The Emerging Roles and Therapeutic Potential of Soluble TREM2 in Alzheimer's Disease

Novel insights into the role of TREM2 in cerebrovascular diseases

Cerebrovascular diseases (CVDs) include conditions such as stroke, cerebral amyloid angiopathy (CAA) and cerebral small vessel disease (CSVD), which contribute significantly to global morbidity and healthcare burden. The pathophysiology of CVD is complex, involving inflammatory, cellular and vascular mechanisms. Recently, research has focused on triggering receptor expressed on myeloid cells 2 (TREM2), an immune receptor predominantly found on microglia. TREM2 interacts with multiple signalling pathways, particularly toll-like receptor 4 (TLR4) and nuclear factor kappa B (NF-κB), inhibiting patients' inflammatory response. This receptor plays an essential role in both immune regulation and neuroprotection. TREM2 deficiency or dysfunction is associated with impaired microglial responses, exacerbated neurodegeneration and neuroinflammation. Up until recently, TREM2 related studies have focused on neurodegenerative diseases (NDs), however a shift in focus towards CVDs is beginning to take place. Advancements in CVD research have focused on developing therapeutic strategies targeting TREM2 to enhance recovery and reduce long-term deficits. These include the exploration of TREM2 agonists and combination therapies with other anti-inflammatory agents, which may synergistically reduce neuroinflammation and promote neuroprotection. The modulation of TREM2 activity holds potential for innovative treatment approaches aimed at improving patient outcomes following cerebrovascular insults. This review compiles current research on TREM2, emphasising its molecular mechanisms, therapeutic potential, and advancements in CNS disease research.

An interpretable generative multimodal neuroimaging-genomics framework for decoding alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia, affecting millions worldwide with a progressive decline in cognitive abilities. The AD continuum encompasses a prodromal stage known as Mild Cognitive Impairment (MCI), where patients may either progress to AD (MCIc) or remain stable (MCInc). Understanding the underlying mechanisms of AD requires complementary analyses relying on different data sources, leading to the development of multimodal deep learning models. In this study, we leveraged structural and functional Magnetic Resonance Imaging (sMRI/fMRI) to investigate the disease-induced grey matter and functional network connectivity changes. Moreover, considering AD's strong genetic component, we introduced Single Nucleotide Polymorphisms (SNPs) as a third channel. Given such diverse inputs, missing one or more modalities is a typical concern of multimodal methods. We hence propose a novel deep learning-based classification framework where a generative module employing Cycle Generative Adversarial Networks (cGAN) was adopted for imputing missing data within the latent space. Additionally, we adopted an Explainable Artificial Intelligence (XAI) method, Integrated Gradients (IG), to extract input features' relevance, enhancing our understanding of the learned representations. Two critical tasks were addressed: AD detection and MCI conversion prediction. Experimental results showed that our framework was able to reach the state-of-the-art in the classification of CN vs AD with an average test accuracy of 0.926 ± 0.02. For the MCInc vs MCIc task, we achieved an average prediction accuracy of 0.711 ± 0.01 using the pre-trained model for CN and AD. The interpretability analysis revealed that the classification performance was led by significant grey matter modulations in cortical and subcortical brain areas well known for their association with AD. Moreover, impairments in sensory-motor and visual resting state network connectivity along the disease continuum, as well as mutations in SNPs defining biological processes linked to endocytosis, amyloid-beta, and cholesterol, were identified as contributors to the achieved performance. Overall, our integrative deep learning approach shows promise for AD detection and MCI prediction, while shading light on important biological insights.

Current understanding on TREM-2 molecular biology and physiopathological functions

Triggering receptor expressed on myeloid cells 2 (TREM-2), a glycosylated receptor belonging to the immunoglobin superfamily and especially expressed in the myeloid cell lineage, is frequently explained as a reminiscent receptor for both adaptive and innate immunity regulation. TREM-2 is also acknowledged to influence NK cell differentiation via the PI3K and PLCγ signaling pathways, as well as the partial activation or direct inhibition of T cells. Additionally, TREM-2 overexpression is substantially linked to cell-specific functions, such as enhanced phagocytosis, reduced toll-like receptor (TLR)-mediated inflammatory cytokine production, increased transcription of anti-inflammatory cytokines, and reshaped T cell function. Whereas TREM-2-deficient cells exhibit diminished phagocytic function and enhanced proinflammatory cytokines production, proceeding to inflammatory injuries and an immunosuppressive environment for disease progression. Despite the growing literature supporting TREM-2+ cells in various diseases, such as neurodegenerative disorders and cancer, substantial facets of TREM-2-mediated signaling remain inadequately understood relevant to pathophysiology conditions. In this direction, herein, we have summarized the current knowledge on TREM-2 biology and cell-specific TREM-2 expression, particularly in the modulation of pivotal TREM-2-dependent functions under physiopathological conditions. Furthermore, molecular regulation and generic biological relevance of TREM-2 are also discussed, which might provide an alternative approach for preventing or reducing TREM-2-associated deformities. At last, we discussed the TREM-2 function in supporting an immunosuppressive cancer environment and as a potential drug target for cancer immunotherapy. Hence, summarized knowledge of TREM-2 might provide a window to overcome challenges in clinically effective therapies for TREM-2-induced diseases in humans.

Osteopontin characterizes bile duct-associated macrophages and correlates with liver fibrosis severity in primary sclerosing cholangitis

BACKGROUND AND AIMS

Primary sclerosing cholangitis (PSC) is an immune-mediated cholestatic liver disease for which pharmacological treatment options are currently unavailable. PSC is strongly associated with colitis and a disruption of the gut-liver axis, and macrophages are involved in the pathogenesis of PSC. However, how gut-liver interactions and specific macrophage populations contribute to PSC is incompletely understood.

