Prior activation state shapes the microglia response to antihuman TREM2 in a mouse model of Alzheimer's disease

Altered immune response is associated with sex difference in vulnerability to Alzheimer's disease in human prefrontal cortex

Alzheimer's disease (AD) is a neurodegenerative disorder with a higher risk incidence in females than in males, and there are also differences in AD pathophysiology between sexes. The role of sex in the pathogenesis of AD may be crucial, yet the cellular and molecular basis remains unclear. Here, we performed a comprehensive analysis using four public transcriptome datasets of AD patients and age-matched control individuals in prefrontal cortex, including bulk transcriptome (295 females and 402 males) and single-nucleus RNA sequencing (snRNA-seq) data (224 females and 219 males). We found that the transcriptomic profile in female control was similar to those in AD. To characterize the key features associated with both the pathogenesis of AD and sex difference, we identified a co-expressed gene module that positively correlated with AD, sex, and aging, and was also enriched with immune-associated pathways. Using snRNA-seq datasets, we found that microglia (MG), a resident immune cell in the brain, demonstrated substantial differences in several aspects between sexes, such as an elevated proportion of activated MG, altered transcriptomic profile and cell-cell interaction between MG and other brain cell types in female control. Additionally, genes upregulated in female MG, such as TLR2, MERTK, SPP1, SLA, ACSL1, and FKBP5, had high confidence to be identified as biomarkers to distinguish AD status, and these genes also interacted with some approved drugs for treatment of AD. These findings underscore the altered immune response in female is associated with sex difference in susceptibility to AD, and the necessity of considering sex factors when developing AD biomarkers and therapeutic strategies, providing a scientific basis for further in-depth studies on sex differences in AD.

Knowing the enemy: strategic targeting of complement to treat Alzheimer disease

The complement system protects against infection, positively responds to tissue damage, clears cell debris, directs and modulates the adaptive immune system, and functions in neuronal development, normal synapse elimination and intracellular metabolism. However, complement also has a role in aberrant synaptic pruning and neuroinflammation - processes that lead to a feedforward loop of inflammation, injury and neuronal death that can contribute to neurodegenerative and neurological disorders, including Alzheimer disease. This Review provides justification, largely from preclinical mouse models but also from correlates with human tissue and biomarkers, for targeting specific complement components for therapeutic intervention in Alzheimer disease. We discuss promising strategies to slow the progression of cognitive loss with minimal undesired effects. The diverse interactions and functions of complement system components can influence biological processes in the healthy and diseased brain; here, these functions are described as a prerequisite to selecting appropriate, safe and effective therapeutic targets for translation to the clinic.

Immune checkpoint TIM-3 regulates microglia and Alzheimer's disease

Microglia are the resident immune cells in the brain and have pivotal roles in neurodevelopment and neuroinflammation1,2. This study investigates the function of the immune-checkpoint molecule TIM-3 (encoded by HAVCR2) in microglia. TIM-3 was recently identified as a genetic risk factor for late-onset Alzheimer's disease3, and it can induce T cell exhaustion4. However, its specific function in brain microglia remains unclear. We demonstrate in mouse models that TGFβ signalling induces TIM-3 expression in microglia. In turn, TIM-3 interacts with SMAD2 and TGFBR2 through its carboxy-terminal tail, which enhances TGFβ signalling by promoting TGFBR-mediated SMAD2 phosphorylation, and this process maintains microglial homeostasis. Genetic deletion of Havcr2 in microglia leads to increased phagocytic activity and a gene-expression profile consistent with the neurodegenerative microglial phenotype (MGnD), also referred to as disease-associated microglia (DAM). Furthermore, microglia-targeted deletion of Havcr2 ameliorates cognitive impairment and reduces amyloid-β pathology in 5×FAD mice (a transgenic model of Alzheimer's disease). Single-nucleus RNA sequencing revealed a subpopulation of MGnD microglia in Havcr2-deficient 5×FAD mice characterized by increased pro-phagocytic and anti-inflammatory gene expression alongside reduced pro-inflammatory gene expression. These transcriptomic changes were corroborated by single-cell RNA sequencing data across most microglial clusters in Havcr2-deficient 5×FAD mice. Our findings reveal that TIM-3 mediates microglia homeostasis through TGFβ signalling and highlight the therapeutic potential of targeting microglial TIM-3 in Alzheimer's disease.

Phase 1, First-In-Human, Single-/Multiple-Ascending Dose Study of Iluzanebart in Healthy Volunteers

OBJECTIVE

To evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics of iluzanebart, a fully human monoclonal antibody TREM2 (triggering receptor expressed on myeloid cells 2) agonist, after single- (SAD) and multiple-ascending-dose (MAD) administration.

METHODS

Healthy adult volunteers (N = 136) received intravenous placebo or iluzanebart 1-60 mg/kg (SAD) or 10-60 mg/kg (MAD) followed by serial pharmacokinetics and safety assessments. Safety assessments included adverse events (AEs), vital signs, electrocardiograms, and clinical laboratory evaluations. Pharmacokinetics were assessed through noncompartmental analysis. The study also included open-label cohorts (3, 10, 20, 40, 60 mg/kg SAD; 10, 20, 40 mg/kg MAD) for cerebrospinal fluid (CSF) collection for exploratory pharmacodynamic biomarker analysis.

RESULTS

Iluzanebart was safe and well tolerated following single and multiple doses of up to 60 mg/kg. Most AEs were mild and resolved spontaneously. The most frequently reported AE was pruritus. No serious AEs or investigational product-related clinically meaningful changes in vital signs, electrocardiograms, or laboratory assessments were reported. Iluzanebart serum exposure was related to dose, with a 29-day half-life that is supportive of monthly dosing and confirmed central nervous system (CNS) exposure (≈0.15% CSF-to-serum ratio). Durable and dose-dependent target engagement, evidenced by marked reductions in soluble TREM2 and increased soluble CSF1R (colony-stimulating factor 1 receptor) and osteopontin/SPP1 (secreted phosphoprotein 1) levels in CSF, was observed, indicating that iluzanebart changes microglial activity following single and repeat dosing.

INTERPRETATION

Iluzanebart demonstrated favorable safety, tolerability, pharmacokinetics, and pharmacological activity in the CNS, supporting further clinical development for adult-onset leukoencephalopathy with axonal spheroids and pigmented glia.

TREM2+ macrophages: a key role in disease development

Triggering receptors expressed on myeloid cells 2 (TREM2), an immune receptor expressed on myeloid cells, has garnered considerable attention in recent years due to its role in unique signaling pathways and diverse biological functions, including phagocytosis, lipid metabolism, cell survival, and inflammatory responses. Although TREM2 is expressed in various cell types, such as macrophages, dendritic cells (DCs), osteoclasts, and others, where it exhibits context-dependent functional characteristics, it is mainly expressed in macrophages. Notably, TREM2 is implicated in the development and progression of multiple diseases, playing dual and often opposing roles in noncancerous diseases and cancers. This review aims to highlight the pivotal role of TREM2 in macrophages and immune-related diseases, elucidate its underlying mechanisms of action, explore its potential as a clinical diagnostic and prognostic marker, and propose therapeutic strategies targeting TREM2 based on current clinical trial data, providing comprehensive guidance and references for clinical practice.

Central Nervous System Macrophages in Health and Disease

The central nervous system (CNS) has a unique set of macrophages that seed the tissue early during embryonic development. Microglia reside in the parenchyma, and border-associated macrophages are present in border regions, including the meninges, perivascular spaces, and choroid plexus. CNS-resident macrophages support brain homeostasis during development and steady state. In the diseased brain, however, the immune landscape is altered, with phenotypic and transcriptional changes in resident macrophages and the invasion of blood-borne monocytes, which differentiate into monocyte-derived macrophages upon entering the CNS. In this review, we focus on the fate and function of the macrophage compartment in health, neurodegenerative conditions such as amyloidosis, and neuroinflammation as observed in multiple sclerosis and infection. We discuss our current understanding that monocyte-derived macrophages contribute to neuropathology whereas native macrophages play a neuroprotective role in disease.

