Liver organoids: from basic research to therapeutic applications

Abstract

Organoid cultures have emerged as an alternative in vitro system to recapitulate tissues in a dish. While mouse models and cell lines have furthered our understanding of liver biology and associated diseases, they suffer in replicating key aspects of human liver tissue, in particular its complex architecture and metabolic functions. Liver organoids have now been established for multiple species from induced pluripotent stem cells, embryonic stem cells, hepatoblasts and adult tissue-derived cells. These represent a promising addition to our toolbox to gain a deeper understanding of this complex organ. In this perspective we will review the advances in the liver organoid field, its limitations and potential for biomedical applications.

Keywords: disease modelling; liver; organoid; personalised medicine.

Figure 1

Organogenesis and stages for organoid progenitor isolation. Schematic depicting key stages of organogenesis timings in mice and humans. Following fertilisation and cleavage of the embryo, the blastocyst is formed in which cells segregate into the outer layer and the inner cell mass (ICM). Cells of the ICM are pluripotent and can be isolated to generate embryonic stem cells. The next key developmental milestone is gastrulation, a process whereby cells derived from the ICM undergo dynamic cell movements and rearrange to form the three germ layers: endoderm, mesoderm and ectoderm. Here we depict a human gastrula which develops as an embryonic disc (note: gastrulation in mice occurs as an egg cylinder). As development progresses, progenitors within each germ layer become specified to give rise to specific tissues and organs. The identities of the progenitors are influenced by their anterior-posterior and dorsal-ventral positions in the embryo. The endoderm becomes patterned along the anterior-posterior axis into the anterior foregut (AF), posterior foregut (PF), midgut (M) and hindgut (H). Illustrated here are a selection of organs that derive from the different endodermal domains: AF—lungs; PF—liver and pancreas; M—small intestine. The hindgut gives rise to more posterior tissues such as the colon. Organoids can be derived from tissue-resident progenitors isolated at both organogenesis stages and from adult tissues.

Figure 2

Liver organoids can be derived from various cells of origin by regulating signalling pathways during in vitro culture. (A) Liver organoids can be formed from tissue-resident cells isolated from biopsies of adult tissues or from embryonic stages during organogenesis. Hepatoblasts (the bipotent embryonic progenitors in vivo which give rise to ductal cells and hepatocytes) can be placed in Matrigel as ECM and generate ductal or hepatocyte organoids depending on the growth factors supplemented in the culture medium. (Bright-field images of mouse embryonic ductal and hepatocyte organoids taken from Prior et al.52) Signalling pathways which are typically modulated to enable organoid formation are listed; the pathways which are essential for different types of liver organoids are in bold. Formation of ductal or hepatocyte organoids from adult tissues requires the isolation of appropriate cells of origin. In order to generate ductal hepatic organoids from adult tissues, ductal fragments or ductal cells can be placed in Matrigel with the optimised media. Formation of adult hepatocyte organoids requires the isolation of mature hepatocytes. (B) Liver organoids can also be generated from pluripotent stem cells (iPSCs and ESCs), usually by a three-stage differentiation process that recapitulates the signalling programmes active during development. iPSCs/ESCs are first directed towards an endodermal fate by exposure to Act A and Wnt. These endoderm cells then progress to a hepatic fate following induction of HGF and FGF signalling. These hepatic progenitors are hepatoblast-like cells. The hepatic progenitors can form hepatocyte-like cells in response to OSM signalling. Conversely, by placing the hepatic progenitors in ECM and modulating FGF, EGF and Act A signalling, ductal organoids can be generated. Act A, Activin A; BMP, bone morphogenetic protein; ECM, extracellular matrix; EGF, epidermal growth factor; ESCs, embryonic stem cells; FGF, fibroblast growth factor; FSK, forskolin; HGF, hepatocyte growth factor; ICM, inner cell mass; iPSCs, induced pluripotent stem cells; OSM, Oncostatin M; TGFbi, transforming growth factor beta inhibitor; TNFa, tumour necrosis factor-alpha.

Figure 3

Applications of liver organoids. (A) Organoids derived from healthy donors or patients can be used as a model in basic research to investigate liver development and function in healthy conditions and to dissect the mechanisms of disease. Liver organoids are also a potential bridging tool towards personalised medicine, allowing for patient-specific drug screening and gene therapy. (B) Organoids can be expanded in vitro and cryopreserved enabling the establishment of biobanks. These can be used on a larger scale for regenerative medicine (including transplants), drug screening (patient-derived organoids can help identify drugs that a cohort of patients are most likely to respond to) and toxicology studies for predicting which potential therapies may induce drug-induced liver injury.