, new neurochemical processes are being evaluated and explored as sources of alternative druggable targets. Among these newly identified targets, neuroinflammation is emerging as a front-runner, and within the realm of neuroinflammation, the inflammasome, particularly the NLRP3 complex, is garnering focussed attention. This review summarizes current data and approaches to understanding the role of the NLRP3 inflammasome in neuroinflammation and AD, and systematically identifies and evaluates multiple targets within the NLRP3 inflammasome cascade as putative drug targets.

Innate immunity stimulation via CpG oligodeoxynucleotides ameliorates Alzheimer's disease pathology in aged squirrel monkeys

Alzheimer's disease is the most common cause of dementia and the only illness among the top 10 causes of death for which there is no disease-modifying therapy. The failure rate of clinical trials is very high, in part due to the premature translation of successful results in transgenic mouse models to patients. Extensive evidence suggests that dysregulation of innate immunity and microglia/macrophages plays a key role in Alzheimer's disease pathogenesis. Activated resident microglia and peripheral macrophages can display protective or detrimental phenotypes depending on the stimulus and environment. Toll-like receptors (TLRs) are a family of innate immune regulators known to play an important role in governing the phenotypic status of microglia. We have shown in multiple transgenic Alzheimer's disease mouse models that harnessing innate immunity via TLR9 agonist CpG oligodeoxynucleotides (ODNs) modulates age-related defects associated with immune cells and safely reduces amyloid plaques, oligomeric amyloid-β, tau pathology, and cerebral amyloid angiopathy (CAA) while promoting cognitive benefits. In the current study we have used a non-human primate model of sporadic Alzheimer's disease pathology that develops extensive CAA-elderly squirrel monkeys. The major complications in current immunotherapeutic trials for Alzheimer's disease are amyloid-related imaging abnormalities, which are linked to the presence and extent of CAA; hence, the prominence of CAA in elderly squirrel monkeys makes them a valuable model for studying the safety of the CpG ODN-based concept of immunomodulation. We demonstrate that long-term use of Class B CpG ODN 2006 induces a favourable degree of innate immunity stimulation without producing excessive or sustained inflammation, resulting in efficient amelioration of both CAA and tau Alzheimer's disease-related pathologies in association with behavioural improvements and in the absence of microhaemorrhages in aged elderly squirrel monkeys. CpG ODN 2006 has been well established in numerous human trials for a variety of diseases. The present evidence together with our earlier, extensive preclinical research, validates the beneficial therapeutic outcomes and safety of this innovative immunomodulatory approach, increasing the likelihood of CpG ODN therapeutic efficacy in future clinical trials.

EGB761 ameliorates chronic cerebral hypoperfusion-induced cognitive dysfunction and synaptic plasticity impairment

Chronic cerebral hypoperfusion (CCH) may lead to the cognitive dysfunction, but the underlying mechanisms are unclear. EGB761, extracted from Ginkgo biloba and as a phytomedicine widely used in the world, has been showed to have various neuroprotective roles and mechanisms, and therapeutic effects in Alzheimer’s disease and other cognitive dysfunctions. However, improvements in cognitive function after CCH, following treatment with EGB761, have not been ascertained yet. In this study, we used the behavior test, electrophysiology, neurobiochemistry, and immunohistochemistry to investigate the EGB761’s effect on CCH-induced cognitive dysfunction and identify its underlying mechanisms. The results showed that EGB761 ameliorates spatial cognitive dysfunction occurring after CCH. It may also improve impairment of the long-term potentiation, field excitable potential, synaptic transmission, and the transmission synchronization of neural circuit signals between the entorhinal cortex and hippocampal CA1. EGB761 may also reverse the inhibition of neural activity and the degeneration of dendritic spines and synaptic structure after CCH; it also prevents the downregulation of synaptic proteins molecules and pathways related to the formation and stability of dendritic spines structures. EGB761 may inhibit axon demyelination and ameliorate the inhibition of the mTOR signaling pathway after CCH to improve protein synthesis. In conclusion, EGB761 treatment after CCH may improve spatial cognitive function by ameliorating synaptic plasticity impairment, synapse degeneration, and axon demyelination by rectifying the inhibition of the mTOR signaling pathway.

