The molecular basis of transdifferentiation
Stem Cell Review Series
The molecular basis of transdifferentiation
Wan-Chun Li #, Wei-Yuan Yu #, Jonathan M. Quinlan, Zoë D. Burke, David Tosh *
Centre for Regenerative Medicine, Department of Biology & Biochemistry, University of Bath, Claverton Down, Bath, UK Received: April 15, 2005; Accepted: August 19, 2005
# These authors contributed equally * Correspondence to: David TOSH, Centre for Regenerative Medicine, Department of Biology & Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK. Tel.: +44 (0) 1225.386532; Fax: +44 (0) 1225.386779 E-mail: [email protected]
There is now excellent experimental evidence demonstrating the remarkable ability of some differentiated cells to convert to a completely different phenotype. The conversion of one cellular phenotype to another is referred to as 'transdifferentiation' and belongs to a wider class of cell-type switches termed 'metaplasias'. Defining the molecular steps in transdifferentiation will help us to understand the developmental biology of the cells that interconvert, as well as help identify key regulatory transcription factors that may be important for the reprogramming of stem cells. Ultimately, being able to produce cells at will offers a compelling new approach to therapeutic transplantation and therefore the treatment and cure of diseases such as diabetes.
Keywords: transdifferentiation • metaplasia • liver • pancreas • stem cells
Source: J. Cell. Mol. Med. Vol 9, No 3, 2005 pp. 569-582
Transdifferentiation is defined as an irreversible switch in postnatal life of one type of already differentiated cell to another type of normal differentiated cell . Normally dedifferentiation and cell division are essential intermediate processes although they may not be obligatory in all cases . Transdifferentiation belongs to a wider class of cell type conversions referred to as 'metaplasias'. By definition, metaplasia is the general name used to describe the conversion of one cell or tissue type to another. Under the definition of metaplasia, stem cells of one tissue type can switch to become those of another . Transdifferentiation and metaplasia are associated with a discrete change in cellular morphology associated with a change in the programme of gene expression. At the molecular level, the cause of transdifferentiation is presumably a change in the expression of a master regulatory (master switch) gene whose normal function is to distinguish the two tissues in normal development [1, 3]. During embryogenesis, different tissue types arise from a common cell sheet because different combinations of master switch genes are switched on in each region in response to inductive signals. Since transdifferentiation appears to be the result of a single-step change, it is logical to assume that tissues between which such changes occur are neighbours in the sense that the combination of selector genes that defines them differ only in the state of one gene. Where the new tissue normally consists of more than one cell type, formed from a common stem cell, the metaplastic foci usually contain all these cell types. This indicates that metaplasia represents a switch of state from one stem cell to another. In the present review we shall discuss the identification of selector, or master switch, genes as well as examples of transdifferentiation in which the molecular basis is at least partly understood.
Identification of master switch genes and MyoD
Identification of master switch genes
A number of genes exist that exemplify the definition of a master switch gene. For example, MyoD is thought to be the master switch gene for myogenesis while peroxisome proliferating activating receptor- γ (PPARγ) and CCAAT-enhancer binding protein- α (C/EBPα) are both involved in adipogenesis. The role of these transcription factors have largely been determined by in vitro studies. Pancreatic transcription factors such as Pancreatic-duodenal homeobox gene (Pdx1), Neurogenin3 (Ngn3), Hairy and Enhancer-of-split-1 (Hes1) and Pancreas transcription factor-1a/p48 (PTF1-p48) can also act as master switch genes. Each of these examples is discussed in more detail below. Identification of MyoD
The master switch gene for muscle, MyoD, was found by a rather unusual experimental approach. It had been known for some time that treatment of the mouse fibroblast cell line 10T1/2 with the hypomethylating agent 5-azacytidine is sufficient to produce three different mesenchymal lineagesmyocyte, chondrocyte or adipocyte clones [4, 5]. All three phenotypes persist after withdrawal of 5- azacytidine, suggesting a stable switch has occurred. To identify which gene(s) was responsible for the switch from a fibroblast to a muscle phenotype, subtracted cDNA probes were initially prepared by isolating cDNA clones that are present in the 5-azacytidine-treated 10T1/2 cells and the mouse myogenic cell line C2C12 but not in the undifferentiated 10T1/2 cells. The myoblast-specific cDNA probes were then positively selected by taking the remaining cDNA probes from either 5- azacytidine-treated or C2C12 myoblasts and screening a myoblast cDNA library. One cDNA was identified which, when transfected into 10T1/2 cells, induced stable myoblast expression . The cDNA was called MyoD. It is now known that at least four myogenic regulatory transcription factors (MRFs) are important for skeletal muscle commitment and myotube formation - Myf5, MyoD, myogenin and MRF4 - and all are basic helix-loop-helix (bHLH) nuclear proteins . To determine whether MyoD was indeed the master switch gene for muscle, the transcription factor was introduced into primary fibroblasts, adipocytes, smooth muscle cells, baby hamster kidney cells, and hepatocytes. Upon MyoD expression, the cells started to adopt the muscle phenotype (myotube formation and fusion) and muscle cell markers myosin heavy chain (MHC) and myosin light chain (MLC2) became expressed. These results suggest that MyoD is indeed the master switch gene for muscle . It is worth noting that in these examples of cell type conversions, germ layer boundaries are crossed; fibroblasts, adipocytes, smooth muscle and kidney cells are mesodermal in origin, but hepatocytes are endodermal. However, not all cells forced to express MyoD caused the muscle differentiation programme to be activated. Expression of MyoD in CV1 (an African green monkey kidney-derived cell) or HeLa (human cervical carcinoma) cells failed to activate differentiated muscle markers . For melanoma and neuroblastoma cells the authors found co-expression of the parental melanocyte or neural markers with the muscle markers. This contrasts with adipocytes, in which the co-expression of adipocyte markers and muscle markers in the transdifferentiated myocytes was absent. This observation is possibly a result of myogenesis inhibiting differentiation pathways that are very close to the muscle lineage (e.g. the adipocyte lineage), but not those that are further away from the normal developmental pathway.
