Plaque-dependent morphological and electrophysiological heterogeneity of microglia in an Alzheimer's disease mouse model

The dichotomous activities of microglia: A potential driver for phenotypic heterogeneity in Alzheimer's disease

Alzheimer's disease (AD) is a leading cause of dementia, characterized by two defining neuropathological hallmarks: amyloid plaques composed of Aβ aggregates and neurofibrillary pathology. Recent research suggests that microglia have both beneficial and detrimental effects in the development of AD. A new theory proposes that microglia play a beneficial role in the early stages of the disease but become harmful in later stages. Further investigations are needed to gain a comprehensive understanding of this shift in microglia's function. This transition is likely influenced by specific conditions, including spatial, temporal, and transcriptional factors, which ultimately lead to the deterioration of microglial functionality. Additionally, recent studies have also highlighted the potential influence of microglia diversity on the various manifestations of AD. By deciphering the multiple states of microglia and the phenotypic heterogeneity in AD, significant progress can be made towards personalized medicine and better treatment outcomes for individuals affected by AD.

Sporoderm-removed Ganoderma lucidum spores ameliorated early depression-like behavior in a rat model of sporadic Alzheimer's disease

Introduction: Ganoderma lucidum: (G. lucidum, Lingzhi) is a medicinal and edible homologous traditional Chinese medicine that is used to treat various diseases, including Alzheimer's disease and mood disorders. We previously reported that the sporoderm-removed G. lucidum spore extract (RGLS) prevented learning and memory impairments in a rat model of sporadic Alzheimer's disease (sAD), but the effect of RGLS on depression-like behaviors in this model and its underlying molecular mechanisms of action remain unclear. Method: The present study investigated protective effects of RGLS against intracerebroventricular streptozotocin (ICV-STZ)-induced depression in a rat model of sAD and its underlying mechanism. Effects of RGLS on depression- and anxiety-like behaviors in ICV-STZ rats were assessed in the forced swim test, sucrose preference test, novelty-suppressed feeding test, and open field test. Results: Behavioral tests demonstrated that RGLS (360 and 720 mg/kg) significantly ameliorated ICV-STZ-induced depression- and anxiety-like behaviors. Immunofluorescence, Western blot and enzyme-linked immunosorbent assay results further demonstrated that ICV-STZ rats exhibited microglia activation and neuroinflammatory response in the medial prefrontal cortex (mPFC), and RGLS treatment reversed these changes, reflected by the normalization of morphological changes in microglia and the expression of NF-κB, NLRP3, ASC, caspase-1 and proinflammatory cytokines. Golgi staining revealed that treatment with RGLS increased the density of mushroom spines in neurons. This increase was associated with elevated expression of brain-derived neurotrophic protein in the mPFC. Discussion: In a rat model of ICV-STZ-induced sAD, RGLS exhibits antidepressant-like effects, the mechanism of which may be related to suppression of the inflammatory response modulated by the NF-κB/NLRP3 pathway and enhancement of synaptic plasticity in the mPFC.

Clearance of β-amyloid and synapses by the optogenetic depolarization of microglia is complement selective

Microglia actively monitor the neighboring brain microenvironments and constantly contact synapses with their unique ramified processes. In neurodegenerative diseases, including Alzheimer's disease (AD), microglia undergo morphological and functional alterations. Whether the direct manipulation of microglia can selectively or concurrently modulate synaptic function and the response to disease-associated factors remains elusive. Here, we employ optogenetic methods to stimulate microglia in vitro and in vivo. Membrane depolarization rapidly changes microglia morphology and leads to enhanced phagocytosis. We found that the optogenetic stimulation of microglia can efficiently promote β-amyloid (Aβ) clearance in the brain parenchyma, but it can also enhance synapse elimination. Importantly, the inhibition of C1q selectively prevents synapse loss induced by microglia depolarization but does not affect Aβ clearance. Our data reveal independent microglia-mediated phagocytosis pathways toward Aβ and synapses. Our results also shed light on a synergistic strategy of depolarizing microglia and inhibiting complement functions for the clearance of Aβ while sparing synapses.

Early amyloid-induced changes in microglia gene expression in male APP/PS1 mice

Alzheimer's disease (AD) is a progressive neurodegenerative disease and the most common cause of dementia, characterized by deposition of extracellular amyloid-beta (Aβ) aggregates and intraneuronal hyperphosphorylated Tau. Many AD risk genes, identified in genome-wide association studies (GWAS), are expressed in microglia, the innate immune cells of the central nervous system. Specific subtypes of microglia emerged in relation to AD pathology, such as disease-associated microglia (DAMs), which increased in number with age in amyloid mouse models and in human AD cases. However, the initial transcriptional changes in these microglia in response to amyloid are still unknown. Here, to determine early changes in microglia gene expression, hippocampal microglia from male APPswe/PS1dE9 (APP/PS1) mice and wild-type littermates were isolated and analyzed by RNA sequencing (RNA-seq). By bulk RNA-seq, transcriptomic changes were detected in hippocampal microglia from 6-months-old APP/PS1 mice. By performing single-cell RNA-seq of CD11c-positive and negative microglia from 6-months-old APP/PS1 mice and analysis of the transcriptional trajectory from homeostatic to CD11c-positive microglia, we identified a set of genes that potentially reflect the initial response of microglia to Aβ.

Exercise mimetics: a novel strategy to combat neuroinflammation and Alzheimer's disease

Neuroinflammation is a pathological hallmark of Alzheimer's disease (AD), characterized by the stimulation of resident immune cells of the brain and the penetration of peripheral immune cells. These inflammatory processes facilitate the deposition of amyloid-beta (Aβ) plaques and the abnormal hyperphosphorylation of tau protein. Managing neuroinflammation to restore immune homeostasis and decrease neuronal damage is a therapeutic approach for AD. One way to achieve this is through exercise, which can improve brain function and protect against neuroinflammation, oxidative stress, and synaptic dysfunction in AD models. The neuroprotective impact of exercise is regulated by various molecular factors that can be activated in the same way as exercise by the administration of their mimetics. Recent evidence has proven some exercise mimetics effective in alleviating neuroinflammation and AD, and, additionally, they are a helpful alternative option for patients who are unable to perform regular physical exercise to manage neurodegenerative disorders. This review focuses on the current state of knowledge on exercise mimetics, including their efficacy, regulatory mechanisms, progress, challenges, limitations, and future guidance for their application in AD therapy.

Amyloid plaques and normal ageing have differential effects on microglial Ca2+ activity in the mouse brain

In microglia, changes in intracellular calcium concentration ([Ca2+]i) may regulate process motility, inflammasome activation, and phagocytosis. However, while neurons and astrocytes exhibit frequent spontaneous Ca2+ activity, microglial Ca2+ signals are much rarer and poorly understood. Here, we studied [Ca2+]i changes of microglia in acute brain slices using Fluo-4-loaded cells and mice expressing GCaMP5g in microglia. Spontaneous Ca2+ transients occurred ~ 5 times more frequently in individual microglial processes than in their somata. We assessed whether microglial Ca2+ responses change in Alzheimer's disease (AD) using AppNL-G-F knock-in mice. Proximity to Aβ plaques strongly affected microglial Ca2+ activity. Although spontaneous Ca2+ transients were unaffected in microglial processes, they were fivefold more frequent in microglial somata near Aβ plaques than in wild-type microglia. Microglia away from Aβ plaques in AD mice showed intermediate properties for morphology and Ca2+ responses, partly resembling those of wild-type microglia. By contrast, somatic Ca2+ responses evoked by tissue damage were less intense in microglia near Aβ plaques than in wild-type microglia, suggesting different mechanisms underlying spontaneous vs. damage-evoked Ca2+ signals. Finally, as similar processes occur in neurodegeneration and old age, we studied whether ageing affected microglial [Ca2+]i. Somatic damage-evoked Ca2+ responses were greatly reduced in microglia from old mice, as in the AD mice. In contrast to AD, however, old age did not alter the occurrence of spontaneous Ca2+ signals in microglial somata but reduced the rate of events in processes. Thus, we demonstrate distinct compartmentalised Ca2+ activity in microglia from healthy, aged and AD-like brains.

