L6 Flashcards
Invertebrate neural induction:
Induction of CNS dependent on competing gradients do Dorsal and Decapentaplegic (DPP/BMP), Dorsal targets high+low affinity (HA+LA) sites in genome, in lateral region only HA activated -> neuroecotderm, LA (suppress HA) in ventral -> mesoderm; Leads to competing gradients of SOG and DPP- at low concentrations Dorsal binds Sog (direct activation), at high binds Twist, upregulates Snail and SNAIL represses Sog
Which genes to Twist, Snail, Sog and DPP activate?
Twist- mesoderm (ventral)
Snail- non-mesodermal (lateral+dorsal)
Sog- neuroectoderm (lateral)
DPP- dorsal ectoderm (dorsal)
Effects of modifying gradients?
Knocking out a gradient or master TF leads to loss of entire layer
Tweaking a gradient leads to shift in the boundary between lawyers
Layer boundaries?
Learn the fucking table
SoxN
One of the earliest genes expressed throughout SOG domain; is a homologous of vertebrate Sox2
After invertebrate neural induction:
SoxN dependent transcription programme in neuroectoderm cells leads to expression of a proneural gene; proneural factor confers a neural fate to a cell; not all cells in neuroectoderm progress to neuroblasts, rest revert to ventral fate and form ventral epidermis due to lateral inhibition
Neural induction in frogs
Generation of main tissue layers Animal pole (future ectoderm), vegetal pole (future endoderm); endoderm signal to ectoderm to transform into mesoderm at equatorial ring; mesoderm signal to ectoderm via Sox2+Oct4 to induce neuroectoderm (at dorsal midline, NSCs now committed); lateral ectoderm signals to lateral neuroectoderm to specify neural crest (thus is NE that subsequently acquires potent mesodermal properties); anterior neuroectoderm signals to anterior ectoderm to specify panplacodal ectoderm
Placodal ectoderm
Forebrain (olfactory placode = nasal epithelium)
Hindbrain (otic placode = auditory epithelium)
Cranial neural crest
Facial bones
Cranial neural crest
Spinal chord (DRG of PNS, ganglionic cells of ANS)
What are the key pathways in vertebrate neural induction?
BMP4/ FGF8
N.B. Early events ouch cells towards primitive germ layers, mesoderm then helps to induce neuroectoderm by blocking weak ectoderm signal (BMP) that allows FGF to drive ectoderm to neuroectoderm (SOX2-OCT4) fate
Gradients of maternal mRNA…
(Neural induction step 1)
Frog
…polarise the egg
Dominant gradient early on is ventral TGFβ, quickly drives vegetal cells to endoderm, anterior BMP is slower acting (takes longer to specific ectoderm fate in animal cells)
Geminin gradient also makes Sox2 gene more accessible in animal cells
The dorsal organiser (nodal gradient) arises…
Vertebrate neural induction step 2
Frog
Signalling in the vegetal plug establishes the dorsal organiser, the resultant Nodal signal now induces animal cells at equator to become mesoderm
The Spemann organiser (BMP inhibitor) arises…
Vertebrate neural induction step 3
Frog
Mesoderm expresses WNT and migrates around the equator to the posterior (i.e above DO), WNT signal combined with high Nodal from DO generates the Spemann organiser
Gastrulation-early
Frog
Cells on dorsal lip of blastopore invaginate and pass under ectoderm (derived from animal cap), invaginating cells are mesoderm
Late gastrulation
Frog
Spemann organiser induces overlying ectoderm to form neural tissue instead of epidermis
Neural induction in culture
Frog
Isolate animal cap from late blastula and culture -> intact epidermis
If same cells isolated -> neural
Thus cell-cell contact prevents ectoderm from becoming neural, and default state for ectoderm is neural
If isolated cells have BMP4 added, become epidermis again
If cells not isolated but have Noggin or Follistatin added, become neural again (blocks signalling between intact ectodermal cells)
Where are Noggin/Follistatin/Chordin expressed?
Dorsal lip of blastopore (Spemann organiser), all three bind BMP4
More recent work has shown that FGF signalling inhibits BMP signalling at Smad1 level, and that FGF upregulates Sox2
Frog
In the animal cap, BMP>Smad pushes cells towards ectoderm, before they fully commit Spemann organiser is established and releases various BMP inhibitors; mesoderm carries these under animal cap to block BMP which allows FGF>MAPK to be upregulated; MAPK now cross inhibits Smad intracellularly (blocking BMP transduction); BMP inhibition allows more potent FGF activation; FGF increase Sox2 to autoregulation -> cells now committed neuroectoderm (neural plate)
Neural induction
Chick
Activation of near primitive streak by FGF4/8, generates most of the neural tube
Expansion driven by Hensen’s node BMP+WNT inhibition, expands the anterior neural tube (forebrain)
Frog VS Chick
Chick has separated roles of BMP inhibition and FGF4/8 activation (+WNT needs to be blocked to form anterior)
[in chick, BMP not present in primitive streak region and a novel mechanism has evolved to modulate chromatin remodelling at Sox2 locus (Bert and Erni, which act via Geminin-Brahma, so evolved from chromatin remodelling mechanism in frogs]
