Identifying targetable mechanisms of TMEM38B/TRIC-B in liver fibrosis

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ABSTRACT: Liver fibrosis impacts millions of people globally, with no pharmacologic therapies available to limit fibrosis progression or promote fibrosis regression. The major fibrogenic cells in the liver are Hepatic Stellate Cells (HSCs). Upon liver injury, activated HSCs produce and secrete fibrogenic proteins, including collagen Iα1 and 1α2, which are major component of fibrotic scarring in the liver. Several therapies have aimed to target HSCs to limit fibrogenesis by preventing HSC activation or promoting HSC apoptosis; however these strategies have not been successful in the clinic. We propose that understand the procollagen I trafficking machinery would provide a unique and viable strategy for limiting fibrosis. We performed single cell RNA sequencing on fibrogenic HSCs from mice and found upregulation of the understudied gene TMEM38B in fibrogenic HSCs. TMEM38B is implicated in procollagen I trafficking in bone-producing osteoblasts, and mutations in TMEM38B leads to brittle bones in mice and humans. We hypothesize that TMEM38B is critical for fibrogenesis through mediating procollagen I trafficking and HSC survival, and that targeting TMEM38B could serve as a viable anti- fibrotic strategy. Our preliminary data highlights a key role for TMEM38B in fibrogenesis. TMEM38B expression is increased in patients where liver fibrosis has progressed to cirrhosis, as well as in murine models of liver fibrosis. Secondly, TMEM38B is increased in primary human and mouse HSCs following activation with the pro-fibrotic cytokine TGFβ. Finally, we show that TMEM38B is negatively regulated by the Polycomb Repressor Complex 2 (PRC), a histone methyltransferase complex dysregulated in fibrosis. Disruption of the PRC2 complex leads to increased TMEM38B expression in HSCs, and increased procollagen I deposition in a murine model where key components of the PRC2 complex are deleted from osteoblasts. The experiments proposed here will increase our understanding of TMEM38B in 3 critical areas: 1) Elucidating the critical contribution of TMEM38B to fibrogenic processes, including procollagen I secretion and HSC survival, 2) interrogating the critical domains and post-translational regulation of TMEM38B channel activity, and 3) revealing mechanisms that regulate TMEM38B expression. Completion of the proposed research will provide crucial insight into TMEM38B function and regulation under both physiological and pathophysiological conditions as well as provide novel reagents to share with the research community, with the long term goal of targeting TMEM38B as an anti-fibrotic strategy.