Isolation of subpopulations of murine epidermal cells using monoclonal antibodies against differentiation-related cell surface molecules

Isolation and long-term expansion of murine epidermal stem-like cells

Epidermis is the most outer layer of the skin and a physical barrier protecting the internal tissues from mechanical and environmental insults. The basal keratinocytes, which, through proliferation and differentiation, supply diverse cell types for epidermal homeostasis and injury repair. Sustainable culture of murine keratinocyte, however, is a major obstacle. Here we developed murine keratinocyte lines using low-Ca²⁺ (0.06 mM) keratinocyte serum-free medium (KSFM-Ca²⁺) without feeder cells. Cells derived in this condition could be subcultured for >70 passages. They displayed basal epithelial cell morphology and expressed keratin (Krt) 14, but lacked the epithelial-characteristic intercellular junctions. Moreover, these cells could be adapted to grow in the Defined-KSFM (DKSFM) media containing 0.15 mM Ca²⁺, and the adapted cells established tight- and adherens-junctions and exhibited increased Krt1/10 expression while retained subculture capacity. Global gene expression studies showed cells derived in KSFM-Ca²⁺ media had enriched stem/proliferation markers and cells adapted in DKSFM media had epithelial progenitor signatures. Correspondingly, KSFM-Ca²⁺-derived cells exhibited a remarkable capacity of clonal expansion, whereas DKSFM-adapted cells could differentiate to suprabasal epithelial cell types in 3-dimentional (3D) organoids. The generation of stem-like murine keratinocyte lines and the conversion of these cells to epithelial progenitors capable of terminal differentiation provide the critically needed resources for skin research.

Isolation and In Vitro Characterization of Epidermal Stem Cells

Colony-forming assays represent prospective methods, where cells isolated from enzymatically dissociated tissues or from tissue cultures are assessed for their proliferative capacity in vitro. Complex tissues such as the epithelial component of the skin (the epidermis) are characterized by a substantial cellular heterogeneity. Analysis of bulk populations of cells by colony-forming assays can consequently be convoluted by a number of factors that are not controlled for in population wide studies. It is therefore advantageous to refine in vitro growth assays by sub-fractionation of cells using flow cytometry. Using markers that define the spatial origin of epidermal cells, it is possible to interrogate the specific characteristics of subpopulations of cells based on their in vivo credentials. Here, we describe how to isolate, culture, and characterize keratinocytes from murine back and tail skin sorted by surface antigens associated with adult stem cell characteristics.

Isolation, culture and identification of epidermal stem cells from newborn mouse skin

In healthy individuals, skin integrity is maintained by epidermal stem cells which self-renew and generate daughter cells that undergo terminal differentiation. Epidermal stem cells represent a promising source of stem cells, and their culture has great potential in scientific research and clinical application. However, no single method has been universally adopted for identifying and isolating epidermal stem cells. Here, we reported the isolation and characterization of putative epidermal stem cells from newborn mouse skin. The keratinocytes were separated enzymatically. Putative epidermal stem cells were selected by rapid adherence on a composite matrix made of type I collagen and fibronectin. Unattached cells were discarded after 10 min, and the attached cells were cultured in a defined culture medium. The isolated cells showed the typical epidermal stem cell morphology. Immunofluorescence indicated that the cells were strongly stained for β1 integrin family of extracellular matrix receptors. In conclusion, mouse putative epidermal stem cells were successfully isolated from newborn mouse epidermis on the basis of high rapid adhesion to extracellular matrix proteins and cultured in vitro.

Plasticity of epidermal stem cells: survival in various environments

The keratinocyte cell compartment in the continuously renewing epidermis of the skin is maintained by undifferentiated, self-renewing stem cells. We show that a small subpopulation of epidermal stem cells (EpiSCs) have the capacity to integrate into multiple tissues. These EpiSCs can change their phenotype in direct response of changes in cytokines in vitro, changes in cocultured cells, after injection into damaged environments in vivo. These changes appear to be unrelated to the age of the EpiSC. Even though we can isolate these cells and show that the age of thses cells appears to be irrelevant to this multipotent function, we still do not know how such cells are defined within a tissue or what the life span of a multipotent stem cell is.

