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，影响着超过一百万的美国人。 这种毁灭性神经退行性疾病的最大风险因素是GBA基因突变， 编码溶酶体酶葡糖脑苷脂酶（GCase）。GBA突变可以削弱GCase的能力， 分解其底物，鞘糖脂葡萄糖神经酰胺（GlcCer），但目前尚不清楚如何缺陷， GlcCer分解增加阿尔茨海默病相关痴呆症的风险。为了研究这个问题，我们 创造了一种缺乏GBA的果蝇直系同源物Gba 1b的果蝇，作为GCase缺陷的模型。我们的Gba 1b突变体 显示GlcCer的积累，并概括了阿尔茨海默病相关痴呆的特征，包括 神经变性、脑蛋白质聚集和年龄相关的认知衰退。我们现在建议使用 Gba 1b突变体，以测试与GCase缺乏相关的神经变性的新模型，其中GlcCer 这种累积通过细胞外囊泡（EV）的变化导致神经炎症。在早期的工作中，我们发现 Gba 1b突变体增加了EV蛋白的丰度和周转， 神经元EV产生改善了突变体表型。最近，RNA-Seq实验显示， Gba 1b突变体具有明显的先天免疫激活。主要先天免疫途径的后续RNAi筛选 发现神经元敲低两种p38 MAPK通路成分，转录因子Atf-2及其受体， 上游激酶独角兽，也抑制Gba 1b突变表型。基于这些和其他发现，我们 假设Gba 1b突变体中免疫介导神经变性是由过量GlcCer驱动的过程 扮演两个不同的角色具体来说，我们假设神经元中过量的GlcCer触发配体非依赖性的 受体酪氨酸激酶活性和p38 MAPK信号传导，刺激EV释放;释放的EV中的GlcCer 然后导致神经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设。第一个将描绘神经元内信号通路， 导致异常EV释放;第二个将调查EV改变的性质;第三个将确定 这些EV如何触发神经胶质细胞的免疫效应分泌。鉴于神经炎症的大量证据 在阿尔茨海默病和相关痴呆症中，我们预期我们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.