Dedifferentiation of committed epithelial cells into stem cells in vivo

Sox9 Activation Highlights a Cellular Pathway of Renal Repair in the Acutely Injured Mammalian Kidney

After acute kidney injury (AKI), surviving cells within the nephron proliferate and repair. We identify Sox9 as an acute epithelial stress response in renal regeneration. Translational profiling after AKI revealed a rapid upregulation of Sox9 within proximal tubule (PT) cells, the nephron cell type most vulnerable to AKI. Descendants of Sox9(+) cells generate the bulk of the nephron during development and regenerate functional PT epithelium after AKI-induced reactivation of Sox9 after renal injury. After restoration of renal function post-AKI, persistent Sox9 expression highlights regions of unresolved damage within injured nephrons. Inactivation of Sox9 in PT cells pre-injury indicates that Sox9 is required for the normal course of post-AKI recovery. These findings link Sox9 to cell intrinsic mechanisms regulating development and repair of the mammalian nephron.

Therapeutic use of human renal progenitor cells for kidney regeneration

The ability of the human kidney to repair itself is limited. Consequently, repeated injury can trigger a maladaptive response that is characterized by fibrosis and loss of renal function. The transcription patterns that characterize nephrogenesis in fetal renal progenitor cells (RPCs) are only partially activated during renal repair in adults. Nevertheless, evidence suggests that segment-restricted progenitor resident cells support renal healing in adults. In this Review, we discuss the evidence for the existence of functional human RPCs in adults and their role in renal repair, and consider the controversial issue of whether RPCs are a fixed population or arise through phenotypical plasticity of tubular cells that is mediated by the microenvironment. We also discuss the strategies for generating renal progenitor cells from pluripotent stem cells or differentiated cells and their use in therapy. Finally, we examine preclinical data on the therapeutic use of human fetal cells, adult progenitor cells and adult renal cells.

Stem Cells Show Parental Control

Stem cells interact with their niche to maintain an undifferentiated state. The study by Pardo-Saganta et al. shows that airway basal stem cells maintain secretory daughter cells in airway epithelia through forward regulation, suggesting that stem cells may serve as a niche for their progeny.

Keeping it together: Pulmonary alveoli are maintained by a hierarchy of cellular programs

The application of in vivo genetic lineage tracing has advanced our understanding of cellular mechanisms for tissue renewal in organs with slow turnover, like the lung. These studies have identified an adult stem cell with very different properties than classically understood ones that maintain continuously cycling tissues such as the intestine. A portrait has emerged of an ensemble of cellular programs that replenish the cells that line the gas exchange (alveolar) surface, enabling a response tailored to the extent of cell loss. A capacity for differentiated cells to undergo direct lineage transitions allows for local restoration of proper cell balance at sites of injury. We present these recent findings as a paradigm for how a relatively quiescent tissue compartment can maintain homeostasis throughout a lifetime punctuated by injuries ranging from mild to life-threatening, and discuss how dysfunction or insufficiency of alveolar repair programs produce serious health consequences like cancer and fibrosis.

The cancer cells-of-origin in the gastrointestinal tract: progenitors revisited

A prominent model of tumor progression posits that normal self-renewing and multipotent stem cells(SCs) are the initial target of transformation. This view has been robustly challenged by the recurring observation that transit-amplifying cells and differentiated progenitors can initiate neoplasia outside the SC zone thus qualifying as cancer cells-of-origin. The emerging concept is that a cancer SC and a cancer cell-of-origin are not necessarily the same cell. Importantly, progenitor cells were shown to possess remarkable plasticity and to revert, on demand, to a SC-like state. The present review revisits our early hypothesis that colonic progenitors acquiring a mutant adenomatous polyposis coli gene after exiting the stem zone may serve as genuine cancer cells-of-origin. New findings consonant with this view are examined, and tenable molecular and cellular mechanisms underpinning the plasticity of progenitor cells in the gastrointestinal tract and in other tissues are discussed. The translational impact of cell plasticity is addressed, and recommendations for future research are advanced.

Scarless wound healing: chasing the holy grail

Over 100 million patients acquire scars in the industrialized world each year, primarily as a result of elective operations. Although undefined, the global incidence of scarring is even larger, extending to significant numbers of burn and other trauma-related wounds. Scars have the potential to exert a profound psychological and physical impact on the individual. Beyond aesthetic considerations and potential disfigurement, scarring can result in restriction of movement and reduced quality of life. The formation of a scar following skin injury is a consequence of wound healing occurring through reparative rather than regenerative mechanisms. In this article, the authors review the basic stages of wound healing; differences between adult and fetal wound healing; various mechanical, genetic, and pharmacologic strategies to reduce scarring; and the biology of skin stem/progenitor cells that may hold the key to scarless regeneration.

Principles and mechanisms of regeneration in the mouse model for wound-induced hair follicle neogenesis

Wound‐induced hair follicle neogenesis (WIHN) describes a regenerative phenomenon in adult mammalian skin wherein fully functional hair follicles regenerate de novo in the center of large excisional wounds. Originally described in rats, rabbits, sheep, and humans in 1940−1960, the WIHN phenomenon was reinvestigated in mice only recently. The process of de novo hair regeneration largely duplicates the morphological and signaling features of normal embryonic hair development. Similar to hair development, WIHN critically depends on the activation of canonical WNT signaling. However, unlike hair development, WNT activation in WIHN is dependent on fibroblast growth factor 9 signaling generated by the immune system's γδ T cells. The cellular bases of WIHN remain to be fully characterized; however, the available evidence leaves open the possibility for a blastema‐like mechanism wherein epidermal and/or dermal wound cells undergo epigenetic reprogramming toward a more plastic, embryonic‐like state. De novo hair follicles do not regenerate from preexisting hair‐fated bulge stem cells. This suggests that hair neogenesis is not driven by preexisting lineage‐restricted progenitors, as is the case for amputation‐induced mouse digit tip regeneration, but rather may require a blastema‐like mechanism. The WIHN model is characterized by several intriguing features, which await further explanation. These include (1) the minimum wound size requirement for activating neogenesis, (2) the restriction of hair neogenesis to the wound's center, and (3) imperfect patterning outcomes, both in terms of neogenic hair positioning within the wound and in terms of their orientation. Future enquiries into the WIHN process, made possible by a wide array of available skin‐specific genetic tools, will undoubtedly expand our understanding of the regeneration mechanisms in adult mammals.

