The Balance of Positive and Negative Effects of TGF-Beta Signaling Regulates the Hemangioblast Development in Human Pluripotent Stem Cells

Abstract 1220

Derived from mesoderm precursors, hemangioblasts are bipotential common progenitors of hematopoietic cells and endothelial cells. The regulatory events controlling human hemangioblast development are largely unknown. Our previous study demonstrated that CD34 progenitors from human embryonic stem cells (hESCs) contain a population of cells that give rise to hematopoietic cells and endothelial cells. In this study, we established a serum-free and feeder-free system to investigate the signals that direct differentiation of human pluripotent stem cells (hPSCs), including hESCs and induced-pluripotent stem cells (hiPSCs). The hemangioblast development in hPSCs was established in three major steps: i) the formation of uniformed embryoid bodies (EBs) in hanging-drops, ii) the development of multipotential mesoderm precursors in cultivation of EBs, and iii) the generation of CD34+CD31+VE-cadherin+ progenitors, which have hemangioblast potential to give rise to both hematopoietic and endothelial cells (via hemogenic endothelium). The hESC-derived hematopoietic cells expressed CD41, CD61, CD117, CD71, and CD235a. The endothelial cells were CD31+ and VE-cadherin+ cells. We found that inhibition of Rho-associated coiled-coil forming protein serine/threonine kinase (ROCK) by Y27632 was critical for hanging-drop EB formation. Transforming growth factor beta (TGF-beta) is a key regulator of hematopoiesis with potent inhibitory effects on progenitor and stem cell proliferation. The expression of TGF-beta was increased during hPSC differentiation. Inhibition of TGF-beta signaling by the TGF-beta inhibitor, SB431542, significantly decreased the efficiency of CD34+ progenitors, suggesting a crucial role of TGF-beta signaling in hPSC differentiation. To further investigate when TGF-beta signaling affects hPSC differentiation to CD34+ progenitors, we added TGF-beta inhibitor at different stages of hPSC differentiation. We found that TGF-beta signaling is required for the early mesoderm induction. Inhibition of TGFb signaling before hemangioblast commitment during PSC differentiation significantly decreases the frequency of hemangioblasts (CD34+/CD31+/VE-cadherin+ cells) and the expression of mesoderm markers, such as Brachyury, Tie1, EOMES, and MIXL1. On the other hand, inhibition of TGF-beta signaling after mesoderm induction increases the number of CD34+/CD31+/VE-cadherin+ cells. EdU cell proliferation assays indicated that the proliferation of CD34+ progenitors were not affected by SB431542, suggesting that TGF-beta signaling suppresses hemangioblast generation. Our study demonstrated that TGF-beta signaling has the double-edged effect on hPSC differentiation to hemangioblasts.