Stem cell plasticity in mammals and transdetermination in Drosophila: common themes?

Lineage Plasticity in Cancer: The Tale of a Skin-Walker

Lineage plasticity, the switching of cells from one lineage to another, has been recognized as a cardinal property essential for embryonic development, tissue repair and homeostasis. However, such a highly regulated process goes awry when cancer cells exploit this inherent ability to their advantage, resulting in tumorigenesis, relapse, metastasis and therapy resistance. In this review, we summarize our current understanding on the role of lineage plasticity in tumor progression and therapeutic resistance in multiple cancers. Lineage plasticity can be triggered by treatment itself and is reported across various solid as well as liquid tumors. Here, we focus on the importance of lineage switching in tumor progression and therapeutic resistance of solid tumors such as the prostate, lung, hepatocellular and colorectal carcinoma and the myeloid and lymphoid lineage switch observed in leukemias. Besides this, we also discuss the role of epithelial-mesenchymal transition (EMT) in facilitating the lineage switch in biphasic cancers such as aggressive carcinosarcomas. We also discuss the mechanisms involved, current therapeutic approaches and challenges that lie ahead in taming the scourge of lineage plasticity in cancer.

Cellular plasticity: A mechanism for homeostasis in the kidney

Cellular plasticity is a topical subject with interest spanning a wide range of fields from developmental biology to regenerative medicine. Even the nomenclature is a subject of debate, and the underlying mechanisms are still under investigation. On top of injury repair, cell plasticity is a constant physiological process in adult organisms and tissues, in response to homeostatic challenges. In this review we discuss two examples of plasticity for the maintenance of homeostasis in the renal system-namely the renin-producing juxtaglomerular cells (JG cells) and cortical collecting duct (CCD) cells. JG cells show plasticity through recruitment mechanisms, answering the demand for an increase in renin production. In the CCD, cells appear to have the ability to transdifferentiate between principal and intercalated cells to help maintain the highly regulated solute transport levels of that segment. These two cases highlight the complexity of plasticity processes and the role they can play in the kidney.

Evolution and Regulation of Limb Regeneration in Arthropods

Regeneration has fascinated both scientists and non-scientists for centuries. Many organisms can regenerate, and arthropod limbs are no exception although their ability to regenerate is a product shaped by natural and sexual selection. Recent studies have begun to uncover cellular and molecular processes underlying limb regeneration in several arthropod species. Here we argue that an evo-devo approach to the study of arthropod limb regeneration is needed to understand aspects of limb regeneration that are conserved and divergent. In particular, we argue that limbs of different species are comprised of cells at distinct stages of differentiation at the time of limb loss and therefore provide insights into regeneration involving both stem cell-like cells/precursor cells and differentiated cells. In addition, we review recent studies that demonstrate how limb regeneration impacts the development of the whole organism and argue that studies on the link between local tissue damage and the rest of the body should provide insights into the integrative nature of development. Molecular studies on limb regeneration are only beginning to take off, but comparative studies on the mechanisms of limb regeneration across various taxa should not only yield interesting insights into development but also answer how this remarkable ability evolved across arthropods and beyond.

Clinical potential of mesenchymal stem/stromal cell-derived extracellular vesicles

Within the last two decades mesenchymal stem/stromal cells (MSCs) emerged after hematopoietic stem cells as the second most investigated and applied somatic stem cell entity so far. MSCs mediate immunosuppressive as well as pro-regenerative activities. Against the initial assumption, MSCs may not primarily exert their therapeutic functions in a cellular but rather in a paracrine manner. Here, extracellular vesicles (EVs), such as exosomes and microvesicles, have been identified as major mediators of these paracrine effects. Meanwhile, MSC-EVs have been applied to an increasing amount of different animal models and were tested in a patient suffering from steroid-refractory acute graft-versus-host disease (acute GvHD) as well as in a patient cohort with chronic kidney disease. So far, the MSC-EV administration appears to be safe in humans and all tested animal models. Improvements were reported in all settings. Thus, MSC-EVs appear as promising novel therapeutic agents which might help to improve disease associated symptoms in millions of patients. Here, we review some of the milestones in the field, briefly discuss challenges and highlight clinical aspects of acute GvHD and its treatment with MSCs and MSC-EVs.

The legacy of Drosophila imaginal discs

The study of Drosophila imaginal discs has contributed to a number of discoveries in developmental and cellular biology. In addition to the elucidation of the role of tissue compartments and organ-specific master regulator genes during development, imaginal discs have also become well established as models for studying cellular interactions and complex genetic pathways. Here, we review key discoveries resulting from investigations of these epithelial precursor organs, ranging from cell fate determination and transdetermination to tissue patterning. Furthermore, the design of increasingly sophisticated genetic tools over the last decades has added value to the use of imaginal discs as model systems. As a result of tissue-specific genetic screens, several components of developmentally regulated signaling pathways were identified and epistasis revealed the levels at which they function. Discs have been widely used to assess cellular interactions in their natural tissue context, contributing to a better understanding of growth regulation, tissue regeneration, and cancer. With the continuous implementation of novel tools, imaginal discs retain significant potential as model systems to address emerging questions in biology and medicine.

Stem cell transplantation in neurological diseases: improving effectiveness in animal models

Neurological diseases afflict a growing proportion of the human population. There are two reasons for this: first, the average age of the population (especially in the industrialized world) is increasing, and second, the diagnostic tools to detect these pathologies are now more sophisticated and can be used on a higher percentage of the population. In many cases, neurological disease has a pharmacological treatment which, as in the case of Alzheimer's disease, Parkinson's disease, Epilepsy, and Multiple Sclerosis can reduce the symptoms and slow down the course of the disease but cannot reverse its effects or heal the patient. In the last two decades the transplantation approach, by means of stem cells of different origin, has been suggested for the treatment of neurological diseases. The choice of slightly different animal models and the differences in methods of stem cell preparation make it difficult to compare the results of transplantation experiments. Moreover, the translation of these results into clinical trials with human subjects is difficult and has so far met with little success. This review seeks to discuss the reasons for these difficulties by considering the differences between human and animal cells (including isolation, handling and transplantation) and between the human disease model and the animal disease model.

