Microglial heparan sulfate in the modulation of APOE function and neurodegeneration

硫酸乙酰肝素(HS)是一种表达在细胞表面和细胞外基质中的硫化多糖，具有很长的 根据间接证据，作为阿尔茨海默病(AD)的推定因素引起了人们的注意 来自体外和临床病理研究的证据。尽管如此，HS在中国的功能意义 AD，特别是占AD病例90%以上的晚发型(LOAD)，是难以捉摸的。近期 人类基因组研究表明载脂蛋白E和小胶质细胞是神经变性的主要贡献者 装填。载脂蛋白E基因是已知的最强的负荷遗传风险因素。累积的数据表明 ApoE蛋白对小胶质细胞具有免疫调节作用，并且ApoE的这种作用至少是 部分由TREM2受体介导，这是另一个由小胶质细胞表达的AD危险因子。在这些人中 人类的三个载脂蛋白E等位基因中，载脂蛋白4等位基因增加了负荷风险，而载脂蛋白2等位基因 与更常见的APOE3等位基因相比，风险降低。这些等位基因的分子基础- 具体的风险变异是AD研究中尚未回答的关键问题之一。在这种情况下，有趣的是 要注意的是，ApoE蛋白以一种异构体特异性的方式结合HS-ApoE4表现出2-3倍的 对HS的亲和力高于APOE2和ApoE3，因此明显与他们的相对AD风险相关。此外， 最近关于一名哥伦比亚妇女的独特病例的报道，她携带了一种非常有害的 PSEN1E280A突变，表明她的APOE3“克赖斯特彻奇”突变，这取消了亲和力 APOE3对于HS，提供强大的保护，防止神经退化和认知障碍。也是 提示HS在LOAD中的功能参与是HS基因关联的反复报道 磺基转移酶基因的负载量。在一系列初步研究中，我们获得了多件 证据表明，HS与ApoE4的强相互作用，相对于其与APOE2的相互作用 和ApoE3，是ApoE4对大脑产生有害影响的关键潜在机制。 值得注意的是，我们发现HS表达的单倍体不足减少导致了 PS19TauP310S小鼠模型中突触丢失和小胶质细胞激活。我们假设HS会玩 在调节载脂蛋白E4对小胶质细胞反应和功能的影响中起着关键作用，而有害的 APOE4等位基因的作用主要是由于它与富含3-O硫酸盐的HS物种有更强的相互作用 表达于小胶质细胞。我们将：(1)确定HS在ApoE-TREM2相互作用和信号转导中的作用 小胶质细胞；(2)确定小胶质细胞HS在ApoE/ABCA1介导的胆固醇外流中的作用；以及(3) 观察小胶质细胞HS对AD模型小鼠脑病理和脑功能的影响。这 研究将为HS在小胶质细胞中的作用提供全新的见解，而且还将提供长期的 关于载脂蛋白E变异体的异构体特定风险的问题。这个项目的成功完成将 揭示ApoE4-HS相互作用是降低APOE4携带者AD风险的有前景的治疗靶点。

Heparan sulfate (HS), a sulfated glycan expressed at the cell surface and in extracellular matrix, has long attracted attention as a putative factor involved in Alzheimer's disease (AD), based on circumstantial evidence from in vitro and clinicopathological studies. Nevertheless, the functional significance of HS in AD, especially in the late-onset type (LOAD) that comprises more than 90% of AD cases, is elusive. Recent human genomic studies implicate ApoE and microglia as dominant contributors to neurodegeneration in LOAD. The APOE gene is the strongest known genetic risk factor for LOAD. Accumulating data indicate that ApoE proteins exert immunomodulatory effects on microglia, and that this function of ApoE is at least partly mediated by the TREM2 receptor, another AD risk factor that is expressed by microglia. Among the three human APOE alleles, the APOE4 allele confers an increased risk for LOAD, while the APOE2 allele confers a decreased risk relative to the more common APOE3 allele. The molecular basis of these allele- specific risk variations is one of the key unanswered issues in AD research. In this context, it is interesting to note that ApoE proteins bind HS in an isoform-specific manner — ApoE4 exhibits 2- to 3-fold greater affinity for HS than ApoE2 and ApoE3, thus apparently correlating with their relative AD risks. Furthermore, a recent report regarding the unique case of a Columbian woman, who carries a highly detrimental PSEN1E280A mutation, suggests that her APOE3 "Christchurch" mutation, which abolishes the affinity of ApoE3 for HS, confers strong protection against neurodegeneration and cognitive impairment. Also suggesting the functional involvement of HS in LOAD are recurrent reports of genetic association of HS sulfotransferase genes with LOAD. In a series of preliminary studies, we have obtained multiple pieces of evidence suggesting that the strong interaction of HS with ApoE4, relative to its interactions with ApoE2 and ApoE3, is the key underlying mechanism by which ApoE4 exerts a detrimental effect on the brain. Significantly, we have found that haploinsufficient reduction in HS expression leads to the mitigation of synapse loss and microglial activation in the PS19 TauP310S mouse model. We hypothesize that HS plays a critical role in mediating the ApoE4 effect on microglial response and function, and that the detrimental effect of the APOE4 allele is mainly due to its stronger interaction with the 3-O sulfate-rich HS species expressed in microglia. We will: (1) determine the role of HS in ApoE-TREM2 interaction and signaling in microglia; (2) determine the role of microglial HS in ApoE/ABCA1-mediated cholesterol efflux; and (3) examine in vivo effects of microglial HS on brain pathology and function in AD mouse models. This research will provide entirely novel insights into the role of HS in microglia, and moreover, into long-term questions regarding the isoform-specific risk of APOE variants. Successful completion of this project will reveal the ApoE4-HS interaction as a promising therapeutic target for reducing AD risk in APOE4 carriers.