Tissue-specific designs of stem cell hierarchies. Nat Cell Biol. 2016;18:349-355. [PMID: 26999737 DOI: 10.1038/ncb3332]

HB-EGF promotes progenitor cell proliferation and sensory neuron regeneration in the zebrafish olfactory epithelium

Maintenance and regeneration of the zebrafish olfactory epithelium (OE) are supported by two distinct progenitor cell populations that occupy spatially discrete stem cell niches and respond to different tissue conditions. Globose basal cells (GBCs) reside at the inner and peripheral margins of the sensory OE and are constitutively active to replace sporadically dying olfactory sensory neurons (OSNs). In contrast, horizontal basal cells (HBCs) are uniformly distributed across the sensory tissue and are selectively activated by acute injury conditions. Here we show that expression of the heparin-binding epidermal growth factor-like growth factor (HB-EGF) is strongly and transiently upregulated in response to OE injury and signals through the EGF receptor (EGFR), which is expressed by HBCs. Exogenous stimulation of the OE with recombinant HB-EGF promotes HBC expansion and OSN neurogenesis in a pattern that resembles the tissue response to injury. In contrast, pharmacological inhibition of HB-EGF membrane shedding, HB-EGF availability, and EGFR signaling strongly attenuate or delay injury-induced HBC activity and OSN restoration without affecting maintenance neurogenesis by GBCs. Thus, HB-EGF/EGFR signaling appears to be a critical component of the signaling network that controls HBC activity and, consequently, repair neurogenesis in the zebrafish OE.

Elevated stress response marks deeply quiescent reserve cells of gastric chief cells

Gastrointestinal tract organs harbor reserve cells, which are endowed with cellular plasticity and regenerate functional units in response to tissue damage. However, whether the reserve cells in gastrointestinal tract exist as long-term quiescent cells remain incompletely understood. In the present study, we systematically examine H2b-GFP label-retaining cells and identify a long-term slow-cycling population in the gastric corpus but not in other gastrointestinal organs. The label-retaining cells, which reside near the basal layers of the corpus, comprise a subpopulation of chief cells. The identified quiescent cells exhibit induction of Atf4 and its target genes including Atf3, a marker of paligenosis, and activation of the unfolded protein response, but do not show elevated expression of Troy, Lgr5, or Mist. External damage to the gastric mucosa induced by indomethacin treatment triggers proliferation of the quiescent Atf4+ population, indicating that the gastric corpus harbors a specific cell population that is primed to facilitate stomach regeneration.

Regulation of cellular contractile force, shape and migration of fibroblasts by oncogenes and Histone deacetylase 6

The capacity of cells to adhere to, exert forces upon and migrate through their surrounding environment governs tissue regeneration and cancer metastasis. The role of the physical contractile forces that cells exert in this process, and the underlying molecular mechanisms are not fully understood. We, therefore, aimed to clarify if the extracellular forces that cells exert on their environment and/or the intracellular forces that deform the cell nucleus, and the link between these forces, are defective in transformed and invasive fibroblasts, and to indicate the underlying molecular mechanism of control. Confocal, Epifluorescence and Traction force microscopy, followed by computational analysis, showed an increased maximum contractile force that cells apply on their environment and a decreased intracellular force on the cell nucleus in the invasive fibroblasts, as compared to normal control cells. Loss of HDAC6 activity by tubacin-treatment and siRNA-mediated HDAC6 knockdown also reversed the reduced size and more circular shape and defective migration of the transformed and invasive cells to normal. However, only tubacin-mediated, and not siRNA knockdown reversed the increased force of the invasive cells on their surrounding environment to normal, with no effects on nuclear forces. We observed that the forces on the environment and the nucleus were weakly positively correlated, with the exception of HDAC6 siRNA-treated cells, in which the correlation was weakly negative. The transformed and invasive fibroblasts showed an increased number and smaller cell-matrix adhesions than control, and neither tubacin-treatment, nor HDAC6 knockdown reversed this phenotype to normal, but instead increased it further. This highlights the possibility that the control of contractile force requires separate functions of HDAC6, than the control of cell adhesions, spreading and shape. These data are consistent with the possibility that defective force-transduction from the extracellular environment to the nucleus contributes to metastasis, via a mechanism that depends upon HDAC6. To our knowledge, our findings present the first correlation between the cellular forces that deforms the surrounding environment and the nucleus in fibroblasts, and it expands our understanding of how cells generate contractile forces that contribute to cell invasion and metastasis.

Induced pluripotent stem cell technology in bone biology

Technologies on the development and differentiation of human induced pluripotent stem cells (hiPSCs) are rapidly improving, and have been applied to create cell types relevant to the bone field. Differentiation protocols to form bona fide bone-forming cells from iPSCs are available, and can be used to probe details of differentiation and function in depth. When applied to iPSCs bearing disease-causing mutations, the pathogenetic mechanisms of diseases of the skeleton can be elucidated, along with the development of novel therapeutics. These cells can also be used for development of cell therapies for cell and tissue replacement.

Molecular Mechanisms of Tumor Cell Stemness Modulation during Formation of Spheroids

Cancer stem cells (CSCs), their properties and interaction with microenvironment are of interest in modern medicine and biology. There are many studies on the emergence of CSCs and their involvement in tumor pathogenesis. The most important property inherent to CSCs is their stemness. Stemness combines ability of the cell to maintain its pluripotency, give rise to differentiated cells, and interact with environment to maintain a balance between dormancy, proliferation, and regeneration. While adult stem cells exhibit these properties by participating in tissue homeostasis, CSCs behave as their malignant equivalents. High tumor resistance to therapy, ability to differentiate, activate angiogenesis and metastasis arise precisely due to the stemness of CSCs. These cells can be used as a target for therapy of different types of cancer. Laboratory models are needed to study cancer biology and find new therapeutic strategies. A promising direction is three-dimensional tumor models or spheroids. Such models exhibit properties resembling stemness in a natural tumor. By modifying spheroids, it becomes possible to investigate the effect of therapy on CSCs, thus contributing to the development of anti-tumor drug test systems. The review examines the niche of CSCs, the possibility of their study using three-dimensional spheroids, and existing markers for assessing stemness of CSCs.

Fatty Acid Metabolites and the Tumor Microenvironment as Potent Regulators of Cancer Stem Cell Signaling

Individual cancer cells are not equal but are organized into a cellular hierarchy in which only a rare few leukemia cells can self-renew in a manner reminiscent of the characteristic stem cell properties. The PI3K/AKT pathway functions in a variety of cancers and plays a critical role in the survival and proliferation of healthy cells under physiologic conditions. In addition, cancer stem cells might exhibit a variety of metabolic reprogramming phenotypes that cannot be completely attributed to the intrinsic heterogeneity of cancer. Given the heterogeneity of cancer stem cells, new strategies with single-cell resolution will become a powerful tool to eradicate the aggressive cell population harboring cancer stem cell phenotypes. Here, this article will provide an overview of the most important signaling pathways of cancer stem cells regarding their relevance to the tumor microenvironment and fatty acid metabolism, suggesting valuable strategies among cancer immunotherapies to inhibit the recurrence of tumors.

