Trem2 Deletion Reduces Late-Stage Amyloid Plaque Accumulation, Elevates the Aβ42:Aβ40 Ratio, and Exacerbates Axonal Dystrophy and Dendritic Spine Loss in the PS2APP Alzheimer's Mouse Model

TREM2 and sTREM2 in Alzheimer's disease: from mechanisms to therapies

Triggering receptor expressed on myeloid cells 2 (TREM2) is an innate immune receptor predominantly expressed by microglia in the brain. Recent studies have established TREM2 as a central immune signaling hub in neurodegeneration, where it triggers immune responses upon sensing pathological development and tissue damages. TREM2 binds diverse ligands and activates downstream pathways that regulate microglial phagocytosis, inflammatory responses, and metabolic reprogramming. Interestingly, TREM2 exists both in its membrane-bound form and as a soluble variant (sTREM2), that latter is generated through proteolytic shedding or alternative splicing and can be detected in cerebrospinal fluid and plasma. Emerging clinical and preclinical evidence underscores the potential of TREM2 and sTREM2 as diagnostic biomarkers and therapeutic targets in Alzheimer's disease (AD). This review provides a comprehensive overview of the molecular functions, regulatory mechanisms, and pathological implications of TREM2 and sTREM2 in AD. Furthermore, we explore their potential roles in diagnostics and therapeutics while suggesting key research directions for advancing TREM2/sTREM2-based strategies in combating AD.

Dynamic changes of hippocampal dendritic spines in Alzheimer's disease mice among the different stages

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β (Aβ) peptides and a progressive decline in cognitive function. Hippocampus as a crucial brain area for learning and memory, is also adversely affected by AD's pathology. The accumulation of Aβ is often associated with the loss of dendritic spines of the hippocampus. However, the dynamic alterations in dendritic spines throughout AD progression are not fully understood. To investigate it, we conducted in-vivo imaging in two mouse models representing the early and late stages of AD pathology: young mice injected with Aβ1-42 oligomers and APP/PS1 transgenic mice. In the early-stage AD model, imaging was conducted at third- and fifth- weeks post-injection. In the late-stage AD model, a four-month imaging began at 14 months old. The imaging results showed spine elimination in both models. Notably, acute Aβ exposure was linked to heightened spine loss on secondary dendrites, while in the late stage the primary effect was on tertiary dendrites. Concurrently, with the metabolism of Aβ, cognition recovered to some extent by five weeks post Aβ1-42 exposure. These findings suggested that dendritic spine plasticity was impaired during the development of AD, as evidenced by increasing spine loss at different levels. However, the cognitive recovery observed in early-stage AD model mice may indicate a compensatory structural reorganization, highlighting the potential of early intervention to mitigate disease progression. Our results provide novel insights into the neurotoxic effects of Aβ1-42 and may contribute to the development of therapeutic strategies for AD.

Treadmill exercise ameliorates hippocampal synaptic injury and recognition memory deficits by TREM2 in AD rat model

OBJECTIVE

The impairment of cognitive function has been associated with Alzheimer's disease (AD). Exercise exerts a positive modulatory effect on cognition by reducing synapse injury. However, limited in vivo evidence is available to validate the neuroprotective effect of TREM2 on synaptic function in this phenomenon. Here, we aim to explore whether physical exercise pretreatment alters Aβ-induced recognition memory impairment in structural synaptic plasticity within the hippocampus in AD rats. METHODS：: In study 1, fifty-two Sprague-Dawley (SD) rats were randomly divided into following four groups: control group (C group, n = 13), Alzheimer's disease group (AD group, n = 13), 4 weeks of physical exercise and Alzheimer's disease group (Exe+AD group, n = 13), 4 weeks of physical exercise and blank group (Exercise group, n = 13). Four weeks of treadmill exercise intervention was performed, and AD model were established by intra-cerebroventricular injection (ICV) injection of Aβ1-42 protein. After 3 weeks, we also conducted a novel object test to evaluate recognition memory in the behavior assessment. Golgi staining and transmission electron microscopy were used to evaluate the morphology and synaptic ultrastructure of neurons. Western blotting was used to measure the expression of hippocampal synaptic proteins. Extracellular neurotransmitters in the hippocampus were detected by microdialysis coupled with high-performance liquid chromatography. In study 2, 33 SD rats were randomly divided into three groups: 4 weeks of physical exercise and Alzheimer's disease group (Exe+AD group, n = 11), AAV-Control and physical exercise and Alzheimer's disease group (AAV-Control+Exe+AD group, n = 11), AAV-TREM2 and physical exercise and Alzheimer's disease group (AAV-TREM2 +Exe+AD group, n = 11). Stereotactic intracerebral injection in the bilateral hippocampus was performed to achieve microglial TREM2 down-expression by using adeno-associated virus (AAV) with CD68 promoter. After 4 weeks treadmill exercise and 3 weeks Aβ injection, all rats received behavior test and molecular experiment, which the same with experiment 2.

RESULTS

Novel recognition index in novel object recognition test significantly decreased, and western blot demonstrate that hippocampal TREM2 protein is significantly decreased (P < 0.001). But physical exercise reversed this phenomenon(P < 0.001). In addition, compared with Con group, the neuron from Exe+AD group exhibited a more complex branching pattern (P < 0.05). And impaired synaptic ultrastructure was observed in AD group. Hippocampal synaptic-related protein (SYX, SYP, GAP43, PSD95) and neurotransmitter (DA, Glu, GABA) was also significantly decreased (P < 0.01) in AD group. But the neuroprotection effect can be found in Exe+AD group, which are associated with the inhibition of synaptic injury by activate hippocampal TREM2 (P < 0.05). However, when blockade of hippocampal TREM2 reduced brain protective effect of exercise in AD rat model, including increased the damage of neuronal dendritic complexity, synaptic ultrastructure, and the decrease of hippocampal synapses-related protein, typical neurotransmitter.

CONCLUSION

Treadmill exercise facilitated recognition memory acquisition via TREM2-mediated structural synaptic plasticity of the hippocampus in an AD rat model.

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.

Asrij/OCIAD1 depletion reduces inflammatory microglial activation and ameliorates Aβ pathology in an Alzheimer's disease mouse model

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the accumulation of amyloid-beta (Aβ) plaques and neurofibrillary tangles, neuroinflammation, and glial activation. Asrij/OCIAD1 (Ovarian Carcinoma Immunoreactive Antigen Domain containing protein 1) is an AD-associated factor. Increased Asrij levels in the brains of AD patients and mouse models are linked to the severity of neurodegeneration. However, the contribution of Asrij to AD progression and whether reducing Asrij levels is sufficient to mitigate Aβ pathology in vivo is unclear.

METHODS

To explore the impact of Asrij on AD pathology, we deleted asrij in the APP/PS1 mouse model of AD and analyzed the effects on AD hallmarks. We used the Morris water maze and open field test to assess behavioral performance. Using immunohistochemistry and biochemical analyses, we evaluated Aβ plaque load, neuronal and synaptic damage, and gliosis. Further, we utilized confocal microscopy imaging, flow cytometry, and RNA sequencing analysis to comprehensively investigate changes in microglial responses to Aβ pathology upon Asrij depletion.