Transdifferentiation of myoblasts to adipocytes
Myoblasts and adipocytes arise from the same germlayer of the embryo, the mesoderm, and recent observations suggests it is possible to directly induce the conversion of myoblasts to adipocytes. G8 myoblasts are a tissue-culture model for myogenesis and can differentiate spontaneously into myotubes when cultured in medium containing fetal calf serum. The transcription factors C/EBPα and PPARγ, when expressed in G8 myoblasts, can suppress the musclespecific transcription factors (Myf5, MyoD, myogenin and MRF4) . Conversely, markers specific for adipocytes, such as aP-2, adipsin, lipoprotein lipase and phosphoenolpyruvate carboxykinase appeared in G8 myoblasts co-expressing both PPARγ and C/EBPα. Both C/EBPβ and C/EBPδ also stimulate adipogenesis in fibroblasts , which probably occurs through up-regulation of PPARγ expression . However, the normal activation of the PPARγ nuclear receptor in adipogenesis still requires ligand binding . There are also cases where expression of C/EBPα in fibroblast cell lines (such as NIH3T3) will promote adipogenesis , but this only occurs with overexpression of C/EBPα. Normally, C/EBPα is expressed later than PPARγ in preadipocyte differentiation, and physiological amounts of C/EBPα can synergize with PPARγ promoting adipogenesis in fibroblasts . It has also been shown that the canonical Wnt signalling pathway is involved in inhibiting preadipocyte differentiation , probably through modulating expression of PPARγ and C/EBPα. Understanding the regulation of C/EBPs and PPARγ is important not only for maintaining the adipocyte differentiation state , but also in developing potential therapies for obesity.Transdifferentiation (metaplasia) of endodermally-derived tissues
One of the best-documented examples of transdifferentiation is the switch between liver and pancreas, a conversion that reflects the close developmental relationship between the two tissues . During development, the liver and pancreas arise from adjacent regions of the embryo called the anterior foregut endoderm and may therefore express common transcription factors during the early stage of development. Transdifferentiation of pancreas and liver is observed following the exposure of animals to certain carcinogens or in humans as preneoplastic alterations [reviewed in 18, 19]. The known molecular events underlying the switch between liver and pancreas and other endodermally-derived tissues, such as lung and intestine, or oesophagus and intestine, are discussed below.
In vivo pancreas to liver transdifferentiation
In vivo pancreas to liver transdifferentiation
Ectopic hepatic cells can be generated in the pancreas under a variety of conditions [19, 20]. Rao et al. treated rats with a copper depletion-repletion protocol and observed the appearance of multiple foci of hepatocytes in the pancreas . The evidence for the presence of hepatocytes in the pancreas was based on the expression of albumin and catalase proteins although the molecular mechanisms mediating the switch in cell phenotype in this model remain elusive. Albumin and α- fetoprotein-positive cells have also been induced in the islets of Langerhans in transgenic mice expressing keratinocyte growth factor (KGF) under the control of the insulin promoter . In vitro transdifferentiation of pancreatic cells to hepatocytes
More recently an in vitro model exhibiting the formation of transdifferentiated hepatocytes in pancreatic-derived AR42J-B13 cells (or B13 cells) has been described (Fig. 1A) and may provide some clues to the mechanisms underlying the changes observed in vivo [23–26]. The authors demonstrated that the transcription factor C/EBPβ is important for the transdifferentiation of pancreatic B13 cells to hepatocytes . Over-expression of C/EBPβ induced the loss of the pancreatic phenotype and gain of the hepatic phenotype. The essential requirement for C/EBPβ in the transdifferentiation was shown by a dominant negative approach. Introduction of liver inhibitory protein (LIP), an alternative posttranscriptional product of C/EBPβ mRNA , prior to the treatment of B13 cells with dexamethasone, inhibited the induction of transdifferentiation.
Conversion of hepatic cells to pancreatic-like cells
Although experimentally less well documented, recent progress in our understanding of the molecular biology of pancreatic development has taken us several steps closer to elucidating the mechanisms involved in the switch from hepatic to pancreatic cell types. Studies using mouse knockouts have demonstrated that certain transcription factors are essential for the steps in β cell formation: first the specification of a pancreatic rudiment in the endodermal epithelium, then the formation of endocrine precursor cells and finally the commitment to the β cell phenotype [28–30]. In the mouse embryo, a single inductive signal, fibroblast growth factor (FGF), distinguishes the region of endoderm that becomes the liver bud from that destined to become the ventral pancreatic bud , suggesting that only one or a few transcription factors need to have their activity switched on or off in order to interconvert the two tissues. Candidate transcription factors acting as master switch genes to determine a pancreatic fate are Pdx1, Ngn3, PTF1-p48 and Hes1. Pdx1 and Ngn3
Two transcription factors known to be particularly important in the normal development of β cells are Pdx1 and Ngn3. Pdx1 is a homeodomain-containing transcription factor expressed in the early pancreatic buds and surrounding duodenum . All pancreatic precursor cells express Pdx1 at an early stage of development , however expression later becomes restricted to β cells where Pdx1 is a specific transcription factor required for the expression of insulin. Interestingly, if the Pdx1 gene is knocked out, pancreatic agenesis results [32, 34], implicating Pdx1 as an essential factor in the determination of the pancreatic cell fate.
Transdifferentiation of liver to pancreas by ectopic expression of Pdx1
Several labs have recently shown that it is possible to induce some degree of pancreatic gene expression in liver cells using unmodified Pdx1 [35–37]. Results from our own lab showed that unmodified Pdx1 has very low activity in converting liver to pancreas, but that the activity can be greatly increased by incorporation of a transcriptional activation domain from the Herpes simplex protein VP16 . In Xenopus transgenics, Pdx1-VP16 is able to reprogram macroscopic areas of the liver to pancreas (Fig. 2). We have shown that the endogenous Pdx1 gene is activated in the ectopic pancreas and that the transthyretin (TTR) promoter, which drives the transgene, is not active in the pancreas. This suggests that the requirement for the presence of the transgene is only temporary and that once the cells have transdifferentiated to pancreas, transgene expression is shut off and the tissue is as stable as the endogenous pancreas. Li et al.  further confirmed the transdifferentiation of human hepatoma HepG2 cells into functional pancreaticlike cells using the same transgene (Fig. 1B). The pancreatic-like cells induced in liver using Pdx1 as a transgene show similar results from different groups and most of them also demonstrate rescue from hyperglycaemia in diabetic animals . Endocrine differentiation
Ngn3, a bHLH transcription factor, is expressed in all endocrine precursor cells. The knockout lacks endocrine cells, and overexpression drives the formation of additional endocrine cells [41, 42]. Ngn3 is also expressed in the developing central nervous system . More recently, Ngn3 was shown to be crucial for differentiation of endocrine cell types in pancreas, intestine and stomach [42, 44]. It is expressed from embryonic day 9.5 (E9.5), peaks at E15.5 and diminishes thereafter to undetectable levels in the neonatal mouse pancreas . It is thought that Ngn3 is probably responsible for the commitment of endocrine cells rather than all pancreatic cell lineages. Ngn3 is also able to drive pancreatic duct cells to a neuroendocrine phenotype with the formation of insulin-positive cells . This mirrors its role in the later stages of pancreas development and suggests that Ngn3 may require another 'central regulator' to change cell fate. More recently, it has been demonstrated that Ngn3 might be a mediator for converting intestinal epithelial cells to insulin-positive cells following treatment with glucagon-like peptide 1 in vitro and in vivo . The bHLH transcription factor PTF1-p48 was first described in exocrine-specific pancreas development and gene expression . Recent results have revised the role of PTF1-p48, with involvement in both exocrine and endocrine cell lineages in murine and zebrafish models [48, 49]. Kawaguchi et al.  demonstrated conversion of pancreatic progenitors into duodenal epithelium through the inactivation of PTF1-p48, thereby suggesting a close developmental relationship between intestine and pancreas (similar to that between liver and pancreas). Also, it provides a possible route for creating new pancreatic cells following the overexpression of PTF1-p48 in other cell types such as intestine.
