Generation of Nkx2-5/CreER transgenic mice for inducible Cre expression in developing hearts.

Nkx2-5 is a homeobox-containing transcriptional regulator that serves as one of the earliest markers of cardiac lineage commitment. To study the role of Nkx2-5-expressing progenitors at specific time points in cardiac development, we have generated a novel and inducible NKX2-5 mouse line by knocking in a CreER cassette into the Nkx2-5 genomic locus, while preserving the endogenous Nkx2-5 gene to avoid haploinsufficiency. We evaluated the specificity and efficiency of CreER activity after 4-OHT injection by crossing Nkx2-5CreER/+ mice with a Rosa26tdT/+ reporter strain. Our immunohistochemistry results confirmed Cre-induced tdTomato expression specifically in cells expressing Nkx2-5. These cells were mainly cardiomyocytes and were observed in the embryonic heart as early as day 9.5. Additionally, quantitative polymerase chain reaction on postnatal hearts showed enriched expression of Nkx2-5 in isolated tdTomato-expressing cells. No tdTomato expression was observed in Nkx2-5CreER/+ ;Rosa26tdT/+ mice in the absence of 4-OHT, confirming the inducible nature of CreER activity. The Nkx2-5/CreER mouse model described in this article will serve as an invaluable tool to trace myocardial lineage and to temporally induce genetic manipulation in a selective population of cardiac progenitors during embryonic development and in the adult heart.