Elucidating the Role of TREM2 in Alzheimer’s Disease

Microglial and TREM2 dialogues in the developing brain

From the migration of precursor cells to the refinement of neural circuits, the immune system plays a critical role in the development of the central nervous system. As the brain resident macrophages, microglia are integral to these processes, influencing key developmental stages and contributing to circuit remodeling. Recent years have brought a wealth of new insights into how microglia regulate key stages of brain development, particularly through their continuous crosstalk with various brain cell types. In this review, we synthesize this growing body of literature on microglia and neurodevelopment, highlighting the involvement of the TREM2 receptor, known for its role in aging and neurodegeneration, which profoundly affects the state of microglia and guides target cells by shaping their transcriptional and functional fate. We examine microglial communication with four major cell types-neural precursors, neurons, astrocytes, and oligodendrocytes-also delving into the described mechanisms that underpin these interactions.

Vascular models of Alzheimer's disease: An overview of recent in vitro models of the blood-brain barrier

Alzheimer's disease (AD) remains an overwhelming epidemiologic and economic burden on our healthcare systems, affecting an estimate of 11 % of individuals aged 65 years and older. Increasing evidence of the role of the blood-brain barrier (BBB) in AD pathology lends support to the vascular hypothesis of AD, which posits that damage to cerebral vasculature and impairments to cerebral blood flow are major contributors to neurodegeneration in AD. While the question remains whether the dysfunction of the BBB is the cause or consequence of the disease, understanding of the relationship between vascular pathology and AD is growing increasingly complex, warranting the need for better tools to study vasculature in AD. This review provides an overview of AD models in the context of studying vascular impairments and their relevance in pathology. Specifically, we summarize opportunities in in vitro models, cell sources, and phenotypic observations in sporadic and familial forms of AD. Further, we describe recent advances in generating models which recapitulate in vivo characteristics of the BBB in AD through the use of microfluidics, induced pluripotent stem cells (iPSC), and organoid technologies. Finally, we provide a searchable database of reported cell-based models of pathogenic AD gene variants.

4,4'-Dimethoxychalcone Mitigates Neuroinflammation Following Traumatic Brain Injury Through Modulation of the TREM2/PI3K/AKT/NF-κB Signaling Pathway

Research on 4,4'-dimethoxychalcone (DMC) in the context of traumatic brain injury (TBI) is extremely limited, and no effective clinical treatments are available to improve outcomes for individuals with TBI. Our study aims to investigate the underlying mechanisms by which DMC may alleviate neuroinflammation and neuronal damage following TBI. This study seeks to provide a theoretical foundation for the development of future pharmacological therapies for TBI. A moderate TBI model was established using the fluid percussion injury (FPI) method. The recovery of neuromotor function following TBI was evaluated using the modified neurological severity score (mNSS), the Morris water maze test, and analysis of cerebral edema. Gene and protein expression levels were quantified using cell viability assays, quantitative real-time polymerase chain reaction (qRT-PCR), Western blotting, enzyme-linked immunosorbent assay (ELISA), immunohistochemistry, and immunofluorescence. Network pharmacology was employed to predict potential targets of DMC, and gene ontology (GO) analysis along with KEGG pathway enrichment was conducted to predict signaling pathways affected by DMC.DMC treatment significantly improved neuromotor deficits in mice after TBI. In both in vivo and in vitro experiments, DMC suppressed microglial activation and decreased the production and release of inflammatory factors. Additionally, DMC reduced neuronal lesions after TBI. DMC notably decreased the elevated expression of triggering receptor expressed on myeloid cells 2 (TREM2) following TBI. Network pharmacological analysis indicated that DMC's therapeutic effects may be mediated through the PI3K/AKT signaling cascade. These findings indicate that DMC has therapeutic potential for TBI, with significant anti-inflammatory and neuroprotective properties likely mediated by the TREM2/PI3K/AKT/NF-κB signaling cascade.

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.

Microglia, Trem2, and Neurodegeneration

Microglia are a specialized type of neuroimmune cells that undergo morphological and molecular changes through multiple signaling pathways in response to pathological protein aggregates, neuronal death, tissue injury, or infections. Microglia express Trem2, which serves as a receptor for a multitude of ligands enhancing their phagocytic activity. Trem2 has emerged as a critical modulator of microglial activity, especially in many neurodegenerative disorders. Human TREM2 mutations are associated with an increased risk of developing Alzheimer disease (AD) and other neurodegenerative diseases. Trem2 plays dual roles in neuroinflammation and more specifically in disease-associated microglia. Most recent developments on the molecular mechanisms of Trem2, emphasizing its role in uptake and clearance of amyloid β (Aβ) aggregates and other tissue debris to help protect and preserve the brain, are encouraging. Although Trem2 normally stimulates defense mechanisms, its dysregulation can intensify inflammation, which poses major therapeutic challenges. Recent therapeutic approaches targeting Trem2 via agonistic antibodies and gene therapy methodologies present possible avenues for reducing the burden of neurodegenerative diseases. This review highlights the promise of Trem2 as a therapeutic target, especially for Aβ-associated AD, and calls for more mechanistic investigations to understand the context-specific role of microglial Trem2 in developing effective therapies against neurodegenerative diseases.

Mapping Endothelial-Macrophage Interactions in Diabetic Vasculature: Role of TREM2 in Vascular Inflammation and Ischemic Response

Diabetes mellitus (DM) significantly accelerates vascular diseases like peripheral arterial disease (PAD). Endothelial cells (ECs) and macrophages (MΦs) singularly and synergistically are important contributors to DM-associated vascular dysfunction. Single-cell (sc) profiling technologies are revealing the true heterogeneity of ECs and MΦs, but how this cellular diversity translates to cell-cell interactions, and consequentially vascular function, remains unknown. We leveraged scRNA sequencing and spatial transcriptome (ST) profiling to analyze human mesenteric arteries from non-diabetic (ND) and type 2 diabetic (T2D) donors. We generated a transcriptome and interactome map encompassing the major arterial cells and highlighted Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) as a top T2D-induced gene in mononuclear phagocytes (MPs), with concomitant increases of TREM2 ligands in ECs. We verified DM-associated TREM2 induction in cell and mouse models, and found that TREM2 inhibition decreases pro-inflammatory responses in MPs and ECs, as well as increases EC migration in vitro. Furthermore, TREM2 inhibition using a neutralizing antibody enhanced ischemic recovery and flow reperfusion in DM mice subjected to hindlimb ischemia, suggesting that TREM2 promotes ischemic injury in DM. Finally, in human PAD, co-existing DM was associated with greater expression of TREM2 and its interaction with ECs, with a further increase in ischemic tissue compared to patient-matched non-ischemic tissue. Collectively, our study presents the first atlas of human diabetic vessels with single cell and spatial resolution, and identifies TREM2-EC interaction as a key driver of diabetic vasculopathies, the targeting of which may offer an opportunity to ameliorate vascular dysfunction associated with DM-PAD.

Decoding microglial immunometabolism: a new frontier in Alzheimer's disease research

Alzheimer's disease (AD) involves a dynamic interaction between neuroinflammation and metabolic dysregulation, where microglia play a central role. These immune cells undergo metabolic reprogramming in response to AD-related pathology, with key genes such as TREM2, APOE, and HIF-1α orchestrating these processes. Microglial metabolism adapts to environmental stimuli, shifting between oxidative phosphorylation and glycolysis. Hexokinase-2 facilitates glycolytic flux, while AMPK acts as an energy sensor, coordinating lipid and glucose metabolism. TREM2 and APOE regulate microglial lipid homeostasis, influencing Aβ clearance and immune responses. LPL and ABCA7, both associated with AD risk, modulate lipid processing and cholesterol transport, linking lipid metabolism to neurodegeneration. PPARG further supports lipid metabolism by regulating microglial inflammatory responses. Amino acid metabolism also contributes to microglial function. Indoleamine 2,3-dioxygenase controls the kynurenine pathway, producing neurotoxic metabolites linked to AD pathology. Additionally, glucose-6-phosphate dehydrogenase regulates the pentose phosphate pathway, maintaining redox balance and immune activation. Dysregulated glucose and lipid metabolism, influenced by genetic variants such as APOE4, impair microglial responses and exacerbate AD progression. Recent findings highlight the interplay between metabolic regulators like REV-ERBα, which modulates lipid metabolism and inflammation, and Syk, which influences immune responses and Aβ clearance. These insights offer promising therapeutic targets, including strategies aimed at HIF-1α modulation, which could restore microglial function depending on disease stage. By integrating metabolic, immune, and genetic factors, this review underscores the importance of microglial immunometabolism in AD. Targeting key metabolic pathways could provide novel therapeutic strategies for mitigating neuroinflammation and restoring microglial function, ultimately paving the way for innovative treatments in neurodegenerative diseases.

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.

