Enhancer-promoter interactions can form independently of genomic distance and be functional across TAD boundaries.

Topologically Associating Domains (TADs) have been suggested to facilitate and constrain enhancer-promoter interactions. However, the role of TAD boundaries in effectively restricting these interactions remains unclear. Here, we show that a significant proportion of enhancer-promoter interactions are established across TAD boundaries in Drosophila embryos, but that developmental genes are strikingly enriched in intra- but not inter-TAD interactions. We pursued this observation using the twist locus, a master regulator of mesoderm development, and systematically relocated one of its enhancers to various genomic locations. While this developmental gene can establish inter-TAD interactions with its enhancer, the functionality of these interactions remains limited, highlighting the existence of topological constraints. Furthermore, contrary to intra-TAD interactions, the formation of inter-TAD enhancer-promoter interactions is not solely driven by genomic distance, with distal interactions sometimes favored over proximal ones. These observations suggest that other general mechanisms must exist to establish and maintain specific enhancer-promoter interactions across large distances.

Combinatorial transcription factor activities on open chromatin induce embryonic heterogeneity in vertebrates.

During vertebrate gastrulation, mesoderm is induced in pluripotent cells, concomitant with dorsal-ventral patterning and establishing of the dorsal axis. We applied single-cell chromatin accessibility and transcriptome analyses to explore the emergence of cellular heterogeneity during gastrulation in Xenopus tropicalis. Transcriptionally inactive lineage-restricted genes exhibit relatively open chromatin in animal caps, whereas chromatin accessibility in dorsal marginal zone cells more closely reflects transcriptional activity. We characterized single-cell trajectories and identified head and trunk organizer cell clusters in early gastrulae. By integrating chromatin accessibility and transcriptome data, we inferred the activity of transcription factors in single-cell clusters and tested the activity of organizer-expressed transcription factors in animal caps, alone or in combination. The expression profile induced by a combination of Foxb1 and Eomes most closely resembles that observed in the head organizer. Genes induced by Eomes, Otx2, or the Irx3-Otx2 combination are enriched for maternally regulated H3K4me3 modifications, whereas Lhx8-induced genes are marked more frequently by zygotically controlled H3K4me3. Taken together, our results show that transcription factors cooperate in a combinatorial fashion in generally open chromatin to orchestrate zygotic gene expression.