Atropine Is a Suppressor of Epithelial–Mesenchymal Transition (EMT) That Reduces Stemness in Drug-Resistant Breast Cancer Cells

Atropine (ATR) is extracted from a belladonna plant that belongs to a class of anticholinergic drugs and is therefore involved in the treatment of the overdose of cholinergic drugs or mushroom poisoning. It is a well-known blocker of muscarinic acetylcholine receptors (mAChRs) that are expressed in various tumor cells, including breast tumors from animal and human origin, but it has yet to be recommended as an anticancer drug. Our in silico docking analysis indicates that atropine has a roust virtual binding, with a stable binding energy, to two major signaling molecules involved in EMT regulation: E-cad and ZEB-2. For both, the gene and the protein expression level results show that atropine is an effective molecule in reducing epithelial–mesenchymal transition (EMT) and colony formation induced by TGF-B or carboplatin in both the mesenchymal-like cell line MDA-MB-231 and the epithelial-like cell line T47D. We conclude that atropine as a potential suppressor of EMT could be co-administrated with other chemotherapeutic drugs to reduce stemness in drug-resistant breast tumor cells.

Disparate progenitor cell populations contribute to maintenance and repair neurogenesis in the zebrafish olfactory epithelium

Olfactory sensory neurons (OSNs) undergo constant turnover under physiological conditions but also regenerate efficiently following tissue injury. Maintenance and repair neurogenesis in the olfactory epithelium (OE) have been attributed to the selective activity of globose (GBCs) and horizontal basal cells (HBCs), respectively. In zebrafish, cells with GBC-like properties are localized to the peripheral margins of the sensory OE and contribute to OSN neurogenesis in the intact OE, while cells that resemble HBCs at the morphological and molecular level are more uniformly distributed. However, the contribution of these cells to the restoration of the injured OE has not been demonstrated. Here, we provide a detailed cellular and molecular analysis of the tissue response to injury and show that a dual progenitor cell system also exists in zebrafish. Zebrafish HBCs respond to the structural damage of the OE and generate a transient population of proliferative neurogenic progenitors that restores OSNs. In contrast, selective ablation of OSNs by axotomy triggers neurogenic GBC proliferation, suggesting that distinct signaling events activate GBC and HBC responses. Molecular analysis of differentially expressed genes in lesioned and regenerating OEs points toward an involvement of the canonical Wnt/β-catenin pathway. Activation of Wnt signaling appears to be sufficient to stimulate mitotic activity, while inhibition significantly reduces, but does not fully eliminate, HBC responses. Zebrafish HBCs are surprisingly active even under physiological conditions with a strong bias toward the zones of constitutive OSN neurogenesis, suggestive of a direct lineage relationship between progenitor cell subtypes.

Ciliary signaling in stem cells in health and disease: Hedgehog pathway and beyond

The primary cilium is a hair-like sensory compartment that protrudes from the cellular surface. The primary cilium is enriched in a variety of signaling molecules that regulate cellular activities. Stem cells have primary cilia. They reside in a specialized environment, called the stem cell niche. This niche contains a variety of secreted factors, and some of their receptors are localized in the primary cilia of stem cells. Here, we summarize the current understanding of the function of cilia in compartmentalized signaling in stem cells. We describe how ciliary signaling regulates stem cells and progenitor cells during development, tissue homeostasis and tumorigenesis. We summarize our understanding of cilia regulated signaling -primary involving the hedgehog pathway- in stem cells in diverse settings that include neuroepithelial cells, radial glia, cerebellar granule neuron precursors, hematopoietic stem cells, hair follicle stem cells, bone marrow mesenchymal stem cells and mammary gland stem cells. Overall, our review highlights a variety of roles that ciliary signaling plays in regulating stem cells throughout life.

Overexpression of IGF-1 During Early Development Expands the Number of Mammary Stem Cells and Primes them for Transformation

Insulin-like growth factor I (IGF-1) has been implicated in breast cancer due to its mitogenic and anti-apoptotic effects. Despite substantial research on the role of IGF-1 in tumor progression, the relationship of IGF-1 to tissue stem cells, particularly in mammary tissue, and the resulting tumor susceptibility has not been elucidated. Previous studies with the BK5.IGF-1 transgenic (Tg) mouse model reveals that IGF-1 does not act as a classical, post-carcinogen tumor promoter in the mammary gland. Pre-pubertal Tg mammary glands display increased numbers and enlarged sizes of terminal end buds, a niche for mammary stem cells (MaSCs). Here we show that MaSCs from both wild type (WT) and Tg mice expressed IGF-1R and that overexpression of Tg IGF-1 increased numbers of MaSCs by undergoing symmetric division, resulting in an expansion of the MaSC and luminal progenitor (LP) compartments in pre-pubertal female mice. This expansion was maintained post-pubertally and validated by mammosphere assays in vitro and transplantation assays in vivo. The addition of recombinant IGF-1 promoted, and IGF-1R downstream inhibitors decreased mammosphere formation. Single-cell transcriptomic profiles generated from two related platforms reveal that IGF-1 stimulated quiescent MaSCs to enter the cell cycle and increased their expression of genes involved in proliferation, plasticity, tumorigenesis, invasion, and metastasis. This study identifies a novel, pro-tumorigenic mechanism, where IGF-1 increases the number of transformation-susceptible carcinogen targets during the early stages of mammary tissue development, and “primes” their gene expression profiles for transformation.

Computational Methods to Identify Cell-Fate Determinants, Identity Transcription Factors, and Niche-Induced Signaling Pathways for Stem Cell Research

The large-scale development of high-throughput sequencing technologies has not only allowed the generation of reliable omics data related to various regulatory layers but also the development of novel computational models in the field of stem cell research. These computational approaches have enabled the disentangling of a complex interplay between these interrelated layers of regulation by interpreting large quantities of biomedical data in a systematic way. In the context of stem cell research, network modeling of complex gene-gene interactions has been successfully used for understanding the mechanisms underlying stem cell differentiation and cellular conversion. Notably, it has proven helpful for predicting cell-fate determinants and signaling molecules controlling such processes. This chapter will provide an overview of various computational approaches that rely on single-cell and/or bulk RNA sequencing data for elucidating the molecular underpinnings of cell subpopulation identities, lineage specification, and the process of cell-fate decisions. Furthermore, we discuss how these computational methods provide the right framework for computational modeling of biological systems in order to address long-standing challenges in the stem cell field by guiding experimental efforts in stem cell research and regenerative medicine.

