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Fig. 2. Wnt-Drl/Ryk mediates axon repulsion in the Drosophila nerve
cord and vertebrate cortico-spinal tract. (A-C) Schematics of a
Drosophila nerve cord (NC) segment, illustrating anterior (ac) and
posterior (pc) commissures. The axons of anterior commissural neurons (ACN)
express derailed (drl; shown in blue) and are repelled by
Wnt5 derived from midline cells (red circles). (A) In wild-type
Drosophila, axons of posterior commissural neurons (PCN) do not
express drl and enter the pc. (B) The misexpression of drl
in PCN forces their axons through the ac. (C) Wnt5 loss of function
(grey circles) suppresses the phenotype induced by drl ectopic
expression, demonstrating that Wnt5, through Drl, is an essential cue for
proper commissural axon projection. (D) Schematic of the cortico-spinal
tracts (cst) in the dorsal portion of the postnatal mouse spinal cord. In
vertebrates, a Ryk-mediated repulsive gradient of Wnt proteins (Wnt1 or Wnt5a)
guides cortico-spinal axons in the AP direction. (E) Injections of
anti-Ryk antibodies into the cervical spinal cord stall cortico-spinal axons
posterior to the site of injection, with a consequent shortening of the cst.
D, dorsal; V, ventral.
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