Biased competition between Lgr5 intestinal stem cells driven by oncogenic mutation induces clonal expansion

The concept of ‘field cancerization’ describes the clonal expansion of genetically altered, but morphologically normal cells that predisposes a tissue to cancer development. Here, we demonstrate that biased stem cell competition in the mouse small intestine can initiate the expansion of such clones. We quantitatively analyze how the activation of oncogenic K-ras in individual Lgr5⁺ stem cells accelerates their cell division rate and creates a biased drift towards crypt clonality. K-ras mutant crypts then clonally expand within the epithelium through enhanced crypt fission, which distributes the existing Paneth cell niche over the two new crypts. Thus, an unequal competition between wild-type and mutant intestinal stem cells initiates a biased drift that leads to the clonal expansion of crypts carrying oncogenic mutations.     

Drosophila midgut homeostasis involves neutral competition between symmetrically dividing intestinal stem cells

The Drosophila adult posterior midgut has been identified as a powerful system in which to study mechanisms that control intestinal maintenance, in normal conditions as well as during injury or infection. Early work on this system has established a model of tissue turnover based on the asymmetric division of intestinal stem cells. From the quantitative analysis of clonal fate data, we show that tissue turnover involves the neutral competition of symmetrically dividing stem cells. This competition leads to stem-cell loss and replacement, resulting in neutral drift dynamics of the clonal population. As well as providing new insight into the mechanisms regulating tissue self-renewal, these findings establish intriguing parallels with the mammalian system, and confirm Drosophila as a useful model for studying adult intestinal maintenance.     

Cancer stem cells--normal stem cells "Jedi" that went over to the "dark side"

Evidence has accumulated that cancer develops from a population of quiescent tissue committed/pluripotent stem cells (TCSC/PSC) or cells developmentally closely related to them that are distributed in various organs. To support this notion, stem cells (SC) are long lived cells and thus may become the subject of accumulating mutations that are crucial for initiation/progression of cancer. More important, they may maintain these mutations and pass them to the daughter stem cells. Therefore, mutations that occur in normal SC, accumulate during the life of an organism at the clonal level in the stem cell compartment committed to a given tissue/organ. As a consequence, this may lead to the malignant transformation of SC and tumor initiation. Furthermore, many biological features of normal and cancer SC such as the physiological trafficking of normal and metastasis of cancer stem cells involve similar molecular mechanisms, and we discuss these similarities here. Therefore, looking both at the origin and behavioral aspects we can envision cancer SC being normal SC "Jedi" that went over to the "dark side".     

Epigenetic regulation in adult stem cells and cancers

Adult stem cells maintain tissue homeostasis by their ability to both self-renew and differentiate to distinct cell types. Multiple signaling pathways have been shown to play essential roles as extrinsic cues in maintaining adult stem cell identity and activity. Recent studies also show dynamic regulation by epigenetic mechanisms as intrinsic factors in multiple adult stem cell lineages. Emerging evidence demonstrates intimate crosstalk between these two mechanisms. Misregulation of adult stem cell activity could lead to tumorigenesis, and it has been proposed that cancer stem cells may be responsible for tumor growth and metastasis. However, it is unclear whether cancer stem cells share commonalities with normal adult stem cells. In this review, we will focus on recent discoveries of epigenetic regulation in multiple adult stem cell lineages. We will also discuss how epigenetic mechanisms regulate cancer stem cell activity and probe the common and different features between cancer stem cells and normal adult stem cells.     

Autophagy in stem cells

Autophagy is a highly conserved cellular process by which cytoplasmic components are sequestered in autophagosomes and delivered to lysosomes for degradation. As a major intracellular degradation and recycling pathway, autophagy is crucial for maintaining cellular homeostasis as well as remodeling during normal development, and dysfunctions in autophagy have been associated with a variety of pathologies including cancer, inflammatory bowel disease and neurodegenerative disease. Stem cells are unique in their ability to self-renew and differentiate into various cells in the body, which are important in development, tissue renewal and a range of disease processes. Therefore, it is predicted that autophagy would be crucial for the quality control mechanisms and maintenance of cellular homeostasis in various stem cells given their relatively long life in the organisms. In contrast to the extensive body of knowledge available for somatic cells, the role of autophagy in the maintenance and function of stem cells is only beginning to be revealed as a result of recent studies. Here we provide a comprehensive review of the current understanding of the mechanisms and regulation of autophagy in embryonic stem cells, several tissue stem cells (particularly hematopoietic stem cells), as well as a number of cancer stem cells. We discuss how recent studies of different knockout mice models have defined the roles of various autophagy genes and related pathways in the regulation of the maintenance, expansion and differentiation of various stem cells. We also highlight the many unanswered questions that will help to drive further research at the intersection of autophagy and stem cell biology in the near future.     