Tolerance-inducing effect and properties of innate immune stimulation on chronic stress-induced behavioral abnormalities in mice

Over-activation of the innate immune system constitutes a risk factor for the development of nervous system disorders but may reduce the severity of these disorders by inducing tolerance effect. Here, we studied the tolerance-inducing effect and properties of innate immune stimulation on chronic social defeat stress (CSDS)-induced behavioral abnormalities in mice. A single injection of the innate immune enhancer lipopolysaccharide (LPS) one day before stress exposure prevented CSDS-induced impairment in social interaction and increased immobility time in the tail suspension test and forced swimming test. This effect was observed at varying doses (100, 500, and 1000 μg/kg) and peaked at 100 μg/kg. A single LPS injection (100 μg/kg) either one or five but not ten days before stress exposure prevented CSDS-induced behavioral abnormalities. A second LPS injection ten days after the first LPS injection, or a 2 × or 4 × LPS injections ten days before stress exposure also induced tolerance against stress-induced behavioral abnormalities. Our results furthermore showed that a single LPS injection one day before stress exposure skewed the neuroinflammatory response in the hippocampus and prefrontal cortex of CSDS-exposed mice toward an anti-inflammatory phenotype. Inhibiting the central innate immune response by pretreatment with minocycline or PLX3397 abrogated the tolerance-inducing effect of LPS preconditioning on CSDS-induced behavioral abnormalities and neuroinflammatory responses in the brain. These results provide evidence for a prophylactic effect of innate immune stimulation on stress-induced behavioral abnormalities via changes in microglial activation, which may help develop novel strategies for the prevention of stress-induced psychological disorders.

Toll-like Receptors as a Novel Therapeutic Target for Natural Products Against Chronic Diseases

Toll-like receptors (TLR) are one among the initial responders of the immune system which participate in the activation inflammatory processes. Several different types of TLR such as TLR2, TLR4, TLR7 and TLR9 have been identified in various cell types, each having distinct ligands like lipids, lipoproteins, nucleic acids and proteins. Though its prime concern is xenobiotic defences, TLR signalling has also recognized as an activator of inflammation and associated development of chronic degenerative disorders (CDDs) including obesity, type 2 diabetes mellitus (T2DM), fatty liver disease, cardiovascular and neurodegenerative disorders as well as various types of cancers. Numerous drugs are in use to prevent these disorders, which specifically inhibit different pathways associated with the development of CDDs. Compared to these drug targets, inhibition of TLR, which specifically responsible for the inflammatory insults has proven to be a better drug target. Several natural products have emerged as inhibitors of CDDs, which specifically targets TLR signalling, among these, many are in the clinical trials. This review is intended to summarize the recent progress on TLR association with CDDs and to list possible use of natural products, their combinations and their synthetic derivative in the prevention of TLR-driven CDD development.

Progression of Alzheimer's disease and effect of scFv-h3D6 immunotherapy in the 3xTg-AD mouse model: An in vivo longitudinal study using Magnetic Resonance Imaging and Spectroscopy

