Lang et al. studied a mechanism by which stem cells can be pluripotent but not totipotent, that is, they can be partially committed to a cell fate but remain undifferentiated. The authors noted that Pax3-positive cells were present in skin cells that also express the melanin biosynthetic enzyme dopachrome tautomerase (Dct). These cells were determined to be melanocyte stem cells, and further analysis indicated that not all of the Pax3-positive cells were also positive for Dct, suggesting that Pax3 may be involved in differentiation. Indeed, Pax3, which is known to stimulate the expression of the gene encoding the transcription factor Mitf, was shown to inhibit expression from a Dct reporter at the same concentration that activated a Mitf reporter. Mitf and Pax3 competed for binding to the Dct enhancer in chromatin immunoprecipitation (ChIP) assays. Pax3 has a binding site for the LEF/TCF family of transcription factors, which are regulated by Wnt signaling, and transfection of active β-catenin into cells expressing Pax3 and the Dct reporter abolished the repressor activity of Pax3 and displaced Pax3 from the Dct enhancer. In vitro, Pax3, Lef1, and the repressor Grg4 formed a complex and the addition of β-catenin blocked the interaction of Pax3 and Grg4 with Lef1. Confirmation that β-catenin signaling was required in vivo for Dct expression came from β-catenin knockout mouse embryos in which Pax3- and Mitf-positive skin cells were present but Dct was absent. Thus, Pax3 appears to serve to commit the cells to a melanocyte lineage through the stimulation of Mitf expression but prevents full differentiation in the absence of Wnt signaling by competing for downstream gene enhancers such as that in the Dct gene.
D. Lang, M. M. Lu, L. Huang, K. A. Engleka, M. Zhang, E. Y. Chu, S. Lipner, A. Skoultchi, S. E. Millar, J. A. Epstein, Pax3 functions at a nodal point in melanocyte stem cell differentiation. Nature 433, 884-887 (2005). [PubMed]