Hedgehog, Wnt Signals and NC Cells

Question 1

Describe what the different hedgehog protein ligands are reponsible for.
Sonic HH
Indian HH
Desert HH

Answer 1

1. Development of the CNS, expressed in notochord and foreplate
2. Development of bone and cartilage
3. Development of peripheral nerves

Question 2

Describe the Normal pathway (unexposed to HH).

Answer 2

1. Patch is unexposed to ligand HH
2. Patch (Ptc) receptor BLOCKS Smoothened (Smo)
3. Gli(rep) is phosphorylated then partially proteolyzed to repress transcription of target genes due to this.
   * This pathway is CONSTITUTIVELY ON as Gli proteins are constantly proteolyzed into repressor

Question 3

Describe the Hedgehog Pathway.

Answer 3

1. HH signal binds to Ptc.
2. Smo is active and endocytosed into the primary cilium and able to ACTIVATE Gli 1 and 2 proteins
3. Gli “de-represses” target transcription as a FULL LENGTH protein

Question 4

Explain why ZPA is important.

Answer 4

“Zone polarizing activity” is another signal (aside from Shh) that functions for the location of digits (fingers). It works with Shh in posterior side of limb buds.

Question 5

Link between Medulloblastoma and HH ligand.

Description, cause, effect, tx

Answer 5

This cancer is a tumor of the cerebellum during development.
Cause: LOSS OF Ptch 1 receptor.
Effect: AMPLIFICATION of Gli 1 and 2 in HH pathway
Treatment: Vismodegib works by inhibiting Smo

Question 6

Describe the Wnt Pathway.

Canonical Wnt Pathway compared to normal

Answer 6

[Canonical Wnt]

1. Wnt binds Frizzled (FZ) receptor on surface
2. This activates Dsh -> Axin + APC + GSK-3 complex
3. Produces MORE Beta-Catenin
4. B-Cat enters nucleus and binds TCF to begin transcription
   * Over-expression of B-Cat or mutation of APC -colorectal cancer)

[Normal]
A. Phosphorylated Beta-catenin is ubiquinated by Axin/APC/GSK3 pathway.
B. B-Cat is destroyed by proteasome

Question 7

How does Shh relate to (HPE) Holoprosencephaly?

Answer 7

LOW Shh levels lead to HIGH Gli3 repressor activity.
This prevents development of ventral CNS
Features: Cyclopia, Single maximililary incisor, cleft palate
Haploinsufficiency of Shh - example of how a single copy ain’t good enough; can lead to incomplete midline formation in forebrain

Question 8

What role does cholesterol play in the HH pathway and what disease does its lack lead to?

Answer 8

1. Cholesterol is needed for self-cleavage of precursor HH to make mature HH for signaling.
2. Smith-Lemli-Opitz Syndrome - loss of function in CHCR7 gene that encodes for cholesterol.
   (Lack of Cholesterol = Lack of Shh = HPE-similar symptoms)

Question 9

What disease(s) result from too much Gli3 repressor?
Too little Gli3?

Answer 9

1. HPE, Cyclopia

2. Pallister-Hall, Postaxial Polydactyly (Haploinsufficiency - extra small digit)

Question 10

Neural Crest cell destinations.

Cranial, Trunk

Answer 10

Cranial: peripheral neurons, ganglia, cartilage, bone, CT
(Facial skeleton; defect in this migration leads to cleft lip)
Trunk: pigment cells, sensory glia of PNS, adrenal medulla

Question 11

Describe the effect of the different levels of BMP affecting ectoderm transformation. (Low, intermediate, high)

Answer 11

Low BMP: expressed near midline to form NEURAL TUBE
Intermediate BMP: expressed in NEURAL CREST for migration to the mesoderm
High BMP: expressed on the NEURAL PLATE to give rise to epidermis

Question 12

What 3 events must occur for migration to begin?

Answer 12

1. “Snail” is expressed for early transcription factors
2. Epithelial-mesenchymal transition - loss off 6B cadherins
3. Secretion of hyaluronic acid - makes space for transition

Question 13

What are some receptors on NC cells and what do these cells give rise to? (Name 2) Name some diseases associated with mutations.

Answer 13

C-kit: binds to “Steele” ligand that is secreted in a gradient and gives rise to pigment cells
*Defect: Piebaldism (Pigment migratory deficit)
C-Ret: binds to “GDNF” ligand that is also a chemoattractant that gives rise to enteric nervous system in the gut
*Defect: Hirschsprung Disease (HSCR)

Question 14

DiGeorge Syndrome (DGS)
(Symptoms, features, genetic cause, mutations)

Answer 14

Symptoms: several organ systems affected
-cardiac outflow defect, cranial anomalies, immune deficiency
Features: cleft lip, small jaws, affected pharynx
Genetic Cause: microdeletions in Ch. 22
Null mutations: Tbx1 (no GFs for NC migration)
-Also Crk1 (expressed on NC cells; pharynx dev’t. defect)

Question 15

Heterozygous Pigment Migratory Deficit (Piebaldism)

Mutation, Effect, symptoms

Answer 15

Mutation: C-kit receptor or Steele ligand
Effect: Pigment cell precursors do not migrate down from trunk NC cells
Symptoms: light pigment spot on forehead

Question 16

Hirschsprung Disease (HSCR)
(Mutations, Effect, symptoms)

Answer 16

Mutations: C-Ret receptor or GDNF ligand
Effect: deficiency of enteric ganglia - NC cells do not fully move down bowels.
Symptoms: constipation, huge colon

Question 17

Fetal Alcohol Syndrome

Cause, Effect, Symptoms/Features

Answer 17

Cause: Excess alcohol (EtOH as a teratogen in week 3)
Effect: Apoptosis of NC cells
Symptoms: 3rd leading cause of mental retardation, congenital heart disease, wide array of abnormalities
Features: low nose, smooth philtrum, cleft palate

Question 18

Describe Shah-Waardenberg Syndrome.

Cause, mutation, normal pathway, symptoms

Answer 18

Cause: truncated protein -> gain of function
MRNA is degraded form haploinsufficiency
Mutated Sox10 or receptor disrupts necessary NC migration.
Snail -> Sox10 -> c-Kit expression.
Symptoms: hearing loss, pimentation and HSCR symptoms (i.e. Constipation)

Question 19

What is Tbx1?

Answer 19

Tbx1 is a T-Box Gene that targets FGF genes downstream and is regulated by Shh signal protein.
It’s mutation is related to DiGeorge Syndrome

Question 20

More levels of Beta Catenin (Wnt Pathway) are found in what parts of the cell: the ________ and ________

Answer 20

Cytoplasm and plasma membrane.