Alzheimer's disease (AD) is an incurable disease that affects most of the 47 million people estimated as living with dementia worldwide. The main histopathological hallmarks of AD are extracellular β-amyloid (Aβ) plaques and intracellular neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein. In recent years, Aβ-immunotherapy has been revealed as a potential tool in AD treatment. One strategy consists of using single-chain variable fragments (scFvs), which avoids the fragment crystallizable (Fc) effects that are supposed to trigger a microglial response, leading to microhemorrhages and vasogenic edemas, as evidenced in clinical trials with bapineuzumab. The scFv-h3D6 generated by our research group derives from this monoclonal antibody, which targets the N-terminal of the Aβ peptide and recognizes monomers, oligomers and fibrils. In this study, 3xTg-AD mice were intraperitoneally and monthly treated with 100 μg of scFv-h3D6 (a dose of ~3.3 mg/kg) or PBS, from 5 to 12 months of age (-mo), the age at which the mice were sacrificed and samples collected for histological and biochemical analyses. During treatments, four monitoring sessions using magnetic resonance imaging and spectroscopy (MRI/MRS) were performed at 5, 7, 9, and 12 months of age. MRI/MRS techniques are widely used in both human and mouse research, allowing to draw an in vivo picture of concrete aspects of the pathology in a non-invasive manner and allowing to monitor its development across time. Compared with the genetic background, 3xTg-AD mice presented a smaller volume in almost all cerebral regions and ages examined, an increase in both the intra and extracellular Aβ_1-42 at 12-mo, and an inflammation process at this age, in both the hippocampus (IL-6 and mIns) and cortex (IL-6). In addition, treatment with scFv-h3D6 partially recovered the values in brain volume, and Aβ, IL-6, and mIns concentrations, among others, encouraging further studies with this antibody fragment.

TLR4 Cross-Talk With NLRP3 Inflammasome and Complement Signaling Pathways in Alzheimer's Disease

Amyloid plaques, mainly composed of abnormally aggregated amyloid β-protein (Aβ) in the brain parenchyma, and neurofibrillary tangles (NFTs), consisting of hyperphosphorylated tau protein aggregates in neurons, are two pathological hallmarks of Alzheimer's disease (AD). Aβ fibrils and tau aggregates in the brain are closely associated with neuroinflammation and synapse loss, characterized by activated microglia and dystrophic neurites. Genome-wide genetic association studies revealed important roles of innate immune cells in the pathogenesis of late-onset AD by recognizing a dozen genetic risk loci that modulate innate immune activities. Furthermore, microglia, brain resident innate immune cells, have been increasingly recognized to play key, opposing roles in AD pathogenesis by either eliminating toxic Aβ aggregates and enhancing neuronal plasticity or producing proinflammatory cytokines, reactive oxygen species, and synaptotoxicity. Aggregated Aβ binds to toll-like receptor 4 (TLR4) and activates microglia, resulting in increased phagocytosis and cytokine production. Complement components are associated with amyloid plaques and NFTs. Aggregated Aβ can activate complement, leading to synapse pruning and loss by microglial phagocytosis. Systemic inflammation can activate microglial TLR4, NLRP3 inflammasome, and complement in the brain, leading to neuroinflammation, Aβ accumulation, synapse loss and neurodegeneration. The host immune response has been shown to function through complex crosstalk between the TLR, complement and inflammasome signaling pathways. Accordingly, targeting the molecular mechanisms underlying the TLR-complement-NLRP3 inflammasome signaling pathways can be a preventive and therapeutic approach for AD.

Class C CpG Oligodeoxynucleotide Immunomodulatory Response in Aged Squirrel Monkey (Saimiri Boliviensis Boliviensis)

One means of stimulating the mammalian innate immune system is via Toll-like receptor 9 (TLR9) being exposed to unmethylated cytosine-phosphate-guanine (CpG) DNA, also known as pathogen-associated molecular patterns (PAMPs) of microbial origin. Synthetic CpG oligodeoxynucleotides (ODNs) with defined CpG motifs possess broad immunostimulatory properties that make CpG ODNs suitable as therapeutic interventions in a variety of human disease conditions, including Alzheimer's disease (AD). Rodent models are often used to preclinically test the effectiveness of CpG ODN therapeutic agents for AD and other disorders. However, the translatability of findings in such models is limited due to the significant difference of the expression of TLR9 between primates and rodents. The squirrel monkey (SQM), a New World non-human primate (NHP), is known to be phylogenetically proximate to humans, and develops extensive age-dependent cerebral amyloid angiopathy (CAA), a key pathological feature of AD. Hence, this model is currently being used to test AD therapeutics. In the present study, we conducted the first examination of Class C CpG ODN's immunomodulatory role in elderly SQMs. We documented the effectiveness of CpG ODN to trigger an immune response in an aged cohort whose immune system is senescent. The specific immune response patterns detected here closely resembled CpG ODN-induced immunostimulatory patterns observed in prior human studies. Overall, our findings provide critical data regarding the immunomodulatory potential of CpG ODN in this NHP model, allowing for future translational studies of innate immunity stimulation via TLR9 agonists for diverse indications, including AD therapeutics.

