Expression of the orphan protein Plet-1 during trichilemmal differentiation of anagen hair follicles

Alveolar macrophage-expressed Plet1 is a driver of lung epithelial repair after viral pneumonia

Influenza A virus (IAV) infection mobilizes bone marrow-derived macrophages (BMDM) that gradually undergo transition to tissue-resident alveolar macrophages (TR-AM) in the inflamed lung. Combining high-dimensional single-cell transcriptomics with complex lung organoid modeling, in vivo adoptive cell transfer, and BMDM-specific gene targeting, we found that transitioning ("regenerative") BMDM and TR-AM highly express Placenta-expressed transcript 1 (Plet1). We reveal that Plet1 is released from alveolar macrophages, and acts as important mediator of macrophage-epithelial cross-talk during lung repair by inducing proliferation of alveolar epithelial cells and re-sealing of the epithelial barrier. Intratracheal administration of recombinant Plet1 early in the disease course attenuated viral lung injury and rescued mice from otherwise fatal disease, highlighting its therapeutic potential.

Distinct mechanisms for sebaceous gland self-renewal and regeneration provide durability in response to injury

Sebaceous glands (SGs) release oils that protect our skin, but how these glands respond to injury has not been previously examined. Here, we report that SGs are largely self-renewed by dedicated stem cell pools during homeostasis. Using targeted single-cell RNA sequencing, we uncovered both direct and indirect paths by which resident SG progenitors ordinarily differentiate into sebocytes, including transit through a Krt5+PPARγ+ transitional basal cell state. Upon skin injury, however, SG progenitors depart their niche, reepithelialize the wound, and are replaced by hair-follicle-derived stem cells. Furthermore, following targeted genetic ablation of >99% of SGs from dorsal skin, these glands unexpectedly regenerate within weeks. This regenerative process is mediated by alternative stem cells originating from the hair follicle bulge, is dependent upon FGFR2 signaling, and can be accelerated by inducing hair growth. Altogether, our studies demonstrate that stem cell plasticity promotes SG durability following injury.

Distinct mechanisms for sebaceous gland self-renewal and regeneration provide durability in response to injury

Sebaceous glands (SGs) release oils that protect our skin, but how these glands respond to injury has not been previously examined. Here, we report that SGs are largely self-renewed by dedicated stem cell pools during homeostasis. Using targeted single cell RNA-sequencing, we uncovered both direct and indirect paths by which these resident SG progenitors ordinarily differentiate into sebocytes, including transit through a PPARγ+Krt5+ transitional cell state. Upon skin injury, however, SG progenitors depart their niche, reepithelialize the wound, and are replaced by hair follicle-derived stem cells. Furthermore, following targeted genetic ablation of >99% of SGs from dorsal skin, these glands unexpectedly regenerate within weeks. This regenerative process is mediated by alternative stem cells originating from the hair follicle bulge, is dependent upon FGFR signaling, and can be accelerated by inducing hair growth. Altogether, our studies demonstrate that stem cell plasticity promotes SG durability following injury.

Wound-Induced Hair Neogenesis: A Portal to the Development of New Therapies for Hair Loss and Wound Regeneration

Adult mammals retain the remarkable ability to regenerate hair follicles after wounding. Wound-induced hair neogenesis (WIHN) in many ways recapitulates embryogenesis. The origin of the stem cells that give rise to a nascent hair follicle after wounding and the role of mesenchymal cells and signaling pathways responsible for this regenerative phenomenon are slowly being elucidated. WIHN provides a potential therapeutic window for manipulating cell fate by the introduction of factors during the wound healing process to enhance hair follicle formation.

IL-12 regulates type 3 immunity through interfollicular keratinocytes in psoriasiform inflammation

[Figure: see text].

Troy/Tnfrsf19 marks epidermal cells that govern interfollicular epidermal renewal and cornification

The skin epidermis is a highly compartmentalized tissue consisting of a cornifying epithelium called the interfollicular epidermis (IFE) and associated hair follicles (HFs). Several stem cell populations have been described that mark specific compartments in the skin but none of them is specific to the IFE. Here, we identify Troy as a marker of IFE and HF infundibulum basal layer cells in developing and adult human and mouse epidermis. Genetic lineage-tracing experiments demonstrate that Troy-expressing basal cells contribute to long-term renewal of all layers of the cornifying epithelium. Single-cell transcriptomics and organoid assays of Troy-expressing cells, as well as their progeny, confirmed stem cell identity as well as the ability to generate differentiating daughter cells. In conclusion, we define Troy as a marker of epidermal basal cells that govern interfollicular epidermal renewal and cornification.

