Microglial phagocytosis of fibrillar beta-amyloid through a beta1 integrin-dependent mechanism

The Role of CXCR3 in Nervous System-Related Diseases

Inflammatory chemokines are a group of G-protein receptor ligands characterized by conserved cysteine residues, which can be divided into four main subfamilies: CC, CXC, XC, and CX3C. The C-X-C chemokine receptor (CXCR) 3 and its ligands, C-X-C chemokine ligands (CXCLs), are widely expressed in both the peripheral nervous system (PNS) and central nervous system (CNS). This comprehensive literature review aims to examine the functions and pathways of CXCR3 and its ligands in nervous system-related diseases. In summary, while the related pathways and the expression levels of CXCR3 and its ligands are varied among different cells in PNS and CNS, the MPAK pathway is the core via which CXCR3 exerts physiological functions. It is not only the core pathway of CXCR3 after activation but also participates in the expression of CXCR3 ligands in the nervous system. In addition, despite CXCR3 being a common inflammatory chemokine receptor, there is no consensus on its precise roles in various diseases. This uncertainty may be attributable to distinct inflammatory characteristics, that inflammation simultaneously possesses the dual properties of damage induction and repair facilitation.

Cell autonomous functions of CD47 in regulating cellular plasticity and metabolic plasticity

CD47 is a ubiquitously expressed cell surface receptor, which is widely known for preventing macrophage-mediated phagocytosis by interacting with signal regulatory protein α (SIRPα) on the surface of macrophages. In addition to its role in phagocytosis, emerging studies have reported numerous noncanonical functions of CD47 that include regulation of various cellular processes such as proliferation, migration, apoptosis, differentiation, stress responses, and metabolism. Despite lacking an extensive cytoplasmic signaling domain, CD47 binds to several cytoplasmic proteins, particularly upon engaging with its secreted matricellular ligand, thrombospondin 1. Indeed, the regulatory functions of CD47 are greatly influenced by its interacting partners. These interactions are often cell- and context-specific, adding a further level of complexity. This review addresses the downstream cell-intrinsic signaling pathways regulated by CD47 in various cell types and environments. Some of the key pathways modulated by this receptor include the PI3K/AKT, MAPK/ERK, and nitric oxide signaling pathways, as well as those implicated in glucose, lipid, and mitochondrial metabolism. These pathways play vital roles in maintaining tissue homeostasis, highlighting the importance of understanding the phagocytosis-independent functions of CD47. Given that CD47 expression is dysregulated in a variety of cancers, improving our understanding of the cell-intrinsic signals regulated by this molecule will help advance the development of CD47-targeted therapies.

Hybrid Amyloid Quantum Dot Nano-Bio Assemblies to Probe Neuroinflammatory Damage

Various oligomeric species of amyloid-beta have been proposed to play different immunogenic roles in the cellular pathology of Alzheimer's Disease. The dynamic interconversion between various amyloid oligomers and fibrillar assemblies makes it difficult to elucidate the role each potential aggregation state may play in driving neuroinflammatory and neurodegenerative pathology. The ability to identify the amyloid species that are key and essential drivers of these pathological hallmarks of Alzheimer's Disease is of fundamental importance for also understanding downstream events including tauopathies that mediate neuroinflammation with neurologic deficits. Here, we report the design and construction of a quantum dot mimetic for larger spherical oligomeric amyloid species as an "endogenously" fluorescent proxy for this cytotoxic assembly of amyloid to investigate its role in inducing inflammatory and stress response states in neuronal and glial cell types. The design parameters and construction protocol developed here may be adapted for developing quantum dot nano-bio assemblies for other biological systems of interest, particularly neurodegenerative diseases involving other protein aggregates.

The role of integrins in brain health and neurodegenerative diseases

Integrins are heterodimeric membrane proteins expressed on the surface of most cells. They mediate adhesion and signaling processes relevant for a wealth of physiological processes, including nervous system development and function. Interestingly, integrins are also recognized therapeutic targets for inflammatory diseases, such as multiple sclerosis. Here, we discuss the role of integrins in brain development and function, as well as in neurodegenerative diseases affecting the brain (Alzheimer's disease, multiple sclerosis, stroke). Furthermore, we discuss therapeutic targeting of these adhesion receptors in inflammatory diseases of the brain.

Cognitive Effects of Simulated Galactic Cosmic Radiation Are Mediated by ApoE Status, Sex, and Environment in APP Knock-In Mice

Cosmic radiation experienced during space travel may increase the risk of cognitive impairment. While simulated galactic cosmic radiation (GCRsim) has led to memory deficits in wildtype (WT) mice, it has not been investigated whether GCRsim in combination with genetic risk factors for Alzheimer's disease (AD) worsens memory further in aging mice. Here, we investigated the central nervous system (CNS) effects of 0 Gy (sham) or 0.75 Gy five-ion GCRsim or 2 Gy gamma radiation (IRR) in 14-month-old female and male APPNL-F/NL-F knock-in (KI) mice bearing humanized ApoE3 or ApoE4 (APP;E3F and APP;E4F). As travel to a specialized facility was required for irradiation, both traveled sham-irradiated C57BL/6J WT and KI mice and non-traveled (NT) KI mice acted as controls for potential effects of travel. Mice underwent four behavioral tests at 20 months of age and were euthanized for pathological and biochemical analyses 1 month later. Fecal samples were collected pre- and post-irradiation at four different time points. GCRsim seemed to impair memory in male APP;E3F mice compared to their sham counterparts. Travel tended to improve cognition in male APP;E3F mice and lowered total Aβ in female and male APP;E3F mice compared to their non-traveled counterparts. Sham-irradiated male APP;E4F mice accumulated more fibrillar amyloid than their APP;E3F counterparts. Radiation exposure had only modest effects on behavior and brain changes, but travel-, sex-, and genotype-specific effects were seen. Irradiated mice had immediate and long-term differences in their gut bacterial composition that correlated to Alzheimer's disease phenotypes.

Downregulation of Fidgetin-Like 2 Increases Microglial Function: The Relationship Between Microtubules, Morphology, and Activity

The microtubule cytoskeleton regulates microglial morphology, motility, and effector functions. The microtubule-severing enzyme, fidgetin-like 2 (FL2), negatively regulates cell motility and nerve regeneration, making it a promising therapeutic target for central nervous system injury. Microglia perform important functions in response to inflammation and injury, but how FL2 affects microglia is unclear. In this study, we investigated the role of FL2 in microglial morphology and injury responses in vitro. We first determined that the pro-inflammatory stimulus, lipopolysaccharide (LPS), induced a dose- and time-dependent reduction in FL2 expression associated with reduced microglial ramification. We then administered nanoparticle-encapuslated FL2 siRNA to knockdown FL2 and assess microglial functions compared to negative control siRNA and vehicle controls. Time-lapse live-cell microscopy showed that FL2 knockdown increased the velocity of microglial motility. After incubation with fluorescently labeled IgG-opsonized beads, FL2 knockdown increased phagocytosis. Microglia were exposed to low-dose LPS after nanoparticle treatment to model injury-induced cytokine secretion. FL2 knockdown enhanced LPS-induced cytokine secretion of IL-1α, IL-1β, and TNFα. These results identify FL2 as a regulator of microglial morphology and suggest that FL2 can be targeted to increase or accelerate microglial injury responses.

Quantitative 3D histochemistry reveals region-specific amyloid-β reduction by the antidiabetic drug netoglitazone

A hallmark of Alzheimer's disease (AD) is the extracellular aggregation of toxic amyloid-beta (Aβ) peptides in form of plaques. Here, we identify netoglitazone, an antidiabetic compound previously tested in humans, as an Aβ aggregation antagonist. Netoglitazone improved cognition and reduced microglia activity in a mouse model of AD. Using quantitative whole-brain three-dimensional histology (Q3D), we precisely identified brain regions where netoglitazone reduced the number and size of Aβ plaques. We demonstrate the utility of Q3D in preclinical drug evaluation for AD by providing a high-resolution brain-wide view of drug efficacy. Applying Q3D has the potential to improve pre-clinical drug evaluation by providing information that can help identify mechanisms leading to brain region-specific drug efficacy.