Modeling the neuroimmune system in Alzheimer's and Parkinson's diseases

Parkinson's disease (PD) and Alzheimer's disease (AD) are neurodegenerative disorders caused by the interaction of genetic, environmental, and familial factors. These diseases have distinct pathologies and symptoms that are linked to specific cell populations in the brain. Notably, the immune system has been implicated in both diseases, with a particular focus on the dysfunction of microglia, the brain's resident immune cells, contributing to neuronal loss and exacerbating symptoms. Researchers use models of the neuroimmune system to gain a deeper understanding of the physiological and biological aspects of these neurodegenerative diseases and how they progress. Several in vitro and in vivo models, including 2D cultures and animal models, have been utilized. Recently, advancements have been made in optimizing these existing models and developing 3D models and organ-on-a-chip systems, holding tremendous promise in accurately mimicking the intricate intracellular environment. As a result, these models represent a crucial breakthrough in the transformation of current treatments for PD and AD by offering potential for conducting long-term disease-based modeling for therapeutic testing, reducing reliance on animal models, and significantly improving cell viability compared to conventional 2D models. The application of 3D and organ-on-a-chip models in neurodegenerative disease research marks a prosperous step forward, providing a more realistic representation of the complex interactions within the neuroimmune system. Ultimately, these refined models of the neuroimmune system aim to aid in the quest to combat and mitigate the impact of debilitating neuroimmune diseases on patients and their families.

Morphometric Analyses of Macrophages

Cell morphology and motility drive the cellular capabilities to interact with the environment. For example, microglia, the longest known tissue-resident macrophages, show a highly branched process tree with which they continuously scan their environment. Computational image analysis allows to quantify morphology and/or motility from images of tissue-resident macrophages. Here, I describe a step-by-step protocol for analyzing the morphology (and motility) of macrophages with our recently described, freely available software MotiQ, which provides a broad band of parameters and thereby serves as a versatile tool for studies of morphology and motility.

Neuroprotection by upregulation of the major histocompatibility complex class I (MHC I) in SOD1G93A mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that progressively affects motoneurons, causing muscle atrophy and evolving to death. Astrocytes inhibit the expression of MHC-I by neurons, contributing to a degenerative outcome. The present study verified the influence of interferon β (IFN β) treatment, a proinflammatory cytokine that upregulates MHC-I expression, in SOD1G93A transgenic mice. For that, 17 days old presymptomatic female mice were subjected to subcutaneous application of IFN β (250, 1,000, and 10,000 IU) every other day for 20 days. Rotarod motor test, clinical score, and body weight assessment were conducted every third day throughout the treatment period. No significant intergroup variations were observed in such parameters during the pre-symptomatic phase. All mice were then euthanized, and the spinal cords collected for comparative analysis of motoneuron survival, reactive gliosis, synapse coverage, microglia morphology classification, cytokine analysis by flow cytometry, and RT-qPCR quantification of gene transcripts. Additionally, mice underwent Rotarod motor assessment, weight monitoring, and neurological scoring. The results show that IFN β treatment led to an increase in the expression of MHC-I, which, even at the lowest dose (250 IU), resulted in a significant increase in neuronal survival in the ALS presymptomatic period which lasted until the onset of the disease. The treatment also influenced synaptic preservation by decreasing excitatory inputs and upregulating the expression of AMPA receptors by astrocytes. Microglial reactivity quantified by the integrated density of pixels did not decrease with treatment but showed a less activated morphology, coupled with polarization to an M1 profile. Disease progression upregulated gene transcripts for pro- and anti-inflammatory cytokines, and IFN β treatment significantly decreased mRNA expression for IL4. Overall, the present results demonstrate that a low dosage of IFN β shows therapeutic potential by increasing MHC-I expression, resulting in neuroprotection and immunomodulation.

The role of signaling crosstalk of microglia in hippocampus on progression of ageing and Alzheimer's disease

Based on single-cell sequencing of the hippocampi of 5× familiar Alzheimer's disease (5× FAD) and wild type mice at 2-, 12-, and 24-month of age, we found an increased percentage of microglia in aging and Alzheimer's disease (AD) mice. Blood brain barrier injury may also have contributed to this increase. Immune regulation by microglia plays a major role in the progression of aging and AD, according to the functions of 41 intersecting differentially expressed genes in microglia. Signaling crosstalk between C-C motif chemokine ligand (CCL) and major histocompatibility complex-1 bridges intercellular communication in the hippocampus during aging and AD. The amyloid precursor protein (APP) and colony stimulating factor (CSF) signals drive 5× FAD to deviate from aging track to AD occurrence among intercellular communication in hippocampus. Microglia are involved in the progression of aging and AD can be divided into 10 functional types. The strength of the interaction among microglial subtypes weakened with aging, and the CCL and CSF signaling pathways were the fundamental bridge of communication among microglial subtypes.

New insights into hypothalamic neurogenesis disruption after acute and intense stress: implications for microglia and inflammation

In recent years, the hypothalamus has emerged as a new neurogenic area, capable of generating new neurons after development. Neurogenesis-dependent neuroplasticity seems to be critical to continuously adapt to internal and environmental changes. Stress is a potent environmental factor that can produce potent and enduring effects on brain structure and function. Acute and chronic stress is known to cause alterations in neurogenesis and microglia in classical adult neurogenic regions such as the hippocampus. The hypothalamus is one of the major brain regions implicated in homeostatic stress and emotional stress systems, but little is known about the effect of stress on the hypothalamus. Here, we studied the impact of acute and intense stress (water immersion and restrain stress, WIRS), which may be considered as an inducer of an animal model of posttraumatic stress disorder, on neurogenesis and neuroinflammation in the hypothalamus of adult male mice, focusing on three nuclei: PVN, VMN and ARC, and also in the periventricular area. Our data revealed that a unique stressor was sufficient to provoke a significant impact on hypothalamic neurogenesis by inducing a reduction in the proliferation and number of immature neurons identified as DCX+ cells. These differences were accompanied by marked microglial activation in the VMN and ARC, together with a concomitant increase in IL-6 levels, indicating that WIRS induced an inflammatory response. To investigate the possible molecular mechanisms responsible for neuroplastic and inflammatory changes, we tried to identify proteomic changes. The data revealed that WIRS induced changes in the hypothalamic proteome, modifying the abundance of three and four proteins after 1 h or 24 h of stress application, respectively. These changes were also accompanied by slight changes in the weight and food intake of the animals. These results are the first to show that even a short-term environmental stimulus such as acute and intense stress can have neuroplastic, inflammatory, functional and metabolic consequences on the adult hypothalamus.

Uncovering neuroinflammation-related modules and potential repurposing drugs for Alzheimer's disease through multi-omics data integrative analysis

Background

Neuroinflammation is one of the key factors leading to neuron death and synapse dysfunction in Alzheimer's disease (AD). Amyloid-β (Aβ) is thought to have an association with microglia activation and trigger neuroinflammation in AD. However, inflammation response in brain disorders is heterogenous, and thus, it is necessary to unveil the specific gene module of neuroinflammation caused by Aβ in AD, which might provide novel biomarkers for AD diagnosis and help understand the mechanism of the disease.

