Plasticity of cell fate: insights from heterokaryons

Functional specialisation and coordination of myonuclei

Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.

Cancer stem cells (CSCs): key player of radiotherapy resistance and its clinical significance

Cancer stem cells (CSCs) are self-renewing and slow-multiplying micro subpopulations in tumour microenvironments. CSCs contribute to cancer's resistance to radiation (including radiation) and other treatments. CSCs control the heterogeneity of the tumour. It alters the tumour's microenvironment cellular singling and promotes epithelial-to-mesenchymal transition (EMT). Current research decodes the role of extracellular vesicles (EVs) and CSCs interlink in radiation resistance. Exosome is a subpopulation of EVs and originated from plasma membrane. It is secreted by several active cells. It involed in cellular communication and messenger of healthly and multiple pathological complications. Exosomal biological active cargos (DNA, RNA, protein, lipid and glycan), are capable to transform recipient cells' nature. The molecular signatures of CSCs and CSC-derived exosomes are potential source of cancer theranostics development. This review discusse cancer stem cells, radiation-mediated CSCs development, EMT associated with CSCs, the role of exosomes in radioresistance development, the current state of radiation therapy and the use of CSCs and CSCs-derived exosomes biomolecules as a clinical screening biomarker for cancer. This review gives new researchers a reason to keep an eye on the next phase of scientific research into cancer theranostics that will help mankind.

Circulating Cells with Macrophage-like Characteristics in Cancer: The Importance of Circulating Neoplastic-Immune Hybrid Cells in Cancer

Simple Summary

In cancer, disseminated neoplastic cells circulating in blood are a source of tumor DNA, RNA, and protein, which can be harnessed to diagnose, monitor, and better understand the biology of the tumor from which they are derived. Historically, circulating tumor cells (CTCs) have dominated this field of study. While CTCs are shed directly into circulation from a primary tumor, they remain relatively rare, particularly in early stages of disease, and thus are difficult to utilize as a reliable cancer biomarker. Neoplastic-immune hybrid cells represent a novel subpopulation of circulating cells that are more reliably attainable as compared to their CTC counterparts. Here, we review two recently identified circulating cell populations in cancer—cancer-associated macrophage-like cells and circulating hybrid cells—and discuss the future impact for the exciting area of disseminated hybrid cells.

Abstract

Cancer remains a significant cause of mortality in developed countries, due in part to difficulties in early detection, understanding disease biology, and assessing treatment response. If effectively harnessed, circulating biomarkers promise to fulfill these needs through non-invasive “liquid” biopsy. While tumors disseminate genetic material and cellular debris into circulation, identifying clinically relevant information from these analytes has proven difficult. In contrast, cell-based circulating biomarkers have multiple advantages, including a source for tumor DNA and protein, and as a cellular reflection of the evolving tumor. While circulating tumor cells (CTCs) have dominated the circulating cell biomarker field, their clinical utility beyond that of prognostication has remained elusive, due to their rarity. Recently, two novel populations of circulating tumor-immune hybrid cells in cancer have been characterized: cancer-associated macrophage-like cells (CAMLs) and circulating hybrid cells (CHCs). CAMLs are macrophage-like cells containing phagocytosed tumor material, while CHCs can result from cell fusion between cancer and immune cells and play a role in the metastatic cascade. Both are detected in higher numbers than CTCs in peripheral blood and demonstrate utility in prognostication and assessing treatment response. Additionally, both cell populations are heterogeneous in their genetic, transcriptomic, and proteomic signatures, and thus have the potential to inform on heterogeneity within tumors. Herein, we review the advances in this exciting field.

Cell-cell fusion of mesenchymal cells with distinct differentiations triggers genomic and transcriptomic remodelling toward tumour aggressiveness

Cell–cell fusion is a physiological process that is hijacked during oncogenesis and promotes tumour evolution. The main known impact of cell fusion is to promote the formation of metastatic hybrid cells following fusion between mobile leucocytes and proliferating tumour cells. We show here that cell fusion between immortalized myoblasts and transformed fibroblasts, through genomic instability and expression of a specific transcriptomic profile, leads to emergence of hybrid cells acquiring dissemination properties. This is associated with acquisition of clonogenic ability by fused cells. In addition, by inheriting parental properties, hybrid tumours were found to mimic the histological characteristics of a specific histotype of sarcomas: undifferentiated pleomorphic sarcomas with incomplete muscular differentiation. This finding suggests that cell fusion, as macroevolution event, favours specific sarcoma development according to the differentiation lineage of parent cells.

Simulation of multiple microenvironments shows a pivot role of RPTPs on the control of Epithelial-to-Mesenchymal Transition

Epithelial-to-Mesenchymal Transition (EMT) is a natural and reversible process involved in embryogenesis, wound healing and thought to participate in the process of metastasis. Multiple signals from the microenvironment have been reported to drive EMT. However, the tight control of this process on physiological scenarios and how it is disrupted during cancer progression is not fully understood. Here, we analysed a regulatory network of EMT accounting for 10 key microenvironment signals focusing on the impact of two cell contact signals on the reversibility of EMT and the stability of resulting phenotypes. The analysis showed that the microenvironment is not enough for stabilizing Hybrid and Amoeboid-like phenotypes, requiring intracellular de-regulations as reported during cancer progression. Our simulations demonstrated that RPTP activation by cell contacts have the potential to inhibit the process of EMT and trigger its reversibility under tissue growth and chronic inflammation scenarios. Simulations also showed that hypoxia inhibits the capacity of RPTPs to control EMT. Our analysis further provided a theoretical explanation for the observed correlation between hypoxia and metastasis under chronic inflammation, and predicted that de-regulations in FAT4 signalling may promote Hybrid stabilization. Taken together, we propose a natural control mechanism of EMT that supports the idea that EMT is tightly regulated by the microenvironment.

