Soluble TREM2 ameliorates pathological phenotypes by modulating microglial functions in an Alzheimer's disease model

Soluble TREM2 ameliorates pathological phenotypes in ischemic stroke models via modulating neuronal and microglial functions

Although the neuroprotective effects of triggering receptor expressed on myeloid cell 2 (TREM2) upregulation after ischemic stroke has been demonstrated, the level change and effect of soluble TREM2 (sTREM2) derived from proteolytic cleavage of the TREM2 extracellular domain in ischemic stroke remain unknown. In our study, the level and function of sTREM2 were detected in neuron-microglia co-cultures subjected to oxygen glucose deprivation (OGD) and in the ischemic striatum of C57BL/6 J mice in a transient middle cerebral artery occlusion (tMCAO) model. sTREM2's effect on neuronal nitric oxide synthase (nNOS)-postsynaptic density protein-95 (PSD-95) interaction was determined by co-immunoprecipitation. The microglial-activated morphology in the striatum was identified by immunohistochemistry. Quantitative real-time polymerase chain reactionwas used to detect the transcriptional levels of TREM2, shorter variant TREM2, insulin-like growth factor 1, interleukin (IL)-4, and IL-13. Levels of sTREM2, generated through the cleavage of full-length TREM2 at the His157-Ser158 peptide bond, declined after OGD and tMCAO. sTREM2 reduced neuronal death after OGD and alleviated brain infarction and neurological deficits after tMCAO by disrupting the nNOS-PSD-95 interaction, promoting microglial activation, and increasing the expression of some cytokines associated with microglial polarization towards an anti-inflammatory phenotype. To the best of our knowledge, this is the first study to suggest that sTREM2 protects against transient cerebral ischemia.

Repurposing pharmaceuticals for Alzheimer's treatment via adjusting the lactoferrin interacting proteins

Alzheimer's disease (AD), the most common neurodegenerative disease in humans, has been a major medical challenge. Lactoferrin (Ltf) in salivary glands might be identified as a potential detectable biomarker in AD and a therapeutic target for AD. Pharmaceutical studies directly addressing this biomarker, though, are scarce. Using a computational strategy for drug repurposing, we explored the proximal neighborhood of Ltf by exploring its interactome and regulatory constellations. We aimed to focus on the discovery of potential therapeutic agents for AD. Based on extensive analytical evaluation comprising structural congruence scales, profiling disease clusters, pathway enrichment analyses as well as molecular docking, SPR, in vivo studies, and immunofluorescence assays, our research identified three candidate repurposed drugs: Lovastatin, SU-11652, and SB-239063. Taken together, these results highlight strong binding affinities of the drug candidates to Ltf. In vitro studies showed that such compounds decrease β-amyloid (Aβ) production by increasing the fluorescence signal emitted by Ltf in N2a-sw cells, and that they act by modulating the expression of amyloidogenic pathway-associated enzymes (BACE1 and APH1α). In addition, in vivo studies showed a concomitant reduction in the expression levels of amyloidogenic pathway-related enzymes (BACE1 or APH1α). Thus, computational studies have focused on Ltf interactions that may recommend drug repurposing strategies and options for AD.

The late-onset Alzheimer's disease risk factor RHBDF2 is a modifier of microglial TREM2 proteolysis

The cell surface receptor TREM2 is a key genetic risk factor and drug target in Alzheimer's disease (AD). In the brain, TREM2 is expressed in microglia, where it undergoes proteolytic cleavage, linked to AD risk, but the responsible protease in microglia is still unknown. Another microglial-expressed AD risk factor is catalytically inactive rhomboid 2 (iRhom2, RHBDF2), which binds to and acts as a non-catalytic subunit of the metalloprotease ADAM17. A potential role in TREM2 proteolysis is not yet known. Using microglial-like BV2 cells, bone marrow-derived macrophages, and primary murine microglia, we identify iRhom2 as a modifier of ADAM17-mediated TREM2 shedding. Loss of iRhom2 increased TREM2 in cell lysates and at the cell surface and enhanced TREM2 signaling and microglial phagocytosis of the amyloid β-peptide (Aβ). This study establishes ADAM17 as a physiological TREM2 protease in microglia and suggests iRhom2 as a potential drug target for modulating TREM2 proteolysis in AD.

Brain-wide microglia replacement using a nonconditioning strategy ameliorates pathology in mouse models of neurological disorders

Growing genetic and pathological evidence has identified microglial dysfunction as a key contributor to the pathogenesis and progression of various neurological disorders, positioning microglia replacement as a promising therapeutic strategy. Traditional bone marrow transplantation (BMT) methods for replenishing brain microglia have limitations, including low efficiency and the potential for brain injury because of preconditioning regimens, such as irradiation or chemotherapy. Moreover, BM-derived cells that migrate to the brain do not recapitulate the phenotypic and functional properties of resident microglia. Here, we present a microglia transplantation strategy devoid of any conditioning, termed "tricyclic microglial depletion for transplantation" (TCMDT). This approach leverages three cycles of microglial depletion using the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX3397, creating an optimal window for efficient engraftment of exogenous microglia. Transplantation of primary cultured microglia by TCMDT successfully restored the identity and functions of endogenous microglia. To evaluate the therapeutic potential of TCMDT, we applied this strategy to two distinct mouse models of neurologic disorder. In a Sandhoff disease model, a neurodegenerative lysosomal storage disorder caused by hexosaminidase subunit beta (Hexb) deficiency, TCMDT effectively replaced deficient microglia, attenuating neurodegeneration and improving motor performance. Similarly, in an Alzheimer's disease (AD)-related amyloid mouse model carrying the triggering receptor expressed on myeloid cells 2 (Trem2) R47H mutation, our transplantation strategy rescued microglial dysfunction and mitigated AD-related pathology. Overall, our study introduces TCMDT as a practical, efficient, and safe approach for microglia replacement, suggesting therapeutic potential for treating neurological disorders associated with microglial dysfunction.

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.

