Unit 3: Neurodevelopment Flashcards
1
Q
What is the ganglionic eminence?
A
A section of the developing cortex that produces inhibitory interneurons that migrate tangentially to radial glial cell progeny.
2
Q
Which are the last nervous system cells to be developed?
A
Glia.
3
Q
Where are oligodendrocytes born?
A
The ganglionic eminence.
4
Q
Where are adult new neurons born from?
A
Stem cells lining the lateral ventricle and sub-ventricular zone.
5
Q
What are somites?
A
Blocks of mesoderm that line the neural tube.
They determine the migratory path of neural crest cells and the axons of spinal nerve cells.
6
Q
What are neural crest cells repulsed by?
A
Ephrin B on the caudal side of somites.
7
Q
What do neural crest cells develop into?
A
Eye, sympathetic ganglion, parasympathetic ganglia, Schwann cells, melanocytes, Schwann cells for all cranial nerve ganglia, aortic arches, dorsal root ganglia, adrenal medulla, and enteric ganglia.
8
Q
How do neural crest cells develop in different locations?
A
Their fate depends on local signals. They can change their differentiation if placed in a different environment.
9
Q
What are some local factors that change neural crest cell fate?
A
BMP, Glial growth factor, and TGF-𝛽.
10
Q
What is induction?
A
The process by which a tissue is instructed to adopt a particular fate.
11
Q
What is determination?
A
When the cell commits to adopting a particular fate.
12
Q
What is differentiation?
A
The process by which a neuron or glial cell attains all the mature properties of the fate to which it has committed. It has become that fate in morphology, gene expression, physiology, and synaptic connectivity.
13
Q
What are progenitor cells?
A
Dividing cells gives rise to many early cells. Typically pluripotent or multipotent.
14
Q
What are post-mitotic cells?
A
Progenitor cells offspring that induce expression of transcription factors that direct different differentiation programs.
15
Q
How can differentiation be induced in cells?
A
Localization of different factors to separate sides of the cell. Ex: Numb and notch.
Expression of different transcription factors.
16
Q
What is the difference between pan-neuronal factors and subtype-specific factors?
A
Pan-neuronal factors only specify a generalized neuronal phenotype.
Subtype-specific factors send neurons down a specific pathway.
17
Q
What is Fezf2?
A
A transcription factor that is specific for layer V neurons. It leads to the expression of Ephb1 which promotes specific axonal guidance.
18
Q
What are MAPs?
A
Microtubule-associated proteins.
19
Q
What is the major MAP in the axon and in the dendrite?
A
Tau is the major MAP in the axon.
MAP-2 is the major MAP in the dendrite.
20
Q
How is it decided which neurite becomes the axon?
A
It depends on which one enters a new substrate first.
21
Q
How do axons get to where they need to go?
A
Axons are attracted to/repulsed by different factors/local guidance factors. Unique combinations of these lead to axons getting to their targets.
22
Q
What are the 4 major cytoskeletal systems?
A
Microtubule
Neurofilament
Microfilament
Actin
23
Q
Are microtubules polarized?
A
Yes.
24
Q
Where are polymerization and depolarization faster in the microtubule?
A
In the + end.
25
How do microtubules become stable?
If they have a high molecular weight.
If they are linked to other cytoskeletal elements.
If they have a tubulin-binding end.
26
Why are neurofilaments cross-linked?
To provide strength and stiffness.
27
Where are neurofilaments used the most?
In the axon.
28
What are the crossbridges in axons?
Neurofilament projections.
29
What are the crossbridges in dendrites?
Microtubules.
30
How do motor MAPs work?
They move along MAPs using ATP-carrying vesicles.
Kinesins move toward the + end.
Dyneins move toward the - end.
31
How are microtubules oriented in axons?
The + ends are distal and away from the cell body.
32
How are microtubules oriented in dendrites?
They are randomly oriented.
33
What is transported in fast anterograde axonal transport?
Organelles and vesicles that are transported via kinesin.
34
What is transported in fast retrograde axonal transport?
Vesicles carrying signaling molecules from axon terminals → cell body.
