Lecture 2: Neuronal Development (cont'd) and Axon Guidance

Question 1

What is Reelin?

Answer 1

a gene that when encoded, responsible for neuronal migration during development. Expressed in Cajal - Retzius cells.

Question 2

What happens in a reelin mutation?

Answer 2

Reelin is expressed in the Cajal - Reztius cells located towards the pial surface. The neurons do not wrap arounf the radial glia. Therefore, no reelin, cells in ventricular zone do not migrate up and layers are inverted.

Question 3

What is the purpose of notch signaling?

Answer 3

Notch signaling helps give rise the neuroblast and drives cell differences. Present in proneural cluster cells.

Question 4

What happens in the neuroblast?

Answer 4

The cell in the proneural cluster with the most Asc will become the neuroblast, inhibiting the other cluster of cells remain where they are. The neuroblast will migrate inside. The signal is local and inhibitory. Notch signaling facilitates this process.

Question 5

What is the purpose of Aschaete - scute?

Answer 5

Aschaete - scute (Asc) is present in drosophila proneural cluster cells. Helps with the creation of the neuroblast.

A gene a part of notch signaling. If absent, no nerve cells.

Question 6

Describe notch signaling.

Answer 6

1. Asc is translated first, protein is created and gives rise to DELTA.
2. Delta, membrane bound ligand, binds to NOTCH receptor on the receiving cell.
3. NOTCH cleaves off, enters the nucleus and binds to ENHANCER OF SPLIT.
4. EOS binds to ASC. ASC is now inhibited and slows down, leading to less transcription of ASC protein.
5. Because ASC slows down, delta is not being produced and cannot bind to notch on the other cell.

One cell ends up with more ASC than the other, making it the neuroblast.

Question 7

What if NOTCH is mutated?

Answer 7

The inhibitory pathway will be gone. You’ll have too much ASC, too many neuroblasts, and too many neurons.

Question 8

What if Asc is mutated?

Answer 8

No neuroblasts because Asc is needed in neural cluster. No neurons.

Question 9

What if Enhancer of Split is mutated?

Answer 9

EOS inhibits Asc, if not present, more Asc means more excess neurons.

Question 10

What if there is a mutation of delta?

Answer 10

There would be no notch signaling but too much Asc, creating more neural cluster cells and too many neurons.

Question 11

What is interkinetic migration?

Answer 11

Involves radial glia cells. The way nucleus migrates along the progenitor glial cells
The nuclei migrate up to the basal section as they progress through the cell cycle.

Question 12

What are the molecules involved with migration along the glial cells?

Answer 12

Reelin and Doublecortin (DBX)

Question 13

What happens in doublecortin mutation?

Answer 13

Doublecortin is expressed in the neurons migrating and is needed in microtubule movement. double cortex layers, misplaced cells. Doesn’t follow typical migration pattern.

Question 14

What other ways do cells migrate to cortex to become neurons?

Answer 14

From the neural crest. Neural crest cells do not rely on glial migration or tangential migration, rely on free migration.

Question 15

How does cell fate get determined in crest cells?

Answer 15

Transcription factor expression. Some are built in with the ability to change based on what the target is. A neuron might have the ability to secrete norepinephrine or AcH based on where they target.

Question 16

What happens to motor neurons over time?

Answer 16

We can observe less motor neurons as time passes with development.

Question 17

What experiment can show you what happens to motor neurons over time and the way they survive?

Answer 17

Neurotrophic Factor Hypothesis Experiment:

A. Remove the limb during development so that the target muscle could not be innervated by motor neuron. There will be an increase in motor neuron death present at spinal cord at the side where the limb was removed.

B. When you add an additional limb bud to the embryo, more motor neurons survived.

C. When the leg was paralyzed, more motor neurons are rescued. Paralyzing the muscle blocks electrical activity in the muscle. This also led to more motor neurons surviving.

Question 18

Describe Neurotropic Factor Hypothesis.

Answer 18

The target of innervation releases a limited amount to neurotropin to neuron, which is essential for the neuron’s survival. Aids in determining the correct number of neurons.

Question 19

Describe neurotrophic factors

Answer 19

Nerve growth factor (NGF), Brain derived neurotrophic factor (BDNF), and neurotrophin -3 (NT-3) all bind to a transmembrane tyrosine kinase receptor (Trk). p75 receptor can bind with all three neurtrophins. Neurotrophic factors delay programmed cell death.

