Lecture 6 - Developmental Neurobiology - Ventralisation of the neural tube

at the same time of neural plate/neural tube formation what else forms?

• the axial mesoderm (composed of notochord and prechordal mesoderm) forms

• axial mesoderm lies below ventral midline of the neural tube

• notochord - positioned under the ventral midline of the neural tube - it extends along most of the body axis posteriorly

• prechordal mesoderm - found anteriorly beneath the forebrain region of the neural tube

what develops in the neural tube ventral midlines above the notochord?

• specialised cells called the floor plate

what does bilateral symmetry of neurons refer to?

• neurons develop with bilateral symmetry around the midline throughout the entire D-V axis

• neurons lie in discrete regions along the D-V axis

• neurons develop in those spatial positions with absolute accuracy

what does the position of the notochord and floor plate suggest?

• that they could make secreted factor (protein) that directs the development of ventral neurons

• these cells are influencers and are the source of the signal

how do we test that the notochord and floor plate cells are the source of the signal?

experiment:

• through gain of function experiments

• take notochord or floor plate from donor and graft it in unusual (ectopic) position in a host

• use markers to detect if extra motor neurons and ventral floor plates are induced

  findings:

• the tissue next to the graft forms:

  1. ectopic (extra) motor neurons

  2. sometimes a new floor plate

  conclusion:

• this shows that signals from the notochord or floor plate are sufficient to induce ventral neural fates - even in region where they don’t normally form

• overall, ectopic floor plate and ventral neurons are induced after graft of notochord or floor plate

as axial mesoderm forms what does it begin to upregulate?

• transcription of a gene called Shh (Sonic hedgehog)

what does Shh code for?

• codes for a secreted protein

where is Shh mRNA detected?

• first detected in the prechordal mesoderm and notochord

• then also detected in the floor plate

beyond the floor plate cells and notochord what other place is Shh protein detected?

• Shh diffuses out of notochord and floor plate

• a gradient of Shh protein can be detected in the ventral neural tube

what does immunohistochemistry show about Shh?

• Shh protein in a gradient emanating away from source cells

• the expression of Shh is less further away from the source cells and higher closer to the source cells

what is the Shh mRNA?

• the instructions to make the Shh protein

• it stays inside the cell that make it

• found only in the cells that produce Shh

what is the Shh protein?

• the actual signal (morphogen)

• it is secreted and spreads out (diffuses) to nearby cells/tissues

why does Shh act as a morphogen?

• to induce different progenitor cells along the D-V axis of the future spinal cord

where are different transcription bands induced?

• in bands of progenitor cells along the D-V axis

• these are readouts of a Shh morphogen gradient along D-V axis

what is a morphogen?

• a secreted diffusible molecule that provides positional information by establishing a gradient that is then translated into distinct fates

how many discrete cell states/fates is a morphogen capable of inducing?

• 2 or more discrete cells states/fates in response to different threshold concentrations

• they are instructive - they directly determine cell state/fate

what is positional information?

• the idea that a cell differentiates according to where it lies relative to neighbours that signal information

• if a cell sees a certain concentration threshold = Shh signalling pathway is stimulated and certain TFs are activated → determining certain cell fate

what is the Lewis Wolpert French flag model of positional information?

• the idea that if you have a field of cells - as long as there’s a source at one end and a sink at the other end - the different cell types are able to read out different threshold concentrations and acquire distinct fates

what is the source and sink?

• source = where the morphogen is produced and released from

• sink = where the morphogen is removed, degraded and/or absorbed

how does Shh act?

1. an extrinsic signal binds to receptor activating a signalling pathway in a responsive cell

2. as a result of this activation - a TF is activated

3. the TF goes into the nucleus to initiate a new transcriptional programme

4. this leads to further differentiation of that cell

what do signals acting as morphogens depend on?

• depending on the strength of initial signal

• different transcriptional profiles are elicited and therefore different progenitor cell fates

what does the transcription factor code dictate?

• lateral differentiation

when the cells (nerves) differentiate how do they move?

• they move laterally (radially)

  → laterally = sideways (rapidly)

• this means the neural tube gets thicker and thicker by adding layers and layers moving sideways

in the ventral regions how are progenitors defined?

• on the basis of the Nkx genes

in the dorsal region how are progenitors defined?

• on the basis of the Pax6 genes

at what stage does Shh act and what does it confer/assign?

• acts at an early stage

• to assign a D-V pattern of TFs on progenitor cells

• these TFs are the upstream master regulators of partial neuronal fate/identity

how many ventral classes of progenitors and how many dorsal classes of progenitors can the vertebrate neural tube be broken down into?

• ventral classes of progenitors - 5 Classes

• dorsal classes of progenitors - 6 Classes

how is the neural tube patterned along its dorsa-ventral axis?

• the BMP and Shh pathway antagonise each other

• because there’s BMP from the roof plate and Shh from the notochord and floor plate - so together the opposing gradients of BMPs and Shh pattern the DV axis

• BMPs from roof plate pattern the dorsal spinal cord

• Shh from the notochord and floor plate pattern the ventral part of the spinal cord/neural tube

what do the BMP antagonists like chordin help contribute to?

• they prevent BMPs from being relayed all the way down to the bottom of the ventral neural tube

progenitor domain made up of proliferating bands of cells are established through what?

• opposing action of BMPs/Wnts (dorsally) and Shh (ventrally)

• over time these differentiate and migrate (radially) to from defined neurons of the spinal cord and they then extend axons