Cis-regulatory basis of sister cell type divergence in the vertebrate retina

Multicellular organisms evolved via repeated functional divergence of transcriptionally related sister cell types, but the mechanisms underlying sister cell type divergence are not well understood. Here, we study a canonical pair of sister cell types, retinal photoreceptors and bipolar cells, to identify key cis-regulatory features that mediate the transcriptomic and functional differences between them. We generated open chromatin maps and transcriptome profiles of mouse ON and OFF bipolar cells isolated by fluorescence-activated cell sorting (FACS) and compared them to similar data from rod and cone photoreceptors. We found that photoreceptors and bipolar cells have divergent transcriptional profiles, and yet share remarkably similar cis-regulatory grammars. The predominant cis-regulatory motifs in the enhancers of both cell classes are K50 homeodomain binding sites, which are required for activity. These motifs are bound by two transcription factors (TFs), CRX and OTX2, which are expressed in both photoreceptors and bipolar cells. In contrast, cell type-specific open chromatin regions are distinguished by enrichment of E-box motifs in bipolar cells, and Q50 homeodomain motifs in photoreceptors. In bipolar cells, Q50 motifs are thought to mediate repression of photoreceptor-specific genes through the bipolar-specific TF VSX2. We show that converting selected K50 motifs to Q50 motifs represses reporter expression in bipolar cells, while photoreceptor expression is maintained. These findings suggest that partitioning of Q50 motifs within cell type-specific CREs was likely a critical step in the divergence of the bipolar transcriptome from that of photoreceptors, facilitating the emergence of the complex interneuronal circuitry of the vertebrate retina. Overall design: ATAC-seq of adult (6-8wk) mouse bipolar cells, including aggregate, ON, and OFF populations (two biological replicates each) purified from transgenic reporter lines via fluorescence-activated cell sorting (FACS). RNA-seq of adult (6-8wk) mouse bipolar cells, including aggregate, ON, and OFF populations (three biological replicates each) purified from transgenic reporter lines via FACS. Photoreceptor samples are re-analyzed from GSE83312.