The role of TREM2 in myelin sheath dynamics: A comprehensive perspective from physiology to pathology

Demyelinating disorders, characterizing by the loss of myelin integrity, present significant challenges due to their impact on neurological function and lack of effective treatments. Understanding the mechanisms underlying myelin damage is crucial for developing therapeutic strategies. Triggering receptor expressed on myeloid cells 2 (TREM2), a pivotal immune receptor predominantly found on microglial cells, plays essential roles in phagocytosis and lipid metabolism, vital processes in neuroinflammation and immune regulation. Emerging evidence indicates a close relationship between TREM2 and various aspects of myelin sheath dynamics, including maintenance, response to damage, and regeneration. This review provides a comprehensive discussion of TREM2's influence on myelin physiology and pathology, highlighting its therapeutic potential and putative mechanisms in the progression of demyelinating disorders.

Acute TREM2 inhibition depletes MAFB-high microglia and hinders remyelination

We investigated the role of Triggering Receptor Expressed on Myeloid cells 2 (TREM2) in myelin regeneration in the brain. TREM2 is a receptor that activates microglia, which are crucial for clearing myelin debris and promoting remyelination. Previous studies in a mouse model of demyelination induced by the copper-chelating agent Cuprizone (CPZ) have shown that stimulation of TREM2 with a monoclonal antibody reduces demyelination, while deleting the Trem2 gene in mice impairs remyelination. Here, we blocked TREM2 function acutely with an antibody during both the demyelination and remyelination phases of the CPZ model and analyzed the impact of the antibody treatment on myelination and gene expression in single cells. We found that blocking TREM2 depleted a distinct population of microglia with high expression of the transcription factor MAFB during remyelination. The loss of these MAFB-high microglia was linked to impaired generation of myelinating oligodendrocytes. Importantly, we identified MAFB+ microglia in acute and acute-chronic brain lesions from individuals with multiple sclerosis (MS), but not in inactive lesions. We conclude that TREM2 is essential for maintaining a population of MAFB-high microglia that is associated with myelin repair. This finding has significant implications for understanding demyelinating diseases like MS and suggests that stimulating TREM2 could be a promising therapeutic approach for myelin repair.

Enhancing TREM2 expression activates microglia and modestly mitigates tau pathology and neurodegeneration

TREM2, a microglia-specific receptor, is strongly associated with Alzheimer's disease (AD) risk, mediating microglial responses to amyloid pathology critical to AD development. However, its role in tau pathology and neurodegeneration remains unclear. Using the PS19 tauopathy mouse model with inducible overexpression of human wild-type TREM2 (TREM2-WT) or the R47H variant (TREM2-R47H), we show that increasing TREM2-WT expression modestly reduces soluble phosphorylated tau levels and mildly preserves neuronal integrity. Single-cell RNA sequencing reveals that TREM2-WT robustly enhances microglial activation, characterized by a disease-associated microglia (DAM) signature. In contrast, TREM2-R47H overexpression exhibits a loss-of-function phenotype, with no significant impact on tau levels, neurodegeneration, or microglial activation. These findings highlight the role of TREM2 in modulating microglial activity and its influence on tau pathology and neurodegeneration, providing important insights for the future development of therapies targeting TREM2 or microglial pathways in AD or other tauopathies.

Microglial Responses to Alzheimer's Disease Pathology: Insights From "Omics" Studies

Human genetics studies lent firm evidence that microglia are key to Alzheimer's disease (AD) pathogenesis over a decade ago following the identification of AD-associated genes that are expressed in a microglia-specific manner. However, while alterations in microglial morphology and gene expression are observed in human postmortem brain tissue, the mechanisms by which microglia drive and contribute to AD pathology remain ill-defined. Numerous mouse models have been developed to facilitate the disambiguation of the biological mechanisms underlying AD, incorporating amyloidosis, phosphorylated tau, or both. Over time, the use of multiple technologies including bulk tissue and single cell transcriptomics, epigenomics, spatial transcriptomics, proteomics, lipidomics, and metabolomics have shed light on the heterogeneity of microglial phenotypes and molecular patterns altered in AD mouse models. Each of these 'omics technologies provide unique information and biological insight. Here, we review the literature on the approaches and findings of these methods and provide a synthesis of the knowledge generated by applying these technologies to mouse models of AD.

The emerging role of the microglia triggering receptor expressed on myeloid cells (TREM) 2 in multiple sclerosis

BACKGROUND

The chronic inflammatory condition known as multiple sclerosis (MS) causes inflammation and demyelination in the central nervous system (CNS). The activation of multiple cell types, including the CNS's resident immune cells called microglia, is a component of the immunological response in MS. Recently, the triggering receptor expressed on myeloid cells (TREM) family has emerged as a crucial player in modulating microglial function and subsequent neuroinflammation. Understanding the role of TREM receptors in MS pathogenesis could provide insightful information on how to develop new therapeutic approaches.

MAIN BODY

The TREM family consists of several receptors, including TREM-1 and TREM-2, which can be expressed on both immune cells, such as myeloid cells and microglia, and non-immune cells. These receptors interact with their respective ligands and regulate signaling pathways, ultimately leading to the control of microglial activation and inflammatory reactions. TREM-2, in particular, has garnered significant interest because of its connection with MS and other neurodegenerative diseases. The activation of microglia through TREM receptors in MS is thought to influence the equilibrium between helpful and detrimental inflammatory responses. TREM receptors can promote the phagocytosis of myelin debris and remove apoptotic cells, thus contributing to tissue repair and regeneration. However, excessive or dysregulated activation of microglia mediated by TREM receptors can lead to the release of pro-inflammatory cytokines and neurotoxic factors, exacerbating neuroinflammation and neurodegeneration in MS.

CONCLUSION

The emerging role of the TREM family in demyelinating diseases highlights the importance of microglia in disease pathogenesis. Understanding the mechanisms by which TREM receptors modulate microglial function can provide valuable insights into the development of targeted therapies for these disorders. By selectively targeting TREM receptors, it may be possible to harness their beneficial effects on tissue repair while dampening their detrimental pro-inflammatory responses. Further research is warranted to elucidate the precise signaling pathways and ligand interactions involved in TREM-mediated microglial activation, which could uncover novel therapeutic avenues for treating MS and other neuroinflammatory disorders.

Rescue of in vitro models of CSF1R-related adult-onset leukodystrophy by iluzanebart: mechanisms and therapeutic implications of TREM2 agonism

Microglia dysfunction is implicated in several neurodegenerative disorders, including a rare microgliopathy; CSF1R-related adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (CSF1R-ALSP). CSF1R-ALSP is caused by heterozygous loss-of-function mutations in the colony stimulating factor 1 receptor (CSF1R) gene, which encodes a receptor required for the differentiation of myeloid cells, as well as for microglial survival and proliferation. Similar functions have also been ascribed to triggering receptor expressed on myeloid cells 2 (TREM2), which shares an analogous microglia enrichment profile and converging intracellular signaling pathway mediated by spleen associated tyrosine kinase (SYK) and phosphoinositide-3-kinase (PI3K). Iluzanebart is a human monoclonal IgG1, human TREM2 (hTREM2) agonist antibody under development for the treatment of CSF1R-ALSP. To explore the therapeutic hypothesis that loss of CSF1R signaling and related microglial hypofunction can be circumvented via activation of TREM2, we evaluated the potential of iluzanebart to compensate for CSF1R loss-of-function. Herein, we demonstrate that iluzanebart is a potent, dose-dependent, and specific activator of TREM2 signaling in human primary cells. Iluzanebart treatment rescued viability of human monocyte-derived macrophages (hMDM) and induced pluripotent stem cell-derived human microglia (iMGL) in multiple in vitro models of CSF1R-ALSP, including in induced pluripotent stem cell (iPSC) differentiated microglia carrying the heterozygous I794T mutation found in CSF1R-ALSP patients. Additionally, iluzanebart treatment in microglia modulated surface levels of CSF1R, resulting in increased receptor activation as measured by phosphorylation of CSF1R. Differentially expressed genes identified in the hippocampus of mice treated with iluzanebart were exemplary of TREM2 activation and were related to cell proliferation, regulation of inflammatory processes, and innate immune response pathways. Proliferation of microglia, changes in protein levels of specific chemokines identified by gene expression analysis, and increased CSF1R levels were also confirmed in vivo. These findings demonstrate that iluzanebart is a potent and selective TREM2 agonistic antibody, with pharmacology that supports the hypothesis that TREM2 activation can compensate for CSF1R dysfunction and its continued clinical development for individuals with CSF1R-ALSP.

