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Genetic Analysis of Ephrin-A2 and Ephrin-A5Show Their Requirement in Multiple Aspects
of Retinocollicular Mapping
Interdisciplinary Program in Brain ScienceEye movement & Vision Lab.
Hwang, JaeWon
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Introduction Topographic Maps Chemoaffinity Theory Eph receptor and Ephrin Gradient vs Competition Model Neuraxis
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Topographic Maps Axon projections in
the vertebrate nervous system are typically organized with nearest neighbor relationships of the projecting neurons maintained in their connections within the target.
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Chemoaffinity Theory
There are labels in gradients across the projecting and target fields and that axons find their correct location by matching up the labels. (Sperry, 1963)
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Eph receptor and Ephrin
Eph receptor- RTK- At least 14 members Ephrin- Eph ligand- At least 8 members- A family & B family- Repellant activity
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Gradient vs Competition
Dual-gradient model There may be two opposing gradients,
for example a repellent and an attractant, with each axon identifying its correct place as the point where the opposing forces cancel out.
Axon-axon competition There may be axon-axon competition
for limiting positive factors in the target or by direct axon-axon interactions.
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Neuraxis
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Experiment Methods Results
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Methods Mice with a disruption in the ephrin-A2
and ephrin-A5 gene. (using homologous recombination in ES cells)
A focal injection of DiI was made in one retina followed by examination of the contralateral midbrain.
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Result - Single Mutant Temporal axons- an additional more post
erior arborization Nasal axons- no defects in ephrin-A2-/-
mice- an additional more anter
ior arborization in ephrin-A5-/- mice
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Result – Double Mutant Homozygote- the ectopic terminati
on extended over all regions
Heterozygote- similar to single mut
ant
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Result – Double Mutant 2
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Result - Ephrin Expression
Ephrin RNA distribution
Ephrin(protein) distribution
(Wild Type)
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Result - Ephrin Expression 2
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Result - EphA Expression
EphA RNA expression Ephrin-A2 protein
EphA5 protein
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Result – Stripe Assays
Axon preference for anterior SC lanes
no preference
strong preference
A BC D
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Result - Summary
Fig 8. Schmatic illustration of retinocollicular mapping phenotypes in mice with disrupted ephrin-A2 or ephrin-A5 genes.
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Discussion Discussion 1 Discussion 2 Discussion 3 Discussion 4 Discussion 5
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Discussion 1-1 Simple repulsion model and two count
erbalanced gradients cannot account for the behavior of nasal axons and double mutant phenotype.
⇒ An alternative is a model involving a repellent gradient of ephrins, in combination with axon-axon competition.
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Discussion 1-2 Incorporating competiton in the model can
explain several aspects of data.1. Nasal axons shift anteriorly.2. Axons are not respecified to a specific ecto
pic position.3. Retinal axons fill the available space in the
target.
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Discussion 2 The termination zones always took the
form of punctate spots in the SC. But why?
⇒ There may be a mechanism that causes neighboring axons in the retina to cluster together in the SC, such as Hebbian activity-dependent refinement.
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Discussion 3 Do axons detect absolute or relative ep
hrin levels?⇒ Data in double heterozygotes show ne
ither of them correct. The ability of axons to discriminate between any two points on the tectum would depend on those two points having a sufficiently great difference in ephrin concentration.
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Discussion 4 Additive and Distinct function of ephrin-A2 an
d ephrin-A5- Double homozygous mutant shows a synergis
tic phenotype. (Additive)- Ephrin-A5 is dominant in posterior tectum. (Di
stinct)- Ephrin-A2-/-, ephrin-A5-/-, and ephrin-A2+/-;ephr
in-A5+/- mice all show a similar temporal axon phenotype in anterior SC. (Additive)
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Discussion 5 Two possible model of dorsoventral
mapping errors in double homozygous mutant.
1. Ephrin acting directly as dorsoventral labels.
2. A secondary effect to the anteroposterior defect.