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The skin provides an anatomical barrier to physical, chemical and biological agents. Hence, it is not surprising that it has well-developed innate immunity. What we find surprising is that the CD49f+/ CD34+ hair follicle stem cells should have an enriched expression profile of so many genes involved in innate immunity. Do these stem cells require extra protection from environmental insults? Or, could there be a new role for these genes? To probe these questions, we first summarize the roles of some key players in epidermal innate immunity. We next focus on their expression in CD49f+/ CD34+ hair follicle stem cells. Then, we consider recent data suggesting a new role for these 'old players' in the regulation and mobilization of haematopoietic and mesenchymal stem cells. Finally, we hypothesize that the 'old players' in these hair follicle stem cells may be playing a 'new game'. [ABSTRACT FROM AUTHOR]

Background: Kruppel-like factor KLF4 is a transcription factor critical for the establishment of the barrier function of the skin. Its function in stem cell biology has been recently recognized. Previous studies have revealed that hair follicle stem cells contribute to cutaneous wound healing. However, expression of KLF4 in hair follicle stem cells and the importance of such expression in cutaneous wound healing have not been investigated. Methodology/Principal Findings: Quantitative real time polymerase chain reaction (RT-PCR) analysis showed higher KLF4 expression in hair follicle stem cell-enriched mouse skin keratinocytes than that in control keratinocytes. We generated KLF4 promoter-driven enhanced green fluorescence protein (KLF4/EGFP) transgenic mice and tamoxifen-inducible KLF4 knockout mice by crossing KLF4 promoter-driven Cre recombinase fused with tamoxifen-inducible estrogen receptor (KLF4/ CreERTM) transgenic mice with KLF4(flox) mice. KLF4/EGFP cells purified from dorsal skin keratinocytes of KLF4/EGFP transgenic mice were co-localized with 5-bromo-2'-deoxyuridine (BrdU)-label retaining cells by flow cytometric analysis and immunohistochemistry. Lineage tracing was performed in the context of cutaneous wound healing, using KLF4/CreERTM and Rosa26RLacZ double transgenic mice, to examine the involvement of KLF4 in wound healing. We found that KLF4 expressing cells were likely derived from bulge stem cells. In addition, KLF4 expressing multipotent cells migrated to the wound and contributed to the wound healing. After knocking out KLF4 by tamoxifen induction of KLF4/CreERTM and KLF4(flox) double transgenic mice, we found that the population of bulge stem cell-enriched population was decreased, which was accompanied by significantly delayed cutaneous wound healing. Consistently, KLF4 knockdown by KLF4-specific small hairpin RNA in human A431 epidermoid carcinoma cells decreased the stem cell population and was accompanied by compromised cell migration. Conclusions/Significance: KLF4 expression in mouse hair bulge stem cells plays an important role in cutaneous wound healing. These findings may enable future development of KLF4-based therapeutic strategies aimed at accelerating cutaneous wound closure. [ABSTRACT FROM AUTHOR]

Understanding keratinocyte stem cell regulation is important in understanding the pathogenesis of wound healing and nonmelanoma skin cancer. We previously used a sensitive and quantitative assay for in vitro keratinocyte colony formation and mapped the keratinocyte stem cell locus (Ksc1) on mouse chromosome 9. Examination of the candidate genes in this locus disclosed a sequence variant in the gene for bone morphogenetic protein 5 (Bmp5). In this report, we used a naturally occurring mouse with a null mutation in this gene to probe stem cell properties in mouse epidermis. We found that the mutant keratinocytes had a significant reduction in the size and number of clonogenic keratinocytes. The mutant mice had a 50% reduction in the number of label-retaining cells when compared with their littermates. Addition of exogenous Bmp5 protein increased the number and size of keratinocyte colonies in the mutant as well as their wild-type littermates. Surprisingly, the mutant mice showed at least a 2-fold increase in skin tumor susceptibility over their littermates. We conclude that a naturally occurring mutation in Bmp5 affects keratinocyte stem cell proliferation, and skin tumor susceptibility, and is a candidate stem cell regulatory gene in the Ksc1 locus. [ABSTRACT FROM AUTHOR]

The discovery of long-lived epithelial stem cells in the bulge region of the hair follicle led to the hypothesis that epidermal renewal and epidermal repair after wounding both depend on these cells. To determine whether bulge cells are necessary for epidermal renewal, here we have ablated these cells by targeting them with a suicide gene encoding herpes simplex virus thymidine kinase (HSV-TK) using a Keratin 1–15 (Krt1-15) promoter. We show that ablation leads to complete loss of hair follicles but survival of the epidermis. Through fate-mapping experiments, we find that stem cells in the hair follicle bulge do not normally contribute cells to the epidermis which is organized into epidermal proliferative units, as previously predicted. After epidermal injury, however, cells from the bulge are recruited into the epidermis and migrate in a linear manner toward the center of the wound, ultimately forming a marked radial pattern. Notably, although the bulge-derived cells acquire an epidermal phenotype, most are eliminated from the epidermis over several weeks, indicating that bulge stem cells respond rapidly to epidermal wounding by generating short-lived 'transient amplifying' cells responsible for acute wound repair. Our findings have implications for both gene therapy and developing treatments for wounds because it will be necessary to consider epidermal and hair follicle stem cells as distinct populations. [ABSTRACT FROM AUTHOR]