Enhanced phagocytosis associated with multinucleated microglia via Pyk2 inhibition in an acute β-amyloid infusion model

Multinucleated microglia have been observed in contexts associated with infection, inflammation, and aging. Though commonly linked to pathological conditions, the larger cell size of multinucleated microglia might enhance their phagocytic functions, potentially aiding in the clearance of brain debris and suggesting a reassessment of their pathological significance. To assess the phagocytic capacity of multinucleated microglia and its implications for brain debris clearance, we induced their formation by inhibiting Pyk2 activity using the pharmacological inhibitor PF-431396, which triggers cytokinesis regression. Multinucleated microglia demonstrate enhanced phagocytic function, as evidenced by their increased capacity to engulf β-amyloid (Aβ) oligomers. Concurrently, the phosphorylation of Pyk2, induced by Aβ peptide, was diminished upon treatment with a Pyk2 inhibitor (Pyk2-Inh, PF-431396). Furthermore, the increased expression of Lamp1, a lysosomal marker, with Pyk2-inh treatment, suggests an enhancement in proteolytic activity. In vivo, we generated an acute Alzheimer's disease (AD) model by infusing Aβ into the brains of Iba-1 EGFP transgenic (Tg) mice. The administration of the Pyk2-Inh led to an increased migration of microglia toward amyloid deposits in the brains of Iba-1 EGFP Tg mice, accompanied by morphological activation, suggesting a heightened affinity for Aβ. In human microglia, lipopolysaccharide (LPS)-induced inflammatory responses showed that inhibition of Pyk2 signaling significantly reduced the transcription and protein expression of pro-inflammatory markers. These results suggest that Pyk2 inhibition can modulate microglial functions, potentially reducing neuroinflammation and aiding in the clearance of neurodegenerative disease markers. This highlights Pyk2 as a promising target for therapeutic intervention in neurodegenerative diseases.

Traumatic brain injury alters the effects of class II invariant peptide (CLIP) antagonism on chronic meningeal CLIP + B cells, neuropathology, and neurobehavioral impairment in 5xFAD mice

BACKGROUND

Traumatic brain injury (TBI) is a significant risk factor for Alzheimer's disease (AD), and accumulating evidence supports a role for adaptive immune B and T cells in both TBI and AD pathogenesis. We previously identified B cell and major histocompatibility complex class II (MHCII)-associated invariant chain peptide (CLIP)-positive B cell expansion after TBI. We also showed that antagonizing CLIP binding to the antigen presenting groove of MHCII after TBI acutely reduced CLIP + splenic B cells and was neuroprotective. The current study investigated the chronic effects of antagonizing CLIP in the 5xFAD Alzheimer's mouse model, with and without TBI.

METHODS

12-week-old male wild type (WT) and 5xFAD mice were administered either CLIP antagonist peptide (CAP) or vehicle, once at 30 min after either sham or a lateral fluid percussion injury (FPI). Analyses included flow cytometric analysis of immune cells in dural meninges and spleen, histopathological analysis of the brain, magnetic resonance diffusion tensor imaging, cerebrovascular analysis, and assessment of motor and neurobehavioral function over the ensuing 6 months.

RESULTS

9-month-old 5xFAD mice had significantly more CLIP + B cells in the meninges compared to age-matched WT mice. A one-time treatment with CAP significantly reduced this population in 5xFAD mice. Importantly, CAP also improved some of the immune, histopathological, and neurobehavioral impairments in 5xFAD mice over the ensuing six months. Although FPI did not further elevate meningeal CLIP + B cells, it did negate the ability of CAP to reduce meningeal CLIP + B cells in the 5xFAD mice. FPI at 3 months of age exacerbated some aspects of AD pathology in 5xFAD mice, including further reducing hippocampal neurogenesis, increasing plaque deposition in CA3, altering microgliosis, and disrupting the cerebrovascular structure. CAP treatment after injury ameliorated some but not all of these FPI effects.

Progress in cancer research on the regulator of phagocytosis CD47, which determines the fate of tumor cells (Review)

Cluster of differentiation 47 (CD47) is a transmembrane protein that is widely and moderately expressed on the surface of various cells and can have an essential role in mediating cell proliferation, migration, phagocytosis, apoptosis, immune homeostasis and other related responses by binding to its ligands, integrins, thrombospondin-1 and signal regulatory protein α. The poor prognosis of cancer patients is closely associated with high expression of CD47 in glioblastoma, ovarian cancer, breast cancer, bladder cancer, colon cancer and hepatocellular carcinoma. Upregulation of CD47 expression facilitates the growth of numerous types of tumor cells, while downregulation of its expression promotes phagocytosis of tumor cells by macrophages, thereby limiting tumor growth. In addition, blocking CD47 activates the cyclic GMP-AMP (cGAMP) synthase/cGAMP/interferon gene stimulating factor signaling pathway and initiates an adaptive immune response that kills tumor cells. The present review describes the structure, function and interactions of CD47 with its ligands, as well as its regulation of phagocytosis and tumor cell fate. It summarizes the therapeutics, mechanisms of action, research advances and challenges of targeting CD47. In addition, this paper provides an overview of the latest therapeutic options for targeting CD47, such as chimeric antigen receptor (CAR) T-cells, CAR macrophages and nanotechnology-based delivery systems, which are essential for future clinical research on targeting CD47.

Therapeutic approaches in proteinopathies

A family of maladies known as amyloid disorders, proteinopathy, or amyloidosis, are characterized by the accumulation of abnormal protein aggregates containing cross-β-sheet amyloid fibrils in many organs and tissues. Often, proteins that have been improperly formed or folded make up these fibrils. Nowadays, most treatments for amyloid illness focus on managing symptoms rather than curing or preventing the underlying disease process. However, recent advances in our understanding of the biology of amyloid diseases have led to the development of innovative therapies that target the emergence and accumulation of amyloid fibrils. Examples of these treatments include the use of small compounds, monoclonal antibodies, gene therapy, and others. In the end, even if the majority of therapies for amyloid diseases are symptomatic, greater research into the biology behind these disorders is identifying new targets for potential therapy and paving the way for the development of more effective treatments in the future.

The Role of Glial Cells in Synaptic Dysfunction: Insights into Alzheimer's Disease Mechanisms

Alzheimer's disease (AD) is a devastating neurodegenerative disorder that impacts a substantial number of individuals globally. Despite its widespread prevalence, there is currently no cure for AD. It is widely acknowledged that normal synaptic function holds a key role in memory, cognitive abilities, and the interneuronal transfer of information. As AD advances, symptoms including synaptic impairment, decreased synaptic density, and cognitive decline become increasingly noticeable. The importance of glial cells in the formation of synapses, the growth of neurons, brain maturation, and safeguarding the microenvironment of the central nervous system is well recognized. However, during AD progression, overactive glial cells can cause synaptic dysfunction, neuronal death, and abnormal neuroinflammation. Both neuroinflammation and synaptic dysfunction are present in the early stages of AD. Therefore, focusing on the changes in glia-synapse communication could provide insights into the mechanisms behind AD. In this review, we aim to provide a summary of the role of various glial cells, including microglia, astrocytes, oligodendrocytes, and oligodendrocyte precursor cells, in regulating synaptic dysfunction. This may offer a new perspective on investigating the underlying mechanisms of AD.

Rat primary cortical cell tri-culture to study effects of amyloid-beta on microglia function

Introduction

The etiology and progression of sporadic Alzheimer's Disease (AD) have been studied for decades. One proposed mechanism is that amyloid-beta (Aβ) proteins induce neuroinflammation, synapse loss, and neuronal cell death. Microglia play an especially important role in Aβ clearance, and alterations in microglial function due to aging or disease may result in Aβ accumulation and deleterious effects on neuronal function. However, studying these complex factors in vivo , where numerous confounding processes exist, is challenging, and until recently, in vitro models have not allowed sustained culture of microglia, astrocytes and neurons in the same culture. Here, we employ a tri-culture model of rat primary neurons, astrocytes, and microglia and compare it to co-culture (neurons and astrocytes) and mono-culture enriched for microglia to study microglial function (i.e., motility and Aβ clearance) and proteomic response to exogenous Aβ.

