A distinct population of clonogenic and multipotent murine follicular keratinocytes residing in the upper isthmus

[Progress in epidermal stem cells]

Mammalian skin epidermis contains different epidermal stem cell pools which contribute to the homeostasis and repair of skin epithelium. Epidermal stem cells possess two essential features common to all stem cells: self-renewal and differentiation. Disturbing the balance between self-renewal and differentiation of epidermal stem cell often causes tumors or other skin diseases. Epidermal stem cell niches provide a special microenvironment that maintains a balance of stem cell quiescence and activity. This review primarily concentrates on the following points of the epidermal stem cells: the existing evidences, the self-renewal and differentiation, the division pattern, the signal pathways regulating self-renewal and differentiation, and the microenvironment (niche) and macroenvironment maintaining the homeostasis of stem cells.

Imaging and analysis of 3D tumor spheroids enriched for a cancer stem cell phenotype

Tumors that display a highly metastatic phenotype contain subpopulations of cells that display characteristics similar to embryonic stem cells. These cells exhibit the ability to undergo self-renewal; slowly replicate to retain a nucleoside analog label, leading to their definition as "label-retaining cells"; express specific surface markers such as CD44(+)/CD24(-/low) and CD133; and can give rise to cells of different lineages (i.e., they exhibit multipotency). Based on these characteristics, as well as their demonstrated ability to give rise to tumors in vivo, these cells have been defined as tumor-initiating cells (TICs), tumor-propagating cells, or cancer stem cells (CSCs). These cells are highly resistant to chemotherapeutic agents and radiation and are believed to be responsible for the development of both primary tumors and metastatic lesions at sites distant from the primary tumor. Established cancer cell lines contain CSCs, which can be propagated in vitro using defined conditions, to form 3D tumor spheroids. Because the vast majority of studies to identify cancer-associated genes and therapeutic targets use adherent cells grown in 2 dimensions on a plastic substrate, the multicellular composition of these 3D tumor spheroids presents both challenges and opportunities for their imaging and characterization. The authors describe approaches to image and analyze the properties of CSCs within 3D tumor spheroids, which can serve as the basis for defining the gene and protein signatures of CSCs and to develop therapeutic strategies that will effectively target this critically important population of cells that may be responsible for tumor progression.

Assaying proliferation and differentiation capacity of stem cells using disaggregated adult mouse epidermis

In this protocol, we describe how to isolate keratinocytes from adult mouse epidermis, fractionate them into different sub-populations on the basis of cell surface markers and examine their function in an in vivo skin reconstitution assay with disaggregated neonatal dermal cells. We also describe how the isolated keratinocytes can be subjected to clonal analysis in vitro and in vivo and how to enrich for hair follicle-inducing dermal papilla cells in the dermal preparation. Using these approaches, it is possible to compare the capacity of different populations of adult epidermal stem cells to proliferate and to generate progeny that differentiate along the different epidermal lineages. Isolating, fractionating and grafting cells for the skin reconstitution assay is normally spread over 2 d. Clonal growth in culture is assessed after 14 d, while evaluation of the grafts is carried out after 4-5 weeks.

The immunosuppressive surface ligand CD200 augments the metastatic capacity of squamous cell carcinoma

CD200 (OX-2) is a cell surface glycoprotein that imparts immune privileges by suppressing alloimmune and autoimmune responses through its receptor, CD200R, expressed primarily on myeloid cells. The ability of CD200 to suppress myeloid cell activation is critical for maintaining normal tissue homeostasis but may also enhance the survival of migratory neoplastic cells. We show that CD200 expression is largely absent in well-differentiated primary squamous cell carcinoma (SCC) of the skin, but is highly induced in SCC metastases to the lymph node and other solid tissues. CD200 does not influence the proliferative or invasive capacity of SCC cells or their ability to reconstitute primary skin tumors. However, loss of CD200 impairs the ability of SCC cells to metastasize and seed secondary tumors, indicating that the survival of CD200(+) SCC cells may depend on their ability to interact with CD200R(+) immune cells. The predominant population of CD200R(+) stromal cells was CD11b(+)Gr-1(+) myeloid-derived suppressor cells, which release elevated levels of granulocyte colony-stimulating factor and granulocyte macrophage colony-stimulating factor when in the presence of SCC cells in a CD200-dependent manner. Collectively, our findings implicate CD200 as a hallmark of SCC metastasis and suggest that the ability of CD200(+) SCC keratinocytes to directly engage and modulate CD200R(+) myeloid-derived suppressor cells is essential to metastatic survival.

