A reserve stem cell population in small intestine renders Lgr5-positive cells dispensable

Unraveling intestinal stem cell behavior with models of crypt dynamics

The definition, regulation and function of intestinal stem cells (ISCs) has been hotly debated. Recent discoveries have started to clarify the nature of ISCs, but many questions remain. This review discusses the current advances and controversies of ISC biology as well as theoretical compartmental models that have been coupled with in vivo experimentation to investigate the mechanisms of ISC dynamics during homeostasis, tumorigenesis, repair and development. We conclude our review by discussing the key lingering questions in the field and proposing how many of these questions can be addressed using both compartmental models and experimental techniques.

Side population sorting separates subfractions of cycling and non-cycling intestinal stem cells

We report here that side population (SP) sorting allows for the simultaneous isolation of two intestinal stem cell (ISC) subsets from wild-type (WT) mice which are phenotypically different and represent cycling and non-cycling pools of cells. Following 5-ethynyl-2'-deoxyuridine (EdU) injection, in the upper side population (USP) the percentage of EdU+ was 36% showing this fraction to be highly proliferative. In the lower side population (LSP), only 0.4% of cells were EdU+, indicating this fraction to be predominantly non-cycling. Using Lgr5-EGFP mice, we show that Lgr5-EGFP(hi) cells, representing actively cycling ISCs, are essentially exclusive to the USP. In contrast, using histone 2B-YFP mice, SP analysis revealed YFP label retaining cells (LRCs) in both the USP and the LSP. Correspondingly, evaluation of the SP fractions for mRNA markers by qRT-PCR showed that the USP was enriched in transcripts associated with both quiescent and active ISCs. In contrast, the LSP expressed mRNA markers of quiescent ISCs while being de-enriched for those of the active ISC. Both the USP and LSP are capable of generating enteroids in culture which include the four intestinal lineages. We conclude that sorting of USP and LSP fractions represents a novel isolation of cycling and non-cycling ISCs from WT mice.

Inhibition of Notch signaling reduces the number of surviving Dclk1+ reserve crypt epithelial stem cells following radiation injury

We have previously reported that doublecortin-like kinase 1 (Dclk1) is a putative intestinal stem cell (ISC) marker. In this report, we evaluated the use of Dclk1 as a marker of surviving ISCs in response to treatment with high-dose total body irradiation (TBI). Both apoptotic and mitotic Dclk1(+) cells were observed 24 h post-TBI associated with a corresponding loss of intestinal crypts observed at 84 h post-TBI. Although the Notch signaling pathway plays an important role in regulating proliferation and lineage commitment within the intestine, its role in ISC function in response to severe genotoxic injury is not yet fully understood. We employed the microcolony assay to functionally assess the effects of Notch inhibition with difluorophenacetyl-l-alanyl-S-phenylglycine t-butyl ester (DAPT) on intestinal crypt stem cell survival following severe (>8 Gy) radiation injury. Following treatment with DAPT, we observed a nearly 50% reduction in the number of surviving Dclk1(+) crypt epithelial cells at 24 h after TBI and similar reduction in the number of surviving small intestinal crypts at 84 h. These data indicate that inhibition of Notch signaling decreases ISC survival following radiation injury, suggesting that the Notch signaling pathway plays an important role in ISC-mediated crypt regeneration. These results also suggest that crypt epithelial cell Dclk1 expression can be used as one potential marker to evaluate the early survival of ISCs following severe radiation injury.

Intestinal stem cells and the colorectal cancer microenvironment

Colorectal cancer (CRC) remains a highly fatal condition in part due to its resilience to treatment and its propensity to spread beyond the site of primary occurrence. One possible avenue for cancer to escape eradication is via stem-like cancer cells that, through phenotypic heterogeneity, are more resilient than other tumor constituents and are key contributors to cancer growth and metastasis. These proliferative tumor cells are theorized to possess many properties akin to normal intestinal stem cells. Not only do these CRC “stem” cells demonstrate similar restorative ability, they also share many cell pathways and surface markers in common, as well as respond to the same key niche stimuli. With the improvement of techniques for epithelial stem cell identification, our understanding of CRC behavior is also evolving. Emerging evidence about cellular plasticity and epithelial mesenchymal transition are shedding light onto metastatic CRC processes and are also challenging fundamental concepts about unidirectional epithelial proliferation. This review aims to reappraise evidence supporting the existence and behavior of CRC stem cells, their relationship to normal stem cells, and their possible dependence on the stem cell niche.

Next-generation regenerative medicine: organogenesis from stem cells in 3D culture

The behavior of stem cells, when they work collectively, can be much more sophisticated than one might expect from their individual programming. This Perspective covers recent discoveries about the dynamic patterning and structural self-formation of complex organ buds in 3D stem cell culture, including the generation of various neuroectodermal and endodermal tissues. For some tissues, epithelial-mesenchymal interactions can also be manipulated in coculture to guide organogenesis. This new area of stem cell research-the spatiotemporal control of dynamic cellular interactions-will open a new avenue for next-generation regenerative medicine.

Adult hair follicle stem cells do not retain the older DNA strands in vivo during normal tissue homeostasis

Tissue stem cells have been proposed to segregate the chromosomes asymmetrically (in a non-random manner), thereby retaining preferentially the older "immortal" DNA strands bearing the stemness characteristics into one daughter cell, whereas the newly synthesized strands are segregated to the other daughter cell that will commit to differentiation. Moreover, this non-random segregation would protect the stem cell genome from accumulating multiple mutations during repeated DNA replication. This long-standing hypothesis remains an active subject of study due to conflicting results for some systems and lack of consistency among different tissue stem cell populations. In this review, we will focus on work done in the hair follicle, which is one of the best-understood vertebrate tissue stem cell system to date. In cell culture analysis of paired cultured keratinocytes derived from hair follicle, stem cells suggested a non-random segregation of chromosome with respect to the older DNA strand. In vivo, the hair follicle stem cells appear to self-renew and differentiate at different phases of their homeostatic cycle. The fate decisions occur in quiescence when some stem cells migrate out of their niche and commit to differentiation without self-renewal. The stem cells left behind in the niche self-renew symmetrically and randomly segregate the chromosomes at each division, making more stem cells. This model seems to apply to at least a few other vertebrate tissue stem cells in vivo.