Methods

We established cortical co-culture (neurons and astrocytes), tri-culture (neurons, astrocytes, and microglia), and mono-culture (microglia) from perinatal rat pups. On days in vitro (DIV) 7 - 14, the cultures were exposed to fluorescently-labeled Aβ (FITC-Aβ) particles for varying durations. Images were analyzed to determine the number of FITC-Aβ particles after specific lengths of exposure. A group of cells were stained for βIII-tubulin, GFAP, and Iba1 for morphological analysis via quantitative fluorescence microscopy. Cytokine profiles from conditioned media were obtained. Live-cell imaging with images acquired every 5 minutes for 4 hours was employed to extract microglia motility parameters (e.g., Euclidean distance, migration speed, directionality ratio).

Results and discussion

FITC-Aβ particles were more effectively cleared in the tri-culture compared to the co-culture. This was attributed to microglia engulfing FITC-Aβ particles, as confirmed via epifluorescence and confocal microscopy. Adding FITC-Aβ significantly increased the size of microglia, but had no significant effect on neuronal surface coverage or astrocyte size. Analysis of the cytokine profile upon FITC-Aβ addition revealed a significant increase in proinflammatory cytokines (TNF-α, IL-1α, IL-1β, IL-6) in tri-culture, but not co-culture. In addition, Aβ addition altered microglia motility marked by swarming-like motion with decreased Euclidean distance yet unaltered speed. These results highlight the importance of cell-cell communication in microglia function (e.g., motility and Aβ clearance) and the utility of the tri-culture model to further investigate microglia dysfunction in AD.

Novel histone post-translational modifications in Alzheimer's disease: current advances and implications

Alzheimer's disease (AD) has a complex pathogenesis, and multiple studies have indicated that histone post-translational modifications, especially acetylation, play a significant role in it. With the development of mass spectrometry and proteomics, an increasing number of novel HPTMs, including lactoylation, crotonylation, β-hydroxybutyrylation, 2-hydroxyisobutyrylation, succinylation, and malonylation, have been identified. These novel HPTMs closely link substance metabolism to gene regulation, and an increasing number of relevant studies on the relationship between novel HPTMs and AD have become available. This review summarizes the current advances and implications of novel HPTMs in AD, providing insight into the deeper pathogenesis of AD and the development of novel drugs.

Myeloid cells in vascular dementia and Alzheimer's disease: Possible therapeutic targets?

Growing evidence supports the suggestion that the peripheral immune system plays a role in different pathologies associated with cognitive impairment, such as vascular dementia (VD) or Alzheimer's disease (AD). The aim of this review is to summarize, within the peripheral immune system, the implications of different types of myeloid cells in AD and VD, with a special focus on post-stroke cognitive impairment and dementia (PSCID). We will review the contributions of the myeloid lineage, from peripheral cells (neutrophils, platelets, monocytes and monocyte-derived macrophages) to central nervous system (CNS)-associated cells (perivascular macrophages and microglia). Finally, we will evaluate different potential strategies for pharmacological modulation of pathological processes mediated by myeloid cell subsets, with an emphasis on neutrophils, their interaction with platelets and the process of immunothrombosis that triggers neutrophil-dependent capillary stall and hypoperfusion, as possible effector mechanisms that may pave the way to novel therapeutic avenues to stop dementia, the epidemic of our time. LINKED ARTICLES: This article is part of a themed issue From Alzheimer's Disease to Vascular Dementia: Different Roads Leading to Cognitive Decline. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.6/issuetoc.

Transcriptomic Profiling Reveals Neuroinflammation in the Corpus Callosum of a Transgenic Mouse Model of Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is a widespread neurodegenerative disorder characterized by progressive cognitive decline, affecting a significant portion of the aging population. While the cerebral cortex and hippocampus have been the primary focus of AD research, accumulating evidence suggests that white matter lesions in the brain, particularly in the corpus callosum, play an important role in the pathogenesis of the disease.

OBJECTIVE

This study aims to investigate the gene expression changes in the corpus callosum of 5xFAD transgenic mice, a widely used AD mouse model.

METHODS

We conducted behavioral tests for spatial learning and memory in 5xFAD transgenic mice and performed RNA sequencing analyses on the corpus callosum to examine transcriptomic changes.

RESULTS

Our results show cognitive decline and demyelination in the corpus callosum of 5xFAD transgenic mice. Transcriptomic analysis reveals a predominance of upregulated genes in AD mice, particularly those associated with immune cells, including microglia. Conversely, downregulation of genes related to chaperone function and clock genes such as Per1, Per2, and Cry1 is also observed.

CONCLUSIONS

This study suggests that activation of neuroinflammation, disruption of chaperone function, and circadian dysfunction are involved in the pathogenesis of white matter lesions in AD. The findings provide insights into potential therapeutic targets and highlight the importance of addressing white matter pathology and circadian dysfunction in AD treatment strategies.

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.

Translational profiling identifies sex-specific metabolic and epigenetic reprogramming of cortical microglia/macrophages in APPPS1-21 mice with an antibiotic-perturbed-microbiome

BACKGROUND

Microglia, the brain-resident macrophages perform immune surveillance and engage with pathological processes resulting in phenotype changes necessary for maintaining homeostasis. In preceding studies, we showed that antibiotic-induced perturbations of the gut microbiome of APPPS1-21 mice resulted in significant attenuation in Aβ amyloidosis and altered microglial phenotypes that are specific to male mice. The molecular events underlying microglial phenotypic transitions remain unclear. Here, by generating 'APPPS1-21-CD11br' reporter mice, we investigated the translational state of microglial/macrophage ribosomes during their phenotypic transition and in a sex-specific manner.

METHODS

Six groups of mice that included WT-CD11br, antibiotic (ABX) or vehicle-treated APPPS1-21-CD11br males and females were sacrificed at 7-weeks of age (n = 15/group) and used for immunoprecipitation of microglial/macrophage polysomes from cortical homogenates using anti-FLAG antibody. Liquid chromatography coupled to tandem mass spectrometry and label-free quantification was used to identify newly synthesized peptides isolated from polysomes.

RESULTS

We show that ABX-treatment leads to decreased Aβ levels in male APPPS1-21-CD11br mice with no significant changes in females. We identified microglial/macrophage polypeptides involved in mitochondrial dysfunction and altered calcium signaling that are associated with Aβ-induced oxidative stress. Notably, female mice also showed downregulation of newly-synthesized ribosomal proteins. Furthermore, male mice showed an increase in newly-synthesized polypeptides involved in FcγR-mediated phagocytosis, while females showed an increase in newly-synthesized polypeptides responsible for actin organization associated with microglial activation. Next, we show that ABX-treatment resulted in substantial remodeling of the epigenetic landscape, leading to a metabolic shift that accommodates the increased bioenergetic and biosynthetic demands associated with microglial polarization in a sex-specific manner. While microglia in ABX-treated male mice exhibited a metabolic shift towards a neuroprotective phenotype that promotes Aβ clearance, microglia in ABX-treated female mice exhibited loss of energy homeostasis due to persistent mitochondrial dysfunction and impaired lysosomal clearance that was associated with inflammatory phenotypes.

CONCLUSIONS

Our studies provide the first snapshot of the translational state of microglial/macrophage cells in a mouse model of Aβ amyloidosis that was subject to ABX treatment. ABX-mediated changes resulted in metabolic reprogramming of microglial phenotypes to modulate immune responses and amyloid clearance in a sex-specific manner. This microglial plasticity to support neuro-energetic homeostasis for its function based on sex paves the path for therapeutic modulation of immunometabolism for neurodegeneration.

