Label retaining cells and cutaneous stem cells

New insights into inflammatory memory of epidermal stem cells

Inflammatory memory, as one form of innate immune memory, has a wide range of manifestations, and its occurrence is related to cell epigenetic modification or metabolic transformation. When re-encountering similar stimuli, executing cells with inflammatory memory function show enhanced or tolerated inflammatory response. Studies have identified that not only hematopoietic stem cells and fibroblasts have immune memory effects, but also stem cells from various barrier epithelial tissues generate and maintain inflammatory memory. Epidermal stem cells, especially hair follicle stem cells, play an essential role in wound healing, immune-related skin diseases, and skin cancer development. In recent years, it has been found that epidermal stem cells from hair follicle can remember the inflammatory response and implement a more rapid response to subsequent stimuli. This review updates the advances of inflammatory memory and focuses on its mechanisms in epidermal stem cells. We are finally looking forward to further research on inflammatory memory, which will allow for the development of precise strategies to manipulate host responses to infection, injury, and inflammatory skin disease.

L-arginine homeostasis governs adult neural stem cell activation by modulating energy metabolism in vivo

Neurogenesis in the developing and adult brain is intimately linked to remodeling of cellular metabolism. However, it is still unclear how distinct metabolic programs and energy sources govern neural stem cell (NSC) behavior and subsequent neuronal differentiation. Here, we found that adult mice lacking the mitochondrial urea metabolism enzyme, Arginase-II (Arg-II), exhibited NSC overactivation, thereby leading to accelerated NSC pool depletion and decreased hippocampal neurogenesis over time. Mechanistically, Arg-II deficiency resulted in elevated L-arginine levels and induction of a metabolic shift from glycolysis to oxidative phosphorylation (OXPHOS) caused by impaired attachment of hexokinase-I to mitochondria. Notably, selective inhibition of OXPHOS ameliorated NSC overactivation and restored abnormal neurogenesis in Arg-II deficient mice. Therefore, Arg-II-mediated intracellular L-arginine homeostasis directly influences the metabolic fitness of neural stem cells that is essential to maintain neurogenesis with age.

D-2-hydroxyglutarate dehydrogenase governs adult neural stem cell activation and promotes histone acetylation via ATP-citrate lyase

The generation of neurons from quiescent radial-glia-like neural stem cells (RGLs) in adult brain goes hand in hand with the modulation of cellular metabolism. However, it is still unclear how the exact metabolic program governs the balance between quiescent and activated RGLs. Here, we find that loss of mitochondrial D-2-hydroxyglutarate dehydrogenase (D2HGDH) leads to aberrant accumulation of D-2-hydroxyglutarate (D-2-HG) and impaired RGL activation. Mechanistically, accumulated D-2-HG bonds directly to ATP-citrate lyase and competitively inhibits its enzymatic activity, thereby reducing acetyl-CoA production and diminishing histone acetylation. However, administration of acetate restores the acetyl-CoA levels via acetyl-CoA synthetase-mediated catabolism and rescues the deficiencies in histone acetylation and RGL activation caused by loss of D2HGDH. Therefore, our findings define the role of cross talk between mitochondria and the nucleus via a mitochondrial metabolite, D-2-HG, the aberrant accumulation of which hinders the regulation of histone acetylation in RGL activation and attenuates continuous neurogenesis in adult mammalian brain.

Human Basal and Suprabasal Keratinocytes Are Both Able to Generate and Maintain Dermo-Epidermal Skin Substitutes in Long-Term In Vivo Experiments

