TREM2-mediated microglial tau clearance enhancement

Triggering receptor expressed on myeloid cells 2 (TREM2) has been suggested to play a crucial role in Alzheimer's disease (AD) pathogenesis, as revealed by genome-wide association studies (GWAS). Since then, rapidly increasing literature related to TREM2 has focused on elucidating its role in AD pathology. In this review, we summarize our understanding of TREM2 biology, explore TREM2 functions in microglia, address the multiple mechanisms of TREM2 in AD, and raise key questions for further investigations to elucidate the detailed roles and molecular mechanisms of TREM2 in microglial responses. A major breakthrough in our understanding of TREM2 is based on our hypothesis suggesting that TREM2 may act as a multifaceted player in microglial functions in AD brain homeostasis. We conclude that TREM2 can not only influence microglial functions in amyloid and tau pathologies but also participate in inflammatory responses and metabolism, acting alone or with other molecules, such as apolipoprotein E (APOE). This review provides novel insight into the broad role of TREM2 in microglial function in AD and enables us to develop new strategies aimed at the immune system to treat AD pathogenesis.

TREM2 is a receptor for lipids expressed in microglia. The R47H variant of human TREM2 impairs ligand binding and increases Alzheimer's disease (AD) risk. In mouse models of amyloid β (Aβ) accumulation, defective TREM2 function affects microglial response to Aβ plaques, exacerbating tissue damage, whereas TREM2 overexpression attenuates pathology. Thus, AD may benefit from TREM2 activation. Here, we examined the impact of an anti-human TREM2 agonistic mAb, AL002c, in a mouse AD model expressing either the common variant (CV) or the R47H variant of TREM2. Single-cell RNA-seq of microglia after acute systemic administration of AL002c showed induction of proliferation in both CV- and R47H-transgenic mice. Prolonged administration of AL002c reduced filamentous plaques and neurite dystrophy, impacted behavior, and tempered microglial inflammatory response. We further showed that a variant of AL002c is safe and well tolerated in a first-in-human phase I clinical trial and engages TREM2 based on cerebrospinal fluid biomarkers. We conclude that AL002 is a promising candidate for AD therapy.

Predisposition to Alzheimer's disease (AD) may arise from lipid metabolism perturbation, however, the underlying mechanism remains elusive. Here, we identify ATPase family AAA-domain containing protein 3A (ATAD3A), a mitochondrial AAA-ATPase, as a molecular switch that links cholesterol metabolism impairment to AD phenotypes. In neuronal models of AD, the 5XFAD mouse model and post-mortem AD brains, ATAD3A is oligomerized and accumulated at the mitochondria-associated ER membranes (MAMs), where it induces cholesterol accumulation by inhibiting gene expression of CYP46A1, an enzyme governing brain cholesterol clearance. ATAD3A and CYP46A1 cooperate to promote APP processing and synaptic loss. Suppressing ATAD3A oligomerization by heterozygous ATAD3A knockout or pharmacological inhibition with DA1 restores neuronal CYP46A1 levels, normalizes brain cholesterol turnover and MAM integrity, suppresses APP processing and synaptic loss, and consequently reduces AD neuropathology and cognitive deficits in AD transgenic mice. These findings reveal a role for ATAD3A oligomerization in AD pathogenesis and suggest ATAD3A as a potential therapeutic target for AD.

Adenosine monophosphate-activated protein kinase (AMPK) activity is stimulated to promote metabolic adaptation upon energy stress. However, sustained metabolic stress may cause cell death. The mechanisms by which AMPK dictates cell death are not fully understood. We report that metabolic stress promoted receptor-interacting protein kinase 1 (RIPK1) activation mediated by TRAIL receptors, whereas AMPK inhibited RIPK1 by phosphorylation at Ser415 to suppress energy stress-induced cell death. Inhibiting pS415-RIPK1 by Ampk deficiency or RIPK1 S415A mutation promoted RIPK1 activation. Furthermore, genetic inactivation of RIPK1 protected against ischemic injury in myeloid Ampkα1-deficient mice. Our studies reveal that AMPK phosphorylation of RIPK1 represents a crucial metabolic checkpoint, which dictates cell fate response to metabolic stress, and highlight a previously unappreciated role for the AMPK-RIPK1 axis in integrating metabolism, cell death, and inflammation.

