Anti-human TREM2 induces microglia proliferation and reduces pathology in an Alzheimer's disease model

Type I Interferon Signalling and Ischemic Stroke: Mechanisms and Therapeutic Potentials

Type I interferon (IFN-I) signalling is intricately involved in the pathogenesis of multiple infectious diseases, autoimmune diseases, and neurological diseases. Acute ischemic stroke provokes overactivation of IFN-I signalling within the injured brain, particularly in microglia. Following cerebral ischemia, damage-associated molecular patterns (DAMPs) released from injured neural cells elicit marked proinflammatory episodes within minutes. Among these, self-nucleic acids, including nuclear DNA and mitochondrial DNA (mtDNA), have been recognized as a critical alarm signal to fan the flames of neuroinflammation, predominantly via inducing IFN-I signalling activation in microglia. The concept of interferon-responsive microglia (IRM), marked by upregulation of a plethora of IFN-stimulated genes, has been emergingly elucidated in ischemic mouse brains, particularly in aged ones. Among the pattern recognition receptors responsible for IFN-I induction, cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) plays integral roles in potentiating microglia-driven neuroinflammation and secondary brain injury after cerebral ischemia. Here, we aim to provide an up-to-date review on the multifaceted roles of IFN-I signalling, the detailed molecular and cellular mechanisms leading to and resulting from aberrant IFN-I signalling activation after cerebral ischemia, and the therapeutic potentials. A thorough exploration of these above points will inform our quest for IFN-based therapies as effective immunomodulatory therapeutics to complement the limited repertoire of thrombolytic agents, thereby facilitating the translation from bench to bedside.

TREM2 in cardiovascular diseases: Mechanisms and therapeutic perspectives

Cardiovascular diseases (CVDs) are the leading cause of global mortality, with immune responses playing a central role in their pathogenesis. Triggering receptor expressed on myeloid cells 2 (TREM2) is a key immune regulator in CVDs, influencing inflammation, lipid metabolism, and tissue repair. This review comprehensively examines TREM2's structure, function, and signaling pathways, highlighting its roles in atherosclerosis, myocardial infarction, hypertension, atrial fibrillation, and heart failure. In atherosclerosis, macrophages with high TREM2 expression (TREM2hi macrophages) promote plaque progression in early stages but enhance plaque stability in advanced stages. In myocardial infarction, TREM2 modulates macrophage diversity and efferocytosis, aiding cardiac repair. TREM2 also plays a protective role in hypertensive heart disease by reducing inflammation and promoting tissue repair. Challenges in targeting TREM2 therapeutically include its context-dependent effects and complex signaling pathways. Future research should focus on elucidating TREM2's mechanisms in CVDs and developing stage-specific therapies.

Refining the interactions between microglia and astrocytes in Alzheimer's disease pathology

Microglia and astrocytes are central to the pathogenesis and progression of Alzheimer's Disease (AD), working both independently and collaboratively to regulate key pathological processes such as β-amyloid protein (Aβ) deposition, tau aggregation, neuroinflammation, and synapse loss. These glial cells interact through complex molecular pathways, including IL-3/IL-3Ra and C3/C3aR, which influence disease progression and cognitive decline. Emerging research suggests that modulating these pathways could offer therapeutic benefits. For instance, recombinant IL-3 administration in mice reduced Aβ plaques and improved cognitive functions, while C3aR inhibition alleviated Aβ and tau pathologies, restored synaptic function, and corrected immune dysregulation. However, the effects of these interactions are context-dependent. Acute C3/C3aR activation enhances microglial Aβ clearance, whereas chronic activation impairs it, highlighting the dual roles of glial signaling in AD. Furthermore, C3/C3aR signaling not only impacts Aβ clearance but also modulates tau pathology and synaptic integrity. Given AD's multifactorial nature, understanding the specific pathological environment is crucial when investigating glial cell contributions. The interplay between microglia and astrocytes can be both neuroprotective and neurotoxic, depending on the disease stage and brain region. This complexity underscores the need for targeted therapies that modulate glial cell activity in a context-specific manner. By elucidating the molecular mechanisms underlying microglia-astrocyte interactions, this research advances our understanding of AD and paves the way for novel therapeutic strategies aimed at mitigating neurodegeneration and cognitive decline in AD and related disorders.

Drug screening targeting TREM2-TYROBP transmembrane binding

TREM2 encodes a microglial membrane receptor involved in the disease-associated microglia (DAM) phenotype whose activation requires the transmembrane interaction with TYROBP. Mutations in TREM2 represent a high-impact risk factor for Alzheimer's disease (AD) which turned TREM2 into a significant drug target. We present a bacterial two-hybrid (B2H) system designed for high-throughput screening of modulators for the TREM2-TYROBP transmembrane interaction. In a pilot study, 315 FDA-approved drugs were analyzed to identify potential binding modifiers. Our pipeline includes multiple filtering steps to ensure candidate specificity. The screening suggested two potential candidates that were finally assayed in the human microglial cell line HMC3. Upon stimulation with anti-TREM2 mAb, pSYK/SYK ratios were calculated in the presence of the candidates. As a result, we found that varenicline, a smoking cessation medication, can be considered as a transmembrane agonist of the TREM2-TYROBP interaction.

Neuroinflammation in Alzheimer disease

Increasing evidence points to a pivotal role of immune processes in the pathogenesis of Alzheimer disease, which is the most prevalent neurodegenerative and dementia-causing disease of our time. Multiple lines of information provided by experimental, epidemiological, neuropathological and genetic studies suggest a pathological role for innate and adaptive immune activation in this disease. Here, we review the cell types and pathological mechanisms involved in disease development as well as the influence of genetics and lifestyle factors. Given the decade-long preclinical stage of Alzheimer disease, these mechanisms and their interactions are driving forces behind the spread and progression of the disease. The identification of treatment opportunities will require a precise understanding of the cells and mechanisms involved as well as a clear definition of their temporal and topographical nature. We will also discuss new therapeutic strategies for targeting neuroinflammation, which are now entering the clinic and showing promise for patients.

