Post on 18-Jan-2018
description
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
Introduction Topographic Maps Chemoaffinity Theory Eph receptor and Ephrin Gradient vs Competition Model Neuraxis
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.
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)
Eph receptor and Ephrin
Eph receptor- RTK- At least 14 members Ephrin- Eph ligand- At least 8 members- A family & B family- Repellant activity
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.
Neuraxis
Experiment Methods Results
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.
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
Result – Double Mutant Homozygote- the ectopic terminati
on extended over all regions
Heterozygote- similar to single mut
ant
Result – Double Mutant 2
Result - Ephrin Expression
Ephrin RNA distribution
Ephrin(protein) distribution
(Wild Type)
Result - Ephrin Expression 2
Result - EphA Expression
EphA RNA expression Ephrin-A2 protein
EphA5 protein
Result – Stripe Assays
Axon preference for anterior SC lanes
no preference
strong preference
A BC D
Result - Summary
Fig 8. Schmatic illustration of retinocollicular mapping phenotypes in mice with disrupted ephrin-A2 or ephrin-A5 genes.
Discussion Discussion 1 Discussion 2 Discussion 3 Discussion 4 Discussion 5
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.
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.
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.
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.
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)
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.