Interleukin-4 induces neural stem cell plasticity by antagonizing Serotonin/HTR1 signaling that suppresses BDNF/NGFRA in adult zebrafish Alzheimer's disease model

In Alzheimer's disease (AD), reduced neural stem cell (NSC) plasticity causes reduced neurogenesis. Therefore, supplying the brain with new neurons using endogenous neurogenic reservoir – NSCs - might counteract the progression of AD. However, the mechanisms that enhance NSC plasticity are unknown. We recently generated an AD model in adult zebrafish brain where NSCs could react by enhanced plasticity and neurogenesis, and identified Interleukin-4 (IL4) as a key factor underlying this ability. Although IL4 directly regulated NSCs through its receptor IL4R, it was not clear how IL4R-negative NSCs would enhance their proliferation. Here, by performing whole tissue and single cell transcriptomics, we report that IL4 regulates IL4R-negative NSCs through modulating tryptophan metabolism by reducing the availability of Serotonin (5-HT) that suppresses NSC plasticity. 5-HT receptor htr1+ is only expressed in periventricular neurons where 5-HT balances the expression of brain-derived neurotrophic factor (bdnf), which promotes NSC proliferation through its receptor NGFRA, which is predominantly expressed in IL4R-negative NSCs. AD conditions induce IL4 that reduce 5-HT levels, and suppresses the suppressive effect on BDNF levels. Elevated levels of BDNF activate, through NGFRA, the proliferative output of the IL4R-negative NSCs. 5-HT overexpression dramatically reduces BDNF and proliferative ability of NSCs. Our results identify a novel IL4-dependent balancing mechanism on NSC proliferation by 5-HT through an intermediary regulatory cascade via HTR1, and BDNF/NGFRA signaling between periventricular neurons and NSCs. Our findings mark the heterogeneity of NSCs in response to direct or indirect regulation by IL4 – a biomarker for neurodegeneration-induced NSC plasticity. Overall design: 6 months old Tg(her4.1:EGFP) adult fish brains were injected with CPP or AB42 using CVMI method (Kizil etal, 2013). 48 hpi brains were dissected, dissocited and non-GFP (GFP-N) and GFP positive (GFP-P) cells were sorted with FACS. Following total RNA isolation, samples were sequenced with triple replicates on Illumina HiSeq 2000 platform (75 bp end-read).

阿尔茨海默病（Alzheimer's disease, AD）中，神经干细胞（neural stem cell, NSC）可塑性降低会导致神经发生受损。因此，利用内源性神经发生储备库——神经干细胞为大脑补充新生神经元，或可延缓阿尔茨海默病的进展。不过，增强神经干细胞可塑性的具体机制仍未明确。我们近期在成年斑马鱼大脑中构建了阿尔茨海默病模型，其中神经干细胞可通过增强可塑性和神经发生作出反应，并鉴定出白细胞介素-4（Interleukin-4, IL4）是介导这一过程的关键因子。尽管IL4可通过其受体IL4R直接调控神经干细胞，但IL4R阴性的神经干细胞如何增强其增殖能力仍不明确。本研究通过全组织转录组学与单细胞转录组学分析，发现IL4可通过调节色氨酸代谢，降低抑制神经干细胞可塑性的血清素（Serotonin, 5-HT）的可用性，从而间接调控IL4R阴性的神经干细胞。血清素受体htr1+仅表达于室管膜下神经元，血清素可通过该神经元平衡脑源性神经营养因子（brain-derived neurotrophic factor, BDNF）的表达；而BDNF可通过其主要表达于IL4R阴性神经干细胞的受体NGFRA，促进神经干细胞增殖。阿尔茨海默病病理状态下，IL4表达上调可降低血清素水平，并削弱血清素对BDNF表达的抑制作用。升高的BDNF可通过NGFRA激活IL4R阴性神经干细胞的增殖活性。血清素过表达则会显著降低BDNF水平与神经干细胞的增殖能力。本研究揭示了一种全新的IL4依赖的调控机制：通过HTR1介导的中间调控级联，以及室管膜下神经元与神经干细胞之间的BDNF/NGFRA信号通路，实现血清素对神经干细胞增殖的平衡调控。我们的研究结果凸显了神经干细胞在应对IL4直接或间接调控时的异质性——IL4可作为神经退行性疾病诱导的神经干细胞可塑性的生物标志物。实验整体设计：将CPP或AB42通过CVMI方法注射至6月龄Tg(her4.1:EGFP)成年斑马鱼大脑中（Kizil等人，2013年）。于注射后48小时（hours post injection, hpi）取脑组织，解离后通过荧光激活细胞分选（Fluorescence-Activated Cell Sorting, FACS）分离非GFP阳性（GFP-N）与GFP阳性（GFP-P）细胞。提取总RNA后，设置三次生物学重复，采用Illumina HiSeq 2000平台进行测序（读长75 bp，末端读段）。