Cartilage regeneration for treatment of osteoarthritis: a paradigm for nonsurgical intervention

Osteoarthritis (OA) is associated with articular cartilage abnormalities and affects people of older age: preventative or therapeutic treatment measures for OA and related articular cartilage disorders remain challenging. In this perspective review, we have integrated multiple biological, morphological, developmental, stem cell and homeostasis concepts of articular cartilage to develop a paradigm for cartilage regeneration. OA is conceptually defined as an injury of cartilage that initiates chondrocyte activation, expression of proteases and growth factor release from the matrix. This regenerative process results in the local activation of inflammatory response genes in cartilage without migration of inflammatory cells or angiogenesis. The end results are catabolic and anabolic responses, and it is the balance between these two outcomes that controls remodelling of the matrix and regeneration. A tantalizing clinical clue for cartilage regrowth in OA joints has been observed in surgically created joint distraction. We hypothesize that cartilage growth in these distracted joints may have a biological connection with the size of organs and regeneration. Therefore we propose a novel, practical and nonsurgical intervention to validate the role of distraction in cartilage regeneration in OA. The approach permits normal wake-up activity while during sleep; the index knee is subjected to distraction with a pull traction device. Comparison of follow-up magnetic resonance imaging (MRI) at 3 and 6 months of therapy to those taken before therapy will provide much-needed objective evidence for the use of this mode of therapy for OA. We suggest that the paradigm presented here merits investigation for treatment of OA in knee joints.

Systems biology approach to developing S2RM-based “systems therapeutics” and naturally induced pluripotent stem cells

The degree to, and the mechanisms through, which stem cells are able to build, maintain, and heal the body have only recently begun to be understood. Much of the stem cell’s power resides in the release of a multitude of molecules, called stem cell released molecules (SRM). A fundamentally new type of therapeutic, namely “systems therapeutic”, can be realized by reverse engineering the mechanisms of the SRM processes. Recent data demonstrates that the composition of the SRM is different for each type of stem cell, as well as for different states of each cell type. Although systems biology has been successfully used to analyze multiple pathways, the approach is often used to develop a small molecule interacting at only one pathway in the system. A new model is emerging in biology where systems biology is used to develop a new technology acting at multiple pathways called “systems therapeutics”. A natural set of healing pathways in the human that uses SRM is instructive and of practical use in developing systems therapeutics. Endogenous SRM processes in the human body use a combination of SRM from two or more stem cell types, designated as S²RM, doing so under various state dependent conditions for each cell type. Here we describe our approach in using state-dependent SRM from two or more stem cell types, S²RM technology, to develop a new class of therapeutics called “systems therapeutics.” Given the ubiquitous and powerful nature of innate S²RM-based healing in the human body, this “systems therapeutic” approach using S²RM technology will be important for the development of anti-cancer therapeutics, antimicrobials, wound care products and procedures, and a number of other therapeutics for many indications.

Progenitor cells in proximal airway epithelial development and regeneration

Multiple distinct epithelial domains are found throughout the airway that are distinguishable by location, structure, function, and cell-type composition. Several progenitor cell populations in the proximal airway have been identified to reside in confined microenvironmental niches including the submucosal glands (SMGs), which are embedded in the tracheal connective tissue between the surface epithelium and cartilage, and basal cells that reside within the surface airway epithelium (SAE). Current research suggests that regulatory pathways that coordinate development of the proximal airway and establishment of progenitor cell niches may overlap with pathways that control progenitor cell responses during airway regeneration following injury. SMGs have been shown to harbor epithelial progenitor cells, and this niche is dysregulated in diseases such as cystic fibrosis. However, mechanisms that regulate progenitor cell proliferation and maintenance within this glandular niche are not completely understood. Here we discuss glandular progenitor cells during development and regeneration of the proximal airway and compare properties of glandular progenitors to those of basal cell progenitors in the SAE. Further investigation into glandular progenitor cell control will provide a direction for interrogating therapeutic interventions to correct aberrant conditions affecting the SMGs in diseases such as cystic fibrosis, chronic bronchitis, and asthma.

Switching roles: the functional plasticity of adult tissue stem cells

Adult organisms have to adapt to survive, and the same is true for their tissues. Rates and types of cell production must be rapidly and reversibly adjusted to meet tissue demands in response to both local and systemic challenges. Recent work reveals how stem cell (SC) populations meet these requirements by switching between functional states tuned to homoeostasis or regeneration. This plasticity extends to differentiating cells, which are capable of reverting to SCs after injury. The concept of the niche, the micro-environment that sustains and regulates stem cells, is broadening, with a new appreciation of the role of physical factors and hormonal signals. Here, we review different functions of SCs, the cellular mechanisms that underlie them and the signals that bias the fate of SCs as they switch between roles.

