The minimal ability of the adult human heart to regenerate lost or damaged cardiomyocytes has led to an intense effort to direct human embryonic stem cells (hESCs) and, more recently, human-induced pluripotent stem cells (hIPSCs) to form cardiomyocytes in order to model human heart disease and develop therapies [31
]. The hESC-derived cardiomyocytes resemble immature human fetal cardiomyocytes by multiple criteria, including electrophysiology [1
], calcium handling [1
], force generation [12
], contractile protein expression, and myofibrillar structure [25
], and we find that cardiomyocytes from hIPSCs appear similar [19
]. Because hESC-derived cardiomyocytes have the potential to engraft into surgical models of heart disease [26
], they have been considered for cardiomyocyte replacement therapy and they might also shed light on regeneration from endogenous stem cell populations in the heart. Despite these encouraging advances, the use of hESC-derived cardiomyocytes for basic developmental research and large-scale applications, such as high-throughput screening, toxicology testing, and large animal studies, has been hindered by their poor yield from typically heterogeneous stem cell cultures.
In this review, we describe the developmental progression from a pluripotent hESC state to cardiomyocyte and discuss the involvement of the signaling pathways that direct the embryonic heart formation. The tight orchestration of diffusible signaling molecules and intracellular mediators that drive progression from an initial cardiac field to a functional heart tube during embryogenesis involves numerous signal transduction proteins and transcription factors that are well conserved across vertebrate and even in some invertebrate species. Judicious testing of naturally occurring, diffusible factors for stimulation of cardiomyogenesis in hESCs has led to optimized, defined conditions for production of cardiomyocytes [26
]. Although such advances quantitatively improved the proportion of the cells that differentiate into cardiomyocytes, in most settings the percentage of cardiomyocytes in a final culture still remains less than 10%. A better understanding of the signaling pathways that control differentiation would provide the key insights that are needed to develop reagents and regimens for enhanced differentiation.
Chemical biology offers one means of discovering novel cellular signaling molecules that mediate stem cell cardiogenesis. The development of phenotypic assays that identify compounds based on the ability to stimulate stem cell cardiomyogenesis, coupled with recent advances in image cytometry and the automation of complex cell-based assays, makes it possible to screen chemical libraries in a moderately high-throughput mode. We describe the current state of assay development, screening, and the challenges inherent with identification of cellular targets along with potential strategies for target identification.