Prevention of Alzheimer Pathology by Blocking Neuregulin Signaling on Microglia

Plaque formation, microglial activation, and synaptic loss are pathologic hallmarks of Alzheimer's disease; however, removing plaques has had little clinical benefit. Here, we show that neuregulin-1, a glial growth factor, induces inflammatory cytokines and promotes phagocytic activity in vitro and augments microglial activation and plaque formation in 5XFAD Alzheimer's mice. Brain-specific targeting of neuregulin-1 by intraventricular delivery of a novel neuregulin-1 fusion protein antagonist, GlyB4, significantly alters microglial morphology and function to a nonpathogenic morphology in early-stage 5XFAD mice and prevents plaques from forming. Once plaques have already formed, GlyB4 reduces new plaque formation and prevents synaptic loss. Selective, targeted disruption of neuregulin-1 signaling on brain microglia with GlyB4 could be a novel "upstream" approach to slow or stop disease progression in Alzheimer's disease.

Role of IL-34 and its receptors in inflammatory diseases

In recent years, IL-34 has been widely discussed as a novel cytokine. IL-34 is a pro-inflammatory cytokine binding four distinct receptors, namely CSF-1R, syndecan-1, PTP-ζ and TREM2. Previous studies have shown that IL-34 and its receptors play important roles in the development and progression of various inflammatory diseases. Therefore, IL-34 has the potential to be a biomarker and therapeutic target for inflammatory diseases. However, further study is still needed to identify the specific mechanism through which IL-34 contributes to illness. In this article, we review the recent advances in the biological roles of IL-34 and its receptors as well as their roles in the development and therapeutic application of inflammatory diseases.

Identification of methylation-regulated genes modulating microglial phagocytosis in hyperhomocysteinemia-exacerbated Alzheimer's disease

BACKGROUND

Hyperhomocysteinemia (HHcy) has been linked to development of Alzheimer's disease (AD) neuropathologically characterized by the accumulation of amyloid β (Aβ). Microglia (MG) play a crucial role in uptake of Aβ fibrils, and its dysfunction worsens AD. However, the effect of HHcy on MG Aβ phagocytosis remains unstudied.

METHODS

We isolated MG from the cerebrum of HHcy mice with genetic cystathionine-β-synthase deficiency (Cbs-/-) and performed bulk RNA-seq. We performed meta-analysis over transcriptomes of Cbs-/- mouse MG, human and mouse AD MG, MG Aβ phagocytosis model, human AD methylome, and GWAS AD genes.

RESULTS

HHcy and hypomethylation conditions were identified in Cbs-/- mice. Through Cbs-/- MG transcriptome analysis, 353 MG DEGs were identified. Phagosome formation and integrin signaling pathways were found suppressed in Cbs-/- MG. By analyzing MG transcriptomes from 4 AD patient and 7 mouse AD datasets, 409 human and 777 mouse AD MG DEGs were identified, of which 37 were found common in both species. Through further combinatory analysis with transcriptome from MG Aβ phagocytosis model, we identified 130 functional-validated Aβ phagocytic AD MG DEGs (20 in human AD, 110 in mouse AD), which reflected a compensatory activation of Aβ phagocytosis. Interestingly, we identified 14 human Aβ phagocytic AD MG DEGs which represented impaired MG Aβ phagocytosis in human AD. Finally, through a cascade of meta-analysis of transcriptome of AD MG, functional phagocytosis, HHcy MG, and human AD brain methylome dataset, we identified 5 HHcy-suppressed phagocytic AD MG DEGs (Flt1, Calponin 3, Igf1, Cacna2d4, and Celsr) which were reported to regulate MG/MΦ migration and Aβ phagocytosis.

CONCLUSIONS

We established molecular signatures for a compensatory response of Aβ phagocytosis activation in human and mouse AD MG and impaired Aβ phagocytosis in human AD MG. Our discoveries suggested that hypomethylation may modulate HHcy-suppressed MG Aβ phagocytosis in AD.

Hybrid Amyloid Quantum Dot Nanoassemblies to Probe Neuroinflammatory Damage

Various oligomeric species of amyloid-beta have been proposed to play different immunogenic roles in the cellular pathology of Alzheimer's Disease. However, investigating the role of a homogenous single oligomeric species has been difficult due to highly dynamic oligomerization and fibril formation kinetics that convert between many species. Here we report the design and construction of a quantum dot mimetic for larger spherical oligomeric amyloid species as an "endogenously" fluorescent proxy for this cytotoxic species to investigate its role in inducing inflammatory and stress response states in neuronal and glial cell types.

Interrogation of human microglial phagocytosis by CRISPR genome editing

Background

Microglia are an integral part of central nervous system, but our understanding of microglial biology is limited due to the challenges in obtaining and culturing primary human microglia. HMC3 is an important cell line for studying human microglia because it is readily accessible and straightforward to maintain in standard laboratories. Although HMC3 is widely used for microglial research, a robust genetic method has not been described. Here, we report a CRISPR genome editing platform, by the electroporation of Cas9 ribonucleoproteins (Cas9 RNP) and synthetic DNA repair templates, to enable rapid and precise genetic modifications of HMC3. For proof-of-concept demonstrations, we targeted the genes implicated in the regulation of amyloid beta (Aβ) and glioblastoma phagocytosis in microglia. We showed that CRISPR genome editing could enhance the phagocytic activities of HMC3.

Methods

We performed CRISPR gene knockout (KO) in HMC3 by the electroporation of pre-assembled Cas9 RNP. Co-introduction of DNA repair templates allowed site-specific knock-in (KI) of an epitope tag, a synthetic promoter and a fluorescent reporter gene. The editing efficiencies were determined genotypically by DNA sequencing and phenotypically by immunofluorescent staining and flow cytometry. The gene-edited HMC3 cells were examined in vitro by fluorescent Aβ and glioblastoma phagocytosis assays.

Results

Our platform enabled robust single (>90%) and double (>70%) KO without detectable off-target editing by high throughput DNA sequencing. We also inserted a synthetic SFFV promoter to efficiently upregulate the expression of endogenous CD14 and TREM2 genes associated with microglial phagocytosis. The CRISPR-edited HMC3 showed stable phenotypes and enhanced phagocytosis of fluorescence-labeled Aβ1-42 peptides. Confocal microscopy further confirmed the localization of Aβ_1-42 aggregates in the acidified lysosomes. HMC3 mutants also changed the phagocytic characteristic toward apoptotic glioblastoma cells.

Conclusion

CRISPR genome editing by Cas9 RNP electroporation is a robust approach to genetically modify HMC3 for functional studies such as the interrogation of Aβ and tumor phagocytosis, and is readily adoptable to investigate other aspects of microglial biology.

Microglia in Alzheimer's disease: pathogenesis, mechanisms, and therapeutic potentials

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by protein aggregation in the brain. Recent studies have revealed the critical role of microglia in AD pathogenesis. This review provides a comprehensive summary of the current understanding of microglial involvement in AD, focusing on genetic determinants, phenotypic state, phagocytic capacity, neuroinflammatory response, and impact on synaptic plasticity and neuronal regulation. Furthermore, recent developments in drug discovery targeting microglia in AD are reviewed, highlighting potential avenues for therapeutic intervention. This review emphasizes the essential role of microglia in AD and provides insights into potential treatments.

Recent Mechanisms of Neurodegeneration and Photobiomodulation in the Context of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease and the world's primary cause of dementia, a condition characterized by significant progressive declines in memory and intellectual capacities. While dementia is the main symptom of Alzheimer's, the disease presents with many other debilitating symptoms, and currently, there is no known treatment exists to stop its irreversible progression or cure the disease. Photobiomodulation has emerged as a very promising treatment for improving brain function, using light in the range from red to the near-infrared spectrum depending on the application, tissue penetration, and density of the target area. The goal of this comprehensive review is to discuss the most recent achievements in and mechanisms of AD pathogenesis with respect to neurodegeneration. It also provides an overview of the mechanisms of photobiomodulation associated with AD pathology and the benefits of transcranial near-infrared light treatment as a potential therapeutic solution. This review also discusses the older reports and hypotheses associated with the development of AD, as well as some other approved AD drugs.

