Deciphering the role of TREM2 in Non-Alcoholic Steatohepatitis

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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.