CMPK2 promotes microglial activation through the cGAS-STING pathway in the neuroinflammatory mechanism

The activation of microglia and the resulting neuroinflammation play crucial regulatory roles in the pathogenesis and progression of neurological diseases, although the specific mechanisms remain incompletely understood. Cytidine monophosphate kinase 2 (CMPK2) is a key mitochondrial nucleotide kinase involved in cellular energy metabolism and nucleotide synthesis. Recent studies suggest that CMPK2 plays a role in microglial-mediated neuroinflammation; however, its specific impact on microglial activation remains unclear. In this study, we hypothesize that CMPK2 promotes microglial-mediated neuroinflammation by activating the cGAS-STING signaling pathway. To investigate this mechanism, we employed lipopolysaccharide (LPS)-treated microglial cells to investigate the detailed mechanisms by which CMPK2 regulates neuroinflammation. Our experimental results indicate that in the BV2 and mouse primary microglial neuroinflammation model, both CMPK2 protein and transcript levels were significantly elevated, accompanied by microglial activation phenotypes such as increased cell size, shortened processes, transformation to round or rod-like shapes, and elevated CD40 expression. Concurrently, there was an increase in pro-inflammatory cytokine levels and a decrease in anti-inflammatory cytokine levels. Further investigation revealed that in the microglial, the expression of cGAS and STING was elevated, along with an increase in oxidative products and inflammatory responses. CMA stimulation further intensified these changes, while cGAS knockdown mitigated them. Finally, we demonstrated that cGAS knockdown inhibited the oxidative stress, cell activation-related changes, and neuroinflammatory responses induced by CMPK2 overexpression in the BV2 neuroinflammation model. Molecular docking experiments showed that CMPK2 stably binds to cGAS at the protein level. These findings suggest that the cGAS-STING pathway mediates CMPK2-induced microglial activation. In summary, our study demonstrates that LPS-induced CMPK2 overactivity promotes microglial activation and neuroinflammatory through the cGAS-STING pathway.

Comparison of Lean, Obese, and Weight-Loss Models Reveals TREM2 Deficiency Attenuates Breast Cancer Growth Uniquely in Lean Mice and Alters Clonal T-cell Populations

Obesity is an established risk factor for breast cancer development and poor prognosis. The adipose environment surrounding breast tumors, which is inflamed in obesity, has been implicated in tumor progression, and triggering receptor expressed on myeloid cells 2 (TREM2), a transmembrane receptor expressed on macrophages in adipose tissue and tumors, is an emerging therapeutic target for cancer. A better understanding of the mechanisms for the obesity-breast cancer association and the potential benefits of weight loss could help inform treatment strategies. In this study, we utilized lean, obese, and weight-loss mouse models to examine the impact of TREM2 deficiency on postmenopausal breast cancer depending on weight history conditions. Trem2 deficiency constrained tumor growth in lean, but not in obese or weight-loss, mice. Single-cell RNA sequencing, in conjunction with variable-diversity-joining sequencing, of tumor and tumor-adjacent mammary adipose tissue immune cells revealed differences in the immune landscapes across the different models. Tumors of lean TREM2-deficient mice exhibited a shift in clonal CD8+ T cells from an exhausted to an effector memory state, accompanied by increased clonality of CD4+ Th1 cells, that was not observed in any other diet-genotype group. Notably, identical T-cell clonotypes were identified in the tumor and tumor-adjacent mammary adipose tissue of the same mouse. Finally, anti-PD-1 therapy restricted tumor growth in lean and weight-loss, but not in obese, mice. These findings indicate that weight history could affect the efficacy of TREM2 inhibition in postmenopausal breast cancer. The reported immunologic interactions between tumors and the surrounding adipose tissue highlight significant differences under obese and weight-loss conditions. Significance: Weight history impacts the immunological landscape of postmenopausal breast cancer and the efficacy of TREM2 modulation and anti-PD-1 therapy, which has implications for personalized medicine.

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.

Soluble TREM2 reflects liver fibrosis status and predicts postoperative liver dysfunction after liver surgery

Background & Aims

Triggering receptor expressed on myeloid cells 2 (TREM2)-expressing macrophages and systemic levels of soluble TREM2 (sTREM2) appear critical in the development of chronic liver disease (CLD) and seem relevant in its detection. The aim of this study was to examine sTREM2 as a marker for early CLD and its potential to predict posthepatectomy liver failure (PHLF) in patients undergoing partial hepatectomy.

Methods

sTREM2 was assessed in the plasma of 108 patients undergoing liver resection. Blood was drawn prior to surgery (preop) and on the first and fifth postoperative day.

Results

Preop sTREM2 levels were similar across different indications for resection (p = 0.091). Higher preop sTREM2 levels were associated with advanced hepatic fibrosis (p = 0.030) and PHLF (p = 0.007). Fibrosis-4 index (FIB-4) (p = 0.619) and model for end-stage liver disease (MELD) (p = 0.590) did not show a difference between patients grouped by their CLD. Comparing the AUC from receiver-operating characteristic analysis, sTREM2 (AUC = 0.708) outperformed FIB-4 (AUC = 0.529), MELD (AUC = 0.587), Child-Pugh grading (AUC = 0.570) and LiMAx (liver maximum capacity test) (AUC = 0.516) in predicting PHLF. Similarly, in uni- and multivariate analysis, only sTREM2 proved predictive for PHLF (p = 0.023). High-risk (p = 0.003) and low-risk (p = 0.011) cut-offs for systemic sTREM2 levels could identify patients at risk for adverse outcomes after surgery. Finally, high sTREM2 was associated with decreased overall survival after liver surgery (p <0.001).

Conclusions

Circulating sTREM2 shows sensitivity for early-stage, asymptomatic liver disease, irrespective of the underlying indication for liver surgery. Assessment of CLD via sTREM2 monitoring could improve early detection of CLD and improve outcomes after liver surgery.

Impact and implications

Soluble TREM2 (sTREM2) has previously been shown to correlate with the degree of chronic liver disease. We found that even in patients undergoing liver resection, who generally do not suffer from end-stage liver disease, sTREM2 reflects liver fibrosis status and predicts postoperative development of liver dysfunction. This is especially relevant for liver surgeons and patients, as postoperative liver dysfunction is the main reason for postoperative mortality. Our findings are also important for hepatologists, as early detection of liver fibrosis and cirrhosis is paramount for overall patient survival and we can show that even in a cohort with a median model for end-stage liver disease score of 6, sTREM2 is able to distinguish patients based on their liver fibrosis status.

