Lgr6 marks stem cells in the hair follicle that generate all cell lineages of the skin

MicroRNA-203 functions as a tumor suppressor in basal cell carcinoma

Basal cell carcinoma (BCC) of the skin represents the most common malignancy in humans. MicroRNAs (miRNAs), small regulatory RNAs with pleiotropic function, are commonly misregulated in cancer. Here we identify miR-203, a miRNA abundantly and preferentially expressed in skin, to be downregulated in BCCs. We show that activation of the Hedgehog (HH) pathway, critically involved in the pathogenesis of BCCs, as well as the EGFR/MEK/ERK/c-JUN signaling pathway suppresses miR-203. We identify c-JUN, a key effector of the HH pathway, as a novel direct target for miR-203 in vivo. Further supporting the role of miR-203 as a tumor suppressor, in vivo delivery of miR-203 mimics in a BCC mouse model results in the reduction of tumor growth. Our results identify a regulatory circuit involving miR-203 and c-JUN, which provides functional control over basal cell proliferation and differentiation. We propose that miR-203 functions as a ‘bona fide' tumor suppressor in BCC, whose suppressed expression contributes to oncogenic transformation via derepression of multiple stemness- and proliferation-related genes, and its overexpression could be of therapeutic value.

Orphan G protein-coupled receptors (GPCRs): biological functions and potential drug targets

The superfamily of G protein-coupled receptors (GPCRs) includes at least 800 seven-transmembrane receptors that participate in diverse physiological and pathological functions. GPCRs are the most successful targets of modern medicine, and approximately 36% of marketed pharmaceuticals target human GPCRs. However, the endogenous ligands of more than 140 GPCRs remain unidentified, leaving the natural functions of those GPCRs in doubt. These are the so-called orphan GPCRs, a great source of drug targets. This review focuses on the signaling transduction pathways of the adhesion GPCR family, the LGR subfamily, and the PSGR subfamily, and their potential functions in immunology, development, and cancers. In this review, we present the current approaches and difficulties of orphan GPCR deorphanization and characterization.

25 years of epidermal stem cell research

This is a chronicle of concepts in the field of epidermal stem cell biology and a historic look at their development over time. The past 25 years have seen the evolution of epidermal stem cell science, from first fundamental studies to a sophisticated science. The study of epithelial stem cell biology was aided by the ability to visualize the distribution of stem cells and their progeny through lineage analysis studies. The excellent progress we have made in understanding epidermal stem cell biology is discussed in this article. The challenges we still face in understanding epidermal stem cells include defining molecular markers for stem and progenitor sub-populations, determining the locations and contributions of the different stem cell niches, and mapping regulatory pathways of epidermal stem cell proliferation and differentiation. However, our rapidly evolving understanding of epidermal stem cells has many potential uses that promise to translate into improved patient therapy.

Epithelial stem cells, wound healing and cancer

It is well established that tissue repair depends on stem cells and that chronic wounds predispose to tumour formation. However, the association between stem cells, wound healing and cancer is poorly understood. Lineage tracing has now shown how stem cells are mobilized to repair skin wounds and how they contribute to skin tumour development. The signalling pathways, including WNT and Hedgehog, that control stem cell behaviour during wound healing are also implicated in tumour formation. Furthermore, tumorigenesis and wound repair both depend on communication between epithelial cells, mesenchymal cells and bone marrow-derived cells. These studies suggest ways to harness stem cells for wound repair while minimizing cancer risk.

Using stem cells in skin regeneration: possibilities and reality

Tissue-engineered skin has a long history of clinical applications, yet current treatments are not capable of completely regenerating normal, uninjured skin. Nonetheless, the field has experienced a tremendous development in the past 10 years, encountering the summit of tissue engineering (TE) and the arising of stem cell research. Since then, unique features of these cells such as self-renewal capacity, multi-lineage differentiation potential, and wound healing properties have been highlighted. However, a realistic perspective of their outcome in skin regenerative medicine applications is still absent. This review intends to discuss the directions that adult and embryonic stem cells (ESCs) can take, strengthening the skin regeneration field. Distinctively, a critical overview of stem cells' differentiation potential onto skin main lineages, along with a highlight of their participation in wound healing mechanisms, is herein provided. We aim to compile and review significant work to allow a better understanding of the best skin TE approaches, enabling the embodiment of the materialization of a new era in skin regeneration to come, with a conscious overview of the current limitations.

Dissecting the bulge in hair regeneration

The adult hair follicle houses stem cells that govern the cyclical growth and differentiation of multiple cell types that collectively produce a pigmented hair. Recent studies have revealed that hair follicle stem cells are heterogeneous and dynamic throughout the hair cycle. Moreover, interactions between heterologous stem cells, including both epithelial and melanocyte stem cells, within the hair follicle are just now being explored. This review will describe how recent findings have expanded our understanding of the development, organization, and regeneration of hair follicle stem cells. At a basic level, this review is intended to help construct a reference point to integrate the surge of studies on the molecular mechanisms that regulate these cells.

Ontogeny and Homeostasis of Adult Epithelial Skin Stem Cells

Mouse epithelial skin stem cells constitute an important model system for understanding the dynamics of stem cell emergence and behavior in an intact vertebrate tissue. Recent published work defined discrete populations of epithelial stem cells in the adult skin epithelium, which reside in the hair follicle bulge and germ, isthmus, sebaceous gland and inter-follicular epidermis. Adult epidermal and hair follicle stem cells seem to adopt mostly symmetric or unidirectional fate decisions of either one of two possible fates: (1) differentiate and be lost from the tissue or (2) expand symmetrically to self-renew. Asymmetric divisions appear to be mostly implicated in differentiation and stratification of the epidermis. While mechanisms of adult stem cell homeostasis begin to be unraveled, the embryonic origin of the adult epithelial skin stem cells is poorly understood. Recent studies reported Sox9, Lgr6, and Runx1 expression in subpopulations of cells in the embryonic hair placode. These subpopulations seem to act as precursors of different classes of adult epithelial stem cells. In particular, Runx1 regulates a Wnt-mediated cross-talk between the nascent adult-type hair follicle stem cells and their environment, which is essential for timely stem cell emergence, proper maturation, long-term differentiation potential, and maintenance. The new data begin to define the basic dynamics and regulatory pathways governing the ontogeny of adult epithelial stem cells.

