The quintessential attribute of stem cells is their ability to develop into multiple cell-types. We seek to understand the molecular basis of this property (multilineage potential) and to realize such potential by developing methods to differentiate stem cells into pure populations of downstream progeny.
A roadmap for early human tissue development
How is the multilineage potential of embryonic stem cells realized as they differentiate into various tissues? Within complex developing embryos, it has been difficult to precisely map how early tissues develop from stem cells. If we truly understand development at the level of causation, we should be able to reconstitute it in vitro. To this end we have taken a reductionist approach to unravel how early human tissue progenitors form. We have systematically mapped out the sequence of binary lineage choices (lineage bifurcations) through which ESCs become diversified into a broad array of 20+ different endodermal and mesodermal cell-types, ranging from liver to intestines to bone to heart (Loh & Ang et al., 2014; Cell Stem Cell; Loh & Chen et al., 2016; Cell; Ang et al., 2018; Cell Reports). At each binary lineage choice, we rigorously identified and tested the minimal combinations of inductive and repressive cues that were sufficient to induce pure populations of various human tissue progenitors from ESCs. First, this identified the minimal combinations of factors sufficient to specify various cell-types ab initio. Second, this provided an informative reference map for early tissue development. Third, this experimental system to induce tissue progenitor fates from cultured stem cells constitutes a causal test of our knowledge of developmental biology in a fashion akin to biochemical reconstitution of a complex process.
Generating pure populations of human tissue progenitors
Identifying the extracellular signals that control stem cell differentiation into one cell-type or the other at each lineage branchpoint provided unique insight into how to differentiate stem cells into pure populations of desired lineages. At each pairwise lineage choice, by providing the inductive signals to specify the desired fate while inhibiting signals that otherwise steered differentiation towards the unwanted fate, we could "force" ESCs to differentiate into a highly pure population of a desired cell-type by blocking off alternate paths. Through this approach, we differentiated ESCs into enriched populations of human liver progenitors that could engraft the mouse liver (Loh & Ang et al., 2014; Cell Stem Cell; Ang et al., 2018; Cell Reports) and we also generated human bone progenitors that could form an ectopic human bone in mice (Loh & Chen et al., 2016; Cell).
Our capacity to produce these pure populations of human tissue progenitors therefore provides a foundation for regenerative medicine. With our collaborators, we also are mapping the transcriptional and chromatin landscapes of these human tissue progenitors to illuminate the stepwise molecular events through which pluripotent cells develop into various tissues (Koh & Sinha et al., 2016; Scientific Data).
The basis of stem-cell multilineage potential
Embryonic stem cells (ESCs) are defined by their ability to differentiate into all bodily cell-types (pluripotency). Paradoxically, prevailing models suggest that stem cell transcription factors block differentiation in order to maintain a "self-renewing" state. We proposed that the pluripotent state is underpinned by competing lineage-specifying transcription factors (e.g., Oct4, Sox2 and Nanog), each of which specifies differentiation to a different lineage (Loh & Lim, 2011; Cell Stem Cell). When all these pluripotency factors are coexpressed in ESCs, they cross-repress one anothers' lineage-specifying activities, therefore maintaining a temporarily "undifferentiated" state where differentiation to any single lineage is kept in check. However, the expression of these lineage-specifying factors in ESCs endows them with the innate ability to differentiate into various lineages, therefore explaining their multilineage potential (Loh et al., 2015; Physiological Reviews). Could a competition between lineage-specifying transcription factors be a general strategy through which diverse types of tissue stem cells are endowed with multilineage potential?
Besides developmental biology, we also have an active interest in tissue renewal (Clevers, Loh & Nusse, 2014; Science), the definition of what a "cell type" really means (Roberts, Loh et al., 2014; Reproduction), mechanisms of ESC self-renewal (Loh & Lim, 2013; EMBO Reports) and mechanisms of cellular reprogramming (Loh & Lim, 2012, 2013, 2015; Nature).