Methods

Transcriptomic datasets of brain region tissues from AD patients and the corresponding normal tissues were first used to identify gene modules through the weighted gene co-expression network analysis (WGCNA) method. Then, key modules highly associated with Aβ accumulation and neuroinflammatory response were pinpointed by combining module expression score and functional information. Meanwhile, the relationship of the Aβ-associated module to the neuron and microglia was explored based on snRNA-seq data. Afterward, transcription factor (TF) enrichment and the SCENIC analysis were performed on the Aβ-associated module to discover the related upstream regulators, and then a PPI network proximity method was employed to repurpose the potential approved drugs for AD.

Results

A total of 16 co-expression modules were primarily obtained by the WGCNA method. Among them, the green module was significantly correlated with Aβ accumulation, and its function was mainly involved in neuroinflammation response and neuron death. Thus, the module was termed the amyloid-β induced neuroinflammation module (AIM). Moreover, the module was negatively correlated with neuron percentage and showed a close association with inflammatory microglia. Finally, based on the module, several important TFs were recognized as potential diagnostic biomarkers for AD, and then 20 possible drugs including ibrutinib and ponatinib were picked out for the disease.

Conclusion

In this study, a specific gene module, termed AIM, was identified as a key sub-network of Aβ accumulation and neuroinflammation in AD. Moreover, the module was verified as having an association with neuron degeneration and inflammatory microglia transformation. Moreover, some promising TFs and potential repurposing drugs were presented for AD based on the module. The findings of the study shed new light on the mechanistic investigation of AD and might make benefits the treatment of the disease.

The Functions and Phenotypes of Microglia in Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease worldwide, but therapeutic strategies to slow down AD pathology and symptoms have not yet been successful. While attention has been focused on neurodegeneration in AD pathogenesis, recent decades have provided evidence of the importance of microglia, and resident immune cells in the central nervous system. In addition, new technologies, including single-cell RNA sequencing, have revealed heterogeneous cell states of microglia in AD. In this review, we systematically summarize the microglial response to amyloid-β and tau tangles, and the risk factor genes expressed in microglia. Furthermore, we discuss the characteristics of protective microglia that appear during AD pathology and the relationship between AD and microglia-induced inflammation during chronic pain. Understanding the diverse roles of microglia will help identify new therapeutic strategies for AD.

Microglial Phagocytosis/Cell Health High-Content Assay

We report a microglial phagocytosis/cell health high-content assay that has been used to test small molecule chemical probes and support our drug discovery projects targeting microglia for Alzheimer's disease therapy. The assay measures phagocytosis and cell health (cell count and nuclear intensity) simultaneously in 384-well plates processed with an automatic liquid handler. The mix-and-read live cell imaging assay is highly reproducible with capacity to meet drug discovery research needs. Assay procedures take 4 days including plating cells, treating cells, adding pHrodo-myelin/membrane debris to cells for phagocytosis, staining cell nuclei before performing high-content imaging, and analysis. Three selected parameters are measured from cells: 1) mean total fluorescence intensity per cell of pHrodo-myelin/membrane debris in phagocytosis vesicles to quantify phagocytosis; 2) cell counts per well (measuring compound effects on proliferation and cell death); and 3) average nuclear intensity (measuring compound induced apoptosis). The assay has been used on HMC3 cells (an immortalized human microglial cell line), BV2 cells (an immortalized mouse microglial cell line), and primary microglia isolated from mouse brains. Simultaneous measurements of phagocytosis and cell health allow for the distinction of compound effects on regulation of phagocytosis from cellular stress/toxicity related changes, a distinguishing feature of the assay. The combination of cell counts and nuclear intensity as indicators of cell health is also an effective way to measure cell stress and compound cytotoxicity, which may have broad applications as simultaneous profiling measurements for other phenotypic assays. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Microglial phagocytosis/cell health high-content assay protocol Support Protocol: Procedures to isolate myelin/membrane debris from mouse brain and label with pHrodo.

The β-adrenergic hypothesis of synaptic and microglial impairment in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease originating partly from amyloid β protein-induced synaptic failure. As damaging of noradrenergic neurons in the locus coeruleus (LC) occurs at the prodromal stage of AD, activation of adrenergic receptors could serve as the first line of defense against the onset of the disease. Activation of β₂ -ARs strengthens long-term potentiation (LTP) and synaptic activity, thus improving learning and memory. Physical stimulation of animals exposed to an enriched environment (EE) leads to the activation of β₂ -ARs and prevents synaptic dysfunction. EE also suppresses neuroinflammation, suggesting that β₂ -AR agonists may play a neuroprotective role. The β₂ -AR agonists used for respiratory diseases have been shown to have an anti-inflammatory effect. Epidemiological studies further support the beneficial effects of β₂ -AR agonists on several neurodegenerative diseases. Thus, I propose that β₂ -AR agonists may provide therapeutic value in combination with novel treatments for AD.

The Fuzzy Border between the Functional and Dysfunctional Effects of Beta-Amyloid: A Synaptocentric View of Neuron-Glia Entanglement

Recent observations from clinical trials using monoclonal antibodies against Aβ seem to suggest that Aβ-targeting is modestly effective and not sufficiently based on an effective challenge of the role of Aβ from physiological to pathological. After an accelerated approval procedure for aducanumab, and more recently lecanemab, their efficacy and safety remain to be fully defined despite previous attempts with various monoclonal antibodies, and both academic institutions and pharmaceutical companies are actively searching for novel treatments. Aβ needs to be clarified further in a more complicated context, taking into account both its accumulation and its biological functions during the course of the disease. In this review, we discuss the border between activities affecting early, potentially reversible dysfunctions of the synapse and events trespassing the threshold of inflammatory, self-sustaining glial activation, leading to irreversible damage. We detail a clear understanding of the biological mechanisms underlying the derangement from function to dysfunction and the switch of the of Aβ role from physiological to pathological. A picture is emerging where the optimal therapeutic strategy against AD should involve a number of allied molecular processes, displaying efficacy not only in reducing the well-known AD pathogenesis players, such as Aβ or neuroinflammation, but also in preventing their adverse effects.

TRPV1 regulates ApoE4-disrupted intracellular lipid homeostasis and decreases synaptic phagocytosis by microglia

Although the ε4 allele of the apolipoprotein E (ApoE4) gene has been established as a genetic risk factor for many neurodegenerative diseases, including Alzheimer's disease, the mechanism of action remains poorly understood. Transient receptor potential vanilloid 1 (TRPV1) was reported to regulate autophagy to protect against foam cell formation in atherosclerosis. Here, we show that ApoE4 leads to lipid metabolism dysregulation in microglia, resulting in enhanced MHC-II-dependent antigen presentation and T-cell activation. Lipid accumulation and inflammatory reactions were accelerated in microglia isolated from TRPV1^flox/flox; Cx3cr1^cre-ApoE4 mice. We showed that metabolic boosting by treatment with the TRPV1 agonist capsaicin rescued lipid metabolic impairments in ApoE4 neurons and defects in autophagy caused by disruption of the AKT-mTOR pathway. TRPV1 activation with capsaicin reversed ApoE4-induced microglial immune dysfunction and neuronal autophagy impairment. Capsaicin rescued memory impairment, tau pathology, and neuronal autophagy in ApoE4 mice. Activation of TRPV1 decreased microglial phagocytosis of synapses in ApoE4 mice. TRPV1 gene deficiency exacerbated recognition memory impairment and tau pathology in ApoE4 mice. Our study suggests that TRPV1 regulation of lipid metabolism could be a therapeutic approach to alleviate the consequences of the ApoE4 allele.

