Characterization and isolation of stem cell-enriched human hair follicle bulge cells

Biology of human hair: know your hair to control it

Hair can be engineered at different levels--its structure and surface--through modification of its constituent molecules, in particular proteins, but also the hair follicle (HF) can be genetically altered, in particular with the advent of siRNA-based applications. General aspects of hair biology are reviewed, as well as the most recent contributions to understanding hair pigmentation and the regulation of hair development. Focus will also be placed on the techniques developed specifically for delivering compounds of varying chemical nature to the HF, indicating methods for genetic/biochemical modulation of HF components for the treatment of hair diseases. Finally, hair fiber structure and chemical characteristics will be discussed as targets for keratin surface functionalization.

Tissue engineering of skin

The engineering of skin substitutes and their application on human patients has become a reality. However, cell biologists, biochemists, technical engineers, and surgeons are still struggling with the generation of complex skin substitutes that can readily be transplanted in large quantities, possibly in only one surgical intervention and without significant scarring. Constructing a dermo-epidermal substitute that rapidly vascularizes, optimally supports a stratifying epidermal graft on a biodegradable matrix, and that can be conveniently handled by the surgeon, is now the ambitious goal. After all, this goal has to be reached coping with strict safety requirements and the harsh rules of the economic market.

High aldehyde dehydrogenase activity: a novel functional marker of murine prostate stem/progenitor cells

We have shown previously that prostatic stem/progenitor cells can be purified from isolated prostate ducts, based on their high expression of the Sca-1 surface antigen. We now report that high levels of aldehyde dehydrogenase (ALDH) activity are present in a subset of prostate epithelial cells that coexpress a number of antigens found on stem/progenitor cells of other origins (CD9, Bcl-2, CD200, CD24, prominin, Oct 3/4, ABCG2, and nestin). Almost all of these cells expressing high levels of ALDH activity also express Sca-1 and a third of them express high levels of this antigen. The cells with high levels of ALDH activity have greater in vitro proliferative potential than cells with low ALDH activity. Importantly, in an in vivo prostate reconstitution assay, the cells expressing high levels of ALDH activity were much more effective in generating prostatic tissue than a population of cells with low enzymatic activity. Thus, a high level of ALDH activity can be considered a functional marker of prostate stem/progenitor cells and allows for simple, efficient isolation of cells with primitive features. The elucidation of the role of ALDH in prostate stem/progenitor cells may lead to the development of rational therapies for treating prostate cancer and benign prostatic hyperplasia.

Tumor mapping in 2 large multigenerational families with CYLD mutations: implications for disease management and tumor induction

OBJECTIVES

To comprehensively ascertain the extent and severity of clinical features in affected individuals from 2 large families with proven heterozygous mutations in the CYLD locus and to correlate these findings with the 3 appendageal tumor predisposition syndromes (familial cylindromatosis, Brooke-Spiegler syndrome, and multiple familial trichoepitheliomas) known to be associated with such germline mutations.

DESIGN

Interfamilial and intrafamilial observational study.

SETTING

Tertiary genetic and dermatology referral center.

PARTICIPANTS

Thirty-four individuals recruited from 2 large multigenerational families with CYLD mutations. Clinical details, history, and tumor maps were obtained from all participants; in 18, the information was corroborated by detailed clinical examination.

MAIN OUTCOME MEASURES

Tumor density, distribution and histologic findings, associated medical conditions, patient symptoms, and impact of disease on quality of life.

RESULTS

The severity of penetrance and phenotype varied within families. Although an approximately equal female to male predisposition was noted, 5 women and 1 man (of 26 patients surveyed [23%]) had undergone total scalp removal. The average age at onset was 16 years (range, 8-30 years). Symptoms reported by affected patients included painful tumors (in 12 of 23 patients [52%] who answered the question), conductive deafness, and sexual dysfunction. Of the 26 surveyed patients, tumors were noted on the scalp in 21 (81%), on the trunk in 18 (69%), and in the pubic area in 11 (42%). Tumor mapping provided clinical evidence that correlated with hormonally stimulated hair follicles being particularly vulnerable to loss of heterozygosity and tumor induction.

CONCLUSIONS

The burden of disease at sites other than the head and neck appears to be underreported in the literature and greatly affects quality of life. Differentiation between the clinical diagnoses has little prognostic or clinical utility in genetic counseling, even within individuals from the same family. Thus, we suggest an encompassing diagnosis of "CYLD cutaneous syndrome." Finally, the clinical distribution of tumors suggests that hormonal factors may play an important role in tumor induction in these patients.

Hedgehog signaling maintains hair follicle stem cell phenotype in young and aged human skin

Skin hair follicles (HF) contain bulge stem cells (SC) that regenerate HFs during hair cycles, and repair skin epithelia following injury. As natural aging is associated with decreased skin repair capacity in humans, we have investigated the impact of age on human scalp HF bulge cell number and function. Here, we isolated human bulge cells, characterized as CD200+/KRT15+/KRT19+ cells of the HF, by dissection-combined CD200 selection in young and aged human skin. Targeted transcriptional profiling indicates that KRT15, KRT19, Dkk3, Dkk4, Tcf3, S100A4, Gas1, EGFR and CTGF/CCN2 are also preferentially expressed by human bulge cells, compared to differentiated HF keratinocytes (KC). Our results demonstrate that aging does not alter expression or localization of these HF SC markers. In addition, we could not detect significant differences in HF density or bulge cell number between young and aged human scalp skin. Interestingly, hedgehog (Hh) signaling is activated in human bulge cells in vivo, and down-regulated in differentiated HF KCs, both in young and aged skin. In addition, activation of Hh signaling by lentivirus-mediated overexpression of transcription factor Gli1 induces transcription of HF SC markers KRT15, KRT19, and Gas1, in cultured KCs. Together with previously reported knock-out mouse results, these data suggest a role for Hh signaling in maintaining bulge cell phenotype in young and aged human skin.