Cytotoxic and Genotoxic effects of Arsenic and Lead on Human Adipose Derived Mesenchymal Stem Cells (AMSCs)

Arsenic and lead, known to have genotoxic and mutagenic effects, are ubiquitously distributed in the environment. The presence of arsenic in drinking water has been a serious health problem in many countries. Human exposure to these metals has also increased due to rapid industrialization and their use in formulation of many products. Liposuction material is a rich source of stem cells. In the present study cytotoxic and genotoxic effects of these metals were tested on adipose derived mesenchymal stem cells (AMSCs). Cells were exposed to 1-10 μg/ml and 10-100 μg/ml concentration of arsenic and lead, respectively, for 6, 12, 24 and 48 h. The cytotoxic effects were measured by neutral red uptake assay, while the genotoxic effects were tested by comet assay. The growth of cells decreased with increasing concentration and the duration of exposure to arsenic. Even the morphology of cells was changed; they became round at 10 μg /ml of arsenic. The cell growth was also decreased after exposure to lead, though it proved to be less toxic when cells were exposed for longer duration. The cell morphology remained unchanged. DNA damage was observed in the metal treated cells. Different parameters of comet assay were investigated for control and treated cells which indicated more DNA damage in arsenic treated cells compared to that of lead. Intact nuclei were observed in control cells. Present study clearly demonstrates that both arsenic and lead have cytotoxic and genotoxic effects on AMSCs, though arsenic compared to lead has more deleterious effects on AMSCs.

Regeneration and transdetermination in Drosophila imaginal discs

The study of regeneration in Drosophila imaginal discs provides an opportunity to use powerful genetic tools to address fundamental problems pertaining to tissue regeneration and cell plasticity. We present a historical overview of the field and describe how the application of modern methods has made the study of disc regeneration amenable to genetic analysis. Discs respond to tissue damage in several ways: (a) Removal of part of the disc elicits localized cell proliferation and regeneration of the missing tissue. (b) Damage at specific locations in the disc can cause cells to generate disc-inappropriate structures (e.g., wing instead of leg), a phenomenon known as transdetermination. (c) Diffuse damage to imaginal discs, results in compensatory proliferation of surviving cells. Candidate-gene approaches have implicated the JNK, Wingless, and Hippo pathways in regeneration. Recently developed systems will enable extensive genetic screens that could provide new insights into tissue regeneration, transdetermination and compensatory proliferation.

The potential of adipose stem cells in regenerative medicine

Adipose stem cells (ASCs) are an attractive and abundant stem cell source with therapeutic applicability in diverse fields for the repair and regeneration of acute and chronically damaged tissues. Importantly, unlike the human bone marrow stromal/stem stem cells (BMSCs) that are present at low frequency in the bone marrow, ASCs can be retrieved in high number from either liposuction aspirates or subcutaneous adipose tissue fragments and can easily be expanded in vitro. ASCs display properties similar to that observed in BMSCs and, upon induction, undergo at least osteogenic, chondrogenic, adipogenic and neurogenic, differentiation in vitro. Furthermore, ASCs have been shown to be immunoprivileged, prevent severe graft-versus-host disease in vitro and in vivo and to be genetically stable in long-term culture. They have also proven applicability in other functions, such as providing hematopoietic support and gene transfer. Due to these characteristics, ASCs have rapidly advanced into clinical trials for treatment of a broad range of conditions. As cell therapies are becoming more frequent, clinical laboratories following good manufacturing practices are needed. At the same time as laboratory processes become more extensive, the need for control in the processing laboratory grows consequently involving a greater risk of complications and possibly adverse events for the recipient. Therefore, the safety, reproducibility and quality of the stem cells must thoroughly be examined prior to extensive use in clinical applications. In this review, some of the aspects of examination on ASCs in vitro and the utilization of ASCs in clinical studies are discussed.

Wnt5a-mediating neurogenesis of human adipose tissue-derived stem cells in a 3D microfluidic cell culture system

In stem cell biology, cell plasticity refers to the ability of stem cells to differentiate into a variety of cell lineages. Recently, cell plasticity has been used to refer to the ability of a given cell type to reversibly de-differentiate, re-differentiate, or transdifferentiate in response to specific stimuli. These processes are regulated by multiple intracellular and extracellular growth and differentiation factors, including low oxygen. Our recent study showed that 3D microfluidic cell culture induces activation of the Wnt5A/β-catenin signaling pathway in hATSCs (human Adipose Tissue-derived Stem Cells). This resulted in self renewal and transdifferentiation of hATSCs into neurons. To improve neurogenic potency of hATSCs in response to low oxygen and other unknown physical factors, we developed a gel-free 3D microfluidic cell culture system (3D-μFCCS). The functional structure was developed for the immobilization of 3D multi-cellular aggregates in a microfluidic channel without the use of a matrix on the chip. Growth of hATSCs neurosphere grown on a chip was higher than the growth of control cells grown in a culture dish. Induction of differentiation in the Chip system resulted in a significant increase in the induction of neuronal-like cell structures and the presentation of TuJ or NF160 positive long neuritis compared to control cells after active migration from the center of the microfluidic channel layer to the outside of the microfluidic channel layer. We also observed that the chip neurogenesis system induced a significantly higher level of GABA secreting neurons and, in addition, almost 60% of cells were GABA + cells. Finally, we observed that 1 month of after the transplantation of each cell type in a mouse SCI lesion, chip cultured and neuronal differentiated hATSCs exhibited the ability to effectively transdifferentiate into NF160 + motor neurons at a high ratio. Interestingly, our CHIP/PCR analysis revealed that HIF1α-induced hATSCs neurogenesis on the chip. This induction was a result of the direct binding of HIF1α to the regulatory regions of the Oct4 and β-catenin genes in nucleus. In the Chip culture of hATSCs that we developed, a low oxygen microenvironment was induced. The low oxygen level induced HIF1α expression, which resulted in increased expression of Wnt5A/β-catenin and Oct4 via the direct binding of HIF1α to the regulatory regions of β-catenin and Oct4.