APPROACH AND RESULTS

We investigated the impact of cholestasis and colitis on the hepatic and colonic microenvironment, and performed an in-depth characterization of hepatic macrophage dynamics and function in models of concomitant cholangitis and colitis. Cholestasis-induced fibrosis was characterized by depletion of resident KCs, and enrichment of monocytes and monocyte-derived macrophages (MoMFs) in the liver. These MoMFs highly express triggering-receptor-expressed-on-myeloid-cells-2 ( Trem2 ) and osteopontin ( Spp1 ), markers assigned to hepatic bile duct-associated macrophages, and were enriched around the portal triad, which was confirmed in human PSC. Colitis induced monocyte/macrophage infiltration in the gut and liver, and enhanced cholestasis-induced MoMF- Trem2 and Spp1 upregulation, yet did not exacerbate liver fibrosis. Bone marrow chimeras showed that knockout of Spp1 in infiltrated MoMFs exacerbates inflammation in vivo and in vitro , while monoclonal antibody-mediated neutralization of SPP1 conferred protection in experimental PSC. In human PSC patients, serum osteopontin levels are elevated compared to control, and significantly increased in advanced stage PSC and might serve as a prognostic biomarker for liver transplant-free survival.

CONCLUSIONS

Our data shed light on gut-liver axis perturbations and macrophage dynamics and function in PSC and highlight SPP1/OPN as a prognostic marker and future therapeutic target in PSC.

Microglia as a Possible Alternative Therapeutic for Dementia

Dementia is a syndrome in which there is deterioration in memory, behavior, and the ability to perform everyday activities. Alzheimer's disease and vascular dementia are the most common forms of dementia. There is evidence supporting the hypothesis that inflammatory and immune mechanisms are involved in dementia. Microglia, the resident macrophage tissues in the central nervous system, play a significant role in neuroinflammation and play an important role in amyloid-β clearance in the brain, and impaired microglial clearance of amyloid-β has also been shown to be involved in the pathogenesis of Alzheimer's disease. However, there is also abundant evidence that microglia have harmful actions in dementia. Once activated, they can mediate uptake at neuronal synapses. They can also exacerbate tau pathology and secrete deleterious inflammatory factors that can directly or indirectly damage neurons. Thus, depending on the stage of the disease, microglia can act both protectively and detrimentally. Therefore, it is still necessary to continue with studies to better understand the role of microglia in the pathology of dementia. Currently available drugs can only improve cognitive symptoms, have no impact on progression and are not curative, so identifying and studying new therapeutic approaches is important. Considering the role played by microglia in this pathology, it has been pointed out as a possible therapeutic approach. This manuscript aims to address the relationship between microglia and dementia and how this relationship could be used for therapeutic purposes.

TREM2 regulates microglial lipid droplet formation and represses post-ischemic brain injury

Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor protein predominantly expressed in microglia within the central nervous system (CNS). TREM2 regulates multiple microglial functions, including lipid metabolism, immune reaction, inflammation, and microglial phagocytosis. Recent studies have found that TREM2 is highly expressed in activated microglia after ischemic stroke. However, the role of TREM2 in the pathologic response after stroke remains unclear. Herein, TREM2-deficient microglia exhibit an impaired phagocytosis rate and cholesteryl ester (CE) accumulation, leading to lipid droplet formation and upregulation of Perilipin-2 (PLIN2) expression after hypoxia. Knockdown of TREM2 results in increased lipid synthesis (PLIN2, SOAT1) and decreased cholesterol clearance and lipid hydrolysis (LIPA, ApoE, ABCA1, NECH1, and NPC2), further impacting microglial phenotypes. In these lipid droplet-rich microglia, the TGF-β1/Smad2/3 signaling pathway is downregulated, driving microglia towards a pro-inflammatory phenotype. Meanwhile, in a neuron-microglia co-culture system under hypoxic conditions, we found that microglia lost their protective effect against neuronal injury and apoptosis when TREM2 was knocked down. Under in vivo conditions, TREM2 knockdown mice express lower TGF-β1 expression levels and a lower number of anti-inflammatory M2 phenotype microglia, resulting in increased cerebral infarct size, exacerbated neuronal apoptosis, and aggravated neuronal impairment. Our work suggests that TREM2 attenuates stroke-induced neuroinflammation by modulating the TGF-β1/Smad2/3 signaling pathway. TREM2 may play a direct role in the regulation of inflammation and also exert an influence on the post-ischemic inflammation and the stroke pathology progression via regulation of lipid metabolism processes. Thus, underscoring the therapeutic potential of TREM2 agonists in ischemic stroke and making TREM2 an attractive new clinical target for the treatment of ischemic stroke and other inflammation-related diseases.

Neurodegeneration and its potential markers in the diagnosing of secondary progressive multiple sclerosis. A review

Approximately 70% of relapsing-remitting multiple sclerosis (RRMS) patients will develop secondary progressive multiple sclerosis (SPMS) within 10-15 years. This progression is characterized by a gradual decline in neurological functionality and increasing limitations of daily activities. Growing evidence suggests that both inflammation and neurodegeneration are associated with various pathological processes throughout the development of MS; therefore, to delay disease progression, it is critical to initiate disease-modifying therapy as soon as it is diagnosed. Currently, a diagnosis of SPMS requires a retrospective assessment of physical disability exacerbation, usually over the previous 6-12 months, which results in a delay of up to 3 years. Hence, there is a need to identify reliable and objective biomarkers for predicting and defining SPMS conversion. This review presents current knowledge of such biomarkers in the context of neurodegeneration associated with MS, and SPMS conversion.

