Lecture 9 Neural induction and NT patterning

Question 1

What do all ectodermal cells make and secrete?

Answer 1

BMP4

Question 2

What does BMP bind to

Answer 2

TGFβ receptors on adjacent cells

Question 3

What does BMP signalling pathway induce

Answer 3

Differentiation into epidermal ectoderm

Question 4

What genes cause the Node to express BMP antagonists

Answer 4

Gsc and siamois

Question 5

Does the Node express BMP antagonists intrinsically or extrinsically

Answer 5

Intrinsically

Question 6

3 examples of BMP antagonists

Answer 6

chordin, noggin, follistatin

Question 7

How do BMP antagonsits work

Answer 7

• They diffuse into the same EC spaces and compete effectively for binding to the BMPs so they can no longer activate their receptors

Question 8

What is the effect on ectoderm cells adjacent to the Node

Answer 8

BMP antagonists result in ectoderm cells changing their fate to neural identity in neural induction

Question 9

What occurs at the same time as neural induction

Answer 9

BMP antagonists act on the rest of the ‘non-organiser’ mesoderm to refine mesodermal fate

Question 10

Describe 4 stages of mesoderm induction and patterning

Answer 10

1. Low level Nodal gives the ventral mesoderm
2. High level Nodal gives the organiser
3. Signals from the organiser acts to inhibit BMPs to dorsalise and pattern adjacent mesoderm
4. At the same time, antagonism of BMPs –> Gives a neural identity

Question 11

Low levels of nodal gives

Answer 11

Ventral mesoderm

Question 12

High levels of nodal gives

Answer 12

organsier

Question 13

Ventral most mesoderm fate

Answer 13

blood then kidneys

lowest conc of BMP antagonists

Question 14

Intermediate mesoderm

Answer 14

somites and heart

Question 15

Experimental proof for neural induction by organiser

Answer 15

• 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.

Question 16

2nd neural tube was derived from what

Answer 16

HOST

host derived shows that the neural tissue was induced from the ectoderm in response to signalling from the organiser

Question 17

Axial mesoderm/AE derived from what

Answer 17

The axial mesoderm (prechordal mesoderm and notochord) and anterior endoderm was donor derived and therefore differentiates from the organiser

Question 18

Define induction

Answer 18

• 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

Question 19

What should induction not be mistaken for

Answer 19

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.

Question 20

Describe one model for the formation of the AP axis

Answer 20

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.

Question 21

What 3 cell types does the organiser differentiate into

Answer 21

• Anterior endoderm (AE)
• Prechordal mesoderm (PM)
• Notochord (NC) mostly

Question 22

What is the end result of this model? What factors are induced?

Answer 22

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

Question 23

Name the 2nd model that is no longer used

Answer 23

Activation-transformation model

Question 24

Activation-transformation model is basis for

Answer 24

• It is the basis for formation of forebrain brain (A) versus hindbrain and spinal cord (P)

Question 25

Describe activation-transformation model

Answer 25

* 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.

Question 26

What do both models agree on

Answer 26

THE SIGNALS THAT COME FROM PRECHORDAL MESODERM AND NOTOCHORD

Question 27

What signals are maintained anteriorly

Answer 27

• BMP antagonists and Wnt antagonists are maintained anteriorly to maintain forebrain TF

Question 28

What signals are maintained posteriorly

Answer 28

• FGF, Wnts, RA expressed posteriorly, promote growth and ‘posteriorise’ (spinal cord identity)

Question 29

How does RA signal

Answer 29

• 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

Question 30

What is a core concept in development

Answer 30

Establish a regional pattern by placing 2 antagonistic molecules at each end of a forming (growing) structure

Question 31

How are different domains of hindbrain and spinal cord defined

Answer 31

through Hox signature i.e. expression of TF known as Hox genes

Question 32

What induces different patterns of Hox transcription and what does this later dictate

Answer 32

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

Question 33

How are some Hox genes expressed

Answer 33

Redundantly i.e. co-expressed

Question 34

What derives from distinct rhombomeres

Answer 34

Nerves

Question 35

Describe RA gradient

Answer 35

* 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

Question 36

What is the result of the hindbrain and forebrain interaction

Answer 36

Induces midbrain cells

Question 37

What events occurs after hindbrain and spinal cord are defined

Answer 37

Regionalisation of forebrain into diencephalon and telecephalon

Question 38

Define neuroepithelium

Answer 38

single cell-thick neural plate

Question 39

What occurs when neuroepithelium is induced

Answer 39

it rolls up into the neural tube

Question 40

What is the result from neurulation in terms of axis

Answer 40

• This transforms the medio-lateral axis into the dorso-ventral axis (what was medial and underlain by notochord becomes ventral - still underlain by notochord).

Question 41

How does the NT form from overlying ectoderm

Answer 41

• 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

Question 42

Where is the axial mesoderm in relation to neural plate after neurulation

Answer 42

• Once neural plate rolls up, axial mesoderm sits under relatively ventral axis

Question 43

What do roof plate cells upregulate

Answer 43

BMPs and Wnts

Question 44

BMP and Wnt are what sort of molecule

Answer 44

Secreted molecules

Question 45

What do BMP and Wnt induce

Answer 45

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.

Question 46

What previously was it believed BMPs acted as

Answer 46

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.

Question 47

What do roof plate cells actually express

Answer 47

But recent work suggests that the roof plate expresses many different BMPs, each of which induces a particular dorsal cell type.

Question 48

BMPs that derive from the …. initially induce their own expression in the ...….of the......

Answer 48

BMPs that derive from the surface ectoderm initially induce their own expression in the immediately adjacent roof plate of the dorsal spinal cord.

Question 49

BMPs act as.... to induce ….

Answer 49

BMPs from the surface ectoderm, the roof plate, or both, then act as local morphogens to induce different sets of progenitor cells

Question 50

What do the BMP induced progenitor cells differentiate into

Answer 50

6 distinct neuronal subsets in dorsal spinal cord

Question 51

Neuronal fate of a cell is dictated by

Answer 51

The combinatorial expression of TF in early progenitor cells

Question 52

Is Shh upregulated before or after BMP

Answer 52

After

Question 53

Where is Shh upregualted

Answer 53

NT

Question 54

Shh in NT induces...

Answer 54

Shh-expressing floor plate cells in the ventral midline of the Neural tube

Question 55

Shh is an example of..

Answer 55

morphogen

Question 56

What is Shh gradient translated into

Answer 56

The Shh gradient is translated into a GliA-GliR gradient in responding cells, which in turn, begin to express particular homeodomain TFs

Question 57

What dictates later differentiation in Shh pathway

Answer 57

The homeodomain transcription factor ‘code’ dictates later differentiation.

Question 58

As cells (nerves) differentiate they move...

Answer 58

Laterally

Question 59

Shh acts at an early stage to confer …..

Answer 59

a DV pattern of TF on progenitor cells

Question 60

What do the TF from Shh do

Answer 60

they are upstream master regulators of particular neuronal fate identity

Question 61

Overall what patterns the DV axis in NT

Answer 61

opposing gradients of BMPs from dorsal region and Shh from ventral region pattern the DV axis