Defining the early steps of cardiovascularlineage segregation by single cell RNA-seq

Mouse heart development arises fromMesp1 expressing cardiovascular progenitors that are specified at the early stage of gastrulation. Lineage tracing and clonal analysisof Mesp1 progenitors revealed that heart development arises from distinct populations of Mesp1+ cardiovascular progenitors (CPs) expressing Mesp1 at different time points during gastrulation that contribute to different heart regions and different cardiovascular lineages. However, it remains unclear what are the molecular mechanisms that control the early regional and lineage segregation of Mesp1 CPs. Here, we performedsingle cell RNA-sequencing of FACS isolated Mesp1 CPs in WT and Mesp1 null embryos at different times to define the cellular and molecular heterogeneity of the Mesp1CPs, identify the role of Mesp1 in regulating cellular heterogeneity and uncover the mechanisms associated with lineage and regional segregation during the early stages of gastrulation.We showed that Mesp1 CPs isolated at E6.75 and E7.25 are molecularly distinct and make the continuum betweenepiblast and later mesodermal cells including hematopoietic progenitors. Single cell transcriptome of Mesp1 deficient CPsshowed that Mesp1 is required for the exitof thepluripotent state and the induction of the cardiovascular gene expression program in vivo.Using dimensional reduction analysis, we identified distinct populations of Mesp1 CPs that correspond to progenitors committed to different celllineages and regions of the heart, identifying the molecular features associated with the early lineage and regional segregation. Notch1CREER lineage tracing, a marker preferentially expressed by one of the different Mesp1CP subpopulations, during the early stage of gastrulation marked almost exclusively ECs, demonstrating theexistence of an early Mesp1 subpopulation committed to the EC fate.This study uncoversthe cellular and molecular heterogeneity associated with early lineage restriction and regional segregation of the heart at the early stage of gastrulation. Overall design: Single cell RNA sequencing of gastrulation E6.75 and E7.25 Mesp1+ cells