Microglia depletion reduces human neuronal APOE4-related pathologies in a chimeric Alzheimer's disease model

Despite strong evidence supporting the important roles of both apolipoprotein E4 (APOE4) and microglia in Alzheimer's disease (AD) pathogenesis, the effects of microglia on neuronal APOE4-related AD pathogenesis remain elusive. To examine such effects, we utilized microglial depletion in a chimeric model with induced pluripotent stem cell (iPSC)-derived human neurons in mouse hippocampus. Specifically, we transplanted homozygous APOE4, isogenic APOE3, and APOE-knockout (APOE-KO) iPSC-derived human neurons into the hippocampus of human APOE3 or APOE4 knockin mice and then depleted microglia in half of the chimeric mice. We found that both neuronal APOE and microglial presence were important for the formation of Aβ and tau pathologies in an APOE isoform-dependent manner (APOE4 > APOE3). Single-cell RNA sequencing analysis identified two pro-inflammatory microglial subtypes with elevated MHC-II gene expression enriched in chimeric mice with human APOE4 neuron transplants. These findings highlight the concerted roles of neuronal APOE, especially APOE4, and microglia in AD pathogenesis.

TREM2 bridges microglia and extracellular microenvironment: Mechanistic landscape and therapeutical prospects on Alzheimer's disease

Neuroinflammation is closely related to the pathogenesis of Alzheimer's disease (AD). One of its prominent cellular components, microglia, is a potent coordinator of neuroinflammation in interplay with the characteristic AD pathological alterations including Aβ, tau, and neuronal defects, which constitute the AD-unique extracellular microenvironment. Mounting evidence implicates Triggering Receptors Expressed on Myeloid Cells 2 (TREM2) in the center of microglial activation, a vital event in the pathogenesis of AD. TREM2 is a pivotal microglial receptor that interacts with specific elements present in the AD microenvironment and induces microglial intracellular signallings contributing to phagocytosis, migration, cytokine production, metabolism, and survival, which shapes the microglial activation profile. It follows that TREM2 builds up a bridge between microglia and the extracellular microenvironment. This review illustrates how TREM2 modulates microglia to affect AD pathogenesis. Mainly presented facets in the review are i. the development of AD-specific microglial phenotypes (disease-associated microglia, DAM), ii. microglial interactions with major AD pathologies, and iii. the underlying intracellular signallings of microglial activation. Also, outstanding controversies regarding the nature of neuroinflammation are discussed. Through our illustration, we attempt to establish a TREM2-centered network of AD pathogenesis, in the hope as well to provide insights into the potential therapeutic strategies based on the underlying mechanisms.

Genetically Engineered Mouse Models for Alzheimer Disease and Frontotemporal Dementia: New Insights from Single-Cell and Spatial Transcriptomics

Neurodegenerative diseases, including Alzheimer disease, frontotemporal dementia, Parkinson disease, Huntington disease, and amyotrophic lateral sclerosis, are often casually linked to protein aggregation and inclusion. As the origins of those proteinopathies have been biochemically traced and genetically mapped, genetically engineered animal models carrying the specific mutations or variants are widely used for investigating the etiology of these diseases, as well as for testing potential therapeutics. This article focuses on the mouse models of Alzheimer disease and closely related frontotemporal dementia, particularly the ones that have provided most valuable knowledge, or are in a trajectory of doing so. More importantly, some of the major findings from these models are summarized, based on the recent single-cell transcriptomics, multiomics, and spatial transcriptomics studies. While no model is perfect, it is hoped that the new insights from these models and the practical use of these models will continue to help to establish a path forward.

Microglia in the aged brain develop a hypoactive molecular phenotype after surgery

BACKGROUND

Microglia, the resident immune cells of the brain, play a crucial role in maintaining homeostasis in the central nervous system (CNS). However, they can also contribute to neurodegeneration through their pro-inflammatory properties and phagocytic functions. Acute post-operative cognitive deficits have been associated with inflammation, and microglia have been implicated primarily based on morphological changes. We investigated the impact of surgery on the microglial transcriptome to test the hypothesis that surgery produces an age-dependent pro-inflammatory phenotype in these cells.

METHODS

Three-to-five and 20-to-22-month-old C57BL/6 mice were anesthetized with isoflurane for an abdominal laparotomy, followed by sacrifice either 6 or 48 h post-surgery. Age-matched controls were exposed to carrier gas. Cytokine concentrations in plasma and brain tissue were evaluated using enzyme-linked immunosorbent assays (ELISA). Iba1+ cell density and morphology were determined by immunohistochemistry. Microglia from both surgically treated mice and age-matched controls were isolated by a well-established fluorescence-activated cell sorting (FACS) protocol. The microglial transcriptome was then analyzed using quantitative polymerase chain reaction (qPCR) and RNA sequencing (RNAseq).

RESULTS

Surgery induced an elevation in plasma cytokines in both age groups. Notably, increased CCL2 was observed in the brain post-surgery, with a greater change in old compared to young mice. Age, rather than the surgical procedure, increased Iba1 immunoreactivity and the number of Iba1+ cells in the hippocampus. Both qPCR and RNAseq analysis demonstrated suppression of neuroinflammation at 6 h after surgery in microglia isolated from aged mice. A comparative analysis of differentially expressed genes (DEGs) with previously published neurodegenerative microglia phenotype (MGnD), also referred to disease-associated microglia (DAM), revealed that surgery upregulates genes typically downregulated in the context of neurodegenerative diseases. These surgery-induced changes resolved by 48 h post-surgery and only a few DEGs were detected at that time point, indicating that the hypoactive phenotype of microglia is transient.

CONCLUSIONS

While anesthesia and surgery induce pro-inflammatory changes in the plasma and brain of mice, microglia adopt a homeostatic molecular phenotype following surgery. This effect seems to be more pronounced in aged mice and is transient. These results challenge the prevailing assumption that surgery activates microglia in the aged brain.

A cross-disease resource of living human microglia identifies disease-enriched subsets and tool compounds recapitulating microglial states

Human microglia play a pivotal role in neurological diseases, but we still have an incomplete understanding of microglial heterogeneity, which limits the development of targeted therapies directly modulating their state or function. Here, we use single-cell RNA sequencing to profile 215,680 live human microglia from 74 donors across diverse neurological diseases and CNS regions. We observe a central divide between oxidative and heterocyclic metabolism and identify microglial subsets associated with antigen presentation, motility and proliferation. Specific subsets are enriched in susceptibility genes for neurodegenerative diseases or the disease-associated microglial signature. We validate subtypes in situ with an RNAscope-immunofluorescence pipeline and high-dimensional MERFISH. We also leverage our dataset as a classification resource, finding that induced pluripotent stem cell model systems capture substantial in vivo heterogeneity. Finally, we identify and validate compounds that recapitulate certain subtypes in vitro, including camptothecin, which downregulates the signature of disease-enriched subtypes and upregulates a signature previously associated with Alzheimer's disease.

Microglia and TREM2

TREM2 is a membrane receptor solely expressed on microglia in normal brain. In this review we outline recent advances in TREM2 biology and its implications for microglial function, with particular emphasis on findings from iPSC-derived microglia (iMG) expressing TREM2 loss-of-function mutations. Alterations in receptor proximal and distal signalling underlie TREM2 risk variants linked to neurodegenerative disease, principally NH-linked FTD, and late-onset AD, but emerging data suggest roles for TREM2 in PD, MS and ALS. TREM2 downstream functions include phagocytosis of myelin debris, amyloid beta peptides, and phosphatidylserine-expressing cells (resulting from damage or stress). Microglial survival, migration, DAMP signalling, inflammasome activation, and intercellular signalling including tau spreading via exosomes, as well as roles for sTREM2 in protection and as a biomarker are discussed. The role of TREM2 in metabolic homeostasis, and immunometabolic switching are discussed regarding microglial responses to damage and protection. The use of iPSC models to investigate the role of TREM2 in AD, PD, MS, ALS, and other neurodegenerative diseases could prove invaluable due to their ability to recapitulate human pathology, allowing a full understanding of TREM2 and microglial involvement in the underlying disease mechanisms and progression. This article is part of the Special Issue on "Microglia".

