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
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. 10.5vegetal view
dorsal lip
St. 11vegetal view
dorsal view
Noggin Expression Pattern
notochordDV
Noggin Rescues UV Ventralized Embryos UV
injectednoggin 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. 10 St. 10.5 St. 13
dorsal blastopore lip
blastoporeaxial mesoderm
prechordal plate
Chordin FunctionChordin 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
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