Keratinocyte stem cells and the targets for nonmelanoma skin cancer

The mammalian skin is a complex dynamic organ composed of thin multilayered epidermis and a thick underlying connective tissue layer dermis. The epidermis undergoes continuous renewal throughout life. The stems cells uniquely express particular surface markers utilized for their identification, isolation and localization in specific niches in epidermis as well as hair follicles (HFs). The two stage skin carcinogenesis model involves stepwise accumulation of genetic alterations and ultimately leading to malignancy. Whereas early research on skin carcinogenesis focused on the molecular nature of carcinogens and tumor promoters, more recent studies have focused on the identification of the target cells and tumor promoting cells for both chemical and physical carcinogens and promoters. Recent studies support the hypothesis that keratinocyte stem cells are the targets in skin carcinogenesis. In this review, we discuss briefly the localization of stem cells in the epidermis and HFs, and review the possibility that skin papillomas and carcinomas are derived from stem cells, as well as from other cells in the cutaneous epithelium whose stem cell properties are not well known.

Targeting MET in cancer: rationale and progress

Uncontrolled cell survival, growth, angiogenesis and metastasis are essential hallmarks of cancer. Genetic and biochemical data have demonstrated that the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, the tyrosine kinase MET, have a causal role in all of these processes, thus providing a strong rationale for targeting these molecules in cancer. Parallel progress in understanding the structure and function of HGF/SF, MET and associated signalling components has led to the successful development of blocking antibodies and a large number of small-molecule MET kinase inhibitors. In this Review, we discuss these advances, as well as results from recent clinical studies that demonstrate that inhibiting MET signalling in several types of solid human tumours has major therapeutic value.

The interfollicular epidermal stem cell saga: sensationalism versus reality check

Adult stem cells in rapidly renewing tissues have been classically defined as rare, relatively quiescent cells with the unique capacity to constantly self-renew and regenerate tissues during homeostasis. Although this view remains firmly embedded in the skin field, particularly in the area of hair follicle stem cell biology, it has been challenged by a number of notable publications in 2007. These papers leave an uncomfortable feeling with the reader if one believes that stem cells and transit amplifying cells are two polar opposites and 'never the twain shall meet.' Even if you do not subscribe to this extreme view, the implications appear to be far-reaching given that the majority of techniques devised for stem cell identification have used the fundamental tenet that the proliferating compartment is comprised of two distinct, mutually exclusive compartments, i.e. a minor proportion of long-lived quiescent stem cells with unlimited self-renewal and a large pool of rapidly cycling, short-lived transient amplifying cells with limited or no self-renewal capacity in normal steady-state conditions. However, these recent findings have resulted in papers that could be described as sensationalistic because they make little or no attempt to reconcile their observations with the large bulk of historical data with direct bearing on the interpretation of stem cell activity in normal steady-state conditions. Here, we offer some explanations that may help to integrate all of the data while presenting a case that both quiescent stem cells and cycling 'transit amplifying' cells contribute to epidermal replacement.

How do they do Wnt they do?: regulation of transcription by the Wnt/β-catenin pathway

Wnt/β-catenin signalling is known to play many roles in metazoan development and tissue homeostasis. Misregulation of the pathway has also been linked to many human diseases. In this review, specific aspects of the pathway's involvement in these processes are discussed, with an emphasis on how Wnt/β-catenin signalling regulates gene expression in a cell and temporally specific manner. The T-cell factor (TCF) family of transcription factors, which mediate a large portion of Wnt/β-catenin signalling, will be discussed in detail. Invertebrates contain a single TCF gene that contains two DNA-binding domains, the high mobility group (HMG) domain and the C-clamp, which increases the specificity of DNA binding. In vertebrates, the situation is more complex, with four TCF genes producing many isoforms that contain the HMG domain, but only some of which possess a C-clamp. Vertebrate TCFs have been reported to act in concert with many other transcription factors, which may explain how they obtain sufficient specificity for specific DNA sequences, as well as how they achieve a wide diversity of transcriptional outputs in different cells.

Endogenous retinoids in the hair follicle and sebaceous gland

Vitamin A and its derivatives (retinoids) are critically important in the development and maintenance of multiple epithelial tissues, including skin, hair, and sebaceous glands, as shown by the detrimental effects of either vitamin A deficiency or toxicity. Thus, precise levels of retinoic acid (RA, active metabolite) are needed. These precise levels of RA are achieved by regulating several steps in the conversion of dietary vitamin A (retinol) to RA and RA catabolism. This review discusses the localization of RA synthesis to specific sites within the hair follicle and sebaceous gland, including their stem cells, during both homeostasis and disease states. It also discusses what is known about the specific roles of RA within the hair follicle and sebaceous gland. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

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.