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

Mammalian epidermis consists of three self-renewing compartments: the hair follicle, the sebaceous gland, and the interfollicular epidermis. We generated knock-in alleles of murine Lgr6, a close relative of the Lgr5 stem cell gene. Lgr6 was expressed in the earliest embryonic hair placodes. In adult hair follicles, Lgr6+ cells resided in a previously uncharacterized region directly above the follicle bulge. They expressed none of the known bulge stem cell markers. Prenatal Lgr6+ cells established the hair follicle, sebaceous gland, and interfollicular epidermis. Postnatally, Lgr6+ cells generated sebaceous gland and interfollicular epidermis, whereas contribution to hair lineages gradually diminished with age. Adult Lgr6+ cells executed long-term wound repair, including the formation of new hair follicles. We conclude that Lgr6 marks the most primitive epidermal stem cell.

The hair follicle-a stem cell zoo

Recent studies on stem cells in the adult hair follicle (HF) have uncovered a veritable menagerie of exceptionally diverse and dynamic keratinocytes with stem cell properties located in distinct regions of the HF. Although endowed with specific functions during normal hair follicle maintenance, the majority of these cells can act as multipotent stem cells in stress situations, such as physical injury, which argues for an unanticipated degree of plasticity of these cells. This review provides an overview of the different epithelial stem cell populations, identified in the mouse HF, and their relationships with one another, and envisions possible cellular mechanisms underlying normal HF maintenance and skin regeneration.

Epidermal stem cell diversity and quiescence

Mammalian epidermis is maintained by self-renewal of stem cells and terminal differentiation of their progeny. New data reveal a diversity amongst stem cells that was previously unrecognized. Different stem cell populations have different locations and differ in whether they are quiescent or actively cycling. During normal epidermal homeostasis, each stem cell population feeds a restricted number of differentiated lineages. However, in response to injury or genetic manipulation the different pools of stem cells demonstrate multi-lineage differentiation ability. While it is well established that Wnt signalling promotes hair follicle (HF) differentiation, new observations suggest a role for EGF receptor signalling in promoting differentiation of interfollicular epidermis. NFATc1 maintains quiescence in the HF, while Lrig1 exerts the same function in the junctional zone. The stage is now set for exploring the relationship between the different epidermal stem cell populations and between quiescence and lineage selection.

Identification of the cell lineage at the origin of basal cell carcinoma

For most types of cancers, the cell at the origin of tumour initiation is still unknown. Here, we used mouse genetics to identify cells at the origin of basal cell carcinoma (BCC), which is one of the most frequently occurring types of cancer in humans, and can result from the activation of the Hedgehog signalling pathway. Using mice conditionally expressing constitutively active Smoothened mutant (SmoM2), we activated Hedgehog signalling in different cellular compartments of the skin epidermis and determined in which compartments Hedgehog activation induces BCC formation. Activation of SmoM2 in hair follicle bulge stem cells and their transient amplifying progenies did not induce cancer formation, demonstrating that BCC does not originate from bulge stem cells, as previously thought. Using clonal analysis, we found that BCC arises from long-term resident progenitor cells of the interfollicular epidermis and the upper infundibulum. Our studies uncover the cells at the origin of BCC in mice and demonstrate that expression of differentiation markers in tumour cells is not necessarily predictive of the cancer initiating cells.

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

The rat mAb 33A10 recognizes an antigen in a variety of mouse epithelial tissues. In this study, we investigated in detail the expression pattern of the 33A10-defined antigen in the hair follicle. We show that 33A10 reactivity is confined to the most differentiated keratinocytes of the outer root sheath (ORS), the companion layer (CL), and to cells of the sebaceous gland duct. In vitro, the 33A10-defined antigen is expressed in keratinocytes derived from the ORS and accumulates on induction of differentiation. Using microarray analysis and transient transfection approaches, we established that the 33A10-defined antigen is the orphan protein, Placenta-expressed transcript (Plet)-1. Biochemical data indicated that Plet-1 is a glycosylphosphatidylinositol-anchored glycoprotein with N-linked carbohydrates in addition to other posttranslational modifications. Although silencing of Plet-1 expression using stable RNA interference in ORS keratinocytes decreased cellular migration, it increased adhesion to collagens I and IV. Immunohistochemical analysis showed that Plet-1 was primarily localized at the leading edge of epidermal wounds, where keratinocytes contacted the eschar. The restricted localization in both differentiated ORS and CL cells contacting the hair fiber and epidermal wounds suggests a role for the Plet-1 protein in regulating the interaction of keratinocytes with inert tissues.