De novo follicular regeneration of the skin by wingless int 3 and bone morphogenetic protein 2 genes introduced into dermal fibroblasts and fibroblast growth factor-2 protein

In this study, we regenerated skin and its appendages by transplanting cultured normal dermal fibroblasts, into which morphogen genes had been introduced. We cultured normal dermal fibroblasts obtained from Fisher 344 rats on the surface of hydroxyapatite beads, and then adsorbed them onto the surface of a collagen sponge, which was transplanted into a full-thickness skin defect prepared on the backs of rats. Before transplantation, genes were introduced into the dermal fibroblasts via adenovirus vector (ad)-bone morphogenetic protein 2 and ad-wingless int 3 genes in addition to fibroblast growth factor-2 protein. By Week 4, the appearance of follicle germs or primitive hair germs was observed only in the ad-bone morphogenetic protein 2+ad-wingless int 3 combined with the fibroblast growth factor-2 protein group. By Week 16, in that same group, hair follicles having mature pilosebaceous systems with equally spaced localization had formed in the ulcer wound.

Characterization of rat hair follicle stem cells selected by vario magnetic activated cell sorting system

Hair follicle stem cells (HfSCs) play crucial roles in hair follicle morphogenesis and hair cycling. These stem cells are self-renewable and have the multi-lineage potential to generate epidermis, sebaceous glands, and hair follicle. The separation and identification of hair follicle stem cells are important for further research in stem cell biology. In this study, we report on the successful enrichment of rat hair follicle stem cells through vario magnetic activated cell sorting (Vario MACS) and the biological characteristics of the stem cells. We chose the HfSCs positive surface markers CD34, α6-integrin and the negative marker CD71 to design four isolation strategies: positive selection with single marker of CD34, positive selection with single marker of α6-integrin, CD71 depletion followed by CD34 positive selection, and CD71 depletion followed by α6-integrin positive selection. The results of flow cytometry analysis showed that all four strategies had ideal effects. Specifically, we conducted a series of researches on HfSCs characterized by their high level of CD34, termed CD34^bri cells, and low to undetectable expression of CD34, termed CD34^dim cells. CD34^bri cells had greater proliferative potential and higher colony-forming ability than CD34^dim cells. Furthermore, CD34^bri cells had some typical characteristics as progenitor cells, such as large nucleus, obvious nucleolus, large nuclear:cytoplasmic ratio and few cytoplasmic organelles. Our findings clearly demonstrated that HfSCs with high purity and viability could be successfully enriched with Vario MACS.

Necl2 regulates epidermal adhesion and wound repair

Differential expression of cell adhesion molecules regulates stem cell location, self-renewal and lineage selection under steady state conditions and during tissue repair. We show that the intercellular adhesion protein nectin-like molecule 2 (Necl2) is highly expressed in bulge stem cells of adult human and mouse hair follicles. Overexpression of Necl2 in cultured human keratinocytes led to upregulation of calcium/calmodulin-associated Ser/Thr kinase (CASK), increased calcium-independent intercellular adhesion, and inhibition of cell motility and in vitro wound healing. Although the rate of cell proliferation was reduced, terminal differentiation was unaffected. To assess the role of Necl2 in vivo, we examined the epidermis of Necl2-null mice and developed transgenic mice that expressed Necl2 in the basal layer of murine epidermis. Necl2 overexpression led to a reduction in S-phase cells and an increase in quiescent cells retaining DNA label in the bulge. Although epidermal homeostasis appeared normal in both transgenic and knockout mice, wound healing was markedly delayed. Necl2 overexpression resulted in reduced proliferation and increased levels of CASK and E-cadherin at the leading edge of healing wounds, consistent with its effects in culture. Our results demonstrate that Necl2 is involved in regulating epidermal stem cell quiescence and location.

Loss-of-function FERMT1 mutations in kindler syndrome implicate a role for fermitin family homolog-1 in integrin activation

Kindler syndrome is an autosomal recessive disorder characterized by skin atrophy and blistering. It results from loss-of-function mutations in the FERMT1 gene encoding the focal adhesion protein, fermitin family homolog-1. How and why deficiency of fermitin family homolog-1 results in skin atrophy and blistering are unclear. In this study, we investigated the epidermal basement membrane and keratinocyte biology abnormalities in Kindler syndrome. We identified altered distribution of several basement membrane proteins, including types IV, VII, and XVII collagens and laminin-332 in Kindler syndrome skin. In addition, reduced immunolabeling intensity of epidermal cell markers such as beta1 and alpha6 integrins and cytokeratin 15 was noted. At the cellular level, there was loss of beta4 integrin immunolocalization and random distribution of laminin-332 in Kindler syndrome keratinocytes. Of note, active beta1 integrin was reduced but overexpression of fermitin family homolog-1 restored integrin activation and partially rescued the Kindler syndrome cellular phenotype. This study provides evidence that fermitin family homolog-1 is implicated in integrin activation and demonstrates that lack of this protein leads to pathological changes beyond focal adhesions, with disruption of several hemidesmosomal components and reduced expression of keratinocyte stem cell markers. These findings collectively provide novel data on the role of fermitin family homolog-1 in skin and further insight into the pathophysiology of Kindler syndrome.