Endothelial to Mesenchymal Transition: Role in Physiology and in the Pathogenesis of Human Diseases

Numerous studies have demonstrated that endothelial cells are capable of undergoing endothelial to mesenchymal transition (EndMT), a newly recognized type of cellular transdifferentiation. EndMT is a complex biological process in which endothelial cells adopt a mesenchymal phenotype displaying typical mesenchymal cell morphology and functions, including the acquisition of cellular motility and contractile properties. Endothelial cells undergoing EndMT lose the expression of endothelial cell-specific proteins such as CD31/platelet-endothelial cell adhesion molecule, von Willebrand factor, and vascular-endothelial cadherin and initiate the expression of mesenchymal cell-specific genes and the production of their encoded proteins including α-smooth muscle actin, extra domain A fibronectin, N-cadherin, vimentin, fibroblast specific protein-1, also known as S100A4 protein, and fibrillar type I and type III collagens. Transforming growth factor-β1 is considered the main EndMT inducer. However, EndMT involves numerous molecular and signaling pathways that are triggered and modulated by multiple and often redundant mechanisms depending on the specific cellular context and on the physiological or pathological status of the cells. EndMT participates in highly important embryonic development processes, as well as in the pathogenesis of numerous genetically determined and acquired human diseases including malignant, vascular, inflammatory, and fibrotic disorders. Despite intensive investigation, many aspects of EndMT remain to be elucidated. The identification of molecules and regulatory pathways involved in EndMT and the discovery of specific EndMT inhibitors should provide novel therapeutic approaches for various human disorders mediated by EndMT.

A systematic review and meta-analysis on the prevalence of Dupuytren disease in the general population of Western countries

BACKGROUND

Dupuytren disease is a fibroproliferative disease of palmar fascia of the hand. Its prevalence has been the subject of several reviews; however, an accurate description of the prevalence range in the general population--and of the relation between age and disease--is lacking.

METHODS

Embase and PubMed were searched using database-specific Medical Subject Headings; titles and abstracts were searched for the words "Dupuytren," "incidence," and "prevalence." Two reviewers independently assessed the articles using inclusion and exclusion criteria, and rated the included studies with a quality assessment instrument. In a meta-analysis, the median prevalence, as a function of age by sex, was estimated, accompanied by 95 percent prediction intervals. The observed heterogeneity in prevalence was investigated with respect to study quality and geographic location.

RESULTS

Twenty-three of 199 unique identified articles were included. The number of participants ranged from 37 to 97,537, and age ranged from 18 to 100 years. Prevalence varied from 0.6 to 31.6 percent. The quality of studies differed but could not explain the heterogeneity among studies. Mean prevalence was estimated as 12, 21, and 29 percent at ages 55, 65, and 75 years, respectively, based on the relation between age and prevalence determined from 10 studies.

CONCLUSIONS

The authors describe a prevalence range of Dupuytren disease in the general population of Western countries. The relation between age and prevalence of Dupuytren disease is given according to sex, including 95 percent prediction intervals. It is possible to determine disease prevalence at a certain age for the total population, and for men and women separately.

Siberian Sturgeon Oocyte Extract Induces Epigenetic Modifications of Porcine Somatic Cells and Improves Developmental Competence of SCNT Embryos

Somatic cell nuclear transfer (SCNT) has generally demonstrated that a differentiated cell can convert into a undifferentiated or pluripotent state. In the SCNT experiment, nuclear reprogramming is induced by exposure of introduced donor nuclei to the recipient cytoplasm of matured oocytes. However, because the efficiency of SCNT still remains low, a combination of SCNT technique with the ex-ovo method may improve the normal development of SCNT embryos. Here we hypothesized that treatment of somatic cells with extracts prepared from the germinal vesicle (GV) stage Siberian sturgeon oocytes prior to their use as nuclear donor for SCNT would improve in vitro development. A reversible permeability protocol with 4 μg/mL of digitonin for 2 min at 4°C in order to deliver Siberian sturgeon oocyte extract (SOE) to porcine fetal fibroblasts (PFFs) was carried out. As results, the intensity of H3K9ac staining in PFFs following treatment of SOE for 7 h at 18°C was significantly increased but the intensity of H3K9me3 staining in PFFs was significantly decreased as compared with the control (p<0.05). Additionally, the level of histone acetylation in SCNT embryos at the zygote stage was significantly increased when reconstructed using SOE-treated cells (p<0.05), similar to that of IVF embryos at the zygote stage. The number of apoptotic cells was significantly decreased and pluripotency markers (Nanog, Oct4 and Sox2) were highly expressed in the blastocyst stage of SCNT embryos reconstructed using SOE-treated cells as nuclear donor (p<0.05). And there was observed a better development to the blastocyst stage in the SOE-treated group (p<0.05). Our results suggested that pre-treatment of cells with SOE could improve epigenetic reprogramming and the quality of porcine SCNT embryos.

