PROJECT SUMMARY In addition to signaling/transcription factor-dependent regulatory mechanisms, lineage-specific gene expression programs in stem cells and their progenies are also regulated epigenetically, i.e., via regulation of covalent DNA/histone modifications and higher-order chromatin remodeling. Epigenetic regulatory machinery maintain the progenitor population and epithelial identity in epidermal keratinocytes, as well as inhibit premature activation of terminal differentiation-associated genes and balance their transcription in differentiating cells. Transposable elements (TEs) constitute a large portion (44%-55%) of the entire mouse or human genomes. Most TEs are transcriptionally inactive under physiological conditions, while their inappropriate activation has been implicated in pathogenesis of autoimmune disorders and carcinogenesis. In keratinocytes, UV exposure activates transcription of various endogenous retroviral sequences which are closely related to the sequences activated in lupus erythematosus patients. Furthermore, increasing evidence of data suggest an involvement of endogenous TEs in the development of pro-inflammatory skin conditions, such as psoriasis. However, there is a number of critical questions that this project seeks to investigate, remain unclear: 1) Which TEs are expressed in normal keratinocytes during skin development, postnatal homeostasis and terminal differentiation, 2) Which mechanisms regulate TE silencing in keratinocytes, and 3) When TE silencing is compromised, how are keratinocyte differentiation and epidermal inflammatory response impacted? In this Multi-PI proposal, we will address the hypothesis that distinct classes of TEs differentially contribute to the control of gene expression in epidermal keratinocytes, mediated by epigenetic regulators Lsh and Setdb1 that serve as critical determinants mediating the TE silencing and preventing pro-inflammatory responses in the epidermis. This hypothesis will be addressed via three Specific Aims: Aim 1. Define the landscape of transposable element expression in normal mouse and human keratinocytes during skin development, postnatal homeostasis and terminal differentiation. Aim 2. Define the roles for Lsh and Setdb1 in the control of transposable element silencing in keratinocytes. Aim 3. Define the impact and mechanisms associated with the transposable element activation in keratinocytes on epidermal inflammatory response. The generated outputs from this application will provide novel insights into fundamental mechanisms underlying keratinocyte differentiation in normal mouse and human skin, as well as will promote the development of novel paradigms for management of disorders of epidermal differentiation and inflammatory responses in humans via modulation of TE activities.