Minireview: beta-cell replacement therapy for diabetes in the 21st century: manipulation of cell fate by directed differentiation

Pancreatic beta-cell failure underlies type 1 diabetes; it also contributes in an essential way to type 2 diabetes. beta-Cell replacement is an important component of any cure for diabetes. The current options of islet and pancreas transplantation are not satisfactory as definitive forms of therapy. Here, we review strategies for induced de novo pancreatic beta-cell formation, which depend on the targeted differentiation of cells into pancreatic beta-cells. With this objective in mind, one can manipulate the fate of three different types of cells: 1) from terminally differentiated cells, e.g. exocrine pancreatic cells, into beta-cells; 2) from multipotent adult stem cells, e.g. hepatic oval cells, into pancreatic islets; and 3) from pluripotent stem cells, e.g. embryonic stem cells and induced pluripotent stem cells, into beta-cells. We will examine the pros and cons of each strategy as well as the hurdles that must be overcome before these approaches to generate new beta-cells will be ready for clinical application.

Neurogenin3 is sufficient for transdetermination of hepatic progenitor cells into neo-islets in vivo but not transdifferentiation of hepatocytes

The transcription factor Neurogenin3 (Ngn3) is required for islet-cell type specification. Here, we show that hepatic gene transfer of Ngn3 transiently induces insulin in terminally differentiated hepatocytes but fails to transdifferentiate them, i.e., switch their lineage into islet cells. However, Ngn3 leads to long-term diabetes reversal in mice due to the emergence of periportal islet-like cell clusters. These neo-islets display glycemia-regulated insulin, beta-cell-specific transcripts, and an islet-specific transcription cascade, and they produce all four major islet hormones. They appear to arise from hepatic progenitor cells, most likely endoderm-derived oval cells. Thus, transfer of a single lineage-defining transcription factor, Ngn3, is sufficient to induce cell-lineage switching from a hepatic to an islet lineage in these progenitor cells, a process consistent with transdetermination, i.e, lineage switching in lineage-determined, but not terminally differentiated, cells. This paradigm of induced transdetermination of receptive progenitor cells in vivo may be generally applicable to therapeutic organogenesis for multiple diseases, including diabetes.

Salt stress-induced cell reprogramming, cell fate switch and adaptive plasticity during root hair development in Arabidopsis

Developmental plasticity defines an adaptive mechanism, which plays a fundamental role in plant development and survival. How intrinsic or extrinsic factors are integrated to specify cell fates and subsequent organ and body building of a plant is still poorly understood. By studying developmental plasticity of Arabidopsis root hair in response to salt stress, we have begun to understand more about the basis of cellular plasticity. This paper summarizes our recent paper in which it described salt stress induced plasticity of root epidermis and root hair development in Arabidopsis. Analysis of gene expression of the homeobox transcription factor GLABRA2 (GL2), which determines hair/non-hair cell fate, showed that salt stress modulates root epidermal cell proliferation and changes the cell fate decisions. Furthermore, by analyzing the salt overly sensitive (sos) mutants, we showed that salt-induced root hair plastic response is caused by ion disequilibrium and it appears to be adaptive mechanism. Based on the most recent discoveries, we propose here that chromatin remodeling and epigenetic control may be the basis for cell fate changes and the ultimately adaptive plasticity in response to transient changes of environmental conditions.

Adipose-derived stem cells: isolation, expansion and differentiation

The emerging field of regenerative medicine will require a reliable source of stem cells in addition to biomaterial scaffolds and cytokine growth factors. Adipose tissue has proven to serve as an abundant, accessible and rich source of adult stem cells with multipotent properties suitable for tissue engineering and regenerative medical applications. There has been increased interest in adipose-derived stem cells (ASCs) for tissue engineering applications. Here, methods for the isolation, expansion and differentiation of ASCs are presented and described in detail. While this article has focused on the isolation of ASCs from human adipose tissue, the procedure can be applied to adipose tissues from other species with minimal modifications.

Biologic properties of mesenchymal stem cells derived from bone marrow and adipose tissue

The biologic characteristics of mesenchymal stem cells (MSCs) isolated from two distinct tissues, bone marrow and adipose tissue were evaluated in these studies. MSCs derived from human and non-human primate (rhesus monkey) tissue sources were compared. The data indicate that MSCs isolated from rhesus bone marrow (rBMSCs) and human adipose tissue (hASCs) had more similar biologic properties than MSCs of rhesus adipose tissue (rASCs) and human bone marrow MSCs (hBMSCs). Analyses of in vitro growth kinetics revealed shorter doubling time for rBMSCs and hASCs. rBMSCs and hASCs underwent significantly more population doublings than the other MSCs. MSCs from all sources showed a marked decrease in telomerase activity over extended culture; however, they maintained their mean telomere length. All of the MSCs expressed embryonic stem cell markers, Oct-4, Rex-1, and Sox-2 for at least 10 passages. Early populations of MSCs types showed similar multilineage differentiation capability. However, only the rBMSCs and hASCs retain greater differentiation efficiency at higher passages. Overall in vitro characterization of MSCs from these two species and tissue sources revealed a high level of common biologic properties. However, the results demonstrate clear biologic distinctions, as well.

Stem cell plasticity: the debate begins to clarify

The stem cell story begins with the recognition of the regenerative powers of the head of the Lernean Hydra and the human liver (Prometheus) by the ancient Greeks. In modern times, the adult human stem cell has been epitomized by the hematopoietic stem cell in the bone marrow. More recently, bone marrow derived cells were reported to contribute to nonhematopoietic organs, suggesting a level of plasticity not previously expected. However, other reports failed to repeat some of these results, resulting in a heated debate on the plasticity of adult stem cells that has crossed over into the public domain and become a matter of political impact on the use of embryonic vs adult stem cells for organ regeneration or gene therapy. This review discusses the current status of the "plasticity" debate and presents existing data on detection methodology, underlying mechanisms, physiological implications, and clinical significance.

