Markers of epidermal stem cell subpopulations in adult mammalian skin

Stem cell dynamics, migration and plasticity during wound healing

Tissue repair is critical for animal survival. The skin epidermis is particularly exposed to injuries, which necessitates rapid repair. The coordinated action of distinct epidermal stem cells recruited from various skin regions together with other cell types, including fibroblasts and immune cells, is required to ensure efficient and harmonious wound healing. A complex crosstalk ensures the activation, migration and plasticity of these cells during tissue repair.

Annurca Apple Polyphenols Ignite Keratin Production in Hair Follicles by Inhibiting the Pentose Phosphate Pathway and Amino Acid Oxidation

Patterned hair loss (PHL) affects around 50% of the adult population worldwide. The negative impact that this condition exerts on people’s life quality has boosted the appearance of over-the-counter products endowed with hair-promoting activity. Nutraceuticals enriched in polyphenols have been recently shown to promote hair growth and counteract PHL. Malus pumila Miller cv. Annurca is an apple native to Southern Italy presenting one of the highest contents of Procyanidin B2. We have recently shown that oral consumption of Annurca polyphenolic extracts (AAE) stimulates hair growth, hair number, hair weight and keratin content in healthy human subjects. Despite its activity, the analysis of the molecular mechanism behind its hair promoting effect is still partially unclear. In this work we performed an unprecedented metabolite analysis of hair follicles (HFs) in mice topically treated with AAE. The metabolomic profile, based on a high-resolution mass spectrometry approach, revealed that AAE re-programs murine HF metabolism. AAE acts by inhibiting several NADPH dependent reactions. Glutaminolysis, pentose phosphate pathway, glutathione, citrulline and nucleotide synthesis are all halted in vivo by the treatment of HFs with AAE. On the contrary, mitochondrial respiration, β-oxidation and keratin production are stimulated by the treatment with AAE. The metabolic shift induced by AAE spares amino acids from being oxidized, ultimately keeping them available for keratin biosynthesis.

Establishment and characterization of a canine keratinocyte organoid culture system

BACKGROUND

Perturbations of epidermal and follicular homeostasis have been attributed to a variety of skin diseases affecting dogs. The availability of an in vitro system to investigate these diseases is important to understand underlying pathomechanisms.

OBJECTIVES

To establish an accurate and reliable in vitro 3D system of canine keratinocyte organoids to lay the basis for studying functional defects in interfollicular epidermis (IFE) and hair follicle (HF) morphogenesis, reconstitution and differentiation that lead to alopecic and epidermal diseases.

ANIMALS

Skin biopsies were obtained from freshly euthanized dogs of different breeds with no skin abnormalities.

METHODS

Cells derived from microdissected IFE and HFs were seeded in Matrigel and keratinocyte organoids were grown and characterized using immunohistochemistry, RT-qPCR and RNA sequencing.

RESULTS

Both organoid lines develop into a basal IFE-like cell type. Gene and protein expression analysis revealed high mRNA and protein levels of keratins 5 and 14, IFE differentiation markers and intercellular molecules. Key markers of HF stem cells were lacking. Withdrawal of growth factors resulted in upregulation of markers such as KRT16, Involucrin, KRT17 and SOX9, showing the potential of the organoids to develop towards more differentiated tissue.

CONCLUSION AND CLINICAL IMPORTANCE

Our 3D in vitro culture system provides the basis to explore epidermal function, to investigate the culture conditions necessary for the development of organoids with a HF signature and to address cutaneous disorders in dogs. However, for induction of HF signatures or hair growth, addition of different growth factors or co-culture with dermal papilla will be required.

The RNA-binding protein YBX1 regulates epidermal progenitors at a posttranscriptional level

The integrity of stratified epithelia depends on the ability of progenitor cells to maintain a balance between proliferation and differentiation. While much is known about the transcriptional pathways underlying progenitor cells’ behavior in the epidermis, the role of posttranscriptional regulation by mRNA binding proteins—a rate-limiting step in sculpting the proteome—remains poorly understood. Here we report that the RNA binding protein YBX1 (Y-box binding protein-1) is a critical effector of progenitors’ function in the epidermis. YBX1 expression is restricted to the cycling keratinocyte progenitors in vivo and its genetic ablation leads to defects in the architecture of the skin. We further demonstrate that YBX1 negatively controls epidermal progenitor senescence by regulating the translation of a senescence-associated subset of cytokine mRNAs via their 3′ untranslated regions. Our study establishes YBX1 as a posttranscriptional effector required for maintenance of epidermal homeostasis.

