Early Xenopus development:a story of two signalingcascadesWnt signal transduction pathwayBMP/TGF-! signal transductionpathway TGF-!What is similar in the way BMP andWnt signaling pathways change theirprotein targets, Smads and !-catenin,respectively?What is different?Name:Student ID#2.1 Developmental history of a frog2.2 Early development of the frogXenopus laevis (Part 1)2.2 Early development of the frogXenopus laevis (Part 2)Fate maps of the Xenopus (frog) blastulahttp://www.xenbase.org/anatomy/static/xenbasefate.jsphttp://www.xenbase.org/xenbase/original/atlas/movies/gastmov.qt10.19 Organization of a secondaryaxis by dorsal blastopore lip tissue(Part 2)Inward movement of the bottle cells forms ablastopore and starts gastrulation in the frogMid-gastrulation in the frog: Archenteronformation, displacement of the blastocoel, andepiboly of ectodermal cellsThree signaling events:• Endoderm => mesoderm (Nodal-relatedfactors)• Organizer =>dorsal structures (BMPantagonists)• Anterior endomesoderm => brain (Wntantagonists)Part I: Endoderm induces mesoderm10.26 ! -catenin, Vg-1 and VegT act synergisticallyto activate the expression of TGF-!-like factors(Xnr’s)Nieuwkoopcenter10.26 The Organizer is formed by theinteraction of !-catenin and TGF-! proteinsEndodermMesodermPart II: Organizer induces dorsalstructures (neural plate, notochord)10.31 Localization of chordin mRNA10.29 Rescue of dorsal structures byNoggin proteinNodal-related factors promote theformation of the Organizer. BMPfactors antagonize its action. Both actvia TGF-! pathway. How can they havesuch different effects?Name:Student ID#Part III: Pharyngeal endoderm and headmesoderm induce the brain10.37 Cerberus mRNA injected into a singleD4 blastomere of a 32-cell Xenopus embryoinduces head structuresFrzb is a small, soluble Wnt receptorthat inhibits Wnt signaling bysequestering WntTake home messages• Cells acquire their fates through interactionswith other cells, via signal transductionpathways• Different amounts of signaling can lead todifferent outcomes (high TGF-! signal =>dorsal mesoderm, low => ventral mesoderm)• Inhibition of a signal can be as important asthe signal itself (organizer inhibits