The basal layer of human interfollicular epidermis has been described to harbour both quiescent keratinocyte stem cells and a transit amplifying cell population that maintains the suprabasal epidermal layers. We performed immunofluorescence analyses and revealed that the main proliferative keratinocyte pool in vivo resides suprabasally. We isolated from the human epidermis two distinct cell populations, the basal and the suprabasal keratinocytes, according to the expression of integrin β4 (iβ4). We compared basal iβ4⁺ or suprabasal iβ4^- keratinocytes with respect to their proliferation and colony-forming ability and their Raman spectral properties. In addition, we generated dermo-epidermal substitutes using freshly isolated and sorted basal iβ4⁺ or suprabasal iβ4^- keratinocytes and transplanted them on immuno-compromised rats. We show that suprabasal iβ4^- keratinocytes acquire a similar proliferative capacity as basal iβ4⁺ keratinocytes after two weeks of culture in vitro, with expression of high levels of iβ4 and downregulation of K10 expression. In addition, both basal iβ4⁺ and suprabasal iβ4^- keratinocytes acquire authentic self-renewing properties during the in vitro 3D-culture phase and are able to generate and maintain a fully stratified epidermis for 16 weeks in vivo. Therefore, against the leading dogma, we propose that human suprabasal keratinocytes can retro-differentiate into true basal stem cells in a wound situation and/or when in contact with the basement membrane.

The evolving definition of salivary gland stem cells

Dysfunction of the salivary gland and irreversible hyposalivation are the main side effects of radiotherapy treatment for head and neck cancer leading to a drastic decrease of the quality of life of the patients. Approaches aimed at regenerating damaged salivary glands have been proposed as means to provide long-term restoration of tissue function in the affected patients. In studies to elucidate salivary gland regenerative mechanisms, more and more evidence suggests that salivary gland stem/progenitor cell behavior, like many other adult tissues, does not follow that of the hard-wired professional stem cells of the hematopoietic system. In this review, we provide evidence showing that several cell types within the salivary gland epithelium can serve as stem/progenitor-like cells. While these cell populations seem to function mostly as lineage-restricted progenitors during homeostasis, we indicate that upon damage specific plasticity mechanisms might be activated to take part in regeneration of the tissue. In light of these insights, we provide an overview of how recent developments in the adult stem cell research field are changing our thinking of the definition of salivary gland stem cells and their potential plasticity upon damage. These new perspectives may have important implications on the development of new therapeutic approaches to rescue radiation-induced hyposalivation.

Epidermal Stem Cells in Hair Follicle Cycling and Skin Regeneration: A View From the Perspective of Inflammation

There are many studies devoted to the role of hair follicle stem cells in wound healing as well as in follicle self-restoration. At the same time, the influence of the inflammatory cells on the hair follicle cycling in both injured and intact skin is well established. Immune cells of all wound healing stages, including macrophages, γδT cells, and T regs, may activate epidermal stem cells to provide re-epithelization and wound-induced hair follicle neogenesis. In addition to the ability of epidermal cells to maintain epidermal morphogenesis through differentiation program, they can undergo de-differentiation and acquire stem features under the influence of inflammatory milieu. Simultaneously, a stem cell compartment may undergo re-programming to adopt another fate. The proportion of skin resident immune cells and wound-attracted inflammatory cells (e.g., neutrophils and macrophages) in wound-induced hair follicle anagen and plucking-induced anagen is still under discussion to date. Experimental data suggesting the role of reactive oxygen species and prostaglandins, which are uncharacteristic of the intact skin, in the hair follicle cycling indicates the role of neutrophils in injury-induced conditions. In this review, we discuss some of the hair follicles stem cell activities, such as wound-induced hair follicle neogenesis, hair follicle cycling, and re-epithelization, through the prism of inflammation. The plasticity of epidermal stem cells under the influence of inflammatory microenvironment is considered. The relationship between inflammation, scarring, and follicle neogenesis as an indicator of complete wound healing is also highlighted. Taking into consideration the available data, we also conclude that there may exist a presumptive interlink between the stem cell activation, inflammation and the components of programmed cell death pathways.