The integrity of the stratified epithelia relies on controlled cell turnover but it is unclear how mRNA binding proteins regulates this. Here, the authors show that the RNA binding protein Y-box binding protein-1 translationally represses cytokines, so preventing senescence and maintaining epidermal homeostasis.

The Hair Follicle: An Underutilized Source of Cells and Materials for Regenerative Medicine

The hair follicle is one of only two structures within the adult body that selectively degenerates and regenerates, making it an intriguing organ to study and use for regenerative medicine. Hair follicles have been shown to influence wound healing, angiogenesis, neurogenesis, and harbor distinct populations of stem cells; this has led to cells from the follicle being used in clinical trials for tendinosis and chronic ulcers. In addition, keratin produced by the follicle in the form of a hair fiber provides an abundant source of biomaterials for regenerative medicine. In this review, we provide an overview of the structure of a hair follicle, explain the role of the follicle in regulating the microenvironment of skin and the impact on wound healing, explore individual cell types of interest for regenerative medicine, and cover several applications of keratin-based biomaterials.

Defining Adult Stem Cells by Function, not by Phenotype

Central to the classical hematopoietic stem cell (HSC) paradigm is the concept that the maintenance of blood cell numbers is exclusively executed by a discrete physical entity: the transplantable HSC. The HSC paradigm has served as a stereotypic template in stem cell biology, yet the search for rare, hardwired professional stem cells has remained futile in most other tissues. In a more open approach, the focus on the search for stem cells as a physical entity may need to be replaced by the search for stem cell function, operationally defined as the ability of an organ to replace lost cells. The nature of such a cell may be different under steady state conditions and during tissue repair. We discuss emerging examples including the renewal strategies of the skin, gut epithelium, liver, lung, and mammary gland in comparison with those of the hematopoietic system. While certain key housekeeping and developmental signaling pathways are shared between different stem cell systems, there may be no general, deeper principles underlying the renewal mechanisms of the various individual tissues.

Epidermal Nbn deletion causes premature hair loss and a phenotype resembling psoriasiform dermatitis

Nijmegen Breakage Syndrome is a disease caused by NBN mutations. Here, we report a novel function of Nbn in skin homeostasis. We found that Nbn deficiency in hair follicle (HF) progenitors promoted increased DNA damage signaling, stimulating p16^Ink4a up-regulation, Trp53 stabilization and cytokines secretion leading to HF-growth arrest and hair loss. At later stages, the basal keratinocytes layer exhibited also enhanced DNA damage response but in contrast to the one in HF progenitor was not associated with pro-inflammatory cytokines expression, but rather increased proliferation, lack of differentiation and immune response resembling psoriasiform dermatitis. Simultaneous Nbn and Trp53 inactivation significantly exacerbated this phenotype, due to the lack of inhibition of pro-inflammatory cytokines secretion by Trp53. Altogether, we demonstrated novel functions of Nbn in HF maintenance and prevention of skin inflammation and we provide a mechanistic explanation that links cell intrinsic DNA maintenance with large scale morphological tissue alterations.

Divergent proliferation patterns of distinct human hair follicle epithelial progenitor niches in situ and their differential responsiveness to prostaglandin D2

Human scalp hair follicles (hHF) harbour several epithelial stem (eHFSC) and progenitor cell sub-populations organised into spatially distinct niches. However, the constitutive cell cycle activity of these niches remains to be characterized in situ. Therefore, the current study has studied these characteristics of keratin 15+ (K15), CD200+ or CD34+ cells within anagen VI hHFs by immunohistomorphometry, using Ki-67 and 5-ethynyl-2'-deoxyuridine (EdU). We quantitatively demonstrate in situ the relative cell cycle inactivity of the CD200+/K15+ bulge compared to other non-bulge CD34+ and K15+ progenitor compartments and found that in each recognized eHFSC/progenitor niche, proliferation associates negatively with eHFSC-marker expression. Furthermore, we also show how prostaglandin D2 (PGD2), which is upregulated in balding scalp, differentially impacts on the proliferation of distinct eHFSC populations. Namely, 24 h organ-cultured hHFs treated with PGD2 displayed reduced Ki-67 expression and EdU incorporation in bulge resident K15+ cells, but not in supra/proximal bulb outer root sheath K15+ progenitors. This study emphasises clear differences between the cell cycle behaviour of spatially distinct stem/progenitor cell niches in the hHF, and demonstrates a possible link between PGD2 and perturbed proliferation dynamics in epithelial stem cells.