Epidermal stem cells: interactions in developmental environments

Homeostasis of continuously renewing adult tissues, such as the epidermis of the skin, is maintained by epidermal stem cells (EpiSC), which are a small population of undifferentiated, self-renewing basal keratinocyte cells that produce daughter transit amplifying (TA) cells to make up the majority of the proliferative basal cell population in the epidermis. We have isolated EpiSC from neonatal and adult skin, and shown that these cells can regenerate an epidermis that lasts long term in vitro and in vivo, and that permanently expresses a recombinant gene in the regenerated tissue (Bickenbach and Dunnwald, 2000; Dunnwald et al., 2001). When we injected murine EpiSC into the developing blastocyst environment of the mouse, we found that both neonatal and adult EpiSC retained some ability to participate in the formation of tissues from all three germ layers (Liang and Bickenbach, 2002; Bickenbach and Chinnathambi, 2004; Liang et al., 2004). Although it appears evident that EpiSC act as pluripotent stem cells, how this reprogramming takes place is not understood. EpiSC might directly transdifferentiate into other cell types or they might first dedifferentiate into a more primitive cell type, and then proceed to develop along a cell lineage pathway. To begin to unravel this, we co-cultured EpiSC with embryonic stem (ES) cells, and found that EpiSC could alter their cell lineage protein expression to that of a more primitive cell type. We also placed EpiSC in a wounded environment and found that EpiSC interacted with the mesenchymal cells repopulating the wound bed. Our findings indicate that the population of cells that we isolate as EpiSC has a pluripotent capability. This has led us to postulate a paradigm shift for somatic stem cells. We propose that tissues maintain a sequestered population of uncommitted stem cells that retain a regenerative response which is enhanced when the cells are exposed to developmental or stress influences.

Keratinocyte stem cell assays: an evolving science

Although the existence of epithelial stem cells in the skin has been known for some decades from cell kinetic studies performed in vivo, attempts to prospectively isolate these cells for further biological characterization have been made possible relatively recently facilitated by the availability of antibodies that detect cell surface markers on epidermal cells. Elegant gene marking studies in vivo have provided confirmation of the patterns of epithelial tissue replacement predicted by classical cell turnover studies. But, the identification of candidate epidermal stem cells ex vivo remains an area of great controversy, requiring the re-evaluation of current experimental approaches that rely of necessity on predicted epidermal stem cell behavior in culture. Here we review the diverse experimental approaches utilized to identify keratinocyte stem cells and their underlying assumptions. We conclude that hair follicles and interfollicular epidermis each have their own self-renewing stem cell populations, contributing to distinct regions of the epithelium during homeostasis, although this is perturbed during wound healing. The need for the development of more rigorous assays for stem cell activity is highlighted given our recent observations using current assays and the discovery of new surface markers that identify putative epidermal stem cells.

As epidermal stem cells age they do not substantially change their characteristics

In this study, we ask the basic question: do stem cells age? We demonstrated that epidermal stem cells isolated from neonatal mice had the capacity to form multiple cell lineages during development. Here we demonstrate the cell lineages are clonal, and that epidermal stem cells isolated from the footpad epithelium of old mice have similar capabilities. Using Hoechst dye exclusion and cell size, we isolated viable homogenous populations of epidermal stem and transit-amplifying (TA) cells from GFP-transgenic mice, and injected these cells into 3.5-d blastocysts. Only the stem-injected blastocysts produced mice with GFP+ cells in their tissues. Furthermore, aged and young stem cells showed similar gene and protein expression profiles that showed some differences from the TA cell profiles. These data suggest that there may be a fundamental difference between somatic stem and TA cells, and that an epidermal stem cell placed in a developmental environment can alter its fate determination no matter what its age.

