Microenvironmental control of stem cell fate in intestinal homeostasis and disease

The conventional model of intestinal epithelial architecture describes a unidirectional tissue organizational hierarchy with stem cells situated at the crypt base and daughter cells proliferating and terminally differentiating as they progress along the vertical (crypt–luminal) axis. In this model, the fate of a cell that has left the niche is determined and its lifespan limited. Evidence is accumulating to suggest that stem cell control and daughter cell fate determination is not solely an intrinsic, cell autonomous property but is heavily influenced by the microenvironment including paracrine, mesenchymal, and endogenous epithelial morphogen gradients. Recent research suggests that in intestinal homeostasis, stem cells transit reversibly between states of variable competence in the niche. Furthermore, selective pressures that disrupt the homeostatic balance, such as intestinal inflammation or morphogen dysregulation, can cause committed progenitor cells and even some differentiated cells to regain stem cell properties. Importantly, it has been recently shown that this disruption of cell fate determination can lead to somatic mutation and neoplastic transformation of cells situated outside the crypt base stem cell niche. This paper reviews the exciting developments in the study of stem cell dynamics in homeostasis, intestinal regeneration, and carcinogenesis, and explores the implications for human disease and cancer therapies. © 2015 Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.     

N-Cadherin is expressed by putative stem/progenitor cells and melanocytes in the human limbal epithelial stem cell niche

Corneal epithelial stem cells are known to be localized to the basal layer of the limbal epithelium, providing a model system for epithelial stem cell biology; however, the mechanisms regarding the maintenance of these stem cells in their specialized niche remain poorly understood. N-cadherin is a member of the classic cadherin family and has previously been demonstrated to be expressed by hematopoietic stem cells. In the present study, we demonstrate that N-cadherin is expressed by putative stem/progenitor cells, as well as melanocytes, in the human limbal epithelial stem cell niche. In addition, we demonstrate that upon in vitro culture using 3T3 feeder layers, loss of N-cadherin expression occurs with cell proliferation. These results indicate that N-cadherin may be a critical cell-to-cell adhesion molecule between corneal epithelial stem/progenitor cells and their corresponding niche cells in the limbal epithelium.     

The cardiogenic niche as a fundamental building block of engineered myocardium

Cardiac muscle engineering is evolving rapidly, aiming at the provision of innovative models for drug development and therapeutic myocardium. The progress in this field will depend crucially on the proper exploitation of stem cell technologies. Understanding the processes governing stem cell differentiation towards a desired phenotype and subsequent maturation in an organotypic manner will be key to ultimately providing realistic tissue models or therapeutics. Cardiogenesis is controlled by milieu factors that collectively constitute a so-called cardiogenic niche. The components of the cardiogenic niche are not yet fully defined but include paracrine factors and instructive extracellular matrix. Both are provided by supportive stromal cells under strict spatial and temporal control. Detailed knowledge on the exact composition and functionality of the dynamic cardiogenic niche during development will likely be instrumental to further advance cardiac muscle engineering. This review will discuss the concept of myocardial tissue engineering from the stem cell/developmental biology perspective and put forward the hypothesis of the cardiogenic niche as a fundamental building block of tissue-engineered myocardium.     

Stem Cell Competition for Niche Occupancy: Emerging Themes and Mechanisms

Tissue-specific adult stem cells are responsible for generating a range of various differentiated cells during their life time; these cells are intimately associated with niche for maintenance and function. Recent studies of germline stem cell niches in Drosophila gonad suggest that stem cells within a niche constantly compete with each other for niche occupancy. Competition within a niche occurs between same type of stem cells as well as different types. In both cases, cell adhesion molecules are critical in mediating the competition.     

A new press-fit stem concept to reduce the risk of end-of-stem pain at revision TKA: a pre-clinical study

PurposeRevision total knee arthroplasty presents numerous technical challenges, with lower patient outcomes compared with those obtained in primary surgery. Extended stems have been used in revision total knee arthroplasty to improve component alignment and fixation. Hybrid fixation with cemented tibial tray and press-fit stem has shown good results. One of the disadvantages of this technique is pain related to the presence of a cementless diaphyseal engaging stem, often designated as end-of-stem pain. Patients with this pain have reported a decrease in overall satisfaction, as well as demonstrate a lower clinical outcome score. Clinical findings suggest that stem material and design are important factors in the development of end-of-stem pain. Therefore, a question can be raised: can a novel press-fit stem concept minimize bone strain changes at the stem tip? The hypothesis here considered lies upon the fact, that if periosteal cortex strain changes are minimized at the stem tip comparatively to the intact situation, the risk of end-of-stem pain might be minimized.ScopeThis pre-clinical study was accomplished using synthetic tibiae to experimentally predict the periosteal cortex strains at the proximal and stem tip regions, with a commercial press-fit stem and a new stem concept.ConclusionsThe results demonstrated that the new stem concept has the ability to minimize strain changes induced by the stem tip at the distal periosteal cortex and consequently, at the periosteal layer of bone tissue, which is highly pain sensitive, probably contributing to the reduction of the risk of end-of-stem pain.Level of evidenceV     