An unsupervised method for physical cell interaction profiling of complex tissues

Cellular identity in complex multicellular organisms is determined in part by the physical organization of cells. However, large-scale investigation of the cellular interactome remains technically challenging. Here we develop cell interaction by multiplet sequencing (CIM-seq), an unsupervised and high-throughput method to analyze direct physical cell-cell interactions between cell types present in a tissue. CIM-seq is based on RNA sequencing of incompletely dissociated cells, followed by computational deconvolution into constituent cell types. CIM-seq estimates parameters such as number of cells and cell types in each multiplet directly from sequencing data, making it compatible with high-throughput droplet-based methods. When applied to gut epithelium or whole dissociated lung and spleen, CIM-seq correctly identifies known interactions, including those between different cell lineages and immune cells. In the colon, CIM-seq identifies a previously unrecognized goblet cell subtype expressing the wound-healing marker Plet1, which is directly adjacent to colonic stem cells. Our results demonstrate that CIM-seq is broadly applicable to unsupervised profiling of cell-type interactions in different tissue types.

Skin Barriers in Dermal Drug Delivery: Which Barriers Have to Be Overcome and How Can We Measure Them?

Although, drugs are required in the various skin compartments such as viable epidermis, dermis, or hair follicles, to efficiently treat skin diseases, drug delivery into and across the skin is still challenging. An improved understanding of skin barrier physiology is mandatory to optimize drug penetration and permeation. The various barriers of the skin have to be known in detail, which means methods are needed to measure their functionality and outside-in or inside-out passage of molecules through the various barriers. In this review, we summarize our current knowledge about mechanical barriers, i.e., stratum corneum and tight junctions, in interfollicular epidermis, hair follicles and glands. Furthermore, we discuss the barrier properties of the basement membrane and dermal blood vessels. Barrier alterations found in skin of patients with atopic dermatitis are described. Finally, we critically compare the up-to-date applicability of several physical, biochemical and microscopic methods such as transepidermal water loss, impedance spectroscopy, Raman spectroscopy, immunohistochemical stainings, optical coherence microscopy and multiphoton microscopy to distinctly address the different barriers and to measure permeation through these barriers in vitro and in vivo.

Absence of integrin α3β1 promotes the progression of HER2-driven breast cancer in vivo

Background

HER2-driven breast cancer is correlated with poor prognosis, especially during its later stages. Numerous studies have shown the importance of the integrin α3β1 during the initiation and progression of breast cancer; however, its role in this disease is complex and often opposite during different stages and in different types of tumors. In this study, we aim to elucidate the role of integrin α3β1 in a genetically engineered mouse model of HER2-driven mammary tumorigenesis.

Methods

To investigate the role of α3β1 in HER2-driven tumorigenesis in vivo, we generated a HER2-driven MMTV-cNeu mouse model of mammary tumorigenesis with targeted deletion of Itga3 (Itga3 KO mice). We have further used several established triple-negative and HER2-overexpressing human mammary carcinoma cell lines and generated ITGA3-knockout cells to investigate the role of α3β1 in vitro. Invasion of cells was assessed using Matrigel- and Matrigel/collagen I-coated Transwell assays under static or interstitial fluid flow conditions. The role of α3β1 in initial adhesion to laminin and collagen was assessed using adhesion assays and immunofluorescence.

Results

Tumor onset in mice was independent of the presence of α3β1. In contrast, the depletion of α3β1 reduced the survival of mice and increased tumor growth and vascularization. Furthermore, Itga3 KO mice were significantly more likely to develop lung metastases and had an increased metastatic burden compared to WT mice. In vitro, the deletion of ITGA3 caused a significant increase in the cellular invasion of HER2-overexpressing SKBR3, AU565, and BT474 cells, but not of triple-negative MDA-MB-231. This invasion suppressing function of α3β1 in HER2-driven cells depended on the composition of the extracellular matrix and the interstitial fluid flow.