Transported via dynein.
35
What is transported in slow axonal transport?
Actin, and soluble metabolic enzymes that are used for glycolysis and neurotransmitter synthesis.
36
What is transported in the slowest axonal transport?
Microtubules and neurofilaments.
37
Is kinesin in all neurites?
Initially yes, but they move to the axon during its specification.
38
Does microtubule stabilization promote axon specification?
Yes.
39
What are the domains of the growth cone?
Peripheral (P) domain
Central (C) domain
Transition (T) domain
40
What is the peripheral (P) domain of the growth cone?
A network of actin filaments (F-actin) that form the filopodia.
41
What is the central (C) domain of the growth cone?
The area that encloses stable microtubules that enter the growth cone from the axon shaft.
42
What is the transition (T) zone of the growth cone?
The area between the P and C domains that forms an actomyosin contractile arc.
43
How do growth cones grow?
Growth cones grow by adding material to the leading edge of the growth cone.
44
What happens to microtubules when they enter the growth cone?
Microtubules are organized into bundles by microtubule-associated proteins (MAPs). Once they center the C domain, they defasciculate. They go through the T zone and then enter the P domain where they become aligned with the F-actin bundle.
45
What happens to F-actin as the growth cone grows?
The terminal of F-actin is in the T domain where it is severed into short filaments. The broken actin parts are then added to the filopodia tip end.
46
What is Filamentous (F)-actin treadmilling?
F-actin is polymerized at the leading edge and severed at the T zone. Subunits are then recycled back to the leading edge.
47
What is F-actin retrograde flow?
Continuous movement of F-actin from the leading edge towards the center of the growth cone, leading to idle growth.
48
What happens with F-actin retrograde is slow?
Propulsion occurs.
This leads to microtubules no longer being cleared from the P domain. The microtubules can now serve as a track for lamellipodal engorgement which enables axon growth and growth cone turning.
49
What is defasciculation?
Axons leave a nerve bundle and enter a target area.
This is the first step of target selection.
50
What happens to microtubules when an attractant is found?
There are exploratory microtubules that act as a scaffold for actin bundles. Once an attractant is found, more microtubules are recruited.
51
What are the stages of axon growth?
A substrate is encountered.
Protrusion = filopodia and lamellipodia-like veils extend forward.
Engorgement = C domain moves forward.
Consolidation = new axon shaft forms.
52
What are the 4 different types of axon-guidance cues?
Contact-mediated repulsion
Contact-mediated attraction
Diffusible chemorepellants
Diffusible chemoattractants
53
What molecules are usually attractants?
Cell-ECM contact
Cell-cell contact with cell adhesion molecules
Cell-cell contact with cadherins.
Netrin family
Netrin-DCC signaling attracts growth cones.
54
What molecules are usually repulsive?
Cell-cell contact with semaphorins
Slit family; Slit-Robo signaling is repulsive.
Cell-cell contact with ephrins
55
What do attractive cues do?
Promote actin filament elongation.
Engage the clutch to stop retrograde flow
Promote microtubule extension.
56
What do repellant cues do?
Dissolution of actin filaments.
Loss of dynamic microtubules.
57
What is the role of Rho GTPases in axon growth? Which ones are involved?
Rho GTPases couple guidance cues to the actin cytoskeleton.
Cdc42, Rac1, and RhoA.
58
What is the role of RhoA in axon growth?
Depolymerizes actin and induces endocytosis to the leading edge of the plasma membrane.
This leads to filopodia and lamellopodia disappearing.
59
What is the role of Rac in axon growth?
Recruited by chemoattractant receptors to polymerize actin and facilitate filopodia and lamellopodia formation.
60
What is the role of Cdc42 in axon growth?
Recruited by chemoattractant receptors to polymerize actin and facilitate filopodia and lamellopodia formation.
61
What is a downstream protein of Rho signaling involved in axon growth and what does it do?
ROCK
Actin polymerization/depolymerization, branching
62
How does Netrin-DCC signaling interact with the Rho family?
Activates Rac/Cdc42, and inhibits Rho.