Rapid tissue engineering of biomimetic human corneal limbal crypts with 3D niche architecture

Limbal epithelial stem cells are responsible for the maintenance of the human corneal epithelium and these cells reside in a specialised stem cell niche. They are located at the base of limbal crypts, in a physically protected microenvironment in close proximity to a variety of neighbouring niche cells. Design and recreation of elements of various stem cell niches have allowed researchers to simplify aspects of these complex microenvironments for further study in vitro. We have developed a method to rapidly and reproducibly create bioengineered limbal crypts (BLCs) in a collagen construct using a simple one-step method. Liquid is removed from collagen hydrogels using hydrophilic porous absorbers (HPAs) that have custom moulded micro-ridges on the base. The resulting topography on the surface of the thin collagen constructs resembles the dimensions of the stromal crypts of the human limbus. Human limbal epithelial cells seeded onto the surface of the constructs populate these BLCs and form numerous layers with a high proportion of the cells lining the crypts expressing putative stem cell marker, p63α. The HPAs are produced using a moulding process that is flexible and can be adapted depending on the requirements of the end user. Creation of defined topographical features using this process could be applicable to numerous tissue-engineering applications where varied 3-dimensional niche architectures are required.     

The gastrointestinal tract stem cell niche

The gastrointestinal epithelium is unique in that cell proliferation, differentiation, and apoptosis occur in an orderly fashion along the crypt-villus axis. The intestinal crypt is mainly a proliferative compartment, is monoclonal and is maintained by stem cells. The villus represents the differentiated compartment, and is polyclonal as it receives cells from multiple crypts. In the small intestine, cell migration begins near the base of the crypt, and cells migrate from here emerging onto the villi. The basal crypt cells at position 5 are candidate stem cells. As the function of stem cells is to maintain the integrity of the intestinal epithelium, it must self-renew, proliferate, and differentiate within a protective niche. This niche is made up of proliferating and differentiating epithelial cells and surrounding mesenchymal cells. These mesenchymal cells promote the epithelial-mesenchymal crosstalk required to maintain the niche. A stochastic model of cell division has been proposed to explain how a single common ancestral stem cell exists from which all stem cells in a niche are descended. Our group has argued that these crypts then clonally expand by crypt fission, forming two daughters’ crypts, and that this is the mechanism by which mutated stem cells or even cancer stem cell clones expand in the colon and in the entire gastrointestinal tract. Until recently, the differentiation potential of stem cells into adult tissues has been thought to be limited to cell lineages in the organ from which they were derived. Bone marrow cells are rare among adult stem cells regarding their abundance and role in the continuous, lifelong, physiological replenishment of circulating cells. In human and mice experiments, we have shown that bone marrow can contribute to the regeneration of intestinal myofibroblasts and thereby after epithelium following damage, through replacing the cells, which maintain the stem cells niche. Little is known about the markers characterizing the stem and transit amplifying populations of the gastrointestinal tract, although musashi-1 and hairy and enhancer of split homolog-1 have been proposed. As the mammalian gastrointestinal tract develops from the embryonic gut, it is made up of an endodermally-derived epithelium surrounded by cells of mesoderm origin. Cell signaling between these two tissue layers plays a critical role in coordinating patterning and organogenesis of the gut and its derivatives. Many lines of evidence have revealed that Wnt signaling is the most dominant force in controlling cell proliferation, differentiation, and apoptosis along the crypt-villus axis. We have found Wnt messenger RNAs expression in intestinal subepithelial myofibroblasts and frizzled messenger RNAs expression in both myofibroblasts and crypt epithelium. Moreover, there are many other factors, for example, bone morphogenetic protein, homeobox, forkhead, hedgehog, homeodomain, and platelet-derived growth factor that are also important to stem cell signaling in the gastrointestinal tract.     

