Transdifferentiation, Metaplasia and Tissue Regeneration

Transdifferentiation is defined as the conversion of one cell type to another. It belongs to a wider class of cell type transformations called metaplasias which also includes cases in which stem cells of one tissue type switch to a completely different stem cell. Numerous examples of transdifferentiation exist within the literature. For example, isolated striated muscle of the invertebrate jellyfish (Anthomedusae) has enormous transdifferentiation potential and even functional organs (e.g., tentacles and the feeding organ (manubrium)) can be generated in vitro. In contrast, the potential for transdifferentiation in vertebrates is much reduced, at least under normal (nonpathological) conditions. But despite these limitations, there are some well-documented cases of transdifferentiation occurring in vertebrates. For example, in the newt, the lens of the eye can be formed from the epithelial cells of the iris. Other examples of transdifferentiation include the appearance of hepatic foci in the pancreas, the development of intestinal tissue at the lower end of the oesophagus and the formation of muscle, chondrocytes and neurons from neural precursor cells. Although controversial, recent results also suggest the ability of adult stem cells from different embryological germlayers to produce differentiated cells e.g., mesodermal stem cells forming ecto- or endodermally-derived cell types. This phenomenon may constitute an example of metaplasia. The current review examines in detail some well-documented examples of transdifferentiation, speculates on the potential molecular and cellular mechanisms that underlie the switches in phenotype, together with their significance to organogenesis and regenerative medicine.Key Words: transdifferentiation, metaplasia, tissue regeneration, stem cells, plasticity, reprogramming, regenerative medicine     

Transdifferentiation-inducing HCCR-1 oncogene

Background

Cell transdifferentiation is characterized by loss of some phenotypes along with acquisition of new phenotypes in differentiated cells. The differentiated state of a given cell is not irreversible. It depends on the up- and downregulation exerted by specific molecules.

Results

We report here that HCCR-1, previously shown to play an oncogenic role in human cancers, induces epithelial-to-mesenchymal transition (EMT) and mesenchymal-to-epithelial transition (MET) in human and mouse, respectively. The stem cell factor receptor CD117/c-Kit was induced in this transdifferentiated (EMT) sarcoma tissues. This MET occurring in HCCR-1 transfected cells is reminiscent of the transdifferentiation process during nephrogenesis. Indeed, expression of HCCR-1 was observed during the embryonic development of the kidney. This suggests that HCCR-1 might be involved in the transdifferentiation process of cancer stem cell.

Conclusions

Therefore, we propose that HCCR-1 may be a regulatory factor that stimulates morphogenesis of epithelia or mesenchyme during neoplastic transformation.     

Transdifferentiation,Metaplasia and Tissue Regeneration

Transdifferentiation is defined as the conversion of one cell type to another. It belongs to a wider class of cell type transformations called metaplasias which also includes cases in which stem cells of one tissue type switch to a completely different stem cell. Numerous examples of transdifferentiation exist within the literature. For example, isolated striated muscle of the invertebrate jellyfish (Anthomedusae) has enormous transdifferentiation potential and even functional organs (e.g. tentacles and the feeding organ (manubrium) can be generated in-vitro. In contrast, the potential for transdifferentiation in vertebrates is much reduced, at least under normal (non-pathological) conditions. But despite these limitations, there are some well-documented cases of transdifferentiation occurring in vertebrates. For example, in the newt, the lens of the eye can be formed from the epithelial cells of the iris. Other examples of transdifferentiation include the appearance of hepatic foci in the pancreas, the development of intestinal tissue at the lower end of the oesophagus and the formation of muscle, chondrocytes and neurons from neural precursor cells. Although controversial, recent results also suggest the ability of adult stem cells from different embryological germlayers to produce differentiated cells e.g. mesodermal stem cells forming ecto- or endodermally-derived cell types. This phenomenon may constitute an example of metaplasia. The current review examines in detail some well-documented examples of transdifferentiation, speculates on the potential molecular and cellular mechanisms that underlie the switches in phenotype, together with their significance to organogenesis and regenerative medicine.     