RESULTS

Asrij depletion ameliorates cognitive impairments, Aβ deposition, neuronal and synaptic damage, and reactive astrogliosis in the AD mouse. Notably, Asrij-deficient microglia exhibit reduced plaque-associated proliferation and decreased phagocytic activation. Transcriptomic analyses of AD microglia reveal upregulation of energy metabolism pathways and downregulation of innate immunity and inflammatory pathways upon Asrij depletion. Mechanistically, loss of Asrij increases mitochondrial activity and impedes the acquisition of a pro-inflammatory disease-associated microglia (DAM) state. Reduced levels of proinflammatory cytokines and decreased STAT3 and NF-κB activation indicate protective changes in AD microglia. Taken together, our results suggest that increased Asrij levels reported in AD, may suppress microglial metabolic activity and promote inflammatory microglial activation, thereby exacerbating AD pathology.

CONCLUSIONS

In summary, we show that Asrij depletion ameliorates Aβ pathology, neuronal and synaptic damage, gliosis, and improves behavioral performance in APP/PS1 mice. This supports that Asrij exacerbates the AD pathology. Mechanistically, Asrij is critical for the development of DAM and promotes neuroinflammatory signaling activation in microglia, thus restricting neuroprotective microglial responses. Hence, reducing Asrij in this context may help retard AD. Our work positions Asrij as a critical molecular regulator that links microglial dysfunction to AD pathogenesis.

Loss of endothelial CD2AP causes sex-dependent cerebrovascular dysfunction

Polymorphisms in CD2-associated protein (CD2AP) predispose to Alzheimer's disease (AD), but the underlying mechanisms remain unknown. Here, we show that loss of CD2AP in cerebral blood vessels is associated with cognitive decline in AD subjects and that genetic downregulation of CD2AP in brain vascular endothelial cells impairs memory function in male mice. Animals with reduced brain endothelial CD2AP display altered blood flow regulation at rest and during neurovascular coupling, defects in mural cell activity, and an abnormal vascular sex-dependent response to Aβ. Antagonizing endothelin-1 receptor A signaling partly rescues the vascular impairments, but only in male mice. Treatment of CD2AP mutant mice with reelin glycoprotein that mitigates the effects of CD2AP loss function via ApoER2 increases resting cerebral blood flow and even protects male mice against the noxious effect of Aβ. Thus, endothelial CD2AP plays critical roles in cerebrovascular functions and represents a novel target for sex-specific treatment in AD.

Innate and adaptive immunity in neurodegenerative disease

Neurodegenerative diseases (NDs) are a group of neurological disorders characterized by the progressive loss of selected neurons. Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common NDs. Pathologically, NDs are characterized by progressive failure of neural interactions and aberrant protein fibril aggregation and deposition, which lead to neuron loss and cognitive and behavioral impairments. Great efforts have been made to delineate the underlying mechanism of NDs. However, the very first trigger of these disorders and the state of the illness are still vague. Existing therapeutic strategies can relieve symptoms but cannot cure these diseases. The human immune system is a complex and intricate network comprising various components that work together to protect the body against pathogens and maintain overall health. They can be broadly divided into two main types: innate immunity, the first line of defense against pathogens, which acts nonspecifically, and adaptive immunity, which follows a defense process that acts more specifically and is targeted. The significance of brain immunity in maintaining the homeostatic environment of the brain, and its direct implications in NDs, has increasingly come into focus. Some components of the immune system have beneficial regulatory effects, whereas others may have detrimental effects on neurons. The intricate interplay and underlying mechanisms remain an area of active research. This review focuses on the effects of both innate and adaptive immunity on AD and PD, offering a comprehensive understanding of the initiation and regulation of brain immunity, as well as the interplay between innate and adaptive immunity in influencing the progression of NDs.

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

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

Neuroglia in neurodegeneration: Alzheimer, Parkinson, and Huntington disease

The conspicuous rise of chronic neurodegenerative diseases, including Alzheimer (AD), Parkinson (PD), and Huntington (HD) diseases, is currently without disease-modifying therapies and accompanied by an excessive rate of unsuccessful clinical trials. This reflects a profound lack of understanding of the pathogenesis of these diseases, indicating that the current paradigms guiding disease modeling and drug development are in need of reconsideration. The role of neuroglia, namely astrocytes, microglial cells, and oligodendrocytes, in the pathogenesis of neurodegenerative diseases emerged during the last decades. This chapter provides the state-of-the-art update on the changes of astrocytes, microglial cells, and oligodendrocytes in AD, PD, and HD. A growing body of evidence suggests that homeostatic and defensive functions of glial cells are compromised at different disease stages, leading to increased susceptibility of neurons to noxious stimuli, eventually resulting in their malfunction and degeneration. Investments are needed in the generation of novel preclinical models suitable for studying glial pathology, in "humanizing" research, and in-depth investigation of glial cell alterations to slow down and, possibly, halt and prevent the rise of neurodegenerative disease. Targeting glial cells opens new therapeutic avenues to treat AD, PD, and HD.

Tau Accumulation Induces Microglial State Alterations in Alzheimer's Disease Model Mice

Unique microglial states have been identified in Alzheimer's disease (AD) model mice and postmortem AD brains. Although it has been well documented that amyloid-β accumulation induces the alteration of microglial states, the relationship between tau pathology and microglial states remains incompletely understood because of a lack of suitable AD models. In the present study, we generated a novel AD model mouse by the intracerebral administration of tau purified from human brains with primary age-related tauopathy into App knock-in mice with humanized tau. Immunohistochemical analyses revealed that Dectin-1-positive disease-associated microglia were increased in the AD model mice after tau accumulation in the brain. We then performed single-nucleus RNA sequencing on the AD model mice to evaluate the differences in microglial states with and without tau propagation and accumulation. By taking advantage of spatial transcriptomics and existing single-cell RNA sequencing datasets, we showed for the first time that tau propagation and accumulation induce a disease-associated microglial phenotype at the expense of an age-related nonhomeostatic counterpart (namely, white matter-associated microglia) in an AD model mouse brain. Future work using spatial transcriptomics at single-cell resolution will pave the way for a more appropriate interpretation of microglial alterations in response to tau pathology in the AD brain.

Preclinical and first-in-human evaluation of AL002, a novel TREM2 agonistic antibody for Alzheimer's disease

BACKGROUND

Variants of the gene triggering receptor expressed on myeloid cells-2 (TREM2) increase the risk of Alzheimer's disease (AD) and other neurodegenerative disorders. Signaling by TREM2, an innate immune receptor expressed by microglia, is thought to enhance phagocytosis of amyloid beta (Aβ) and other damaged proteins, promote microglial proliferation, migration, and survival, and regulate inflammatory signaling. Thus, TREM2 activation has potential to alter the progression of AD. AL002 is an investigational, engineered, humanized monoclonal immunoglobulin G1 (IgG1) antibody designed to target TREM2. In AD mouse models, an AL002 murine variant has been previously shown to induce microglial proliferation and reduce filamentous Aβ plaques and neurite dystrophy.

