TREM2 Binds to Apolipoproteins, Including APOE and CLU/APOJ, and Thereby Facilitates Uptake of Amyloid-Beta by Microglia

Neuronal guidance signaling in neurodegenerative diseases: Key regulators that function at neuron-glia and neuroimmune interfaces

The nervous system processes a vast amount of information, performing computations that underlie perception, cognition, and behavior. During development, neuronal guidance genes, which encode extracellular cues, their receptors, and downstream signal transducers, organize neural wiring to generate the complex architecture of the nervous system. It is now evident that many of these neuroguidance cues and their receptors are active during development and are also expressed in the adult nervous system. This suggests that neuronal guidance pathways are critical not only for neural wiring but also for ongoing function and maintenance of the mature nervous system. Supporting this view, these pathways continue to regulate synaptic connectivity, plasticity, and remodeling, and overall brain homeostasis throughout adulthood. Genetic and transcriptomic analyses have further revealed many neuronal guidance genes to be associated with a wide range of neurodegenerative and neuropsychiatric disorders. Although the precise mechanisms by which aberrant neuronal guidance signaling drives the pathogenesis of these diseases remain to be clarified, emerging evidence points to several common themes, including dysfunction in neurons, microglia, astrocytes, and endothelial cells, along with dysregulation of neuron-microglia-astrocyte, neuroimmune, and neurovascular interactions. In this review, we explore recent advances in understanding the molecular and cellular mechanisms by which aberrant neuronal guidance signaling contributes to disease pathogenesis through altered cell-cell interactions. For instance, recent studies have unveiled two distinct semaphorin-plexin signaling pathways that affect microglial activation and neuroinflammation. We discuss the challenges ahead, along with the therapeutic potentials of targeting neuronal guidance pathways for treating neurodegenerative diseases. Particular focus is placed on how neuronal guidance mechanisms control neuron-glia and neuroimmune interactions and modulate microglial function under physiological and pathological conditions. Specifically, we examine the crosstalk between neuronal guidance signaling and TREM2, a master regulator of microglial function, in the context of pathogenic protein aggregates. It is well-established that age is a major risk factor for neurodegeneration. Future research should address how aging and neuronal guidance signaling interact to influence an individual's susceptibility to various late-onset neurological diseases and how the progression of these diseases could be therapeutically blocked by targeting neuronal guidance pathways.

Protein recognition is chiefly mediated by the CDR2 region in TREM2 - an in silico characterization

The Triggering Receptor Expressed on Myeloid cells 2 (TREM2) is an immune receptor with three complementarity-determining regions (CDR1-3) that primarily interact with the receptor's ligands. Aside from its role in reducing inflammation, enhancing phagocytosis, and contributing to cellular maturation and survival, TREM2 also contributes to the pathophysiology of neurodegenerative disorders, cancer, and metabolic diseases. Therefore, understanding how the receptor interacts with its ligands is essential to mitigate its adverse effects and/or to foster the development of new therapeutic approaches. Thus, our research focused on understanding the interactions between TREM2 and its protein ligands: APOA1, APOA2, APOE3, APOE4, APOJ, C1q, Galectin-3, cyclophilin A, Heat shock protein 60 (HSP60), IL-34, IL-4, the SARS-CoV-2 membrane protein and the cholera toxin subunit B, TDP-43 using in silico methods, such as molecular docking and molecular dynamics simulations. TREM2 showed a higher affinity and stability with HSP60, APOA2, Cyclophilin A, Galectin-3, TDP-43 and C1q when compared to the other protein ligands. Notably, our data suggest that TREM2 interacts with its ligands predominantly through the CDR2 region by the following residues: N68, L69, W70, L71, L72, F74 and R76. Our findings indicate that the CDR2 region can be a crucial target for the development of inhibitory or agonistic approaches targeting the receptor's activity.

cGAS-STING and neurodegenerative diseases: A molecular crosstalk and therapeutic perspective

Neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), Amyotrophic Lateral Sclerosis (ALS), Multiple Sclerosis (MS) and Frontotemporal Dementia (FTD) share key pathological features, including neuroinflammation, oxidative stress, mitochondrial dysfunction, autophagic dysfunction, and DNA damage. By identifying cytosolic DNA and triggering the type I interferon response, the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway regulates neuroinflammation. Dysregulated cGAS-STING signaling has been linked to neuroinflammation and neuronal degeneration across multiple neurodegenerative conditions. In many neurodegenerative disorders, neuroinflammation is mediated by the cGAS-STING pathway. Mitochondrial malfunction and impaired autophagy cause cytosolic DNA buildup in Huntington's, Parkinson's, and Alzheimer's diseases, which activates cGAS-STING and drives chronic inflammation. This pathway is triggered by TDP-43 pathology and nucleic acid dysregulation in ALS and FTD, which leads to neuronal destruction. Both central demyelination and peripheral immunological responses are linked to cGAS-STING activation in multiple sclerosis. Various inhibitors, such as RU.521, H-151, and naturally occurring compounds like metformin, potentially attenuate cGAS-STING-mediated neuroinflammation and associated pathologies. H-151 significantly decreased the expression of pro-inflammatory markers in murine macrophage J774 cells activated with cGAMP: TNF-α by 68 %, IFN-β by 84 %, and CXCL10 by 96 %. cGAS-STING inhibitors target neuroinflammation, offering a disease-modifying approach unlike current symptomatic treatments. However, challenges like blood-brain barrier penetration, off-target effects, and immune suppression hinder clinical translation, necessitating optimized drug delivery and immune modulation. With a focus on its potential for future clinical applications, this review explores the role of the cGAS-STING pathway in neurodegeneration and new treatment approaches.

TREM2-expressing macrophages in liver diseases

Metabolic dysfunction-associated steatotic liver disease (MASLD) affects over 30% of the global population and spans a spectrum of liver abnormalities, including simple steatosis, inflammation, fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Recent studies have identified triggering receptors expressed on myeloid cells 2 (TREM2)-expressing macrophages as key regulators of MASLD progression. TREM2 plays a pivotal role in regulating macrophage-mediated processes such as efferocytosis, inflammatory control, and fibrosis resolution. Additionally, soluble TREM2 (sTREM2) was proposed as a noninvasive biomarker for diagnosing and monitoring MASLD progression. However, the molecular mechanisms through which TREM2 influences MASLD pathogenesis remain incompletely understood. This review summarizes the current understanding of TREM2-expressing macrophages in MASLD, with the goal of illuminating future research and guiding the development of innovative therapeutic strategies targeting TREM2 signaling pathways.

Evidence suggesting that microglia make amyloid from neuronally expressed APP: a hypothesis

While APP is largely neuonally expressed, Aβ amyloid is largely produced by microglia as the clearance mechanisms for damaged membranes becomes overwhelmed.

Alzheimer's disease protective allele of Clusterin modulates neuronal excitability through lipid-droplet-mediated neuron-glia communication

BACKGROUND

Genome-wide association studies (GWAS) of Alzheimer's disease (AD) have identified a plethora of risk loci. However, the disease variants/genes and the underlying mechanisms have not been extensively studied.

