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Summary: Contractile to synthetic phenotypic switching of smooth muscle cells (SMCs) contributes to stenosis in vascular disease and vascular transplants. To generate more contractile SMCs, we performed a high-throughput differentiation screen using a MYH11-NLuc-tdTomato human embryonic stem cell reporter cell line. We identified RepSox as a factor that promotes differentiation of MYH11-positive cells by promoting NOTCH signaling. RepSox induces SMCs to exhibit a more contractile phenotype than SMCs generated using PDGF-BB and TGF-β1, two factors previously used for SMC differentiation but which also cause intimal hyperplasia. In addition, RepSox inhibited intimal hyperplasia caused by contractile to synthetic phenotypic switching of SMCs in a rat balloon injury model. Thus, in addition to providing more contractile SMCs that could prove useful for constructing artificial blood vessels, this study suggests a strategy for identifying drugs for inhibiting intimal hyperplasia that act by driving contractile differentiation rather than inhibiting proliferation non-specifically. : Thomson, Zhang, and colleagues report a high-throughput screen that can be used for optimization of the fully defined differentiation of contractile smooth muscle cells and identification of intimal hyperplasia inhibitors. Both in vitro and in vivo evidence revealed that RepSox is better than PDGF-BB and TGF-β1 in inducing contractile phenotype of smooth muscle cells and reducing the risk of intimal hyperplasia. Keywords: pluripotent stem cells, contractile smooth muscle cells, differentiation, maturation, RepSox, NOTCH, intima hyperplasia, MYH11-NLuc-tdTomato reporter cell line, high-throughput screen, restenosis

Hemogenic endothelium (HE) has been recognized as a source of hematopoietic stem cells (HSCs) in the embryo. Access to human HE progenitors (HEPs) is essential for enabling the investigation of the molecular determinants of HSC specification. Here, we show that HEPs capable of generating definitive hematopoietic cells can be obtained from human pluripotent stem cells (hPSCs) and identified precisely by a VE-cadherin+CD73−CD235a/CD43− phenotype. This phenotype discriminates true HEPs from VE-cadherin+CD73+ non-HEPs and VE-cadherin+CD235a+CD41a− early hematopoietic cells with endothelial and FGF2-dependent hematopoietic colony-forming potential. We found that HEPs arise at the post-primitive-streak stage of differentiation directly from VE-cadherin-negative KDRbrightAPLNR+PDGFRαlow/− hematovascular mesodermal precursors (HVMPs). In contrast, hemangioblasts, which are capable of forming endothelium and primitive blood cells, originate from more immature APLNR+PDGFRα+ mesoderm. The demarcation of HEPs and HVMPs provides a platform for modeling blood development from endothelium with a goal of facilitating the generation of HSCs from hPSCs.