Trajectory mapping of human embryonic stem cell cardiogenesis reveals lineage branch points and an ISL1 progenitor‐derived cardiac fibroblast lineage

A family of multipotent heart progenitors plays a central role in the generation of diverse myogenic and nonmyogenic lineages in the heart. Cardiac progenitors in particular play a significant role in lineages involved in disease, and have also emerged to be a strong therapeutic candidate. Based on this premise, we aimed to deeply characterize the progenitor stage of cardiac differentiation at a single-cell resolution. Integrated comparison with an embryonic 5-week human heart transcriptomic data set validated lineage identities with their late stage in vitro counterparts, highlighting the relevance of an in vitro differentiation for progenitors that are developmentally too early to be accessed in vivo. We utilized trajectory mapping to elucidate progenitor lineage branching points, which are supported by RNA velocity. Nonmyogenic populations, including cardiac fibroblast-like cells and endoderm, were found, and we identified TGFBI as a candidate marker for human cardiac fibroblasts in vivo and in vitro. Both myogenic and nonmyogenic populations express ISL1, and its loss redirected myogenic progenitors into a neural-like fate. Our study provides important insights into processes during early heart development. © AlphaMed Press 2020 SIGNIFICANCE STATEMENT: Despite the large interest in defining the transcriptional profiles of human cardiac cells in single-cell resolution, the transient stages of early human cardiac progenitors have remained elusive. This article defines the differentiation and lineage branching points of human embryonic stem cells into cardiomyocytes, endoderm, and cardiac fibroblast-like cells, and elucidates the role of ISL1 in the development of these cell types. Genetic perturbation studies found that ISL1 may be required to suppress a neural-like cell fate in myogenic progenitors.