METHODS

Bulk ATAC-seq was performed in induced pluripotent stem cells (iPSCs) differentiated various brain cell types to identify allele-specific open chromatin (ASoC) SNPs. CRISPR-Cas9 editing generated isogenic pairs, which were then differentiated into glutamatergic neurons (iGlut). Transcriptomic analysis and functional studies of iGlut co-cultured with mouse astrocytes assessed neuronal excitability and lipid droplet formation.

RESULTS

We identified a putative causal SNP of CLU that impacted neuronal chromatin accessibility to transcription-factor(s), with the AD protective allele upregulating neuronal CLU and promoting neuron excitability. And, neuronal CLU facilitated neuron-to-glia lipid transfer and astrocytic lipid droplet formation coupled with reactive oxygen species (ROS) accumulation. These changes caused astrocytes to uptake less glutamate thereby altering neuron excitability.

CONCLUSIONS

For a strong AD-associated locus near Clusterin (CLU), we connected an AD protective allele to a role of neuronal CLU in promoting neuron excitability through lipid-mediated neuron-glia communication. Our study provides insights into how CLU confers resilience to AD through neuron-glia interactions.

Discovery of a small molecule secreted clusterin enhancer that improves memory in Alzheimer's disease mice

Despite substantial research and drug discovery efforts, Alzheimer's Disease (AD) remains the sixth leading cause of death in the United States, underscoring the urgent need for novel therapeutic targets. A mutation in the clusterin (CLU) gene that hinders expression of the cyto-protective secreted isoform of clusterin (sCLU) that affects the aggregation and clearance of two key proteins implicated in AD, Aβ and tau, is the third most significant genetic risk factor for late-onset AD. Here, we present findings from our drug discovery program to identify small molecules that enhance sCLU levels and assess their impact on AD pathology and cognition in a murine model of AD. A high-throughput screening campaign identified two classes of epigenetic modulators that increase sCLU levels with subsequent medicinal chemistry efforts leading to bromodomain and extra-terminal (BET) inhibitor new chemical entities (NCEs) with enhanced potency, drug-like properties, and oral brain bioavailability. The lead candidate NCE, DDL-357, increased brain sCLU in the murine ApoE4TR-5XFAD model of AD in a subchronic study. In a follow-up chronic study in the murine 3xTg-AD model, DDL-357 reduced phospho-tau in brain and led to improvements in mouse performance and memory in the Barnes maze testing paradigm. Proteomic analysis of brain tissue from both AD models revealed changes in proteins involved in mitochondrial function and synaptic plasticity. These findings reveal the potential of sCLU enhancement as a target for therapeutic development in AD and support the continued development of the preclinical lead candidate.

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

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

Preventing the antigen-presenting function of retinal microglia blocks autoimmune neuroinflammation by dendritic cell-primed CD4+ T cells

Autoimmune uveitis is a major cause of blindness and experimental autoimmune uveitis (EAU) is mediated by interphotoreceptor retinoid-binding protein specific effector CD4+ T cells that infiltrate the retina. At least two MHC Class II (MHC II) antigen-presenting cell (APC) events are required for uveitis to develop. The first occurs in the secondary lymphoid organs when dendritic cells (DCs) activate and expand effector CD4+ T cells that enter the circulation and migrate systemically. The second APC event occurs when DC-primed effector CD4+ T cells infiltrate the retina and are restimulated by the relevant autoantigen. Importantly, if this second restimulation does not occur, then uveitis does not develop. However, it is still unclear which cell type(s) function as APCs within the retina. There are two candidate MHC II+ cell types-resident microglia and infiltrating DCs. We used the inducible Cre-lox approach to develop mouse strains in which MHC II was knocked out specifically on microglia using either the P2ry12 or Tmem119 gene to drive recombination. We also used Itgax (CD11c encoding gene) to drive recombination in DCs. Using this approach, we uncovered that the second APC event was mediated by MHC II+ microglia and not infiltrating MHC II+ DCs. Therefore, microglia are an important therapeutic target that can prevent and/or diminish uveitis even in the presence of circulating retinal autoantigen-specific effector CD4+ T cells.

Cholesterol Metabolism in CNS Diseases: The Potential of SREBP2 and LXR as Therapeutic Targets

The brain is the organ with the highest cholesterol content in the body. Cholesterol in the brain plays a crucial role in maintaining the integrity of synapses and myelin sheaths to ensure normal brain function. Disruptions in cholesterol metabolism are closely associated with various central nervous system (CNS) diseases, including Alzheimer's disease (AD), Huntington's disease (HD), and multiple sclerosis (MS). In this review, we explore the synthesis, regulation, transport, and functional roles of cholesterol in the CNS. We discuss in detail the associations between cholesterol homeostasis imbalance and CNS diseases including AD, HD, and MS, highlighting the significant role of cholesterol metabolism abnormalities in the development of these diseases. Sterol regulatory element binding protein-2 (SREBP2) and liver X receptor (LXR) are two critical transcription factors that play central roles in cholesterol synthesis and reverse transport, respectively. Their cooperative interaction finely tunes the balance of brain cholesterol metabolism, presenting potential therapeutic value for preventing and treating CNS diseases. We particularly emphasize the alterations in SREBP2 and LXR under pathological conditions and their impacts on disease progression. This review summarizes current therapeutic agents targeting these two pathways, with the hope of broadening the perspectives of CNS drug developers and encouraging further study into SREBP2 and LXR-related therapies for CNS diseases.

A single fibril study reveals that ApoE inhibits the elongation of Aβ42 fibrils in an isoform-dependent manner

ApoE-ε4 is the strongest genetic risk factor for late-onset Alzheimer's disease (AD), linked to increased amyloid-β (Aβ) deposition in the brain. In AD mouse models, microglial expression of apoE3 reduces amyloid plaque burden through enhanced phagocytosis, whereas apoE4 is associated with impaired Aβ clearance. However, the isoform-specific interactions of apoE with Aβ aggregates and the molecular mechanisms by which these isoforms influence Aβ aggregation and clearance remain poorly understood, which is critical for developing potential therapeutic interventions. Here, we employed TIRFM, superresolution microscopy, and single-molecule photobleaching techniques to investigate the isoform-specific effects of apoE on the rate constants of Aβ42 aggregation at the single-fibril level, as well as to quantify the binding affinity and specificity of apoE isoforms to individual Aβ fibril ends. Our results show that apoE4 is ca. 4-5 times less effective than apoE3 and apoE2 in inhibiting fibril elongation, while secondary nucleation is largely unaffected by any of the isoforms. Furthermore, apoE3 exhibits stronger and more specific binding to fibril ends compared to apoE4. These findings suggest that apoE4's reduced affinity for growing fibril ends may impair microglial clearance and increase amyloid deposition through a higher elongation rate in the brain of ApoE-ε4 carriers.