METHODS

Preclinical studies assessed the safety, tolerability, pharmacokinetics, and pharmacodynamics of AL002 in cynomolgus monkeys. INVOKE-1 (NCT03635047) was a first-in-human phase 1, randomized, placebo-controlled, double-blind study assessing the safety, tolerability, PK, and PD of AL002 administered as single ascending doses (SAD) in healthy volunteers.

RESULTS

In cynomolgus monkeys, weekly intravenous injections of AL002 for 4 weeks were well tolerated, dose-dependently decreased soluble TREM2 (sTREM2) in cerebrospinal fluid (CSF) and total TREM2 in hippocampus and frontal cortex, and increased biomarkers of TREM2 signaling in CSF and brain. In the phase 1 study of 64 healthy volunteers, a single intravenous infusion of AL002 demonstrated brain target engagement based on a dose-dependent reduction of sTREM2 in CSF and parallel increases in biomarkers of TREM2 signaling and microglia recruitment. Single-dose AL002 showed central nervous system penetrance and was well tolerated, with no treatment-related serious adverse events over 12 weeks.

CONCLUSIONS

These findings support the continued clinical development of AL002 for AD and other neurodegenerative diseases in which TREM2 activation may be beneficial. AL002 is currently being tested in a phase 2, randomized, double-blind, placebo-controlled study in early AD.

TRIAL REGISTRATION

Clinicaltrials.gov, NCT03635047. Registered on August 15, 2018, https://www.

CLINICALTRIALS

gov/study/NCT03635047 .

Adhesion G protein-coupled receptor ADGRG1 promotes protective microglial response in Alzheimer's disease

Germline genetic architecture of Alzheimer's disease (AD) indicates microglial mechanisms of disease susceptibility and outcomes. However, the mechanisms that enable microglia to mediate protective responses to AD pathology remain elusive. Adgrg1 is specifically expressed in yolk-sac-derived microglia. This study reveals the role of yolk-sac-derived microglia in AD pathology, highlighting the function of ADGRG1 in modulating microglial protective responses to amyloid deposition. Utilizing both constitutive and inducible microglial Adgrg1 knockout 5xFAD models, we demonstrate that Adgrg1 deficiency leads to increased amyloid deposition, exacerbated neuropathology, and accelerated cognitive impairment. Transcriptomic analyses reveal a distinct microglial state characterized by downregulated genes associated with homeostasis, phagocytosis, and lysosomal functions. Functional assays in mouse models and human embryonic stem cells-derived microglia support that microglial ADGRG1 is required for efficient Aβ phagocytosis. Together, these results uncover a GPCR-dependent microglial response to Aβ, pointing towards potential therapeutic strategies to alleviate disease progression by enhancing microglial functional competence.

Coupling of Alzheimer's Disease Genetic Risk Factors with Viral Susceptibility and Inflammation

Alzheimer's disease (AD) is a neurodegenerative disease characterized by persistent cognitive decline. Amyloid plaque deposition and neurofibrillary tangles are the main pathological features of AD brain, though mechanisms leading to the formation of lesions remain to be understood. Genetic efforts through genome-wide association studies (GWAS) have identified dozens of risk genes influencing the pathogenesis and progression of AD, some of which have been revealed in close association with increased viral susceptibilities and abnormal inflammatory responses in AD patients. In the present study, we try to present a list of AD candidate genes that have been shown to affect viral infection and inflammatory responses. Understanding of how AD susceptibility genes interact with the viral life cycle and potential inflammatory pathways would provide possible therapeutic targets for both AD and infectious diseases.

Edaravone Dexborneol ameliorates the cognitive deficits of APP/PS1 mice by inhibiting TLR4/MAPK signaling pathway via upregulating TREM2

Currently, there are no effective therapeutic agents available to treat Alzheimer's disease (AD). However, edaravone dexborneol (EDB), a novel composite agent used to treat acute ischemic stroke, has recently been shown to exert efficacious neuroprotective effects. However, whether EDB can ameliorate cognitive deficits in AD currently remains unclear. To this end, we explored the effects of EDB on AD and its potential mechanisms using an AD animal model (male APP/PS1 mice) treated with EDB for 10 weeks starting at 6 months of age. Subsequent analyses revealed that EDB-treated APP/PS1 mice exhibited improved cognitive abilities compared to untreated APP/PS1 mice. Administration of EDB in APP/PS1 mice further alleviated neuropathological alterations of the hippocampus, including Aβ deposition, pyramidal cell karyopyknosis, and oxidative damage, and significantly decreased the levels of inflammatory cytokines (IL-1β, IL-6 and TNF-α) and COX-2 in the hippocampus of APP/PS1 mice. Transcriptome sequencing analysis demonstrated the critical role of the inflammatory reaction in EDB treatment in APP/PS1 mice, indicating that the alleviation of the inflammatory reaction by EDB in the hippocampus of APP/PS1 mice was linked to the action of the TREM2/TLR4/MAPK signaling pathway. Further in vitro investigations showed that EDB suppressed neuroinflammation in LPS-stimulated BV2 cells by inhibiting the TLR4/MAPK signaling pathway and upregulating TREM2 expression. Thus, the findings of the present study demonstrate that EDB is a promising therapeutic agent for AD-related cognitive dysfunction.

Therapeutic potential of plant-derived natural compounds in Alzheimer's disease: Targeting microglia-mediated neuroinflammation

Microglia are resident immune cells of the central nervous system (CNS) with roles in sensing, housekeeping, and defense. Exploring the role of microglia in the occurrence and development of Alzheimer's disease (AD) and the possible therapeutic mechanism of plant-derived natural compounds (PDNCs) that regulate microglia-associated neuroinflammation may potentially help in elucidating the pathogenesis of AD and provide novel insights for its treatment. This review explores the role of abnormal microglial activation and its dominant neuroinflammatory response, as well as the activation of their target receptors and signaling pathways in AD pathogenesis. Additionally, we report an update on the potential pharmacological mechanisms of multiple PDNCs in modulating microglia-associated neuroinflammation in AD treatment. Dysregulated activation of microglial receptors and their downstream pathways impaired immune homeostasis in animal models of AD. Multiple signaling pathways, such as mitogen-activated protein kinase (MAPK), nuclear factor kappa light chain enhancer of activated B cells (NF-κB), and Toll-like receptors, play important roles in microglial activation and can exacerbate microglia-mediated neuroinflammation. PDNCs, such as magnolol, stigmasterol, matrine, naringenin, naringin, and resveratrol, can delay the progression of AD by inhibiting the proinflammatory receptors of microglia, activating its anti-inflammatory receptors, regulating the receptors related to β-amyloid (Aβ) clearance, reversing immune dysregulation, and maintaining the immune homeostasis of microglial downstream pathways. This review summarizes the mechanisms by which microglia cause chronic inflammation in AD and evaluates the beneficial effects of PDNCs on immune regulation in AD by regulating microglial receptors and their downstream pathways.