The use of adipose-derived stem cells in cell assisted lipotransfer as potential regenerative therapy in breast reconstruction

Breast reconstruction for breast cancer patients is performed as a standard of care to improve patients' quality of life, physical and psychosocial well-being. Stem cell therapy holds a promise in regenerative medicine, including in breast reconstruction. This review explores the potential use of adipose-derived stem cells (ADSCs) in cell assisted lipotransfer (CAL) for reconstruction of the breast. The review of literature was done using electronic databases using appropriate keywords, including "adipose-derived stem cell", "stem cell therapy", "adipose-derived stem cell", "cell-assisted lipotransfer", "regenerative therapy", "breast cancer" and "breast reconstruction", with literatures limited to ten years post publication. Adipose-derived stem cells are multipotent cells with angiogenic and immunomodulatory potential. Several studies reveal ADSCs use in CAL results in long-term breast volume retention suggesting improved fat graft survival. Some conflicting outcomes are also discussed, potentially related to numbers of cells enriched and factors affecting the cells' microenvironment. The use of ADSCs in CAL may be beneficial for therapy of breast reconstruction in breast cancer patients after surgical management. Further investigation would be needed to improve the confidence of its clinical use.

Exploring microenvironment strategies to delay mesenchymal stem cell senescence

Mesenchymal stem cells (MSCs) have recently emerged as an important candidate for cell therapy and tissue regeneration. However, some limitations in translational research and therapies still exist, such as insufficient cell supply, inadequate differentiation potential, and decreased immune capacity, all of which result from replicative senescence during long-term in vitro culture. In vitro, stem cells lack a protective microenvironment owing to the absence of physical and biochemical cues compared with the in vivo niche, which provides dynamic physicochemical and biological cues. This difference results in accelerated aging after long-term in vitro culture. Therefore, it remains a great challenge to delay replicative senescence in culture. Constructing a microenvironment to delay replicative senescence of MSCs by maintaining their phenotypes, properties, and functions is a feasible strategy to solve this problem and has made measurable progress both in preclinical studies and clinical trials. Here, we review the current knowledge on the characteristics of senescent MSCs, explore the molecular mechanisms of MSCs senescence, describe the niche of MSCs, and discuss some current microenvironment strategies to delay MSCs replicative senescence that can broaden their range of therapeutic applications.

Stem Cell-Based Therapies: What Interventional Radiologists Need to Know

As the basic units of biological organization, stem cells and their progenitors are essential for developing and regenerating organs and tissue systems using their unique self-renewal capability and differentiation potential into multiple cell lineages. Stem cells are consistently present throughout the entire human development, from the zygote to adulthood. Over the past decades, significant efforts have been made in biology, genetics, and biotechnology to develop stem cell-based therapies using embryonic and adult autologous or allogeneic stem cells for diseases without therapies or difficult to treat. Stem cell-based therapies require optimum administration of stem cells into damaged organs to promote structural regeneration and improve function. Maximum clinical efficacy is highly dependent on the successful delivery of stem cells to the target tissue. Direct image-guided locoregional injections into target tissues offer an option to increase therapeutic outcomes. Interventional radiologists have the opportunity to perform a key role in delivering stem cells more efficiently using minimally invasive techniques. This review discusses the types and sources of stem cells and the current clinical applications of stem cell-based therapies. In addition, the regulatory considerations, logistics, and potential roles of interventional Radiology are also discussed with the review of the literature.

The butyrophilin 1a1 knockout mouse revisited: Ablation of Btn1a1 leads to concurrent cell death and renewal in the mammary epithelium during lactation

Butyrophilin 1A1 (BTN1A1) is implicated in the secretion of lipid droplets from mammary epithelial cells as a membrane receptor, which forms a secretion complex with the redox enzyme, xanthine oxidoreductase (XDH). The first evidence that BTN1A1 functions in this process was the generation of Btn1a1 -/- mouse lines, in which lipid secretion was disrupted and large unstable droplets were released into alveolar spaces with fragmented surface membranes. We have revisited one of these mutant mouse lines using RNAseq and proteomic analysis to assess the consequences of ablating the Btn1a1 gene on the expression of other genes and proteins. Disruption of intact Btn1a1 protein expression led to a large build-up of Xdh in the cytoplasm, induction of acute phase response genes and Lif-activation of Stat3 phosphorylation. At peak lactation, approx. 10% of the cells were dying, as assessed by TUNEL-analysis of nuclear DNA. Possible cell death pathways included expression of caspase 8 and activated caspase 3, autophagy, Slc5a8-mediated inactivation of survivin (Birc5), and pStat3-mediated lysosomal lysis, the latter of which is the principal death route in involuting wild type cells. Milk secretion was prolonged by renewal of the secretory epithelium, as evidenced by the upregulation of Ki67 in approx. 10% of cell nuclei and expression of cyclins and Fos/Jun. These data highlight the plasticity of the mammary epithelium and the importance of functional BTN1A1 expression for maintenance of terminally differentiated secretory cells and optimal milk production throughout lactation.

The mutational landscape of human somatic and germline cells

Over the course of an individual's lifetime, normal human cells accumulate mutations1. Here we compare the mutational landscape in 29 cell types from the soma and germline using multiple samples from the same individuals. Two ubiquitous mutational signatures, SBS1 and SBS5/40, accounted for the majority of acquired mutations in most cell types, but their absolute and relative contributions varied substantially. SBS18, which potentially reflects oxidative damage2, and several additional signatures attributed to exogenous and endogenous exposures contributed mutations to subsets of cell types. The rate of mutation was lowest in spermatogonia, the stem cells from which sperm are generated and from which most genetic variation in the human population is thought to originate. This was due to low rates of ubiquitous mutational processes and may be partially attributable to a low rate of cell division in basal spermatogonia. These results highlight similarities and differences in the maintenance of the germline and soma.

Dynamic regulation of mitochondrial-endoplasmic reticulum crosstalk during stem cell homeostasis and aging

Cellular therapy exerts profound therapeutic potential for curing a broad spectrum of diseases. Adult stem cells reside within a specified dynamic niche in vivo, which is essential for continuous tissue homeostatic maintenance through balancing self-renewal with lineage selection. Meanwhile, adult stem cells may be multipotent or unipotent, and are present in both quiescent and actively dividing states in vivo of the mammalians, which may switch to each other state in response to biophysical cues through mitochondria-mediated mechanisms, such as alterations in mitochondrial respiration and metabolism. In general, stem cells facilitate tissue repair after tissue-specific homing through various mechanisms, including immunomodulation of local microenvironment, differentiation into functional cells, cell "empowerment" via paracrine secretion, immunoregulation, and intercellular mitochondrial transfer. Interestingly, cell-source-specific features have been reported between different tissue-derived adult stem cells with distinct functional properties due to the different microenvironments in vivo, as well as differential functional properties in different tissue-derived stem cell-derived extracellular vehicles, mitochondrial metabolism, and mitochondrial transfer capacity. Here, we summarized the current understanding on roles of mitochondrial dynamics during stem cell homeostasis and aging, and lineage-specific differentiation. Also, we proposed potential unique mitochondrial molecular signature features between different source-derived stem cells and potential associations between stem cell aging and mitochondria-endoplasmic reticulum (ER) communication, as well as potential novel strategies for anti-aging intervention and healthy aging.