63
How does Slit-Robo interact with the Rho family?
Activates Rho and inhibits Rac.
64
What molecule increases in the growth cone in response to attractants?
Ca2+
65
What can block response to attractants and how does it do this?
SKF blocks TRPC Ca2+ channels. So, this leads to turning towards attractants to be inhibited.
66
What role does tension play in axon growth?
Mechanical tension causes axon growth.
The process requires intact microtubules.
Tension leads to the microtubule segments to accumulate. They then thin out as the axon elongates.
67
What is the role of pioneer axons?
Pioneer axons are guidance posts for follower axons.
68
What was the first molecule identified to play a role in selective fasciculation?
FasII
69
Are growth and adhesivity correlated?
Not really for physiological substrates.
70
What happens when a growth cone encounters a repulsive signal?
It stops, the growth cone collapses and the growth cone turns and grows in a random direction.
71
What type of guidance molecule is EphrinB?
A repulsive signal.
72
What axons does Sema3A guide?
It guides LMCm (lateral motor column) toward it as they are insensitive to it.
It guides LMCI away from it as LMCm is going there to the ventral limb.
73
What axons does Sema3F guide?
It guides LMCm axons away from it.
It guides LMCI axons toward it into the dorsal limb as they are insensitive to Sema3F.
74
What sort of guidance molecule is netrin?
A chemoattractant for pioneer axons to the floorplate.
75
What sort of guidance molecule is slit?
A chemorepellant for pioneer axons on the floor plate.
76
What is the receptor for netrin?
DCC.
77
What is the receptor for slit?
Robo
78
What occurs if netrin is KO?
No commissure forms.
79
What is the behavior of retinal ganglion cell growth cones?
The growth cones extend rapidly across the midline in the optic chiasm.
The growth cone pauses in the midline of the optic chiasm and then advances rapidly into the midline after the pause.
80
What is the resonance hypothesis?
Paul Weiss proposed this in 1939.
Axons are guided solely by mechanical cues.
Then, sensory/motor experience allows the connections to be refined.
81
Is the resonance hypothesis supported?
No.
After innervation was changed, the correct behaviors were never learned.
82
How are retinotopic maps organized?
By Ephrin-Eph signaling gradients.
83
What characteristics make up an NMJ?
100s of active zones
There is a certainty of a postsynaptic response.
Acetylcholine is the main neurotransmitter
No inhibitory synapses are present
The presynaptic terminal is at the end of the axon.
84
What characteristics make up a CNS synapse?
One active zone
A postsynaptic response is not certain.
Glutamate and GABA are the primary neurotransmitters.
The presynaptic terminal is often along the shaft.
Agrin is not critical to CNS synapse development.
CNS synapses are much more varied in structure and molecular components.
No basal lamina is present.
85
How does the presynaptic cell contribute to building a synapse?
The growth cone develops into a terminal bouton.
Makes stable contact with the postsynaptic cell
Synaptic vesicles and active zone machinery are prepared.
86
How does the postsynaptic cell contribute to building a synapse?
Forms the basal lamina
Neurotransmitter receptors are clustered
PSD is formed.
87
What is a motor unit?
One motor axon and all of the junctions it innervates.
88
How does neural activity affect NMJ postsynaptic receptor clustering?
Neural activity activates nuclei close to it to express AChR; nuclei farther away are repressed from expressing AChR.
89
What is the agrin hypothesis?
Agrin is released by motor terminals and is incorporated into the basal lamina to stimulate AChR clustering.
90
What does agrin do?
Agrin is a proteoglycan that binds to a MuSK/Lrp4.
MuSK activates Rapsyn, a cytoplasmic protein that binds AChR to the membrane and aggregates them into receptor clusters.
Agrin is only available at the synaptic site to stabilize the AChRs there.
91
Is the agrin hypothesis supported?
It’s not the full picture.
AChR receptors are clustered along the postsynapse before the presynaptic cell arrives.
92
How are AChRs clustered without agrin?
Dok-7, a cytoplasmic muscle protein binds MuSK, activates it, and induces AChR clustering. This happens without needing agrin.
