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Axon guidance Flashcards

1
Q

Define axon guidance

A

The process by which neurons are able to find their synaptic targets

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2
Q

The growth cone of an axon is made of primarily this cytoskeletal element

A

Filopodia

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3
Q

How do axons grow?

A

By extension and retraction of filopodia

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4
Q

Axons in their immature state are often referred to as…

A

Neurites

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5
Q

What kinds of glycoproteins and proteoglycans are present on the extracellular matrix? (5)

A
  1. laminin
  2. fibronectin
  3. collagen
  4. tenascin
  5. heparin sulfate PG
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6
Q

Notably, the growth cone contains receptors for…

A

Members of the integrin family (a and b subunits)

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7
Q

What are integrins?

A

The primary proteins that animal cells use to bind to the ECM

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8
Q

Filopodia are rich in this protein

A

Actin

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9
Q

What proteins are responsible for mediating an interaction between the ECM and the cytoskeleton (specifically, actin)

A

Integrins

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10
Q

ECm glycoproteins and proteoglycans are (secreted/stationary)

A

Secreted

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11
Q

Where are integrins found?

A

On cell surfaces (transmembrane)

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12
Q

What is the role of talin?

A

Bridges actin and integrin

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13
Q

What is the role of FAK and Src?

A

They are kinases which mediate the binding of actin and integrin

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14
Q

What are vinculin and paxilin?

A

Adaptor proteins that aid in the correct spatial binding of actin and integrin

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15
Q

Describe what is happening here, what is the assay?

A

Assay: looking for growth of neurites along the laminin strip.

Laminin strips applied to a developing neuron and see where the axons start growing – testing to see if laminin is an appropriate axon growth substrate

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16
Q

Immunostaining reveals that the soma and axon of developing neurons are rich in (actin/microtubules) and the growth cone is rich in (actin/microtubules)

A
  1. Microtubules
  2. Actin
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17
Q

Why is aplysia a good model organism for studying axon guidance? (3)

A
  1. They have large neurons
  2. Well-characterized nervous system
  3. Easy to examine in culture
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18
Q

If you wanted to knock out the growth of axons, what kinds of molecules might you want to target?

A

Actin inhibitors – inhibit the formation of filopodia

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19
Q

What is cytochalasin B?

A

An inhibitor of actin polymerization

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20
Q

What happens when cytochalasin B is applied to growing axons?

A

There is a complete retraction of filopodia

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21
Q

What are the key molecular switches that regulate the cytoskeleton?

A

Rho family GTPases

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22
Q

What are the 3 key Rho GTPases studied in this lecture?