Regeneration of multilineage skin epithelia by differentiated keratinocytes

Although homeostasis of rapidly renewing tissues like skin epithelia is maintained by stem cells, the committed progeny of stem cells in the basal layer of epidermis retain regenerative potential and are capable of forming epidermis in response to environmental cues. It is not clear, however, at what point within the epidermal lineage keratinocytes lose this regenerative potential. In this study, we examined the extent of tissue formation by post-mitotic differentiated keratinocytes. We show that cultures of mouse keratinocytes that were, by all measures, differentiated were able to reform a self-renewing, hair-bearing skin when transplanted onto suitable sites in vivo. Genetic labeling and lineage-tracing studies in combination with an involucrin-driven Cre/lox reporter system confirmed that transplanted differentiated keratinocytes were indeed the source of the regenerated skin. More importantly, analysis of early stages of skin regeneration showed hallmarks of dedifferentiation of transplanted differentiated keratinocytes. These data indicate that commitment to differentiation does not prohibit cells from re-entering the cell cycle, de-differentiating, and acquiring "stemness". These findings suggest that epidermis can use different strategies for homeostasis and tissue regeneration.

Targeted disruption of Bcl-xL in mouse keratinocytes inhibits both UVB- and chemically induced skin carcinogenesis

Bcl-x(L) is one of several antiapoptotic proteins regulated by signal transducer and activator of transcription 3 (Stat3). We have recently shown that Stat3 is required for chemically induced and ultraviolet B (UVB)-induced skin carcinogenesis. In this study, the functional role of Bcl-x(L) in skin carcinogenesis was investigated using skin-specific Bcl-x(L)-deficient mice. In this model, Bcl-x(L) expression is disrupted in the basal compartment of mouse epidermis using the bovine keratin 5 (K5) promoter to drive expression of Cre recombinase (K5.Cre x Bcl-x(L) (fl/fl) mice). A significant increase in apoptosis induced by either UVB irradiation or 7,12-dimethylbenz[a]anthracene (DMBA) treatment was observed in the epidermis of Bcl-x(L)-deficient mice. Furthermore, an increase in apoptotic cells was noted in hair follicle keratinocytes, including those located in the bulge region. Cell proliferation was not affected by Bcl-x(L) deficiency following exposure to either UVB or 12-O-tetradecanoylphorbol-13-acetate (TPA). Bcl-x(L)-deficient mice were more resistant than wild-type controls to skin tumor development with delayed onset and reduced number of tumors using either UVB or the DMBA/TPA two-stage regimen. Moreover, Bcl-2, Mcl-1, and survivin protein levels were increased in the epidermis of Bcl-x(L)-deficient mice in the absence of stimuli. Furthermore, levels of these antiapoptotic proteins were also high in skin tumors from Bcl-x(L)-deficient mice that developed in response to either UVB or two-stage carcinogenesis protocols. Collectively, these studies demonstrate that Bcl-x(L) plays a role early in skin carcinogenesis through its anti-apoptotic functions to enhance survival of keratinocytes, including bulge region keratinocyte stem cells, following DNA damage.

Targeted disruption of stat3 reveals a major role for follicular stem cells in skin tumor initiation

The initiation stage of mouse skin carcinogenesis involves the induction of mutations in keratinocyte stem cells (KSC), which confers a selective growth advantage allowing clonal expansion during tumor promotion. Targeted disruption of signal transducer and activator of transcription 3 (Stat3) in bulge region KSCs was achieved by treating K15.CrePR1 x Stat3(fl/fl) mice with RU486. Deletion of Stat3 prior to skin tumor initiation with 7,12-dimethylbenz(a)anthracene significantly increased the number of apoptotic KSCs and decreased the frequency of Ha-ras codon 61 A(182)-->T transversion mutations in this cell population compared with wild-type littermates. Targeted disruption of Stat3 in bulge region KSCs at the time of initiation also dramatically reduced the number of skin tumors (by approximately 80%) produced following promotion with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. These results show that Stat3 is required for the survival of bulge region KSCs during tumor initiation. Furthermore, these data provide direct evidence that bulge region KSCs are the primary targets for the initiation of skin tumors in this model system.

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.

Lrig1 expression defines a distinct multipotent stem cell population in mammalian epidermis

Lrig1 is a marker of human interfollicular epidermal stem cells and helps maintain stem cell quiescence. We show that, in mouse epidermis, Lrig1 defines the hair follicle junctional zone adjacent to the sebaceous glands and infundibulum. Lrig1 is a Myc target gene; loss of Lrig1 increases the proliferative capacity of stem cells in culture and results in epidermal hyperproliferation in vivo. Lrig1-expressing cells can give rise to all of the adult epidermal lineages in skin reconstitution assays. However, during homeostasis and on retinoic acid stimulation, they are bipotent, contributing to the sebaceous gland and interfollicular epidermis. beta-catenin activation increases the size of the junctional zone compartment, and loss of Lrig1 causes a selective increase in beta-catenin-induced ectopic hair follicle formation in the interfollicular epidermis. Our results suggest that Lrig1-positive cells constitute a previously unidentified reservoir of adult mouse interfollicular epidermal stem cells.

