Lecture 4- Induction of the Nervous System II Flashcards
How was the organizer molecule found?
- the organizer molecule that can induce neural fate found using molecular approaches -cDNA “library” from genes turned on in organizer
- library screened to look for cDNA that could rescue UV treated ventralized frog embryos
How is the cDNA library created?
-
What is a UV treated ventralized embryo?
- treat embryo with UV at the right time, no ventral structures form
- can inject cDNA and induce formation of those structures
What are the factors that were identified as neural inducers from organizer? (5)
- first to be identified was Noggin
- then Chordin
- Follistatin
- Cerberus
- Xenopus nodal related 3 (Xnr3)
What is Follistatin and what does it do?
- member of the TGF-beta family
- inhibits Activin
How did Follistatin provide information about how the neural inducers work?
- Follistatin= activin inhibitor
- blocking Activin signalling using non-signalling receptor (dominant negative or DN) induced neural tissue in the animal cap assay
- activin is TGF-beta family member: first evidence that TGF-beta family proteins are epidermalising factors
What are some other TGF-beta (Transforming Growth Factor= TGF) family members apart from Activin?
- Bone Morphogenetic Proteins (BMP’s)
- Nodal
What does the presence of TGF-beta proteins suggest?
- epidermal fate is specified by TGF-beta signalling?
- neural induction involves blocking of TGF-beta signals?
What does the BMP signal transduction pathway look like?
-
What does truncated activin receptor do?
-Truncated activin receptor later found to inhibit both activin (which actually works by inducing mesoderm) and BMP signalling
What are the best candidates for native epidermalising factors?
- Bone Morphogenic Proteins (BMPs)
- particularly BMP-2, BMP-4, BMP-7
What happens to dissociated animal cap cells when treated with BMP-4?
- lose their neural fate
- become epidermis
- suggests that if you have BMP signalling you will develop epidermis, thus blocking of BMP signalling leads to neural fate
What does blocking of BMP signalling do?
-induces neural fate
What are the BMP inhibitors and what do they do?
- Noggin, Chordin, Follistatin secreted from the organizer
- block BMP signalling thus inducing neural fate in the neurogenic region
What do Chordin and Noggin bind?
- BMP-4 with high affinity
- blocks BMP-4 from activating receptor
What does Follistatin bind?
- BMP-7 and activin
- binding these blocks the BMP from activating the receptor
Do BMP inhibitors act alone or are the combined actions of several inhibitors required?
- need combined action of all inhibitors
- if you knock out just one or even two there is formation of the neuroectodermal tissue but diminished from normal (by injecting morpholinos can test this)
What is the default fate for ectodermal tissue?
-neural (seen when cells are dissociated)
What happens when ectodermal cells are in close proximity to one another?
- BMP signalling inhibits neural fate and the cells become epidermal EXCEPT where they are exposed to organizer signals ( from mesodermal cells that have passed through organizer region- IMZ/DLB during involution)
- the part of the ectoderm above the IMZ is exposed to BMP inhibitors and is induced to become neural and adopts neural fate
Does the deafult model apply to other vertebrates?
- no
- evidence for additional neural inducers in mammals and birds
- follistatin, chordin, noggin, cerberus single-gene knoc-out mice have been produced and all showed normal sized neural plate
What happens in a mouse with both chordin and noggin knocked out?
-elimination of mouse forebrain but some neural tissue still forms
When does neural induction occur in birds?
- prior to gastrulation and before appearance of noggin, chordin or follistatin
- different to the Xenopus model
What induces neural fate in a chick embryo and how does it happen?
- happens in pregastrular stage
- Fibroblast Growth Factors (FGF’s) induce the neural fate in pregastrular stage in chick embryo
- Fgf3 mRNA is expressed in presumptive neuroectoderm in pregastrula stage
- BMP expression is normally down-regulated in this region
- inhibition of FGF signalling at this stage prevents down-regulation of BMP expression and shifts cells from a neural to epidermal fate (evidence for the FGF’s role in neural induction)
How does FGF inhibition of BMP signalling work? (chick embryo)
- FGF signalling blocks BMP signalling via P-SMAD and via increased Noggin (an FGF target gene)
- inhibits BMP gene transcription
What is the mechanism of repression of BMP in Xenopus? (diagram)
-the mechanism differs in different vertebrate groups
What is the mechanism for repression of BMP signalling in chick embryos?
-
What does signalling between ectodermal cells repress?
-default neural fate and cells acquire epidermal fate
What does epidermal fate result from?
-Bone Morphogenetic Protein (BMP) signalling
What does neural induction occur via?
- inhibition of BMP signalling
- in frogs, BMP inhibitors secretes from organizer (Noggin, Chordin, Follistatin) induce neuroectoderm during gastrulation
- in chick embryos, the first stage of neural induction appears to be dependent on Fibroblast Growth Factor (FGF) signalling at the pre-gastrula stage
What do inducers from node help do (chick embryo)?
- Primitive streak (ps) induced node (hn-Hensen’s node)
- node induces stable neural fate via Chordin inhibition of BMP signalling