Corticosterone effects induced by stress and immunity and inflammation: mechanisms of communication

The body instinctively responds to external stimuli by increasing energy metabolism and initiating immune responses upon receiving stress signals. Corticosterone (CORT), a glucocorticoid (GC) that regulates secretion along the hypothalamic-pituitary-adrenal (HPA) axis, mediates neurotransmission and humoral regulation. Due to the widespread expression of glucocorticoid receptors (GR), the effects of CORT are almost ubiquitous in various tissue cells. Therefore, on the one hand, CORT is a molecular signal that activates the body's immune system during stress and on the other hand, due to the chemical properties of GCs, the anti-inflammatory properties of CORT act as stabilizers to control the body's response to stress. Inflammation is a manifestation of immune activation. CORT plays dual roles in this process by both promoting inflammation and exerting anti-inflammatory effects in immune regulation. As a stress hormone, CORT levels fluctuate with the degree and duration of stress, determining its effects and the immune changes it induces. The immune system is essential for the body to resist diseases and maintain homeostasis, with immune imbalance being a key factor in the development of various diseases. Therefore, understanding the role of CORT and its mechanisms of action on immunity is crucial. This review addresses this important issue and summarizes the interactions between CORT and the immune system.

The night's watch: Exploring how sleep protects against neurodegeneration

Sleep loss is often regarded as an early manifestation of neurodegenerative diseases given its common occurrence and link to cognitive dysfunction. However, the precise mechanisms by which sleep disturbances contribute to neurodegeneration are not fully understood, nor is it clear why some individuals are more susceptible to these effects than others. This review addresses critical unanswered questions in the field, including whether sleep disturbances precede or result from neurodegenerative diseases, the functional significance of sleep changes during the preclinical disease phase, and the potential role of sleep homeostasis as an adaptive mechanism enhancing resilience against cognitive decline and neurodegeneration.

Molecular Biomarkers in Neurological Diseases: Advances in Diagnosis and Prognosis

Neurological diseases contribute significantly to disability and mortality, necessitating improved diagnostic and prognostic tools. Advances in molecular biomarkers at genomic, transcriptomic, epigenomic, and proteomic levels have facilitated early disease detection. Notably, neurofilament light chain (NfL) serves as a key biomarker of neurodegeneration, while liquid biopsy techniques enable non-invasive monitoring through exosomal tau, α-synuclein, and inflammatory markers. Artificial intelligence (AI) and multi-omics integration further enhance biomarker discovery, promoting precision medicine. A comprehensive literature review was conducted using PubMed, Scopus, and Web of Science to identify studies (2010-2024) on molecular biomarkers in neurodegenerative and neuroinflammatory disorders. Key findings on genomic mutations, transcriptomic signatures, epigenetic modifications, and protein-based biomarkers were analyzed. The findings highlight the potential of liquid biopsy and multi-omics approaches in improving diagnostic accuracy and therapeutic stratification. Genomic, transcriptomic, and proteomic markers demonstrate utility in early detection and disease monitoring. AI-driven analysis enhances biomarker discovery and clinical application. Despite advancements, challenges remain in biomarker validation, standardization, and clinical implementation. Large-scale longitudinal studies are essential to ensure reliability. AI-powered multi-omics analysis may accelerate biomarker application, ultimately improving patient outcomes in neurological diseases.

Oligodendrocytes in Alzheimer's disease pathophysiology

Our understanding of Alzheimer's disease (AD) has transformed from a purely neuronal perspective to one that acknowledges the involvement of glial cells. Despite remarkable progress in unraveling the biology of microglia, astrocytes and vascular elements, the exploration of oligodendrocytes in AD is still in its early stages. Contrary to the traditional notion of oligodendrocytes as passive bystanders in AD pathology, emerging evidence indicates their active participation in and reaction to amyloid and tau pathology. Oligodendrocytes undergo a functional transition to a disease-associated state, engaging in immune modulation, stress responses and cellular survival. Far from being inert players, they appear to serve a dual role in AD pathogenesis, potentially offering defense mechanisms against pathology while also contributing to disease progression. This Review explores recent advancements in understanding the roles of oligodendrocytes and their myelin sheaths in the context of AD, shedding light on their complex interactions within the disease pathology.

Predicting the impact of missense mutations on an unresolved protein's stability, structure, and function: A case study of Alzheimer's disease-associated TREM2 R47H variant

AlphaFold2 (AF2) has spurred a revolution in predicting unresolved structures of wild-type proteins with high accuracy. However, AF2 falls short of predicting the effects of missense mutations on unresolved protein structures that may be informative to efforts in personalized medicine. Over the last decade, countless in-silico methods have been developed to predict the pathogenicity of point mutations on resolved structures, but no studies have evaluated their capabilities on unresolved protein structures predicted by AF2. Herein, we investigated Alzheimer's disease (AD)-causing coding variants of the triggering receptor expressed on myeloid cells 2 (TREM2) receptor using in-silico mutagenesis techniques on the AF2-predicted structure. We first demonstrated that the predicted structure retained a high accuracy in critical regions of the extracellular domain and subsequently validated the in-silico mutagenesis methods by evaluating the effects of the strongest risk variant R47H of TREM2. After validation of the R47H variant, we predicted the molecular basis and effects on protein stability and ligand-binding affinity of the R62H and D87N variants that remain unknown in current literature. By comparing it with the R47H variant, our analysis reveals that R62H and D87N variants exert a much less pronounced effect on the structural stability of TREM2. These in-silico findings show the possibility that the R62H and D87N mutations are likely less pathogenic than the R47H AD. Lastly, we investigated the Nasu-Hakola (NHD)-causing Y38C and V126G TREM2 as a comparison and found that they imposed greater destabilization compared to AD-causing variants. We believe that the in-silico mutagenesis methods described here can be applied broadly to evaluate the ever-growing numbers of protein mutations/variants discovered in human genetics study for their potential in diseases, ultimately facilitating personalized medicine.

Blood-Based Biomarkers and Risk of Onset of Mild Cognitive Impairment Over the Short and Long Term

BACKGROUND AND OBJECTIVES

Blood-based biomarkers of amyloid and tau have been shown to predict Alzheimer disease (AD) dementia. Much less is known about their ability to predict risk of mild cognitive impairment (MCI), an earlier disease stage. This study examined whether levels of blood biomarkers of amyloid (Aβ42/Aβ40 ratio), tau (p-tau181), neurodegeneration (NfL), and glial activation and neuroinflammation (glial fibrillary acidic protein [GFAP], YKL40, soluble triggering receptor expressed on myeloid cells 2 [sTREM2]) collected when participants were cognitively normal are associated with the time to onset of MCI.

METHODS

Cognitively unimpaired participants from the longitudinal observational BIOCARD study provided blood plasma at their baseline evaluation ("baseline 1"). A second "baseline" specimen (collected using slightly different procedures) was evaluated for participants who were still cognitively normal approximately 7 years later. The plasma assays were based on the NeuroToolKit (cobas Elecsys assays, Roche Diagnostics). Cox regression models tested the association of biomarker levels with time to MCI symptom onset, separately for both baselines.

RESULTS

Participants included 271 individuals at "baseline 1" (mean age = 57.5 years, 60.5% female, including 82 who progressed to MCI/dementia) and 202 individuals at "baseline 2" (mean age = 64.5 years, 62.4% female, including 31 progressors). The mean clinical follow-up was 15.5 years for "baseline 1" and 9.9 years for "baseline 2." For both baselines, lower plasma Aβ42/Aβ40 ratio (both hazard ratios, HRs ≤ 0.69, 95% CIs ≤ 0.55-0.87, p ≤ 0.034), higher GFAP (HRs ≥ 1.83, CIs ≥ 1.28-2.60, p < 0.002), and a higher ratio of p-tau181/(Aβ42/Aβ40) (HRs ≥ 1.64, CIs ≥ 1.25-2.13, p ≤ 0.001) were each associated with an earlier time to MCI symptom onset. For baseline 2, higher p-tau181 (HR = 2.07, CI = 1.12-3.83, p = 0.021) and higher NfL (HR = 1.75, CI = 0.99-3.10, p = 0.05) were also associated with earlier MCI symptom onset for progression within 7 years. When combining biomarkers, neither GFAP nor NFL was associated with MCI symptom onset after accounting for AD biomarker levels (e.g., p-tau181/(Aβ42/Aβ40)), which remained significant. YKL40 and sTREM2 were not associated with MCI onset.

DISCUSSION

Results indicate that during preclinical AD, more abnormal blood biomarker levels of amyloid (Aβ42/Aβ40), p-tau181, neurodegeneration (NfL), and neuroinflammation (GFAP) individually are associated with progression from normal cognition to MCI, but the AD-nonspecific neurodegeneration and inflammation markers were not associated with symptom onset after accounting for amyloid and p-tau levels.