Therapeutic targeting of immunometabolism reveals a critical reliance on hexokinase 2 dosage for microglial activation and Alzheimer's progression

Neuroinflammation is a prominent feature of Alzheimer's disease (AD). Activated microglia undergo a reprogramming of cellular metabolism necessary to power their cellular activities during disease. Thus, selective targeting of microglial immunometabolism might be of therapeutic benefit for treating AD. In the AD brain, the levels of microglial hexokinase 2 (HK2), an enzyme that supports inflammatory responses by promoting glycolysis, are significantly increased. In addition, HK2 displays non-metabolic activities that extend its inflammatory role beyond glycolysis. The antagonism of HK2 affects microglial phenotypes and disease progression in a gene-dose-dependent manner. HK2 complete loss fails to improve pathology by exacerbating inflammation, while its haploinsufficiency reduces pathology in 5xFAD mice. We propose that the partial antagonism of HK2 is effective in slowing disease progression by modulating NF-κB signaling through its cytosolic target, IKBα. The complete loss of HK2 affects additional inflammatory mechanisms related to mitochondrial dysfunction.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Alzheimer's disease associated isoforms of human CD33 distinctively modulate microglial cell responses in 5XFAD mice

Microglia play diverse pathophysiological roles in Alzheimer's disease (AD), with genetic susceptibility factors skewing microglial cell function to influence AD risk. CD33 is an immunomodulatory receptor associated with AD susceptibility through a single nucleotide polymorphism that modulates mRNA splicing, skewing protein expression from a long protein isoform (CD33M) to a short isoform (CD33m). Understanding how human CD33 isoforms differentially impact microglial cell function in vivo has been challenging due to functional divergence of CD33 between mice and humans. We address this challenge by studying transgenic mice expressing either of the human CD33 isoforms crossed with the 5XFAD mouse model of amyloidosis and find that human CD33 isoforms have opposing effects on the response of microglia to amyloid-β (Aβ) deposition. Mice expressing CD33M have increased Aβ levels, more diffuse plaques, fewer disease-associated microglia, and more dystrophic neurites compared to 5XFAD control mice. Conversely, CD33m promotes plaque compaction and microglia-plaque contacts, and minimizes neuritic plaque pathology, highlighting an AD protective role for this isoform. Protective phenotypes driven by CD33m are detected at an earlier timepoint compared to the more aggressive pathology in CD33M mice that appears at a later timepoint, suggesting that CD33m has a more prominent impact on microglia cell function at earlier stages of disease progression. In addition to divergent roles in modulating phagocytosis, scRNAseq and proteomics analyses demonstrate that CD33m+ microglia upregulate nestin, an intermediate filament involved in cell migration, at plaque contact sites. Overall, our work provides new functional insights into how CD33, as a top genetic susceptibility factor for AD, modulates microglial cell function.

Microglia and amyloid plaque formation in Alzheimer's disease - Evidence, possible mechanisms, and future challenges

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive decline that severely affects patients and their families. Genetic and environmental risk factors, such as viral infections, synergize to accelerate the aging-associated neurodegeneration. Genetic risk factors for late-onset AD (LOAD), which accounts for most AD cases, are predominantly implicated in microglial and immune cell functions. As such, microglia play a major role in formation of amyloid beta (Aβ) plaques, the major pathological hallmark of AD. This review aims to provide an overview of the current knowledge regarding the role of microglia in Aβ plaque formation, as well as their impact on morphological and functional diversity of Aβ plaques. Based on this discussion, we seek to identify challenges and opportunities in this field with potential therapeutic implications.

The Intersection of cerebral cholesterol metabolism and Alzheimer's disease: Mechanisms and therapeutic prospects

Alzheimer's disease (AD) is a common neurodegenerative disease in the elderly, the exact pathogenesis of which remains incompletely understood, and effective preventive and therapeutic drugs are currently lacking. Cholesterol plays a vital role in cell membrane formation and neurotransmitter synthesis, and its abnormal metabolism is associated with the onset of AD. With the continuous advancement of imaging techniques and molecular biology methods, researchers can more accurately explore the relationship between cholesterol metabolism and AD. Elevated cholesterol levels may lead to vascular dysfunction, thereby affecting neuronal function. Additionally, abnormal cholesterol metabolism may affect the metabolism of β-amyloid protein, thereby promoting the onset of AD. Brain cholesterol levels are regulated by multiple factors. This review aims to deepen the understanding of the subtle relationship between cholesterol homeostasis and AD, and to introduce the latest advances in cholesterol-regulating AD treatment strategies, thereby inspiring readers to contemplate deeply on this complex relationship. Although there are still many unresolved important issues regarding the risk of brain cholesterol and AD, and some studies may have opposite conclusions, further research is needed to enrich our understanding. However, these findings are expected to deepen our understanding of the pathogenesis of AD and provide important insights for the future development of AD treatment strategies targeting brain cholesterol homeostasis.

Bacteroidota inhibit microglia clearance of amyloid-beta and promote plaque deposition in Alzheimer's disease mouse models

The gut microbiota and microglia play critical roles in Alzheimer's disease (AD), and elevated Bacteroides is correlated with cerebrospinal fluid amyloid-β (Aβ) and tau levels in AD. We hypothesize that Bacteroides contributes to AD by modulating microglia. Here we show that administering Bacteroides fragilis to APP/PS1-21 mice increases Aβ plaques in females, modulates cortical amyloid processing gene expression, and down regulates phagocytosis and protein degradation microglial gene expression. We further show that administering Bacteroides fragilis to aged wild-type male and female mice suppresses microglial uptake of Aβ1-42 injected into the hippocampus. Depleting murine Bacteroidota with metronidazole decreases amyloid load in aged 5xFAD mice, and activates microglial pathways related to phagocytosis, cytokine signaling, and lysosomal degradation. Taken together, our study demonstrates that members of the Bacteroidota phylum contribute to AD pathogenesis by suppressing microglia phagocytic function, which leads to impaired Aβ clearance and accumulation of amyloid plaques.

APOE2 gene therapy reduces amyloid deposition and improves markers of neuroinflammation and neurodegeneration in a mouse model of Alzheimer disease

Epidemiological studies show that individuals who carry the relatively uncommon APOE ε2 allele rarely develop Alzheimer disease, and if they do, they have a later age of onset, milder clinical course, and less severe neuropathological findings than people without this allele. The contrast is especially stark when compared with the major genetic risk factor for Alzheimer disease, APOE ε4, which has an age of onset several decades earlier, a more aggressive clinical course and more severe neuropathological findings, especially in terms of the amount of amyloid deposition. Here, we demonstrate that brain exposure to APOE ε2 via a gene therapy approach, which bathes the entire cortical mantle in the gene product after transduction of the ependyma, reduces Aβ plaque deposition, neurodegenerative synaptic loss, and, remarkably, reduces microglial activation in an APP/PS1 mouse model despite continued expression of human APOE ε4. This result suggests a promising protective effect of exogenous APOE ε2 and reveals a cell nonautonomous effect of the protein on microglial activation, which we show is similar to plaque-associated microglia in the brain of Alzheimer disease patients who inherit APOE ε2. These data increase the potential that an APOE ε2 therapeutic could be effective in Alzheimer disease, even in individuals born with the risky ε4 allele.

Silencing Apoe with divalent-siRNAs improves amyloid burden and activates immune response pathways in Alzheimer's disease

INTRODUCTION

The most significant genetic risk factor for late-onset Alzheimer's disease (AD) is APOE4, with evidence for gain- and loss-of-function mechanisms. A clinical need remains for therapeutically relevant tools that potently modulate APOE expression.