Conclusion

Downregulation of α3β1 in a HER2-driven mouse model and in HER2-overexpressing human mammary carcinoma cells promotes progression and invasiveness of tumors. The invasion-suppressive role of α3β1 was not observed in triple-negative mammary carcinoma cells, illustrating the tumor type-specific and complex function of α3β1 in breast cancer.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1146-8) contains supplementary material, which is available to authorized users.

Nrf2-Mediated Expansion of Pilosebaceous Cells Accelerates Cutaneous Wound Healing

The nuclear factor (erythroid-derived 2)-like 2 (Nrf2) transcription factor is a key regulator of the cellular stress response. Therefore, pharmacologic Nrf2 activation is a promising strategy for skin protection and cancer prevention. This study found that genetic Nrf2 activation in keratinocytes accelerates wound repair. Enhanced proliferation of cells of the pilosebaceous unit peripheral to the wound and a concomitant acceleration of re-epithelialization were identified as the underlying mechanism. Nrf2 specifically promoted the expansion of pilosebaceous cells expressing markers of junctional zone and upper isthmus follicular stem cells. This may result, at least in part, from the up-regulation of the direct Nrf2 target epigen and a concomitant increase in epidermal growth factor receptor signaling. The increase in pilosebaceous cells provided a larger pool of keratinocytes that migrate into the wound, resulting in faster wound closure. These results unravel a novel function of Nrf2 in wound repair and suggest the use of NRF2-activating compounds in patients with impaired healing.

Expression of Plet1 controls interstitial migration of murine small intestinal dendritic cells

Under homeostatic conditions, dendritic cells (DCs) continuously patrol the intestinal lamina propria. Upon antigen encounter, DCs initiate C‐C motif chemokine receptor 7 (CCR7) expression and migrate into lymph nodes to direct T cell activation and differentiation. The mechanistic underpinnings of DC migration from the tissues to lymph nodes have been largely elucidated, contributing greatly to our understanding of DC functionality and intestinal immunity. In contrast, the molecular mechanisms allowing DCs to efficiently migrate through the complex extracellular matrix of the intestinal lamina propria prior to antigen encounter are still incompletely understood. Here we show that small intestinal murine CD11b⁺CD103⁺ DCs express Placenta‐expressed transcript 1 (Plet1), a glycophoshatidylinositol (GPI)‐anchored surface protein involved in migration of keratinocytes during wound healing. In the absence of Plet1, CD11b⁺CD103⁺ DCs display aberrant migratory behavior, and accumulate in the small intestine, independent of CCR7 responsiveness. RNA‐sequencing indicated involvement of Plet1 in extracellular matrix‐interactiveness, and subsequent in‐vitro migration assays revealed that Plet1 augments the ability of DCs to migrate through extracellular matrix containing environments. In conclusion, our findings reveal that expression of Plet1 facilitates homeostatic interstitial migration of small intestinal DCs.

Tight Junction barriers in human hair follicles - role of claudin-1

Barrier function of hair follicles (HFs) is of great interest because they might be an entry port for allergens/pathogens, but could on the other hand be used for drug delivery or vaccination. Therefore we investigated tight junction (TJ) barrier function in human HFs. We show that there is a TJ barrier in the outermost living layer bordering to the environment from the infundibulum to the lower central part and between Henle’s and Huxles layer of anagen HFs. In club hair typical for catagen and telogen HFs a TJ barrier is found surrounding the club. This demonstrates that there is a continuous TJ barrier along interfollicular epidermis and HFs in different phases of HF cycle. However, interestingly, in cell culture experiments we can show that barrier is less tight in HF keratinocytes compared to interfollicular keratinocytes. Knock-down of the TJ protein claudin-1, which we demonstrate here to be less expressed in HFs of lesional atopic dermatitis skin, results in impaired barrier function, decreased proliferation and increased apoptosis of hair keratinocytes. This is in line with a hair growth phenotype in claudin-1 deficient patients (NISCH syndrome) and corresponding knock-out mice and indicates an important role of claudin-1 in HF barrier function and growth.