The Reconstructed Human Epidermis in vitro — a Model for Basic and Applied Research of Human Skin

Background. The reconstructed human epidermis (RE) is an in vitro tissue-engineering construct similar to the native epidermis. Objective. To develop a full-layer RE. Describe its structure: determine the presence of all layers of the epidermal component, including basal, spinous and granular layers and stratum corneum of the epidermis; detect the basement membrane, the border between the epidermal and mesenchymal component. Materials and methods. Isolation of keratinocytes and fibroblasts from human donor skin. Cultivation of keratinocytes and fibroblasts in vitro under 2D conditions, cell subculturing and 3D modeling of RE, obtaining cryosections, histological staining, immunohistochemical (IHC) study with antibodies to cytokeratins 14 and 10, Ki67 protein, loricrin, laminin 5 and plectin. Results. A technique was developed for the formation of RE. Histological examination showed that the stratification of keratinocyte layers occurs during the formation of RE. Layers are formed including basal, spinous and granular layers and stratum corneum. The IHC study has shown the proliferative activity of keratinocytes of the basal layer and has detected the presence of marker proteins of keratinocytes at different stages of differentiation. RE basal keratinocytes, like native ones, form hemidesmosomes and synthesize basement membrane proteins. Conclusions. A full-layer human RE was obtained in vitro. RE meets all the characteristics of the native epidermis and it is suitable for basic and practical research in the field of skin biology, dermatology, and cosmetology.

Skin Stem Cells, Their Niche and Tissue Engineering Approach for Skin Regeneration

Skin is the main organ that covers the human body and acts as a protective barrier between the human body and the environment. Skin tissue as a stem cell source can be used for transplantation in therapeutic application in terms of its properties such as abundant, easy to access, high plasticity and high ability to regenerate. The immunological profile of these cells makes it a suitable resource for autologous and allogeneic applications. The lack of major histo-compatibility complex 1 is also advantageous in its use. Epidermal stem cells are the main stem cells in the skin and are suitable cells in tissue engineering studies for their important role in wound repair. In the last 30 years, many studies have been conducted to develop substitutions that mimic human skin. Stem cell-based skin substitutions have been developed to be used in clinical applications, to support the healing of acute and chronic wounds and as test systems for dermatological and pharmacological applications. In this chapter, tissue specific properties of epidermal stem cells, composition of their niche, regenerative approaches and repair of tissue degeneration have been examined.

Salivary gland stem cells: A review of development, regeneration and cancer

Salivary glands are responsible for maintaining the health of the oral cavity and are routinely damaged by therapeutic radiation for head and neck cancer as well as by autoimmune diseases such as Sjögren's syndrome. Regenerative approaches based on the reactivation of endogenous stem cells or the transplant of exogenous stem cells hold substantial promise in restoring the structure and function of these organs to improve patient quality of life. However, these approaches have been hampered by a lack of knowledge on the identity of salivary stem cell populations and their regulators. In this review we discuss our current knowledge on salivary stem cells and their regulators during organ development, homeostasis and regeneration. As increasing evidence in other systems suggests that progenitor cells may be a source of cancer, we also review whether these same salivary stem cells may also be cancer initiating cells.

Hair Germ Model In Vitro via Human Postnatal Keratinocyte-Dermal Papilla Interactions: Impact of Hyaluronic Acid

Hair follicle (HF) reconstruction in vitro is a promising field in alopecia treatment and human HF development research. Here, we combined postnatal human dermal papilla (DP) cells and skin epidermal keratinocytes (KCs) in a hanging drop culture to develop an artificial HF germ. The method is based on DP cell hair-inducing properties and KC self-organization. We evaluated two protocols of aggregate assembling. Mixed HF germ-like structures demonstrated the initiation of epithelial-mesenchymal interaction, including WNT pathway activation and expression of follicular markers. We analyzed the influence of possible DP cell niche components including soluble factors and extracellular matrix (ECM) molecules in the process of the organoid assembling and growth. Our results demonstrated that soluble factors had little impact on HF germ generation and Ki67⁺ cell score inside the organoids although BMP6 and VD3 maintained effectively the DP identity in the monolayer culture. Aggrecan, biglycan, fibronectin, and hyaluronic acid (HA) significantly stimulated cell proliferation in DP cell monolayer culture without any effect on DP cell identity. Most of ECM compounds prevented the formation of cell aggregates while HA promoted the formation of larger organoids. In conclusion, our model could be suitable to study cell-cell and cell-niche interactions during HF reconstruction in vitro.