Two- and Three-Dimensional In Vitro Models of Parkinson's and Alzheimer's Diseases: State-of-the-Art and Applications

In vitro models play a pivotal role in advancing our understanding of neurodegenerative diseases (NDs) such as Parkinson's and Alzheimer's disease (PD and AD). Traditionally, 2D cell cultures have been instrumental in elucidating the cellular mechanisms underlying these diseases. Cultured cells derived from patients or animal models provide valuable insights into the pathological processes at the cellular level. However, they often lack the native tissue environment complexity, limiting their ability to fully recapitulate their features. In contrast, 3D models offer a more physiologically relevant platform by mimicking the 3D brain tissue architecture. These models can incorporate multiple cell types, including neurons, astrocytes, and microglia, creating a microenvironment that closely resembles the brain's complexity. Bioengineering approaches allow researchers to better replicate cell-cell interactions, neuronal connectivity, and disease-related phenotypes. Both 2D and 3D models have their advantages and limitations. While 2D cultures provide simplicity and scalability for high-throughput screening and basic processes, 3D models offer enhanced physiological relevance and better replicate disease phenotypes. Integrating findings from both model systems can provide a better understanding of NDs, ultimately aiding in the development of novel therapeutic strategies. Here, we review existing 2D and 3D in vitro models for the study of PD and AD.

TREM2 signaling in Parkinson's disease: Regulation of microglial function and α-synuclein pathology

BACKGROUND

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons, abnormal accumulation of α-synuclein (α-syn), and microglial activation. Triggering receptor expressed on myeloid cells 2 (TREM2) regulates multiple functions of microglia in the brain, and several studies have shown that TREM2 variant R47H is a risk factor for PD. However, the regulation of microglia by TREM2 in PD remains poorly understood.

METHODS

We constructed PD cell and animal models using α-syn preformed fibrils. siRNA knockdown and lentiviral overexpression were used to perturb TREM2 levels in cells, and TREM2 knockout mice and lentiviral overexpression was used in animal models to investigate the effects of TREM2 on microglial function, α-syn-related pathology, and dopaminergic neuron degeneration.

RESULTS

Microglia phagocytosed α-syn preformed fibrils in a concentration- and time-dependent manner, with some capacity to degrade α-syn aggregates. TREM2 expression increased in PD. In the context of PD, TREM2 knockout mice exhibited worsened pathological α-syn spread, decreased microglial reactivity, and increased loss of TH-positive neurons in the substantia nigra compared to wild-type mice. TREM2 overexpression enhanced reactive microglial aggregation towards the pathological site. Cellular experiments revealed that reduced TREM2 impaired microglial phagocytosis and proliferation, but enhanced autophagy via the PI3K/AKT/mTOR pathway.

CONCLUSION

TREM2 signaling in PD maintains microglial phagocytosis, proliferation, and reactivity, stabilizing autophagy and proliferation via the PI3K/AKT/mTOR pathway. Regulating TREM2 levels may be beneficial in PD treatment.

Sulfatide deficiency-induced astrogliosis and myelin lipid dyshomeostasis are independent of Trem2-mediated microglial activation

Disrupted lipid homeostasis and neuroinflammation often co-exist in neurodegenerative disorders including Alzheimer's disease (AD). However, the intrinsic connection and causal relationship between these deficits remain elusive. Our previous studies show that the loss of sulfatide (ST), a class of myelin-enriched lipids, causes AD-like neuroinflammatory responses, cognitive impairment, bladder enlargement, as well as lipid dyshomeostasis. To better understand the relationship between neuroinflammation and lipid disruption induced by ST deficiency, we established a ST-deficient mouse model with constitutive Trem2 knockout and studied the impact of Trem2 in regulating ST deficiency-induced microglia-mediated neuroinflammation, astrocyte activation and lipid disruption. Our study demonstrates that Trem2 regulates ST deficiency-induced microglia-mediated neuroinflammatory pathways and astrogliosis at the transcriptomic level, but not astrocyte activation at the protein level, suggesting that Trem2 is indispensable for ST deficiency-induced microglia-mediated neuroinflammation but not astrogliosis. Meanwhile, ST loss-induced lipidome disruption and free water retention were consistently observed in the absence of Trem2 . Collectively, these results emphasize the essential role of Trem2 in mediating lipid loss-associated microglia-mediated neuroinflammation, but not both astrogliosis and myelin lipid disruption. Moreover, we demonstrated that attenuating neuroinflammation has a limited impact on brain ST loss-induced lipidome alteration or AD-like peripheral disorders. Our findings suggest that preserving lipidome and astrocyte balance may be crucial in decelerating the progression of AD.

Apolipoprotein E aggregation in microglia initiates Alzheimer's disease pathology by seeding β-amyloidosis

The seeded growth of pathogenic protein aggregates underlies the pathogenesis of Alzheimer's disease (AD), but how this pathological cascade is initiated is not fully understood. Sporadic AD is linked genetically to apolipoprotein E (APOE) and other genes expressed in microglia related to immune, lipid, and endocytic functions. We generated a transgenic knockin mouse expressing HaloTag-tagged APOE and optimized experimental protocols for the biochemical purification of APOE, which enabled us to identify fibrillary aggregates of APOE in mice with amyloid-β (Aβ) amyloidosis and in human AD brain autopsies. These APOE aggregates that stained positive for β sheet-binding dyes triggered Aβ amyloidosis within the endo-lysosomal system of microglia, in a process influenced by microglial lipid metabolism and the JAK/STAT signaling pathway. Taking these observations together, we propose a model for the onset of Aβ amyloidosis in AD, suggesting that the endocytic uptake and aggregation of APOE by microglia can initiate Aβ plaque formation.

Prevention and Treatment Strategies for Alzheimer's Disease: Focusing on Microglia and Astrocytes in Neuroinflammation

Alzheimer's disease (AD) is a fatal neurodegenerative disease characterized by its insidious onset and progressive development, making it the most common form of dementia. Despite its prevalence, the exact causes and mechanisms responsible for AD remain unclear. Recent studies have highlighted that inflammation in the central nervous system (CNS) plays a crucial role in both the initiation and progression of AD. Neuroinflammation, an immune response within the CNS triggered by glial cells in response to various stimuli, such as nerve injury, infection, toxins, or autoimmune reactions, has emerged as a significant factor alongside amyloid deposition and neurofibrillary tangles (NFTs) commonly associated with AD. This article aims to provide an overview of the most recent research regarding the involvement of neuroinflammation in AD, with a particular focus on elucidating the specific mechanisms involving microglia and astrocytes. By exploring these intricate processes, a new theoretical framework can be established to further probe the impact of neuroinflammation on the development and progression of AD. Through a deeper understanding of these underlying mechanisms, potential targets for therapeutic interventions and novel treatment strategies can be identified in the ongoing battle against AD.

Transcriptome and proteome profiling reveals TREM2-dependent and -independent glial response and metabolic perturbation in an Alzheimer's mouse model

Elucidating the intricate molecular mechanisms of Alzheimer's disease (AD) requires a multidimensional analysis incorporating various omics data. In this study, we employed transcriptome and proteome profiling of AppNL-G-F, a human APP knock-in model of amyloidosis, at the early and mid-stages of amyloid-beta (Aβ) pathology to delineate the impacts of Aβ deposition on brain cells. By contrasting AppNL-G-F mice with TREM2 (Triggering receptor expressed on myeloid cells 2) knockout models, our study further investigates the role of TREM2, a well-known AD risk gene, in influencing microglial responses to Aβ pathology. Our results highlight altered microglial states as a central feature of Aβ pathology, characterized by the significant upregulation of microglia-specific genes related to immune responses such as complement system and antigen presentation, and catabolic pathways such as phagosome formation and lysosome biogenesis. The absence of TREM2 markedly diminishes the induction of these genes, impairs Aβ clearance, and exacerbates dystrophic neurite formation. Importantly, TREM2 is required for the microglial engagement with Aβ plaques and the formation of compact Aβ plaque cores. Furthermore, this study reveals substantial disruptions in energy metabolism and protein synthesis, signaling a shift from anabolism to catabolism in response to Aβ deposition. This metabolic alteration, coupled with a decrease in synaptic protein abundance, occurs independently of TREM2, suggesting the direct effects of Aβ deposition on synaptic integrity and plasticity. In summary, our findings demonstrate altered microglial states and metabolic disruption following Aβ deposition, offering mechanistic insights into Aβ pathology and highlighting the potential of targeting these pathways in AD therapy.

The role of microglial TREM2 in development: A path toward neurodegeneration?

The nervous and the immune systems undergo a continuous cross talk, starting from early development and continuing throughout adulthood and aging. Defects in this cross talk contribute to neurodevelopmental and neurodegenerative diseases. Microglia are the resident immune cells in the brain that are primarily involved in this bidirectional communication. Among the microglial genes, trem2 is a key player, controlling the functional state of microglia and being at the forefront of many processes that require interaction between microglia and other brain components, such as neurons and oligodendrocytes. The present review focuses on the early developmental window, describing the early brain processes in which TREM2 is primarily involved, including the modulation of synapse formation and elimination, the control of neuronal bioenergetic states as well as the contribution to myelination processes and neuronal circuit formation. By causing imbalances during these early maturation phases, dysfunctional TREM2 may have a striking impact on the adult brain, making it a more sensitive target for insults occurring during adulthood and aging.

