Pax3 repairs a neural circuit through a program of directed axon outgrowth

Repairing damaged or dysfunctional human brain circuits remains an ongoing challenge for biomedical science. While surviving neuronal networks can be reorganised after lesion, for example by neurotrophins, these new connections are disorganised and rarely produce clinical improvement. Here we investigate how to promote axonal growth while retaining correct cellular targeting. We show that, in response to brain-derived neurotrophic factor (BDNF) in target-tissue, potential reinnervating neurons upregulate Pax3. Pax3 in turn increases polysialic acid-neural cell adhesion molecule (PSA-NCAM) on their axon terminals, facilitating their outgrowth and pathfinding, and resulting in correctly-targeted neural circuit repair in the mature nervous system. This is a novel role for Pax3, which we confirmed by showing its expression in afferent neurons is essential for spontaneous and BDNF-induced reinnervation in the developing and mature brains, respectively. Together these results suggest that Pax3 contributes to a repair program, in which axon growth is promoted and direction signaling maintained. These data advance our understanding for accurately rebuilding neural circuits: restricting growth-promotion to potential afferent neurons, as opposed to stimulating the whole circuit, allows axon growth without impairing its guidance.