SNAP23 is selectively expressed in airway secretory cells and mediates baseline and stimulated mucin secretion

Airway mucin secretion is important pathophysiologically and as a model of polarized epithelial regulated exocytosis. We find the trafficking protein, SNAP23 (23-kDa paralogue of synaptosome-associated protein of 25 kDa), selectively expressed in secretory cells compared with ciliated and basal cells of airway epithelium by immunohistochemistry and FACS, suggesting that SNAP23 functions in regulated but not constitutive epithelial secretion. Heterozygous SNAP23 deletant mutant mice show spontaneous accumulation of intracellular mucin, indicating a defect in baseline secretion. However mucins are released from perfused tracheas of mutant and wild-type (WT) mice at the same rate, suggesting that increased intracellular stores balance reduced release efficiency to yield a fully compensated baseline steady state. In contrast, acute stimulated release of intracellular mucin from mutant mice is impaired whether measured by a static imaging assay 5 min after exposure to the secretagogue ATP or by kinetic analysis of mucins released from perfused tracheas during the first 10 min of ATP exposure. Together, these data indicate that increased intracellular stores cannot fully compensate for the defect in release efficiency during intense stimulation. The lungs of mutant mice develop normally and clear bacteria and instilled polystyrene beads comparable to WT mice, consistent with these functions depending on baseline secretion that is fully compensated.

Dynamic stem cell heterogeneity

Recent lineage-tracing studies based on inducible genetic labelling have emphasized a crucial role for stochasticity in the maintenance and regeneration of cycling adult tissues. These studies have revealed that stem cells are frequently lost through differentiation and that this is compensated for by the duplication of neighbours, leading to the consolidation of clonal diversity. Through the combination of long-term lineage-tracing assays with short-term in vivo live imaging, the cellular basis of this stochastic stem cell loss and replacement has begun to be resolved. With a focus on mammalian spermatogenesis, intestinal maintenance and the hair cycle, we review the role of dynamic heterogeneity in the regulation of adult stem cell populations.

Plasticity of Hopx(+) type I alveolar cells to regenerate type II cells in the lung

The plasticity of differentiated cells in adult tissues undergoing repair is an area of intense research. Pulmonary alveolar type II cells produce surfactant and function as progenitors in the adult, demonstrating both self-renewal and differentiation into gas exchanging type I cells. In vivo, type I cells are thought to be terminally differentiated and their ability to give rise to alternate lineages has not been reported. Here we show that Hopx becomes restricted to type I cells during development. However, unexpectedly, lineage-labelled Hopx(+) cells both proliferate and generate type II cells during adult alveolar regrowth following partial pneumonectomy. In clonal 3D culture, single Hopx(+) type I cells generate organoids composed of type I and type II cells, a process modulated by TGFβ signalling. These findings demonstrate unanticipated plasticity of type I cells and a bidirectional lineage relationship between distinct differentiated alveolar epithelial cell types in vivo and in single-cell culture.

Stem cell state and the epithelial-to-mesenchymal transition: Implications for cancer therapy

The cancer stem cell paradigm, the epithelial-to-mesenchymal transition and its converse, the mesenchymal-to-epithelial transition, have reached convergence. Implicit in this understanding is the notion that cancer cells can change state, and with such change come bidirectional alterations in motility, proliferative activity, and drug resistance. As such, tumors present a moving target for antineoplastic therapy. This article will review the evolving adult stem cell paradigm and how changes in our understanding of the bidirectional nature of cancer cell differentiation may affect the selection and timing of antineoplastic therapy. The goal is to determine how to best administer therapies potentially targeted against the cancer stem cell state in the context of established treatment regimens, and to evaluate long-term effects beyond tumor regression.

Retinoic acid signaling and neuronal differentiation

The identification of neurological symptoms caused by vitamin A deficiency pointed to a critical, early developmental role of vitamin A and its metabolite, retinoic acid (RA). The ability of RA to induce post-mitotic, neural phenotypes in various stem cells, in vitro, served as early evidence that RA is involved in the switch between proliferation and differentiation. In vivo studies have expanded this "opposing signal" model, and the number of primary neurons an embryo develops is now known to depend critically on the levels and spatial distribution of RA. The proneural and neurogenic transcription factors that control the exit of neural progenitors from the cell cycle and allow primary neurons to develop are partly elucidated, but the downstream effectors of RA receptor (RAR) signaling (many of which are putative cell cycle regulators) remain largely unidentified. The molecular mechanisms underlying RA-induced primary neurogenesis in anamniote embryos are starting to be revealed; however, these data have been not been extended to amniote embryos. There is growing evidence that bona fide RARs are found in some mollusks and other invertebrates, but little is known about their necessity or functions in neurogenesis. One normal function of RA is to regulate the cell cycle to halt proliferation, and loss of RA signaling is associated with dedifferentiation and the development of cancer. Identifying the genes and pathways that mediate cell cycle exit downstream of RA will be critical for our understanding of how to target tumor differentiation. Overall, elucidating the molecular details of RAR-regulated neurogenesis will be decisive for developing and understanding neural proliferation-differentiation switches throughout development.

Metastasis prevention by targeting the dormant niche

Despite considerable advancements that shattered previously held dogmas about the metastatic cascade, the evolution of therapies to treat metastatic disease has not kept up. In this Opinion article, I argue that, rather than waiting for metastases to emerge before initiating treatment, it would be more effective to target metastatic seeds before they sprout. Specifically, I advocate directing therapies towards the niches that harbour dormant disseminated tumour cells to sensitize them to cytotoxic agents. Treatment sensitization, achieved by disrupting reservoirs of leukaemic stem cells and latent HIV, argues that this approach, although unconventional, could succeed in improving patient survival by delaying or even preventing metastasis.

Origin of cardiomyocytes in the adult heart

This review article discusses the mechanisms of cardiomyogenesis in the adult heart. They include the re-entry of cardiomyocytes into the cell cycle; dedifferentiation of pre-existing cardiomyocytes, which assume an immature replicating cell phenotype; transdifferentiation of hematopoietic stem cells into cardiomyocytes; and cardiomyocytes derived from activation and lineage specification of resident cardiac stem cells. The recognition of the origin of cardiomyocytes is of critical importance for the development of strategies capable of enhancing the growth response of the myocardium; in fact, cell therapy for the decompensated heart has to be based on the acquisition of this fundamental biological knowledge.

