Download Axis formation in Amphibians

The Organizer
Back to the Organizer: “Primary”
Induction
What Does Spemann’s Organizer
Do?
• Mesoderm that dorsalizes the embryo:
– Becomes dorsal mesoderm (prechordal plate,
chordamesoderm)
– Induces nearby mesoderm to form lateral mesoderm
(rather than ventral)
– Induces ectoderm into neural ectoderm
– Initiates gastrulation
– Induces neural plate to become neural tube
How to Find the Real Organizer
Molecules?
• What are the molecules responsible for these
inductions/organizations?
– Some biochemical experiments (1930s -1940s) seemed to belittle
the all-powerful organizer
•
•
•
•
Dead organizer tissue
pH or ionic composition
Turpentine, alcohol
But only live tissue of organizer could do it
– Suggested that responding tissue (not inducer) had specificity but
something unleashed it
• Molecular approaches to find the real signals
There Are Many
Isolation of First Soluble
Organizer Molecule: Noggin
– Inhibits ventralization
• Extract mRNAs
– copy into DNA and amplify
– Inject pools into ventralized
embryos (UV)
• Look for dorsal tissue
formation
Increasing noggin mRNA
• Li+ treated embryos
(dorsalized)
Where is Noggin mRNA Localized?
gastrula
dorsal marginal
zone
dorsal blastopore lip
notochord
What Does Noggin Do?
• Secreted protein
– Induces dorsal ectoderm -> neural tissue
– Dorsalizes mesoderm
• Acts by binding other secreted proteins
– BMP4 and BMP2
– Prevent receptor interaction
– So dorsalization by inhibiting other signals?
New Concept: Epidermal
Induction
• Is neural fate the “default” ectoderm fate?
– Epidermal fate the “induced” fate?
• Cell dissociation studies
• Dissociated animal cap cells --> neural tissue!
• Suggests neural fate = default fate
• The organizer prevents induction in preneural ectoderm
Then What Is the Inducing Molecule for
Ventral Ectoderm and Mesoderm?
• BMP-4 is present in the
marginal zone except in
dorsal-most region
• Injection into embryos has
ventralizing effect
– all mesoderm develops as
ventrolateral mesoderm with
no gastrulation)
Other Diffusible Factors are Also
BMP Antagonists
• Inhibition by chordin
– mRNAs in dorsalized not ventralized embryos
– Injection to produce 2o axis
– Binds to bmp2, bmp4
• Inhibition by Xnr-3 (nodal) same
• Inhibition by follistatin
– Inhibits activin related proteins
• All prevent BMP binding to receptors
BMP Antagonism Model of
Organizer Action
How Are Different Mesodermal
Regions Specified?
• BMP-4 activates genes in
a concentrationdependent fashion
• Rough model
–
–
–
–
[High] -> blood
[Med] -> kidney
[Low] -> muscle
[None] -> notochord
The Head Region
• The most anterior region is induced not
by notochord but by endoderm and
head mesoderm
• Signals here are BMP and Wnt8
(Xwnt8)
• What antagonizes Wnt8 in the head
region?
The Anterior Endomesoderm
Two Types of Inhibitors
Dorsal Posterior
BMP inhibitors
Dorsal Anterior
Wnt inhibitors
What are the Wnt Inhibitors?
• Cerberus
– Expressed in endomesodermal cells of
early blastopore lip
– Induces ectopic head structures if injected
into ventral vegetal region of blastula
– Promotes formation of anterior-most
ectoderm
• cement gland
• eyes
• olfactory placodes
– Can bind to BMPs and Xwnt8
What are the Wnt Inhibitors?
• Frzb and Dickkopf
– Soluble proteins that also interact with Wnt
proteins
• Expressed in endomesoderm beneath the
future head
• If overexpressed -> “big head” embryos
– Wnt signaling is prevented everywhere and
therefore excessive head induction is allowed to
occur
An Ultimate Frzb Game
Extra Frzb blocks Wnt signal
Control
Injection of Frzb mRNA
Wnt Antagonism Model of Head
Organizer Action
Model of Mesoderm Induction:
Gradients of BMP4 and Xwnt8 Set Up by
Xnr Gradient
V + b-cat give
nodal gradient
Organizer Paracrine Factors
Antagonists
• Battles in extracellular space!
– Head + brain: no BMP or Wnt
– Trunk: Wnt but not BMP
– Epidermis: Wnt + BMP
• Graded series of inhibitors
these tissues
secrete
these factors
Outside Reading
• http://9e.devbio.com/article.php?ch=7&id=123
• http://9e.devbio.com/article.php?ch=7&id=110
• http://9e.devbio.com/article.php?ch=7&id=114
• Please write a page about the history of the search for
the “organizer” molecules and comment on the
nature of “proof” in search for developmental
mechanisms and the possibility of misleading results