Immune modulation to treat Alzheimer's disease

Immune mechanisms play a fundamental role in Alzheimer's disease (AD) pathogenesis, suggesting that approaches which target immune cells and immunologically relevant molecules can offer therapeutic opportunities beyond the recently approved amyloid beta monoclonal therapies. In this review, we provide an overview of immunomodulatory therapeutics in development, including their preclinical evidence and clinical trial results. Along with detailing immune processes involved in AD pathogenesis and highlighting how these mechanisms can be therapeutically targeted to modify disease progression, we summarize knowledge gained from previous trials of immune-based interventions, and provide a series of recommendations for the development of future immunomodulatory therapeutics to treat AD.

The various roles of TREM2 in cardiovascular disease

Triggering receptor expressed on myeloid cells-2 (TREM2) is a transmembrane immune receptor that is expressed mainly on macrophages. As a pathology-induced immune signaling hub, TREM2 senses tissue damage and activates immune remodeling in response. Previous studies have predominantly focused on the TREM2 signaling pathway in Alzheimer's disease, metabolic syndrome, and cancer. Recent research has indicated that TREM2 signaling is also activated in various cardiovascular diseases. In this review, we summarize the current understanding and the unanswered questions regarding the role of TREM2 signaling in mediating the metabolism and function of macrophages in atherosclerosis and various models of heart failure. In the context of atherosclerosis, TREM2 signaling promotes foam cell formation and is crucial for maintaining macrophage survival and plaque stability through efferocytosis and cholesterol efflux. Recent studies on myocardial infarction, sepsis-induced cardiomyopathy, and hypertensive heart failure also implicated the protective role of TREM2 signaling in cardiac macrophages through efferocytosis and paracrine functions. Additionally, we discuss the clinical significance of elevated soluble TREM2 (sTREM2) in cardiovascular disease and propose potential therapies targeting TREM2. The overall aim of this review is to highlight the various roles of TREM2 in cardiovascular diseases and to provide a framework for therapeutic strategies targeting TREM2.

Peripheral and central neuroimmune mechanisms in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) poses a growing global health challenge as populations age. Recent research highlights the crucial role of peripheral immunity in AD pathogenesis. This review explores how blood-brain barrier disruption allows peripheral immune cells to infiltrate the central nervous system (CNS), worsening neuroinflammation and disease progression. We examine recent findings on interactions between peripheral immune cells and CNS-resident microglia, forming a self-perpetuating inflammatory cycle leading to neuronal dysfunction. Moreover, this review emphasizes recent developments in the dysregulation of immune factors from both the periphery and CNS, and their impact on AD progression. With ongoing research and development of new therapeutic strategies, this review underscores the importance of modulating interactions between the peripheral immune system and CNS in AD therapy.

RhoA/ROCK/GSK3β Signaling: A Keystone in Understanding Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive cognitive decline and loss of neuronal integrity. Emerging evidence suggests that RhoA, Rho-associated coiled-coil kinase (ROCK), and their downstream effector molecule glycogen synthase 3β (GSK3β) interact within a complex signaling pathway (RhoA/ROCK/GSK3β) that plays a crucial role in the pathogenesis of AD. RhoA, a small GTPase, along with its downstream effector, ROCK, regulates various cellular processes, including actin cytoskeleton dynamics, apoptosis, and synaptic plasticity. GSK3β, a serine/threonine kinase, plays a key role in neuronal function and AD pathology, including the regulation of tau phosphorylation and amyloid-beta cleavage. Overactive GSK3β has been closely linked to tau hyperphosphorylation, neurodegeneration, and the progression of AD. Thus, GSK3β has been considered as a promising therapeutic target for treating AD and mitigating cognitive impairment. However, clinical trials of GSK3β in AD have faced considerable challenges due to the complexity of the specific neuronal inhibition of GSK3β. In this review, we summarize the literature regarding the relationship of RhoA/ROCK and GSK3β signaling pathways in AD pathogenesis. We further discuss recent findings of the sTREM2-transgelin-2 (TG2) axis as a potential mediator of this complex pathway and provide our review on a novel targeting strategy for AD.

Modulation of β secretase and neuroinflammation by biomimetic nanodelivery system for Alzheimer's disease therapy

Alzheimer's disease (AD) is an irreversible and progressive neurodegenerative disorder. The vicious circle between amyloid-β peptide (Aβ) overgeneration and microglial dysfunction is an important pathological event that promotes AD progression. However, therapeutic strategies toward only Aβ or microglial modulation still have many problems. Herein, inspired by the Aβ transportation, an Aβ-derived peptide (CKLVFFAED) engineered biomimetic nanodelivery system (MK@PC-R NPs) is reported for realizing BBB penetration and reprogram neuron and microglia in AD lesion sites. This hollow mesoporous Prussian blue-based MK@PC-R NPs carrying curcumin and miRNA-124 can down-regulate β secretase expression, thereby inhibiting Aβ production and reducing Aβ-induced neurotoxicity. Meanwhile, MK@PC-R NPs with excellent antioxidant and anti-inflammatory properties could normalize the microglial phenotype and promote Aβ degradation, providing neuroprotection. As expected, after treatment with MK@PC-R NPs, the Aβ burdens, neuron damages, neuroinflammation, and memory deficits of transgenic AD mice (APP/PS1 mice) are significantly attenuated. Overall, this biomimetic nanodelivery system with anti-Aβ and anti-inflammatory properties provides a promising strategy for the multi-target therapy of early AD.

HEBE: A novel chimeric chronokine for ameliorating memory deficits in Alzheimer's disease

Alzheimer's disease (AD) is a prevalent neurodegenerative disorder characterized by amyloid-β and Tau protein depositions, with treatments focusing on single proteins have shown limited success due to the complexity of pathways involved. This study explored the potential of chronokines -proteins that modulate aging-related processes- as an alternative therapeutic approach. Specifically, we focused on a novel pleiotropic chimeric protein named HEBE, combining s-KL, sTREM2 and TIMP2, guided by bioinformatic analyses to ensure the preservation of each protein's conformation, crucial for their functions. In vitro studies confirmed HEBE's stability and enzymatic activities, even suggesting it has different activities compared to the individual chronokines. In vivo experiments on APP/Tau mice revealed improved learning and memory functions with HEBE treatment, along with decreased levels of phosphorylated Tau and minor effects on amyloid-β levels. These findings suggest that HEBE is as a promising therapeutic candidate for ameliorating memory deficits and reducing pTau in an AD mouse model.

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.

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.

