Hörndli C.S., and Chien, C.B. (2012) Sonic hedgehog is indirectly required for intraretinal axon pathfinding by regulating chemokine expression in the optic stalk. Development (Cambridge, England). 139(14):2604-2613.
Successful axon pathfinding requires both correct patterning of tissues, which will later harbor axonal tracts, and precise
localization of axon guidance cues along these tracts at the time of axon outgrowth. Retinal ganglion cell (RGC) axons grow
towards the optic disc in the central retina, where they turn to exit the eye through the optic nerve. Normal patterning of
the optic disc and stalk and the expression of guidance cues at this choice point are necessary for the exit of RGC axons
out of the eye. Sonic hedgehog (Shh) has been implicated in both patterning of ocular tissue and direct guidance of RGC axons.
Here, we examine the precise spatial and temporal requirement for Hedgehog (Hh) signaling for intraretinal axon pathfinding
and show that Shh acts to pattern the optic stalk in zebrafish but does not guide RGC axons inside the eye directly. We further
reveal an interaction between the Hh and chemokine pathways for axon guidance and show that cxcl12a functions downstream of Shh and depends on Shh for its expression at the optic disc. Together, our results support a model
in which Shh acts in RGC axon pathfinding indirectly by regulating axon guidance cues at the optic disc through patterning
of the optic stalk.