Question 20

How do cell dies?

Answer 20

Apoptosis is programmed cell death. If the cell does not receive the right signal, it will die based of of preprogrammed instructions. Apoptosis appears to be the neuron’s default state. This process is mediated by the protein capases.

Question 21

What are the pathways in which cells can die?

Answer 21

Extrinsically and intrinsically (intracellular events).

Question 22

What are the four stages of neuronal growth?

Answer 22

Lamellipodia formation, neurite formation, neurite forms axon, and other neurites become dendrites.

Question 23

What is the purpose of lamellipodia?

Answer 23

Composed of actin filaments and are necessary for cell migration, as it is responsible for cell mobility.

Question 24

What are the molecules responsible for axon and dendritic growth, respectively?

Answer 24

Tau protein localized in axon and MAP2 protein in dendrites (SAD Kinases).

Question 25

Neurons have a stereotypical orientation. What determines this?

Answer 25

Extracellular factors. Semaphroin -3A (Sema3A) is secreted by cells near the pial surface, which neurons are attracted to and will grow dendrites. Without it, neurons lose their parallel orientation. Sema 3A is responsible for neuron polarity.

Question 26

How do dendrites grow?

Answer 26

Dendritic formation is determined by intrinsic programming set up during development.

STEPS: Initiation, outgrowth, branching, spine formation, stopping/pruning (in order to have sufficient amount of dendrites). They do not bunch together because of repellant signaling between dendrites.

Question 27

Define axon guidance.

Answer 27

The way an growth cone moves to find a post synaptic target.

Question 28

What is the growth cone?

Answer 28

The growth cone is a structure at the tip of the axon that moves through the embryo sensing the environment for different substrates. It pulls the axon forward.

Question 29

What makes up the growth cone?

Answer 29

Central core: made up of microtubules and other organelles.
Filopodia: project from cell body, receives signals from the environment that stimulates the growth cone.
Lamellipodia: between the filopodia.

Question 30

Describe Roger Sperry’s Chemoaffinity Hypothesis.

Answer 30

Neurons are predetermined during development to go to a specific location (think location, dorsal/ventral). When the frog’s eye was removed and rotated 180 degrees, the cells grew back in their place of origin, but the location was rearranged (up perceived as down, left perceived as right). When the cells grew back, retinal cells grew towards their original target on tectum cells, not taking into account the rotation.

This experiment showed that the axon on the retinal cells recognized its targets in the tectum because of guidance cues. This confirms axon guidance.

Question 31

Provide a molecular explanation of the chemoaffinity hypothesis.

Answer 31

Researchers discovered posterior retinal tissue grew only on anterior tectal membrane. This may be possible because posterior retinal tissue is repulsive towards posterior tectal membrane and prefers to grow on anterior tectal membrane.

Question 32

How do more neurons develop?

Answer 32

Progenitor cells in the ventricular zone of the neural tube. They operate like stem cells and can create copies of themselves as well as give rise to differentiate neurons and glial cells.

Question 33

Radial glia cells

Answer 33

• cell body located in ventricular zone
• give rise to different neurons
• differentiate into astrocytes

Question 34

Describe tangential migration.

Answer 34

In tangential migration, neurons use axonal tracts to guide where they need to go. This happens in LGE and MGE.

Question 35

Describe the role of second messengers.

Answer 35

Second messengers influence how the cytoskeleton is organized and regulates the direction and growth rate of the growth cone. PKA activity determines the growth cone’s response to an extracellular factor, like netrin. High PKA activity -> elevation in cAMP, allowing the growth cone to be attracted to netrin.

Question 36

What did the strip assay experiment show?

Answer 36

Posterior cells will secrete something that acts like a repelent so that later axons don’t bump into each other. Posterior retinal cells are closer proximity to tectal anterior cells, therefore, reaches its target first.

Question 37

What happened to the paralyzed limb bud?

Answer 37

When you inhibit electrical activity, you don’t get the scrambling of receptors on the motor neuron. This scrambling helps with the pruning of motor neurons. When you don’t get electrical activity, you don’t get the pruning of the neurons, resulting in 75% neurons remaining.

Question 38

What is the clutch hypothesis?

Answer 38

States that during axon extension you need a substrate to ground the axon as it moves How the growth cone can latch on to a substrate to pull the entire cell forward. Determines how they can turn away from different chemicals.