The concept of mesenchymal stem cells

In this chapter we examine whether criteria usually defining adult tissue stem cells apply to mesenchymal stem cells (MSCs) that give rise to cells of the skeletal connective tissues. MSCs appear to constitute a heterogeneous population of undifferentiated and committed, lineage-primed cells, capable of: homing upon engraftment to a number of growth microenvironments, extensive proliferation, producing large numbers of differentiated progeny, and functional tissue repair after injury. In addition, MSCs are extensively distributed throughout tissues, and bone marrow MSCs provide the stromal component of the niche of hematopoietic stem cells. The capacity of apparently differentiated mesenchymal cells to shift their differentiation pathway with changing microenvironmental conditions (known as differentiation plasticity) may be due to de-differentiation and reprogramming in MSCs. Because they present several features setting them apart from other stem cells, MSCs may constitute another paradigm for stem cell systems, where self-renewal and hierarchy are no longer essential, but where plasticity is the major characteristic.     

Distribution and homing pattern of c-kit+ Sca-1+ CXCR4+ resident cardiac stem cells in neonatal,postnatal, and adult mouse heart

IntroductionThe origin of heart-forming cells and their roles in organ development have fascinated biologists for over a century. C-X-C chemokine receptor type 4 plays a crucial role during embryonic development and in maintaining the stem cell niche and homing. The aim of the present was to study the expression pattern of resident cardiac stem cell markers and their homing factor in neonatal, postnatal, and adult mouse heart.MethodsCardiac stem cell protein expression was analyzed using immunofluorescence, immunohistochemistry, and Western blotting. The messenger ribonucleic acid expression of cardiac stem cell markers c-kit, stem cell antigen-1, and homing factor C-X-C chemokine receptor type 4 was quantitatively analyzed using quantitative polymerase chain reaction. Data were analyzed using Student's t test and two-way analysis using SPSS software.ResultsStem cell antigen-1- and c-kit-positive cell populations were heterogeneously distributed in the adult and postnatal hearts but scattered in the neonatal heart. The expression of c-kit showed a significant difference between right and left atrium, though it was higher compared to ventricles. The homing factor C-X-C chemokine receptor type 4 expression was higher in the neonatal heart than in the postnatal heart but was not detectable in the adult heart.ConclusionsThe present study reveals the distribution of cardiac stem cells in the different compartments of the heart and significant reduction in their number in adult heart. Cardiac stem cells are higher in the atrium than in the ventricle, suggesting the atria as the source of cardiac stem cell.     

Gremlin 1 — small protein,big impact: the multiorgan consequences of disrupted BMP antagonism†

Highly conserved, complex and interacting morphogen signalling pathways regulate adult stem cells and control cell fate determination across numerous different organs. In homeostasis, the bone morphogenetic protein (BMP) pathway predominantly promotes cell differentiation. Localised expression of ligand sequestering BMP antagonists, such as Gremlin 1 (Grem1), necessarily restricts BMP activity within the stem cell niche and facilitate stemness and self-renewal. In a new paper, Rowan, Jahns et al show that acute deletion of Grem1 in adult mice, using a ubiquitous ROSA26-Cre recombinase, induced not only severe intestinal enteropathy but also hypocellular bone marrow failure suggestive of stem cell niche collapse in both tissues. Grem1 has an increasingly recognised pleiotrophic role in a number of organ systems and is implicated across a wide range of disease states. Although the importance of Grem1 in intestinal stem cell regulation has been well described, a putative function in haematopoietic niche maintenance is novel and requires further exploration. Moreover, the complex and context-specific regulation of Grem1, among a host of functionally convergent but structurally disparate BMP antagonists, warrants further research as we learn more about the pathogenic consequences of deranged expression of this small, but important, protein. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.     

Gremlin 1 depletion in vivo causes severe enteropathy and bone marrow failure

The intestinal epithelium is perpetually renewed from a stem cell niche in the base of crypts to maintain a healthy bowel mucosa. Exit from this niche and maturation of epithelial cells requires tightly controlled gradients in BMP signalling, progressing from low BMP signalling at the crypt base to high signalling at the luminal surface. The BMP antagonist gremlin 1 (Grem1) is highly expressed by subepithelial myofibroblasts adjacent to the intestinal crypts but its role in regulating the stem cell niche and epithelial renewal in vivo has not been explored. To explore the effects of Grem1 loss in adulthood following normal growth and development, we bred mice (ROSA26CreER-Grem1 ^flx/flx) in which Grem1 could be deleted by tamoxifen administration. While Grem1 remained intact, these mice were healthy, grew normally, and reproduced successfully. Following Grem1 depletion, the mice became unwell and were euthanised (at 7–13 days). Post-mortem examination revealed extensive mucosal abnormalities throughout the small and large intestines with failure of epithelial cell replication and maturation, villous atrophy, and features of malabsorption. Bone marrow hypoplasia was also observed with associated early haematopoietic failure. These results demonstrate an essential homeostatic role for gremlin 1 in maintaining normal bowel epithelial function in adulthood, suggesting that abnormalities in gremlin 1 expression can contribute to enteropathies. We also identified a previously unsuspected requirement for gremlin 1 in normal haematopoiesis. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.     

