Heart tube morphogenesis is regulated by segment-specific gene regulatory networks controlled by MEF2C  [snRNA-Seq]
The transcription factor MEF2C plays a critical role in the development of the linear heart tube, but the specific transcriptional networks controlled by MEF2C remain largely undefined. To address this, we performed combined single-nucleus RNA- and ATAC- sequencing on wild type and MEF2C-null embryos at distinct stages of development. We identified a broadly “posteriorized” cardiac gene signature and chromatin landscape throughout the heart tube in the absence of MEF2C. By integrating our gene expression and chromatin accessibility data in a deep-learning based model, we were able to construct developmental trajectories for each of the outflow tract, ventricular, and inflow tract lineages and determined how each of these segment-specific trajectories were distinctly altered in the MEF2C-null embryos. We computationally identified potential segment-specific MEF2C-dependent enhancers. Using transgenesis in zebrafish embryos, we identified novel MEF2C-dependent enhancers with activity in the developing heart tube from among these candidates. Finally, using inferred gene regulatory networks we discovered a genetic interaction between MEF2C and the atrial nuclear hormone receptor NR2F2, revealing that the MEF2C-null heart malformations are partly driven by a transcriptional network with increased NR2F2 activity. These studies not only provide a rich description of the genomic regulation of early heart tube development, but provide a generalizable framework for using genetic mutants to dissect the transcriptional networks that govern developmental processes. Overall design: We collected WT and MEF2C KO mouse embryos at E7.75, E8.5, and E9. At each developmental stage we collected two replicate WT and two replicate MEF2C KO embryos. Embryos were processed through the 10x Multiome kit to produce snRNA-seq and snATAC-seq libraries. Each sample in the dataset corresponds to an individual embryo. This dataset contains the snRNA-seq data from that experiment.