Stem cell plasticity, cell fusion, and transdifferentiation

One of the most contentious issues in biology today concerns the existence of stem cell plasticity. The term "plasticity" refers to the capacity of tissue-derived stem cells to exhibit a phenotypic potential that extends beyond the differentiated cell phenotypes of their resident tissue. Although evidence of stem cell plasticity has been reported by multiple laboratories, other scientists have not found the data persuasive and have remained skeptical about these new findings. This review will provide an overview of the stem cell plasticity controversy. We will examine many of the major objections that have been made to challenge the stem cell plasticity data. This controversy will be placed in the context of the traditional view of stem cell potential and cell phenotypic diversification. What the implications of cell plasticity are, and how its existence may modulate our present understanding of stem cell biology, will be explored. In addition, we will examine a topic that is usually not included within a discussion of stem cell biology--the direct conversion of one differentiated cell type into another. We believe that these observations on the transdifferentiation of differentiated cells have direct bearing on the issue of stem cell plasticity, and may provide insights into how cell phenotypic diversification is realized in the adult and into the origin of cell phenotypes during evolution.     

Transdifferentiation: a cell and molecular reprogramming process

Evidence has emerged recently indicating that differentiation is not entirely a one-way process, and that it is possible to convert one cell type to another, both in vitro and in vivo. This phenomenon is called transdifferentiation, and is generally defined as the stable switch of one cell type to another. Transdifferentiation plays critical roles during development and in regeneration pathways in nature. Although this phenomenon occurs rarely in nature, recent studies have been focused on transdifferentiation and the reprogramming ability of cells to produce specific cells with new phenotypes for use in cell therapy and regenerative medicine. Thus, understanding the principles and the mechanism of this process is important for producing desired cell types. Here some well-documented examples of transdifferentiation, and their significance in development and regeneration are reviewed. In addition, transdifferentiation pathways are considered and their potential molecular mechanisms, especially the role of master switch genes, are considered. Finally, the significance of transdifferentiation in regenerative medicine is discussed.     

多能干细胞诱导分化及体细胞转分化为心肌细胞的研究进展

心血管疾病是威胁人类健康的重大疾病,而心肌细胞数量逐渐减少,甚至衰竭是其核心病变。心肌细胞补偿性替代治疗是未来用于治疗这类疾病的重要手段,因此,心肌细胞的来源和有效治疗将成为关键。目前,心肌细胞构建的主要方法有多能干细胞诱导分化成心肌祖细胞或心肌细胞、心源性心肌祖细胞,以及体细胞重编程等。其中,多能干细胞向心肌细胞分化是最常用的方法;而体细胞转分化技术相较于传统的诱导多潜能干细胞衍生心肌细胞缩短了时间窗,为潜在的心血管疾病治疗提供了另一种思路。随着获取心肌细胞效率及其质量的提升,未来心血管疾病的治疗将有望获得重大突破。     

Epigenetic modulators promote mesenchymal stem cell phenotype switches

Discoveries in recent years have suggested that some tissue specific adult stem cells in mammals might have the ability to differentiate into cell types from different germ layers. This phenomenon has been referred to as stem cell transdifferentiation or plasticity. Despite controversy, the current consensus holds that transdifferentiation does occur in mammals, but only within a limited range. Understanding the mechanisms that underlie the switches in phenotype and development of the methods that will promote such type of conversions can open up endless possibilities for regenerative medicine. Epigenetic control contributes to various processes that lead to cellular plasticity and DNA and histone covalent modifications play a key role in these processes. Recently, we have been able to convert human mesenchymal stem cells (hMSCs) into neural-like cells by exposing cells to epigenetic modifiers and neural inducing factors. The goal of this study was to investigate the stability and plasticity of these transdifferentiated cells. To this end, neurally induced MSCs (NI-hMSCs) were exposed to adipocyte inducing factors. Grown for 24–48 h in fat induction media NI-hMSCs reversed their morphology into fibroblast-like cells and regained their proliferative properties. After 3 weeks approximately 6% of hMSCs differentiated into multilocular or plurivacuolar adipocyte cells that demonstrated by Oil Red O staining. Re-exposure of these cultures or the purified adipocytes to neural induction medium induced the cells to re-differentiate into neuronal-like cells. These data suggest that cell plasticity can be manipulated by the combination of small molecule modulators of chromatin modifying enzymes and specific cell signaling pathways.     

