Herpes simplex virus type 1, apolipoprotein E, and cholesterol: A dangerous liaison in Alzheimer’s disease and other disorders

Host response to Aplysia Abyssovirus 1 in nervous system and gill

The California sea hare (Aplysia californica) is a model for age associated cognitive decline. Recent researched identified a novel nidovirus, Aplysia Abyssovirus 1, with broad tropism enriched in the Aplysia nervous system. This virus is ubiquitous in wild and maricultured, young and old animals without obvious pathology. Here we re-evaluated gene expression data from several previous studies to investigate differential expression in the nervous system and gill in response to virus and aging as well as the mutational spectrum observed in the viral sequences obtained from these datasets. Viral load and age were highly correlated, indicating persistent infection. Upregulated genes in response to virus were enriched for immune genes and signatures of ER and proteostatic stress, while downregulated genes were enriched for mitochondrial metabolism. Differential expression with respect to age suggested increased iron accumulation and decreased glycolysis, fatty acid metabolism, and proteasome function. Interaction of gene expression trends associated with viral infection and aging suggest that viral infection likely plays a role in aging in the Aplysia nervous system. Mutation analysis of viral RNA identified signatures suggesting ADAR and AID/APOBEC like deaminase act as part of Aplysia anti-viral defense.

A review of the roles of pathogens in Alzheimer's disease

Alzheimer's disease (AD) is one of the leading causes of dementia and is characterized by memory loss, mental and behavioral abnormalities, and impaired ability to perform daily activities. Even as a global disease that threatens human health, effective treatments to slow the progression of AD have not been found, despite intensive research and significant investment. In recent years, the role of infections in the etiology of AD has sparked intense debate. Pathogens invade the central nervous system through a damaged blood-brain barrier or nerve trunk and disrupt the neuronal structure and function as well as homeostasis of the brain microenvironment through a series of molecular biological events. In this review, we summarize the various pathogens involved in AD pathology, discuss potential interactions between pathogens and AD, and provide an overview of the promising future of anti-pathogenic therapies for AD.

Recent Aspects of Periodontitis and Alzheimer's Disease-A Narrative Review

Periodontitis is an inflammatory condition affecting the supporting structures of the teeth. Periodontal conditions may increase the susceptibility of individuals to various systemic illnesses, including Alzheimer's disease. Alzheimer's disease is a neurodegenerative condition characterized by a gradual onset and progressive deterioration, making it the primary cause of dementia, although the exact cause of the disease remains elusive. Both Alzheimer's disease and periodontitis share risk factors and clinical studies comparing the associations and occurrence of periodontitis among individuals with Alzheimer's disease have suggested a potential correlation between these conditions. Brains of individuals with Alzheimer's disease have substantiated the existence of microorganisms related to periodontitis, especially Porphyromonas gingivalis, which produces neurotoxic gingipains and may present the capability to breach the blood-brain barrier. Treponema denticola may induce tau hyperphosphorylation and lead to neuronal apoptosis. Lipopolysaccharides-components of bacterial cell membranes and mediators of inflammation-also have an impact on brain function. Further research could unveil therapeutic approaches targeting periodontal pathogens to potentially alleviate AD progression.

Viruses - a major cause of amyloid deposition in the brain

INTRODUCTION

Clinically, Alzheimer's disease (AD) is a syndrome with a spectrum of various cognitive disorders. There is a complete dissociation between the pathology and the clinical presentation. Therefore, we need a disruptive new approach to be able to prevent and treat AD.

AREAS COVERED

In this review, the authors extensively discuss the evidence why the amyloid beta is not the pathological cause of AD which makes therefore the amyloid hypothesis not sustainable anymore. They review the experimental evidence underlying the role of microbes, especially that of viruses, as a trigger/cause for the production of amyloid beta leading to the establishment of a chronic neuroinflammation as the mediator manifesting decades later by AD as a clinical spectrum. In this context, the emergence and consequences of the infection/antimicrobial protection hypothesis are described. The epidemiological and clinical data supporting this hypothesis are also analyzed.

EXPERT OPINION

For decades, we have known that viruses are involved in the pathogenesis of AD. This discovery was ignored and discarded for a long time. Now we should accept this fact, which is not a hypothesis anymore, and stimulate the research community to come up with new ideas, new treatments, and new concepts.

Systemic Inflammation Causes Microglial Dysfunction With a Vascular AD phenotype

Background

Studies in rodents and humans have indicated that inflammation outside CNS (systemic inflammation) affects brain homeostasis contributing to neurodevelopmental disorders. Itis becoming increasingly evident that such early insults may also belinked to neurodegenerative diseases like late-onset Alzheimer's disease (AD). Importantly, lifestyle and stress, such as viral or bacterial infection causing chronic inflammation, may contribute to neurodegenerative dementia. Systemic inflammatory response triggers a cascade of neuroinflammatory responses, altering brain transcriptome, cell death characteristic of AD, and vascular dementia. Our study aimed to assess the temporal evolution of the pathological impact of systemic inflammation evoked by prenatal and early postnatal peripheral exposure of viral mimetic Polyinosinic:polycytidylic acid (PolyI:C) and compare the hippocampal transcriptomic changes with the profiles of human post-mortem AD and vascular dementia brain specimens.

Methods

We have engineered the PolyI:C sterile infection model in wildtype C57BL6 mice to achieve chronic low-grade systemic inflammation. We have conducted a cross-sectional analysis of aging PolyI:C and Saline control mice (3 months, 6 months, 9 months, and 16 months), taking the hippocampus as a reference brain region, and compared the brain aging phenotype to AD progression in humans with mild AD, severe AD, and Controls (CTL), in parallel to Vascular dementia (VaD) patients' specimens.

Results

We found that PolyI:C mice display both peripheral and central inflammation with a peak at 6 months, associated with memory deficits. The hippocampus is characterized by a pronounced and progressive tauopathy. In PolyI:C brains, microglia undergo aging-dependent morphological shifts progressively adopting a phagocytic phenotype. Transcriptomic analysis reveals a profound change in gene expression throughout aging, with a peak in differential expression at 9 months. We show that the proinflammatory marker Lcn2 is one of the genes with the strongest upregulation in PolyI:C mice upon aging. Validation in brains from patients with increasing severity of AD and VaD shows the reproducibility of some gene targets in vascular dementia specimens as compared to AD ones.

Conclusions

The PolyI:C model of sterile infection demonstrates that peripheral chronic inflammation causes progressive tau hyperphosphorylation, changes in microglia morphology, astrogliosis and gene reprogramming reflecting increased neuroinflammation, vascular remodeling, and the loss of neuronal functionality seen to some extent in human AD and Vascular dementia suggesting early immune insults could be crucial in neurodegenerative diseases.

