Liver Regeneration From Induced Pluripotent Stem Cells
Authors: Xin Cheng, Paul Gadue
Journal: Molecular Therapy (2010)

Are induced pluripotent stem (iPS) cells capable of regenerating an injured organ? A recent study by Espejel et al.1Espejel S Roll GR McLaughlin KJ Lee AY Zhang JY Laird DJ et al.Induced pluripotent stem cell-derived hepatocytes have the functional and proliferative capabilities needed for liver regeneration in mice.J Clin Invest. 2010; 120: 3120-3126Crossref PubMed Scopus (149) Google Scholar published in the Journal of Clinical Investigation indicates that the answer to this question is yes. In this study, hepatocytes derived from chimeric mice generated with iPS cells were analyzed functionally in vivo. IPS cell–derived hepatocytes were found to regenerate after a two-thirds hepatectomy in a manner similar to that of normal hepatocytes and displayed repopulation ability equivalent to that of normal hepatocytes in a competitive liver-reconstitution model. These findings indicate the full potency of iPS cell–derived hepatocytes in liver regeneration and function. Although exciting as a proof of principle, these studies utilized a chimeric mouse model to generate the hepatocytes, which is not a viable option for the generation of hepatocytes from human iPS cells and highlights the need to optimize methods for the in vitro generation of functional hepatocytes from iPS cells before clinical use of these cells can become a reality. The advent of iPS cell technology in mid-2006 has made possible for the first time the generation of patient-specific pluripotent stem cells for cell-based therapy (reviewed by Nishikawa et al.2Nishikawa S-I Goldstein RA Nierras CR The promise of human induced pluripotent stem cells for research and therapy.Nat Rev Mol Cell Biol. 2008; 9: 725-729Crossref PubMed Scopus (355) Google Scholar). These cells should circumvent the need for immune suppression and at the same time alleviate the ethical considerations of using human embryonic stem (ES) cells. The most convincing data for the developmental potential of iPS cells are in the mouse system, in which germline-competent chimeric mice can be generated,3Okita K Ichisaka T Yamanaka S Generation of germline-competent induced pluripotent stem cells.Nature. 2007; 448: 313-317Crossref PubMed Scopus (3506) Google Scholar and more recently it was shown that viable mice could be produced that were completely derived from iPS cells using the tetraploid complementation system.4Zhao X Li W Liu L Tong M Hai T Hao J et al.iPS cells produce viable mice through tetraploid complementation.Nature. 2009; 461: 341Crossref Scopus (638) Google Scholar However, other work suggests that iPS cells may have some limitations with regard to developmental potential and/or proliferative capacity of their differentiated progeny.5Hu B-Y Weick JP Yu J Ma LX Zhang XQ Thomson JA et al.Neural differentiation of human induced pluripotent stem cells follows developmental principles but with variable potency.Proc Natl Acad Sci USA. 2010; 107: 4335-4340Crossref PubMed Scopus (803) Google Scholar,6Feng Q Lu SJ Klimanskaya I Gomes I Kim D Chung Y et al.Hemangioblastic derivatives from human induced pluripotent stem cells exhibit limited expansion and early senescence.Stem Cells. 2010; 28: 704-712Crossref PubMed Scopus (323) Google Scholar,7Kim K Doi A Wen B Ng K Zhao R Cahan P et al.Epigenetic memory in induced pluripotent stem cells.Nature. 2010; 467: 285-290Crossref PubMed Scopus (1701) Google Scholar This is of concern if iPS cells are to be used to reconstitute the function of an entire organ, which would require both efficient differentiation and proliferative ability. The work described by Espejel and colleagues addresses not only whether iPS cells could generate a liver but also whether the iPS-derived hepatocytes had the proliferative capacity to both regenerate a functional liver and reconstitute a genetic mouse model of liver damage. To generate chimeric mice such that the entire liver is derived from iPS cells, blastocysts from the fumarylacetoacetate hydrolase (FAH)–deficient mouse model were injected with wild-type iPS cells. FAH-mutant mice develop tyrosinemia and die from progressive liver failure8Grompe M Lindstedt S al-Dhalimy M Kennaway NG Papaconstantinou J Torres-Ramos CA et al.Pharmacological correction of neonatal lethal hepatic dysfunction in a murine model of hereditary tyrosinaemia type I.Nat Genet. 1995; 10: 453-460Crossref PubMed Scopus (262) Google Scholar unless treated by the administration of 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC), a drug that mitigates hepatocyte toxicity in FAH-deficient animals. Therefore, the wild-type iPS cells have a competitive advantage and could achieve nearly 100% liver contribution in some chimeric mice after NTBC withdrawal. Consistent with these repopulation rates, blood analyses of liver function in the chimeric animals indicated completely restored liver function within a month after NTBC withdrawal. The liver function remained normal in these animals after 10 months of NTBC withdrawal, demonstrating that iPS cell–derived hepatocytes can repopulate the damaged liver efficiently and restore liver function over the long term. Using chimeric mice whose livers were entirely repopulated by iPS cell–derived cells, Espejel et al. were then able to evaluate the proliferative capabilities of iPS cell–derived hepatocytes. Either by two-thirds hepatectomy or by