A systematic review and meta-analysis on the prevalence of Dupuytren disease in the general population of Western countries

BACKGROUND

Dupuytren disease is a fibroproliferative disease of palmar fascia of the hand. Its prevalence has been the subject of several reviews; however, an accurate description of the prevalence range in the general population--and of the relation between age and disease--is lacking.

METHODS

Embase and PubMed were searched using database-specific Medical Subject Headings; titles and abstracts were searched for the words "Dupuytren," "incidence," and "prevalence." Two reviewers independently assessed the articles using inclusion and exclusion criteria, and rated the included studies with a quality assessment instrument. In a meta-analysis, the median prevalence, as a function of age by sex, was estimated, accompanied by 95 percent prediction intervals. The observed heterogeneity in prevalence was investigated with respect to study quality and geographic location.

RESULTS

Twenty-three of 199 unique identified articles were included. The number of participants ranged from 37 to 97,537, and age ranged from 18 to 100 years. Prevalence varied from 0.6 to 31.6 percent. The quality of studies differed but could not explain the heterogeneity among studies. Mean prevalence was estimated as 12, 21, and 29 percent at ages 55, 65, and 75 years, respectively, based on the relation between age and prevalence determined from 10 studies.

CONCLUSIONS

The authors describe a prevalence range of Dupuytren disease in the general population of Western countries. The relation between age and prevalence of Dupuytren disease is given according to sex, including 95 percent prediction intervals. It is possible to determine disease prevalence at a certain age for the total population, and for men and women separately.

A blueprint for engineering cell fate: current technologies to reprogram cell identity

Human diseases such as heart failure, diabetes, neurodegenerative disorders, and many others result from the deficiency or dysfunction of critical cell types. Strategies for therapeutic tissue repair or regeneration require the in vitro manufacture of clinically relevant quantities of defined cell types. In addition to transplantation therapy, the generation of otherwise inaccessible cells also permits disease modeling, toxicology testing and drug discovery in vitro. In this review, we discuss current strategies to manipulate the identity of abundant and accessible cells by differentiation from an induced pluripotent state or direct conversion between differentiated states. We contrast these approaches with recent advances employing partial reprogramming to facilitate lineage switching, and discuss the mechanisms underlying the engineering of cell fate. Finally, we address the current limitations of the field and how the resulting cell types can be assessed to ensure the production of medically relevant populations.

Direct lineage conversions: unnatural but useful?

Classic experiments such as somatic cell nuclear transfer into oocytes and cell fusion demonstrated that differentiated cells are not irreversibly committed to their fate. More recent work has built on these conclusions and discovered defined factors that directly induce one specific cell type from another, which may be as distantly related as cells from different germ layers. This suggests the possibility that any specific cell type may be directly converted into any other if the appropriate reprogramming factors are known. Direct lineage conversion could provide important new sources of human cells for modeling disease processes or for cellular-replacement therapies. For future applications, it will be critical to carefully determine the fidelity of reprogramming and to develop methods for robustly and efficiently generating human cell types of interest.

Pancreatic ductal cells acquire mesenchymal characteristics through cell fusion with bone marrow-derived mesenchymal stem cells and SIRT1 attenuates the apoptosis of hybrid cells

Bone marrow-derived mesenchymal stem cells (bMSCs) contribute to tissue repair and regeneration. Cell fusion between somatic cells and bMSCs to form hybrid cells may have an important role in tissue repair through the subsequent reprogramming of the somatic cell nucleus. Few studies have assessed the mesenchymal characteristics of fusion-induced hybrid cells and their survival mechanisms. In this study, we investigated the effect of cell fusion on the biological characteristics of pancreatic ductal cells (PDCs) and on the survival mechanism of hybrid cells. To this end, we generated mouse-mouse hybrid cells in vitro by polyethylene glycol-mediated fusion of primary mouse bMSCs with primary mouse PDCs. Hybrid cells showed an enhanced capacity for proliferation and self-renewal compared with PDCs. No PDC had the capacity for anchorage-independent growth or invasion into Matrigel, but some hybrid cells were able to form colonies in soft agar and invade Matrigel. Expression of the tumor suppressor protein p53, which initiates apoptosis, was detected in hybrid cells but not in PDCs or bMSCs. However, the p53 deacetylase, sirtuin 1 (SIRT1), was also detected in hybrid cells, and the level of acetylated p53, the active form, was low. The addition of nicotinamide (Nam) inhibited the deacetylation activity of SIRT1 on p53 and induced cell apoptosis in hybrid cells. This study demonstrated that PDCs could obtain high proliferation rates, self-renewal capabilities, and mesenchymal characteristics by fusion with bMSCs. SIRT1 expression in the hybrid cells attenuated their apoptosis.