Hes1 as a suppressor of Ngn3
Hes1 encodes a bHLH transcription factor that acts as a transcriptional repressor of genes such as Ngn3 . Hes1 is required for normal repression of neurogenic programming and expansion of neuronal progenitors . More recently, analysis of Hes1-/- animals has suggested that Hes1 may have a role in repressing endocrine cell development . The elevated expression of Ngn3 in the stomach, small and large intestine in Hes1-/- mice confirmed that Hes1 and Ngn3 counteract in the determination of endocrine cell fate . Based on the pro-endocrine action of Ngn3, the impaired expression of Hes1 would therefore be predicted to influence the differentiation of certain cell types such as pancreatic endocrine cells. To further support this idea, conversion of the developing biliary system to pancreatic tissue in Hes1 deficient mice has recently been described [54, 55]. The experiments also claimed that the conversion from biliary system to pancreatic-like cells might be through the re-activation of Ngn3 that is initially restrained by Hes1. Intestine in the lung
During development, the lung arises as an outgrowth of the ventral foregut endoderm, a little later than the appearance of the liver (around E9- 9.5 in the mouse) . The developing foregut tube separates to form (1) the future oesophagus and (2) the trachea that bifurcates to give rise to two ventrolateral buds that will become the bronchi. Following a period of morphogenesis involving extensive branching the typical 'treelike' structure characteristic of the lungs emerges and differentiation of the lung epithelia gives rise to several specialised pulmonary cell types; ciliated cells, mucus secreting goblet cells, pulmonary neuroendocrine cells, Clara cells and the alveolar type 1 and type 2 pneumocytes. The signalling events that mediate branching of the lung epithelia are now relatively well understood and involve interplay between FGF and Sonic hedgehog (Shh) pathways . In contrast, the mechanisms regulating determination of the different cell lineages within the lung are not clear. There is some evidence to suggest that development along the pulmonary neuroendocrine lineage requires expression of the Achaete-scute homolog-1, Mash1 , whereas Hes1 (Notch signalling pathway) may be involved in the generation of non-neuroendocrine cell types [59, 60]
The Wnts are soluble secreted glycoproteins that interact with cell surface receptors belonging to the frizzled family. Wnt receptor activation leads to the inhibition of the threonine/serine kinase glycogen synthase kinase-3β (GSK-3β) and the stabilisation of β-catenin, which in turn can bind to transcription factors of the TCF/LEF-1 family and translocate to the nucleus to modulate expression of target genes . Interestingly, several components of the Wnt signalling pathway are dynamically expressed during lung development. During epithelial branching, Wnt2 is expressed in the mesoderm adjacent to the tips of the branching endoderm which express Wnt7b , while the transcription factors TCF/LEF-1 are expressed in both mesoderm and endoderm . In order to further study the role of Wnt signalling in the lung, Okubo and Hogan developed a transgenic mouse model in which an activated β-catenin-LEF-1 fusion protein was constitutively expressed under the control of the human surfactant protein C gene promoter . Despite looking grossly normal, the transgenic lungs exhibited a drastically reduced number of Clara cells, a lack of fully differentiated alveolar type 1 and 2 cells and abnormally high proliferation of the bronchial epithelium. Subsequent microarray analysis and in situ hybridisation revealed the expression of multiple genes (trefoil factor 3, defensin related cryptidins 5, 6 and 15), characteristic of intestinal cell types such as Paneth and goblet cells, within the pulmonary epithelium of transgenic lungs. Furthermore, transgenic lungs showed induction or upregulation in the expression of a number of transcription factors that are components or targets of the Notch signalling pathway. These factors include Atoh 1 (Math1), not normally expressed in the lung but active in progenitor cells of the intestine , Gfi1, found in the neuroendocrine precursors of the gut and the lung  and increased levels of the Delta/Notch components Mash1, NeuroD4 and Delta-like 3. It appears that overactivation of Wnt signalling in the pulmonary epithelium leads to the respecification of pulmonary precursors towards cells of an intestinal cell fate, a process known as transdetermination. Wnt signalling is required in the developing intestine for the proliferation, maintenance and cell fate determination of epithelial stem cells in combination with the activity of the Notch signalling pathway [44, 65, 67]. Given that overactivation of the Wnt signalling pathway in the pulmonary epithelium leads to upregulation of several Notch signalling components, it is possible that the observed switch from a pulmonary to intestinal cell fate is dependent upon the convergence of these two pathways in a dose-dependent manner. The work of Okubo and Hogan highlights the plasticity of tissue-specific precursors when the dose of certain signalling molecules is altered within the developing endoderm and lends further support to the potential of manipulating such signalling pathways in stem cells for cellular replacement therapies in conditions such as diabetes and metabolic liver disease.
