ABSTRACT The skin epidermis provides a critical first line of environmental defense. However, the mechanisms mediating how epidermal keratinocytes adapt and respond to the external environment are poorly understood. Involucrin (IVL) is the major scaffold protein of the cornified envelope surrounding the differentiated keratinocyte. Our previous work supports a functional role for involucrin in epidermal adaptive responses to the environment, acute microbial exposure, and inflammation. (1) We discovered recent selective sweep for an IVL haplotype in the northern European (CEU) population. This IVL CEU haplotype is associated with increased IVL expression in sun-exposed skin compared to that in non-sun-exposed skin [according to Genotype-Tissue Expression Project (GTEx) data]. Positive selection of IVL CEU is directly correlated with increasingly northern latitudes, suggesting a fitness benefit of increased IVL expression in northern clines. (2) IVL was the only large Epidermal Differentiation Complex (EDC)-encoded skin barrier protein that exhibited significantly higher levels in the epidermis of conventionally raised (CR) lab mice compared to those in germ-free (GF) and reconstituted skin microbiome (RSM) mice (GF mice acutely exposed to CR commensal microbiota), suggesting an adaptive keratinocyte response to microbial interactions. (3) Ivl-/- mouse skin exhibited a protective effect against MC903-induced inflammation with reduced vitamin D receptor expression, suggesting a regulatory role for IVL in vitamin D signaling in the epidermis. We hypothesize that IVL levels are specifically calibrated to generate an adaptive skin barrier that is finely tuned in response to the environment. We will test this hypothesis by performing three aims. Aim 1. Determine the causative genetic variants that increase IVL gene expression levels. IVL CEU includes enhancer 923 that is required for involucrin target gene expression as evidenced by loss of Ivl expression in 923-/- mice. Cellular and molecular assays in keratinocytes will be performed to identify the molecular mechanisms by which enhancer variation(s) modulate target gene expression. Aim 2. Determine the role of involucrin in the skin adaptive response to commensal microbiota. We will examine the skin and inflammatory responses in rederived Ivl-/- CV mice as a functional measurement of skin adaptation. Aim 3. Determine the molecular mechanism by which involucrin regulates Vitamin D receptor expression and signaling in the epidermis. MC903-treated Ivl-/- ear skins did not exhibit severe inflammation and exhibited reduced Vdr expression. We hypothesize a role for Involucrin to positively regulate Vdr expression and signaling via phosphorylated p38δ, a known VDR activator in other human cell types. Our hypothesis will be rigorously tested using constitutively active p38δ and knockdown in vitro studies with anticipated results for increased and decreased Vdr expression and signaling, respectively. This stud...