Rod-shaped microglia interact with neuronal dendrites to regulate cortical excitability in TDP-43 related neurodegeneration

Motor cortical hyperexcitability is well-documented in the presymptomatic stage of amyotrophic lateral sclerosis (ALS). However, the mechanisms underlying this early dysregulation are not fully understood. Microglia, as the principal immune cells of the central nervous system, have emerged as important players in sensing and regulating neuronal activity. Here we investigated the role of microglia in the motor cortical circuits in a mouse model of TDP-43 neurodegeneration (rNLS8). Utilizing multichannel probe recording and longitudinal in vivo calcium imaging in awake mice, we observed neuronal hyperactivity at the initial stage of disease progression. Spatial and single-cell RNA sequencing revealed that microglia are the primary responders to motor cortical hyperactivity. We further identified a unique subpopulation of microglia, rod-shaped microglia, which are characterized by a distinct morphology and transcriptional profile. Notably, rod-shaped microglia predominantly interact with neuronal dendrites and excitatory synaptic inputs to attenuate motor cortical hyperactivity. The elimination of rod-shaped microglia through TREM2 deficiency increased neuronal hyperactivity, exacerbated motor deficits, and further decreased survival rates of rNLS8 mice. Together, our results suggest that rod-shaped microglia play a neuroprotective role by attenuating cortical hyperexcitability in the mouse model of TDP-43 related neurodegeneration.

Association between sTREM2, an immune biomarker of microglial activation, and frontal lobe function in community-dwelling older adults: a cross-sectional study

Objective: Identifying the peripheral biomarkers related to the prevention or modification of unhealthy mental conditions in older adults is extremely beneficial. This study aimed to evaluate the serum levels of soluble triggering receptor expressed on myeloid cells 2 (sTREM2), a soluble form of an innate immune receptor expressed on microglia, in older adults living in a rural community, and their association with cognitive function. Materials and Methods: This survey was conducted between November 2016 and September 2017 in Kurokawa-cho, Imari, Saga Prefecture, Japan, among people aged ≥65 years. Blood samples were collected from the participants for serum sTREM2 level analysis using a peptide enzyme immunoassay. The participants underwent cognitive function assessments, including the Mini-Mental State Examination, Clinical Dementia Rating, and Frontal Assessment Battery. Therefore, we examined the association between serum sTREM2 levels and cognitive function. Results: Of the 95 participants, 25 were men and 70 were women with a mean age 78.24 ± 3.85 years and 77.96 ± 5.52 years, respectively. Serum sTREM2 levels were negatively associated with Frontal Assessment Battery scores, even after adjusting for age, sex, years of education, and serum high-sensitivity C-reactive protein levels. Conclusion: Serum sTREM2 levels may be associated with frontal lobe function in adults aged ≥65 years.

Advances in the Understanding of the Correlation Between Neuroinflammation and Microglia in Alzheimer's Disease

Alzheimer's disease (AD) is a fatal neurodegenerative disease with a subtle and progressive onset and is the most common type of dementia. However, its etiology and pathogenesis have not yet been fully elucidated. The common pathological manifestations of AD include extraneuronal β-amyloid deposition (Aβ), intraneuronal tau protein phosphorylation leading to the formation of 'neurofibrillary tangles' (NFTs), neuroinflammation, progressive loss of brain neurons/synapses, and glucose metabolism disorders. Current treatment approaches for AD primarily focus on the 'Aβ cascade hypothesis and abnormal aggregation of hyperphosphorylation of tau proteins', but have shown limited efficacy. Therefore, there is an ongoing need to identify more effective treatment targets for AD. The central nervous system (CNS) inflammatory response plays a key role in the occurrence and development of AD. Neuroinflammation is an immune response activated by glial cells in the CNS that usually occurs in response to stimuli such as nerve injury, infection and toxins or in response to autoimmunity. Neuroinflammation ranks as the third most prominent pathological feature in AD, following Aβ and NFTs. In recent years, the focus on the role of neuroinflammation and microglia in AD has increased due to the advancements in genome-wide association studies (GWAS) and sequencing technology. Furthermore, research has validated the pivotal role of microglia-mediated neuroinflammation in the progression of AD. Therefore, this article reviews the latest research progress on the role of neuroinflammation triggered by microglia in AD in recent years, aiming to provide a new theoretical basis for further exploring the role of neuroinflammation in the process of AD occurrence and development.

Identification of senescent, TREM2-expressing microglia in aging and Alzheimer's disease model mouse brain

Alzheimer's disease (AD) and dementia in general are age-related diseases with multiple contributing factors, including brain inflammation. Microglia, and specifically those expressing the AD risk gene TREM2, are considered important players in AD, but their exact contribution to pathology remains unclear. In this study, using high-throughput mass cytometry in the 5×FAD mouse model of amyloidosis, we identified senescent microglia that express high levels of TREM2 but also exhibit a distinct signature from TREM2-dependent disease-associated microglia (DAM). This senescent microglial protein signature was found in various mouse models that show cognitive decline, including aging, amyloidosis and tauopathy. TREM2-null mice had fewer microglia with a senescent signature. Treating 5×FAD mice with the senolytic BCL2 family inhibitor ABT-737 reduced senescent microglia, but not the DAM population, and this was accompanied by improved cognition and reduced brain inflammation. Our results suggest a dual and opposite involvement of TREM2 in microglial states, which must be considered when contemplating TREM2 as a therapeutic target in AD.

SHIP inhibition mediates select TREM2-induced microglial functions

Microglia play a pivotal role in the pathology of Alzheimer's Disease (AD), with the Triggering Receptor Expressed on Myeloid cells 2 (TREM2) central to their neuroprotective functions. The R47H variant of TREM2 has emerged as a significant genetic risk factor for AD, leading to a loss-of-function phenotype in mouse AD models. This study elucidates the roles of TREM2 in human microglia-like HMC3 cells and the regulation of these functions by SH2-containing inositol-5'-phosphatase 1 (SHIP1). Using stable cell lines expressing wild-type TREM2, the R47H variant, and TREM2-deficient lines, we found that functional TREM2 is essential for the phagocytosis of Aβ, lysosomal capacity, and mitochondrial activity. Notably, the R47H variant displayed increased phagocytic activity towards apoptotic neurons. Introducing SHIP1, known to modulate TREM2 signaling in other cells, revealed its role as a negative regulator of these TREM2-mediated functions. Moreover, pharmacological inhibition of both SHIP1 and its isoform SHIP2 amplified Aβ phagocytosis and lysosomal capacity, independently of TREM2 or SHIP1 expression, suggesting a potential regulatory role for SHIP2 in these functions. The absence of TREM2, combined with the presence of both SHIP isoforms, suppressed mitochondrial activity. However, pan-SHIP1/2 inhibition enhanced mitochondrial function in these cells. In summary, our findings offer a deeper understanding of the relationship between TREM2 variants and SHIP1 in microglial functions, and emphasize the therapeutic potential of targeting the TREM2 and SHIP1 pathways in microglia for neurodegenerative diseases.

Cell autonomous microglia defects in a stem cell model of frontotemporal dementia

Neuronal dysfunction has been extensively studied as a central feature of neurodegenerative tauopathies. However, across neurodegenerative diseases, there is strong evidence for active involvement of immune cells like microglia in driving disease pathophysiology. Here, we demonstrate that tau mRNA and protein are expressed in microglia in human brains and in human induced pluripotent stem cell (iPSC)-derived microglia like cells (iMGLs). Using iMGLs harboring the MAPT IVS10+16 mutation and isogenic controls, we demonstrate that a tau mutation is sufficient to alter microglial transcriptional states. We discovered that MAPT IVS10+16 microglia exhibit cytoskeletal abnormalities, stalled phagocytosis, disrupted TREM2/TYROBP networks, and altered metabolism. Additionally, we found that secretory factors from MAPT IVS10+16 iMGLs impact neuronal health, reducing synaptic density in neurons. Key features observed in vitro were recapitulated in human brain tissue and cerebrospinal fluid from MAPT mutations carriers. Together, our findings that MAPT IVS10+16 drives cell-intrinsic dysfunction in microglia that impacts neuronal health has major implications for development of therapeutic strategies.

