Glycogen synthase kinase-3 and Axin function in a β-catenin-independent pathway that regulates neurite outgrowth in neuroblastoma cells

Abstract

We have sought to determine the roles of β-catenin and the Wnt signaling pathway in neurite outgrowth using a model cell system, the Neuro-2a neuroblastoma cell line. Activation of the Wnt signaling pathway disrupts a multiprotein complex that includes β-catenin, Axin, and glycogen synthase kinase-3 (GSK-3), which would otherwise promote the phosphorylation and degradation of β-catenin. Stabilized β-catenin accumulates in the cytosol and in the nucleus; in the nucleus it binds to TCF family transcription factors, forming a bipartite transcriptional activator of Wnt target genes. These events can be mimicked by lithium (Li+), which inhibits GSK-3 activity. Both Li+ and the GSK-3 inhibitor SB415286 induced neurite outgrowth of Neuro-2a cells. Li+-induced neurite outgrowth did not require β-catenin-/TCF-dependent transcription, and increasing levels of β-catenin either by transfection or using Wnt-3A was not sufficient to induce neurite outgrowth. Interestingly, Axin, which is also a substrate for GSK-3, was destabilized by Li+ and ectopic expression of Axin inhibited Li+-induced neurite outgrowth. Deletion analysis of Axin indicated that this inhibition required the GSK-3 binding site, but not the β-catenin binding site. Our results suggest that a signaling pathway involving Axin and GSK-3, but not β-catenin, regulates Li+-induced neurite outgrowth in Neuro-2a cells.