Parkinson's disease (PD) is a neurodegenerative disease characterized by the death of dopaminergic neurons in the substantia nigra and the formation of Lewy bodies that are composed of aggregated α-synuclein (α-Syn). However, the factors that regulate α-Syn pathology and nigrostriatal dopaminergic degeneration remain poorly understood. Previous studies demonstrate cholesterol 24-hydroxylase (CYP46A1) increases the risk for PD. Moreover, 24-hydroxycholesterol (24-OHC), a brain-specific oxysterol that is catalyzed by CYP46A1, is elevated in the cerebrospinal fluid of PD patients. Herein, we show that the levels of CYP46A1 and 24-OHC are elevated in PD patients and increase with age in a mouse model. Overexpression of CYP46A1 intensifies α-Syn pathology, whereas genetic removal of CYP46A1 attenuates α-Syn neurotoxicity and nigrostriatal dopaminergic degeneration in the brain. Moreover, supplementation with exogenous 24-OHC exacerbates the mitochondrial dysfunction induced by α-Syn fibrils. Intracerebral injection of 24-OHC enhances the spread of α-Syn pathology and dopaminergic neurodegeneration via elevated X-box binding protein 1 (XBP1) and lymphocyte-activation gene 3 (LAG3) levels. Thus, elevated CYP46A1 and 24-OHC promote neurotoxicity and the spread of α-Syn via the XBP1-LAG3 axis. Strategies aimed at inhibiting the CYP46A1-24-OHC axis and LAG3 could hold promise as disease-modifying therapies for PD.

Triggering receptor expressed on myeloid cell 2 (TREM2) is linked to risk of neurodegenerative disease. However, the function of TREM2 in neurodegeneration is still not fully understood. Here, we investigated the role of microglial TREM2 in TAR DNA-binding protein 43 (TDP-43)-related neurodegeneration using virus-mediated and transgenic mouse models. We found that TREM2 deficiency impaired phagocytic clearance of pathological TDP-43 by microglia and enhanced neuronal damage and motor impairments. Mass cytometry analysis revealed that human TDP-43 (hTDP-43) induced a TREM2-dependent subpopulation of microglia with high CD11c expression and phagocytic ability. Using mass spectrometry (MS) and surface plasmon resonance (SPR) analysis, we further demonstrated an interaction between TDP-43 and TREM2 in vitro and in vivo as well as in human tissues from individuals with amyotrophic lateral sclerosis (ALS). We computationally identified regions within hTDP-43 that interact with TREM2. Our data highlight that TDP-43 is a possible ligand for microglial TREM2 and that this interaction mediates neuroprotection of microglia in TDP-43-related neurodegeneration.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder driven by complex interactions between neuroinflammation, immune dysregulation, metabolic impairment, and disrupted synaptic plasticity. Emerging evidence highlights maladaptive microglial activation, chronic cytokine signaling (including IL-1β, TNF-α, and IL-6), and hypothalamic-pituitary-adrenal (HPA) axis hyperactivity as pivotal contributors to neuronal damage and cognitive decline. Genetic studies further underscore the importance of immune and metabolic pathways, implicating key risk genes such as APOE, TREM2, and CR1, while deficits in autophagy exacerbate pathological protein aggregation, including amyloid-β and tau, ultimately accelerating synaptic loss. In this review, we synthesize molecular, genetic, and cellular evidence to clarify the mechanisms driving AD pathogenesis. We discuss genome-wide association study (GWAS) findings that define the genetic architecture of the disease, the neuroimmune crosstalk affecting memory-related brain regions, the link between chronic stress and amyloid pathology through HPA-axis dysregulation, and metabolic reprogramming in neurons, astrocytes, and microglia. Together, these interconnected processes highlight how dysregulated immunity and impaired protein clearance contribute to neuronal dysfunction and the progressive cognitive decline characteristic of AD.

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline and pathological hallmarks, including amyloid plaques, tau tangles, microgliosis, and chronic neuroinflammation. Over the past decade, advances in human genetics have revealed microglia and the innate immune pathways are central determinants of AD susceptibility, resilience, and progression, fundamentally redefining the recent conceptual framework of AD research. Genome-wide association studies (GWAS) implicate microglia-enriched genes including triggering receptor expressed on myeloid cells 2 (TREM2), phospholipase-C gamma 2 (PLCG2), and inositol polyphosphate-5-phosphatase D (INPP5D). Among these, the rare PLCG2 P522R variant is associated with reduced AD risk, enhanced microglial responsiveness, and enrichment in cognitively healthy centenarians. Single-cell and spatial transcriptomic studies have uncovered substantial microglial heterogeneity and pronounced region-specific alterations across age and disease progression. These analyses show that microglia transition through a spectrum of transcriptionally distinct states regulated by coordinated remodeling of lipid metabolic, phagocytic and lysosomal pathways, as well as cytokine-receptor signaling networks. Depending on the direction of these state transitions, microglia may engage neuroprotective programs that enhance debris clearance, maintain tissue homeostasis, and support repair, or alternatively, enter maladaptive