TREM2 Deficiency Aggravates NLRP3 Inflammasome Activation and Pyroptosis in MPTP-Induced Parkinson's Disease Mice and LPS-Induced BV2 Cells

Microglia-mediated neuroinflammation plays a crucial role in the pathogenesis of Parkinson's disease (PD). Triggering receptor expressed on myeloid cells 2 (TREM2) confers strong neuroprotective effects in PD by regulating the phenotype of microglia. Recent studies suggest that TREM2 regulates high glucose-induced microglial inflammation through the NLRP3 signaling pathway. This study aimed to investigate the effect of TREM2 on NLRP3 inflammasome activation and neuroinflammation in PD. Mice were injected with AAV-TREM2-shRNA into both sides of the substantia nigra using a stereotactic injection method, followed by intraperitoneal injection of MPTP to establish chronic PD mouse model. Behavioral assessments including the pole test and rotarod test were conducted to evaluate the effects of TREM2 deficiency on MPTP-induced motor dysfunction. Immunohistochemistry of TREM2 and tyrosine hydroxylase (TH), immunohistochemistry and immunofluorescence Iba1, Western blot of NLRP3 inflammasome and its downstream inflammatory factors IL-1β and IL-18, and the key pyroptosis factors GSDMD and GSDMD-N were performed to explore the effect of TREM2 on NLRP3 inflammasome and neuroinflammation. In an in vitro experiment, lentivirus was used to interfere with the expression of TREM2 in BV2 microglia, and then lipopolysaccharide (LPS) and adenopterin nucleoside triphosphate (ATP) were used to stimulate inflammation to construct a cellular inflammation model. The expression differences of NLRP3 inflammasome and its components were detected by qPCR and Western blot. In vivo, TREM2 knockdown aggravated the loss of dopaminergic neuron and the decline of motor function. After TREM2 knockdown, the number of activated microglia was significantly increased, and the expression of cleaved caspase-1, NLRP3 inflammasome, IL-1β, GSDMD, and GSDMD-N was increased. In vitro, TREM2 knockdown aggravated the inflammatory response of BV2 cells stimulated by LPS and promoted the activation of NLRP3 inflammasome through the NF-κB pathway. In addition, TREM2 knockdown also promoted the expression of TLR4/MyD88, an upstream factor of the NF-κB pathway. Our vivo and vitro data showed that TREM2 knockdown promoted NLRP3 inflammasome activation and downstream inflammatory response, promoted pyroptosis, and aggravated dopaminergic neuron loss. TREM2 acts as an anti-inflammatory in PD through the TLR4/MyD88/NF-κB pathway, which extends previous findings and supports the notion that TREM2 ameliorates neuroinflammation in PD.

Taurine reduces microglia activation in the brain of aged senescence-accelerated mice by increasing the level of TREM2

Alzheimer's disease (AD), a chronic neurodegenerative disorder, is the leading cause of dementia. Over-activated microglia is related to amyloid-beta (Aβ) and phosphorylated tau (phospho-tau) accumulation in the AD brain. Taurine is an amino acid with multiple physiological functions including anti-inflammatory effects, and has been reported to be neuroprotective in AD. However, the role of taurine in microglia-mediated AD remains unclear. Here, we examined the effects of taurine on the brains of senescence-accelerated mouse prone 8 (SAMP8) mice by comparing those administered 1% taurine water with those administered distilled water (DW). We observed increased levels of taurine and taurine transporter (TAUT) in the brains of the taurine-treated mice compared with those of control mice. Immunohistochemical and Western blot analyses revealed that taurine significantly reduced the number of activated microglia, levels of phospho-tau and Aβ deposit in the hippocampus and cortex. Triggering receptors expressed on myeloid cells-2 (TREM2) are known to protect against AD pathogenesis. Taurine upregulated TREM2 expression in the hippocampus and cortex. In conclusion, the present study suggests that taurine treatment may upregulate TREM2 to protect against microglia over-activation by decreasing the accumulation of phospho-tau and Aβ; providing an insight into a novel preventive strategy in AD.

MHC-I in the hippocampus promotes comorbid depressive symptoms in bone cancer pain via the upregulation of microglial TREM2/DAP12 signaling

Pain and depression comorbidity affects patients' physical and mental health, as well as quality of life. Comorbid depressive symptoms in cancer pain have a severe impact on the recognition and treatment of pain. Similarly, cancer pain patients with depression are inclined towards more despair and greater impairment. The mechanisms responsible for the comorbid depressive symptoms in bone cancer pain (BCP) have not been fully delineated. Here, it was reported that the implantation of carcinoma cells into the femoral cavity of mice led to the upregulation of major histocompatibility complex class I (MHC-I) in the hippocampus. This was associated with the activation of microglial signaling pathway mediated by the triggering receptor expressed on myeloid cells 2 protein (TREM2) and DNAX-activating protein of 12 kDa (DAP12). Pain and depression-like behaviors were reversed by the knockdown of hippocampal MHC-I via a lentiviral vector harboring ribonucleic acid interference (RNAi) sequence. Moreover, MHC-I knockdown exhibited a marked reduction in the expression of TREM2 and DAP12. These results suggested that hippocampal MHC-I was involved in BCP and depression comorbidity via upregulating the signals mediated by TREM2/DAP12 in microglia. The suppression of MHC-I could be a potential therapeutic target for BCP.

TREM2 deficiency impairs the energy metabolism of Schwann cells and exacerbates peripheral neurological deficits

Triggering receptor expressed on myeloid cells-2 (TREM2) has been implicated in susceptibility to neurodegenerative disease. Schwann cells (SCs), the predominant glial cell type in the peripheral nervous system (PNS), play a crucial role in myelination, providing trophic support for neurons and nerve regeneration. However, the function of TREM2 in SCs has not been fully elucidated. Here, we found that TREM2 is expressed in SCs but not in neurons in the PNS. TREM2 deficiency leads to disruption of glycolytic flux and oxidative metabolism in SCs, impairing cell proliferation. The energy crisis caused by TREM2 deficiency triggers mitochondrial damage and autophagy by activating AMPK and impairing PI3K-AKT-mTOR signaling. Combined metabolomic analysis demonstrated that energic substrates and energy metabolic pathways were significantly impaired in TREM2-deficient SCs. Moreover, TREM2 deficiency impairs energy metabolism and axonal growth in sciatic nerve, accompanied by exacerbation of neurological deficits and suppression of nerve regeneration in a mouse model of acute motor axonal neuropathy. These results indicate that TREM2 is a critical regulator of energy metabolism in SCs and exerts neuroprotective effects on peripheral neuropathy. TREM2 deficiency impairs glycolysis and oxidative metabolism in Schwann cells, resulting in compromised cell proliferation. The energy crisis caused by TREM2 deficiency induces mitochondrial damage and autophagy by activating AMPK and impairing PI3K-AKT-mTOR signaling. Moreover, TREM2 deficiency disrupts the energy metabolism of the sciatic nerve and impairs support for axonal regeneration, accompanied by exacerbation of neurological deficits and suppression of nerve regeneration in a mouse model of acute motor axonal neuropathy (by FigDraw).

TREM2 regulates BV2 microglia activation and influences corticosterone-induced neuroinflammation in depressive disorders

Depressive disorders is a serious mental illness, and its underlying pathological mechanisms remain unclear. The overactivation of microglia and neuroinflammation are thought to play an essential role in the occurrence and development of depressive disorders. TREM2, an immune protein mainly expressed in microglia, is an important part of nerve cells involved in inflammatory response. Corticosterone (CORT) is often referred to as a stress hormone and plays a role in the immune system and stress response. Therefore, this study investigated the role of TREM2 in CORT-induced BV2 cell damage and preliminarily analyzed the effects of TREM2 on JAK2/STAT3 signaling pathway and microglia polarization. The cell model of CORT-induced depression in vitro was established, and the effect of CORT on the activity of BV2 microglia was detected by CCK8. Plasmid transfection was used to overexpress and interfere with TREM2 in BV2 cells cultured by CORT. Western blotting, PCR, and ELISA analyzed the expression of related proteins and inflammatory factors. The results showed that CORT could affect BV2 cell proliferation and TREM2 levels. In the presence of CORT, overexpression of TREM2 decreased the levels of TNF-α, IL-1β, and IL-6 and increased the levels of IL-10. Interference with TREM2 increased the levels of TNF-α, IL-1β, and IL-6 and decreased the levels of IL-10. TREM2 can affect the release of inflammatory factors through the JAK2/STAT3 signaling pathway and regulate the M1/M2 phenotypic transformation of microglia. TREM2 plays a role in regulating CORT-induced inflammatory responses, revealing the influence of TREM2 on the neuroinflammatory pathogenesis of depressive disorders and suggesting that TREM2 may be a new target for the prevention and treatment of depressive disorders.

