Rationale The developing heart requires both mechanical load and vascularization to reach its proper size, yet the regulation of human heart growth by these processes is poorly understood. Objective We seek to elucidate the responses of immature human myocardium to mechanical load and vascularization using tissue engineering approaches. Methods and Results Using human embryonic stem cell and human induced pluripotent stem cell-derived cardiomyocytes in a three dimensional collagen matrix, we show that uniaxial mechanical stress conditioning promotes 2-fold increases in cardiomyocyte and matrix fiber alignment and enhances myofibrillogenesis and sarcomeric banding. Furthermore, cyclic strain markedly increases cardiomyocyte hypertrophy (2.2-fold) and proliferation rates (21%) vs. unstrained constructs. Addition of endothelial cells enhances cardiomyocyte proliferation under all stress conditions (14% to 19%), and addition of stromal supporting cells enhances formation of vessel-like structures by ~10-fold. Furthermore, these optimized human cardiac tissue constructs generate Starling curves, increasing their active force in response to increased resting length. When transplanted onto hearts of athymic rats, the human myocardium survives and forms grafts closely apposed to host myocardium. The grafts contain human microvessels that are perfused by the host coronary circulation. Conclusions Our results indicate that both mechanical load and vascular cell co-culture control cardiomyocyte proliferation, and that mechanical load further controls the hypertrophy and architecture of engineered human myocardium. Such constructs may be useful for studying human cardiac development as well as for regenerative therapy.

Background Tissue engineering enables the generation of functional human cardiac tissue using cells derived in vitro in combination with biocompatible materials. Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes provide a cell source for cardiac tissue engineering; however, their immaturity limits their potential applications. Here we sought to study the effect of mechanical conditioning and electrical pacing on the maturation of hiPSC-derived cardiac tissues. Methods Cardiomyocytes derived from hiPSCs were used to generate collagen-based bioengineered human cardiac tissue. Engineered tissue constructs were subjected to different mechanical stress and electrical pacing conditions. Results The engineered human myocardium exhibits Frank-Starling-type force-length relationships. After 2 weeks of static stress conditioning, the engineered myocardium demonstrated increases in contractility (0.63±0.10 mN/mm2 vs 0.055±0.009 mN/mm2 for no stress), tensile stiffness, construct alignment, and cell size. Stress conditioning also increased SERCA2 expression, which correlated with a less negative force-frequency relationship. When electrical pacing was combined with static stress conditioning, the tissues showed an additional increase in force production (1.34±0.19 mN/mm2), with no change in construct alignment or cell size, suggesting maturation of excitation-contraction coupling. Supporting this notion, we found expression of RYR2 and SERCA2 further increased by combined static stress and electrical stimulation. Conclusions These studies demonstrate that electrical pacing and mechanical stimulation promote maturation of the structural, mechanical and force generation properties of hiPSC-derived cardiac tissues.

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...

Objective-Human embryonic stem cells (hESCs) offer a sustainable source of endothelial cells for therapeutic vascularization and tissue engineering, but current techniques for generating these cells remain inefficient. We endeavored to induce and isolate functional endothelial cells from differentiating hESCs. Methods and Results-To enhance endothelial cell differentiation above a baseline of Ϸ2% in embryoid body (EB) spontaneous differentiation, 3 alternate culture conditions were compared. Vascular endothelial growth factor (VEGF) treatment of EBs showed the best induction, with markedly increased expression of endothelial cell proteins CD31, VE-Cadherin, and von Willebrand Factor, but not the hematopoietic cell marker CD45. CD31 expression peaked around days 10 to 14. Continuous VEGF treatment resulted in a 4-to 5-fold enrichment of CD31 ϩ cells but did not increase endothelial proliferation rates, suggesting a primary effect on differentiation. CD31 ϩ cells purified from differentiating EBs upregulated ICAM-1 and VCAM-1 in response to TNF␣, confirming their ability to function as endothelial cells. These cells also expressed multiple endothelial genes and formed lumenized vessels when seeded onto porous poly(2-hydroxyethyl methacrylate) scaffolds and implanted in vivo subcutaneously in athymic rats. Collagen gel constructs containing hESC-derived endothelial cells and implanted into infarcted nude rat hearts formed robust networks of patent vessels filled with host blood cells. A persisting challenge to the application of cell-based therapies is the sourcing of specific cells of interest. Because many mature tissues cannot be rebuilt using adult cells derived from biopsies, human embryonic stem cells (hESCs) could be instrumental in regenerative tissue engineering. Their immense proliferative and differentiation potential could provide extensive banks of cells-in quantity as well as type-for therapeutic applications. Conclusions-VEGFNatural and engineered tissues more than Ϸ200 m thick require suitable vascular support to survive and function properly. Although growth of host vessels into tissue engineering scaffolds has been achieved via controlled release of angiogenic molecules, this strategy requires many days to produce mature vessels. Further, host-derived vessel formation may be compromised by conditions that reduce angiogenesis such as diabetes and radiation therapy. 1 As angiogenesis is directed by a series of cytokines in a precise temporal sequence, adding 1 or even 2 cytokines may lead to incomplete blood vessel formation. Providing scaffolds with exogenous vascular cells before implantation may increase both the speed and extent of vascularization of engineered tissues. Before this approach can be successful, it will be critical to develop reliable methods to generate sufficient endothelial cells.Differentiation of hESCs in embryoid bodies treated with serum proceeds in a complex and largely uncontrolled manner. Methods to guide hESC differentiation into a specific lineage would therefore prov...