Apolipoprotein E in Alzheimer's disease: molecular insights and therapeutic opportunities

Apolipoprotein E (APOE- gene; apoE- protein) is the strongest genetic modulator of late-onset Alzheimer's disease (AD), with its three major isoforms conferring risk for disease ε2 < ε3 < ε4. Emerging protective gene variants, such as APOE Christchurch and the COLBOS variant of REELIN, an alternative target of certain apoE receptors, offer novel insights into resilience against AD. In recent years, the role of apoE has been shown to extend beyond its primary function in lipid transport, influencing multiple biological processes, including amyloid-β (Aβ) aggregation, tau pathology, neuroinflammation, autophagy, cerebrovascular integrity and protection from lipid peroxidation and the resulting ferroptotic cell death. While the detrimental influence of apoE ε4 on these and other processes has been well described, the molecular mechanisms underpinning this disadvantage require further enunciation, particularly to realize therapeutic opportunities related to apoE. This review explores the multifaceted roles of apoE in AD pathogenesis, emphasizing recent discoveries and translational approaches to target apoE-mediated pathways. These findings underscore the potential for apoE-based therapeutic strategies to prevent or mitigate AD in genetically at-risk populations.

CLU alleviates Alzheimer's disease-relevant processes by modulating astrocyte reactivity and microglia-dependent synaptic density

Genetic studies implicate clusterin (CLU) in the pathogenesis of Alzheimer's disease (AD), yet its precise molecular impact remains unclear. Through unbiased proteomic profiling and functional validation in CLU-deficient astrocytes, we identify increased nuclear factor κB (NF-κB)-dependent signaling and complement C3 secretion. Reduction of astrocyte CLU induced microglia-dependent modulation of extracellular apolipoprotein E (APOE) and phosphorylated tau, as well as increased microglial phagocytosis and reduced synapse numbers. By integrating mouse and human cellular models with comprehensive analyses of human plasma and brain tissue, we demonstrate that CLU AD-risk alleles are associated with reduced CLU protein and heightened inflammatory profiles. These findings establish a mechanistic link between AD genetic risk factors, astrocyte reactivity, and microglia-mediated effects on synaptic integrity. Collectively, these results support a model in which CLU upregulation in response to neuropathology is associated with maintenance of cognitive function, while diminished astrocyte CLU levels heighten disease susceptibility.

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.

Huang-Lian-Jie-Du decoction alleviates cognitive deficits in Alzheimer's disease model 5xFAD mice by inhibiting Trem2/Dap12 signaling pathway

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder predominantly affecting the elderly population. It is characterized by cognitive deficits associated with the accumulation of amyloid-beta plaques and neurofibrillary tangles. Huang-Lian-Jie-Du (HLJD) decoction, recognized as a representative formulation with heat-clearing and detoxification effects, has been demonstrated to be effective in treating AD. However, the underlying mechanisms require further investigation.

METHODS

5xFAD mice were administrated low and high doses of HLJD. The Morris water maze test was conducted to assess the effects of HLJD. Aβ42 and total tau protein levels were evaluated. Additionally, network pharmacology analysis was performed to identify therapeutic targets of HLJD's active components and their relevance to AD. ELISA, qPCR, Western Blot, and immunofluorescence assays were employed to confirm the identified pathways. Finally, primary microglia isolated from 5xFAD mice were used to validate the candidate targets of HLJD.

RESULTS

HLJD improved cognitive deficits in 5xFAD mice and reduced amyloid plaque deposition and tau protein levels. Network pharmacology analysis indicated that HLJD influences the neuroinflammatory response, particularly through the Dap12 signaling pathway. This was confirmed by reduced levels of neuroinflammation markers, including TNF-α, IL-1β, IL-6, and indicators of microglial activation and polarization. The expression of Trem2 and Dap12 in the hippocampus (HIP) of 5xFAD mice, as well as in the isolated primary microglia, were downregulated following HLJD treatment.

CONCLUSION

Our study indicates that HLJD alleviates cognitive deficits in AD by suppressing the Trem2/Dap12 signaling pathway in the HIP of 5xFAD mice, thereby inhibiting microglial neuroinflammation.

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.

Synergistic reduction in interfacial flexibility of TREM2R47H and ApoE4 may underlie AD pathology

BACKGROUND

The strongest genetic drivers of late-onset Alzheimer's disease (AD) are apolipoprotein E4 (ApoE4) and TREM2R47H. Despite their critical roles, the mechanisms underlying their interactions remain poorly understood.

METHODS

We conducted microsecond-long molecular dynamics simulations of TREM2-ApoE complexes, including TREM2R47H, validating our findings through comparison with published experimental data on TREM2-ApoE binding interactions.

RESULTS

Our simulations reveal TREM2WT can sample an "open" CDR2 conformation, challenging the prevailing notion that this conformation is pathogenic. TREM2WT exhibits greater flexibility, accessing diverse CDR2 conformations, while rigidity in TREM2R47H's CDR2 may explain its reduced ligand-binding properties. ApoE2 facilitates dynamic reconfiguration of TREM2-ApoE2 complexes, which is absent with ApoE4. TREM2R47H and ApoE4 mutually rigidify each other, suppressing interfacial flexibility.

DISCUSSION

Our findings suggest mechanisms underlying ApoE2's neuroprotective functions, ApoE4's pathogenicity, and the synergistic effects of ApoE4 and TREM2R47H in AD. TREM2WT's flexibility and reconfiguration with ApoE2 may support microglial activation, while rigidity in TREM2R47H-ApoE4 interactions may drive pathogenic signaling.

HIGHLIGHTS

TREM2WT samples diverse CDR2 conformations, challenging prior assumptions that an "open" CDR2 state is solely pathogenic. ApoE2 promotes dynamic reconfiguration of TREM2-ApoE2 complexes, preserving TREM2WT's flexibility. ApoE4's hinge forms a unique binding pocket that enhances TREM2 binding. The TREM2R47H-ApoE4 complex exhibits mutual rigidity, suppressing CDR2 and hinge flexibility.

Amelioration of signaling deficits underlying metabolic shortfall in TREM2R47H human iPSC-derived microglia

The microglial triggering receptor expressed on myeloid cells 2 (TREM2) is required for diverse microglia responses in neurodegeneration, including immunometabolic plasticity, phagocytosis, and survival. We previously identified that patient iPSC-derived microglia (iPS-Mg) harboring the Alzheimer's disease (AD) TREM2R47H hypomorph display several functional deficits linked to metabolism. To investigate whether these deficits are associated with disruptions in metabolite signaling, we generated common variant, TREM2R47H and TREM2-/- variant human iPS-Mg. We assessed the ability of supplementation with citrate or succinate, key metabolites and cell cycle breaking points upon microglia activation, to overcome these functional deficits with potential impact on neurons. Succinate supplementation was more effective than citrate at overcoming mitochondrial deficits in OXPHOS and did not promote a glycolytic switch. Citrate enhanced the lipid content of TREM2R47H iPS-Mg and was more effective at overcoming Αβ phagocytic deficits, whereas succinate increased lipid content and phagocytic capacity in TREM2-/- iPS-Mg. Microglia cytokine secretion upon pro-inflammatory activation was moderately affected by citrate or succinate showing a condition-dependent increasing trend. Neither metabolite altered basal levels of soluble TREM2 shedding. In addition, neither citrate nor succinate enhanced glycolysis; instead, drove their effects through oxidative phosphorylation. IPS-neurons exposed to conditioned medium from TREM2 variant iPS-Mg showed changes in oxidative phosphorylation, which could be ameliorated when iPS-Mg were first treated with citrate or succinate. Our data point to discrete pathway linkage between microglial metabolism and functional outcomes with implications for AD pathogenesis and treatments.