Mouse epidermal stem cells proceed through the cell cycle

The epidermis is a continuously renewing tissue maintained by undifferentiated stem cells. For decades it has been assumed that epidermal stem cells (ESCs) were held in the G0 phase of the cell cycle and that they only entered the cell cycle when needed. Previously, we showed that ESCs retained nuclear label for long periods, indicating that these cells did not proceed through the cell cycle at the same rate as the other proliferative basal cells. However, their exact cell-cycle profile has not been determined because a pure population of ESCs has not been available. In this study, we sorted stem and transient amplifying (TA) cells from murine neonatal back skin, and adult ear, footpad, and back skin, using our recently developed method. We found that neonatal back skin had two times the number of ESCs as the adult tissues. Despite the age and anatomical difference, these ESC populations exhibited similar cell cycle profiles with approximately 96% in G0/G1 and 4% in S-G2/M. The cell cycle profiles of the TA cells from neonatal back skin and adult footpad also showed a profile similar to each other (85% in G1 and 15% in S-G2/M). Examination of genes on a cell cycle chip showed that proliferation associated genes and only p57 were upregulated in the TA cell and ESC population, respectively. We found BrdU positive and cyclin B1 positive cells in all groups, confirming that both ESCs and TA cells were cycling. These data demonstrate that there are more TA cells dividing than ESCs, that the cell cycle profile of adult TA cells is related to the proliferative state of the tissue in which they reside, and that ESC proceed through the cell cycle.

Somatic epidermal stem cells can produce multiple cell lineages during development

It has been demonstrated that several types of somatic stem cells have the remarkable capacity to differentiate into other types of tissues. We demonstrate here that stem cells from the skin, the largest organ of the body, have the capacity to form multiple cell lineages during development. Using our recently developed sorting technique, we isolated viable homogeneous populations of somatic epidermal stem and transient amplifying cells from the skin of 3-day old transgenic mice, who carried the enhanced green fluorescent protein transgene, and injected stem, TA, or unsorted basal epidermal cells into 3.5-day C57BL/6 blastocysts. Only the stem-injected blastocysts produced mice with GFP(+) cells in their tissues. We found GFP(+) cells in ectodermal, mesenchymal, and neural-crest-derived tissues in E13.5 embryos, 13-day-old neonates, and 60-day-old adult mice, proving that epidermal stem cells survived the blastocyst injection and multiplied during development. Furthermore, the injected stem cells altered their epidermal phenotype and expressed the appropriate proteins for the tissues into which they developed, demonstrating that somatic epidermal stem cells have the ability to produce cells of different lineages during development. These data suggest that somatic epidermal stem cells may show a generalized plasticity expected only of embryonic stem cells and that environmental (extrinsic) factors may influence the lineage pathway for somatic stem cells. Thus, the skin could be a source of easily accessible stem cells that are able to be reprogrammed to form multiple cell lineages.

Murine epidermal label-retaining cells isolated by flow cytometry do not express the stem cell markers CD34, Sca-1, or Flk-1

Keratinocyte stem cells are present in the murine epidermis, based on both in vitro and in vivo evidence, and better characterization of these cells remains an active goal. Because keratinocyte stem cells are believed to cycle slowly, a good method for identification is based on their ability to retain nucleoside analog, such as bromodeoxyuridine. Adult stem cells have been identified in other tissues, including hematopoietic, neural, and skeletal muscle, and stem cell surface markers have been characterized. We wanted to determine if cell-surface markers present on both hematopoietic and skeletal muscle stem cells (CD34, Sca-1, and Flk-1) were also present on keratinocyte stem cells, and could be used to identify them. The cell-surface expression of cells that retained bromodeoxyuridine label for at least 21 d was compared with that of nonlabel-retaining cells. Double-labeling for flow cytometric analysis was employed, and label-retaining cells were found to lack expression of the tested markers. Beta1 integrin levels were also evaluated, and although high expression was found on label-retaining cells, it was not specific for these cells.