TREM2 receptor protects against complement-mediated synaptic loss by binding to complement C1q during neurodegeneration

Triggering receptor expressed on myeloid cells 2 (TREM2) is strongly linked to Alzheimer's disease (AD) risk, but its functions are not fully understood. Here, we found that TREM2 specifically attenuated the activation of classical complement cascade via high-affinity binding to its initiator C1q. In the human AD brains, the formation of TREM2-C1q complexes was detected, and the increased density of the complexes was associated with lower deposition of C3 but higher amounts of synaptic proteins. In mice expressing mutant human tau, Trem2 haploinsufficiency increased complement-mediated microglial engulfment of synapses and accelerated synaptic loss. Administration of a 41-amino-acid TREM2 peptide, which we identified to be responsible for TREM2 binding to C1q, rescued synaptic impairments in AD mouse models. We thus demonstrate a critical role for microglial TREM2 in restricting complement-mediated synaptic elimination during neurodegeneration, providing mechanistic insights into the protective roles of TREM2 against AD pathogenesis.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Investigation of Potential Drug Targets for Cholesterol Regulation to Treat Alzheimer's Disease

Despite extensive research and seven approved drugs, the complex interplay of genes, proteins, and pathways in Alzheimer's disease remains a challenge. This implies the intricacies of the mechanism for Alzheimer's disease, which involves the interaction of hundreds of genes, proteins, and pathways. While the major hallmarks of Alzheimer's disease are the accumulation of amyloid plaques and tau protein tangles, excessive accumulation of cholesterol is reportedly correlated with Alzheimer's disease patients. In this work, protein-protein interaction analysis was conducted based upon the genes from a clinical database to identify the top protein targets with most data-indicated involvement in Alzheimer's disease, which include ABCA1, CYP46A1, BACE1, TREM2, GSK3B, and SREBP2. The reactions and pathways associated with these genes were thoroughly studied for their roles in regulating brain cholesterol biosynthesis, amyloid beta accumulation, and tau protein tangle formation. Existing clinical trials for each protein target were also investigated. The research indicated that the inhibition of SREBP2, BACE1, or GSK3B is beneficial to reduce cholesterol and amyloid beta accumulation, while the activation of ABCA1, CYP46A1, or TREM2 has similar effects. In this study, Sterol Regulatory Element-Binding Protein 2 (SREBP2) emerged as the primary protein target. SREBP2 serves a pivotal role in maintaining cholesterol balance, acting as a transcription factor that controls the expression of several enzymes pivotal for cholesterol biosynthesis. Novel studies suggest that SREBP2 performs a multifaceted role in Alzheimer's disease. The hyperactivity of SREBP2 may lead to heightened cholesterol biosynthesis, which suggested association with the pathogenesis of Alzheimer's disease. Lowering SREBP2 levels in an Alzheimer's disease mouse model results in reduced production of amyloid-beta, a major contributor to Alzheimer's disease progression. Moreover, its thoroughly analyzed crystal structure allows for computer-aided screening of potential inhibitors; SREBP2 is thus selected as a prospective drug target. While more protein targets can be added onto the list in the future, this work provides an overview of key proteins involved in the regulation of brain cholesterol biosynthesis that may be further investigated for Alzheimer's disease intervention.

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

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

The metal ion hypothesis of Alzheimer's disease and the anti-neuroinflammatory effect of metal chelators

Alzheimer's disease (AD), characterized by the β-amyloid protein (Aβ) deposition and tau hyperphosphorylation, is the most common dementia with uncertain etiology. The clinical trials of Aβ monoclonal antibody drugs have almost failed, giving rise to great attention on the other etiologic hypothesis regarding AD such as metal ions dysmetabolism and chronic neuroinflammation. Mounting evidence revealed that the metal ions (iron, copper, and zinc) were dysregulated in the susceptible brain regions of AD patients, which was highly associated with Aβ deposition, tau hyperphosphorylation, neuronal loss, as well as neuroinflammation. Further studies uncovered that iron, copper and zinc could not only enhance the production of Aβ but also directly bind to Aβ and tau to promote their aggregations. In addition, the accumulation of iron and copper could respectively promote ferroptosis and cuproptosis. Therefore, the metal ion chelators were recognized as promising agents for treating AD. This review comprehensively summarized the effects of metal ions on the Aβ dynamics and tau phosphorylation in the progression of AD. Furthermore, taking chronic neuroinflammation contributes to the progression of AD, we also provided a summary of the mechanisms concerning metal ions on neuroinflammation and highlighted the metal ion chelators may be potential agents to alleviate neuroinflammation under the condition of AD. Nevertheless, more investigations regarding metal ions on neuroinflammation should be taken into practice, and the effects of metal ion chelators on neuroinflammation should gain more attention. Running title: Metal chelators against neuroinflammation.

Anesthesia and surgery-induced elevation of CSF sTREM2 is associated with early cognitive dysfunction after thoracoabdominal aortic dissection surgery

PURPOSE

Soluble triggering receptor expressed on myeloid cells 2 (sTREM2) concentration is increased in cerebrospinal fluid (CSF) in early symptomatic phase of Alzheimer's disease (AD). This study investigated whether CSF sTREM2 has a relationship with early cognitive dysfunction following surgery in cardiac surgery patients.

METHODS

A total of 82 patients undergoing thoracoabdominal aortic replacement were recruited in this study. Neuropsychological testing battery was conducted before and after surgery. Postoperative cognitive dysfunction (POCD) was defined as a Z-score > 1.96 on at least 2 different tests or Telephone Interviews for Cognitive Status-Modified (TICS-M) score < 27. The CSF and serum sTREM2, Aβ₄₂, T-tau and P-tau were collected and measured by ELISA on day before surgery and postoperative day 3.