Neuroprotective Effects of Eugenol Acetate Against Ischemic Stroke

Objective

To explore the neuroprotective effect of Eugenol Acetate (EA) on post-stroke neuroinflammation and investigate the underlying mechanisms.

Methods

For in vitro experiments, primary microglia were pre-incubated with EA for 2 hours, followed by lipopolysaccharide (LPS) stimulation for 24 hours or Oxygen-Glucose Deprivation (OGD) treatment for 4 hours. Real-time quantitative PCR, enzyme-linked immunosorbent assay (ELISA) and Western blot were performed to examine the expression levels of inflammatory cytokines in primary microglia. The activation of NF-κB signaling pathway was evaluated by immunofluorescence staining and Western blot. For in vivo experiments, middle cerebral artery occlusion (MCAO) was constructed to mimic ischemic brain injury on 8-week-old male C57BL/6J mice. The mice were continuously injected intraperitoneally with EA or vehicle after MCAO. Neurobehavioral tests and TTC staining were conducted to estimate the neurological deficits and infarct area. Moreover, the white matter integrity after MCAO was observed via immunofluorescence staining.

Results

EA significantly reduced the expression of pro-inflammatory cytokines in LPS or OGD treated primary microglia, and inhibited LPS-induced activation of the NF-κB signaling pathway. In addition, EA alleviated ischemic brain injury and improved neuromotor function of MCAO mice. Furthermore, long-term neurological deficits and white matter integrity were improved by EA treatment after MCAO.

Conclusion

EA alleviated ischemic injury and restored white matter integrity in MCAO mice, which might be associated with the inhibition of NF-κB signaling pathway in microglia. Therefore, EA might be a promising candidate for the treatment of ischemic stroke.

Soluble TREM2 drives triple-negative breast cancer progression via activation of the AKT pathway

Triggering receptor expressed on myeloid cells 2 (TREM2) plays a key role in immune regulation, particularly within tumor-associated macrophages (TAMs). In triple-negative breast cancer (TNBC), TREM2+ TAMs have been shown to modulate the tumor microenvironment, but the role of its soluble form: soluble triggering receptor expressed on myeloid cells 2 (sTREM2), produced through proteolytic cleavage, remains unclear. In this study, we investigated the effects of sTREM2 on TNBC progression. In vitro, treatment of TNBC cells with recombinant sTREM2 or sTREM2-containing culture supernatant significantly enhanced cell proliferation, invasion, and migration. These effects were further confirmed by the use of TREM2-neutralizing antibodies, which abrogated sTREM2's tumor-promoting activities. In vivo, peri-tumoral injections of recombinant sTREM2 led to a notable acceleration of tumor growth in mouse models. Mechanistically, we found that the effects of sTREM2 were mediated through its binding to the TG2 protein in 4T1 cells, thereby activating the AKT signaling pathway. Collectively, our findings suggest that sTREM2 drives TNBC progression by enhancing critical tumor cell functions, positioning it as a potential therapeutic target for TNBC treatment.

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.

TLR4 Targeting: A Promising Therapeutic Approach Across Multiple Human Diseases

TLR4 stands at the forefront of innate immune responses, recognizing various pathogen- associated molecular patterns and endogenous ligands, thus serving as a pivotal mediator in the immune system's defense against infections and tissue damage. Beyond its canonical role in infection, emerging evidence highlights TLR4's involvement in numerous non-infectious human diseases, ranging from metabolic disorders to neurodegenerative conditions and cancer. Targeting TLR4 signaling pathways presents a promising therapeutic approach with broad applicability across these diverse pathological states. In metabolic disorders such as obesity and diabetes, dysregulated TLR4 activation contributes to chronic low-grade inflammation and insulin resistance, driving disease progression. In cardiovascular diseases, TLR4 signaling promotes vascular inflammation and atherogenesis, implicating its potential as a therapeutic target to mitigate cardiovascular risk. Neurodegenerative disorders, including Alzheimer's and Parkinson's diseases, exhibit aberrant TLR4 activation linked to neuroinflammation and neuronal damage, suggesting TLR4 modulation as a strategy to attenuate neurodegeneration. Additionally, in cancer, TLR4 signaling within the tumor microenvironment promotes tumor progression, metastasis, and immune evasion, underscoring its relevance as a target for anticancer therapy. Advances in understanding TLR4 signaling cascades and their contributions to disease pathogenesis have spurred the development of various pharmacological agents targeting TLR4. These agents range from small molecule inhibitors to monoclonal antibodies, with some undergoing preclinical and clinical evaluations. Furthermore, strategies involving TLR4 modulation through dietary interventions and microbiota manipulation offer additional avenues for therapeutic exploration. Hence, targeting TLR4 holds significant promise as a therapeutic strategy across a spectrum of human diseases, offering the potential to modulate inflammation, restore immune homeostasis, and impede disease progression.

Role of PI3Kγ in the polarization, migration, and phagocytosis of microglia

Phosphoinositide 3-kinase γ (PI3Kγ) is a signaling protein that is constitutively expressed in immune competent cells and plays a crucial role in cell proliferation, apoptosis, migration, deformation, and immunology. Several studies have shown that high expression of PI3Kγ can inhibit the occurrence of inflammation in microglia while also regulating the polarization of microglia to inhibit inflammation and enhance microglial migration and phagocytosis. It is well known that the regulation of microglial polarization, migration, and phagocytosis is key to the treatment of most neurodegenerative diseases. Therefore, in this article, we review the important regulatory role of PI3Kγ in microglia to provide a basis for the treatment of neurodegenerative diseases.

The Interplay Between Accumulation of Amyloid-Beta and Tau Proteins, PANoptosis, and Inflammation in Alzheimer's Disease

Alzheimer's disease (AD) is a common progressive neurodegenerative disorder, and the vast majority of cases occur in elderly patients. Recently, the accumulation of Aβ and tau proteins has drawn considerable attention in AD research. This review explores the multifaceted interactions between these proteins and their contribution to the pathological landscape of AD, encompassing synaptic dysfunction, neuroinflammation, and PANoptosis. PANoptosis is a collective term for programmed cell death (PCD) modalities that encompass elements of apoptosis, pyroptosis, and necroptosis. The accumulation of Aβ peptides and tau proteins, along with the immune response in brain cells, may trigger PANoptosis, thus advancing the progression of the disease. Recent advancements in molecular imaging and genetics have provided deeper insights into the interactions between Aβ peptides, tau proteins, and the immune response. The review also discusses the role of mitochondrial dysregulation in AD. The exploration of the interplay between neurodegeneration, immune responses, and cell death offers promising avenues for the development of innovative treatments.

