Cellular and molecular mechanisms underlying the maintenance of genomic integrity in epidermal stem cells

Adult Stem Cells (SCs) have been found in almost every organ. They are responsible forhomeostasis and tissue repair after injury. SCs reside and self-renew in the adult bodythroughout the life of the organism. In rapid self-renewing organs, such as the skin, theintestine and the blood, SCs divide many times during the life of the animal in order to sustainthe homeostatic needs of the tissue.All cells of the body, including SCs, are constantly subjected to DNA assaults arising fromendogenous sources, such as reactive oxygen species (ROS) generated by cellularmetabolism, or exogenous assaults arising from the environment. The DNA damage response(DDR) and DNA repair mechanisms protect cells from accumulating DNA damage byinducing transient cell cycle arrest allowing DNA repair, triggering senescence or apoptosis.DNA damages trigger the activation of the effectors of the DDR inducing a transient cellcycle arrest, allowing DNA repair, or triggering a permanent arrest of the cell cycle orapoptosis if damages are too extensive.As skin is the outermost barrier of the body, epidermal cells, including SCs, arecontinuously subjected to genotoxic stress, such as UV rays, ionizing radiation (IR) andchemicals. The skin epidermis is composed of hair follicles (HFs), its associated sebaceousgland (SG) and the surrounding inter-follicular epidermis (IFE). Different types of SCsmaintain the homeostasis of the skin; multipotent adult bulge SCs ensure the cyclicregeneration of the HF and the repair of the epidermis after injury, while individual unipotentSCs ensure homeostasis of the SG and the IFE.In tissues with high cellular turnover, such as the epidermis, the numerous divisions that aSC undergoes could result in the accumulation of replication-associated DNA damage. It hasbeen suggested that adult SCs may undergo asymmetric divisions in which the daughter SCretains the older (thus “immortal”) DNA strand, while the daughter cell committed todifferentiation inherits the newly synthesized strand that may have incorporated replicationderivedmutations. The in vivo relevance of this mechanism is still a matter of intense debate.We used multiple in vivo experimental approaches to investigate precisely how bulge SCssegregate their chromosomes during HF morphogenesis, SC activation and skin homeostasis.Using pulse-chase experiments with two different uridine analogs together with DNAindependentchromatin labelling, we showed that multipotent HF SCs segregate theirchromosomes randomly, and that the label-retention observed in the skin epidermis derivessolely from relative quiescence of skin SCs 1.We investigated the in vivo response of multipotent adult HF bulge SCs to DNA damageinduced by IR. We showed that bulge SCs are profoundly resistant to DNA damage-inducedcell death compared to their more mature counterparts. Interestingly, we demonstrated thatresistance of bulge SCs to IR-induced apoptosis does not rely on their relative quiescence.Moreover, we showed that DDR in SCs does not lead to premature senescence. We found thattwo intrinsic cellular mechanisms participate in the resistance of bulge SCs to DNA damageinducedcell death. Bulge SCs express higher level of the anti-apoptotic Bcl-2 and presentmore transient activation of p53 due to a faster DNA repair activity mediated by a nonhomologousend joining (NHEJ) mechanism. Since NHEJ is not error free, this propertymight be a double-edged sword, supporting short-term survival of bulge SCs but impairinglong-term genomic integrity 2.While we unveiled the relevance of DSBs repair by NHEJ in the skin epidermis, little isknown about the role of homologous recombination (HR) during the morphogenesis of theskin epidermis. Brca1 is an essential protein for HR. Conditional deletion of Brca1 in thedeveloping epidermis leads to congenital alopecia accompanied by a decreased density of hairplacodes. The remaining HFs never produce mature hair and progressively degenerate due tohigh levels of apoptosis. Multipotent adult HF bulge SCs cannot be detected in adult HF inthe Brca1 cKO epidermis. Brca1 deletion in the epidermis triggers p53 activation throughoutthe epidermis, which activates apoptosis. Interestingly, IFE and the isthmus region of the HFdo not present any pathological phenotype by constitutive deletion of Brca1. Our resultsdemonstrated the critical role of Brca1 during HF morphogenesis. Future studies will berequired to understand the molecular mechanisms controlling this phenotype