J Mol Med (2017) 95:695–703 DOI 10.1007/s00109-017-1550-4 REVIEW The novel tool of cell reprogramming for applications in molecular medicine Moritz Mall1 & Marius Wernig1 Received: 1 May 2017 /Revised: 15 May 2017 /Accepted: 19 May 2017 /Published online: 8 June 2017 # The Author(s) 2017. This article is an open access publication Abstract Recent discoveries in the field of stem cell biology Keywords Cell fate . Reprogramming . Stem cell biology have enabled scientists to Breprogram^ cells from one type to another. For example, it is now possible to place adult skin or blood cells in a dish and convert them into neurons, liver, or Introduction heart cells. It is also possible to literally Brejuvenate^ adult cells by reprogramming them into embryonic-like stem cells, The fate of a cell is an integral of its morphological and func- which in turn can be differentiated into every tissue and cell tional makeup that is in turn dictated by its transcriptional, type of the human body. Our ability to reprogram cell types epigenetic, proteomic, and metabolic configuration. Cellular has four main implications for medicine: (1) scientists can fate is changing during development as the multicellular or- now take skin or blood cells from patients and convert them ganism develops from a single totipotent cell to yield billions to other cells to study disease processes. This disease model- of specialized cells that make up the human body. Ever since ing approach has the advantage over animal models because it Hans Spemann showed in 1923 that the blastomeres of a 16- is directly based on human patient cells. (2) Reprogramming cell salamander embryo are all equivalent to the totipotent could also be used as a Bclinical trial in a dish^ to evaluate the zygote, it remained an open question whether more differen- general efficacy and safety of newly developed drugs on hu- tiated cells irreversibly lose this developmental potential [1]. It man patient cells before they would be tested in animal was debated whether perhaps even genetic material might be models or people. (3) In addition, many drugs have deleteri- lost during differentiation, which would eliminate the totipo- ous side effects like heart arrhythmias in only a small and tent potential of specialized cells. unpredictable subpopulation of patients. Reprogramming One of the first decisive experiments was the nuclear trans- could facilitate precision medicine by testing the safety of fer of specialized cell nuclei into oocytes (Fig. 1a). These already approved drugs first on reprogrammed patient cells experiments first done in frogs showed that specialized cells in a personalized manner prior to administration. For example, can be reprogrammed to totipotency and can give rise to a new drugs known to sometimes cause arrhythmias could be first animal [2, 3]. Thus, even specialized cells can activate the tested on reprogrammed heart cells from individual patients. entire program of embryonic development. In addition, adult (4) Finally, reprogramming allows the generation of new tis- cells can adapt and change quite dramatically upon certain sues that could be grafted therapeutically to regenerate lost or environmental conditions. For example, the respiratory epi- damaged cells. thelium in the lungs of smokers can convert into squamous cells, and the esophagus epithelium can adopt the morphology of gastric epithelium in a process called metaplasia [4]. But also in hematopoietic tumors, cells have been found to * Marius Wernig [email protected] transdifferentiate from one blood lineage to another [5, 6]. There is also evidence that pancreatic α or δ cells can change β 1 Department of Pathology and Institute for Stem Cell Biology and to cells upon injury [7, 8]. An additional example for in- Regenerative Medicine, Stanford University School of Medicine, duced lineage plasticity was provided by cell fusion experi- Stanford, CA 94305, USA ments (Fig. 1b) [9, 10]. 696 J Mol Med (2017) 95:695–703 A mature fertile animals by transplanting nuclei of cultured in- oocyte testinal cells of Xenopus tadpoles into enucleated oocytes transfer somatic nucleus (Fig. 1a) [3]. Subsequent experiments using different adult into enucleated oocyte cell types only yielded swimming tadpoles, suggesting that the starting cell population may be restricted in their degree of plasticity or that reprogramming was not complete [16]. For many years, similar experiments in higher vertebrates failed pluripotent cells and the field was dominated by the notion that mammalian cells have lost the plasticity of amphibian cells. It was only in fibroblast 1996 when Ian Wilmut and his co-workers demonstrated that B it is possible to generate live animals by nuclear transfer of mouse myocytes adult mammalian cells when