Microbiota-gut-brain axis and toll-like receptors in Alzheimer's disease

Alzheimer’s disease (AD) is a multifactorial disease which involves both the periphery and central nervous system (CNS). It has been recently recognized that gut microbiota interacts with the gut and brain (microbiota-gut-brain axis), contributing to the pathogenesis of neurodegenerative diseases, such as AD. Dysbiosis of gut microbiota can induce increased intestinal permeability and systemic inflammation, which may lead to the development of AD pathologies and cognitive impairment via the neural, immune, endocrine, and metabolic pathways. Toll-like receptors (TLRs) play an important role in the innate immune system via recognizing microbes-derived pathogens and initiating the inflammatory process. TLRs have also been found in the brain, especially in the microglia, and have been indicated in the development of AD. In this review, we summarized the relationship between microbiota-gut-brain axis and AD, as well as the complex role of TLRs in AD. Intervention of the gut microbiota or modulation of TLRs properly might emerge as promising preventive and therapeutic strategies for AD.

Pharmacological modulation of nucleic acid sensors - therapeutic potential and persisting obstacles

Nucleic acid sensors, primarily TLR and RLR family members, as well as cGAS-STING signalling, play a critical role in the preservation of cellular and organismal homeostasis. Accordingly, deregulated nucleic acid sensing contributes to the origin of a diverse range of disorders, including infectious diseases, as well as cardiovascular, autoimmune and neoplastic conditions. Accumulating evidence indicates that normalizing aberrant nucleic acid sensing can mediate robust therapeutic effects. However, targeting nucleic acid sensors with pharmacological agents, such as STING agonists, presents multiple obstacles, including drug-, target-, disease- and host-related issues. Here, we discuss preclinical and clinical data supporting the potential of this therapeutic paradigm and highlight key limitations and possible strategies to overcome them.

A cross-species approach to disorders affecting brain and behaviour

Structural and functional elements of biological systems are highly conserved across vertebrates. Many neurological and psychiatric conditions affect both humans and animals. A cross-species approach to the study of brain and behaviour can advance our understanding of human disorders via the identification of unrecognized natural models of spontaneous disorders, thus revealing novel factors that increase vulnerability or resilience, and via the assessment of potential therapies. Moreover, diagnostic and therapeutic advances in human neurology and psychiatry can often be adapted for veterinary patients. However, clinical and research collaborations between physicians and veterinarians remain limited, leaving this wealth of comparative information largely untapped. Here, we review pain, cognitive decline syndromes, epilepsy, anxiety and compulsions, autoimmune and infectious encephalitides and mismatch disorders across a range of animal species, looking for novel insights with translational potential. This comparative perspective can help generate novel hypotheses, expand and improve clinical trials and identify natural animal models of disease resistance and vulnerability.

Rodent models for Alzheimer disease

Animal models are indispensable tools for Alzheimer disease (AD) research. Over the course of more than two decades, an increasing number of complementary rodent models has been generated. These models have facilitated testing hypotheses about the aetiology and progression of AD, dissecting the associated pathomechanisms and validating therapeutic interventions, thereby providing guidance for the design of human clinical trials. However, the lack of success in translating rodent data into therapeutic outcomes may challenge the validity of the current models. This Review critically evaluates the genetic and non-genetic strategies used in AD modelling, discussing their strengths and limitations, as well as new opportunities for the development of better models for the disease.