Characterization of Pax3-expressing cells from adult blood vessels

We report expression of Pax3, an important regulator of skeletal muscle stem cell behaviour, in the brachial and femoral arteries of adult mice. In these contractile arteries of the limb, but not in the elastic arteries of the trunk, bands of GFP-positive cells were observed in Pax3(GFP/+) mice. Histological and biochemical examination of the vessels, together with clonal analysis after purification of Pax3-GFP-positive cells by flow cytometry, established their vascular smooth muscle identity. These blood-vessel-derived cells do not respond to inducers of other mesodermal cell types, such as bone, however, they can contribute to muscle fibre formation when co-cultured with skeletal muscle cells. This myogenic conversion depends on the expression of Pax3, but is rare and non-cell autonomous as it requires cell fusion. Myocardin, which promotes acquisition of a mature smooth muscle phenotype in these Pax3-GFP-positive cells, antagonises their potential for skeletal muscle differentiation. Genetic manipulation shows that myocardin is, however, positively regulated by Pax3, unlike genes for other myocardin-related factors, MRTFA, MRTFB or SRF. Expression of Pax3 overlaps with that reported for Msx2, which is required for smooth muscle differentiation of blood vessel-derived multipotent mesoangioblasts. These observations are discussed with respect to the origin and function of Pax3-expressing cells in blood vessels, and more general questions of cell fate determination and adult cell plasticity and reprogramming.

V-fusion: a convenient, nontoxic method for cell fusion

Cell-to-cell fusion (cell fusion) is a fundamental biological process that also has been used as a versatile experimental tool to dissect a variety of cellular mechanisms, including the consequences of cell fusion itself, and to produce cells with desired properties, such as hybridomas and reprogrammed progenitors. However, current methods of cell fusion are not satisfactory because of their toxicity, inefficiency, or lack of flexibility. We describe a simple, versatile, scalable, and nontoxic approach that we call V-fusion, as it is based on the ability of the vesicular stomatitis virus G protein (VSV-G), a viral fusogen of broad tropism, to become rapidly and reversibly activated. We suggest that this approach will benefit a broad array of studies that investigate consequences of cell fusion or use cell fusion as an experimental tool.

Reprogramming of embryonic human fibroblasts into fetal hematopoietic progenitors by fusion with human fetal liver CD34+ cells

Experiments with somatic cell nuclear transfer, inter-cellular hybrid formation_ENREF_3, and ectopic expression of transcription factors have clearly demonstrated that cell fate can be dramatically altered by changing the epigenetic state of cell nuclei. Here we demonstrate, using chemical fusion, direct reprogramming of the genome of human embryonic fibroblasts (HEF) into the state of human fetal liver hFL CD34+ (hFL) hematopoietic progenitors capable of proliferating and differentiating into multiple hematopoietic lineages. We show that hybrid cells retain their ploidy and can differentiate into several hematopoietic lineages. Hybrid cells follow transcription program of differentiating hFL cells as shown by genome-wide transcription profiling. Using whole-genome single nucleotide polymorphism (SNP) profiling of both donor genomes we demonstrate reprogramming of HEF genome into the state of hFL hematopoietic progenitors. Our results prove that it is possible to convert the fetal somatic cell genome into the state of fetal hematopoietic progenitors by fusion. This suggests a possibility of direct reprogramming of human somatic cells into tissue specific progenitors/stem cells without going all the way back to the embryonic state. Direct reprogramming of terminally differentiated cells into the tissue specific progenitors will likely prove useful for the development of novel cell therapies.

Cell-fusion-mediated reprogramming: pluripotency or transdifferentiation? Implications for regenerative medicine

Cell-cell fusion is a natural process that occurs not only during development, but as has emerged over the last few years, also with an important role in tissue regeneration. Interestingly, in-vitro studies have revealed that after fusion of two different cell types, the developmental potential of these cells can change. This suggests that the mechanisms by which cells differentiate during development to acquire their identities is not irreversible, as was considered until a few years ago. To date, it is well established that the fate of a cell can be changed by a process known as reprogramming. This mainly occurs in two different ways: the differentiated state of a cell can be reversed back into a pluripotent state (pluripotent reprogramming), or it can be switched directly to a different differentiated state (lineage reprogramming). In both cases, these possibilities of obtaining sources of autologous somatic cells to maintain, replace or rescue different tissues has provided new and fundamental insights in the stem-cell-therapy field. Most interestingly, the concept that cell reprogramming can also occur in vivo by spontaneous cell fusion events is also emerging, which suggests that this mechanism can be implicated not only in cellular plasticity, but also in tissue regeneration. In this chapter, we will summarize the present knowledge of the molecular mechanisms that mediate the restoration of pluripotency in vitro through cell fusion, as well as the studies carried out over the last 3 decades on lineage reprogramming, both in vitro and in vivo. How the outcome of these studies relate to regenerative medicine applications will also be discussed.

Biomarkers for epithelial-mesenchymal transitions

Somatic cells that change from one mature phenotype to another exhibit the property of plasticity. It is increasingly clear that epithelial and endothelial cells enjoy some of this plasticity, which is easily demonstrated by studying the process of epithelial-mesenchymal transition (EMT). Published reports from the literature typically rely on ad hoc criteria for determining EMT events; consequently, there is some uncertainty as to whether the same process occurs under different experimental conditions. As we discuss in this Personal Perspective, we believe that context and various changes in plasticity biomarkers can help identify at least three types of EMT and that using a collection of criteria for EMT increases the likelihood that everyone is studying the same phenomenon - namely, the transition of epithelial and endothelial cells to a motile phenotype.

