In terms of human heart development, the exact role of one of the first discovered microRNAs (miRNAs), the let-7 family has not yet been fully elucidated. Established protocols to differentiate human induced pluripotent stem cells (hiPSCs) into cardiomyocytes (CMs) provide a good model to study their implication at different time windows: from hiPSCs through mesodermal cells, cardiac progenitors (Pgs), immature CMs and progressively more mature CMs.

To have an overview, we differentiated different hiPSCs lines into CMs using a modulation of Wnt signaling pathway by small molecules. All members of the let-7 family were then screened by qRT-PCR throughout 35 days. Results exhibited similar expression profiles for the all-set characterized by an early transient increase at the time of mesoderm formation. Subsequently, their expression vanished to only increase gradually, later on, during maturation of CMs.

Following those results, we proceeded to 5 days modulations studies on different cellular contexts via transfections of let-7 mimics and all family inhibitors. In the context of Pgs, let-7g mimic downregulated a number of Pgs markers (Isl1, Mef2c, Tbx5) as well as contractile proteins (cTnT, Myh6, Myh7) at the mRNA level. Antagonistically miR-1, the most robustly expressed cardiac miRNA, started to be upregulated at the time of Pgs and showed a propensity to induce those markers when transfected.

Collectively, our results indicate that let-7 family has to be repressed after mesodermal induction in order to give rise to cardiac Pgs and later on also, to allow differentiation of Pgs into CMs exhibiting contractile machinery. Very interestingly, reseeding appeared as a way to promote Pgs progression into differentiation and CMs maturation reflected by the accompanied let-7 family upregulation.