Potential Novel Approaches to Understand the Pathogenesis and Treat Alzheimer's Disease

There is growing genetic and proteomic data highlighting the complexity of Alzheimer's disease (AD) pathogenesis. Greater use of unbiased "omics" approaches is being increasingly recognized as essential for the future development of effective AD research, that need to better reflect the multiple distinct pathway abnormalities that can drive AD pathology. The track record of success in AD clinical trials thus far has been very poor. In part, this high failure rate has been related to the premature translation of highly successful results in animal models that mirror only limited aspects of AD pathology to humans. We highlight our recent efforts to increase use of human tissue to gain a better understanding of the AD pathogenesis subtype variety and to develop several distinct therapeutic approaches tailored to address this diversity. These therapeutic approaches include the blocking of the Aβ/apoE interaction, stimulation of innate immunity, and the simultaneous blocking of Aβ/tau oligomer toxicity. We believe that future successful therapeutic approaches will need to be combined to better reflect the complexity of the abnormal pathways triggered in AD pathogenesis.

Exploiting microglial and peripheral immune cell crosstalk to treat Alzheimer's disease

Neuroinflammation is considered one of the cardinal features of Alzheimer’s disease (AD). Neuritic plaques composed of amyloid β and neurofibrillary tangle-laden neurons are surrounded by reactive astrocytes and microglia. Exposure of microglia, the resident myeloid cell of the CNS, to amyloid β causes these cells to acquire an inflammatory phenotype. While these reactive microglia are important to contain and phagocytose amyloid plaques, their activated phenotype impacts CNS homeostasis. In rodent models, increased neuroinflammation promoted by overexpression of proinflammatory cytokines can cause an increase in hyperphosphorylated tau and a decrease in hippocampal function. The peripheral immune system can also play a detrimental or beneficial role in CNS inflammation. Systemic inflammation can increase the risk of developing AD dementia, and chemokines released directly by microglia or indirectly by endothelial cells can attract monocytes and T lymphocytes to the CNS. These peripheral immune cells can aid in amyloid β clearance or modulate microglia responses, depending on the cell type. As such, several groups have targeted the peripheral immune system to modulate chronic neuroinflammation. In this review, we focus on the interplay of immunomodulating factors and cell types that are being investigated as possible therapeutic targets for the treatment or prevention of AD.

Drugs for Targeted Therapies of Alzheimer's Disease

Alzheimer's disease (AD) is one type of neurodegenerative diseases, which is prevalent in the elderly. Beta-amyloid (Aβ) plaques and phosphorylated tau-induced neurofibrillary tangles are two pathological hallmarks of this disease and the corresponding pathological pathways of these hallmarks are considered as the therapeutic targets. There are many drugs scheduled for pre-clinical and clinical trial that target to inhibit the initiators of pathological Aβ and tau aggregates as well as critical Aβ secretases and kinases in tau hyperphosphorylation. In addition, studies in disease gene variations, and detection of key prognostic effectors in early development are also important for AD control. The discovery of potential drug targets contributed to targeted therapy in a stage-dependent manner, However, there are still some issues that cause concern such as the low bioavailability and low efficacy of candidate drugs from clinical trial reports. Therefore, modification of drug candidates and development of delivery agents are essential and critical. With other medical advancements like cell replacement therapy, there is hope for the cure of Alzheimer's disease in the foreseeable future.

