DNA methylation in the mouse cochlea promotes maturation of supporting cells and contributes to the failure of hair cell regeneration. DNA methylation in the mouse cochlea promotes maturation of supporting cells and contributes to the failure of hair cell regeneration

Purpose - To profile epigenetic changes in cochlear progenitor cells, hair cells, and supporting cells across postnatal development. Methods- Cochlear cell types were obtained from fluorescently labeled transgenic mice. E13.5 cochlear prosensory progenitor cells (E13.5_PG, p27Kip1-GFP+), P1 hair cells (P1_HC, Atoh1-GFP+), P1 supporting cells (P1_SC, Lfng-GFP+), and P6, P8, P21 supporting cells (P6_SC, P8_SC, P21_SC, Lfng-creER, NuTRAP lineage-traced) were FACS purified for epigenetic profiling using WGBS (DNA methylation), CUT&Tag (H3K4me1, H3K4me3, H3K27me3, CUTAC H3K4me2). DNA demethylation of hair cell-specific promoters was interrogated by comparing % mCpG profiles of transdifferentiating supporting cells (P1_SC_tdt_gfp, Lfng-creER/TdTomato+, Atoh1-GFP+) against non-transdifferentiating supporting cells (P1_SC_tdt, P6_SC_tdt, Lfng-creER/TdTomato+) purified via FACS. P1 and P8 wildtype cochleas were enzymatically dissociated and used directly for scMultiome simultaneous profiling of RNA and ATAC at the single cell level (P1_scMultiome, P8_scMultiome, 10x Genomics). For scMultiome of P70 wildtype (P70_wt_scMultiome, Lfng-CreERT2, NuTRAP) and P70 deafened (P70_deaf_scMultiome, Lfng-CreERT2, NuTRAP, Pou4f3DTR) supporting cells, mice received 100 mg/kg tamoxifen (Sigma-Aldrich T5648) at P21, 0.01 mg/kg Diptheria toxin (Sigma-Aldrich D0564) at P28, and allowed to mature to P70, at which point cochlear tissues were harvested, FACS purified for NuTRAP+ supporting cells, and inputted into a scMultiome reaction. Results- WGBS highlighted differentially methylated regions (DMR) across E13.5_PG, P1_HC, P1_SC, P6_SC, and P21_SC. Pre-established DMRs become hypermethylated in supporting cells between P1 and P21. This corresponds with increasing heterochromatin characteristics, such as loss of accessibility (CUTAC), loss of H3K4me1, and switch between H3K27me3-mediated repression to DNA methylation-mediated silencing. WGBS of P1_SC_tdt_gfp compared to P1_SC_tdt and P6_SC_tdt showed DNA demethylation of hair cell-specific DMRs, suggesting that DNA demethylation is required for transdifferentiation of supporting cells into hair cells. P1_scMultiome, P8_scMultiome, P70_wt_scMultiome, and P70_deaf_scMultiome revealed changes in chromatin accessibility associated with age, as well as from long-term deafening at the single cell level. scMultiome datasets also highlight cell type-specific enhancers active during different stages of postnatal development. Overall design: Examination of epigenetic modifications of FACS purified mouse cochlear cells at P1, P6, P8, and P21. scMultiome ATAC + Gene Expression (10x genomics) experiments on P1 and P8 wildtype, as well as P70 wildtype and long-deafened cochlear sensory epithelia.