IL-17A-Induced PLET1 Expression Contributes to Tissue Repair and Colon Tumorigenesis

This study identifies a novel mechanism linking IL-17A with colon tissue repair and tumor development. Abrogation of IL-17A signaling in mice attenuated tissue repair of dextran sulfate sodium (DSS)-induced damage in colon epithelium and markedly reduced tumor development in an azoxymethane/DSS model of colitis-associated cancer. A novel IL-17A target gene, PLET1 (a progenitor cell marker involved in wound healing), was highly induced in DSS-treated colon tissues and tumors in an IL-17RC-dependent manner. PLET1 expression was induced in LGR5⁺ colon epithelial cells after DSS treatment. LGR5⁺PLET1⁺ marks a highly proliferative cell population with enhanced expression of IL-17A target genes. PLET1 deficiency impaired tissue repair of DSS-induced damage in colon epithelium and reduced tumor formation in an azoxymethane/DSS model of colitis-associated cancer. Our results suggest that IL-17A-induced PLET1 expression contributes to tissue repair and colon tumorigenesis.

Cellular Localization and Regulation of Expression of the PLET1 Gene in Porcine Placenta

The placenta expressed transcript 1 (PLET1) gene, which is expressed in placentas of pigs and mice, has been found to have a potential role in trophoblast cell fate decision in mice. Results of this study showed that the porcine PLET1 mRNA and protein were expressed exclusively in trophoblast cells on Days 15, 26, 50, and 95 of gestation (gestation length in the pig is 114 days), indicating that the PLET1 could be a useful marker for porcine trophoblast cells. Additionally, PLET1 protein was found to be redistributed from cytoplasm to the apical side of trophoblast cells as gestation progresses, which suggests a role of PLET1 in the establishment of a stable trophoblast and endometrial epithelial layers. In addition, two transcripts that differ in the 3′ UTR length but encode identical protein were identified to be generated by the alternative cleavage and polyadenylation (APA), and the expression of PLET1-L transcript was significantly upregulated in porcine placentas as gestation progresses. Furthermore, we demonstrated the interaction between the miR-365-3p and PLET1 gene using luciferase assay system. Our findings imply an important role of PLET1 in the placental development in pigs.

Plet1 is an epigenetically regulated cell surface protein that provides essential cues to direct trophoblast stem cell differentiation

Gene loci that are hypermethylated and repressed in embryonic (ESCs) but hypomethylated and expressed in trophoblast (TSCs) stem cells are very rare and may have particularly important roles in early developmental cell fate decisions, as previously shown for Elf5. Here, we assessed another member of this small group of genes, Placenta Expressed Transcript 1 (Plet1), for its function in establishing trophoblast lineage identity and modulating trophoblast differentiation. We find that Plet1 is tightly repressed by DNA methylation in ESCs but expressed on the cell surface of TSCs and trophoblast giant cells. In hypomethylated ESCs that are prone to acquire some trophoblast characteristics, Plet1 is required to confer a trophoblast-specific gene expression pattern, including up-regulation of Elf5. Plet1 displays an unusual biphasic expression profile during TSC differentiation and thus may be pivotal in balancing trophoblast self-renewal and differentiation. Furthermore, overexpression and CRISPR/Cas9-mediated knockout in TSCs showed that high Plet1 levels favour differentiation towards the trophoblast giant cell lineage, whereas lack of Plet1 preferentially induces syncytiotrophoblast formation. Thus, the endogenous dynamics of Plet1 expression establish important patterning cues within the trophoblast compartment by promoting differentiation towards the syncytiotrophoblast or giant cell pathway in Plet1-low and Plet1-high cells, respectively.

Identification of a Bipotent Epithelial Progenitor Population in the Adult Thymus

Thymic epithelial cells (TECs) are critically required for T cell development, but the cellular mechanisms that maintain adult TECs are poorly understood. Here, we show that a previously unidentified subpopulation, EpCam(+)UEA1(-)Ly-51(+)PLET1(+)MHC class II(hi), which comprises <0.5% of adult TECs, contains bipotent TEC progenitors that can efficiently generate both cortical (c) TECs and medullary (m) TECs. No other adult TEC population tested in this study contains this activity. We demonstrate persistence of PLET1(+)Ly-51(+) TEC-derived cells for 9 months in vivo, suggesting the presence of thymic epithelial stem cells. Additionally, we identify cTEC-restricted short-term progenitor activity but fail to detect high efficiency mTEC-restricted progenitors in the adult thymus. Our data provide a phenotypically defined adult thymic epithelial progenitor/stem cell that is able to generate both cTECs and mTECs, opening avenues for improving thymus function in patients.