Childhood neglect and deprivation are the most common forms of early adversity, yet their biological impact on cognitive development-and how enrichment mitigates these effects-remains poorly understood. Using limited bedding (LB) as a mouse model of deprivation, we previously showed that abnormal microglia-mediated synaptic pruning during the second and third postnatal weeks impairs synaptic connectivity and hippocampal function, particularly in males. However, the molecular basis of this microglial dysfunction is unclear. Here, we demonstrate that LB reduces expression of Triggering Receptor Expressed on Myeloid cells 2 (TREM2) across multiple mouse strains and that TREM2 deficiency accounts for roughly half of the phagocytic deficit. Overexpressing TREM2 restores microglial phagocytic function and rescues deficits in hippocampal connectivity and fear learning later in life. Brief postnatal enrichment normalizes synaptic pruning in a TREM2-dependent manner and restores contextual fear conditioning in adolescent LB male mice. Together, these findings identify TREM2 activity during early development as a key mediator of the long-term impact of deprivation and enrichment on synaptic connectivity and cognitive function.

Microglia are tissue-resident macrophages of the central nervous system (CNS). In the CNS, microglia play an important role in the monitoring and intervention of synaptic and neuron-level activities. Interventions targeting microglia have been shown to improve the prognosis of various neurological diseases. Recently, studies have observed the activation of microglia in different cardiovascular diseases. In addition, different approaches that regulate the activity of microglia have been shown to modulate the incidence and progression of cardiovascular diseases. The change in autonomic nervous system activity after neuroinflammation may be a potential intermediate link between microglia and cardiovascular diseases. Here, in this review, we will discuss recent updates on the regulatory role of microglia in hypertension, myocardial infarction and ischemia/reperfusion injury. We propose that microglia serve as neuroimmune modulators and potential targets for cardiovascular diseases.

Triggering receptor expressed on myeloid cells 2 (TREM2) has been suggested to play a crucial role in Alzheimer's disease (AD) pathogenesis, as revealed by genome-wide association studies (GWAS). Since then, rapidly increasing literature related to TREM2 has focused on elucidating its role in AD pathology. In this review, we summarize our understanding of TREM2 biology, explore TREM2 functions in microglia, address the multiple mechanisms of TREM2 in AD, and raise key questions for further investigations to elucidate the detailed roles and molecular mechanisms of TREM2 in microglial responses. A major breakthrough in our understanding of TREM2 is based on our hypothesis suggesting that TREM2 may act as a multifaceted player in microglial functions in AD brain homeostasis. We conclude that TREM2 can not only influence microglial functions in amyloid and tau pathologies but also participate in inflammatory responses and metabolism, acting alone or with other molecules, such as apolipoprotein E (APOE). This review provides novel insight into the broad role of TREM2 in microglial function in AD and enables us to develop new strategies aimed at the immune system to treat AD pathogenesis.

The diverse leukocyte infiltrate in atherosclerotic mouse aortas was recently analyzed in 9 single-cell RNA sequencing and 2 mass cytometry studies. In a comprehensive meta-analysis, we confirm 4 known macrophage subsets-resident, inflammatory, interferon-inducible cell, and Trem2 (triggering receptor expressed on myeloid cells-2) foamy macrophages-and identify a new macrophage subset resembling cavity macrophages. We also find that monocytes, neutrophils, dendritic cells, natural killer cells, innate lymphoid cells-2, and CD (cluster of differentiation)-8 T cells form prominent and separate immune cell populations in atherosclerotic aortas. Many CD4 T cells express IL (interleukin)-17 and the chemokine receptor CXCR (C-X-C chemokine receptor)-6. A small number of regulatory T cells and T helper 1 cells is also identified. Immature and naive T cells are present in both healthy and atherosclerotic aortas. Our meta-analysis overcomes limitations of individual studies that, because of their experimental approach, over- or underrepresent certain cell populations. Mass cytometry studies demonstrate that cell surface phenotype provides valuable information beyond the cell transcriptomes. The present analysis helps resolve some long-standing controversies in the field. First, Trem2+ foamy macrophages are not proinflammatory but interferon-inducible cell and inflammatory macrophages are. Second, about half of all foam cells are smooth muscle cell-derived, retaining smooth muscle cell