METHODS

We optimized small interfering RNAs (di-siRNA, GalNAc) to potently silence brain or liver Apoe and evaluated the impact of each pool of Apoe on pathology.

RESULTS

In adult 5xFAD mice, siRNAs targeting CNS Apoe efficiently silenced Apoe expression and reduced amyloid burden without affecting systemic cholesterol, confirming that potent silencing of brain Apoe is sufficient to slow disease progression. Mechanistically, silencing Apoe reduced APOE-rich amyloid cores and activated immune system responses.

DISCUSSION

These results establish siRNA-based modulation of Apoe as a viable therapeutic approach, highlight immune activation as a key pathway affected by Apoe modulation, and provide the technology to further evaluate the impact of APOE silencing on neurodegeneration.

The Single Toxin Origin of Alzheimer's Disease and Other Neurodegenerative Disorders Enables Targeted Approach to Treatment and Prevention

New data suggest that the aggregation of misfolded native proteins initiates and drives the pathogenic cascade that leads to Alzheimer's disease (AD) and other age-related neurodegenerative disorders. We propose a unifying single toxin theory of brain neurodegeneration that identifies new targets and approaches to the development of disease-modifying treatments. An extensive body of genetic evidence suggests soluble aggregates of beta-amyloid (Aβ) as the primary neurotoxin in the pathogenesis of AD. New insights from fluid biomarkers, imaging, and clinical studies provide further evidence for the decisive impact of toxic Aβ species in the initiation and progression of AD. Understanding the distinct roles of soluble and insoluble amyloid aggregates on AD pathogenesis has been the key missing piece of the Alzheimer's puzzle. Data from clinical trials with anti-amyloid agents and recent advances in the diagnosis of AD demonstrate that the driving insult in biologically defined AD is the neurotoxicity of soluble Aβ aggregates, called oligomers and protofibrils, rather than the relatively inert insoluble mature fibrils and amyloid plaques. Amyloid oligomers appear to be the primary factor causing the synaptic impairment, neuronal stress, spreading of tau pathology, and eventual cell death that lead to the clinical syndrome of AD dementia. All other biochemical effects and neurodegenerative changes in the brain that are observed in AD are a response to or a downstream effect of this initial toxic insult by oligomers. Other neurodegenerative disorders follow a similar pattern of pathogenesis, in which normal brain proteins with important biological functions become trapped in the aging brain due to impaired clearance and then misfold and aggregate into neurotoxic species that exhibit prion-like behavior. These aggregates then spread through the brain and cause disease-specific neurodegeneration. Targeting the inhibition of this initial step in neurodegeneration by blocking the misfolding and aggregation of healthy proteins has the potential to slow or arrest disease progression, and if treatment is administered early in the course of AD and other neurodegenerative disorders, it may delay or prevent the onset of clinical symptoms.

A case for seeking sex-specific treatments in Alzheimer's disease

There is no satisfactory explanation for the sex-related differences in the incidence of many diseases and this is also true of Alzheimer's disease (AD), where females have a higher lifetime risk of developing the disease and make up about two thirds of the AD patient population. The importance of understanding the cause(s) that account for this disproportionate distribution cannot be overestimated, and is likely to be a significant factor in the search for therapeutic strategies that will combat the disease and, furthermore, potentially point to a sex-targeted approach to treatment. This review considers the literature in the context of what is known about the impact of sex on processes targeted by drugs that are in clinical trial for AD, and existing knowledge on differing responses of males and females to these drugs. Current knowledge strongly supports the view that trials should make assessing sex-related difference in responses a priority with a focus on exploring the sex-stratified treatments.

Passive immunotherapy for Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease characterized by cognitive impairment with few therapeutic options. Despite many failures in developing AD treatment during the past 20 years, significant advances have been achieved in passive immunotherapy of AD very recently. Here, we review characteristics, clinical trial data, and mechanisms of action for monoclonal antibodies (mAbs) targeting key players in AD pathogenesis, including amyloid-β (Aβ), tau and neuroinflammation modulators. We emphasized the efficacy of lecanemab and donanemab on cognition and amyloid clearance in AD patients in phase III clinical trials and discussed factors that may contribute to the efficacy and side effects of anti-Aβ mAbs. In addition, we provided important information on mAbs targeting tau or inflammatory regulators in clinical trials, and indicated that mAbs against the mid-region of tau or pathogenic tau have therapeutic potential for AD. In conclusion, passive immunotherapy targeting key players in AD pathogenesis offers a promising strategy for effective AD treatment.

Microglia in Neurodegenerative Diseases

Neurodegenerative diseases are manifested by a progressive death of neural cells, resulting in the deterioration of central nervous system (CNS) functions, ultimately leading to specific behavioural and cognitive symptoms associated with affected brain regions. Several neurodegenerative disorders are caused by genetic variants or mutations, although the majority of cases are sporadic and linked to various environmental risk factors, with yet an unknown aetiology. Neuroglial changes are fundamental and often lead to the pathophysiology of neurodegenerative diseases. In particular, microglial cells, which are essential for maintaining CNS health, become compromised in their physiological functions with the exposure to environmental risk factors, genetic variants or mutations, as well as disease pathology. In this chapter, we cover the contribution of neuroglia, especially microglia, to several neurodegenerative diseases, including Nasu-Hakola disease, Parkinson's disease, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's disease, infectious disease-associated neurodegeneration, and metal-precipitated neurodegeneration. Future research perspectives for the field pertaining to the therapeutic targeting of microglia across these disease conditions are also discussed.

Targeting synapse function and loss for treatment of neurodegenerative diseases

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

Endothelial senescence alleviates cognitive impairment in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is among the most prevalent age-related neurodegenerative diseases. Endothelial cell (EC) senescence was discovered in the AD brain, but its function in AD pathogenesis was unidentified. Here we created an AD mouse model with EC senescence (APP/PS1;TERF2DN mice) by intercrossing APP/PS1 mice with Tie2 promoter-driven dominant negative telomeric repeat-binding factor 2 transgenic mice (TERF2DN-Tg mice). We evaluated cognitive functions and AD brain pathology in APP/PS1;TERF2DN mice. Surprisingly, compared with the control APP/PS1 mice, APP/PS1;TERF2DN mice demonstrated the attenuation of cognitive impairment and amyloid-β (Aβ) pathology, accompanied by the compaction of Aβ plaques with increased microglial coverage and reduced neurite dystrophy. Moreover, we evaluated whether EC senescence could affect microglial morphology and phagocytosis of Aβ. Compared with wild-type mice, microglia in TERF2DN-Tg mice display increased numbers of endpoints (a morphometric parameter to quantify the number of processes) and Aβ phagocytosis and related gene expression. Single-cell RNA-sequencing analysis showed that compared with APP/PS1 mouse microglia, APP/PS1;TERF2DN mouse microglia displayed a modest decline in disease-associated microglia, accompanied by an altered direction of biological process branching from antigen synthesis and arrangement to ribonucleoprotein complex biogenesis. Our outcomes indicate that EC senescence alters microglia toward a protective phenotype with a rise in phagocytic and barrier roles, and may offer a clue to create a novel preventive/therapeutic method to treat AD.