Stem Cells in Skin Regeneration, Wound Healing, and Their Clinical Applications

The skin is the largest organ of the body and has an array of functions. Skin compartments, epidermis, and hair follicles house stem cells that are indispensable for skin homeostasis and regeneration. These stem cells also contribute to wound repair, resulting in restoration of tissue integrity and function of damaged tissue. Unsuccessful wound healing processes often lead to non-healing wounds. Chronic wounds are caused by depletion of stem cells and a variety of other cellular and molecular mechanisms, many of which are still poorly understood. Current chronic wound therapies are limited, so the search to develop better therapeutic strategies is ongoing. Adult stem cells are gaining recognition as potential candidates for numerous skin pathologies. In this review, we will discuss epidermal and other stem cells present in the skin, and highlight some of the therapeutic applications of epidermal stem cells and other adult stem cells as tools for cell/scaffold-based therapies for non-healing wounds and other skin disorders. We will also discuss emerging concepts and offer some perspectives on how skin tissue-engineered products can be optimized to provide efficacious therapy in cutaneous repair and regeneration.

Interfering with stem cell-specific gatekeeper functions controls tumour initiation and malignant progression of skin tumours

Epithelial cancer constitutes a major clinical challenge and molecular mechanisms underlying the process of tumour initiation are not well understood. Here we demonstrate that hair follicle bulge stem cells (SCs) give rise to well-differentiated sebaceous tumours and show that SCs are not only crucial in tumour initiation, but are also involved in tumour plasticity and heterogeneity. Our findings reveal that SC-specific expression of mutant Lef1, which mimics mutations found in human sebaceous tumours, drives sebaceous tumour formation. Mechanistically, we demonstrate that mutant Lef1 abolishes p53 activity in SCs. Intriguingly, mutant Lef1 induces DNA damage and interferes with SC-specific gatekeeper functions normally protecting against accumulations of DNA lesions and cell loss. Thus, normal control of SC proliferation is disrupted by mutant Lef1, thereby allowing uncontrolled propagation of tumour-initiating SCs. Collectively, these findings identify underlying molecular and cellular mechanisms of tumour-initiating events in tissue SCs providing a potential target for future therapeutic strategies.

The presence of multiple stem and progenitor cells in the skin has a major impact on the formation of different epidermal tumours. Here the authors identify bulge stem cells as the cells of origin of sebaceous tumours through genetic lineage tracing and molecular studies in a mouse model.

Heterogeneity and plasticity of epidermal stem cells

The epidermis is an integral part of our largest organ, the skin, and protects us against the hostile environment. It is a highly dynamic tissue that, during normal steady-state conditions, undergoes constant turnover. Multiple stem cell populations residing in autonomously maintained compartments facilitate this task. In this Review, we discuss stem cell behaviour during normal tissue homeostasis, regeneration and disease within the pilosebaceous unit, an integral structure of the epidermis that is responsible for hair growth and lubrication of the epithelium. We provide an up-to-date view of the pilosebaceous unit, encompassing the heterogeneity and plasticity of multiple discrete stem cell populations that are strongly influenced by external cues to maintain their identity and function.

Overexpression of epigen during embryonic development induces reversible, epidermal growth factor receptor-dependent sebaceous gland hyperplasia

The epidermal growth factor receptor (EGFR) system is a key regulator of epithelial development and homeostasis. Its functions in the sebaceous gland (SG), however, remain poorly characterized. In this study, using a transgenic mouse line with tissue-specific and inducible expression of the EGFR ligand epigen, we showed that increased activation of the EGFR in skin keratinocytes results in enlarged SGs and increased sebum production. The phenotype can be reverted by interrupting transgene expression and is EGFR dependent, as gland size and sebum levels return to normal values after crossing to the EGFR-impaired mouse line Wa5. Intriguingly, however, the SG enlargement appears only if EGFR activation occurs before birth. Importantly, the enlarged sebaceous glands are associated with an increased expression of the transcription factor MYC and of the transmembrane proteins LRIG1, an established negative-feedback regulator of the EGFR/ERBB tyrosine kinase receptors and a stem cell marker. Our findings identify EGFR signaling as a major pathway determining SG activity and suggest a functional relationship between the EGFR/ERBB system and MYC/LRIG1 in the commitment of stem cells toward specific progenitor cell types, with implications for our understanding of their role in tissue development, homeostasis, and disease.

