Genetic regulation of mouse stem cells: identification of two keratinocyte stem cell regulatory loci

25 years of epidermal stem cell research

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

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.

Tiers of clonal organization in the epidermis: the epidermal proliferation unit revisited

As one of the most proliferative tissues in adult mammals, the epidermis is a good example of the precise regulation necessary between stem cell self-renewal and differentiation. The epidermis is derived from ectodermal progenitor cells and contains three distinct classes of cells: epidermal stem cells which are capable of infinite rounds of cell division; their immediate descendants, transient amplifying cells, which are capable of numerous but finite rounds of cell division; and finally, non-dividing, differentiating cells (Aberdam in Cell and Tissue Research 331:103-107, 2008). This proliferative hierarchy must be tightly regulated both temporally and spatially during epidermal development and homeostasis in order to prevent uncontrolled growth leading to hyperproliferative states and/or tumorigenesis. Historically, the most basic unit of epidermal proliferation has been described as the epidermal proliferation unit (EPU). The EPU, as originally characterized by Christophers, Potten and Mackenzie, is a proliferation unit consisting of approximately 10 basal cells with a clonogenic cell in the center and overlaid by the suprabasal and corneocyte progeny (reviewed in Potten, C. S. (1974). The epidermal proliferative unit: the possible role of the central basal cell. Cell and Tissue Kinetics, 7(1), 77-88). Numerous researchers have identified this classical EPU structure, consisting of approximately 20 cells, in a variety of mammalian skin sources. Recently however, lineage analyses have provided evidence for much larger clonal epidermal units consisting of hundreds to thousands of cells. Furthermore, cutaneous mosaicism as well as a variety of cutaneous pathologies indicate that clonal areas extend to whole patches of mammalian skin many centimeters across. In this review we revisit four decades of experimental evidence and put forward a model of clonal units derived from multiple classes of epidermal progenitors ranging from the largest and most primitive units, clonal ectodermal units, to epidermal stem cell units, and finally, to the most basic structural unit, the EPU.

The in vitro behaviour and patterns of colony formation of murine epithelial stem cells

OBJECTIVE

The mechanisms of renewal of skin and mucosal epithelia in vivo are associated with hierarchies of stem and amplifying cells organized in distinct spatial patterns. Stem and amplifying characteristics persist after isolation and growth of human keratinocytes in vitro but the pattern for murine keratinocytes has been less clear.

MATERIALS AND METHODS

Murine keratinocytes were grown in low calcium media and examined for their patterns of colony morphologies.

RESULTS

We consistently identified three types of colonies, one of which contains concentric zones of amplifying and differentiated cells surrounding a central zone of cells that have patterns of expression and behavioural characteristic of stem cells. This zonal organization facilitated analysis of stem cell formation and loss. Cells in the central stem cell zone undergo rapid symmetric divisions but expansion of this population is partially limited by their peripheral transition into amplifying cells. A striking feature of central zone cells is their enhanced apoptotic susceptibility and stem cell expansion limited by consistently high background rates of apoptosis. This occasionally reaches catastrophic levels with elimination of the entire central zone.

CONCLUSION

In vitro amplification of stem cells for the generation of engineered tissue has tended to focus on control of asymmetric division but these findings suggest that development of mechanisms protecting stem cells from apoptotic changes are also likely to be of particular value.

Rapid adhesion to collagen isolates murine keratinocytes with limited long-term repopulating ability in vivo despite high clonogenicity in vitro

A prevalent belief in epidermal biology is that stem cells are highly clonogenic; that is, they have the ability to produce many large colonies in vitro. However, it has been well-established in hematology, and recently suggested in epithelial biology, that short-term in vitro clonogenic assays may not be reliable predictors of long-term in vivo repopulating ability. Numerous groups have shown that rapid adhesion to collagen selects for highly clonogenic keratinocytes, but it has not been demonstrated whether this subpopulation is enriched in stem cells as defined by long-term repopulating ability in vivo. We found that although rapid adhesion to collagen (within 5 minutes) selected for cells with increased short-term colony forming ability in vitro, these cells were not enriched in long-term proliferative ability in vitro or in repopulating ability in vivo after 9 weeks. Conversely, keratinocytes that did not adhere to collagen (after 20 minutes) were less clonogenic in short-term assays but possessed equivalent long-term proliferative ability in vitro and superior long-term repopulating ability in vivo. Both the rapidly adherent cell and not rapidly adherent cell populations contained small, noncomplex basaloid cells, expressed integrin alpha2 (a collagen IV receptor), and expressed the putative epidermal stem cell phenotype integrin alpha6(hi)CD71(lo). Our results indicate that the superior short-term colony forming ability of collagen-adherent murine keratinocytes does not correlate with long-term repopulating ability in vitro or in vivo and that proliferation in vitro is not a reliable surrogate for stem cell behavior in vivo.