High resolution 16S rRNA gene Next Generation Sequencing study of brain areas associated with Alzheimer's and Parkinson's disease

Introduction

Alzheimer's (AD) and Parkinson's disease (PD) are neurodegenerative conditions characterized by incremental deposition of β-amyloid (Aβ) and α-synuclein in AD and PD brain, respectively, in relatively conserved patterns. Both are associated with neuroinflammation, with a proposed microbial component for disease initiation and/or progression. Notably, Aβ and α-synuclein have been shown to possess antimicrobial properties. There is evidence for bacterial presence within the brain, including the oral pathobiont Porphyromonas gingivalis, with cognitive impairment and brain pathology being linked to periodontal (gum) disease and gut dysbiosis.

Methods

Here, we use high resolution 16S rRNA PCR-based Next Generation Sequencing (16SNGS) to characterize bacterial composition in brain areas associated with the early, intermediate and late-stage of the diseases.

Results and discussion

This study reveals the widespread presence of bacteria in areas of the brain associated with AD and PD pathology, with distinctly different bacterial profiles in blood and brain. Brain area profiles were overall somewhat similar, predominantly oral, with some bacteria subgingival and oronasal in origin, and relatively comparable profiles in AD and PD brain. However, brain areas associated with early disease development, such as the locus coeruleus, were substantially different in bacterial DNA content compared to areas affected later in disease etiology.

Viral Clearance and Neuroinflammation in Acute TMEV Infection Vary by Host Genetic Background

A wide range of viruses cause neurological manifestations in their hosts. Infection by neurotropic viruses as well as the resulting immune response can irreversibly disrupt the complex structural and functional architecture of the brain, depending in part on host genetic background. The interaction between host genetic background, neurological response to viral infection, and subsequent clinical manifestations remains poorly understood. In the present study, we used the genetically diverse Collaborative Cross (CC) mouse resource to better understand how differences in genetic background drive clinical signs and neuropathological manifestations of acute Theiler's murine encephalomyelitis virus (TMEV) infection. For the first time, we characterized variations of TMEV viral tropism and load based on host genetic background, and correlated viral load with microglial/macrophage activation. For five CC strains (CC002, CC023, CC027, CC057, and CC078) infected with TMEV, we compared clinical signs, lesion distribution, microglial/macrophage response, expression, and distribution of TMEV mRNA, and identified genetic loci relevant to the early acute (4 days post-infection [dpi]) and late acute (14 dpi) timepoints. We examined brain pathology to determine possible causes of strain-specific differences in clinical signs, and found that fields CA1 and CA2 of the hippocampal formation were especially targeted by TMEV across all strains. Using Iba-1 immunolabeling, we identified and characterized strain- and timepoint-specific variation in microglial/macrophage reactivity in the hippocampal formation. Because viral clearance can influence disease outcome, we used RNA in situ hybridization to quantify viral load and TMEV mRNA distribution at both timepoints. TMEV mRNA expression was broadly distributed in the hippocampal formation at 4 dpi in all strains but varied between radiating and clustered distribution depending on the CC strain. We found a positive correlation between microglial/macrophage reactivity and TMEV mRNA expression at 4 dpi. At 14 dpi, we observed a dramatic reduction in TMEV mRNA expression, and localization to the medial portion of field CA1 and field CA2. To better understand how host genetic background can influence pathological outcomes, we identified quantitative trait loci associated with frequency of lesions in a particular brain region and with microglial/macrophage reactivity. These QTL were located near several loci of interest: lysosomal trafficking regulator (Lyst) and nidogen 1 (Nid1), and transmembrane protein 106 B (Tmem106b). Together, these results provide a novel understanding about the influences of genetic variation on the acute neuropathological and immunopathological environment and viral load, which collectively lead to variable disease outcomes. Our findings reveal possible avenues for future investigation which may lead to more effective intervention strategies and treatment regimens.

Antiherpetic drugs: a potential way to prevent Alzheimer's disease?

Background

Considering the growing body of evidence suggesting a potential implication of herpesviruses in the development of dementia, several authors have questioned a protective effect of antiherpetic drugs (AHDs) which may represent a new means of prevention, well tolerated and easily accessible. Subsequently, several epidemiological studies have shown a reduction in the risk of dementia in subjects treated with AHDs, but the biological plausibility of this association and the impact of potential methodological biases need to be discussed in more depth.

Methods

Using a French medico-administrative database, we assessed the association between the intake of systemic AHDs and the incidence of (i) dementia, (ii) Alzheimer’s disease (AD), and (iii) vascular dementia in 68,291 subjects over 65 who were followed between 2009 and 2017. Regarding potential methodological biases, Cox models were adjusted for numerous potential confounding factors (including proxies of sociodemographic status, comorbidities, and use of healthcare) and sensitivity analyses were performed in an attempt to limit the risk of indication and reverse causality biases.

Results

9.7% of subjects (n=6642) had at least one intake of systemic AHD, and 8883 incident cases of dementia were identified. Intake of at least one systemic AHD during follow-up was significantly associated with a decreased risk of AD (aHR 0.85 95% confidence interval [0.75–0.96], p=0.009) and, to a lesser extent with respect to p values, to both dementia from any cause and vascular dementia. The association with AD remained significant in sensitivity analyses. The number of subjects with a regular intake was low and prevented us from studying its association with dementia.

Conclusions

Taking at least one systemic AHD during follow-up was significantly associated with a 15% reduced risk of developing AD, even after taking into account several potential methodological biases. Nevertheless, the low frequency of subjects with a regular intake questions the biological plausibility of this association and highlights the limits of epidemiological data to evaluate a potential protective effect of a regular treatment by systemic AHDs on the incidence of dementia

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-021-00950-0.

The Role of Chronic Infection in Alzheimer's Disease: Instigators, Co-conspirators, or Bystanders?

PURPOSE OF REVIEW

Herein, we provide a critical review of the clinical and translational research examining the relationship between viral and bacterial pathogens and Alzheimer's disease. In addition, we provide an overview of the biological pathways through which chronic infection may contribute to Alzheimer's disease.

RECENT FINDINGS

Dementia due to Alzheimer's disease is a leading cause of disability among older adults in developed countries, yet knowledge of the causative factors that promote Alzheimer's disease pathogenesis remains incomplete. Over the past several decades, numerous studies have demonstrated an association of chronic viral and bacterial infection with Alzheimer's disease. Implicated infectious agents include numerous herpesviruses (HSV-1, HHV-6, HHV-7) and various gastric, enteric, and oral bacterial species, as well as Chlamydia pneumonia and multiple spirochetes.

SUMMARY

Evidence supports the association between multiple pathogens and Alzheimer's disease risk. Whether these pathogens play a causal role in Alzheimer's pathophysiology remains an open question. We propose that the host immune response to active or latent infection in the periphery or in the brain triggers or accelerates the Alzheimer's disease processes, including the accumulation of amyloid-ß and pathogenic tau, and neuroinflammation. While recent research suggests that such theories are plausible, additional longitudinal studies linking microorganisms to Aß and phospho-tau development, neuroinflammation, and clinically defined Alzheimer's dementia are needed.