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

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

Microglial efferocytosis: Diving into the Alzheimer's disease gene pool

Genome-wide association studies and functional genomics studies have linked specific cell types, genes, and pathways to Alzheimer's disease (AD) risk. In particular, AD risk alleles primarily affect the abundance or structure, and thus the activity, of genes expressed in macrophages, strongly implicating microglia (the brain-resident macrophages) in the etiology of AD. These genes converge on pathways (endocytosis/phagocytosis, cholesterol metabolism, and immune response) with critical roles in core macrophage functions such as efferocytosis. Here, we review these pathways, highlighting relevant genes identified in the latest AD genetics and genomics studies, and describe how they may contribute to AD pathogenesis. Investigating the functional impact of AD-associated variants and genes in microglia is essential for elucidating disease risk mechanisms and developing effective therapeutic approaches.

Comparative analysis of transcriptome remodeling in plaque-associated and plaque-distant microglia during amyloid-β pathology progression in mice

Background

Research in recent years firmly established that microglial cells play an important role in the pathogenesis of Alzheimer's disease (AD). In parallel, a series of studies showed that, under both homeostatic and pathological conditions, microglia are a heterogeneous cell population. In AD, amyloid-β (Aβ) plaque-associated microglia (PAM) display a clearly distinct phenotype compared to plaque-distant microglia (PCM), suggesting that these two microglia subtypes likely differently contribute to disease progression. So far, molecular characterization of PAM was performed indirectly using single cell RNA sequencing (scRNA-seq) approaches or based on markers that are supposedly up-regulated in this microglia subpopulation.

Methods

In this study based on a well-characterized AD mouse model, we combined cell-specific laser capture microdissection and RNA-seq analysis to i) identify, without preconceived notions of the molecular and/or functional changes that would affect these cells, the genes and gene networks that are dysregulated in PAM or PCM at three critical stages of the disease, and ii) to investigate the potential contribution of both plaque-associated and plaque-distant microglia.

Results

First, we established that our approach allows selective isolation of microglia, while preserving spatial information and preventing transcriptome changes induced by classical purification approaches. Then, we identified, in PAM and PCM subpopulations, networks of co-deregulated genes and analyzed their potential functional roles in AD. Finally, we investigated the dynamics of microglia transcriptomic remodeling at early, intermediate and late stages of the disease and validated select findings in postmortem human AD brain.

Conclusions

Our comprehensive study provides useful transcriptomic information regarding the respective contribution of PAM and PCM across the Aβ pathology progression. It highlights specific pathways that would require further study to decipher their roles across disease progression. It demonstrates that the proximity of microglia to Aβ-plaques dramatically alters the microglial transcriptome and reveals that these changes can have both positive and negative impacts on the surrounding cells. These opposing effects may be driven by local microglia heterogeneity also demonstrated by this study. Our approach leads to molecularly define the less well studied plaque-distant microglia. We show that plaque-distant microglia are not bystanders of the disease, although the transcriptomic changes are far less striking compared to what is observed in plaque-associated microglia. In particular, our results suggest they may be involved in Aβ oligomer detection and in Aβ-plaque initiation, with increased contribution as the disease progresses.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02581-0.

Pharmacological targeting of microglia dynamics in Alzheimer's disease: Preclinical and clinical evidence

Numerous clinical trials of anti-amyloid agents for Alzheimer's disease (AD) were so far unsuccessful thereby challenging the validity of the amyloid hypothesis. This lack of progress has encouraged researchers to investigate alternative mechanisms in non-neuronal cells, among which microglia represent nowadays an attractive target. Microglia play a key role in the developing brain and contribute to synaptic remodeling in the mature brain. On the other hand, the intimate relationship between microglia and synapses led to the so-called synaptic stripping hypothesis, a process in which microglia selectively remove synapses from injured neurons. Synaptic stripping, along with the induction of a microglia-mediated chronic neuroinflammatory environment, promote the progressive synaptic degeneration in AD. Therefore, targeting microglia may pave the way for a new disease modifying approach. This review provides an overview of the pathophysiological roles of the microglia cells in AD and describes putative targets for pharmacological intervention. It also provides evidence for microglia-targeted strategies in preclinical AD studies and in early clinical trials.

All roads lead to heterogeneity: The complex involvement of astrocytes and microglia in the pathogenesis of Alzheimer’s disease

In recent years, glial cells have been acknowledged as key players in the pathogenesis of Alzheimer’s disease (AD), a neurodegenerative condition in which an accumulation of intracellular neurofibrillary tangles and extracellular fibrillar amyloid beta is notably observed in the central nervous system. Genome-wide association studies have shown, both in microglia and astrocytes, an increase in gene variants associated with a higher risk of developing late-onset AD. Microglia, the resident innate immune cells of the brain, and astrocytes, glial cells crucial for vascular integrity and neuronal support, both agglomerate near amyloid beta plaques and dystrophic neurites where they participate in the elimination of these harmful parenchymal elements. However, their role in AD pathogenesis has been challenging to resolve due to the highly heterogeneous nature of these cell populations, i.e., their molecular, morphological, and ultrastructural diversity, together with their ever-changing responsiveness and functions throughout the pathological course of AD. With the recent expansions in the field of glial heterogeneity through innovative advances in state-of-the-art microscopy and -omics techniques, novel concepts and questions arose, notably pertaining to how the diverse microglial and astrocytic states interact with each other and with the AD hallmarks, and how their concerted efforts/actions impact the progression of the disease. In this review, we discuss the recent advances and findings on the topic of glial heterogeneity, particularly focusing on the relationships of these cells with AD hallmarks (e.g., amyloid beta plaques, neurofibrillary tangles, synaptic loss, and dystrophic neurites) in murine models of AD pathology and post-mortem brain samples of patients with AD.

Neuroinflammation in neurodegeneration via microbial infections

Recent epidemiological studies show a noticeable correlation between chronic microbial infections and neurological disorders. However, the underlying mechanisms are still not clear due to the biological complexity of multicellular and multiorgan interactions upon microbial infections. In this review, we show the infection leading to neurodegeneration mediated by multiorgan interconnections and neuroinflammation. Firstly, we highlight three inter-organ communications as possible routes from infection sites to the brain: nose-brain axis, lung-brain axis, and gut-brain axis. Next, we described the biological crosstalk between microglia and astrocytes upon pathogenic infection. Finally, our study indicates how neuroinflammation is a critical player in pathogen-mediated neurodegeneration. Taken together, we envision that antibiotics targeting neuro-pathogens could be a potential therapeutic strategy for neurodegeneration.

Effects of α5 GABAA receptor modulation on social interaction, memory, and neuroinflammation in a mouse model of Alzheimer's disease

Aims

GABAergic modulation involved in cognitive processing appears to be substantially changed in Alzheimer's disease (AD). In a widely used 5xFAD model of AD, we aimed to assess if negative and positive allosteric modulators of α5 GABA_A receptors (NAM and PAM, respectively) would affect social interaction, social, object and spatial memory, and neuroinflammation.

Methods

After 10‐day treatment with PAM, NAM, or solvent, 6‐month‐old transgenic and non‐transgenic 5xFAD mice underwent testing in a behavioral battery. Gene expressions of IL‐1β, IL‐6, TNF‐α, GFAP, and IBA‐1 were determined in hippocampus and prefrontal cortex by qPCR.