RESULTS

Patients were classified into POCD (n = 34) and non-POCD (n = 48) groups according to Z-score. Compared to non-POCD group, the levels of CSF sTREM2 (p < 0.001) and serum sTREM2 (p = 0.001) were significantly higher in POCD group on postoperative day 3. The levels of Aβ₄₂ (p = 0.005) and Aβ₄₂/T-tau ratio (p = 0.036) were significantly lower in POCD group on postoperative day 3. Multivariate logistic regression analysis revealed that higher value of postoperative CSF sTREM2 (odds ratio: 1.06, 95% confidence interval: 1.02-1.11, p = 0.009), age (OR: 1.15, 95%CI: 1.03-1.28, p = 0.014) and POD duration (OR: 2.47, 95%CI: 1.15-5.29, p = 0.02) were the risk factors of POCD.

CONCLUSION

This study indicates that anesthesia and surgery-induced elevation of CSF sTREM2 is associated with an increased risk of early cognitive dysfunction following surgery.

Immunotherapies for Alzheimer’s Disease—A Review

Alzheimer’s disease (AD) is a chronic neurodegenerative disorder that falls under the umbrella of dementia and is characterised by the presence of highly neurotoxic amyloid-beta (Aβ) plaques and neurofibrillary tangles (NFTs) of tau protein within the brain. Historically, treatments for AD have consisted of medications that can slow the progression of symptoms but not halt or reverse them. The shortcomings of conventional drugs have led to a growing need for novel, effective approaches to the treatment of AD. In recent years, immunotherapies have been at the forefront of these efforts. Briefly, immunotherapies utilise the immune system of the patient to treat a condition, with common immunotherapies for AD consisting of the use of monoclonal antibodies or vaccines. Most of these treatments target the production and deposition of Aβ due to its neurotoxicity, but treatments specifically targeting tau protein are being researched as well. These treatments have had great variance in their efficacy and safety, leading to a constant need for the research and development of new safe and effective treatments.

Distinct microglia alternative splicing in Alzheimer's disease

Numerous alternative splicing (AS) events have been documented in Alzheimer's disease (AD). However, cell type-specific AS analysis is still lacking. We described AS events in the hippocampal microglia sorted by CD45 and CD11b from Aβ precursor protein (APP) and non-transgenic (Ntg) mice. GSE171195 dataset was downloaded from GEO database, aligned to GRCm39 genome. Skipped exon (SE), alternative 3'SS (A3SS), retained intron (RI), alternative 5'SS (A5SS), and mutually exclusive exons (MXE) were evaluated using rMATS and maser. Differential expressed genes or transcripts were analyzed via limma. Gene ontology and correlation analyses were performed with clusterProfiler and ggcorrplot R packages. 36,340 raw counts of AS were identified, and 95 significant AS events were eventually selected with strict criteria: (1) average coverage >5; (2) delta percent spliced in >0.1. SE was the most common AS events (68.42%), followed by A3SS and RI. Autophagy genes were mainly spliced in SE events, actin depolymerization genes spliced in A3SS events, while synaptic plasticity related genes were mainly spliced in RI pattern. These significant AS events may be regulated by dysregulated splicing factors in AD. In conclusion, we revealed microglia specific AS events in AD, and our study provides novel pathological mechanisms in the pathogenesis of AD.

TREM2 Gene Compound Heterozygosity in Neurodegenerative Disorders

BACKGROUND

Homozygous variants of the TREM2 and TYROBP genes have been shown to be causative for multiple bone cysts and neurodegeneration leading to progressive dementia (NHD, Nasu-Hakola disease).

OBJECTIVE

To determine if biallelic variants of these genes and/or oligogenic inheritance could be responsible for a wider spectrum of neurodegenerative conditions.

METHODS

We analyzed 52 genes associated with neurodegenerative disorders using targeted next generation sequencing in a selected group of 29 patients (n = 14 Alzheimer's disease, n = 8 frontotemporal dementia, n = 7 amyotrophic lateral sclerosis) carrying diverse already determined rare variants in exon 2 of TREM2. Molecular modeling was used to get an insight into the potential effects of the mutation.

RESULTS

We identified a novel mutation c.401_406delinsTCTAT; p.(Asp134Valfs*55) in exon 3 of TREM2 in an Alzheimer's disease patient also carrying the p.Arg62His TREM2 variant. Molecular modeling revealed that the identified mutation prevents anchoring of the TREM2 protein in the membrane, leaving the core of the Ig-like domain intact.

CONCLUSION

Our results expand the spectrum of neurodegenerative diseases, where the carriers of biallelic mutations in TREM2 have been described for Alzheimer's disease, and highlight the impact of variant burden in other genes on phenotypic heterogeneity.

Recent advances in pre-clinical diagnosis of Alzheimer's disease

Alzheimer's disease (AD) is the most common dementia with currently no known cures or disease modifying treatments (DMTs), despite much time and effort from the field. Diagnosis and intervention of AD during the early pre-symptomatic phase of the disease is thought to be a more effective strategy. Therefore, the detection of biomarkers has emerged as a critical tool for monitoring the effect of new AD therapies, as well as identifying patients most likely to respond to treatment. The establishment of the amyloid/tau/neurodegeneration (A/T/N) framework in 2018 has codified the contexts of use of AD biomarkers in neuroimaging and bodily fluids for research and diagnostic purposes. Furthermore, a renewed drive for novel AD biomarkers and innovative methods of detection has emerged with the goals of adding additional insight to disease progression and discovery of new therapeutic targets. The use of biomarkers has accelerated the development of AD drugs and will bring new therapies to patients in need. This review highlights recent methods utilized to diagnose antemortem AD.

The association between plasma soluble triggering receptor expressed on myeloid cells 2 and cognitive impairment after acute ischemic stroke

BACKGROUND

Previous researches have suggested that soluble triggering receptor expressed on myeloid cells 2 (sTREM2) plays a pivotal role in central nervous system pathologies and the development of neurodegenerative disorders. This study aimed to prospectively investigate the association between plasma sTREM2 levels and post-stroke cognitive impairment (PSCI).