Recent advances on skin-resident stem/progenitor cell functions in skin regeneration, aging and cancers and novel anti-aging and cancer therapies

Recent advances in skin-resident adult stem/progenitor cell research have revealed that these immature and regenerative cells with a high longevity provide critical functions in maintaining skin homeostasis and repair after severe injuries along the lifespan of individuals. The establishment of the functional properties of distinct adult stem/progenitor cells found in skin epidermis and hair follicles and extrinsic signals from their niches, which are deregulated during their aging and malignant transformation, has significantly improved our understanding on the etiopathogenesis of diverse human skin disorders and cancers. Particularly, enhanced ultraviolet radiation exposure, inflammation and oxidative stress and telomere attrition during chronological aging may induce severe DNA damages and genomic instability in the skin-resident stem/progenitor cells and their progenies. These molecular events may result in the alterations in key signalling components controlling their self-renewal and/or regenerative capacities as well as the activation of tumour suppressor gene products that trigger their growth arrest and senescence or apoptotic death. The progressive decline in the regenerative functions and/or number of skin-resident adult stem/progenitor cells may cause diverse skin diseases with advancing age. Moreover, the photoaging, telomerase re-activation and occurrence of different oncogenic events in skin-resident adult stem/progenitor cells may also culminate in their malignant transformation into cancer stem/progenitor cells and skin cancer initiation and progression. Therefore, the anti-inflammatory and anti-oxidant treatments and stem cell-replacement and gene therapies as well as the molecular targeting of their malignant counterpart, skin cancer-initiating cells offer great promise to treat diverse skin disorders and cancers.

Control of stem cell fate by physical interactions with the extracellular matrix

A diverse array of environmental factors contributes to the overall control of stem cell activity. In particular, new data continue to mount on the influence of the extracellular matrix (ECM) on stem cell fate through physical interactions with cells, such as the control of cell geometry, ECM geometry/topography at the nanoscale, ECM mechanical properties, and the transmission of mechanical or other biophysical factors to the cell. Here, we review some of the physical processes by which cues from the ECM can influence stem cell fate, with particular relevance to the use of stem cells in tissue engineering and regenerative medicine.

Exploring the role of stem cells in cutaneous wound healing

The skin offers a perfect model system for studying the wound healing cascade, which involves a finely tuned interplay between several cell types, pathways and processes. The dysregulation of these factors may lead to wound healing disorders resulting in chronic wounds, as well as abnormal scars such as hypertrophic and keloid scars. As the contribution of stem cells towards tissue regeneration and wound healing is increasingly appreciated, a rising number of stem cell therapies for cutaneous wounds are currently under development, encouraged by emerging preliminary findings in both animal models and human studies. However, we still lack an in-depth understanding of the underlying mechanisms through which stem cells contribute to cutaneous wound healing. The aim of this review is, therefore, to present a critical synthesis of our current understanding of the role of stem cells in normal cutaneous wound healing. In addition to summarizing wound healing principles and related key molecular and cellular players, we discuss the potential participation of different cutaneous stem cell populations in wound healing, and list corresponding stem cells markers. In summary, this review delineates current strategies, future applications, and limitations of stem cell-based or stem cell-targeted therapy in the management of acute and chronic skin wounds.

Expression pattern of GATA-3 in embryonic and fetal human skin suggests a role in epidermal and follicular morphogenesis

BACKGROUND

The transcription factor GATA-3 was recently identified as a master regulator in the specification of the inner root sheath. Additionally, it seems to play a role in skin barrier physiology. p63 binds and transactivates the GATA-3 promoter. While the expression profile of GATA-3 is delineated for the mouse, little is known about its expression in the adult human hair follicle and no studies are published about its distribution during human cutaneous embryogenesis.

METHODS

We examined samples from embryonic, fetal and adult human skin for the expression of GATA-3 using immunohistochemistry.

RESULTS

GATA-3 is expressed late during human skin development. Its expression pattern is comparable to the mouse and confined to the Huxley layer and inner root sheath cuticle but sparing the Henle layer. In addition, GATA-3 localizes to the spinous cell layer of the interfollicular epidermis.

CONCLUSIONS

From the described expression pattern, it is highly probable that GATA-3 plays a role in follicular and epidermal morphogenesis. What the anatomically confined expression of GATA-3 to the spinous layer means biologically for the physiology of the skin is still unclear. Likewise, it still needs to be shown if GATA-3 could be exploited in the diagnosis of adnexal neoplasms.

Expression and function of glycogen synthase kinase-3 in human hair follicles

Beta-catenin is involved in the hair follicle morphogenesis and stem cell differentiation, and inhibition of glycogen synthase kinase-3 (GSK-3) increases beta-catenin concentration in the cytoplasm. To examine the effects of GSK-3 inhibition on the hair follicle epithelium, we first examined the expression of GSK-3 in plucked human hair follicles by RT-PCR and found GSK-3 expression in hair follicles. Western blotting with a GSK-3beta-specific antibody, Y174, also demonstrated GSK-3beta expression in the follicles. Moreover, GSK-3beta immunostaining with Y174 showed that GSK-3beta colocalized with hair follicle bulge markers. Contrary to GSK-3beta, GSK-3 alpha was widely expressed throughout the follicles when immunostained with a specific antibody, EP793Y. We then investigated the influence of GSK-3 inhibition. A GSK-3 inhibitor, BIO, promoted the growth of human outer root sheath cells, which could be cultured for up to four passages. The BIO-treated cells exhibited smaller and more undifferentiated morphology than control cells. Moreover, in organ culture of plucked human hair, outer root sheath cells in the middle of a hair follicle proliferated when cultured with BIO. These results indicate that GSK-3beta is expressed in hair bulge stem cells and BIO promotes the growth of ORS cells, possibly by regulating the GSK-3 signaling pathway.

