Nat Aging—Alzheimer's mouse glial cells participate in synaptic clearance through the complement pathway

Written by Jane Cheng, Editor-in-Chief—Sizhen Wang—Chronic neuroinflammation in Xia Ye is a hallmark of Alzheimer's Disease (AD), manifested by glial hyperplasia, elevated levels of pro-inflammatory factors, and synaptic loss[1].

。 In mouse models and in the brains of AD patients, activation of the Classic Complement Pathway (CCP) leads to neuronal damage and synaptic loss, abnormally elevated CCP factors in the cerebrospinal fluid of the brains of patients, and human genomics studies have also supported the involvement of the CCP pathway in the pathogenesis of AD[2].

In a mouse model of AD, pharmacological or genetically inhibitory complement pathways improve neurodegenerative degeneration and synaptic loss [3-4].

During development, complement C1q binds to pathogens, apoptotic cells, and microglia engorge complement-tagged synapses[5
].
In addition to microglia, astrocytes can also play a synaptic clearance role during development, in the adult brain, and in disease [6-8], however, the molecular mechanism of the synaptic clearance they exert is unknown
.

Recently, the Borislav Dejanovic team from the Broad Institute in the United States and the Jesse E.
Hanson team from Genentech in Southern California presented a collaboration in Nature Aging titled "Complement C1q-dependent excitatory and inhibitory synapse elimination by.
" The article astrocytes and microglia in Alzheimer's disease mouse models, combined with immunostaining through deep proteomic analysis, revealed that in a mouse model of AD, microglia and astrocytes work together to clear excitatory and sexually inhibitory synapses through the C1q pathway, and suggest that complement C1q may be a target for the treatment of AD
.


The researchers applied the AD model P301S transgenic mice as the research object, and found that the whole brain volume and hippocampus volume of the 9-month P301S mice were significantly reduced, while the knockout C1q gene expression (P301S; The AD mice of C1q^KO) are almost close to normal mice; Correspondingly, P301S mice exhibit low locomotor activity, while P301S exhibited low activity; C1q^KO mice are normally active (Figure 1), suggesting that knockout C1q has a protective effect
on neurodegeneration.
In addition, C1q deletion did not have a significant effect
on Tau pathology, glial cell hyperplasia, and glial transcription changes in P301S mice.

Figure 1 C1q deletion can reduce nerve degeneration in P301S mice (Source: Dejanovic B, et al.
, Nat Aging, 2022)
So how does C1q deletion play a protective role in neurodegeneration? The researchers compared synaptoproteomic changes
at different development times in different transgenic mice through multiple tandem mass tag (TMT) proteomic analysis.
It was found that there were 108 down-regulated and 68 up-regulated differentially expressed proteins (DE) in the 6-month-old P301S synapse; At 9 months, there were 253 down-regulated and 434 up-regulated DE (16.
5% of total protein).

In contrast, the P301S; The synapses of the hippocampus in C1q^KO mice showed only 17 down-regulated and 19 up-regulated DEs (0.
5% of total proteins) at 6 months, and 79 down-regulated and 224 upregulated DEs at 9 months (7% of total proteins), and C1q deletions did not affect Tau levels in the synapses of the P301S brain (Figure 2).

。 Thus, these results suggest that C1q deletion weakens the age-dependent changes induced by Tau pathology; The deletion of C1q did not significantly alter the transcriptome changes in the P301S brain, suggesting that changes in the synaptic proteome are likely caused
by local protein changes in the synapse.

Figure 2 C1q deletion reduces changes in synaptic proteins in the hippocampus of P301S mice (Source: Dejanovic B, et al.
, Nat Aging, 2022).

In addition, the researchers noticed that many typical astrocyte-specific proteins, such as Aqp4, Mlc1, and Slc1a4, increased in a C1q-dependent manner in the 9-month-old P301S synapse (Figure 2d), and then used pseudobulk single-cell RNA sequencing analysis to find that most of the upregulated proteins were mainly expressed by glial cells and were highly correlated
with whether or not to express C1q 。 KEGG pathway analysis showed that P301S synaptic DE protein was mainly enriched in the "metabolic pathway", "fatty acid degradation", "peroxisome", "peroxisome proliferative activation receptor" signaling pathway, and the "Alzheimer's disease" and "ion homeostasis" related signals were significantly enriched at 9 months of age.
However, in the P301S; In the C1q^KO synapse, the enrichment of these pathways is relatively weak (Figure 2g
).
Further, by immunoelectron microscopy techniques found that GFAP-Homer1 contact increased significantly in the hippocampal synapses of P301S mice, suggesting an increased
link between astrocytes and excitatory neurons.
And in the P301S; C1qKO mice were no different from normal mice (3), saying that star glial cells increase contact
with synapses in a C1q-dependent manner.

Figure 3 The absence of C1q reduces the contact between the star gum and the synapse (Source: Dejanovic B, et al.
, Nat Aging, 2022).

Interestingly, glial proteins elevated in the hippocampal synapses of P301S mice, including complement factors C1q and C4, astrocyte-labeled proteins MLC1 and GFAP, microglia GPNMB and ANHAK, and annexin are the proteins that are most added in AD synaptic neurosomes (Figure 4a); The researchers also found a significant increase in total C4 concentrations and processing (lysis and activation) C4 concentrations in the cerebrospinal fluid of AD patients (Figure 4b), suggesting that the CCP pathway is strongly associated
with neural degeneration in the AD process.

