show Abstracthide AbstractThe role of POU4F3 in Merkel cell maturation, but not induction, is similar to its role in hair cells. This prompted us to investigate whether POU4F3 also regulates the accessibility of ATOH1 targets in a feed-forward manner during the maturation of Merkel cells. Our results suggest that, just as in hair cells, the pioneer factor activity of POU4F3 is also required for ATOH1 to correctly coordinate mechanosensory differentiation in Merkel cells. Overall design: Epithelial progenitors surrounding Merkel cells in touch domes in E17.5 epidermis can be isolated by their combinatory expression pattern of Integrin-a6, Cd34, Sca-1, and CD200 (Doucet et al., 2013), whereas Merkel cells can be isolated by their expression of ATOH1-GFP. We used ATOH1-GFP mice and ATOH1-GFP; Pou4f3 mutant mice to purify touch dome epithelial progenitors and Merkel cells from wild type and Pou4f3-mutant epidermis. We analyzed accessible chromatin with µATACseq and identified elements binding ATOH1 with CUT&RUN. We identified 4,683 ATOH1 target elements that were accessible in nascent Merkel cells, but not epithelial progenitors. We found that the accessibility of 710 of these 4,683 de novo elements was dependent on POU4F3. Many of these POU4F3-dependent de novo ATOH1-target elements were near genes critical for Merkel cell maturation and mechanotransduction, such as Krt20, Rab3c (Haeberle et al., 2004; Perdigoto et al., 2014), and the light-touch mechanotransduction channel Piezo2 (Ikeda et al., 2014; Maksimovic et al., 2014; Ranade et al., 2014; Woo et al., 2014). RNAseq confirmed that expression of Piezo2, Rab3c and Krt20 was significantly reduced in Pou4f3 mutant Merkel cells (FDR < 0.1).