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Neuro M101B > Module 1 Lecture 4: Axon Guidance > Flashcards

Module 1 Lecture 4: Axon Guidance Flashcards

1
Q

main characteristic of the growth cone

A

engine and navigator of axon guidance

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2
Q

what major cytoskeletal systems are essential for dynamics

A
  1. microtubules (MTs) (common in cell division - spindle)
  2. actin (makes up thin filaments in muscles to shorten them)
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3
Q

how are microtubules and actin assembled

A

from monomers into polymers

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4
Q

what are the two poles of the polymers

A

fast growing plus and slow growing minus ends
- structurally and kinetically distinct

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5
Q

where does the plus end of the polymer point

A

toward the leading edge of growth cone

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6
Q

what drives growth cone movement

A

new polymer assembly

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7
Q

what composes the peripheral domain

A
  • long, bundled actin filaments in filopodia (exploratory structures)
  • crosslinked actin networks in veil-like structures between filopodia
  • individual, dynamic ‘pioneer’ microtubules which reach along actin bundles
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8
Q

what composes the central domain

A
  • stable, bundled microtubules enter the growth cone from the axon shaft
  • organelles, vesicles
  • central actin bundles
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9
Q

what is the transition zone in the growth cone composed of

A
  • interface between peripheral domain and central domain
  • ‘actin arcs’: contractile actin-myosin structures (similar to muscles); perpendicular to actin bundles of the peripheral domain
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10
Q

treadmilling

A

internal retrograde flow of actin with no net growth cone movement

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11
Q

how do microtubules contribute to axon guidance

A

necessary for movement
- no new microtubule polymerization = no forward movement
- dynamic microtubules (pioneers out in peripheral domain) also important for steering

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12
Q

how does actin contribute to axon guidance

A

necessary for guidance
- growth cone without actin dynamics can move but cannot navigate
- growth cones turn in the direction of net actin assembly

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13
Q

retrograde flow (‘idling engine’)

A

actin assembled at the leading edge but pulled back at the transition zone by myosin
- no (or little) net movement

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14
Q

what is a clutch

A

in a car, a mechanical system that connects rotational motion produced by the engine to the wheels

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15
Q

what is the clutch made up of in the growth cone

A

protein assemblies that physically link the actin cytoskeleton to the substrate

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16
Q

how does the clutch affect growth cone movement

A

actin assembly pushes the leading edge forward and myosin pulls the rest of the growth cone to follow
- movement! treadmilling stops

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17
Q

what is the adhesive substrate-bound cues (the roadway)

A
  • cell adhesive molecules (CAMs) and cadherins
  • extracellular matrix (laminin and fibronectin)
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18
Q

what are the repellent substrate-bound cues (the roadway guard rails)

A
  • slits and ephrins
  • chondroitin sulphate proteoglycans
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19
Q

what is the receptor for the substrate-bound, attractive, ECM (laminin and fibronectin)

A

integrin

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20
Q

integrin receptor function

A

links extracellular matrix to actin (clutch component)
- growth cones can switch integrin composition through development to alter their responses

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21
Q

what is the receptor for the substrate-bound, repulsive, extracellular matrix (Chondroitin sulphate proteoglycan)

A

protein tyrosine phosphatase (PTP) sigma

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22
Q

protein tyrosine phosphatase sigma receptor function

A

Chondroitin sulphate proteoglycan binding to protein tyrosine phosphatase sigma inhibits axon growth through signaling to the actin cytoskeleton

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23
Q

what are the substrate-bound, attractive, cell surface molecules (the surface is another cell, not extracellular matrix)

A
  • cell adhesion molecules (CAMs)
  • cadherins
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24
Q

cell adhesion molecules characteristics

A
  • bind other CAMs on other cells
  • binding can be with same (homophilic) or different CAM (heterophilic)
  • intracellular domain links to cytoskeleton (clutch component)
    -eg NCAM, L1-CAM, APCAM
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25
Q
A
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26
Q

characteristics of cadherins

A
  • Ca2+ dependent adhesion molecule
  • homophilic binder
  • clutch component
  • eg Cadherin, N-Cadherin
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27
Q

what does APCAM coated bead placed on growth cone cause

A

engagement of clutch

28
Q

types of axon-axon interactions

A

fasciculation, defasciculation, selective fasciculation, selective defasciculation

29
Q

Post Optic Commissure (POC) function

A

connects two clusters of ventral neurons in the forebrain of zebrafisu

30
Q

characteristics of pioneers and followers in the POC

A

pioneers and followers move at the same rate except at the midline (a decision point)
- at the midline, pioneers slow down, followers do not
- pioneers and followers have different shaped growth cones