Lhx2 differentially regulates Sox9, Tcf4 and Lgr5 in hair follicle stem cells to promote epidermal regeneration after injury

The Lhx2 transcription factor plays essential roles in morphogenesis and patterning of ectodermal derivatives as well as in controlling stem cell activity. Here, we show that during murine skin morphogenesis, Lhx2 is expressed in the hair follicle (HF) buds, whereas in postnatal telogen HFs Lhx2(+) cells reside in the stem cell-enriched epithelial compartments (bulge, secondary hair germ) and co-express selected stem cell markers (Sox9, Tcf4 and Lgr5). Remarkably, Lhx2(+) cells represent the vast majority of cells in the bulge and secondary hair germ that proliferate in response to skin injury. This is functionally important, as wound re-epithelization is significantly retarded in heterozygous Lhx2 knockout (+/-) mice, whereas anagen onset in the HFs located closely to the wound is accelerated compared with wild-type mice. Cell proliferation in the bulge and the number of Sox9(+) and Tcf4(+) cells in the HFs closely adjacent to the wound in Lhx2(+/-) mice are decreased in comparison with wild-type controls, whereas expression of Lgr5 and cell proliferation in the secondary hair germ are increased. Furthermore, acceleration of wound-induced anagen development in Lhx2(+/-) mice is inhibited by administration of Lgr5 siRNA. Finally, Chip-on-chip/ChIP-qPCR and reporter assay analyses identified Sox9, Tcf4 and Lgr5 as direct Lhx2 targets in keratinocytes. These data strongly suggest that Lhx2 positively regulates Sox9 and Tcf4 in the bulge cells, and promotes wound re-epithelization, whereas it simultaneously negatively regulates Lgr5 in the secondary hair germ and inhibits HF cycling. Thus, Lhx2 operates as an important regulator of epithelial stem cell activity in the skin response to injury.

Signaling circuitries controlling stem cell fate: to be or not to be

The integration of extrinsic and intrinsic signals is required to preserve the self-renewal and tissue regenerative capacity of adult stem cells, while protecting them from malignant conversion or loss of proliferative potential by death, differentiation or senescence. Here we review emerging signaling circuitries regulating stem cell fate, with emphasis on epithelial stem cells. Wnt, mTOR, GPCRs, Notch, Rho GTPases, YAP and DNA and histone methylases are some of the mechanisms that allow stem cells to balance their regenerative potential and the initiation of terminal differentiation programs, guaranteeing appropriate tissue homeostasis. Understanding the signaling circuitries regulating stem cell fate decisions might provide important insights into cancer initiation and numerous human pathologies that involve the progressive loss of tissue-specific adult stem cells.

Primary cicatricial alopecia: recent advances in understanding and management

Primary cicatricial alopecias (PCA) are a rare group of disorders, in which the hair follicle is the main target of destructive inflammation resulting in irreversible hair loss with scarring of affected lesions. The most typical clinical manifestation of PCA is the loss of visible follicular ostia. The histopathological hallmark of a fully developed lesion is the replacement of the hair follicle structure by fibrous tissue. PCA could share similar clinical manifestations and eventually lead to "burn-out" alopecia. Some subsets are hardly distinguishable histopathologically and the mechanisms that elicit such a destructive reaction have not been fully elucidated. Thus, the management of PCA represents one of the most challenging clinical problems for dermatologists. The aim of this review is to provide a concise and comprehensive summary of recent advances in PCA management, especially focusing on novel methodologies to aid diagnosis, and updates on our understanding of the etiopathogenesis. Dermoscopy, a new pathological preparation technique and direct immunofluorescence analysis enable more accurate clinicopathological diagnosis of PCA. Microarray analysis may be beneficial to distinguish PCA subtypes. Currently suggested mechanisms underlying PCA include loss of immune protection of stem cells, impaired stem cell self-maintenance, enhanced autoimmunity by pro-inflammatory cytokines and environmental/genetic predispositions. Interestingly, recent data indicates the association between lipid metabolism dysregulation and PCA development, implying an important role of the sebaceous gland dysfunction in the etiopathogenesis. Based on that hypothesis and observations, novel therapeutic approaches have been proposed, including the use of peroxisome proliferator-activated receptor-γ agonist for lichen planopilaris.

Nestin-positive hair follicle pluripotent stem cells can promote regeneration of impinged peripheral nerve injury

Nestin-positive, keratin 15 (K15)-negative multipotent hair follicle stem cells are located above the hair follicle bulge. We have termed this location the hair follicle pluripotent stem cell area. We have previously shown that transplantation of nestin-expressing hair follicle stem cells can regenerate peripheral nerve and spinal cord injuries. In the present study, we regenerated the impinged sciatic nerve by transplanting hair follicle pluripotent stem cells. Human hair follicle stem cells were transplanted around the impinged sciatic nerve of ICR nude (nu/nu) mice. The hair follicle stem cells were transplanted between impinged sciatic nerve fragments of the mouse where they differentiated into glial fibrillary acidic protein-positive Schwann cells and promoted the recovery of pre-existing axons. The regenerated sciatic nerve functionally recovered. These multipotent hair follicle stem cells thereby provide a potential accessible, autologous source of stem cells for regeneration therapy of nerves degenerated by compression between bony or other hard surfaces.

The circadian molecular clock creates epidermal stem cell heterogeneity

Murine epidermal stem cells undergo alternate cycles of dormancy and activation, fuelling tissue renewal. However, only a subset of stem cells becomes active during each round of morphogenesis, indicating that stem cells coexist in heterogeneous responsive states. Using a circadian-clock reporter-mouse model, here we show that the dormant hair-follicle stem cell niche contains coexisting populations of cells at opposite phases of the clock, which are differentially predisposed to respond to homeostatic cues. The core clock protein Bmal1 modulates the expression of stem cell regulatory genes in an oscillatory manner, to create populations that are either predisposed, or less prone, to activation. Disrupting this clock equilibrium, through deletion of Bmal1 (also known as Arntl) or Per1/2, resulted in a progressive accumulation or depletion of dormant stem cells, respectively. Stem cell arrhythmia also led to premature epidermal ageing, and a reduction in the development of squamous tumours. Our results indicate that the circadian clock fine-tunes the temporal behaviour of epidermal stem cells, and that its perturbation affects homeostasis and the predisposition to tumorigenesis.