Epithelial stem cells and intestinal cancer

The mammalian intestine is comprised of an epithelial layer that serves multiple functions in order to maintain digestive activity as well as intestinal homeostasis. This epithelial layer contains highly proliferative stem cells which facilitate its characteristic rapid regeneration. How these stem cells contribute to tissue repair and normal homeostasis are actively studied, and while we have a greater understanding of the molecular mechanisms and cellular locations that underlie stem cell regulation in this tissue, much still remains undiscovered. This review describes epithelial stem cells in both intestinal and non-intestinal tissues, as well as the strategies that have been used to further characterize the cells. Through a discussion of the current understanding of intestinal self-renewal and tissue regeneration in response to injury, we focus on how dysregulation of critical signaling pathways results in potentially oncogenic aberrations, and highlight issues that should be addressed in order for effective intestinal cancer therapies to be devised.

Inflammatory signals that regulate intestinal epithelial renewal, differentiation, migration and cell death: Implications for necrotizing enterocolitis

Necrotizing enterocolitis is a disease entity with multiple proposed pathways of pathogenesis. Various combinations of these risk factors, perhaps based on genetic predisposition, possibly lead to the mucosal and epithelial injury that is the hallmark of NEC. Intestinal epithelial integrity is controlled by a tightly regulated balance between proliferation and differentiation of epithelium from intestinal epithelial stem cells and cellular loss by apoptosis. various signaling pathways play a key role in creating and maintaining this balance. The aim of this review article is to outline intestinal epithelial barrier development and structure and the impact of these inflammatory signaling and regulatory pathways as they pertain to the pathogenesis of NEC.

Intestinal crypt homeostasis revealed at single-stem-cell level by in vivo live imaging

The rapid turnover of the mammalian intestinal epithelium is supported by stem cells located around the base of the crypt¹. Alongside Lgr5, intestinal stem cells have been associated with various markers, which are expressed heterogeneously within the crypt base region^1-6. Previous quantitative clonal fate analyses have proposed that homeostasis occurs as the consequence of neutral competition between dividing stem cells^7-9. However, the short-term behaviour of individual Lgr5⁺ cells positioned at different locations within the crypt base compartment has not been resolved. Here, we established the short-term dynamics of intestinal stem cells using a novel approach of continuous intravital imaging of Lgr5-Confetti mice. We find that Lgr5⁺ cells in the upper part of the niche (termed ‘border cells’) can be passively displaced into the transit-amplifying (TA) domain, following division of proximate cells, implying that determination of stem cell fate can be uncoupled from division. Through the quantitative analysis of individual clonal lineages, we show that stem cells at the crypt base, termed ‘central cells’, experience a survival advantage over border stem cells. However, through the transfer of stem cells between the border and central regions, all Lgr5⁺ cells are endowed with long-term self-renewal potential. These findings establish a novel paradigm for stem cell maintenance in which a dynamically heterogeneous cell population is able to function long-term as a single stem cell pool.

On the cutting edge of organ renewal: Identification, regulation, and evolution of incisor stem cells

The rodent incisor is one of a number of organs that grow continuously throughout the life of an animal. Continuous growth of the incisor arose as an evolutionary adaptation to compensate for abrasion at the distal end of the tooth. The sustained turnover of cells that deposit the mineralized dental tissues is made possible by epithelial and mesenchymal stem cells residing at the proximal end of the incisor. A complex network of signaling pathways and transcription factors regulates the formation, maintenance, and differentiation of these stem cells during development and throughout adulthood. Research over the past 15 years has led to significant progress in our understanding of this network, which includes FGF, BMP, Notch, and Hh signaling, as well as cell adhesion molecules and micro-RNAs. This review surveys key historical experiments that laid the foundation of the field and discusses more recent findings that definitively identified the stem cell population, elucidated the regulatory network, and demonstrated possible genetic mechanisms for the evolution of continuously growing teeth.

Modeling of stem cell dynamics in human colonic crypts in silico

BACKGROUND

Several possible scenarios of cellular dynamics in human colonic crypts have been inferred from transgenic animal experiments. However, because of the discrepancy in size and physiology between humans and animals, quantitative predictions of tissue renewal and cancer development are difficult to execute.

METHODS

A two-dimensional individual based model was developed for the first time to predict cellular dynamics in human colonic crypts. A simple scenario, in which stem cells were not fixed positionally, divide symmetrically and asymmetrically in a stochastic fashion in the lower part of the crypt, was proposed and implemented in the developed model. Numerical simulations of the model were executed in silico.

RESULTS

By comparing the results of computational simulations with available experimental data, the presented scenario was consistent with various experimental evidence. Using this scenario, we simulated and visualized monoclonal conversion in the human colonic crypt. We also predicted that the propensity for monoclonal expansion of a mutant cell was largely dependent on the phenotype, the cell type, the position and the state of the crypt.

CONCLUSIONS

Using the computational framework developed in this study, model users can verify possible scenarios of stem cell dynamics occurring in human colonic crypts and quantitatively predict cell behavior. Its applicability in scenario verification and predictability makes it a valuable tool for elucidation of stem cell dynamics in human colonic crypts.

Role of Hippo signaling in cancer stem cells

Cancer stem cells (CSCs) have been proposed and evidenced as the initiator of tumor formation and the seeds of metastases. Thereby, the molecular mechanisms regarding modulation of CSCs have been widely explored, aimed to improve treatment for cancer patients. Recent progress has highlighted the effects of Hippo signaling in tumorigenesis and cancer development, including its crucial role in CSC regulation. Although the kernel Hippo signaling cascade has been well studied, its upstream inputs and downstream transcriptional regulation still remain elusive. In this review, we summarize the current understanding of the mechanism and regulatory function of Hippo signaling in CSCs, with emphasis on its possible roles in regulation of CSC self-renewal, differentiation and tumorigenesis.

Postembryonic organogenesis of the digestive tube: why does it occur in worms and sea cucumbers but fail in humans?

We provide an integrative view of mechanisms that enable regeneration of the digestive tube in various animal models, including vertebrates, tunicates, echinoderms, insects, and flatworms. Two main strategies of regeneration of the endodermal luminal (mucosal) epithelium have evolved in metazoans. One of them involves proliferation of resident epithelial cells, while the other relies on recruitment of cells from extramucosal sources. In any of these two scenarios, either pluri-/multipotent stem cells or specialized differentiated cells can be used as the starting material. Posttraumatic visceral regeneration shares some common mechanisms with normal embryonic development as well as with organ homeostatic maintenance, but there are signaling pathways and/or cellular pools that are specific to the regenerative phenomena. Comparative analysis of the literature suggests that mammals share with spontaneously regenerating animals many of the regeneration-related adaptations and are able to efficiently repair components of their digestive tube at the level of individual tissues, but fail to do so at the whole-organ scale. We review what might cause this failure in the context of the current state of knowledge about various regenerative models.