Strategies to induce nuclear reprogramming

The cloning of mammals from adult donor cells has demonstrated that the oocyte can reprogram a differentiated nucleus into a pluripotent embryonic state. Reprogramming of committed cells into pluripotent cells can also be achieved by the explantation of germ line cells and by the fusion of differentiated cells with embryonic cells. The future challenge will be to stably convert a differentiated cell into embryonic stem (ES) cells by the transient expression of defined genes. Recent findings suggest that the exposure of adult cells to a few defined factors can indeed induce a pluripotent-like state resembling that of ES cells. This approach may allow for the generation of patient-specific stem cells in order to study and treat degenerative diseases without recourse to nuclear transfer.

Asymmetry of stem cell fate and the potential impact of the niche: observations, simulations, and interpretations

Asymmetric cell division is a common concept to explain the capability of stem cells to simultaneously produce a continuous output of differentiated cells and to maintain their own population of undifferentiated cells. Whereas for some stem cell systems, an asymmetry in the division process has explicitly been demonstrated, no evidence for such a functional asymmetry has been shown for hematopoietic stem cells (HSC) so far. This raises the question regarding whether asymmetry of cell division is a prerequisite to explain obvious heterogeneity in the cellular fate of HSC. Through the application of a mathematical model based on self-organizing principles, we demonstrate that the assumption of asymmetric stem cell division is not necessary to provide a consistent account for experimentally observed asymmetries in the development of HSC. Our simulation results show that asymmetric cell fate can alternatively be explained by a reversible expression of functional stem cell potentials, controlled by changing cell-cell and cell-microenvironment interactions. The proposed view on stem cell organization is pointing to the potential role of stem cell niches as specific signaling environments, which induce developmental asymmetries and therefore, generate cell fate heterogeneity. The self-organizing concept is fully consistent with the functional definition of tissue stem cells. It naturally includes plasticity phenomena without contradicting a hierarchical appearance of the stem cell population. The concept implies that stem cell fate is only predictable in a probabilistic sense and that retrospective categorization of stem cell potential, based on individual cellular fates, provides an incomplete picture.

On the use of the word ‘epigenetic’

Over the past few years we have seen an odd change, or extension, in the use of the word 'epigenetic' when describing matters of gene regulation in eukaryotes. Although it may generally be that it is not worth arguing over definitions, this is true only insofar as the participants in the discussion know what each other means. I believe the altered use of the term carries baggage from the standard definition that can have misleading implications. Here I wish to probe our use of language in this way, and to show how such a discussion leads to some more general considerations concerning gene regulation.

'Open minded' cells: how cells can change fate

It has long intrigued researchers why some but not all organisms can regenerate missing body parts. Plants are remarkable in that they can regenerate the entire organism from a small piece of tissue, or even a single cell. Epigenetic mechanisms that control chromatin organization are now known to regulate the cellular plasticity and reprogramming necessary for regeneration. Interestingly, although animals and plants have evolved different strategies and mechanisms to control developmental processes, they have maintained many similarities in the way they regulate chromatin organization. Given that plants can rapidly switch fate, we propose that an understanding of the mechanisms regulating this process in plant cells could provide a new perspective on cellular dedifferentiation in animals.

Cell fusion for reprogramming pluripotency

Embryonic stem cell (ESC) technology should enable the generation of specific cell types for the study and treatment of human diseases. Therapeutic cloning provides a way to generate ESCs genetically matched to diseased individuals through nuclear reprogramming of the somatic genome. However, practical and ethical limitations associated with therapeutic cloning are calling for the development of oocyte- and-embryo-free alternatives for obtaining of autologous pluripotent cells for transplantation therapy. An alternative approach to reprogram the somatic genome involves fusion between somatic and pluripotent cells. Potential fusion partners with reprogramming activities include embryonal carcinoma cells, embryonic germ cells, and ESCs. Experimental evidence is now available, which demonstrates that mouse and human somatic cells can be reprogrammed by fusion to form pluripotent hybrid cells. Recent progress infusion-based reprogramming is reviewed with reference to the developmental potency of hybrid cells as well as genetic and epigenetic correlates of reprogramming. However, hybrid cells lack therapeutic potential because of their abnormal ploidy and the presence of nonautologous genes from the pluripotent parent. We discuss the potential of fusion-based reprogramming for the generation of diploid, autologous pluripotent cells using two alternative routes: the enucleation of ESCs and the fusion of such cytoplasts to somatic cell karyoplasts or intact somatic cells, and the selective elimination of the pluripotent genome following fusion to the somatic partner. Finally, these approaches are discussed in the light of recent progress showing that overexpression of embryonic transcription factors can restore a state of pluripotency to somatic cells.