Lgr5 and Lgr6 as markers to study adult stem cell roles in self-renewal and cancer

The extended longevity of many mammals imposes the need for an effective tissue renewal capacity within the vital organs to maintain optimal function. Resident adult stem cells are instrumental in delivering this renewal capacity by virtue of their characteristic ability to maintain themselves long-term as a population (self-renewal), whilst also supplying all functional cell-lineages of the respective tissue (multipotency). The homeostatic activity of these adult stem cell reservoirs is tailored to meet the specific renewal requirements of individual tissues through a combination of intrinsic genetic programming and local cues delivered from the surrounding environment (the niche). Considerable phenotypic diversity therefore exists between adult stem cell populations in different organs, making it a considerable challenge to identify broadly applicable markers that facilitate their identification and characterization. However, the 7-transmembrane receptor, Lgr5 has recently gained prominence as a marker of Wnt-regulated adult stem cell populations in the hair-follicle, intestine and stomach. A closely-related protein, Lgr6 marks adult stem cells responsible for fueling the renewal of the sebaceous gland and skin. The discovery of these markers has already greatly improved our understanding of stem cell biology in these rapidly renewing tissues and has major implications for the identification and isolation of human adult stem cell populations for exploitation of their regenerative medicine potential in the clinic.

Nerve-derived sonic hedgehog defines a niche for hair follicle stem cells capable of becoming epidermal stem cells

In adult skin, stem cells in the hair follicle bulge cyclically regenerate the follicle, whereas a distinct stem cell population maintains the epidermis. The degree to which all bulge cells have equal regenerative potential is not known. We found that Sonic hedgehog (Shh) from neurons signals to a population of cells in the telogen bulge marked by the Hedgehog response gene Gli1. Gli1-expressing bulge cells function as multipotent stem cells in their native environment and repeatedly regenerate the anagen follicle. Shh-responding perineural bulge cells incorporate into healing skin wounds where, notably, they can change their lineage into epidermal stem cells. The perineural niche (including Shh) is dispensable for follicle contributions to acute wound healing and skin homeostasis, but is necessary to maintain bulge cells capable of becoming epidermal stem cells. Thus, nerves cultivate a microenvironment where Shh creates a molecularly and phenotypically distinct population of hair follicle stem cells.

Dynamic signals for hair follicle development and regeneration

Hair follicles form during embryonic development and, after birth, undergo recurrent cycling of growth, regression, and relative quiescence. As a functional mini-organ, the hair follicle develops in an environment with dynamic and alternating changes of diverse molecular signals. Over the past decades, genetically engineered mouse models have been used to study hair follicle morphogenesis and significant advances have been made toward the identification of key signaling pathways and the regulatory genes involved. In contrast, much less is understood in signals regulating hair follicle regeneration. Like hair follicle development, hair follicle regeneration probably relies on populations of stem cells that undergo a highly coordinated and stepwise program of differentiation to produce the completed structure. Here, we review recent advances in the understanding of the molecular signals underlying hair follicle morphogenesis and regeneration, with a focus on the initiation of the primary hair follicle structure placode. Knowledge about hair follicle morphogenesis may help develop novel therapeutic strategies to enhance cutaneous regeneration and improve wound healing.

Specific microRNAs are preferentially expressed by skin stem cells to balance self-renewal and early lineage commitment

Increasing evidence suggests that microRNAs may play important roles in regulating self-renewal and differentiation in mammalian stem cells (SCs). Here, we explore this issue in skin. We first characterize microRNA expression profiles of skin SCs versus their committed proliferative progenies and identify a microRNA subset associating with "stemness." Of these, miR-125b is dramatically downregulated in early SC progeny. We engineer an inducible mice system and show that when miR-125b is sustained in SC progenies, tissue balance is reversibly skewed toward stemness at the expense of epidermal, oil-gland, and HF differentiation. Using gain- and loss-of-function in vitro, we further implicate miR-125b as a repressor of SC differentiation. In vivo, transcripts repressed upon miR-125b induction are enriched >700% for predicted miR-125b targets normally downregulated upon SC-lineage commitment. We verify some of these miR-125b targets, and show that Blimp1 and VDR in particular can account for many tissue imbalances we see when miR-125b is deregulated.

Biomimetic platforms for human stem cell research

Stem cells are central to developing new treatment options for tissue regeneration and constructing controllable models for biological research. Bioengineered cell culture environments that combine microenvironmental control with tissue-specific transport and signaling are critical tools in our efforts to study tissue development, regeneration, and disease under conditions that predict the human in vivo context. We propose that experimentation at the interfaces of biology, engineering, and medical sciences is critical for unlocking the full potential of stem cells. Here, we focus on the design and utilization of in vitro platforms that recapitulate the environments associated with tissue development, disease, and regeneration.

Strategies for homeostatic stem cell self-renewal in adult tissues

In adult tissues, an exquisite balance exists between stem cell proliferation and the generation of differentiated offspring. Classically, it has been argued that this balance is obtained at the level of a single stem cell, which divides strictly into a new stem cell and a progenitor. However, recent evidence suggests that balance can also be achieved at the level of the stem cell population. Some stem cells might be lost due to differentiation or damage, whereas others divide symmetrically to fill this gap. Here, we consider the general strategies for stem cell self-renewal and review the evidence for stochastic stem cell fate in adult tissues across a range of tissue types and organisms.

Slide preparation for single-cell-resolution imaging of fluorescent proteins in their three-dimensional near-native environment

In recent years, many mouse models have been developed to mark and trace the fate of adult cell populations using fluorescent proteins. High-resolution visualization of such fluorescent markers in their physiological setting is thus an important aspect of adult stem cell research. Here we describe a protocol to produce sections (150-200 μm) of near-native tissue with optimal tissue and cellular morphology by avoiding artifacts inherent in standard freezing or embedding procedures. The activity of genetically expressed fluorescent proteins is maintained, thereby enabling high-resolution three-dimensional (3D) reconstructions of fluorescent structures in virtually all types of tissues. The procedure allows immunofluorescence labeling of proteins to depths up to 50 μm, as well as a chemical 'Click-iT' reaction to detect DNA-intercalating analogs such as ethynyl deoxyuridine (EdU). Generation of near-native sections ready for imaging analysis takes approximately 2-3 h. Postsectioning processes, such as antibody labeling or EdU detection, take up to 10 h.