TGF-β family signaling in stem cells

Background

The diversity of cell types and tissue types that originate throughout development derives from the differentiation potential of embryonic stem cells and somatic stem cells. While the former are pluripotent, and thus can give rise to a full differentiation spectrum, the latter have limited differentiation potential but drive tissue remodeling. Additionally cancer tissues also have a small population of self-renewing cells with stem cell properties. These cancer stem cells may arise through dedifferentiation from non-stem cells in cancer tissues, illustrating their plasticity, and may greatly contribute to the resistance of cancers to chemotherapies.

Scope of review

The capacity of the different types of stem cells for self-renewal, the establishment and maintenance of their differentiation potential, and the selection of differentiation programs are greatly defined by the interplay of signaling molecules provided by both the stem cells themselves, and their microenvironment, the niche. Here we discuss common and divergent roles of TGF-β family signaling in the regulation of embryonic, reprogrammed pluripotent, somatic, and cancer stem cells.

Major conclusions

Increasing evidence highlights the similarities between responses of normal and cancer stem cells to signaling molecules, provided or activated by their microenvironment. While TGF-β family signaling regulates stemness of normal and cancer stem cells, its effects are diverse and depend on the cell types and physiological state of the cells.

General significance

Further mechanistic studies will provide a better understanding of the roles of TGF-β family signaling in the regulation of stem cells. These basic studies may lead to the development of a new therapeutic or prognostic strategies for the treatment of cancers. This article is part of a Special Issue entitled Biochemistry of Stem Cells.     

Botryllus schlosseri,an emerging model for the study of aging,stem cells,and mechanisms of regeneration

The decline of tissue regenerative potential with the loss of stem cell function is a hallmark of mammalian aging. We study Botryllus schlosseri, a colonial chordate which exhibits robust stem cell-mediated regeneration capacities throughout life. Larvae, derived by sexual reproduction and chordate development, metamorphose to clonal founders that undergo weekly formation of new individuals by budding from stem cells. Individuals are transient structures which die through massive apoptosis, and successive buds mature to replicate an entire new body. As a result, their stem cells, which are the only self-renewing cells in a tissue, are the only cells which remain through the entire life of the genotype and retain the effects of time. During aging, a significant decrease in the colonies’ regenerative potential is observed and both sexual and asexual reproductions will eventually halt. When a parent colony is experimentally separated into a number of clonal replicates, they frequently undergo senescence simultaneously, suggesting a heritable factor that determines lifespan in these colonies. The availability of the recently published B. schlosseri genome coupled with its unique life cycle features promotes the use of this model organism for the study of the evolution of aging, stem cells, and mechanisms of regeneration.     

A mathematical model of breast cancer development, local treatment and recurrence

Cancer development is a stepwise process through which normal somatic cells acquire mutations which enable them to escape their normal function in the tissue and become self-sufficient in survival. The number of mutations depends on the patient's age, genetic susceptibility and on the exposure of the patient to carcinogens throughout their life. It is believed that in every malignancy 4-6 crucial similar mutations have to occur on cancer-related genes. These genes are classified as oncogenes and tumour suppressor genes (TSGs) which gain or lose their function respectively, after they have received one mutative hit or both of their alleles have been knocked out. With the acquisition of each of the necessary mutations the transformed cell gains a selective advantage over normal cells, and the mutation will spread throughout the tissue via clonal expansion. We present a simplified model of this mutation and expansion process, in which we assume that the loss of two TSGs is sufficient to give rise to a cancer. Our mathematical model of the stepwise development of breast cancer verifies the idea that the normal mutation rate in genes is only sufficient to give rise to a tumour within a clinically observable time if a high number of breast stem cells and TSGs exist or genetic instability is involved as a driving force of the mutation pathway. Furthermore, our model shows that if a mutation occurred in stem cells pre-puberty, and formed a field of cells with this mutation through clonal formation of the breast, it is most likely that a tumour will arise from within this area. We then apply different treatment strategies, namely surgery and adjuvant external beam radiotherapy and targeted intraoperative radiotherapy (TARGIT) and use the model to identify different sources of local recurrence and analyse their prevention.     

