Esther Hoste

Epithelial stem cells, wound healing and cancer

It is well established that tissue repair depends on stem cells and that chronic wounds predispose to tumour formation. However, the association between stem cells, wound healing and cancer is poorly understood. Lineage tracing has now shown how stem cells are mobilized to repair skin wounds and how they contribute to skin tumour development. The signalling pathways, including WNT and Hedgehog, that control stem cell behaviour during wound healing are also implicated in tumour formation. Furthermore, tumorigenesis and wound repair both depend on communication between epithelial cells, mesenchymal cells and bone marrow-derived cells. These studies suggest ways to harness stem cells for wound repair while minimizing cancer risk.

Caspase-14 protects against epidermal UVB photodamage and water loss.

Caspase-14 belongs to a conserved family of aspartate-specific proteinases. Its expression is restricted almost exclusively to the suprabasal layers of the epidermis and the hair follicles. Moreover, the proteolytic activation of caspase-14 is associated with stratum corneum formation, implicating caspase-14 in terminal keratinocyte differentiation and cornification. Here, we show that the skin of caspase-14-deficient mice was shiny and lichenified, indicating an altered stratum-corneum composition. Caspase-14-deficient epidermis contained significantly more alveolar keratohyalin F-granules, the profilaggrin stores. Accordingly, caspase-14-deficient epidermis is characterized by an altered profilaggrin processing pattern and we show that recombinant caspase-14 can directly cleave profilaggrin in vitro. Caspase-14-deficient epidermis is characterized by reduced skin-hydration levels and increased water loss. In view of the important role of filaggrin in the structure and moisturization of the skin, the knockout phenotype could be explained by an aberrant processing of filaggrin. Importantly, the skin of caspase-14-deficient mice was highly sensitive to the formation of cyclobutane pyrimidine dimers after UVB irradiation, leading to increased levels of UVB-induced apoptosis. Removal of the stratum corneum indicate that caspase-14 controls the UVB scavenging capacity of the stratum corneum.

Caspase-14 Is Required for Filaggrin Degradation to Natural Moisturizing Factors in the Skin

Caspase-14 is a protease that is mainly expressed in suprabasal epidermal layers and activated during keratinocyte cornification. Caspase-14-deficient mice display reduced epidermal barrier function and increased sensitivity to UVB radiation. In these mice, profilaggrin, a protein with a pivotal role in skin barrier function, is processed correctly to its functional filaggrin (FLG) repeat unit, but proteolytic FLG fragments accumulate in the epidermis. In wild-type stratum corneum, FLG is degraded into free amino acids, some of which contribute to generation of the natural moisturizing factors (NMFs) that maintain epidermal hydration. We found that caspase-14 cleaves the FLG repeat unit and identified two caspase-14 cleavage sites. These results indicate that accumulation of FLG fragments in caspase-14(-/-) mice is due to a defect in the terminal FLG degradation pathway. Consequently, we show that the defective FLG degradation in caspase-14-deficient skin results in substantial reduction in the amount of NMFs, such as urocanic acid and pyrrolidone carboxylic acid. Taken together, we identified caspase-14 as a crucial protease in FLG catabolism.

Epidermal barrier defects link atopic dermatitis with altered skin cancer susceptibility

Atopic dermatitis can result from loss of structural proteins in the outermost epidermal layers, leading to a defective epidermal barrier. To test whether this influences tumour formation, we chemically induced tumours in EPI−/− mice, which lack three barrier proteins—Envoplakin, Periplakin, and Involucrin. EPI−/− mice were highly resistant to developing benign tumours when treated with 7,12-dimethylbenz(a)anthracene (DMBA) and 12-O-tetradecanoylphorbol-13-acetate (TPA). The DMBA response was normal, but EPI−/− skin exhibited an exaggerated atopic response to TPA, characterised by abnormal epidermal differentiation, a complex immune infiltrate and elevated serum thymic stromal lymphopoietin (TSLP). The exacerbated TPA response could be normalised by blocking TSLP or the immunoreceptor NKG2D but not CD4+ T cells. We conclude that atopy is protective against skin cancer in our experimental model and that the mechanism involves keratinocytes communicating with cells of the immune system via signalling elements that normally protect against environmental assaults. DOI: http://dx.doi.org/10.7554/eLife.01888.001

