Lecture 2: Neuronal Development (cont'd) and Axon Guidance Flashcards
What is Reelin?
a gene that when encoded, responsible for neuronal migration during development. Expressed in Cajal - Retzius cells.
What happens in a reelin mutation?
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.
What is the purpose of notch signaling?
Notch signaling helps give rise the neuroblast and drives cell differences. Present in proneural cluster cells.
What happens in the neuroblast?
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.
What is the purpose of Aschaete - scute?
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.
Describe notch signaling.
- Asc is translated first, protein is created and gives rise to DELTA.
- Delta, membrane bound ligand, binds to NOTCH receptor on the receiving cell.
- NOTCH cleaves off, enters the nucleus and binds to ENHANCER OF SPLIT.
- EOS binds to ASC. ASC is now inhibited and slows down, leading to less transcription of ASC protein.
- 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.
What if NOTCH is mutated?
The inhibitory pathway will be gone. You’ll have too much ASC, too many neuroblasts, and too many neurons.
What if Asc is mutated?
No neuroblasts because Asc is needed in neural cluster. No neurons.
What if Enhancer of Split is mutated?
EOS inhibits Asc, if not present, more Asc means more excess neurons.
What if there is a mutation of delta?
There would be no notch signaling but too much Asc, creating more neural cluster cells and too many neurons.
What is interkinetic migration?
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.
What are the molecules involved with migration along the glial cells?
Reelin and Doublecortin (DBX)
What happens in doublecortin mutation?
Doublecortin is expressed in the neurons migrating and is needed in microtubule movement. double cortex layers, misplaced cells. Doesn’t follow typical migration pattern.
What other ways do cells migrate to cortex to become neurons?
From the neural crest. Neural crest cells do not rely on glial migration or tangential migration, rely on free migration.
How does cell fate get determined in crest cells?
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.
What happens to motor neurons over time?
We can observe less motor neurons as time passes with development.
What experiment can show you what happens to motor neurons over time and the way they survive?
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.
Describe Neurotropic Factor Hypothesis.
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.
Describe neurotrophic factors
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.
How do cell dies?
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.
What are the pathways in which cells can die?
Extrinsically and intrinsically (intracellular events).
What are the four stages of neuronal growth?
Lamellipodia formation, neurite formation, neurite forms axon, and other neurites become dendrites.
What is the purpose of lamellipodia?
Composed of actin filaments and are necessary for cell migration, as it is responsible for cell mobility.
What are the molecules responsible for axon and dendritic growth, respectively?
Tau protein localized in axon and MAP2 protein in dendrites (SAD Kinases).
Neurons have a stereotypical orientation. What determines this?
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.
How do dendrites grow?
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.
Define axon guidance.
The way an growth cone moves to find a post synaptic target.
What is the growth cone?
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.
What makes up the growth cone?
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.
Describe Roger Sperry’s Chemoaffinity Hypothesis.
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.
Provide a molecular explanation of the chemoaffinity hypothesis.
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.
How do more neurons develop?
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.
Radial glia cells
- cell body located in ventricular zone
- give rise to different neurons
- differentiate into astrocytes
Describe tangential migration.
In tangential migration, neurons use axonal tracts to guide where they need to go. This happens in LGE and MGE.
Describe the role of second messengers.
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.
What did the strip assay experiment show?
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.
What happened to the paralyzed limb bud?
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.
What is the clutch hypothesis?
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.
