| Development
of Neuronal Connections in the Visual System using
the Marsupial Mammal, the Wallaby, as a Model.
Dr.
Lauren Marotte, Central Nervous Stability and
Degeneration Group, RSBS.
The protracted and largely postnatal
development of the visual system in the wallaby, combined
with features it shares with higher placental mammals,
make it an excellent alternative model for developmental
studies. For example, at birth the first axons of
retinal ganglion cells have only just left the eye
and are yet to make connections with visual centres
in the brain. This model permits unrivalled access
for experimental manipulation at early stages of development
that in placental mammals occur in utero, where manipulation
is extremely difficult or impossible.
Particular areas of interest are
the molecular mechanisms involved in the establishment
of topographic maps made by retinal axons in the brain
and in the establishment of cortical areas and their
connections. Molecules being examined include brain-derived
neurotrophic factor and Ten_m3, a transmembrane protein.
Approaches being used
are described below (in collaboration with Dr C Leamey
of the University of Sydney) and could form the basis
of a project.
1. We have recently described a novel gradient of
brain-derived neurotrophic factor (BDNF) in the midbrain
superior colliculus (SC) during development of the
topographic map. This molecule is a candidate for
a postulated attractant/branching signal for retinal
axons which to date has not been identified. There
is also a striking decrease in expression of BDNF
during the time of axon refinement. Its role will
be tested by locally up-regulating the levels of BDNF
in the SC in vivo at selected developmental stages
to interfere with the gradient. This will be done
by electroporation of a construct encoding BDNF and
green fluorescent protein. The effect on retinal axon
ingrowth, topographic branching and refinement will
be assessed anatomically, by labelling retinal axons
from different quadrants of the retina and examining
their distribution in the SC.
2. Preliminary data using reverse transcriptase polymerase
chain reaction has shown that messenger RNA for Ten_m3
is in a high ventral low dorsal distribution in the
retina and a high medial low lateral distribution
in the SC early in development in the wallaby. Since
the retinal region high in Ten_m3 connects with the
region of SC also high in Ten_m3 and Ten_m3 has been
found to form homodimers, this suggests a novel role
for this molecule in mapping along the mediolateral
axis of the SC. The cellular distribution of messenger
RNA for Ten_m3 in tissue sections of the retina and
SC will be examined throughout development using in
situ hybridisation. Levels will be up-regulated and
the effect on topographic mapping assessed as described
above.
3. A knockout mouse which lacks the gene for Ten_m3
is available. Nothing is known of the effects of this
gene knockout on brain development at a general morphological
level, on synaptic connectivity or on the development
of topographic maps. A number of approaches to investigate
this could include a general histological survey of
brain structure, immunohistochemistry, electron microscopy
and neuronal tracing to examine mapping as described
in (1) above.
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