Role of aPKCl in epidermal morphogenesis and homeostasis

Cell polarity, defined as the asymmetric distribution of proteins, lipids and structural elements within a cell, is crucial for tissue homeostasis by regulating a variety of functions such as barrier formation, cell fate, migration and cell shape. The overall aim of this study was to examine how polarity pathways regulate epidermal morphogenesis and homeostasis and more specifically, the balance between differentiation and proliferation in a self-renewing stratified epithelium. Atypical protein kinase C (aPKC) is a key player in the regulation of polarity, although most of its functions have been studied in lower organisms or in mammalian simple epithelial cells. In mammals, two aPKC isoforms have been identified, aPKCλ and aPKCζ. To elucidate the role of aPKCλ in the stratifying epithelium of mice, aPKCλ was deleted specifically in all epidermal lineages (aPKCλepi-/- mice). Loss of aPKCλ resulted in a progressive phenotype characterized by a hyperthickened interfollicular epidermis (IFE), enlarged sebaceous glands and deformed hair follicles (HFs). We observed increased differentiation of sebaceous glands and IFE whereas HF differentiation was perturbed after aPKCλ deletion, suggesting cell fate changes within the epidermal lineage. aPKCλepi-/- HFs increasingly failed to enter resting phases of the hair cycle, which was associated with increased proliferation and a loss of quiescence in HF bulge stem cells. Concomitantly with inappropriate HF stem cell activation, immunohistochemical and FACS analyses revealed a gradual loss of bulge stem cell marker expression, indicating a depletion of bulge stem cells over time in aPKCλepi-/- mice. This was accompanied by a gradually increased number of cells expressing markers of more committed progenitors suggesting altered cellular fate of aPKCλ-/- HF stem cells. The gradual loss of stem cell function was confirmed in vitro by a loss of proliferative potential of adult keratinocytes. Moreover, aged mice showed complete alopecia and a premature aging phenotype indicating exhaustion of stem cells. Together, these data demonstrate that loss of aPKCλ exhausts the bulge stem cell compartment over time. Cellular analyses revealed that epidermal deletion of aPKCλ increased the ratio of asymmetric versus symmetric divisions in the IFE as well as HFs providing a mechanistic explanation for how aPKC regulates epidermal cell fate decisions. On the molecular level this was linked to altered Rac1 activation and changes in the expression and in vitro localization of the cell fate protein Numb. Moreover, loss of aPKCλ increased Wnt/β-catenin signaling in keratinocytes, which provides a potential link to altered HF cycling. In summary, this work revealed that aPKCλ is crucial for epidermal homeostasis and HF stem cell maintenance and demonstrates a key role for aPKCλ in the regulation of epidermal cell fate decisions, likely by controlling the balance of asymmetric versus symmetric cell divisions