Current and Future Perspectives of Stem Cell Therapy in Dermatology

Stem cells are undifferentiated cells capable of generating, sustaining, and replacing terminally differentiated cells and tissues. They can be isolated from embryonic as well as almost all adult tissues including skin, but are also generated through genetic reprogramming of differentiated cells. Preclinical and clinical research has recently tremendously improved stem cell therapy, being a promising treatment option for various diseases in which current medical therapies fail to cure, prevent progression or relieve symptoms. With the main goal of regeneration or sustained genetic correction of damaged tissue, advanced tissue-engineering techniques are especially applicable for many dermatological diseases including wound healing, genodermatoses (like the severe blistering disorder epidermolysis bullosa) and chronic (auto-)inflammatory diseases. This review summarizes general aspects as well as current and future perspectives of stem cell therapy in dermatology.

Current Understanding of the Pathways Involved in Adult Stem and Progenitor Cell Migration for Tissue Homeostasis and Repair

With the advancements in the field of adult stem and progenitor cells grows the recognition that the motility of primitive cells is a pivotal aspect of their functionality. There is accumulating evidence that the recruitment of tissue-resident and circulating cells is critical for organ homeostasis and effective injury responses, whereas the pathobiology of degenerative diseases, neoplasm and aging, might be rooted in the altered ability of immature cells to migrate. Furthermore, understanding the biological machinery determining the translocation patterns of tissue progenitors is of great relevance for the emerging methodologies for cell-based therapies and regenerative medicine. The present article provides an overview of studies addressing the physiological significance and diverse modes of stem and progenitor cell trafficking in adult mammalian organs, discusses the major microenvironmental cues regulating cell migration, and describes the implementation of live imaging approaches for the exploration of stem cell movement in tissues and the factors dictating the motility of endogenous and transplanted cells with regenerative potential.

HPV8 Field Cancerization in a Transgenic Mouse Model Is due to Lrig1+ Keratinocyte Stem Cell Expansion

β-Human papillomaviruses (HPVs) cause near ubiquitous latent skin infection within long-lived hair follicle (HF) keratinocyte stem cells. In patients with epidermodysplasia verruciformis, β-HPV viral replication is associated with skin keratosis and cutaneous squamous cell carcinoma. To determine the role of HF keratinocyte stem cells in β-HPV-induced skin carcinogenesis, we utilized a transgenic mouse model in which the keratin 14 promoter drives expression of the entire HPV8 early region (HPV8tg). HPV8tg mice developed thicker skin in comparison with wild-type littermates consistent with a hyperproliferative epidermis. HF keratinocyte proliferation was evident within the Lrig1+ keratinocyte stem cell population (69 vs. 55%, P < 0.01, n = 7), and not in the CD34+, LGR5+, and LGR6+ keratinocyte stem cell populations. This was associated with a 2.8-fold expansion in Lrig1+ keratinocytes and 3.8-fold increased colony-forming efficiency. Consistent with this, we observed nuclear p63 expression throughout this population and the HF infundibulum and adjoining interfollicular epidermis, associated with a switch from p63 transcriptional activation isoforms to ΔNp63 isoforms in HPV8tg skin. Epidermodysplasia verruciformis keratosis and in some cases actinic keratoses demonstrated similar histology associated with β-HPV reactivation and nuclear p63 expression within the HF infundibulum and perifollicular epidermis. These findings would suggest that β-HPV field cancerization arises from the HF junctional zone and predispose to squamous cell carcinoma.

