A study of Xlim1 function in the Spemann-Mangold organizer

The Spemann-Mangold organizer is required in amphibian embryos to coordinat e cell fate specification, differentiation of dorsal cell types and morphog enetic movements at early stages of development. A great number of genes ar e specifically expressed within the organizer, most of them encoding secret ed proteins and transcription factors. The challenge is now to uncover gene tic cascades and networks of interactions between these genes, in order to understand how the organizer functions. The task is immense and requires lo ss-of-function approaches to test the requirement for a given factor in a s pecific process. For transcription factors, it is possible to generate inhi bitory molecules by fusing the DNA binding region to a repressor or activat or domain, which should in principle antagonize the activity of the endogen ous protein at the level of the DNA targets. We used this strategy to desig n activated and inhibitory forms of the LIM homeodomain transcription facto r Lim1,which is encoded by an organizer gene involved in head development, as revealed by analyses of knockout mice. We found that Lim1 is a transcrip tional activator, and can trigger dorso-anterior development upon ventral e xpression of hyperactive forms, in which Ldb1 is fused to Lim1. Using inhib itory Lim1 fusion proteins, we found that Lim1, or genes closely related to it, is required for head formation as well as for notochord development. G o-expression experiments revealed that Lim1 is required downstream of the e arly organizer factor Siamois, first, to establish the genetic program of t he organizer and second, to mediate the action of organizer agents that are responsible for blocking ventralizing activities in the gastrula.