As a stem cell biologist, my overall goal is to understand the mechanisms through which stem cells differentiate into progressively-specialized cell-types and to harness this knowledge to artificially generate pure populations of desired cell-types from stem cells. My work over the past 10 years has centered on pluripotent stem cells (PSCs, which include embryonic and pluripotent stem cells), which have the remarkable ability to generate any of the hundreds of diverse cell-types in the body. However, it has been notoriously difficult to guide PSCs to differentiate into a pure population of a given cell-type. Current differentiation strategies typically generate heterogeneous cell populations unsuitable for basic research or clinical applications. To address this challenge, I mapped the cascade of branching lineage choices through which PSCs differentiate into a variety of endodermal and mesodermal cell-types. I then developed effective methods to differentiate PSCs into specific lineages by providing the extracellular signal(s) that specify a given lineage while inhibiting the signals that induce the alternate fate(s), enabling the generation of highly-pure human heart, bone (Loh & Chen et al., 2016; Cell) and liver (Loh & Ang et al., 2014; Cell Stem Cell) from PSCs. In particular, I have focused on generating pure populations of liver progenitors from PSCs; these PSC-derived human liver progenitors regenerated human liver tissue, and improved the survival of, mouse models of liver failure (Ang et al., 2018; Cell Reports). My goal is to complete the preclinical development of PSC-derived liver progenitors as a potential cellular replacement therapy for liver failure. This project will be facilitated by my experience with PSC differentiation, assays of liver cell identity and function, and mouse models of liver failure.
I earned my Ph.D. jointly from the University of Cambridge and A*STAR and was subsequently appointed as a Research Fellow, and later, a Senior Research Fellow, at the Genome Institute of Singapore. At Singapore, I was an independent group leader and received extramural funding support as PI or co-PI on three government grants. In April 2018, I moved my laboratory to Stanford University as a Siebel Investigator and Instructor at the Stanford Institute for Stem Cell Biology & Regenerative Medicine. My laboratory is supported by the Siebel Investigatorship and two grants from the California Institute for Regenerative Medicine.
A Roadmap for Human Liver Differentiation from Pluripotent Stem Cells
2018; 22 (8): 2190–2205
How are closely related lineages, including liver, pancreas, and intestines, diversified from a common endodermal origin? Here, we apply principles learned from developmental biology to rapidly reconstitute liver progenitors from human pluripotent stem cells (hPSCs). Mapping the formation of multiple endodermal lineages revealed how alternate endodermal fates (e.g., pancreas and intestines) are restricted during liver commitment. Human liver fate was encoded by combinations of inductive and repressive extracellular signals at different doses. However, these signaling combinations were temporally re-interpreted: cellular competence to respond to retinoid, WNT, TGF-β, and other signals sharply changed within 24 hr. Consequently, temporally dynamic manipulation of extracellular signals was imperative to suppress the production of unwanted cell fates across six consecutive developmental junctures. This efficiently generated 94.1% ± 7.35% TBX3+HNF4A+ human liver bud progenitors and 81.5% ± 3.2% FAH+ hepatocyte-like cells by days 6 and 18 of hPSC differentiation, respectively; the latter improved short-term survival in the Fah-/-Rag2-/-Il2rg-/- mouse model of liver failure.
View details for PubMedID 29466743
- Isolation and 3D expansion of multipotent Sox9(+) mouse lung progenitors NATURE METHODS 2017; 14 (12): 1205-+
Evaluating the regenerative potential and functionality of human liver cells in mice
2017; 98: 25–34
Liver diseases afflict millions of patients worldwide. Currently, the only long-term treatment for liver failure is the transplantation of a new liver. However, intravenously transplanting a suspension of human hepatocytes might be a less-invasive approach to partially reconstitute lost liver functions in human patients as evinced by promising outcomes in clinical trials. The purpose of this essay is to emphasize outstanding questions that continue to surround hepatocyte transplantation. While adult primary human hepatocytes are the gold standard for transplantation, hepatocytes are heterogeneous. Whether all hepatocytes engraft equally and what specifically defines an "engraftable" hepatocyte capable of long-term liver reconstitution remains unclear. To this end, mouse models of liver injury enable the evaluation of human hepatocytes and their behavior upon transplantation into a complex injured liver environment. While mouse models may not be fully representative of the injured human liver and human hepatocytes tend to engraft mice less efficiently than mouse hepatocytes, valuable lessons have nonetheless been learned from transplanting human hepatocytes into mouse models. With an eye to the future, it will be crucial to eventually detail the optimal biological source (whether in vivo- or in vitro-derived) and presumptive heterogeneity of human hepatocytes and to understand the mechanisms through which they engraft and regenerate liver tissue in vivo.
View details for PubMedID 29078082
Efficient endoderm induction from human pluripotent stem cells by logically directing signals controlling lineage bifurcations.
Cell stem cell
2014; 14 (2): 237-252
Human pluripotent stem cell (hPSC) differentiation typically yields heterogeneous populations. Knowledge of signals controlling embryonic lineage bifurcations could efficiently yield desired cell types through exclusion of alternate fates. Therefore, we revisited signals driving induction and anterior-posterior patterning of definitive endoderm to generate a coherent roadmap for endoderm differentiation. With striking temporal dynamics, BMP and Wnt initially specified anterior primitive streak (progenitor to endoderm), yet, 24 hr later, suppressed endoderm and induced mesoderm. At lineage bifurcations, cross-repressive signals separated mutually exclusive fates; TGF-β and BMP/MAPK respectively induced pancreas versus liver from endoderm by suppressing the alternate lineage. We systematically blockaded alternate fates throughout multiple consecutive bifurcations, thereby efficiently differentiating multiple hPSC lines exclusively into endoderm and its derivatives. Comprehensive transcriptional and chromatin mapping of highly pure endodermal populations revealed that endodermal enhancers existed in a surprising diversity of "pre-enhancer" states before activation, reflecting the establishment of a permissive chromatin landscape as a prelude to differentiation.
View details for DOI 10.1016/j.stem.2013.12.007
View details for PubMedID 24412311