Lgr6 is a stem cell marker in mouse skin squamous cell carcinoma

The G-protein-coupled receptors Lgr4/5/6 are Wnt signalling mediators, but their functions in squamous carcinomas (SCCs) are unclear. Using lineage tracing in Lgr5-EGFP-CreERT2- and Lgr6-EGFP-CreERT2- Rosa26/Tomato reporter mice, we demonstrate that Lgr6, but not Lgr5, acts as an epithelial stem cell marker in vivo in SCCs. We identify, by single molecule in situ hybridisation and cell sorting, rare Lgr6-positive cells in immortalised keratinocytes, and show that their frequency increases in advanced SCCs. Lgr6 expression is enriched in cells with stem cell characteristics, and Lgr6 downregulation in vivo causes increased epidermal proliferation, with expanded lineage tracing from Lgr6⁺ epidermal stem cells. Surprisingly, Lgr6 germline knockout mice are predisposed to SCC development, by a mechanism that includes compensatory upregulation of Lgr5. These data provide a model for human patients with germline loss of function mutations in WNT pathway genes RSPO1 or LGR4, who show increased susceptibility to squamous tumour development.

Concise Review: Mechanisms of Quiescent Hair Follicle Stem Cell Regulation

Maintaining a pool of adult stem cells is essential for tissue homeostasis and wound repair. In mammalian tissues, notably hair follicles, blood, and muscle, stem cells acquire quiescence and infrequently divide for self-renewal. Mechanistic understanding of stem cell quiescence is critical for applying these multipotent cells in regenerative medicine and interrogating their roles in human diseases such as cancer. Quiescent and dividing epithelial stem cells located in hair follicle are conspicuously organized in a spatiotemporally specific manner, allowing them to be studied at a considerable depth. Recent advancements in mouse genetics, genomics, and imaging have revealed unprecedented insights into establishment, maintenance, and regulation of quiescent hair follicle stem cells. This concise review summarizes the progress with a focus on mechanisms mediated by signaling pathways and transcription factors and discusses their implications in the understanding of stem cell biology. Stem Cells 2017;35:2323-2330.

Epidermal Stem Cells in Skin Wound Healing

Significance: Skin serves as a protective barrier for mammals. Epidermal stem cells are responsible for maintaining skin homeostasis. When cutaneous injuries occur, skin homeostasis and integrity are damaged, leading to dire consequences such as acute, chronic, or infected wounds. Skin wound healing is an intrinsic self-saving chain reaction, which is crucial to facilitating the replacement of damaged or lost tissue. Recent Advances: An immense amount of research has uncovered the underlying mechanisms behind the complex and highly regulated wound healing process. In this review, we will dissect the biological process of adult skin wound healing and emphasize the importance of epidermal stem cells during the wound healing. Critical Issues: We will comprehensively discuss the current clinical practices used on patients with cutaneous wounds, including both traditional skin grafting procedures and advanced grafting techniques with cultured skin stem cells. The majority of these leading techniques still retain some deficiencies during clinical use. Moreover, the regeneration of skin appendages after severe injuries remains a challenge in treatment. Future Directions: Understanding epidermal stem cells and their essential functions during skin wound healing are fundamental components behind the development of clinical treatment on patients with cutaneous wounds. It is important to improve the current standard of care and to develop novel techniques improving patient outcomes and long-term rehabilitation, which should be the goals of future endeavors in the field of skin wound healing.

Molecular Mechanisms of Human Papillomavirus Induced Skin Carcinogenesis

Infection of the cutaneous skin with human papillomaviruses (HPV) of genus betapapillomavirus (βHPV) is associated with the development of premalignant actinic keratoses and squamous cell carcinoma. Due to the higher viral loads of βHPVs in actinic keratoses than in cancerous lesions, it is currently discussed that these viruses play a carcinogenic role in cancer initiation. In vitro assays performed to characterize the cell transforming activities of high-risk HPV types of genus alphapapillomavirus have markedly contributed to the present knowledge on their oncogenic functions. However, these assays failed to detect oncogenic functions of βHPV early proteins. They were not suitable for investigations aiming to study the interactive role of βHPV positive epidermis with mesenchymal cells and the extracellular matrix. This review focuses on βHPV gene functions with special focus on oncogenic mechanisms that may be relevant for skin cancer development.