Metabolic response of microglia to amyloid deposition during Alzheimer's disease progression in a mouse model

Alzheimer's disease (AD) drives metabolic changes in the central nervous system (CNS). In AD microglia are activated and proliferate in response to amyloid β plaques. To further characterize the metabolic changes in microglia associated with plaque deposition in situ, we examined cortical tissue from 2, 4, and 8-month-old wild type and 5XFAD mice, a mouse model of plaque deposition. 5XFAD mice exhibited progressive microgliosis and plaque deposition as well as changes in microglial morphology and neuronal dystrophy. Multiphoton-based fluorescent lifetime imaging microscopy (FLIM) metabolic measurements showed that older mice had an increased amount of free NAD(P)H, indicative of a shift towards glycolysis. Interestingly in 5XFAD mice, we also found an abundant previously undescribed third fluorescence component that suggests an alternate NAD(P)H binding partner associated with pathology. This work demonstrates that FLIM in combination with other quantitative imaging methods, is a promising label-free tool for understanding the mechanisms of AD pathology.

INPP5D modulates TREM2 loss-of-function phenotypes in a β-amyloidosis mouse model

The genetic associations of TREM2 loss-of-function variants with Alzheimer disease (AD) indicate the protective roles of microglia in AD pathogenesis. Functional deficiencies of TREM2 disrupt microglial clustering around amyloid β (Aβ) plaques, impair their transcriptional response to Aβ, and worsen neuritic dystrophy. However, the molecular mechanism underlying these phenotypes remains unclear. In this study, we investigated the pathological role of another AD risk gene, INPP5D, encoding a phosphoinositide PI(3,4,5)P3 phosphatase expressed in microglia. In a Tyrobp-deficient TREM2 loss-of-function mouse model, Inpp5d haplodeficiency restored the association of microglia with Aβ plaques, partially restored plaque compaction, and astrogliosis, and reduced phosphorylated tau+ dystrophic neurites. Mechanistic analyses suggest that TREM2/TYROBP and INPP5D exert opposing effects on PI(3,4,5)P3 signaling pathways as well as on phosphoproteins involved in the actin assembly. Our results suggest that INPP5D acts downstream of TREM2/TYROBP to regulate the microglial barrier against Aβ toxicity, thereby modulates Aβ-dependent pathological conversion of tau.

Retinal pathological features and proteome signatures of Alzheimer's disease

Alzheimer's disease (AD) pathologies were discovered in the accessible neurosensory retina. However, their exact nature and topographical distribution, particularly in the early stages of functional impairment, and how they relate to disease progression in the brain remain largely unknown. To better understand the pathological features of AD in the retina, we conducted an extensive histopathological and biochemical investigation of postmortem retina and brain tissues from 86 human donors. Quantitative examination of superior and inferior temporal retinas from mild cognitive impairment (MCI) and AD patients compared to those with normal cognition (NC) revealed significant increases in amyloid β-protein (Aβ42) forms and novel intraneuronal Aβ oligomers (AβOi), which were closely associated with exacerbated retinal macrogliosis, microgliosis, and tissue atrophy. These pathologies were unevenly distributed across retinal layers and geometrical areas, with the inner layers and peripheral subregions exhibiting most pronounced accumulations in the MCI and AD versus NC retinas. While microgliosis was increased in the retina of these patients, the proportion of microglial cells engaging in Aβ uptake was reduced. Female AD patients exhibited higher levels of retinal microgliosis than males. Notably, retinal Aβ42, S100 calcium-binding protein B+ macrogliosis, and atrophy correlated with severity of brain Aβ pathology, tauopathy, and atrophy, and most retinal pathologies reflected Braak staging. All retinal biomarkers correlated with the cognitive scores, with retinal Aβ42, far-peripheral AβOi and microgliosis displaying the strongest correlations. Proteomic analysis of AD retinas revealed activation of specific inflammatory and neurodegenerative processes and inhibition of oxidative phosphorylation/mitochondrial, and photoreceptor-related pathways. This study identifies and maps retinopathy in MCI and AD patients, demonstrating the quantitative relationship with brain pathology and cognition, and may lead to reliable retinal biomarkers for noninvasive retinal screening and monitoring of AD.

Immune Regulatory Functions of Macrophages and Microglia in Central Nervous System Diseases

Macrophages can be characterized as a very multifunctional cell type with a spectrum of phenotypes and functions being observed spatially and temporally in various disease states. Ample studies have now demonstrated a possible causal link between macrophage activation and the development of autoimmune disorders. How these cells may be contributing to the adaptive immune response and potentially perpetuating the progression of neurodegenerative diseases and neural injuries is not fully understood. Within this review, we hope to illustrate the role that macrophages and microglia play as initiators of adaptive immune response in various CNS diseases by offering evidence of: (1) the types of immune responses and the processes of antigen presentation in each disease, (2) receptors involved in macrophage/microglial phagocytosis of disease-related cell debris or molecules, and, finally, (3) the implications of macrophages/microglia on the pathogenesis of the diseases.

Inflammasome activation under high cholesterol load triggers a protective microglial phenotype while promoting neuronal pyroptosis

BACKGROUND

Persistent inflammatory response in the brain can lead to tissue damage and neurodegeneration. In Alzheimer's disease (AD), there is an aberrant activation of inflammasomes, molecular platforms that drive inflammation through caspase-1-mediated proteolytic cleavage of proinflammatory cytokines and gasdermin D (GSDMD), the executor of pyroptosis. However, the mechanisms underlying the sustained activation of inflammasomes in AD are largely unknown. We have previously shown that high brain cholesterol levels promote amyloid-β (Aβ) accumulation and oxidative stress. Here, we investigate whether these cholesterol-mediated changes may regulate the inflammasome pathway.

METHODS

SIM-A9 microglia and SH-SY5Y neuroblastoma cells were cholesterol-enriched using a water-soluble cholesterol complex. After exposure to lipopolysaccharide (LPS) plus muramyl dipeptide or Aβ, activation of the inflammasome pathway was analyzed by immunofluorescence, ELISA and immunoblotting analysis. Fluorescently-labeled Aβ was employed to monitor changes in microglia phagocytosis. Conditioned medium was used to study how microglia-neuron interrelationship modulates the inflammasome-mediated response.

RESULTS

In activated microglia, cholesterol enrichment promoted the release of encapsulated IL-1β accompanied by a switch to a more neuroprotective phenotype, with increased phagocytic capacity and release of neurotrophic factors. In contrast, in SH-SY5Y cells, high cholesterol levels stimulated inflammasome assembly triggered by both bacterial toxins and Aβ peptides, resulting in GSDMD-mediated pyroptosis. Glutathione (GSH) ethyl ester treatment, which recovered the cholesterol-mediated depletion of mitochondrial GSH levels, significantly reduced the Aβ-induced oxidative stress in the neuronal cells, resulting in lower inflammasome activation and cell death. Furthermore, using conditioned media, we showed that neuronal pyroptosis affects the function of the cholesterol-enriched microglia, lowering its phagocytic activity and, therefore, the ability to degrade extracellular Aβ.

CONCLUSIONS

Changes in intracellular cholesterol levels differentially regulate the inflammasome-mediated immune response in microglia and neuronal cells. Given the microglia-neuron cross-talk in the brain, cholesterol modulation should be considered a potential therapeutic target for AD treatment, which may help to block the aberrant and chronic inflammation observed during the disease progression.

The Pathological Activation of Microglia Is Modulated by Sexually Dimorphic Pathways

Microglia are the primary immunocompetent cells of the central nervous system (CNS). Their ability to survey, assess and respond to perturbations in their local environment is critical in their role of maintaining CNS homeostasis in health and disease. Microglia also have the capability of functioning in a heterogeneous manner depending on the nature of their local cues, as they can become activated on a spectrum from pro-inflammatory neurotoxic responses to anti-inflammatory protective responses. This review seeks to define the developmental and environmental cues that support microglial polarization towards these phenotypes, as well as discuss sexually dimorphic factors that can influence this process. Further, we describe a variety of CNS disorders including autoimmune disease, infection, and cancer that demonstrate disparities in disease severity or diagnosis rates between males and females, and posit that microglial sexual dimorphism underlies these differences. Understanding the mechanism behind differential CNS disease outcomes between men and women is crucial in the development of more effective targeted therapies.

