Urokinase-Type Plasminogen Activator Triggers Wingless/Int1-Independent Phosphorylation of the Low-Density Lipoprotein Receptor-Related Protein-6 in Cerebral Cortical Neurons

Background: Urokinase-type plasminogen activator (uPA) is a serine proteinase found in excitatory synapses located in the II/III and V cortical layers. The synaptic release of uPA promotes the formation of synaptic contacts and the repair of synapses damaged by various forms of injury, and its abundance is decreased in the synapse of Alzheimer's disease (AD) patients. Inactivation of the Wingless/Int1 (Wnt)-beta-catenin pathway plays a central role in the pathogenesis of AD. Soluble amyloid-beta (A beta) prevents the phosphorylation of the low-density lipoprotein receptor-related protein-6 (LRP6), and the resultant inactivation of the Wnt-beta-catenin pathway prompts the amyloidogenic processing of the amyloid-beta protein precursor (A beta PP) and causes synaptic loss. Objective: To study the role of neuronal uPA in the pathogenesis of AD. Methods: We used in vitro cultures of murine cerebral cortical neurons, a murine neuroblastoma cell line transfected with the APP-695 Swedish mutation (N2asw), and mice deficient on either plasminogen, or uPA, or its receptor (uPAR). Results: We showthat uPAactivates the Wnt-beta-catenin pathway in cerebral cortical neurons by triggering the phosphorylation of LRP6 via a plasmin-independent mechanism that does not require binding of Wnt ligands (Wnts). Our data indicate that uPA-induced activation of the Wnt-beta-catenin pathway protects the synapse from the harmful effects of soluble A beta and prevents the amyloidogenic processing of A beta PP by inhibiting the expression of beta-secretase 1 (BACE1) and the ensuing generation of A beta 40 and A beta 42 peptides. Conclusion: uPA protects the synapse and antagonizes the inhibitory effect of soluble A beta on the Wnt-beta-catenin pathway by providing an alternative pathway for LRP6 phosphorylation and beta-catenin stabilization.