Disruption of epidermal specific gene expression and delayed skin development in AP-2 gamma mutant mice

Summary Sentence: Conditional ablation of AP-2γ results in a delay in skin development and abnormal expression of p63, K14, K1, filaggrin, repetin and secreted Ly6/Plaur domain containing 1, key genes required for epidermal development and differentiation.The development of the epidermis, a stratified squamous epithelium, is dependent on the regulated differentiation of keratinocytes. Differentiation begins with the initiation of stratification, a process tightly controlled through proper gene expression. AP-2γ is expressed in skin and previous research suggested a pathway where p63 gene induction results in increased expression of AP-2γ, which in turn is responsible for induction of K14. This study uses a conditional gene ablation model to further explore the role of AP-2γ in skin development. Mice deficient for AP-2γ exhibited delayed expression of p63, K14, and K1, key genes required for development and differentiation of the epidermis. In addition, microarray analysis of E16.5 skin revealed delayed expression of additional late epidermal differentiation genes: filaggrin, repetin and secreted Ly6/Plaur domain containing 1, in mutant mice. The genetic delay in skin development was further confirmed by a functional delay in the formation of an epidermal barrier. These results document an important role for AP-2γ in skin development, and reveal the existence of regulatory factors that can compensate for AP-2γ in its absence.     

Forward genetics identifies Kdf1/1810019J16Rik as an essential regulator of the proliferation–differentiation decision in epidermal progenitor cells

Cell fate decisions during embryogenesis and adult life govern tissue formation, homeostasis and repair. Two key decisions that must be tightly coordinated are proliferation and differentiation. Overproliferation can lead to hyperplasia or tumor formation while premature differentiation can result in a depletion of proliferating cells and organ failure. Maintaining this balance is especially important in tissues that undergo rapid turnover like skin however, despite recent advances, the genetic mechanisms that balance cell differentiation and proliferation are still unclear. In an unbiased genetic screen to identify genes affecting early development, we identified an essential regulator of the proliferation–differentiation balance in epidermal progenitor cells, the Keratinocyte differentiation factor 1 (Kdf1; 1810019J16Rik) gene. Kdf1 is expressed in epidermal cells from early stages of epidermis formation through adulthood. Specifically, Kdf1 is expressed both in epidermal progenitor cells where it acts to curb the rate of proliferation as well as in their progeny where it is required to block proliferation and promote differentiation. Consequently, Kdf1 mutants display both uncontrolled cell proliferation in the epidermis and failure to develop terminal fates. Our findings reveal a dual role for the novel gene Kdf1 both as a repressive signal for progenitor cell proliferation through its inhibition of p63 and a strong inductive signal for terminal differentiation through its interaction with the cell cycle regulator Stratifin.     

Epidermal insulin/IGF-1 signalling control interfollicular morphogenesis and proliferative potential through Rac activation

The lifelong self-renewal of the epidermis is driven by a progenitor cell population with high proliferative potential. To date, the upstream signals that determine this potential have remained largely elusive. Here, we find that insulin and insulin-like growth factor receptors (IR and IGF-1R) determine epidermal proliferative potential and cooperatively regulate interfollicular epidermal morphogenesis in a cell autonomous manner. Epidermal deletion of either IR or IGF-1R or both in mice progressively decreased epidermal thickness without affecting differentiation or apoptosis. Proliferation was temporarily reduced at E17.5 in the absence of IGF-1R but not IR. In contrast, clonogenic capacity was impaired in both IR- and IGF-1R-deficient primary keratinocytes, concomitant with an in vivo loss of keratin 15. Together with a reduction in label-retaining cells in the interfollicular epidermis, this suggests that IR/IGF-1R regulate progenitor cells. The expression of dominant active Rac rescued clonogenic potential of IR/IGF-1R-negative keratinocytes and reversed epidermal thinning in vivo. Our results identify the small GTPase Rac as a key target of epidermal IR/IGF-1R signalling crucial for proliferative potential and interfollicular morphogenesis.     

Jarid2 regulates mouse epidermal stem cell activation and differentiation

Jarid2 is required for the genomic recruitment of the polycomb repressive complex-2 (PRC2) in embryonic stem cells. However, its specific role during late development and adult tissues remains largely uncharacterized. Here, we show that deletion of Jarid2 in mouse epidermis reduces the proliferation and potentiates the differentiation of postnatal epidermal progenitors, without affecting epidermal development. In neonatal epidermis, Jarid2 deficiency reduces H3K27 trimethylation, a chromatin repressive mark, in epidermal differentiation genes previously shown to be targets of the PRC2. However, in adult epidermis Jarid2 depletion does not affect interfollicular epidermal differentiation but results in delayed hair follicle (HF) cycling as a consequence of decreased proliferation of HF stem cells and their progeny. We conclude that Jarid2 is required for the scheduled proliferation of epidermal stem and progenitor cells necessary to maintain epidermal homeostasis.