Osr1-mediated mesothelial transition of liver mesenchymal cells exacerbates fibrotic liver damage

In chronic liver diseases, hepatic stellate cells (HSCs) are induced to form the myofibroblasts responsible for scar formation, leading to liver fibrosis and cirrhosis. Here, single-cell RNA sequencing with in vivo lineage tracing in non-alcoholic steatohepatitis (NASH) model mice reveals a subpopulation of HSCs transitioning back to a state resembling their developmental precursors, mesothelial cells (MCs), after liver injury. These damage-associated intermediates between HSCs and MCs (DIHMs) can be traced with a dual recombinase system by labeling Krt19-expressing cells within pre-labeled Pdgfrb-positive HSCs, and DIHMs highly express inflammation- and fibrosis-associated genes. Cre and Dre-inducible depletion of DIHMs by administering diphtheria toxin reduces liver fibrosis and alleviates liver damage in NASH model mice. Importantly, knockdown of Osr1, a zinc finger transcription factor of the OSR gene family, can block DIHM induction in vitro. Conditional knockout Osr1 in Pdgfrb-expressing mesenchymal cells in NASH model mice can reduce liver fibrosis in vivo. Our study collectively uncovers an injury-induced developmental reversion process wherein HSCs undergo what we term a mesenchymal-to-mesothelial transition (MMesoT), which can be targeted to develop interventions to treat chronic liver diseases. To characterize the molecular identities and cell dynamics of myofibroblasts or activated HSC underlying the development, progression, and resolution of non-alcoholic steatohepatitis (NASH), we induced a mouse model of NASH using a methionine- and choline-deficient diet combined with a 45% high-fat diet and supplemented with 0.1% L-methionine in drinking water (MCDHF) for eight weeks. In addition, Col1a2-CreERT2; R26RtdTomato reporter mice were used to enrich Col1a2 positive myofibroblasts as an individual sample.