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Lecture 9

CSB332H1 Lecture Notes - Lecture 9: Superior Colliculus, Netrin, Growth Cone

Cell and Systems Biology
Course Code
Melanie Woodin

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CSB332H1S L9; Feb. 8, 2012
Axon Guidance
Read: Pg. 558-562
GCs of cultured Xenopus spinal neurons exhibit adaptation
during chemotactic migration, undergoing consecutive phases
of desensitization & resensitization in presence of guidance
factor netrin-1
With more time (90min) resensitizes to gradient now
responsive to the chemoattractant Adapt to envt
Large grad of Ca coming from neuron but not important for act
pot important for many other things
Images of Ca-binding growth cone been bathed in fluorescent
Ca-sensitive indicator
Calcium signals induced by a netrin-1 gradient correlate w
Fluorescence images of a GC loaded w a Ca2+-sensitive
fluorescent indicator (Oregon green BAPTA-1-dextran).
Neuron was incubated w netrin-1 (5 ng ml-1) in the bath 30
min (a) or 90 min (b) before exposure to a netrin-1 gradient.
A: apply grad @ t=0
o Desensitized to netrin so growth cone not turned towards
gradient, overall no global change in amt of Ca
B: resensitzation since more time passed increase in yellow
signal indicates Ca levels increased in growth cone so
growth cone grows towards gradient
Ca = important in signaling other molecs (ex. prots) for gene
Black ovals = rec for chemoattractant
In encountering a shallow gradient of guidance molecs (yellow
cloud) shallow gradient of rec activation (red oval)
clustering of activated recs.
o More red ovals close to gradient
Initial clustering triggers recruitment & further clustering of
cytoplasmic adaptors & effectors eventually orientated
assembly of actin filaments (and other prots needed) on 1 side
of growth cone.
The cascade of local clustering & consequent depletion of
signaling molecs @ rest of cell
o Provide local activation & global inhibition
Required for amplification of gradient signal.
respond more effectively on upper-left side, also
decreases response in regions furthest from gradient
(global inhibition on right side)
Prots get localized growth cone turning
Gradient of Ca is important
Hign Ca grad attraction
Low Ca grad repulsion
For both attraction &
repulsion, Ca level above
baseline, just more so for
Activate cascades
o Ex. Phosphorylation or
prot kinases need
certain levels of Ca to
Retina projections ganglia to optic tectum (analygous to LGN in
vertebrates?) in midbrain
Retina divided in nasal & temporal regions
Optic tectum divided btwn anterior & posterior
Nasal innervates posterior
Temporal innverates anterior
o Due to conc grads
Eph = rec
So growth growth growing from temporal region has much
higher conc of Eph recs
Nasal region has lower expression of Eph recs
o Ligands that activate Eph recs = Ephrins, found in optic
Critically important: when ephrin binds to rec repulsive,
inhibitory reaction growth cone moves away from that
Ganglion cell in temporal retina
o If grow into tectum, will choose anterior region since less
ligands there
Ganglion cell in nasal region
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