Neural crest part 2

Question 1

What do vagal and sacral neural crests both contribute to?

Answer 1

gut enteric ganglia -> innervation of the gut and therefore control the intestinal peristalsis (muscular contractions of the gut that allow faeces to leave the body).

Question 2

What is Hirschsprung’s disease?

What causes it?

What are the complications?

Answer 2

• This disease arises when neural crest cells no not populate parts of the gut. Normally the neural crest cells tell the colon, rectum and anus to work together to push waste out the body. However with Hirschsprung’s disease a section of the colon is missing nerves because neural crest have failed to populate this section during embryonic development.
• This leads to a portion of the gut being less smaller than it should be and a large section above full of waste.
• This disease leads to constipation and can lead to life threatening infection. It requires surgery where the aganglionic part of the gut is removed and the gut is joined together.

Question 3

What controls enteric NC migration?

Answer 3

• Ednrb/Sox10+ NCC migrate into Edn3+ve foregut
• The gut mesenchyme expresses GDNF
• NCC express Ret (GDNF receptor) and migrate towards GDNF
• High levels of GDNF in caecum cause differentiation
• GDNF and Ret deficiency results in Hirschsprung disease

Question 4

How do cranial neural crests migrate?

Answer 4

• Cranial neural crest migrate between the dorsal neural tube and the overlying ectoderm or between the dorsal neural tube and underlying somites.

Question 5

How do the trunk neural crests migrate?

Answer 5

• Trunk neural crest have a variety of different migration pathways. Early migration takes place between the somites or between the dorsal neural tube and the ectoderm.
• Mid stage neural crest migration takes places takes the ventral lateral pathway through the rostral sclerotome (anterior part of the somite) where they migrate to the dorsal aorta.
• Late trunk neural crest migration is within the somite and these neural crest will give rise to dorsal root ganglia and those that make it to the dorsal aorta will give rise to sympathetic ganglia.
• There is also a late trunk neural crest migration called the dorsolateral pathway which moves over the dermomyotome of the somite.
• These pathways are regulate by different members of the neuropylin and semaphorin cell signalling family.
• Trunk neural crest migrating via ventral pathways, between or through somites at early and mid-stages of migration give rise to several different types of derivatives:

Question 6

What causes Neuroblastoma?

Answer 6

• Rare childhood cancer: heterogenic with variable severity and mortality
• manifests anywhere along the sympathetic nervous system, but frequently in the abdomen along the sympathetic chain and in the adrenal gland medullary region
• Precise origin still unclear but probably neural-crest-derived sympathoadrenal progenitor cells that differentiate to sympathetic ganglion cells and adrenal catecholamine-secreting chromaffin cells
• Dysregulation of neural crest EMT can give rise to hyperneoplastic lesions that can result in neuroblastoma.
• Dysregulated expression of MYCN is the most powerful oncogenic driver for neuroblastoma and induces proliferation and inhibits apoptosis of sympathoadrenal lineage cells
• LIN28B control MYCN expression via regulating Let-7 miRNA.
• Anaplastic lymphoma kinase (ALK) and paired-like homeobox 2B (PHOX2B) are germline mutations found in neuroblastoma.
• Rho signaling is crucial for neural crest migration, the controlling contact inhibition of locomotion.
• SWI/SNF complexes are tumor suppressors which influences the transcriptional output, DNA replication and repair.

Question 7

What do cells specifically do during a EMT transition (at a cellular level)?

Answer 7

• This involves the removal of cell to cell junctions and reduction of cell to cell adhesions
• They also need to re-arrange the cytoskeleton to allow cell motility and to be able to remodel the extracellular matrix
• These all enable the mesenchymal phenotypic switch where cells lose their epithelial characteristic of being bound through tight epithelial sheets to a more mesenchymal migratory characteristic involving modulating cell shape to allow increased cell motility

Question 8

Compare neural crest migration to cancer metastasis

Answer 8

• Both cell types undergo EMT. These are initiated by signalling pathways which are in common between cancer and neural crest formation
• The transcription factors that regulate the switch to a more migratory cell type include common TFs between the two sets.
• This is followed by a cadherin switch (cell adhesion molecules that regulate the formation of adherons junctions between cells). A switch is needed in both groups
• Extracellular remodelling regulated by MMPs and ADAM proteins
• The migratory cells make use of pre-migratory structures in order to spread such as metastasis cells and nerve pathways to migrate and basement membranes.

Question 9

How do cells migrate?

What process mediates formation of cell-cell contacts?

