Lecture 9 Neural induction and NT patterning Flashcards
What do all ectodermal cells make and secrete?
BMP4
What does BMP bind to
TGFβ receptors on adjacent cells
What does BMP signalling pathway induce
Differentiation into epidermal ectoderm
What genes cause the Node to express BMP antagonists
Gsc and siamois
Does the Node express BMP antagonists intrinsically or extrinsically
Intrinsically
3 examples of BMP antagonists
chordin, noggin, follistatin
How do BMP antagonsits work
• They diffuse into the same EC spaces and compete effectively for binding to the BMPs so they can no longer activate their receptors
What is the effect on ectoderm cells adjacent to the Node
BMP antagonists result in ectoderm cells changing their fate to neural identity in neural induction
What occurs at the same time as neural induction
BMP antagonists act on the rest of the ‘non-organiser’ mesoderm to refine mesodermal fate
Describe 4 stages of mesoderm induction and patterning
- Low level Nodal gives the ventral mesoderm
- High level Nodal gives the organiser
- Signals from the organiser acts to inhibit BMPs to dorsalise and pattern adjacent mesoderm
- At the same time, antagonism of BMPs –> Gives a neural identity
Low levels of nodal gives
Ventral mesoderm
High levels of nodal gives
organsier
Ventral most mesoderm fate
blood then kidneys
lowest conc of BMP antagonists
Intermediate mesoderm
somites and heart
Experimental proof for neural induction by organiser
- 1920s Organiser graft experiment, Spemann + Mangold
- Grafted an organiser (the bit of induced mesoderm that lies directly above the Nieuwkoop centre) from a donor to a host newt (similar to a Xenopus), and found that a ‘twinned’ embryo developed - with a complete secondary neural axis.
2nd neural tube was derived from what
HOST
host derived shows that the neural tissue was induced from the ectoderm in response to signalling from the organiser
Axial mesoderm/AE derived from what
The axial mesoderm (prechordal mesoderm and notochord) and anterior endoderm was donor derived and therefore differentiates from the organiser
Define induction
- Induction is an example of a change in fate mediated by extrinsic/non cell-autonomous event
- It can be from a signal or lack or signal e.g. lack of BMP neural
What should induction not be mistaken for
This is different to a change in fate from cell autonomous/cell intrinsic differentiation where there is a different distribution of cytoplasmic determinants, causing the cell to divide asymmetrically. One daughter stays same fate as mother. Second daughter inherits different components and cell-autonomously differentiates to alternate Cell fate X.
Describe one model for the formation of the AP axis
The anterior-posterior axis becomes apparent as cells in the organiser autonomously differentiates into the axial mesendoderm
This causes the dorsal mesoderm to involutes and undergo convergent extension to lie under the ectodermal layer of the embryo. Convergent extension (gastrulation) causes this change in shape to form a long thin rod of axial mesoderm cells sitting under the ventral midline of the neural tube.
As these processes occur, the neural plate grows and elongates along the A-P axis, and rolls into the neural tube.
What 3 cell types does the organiser differentiate into
- Anterior endoderm (AE)
- Prechordal mesoderm (PM)
- Notochord (NC) mostly
What is the end result of this model? What factors are induced?
It means that the A end of NP is underlaid by different cell types to P end. Anterior endoderm/prechordal mesoderm induce ‘brain-like’ neural transcription factors e.g. Otx2; notochord induces ‘spinal cord-like’ neural TFs
Name the 2nd model that is no longer used
Activation-transformation model
Activation-transformation model is basis for
• It is the basis for formation of forebrain brain (A) versus hindbrain and spinal cord (P)
Describe activation-transformation model
- Initially, entire neural plate is ‘brain-like’ under influence of AE/PM
- Signals from the notochord cause cells at the back end of the neural plate to proliferate and ‘transform’ these cells from an anterior to a posterior identity – i.e. turn off TFs that dictate A identity and turn on TFs that dictate P identity.