Cognitive resilience in Alzheimer's disease: Mechanism and potential clinical intervention

Alzheimer's disease (AD) is a globally recognized neurodegenerative disorder that severely impairs cognitive function and imposes substantial psychological and financial burdens on patients and their families. The hallmark pathological features of AD include progressive neurodegeneration, extracellular beta-amyloid (Aβ) plaque accumulation, and intracellular hyperphosphorylated tau protein tangles. However, recent studies have identified a subset of patients exhibiting cognitive resilience, characterized by a slower cognitive decline or the preservation of high cognitive function despite the presence of AD pathology. Cognitive resilience is influenced by a complex interplay of genetic, environmental, and lifestyle factors. In addition, cognitive resilience contributes to the new perspectives on the diagnosis and personalized treatment of AD. This review aims to provide a comprehensive analysis of current studies on cognitive resilience in AD and to explore future research directions of AD diagnosis and treatment.

TREM2 as a Therapeutic Target in Atherosclerosis

Atherosclerosis is driven by the expansion of cholesterol-loaded foamy macrophages in the arterial intima. Single-cell RNA sequencing has recently revealed the transcriptional landscape of macrophages in these atherosclerotic plaques and uncovered a population of foamy cell-like myeloid cells expressing triggering receptor expressed on myeloid cells-2 (TREM2)-TREM2hi macrophages. Fundamental research has brought essential insight into the significance of TREM2 for foam macrophage survival and atherosclerosis progression, making TREM2 as a therapeutic target in atherosclerosis possible. This review retraces TREM2's winding route from pure knowledge to therapeutic interventions, as well as the potential feasibility of its clinical application for atherosclerosis.

Lipid metabolism, remodelling and intercellular transfer in the CNS

Lipid metabolism encompasses the catabolism and anabolism of lipids, and is fundamental for the maintenance of cellular homeostasis, particularly within the lipid-rich CNS. Increasing evidence further underscores the importance of lipid remodelling and transfer within and between glial cells and neurons as key orchestrators of CNS lipid homeostasis. In this Review, we summarize and discuss the complex landscape of processes involved in lipid metabolism, remodelling and intercellular transfer in the CNS. Highlighted are key pathways, including those mediating lipid (and lipid droplet) biogenesis and breakdown, lipid oxidation and phospholipid metabolism, as well as cell-cell lipid transfer mediated via lipoproteins, extracellular vesicles and tunnelling nanotubes. We further explore how the dysregulation of these pathways contributes to the onset and progression of neurodegenerative diseases, and examine the homeostatic and pathogenic impacts of environment, diet and lifestyle on CNS lipid metabolism.

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.

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.

Ginsenoside 1 mitigates postoperative cognitive dysfunction by enhancing microglial Aβ clearance through the endo-lysosomal pathway

BACKGROUND

Postoperative cognitive dysfunction (POCD) is a common complication in patients after surgery, especially in the elderly. The incidence of POCD not only impaired learning and memory, but also increased morbidity and mortality in patients. However, the exploration of therapeutic agents is limited. Ginsenoside 1 (Rg1) is one of the main compounds of ginseng, which exhibits bioactive and neuroprotective efficiency. In the present study, we aimed to investigate the effects of Rg1 on POCD.

METHODS

The POCD model was established by performing aseptic laparotomy surgery under anesthesia in 18-month mice. The cognition and anxiety of mice were assessed with MWM, OFT, and NOR tests. An in vitro model was performed on BV2 microglial cells. RNA sequencing, Western blotting, electrophysiology, Golgi staining, engulfment, and immunofluorescence analysis were performed.

RESULTS

Our results showed that Rg1 effectively alleviated the cognitive dysfuncion and anxiety of POCD mice. Transcriptomic sequencing data in microglia indicated that Rg1 mainly affects endosomes and lysosomes. By upregulating Rab7 and TFEB expression, Rg1 promoted microglial engulfment of Aβ through the endo-lysosomal pathway. Additionally, Rg1 reduced inflammatory levels, increased synaptic plasticity, and mitigated neuronal damage caused by Aβ. Moreover, the effects of Rg1 on TFEB depended on MEK/ERK signaling, while activation of MEK reversed Rg1's protective effects.

CONCLUSIONS

In conclusion, our study demonstrates that Rg1 can effectively ameliorate cognitive and synaptic deficit by enhancing microglial Aβ clearance through the endo-lysosomal pathway in aged POCD mice, which provides a potential strategy for the prevention of POCD.

Soluble TREM2 is a biomarker but not a mediator of fibrosing steatohepatitis

Background

Triggering receptor expressed on myeloid cells 2 (TREM2), a transmembrane, lipid-sensing protein expressed by Kupffer cells, is thought to play a role in metabolic dysfunction-associated steatotic liver disease (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH). Plasma levels of the TREM2 cleavage product, soluble TREM2 (sTREM2), are strongly associated with MASLD severity. We investigated the role of TREM2 in MASH pathogenesis and whether sTREM2 acts both as biomarker and mediator of MASH.

Methods

Adult C57BL/6J mice were assigned to normal, high-fat (HF), or high-fat and high-cholesterol (HFHC) diets for 15, 30, 90, and 180 days. Plasma sTrem2 levels, liver pathology, and hepatic RNA expression were assessed. To test whether sTrem2 is a mediator of MASH, C57BL/6J mice were injected retro-orbitally with a liver-targeted adeno-associated virus, TBG-AAV8-sTrem2, resulting in secretion of sTrem2 by hepatocytes, or empty TBG-AAV8-control, and subsequently fed HFHC diet for 15, 90, or 180 days.

Results

HFHC-fed mice developed fibrosing steatohepatitis at 180 days together with a 15-fold increase in plasma sTrem2 levels. In the livers of HFHC-fed mice, crown-like structures (CLSs) consisting of TREM2+ macrophages surrounded necrotic, steatotic hepatocytes and their remnant lipid droplets, which contained prominent crystals containing cholesterol. TBG-AAV8-sTrem2-injected mice had higher levels of plasma sTrem2 than TBG-AAV8-control-injected mice, but there were no differences in liver weight, body weight, hepatic fibrosis, hepatic inflammation, hepatic cholesterol crystals, or plasma cholesterol levels.