Acute kidney injury promotes development of papillary renal cell adenoma and carcinoma from renal progenitor cells

Acute tissue injury causes DNA damage and repair processes involving increased cell mitosis and polyploidization, leading to cell function alterations that may potentially drive cancer development. Here, we show that acute kidney injury (AKI) increased the risk for papillary renal cell carcinoma (pRCC) development and tumor relapse in humans as confirmed by data collected from several single-center and multicentric studies. Lineage tracing of tubular epithelial cells (TECs) after AKI induction and long-term follow-up in mice showed time-dependent onset of clonal papillary tumors in an adenoma-carcinoma sequence. Among AKI-related pathways, NOTCH1 overexpression in human pRCC associated with worse outcome and was specific for type 2 pRCC. Mice overexpressing NOTCH1 in TECs developed papillary adenomas and type 2 pRCCs, and AKI accelerated this process. Lineage tracing in mice identified single renal progenitors as the cell of origin of papillary tumors. Single-cell RNA sequencing showed that human renal progenitor transcriptome showed similarities to PT1, the putative cell of origin of human pRCC. Furthermore, NOTCH1 overexpression in cultured human renal progenitor cells induced tumor-like 3D growth. Thus, AKI can drive tumorigenesis from local tissue progenitor cells. In particular, we find that AKI promotes the development of pRCC from single progenitors through a classical adenoma-carcinoma sequence.

Epithelial cell plasticity: breaking boundaries and changing landscapes

Epithelial tissues respond to a wide variety of environmental and genotoxic stresses. As an adaptive mechanism, cells can deviate from their natural paths to acquire new identities, both within and across lineages. Under extreme conditions, epithelial tissues can utilize "shape-shifting" mechanisms whereby they alter their form and function at a tissue-wide scale. Mounting evidence suggests that in order to acquire these alternate tissue identities, cells follow a core set of "tissue logic" principles based on developmental paradigms. Here, we review the terminology and the concepts that have been put forward to describe cell plasticity. We also provide insights into various cell intrinsic and extrinsic factors, including genetic mutations, inflammation, microbiota, and therapeutic agents that contribute to cell plasticity. Additionally, we discuss recent studies that have sought to decode the "syntax" of plasticity-i.e., the cellular and molecular principles through which cells acquire new identities in both homeostatic and malignant epithelial tissues-and how these processes can be manipulated for developing novel cancer therapeutics.

Stem cells of aquatic invertebrates as an advanced tool for assessing ecotoxicological impacts

Environmental stressors are assessed through methods that quantify their impacts on a wide range of metrics including species density, growth rates, reproduction, behaviour and physiology, as on host-pathogen interactions and immunocompetence. Environmental stress may induce additional sublethal effects, like mutations and epigenetic signatures affecting offspring via germline mediated transgenerational inheritance, shaping phenotypic plasticity, increasing disease susceptibility, tissue pathologies, changes in social behaviour and biological invasions. The growing diversity of pollutants released into aquatic environments requires the development of a reliable, standardised and 3R (replacement, reduction and refinement of animals in research) compliant in vitro toolbox. The tools have to be in line with REACH regulation 1907/2006/EC, aiming to improve strategies for potential ecotoxicological risks assessment and monitoring of chemicals threatening human health and aquatic environments. Aquatic invertebrates' adult stem cells (ASCs) are numerous and can be pluripotent, as illustrated by high regeneration ability documented in many of these taxa. This is of further importance as in many aquatic invertebrate taxa, ASCs are able to differentiate into germ cells. Here we propose that ASCs from key aquatic invertebrates may be harnessed for applicable and standardised new tests in ecotoxicology. As part of this approach, a battery of modern techniques and endpoints are proposed to be tested for their ability to correctly identify environmental stresses posed by emerging contaminants in aquatic environments. Consequently, we briefly describe the current status of the available toxicity testing and biota-based monitoring strategies in aquatic environmental ecotoxicology and highlight some of the associated open issues such as replicability, consistency and reliability in the outcomes, for understanding and assessing the impacts of various chemicals on organisms and on the entire aquatic environment. Following this, we describe the benefits of aquatic invertebrate ASC-based tools for better addressing ecotoxicological questions, along with the current obstacles and possible overhaul approaches.

A stem cell population at the anorectal junction maintains homeostasis and participates in tissue regeneration

At numerous locations of the body, transition zones are localized at the crossroad between two types of epithelium and are frequently associated with neoplasia involving both type of tissues. These transition zones contain cells expressing markers of adult stem cells that can be the target of early transformation. The mere fact that transition zone cells can merge different architecture with separate functions implies for a unique plasticity that these cells must display in steady state. However, their roles during tissue regeneration in normal and injured state remain unknown. Here, by using in vivo lineage tracing, single-cell transcriptomics, computational modeling and a three-dimensional organoid culture system of transition zone cells, we identify a population of Krt17+ basal cells with multipotent properties at the squamo-columnar anorectal junction that maintain a squamous epithelium during normal homeostasis and can participate in the repair of a glandular epithelium following tissue injury.

Linking EMT programmes to normal and neoplastic epithelial stem cells

Epithelial stem cells serve critical physiological functions in the generation, maintenance and repair of diverse tissues through their ability to self-renew and spawn more specialized, differentiated cell types. In an analogous fashion, cancer stem cells have been proposed to fuel the growth, progression and recurrence of many carcinomas. Activation of an epithelial-mesenchymal transition (EMT), a latent cell-biological programme involved in development and wound healing, has been linked to the formation of both normal and neoplastic stem cells, but the mechanistic basis underlying this connection remains unclear. In this Perspective, we outline the instances where aspects of an EMT have been implicated in normal and neoplastic epithelial stem cells and consider the involvement of this programme during tissue regeneration and repair. We also discuss emerging concepts and evidence related to the heterogeneous and plastic cell states generated by EMT programmes and how these bear on our understanding of cancer stem cell biology and cancer metastasis. A more comprehensive accounting of the still-elusive links between EMT programmes and the stem cell state will surely advance our understanding of both normal stem cell biology and cancer pathogenesis.

