Fig. 4: Compromised epidermal integrity in Grhl3^–/– skin is improved through application of 5ASA.

Back skins from E16.5 WT and Grhl3^–/– embryo were cultured together with vehicle control (a, c) or in the presence of 1 mM 5ASA (b, d). No significant changes in WT epidermal morphology were observed following 5ASA treatment of WT skin, however, treatment of the Grhl3^–/– skin (c, d) restored granular layer formation. The epidermal differentiation marker, K10, was also specifically increased in Grhl3^–/– skins following 5ASA treatment (e-h), and expression of the structural barrier integrity protein PPL (i-l) was restored. 5ASA did not affect the skin from WT animals (m, n) but reduced the expression of TARC in Grhl3^–/– skin (o, p). The treatment of E16.5 WT skin with recombinant TARC demonstrates expansion of the basal layer Keratin 14-positive compartment and reduced keratinocyte compaction towards the granular layer (q-v). The granular layer is less pronounced in H&Es (w, x). Scale bars correspond to 100 μm. These data are summarised in the proposed functional model (y) suggesting that loss of GRHL3 in differentiated cells leads to a compromised epidermal barrier with concomitant TARC production and secretion to stimulate proliferation of basal keratinocytes. This triggers additional TARC expression from basal cells amplifying the proliferative signal