Plant-Based Natural Products and Extracts: Potential Source to Develop New Antiviral Drug Candidates

Viral infections are among the most complex medical problems and have been a major threat to the economy and global health. Several epidemics and pandemics have occurred due to viruses, which has led to a significant increase in mortality and morbidity rates. Natural products have always been an inspiration and source for new drug development because of their various uses. Among all-natural sources, plant sources are the most dominant for the discovery of new therapeutic agents due to their chemical and structural diversity. Despite the traditional use and potential source for drug development, natural products have gained little attention from large pharmaceutical industries. Several plant extracts and isolated compounds have been extensively studied and explored for antiviral properties against different strains of viruses. In this review, we have compiled antiviral plant extracts and natural products isolated from plants reported since 2015.

Exploring ER stress response in cellular aging and neuroinflammation in Alzheimer's disease

One evident hallmark of Alzheimer's disease (AD) is the irregular accumulation of proteins due to changes in proteostasis involving endoplasmic reticulum (ER) stress. To alleviate ER stress and reinstate proteostasis, cells undergo an integrated signaling cascade called the unfolded protein response (UPR) that reduces the number of misfolded proteins and inhibits abnormal protein accumulation. Aging is associated with changes in the expression of ER chaperones and folding enzymes, leading to the impairment of proteostasis, and accumulation of misfolded proteins. The disrupted initiation of UPR prevents the elimination of unfolded proteins, leading to ER stress. In AD, the accumulation of misfolded proteins caused by sustained cellular stress leads to neurodegeneration and neuronal death. Current research has revealed that ER stress can trigger an inflammatory response through diverse transducers of UPR. Although the involvement of a neuroinflammatory component in AD has been documented for decades, whether it is a contributing factor or part of the neurodegenerative events is so far unknown. Besides, a feedback loop occurs between neuroinflammation and ER stress, which is strongly associated with neurodegenerative processes in AD. In this review, we focus on the current research on ER stress and UPR in cellular aging and neuroinflammatory processes, leading to memory impairment and synapse dysfunction in AD.

Porphyromonas gingivalis is a Strong Risk Factor for Alzheimer's Disease

Porphyromonas gingivalis (P. gingivalis) is one of the several important bacterial pathogens associated with the sporadic Alzheimer’s disease (AD). Different serotypes are either capsulated or are non-capsulated. It has been demonstrated that P. gingivalis (non-capsulated) can reproduce the neurodegenerative AD-like changes in vitro, and a capsular P. gingivalis (strain W83) could reproduce the cardinal hallmark lesions of AD in a wild-type mouse model. All P. gingivalis forms express proteolytically active proteases that enable cleavage of the amyloid-β protin precursor (AβPP) and tau resulting in the formation of amyloid-β and neurofibrillary tangles. Tau is an established substrate for gingipains, which can cleave tau into various peptides. Some of the P. gingivalis fragmented tau protein peptides contain “VQIINK” and “VQIVYK” hexapeptide motifs which map to the flanking regions of the microtubule binding domains and are also found in paired helical filaments that form NFTs. P. gingivalis can induce peripheral inflammation in periodontitis and can also initiate signaling pathways that activate kinases, which in turn, phosphorylate neuronal tau. Periodontal disease related inflammation has metabolic implications for an individual’s peripheral and brain health as patients suffering from generalized periodontitis often have related co-morbidities and are “at risk” of developing AD. The aim here is to discuss the role of P. gingivalis behind such associations with the backdrop of huge efforts to test P. gingivalis virulence factors clinically (GAIN Trial: Phase 2/3 Study of COR388 in Subjects with AD) with inhibitors, which may lead to an intervention by reducing the pathogenic bacterial load.

Interaction between APOE4 and herpes simplex virus type 1 in Alzheimer's disease

INTRODUCTION

Numerous results suggest the implication of infectious agents in the onset of Alzheimer's disease (AD).

METHODS

In the Bordeaux-3C prospective cohort, we assessed the impact of herpes simplex virus type 1 (HSV-1) infection on the incidence of AD according to apolipoprotein E (APOE) status, a genetic susceptibility factor. Cox models were performed to estimate the 10-year risk of AD associated with anti-HSV antibodies in 1037 participants according to APOE4 status.

RESULTS

Among APOE4 carriers, subjects for whom the frequency of HSV-1 reactivation is supposed to be high, that is, immunoglobulin M (IgM) positive or elevated levels of IgG, had an increased risk of AD with adjusted hazard ratios (HRs) of 3.68 (1.08-12.55) and 3.28 (1.19-9.03), respectively. No significant association was found in APOE4-negative subjects.

DISCUSSION

These results, in accordance with a solid pathophysiological rationale, suggest a role for HSV-1 in AD development among subjects with a genetic susceptibility factor, the APOE4 allele.

Alzheimer's disease as a systems network disorder: chronic stress/dyshomeostasis, innate immunity, and genetics

Ineffective results of clinical trials of over 200 anti-Alzheimer's drug candidates, with a 99.6% attrition rate, suggest that the current paradigm of Alzheimer's disease (AD) may be incomplete, necessitating exploration of alternative and complementary frameworks.Using algorithms for hypothesis independent search and expert-assisted synthesis of heterogeneous data, we attempted to reconcile multimodal clinical profiles of early-stage AD patients and accumulated research data within a parsimonious framework. Results of our analysis suggest that Alzheimer's may not be a brain disease but a progressive system-level network disorder, which is driven by chronic network stress and dyshomeostasis. The latter can be caused by various endogenous and exogenous factors, such as chronic inflammatory conditions, infections, vascular dysfunction, head trauma, environmental toxicity, and immune disorders. Whether originating in the brain or on the periphery, chronic stress, toxicity, and inflammation are communicated to the central nervous system (CNS) via humoral and neural routes, preferentially targeting high-centrality regulatory nodes and circuits of the nervous system, and eventually manifesting as a neurodegenerative CNS disease.In this report, we outline an alternative perspective on AD as a systems network disorder and discuss biochemical and genetic evidence suggesting the central role of chronic tissue injury/dyshomeostasis, innate immune reactivity, and inflammation in the etiopathobiology of Alzheimer's disease.

Independent and Correlated Role of Apolipoprotein E ɛ4 Genotype and Herpes Simplex Virus Type 1 in Alzheimer's Disease

The ɛ4 allele of the Apolipoprotein E (APOE) gene in individuals infected by Herpes simplex virus type 1 (HSV-1) has been demonstrated to be a risk factor in Alzheimer's disease (AD). APOE-ɛ4 reduces the levels of neuronal cholesterol, interferes with the transportation of cholesterol, impairs repair of synapses, decreases the clearance of neurotoxic peptide amyloid-β (Aβ), and promotes the deposition of amyloid plaque, and eventually may cause development of AD. HSV-1 enters host cells and can infect the olfactory system, trigeminal ganglia, entorhinal cortex, and hippocampus, and may cause AD-like pathological changes. The lifecycle of HSV-1 goes through a long latent phase. HSV-1 induces neurotropic cytokine expression with pro-inflammatory action and inhibits antiviral cytokine production in AD. It should be noted that interferons display antiviral activity in HSV-1-infected AD patients. Reactivated HSV-1 is associated with infectious burden in cognitive decline and AD. Finally, HSV-1 DNA has been confirmed as present in human brains and is associated with APOEɛ4 in AD. HSV-1 and APOEɛ4 increase the risk of AD and relate to abnormal autophagy, higher concentrations of HSV-1 DNA in AD, and formation of Aβ plaques and neurofibrillary tangles.

