Hedgehog and Wnt pathways

Question 1

What are the members of the HH family and their roles in development?

Answer 1

Indian - cartilage

Sonic - CNS

Desert - PNS

Question 2

Describe the members of the HH pathway

Answer 2

Hedgehog ligands are hydrophobic proteins that are secreted by specific groups of cells.

Patched receptor binds the ligands, 12-span transmembrane protein.

Smoothened doesn’t bind HH ligands and is a 7-alpha-helix G protein coupled transmembrane protein.

The Gli/Ci proteins are transcription factors that can either activate or repress expression based on their status and through their zinc finger DNA binding domain.

PKA - phosphorylates the Gli/Ci proteins and targets them for degradation.

Question 3

Describe the mechanisms for the on and off HH pathways.

Answer 3

With the lack of HH ligand, the patched receptor is constitutively inhibiting Smoothened. It is thought that in primary cilia, patched prevents the movement of smoothened there and so signals can’t be received. Gli proteins will be targeted for partial proteolysis and enter the nucleus as a repressor.

When HH does bind to Patched receptor, the complex gets ingested and degraded, allowing for the smoothed to block proteolysis allowing for the activation of target genes.

Question 4

What are the members of the HH family and their roles in development?

Answer 4

Indian - cartilage

Sonic - CNS

Desert - PNS

Question 5

Describe how digit pattern is formed?

Answer 5

Via gradients of Shh with the highest concentration in the ZPA in the posterior. This correlates to levels of GliAct and counters the gradient of Gli repressor highest in the anterior.

Question 6

What is the effect of Shh in the dentate gyrus (part of the hippocampus)

Answer 6

If you knock out HH, you get a reduction in dentate gyrus cells.

If you use an HH agonist, you will get higher activity.

Question 7

What is HPE and the role of Shh.

Answer 7

Holoprosencephaly: defective midline formation and brain development

Symptoms: cyclopedia, single forebrain vesicle, single maxillary midline incisor.

Cause: Shh protein haploinsufficiency (only one copy of the gene). The patient is limited to brain and face defects suggesting different structures have different sensitivities developmentally to reduced Shh.

This could arise from mutations in Shh, GLI2, Smoothened, Patch, any step in the pathway that increases GLi2 repressor.

Question 8

Describe the role of cholesterol in SHH.

Answer 8

Mature SHH protein is made from a cleavage of a longer precursor and then the addition of a cholesterol to the 3’ end.

If the cleavage or the cholesterol addition is mutated, this can lead to HPE and Smith-Lemli-Opitz syndrome.

A lack of cholesterol produces the same types of phenotypes so cholesterol reducing drugs ought to be carefully used.

Question 9

Describe the situation in those with medulloblastoma.

Answer 9

Some of them may have Patch1 mutations. The pathway would be disinhibited even in the absence of SHH.

This leads to amplification of Gli1 and Gli2.

Medulloblastoma is a tumor arising in the cerebellum. It arises from granule neuron progenitors.

So a mode of treatment would be to selectively inhibit smoothened in those cells.

Question 10

Describe Wnt pathway.

Answer 10

Wnt is not hydrophobic so it needs to bind to receptor.

Canonical Wnt pathway activates beta catenin.

Question 11

How can the Wnt pathway cause cancer?

Answer 11

mutations in beta-catenin that prevent phosphorylation would have pathways constitutively on.

APC (member of the degradation complex) mutations has strong link to colorectal cancer in FAP.

So Axin,Gsk, Apc are tumor supressor genes.

Beta catenin and Wnt are prooncogenes.

Question 12

How is the neural tube formed by the activites of BMP and noggin, chordin, FGF

Answer 12

Noggin, chordin, FGF suppress BMP activity near the midline.

Area of low BMP forms the neural plate.

High BMP will form the overlying epidermis.

And intermediate BMP forms the neural crest cells. After the tube is formed from the neural plate, the intermediate zone will migrate.

Question 13

What cell types derive from neural crest cells.

Answer 13

Neurons and cranial ganglia

Cartilage and bone

Connective tissue

Adrenal cells

Sensory neurons in the gut

And pigment cells.

Question 14

How do intermediate levels of BMP signal neural crest cells?

Answer 14

Intermediate BMP with FGF will activate snail

Question 15

What two things must occur for NC cells to migrate?

Answer 15

NC must change from epithelium to mesenchyme via loss of 6B class of cadherins

And they need space to migrate so hyaluronic acid is secreted to generate space.

Question 16

What are dominant spotting and steel conditions?

Answer 16

They alter pigmentation and anemia, sterility.

The mutation is on a receptor or the ligand that is produced by cells in the migratory pathway.

C-kit receptor mutation (dominant spotting) will not bind Steel which is a chemoattractant for receptor expressing cells in the migratory pathway.

Question 17

How is NC cells into the gut regulated and why is it clinically relevant.

Answer 17

C-ret is the receptor expressed in neural crest cells and GDNF is the chemoattractant.

Mutation in either will lead to Hirchsprung where sensory ganglia can’t enter the gut and peristalsis can’t occur.

There are two expressions of GDNF, GDNF first reaches a part of the gut and then a second larger expression occurs in the more posterior region and recruits the ganglia further down.

Question 18

What is DiGeorge Syndrome?

Answer 18

It is caused by defects in neural crest cell migration.

You have cardiac outflow tract and septal defects, immunedefficiency, craniofacial anomalies, cleft palate, small jaw, and low-set abnormal ears.

So they can be divided into defects in pharyngeal pouch (larynx) and neural crest derivatives.

So there is interaction between these two tissues and a failure of either tissue could lead to DGS. Neural crest cells invade the pharyngeal arches and gives rise to thymus, parathyroid and cardiac outflow tract.

Likely the gene being mutated is Tbx1 (found in pharyngeal arches) and Crk1 gene (Neural crest specific).

Question 19

What is Tbx1?

Answer 19

At midgestation, Tbx1 is expressed in pharynx cells but not migrated neural crest cells.

Tbx1 is regulated by Shh and it affects FGF family genes. Mutations in any can lead to the phenotypic symptoms: abnormal aortic arches, hypoplastic thymus, septal defects, cleft palate, low set ears.

Question 20

How does Tbx1 and Crk1 work together?

Answer 20

So Tbx1 expressed in the pharyngeal tissue will activate FGF genes which promotes survival of the migrating neural crest cells. Essentially Tbx1 is responsible for the environment neural crest cells find themselves in.

From there Crk1, is responsible for the completion of the neural crest developmental pathways.

Question 21

What is the effect of alcohol on development.

Answer 21

It leads to apoptosis in NC cells.

You get smooth philtrum, mental retardation, low nose bridge,

2-3 oz hard liquor a day or a single binging episode.