Cerebrospinal fluid proteomics in patients with Alzheimer's disease reveals five molecular subtypes with distinct genetic risk profiles

Alzheimer's disease (AD) is heterogenous at the molecular level. Understanding this heterogeneity is critical for AD drug development. Here we define AD molecular subtypes using mass spectrometry proteomics in cerebrospinal fluid, based on 1,058 proteins, with different levels in individuals with AD (n = 419) compared to controls (n = 187). These AD subtypes had alterations in protein levels that were associated with distinct molecular processes: subtype 1 was characterized by proteins related to neuronal hyperplasticity; subtype 2 by innate immune activation; subtype 3 by RNA dysregulation; subtype 4 by choroid plexus dysfunction; and subtype 5 by blood-brain barrier impairment. Each subtype was related to specific AD genetic risk variants, for example, subtype 1 was enriched with TREM2 R47H. Subtypes also differed in clinical outcomes, survival times and anatomical patterns of brain atrophy. These results indicate molecular heterogeneity in AD and highlight the need for personalized medicine.

Microglial Senescence and Activation in Healthy Aging and Alzheimer's Disease: Systematic Review and Neuropathological Scoring

The greatest risk factor for neurodegeneration is the aging of the multiple cell types of human CNS, among which microglia are important because they are the "sentinels" of internal and external perturbations and have long lifespans. We aim to emphasize microglial signatures in physiologic brain aging and Alzheimer's disease (AD). A systematic literature search of all published articles about microglial senescence in human healthy aging and AD was performed, searching for PubMed and Scopus online databases. Among 1947 articles screened, a total of 289 articles were assessed for full-text eligibility. Microglial transcriptomic, phenotypic, and neuropathological profiles were analyzed comprising healthy aging and AD. Our review highlights that studies on animal models only partially clarify what happens in humans. Human and mice microglia are hugely heterogeneous. Like a two-sided coin, microglia can be protective or harmful, depending on the context. Brain health depends upon a balance between the actions and reactions of microglia maintaining brain homeostasis in cooperation with other cell types (especially astrocytes and oligodendrocytes). During aging, accumulating oxidative stress and mitochondrial dysfunction weaken microglia leading to dystrophic/senescent, otherwise over-reactive, phenotype-enhancing neurodegenerative phenomena. Microglia are crucial for managing Aβ, pTAU, and damaged synapses, being pivotal in AD pathogenesis.

Soluble TREM2 ameliorates tau phosphorylation and cognitive deficits through activating transgelin-2 in Alzheimer's disease

Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane protein that is predominantly expressed by microglia in the brain. The proteolytic shedding of TREM2 results in the release of soluble TREM2 (sTREM2), which is increased in the cerebrospinal fluid of patients with Alzheimer's disease (AD). It remains unknown whether sTREM2 regulates the pathogenesis of AD. Here we identified transgelin-2 (TG2) expressed on neurons as the receptor for sTREM2. The microglia-derived sTREM2 binds to TG2, induces RhoA phosphorylation at S188, and deactivates the RhoA-ROCK-GSK3β pathway, ameliorating tau phosphorylation. The sTREM2 (77-89) fragment, which is the minimal active sequence of sTREM2 to activate TG2, mimics the inhibitory effect of sTREM2 on tau phosphorylation. Overexpression of sTREM2 or administration of the active peptide rescues tau pathology and behavioral defects in the tau P301S transgenic mice. Together, these findings demonstrate that the sTREM2-TG2 interaction mediates the cross-talk between microglia and neurons. sTREM2 and its active peptide may be a potential therapeutic intervention for tauopathies including AD.

Effects of autologous serum on TREM2 and APOE in a personalized monocyte-derived macrophage assay of late-onset Alzheimer's patients

BACKGROUND

Age-associated deterioration of the immune system contributes to a chronic low-grade inflammatory state known as "inflammaging" and is implicated in the pathogenesis of late-onset Alzheimer's disease (LOAD). Whether changes in the tissue environment caused by circulatory factors associated with aging may alter the innate immune response is unknown. Monocyte-derived macrophages (Mo-MФs) infiltrating the brain alongside microglia are postulated to play a modulatory role in LOAD and both express triggering receptor expressed on myeloid cells 2 (TREM2). Apolipoprotein E (APOE) acts as a ligand for TREM2, and their role in amyloid beta (Aβ) clearance highlights their importance in LOAD. However, the influence of the patient's own milieu (autologous serum) on the synthesis of TREM2 and APOE in infiltrating macrophages remains unknown.

OBJECTIVES

To functionally assess patient-specific TREM2 and APOE synthesis, we designed a personalized assay based on Mo-MФs using monocytes from LOAD patients and matched controls (CO). We assessed the influence of each participant's own milieu, by examining the effect of short- (1 day) and long- (10 days) term differentiation of the cells in the presence of the donor´s autologous serum (AS) into M1-, M2- or M0-macrophages. Additionally, sex differences and Aβ-uptake ability in short- and long-term differentiated Mo-MФs were assessed.

RESULTS

We showed a time-dependent increase in TREM2 and APOE protein levels in LOAD- and CO-derived cells. While AS did not differentially modulate TREM2 compared to standard fetal calf serum (FCS), AS decreased APOE levels in M2 macrophages but increased levels in M1 macrophages. Interestingly, higher levels of TREM2 and lower levels of APOE were detected in female- than in male- LOAD patients. Finally, we report decreased Aβ-uptake in long-term differentiated CO- and LOAD-derived cells, particularly in APOEε4(+) carriers.

CONCLUSIONS

We demonstrate for the first time the suitability of a personalized Mo-MФ cell culture-based assay for studying functional TREM2 and APOE synthesis in a patient's own aged milieu. Our strategy may thus provide a useful tool for future research on diagnostic and therapeutic aspects of personalized medicine.

Tipping points in neurodegeneration

In Alzheimer's disease (AD), Aβ deposits form slowly, several decades before further pathological events trigger neurodegeneration and dementia. However, a substantial proportion of affected individuals remains non-demented despite AD pathology, raising questions about the underlying factors that determine the transition to clinical disease. Here, we emphasize the critical function of resilience and resistance factors, which we extend beyond the concept of cognitive reserve to include the glial, immune, and vascular system. We review the evidence and use the metaphor of "tipping points" to illustrate how gradually forming AD neuropathology in the preclinical stage can transition to dementia once adaptive functions of the glial, immune, and vascular system are lost and self-reinforcing pathological cascades are unleashed. Thus, we propose an expanded framework for pathomechanistic research that focuses on tipping points and non-neuronal resilience mechanisms, which may represent previously untapped therapeutic targets in preclinical AD.

Alzheimer risk-increasing TREM2 variant causes aberrant cortical synapse density and promotes network hyperexcitability in mouse models

The R47H variant of triggering receptor expressed on myeloid cells 2 (TREM2) increases the risk of Alzheimer's disease (AD). To investigate potential mechanisms, we analyzed knockin mice expressing human TREM2-R47H from one mutant mouse Trem2 allele. TREM2-R47H mice showed increased seizure activity in response to an acute excitotoxin challenge, compared to wildtype controls or knockin mice expressing the common variant of human TREM2. TREM2-R47H also increased spontaneous thalamocortical epileptiform activity in App knockin mice expressing amyloid precursor proteins bearing autosomal dominant AD mutations and a humanized amyloid-β sequence. In mice with or without such App modifications, TREM2-R47H increased the density of putative synapses in cortical regions without amyloid plaques. TREM2-R47H did not affect synaptic density in hippocampal regions with or without plaques. We conclude that TREM2-R47H increases AD-related network hyperexcitability and that it may do so, at least in part, by causing an imbalance in synaptic densities across brain regions.

Frontotemporal dementia presentation in patients with heterozygous p.H157Y variant of TREM2

BACKGROUND

The triggering receptor expressed on myeloid cell 2 (TREM2) is a major regulator of neuroinflammatory processes in neurodegeneration. To date, the p.H157Y variant of TREM2 has been reported only in patients with Alzheimer's disease. Here, we report three patients with frontotemporal dementia (FTD) from three unrelated families with heterozygous p.H157Y variant of TREM2: two patients from Colombian families (study 1) and a third Mexican origin case from the USA (study 2).

METHODS

To determine if the p.H157Y variant might be associated with a specific FTD presentation, we compared in each study the cases with age-matched, sex-matched and education-matched groups-a healthy control group (HC) and a group with FTD with neither TREM2 mutations nor family antecedents (Ng-FTD and Ng-FTD-MND).

RESULTS

The two Colombian cases presented with early behavioural changes, greater impairments in general cognition and executive function compared with both HC and Ng-FTD groups. These patients also exhibited brain atrophy in areas characteristic of FTD. Furthermore, TREM2 cases showed increased atrophy compared with Ng-FTD in frontal, temporal, parietal, precuneus, basal ganglia, parahippocampal/hippocampal and cerebellar regions. The Mexican case presented with FTD and motor neuron disease (MND), showing grey matter reduction in basal ganglia and thalamus, and extensive TDP-43 type B pathology.

CONCLUSION

In all TREM2 cases, multiple atrophy peaks overlapped with the maximum peaks of TREM2 gene expression in crucial brain regions including frontal, temporal, thalamic and basal ganglia areas. These results provide the first report of an FTD presentation potentially associated with the p.H157Y variant with exacerbated neurocognitive impairments.