Barrett's metaplasia is a pathological condition characterised by a phenotypic switch in oesophageal epithelium from stratified squamous to intestinal type columnar epithelium containing mucin-secreting goblet cells . This switch predisposes to oesophageal adenocarcinoma , the incidence of which has increased rapidly in the last three decades  and carries a poor prognosis with a 5-year mortality exceeding 80% . The development of Barrett's oesophagus is related to reflux of gastro-duodenal contents, with an annual rate of progression to adenocarcinoma of approximately 0.5% . The neoplastic change follows a metaplasiadysplasia- carcinoma sequence, with dysplasia classified into low grade or high grade depending on the severity of atypical cytological alterations and nuclear polymorphism. The histological assessment of dysplasia and prediction of cancer risk is subjective and hampered by inter-observer variability . There has been considerable interest in using somatic mutations such as aneuploidy, p53 mutations, cyclin D1 overexpression, decreased E-cadherin expression and APC inactivation as markers to predict neoplastic progression [reviewed in 74]. By contrast, the molecular events prompting the initial metaplastic switch from stratified squamous to intestinal type columnar epithelium are at present poorly understood.Role of cdx2 in Barrett's metaplasia
Recently, there has been interest in the homeodomain transcription factor cdx2 as an early marker of Barrett's metaplasia. cdx2 is located on chromosome 13 in humans and is a member of the Parahox cluster . Various recent results suggest that cdx2 is a master switch gene whose expression normally distinguishes between the upper and lower regions of alimentary canal epithelia. First, it is normally expressed in the postgastric epithelium during embryonic development and into adult life [76, 77]. Second, ectopic expression in the stomach can provoke intestinal metaplasia. Silberg and colleagues directed ectopic cdx2 expression to the stomach in transgenic mice using cis-regulatory elements of the foxa3 promoter . The induction of intestinal metaplasia was confirmed both histologically and by demonstrating intestine-specific genes within the gastric mucosa. Mutoh and colleagues used the H+/K+ ATPase β-subunit gene promoter in a transgenic mouse to drive gastric cdx2 expression and similarly demonstrated intestinal metaplasia . As discussed by Beck, the proton pump functions postnatally and cdx2 may therefore have been expressed in differentiated gastric epithelium . This raises the question of whether there has been a transdifferentiation to intestinal type epithelium or whether a subset of parietal cells retain progenitor potential. Evidence for the master switch function of cdx2 is further supported by loss of function in the intestine leading to transformations to squamous epithelium resembling the oesophagus and forestomach. This is seen in focal colonic lesions of cdx2 +/- mice within which cdx2 expression has been completely lost . Interestingly, ectopic squamous patches in the colon are bordered by regions of gastric and small intestinal type epithelium . The appearance of 'intermediate' tissue types is ascribed to intercalary regeneration, which occurs at a junction between experimentally joined body parts resulting in the formation of parts which normally lie between them , and may reflect a gradient in developmental commitment between the focus of forestomach and surrounding colonic epithelium. Faller and colleagues demonstrated that gastric metaplasia in the duodenum was associated with loss of cdx2 expression and its target gene product sucraseisomaltase . These observations together suggest that cdx2 directs intestinal development towards a caudal phenotype and that the 'default' state resembles forestomach epithelium. cdx2 has been demonstrated in Barrett's metaplasia, oesophagitis and intestinal metaplasia of the stomach in humans [84–87]. Moons and colleagues demonstrated cdx2 mRNA in the squamous epithelium above the Barrett's segment in one third of patients and speculate that its expression may precede the switch in phenotype . There is mounting evidence that cdx2 may be involved in the transition from squamous epithelium to intestinal columnar epithelium in Barrett's metaplasia. Whether this is a causative effect or merely an association remains to be elucidated. The situation is further complicated by the recent report by Mutoh and colleagues using a transgenic mouse with ectopic cdx1 expression in the stomach under the H+/K+ ATPase β-subunit gene promoter which provoked similar, although not identical, intestinal metaplasia to their transgenic cdx2 mouse model . The molecular events in the development of Barrett's metaplasia may therefore be controlled by one, or a small number, of master switch genes. The exact mechanism of action of these transcription factors remains to be clarified.
Bone marrow transdifferentiation - fact or artefact?
Bone marrow transdifferentiation - fact or artefact?
Bone marrow contains two populations of stem cells - (1) haematopoietic stem cells (HSCs), which normally give rise to all mature lineages of blood, and (2) mesenchymal stem cells (MSCs), which can differentiate to bone, cartilage and fat. Previously, bone marrow-derived cells (BMDCs) were thought to be pluripotent and have the ability to transdifferentiate directly to non-haematopoietic cell types [reviewed in 89]. For example, both pancreatic cells [90–92] and liver cells [93–95] have been shown to arise from BMDCs. While these examples were first thought to provide evidence for direct transdifferentiation it is now recognised that (at least in some examples) BMDCs form fusion heterokaryons with cells in the target organs. So far, BMDCs have been shown to fuse with liver, skeletal muscle, cardiac muscle and neurons (Fig. 3) . While there is evidence of metaplasia when BMDCs convert to satellite cells in irradiated or exercise-induced muscle damage , there is also evidence that formation of new functional myofibres involves fusion of myeloid cells with the injured muscle fibres . However, one disadvantage of using muscle to study metaplasia is the difficulty in distinguishing between the mature multi-nucleated muscle fibre and nuclei incorporated during fusion.Criteria for transdifferentiation
In order to unambiguously demonstrate that transdifferentiation has occurred, two criteria must be fulfilled. The first requires that the differentiated state before and after transdifferentiation is demonstrated. The second is to demonstrate the ancestordescendant relationship between the two cell types One approach to addressing the ancestor-descendant relationship is to use the Cre-lox system. In the Cre-lox system, a mouse is engineered to contain two transgenes. Expression of the Cre recombinase is driven by a tissue-specific promoter. A reporter gene is driven by a ubiquitous promoter, but only after the Cre enzyme has excised an inhibitory sequence flanked by loxP sites. This means that the reporter becomes permanently active in the lineages that have previously activated the tissue-specific promoter. The Cre-lox system has been used to detect cell fusion events. Alvarez-Dolado et al. found evidence of BMDC fusion with hepatocytes and cardiomyocytes in vivo . There is also the example of BMDC incorporation into fully differentiated, non-proliferative Purkinje neurons in both humans  and mice [98, 100]. If fusion can occur in non-dividing cells, such as Purkinje neurons, or damaged tissues, such as cardiomyocytes following heart failure, there may be potential in the use of cell fusion as a mechanism for cell therapies. While the Cre-lox system is a powerful method for tracing cell lineages, other lineage tracing methods include transplanting male (Y chromosome) donor cells into female recipients or simply labelling the cells of interest with lipophilic dyes such as DiI. Evidence from transplantation studies demonstrates that HSCs can rescue liver function in a mouse model of hereditary tyrosinaemia type I . These mice lack the enzyme fumaryl-acetoacetate hydrolase (FAH) and can normally only survive if treated with the drug 2-(2-nitro-4-trifluoro- methylbenzyol)-1,3-cyclohexanedione (NBTC). HSCs derived from Fah+/+ mice reconstituted the Fah-/- liver by fusing with the existing hepatocytes [102, 103]. The authors excluded the possibility that HSCs from Fah+/+ mice converted to hepatocytes prior to fusion  and subsequently established through analysis of the genotype and phenotype of the reconstituted hepatocytes that a specific lineage of HSCs were responsible for fusion. This was confirmed when granulocyte macrophage progenitors (GMP, the colony forming units of progenitors for ganulocytes, macrophages and dendritic cells) or bone marrow-derived macrophages were transplanted into Fah-/- mice, the cells fused with the host hepatocytes and rescued the mutant phenotype . Granulocyte macrophage progenitors may therefore prove potentially useful in the development of therapeutic strategies for cell and organ replacement. Based on these examples, it is important to distinguish between the mechanisms i.e. direct transdifferentiation versus cell fusion. In order to distinguish these possibilities, the number of nuclei or chromosomes in the transdifferentiated cells can be determined. Although tissue damage is thought to be a primary factor for BMDC fusion with target cells, the precise signals regulating fusion and why only some cell types can fuse and others not is still unclear . There is evidence that fused cells become mononucleated again, either by nuclear fusion or by supernumerary nuclei elimination [98, 102]. Whether the fusion product - the stable heterokaryons - can also re-activate post-mitotic, terminally differentiated cell types to undergo cell division is currently under investigation. Despite the limitations of cell fusion rates this mechanism may be considered as a potential avenue for tissue repair.
