| Human embryonic stem (hES) cells offer developmental biologists a new tool for understanding the molecular mechanisms underlying human development. Using hES cells we have developed a model of human neural specification that is chemically defined, and temporally and morphologically consistent with in vivo development. Using this model system I found that hES cells are efficiently converted to neuroepithelial cells in the absence of Bone Morphogenetic Protein (BMP) antagonists. Further, when these cells are exposed to high concentrations of exogenous BMP4, a large percentage of differentiating hES cells still adopted a neural fate. I established that an intrinsic program of BMP signaling inhibition, employed by differentiating hES cells, explained this paradoxical finding. This intrinsic program was found to inhibit BMP signaling at multiple levels of the signaling cascade; extracellularly through increased expression of secreted BMP antagonists, in the cytoplasm by retention of the BMP activated transcription factor Smad1, and in the nucleus through increased expression of transcriptional repressors of BMP gene activation. I demonstrated that the intrinsic program of neural specification used by differentiating hES extended to the FGF signaling pathway as well. FGF signaling was found to be required early in the process of neural specification of hES cells, but was down regulated at later stages of the process. I also discovered that, unlike in developing Xenopus, the opposing effects of these two signaling pathways did not converge to regulate neural specification through the differential phosphorylation of Smad1, but acted independently. These results provide a clear rationale for studying fundamental developmental processes in a human system. |
