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Cell Bio & Developmental Genetics > Lecture 12 (6) - Mechanisms of Axonal Pathfinding > Flashcards

Lecture 12 (6) - Mechanisms of Axonal Pathfinding Flashcards

1
Q

Attractive signal

A

microfilaments and microtubules accumulate at the contact site

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2
Q

Growth cone at tip

A

webby
• different regions, filopodia stabilized by actin
–> growth cone to attractive

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3
Q

Mechanisms of axonal pathfinding

A
  • stereotropism
  • haptotaxis
  • chemotropism
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4
Q

Stereotropism

A

physical barriers
• axons move around physical objects
• some artificial substrates
• repair tissues/nerves

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5
Q

Haptotaxis

A

movement along substrate bound molecules
• most common
• bound to cell surface

  • molecules of the extracellular matrix and cell surface molecules
  • selective fasciculation of axons : labelled pathways hypothesis
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6
Q

Chemotropism

A

diffusabe factors

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7
Q
  1. Stereotropism - physical barriers
A
  • collagen fibers
  • plastic barrier
  • artificial substrate
  • tunnel
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8
Q
  1. Haptotaxis - movement of growth cones along substrate bound molecules
A

ECM: glycoproteins and proteoglycans

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9
Q

Cell adhesion proteins control

A

mobility and guide toward movement
• cell-cell adhesion
• cell-matrix adhesion

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10
Q

Cell-cell adhesion

A
  • classical cadherins (E, N, P, VE), homophilic binding, associated with cytoskeleton (actin filaments) via catenins
  • Ig family members (N-CAM, ICAM), homo- and heterophilic binding
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11
Q

Cell-matrix adhesion

A
  • Integrins (many types), heterophilic binding associated with cytoskeletion (actin filaments), via talin, paxillin, filamin, etc.
  • transmembrane proteoglycans (syndecans), heterophilic binding
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12
Q

To find molecules that axons like, use

A

cell culture assays

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13
Q

Cell culture as guidance molecules - assays are used for identifying
1. cell-adhesion assay

A

• can the neuron stick to other cells or a given substrate

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14
Q

Cell culture assays for identifying guidance

2. stripe assay

A

• petri dish - different areas with different molecules
• neuron in, where its axon grows =
which stripe is attractive or repulsive

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15
Q

Haptotaxis I - ECM-molecules facilitate or inhibit axonal growth

A

CSPG - chondroitin-sulfatproteoglycan
• dorsal roof plate of spinal cord
• when growth cone reaches CSPG, is repulsed and turns away

laminin attracts

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16
Q

Haptotaxis II - cell adhesion molecules are involved in axonal outgrowth, fasciculation and pathfinding

A

most growth cones grow on the surface of other axons or cells
• only if 2 molecules recognize each other
• floor plate instructs axons to grow along (cross midline) or repulses
• the midline is transient in embryos

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17
Q

Midline - axons express

A

axonin1 on surface

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18
Q

Cells of the floor plate have

A

NRCAm - both adhesions and bind to each other

• axon guided to cross midline

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19
Q

Incubate floor plate with

A

antibodies = 1 or 2 can’t pass

• antibodies block sites where 1 or other cross = can’t bind

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20
Q
  1. Selective fasciculation
A

the “labeled pathways” hypothesis
• guidance on axon
• G - neurons grow where nothing else arond axon
• recognizes molecules on axon P
• grows to decision point and sees P missing
–> stalls
= absence of whole axon tracks

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21
Q

Fasciculation is affected in

A

fasciclin II mutant flies

• FasII is expressed on pioneer axons of longitudinal tract

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22
Q
  1. Chemotropism
A

guidance by diffusable factors
• repulsive factors lead to growth cone collapse
• collapsin = growth cone turns away and collapses
• growth cone collapses, grows, tries again

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23
Q

Collapsin

A

growth cone turns away and collapses

• collapses, grows, tries again

24
Q

Pathfinding of the DRG-axons is based on

A

repulsive factors
• spinal cord
• DRG cells grow straight bc repulsed by secreted substance

25
Q
  1. Chemotropism
A
  • NGF - nerve growth facctor
  • all axons like NGF
  • follows this diffusible factor
26
Q

Both in vertebrates and invertebrates the CNS midline plays an important role in

A

organizing the axonal scaffold
• vertebrates - floor plate = epithelial cells, transient
• in embryo, guides axons in midline (attracts/repulses)
• cross = turn ant/post