There are now a growing number of models, both in vitro and in vivo demonstrating the plasticity of differentiated cells. In some cases, questions remain concerning the underlying molecular mechanisms and whether these represent true transdifferentiations or alternative events such as cell fusions. The therapeutic potentials of tissue reprogramming and cell replacement therapies offer exciting possibilities for new treatment strategies in degenerative and autoimmune diseases. As our understanding of the regulation of tissue development and maintenance increases, the promise of harnessing gene expression to influence cell phenotype and reprogramme tissues presents a worthy challenge. Acknowledgements
The work in the Tosh lab is funded by the Biotechnology and Biological Sciences Research Council, the Medical Research Council, Cancer Research UK, the Sulis Seedcorn Fund and the Wellcome Trust.
1. Tosh D, Slack JMW. How cells change their phenotype. Nat Rev Mol Cell Biol. 2002; 3:187–94.
2. Beresford WA. Direct transdifferentiation: can cells change their phenotype without dividing? Cell Differ Dev. 1990; 29:81–93.
3. Slack JMW, Tosh D. Transdifferentiation and metaplasia - switching cell types. Curr Opin Genet Dev. 2001; 11: 581–6.
4. Taylor SM, Jones PA. Multiple new phenotypes induced in 10T1/2 and 3T3 cells treated with 5-azacytidine. Cell 1979; 17: 771–9.
5. Konieczny SF, Emerson CP Jr. 5-Azacytidine induction of stable mesodermal stem cell lineages from 10T1/2 cells: evidence for regulatory genes controlling determination, Cell 1984; 38: 791–800.
6. Davis RL, Weintraub H, Lassar AB. Expression of a single transfected cDNA converts fibroblasts to myoblasts. Cell 1987; 51: 987–1000.
7. Parker MH, Seale P, Rudnicki MA. Looking back to the embryo: defining transcriptional networks in adult myogenesis. Nat Rev Genet. 2003; 4: 497–507.
8. Weintraub H, Tapscott SJ, Davis RL, Thayer MJ, Adam MA, Lassar AB, Miller AD. Activation of musclespecific genes in pigment, nerve, fat, liver, and fibroblast cell lines by forced expression of MyoD. Proc Natl Acad Sci USA. 1989; 86: 5434–8.
9. Hu E, Tontonoz P, Spiegelman BM. Transdifferentiation of myoblasts by the adipogenic transcription factors PPAR gamma and C/EBPα. Proc Natl Acad Sci USA. 1995; 92: 9856–60.
10. Yeh WC, Cao Z, Classon M, McKnight SL. Cascade regulation of terminal adipocyte differentiation by three members of the C/EBP family of leucine zipper proteins. Genes Dev. 1995; 9: 168–81.
11. Wu Z, Xie Y, Bucher NL, Farmer SR. Conditional ectopic expression of C/EBPβ in NIH-3T3 cells induces PPARγ and stimulates adipogenesis. Genes Dev. 1995; 9: 2350–63.
12. Wu Z, Bucher NL, Farmer SR. Induction of peroxisome proliferator-activated receptor γ during the conversion of 3T3 fibroblasts into adipocytes is mediated by C/EBPβ, C/EBPδ, and glucocorticoids. Mol Cell Biol. 1996; 16: 4128–36.
13. Freytag SO, Paielli DL, Gilbert JD. Ectopic expression of the CCAAT/enhancer-binding protein α promotes the adipogenic program in a variety of mouse fibroblastic cells. Genes Dev. 1994; 8: 1654–63.
14. Tontonoz P, Hu E, Spiegelman BM. Stimulation of adipogenesis in fibroblasts by PPAR γ 2, a lipid-activated transcription factor. Cell 1994; 79: 1147–56.
15. Ross SE, Hemati N, Longo KA, Bennett CN, Lucas PC, Erickson RL, MacDougald OA. Inhibition of adipogenesis by Wnt signaling. Science 2000; 289: 950–3.
16. Rosen ED, Walkey CJ, Puigserver P, Spiegelman BM. Transcriptional regulation of adipogenesis. Genes Dev. 2000; 14: 1293–307.
17. Wells JM, Melton DA. Vertebrate endoderm development. Annu Rev Cell Dev Biol. 1999; 15: 393–410. 18. Slack JMW. Homoeotic transformations in man: implications for the mechanism of embryonic development and for the organization of epithelia. J Theor Biol. 1985; 114: 463–90.
19. Shen CN, Horb ME, Slack JMW, Tosh D. Transdifferentiation of pancreas to liver. Mech Dev. 2003;
20. Grompe M. Pancreatic-hepatic switches in vivo. Mech Dev. 2003; 120: 99–106.
21. Rao MS, Subbarao V, Reddy JK. Induction of hepatocytes in the pancreas of copper-depleted rats following copper repletion. Cell Differ. 1986; 18: 109–17.
22. Krakowski ML, Kritzik MR, Jones EM, Krahl T, Lee J, Arnush M, Gu D, Sarvetnick N. Pancreatic expression of keratinocyte growth factor leads to differentiation of islet hepatocytes and proliferation of duct cells. Am J Pathol. 1999; 154: 683–91.
23. Shen CN, Slack JMW, Tosh D. Molecular basis of transdifferentiation of pancreas to liver. Nat Cell Biol. 2000; 2: 879–87.
24. Tosh D, Shen CN, Slack JMW. Conversion of pancreatic cells to hepatocytes. Biochem Soc Trans. 2002; 30: 51–5.
25. Tosh D, Shen CN, Slack JMW. Differentiated properties of hepatocytes induced from pancreatic cells. Hepatology 2002; 36: 534–43.
26. Kurash JK, Shen CN, Tosh D. Induction and regulation of acute phase proteins in transdifferentiated hepatocytes. Exp Cell Res. 2004; 292: 342–58.
27. Descombes P, Schibler U. A liver-enriched transcriptional activator protein, LAP, and a transcriptional inhibitory protein, LIP, are translated from the same mRNA. Cell 1991; 67: 569–79.