Immune checkpoint TIM-3 regulates microglia and Alzheimer's disease

Microglia are the resident immune cells in the brain and have pivotal roles in neurodevelopment and neuroinflammation1,2. This study investigates the function of the immune-checkpoint molecule TIM-3 (encoded by HAVCR2) in microglia. TIM-3 was recently identified as a genetic risk factor for late-onset Alzheimer's disease3, and it can induce T cell exhaustion4. However, its specific function in brain microglia remains unclear. We demonstrate in mouse models that TGFβ signalling induces TIM-3 expression in microglia. In turn, TIM-3 interacts with SMAD2 and TGFBR2 through its carboxy-terminal tail, which enhances TGFβ signalling by promoting TGFBR-mediated SMAD2 phosphorylation, and this process maintains microglial homeostasis. Genetic deletion of Havcr2 in microglia leads to increased phagocytic activity and a gene-expression profile consistent with the neurodegenerative microglial phenotype (MGnD), also referred to as disease-associated microglia (DAM). Furthermore, microglia-targeted deletion of Havcr2 ameliorates cognitive impairment and reduces amyloid-β pathology in 5×FAD mice (a transgenic model of Alzheimer's disease). Single-nucleus RNA sequencing revealed a subpopulation of MGnD microglia in Havcr2-deficient 5×FAD mice characterized by increased pro-phagocytic and anti-inflammatory gene expression alongside reduced pro-inflammatory gene expression. These transcriptomic changes were corroborated by single-cell RNA sequencing data across most microglial clusters in Havcr2-deficient 5×FAD mice. Our findings reveal that TIM-3 mediates microglia homeostasis through TGFβ signalling and highlight the therapeutic potential of targeting microglial TIM-3 in Alzheimer's disease.

Microglial mechanisms drive amyloid-β clearance in immunized patients with Alzheimer's disease

Alzheimer's disease (AD) therapies utilizing amyloid-β (Aβ) immunization have shown potential in clinical trials. Yet, the mechanisms driving Aβ clearance in the immunized AD brain remain unclear. Here, we use spatial transcriptomics to explore the effects of both active and passive Aβ immunization in the AD brain. We compare actively immunized patients with AD with nonimmunized patients with AD and neurologically healthy controls, identifying distinct microglial states associated with Aβ clearance. Using high-resolution spatial transcriptomics alongside single-cell RNA sequencing, we delve deeper into the transcriptional pathways involved in Aβ removal after lecanemab treatment. We uncover spatially distinct microglial responses that vary by brain region. Our analysis reveals upregulation of the triggering receptor expressed on myeloid cells 2 (TREM2) and apolipoprotein E (APOE) in microglia across immunization approaches, which correlate positively with antibody responses and Aβ removal. Furthermore, we show that complement signaling in brain myeloid cells contributes to Aβ clearance after immunization. These findings provide new insights into the transcriptional mechanisms orchestrating Aβ removal and shed light on the role of microglia in immune-mediated Aβ clearance. Importantly, our work uncovers potential molecular targets that could enhance Aβ-targeted immunotherapies, offering new avenues for developing more effective therapeutic strategies to combat 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.

Phase 1, First-In-Human, Single-/Multiple-Ascending Dose Study of Iluzanebart in Healthy Volunteers

OBJECTIVE

To evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics of iluzanebart, a fully human monoclonal antibody TREM2 (triggering receptor expressed on myeloid cells 2) agonist, after single- (SAD) and multiple-ascending-dose (MAD) administration.

METHODS

Healthy adult volunteers (N = 136) received intravenous placebo or iluzanebart 1-60 mg/kg (SAD) or 10-60 mg/kg (MAD) followed by serial pharmacokinetics and safety assessments. Safety assessments included adverse events (AEs), vital signs, electrocardiograms, and clinical laboratory evaluations. Pharmacokinetics were assessed through noncompartmental analysis. The study also included open-label cohorts (3, 10, 20, 40, 60 mg/kg SAD; 10, 20, 40 mg/kg MAD) for cerebrospinal fluid (CSF) collection for exploratory pharmacodynamic biomarker analysis.

RESULTS

Iluzanebart was safe and well tolerated following single and multiple doses of up to 60 mg/kg. Most AEs were mild and resolved spontaneously. The most frequently reported AE was pruritus. No serious AEs or investigational product-related clinically meaningful changes in vital signs, electrocardiograms, or laboratory assessments were reported. Iluzanebart serum exposure was related to dose, with a 29-day half-life that is supportive of monthly dosing and confirmed central nervous system (CNS) exposure (≈0.15% CSF-to-serum ratio). Durable and dose-dependent target engagement, evidenced by marked reductions in soluble TREM2 and increased soluble CSF1R (colony-stimulating factor 1 receptor) and osteopontin/SPP1 (secreted phosphoprotein 1) levels in CSF, was observed, indicating that iluzanebart changes microglial activity following single and repeat dosing.

INTERPRETATION

Iluzanebart demonstrated favorable safety, tolerability, pharmacokinetics, and pharmacological activity in the CNS, supporting further clinical development for adult-onset leukoencephalopathy with axonal spheroids and pigmented glia.

Unraveling the Immune Puzzle: Role of Immunomodulation in Alzheimer's Disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder with growing evidence highlighting the dual role of immunomodulation in its pathogenesis and potential therapeutic strategies. Disturbance in the immune system increases the inflammatory cytokines that cause tau hyperphosphorylation and neuroinflammation. Also, immune checkpoint inhibition further increases the amyloid-beta deposition. Therefore, this review examines the intricate interplay between the immune system and AD, focusing on how immunomodulatory mechanisms influence key pathological hallmarks, including amyloid-beta aggregation, tau hyperphosphorylation, neuroinflammation, and cholinergic dysfunction. We analyse critical signaling pathways involved in immune regulation, such as Toll-like receptor (TLR), Janus kinase/signal transducer and activator of transcription (JAK/STAT), phosphoinositide 3-kinase/Akt (PI3K/Akt), Wnt/β-catenin, tumor necrosis factor (TNF), and triggering receptor expressed on myeloid cells (TREM), along with immune checkpoints like programmed cell death protein 1 (PD-1). Preclinical studies of immunomodulatory agents, including salidroside, festidinol, astragalin, sulforaphane, BM-MSC, simvastatin, Ab-T1, hTREM2, and XENP345, demonstrate promising effects. Additionally, clinical investigations of drugs such as simufilam, AL002, TB006, VGL101, DNL919, XPro1595, astragalus, and IBC-Ab002 underscore the therapeutic potential of targeting immune pathways in AD. This review emphasizes how neuroinflammation, microglial activation, and peripheral immune responses contribute to disease progression. By exploring immunomodulatory mechanisms, the article sheds light on potential therapeutic targets that could help mitigate AD pathology which may pave the way for novel interventions preventing neurodegeneration in AD.