Wnt signaling and mammary stem cells

Wnt signaling is an important morphogenetic signaling pathway best known for its essential role in determining embryonic cell fates; it is often activated to re-specify fetal cells or to maintain the lineage flexibility of somatic stem cells. In this review, we consider the role of this pathway in the remarkable process of differentiation, growth and morphogenesis of the mammary gland during embryogenesis, ductal outgrowth and pregnancy. Specifically, mammary stem cells are compared with stem cells from other tissues, to identify commonalities and differences. Wnt signaling is known to be required to maintain the bipotent basal stem cell present in adult mammary ductal trees, however, the absence of this stem cell has little effect on growth or morphogenesis, and Wnt signaling is not induced during the ductal/alveolar expansion during pregnancy. The evidence for pre-determined hierarchies of mammary epithelial cells is reviewed, together with the role of signaling between mixtures of specified mammary epithelial cells in the maintenance of Wnt-dependent clonagenic stem cells. The dazzling variety of Wnt signaling components expressed by mammary epithelial cells is presented, along with some potential stromal sources of Wnt proteins that may be important starting points for the induction of plasticity in the epithelium.

Expression profile of intestinal stem cell and cancer stem cell markers in gastric cancers with submucosal invasion

Cancer stem cells (CSCs) are believed to be responsible for tumor growth, invasion, and metastasis. Submucosal invasion, which greatly enhances metastasis risk, is a critical step in gastric cancer (GC) progression. To identify stem cell-related markers associated with submucosal invasion and lymph node (LN) metastasis in GCs, we investigated the expression of candidate CSC markers (CD133, CD44, and ALDH1A) and intestinal stem cell (ISC) markers (EPHB2, OLFM4, and LGR5) in early GCs that manifested submucosal invasion. We discovered that EPHB2 and LGR5 expression was frequently confined to the basal area of the lamina propria (basal pattern) in mucosal cancer, and the proportion of stem cell marker-positive cells substantially increased during submucosal invasion. CD44 expression showed a focal pattern, ALDH1A was predominantly expressed diffusely, and there was no expansion of CD44 or ALDH1A expression in the submucosal cancer cells. Unexpectedly, no CSC markers showed any associations with LN metastasis, and only loss of EPHB2 expression was associated with increased LN metastasis. Treatment of RSPO2, a niche factor, along with Wnt 3a, to GC cells led to increased EPHB2 and LGR5 mRNA levels. RNA in situ hybridization confirmed specific RSPO2 expression in the smooth muscle cells of the muscularis mucosa, suggesting that RSPO2 is responsible for the increased expression of ISC markers in GC cells at the basal areas. In summary, no stem cell markers were associated with increased LN metastasis in early GCs. Conversely, isolated EPHB2 expression was associated with lower LN metastasis. EPHB2 and LGR5 showed a basal distribution pattern along with enhanced expression in submucosal invading cells in early GCs, which was induced by a niche factor, RSPO2, from the muscularis mucosa.

Pluripotent Stem Cells: Cancer Study, Therapy, and Vaccination

INTRODUCTION

Pluripotent stem cells (PSCs) are promising tools for modern regenerative medicine applications because of their stemness properties, which include unlimited self-renewal and the ability to differentiate into all cell types in the body. Evidence suggests that a rare population of cells within a tumor, termed cancer stem cells (CSCs), exhibit stemness and phenotypic plasticity properties that are primarily responsible for resistance to chemotherapy, radiotherapy, metastasis, cancer development, and tumor relapse. Different therapeutic approaches that target CSCs have been developed for tumor eradication.

RESULTS AND DISCUSSION

In this review, we first provide an overview of different viewpoints about the origin of CSCs. Particular attention has been paid to views believe that CSCs are probably appeared through dysregulation of very small embryonic-like stem cells (VSELs) which reside in various tissues as the main candidate for tissue-specific stem cells. The expression of pluripotency markers in these two types of cells can strengthen the validity of this theory. In this regard, we discuss the common properties of CSCs and PSCs, and highlight the potential of PSCs in cancer studies, therapeutic applications, as well as educating the immune system against CSCs.

CONCLUSION

In conclusion, the resemblance of CSCs to PSCs can provide an appropriate source of CSC-specific antigens through cultivation of PSCs which brings to light promising ideas for prophylactic and therapeutic cancer vaccine development.

[Roles of autophagy onself-renewal and differentiation of mesenchymal stem cells]

Mesenchymal stem cells (MSCs), which have the potential of self-replication and differentiation, are a very valuable cell source for stem cell-based medical therapy. Their application has opened up a new way for disease research. Although MSCs can maintain cell stemness through self-renewal, with the prolongation of cell passage and culture time, the stemness of MSCs gradually decays, and the cell aging and differentiation potential decreases gradually. Autophagy is a highly conserved cytological process that degrades the modified, excess, and deleterious cytoplasmic components in autophagosomes, which are then degraded by fusion with lysosomes. As the main intracellular degradation and recycling pathway, autophagy plays an active role in maintaining cell homeostasis, self-renewal and pluripotency. In this paper, the role of autophagy in self-renewal and maintenance of multidirectional differentiation potential of MSCs was reviewed, which laid a theoretical foundation and practical basis for the research and application of MSCs.

The effect of radiation on the ability of rat mammary cells to form mammospheres

As classical transplantation repopulation assays for studying the radiobiology of rat mammary stem/progenitor cells are extremely time-consuming, this study aimed to characterize the radiobiological properties of mammospheres, spherical clumps of mammary cells formed under non-adherent culture conditions, which are a simple and widely used technique for assessing progenitor cell activity. Rat mammary cells were dissociated and used in transplantation repopulation assays and for the formation of mammospheres. Immunofluorescence for cytokeratin 14 and 18 was used to identify basal and luminal mammary epithelial cells, respectively. Incorporation of 5-bromo-2'-deoxyuridine was used to evaluate cell proliferation. The repopulating activity of the transplanted primary rat mammary cells demonstrated their radiosensitivity, reproducing previous data, with a significant reduction in repopulating activity at ≥ 2 Gy. Cells constituting rat mammospheres were positive for either cytokeratin 14 or 18, with occasional double-positive cells. Both proliferation and aggregation contributed to sphere formation. Cells obtained from the spheres showed lower repopulating activity after transplantation than primary cells. When primary cells were irradiated and then used for sphere formation, the efficiency of sphere formation was significantly decreased at 8 Gy but not at ≤ 6 Gy, indicating radioresistance of the formation process. Irradiation at 8 Gy reduced the proliferation of cells during sphere formation, whereas the cellular composition of the resulting spheres was unaffectes. Thus, mammosphere formation assays may measure a property of putative mammary progenitors that is different from what is measured in the classic transplantation repopulation assay in radiobiology.

One stem cell program to rule them all?