Affibody-Mediated Sequestration of Amyloid β Demonstrates Preventive Efficacy in a Transgenic Alzheimer's Disease Mouse Model

Different strategies for treatment and prevention of Alzheimer’s disease (AD) are currently under investigation, including passive immunization with anti-amyloid β (anti-Aβ) monoclonal antibodies (mAbs). Here, we investigate the therapeutic potential of a novel type of Aβ-targeting agent based on an affibody molecule with fundamentally different properties to mAbs. We generated a therapeutic candidate, denoted Z_SYM73-albumin-binding domain (ABD; 16.8 kDa), by genetic linkage of the dimeric Z_SYM73 affibody for sequestering of monomeric Aβ-peptides and an ABD for extension of its in vivo half-life. Amyloid precursor protein (APP)/PS1 transgenic AD mice were administered with Z_SYM73-ABD, followed by behavioral examination and immunohistochemistry. Results demonstrated rescued cognitive functions and significantly lower amyloid burden in the treated animals compared to controls. No toxicological symptoms or immunology-related side-effects were observed. To our knowledge, this is the first reported in vivo investigation of a systemically delivered scaffold protein against monomeric Aβ, demonstrating a therapeutic potential for prevention of AD.

Tripchlorolide May Improve Spatial Cognition Dysfunction and Synaptic Plasticity after Chronic Cerebral Hypoperfusion

Chronic cerebral hypoperfusion (CCH) is a common pathophysiological mechanism that underlies cognitive decline and degenerative processes in dementia and other neurodegenerative diseases. Low cerebral blood flow (CBF) during CCH leads to disturbances in the homeostasis of hemodynamics and energy metabolism, which in turn results in oxidative stress, astroglia overactivation, and synaptic protein downregulation. These events contribute to synaptic plasticity and cognitive dysfunction after CCH. Tripchlorolide (TRC) is an herbal compound with potent neuroprotective effects. The potential of TRC to improve CCH-induced cognitive impairment has not yet been determined. In the current study, we employed behavioral techniques, electrophysiology, Western blotting, immunofluorescence, and Golgi staining to investigate the effect of TRC on spatial learning and memory impairment and on synaptic plasticity changes in rats after CCH. Our findings showed that TRC could rescue CCH-induced spatial learning and memory dysfunction and improve long-term potentiation (LTP) disorders. We also found that TRC could prevent CCH-induced reductions in N-methyl-D-aspartic acid receptor 2B, synapsin I, and postsynaptic density protein 95 levels. Moreover, TRC upregulated cAMP-response element binding protein, which is an important transcription factor for synaptic proteins. TRC also prevented the reduction in dendritic spine density that is caused by CCH. However, sham rats treated with TRC did not show any improvement in cognition. Because CCH causes disturbances in brain energy homeostasis, TRC therapy may resolve this instability by correcting a variety of cognitive-related signaling pathways. However, for the normal brain, TRC treatment led to neither disturbance nor improvement in neural plasticity. Additionally, this treatment neither impaired nor further improved cognition. In conclusion, we found that TRC can improve spatial learning and memory, enhance synaptic plasticity, upregulate the expression of some synaptic proteins, and increase the density of dendritic spines. Our findings suggest that TRC may be beneficial in the treatment of cognitive impairment induced by CCH.

Future horizons in Alzheimer's disease research

There are growing genetic, transcriptomic and proteomic data pointing to the complexity of Alzheimer's disease (AD) pathogenesis. Unbiased "omics" approaches are essential for the future development of effective AD research, which will need to be combined and personalized, given that multiple distinct pathways can drive AD pathology. It is essential to gain a better understanding of the AD pathogenesis subtype variety and to develop several distinct therapeutic approaches tailored to address this diversity, as well as the common presence of mixed pathologies. These nonmutually exclusive therapeutic approaches include the targeting of multiple toxic oligomeric species concurrently, targeting the apolipoprotein E/amyloid β interaction and the modulation of innate immunity, as well as more "out of the box" ideas such as targeting infectious agents that may play a role in AD.