The potential and challenges of TREM2-targeted therapy in Alzheimer's disease: insights from the INVOKE-2 study

Alzheimer's disease (AD) is a severe neurodegenerative disorder with a growing global burden. With the rising incidence of AD, the need for novel therapeutic targets has become increasingly critical. TREM2, a receptor expressed on microglial cells, plays a crucial role in modulating neuroinflammation and clearing pathological substrates, making it a promising candidate for AD therapy. However, the recent clinical trial INVOKE-2 failed to demonstrate significant clinical benefits of the TREM2-targeted antibody AL002, raising doubts about the efficacy of TREM2-targeted methods. This article examines the role of TREM2 in AD pathogenesis, evaluates potential reasons for the disappointing outcomes of the INVOKE-2 trial, and discusses future directions for TREM2-based therapies. Factors such as treatment timing, dosage optimization, patient genetic variability, and combination therapy strategies are identified as critical determinants of therapeutic success. Future studies should aim to refine treatment strategies, identify precise indications, and explore the potential for combination therapies to enhance efficacy.

Blockage of CCL3 with neutralizing antibody reduces neuroinflammation and reverses Alzheimer disease phenotypes

BACKGROUND

Accumulating evidence indicates that neuroinflammation is involved in the pathogenesis of Alzheimer's disease (AD). According to RNA sequencing and quantitative PCR (qPCR), we found that chemokine CCL3 mRNA expression was abnormally upregulated in the brains of AD transgenic mice. Moreover, the levels of CCL3 in the serum of AD patients were significantly elevated and negatively correlated with their cognitive abilities. However, the role of CCL3 in AD neuroinflammation and pathological damages remains elusive.

METHODS

Using behavioral, histological, and biochemical methods, outcomes of CCL3 antibody treatment on neuropathology and cognitive deficits were studied in the APPswe/PS1dE9 mice.

RESULTS

In the present study, we reported that CCL3 protein expression was increased in the APPswe/PS1dE9 mice, whereas blockage of CCL3 with neutralizing antibody potently inhibited CCL3 activation in the APPswe/PS1dE9 mice down to the levels of wild-type mice. Specifically, CCL3 antibody significantly improved the learning and memory abilities of APPswe/PS1dE9 mice. In addition, CCL3 antibody treatment decreased cerebral amyloid-β (Aβ) levels and plaque burden via inhibiting amyloid precursor protein (APP) processing by reducing beta-site APP cleaving enzyme 1 (BACE1) expression in the APPswe/PS1dE9 mice. We also found that CCL3 antibody treatment alleviated neuroinflammation and reduced synaptic defects in the APPswe/PS1dE9 mice. Furthermore, the activated NF-κB signaling pathway in APPswe/PS1dE9 mice was inhibited by CCL3 antibody treatment.

CONCLUSIONS

Collectively, our findings provide evidence that CCL3 activation may contribute to the AD pathogenesis and may serve as a novel therapeutic target in the treatment of AD.

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.

Loss of ATG7 in microglia impairs UPR, triggers ferroptosis, and weakens amyloid pathology control

Microglia impact brain development, homeostasis, and pathology. One important microglial function in Alzheimer's disease (AD) is to contain proteotoxic amyloid-β (Aβ) plaques. Recent studies reported the involvement of autophagy-related (ATG) proteins in this process. Here, we found that microglia-specific deletion of Atg7 in an AD mouse model impaired microglia coverage of Aβ plaques, increasing plaque diffusion and neurotoxicity. Single-cell RNA sequencing, biochemical, and immunofluorescence analyses revealed that Atg7 deficiency reduces unfolded protein response (UPR) while increasing oxidative stress. Cellular assays demonstrated that these changes lead to lipoperoxidation and ferroptosis of microglia. In aged mice without Aβ buildup, UPR reduction and increased oxidative damage induced by Atg7 deletion did not impact microglia numbers. We conclude that reduced UPR and increased oxidative stress in Atg7-deficient microglia lead to ferroptosis when exposed to proteotoxic stress from Aβ plaques. However, these microglia can still manage misfolded protein accumulation and oxidative stress as they age.

Microglia heterogeneity, modeling and cell-state annotation in development and neurodegeneration

Within the CNS, microglia execute various functions associated with brain development, maintenance of homeostasis and elimination of pathogens and protein aggregates. This wide range of activities is closely associated with a plethora of cellular states, which may reciprocally influence or be influenced by their functional dynamics. Advancements in single-cell RNA sequencing have enabled a nuanced exploration of the intricate diversity of microglia, both in health and disease. Here, we review our current understanding of microglial transcriptional heterogeneity. We provide an overview of mouse and human microglial diversity encompassing aspects of development, neurodegeneration, sex and CNS regions. We offer an insight into state-of-the-art technologies and model systems that are poised to improve our understanding of microglial cell states and functions. We also provide suggestions and a tool to annotate microglial cell states on the basis of gene expression.

