**Single Cell RNA Sequencing Identifies Subsets of Hepatic Stellate Cells and Myofibroblasts in Liver Fibrosis**

#### **Oliver Krenkel 1,2 , Jana Hundertmark 1,3 , Thomas P. Ritz <sup>1</sup> , Ralf Weiskirchen <sup>4</sup> and Frank Tacke 3, \***


Received: 1 May 2019; Accepted: 22 May 2019; Published: 24 May 2019

**Abstract:** Activation of hepatic stellate cells (HSCs) and their trans-differentiation towards collagen-secreting myofibroblasts (MFB) promote liver fibrosis progression. During chronic liver disease, resting HSCs become activated by inflammatory and injury signals. However, HSCs/MFB not only produce collagen, but also secrete cytokines, participate in metabolism, and have biomechanical properties. We herein aimed to characterize the heterogeneity of these liver mesenchymal cells by single cell RNA sequencing. In vivo resting HSCs or activated MFB were isolated from C57BL6/J mice challenged by carbon tetrachloride (CCl4) intraperitoneally for 3 weeks to induce liver fibrosis and compared to in vitro cultivated MFB. While resting HSCs formed a homogenous population characterized by high platelet derived growth factor receptor β (PDGFRβ) expression, in vivo and in vitro activated MFB split into heterogeneous populations, characterized by α-smooth muscle actin (α-SMA), collagens, or immunological markers. S100 calcium binding protein A6 (S100A6) was a universal marker of activated MFB on both the gene and protein expression level. Compared to the heterogeneity of in vivo MFB, MFB in vitro sequentially and only transiently expressed marker genes, such as chemokines, during culture activation. Taken together, our data demonstrate the heterogeneity of HSCs and MFB, indicating the existence of functionally relevant subsets in hepatic fibrosis.

**Keywords:** hepatic stellate cells; myofibroblasts; liver fibrosis; scRNASeq
