Deletion of Mediator 1 suppresses TGFb signaling leading to changes in epidermal lineages and regeneration
Epidermal lineages and injury induced regeneration are controlled by transcriptional programs coordinating cellular signaling and epigenetic regulators, but the mechanism remains unclear. Previous studies showed that conditional deletion of the transcriptional coactivator Mediator 1 (Med1) changes epidermal lineages and accelerates wound re-epithelialization. Here, we studied a molecular mechanism by which Med1 facilitates these processes, in particular, by focusing on TGFb signaling through genome wide transcriptome analysis.  The expression of the TGF ligands (Tgfb1/b2) and their downstream target genes is decreased in both normal and wounded Med1 null skin. Med1 silencing in cultured keratinocytes likewise reduces the expression of the ligands (TGFb1/b2) and diminishes activity of TGFb signaling as shown by decreased p-Smad2/3. Silencing Med1 increases keratinocyte proliferation and migration in vitro. Epigenetic studies using chromatin immuno-precipitation and next generation DNA sequencing reveals that Med1 regulates transcription of TGFb components by forming large clusters of enhancers called super-enhancers at the regulatory regions of the TGFb ligand and SMAD3 genes. These results demonstrate that Med1 is required for the maintenance of the TGFb  signaling pathway. Finally, we show that pharmacological inhibition of TGFb signaling enhances epidermal lineages and accelerates wound re-epithelialization in skin similar to that seen in the Med1 null mice, providing new insights into epidermal regeneration. Overall design: Identification of super-enhancers by Med1 with  CTCF and active histone modification (H3K27ac) in primary epidermal keratinocytes.