A resource for discovering specific and universal biomarkers for distributed stem cells

Specific and universal biomarkers for distributed stem cells (DSCs) have been elusive. A major barrier to discovery of such ideal DSC biomarkers is difficulty in obtaining DSCs in sufficient quantity and purity. To solve this problem, we used cell lines genetically engineered for conditional asymmetric self-renewal, the defining DSC property. In gene microarray analyses, we identified 85 genes whose expression is tightly asymmetric self-renewal associated (ASRA). The ASRA gene signature prescribed DSCs to undergo asymmetric self-renewal to a greater extent than committed progenitor cells, embryonic stem cells, or induced pluripotent stem cells. This delineation has several significant implications. These include: 1) providing experimental evidence that DSCs in vivo undergo asymmetric self-renewal as individual cells; 2) providing an explanation why earlier attempts to define a common gene expression signature for DSCs were unsuccessful; and 3) predicting that some ASRA proteins may be ideal biomarkers for DSCs. Indeed, two ASRA proteins, CXCR6 and BTG2, and two other related self-renewal pattern associated (SRPA) proteins identified in this gene resource, LGR5 and H2A.Z, display unique asymmetric patterns of expression that have a high potential for universal and specific DSC identification.

The human urothelium consists of multiple clonal units, each maintained by a stem cell

Little is known about the clonal architecture of human urothelium. It is likely that urothelial stem cells reside within the basal epithelial layer, yet lineage tracing from a single stem cell as a means to show the presence of a urothelial stem cell has never been performed. Here, we identify clonally related cell areas within human bladder mucosa in order to visualize epithelial fields maintained by a single founder/stem cell. Sixteen frozen cystectomy specimens were serially sectioned. Patches of cells deficient for the mitochondrially encoded enzyme cytochrome c oxidase (CCO) were identified using dual-colour enzyme histochemistry. To show that these patches represent clonal proliferations, small CCO-proficient and -deficient areas were individually laser-capture microdissected and the entire mitochondrial genome (mtDNA) in each area was PCR amplified and sequenced to identify mtDNA mutations. Immunohistochemistry was performed for the different cell layers of the urothelium and adjacent mesenchyme. CCO-deficient patches could be observed in normal urothelium of all cystectomy specimens. The two-dimensional length of these negative patches varied from 2-3 cells (about 30 µm) to 4.7 mm. Each cell area within a CCO-deficient patch contained an identical somatic mtDNA mutation, indicating that the patch was a clonal unit. Patches contained all the mature cell differentiation stages present in the urothelium, suggesting the presence of a stem cell. Our results demonstrate that the normal mucosa of human bladder contains stem cell-derived clonal units that actively replenish the urothelium during ageing. The size of the clonal unit attributable to each stem cell was broadly distributed, suggesting replacement of one stem cell clone by another.

Lgr5 homologues associate with Wnt receptors and mediate R-spondin signalling

The adult stem cell marker Lgr5 and its relative Lgr4 are often co-expressed in Wnt-driven proliferative compartments. We find that conditional deletion of both genes in the mouse gut impairs Wnt target gene expression and results in the rapid demise of intestinal crypts, thus phenocopying Wnt pathway inhibition. Mass spectrometry demonstrates that Lgr4 and Lgr5 associate with the Frizzled/Lrp Wnt receptor complex. Each of the four R-spondins, secreted Wnt pathway agonists, can bind to Lgr4, -5 and -6. In HEK293 cells, RSPO1 enhances canonical WNT signals initiated by WNT3A. Removal of LGR4 does not affect WNT3A signalling, but abrogates the RSPO1-mediated signal enhancement, a phenomenon rescued by re-expression of LGR4, -5 or -6. Genetic deletion of Lgr4/5 in mouse intestinal crypt cultures phenocopies withdrawal of Rspo1 and can be rescued by Wnt pathway activation. Lgr5 homologues are facultative Wnt receptor components that mediate Wnt signal enhancement by soluble R-spondin proteins. These results will guide future studies towards the application of R-spondins for regenerative purposes of tissues expressing Lgr5 homologues.

The recruitment of bone marrow-derived cells to skin wounds is independent of wound size

Wounded skin recruits progenitor cells, which repair the tissue defect. These cells are derived from stem cells in several niches in the skin. In addition, bone marrow-derived cells (BMDCs) are recruited and contribute to wound repair. We hypothesized that larger wounds recruit more cells from the bone marrow. Wild-type rats were lethally irradiated and transplanted with bone marrow cells from green fluorescent protein (GFP)-transgenic rats. Seven weeks later, 4, 10, and 20 mm wounds were created. The wound tissue was harvested after 14 days. The density of GFP-positive cells in the wounds and the adjacent tissues was determined, as well as in normal skin from the flank. Bone marrow-derived myofibroblasts, activated fibroblasts, and macrophages were also quantified. After correction for cell density, the recruitment of BMDCs (23±11%) was found to be independent of wound size. Similar fractions of GFP-positive cells were also detected in nonwounded adjacent tissue (29±11%), and in normal skin (26±19%). The data indicate that BMDCs are not preferentially recruited to skin wounds. Furthermore, wound size does not seem to affect the recruitment of BMDCs.

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.