Reprogramming somatic gene activity by fusion with pluripotent cells

Fertilized eggs and early blastomeres, that have the potential to develop to fetuses when placed into a uterus, are totipotent. Those cells in the embryo, that can give rise to all cell types of an organism, but not to an organism itself, are pluripotent. Embryonic stem (ES), embryonic carcinoma (EC), and embryonic germ (EG) cells are powerful in vitro artifacts derived from different embryonic stages and are pluripotent. Totipotent and pluripotent cells have the potential to greatly benefit biological research and medicine. One powerful feature is that the genetic program of somatic cells can be converted into that of totipotent or pluripotent cells, as shown by nuclear transfer or cell fusion experiments. During reprogramming by cell fusion various features of pluripotent cells are acquired. These include the typical morphology of the respective pluripotent fusion partner, a specific epigenetic state, a specific gene profile, inactivation of tissue-specific genes expressed in the somatic fusion partner, and the developmental as well as differentiation potential of pluripotent cells. In this review, we will discuss what is known about the reprogramming process mediated by cell fusion and the potential use of fusion-induced reprogramming for therapeutic applications.

Nuclear reprogramming and pluripotency

The cloning of mammals from differentiated donor cells has refuted the old dogma that development is an irreversible process. It has demonstrated that the oocyte can reprogramme an adult nucleus into an embryonic state that can direct development of a new organism. The prospect of deriving patient-specific embryonic stem cells by nuclear transfer underscores the potential use of this technology in regenerative medicine. The future challenge will be to study alternatives to nuclear transfer in order to recapitulate reprogramming in a Petri dish without the use of oocytes.

In vitro reprogramming of nuclei and cells

Directly turning a somatic cell type into another would be beneficial for producing replacement cells for therapeutic purposes. To this end, novel cell reprogramming strategies are being developed. We describe here methods for functionally reprogramming a somatic cell using an extract derived from another somatic cell type. The procedure involves reversible permeabilization of 293T fibroblasts, incubation of the permeabilized cells in a nuclear and cytoplasmic extract of T-cells, resealing of the "reprogrammed" cells, and culture for assessment of reprogramming. Reprogramming has been evidenced by nuclear uptake and assembly of transcription factors, induction of activity of a chromatin remodeling complex, changes in chromatin composition, activation of lymphoid cell-specific genes, and expression of T-cell-specific surface molecules. The system is likely to constitute a powerful tool to examine the processes of nuclear reprogramming, at least as they occur in vitro.

Cellular reprogramming

The concept of reprogramming a cell is very intriguing and has immense therapeutic potential. Examples from physiology and developmental biology suggest that it may well be possible. Experimental approaches are beginning to suggest this also, in particular the initially astonishing accomplishment of somatic cell nuclear transfer and cloning. This chapter reviews current strategies and describes emerging methods for the proposition of reprogramming cells with cell extracts.

Nuclear reprogramming by human embryonic stem cells

Embryonic stem cells have two unique properties. They are capable of indefinite self-renewal and, being pluripotent, they can differentiate into all possible cell types, including germ cells. A new study by Cowan et al. (2005) published in Science shows that human embryonic stem cells are able to reprogram the nuclei of fully differentiated human somatic cells, apparently conferring on them a pluripotent state.

Opinion: the origin of the cancer stem cell: current controversies and new insights

Most tumours are derived from a single cell that is transformed into a cancer-initiating cell (cancer stem cell) that has the capacity to proliferate and form tumours in vivo. However, the origin of the cancer stem cell remains elusive. Interestingly, during development and tissue repair the fusion of genetic and cytoplasmic material between cells of different origins is an important physiological process. Such cell fusion and horizontal gene-transfer events have also been linked to several fundamental features of cancer and could be important in the development of the cancer stem cell.

Biological implications of cell fusion

Until recently, cells were thought to be integral and discrete components of tissues, and their state was determined by cell differentiation. However, under some conditions, stem cells or their progeny can fuse with cells of other types, mixing cytoplasmic and even genetic material of different (heterotypic) origins. The fusion of heterotypic cells could be of central importance for development, repair of tissues and the pathogenesis of disease.

Unveiling the mechanisms of cell-cell fusion

Cell-cell fusion is fundamental to the development and physiology of multicellular organisms, but little is known of its mechanistic underpinnings. Recent studies have revealed that many proteins involved in cell-cell fusion are also required for seemingly unrelated cellular processes such as phagocytosis, cell migration, axon growth, and synaptogenesis. We review advances in understanding cell-cell fusion by contrasting it with virus-cell and intracellular vesicle fusion. We also consider how proteins involved in general aspects of membrane dynamics have been co-opted to control fusion of diverse cell types by coupling with specialized proteins involved in cell-cell recognition, adhesion, and signaling.

Nuclei of embryonic stem cells reprogram somatic cells

The restricted potential of a differentiated cell can be reverted back to a pluripotent state by cell fusion; totipotency can even be regained after somatic cell nuclear transfer. To identify factors involved in resetting the genetic program of a differentiated cell, we fused embryonic stem cells (ESCs) with neurosphere cells (NSCs). The fusion activated Oct4, a gene essential for pluripotency, in NSCs. To further identify whether cytoplasmic or nuclear factors are responsible for its reactivation, we fused either karyoplasts or cytoplasts of ESCs with NSCs. Our results show that ESC karyoplasts could induce Oct4 expression in the somatic genome, but cytoplasts lacked this ability. In addition, mitomycin C-treated ESCs, although incapable of DNA replication and cell division, could reprogram 5-azacytidine-treated NSCs. We therefore conclude that the Oct4 reprogramming capacity resides in the ESC karyoplast and that gene reactivation is independent of DNA replication and cell division.