Isolating a pure population of epidermal stem cells for use in tissue engineering

Continuously renewing tissues, such as the epidermis, are maintained by stem cells that slowly proliferate and remain in the tissue for life. Although it has been known for decades that epithelial stem cells can be identified as label-retaining cells (LRCs) by long term retention of a nuclear label, isolating a pure population of stem cells has been problematic. Using a Hoechst and propidium iodide dye combination and specifically defined gating, we sorted mouse epidermal basal cells into three fractions, which we have now identified as stem, transient amplifying (TA), and non-proliferative basal cells. More than 90% of freshly isolated stem cells showed a G0/G1 cell cycle profile, while greater than 20% of the TA cells were actively dividing. Both stem and TA cells retained proliferative capacity, but the stem cells formed larger, more expandable colonies in culture. Both populations could be transduced with a retroviral vector and used to bioengineer an epidermis. However, only the epidermis from the stem cell population continued to grow and express the reporter gene for 6 months in organotypic culture. The epidermis from the transient amplifying cell fraction completely differentiated by 2 months. This novel sorting method yields pure viable epithelial stem cells that can be used to bioengineer a tissue and to test permanent recombinant gene expression.

Enrichment for murine keratinocyte stem cells based on cell surface phenotype

The identification and physical isolation of epithelial stem cells is critical to our understanding of their growth regulation during homeostasis, wound healing, and carcinogenesis. These stem cells remain poorly characterized because of the absence of specific molecular markers that permit us to distinguish them from their progeny, the transit amplifying (TA) cells, which have a more restricted proliferative potential. Cell kinetic analyses have permitted the identification of murine keratinocyte stem cells (KSCs) as slowly cycling cells that retain [(3)H]thymidine ([(3)H]Tdr) label, termed label-retaining cells (LRCs), whereas TA cells are visualized as rapidly cycling cells after a single pulse of [(3)H]Tdr, termed pulse-labeled cells (PLCs). Here, we report on the successful separation of KSCs from TA cells through the combined use of in vivo cell kinetic analysis and fluorescence-activated cell sorting. Specifically, we demonstrate that murine dorsal keratinocytes characterized by their high levels of alpha(6) integrin and low to undetectable expression of the transferrin receptor (CD71) termed alpha(6)(bri)CD71(dim) cells, are enriched for epithelial stem cells because they represent a minor ( approximately 8%) and quiescent subpopulation of small blast-like cells, with a high nuclear:cytoplasmic ratio, containing approximately 70% of label-retaining cells, the latter being a well documented characteristic of stem cells. Conversely, TA cells could be enriched in a phenotypically distinct subpopulation termed alpha(6)(bri)CD71(bri), representing the majority ( approximately 60%) of basal keratinocytes that are actively cycling, and importantly contain approximately 70% of [(3)H]Tdr pulse-labeled cells. Importantly, immunostaining of dorsal skin revealed the presence of CD71(dim) cells in the hair follicle bulge region, a well documented location for KSCs.

Adhesive properties of human basal epidermal cells: an analysis of keratinocyte stem cells, transit amplifying cells, and postmitotic differentiating cells

The basal layer of human epidermis is a heterogeneous population of proliferative and differentiating cells that can be divided into at least three functionally discrete compartments: keratinocyte stem cells, transit amplifying cells, and postmitotic differentiating cells. Basal cells adhere to the underlying basement membrane via integrins, and although decreased adhesion is a key event in epidermal differentiation, the specific role of particular integrins is poorly understood. We report here on the comparative expression and function of the beta1 versus alpha6beta4 integrins in keratinocyte stem cells, transit amplifying cells, and postmitotic differentiating cells of neonatal human foreskin epidermis. Adhesion assays demonstrate that both keratinocyte stem cells and transit amplifying cells comprise rapidly adhering cells that exhibit high levels of functional beta1 and alpha6beta4 integrins. Interestingly, a proportion of basal cells that have begun to differentiate in vivo within the basal layer as determined by their expression of the differentiation-specific markers K10 and involucrin also retain high levels of activated beta1 integrin, but downregulate alpha6beta4 expression selectively (termed alpha6dimbeta1bri). These cells also retain their adhesive capacity, indicating that induction of differentiation in vivo does not correlate with decreased beta1 integrin expression or function. We have previously reported on the use of alpha6 integrin in conjunction with a proliferation associated marker (10G7 ag) to separate keratinocyte stem cells (phenotype alpha6bri10G7dim) from other basal cells (Li et al. Proc Natl Acad Sci 95:3902-3907 1998). A comparison of the long-term proliferative potential of beta1bri10G7dim cells with alpha6bri10G7dim showed that selection of alpha6bri10G7dim allows the isolation of a purer fraction of keratinocyte stem cells.

