Expressed during gastrulation: Encodes a protein that is responsible for induction of ectopic heads in absence of trunk. Acts as an antagonist to Wnt, BMP and Nodal proteins by binding to them at INDEPENDENT sites
Inhibition of trunk mesoderm
Neutralization of ectoderm
Cerebus Fig 1
Main point: Cer-L is able to bind to Xnr-1, BMP and Xwnt-8 at individual sites in solution
First Cer tagged with FLAG: 2 products found, Cer-S and Cer-L
Looked at binding of Cer to mesoderm inducing factors Xnr1, Vg1 and Activin: IP showed Xnr1 binds, not the other 2
Then looked at Cer in presence of all 3 above, and saw that Xbra was decreased with only Xnr1 present: Cer inactivates Xnr1 signaling
Cer-L (not Cer-S) tagged with FLAG showed binding to BMP4 (Blocks Cer binding) AND also NEUTRALIZATION activity, whereas Cer-S did NOT
FULL length cer mRNA INHIBITS Xwnt8 protein, and to test if the binding of Xwnt to Cer is extracellular, Flag-tagged CerS and Cer-L and then added Xwnt-8-HA protein and saw IP with Cer-L
Xnr1 binding NOT displaced by BMP4, and Wnt NOT displaced by excess BMP4 Xnr1, so binds at independent sites to all 3 signaling factors
Cerebus Fig 2
Main point: Cer-S inhibits nodal related signals to lead to ectopic head formation
Xenopus embryo used to see if Cer-S mRNA injected causes blocking of expression: Mesoderm markers Xnr1 and Xwnt-8 BLOCKED with Cer.
Expression of endogenous cer was BLOCKED by cer-smRNA
Nodal signal Xnr1 induced endogenous cerebus, but only early on (RNA level), not at the DNA level of Xnr1
Looked at BMP signals: tBR blocks BMP signals, and when this occurred with cer-S, head like formation occurred *similar to when full length cer inserted.
Did not do anti-Wnt activity experiments, except for looking at loss of Xwnt in embryos in beginning figs.
Nodal activity sufficient in Cer-S
NOT Sufficient to induce head-like structures alone
Main point: All three inhibitory activities of cerebus are REQUIRED for ectopic head formation
Looked at whole embryos and added antagonists
Adding chd and cer-s: able to see ectopic head structures
When they added Xwnt-8 DNA ectopic head BLOCKED
ALK-4 receptor: blocks anti-nodal activity of cerebus mRNA, resulted in ABOLISHED ectopic head formation
BMPR: BMP receptor, when added it stopped ectopic head formation
Frzb-1: anti-Wnt (inhibitor for Wnt protein)
When chd (anti-BMP) and frzb-1 were added, ectopic head formation occurred
Xnr-1 DNA added (nodal signal)
Role of anterior endoderm with regards to head development (chart)
Early endoderm signals Vg1 and Nodal come in, as well as antagonists chordin(a-BMP), Frzb1 (a-wnt) activate Cerebus during gastrulation.
Head formation occurs only if all three (Nodal, Wnt and BMP) proteins are inhibited.