Biomarker-guided decision making in clinical drug development for neurodegenerative disorders

Neurodegenerative disorders are characterized by complex neurobiological changes that are reflected in biomarker alterations detectable in blood, cerebrospinal fluid (CSF) and with brain imaging. As accessible proxies for processes that are difficult to measure, biomarkers are tools that hold increasingly important roles in drug development and clinical trial decision making. In the past few years, biomarkers have been the basis for accelerated approval of new therapies for Alzheimer disease and amyotrophic lateral sclerosis as surrogate end points reasonably likely to predict clinical benefit.Blood-based biomarkers are emerging for Alzheimer disease and other neurodegenerative disorders (for example, Parkinson disease, frontotemporal dementia), and some biomarkers may be informative across multiple disease states. Collection of CSF provides access to biomarkers not available in plasma, including markers of synaptic dysfunction and neuroinflammation. Molecular imaging is identifying an increasing array of targets, including amyloid plaques, neurofibrillary tangles, inflammation, mitochondrial dysfunction and synaptic density. In this Review, we consider how biomarkers can be implemented in clinical trials depending on their context of use, including providing information on disease risk and/or susceptibility, diagnosis, prognosis, pharmacodynamic outcomes, monitoring, prediction of response to therapy and safety. Informed choice of increasingly available biomarkers and rational deployment in clinical trials support drug development decision making and de-risk the drug development process for neurodegenerative disorders.

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.

Central Nervous System Macrophages in Health and Disease

The central nervous system (CNS) has a unique set of macrophages that seed the tissue early during embryonic development. Microglia reside in the parenchyma, and border-associated macrophages are present in border regions, including the meninges, perivascular spaces, and choroid plexus. CNS-resident macrophages support brain homeostasis during development and steady state. In the diseased brain, however, the immune landscape is altered, with phenotypic and transcriptional changes in resident macrophages and the invasion of blood-borne monocytes, which differentiate into monocyte-derived macrophages upon entering the CNS. In this review, we focus on the fate and function of the macrophage compartment in health, neurodegenerative conditions such as amyloidosis, and neuroinflammation as observed in multiple sclerosis and infection. We discuss our current understanding that monocyte-derived macrophages contribute to neuropathology whereas native macrophages play a neuroprotective role in disease.

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 functions in Alzheimer's disease: insights from human models

Microglia, key orchestrators of the brain's immune responses, play a pivotal role in the progression of Alzheimer's disease (AD). Emerging human models, including stem cell-derived microglia and cerebral organoids, are transforming our understanding of microglial contributions to AD pathology. In this review, we highlight how these models have uncovered human-specific microglial responses to amyloid plaques and their regulation of neuroinflammation, which are not recapitulated in animal models. We also illustrate how advanced human models that better mimic brain physiology and AD pathology are providing unprecedented insights into the multifaceted roles of microglia. These innovative approaches, combined with sophisticated technologies for cell editing and analysis, are shaping AD research and opening new avenues for therapeutic interventions targeting microglia.

Acute TREM2 inhibition depletes MAFB-high microglia and hinders remyelination

We investigated the role of Triggering Receptor Expressed on Myeloid cells 2 (TREM2) in myelin regeneration in the brain. TREM2 is a receptor that activates microglia, which are crucial for clearing myelin debris and promoting remyelination. Previous studies in a mouse model of demyelination induced by the copper-chelating agent Cuprizone (CPZ) have shown that stimulation of TREM2 with a monoclonal antibody reduces demyelination, while deleting the Trem2 gene in mice impairs remyelination. Here, we blocked TREM2 function acutely with an antibody during both the demyelination and remyelination phases of the CPZ model and analyzed the impact of the antibody treatment on myelination and gene expression in single cells. We found that blocking TREM2 depleted a distinct population of microglia with high expression of the transcription factor MAFB during remyelination. The loss of these MAFB-high microglia was linked to impaired generation of myelinating oligodendrocytes. Importantly, we identified MAFB+ microglia in acute and acute-chronic brain lesions from individuals with multiple sclerosis (MS), but not in inactive lesions. We conclude that TREM2 is essential for maintaining a population of MAFB-high microglia that is associated with myelin repair. This finding has significant implications for understanding demyelinating diseases like MS and suggests that stimulating TREM2 could be a promising therapeutic approach for myelin repair.

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.

Microglia efferocytosis: an emerging mechanism for the resolution of neuroinflammation in Alzheimer's disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by significant neuroinflammatory responses. Microglia, the immune cells of the central nervous system, play a crucial role in the pathophysiology of AD. Recent studies have indicated that microglial efferocytosis is an important mechanism for clearing apoptotic cells and cellular debris, facilitating the resolution of neuroinflammation. This review summarizes the biological characteristics of microglia and the mechanisms underlying microglial efferocytosis, including the factors and signaling pathways that regulate efferocytosis, the interactions between microglia and other cells that influence this process, and the role of neuroinflammation in AD. Furthermore, we explore the role of microglial efferocytosis in AD from three perspectives: its impact on the clearance of amyloid plaques, its regulation of neuroinflammation, and its effects on neuroprotection. Finally, we summarize the current research status on enhancing microglial efferocytosis to alleviate neuroinflammation and improve AD, as well as the future challenges of this approach as a therapeutic strategy for AD.

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.

Deer antler polypeptides inhibit microglial activation via TREM2 to improve behavior and neuroinflammation in CUMS mice

To investigate the effects and mechanisms of deer antler total polypeptides (VAP-T) and its active component Y (VAP-Y) in treating depression in CUMS mice.VAP-T improved depression-like behavior in CUMS mice, reduced microglial activation, and tissue inflammation. VAP-Y showed better improvement in depression-like behavior in CUMS mice compared to VAP-T, significantly inhibiting microglial activation and tissue inflammation. Chromatographic analysis of VAP-Y revealed that short peptides had good binding activity with TREM2. VAP-T and VAP-Y have significant improvement effects on depression-like behavior in CUMS mice, which is related to the inhibition of TREM2-mediated microglial activation. VAP-Y has better activity than the total polypeptides and is a polypeptide with potential for treating depression.

Enhancing TREM2 expression activates microglia and modestly mitigates tau pathology and neurodegeneration

TREM2, a microglia-specific receptor, is strongly associated with Alzheimer's disease (AD) risk, mediating microglial responses to amyloid pathology critical to AD development. However, its role in tau pathology and neurodegeneration remains unclear. Using the PS19 tauopathy mouse model with inducible overexpression of human wild-type TREM2 (TREM2-WT) or the R47H variant (TREM2-R47H), we show that increasing TREM2-WT expression modestly reduces soluble phosphorylated tau levels and mildly preserves neuronal integrity. Single-cell RNA sequencing reveals that TREM2-WT robustly enhances microglial activation, characterized by a disease-associated microglia (DAM) signature. In contrast, TREM2-R47H overexpression exhibits a loss-of-function phenotype, with no significant impact on tau levels, neurodegeneration, or microglial activation. These findings highlight the role of TREM2 in modulating microglial activity and its influence on tau pathology and neurodegeneration, providing important insights for the future development of therapies targeting TREM2 or microglial pathways in AD or other tauopathies.