Epithelial stem cells in the skin: definition, markers, localization and functions

In recent years, cutaneous epithelial stem cells have attained a genuine celebrity status. They are considered the key resource for epidermal and skin appendage regeneration, and are proposed as a preferential target of cutaneous gene therapy. Follicular epithelial stem cells may also give rise to a large variety of epithelial tumors, and cutaneous epithelial stem cells likely are crucial targets for physical or chemical agents (including carcinogens) that damage the skin and its appendages. However, as this Controversies feature illustrates, few experts can agree on how exactly to define and identify these elusive cells, or on where precisely in the skin they are localized. Given their potential importance in skin biology, pathology and future dermatological therapy, it is, therefore, timely to carefully reconsider the basic questions: What exactly is a stem cell, and how can we reliably identify epithelial stem cells? How many different kinds are there, and how do they differ functionally? Where exactly in the skin epithelium is each of the putative stem cell subpopulations located, and can we selectively manipulate any of them?

Selection and extended growth of murine epidermal stem cells in culture

Continuously renewing epithelia contain small undifferentiated stem cells capable of self-renewal and maintenance of the differentiating cell population. In murine epidermis stem cells have been identified as label-retaining cells (LRCs) by long-term retention of tritiated thymidine or BrdU. It has been suggested that epidermal stem cells adhere to basement membranes through differential expression of specific integrins. To determine whether we could use a specific integrin to enrich for murine epidermal stem cells, we tested adherence of LRCs to several substrates. Regardless of the substrate used, approximately 10% of total basal cells and 100% of LRCs adhered in 10 min. In our medium specifically formulated for murine keratinocytes, rapidly adherent stem cells formed large colonies and could be used to form a structurally complete epidermis in organotypic culture. They showed a fivefold greater transient transfection efficiency than total basal cells, and when individual adherent cells were transduced with a retroviral vector, they formed large clones. Although these stem cells grew more slowly than the total basal cell population, they could be subcultured more times. Our results indicate that murine epidermal stem cells can be selected by rapid attachment to a substrate, but not by one specific integrin, and that they can be expanded in culture if the appropriate conditions are maintained.

Identification and isolation of candidate human keratinocyte stem cells based on cell surface phenotype

Despite the central role of human epidermal stem cells in tissue homeostasis, wound repair, and neoplasia, remarkably little is known about these cells, largely due to the absence of molecular markers that distinguish them from other proliferative cells within the germinative/basal layer. Epidermal stem cells can be distinguished from other cells in the basal layer by their quiescent nature in vivo and their greater overall proliferative capacity. In this study, we demonstrate enrichment and isolation of a subpopulation of basal epidermal cells from neonatal human foreskin based on cell surface phenotype, which satisfy these criteria. These putative stem cells are distinguished from other basal cells by their characteristic expression of high levels of the adhesion molecule alpha6, a member of the integrin family (alpha6bri), and low levels of a proliferation-associated cell surface marker recognized by recently described mAb 10G7 (10G7(dim)). We conclude that cells with the phenotype alpha6bri10G7(dim) represent the epidermal stem cell population based on the demonstration that these cells (i) exhibit the greatest regenerative capacity of any basal cells, (ii) represent a minor subpopulation (approximately 10%) of immature epidermal cells, which (iii) are quiescent at the time of isolation from the epidermis, as determined by cell cycle analysis.