Caspase-14-Deficient Mice Are More Prone to the Development of Parakeratosis

Caspase-14 is an important protease in the proper formation of a fully functional skin barrier. Newborn mice that are deficient in caspase-14 exhibit increased transepidermal water loss and are highly sensitive to UVB-induced photodamage. Decreased caspase-14 expression and incomplete caspase-14 processing in lesional psoriatic parakeratotic stratum corneum has been reported previously. In this study, we show that caspase-14-deficient skin frequently displays incompletely cornified cells in the transitional zone between the granular and the cornified layers, pointing to a delay in cornification. We also demonstrate that after challenge of epidermal permeability barrier function by repetitive acetone treatment, a higher incidence of large parakeratotic plaques was observed in caspase-14-deficient skin. Furthermore, caspase-14-deficient mice are more prone than control mice to the development of parakeratosis upon induction of psoriasis-like dermatitis by imiquimod treatment. These results show that lack of caspase-14 expression predisposes to the development of parakeratosis and that caspase-14 has an important role in keratinocyte terminal differentiation and the maintenance of normal stratum corneum, especially in conditions causing epidermal hyperproliferation.

Wounding induces dedifferentiation of epidermal Gata6 + cells and acquisition of stem cell properties

The epidermis is maintained by multiple stem cell populations whose progeny differentiate along diverse, and spatially distinct, lineages. Here we show that the transcription factor Gata6 controls the identity of the previously uncharacterized sebaceous duct (SD) lineage and identify the Gata6 downstream transcription factor network that specifies a lineage switch between sebocytes and SD cells. During wound healing differentiated Gata6+ cells migrate from the SD into the interfollicular epidermis and dedifferentiate, acquiring the ability to undergo long-term self-renewal and differentiate into a much wider range of epidermal lineages than in undamaged tissue. Our data not only demonstrate that the structural and functional complexity of the junctional zone is regulated by Gata6, but also reveal that dedifferentiation is a previously unrecognized property of post-mitotic, terminally differentiated cells that have lost contact with the basement membrane. This resolves the long-standing debate about the contribution of terminally differentiated cells to epidermal wound repair.

Fate of Prominin-1 Expressing Dermal Papilla Cells During Homeostasis, Wound Healing and Wnt Activation

Prominin-1/CD133 (Prom1) is expressed by fibroblasts in the dermal papilla (DP) of the hair follicle (HF). By examining endogenous Prom1 expression and expression of LacZ in the skin of Prom1CreERLacZ (Prom1C-L) mice, in which a CreERT2-IRES-nuclear LacZ cassette is knocked into the first ATG codon of Prom1, we confirmed that Prom1 is expressed in the DP of all developing HFs and also by postnatal anagen follicles. To analyze the fate of Prom1+ DP cells, we crossed Prom1C-L mice with Rosa26-CAG flox/stop/flox tdTomato reporter mice and applied 4-hydroxytamoxifen (4OHT) to back skin at postnatal day (P) 1 and P2. We detected tdTomato+ cells in ~50% of DPs. The proportion of labeled cells per DP increased between P5 and P63, while the total number of cells per DP declined. Following full thickness wounding, there was no migration of tdTomato-labeled cells out of the DP. When β-catenin was activated in Prom1+ DP cells there was an increase in the size of anagen and telogen DP, but the proportion of tdTomato-labeled cells did not increase. We conclude that Prom1+ DP cells do not contribute to dermal repair but are nevertheless capable of regulating DP size via β-catenin-mediated intercellular communication.

