Frizzled3 controls axonal polarity and intermediate target entry during striatal pathway development

The striatum is a large brain nucleus with an important role in the control of movement and emotions.Mediumspiny neurons (MSNs) are striatal output neurons forming prominent descending axon tracts that target different brain nuclei. However, how MSN axon tracts in the forebrain develop remains poorly understood. Here, we implicate the Wnt binding receptor Frizzled3 in several uncharacterized aspects of MSN pathway formation [i.e., anterior–posterior guidance of MSN axons in the striatum and their subsequent growth into the globus pallidus (GP), an important (intermediate) target]. In Frizzled3 knock-out mice, MSN axons fail to extend along the anterior–posterior axis of the striatum, andmanydo not reach the GP. Wnt5a acts as an attractant forMSNaxons in vitro, is expressed in a posterior high, anterior low gradient in the striatum, and Wnt5a knock-out mice phenocopy striatal anterior–posterior defects observed in Frizzled3 mutants. This suggests that Wnt5a controls anterior–posterior guidance ofMSNaxons through Frizzled3. Axons that reach the GP in Frizzled3 knock-out mice fail to enter this structure. Surprisingly, entry ofMSNaxons into the GP non–cell-autonomously requires Frizzled3, and our data suggest that GP entry may be contingent on the correct positioning of “corridor” guidepost cells for thalamocortical axons by Frizzled3. Together, these data dissect MSN pathway development and reveal (non)cell-autonomous roles for Frizzled3 in MSN axon guidance. Further, they are the first to identify a gene that provides anterior–posterior axon guidance in a large brain nucleus and link Frizzled3 to corridor cell development.