Vol 462j3 December 2009jdoi:10.1038/nature08533 REVIEWS Forcing cells to change lineages Thomas Graf1 & Tariq Enver2 The ability to produce stem cells by induced pluripotency (iPS reprogramming) has rekindled an interest in earlier studies showing that transcription factors can directly convert specialized cells from one lineage to another. Lineage reprogramming has become a powerful tool to study cell fate choice during differentiation, akin to inducing mutations for the discovery of gene functions. The lessons learnt provide a rubric for how cells may be manipulated for therapeutic purposes. eemingly at odds with the stability of the differentiated state in transparent lens cells consisting of specialized keratinocytes (reviewed metazoa are cell fusion and nuclear transfer experiments, in ref. 9). In another well-studied system of regeneration, limb rege- which have shown that the epigenomes of differentiated cells neration in axolotls, it has long been assumed that the blastema that S can be remarkably plastic. Experiments performed several forms in response to injury contains de- and re-differentiating cells. decades ago showed that dormant gene expression programs can be However, recent work indicates that only dermis cells can ‘transdiffer- dominantly awakened in differentiated cells by the fusion of different entiate’ into cartilage and tendons, whereas cartilage, muscle and neur- pairs of cell types1. Subsequently, lineage conversions could be effected onal precursors within the blastema do not change identity before simply through the introduction of defined transcription factors2,3 generating a new limb11. Several types of metaplasia have been attri- (Fig. 1a, b). Parallel experiments, conducted in a number of different buted to transdifferentiation9, and epithelial mesenchymal transitions species, showed that transfer of nuclei from both embryonic and adult may be involved in the formation of metastatic breast cancers10.Here, somatic cell types can lead to the formation of all three germ layers and as during normal epithelial mesenchymal transitions, activation of the even to the generation of entire new animals4–7, unequivocally demon- transcription factors Snail, Slug and Twist are essential9,10.Withthe strating that the identity of differentiated cells can be fully reversed. rapidly growing arsenal of lineage tracing tools it seems likely that The latest and most dramatic development is the demonstration that many more physiological or pathogenic cell conversions will be dis- somatic cells can be reprogrammed to a pluripotent state by the covered in the future. expression of a transcription factor cocktail, generating induced Perhaps the best evidence that functionally differentiated cells can pluripotent stem (iPS, for nomenclature see Box 1) cells8 (Fig. 1c). change fate during normal development comes from studies on the The facility with which cell fates can be altered experimentally raises origin of blood. Fetal blood cells originate in the dorsal aorta after the question as to whether such interconversions occur physiologically activation of Scl (also known as Tal1) and Runx112,13, two transcrip- or in the context of disease. Arguably, gastrulation provides a first tion factors essential for haematopoietic stem cell formation14. Most example of transdetermination, where an invagination of the ectoderm strikingly, time-lapse experiments recently showed that a small pro- produces mesoderm (reviewed in refs 9, 10). Transdetermination and portion of cells within cultured endothelial sheet colonies derived transdifferentiation may also have a role in regeneration, metaplasia from embryonic stem (ES) cells undo their tight junctions, round up and cancer. For example, removing the eye lens of a newt leads to and begin to express erythroid and monocytic haematopoietic anti- depigmentation of dorsal iris cells and their redifferentiation into gens15 (Fig. 2). This process, which is unique to embryonic as abc de f Fibroblasts Monocytic Fibroblasts B cells B cells Exocrine cells precursors Oct4 MyoD Pdx1 Sox2 Pax5 GATA1 C/EBPα Ngn3 Klf4 ablation MafA Myc Erythroid- Muscle T cells, megakaryocytic iPS cells Macrophages Islet β-cells cells macrophages cells, eosinophils Figure 1 | Examples of transcription factor overexpression or ablation experiments that result in cell fate changes. For explanation of panels a–f see text. 1Center for Genomic Regulation and ICREA, 08003 Barcelona, Spain. 