Tryptophan Metabolism in Alzheimer's Disease with the Involvement of Microglia and Astrocyte Crosstalk and Gut-Brain Axis

Alzheimer's disease (AD) is an age-dependent neurodegenerative disease characterized by extracellular Amyloid Aβ peptide (Aβ) deposition and intracellular Tau protein aggregation. Glia, especially microglia and astrocytes are core participants during the progression of AD and these cells are the mediators of Aβ clearance and degradation. The microbiota-gut-brain axis (MGBA) is a complex interactive network between the gut and brain involved in neurodegeneration. MGBA affects the function of glia in the central nervous system (CNS), and microbial metabolites regulate the communication between astrocytes and microglia; however, whether such communication is part of AD pathophysiology remains unknown. One of the potential links in bilateral gut-brain communication is tryptophan (Trp) metabolism. The microbiota-originated Trp and its metabolites enter the CNS to control microglial activation, and the activated microglia subsequently affect astrocyte functions. The present review highlights the role of MGBA in AD pathology, especially the roles of Trp per se and its metabolism as a part of the gut microbiota and brain communications. We (i) discuss the roles of Trp derivatives in microglia-astrocyte crosstalk from a bioinformatics perspective, (ii) describe the role of glia polarization in the microglia-astrocyte crosstalk and AD pathology, and (iii) summarize the potential of Trp metabolism as a therapeutic target. Finally, we review the role of Trp in AD from the perspective of the gut-brain axis and microglia, as well as astrocyte crosstalk, to inspire the discovery of novel AD therapeutics.

Orchestration of antiviral responses within the infected central nervous system

Many newly emerging and re-emerging viruses have neuroinvasive potential, underscoring viral encephalitis as a global research priority. Upon entry of the virus into the CNS, severe neurological life-threatening conditions may manifest that are associated with high morbidity and mortality. The currently available therapeutic arsenal against viral encephalitis is rather limited, emphasizing the need to better understand the conditions of local antiviral immunity within the infected CNS. In this review, we discuss new insights into the pathophysiology of viral encephalitis, with a focus on myeloid cells and CD8+ T cells, which critically contribute to protection against viral CNS infection. By illuminating the prerequisites of myeloid and T cell activation, discussing new discoveries regarding their transcriptional signatures, and dissecting the mechanisms of their recruitment to sites of viral replication within the CNS, we aim to further delineate the complexity of antiviral responses within the infected CNS. Moreover, we summarize the current knowledge in the field of virus infection and neurodegeneration and discuss the potential links of some neurotropic viruses with certain pathological hallmarks observed in neurodegeneration.

A systematic review of the role of TREM2 in Alzheimer's disease

BACKGROUND

Given the established genetic linkage between triggering receptors expressed on myeloid cells 2 (TREM2) and Alzheimer's disease (AD), an expanding research body has delved into the intricate role of TREM2 within the AD context. However, a conflicting landscape of outcomes has emerged from both in vivo and in vitro investigations. This study aimed to elucidate the multifaceted nuances and gain a clearer comprehension of the role of TREM2.

METHODS

PubMed database was searched spanning from its inception to January 2022. The search criteria took the form of ("Alzheimer's disease" OR "AD") AND ("transgenic mice model" OR "transgenic mouse model") AND ("Triggering receptor expressed on myeloid cells" OR "TREM2"). Inclusion criteria consisted of the following: (1) publication of original studies in English; (2) utilization of transgenic mouse models for AD research; and (3) reports addressing the subject of TREM2.

RESULTS

A total of 43 eligible articles were identified. Our analysis addresses four pivotal queries concerning the interrelation of TREM2 with microglial function, Aβ accumulation, tau pathology, and inflammatory processes. However, the diverse inquiries posed yielded inconsistent responses. Nevertheless, the inconsistent roles of TREM2 within these AD mouse models potentially hinge upon factors such as age, sex, brain region, model type, and detection methodologies.

CONCLUSIONS

This review substantiates the evolving understanding of TREM2's disease progression-dependent impacts. Furthermore, it reviews the interplay between TREM2 and its effects across diverse tissues and temporal stages.

Neutral or Detrimental Effects of TREM2 Agonist Antibodies in Preclinical Models of Alzheimer's Disease and Multiple Sclerosis

Human genetics and preclinical studies have identified key contributions of TREM2 to several neurodegenerative conditions, inspiring efforts to modulate TREM2 therapeutically. Here, we characterize the activities of three TREM2 agonist antibodies in multiple mixed-sex mouse models of Alzheimer's disease (AD) pathology and remyelination. Receptor activation and downstream signaling are explored in vitro, and active dose ranges are determined in vivo based on pharmacodynamic responses from microglia. For mice bearing amyloid-β (Aβ) pathology (PS2APP) or combined Aβ and tau pathology (TauPS2APP), chronic TREM2 agonist antibody treatment had limited impact on microglia engagement with pathology, overall pathology burden, or downstream neuronal damage. For mice with demyelinating injuries triggered acutely with lysolecithin, TREM2 agonist antibodies unexpectedly disrupted injury resolution. Likewise, TREM2 agonist antibodies limited myelin recovery for mice experiencing chronic demyelination from cuprizone. We highlight the contributions of dose timing and frequency across models. These results introduce important considerations for future TREM2-targeting approaches.

Flow Cytometry Analysis of Microglial Phenotypes in the Murine Brain During Aging and Disease

Microglia, the brain's primary resident immune cell, exists in various phenotypic states depending on intrinsic and extrinsic signaling. Distinguishing between these phenotypes can offer valuable biological insights into neurodevelopmental and neurodegenerative processes. Recent advances in single-cell transcriptomic profiling have allowed for increased granularity and better separation of distinct microglial states. While techniques such as immunofluorescence and single-cell RNA sequencing (scRNA-seq) are available to differentiate microglial phenotypes and functions, these methods present notable limitations, including challenging quantification methods, high cost, and advanced analytical techniques. This protocol addresses these limitations by presenting an optimized cell preparation procedure that prevents ex vivo activation and a flow cytometry panel to distinguish four distinct microglial states from murine brain tissue. Following cell preparation, fluorescent antibodies were applied to label 1) homeostatic, 2) disease-associated (DAM), 3) interferon response (IRM), and 4) lipid-droplet accumulating (LDAM) microglia, based on gene markers identified in previous scRNA-Seq studies. Stained cells were analyzed by flow cytometry to assess phenotypic distribution as a function of age and sex. A key advantage of this procedure is its adaptability, allowing the panel provided to be enhanced using additional markers with an appropriate cell analyzer (i.e., Cytek Aurora 5 laser spectral flow cytometer) and interrogating different brain regions or disease models. Additionally, this protocol does not require microglial cell sorting, resulting in a relatively quick and straightforward experiment. Ultimately, this protocol can compare the distribution of microglial phenotypic states between various experimental groups, such as disease state or age, with a lower cost and higher throughput than scRNA-seq. Key features • Analysis of microglial phenotypes from murine brain without the need for cell sorting, imaging, or scRNA-seq. • This protocol can distinguish between homeostatic, disease-associated (DAM), lipid-droplet accumulating (LDAM), and interferon response (IRM) microglia from any murine brain region and/or disease model of interest. • This protocol can be modified to incorporate additional markers of interest or dyes when using a cell analyzer capable of multiple color detections.