28. St-Onge L, Wehr R, Gruss P. Pancreas development and diabetes. Curr Opin Genet Dev. 1999; 9: 295–300.
29. Wilson ME, Scheel D, German MS. Gene expression cascades in pancreatic development. Mech Dev. 2003; 120: 65–80.
30. Kumar M, Melton D. Pancreas specification: a budding question. Curr Opin Genet Dev. 2003; 13: 401–7.
31. Deutsch G, Jung J, Zheng M, Lora J, Zaret KS. A bipotential precursor population for pancreas and liver within the embryonic endoderm. Development 2001; 128: 871–81.
32. Offield MF, Jetton TL, Labosky PA, Ray M, Stein RW, Magnuson MA, Hogan BL, Wright CV. PDX-1 is required for pancreatic outgrowth and differentiation of the rostral duodenum. Development 1996; 122: 983–95.
33. Gu G, Dubauskaite J, Melton DA. Direct evidence for the pancreatic lineage: NGN3+ cells are islet progenitors and are distinct from duct progenitors. Development 2002; 129: 2447–57.
34. Stoffers DA, Zinkin NT, Stanojevic V, Clarke WL, Habener JF. Pancreatic agenesis attributable to a single nucleotide deletion in the human IPF1 gene coding sequence. Nat Genet. 1997; 15: 106–10.
35. Ferber S, Halkin A, Cohen H, Ber I, Einav Y, Goldberg I, Barshack I, Seijffers R, Kopolovic J, Kaiser N, Karasik A. Pancreatic and duodenal homeobox gene 1 induces expression of insulin genes in liver and ameliorates streptozotocin-induced hyperglycemia. Nat Med. 2000; 6: 568–72.
36. Ber I, Shternhall K, Perl S, Ohanuna Z, Goldberg I, Barshack I, Benvenisti-Zarum L, Meivar-Levy I, Ferber S. Functional, persistent, and extended liver to pancreas transdifferentiation. J Biol Chem. 2003; 278: 31950–7.
37. Zalzman M, Gupta S, Giri RK, Berkovich I, Sappal BS, Karnieli O, Zern MA, Fleischer N, Efrat S. Reversal of hyperglycemia in mice by using human expandable insulin-producing cells differentiated from fetal liver progenitor cells. Proc Natl Acad Sci USA 2003; 100: 7253–8.
38. Horb ME, Shen CN, Tosh D, Slack JM. Experimental conversion of liver to pancreas. Curr Biol. 2003; 13: 105–15.
39. Li WC, Horb ME, Tosh D, Slack JMW. In vitro transdifferentiation of hepatoma cells into functional pancreatic cells. Mech Dev. 2005; 122: 835–47.
40. Imai J, Katagiri H, Yamada T, Ishigaki Y, Ogihara T, Uno K, Hasegawa Y, Gao J, Ishihara H, Sasano H, Mizuguchi H, Asano T, Oka Y. Constitutively active PDX-1 induced efficient insulin production in adult murine liver. Biochem Biophys Res Commun. 2005; 326: 402–9.
41. Apelqvist A, Li H, Sommer L, Beatus P, Anderson DJ, Honjo T, Hrabe de Angelis M, Lendahl U, Edlund H. Notch signalling controls pancreatic cell differentiation. Nature 1999; 400: 877–81.
42. Gradwohl G, Dierich A, LeMeur M, Guillemot F. Neurogenin3 is required for the development of the four endocrine cell lineages of the pancreas. Proc Natl Acad Sci USA 2000; 97: 1607–11.
43. Sommer L, Ma Q, Anderson DJ. Neurogenins, a novel family of atonal-related bHLH transcription factors, are putative mammalian neuronal determination genes that reveal progenitor cell heterogeneity in the developing CNS and PNS. Mol Cell Neurosci. 1996; 8: 221–41.
44. Jenny M, Uhl C, Roche C, Duluc I, Guillermin V, Guillemot F, Jensen J, Kedinger M, Gradwohl G. Neurogenin3 is differentially required for endocrine cell fate specification in the intestinal and gastric epithelium. EMBO J. 2002; 21: 6338–47.
45. Heremans Y, Van De Casteele M, in't Veld P, Gradwohl G, Serup P, Madsen O, Pipeleers D, Heimberg H. Recapitulation of embryonic neuroendocrine differentiation in adult human pancreatic duct cells expressing neurogenin 3. J Cell Biol. 2002; 159: 303–12.
46. Suzuki A, Nakauchi H, Taniguchi H. Glucagon-like peptide 1 (1–37) converts intestinal epithelial cells into insulin-producing cells. Proc Natl Acad Sci USA 2003; 100: 5034–9.
47. Krapp A, Knofler M, Frutiger S, Hughes GJ, Hagenbuchle O, Wellauer PK. The p48 DNA-binding subunit of transcription factor PTF1 is a new exocrine pancreas- specific basic helix-loop-helix protein. EMBO J. 1996; 15: 4317–29.
48. Chiang MK, Melton DA. Single-cell transcript analysis of pancreas development. Dev Cell. 2003; 4: 383–93.
49. Lin JW, Biankin AV, Horb ME, Ghosh B, Prasad NB, Yee NS, Pack MA, Leach SD. Differential requirement for ptf1a in endocrine and exocrine lineages of developing zebrafish pancreas. Dev Biol. 2004; 270: 474–86.
50. Kawaguchi Y, Cooper B, Gannon M, Ray M, MacDonald RJ, Wright CV. The role of the transcriptional regulator Ptf1a in converting intestinal to pancreatic progenitors. Nat Genet. 2002; 32: 128–34.
51. Ishibashi M, Ang SL, Shiota K, Nakanishi S, Kageyama R, Guillemot F. Targeted disruption of mammalian hairy and Enhancer of split homolog-1 (HES-1) leads to up-regulation of neural helix-loop-helix factors, premature neurogenesis, and severe neural tube defects. Genes Dev. 1995; 9: 3136–48.
52. Kageyama R, Ohtsuka T. The Notch-Hes pathway in mammalian neural development. Cell Res. 1999; 9: 179–88.
53. Jensen J, Pedersen EE, Galante P, Hald J, Heller RS, Ishibashi M, Kageyama R, Guillemot F, Serup P, Madsen OD. Control of endodermal endocrine development by Hes-1. Nat Genet. 2000; 24: 36–44.
54. Sumazaki R, Shiojiri N, Isoyama S, Masu M, Keino- Masu K, Osawa M, Nakauchi H, Kageyama R, Matsui A. Conversion of biliary system to pancreatic tissue in Hes1-deficient mice. Nat Genet. 2004; 36: 83–7.
55. Burke ZD, Shen CN, Tosh D. Bile ducts as a source of pancreatic β cells. Bioessays 2004; 26: 932–7.
56. Cardoso WV. Lung morphogenesis revisited: old facts, current ideas. Dev Dyn. 2000; 219: 121–30.
57. Warburton D, Schwarz M, Tefft D, Flores-Delgado G, Anderson KD, Cardaso WV. The molecular basis of lung morphogenesis. Mech Dev. 2000; 92: 55–81.