Many species of animals have stem cells that they maintain throughout their lives, which suggests that stem cells are an ancestral feature of all animals. From this, we take the viewpoint that cells with the biological properties of 'stemness'-self-renewal and multipotency-may share ancestral genetic circuitry. However, in practice is it very difficult to identify and compare stemness gene signatures across diverse animals and large evolutionary distances? First, it is critical to experimentally demonstrate self-renewal and potency. Second, genomic methods must be used to determine specific gene expression in stem cell types compared with non-stem cell types to determine stem cell gene enrichment. Third, gene homology must be mapped between diverse animals across large evolutionary distances. Finally, conserved genes that fulfill these criteria must be tested for role in stem cell function. It is our viewpoint that by comparing stem cell-specific gene signatures across evolution, ancestral programs of stemness can be uncovered, and ultimately, the dysregulation of stemness programs drives the state of cancer stem cells.

The science and engineering of stem cell-derived organoids-examples from hepatic, biliary, and pancreatic tissues

The field of organoid engineering promises to revolutionize medicine with wide-ranging applications of scientific, engineering, and clinical interest, including precision and personalized medicine, gene editing, drug development, disease modelling, cellular therapy, and human development. Organoids are a three-dimensional (3D) miniature representation of a target organ, are initiated with stem/progenitor cells, and are extremely promising tools with which to model organ function. The biological basis for organoids is that they foster stem cell self-renewal, differentiation, and self-organization, recapitulating 3D tissue structure or function better than two-dimensional (2D) systems. In this review, we first discuss the importance of epithelial organs and the general properties of epithelial cells to provide a context and rationale for organoids of the liver, pancreas, and gall bladder. Next, we develop a general framework to understand self-organization, tissue hierarchy, and organoid cultivation. For each of these areas, we provide a historical context, and review a wide range of both biological and mathematical perspectives that enhance understanding of organoids. Next, we review existing techniques and progress in hepatobiliary and pancreatic organoid engineering. To do this, we review organoids from primary tissues, cell lines, and stem cells, and introduce engineering studies when applicable. We discuss non-invasive assessment of organoids, which can reveal the underlying biological mechanisms and enable improved assays for growth, metabolism, and function. Applications of organoids in cell therapy are also discussed. Taken together, we establish a broad scientific foundation for organoids and provide an in-depth review of hepatic, biliary and pancreatic organoids.

Erk and MAPK signaling is essential for intestinal development through Wnt pathway modulation

Homeostasis of intestinal stem cells (ISCs) is maintained by the orchestration of niche factors and intrinsic signaling networks. Here, we have found that deletion of Erk1 and Erk2 (Erk1/2) in intestinal epithelial cells at embryonic stages resulted in an unexpected increase in cell proliferation and migration, expansion of ISCs, and formation of polyp-like structures, leading to postnatal death. Deficiency of epithelial Erk1/2 results in defects in secretory cell differentiation as well as impaired mesenchymal cell proliferation and maturation. Deletion of Erk1/2 strongly activated Wnt signaling through both cell-autonomous and non-autonomous mechanisms. In epithelial cells, Erk1/2 depletion resulted in loss of feedback regulation, leading to Ras/Raf cascade activation that transactivated Akt activity to stimulate the mTor and Wnt/β-catenin pathways. Moreover, Erk1/2 deficiency reduced the levels of Indian hedgehog and the expression of downstream pathway components, including mesenchymal Bmp4 - a Wnt suppressor in intestines. Inhibition of mTor signaling by rapamycin partially rescued Erk1/2 depletion-induced intestinal defects and significantly prolonged the lifespan of mutant mice. These data demonstrate that Erk/Mapk signaling functions as a key modulator of Wnt signaling through coordination of epithelial-mesenchymal interactions during intestinal development.

Single-cell RNA-sequencing reveals distinct patterns of cell state heterogeneity in mouse models of breast cancer

Breast cancer stem cells (BCSCs) contribute to intra-tumoral heterogeneity and therapeutic resistance. However, the binary concept of universal BCSCs co-existing with bulk tumor cells is over-simplified. Through single-cell RNA-sequencing, we found that Neu, PyMT and BRCA1-null mammary tumors each corresponded to a spectrum of minimally overlapping cell differentiation states without a universal BCSC population. Instead, our analyses revealed that these tumors contained distinct lineage-specific tumor propagating cells (TPCs) and this is reflective of the self-sustaining capabilities of lineage-specific stem/progenitor cells in the mammary epithelial hierarchy. By understanding the respective tumor hierarchies, we were able to identify CD14 as a TPC marker in the Neu tumor. Additionally, single-cell breast cancer subtype stratification revealed the co-existence of multiple breast cancer subtypes within tumors. Collectively, our findings emphasize the need to account for lineage-specific TPCs and the hierarchical composition within breast tumors, as these heterogenous sub-populations can have differential therapeutic susceptibilities.

Adipogenesis, Osteogenesis, and Chondrogenesis of Human Mesenchymal Stem/Stromal Cells: A Comparative Transcriptome Approach

Adipogenesis, osteogenesis and chondrogenesis of human mesenchymal stem/stromal cells (MSC) are complex and highly regulated processes. Over the years, several studies have focused on understanding the mechanisms involved in the MSC commitment to the osteogenic, adipogenic and/or chondrogenic phenotypes. High-throughput methodologies have been used to investigate the gene expression profile during differentiation. Association of data analysis of mRNAs, microRNAs, circular RNAs and long non-coding RNAs, obtained at different time points over these processes, are important to depict the complexity of differentiation. This review will discuss the results that were highlighted in transcriptome analyses of MSC undergoing adipogenic, osteogenic and chondrogenic differentiation. The focus is to shed light on key molecules, main signaling pathways and biological processes related to different time points of adipogenesis, osteogenesis and chondrogenesis.

Single-cell transcriptional diversity is a hallmark of developmental potential

Single-cell RNA sequencing (scRNA-seq) is a powerful approach for reconstructing cellular differentiation trajectories. However, inferring both the state and direction of differentiation is challenging. Here, we demonstrate a simple, yet robust, determinant of developmental potential-the number of expressed genes per cell-and leverage this measure of transcriptional diversity to develop a computational framework (CytoTRACE) for predicting differentiation states from scRNA-seq data. When applied to diverse tissue types and organisms, CytoTRACE outperformed previous methods and nearly 19,000 annotated gene sets for resolving 52 experimentally determined developmental trajectories. Additionally, it facilitated the identification of quiescent stem cells and revealed genes that contribute to breast tumorigenesis. This study thus establishes a key RNA-based feature of developmental potential and a platform for delineation of cellular hierarchies.