Stem cell therapy to reduce radiation-induced normal tissue damage

Normal tissue damage after radiotherapy is still a major problem in cancer treatment. Stem cell therapy may provide a means to reduce radiation-induced side effects and improve the quality of life of patients. This review discusses the current status in stem cell research with respect to their potential to reduce radiation toxicity. A number of different types of stem cells are being investigated for their potential to treat a variety of disorders. Their current status, localization, characterization, isolation, and potential in stem cell-based therapies are addressed. Although clinical adult stem cell research is still at an early stage, preclinical experiments show the potential these therapies may have. Based on the major advances made in this field, stem cell-based therapy has great potential to allow prevention or treatment of normal tissue damage after radiotherapy.

Differential expression of stem-cell-associated markers in human hair follicle epithelial cells

Several putative biomarkers have been suggested for identifying murine follicular stem cells; however, human hair follicles have a different pattern of biomarker expression, and follicular stem cell isolation methods have not been established. To isolate a stem cell population applicable to clinical settings, we conducted a comprehensive survey of the expression of stem-cell-associated (K15, CD200, CD34, and CD271) and other biomarkers (K1, K14, CD29, and CD49f) in immunohistological sections of the human epidermis and follicular outer root sheath (ORS). We also examined freshly isolated and cultured epidermal or follicular cells with single- and multicolor flow cytometry or immunocytochemistry. After sorting cells by CD200, CD34, and forward scatter (FSC) values (cell size), colony-forming assays were performed. We found that biomarkers were differentially expressed in the epidermis and ORS. Basal bulge cells were mainly K15+CD200+CD34(-)CD271(-), and suprabasal cells were K15(-)CD200+CD34(-)CD271(-). We categorized follicular cells into nine subpopulations according to biomarker expression profiles. The CD200+CD34(-) bulge cells had much higher colony-forming abilities than the CD34+ population, and were divided into two subpopulations: a CD200+CD34(-)FSC(high) (K15-rich, basal) and a CD200+CD34(-)FSC(low) (K15-poor, suprabasal) population. The former formed fewer but larger-sized colonies than the latter. Follicular epithelial cell cultivation resulted in loss of K15, CD200, CD34, and CD271 expression, but maintenance of K14, CD29, and CD49f expression. We found that the bulge contained two populations with different localizations, cell sizes, and colony-forming abilities. We showed that K15, CD200, CD34, and CD271 were useful biomarkers for characterizing freshly isolated human follicular epithelial cells in diverse stages of differentiation.

Leptin and the skin: a new frontier

Here, we examine the currently available information which supports that the adipokine, leptin, is a major player in the biology and pathology of mammalian skin and its appendages. Specifically, the potent metabolic effects of leptin and its mimetics may be utilized to improve, preserve and restore skin regeneration and hair cycle progression, and may halt or even partially reverse some aspects of skin ageing. Since leptin can enhance mitochondrial activity and biogenesis, this may contribute to the wound healing-promoting and hair growth-modulatory effects of leptin. Leptin dependent intracellular signalling by the Janus kinase 2 dependent signal transducer and activator of transcription 3, adenosine monophosphate kinase, and peroxisome proliferator-activated receptor (PPAR) gamma coactivator/PPAR converges to mediate mitochondrial metabolic activation and enhanced cell proliferation which may orchestrate the potent developmental, trophic and protective effects of leptin. Since leptin and leptin mimetics have already been clinically tested, investigative dermatology is well-advised to place greater emphasis on the systematic exploration of the cutaneous dimensions and dermatological potential of this pleiotropic hormone.

Distribution and quantity of label-retaining cells in rat oral epithelia

BACKGROUND

Label-retaining cells (LRCs), the presumptive stem cells, have been detected in the mouse and hamster oral epithelia, but the data on LRCs in rat oral epithelia have not been available yet. The aim of this study was to identify LRCs in oral squamous epithelia of rat.

METHODS

Fifty-four two-week-old Sprague-Dawley rats were injected intraperitoneally with BrdU twice daily for four consecutive days. The BrdU-labeled rats were sacrificed at 2 h, week 2, 4, 6, 8, 10, 12, 14 and 16 after the last BrdU injection. The tissues of cheek, tongue and palate were analyzed by techniques of fluorescence-activated cell sorting (FACS) and immunohistochemistry.

RESULTS

The number of BrdU-labeled cells quantified by FACS increased within 2 weeks after labeling, then, decreased gradually until week 10. After week 10, the BrdU-labeled cells were found to locate mainly in basal layer and their number kept consistent at 3-7% of total oral epithelial cells. The number of BrdU-labeled epithelial cells in palate was statistically higher than that in cheek or tongue at the same time point.

CONCLUSIONS

This study showed that the number and the distribution of BrdU-labeled epithelial cells stabilized from 10 weeks after labeling. Therefore, these BrdU-labeled cells after a 10-week chase were considered oral epithelial LRCs.

The hair follicle barrier to involvement by malignant melanoma

BACKGROUND

Melanoma characteristically grows within the epidermis along the dermal-epidermal junction, sometimes extending outward up to several centimeters beyond the foci of invasive tumors. Although follicular involvement by malignant melanoma is widely recognized, to the authors' knowledge no previously published data address this phenomenon.

METHODS

To examine the growth characteristics of in situ melanomas in relation to the hair follicle microanatomy, the authors analyzed 100 cases of primary cutaneous melanomas (61 in situ and 39 invasive melanomas with significant in situ components) obtained from pathology clinical archives.