Adoptive Transfer of CX3CR1-Transduced Tregs Homing to the Forebrain in Lipopolysaccharide-Induced Neuroinflammation and 3xTg Alzheimer's Disease Models

CX3CR1-transduced regulatory T cells (Tregs) have shown potential in reducing neuroinflammation by targeting microglial activation. Reactive microglia are implicated in neurological disorders, and CX3CR1-CX3CL1 signaling modulates microglial activity. The ability of CX3CR1-transduced Tregs to inhibit LPS-induced neuroinflammation was assessed in animal models. CX3CR1 Tregs were administered to LPS-induced and 3xTg Alzheimer's mouse models, resulting in reduced proinflammatory marker expression in both the cortices and hippocampi. In the 3xTg Alzheimer's model, neuroinflammation was significantly reduced, demonstrating the efficacy of CX3CR1 Tregs even in chronic neuroinflammatory conditions. These findings highlight the therapeutic potential of CX3CR1 Treg therapy in modulating microglial activity and offer promising treatment strategies for neurodegenerative diseases.

Current status of fluid biomarkers for early Alzheimer's disease and FDA regulation implications

Many changes can now be seen in the development and use of tests, especially those incorporating fluid biomarkers, to diagnose Alzheimer's disease (AD), a devastating disease caused by the progressive but rapid degeneration of cortical tissue. Some biomarkers we already know have a significant association with AD, such as amyloid beta (Aβ) and tau, as well as the ratio of concentrations of other Aβ isoforms. In addition, several novel biomarkers are emerging that can also be used as diagnostic fluid biomarkers for AD, but many studies are still needed before we can consider them reliable. The U.S. Federal Food and Drug Administration recently announced its final ruling to regulate laboratory-developed tests (LDTs) as medical devices, which can significantly impact LDT development. In this narrative review, we discuss the current status of fluid biomarkers used to diagnose early AD, their potential and limitations, and the impact caused by the FDA's decision and strategies to help developers navigate the complex changes in the regulatory landscape of LDTs.

Biological and clinical role of TREM2 in liver diseases

Liver diseases constitute a major health burden worldwide, accounting for more than 4% of all disease-related mortalities. While the incidence of viral hepatitis is expected to decrease, metabolic liver disorders are increasingly diagnosed. Liver pathology is diverse, with functional and molecular alterations in both parenchymal and mesenchymal cells, including immune cells. Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor of the immunoglobulin superfamily and mainly expressed on myeloid cells. Several studies have demonstrated that TREM2 plays a critical role in tissue physiology and various pathological conditions. TREM2 is recognized as being associated with the development of liver diseases by regulating tissue homeostasis and the immune microenvironment. The biological and clinical impact of TREM2 is complex, given its diverse context-dependent functions. This review aims to summarize recent progress in understanding the association between TREM2 and different liver disorders and shed light on the clinical significance of targeting TREM2.

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.

Therapeutic approaches to microglial dysfunction in Alzheimer's disease: Enhancing phagocytosis and metabolic regulation

Microglia are essential in neurogenesis, synaptic pruning, and homeostasis. Nevertheless, aging, and cellular senescence may modify their role, causing them to shift from being shields to being players of neurodegeneration. In the aging brain, the population of microglia increases, followed by enhanced activity of genes related to neuroinflammation. This change increases their ability to cause inflammation, resulting in a long-lasting state of inflammation in the brain that harms the condition of neurons. In Alzheimer's Disease (AD), microglia are located inside amyloid plaques and exhibit an inflammatory phenotype characterized by a diminished ability to engulf and remove waste material, worsening the illness's advancement. Genetic polymorphisms in TREM2, APOE, and CD33 highlight the significant impact of microglial dysfunction in AD. This review examines therapeutic approaches that aim to address microglial dysfunction, such as enhancing the microglial capability to engulf and remove amyloid-β clumps and regulating microglial metabolism and mitochondrial activity. Microglial transplanting and reprogramming advancements show the potential to restore their ability to reduce inflammation. Although there has been notable advancement, there are still voids in our knowledge of microglial biology, including their relationships with other brain cells. Further studies should prioritize the improvement of human AD models, establish standardized methods for characterizing microglia, and explore how various factors influence microglial responses. It is essential to tackle these problems to create effective treatment plans that focus on reducing inflammation in the brain and protecting against damage in age-related neurodegenerative illnesses.

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

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

Microglia and TREM2

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

Trem2 deficiency attenuates breast cancer tumor growth in lean, but not obese or weight loss, mice and is associated with alterations of clonal T cell populations

Obesity is an established risk factor for breast cancer development and worsened prognosis; however, the mechanisms for this association - and the potential benefits of weight loss - have not been fully explored. The adipose environment surrounding breast tumors, which is inflamed in obesity, has been implicated in tumor progression. An emerging therapeutic target for cancer is TREM2, a transmembrane receptor of the immunoglobulin superfamily that is expressed on macrophages in adipose tissue and tumors. We utilized genetic loss of function (Trem2 +,+ and Trem2 -/-) models and dietary (lean, obese, and weight loss) intervention approaches to examine impacts on postmenopausal breast cancer. Remarkably, Trem2 deficiency ameliorated tumor growth in lean, but not obese or weight loss mice. Single-cell RNA sequencing, in conjunction with VDJ sequencing of tumor and tumor-adjacent mammary adipose tissue (mATTum-adj) immune cells, revealed that tumors of lean Trem2 -/- mice exhibited a shift in clonal CD8+ T cells from an exhausted to an effector memory state, accompanied with increased clonality of CD4+ Th1 cells, that was not observed in any other diet-genotype group. Notably, identical T cell clonotypes were identified in the tumor and mATTum-adj of the same mouse. Finally, an immune checkpoint study demonstrated that αPD-1 therapy restricted tumor growth in lean and weight loss, but not obese mice. We conclude that weight history is relevant when considering potential efficacy of TREM2 inhibition in postmenopausal breast cancer. This work reveals immunological interactions between tumors and surrounding adipose tissue, highlighting significant differences under obese and weight loss conditions.

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.