Keratin 79 identifies a novel population of migratory epithelial cells that initiates hair canal morphogenesis and regeneration

The formation of epithelial tubes underlies the development of diverse organs. In the skin, hair follicles resemble tube-like structures with lumens that are generated through poorly understood cellular rearrangements. Here, we show that creation of the hair follicle lumen is mediated by early outward movement of keratinocytes from within the cores of developing hair buds. These migratory keratinocytes express keratin 79 (K79) and stream out of the hair germ and into the epidermis prior to lumen formation in the embryo. Remarkably, this process is recapitulated during hair regeneration in the adult mouse, when K79(+) cells migrate out of the reactivated secondary hair germ prior to formation of a new hair canal. During homeostasis, K79(+) cells line the hair follicle infundibulum, a domain we show to be multilayered, biochemically distinct and maintained by Lrig1(+) stem cell-derived progeny. Upward movement of these cells sustains the infundibulum, while perturbation of this domain during acne progression is often accompanied by loss of K79. Our findings uncover previously unappreciated long-distance cell movements throughout the life cycle of the hair follicle, and suggest a novel mechanism by which the follicle generates its hollow core through outward cell migration.

Epidermal Snail expression drives skin cancer initiation and progression through enhanced cytoprotection, epidermal stem/progenitor cell expansion and enhanced metastatic potential

Expression of the EMT-inducing transcription factor Snail is enhanced in different human cancers. To investigate the in vivo role of Snail during progression of epithelial cancer, we used a mouse model with skin-specific overexpression of Snail. Snail transgenic mice spontaneously developed distinct histological subtypes of skin cancer, such as basal cell carcinoma, squamous cell carcinoma and sebaceous gland carcinoma. Development of sebaceous gland carcinomas strongly correlated with the direct and complete repression of Blimp-1, a central regulator of sebocyte homeostasis. Snail expression in keratinocyte stem cells significantly promotes their proliferation associated with an activated FoxM1 gene expression signature, resulting in a larger pool of Mts24-marked progenitor cells. Furthermore, primary keratinocytes expressing Snail showed increased survival and strong resistance to genotoxic stress. Snail expression in a skin-specific p53-null background resulted in accelerated formation of spontaneous tumours and enhanced metastasis. Our data demonstrate that in vivo expression of Snail results in de novo epithelial carcinogenesis by allowing enhanced survival, expansion of the cancer stem cell pool with accumulated DNA damage, a block in terminal differentiation and increased proliferation rates of tumour-initiating cells.

Indian hedgehog controls proliferation and differentiation in skin tumorigenesis and protects against malignant progression

Mutations in the hedgehog pathway drive the formation of tumors in many different organs, including the development of basal cell carcinoma in the skin. However, little is known about the role of epidermal Indian hedgehog (Ihh) in skin physiology. Using mouse genetics, we identified overlapping and distinct functions of Ihh in different models of epidermal tumorigenesis. Epidermal deletion of Ihh resulted in increased formation of benign squamous papilloma. Strikingly, Ihh-deficient mice showed an increase in malignant squamous cell carcinoma and developed lung and lymph node metastases. In a sebaceous gland tumor model, Ihh deficiency inhibited tumor cell differentiation. More mechanistically, IHH stimulated cell proliferation by activating the transcription factor GLI2 in human keratinocytes and human tumors. Thus, our results uncover important functions for Ihh signaling in controlling proliferation, differentiation, malignant progression, and metastasis of epithelial cancer, establishing Ihh as a gatekeeper for controlling the grade of tumor malignancy.