Microglial Piezo1 senses Aβ fibril stiffness to restrict Alzheimer's disease

The pathology of Alzheimer's disease (AD) is featured with extracellular amyloid-β (Aβ) plaques, whose impact on the mechanical properties of the surrounding brain tissues is unclear. Microglia sense and integrate biochemical cues of the microenvironment. However, whether the microglial mechanosensing pathways influence AD pathogenesis is unknown. Here, we surveyed the elevated stiffness of Aβ-plaque-associated tissues and observed the selective upregulation of the mechanosensitive ion channel Piezo1 in Aβ-plaque-associated microglia. Piezo1 sensed the stiffness stimuli of Aβ fibrils and subsequently induced Ca²⁺ influx for microglial clustering, phagocytosis, and compacting of Aβ plaques. Microglia lacking Piezo1 led to the exacerbation of Aβ pathology and cognitive decline, whereas pharmacological activation of microglial Piezo1 ameliorated brain Aβ burden and cognitive impairment in 5 × FAD mice. Together, our results reveal that Piezo1, a mechanosensor of Aβ fibril stiffness in microglia, represents a potential therapeutic target for AD.

Measuring Phagosomal pH by Fluorescence Microscopy

Dual-wavelength and dual-fluorophore ratiometric imaging has become a powerful tool for the study of pH in intracellular compartments. It allows for the dynamic imaging of live cells while accounting for changes in the focal plane, differential loading of the fluorescent probe, and photobleaching caused by repeated image acquisitions. Ratiometric microscopic imaging has the added advantage over whole-population methods of being able to resolve individual cells and even individual organelles. In this chapter, we provide a detailed discussion of the basic principles of ratiometric imaging and its application to the measurement of phagosomal pH, including probe selection, the necessary instrumentation, and calibration methods.

Pathological Roles of INPP5D in Alzheimer's Disease

Current hypothesis of Alzheimer's disease (AD) postulates that amyloid β (Aβ) deposition in the brain causes tau inclusion in neurons and leads to cognitive decline. The discovery of the genetic association between triggering receptor expressed on myeloid cells 2 (TREM2) with increased AD risk points to a causal link between microglia and AD pathogenesis, and revealed a crucial role of TREM2-dependent clustering of microglia around amyloid plaques that prevents Aβ toxicity to facilitate tau deposition near the plaques. Here we review the physiological and pathological roles of another AD risk gene expressed in microglia, inositol polyphosphate-5-polyphosphatase D (INPP5D), which encodes a phosphoinositide phosphatase. Evidence suggests that its risk polymorphisms alter the expression level and/or function of INPP5D, while concomitantly affecting tau levels in cerebrospinal fluids. In β-amyloidosis mice, INPP5D was upregulated upon Aβ deposition and negatively regulated the microglial clustering toward amyloid plaques. INPP5D seems to exert its function by acting antagonistically at downstream of the TREM2 signaling pathway, suggesting that it is a novel regulator of the protective barrier by microglia. Further studies to elucidate INPP5D's role in AD may help in developing new therapeutic targets for AD treatment.

Src family kinases (SFKs): critical regulators of microglial homeostatic functions and neurodegeneration in Parkinson's and Alzheimer's diseases

c-Src was the first protein kinase to be described as capable of phosphorylating tyrosine residues. Subsequent identification of other tyrosine-phosphorylating protein kinases with a similar structure to c-Src gave rise to the concept of Src family kinases (SFKs). Microglia are the resident innate immune cell population of the CNS. Under physiological conditions, microglia actively participate in brain tissue homeostasis, continuously patrolling the neuronal parenchyma and exerting neuroprotective actions. Activation of pathogen-associated molecular pattern (PAMP) and damage-associated molecular pattern (DAMP) receptors induces microglial proliferation, migration toward pathological foci, phagocytosis, and changes in gene expression, concurrent with the secretion of cytokines, chemokines, and growth factors. A significant body of literature shows that SFK stimulation positively associates with microglial activation and neuropathological conditions, including Alzheimer's and Parkinson's diseases. Here, we review essential microglial homeostatic functions regulated by SFKs, including phagocytosis, environmental sensing, and secretion of inflammatory mediators. In addition, we discuss the potential of SFK modulation for microglial homeostasis in Parkinson's and Alzheimer's diseases.

Synergistic regulation of microglia differentiation by CD93 and integrin β1 in the rat pneumococcal meningitis model

Streptococcus pneumoniae is the main bacterial pathogen of meningitis worldwide, which has a high mortality rate and survivors are prone to central nervous system (CNS) sequelae. In this regard, microglia activation has been associated with injury to the CNS. The aim of this study was to investigate the relationship between CD93, integrin β1, and microglia activation. In the rat pneumococcal meningitis model, we found significant increases of CD93 and integrin β1 expression and differentiation of M1 phenotype microglia. Furthermore, we showed in vitro siRNA-mediated downregulation of CD93 and integrin β1 expression after infecting highly aggressive proliferating immortalized (HAPI) microglia cells with S. pneumoniae. We observed differentiation of S. pneumonia-infected HAPI microglia cells to the M1 phenotype and significant release of soluble CD93 (sCD93) and integrin β1 expression. Complement C1q and metalloproteinases promoted sCD93 release. We also showed that downregulation of CD93 significantly reduced differentiation to M1 microglia and increased differentiation to M2 microglia. However, addition of recombinant CD93 may regulate microglia differentiation to the M1 phenotype. Furthermore, the downregulation of integrin β1 resulted in downregulation of the CD93 protein. In conclusion, interaction between integrin β1 and CD93 promotes differentiation of microglia to the M1 phenotype, increases the release of pro-inflammatory factors, and leads to nervous system injury in pneumococcal meningitis.

Microglia dynamics in aging-related neurobehavioral and neuroinflammatory diseases

Microglia represent the first line of immune feedback in the brain. Beyond immune surveillance, they are essential for maintaining brain homeostasis. Recent research has revealed the microglial cells' spatiotemporal heterogeneity based on their local and time-based functions in brain trauma or disease when homeostasis is disrupted. Distinct "microglial signatures" have been recorded in physiological states and brain injuries, with discrete or sometimes overlapping pro- and anti-inflammatory functions. Microglia are involved in the neurological repair processes, such as neurovascular unit restoration and synaptic plasticity, and manage the extent of the damage due to their phenotype switching. The versatility of cellular phenotypes beyond the classical M1/M2 classification, as well as the double-edge actions of microglia in neurodegeneration, indicate the need for further exploration of microglial cell dynamics and their contribution to neurodegenerative processes. This review discusses the homeostatic functions of different microglial subsets focusing on neuropathological conditions. Also, we address the feasibility of targeting microglia as a therapeutic strategy in neurodegenerative diseases.

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

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

Selecting Multitarget Peptides for Alzheimer's Disease

Alzheimer's disease (AD) is a multifactorial disease with a complex pathogenesis. Developing multitarget drugs could be a powerful strategy to impact the progressive loss of cognitive functions in this disease. The purpose of this study is to select a multitarget lead peptide candidate among a series of peptide variants derived from the neutrophil granule protein cathepsin G. We screened eight peptide candidates using the following criteria: (1) Inhibition and reversion of amyloid beta (Aβ) oligomers, quantified using an enzyme-linked immunosorbent assay (ELISA); (2) direct binding of peptide candidates to the human receptor for advanced glycation end-products (RAGE), the Toll-like receptor 4 (TLR4) and the S100 calcium-binding protein A9 (S100A9), quantified by ELISA; (3) protection against Aβ oligomer-induced neuronal cell death, using trypan blue to measure cell death in a murine neuronal cell line; (4) inhibition of TLR4 activation by S100A9, using a human TLR4 reporter cell line. We selected a 27-mer lead peptide that fulfilled these four criteria. This lead peptide is a privileged structure that displays inherent multitarget activity. This peptide is expected to significantly impact cognitive decline in mouse models of Alzheimer's disease, by targeting both neuroinflammation and neurodegeneration.