they successfully cloned the fa- human mous sheep Dolly [17]. In the coming years, many other amniocyte cell fusion mammalian species were successfully cloned including mice, rats, cats, dogs, cows, and others [18]. But for a long time, it remained unclear whether nuclear transfer could also repro- gram human cells. Just a few years ago, this question was resolved and several groups convincingly showed that adult heterokaryon expressing human muslce genes human fibroblasts can be transferred into human oocytes, which can give rise to blastocysts containing expandable plu- – C transcription factor-mediated ripotent cells [19 21]. reprogramming (OSKM) The power of transcription factors: direct cell fate reprogramming fibroblast induced pluripotent cells Developmental biology has focused on identification of tran- Fig. 1 Common technologies to reprogram cell fate. a Somatic cell scription factors that are essential to induce cell type specific nuclear transfer (SCNT), in which an oocyte is enucleated to receive a genetic programs; those factors are often expressed at distinct nucleus from a donor cell such as a fibroblast uses the cytoplasmic machinery to reprogram the donor cell to pluripotency. Similar methods stages during differentiation to activate the desired genetic were used to clone entire animals such as Dolly the sheep and generate programs and are termed Bselector genes.^ Onesuchexample human stem cell lines. b Analogous to SCNT diffusible factors can is the Drosophila eyeless gene (Pax6 in mammals) that is reprogram the expression program of a donor cell such as a human required for eye development [22]. Strikingly, Pax6 overex- amniocyte upon induced cell fusion with heterologous cells such as mouse myocytes to induce the expression of human muscle genes. c pression can induce the formation of eye structures in various Alternatively, strong cell fate determination transcription factors can be appendages of the fly [12]. Similar effects have been observed overexpressed using different methods to change a cell fate. For example, using other selector genes, including the Hox family members the transcription factors Oct4, Sox2, Klf4, and c-Myc (OSKM) can con- distalless and vestigial (reviewed in [23]). vert a fibroblast into an induced pluripotent stem cell A different class are the so called Bterminal selector genes^ that regulate the identity of specific neuronal subtypes in More recently, specific transcription factors or combina- C. elegans [24]. Terminal selector genes are transcription fac- tions thereof have been identified to induce such lineage con- tors that are either alone or in combination specifically in- versions (Fig. 1c). Among others, MyoD was found to induce duced as the corresponding neuronal subtype is generated. muscle fates in fibroblasts [11], Pax6 was shown to induce Unlike classical selector genes, they stay expressed in these entire ectopic eyes [12], and several factors in combination cells throughout the life of the animal and not only induce but could induce insulin-producing cells from exocrine pancreas also maintain subtype identity by activating key transcription- cells [13]. These efforts culminated in the discovery that adult al modules necessary for the cell’s function and by repressing somatic cells can be reprogrammed to pluripotency and con- other terminal selector genes. verted into distantly related cell types as diverse as lineages The basic helix-loop-helix (bHLH) transcription factor representing different germ layers [14, 15]. MyoD was the first factor identified that has the power to induce a cell lineage program in an unrelated cell type. The power of oocytes: installment of pluripotency Following a subtractive cDNA library screen, Harold Weintraub and colleagues cloned the cDNA coding for As briefly stated in the introduction, John Gurdon showed that MyoD, which was sufficient to convert cultured mouse fibro- even somatic cells can be reactivated by the oocyte to form blasts into beating muscle cells [11]. This work sparked the J Mol Med (2017) 95:695–703 697 search for similar Bmaster^ lineage regulators for other cell endodermal origin by converting terminally differentiated types. By and large, however, this search was initially unsuc- hepatocytes [30]. cessful, and for many years, it was assumed that MyoD is This work sparked great interest in the field and triggered unique. several labs to further develop iN cell reprogramming tech- Nevertheless, work in hematopoietic lineages continued to niques [31]. The successful generation of iN cells also inspired provide evidence for the existence of individual powerful lin- scientists to apply similar strategies to other cell lineages. To eage determination factors. Thomas Graf showed that the my- date, many