RESULTS

Eighty-two (82%) cases of melanoma in situ demonstrated tumor cells within >or=1 hair follicles. Of those, 57 (69.5%) cases demonstrated the tumor cells only within the infundibulum. Extension of the tumor cells down to the isthmus was observed in 24 cases (29.3%). In only 1 exceptional case (1%) were tumor cells detected beneath the level of the hair follicle bulge.

CONCLUSIONS

The authors postulate that a physiologic barrier restricts the intraepithelial spread of melanoma tumor cells at or beyond the level of the stem cell niche in the hair follicle bulge. Although the nature of this barrier remains to be elucidated, the distinct biologic characteristics of the hair follicle bulge may provide clues to understanding this phenomenon.

Hair regeneration from transected follicles in duplicative surgery: rate of success and cell populations involved

BACKGROUND

The use of bisected hair follicles in hair transplantation has been previously reported, but the capacity of each half to regenerate the entire hair has not been clarified.

OBJECTIVE

To evaluate duplicative surgery rate of success and to analyze the cell populations involved in hair regeneration.

METHODS

We screened 28 patients undergoing duplicative surgery. Approximately 100 hair follicles from each patient were horizontally bisected and implanted. Upper and lower portions were stained for the known epithelial stem cell markers CD200, p63, beta1-integrin, CD34, and K19.

RESULTS

Similar percentages of hair regrowth after 12 months were observed when implanting the upper (72.7 +/- 0.4%) and lower (69.2 +/- 1.1%) portions. Expression of CD200, p63, and beta1-integrin was detected in both portions, whereas K19 and CD34 stained different cell populations in the upper and lower fragment, respectively.

CONCLUSION

Duplicative surgery might represent a successful alternative for hair transplantation, because both portions are capable of regenerating a healthy hair. Moreover, our results suggest the possible presence of stem cells in both halves of the follicle.

The hair follicle as a dynamic miniorgan

Hair is a primary characteristic of mammals, and exerts a wide range of functions including thermoregulation, physical protection, sensory activity, and social interactions. The hair shaft consists of terminally differentiated keratinocytes that are produced by the hair follicle. Hair follicle development takes place during fetal skin development and relies on tightly regulated ectodermal-mesodermal interactions. After birth, mature and actively growing hair follicles eventually become anchored in the subcutis, and periodically regenerate by spontaneously undergoing repetitive cycles of growth (anagen), apoptosis-driven regression (catagen), and relative quiescence (telogen). Our molecular understanding of hair follicle biology relies heavily on mouse mutants with abnormalities in hair structure, growth, and/or pigmentation. These mice have allowed novel insights into important general molecular and cellular processes beyond skin and hair biology, ranging from organ induction, morphogenesis and regeneration, to pigment and stem cell biology, cell proliferation, migration and apoptosis. In this review, we present basic concepts of hair follicle biology and summarize important recent advances in the field.

Epidermal stem cells: practical perspectives and potential uses

Throughout adult life, the epidermis and the hair follicle undergo a perpetual cycle of growth, regression and rest. Stem cells in the epidermis not only ensure the maintenance of epidermal homeostasis and hair regeneration, but also contribute to repair of the epidermis after injury. These stem cells lie within specific niches in the hair follicle and the epidermis. The availability of monoclonal antibodies that can be used on formalin-fixed paraffin-embedded tissue has greatly facilitated the use of this methodology as an adjunct to uncovering stem cell niches. In this review, we attempt to provide an overview of the potential markers available to identify and study stem cells in an effort to providing a better understanding of the pathogenesis of skin diseases including disorders of hair loss and malignancies. The potential uses of these markers in prognosis and in expanding the therapeutic options in several disorders will also be addressed.

Expression of hair follicle stem cells detected by cytokeratin 15 stain: implications for pathogenesis of the scarring process in cutaneous lupus erythematosus

BACKGROUND

Discoid lupus erythematosus (DLE) is a scarring disease. Although the scarring and deformity may affect any part of the body, such changes have been reported to be most obvious on the face and scalp. The pathogenesis behind this scarring process is not well understood. Once lesions have scarred, recurrent disease tends to occur at the edge of the scarred lesions but not within them.

OBJECTIVES

The fact that inflammation in DLE generally involves the bulge area of the follicles raises the possibility that damage to the stem cells of the bulge region may be one process leading to the permanent loss of follicles. The aim of this study was to investigate the role of the hair follicle stem cells which reside in the bulge region in the scarring process in cutaneous lupus erythematosus (CLE).

METHODS

We studied the reactivity of an antibody to the CD8 antigen (C8/144B), which recognizes cytokeratin (CK) 15 and preferentially immunostains hair follicle stem cells without staining the remaining hair follicle, on skin biopsies (scalp and body lesions) from patients with CLE (36 with discoid lesions and 10 with subacute lesions). Normal scalp and body biopsy specimens served as controls. The correlation between the extent of the cytotoxic inflammatory cell infiltrate (CD8+) and the presence of stem cells was investigated. Results were analysed semiquantitatively.

RESULTS

The expression of CK15 in hair follicle stem cells was variable in the DLE lesions; there was normal to moderate CK15 expression at the bulge region of hair follicles when surrounded by mild or moderate inflammatory infiltrate (CD8+), but in cases of severe inflammation, CK15 expression was weak or absent.

CONCLUSIONS

The bulge region appears to be involved in this disease as part of a broader involvement of the hair follicles; it is secondarily affected by the surrounding inflammatory cell infiltrate. Expression of C8/144B diminished and was then absent, indicating either damage to stem cells or differentiation to help in the repair process. Damage to follicular stem cells may help to explain the irreversible alopecia and the scarring process which characterize this disease.

