ABSTRACT: 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.