Lineage tracing of hair follicle stem cells in epidermal whole mounts

Lineage tracing of tissue stem cells represents a powerful tool to address fundamental questions of deve-lopment, differentiation and cellular renewal in a natural tissue environment. The Cre/lox site-specific recombination system is increasingly used to genetically label specific cell populations to perform cell lineage tracing or fate mapping experiments in sophisticated mouse models. Here we describe a method of labeling and subsequent tracking stem cells of the hair follicle bulge region in mouse skin. Hair follicle stem cells are specifically labeled by expressing the Cre recombinase under control of keratin15 (K15) regulatory sequences and by crossing the Cre-containing animals with Cre-sensitive Rosa26R (R26R) reporter mice. To achieve a temporal control of recombinase activity in stem cells, Cre is fused to a modified estrogen receptor (CreER(G)T2). In the K15CreER(G)T2/R26R mouse model, hair follicle stem cells (HFSCs) are specifically labeled after Cre activation upon treatment of mice with tamoxifen. By analyzing the skin tissue at different time points following genetic labeling, important information on stem cell behavior and contribution of labeled stem cells to epidermal structures during tissue homeostasis and hair follicle regeneration are obtained. Combining the lineage tracing approach with the whole mount technique allows examining large areas of the epidermis containing many hair follicles and sebaceous glands and reveals the complex three-dimensional relationship of labeled stem cell clones within the tissue.

Mechanisms regulating epidermal stem cells

The skin epidermis contains different appendages such as the hair follicle and the sebaceous glands. Recent studies demonstrated that several types of stem cells (SCs) exist in different niches within the epidermis and maintain discrete epidermal compartments, but the exact contribution of each SC populations under physiological conditions is still unclear. In addition, the precise mechanisms controlling the balance between proliferation and differentiation of epidermal SC still remain elusive. Recent studies provide new insights into these important questions by showing the contribution of hair follicle SC to the sebaceous lineage and the importance of chromatin modifications and micro-RNAs (miRs) in regulating epidermal SCs renewal and differentiation. In this review, we will discuss the importance of these papers to our understanding of the mechanisms that control epidermal SC functions.

Stem cell dynamics in sebaceous gland morphogenesis in mouse skin

The hair follicle (HF) and the sebaceous gland (SG) constitute the two integral parts of the pilosebaceous unit and significantly contribute to the barrier function of mammalian skin. Considerable progress has been made in our understanding how HF formation is regulated. However, the development of the SG is poorly understood, both at the molecular and cellular level. Here, we investigate the process of SG morphogenesis and the dynamics of its cellular organisation in more detail. The spatial and temporal organisation of distinct stem and progenitor compartments was analysed during morphogenesis of the pilosebaceous unit in mouse tail epidermis. Our experiments reveal a dynamic expression pattern for diverse HF stem cell marker molecules including Sox9 and Lrig1. Surprisingly, Sox9 and Lrig1 are initially coexpressed by epidermal progenitor cells and are confined to different regions within the pilosebaceous unit when the specification of the sebocyte cell lineage takes place. We demonstrate that SG development at the distal part of the HF is driven by asymmetric cell fate decision of Lrig1 positive stem cells, whereas MTS24/Plet1 positive precursor cells seem not to play a role in this process. Importantly, our data clearly show that distinct stem and progenitor compartments are established at different time points of development. By studying the process of SG morphogenesis more precisely, we discovered that the two prominent SGs attached to one tail HF originate from one small cluster of sebocyte cells. Finally, we show regional specificity for HF patterning and spatio-temporal control of the underlying molecular signals initiating the development of the pilosebaceous unit.

TCF/Lef1 activity controls establishment of diverse stem and progenitor cell compartments in mouse epidermis

This work investigates cell fate and lineages of hair follicles and sebaceous glands (SG) in the mouse epidermis. The combination of transgenic models with complementary tracing techniques provides unequivocal evidence for a direct contribution of bulge stem cells to the SG units as well as stem cell niches within the isthmus region.

Mammalian epidermis consists of the interfollicular epidermis, hair follicles (HFs) and associated sebaceous glands (SGs). It is constantly renewed by stem and progenitor cell populations that have been identified and each compartment features a distinct mechanism of cellular turnover during renewal. The functional relationship between the diverse stem cell (SC) pools is not known and molecular signals regulating the establishment and maintenance of SC compartments are not well understood. Here, we performed lineage tracing experiments to demonstrate that progeny of HF bulge SCs transit through other SC compartments, suggesting a hierarchy of competent multipotent keratinocytes contributing to tissue renewal. The bulge was identified as a bipotent SC compartment that drives both cyclic regeneration of HFs and continuous renewal of SGs. Our data demonstrate that aberrant signalling by TCF/Lef1, transcription factors crucial for bulge SC activation and hair differentiation, results in development of ectopic SGs originating from bulge cells. This process of de novo SG formation is accompanied by the establishment of new progenitor niches. Detailed molecular analysis suggests the recapitulation of steps of tissue morphogenesis.