BAC Transgenic Expression of Human TREM2-R47H Remodels Amyloid Plaques but Unable to Reprogram Plaque-associated Microglial Reactivity in 5xFAD Mice

Background

Genetic study of late-onset Alzheimer's disease (AD) reveals that a rare Arginine-to-Histamine mutation at amino acid residue 47 (R47H) in Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) results in increased disease risk. TREM2 plays critical roles in regulating microglial response to amyloid plaques in AD, leading to their clustering and activation surrounding the plaques. We previously showed that increasing human TREM2 gene dosage exerts neuroprotective effects against AD-related deficits in amyloid depositing mouse models of AD. However, the in vivo effects of the R47H mutation on human TREM2-mediated microglial reprogramming and neuroprotection remains poorly understood.

Method

Here we created a BAC transgenic mouse model expressing human TREM2 with the R47H mutation in its cognate genomic context (BAC-TREM2-R47H). Importantly, the BAC used in this study was engineered to delete critical exons of other TREM-like genes on the BAC to prevent confounding effects of overexpressing multiple TREM-like genes. We crossed BAC-TREM2- R47H mice with 5xFAD [1], an amyloid depositing mouse model of AD, to evaluate amyloid pathologies and microglial phenotypes, transcriptomics and in situ expression of key TREM2 -dosage dependent genes. We also compared the key findings in 5xFAD/BAC-TREM2-R47H to those observed in 5xFAD/BAC-TREM2 mice.

Result

Both BAC-TREM2 and BAC-TREM2-R47H showed proper expression of three splicing isoforms of TREM2 that are normally found in human. In 5xFAD background, elevated TREM2-R47H gene dosages significantly reduced the plaque burden, especially the filamentous type. The results were consistent with enhanced phagocytosis and altered NLRP3 inflammasome activation in BAC- TREM2-R47H microglia in vitro. However, unlike TREM2 overexpression, elevated TREM2- R47H in 5xFAD failed to ameliorate cognitive and transcriptomic deficits. In situ analysis of key TREM2 -dosage dependent genes and microglial morphology uncovered that TREM2-R47H showed a loss-of-function phenotype in reprogramming of plaque-associated microglial reactivity and gene expression in 5xFAD.

Conclusion

Our study demonstrated that the AD-risk variant has a previously unknown, mixture of partial and full loss of TREM2 functions in modulating microglial response in AD mouse brains. Together, our new BAC-TREM2-R47H model and prior BAC-TREM2 mice are invaluable resource to facilitate the therapeutic discovery that target human TREM2 and its R47H variant to ameliorate AD and other neurodegenerative disorders.

Pushing the boundaries of brain organoids to study Alzheimer's disease

Progression of Alzheimer's disease (AD) entails deterioration or aberrant function of multiple brain cell types, eventually leading to neurodegeneration and cognitive decline. Defining how complex cell-cell interactions become dysregulated in AD requires novel human cell-based in vitro platforms that could recapitulate the intricate cytoarchitecture and cell diversity of the human brain. Brain organoids (BOs) are 3D self-organizing tissues that partially resemble the human brain architecture and can recapitulate AD-relevant pathology. In this review, we highlight the versatile applications of different types of BOs to model AD pathogenesis, including amyloid-β and tau aggregation, neuroinflammation, myelin breakdown, vascular dysfunction, and other phenotypes, as well as to accelerate therapeutic development for AD.

Soluble TREM2 Concentrations in the Cerebrospinal Fluid Correlate with the Severity of Neurofibrillary Degeneration, Cognitive Impairment, and Inflammasome Activation in Alzheimer's Disease

BACKGROUND

Individuals with specific TREM2 gene variants that encode for a Triggering Receptor Expressed on Myeloid cells 2 have a higher prevalence of Alzheimer's disease (AD). By interacting with amyloid and apolipoproteins, the TREM2 receptor regulates the number of myeloid cells, phagocytosis, and the inflammatory response. Higher TREM2 expression has been suggested to protect against AD. However, it is extremely difficult to comprehend TREM2 signaling in the context of AD. Previous results are variable and show distinct effects on diverse pathological changes in AD, differences between soluble and membrane isoform signaling, and inconsistency between animal models and humans. In addition, the relationship between TREM2 and inflammasome activation pathways is not yet entirely understood.

OBJECTIVE

This study aimed to determine the relationship between soluble TREM2 (sTREM2) levels in cerebrospinal fluid (CSF) and plasma samples and other indicators of AD pathology.

METHODS

Using the Enzyme-Linked Immunosorbent Assay (ELISA), we analyzed 98 samples of AD plasma, 35 samples of plasma from individuals with mild cognitive impairment (MCI), and 11 samples of plasma from healthy controls (HC), as well as 155 samples of AD CSF, 90 samples of MCI CSF, and 50 samples of HC CSF.

RESULTS

CSF sTREM2 levels were significantly correlated with neurofibrillary degeneration, cognitive decline, and inflammasome activity in AD patients. In contrast to plasma sTREM2, CSF sTREM2 levels in the AD group were higher than those in the MCI and HC groups. Moreover, concentrations of sTREM2 in CSF were substantially higher in the MCI group than in the HC group, indicating that CSF sTREM2 levels could be used not only to distinguish between HC and AD patients but also as a biomarker to detect earlier changes in the MCI stage.

CONCLUSIONS

The results indicate CSF sTREM2 levels reliably predict neurofibrillary degeneration, cognitive decline, and inflammasome activation, and also have a high diagnostic potential for distinguishing diseased from healthy individuals. To add sTREM2 to the list of required AD biomarkers, future studies will need to include a larger number of patients and utilize a standardized methodology.

Advances in proteomic phenotyping of microglia in neurodegeneration

Microglia are dynamic resident immune cells of the central nervous system (CNS) that sense, survey, and respond to changes in their environment. In disease states, microglia transform from homeostatic to diverse molecular phenotypic states that play complex and causal roles in neurologic disease pathogenesis, as evidenced by the identification of microglial genes as genetic risk factors for neurodegenerative disease. While advances in transcriptomic profiling of microglia from the CNS of humans and animal models have provided transformative insights, the transcriptome is only modestly reflective of the proteome. Proteomic profiling of microglia is therefore more likely to provide functionally and therapeutically relevant targets. In this review, we discuss molecular insights gained from transcriptomic studies of microglia in the context of Alzheimer's disease as a prototypic neurodegenerative disease, and highlight existing and emerging approaches for proteomic profiling of microglia derived from in vivo model systems and human brain.

CARD9 attenuates Aβ pathology and modifies microglial responses in an Alzheimer's disease mouse model

Recent advances have highlighted the importance of several innate immune receptors expressed by microglia in Alzheimer's disease (AD). In particular, mounting evidence from AD patients and experimental models indicates pivotal roles for TREM2, CD33, and CD22 in neurodegenerative disease progression. While there is growing interest in targeting these microglial receptors to treat AD, we still lack knowledge of the downstream signaling molecules used by these receptors to orchestrate immune responses in AD. Notably, TREM2, CD33, and CD22 have been described to influence signaling associated with the intracellular adaptor molecule CARD9 to mount downstream immune responses outside of the brain. However, the role of CARD9 in AD remains poorly understood. Here, we show that genetic ablation of CARD9 in the 5xFAD mouse model of AD results in exacerbated amyloid beta (Aβ) deposition, increased neuronal loss, worsened cognitive deficits, and alterations in microglial responses. We further show that pharmacological activation of CARD9 promotes improved clearance of Aβ deposits from the brains of 5xFAD mice. These results help to establish CARD9 as a key intracellular innate immune signaling molecule that regulates Aβ-mediated disease and microglial responses. Moreover, these findings suggest that targeting CARD9 might offer a strategy to improve Aβ clearance in AD.

Association of rs2062323 in the TREM1 gene with Alzheimer's disease and cerebrospinal fluid-soluble TREM2

INTRODUCTION AND AIMS

Genetic variations play a significant role in determining an individual's AD susceptibility. Research on the connection between AD and TREM1 gene polymorphisms (SNPs) remained lacking. We sought to examine the associations between TREM1 SNPs and AD.

METHODS

Based on the 1000 Genomes Project data, linkage disequilibrium (LD) analyses were utilized to screen for candidate SNPs in the TREM1 gene. AD cases (1081) and healthy control subjects (870) were collected and genotyped, and the associations between candidate SNPs and AD risk were analyzed. We explored the associations between target SNP and AD biomarkers. Moreover, 842 individuals from ADNI were selected to verify these results. Linear mixed models were used to estimate associations between the target SNP and longitudinal cognitive changes.