Aβ plaques do not protect against HSV-1 infection in a mouse model of familial Alzheimer's disease, and HSV-1 does not induce Aβ pathology in a model of late onset Alzheimer's disease

The possibility that the etiology of late onset Alzheimer's disease is linked to viral infections of the CNS has been actively debated in recent years. According to the antiviral protection hypothesis, viral pathogens trigger aggregation of Aβ peptides that are produced as a defense mechanism in response to infection to entrap and neutralize pathogens. To test the causative relationship between viral infection and Aβ aggregation, the current study examined whether Aβ plaques protect the mouse brain against Herpes Simplex Virus 1 (HSV-1) infection introduced via a physiological route and whether HSV-1 infection triggers formation of Aβ plaques in a mouse model of late-onset AD that does not develop Aβ pathology spontaneously. In aged 5XFAD mice infected via eye scarification, high density of Aβ aggregates did not improve survival time or rate when compared with wild type controls. In 5XFADs, viral replication sites were found in brain areas with a high density of extracellular Aβ deposits, however, no association between HSV-1 and Aβ aggregates could be found. To test whether HSV-1 triggers Aβ aggregation in a mouse model that lacks spontaneous Aβ pathology, 13-month-old hAβ/APOE4/Trem2*R47H mice were infected with HSV-1 via eye scarification with the McKrae HSV-1 strain, intracranial inoculation with McKrae, intracranial inoculation after priming with LPS for 6 weeks, or intracranial inoculation with high doses of McKrae or 17syn + strains that represent different degrees of neurovirulence. No signs of Aβ aggregation were found in any of the experimental groups. Instead, extensive infiltration of peripheral leukocytes was observed during the acute stage of HSV-1 infection, and phagocytic activity of myeloid cells was identified as the primary defense mechanism against HSV-1. The current results argue against a direct causative relationship between HSV-1 infection and Aβ pathology.

Coagulansin-A improves spatial memory in 5xFAD Tg mice by targeting Nrf-2/NF-κB and Bcl-2 pathway

The present study was designed to investigate the underlying molecular signaling of Coagulansin-A (Coag-A) as a therapeutic agent against Alzheimer's disease (AD). Preliminarily, it exhibited a neuroprotective effect against H₂O₂-induced oxidative stress in HT-22 cells. The in vivo studies were performed by administering Coag-A (0.1, 1, and 10 mg/kg) intraperitoneally to 5xFAD transgenic (Tg) mouse model. Coag-A (10 mg/kg) significantly attenuated the cognitive decline compared to Tg mice group in the shallow water maze (SWM) and Y-maze test paradigms. The anti-aggregation potential of Coag-A was determined by performing Fourier transform-infrared (FT-IR) spectroscopy and differential scanning calorimeter (DSC) analysis in the prefrontal cortex (PFC) and hippocampal (HC) regions of mice brain. The FT-IR spectra demonstrated the inhibition of amyloid beta (Aβ) through a decrease in β-sheet aggregation, along with the inhibition of changes in the lipids, proteins, and phospholipids. The DSC analysis displayed a low-temperature exotherm associated with the reversible process of aggregation of soluble protein fractions prior to denaturation. Furthermore, Coag-A treatment displayed a regular density of granule cells in H&E stained sections, along with a reduced amyloid load and PAS-positive granules in all the regions of interest in mice brain. The real-time polymerase chain reaction (q-PCR), western blot and immunohistochemical (IHC) analysis demonstrated antioxidant, anti-inflammatory, and anti-apoptotic effect of Coag-A by enhancing the expression of nuclear factor erythroid-2-related factor (Nrf-2) and reducing nuclear factor kappa B (NF-κB) and Bax protein expression. In addition, Coag-A significantly increased the antioxidant enzymes and proteins level, along with a reduced pro-inflammatory cytokines production.

Amyloid precursor protein (APP) and amyloid β (Aβ) interact with cell adhesion molecules: Implications in Alzheimer’s disease and normal physiology

Alzheimer’s disease (AD) is an incurable neurodegenerative disorder in which dysfunction and loss of synapses and neurons lead to cognitive impairment and death. Accumulation and aggregation of neurotoxic amyloid-β (Aβ) peptides generated via amyloidogenic processing of amyloid precursor protein (APP) is considered to play a central role in the disease etiology. APP interacts with cell adhesion molecules, which influence the normal physiological functions of APP, its amyloidogenic and non-amyloidogenic processing, and formation of Aβ aggregates. These cell surface glycoproteins also mediate attachment of Aβ to the neuronal cell surface and induce intracellular signaling contributing to Aβ toxicity. In this review, we discuss the current knowledge surrounding the interactions of cell adhesion molecules with APP and Aβ and analyze the evidence of the critical role these proteins play in regulating the processing and physiological function of APP as well as Aβ toxicity. This is a necessary piece of the complex AD puzzle, which we should understand in order to develop safe and effective therapeutic interventions for AD.

A Dichotomous Role for FABP7 in Sleep and Alzheimer's Disease Pathogenesis: A Hypothesis

Fatty acid binding proteins (FABPs) are a family of intracellular lipid chaperone proteins known to play critical roles in the regulation of fatty acid uptake and transport as well as gene expression. Brain-type fatty acid binding protein (FABP7) is enriched in astrocytes and has been implicated in sleep/wake regulation and neurodegenerative diseases; however, the precise mechanisms underlying the role of FABP7 in these biological processes remain unclear. FABP7 binds to both arachidonic acid (AA) and docosahexaenoic acid (DHA), resulting in discrete physiological responses. Here, we propose a dichotomous role for FABP7 in which ligand type determines the subcellular translocation of fatty acids, either promoting wakefulness aligned with Alzheimer's pathogenesis or promoting sleep with concomitant activation of anti-inflammatory pathways and neuroprotection. We hypothesize that FABP7-mediated translocation of AA to the endoplasmic reticulum of astrocytes increases astrogliosis, impedes glutamatergic uptake, and enhances wakefulness and inflammatory pathways via COX-2 dependent generation of pro-inflammatory prostaglandins. Conversely, we propose that FABP7-mediated translocation of DHA to the nucleus stabilizes astrocyte-neuron lactate shuttle dynamics, preserves glutamatergic uptake, and promotes sleep by activating anti-inflammatory pathways through the peroxisome proliferator-activated receptor-γ transcriptional cascade. Importantly, this model generates several testable hypotheses applicable to other neurodegenerative diseases, including amyotrophic lateral sclerosis and Parkinson's disease.

LILRB2-mediated TREM2 signaling inhibition suppresses microglia functions

Background

Microglia plays crucial roles in Alzheimer’s disease (AD) development. Triggering receptor expressed on myeloid cells 2 (TREM2) in association with DAP12 mediates signaling affecting microglia function. Here we study the negative regulation of TREM2 functions by leukocyte immunoglobulin-like receptor subfamily B member 2 (LILRB2), an inhibitory receptor bearing ITIM motifs.

Methods

To specifically interrogate LILRB2-ligand (oAβ and PS) interactions and microglia functions, we generated potent antagonistic LILRB2 antibodies with sub-nanomolar level activities. The biological effects of LILRB2 antagonist antibody (Ab29) were studied in human induced pluripotent stem cell (iPSC)–derived microglia (hMGLs) for migration, oAβ phagocytosis, and upregulation of inflammatory cytokines. Effects of the LILRB2 antagonist antibody on microglial responses to amyloid plaques were further studied in vivo using stereotaxic grafted microglia in 5XFAD mice.

Results

We confirmed the expression of both LILRB2 and TREM2 in human brain microglia using immunofluorescence. Upon co-ligation of the LILRB2 and TREM2 by shared ligands oAβ or PS, TREM2 signaling was significantly inhibited. We identified a monoclonal antibody (Ab29) that blocks LILRB2/ligand interactions and prevents TREM2 signaling inhibition mediated by LILRB2. Further, Ab29 enhanced microglia phagocytosis, TREM2 signaling, migration, and cytokine responses to the oAβ-lipoprotein complex in hMGL and microglia cell line HMC3. In vivo studies showed significantly enhanced clustering of microglia around plaques with a prominent increase in microglial amyloid plaque phagocytosis when 5XFAD mice were treated with Ab29.

