Role of p63 in epidermal development

Development of the epidermis, the outer layer of the skin formed of keratinocytes, requires the activity of the transcription factor p63, since p63-/mice are born without epidermis and consequently die at birth. The TP63 gene contains two distinct promoters, which drive the expression of two proteins, one with an amino-terminal trans-activation domain (TAp63) and one without (Np63), although their relative contribution to epidermal formation and homeostasis is not known. To address this issues, we reintroduced TAp63 and/or Np63 into p63-/mice by in vivo genetic complementation. While extremely rare patches of differentiated keratinocytes were detected in p63-/and p63-/-;TA mice, there were significant signs of epidermal basal layer formation in p63-/-;N mice. When both TAp63/Np63 were re-introduced in p63-/mice, a still greater number of patches of differentiated skin appeared. In addition, microarray analysis indicates that Np63 induces the expression of basal layer markers, while TAp63 drives the expression of upper layer markers. Indeed, Np63 directly transactivates the keratin 14 promoter; we also show that TAp63 action is mediated at the molecular level by direct transactivation of IKK. The action of p63 is not restricted to the epidermis, as the p63-/mice have an abnormally small thymus with very low numbers of thymocytes, although the expression of K5/K8 differentiation markers appears normal in thymic epithelial cells. In p63-/-;N mice, and even more in p63-/-;N;TA, but not in p63-/-;TA, the thymus was bigger, with a significantly increased cellularity in comparison to p63-/-. The data presented are consistent with a role for Np63 in controlling the expansion of epithelial cells from progenitor precursors in epidermis and thymus, to allow TAp63, acting subsequently and synergistically to Np63, to control epithelial differentiation via IKK.