Regulation of MAPK activation, AP-1 transcription factor expression and keratinocyte differentiation in wounded fetal skin

Fetal epithelium retains the ability to re-epithelialize a wound in organotypic culture in a manner not dependent on the presence of underlying dermal substrata. This capacity is lost late in the third trimester of gestation or after embryonic day 17 (E₁₇) in the rat such that embryonic day 19 (E₁₉) wounds do not re-epithelialize. Moreover, wounds created in E₁₇ fetuses in utero heal in a regenerative, scar-free fashion. To investigate the molecular events regulating re-epithelialization in fetal skin, the wound-induced expression profile and tissue localization of activator protein 1 (AP-1) transcription factors c-Fos and c-Jun was characterised in E₁₇ and E₁₉ skin using organotypic fetal cultures. The involvement of mitogen-activated protein kinase (MAPK) signaling in mediating wound-induced transcription factor expression and wound re-epithelialization was assessed, with the effect of wounding on the expression of keratinocyte differentiation markers determined. Our results show that expression of AP-1 transcription factors was induced immediately by wounding and localized predominantly to the epidermis in E₁₇ and E₁₉ skin, c-fos and c-jun induction was transient in E₁₇ skin with MAPK-dependent c-fos expression necessary for the re-epithelialization of an excisional wound in organotypic culture. In E₁₉ skin, AP-1 expression persisted beyond 12 h post-wounding, and marked upregulation of the keratinocyte differentiation markers keratin 10 and loricrin was observed. No such changes in the expression of keratin 10 or loricrin occurred in E₁₇ skin. These findings indicate that re-epithelialization in fetal skin is regulated by wound-induced AP-1 transcription factor expression via MAPK and the differentiation status of keratinocytes.