The niche as a target for hematopoietic manipulation and regeneration

Hematopoietic stem cells (HSCs), rare primitive cells capable of reconstituting all blood cell lineages, are the only stem cells currently routinely used for therapeutic purposes. Clinical experience has shown that HSC number is an important limiting factor in treatment success. Strategies to expand HSCs are of great clinical appeal, as they would improve therapeutic use of these cells in stem cell transplantation and in conditions of bone marrow failure. The microenvironment in which HSCs reside, known as the niche, has long been considered a critical regulator of HSCs. Data accumulated over the past decade strongly confirm the importance of the niche in HSC behavior. A number of niche components as well as signaling pathways, such as Notch, have been implicated in the interaction of the microenvironment with HSCs and continue to be genetically evaluated in the hope of defining the critical elements that are required and which, if modified, can initiate HSC behaviors. In this review, we highlight the known characteristics of HSCs, challenges in their expansion, the niche phenomenon, and explain why niche stimulated HSC expansion is of utmost interest in the field, while beginning to bring to the fore potential caveats of niche manipulation. Lastly, the potential pitfalls of avoiding malignancy and controlling self-renewal versus differentiation will be briefly reviewed.     

Nasal ectomesenchymal stem cells: Multi-lineage differentiation and transformation effects on fibrin gels

Ectomesenchymal stem cells (EMSCs) are novel adult stem cells derived from the cranial neural crest. However, their stemness and multi-lineage differentiation potential on three-dimensional fibrin gels has not yet been explored. The objective of this study was to investigate induced differentiation of EMSCs on fibrin gels and their remodeling effects on the scaffolds during the induced differentiation process. The results indicated that CD133⁺/nestin⁺/CD44⁺ EMSCs were extensively distributed in the lamina propria of the nasal mucosa. The passaged cells could be induced to differentiate to a greater degree into neurons, Schwann cells and osteoblasts on three-dimensional fibrin gels than on two-dimensional glass slides. More importantly, the induced Schwann cells and osteoblasts exerted channelized and calcified remodeling effects, respectively, on the fibrin gels. Thus, these reshaped scaffolds have desirable biological properties, such as good cell adhesion, biocompatibility and guidance over the cell behavior, providing a tissue-committed niche for specific tissue generation.     

Exploring the Origins of the Normal Prostate and Prostate Cancer Stem Cell

Prostate epithelial stem cells (PSCs) are primed by the urogenital mesenchyme to initiate bud formation and branching morphogenesis, ultimately culminating in a glandular structure composed of luminal, basal and neuroendocrine cells. Identity of this cell has remained elusive however cell populations enriched for cells exhibiting stem cell characteristics express the stem cell markers CD133⁺, α2β1^hi, CD44 and Sca-1 along with embryonic stem cell factors including Oct-1, Nanog, Sox2 and nestin. Androgens are critical to prostate organogenesis and play a major role in normal prostate function and the development of prostate cancer. Cell lineage is another variable in the development of prostate cancer. This review discusses the embryonic prostate stem cell niche, normal prostate development, isolation and characterization of normal prostate and prostate cancer stem cells, and current concepts on the origin of prostate cancer. Funding for this work was provided by the National Institute of Diabetes and Digestive and Kidney Diseases (R01DK60957) and the Frances Williams Preston Laboratories of the T.J. Martell Foundation. An erratum to this article can be found at     

Studies on the phenotype of migrant thymic stem cells

Stem cells first enter the thymus around the 11^th to 12^th days of gestation in BALB/c mouse embryos. The phenotype of these stem cells has been difficult to determine because their entry occurs when the thymic primordium is very small and involves too few stem cells to allow studies by flow cytometry. We have been able to microdissect the thymus from embryos during this stage and immunophenotype cells in sections using a sensitive tyramide amplification system. Our results show that migrant stem cells express CD45, c‐kit, CD44, CD34 and α₄ integrin, but other markers such as CD62L, CD25, Thy‐1.2, CD3ϵ, α₅ integrin and RAG‐1 expression are detected only after stem cell entry. These results should help to improve the isolation and characterization of migrant thymic stem cells.     