Effects of environmentally relevant concentration of short-chain chlorinated paraffins on BV2 microglia activation and lipid metabolism, implicating altered neurogenesis

Short-chain chlorinated paraffins (SCCPs), a class of persistent organic pollutants, have been found to cause diverse organ and systemic toxicity. However, little is known about their neurotoxic effects. In this study, we exposed BV2, a mouse microglia cell line, to environmentally relevant concentration of SCCPs (1 μg/L, 10 μg/L, 100 μg/L) for 24 h to investigate their impacts on the nervous system. Our observations revealed that SCCPs induced the activation of BV2 microglia, as indicated by altered morphology, stimulated cell proliferation, enhanced phagocytic and migratory capabilities. Analysis at the mRNA level confirmed the activation status, with the downregulation of TMEM119 and Tgfbr1, and upregulation of Iba1 and CD11b. The upregulated expression of genes such as cenpe, mki67, Axl, APOE and LPL also validated alterations in cell functions. Moreover, BV2 microglia presented an M2 alternative phenotype upon SCCPs exposure, substantiated by the reduction of NF-κB, TNF-α, IL-1β, and the elevation of TGF-β. Additionally, SCCPs caused lipid metabolic changes in BV2 microglia, characterized by the upregulations of long-chain fatty acids and acylcarnitines, reflecting an enhancement of β-oxidation. This aligns with our findings of increased ATP production upon SCCPs exposure. Intriguingly, cell activation coincided with elevated levels of omega-3 polyunsaturated fatty acids. Furthermore, activated microglial medium remarkably altered the proliferation and differentiation of mouse neural stem cells. Collectively, exposure to environmentally relevant concentrations of SCCPs resulted in activation and lipid metabolic alterations in BV2 microglia, potentially impacting neurogenesis. These findings provide valuable insights for further research on the neurotoxic effect of SCCPs.

Targeting dysregulated lipid metabolism for the treatment of Alzheimer's disease and Parkinson's disease: Current advancements and future prospects

Alzheimer's and Parkinson's diseases are two of the most frequent neurological diseases. The clinical features of AD are memory decline and cognitive dysfunction, while PD mainly manifests as motor dysfunction such as limb tremors, muscle rigidity abnormalities, and slow gait. Abnormalities in cholesterol, sphingolipid, and glycerophospholipid metabolism have been demonstrated to directly exacerbate the progression of AD by stimulating Aβ deposition and tau protein tangles. Indirectly, abnormal lipids can increase the burden on brain vasculature, induce insulin resistance, and affect the structure of neuronal cell membranes. Abnormal lipid metabolism leads to PD through inducing accumulation of α-syn, dysfunction of mitochondria and endoplasmic reticulum, and ferroptosis. Great progress has been made in targeting lipid metabolism abnormalities for the treatment of AD and PD in recent years, like metformin, insulin, peroxisome proliferator-activated receptors (PPARs) agonists, and monoclonal antibodies targeting apolipoprotein E (ApoE). This review comprehensively summarizes the involvement of dysregulated lipid metabolism in the pathogenesis of AD and PD, the application of Lipid Monitoring, and emerging lipid regulatory drug targets. A better understanding of the lipidological bases of AD and PD may pave the way for developing effective prevention and treatment methods for neurodegenerative disorders.

Maternal immunoglobulin G affects brain development of mouse offspring

Maternal immunoglobulin (Ig)G is present in breast milk and has been shown to contribute to the development of the immune system in infants. In contrast, maternal IgG has no known effect on early childhood brain development. We found maternal IgG immunoreactivity in microglia, which are resident macrophages of the central nervous system of the pup brain, peaking at postnatal one week. Strong IgG immunoreactivity was observed in microglia in the corpus callosum and cerebellar white matter. IgG stimulation of primary cultured microglia activated the type I interferon feedback loop by Syk. Analysis of neonatal Fc receptor knockout (FcRn KO) mice that could not take up IgG from their mothers revealed abnormalities in the proliferation and/or survival of microglia, oligodendrocytes, and some types of interneurons. Moreover, FcRn KO mice also exhibited abnormalities in social behavior and lower locomotor activity in their home cages. Thus, changes in the mother-derived IgG levels affect brain development in offsprings.

Xenografted human microglia display diverse transcriptomic states in response to Alzheimer's disease-related amyloid-β pathology

Microglia are central players in Alzheimer's disease pathology but analyzing microglial states in human brain samples is challenging due to genetic diversity, postmortem delay and admixture of pathologies. To circumvent these issues, here we generated 138,577 single-cell expression profiles of human stem cell-derived microglia xenotransplanted in the brain of the AppNL-G-F model of amyloid pathology and wild-type controls. Xenografted human microglia adopt a disease-associated profile similar to that seen in mouse microglia, but display a more pronounced human leukocyte antigen or HLA state, likely related to antigen presentation in response to amyloid plaques. The human microglial response also involves a pro-inflammatory cytokine/chemokine cytokine response microglia or CRM response to oligomeric Aβ oligomers. Genetic deletion of TREM2 or APOE as well as APOE polymorphisms and TREM2R47H expression in the transplanted microglia modulate these responses differentially. The expression of other Alzheimer's disease risk genes is differentially regulated across the distinct cell states elicited in response to amyloid pathology. Thus, we have identified multiple transcriptomic cell states adopted by human microglia in a multipronged response to Alzheimer's disease-related pathology, which should be taken into account in translational studies.

Antibody-mediated targeting of human microglial leukocyte Ig-like receptor B4 attenuates amyloid pathology in a mouse model

Microglia help limit the progression of Alzheimer's disease (AD) by constraining amyloid-β (Aβ) pathology, effected through a balance of activating and inhibitory intracellular signals delivered by distinct cell surface receptors. Human leukocyte Ig-like receptor B4 (LILRB4) is an inhibitory receptor of the immunoglobulin (Ig) superfamily that is expressed on myeloid cells and recognizes apolipoprotein E (ApoE) among other ligands. Here, we find that LILRB4 is highly expressed in the microglia of patients with AD. Using mice that accumulate Aβ and carry a transgene encompassing a portion of the LILR region that includes LILRB4, we corroborated abundant LILRB4 expression in microglia wrapping around Aβ plaques. Systemic treatment of these mice with an anti-human LILRB4 monoclonal antibody (mAb) reduced Aβ load, mitigated some Aβ-related behavioral abnormalities, enhanced microglia activity, and attenuated expression of interferon-induced genes. In vitro binding experiments established that human LILRB4 binds both human and mouse ApoE and that anti-human LILRB4 mAb blocks such interaction. In silico modeling, biochemical, and mutagenesis analyses identified a loop between the two extracellular Ig domains of LILRB4 required for interaction with mouse ApoE and further indicated that anti-LILRB4 mAb may block LILRB4-mApoE by directly binding this loop. Thus, targeting LILRB4 may be a potential therapeutic avenue for AD.

TREM2 in Alzheimer's disease: Structure, function, therapeutic prospects, and activation challenges

Triggering receptor expressed on myeloid cells 2 (TREM2) is a membrane glycoprotein that plays a crucial role in the regulation of microglial survival, activation, phagocytosis, as well as in the maintenance of brain homeostasis and the inflammatory response to injury or neurodegeneration. This review provides a comprehensive overview of TREM2 structure and functions, highlighting the role of its variants in the development and progression of Alzheimer's disease (AD), a devastating neurodegenerative disease that affects millions of people worldwide. Additionally, the article discusses the potential of TREM2 as a therapeutic target in AD, analyzing the current state of research and future prospects. Given the significant challenges associated with the activation of TREM2, particularly due to its diverse isoforms and the delicate balance required to modulate the immune response without triggering hyperactivation, this review aims to enhance our understanding of TREM2 in AD and inspire further research into this promising yet challenging therapeutic target.