Culture of mouse neural stem cell precursors

Primary neural stem cell cultures are useful for studying the mechanisms underlying central nervous system development. Stem cell research will increase our understanding of the nervous system and may allow us to develop treatments for currently incurable brain diseases and injuries. In addition, stem cells should be used for stem cell research aimed at the detailed study of mechanisms of neural differentiation and transdifferentiation and the genetic and environmental signals that direct the specialization of the cells into particular cell types. This video demonstrates a technique used to disaggregate cells from the embryonic day 12.5 mouse dorsal forebrain. The dissection procedure includes harvesting E12.5 mouse embryos from the uterus, removing the "skin" with fine dissecting forceps and finally isolating pieces of cerebral cortex. Following the dissection, the tissue is digested and mechanically dissociated. The resuspended dissociated cells are then cultured in "stem cell" media that favors growth of neural stem cells.     

Cloned Myogenic Cells Can Transdifferentiate In Vivo into Neuron-Like Cells

Background

The question of whether intact somatic cells committed to a specific differentiation fate, can be reprogrammed in vivo by exposing them to a different host microenvironment is a matter of controversy. Many reports on transdifferentiation could be explained by fusion with host cells or reflect intrinsic heterogeneity of the donor cell population.

Methodology/Principal Findings

We have tested the capacity of cloned populations of mouse and human muscle progenitor cells, committed to the myogenic pathway, to transdifferentiate to neurons, following their inoculation into the developing brain of newborn mice. Both cell types migrated into various brain regions, and a fraction of them gained a neuronal morphology and expressed neuronal or glial markers. Likewise, inoculated cloned human myogenic cells expressed a human specific neurofilament protein. Brain injected donor cells that expressed a YFP transgene controlled by a neuronal specific promoter, were isolated by FACS. The isolated cells had a wild-type diploid DNA content.

Conclusions

These and other results indicate a genuine transdifferentiation phenomenon induced by the host brain microenvironment and not by fusion with host cells. The results may potentially be relevant to the prospect of autologous cell therapy approach for CNS diseases.     

Therapeutic potential of bone marrow stem cells for liver diseases

Stem cells of the bone marrow, including hematopoietic stem cells (HSC), mesenchymal stem cells (MSC) and hepatic progenitors were reported to give rise to hepatocytes by both transdifferentiation and cellular fusion. Transdifferentiation was observed without liver damage although significant numbers of stem cell derived hepatocytes were not described. Cellular fusion was demonstrated in the presence of a proliferation stimulus in conjunction with impaired intrinsic liver regeneration capacity. Here, we review potential therapeutic applications of stem cell derived hepatocytes depending on how they emerge. Stem cells turning into hepatocytes by transdifferentiation introduce new functioning liver cells into a diseased organ, which can support intrinsic liver regeneration or bridge the time gap until a definitive treatment is available. When cellular fusion is the mechanism behind stem cell plasticity, however, no new cells emerge in the first place, whereas new genetic material is introduced. The fusion cell thereby acquires a selective advantage over resident hepatocytes allowing for extensive proliferation and liver repopulation. Therefore genetic deficiencies might be the predominant target for cell fusion therapies. We conclude that transdifferentiation and cellular fusion might be powerful tools for the therapy of liver diseases in the future and we propose the introduction of artificial cell fusion as well as stem cell differentiation as therapeutic options.     

Bone Marrow Transplantation Results in Human Donor Blood Cells Acquiring and Displaying Mouse Recipient Class I MHC and CD45 Antigens on Their Surface

Background

Mouse models of human disease are invaluable for determining the differentiation ability and functional capacity of stem cells. The best example is bone marrow transplants for studies of hematopoietic stem cells. For organ studies, the interpretation of the data can be difficult as transdifferentiation, cell fusion or surface antigen transfer (trogocytosis) can be misinterpreted as differentiation. These events have not been investigated in hematopoietic stem cell transplant models.

