Regeneration of skin and hair follicles after wounding - a process known as wound-induced hair neogenesis (WIHN) - is a rare example of adult organogenesis in mammals. As such, WIHN provides a unique model system for deciphering mechanisms underlying mammalian regeneration. Here, we show that dsRNA, which is released from damaged skin, activates Toll-Like Receptor 3 (TLR3) and its downstream effectors IL6 and STAT3 to promote hair follicle regeneration. Conversely, TLR3-deficient animals fail to initiate WIHN. TLR3 activation promotes expression of hair follicle stem cell markers and induces elements of the core hair morphogenetic program, including EDAR and the Wnt and Shh pathways. Our results therefore show that dsRNA and TLR3 link the earliest events of mammalian skin wounding to regeneration and suggest potential therapeutic approaches for promoting hair neogenesis.

With the advent of the Xenopus tropicalis genome project, we analyzed scaffolds containing MHC genes. On eight scaffolds encompassing 3.65 Mbp, 122 MHC genes were found of which 110 genes were annotated. Expressed sequence tag database screening showed that most of these genes are expressed. In the extended class II and class III regions the genomic organization, excluding several block inversions, is remarkably similar to that of the human MHC. Genes in the human extended class I region are also well conserved in Xenopus, excluding the class I genes themselves. As expected from previous work on the Xenopus MHC, the single classical class I gene is tightly linked to immunoproteasome and transporter genes, defining the true class I region, present in all nonmammalian jawed vertebrates studied to date. Surprisingly, the immunoproteasome gene PSMB10 is found in the class III region rather than in the class I region, likely reflecting the ancestral condition. Xenopus DMα, DMβ, and C2 genes were identified, which are not present or not clearly identifiable in the genomes of any teleosts. Of great interest are novel V-type Ig superfamily (Igsf) genes in the class III region, some of which have inhibitory motifs (ITIM) in their cytoplasmic domains. Our analysis indicates that the vertebrate MHC experienced a vigorous rearrangement in the bony fish and bird lineages, and a translocation and expansion of the class I genes in the mammalian lineage. Thus, the amphibian MHC is the most evolutionary conserved MHC so far analyzed.

Despite similar components, the heterogeneity of skin characteristics across the human body is enormous. It is classically believed that site-specific fibroblasts in the dermis control postnatal skin identity by modulating the behavior of the surface-overlying keratinocytes in the epidermis. To begin testing this hypothesis, we characterized the gene expression differences between volar (ventral; palmoplantar) and nonvolar (dorsal) human skin. We show that KERATIN 9 (KRT9) is the most uniquely enriched transcript in volar skin, consistent with its etiology in genetic diseases of the palms and soles. In addition, ectopic KRT9 expression is selectively activated by volar fibroblasts. However, KRT9 expression occurs in the absence of all fibroblasts, although not to the maximal levels induced by fibroblasts. Through gain-of-function and loss-of-function experiments, we demonstrate that the mechanism is through overlapping paracrine or autocrine canonical WNTeb-catenin signaling in each respective context. Finally, as an in vivo example of ectopic expression of KRT9 independent of volar fibroblasts, we demonstrate that in the human skin disease lichen simplex chronicus, WNT5a and KRT9 are robustly activated outside of volar sites. These results highlight the complexities of site-specific gene expression and its disruption in skin disease. Site-specific epidermal differentiation programs define the heterogeneity of skin identity across the human body. It is generally believed that positional skin identity is regulated by epithelial (keratinocytes) and mesenchymal (fibroblasts) interactions. 1e5 Indeed, three-dimensional skin equivalent models and transplantation experiments show that epidermal stratification during the keratinocyte differentiation program is remarkably disrupted in the absence of fibroblasts, 2,4,5 demonstrating that epidermal development and homeostasis are regulated by extrinsic factors released by fibroblasts. Such interactions have been further clarified through an analysis of the differential localization of epidermal keratins in distinct epidermal layers and body sites. Volar (palmoplantar) skin is characterized by thick epidermal layers, less pigmentation, and lack of hair. In addition to these features, cytoskeleton KERATIN (KRT) 9 6 has been thought to be almost exclusively localized to volar keratinocytes, 1,6e9 indicating that KRT9 can be a unique marker for volar skin. To date, many clinical studies have reported that KRT9 mutations cause epidermolytic palmoplantar keratoderma, characterized by compensatory thickened epidermal layers in the palms and soles. 10 Conditional deletion of Krt9 in a murine model demonstrated that Krt9 is responsible for maintaining mechanical integrity and terminal differentiation of volar skin.

T cell factor (TCF) family of transcription factors and beta-catenin critically regulate T cell development as demonstrated by the deletion of the tcf gene, which results in a block early in development that becomes complete in mice bearing tcf/lef double deletion. However, the role of beta-catenin, a major TCF cofactor, remains controversial. To directly address this, we have generated transgenic mice expressing Inhibitor of beta-catenin and TCF (ICAT), a naturally occurring inhibitor that specifically disrupts TCF and beta-catenin interactions. In this report, we demonstrate that disrupting the interaction of beta-catenin with TCF renders adult thymocytes and activated T cells highly susceptible to apoptosis. In contrast to previously reported observations during fetal thymocyte development, these data show that in adult mice, survival and not differentiation of thymocytes, depends on transcription by TCF and beta-catenin. Indeed, we demonstrate that expression of ICAT impedes thymocyte survival by reducing the expression of Bcl(xL) in thymocytes below a critical threshold. Survival of activated mature T cells was also impaired due to diminished expression of activation-induced Bcl(xL). Accordingly, expression of transgenic Bcl-2 rescued activated ICAT-Tg CD4 T cells from apoptosis. Thus, disruption of TCF-beta-catenin interactions specifically impairs the survival of thymocytes and activated T cells.