Building the liver, from human pluripotent stem cells, is an enormous scientific challenge and could help for chronic liver diseases like cirrhosis for which is there is no medical treatment.
The field of liver regenerative medicine is a mature field that has expanded into several arenas, including liver regeneration, liver cell therapy, liver development and molecular control, in vitro hepatic culture, bioartificial livers, human-in-mouse livers, microphysiological systems, human adult and pluripotent stem cells (hPSC), and hepatic organoids. Tools and technologies that employ human hepatocytes (hHEPs) are limited because donor hepatocytes are scarce and of variable quality; similarly, tools that utilize three dimensional (3D) liver tissue, like organ transplantation, are limited by human donor organs. In vitro systems and hepatic organoids, which are critical for drug development, are not only limited by the lack of hHEPs, but also the lack of an intact hepatobiliary system, the lack 3D hepatic architecture (hepatic cords), the lack of hepatocyte proliferation, and the inability to rapidly expand, as occurs during liver development. Furthermore, human-in-mouse livers are limited because non-parenchymal tissue is of mouse, not human, origin. Liver cell therapies are limited because the lack of engraftment but also because replacing liver mass would require 20-200 million hHEPs, which is not practical to obtain or deliver. Finally, driving self-renewable PSC to form hHEPs could solve several problems, but currently they are functionally immature.
We propose re-examining developmental programs, and learning about and applying principles of liver development may enable us to address limitations within liver regenerative medicine. The liver arises from the endoderm cell germ layer, which is divided into foregut, midgut, and hindgut. The ventral (anterior) foregut endoderm, an epithelial sheet which forms a tube and thickens at E8.5 in mouse, not only gives rise to the liver, but also the pancreas, hepatobiliary system, lung, ventral pancreas, and thyroid gland. In the mouse, hepatic endoderm, with albumin gene activated and expressing HNF4a, AFP, and other liver transcription factors, receives signals from an apposing layer of endothelial cells, cardiac mesoderm (Fgf2), septum transversum mesenchyme (BMP4), and delaminate into the mesenchyme, forming hepatoblasts. These hepatoblasts express the master factors Hex, Prox1, Hlx, Tbx3, and c-Met. Reciprocal interactions between hepatoblasts and septum transversum mesenchyme lead to rapid growth and formation of the hepatic cords within the embryonic liver by E11.5. From E12.5-E18, the liver becomes the sight of fetal hematopoiesis, which coincides with both massive proliferation (106 fold increase in 8 days) and maturation into fetal hepatocytes.
If we could understand how to engineer 3D hepatic and hepatobilliary tissue from hPSC in vitro and in vivo, we could address nearly all of these issues. We are designing hPSC-derived hepatic organoids that are bioinspired by developmental principles to achieve these goals. In these next generation hepatic organoids, we are evaluating critical aspects of epithelial cell biology, cell and tissue morphogenesis, self-assembly, cell-mesenchymal interactions, and 3D growth. Further, these organoids are being evaluated for in vitro hepatobiliary differentiation, hepatic cord formation, hepatic functional maturity, and for 3D liver tissue generation in vitro and in vivo. Finally, we are developing these organoids for disease modeling applications, and developing diagnostics tests for liver fibrosis. Overall, we expect that these bioinspired hepatic organoids are expected to address several limitations within liver regenerative medicine.