Type XVII collagen coordinates proliferation in the interfollicular epidermis

Type XVII collagen (COL17) is a transmembrane protein located at the epidermal basement membrane zone. COL17 deficiency results in premature hair aging phenotypes and in junctional epidermolysis bullosa. Here, we show that COL17 plays a central role in regulating interfollicular epidermis (IFE) proliferation. Loss of COL17 leads to transient IFE hypertrophy in neonatal mice owing to aberrant Wnt signaling. The replenishment of COL17 in the neonatal epidermis of COL17-null mice reverses the proliferative IFE phenotype and the altered Wnt signaling. Physical aging abolishes membranous COL17 in IFE basal cells because of inactive atypical protein kinase C signaling and also induces epidermal hyperproliferation. The overexpression of human COL17 in aged mouse epidermis suppresses IFE hypertrophy. These findings demonstrate that COL17 governs IFE proliferation of neonatal and aged skin in distinct ways. Our study indicates that COL17 could be an important target of anti-aging strategies in the skin.

Effects of gamma rays on the regeneration of murine hair follicles in the natural hair cycle

PURPOSE

This review evaluates the effects of γ-rays on the regeneration of murine hair follicles in the natural hair cycle. A series of studies were performed to investigate this issue, in which the whole bodies of C57BL/10JHir mice in the 1st telogen phase of the hair cycle were irradiated with γ-rays.

RESULTS

The dermis of the irradiated skin showed a decrease in hair follicle density and induction of curved hair follicles along with the presence of white hairs and hypopigmented hair bulbs in the 2nd and 3rd anagen phases. An increased frequency of hypopigmented hair bulbs was still observed in the later hair cycle at postnatal day 200. There was no significant difference in the number of stem cells in the hair bulge region between control and irradiated skin.

CONCLUSIONS

These results show that the effects of γ-rays on the pigmentation of murine hair follicles are persistently carried over to later hair cycles, although those on the number and structure of hair follicles appear to be hidden by the effects of aging. Our findings may be important for understanding the mechanisms of the actions of stem cells on hair regeneration in connection with age-related phenotypes.

Wounding induces dedifferentiation of epidermal Gata6+ cells and acquisition of stem cell properties

The epidermis is maintained by multiple stem cell populations whose progeny differentiate along diverse, and spatially distinct, lineages. Here we show that the transcription factor Gata6 controls the identity of the previously uncharacterized sebaceous duct (SD) lineage and identify the Gata6 downstream transcription factor network that specifies a lineage switch between sebocytes and SD cells. During wound healing differentiated Gata6⁺ cells migrate from the SD into the interfollicular epidermis and dedifferentiate, acquiring the ability to undergo long-term self-renewal and differentiate into a much wider range of epidermal lineages than in undamaged tissue. Our data not only demonstrate that the structural and functional complexity of the junctional zone is regulated by Gata6, but also reveal that dedifferentiation is a previously unrecognized property of post-mitotic, terminally differentiated cells that have lost contact with the basement membrane. This resolves the long-standing debate about the contribution of terminally differentiated cells to epidermal wound repair.

A genome-wide screen identifies YAP/WBP2 interplay conferring growth advantage on human epidermal stem cells

Individual human epidermal cells differ in their self-renewal ability. To uncover the molecular basis for this heterogeneity, we performed genome-wide pooled RNA interference screens and identified genes conferring a clonal growth advantage on normal and neoplastic (cutaneous squamous cell carcinoma, cSCC) human epidermal cells. The Hippo effector YAP was amongst the top positive growth regulators in both screens. By integrating the Hippo network interactome with our data sets, we identify WW-binding protein 2 (WBP2) as an important co-factor of YAP that enhances YAP/TEAD-mediated gene transcription. YAP and WPB2 are upregulated in actively proliferating cells of mouse and human epidermis and cSCC, and downregulated during terminal differentiation. WBP2 deletion in mouse skin results in reduced proliferation in neonatal and wounded adult epidermis. In reconstituted epidermis YAP/WBP2 activity is controlled by intercellular adhesion rather than canonical Hippo signalling. We propose that defective intercellular adhesion contributes to uncontrolled cSCC growth by preventing inhibition of YAP/WBP2.

