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. 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