ABSTRACT: Studies of the Xenopus organizer have laid the foundation for our understanding of the conserved signaling pathways that pattern vertebrate embryos during gastrulation. Here, we use this wealth of knowledge as leverage in the design and analysis of a genomic visualization of organizer-related gene transcription. Using ectopic expression of the two major activities of the organizer, BMP and Wnt inhibition, as well as endogenous tissues, we generate a focused set of samples that represent different aspects of organizer signaling. The genomic expression values of each sample are then measured with oligonucleotide arrays. From this data, genes regulated by organizer signaling are selected and then clustered by their patterns of regulation. A new GO biological process annotation of the Xenopus genome allows us to rapidly identify clusters that are highly enriched for known gastrula patterning genes. Within these clusters, we can predict the expression patterns of unknown genes with remarkable accuracy, leading to the discovery of new organizer-related gastrula stage expression patterns for 19 genes. Moreover, the patterns of gene response observed within these clusters allow us to parse apart the contributions of BMP and Wnt inhibition in organizer function. We find that the majority of gastrula patterning genes respond transcriptionally to these activities according to only a few stereotyped patterns, allowing us to describe suites of genes that are likely to share similar regulatory mechanisms. These suites of genes demonstrate a mechanism where BMP inhibition initiates the organizer program before gastrulation, and Wnt inhibition maintains and drives the organizer program during gastrulation. Experiment Overall Design: In order to describe and separate the genomic expression changes induced by the two main organizing activities, BMP inhibition and Wnt inhibition, we created a panel of gastrula stage ventral tissues that ectopically overexpressed one or both of these activities, and compared these samples to endogenous dorsal and ventral gastrula stage tissue. Two well-studied organizer secreted factors, Noggin and Dickkopf-1 (Dkk-1), were used to ectopically inhibit BMP and Wnt signaling, respectively. Four different overexpression mixtures were injected ventrally into 4-cell embryos: noggin and eGFP (anti-BMP); noggin, dkk-1, and eGFP (anti-BMP and anti-Wnt); dkk-1 and eGFP (anti-Wnt); and eGFP alone. eGFP mRNA was used to trace targeting. Plasmid DNA was used for noggin and dkk-1 overexpression, instead of mRNA, in order to limit ectopic expression of these genes to post-MBT stages, more closely mimicking the endogenous regulation of these genes. The ventrally injected embryos were grown to early stage 10, sorted for appropriately targeted eGFP florescence, and then bisected between the dorsal and ventral halves at either stage 10 (early gastrula) or stage 11.5 (late gastrula). For the noggin and/or dkk-1 injected embryos, only the ventral halves of the embryos were saved, eliminating endogenous organizer tissues. For the embryos injected with only eGFP, both the ventral and dorsal halves were saved, creating separate ventral and dorsal control conditions. These five conditions were each generated twice at both stage 10 and 11.5, with each set of five samples coming from a single clutch of embryos, creating twenty total tissue samples from 4 different clutches. For each batch of injections some sorted embryos were allowed to develop through tailbud stages in order to validate the phenotypes induced by our constructs. Total RNA was then isolated from the twenty tissue samples and applied to Affymetrix oligonucleotide arrays.
