Nat Aging︱Alzheimer’s disease new discovery: a regulatory mechanism of bone marrow-derived macrophages that do not rely on the TREM2 pathway

Author ︱Tong responsible editor︱ Wang Sizhen Alzheimer’s disease (AD) is a very serious neurodegenerative disease and the most common cause of dementia
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The pathological features of AD include the deposition of β-amyloid (Aβ) and neurofibrillary tangles, accompanied by neuronal dystrophy and neuroinflammation; clinical manifestations include cognitive impairment and progressive memory loss
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The extracellular domain of amyloid precursor protein is cleaved by enzymes to produce variable-length Aβ polypeptides.
These peptides have different self-aggregation tendencies and neurotoxicity, and can aggregate to form insoluble and synaptic-toxic oligomers.
Body [1], can also aggregate to form fibers, and then form plaques [2]
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In recent years, the treatment of pathological plaques and tangles has not made very effective progress.
Therefore, people have turned their attention to microglia, which are responsible for regulating brain homeostasis and having a clearing function
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Microglia are activated under the stimulation of the pathological environment
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Researchers defined the microglia in the 5xFAD amyloidosis (amyloidosis) mouse model as disease-associated microglia (DAM) [3, 4]
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The conversion of microglia from steady state to DAM relies on triggering receptor expressed on myeloid cells 2 (TREM2) [3], which is considered to be a risk factor for late-onset AD
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 Under pathological conditions, due to the destruction of the blood-brain barrier, monocyte-derived macrophages (MDM) derived from peripheral monocytes will infiltrate the central nervous system and play a vital role in the repair of central nervous system damage.
Role
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In the AD model, targeting the programmed death receptor 1 (PD-L1) pathway can improve cognitive function and improve pathological changes in the brain [5-7]
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However, the changes of MDM in this process are still unclear, and the relationship between MDM and the innate immune cells in the brain-microglia is also unclear
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 On December 20, 2021, the research group of Assaf Weiner, Ido Amit, and Michal Schwartz of the Weizmann Institute of Science in Rehovot, Israel jointly published an article entitled "Alzheimer's disease modification mediated by bone marrow-derived" on Nature Aging.
Macrophages via a TREM2-independent pathway in mouse model of amyloidosis” found that targeting PD-L1 can improve the cognitive function of AD mice and reduce the level of soluble Aβ, and the activation of MDM in this process has nothing to do with the TREM2 receptor pathway
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In order to explain the contribution of DAM and MDM to the process of AD, the researchers first observed whether the treatment of PD-L1 affects microglia
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Therefore, they used PD-L1 antibody and IgG2b antibody to treat Trem2+/+5xFAD mice, and performed single-cell sequencing of CD45+ cells in brain tissue 14 days after treatment (Figure 1a), and the small glue in it Plasma cells were further analyzed (Figure 1 b, c)
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The results showed that compared with the control group, the DAM level of the PD-L1 group mice was significantly higher (Figure 1 d)
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So after MDM infiltrates into the brain tissue, will it show the phenotype of DAM like the inherent microglia? Therefore, the researchers transplanted mouse bone marrow with green fluorescent markers into Trem2+/+5xFAD mice, injected anti-PD-L1 antibodies in the same way, and analyzed them by flow cytometry 14 days after treatment.
CD45+ GFP+ and CD45+ cells were selected and single-cell sequencing was performed (Figure 1e)
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The results showed that the DAM phenotype was not observed in all CD45+GFP+ cells (Figure 1g)
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This means that after MDM infiltrates the brain, it does not show the phenotype of DAM, and the transcriptomic characteristics of MDM are inconsistent with the phenotype of any kind of microglia
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Figure 1 DAM levels increased after anti-PD-L1 treatment (Source: Dvir-Szternfeld, R.
, et al.
, Nat Aging, 2021) In order to explore the role of MDM in AD models, researchers used anti-PD- Trem2-/-5xFAD mice (without DAM) aged 6-9 months were treated with L1 or IgG2b antibody, and behavioral observations were performed on the mice 27-30 days after the antibody injection (Figure 2a)
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It was found that after anti-PD-L1 antibody treatment, Trem2-/-5xFAD mice found a platform in the water maze experiment (RAWM) shorter than the control group, and the new object recognition experiment (NOR) was more effective in exploring new objects.
It took longer than the control group (Figure 2 b, c)
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These results indicate that anti-PD-L1 treatment improves the cognitive ability of Trem2-/-5xFAD mice, and that the improvement of MDM on the cognitive ability of mice does not depend on TREM2
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 Figure 2 Anti-PD-L1 treatment improved the learning and cognitive function of Trem2-/-5xFAD and Trem2+/+5xFAD mice (Source: Dvir-Szternfeld, R.
, et al.
, Nat Aging, 2021).
Previous studies It shows that although there is no significant difference in the level of soluble Aβ in the brain of Trem2+/+5xFAD and Trem2-/-5xFAD mice, the clearance of Aβ depends on the TREM2 receptor [8, 9]
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Here, the authors found that after anti-PD-L1 antibody treatment, the levels of soluble Aβ in the brains of Trem2+/+5xFAD and Trem2-/-5xFAD mice were significantly reduced (Figure 3 a, b), and the content of soluble Aβ It is correlated with the performance of mice in the water maze experiment and new object recognition experiment (Figure 3 c, d)
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This suggests that there may be a potential connection between the level of soluble Aβ and the cognitive ability of mice
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Figure 3 The level of soluble Aβ in the brain of mice in the anti-PD-L1 treatment group decreased (Source: Dvir-Szternfeld, R.
, et al.