Results

PAM treatment impaired spatial learning in transgenic females compared to solvent‐treated transgenic females, and social recognition in transgenic and non‐transgenic males. NAM treatment declined social interaction in transgenic and non‐transgenic males, while had beneficial effect on cognitive flexibility in non‐transgenic males compared to solvent‐treated non‐transgenic males. Transgenic animals have not fully displayed cognitive symptoms, but neuroinflammation was confirmed. NAM reduced proinflammatory gene expressions in transgenic females and astrogliosis in transgenic males compared to pathological controls.

Conclusion

PAM and NAM failed to exert favorable behavioral effects in transgenic animals. Suppression of neuroinflammation obtained with NAM calls for more studies with GABAergic ligands in amyloid beta‐ and/or tau‐dependent models with prominent neuroinflammation.

Microglia Phenotypes in Aging and Neurodegenerative Diseases

Neuroinflammation is a hallmark of many neurodegenerative diseases (NDs) and plays a fundamental role in mediating the onset and progression of disease. Microglia, which function as first-line immune guardians of the central nervous system (CNS), are the central drivers of neuroinflammation. Numerous human postmortem studies and in vivo imaging analyses have shown chronically activated microglia in patients with various acute and chronic neuropathological diseases. While microglial activation is a common feature of many NDs, the exact role of microglia in various pathological states is complex and often contradictory. However, there is a consensus that microglia play a biphasic role in pathological conditions, with detrimental and protective phenotypes, and the overall response of microglia and the activation of different phenotypes depends on the nature and duration of the inflammatory insult, as well as the stage of disease development. This review provides a comprehensive overview of current research on the various microglia phenotypes and inflammatory responses in health, aging, and NDs, with a special emphasis on the heterogeneous phenotypic response of microglia in acute and chronic diseases such as hemorrhagic stroke (HS), Alzheimer’s disease (AD), and Parkinson’s disease (PD). The primary focus is translational research in preclinical animal models and bulk/single-cell transcriptome studies in human postmortem samples. Additionally, this review covers key microglial receptors and signaling pathways that are potential therapeutic targets to regulate microglial inflammatory responses during aging and in NDs. Additionally, age-, sex-, and species-specific microglial differences will be briefly reviewed.

The Shape of μ—How Morphological Analyses Shape the Study of Microglia

Microglia, the innate immune cells of the CNS parenchyma, serve as the first line of defense in a myriad of neurodevelopmental, neurodegenerative, and neuroinflammatory conditions. In response to the peripheral inflammation, circulating mediators, and other external signals that are produced by these conditions, microglia dynamically employ different transcriptional programs as well as morphological adaptations to maintain homeostasis. To understand these cells’ function, the field has established a number of essential analysis approaches, such as gene expression, cell quantification, and morphological reconstruction. Although high-throughput approaches are becoming commonplace in regard to other types of analyses (e.g., single-cell scRNA-seq), a similar standard for morphological reconstruction has yet to be established. In this review, we offer an overview of microglial morphological analysis methods, exploring the advantages and disadvantages of each, highlighting a number of key studies, and emphasizing how morphological analysis has significantly contributed to our understanding of microglial function in the CNS parenchyma. In doing so, we advocate for the use of unbiased, automated morphological reconstruction approaches in future studies, in order to capitalize on the valuable information embedded in the cellular structures microglia inhabit.

Targeting Microglia in Alzheimer’s Disease: From Molecular Mechanisms to Potential Therapeutic Targets for Small Molecules

Alzheimer’s disease (AD) is a common, progressive, and devastating neurodegenerative disorder that mainly affects the elderly. Microglial dysregulation, amyloid-beta (Aβ) plaques, and intracellular neurofibrillary tangles play crucial roles in the pathogenesis of AD. In the brain, microglia play roles as immune cells to provide protection against virus injuries and diseases. They have significant contributions in the development of the brain, cognition, homeostasis of the brain, and plasticity. Multiple studies have confirmed that uncontrolled microglial function can result in impaired microglial mitophagy, induced Aβ accumulation and tau pathology, and a chronic neuroinflammatory environment. In the brain, most of the genes that are associated with AD risk are highly expressed by microglia. Although it was initially regarded that microglia reaction is incidental and induced by dystrophic neurites and Aβ plaques. Nonetheless, it has been reported by genome-wide association studies that most of the risk loci for AD are located in genes that are occasionally uniquely and highly expressed in microglia. This finding further suggests that microglia play significant roles in early AD stages and they be targeted for the development of novel therapeutics. In this review, we have summarized the molecular pathogenesis of AD, microglial activities in the adult brain, the role of microglia in the aging brain, and the role of microglia in AD. We have also particularly focused on the significance of targeting microglia for the treatment of AD.

Microglial Priming in Infections and Its Risk to Neurodegenerative Diseases

Infectious diseases of different etiologies have been associated with acute and long-term neurological consequences. The primary cause of these consequences appears to be an inflammatory process characterized primarily by a pro-inflammatory microglial state. Microglial cells, the local effectors’ cells of innate immunity, once faced by a stimulus, alter their morphology, and become a primary source of inflammatory cytokines that increase the inflammatory process of the brain. This inflammatory scenario exerts a critical role in the pathogenesis of neurodegenerative diseases. In recent years, several studies have shown the involvement of the microglial inflammatory response caused by infections in the development of neurodegenerative diseases. This has been associated with a transitory microglial state subsequent to an inflammatory response, known as microglial priming, in which these cells are more responsive to stimuli. Thus, systemic inflammation and infections induce a transitory state in microglia that may lead to changes in their state and function, making priming them for subsequent immune challenges. However, considering that microglia are long-lived cells and are repeatedly exposed to infections during a lifetime, microglial priming may not be beneficial. In this review, we discuss the relationship between infections and neurodegenerative diseases and how this may rely on microglial priming.

A breakdown in microglial metabolic reprogramming causes internalization dysfunction of α-synuclein in a mouse model of Parkinson’s disease

Background

The α-synuclein released by neurons activates microglia, which then engulfs α-synuclein for degradation via autophagy. Reactive microglia are a major pathological feature of Parkinson’s disease (PD), although the exact role of microglia in the pathogenesis of PD remains unclear. Transient receptor potential vanilloid type 1 (TRPV1) channels are nonselective cation channel protein that have been proposed as neuroprotective targets in neurodegenerative diseases.

Methods

Using metabolic profiling, microglia energy metabolism was measured including oxidative phosphorylation and aerobic glycolysis. The mRFP-GFP-tagged LC3 reporter was introduced to characterize the role of TRPV1 in microglial autophagy. α-synuclein preformed fibril (PFF) TRPV1^flox/flox; Cx3cr1^Cre mouse model of sporadic PD were employed to study the capacity of TRPV1 activation to attenuate neurodegeneration process.

Results

We found that acute exposure to PFF caused microglial activation as a result of metabolic reprogramming from oxidative phosphorylation to aerobic glycolysis via the AKT–mTOR–HIF-1α pathway. Activated microglia eventually reached a state of chronic PFF-tolerance, accompanied by broad defects in energy metabolism. We showed that metabolic boosting by treatment with the TRPV1 agonist capsaicin rescued metabolic impairments in PFF-tolerant microglia and also defects in mitophagy caused by disruption of the AKT–mTOR–HIF-1α pathway. Capsaicin attenuated phosphorylation of α-synuclein in primary neurons by boosting phagocytosis in PFF-tolerant microglia in vitro. Finally, we found that behavioral deficits and loss of dopaminergic neurons were accelerated in the PFF TRPV1^flox/flox; Cx3cr1^Cre mouse model of sporadic PD. We identified defects in energy metabolism, mitophagy and phagocytosis of PFF in microglia from the substantia nigra pars compacta of TRPV1^flox/flox; Cx3cr1^Cre mice.