Heterogeneity of the level of activity of lgr5+ intestinal stem cells

Intestinal stem cells (ISCs) are a group of rare cells located in the intestinal crypts which are responsible for the maintenance of the intestinal epithelial homeostasis and regeneration following injury or inflammation. Lineage tracing experiments in mice have proven that ISCs can repopulate the entire intestinal crypt. It is noteworthy that in such experiments, only a subset of intestinal crypts is marked by the specific marker. This is suggestive of different levels of activity of stem cells in different crypts i.e. intracryptal variation. Niche succession i.e. dominating the entire crypt by the progenies of one stem cell is also suggestive of the intercryptal stem cell heterogeneity. Regional differences in crypt size, proliferative index, and distribution of proliferative cells along the crypt axis have been reported. It is conceivable that ISCs are heterogeneous in terms of their levels of activity. Appreciation of such heterogeneity will significantly challenge the way in which ISCs are investigated. A better understanding of ISC biology will in turn improve our mechanistic understanding of major intestinal disease including inflammatory bowel disease and colorectal cancer.

Molecular and cellular regulation of skeletal myogenesis

Since the seminal discovery of the cell-fate regulator Myod, studies in skeletal myogenesis have inspired the search for cell-fate regulators of similar potential in other tissues and organs. It was perplexing that a similar transcription factor for other tissues was not found; however, it was later discovered that combinations of molecular regulators can divert somatic cell fates to other cell types. With the new era of reprogramming to induce pluripotent cells, the myogenesis paradigm can now be viewed under a different light. Here, we provide a short historical perspective and focus on how the regulation of skeletal myogenesis occurs distinctly in different scenarios and anatomical locations. In addition, some interesting features of this tissue underscore the importance of reconsidering the simple-minded view that a single stem cell population emerges after gastrulation to assure tissuegenesis. Notably, a self-renewing long-term Pax7+ myogenic stem cell population emerges during development only after a first wave of terminal differentiation occurs to establish a tissue anlagen in the mouse. How the future stem cell population is selected in this unusual scenario will be discussed. Recently, a wealth of information has emerged from epigenetic and genome-wide studies in myogenic cells. Although key transcription factors such as Pax3, Pax7, and Myod regulate only a small subset of genes, in some cases their genomic distribution and binding are considerably more promiscuous. This apparent nonspecificity can be reconciled in part by the permissivity of the cell for myogenic commitment, and also by new roles for some of these regulators as pioneer transcription factors acting on chromatin state.

Lgr5+ stem cells are indispensable for radiation-induced intestinal regeneration

The intestinal epithelium continually self-renews and can rapidly regenerate after damage. Lgr5 marks mitotically active intestinal stem cells (ISCs). Importantly, intestinal homeostasis can be maintained after depletion of Lgr5(+) cells due to the activation of Lgr5(-) reserve ISCs. The Lgr5(-) ISC populations are thought to play a similar role during intestinal regeneration following radiation-induced damage. We tested this regeneration hypothesis by combining depletion of Lgr5(+) ISCs with radiation exposure. In contrast to the negligible effect of Lgr5(+) ISC loss during homeostasis, depletion of Lgr5(+) cells during radiation-induced damage and subsequent repair caused catastrophic crypt loss and deterioration of crypt-villus architecture. Interestingly though, we found that crypts deficient for Lgr5(+) cells are competent to undergo hyperplasia upon loss of Apc. These data argue that Lgr5(-) reserve stem cells are radiosensitive and that Lgr5(+) cells are crucial for robust intestinal regeneration following radiation exposure but are dispensable for premalignant hyperproliferation.

Adult intestinal stem cells: critical drivers of epithelial homeostasis and regeneration

Small populations of adult stem cells are responsible for the remarkable ability of the epithelial lining of the intestine to be efficiently renewed and repaired throughout life. The recent discovery of specific markers for these stem cells, together with the development of new technologies to track endogenous stem cell activity in vivo and to exploit their ability to generate new epithelia ex vivo, has greatly improved our understanding of stem cell-driven homeostasis, regeneration and cancer in the intestine. These exciting new insights into the biology of intestinal stem cells have the potential to accelerate the development of stem cell-based therapies and ameliorate cancer treatments.

Distribution of LGR5+ cells and associated implications during the early stage of gastric tumorigenesis

Lgr5 was identified as a promising gastrointestinal tract stem cell marker in mice. Lineage tracing indicates that Lgr5⁺ cells may not only be the cells responsible for the origin of tumors; they may also be the so-called cancer stem cells. In the present study, we investigated the presence of Lgr5⁺ cells and their biological significance in normal human gastric mucosa and gastric tumors. RNAscope, a newly developed RNA in situ hybridization technique, specifically labeled Lgr5⁺ cells at the basal glands of the gastric antrum. Notably, the number of Lgr5⁺ cells was remarkably increased in intestinal metaplasia. In total, 76% of gastric adenomas and 43% of early gastric carcinomas were positive for LGR5. Lgr5⁺ cells were found more frequently in low-grade tumors with active Wnt signaling and an intestinal gland type, suggesting that LGR5 is likely involved in the very early stages of Wnt-driven tumorigenesis in the stomach. Interestingly, similar to stem cells in normal tissues, Lgr5⁺ cells were often restricted to the base of the tumor glands, and such Lgr5⁺ restriction was associated with high levels of intestinal stem cell markers such as EPHB2, OLFM4, and ASCL2. Thus, our findings show that Lgr5⁺ cells are present at the base of the antral glands in the human stomach and that this cell population significantly expands in intestinal metaplasias. Furthermore, Lgr5⁺ cells are seen in a large number of gastric tumors ; their frequent basal arrangements and coexpression of ISC markers support the idea that Lgr5⁺ cells act as stem cells during the early stage of intestinal-type gastric tumorigenesis.

Wnt signaling in adult intestinal stem cells and cancer

Signaling initiated by secreted glycoproteins of the Wnt family regulates many aspects of embryonic development and it is involved in homeostasis of adult tissues. In the gastrointestinal (GI) tract the Wnt pathway maintains the self-renewal capacity of epithelial stem cells. The stem cell attributes are conferred by mutual interactions of the stem cell with its local microenvironment, the stem cell niche. The niche ensures that the threshold of Wnt signaling in the stem cell is kept in physiological range. In addition, the Wnt pathway involves various feedback loops that balance the opposing processes of cell proliferation and differentiation. Today, we have compelling evidence that mutations causing aberrant activation of the Wnt pathway promote expansion of undifferentiated progenitors and lead to cancer. The review summarizes recent advances in characterization of adult epithelial stem cells in the gut. We mainly focus on discoveries related to molecular mechanisms regulating the output of the Wnt pathway. Moreover, we present novel experimental approaches utilized to investigate the epithelial cell signaling circuitry in vivo and in vitro. Pivotal aspects of tissue homeostasis are often deduced from studies of tumor cells; therefore, we also discuss some latest results gleaned from the deep genome sequencing studies of human carcinomas of the colon and rectum.