Answer 9

• This involves the asymmetric organisation of the cell cytoskeleton
• The cell stretches to form a leading and trailing edge
• Trailing adhesions then dissolves and myosin base contractility occurs

Refer to Rho-GTPase slide

Question 10

Define chemotaxis

Answer 10

• This is the movement of a motile cell organism or part of an organism in a direction corresponding to a gradient of increasing or decreasing concentration of a particualar substance

Question 11

How is polarisation generated in a migrating cell?

Answer 11

• This is done via expression of RAC GTP at the leading end of the cell and RhoA GTP at the rear of the cell.
• They both repress each other causing polarisation of the cell

Question 12

What other mechanisms exist for separating neural crest cells into there separate streams?

Answer 12

• As mentioned before neural crest stream will separate from each other.
• Along with previous molecules described other molecules are involved in the restriction of the neural crest zones such as Semaphorins and there receptor neuropilins (non-tyrosine kinase receptors) and the slit secreted glycoprotein ligands which bind to Roboimmunoglobin receptors.
• These are all expressed in lateral domains in the developing embryo and help restrict the neural crest into there separate streams.

Question 13

What explains the reason that allows neural crest cells to migrate from the dorsal to the ventral direction from the neural tube?

Answer 13

• There are 2 main hypothesis for this based on chemoataxis:
• Positive chemotaxis involves high levels of expression to which molecules are attracted to because of matching receptor ligand molecules they express themselves. However no evidence of these chemoattractants has been found.
• Negative chemotaxis involves the neural tube expressing molecules that repel neural crest cells because of there molecular signature and therefore migrate away from the neural tube. However, this was dispropen by Erickson in 1985 by showing grafted neural crest can move in the reverse direction against the normal migration pathway.
• Therefore, we can draw the conclusion that only the leader cells are polarised

Question 14

How do cells move in a group?

Answer 14

• This image shows xenopus neural crest cells cultured as either a group or a singular cell. The blue line is tracking the movement of the cell in the culture.
• Cells at the edge of the group become polarised and move away from each other while the internal cells are moving more randomly.
• The isolated cell don’t migrate well with little directional progress which suggests that directional movement of neural crest cells depends on cell-cell contacts which is likely to inhibit the formation of cell protrusions which lead to cell-polarisation in internal cells of the group. However, this is possible for cells at the edge of the group which allows the crest migrate as a collective group.

Question 15

Describe the cellular pathways that occur with a cell-cell collision and how they migrate away from each other

Answer 15

• In the image on the left we can see increasing levels of RhoA during a collision which increases localise to the regions of cell-cell contact.
• This will lead to the repression of Rac and the collapse of cell protrusions in this region leading to a change in cell polarity
• In the next image there is a downregulation of Rac at the collision imterface.
• The activation of RhoA is mediated by the non-cononical planar cell polarity cell polarity mediated by the expression of the gene Dsh. At the same time N-cadherin inhibits Rac-1 at the position of cell contact and increases Rac expression at the free egde of the cell, changing cell polarisation.
• RhoA and Rac then repress each other re-inforcing polarisation of the cells follow cell-cell collision

Question 16

Model of NCC directional migration

Answer 16

• Neural crest move as a collective mass of cells with high directionality in streams of limited width.
• As previously mentioned neural crest inhibitor signals keep neural crest cells within there stream.
• The planar cell polarity PCP wnt pathway is needed for this directionality.
• PCP signals direct the expression of RhoA to prevent the formation of projections and polarisation of the cell
• Neural crest in the main body of the stream are fully surrounded by other cells and express PCP at all contact points and therefore are not polarised. Leading cells at the edge of the stream are not fully surrounded and have a free edge where polarisation can occur via regulation of Rac and formation of cell protrusions.
• Lateral repulsive signals prevent sideways migration so the leading neural crest cells move together in one direction, with the rest of the neural crest stream following.

Question 17

Neural crest are attracted by placodes and placodes are repelled by neural crest

Answer 17

• In an experiment where neural crest cells and placodes are cultured together they engage in chase and run behaviour
• When cultured alone neural crest cells move randomly and placode cells hardly move at all.
• When they are both present placodes switch to a directional movement away from the neural crest cells and the neural crest express directional migration towards the placodes.
• Therefore neural crest cells are attracted towards the placodes, whereas the placodes are repelled by neural crest cells.
• This behaviour is mediated by placode expression of Sdf1 – the ligand for cxcr4
• Placode are a source of Sdf1 cell in vivo and establish an interaction where neural crest cells chase sdf1 positive placodes precursor cells but are also repelled by them.
• Contact between the neural crest cells and placodes induce a Ncadh mediated contact inhibition of locomotion. Protrusions are inhibited in placodes at contact sites, breaking symmetry allowing the polarisation of placode cells and directional movement away from neural crest cells.
• The whole system sustains due to chemotaxis and contact inhibition of locomotion