Triggering receptor expressed on myeloid cells 2 (TREM2) is a single-pass transmembrane immune receptor that is mainly expressed on microglia in the brain and macrophages in the periphery. Recent studies have identified TREM2 as a risk factor for Alzheimer's disease (AD). Increasing evidence has shown that TREM2 can affect lipid metabolism both in the central nervous system (CNS) and in the periphery. In the CNS, TREM2 affects the metabolism of cholesterol, myelin, and phospholipids and promotes the transition of microglia into a disease-associated phenotype. In the periphery, TREM2 influences lipid metabolism by regulating the onset and progression of obesity and its complications, such as hypercholesterolemia, atherosclerosis, and nonalcoholic fatty liver disease. All these altered lipid metabolism processes could influence the pathogenesis of AD through several means, including affecting inflammation, insulin resistance, and AD pathologies. Herein, we will discuss a potential pathway that TREM2 mediates lipid metabolism to influence the pathogenesis of AD in both the CNS and periphery. Moreover, we discuss the possibility that TREM2 may be a key factor that links central and peripheral lipid metabolism under disease conditions, including AD. This link may be due to impacts on the integrity of the blood-brain barrier, and we introduce potential pathways by which TREM2 affects the blood-brain barrier. Moreover, we discuss the role of lipids in TREM2-associated treatments for

Single-cell or single-nucleus transcriptomics is a powerful tool for identifying cell types and cell states. However, hypotheses derived from these assays, including gene expression information, require validation, and their functional relevance needs to be established. The choice of validation depends on numerous factors. Here, we present types of orthogonal and functional validation experiment to strengthen preliminary findings obtained using single-cell and single-nucleus transcriptomics as well as the challenges and limitations of these approaches.

Alzheimer's disease (AD) is traditionally defined by Amyloid-β (Aβ) plaques and tau neurofibrillary tangles, yet these proteinopathies alone fail to explain disease heterogeneity, progression, and cognitive decline. Emerging evidence identifies chronic neuroinflammation as a central integrator that converts molecular pathology into synaptic failure and neurodegeneration. In this context, Aβ acts as a danger-associated molecular pattern that activates microglial and astrocytic immune programs through receptors such as TREM2, TLRs, and RAGE, leading to inflammasome activation, cytokine release, and oxidative stress. These responses pathologically re-engage developmental complement pathways (C1q-C3-CR3), driving excessive synaptic pruning that correlates more closely with cognitive impairment than neuronal loss. Reactive astrocytes further amplify dysfunction by impairing glutamate and potassium homeostasis, promoting excitotoxic and metabolic stress, while inflammatory glia facilitate prion-like tau propagation via extracellular vesicles. Concurrent neurovascular inflammation disrupts blood-brain barrier integrity and cerebral perfusion, reinforcing immune-metabolic failure. Importantly, neuroinflammatory biomarkers (GFAP, sTREM2, YKL-40, cytokines, complement, and TSPO-PET) provide dynamic readouts of disease activity and therapeutic response. Together, these findings position AD as a disorder of failed immune resolution and support precision immunomodulatory and pro-resolving

Parkinson's disease (PD) is a complex neurodegenerative disorder that poses significant burden on patients and families. Its exact cause is unknown, resulting in limited effective treatments. Mitochondrial dysfunction, linked to genetics, aging, oxidative stress, and environmental factors, is central to PD. Healthy elderly individuals have a compensatory mitochondrial DNA (mtDNA) mechanism in brain cells, but this mechanism is impaired in PD patients, leading to mtDNA reduction, respiratory chain dysfunction, decreased adenosine triphosphate (ATP) synthesis, and inadequate neuron energy. Aging increases oxidative stress, impairing mitochondrial function. Mitochondrial dysfunction in the dopaminergic neurons of the substantia nigra causes neuronal loss and disease progression. Aging microglia also play a crucial role, with a reduced capacity to clear neurotoxic substances, especially in the substantia nigra. A decrease in triggering receptor expressed on myeloid cells 2 (TREM2) gene expression shifts microglia to a pro-inflammatory phenotype, exacerbating neuroinflammatory responses and protein deposition. Down-regulation of the C-X3-C motif chemokine ligand 1 (CX3CL1)/C-X3-C chemokine receptor 1 (CX3CR1) signaling pathway increases the expression of pro-inflammatory cytokines, accelerating neuronal loss and disease progression. Recent research has identified a new astrocyte aging regulatory mechanism involving the cyclic GMP‒AMP synthase (cGAS)/stimulator of interferon genes