Significance Robust and predictive in vitro models of human cardiac tissue function could have transformative impact on our ability to test new drugs and understand cardiac disease. Despite significant effort, the generation of high-fidelity adult-like human cardiac tissue analogs remains challenging. In this paper, we systematically explore the design criteria for pluripotent stem cell-derived engineered cardiac tissue. Parameters such as biomechanical stress during tissue remodeling, input-cell composition, electrical stimulation, and tissue geometry are evaluated. Our results suggest that a specified combination of a 3D matrix-based microenvironment, uniaxial mechanical stress, and mixtures of cardiomyocytes and fibroblasts improves the performance and maturation state of in vitro engineered cardiac tissue.

The epicardium supports cardiomyocyte proliferation early in development and provides fibroblasts and vascular smooth muscle cells to the developing heart. The epicardium has been shown to play an important role during tissue remodeling after cardiac injury, making access to this cell lineage necessary for the study of regenerative medicine. Here we describe the generation of epicardial lineage cells from human pluripotent stem cells by stage-specific activation of the BMP and WNT signaling pathways. These cells display morphological characteristics and express markers of the epicardial lineage, including the transcription factors WT1 and TBX18 and the retinoic acid–producing enzyme ALDH1A2. When induced to undergo epicardial-tomesenchymal transition, the cells give rise to populations that display characteristics of the fibroblast and vascular smooth muscle lineages. These findings identify BMP and WNT as key regulators of the epicardial lineage in vitro and provide a model for investigating epicardial function in human development and disease.

Targeting of activated plasma membrane receptors to endocytic pathways is important in determining the outcome of growth factor signaling. However, the molecular mechanisms are still poorly understood. Here, we show that the synaptotagmin-related membrane protein E-Syt2 is essential for rapid endocytosis of the activated FGF receptor and for functional signal transduction during Xenopus development. E-Syt2 depletion prevents an early phase of activated FGF receptor endocytosis that we show is required for ERK activation and the induction of the mesoderm. E-Syt2 interacts selectively with the activated FGF receptor and with Adaptin-2, and is required upstream of Ras activation and of receptor autophosphorylation for ERK activation and the induction of the mesodermal marker Xbra. The data identify E-Syt2 as an endocytic adaptor for the clathrin-mediated pathway whose function is conserved in human and suggest a broader role for the E-Syt subfamily in growth factor signaling.

Highlights d Endocardial cells can be generated from hPSC-derived cardiovascular mesoderm d BMP10 is required for specification and maintenance of human endocardial cells d Human endocardial cells can induce a trabecular fate in hPSC-derived cardiomyocytes d hPSC-derived endocardial cells can generate valvular interstitial-like cells