Corneal limbal microenvironment can induce transdifferentiation of hair follicle stem cells into corneal epithelial-like cells

The aim of this study was to investigate the transdifferentiation potential of murine vibrissa hair follicle (HF) stem cells into corneal epithelial-like cells through modulation by corneal- or limbus-specific microenvironmental factors. Adult epithelial stem cells were isolated from the HF bulge region by mechanical dissection or fluorescence-activated cell sorting using antibodies to alpha6 integrin, enriched by clonal expansion, and subcultivated on various extracellular matrices (type IV collagen, laminin-1, laminin-5, fibronectin) and in different conditioned media derived from central and peripheral corneal fibroblasts, limbal stromal fibroblasts, and 3T3 fibroblasts. Cellular phenotype and differentiation were evaluated by light and electron microscopy, real-time reverse transcription-polymerase chain reaction, immunocytochemistry, and Western blotting, using antibodies against putative stem cell markers (K15, alpha6 integrin) and differentiation markers characteristic for corneal epithelium (K12, Pax6) or epidermis (K10). Using laminin-5, a major component of the corneo-limbal basement membrane zone, and conditioned medium from limbal stromal fibroblasts, clonally enriched HF stem and progenitor cells adhered rapidly and formed regularly arranged stratified cell sheets. Conditioned medium derived from limbal fibroblasts markedly upregulated expression of cornea-specific K12 and Pax6 on the mRNA and protein level, whereas expression of the epidermal keratinocyte marker K10 was strongly downregulated. These findings suggest that adult HF epithelial stem cells are capable of differentiating into corneal epithelial-like cells in vitro when exposed to a limbus-specific microenvironment. Therefore, the HF may be an easily accessible alternative therapeutic source of autologous adult stem cells for replacement of the corneal epithelium and restoration of visual function in patients with ocular surface disorders.

Attributes of adult stem cells

While cultured embryonic stem (ES) cells can be harvested in abundance and appear to be the most versatile of cells for regenerative medicine, adult stem cells also hold promise, but the identity and subsequent isolation of these comparatively rare cells remains problematic in most tissues, perhaps with the notable exception of the bone marrow. The ability to continuously self-renew and produce the differentiated progeny of the tissue of their location are their defining properties. Identifying surface molecules (markers) that would aid in stem cell isolation is a major goal. Considerable overlap exists between different putative organ-specific stem cells in their repertoire of gene expression, often related to self-renewal, cell survival and cell adhesion. More robust tests of 'stemness' are now being employed, using lineage-specific genetic marking and tracking to show production of long-lived clones and multipotentiality in vivo. Moreover, the characterization of normal stem cells in specific tissues may provide a dividend for the treatment of cancer. The successful treatment of neoplastic disease may well require the specific targeting of neoplastic stem cells, cells that may well have many of the characteristics of their normal counterparts.

Hair follicle stem cells in the pathogenesis of the scarring process in cutaneous lupus erythematosus

Lupus erythematosus (LE) is an autoimmune disease that can affect one or more internal organs (systemic LE [SLE]) as well as the skin (CLE). Common cutaneous subtypes of CLE are chronic CLE (CCLE) and subacute CLE (SCLE). CCLE is the only type of CLE which heals with scarring and this may affect any site in the body. The fact that inflammation in CCLE generally involves the bulge area of the follicles (where the stem cells reside) raises the possibility that damage to the stem cells may be one process leading to permanent loss of follicles. One of the most useful distinctive markers of the stem cells is cytokeratin 15 (CK15) and this has been used in some studies to demonstrate the involvement of the bulge region in the scarring process in primary cicatricial alopecia and DLE. The bulge region appears to be involved in the scarring process in CLE and other types of cicatricial alopecia as part of broader involvement of the hair follicles; it is secondarily affected by the surrounding inflammatory cell infiltrate. Expression of the stem cell marker CK15 diminished and was then absent indicating either damage to stem cells or differentiation to help in the repair process.

Characterization of human skin-derived mesenchymal stem cell proliferation rate in different growth conditions

This study investigated conditions for optimal in vitro propagation of human skin-derived mesenchymal stem cells (S-MSC). Forty primary skin-derived precursor cell (SKP) cultures were established from both male and female donors (age 29-65 years) and eight of them were randomly selected for in-depth characterization. Effects of basic fibroblast growth factor (FGF-2), epidermal growth factor (EGF), leukemia inhibiting factor (LIF) and dibutyryl-cyclic adenosine monophosphate (db-cAMP) on S-MSC proliferation were investigated. Primary SKP cultures were >95% homogenous for CD90, CD73, and CD105 marker expression enabling to classify these cells as S-MSC. FGF-2 dose-dependent stimulation was observed in low serum medium only, whereas EGF neither stimulated S-MSC proliferation nor potentates the effect of FGF-2. Pronounced donor to donor differences among S-MSC cultures were observed in 3-day proliferation assay. This study demonstrates that homogenous S-MSC populations can be reproducibly isolated from individual donors of different age. Optimal cell culture conditions for in vitro propagation of S-MSC are B27 supplemented or low serum media with FGF-2 (4 ng/ml). EGF and LIF as well as db-cAMP are dispensable for S-MSC proliferation.

Evidence that the bulge region is a site of relative immune privilege in human hair follicles

BACKGROUND

Recent gene profiling data suggest that, besides the anagen hair bulb, the epithelial stem cell region in the outer root sheath of hair follicles (HFs), termed the bulge, may also represent an area of relative immune privilege (IP).

OBJECTIVES

To investigate whether the human HF bulge is a site of relative IP within anagen VI HFs.

METHODS

Anagen VI HFs from normal human scalp skin were analysed using immunohistological staining techniques, quantitative histomorphometry and statistical analysis. For functional evidence we performed full-thickness human scalp skin organ cultures to investigate whether interferon (IFN)-gamma, a key inducer of IP collapse in hair bulbs, has a similar effect on the putative bulge IP.