Plasticity of epidermal stem cells: survival in various environments

The keratinocyte cell compartment in the continuously renewing epidermis of the skin is maintained by undifferentiated, self-renewing stem cells. We show that a small subpopulation of epidermal stem cells (EpiSCs) have the capacity to integrate into multiple tissues. These EpiSCs can change their phenotype in direct response of changes in cytokines in vitro, changes in cocultured cells, after injection into damaged environments in vivo. These changes appear to be unrelated to the age of the EpiSC. Even though we can isolate these cells and show that the age of thses cells appears to be irrelevant to this multipotent function, we still do not know how such cells are defined within a tissue or what the life span of a multipotent stem cell is.

Transdetermination: Drosophila imaginal disc cells exhibit stem cell-like potency

Drosophila imaginal discs, the primordia of the adult fly appendages, are an excellent system for studying developmental plasticity. Cells in the imaginal discs are determined for their disc-specific fate (wingness, legness) during embryogenesis. Disc cells maintain their determination during larval development, a time of extensive growth and proliferation. Only when prompted to regenerate do disc cells exhibit lability in their determined identity. Regeneration in the disc is mediated by a localized region of cell division, known as the regeneration blastema. Most regenerating disc cells strictly adhere to their disc-specific identity; some cells however, switch fate in a phenomenon known as transdetermination. Similar regeneration and transdetermination events can be induced in situ by misexpression of the signaling molecule wingless. Recent studies indicate that the plasticity of disc cells during regeneration is associated with high morphogen activity and the reorganization of chromatin structure. Here we provide both a historical perspective of imaginal disc transdetermination, as well as discuss recent findings on how imaginal disc cells acquire developmental plasticity and multipotency. We also highlight how an understanding of imaginal disc transdetermination can enhance an understanding of developmental potency exhibited by stem cells.

Epidermal stem cells have the potential to assist in healing damaged tissues

Homeostasis of continuously renewing tissues, such as the epidermis, is maintained by somatic undifferentiated, self-renewing stem cells, which are thought to persist throughout life. Through a series of labeling experiments, we previously showed that stem cells from mouse skin did not divide often, but they did divide at a steady rate in vivo. Using our recently redefined sorting method, we isolated epidermal stem and transit amplifying (TA) cells from mouse skin. When injected into a developing blastocyst or into damaged tissues, the stem cells, but not the TA cells, could participate in the formation of new tissues. We hypothesize that all tissues contain reserved undifferentiated stem cells that are primed to react if needed. These reserve stem cells could restore the tissue in which they reside or they could be called upon to help restore another tissue that was severely damage.

Potential application for mesenchymal stem cells in the treatment of cardiovascular diseases

Stem cells isolated from various sources have been shown to vary in their differentiation capacity or pluripotentiality. Two groups of stem cells, embryonic and adult stem cells, may be capable of differentiating into any desired tissue or cell type, which offers hope for the development of therapeutic applications for a large number of disorders. However, major limitations with the use of embryonic stem cells for human disease have led researchers to focus on adult stem cells as therapeutic agents. Investigators have begun to examine postnatal sources of pluripotent stem cells, such as bone marrow stroma or adipose tissue, as sources of mesenchymal stem cells. The following review focuses on recent research on the use of stem cells for the treatment of cardiovascular and pulmonary diseases and the future application of mesenchymal stem cells for the treatment of a variety of cardiovascular disorders.

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.

Aberrant expression of TTF-1 and forkhead factor HFH-4 in atrophic gastritis and ciliated metaplasia suggests gastric broncho-pulmonary transdetermination

Ciliated metaplasia (CM) in the stomach is mainly found in gastric mucosa that harbours gastric cancer. The true nature of this lesion and the regulatory factors responsible for the formation of CM are unknown. Broncho-pulmonary differentiation is controlled by the homeodomain transcription factor TTF-1 and ciliogenesis by the forkhead transcription factor HFH-4, respectively. Using immunohistochemistry, the present study shows that gastric CM is associated with the expression of TTF-1 and HFH-4. Furthermore, TTF-1 expression was found in non-ciliated cells in 50% of cases with atrophic gastritis, whereas TTF-1 and HFH-4 were not expressed in normal gastric mucosa or in non-atrophic gastritis. These data suggest that CM in the gastric mucosa can be regarded as gastric broncho-pulmonary transdetermination. Evidence for this particular transdetermination is frequently found in atrophic gastritis even without fully developed ciliated cells.

Epidermal stem cells: interactions in developmental environments

Homeostasis of continuously renewing adult tissues, such as the epidermis of the skin, is maintained by epidermal stem cells (EpiSC), which are a small population of undifferentiated, self-renewing basal keratinocyte cells that produce daughter transit amplifying (TA) cells to make up the majority of the proliferative basal cell population in the epidermis. We have isolated EpiSC from neonatal and adult skin, and shown that these cells can regenerate an epidermis that lasts long term in vitro and in vivo, and that permanently expresses a recombinant gene in the regenerated tissue (Bickenbach and Dunnwald, 2000; Dunnwald et al., 2001). When we injected murine EpiSC into the developing blastocyst environment of the mouse, we found that both neonatal and adult EpiSC retained some ability to participate in the formation of tissues from all three germ layers (Liang and Bickenbach, 2002; Bickenbach and Chinnathambi, 2004; Liang et al., 2004). Although it appears evident that EpiSC act as pluripotent stem cells, how this reprogramming takes place is not understood. EpiSC might directly transdifferentiate into other cell types or they might first dedifferentiate into a more primitive cell type, and then proceed to develop along a cell lineage pathway. To begin to unravel this, we co-cultured EpiSC with embryonic stem (ES) cells, and found that EpiSC could alter their cell lineage protein expression to that of a more primitive cell type. We also placed EpiSC in a wounded environment and found that EpiSC interacted with the mesenchymal cells repopulating the wound bed. Our findings indicate that the population of cells that we isolate as EpiSC has a pluripotent capability. This has led us to postulate a paradigm shift for somatic stem cells. We propose that tissues maintain a sequestered population of uncommitted stem cells that retain a regenerative response which is enhanced when the cells are exposed to developmental or stress influences.