Aducanumab anti-amyloid immunotherapy induces sustained microglial and immune alterations

Aducanumab, an anti-amyloid immunotherapy for Alzheimer's disease, efficiently reduces Aβ, though its plaque clearance mechanisms, long-term effects, and effects of discontinuation are not fully understood. We assessed the effect of aducanumab treatment and withdrawal on Aβ, neuritic dystrophy, astrocytes, and microglia in the APP/PS1 amyloid mouse model. We found that reductions in amyloid and neuritic dystrophy during acute treatment were accompanied by microglial and astrocytic activation, and microglial recruitment to plaques and adoption of an aducanumab-specific pro-phagocytic and pro-degradation transcriptomic signature, indicating a role for microglia in aducanumab-mediated Aβ clearance. Reductions in Aβ and dystrophy were sustained 15 but not 30 wk after discontinuation, and reaccumulation of plaques coincided with loss of the microglial aducanumab signature and failure of microglia to reactivate. This suggests that despite the initial benefit from treatment, microglia are unable to respond later to restrain plaque reaccumulation, making further studies on the effect of amyloid-directed immunotherapy withdrawal crucial for assessing long-term safety and efficacy.

Single-cell transcriptomics analysis of cellular heterogeneity and immune mechanisms in neurodegenerative diseases

Single-cell transcriptomics analysis is an advanced technology that can describe the intracellular transcriptome in complex tissues. It profiles and analyses datasets by single-cell RNA sequencing. Neurodegenerative diseases are identified by the abnormal apoptosis of neurons in the brain with few or no effective therapy strategies at present, which has been a growing healthcare concern and brought a great burden to society. The transcriptome of individual cells provides deep insights into previously unforeseen cellular heterogeneity and gene expression differences in neurodegenerative disorders. It detects multiple cell subsets and functional changes during pathological progression, which deepens the understanding of the molecular underpinnings and cellular basis of neurodegenerative diseases. Furthermore, the transcriptome analysis of immune cells shows the regulation of immune response. Different subtypes of immune cells and their interaction are found to contribute to disease progression. This finding enables the discovery of novel targets and biomarkers for early diagnosis. In this review, we emphasize the principles of the technology, and its recent progress in the study of cellular heterogeneity and immune mechanisms in neurodegenerative diseases. The application of single-cell transcriptomics analysis in neurodegenerative disorders would help explore the pathogenesis of these diseases and develop novel therapeutic methods.

Pyrolae herba alleviates cognitive impairment via hippocampal TREM2 signaling modulating neuroinflammation and neurogenesis in lipopolysaccharide-treated mice

ETHNOPHARMACOLOGICAL RELEVANC

Pyrolae herba (PH), a kind of Chinese herb, has been identified to have an anti-inflammatory effect, while the potential for treating cognitive impairment (CI), as well as the underlying mechanisms, is unclear. Currently, the interaction between neuroinflammation and neural function play a critical role in pathophysiology of CI.

AIM OF THE STUDY

To elucidate therapeutic effect of PH for CI as well as its underlying mechanisms with LPS-treated mice model.

METHODS AND MATERIALS

In this study, male C57BL6/J mice received lipopolysaccharide (LPS) injection for 10 days to establish CI model and were administrated with PH for 14 days. We used piracetam as a positive control. Memory and spatial function was tested by Morris water maze (MWM). The level of inflammation-related cytokines (TNF-α, IL-1β, IL-10, IL-6) were determined by enzyme-linked immunosorbent assay (ELISA) in serum and western blot in hippocampus. Immunofluorescence (IF) was used to measure the levels of ionized calcium binding linker molecule 1 (IBA-1), glial fibrillary acidic protein (GFAP), BrdU, Ki67 and doublecortin (DCX) in hippocampus. The mRNA sequencing was used to screen the potential target of PH with therapeutic CI. Reverse transcription-polymerase chain reaction (RT-PCR) was used to determine the gene alteration of triggering receptor expressed on myeloid cells 2 (TREM2) in hippocampus. We used western blot to determine protein expressions of TREM2 and its related signaling, as well as synaptic proteins in hippocampus.

RESULTS

The results revealed that LPS contributed to CI, and PH or piracetam treatment significantly ameliorated CI in MWM test. LPS contributed to increasing expressions of TNF-α and IL-1β in serum and hippocampus, which both reversed by PH or piracetam. PH or piracetam could inhibit the activation of glial cells including microglia and astrocyte in the hippocampus in LPS-induced CI model. The mRNA sequencing and RT-PCR results showed that LPS significantly increased the gene expression of TREM2, which was reversed by PH. The alteration of TREM2 expression was the most significant among the 10 genes (TREM2, Slc24a2, Ptch2, Gck, Il1rapl1, Cadps2, Btbd11, Secisbp2l, Tenm3 and Prepl) in hippocampus. Protein results showed that LPS upregulated the expressions of TREM2 and its related proteins including DAP12, spleen tyrosine kinase (SYK) phosphorylation and ADAM 10, which were all reversed by PH or piracetam in hippocampus. Furthermore, LPS was capable of reducing the expression of BrdU and DCX co-labeled positive cells in hippocampal dentate gyrus (DG), which was reversed only by PH. Moreover, PH or piracetam treatment significantly increased the expression of Ki67 and DCX co-labeled positive cells in hippocampal DG. The expression of synapsin1 was obviously decreased by LPS and was significantly reversed by PH or piracetam.

CONCLUSIONS

PH could alleviate CI by suppressing the secretion of pro-inflammatory cytokines and mitigating astrocyte activity by restraining microglia's activation in hippocampus, further facilitating neurogenesis and proliferation, thereby enhancing pre-synaptic protein. This study highlighted on the clinical application of PH, which might promote the use of phytomedicine in CI patients.

Immunological dimensions of neuroinflammation and microglial activation: exploring innovative immunomodulatory approaches to mitigate neuroinflammatory progression

The increasing life expectancy has led to a higher incidence of age-related neurodegenerative conditions. Within this framework, neuroinflammation emerges as a significant contributing factor. It involves the activation of microglia and astrocytes, leading to the release of pro-inflammatory cytokines and chemokines and the infiltration of peripheral leukocytes into the central nervous system (CNS). These instances result in neuronal damage and neurodegeneration through activated nucleotide-binding domain and leucine-rich repeat containing (NLR) family pyrin domain containing protein 3 (NLRP3) and nuclear factor kappa B (NF-kB) pathways and decreased nuclear factor erythroid 2-related factor 2 (Nrf2) activity. Due to limited effectiveness regarding the inhibition of neuroinflammatory targets using conventional drugs, there is challenging growth in the search for innovative therapies for alleviating neuroinflammation in CNS diseases or even before their onset. Our results indicate that interventions focusing on Interleukin-Driven Immunomodulation, Chemokine (CXC) Receptor Signaling and Expression, Cold Exposure, and Fibrin-Targeted strategies significantly promise to mitigate neuroinflammatory processes. These approaches demonstrate potential anti-neuroinflammatory effects, addressing conditions such as Multiple Sclerosis, Experimental autoimmune encephalomyelitis, Parkinson's Disease, and Alzheimer's Disease. While the findings are promising, immunomodulatory therapies often face limitations due to Immune-Related Adverse Events. Therefore, the conduction of randomized clinical trials in this matter is mandatory, and will pave the way for a promising future in the development of new medicines with specific therapeutic targets.

Roles of TREM2 in the Pathological Mechanism and the Therapeutic Strategies of Alzheimer's Disease

Alzheimer's disease (AD) is an age-related degenerative disease, which is characteristic by the deposition of senile plaques (SP) outside the cells, the neurofibrillary tangles (NFTs) inside the neurons, and the loss of synapse and neurons. Neuroinflammation may play an important role in the pathogenesis of AD. Microglia are the immune cells in the central nervous system. However, microglia might become disease-related microglia (DAMs) when stimulated by the external environment. DAMs have been shown to be involved in a series of events of AD development including Aβ accumulation and tau phosphorylation. The triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor that is mainly expressed by microglia in the central nervous system (CNS). TREM2 plays an important role in the physiological function of microglia, and the dyshomeostasis of TREM2 is related to the development of late-onset AD. This article summarized the latest advances in TREM2 biology and its impact on the roles of microglia in AD development, with a particular emphasis on the structure, ligands, signal transduction, and the agonistic antibodies of TREM2 for AD treatment. We further discussed the survival, migration, phagocytosis, inflammation, and cellular metabolism of microglia, as well as the role of sTREM2 in neuroprotection and as a biomarker for AD. It provides a reference for further research on the molecular mechanism of microglial TREM2 in the occurrence and development of AD and on the therapeutic strategies targeted on the microglial TREM2.