Is there a link between genetic defects in the complement cascade and Porphyromonas gingivalis in Alzheimer's disease?

Defects, as determined by Genome-Wide Association Studies (GWAS), in the complement cascade of innate immunity have been suggested to play a key role in Alzheimer's disease (AD). These defective genes encode sub-component 1s (C1s), complement receptor 1, complement component 9, and clusterin, a fluid-phase regulatory protein. A dysregulated complement cascade has been shown to relate to cell activation, defective complement mediated clearance and possible cognitive decline in AD patients. Porphyromonas gingivalis, a putative keystone pathogen of periodontal disease, has been reported to be associated with human AD. The inflammatory burden following experimental oral infection in mice and putative entry of this bacterium into the brain appears to drive the formation of amyloid-beta plaques and neurofibrillary tangles with loss of cognition. P. gingivalis is a master of immune subversion in this inflammatory cascade and may establish microbial dysbiosis where it is located. Here we discuss if P. gingivalis may enhance the detrimental effects of the defective GWAS complement cascade protein genes.

Herpes simplex virus type 1 and Alzheimer's disease: link and potential impact on treatment

Introduction: Alzheimer's disease (AD), the most common form of dementia worldwide, is a multifactorial disease with a still unknown etiology. Herpes simplex virus 1 (HSV-1) has long been suspected to be one of the factors involved in the pathogenesis of the disease. Areas covered: We review the literature focusing on viral characteristics of HSV-1, the mechanisms this virus uses to infect neural cells, its interaction with the host immune system and genetic background and summarizes results and research that support the hypothesis of an association between AD and HSV-1. The possible usefulness of virus-directed pharmaceutical approaches as potential treatments for AD will be discussed as well. Expert opinion: We highlight crucial aspects that must be addressed to clarify the possible role of HSV-1 in the pathogenesis of the disease, and to allow the design of new therapeutical approaches for AD.

Herpes Simplex Virus, Alzheimer's Disease and a Possible Role for Rab GTPases

Herpes simplex virus (HSV) is a common pathogen, infecting 85% of adults in the United States. After reaching the nucleus of the long-lived neuron, HSV may enter latency to persist throughout the life span. Re-activation of latent herpesviruses is associated with progressive cognitive impairment and Alzheimer's disease (AD). As an enveloped DNA virus, HSV exploits cellular membrane systems for its life cycle, and thereby comes in contact with the Rab family of GTPases, master regulators of intracellular membrane dynamics. Knock-down and overexpression of specific Rabs reduce HSV production. Disheveled membrane compartments could lead to AD because membrane sorting and trafficking are crucial for synaptic vesicle formation, neuronal survival signaling and Abeta production. Amyloid precursor protein (APP), a transmembrane glycoprotein, is the parent of Abeta, the major component of senile plaques in AD. Up-regulation of APP expression due to HSV is significant since excess APP interferes with Rab5 endocytic trafficking in neurons. Here, we show that purified PC12-cell endosomes transport both anterograde and retrograde when injected into the squid giant axon at rates similar to isolated HSV. Intracellular HSV co-fractionates with these endosomes, contains APP, Rab5 and TrkA, and displays a second membrane. HSV infected PC12 cells up-regulate APP expression. Whether interference with Rabs has a specific effect on HSV or indirectly affects membrane compartment dynamics co-opted by virus needs further study. Ultimately Rabs, their effectors or their membrane-binding partners may serve as handles to reduce the impact of viral re-activation on cognitive function, or even as more general-purpose anti-microbial therapies.

Can Better Management of Periodontal Disease Delay the Onset and Progression of Alzheimer's Disease?

A risk factor relationship exists between periodontal disease and Alzheimer's disease (AD) via tooth loss, and improved memory following dental intervention. This links the microbial contribution from indigenous oral periodontal pathogens to the manifestation of chronic conditions, such as AD. Here, we use Porphyromonas gingivalis infection to illustrate its effect on mental health. P. gingivalis infection, in its primary sub-gingival niche, can cause polymicrobial synergy and dysbiosis. Dysbiosis describes the residency of select commensals from the oral cavity following co-aggregation around the dominant keystone pathogen, such as P. gingivalis, to gain greater virulence. The initial process involves P. gingivalis disturbing neutrophil mediated innate immune responses in the healthy gingivae and then downregulating adaptive immune cell differentiation and development to invade, and subsequently, establish new dysbiotic bacterial communities. Immune responses affect the host in general and functionally via dietary adjustments caused by tooth loss. Studies from animals orally infected with P. gingivalis confirm this bacterium can transmigrate to distant organ sites (the brain) and contribute toward peripheral and intracerebral inflammation, and compromise vascular and microvascular integrity. In another study, P. gingivalis infection caused sleep pattern disturbances by altering glial cell light/dark molecular clock activity, and this, in turn, can affect the clearance of danger associated molecular patterns, such as amyloid-β, via the glymphatic system. Since P. gingivalis can transmigrate to the brain and modulate organ-specific inflammatory innate and adaptive immune responses, this paper explores whether better management of indigenous periodontal bacteria could delay/prevent the onset and/or progression of dementia.

Molecular Mechanisms for Herpes Simplex Virus Type 1 Pathogenesis in Alzheimer's Disease

This review focuses on research in the areas of epidemiology, neuropathology, molecular biology and genetics that implicates herpes simplex virus type 1 (HSV-1) as a causative agent in the pathogenesis of sporadic Alzheimer’s disease (AD). Molecular mechanisms whereby HSV-1 induces AD-related pathophysiology and pathology, including neuronal production and accumulation of amyloid beta (Aβ), hyperphosphorylation of tau proteins, dysregulation of calcium homeostasis, and impaired autophagy, are discussed. HSV-1 causes additional AD pathologies through mechanisms that promote neuroinflammation, oxidative stress, mitochondrial damage, synaptic dysfunction, and neuronal apoptosis. The AD susceptibility genes apolipoprotein E (APOE), phosphatidylinositol binding clathrin assembly protein (PICALM), complement receptor 1 (CR1) and clusterin (CLU) are involved in the HSV lifecycle. Polymorphisms in these genes may affect brain susceptibility to HSV-1 infection. APOE, for example, influences susceptibility to certain viral infections, HSV-1 viral load in the brain, and the innate immune response. The AD susceptibility gene cholesterol 25-hydroxylase (CH25H) is upregulated in the AD brain and is involved in the antiviral immune response. HSV-1 interacts with additional genes to affect cognition-related pathways and key enzymes involved in Aβ production, Aβ clearance, and hyperphosphorylation of tau proteins. Aβ itself functions as an antimicrobial peptide (AMP) against various pathogens including HSV-1. Evidence is presented supporting the hypothesis that Aβ is produced as an AMP in response to HSV-1 and other brain infections, leading to Aβ deposition and plaque formation in AD. Epidemiologic studies associating HSV-1 infection with AD and cognitive impairment are discussed. Studies are reviewed supporting subclinical chronic reactivation of latent HSV-1 in the brain as significant in the pathogenesis of AD. Finally, the rationale for and importance of clinical trials treating HSV-1-infected MCI and AD patients with antiviral medication is discussed.