Association of TGFβ signaling with the maintenance of a quiescent stem cell niche in human oral mucosa

A dogma in squamous epithelial biology is that proliferation occurs in the basal cell layer. Notable exceptions are squamous epithelia of the human oral cavity, esophagus, ectocervix, and vagina. In these human epithelia, proliferation is rare in the basal cell layer, and the vast majority of cells positive for Ki67 and other proliferation markers are found in para- and suprabasal cell layers. This unique human feature of a generally quiescent basal cell layer overlaid by highly proliferative cells offers the rare opportunity to study the molecular features of undifferentiated, quiescent, putative stem cells in their natural context. Here, we show that the quiescent human oral mucosa basal cell layer expresses putative markers of stemness, while para- and suprabasal cells are characterized by cell cycle genes. We identified a TGFβ signature in this quiescent basal cell layer. In in vitro organotypic cultures, human keratinocytes could be induced to express markers of these quiescent basal cells when TGFβ signaling is activated. The study suggests that the separation of basal cell layer and proliferation in human oral mucosa may function to accommodate high proliferation rates and the protection of a quiescent reserve stem cell pool. Psoriasis, an epidermal inflammatory hyperproliferative disease, exhibits features of a quiescent basal cell layer mimicking normal oral mucosa. Our data indicate that structural changes in the organization of epithelial proliferation could contribute to longevity and carcinogenesis.

Return to quiescence of mouse neural stem cells by degradation of a proactivation protein

Quiescence is essential for long-term maintenance of adult stem cells. Niche signals regulate the transit of stem cells from dormant to activated states. Here, we show that the E3-ubiquitin ligase Huwe1 (HECT, UBA, and WWE domain-containing 1) is required for proliferating stem cells of the adult mouse hippocampus to return to quiescence. Huwe1 destabilizes proactivation protein Ascl1 (achaete-scute family bHLH transcription factor 1) in proliferating hippocampal stem cells, which prevents accumulation of cyclin Ds and promotes the return to a resting state. When stem cells fail to return to quiescence, the proliferative stem cell pool becomes depleted. Thus, long-term maintenance of hippocampal neurogenesis depends on the return of stem cells to a transient quiescent state through the rapid degradation of a key proactivation factor.

Maintenance of sweat glands by stem cells located in the acral epithelium

The skin is responsible for a variety of physiological functions and is critical for wound healing and repair. Therefore, the regenerative capacity of the skin is important. However, stem cells responsible for maintaining the acral epithelium had not previously been identified. In this study, we identified the specific stem cells in the acral epithelium that participate in the long-term maintenance of sweat glands, ducts, and interadnexal epidermis and that facilitate the regeneration of these structures following injury. Lgr6-positive cells and Bmi1-positive cells were found to function as long-term multipotent stem cells that maintained the entire eccrine unit and the interadnexal epidermis. However, while Lgr6-positive cells were rapidly cycled and constantly supplied differentiated cells, Bmi1-positive cells were slow to cycle and occasionally entered the cell cycle under physiological conditions. Upon irradiation-induced injury, Bmi1-positive cells rapidly proliferated and regenerated injured epithelial tissue. Therefore, Bmi1-positive stem cells served as reservoir stem cells. Lgr5-positive cells were rapidly cycled and maintained only sweat glands; therefore, we concluded that these cells functioned as lineage-restricted progenitors. Taken together, our data demonstrated the identification of stem cells that maintained the entire acral epithelium and supported the different roles of three cellular classes.