Unlocking the Neurogenic Potential of Mammalian Müller Glia

Müller glia (MG) are the primary support cells in the vertebrate retina, regulating homeostasis in one of the most metabolically active tissues. In lower vertebrates such as fish, they respond to injury by proliferating and reprogramming to regenerate retinal neurons. In mammals, MG may also react to injury by proliferating, but they fail to initiate regeneration. The barriers to regeneration could be intrinsic to mammalian MG or the function of the niche that cannot support the MG reprogramming required for lineage conversion or both. Understanding these mechanisms in light of those being discovered in fish may lead to the formulation of strategies to unlock the neurogenic potential of MG and restore regeneration in the mammalian retina.

Single-Cell Transcriptomics Reveals that Differentiation and Spatial Signatures Shape Epidermal and Hair Follicle Heterogeneity

The murine epidermis with its hair follicles represents an invaluable model system for tissue regeneration and stem cell research. Here we used single-cell RNA-sequencing to reveal how cellular heterogeneity of murine telogen epidermis is tuned at the transcriptional level. Unbiased clustering of 1,422 single-cell transcriptomes revealed 25 distinct populations of interfollicular and follicular epidermal cells. Our data allowed the reconstruction of gene expression programs during epidermal differentiation and along the proximal-distal axis of the hair follicle at unprecedented resolution. Moreover, transcriptional heterogeneity of the epidermis can essentially be explained along these two axes, and we show that heterogeneity in stem cell compartments generally reflects this model: stem cell populations are segregated by spatial signatures but share a common basal-epidermal gene module. This study provides an unbiased and systematic view of transcriptional organization of adult epidermis and highlights how cellular heterogeneity can be orchestrated in vivo to assure tissue homeostasis.

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Single-cell RNA-seq analysis identifies 25 populations of epidermal cells

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Differentiation and spatial gene expression signatures can be defined

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Interplay of differentiation and spatial signatures explains most heterogeneity

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Stem cell populations are divided by spatial signatures and only share basal identity

β-Catenin Stabilization in Skin Fibroblasts Causes Fibrotic Lesions by Preventing Adipocyte Differentiation of the Reticular Dermis

The Wnt/β-catenin pathway plays a central role in epidermal homeostasis and regeneration, but how it affects fibroblast fate decisions is unknown. We investigated the effect of targeted β-catenin stabilization in dermal fibroblasts. Comparative gene expression profiling of stem cell antigen 1^- (Sca1^-) and Sca1⁺ neonatal fibroblasts from upper and lower dermis, respectively, confirmed that Sca1⁺ cells had a preadipocyte signature and showed differential expression of Wnt/β-catenin–associated genes. By targeting all fibroblasts or selectively targeting Dlk1⁺ lower dermal fibroblasts, we found that β-catenin stabilization between developmental stages E16.5 and P2 resulted in a reduction in the dermal adipocyte layer with a corresponding increase in dermal fibrosis and an altered hair cycle. The fibrotic phenotype correlated with a reduction in the potential of Sca1⁺ fibroblasts to undergo adipogenic differentiation ex vivo. Our findings indicate that Wnt/β-catenin signaling controls adipogenic cell fate within the lower dermis, which potentially contributes to the pathogenesis of fibrotic skin diseases.

Survivin Modulates Squamous Cell Carcinoma-Derived Stem-Like Cell Proliferation, Viability and Tumor Formation in Vivo

Squamous Cell Carcinoma-derived Stem-like Cells (SCC-SC) originate from alterations in keratinocyte stem cells (KSC) gene expression and sustain tumor development, invasion and recurrence. Since survivin, a KSC marker, is highly expressed in SCC-SC, we evaluate its role in SCC-SC cell growth and SCC models. Survivin silencing by siRNA decreases clonal growth of SCC keratinocytes and viability of total, rapidly adhering (RAD) and non-RAD (NRAD) cells from primary SCC. Similarly, survivin silencing reduces the expression of stem cell markers (OCT4, NOTCH1, CD133, β₁-integrin), while it increases the level of differentiation markers (K10, involucrin). Moreover, survivin silencing improves the malignant phenotype of SCC 3D-reconstruct, as demonstrated by reduced epidermal thickness, lower Ki-67 positive cell number, and decreased expression of MMP9 and psoriasin. Furthermore, survivin depletion by siRNA in Ras^G12V-IκBα-derived tumors leads to smaller tumor formation characterized by lower mitotic index and reduced expression of the tumor-associated marker HIF1α, VEGF and CD51. Therefore, our results indicate survivin as a key gene in regulating SCC cancer stem cell formation and cSCC development.