Tracing tumorigenesis in a solid tumor model at single-cell resolution

Characterizing the complex composition of solid tumors is fundamental for understanding tumor initiation, progression and metastasis. While patient-derived samples provide valuable insight, they are heterogeneous on multiple molecular levels, and often originate from advanced tumor stages. Here, we use single-cell transcriptome and epitope profiling together with pathway and lineage analyses to study tumorigenesis from a developmental perspective in a mouse model of salivary gland squamous cell carcinoma. We provide a comprehensive cell atlas and characterize tumor-specific cells. We find that these cells are connected along a reproducible developmental trajectory: initiated in basal cells exhibiting an epithelial-to-mesenchymal transition signature, tumorigenesis proceeds through Wnt-differential cancer stem cell-like subpopulations before differentiating into luminal-like cells. Our work provides unbiased insights into tumor-specific cellular identities in a whole tissue environment, and emphasizes the power of using defined genetic model systems.

Understanding tumour development at a granular level is a challenge in solid tumours. Here, the authors provide a cell atlas across tumour development in a genetic model of salivary gland squamous cell carcinoma using single-cell transcriptome and epitope profiling.

Endogenous neural precursor cells in health and disease

Neurogenesis persists in the adult brain of mammals in the subventricular zone (SVZ) of the lateral ventricles and in the subgranular zone (SGZ) of the dentate gyrus (DG). The complex interactions between intrinsic and extrinsic signals provided by cells in the niche but also from distant sources regulate the fate of neural stem/progenitor cells (NPCs) in these sites. This fine regulation is perturbed in aging and in pathological conditions leading to a different NPC behavior, tailored to the specific physio-pathological features. Indeed, NPCs exert in physiological and pathological conditions important neurogenic and non-neurogenic regulatory functions and participate in maintaining and protecting brain tissue homeostasis. In this review, we discuss intrinsic and extrinsic signals that regulate NPC activation and NPC functional role in various homeostatic and non-homeostatic conditions.

Environmental exposures, stem cells, and cancer

An estimated 70-90% of all cancers are linked to exposure to environmental risk factors. In parallel, the number of stem cells in a tissue has been shown to be a strong predictor of risk of developing cancer in that tissue. Tumors themselves are characterized by an acquisition of "stem cell" characteristics, and a growing body of evidence points to tumors themselves being sustained and propagated by a stem cell-like population. Here, we review our understanding of the interplay between environmental exposures, stem cell biology, and cancer. We provide an overview of the role of stem cells in development, tissue homeostasis, and wound repair. We discuss the pathways and mechanisms governing stem cell plasticity and regulation of the stem cell state, and describe experimental methods for assessment of stem cells. We then review the current understanding of how environmental exposures impact stem cell function relevant to carcinogenesis and cancer prevention, with a focus on environmental and occupational exposures to chemical, physical, and biological hazards. We also highlight key areas for future research in this area, including defining whether the biological basis for cancer disparities is related to effects of complex exposure mixtures on stem cell biology.

Designing stem cell niches for differentiation and self-renewal

Mesenchymal stem cells, characterized by their ability to differentiate into skeletal tissues and self-renew, hold great promise for both regenerative medicine and novel therapeutic discovery. However, their regenerative capacity is retained only when in contact with their specialized microenvironment, termed the stem cell niche Niches provide structural and functional cues that are both biochemical and biophysical, stem cells integrate this complex array of signals with intrinsic regulatory networks to meet physiological demands. Although, some of these regulatory mechanisms remain poorly understood or difficult to harness with traditional culture systems. Biomaterial strategies are being developed that aim to recapitulate stem cell niches, by engineering microenvironments with physiological-like niche properties that aim to elucidate stem cell-regulatory mechanisms, and to harness their regenerative capacity in vitro In the future, engineered niches will prove important tools for both regenerative medicine and therapeutic discoveries.

Stem Cells and the Differentiation Hierarchy in Mammary Gland Development

The mammary gland is a highly dynamic organ that undergoes profound changes within its epithelium during puberty and the reproductive cycle. These changes are fueled by dedicated stem and progenitor cells. Both short- and long-lived lineage-restricted progenitors have been identified in adult tissue as well as a small pool of multipotent mammary stem cells (MaSCs), reflecting intrinsic complexity within the epithelial hierarchy. While unipotent progenitor cells predominantly execute day-to-day homeostasis and postnatal morphogenesis during puberty and pregnancy, multipotent MaSCs have been implicated in coordinating alveologenesis and long-term ductal maintenance. Nonetheless, the multipotency of stem cells in the adult remains controversial. The advent of large-scale single-cell molecular profiling has revealed striking changes in the gene expression landscape through ontogeny and the presence of transient intermediate populations. An increasing number of lineage cell-fate determination factors and potential niche regulators have now been mapped along the hierarchy, with many implicated in breast carcinogenesis. The emerging diversity among stem and progenitor populations of the mammary epithelium is likely to underpin the heterogeneity that characterizes breast cancer.

Applicability of adipose-derived mesenchymal stem cells in treatment of patients with type 2 diabetes

Type 2 diabetes mellitus (T2DM) is mainly characterized by insulin resistance (IR) and impaired insulin secretion. The chronic inflammatory process contributed to IR and could also hamper pancreatic β cell function. However, currently applied treatment cannot reverse β cell damage or alleviate inflammation. Mesenchymal stem cells (MSCs), the cell-based therapy for their self-renewable, differentiation potential, and immunosuppressive properties, have been demonstrated in displaying therapeutic effects in T2DM. Adipose-derived MSCs (AD-MSCs) attracted more attention due to less harvested inconvenience and ethical issues commonly accompany with bone marrow-derived MSCs (BM-MSCs) and fetal annex-derived MSCs. Both AD-MSC therapy studies and mechanism explorations in T2DM animals presented that AD-MSCs could translate to clinical application. However, hyperglycemia, hyperinsulinemia, and metabolic disturbance in T2DM are crucial for impairment of AD-MSC function, which may limit the therapeutical effects of MSCs. This review focuses on the outcomes and the molecular mechanisms of MSC therapies in T2DM which light up the hope of AD-MSCs as an innovative strategy to cure T2DM.

3D printing with peptide-polymer conjugates for single-step fabrication of spatially functionalized scaffolds

Biodegradable polymer-based scaffolds are widely used to provide support during early stages of regeneration and can be functionalized with various chemical groups or bioactive cues to promote desired cellular behavior. However, these scaffolds are often modified post-fabrication, which can lead to undesired changes and homogeneously distributed chemistries that fail to mimic the spatial biochemical organization found in native tissues. To address these challenges, surface functionalization can be achieved by 3D printing with pre-functionalized biodegradable polymers, such as peptide-modified polymer conjugates, to control the deposition of preferred chemistries. Peptide-PCL conjugates were synthesized with the canonical cell adhesion peptide motif RGDS or its negative control RGES and 3D printed into scaffolds displaying one or both peptides. The peptides were also modified with bioorthogonal groups, biotin and azide, to visualize peptide concentration and location by labeling with complementary fluorophores. Peptide concentration on the scaffold surface increased with increasing peptide-PCL conjugate concentration added to the ink prior to 3D printing, and scaffolds printed with the highest RGDS(biotin)-PCL concentrations showed a significant increase in NIH3T3 fibroblast adhesion. To demonstrate spatial control of peptide functionalization, multiple printer heads were used to print both peptide-PCL conjugates into the same construct in alternating patterns. Cells preferentially attached and spread on RGDS(biotin)-PCL fibers compared to RGES(azide)-PCL fibers, illustrating how spatial functionalization can be used to influence local cell behavior within a single biomaterial. This presents a versatile platform to generate multifunctional biomaterials that can mimic the biochemical organization found in native tissues to support functional regeneration.

