Dietary Restriction Delays But Cannot Heal Irradiation-Induced Hair Graying by Preserving Hair Follicle Stem Cells in Quiescence

DNA damage represents one of the cell intrinsic causes of stem cell aging, which leads to differentiation induced removal of damaged stem cells in skin and blood. Dietary restriction (DR) retards aging across various species including several strains of laboratory mice. Whether, DR has the potential to ameliorate DNA damage driven stem cell exhaustion remains incompletely understood. Here, we show that DR strongly extends the time to hair graying in response to γ–irradiation (IR) induced DNA damage of C57BL/6J mice. The study shows that DR prolongs quiescence of hair follicle stem cells (HFSCs) by silencing gene regulatory networks and metabolic switches that control proliferation and tissue regeneration. DR-mediated prolongation of HSFC quiescence blocks hair growth and prevents the depletion of HFSCs and ckit + melanoblasts in response to IR. However, prolongation of HSFCs quiescence also leads to a suppression of DNA repair pathways and cannot prevent melanoblast loss and hair graying in the long run, when hair cycling is re-initiated even after extended periods of time. Together, these results support a model indicating that nutrient deprivation can delay but not heal DNA damage driven extinction of HFSCs and melanoblasts by stalling HSFCs in a prolonged state of quiescence coupled with inhibition of DNA repair. Disconnecting these two types of responses to DR could have the potential to delay stem cell aging.