The circadian clock, an ancient, evolutionary conserved timing system required for optimal function of organs and organismal lifespan, is active in peripheral tissues, including the skin. Clocks in peripheral organs are coordinated by the central clock in the suprachiasmatic nucleus, but we also know that time-restricted feeding affects circadian clocks, gene expression, and homeostasis in peripheral tissues. Although new insights are emerging, especially from studies in metabolic organs like the liver, the interplay between feeding time, clocks, and tissue health in epithelia is unclear. In particular, we don't know how time-restricted feeding affects the regenerative function of epidermal stem cells and skin aging. In mice, the circadian clock coordinates progression of the cell cycle and DNA excision repair with intermediary metabolism, as reflected in the redox state of epidermal stem cells. Intriguingly, daytime-restricted feeding shifts the phase and decreases the amplitude of the skin circadian clock, and it shifts the expression of the metabolism-related transcriptome without altering the phase of the diurnal oscillations in DNA synthesis. Daytime-restricted feeding, then, disrupts the coordination between metabolism and cell cycle progression in epidermal stem cells. Whereas these cycles in epidermal stem cells are known to modulate the sensitivity to UVB-induced DNA damage, their role in homeostasis of epidermal stem cells remains otherwise unknown. Here, we will investigate the idea that the clock coordinates oscillations of metabolism-generated ROS levels with the cell cycle and the DNA repair machinery to maximize the health and function of epidermal stem cells. Specifically, we hypothesize that this regulation minimizes metabolism- generated ROS when most epidermal stem cells are undergoing DNA replication, the cell cycle stage most sensitive to oxidative DNA damage. This hypothesis predicts that daytime feeding-induced circadian misalignment in epidermal stem cells causes asynchrony between oxidative metabolism and the cell cycle, leading to increased ROS-induced DNA mutations, epidermal stem cell dysfunction, and skin aging. To test this hypothesis, we propose two aims. First, we will define the gene-regulatory mechanisms underlying time- restricted feeding modulation of the circadian clock and metabolism in epidermal stem cells. Second, we will determine how time-restricted feeding modulates epidermal stem cell function and affects the rate of age- associated DNA mutations in epidermal stem cells. The proposal is significant because it tests a new model of how the circadian clock coordinates the timing of intermediary metabolism and the cell cycle in epithelial stem cells to minimize the accumulation of somatic DNA mutations, and how time-restricted feeding can enforce or disrupt this coordination. The proposal is innovative because it pursues a new idea about the role of dietary intervention and the circadian clock in skin aging, and it uses state of the art approaches, including duplex DNA- sequencing, fluorescence lifetime imaging, and single cell RNA-sequencing--approaches not previously applied to skin aging.
We test fundamental ideas about how the circadian clock modulates skin regeneration and aging and how these processes are affected by time of feeding. Skin aging causes skin fragility and delayed wound healing, a common cause of chronic wounds in the elderly, which poses a major public health problem in the aging population. This work may lead to new ideas about how to maintain skin strength and regenerative ability into old age.
