Mechanism of innate immune activation in a Drosophila model of Alzheimer's disease related dementia

摘要： 路易体痴呆症是一种与阿尔茨海默病相关的痴呆症，影响着100多万美国人。 这种毁灭性的神经退行性疾病的最大风险因素是GBA基因的突变，这是 编码溶酶体酶葡萄糖脑苷酶(GCase)。GBA突变会削弱GCase 分解其底物，神经鞘糖脂(GlcCer)，但尚不清楚缺陷是如何在 GlcCer的崩溃增加了阿尔茨海默病相关痴呆的风险。为了研究这个问题，我们 创造了一种缺少Gba的果蝇同源基因Gba1b的果蝇，作为GCase缺乏症的模型。我们的Gba1b突变体 显示了GlcCer的积聚，并概括了阿尔茨海默病相关痴呆的特征，包括 神经退行性变，大脑蛋白质聚集体，以及与年龄相关的认知衰退。我们现在建议使用 Gba1b突变体测试与GCase缺陷相关的神经退行性变的新模型，其中GlcCer 积聚通过细胞外小泡(EVS)的变化导致神经炎症。在早期的研究中，我们发现 Gba1b突变体增加了EV蛋白的丰度和周转，并从基因上抑制了 神经元EV的产生改善了突变表型。最近，RNA-Seq实验揭示了 Gba1b突变体具有明显的天然免疫激活作用。主要先天免疫途径的RNAi追踪筛查 发现神经元下调p38MAPK通路的两个组成部分，转录因子ATF-2和其 上游的甘草酸激酶，也抑制Gba1b突变表型。基于这些和其他发现，我们 假设Gba1b突变体的免疫介导的神经变性是由过量的GlcCer驱动的过程 扮演两个不同的角色。具体地说，我们假设神经元中过多的GlcCer触发非配体依赖 受体酪氨酸激酶活性和p38MAPK信号，刺激EV释放；释放EV中的GlcCer 然后使胶质细胞分泌免疫效应蛋白，导致蛋白质聚集和神经变性。我们 提出三个目标来解决这些假说。第一个将描绘神经元内信号通路， 导致EV异常释放；第二个将调查EV改变的性质；第三个将确定 这些电动汽车是如何触发神经胶质细胞分泌免疫效应器的。考虑到大量的神经炎症证据 在阿尔茨海默病和相关痴呆症方面，我们预计我们的工作将具有广泛的医学意义。

ABSTRACT: Lewy body dementia is an Alzheimer’s disease–related dementia that affects more than one million Americans. The strongest risk factor for this devastating neurodegenerative disease is mutation of the GBA gene, which encodes the lysosomal enzyme glucocerebrosidase (GCase). GBA mutations can impair the ability of GCase to break down its substrate, the glycosphingolipid glucosylceramide (GlcCer), but it is not clear how defects in GlcCer breakdown increase the risk of Alzheimer’s disease–related dementias. To study this problem, we created a fly lacking the Drosophila ortholog of GBA, Gba1b, as a model of GCase deficiency. Our Gba1b mutant shows accumulation of GlcCer and recapitulates features of Alzheimer’s disease–related dementias including neurodegeneration, brain protein aggregates, and age-related cognitive decline. We now propose to use the Gba1b mutant to test a novel model of neurodegeneration associated with GCase deficiency, in which GlcCer accumulation leads to neuroinflammation via changes in extracellular vesicles (EVs). In earlier work, we found that Gba1b mutants had increased abundance and turnover of EV proteins, and that genetically suppressing neuronal EV production ameliorated mutant phenotypes. More recently, RNA-Seq experiments revealed that Gba1b mutants had marked innate immune activation. A followup RNAi screen of major innate immune pathways found that neuronal knockdown of two p38 MAPK pathway components, the transcription factor Atf-2 and its upstream kinase licorne, also suppressed Gba1b mutant phenotypes. Based on these and other findings, we hypothesize that immune-mediated neurodegeneration in Gba1b mutants is a process driven by excess GlcCer in two separate roles. Specifically, we hypothesize that excess GlcCer in neurons triggers ligand-independent receptor tyrosine kinase activity and p38 MAPK signaling, stimulating EV release; GlcCer in the released EVs then causes glia to secrete immune effector proteins, leading to protein aggregation and neurodegeneration. We propose three aims to address these hypotheses. The first will delineate the intraneuronal signaling pathway that leads to abnormal EV release; the second will investigate the nature of the EV alterations; the third will determine how those EVs trigger immune effector secretion from glia. Given the abundant evidence for neuroinflammation in Alzheimer’s disease and related dementias, we anticipate that our work will have broad medical significance.