A Computational Approach in the Systematic Search of the Interaction Partners of Alternatively Spliced TREM2 Isoforms

Alzheimer's disease is the most common form of dementia, characterized by the pathological accumulation of amyloid-beta (Aβ) plaques and tau neurofibrillary tangles. Triggering receptor expressed on myeloid cells 2 (TREM2) is increasingly recognized as playing a central role in Aβ clearance and microglia activation in AD. The TREM2 gene transcriptional product is alternatively spliced to produce three different protein isoforms. The canonical TREM2 isoform binds to DAP12 to activate downstream pathways. However, little is known about the function or interaction partners of the alternative TREM2 isoforms. The present study utilized a computational approach in a systematic search for new interaction partners of the TREM2 isoforms by integrating several state-of-the-art structural bioinformatics tools from initial large-scale screening to one-on-one corroborative modeling and eventual all-atom visualization. CD9, a cell surface glycoprotein involved in cell-cell adhesion and migration, was identified as a new interaction partner for two TREM2 isoforms, and CALM, a calcium-binding protein involved in calcium signaling, was identified as an interaction partner for a third TREM2 isoform, highlighting the potential role of cell adhesion and calcium regulation in AD.

A Risk Variant rs6922617 in TREM Is Discrepantly Associated With Defining Neuropathological Hallmarks in the Alzheimer's Continuum

The single nucleotide polymorphism (SNP)-rs6922617 in the triggering receptor expressed on myeloid cells (TREM) gene cluster is a potential risk factor for Alzheimer's disease (AD). Here, we examined whether rs6922617 is associated with AD-defining neuropathological hallmarks and memory performance. We assessed the interaction between the variant rs6922617 and levels of beta-amyloid (Aβ), tau pathology, neurodegeneration, namely amyloid-tau-neurodegeneration framework, and cognition functions in 660 healthy controls, 794 mild cognitively impaired, and 272 subjects with AD. We employed linear regression and linear mixed models to examine the association. Here we find that the SNP-rs6922617 in the TREM gene cluster is associated with a higher global amyloid-ligands positron emission tomography (Aβ-PET) burden and lower fluorodeoxyglucose positron emission tomography (FDG-PET) load. Interestingly, rs6922617 risk allele carriers exhibit a significantly reduced tau accumulation compared to the non-carriers, indicating a discrepant association with Aβ and tau pathologies. Though the participants carrying the rs6922617 risk allele do not show a correlation with poorer cognitive performance, stronger neuropathological phenotypes, and memory impairments are evident in ApoE ε4 carriers with the rs6922617 risk allele. These results support the notion that the SNP-rs6922617 in the TREM gene cluster is associated with AD-related neuropathological hallmarks, such as Aβ and FDG-mediated neurodegeneration, rather than tau accumulation. Although the direct association with memory impairment in the Alzheimer's continuum remains inconclusive, our findings suggest a potential role of rs6922617 in facilitating neuropathology hallmarks.

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.

Mechanistic Approach to Immunity and Immunotherapy of Alzheimer's Disease: A Review

Alzheimer's disease (AD) is a debilitating neurodegenerative condition characterized by progressive cognitive decline and memory loss, affecting millions of people worldwide. Traditional treatments, such as cholinesterase inhibitors and NMDA receptor antagonists, offer limited symptomatic relief without addressing the underlying disease mechanisms. These limitations have driven the development of more potent and effective therapies. Recent advances in immunotherapy present promising avenues for AD treatment. Immunotherapy strategies, including both active and passive approaches, harness the immune system to target and mitigate AD-related pathology. Active immunotherapy stimulates the patient's immune response to produce antibodies against AD-specific antigens, while passive immunotherapy involves administering preformed antibodies or immune cells that specifically target amyloid-β (Aβ) or tau proteins. Monoclonal antibodies, such as aducanumab and lecanemab, have shown potential in reducing Aβ plaques and slowing cognitive decline in clinical trials, despite challenges related to adverse immune responses and the need for precise targeting. This comprehensive review explores the role of the immune system in AD, evaluates the current successes and limitations of immunotherapeutic approaches, and discusses future directions for enhancing the treatment efficacy.

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.

Targeting Microglia in Alzheimer's Disease: Pathogenesis and Potential Therapeutic Strategies

Microglia, as resident macrophages in the central nervous system, play a multifunctional role in the pathogenesis of Alzheimer's disease (AD). Their clustering around amyloid-β (Aβ) deposits is a core pathological feature of AD. Recent advances in single-cell RNA sequencing (scRNA-seq) and single-nucleus RNA sequencing (snRNA-seq) have revealed dynamic changes in microglial phenotypes over time and across different brain regions during aging and AD progression. As AD advances, microglia primarily exhibit impaired phagocytosis of Aβ and tau, along with the release of pro-inflammatory cytokines that damage synapses and neurons. Targeting microglia has emerged as a potential therapeutic approach for AD. Treatment strategies involving microglia can be broadly categorized into two aspects: (1) enhancing microglial function: This involves augmenting their phagocytic ability against Aβ and cellular debris and (2) mitigating neuroinflammation: Strategies include inhibiting TNF-α signaling to reduce the neuroinflammatory response triggered by microglia. Clinical trials exploring microglia-related approaches for AD treatment have garnered attention. Additionally, natural products show promise in enhancing beneficial effects and suppressing inflammatory responses. Clarifying microglial dynamics, understanding their roles, and exploring novel therapeutic approaches will advance our fight against AD.

Psilocybin for the treatment of Alzheimer's disease

Alzheimer's disease (AD) stands as a formidable neurodegenerative ailment and a prominent contributor to dementia. The scarcity of available therapies for AD accentuates the exigency for innovative treatment modalities. Psilocybin, a psychoactive alkaloid intrinsic to hallucinogenic mushrooms, has garnered attention within the neuropsychiatric realm due to its established safety and efficacy in treating depression. Nonetheless, its potential as a therapeutic avenue for AD remains largely uncharted. This comprehensive review endeavors to encapsulate the pharmacological effects of psilocybin while elucidating the existing evidence concerning its potential mechanisms contributing to a positive impact on AD. Specifically, the active metabolite of psilocybin, psilocin, elicits its effects through the modulation of the 5-hydroxytryptamine 2A receptor (5-HT2A receptor). This modulation causes heightened neural plasticity, diminished inflammation, and improvements in cognitive functions such as creativity, cognitive flexibility, and emotional facial recognition. Noteworthy is psilocybin's promising role in mitigating anxiety and depression symptoms in AD patients. Acknowledging the attendant adverse reactions, we proffer strategies aimed at tempering or mitigating its hallucinogenic effects. Moreover, we broach the ethical and legal dimensions inherent in psilocybin's exploration for AD treatment. By traversing these avenues, We propose therapeutic potential of psilocybin in the nuanced management of Alzheimer's disease.

