Facilitation of axon outgrowth via a Wnt5a-CaMKK-CaMKIα pathway during neuronal polarization

Background: Wnt5a, originally identified as a guidance cue for commissural axons, activates a non-canonical pathway critical for cortical axonal morphogenesis. The molecular signaling cascade underlying this event remains obscure. Results: Through Ca2+ imaging in acute embryonic cortical slices, we tested if radially migrating cortical excitatory neurons that already bore primitive axons were sensitive to Wnt5a. While Wnt5a only evoked brief Ca2+ transients in immature neurons present in the intermediate zone (IZ), Wnt5a-induced Ca2+ oscillations were sustained in neurons that migrated out to the cortical plate (CP). We wondered whether this early Wnt5a-Ca2+ signaling during neuronal polarization has a morphogenetic consequence. During transition from round to polarized shape, Wnt5a administration to immature cultured cortical neurons specifically promoted axonal, but not dendritic, outgrowth. Pharmacological and genetic inhibition of the CaMKK-CaMKI alpha pathway abolished Wnt5a-induced axonal elongation, and rescue of CaMKI alpha in CaMKI alpha-knockdown neurons restored Wnt5a-mediated axon outgrowth. Conclusions: This study suggests that Wnt5a activates Ca2+ signaling during a neuronal morphogenetic time window when axon outgrowth is critically facilitated. Furthermore, the CaMKK-CaMKI alpha cascade is required for the axonal growth effect of Wnt5a during neuronal polarization.