Methodology/Principal Findings

In this study we investigated fusion and trogocytosis involving blood cells during bone marrow transplantation using a xenograft model. We report that using a standard SCID repopulating assay almost 100% of the human donor cells appear as hybrid blood cells containing both mouse and human surface antigens.

Conclusion/Significance

Hybrid cells are not the result of cell-cell fusion events but appear to be due to efficient surface antigen transfer, a process referred to as trogocytosis. Antigen transfer appears to be non-random and includes all donor cells regardless of sub-type. We also demonstrate that irradiation preconditioning enhances the frequency of hybrid cells and that trogocytosis is evident in non-blood cells in chimera mice.     

The development of cell lineages: A sequential model

Abstract. The concept of cell lineage and the empirical characterization of specific lineages provide valuable insight into the problems of developmental biology. Of central interest is the decision-making process that results in the diversification of cell lines. Studies of the haemopoietic system, in which stem cells can be committed to one of at least six pathways of differentiation, have suggested that the restriction of differentiation potentials is a progressive and stochastic process. We have recently proposed an alternative model which hypothesizes that lineage potentials during haemopoiesis are expressed individually and in a predetermined sequence as progenitor cells mature. The model first arises from experimental studies which show that both normal myeloid progenitor cells and a human promyeloid cell line, which are able to differentiate towards either neutrophils or monocytes, express these potentials sequentially in culture. The close linear relationship between other haemopoietic progenitor cells is inferred from collective data from studies of bipotent progenitor cells and of haemopoietic proliferative disorders. If the development of haemopoietic cell lineages shows a tendency to follow a particular program, such a mechanism is likely to operate throughout development. In this paper we consider the evidence in favour of programmed events within progenitor cells implementing diversification, and the implications of predetermined and restricted pathways of embryonic development.     

BM stem cells and cardiac repair: where do we stand in 2004?

Adult BM stem cells are being investigated for their potential to regenerate injured tissues by a process referred to as plasticity or transdifferentiation. Although data supporting stem cell plasticity is extensive, a controversy has emerged based on findings that propose cell-cell fusion as a more appropriate interpretation for this phenomenon. A major focus of this controversy is the claim that acutely infarcted myocardium in adult hearts can be regenerated by BM stem cells. Many researchers consider the adult heart to be a post-mitotic organ, whereas others believe that a low level of cardiomyocyte renewal occurs throughout life. If renewal occurs, it may be in response to cardiac stem cell activity or to stem cells that migrate from distant tissues. Post-mortem microscopic analysis of experimentally induced myocardial infarctions in several rodent models suggests that cardiomyocyte renewal is achieved by stem cells that infiltrate the damaged tissue. For a better understanding of the possible involvement of stem cells in myocardial regeneration, it is important to develop appropriate technologies to monitor myocardial repair over time with an emphasis on large animal models. Studies on non-human primate, swine and canine models of acute myocardial infarctions would enable investigators to utilize clinical quality cell-delivery devices, track labeled donor cells after precision transplantation and utilize non-invasive imaging for functional assays over time with clinical accuracy. In addition, if stem cell plasticity is to reach the next level of acceptance, it is important to identify the environmental cues needed for stem cell trafficking and to define the genetic and cellular mechanisms that initiate transdifferentiation. Only then will it be possible to determine if, and to what extent, BM stem cells are involved in myocardial regeneration and to begin to regulate precisely tissue repair.     

Transdifferentiation in neoplastic development and its pathological implication

Transdifferentiation is a process in which a cell committed to a particular specialization changes to another quite distinct type. It occurs during embryological development and some pathological processes, and causes the tumor cells to express a phenotype different from that of their normal progenitors. Neoplastic transdifferentiation involves pathogenesis of cancer subtype, transition between neoplastic epithelia and neuroendocrine cell, transition between neoplastic epithelia and mesenchyme, as well as transition between non-neuroectodermal and neuroectodermal cells. We propose that differentiation disturbance of cancer cells should include not only lower-, un-, or de-differentiation, but also transdifferentiation. Tumor cell transdifferentiation results from genetic instabilities. In some type of neoplastic transition, the initiation may be induced by extracellular matrix and growth factors.     

