16S rRNA V1-V3 sequencing of mouse skin microbiome Metagenome

Terminally differentiated keratinocytes are critical for skin barrier function and are surrounded by an involucrin (IVL)-rich cornified envelope. Increased IVL expression in the epidermis is associated with a recent selective sweep in Europe, yet the functional significance is unclear. We hypothesize the evolutionary role of Ivl to modulate Vitamin D receptor (Vdr) activity whose function in the epidermis is environmentally adaptive. We tested Vdr function on Ivl -/- mice with vitamin D agonist (MC903) treatment and comprehensively determined the cellular response using scRNA-seq and 16S bacterial phylotyping. MC903-mediated Vdr activation in Ivl-/- skin was reduced with dampened skin inflammation marked by a significant reduction in CD4+ T cells, basophils, macrophages, and type II basal keratinocytes and increased suprabasal keratinocytes as determined by scRNA-seq. Ivl-/- mice also exhibited dysbiosis with a decrease in Bacteroidetes family, Muribaculaceae, and conversely enriched for Firmicutes genera, Aerococcus and Streptococcus, that persisted regardless of MC903 exposure. This is in contrast to WT skin, which exhibited a MC903- responsive microbial shift for increased Bacteroidetes and decreased Firmicutes. Cellular assays further revealed the dampening of Vdr function in Ivl-/- keratinocytes owing to loss of Vdr nuclear localization and downstream decreased Tslp activation upon MC903-treatment. Together, our studies in Ivl-/- mice identifies a functional role for Involucrin to positively regulate Vdr function and impact microbial niche composition relevant to epidermal adaptation.