Vertebrate Embryonic Patterning 3 Molecular Basis of Spemann’s Organizer

Inducing Centers in the Blastula “Spemann” (General)

Fate Map of Organizer and Nieuwkoop’s Center

Molecular Nature of the Organizer Transcription Factors Diffusible Molecules –BMP inhibitors –Wnt inhibitors

Goosecoid named based on homology to Drosophila gooseberry and bicoid homeodomain expressed in the organizer – dorsal blastopore lip activates txn of other organizer-specific genes e.g. Xotx2, Xnot, Xlim1, chordin, noggin, Xnr 1, 2, 4

Goosecoid Expression Pattern Visit the Xenopus Molecular Marker Page

Targeted Injections of Xenopus Embryos

Goosecoid Induces Secondary Axes Injection of gsc into ventral vegetal blastomere at 8-16 cell stage induces a secondary axis. Both axes begin with the concentration of pigment at a blastopore lip (arrows). The 2° axis forms anterior head structures (cement gland)

BMP Inhibitors ORGANIZER MOLECULES Noggin – expressed in deep mesoderm of organizer that ultimately forms notochord Chordin – expressed in deep mesoderm & pharyngeal endoderm that ultimately underlies head Cerberus – expressed in organizer & endoderm ultimately forming head

BMP-4 Expression Pattern St vegetal view dorsal lip St. 11 vegetal view dorsal view

Noggin Expression Pattern notochord D V

Noggin Rescues UV Ventralized Embryos UV injected noggin mRNA lo hi Noggin cloned through a functional screen by Richard Harland. Protein functions by binding to BMP2 & 4 and prevents their binding to their receptors.

Chordin Expression Pattern St. 10St. 10.5St. 13 dorsal blastopore lip blastopore axial mesoderm prechordal plate

Chordin Function Chordin also binds & sequesters BMP4

BMP Gradient formed by Opposing Activities of Chordin & Xolloid

Cerberus Expression Pattern

Cerberus Duplicates Axes! Who’d a Thunk It?

Cerberus Function Cerberus binds to –Xwnt-8 (ventral mesoderm) –BMP-4 (ventral ectoderm) –Xnr-1 (dorsal mesoderm) Cerberus

Cer Double Axes Enhanced by Blocking Wnt, BMP, and Xnr Pathways

Follistatin –binds to TGF-  family ligands –blocks their interaction with receptors

Wnt Inhibitors Frisbee – Soluble protein that mimics the frizzled wnt binding domain. Dickkopf – functions by sequestering wnts Cerberus – binds to wnts

Frisbee Expression

Frzb Function

Frisbee Causes Anteriorization

Frz & Fz are Members of Same Protein Superfamily Family CRD= cysteine rich domain: conserved set of cys residues that give protein distinct 2° structure