Conclusion

The findings suggest that modulating microglial metabolism might be a new therapeutic strategy for PD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02484-0.

Microglial Endocannabinoid Signalling in AD

Chronic inflammation in Alzheimer's disease (AD) has been recently identified as a major contributor to disease pathogenesis. Once activated, microglial cells, which are brain-resident immune cells, exert several key actions, including phagocytosis, chemotaxis, and the release of pro- or anti-inflammatory mediators, which could have opposite effects on brain homeostasis, depending on the stage of disease and the particular phenotype of microglial cells. The endocannabinoids (eCBs) are pleiotropic bioactive lipids increasingly recognized for their essential roles in regulating microglial activity both under normal and AD-driven pathological conditions. Here, we review the current literature regarding the involvement of this signalling system in modulating microglial phenotypes and activity in the context of homeostasis and AD-related neurodegeneration.

Stress vulnerability shapes disruption of motor cortical neuroplasticity

Chronic stress is a major cause of neuropsychiatric conditions such as depression. Stress vulnerability varies individually in mice and humans, measured by behavioral changes. In contrast to affective symptoms, motor retardation as a consequence of stress is not well understood. We repeatedly imaged dendritic spines of the motor cortex in Thy1-GFP M mice before and after chronic social defeat stress. Susceptible and resilient phenotypes were discriminated by symptom load and their motor learning abilities were assessed by a gross and fine motor task. Stress phenotypes presented individual short- and long-term changes in the hypothalamic-pituitary-adrenal axis as well as distinct patterns of altered motor learning. Importantly, stress was generally accompanied by a marked reduction of spine density in the motor cortex and spine dynamics depended on the stress phenotype. We found astrogliosis and altered microglia morphology along with increased microglia-neuron interaction in the motor cortex of susceptible mice. In cerebrospinal fluid, proteomic fingerprints link the behavioral changes and structural alterations in the brain to neurodegenerative disorders and dysregulated synaptic homeostasis. Our work emphasizes the importance of synaptic integrity and the risk of neurodegeneration within depression as a threat to brain health.

TRPV1-Mediated Microglial Autophagy Attenuates Alzheimer’s Disease-Associated Pathology and Cognitive Decline

Autophagy is a major regulator of the ageing process of the central nervous system and neurodegeneration. Autophagy dysfunction has been implicated in the pathogenesis of Alzheimer’s disease (AD). TRPV1 was reported to regulate autophagy to protect against foam cell formation and reduce the release of inflammatory factors in atherosclerosis. In this study, pharmacological activation of TRPV1 with the TRPV1 agonist capsaicin induced autophagy in a TRPV1-dependent manner in both primary microglia and BV2 cells. TRPV1-mediated autophagy regulated glycolysis and oxidative phosphorylation by controlling the expression of genes required for aerobic glycolysis and mitochondrial respiration in primary microglia. TRPV1 agonist capsaicin decreased amyloid and phosphorylated tau pathology and reversed memory deficits by promoting microglia activation, metabolism, and autophagy in 3xTg mice. These results indicate that TRPV1 was a potential therapeutic target for AD, which suggests that capsaicin should be further assessed as a possible treatment for AD.

The Effects of Modified Curcumin Preparations on Glial Morphology in Aging and Neuroinflammation

Neuroinflammation is characterized by reactive microglia and astrocytes (collectively called gliosis) in the central nervous system and is considered as one of the main pathological hallmarks in different neurodegenerative diseases such as Alzheimer's disease, age-related dementia, and multiple sclerosis. Upon activation, glia undergoes structural and morphological changes such as the microglial cells swell in size and astrocytes become bushy, which play both beneficial and detrimental roles. Hence, they are unable to perform the normal physiological role in brain immunity. Curcumin, a cytokine suppressive anti-inflammatory drug, has a high proven pre-clinical potency and efficacy to reverse chronic neuroinflammation by attenuating the activation and morphological changes that occur in the microglia and astrocytes. This review will highlight the recent findings on the tree structure changes of microglia and astrocytes in neuroinflammation and the effects of curcumin against the activation and morphology of glial cells.

Microglial Potassium Channels: From Homeostasis to Neurodegeneration

The growing interest in the role of microglia in the progression of many neurodegenerative diseases is developing in an ever-expedited manner, in part thanks to emergent new tools for studying the morphological and functional features of the CNS. The discovery of specific biomarkers of the microglia phenotype could find application in a wide range of human diseases, and creates opportunities for the discovery and development of tailored therapeutic interventions. Among these, recent studies highlight the pivotal role of the potassium channels in regulating microglial functions in physiological and pathological conditions such as Alzheimer's Disease, Parkinson's Disease, and Amyotrophic Lateral Sclerosis. In this review, we summarize the current knowledge of the involvement of the microglial potassium channels in several neurodegenerative diseases and their role as modulators of microglial homeostasis and dysfunction in CNS disorders.

Glutamate and GABA in Microglia-Neuron Cross-Talk in Alzheimer's Disease

The physiological balance between excitation and inhibition in the brain is significantly affected in Alzheimer’s disease (AD). Several neuroactive compounds and their signaling pathways through various types of receptors are crucial in brain homeostasis, among them glutamate and γ-aminobutyric acid (GABA). Activation of microglial receptors regulates the immunological response of these cells, which in AD could be neuroprotective or neurotoxic. The novel research approaches revealed the complexity of microglial function, including the interplay with other cells during neuroinflammation and in the AD brain. The purpose of this review is to describe the role of several proteins and multiple receptors on microglia and neurons, and their involvement in a communication network between cells that could lead to different metabolic loops and cell death/survival. Our review is focused on the role of glutamatergic, GABAergic signaling in microglia–neuronal cross-talk in AD and neuroinflammation. Moreover, the significance of AD-related neurotoxic proteins in glutamate/GABA-mediated dialogue between microglia and neurons was analyzed in search of novel targets in neuroprotection, and advanced pharmacological approaches.

Cannabinoid Receptor 2 Alters Social Memory and Microglial Activity in an Age-Dependent Manner

Physiological brain aging is characterized by gradual, substantial changes in cognitive ability, accompanied by chronic activation of the neural immune system. This form of inflammation, termed inflammaging, in the central nervous system is primarily enacted through microglia, the resident immune cells. The endocannabinoid system, and particularly the cannabinoid receptor 2 (CB2R), is a major regulator of the activity of microglia and is upregulated under inflammatory conditions. Here, we elucidated the role of the CB2R in physiological brain aging. We used CB2R-/- mice of progressive ages in a behavioral test battery to assess social and spatial learning and memory. This was followed by detailed immunohistochemical analysis of microglial activity and morphology, and of the expression of pro-inflammatory cytokines in the hippocampus. CB2R deletion decreased social memory in young mice, but did not affect spatial memory. In fact, old CB2R-/- mice had a slightly improved social memory, whereas in WT mice we detected an age-related cognitive decline. On a cellular level, CB2R deletion increased lipofuscin accumulation in microglia, but not in neurons. CB2R-/- microglia showed an increase of activity markers Iba1 and CD68, and minor upregulation in tnfa and il6 expression and downregulation of ccl2 with age. This was accompanied by a change in morphology as CB2R-/- microglia had smaller somas and lower polarity, with increased branching, cell volume, and tree length. We present that CB2Rs are involved in cognition and age-induced microglial activity, but may also be important for microglial activation itself.