Neural stem cells and their use as therapeutic tool in neurological disorders

Spontaneous neural tissue repair occurs in patients affected by inflammatory and degenerative disorders of the central nervous system (CNS). However, this process is not robust enough to promote a functional and stable recovery of the CNS architecture. The development of cell-based therapies aimed at promoting brain repair, through damaged cell-replacement, is therefore foreseen. Several experimental cell-based strategies aimed at replacing damaged neural cells have been developed in the last 30 years. Although successful in promoting site-specific repair in focal CNS disorders, most of these therapeutic approaches have failed to foster repair in multifocal CNS diseases where the anatomical and functional damage is widespread. Stem cell-based therapies have been recently proposed and might represent in the near future a plausible alternative strategy in these disorders. However, before envisaging any human applications of stem cell-based therapies in neurological diseases, we need to consider some preliminary and still unsolved issues: (i) the ideal stem cell source for transplantation, (ii) the most appropriate route of stem cell administration, and, last but not least, (iii) the best approach to achieve an appropriate, functional, and long-lasting integration of transplanted stem cells into the host tissue.

Conceptual models to understand tissue stem cell organization

PURPOSE OF REVIEW

Theoretic and, in particular, mathematic models can help biologists to select and design experiments, to highlight general principles, to discriminate similar and to link different phenomena, and to predict novel features. Specifically, they contribute to an understanding of latent mechanisms and crucial parameters of biologic processes. The following review gives an overview of recent developments in the field of hematopoietic tissue stem cell modeling.

RECENT FINDINGS

A number of experimental findings on heterogeneity, flexibility, and plasticity of hematopoietic and other tissue stem cells are challenging the classic stem cell concept of a predefined intrinsic stem cell program. Self-organizing systems provide a more elegant and comprehensive alternative to explain experimental data.

SUMMARY

Within the last few decades, modeling approaches in stem cell biology have evolved and now encompass a broad spectrum of phenomena, ranging from the cellular level to the tissue level. The application of theoretic models is currently suggesting that we abandon the classic assumption of a strict developmental hierarchy and understand stem cell organization as a dynamic, functional process. Such a perspective has implications for a prospective characterization of tissue stem cells (eg, regarding gene expression profiles and genetic regulation patterns).

Human skin-derived stem cells migrate throughout forebrain and differentiate into astrocytes after injection into adult mouse brain

Recent evidence indicates that neural stem cell properties can be found among a mammalian skin-derived multipotent population. A major barrier in the further characterization of the human skin-derived neural progenitors is the inability to isolate this population based on expression of cell surface markers. Our work has been devoted to purified human skin-derived stem cells that are capable of neural differentiation, based on the presence or absence of the AC133 cell surface marker. The enriched skin-derived AC133(+) cells express the CD34 and Thy-1 antigens. These cells cultured in a growth medium containing epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) proliferate, forming spheres, and differentiate in vitro into neurons, astrocytes, and rarely into oligodendrocytes. Single cells from sphere cultures initiated from human purified AC133(+) cells were replated as single cells and were able to generate new spheres, demonstrating the self-renewing ability of these stem cell populations. Brain engraftment of cells obtained from human purified AC133(+)-derived spheres generated different neural phenotypes: immature neurons and a most abundant population of well differentiated astrocytes. The AC133-derived astrocytes assumed perivascular locations in the frontal cortex. No donor-derived oligodendrocytes were found in the transplanted mouse brains. Several donor small, rounded cells that expressed endothelial markers were found close to the host vessel and near the subventricular zone. Thus, mammalian skin AC133-derived cells behave as a multipotent population with the capacity to differentiate into neural lineages in vitro and, prevalently, endothelium and astrocytes in vivo, demonstrating the great plasticity of these cells and suggesting potential clinical application.

As epidermal stem cells age they do not substantially change their characteristics

In this study, we ask the basic question: do stem cells age? We demonstrated that epidermal stem cells isolated from neonatal mice had the capacity to form multiple cell lineages during development. Here we demonstrate the cell lineages are clonal, and that epidermal stem cells isolated from the footpad epithelium of old mice have similar capabilities. Using Hoechst dye exclusion and cell size, we isolated viable homogenous populations of epidermal stem and transit-amplifying (TA) cells from GFP-transgenic mice, and injected these cells into 3.5-d blastocysts. Only the stem-injected blastocysts produced mice with GFP+ cells in their tissues. Furthermore, aged and young stem cells showed similar gene and protein expression profiles that showed some differences from the TA cell profiles. These data suggest that there may be a fundamental difference between somatic stem and TA cells, and that an epidermal stem cell placed in a developmental environment can alter its fate determination no matter what its age.

Neurogenesis of Rhesus adipose stromal cells

In this study, we isolated and characterized a population of non-human primate adipose tissue stromal cells (pATSCs) containing multipotent progenitor cells. We show that these pATSCs can differentiate into several mesodermal lineages, as well as neural lineage cells. For neural induction of pATSCs and non-human primate bone marrow stromal cells (pBMSCs), the cells were cultured in Neurobasal (NB) media supplemented with B27, basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF) and epidermal growth factor (EGF). After 4 days in culture, the pATSCs form compact, spheroid bodies that ultimately become neurospheres (NS). Free-floating neurospheres undergo extensive differentiation when cultured on PDL-laminin. Our data suggest that the neurogenic potential of pATSCs is markedly higher than that of pBMSCs. We have also performed microarray analysis and characterized the gene expression patterns in undifferentiated pATSCs. The direct comparison of gene expression profiles in undifferentiated pATSCs and pATSC-NS, and delineated specific members of important growth factor, signaling, cell adhesion and transcription factors families. Our data indicate that adipose tissue may be an alternative source of stem cells for therapy of central nervous system (CNS) defects.