METHODS

A sample of 599 consecutive acute ischemic stroke patients with sTREM2 measurements from the China Antihypertensive Trial in Acute Ischemic Stroke were eventually included in this analysis. Cognitive impairment was defined as a score of <25 for Mini-Mental State Examination, measured at 3-month follow up. Binary logistic regression was used to estimate the odds ratios (ORs) of plasma sTREM2 levels on the risk of PSCI.

RESULTS

Of the 599 participants (mean age, 60.0 ± 10.4 years; male, 70.5%), 228 (38.1%) patients were diagnosed as PSCI. The risk of PSCI elevated significantly with higher plasma sTREM2 levels (p for trend <0.01). After adjusting for several confounding factors, the ORs for the highest quartile of sTREM2 compared with the lowest quartile was 2.06 (95% confidence interval, 1.20-3.53) for PSCI. Moreover, the addition of sTREM2 to the conventional model with established risk factors significantly improved risk discrimination (C-statistics increased from 0.668 to 0.691, p = 0.02) and reclassification (net reclassification improvement: 32.2%, p < 0.001; integrated discrimination improvement: 1.3%, p = 0.01) for PSCI.

LIMITATIONS

Results might be subject to selective bias and potential confounding.

CONCLUSIONS

The present study demonstrated that elevated level of plasma sTREM2 may be associated with PSCI, and sTREM2 has potential value in predicting PSCI.

Does Soluble TREM2 Protect Against Alzheimer's Disease?

Triggering Receptor Expressed in Myeloid Cells 2 (TREM2) is a pattern recognition receptor on myeloid cells, and is upregulated on microglia surrounding amyloid plaques in Alzheimer's disease (AD). Rare, heterozygous mutations in TREM2 (e.g., R47H) increase AD risk several fold. TREM2 can be cleaved at the plasma membrane by metalloproteases to release the ectodomain as soluble TREM2 (sTREM2). Wild-type sTREM2 binds oligomeric amyloid beta (Aβ) and acts as an extracellular chaperone, blocking and reversing Aβ oligomerization and fibrillization, and preventing Aβ-induced neuronal loss in vitro. Whereas, R47H sTREM2 increases Aβ fibrillization and neurotoxicity. AD brains expressing R47H TREM2 have more fibrous plaques with more neuritic pathology around these plaques, consistent with R47H sTREM2 promoting Aβ fibrillization relative to WT sTREM2. Brain expression or injection of wild-type sTREM2 reduces pathology in amyloid models of AD in mice, indicating that wild-type sTREM2 is protective against amyloid pathology. Levels of sTREM2 in cerebrospinal fluid (CSF) fall prior to AD, rise in early AD, and fall again in late AD. People with higher sTREM2 levels in CSF progress more slowly into and through AD than do people with lower sTREM2 levels, suggesting that sTREM2 protects against AD. However, some of these experiments can be interpreted as full-length TREM2 protecting rather than sTREM2, and to distinguish between these two possibilities, we need more experiments testing whether sTREM2 itself protects in AD and AD models, and at what stage of disease. If sTREM2 is protective, then treatments could be designed to elevate sTREM2 in AD.

Identification of the minimal active soluble TREM2 sequence for modulating microglial phenotypes and amyloid pathology

Background

TREM2 is a microglial receptor genetically linked to the risk for Alzheimer’s disease (AD). The cerebrospinal fluid (CSF) levels of soluble TREM2 (sTREM2) have emerged as a valuable biomarker for the disease progression in AD and higher CSF levels of sTREM2 are linked to slower cognitive decline. Increasing sTREM2 in mouse models of amyloidosis reduces amyloid-related pathology through modulating microglial functions, suggesting a beneficial role of sTREM2 in microglia biology and AD pathology.

Methods

In the current study, we performed serial C- and N-terminal truncations of sTREM2 protein to define the minimal sequence requirement for sTREM2 function. We initially assessed the impacts of sTREM2 mutants on microglial functions by measuring cell viability and inflammatory responses. The binding of the sTREM2 mutants to oligomeric Aβ was determined by solid-phase protein binding assay and dot blot assay. We further evaluated the impacts of sTREM2 mutants on amyloid-related pathology by direct stereotaxic injection of sTREM2 proteins into the brain of 5xFAD mice.

Results

We found that both sTREM2 fragments 41–81 and 51–81 enhance cell viability and inflammatory responses in primary microglia. However, the fragment 51–81 exhibited impaired affinity to oligomeric Aβ. When administrated to the 5xFAD mice brain, the sTREM2 fragment 41–81, but not 51–81, increased the number of plaque-associated microglia and reduced the plaque deposition. Interestingly, the fragment 41–81 was more efficient than the physiological form of sTREM2 in ameliorating Aβ-related pathology.

Conclusions

Our results indicate that the interaction of sTREM2 truncated variants with Aβ is essential for enhancing microglial recruitment to the vicinity of an amyloid plaque and reducing the plaque load. Importantly, we identified a 41-amino acid sequence of sTREM2 that is sufficient for modulating microglial functions and more potent than the full-length sTREM2 in reducing the plaque load and the plaque-associated neurotoxicity. Taken together, our data provide more insights into the mechanisms underlying sTREM2 function and the minimal active sTREM2 sequence represents a promising candidate for AD therapy.