High-dimensional switches and the modelling of cellular differentiation

Many genes have been identified as driving cellular differentiation, but because of their complex interactions, the understanding of their collective behaviour requires mathematical modelling. Intriguingly, it has been observed in numerous developmental contexts, and particularly haematopoiesis, that genes regulating differentiation are initially co-expressed in progenitors despite their antagonism, before one is upregulated and others downregulated. We characterise conditions under which three classes of generic "master regulatory networks", modelled at the molecular level after experimentally observed interactions (including bHLH protein dimerisation), and including an arbitrary number of antagonistic components, can behave as a "multi-switch", directing differentiation in an all-or-none fashion to a specific cell-type chosen among more than two possible outcomes. bHLH dimerisation networks can readily display coexistence of many antagonistic factors when competition is low (a simple characterisation is derived). Decision-making can be forced by a transient increase in competition, which could correspond to some unexplained experimental observations related to Id proteins; the speed of response varies with the initial conditions the network is subjected to, which could explain some aspects of cell behaviour upon reprogramming. The coexistence of antagonistic factors at low levels, early in the differentiation process or in pluripotent stem cells, could be an intrinsic property of the interaction between those factors, not requiring a specific regulatory system.

Cardiomyocytes fuse with surrounding noncardiomyocytes and reenter the cell cycle

The concept of the plasticity or transdifferentiation of adult stem cells has been challenged by the phenomenon of cell fusion. In this work, we examined whether neonatal cardiomyocytes fuse with various somatic cells including endothelial cells, cardiac fibroblasts, bone marrow cells, and endothelial progenitor cells spontaneously in vitro. When cardiomyocytes were cocultured with endothelial cells or cardiac fibroblasts, they fused and showed phenotypes of cardiomyocytes. Furthermore, cardiomyocytes reentered the G2-M phase in the cell cycle after fusing with proliferative noncardiomyocytes. Transplanted endothelial cells or skeletal muscle–derived cells fused with adult cardiomyocytes in vivo. In the cryoinjured heart, there were Ki67-positive cells that expressed both cardiac and endothelial lineage marker proteins. These results suggest that cardiomyocytes fuse with other cells and enter the cell cycle by maintaining their phenotypes.

Adult stem cells--reprogramming neurological repair?

Much excitement has surrounded recent breakthroughs in embryonic stem-cell research. Of lower profile, but no less exciting, are the advances in the field of adult stem-cell research, and their implications for cell therapy. Clinical experience from use of adult haemopoietic stem cells in haematology will facilitate and hasten transition from laboratory to clinic--indeed, clinical trials using adult human stem cells are already in progress in some disease states, including myocardial ischaemia. Here, with particular reference to neurology, we review processes that might underlie apparent changes in adult cell phenotype. We discuss implications these processes might have for the development of new therapeutic strategies using adult stem cells.

Epithelial-mesenchymal transition and its implications for fibrosis

Epithelial to mesenchymal transition (EMT) is a central mechanism for diversifying the cells found in complex tissues. This dynamic process helps organize the formation of the body plan, and while EMT is well studied in the context of embryonic development, it also plays a role in the genesis of fibroblasts during organ fibrosis in adult tissues. Emerging evidence from studies of renal fibrosis suggests that more than a third of all disease-related fibroblasts originate from tubular epithelia at the site of injury. This review highlights recent advances in the process of EMT signaling in health and disease and how it may be attenuated or reversed by selective cytokines and growth factors.

Epithelial-mesenchymal transition and its implications for fibrosis

Epithelial to mesenchymal transition (EMT) is a central mechanism for diversifying the cells found in complex tissues. This dynamic process helps organize the formation of the body plan, and while EMT is well studied in the context of embryonic development, it also plays a role in the genesis of fibroblasts during organ fibrosis in adult tissues. Emerging evidence from studies of renal fibrosis suggests that more than a third of all disease-related fibroblasts originate from tubular epithelia at the site of injury. This review highlights recent advances in the process of EMT signaling in health and disease and how it may be attenuated or reversed by selective cytokines and growth factors.

Plasticity of marrow-derived stem cells

Bone marrow (BM) contains hematopoietic stem cells (HSCs), which differentiate into every type of mature blood cell; endothelial cell progenitors; and marrow stromal cells, also called mesenchymal stem cells (MSCs), which can differentiate into mature cells of multiple mesenchymal tissues including fat, bone, and cartilage. Recent findings indicate that adult BM also contains cells that can differentiate into additional mature, nonhematopoietic cells of multiple tissues including epithelial cells of the liver, kidney, lung, skin, gastrointestinal (GI) tract, and myocytes of heart and skeletal muscle. Experimental results obtained in vitro and in vivo are the subject of this review. The emphasis is on how these experiments were performed and under what conditions differentiation from bone marrow to epithelial and neural cells occurs. Questions arise regarding whether tissue injury is necessary for this differentiation and the mechanisms by which it occurs. We also consider which bone marrow subpopulations are capable of this differentiation. Only after we have a better understanding of the mechanisms involved and of the cells required for this differentiation will we be able to fully harness adult stem cell plasticity for clinical purposes.

