Sonic hedgehog regulates the pathfinding of descending serotonergic axons in hindbrain in collaboration with Wnt5a and secreted frizzled-related protein 1.

Previous studies have demonstrated that both Wnt5a and Sonic hedgehog (Shh) are involved in regulating the pathfinding of descending serotonergic (5-HT, 5-hydroxytryptamine) axons in an opposite manner in the brainstem. Shh and Wnt signaling pathways interact to guide post-crossing commissural axons, where Shh acts as a repellent directly and shaping the Wnt gradient indirectly by regulating the gradient expression of the frizzled-related protein 1 (Sfrp1). Whether such a mechanism functions in descending 5-HT axon guidance remains unknown. Here, we found that the core components of the Shh and Wnt planar cell polarity signaling pathways are expressed in caudal 5-HT neurons, and the expression gradients of Shh, Sfrp1, and Wnt5a exist simultaneously in hindbrain. Dunn chamber assays revealed that Sfrp1 suppressed the attractive Wnt gradient. Moreover, we found that Shh overexpression led to pathfinding defects in 5-HT axon descending, and the axonal pathfinding defects could be partially rescued by administration of an Sfrp1 antagonist in vivo. Biochemical evidence showed Shh overexpression upregulated the expression of the Sfrp1 gene and interrupted Wnt5a binding to Frizzled-3. Taken together, our results indicate that Shh, Sfrp1, and Wnt5a collaborate to direct the pathfinding of descending 5-HT axons in the brainstem.