Differential expression of Triggering Receptor Expressed on Myeloid cells 2 (Trem2) in tissue eosinophils

No longer regarded simply as end-stage cytotoxic effectors, eosinophils are now recognized as complex cells with unique phenotypes that develop in response stimuli in the local microenvironment. In our previous study, we documented eosinophil infiltration in damaged muscle characteristic of dystrophin-deficient (mdx) mice that model Duchenne muscular dystrophy. Specifically, we found that eosinophils did not promote the generation of muscle lesions, as these persisted in eosinophil-deficient mdx.PHIL mice. To obtain additional insight into these findings, we performed RNA sequencing of eosinophils isolated from muscle tissue of mdx, IL5tg, and mdx.IL5tg mice. We observed profound up-regulation of classical effector proteins (major basic protein-1, eosinophil peroxidase, and eosinophil-associated ribonucleases) in eosinophils isolated from lesion-free muscle from IL5tg mice. By contrast, we observed significant up-regulation of tissue remodeling genes, including proteases, extracellular matrix components, collagen, and skeletal muscle precursors, as well as the immunomodulatory receptor, Trem2, in eosinophils isolated from skeletal muscle tissue from the dystrophin-deficient mdx mice. Although the anti-inflammatory properties of Trem2 have been described in the monocyte/macrophage lineage, no previous studies have documented its expression in eosinophils. We found that Trem2 was critical for full growth and differentiation of bone marrow-derived eosinophil cultures and full expression of TLR4. Immunoreactive Trem2 was also detected on human peripheral blood eosinophils at levels that correlated with donor body mass index and total leukocyte count. Taken together, our findings provide important insight into the immunomodulatory and remodeling capacity of mouse eosinophils and the flexibility of their gene expression profiles in vivo.

Plasma soluble TREM2 is associated with white matter lesions independent of amyloid and tau

Cerebral small vessel disease is one of the most common causes of cognitive decline and stroke. While several lines of evidence have established a relationship between inflammation and cerebrovascular pathology, the mechanistic link has not yet been elucidated. Recent studies suggest activation of immune mediators, including the soluble form of triggering receptor expressed on myeloid cells 2 (TREM2), may be critical regulators. In this study, we compared the plasma levels of soluble TREM2 and its correlations with neuroimaging markers and cerebral amyloid load in ten patients with Alzheimer's disease and 66 survivors of spontaneous intracerebral haemorrhage with cerebral amyloid angiopathy or hypertensive small vessel disease, two of the most common types of sporadic small vessel disease. We performed brain MRI and 11C-Pittsburgh compound B PET for all participants to evaluate radiological small vessel disease markers and cerebral amyloid burden, and 18F-T807 PET in a subgroup of patients to evaluate cortical tau pathology. Plasma soluble TREM2 levels were comparable between patients with Alzheimer's disease and small vessel disease (P=0.690). In patients with small vessel disease, plasma soluble TREM2 was significantly associated with white matter hyperintensity volume (P<0.001), but not with cerebral amyloid load. Among patients with Alzheimer's disease and cerebral amyloid angiopathy, plasma soluble TREM2 was independently associated with a tau-positive scan (P=0.001) and white matter hyperintensity volume (P=0.013), but not amyloid load (P=0.221). Our results indicate plasma soluble TREM2 is associated with white matter hyperintensity independent of amyloid and tau pathology. These findings highlight the potential utility of plasma soluble TREM2 as a strong predictive marker for small vessel disease-related white matter injury and hold clinical implications for targeting the innate immune response when treating this disease.

Alternative Splicing Regulation of Low-Frequency Genetic Variants in Exon 2 of TREM2 in Alzheimer's Disease by Splicing-Based Aggregation

TREM2 is among the most well-known Alzheimer’s disease (AD) risk genes; however, the functional roles of its AD-associated variants remain to be elucidated, and most known risk alleles are low-frequency variants whose investigation is challenging. Here, we utilized a splicing-guided aggregation method in which multiple low-frequency TREM2 variants were bundled together to investigate the functional impact of those variants on alternative splicing in AD. We analyzed whole genome sequencing (WGS) and RNA-seq data generated from cognitively normal elderly controls (CN) and AD patients in two independent cohorts, representing three regions in the frontal lobe of the human brain: the dorsolateral prefrontal cortex (CN = 213 and AD = 376), frontal pole (CN = 72 and AD = 175), and inferior frontal (CN = 63 and AD = 157). We observed an exon skipping event in the second exon of TREM2, with that exon tending to be more frequently skipped (p = 0.0012) in individuals having at least one low-frequency variant that caused loss-of-function for a splicing regulatory element. In addition, genes differentially expressed between AD patients with high vs. low skipping of the second exon (i.e., loss of a TREM2 functional domain) were significantly enriched in immune-related pathways. Our splicing-guided aggregation method thus provides new insight into the regulation of alternative splicing of the second exon of TREM2 by low-frequency variants and could be a useful tool for further exploring the potential molecular mechanisms of multiple, disease-associated, low-frequency variants.

Understanding microglial diversity and implications for neuronal function in health and disease

Genetic data implicate microglia as central players in brain health and disease, urging the need to better understand what microglia do in the brain. Microglia are critical partners in neuronal wiring and function during development and disease. Emerging literature suggests that microglia have diverse functional roles, raising the intriguing question of which functions of microglia become impaired in disease to undermine proper neuronal function. It is also becoming increasingly clear that microglia exist in heterogeneous cell states. Microglial cell states appear context-dependent, that is, age, sex, location, and health of their microenvironment; these are further influenced by external signaling factors including gut microbiota and lipid metabolites. These data altogether suggest that microglia exist in functional clusters that impact, and are impacted by, surrounding neuronal microenvironment. However, we still lack understanding of how we can translate microglia cell states into function. Here, we summarize the state-of-the-art on the diverse functions of microglia in relation to neuronal health. Then, we discuss heterogeneity during developing, healthy adult and diseased brains, and whether this may be predetermined by origin and/or regulated by local milieu. Finally, we propose that it is critical to gain high-resolution functional discernment into microglia-neuron interactions while preserving the spatial architecture of the tissue. Such insight will reveal specific targets for biomarker and therapeutic development toward microglia-neuron crosstalk in disease.