Compartmentalized Epidermal Activation of β-Catenin Differentially Affects Lineage Reprogramming and Underlies Tumor Heterogeneity

Wnt/β-catenin activation in adult epidermis can induce new hair follicle formation and tumor development. We used lineage tracing to uncover the relative contribution of different stem cell populations. LGR6(+) and LRIG1(+) stem cells contributed to ectopic hair follicles formed in the sebaceous gland upon β-catenin activation, whereas LGR5(+) cells did not. Lgr6, but not Lrig1 or Lgr5, was expressed in a subpopulation of interfollicular epidermal cells that were competent to form new hair follicles. Oncogenic β-catenin expression in LGR5(+) cells led to formation of pilomatricomas, while LRIG1(+) cells formed trichoadenomas and LGR6(+) cells formed dermatofibromas. Tumor formation was always accompanied by a local increase in dermal fibroblast density and transient extracellular matrix remodeling. However, each tumor had a distinct stromal signature in terms of immune cell infiltrate and expression of CD26 and CD44. We conclude that compartmentalization of epidermal stem cells underlies different responses to β-catenin and skin tumor heterogeneity.

Dynamics of Lgr6⁺ Progenitor Cells in the Hair Follicle, Sebaceous Gland, and Interfollicular Epidermis

The dynamics and interactions between stem cell pools in the hair follicle (HF), sebaceous gland (SG), and interfollicular epidermis (IFE) of murine skin are still poorly understood. In this study, we used multicolor lineage tracing to mark Lgr6⁺ -expressing basal cells in the HF isthmus, SG, and IFE.We show that these Lgr6⁺ cells constitute long-term self-renewing populations within each compartment in adult skin. Quantitative analysis of clonal dynamics revealed that the Lgr6⁺ progenitor cells compete neutrally in the IFE, isthmus, and SG, indicating population asymmetry as the underlying mode of tissue renewal. Transcriptional profiling of Lgr6⁺ and Lgr6⁺ cells did not reveal a distinct Lgr6⁺ -associated gene expression signature, raising the question of whether Lgr6⁺ expression requires extrinsic niche signals. Our results elucidate the interrelation and behavior of Lgr6⁺ populations in the IFE, HF, and SG and suggest population asymmetry as a common mechanism for homeostasis in several epithelial skin compartments.

The Androgen Receptor Antagonizes Wnt/β-Catenin Signaling in Epidermal Stem Cells

Activation of Wnt/β-catenin signaling in adult mouse epidermis leads to expansion of the stem cell compartment and redirects keratinocytes in the interfollicular epidermis and sebaceous glands (SGs) to differentiate along the hair follicle (HF) lineages. Here we demonstrate that during epidermal development and homeostasis there is reciprocal activation of the androgen receptor (AR) and β-catenin in cells of the HF bulb. AR activation reduced β-catenin-dependent transcription, blocked β-catenin-induced induction of HF growth, and prevented β-catenin-mediated conversion of SGs into HFs. Conversely, AR inhibition enhanced the effects of β-catenin activation, promoting HF proliferation and differentiation, culminating in the formation of benign HF tumors and a complete loss of SG identity. We conclude that AR signaling has a key role in epidermal stem cell fate selection by modulating responses to β-catenin in adult mouse skin.

Exercise and Stem Cells

Stem cells are traditionally studied in the context of embryonic development, yet studies confirm that a fraction remains in the adult organism for the purpose of daily remodeling and rejuvenation of multiple tissues following injury. Adult stem cells (ASCs) are found in close proximity to vessels and respond to tissue-specific cues in the microenvironment that dictate their fate and function. Exercise can dramatically alter strain sensing, extracellular matrix composition, and inflammation, and such changes in the niche likely alter ASC quantity and function postexercise. The field of stem cell biology is still in its infancy and identification and terminology of ASCs continues to evolve; thus, current information regarding exercise and stem cells is lacking. This chapter summarizes the literature that reports on the ASC response to acute exercise and exercise training, with particular emphasis on hematopoietic stem cells, endothelial progenitor cells, and mesenchymal stem cells.