Cannabinoid receptor 2 is necessary to induce toll-like receptor-mediated microglial activation

The tight regulation of microglia activity is key for precise responses to potential threats, while uncontrolled and exacerbated microglial activity is neurotoxic. Microglial toll-like receptors (TLRs) are indispensable for sensing different types of assaults and triggering an innate immune response. Cannabinoid receptor 2 (CB2) signaling is a key pathway to control microglial homeostasis and activation, and its activation is connected to changes in microglial activity. We aimed to investigate how CB2 signaling impacts TLR-mediated microglial activation. Here, we demonstrate that deletion of CB2 causes a dampened transcriptional response to prototypic TLR ligands in microglia. Loss of CB2 results in distinct microglial gene expression profiles, morphology, and activation. We show that the CB2-mediated attenuation of TLR-induced microglial activation is mainly p38 MAPK-dependent. Taken together, we demonstrate that CB2 expression and signaling are necessary to fine-tune TLR-induced activation programs in microglia.

Microglial morphology in Alzheimer's disease and after Aβ immunotherapy

Microglia are the brain immune cells and their function is highly dependent on cell motility. It was hypothesised that morphological variability leads to differences in motility, ultimately impacting on the microglial function. Here, we assessed microglial morphology in 32 controls, 44 Alzheimer's disease (AD) cases and 16 AD cases from patients immunised against Aβ42 (iAD) using 2D and 3D approaches. Our 2D assessment showed an increased number of microglia in iAD vs. AD (P = 0.032) and controls (P = 0.018). Ramified microglia were fewer in AD vs. controls (P = 0.041) but increased in iAD compared to AD (P < 0.001) and controls (P = 0.006). 3D reconstructions highlighted larger cell bodies in AD vs. controls (P = 0.049) and increased total process length in iAD vs. AD (P = 0.032), with negative correlations detected for pan-Aβ load with total process length (P < 0.001) in AD and number of primary processes (P = 0.043) in iAD. In summary, reactive/amoeboid microglia are the most represented population in the aged human brain. AD does not affect the number of microglia, but the ramified population is decreased adopting a more reactive morphology. Aβ removal by immunotherapy leads to increased ramified microglia, implying that the cells retain plasticity in an aged disease brain meriting further investigation.

Evolving Models and Tools for Microglial Studies in the Central Nervous System

Microglia play multiple roles in such processes as brain development, homeostasis, and pathology. Due to their diverse mechanisms of functions, the complex sub-classifications, and the large differences between different species, especially compared with humans, very different or even opposite conclusions can be drawn from studies with different research models. The choice of appropriate research models and the associated tools are thus key ingredients of studies on microglia. Mice are the most commonly used animal models. In this review, we summarize in vitro and in vivo models of mouse and human-derived microglial research models, including microglial cell lines, primary microglia, induced microglia-like cells, transgenic mice, human-mouse chimeric models, and microglial replacement models. We also summarize recent developments in novel single-cell and in vivo imaging technologies. We hope our review can serve as an efficient reference for the future study of microglia.

Towards PET imaging of the dynamic phenotypes of microglia

There is increasing evidence showing the heterogeneity of microglia activation in neuroinflammatory and neurodegenerative diseases. It has been hypothesized that pro‐inflammatory microglia are detrimental and contribute to disease progression, while anti‐inflammatory microglia play a role in damage repair and remission. The development of therapeutics targeting the deleterious glial activity and modulating it into a regenerative phenotype relies heavily upon a clearer understanding of the microglia dynamics during disease progression and the ability to monitor therapeutic outcome in vivo. To that end, molecular imaging techniques are required to assess microglia dynamics and study their role in disease progression as well as to evaluate the outcome of therapeutic interventions. Positron emission tomography (PET) is such a molecular imaging technique, and provides unique capabilities for non‐invasive quantification of neuroinflammation and has the potential to discriminate between microglia phenotypes and define their role in the disease process. However, several obstacles limit the possibility for selective in vivo imaging of microglia phenotypes mainly related to the poor characterization of specific targets that distinguish the two ends of the microglia activation spectrum and lack of suitable tracers. PET tracers targeting translocator protein 18 kDa (TSPO) have been extensively explored, but despite the success in evaluating neuroinflammation they failed to discriminate between microglia activation statuses. In this review, we highlight the current knowledge on the microglia phenotypes in the major neuroinflammatory and neurodegenerative diseases. We also discuss the current and emerging PET imaging targets, the tracers and their potential in discriminating between the pro‐ and anti‐inflammatory microglia activation states.

TRPV1 sustains microglial metabolic reprogramming in Alzheimer's disease

As the brain-resident innate immune cells, reactive microglia are a major pathological feature of Alzheimer's disease (AD). However, the exact role of microglia is still unclear in AD pathogenesis. Here, using metabolic profiling, we show that microglia energy metabolism is significantly suppressed during chronic Aβ-tolerant processes including oxidative phosphorylation and aerobic glycolysis via the mTOR-AKT-HIF-1α pathway. Pharmacological activation of TRPV1 rescues Aβ-tolerant microglial dysfunction, the AKT/mTOR pathway activity, and metabolic impairments and restores the immune responses including phagocytic activity and autophagy function. Amyloid pathology and memory impairment are accelerated in microglia-specific TRPV1-knockout APP/PS1 mice. Finally, we showed that metabolic boosting with TRPV1 agonist decreases amyloid pathology and reverses memory deficits in AD mice model. These results indicate that TRPV1 is an important target regulating metabolic reprogramming for microglial functions in AD treatment.

Glial Purinergic Signaling in Neurodegeneration

Purinergic signaling regulates neuronal and glial cell functions in the healthy CNS. In neurodegenerative diseases, purinergic signaling becomes dysregulated and can affect disease-associated phenotypes of glial cells. In this review, we discuss how cell-specific expression patterns of purinergic signaling components change in neurodegeneration and how dysregulated glial purinergic signaling and crosstalk may contribute to disease pathophysiology, thus bearing promising potential for the development of new therapeutical options for neurodegenerative diseases.

Cannabinoid receptor 1 signalling modulates stress susceptibility and microglial responses to chronic social defeat stress

Psychosocial stress is one of the main environmental factors contributing to the development of psychiatric disorders. In humans and rodents, chronic stress is associated with elevated inflammatory responses, indicated by increased numbers of circulating myeloid cells and activation of microglia, the brain-resident immune cells. The endocannabinoid system (ECS) regulates neuronal and endocrine stress responses via the cannabinoid receptor 1 (CB1). CB1-deficient mice (Cnr1^-/-) are highly sensitive to stress, but if this involves altered inflammatory responses is not known. To test this, we exposed Cnr1^+/+ and Cnr1^-/- mice to chronic social defeat stress (CSDS). Cnr1^-/- mice were extremely sensitive to a standard protocol of CSDS, indicated by an increased mortality rate. Therefore, a mild CSDS protocol was established, which still induced a behavioural phenotype in susceptible Cnr1^-/- mice. These mice also showed altered glucocorticoid levels after mild CSDS, suggesting dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis. Mild CSDS induced weak myelopoiesis in the periphery, but no recruitment of myeloid cells to the brain. In contrast, mild CSDS altered microglial activation marker expression and morphology in Cnr1^-/- mice. These microglial changes correlated with the severity of the behavioural phenotype. Furthermore, microglia of Cnr1^-/- mice showed increased expression of Fkbp5, an important regulator of glucocorticoid signalling. Overall, the results confirm that CB1 signalling protects the organism from the physical and emotional harm of social stress and implicate endocannabinoid-mediated modulation of microglia in the development of stress-related pathologies.

Neuroinflammation and microglial activation in Alzheimer disease: where do we go from here?