Nuclear reprogramming in cell-free extracts

Methods for directly turning a somatic cell type into another type (a process referred to as transdifferentiation) would be beneficial for producing replacement cells for therapeutic applications. Adult stem cells have been shown to display a broader differentiation potential than anticipated and may contribute to tissues other than those in which they reside. In addition, novel transdifferentiation strategies are being developed. I report recent results on the functional reprogramming of a somatic cell using a nuclear and cytoplasmic extract derived from another somatic cell type. The reprogramming of 293T fibroblasts in an extract from T cells is evidenced by nuclear uptake and the assembly of transcription factors, induction of activity of a chromatin remodelling complex, changes in chromatin composition and activation of lymphoid cell-specific genes. The reprogrammed cells express T-cell-specific surface molecules and a complex regulatory function. Reprogramming cells in cell-free extracts may create possibilities for producing replacement cells for therapeutic applications. The system may also constitute a powerful tool to examine the mechanisms of nuclear reprogramming, at least as they occur in vitro.

Ceramic hydroxyapatite coating on titanium implants drives selective bone marrow stromal cell adhesion

The aim of this study was to determine the cell characteristics that regulate implant osseointegration. The heterogeneity of bone marrow stromal cells obtained from 11 donors was assessed by measuring the expression of a large panel of adhesion molecules. Large differences in expression of adhesion molecules were detected depending on the culture conditions used. Cells cultured in fetal bovine serum induced the expression of different adhesion molecules from cells cultured in human serum. Donor-to-donor variation was determined by measuring the expression of adhesion molecules for stromal cells obtained from different donors that were processed identically. Fat adherent cells but also loose bone marrow cells showed large differences in expression of some but not all adhesion molecules. The flow cytometric data demonstrated large heterogeneity in expression of adhesion molecules, and this heterogeneity was influenced by culture conditions and varied from donor to donor. This demonstrates that the implant encounters different cell types, which could lead to different levels of integration. Surprisingly, in vitro only a subfraction of bone marrow stromal cells attached to titanium coated with ceramic hydroxyapatite. Adaptation of all cell types present in heterogeneous bone marrow to a coated surface is apparently not possible. Differential binding was not caused by aberrant staining of the stromal cells as the results were confirmed with bone marrow cells obtained from transgenic GFP mice. These results demonstrate that hydroxyapatite ceramics are selective in cell recruitment from the bone marrow, explaining the differences found in vivo for these coatings compared with titanium.

Transdetermination in Drosophila imaginal discs: a model for understanding pluripotency and selector gene maintenance

Drosophila imaginal disc cells have the ability to undergo transdetermination, a process whereby determined disc cells change fate to that of another disc identity. For example, leg disc cells can transdetermine to develop as wing cells. Such events can occur after mechanical disc fragmentation and subsequent regeneration. A subset of transdetermination events can be induced in situ by misexpression of the signaling gene wingless. Both fragmentation and wingless induce transdetermination by altering the expression of selector genes, which drive disc-specific developmental programs. An important future goal is to address how signaling pathways interact with chromatin structure to regulate and maintain the proper expression of selector genes.

Teaching cells new tricks

The direct conversion of one differentiated cell type into another--a process referred to as transdifferentiation--would be beneficial for producing isogenic (patient's own) cells to replace sick or damaged cells or tissue. Adult stem cells display a broader differentiation potential than anticipated and might contribute to tissues other than those in which they reside. As such, they could be worthy therapeutic agents. Recent advances in transdifferentiation involve nuclear transplantation, manipulation of cell culture conditions, induction of ectopic gene expression and uptake of molecules from cellular extracts. These approaches open the doors to new avenues for engineering isogenic replacement cells. To avoid unpredictable tissue transformation, nuclear reprogramming requires controlled and heritable epigenetic modifications. Considerable efforts remain to unravel the molecular processes underlying nuclear reprogramming and evaluate stable of the changes in reprogrammed cells.

Novel approaches to transdifferentiation

Ways of directly turning a somatic cell into another (a process known as transdifferentiation) would alleviate difficulties associated with current nuclear transplantation procedures and be beneficial for producing replacement cells for therapeutic purposes. Adult stem cells have been shown to display a broader differentiation potential than anticipated and may contribute to tissues other than those in which they reside. In addition, novel transdifferentiation strategies are being developed. We illustrate here a functional reprogramming of a somatic cell using a nuclear and cytoplasmic extract derived from another somatic cell type. Reprogramming of 293T fibroblasts in an extract from T cells is evidenced by nuclear uptake and assembly of transcription factors, induction of activity of a chromatin remodeling complex, changes in chromatin composition and activation of lymphoid cell-specific genes. The reprogrammed cells expressed T cell-specific surface molecules and a complex regulatory function. We propose that in vitro cell reprogramming may create possibilities for producing isogenic replacement cells for therapeutic applications. The system is also likely to constitute a powerful tool to examine the mechanisms of nuclear reprogramming as they occur in vitro.