Stem cell: balancing aging and cancer

Stem cells are defined by their self-renewing capacity and the ability to differentiate into one or more cell types. Stem cells can be divided, depending on their origin, into embryonic or adult. Embryonic stem cells derive from early stage embryos and can give rise to cells from all three germ layers. Adult stem cells, first identified in hematopoietic tissue, reside in a variety of adult tissues. Under normal physiologic conditions, adult stem cells are capable of differentiating into the limited cell types that comprise the particular tissue or organ. Adult stem cells are responsible for tissue renewal and exhaustion of their replicative capacity may contribute to tissue aging. Loss of unlimited proliferative capacity in some of the adult stem cells and/or their progenitors may have involved the evolutionary trade-off: senescence prevents cancer but may promote aging. Embryonic stem cells exhibit unlimited self-renewal capacity due to the expression of telomerase. Although they possess some cancer cell characteristics, embryonic stem cells exhibit a remarkable resistance to genomic instability and malignant transformation. Understanding the tumor suppressive mechanisms employed by embryonic stem cells may contribute to the development of novel cancer treatments and safe cell-based therapies for age-related diseases.     

Regulation of reactive oxygen species in stem cells and cancer stem cells

Stem cells are defined by their ability to self-renew and their multi-potent differentiation capacity. As such, stem cells maintain tissue homeostasis throughout the life of a multicellular organism. Aerobic metabolism, while enabling efficient energy production, also generates reactive oxygen species (ROS), which damage cellular components. Until recently, the focus in stem cell biology has been on the adverse effects of ROS, particularly the damaging effects of ROS accumulation on tissue aging and the development of cancer, and various anti-oxidative and anti-stress mechanisms of stem cells have been characterized. However, it has become increasingly clear that, in some cases, redox status plays an important role in stem cell maintenance, i.e., regulation of the cell cycle. An active area of current research is redox regulation in various cancer stem cells, the malignant counterparts of normal stem cells that are viewed as good targets of cancer therapy. In contrast to cancer cells, in which ROS levels are increased, some cancer stem cells maintain low ROS levels, exhibiting redox patterns that are similar to the corresponding normal stem cell. To fully elucidate the mechanisms involved in stem cell maintenance and to effectively target cancer stem cells, it is essential to understand ROS regulatory mechanisms in these different cell types. Here, the mechanisms of redox regulation in normal stem cells, cancer cells, and cancer stem cells are reviewed.     

Fibroblast growth factor signaling in embryonic and cancer stem cells

Cancer stem cells are cancer cells that originate from the transformation of normal stem cells. The most important property of any stem cell is the ability to self-renew. Through this property, there are striking parallels between normal stem cells and cancer stem cells. Both cell types share various markers of “stemness”. In particular, normal stem cells and cancer stem cells utilize similar molecular mechanisms to drive self-renewal, and similar signaling pathways may induce their differentiation.The fibroblast growth factor 2 (FGF-2) pathway is one of the most significant regulators of human embryonic stem cell (hESC) self-renewal and cancer cell tumorigenesis. Here we summarize recent data on the effects of FGF-2 and its receptors on hESCs and leukemic stem/progenitor cells. Also, we discuss the similarities of these findings with stem cell renewal and differentiation phenotypes.     

Clonal expansion of ovarian germline stem cells during niche formation in Drosophila

Stem cell niches are specific regulatory microenvironments formed by neighboring stromal cells. Owing to difficulties in identifying stem cells and their niches in many systems, mechanisms that control niche formation and stem cell recruitment remain elusive. In the Drosophila ovary, two or three germline stem cells (GSCs) have recently been shown to reside in a niche, in which terminal filaments (TFs) and cap cells are two major components. We report that signals from newly formed niches promote clonal expansion of GSCs during niche formation in the Drosophila ovary. After the formation of TFs and cap cells, anterior primordial germ cells (PGCs) adjacent to TFs/cap cells can develop into GSCs at the early pupal stage while the rest directly differentiate. The anterior PGCs are very mitotically active and exhibit two division patterns with respect to cap cells. One of these patterns generates two daughters that both contact cap cells and potentially become GSCs. Our lineage tracing study confirms that one PGC can generate two or three GSCs to occupy a whole niche ('clonal expansion'). decapentaplegic (dpp), the Drosophila homolog of human bone morphogenetic protein 2/4, is expressed in anterior somatic cells of the gonad, including TFs/cap cells. dpp overexpression promotes PGC proliferation and causes the accumulation of more PGCs in the gonad. A single PGC mutant for thick veins, encoding an essential dpp receptor, loses the ability to clonally populate a niche. Therefore, dpp is probably one of the mitotic signals that promote the clonal expansion of GSCs in a niche. This study also suggests that signals from newly formed niche cells are important for expanding stem cells and populating niches.     