58. Ball DW. Achaete-scute homolog-1 and Notch in lung neuroendocrine development and cancer. Cancer Lett. 2004; 204: 159–69.
59. Ito T, Ukada N, Yazawa T, Okudela K, Hayashi H, Sudo T, Guillemot F, Kageyama R, Kitamura H. Basic helix-loop-helix transcription factors regulate the neuroendocrine differentiation of fetal mouse pulmonary epithelium. Development 2000; 127: 3913–21.
60. Borges M, Linnoila RI, van de Velde HJ, Chen H, Nelkin BD, Mabry M, Baylin SB, Ball DW. An achaetescute homologue essential for neuroendocrine differentiation in the lung. Nature 1997; 386: 852–5.
61. Logan CY, Nusse R. The Wnt signaling pathway in development and disease. Annu Rev Cell Dev Biol. 2004; 20: 781–810.
62. Shu W, Jiang YQ, Lu MM, Morrisey EE. Wnt7b regulates mesenchymal proliferation and vascular development in the lung. Development 2002; 129:4831–42.
63. Tebar M, Destree O, de Vree WJ, Ten Have-Opbroek AA. Expression of Tcf/Lef and sFrp and localization of β- catenin in the developing mouse lung. Mech Dev. 2001; 109: 437–40.
64. Okubo T, Hogan BLM. Hyperactive Wnt signaling changes the developmental potential of embryonic lung endoderm. J Biol. 2004; 3:11.
65. Yang Q, Bermingham NA, Finegold MJ, Zoghby HY. Requirement of Math1 for secretory cell lineage commitment in the mouse intestine. Science 2001; 294: 2155–8.
66. Wallis D, Hamblen M, Zhou Y, Venken KJ, Schumacher A, Grimes HL, Zoghbi HY, Orkin SH, Bellen HJ. The zinc finger transcription factor Gfi1, implicated in lymphomagenesis, is required for inner ear hair cell differentiation and survival. Development 2003; 130: 221–32.
67. Korinek V, Barker N, Moerer P, van Donselaar E, Huls G, Peters PJ, Clevers H. Depletion of epithelial stem-cell compartments in the small intestine of mice lacking Tcf-4. Nat Genet. 1998; 19: 379–83.
68. Sharma P, McQuaid K, Dent J, Fennerty MB, Sampliner R, Spechler S, Cameron A, Corley D, Falk G, Goldblum J, Hunter J, Jankowski J, Lundell L, Reid B, Shaheen NJ, Sonnenberg A, Wang K, Weinstein W. A critical review of the diagnosis and management of Barrett's esophagus: the AGA Chicago workshop. Gastroenterology 2004; 127: 310–30.
69. Haggit RC, Tryzelaar J, Ellis FH, Colcher H. Adenocarcinoma complicating columnar epithelium-lined (Barrett's) esophagus. Am J Clin Pathol. 1978; 70: 1-5.
70. Bollschweiler E, Wolfgarten E, Gutschow C, Holscher AH. Demographic variations in the rising incidence of esophageal adenocarcinoma in white males. Cancer 2001; 92: 549–55.
71. Tytgat GNJ, Bartelink H, Bernards R, Giaccone G, van Lanschot JJB, Offerhaus GJA, Peters GJ. Cancer of the esophagus and gastric cardia: recent advances. Diseases of the Esophagus 2004; 17: 10–26.
72. Murray L, Watson P, Johnston B, Sloan J, Mainie IM, Gavin A. Risk of adenocarcinoma in Barrett's oesophagus: population based study. Br Med J. 2003; 327: 534–5.
73. Montgomery E, Bronner MP, Goldblum JR, Greenson JK, Haber MM, Hart J, Lamps LW, Lauwers GY, Lazenby AJ, Lewin DN, Robert ME, Toleadano AY, Washington K. Reproducibility of the diagnosis of dysplasia in Barrett's esophagus: a reaffirmation. Hum Pathol. 2001; 32: 368–78.
74. Jenkins GJS, Doak SH, Parry JM, D'Souza FR, Griffiths AP, Baxter JN. Genetic pathways involved in the progression of Barrett's metaplasia to adenocarcinoma. Br J Surg. 2002; 89: 824–37.
75. German MS, Wang J, Fernald AA, Espinosa R. Localisation of the genes encoding two transcription factors, LMX1 and CDX3, regulating insulin gene expression to human chromosomes 1 and 13. Genomics 1994; 24:403–4.
76. Beck F, Erler T, Russell A, James R. Expression of Cdx-2 in the mouse embryo and placenta: possible role in the patterning of the extra-embryonic membranes. Dev Dyn. 1995; 204: 219–27.
77. James R, Kazenwadel J. Homeobox gene expression in the intestinal epithelium of adult mice. J Biol Chem. 1991; 266: 3246–51.
78. Silberg DG, Sullivan J, Kang E, Swain GP, Moffett J, Sund NJ, Sackett SD, Kaestner KH. Cdx2 ectopic expression induces gastric intestinal metaplasia in transgenic mice. Gastroenterology 2002; 122: 689–96.
79. Mutoh H, Hakamata Y, Sato K, Eda A, Yanaka I, Honda S, Osawa H, Kaneko Y, Sugano K. Conversion of gastric mucosa to intestinal metaplasia in Cdx2-expressing transgenic mice. Biochem Biophys Res Comm. 2002; 294: 470–9.
80. Beck F., The role of Cdx genes in the mammalian gut. Gut 2004; 53: 1394–6.
81. Chawengsaksophak K, James R, Hammond VE, Köntgen F., Beck F., Homeosis and intestinal tumours in Cdx2 mutant mice. Nature 1997; 386: 84–7.
82. Beck F, Chawengsaksophak K, Waring P, Playford RJ, Furness JB. Reprogramming of intestinal differentiation and intercalary regeneration in Cdx2 mutant mice. Proc Natl Acad Sci USA 1999; 96: 7318–23.
83. Faller G, Dimmler A, Rau T, Spaderna S, Hlubek F, Jung A, Kirchner T. Evidence for acid-induced loss of Cdx2 expression in duodenal gastric metaplasia. J Pathol. 2004; 203: 904–8.
84. Eda A, Osawa H, Yanaka I, Satoh K, Mutoh H, Kihira K, Sugano K. Expression of homeobox gene CDX2 precedes that of CDX1 during the progression of intestinal metaplasia. J Gastroenterol. 2002; 37: 94–100.
85. Eda A, Osawa H, Satoh K, Yanaka I, Kihira K, Ishino Y, Mutoh H, Sugano K. Aberrant expression of CDX2 in Barrett's epithelium and inflammatory esophageal mucosa. J Gastroenterol. 2003; 38: 14–22.
