Success of human myocardial tissue engineering for cardiac repair has been limited by adverse effects of scaffold materials, necrosis at the tissue core, and poor survival after transplantation due to ischemic injury. Here, we report the development of scaffold-free prevascularized human heart tissue that survives in vivo transplantation and integrates with the host coronary circulation. Human embryonic stem cells (hESCs) were differentiated to cardiomyocytes by using activin A and BMP-4 and then placed into suspension on a rotating orbital shaker to create human cardiac tissue patches. Optimization of patch culture medium significantly increased cardiomyocyte viability in patch centers. These patches, composed only of enriched cardiomyocytes, did not survive to form significant grafts after implantation in vivo. To test the hypothesis that ischemic injury after transplantation would be attenuated by accelerated angiogenesis, we created ''second-generation,'' prevascularized, and entirely human patches from cardiomyocytes, endothelial cells (both human umbilical vein and hESC-derived endothelial cells), and fibroblasts. Functionally, vascularized patches actively contracted, could be electrically paced, and exhibited passive mechanics more similar to myocardium than patches comprising only cardiomyocytes. Implantation of these patches resulted in 10-fold larger cell grafts compared with patches composed only of cardiomyocytes. Moreover, the preformed human microvessels anastomosed with the rat host coronary circulation and delivered blood to the grafts. Thus, inclusion of vascular and stromal elements enhanced the in vitro performance of engineered human myocardium and markedly improved viability after transplantation. These studies demonstrate the importance of including vascular and stromal elements when designing human tissues for regenerative therapies.angiogenesis ͉ human embryonic stem cells ͉ tissue engineering ͉ myocardial infarction ͉ cardiomyocyte

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...

The ability to control proliferation of grafted cells in the heart and consequent graft size could dramatically improve the efficacy of cell therapies for cardiac repair. To achieve targeted graft cell proliferation, we created a chimeric receptor (F36Vfgfr-1) composed of a modified FK506-binding protein (F36V) fused with the cytoplasmic domain of the fibroblast growth factor receptor-1 (FGFR-1). We retrovirally transduced mouse C2C12 and MM14 skeletal myoblasts with this construct and treated them with AP20187, a dimeric F36V ligand ("dimerizer"), in vitro and in vivo to induce receptor dimerization. Dimerizer treatment in vitro activated the mitogen-activated protein kinase pathway and induced proliferation in myoblasts expressing F36Vfgfr-1 comparable with the effects of basic FGF. Wild-type myoblasts did not respond to dimerizer. Subcutaneous grafts composed of myoblasts expressing F36Vfgfr-1 showed a dose-dependent increase in DNA synthesis with dimerizer treatment. When myoblasts expressing F36Vfgfr-1 were injected into infarcted hearts of nude mice, dimerizer treatment resulted in a dose-dependent increase in graft size, from 20 +/- 3 to 42.9 +/- 4.3% of the left ventricle. Blinded echocardiographic analysis demonstrated that larger graft size was associated with a dose-dependent reduction in ventricular dilation after myocardial infarction, although animals with the largest grafts showed an increased incidence of ventricular tachycardia. Thus, selective proliferation of genetically modified graft cells can be induced with a systemically administered synthetic molecule in vitro or in vivo. Control of intramyocardial graft size by this approach may allow optimization of cell-based therapy to obtain desired cardiac function postinfarction.

Basic fibroblast growth factor (bFGF) is a potent angiogenic molecule, but its therapeutic use is limited by mitogenic effects on multiple cell types. To specifically activate FGF signaling in endothelial cells, a chimeric FGF receptor was generated that contained a modified FK506 drug-binding domain (F36V) fused to the FGF receptor-1 (FGFR1) cytoplasmic domain. Human umbilical vein endothelial cells (HUVECs) and human microvascular endothelial cells were retrovirally transduced with this chimeric receptor, and the effects of administering synthetic receptor-dimerizing ligands were studied. As expected, both control and transduced cells proliferated in response to bFGF treatment; however, only transduced endothelial cells exhibited dose-dependent proliferative responses to dimerizer treatment. Dimerizer-induced proliferation was MEK-dependent and was accompanied by MAP kinase phosphorylation, indicating that the chimeric receptor utilizes signaling pathways similar to endogenous FGFR1. Although bFGF stimulated wound re-epithelialization in HUVECs (which natively express FGFR1 and FGFR4), chemical dimerization of FGFR1 did not; this suggests FGFR4 may control migration in these cells. The ability to selectively activate receptor subtypes should facilitate the study of signaling pathways in vitro and in vivo beyond what can be accomplished with nonselective natural ligands, and it may eventually permit stimulation of graft cell angiogenesis without driving overgrowth of host cells.