Skin cancers as seen in the clinic are the result of a long history of events of which only the final stages are easily observed. As normal cells progress to the neoplastic and later metastatic stages, multiple changes in gene expression and cellular phenotypes occur. Nevertheless, the early events in the pathway leading from the first exposure to carcinogenic or mutagenic agents to a frank tumor are thought to involve a two-step process of tumor initiation and tumor promotion. In experimental two-stage skin carcinogenesis in mice, benign and malignant neoplasms can be induced on the backs of mice following a low, or sub-threshold, exposure to a carcinogen (initiation) and subsequent chronic regenerative epidermal hyperplasia caused by a variety of physical, chemical, or biological agents (promotion). Tumor initiation is thought to involve conversion of some of the epidermal cells into latent neoplastic cells, whereas promotion elicits expression of the neoplastic change. Many questions remain about this process, in particular the identity and biological properties of the cells that are specifically the targets of tumor initiation and promotion. Conceivably, any proliferative cell could become and remain initiated; however, these rare cells in the cutaneous epithelium able to become neoplastic cells after exposure to carcinogens and tumor promoters have many of the properties of stem cells. Although this concept that stem cells are the target cells in the development of cancer is not new, I will consider here the evidence that the target cells are indeed stem cells in the cutaneous epithelium. [ABSTRACT FROM AUTHOR]

The hair follicle bulge possesses putative epithelial stem cells. Characterization of these cells has been hampered by the inability to target bulge cells genetically. Here, we use a Keratin1-15 (Krt1-15, also known as K15) promoter to target mouse bulge cells with an inducible Cre recombinase construct or with the gene encoding enhanced green fluorescent protein (EGFP), which allow for lineage analysis and for isolation of the cells. We show that bulge cells in adult mice generate all epithelial cell types within the intact follicle and hair during normal hair follicle cycling. After isolation, adult Krt1-15-EGFP-positive cells reconstituted all components of the cutaneous epithelium and had a higher proliferative potential than Krt1-15-EGFP-negative cells. Genetic profiling of hair follicle stem cells revealed several known and unknown receptors and signaling pathways important for maintaining the stem cell phenotype. Ultimately, these findings provide potential targets for the treatment of hair loss and other disorders of skin and hair. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Charruyer and colleagues (this issue) report two significant advances to the field of cutaneous keratinocyte stem cells: a pair of new selectable markers that recognize a subset of α6+CD34+ label-retaining cells, and an in vivo limiting dilution assay for keratinocyte stem cells with long-term repopulating ability. This work has important implications for keratinocyte stem cell identification and assay, as well as for the identification of target cells in non-melanoma skin cancer. [ABSTRACT FROM AUTHOR]

Mouse keratinocyte stem cells originate from the bulge of hair follicle, and, according to definition, possess a clonogenic activity in vitro. We have investigated seven inbred (C57BL/6, C3H, DBA/2, BALB/c, FVB) and outbred (SENCAR, CD-1) mouse strains and found that three genetically distinct subsets of mouse strains differ significantly in the frequency of clonogenic activity in vitro. The analysis of keratinocyte colonies in two reciprocal backcross [C57BL/6 × (BALB/c × C57BL/6); BALB/c × (BALB/c × C57BL/6)] and intercross [(BALB/c × C57BL/6)F2] of BALB/c and C57BL/6 mice allowed us to identify two subpopulations of clonogenic keratinocytes able to produce small (less than 2 mm2) and large (more than 2 mm2) colonies. We conducted linkage analysis and found that small colonies associated with mouse chromosomes 1, 6, 7, 8, and 9; but large colonies—with the chromosome 4. We defined locus on the chromosome 9 that associated with small colonies as keratinocyte stem cell locus 1 ( Ksc1), and locus on the mouse chromosome 4 associated with large colonies-keratinocyte stem cell locus 2 ( Ksc2). Ksc1 and loci on chromosomes 6 and 7 are close if not equal to loci associated with sensitivity to skin carcinogenesis. We conclude that two subpopulations of stem cells able to produce small and large colonies regulated by different genes and genes regulating small colonies might be responsible for sensitivity to skin carcinogenesis. [ABSTRACT FROM AUTHOR]