Deciphering the role of TREM2 in Non-Alcoholic Steatohepatitis

项目摘要 非酒精性脂肪性肝炎(NASH)是非酒精性脂肪性肝病(NAFLD)的一种侵袭性形式， 以肝脏脂肪堆积、肝脏损伤、炎症和纤维化为特征。NASH的流行已经 在过去的十年里，它急剧上升，成为与肝脏相关的发病率和死亡率的主要原因。 这是全球范围内的一个主要原因，也是肝移植的主要原因。饮食肥胖是NAFLD的诱因 肝脏中脂质过度堆积，导致肝细胞死亡和随后宿主来源的释放 损伤相关的分子模式，进而激活肝巨噬细胞，点燃肝脏炎症。是这样的 炎症的特征是慢性产生促炎细胞因子，包括肿瘤坏死因子、白介素6和白介素1b。 过去十年的几项里程碑式的研究共同表明，慢性肝炎是关键 将单纯性脂肪变性转变为NASH。然而，饮食肥胖是如何促进 肝脏慢性炎症的建立仍然难以捉摸。最近，多个单细胞转录本 研究表明，在髓系细胞2(TREM2)中表达的触发受体的出现 NASH、肝硬变和肝细胞癌患者体内高度浓缩的巨噬细胞群。 为了研究巨噬细胞TREM2在NASH发病机制中的作用，我们产生了髓系细胞特异性TREM2 并将它们置于西方饮食诱导的NASH模型中。我们发现巨噬细胞 TREM2保护小鼠免受NASH发展。值得注意的是，我们意外地发现，尽管它的信使核糖核酸 在整个NASH进展过程中，TREM2蛋白不断上调，只有在单纯性脂肪变性中才会增加，但 差点在纳什被淘汰。我们进一步证明了NASH中TREM2蛋白的急剧下降 是由于巨噬细胞表面存在的全长TREM2被蛋白水解性切割所致。这样做的总体目标是 建议综合研究(1)TREM2在NASH致病过程中的表达调控 进展，(2)TREM2在巨噬细胞中起什么作用来限制NASH的发展，以及(3)是否阻止 TREM2裂解可抑制NASH进展。为实现这一目标，我们将落实以下三项具体工作 目标。在目标1中，我们将破译TREM2被动态调节的分子机制 纳什进展。具体地说，我们将确定负责TREM2上调和下调的关键信号通路- 分别在单纯性脂肪变性和NASH阶段进行调节。在目标2中，我们将测试TREM2是否起到关键作用 在巨噬细胞吞噬脂质诱导的肝细胞凋亡中的作用从而限制慢性肝脏 炎症和NASH的发展。最后，在目标3中，通过利用抗切割的TREM2敲入(TREM2- IPD)小鼠，我们将在体内进行概念验证测试，以确定是否阻止TREM2裂解以恢复 巨噬细胞泡腾作用可抑制NASH。这项研究的完成不仅将提供急需的 解释长期高营养如何导致慢性肝炎的机械论见解，但也会 为设计抗TREM2裂解途径治疗NASH奠定了具体基础。

Project Summary Nonalcoholic steatohepatitis (NASH), an aggressive form of nonalcoholic fatty liver disease (NAFLD), is characterized by hepatic lipid buildup, liver damage, inflammation, and fibrosis. The prevalence of NASH has skyrocketed during the past decade, making it the leading cause of liver-related morbidity and mortality worldwide and a primary reason for liver transplantation. Dietary obesity, the trigger of NAFLD, induces excessive lipid accumulation in the liver, causing hepatocyte death and subsequent release of host-derived damage-associated molecular patterns that in turn activate liver macrophage to ignite hepatic inflammation. Such inflammation is featured by chronic production of proinflammatory cytokines, including TNF, IL-6, and IL-1b. Several landmark studies in the past decade have collectively shown that chronic liver inflammation is the key switch mediating simple steatosis transition into NASH. However, how dietary obesity promotes the establishment of chronic inflammation in the liver remains elusive. Recently, multiple single-cell transcriptomic studies revealed the emergence of a triggering receptor expressed in myeloid cell 2 (TREM2)-expressing macrophage population that is highly enriched in patients with NASH, cirrhosis and hepatocellular carcinoma. To study the role of macrophage TREM2 in NASH pathogenesis, we generated myeloid cell-specific Trem2 knockout mice and subjected them to a western diet-induced NASH model. We discovered that macrophage TREM2 protects mice against NASH development. Of note, we unexpectedly found that despite its mRNA being continuously upregulated throughout NASH progression, TREM2 protein only increases in simple steatosis but almost gets eliminated at NASH. We further demonstrated that the dramatic decline of TREM2 protein in NASH is due to proteolytic cleavage of full-length TREM2 present on macrophage surface. The overall objective of this proposal is to comprehensively investigate (1) how TREM2 expression is regulated during NASH pathogenic progression, (2) what TREM2 does in macrophages to restrict NASH development, and (3) whether blocking TREM2 cleavage can inhibit NASH progression. To achieve this goal, we will pursue the following three specific aims. In Aim 1, we will decipher the molecular mechanism by which TREM2 is dynamically regulated during NASH progression. Specifically, we will identify key signaling pathways responsible for TREM2 up- and down- regulation at simple steatosis and NASH stages, respectively. In Aim 2, we will test whether TREM2 plays a key role in macrophage efferocytosis of lipid-laden apoptotic hepatocytes and thereby restrict chronic liver inflammation and NASH development. Lastly, in Aim 3, by utilizing a cleavage-resistant Trem2 knock-in (Trem2- IPD) mice, we will perform a proof-of-concept in vivo test to determine if blocking TREM2 cleavage to restore macrophage efferocytosis can inhibit NASH. Completion of this study will not only provide much-needed mechanistic insights explaining how prolonged hypernutrition results in chronic liver inflammation, but also will establish a concrete foundation for designing anti TREM2 cleavage approaches to treat NASH.