A COUP-TFII Human Embryonic Stem Cell Reporter Line to Identify and Select Atrial Cardiomyocytes

Reporter cell lines have already proven valuable in identifying, tracking, and purifying cardiac subtypes and progenitors during differentiation of human pluripotent stem cells (hPSCs). We previously showed that chick ovalbumin upstream promoter transcription factor II (COUP-TFII) is highly enriched in human atrial cardiomyocytes (CMs), but not ventricular. Here, we targeted mCherry to the COUP-TFII genomic locus in hPSCs expressing GFP from the NKX2.5 locus. This dual atrial NKX2.5^EGFP/+-COUP-TFII^mCherry/+ reporter line allowed identification and selection of GFP⁺ (G⁺)/mCherry⁺ (M⁺) CMs following cardiac differentiation. These cells exhibited transcriptional and functional properties of atrial CMs, whereas G⁺/M⁻ CMs displayed ventricular characteristics. Via CRISPR/Cas9-mediated knockout, we demonstrated that COUP-TFII is not required for atrial specification in hPSCs. This new tool allowed selection of human atrial and ventricular CMs from mixed populations, of relevance for studying cardiac specification, developing human atrial disease models, and examining distinct effects of drugs on the atrium versus ventricle. In this article, Passier and colleagues developed an atrial fluorescent stem cell reporter by CRISPR/Cas9-mediated knockin of mCherry at the genomic locus of COUP-TFII in a cardiac NKX2.5^EGFP/w reporter line. As validated at the transcriptional and functional level, the nuclear receptor COUP-TFII successfully marks atrial cardiomyocytes but is not required for atrial specification during in vitro differentiation of hPSCs.