Amelioration of β-amyloid-induced cognitive dysfunction and hippocampal axon degeneration by curcumin is associated with suppression of CRMP-2 hyperphosphorylation

Abstract

The Alzheimer's disease (AD) brain is characterized by β-amyloid deposition, hyperphosphorylation of microtubule-associated proteins, formation of senile plaques and neurofibrillary tangles, and degeneration of specific neuronal populations. Collapsin response mediator protein 2 (CRMP-2) hyperphosphorylation has been implicated in AD-associated neural process regression and neurofibrillary tangle formation. Curcumin is a promising AD drug with incompletely defined therapeutic mechanisms. One possibility is that curcumin prevents β-amyloid-induced CRMP-2 hyperphosphorylation, thereby protecting against axonal regression and (or) promoting axonal regrowth. We examined spatial learning in the Morris water maze, hippocampal expression levels of CRMP-2 and phosphorylated CRMP-2 (p-CRMP-2) by Western blot, and NF-200 (an axon-specific marker) by immunohistochemistry in Sprague-Dawley rats subjected to a single intrahippocampal injection of Aβ1–40 alone or Aβ1–40 followed by curcumin (i.p. daily for 7 days). Compared to controls, spatial learning was significantly impaired in these Aβ1–40-injected AD model rats (P<0.05). In addition, hippocampal expression levels of CRMP-2 and NF-200 were reduced while p-CRMP-2 expression was significantly enhanced (P<0.05 for all). Overexpression of p-CRMP-2 was correlated with NF-200 underexpression (r2=−0.67308, P<0.05), suggesting that Aβ1–40 damaged hippocampal axons. Spatial learning deficits were reversed, CRMP-2 and NF-200 expression levels increased, and p-CRMP-2 expression reduced in curcumin-treated rats (all P<0.05). We propose that curcumin improves spatial learning by inhibiting CRMP-2 hyperphosphorylation, thus protecting against β-amyloid-induced hippocampal damage or promoting regeneration.