27
Q

Commissural

A

crosses ventral midline

28
Q

Ipsilateral

A

doesn’t cross ventral midline

29
Q

Netrin is expressed in the

A

CNS midline of Drosophila
• CNS midline - explant of growth plate to Dci
• Dci attracted to plate over distance
–> shows diffusible factor (later called netrin)

30
Q

Netrin exists in

A

vertebrates and invertebrates

31
Q

In Drosophila, the ventral midline derives from the

A

mesectoderm

32
Q

3 types of midline precursors give rise to the

A

unpaired midline neurons and glia

33
Q

Axon scaffold

A

commissural axons cross betwen midline glial cells and neural cells
• 8 midline precursors per segment (gastrulation –> germ band elongation)
• 16 during division
• midline glial cells separate fasic. into ant and post commissural

34
Q

Midline cells contribute to the

A

formation of the axonal scaffold

35
Q

The ventral midline of crustaceans derive from

A

a medial column of cells that separates the neuroectoderm
• crustaceans - netrin in midline for axonal patterning
• no bilateral midline, have unpaired medial midline bc no welerm (?) band forms

36
Q

Despite the different origin, individual midline progenitorsmight be

A

homologous in malocostraccans and insects

• similar position of midline precursors and cells

37
Q

The ventral midline epithelium is

A

a transient structure that does not express any neural genes
• midline in centi and milli
• transient midline in spiders (like vertebrates)
• spiders split into 2 –> dorsal, come together when yolk into abdomen
= transient midline

38
Q

Commissural axons cross the ventral midline despite

A

the gap between the 2 halves
• the axonal guidance molecule Netrin is expressed in the epithelial ventral midline
• commissural crosses on Netrin-positive cells

39
Q

Netrin is required for

A

axonal guidance at the ventral midline

• same phenotype in Drosophila, spider, and in vertebrates - reduction of commissural axons

40
Q

Pattern of axons at the ventral midline of Drosophila

A

• anterior and posterior commissures connected by longitudinal tracts

41
Q

A large-scale screen for axonal pathfinding mutants resulted in the discovery of the

A

midline guidance system in Drosophila
• blue = midline glial cells
• slit = collapsed at midline
• Robo = roundabout, axons to midline, cross, go back

42
Q

Robo

A

roundabout

• axons to midline, cross, go back

43
Q

The commissures are absent in

A

commissureless mutants
• in axons meant to cross midline, commisureless need to sort Robo to lysosomes
- not on surface growth, can’t sense repulsive (slit)
- Robo on surface of growth cone and repelled by slit = doesn’t cross

44
Q

High robo

A

strongly repelled by Slit

45
Q

Low Robo

A

weakly repelled by Slit

46
Q

comm ON

A

Robo sorted to lysosomes

47
Q

comm OFF

A

Robo delivered to growth cone

48
Q

Commissureless is expressed in axons that

A

cross the midline
• comm sort Robo (receives repulsive Slit)
• comm expressed = no Robo in growth cone
= can’t tell repulsive slit (repulse midline) –> cross midline
• comm off = high Robo expression

49
Q

Commissureless sorts Robo to

A

allow midline crossing

50
Q

In vertebrates, Robo3 functions

A

like comm in Drosophila
• no comm
• Robo3 represses Robo
• Robo3 expressed = Robo low = cross midline
• growth cone doesn’t recross, recognizes midline as repulsive
• Netrin attracts to CNS midline - doesn’t find Netrin attractive on other side

51
Q

The human syndrome “horizontal gaze palsy and progressive scoliosis) is associated with mutations in

A

the Robo3 gene
• aberrant psilateral projections of major ascending and descending axons pathways
• failure of these axons to cross the midline in the hindbrain
• no horizontal eye movement
(can’t see sideways, only up and down)

Ips = on same side
(axons don’t cross midline in hindbrain)

52
Q

Slit activates Robo in the midline resulting in

A

silencing of the netrin receptro DCC

53
Q

In vertebrates, Robo3 functions like comm in Drosophila

A

• attracted to midline by Netrin
- have growth cone receptor for Netrin (dcc)
- bind Netrin and guided
• slit (repressor) in midline affects Robo in crossing
- Robo switches on and binds dcc receptor –> can’t bind attractive signal
(Robo prevents Netrin binding)
- growth cone crossed, dcc receptor no longer functional
• Robo mutant = recross
- plays role in silencing attractive signal

54
Q

Growth cones guide axons through

A

the developing embryo to the target

55
Q

Soluble ans substrate bound factors guide growth cones by

A

attraction and repulsion

56
Q

A complex midline guidance system leads

A

commissural axons across the midline and prevents ipsilateral axons from crossing

Cell Bio & Developmental Genetics (27 decks)

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