Hair follicle morphogenesis and epidermal homeostasis in we/we wal/wal mice with postnatal alopecia

Mice with skin and hair follicle (HF) defects are common models of human skin disorders. A mutant strain with the we/we wal/wal genotype develops alopecia. We found the hair shaft structure in the pelage of mutant mice to have significant defects. Although these mice lose their hair at 21 days, a label-retaining cell population persists in HFs until at least day 54. Depilation-induced anagen was accomplished in we/we wal/wal mutants but the resulting hair shafts were short and extremely deformed. Serious abnormalities in epidermis stratification and HF morphogenesis exist in we/we wal/wal homozygous E18.5 embryos. There were significantly fewer HF primordia in this mutant compared with wild type. We discovered specific structures, identified as invalid placodes, positive for ectodysplasin A1 receptor, nuclear β-catenin, and LEF1, which failed to invaginate, produced a double basal-like layer of epidermal cells, and lacked cylindrical keratinocytes. Specification of dermal papillae (DP) was impaired, and the papillary dermis expressed alkaline phosphatase and LEF1. We also detected DP-like groups of intensively stained cells in the absence of visible signs of folliculogenesis in the epidermis. We showed differentiation disturbances in the mutant embryonic E18.5 epidermis and HFs: The cornified layer was absent, the width of the spinous layer was reduced, and HFs lacked LEF1-positive precortex cells. In this study, we used a very interesting and useful mouse model of alopecia. The presence of symptoms of skin disorders in we/we wal/wal murine embryos correlates with the postnatal skin phenotype. This correlation may help to evaluate reasons of alopecia.

Label-retaining cells in the adult murine salivary glands possess characteristics of adult progenitor cells

Radiotherapy is the primary treatment for patients with head and neck cancer, which account for roughly 500,000 annual cases worldwide. Dysfunction of the salivary glands and associated conditions like xerostomia and dysphagia are often developed by these patients, greatly diminishing their life quality. Current preventative and palliative care fail to deliver an improvement in the quality of life, thus accentuating the need for regenerative therapies. In this study, a model of label retaining cells (LRCs) in murine salivary glands was developed, in which LRCs demonstrated proliferative potential and possessed markers of putative salivary progenitors. Mice were labeled with 5-Ethynyl-2′-deoxyuridine (EdU) at postnatal day 10 and chased for 8 weeks. Tissue sections from salivary glands obtained at the end of chase demonstrated co-localization between LRCs and the salivary progenitor markers keratin 5 and keratin 14, as well as kit mRNA, indicating that LRCs encompass a heterogeneous population of salivary progenitors. Proliferative potential of LRCs was demonstrated by a sphere assay, in which LRCs were found in primary and secondary spheres and they co-localized with the proliferation marker Ki67 throughout sphere formation. Surprisingly, LRCs were shown to be radio-resistant and evade apoptosis following radiation treatment. The clinical significance of these findings lie in the potential of this model to study the mechanisms that prevent salivary progenitors from maintaining homeostasis upon exposure to radiation, which will in turn facilitate the development of regenerative therapies for salivary gland dysfunction.

Label-retaining, quiescent globose basal cells are found in the olfactory epithelium

The vertebrate olfactory epithelium (OE) is known for its ability to renew itself throughout life as well as to reconstitute after injury. Although this remarkable capacity demonstrates the persistence of stem cells and multipotent progenitor cells, their nature in the OE remains undefined and controversial, as both horizontal basal cells (HBCs) and globose basal cells (GBCs) have features in common with each other and with stem cells in other tissues. Here, we investigate whether some among the population of GBCs satisfy a key feature of stem cells, i.e., mitotic quiescence with retention of thymidine analogue label and activation by injury. Accordingly, we demonstrate that some GBCs express p27(Kip1) , a member of the Kip/Cip family of cyclin-dependent kinase inhibitors. In addition, some GBCs retain bromodeoxyuridine or ethynyldeoxyuridine for an extended period when the pulse is administered in neonates followed by a 1-month chase. Their identity as GBCs was confirmed by electron microscopy. All spared GBCs express Ki-67 in the methyl bromide (MeBr)-lesioned OE initially after lesion, indicating that the label-retaining (LR) GBCs are activated in response to injury. LR-GBCs reappear during the acute recovery period following MeBr exposure, as demonstrated with 2- or 4-week chase periods after labeling. Taken together, our data demonstrate the existence of LR-GBCs that are seemingly activated in response to epithelial injury and then re-established after the initial phase of recovery is completed. In this regard, some among the GBCs satisfy a common criterion for functioning like stem cells.