Infection of microglia with Porphyromonas gingivalis promotes cell migration and an inflammatory response through the gingipain-mediated activation of protease-activated receptor-2 in mice

Despite a clear correlation between periodontitis and cognitive decline in Alzheimer's disease, the precise mechanism underlying the relationship remains unclear. The periodontal pathogen Porphyromonas gingivalis produces a unique class of cysteine proteinases termed gingipains that comprises Arg-gingipain (Rgp) and Lys-gingipain (Kgp). Rgp and Kgp are important in the bacterial mediated host cell responses and the subsequent intracellular signaling in infected cells. In the present study, we attempted to clarify the potential effects of Rgp and Kgp on the cellular activation of brain-resident microglia. We provide the first evidence that Rgp and Kgp cooperatively contribute to the P. gingivalis-induced cell migration and expression of proinflammatory mediators through the activation of protease-activated receptor 2. The subsequent activation of phosphoinositide 3-kinase/Akt and mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase/ERK pathways contributes to cell migration and inflammatory response of microglia.

Periodontitis: a potential risk factor for Alzheimer's disease

The role of periodontitis as a risk factor for multiple systemic diseases is widely accepted and there is growing evidence of an association between periodontitis and sporadic late onset Alzheimer's disease (SLOAD). Recent epidemiologic, microbiologic and inflammatory findings strengthen this association, indicating that periodontal pathogens are possible contributors to neural inflammation and SLOAD. The aim of this article is to present contemporary evidence of this association.

On the translocation of bacteria and their lipopolysaccharides between blood and peripheral locations in chronic, inflammatory diseases: the central roles of LPS and LPS-induced cell death

We have recently highlighted (and added to) the considerable evidence that blood can contain dormant bacteria. By definition, such bacteria may be resuscitated (and thus proliferate). This may occur under conditions that lead to or exacerbate chronic, inflammatory diseases that are normally considered to lack a microbial component. Bacterial cell wall components, such as the endotoxin lipopolysaccharide (LPS) of Gram-negative strains, are well known as potent inflammatory agents, but should normally be cleared. Thus, their continuing production and replenishment from dormant bacterial reservoirs provides an easy explanation for the continuing, low-grade inflammation (and inflammatory cytokine production) that is characteristic of many such diseases. Although experimental conditions and determinants have varied considerably between investigators, we summarise the evidence that in a great many circumstances LPS can play a central role in all of these processes, including in particular cell death processes that permit translocation between the gut, blood and other tissues. Such localised cell death processes might also contribute strongly to the specific diseases of interest. The bacterial requirement for free iron explains the strong co-existence in these diseases of iron dysregulation, LPS production, and inflammation. Overall this analysis provides an integrative picture, with significant predictive power, that is able to link these processes via the centrality of a dormant blood microbiome that can resuscitate and shed cell wall components.

Diurnal dynamic behavior of microglia in response to infected bacteria through the UDP-P2Y6 receptor system

It has long been believed that microglia morphologically transform into the activated state by retracting their long processes and consuming pathogens when bacteria infect into the brain parenchyma. In the present study, however, we showed for the first time that murine cortical microglia extend their processes towards focally injected Porphyromonas gingivalis. This P. gingivalis-induced microglial process extension was significantly increased during the light (sleeping) phase than the dark (waking) phase. In contrast, focally injected ATP-induced microglial process extension was significantly increased during the dark phase than the light phase. Furthermore, in contrast to the P2Y₁₂ receptor-mediated mechanism of ATP-induced microglial process extension, the P. gingivalis-mediated microglial process extension was mediated by P2Y₆ receptors. The infection of bacteria such as P. gingivalis to the brain parenchyma may induce the secretion of UDP from microglia at the site of infection, which in turn induces the process extension of the neighboring microglia.

Herpes Simplex Virus Type 1 and Other Pathogens are Key Causative Factors in Sporadic Alzheimer's Disease

This review focuses on research in epidemiology, neuropathology, molecular biology, and genetics regarding the hypothesis that pathogens interact with susceptibility genes and are causative in sporadic Alzheimer's disease (AD). Sporadic AD is a complex multifactorial neurodegenerative disease with evidence indicating coexisting multi-pathogen and inflammatory etiologies. There are significant associations between AD and various pathogens, including Herpes simplex virus type 1 (HSV-1), Cytomegalovirus, and other Herpesviridae, Chlamydophila pneumoniae, spirochetes, Helicobacter pylori, and various periodontal pathogens. These pathogens are able to evade destruction by the host immune system, leading to persistent infection. Bacterial and viral DNA and RNA and bacterial ligands increase the expression of pro-inflammatory molecules and activate the innate and adaptive immune systems. Evidence demonstrates that pathogens directly and indirectly induce AD pathology, including amyloid-β (Aβ) accumulation, phosphorylation of tau protein, neuronal injury, and apoptosis. Chronic brain infection with HSV-1, Chlamydophila pneumoniae, and spirochetes results in complex processes that interact to cause a vicious cycle of uncontrolled neuroinflammation and neurodegeneration. Infections such as Cytomegalovirus, Helicobacter pylori, and periodontal pathogens induce production of systemic pro-inflammatory cytokines that may cross the blood-brain barrier to promote neurodegeneration. Pathogen-induced inflammation and central nervous system accumulation of Aβ damages the blood-brain barrier, which contributes to the pathophysiology of AD. Apolipoprotein E4 (ApoE4) enhances brain infiltration by pathogens including HSV-1 and Chlamydophila pneumoniae. ApoE4 is also associated with an increased pro-inflammatory response by the immune system. Potential antimicrobial treatments for AD are discussed, including the rationale for antiviral and antibiotic clinical trials.