Tumor-initiating stem cells of squamous cell carcinomas and their control by TGF-β and integrin/focal adhesion kinase (FAK) signaling

Cancer stem cells (CSCs) sustain tumor growth through their ability to self-renew and to generate differentiated progeny. These functions endow CSCs with the potential to initiate secondary tumors bearing characteristics similar to those of the parent. Recently the hair follicle stem cell marker CD34 was used to purify a CSC-like cell population from early skin tumors arising from treatment with 7,12-dimethylbenz[α]anthracene/12-o-tetradecanoylphorbol-13-acetate, which typically generates benign papillomas that occasionally progress to squamous cell carcinomas (SCCs). In the present study, we identify and characterize CSCs purified from malignant SCCs. We show that SCCs contain two highly tumorigenic CSC populations that differ in CD34 levels but are enriched for integrins and coexist at the SCC-stroma interface. Intriguingly, whether CD34(lo) or CD34(hi), α6(hi)β1(hi) populations can initiate secondary tumors by serial limit-dilution transplantation assays, but α6(lo)β1(lo) populations cannot. Moreover, secondary tumors generated from a single CSC of either subtype contain both CD34(lo) and CD34(hi) α6(hi)β1(hi)CSCs, indicating their nonhierarchical organization. Genomic profiling and hierarchical cluster analysis show that these two CSC subtypes share a molecular signature distinct from either the CD34(-) epidermal or the CD34(hi) hair follicle stem cell signature. Although closely related, α6(hi)β1(hi)CD34(lo) and α6(hi)β1(hi)CD34(hi) CSCs differ in cell-cycle gene expression and proliferation characteristics. Indeed, proliferation and expansion of α6(hi)β1(hi)CD34(hi) CSCs is sensitive to whether they can initiate a TGF-β receptor II-mediated response to counterbalance elevated focal adhesion kinase-mediated integrin signaling within the tumor. Overall, the coexistence and interconvertibility of CSCs with differing sensitivities to their microenvironment pose challenges and opportunities for SCC cancer therapies.

[What's new in dermatological treatments?]

Although the isolated clinical cases published are sometimes helpful in individual situations in which the therapeutic options have been exhausted, this type of publication cannot be generalized. For this reason, the selection presented covering the period from November 2009 to October 2010 is to a very large extent based on controlled trials, either because they contribute important information or because they raise great hope for a significant number of patients. For the first time in cutaneous oncology, a treatment (ipilimumab) has significantly increased overall survival in patients with metastatic melanoma (phase III), although this gain remains modest (4-6 months) and adverse immunological effects are frequent (30-40%). A phase I trial with treatment specifically targeting the mutant BRAF protein has shown an objective response in 81% of the patients treated in the metastatic phase of melanoma, thus allowing its development to be pursued. Grouping two studies in a rare tumor such as dermatofibrosarcoma also gives hope with imatinib as a neoadjuvant treatment when the initial tumor is inoperable, with, however, an inconsistent response of approximately 50% and only if the tumor presents reorganization of chromosomes 17 and 22. Cutaneous inflammatory diseases are still dominated by dual therapies in psoriasis, with, notably, an effectiveness trial on etanercept at different doses not showing a difference in efficacy depending on dose for the joint component of psoriasis, but also by the publication of a direct comparison of two dual therapies, ustekinumab versus etanercept. In atopic dermatitis, a controversial article invites one to reflect upon the progress made in the management of children by clinical nurses, as in the Netherlands and in Great Britain, in an attempt to contend with the shortage of dermatologists. Since the use of biotherapies is not the prerogative of psoriasis, infliximab was assessed in a phase II trial in Verneuil disease without demonstrating significant efficacy on the main criterion, but it did show a tendency to reduce the score used. This trial suffered from a weakness both in methodology and statistical power, thus precluding any conclusion. The rarity of therapeutic trials on drug eruptions warrants their mention. A French phase II study gives a glimpse of a trend toward efficacy in terms of survival in the treatment of toxic epidermal necrolysis with cyclosporine. As for infectious dermatosis and sexually transmitted infections, a French multicenter study has shown significantly higher efficacy with ivermectin than with malathion in treating pediculosis without increasing the side effects. Today, however, this systemic treatment cannot be a first-line treatment outside of certain specific situations. A large cohort study (somewhat unsatisfactory in its methodology) has not demonstrated the teratogenicity of antiherpes treatments in 830,000 infants. In prevention of HIV transmission, no microbicidal gel had shown efficacy to date. This has now been accomplished in South Africa with a 1% tenofovir gel. The results of a preliminary trial on therapeutic vaccination against HPV16 proposed to women who are carriers of cervical intraepithelial neoplasia opens the way for wide vaccine therapy of cutaneous virus-induced neoplasia. In conclusion, several articles analyzing the dermatology literature provide an opportunity to reflect on the quality of such articles, Boutron's being absolutely in-dis-pen-sa-ble!

Expression patterns of lgr4 and lgr6 during zebrafish development

Leucine-rich repeat (LRR)-containing G protein-coupled receptors (LGRs) belong to the superfamily of G protein-coupled receptors, and are characterized by the presence of seven transmembrane domains and an extracellular domain that contains a series of LRR motifs. Three Lgr proteins - Lgr4, Lgr5, and Lgr6 - were identified as members of the LGR subfamily. Mouse Lgr4 has been implicated in the formation of various organs through regulation of cell proliferation during development, and Lgr5 and Lgr6 are stem cell markers in the intestine or skin. Although the expression of these three genes has already been characterized in adult mice, their expression profiles during the embryonic and larval development of the organism have not yet been defined. We cloned two zebrafish lgr genes using the zebrafish genomic database. Phylogenetic analyses showed that these two genes are orthologs of mammalian Lgr4 and Lgr6. Zebrafish lgr4 is expressed in the neural plate border, Kupffer's vesicle, neural tube, otic vesicles, midbrain, eyes, forebrain, and brain ventricular zone by 24h post-fertilization (hpf). From 36 to 96hpf, lgr4 expression is detected in the midbrain-hindbrain boundary, otic vesicles, pharyngeal arches, cranial cartilages such as Meckel's cartilages, palatoquadrates, and ceratohyals, cranial cavity, pectoral fin buds, brain ventricular zone, ciliary marginal zone, and digestive organs such as the intestine, liver, and pancreas. In contrast, zebrafish lgr6 is expressed in the notochord, Kupffer's vesicle, the most anterior region of diencephalon, otic vesicles, and the anterior and posterior lateral line primordia by 24hpf. From 48 to 72hpf, lgr6 expression is confined to the anterior and posterior neuromasts, otic vesicles, pharyngeal arches, pectoral fin buds, and cranial cartilages such as Meckel's cartilages, ceratohyals, and trabeculae. Our results provide a basis for future studies aimed at analyzing the functions of zebrafish Lgr4 and Lgr6 in cell differentiation and proliferation during organ development.