A

Rho
Rac
Cdc42

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23
Q

Rho GTPases are active when bound to…

A

GTP

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24
Q

Rho GTPases are inactive when bound to…

A

GDP

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25
Rho-bound GTP is converted to GDP by...
GTPases activating proteins (GAPs)
26
Rho-bound GDP is activated by...
Gunanine nucleotide exchange factor (GEF)
27
When bound to Rho GTPases, GDP is phosphorylated to GTP...
NEVER
28
Active Rho-GTPases commonly interact with (2)
Protein kinases Actin-binding proteins
29
Activated Rho-GTPases directly or indirectly affect local assembly and disassembly of...
F-actin (filamentous actin, including filopodia)
30
What are focal adhesions?
Actin aggregates which anchor the growth cone to the base of the developing axon
31
What are stress fibers?
Actin filaments within the growth cone which interact with non-muscle myosin -- have a contractile function
32
Rac mediates the formation of...
Focal complexes
33
What are focal complexes?
Actin and integrin complexes -- Interaction area with ECM
34
Rac impacts the formation of this actin structure
Lamellopodia (flat sheets of actin at the edges of the growth cone)
35
How does Cdc42 impact growth cone movement?
Controls the outgrowth of filopodia, which interact with non-muscle myosin to have contractile functions
36
Who was one of the first individuals to propose the existence of molecular mechanisms controlling axon guidance?
Roger Sperry (1939)
37
Roger Sperry postulated that (1) controlled axon guidance
Embryonic gradients
38
What would happen to these neurons if the limb bud were to be truncated?
Neurons would die
39
Describe how Rita Levi-Montalcini "got lucky" with her discoveries
The lab directly next door to her just happened to be a tumour lab and they shared tumours with her. Had she not had easy access to these she may not have made her discoveries Don't get me wrong she slays and wasn't "lucky" she deserves all the credit she gets
40
What inspired Rita to use tumours to see if truncated limb buds could still have live neurons?
She knew tumours had unique growth properties and growth factors which might "inspire" neurons not to die
41
What occurred to peripheral neurons in Rita's experiments when tumours were placed near the truncated limb bud?
Neurons did not die and grew to innervate the tumour
42
Rita hypothesized that truncated limb buds in the presence of tumours could grow to be innervated due to the presence of (1) in the tumours
Neuron survival factors/signals
43
Describe the assay that Rita developed to isolate "nerve growth factor"
Isolated sensory and sympathetic ganglia from chick embryos (which are relatively easy to work with) and combine with fragments or extracts of a tumour Readout: Observation of neuron growth
44
Describe the contribution of Stanley Cohen to Rita's work
He fractionated tumour extract and applied it to growing neurons to isolate the compound which induced significant neuron growth
45
Who were the two people who shared in the nobel prize in physiology and medicine for the isolation of nerve growth factor?
Rita Levi-Montalcini and Stanley Cohen
46
What are the primary receptors for NGF? (2)
TRKA p75
47
Downstream effects of NDF/TRKA activation impact this important molecular switch
Rho GTPases
48
Describe the general steps of the NDF pathway leading to neurite growth
1. NGF binds to surface receptor 2. Activation of Rho GTPases 3. Increase in actin polymerization 4. Neurite outgrowth and differentiation
49
Name 5 neurotrophins
NGF neurotrophin-3 BDNF GDNF CNTF
50
Define a neurotrophin
A molecule which promotes axon outgrowth, survival, and differentiation of neurons
51
True or false: NGF is capable of specifying the correct synaptic targets for an axon
False: it helps guide the axons to the correct general area but NGF alone is not capable of specifying targets
52
Name 4 non-neurotrophin axon guidance molecules
Ephrins, netrins, slit, and semaphorins
53
The primary intracellular interaction of ephrins and netrins are with...
GEFs
54
The primary intracellular interaction of Slit and Semaphorins are with...
GAPs
55
Downstream of Ephrins and Netrins, GEFs activate...
RhoA and Rac1
56
Downstream of Slit and Semaphorins, GAPs inhibit...
RhoA, Rac1, and Cdc42
57
Ephrins and Netrins act as (attractive/repulsive) cues for axons
Repulsive
58
Slit and Semaphorins act as (attractive/repulsive) cues for axons
Attractive
59
Describe the "stop and go" mechanism of axon growth
Different attractive and repulsive signals dictate whether axon growth into a region will stop or go
60
Describe why NT-3 responsive neurons can project to the ventral spinal cord, but NGF-responsibe mechanoreceptors and nociceptors cannot
The ventral region expresses Semaphorin-3, which is an attrative cue for NT-3 neurons, but which causes NGF-responsive neurons to halt their growth
61
What is happening in this image?
Looking at NGF-responsive neurons, when NGF present get robust outgrowth of neurites
62
What is happening in this image?
NGF-responsive neurons are exposed to sema-3, and when this is done there is robust repulsion of neurites around the source of sema-3 NGF RESPONSIVE NEURONS CONTAIN SEMA RECEPTORS, SEMA IS A REPULSIVE CUE WHICH IS WHY WE SEE RETRACTION FROM THE AREA OF SEMA
63
What is happening in these images?
NT3 responsive neurons are being cultured in the presence (right) and absence (left) of Sema 3, when Sema-3 is present, there is an attraction of neurites towards the source NT3 RESPONSIVE NEURONS DO NOT CONTAIN SEMA RECEPTORS, SEMA IS A REPULSIVE CUE WHICH IS WHY WE SEE GROWTH INTO THE AREA OF SEMA
64
Who discovered the somatosensory cortical map?
Wilder Penfield
65
Three examples of sensory maps in the brain include
The somatosensory cortex The auditory map on the basillary membrane Retina
66
Describe the geniculate visual pathway in rodents
Projections to the LGN (lateral geniculate nucleus) to the V1 cortex
67
Describe the extrageniculate pathway in rodents
Projections to the superior colliculus to the V2 cortex
68
What is the primary role of the geniculate pathway?
Processing info about static and moving objects, good at pattern recognition
69
What is the primary role of the extrageniculate pathway?
Directs behavioural responses toward specific points in "body-centered" space (eye movements, for example)
70
The focus of the following discussion paper examines these projections
ganglion cell axons from the retina projecting to the superior colliculus (start of the extrageniculate pathway)
71
Eph is the (receptor/ligand) and ephrin is the (receptor/ligand)
Receptor Ligand
72
Ephrin A's are linked to the membrane via...
GPI linkages (re: these are LIGANDS)
73
Ephrin B's are linked to the membrane via...
Transmembrane domains (re: these are LIGANDS)
74
What is the full name for a GPI anchor? What are they?
Glycophosphatidilinositol Glycolipid that can be attached to the C-term of a protein thru post-translational modification
75
EphA5 and A6 are highest in the (nasal/temporal) retina
Temporal
76
Ephrin A's are highest in the (posterior-caudal/anterior-rostral) superior colliculus
Posterior-caudal
77
Ephs and ephrins are expressed in this motif
Concentration gradients

Biology 467 (20 decks)

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