Role of triggering receptor expressed on myeloid cells 2 (TREM2) in neurodegenerative dementias

Neurodegeneration is a debilitating condition that causes nerve cell degeneration or death. Neurodegenerative diseases (NDDs) such as Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD), and Lewy body dementia (LBD) are posing a larger population burden of dementia worldwide. Neurodegenerative dementia is one of the main challenges in public health with its main characteristics being permanent loss of memory, impairment in cognition, and impaired daily functions. The published literature about genetic studies of these disorders suggests genetic underpinning in the pathogenesis of neurodegenerative dementia. In the process of underlining the pathogenesis of NDD, growing evidence has related genetic variations in the triggering receptor expressed on myeloid cells 2 (TREM2). This review paper aims to provide a detailed information regarding the association of TREM2 and NDDs leading to dementia. A central consideration is AD that accounts for almost 50%-70% of all late-life dementias alone or in combination with other neurological disorders. Other prevalent neurodegenerative conditions that lead to dementia are also discussed. Such studies are important as they can give a comprehensive knowledge of TREM2's role in various NDDs, in order to maximize the potential for developing new therapeutic approaches.

Lack of association between TREM2 rs75932628 variant and amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a multifactorial neurodegenerative disease. Inflammatory processes are among the mechanisms that are implicated in ALS pathogenesis. The TREM2 rs75932628 T variant may influence the regulatory effect of TREM2 on inflammation. Studies regarding the role of the rs75932628 variant in ALS have yielded inconsistent results, so far. To assess the role of TREM2 rs75932628 on ALS risk. We genotyped 155 patients with sporadic ALS and 155 healthy controls for TREM2 rs75932628. We also merged and meta-analyzed our data with data from previous studies (with a total of 7524 ALS cases and 14,675 controls), regarding TREM2 rs75932628 and ALS. No ALS or healthy subjects carried the TREM2 rs75932628-T variant. Results from meta-analyses (overall approach and sensitivity analyses) yielded no significant results for possible connection between TREM2 rs75932628-T variant and ALS. Based on our results, TREM2 rs75932628 does not seem to play a determining role to the pathophysiology of ALS.

The sTREM2 Concentrations in the Blood: A Marker of Neurodegeneration?

Microglia performs a variety of functions during brain development designed to maintain brain homeostasis. Triggering receptor expressed on myeloid cells 2 (TREM2) is expressed in microglial cells modulating phagocytosis, cytokine production, cell proliferation, and cell survival. Interestingly, the levels of soluble TREM2 (the secreted ectodomain of TREM2, sTREM2) were higher in cerebrospinal fluid (CSF) from Alzheimer's disease (AD) patients than subjects without cognitive decline. It is noteworthy that, while CSF sTREM2 levels have been extensively studied, few studies have investigated sTREM2 in blood producing conflicting results. We aimed to investigate the levels of sTREM2 in CSF and blood from a cohort of well-characterized AD comparing the results to those obtained in patients suffering from idiopathic normal pressure hydrocephalus (iNPH), a potentially reversible cognitive impairment. Our findings underlined a significantly lower plasma sTREM2 concentration in AD patients compared to iNPH subjects [39.1 ng/mL (standard deviation (SD), 15.0) and 47.2 ng/mL (SD, 19.5), respectively; p = 0.01], whereas no difference was revealed between the two groups in the CSF sTREM2 levels. The adjusted regression analyses evidenced in AD patients an association between plasma and CSF sTREM2 levels [B = 0.411; 95% confidence interval (CI), 0.137-0.685, p = 0.004], as well as β-amyloid concentrations (B = 0.035; 95% CI, 0.007-0.063, p = 0.01) and an association between CSF sTREM2 and phospho-Tau concentrations (B = 0.248; 95% CI, 0.053-0.443; p = 0.01). No significant relation was found in iNPH patients. In conclusion, these differences in sTREM2 profiles between AD and iNPH reinforce the notion that this receptor has a role in neurodegeneration.

Microglial Hyperreactivity Evolved to Immunosuppression in the Hippocampus of a Mouse Model of Accelerated Aging and Alzheimer's Disease Traits

Neuroinflammation is a risk factor for Alzheimer's disease (AD). We sought to study the glial derangement in AD using diverse experimental models and human brain tissue. Besides classical pro-inflammatory cytokines, we analyzed chitinase 3 like 1 (CHI3L1 or YKL40) and triggering receptor expressed on myeloid cells 2 (TREM2) that are increasingly being associated with astrogliosis and microgliosis in AD, respectively. The SAMP8 mouse model of accelerated aging and AD traits showed elevated pro-inflammatory cytokines and activated microglia phenotype. Furthermore, 6-month-old SAMP8 showed an exacerbated inflammatory response to peripheral lipopolysaccharide in the hippocampus and null responsiveness at the advanced age (for this strain) of 12 months. Gene expression of TREM2 was increased in the hippocampus of transgenic 5XFAD mice and in the cingulate cortex of autosomal dominant AD patients, and to a lesser extent in aged SAMP8 mice and sporadic early-onset AD patients. However, gene expression of CHI3L1 was increased in mice but not in human AD brain samples. The results support the relevance of microglia activation in the pathways leading to neurodegeneration and suggest diverse neuroinflammatory responses according to the AD process. Therefore, the SAMP8 mouse model with marked alterations in the dynamics of microglia activation and senescence may provide a complementary approach to transgenic mouse models for the study of the neuroinflammatory mechanisms underlying AD risk and progression.