Apolipoprotein E and protection against hepatitis E viral infection in American non-Hispanic blacks

Hepatitis E viral (HEV) infection imposes a heavy health burden worldwide and is common in the United States. Previous investigations of risks addressed environmental and host behavioral/lifestyle factors, but host genetic factors have not been examined. We assessed strength of associations between antibody to HEV (anti-HEV) immunoglobulin G seropositivity indicating past or recent HEV infection and human genetic variants among three major racial/ethnic populations in the United States, involving 2434 non-Hispanic whites, 1919 non-Hispanic blacks, and 1919 Mexican Americans from the Third National Health and Nutrition Examination Survey, 1991-1994. We studied 497 single-nucleotide polymorphisms across 190 genes (particularly those associated with lipid metabolism). The genomic control method was used to adjust for potential population stratification. Non-Hispanic blacks had the lowest seroprevalence of anti-HEV immunoglobulin G (15.3%, 95% confidence interval [CI] 12.3%-19.0%) compared with non-Hispanic whites (22.3%, 95% CI 19.1%-25.7%) and Mexican Americans (21.8%, 95% CI 19.0%-25.3%; P<0.01). Non-Hispanic blacks were the only population that showed association between anti-HEV seropositivity and functional ε3 and ε4 alleles of the apolipoprotein E (APOE) gene, encoding the apolipoprotein E protein that mediates lipoprotein metabolism. Seropositivity was significantly lower in participants carrying APOE ε4 (odds ratio=0.5, 95% CI 0.4-0.7; P=0.00004) and ε3 (odds ratio=0.6, 95% CI 0.4-0.8; P=0.001) compared to those carrying APOE ε2. No significant associations were observed between other single-nucleotide polymorphisms and anti-HEV seropositivity in non-Hispanic blacks or between any single-nucleotide polymorphisms and anti-HEV seropositivity in non-Hispanic whites or Mexican Americans.

CONCLUSION

Both APOE ε3 and ε4 are significantly associated with protection against HEV infection in non-Hispanic blacks; additional studies are needed to understand the basis of protection so that preventive services can be targeted to at-risk persons.

Immune aging, dysmetabolism, and inflammation in neurological diseases

As we age, the immune system undergoes a process of senescence accompanied by the increased production of proinflammatory cytokines, a chronic subclinical condition named as “inflammaging”. Emerging evidence from human and experimental models suggest that immune senescence also affects the central nervous system and promotes neuronal dysfunction, especially within susceptible neuronal populations. In this review we discuss the potential role of immune aging, inflammation and metabolic derangement in neurological diseases. The discovery of novel therapeutic strategies targeting age-linked inflammation may promote healthy brain aging and the treatment of neurodegenerative as well as neuropsychiatric disorders.

Microglial priming in neurodegenerative disease

Under physiological conditions, the number and function of microglia--the resident macrophages of the CNS--is tightly controlled by the local microenvironment. In response to neurodegeneration and the accumulation of abnormally folded proteins, however, microglia multiply and adopt an activated state--a process referred to as priming. Studies using preclinical animal models have shown that priming of microglia is driven by changes in their microenvironment and the release of molecules that drive their proliferation. Priming makes the microglia susceptible to a secondary inflammatory stimulus, which can then trigger an exaggerated inflammatory response. The secondary stimulus can arise within the CNS, but in elderly individuals, the secondary stimulus most commonly arises from a systemic disease with an inflammatory component. The concept of microglial priming, and the subsequent exaggerated response of these cells to secondary systemic inflammation, opens the way to treat neurodegenerative diseases by targeting systemic disease or interrupting the signalling pathways that mediate the CNS response to systemic inflammation. Both lifestyle changes and pharmacological therapies could, therefore, provide efficient means to slow down or halt neurodegeneration.

Perspectives in molecular imaging using staging biomarkers and immunotherapies in Alzheimer's disease

Sporadic Alzheimer's disease (AD) is an emerging chronic illness characterized by a progressive pleiotropic pathophysiological mode of actions triggered during the senescence process and affecting the elderly worldwide. The complex molecular mechanisms of AD not only are supported by cholinergic, beta-amyloid, and tau theories but also have a genetic basis that accounts for the difference in symptomatology processes activation among human population which will evolve into divergent neuropathological features underlying cognitive and behaviour alterations. Distinct immune system tolerance could also influence divergent responses among AD patients treated by immunotherapy. The complexity in nature increases when taken together the genetic/immune tolerance with the patient's brain reserve and with neuropathological evolution from early till advance AD clinical stages. The most promising diagnostic strategies in today's world would consist in performing high diagnostic accuracy of combined modality imaging technologies using beta-amyloid 42 peptide-cerebrospinal fluid (CSF) positron emission tomography (PET), Pittsburgh compound B-PET, fluorodeoxyglucose-PET, total and phosphorylated tau-CSF, and volumetric magnetic resonance imaging hippocampus biomarkers for criteria evaluation and validation. Early diagnosis is the challenge task that needs to look first at plausible mechanisms of actions behind therapies, and combining them would allow for the development of efficient AD treatment in a near future.

HSV, axonal transport and Alzheimer's disease: in vitro and in vivo evidence for causal relationships

HSV, a neurotropic virus, travels within neuronal processes by fast axonal transport. During neuronal infection HSV travels retrograde from the sensory nerve terminus to the neuronal cell body, where it replicates or enters latency. During replication HSV travels anterograde from the cell body to the nerve terminus. Postmortem studies find a high frequency of HSV DNA in the trigeminal ganglia as well as the brain. Studies correlating HSV with Alzheimer's disease (AD) have been controversial. Here we review clinical evidence supporting such a link. Furthermore, the author describes experimental data showing physical interactions between nascent HSV particles and host transport machinery implicated in AD. The author concludes that the complexity of this relationship has been insufficiently explored, although the relative ease and nontoxicity of a potential anti-HSV treatment for AD demands further study.

Cytosolic lipid inclusions formed during infection by viral and bacterial pathogens

Lipid inclusions play an important role in several pathological processes. Intracellular bacterial pathogens, such as members of the Mycobacterium and Chlamydia species are able to trigger the formation of lipid-laden foamy macrophages. Lipid droplet accumulation in the host constitutes a reservoir used by the bacilli for long-term persistence. Viruses need lipid droplets as assembly platform. We present the current knowledge about structural, functional and regulatory aspects of lipid inclusions.

Herpes simplex virus type 1 induces simultaneous activation of Toll-like receptors 2 and 4 and expression of the endogenous ligand serum amyloid A in astrocytes

Herpes simplex virus type 1 (HSV-1) is the most common pathogenic cause of sporadic acute encephalitis and it produces latent persistent infection lifelong in infected individuals. Brain inflammation is associated with activation of glial cells, which can detect pathogen-associated molecular patterns (PAMPs) through a variety of pattern-recognition receptors (PRR), including Toll-like receptors (TLRs). In this study, we evaluated the expression and activation of TLR2, TLR3, and TLR4 in HSV-1-infected astrocyte and neuronal primary cultures. Our results showed a clear induction in TLR2 and TLR4 expression in astrocytes as early as 1 h after HSV-1 infection, whereas no significant change was observed in neurons. In addition, infected astrocytes showed increased levels of interferon regulatory factors IRF3 and IRF7, interferon β (INFβ), interleukin 6 (IL6), and serum amyloid A (SAA3) transcripts, as well as phospho-IRF3 protein. These effects seemed to be dependent on viral replication since previous treatment of the cells with acyclovir resulted in low levels of TLRs expression and activation even after 4 h post-infection. These results suggest that reactivation of HSV-1 at the central nervous system (CNS) would likely induce and activate TLR2 and TLR4 receptors directly through interaction of astrocytes with the pathogen and also indirectly by endogenous ligands produced locally, such as serum amyloid protein, potentiating the neuroinflammatory response.