The Dynamic Duo: Niche/Stem Cell Interdependency

Most tissues in our bodies undergo constant cellular turnover. This process requires a dynamic balance between cell production and elimination. Stem cells have been shown in many of these tissues to be the major source of new cells. However, despite the tremendous advances made, it still remains unclear how stem cell behavior and activity are regulated in vivo. Furthermore, we lack basic understanding for the mechanisms that coordinate niche/stem cell interactions to maintain normal tissue homeostasis. Our lab has established a novel imaging approach in live mice using the skin as a model system to investigate these fundamental processes in both physiological and pathological settings such as cancer, with the goal of understanding how tissues successfully orchestrate tissue regeneration throughout the lifetime of an organism.

Mammalian skin cell biology: at the interface between laboratory and clinic

Mammalian skin research represents the convergence of three complementary disciplines: cell biology, mouse genetics, and dermatology. The skin provides a paradigm for current research in cell adhesion, inflammation, and tissue stem cells. Here, I discuss recent insights into the cell biology of skin. Single-cell analysis has revealed that human epidermal stem cells are heterogeneous and differentiate in response to multiple extrinsic signals. Live-cell imaging, optogenetics, and cell ablation experiments show skin cells to be remarkably dynamic. High-throughput, genome-wide approaches have yielded unprecedented insights into the circuitry that controls epidermal stem cell fate. Last, integrative biological analysis of human skin disorders has revealed unexpected functions for elements of the skin that were previously considered purely structural.

BLIMP1 is required for postnatal epidermal homeostasis but does not define a sebaceous gland progenitor under steady-state conditions

B-lymphocyte-induced nuclear maturation protein 1 (BLIMP1) was previously reported to define a sebaceous gland (SG) progenitor population in the epidermis. However, the recent identification of multiple stem cell populations in the hair follicle junctional zone has led us to re-evaluate its function. We show, in agreement with previous studies, that BLIMP1 is expressed by postmitotic, terminally differentiated epidermal cells within the SG, interfollicular epidermis, and hair follicle. Epidermal overexpression of c-Myc results in loss of BLIMP1(+) cells, an effect modulated by androgen signaling. Epidermal-specific deletion of Blimp1 causes multiple differentiation defects in the epidermis in addition to SG enlargement. In culture, BLIMP1(+) sebocytes have no greater clonogenic potential than BLIMP1(-) sebocytes. Finally, lineage-tracing experiments reveal that, under steady-state conditions, BLIMP1-expressing cells do not divide. Thus, rather than defining a sebocyte progenitor population, BLIMP1 functions in terminally differentiated cells to maintain homeostasis in multiple epidermal compartments.

Novel skin phenotypes revealed by a genome-wide mouse reverse genetic screen

Permanent stop-and-shop large-scale mouse mutant resources provide an excellent platform to decipher tissue phenogenomics. Here we analyse skin from 538 knockout mouse mutants generated by the Sanger Institute Mouse Genetics Project. We optimize immunolabelling of tail epidermal wholemounts to allow systematic annotation of hair follicle, sebaceous gland and interfollicular epidermal abnormalities using ontology terms from the Mammalian Phenotype Ontology. Of the 50 mutants with an epidermal phenotype, 9 map to human genetic conditions with skin abnormalities. Some mutant genes are expressed in the skin, whereas others are not, indicating systemic effects. One phenotype is affected by diet and several are incompletely penetrant. In-depth analysis of three mutants, Krt76, Myo5a (a model of human Griscelli syndrome) and Mysm1, provides validation of the screen. Our study is the first large-scale genome-wide tissue phenotype screen from the International Knockout Mouse Consortium and provides an open access resource for the scientific community.

Large-scale efforts are put into the generation of knockout mutant mice for many individual genes. Here, the authors systematically screen skin from 538 mutant mice and identify 50 mutants with epidermal phenotypes, of which 9 are also associated with human skin defects.