(164av) Human Liver Diverticulum Triggers Liver Organogenesis

Many patients suffer from chronic hepatocellular injury, leading to fibrosis and cirrhosis, which ultimately overwhelms the adult liver’s regenerative capacities. Unfortunately, these chronic diseases are escalating globally, in both developing and developed countries, and currently affect more than 800 million people worldwide. The only accepted treatment is orthotopic liver transplantation, but there are several potential therapeutic solutions within the field of liver regenerative medicine. Of these, liver organogenesis has great potential, and has evolved from original studies in mice. Liver organogenesis begins with the liver diverticulum (LD), an out-pocketing of gut that is surrounded by mesoderm (MES) tissue. The role of the LD in liver organogenesis has been largely ignored in current mouse and human studies. We hypothesize that the LD and surrounding MES form a functional tissue complex (LD-MESC) that triggers organogenesis. To elucidate these triggers and better understand the dynamics of global transcriptomic changes in early liver organogenesis, we performed an in-depth bioinformatics analysis, which demonstrated that the murine LD-MESC integrates diverse signals to drive transcription and migration. Our analysis utilized single-cell transcriptomic data from fetal mice, which allowed us to identify and compare several unique cell types. We focused primarily on the transition from definite endoderm (DE, E75), gut tube endoderm (GT, E8.5), migrating hepatoblasts (MH, 9.5), hepatoblasts (HB, E10.5), and hepatomesenchyme (HM, E10.5). After performing quality control and normalization, we were able to analyze the change in gene expression throughout this time period. The data showed several thousand differentially expressed genes between the different cell types, particularly between MH and GT and HB (2102 differentially expressed genes). These differentially expressed gene lists were then able to be further analyzed with genomics tools from DAVID, REACTOME, ENRICHR, as well as our own scoring system. The several analyses used confirmed the significant enrichment of migration gene sets in the MH population, including cell migration (FDR=0.0032), cell adhesion (FDR=6.1E-19), and covalent chromatin modification (FDR=4E-20). Interestingly, we found significant enrichment of important signaling pathways in the MH compared to GT and HB, including Hippo (FDR=0.0039), FoxO (FDR=0.0092), pluripotency signaling (FDR=0.0073), Wnt (FDR=0.051), and TGF-beta (FDR=0.11), indicating a possible role of signaling pathways in migration. We further found significant changes in several metabolic pathways throughout differentiation. Oxidative phosphorylation was found to be a significant downregulated (FDR=1.4E-35) pathway compared to both GT and HB. This same result was found to be significant (p<0.001) when utilizing our scoring system, which considered all genes within the pathway. Many of these same gene dynamics were also observed and confirmed by analyzing additional transcriptomic data sets. A similar cell type at E9.5 was found in the Mu et al. dataset with both migration gene enrichment and signaling pathway enrichment. A few of these gene sets include, cell migration (FDR=0.0005), cell adhesion (1.7E-10), hippo signaling (FDR=3.9E-6), and wnt signaling (FDR=0.00076). Taken together, these studies elucidate details of genes expressed during differentiation and a role of signaling from the LD-MESC in triggering early liver organogenesis. These results will have broad implications in liver regenerative medicine.