ApoE genotype: from geographic distribution to function and responsiveness to dietary factors

ApoE is a key protein in lipid metabolism with three major isoforms.ApoEallele frequencies show non-random global distribution especially in Europe with highapoEε3frequency in the Mediterranean area, whereas theapoEε4genotype is enriched in Northern Europe. TheapoEε4genotype is one of the most important genetic risk factors for age-dependent chronic diseases, including CVD and Alzheimer's disease (AD). The apoE polymorphism has been shown to impact on blood lipids, biomarkers of oxidative stress and chronic inflammation, which all may contribute to the isoform-dependent disease risk. Studies in mice and human subjects indicate that theapoEε3but not theapoEε4genotype may significantly benefit from dietary flavonoids (e.g. quercetin) andn-3 fatty acids. Metabolism of lipid soluble vitamins E and D is likewise differentially affected by theapoEgenotype. Epidemiological and experimental evidence suggest a better vitamin D status inapoEε4than ε3subjects indicating a certain advantage of ε4over ε3. The present review aims at evaluation of current data available on interactions between apoE polymorphism and dietary responsiveness to flavonoids, fat soluble vitamins andn-3 fatty acids. Likewise, distinct geographic distribution and chronic disease risk of the different apoE isoforms are addressed.

Periodontitis and Alzheimer's disease: oral systemic link still on the rise?

Over the past few years, there has been a rapid rise in the older segments of the world population, which has brought along with it a major health concern: dementia. Alzheimer's disease, considered to be the most common cause of dementia, has become a prospect feared by the elderly. Inflammation of the brain is strongly implicated in Alzheimer's disease which could be enhanced by systemic inflammation. Periodontitis being a chronic inflammatory condition, which can cause systemic inflammation, the question is whether chronic periodontitis can initiate or hasten the rate of progression of Alzheimer's disease in susceptible individuals. In this article, the authors outline the proposed oral systemic link between periodontitis and Alzheimer's disease.

Association between IgM anti-herpes simplex virus and plasma amyloid-beta levels

Objective

Herpes simplex virus (HSV) reactivation has been identified as a possible risk factor for Alzheimer's disease (AD) and plasma amyloid-beta (Aβ) levels might be considered as possible biomarkers of the risk of AD. The aim of our study was to investigate the association between anti-HSV antibodies and plasma Aβ levels.

Methods

The study sample consisted of 1222 subjects (73.9 y in mean) from the Three-City cohort. IgM and IgG anti-HSV antibodies were quantified using an ELISA kit, and plasma levels of Aβ_1–40 and Aβ_1–42 were measured using an xMAP-based assay technology. Cross-sectional analyses of the associations between anti-HSV antibodies and plasma Aβ levels were performed by multi-linear regression.

Results

After adjustment for study center, age, sex, education, and apolipoprotein E-e4 polymorphism, plasma Aβ_1–42 and Aβ_1–40 levels were specifically inversely associated with anti-HSV IgM levels (β = −20.7, P = 0.001 and β = −92.4, P = 0.007, respectively). In a sub-sample with information on CLU- and CR1-linked SNPs genotyping (n = 754), additional adjustment for CR1 or CLU markers did not modify these associations (adjustment for CR1 rs6656401, β = −25.6, P = 0.002 for Aβ_1–42 and β = −132.7, P = 0.002 for Aβ_1–40; adjustment for CLU rs2279590, β = −25.6, P = 0.002 for Aβ_1–42 and β = −134.8, P = 0.002 for Aβ_1–40). No association between the plasma Aβ_1–42-to-Aβ_1–40 ratio and anti-HSV IgM or IgG were evidenced.

Conclusion

High anti-HSV IgM levels, markers of HSV reactivation, are associated with lower plasma Aβ_1–40 and Aβ_1–42 levels, which suggest a possible involvement of the virus in the alterations of the APP processing and potentially in the pathogenesis of AD in human.

Ocular herpes simplex virus: how are latency, reactivation, recurrent disease and therapy interrelated?

Most humans are infected with herpes simplex virus (HSV) type 1 in early childhood and remain latently infected throughout life. While most individuals have mild or no symptoms, some will develop destructive HSV keratitis. Ocular infection with HSV-1 and its associated sequelae account for the majority of corneal blindness in industrialized nations. Neuronal latency in the peripheral ganglia is established when transcription of the viral genome is repressed (silenced) except for the latency-associated transcripts and microRNAs. The functions of latency-associated transcripts have been investigated since 1987. Roles have been suggested relating to reactivation, establishment of latency, neuronal protection, antiapoptosis, apoptosis, virulence and asymptomatic shedding. Here, we review HSV-1 latent infections, reactivation, recurrent disease and antiviral therapies for the ocular HSV diseases.

Systemic inflammation and Alzheimer's disease

A number of studies demonstrate disturbances of the central innate immune system in AD (Alzheimer's disease). In animal and human studies, there is evidence of close communication between systemic and central innate immune systems. Animal models of neurodegeneration show evidence of an exaggerated central innate immune response following systemic inflammation. Clinical studies of AD show evidence of increased cognitive decline and exaggerated sickness behaviour in response to systemic inflammation. Recognition of this communication pathway offers alternative explanations for a number of recognized risk factors in the development and progression of AD and highlights the potential of the manipulation of systemic innate immunity as a novel therapeutic approach.

Emerging roles of pathogens in Alzheimer disease

Chronic spirochetal infection can cause slowly progressive dementia, cortical atrophy and amyloid deposition in the atrophic form of general paresis. There is a significant association between Alzheimer disease (AD) and various types of spirochete (including the periodontal pathogen Treponemas and Borrelia burgdorferi), and other pathogens such as Chlamydophyla pneumoniae and herpes simplex virus type-1 (HSV-1). Exposure of mammalian neuronal and glial cells and organotypic cultures to spirochetes reproduces the biological and pathological hallmarks of AD. Senile-plaque-like beta amyloid (Aβ) deposits are also observed in mice following inhalation of C. pneumoniae in vivo, and Aβ accumulation and phosphorylation of tau is induced in neurons by HSV-1 in vitro and in vivo. Specific bacterial ligands, and bacterial and viral DNA and RNA all increase the expression of proinflammatory molecules, which activates the innate and adaptive immune systems. Evasion of pathogens from destruction by the host immune reactions leads to persistent infection, chronic inflammation, neuronal destruction and Aβ deposition. Aβ has been shown to be a pore-forming antimicrobial peptide, indicating that Aβ accumulation might be a response to infection. Global attention and action is needed to support this emerging field of research because dementia might be prevented by combined antibiotic, antiviral and anti-inflammatory therapy.

CRP gene variation affects early development of Alzheimer's disease-related plaques

Introduction

We used the Tampere Autopsy Study (TASTY) series (n = 603, age 0-97 yrs), representing an unselected population outside institutions, to investigate the pathogenic involvement of inflammation in Alzheimer's disease-related lesions.