Monocyte-derived macrophages act as reinforcements when microglia fall short in Alzheimer's disease

The central nervous system (CNS) is endowed with its own resident innate immune cells, the microglia. They constitute approximately 10% of the total cells within the CNS parenchyma and act as 'sentinels', sensing and mitigating any deviation from homeostasis. Nevertheless, under severe acute or chronic neurological injury or disease, microglia are unable to contain the damage, and the reparative activity of monocyte-derived macrophages (MDMs) is required. The failure of the microglia under such conditions could be an outcome of their prolonged exposure to hostile stimuli, leading to their exhaustion or senescence. Here, we describe the conditions under which the microglia fall short, focusing mainly on the context of Alzheimer's disease, and shed light on the functions performed by MDMs. We discuss whether and how MDMs engage in cross-talk with the microglia, why their recruitment is often inadequate, and potential ways to augment their homing to the brain in a well-controlled manner.

Lipid droplets in the nervous system: involvement in cell metabolic homeostasis

Lipid droplets serve as primary storage organelles for neutral lipids in neurons, glial cells, and other cells in the nervous system. Lipid droplet formation begins with the synthesis of neutral lipids in the endoplasmic reticulum. Previously, lipid droplets were recognized for their role in maintaining lipid metabolism and energy homeostasis; however, recent research has shown that lipid droplets are highly adaptive organelles with diverse functions in the nervous system. In addition to their role in regulating cell metabolism, lipid droplets play a protective role in various cellular stress responses. Furthermore, lipid droplets exhibit specific functions in neurons and glial cells. Dysregulation of lipid droplet formation leads to cellular dysfunction, metabolic abnormalities, and nervous system diseases. This review aims to provide an overview of the role of lipid droplets in the nervous system, covering topics such as biogenesis, cellular specificity, and functions. Additionally, it will explore the association between lipid droplets and neurodegenerative disorders. Understanding the involvement of lipid droplets in cell metabolic homeostasis related to the nervous system is crucial to determine the underlying causes and in exploring potential therapeutic approaches for these diseases.

Unraveling the Controversy: The Causal Link Between Osteoarthritis and Alzheimer's Disease

OBJECTIVES

Research on osteoarthritis (OA) and Alzheimer's disease (AD) is currently highly controversial, and the upstream and downstream relationships between them remain unclear. This study aimed to assess the association between OA and AD using Mendelian randomization (MR).

METHOD

Summary data from genome-wide association studies (GWAS) were obtained for OA and AD. Single nucleotide polymorphisms (SNPs) were selected as instrumental variables (IVs), and significant (p < 5.0 × 10-8) and independent (r2 < 0.001) SNPs were extracted for two-sample MR analyses. Inverse variance weighting (IVW) was used to assess these causal relationships, and meta-analysis was used to combine MR results from multiple IVWs. Confounders were assessed by multivariate Mendelian randomization (MVMR). Results were reported as odds ratios (OR). Heterogeneity was then tested using Cochran's Q test, multiplicity was tested using the MR-Egger intercept and MR-PRESSO, and sensitivity analyses were performed using the leave-one-out sensitivity test.

RESULTS

The MR results showed a positive causal effect of AD and OA (IVW OR = 19.89, 95% CI = 2.90-136.57, p = 0.002; OR = 1.28, 95% CI = 1.11-1.47, p = 0.017; OR = 1.27, 95% CI = 1.11-1.46, p = 0.017) and no significance of the reverse MR results (p > 0.05). Meta-analysis of the MR results confirmed this finding and was significant in all population subgroups (OR = 1.29, 95% CI = 1.18-1.40). The findings were maintained after controlling confounders using MVMR (OR = 6.75, 95% CI = 1.50-30.44, p = 0.013). These analyses were confirmed to be reliable and stable by sensitivity testing.

CONCLUSIONS

Our study found a positive causal effect of OA and AD, which was confirmed by the highest levels of evidence-based medicine. It may provide meaningful evidence for the current controversy.

Circadian rhythm, microglia-mediated neuroinflammation, and Alzheimer's disease

Microglia, the brain's resident macrophages, are key mediators of neuroinflammation, responding to immune pathogens and toxins. They play a crucial role in clearing cellular debris, regulating synaptic plasticity, and phagocytosing amyloid-β (Aβ) plaques in Alzheimer's disease (AD). Recent studies indicate that microglia not only exhibit intrinsic circadian rhythms but are also regulated by circadian clock genes, influencing specific functions such as phagocytosis and the modulation of neuroinflammation. Disruption of the circadian rhythm is closely associated with AD pathology. In this review, we will provide an overview of how circadian rhythms regulate microglia-mediated neuroinflammation in the progression of AD, focusing on the pathway from the central nervous system (CNS) and the peripheral immune system. We also discuss potential therapeutic targets, including hormone modulation, lifestyle interventions, and anti-inflammatory therapies, aimed at maintaining brain health in AD. This will shed light on the involvement of circadian rhythm in AD and explore new avenues for AD treatment.

The impact of rare genetic variants on Alzheimer disease

Alzheimer disease (AD) is a progressive neurodegenerative disease with a strong genetic component. Although autosomal dominant mutations and common risk variants in AD risk have been extensively studied, the genetic underpinning of polygenic AD remains incompletely understood. Rare variants could elucidate part of the missing heritability in AD. Rare variant research gained momentum with the discovery of a rare variant in TREM2, along with loss-of-function variants in ABCA7 and SORL1, and has come into full bloom in recent years. Not only has the number of rare variant discoveries increased through large-scale whole-exome and genome sequencing studies, improved imputation in genome-wide association studies and increased focus on understudied populations, the number of studies mapping the functional effects of several of these rare variants has also significantly increased, leading to insights in the pathogenesis of AD and drug development. Here we provide a comprehensive overview of the known and novel rare variants implicated in AD risk, highlighting how they shine new light on AD pathophysiology and provide new inroads for drug development. We will review their impact on individual, familial and population levels, and discuss the potential and challenges of rare variants in genetic risk prediction.