Human epithelial hair follicle stem cells and their progeny: current state of knowledge, the widening gap in translational research and future challenges

Epithelial hair follicle stem cells (eHFSCs) are required to generate, maintain and renew the continuously cycling hair follicle (HF), supply cells that produce the keratinized hair shaft and aid in the reepithelialization of injured skin. Therefore, their study is biologically and clinically important, from alopecia to carcinogenesis and regenerative medicine. However, human eHFSCs remain ill defined compared to their murine counterparts, and it is unclear which murine eHFSC markers really apply to the human HF. We address this by reviewing current concepts on human eHFSC biology, their immediate progeny and their molecular markers, focusing on Keratin 15 and 19, CD200, CD34, PHLDA1, and EpCAM/Ber-EP4. After delineating how human eHFSCs may be selectively targeted experimentally, we close by defining as yet unmet key challenges in human eHFSC research. The ultimate goal is to transfer emerging concepts from murine epithelial stem cell biology to human HF physiology and pathology.

PKH(high) cells within clonal human nephrospheres provide a purified adult renal stem cell population

The existence and identification of adult renal stem cells is a controversial issue. In this study, renal stem cells were identified from cultures of clonal human nephrospheres. The cultured nephrospheres exhibited the activation of stem cell pathways and contained cells at different levels of maturation. In each nephrosphere the presence of 1.12-1.25 cells mirroring stem cell properties was calculated. The nephrosphere cells were able to generate three-dimensional tubular structures in 3D cultures and in vivo. In clonal human nephrospheres a PKH(high) phenotype was isolated using PKH26 epifluorescence, which can identify quiescent cells within the nephrospheres. The PKH(high) cells, capable of self-renewal and of generating a differentiated epithelial, endothelial and podocytic progeny, can also survive in vivo maintaining the undifferentiated status. The PKH(high) status, together with a CD133(+)/CD24(-) phenotype, identified a homogeneous cell population displaying in vitro self-renewal and multipotency capacity. The resident adult renal stem cell population isolated from nephrospheres can be used for the study of mechanisms that regulate self-renewal and differentiation in adult renal tissue as well as in renal pathological conditions.

Identification and analysis of epidermal stem cells from primary mouse keratinocytes

The skin, one of the largest organs of the body, is a dynamic tissue in which terminally differentiated keratinocytes are replaced by the proliferation and differentiation of epidermal stem cells. Epidermal stem cells are relatively undifferentiated, retain a high capacity for self-renewal throughout their lifetime, and normally have a slow cell division cycle in vivo. Furthermore, they have a high proliferation potential in vitro, and it is often desirable to isolate and culture them from adult mice to use in conjunction with in vivo studies. However, the isolation of these cells has been problematic. Here, we describe reliable methods for identifying a population of isolated bulge stem cells by flow cytometry and for measuring the growth and differentiation potential of primary mouse keratinocytes by clonal analysis.

Tissue stem cells: new tools and functional diversity

The detailed understanding of adult tissue stem cells has significance for both regenerative medicine and oncology. This perspective will discuss how major advances in our ability to identify and monitor these cells, which include genetic lineage tracing, FACS purification, and robust in vitro clonogenic assays, have changed our view of their roles in many organs. Label retention and quiescence are no longer considered obligatory stem cell features. Furthermore, some tissues have more than one type of stem cell, each used in only particular situations of regenerative stress. Thus, there is no "one size fits all" adult tissue stem cell paradigm.