Wiring Eye to Brain Flashcards
3 main functions of growth cones
guidance
extension of neurite
form pre-synaptic element
RGC and netrin
attracted and later repelled
2 ways to study guidance mechanisms
expression analyses
culture approaches
expression analyses
receptors expressed by neurons
ligands along path - present at right time?
culture approaches
axon respond to signalling molecule? changes in rate and direction of growth
why is optic projection ideal model for studying guidance mechanisms?
formed by axons of single neuronal type - RGC
accessible
navigated via intermediate targets
3 important signalling molecules
robos/slits, ephs/ephrins and neuropilins/sema/vegf
robos
receptors
1-4
slits
ECM molecules
usually inhibit axon outgrowth
slit 1-3
little receptor specificty
Ephs
receptor
tyrosine kinases
ephA bind ephrin A and EphB binds Ephrin B
ephrins
cell surface associated molecules - inhibitory
ephrins - reverse signalling
ephrins can be receptors and ephs as ligands
neuropilin + flk1/flt1 co-receptors
VEGF
neuropilin + plexins co-receptors
class 3 semaphorins
growth into OFL
cells divide, cell bodies of RGC migrate up to RGC layer and extend into OFL
attraction and repulsion - growth into OFL
attraction of OFL ECM
repulsion from outer retina
Molecules help to repulse RGC from OFL
slit/robo
Where are slits expressed and what to RGC express?
slit 1 and 2 in inner retina
RGC express robo 2
mice lacking slit 1/2 - Where do RGC grow?
outer retina - not all but a subset of axons make mistakes - other important molecules
growth to optic dis - robo/slit
KO slit 1/2 = axons grow disorganised
some grow peripherally then to optic disc
initial polarity lost
What attracts neurons centrally to optic disc?
shh
shh expression pattern in retina
low peripheral and high retina
what is required for growth out of eye?
netrin/DD signal
where is netrin expressed in optic disc?
by glia
What axons express DCC
retinal axons
no netrin
axons fail to exit eye
small optic nerves
binocular disparity
image projected on each eye slightly differently
what is size of ipsilateral projection related to?
degree of binocular overlap in visual field
binocular overlap human vs mice
human 55:45
mouse 97:3
where do ipsilateral axons always arise from?
temporal retina
albinism and guidance at optic chiasm
50% fewer ipsilateral axons
lack of melanin in RPE
non-decussating retinal fugal fibre syndrome
no chiasm, very rare
robos/slit path defining
expressed around RGC and absent from regions where they grow
barrier function
no slit 1,2 - optic chiasm
cross in wrong place, grow into other nerve, wander away from path
what helps axons project contralaterally at chiasm?
NRP1-VEGFA
marker of contralaterally projecting neurons?
NRP1
mice lacking NRP1 - crossing at midline
many axons fail to cross the midline
3 VEGF isoforms
VEGF 188, 164 and 120
differences between VEGFA isoforms
exon 6 and 7
importance of exon 6 and 7 in vegf a
influence neuropilin binding
isoform VEGFa receptors
flt1,flk1 and NRP1/2
what vegfa isoform has exon 6 and 7
188
cant KO VEGFA in animal models - what is done?
use VEGF120 as no exon 7 for NRP1
main NRP1 ligand in nervous system
sema3
is sema3 required for axon crossing at chiasm?
no
Mice lacking VEGFA/NRP1 - chiasm
many RGC axons fail to project contralaterally
does VEGFA attract or repel growth cones?
attract
Effect of KO NRP1 in RGC and epithelial cells
RGC - axons go ipsilaterally
no effect on epithelial cells
What stops ipsilateral crossing?
lack NRP1
ephrins/ephs
xenopus - stage specific ephrin B at midline
lateral - eyes completely crossed projection, no ephrin B
binocular vision - ephrin B at midline
ephB in tadpole - precocious ipsilateral projection
Is ephrin B necessary for ipsilateral growth?
yes - EphBs expressed by ipsis
no ephb1 = fewer ipsis
ephB2 expressed at chiasm
no ephB2 = fewer ipsis
what specifies ipsilateral RGCs?
zic2
zic2 in fish/humans
none in fish - crossed
binocular species - zic2 in region that gives rise to ipsis
in utero electroporation
tease embryo out of uterus
inject DNA
use small electric current to drive DNA into tissue of interest
what happens when zic 2 electroporated?
drive cells ipsilaterally in optic chiasm
what does zic2 do to ephb1?
induces expression
which is expressed first - zic2 or ephb?
ephb
electroporate zic2 into central retina in ephb1 KO mice
size of ectopic ipsilateral tract decreased substantially
