Members of the transforming growth factor- superfamily play essential roles in both the pluripotency and differentiation of embryonic stem (ES) cells. Although bone morphogenic proteins (BMPs) maintain pluripotency of undifferentiated mouse ES cells, the role of autocrine Nodal signaling is less clear. Pharmacological, molecular, and genetic methods were used to further understand the roles and potential interactions of these pathways. Treatment of undifferentiated ES cells with SB431542, a pharmacological inhibitor of Smad2 signaling, resulted in a rapid reduction of phosphorylated Smad2 and altered the expression of several putative downstream targets. Unexpectedly, inhibition of the Nodal signaling pathway resulted in enhanced BMP signaling, as assessed by Smad1/5 phosphorylation. SB431542-treated cells also demonstrated significant induction of the Id genes, which are known direct targets of BMP signaling and important factors in ES cell pluripotency. Inhibition of BMP signaling decreased the SB431542-mediated phosphorylation of Smad1/5 and induction of Id genes, suggesting that BMP signaling is necessary for some Smad2-mediated activity. Because Smad7, a known inhibitory factor to both Nodal and BMP signaling, was down-regulated following inhibition of Nodal-Smad2 signaling, the contribution of Smad7 to the cross-talk between the transforming growth factor- pathways in ES cells was examined. Biochemical manipulation of Smad7 expression, through shRNA knockdown or inducible gene expression, significantly reduced the SB431542-mediated phosphorylation of Smad1/5 and induction of the Id genes. We conclude that autocrine Nodal signaling in undifferentiated mouse ES cells modulates the vital pluripotency pathway of BMP signaling.
Mouse embryonic stem (ES)2 cells are derived from the inner cell mass of the early blastocyst. It is from the inner cell mass that the germ layers arise to produce all cell types of the adult. The capacity of ES cells to either self-renew or differentiate into cells of the three germ layers provides an excellent tool for studying early embryonic development, including self-renewal and pluripotency (1). Some of the essential factors controlling ES cell pluripotency have been identified; however, additional work is necessary to identify other functional signaling pathways that impinge upon the ES cell phenotype, to determine genes that are regulated by these pathways, and to discover how the various pathways interact. Applications of this work will yield insights into the promising use of stem cells for regenerative therapies.ES cells can be maintained as pluripotent cells when grown on mitotically inactivated embryonic fibroblasts or when grown in media supplemented with leukemia inhibitory factor (LIF (2, 3)). Additionally, under serum-free culture conditions, signaling through the bone morphogenetic protein (BMP) pathway is essential for maintaining mouse ES cell pluripotency (4, 5). The combination of LIF and BMP4 inhibits multiple differentiation-inductive signals, a capability that...