Carboxymethylcellulose encapsulated fingolimod, siRNA@ZnO hybrid nanocomposite as a new anti-Alzheimer's material

Alzheimer's disease (AD) is a fatal neurological disorder that causes cognitive and memory function to deteriorate. A critical pathogenic event that speeds up the development of AD is the interaction between dysfunctional microglia and amyloid-β (Aβ). We have developed a hybrid nanocomposite material to treat AD by normalizing the dysfunctional microglia. The material is based on carboxymethylcellulose (CMC) encapsulated fingolimod, siRNA, and zinc oxide (ZnO) with variable loading (CMC-Fi-siRNA@ZnO a-d ). The material was characterized using different techniques including FTIR, XRD, thermal analysis, SEM with EDX, and TEM micrographs. The chemical structure was confirmed by FTIR and XRD analyses, which indicated the successful integration of ZnO nanoparticles (NPs) into the polymer matrix, signifying a well-formed composite structure. The thermal stability order at 10% weight loss was CMC-Fi-siRNA@ZnO c > CMC-Fi-siRNA@ZnO b > CMC-Fi-siRNA@ZnO d > CMC-Fi-siRNA@ZnO a . The CMC-Fi-siRNA@ZnO d dramatically alleviates the priming of microglia by lowering the level of proinflammatory mediators and increasing the secretion of BDNF. This considerably improves the phagocytosis of Aβ. In the cell viability test in immortalized microglia cells (IMG), the hybrid nanocomposite (NP) exhibited no significant effect on cell survival after 48 hours of incubation. The NP also decreased the cytotoxicity caused by Aβ. Therefore, the CMC-hybrid NP has high potential as a drug delivery system in the development of therapeutic strategies for AD.

Neuroimaging biomarkers and CSF sTREM2 levels in Alzheimer's disease: a longitudinal study

Understanding the exact pathophysiological mechanisms underlying the involvement of triggering receptor expressed on myeloid cells 2 (TREM2) related microglia activation is crucial for the development of clinical trials targeting microglia activation at different stages of Alzheimer's disease (AD). Given the contradictory findings in the literature, it is imperative to investigate the longitudinal alterations in cerebrospinal fluid (CSF) soluble TREM2 (sTREM2) levels as a marker for microglia activation, and its potential association with AD biomarkers, in order to address the current knowledge gap. In this study, we aimed to assess the longitudinal changes in CSF sTREM2 levels within the framework of the A/T/N classification system for AD biomarkers and to explore potential associations with AD pathological features, including the presence of amyloid-beta (Aβ) plaques and tau aggregates. The baseline and longitudinal (any available follow-up visit) CSF sTREM2 levels and processed tau-PET and Aβ-PET data of 1001 subjects were recruited from the ADNI database. The participants were classified into four groups based on the A/T/N framework: A+ /TN+ , A+ /TN- , A- /TN+ , and A- /TN- . Linear regression analyses were conducted to assess the relationship between CSF sTREM2 with cognitive performance, tau and Aβ-PET adjusting for age, gender, education, and APOE ε4 status. Based on our analysis there was a significant difference in baseline and rate of change of CSF sTREM2 between ATN groups. While there was no association between baseline CSF sTREM2 and cognitive performance (ADNI-mem), we found that the rate of change of CSF sTREM2 is significantly associated with cognitive performance in the entire cohort but not the ATN groups. We found that the baseline CSF sTREM2 is significantly associated with baseline tau-PET and Aβ-PET rate of change only in the A+ /TN+ group. A significant association was found between the rate of change of CSF sTREM2 and the tau- and Aβ-PET rate of change only in the A+ /TN- group. Our study suggests that the TREM2-related microglia activation and their relations with AD markers and cognitive performance vary the in presence or absence of Aβ and tau pathology. Furthermore, our findings revealed that a faster increase in the level of CSF sTREM2 might attenuate future Aβ plaque formation and tau aggregate accumulation only in the presence of Aβ pathology.

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

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

The longitudinal trajectory of CSF sTREM2: the alzheimer's disease neuroimaging initiative

BACKGROUND

The soluble triggering receptor expressed on myeloid cells 2 (sTREM2) in cerebrospinal fluid (CSF) is considered a biomarker of microglia activity. The objective of this study was to investigate the trajectory of CSF sTREM2 levels over time and examine its association with sex.

METHODS

A total of 1,017 participants from the Alzheimer's Disease Neuroimaging Initiative Study (ADNI) with at least one CSF sTREM2 record were included. The trajectory of CSF sTREM2 was analyzed using a growth curve model. The association between CSF sTREM2 levels and sex was assessed using linear mixed-effect models.

RESULTS

CSF sTREM2 levels were increased with age over time (P < 0.0001). No significant sex difference was observed in sTREM2 levels across the entire sample; however, among the APOE ε4 allele carriers, women exhibited significantly higher sTREM2 levels than men (β = 0.146, P = 0.002).

CONCLUSION

Our findings highlight the association between CSF sTREM2 levels and age-related increments, underscoring the potential influence of aging on sTREM2 dynamics. Furthermore, our observations indicate a noteworthy association between sex and CSF sTREM2 levels, particularly in individuals carrying the APOE ε4 allele.

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

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

Decoding sTREM2: its impact on Alzheimer's disease - a comprehensive review of mechanisms and implications

Soluble Triggering Receptor Expressed on Myeloid Cells 2 (sTREM2) plays a crucial role in the pathogenesis of Alzheimer's disease (AD). This review comprehensively examines sTREM2's involvement in AD, focusing on its regulatory functions in microglial responses, neuroinflammation, and interactions with key pathological processes. We discuss the dynamic changes in sTREM2 levels in cerebrospinal fluid and plasma throughout AD progression, highlighting its potential as a therapeutic target. Furthermore, we explore the impact of genetic variants on sTREM2 expression and its interplay with other AD risk genes. The evidence presented in this review suggests that modulating sTREM2 activity could influence AD trajectory, making it a promising avenue for future research and drug development. By providing a holistic understanding of sTREM2's multifaceted role in AD, this review aims to guide future studies and inspire novel therapeutic strategies.

