Role of the core clock component BMAL1 in the regulation of the mouse hair follicle cycle, epidermal proliferation and UVB-induced DNA damage susceptibility

Mouse skin is a complex organ made up of epidermis, dermis and subcutaneous fat layers and populated by pilosebaceous units also known as hair follicles. While interfollicular epidermis renews on a continuous basis, hair follicles renew in cyclical fashion via a process referred to as the hair follicle cycle. We set out to investigate the molecular players involved in the timing of the hair follicle growth cycle and found that the expression of circadian clock components and target genes is enhanced in telogen. Furthermore, the circadian clock functions in the regulation of telogen to anagen transition in hair follicle regeneration via regulation of the G1-S transition checkpoint through CDKN1A (P21). Hair follicles lacking Bmal1, an essential circadian regulator, are arrested in the early stage of anagen.;In addition to the genes circadian clock pathway, expression of more than 400 genes is enriched in telogen, commonly thought of as a dormant stage of the hair cycle. These genes are enriched in proteolysis, cholesterol metabolism, RNA processing, immune response, cell cycle inhibition and other biological categories. Furthermore, more than 300 genes are dynamic within the prolonged second telogen, such that their expression varies significantly within the prolonged second telogen.;In order to understand skin chronobiology we described the circadian transcriptome in whole mouse skin during anagen and telogen. More than 1000 transcripts are expressed in circadian fashion in telogen and about 430 are circadian in anagen. Furthermore, in telogen, circadian metabolic genes and cell cycle regulators peak at different times - the former during the day and the latter at night. Consistently, ROS accumulation in skin is time-dependent and peaks at night. As opposed to the cyclically regenerating hair follicles, basal keratinocytes of the interfollicular epidermis divide continuously. The circadian clock mechanism plays a role in this division since a percentage of the dividing keratinocytes follows a time-dependent rhythm, at night, that is abolished in Bmal1-/- mice. Interestingly, when Bmal1 is deleted specifically in the epidermis, the hair follicle cycle delay observed in the globally deleted mice does not occur, while the time-dependent S-phase rhythm in the interfollicular epidermis is abolished. This suggests that either central clock output or extrafollicular clock-regulated factors are required for the hair follicle cycle progression. Consistent with time-dependent and BMAL1- dependent S-phase rhythm in the interfollicular epidermis, UVB-induced DNA damage is also time and BMAL1- dependent. Based on the anti-phasic oxidative phosphorylation and S-phase rhythmicity, we speculate that the clock may function in skin to temporally segregate oxidative metabolism from genomic damage incurred during S-phase of the cell cycle.