Human embryonic (ESC) and induced pluripotent stem cells (iPSC) present exciting opportunities for studying development and in vitro disease modeling. However, reported variability in iPSC behavior has called their utility into question. We therefore constituted a test set of 16 iPSCs lines from 7 individuals of varying gender and health status, characterized them extensively for pluripotency, and evaluated their ability to terminally differentiate. Using standardized procedures in two independent laboratories, 13 of the iPSC lines gave rise to functional motor neurons with a range of efficiencies similar to ESCs. Although three iPSC lines were resistant to neural differentiation, early neuralization rescued their performance. Therefore, all lines in the test set passed a stringent test of differentiation capacity despite variations in expression of early pluripotency markers, transgenes and karyotype. This novel iPSC/ESC test set is a robust resource for those interested in the basic biology of stem cells and their applications.

In embryonic stem (ES) cells, a well-characterized transcriptional network promotes pluripotency and represses gene expression required for differentiation. In comparison, the transcriptional networks that promote differentiation of ES cells and the blastocyst inner cell mass are poorly understood. Here, we show that Sox17 is a transcriptional regulator of differentiation in these pluripotent cells. ES cells deficient in Sox17 fail to differentiate into extraembryonic cell types and maintain expression of pluripotency-associated transcription factors, including Oct4, Nanog, and Sox2. In contrast, forced expression of Sox17 down-regulates ES cellassociated gene expression and directly activates genes functioning in differentiation toward an extraembryonic endoderm cell fate. We show these effects of Sox17 on ES cell gene expression are mediated at least in part through a competition between Sox17 and Nanog for common DNA-binding sites. By elaborating the function of Sox17, our results provide insight into how the transcriptional network promoting ES cell self-renewal is interrupted, allowing cellular differentiation.[Keywords: Differentiation; self-renewal; transcriptional network; primitive endoderm; stem cell; Sox17] Supplemental material is available at http://www.genesdev.org.

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