Lee C Y Daniel, De La Rocha Amberlene J, Inouye Kellie, Langfelder Peter, Daggett Anthony, Gu Xiaofeng, Jiang Lu-Lin, Pamonag Zoe, Vaca Raymond G, Richman Jeffrey, Kawaguchi Riki, Gao Fuying, Xu Huaxi, Yang X William
bioRxiv. 2023 Aug 5:2023.08.03.551881. doi: 10.1101/2023.08.03.551881.
Genetic study of late-onset Alzheimer's disease (AD) reveals that a rare Arginine-to-Histamine mutation at amino acid residue 47 (R47H) in Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) results in increased disease risk. TREM2 plays critical roles in regulating microglial response to amyloid plaques in AD, leading to their clustering and activation surrounding the plaques. We previously showed that increasing human gene dosage exerts neuroprotective effects against AD-related deficits in amyloid depositing mouse models of AD. However, the effects of the R47H mutation on human TREM2-mediated microglial reprogramming and neuroprotection remains poorly understood.
Here we created a BAC transgenic mouse model expressing human TREM2 with the R47H mutation in its cognate genomic context (BAC-TREM2-R47H). Importantly, the BAC used in this study was engineered to delete critical exons of other TREM-like genes on the BAC to prevent confounding effects of overexpressing multiple TREM-like genes. We crossed BAC-TREM2- R47H mice with 5xFAD [1], an amyloid depositing mouse model of AD, to evaluate amyloid pathologies and microglial phenotypes, transcriptomics and expression of key -dosage dependent genes. We also compared the key findings in 5xFAD/BAC-TREM2-R47H to those observed in 5xFAD/BAC-TREM2 mice.
Both BAC-TREM2 and BAC-TREM2-R47H showed proper expression of three splicing isoforms of that are normally found in human. In 5xFAD background, elevated TREM2-R47H gene dosages significantly reduced the plaque burden, especially the filamentous type. The results were consistent with enhanced phagocytosis and altered NLRP3 inflammasome activation in BAC- TREM2-R47H microglia in vitro. However, unlike TREM2 overexpression, elevated TREM2- R47H in 5xFAD failed to ameliorate cognitive and transcriptomic deficits. analysis of key -dosage dependent genes and microglial morphology uncovered that TREM2-R47H showed a loss-of-function phenotype in reprogramming of plaque-associated microglial reactivity and gene expression in 5xFAD.
Our study demonstrated that the AD-risk variant has a previously unknown, mixture of partial and full loss of TREM2 functions in modulating microglial response in AD mouse brains. Together, our new BAC-TREM2-R47H model and prior BAC-TREM2 mice are invaluable resource to facilitate the therapeutic discovery that target human TREM2 and its R47H variant to ameliorate AD and other neurodegenerative disorders.
晚发性阿尔茨海默病（AD）的遗传学研究表明，髓样细胞触发受体2（TREM2）氨基酸残基47处罕见的精氨酸到组氨酸突变（R47H）会增加患病风险。TREM2在调节AD中微胶质细胞对淀粉样斑块的反应中起关键作用，导致它们在斑块周围聚集并激活。我们之前表明，增加人类基因剂量对AD淀粉样沉积小鼠模型中与AD相关的缺陷具有神经保护作用。然而，R47H突变对人类TREM2介导的小胶质细胞重编程和神经保护的影响仍知之甚少。
在这里，我们创建了一个BAC转基因小鼠模型，在其同源基因组背景中表达具有R47H突变的人类TREM2（BAC-TREM2-R47H）。重要的是，本研究中使用的BAC经过工程改造，删除了BAC上其他TREM样基因的关键外显子，以防止过表达多个TREM样基因产生混淆效应。我们将BAC-TREM2-R47H小鼠与5xFAD[1]（一种AD淀粉样沉积小鼠模型）杂交，以评估淀粉样病理和小胶质细胞表型、转录组学以及关键剂量依赖性基因的表达。我们还比较了5xFAD/BAC-TREM2-R47H与5xFAD/BAC-TREM2中的关键发现。
BAC-TREM2和BAC-TREM2-R47H均显示出人类中通常发现的三种剪接异构体的正常表达。在5xFAD背景下，升高的TREM2-R47H基因剂量显著降低了斑块负担，尤其是丝状斑块类型。结果与体外BAC-TREM2-R47H小胶质细胞中吞噬作用增强和NLRP3炎性小体激活改变一致。然而，与TREM2过表达不同，5xFAD中升高的TREM2-R47H未能改善认知和转录组缺陷。对关键剂量依赖性基因和小胶质细胞形态的分析发现，TREM2-R47H在5xFAD中斑块相关小胶质细胞反应性和基因表达的重编程中表现出功能丧失表型。
我们的研究表明，AD风险变体在调节AD小鼠大脑中的小胶质细胞反应方面具有先前未知的、部分和完全丧失TREM2功能的混合情况。总之，我们新的BAC-TREM2-R47H模型和先前的BAC-TREM2小鼠是促进治疗发现的宝贵资源，这些发现以人类TREM2及其R47H变体为靶点，以改善AD和其他神经退行性疾病。