Basal cell carcinomas in mice arise from hair follicle stem cells and multiple epithelial progenitor populations

Uncontrolled Hedgehog (Hh) signaling leads to the development of basal cell carcinoma (BCC), the most common human cancer, but the cell of origin for BCC is unclear. While Hh pathway dysregulation is common to essentially all BCCs, there exist multiple histological subtypes, including superficial and nodular variants, raising the possibility that morphologically distinct BCCs may arise from different cellular compartments in skin. Here we have shown that induction of a major mediator of Hh signaling, GLI2 activator (GLI2ΔN), selectively in stem cells of resting hair follicles in mice, induced nodular BCC development from a small subset of cells in the lower bulge and secondary hair germ compartments. Tumorigenesis was markedly accelerated when GLI2ΔN was induced in growing hair follicles. In contrast, induction of GLI2ΔN in epidermis led to the formation of superficial BCCs. Expression of GLI2ΔN at reduced levels in mice yielded lesions resembling basaloid follicular hamartomas, which have previously been linked to low-level Hh signaling in both mice and humans. Our data show that the cell of origin, tissue context (quiescent versus growing hair follicles), and level of oncogenic signaling can determine the phenotype of Hh/Gli-driven skin tumors, with high-level signaling required for development of superficial BCC-like tumors from interfollicular epidermis and nodular BCC-like tumors from hair follicle stem cells.

Defining the origins of Ras/p53-mediated squamous cell carcinoma

The precise identity of cancer cells of origin and the molecular events of tumor initiation in epidermal squamous cell carcinoma (SCC) are unknown. Here we show that malignancy potential is related to the developmental capacity of the initiating cancer cell in a genetically defined, intact, and inducible in vivo model. Specifically, these data demonstrate that SCCs can originate from inside the hair follicle stem cell (SC) niche or from immediate progenitors, whereas more developmentally restricted progeny, the transit amplifying (TA) cells, are unable to generate even benign tumors in the same genetic context. Using a temporal model of tumorigenesis in situ, we highlight the phenotypes of cancer progression from the hair follicle SC niche, including hyperplasia, epithelial to mesenchymal transition, and SCC formation. Furthermore, we provide insights into the inability of hair follicle TA cells to respond to tumorigenic stimuli.

Identifying the cellular origin of squamous skin tumors

Squamous cell carcinoma (SCC) is the second most frequent skin cancer. The cellular origin of SCC remains controversial. Here, we used mouse genetics to determine the epidermal cell lineages at the origin of SCC. Using mice conditionally expressing a constitutively active KRas mutant (G12D) and an inducible CRE recombinase in different epidermal lineages, we activated Ras signaling in different cellular compartments of the skin epidermis and determined from which epidermal compartments Ras activation induces squamous tumor formation. Expression of mutant KRas in hair follicle bulge stem cells (SCs) and their immediate progeny (hair germ and outer root sheath), but not in their transient amplifying matrix cells, led to benign squamous skin tumor (papilloma). Expression of KRas(G12D) in interfollicular epidermis also led to papilloma formation, demonstrating that squamous tumor initiation is not restricted to the hair follicle lineages. Whereas no malignant tumor was observed after KRas(G12D) expression alone, expression of KRas(G12D) combined with the loss of p53 induced invasive SCC. Our studies demonstrate that different epidermal lineages including bulge SC are competent to initiate papilloma formation and that multiple genetic hits in the context of oncogenic KRas are required for the development of invasive SCC.

Isolation and in vitro expansion of Lgr6-positive multipotent hair follicle stem cells

Hair follicles (HFs) are a well-known niche of multipotent stem cells. Recently, HF cells expressing leucine-rich orphan G protein-coupled receptors (Lgr) have been described as multipotent adult stem cells with a high potential for regenerative therapies. We have now established the conditions for the isolation and long-term expansion of stem cells from HFs (HFSCs) and analyzed their characteristics (reverse transcription with the polymerase chain reaction, immunohistochemistry) and multipotent capacity (differentiation assays). HFSCs possess a high self-renewal capacity and share characteristics of putative HF epithelial stem cells, such as the expression of Lgr6, cytokeratins (Ck18, Ck19), and multipotent stem cell markers (Sca-1, Bcrp1, nestin, P75NTR). Under defined cell culture conditions, HFSCs are able to differentiate into mesenchymal lineages (adipocytes, chondrocytes, muscle cells) or neurons (βIII-tubulin). We report, for the first time, an in vitro culture method to isolate Lgr6-positive stem cells from HFs. HFSCs represent a unique tool for studying the cell characteristics of Lgr6-positive cells and should provide a novel easily accessible source for regenerative therapies.

Delayed re-epithelialization in periostin-deficient mice during cutaneous wound healing

Background

Matricellular proteins, including periostin, are important for tissue regeneration.

Methods and Findings

Presently we investigated the function of periostin in cutaneous wound healing by using periostin-deficient (−/−) mice. Periostin mRNA was expressed in both the epidermis and hair follicles, and periostin protein was located at the basement membrane in the hair follicles together with fibronectin and laminin γ2. Periostin was associated with laminin γ2, and this association enhanced the proteolytic cleavage of the laminin γ2 long form to produce its short form. To address the role of periostin in wound healing, we employed a wound healing model using WT and periostin−/− mice and the scratch wound assay in vitro. We found that the wound closure was delayed in the periostin−/− mice coupled with a delay in re-epithelialization and with reduced proliferation of keratinocytes. Furthermore, keratinocyte proliferation was enhanced in periostin-overexpressing HaCaT cells along with up-regulation of phosphorylated NF-κB.

Conclusion

These results indicate that periostin was essential for keratinocyte proliferation for re-epithelialization during cutaneous wound healing.