Methods

We studied senile plaque (SP), neurofibrillary tangles (NFT) and SP phenotype associations with 6 reported haplotype tagging single nucleotide polymorphisms (SNPs) in the CRP gene. CRP and Aβ immunohistochemistry was assessed using brain tissue microarrays.

Results

In multivariate analyses (age- and APOE-adjusted), non-neuritic SP were associated with the high-CRP TA-genotype (3.0% prevalence) of rs3091244 and CA-genotype (10.8%) of rs3093075 compared to common genotypes. Conversely, the low-CRP C allele (39.3%) of rs2794521 reduced the risk of harbouring early non-neuritic SP, compared to the TT genotype. CRP haplotype TAGCC (high) associated with non-neuritic SP, whereas haplotype CCGCC offered protection. TT genotypes (high) of rs3091244 and rs1130864 were associated with CRP staining. There were no associations between SNPs or haplotypes and NFT. CRP staining of the hippocampal CA1/2 region correlated with Aβ staining.

Conclusions

CRP gene variation affects early SP development in prodromal Alzheimer's disease, independent of APOE genotype.

Cross-talk between apolipoprotein E and cytokines

Apolipoprotein E (apoE) is a multifunctional glycosylated protein characterized by its wide tissue distribution. Despite its importance in lipid transport and atherosclerosis pathogenesis, apoE is associated with neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson disease, and autoimmune disorders such as multiple sclerosis and psoriasis. Among others, the role of apoE in modulating inflammation and oxidation is crucial in elucidating the risk factors of the above diseases since the function of apoE is closely linked with both proinflammatory and antiinflammatory cytokines. Moreover, apoE modulates inflammatory and immune responses in an isoform-dependent manner. Correspondingly, inflammatory cytokines can either upregulate or downregulate the production of apoE in various tissue types. However, studies on the interactions between apoE and cytokines occasionally yield conflicting results, highlighting the complex roles of apoE and cytokines in various disorders. The present paper summarizes the current knowledge about the cross-talk between apoE and cytokines, with emphasis on the effects of apoE on the Th1/Th2 balance.

Systemic and central immunity in Alzheimer's disease: therapeutic implications

Clinical pharmaceutical trials aimed at modulating the immune system in Alzheimer's Disease have largely focused on either dampening down central proinflammatory innate immunity or have manipulated adaptive immunity to facilitate the removal of centrally deposited beta amyloid. To date, these trials have had mixed clinical therapeutic effects. However, a number of clinical studies have demonstrated disturbances of both systemic and central innate immunity in Alzheimer's Disease and attention has been drawn to the close communication pathways between central and systemic immunity. This paper highlights the need to take into account the potential systemic effects of drugs aimed at modulating central immunity and the possibility of developing novel therapeutic approaches based on the manipulation of systemic immunity and its communication with the central nervous system.

HSV1 in Alzheimer’s disease: Myth or reality?

Alzheimer’s disease (AD) is the most frequent cause of dementia in the elderly, characterized by the presence of cerebral amyloid plaques and neurofibrillary tangles. The causes of the disease are not well understood, especially considering that more than 95% of AD patients are non-familial. Due to the similarity of brain regions affected in herpes simplex encephalitis to those mainly affected in AD, and owing to the very high prevalence of latent herpes simplex virus type 1 (HSV1) infection, reactivation of HSV1 was proposed as one of the possible causes of AD. The trigeminal ganglion, located only a few millimeters from the entorhinal cortex, is the primary site of HSV1 latency, although other sites including the sensory neurons, the nodose ganglion of the vagus nerve and other regions of the brain may be involved, possibly in relation to very early neurofibrillary AD changes in the dorsal raphe, locus coeruleus and other brainstem nuclei. Novel data obtained upon infection of cultured neuronal cells and mouse brain with HSV1 further show that HSV1 infection causes intracellular amyloid-beta protein accumulation, as well as abnormal phosphorylation of tau protein, the major component of tangles. Another interesting fact is the existence of a significant degree of homology between HSV1 components and AD susceptibility genes. In this review we summarize findings that reveal connections between the two conditions, as well as different suggestions for the mechanisms of HSV1-induced AD. As most of the available results support a connection of AD and HSV1 infection, antiviral therapy should be taken into consideration for AD treatment following early diagnosis.

Apolipoprotein E genotype and hepatitis C, HIV and herpes simplex disease risk: a literature review

Apolipoprotein E is a polymorphic and multifunctional protein with numerous roles in lipoprotein metabolism. The three common isoforms apoE2, apoE3 and apoE4 show isoform-specific functional properties including different susceptibilities to diseases. ApoE4 is an accepted risk factor for Alzheimer's disease and cardiovascular disorders. Recently, associations between apoE4 and infectious diseases have been demonstrated. This review summarises how apoE4 may be involved in the infection incidence and associated pathologies of specific infectious diseases, namely hepatitis C, human immunodeficiency virus disease and herpes simplex.ApoE4 seems to be protective against chronic hepatitis C virus infection and retards fibrosis progression. In contrast apoE4 enhances the fusion rate of human immunodeficiency virus with target cell membranes, resulting in accelerated cell entry and faster disease progression. Its association with human immunodeficiency virus-associated dementia remains controversial. Regarding herpes simplex virus infection, apoE4 intensifies virus latency and is associated with increased oxidative damage of the central nervous system, and there is some evidence that herpes simplex virus infection in combination with the apoE4 genotype may be associated with an increased risk of Alzheimer's disease. In addition to reviewing available data from human trials, evidence derived from a variety of cell culture and animal models are considered in this review in order to provide mechanistic insights into observed association between apoE4 genotype and viral disease infection and pathology.

ER stress in Alzheimer's disease: a novel neuronal trigger for inflammation and Alzheimer's pathology

The endoplasmic reticulum (ER) is involved in several crucial cellular functions, e.g. protein folding and quality control, maintenance of Ca²⁺ balance, and cholesterol synthesis. Many genetic and environmental insults can disturb the function of ER and induce ER stress. ER contains three branches of stress sensors, i.e. IRE1, PERK and ATF6 transducers, which recognize the misfolding of proteins in ER and activate a complex signaling network to generate the unfolded protein response (UPR). Alzheimer's disease (AD) is a progressive neurodegenerative disorder involving misfolding and aggregation of proteins in conjunction with prolonged cellular stress, e.g. in redox regulation and Ca²⁺ homeostasis. Emerging evidence indicates that the UPR is activated in neurons but not in glial cells in AD brains. Neurons display pPERK, peIF2α and pIRE1α immunostaining along with abundant diffuse staining of phosphorylated tau protein. Recent studies have demonstrated that ER stress can also induce an inflammatory response via different UPR transducers. The most potent pathways are IRE1-TRAF2, PERK-eIF2α, PERK-GSK-3, ATF6-CREBH, as well as inflammatory caspase-induced signaling pathways. We will describe the mechanisms which could link the ER stress of neurons to the activation of the inflammatory response and the evolution of pathological changes in AD.