δ-Catenin/NPRAP: A New Member of the Glycogen Synthase Kinase-3β Signaling Complex That Promotes β-Catenin Turnover in Neurons

Through a multiprotein complex, glycogen synthase kinase-3 beta (GSK-3 beta) phosphorylates and destabilizes p-catenin, an important signaling event for neuronal growth and proper synaptic function. delta-Catenin, or NPRAP (CTNND2), is a neural enriched member of the p-catenin superfamily and is also known to modulate neurite outgrowth and synaptic activity. In this study, we investigated the possibility that 8-catenin expression is also affected by GSK-3 beta signaling and participates in the molecular complex regulating p-catenin turnover in neurons. lmmunofluorescent light microscopy revealed colocalization of delta-catenin with members of the molecular destruction complex: GSK-3 beta, beta-catenin, and adenomatous polyposis coli proteins in rat primary neurons. GSK-3 beta formed a complex with delta-catenin, and its inhibition resulted in increased 8-catenin and p-catenin expression levels. LY294002 and amyloid peptide, known activators of GSK-3 beta signaling, reduced delta-catenin expression levels. Furthermore, delta-catenin immunoreactivity increased and protein turnover decreased when neurons were treated with proteasome inhibitors, suggesting that the stability of delta-catenin, like that of beta-catenin, is regulated by proteasome-mediated degradation. Coimmunoprecipitation experiments showed that 8-catenin overexpression promoted GSK-3 beta and beta-catenin interactions. Primary cortical neurons and PC12 cells expressing delta-catenin treated with proteasome inhibitors showed increased ubiquitinated beta-catenin forms. Consistent with the hypothesis that delta-catenin promotes the interaction of the destruction complex molecules, cycloheximide treatment of cells overexpressing delta-catenin showed enhanced beta-catenin turnover. These studies identify 8-catenin as a new member of the GSK-3 beta signaling pathway and further suggest that delta-catenin is potentially involved in facilitating the interaction, ubiquitination, and subsequent turnover of beta-catenin in neuronal cells. (C) 2010 Wiley-Liss, Inc.