Retroviral transduction of murine epidermal stem cells demonstrates clonal units of epidermal structure

It has been suggested that the number and position of epidermal stem cells are related to the units of columnar structure in the upper epidermal strata and that the cells of each unit are derived from a single stem cell. Studies of cell lineage in developing tissues have been facilitated by the use of retroviral transduction to provide inherited expression of a histochemically demonstrable foreign gene product. To provide direct evidence about the clonal nature of epidermal units, murine epidermal keratinocytes were transduced with a replication-deficient retroviral vector carrying the beta-galactosidase gene. Subepidermal injection of virus in vivo led to infrequent transduction with only transient presence of beta-gal-staining keratinocytes within the epidermis. Transduction of keratinocytes in vitro and transplantation back to in vivo sites permitted demonstration of the transduced gene in clusters of cells within the reformed epidermis throughout a 12-wk period. The epidermis redeveloped an ordered columnar structure with restriction of transduced cells to individual columnar units. This clonal appearance is compatible with derivation of each epidermal unit from a single stem cell but is not compatible with a random pattern of cell proliferation. Transduced epidermal sheets that were recombined with oral mucosal connective tissue also redeveloped normal columnar structure with restriction of beta-gal staining to individual columnar units. These data suggest that the establishment of an epidermal stem cell pattern related to units of structure is an intrinsic property of the epithelium and is not dependent on regionally-specific connective tissue influences.

Current status of histogenetic and morphogenetic concepts of salivary gland tumorigenesis

Because of their complexity and relative infrequency, salivary gland tumors commonly result in diagnostic problems. Histogenetic and morphogenetic concepts of tumorigenesis in these glands are reviewed and their relevance to routine diagnosis and classification of salivary gland tumors evaluated. Evidence is presented from animal and human studies that under steady-state and pathophysiological conditions, all cell types present in the normal gland, including acinar cells, are capable of rapidly entering the cell cycle and are, therefore, possible targets for neoplastic transformation.

Monoclonal antibody G6K12 specific for membrane-associated differentiation marker of human stratified squamous epithelia and squamous cell carcinoma

Monoclonal antibody (G6K12) specific for differentiated keratinocytes was developed using in vitro immunization against the SCC-25 cell line. G6K12 only recognized stratified portions of cultured SCC-25 cells. Immunohistochemical examination using normal human oral mucosa showed that specific G6K12-reactivity was limited to the lower spinous-cell layers, while this antibody weakly bound to cells in basal-cell layers as well as in the upper spinous, granular and cornified-cell layers. G6K12 was also reactive to keratinocytes in most moderately- and well-differentiated SCC tissues. Immunoelectron microscopic examination further demonstrated that the G6K12-immunoreactive area was at the outer surface of the entire plasma membrane, including the microvilli of stratified SCC-25. G6K12-binding was reduced 50% by the treatment of native cells with glycoendoceramidase for 2 h. These results suggest that G6K12 recognizes a plasma membrane-anchored glycoconjugate which is specific for differentiated keratinocytes.

Characteristics of homogeneously small keratinocytes from newborn rat skin: possible epidermal stem cells

The aims of the present study were to characterize the phenotype, growth kinetics, and proliferative activation in culture of a population of poorly differentiated homogeneously small (HS) keratinocytes. These slow-cycling cells were separated by unit gravity sedimentation from a population of actively proliferating basal keratinocytes in newborn rat skin. This population (approximately 1% of the total basal keratinocytes) consisted of extremely small cells with little cytoplasm or RNA. Their positive KL4 staining demonstrates that they were keratinocytes. HS keratinocytes did not, however, contain epidermal calcium binding protein. Acridine orange, bivariate Hoechst, and ethidium bromide flow cytometry of in vitro bromodeoxyuridine-labeled cells as well as Ki67 staining showed that HS keratinocytes were in the G0 stage of the cell cycle and did not actively proliferate in vivo. [3H]thymidine label-retaining cells were found only in the HS cell population, showing that HS cells may originate from a central position in the epidermal proliferative unit. Growth of HS cells in vitro was characterized by a delayed but progressive increase in RNA before entry into the cell cycle. The clonogenic efficiency of HS cells in primary culture was much less than that of larger cells. Subclones of HS cell colonies exceeded primary colonies in their cloning efficiency and proliferative potential, suggesting that HS cells, although normally prevented from dividing, retain a high self-renewal capacity. They also maintain the ability to differentiate. The results are consistent with the concept that HS cell population may represent the epidermal-specific progenitor cells which act as stem cells in this tissue.