Recent advances in stem cell neurobiology

1. Neural stem cells can be cultured from the CNS of different mammalian species at many stages of development. They have an extensive capacity for self-renewal and will proliferate ex vivo in response to mitogenic growth factors or following genetic modification with immortalising oncogenes. Neural stem cells are multipotent since their differentiating progeny will give rise to the principal cellular phenotypes comprising the mature CNS: neurons, astrocytes and oligodendrocytes. 2. Neural stem cells can also be derived from more primitive embryonic stem (ES) cells cultured from the blastocyst. ES cells are considered to be pluripotent since they can give rise to the full cellular spectrum and will, therefore, contribute to all three of the embryonic germ layers: endoderm, mesoderm and ectoderm. However, pluripotent cells have also been derived from germ cells and teratocarcinomas (embryonal carcinomas) and their progeny may also give rise to the multiple cellular phenotypes contributing to the CNS. In a recent development, ES cells have also been isolated and grown from human blastocysts, thus raising the possibility of growing autologous stem cells when combined with nuclear transfer technology. 3. There is now an emerging recognition that the adult mammalian brain, including that of primates and humans, harbours stem cell populations suggesting the existence of a previously unrecognised neural plasticity to the mature CNS, and thereby raising the possibility of promoting endogenous neural reconstruction. 4. Such reports have fuelled expectations for the clinical exploitation of neural stem cells in cell replacement or recruitment strategies for the treatment of a variety of human neurological conditions including Parkinson's disease (PD), Huntington's disease, multiple sclerosis and ischaemic brain injury. Owing to their migratory capacity within the CNS, neural stem cells may also find potential clinical application as cellular vectors for widespread gene delivery and the expression of therapeutic proteins. In this regard, they may be eminently suitable for the correction of genetically-determined CNS disorders and in the management of certain tumors responsive to cytokines. Since large numbers of stem cells can be generated efficiently in culture, they may obviate some of the technical and ethical limitations associated with the use of fresh (primary) embryonic neural tissue in current transplantation strategies. 5. While considerable recent progress has been made in terms of developing new techniques allowing for the long-term culture of human stem cells, the successful clinical application of these cells is presently limited by our understanding of both (i) the intrinsic and extrinsic regulators of stem cell proliferation and (ii) those factors controlling cell lineage determination and differentiation. Although such cells may also provide accessible model systems for studying neural development, progress in the field has been further limited by the lack of suitable markers needed for the identification and selection of cells within proliferating heterogeneous populations of precursor cells. There is a further need to distinguish between the committed fate (defined during normal development) and the potential specification (implying flexibility of fate through manipulation of its environment) of stem cells undergoing differentiation. 6. With these challenges lying ahead, it is the opinion of the authors that stem-cell therapy is likely to remain within the experimental arena for the foreseeable future. In this regard, few (if any) of the in vivo studies employing neural stem cell grafts have shown convincingly that behavioural recovery can be achieved in the various model paradigms. Moreover, issues relating to the quality control of cultured cells and their safety following transplantation have only begun to be addressed. 7. While on the one hand cell biotechnologists have been quick to realise the potential commercial value, human stem cell research and its clinical applications has been the subject of intense ethical and legislative considerations. The present chapter aims to review some recent aspects of stem cell research applicable to developmental neurobiology and the potential applications in clinical neuroscience.

Plasticity revisited

Despite some recent setbacks, it remains clear that adult stem cells under appropriate experimental conditions can at some frequency exhibit a wider range of differentiation potentials than previously appreciated. This is underscored by the recent demonstration of the extensive differentiation potential of mesenchymal stem cells. In terms of mechanism, it remains unclear in many cases to what extent plasticity reflects in vitro adaptation, transdifferentiation/cell-type switching or the persistence in adult tissues of stem cells with extensive endogenous or bona fide developmental potentials. These issues will need to be resolved before the full therapeutic potential of adult-derived stem cells can be realised.

Adult bone marrow stromal stem cells express germline, ectodermal, endodermal, and mesodermal genes prior to neurogenesis

Bone marrow stromal stem cells (MSCs) normally differentiate into mesenchymal derivatives but recently have also been converted into neurons, classical ectodermal cells. To begin defining underlying mechanisms, we extended our characterization of MSCs and the differentiated neurons. In addition to expected mesodermal mRNAs, populations and clonal lines of MSCs expressed germinal, endodermal, and ectodermal genes. Thus, the MSCs are apparently "multidifferentiated" in addition to being multipotent. Conversely, the differentiating neurons derived from populations and clonal lines of MSCs expressed the specific markers beta-III tubulin, tau, neurofilament-M, TOAD-64, and synaptophysin de novo. The transmitter enzymes tyrosine hydroxylase and choline acetyltransferase were localized to neuronal subpopulations. Our observations suggest that MSCs are already multidifferentiated and that neural differentiation comprises quantitative modulation of gene expression rather than simple on-off switching of neural-specific genes.

Hematopoietic stem cells and their precursors: developmental diversity and lineage relationships

Within the context of the developing embryo, restrictions in cell lineage potential occur through cell-cell interactions and signaling molecules, leading to changes in genetic programs and to the emergence of disparate tissues containing functionally distinct cell types including somatic stem cells. Tissue maintenance in the adult is thought to occur through specific stem cells, and in the case of the hematopoietic system, through hematopoietic stem cells (HSCs). These cells arise in midgestation within the region of the embryo containing the dorsal aorta, gonads, and mesonephros (AGM) and are thought to maintain a distinct hematopoietic lineage-restricted fate. However, recent transplantation experiments suggest that within the adult, HSCs previously thought to be restricted can, under certain circumstances, display unexpected lineage potentials. With these surprising and controversial results, it is becoming apparent that a better understanding of the developmental processes, molecular programs and lineage relationships leading to the emergence of adult stem cells will provide insight into the incremental steps involved in lineage determination, and perhaps possibilities for the manipulated differentiation of stem cells. The most widely studied, accessible stem cell and cellular differentiation hierarchy is that of the hematopoietic system. With the issue of stem cell potential in the forefront, the focus of this review is on the development of the hematopoietic system: how HSCs arise in the embryo, the lineage relationships of hematopoietic cells as they are generated, and the identification of precursor cells fated to the hematopoietic lineage throughout ontogeny.

Plasticity and hematopoiesis: Circe's transforming potion?

It has long been believed that mammalian stem cells are irreversibly committed to the individual tissue in which they reside; however, several recent studies have challenged this assertion and suggest a remarkable plasticity of stem cells derived from various adult tissues. Hematopoietic stem cells have been central to this paradigm shift, and in this review, the authors discuss the recent advances in this rapidly growing field. Although several exciting findings in rodents have already led to clinical trials in humans, true stem cell plasticity has not rigorously been established in most, if not all, studies to date, and a number of issues remain unresolved. Large animal models should prove invaluable to the progress of the field.

Downregulation of beta2-microglobulin in human cord blood somatic stem cells after transplantation into livers of SCID-mice: an escape mechanism of stem cells?

