The mammalian heart forms early in embryogenesis, and defects in its formation are at the root of congenital
heart defects (CHDs), affecting 1–2% of live births. The gene regulatory networks that are disturbed in CHDs
have begun to be elucidated from large-scale human genetic studies. Many of the genes causing CHD are
transcriptional regulators, and several of these are also associated with genetics of adult heart disease, such as
cardiomyopathy and arrhythmias. A complete understanding of the birth of cardiac progenitors and the first steps
that lead to the formation of the embryonic heart has implications for both CHD and adult heart disease.
The origins of the heart in embryogenesis have been defined as beginning in mesoderm that arises during
gastrulation. We previously defined a specific enhancer of Smarcd3 that labels a very restricted subpopulation
of Mesp1 lineage-labeled mesoderm, which contribute almost exclusively to the heart. Embryogenesis is a highly
dynamic process, whereby complex coordinated cell movements position organ precursors for precise
morphogenesis, and control the response to potent morphogenetic signaling gradients. While aspects of dynamic
cardiac morphogenesis have been visualized in the avian embryo, our knowledge of cardiac progenitor
emergence and behaviour in the mouse embryo is lacking. We have been able to culture and image live lineage-
labeled mouse embryos, and will leverage this exciting approach to investigate mammalian cardiac development.
We hypothesize that differentiation, migration, and contribution to early heart formation of early cardiac
progenitors are regulated by a specific gene regulatory network composed of temporally-modulated DNA
regulatory elements and the coordinated function of a cascade of transcriptional regulators. We will test this
hypothesis in two Specific Aims.
Specific Aim 1: To define genome-wide the gene regulatory networks that control the emergence of early
cardiac progenitors. We will define genome-wide regulatory networks that control the specification, migration,
and differentiation of early cardiac precursor using transgenic reporter lines combined with single cell RNA-seq
and single cell ATACseq (for chromatin accessibility) in normal and mutant embryonic heart development
Specific Aim 2. To visualize the birth and migration of early cardiac progenitors with live embryo imaging.
In this aim, we will use live embryo light sheet microscopic imaging of lineage-labeled mouse embryos to
visualize in 4D the live dynamic emergence of cardiac progenitors, their migration, and their collective behaviours
during early cardiac morphogenesis. We will further image in live embryos in 4D early cardiac progenitors in the
context of impaired migration (Mesp1 knockout embryos) and defective differentiation of first or second heart
fields (Tbx5 and Mef2c knockout embryos).
From transcription factor footprints in the genome, to transcriptomes, to live cells in vivo, our project will elucidate
at the single cell level the molecular nature of the earliest cardiac precursors.