86. Phillips RW, Frierson HF, Moskaluk CA. Cdx2 as a marker of epithelial intestinal differentiation in the esophagus. Am J Surg Pathol. 2003; 27: 1442–7.
87. Moons LMG, Bax DA, Kuipers EJ, van Dekken H, Haringsma J, van Vliet AHM, Siersema PD, Kusters JG. The homeodomain protein CDX2 is an early marker of Barrett's oesophagus. J Clin Pathol. 2003; 57: 1063–8.
88. Mutoh H, Sakurai S, Satoh K, Osawa H, Hakamata Y, Takeuchi T, Sugano K. Cdx1 induced intestinal metaplasia in the transgenic mouse stomach: comparative study with Cdx2 transgenic mice. Gut 2004; 53: 1416–23.
89. Pomerantz J, Blau HM. Nuclear reprogramming: a key to stem cell function in regenerative medicine. Nat Cell Biol. 2004; 6: 810–6.
90. Ianus A, Holz GG, Theise ND, Hussain MA. In vivo derivation of glucose-competent pancreatic endocrine cells from bone marrow without evidence of cell fusion. J Clin Invest. 2003; 111: 843–50.
91. Ruhnke M, Ungefroren H, Nussler A, Martin F, Brulport M, Schormann W, Hengstler JG, Klapper W, Ulrichs K, Hutchinson JA, Soria B, Parwaresch RM, Heeckt P, Kremer B, Fandrich F. Differentiation of in vitro-modified human peripheral blood monocytes into hepatocyte-like and pancreatic islet-like cells. Gastroenterology 2005; 128: 1774–86.
92. Zhang Y-Q, Kritzik M, Sarvetnick N. Identification and expansion of pancreatic stem/progenitor cells. J Cell Mol Med. 2005; 9: 331–44.
93. Jang YY, Collector MI, Baylin SB, Diehl AM, Sharkis SJ. Hematopoietic stem cells convert into liver cells with-in days without fusion. Nat Cell Biol. 2004; 6: 532-539, 2004
94. Ishikawa F, Drake CJ, Yang S, Fleming PA, Minamiguchi H, Visconti RP, Crosby CV, Argraves WS, Harada M, Key LL, Livingston AG, Wingard JR, Ogawa M. Transplanted human cord blood cells give rise to hepatocytes in engrafted mice. Hematopoietic stem cells 2002: Ann NY Acad Sci. 2003; 996: 174–85.
95. Newsome PN, Johannessen I, Boyle S, Dalakas E, McAulay KA, Samuel K, Rae F, Forrester L, Turner ML, Hayes PC, Harrison DJ, Bickmore WA, Plevris JN. Human cord blood-derived cells can differentiate into hepatocytes in the mouse liver with no evidence of cellular fusion. Gastroenterology 2003; 124: 1891–900.
96. LaBarge MA, Blau HM. Biological progression from adult bone marrow to mononucleate muscle stem cell to multinucleate muscle fiber in response to injury. Cell 2002; 111: 589–601.
97. Corbel SY, Lee A, Yi L, Duenas J, Brazelton TR, Blau HM, Rossi FM. Contribution of hematopoietic stem cells to skeletal muscle. Nat Med. 2003; 9: 1528–32.
98. Alvarez-Dolado M, Pardal R, Garcia-Verdugo JM, Fike JR, Lee HO, Pfeffer K, Lois C, Morrison SJ, Alvarez-Buylla A. Fusion of bone-marrow-derived cells with Purkinje neurons, cardiomyocytes and hepatocytes. Nature 2003; 425: 968–73.
99. Weimann JM, Charlton CA, Brazelton TR, Hackman RC, Blau HM. Contribution of transplanted bone marrow cells to Purkinje neurons in human adult brains. Proc Natl Acad Sci USA 2003; 100: 2088–93.
100.Weimann JM, Johansson CB, Trejo A, Blau HM. Stable reprogrammed heterokaryons form spontaneously in Purkinje neurons after bone marrow transplant. Nat Cell Biol. 2003; 5: 959–66.
101. Lagasse E, Connors H, Al-Dhalimy M, Reitsma M, Dohse M, Osborne L, Wang X, Finegold M, Weissman IL, Grompe M. Purified hematopoietic stem cells can differentiate into hepatocytes in vivo. Nat Med. 2000; 6: 1229–34.
102.Wang X, Willenbring H, Akkari Y, Torimaru Y, Foster M, Al-Dhalimy M, Lagasse E, Finegold M, Olson S., Grompe M. Cell fusion is the principal source of bonemarrow- derived hepatocytes. Nature 2003; 422: 897–901.
103.Vassilopoulos G, Wang PR, Russell DW. Transplanted bone marrow regenerates liver by cell fusion. Nature 2003; 422: 901–4.
104.Willenbring H, Bailey AS, Foster M, Akkari Y, Dorrell C, Olson S, Finegold M, Fleming WH, Grompe M. Myelomonocytic cells are sufficient for therapeutic cell fusion in liver. Nat Med. 2004; 10: 744–8.
Fig. 1 In vitro transdifferentiation of pancreas and liver. (A) Hepatocytes can be induced from pancreatic AR42JB13 cells following treatment with the synthetic glucocorticoid dexamethasone. The transdifferentiation is observed by (1) the alteration to a flattened cell morphology, (2) the repression of pancreatic markers such as amylase and (3) the induction of liver proteins. (B) Introduction of an activated form of Pdx1, Xlhbox8VP16, into HepG2 human hepatoma cells induces multiple pancreatic markers .
Fig. 2 Transdifferentiation of liver to pancreas in Xenopus tadpoles. (A) Green fluorescent protein (GFP) under the control of the pancreas-specific elastase promoter (Elas) is expressed in the pancreas of transgenic tadpoles. P, pancreas; L, liver. (B) Proposed scheme for transdifferentiation in transgenic Xenopus. Xlhbox8VP16 is expressed in the liver of TTR-Xlhbox8VP16; Elas-GFP transgenic tadpoles under the control of the hepatic-specific transthyretin (TTR) promoter. The ectopic pancreas (EP) in the liver is visualized by GFP. Persistent expression of GFP in older tadpoles indicates the stability of the transdifferentiated pancreas. Figure adapted with permission from reference 38.
Fig. 3 Cell fusion and direct transdifferentiation of bone marrow-derived cells. Bone marrowderived cells may represent a new avenue for regenerative medicine. There are now examples that bone marrow-derived stem cells (BMDCs) can either fuse (neurons, liver, skeletal and cardiac muscle cells) or directly transdifferentiate to the target tissues (pancreas, hepatocytes and muscle stem cells).
Source: J. Cell. Mol. Med. Vol 9, No 3, 2005 pp. 569-582