Cellular organization of the central canal ependymal zone,a niche of latent neural stem cells in the adult mammalian spinal cord

A stem cell's microenvironment, or “niche,” is a critical regulator of its behaviour. In the adult mammalian spinal cord, central canal ependymal cells possess latent neural stem cell properties, but the ependymal cell niche has not yet been described. Here, we identify important similarities and differences between the central canal ependymal zone and the forebrain subventricular zone (SVZ), a well-characterized niche of neural stem cells. First, direct immunohistochemical comparison of the spinal cord ependymal zone and the forebrain SVZ revealed distinct patterns of neural precursor marker expression. In particular, ependymal cells in the spinal cord were found to be bordered by a previously uncharacterized sub-ependymal layer, which is relatively less elaborate than that of the SVZ and comprised of small numbers of astrocytes, oligodendrocyte progenitors and neurons. Cell proliferation surrounding the central canal occurs in close association with blood vessels, but unlike in the SVZ, involves mainly ependymal rather than sub-ependymal cells. These proliferating ependymal cells typically self-renew rather than produce transit-amplifying progenitors, as they generate doublets of progeny that remain within the ependymal layer and show no evidence of a lineage relationship to sub-ependymal cells. Interestingly, the dorsal pole of the central canal was found to possess a sub-population of tanycyte-like cells that express markers of both ependymal cells and neural precursors, and their presence correlates with higher numbers of dorsally proliferating ependymal cells. Together, these data identify key features of the spinal cord ependymal cell niche, and suggest that dorsal ependymal cells possess the potential for stem cell activity. This work provides a foundation for future studies aimed at understanding ependymal cell regulation under normal and pathological conditions.     

Building stem cell niches from the molecule up through engineered peptide materials

The native stem cell niche is a dynamic and complex microenvironment. Recapitulating this niche is a critical focus within the fields of stem cell biology, tissue engineering, and regenerative medicine and requires the development of well-defined, tunable materials. Recent biomaterial design strategies seek to create engineered matrices that interact with cells at the molecular scale and allow on-demand, cell-triggered matrix modifications. Peptide and protein engineering can accomplish these goals through the molecular-level design of bioinductive and bioresponsive materials. This brief review focuses on engineered peptide and protein materials suitable for use as in vitro neural stem cell niche mimics and in vivo central nervous system repair. A key hallmark of these materials is the immense design freedom to specify the exact amino acid sequence leading to multi-functional bulk materials with tunable properties. These advanced materials are engineered using rational design strategies to recapitulate key aspects of the native neural stem cell niche. The resulting materials often combine the advantages of biological matrices with the engineering control of synthetic polymers. Future design strategies are expected to endow these materials with multiple layers of bi-directional feedback between the cell and the matrix, which will lead to more advanced mimics of the highly dynamic neural stem cell niche.     

The hematopoietic stem cell in its place

A signature characteristic of stem cells is their ability to self-renew, affording a theoretically limitless ability to produce daughter cells and their descendents. This near-timeless dimension of stem cell function is not free of the constraints of place. The idea that highly specialized 'microenvironmental' cues participate in the regulation of stem cells has evidence in classic embryology and more recently in adult stem cells through the use of model organisms. There is now ample evidence that an anatomically defined, specifically constituted place represents the niche for hematopoietic and other tissue-specific stem cells. This review provides a conceptual framework and detailed account of the hematopoietic stem cell niche as defined at present. The components are assembling into a more complex view of the niche and may now be amenable to examination as a system and possibly to alteration to affect outcomes in immune regeneration.     

A profusion of neural stem cells in the brain of the spiny mouse,Acomys cahirinus

The vast majority of neural stem cell studies have been conducted on the brains of mice and rats, the classical model rodent. Non-model organisms may, however, give us some important insights into how to increase neural stem cell numbers for regenerative purposes and with this in mind we have characterized these cells in the brain of the spiny mouse, Acomys cahirinus. This unique mammal is highly regenerative and damaged tissue does not scar or fibrose. We find that there are more than three times as many stem cells in the SVZ and more than 3 times as many proliferating cells compared to the CD-1 outbred strain of lab mouse. These additional cells create thick stem cell regions in the wall of the SVZ and very obvious columns of cells moving into the rostral migratory stream. In the dentate gyrus, there are more than 10 times as many cells proliferating in the sub-granular layer and twice the number of doublecortin expressing neuroblasts. A preliminary analysis of some stem cell niche genes has identified Sox2, Notch1, Shh, and Noggin as up-regulated in the SVZ of Acomys and Bmp2 as being down-regulated. The highly increased neural stem cell numbers in Acomys may endow this animal with increased regenerative properties in the brain or improved physiological performance important for its survival.