Cell fusion and plasticity

Cell plasticity is a central issue in stem cell biology. In many recent discussions, observation of cell fusion has been seen as a confounding factor which calls into question published results concerning cell plasticity of, particularly, adult stem cells. An examination of the voluminous literature of "somatic cell fusion" suggests the relatively frequent occurrence of "spontaneous" cell fusion and shows that the complicated cellular phenotypes which it can give rise to have long been recognized. Here, a brief overview of this field is presented, with emphasis on studies of special relevance to current work on cell plasticity.

Somatic plasticity of neural stem cells: fact or fancy?

Several studies have described the potential for embryonic and adult neural stem cells to differentiate into non-neural cells such as muscle and blood, tissues that are derived from non-neuroectodermal germ layers. This raised the exciting possibility that these cells possessed a broader range of differentiation potential than originally thought and raised interesting prospects for possible transplantation utilization. However, a number of recent reports have raised questions about whether the phenomena observed actually represented true somatic plasticity. In this review, we critically analyze these studies with the aim of providing some criteria by which future studies that address this important problem may be evaluated.

New principles of cell plasticity

Recent discoveries demonstrating surprising cell plasticity in animals and humans call into question many long held assumptions regarding differentiative potential of adult cells. These assumptions reflect a classical paradigm of cell lineage development projected onto both prenatal development and post-natal maintenance and repair of tissues. The classical paradigm describes unidirectional, hierarchical lineages proceedings step-wise from totipotent or pluripotent stem cells through intermediate, ever more restricted progenitor cells, leading finally to 'terminally differentiated' cells. However, in light of both the recent discoveries and older clinical or experimental findings, we have suggested principles comprising a new paradigm of cell plasticity, summarized here.

From intestine to muscle: nuclear reprogramming through defective cloned embryos

Nuclear transplantation is one of the very few ways by which the genetic content and capacity for nuclear reprogramming can be assessed in individual cells of differentiated somatic tissues. No more than 6% of the cells of differentiated tissues have thus far been shown to have nuclei that can be reprogrammed to elicit the formation of unrelated cell types. In Amphibia, about 25% of such nuclear transfers form morphologically abnormal partial blastulae that die within 24 h. We have investigated the genetic content and capacity for reprogramming of those nuclei that generate partial blastulae, using as donors the intestinal epithelium cells of feeding Xenopus larvae. We have analyzed single nuclear transplant embryos obtained directly from intestinal tissue, thereby avoiding any genetic or epigenetic changes that might accumulate during cell culture. The expression of the intestine-specific gene intestinal fatty acid binding protein is extinguished by at least 10(4) times, within a few hours of nuclear transplantation. At the same time several genes that are normally expressed only in early embryos are very strongly activated in nuclear transplant embryos, but to an unregulated extent. Remarkably, cells from intestine-derived partial blastulae, when grafted to normal host embryos, contribute to several host tissues and participate in the normal 100-fold increase in axial muscle over several months. Thus, cells of defective cloned embryos unable to survive for more than 1 day can be reprogrammed to participate in new directions of differentiation and to maintain indefinite growth, despite the abnormal expression of early genes.

Reprogrammed gene expression in a somatic cell-free extract

We have developed a somatic cell-free system that remodels chromatin and activates gene expression in heterologous differentiated nuclei. Extracts of stimulated human T cells elicit chromatin binding of transcriptional activators of the interleukin-2 (IL-2) gene, anchoring and activity of a chromatin-remodeling complex and hyperacetylation of the IL-2 promoter in purified exogenous resting T-cell nuclei. The normally repressed IL-2 gene is transcribed in nuclei from quiescent human T cells and from various non-T-cell lines. This demonstrates that somatic cell extracts can be used to reprogram gene expression in differentiated nuclei. In vitro reprogramming may be useful for investigating regulation of gene expression and for producing replacement cells for the treatment of a wide variety of diseases.

Toward a new paradigm of cell plasticity

The standard paradigm of embryologic development and adult tissue reconstitution posits unidirectional, hierarchical lineages. The presumed mechanisms underlying these differentiative pathways are gene restrictions, such as methylation and heterochromatin formation, which are commonly described as irreversible. However, recent discoveries regarding multi-organ stem cells demonstrate that 'true plasticity' exists, with cells of one organ turning into cells of other organs, including differentiative transformations that cross barriers between tissues derived from different primitive germ layers. These findings, along with earlier experiments into heterokaryon formation and longstanding recognition of reactive and neoplastic lesions in humans and animals, suggest that lineage pathways are not, in fact, unidirectional. Moreover, physiologic mechanisms of reversal of gene restrictions have been recognized. Therefore, in response to these observations, we suggest a new paradigm of cell plasticity, elucidating three guiding principles of 'genomic completeness', 'uncertainty of cell characterization', and 'stochastic nature of cell origins and fates'. These principles imply a change in the way data can be interpreted and could alter subsequent hypothesis formation. This new paradigm will hopefully lead us forward to a more flexible and creative exploration of the potential of adult vertebrate cells.