MafA is critical for maintenance of the mature beta cell phenotype in mice

AIMS/HYPOTHESIS

The plasticity of adult somatic cells allows for their dedifferentiation or conversion to different cell types, although the relevance of this to disease remains elusive. Perturbation of beta cell identity leading to dedifferentiation may be implicated in the compromised functions of beta cells in diabetes, which is a current topic of islet research. This study aims to investigate whether or not v-Maf musculoaponeurotic fibrosarcoma oncogene family, protein A (MafA), a mature beta cell marker, is involved in maintaining mature beta cell phenotypes.

METHODS

The fate and gene expression of beta cells were analysed in Mafa knockout (KO) mice and mouse models of diabetes in which the expression of MafA was reduced in the majority of beta cells.

RESULTS

Loss of MafA reduced the beta to alpha cell ratio in pancreatic islets without elevating blood glucose to diabetic levels. Lineage tracing analyses showed reduced/lost expression of insulin in most beta cells, with a minority of the former beta cells converted to glucagon-expressing cells in Mafa KO mice. The upregulation of genes that are normally repressed in mature beta cells or transcription factors that are transiently expressed in endocrine progenitors was identified in Mafa KO islets as a hallmark of dedifferentiation. The compromised beta cells in db/db and multiple low-dose streptozotocin mice underwent similar dedifferentiation with expression of Mafb, which is expressed in immature beta cells.

CONCLUSIONS/INTERPRETATION

The maturation factor MafA is critical for the homeostasis of mature beta cells and regulates cell plasticity. The loss of MafA in beta cells leads to a deeper loss of cell identity, which is implicated in diabetes pathology.

Airway basal cells. The "smoking gun" of chronic obstructive pulmonary disease

The earliest abnormality in the lung associated with smoking is hyperplasia of airway basal cells, the stem/progenitor cells of the ciliated and secretory cells that are central to pulmonary host defense. Using cell biology and 'omics technologies to assess basal cells isolated from bronchoscopic brushings of nonsmokers, smokers, and smokers with chronic obstructive pulmonary disease (COPD), compelling evidence has been provided in support of the concept that airway basal cells are central to the pathogenesis of smoking-associated lung diseases. When confronted by the chronic stress of smoking, airway basal cells become disorderly, regress to a more primitive state, behave as dictated by their inheritance, are susceptible to acquired changes in their genome, lose the capacity to regenerate the epithelium, are responsible for the major changes in the airway that characterize COPD, and, with persistent stress, can undergo malignant transformation. Together, these observations led to the conclusion that accelerated loss of lung function in susceptible individuals begins with disordered airway basal cell biology (i.e., that airway basal cells are the "smoking gun" of COPD, a potential target for the development of therapies to prevent smoking-related lung disorders).

Ezh2 represses the basal cell lineage during lung endoderm development

The development of the lung epithelium is regulated in a stepwise fashion to generate numerous differentiated and stem cell lineages in the adult lung. How these different lineages are generated in a spatially and temporally restricted fashion remains poorly understood, although epigenetic regulation probably plays an important role. We show that the Polycomb repressive complex 2 component Ezh2 is highly expressed in early lung development but is gradually downregulated by late gestation. Deletion of Ezh2 in early lung endoderm progenitors leads to the ectopic and premature appearance of Trp63+ basal cells that extend the entire length of the airway. Loss of Ezh2 also leads to reduced secretory cell differentiation. In their place, morphologically similar cells develop that express a subset of basal cell genes, including keratin 5, but no longer express high levels of either Trp63 or of standard secretory cell markers. This suggests that Ezh2 regulates the phenotypic switch between basal cells and secretory cells. Together, these findings show that Ezh2 restricts the basal cell lineage during normal lung endoderm development to allow the proper patterning of epithelial lineages during lung formation.

The innate immune function of airway epithelial cells in inflammatory lung disease

The airway epithelium is now considered to be central to the orchestration of pulmonary inflammatory and immune responses, and is also key to tissue remodelling. It acts as the first barrier in the defence against a wide range of inhaled challenges, and is critically involved in the regulation of both innate and adaptive immune responses to these challenges. Recent progress in our understanding of the developmental regulation of this tissue, the differentiation pathways, recognition of pathogens and antimicrobial responses is now exploited to help understand how epithelial cell function and dysfunction contributes to the pathogenesis of a variety of inflammatory lung diseases. Herein, advances in our knowledge of the biology of airway epithelium, as well as its role and (dys)function in asthma, chronic obstructive pulmonary fibrosis and cystic fibrosis will be discussed.

Impact of Sox9 dosage and Hes1-mediated Notch signaling in controlling the plasticity of adult pancreatic duct cells in mice

In the adult pancreas, there has been a long-standing dispute as to whether stem/precursor populations that retain plasticity to differentiate into endocrine or acinar cell types exist in ducts. We previously reported that adult Sox9-expressing duct cells are sufficiently plastic to supply new acinar cells in Sox9-IRES-CreERT2 knock-in mice. In the present study, using Sox9-IRES-CreERT2 knock-in mice as a model, we aimed to analyze how plasticity is controlled in adult ducts. Adult duct cells in these mice express less Sox9 than do wild-type mice but Hes1 equally. Acinar cell differentiation was accelerated by Hes1 inactivation, but suppressed by NICD induction in adult Sox9-expressing cells. Quantitative analyses showed that Sox9 expression increased with the induction of NICD but did not change with Hes1 inactivation, suggesting that Notch regulates Hes1 and Sox9 in parallel. Taken together, these findings suggest that Hes1-mediated Notch activity determines the plasticity of adult pancreatic duct cells and that there may exist a dosage requirement of Sox9 for keeping the duct cell identity in the adult pancreas. In contrast to the extended capability of acinar cell differentiation by Hes1 inactivation, we obtained no evidence of islet neogenesis from Hes1-depleted duct cells in physiological or PDL-induced injured conditions.