Mammary stem cells, where art thou?

Tremendous progress has been made in the field of stem cell biology. This is in part due to the emergence of various vertebrate organs, including the mammary gland, as an amenable model system for adult stem cell studies and remarkable technical advances in single cell technology and modern genetic lineage tracing. In the current review, we summarize the recent progress in mammary gland stem cell biology at both the adult and embryonic stages. We discuss current challenges and controversies, and potentially new and exciting directions for future research. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cell Differentiation and Reversion Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration.

Sexual dimorphism of niche architecture and regulation of the Caenorhabditis elegans germline stem cell pool

Stem cell maintenance by niche signaling is a common theme across phylogeny. In the Caenorhabditis elegans gonad, the broad outlines of germline stem cell (GSC) regulation are the same for both sexes: GLP-1/Notch signaling from the mesenchymal distal tip cell niche maintains GSCs in the distal gonad of both sexes and does so via two key stem cell regulators, SYGL-1 and LST-1. Yet most recent analyses of niche signaling and GSC regulation have focused on XX hermaphrodites, an essentially female sex making sperm in larvae and oocytes in adults. Here we focus on GSC regulation in XO males. Sexual dimorphism of niche architecture, reported previously, suggested that the molecular responses to niche signaling or numbers of GSCs might also be sexually distinct. Remarkably, this is not the case. This work extends our understanding of the sexually dimorphic niche architecture, but also demonstrates that the dimorphic niches drive a similar molecular response and maintain a similar number of GSCs in their stem cell pools.

The invasion of de-differentiating cancer cells into hierarchical tissues

Many fast renewing tissues are characterized by a hierarchical cellular architecture, with tissue specific stem cells at the root of the cellular hierarchy, differentiating into a whole range of specialized cells. There is increasing evidence that tumors are structured in a very similar way, mirroring the hierarchical structure of the host tissue. In some tissues, differentiated cells can also revert to the stem cell phenotype, which increases the risk that mutant cells lead to long lasting clones in the tissue. However, it is unclear under which circumstances de-differentiating cells will invade a tissue. To address this, we developed mathematical models to investigate how de-differentiation is selected as an adaptive mechanism in the context of cellular hierarchies. We derive thresholds for which de-differentiation is expected to emerge, and it is shown that the selection of de-differentiation is a result of the combination of the properties of cellular hierarchy and de-differentiation patterns. Our results suggest that de-differentiation is most likely to be favored provided stem cells having the largest effective self-renewal rate. Moreover, jumpwise de-differentiation provides a wider range of favorable conditions than stepwise de-differentiation. Finally, the effect of de-differentiation on the redistribution of self-renewal and differentiation probabilities also greatly influences the selection for de-differentiation.

How can a tissue such as the blood system or the skin, which constantly produces a huge number of cells, avoids that errors accumulate in the cells over time? Such tissues are typically organized in cellular hierarchies, which induce a directional relation between different stages of cellular differentiation, minimizing the risk of retention of mutations. However, recent evidence also shows that some differentiated cells can de-differentiate into the stem cell phenotype. Why does de-differentiation arise in some tumors, but not in others? We developed a mathematical model to study the growth competition between de-differentiating mutant cell populations and non de-differentiating resident cell population. Our results suggest that the invasion of de-differentiation is jointly influenced by the cellular hierarchy (e.g. number of cell compartments, inherent cell division pattern) and the de-differentiation pattern, i.e. how exactly cells acquire their stem-cell like properties.

Concise Review: A Critical Evaluation of Criteria Used to Define Salivary Gland Stem Cells

In the effort to develop cell-based therapies to treat salivary gland dysfunction, many different populations of cells in the adult salivary glands have been proposed as stem cells. These cell populations vary, depending on the assay used, and are often nonoverlapping, leading to the conclusion that salivary glands harbor multiple stem cells. The goal of this review is to critically appraise the assays and properties used to identify stem cells in the adult salivary gland, and to consider the caveats of each. Re-evaluation of the defining criteria may help to reconcile the many potential stem cell populations described in the salivary gland, in order to increase comparability between studies and build consensus in the field. Stem Cells 2019;37:1144-1150.

Ex vivo HSC expansion challenges the paradigm of unidirectional human hematopoiesis

Understanding mechanisms that determine the behavior of human hematopoietic stem cells (HSCs) is essential for developing novel strategies to expand ex vivo the number of fully functional HSCs. In this review, we focus on the complex interplay between intrinsic mechanisms regulated by transcriptional and mitochondrial networks and extrinsic signals imposed by the bone marrow microenvironment, which in concert regulate the balance between HSC self‐renewal and differentiation. Such integrated signaling mechanisms that dictate the fate of HSCs in vivo must be recapitulated ex vivo to achieve successful expansion of clinically relevant HSCs. We also highlight some of the most recent ex vivo HSC expansion strategies that have currently entered clinical development. Finally, based on the evidence reviewed here and lessons learned from ex vivo HSC expansion, we raise some critical questions regarding HSC fate and the cellular plasticity of hematopoietic cells that challenge the unidirectional model of human hematopoiesis.

Cancer stem cells in breast and prostate: Fact or fiction?

Since the introduction of the cancer stem cell (CSC) hypothesis, accumulating evidence shows that most cancers present stem-like niches. However, therapies aimed at targeting this niche have not been as successful as expected. New evidence regarding CSCs hierarchy, similarities with normal tissue stem cells and cell plasticity might be key in understanding their role in cancer biology and how to efficiently eliminate them. In this Chapter, we discuss what is known in breast and prostate CSCs from their initial discoveries to the current therapeutic efforts in the field. Future challenges towards better CSC identification and isolation strategies will be key to shed light into how CSCs could accurately be targeted in combination to traditional therapies to ultimately prolong patient survival.

Prostate Stem Cells and Cancer Stem Cells

Stem/progenitor cells play central roles in processes of organogenesis and tissue maintenance, whereas cancer stem cells (CSCs) are thought to drive tumor malignancy. Here, we review recent progress in the identification and analysis of normal prostate stem/progenitor cells as well as putative CSCs in both genetically engineered mouse models as well as in human tissue. We also discuss studies that have investigated the cell type of origin for prostate cancer. In addition, we provide a critical assessment of methodologies used in stem cell analyses and outline directions for future research.