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.

Trisomy 21 results in gene-dosage imbalance during embryogenesis and throughout life, ultimately causing multiple anomalies that contribute to the clinical manifestations of Down syndrome. Down syndrome is associated with manifestations of variable severity (e.g., heart anomalies, reduced growth, dental anomalies, shortened life-span). Craniofacial dysmorphology and cognitive dysfunction are consistently observed in all people with Down syndrome. Mouse models are useful for studying the effects of gene-dosage imbalance on development. We investigated quantitative changes in the skull and brain of the Dp(16) 1Yey Down syndrome mouse model and compared these mice to Ts65Dn and Ts1Cje mouse models. Three-dimensional microcomputed tomography images of Dp(16)1Yey and euploid mouse crania were morphometrically evaluated. Cerebellar cross-sectional area, Purkinje cell linear density, and granule cell density were evaluated relative to euploid littermates. Skulls of Dp(16)1Yey and Ts65Dn mice displayed similar changes in craniofacial morphology relative to their respective euploid littermates. Trisomy-based differences in brain morphology were also similar in Dp(16)1Yey and Ts65Dn mice. These results validate examination of the genetic basis for craniofacial and brain phenotypes in Dp(16)1Yey mice and suggest that they, like Ts65Dn mice, are valuable tools for modeling the effects of trisomy 21 on development.

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.