Identification of a Novel Axon Regeneration Role for Non-Canonical Wnt Signaling in the Adult Retina After Injury

Abstract

Canonical and non-canonical Wnt signaling pathways are essential for development and maintenance of the central nervous system (CNS). Whereas the roles of canonical Wnt pathways in neuronal survival and axonal regeneration in adult CNS have been described, the functions of non-canonical Wnt pathways are not well understood. Furthermore, the role of non-canonical Wnt ligands in the adult retina has not been investigated. Non-canonical Wnt signaling shares receptors with canonical Wnt ligands but functions through calcium and JNK signaling pathways. Non-canonical ligands, such as the prototypic ligand Wnt5a, have varying effects in the developing CNS, including inhibiting or promoting axonal growth. To identify a role for non-canonical Wnt signaling in the developed retina after injury, we characterized the effect of Wnt5a on neurite outgrowth in cultured retinal ganglion cell (RGC) neurons and on axonal regeneration in the injured optic nerve in the mouse. Endogenous Wnt5a was upregulated after injury and exogenous Wnt5a significantly enhanced neurite growth of primary RGCs and led to extensive axonal regeneration after optic nerve crush (ONC) injury. Wnt5a also significantly increased RGC survival. Furthermore, Wnt5a induced phosphorylation of CamKII and JNK and induced expression of their downstream pathway components. Therefore, these results demonstrate for the first time that Wnt5a promotes axonal growth and protects RGCs in the adult retina.

Significance Statement

Adult RGCs have poor regenerative abilities after axonal injury. Molecules and signaling mechanisms that induce RGC survival, RGC axon growth and optic nerve regeneration have potential to treat optic neuropathies. In the current study, we identified the neurodevelopment factor Wnt5a as a novel regulator of RGC neurite outgrowth in primary RGC cultures and showed that it induces RGC survival and optic nerve regeneration in an adult optic nerve crush mouse model. Furthermore, we demonstrated that Wnt5a activates multiple prosurvival and proregenerative pathways in RGCs and retina. Therefore, these results demonstrate for the first time that Wnt5a, a non-canonical Wnt ligand critical for CNS development, also promotes axonal growth and protects RGCs in the adult retina.