transplanting iPS cell–derived hepatocytes into FAH-deficient adult mice, the authors found comparable proliferation rates of the iPS cell–derived liver cells as compared with those of normal hepatocytes. Finally, when performing a competitive reconstitution assay with equal numbers of age-matched normal hepatocytes transplanted into immune-deficient, FAH-deficient mice, the iPS cell–derived hepatocytes, in response to NTBC withdrawal, gave rise to nodules in the host liver that were similar in size and number to those from the normal hepatocytes. These results established that iPS cell–derived hepatocytes are equivalent to normal hepatocytes in proliferative capacity when regenerating an injured liver and demonstrate the great promise of using iPS cells as a source of hepatocytes for cellular therapies of the liver. As an extension to their findings, Espejel et al. showed that iPS cell–derived progeny were also able to repopulate FAH-deficient kidney and correct the renal dysfunction, which suggests that the regenerative capacity of iPS-derived cells is not restricted to liver. The results of this study raise the question why the iPS cell–derived hepatocytes have normal function and proliferative capacity whereas studies examining neural differentiation and blood differentiation from iPS cells have shown deficiencies.5Hu B-Y Weick JP Yu J Ma LX Zhang XQ Thomson JA et al.Neural differentiation of human induced pluripotent stem cells follows developmental principles but with variable potency.Proc Natl Acad Sci USA. 2010; 107: 4335-4340Crossref PubMed Scopus (803) Google Scholar,6Feng Q Lu SJ Klimanskaya I Gomes I Kim D Chung Y et al.Hemangioblastic derivatives from human induced pluripotent stem cells exhibit limited expansion and early senescence.Stem Cells. 2010; 28: 704-712Crossref PubMed Scopus (323) Google Scholar,7Kim K Doi A Wen B Ng K Zhao R Cahan P et al.Epigenetic memory in induced pluripotent stem cells.Nature. 2010; 467: 285-290Crossref PubMed Scopus (1701) Google Scholar First, it is possible that certain lineages derived from iPS cells may be more prone to these problems and hepatocytes are not affected. Consistent with this idea, it has recently been found that several human iPS cell lines have hepatocyte differentiation potential similar to that of ES cells.9Rashid ST Corbineau S Hannan N Marciniak SJ Miranda E Alexander G et al.Modeling inherited metabolic disorders of the liver using human induced pluripotent stem cells.J Clin Invest. 2010; 120: 3127-3136Crossref PubMed Scopus (485) Google Scholar It should be noted that this study did not examine proliferative capacity of the iPS cell–derived hepatocytes. It is also possible that the cell type used to generate the iPS cell line could affect subsequent differentiation and function owing to epigenetic memory. One recent study seems to support this idea, in that blood-derived iPS cells had difficulty differentiating into bone cells whereas fibroblast-derived iPS cells had trouble differentiating into blood cells.7Kim K Doi A Wen B Ng K Zhao R Cahan P et al.Epigenetic memory in induced pluripotent stem cells.Nature. 2010; 467: 285-290Crossref PubMed Scopus (1701) Google Scholar Another study showed that this effect was dependent on passage number, such that higher-passage fibroblast-derived iPS cells regained the ability to efficiently differentiate into blood cells.10Polo JM Liu S Figueroa ME Kulalert W Eminli S Tan KY et al.Cell type of origin influences the molecular and functional properties of mouse induced pluripotent stem cells.Nat Biotechnol. 2010; 28: 848-855Crossref PubMed Scopus (933) Google Scholar These changes in developmental potential correlated with changes in both genome-wide methylation and gene expression. The findings would then suggest that epigenetic memory may be overcome and that generation of iPS cells with full competency may require several passages to remove epigenetic memory from the somatic cell of origin. Espejel and colleagues utilized a chimeric mouse model that could also mask possible iPS cell deficiencies. There could be selection pressure in vivo for the most potent iPS cells to contribute to the embryo. Future work will need to be performed to fully determine the capabilities of iPS cells and to optimize the generation of iPS cell lines with full developmental and proliferative potential. To generate functional hepatocytes, Espejel et al. used a chimeric animal model that is unavailable for use with human iPS cells. Thus far, none of the hepatocyte-like populations generated in vitro from either human ES cells or iPS cells could provide function comparable to those of primary hepatocytes, although some levels of repopulation have been demonstrated in xenotransplant animal models with ES cell–derived hepatocytes.11Basma H Soto-Gutiérrez A Yannam GR Liu L Ito R Yamamoto T et al.Differentiation and transplantation of human embryonic stem cell-derived hepatocytes.Gastroenterology. 2009; 136: 990-999Abstract Full Text Full Text PDF PubMed Scopus (419) Google Scholar One method of enhancing hepatocyte formation from iPS cells is to utilize our understanding of developmental biology and organogenesis to develop well-defined, step-by-step in vitro differentiation protocols that mimic embryogenesis. A better understanding of in vivo hepatic development is needed to facilitate the generation of fully functional hepatocytes from iPS cells for use in the study and treatment of diseases of the liver.