, Nat Aging, 2021) In order to further clarify the function of MDM, researchers started from 7 CD45+CD11b+Tmem119neg/low cells were isolated from the anti-PD-L1 treatment group and the control group Trem2+/+5xFAD and Trem2-/-5xFAD mice of -9 months, and performed MARS-seq (massively parallel single-cell RNA sequencing) and Metacell analysis
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The results showed that there was no difference between MDM and monocyte subpopulations in the brains of Trem2+/+5xFAD and Trem2-/-5xFAD (Figure 4 ac), and there was no DAM in Trem2-/-5xFAD
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According to the characteristics of transcriptomics, the author divides MDM into three cell subgroups.
The three subgroups have similar transcriptomics characteristics, and only the expression levels of some genes are different (Figure 4d)
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The high expression of MDM-1 subgroups includes genes encoding clearance receptors, such as Stab1, Cd163, Mrcl and Msr1 (Figure 4 d, e), which may help clear misfolded neurotoxic proteins from the brain and reduce local inflammation
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The MDM-3 subgroup highly expresses Ccr2, indicating that the MDM-3 subgroup may be new cells that have migrated into the brain
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The MDM-2 subgroup is an intermediate state between MDM-1 and MDM-3 (Figure 4d)
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Figure 4 MDM has similar transcriptomic characteristics in Trem2-/-5xFAD and Trem2+/+5xFAD mice (Source: Dvir-Szternfeld, R.
, et al.
, Nat Aging, 2021).
Given that MDM is in Trem2-/ The phenotypes of -5xFAD and Trem2+/+5xFAD mice are similar, so the researchers further used Trem2-/-5xFAD mice to study the effect of MDM on AD without relying on Trem2, especially the effect on DAM
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They hypothesized that anti-PD-L1 therapy mainly promotes the infiltration of MDM into the brain by affecting peripheral effects, which in turn leads to improvement of the disease
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Simultaneously treat 6-9 month-old Trem2-/-5xFAD mice with anti-PD-L1 and anti-Ccr2 antibodies (Figure 5a)
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The results showed that after the use of anti-Ccr2 antibody, the performance of mice in new object recognition and water maze experiments was consistent with that of the IgG2b control group (Figure 5 be)
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This means that anti-Ccr2 treatment eliminates the beneficial effects of anti-PD-L1 antibodies
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Figure 5 Anti-Ccr2 treatment eliminates the beneficial effects of anti-PD-L1 antibody on Trem2-/-5xFAD and Trem2+/+5xFAD mice (Source: Dvir-Szternfeld, R.
, et al.
, Nat Aging, 2021) Conclusion of the article Discussion, inspiration and outlook In short, microglia and monocyte-derived macrophages (MDM) play an important role in AD
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Previous studies have shown that the activation of microglia depends on the TREM2 receptor [8, 9]
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This study proved that the activation of MDM has nothing to do with TREM2, blocking the migration of monocytes by anti-Ccr2 antibody can completely eliminate the cognitive improvement effect of anti-PD-L1 treatment on Trem2-/-5xFAD mice
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 This study supports the idea that the immune system promotes the migration of MDM into the brain, and suggests that targeting MDM may be effective for patients with or without Trem2 gene mutations, so using MDM as a target of the peripheral immune system may also be a treatment for AD One of
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However, this study also has certain limitations
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The results of the article are based on the use of anti-PD-L1 antibodies, so when anti-PD-L1 antibodies are not used, does anti-Ccr2 antibodies have a similar effect? It remains to be further verified
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Original link: https:// Selected previous articles [1] Sci Transl Med︱ New evidence of delayed onset of monoaminergic antidepressants: hippocampal cAMP regulation The function of HCN channel affects mouse behavior and memory [2] Nat Methods︱ Peng Hanchuan's research team develops cross-modal brain registration technology, which provides important support for brain atlas construction and single-cell precision whole brain mapping research [3] Nat Commun︱ star Glial cells inhibit the infiltration of peripheral macrophages in non-human primate cerebral ischemia injury [4] Cereb Cortex | Peng Ziwen/Chen Qi's group reveals the multimodal neuroimaging features of compulsive behavior acquisition [5] Neurosci Bull︱ Li Yunqing’s research team revealed that changes in nerve plasticity in the anterior cingulate gyrus are related to hyperalgesia and anxiety in chronic pancreatitis [6] Neuron︱ Cao Peng’s laboratory discovered the closed-loop neural mechanism of repetitive stereotyped behavior [7] EMBO Rep｜Kang Jiuhong’s research group Discovery of a new mechanism for lncRNA SOX1-OT to regulate human ESC neurogenesis [8] Neuron︱ Li Yulong’s laboratory developed a new fluorescent probe to detect the spatiotemporal dynamics of extracellular ATP [9] Neurosci BullGuangyin Xu’s research team revealed that through DNA A new mechanism for the combination of GATA1 and P2x7r in the spinal cord astrocytes to relieve visceral pain [10] Nat Rev Neurosci Opinion Article ︱Probability Model of Alzheimer's Disease: Modification of the Amyloid Cascade Hypothesis, a high-quality scientific research training course Recommendation [1] Common experimental paradigms for cognitive control and executive functions [2] Single-cell sequencing and spatial transcriptomics data analysis seminar book donation activity [1] Book donation︱ Oxford Science Series "Brain"-Insomnia, Anxiety, Do you really understand your brain? References (swipe up and down to view) [1] Ferreira, ST, Lourenco, MV, Oliveira, MM, De & Felice, FG Soluble amyloid-beta oligomers as synaptotoxins leading to cognitive impairment in Alzheimer's disease.
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