Chronic lung injury by constitutive expression of activation-induced cytidine deaminase leads to focal mucous cell metaplasia and cancer

Activation-induced cytidine deaminase (AID) is an enzyme required for antibody diversification, and it causes DNA mutations and strand breaks. Constitutive AID expression in mice invariably caused lung lesions morphologically similar to human atypical adenomatous hyperplasia (AAH), which can be a precursor of bronchioloalveolar carcinoma. Similar to AAH, mouse AAH-like lesion (MALL) exhibited signs of alveolar differentiation, judging from the expression of alveolar type II (AT2) cell marker surfactant protein C (SP-C). However, electron microscopy indicated that MALL, which possessed certain features of a mucous cell, is distinct from an AAH or AT2 cell. Although MALL developed in all individuals within 30 weeks after birth, lung tumors occurred in only 10%; this suggests that the vast majority of MALLs fail to grow into visible tumors. MALL expressed several recently described markers of lung alveolar regeneration such as p63, keratin 5, keratin 14, leucine-rich repeat containing G protein-coupled receptor 5 (Lgr5), and Lgr6. Increased cell death was observed in the lungs of AID transgenic mice compared with wild-type mice. Based on these observations, we speculate that MALL is a regenerating tissue compensating for cellular loss caused by AID cytotoxicity. AID expression in such regenerating tissue should predispose cells to malignant transformation via its mutagenic activity.

The analysis, roles and regulation of quiescence in hematopoietic stem cells

Tissue homeostasis requires the presence of multipotent adult stem cells that are capable of efficient self-renewal and differentiation; some of these have been shown to exist in a dormant, or quiescent, cell cycle state. Such quiescence has been proposed as a fundamental property of hematopoietic stem cells (HSCs) in the adult bone marrow, acting to protect HSCs from functional exhaustion and cellular insults to enable lifelong hematopoietic cell production. Recent studies have demonstrated that HSC quiescence is regulated by a complex network of cell-intrinsic and -extrinsic factors. In addition, detailed single-cell analyses and novel imaging techniques have identified functional heterogeneity within quiescent HSC populations and have begun to delineate the topological organization of quiescent HSCs. Here, we review the current methods available to measure quiescence in HSCs and discuss the roles of HSC quiescence and the various mechanisms by which HSC quiescence is maintained.

Wound healing and cancer stem cells: inflammation as a driver of treatment resistance in breast cancer

The relationship between wound healing and cancer has long been recognized. The mechanisms that regulate wound healing have been shown to promote transformation and growth of malignant cells. In addition, chronic inflammation has been associated with malignant transformation in many tissues. Recently, pathways involved in inflammation and wound healing have been reported to enhance cancer stem cell (CSC) populations. These cells, which are highly resistant to current treatments, are capable of repopulating the tumor after treatment, causing local and systemic recurrences. In this review, we highlight proinflammatory cytokines and developmental pathways involved in tissue repair, whose deregulation in the tumor microenvironment may promote growth and survival of CSCs. We propose that the addition of anti-inflammatory agents to current treatment regimens may slow the growth of CSCs and improve therapeutic outcomes.

Is reduction of tumor burden sufficient for the 21st century?

Currently, animal models are used to test the efficacy of tumor treatment. A significant reduction of tumor mass is lauded as great improvement. As we begin the 21st century, one wonders if this is sufficient and acceptable for cancer treatment. Although the presence of cancer stem cell (CSCs) is not a new phenomenon, their role in the initiation of the tumor for clinical resurgence is mostly ignored when testing drugs. The current treatment then poses a major limitation to aggressively target the cells most responsible for tumor initiation and resurgence. The review does not trivialize the problem since it is acknowledged that the tumors and cells within the tissue microenvironment would interact through complex mechanisms. It is quite possible that the interaction by CSCs and the microenvironment will vary, depending on the tissue, e.g., bone marrow versus brain. Research studies are needed to investigate if CSCs from the same organ differ after migrating to other tissues. If so, this will pose an economic dilemma for targeted drug development. It will not be feasible to develop drugs for each organ. Besides, the cost, there could be problems to effectively deliver the drugs to all organs, problems to assess drug distribution to particular tissues and toxicity for specific drugs. If multiple drugs are required to eradicate CSCs in different tissues, there is a problem of possible untoward effect for the simultaneous delivery of multiple drugs to a single cancer patient. As new drugs are developed, the investigators will need to pay attention for dedifferentiation of non-CSCs to CSCs. The metabolic pathways will have to be given equal attention as the stem cells genes since their pathways might show major differences rather than the stem cells genes, which are shared by the normal stem cells.

Platelet-derived SDF-1 primes the pulmonary capillary vascular niche to drive lung alveolar regeneration

The lung alveoli regenerate after surgical removal of the left lobe by pneumonectomy (PNX). How this alveolar regrowth/regeneration is initiated remains unknown. We found that platelets trigger lung regeneration by supplying stromal-cell-derived factor-1 (SDF-1, also known as CXCL12). After PNX, activated platelets stimulate SDF-1 receptors CXCR4 and CXCR7 on pulmonary capillary endothelial cells (PCECs) to deploy the angiocrine membrane-type metalloproteinase MMP14, stimulating alveolar epithelial cell (AEC) expansion and neo-alveolarization. In mice lacking platelets or platelet Sdf1, PNX-induced alveologenesis was diminished. Reciprocally, infusion of Sdf1(+/+) but not Sdf1-deficient platelets rescued lung regeneration in platelet-depleted mice. Endothelial-specific ablation of Cxcr4 and Cxcr7 in adult mice similarly impeded lung regeneration. Notably, mice with endothelial-specific Mmp14 deletion exhibited impaired expansion of AECs but not PCECs after PNX, which was not rescued by platelet infusion. Therefore, platelets prime PCECs to initiate lung regeneration, extending beyond their haemostatic contribution. Therapeutic targeting of this haemo-vascular niche could enable regenerative therapy for lung diseases.