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

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

Critical thinking of Alzheimer's transgenic mouse model: current research and future perspective

Transgenic models are useful tools for studying the pathogenesis of and drug development for Alzheimer's Disease (AD). AD models are constructed usually using overexpression or knock-in of multiple pathogenic gene mutations from familial AD. Each transgenic model has its unique behavioral and pathological features. This review summarizes the research progress of transgenic mouse models, and their progress in the unique mechanism of amyloid-β oligomers, including the first transgenic mouse model built in China based on a single gene mutation (PSEN1 V97L) found in Chinese familial AD. We further summarized the preclinical findings of drugs using the models, and their future application in exploring the upstream mechanisms and multitarget drug development in AD.

Alzheimer's genes in microglia: a risk worth investigating

Despite expressing many key risk genes, the role of microglia in late-onset Alzheimer's disease pathophysiology is somewhat ambiguous, with various phenotypes reported to be either harmful or protective. Herein, we review some key findings from clinical and animal model investigations, discussing the role of microglial genetics in mediating perturbations from homeostasis. We note that impairment to protective phenotypes may include prolonged or insufficient microglial activation, resulting in dysregulated metabolomic (notably lipid-related) processes, compounded by age-related inflexibility in dynamic responses. Insufficiencies of mouse genetics and aggressive transgenic modelling imply severe limitations in applying current methodologies for aetiological investigations. Despite the shortcomings, widely used amyloidosis and tauopathy models of the disease have proven invaluable in dissecting microglial functional responses to AD pathophysiology. Some recent advances have brought modelling tools closer to human genetics, increasing the validity of both aetiological and translational endeavours.

Loss of TREM2 exacerbates parenchymal amyloid pathology but diminishes CAA in Tg-SwDI mice

Alzheimer's disease (AD) is a progressive neurodegenerative disease, and it is the most common cause of dementia worldwide. Recent genome-wide association studies (GWAS) identified TREM2 (triggering receptor expressed on myeloid cells 2) as one of the major risk factors for AD. TREM2 is a surface receptor expressed on microglia and largely mediates microglial functions and immune homeostasis in the brain. The functions of TREM2 in AD pathogenesis, including in the formation of the key pathology parenchymal amyloid-β (Aβ) plaques, have been investigated by introducing Trem2 deficiency in AD mouse models. However, the role of TREM2 in cerebrovascular amyloidosis, in particular cerebral amyloid angiopathy (CAA) remains unexplored. CAA features Aβ deposition along the cerebral vessels, signifying an intersection between AD and vascular dysfunction. Using a well-characterized CAA-prone, transgenic mouse model of AD, Tg-SwDI (SwDI), we found that loss of TREM2 led to a marked increase in overall Aβ load in the brain, but a dramatic decrease in CAA in microvessel-rich regions, along with reduced microglial association with CAA. Transcriptomic analysis revealed that in the absence of Trem2 , microglia were activated but trapped in transition to the fully reactive state. Like microglia, perivascular macrophages were activated with upregulation of cell junction related pathways in Trem2 -deficient SwDI mice. In addition, vascular mural cells and astrocytes exhibited distinct responses to Trem2 deficiency, contributing to the pathological changes in the brain of Trem2 -null SwDI mice. Our study provides the first evidence that TREM2 differentially modulates parenchymal and vascular Aβ pathologies, which may have significant implications for both TREM2- and Aβ-targeting therapies for AD.

Polarized microtubule remodeling transforms the morphology of reactive microglia and drives cytokine release

Microglial reactivity is a pathological hallmark in many neurodegenerative diseases. During stimulation, microglia undergo complex morphological changes, including loss of their characteristic ramified morphology, which is routinely used to detect and quantify inflammation in the brain. However, the underlying molecular mechanisms and the relation between microglial morphology and their pathophysiological function are unknown. Here, proteomic profiling of lipopolysaccharide (LPS)-reactive microglia identifies microtubule remodeling pathways as an early factor that drives the morphological change and subsequently controls cytokine responses. We find that LPS-reactive microglia reorganize their microtubules to form a stable and centrosomally-anchored array to facilitate efficient cytokine trafficking and release. We identify cyclin-dependent kinase 1 (Cdk-1) as a critical upstream regulator of microtubule remodeling and morphological change in-vitro and in-situ. Cdk-1 inhibition also rescues tau and amyloid fibril-induced morphology changes. These results demonstrate a critical role for microtubule dynamics and reorganization in microglial reactivity and modulating cytokine-mediated inflammatory responses.

Inflammasome assembly in neurodegenerative diseases

Neurodegenerative disorders are characterized by the progressive dysfunction and death of selectively vulnerable neuronal populations, often associated with the accumulation of aggregated host proteins. Sustained brain inflammation and hyperactivation of inflammasome complexes have been increasingly demonstrated to contribute to neurodegenerative disease progression. Here, we review molecular mechanisms leading to inflammasome assembly in neurodegeneration. We focus primarily on four degenerative brain disorders in which inflammasome hyperactivation has been well documented: Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), and the spectrum of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). We discuss shared and divergent principles of inflammasome assembly across these disorders, and underscore the differences between neurodegeneration-associated inflammasome activation pathways and their peripheral-immune counterparts. We examine how aberrant assembly of inflammasome complexes may amplify pathology in neurodegeneration, including misfolded protein aggregation, and highlight prospects for neurotherapeutic interventions based on targeting inflammasome pathways.

miR155, TREM2, INPP5D: Disease stage and cell type are essential considerations when targeting clinical interventions based on mouse models of Alzheimer's amyloidopathy

Studies of microglial gene manipulation in mouse models of Alzheimer's disease (AD) amyloidopathy can cause unpredictable effects on various key endpoints, including amyloidosis, inflammation, neuritic dystrophy, neurodegeneration, and learning behavior. In this Correspondence, we discuss three examples, microRNA 155 (miR155), TREM2, and INPP5D, in which observed results have been difficult to reconcile with predicted results based on precedent, because these six key endpoints do not reliably track together. The pathogenesis of AD involves multiple cell types and complex events that may change with disease stage. We propose that cell-type targeting and timing of intervention are responsible for the sometimes impossibility of predicting whether any prospective therapeutic intervention should aim at increasing or decreasing the level or activity of a particular molecular target.

TREM2: Potential therapeutic targeting of microglia for Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease, resulting in the loss of cognitive ability and memory. However, there is no specific treatment to mechanistically inhibit the progression of Alzheimer's disease, and most drugs only provide symptom relief and do not fundamentally reverse AD. Current studies show that triggering receptor expressed on myeloid cells 2 (TREM2) is predominantly expressed in microglia of the central nervous system (CNS) and is involved in microglia proliferation, survival, migration and phagocytosis. The current academic view suggests that TREM2 and its ligands have CNS protective effects in AD. Specifically, TREM2 acts by regulating the function of microglia and promoting the clearance of neuronal toxic substances and abnormal proteins by microglia. In addition, TREM2 is also involved in regulating inflammatory response and cell signaling pathways, affecting the immune response and regulatory role of microglia. Although the relationship between TREM2 and Alzheimer's disease has been extensively studied, its specific mechanism of action is not fully understood. The purpose of this review is to provide a comprehensive analysis of the research of TREM2, including its regulation of the inflammatory response, lipid metabolism and phagocytosis in microglia of CNS in AD, and to explore the potential application prospects as well as limitations of targeting TREM2 for the treatment of AD.