PDGF,NT-3 and IGF-2 in Combination Induced Transdifferentiation of Muscle-Derived Stem Cells into Schwann Cell-Like Cells

Muscle-derived stem cells (MDSCs) are multipotent stem cells with a remarkable long-term self-renewal and regeneration capacity. Here, we show that postnatal MDSCs could be transdifferentiated into Schwann cell-like cells upon the combined treatment of three neurotrophic factors (PDGF, NT-3 and IGF-2). The transdifferentiation of MDSCs was initially induced by Schwann cell (SC) conditioned medium. MDSCs adopted a spindle-like morphology similar to SCs after the transdifferentiation. Immunocytochemistry and immunoblot showed clearly that the SC markers S100, GFAP and p75 were expressed highly only after the transdifferentiation. Flow cytometry assay showed that the portion of S100 expressed cells was more than 60 percent and over one fourth of the transdifferentiated cells expressed all the three SC markers, indicating an efficient transdifferentiation. We then tested neurotrophic factors in the conditioned medium and found it was PDGF, NT-3 and IGF-2 in combination that conducted the transdifferentiation. Our findings demonstrate that it is possible to use specific neurotrophic factors to transdifferentiate MDSCs into Schwann cell-like cells, which might be therapeutically useful for clinical applications.     

Adult reserve stem cells and their potential for tissue engineering

Tissue restoration is the process whereby multiple damaged cell types are replaced to restore the histoarchitecture and function to the tissue. Several theories, have been proposed to explain the phenomenon of tissue restoration in amphibians and in animals belonging to higher order. These theories include dedifferentiation of damaged tissues, transdifferentiation of lineage-committed progenitor cells, and activation of reserve, precursor cells. Studies by Young et al. and others demonstrated that connective tissue compartments throughout postnatal individuals contain reserve precursor cells. Subsequent repetitive single cell-cloning and cell-sorting studies revealed that these reserve precursor cells consisted of multiple populations of cells, including, tissue-specific progenitor cells, germ-layer lineage stem cells, and pluripotent stem cells. Tissue-specific progenitor cells display various capacities for differentiation, ranging from unipotency (forming a single cell type) to multipotency (forming multiple cell types). However, all progenitor cells demonstrate a finite life span of 50 to 70 population doublings before programmed cell senescence and cell death occurs. Germ-layer lineage stem cells can form a wider range of cell types than a progenitor cell. An individual germ-layer lineage stem cell can form all cells types within its respective germ-layer lineage (i.e., ectoderm, mesoderm, or endoderm). Pluripotent stem cells can form a wider range of cell types than a single germ-layer lineage stem cell. A single pluripotent stem cell can form cells belonging to all three germ layer lineages. Both germ-layer lineage stem cells and pluripotent stem cells exhibit extended capabilities for self-renewal, far surpassing the limited life span of progenitor cells (50–70 population doublings). The authors propose that the activation of quiescent tissue-specific progenitor cells, germ-layer lineage stem cells, and/or pluripotent stem cells may be a potential explanation, along with dedifferentiation and transdifferentiation, for the process of tissue restoration. Several model systems are currently being investigated to determine the possibilities of using these adult quiescent reserve precursor cells for tissue engineering.     

Switching from bone marrow-derived neurons to epithelial cells through dedifferentiation and translineage redifferentiation

While the ability of stem cells to switch lineages has been suggested, the route(s) through which this may happen is unclear. To date, the best characterized adult stem cell population considered to possess transdifferentiation capacity is BM-MSCs (bone marrow mesenchymal stem cells). We investigated whether BM-MSCs that had terminally differentiated into the neural or epithelial lineage could be induced to transdifferentiate into the other phenotype in vitro. Our results reveal that neuronal phenotypic cells derived from adult rat bone marrow cells can be switched to epithelial phenotypic cells, or vice versa, by culture manipulation allowing the differentiated cells to go through, first, dedifferentiation and then redifferentiation to another phenotype. Direct transdifferentiation from differentiated neuronal or epithelial phenotype to the other differentiated phenotype cannot be observed even when appropriate culture conditions are provided. Thus, dedifferentiation appears to be a prerequisite for changing fate and differentiating into a different lineage from a differentiated cell population.