Self-renewal as a therapeutic target in human colorectal cancer

Tumor recurrence following treatment remains a major clinical challenge. Evidence from xenograft models and human trials indicates selective enrichment of cancer-initiating cells (CICs) in tumors that survive therapy. Together with recent reports showing that CIC gene signatures influence patient survival, these studies predict that targeting self-renewal, the key 'stemness' property unique to CICs, may represent a new paradigm in cancer therapy. Here we demonstrate that tumor formation and, more specifically, human colorectal CIC function are dependent on the canonical self-renewal regulator BMI-1. Downregulation of BMI-1 inhibits the ability of colorectal CICs to self-renew, resulting in the abrogation of their tumorigenic potential. Treatment of primary colorectal cancer xenografts with a small-molecule BMI-1 inhibitor resulted in colorectal CIC loss with long-term and irreversible impairment of tumor growth. Targeting the BMI-1-related self-renewal machinery provides the basis for a new therapeutic approach in the treatment of colorectal cancer.

Engineering the niche for stem cells

Much has been made about the potential for stem cells in regenerative medicine but the reality is that the development of actual therapies has been slow. Adult stem cells rely heavily on the assortment of biochemical and biophysical elements that constitute the local microenvironment in which they exist. One goal of biomedicine is to create an artificial yet biofunctional niche to support multipotency, differentiation and proliferation. Such tools would facilitate more conclusive experimentation by biologists, pharmaceutical scientists and tissue engineers. While many bioengineering techniques and platforms are already in use, technological innovations now allow this to be done at a higher resolution and specificity. Ultimately, the multidisciplinary integration of engineering and biology will allow the niche to be generated at a scale that can be clinically exploited. Using the systems that constitute the intestinal, hematopoietic and epidermal tissues, this article summarizes the various approaches and tools currently employed to recreate stem cell niches and also explores recent advances in the field.

Host-microbiota interaction and intestinal stem cells in chronic inflammation and colorectal cancer

Inflammatory bowel disease (IBD) and colorectal cancer (CRC) are the major diseases of the lower gastrointestinal tract. The intestinal epithelium plays a critical role in the host's interactions with the large communities of resident luminal bacteria. Epithelial cells recognize the bacterial components via pattern-recognition receptors. Toll-like receptors (TLRs) are a major class of pattern-recognition receptors that are present on intestinal epithelial cells, including putative stem cells. Stem cells are responsible for tissue homeostasis and regeneration after injury including IBD. Stem cells are also implicated in the pathogenesis of CRC. In susceptible individuals, disruption of normal homeostatic balance between the host's mucosal cells and enteric microflora is believed to result in aberrant immune responses against the resident commensal bacteria, leading to IBD. Microbiological analyses have revealed that the composition and localization of microbiota is altered in CRC and IBD. It is plausible that stem cells directly sense and respond to microbiota. This review aims to summarize the current knowledge on the effect of microbiota and TLR signaling on intestinal stem cells. It also describes how TLR signaling could affect the stem cell regulatory pathways.

A SCARECROW-RETINOBLASTOMA protein network controls protective quiescence in the Arabidopsis root stem cell organizer

Ben Scheres and colleagues report that in the growing tip of plant roots, a gene regulatory network that includes the plant homologue of Retinoblastoma regulates the divisions of long-term stem cells to replenish tissue and to protect the root stem cell niche.

Quiescent long-term somatic stem cells reside in plant and animal stem cell niches. Within the Arabidopsis root stem cell population, the Quiescent Centre (QC), which contains slowly dividing cells, maintains surrounding short-term stem cells and may act as a long-term reservoir for stem cells. The RETINOBLASTOMA-RELATED (RBR) protein cell-autonomously reinforces mitotic quiescence in the QC. RBR interacts with the stem cell transcription factor SCARECROW (SCR) through an LxCxE motif. Disruption of this interaction by point mutation in SCR or RBR promotes asymmetric divisions in the QC that renew short-term stem cells. Analysis of the in vivo role of quiescence in the root stem cell niche reveals that slow cycling within the QC is not needed for structural integrity of the niche but allows the growing root to cope with DNA damage.

In the plant Arabidposis thaliana, root meristems (in the growing tip of the root) contain slowly dividing cells that act as an organizing center for the root stem cells that surround them. This centre is called the quiescent centre (QC). In this study, we show that the slow rate of division in the QC is regulated by the interaction between two proteins: Retinoblastoma homolog (RBR) and SCARECROW (SCR), a transcription factor that controls stem cell maintenance. RBR and SCR regulate quiescence in the QC by repressing an asymmetric cell division that generates short-term stem cells. Here we genetically manipulate the cells in the QC to alter their quiescence by regulating the RBR/SCR interaction to demonstrate that quiescence is not needed for the organizing capacity of the QC but instead provides cells with a higher resistance to genotoxic stress, allowing stem cells in the QC to survive even if more rapidly cycling stem cells are damaged. A role for mitotic quiescence has been reported in animal stem cells, in which Rb has been implicated. These findings indicate that it might serve a similar role in plant stem cells.

A calibrated agent-based computer model of stochastic cell dynamics in normal human colon crypts useful for in silico experiments

BACKGROUND

Normal colon crypts consist of stem cells, proliferating cells, and differentiated cells. Abnormal rates of proliferation and differentiation can initiate colon cancer. We have measured the variation in the number of each of these cell types in multiple crypts in normal human biopsy specimens. This has provided the opportunity to produce a calibrated computational model that simulates cell dynamics in normal human crypts, and by changing model parameter values, to simulate the initiation and treatment of colon cancer.

RESULTS

An agent-based model of stochastic cell dynamics in human colon crypts was developed in the multi-platform open-source application NetLogo. It was assumed that each cell's probability of proliferation and probability of death is determined by its position in two gradients along the crypt axis, a divide gradient and in a die gradient. A cell's type is not intrinsic, but rather is determined by its position in the divide gradient. Cell types are dynamic, plastic, and inter-convertible. Parameter values were determined for the shape of each of the gradients, and for a cell's response to the gradients. This was done by parameter sweeps that indicated the values that reproduced the measured number and variation of each cell type, and produced quasi-stationary stochastic dynamics. The behavior of the model was verified by its ability to reproduce the experimentally observed monocolonal conversion by neutral drift, the formation of adenomas resulting from mutations either at the top or bottom of the crypt, and by the robust ability of crypts to recover from perturbation by cytotoxic agents. One use of the virtual crypt model was demonstrated by evaluating different cancer chemotherapy and radiation scheduling protocols.