Bace1 Deletion in the Adult Reverses Epileptiform Activity and Sleep-wake Disturbances in AD Mice

Alzheimer's disease (AD) increases the risk for seizures and sleep disorders. We show here that germline deletion of β-site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1) in neurons, but not in astrocytes, increased epileptiform activity. However, Bace1 deletion at adult ages did not alter the normal EEG waveform, indicating less concern for BACE1 inhibition in patients. Moreover, we showed that deletion of Bace1 in the adult was able to reverse epileptiform activity in 5xFAD mice. Intriguingly, treating 5xFAD and APPNL-G-F/NL-G-F (APP KI) mice of either sex with one BACE1 inhibitor Lanabecestat (AZD3293) dramatically increased epileptiform spiking, likely resulting from an off-target effect. We also monitored sleep-wake pathologies in these mice and showed increased wakefulness, decreased non-rapid eye movement sleep, and rapid eye movement sleep in both 5xFAD and APP KI mice; BACE1 inhibition in the adult 5xFAD mice reversed plaque load and sleep disturbances, but this was not seen in APP KI mice. Further studies with and without BACE1 inhibitor treatment showed different levels of plaque-associated microgliosis and activated microglial proteins in 5xFAD mice compared with APP KI mice. Together, BACE1 inhibition should be developed to avoid off-target effect for achieving benefits in reducing epileptic activity and sleep disturbance in Alzheimer's patients.SIGNIFICANCE STATEMENT BACE1 is widely recognized as a therapeutic target for treating Alzheimer's disease patients. However, BACE1 inhibitors failed in clinical trials because of inability to show cognitive improvement in patients. Here we show that BACE1 inhibition actually reduces sleep disturbances and epileptic seizures; both are seen in AD patients. We further showed that one of clinically tested BACE1 inhibitors does have off-target effects, and development of safer BACE1 inhibitors will be beneficial to AD patients. Results from this study will provide useful guidance for additional drug development.

APOE2 gene therapy reduces amyloid deposition, and improves markers of neuroinflammation and neurodegeneration in a mouse model of Alzheimer disease

Epidemiological studies show that individuals who carry the relatively uncommon APOE ε2 allele rarely develop Alzheimer disease, and if they do they have a later age of onset, milder clinical course, and less severe neuropathological findings than others with Alzheimer disease. The contrast is especially stark in comparison to the phenotype associated with the major genetic risk factor for Alzheimer disease, APOE ε4, which has an age of onset several decades earlier, as well as a more aggressive clinical course and notably more severe neuropathological findings, especially in terms of the amount of amyloid deposition. Even one APOE ε2 allele improves phenotype, but it is uncertain if that is due to the replacement of a more toxic allele by APOE ε2, or if APOE ε2 has a protective, neuro-modulatory effect. Here, we demonstrate that brain exposure to APOE2 via a gene therapy approach which bathes the entire cortical mantle in the gene product after transduction of the ependyma, rapidly ameliorates established Aβ plaque deposition, neurodegenerative synaptic loss, and, remarkably, reduces microglial activation in an APP/PS1 mouse model despite continued expression of human APOE4. This result suggests a promising protective effect of exogenous APOE2, revealing a cell non-autonomous effect of the protein on microglial activation. We also show that plaque associated microglia in the brain of patients who inherit APOE2 similarly have less microglial reactivity to plaques. These data raise the potential that an APOE2 therapeutic could be effective in Alzheimer disease even in individuals born with the risk ε4 allele.

One Sentence Summary

Introduction of ApoE2 using an AAV that transduces the ependymal cells of the ventricle causes a reduction in amyloid load and plaque associated synapse loss, and reduces neuroinflammation by modulating microglial responsiveness to plaques.

Abundant Aβ fibrils in ultracentrifugal supernatants of aqueous extracts from Alzheimer's disease brains

Soluble oligomers of amyloid β-protein (Aβ) have been defined as aggregates in supernatants following ultracentrifugation of aqueous extracts from Alzheimer's disease (AD) brains and are believed to be upstream initiators of synaptic dysfunction, but little is known about their structures. We now report the unexpected presence of Aβ fibrils in synaptotoxic high-speed supernatants from AD brains extracted by soaking in an aqueous buffer. The fibrils did not appear to form during preparation, and their counts by EM correlated with Aβ ELISA quantification. Cryo-EM structures of aqueous Aβ fibrils were identical to those from sarkosyl-insoluble homogenates. The fibrils in aqueous extracts were labeled by lecanemab, an Aβ aggregate-directed antibody reported to improve AD cognitive outcomes. Lecanemab provided protection against aqueous fibril synaptotoxicity. We conclude that fibrils are abundant in aqueous extracts from AD brains and have the same structures as those from plaques. These findings have implications for AD pathogenesis and drug design.

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

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

Lipid nanoparticle delivery limits antisense oligonucleotide activity and cellular distribution in the brain after intracerebroventricular injection

Antisense oligonucleotide (ASO) therapeutics are being investigated for a broad range of neurological diseases. While ASOs have been effective in the clinic, improving productive ASO internalization into target cells remains a key area of focus in the field. Here, we investigated how the delivery of ASO-loaded lipid nanoparticles (LNPs) affects ASO activity, subcellular trafficking, and distribution in the brain. We show that ASO-LNPs increase ASO activity up to 100-fold in cultured primary brain cells as compared to non-encapsulated ASO. However, in contrast to the widespread ASO uptake and activity observed following free ASO delivery in vivo, LNP-delivered ASOs did not downregulate mRNA levels throughout the brain after intracerebroventricular injection. This lack of activity was likely due to ASO accumulation in cells lining the ventricles and blood vessels. Furthermore, we reveal a formulation-dependent activation of the immune system post dosing, suggesting that LNP encapsulation cannot mask cellular ASO backbone-mediated toxicities. Together, these data provide insights into how LNP encapsulation affects ASO distribution as well as activity in the brain, and a foundation that enables future optimization of brain-targeting ASO-LNPs.

TREM2-Deficient Microglia Attenuate Tau Spreading In Vivo

The role of TREM2 in Alzheimer's disease (AD) is not fully understood. Previous studies investigating the effect of TREM2 deletion on tauopathy mouse models without the contribution of b-amyloid have focused only on tau overexpression models. Herein, we investigated the effects of TREM2 deficiency on tau spreading using a mouse model in which endogenous tau is seeded to produce AD-like tau features. We found that Trem2-/- mice exhibit attenuated tau pathology in multiple brain regions concomitant with a decreased microglial density. The neuroinflammatory profile in TREM2-deficient mice did not induce an activated inflammatory response to tau pathology. These findings suggest that reduced TREM2 signaling may alter the response of microglia to pathological tau aggregates, impairing their activation and decreasing their capacity to contribute to tau spreading. However, caution should be exercised when targeting TREM2 as a therapeutic entry point for AD until its involvement in tau aggregation and propagation is better understood.

