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

Lecture 12 (6) - Mechanisms of Axonal Pathfinding Flashcards

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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

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A

growth cone turns away and collapses

• collapses, grows, tries again

24
Q

Pathfinding of the DRG-axons is based on

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A

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

25
4. Chemotropism
* NGF - nerve growth facctor * all axons like NGF * follows this diffusible factor
26
Both in vertebrates and invertebrates the CNS midline plays an important role in
organizing the axonal scaffold • vertebrates - floor plate = epithelial cells, transient • in embryo, guides axons in midline (attracts/repulses) • cross = turn ant/post
27
Commissural
crosses ventral midline
28
Ipsilateral
doesn't cross ventral midline
29
Netrin is expressed in the
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
Netrin exists in
vertebrates and invertebrates
31
In Drosophila, the ventral midline derives from the
mesectoderm
32
3 types of midline precursors give rise to the
unpaired midline neurons and glia
33
Axon scaffold
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
Midline cells contribute to the
formation of the axonal scaffold
35
The ventral midline of crustaceans derive from
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
Despite the different origin, individual midline progenitorsmight be
homologous in malocostraccans and insects | • similar position of midline precursors and cells
37
The ventral midline epithelium is
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
Commissural axons cross the ventral midline despite
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
Netrin is required for
axonal guidance at the ventral midline | • same phenotype in Drosophila, spider, and in vertebrates - reduction of commissural axons
40
Pattern of axons at the ventral midline of Drosophila
• anterior and posterior commissures connected by longitudinal tracts
41
A large-scale screen for axonal pathfinding mutants resulted in the discovery of the
midline guidance system in Drosophila • blue = midline glial cells • slit = collapsed at midline • Robo = roundabout, axons to midline, cross, go back
42
Robo
roundabout | • axons to midline, cross, go back
43
The commissures are absent in
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
High robo
strongly repelled by Slit
45
Low Robo
weakly repelled by Slit
46
comm ON
Robo sorted to lysosomes
47
comm OFF
Robo delivered to growth cone
48
Commissureless is expressed in axons that
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
Commissureless sorts Robo to
allow midline crossing
50
In vertebrates, Robo3 functions
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
The human syndrome "horizontal gaze palsy and progressive scoliosis) is associated with mutations in
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
Slit activates Robo in the midline resulting in
silencing of the netrin receptro DCC
53
In vertebrates, Robo3 functions like comm in Drosophila
• 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
Growth cones guide axons through
the developing embryo to the target
55
Soluble ans substrate bound factors guide growth cones by
attraction and repulsion
56
A complex midline guidance system leads
commissural axons across the midline and prevents ipsilateral axons from crossing

Cell Bio & Developmental Genetics (27 decks)

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