RESULTS

The rs2062323 was identified to be associated with AD risk in the Han population, and rs2062323T carriers had a lower AD risk (co-dominant model: OR, 0.67, 95% CI, 0.51-0.88, p = 0.0037; additive model: OR, 0.82, 95% CI, 0.72-0.94, p = 0.0032). Cerebrospinal fluid (CSF) sTREM2 levels were significantly increased in middle-aged rs2062323T carriers (additive model: β = 0.18, p = 0.0348). We also found significantly elevated levels of CSF sTREM2 in the ADNI. The rate of cognitive decline slowed down in rs2062323T carriers.

CONCLUSIONS

This study is the first to identify significant associations between TREM1 rs2062323 and AD risk. The rs2062323T may be involved in AD by regulating the expression of TREM1, TREML1, TREM2, and sTREM2. The TREM family is expected to be a potential therapeutic target for AD.

TREM2 gene expression associations with Alzheimer's disease neuropathology are region-specific: implications for cortical versus subcortical microglia

Previous post-mortem assessments of TREM2 expression and its association with brain pathologies have been limited by sample size. This study sought to correlate region-specific TREM2 mRNA expression with diverse neuropathological measures at autopsy using a large sample size (N = 945) of bulk RNA sequencing data from the Religious Orders Study and Rush Memory and Aging Project (ROS/MAP). TREM2 gene expression of the dorsolateral prefrontal cortex, posterior cingulate cortex, and caudate nucleus was assessed with respect to core pathology of Alzheimer's disease (amyloid-β, and tau), cerebrovascular pathology (cerebral infarcts, arteriolosclerosis, atherosclerosis, and cerebral amyloid angiopathy), microglial activation (proportion of activated microglia), and cognitive performance. We found that cortical TREM2 levels were positively related to AD diagnosis, cognitive decline, and amyloid-β neuropathology but were not related to the proportion of activated microglia. In contrast, caudate TREM2 levels were not related to AD pathology, cognition, or diagnosis, but were positively related to the proportion of activated microglia in the same region. Diagnosis-stratified results revealed caudate TREM2 levels were inversely related to AD neuropathology and positively related to microglial activation and longitudinal cognitive performance in AD cases. These results highlight the notable changes in TREM2 transcript abundance in AD and suggest that its pathological associations are brain-region-dependent.

Serum soluble triggering receptor expressed on myeloid cells-2 was not altered by rTMS in patients with treatment-resistant depression

AIM

Repetitive transcranial magnetic stimulation (rTMS) is one of the most effective and minimally invasive treatments for treatment-resistant depression (TRD). However, the mechanism underlying the therapeutic effects of rTMS in patients with TRD remains unclear. In recent years, the pathogenesis of depression has been closely associated with chronic inflammation and microglia are believed to play an important role in chronic inflammation. Triggering receptor expressed on myeloid cells-2 (TREM2) plays an important role in microglial neuroinflammatory regulation. In this study, we investigated the changes in peripheral soluble TREM2 (sTREM2) before and after rTMS treatment in patients with TRD.

METHODS

Twenty-six patients with TRD were enrolled in this frequency (10 Hz) rTMS study. Depressive symptoms, cognitive function, and serum sTREM2 concentrations were measured at baseline and the end of the 6-week rTMS treatment.

RESULTS

This study showed that rTMS ameliorated depressive symptoms and partially improved cognitive dysfunction in TRD. However, rTMS treatment did not alter serum sTREM2 levels.

CONCLUSIONS

This is the first sTREM2 study in patients with TRD who underwent rTMS treatment. These results suggest that serum sTREM2 may not be relevant for the mechanism underlying the therapeutic effect of rTMS in patients with TRD. Future studies should confirm the present findings using a larger patient sample and a sham rTMS procedure, as well as CSF sTREM2. Furthermore, a longitudinal study should be conducted to clarify the effects of rTMS on sTREM2 levels.

Moving beyond amyloid and tau to capture the biological heterogeneity of Alzheimer's disease

Alzheimer's disease (AD) manifests along a spectrum of cognitive deficits and levels of neuropathology. Genetic studies support a heterogeneous disease mechanism, with around 70 associated loci to date, implicating several biological processes that mediate risk for AD. Despite this heterogeneity, most experimental systems for testing new therapeutics are not designed to capture the genetically complex drivers of AD risk. In this review, we first provide an overview of those aspects of AD that are largely stereotyped and those that are heterogeneous, and we review the evidence supporting the concept that different subtypes of AD are important to consider in the design of agents for the prevention and treatment of the disease. We then dive into the multifaceted biological domains implicated to date in AD risk, highlighting studies of the diverse genetic drivers of disease. Finally, we explore recent efforts to identify biological subtypes of AD, with an emphasis on the experimental systems and data sets available to support progress in this area.

Six mitophagy-related hub genes as peripheral blood biomarkers of Alzheimer's disease and their immune cell infiltration correlation

Background

Alzheimer's disease (AD), a neurodegenerative disorder with progressive symptoms, seriously endangers human health worldwide. AD diagnosis and treatment are challenging, but molecular biomarkers show diagnostic potential. This study aimed to investigate AD biomarkers in the peripheral blood.

Method

Utilizing three microarray datasets, we systematically analyzed the differences in expression and predictive value of mitophagy-related hub genes (MRHGs) in the peripheral blood mononuclear cells of patients with AD to identify potential diagnostic biomarkers. Subsequently, a protein-protein interaction network was constructed to identify hub genes, and functional enrichment analyses were performed. Using consistent clustering analysis, AD subtypes with significant differences were determined. Finally, infiltration patterns of immune cells in AD subtypes and the relationship between MRHGs and immune cells were investigated by two algorithms, CIBERSORT and single-sample gene set enrichment analysis (ssGSEA).

Results

Our study identified 53 AD- and mitophagy-related differentially expressed genes and six MRHGs, which may be potential biomarkers for diagnosing AD. Functional analysis revealed that six MRHGs significantly affected biologically relevant functions and signaling pathways such as IL-4 Signaling Pathway, RUNX3 Regulates Notch Signaling Pathway, IL-1 and Megakaryocytes in Obesity Pathway, and Overview of Leukocyteintrinsic Hippo Pathway. Furthermore, CIBERSORT and ssGSEA algorithms were used for all AD samples to analyze the abundance of infiltrating immune cells in the two disease subtypes. The results showed that these subtypes were significantly related to immune cell types such as activated mast cells, regulatory T cells, M0 macrophages, and neutrophils. Moreover, specific MRHGs were significantly correlated with immune cell levels.

Conclusion

Our findings suggest that MRHGs may contribute to the development and prognosis of AD. The six identified MRHGs could be used as valuable diagnostic biomarkers for further research on AD. This study may provide new promising diagnostic and therapeutic targets in the peripheral blood of patients with AD.

Acute and Chronic Macrophage Differentiation Modulates TREM2 in a Personalized Alzheimer's Patient-Derived Assay

Neuroinflammation plays a pivotal role in the pathogenesis of Alzheimer`s disease (AD). Brain macrophage populations differentially modulate the immune response to AD pathology according to the disease stage. Triggering receptor expressed on myeloid cells 2 (TREM2) is known to play a protective role in AD and has been postulated as a putative therapeutic target. Whether, and to which extent TREM2 expression can be modulated in the aged macrophage population of the brain is unknown, emphasizing the need for a human, patient-specific model. Using cells from AD patients and matched controls (CO) we designed an assay based on monocyte-derived macrophages to mimic brain-infiltrating macrophages and to assess the individualized TREM2 synthesis in vitro. We systematically assessed the effects of short-term (acute-2 days) and long-term (chronic-10 days) M1- (LPS), M2- (IL-10, IL-4, TGF-β), and M0- (vehicle) macrophage differentiation on TREM2 synthesis. Moreover, the effects of retinoic acid (RA), a putative TREM2 modulator, on individualized TREM2 synthesis were assessed. We report increased TREM2 synthesis after acute M2- compared to M1-differentiation in CO- but not AD-derived cells. Chronic M2- and M0-differentiation however resulted in an increase of TREM2 synthesis in both AD- and CO-derived cells while chronic M1-differentiation increased TREM2 in AD-derived cells only. Moreover, chronic M2- and M0-differentiation improved the amyloid-β (Aβ) uptake of the CO-derived whereas M1-differentiation of the AD-derived cells. Interestingly, RA-treatment did not modulate TREM2. In the age of personalized medicine, our individualized model could be used to screen for potential drug-mediated treatment responses in vitro. Triggering receptor expressed on myeloid cells 2 (TREM2) has been postulated as a putative therapeutic target in Alzheimer's disease (AD). Using cells from AD patients and matched controls (CO), we designed a monocyte-derived macrophages (Mo-MФs) assay to assess the individualized TREM2 synthesis in vitro. We report increased TREM2 synthesis after acute M2- compared to M1- macrophage differentiation in CO- but not AD-derived cells. Chronic M2- and M0- differentiation however resulted in an increase of TREM2 synthesis in both AD- and CO-derived cells while chronic M1-differentiation increased TREM2 in AD-cells only.