Role of triggering receptor expressed on myeloid cells 2 in the pathogenesis of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a progressive disease. Without effective interventions, NAFLD can gradually develop to non-alcoholic steatohepatitis, fatty liver fibrosis, liver cirrhosis and even hepatocellular carcinoma. It is still to investigate the precise molecular mechanism behind the pathophysiology of NAFLD. Triggering receptor expressed on myeloid cells 2 (TREM2) can sense tissue injury and mediate immune remodeling, thereby inducing phagocytosis, lipid metabolism, and metabolic transfer, promoting cell survival and combating inflammatory activation. NAFLD might develop as a result of TREM2's regulatory role. We here briefly summarize the biological characteristics of TREM2 and its functions in the disease progression of NAFLD. Moreover, we propose to broaden the therapeutic strategy for NAFLD by targeting TREM2.

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.

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.

Pathogenesis and therapeutic applications of microglia receptors in Alzheimer's disease

Microglia, the resident immune cells of the central nervous system, continuously monitor the brain's microenvironment through their array of specific receptors. Once brain function is altered, microglia are recruited to specific sites to perform their immune functions, including phagocytosis of misfolded proteins, cellular debris, and apoptotic cells to maintain homeostasis. When toxic substances are overproduced, microglia are over-activated to produce large amounts of pro-inflammatory cytokines, which induce chronic inflammatory responses and lead to neurotoxicity. Additionally, microglia can also monitor and protect neuronal function through microglia-neuron crosstalk. Microglia receptors are important mediators for microglia to receive external stimuli, regulate the functional state of microglia, and transmit signals between cells. In this paper, we first review the role of microglia-expressed receptors in the pathogenesis and treatment of Alzheimer's disease; moreover, we emphasize the complexity of targeting microglia for therapeutic interventions in neurodegenerative disorders to inform the discovery of new biomarkers and the development of innovative therapeutics.

Immune Modulation in Alzheimer's Disease: From Pathogenesis to Immunotherapy

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the leading cause of dementia, affecting a significant proportion of the elderly population. AD is characterized by cognitive decline and functional impairments due to pathological hallmarks like amyloid β-peptide (Aβ) plaques and neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau. Microglial activation, chronic neuroinflammation, and disruptions in neuronal communication further exacerbate the disease. Emerging research suggests that immune modulation could play a key role in AD treatment given the significant involvement of neuroinflammatory processes. This review focuses on recent advancements in immunotherapy strategies aimed at modulating immune responses in AD, with a specific emphasis on microglial behavior, amyloid clearance, and tau pathology. By exploring these immunotherapeutic approaches, we aim to provide insights into their potential to alter disease progression and improve patient outcomes, contributing to the evolving landscape of AD treatment.

Immune mechanisms and shared immune targets in neurodegenerative diseases

The immune system plays a major part in neurodegenerative diseases. In some, such as multiple sclerosis, it is the primary driver of the disease. In others, such as Alzheimer disease, amyotrophic lateral sclerosis and Parkinson disease, it has an amplifying role. Immunotherapeutic approaches that target the adaptive and innate immune systems are being explored for the treatment of almost all neurological diseases, and the targets and approaches are often common across diseases. Microglia are the primary immune cells in the brain that contribute to disease pathogenesis, and are consequently a common immune target for therapy. Other therapeutic approaches target components of the peripheral immune system, such as regulatory T cells and monocytes, which in turn act within the CNS. This Review considers in detail how microglia, monocytes and T cells contribute to the pathogenesis of multiple sclerosis, Alzheimer disease, amyotrophic lateral sclerosis and Parkinson disease, and their potential as shared therapeutic targets across these diseases. The microbiome is also highlighted as an emerging therapeutic target that indirectly modulates the immune system. Therapeutic approaches being developed to target immune function in neurodegenerative diseases are discussed, highlighting how immune-based approaches developed to treat one disease could be applicable to multiple other neurological diseases.

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.

The etiology and prevention of early-stage tau pathology in higher cortical circuits: Insights from aging rhesus macaques

Aging rhesus macaques provide a unique model for learning how age and inflammation drive early-stage pathology in sporadic Alzheimer's disease, and for testing potential therapeutics. Unlike mice, aging macaques have extensive association cortices and inflammatory signaling similar to humans, are apolipoprotein E ε4 homozygotes, and naturally develop tau and amyloid pathology with marked cognitive deficits. Importantly, monkeys provide the unique opportunity to study early-stage, soluble hyperphosphorylated tau (p-tau), including p-tau217. As soluble p-tau is rapidly dephosphorylated post mortem, it is not captured in human brains except with biopsy material. However, new macaque data show that soluble p-tau is toxic to neurons and capable of seeding across cortical circuits. Extensive evidence indicates that age-related inflammatory signaling contributes to calcium dysregulation, which drives tau hyperphosphorylation and amyloid beta generation. Pharmacological studies in aged macaques suggest that inhibiting inflammation and restoring calcium regulation can reduce tau hyperphosphorylation with minimal side effects, appropriate for potential preventive therapeutics. HIGHLIGHTS: Aging monkeys provide a unique window into early stage, soluble phosphorylated tau (p-tau). Inflammation with advancing age leads to calcium dysregulation, p-tau, and amyloid beta (Aβ). Macaque research shows p-tau undergoes transsynaptic seeding early in the cortex. p-tau traps amyloid precursor protein-containing endosomes, which may increase Aβ and drive vicious cycles. Restoring calcium regulation in cortex reduced p-tau217 levels in aged macaques.