Conclusion

TREM2+ macrophages characterize the CLSs that surround necrotic hepatocytes and their remnant lipid droplets and cholesterol crystals in fibrosing steatohepatitis-MASH. Plasma sTrem2 is a biomarker, but not a causal mediator, of MASH.

Maternal immune activation followed by peripubertal stress combinedly produce reactive microglia and confine cerebellar cognition

The functional alteration of microglia arises in brains exposed to external stress during early development. Pathophysiological findings of neurodevelopmental disorders such as schizophrenia and autism spectrum disorder suggest cerebellar functional deficits. However, the link between stress-induced microglia reactivity and cerebellar dysfunction is missing. Here, we investigate the developmental immune environment in translational mouse models that combine two risk factors: maternal infection and repeated social defeat stress (2HIT). We find the synergy of inflammatory stress insults, leading to microglial increase specifically in the cerebellum of both sexes. Microglial turnover correlates with the Purkinje neuron loss in 2HIT mice. Highly multiplexed imaging-mass-cytometry identifies a cell transition to TREM2(+) stress-associated microglia in the cerebellum. Single-cell-proteomic clustering reveals IL-6- and TGFβ-signaling association with microglial cell transitions. Reduced excitability of remaining Purkinje cells, cerebellum-involved brain-wide functional dysconnectivity, and behavioral abnormalities indicate cerebellar cognitive dysfunctions in 2HIT animals, which are ameliorated by both systemic and cerebellum-specific microglia replacement.

Identification of potential therapeutic targets for Alzheimer's disease from the proteomes of plasma and cerebrospinal fluid in a multicenter Mendelian randomization study

BACKGROUND

Certain peripheral proteins are believed to be involved in the development of Alzheimer's disease (AD), but the roles of other new protein biomarkers are still unclear. Current treatments aim to manage symptoms, but they are not effective in stopping the progression of the disease. New drug targets are needed to prevent Alzheimer's disease.

METHODS

We used Mendelian randomization (MR) to study drug targets for Alzheimer's disease. We analyzed data from the European Alzheimer's and Dementia Biobank consortium and replicated our findings in GWAS data from IGAP and FinnGen cohorts. We identified genetic instruments for plasma and cerebrospinal fluid (CSF) proteins and conducted sensitivity analyses using various methods. Additionally, a comparison and analysis of protein-protein interactions (PPI) were conducted to identify potential causal proteins. The implications of these findings were further explored through an examination of existing AD drugs and their respective targets.

RESULTS

Through MR analysis, 10 protein AD pairs were identified as statistically significant at the Bonferroni level (P < 6.35 × 10-5). The specific findings indicate that elevated levels of plasma cathepsin H (CTSH) (OR = 1.06, 95%CI: 1.03-1.08, p = 6.12 × 10-6), plasma signal regulatory protein alpha (SIRPA) (OR = 1.03, 95%CI: 1.02-1.05, p = 1.37 × 10-5), plasma TMEM106B (OR = 1.16, 95%CI: 1.09-1.23, p = 1.92 × 10-6), and CSF bone sialoprotein (BSP) (OR = 1.33, 95%CI: 1.17-1.51, p = 9.34 × 10-6), CSF Interleukin-34 (IL-34) (OR = 2.13, 95%CI: 1.51-3.01, p = 1.85 × 10-5), CSF immunoglobulin-like transcript 2 (ILT-2) (OR = 1.33, 95%CI: 1.17-1.51, p = 9.34 × 10-6) are associated with an increased risk of AD, while increased levels of plasma progranulin gene (GRN) (OR = 0.79, 95%CI: 0.74-0.84, p = 2.19 × 10-12), plasma triggering receptor expressed on myeloid cells 2 (TREM2) (OR = 0.67, 95%CI: 0.58-0.78, p = 6.95 × 10-8), plasma sialic acid-bind immunoglobulin-like lectins (SIGLEC)-9 (OR = 0.67, 95%CI: 0.58-0.78, p = 6.95 × 10-8), and CSF SIGLEC7 (OR = 0.42, 95%CI: 0.28-0.64, p = 4.30 × 10-5) are associated with a decreased risk of AD. Bayesian colocalization found that the above protein-related genes shared the same mutation as AD.

CONCLUSION

Increased levels of plasma CTSH, SIRPA, TMEM106B, CSF BSP, CSF IL-34, and CSF ILT-2 have been found to be correlated with an elevated risk of AD, whereas elevated levels of plasma GRN, TREM2, SIGLEC9, and CSF SIGLEC7 are associated with a decreased risk of developing AD. Further investigation through clinical trials is needed.

DNA methylation patterns contribute to changes of cellular differentiation pathways in leukocytes with LOY from patients with Alzheimer´s disease

Alzheimer's disease (AD) is a common and increasing societal problem due to the extending human lifespan. In males, loss of chromosome Y (LOY) in leukocytes is strongly associated with AD. We studied here DNA methylation and RNA expression in sorted monocytes and granulocytes with and without LOY from male AD patients. Through multi-omic analysis, we identified new candidate genes along with those previously associated with AD. Global analyses of DNA methylation in samples with LOY vs. normal state showed that hypomethylation dominated both in granulocytes and monocytes. Our findings highlight LOY-related differences in DNA methylation that occur in gene regulatory regions. Specifically, we observed alterations in key genes involved in leukocyte differentiation: FLI1, involved in early hematopoiesis; RUNX1, essential for blood cell development; RARA, regulating gene expression in response to retinoic acid; CANX, crucial for protein folding; CEBPB, a transcription factor important for immune responses; and MYADM, implicated in cell adhesion and migration. Moreover, protein-protein interaction analysis in granulocytes identified that products of two of these genes, CANX and CEBPB, are key hub proteins. This research underscores the potential of multi-omic approach in pure hematopoietic cell populations to uncover the molecular underpinnings of AD. Finally, our results link previous analysis showing impact of LOY on leukocyte differentiation, LOY-associated transcriptional dysregulation and GWAS studies of LOY.

Microglial Autophagic Dysregulation in Traumatic Brain Injury: Molecular Insights and Therapeutic Avenues

Traumatic brain injury (TBI) is a complex and multifaceted condition that can result in cognitive and behavioral impairments. One aspect of TBI that has received increasing attention in recent years is the role of microglia, the brain-resident immune cells, in the pathophysiology of the injury. Specifically, increasing evidence suggests that dysfunction in microglial autophagy, the process by which cells degrade and recycle their own damaged components, may contribute to the development and progression of TBI-related impairments. Here, we unravel the pathways by which microglia autophagic dysregulation predisposes the brain to secondary damage and neurological deficits following TBI. An overview of the role of autophagic dysregulation in perpetuation and worsening of the inflammatory response, neuroinflammation, and neuronal cell death in TBI follows. Further, we have evaluated several signaling pathways and processes that contribute to autophagy dysfunction-mediated inflammation, neurodegeneration, and poor outcome in TBI. Additionally, a discussion on the small molecule therapeutics employed to modulate these pathways and mechanisms to treat TBI have been presented. However, additional research is required to fully understand the processes behind these underlying pathways and uncover potential therapeutic targets for restoring microglial autophagic failure in TBI.