Adherently growing, non-hematopoietic somatic stem cells isolated from human cord blood were stained with the fluorescent dye PKH26 and transplanted into livers of SCID-mice to examine a possible cell fate transition. Already 7 days after transplantation stem cells were well integrated into the liver tissue. Human albumin that was not expressed by the stem cells before transplantation was detectable in the host's livers after injection of cord blood stem cells. Human alpha1-antitrypsin was detectable in stem cells already before transplantation and remained positive in the mouse liver. The most interesting observation in this study was the downregulation of human beta2-microglobulin (beta2M) in the stem cells after transplantation: beta2M is expressed constitutively in our cord blood stem cells. However, beta2M was no longer detectable by RT-PCR in all tissues where human albumin and alpha1-antitrypsin were expressed after stem cell transplantation. beta2M is known to participate as an integral part of the major histocompatibility complex. Absence of beta2M makes the residual heavy chain inactive as an antigen. Thus, downregulation of beta2M may represent an escape mechanism from killer-T cells and may be a molecular mechanism explaining the recently described "immunological blindness" [37] of stem cells. In contrast to the results obtained after direct injection of stem cells as a suspension, no consistent downregulation of beta2M was observed after transplantation of stem cells encapsulated in alginate beads to generate a compartment where stem cells are protected from the host's natural killer cells. No expression of human genes was observed after transplantation of human cord blood derived mononuclear cells (MNC) that were used as a negative control. In conclusion, we have shown that human cord blood somatic stem cells survive and are reprogrammed after transplantation into mouse livers, although a complete transdifferentiation to hepatocytes did not occur within 7 days, since some marker genes (GATA4 and alpha-fetoprotein) were still negative. Switching off expression of beta2M may be part of an intriguing and novel mechanism explaining why stem cells escape the host's immune system.

Germ line stem cell competition in postnatal mouse testes

Niche is believed to affect stem cell behavior. In self-renewing systems for which functional transplantation assays are available, it has long been assumed that stem cells are fixed in the niche and that ablative treatments to remove endogenous stem cells are required for successful donor engraftment. Our results demonstrate that enriched populations of donor stem cells can produce long-lasting spermatogenic colonies in testes of immature and mature, nonablated mice, albeit at a lower frequency than in ablated mice. Colonization of nonablated recipient testes by neonate, pup, and cryptorchid adult donor spermatogonial stem cells demonstrates that competition for niche begins soon after birth and that endogenous stem cells influence the degree and pattern of donor cell colonization. Thus, a dynamic relationship between stem cell and niche exists in the testis, as has been suggested for hematopoiesis. Therefore, similar competitive properties of donor stem cells may be characteristic of all self-renewing systems.

Somatic epidermal stem cells can produce multiple cell lineages during development

It has been demonstrated that several types of somatic stem cells have the remarkable capacity to differentiate into other types of tissues. We demonstrate here that stem cells from the skin, the largest organ of the body, have the capacity to form multiple cell lineages during development. Using our recently developed sorting technique, we isolated viable homogeneous populations of somatic epidermal stem and transient amplifying cells from the skin of 3-day old transgenic mice, who carried the enhanced green fluorescent protein transgene, and injected stem, TA, or unsorted basal epidermal cells into 3.5-day C57BL/6 blastocysts. Only the stem-injected blastocysts produced mice with GFP(+) cells in their tissues. We found GFP(+) cells in ectodermal, mesenchymal, and neural-crest-derived tissues in E13.5 embryos, 13-day-old neonates, and 60-day-old adult mice, proving that epidermal stem cells survived the blastocyst injection and multiplied during development. Furthermore, the injected stem cells altered their epidermal phenotype and expressed the appropriate proteins for the tissues into which they developed, demonstrating that somatic epidermal stem cells have the ability to produce cells of different lineages during development. These data suggest that somatic epidermal stem cells may show a generalized plasticity expected only of embryonic stem cells and that environmental (extrinsic) factors may influence the lineage pathway for somatic stem cells. Thus, the skin could be a source of easily accessible stem cells that are able to be reprogrammed to form multiple cell lineages.

Functional analysis of stem cells in the adult rat testis

Adult stem cells maintain several self-renewing systems and processes in the body, including the epidermis, hematopoiesis, intestinal epithelium, and spermatogenesis. However, studies on adult stem cells are hampered by their low numbers, lack of information about morphologic or biochemical characteristics, and absence of functional assays, except for hematopoietic and spermatogonial stem cells. We took advantage of the recently developed spermatogonial transplantation technique to analyze germ line stem cells of the rat testis. The results indicate that the stem cell concentration in rat testes is 9.5-fold higher than that in mouse testes, and spermatogenic colonies derived from rat donor testis cells are 2.75 times larger than mouse-derived colonies by 3 mo after transplantation. Therefore, the extent of spermatogenesis from rat stem cells was 26-fold greater than that from mouse stem cells at the time of recipient testis analysis. Attempts to enrich spermatogonial stem cells in rat testis populations using the experimental cryptorchid procedure were not successful, but selection by attachment to laminin-coated plates resulted in 8.5-fold enrichment. Spermatogonial stem cells are unique among adult stem cells because they pass genetic information to the next generation. The high concentration of stem cells in the rat testis and the rapid expansion of spermatogenesis after transplantation will facilitate studies on stem cell biology and the introduction of genetic modifications into the male germ line. The functional differences between spermatogonial stem cells of rat vs. mouse origin after transplantation suggest that the potential of these cells may vary greatly among species.

Eye specification in Drosophila: perspectives and implications

The discovery that Drosophila eyeless is homologous to vertebrate Pax6 produced enormous interest in eye specification and a reappraisal of eye evolution. While the transcription factor Eyeless/Pax6 is necessary and in some circumstances sufficient to induce eye development the simple story of eye specification has become more epic than haiku. At least seven other nuclear proteins act with Eyeless/Pax6 to induce the eye and, furthermore, extrinsic developmental signals are required. Some striking similarities between later events of retinal patterning in vertebrates and insects have led to a deeper debate on the evolutionary path to these apparently quite different organs.