What do both models agree on
THE SIGNALS THAT COME FROM PRECHORDAL MESODERM AND NOTOCHORD
What signals are maintained anteriorly
• BMP antagonists and Wnt antagonists are maintained anteriorly to maintain forebrain TF
What signals are maintained posteriorly
• FGF, Wnts, RA expressed posteriorly, promote growth and ‘posteriorise’ (spinal cord identity)
How does RA signal
• RA is small/lipophilic so diffuses through cell membrane into cells where it binds to a cytosolic receptor - complex RA-R able to enter the nucleus where it directly influences gene expression
What is a core concept in development
Establish a regional pattern by placing 2 antagonistic molecules at each end of a forming (growing) structure
How are different domains of hindbrain and spinal cord defined
through Hox signature i.e. expression of TF known as Hox genes
What induces different patterns of Hox transcription and what does this later dictate
Retinoic acid gradient induces different patterns of Hox transcription that then dictate distinct segmental and neuronal identities along AP axis
As a result, different regions of the neuraxis come to express different Hox genes
How are some Hox genes expressed
Redundantly i.e. co-expressed
What derives from distinct rhombomeres
Nerves
Describe RA gradient
- RA is made at high ends at caudal end to establish gradient. You turn on TF along the developing AP axis in response to different threshold concentrations of RA, in this case the HOX genes
- Varying amount of RA gives particular type of neuron made in a dish these days
What is the result of the hindbrain and forebrain interaction
Induces midbrain cells
What events occurs after hindbrain and spinal cord are defined
Regionalisation of forebrain into diencephalon and telecephalon
Define neuroepithelium
single cell-thick neural plate
What occurs when neuroepithelium is induced
it rolls up into the neural tube
What is the result from neurulation in terms of axis
• This transforms the medio-lateral axis into the dorso-ventral axis (what was medial and underlain by notochord becomes ventral - still underlain by notochord).
How does the NT form from overlying ectoderm
• Lateral edges fuse to become dorsal part of neural tube (remember ‘dorsal’ and ‘ventral’ are all relative terms). • The neural tube ‘pinches off’ from the overlying surface ectoderm
Where is the axial mesoderm in relation to neural plate after neurulation
• Once neural plate rolls up, axial mesoderm sits under relatively ventral axis
What do roof plate cells upregulate
BMPs and Wnts
BMP and Wnt are what sort of molecule
Secreted molecules
What do BMP and Wnt induce
They induce expression of a set of transcription factors (Pax6, Pac7, Pax3, Lim1) that cause neural tube progenitors to acquire ‘dorsal identities’. They define stripes of cells with distinct identities.
What previously was it believed BMPs acted as
Until recently, it was thought that BMPs coming from the roof plate act as morphogens to induce different types of dorsal cells – like French Wolpert model – in response to particular cell threshold, cells adopt certain fates.
What do roof plate cells actually express
But recent work suggests that the roof plate expresses many different BMPs, each of which induces a particular dorsal cell type.
BMPs that derive from the …. initially induce their own expression in the …….of the……
BMPs that derive from the surface ectoderm initially induce their own expression in the immediately adjacent roof plate of the dorsal spinal cord.
BMPs act as…. to induce ….
BMPs from the surface ectoderm, the roof plate, or both, then act as local morphogens to induce different sets of progenitor cells
What do the BMP induced progenitor cells differentiate into
6 distinct neuronal subsets in dorsal spinal cord
Neuronal fate of a cell is dictated by
The combinatorial expression of TF in early progenitor cells
Is Shh upregulated before or after BMP
After
Where is Shh upregualted
NT
Shh in NT induces…
Shh-expressing floor plate cells in the ventral midline of the Neural tube
Shh is an example of..
morphogen
What is Shh gradient translated into
The Shh gradient is translated into a GliA-GliR gradient in responding cells, which in turn, begin to express particular homeodomain TFs
What dictates later differentiation in Shh pathway
The homeodomain transcription factor ‘code’ dictates later differentiation.
As cells (nerves) differentiate they move…
Laterally
Shh acts at an early stage to confer …..
a DV pattern of TF on progenitor cells
What do the TF from Shh do
they are upstream master regulators of particular neuronal fate identity
Overall what patterns the DV axis in NT
opposing gradients of BMPs from dorsal region and Shh from ventral region pattern the DV axis