2MRC Molecular Haematology Unit, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Headington, Oxford OX3 9DS, UK. 587 ©2009 Macmillan Publishers Limited. All rights reserved REVIEWS NATUREjVol 462j3 December 2009 with classic ‘forward’ differentiation. This is where the fields of Box 1 Glossary j induced lineage conversions and developmental biology merge: we Lineage Cells of the same developmental origin sharing a similar propose that the cell interconversions elicited experimentally by tran- phenotype/function. scription factors may mimic specific physiological cell fate transitions Cell differentiation Process by which cells become more specialized, acquiring new identities. and that the two processes are fundamentally similar. Cell determination Commitment to a lineage. In this review we briefly chart the evolution of the transcription Commitment Stable activation of a gene expression program factor perturbation experiments and discuss how they have provided characteristic of a lineage. fundamental insights into the process of lineage specification. They Pluripotent Potential of a cell to generate all cell types except extra- identified lineage-instructive regulators, revealed the principle of embryonic tissue. Examples: embryonic stem cells and induced transcription factor cross-antagonisms in binary lineage decisions pluripotent stem (iPS) cells. and helped explain the dynamic behaviour of regulatory networks. Multipotent Potential of a cell to form several lineages within a tissue. We also discuss more generally what lineage reprogramming has Example: haematopoietic stem cells. shown about mechanisms of development (see also ref. 18) and place Progenitor Cell with the capacity to differentiate and divide but with limited self-renewal potential. them in the context of emerging epigenetic landscape models. iPS cell reprogramming Induced conversion of somatic cells into Finally, we compare transcription-factor-mediated lineage repro- pluripotent stem cells. gramming to iPS cell reprogramming and discuss their potential Lineage reprogramming Conversion of cells from one lineage to for regenerative therapy. another. Term covers both transdifferentiation and transdetermination. Charting the beginnings Transdifferentiation Reprogramming of one specialized cell type into The instructive role of transcription factors in lineage specification another, without reversion to pluripotent cells. Also called ‘lineage was demonstrated in the 1980s, when Harold Weintraub’s laboratory switching’ or ‘lineage conversion’. discovered that forced expression of MyoD can induce myotube Transdetermination Reprogramming of a committed, but not yet fully 2 differentiated, cell type into another. formation in a fibroblast cell line (Fig. 1a). Evidence for the reciprocal Lineage priming Promiscuous expression in progenitors of regulation of lineage-restricted genes came from the blood system, transcriptional programs associated with different lineages. which, with its diversity of well-defined cell lineages and prospectively Epigenome/epigenetics Changes in phenotype or gene expression isolatable intermediate progenitors, is an ideal venue for lineage- caused by mechanisms other than changes in the underlying DNA. conversion experiments. Thus, when ectopically expressed in cell lines Cell regeneration Replacement of cells lost by injury or attrition. of monocytes (macrophage precursors) at high levels, the erythroid- Plasticity Ability of a cell to convert into another cell type either megakaryocyte-affiliated transcription factor GATA1 not only spontaneously, by external cues or by gene perturbation experiments. induced the expression of erythroid-megakaryocyte lineage markers, Metaplasia Replacement in a tissue of one differentiated cell type with but also downregulated monocytic markers3,19. Lower levels of another, generally caused by an abnormal chronic stimulus. GATA1 induced the formation of eosinophils, in line with its levels in normal eosinophils3 (Fig. 1a). The monocytic to erythroid switch opposed to adult endothelium, is exacerbated by shear stress (mim- could also be effected in the opposite direction: expression of PU.1 icking blood flow) through production of nitric oxide, upregulation (also known as Sfpi1) in an erythroid-megakaryocytic cell line of Runx1, c-Myb and Klf2 (refs 16,17). Interestingly, c-Myb, like induced its conversion to the monocytic lineage, repressing GATA1 Runx1 and Scl, is a transcription factor that is also required for the (ref. 20). A potential caveat of these studies was their reliance on formation of definitive blood cells14. The re-specification of endo- cell lines, which may be more inherently plastic than their normal thelium into haematopoietic cells supports the notion that transdif- counterparts. This objection was dismissed when ectopic expression ferentiation may occur during normal development and alternates of GATA1 produced erythroid-megakaryocytic-eosinophil-basophil output from granulocyte-macrophage progenitors freshly isolated from normal bone marrow21. More recently it was