Regulated genes in mesenchymal stem cells and gastric cancer

AIM: To investigate the genes regulated in mesenchymal stem cells (MSCs) and diffuse-type gastric cancer (GC), gene expression was analyzed.

METHODS: Gene expression of MSCs and diffuse-type GC cells were analyzed by microarray. Genes related to stem cells, cancer and the epithelial-mesenchymal transition (EMT) were extracted from human gene lists using Gene Ontology and reference information. Gene panels were generated, and messenger RNA gene expression in MSCs and diffuse-type GC cells was analyzed. Cluster analysis was performed using the NCSS software.

RESULTS: The gene expression of regulator of G-protein signaling 1 (RGS1) was up-regulated in diffuse-type GC cells compared with MSCs. A panel of stem-cell related genes and genes involved in cancer or the EMT were examined. Stem-cell related genes, such as growth arrest-specific 6, musashi RNA-binding protein 2 and hairy and enhancer of split 1 (Drosophila), NOTCH family genes and Notch ligands, such as delta-like 1 (Drosophila) and Jagged 2, were regulated.

CONCLUSION: Expression of RGS1 is up-regulated, and genes related to stem cells and NOTCH signaling are altered in diffuse-type GC compared with MSCs.

A switch from CD44⁺ cell to EMT cell drives the metastasis of prostate cancer

Epithelial-mesenchymal transition (EMT) has been linked to cancer stem-like (CD44+) cell in the prostate cancer (PCa) metastasis. However, the molecular mechanism remains elusive. Here, we found EMT contributed to metastasis in PCa patients failed in androgen deprivation therapy (ADT). Castration TRAMP model also proved PCa treated with ADT promoted EMT with increased CD44+ stem-like cells. Switched CD44+ cell to EMT cell is a key step for luminal PCa cell metastasis. Our results also suggested ADT might go through promoting TGFβ1-CD44 signaling to enhance swift to EMT. Targeting CD44 with salinomycin and siRNA could inhibit cell transition and decrease PCa invasion. Together, cancer stem-like (CD44+) cells could be the initiator cells of EMT modulated by TGFβ1-CD44 signaling. Combined therapy of ADT with anti-CD44 may become a new potential therapeutic approach to battle later stage PCa.

Mechanisms underlying vertebrate limb regeneration: lessons from the salamander

Limb regeneration in adult salamanders proceeds by formation of a mound of progenitor cells called the limb blastema. It provides several pointers for regenerative medicine. These include the role of differentiated cells in the origin of the blastema, the role of regenerating axons of peripheral nerves and the importance of cell specification in conferring morphogenetic autonomy on the blastema. One aspect of regeneration that has received less attention is the ability to undergo multiple episodes without detectable change in the outcome, and with minimal effect of aging. We suggest that, although such pointers are valuable, it is important to understand why salamanders are the only adult tetrapod vertebrates able to regenerate their limbs. Although this remains a controversial issue, the existence of salamander-specific genes that play a significant role in the mechanism of regeneration provides evidence for the importance of local evolution, rather than a purely ancestral mechanism. The three-finger protein called Prod1 is discussed in the present article as an exemplar of this approach.

Involvement of Plant Stem Cells or Stem Cell-Like Cells in Dedifferentiation

Dedifferentiation is the transformation of cells from a given differentiated state to a less differentiated or stem cell-like state. Stem cell-related genes play important roles in dedifferentiation, which exhibits similar histone modification and DNA methylation features to stem cell maintenance. Hence, stem cell-related factors possibly synergistically function to provide a specific niche beneficial to dedifferentiation. During callus formation in Arabidopsis petioles, cells adjacent to procambium cells (stem cell-like cells) are dedifferentiated and survive more easily than other cell types. This finding indicates that stem cells or stem cell-like cells may influence the dedifferentiating niche. In this paper, we provide a brief overview of stem cell maintenance and dedifferentiation regulation. We also summarize current knowledge of genetic and epigenetic mechanisms underlying the balance between differentiation and dedifferentiation. Furthermore, we discuss the correlation of stem cells or stem cell-like cells with dedifferentiation.

Interaction between pancreatic cancer cells and tumor-associated macrophages promotes the invasion of pancreatic cancer cells and the differentiation and migration of macrophages

In this study, the impact of pancreatic cancer cell interaction with macrophages on the differentiation and function of macrophages and the behaviors of pancreatic cancer cells in vitro is evaluated. The expression of immunocompetent cell-associated markers in 22 pancreatic cancer specimens was characterized by immunohistochemistry. The impact of pancreatic cancer cells (PANC-1 and BxPC-3) on the differentiation and migration of human U937 monocytes and the effect of U937-derived macrophages on the proliferation and invasion of PANC-1 and BxPC-3 were determined by transwell assays. The potential effect on U937-derived macrophages or on the behaviors of pancreatic cancer cells following coculture in a transwell system was analyzed by quantitative real-time polymerase chain reaction. The high levels of macrophage-related CD68 and CD163 expression were detected in the pancreatic cancer specimens. Pancreatic cancer cells promoted the differentiation of U937 cells and migration of U937-derived macrophages, but decreased the mRNA transcripts of macrophage polarization-related genes of interleukin (IL)-10, IL-12p40, inducible nitric oxide synthase (iNOS), and CD163, particularly for iNOS. Furthermore, U937-derived M2 macrophages inhibited the proliferation of pancreatic cancer cells, but promoted their invasion. Coculture of pancreatic cancer cells with U937-derived macrophages upregulated the mRNA expression of genes associated with the epithelial-mesenchymal transition process, angiogenesis, and stemness of pancreatic cancer, but downregulated the expression of E-cadherin in pancreatic cancer cells. The interaction between pancreatic cancer cells and tumor-associated macrophages may play a pivotal role in the progression of pancreatic cancer.