RESULTS

Major histocompatibility complex (MHC) class Ia, beta(2)-microglobulin and MHC class II immunoreactivity are downregulated in the human bulge. The immunosuppressants alpha-melanocyte stimulating hormone, transforming growth factor-beta2, macrophage migration inhibitory factor and indoleamine-2,3-dioxygenase (IDO) are upregulated in the CD200+, stem cell-rich bulge region. These CD200+ cells also co-express HLA-E. Furthermore, IFN-gamma induces significant ectopic MHC class Ia expression in bulge cells of organ-cultured human scalp skin.

CONCLUSIONS

These data suggest that the bulge of human anagen HFs represents a hitherto unrecognized site of relative IP in human skin. Simultaneously, we present the first evidence of IDO and HLA-E protein expression in normal human HFs. Bulge IP presumably protects the HF epithelial stem cell reservoir from autoaggressive immune attack whereas a loss of bulge IP may play a central role in the pathogenesis of cicatricial alopecias.

Human skin stem cells and the ageing process

In healthy individuals, skin integrity is maintained by epidermal stem cells which self-renew and generate daughter cells that undergo terminal differentiation. Despite accumulation of senescence markers in aged skin, epidermal stem cells are maintained at normal levels throughout life. Therefore, skin ageing is induced by impaired stem cell mobilisation or reduced number of stem cells able to respond to proliferative signals. In the skin, existence of several distinct stem cell populations has been reported. Genetic labelling studies detected multipotent stem cells of the hair follicle bulge to support regeneration of hair follicles but not been responsible for maintaining interfollicular epidermis, which exhibits a distinct stem cell population. Hair follicle epithelial stem cells have at least a dual function: hair follicle remodelling in daily life and epidermal regeneration whenever skin integrity is severely compromised, e.g. after burns. Bulge cells, the first adult stem cells of the hair follicle been identified, are capable of forming hair follicles, interfollicular epidermis and sebaceous glands. In addition, -- at least in murine hair follicles -- they can also give rise to non-epithelial cells, indicating a lineage-independent pluripotent character. Multipotent cells (skin-derived precursor cells) are present in human dermis; dermal stem cells represent 0.3% among human dermal foreskin fibroblasts. A resident pool of progenitor cells exists within the sebaceous gland, which is able to differentiate into both sebocytes and interfollicular epidermis. The self-renewal and multi-lineage differentiation of skin stem cells make these cells attractive for ageing process studies but also for regenerative medicine, tissue repair, gene therapy and cell-based therapy with autologous adult stem cells not only in dermatology. In addition, they provide in vitro models to study epidermal lineage selection and its role in the ageing process.

Cancer stem cells, CD200 and immunoevasion

The limited success seen in cancer immunotherapy signifies that an alternative approach is required. Advances in cancer biology have identified a biologically unique subpopulation of cells, termed cancer stem cells (CSC), that survive after conventional therapy. CSCs are the putative cancer-initiating cells responsible for tumor initiation, progression and metastasis. CSCs might be able to evade the immune system by generating a tolerogenic response facilitated by the immunosuppressive factor CD200. This article reviews the biological importance of CSCs and the potentially important role of CD200 in tumor immunology. Moreover, we discuss the prospective role CD200 plays in the ability of a CSC to escape the immune system. Future immunotherapy must consider targeting CSCs to achieve curative responses.

Immunophenotyping of the human bulge region: the quest to define useful in situ markers for human epithelial hair follicle stem cells and their niche

Since the discovery of epithelial hair follicle stem cells (eHFSCs) in the bulge of human hair follicles (HFs) an important quest has started: to define useful markers. In the current study, we contribute to this by critically evaluating corresponding published immunoreactivity (IR) patterns, and by attempting to identify markers for the in situ identification of human eHFSCs and their niche. For this, human scalp skin cryosections of at least five different individuals were examined, employing standard immunohistology as well as increased sensitivity methods. Defined reference areas were compared by quantitative immunohistochemistry for the relative intensity of their specific IR. According to our experience, the most useful positive markers for human bulge cells turned out to be cytokeratin 15, cytokeratin 19 and CD200, but were not exclusive, while beta1 integrin and Lhx2 IR were not upregulated by human bulge keratinocytes. Absent IR for CD34, connexin43 and nestin on human bulge cells may be exploited as negative markers. alpha6 integrin, fibronectin, nidogen, fibrillin-1 and latent transforming growth factor (TGF)-beta-binding protein-1 were expressed throughout the connective tissue sheath of human HFs. On the other hand, tenascin-C was upregulated in the bulge and may thus constitute a component of the bulge stem cell niche of human HFs. These immunophenotyping results shed further light on the in situ expression patterns of claimed follicular 'stem cell markers' and suggest that not a single marker alone but only the use of a limited corresponding panel of positive and negative markers may offer a reasonable and pragmatic compromise for identifying human bulge stem cells in situ.