Microglial TREM2 at the Intersection of Brain Aging and Alzheimer's Disease

As resident immune cells of the brain, microglia serve pivotal roles in regulating neuronal function under both physiological and pathological conditions, including aging and the most prevalent neurodegenerative disease, Alzheimer's disease (AD). Instructed by neurons, microglia regulate synaptic function and guard brain homeostasis throughout life. Dysregulation of microglial function, however, can lead to dire consequences, including aggravated cognitive decline during aging and exacerbated neuropathology in diseases. The triggering receptor expressed on myeloid cells 2 (TREM2) is a key regulator of microglial function. Loss-of-function variants of TREM2 are associated with an increased risk of AD. TREM2 orchestrates the switch of microglial transcriptome programming that modulates microglial chemotaxis, phagocytosis, and inflammatory responses, as well as microglial regulation of synaptic function in health and disease. Intriguingly, the outcome of microglial/TREM2 function is influenced by age and the context of neuropathology. This review summarizes the rapidly growing research on TREM2 under physiological conditions and in AD, particularly highlighting the impact of TREM2 on neuronal function.

Microglial INPP5D limits plaque formation and glial reactivity in the PSAPP mouse model of Alzheimer's disease

INTRODUCTION

The inositol polyphosphate-5-phosphatase D (INPP5D) gene encodes a dual-specificity phosphatase that can dephosphorylate both phospholipids and phosphoproteins. Single nucleotide polymorphisms in INPP5D impact risk for developing late onset sporadic Alzheimer's disease (LOAD).

METHODS

To assess the consequences of inducible Inpp5d knockdown in microglia of APPKM670/671NL /PSEN1Δexon9 (PSAPP) mice, we injected 3-month-old Inpp5dfl/fl /Cx3cr1CreER/+ and PSAPP/Inpp5dfl/fl /Cx3cr1CreER/+ mice with either tamoxifen (TAM) or corn oil (CO) to induce recombination.

RESULTS

At age 6 months, we found that the percent area of 6E10+ deposits and plaque-associated microglia in Inpp5d knockdown mice were increased compared to controls. Spatial transcriptomics identified a plaque-specific expression profile that was extensively altered by Inpp5d knockdown.

DISCUSSION

These results demonstrate that conditional Inpp5d downregulation in the PSAPP mouse increases plaque burden and recruitment of microglia to plaques. Spatial transcriptomics highlighted an extended gene expression signature associated with plaques and identified CST7 (cystatin F) as a novel marker of plaques.

HIGHLIGHTS

Inpp5d knockdown increases plaque burden and plaque-associated microglia number. Spatial transcriptomics identifies an expanded plaque-specific gene expression profile. Plaque-induced gene expression is altered by Inpp5d knockdown in microglia. Our plaque-associated gene signature overlaps with human Alzheimer's disease gene networks.

APOEε4 and risk of Alzheimer's disease - time to move forward

The inheritance of Apolipoprotein E4 (APOEε4) brings the highest genetic risk of Alzheimer's disease (AD), arguably the highest genetic risk in human pathology. Since the discovery of the association, APOE protein isoforms have been at the center of tens of thousands of studies and reports. While, without a doubt, our knowledge about the normal physiological function of APOE isoforms in the brain has increased tremendously, the questions of how the inheritance of the APOEε4 allele translates into a risk of AD, and the risk is materialized, remain unanswered. Moreover, the knowledge about the risk associated with APOEε4 has not helped design a meaningful preventative or therapeutic strategy. Animal models with targeted replacement of Apoe have been generated and, thanks to the recent NIH/NIA/Alzheimer's disease Association initiative, are now freely available to AD researchers. While helpful in many aspects, none of the available models recapitulates normal physiological transcriptional regulation of the human APOE gene cluster. Changes in epigenetic regulation of APOE alleles in animal models in response to external insults have rarely been if ever, addressed. However, these animal models provide a useful tool to handle questions and investigate protein-protein interactions with proteins expressed by other recently discovered genes and gene variants considered genetic risk factors of AD, like Triggering Receptor expressed on Myeloid cells 2 (TREM2). In this review, we discuss genetic and epigenetic regulatory mechanisms controlling and influencing APOE expression and focus on interactions of APOE and TREM2 in the context of microglia and astrocytes' role in AD-like pathology in animal models.

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

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

The Multifaceted Role of WNT Signaling in Alzheimer's Disease Onset and Age-Related Progression

The evolutionary conserved WNT signaling pathway orchestrates numerous complex biological processes during development and is critical to the maintenance of tissue integrity and homeostasis in the adult. As it relates to the central nervous system, WNT signaling plays several roles as it relates to neurogenesis, synaptic formation, memory, and learning. Thus, dysfunction of this pathway is associated with multiple diseases and disorders, including several neurodegenerative disorders. Alzheimer's disease (AD) is characterized by several pathologies, synaptic dysfunction, and cognitive decline. In this review, we will discuss the various epidemiological, clinical, and animal studies that demonstrate a precise link between aberrant WNT signaling and AD-associated pathologies. In turn, we will discuss the manner in which WNT signaling influences multiple molecular, biochemical, and cellular pathways upstream of these end-point pathologies. Finally, we will discuss how merging tools and technologies can be used to generate next generation cellular models to dissect the relationship between WNT signaling and AD.

Cerebrospinal Fluid sTREM-2, GFAP, and β-S100 in Symptomatic Sporadic Alzheimer's Disease: Microglial, Astrocytic, and APOE Contributions Along the Alzheimer's Disease Continuum

BACKGROUND

Many transversal mechanisms act synergistically at different time-points in the cascade of Alzheimer's disease (AD), since amyloid-β (Aβ) deposition, tau pathology, and neuroinflammation influence each other.

OBJECTIVE

We explored the contributions of microglia and astrocytes in patients with symptomatic sporadic AD stratified according to AT(N) system and APOE genotype.

METHODS

We compared the cerebrospinal fluid (CSF) levels of sTREM-2 and markers of astrocytic activation (GFAP; β-S100) from 71 patients with AD (23 A+T-,48 A+T+; 38 APOEɛ3, 33 APOEɛ4) and 30 healthy controls (HC). With multivariate analyses we investigated associations between glial biomarkers, Aβ42, and p-tau in all subgroups.

RESULTS

CSF sTREM-2 was higher in A+T+ [1.437 (0.264)] and A+T- [1.355 (0.213)] than in HC [1.042 (0.198); both p < 0.001]; GFAP and β-S100 were comparable across groups. Considering all patients, sTREM-2 positively associated with Aβ 42 (p = 0.04) and p-tau (=0.016), with the first being present only in the A+T- subgroup (p = 0.023). GFAP positively associated with Aβ 42 in all patients (p = 0.020) and in the A+T+ subgroup (p = 0.04). Stratifying by APOE, a positive association of sTREM-2 and p-tau was confirmed selectively in carriers of ɛ4 (p = 0.018). Finally, sTREM-2 positively correlated with β-S100 in all subgroups, and with GFAP in A+T+ (p = 0.042).

CONCLUSION

Our results confirm the increase of CSF sTREM-2 in AD, which associates with reduced amyloidopathy in A+T- patients. Moreover, microglial activation seems to increase CSF tau levels in carriers of APOEɛ4, is associated with astrocytic reactivity (GFAP) in A+T+, and likely leads the acquisition of a more neurotoxic astrocytic phenotype (β-S100).