Targeting synapse function and loss for treatment of neurodegenerative diseases

Synapse dysfunction and loss are hallmarks of neurodegenerative diseases that correlate with cognitive decline. However, the mechanisms and therapeutic strategies to prevent or reverse synaptic damage remain elusive. In this Review, we discuss recent advances in understanding the molecular and cellular pathways that impair synapses in neurodegenerative diseases, including the effects of protein aggregation and neuroinflammation. We also highlight emerging therapeutic approaches that aim to restore synaptic function and integrity, such as enhancing synaptic plasticity, preventing synaptotoxicity, modulating neuronal network activity and targeting immune signalling. We discuss the preclinical and clinical evidence for each strategy, as well as the challenges and opportunities for developing effective synapse-targeting therapeutics for neurodegenerative diseases.

Emerging Microglial Therapies and Targets in Clinical Trial

Modulation of microglia function for treatment of neurodegenerative and neuropsychiatric disorders is an emerging field of neuroscience drug development. This is largely attributed to human genetic association studies combined with biological evidence indicating that the innate immune system acts as a causal contributor superimposed on the reactive component of neuronal loss in neurological dysfunction. The identification of disease risk gene variants that encode immune-modulatory proteins in microglia provides tools to evaluate how microglia cellular function or dysfunction affect neuronal health. The development of clinical stage therapeutic compounds that modify myeloid cell function enables us to investigate how modulating microglia function could become a transformational approach to mitigate neurological disorders. Improving our ability to boost microglia-promoting homeostatic and reparative functions hopefully will translate into achieving a better outcome for patients affected by neurological diseases. In this chapter, we aim to provide an overview of the microglial emerging therapies and targets being studied in current clinical trials.

Comparative Insight into Microglia/Macrophages-Associated Pathways in Glioblastoma and Alzheimer's Disease

Microglia and macrophages are pivotal to the brain's innate immune response and have garnered considerable attention in the context of glioblastoma (GBM) and Alzheimer's disease (AD) research. This review delineates the complex roles of these cells within the neuropathological landscape, focusing on a range of signaling pathways-namely, NF-κB, microRNAs (miRNAs), and TREM2-that regulate the behavior of tumor-associated macrophages (TAMs) in GBM and disease-associated microglia (DAMs) in AD. These pathways are critical to the processes of neuroinflammation, angiogenesis, and apoptosis, which are hallmarks of GBM and AD. We concentrate on the multifaceted regulation of TAMs by NF-κB signaling in GBM, the influence of TREM2 on DAMs' responses to amyloid-beta deposition, and the modulation of both TAMs and DAMs by GBM- and AD-related miRNAs. Incorporating recent advancements in molecular biology, immunology, and AI techniques, through a detailed exploration of these molecular mechanisms, we aim to shed light on their distinct and overlapping regulatory functions in GBM and AD. The review culminates with a discussion on how insights into NF-κB, miRNAs, and TREM2 signaling may inform novel therapeutic approaches targeting microglia and macrophages in these neurodegenerative and neoplastic conditions. This comparative analysis underscores the potential for new, targeted treatments, offering a roadmap for future research aimed at mitigating the progression of these complex diseases.

Cytokine enrichment in deep cerebellar nuclei is contributed by multiple glial populations and linked to reduced amyloid plaque pathology

Alzheimer's disease (AD) pathology and amyloid-beta (Aβ) plaque deposition progress slowly in the cerebellum compared to other brain regions, while the entorhinal cortex (EC) is one of the most vulnerable regions. Using a knock-in AD mouse model (App KI), we show that within the cerebellum, the deep cerebellar nuclei (DCN) has particularly low accumulation of Aβ plaques. To identify factors that might underlie differences in the progression of AD-associated neuropathology across regions, we profiled gene expression in single nuclei (snRNAseq) across all cell types in the DCN and EC of wild-type (WT) and App KI male mice at age 7 months. We found differences in expression of genes associated with inflammatory activation, PI3K-AKT signalling, and neuron support functions between both regions and genotypes. In WT mice, the expression of interferon-response genes in microglia is higher in the DCN than the EC and this enrichment is confirmed by RNA in situ hybridisation, and measurement of inflammatory cytokines by protein array. Our analyses also revealed that multiple glial populations are responsible for establishing this cytokine-enriched niche. Furthermore, homogenates derived from the DCN induced inflammatory gene expression in BV2 microglia. We also assessed the relationship between the DCN microenvironment and Aβ pathology by depleting microglia using a CSF1R inhibitor PLX5622 and saw that, surprisingly, the expression of a subset of inflammatory cytokines was increased while plaque abundance in the DCN was further reduced. Overall, our study revealed the presence of a cytokine-enriched microenvironment unique to the DCN that when modulated, can alter plaque deposition.

Microglia Depletion Reduces Human Neuronal APOE4-Driven Pathologies in a Chimeric Alzheimer's Disease Model

Despite strong evidence supporting the involvement of both apolipoprotein E4 (APOE4) and microglia in Alzheimer's Disease (AD) pathogenesis, the effects of microglia on neuronal APOE4-driven AD pathogenesis remain elusive. Here, we examined such effects utilizing microglial depletion in a chimeric model with human neurons in mouse hippocampus. Specifically, we transplanted homozygous APOE4, isogenic APOE3, and APOE-knockout (APOE-KO) induced pluripotent stem cell (iPSC)-derived human neurons into the hippocampus of human APOE3 or APOE4 knock-in mice, and depleted microglia in half the chimeric mice. We found that both neuronal APOE and microglial presence were important for the formation of Aβ and tau pathologies in an APOE isoform-dependent manner (APOE4 > APOE3). Single-cell RNA-sequencing analysis identified two pro-inflammatory microglial subtypes with high MHC-II gene expression that are enriched in chimeric mice with human APOE4 neuron transplants. These findings highlight the concerted roles of neuronal APOE, especially APOE4, and microglia in AD pathogenesis.

Stimulation of TREM2 with agonistic antibodies-an emerging therapeutic option for Alzheimer's disease

Neurodegenerative disorders, including Alzheimer's disease, are associated with microgliosis. Microglia have long been considered to have detrimental roles in Alzheimer's disease. However, functional analyses of genes encoding risk factors that are linked to late-onset Alzheimer's disease, and that are enriched or exclusively expressed in microglia, have revealed unexpected protective functions. One of the major risk genes for Alzheimer's disease is TREM2. Risk variants of TREM2 are loss-of-function mutations affecting chemotaxis, phagocytosis, lipid and energy metabolism, and survival and proliferation. Agonistic anti-TREM2 antibodies have been developed to boost these protective functions in patients with intact TREM2 alleles. Several anti-TREM2 antibodies are in early clinical trials, and current efforts aim to achieve more efficient transport of these antibodies across the blood-brain barrier. PET imaging could be used to monitor target engagement. Data from animal models, and biomarker studies in patients, further support a rationale for boosting TREM2 functions during the preclinical stage of Alzheimer's disease.

The Dual Nature of Microglia in Alzheimer's Disease: A Microglia-Neuron Crosstalk Perspective

Microglia are critical players in the neuroimmune system, and their involvement in Alzheimer's disease (AD) pathogenesis is increasingly being recognized. However, whether microglia play a positive or negative role in AD remains largely controversial and the precise molecular targets for intervention are not well defined. This partly results from the opposing roles of microglia in AD pathology, and is mainly reflected in the microglia-neuron interaction. Microglia can prune synapses resulting in excessive synapse loss and neuronal dysfunction, but they can also promote synapse formation, enhancing neural network plasticity. Neuroimmune crosstalk accelerates microglial activation, which induces neuron death and enhances the microglial phagocytosis of β-amyloid to protect neurons. Moreover, microglia have dual opposing roles in developing the major pathological features in AD, such as amyloid deposition and blood-brain barrier permeability. This review summarizes the dual opposing role of microglia in AD from the perspective of the interaction between neurons and microglia. Additionally, current AD treatments targeting microglia and the advantages and disadvantages of developing microglia-targeted therapeutic strategies are discussed.