Osteopontin in Alzheimer's Disease: A Double-Edged Sword in Neurodegeneration and Neuroprotection-A Systematic Review

BACKGROUND

Osteopontin (OPN) has emerged as a pivotal molecule in Alzheimer's disease (AD), with studies indicating its potential to act as both a neuroprotective agent and a contributor to neurodegeneration. This systematic review aims to elucidate the roles of OPN in AD pathogenesis through inflammatory pathways.

METHODS

We conducted a comprehensive analysis of current literature on OPN's involvement in AD, focusing on its signaling pathways, cellular interactions, and regulatory mechanisms. We searched PubMed, EMBASE, and Scopus databases by the keyword of Alzheimer's Disease and Osteopontin. Our date search was in 1990 until July 1, 2024 with no language limitation.

RESULTS

In a review of 758 studies, a total of 15 reports met the eligibility criteria and were included. Among the findings, four studies provided evidence supporting the protective mechanism of OPN within the context of AD. Eleven studies explain the inflammatory role of OPN. OPN has been shown to play a role in synaptic pruning, microglial activation, and the inflammatory processes associated with AD. Additionally, OPN is implicated in facilitating cellular communication and serves as a chemotactic molecule. It is suggested that the protective effects of OPN are predominantly mediated by the c fragment of the protein and are most prominent in the early stages of AD progression.

CONCLUSION

OPN in AD has dual effects-protecting neurons and contributing to their degeneration. Future research should enhance its protective mechanisms, target specific signaling pathways, and develop therapies to slow AD progression.

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.

Roles of blood monocytes carrying TREM2R47H mutation in pathogenesis of Alzheimer's disease and its therapeutic potential in APP/PS1 mice

INTRODUCTION

The triggering receptor expressed on myeloid cells 2 (TREM2) arginine-47-histidine (R47H) mutation is a significant risk for Alzheimer's disease (AD) with unclear mechanisms. Previous studies focused on microglial amyloid-β (Aβ) phagocytosis with less attention on the impact of TREM2R47H mutation on blood monocytes.

METHODS

Bone marrow transplantation (BMT) models were used to assess the contribution of blood monocytes carrying TREM2R47H mutation to AD.

RESULTS

Aβ phagocytosis was compromised in mouse monocytes carrying the TREM2R47H mutation. Transplantation of bone marrow cells (BMCs) carrying TREM2R47H mutation increased cerebral Aβ burden and aggravated AD-type pathologies. Moreover, the replacement of TREM2R47H-BMCs restored monocytic Aβ phagocytosis, lowered Aβ levels in the blood and brain, and improved cognitive function.

DISCUSSION

Our study reveals that blood monocytes carrying the TREM2R47H mutation substantially contribute to the pathogenesis of AD, and correcting the TREM2R47H mutation in BMCs would be a potential therapeutic approach for those carrying this mutation.

HIGHLIGHTS

TREM2R47H mutation compromises the Aβ phagocytosis of blood monocytes. Blood monocytes carrying TREM2R47H mutation contribute substantially to AD pathogenesis. Correction of the TREM2R47H mutation in bone marrow cells ameliorates AD pathologies and cognitive impairments.

Rescue of in vitro models of CSF1R-related adult-onset leukodystrophy by iluzanebart: mechanisms and therapeutic implications of TREM2 agonism

Microglia dysfunction is implicated in several neurodegenerative disorders, including a rare microgliopathy; CSF1R-related adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (CSF1R-ALSP). CSF1R-ALSP is caused by heterozygous loss-of-function mutations in the colony stimulating factor 1 receptor (CSF1R) gene, which encodes a receptor required for the differentiation of myeloid cells, as well as for microglial survival and proliferation. Similar functions have also been ascribed to triggering receptor expressed on myeloid cells 2 (TREM2), which shares an analogous microglia enrichment profile and converging intracellular signaling pathway mediated by spleen associated tyrosine kinase (SYK) and phosphoinositide-3-kinase (PI3K). Iluzanebart is a human monoclonal IgG1, human TREM2 (hTREM2) agonist antibody under development for the treatment of CSF1R-ALSP. To explore the therapeutic hypothesis that loss of CSF1R signaling and related microglial hypofunction can be circumvented via activation of TREM2, we evaluated the potential of iluzanebart to compensate for CSF1R loss-of-function. Herein, we demonstrate that iluzanebart is a potent, dose-dependent, and specific activator of TREM2 signaling in human primary cells. Iluzanebart treatment rescued viability of human monocyte-derived macrophages (hMDM) and induced pluripotent stem cell-derived human microglia (iMGL) in multiple in vitro models of CSF1R-ALSP, including in induced pluripotent stem cell (iPSC) differentiated microglia carrying the heterozygous I794T mutation found in CSF1R-ALSP patients. Additionally, iluzanebart treatment in microglia modulated surface levels of CSF1R, resulting in increased receptor activation as measured by phosphorylation of CSF1R. Differentially expressed genes identified in the hippocampus of mice treated with iluzanebart were exemplary of TREM2 activation and were related to cell proliferation, regulation of inflammatory processes, and innate immune response pathways. Proliferation of microglia, changes in protein levels of specific chemokines identified by gene expression analysis, and increased CSF1R levels were also confirmed in vivo. These findings demonstrate that iluzanebart is a potent and selective TREM2 agonistic antibody, with pharmacology that supports the hypothesis that TREM2 activation can compensate for CSF1R dysfunction and its continued clinical development for individuals with CSF1R-ALSP.

Multi-target-directed therapeutic strategies for Alzheimer's disease: controlling amyloid-β aggregation, metal ion homeostasis, and enzyme inhibition

Alzheimer's disease (AD) is the most prevalent neurodegenerative dementia, marked by progressive cognitive decline and memory impairment. Despite advances in therapeutic research, single-target-directed treatments often fall short in addressing the complex, multifactorial nature of AD. This arises from various pathological features, including amyloid-β (Aβ) aggregate deposition, metal ion dysregulation, oxidative stress, impaired neurotransmission, neuroinflammation, mitochondrial dysfunction, and neuronal cell death. This review illustrates their interrelationships, with a particular emphasis on the interplay among Aβ, metal ions, and AD-related enzymes, such as β-site amyloid precursor protein cleaving enzyme 1 (BACE1), matrix metalloproteinase 9 (MMP9), lysyl oxidase-like 2 (LOXL2), acetylcholinesterase (AChE), and monoamine oxidase B (MAOB). We further underscore the potential of therapeutic strategies that simultaneously inhibit Aβ aggregation and address other pathogenic mechanisms. These approaches offer a more comprehensive and effective method for combating AD, overcoming the limitations of conventional therapies.