Hair follicles are required for optimal growth during lateral skin expansion

The hair follicles (HFs) and the interfollicular epidermis (IFE) of intact mature skin are maintained by distinct stem cell populations. Upon wounding, however, emigration of HF keratinocytes to the IFE plays a role in acute stages of healing. In addition to this repair function, rapidly cycling cells of the upper HF have been observed transiting to the IFE in neonatal skin. Here we report that an absence of HF development leads to shortening and kinking of the mouse tail. These skeletal defects are reduced by stimulating keratinocyte proliferation, suggesting that they arise from impaired epidermal expansion. We confirm that rapidly cycling cells of the HF emigrate to the IFE of the neonatal tail. These results suggest that an absence of HFs results in impaired skin growth that is unable to keep pace with the rapidly elongating axial skeleton of the tail. Thus, in addition to their role in wound repair, HFs can make a significant contribution to lateral expansion of the IFE in the absence of trauma.

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.

Epidermal homeostasis: a balancing act of stem cells in the skin

The skin epidermis and its array of appendages undergo ongoing renewal by a process called homeostasis. Stem cells in the epidermis have a crucial role in maintaining tissue homeostasis by providing new cells to replace those that are constantly lost during tissue turnover or following injury. Different resident skin stem cell pools contribute to the maintenance and repair of the various epidermal tissues of the skin, including interfollicular epidermis, hair follicles and sebaceous glands. Interestingly, the basic mechanisms and signalling pathways that orchestrate epithelial morphogenesis in the skin are reused during adult life to regulate skin homeostasis.

Stem cells in urology

The shortage of donors for organ transplantation has stimulated research on stem cells as a potential resource for cell-based therapy in all human tissues. Stem cells have been used for regenerative medicine applications in many organ systems, including the genitourinary system. The potential applications for stem cell therapy have, however, been restricted by the ethical issues associated with embryonic stem cell research. Instead, scientists have explored other cell sources, including progenitor and stem cells derived from adult tissues and stem cells derived from the amniotic fluid and placenta. In addition, novel techniques for generating stem cells in the laboratory are being developed. These techniques include somatic cell nuclear transfer, in which the nucleus of an adult somatic cell is placed into an oocyte, and reprogramming of adult cells to induce stem-cell-like behavior. Such techniques are now being used in tissue engineering applications, and some of the most successful experiments have been in the field of urology. Techniques to regenerate bladder tissue have reached the clinic, and exciting progress is being made in other areas, such as regeneration of the kidney and urethra. Cell therapy as a treatment for incontinence and infertility might soon become a reality. Physicians should be optimistic that regenerative medicine and tissue engineering will one day provide mainstream treatment options for urologic disorders.

Human papillomavirus E6/E7 oncogenes promote mouse ear regeneration by increasing the rate of wound re-epithelization and epidermal growth

Mammals have limited regeneration capacity. We report here that, in transgenic mice (Tg(bK6-E6/E7)), the expression of the E6/E7 oncogenes of human papilloma virus type 16 (HPV16) under the control of the bovine keratin 6 promoter markedly improves the mouse's capacity to repair portions of the ear after being wounded. Increased repair capacity correlates with an increased number of epidermal proliferating cells. In concordance with the expected effects of the E6 and E7 oncogenes, levels of p53 decreased and those of p16 in epidermal cells increased. In addition, we observed that wound re-epithelization proceeded faster in transgenic than in wild-type animals. After the initial re-epithelization, epidermal cell migration from the intact surrounding tissue appears to be a major contributor to the growing epidermis, especially in the repairing tissue of transgenic mice. We also found that there is a significantly higher number of putative epidermal stem cells in Tg(bK6-E6/E7) than in wild-type mice. Remarkably, hair follicles and cartilage regenerated within the repaired ear tissue, without evidence of tumor formation. We propose that the ability to regenerate ear portions is limited by the capacity of the epidermis to repair itself and grow.

The skin: a home to multiple classes of epithelial progenitor cells

To maintain homeostasis in the adult skin, epithelial keratinocyte stem cells are thought to divide infrequently giving rise to short-lived (transit amplifying) cells that undergo a limited number of cell divisions and ultimately terminal differentiation. This model for the epidermal stem cell niche has increased in complexity by the multiple putative progenitor keratinocyte populations that have recently been identified in distinct regions of the interfollicular epidermis and hair follicle appendages. Under normal conditions, these progenitor populations are long-lived and able to sustain the cellular input to certain epidermal structures including the interfollicular epidermis and sebaceous gland. Other putative epithelial progenitors derived from the hair follicle possess high in vitro proliferative capacity and are able to regenerate skin, hair and sebaceous lineages in transplantation studies. These new findings present the cutaneous epithelium as a highly compartmentalized structure potentially maintained by multiple classes of progenitor cells. In this review, we will discuss the implications of these new putative epithelial progenitor populations and their potential to be influenced by external stimuli for skin homeostasis and carcinogenesis.