An Epidermal Barrier Wound Repair Pathway in Drosophila Is Mediated by grainy head

Abstract

We used wounded Drosophila embryos to define an evolutionarily conserved pathway for repairing the epidermal surface barrier. This pathway includes a wound response enhancer from the Ddc gene that requires grainy head (grh) function and binding sites for the Grh transcription factor. At the signaling level, tyrosine kinase and extracellular signal-regulated kinase (ERK) activities are induced in epidermal cells near wounds, and activated ERK is required for a robust wound response. The conservation of this Grh-dependent pathway suggests that the repair of insect cuticle and mammal skin is controlled by an ancient, shared control system for constructing and healing the animal body surface barrier.

Phosphorylation of Grainy head by ERK is essential for wound-dependent regeneration but not for development of an epidermal barrier

“…The barrier repair defects presumably results from a reduction of transcription from a variety of GRH-target genes, one of which is Ddc (4). The MAP kinase ERK is strongly activated in epidermal cells surrounding wounded sites, and ERK inhibition silences GRHmediated Ddc expression around epidermal wounds (4). From this result, we inferred that ERK is in the signaling pathway from injury to activation of GRH-dependent wound enhancers.…”

“…In both Drosophila and mouse, transcription factors of the Grainy head (GRH) family have been shown to be essential for the development of epithelial barriers as well as the repair of barriers after wounding (3)(4)(5)(6)(7)(8). There is also evidence from studies in Caenorhabditis elegans, Xenopus laevis, and Danio rerio indicating that GRH proteins have an evolutionarily conserved role in the development and maintenance of epidermal barrier structure (9)(10)(11).…”

“…GRH target genes such as Ddc and misshapen (msn) are rapidly transcribed around epidermal wounds where ERK is activated (4,26). Because ERK is also activated in cells surrounding wound sites (4), we hypothesized that phosphorylation of GRH by ERK might be required for activation of epidermal wound response genes.…”

“…Ddc is required to produce the quinones that cross-link the specialized apical extracellular matrix molecules that make up the cuticle (4,6). GRH also is required for normal expression levels of Fasciclin 3 and Coracle proteins, which are directly involved in mediating epidermal cell adhesion (4,6,14). A mutation of human Grhl2 results in progressive hereditary deafness, which is presumably due to defective epithelia in the cochlea, where Grhl2 is abundantly expressed (15).…”

“…Analogous to the role of Grhl1 and Grhl3 proteins in mouse, GRH protein in Drosophila activates genes such as Dopa decarboxylase (Ddc). Ddc is required to produce the quinones that cross-link the specialized apical extracellular matrix molecules that make up the cuticle (4,6). GRH also is required for normal expression levels of Fasciclin 3 and Coracle proteins, which are directly involved in mediating epidermal cell adhesion (4,6,14).…”