Neural Induction 2 Flashcards
Where did the search for the neural inducer start?
activin
activin
member of TGFb family
indices mesoderm
found in mesoderm of early embryos and brain/notochord later
Activin in ectodermal cells
bind receptor, dimerises, activates cytoplasmic c kinase domain = signal
activin in ectoderm explants
activin bind receptor, signalling occurs
secondary signal from mesoderm converts ectodermal fate to neuonal fate
activin receptor with no signalling domain
still bind activin and dimerise but no signal
Experiment - activin receptor
mRNA encoding mutant receptor injected into both cells of 2 cell embryo - neural tissue?? would expect epidermal tissue as no signal
finding of truncated activin receptor - neural tissue
neural tissue when to active signalling and no secondary signalling from mesoderm
conclusion from truncated activin receptor experiment
activin inhibition leads to formation of neural tissue
3 endogenous activin inhibitors
follistatin
noggin
chordin
Follistatin - what is it?
activin binding protein
inhibits mesoderm producing activity of activin
promotes neural differentiation
where is follistatin found?
gastrula stage: mRNA in spemann organiser and neural inducing tissues
Follistatin experiment
overexpress follistatin
bin activin and prevent receptor signal
neural tissue
conclusions from follistatin experiment
trunacted activin receptor and this experiment - negative role for activin signalling in neural induction
Noggin
protein - mRNA in neural inducing tissues
what does noggin bind to and prevent signalling?
BMP
Noggin experiment
Inject mRNA into ventral blastomere - partial 2nd neural axis induces neural tissue directly from ectoderm
noggin - direct and indirect neural induction
ectoderm + purified noggin = direct neural tissue
overexpress noggin = mesoderm intermediate
noggin conclusion
noggin present right time and place to be neural inducer
induce neural fate directly or indirectly
chordin - what is it, what does it do and where is it found?
protein, binds BMP
mRNA spemann organiser
chordin - direct or indirect?
directly induces neural tissue
evidence for chordin, follistatin and noggin working together
inhibit 1 = minor neural plae defect
all = substantial defect
stain for sox2
similarities of 3 neural inducers
present right time and place
soluble proteins
induce neural tissue by blocking BMP binding
what does BMP-4 induce?
epidermis
What is BMP-4?
protein - TGFb family
binds to activin receptor
mRNA in ectoderm/epidermis not in spemann organiser
BMP-4 experiment
inject mRNA - ventralises, mRNA to inhibit BMP-4 dorsalises
inject with mRNA against BMP-4 receptor (ventral injection) - partial second axis
what does BMP-4 regulate?
dorsal/ventral patterning in xenopus
what does BMP-4 inhibit?
neural tissue
animal cap + BMP-4/mutant BMP-4 findings
animal cap + mutant BMP-4 or receptor or BMP-4 inhibitor = neural tissue
dissociate cells - no BMP-4 = neural
+BMP-4 = epidermis
challenges to default model
neither chordin nor noggin can induce neural tissue if FGF signalling blocked
BMP signalling necessary but not sufficient
Evidence - challenge to neural default model
inject mRNA BMP antagonist into blastomere contribute to epidermis but not neural plate - no neural tissue
co-inject with FGF = neural tissue
What regions of embryo is BMP-4 not produced?
those with high nuclear b-catenin (controlled by wnt)
Anterior patterning of neural plate
activin, follistatin, chordin, noggin, BMP
What signals posteriorise neural plate?
wnt and FGF
ectoderm - what happens if block BMP-4?
anterior CNS
ectoderm - block BMP-4 and add FGF?
posterior CNS
wnt
class of secreted glycoproteins that regulate b-catenin activity and gene transcription
what happens if wnt overexpressed?
over-dorsalised embryo
explants undergo neural differentiation
evidence wnt posteriorises
gradient of nuclear b-catenin high posteriorly
wnt induces hindbrain markers in neurulised animal caps
3 models of A/P patterning of neuroectoderm
ectoderm pre-patterned - signals from mesoderm
ectoderm homogeneous and mesoderm prepatterned
2 signal model - ectoderm induced to become anterior neural tissue, then posteriorised from posterior mesoderm
diffusible factor from roof plate inducing commissural neurons
BMPs, dorsalin 1
Induction of motor neurons and floorplate by notochord
motor neurons always form at set distance from floor plate inducing signal
induction by floor plate
notochord between edges of neural folds at time of fusion = floor plate where roof plate should be
extra lateral floor plate = ectopic lateral floorplate
what does floorplate secrete?
shh
what does roofplate secrete?
BMP/wnt
netrin affect on commissural axons
protein in spinal cord stimulate outgrowth of commissural axons
attract axons toward floor plate
where is netrin 1 expressed?
floor plate
where is netrin 2 expressed?
ventral 2/3 of spinal cord but not floor plate
shh
produces protein
ventralising factor in vertebrae spinal cord
commisural axons - wnt and shh
attracted to floorplate by shh
ventral midline, switch to repulsion, steer growth cones anteriorly
attract anteriorly to wnt
Evidence that shh indices floor plate and motor neurons
embryos - shh in notochord
shh ectopic = ectopic floor plate
neural tube cells made to express shh - motor neurons
how can shh form floor plate and motor neurons?
floor plate induction by amino-terminal cleavage product - contact dependent
motor neurons - diffusible factor
