Indiana University-Purdue University Indianapolis (IUPUI) / The maternal liver shows marked growth during pregnancy to accommodate the development and metabolic needs of the placenta and fetus. Previous study has shown that the maternal liver grows proportionally to the increase in body weight during gestation by hyperplasia and hypertrophy of hepatocytes. As the maternal liver is enlarged, the transcript level of Ascl1, a transcription factor essential to progenitor cells of the central nervous system and peripheral nervous system, is highly upregulated. The aims of the study were to (1) identify hepatic Ascl1-expressing cells, and (2) study the functions of Ascl1 in maternal liver during pregnancy. In situ hybridization shows that most cell types (parenchymal, nonparenchymal, and mesothelial cells) express Ascl1 mRNA in maternal livers during gestation and in male regenerating livers. Notably, hepatic mesothelial cells abundantly express Ascl1 during pregnancy and liver regeneration. Inducible ablation of Ascl1 gene during pregnancy results in maternal liver enlargement, litter size reduction, and fetal growth retardation. In addition, maternal hepatocytes deficient in Ascl1 gene lack majority of their cytosols and exhibit β-catenin nuclear translocation, while maintaining their cellular boundary and identity. In summary, in both maternal liver during pregnancy and regenerating liver, the expression of Ascl1 is induced in most cell types. Mesothelial cells are potential origin of Ascl1-expressing cells. Ascl1 gene is essential for the progression of normal pregnancy

Indiana University-Purdue University Indianapolis (IUPUI) / During pregnancy, maternal liver exhibits robust growth to meet the metabolic demands of the developing placenta and fetus. Although hepatocyte hypertrophy and hyperplasia are seen in the maternal liver, the molecular and cellular mechanisms mediating the maternal hepatic adaptations to pregnancy is poorly understood. Previous microarray analysis revealed a most upregulated gene named Ascl1, a transcription factor essential for neural development, in the maternal liver at mid-gestation. The aims of the study were to (1) validate the activation of Ascl1 gene; (2) identify Ascl1-expressing cells; and (3) determine the fate of Ascl1-expressing cells, in the maternal liver during the course of gestation. Timed pregnancy was setup in mice and the maternal livers were collected at various stages of gestation. Maternal hepatic Ascl1 mRNA expression was evaluated by qRT-PCR and northern blotting. The results demonstrated that the transcript level of maternal hepatic Ascl1 is exponentially increased during the second half of pregnancy in comparison with a non-pregnant state. Using a Ascl1-GFP mouse model generated by others to monitor the behavior of neural progenitor cells, we found that maternal hepatic Ascl1-expressing cells are non-parenchymal cells, very small in size, and expanding during pregnancy. To map the fate of this cell population, we generated an in vivo tracing mouse model named Ascl1-CreERT2/ROSA26-LacZ. Using this model, we permanently labeled maternal hepatic Ascl1-expressing cells at midgestation by giving tamoxifen and analyzed the labeled cells in the maternal liver prior to parturition. We observed that the initial small Ascl1-expressing cells undergoing expansion at mid-gestation eventually became hepatocyte-like cells at the end stage of pregnancy. Taken together, our findings strongly suggest that Ascl1-expressing cells represent a novel population of hepatic progenitor cells and they can differentiate along hepatocyte lineage and contribute to pregnancy-induced maternal liver growth. Further studies are needed to firmly establish the nature and property of maternal hepatic Ascl1-expressing cells. At this stage, we have gained significant insights into the cellular mechanism by which the maternal liver adapts to pregnancy.

Indiana University-Purdue University Indianapolis (IUPUI) / To cope with the high metabolic demands of the body during pregnancy, the maternal liver adapts by increasing its mass and size. This increase is proportional to the increase in total body weight during the course of gestation. The pregnancy-induced maternal liver growth is a result of both hepatocyte hypertrophy and hyperplasia. Microarray analysis of pregnant maternal livers shows markedly different gene expression profiles when compared to a non-pregnant state. Most interesting was the 2,500-fold up-regulation in the mRNA expression of Ascl1, a transcription factor responsible for the differentiation of neural progenitor cells into various neuronal types, during the second half of pregnancy. Our investigation aimed at (1) characterizing the identity of maternal hepatic Ascl1-expressing cells and (2) tracing the fate of Ascl1-expressing cells in the maternal liver during pregnancy. Timed pregnancies were generated and non-pregnant (NP) and pregnant maternal livers were harvested and analysed. To identify the maternal hepatic Ascl1-expressing cells we used the Ascl1GFP/+ reporter mouse line. NP and gestation day 15 (D15) maternal livers were immunostained for green fluorescent protein (GFP). The result shows that GFP-positive, Ascl1-expressing cells are hepatocyte-like cells, which are present in D15 maternal livers, but absent in NP livers. The Rosa26floxstopLacZ/ floxstopLacZ;Ascl1CreERT2/+ mouse line was used to trace the fate of Ascl1-expressing cells during pregnancy. LacZ staining of gestation day 13 (D13) and 18 (D18) maternal livers demonstrates that D13 hepatic Ascl1-expressing cells (labeled with LacZ) undergo hyperplasia to repopulate a large portion of D18 maternal livers. Furthermore, LacZ and HNF4α co-staining of D13 and D18 maternal livers shows the presence of two populations of LacZ-expressing cells: HNF4α+ population and HNF4α- population. HNF4α+ LacZ-expressing cells represent hepatocyte lineage cells that are derived from Ascl1-expressing cells. We observe that, towards the end of pregnancy, a considerable portion of the maternal liver is comprised of hepatocytes derived from Ascl1-expressing cells. Taken together, our preliminary study suggests that pregnancy induces maternal liver turnover via Ascl1-expressing cells.