Skin and hair: models for exploring organ regeneration

Skin is an excellent model to study the basic biology of organ regeneration and translational approaches to regenerative medicine. Because of the accessibility of the skin, a long history of regenerative approaches already exists. Identifying the commonalities between skin regeneration and the regeneration of other organs could provide major breakthroughs in regenerative medicine. The hair follicle represents a miniature organ with readily accessible stem cells, multiple cell lineages, and signaling centers. During the normal lifespan of a human, this miniature organ regenerates itself more than 10 times. The cells responsible for this remarkable process are called bulge stem cells. A plethora of molecular and genetic tools have been developed to follow their fate and to explore their ontogeny. Major advances have been made toward understanding the normal cell fate of bulge stem cells and their developmental plasticity. Recent studies suggest the epidermis and hair may have an untapped potential to form other organs. Understanding the mechanisms that regulate adult stem-cell proliferation is a major goal for regenerative medicine. In the hair follicle, pharmacologic agents, recombinant proteins, and artificial cell-permeable proteins have been developed to manipulate the proliferation of the quiescent bulge stem cells. These advances illustrate a potential roadmap for regenerative medicine using molecular tools developed for skin biology to promote organ regeneration by manipulating adult stem cells in situ.

Transcriptional profiling of putative human epithelial stem cells

Background

Human interfollicular epidermis is sustained by the proliferation of stem cells and their progeny, transient amplifying cells. Molecular characterization of these two cell populations is essential for better understanding of self renewal, differentiation and mechanisms of skin pathogenesis. The purpose of this study was to obtain gene expression profiles of alpha 6⁺/MHCI⁺, transient amplifying cells and alpha 6⁺/MHCI^-, putative stem cells, and to compare them with existing data bases of gene expression profiles of hair follicle stem cells. The expression of Major Histocompatibility Complex (MHC) class I, previously shown to be absent in stem cells in several tissues, and alpha 6 integrin were used to isolate MHCI positive basal cells, and MHCI low/negative basal cells.

Results

Transcriptional profiles of the two cell populations were determined and comparisons made with published data for hair follicle stem cell gene expression profiles. We demonstrate that presumptive interfollicular stem cells, alpha 6⁺/MHCI^- cells, are enriched in messenger RNAs encoding surface receptors, cell adhesion molecules, extracellular matrix proteins, transcripts encoding members of IFN-alpha family proteins and components of IFN signaling, but contain lower levels of transcripts encoding proteins which take part in energy metabolism, cell cycle, ribosome biosynthesis, splicing, protein translation, degradation, DNA replication, repair, and chromosome remodeling. Furthermore, our data indicate that the cell signaling pathways Notch1 and NF-κB are downregulated/inhibited in MHC negative basal cells.

Conclusion

This study demonstrates that alpha 6⁺/MHCI^- cells have additional characteristics attributed to stem cells. Moreover, the transcription profile of alpha 6⁺/MHCI^- cells shows similarities to transcription profiles of mouse hair follicle bulge cells known to be enriched for stem cells. Collectively, our data suggests that alpha 6⁺/MHCI^- cells may be enriched for stem cells. This study is the first comprehensive gene expression profile of putative human epithelial stem cells and their progeny that were isolated directly from neonatal foreskin tissue. Our study is important for understanding self renewal and differentiation of epidermal stem cells, and for elucidating signaling pathways involved in those processes. The generated data base may serve those working with other human epithelial tissue progenitors.

CD10 and CD34 in fetal and adult human hair follicles: dynamic changes in their immunohistochemical expression during embryogenesis and hair cycling

BACKGROUND

CD10 and CD34 have been detected in both epithelial and mesenchymal components of anagen human hair follicles.

OBJECTIVES

To analyse the expression of CD10 and CD34 in human hair follicle development as well as in different phases of the hair cycle.

METHODS

Fetal and adult hair follicles at different stages of the hair cycle were examined by immunohistochemistry for CD10 and CD34.

RESULTS

In fetal follicles, CD10 is expressed by the cells of the placodes, and CD34 by the mesenchymal cells of the dermal condensate. As the follicle matures, CD10 can be seen in the matrix cells, inner root sheath and dermal sheath. In adult follicles, the expression of CD10 in the follicular epithelium is present in anagen follicles, but tends to disappear in catagen, and is not detected in telogen. The CD10 positivity of the dermal sheath is more intense in catagen than in anagen follicles. CD34 immunostaining of the external root sheath was seen in adult anagen follicles but not in fetal follicles. This staining of the anagen outer sheath tends to disappear in catagen and is not detected in telogen.

CONCLUSIONS

CD10 and CD34 are not proteins constantly present in a specific cell type of the hair follicle, but are proteins that can be expressed by both epithelial and mesenchymal cells depending on the stage of development and hair cycle. The distribution of the immunoreactivity to CD10 in the placode and CD34 in the dermal condensate suggests a role of these proteins in initial stages of hair formation.

(Neuro-)endocrinology of epithelial hair follicle stem cells

The hair follicle is a repository of different types of somatic stem cells. However, even though the hair follicle is both a prominent target organ and a potent, non-classical site of production and/or metabolism of numerous polypetide- and steroid hormones, neuropeptides, neurotransmitters and neurotrophins, the (neuro-)endocrine controls of hair follicle epithelial stem cell (HFeSC) biology remain to be systematically explored. Focussing on HFeSCs, we attempt here to offer a "roadmap through terra incognita" by listing key open questions, by exploring endocrinologically relevant HFeSC gene profiling and mouse genomics data, and by sketching several clinically relevant pathways via which systemic and/or locally generated (neuro-)endocrine signals might impact on HFeSC. Exemplarily, we discuss, e.g. the potential roles of glucocorticoid and vitamin D receptors, the hairless gene product, thymic hormones, bone morphogenic proteins (BMPs) and their antagonists, and Skg-3 in HFeSC biology. Furthermore, we elaborate on the potential role of nerve growth factor (NGF) and substance P-dependent neurogenic inflammation in HFeSC damage, and explore how neuroendocrine signals may influence the balance between maintenance and destruction of hair follicle immune privilege, which protects these stem cells and their progeny. These considerations call for a concerted research effort to dissect the (neuro-)endocrinology of HFeSCs much more systematically than before.