Mitochondrial DAMPs: Key mediators in neuroinflammation and neurodegenerative disease pathogenesis

Neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS) are increasingly linked to mitochondrial dysfunction and neuroinflammation. Central to this link are mitochondrial damage-associated molecular patterns (mtDAMPs), including mitochondrial DNA, ATP, and reactive oxygen species, released during mitochondrial stress or damage. These mtDAMPs activate inflammatory pathways, such as the NLRP3 inflammasome and cGAS-STING, contributing to the progression of neurodegenerative diseases. This review delves into the mechanisms by which mtDAMPs drive neuroinflammation and discusses potential therapeutic strategies targeting these pathways to mitigate neurodegeneration. Additionally, it explores the cross-talk between mitochondria and the immune system, highlighting the complex interplay that exacerbates neuronal damage. Understanding the role of mtDAMPs could pave the way for novel treatments aimed at modulating neuroinflammation and slowing disease progression, ultimately improving patient outcome.

Unraveling the Complex Interplay Between Neuroinflammation and Depression: A Comprehensive Review

The relationship between neuroinflammation and depression is a complex area of research that has garnered significant attention in recent years. Neuroinflammation, characterized by the activation of glial cells and the release of pro-inflammatory cytokines, has been implicated in the pathophysiology of depression. The relationship between neuroinflammation and depression is bidirectional; not only can inflammation contribute to the onset of depressive symptoms, but depression itself can also exacerbate inflammatory responses, creating a vicious cycle that complicates treatment and recovery. The present comprehensive review aimed to explore the current findings on the interplay between neuroinflammation and depression, as well as the mechanisms, risk factors, and therapeutic implications. The mechanisms by which neuroinflammation induces depressive-like behaviors are diverse. Neuroinflammation can increase pro-inflammatory cytokines, activate the hypothalamus-pituitary-adrenal (HPA) axis, and impair serotonin synthesis, all of which contribute to depressive symptoms. Furthermore, the activation of microglia has been linked to the release of inflammatory mediators that can disrupt neuronal function and contribute to mood disorders. Stress-induced neuroinflammatory responses can lead to the release of pro-inflammatory cytokines that not only affect brain function but also influence behavior and mood. Understanding these mechanisms is crucial for developing targeted therapies that can mitigate the effects of neuroinflammation on mood disorders.

Cell signaling pathways discovery from multi-modal data

Deciphering cell signaling pathways is essential for advancing our understanding of basic biology, disease mechanisms, and the development of innovative therapeutic interventions. Recent advancements in multi-omics technologies enable us to capture cell signaling information in a more meaningful context. However, omics data is inherently complex-high-dimensional, heterogeneous, and extensive-making it challenging for human interpretation. Currently, computational tools capable of inferring cell signaling pathways from multi-omics data are very limited, underscoring the urgent need to develop such methods. To address this challenge, we developed Incytr, a method that facilitates the efficient discovery of cell signaling pathways by integrating diverse data modalities, including transcriptomics, proteomics, phosphoproteomics, and kinomics. We demonstrate Incytr's application in elucidating cell signaling within the contexts of COVID-19, Alzheimer's disease, and cancer. Incytr successfully rediscovered known subpathways in these diseases and generated novel hypotheses for cell-type-specific signaling pathways supported by multiple data modalities. We illustrate how overlaying Incytr-identified pathways with prior knowledge from biomarker and small molecule drug databases can be used to facilitate target and drug discovery. Overall, as we demonstrated here, with the use of simple natural language processing AI models, these pathways could serve as a discovery tool to deepen our understanding of cell-cell communication semantics and co-evolution.

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

BACKGROUND

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

MAIN BODY

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

CONCLUSION

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

Alzheimer's disease: insights into pathology, molecular mechanisms, and therapy

Alzheimer's disease (AD), the leading cause of dementia, is characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain. This condition casts a significant shadow on global health due to its complex and multifactorial nature. In addition to genetic predispositions, the development of AD is influenced by a myriad of risk factors, including aging, systemic inflammation, chronic health conditions, lifestyle, and environmental exposures. Recent advancements in understanding the complex pathophysiology of AD are paving the way for enhanced diagnostic techniques, improved risk assessment, and potentially effective prevention strategies. These discoveries are crucial in the quest to unravel the complexities of AD, offering a beacon of hope for improved management and treatment options for the millions affected by this debilitating disease.

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.

APOE Christchurch enhances a disease-associated microglial response to plaque but suppresses response to tau pathology

BACKGROUND

Apolipoprotein E ε4 (APOE4) is the strongest genetic risk factor for late-onset Alzheimer's disease (LOAD). A recent case report identified a rare variant in APOE, APOE3-R136S (Christchurch), proposed to confer resistance to autosomal dominant Alzheimer's Disease (AD). However, it remains unclear whether and how this variant exerts its protective effects.

METHODS

We introduced the R136S variant into mouse Apoe (ApoeCh) and investigated its effect on the development of AD-related pathology using the 5xFAD model of amyloidosis and the PS19 model of tauopathy. We used immunohistochemical and biochemical analysis along with single-cell spatial omics and bulk proteomics to explore the impact of the ApoeCh variant on AD pathological development and the brain's response to plaques and tau.

RESULTS

In 5xFAD mice, ApoeCh enhances a Disease-Associated Microglia (DAM) phenotype in microglia surrounding plaques, and reduces plaque load, dystrophic neurites, and plasma neurofilament light chain. By contrast, in PS19 mice, ApoeCh suppresses the microglial and astrocytic responses to tau-laden neurons and does not reduce tau accumulation or phosphorylation, but partially rescues tau-induced synaptic and myelin loss. We compared how microglia responses differ between the two mouse models to elucidate the distinct DAM signatures induced by ApoeCh. We identified upregulation of antigen presentation-related genes in the DAM response in a PS19 compared to a 5xFAD background, suggesting a differential response to amyloid versus tau pathology that is modulated by the presence of ApoeCh. Bulk proteomics show upregulated mitochondrial protein abundance with ApoeCh in 5xFAD mice, but reductions in mitochondrial and translation associated proteins in PS19 mice.

CONCLUSIONS

These findings highlight the ability of the ApoeCh variant to modulate microglial responses based on the type of pathology, enhancing DAM reactivity in amyloid models and dampening neuroinflammation to promote protection in tau models. This suggests that the Christchurch variant's protective effects likely involve multiple mechanisms, including changes in receptor binding and microglial programming.