Dok-7 basically gets them ready and innervation refines their clustering.
93
What happens when agrin is KO’d?
Early clustering is present. Clustering after innervation is lost.
94
What rescues the agrin KO phenotype?
KO of presynaptic choline acetyltransferase as well. If there is no innervation signal, then the clustering signal is not sent.
95
What happens when Dok-7 is KO’d?
Early and late innervation is lost.
96
How do presynapses find their location after losing their partner or having their axon cut?
Signals in the ECM basal lamina, such as different integrins, help them find their location.
97
What type of laminin is available at synapses?
S-laminin is only present at synaptic sites. Presynapses are attracted to this as S-laminin marks the site for innervation.
98
What happens when ⍺4 laminin is KO’d?
There are less active zones at junctional folds.
99
What happens when 𝛽2 laminin is KO’d?
Loss of folds and active zones.
100
What does synaptic laminin do?
Binds presynaptic Ca2+ channels and organizes presynaptic axon terminals.
101
How are neurotransmitter receptors clustered in CNS synapses?
PDZ domain proteins bind to receptors and these PDZ scaffold proteins cluster them together.
102
What does PSD-95 do?
PSD-95 is a scaffold for postsynaptic densities at excitatory synapses.
103
What does gephyrin do?
It is a scaffold protein at inhibitory synapses.
Gephyrin is not a PDZ protein but it forms a hexagonal array that anchors glycine and GABA receptors.
104
What happens when gephyrin is KO’d?
Inhibitory synapses do not form.
105
Where are excitatory synapses formed?
Mostly (>80%) on dendritic spines.
These become stabilized by contact with the presynaptic terminal.
106
What is the timeline for synaptogenesis?
Initial contact
-Dendritic filopodia come into contact with nascent presynaptic zones.
Assembly of presynaptic machinery
-Receptors cluster
-Presynaptic vesicles and release machinery assemble
Stabilization of the synapse through neural activity.
107
How do presynaptic synapse machinery develop so quickly?
There are already prefabricated transport packages that just need to the sent to the site.
108
Which side of the synapse develops first?
The presynapse.
109
What happens when Munc18 is KO’d?
Synapses can form but they quickly degenerate because there is a lack of neural activity.
110
What is the role of cadherins in synaptogenesis?
Recognize synaptic sites by binding to postsynaptic scaffolding proteins.
They also provide synapse stability.
111
What happens if there is a loss in cadherin expression?
Axons fail to stop at their appropriate targets and overshoot.
112
What are key synapse inductive factors?
Neurexin
Neuroligin
Neuregulin
SynCAM
EphrinB/EphBR
113
How do astrocytes promote synaptogenesis?
They produce thrombospondin, an ECM component.
114
What happens with a gain of function mutation of Neuroligin/Neurexin?
There is an increase in synapse number.
115
Why does apoptosis in developmental cells happen?
To remove unwanted, or unneeded cells.
116
Why are more cells made during development than there are needed?
It is better to have too many and just have to kill some rather than not have enough.
117
What causes target-dependent cell death?
No postsynaptic target exists.
Axons project to the wrong target
There are too many cells and not enough targets.
118
How is the neuron number controlled by the target?
If the target is removed, more neurons will die as there isn’t enough target left.
If the target is bigger, more neurons will survive.
119
What is the central dogma of the neurotrophic support hypothesis?
Neuron number is controlled by competition for neurotrophic factors given by the target. If a cell is not able to get enough neurotrophic factors, it will undergo apoptosis.
120
What needs to happen for caspases to be active?
The proenzyme (zymogen) is inactive. Caspases only become active once the active subunit has been released by proteolytic cleave by an upstream initiator caspase.
121
What happens in caspase 9 is KO’d?
Neurons are not killed and the fetal brain becomes too big to fit inside the skull.
122
What members of the Bcl-2 family proteins are pro-apoptotic?
Bax and Bak
123
What members of the Bcl-2 family proteins are anti-apoptotic?
Bcl-2 and Bcl-XL
124
What is the mitochondrial pathway for apoptosis?