CONCLUSIONS

A virtual crypt has been developed that simulates the quasi-stationary stochastic cell dynamics of normal human colon crypts. It is unique in that it has been calibrated with measurements of human biopsy specimens, and it can simulate the variation of cell types in addition to the average number of each cell type. The utility of the model was demonstrated with in silico experiments that evaluated cancer therapy protocols. The model is available for others to conduct additional experiments.

The Lgr5 transgene is expressed specifically in glycinergic amacrine cells in the mouse retina

Retinal amacrine cells are a diverse set of interneurons within the inner nuclear layer. The canonical Wnt pathway is highly active within mature amacrine cells, but its role remains unclear. Leucine-rich repeat containing G-protein receptor 5 (Lgr5) is a newly identified component of the Wnt receptor complex that potentiates beta-catenin signaling. In multiple epithelial organs Lgr5 marks adult tissue stem cells. We investigated the expression of this gene using Lgr5-eGFP-IRES-CreER transgenic reporter mice. In the eye, Lgr5 was exclusively expressed in glycinergic amacrine cells in adult mice. Amacrine cells are post-mitotic and represent the first neuronal and non-stem cell lineage to express Lgr5. We further interrogated the spatiotemporal labeling of individual amacrine cells with controlled fluorophore expression. This "fluorofilling" technique provides a tool to study amacrine morphology and dissect neural networks.

Trafficking of epidermal growth factor receptor ligands in polarized epithelial cells

A largely unilamellar epithelial layer lines body cavities and organ ducts such as the digestive tract and kidney tubules. This polarized epithelium is composed of biochemically and functionally separate apical and basolateral surfaces. The epidermal growth factor receptor (EGFR) signaling pathway is a critical regulator of epithelial homeostasis and is perturbed in a number of epithelial disorders. It is underappreciated that in vivo EGFR signaling is most often initiated by cell-surface delivery and processing of one of seven transmembrane ligands, resulting in release of the soluble form that binds EGFR. In polarized epithelial cells, EGFR is restricted largely to the basolateral surface, and apical or basolateral ligand delivery therefore has important biological consequences. In vitro approaches have been used to study the biosynthesis, cell-surface delivery, proteolytic processing, and release of soluble EGFR ligands in polarized epithelial cells. We review these results, discuss their relevance to normal physiology, and demonstrate the pathophysiological consequences of aberrant trafficking. These studies have uncovered a rich diversity of apico-basolateral trafficking mechanisms among the EGFR ligands, provided insights into the pathogenesis of an inherited magnesium-wasting disorder of the kidney (isolated renal hypomagnesemia), and identified a new mode of EGFR ligand signaling via exosomes.

An APC:WNT Counter-Current-Like Mechanism Regulates Cell Division Along the Human Colonic Crypt Axis: A Mechanism That Explains How APC Mutations Induce Proliferative Abnormalities That Drive Colon Cancer Development

APC normally down-regulates WNT signaling in human colon, and APC mutations cause proliferative abnormalities in premalignant crypts leading to colon cancer, but the mechanisms are unclear at the level of spatial and functional organization of the crypt. Accordingly, we postulated a counter-current-like mechanism based on gradients of factors (APC;WNT) that regulate colonocyte proliferation along the crypt axis. During crypt renewal, stem cells (SCs) at the crypt bottom generate non-SC daughter cells that proliferate and differentiate while migrating upwards. The APC concentration is low at the crypt bottom and high at the top (where differentiated cells reside). WNT signaling, in contrast, is high at the bottom (where SCs reside) and low at the top. Given that WNT and APC gradients are counter to one another, we hypothesized that a counter-current-like mechanism exists. Since both APC and WNT signaling components (e.g., survivin) are required for mitosis, this mechanism establishes a zone in the lower crypt where conditions are optimal for maximal cell division and mitosis orientation (symmetric versus asymmetric). APC haploinsufficiency diminishes the APC gradient, shifts the proliferative zone upwards, and increases symmetric division, which causes SC overpopulation. In homozygote mutant crypts, these changes are exacerbated. Thus, APC-mutation-induced changes in the counter-current-like mechanism cause expansion of proliferative populations (SCs, rapidly proliferating cells) during tumorigenesis. We propose this mechanism also drives crypt fission, functions in the crypt cycle, and underlies adenoma development. Novel chemoprevention approaches designed to normalize the two gradients and readjust the proliferative zone downwards, might thwart progression of these premalignant changes.

New cancer diagnostics and therapeutics from a ninth 'hallmark of cancer': symmetric self-renewal by mutated distributed stem cells

A total of eight cellular alterations associated with human carcinogenesis have been framed as the 'hallmarks of cancer'. This representation overlooks a ninth hallmark of cancer: the requirement for tumor-originating distributed stem cells to shift sufficiently from asymmetric to symmetric self-renewal kinetics for attainment of the high cell production rate necessary to form clinically significant tumors within a human lifespan. Overlooking this ninth hallmark costs opportunities for discovery of more selective molecular targets for development of improved cancer therapeutics and missing cancer stem cell biomarkers of greater specificity. Here, the biological basis for the ninth hallmark of cancer is considered toward highlighting its importance in human carcinogenesis and, as such, its potential for revealing unique molecules for targeting cancer diagnostics and therapeutics.

Identification of Lgr5-independent spheroid-generating progenitors of the mouse fetal intestinal epithelium

Immortal spheroids were generated from fetal mouse intestine using the culture system initially developed to culture organoids from adult intestinal epithelium. Spheroid proportion progressively decreases from fetal to postnatal period, with a corresponding increase in production of organoids. Like organoids, spheroids show Wnt-dependent indefinite self-renewing properties but display a poorly differentiated phenotype reminiscent of incompletely caudalized progenitors. The spheroid transcriptome is strikingly different from that of adult intestinal stem cells, with minimal overlap of Wnt target gene expression. The receptor LGR4, but not LGR5, is essential for their growth. Trop2/Tacstd2 and Cnx43/Gja1, two markers highly enriched in spheroids, are expressed throughout the embryonic-day-14 intestinal epithelium. Comparison of in utero and neonatal lineage tracing using Cnx43-CreER and Lgr5-CreERT2 mice identified spheroid-generating cells as developmental progenitors involved in generation of the prenatal intestinal epithelium. Ex vivo, spheroid cells have the potential to differentiate into organoids, qualifying as a fetal type of intestinal stem cell.