"Current and emerging drug therapies in Alzheimer's disease: A pathophysiological Perspective"

The analytical and experimental investigation of several targets and biomarkers that help in explaining significant cognitive deficits, covering drug development and precision medicine aimed at different chronic neurodegenerative conditions such as Alzheimer's disease (AD), Parkinson's disease, synaptic dysfunction, brain damage from neuronal apoptosis, and other disease pathologies; this served as the foundation for all phase studies. The focus of current therapeutic approaches is on developing humanized antibodies, agonist and antagonist drugs, receptors, signaling molecules, major targeted drug-metabolizing enzymes, and other metabolites to treat neurodegeneration in the AD brain brought on by tau hyperphosphorylation, amyloid plagues, or other cholinergic effects. The five A's-amnesia, agnosia, aphasia, apraxia, and anomia-are the typical symptoms associated with AD. While the main goal of drug therapeutics studies is modified amino acids acting as pro-drugs, pharmacokinetics studies and trends in evaluating drug-drug interactions focus on interactions between drugs and antibodies, drugs and therapeutic biologics like metabolites, herbs, interleukin-based, and gene silencing mechanism-based. Studies on the biotransformation of xenobiotic compounds and the metabolism of exogenous and endogenous substances are conducted under Phase I, Phase II, and Phase III trials because the pivotal pharmacokinetic properties of drugs, such as absorption, distribution, metabolism, and excretion (ADME), aid in understanding variations in the crucial improvement of various target drugs. This review also highlights the developments in soon-to-be genetically created targeted medications that may serve as ground-breaking treatments for cholinergic illnesses in the brains of AD patients and other neurodegenerative conditions.

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.

Uncovering the epigenetic regulatory clues of PRRT1 in Alzheimer's disease: a strategy integrating multi-omics analysis with explainable machine learning

BACKGROUND

Alzheimer's disease (AD) is a complex neurodegenerative disorder with a largely unexplored epigenetic landscape.

OBJECTIVE

This study employs an innovative approach that integrates multi-omics analysis and explainable machine learning to explore the epigenetic regulatory mechanisms underlying the epigenetic signature of PRRT1 implicated in AD.

METHODS

Through comprehensive DNA methylation and transcriptomic profiling, we identified distinct epigenetic signatures associated with gene PRRT1 expression in AD patient samples compared to healthy controls. Utilizing interpretable machine learning models and ELMAR analysis, we dissected the complex relationships between these epigenetic signatures and gene expression patterns, revealing novel regulatory elements and pathways. Finally, the epigenetic mechanisms of these genes were investigated experimentally.

RESULTS

This study identified ten epigenetic signatures, constructed an interpretable AD diagnostic model, and utilized various bioinformatics methods to create an epigenomic map. Subsequently, the ELMAR R package was used to integrate multi-omics data and identify the upstream transcription factor MAZ for PRRT1. Finally, experiments confirmed the interaction between MAZ and PRRT1, which mediated apoptosis and autophagy in AD.

CONCLUSION

This study adopts a strategy that integrates bioinformatics analysis with molecular experiments, providing new insights into the epigenetic regulatory mechanisms of PRRT1 in AD and demonstrating the importance of explainable machine learning in elucidating complex disease mechanisms.

Exploring the orphan immune receptor TREM2 and its non-protein ligands: In silico characterization

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immunoreceptor that interacts with a wide range of non-protein ligands, and it has been implicated in infectious and non-infectious diseases. However, there is a limited understanding on how this receptor interacts with non-protein ligands and the potential of such information to develop new therapeutic drugs. Therefore, our study aimed to elucidate the interactions between TREM2 and its non-protein ligands. First, we searched PubChem and Protein Data Bank (PDB) for TREM2 structures and their corresponding non-protein ligands. Subsequently, these structures were employed in molecular docking and MM/GBSA simulations with the Maestro software and molecular dynamics in GROMACS software. TREM2 was subsequently subjected to druggable site prediction using CavityPlus and receptor-based drug repositioning via the DrugRep server. TREM2 interacts with high affinity with its 12 non-protein ligands, with affinity values ranging from -33.01 kcal/mol for phosphatidylserine to -80.87 kcal/mol for cardiolipin (CLP). In molecular dynamics simulations, homodimeric TREM2 bound more stably to its lipid ligands, such as CLP and PSF, whereas it was unstable when unbound. The interactions between the receptor and its non-protein ligands were driven by the complementarity determining regions (CDR) 1 and 2, that are present in the hydrophobic and positively charged regions, highlighting that the Y38-R98 region is fundamental for drugs targeting TREM2. Our data underscore the significance of TREM2's CDRs in recognizing its ligands, suggesting they as promising targets for prospective drug design studies.

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.

TREM2 in Regulating Macrophage Inflammatory Responses and Disease Pathogenesis

Triggering receptor expressed on myeloid cells 2 (TREM2) is a cell surface receptor belonging to the TREM family that is predominantly expressed on myeloid cells such as granulocytes, monocytes, osteocytes, macrophages, and microglia. Although much of the functionality of TREM2 is not well understood at the molecular level, it is well established that TREM2 plays a significant role in the regulation of a broad definition of macrophage inflammatory responses. Dysregulation of TREM2 has been implicated in a large number of diseases including Alzheimer's disease, Nasu-Hakola disease, bone-related diseases, and atherosclerosis. The TREM2 gene is highly conserved evolutionarily and at the level of controlling its expression. The function of TREM2 is highly conserved across the broad definition of macrophages, including microglia, osteoclasts, and vascular macrophages. This genetic and physiological "niche conservatism" strongly suggests its pivotal role in regulating inflammatory responses. This mini-review summarizes our current understanding of the structure, expression, and function of TREM2 in the pathogenesis of macrophage-mediated diseases.