The Impact of the Circadian Clock and Feeding Time on Skin Function

The circadian clock regulates many important aspects of physiology, allowing organisms to anticipate daily environmental changes in addition to gating cellular processes. The circadian clock in the skin regulates biological functions ranging from cell renewal, protection against DNA damage, and defense against pathogens. The goal of this dissertation is to explore the effect of the circadian clock and feeding time on skin biological functions. We find that approximately 10% of genes are diurnal in the skin, and the expression of approximately 2,000 genes are acutely affected by food-intake. While daytime-restricted feeding does not shift the phase of epidermal cell proliferation, it reverses the diurnal sensitivity to UVB-induced DNA damage. Next, we sought to investigate the role of the circadian clock in the Toll-like receptor 7 (TLR7)-induced immune responses in the skin. The TLR7 pathway, in coordination with the type I IFN pathway, mediates the expression of key genes, termed “Interferon-sensitive genes” (ISGs) which encode important mediators for host defense against pathogens. We find that topical application of the TLR7 agonist Imiquimod (IMQ) during the daytime elicits a greater magnitude of ISG expression. In addition, shifting the clock by daytime-restricted feeding reverses the diurnal rhythm of IMQ-induced ISG induction. Epidermal immune cells, and not keratinocytes, exhibit time-of-day dependent differences in the induction of the key ISG transcription factor Interferon Regulatory Factor 7 (Irf7) after IMQ treatment. In parallel, IMQ-induced IRF7 activation, as measured by nuclear translocation of IRF7 and phosphorylated IRF7, is greater after IMQ treatment during the daytime in epidermal T cells and monocytes. Further supporting the clock’s role in ISG responses, systemic Bmal1 deletion, but not in keratinocyte- or dendritic cell-specific Bmal1 deletion, results in exacerbated IMQ-induced ISG expression. We also investigate whether the circadian clock modulates IMQ-induced inflammation. Under homeostasis and/or after one IMQ dose, diurnal rhythms exist in epidermal cell proliferation and thickness, neutrophil recruitment, and spleen weight; however, these rhythms are ablated by repeated IMQ doses. Interestingly, systemic Bmal1 deletion does not alter immune infiltration to the skin, splenomegaly, or epidermal proliferation after repeated IMQ doses. These results suggest that, while the circadian clock is a critical mediator of the early ISG response to IMQ, it does not significantly impact the robust inflammation observed after repeated IMQ doses