Modulating Neuroinflammation as a Prospective Therapeutic Target in Alzheimer's Disease

The recent approval of lecanemab highlights that the amyloid beta (Aβ) protein is an important pathological target in Alzheimer's disease (AD) and further emphasizes the significance of neuroinflammatory pathways in regulating Aβ accumulation. Indeed, Aβ accumulation triggers microglia activation, which are key mediators in neuroinflammation. The inflammatory responses in this process can lead to neuronal damage and functional decline. Microglia secrete proinflammatory cytokines that accelerate neuronal death and release anti-inflammatory cytokines and growth factors contributing to neuronal recovery and protection. Thus, microglia play a dual role in neurodegeneration and neuroprotection, complicating their function in AD. Therefore, elucidating the complex interactions between Aβ protein, microglia, and neuroinflammation is essential for developing new strategies for treating AD. This review investigates the receptors and pathways involved in activating microglia and aims to enhance understanding of how these processes impact neuroinflammation in AD, as well as how they can be regulated. This review also analyzed studies reported in the existing literature and ongoing clinical trials. Overall, these studies will contribute to understanding the regulatory mechanisms of neuroinflammation and developing new therapies that can slow the pathological progression of AD.

Extracellular domain of TREM2 possess two distinct ligand recognition sites: Insights from machine-learning guided docking and all-atoms molecular dynamics simulations

The myeloid-specific triggering receptors expressed on myeloid cells 2 (TREM2) is a group of class I receptors expressed in brain microglia plays a decisive role in neurodegenerative diseases such as Alzheimer's disease (AD) and Nasu Hakola disease (NHD). The extracellular domain (ECD) of TREM2 interacts with a wide-range of ligands, yet the molecular mechanism underlying recognition of such ligands to this class I receptor remains underexplored. Herein, we undertook a systematic investigation for exploring the mode of ligand recognition in immunoglobulin-like ectodomain by employing both knowledge-based and machine-learning guided molecular docking approach followed by the state-of-the-art all atoms molecular dynamics (MD) simulations. Besides the known binding site formed by complementarity-determining regions (CDR) 1 and CDR2 loops, which enables the binding of different anionic ligands, our study identifies the presence of second binding site formed by β-strands towards the C-terminal end. We observe a dense network of hydrophobic contacts formed between the explored ligands and CDR loops and β-strands, specifically CDR1, CDR2, β-strand C', loop connecting β-strand D and E, and loop connecting β-strand E and F. Ligand binding in immunoglobulin-like ectodomain increases the conformational flexibility of CDR2 loop, thus most frequently observed pathogenic variants i.e. R47H and R62H in TREM2 may affect the development and progression of AD. Our knowledge-based and machine-learning guided docking and physics-based simulations study unveils deep insights into the endogenous ligand recognition by the positive surface ligand binding site and distant core site pave the way for exploration of other small molecules towards development of novel therapeutics against Alzheimer's disease.

Biophysical mapping of TREM2-ligand interactions reveals shared surfaces for engagement of multiple Alzheimer's disease ligands

TREM2 is a signaling receptor expressed on microglia that has emerged as an important drug target for Alzheimer's disease and other neurodegenerative diseases. While a number of TREM2 ligands have been identified, little is known regarding the structural details of how they engage. To better understand this, we created a protein library of 28 different TREM2 variants that could be used to map interactions with various ligands using biolayer interferometry. The variants are located in previously identified putative binding surfaces on TREM2 called the hydrophobic site, basic site, and site 2. We found that mutations to the hydrophobic site ablated binding to apoE4 and TDP-43. Competition binding experiments indicated that apoE4 and oAβ42 share overlapping binding sites on TREM2. In contrast, binding to C1q was disrupted most strongly by mutations to the basic site, including R46, with some mutations to the hydrophobic site also attenuating binding, thus suggesting a broader mediation of binding across the two sites. Supporting this, competition experiments indicated that C1q binding could be blocked by both apoE and oAβ42. TREM2 binding to IL-34 was mediated by the basic site at a surface centering on R76. Competition binding experiments validated the unique site for IL-34, showing little to no competition with either oAβ42 or apoE4. However, competition experiments between C1q and IL34 suggest that the ligands compete for binding at the basic site. Altogether, our results suggest that TREM2 utilizes the hydrophobic site (consisting of CDR1, CDR2, and CDR3) as a common site to engage multiple ligands, and uses distinct basic sites to engage others. Our findings imply that pharmaceutical strategies targeting these surfaces might be effective to modulate TREM2 functions.

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.

Role of TREM2 in immune and neurological diseases: Structure, function, and implications

Triggering Receptor Expressed on Myeloid Cells 2 (TREM2), a transmembrane receptor initially linked to neurodegenerative diseases, has recently emerged as a key player in conditions such as obesity and cancer. This review explores the structure, function, and mechanisms of TREM2 across these diverse pathological contexts, with a particular focus on its critical roles in immune regulation and neuroprotection. TREM2 primarily modulates cellular activity by binding extracellular ligands, thereby activating downstream signaling pathways and exerting immunomodulatory effects. Additionally, the therapeutic potential of targeting TREM2 is discussed, emphasizing its promise as a future treatment strategy for various diseases.

Alzheimer's disease-linked risk alleles elevate microglial cGAS-associated senescence and neurodegeneration in a tauopathy model

The strongest risk factors for late-onset sporadic Alzheimer's disease (AD) include the ε4 allele of apolipoprotein E (APOE), the R47H variant of triggering receptor expressed on myeloid cells 2 (TREM2), and female sex. Here, we combine APOE4 and TREM2R47H (R47H) in female P301S tauopathy mice to identify the pathways activated when AD risk is the strongest, thereby highlighting detrimental disease mechanisms. We find that R47H induces neurodegeneration in 9- to 10-month-old female APOE4 tauopathy mice. The combination of APOE4 and R47H (APOE4-R47H) worsened hyperphosphorylated tau pathology in the frontal cortex and amplified tauopathy-induced microglial cyclic guanosine monophosphate (GMP)-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling and downstream interferon response. APOE4-R47H microglia displayed cGAS- and BAX-dependent upregulation of senescence, showing association between neurotoxic signatures and implicating mitochondrial permeabilization in pathogenesis. By uncovering pathways enhanced by the strongest AD risk factors, our study points to cGAS-STING signaling and associated microglial senescence as potential drivers of AD risk.

Mouse and human macrophages and their roles in cardiovascular health and disease

The past 15 years have witnessed a leap in understanding the life cycle, gene expression profiles, origins and functions of mouse macrophages in many tissues, including macrophages of the artery wall and heart that have critical roles in cardiovascular health. Here, we review the phenotypical and functional diversity of macrophage populations in multiple organs and discuss the roles that proliferation, survival, and recruitment and replenishment from monocytes have in maintaining macrophages in homeostasis and inflammatory states such as atherosclerosis and myocardial infarction. We also introduce emerging data that better characterize the life cycle and phenotypic profiles of human macrophages. We discuss the similarities and differences between murine and human macrophages, raising the possibility that tissue-resident macrophages in humans may rely more on bone marrow-derived monocytes than in mouse.