Epicutaneous sensitization to house dust mite allergen requires interferon regulatory factor 4–dependent dermal dendritic cells

Background Exposure to allergens, such as house dust mite (HDM), through the skin often precedes allergic inflammation in the lung. It was proposed that TH2 sensitization through the skin occurs when skin barrier function is disrupted by, for example, genetic predisposition, mechanical damage, or the enzymatic activity of allergens. Objective We sought to study how HDM applied to unmanipulated skin leads to TH2 sensitization and to study which antigen‐presenting cells mediate this process. Methods HDM was applied epicutaneously by painting HDM on unmanipulated ear skin or under an occlusive tape. HDM challenge was through the nose. Mouse strains lacking different dendritic cell (DC) populations were used, and 1‐DER T cells carrying a transgenic T‐cell receptor reactive to Der p 1 allergen were used as a readout for antigen presentation. The TH2‐inducing capacity of sorted skin‐derived DC subsets was determined by means of adoptive transfer to naive mice. Results Epicutaneous HDM application led to TH2 sensitization and eosinophilic airway inflammation upon intranasal HDM challenge. Skin sensitization did not require prior skin damage or enzymatic activity within HDM extract, yet was facilitated by applying the allergen under an occlusive tape. Primary proliferation of 1‐DER T cells occurred only in the regional skin‐draining lymph nodes. Epicutaneous sensitization was found to be driven by 2 variants of interferon regulatory factor 4–dependent dermal type 2 conventional DC subsets and not by epidermal Langerhans cells. Conclusion These findings identify skin type 2 conventional DCs as crucial players in TH2 sensitization to common inhaled allergens that enter the body through the skin and can provoke features of allergic asthma.

Epidermal Cadm1 Expression Promotes Autoimmune Alopecia via Enhanced T Cell Adhesion and Cytotoxicity

Autoimmune alopecia is characterized by an extensive epidermal T cell infiltrate that mediates hair follicle destruction. We have investigated the role of cell adhesion molecule 1 (Cadm1; Necl2) in this disease. Cadm1 is expressed by epidermal cells and mediates heterotypic adhesion to lymphocytes expressing class 1-restricted T cell-associated molecule (CRTAM). Using a murine autoimmune alopecia model, we observed an increase in early-activated cytotoxic (CD8-restricted, CRTAM-expressing) T cells, which preferentially associated with hair follicle keratinocytes expressing Cadm1. Coculture with Cadm1-transduced MHC-matched APCs stimulated alopecic lymph node cells to release IL-2 and IFN-γ. Overexpression of Cadm1 in cultured human keratinocytes did not promote cytokine secretion, but led to increased adhesion of alopecic cytotoxic T cells and enhanced T cell cytotoxicity in an MHC-independent manner. Epidermal overexpression of Cadm1 in transgenic mice led to increased autoimmune alopecia susceptibility relative to nontransgenic littermate controls. Our findings reveal that Cadm1 expression in the hair follicle plays a role in autoimmune alopecia.

Langerhans cells : sensing the environment in health and disease

In the last few decades, our understanding of Langerhans cells (LCs) has drastically changed based on novel findings regarding the developmental origin and biological functions of these epidermis-specific resident immune cells. It has become clear that LCs not only exert pivotal roles in immune surveillance and homeostasis but also impact on pathology by either inducing tolerance or mediating inflammation. Their unique capabilities to self-renew within the epidermis, while also being able to migrate to lymph nodes in order to present antigen, place LCs in a key position to sample the local environment and decide on the appropriate cutaneous immune response. Exciting new data distinguishing LCs from Langerin+ dermal dendritic cells (DCs) on a functional and ontogenic level reveal crucial roles for LCs in trauma and various skin pathologies, which will be thoroughly discussed here. However, despite rapid progress in the field, the exact role of LCs during immune responses has not been completely elucidated. This review focuses on what mouse models that have been developed in order to enable the study of murine LCs and other Langerin-expressing DCs have taught us about LC development and function.