In metazoans, transition from fetal to adult heart is accompanied by a switch in energy metabolism-glycolysis to fatty acid oxidation. The molecular factors regulating this metabolic switch remain largely unexplored. We first demonstrate that the molecular signatures in 1-year (y) matured human embryonic stem cell-derived cardiomyocytes (hESC-CMs) are similar to those seen in in vivo-derived mature cardiac tissues, thus making them an excellent model to study human cardiac maturation. We further show that let-7 is the most highly up-regulated microRNA (miRNA) family during in vitro human cardiac maturation. Gain-and loss-of-function analyses of let-7g in hESC-CMs demonstrate it is both required and sufficient for maturation, but not for early differentiation of CMs. Overexpression of let-7 family members in hESC-CMs enhances cell size, sarcomere length, force of contraction, and respiratory capacity. Interestingly, large-scale expression data, target analysis, and metabolic flux assays suggest this let-7-driven CM maturation could be a result of down-regulation of the phosphoinositide 3 kinase (PI3K)/AKT protein kinase/insulin pathway and an up-regulation of fatty acid metabolism. These results indicate let-7 is an important mediator in augmenting metabolic energetics in maturing CMs. Promoting maturation of hESC-CMs with let-7 overexpression will be highly significant for basic and applied research.everal coronary heart diseases (CHDs) are characterized by cardiac dysfunctions predominantly manifested during cardiac maturation (1, 2). Dramatic changes in energy metabolism occur during this postnatal cardiac maturation (3). At early embryonic development, glycolysis is a major source of energy for cardiomyocytes (CMs) (4, 5). However, as the cardiomyocytes mature, mitochondrial oxidative metabolism increases with fatty acid oxidation, providing 90% of the heart's energy demands (6-8). This switch in cardiac metabolism has been shown to have important implications during in vivo cardiac maturation (9). In contrast to the relatively advanced knowledge of the genetic network that contributes to heart development during embryogenesis (10, 11), molecular factors that regulate peri-and postnatal cardiac maturation, particularly in relation to the metabolic switch, remain largely unclear. So far, studies to understand the transition of the glycolysisdependent fetal heart to oxidative metabolism in the adult heart have been mostly related to the peroxisome proliferatoractivated receptor (PPAR)/estrogen-related receptor/PPARγ coactivator-1α circuit (7,8,12). However, it is currently unknown what other factors act upstream or in synergy with this pathway in controlling cardiac energetics.miRNAs have emerged as key factors in controlling the complex regulatory network in a developing heart (13). Genetic studies that enrich or deplete miRNAs in specific cardiac tissue types and large-scale gene expression studies have demonstrated that they achieve such complex control at the level of cardiac gene expression (14-16). We sou...