Download Lect 11 Crossing the midline

Crossing the midline
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Lect 11
Many species have neurons that project
in two directions:
1. Those that cross the midline (contralateral)
2. Those that don’t (ipsilateral)
Netrin is important for commissure
development
Netrin RNA expression
Netrin mutant
Mitchell et al. 1996 Neuron 17:203-215
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Answers to all 3 questions provided by
considering Slit / Robo
Drosophila embryo CNS
1. Why do commissural axons cross the midline
but not other axons?
2. If the midline is attractive to commissural
axons, why do they leave the midline?
3. Why do commissural axons make longitudinal
projections, rather than turning back into the
midline?
Slit mut
commisures
longitudinal
Analysis of Robo
1. Robo RNA expression is uniform across
all neurons
2. Robo protein is not uniform
Robo mut
Too many axons
enter the midline
& stay there
axons collapse
into the midline
• Longitudinal growth cones have Robo, but
growth cones in the commissures do not
• Suggests that maybe Robo is dynamically
regulated in commissural growth cones
During commissure formation there is little Robo in the
commissures, but lots in the longitudinals
Kidd et al., 1998 Cell 92:205-215
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• What keeps Robo levels low in
commissural axons, allowing them to
cross the midline?
Comm is expressed in neurons that cross the
midline, but not in other neurons
Red = neural marker
Green = Comm RNA
• Identification of commissureless (comm)
– A Drosophila mutant lacking commissures
WT
Comm mutant
Comm protein in WT
Tear et al. 1996 Neuron 16:501-514
Yellow indicates
comm expressed in
those neurons
Keleman et al. 2002, Cell 110:415-427
Model for Comm regulation
of Robo
To keep Robo low, Comm
diverts Robo delivery
from the growth cone into
endosomes
– Secretion of Slit is by midline glial cells
After growth cones cross
the midline, Comm
function /expression is
reduced, so Robo
delivery to the growth
cone increases
Robo mutant
missing midline glia
Dickson & Gilestro, Ann. Rev. Cell Biol 2006, 22:651-75
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Short Summary
• What controls crossing the midline?
– Ipsilateral axons are always repelled from
midline by slit / robo
– Contralateral axons are initially insensitive to
Slit / Robo (controlled by Comm) and are
attracted to midline by Netrin
– After crossing, Robo presence on
contralateral axons increases and axons are
prevented from re-crossing
Open Questions:
1. Do (and how) contralateral axons lose
their attaction to the midline after
crossing?
2. How do contralateral axons acquire
sensitivity to Slit after crossing? (not
likely a Comm regulated event)
What about vertebrates?
• General principles seem similar
– Differential sensitivity to repellents
• But, there doesn’t seem to be a vertebrate
equivalent of Comm!
WT
Robo3 mutant
• Vert have 3 Robo orthologs
Sabatier et al. Cell 2004, 117:157-169
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