Enteroendocrine cell types revisited

The GI-tract is profoundly involved in the control of metabolism through peptide hormones secreted from enteroendocrine cells scattered throughout the gut mucosa. A large number of recently generated transgenic reporter mice have allowed for direct characterization of biochemical and cell biological properties of these previously highly elusive enteroendocrine cells. In particular the surprisingly broad co-expression of six functionally related hormones in the intestinal enteroendocrine cells indicates that it should be possible to control not only the hormone secretion but also the type and number of enteroendocrine cells. However, this will require a more deep understanding of the factors controlling differentiation, gene expression and specification of the enteroendocrine cells during their weekly renewal from progenitor cells in the crypts of the mucosa.

If a stem cell dies in the crypt, and no one is around to see it…

Two recent studies continue the debate regarding lineage and hierarchy in the intestinal epithelium. One reports that quiescent crypt cells are Paneth cell precursors (Buczacki et al., 2013). The second shows that tamoxifen induces apoptosis in crypt cells and that suppressing apoptosis alters lineage tracing patterns (Zhu et al., 2013).

Differentiated Troy+ chief cells act as reserve stem cells to generate all lineages of the stomach epithelium

Proliferation of the self-renewing epithelium of the gastric corpus occurs almost exclusively in the isthmus of the glands, from where cells migrate bidirectionally toward pit and base. The isthmus is therefore generally viewed as the stem cell zone. We find that the stem cell marker Troy is expressed at the gland base by a small subpopulation of fully differentiated chief cells. By lineage tracing with a Troy-eGFP-ires-CreERT2 allele, single marked chief cells are shown to generate entirely labeled gastric units over periods of months. This phenomenon accelerates upon tissue damage. Troy(+) chief cells can be cultured to generate long-lived gastric organoids. Troy marks a specific subset of chief cells that display plasticity in that they are capable of replenishing entire gastric units, essentially serving as quiescent "reserve" stem cells. These observations challenge the notion that stem cell hierarchies represent a "one-way street."

Characterization of the stem cell niche and its importance in radiobiological response

Normal tissues are organized hierarchically with a small number of stem cells, able to self-renew and give rise to all the differentiated cells found in the respective specialized tissues. The undifferentiated, multipotent state of normal stem cells is codetermined by the constituents of a specific anatomical space that hosts the normal stem cell population, called the "stem cell niche." Radiation interferes not only with the stem cell population but also with the stem cell niche, thus modulating a complex regulatory network. There is now mounting experimental evidence that many solid cancers share this hierarchical organization with their tissue of origin, with the cancer stem cells also occupying specialized niches. In this review, we highlight some of the best-characterized aspects of normal tissue stem cells, cancer stem cells, and their niches in the bone marrow, gut, and brain, as well as their responses to ionizing radiation.

Regulation of intestinal stem cells by Wnt and Notch signalling

The mammalian intestine is lined by an epithelial cell layer that is constantly renewed via a population of stem cells that reside in a specialised niche within intestinal crypts. The recent development of tools that permit genetic manipulation and lineage tracing of cells in vivo combined with culture methods in vitro has made the intestine particularly amenable for the study of signals that regulate stem cell function. Both Wnt and Notch signalling are critical regulators of stem cell fate. Gene knockout and transgenic expression analysis combined with meticulous analysis of lineage tracing and molecular characterisation has contributed to the definition of the mechanisms by which these pathways act during normal homeostasis and in disease states.

ER stress transcription factor Xbp1 suppresses intestinal tumorigenesis and directs intestinal stem cells

X-box–binding protein 1 suppresses tumor formation in the gut by regulating Ire1α and Stat3-mediated regenerative responses in the epithelium as a consequence of ER stress.

Unresolved endoplasmic reticulum (ER) stress in the epithelium can provoke intestinal inflammation. Hypomorphic variants of ER stress response mediators, such as X-box–binding protein 1 (XBP1), confer genetic risk for inflammatory bowel disease. We report here that hypomorphic Xbp1 function instructs a multilayered regenerative response in the intestinal epithelium. This is characterized by intestinal stem cell (ISC) expansion as shown by an inositol-requiring enzyme 1α (Ire1α)–mediated increase in Lgr5⁺ and Olfm4⁺ ISCs and a Stat3-dependent increase in the proliferative output of transit-amplifying cells. These consequences of hypomorphic Xbp1 function are associated with an increased propensity to develop colitis-associated and spontaneous adenomatous polyposis coli (APC)–related tumors of the intestinal epithelium, which in the latter case is shown to be dependent on Ire1α. This study reveals an unexpected role for Xbp1 in suppressing tumor formation through restraint of a pathway that involves an Ire1α- and Stat3-mediated regenerative response of the epithelium as a consequence of ER stress. As such, Xbp1 in the intestinal epithelium not only regulates local inflammation but at the same time also determines the propensity of the epithelium to develop tumors.

Cell organisation in the colonic crypt: a theoretical comparison of the pedigree and niche concepts

The intestinal mucosa is a monolayer of rapidly self-renewing epithelial cells which is not only responsible for absorption of water and nutrients into the bloodstream but also acts as a protective barrier against harmful microbes entering the body. New functional epithelial cells are produced from stem cells, and their proliferating progeny. These stem cells are found within millions of crypts (tubular pits) spaced along the intestinal tract. The entire intestinal epithelium is replaced every 2–3 days in mice (3–5 days in humans) and hence cell production, differentiation, migration and turnover need to be tightly regulated. Malfunctions in this regulation are strongly linked to inflammatory bowel diseases and to the formation of adenomas and ultimately cancerous tumours. Despite a great deal of biological experimentation and observation, precisely how colonic crypts are regulated to produce mature colonocytes remains unclear. To assist in understanding how cell organisation in crypts is achieved, two very different conceptual models of cell behaviour are developed here, referred to as the ‘pedigree’ and the ‘niche’ models. The pedigree model proposes that crypt cells are largely preprogrammed and receive minimal prompting from the environment as they move through a routine of cell differentiation and proliferation to become mature colonocytes. The niche model proposes that crypt cells are primarily influenced by the local microenvironments along the crypt, and that predetermined cell behaviour plays a negligible role in their development. In this paper we present a computational model of colonic crypts in the mouse, which enables a comparison of the quality and controllability of mature coloncyte production by crypts operating under these two contrasting conceptual models of crypt regulation.