Maturing from embryonic to adult policy on stem cell therapeutics

The National Institutes of Health (NIH) closure of the agency's Center for Regenerative Medicine (CRM), which focused on therapeutic development of induced pluripotent stem cells (iPS), was caused by the lack of progress in practical development of the iPSs for use in human therapies. As the NIH evaluates priorities in future stem cell therapeutic development, adult stem cell processes in the human body need to be prioritized for a number of key reasons, including (1) adult stem cells release many types of molecules that provide much of the therapeutic benefit of stem cells and (2) adult stem cells and somatic cells exist in a state of dynamic transition between different potency levels and can be naturally driven by the microenvironment to a state of pluripotency. Thus, the study and development of adult stems for therapeutic use can include naturally induced pluripotent stem cells (NiPSs) that lack the problematic genetic and epigenetic reprogramming errors found in iPSs.

Subcellular distribution and activity of mechanistic target of rapamycin in aged retinal pigment epithelium

PURPOSE

Inhibiting mechanistic target of rapamycin (mTOR) by pharmacological or genetic approaches can extend lifespan in mammals. The kinase activity of mTOR is controlled by upstream regulatory proteins and its subcellular localization. The purpose of this study was to characterize age-related alterations and functional consequences of mTOR signaling in the postmitotic RPE cells.

METHODS

Activity of mTOR complex 1 (mTORC1) was monitored by measuring phosphorylation status of its downstream effector protein S6, in either cultured human RPE cells or RPE explants prepared from mice at different ages. Subcellular distribution of mTOR was investigated by immunofluorescent staining of RPE culture or flatmount. The signaling of mTORC1 was modulated by either overexpression of a small guanosine triphosphatase, Ras homolog enriched in brain (Rheb), or disruption of the Ragulator complex with small interference RNA targeting p18. The effects of mTOR pathway on degradation of phagocytosed photoreceptor outer segments (POS) were determined by measuring the turnover rate of rhodopsin.

RESULTS

Aged RPE cells had more lysosome-associated mTOR and had increased response to amino acid stimulation. The lysosome distribution was essential for mTORC1 function, as disruption of the Ragulator complex abolished mTORC1 activation by amino acids. Increased mTORC1 activity caused decreased rate of degradation of internalized POS in the RPE.

CONCLUSIONS

Aging changes the subcellular localization and function of mTOR in the RPE. Increased mTORC1 inhibits POS degradation and may further exacerbate lysosome dysfunction of aged RPE.

Role of testicular luminal factors on Basal cell elongation and proliferation in the mouse epididymis

A subset of basal cells (BCs) in the initial segment (IS) of the mouse epididymis has a slender body projection between adjacent epithelial cells. We show here that these projections occasionally cross the apical tight junctions and are in contact with the luminal environment. Luminal testicular factors are critical for the establishment of the IS epithelium, and we investigated their role in the regulation of this luminal sensing property. Efferent duct ligation (EDL) was performed to block luminal flow from the testis without affecting blood flow. Cytokeratin 5 (KRT5) labeling showed a time-dependent reduction of the percentage of BCs with intercellular projections from 1 to 5 days after EDL, compared to controls. Double labeling for caspase-3 and KRT5 showed that a subset of BCs undergoes apoptosis 1 day after EDL. Ki67/KRT5 double labeling showed a low rate of BC proliferation under basal conditions. However, EDL induced a marked increase in the proliferation rate of a subset of BCs 2 days after EDL. A 2-wk treatment with the androgen receptor antagonist flutamide did not affect the number of BCs with intercellular projections, but reduced BC proliferation. Flutamide treatment also reduced the increase in BC proliferation induced 2 days after EDL. We conclude that, in the adult mouse IS, 1) luminal testicular factors play an important role in the ability of BCs to extend their body projection towards the lumen, and are essential for the survival of a subset of BCs; 2) androgens play an important role in the proliferation of some of the BCs that survive the initial insult induced by EDL; and 3) the formation and elongation of BC intercellular projections do not depend on androgens.

Tracheal bioengineering: the next steps. Proceeds of an International Society of Cell Therapy Pulmonary Cellular Therapy Signature Series Workshop, Paris, France, April 22, 2014

There has been significant and exciting recent progress in the development of bioengineering approaches for generating tracheal tissue that can be used for congenital and acquired tracheal diseases. This includes a growing clinical experience in both pediatric and adult patients with life-threatening tracheal diseases. However, not all of these attempts have been successful, and there is ongoing discussion and debate about the optimal approaches to be used. These include considerations of optimal materials, particularly use of synthetic versus biologic scaffolds, appropriate cellularization of the scaffolds, optimal surgical approaches and optimal measure of both clinical and biologic outcomes. To address these issues, the International Society of Cell Therapy convened a first-ever meeting of the leading clinicians and tracheal biologists, along with experts in regulatory and ethical affairs, to discuss and debate the issues. A series of recommendations are presented for how to best move the field ahead.

Evolution of the cancer stem cell model

Genetic analyses have shaped much of our understanding of cancer. However, it is becoming increasingly clear that cancer cells display features of normal tissue organization, where cancer stem cells (CSCs) can drive tumor growth. Although often considered as mutually exclusive models to describe tumor heterogeneity, we propose that the genetic and CSC models of cancer can be harmonized by considering the role of genetic diversity and nongenetic influences in contributing to tumor heterogeneity. We offer an approach to integrating CSCs and cancer genetic data that will guide the field in interpreting past observations and designing future studies.