31
Q

why do pioneers slow down at the midline

A

its growth cone samples the guidance cues

32
Q

why do followers not slow down at the midline

A
  1. more compact growth cone does not engage as many environmental cues
  2. receives additional guidance cues directly from pioneer axon
33
Q

what is another possibility for the different rates of pioneers and followers

A

may be intrinsic (molecular) differences between pioneers and followers that make pioneers more capable pathfinders

34
Q

what are types of substrate-bound, repulsive, secreted or cell surface molecules

A

slits, and ephrins

35
Q

slit characteristics

A

secreted, but remain surface bound (minimal diffusion)
- receptor: Robo (short for roundabout)

36
Q

ephrin characteristics

A

cell surface molecule
- receptor: Eph

37
Q

what does signaling through Robo and Eph receptors effect

A

cytoskeletal dynamics
- not clutch components

38
Q

what does bath application of slit cause

A

growth cone collapse and retraction

39
Q

what are the diffusible chemotropic cues (the road signs)

A
  • classic guidance molecules (netrins and semaphorins)
  • morphogens and growth factors (Wnt, Shh, BMP, adnBDNF)
  • neurotransmitters
  • secreted transcription factors
40
Q

netrin characteristics

A

attractive cue
- receptor: DCC

41
Q

semaphorin characteristics

A

repulsive cue
- receptor: Plexin

42
Q

what does signaling through DCC and semaphorin receptors affect

A

cytoskeletal dynamics

43
Q

what does bath application of Sema cause

A

growth cone collapse and retraction

44
Q

what does Netrin application from the left cause

A

growth cone turning to the left

45
Q

are commissural axons guided exclusively by FP-derived netrin 1 gradient?

A

no

46
Q

how does Netrin affect commissural axons in dorsal spinal cord explants

A

they will orient toward a cell culture aggregate expressing netrin (in vitro)

47
Q

how does Netrin affect isolated growth cones

A

will turn toward a source of soluble Netrin (in vitro)

48
Q

what does a floor plate graft cause

A

causes commissural axons to exit the spinal cord and grow toward it (in vivo, gain of function experiment)

49
Q

where is Netrin (mRNA) expressed

A

both at the floor plate and in the ventricular zone

50
Q

how does genetic ablation of the floor plate affect ventricular zone netrin

A

does not affect it

51
Q

is netrin in floorplate crucial?

A

no, it is dispensible
-ventricular zone netrin is sufficient to support midline crossing

52
Q

characteristics of netrin in wild-type

A

netrin is produced by progenitor cells in the ventricular zone
-those progenitor cells deposit netrin protein to the pia surface
- commissural axons use this surface bound netrin path to find their way down to the plate

53
Q

characteristics of FP-netrin1-/-

A

does not cause an overt phenotype wrt midline crossing

54
Q

characteristics of VZ-netrin1-/- (while keeping floorplate source)

A

leads to disorganization of commissural axons at the dorsal spinal cord and significant reduction in midline crossing

55
Q

what happens if a growth cone is co-expressing the DCC and UNC5receptors

A

Netrin becomes repulsive

56
Q

commissural neuron path

A
  1. move away from roof plate
  2. move ventrally
  3. find the floor plate and cross
  4. do not cross back
  5. move anteriorly toward brain
57
Q

what cue does the commissural neuron depend on when moving away from roof plate

A

BMP (from roof plate)
- receptor: BMPR1B
- repulsive

58
Q

what cue does the commissural neuron dependon when moving ventrally

A

Netrin (follows pia bound netrin)
- receptor: DCC
- attractive

59
Q

what cue does the commissural neuron depend on when finding the floor plate and crossing

A

Netrin, Shh (Shh from floor plate important for crossing; local Netrin activity may still be required for error-free crossing)
- receptor: DCC, BOC
- attractive

60
Q

what cue does the commissural neuron dependon when not crossing back

A

slit (floor plate expresses slit. pre-crossing axons unresponsive; become responsive post-crossing)
- receptor: Robo
- repulsive

61
Q

what cue do the commissural neuron depend on when moving anteriorly toward brain

A

Wnt4 (anterior (high) – posterior (low) gradient in the floor plate. different Wnt and direction compared to AP patterning)
- receptor: Fz3
- attractive

62
Q

what do the Drosophila midline glia express

A

Netrin and SLit

63
Q

when do commissural neurons respond to Netrin

A

with receptor Frazzled (DCC homolog) requires contacting surface-bound Netrin

64
Q

when is Robo active vs downregulated

A
  • downregulated precrossing –> no repulsion
  • active post- crossing –> no turning back
65
Q

Commissureless (Comm) function

A

down-regulates pre-crossing robo; Comm itself is down-regulated post-crossing

66
Q

what happens with no Comm

A

no crossing

67
Q

what happens with no robo

A

go round and round the midline (roundabout)

Neuro M101B (4 decks)

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