Pituitary stem/progenitor cells: embryonic players in the adult gland?

The pituitary gland represents the endocrine core of the body, and its hormonal output governs many key physiological processes. Because endocrine demands frequently change, the pituitary has to flexibly remodel its hormone-producing cell compartment. One mechanism of pituitary plasticity may rely on the generation of new hormonal cells from resident stem/progenitor cells. Existence of such 'master' cells in the pituitary has in the past repeatedly been postulated. Only recently, however, very plausible candidates have been identified that express stem cell-associated markers and signalling factors, and display the stem/progenitor cell characteristics of multipotency, efflux capacity (side population phenotype) and niche-like organization. In other adult tissues, stem cells recapitulate the embryonic developmental path on their course towards mature specialized cells. Interestingly, the pituitary stem/progenitor cell compartment shows prominent expression of transcriptional regulators and signalling factors that play a pivotal role during pituitary embryogenesis. This review summarizes the recent progress in pituitary stem/progenitor cell identification, highlights their potential embryonic phenotype, sketches a tentative stem/progenitor cell model, and discusses further research and challenges. Recognizing and scrutinizing the pituitary stem/progenitor cells as embryonic players in the adult gland may profoundly impact on our still poor understanding of the mechanisms underlying pituitary cell turnover and plasticity.

Tissue-resident adult stem cell populations of rapidly self-renewing organs

The epithelial lining of the intestine, stomach, and skin is continuously exposed to environmental assault, imposing a requirement for regular self-renewal. Resident adult stem cell populations drive this renewal, and much effort has been invested in revealing their identity. Reliable adult stem cell biomarkers would accelerate our understanding of stem cell roles in tissue homeostasis and cancer. Membrane-expressed markers would also facilitate isolation of these adult stem cell populations for exploitation of their regenerative potential. Here, we review recent advances in adult stem cell biology, highlighting the promise and pitfalls of the candidate biomarkers of the various stem cell populations.

Outgrowth of drug-resistant carcinomas expressing markers of tumor aggression after long-term TβRI/II kinase inhibition with LY2109761

TGF-β is produced excessively by many solid tumors and can drive malignant progression through multiple effects on the tumor cell and microenvironment. TGF-β signaling pathway inhibitors have shown efficacy in preclinical models of metastatic cancer. Here, we investigated the effect of systemic LY2109761, a TGF-β type I/II receptor (TβRI/TβRII) kinase inhibitor, in both a tumor allograft model and the mouse skin model of de novo chemically induced carcinogenesis in vivo. Systemic LY2109761 administration disrupted tumor vascular architecture and reduced myofibroblast differentiation of E4 skin carcinoma cells in a tumor allograft. In the 7,12-dimethyl-benzanthracene plus phorbol myristate acetate-induced skin chemical carcinogenesis model, acute dosing of established naive primary carcinomas with LY2109761 (100 mg/kg) every 8 hours for 10 days (100 mg/kg) diminished phospho-Smad2 (P-Smad2) levels and marginally decreased the expression of inflammatory and invasive markers. Sustained exposure to LY2109761 (100 mg/kg/d) throughout the tumor outgrowth phase had no effect on carcinoma latency or incidence. However, molecular analysis of resultant carcinomas by microarray gene expression, Western blotting, and immunohistochemistry suggests that long-term LY2109761 exposure leads to the outgrowth of carcinomas with elevated P-Smad2 levels that do not respond to drug. This is the first description of acquired resistance to a small-molecule inhibitor of the TβRI/TβRII kinase. Resultant carcinomas were more aggressive and inflammatory in nature, with delocalized E-cadherin and elevated expression of Il23a, laminin V, and matrix metalloproteinases. Therefore, TGF-β inhibitors might be clinically useful for applications requiring acute administration, but long-term patient exposure to such drugs should be undertaken with caution.

Wounding enhances epidermal tumorigenesis by recruiting hair follicle keratinocytes

Chronic wounds and acute trauma constitute well-established risk factors for development of epithelial-derived skin tumors, although the underlying mechanisms are largely unknown. Basal cell carcinomas (BCCs) are the most common skin cancers displaying a number of features reminiscent of hair follicle (HF)-derived cells and are dependent on deregulated Hedgehog (Hh)/GLI signaling. Here we show, in a mouse model conditionally expressing GLI1 and in a model with homozygous inactivation of Ptch1, mimicking the situation in human BCCs, that the wound environment accelerates the initiation frequency and growth of BCC-like lesions. Lineage tracing reveals that both oncogene activation and wounding induce emigration of keratinocytes residing in the lower bulge and the nonpermanent part of the HFs toward the interfollicular epidermis (IFE). However, only oncogene activation in combination with a wound environment enables the participation of such cells in the initiation of BCC-like lesions at the HF openings and in the IFE. We conclude that, in addition to the direct enhancement of BCC growth, the tumor-promoting effect of the wound environment is due to recruitment of tumor-initiating cells originating from the neighboring HFs, establishing a link between epidermal wounds and skin cancer risk.

Dynamics between stem cells, niche, and progeny in the hair follicle

Here, we exploit the hair follicle to define the point at which stem cells (SCs) become irreversibly committed along a differentiation lineage. Employing histone and nucleotide double-pulse-chase and lineage tracing, we show that the early SC descendents en route to becoming transit-amplifying cells retain stemness and slow-cycling properties and home back to the bulge niche when hair growth stops. These become the primary SCs for the next hair cycle, whereas initial bulge SCs become reserves for injury. Proliferating descendents further en route irreversibly lose their stemness, although they retain many SC markers and survive, unlike their transit-amplifying progeny. Remarkably, these progeny also home back to the bulge. Combining purification and gene expression analysis with differential ablation and functional experiments, we define critical functions for these non-SC niche residents and unveil the intriguing concept that an irreversibly committed cell in an SC lineage can become an essential contributor to the niche microenvironment.