Roles for Hedgehog signaling in adult organ homeostasis and repair

The hedgehog (HH) pathway is well known for its mitogenic and morphogenic functions during development, and HH signaling continues in discrete populations of cells within many adult mammalian tissues. Growing evidence indicates that HH regulates diverse quiescent stem cell populations, but the exact roles that HH signaling plays in adult organ homeostasis and regeneration remain poorly understood. Here, we review recently identified functions of HH in modulating the behavior of tissue-specific adult stem and progenitor cells during homeostasis, regeneration and disease. We conclude that HH signaling is a key factor in the regulation of adult tissue homeostasis and repair, acting via multiple different routes to regulate distinct cellular outcomes, including maintenance of plasticity, in a context-dependent manner.

Vitamin D is a determinant of mouse intestinal Lgr5 stem cell functions

Lgr5+ intestinal crypt base columnar cells function as stem cells whose progeny populate the villi, and Lgr5+ cells in which Apc is inactivated can give rise to tumors. Surprisingly, these Lgr5+ stem cell properties were abrogated by the lower dietary vitamin D and calcium in a semi-purified diet that promotes both genetically initiated and sporadic intestinal tumors. Inactivation of the vitamin D receptor in Lgr5+ cells established that compromise of Lgr5 stem cell function was a rapid, cell autonomous effect of signaling through the vitamin D receptor. The loss of Lgr5 stem cell function was associated with presence of Ki67 negative Lgr5+ cells at the crypt base. Therefore, vitamin D, a common nutrient and inducer of intestinal cell maturation, is an environmental factor that is a determinant of Lgr5+ stem cell functions in vivo. Since diets used in reports that establish and dissect mouse Lgr5+ stem cell activity likely provided vitamin D levels well above the range documented for human populations, the contribution of Lgr5+ cells to intestinal homeostasis and tumor formation in humans may be significantly more limited, and variable in the population, then suggested by published rodent studies.

Cellular plasticity regulated cancer stem cell niche: a possible new mechanism of chemoresistance

The cancer stem cell (CSC) theory is an emerging concept that proposes a hierarchical nature of carcinogenesis, where a small number of tumor cells are capable of driving tumor growth. Despite many unanswered questions surrounding the cancer stem cell model, the hypothesis has rejuvenated hopes for formulating a novel therapeutic strategy for targeting the roots of cancer. This model predicts that cancer stem cells have the capacity to resist conventional radio- and chemotherapy and initiate disease recurrence. We recently investigated the mechanisms of chemoresistance in glioblastoma (GBM), the most common and aggressive adult human brain tumor. Exposure of patient derived glioma xenograft lines to a therapeutic dose of temolozolomide (TMZ), the most commonly used chemotherapy for patients with GBM, consistently increased the glioma stem cell (GSC) frequency over time. Lineage tracing analysis at the single sell level revealed unprecedented cellular plasticity within the glioma cells, allowing them to reprogram from a differentiated state to an undifferentiated CSC-like state. This reprogramming, mediated by cellular plasticity, is driven by TMZ-induced hypoxia inducible factors (HIFs), and provides a novel mechanism for chemoresistance acquisition. We herein discuss the possible role of temozolomide in regulating a cancer stem cell niche that supports GSC resistance, proliferation, and subsequent therapeutic relapse.

Reprogramming by lineage specifiers: blurring the lines between pluripotency and differentiation

The generation of human induced pluripotent stem cells (iPS) has raised enormous expectations within the biomedical community due to their potential vast implications in regenerative and personalized medicine. However, reprogramming to iPS is still not fully comprehended. Difficulties found in ascribing specific molecular patterns to pluripotent cells (PSCs), and inherent inter-line and intra-line variability between different PSCs need to be resolved. Additionally, and despite multiple assumptions, it remains unclear whether the current in vitro culturing conditions for the maintenance and differentiation of PSCs do indeed recapitulate the developmental processes observed in vivo. As a consequence, basic questions such as what is the actual nature of PSCs remain unanswered and different theories have emerged in regards to the identity of these valuable cell population. Here we discuss on the published theories for defining PSC identity, the implications that the different postulated models have for the reprogramming field as well as speculate on potential future directions that might be opened once a precise knowledge on the nature of PSCs is accomplished.

Epithelial ovarian cancer stem cells: underlying complexity of a simple paradigm

The lack of significant progress in the treatment of epithelial ovarian cancer (EOC) underscores the need to gain a better understanding of the processes that lead to chemoresistance and recurrence. The cancer stem cell (CSC) hypothesis offers an attractive explanation of how a subpopulation of cells within a patient's tumour might remain refractory to treatment and subsequently form the basis of recurrent chemoresistant disease. This review examines the literature defining somatic stem cells of the ovary and fallopian tube, two tissues that give rise to EOC. In addition, considerable research has been reviewed, that has identified subpopulations of EOC cells, based on marker expression (CD133, CD44, CD117, CD24, epithelial cell adhesion molecule, LY6A, ALDH1 and side population (SP)), which are enriched for tumour initiating cells (TICs). While many studies identified either CD133 or CD44 as markers useful for enriching for TICs, there is little consensus. This suggests that EOC cells may have a phenotypic plasticity that may preclude the identification of universal markers defining a CSC. The assay that forms the basis of quantifying TICs is the xenograft assay. Considerable controversy surrounds the xenograft assay and it is essential that some of the potential limitations be examined in this review. Highlighting such limitations or weaknesses is required to properly evaluate data and broaden our interpretation of potential mechanisms that might be contributing to the pathogenesis of ovarian cancer.