DCLK1 regulates pluripotency and angiogenic factors via microRNA-dependent mechanisms in pancreatic cancer

Stem cell pluripotency, angiogenesis and epithelial-mesenchymal transition (EMT) have been shown to be significantly upregulated in pancreatic ductal adenocarcinoma (PDAC) and many other aggressive cancers. The dysregulation of these processes is believed to play key roles in tumor initiation, progression, and metastasis, and is contributory to PDAC being the fourth leading cause of cancer-related deaths in the US. The tumor suppressor miRNA miR-145 downregulates critical pluripotency factors and oncogenes and results in repressed metastatic potential in PDAC. Additionally, the miR-200 family regulates several angiogenic factors which have been linked to metastasis in many solid tumors. We have previously demonstrated that downregulation of DCLK1 can upregulate critical miRNAs in both in vitro and in vivo cancer models and results in downregulation of c-MYC, KRAS, NOTCH1 and EMT-related transcription factors. A recent report has also shown that Dclk1 can distinguish between normal and tumor stem cells in Apc (min/+) mice and that ablation of Dclk1(+) cells resulted in regression of intestinal polyps without affecting homeostasis. Here we demonstrate that the knockdown of DCLK1 using poly(lactide-co-glycolide)-encapsulated-DCLK1-siRNA results in AsPC1 tumor growth arrest. Examination of xenograft tumors revealed, (a) increased miR-145 which results in decreased pluripotency maintenance factors OCT4, SOX2, NANOG, KLF4 as well as KRAS and RREB1; (b) increased let-7a which results in decreased pluripotency factor LIN28B; and (c) increased miR-200 which results in decreased VEGFR1, VEGFR2 and EMT-related transcription factors ZEB1, ZEB2, SNAIL and SLUG. Specificity of DCLK1 post-transcriptional regulation of the downstream targets of miR-145, miR-200 and let-7a was accomplished utilizing a luciferase-based reporter assay. We conclude that DCLK1 plays a significant master regulatory role in pancreatic tumorigenesis through the regulation of multiple tumor suppressor miRNAs and their downstream pro-tumorigenic pathways. This novel concept of targeting DCLK1 alone has several advantages over targeting single pathway or miRNA-based therapies for PDAC.

LGR5-positive colon cancer stem cells interconvert with drug-resistant LGR5-negative cells and are capable of tumor reconstitution

The cancer stem cell (CSC) concept has been proposed as an attractive theory to explain cancer development, and CSCs themselves have been considered as targets for the development of diagnostics and therapeutics. However, many unanswered questions concerning the existence of slow cycling/quiescent, drug-resistant CSCs remain. Here we report the establishment of colon cancer CSC lines, interconversion of the CSCs between a proliferating and a drug-resistant state, and reconstitution of tumor hierarchy from the CSCs. Stable cell lines having CSC properties were established from human colon cancer after serial passages in NOD/Shi-scid, IL-2Rγ(null) (NOG) mice and subsequent adherent cell culture of these tumors. By generating specific antibodies against LGR5, we demonstrated that these cells expressed LGR5 and underwent self-renewal using symmetrical divisions. Upon exposure to irinotecan, the LGR5(+) cells transitioned into an LGR5(-) drug-resistant state. The LGR5(-) cells converted to an LGR5(+) state in the absence of the drug. DNA microarray analysis and immunohistochemistry demonstrated that HLA-DMA was specifically expressed in drug-resistant LGR5(-) cells, and epiregulin was expressed in both LGR5(+) and drug-resistant LGR5(-) cells. Both cells sustained tumor initiating activity in NOG mice, giving rise to a tumor tissue hierarchy. In addition, anti-epiregulin antibody was found to be efficacious in a metastatic model. Both LGR5(+) and LGR5(-) cells were detected in the tumor tissues of colon cancer patients. The results provide new biological insights into drug resistance of CSCs and new therapeutic options for cancer treatment.

Bmi1 is required for tumorigenesis in a mouse model of intestinal cancer

The epigenetic regulator BMI1 is upregulated progressively in a wide variety of human tumors including colorectal cancer. In this study, we assessed the requirement for Bmi1 in intestinal tumorigenesis using an autochthonous mouse model in which Apc was conditionally ablated in the intestinal epithelium. Germline mutation of Bmi1 significantly reduced both the number and size of small intestinal adenomas arising in this model, and it acted in a dose-dependent manner. Moreover, in contrast to wild-type controls, Bmi1(-/-) mice showed no increase in median tumor size, and a dramatic decrease in tumor number, between 3 and 4 months of age. Thus, Bmi1 is required for both progression and maintenance of small intestinal adenomas. Importantly, Bmi1 deficiency did not disrupt oncogenic events arising from Apc inactivation. Instead, the Arf tumor suppressor, a known target of Bmi1 epigenetic silencing, was upregulated in Bmi1 mutant tumors. This was accompanied by significant upregulation of p53, which was confirmed by sequencing to be wild-type, and also elevated apoptosis within the smallest Bmi1(-/-) adenomas. By crossing Arf into this cancer model, we showed that Arf is required for the induction of both p53 and apoptosis, and it is a key determinant of the ability of Bmi1 deficiency to suppress intestinal tumorigenesis. Finally, a conditional Bmi1 mutant strain was generated and used to determine the consequences of deleting Bmi1 specifically within the intestinal epithelium. Strikingly, intestinal-specific Bmi1 deletion suppressed small intestinal adenomas in a manner that was indistinguishable from germline Bmi1 deletion. Thus, we conclude that Bmi1 deficiency impairs the progression and maintenance of small intestinal tumors in a cell autonomous and highly Arf-dependent manner.

Developmental changes in the in vitro activated regenerative activity of primitive mammary epithelial cells

Mouse fetal mammary cells display greater regenerative activity than do adult mammary cells when stimulated to proliferate in a new system that supports the production of transplantable mammary stem cells ex vivo.

Many normal adult tissues contain rare stem cells with extensive self-maintaining regenerative potential. During development, the stem cells of the hematopoietic and neural systems undergo intrinsically specified changes in their self-renewal potential. In the mouse, mammary stem cells with transplantable regenerative activity are first detectable a few days before birth. They share some phenotypic properties with their adult counterparts but are enriched in a subpopulation that displays a distinct gene expression profile. Here we show that fetal mammary epithelial cells have a greater direct and inducible growth potential than their adult counterparts. The latter feature is revealed in a novel culture system that enables large numbers of in vitro clonogenic progenitors as well as mammary stem cells with serially transplantable activity to be produced within 7 days from single fetal or adult input cells. We further show that these responses are highly dependent on novel factors produced by fibroblasts. These findings provide new avenues for elucidating mechanisms that regulate normal mammary epithelial stem cell properties at the single-cell level, how these change during development, and how their perturbation may contribute to transformation.