Embryonic Neocortical Microglia Express Toll-Like Receptor 9 and Respond to Plasmid DNA Injected into the Ventricle: Technical Considerations Regarding Microglial Distribution in Electroporated Brain Walls

Microglia, the resident immune cells in the CNS, play multiple roles during development. In the embryonic cerebral wall, microglia modulate the functions of neural stem/progenitor cells through their distribution in regions undergoing cell proliferation and/or differentiation. Previous studies using CX3CR1-GFP transgenic mice demonstrated that microglia extensively survey these regions. To simultaneously visualize microglia and neural-lineage cells that interact with each other, we applied the in utero electroporation (IUE) technique, which has been widely used for gene-transfer in neurodevelopmental studies, to CX3CR1-GFP mice (males and females). However, we unexpectedly faced a technical problem: although microglia are normally distributed homogeneously throughout the mid-embryonic cortical wall with only limited luminal entry, the intraventricular presence of exogenously derived plasmid DNAs induced microglia to accumulate along the apical surface of the cortex and aggregate in the choroid plexus. This effect was independent of capillary needle puncture of the brain wall or application of electrical pulses. The microglial response occurred at plasmid DNA concentrations lower than those routinely used for IUE, and was mediated by activation of Toll-like receptor 9 (TLR9), an innate immune sensor that recognizes unmethylated cytosine-phosphate guanosine motifs abundant in microbial DNA. Administration of plasmid DNA together with oligonucleotide 2088, the antagonist of TLR9, partially restored the dispersed intramural localization of microglia and significantly decreased luminal accumulation of these cells. Thus, via TLR9, intraventricular plasmid DNA administration causes aberrant distribution of embryonic microglia, suggesting that the behavior of microglia in brain primordia subjected to IUE should be carefully interpreted.

Increased expression of toll-like receptor 3, an anti-viral signaling molecule, and related genes in Alzheimer's disease brains

The focus of this study is the expression of Toll-like receptor-3 (TLR-3), a receptor for double-stranded RNA, in human brains affected by Alzheimer's disease (AD) pathology. Toll-like receptors are a family of pattern recognition molecules primarily involved in host defenses to microbial pathogens, but roles in neurodegenerative disease have also been shown, as amyloid beta (Aβ) can be a ligand for TLR-2 and -4 and α-synuclein for TLR-1 and TLR-2, while TLR-9 activation promotes Aβ removal. However, involvement of TLR-3 in AD has not been rigorously studied. Immunohistochemical analyses in human temporal cortical sections with a validated antibody for TLR-3 predominantly identified microglia, particularly strongly in cells associated with amyloid plaques, also brain vascular endothelial cells and subsets of astrocytes, but not neurons or p62-immunoreactive structures. Microglial TLR-3 colocalized with the endosomal/lysosomal marker CD68, which identifies phagocytic cells. Quantitative analyses of neuropathologically-staged human brain middle temporal gyrus samples using immunohistochemistry and mRNA expression methods demonstrated increased TLR-3 immunoreactivity and increased TLR-3 mRNA in AD compared to non-demented cases. There were significant positive correlations between TLR-3 mRNA levels and plaque or tangle loads in both series of samples. Increased expression of interferon beta (IFN-β) and interferon regulatory factor (IRF)-3 mRNA, two factors induced by TLR-3 signaling, were detected in the AD cases. Increased expression of TLR-4 and TLR-9 mRNA was also observed in these same samples, but not TLR-2. In vitro cultured human brain microglia responses to Aβ inflammatory activation were not altered by TLR-3 activation with activator polyinosinic;polycytidylic acid (poly I:C), while human brain endothelial cells showed reduction in responses when stimulated with both agents. Treatment of microglia with poly I:C did not increase their uptake and breakdown of Aβ.

A Novel Peroxidase Mimics and Ameliorates Alzheimer's Disease-Related Pathology and Cognitive Decline in Mice