Alzheimer disease (AD) is the most common form of neurodegenerative disease, estimated to contribute 60-70% of all cases of dementia worldwide. According to the prevailing amyloid cascade hypothesis, amyloid-β (Aβ) deposition in the brain is the initiating event in AD, although evidence is accumulating that this hypothesis is insufficient to explain many aspects of AD pathogenesis. The discovery of increased levels of inflammatory markers in patients with AD and the identification of AD risk genes associated with innate immune functions suggest that neuroinflammation has a prominent role in the pathogenesis of AD. In this Review, we discuss the interrelationships between neuroinflammation and amyloid and tau pathologies as well as the effect of neuroinflammation on the disease trajectory in AD. We specifically focus on microglia as major players in neuroinflammation and discuss the spatial and temporal variations in microglial phenotypes that are observed under different conditions. We also consider how these cells could be modulated as a therapeutic strategy for AD.

Space-Dependent Glia-Neuron Interplay in the Hippocampus of Transgenic Models of β-Amyloid Deposition

This review is focused on the description and discussion of the alterations of astrocytes and microglia interplay in models of Alzheimer's disease (AD). AD is an age-related neurodegenerative pathology with a slowly progressive and irreversible decline of cognitive functions. One of AD's histopathological hallmarks is the deposition of amyloid beta (Aβ) plaques in the brain. Long regarded as a non-specific, mere consequence of AD pathology, activation of microglia and astrocytes is now considered a key factor in both initiation and progression of the disease, and suppression of astrogliosis exacerbates neuropathology. Reactive astrocytes and microglia overexpress many cytokines, chemokines, and signaling molecules that activate or damage neighboring cells and their mutual interplay can result in virtuous/vicious cycles which differ in different brain regions. Heterogeneity of glia, either between or within a particular brain region, is likely to be relevant in healthy conditions and disease processes. Differential crosstalk between astrocytes and microglia in CA1 and CA3 areas of the hippocampus can be responsible for the differential sensitivity of the two areas to insults. Understanding the spatial differences and roles of glia will allow us to assess how these interactions can influence the state and progression of the disease, and will be critical for identifying therapeutic strategies.

[Immunological variants of amnestic mild cognitive impairment]

BACKGROUND

Amnestic mild cognitive impairment (aMCI) is considered as a possible earliest pre-dementia clinical stage of Alzheimer's disease (AD). Taking into account the prominent role of neuroinflammation in the pathogenesis of AD, it is quite important to study possible immunological markers of the risk of aMCI progression and the changes in immune parameters in patients.

OBJECTIVE

To study the immunological variants of aMCI and AD based on the parameters of humoral and cell immunity, levels of key cytokines and presence of systemic inflammation, and to explore the link between changes in the immune parameters and clinical prognosis.

MATERIAL AND METHODS

One hundred patients with a diagnosis of aMCI, 45 patients with AD at the stage of mild to moderate dementia and 40 people without cognitive impairment (the control group) were enrolled into the study. Immunological assessment included determination of the concentration of key cytokines, C-reactive protein, circulating immune complexes and immunoglobulins (Ig A, M, G) in blood serum by ELISA, determination of the main subpopulations of lymphocytes by flow cytometry.

RESULTS AND CONCLUSION

Four main immunological variants of aMCI syndrome associated with clinical prognosis were identified. The detected changes in immune parameters are important for further studies to assess an effect of viral and bacterial infections, intestinal microflora disorders on a clinical prognosis in patients with different immunological variants of aMCI syndrome.

Textured nanofibrils drive microglial phenotype

Microglia are highly plastic cells that change their properties in response to their microenvironment. By using immunofluorescence, live-cell imaging, electrophysiological recordings and RNA sequencing, we investigated the regulation of modified bacterial cellulose (mBC) nanofibril substrates on microglial properties. We demonstrate that mBC substrates induce ramified microglia with constantly extending and retracting processes, reminiscent of what is observed in vivo. Patch-clamp recordings show that microglia acquire a more negative resting membrane potential and have increased inward rectifier K⁺ currents, caused by an upregulation of Kir2.1 channels. Transcriptome analysis shows upregulation of genes involved in the immune response and downregulation of genes linked to cell adhesion and cell motion. Furthermore, Arp2/3 complex activation and integrin-mediated signaling modulate microglial morphology and motility. Our studies demonstrate that mBC nanofibril substrates modulate microglial phenotype, paving the way for a microglia-material interface that may be very valuable for anti-neuroinflammatory drug screening.

Mouse Astrocytes Promote Microglial Ramification by Releasing TGF-β and Forming Glial Fibers

The morphology of microglial cells is often closely related to their functions. The mechanisms that regulate microglial ramification are not well understood. Here we reveal the biological mechanisms by which astrocytes regulate microglial ramification. Morphological variation in mouse microglial cultures was measured in terms of cell area as well as branch number and length. Effects on microglial ramification were analyzed after microinjecting the toxin L-alpha-aminoadipic acid (L-AAA) in the mouse cortex or hippocampus to ablate astrocytes, and after culturing microglia on their own in an astrocyte-conditioned medium (ACM) or together with astrocytes in coculture. TGF-β expression was determined by Western blotting, immunohistochemistry, and ELISA. The TGF-β signaling pathway was blocked by the TGF-β antibody to assess the role of TGF-β on microglial ramification. The results showed that microglia had more and longer branches and smaller cell bodies in brain areas where astrocytes were abundant. In the mouse cortex and hippocampus, ablation of astrocytes by L-AAA decreased number and length of microglial branches and increased the size of cell bodies. Similar results were obtained with isolated microglia in culture. However, isolated microglia were able to maintain their multibranched structure for a long time when cultured on astrocyte monolayers. Ameboid microglia isolated from P0 to P3 mice showed increased ramification when cultured in ACM or on astrocyte monolayers. Microglia cultured on astrocyte monolayers showed more complex branching structures than those cultured in ACM. Blocking astrocyte-derived TGF-β decreased microglial ramification. Astrocytes induced the formation of protuberances on branches of microglia by forming glial fibers that increased traction. These experiments in mice suggest that astrocytes promote microglial ramification by forming glial fibers to create traction and by secreting soluble factors into the surroundings. For example, astrocyte-secreted TGF-β promotes microglia to generate primitive branches, whose ramification is refined by glial fibers.

Amyloid β induces interneuron-specific changes in the hippocampus of APPNL-F mice

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by cognitive decline and amyloid-beta (Aβ) depositions generated by the proteolysis of amyloid precursor protein (APP) in the brain. In APPNL-F mice, APP gene was humanized and contains two familial AD mutations, and APP-unlike other mouse models of AD-is driven by the endogenous mouse APP promoter. Similar to people without apparent cognitive dysfunction but with heavy Aβ plaque load, we found no significant decline in the working memory of adult APPNL-F mice, but these mice showed decline in the expression of normal anxiety. Using immunohistochemistry and 3D block-face scanning electron microscopy, we found no changes in GABAA receptor positivity and size of somatic and dendritic synapses of hippocampal interneurons. We did not find alterations in the level of expression of perineuronal nets around parvalbumin (PV) interneurons or in the density of PV- or somatostatin-positive hippocampal interneurons. However, in contrast to other investigated cell types, PV interneuron axons were occasionally mildly dystrophic around Aβ plaques, and the synapses of PV-positive axon initial segment (AIS)-targeting interneurons were significantly enlarged. Our results suggest that PV interneurons are highly resistant to amyloidosis in APPNL-F mice and amyloid-induced increase in hippocampal pyramidal cell excitability may be compensated by PV-positive AIS-targeting cells. Mechanisms that make PV neurons more resilient could therefore be exploited in the treatment of AD for mitigating Aβ-related inflammatory effects on neurons.