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Stem Cells and Their Lineages in Skin
Summary: Elaine Fuchs is interested in understanding the molecular mechanisms underlying the ability of multipotent stem cells to produce the skin's epidermis and its appendages. She utilizes mammalian epithelial stem cell culture and mouse genetics as model systems. Her studies bridge an understanding of the normal biology of skin stem cells with an understanding of how these processes go awry in human diseases of the skin, including genetic diseases, skin cancers, and proinflammatory disorders.
Our central objective is to explore the mechanisms governing skin stem cells and their remarkable ability to both self-renew and to commit to proliferate and differentiate along a particular lineage. Skin is one of the few systems where adult stem cells can be maintained and propagated in the laboratory, a feature that greatly facilitates our studies. We also use technologies for targeting foreign genes to epidermis and hairs of transgenic animals, and for removing single epidermal or hair genes from the chromosomes of either mouse embryos (knockout) or skin (conditional inducible knockout). By taking a molecular approach to skin biology and stem cell research, we aim to apply this knowledge to human genetics, cancers, and medicine.
During development, skin begins as a single layer of multipotent embryonic stem cells that are able to produce either hair follicles or epidermis. If they receive a signal from underlying mesenchymal cells, skin stem cells respond by developing into hair follicles, which then produce sebaceous glands; in the absence of this cue, the cells stratify to make epidermis. While fully grown adult skin does not need nearly as many stem cells as embryonic skin, the epidermis must repair wounds, and hairs must regrow periodically throughout life. Thus, the upper portion of every adult hair follicle maintains a tiny reservoir (niche) of infrequently dividing, multipotent stem cells, which are used for these purposes. What activates the niche to mobilize its precious residents?
The initiation of new hair growth and the characteristics of the hair and its cycle are determined in part by interactions between the epithelial stem cells and specialized mesenchymal stem cells, referred to as dermal papillae. Similar to the epithelial-mesenchymal interactions that occur in embryonic skin, signals from the dermal papilla to the epithelial stem cell niche are important to start a new hair cycle. By contrast, upon injury, stem cells respond to growth factors released by damaged tissue.
To monitor the divisions and movements of adult skin stem cells, we devised methods to specifically label epithelial skin stem cells and dermal papillae with fluorescently tagged proteins. Using these methods, we learned that within their niche, most stem cells divide relatively infrequently. However, following stimulation by the dermal papilla, stem cells exit from the base of the niche and proliferate to re-form a new hair follicle. As the hair grows, stem cells continue to exit the niche, and as they reach the base of the hair follicle, they fuel the production of hair. Following this growth phase, the stem cell niche remains dormant until the next hair cycle or injury. In this way, most of the skin stem cells are safely sequestered in their niche and are subjected less frequently to chromosomal replication, which might introduce genetic errors. We recently identified a transcription factor that appears to be essential for controlling the frequency of divisions within the stem cell niche, and the mouse model we generated will now be useful in determining directly the extent to which slow cycling is an essential feature of stem cell maintenance and integrity.
Continuing our investigation of these stem cells, we also have used adult skin stem cells to clone mice. Referred to as nuclear transfer, this technology can also be used to generate embryonic stem (ES) cells. If this technology can be successfully adapted to humans, ES cells might be able to be generated from a patient's skin without genetically altering the skin cell's DNA. Additionally, the recent demonstration that skin fibroblasts can be genetically reprogrammed to make induced pluripotent stem (iPS) cells is particularly intriguing, as it suggests that in the future, it may be possible to turn an adult skin stem cell into an ES cell by epigenetically reprogramming only a small number of genes that are expressed by the ES cell and not the adult skin stem cell.
While multipotent stem cells are carefully guarded within their specialized niche in the hair follicle, the proliferative innermost (basal) layer of epidermal cells also harbors stem cells that produce the stratified, terminally differentiated layers that are continually shed from the skin surface. We discovered that in embryonic skin, stratification involves a reorientation of the mitotic spindle, causing basal cells to undergo asymmetric cell divisions. In this process, the epidermis appears to use a set of evolutionarily conserved genes, which are also used in asymmetric divisions in fruit flies and worms. As an asymmetrically dividing cell enters mitosis, an apical crescent of proteins assembles and anchors one of the two spindle poles. On the basal side of the cell, integrins and growth factor receptors maintain contact with the underlying basement membrane. By orienting the spindle in this way, the innermost daughter retains its ability to divide while the detached, uppermost daughter differentiates and continues its journey outward.