Alzheimer's disease (AD) is the most common neurodegenerative disorder in the elderly, which is characterized by the accumulation of amyloid β (Aβ) plaques, oxidative stress, and neuronal loss. Therefore, clearing Aβ aggregates and reducing oxidative stress could be an effective therapeutic strategy for AD. Deuterohemin-AlaHisThrValGluLys (DhHP-6), a novel deuterohemin-containing peptide mimetic of the natural microperoxidase-11 (MP-11), shows higher antioxidant activity and stability compared to the natural microperoxidases. DhHP-6 possesses the ability of extending lifespan and alleviating paralysis in the Aβ1-42 transgenic Caenorhabditis elegans CL4176 model of AD, as shown in our previous study. Therefore, this study was aimed at exploring the neuroprotective effect of DhHP-6 in the APPswe/PSEN1dE9 transgenic mouse model of AD. DhHP-6 reduced the diameter and fiber structure of Aβ1-42 aggregation in vitro, as shown by dynamic light scattering and transmission electron microscope. DhHP-6 exerted its neuroprotective effect by inhibiting Aβ aggregation and plaque formation, and by reducing Aβ1-42 oligomers-induced neurotoxicity on HT22 (mouse hippocampal neuronal) and SH-SY5Y (human neuroblastoma) cells. In the AD mouse model, DhHP-6 significantly ameliorated cognitive decline and improved spatial learning ability in behavioral tests including the Morris water maze, Y-maze, novel object recognition, open field, and nest-building test. Moreover, DhHP-6 reduced the deposition of Aβ plaques in the cerebral cortex and hippocampus. More importantly, DhHP-6 restored the morphology of astrocytes and microglia, and significantly reduced the levels of pro-inflammatory cytokines. Our findings provide a basis for considering the non-toxic, peroxidase mimetic DhHP-6 as a new candidate drug against AD.

Recent advancements toward therapeutic vaccines against Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is a devastating neurodegenerative disease characterized by protein aggregates of amyloid β (Aβ) and tau. These proteins have normal physiological functions, but in AD, they undergo a conformational change and aggregate as toxic oligomeric and fibrillar species with a high β-sheet content.

AREAS COVERED

Active and passive immunotherapeutic approaches are among the most attractive methods for targeting misfolded Aβ and tau. Promising preclinical testing of various immunotherapeutic approaches has yet to translate to cognitive benefits in human clinical trials. Knowledge gained from these past failures has led to the development of second-generation Aβ-active immunotherapies, anti-Aβ monoclonal antibodies targeting a wide array of Aβ conformations, and to a number of immunotherapies targeting pathological tau. This review covers the more recent advances in vaccine development for AD from 2016 to present.

EXPERT COMMENTARY

Due to the complex pathophysiology of AD, greatest clinical efficacy will most likely be achieved by concurrently targeting the most toxic forms of both Aβ and tau.

NGF steers microglia toward a neuroprotective phenotype

Microglia are the sentinels of the brain but a clear understanding of the factors that modulate their activation in physiological and pathological conditions is still lacking. Here we demonstrate that Nerve Growth Factor (NGF) acts on microglia by steering them toward a neuroprotective and anti-inflammatory phenotype. We show that microglial cells express functional NGF receptors in vitro and ex vivo. Our transcriptomic analysis reveals how, in primary microglia, NGF treatment leads to a modulation of motility, phagocytosis and degradation pathways. At the functional level, NGF induces an increase in membrane dynamics and macropinocytosis and, in vivo, it activates an outward rectifying current that appears to modulate glutamatergic neurotransmission in nearby neurons. Since microglia are supposed to be a major player in Aβ peptide clearance in the brain, we tested the effects of NGF on its phagocytosis. NGF was shown to promote TrkA-mediated engulfment of Aβ by microglia, and to enhance its degradation. Additionally, the proinflammatory activation induced by Aβ treatment is counteracted by the concomitant administration of NGF. Moreover, by acting specifically on microglia, NGF protects neurons from the Aβ-induced loss of dendritic spines and inhibition of long term potentiation. Finally, in an ex-vivo setup of acute brain slices, we observed a similar increase in Aβ engulfment by microglial cells under the influence of NGF. Our work substantiates a role for NGF in the regulation of microglial homeostatic activities and points toward this neurotrophin as a neuroprotective agent in Aβ accumulation pathologies, via its anti-inflammatory activity on microglia.