Figure 4 AD patients with increased glial protein expression in synapses and elevated C4 levels in CSF (Source: Dejanovic B, et al.
, Nat Aging, 2022)
Based on proteomics, immunoelectron microscopy, and immunohistochemistry experimental data, the researchers hypothesized that astrocytes may interact
with synapses in a C1q-dependent manner during synaptic phagocytosis.
By co-staining astrocytes GFAP+, gum Iba1+, lysosomal Lamp1^+, excitatory postsynaptic Homer1, inhibitory synapses gephyrin (Figure 5), it was found that the number of excitatory synapses in P301S mice was much higher than that in P301S; C1q^KO mice
.
Surprisingly, the number of excitatory synapses in the lysosomes of astrocytes in P301S mice was also higher than that of P301S; C1q^KO mice, said star glial cells and microglia clear the synapses
of P301S mice in a complement-dependent manner.
C3 is an important complement component downstream of C1q
.
The percentage of C3⁺ excitatory postsynaptic spots (puncta) in the hippocampus of P301S mice was significantly increased compared to wild-type mice; P301S; The number of postsynaptic spots of C3⁺ excitatory synapse decreased significantly in C1q^KO mice compared with P301S mice, but there was no significant difference in the total C3 puncta percentage, indicating that the absence of C1q particularly affected the accumulation of C3 at synapses; It is also further stated that CCP promotes astrocytes and microglia elimination of synapses
.

Figure 5 Star gum and gum remove synapses from P301S mice in a complement-dependent manner (Source: Dejanovic B, et al.
, Nat Aging, 2022) So how does synaptic clearance occur when the phagocytic function of astrocytes (star gum) or microglia (gum)
is impaired? Small gum-specific TREM2 deletions in AD mouse models are known to inhibit microglia activation, migration to β-amyloid (Aβ) plaques, and phagocytic activity
.
Therefore, in order to determine whether the microgum dysfunction affects the function of the star gum on synaptic clearance, the researchers analyzed the effect of TREM2 deletion in the AD model TauPS2APP mice, which contained both Aβ and Tau pathological features
.
Similarly, using the method of immunoco-contamination found that in the TauPS2APP brain, compared with the plaque-free area, the gum and star gum are near the plaqueDevouring more synapses (Figure 6); Compared with P301S mice, TREM2 deletion caused more
excitatory and inhibitory synapses in the star gum lysosomes than in the gum.
Thus, when the phagocytosis function of the gum is impaired, the star gum can compensatively partially enphagoguate the inhibitory synapse
.

Figure 6 Synaptic clearance near plaques after Trem2 deletion (Source: Dejanovic B, et al.
, Nat Aging, 2022)
Conclusion and discussion, inspiration and prospect In summary, the researchers deeply resolved the proteome data and discovered the new role of astrocytes in complement-dependent excitatory and inhibitory synaptic clearance, Experiments have also shown the accompaniment/coordination role
of astrocytes and microglia in synaptic phagocytosis during pathophysiological processes.
However, this study was immunostained in fixed AD model mouse tissues, so it is not possible to dynamically distinguish whether two types of glial cells are pruning or phagocytic on synapses
.
Further studies on glial cells and neuronal function have yet to be verified
.
First, although studies have shown that the absence of C1q particularly affects the accumulation of C3, an important complementary component downstream of C1q, at synapses, however; The presence or absence of specific astrocyte receptors that can directly detect complement deposition on neurons, or the possibility of indirect complement-dependent signaling to trigger local activation of perisynaptic astrocytes, further experimental evidence is
needed 。 Second, although this study suggests that astrocytes can compensatively partially enphagoguate inhibitory synapses when microglia phagocytosis is impaired, suggesting that astrocytes and microglia may have a synergistic effect during tissue reconstruction of damaged tissues, studies have shown that in mouse models with multiple sclerosis, microglia rather than astrocytes eliminate synapses through complement pathways [9
].
Therefore, astrocytes and microglia have the potential to sense different complement molecules at synapses
.
Original link: selected articles from previous issues

[1] The NPP-Luo Xiongjian research group used Mendelian randomization to screen potential drug targets for the treatment of mental illness

[2] Adv Sci-Chai Renjie's team made important progress in the regeneration of functional hair cells in cochlear organs

[3] J Neuroinflammation—Tang Yamei's team discovered the mechanism by which pregabalin mitigates microglial activation and neuronal damage in radioactive brain injury

[4] Transl Psychiatry—Accelerated aging of brain function in patients with major depression: evidence from large Chinese participants

[5] Nat Commun—Xu Tianle/Li Weiguang/Zhang Siyu teamwork reveals the neuronal cluster organization of fear and regression memory competition

[6] The Brain-Yi Chenju/Niu Jianqin team found that activating the Wnt/β-catenin pathway could alleviate the blood-brain barrier dysfunction in Alzheimer's disease

[7] Prog Neurobiol Frontier ThinkingEffects of genetic factors, aging and intestinal microbial disorders on immune responses in "dry" and "wet" retinal degeneration

[8] Sci Adv-Xi Zhengxiong's team discovered a new mechanism of sports reward: the midbrain red nucleus-ventral covered area glutamate neural pathway

[9] Mol Psychiatry—Jianqin Niu/Lan Xiao's team found that the variable shearing of oligodendroglial precursor cells DISC-Δ3 inhibits excitatory synaptic growth leading to schizophrenia

【10】PNAS | Sun Bo's research group and collaborators found that time signaling is the main factor in regulating multicellular information networks

Recommended high-quality scientific research training courses[1] Seminar on Single Cell Sequencing and Spatial Transcriptomics Data Analysis (October 29-30, Tencent Online Conference)【2】Seminar on Patch Clamp and Optogenetics and Calcium Imaging Technology (October 15-16, 2022, Tencent Conference) Conference/Forum Preview & Review

[1] Trailer | Conference on Neuromodulation and Brain-Computer Interface (U.
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Welcome to "Logical Neuroscience" [1] Talent Recruitment - "Logical Neuroscience" to recruit article interpreter/writing positions (network part-time, online office).

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