Bax and Bak of the Bcl-2 family bind to the mitochondria’s outer membrane making pores to release more pro-apoptopic proteins.
The mitochondria’s outer membrane is permeabilized and pro-apoptotic proteins are released.
Cytochrome c is released. It binds to Apaf-1 (apoptotic protease-activating factor-1) and procaspase 9 forming the apoptosome complex.
The apoptosome is activated and activates caspase 9 leading to the activation of other caspases.
125
What mutation do people with spinal muscular atrophy have and how does it relate to their disease?
They have loss of function mutations in inhibitors of apoptosis proteins leading to an increased amount of apoptosis.
126
What inhibits inhibitors of apoptosis proteins?
Smac/Diablo.
127
What controls the translocation of proapoptotic regulators to the mitochondria?
Akt, Protein Kinase B, and ERK
128
What is the Bad protein and how is it regulated?
Bad is a proapoptotic factor.
Neurotrophic factors phosphorylate it which leads it to be sequestered in the cytoplasm.
Dephosphorylation leads to its release.
129
How is Bax regulated?
Stress or neurotrophic factor withdrawal activates JNK.
JNK activates Jun a transcription factor.
Bax, a pro-apoptotic factor, is transcribed.
130
What is Bim and how is it regulated?
Jun, FoxO, and Myb pathways must be activated for Bim to be transcribed.
Bim is a proapoptopic protein so it having such a high bar to be transcribed helps prevent accidental apoptosis.
131
How do neurotrophic factors prevent apoptosis?
Activate ERKs which activates CREB which transcribes anti-apoptotic factors.
132
What other factors can promote anti-apoptotic factor transcription?
Neural activity, membrane depolarization, and synaptic activity.
133
What is NGF?
A neurotrophic factor that is crucial to neuron’s survival and growth.
134
What are examples of neurotrophic factors?
Nerve Growth Factor (NGF)
Brain-derived neurotrophic Factor (BDNF)
Neurotrophin 3 (NT-3)
Neurotrophin 4 (NT-4)
135
What are the anti-apoptotic receptors of neurotrophins?
Trk receptors.
136
What are the pro-apoptotic receptors of neurotrophins?
p75NTR
137
What happens after the ligand binds to a receptor protein-tyrosine kinase?
Receptors dimerize
The enzyme active sites come into contact and are now active.
The receptors phosphorylate each other.
Targets bind to the receptor and become phosphorylated.
138
What does ciliary neurotrophic factor do?
It binds to IL-6 receptors leading to the activation of JAK.
JAK activates Akt, ERKs, and STAT transcription factors.
139
Are neurons always dependent on neurotrophic factors?
Not always. Many mature neurons do lose their dependence.
140
What are the mechanisms that contribute to mature neuronal survival?
Neural activity
Autocrine/paracrine neurotrophic factors
Downregulation of apoptotic machinery
141
What are the targets of NGF receptor TrkA? What do those targets do?
Protein kinase B: survival growth
ERK/MAP kinase: differentiation
Rac/Rho: growth cones and spine
Phospholipase C-𝛾: Ca2+ signaling
142
How do neurotrophic factors prevent apoptosis?
Phosphorylation proapoptotic factors like Bad.
Upregulate synthesis of antiapoptotic proteins.
143
What kind of ERK activity drives neuronal differentiation?
Prolonged activity.
144
How are neurotrophins trafficked?
They use retrograde transport up the axons using microtubules.
145
What kinds of neurotrophins bind to p75NTR?
Proneurotrophins.
146
What factors are key for neurotrophins to promote axon growth?
ERK and Akt protein kianses.
Rho family
147
What do target-derived neurotrophic factors do?
Promote survival, growth cone growth, and synaptogenesis after being retrogradely transported to the soma.
148
What kind of neurotrophin activity is required to promote survival in early cells?
Persistent activity.
149
What does BDNF do?
Promotes dendrite growth by recruiting Akt/PKB, ERKs, and phospholipase C-𝛾.
150
What is needed for BDNF to promote dendrite growth?
NMDAR activity.