This newfound mechanism for controlling epidermal growth and differentiation provides insights into how the delicate balance of growth and differentiation may be influenced. We are learning that when this balance is tipped by gene mutations that affect epidermal polarity, invasive squamous cell carcinoma (SQCC) is observed. One such gene encodes α-catenin, a cell-cell junction and regulatory protein.
Resident progenitor cells also exist at the base of the sebaceous gland. These cells are marked by expression of Blimp1, a transcriptional repressor for the gene encoding the cell cycle regulator c-Myc. When c-Myc is overexpressed, either transgenically or through ablation of the Blimp1 gene, sebaceous gland tumors develop. In the absence of Blimp1, the multipotent stem cells in the hair follicle are mobilized to fuel production of gland cells, suggesting a precursor-product relationship between multipotent and resident stem cells in the skin.
We are also learning more about the multipotent and resident skin stem cells and their differentiating progeny by exploiting fluorescence tagging to isolate and transcriptionally profile these cells. By identifying genes that are preferentially expressed in each of these cell populations, we are beginning to decipher the unique properties of skin stem cells and how they relate to other stem cells of the body. We have also developed methods to isolate pure populations of dermal papilla cells to learn how these cells communicate with the epithelial stem cells to stimulate rounds of new hair growth. While we are still in the midst of these explorations, the ability to culture adult skin stem cells in the laboratory is a major advantage in evaluating how stem cells maintain a growth- and differentiation-inhibited environment in vivo, and how they become mobilized outside their niche.
We have learned that stem cells express TCF3, a DNA-binding protein that inhibits differentiation. In stem cell activation to make a hair follicle, TCF3 is converted from a repressor to an activator. Although the exact details remain to be elucidated, this switch involves the protein β-catenin, which is stabilized and becomes nuclear upon convergent receipt of activating Wnt signals and inhibiting BMP signals. Among the genes that are subsequently transactivated are those involved in cell proliferation and migration. Intriguingly, when Wnt signaling is overactivated or when BMP signaling is irreversibly compromised, hair follicle tumorigenesis arises. Overall, we are learning that the very pathways involved in the activation of the stem cells of epidermis, sebaceous gland, and hair follicles may contribute to cancer when deregulated. In the future, we will pursue the relation between stem cells and cancers in the skin.
As we continue to tackle our understanding of the environmental signaling and transcriptional changes that take place when stem cells become activated to produce epidermis or hair follicles, we are pursuing our long-standing interest in how the extraordinarily distinct architectures of these two structures are established. Without the proper contacts between neighboring cells and extracellular matrix, epidermis is unable to keep harmful microbes out and body fluids in, hairs are not produced, and wounds cannot be repaired. Genetic defects in cell adhesion genes are prevalent in a variety of disorders, including human cancers, where tissue architecture is reduced to distorted masses of cells.
Over two decades, we have uncovered many molecules that epidermal cells use to make adhesive connections and their associated mechanical frameworks (cytoskeletons). These investigations guided us to the genetic bases of blistering skin diseases as well as hyperproliferative disorders that resemble precancerous lesions. We are now gaining insights into how skin cells control and change the organization of their actin and microtubule cytoskeletons to achieve dynamic tissue organization. In this regard, we again focus on β-catenin and a relative, p120-catenin, this time in the context of their other well-known interacting partners, E-cadherin and α-catenin. These four proteins form the central components of cell-cell junctions, known as adherens junctions.
By integrating cell junctions and cytoskeletal dynamics, cells within the tissue can coordinate contractions and movements. Actin-microtubule-cell junction connections also provide the potential to polarize microtubule-based cargo transport and orient the mitotic spindle. Such connections could be important in orchestrating the orientation of cell divisions in skin and in controlling cellular movements upon stem cell activation or during wound repair.
We have begun to unravel the mechanisms governing these cellular dynamics and have uncovered new insights and new proteins involved. Of particular interest is that the integral components of adherens junctions each appear to have alternate roles. β-Catenin can function as a bipartite transcription factor in Wnt signaling, and p120-catenin and α-catenin regulate actin dynamics and proinflammatory and proliferative responses. We plan to continue to unfold the molecular mechanisms underlying these complex communication networks.
In closing, as we probe more deeply into an understanding of mammalian skin stem cells and tissue morphogenesis, we are progressively learning more about how defects in this process can lead to various human genetic skin disorders, including skin cancers and inflammation.
Grants from the National Institutes of Health provided partial support for some of these projects.
Last updated: June 20, 2008
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