BACKGROUND Accumulating data suggest a central role for brain microglia in mediating cortical neuronal death in Alzheimer's disease (AD), and for Toll-like receptor 2 (TLR2) in their toxic activation. Amyloid deposition in preclinical AD is associated with microglial activation but not directly with neurodegeneration. We examined in transgenic 5xFAD mice the hypothesis that systemic TLR2 agonists, derived from common infectious agents, may accelerate neurodegeneration in AD. METHODS Microbial wall-derived TLR2 agonists zymosan and lipoteichoic acid were administered intraperitoneally or intracerebroventricularly to 7-month-old wild-type or 5xFAD mice. Immunofluorescent stainings were used to quantify cortical neurons and evaluate tissue reaction. Microglial activation was assessed using functional assays, RNA expression, and FACS analysis. RESULTS Repeated low-dose systemic administration of zymosan or lipoteichoic acid killed cortical neurons in 5xFAD mice but not in wild-type mice. Direct CNS delivery of a selective TLR2 antagonist blocked the neurotoxicity of systemically administered zymosan, indicating that CNS TLR2 mediates this effect. Systemically administered zymosan crossed the disrupted blood-brain barrier in 5xFAD mice and entered brain parenchyma. By intracerebroventricular delivery, we found a dose- and exposure time-dependent acute neurotoxic effect of the microbial TLR2 agonist, killing cortical neurons. 5xFAD mice exhibited significantly increased vulnerability to TLR2 agonist-induced neuronal loss as compared to wild-type mice. Microbial TLR2-induced neurodegeneration was abolished by inhibiting microglia. The vulnerability of 5xFAD mice brains was mediated by an increase in number and neurotoxic phenotype of TLR2-expressing microglia. CONCLUSIONS We suggest that repeated exposure to microbial TLR2 agonists may facilitate neurodegeneration in AD by their microglial-mediated toxicity to the hyper-vulnerable environment of the AD brain.

中文翻译：

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系统性微生物TLR2激动剂在阿尔茨海默氏病小鼠中诱导神经变性。

背景技术越来越多的数据表明，脑小胶质细胞在介导阿尔茨海默氏病（AD）中的皮质神经元死亡中起着重要作用，而Toll样受体2（TLR2）在其毒性激活中起着重要作用。临床前AD中的淀粉样蛋白沉积与小胶质细胞活化有关，但与神经变性不直接相关。我们在转基因的5xFAD小鼠中检查了以下假设：源自常见感染因子的全身性TLR2激动剂可能会加速AD的神经变性。方法对7个月大的野生型或5xFAD小鼠腹膜内或脑室内给予微生物源性TLR2激动剂酵母聚糖和脂蛋白酸。免疫荧光染色用于定量皮层神经元并评估组织反应。使用功能测定，RNA表达和FACS分析评估小胶质细胞的激活。结果反复低剂量全身性给予酵母聚糖或脂磷壁酸可杀死5xFAD小鼠的皮质神经元，但不会杀死野生型小鼠。选择性TLR2拮抗剂的直接CNS传递阻断了全身施用的酵母聚糖的神经毒性，表明CNS TLR2介导了这一作用。全身施用的酵母聚糖穿越了5xFAD小鼠的血脑屏障，并进入了脑实质。通过脑室内递送，我们发现了微生物TLR2激动剂的剂量和暴露时间依赖性的急性神经毒性作用，可杀死皮质神经元。与野生型小鼠相比，5xFAD小鼠对TLR2激动剂诱导的神经元丢失表现出明显增加的脆弱性。通过抑制小胶质细胞消除了微生物TLR2诱导的神经变性。5xFAD小鼠大脑的脆弱性由表达TLR2的小胶质细胞的数量和神经毒性表型增加介导。结论我们建议，反复暴露于微生物TLR2激动剂可能通过其对神经胶质细胞的高脆弱环境介导的毒性而促进AD的神经变性。