Inflammation and DNA methylation in Alzheimer's disease: mechanisms of epigenetic remodelling by immune cell oxidants in the ageing brain

Alzheimer's disease is a neurodegenerative disease involving memory impairment, confusion, and behavioural changes. The disease is characterised by the accumulation of amyloid beta plaques and neurofibrillary tangles in the brain, which disrupt normal neuronal function. There is no known cure for Alzheimer's disease and due to increasing life expectancy, occurrence is projected to rise over the coming decades. The causes of Alzheimer's disease are multifactorial with inflammation, oxidative stress, genetic and epigenetic variation, and cerebrovascular abnormalities among the strongest contributors. We review the current literature surrounding inflammation and epigenetics in Alzheimer's disease, with a focus on how oxidants from infiltrating immune cells have the potential to alter DNA methylation profiles in the ageing brain.

Huang-Lian-Jie-Du decoction alleviates cognitive impairment in high-fat diet-induced obese mice via Trem2/Dap12/Syk pathway

BACKGROUND

Cognitive impairment induced by a high-fat diet (HFD) is common, but its mechanism is largely unknown. Huang-Lian-Jie-Du (HLJD) decoction is a classical and powerful prescription in China. It consists of four medicinal plants and is widely used in traditional Chinese medicines (TCM). Studies have shown that HLJD decoction is effective in treating obesity, depression, and so on. However, the therapeutic mechanism of HLJD is still poorly understood.

PURPOSE

Our study aimed to explore whether inflammatory factors and Trem2/Dap12/Syk pathway are involved in this process and whether HLJD treatment can repair cognitive impairment in HFD-induced obesity.

METHODS

To obtain the obese mice, male mice were treated with HFD (60 Kcal% fat) for 16 weeks. After an additional eight weeks, HLJD decoction was administered orally at doses of 4 and 8 g/kg daily for eight weeks. The mice were then subjected to four behavior tests. Aβ42, total Tau, inflammatory-related, and microglial dysregulation-related markers expression were measured. Molecular docking analysis was also conducted to predict the interaction of the chemical constituents of HLJD with human TREM2, DAP12, and SYK. HLJD at doses of 12.5, 25, and 50 µg/mL or limonin at concentrations of 12.5, 25, and 50 µM were used to treat BV2 cells for 24 h. CCK8 assay and Trem2, Dap12, Syk, and p-Syk expression were measured.

RESULTS

Our study revealed that cognitive impairment was evident in mice treated with HFD, indicating the impact of obesity on cognitive function. The expression of Aβ42 and total Tau in the hippocampus (HIP) was significantly higher in obese (HFD-V) mice compared to normal control (NC-V) mice. The Il6, Il1b, and Il10 mRNA expression levels were also markedly increased in the HIP of obese mice. Furthermore, Trem2, Dap12, p-Syk, and Iba1 expression were elevated in the HIP of obese mice. Importantly, HLJD treatment was found to repair cognitive impairment and lower the protein expression of Aβ42, Tau, Trem2, Dap12, p-Syk, and the expression of Il6, Il1b, and Il10 mRNA in HIP of HFD-V mice. The increased expression of Trem2, Dap12, p-Syk, and Iba1 in HIP after HFD consumption could be reduced after receiving HLJD decoction. The compound Limonin showed a well-predicted binding energy with TREM2, DAP12, and SYK. BV2 cells with HLJD or limonin detected the mRNA expressions of Trem2/Dap12. HLJD at 25 and 50 µg/mL decreased Trem2, Dap12, and p-Syk protein levels in BV2 cells.

CONCLUSION

These results reveal that HLJD treatment could alleviate cognitive impairment in HFD-induced obese mice by controlling the activation of the Trem2/Dap12 pathway and reducing Syk phosphorylation in HIP microglia. HLJD and limonin suppressed Trem2/Dap12/Syk signaling pathway in BV2 cells. HLJD therapy might represent a novel treatment for patients with cognitive impairment induced by obesity.

Protein profile of mouse endolymph suggests a role in controlling cochlear homeostasis

The cochlea contains two extracellular fluids, perilymph and endolymph. Endolymph exhibits high potential of approximately +80 to +110 mV (depending on species), which sensitizes sensory hair cells. Other properties of this unique fluid remain elusive, owing to its minuscule volume in rodent cochlea. We therefore developed a technique to collect high-purity endolymph from mouse cochleae. Comprehensive proteomic analysis of sampled endolymph using liquid chromatography with mass spectrometry identified 301 proteins, dominated by molecules engaged in immunity and proteostasis. Approximately 30% of these proteins were undetectable in our perilymph. A combination of mass spectrometry and different approaches revealed that, compared to perilymph, endolymph was enriched with α2-macroglobulin, osteopontin, apolipoprotein D, apolipoprotein E, and apolipoprotein J/clusterin. In other cells or tissues, α2-macroglobulin, apolipoprotein E, and apolipoprotein J contribute to the clearance of degraded proteins from extracellular fluid. Altogether, with the proteins described here, endolymph may play a protective role in stabilizing cochlear homeostasis.

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.

The double-edged role and therapeutic potential of TREM2 in atherosclerosis

Atherosclerosis is a chronic lipid-driven inflammatory disease characterized by infiltration of large numbers of macrophages. The progression of the disease is closely related to the status of macrophages in atherosclerotic plaques. Recent advances in plaque analysis have revealed a subpopulation of macrophages that express high levels of triggering receptor expressed on myeloid cells 2 (TREM2). Although TREM2 is known to play a critical role in inflammation, lipid metabolism, and tissue repair, its role in atherosclerosis is still not fully understood. Recent studies have shown that TREM2 promotes macrophage cholesterol uptake and efflux, enhances efferocytosis function, regulates inflammation and metabolism, and promotes cell survival, all of which are significant functions in atherosclerosis. In early plaques TREM2 promotes lipid uptake and increases lesion size. In advanced plaques TREM2 promotes macrophage survival and increases plaque stability. The dualistic nature of TREM2 in atherosclerosis, where it can exert both protective effect and a side effect of increased lesion size, presents a complex but crucial area of study. Understanding these dual roles could help in the development of new therapeutic strategies to modulate TREM2 activity and utilize its atheroprotective function while mitigating its deleterious effects. In this review, we discuss the roles and mechanisms of TREM2 during different stages of atherosclerotic plaques, as well as the potential applications of TREM2 in the diagnosis and treatment of atherosclerosis.

Pathogenesis, diagnostics, and therapeutics for Alzheimer's disease: Breaking the memory barrier

Alzheimer's disease (AD) is the most common cause of dementia and accounts for 60-70 % of all cases. It affects millions of people worldwide. AD poses a substantial economic burden on societies and healthcare systems. AD is a progressive neurodegenerative disorder characterized by cognitive decline, memory loss, and impaired daily functioning. As the prevalence of AD continues to increase, understanding its pathogenesis, improving diagnostic methods, and developing effective therapeutics have become paramount. This comprehensive review delves into the intricate mechanisms underlying AD, explores the current state of diagnostic techniques, and examines emerging therapeutic strategies. By revealing the complexities of AD, this review aims to contribute to the growing body of knowledge surrounding this devastating disease.