Role of IGF-1R/IR signaling in epidermal development and morphogenesis

The epidermis protects the organism from external challenges and from dehydration. The establishment, maintenance and restoration of this epithelial barrier are driven by a balance between self-renewal and differentiation of interfollicular epidermal (IFE) progenitor cells. However, the upstream signals that regulate this balance are largely unknown. The major aim of this thesis was to identify and characterize the in vivo role of epidermal insulin/IGF signaling, which have been identified as key regulators of growth, metabolism and stem cell behavior in other tissues. Inactivating the insulin receptor (IR-/-), the IGF-1 receptor (IGF-1R-/-) or both (dko) specifically in the epidermis resulted in a progressive decrease in the number of suprabasal layers, first obvious at E16.5, although differentiation and apoptosis were unaltered. Instead, the phenotype was directly associated with a strong decrease in proliferative potential in vitro and altered IFE progenitor cell behavior in vivo. Furthermore, rescue experiments identified the small GTPase Rac1 as a key target of IGF-1R/IR signaling that regulates epidermal morphogenesis in vivo and proliferative potential in vitro. To address how IR/IGF-1R regulates epidermal morphogenesis, we focused on E16.5 dko embryos, when the phenotype is first obvious. Whereas no change was observed in expression of the proliferation marker Ki67, a reduced number of anaphase spindles and an increased number of metaphase spindles were found in dko mice compared to control, and in vitro in cultured keratinocytes. This indicated an arrest in the spindle check point of the cell cycle in the absence of IR/IGF-1R. This was associated with disturbed spindle formation in IGF-1R-/- metaphase cells and reduced expression of AuroraB, a key regulator of mitosis, as well as other cell cycle regulators, such as p53 and stratifin (14-3-3σ). Interestingly, the reduction in cell divisions upon loss of IR/IGF-R was predominantly on the expense of asymmetric divisions and correlated with reduced p63 expression, a known regulator of this process. Since asymmetric cell divisions have been implicated in regulating epidermal stratification, the results indicate that epidermal Insulin/IGF1 signaling couples cell cycle progression to asymmetric divisions thereby regulating the number of suprabasal layers in the IFE during morphogenesis. In short, the results in this thesis have identified epidermal IR/IGF-1R as key regulators of epidermal morphogenesis, proliferative potential, IFE progenitor cells and asymmetric cell divisions