George-Hyslop P, Fox NC, Suzuki K, Okhravi HR, Chhatwal J, Levin J, Jucker M, Sims JR, Holdridge KC, Proctor NK, Yaari R, Andersen SW, Mancini M, Llibre-Guerra J, Bateman RJ, McDade E. Downstream Biomarker Effects of Gantenerumab or Solanezumab in Dominantly Inherited Alzheimer Disease: The DIAN-TU-001 Randomized Clinical Trial

Importance

Effects of antiamyloid agents, targeting either fibrillar or soluble monomeric amyloid peptides, on downstream biomarkers in cerebrospinal fluid (CSF) and plasma are largely unknown in dominantly inherited Alzheimer disease (DIAD).

Objective

To investigate longitudinal biomarker changes of synaptic dysfunction, neuroinflammation, and neurodegeneration in individuals with DIAD who are receiving antiamyloid treatment.

Design, Setting, and Participants

From 2012 to 2019, the Dominantly Inherited Alzheimer Network Trial Unit (DIAN-TU-001) study, a double-blind, placebo-controlled, randomized clinical trial, investigated gantenerumab and solanezumab in DIAD. Carriers of gene variants were assigned 3:1 to either drug or placebo. The present analysis was conducted from April to June 2023. DIAN-TU-001 spans 25 study sites in 7 countries. Biofluids and neuroimaging from carriers of DIAD gene variants in the gantenerumab, solanezumab, and placebo groups were analyzed.

Interventions

In 2016, initial dosing of gantenerumab, 225 mg (subcutaneously every 4 weeks) was increased every 8 weeks up to 1200 mg. In 2017, initial dosing of solanezumab, 400 mg (intravenously every 4 weeks) was increased up to 1600 mg every 4 weeks.

Main Outcomes and Measures

Longitudinal changes in CSF levels of neurogranin, soluble triggering receptor expressed on myeloid cells 2 (sTREM2), chitinase 3-like 1 protein (YKL-40), glial fibrillary acidic protein (GFAP), neurofilament light protein (NfL), and plasma levels of GFAP and NfL.

Results

Of 236 eligible participants screened, 43 were excluded. A total of 142 participants (mean [SD] age, 44 [10] years; 72 female [51%]) were included in the study (gantenerumab, 52 [37%]; solanezumab, 50 [35%]; placebo, 40 [28%]). Relative to placebo, gantenerumab significantly reduced CSF neurogranin level at year 4 (mean [SD] β = -242.43 [48.04] pg/mL; P < .001); reduced plasma GFAP level at year 1 (mean [SD] β = -0.02 [0.01] ng/mL; P = .02), year 2 (mean [SD] β = -0.03 [0.01] ng/mL; P = .002), and year 4 (mean [SD] β = -0.06 [0.02] ng/mL; P < .001); and increased CSF sTREM2 level at year 2 (mean [SD] β = 1.12 [0.43] ng/mL; P = .01) and year 4 (mean [SD] β = 1.06 [0.52] ng/mL; P = .04). Solanezumab significantly increased CSF NfL (log) at year 4 (mean [SD] β = 0.14 [0.06]; P = .02). Correlation analysis for rates of change found stronger correlations between CSF markers and fluid markers with Pittsburgh compound B positron emission tomography for solanezumab and placebo.

Conclusions and Relevance

This randomized clinical trial supports the importance of fibrillar amyloid reduction in multiple AD-related processes of neuroinflammation and neurodegeneration in CSF and plasma in DIAD. Additional studies of antiaggregated amyloid therapies in sporadic AD and DIAD are needed to determine the utility of nonamyloid biomarkers in determining disease modification.

Trial Registration

ClinicalTrials.gov Identifier: NCT04623242.

Variants in the MS4A cluster interact with soluble TREM2 expression on biomarkers of neuropathology

Recent evidence suggests that Alzheimer's disease (AD) genetic risk variants (rs1582763 and rs6591561) of the MS4A locus are genome-wide significant regulators of soluble TREM2 levels such that the minor allele of the protective variant (rs1582763) is associated with higher sTREM2 and lower AD risk while the minor allele of (rs6591561) relates to lower sTREM2 and higher AD risk. Our group previously found that higher sTREM2 relates to higher Aβ40, worse blood-brain barrier (BBB) integrity (measured with the CSF/plasma albumin ratio), and higher CSF tau, suggesting strong associations with amyloid abundance and both BBB and neurodegeneration complicate interpretation. We expand on this work by leveraging these common variants as genetic tools to tune the interpretation of high CSF sTREM2, and by exploring the potential modifying role of these variants on the well-established associations between CSF sTREM2 as well as TREM2 transcript levels in the brain with AD neuropathology. Biomarker analyses leveraged data from the Vanderbilt Memory & Aging Project (n = 127, age = 72 ± 6.43) and were replicated in the Alzheimer's Disease Neuroimaging Initiative (n = 399, age = 73 ± 7.39). Autopsy analyses were performed leveraging data from the Religious Orders Study and Rush Memory and Aging Project (n = 577, age = 89 ± 6.46). We found that the protective variant rs1582763 attenuated the association between CSF sTREM2 and Aβ40 (β = -0.44, p-value = 0.017) and replicated this interaction in ADNI (β = -0.27, p = 0.017). We did not observe this same interaction effect between TREM2 mRNA levels and Aβ peptides in brain (Aβ total β = -0.14, p = 0.629; Aβ1-38, β = 0.11, p = 0.200). In contrast to the effects on Aβ, the minor allele of this same variant seemed to enhance the association with blood-brain barrier dysfunction (β = 7.0e-4, p = 0.009), suggesting that elevated sTREM2 may carry a much different interpretation in carriers vs. non-carriers of this allele. When evaluating the risk variant (rs6591561) across datasets, we did not observe a statistically significant interaction against any outcome in VMAP and observed opposing directions of associations in ADNI and ROS/MAP on Aβ levels. Together, our results suggest that the protective effect of rs1582763 may act by decoupling the associations between sTREM2 and amyloid abundance, providing important mechanistic insight into sTREM2 changes and highlighting the need to incorporate genetic context into the analysis of sTREM2 levels, particularly if leveraged as a clinical biomarker of disease in the future.