Conclusions

This study revealed for the first time the molecular mechanisms of LILRB2-mediated inhibition of TREM2 signaling in microglia and demonstrated a novel approach of enhancing TREM2-mediated microglia functions by blocking LILRB2-ligand interactions. Translationally, a LILRB2 antagonist antibody completely rescued the inhibition of TREM2 signaling by LILRB2, suggesting a novel therapeutic strategy for improving microglial functions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-022-00550-y.

Multifunctional Anti-Alzheimer’s Disease Effects of Natural Xanthone Derivatives: A Primary Structure-Activity Evaluation

Background: A series of α-Mangostin (α-M) derivatives were designed and synthesized. α-M and four analogues were evaluated for their multifunctional anti-Alzheimer’s disease (anti-AD) effects on fibrillogenesis, microglial uptake, microglial degradation, and anti-neurotoxicity of Aβ, as well as LPS-induced neuroinflammation. The differences in bioactivities were analyzed to understand the structure-activity relationship for further modifications.

Purpose: This study aims to investigate the anti-AD effects of α-M and elucidate its structure-activity relationship by comparing difference between α-M and several analogues.

Methods: Aβ fibrillogenesis was detected by Thioflavin T fluorometric assay. The levels of Aβ_1-42 and inflammatory cytokines were evaluated by enzyme-linked immunosorbent assay. Neuron viability was examined by the CCK-8 assay. The morphology of ZO-1 of bEnd.3 cultured in BV-2-conditioned medium was evaluated by immunofluorescence staining.

Results: Aβ fibrillogenesis was significantly inhibited by co-incubation with α-M, Zcbd-2 or Zcbd-3. α-M, Zcbd-2, Zcbd-3, and Zcbd-4 decreased the levels of Aβ_1-42 and inflammatory cytokines, and promoted Aβ uptake, degradation and anti-inflammation effects inflammation in microglia. α-M and Zcbd-3 protected neuron viability from Aβ-induced neurotoxicity, and preserved tight junction integrity of bEnd.3 against LPS-induced neuroinflammation.

Conclusion: Zcbd-3 acted as α-M almost in all effects. The structure-activity analysis indicated that the 3-methyl-2-butenyl group at C-8 is essential for the bioactivity of α-M, while modifying the double hydroxylation at the C-2 position may improve the multifunctional anti-AD effects.

Feature Fusion and Detection in Alzheimer's Disease Using a Novel Genetic Multi-Kernel SVM Based on MRI Imaging and Gene Data

Voxel-based morphometry provides an opportunity to study Alzheimer's disease (AD) at a subtle level. Therefore, identifying the important brain voxels that can classify AD, early mild cognitive impairment (EMCI) and healthy control (HC) and studying the role of these voxels in AD will be crucial to improve our understanding of the neurobiological mechanism of AD. Combining magnetic resonance imaging (MRI) imaging and gene information, we proposed a novel feature construction method and a novel genetic multi-kernel support vector machine (SVM) method to mine important features for AD detection. Specifically, to amplify the differences among AD, EMCI and HC groups, we used the eigenvalues of the top 24 Single Nucleotide Polymorphisms (SNPs) in a p-value matrix of 24 genes associated with AD for feature construction. Furthermore, a genetic multi-kernel SVM was established with the resulting features. The genetic algorithm was used to detect the optimal weights of 3 kernels and the multi-kernel SVM was used after training to explore the significant features. By analyzing the significance of the features, we identified some brain regions affected by AD, such as the right superior frontal gyrus, right inferior temporal gyrus and right superior temporal gyrus. The findings proved the good performance and generalization of the proposed model. Particularly, significant susceptibility genes associated with AD were identified, such as CSMD1, RBFOX1, PTPRD, CDH13 and WWOX. Some significant pathways were further explored, such as the calcium signaling pathway (corrected p-value = 1.35 × 10-6) and cell adhesion molecules (corrected p-value = 5.44 × 10-4). The findings offer new candidate abnormal brain features and demonstrate the contribution of these features to AD.

The niacin receptor HCAR2 modulates microglial response and limits disease progression in a mouse model of Alzheimer's disease

Increased dietary intake of niacin has been correlated with reduced risk of Alzheimer's disease (AD). Niacin serves as a high-affinity ligand for the receptor HCAR2 (GPR109A). In the brain, HCAR2 is expressed selectively by microglia and is robustly induced by amyloid pathology in AD. The genetic inactivation of Hcar2 in 5xFAD mice, a model of AD, results in impairment of the microglial response to amyloid deposition, including deficits in gene expression, proliferation, envelopment of amyloid plaques, and uptake of amyloid-β (Aβ), ultimately leading to exacerbation of amyloid burden, neuronal loss, and cognitive deficits. In contrast, activation of HCAR2 with an FDA-approved formulation of niacin (Niaspan) in 5xFAD mice leads to reduced plaque burden and neuronal dystrophy, attenuation of neuronal loss, and rescue of working memory deficits. These data provide direct evidence that HCAR2 is required for an efficient and neuroprotective response of microglia to amyloid pathology. Administration of Niaspan potentiates the HCAR2-mediated microglial protective response and consequently attenuates amyloid-induced pathology, suggesting that its use may be a promising therapeutic approach to AD that specifically targets the neuroimmune response.

Astrocyte-derived TNF and glutamate critically modulate microglia activation by methamphetamine

Methamphetamine (Meth) is a powerful illicit psychostimulant, widely used for recreational purposes. Besides disrupting the monoaminergic system and promoting oxidative brain damage, Meth also causes neuroinflammation, contributing to synaptic dysfunction and behavioral deficits. Aberrant activation of microglia, the largest myeloid cell population in the brain, is a common feature in neurological disorders triggered by neuroinflammation. In this study, we investigated the mechanisms underlying the aberrant activation of microglia elicited by Meth in the adult mouse brain. We found that binge Meth exposure caused microgliosis and disrupted risk assessment behavior (a feature that usually occurs in individuals who abuse Meth), both of which required astrocyte-to-microglia crosstalk. Mechanistically, Meth triggered a detrimental increase of glutamate exocytosis from astrocytes (in a process dependent on TNF production and calcium mobilization), promoting microglial expansion and reactivity. Ablating TNF production, or suppressing astrocytic calcium mobilization, prevented Meth-elicited microglia reactivity and re-established risk assessment behavior as tested by elevated plus maze (EPM). Overall, our data indicate that glial crosstalk is critical to relay alterations caused by acute Meth exposure.

Dietary Regulation of Gut-Brain Axis in Alzheimer's Disease: Importance of Microbiota Metabolites

Alzheimer's disease (AD) is a neurodegenerative disease that impacts 45 million people worldwide and is ranked as the 6th top cause of death among all adults by the Centers for Disease Control and Prevention. While genetics is an important risk factor for the development of AD, environment and lifestyle are also contributing risk factors. One such environmental factor is diet, which has emerged as a key influencer of AD development/progression as well as cognition. Diets containing large quantities of saturated/trans-fats, refined carbohydrates, limited intake of fiber, and alcohol are associated with cognitive dysfunction while conversely diets low in saturated/trans-fats (i.e., bad fats), high mono/polyunsaturated fats (i.e., good fats), high in fiber and polyphenols are associated with better cognitive function and memory in both humans and animal models. Mechanistically, this could be the direct consequence of dietary components (lipids, vitamins, polyphenols) on the brain, but other mechanisms are also likely to be important. Diet is considered to be the single greatest factor influencing the intestinal microbiome. Diet robustly influences the types and function of micro-organisms (called microbiota) that reside in the gastrointestinal tract. Availability of different types of nutrients (from the diet) will favor or disfavor the abundance and function of certain groups of microbiota. Microbiota are highly metabolically active and produce many metabolites and other factors that can affect the brain including cognition and the development and clinical progression of AD. This review summarizes data to support a model in which microbiota metabolites influence brain function and AD.