Many adult tissues are maintained by a rare subset of undifferentiated stem cells that can self-renew and give rise to specialized daughter cells that have a more limited regenerative ability. The recent identification of cells in the fetal and adult mammary gland that display the properties of stem cells provides a foundation for investigating their self-renewal and differentiation control. We now show that these stem cell properties can be elicited from single mouse mammary cells placed in 3D cultures if novel factors produced by fibroblasts are present. Moreover, a comparison of the clonal outputs of fetal and adult mammary cells in this in vitro system shows that the fetal mammary cells have superior regenerative activity relative to their adult counterparts. The ability to activate and quantify the regenerative capacity of single mouse mammary epithelial cells in vitro sets the stage for further investigations of the timing and mechanisms that alter their stem cell properties during development, the potential relevance of these events to other normal epithelial tissues, and how these processes might be involved in the genesis of breast cancer.

Beyond the niche: tissue-level coordination of stem cell dynamics

Adult animals rely on populations of stem cells to ensure organ function throughout their lifetime. Stem cells are governed by signals from stem cell niches, and much is known about how single niches promote stemness and direct stem cell behavior. However, most organs contain a multitude of stem cell-niche units, which are often distributed across the entire expanse of the tissue. Beyond the biology of individual stem cell-niche interactions, the next challenge is to uncover the tissue-level processes that orchestrate spatial control of stem-based renewal, repair, and remodeling throughout a whole organ. Here we examine what is known about higher order mechanisms for interniche coordination in epithelial organs, whose simple geometry offers a promising entry point for understanding the regulation of niche number, distribution, and activity. We also consider the potential existence of stem cell territories and how tissue architecture may influence niche coordination.

Isolation and characterization of intestinal stem cells based on surface marker combinations and colony-formation assay

BACKGROUND & AIMS

Identification of intestinal stem cells (ISCs) has relied heavily on the use of transgenic reporters in mice, but this approach is limited by mosaic expression patterns and difficult to directly apply to human tissues. We sought to identify reliable surface markers of ISCs and establish a robust functional assay to characterize ISCs from mouse and human tissues.

METHODS

We used immunohistochemistry, real-time reverse-transcription polymerase chain reaction, and fluorescence-activated cell sorting (FACS) to analyze intestinal epithelial cells isolated from mouse and human intestinal tissues. We compared different combinations of surface markers among ISCs isolated based on expression of Lgr5-green fluorescent protein. We developed a culture protocol to facilitate the identification of functional ISCs from mice and then tested the assay with human intestinal crypts and putative ISCs.

RESULTS

CD44(+)CD24(lo)CD166(+) cells, isolated by FACS from mouse small intestine and colon, expressed high levels of stem cell-associated genes. Transit-amplifying cells and progenitor cells were then excluded based on expression of GRP78 or c-Kit. CD44(+)CD24(lo)CD166(+) GRP78(lo/-) putative stem cells from mouse small intestine included Lgr5-GFP(hi) and Lgr5-GFP(med/lo) cells. Incubation of these cells with the GSK inhibitor CHIR99021 and the E-cadherin stabilizer Thiazovivin resulted in colony formation by 25% to 30% of single-sorted ISCs.

CONCLUSIONS

We developed a culture protocol to identify putative ISCs from mouse and human tissues based on cell surface markers. CD44(+)CD24(lo)CD166(+), GRP78(lo/-), and c-Kit(-) facilitated identification of putative stem cells from the mouse small intestine and colon, respectively. CD44(+)CD24(-/lo)CD166(+) also identified putative human ISCs. These findings will facilitate functional studies of mouse and human ISCs.

Mesenchymal stem cell therapy stimulates endogenous host progenitor cells to improve colonic epithelial regeneration

Patients who undergo pelvic radiotherapy may develop severe and chronic complications resulting from gastrointestinal alterations. The lack of curative treatment highlights the importance of novel and effective therapeutic strategies. We thus tested the therapeutic benefit of mesenchymal stem cells (MSC) treatment and proposed molecular mechanisms of action. MSC efficacy was tested in an experimental model of radiation-induced severe colonic ulceration histologically similar to that observed in patients. In this model, MSC from bone marrow were administered intravenously, immediately or three weeks (established lesions) after irradiation. MSC therapy reduces radiation-induced colonic ulceration and increases animal survival. MSC treatment induces therapeutic efficacy whatever the time of cell infusion. Infused-MSC engraft in the colon but also increase endogenous MSC mobilization in blood that have lasting benefits over time. In vitro analysis demonstrates that the MSC effect is mediated by paracrine mechanisms through the non-canonical WNT (Wingless integration site) pathway. In irradiated rat colons, MSC treatment increases the expression of the non-canonical WNT4 ligand by epithelial cells. The epithelial regenerative process is improved after MSC injection by stimulation of colonic epithelial cells positive for SOX9 (SRY-box containing gene 9) progenitor/stem cell markers. This study demonstrates that MSC treatment induces stimulation of endogenous host progenitor cells to improve the regenerative process and constitutes an initial approach to arguing in favor of the use of MSC to limit/reduce colorectal damage induced by radiation.

Oral epithelial stem cells in tissue maintenance and disease: the first steps in a long journey

The identification and characterization of stem cells is a major focus of developmental biology and regenerative medicine. The advent of genetic inducible fate mapping techniques has made it possible to precisely label specific cell populations and to follow their progeny over time. When combined with advanced mathematical and statistical methods, stem cell division dynamics can be studied in new and exciting ways. Despite advances in a number of tissues, relatively little attention has been paid to stem cells in the oral epithelium. This review will focus on current knowledge about adult oral epithelial stem cells, paradigms in other epithelial stem cell systems that could facilitate new discoveries in this area and the potential roles of epithelial stem cells in oral disease.

Hormonal control of stem cell systems

Many organs respond to physiological challenges by changing tissue size or composition. Such changes may originate from tissue-specific stem cells and their supportive environment (niche). The endocrine system is a major effector and conveyor of physiological changes and as such could alter stem cell behavior in various ways. In this review, we examine how hormones affect stem cell biology in four different organs: the ovary, intestine, hematopoietic system, and mammary gland. Hormones control every stage of stem cell life, including establishment, expansion, maintenance, and differentiation. The effects can be cell autonomous or non-cell autonomous through the niche. Moreover, a single hormone can affect different stem cells in different ways or affect the same stem cell differently at various developmental times. The vast complexity and diversity of stem cell responses to hormonal cues allow hormones to coordinate the body's reaction to physiological challenges.