Interaction between genetic factors, Porphyromonas gingivalis and microglia to promote Alzheimer's disease

In late-onset Alzheimer disease (AD) pathogenesis, genes, infections and immunity could be significant factors. We have reviewed if the keystone periodontal pathogen Porphyromonas gingivalis may affect genes and microglia (primary immune cells in the brain) to promote AD development. Genes for apolipoprotein, clusterin, CD33, triggering receptor expressed on myeloid cells-2 (TREM-2), tyrosine kinase binding protein (TYR-OBP), and complement receptors can affect microglia. Most of these genes can also be affected by P. gingivalis via its mastering of immune suppression. Besides, P. gingivalis can affect microglia directly in several ways. Taken together, genetic predisposition, P. gingivalis infection and microglia could promote neurodegeneration typical of that reported for AD.

Quinoline-Pyrazole Scaffold as a Novel Ligand of Galectin-3 and Suppressor of TREM2 Signaling

Galectin-3 has been identified as a critical player in driving the neuroinflammatory responses in Alzheimer's disease (AD). A key feature of this function of galectin-3 is associated with its interaction with the triggering receptor expressed on myeloid cells-2 (TREM2). Herein, we report a high-throughput screening (HTS) platform that can be used for the identification of inhibitors of TREM2 and galectin-3 interaction. We have utilized this HTS assay to screen a focused library of compounds optimized for the central nervous system (CNS)-related diseases. MG-257 was identified from this screen as the first example of a small molecule that can attenuate TREM2 signaling based on its high affinity to galectin-3 (endogenous ligand of TREM2). Remarkably, MG-257 reduced the levels of proinflammatory cytokines in activated microglial cells, which highlights its ability to inhibit the neuroinflammatory response associated with AD.

Microglia-targeting nanotherapeutics for neurodegenerative diseases

Advances in nanotechnology have enabled the design of nanotherapeutic platforms that could address the challenges of targeted delivery of active therapeutic agents to the central nervous system (CNS). While the majority of previous research studies on CNS nanotherapeutics have focused on neurons and endothelial cells, the predominant resident immune cells of the CNS, microglia, are also emerging as a promising cellular target for neurodegeneration considering their prominent role in neuroinflammation. Under normal physiological conditions, microglia protect neurons by removing pathological agents. However, long-term exposure of microglia to stimulants will cause sustained activation and lead to neuronal damage due to the release of pro-inflammatory agents, resulting in neuroinflammation and neurodegeneration. This Perspective highlights criteria to be considered when designing microglia-targeting nanotherapeutics for the treatment of neurodegenerative disorders. These criteria include conjugating specific microglial receptor-targeting ligands or peptides to the nanoparticle surface to achieve targeted delivery, leveraging microglial phagocytic properties, and utilizing biocompatible and biodegradable nanomaterials with low immune reactivity and neurotoxicity. In addition, certain therapeutic agents for the controlled inhibition of toxic protein aggregation and for modulation of microglial activation pathways can also be incorporated within the nanoparticle structure without compromising stability. Overall, considering the multifaceted disease mechanisms of neurodegeneration, microglia-targeted nanodrugs and nanotherapeutic particles may have the potential to resolve multiple pathological determinants of the disease and to guide a shift in the microglial phenotype spectrum toward a more neuroprotective state.

IL-4 and IL-10 promotes phagocytic activity of microglia by up-regulation of TREM2

Triggering receptor expressed on myeloid cells-2 (TREM2) is an innate immune receptor that promotes phagocytosis by microglia. However, whether TREM2 is related to the stimulus-dependent phagocytic activity of microglia is unclear. In this study, the primary cultured microglia were stimulated with interferon (IFN)-γ, interleukin (IL)-4, and interleukin (IL)-10, respectively, and their phagocytic activity against microbeads and apoptotic neural stem cells (NSCs) was measured. TREM2 of microglia was detected by qPCR and western blotting. The TREM2 signal was blocked in microglia using the siRNA technique. The results showed that IL-4 or IL-10 treatment significantly increased the number of microglia gathered around the apoptotic neurosphere. IL-4 and IL-10 treatment also promoted phagocytosis of microbeads and apoptotic NSCs by primary cultured microglia. The TREM2 expression was up-regulated in IL-4- or IL-10- treated microglia. TREM2 siRNA treatment blocked the phagocytic activity of IL-4- or IL-10-treated microglia. In conclusion, these results indicated that IL-4 and IL-10 promote the phagocytic activity of microglia by the up-regulation of TREM2, which suggested a new potential therapeutic target for neurodegenerative disease.

TREM2 Overexpression Attenuates Cognitive Deficits in Experimental Models of Vascular Dementia

Neuroinflammation plays a prominent role in the pathogenesis of vascular dementia (VD). Triggering receptor expressed on myeloid cells 2 (TREM2) is a transmembrane receptor mainly expressed on microglia and has been known for its anti-inflammatory properties during immune response. However, data evaluating the effects of TREM2 in VD are lacking. Therefore, the present study is aimed at investigating the role of TREM2 in VD. In this study, the mouse model of VD was induced by transient bilateral common carotid artery occlusion (BCCAO). We compared the hippocampal gene and protein expressions of TREM2 between the VD mice and sham-operated mice at different time points. The TREM2 mRNA and protein expression levels in the VD mice were higher than those in the sham-operated mice. The cognitive deficits of VD mice were observed in the Morris water maze test. Interestingly, overexpression of TREM2 by intracerebroventricular injection of a lentiviral vector that encoded TREM2 (LV-TREM2) significantly improved the spatial learning and memory and attenuated the hippocampal neural loss in VD mice. Further mechanistic study revealed that overexpression of TREM2 significantly inhibited microglia M1 polarization by decreasing inducible nitric oxide synthase (iNOS) and proinflammatory cytokines expression levels and conversely enhanced microglia M2 polarization by increasing Arginase-1 (Arg-1) and anti-inflammatory cytokine expression levels. These results strongly suggest that TREM2 provides a protective effect in VD via modulating the phenotype of activated microglia and may serve as a novel potential therapeutic target for VD.