Human microglial state dynamics in Alzheimer's disease progression

Altered microglial states affect neuroinflammation, neurodegeneration, and disease but remain poorly understood. Here, we report 194,000 single-nucleus microglial transcriptomes and epigenomes across 443 human subjects and diverse Alzheimer's disease (AD) pathological phenotypes. We annotate 12 microglial transcriptional states, including AD-dysregulated homeostatic, inflammatory, and lipid-processing states. We identify 1,542 AD-differentially-expressed genes, including both microglia-state-specific and disease-stage-specific alterations. By integrating epigenomic, transcriptomic, and motif information, we infer upstream regulators of microglial cell states, gene-regulatory networks, enhancer-gene links, and transcription-factor-driven microglial state transitions. We demonstrate that ectopic expression of our predicted homeostatic-state activators induces homeostatic features in human iPSC-derived microglia-like cells, while inhibiting activators of inflammation can block inflammatory progression. Lastly, we pinpoint the expression of AD-risk genes in microglial states and differential expression of AD-risk genes and their regulators during AD progression. Overall, we provide insights underlying microglial states, including state-specific and AD-stage-specific microglial alterations at unprecedented resolution.

The biology of TREM receptors

Triggering receptors expressed on myeloid cells (TREMs) encompass a family of cell-surface receptors chiefly expressed by granulocytes, monocytes and tissue macrophages. These receptors have been implicated in inflammation, neurodegenerative diseases, bone remodelling, metabolic syndrome, atherosclerosis and cancer. Here, I review the structure, ligands, signalling modes and functions of TREMs in humans and mice and discuss the challenges that remain in understanding TREM biology.

The effect of different sources of mesenchymal stem cells on microglia states

Microglial reaction plays a key role in the prognosis of traumatic CNS injuries (TBI and SCI). A growing number of studies have shown that mesenchymal stem cells (MSCs) play an important role in regulating microglial states. This review summarizes the effects and mechanisms of different sources of MSCs on microglial states in the last 5 years. In general, bone marrow-derived mesenchymal stem cells are the most accessible and widely used, and can produce immunosuppressive effects on a variety of brain injuries including TBI through tissue engineering in situ implantation; MSCs mainly regulate inflammatory pathways and promote the states of microglia in the anti-inflammatory direction, which also secrete certain cytokines or extracellular vesicles to affect apoptotic pathways, such as the extracellular vesicles miR-21-5p, acting as a neuronal protector.

Identification of a protective microglial state mediated by miR-155 and interferon-γ signaling in a mouse model of Alzheimer's disease

Microglia play a critical role in brain homeostasis and disease progression. In neurodegenerative conditions, microglia acquire the neurodegenerative phenotype (MGnD), whose function is poorly understood. MicroRNA-155 (miR-155), enriched in immune cells, critically regulates MGnD. However, its role in Alzheimer's disease (AD) pathogenesis remains unclear. Here, we report that microglial deletion of miR-155 induces a pre-MGnD activation state via interferon-γ (IFN-γ) signaling, and blocking IFN-γ signaling attenuates MGnD induction and microglial phagocytosis. Single-cell RNA-sequencing analysis of microglia from an AD mouse model identifies Stat1 and Clec2d as pre-MGnD markers. This phenotypic transition enhances amyloid plaque compaction, reduces dystrophic neurites, attenuates plaque-associated synaptic degradation and improves cognition. Our study demonstrates a miR-155-mediated regulatory mechanism of MGnD and the beneficial role of IFN-γ-responsive pre-MGnD in restricting neurodegenerative pathology and preserving cognitive function in an AD mouse model, highlighting miR-155 and IFN-γ as potential therapeutic targets for AD.

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.

Phagocytosis increases an oxidative metabolic and immune suppressive signature in tumor macrophages

Phagocytosis is a key macrophage function, but how phagocytosis shapes tumor-associated macrophage (TAM) phenotypes and heterogeneity in solid tumors remains unclear. Here, we utilized both syngeneic and novel autochthonous lung tumor models in which neoplastic cells express the fluorophore tdTomato (tdTom) to identify TAMs that have phagocytosed neoplastic cells in vivo. Phagocytic tdTompos TAMs upregulated antigen presentation and anti-inflammatory proteins, but downregulated classic proinflammatory effectors compared to tdTomneg TAMs. Single-cell transcriptomic profiling identified TAM subset-specific and common gene expression changes associated with phagocytosis. We uncover a phagocytic signature that is predominated by oxidative phosphorylation (OXPHOS), ribosomal, and metabolic genes, and this signature correlates with worse clinical outcome in human lung cancer. Expression of OXPHOS proteins, mitochondrial content, and functional utilization of OXPHOS were increased in tdTompos TAMs. tdTompos tumor dendritic cells also display similar metabolic changes. Our identification of phagocytic TAMs as a distinct myeloid cell state links phagocytosis of neoplastic cells in vivo with OXPHOS and tumor-promoting phenotypes.

Alzheimer's disease and neuroinflammation: will new drugs in clinical trials pave the way to a multi-target therapy?

Despite extensive research, no disease-modifying therapeutic option, able to prevent, cure or halt the progression of Alzheimer's disease [AD], is currently available. AD, a devastating neurodegenerative pathology leading to dementia and death, is characterized by two pathological hallmarks, the extracellular deposits of amyloid beta (Aβ) and the intraneuronal deposits of neurofibrillary tangles (NFTs) consisting of altered hyperphosphorylated tau protein. Both have been widely studied and pharmacologically targeted for many years, without significant therapeutic results. In 2022, positive data on two monoclonal antibodies targeting Aβ, donanemab and lecanemab, followed by the 2023 FDA accelerated approval of lecanemab and the publication of the final results of the phase III Clarity AD study, have strengthened the hypothesis of a causal role of Aβ in the pathogenesis of AD. However, the magnitude of the clinical effect elicited by the two drugs is limited, suggesting that additional pathological mechanisms may contribute to the disease. Cumulative studies have shown inflammation as one of the main contributors to the pathogenesis of AD, leading to the recognition of a specific role of neuroinflammation synergic with the Aβ and NFTs cascades. The present review provides an overview of the investigational drugs targeting neuroinflammation that are currently in clinical trials. Moreover, their mechanisms of action, their positioning in the pathological cascade of events that occur in the brain throughout AD disease and their potential benefit/limitation in the therapeutic strategy in AD are discussed and highlighted as well. In addition, the latest patent requests for inflammation-targeting therapeutics to be developed in AD will also be discussed.

Novel CH25H+ and OASL+ microglia subclusters play distinct roles in cerebral ischemic stroke

BACKGROUND

Microglial polarization is one of the most promising therapeutic targets for multiple central nervous system (CNS) disorders, including ischemic stroke. However, detailed transcriptional alteration of microglia following cerebral ischemic stroke remains largely unclear.

METHODS

Focal cerebral ischemia was induced by transient middle cerebral artery occlusion (tMCAO) for 60 min in mice. Single-cell RNA sequencing (scRNA-seq) was performed using ischemic brain tissues from tMCAO and sham mice 3 days after surgery. Ch25h^-/- mice were used to investigate the role of specific microglia subcluster on post-stroke infarct volume and neuroinflammation.

RESULTS

We identified a relatively homeostatic subcluster with enhanced antigen processing and three "ischemic stroke associated microglia" (ISAM): MKI67⁺, CH25H⁺ and OASL⁺ subclusters. We found the MKI67⁺ subcluster undergo proliferation and differentiation into CH25H⁺ and OASL⁺ subclusters. CH25H⁺ microglia was a critical subcluster of ISAM that exhibited increased phagocytosis and neuroprotective property after stroke. Ch25h^-/- mice developed significantly increased infarct volume following ischemic stroke compared to Ch25h^+/-. Meanwhile, the OASL⁺ subcluster accumulated in the ischemic brain and was associated with the evolving of neuroinflammation after stroke, which was further aggravated in the aged mice brain.

CONCLUSIONS

Our data reveal previously unrecognized roles of the newly defined CH25H⁺ and OASL⁺ microglia subclusters following ischemic stroke, with novel insights for precise microglia modulation towards stroke therapy.