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Vol. 15, No. 7, pp. 877-888, April 1, 2001
1 European Molecular Biology Laboratory, D-69117
Heidelberg, Germany; 2 Regeneron Pharmaceuticals,
Inc.,Tarrytown, New York 10591-6707, USA
Growing axons follow highly stereotypical pathways, guided by a
variety of attractive and repulsive cues, before establishing specific
connections with distant targets. A particularly well-known example
that illustrates the complexity of axonal migration pathways involves
the axonal projections of motor neurons located in the motor cortex.
These projections take a complex route during which they first cross
the midline, then form the corticospinal tract, and ultimately connect
with motor neurons in the contralateral side of the spinal cord. These
obligatory contralateral connections account for why one side of the
brain controls movement on the opposing side of the body. The netrins
and slits provide well-known midline signals that regulate axonal
crossings at the midline. Herein we report that a member of the ephrin
family, ephrin-B3, also plays a key role at the midline to regulate
axonal crossing. In particular, we show that ephrin-B3 acts as the
midline barrier that prevents corticospinal tract projections from
recrossing when they enter the spinal gray matter. We report that in
ephrin-B3
/ mice, corticospinal tract projections freely
recross in the spinal gray matter, such that the motor cortex on one
side of the brain now provides bilateral input to the spinal cord. This
neuroanatomical abnormality in ephrin-B3/ mice correlates
with loss of unilateral motor control, yielding mice that
simultaneously move their right and left limbs and thus have a peculiar
hopping gait quite unlike the alternate step gait displayed by normal
mice. The corticospinal and walking defects in ephrin-B3/
mice resemble those recently reported for mice lacking the EphA4 receptor, which binds ephrin-B3 as well as other ephrins, suggesting that the binding of EphA4-bearing axonal processes to ephrin-B3 at the
midline provides the repulsive signal that prevents corticospinal tract
projections from recrossing the midline in the developing spinal cord.
[Key Words: Eph; ephrin; receptor tyrosine kinase; corticospinal tract; axon guidance; floor plate]
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