The beta-amyloid protein of Alzheimers disease increases neuronal CRMP-2 phosphorylation by a Rho-GTP mechanism

Neuritic abnormalities are a major hallmark of Alzheimers disease (AD) pathology. Accumulation of -amyloid protein (A beta) in the brain causes changes in neuritic processes in individuals with this disease. In this study, we show that A beta decreases neurite outgrowth from SH-SY5Y human neuroblastoma cells. To explore molecular pathways by which A beta alters neurite outgrowth, we examined the activation and localization of RhoA and Racl which regulate the level and phosphorylation of the collapsin response mediator protein-2 (CRMP-2). A increased the levels of the GTP-bound (active) form of RhoA in SH-SY5Y cells. This increase in GTP-RhoA correlated with an increase in an alternatively spliced form of CRMP-2 (CRMP-2A) and its threonine phosphorylated form. Both a constitutively active form of Racl (CA-Racl) and the Rho kinase inhibitor, Y27632, decreased levels of the CRMP-2A variant and decreased threonine phosphorylation caused by A beta stimulation. The amount of tubulin bound to CRMP-2 was decreased in the presence of A beta but Y27632 increased the levels of tubulin bound to CRMP-2. Increased levels of both RhoA and CRMP-2 were found in neurons surrounding amyloid plaques in the cerebral cortex of the APP(Swe) Tg2576 mice. We found that there was an increase in threonine phosphorylation of CRMP-2 in Tg2576 mice and the increase correlated with a decrease in the ability of CRMP-2 to bind tubulin. The results suggest that A beta-induced neurite outgrowth inhibition may be initiated through a mechanism in which A beta causes an increase in Rho GTPase activity which, in turn, phosphorylates CRMP-2 to interfere with tubulin assembly in neurites.