Modeling the clonal heterogeneity of stem cells

Recent experimental studies suggest that tissue stem cell pools are composed of functionally diverse clones. Metapopulation models in ecology concentrate on collections of populations and their role in stabilizing coexistence and maintaining selected genetic or epigenetic variation. Such models are characterized by expansion and extinction of spatially distributed populations. We develop a mathematical framework derived from the multispecies metapopulation model of Tilman et al (1994) to study the dynamics of heterogeneous stem cell metapopulations. In addition to normal stem cells, the model can be applied to cancer cell populations and their response to treatment. In our model disturbances may lead to expansion or contraction of cells with distinct properties, reflecting proliferation, apoptosis, and clonal competition. We first present closed-form expressions for the basic model which defines clonal dynamics in the presence of exogenous global disturbances. We then extend the model to include disturbances which are periodic and which may affect clones differently. Within the model framework, we propose a method to devise an optimal strategy of treatments to regulate expansion, contraction, or mutual maintenance of cells with specific properties.     

Dual isotype expressing B cells [kappa(+)/lambda(+)] arise during the ontogeny of B cells in the bone marrow of normal nontransgenic mice

Central to the clonal selection theory is the tenet that a single B cell expresses a single receptor with a single specificity. Previously, based on our work in anti-phosphocholine transgenic mouse models, we suggested that B cells escaped clonal deletion by coexpression of more than one receptor on their cell surface. We argued that "receptor dilution" was necessary when: (i) the expressed immunoglobulin receptor is essential for immune protection against pathogens and (ii) this protective receptor is autoreactive and would be clonally deleted, leaving a hole in the B cell repertoire. Here, we demonstrate that dual isotype expressing B cells arise during the normal ontogeny of B cells in the bone marrow and populate both the spleen and peritoneal cavity of nontransgenic mice. Furthermore, single cell analysis of the expressed immunoglobulin light chains suggests that receptor editing may play a role in the generation of a significant fraction of dual isotype expressing B cells.     

A Reliable Parameter to Standardize the Scoring of Stem Cell Spheres

Sphere formation assay is widely used in selection and enrichment of normal stem cells or cancer stem cells (CSCs), also known as tumor initiating cells (TICs), based on their ability to grow in serum-free suspension culture for clonal proliferation. However, there is no standardized parameter to accurately score the spheres, which should be reflected by both the number and size of the spheres. Here we define a novel parameter, designated as Standardized Sphere Score (SSS), which is expressed by the total volume of selected spheres divided by the number of cells initially plated. SSS was validated in quantification of both tumor spheres from cancer cell lines and embryonic bodies (EB) from mouse embryonic stem cells with high sensitivity and reproducibility.     

Cellular signaling in normal and cancerous stem cells

Self-renewing divisions of normal and cancerous stem cells are responsible for the initiation and maintenance of normal and certain cancerous tissues, respectively. Recent findings suggest that tumor surveillance mechanisms can reduce regenerative capacity and frequency of normal stem cells, thereby contributing to tissue aging. Signaling pathways promoting self-renewal of stem cells can also drive proliferation in cancer. The BMI-1 proto-oncogene is required for the maintenance of tissue-specific stem cells and is involved in carcinogenesis within the same tissues. BMI-1 promotes self-renewal of stem cells largely by interfering with two central cellular tumor suppressor pathways, p16^Ink4a/retinoblastoma protein (Rb) and ARF/p53, whose disruption is a hallmark of cancer. Nucleolin, an Rb-associated protein, is abundant in proliferating cancerous cells and likely contributes to the maintenance of human CD34-positive stem/progenitor cells of hematopoiesis. Elucidation of the involvement of proto-oncogenes and tumor suppressors in the maintenance of stem cells might have therapeutic implications.     

Cancer initiation and progression: involvement of stem cells and the microenvironment

The molecular events that lead to the cancer-initiating cell involve critical mutations in genes regulating normal cell growth and differentiation. Cancer stem cells, or cancer initiating cells have been described in the context of acute myeloid leukemia, breast, brain, bone, lung, melanoma and prostate. These cells have been shown to be critical in tumor development and should harbor the mutations needed to initiate a tumor. The origin of the cancer stem cells is not clear. They may be derived from stem cell pools, progenitor cells or differentiated cells that undergo trans-differentiation processes. It has been suggested that cell fusion and/or horizontal gene transfer events, which may occur in tissue repair processes, also might play an important role in tumor initiation and progression. Fusion between somatic cells that have undergone a set of specific mutations and normal stem cells might explain the extensive chromosomal derangements seen in early tumors. Centrosome deregulation can be an integrating factor in many of the mechanisms involved in tumor development. The regulation of the balance between cell renewal and cell death is critical in cancer. Increased knowledge of developmental aspects in relation to self-renewal and differentiation, both under normal and deregulated conditions, will probably shed more light on the mechanisms that lead to tumor initiation and progression.