Advanced Glycation Endproducts Induce Alzheimer-like Pathology And Underlying Mechanisms

Background The hallmark lesions observed in Alzheimer disease (AD) brain is the formation of numerous neurofibrillary tangles (NFTs) and senile plaques (SPs), which are respectively composed of the hyperphosphorylated tau and (3-amyloid (Aβ). The advanced glycation endproducts (AGEs) are produced from a complex nonenzymatic multistep reaction of reducing sugars or dicarboneyl compounds, such as glyoxal and methylglyoxal, with the amino groups of proteins, especially the N-terminal amino groups and side chains of lysine and arginine. AGEs are elevated in the AD brains and the AGEs can stimulateβ-amyloid production and colocalize with NFTs and SPs, suggesting that AGEs play an important role in pathogenesis of AD. However, whether and how AGEs may cause AD-like tau hyperphosphorylation, whether elevated AGEs could induce spatial memory impairment, and whether early decreasing AGEs could reverse spatial memory defict of AD model are not reported.Objective It was to investigate the effects of AGEs on tau phosphorlation, Aβmetabolism and spatial memory, and whether decreasing AGEs accumulation could prevent memory deficits and AD-like pathologies in Tg2576 mice.Methods In vitro, we used exogenous AGEs to treat SK-N-SH cells and primary hippocampal neurons. RAGE antibody or inhibitors of GSK-3 (SB216763, LiC1), Erk1/2 (PD98059) and p38 (SB202190) were simultaneously co-incubated with AGEs. In vivo, we injected AGEs with or without RAGE antibody or LiCl into the hippocampus of SD rats. In order to study early inhibition of AGEs on AD model,6-m Tg2576 mice were used and subcutaneously injected with 0.9% normal saline (NS) or aminoguanidine (AG) for 3 m. Tau phosphorylation, protein kinases and phosphoesterase, synapse proteins were measured with western blotting or immunohischemistry. The level of A(3 and AGEs were measured with ELISA or dot blot. The interaction of Aβand AGEs was investigated with co-immunoprecipitation. Synapse plasticity was examined with long term potentiation (LTP). Spatial memory was investigated with Morris water maze. Results(1) AGEs could induce tau hyperphosphorylation through RAGE/GSK-3 pathway.First, we found AGEs induced tau hyperphosphorylation in SK-N-SH cells, primary hippocampal neurons and SD rats. Then, to study the underlying mechanism, we used the optimal concentration of AGEs (50μg/ml) to treat SK-N-SH cells for the optimal time (24 h). We observed that, with AGEs-induced tau hyperphosphorylation, mRNA and protein level of RAGE (receptor for AGEs) were elevated, the activity of akt was inhibited, and GSK-3, p38 and Erk1/2 were activated. At the last, to further indentify the role of the above chanded proteins in AGEs-induced tau hyperphosphorylation, we used Co-incubation of RAGE antibody or GSK-3 inhibitor with AGE to treat cells. We found that only RAGE antibody and GSK-3 inhibitor could insignificantly reverse tau hyperphosphorylation induced by AGEs, suggesting AGEs could induce tau hyperphosphorylation through RAGE/GSK-3 pathway.(2) AGEs could induce spatial memory deficit of SD rats, with tau hyperphosphorylation, inhibition of LTP, decline of synapse-related proteins through RAGE/GSK-3 pathway.First, we found that AGEs induced spatial memory deficit of SD rats. Then, to explore the possible mechanism, we investigated memory-related markers. We found that, with the impairment of memory, tau was hyperphosphorylated, LTP was inhibited, and synaptic proteins, especially postsynaptic proteins, were decreased. Meanwhile, with the hyperphosphorylated tau, RAGE was elevated, akt was inhibited, and GSK-3 was activated. And then, to further study the underlying mechanism of memory deficit, rats were co-injected with AGEs plus RAGE antibody or GSK-3 inhibitor, and found that co-injection not only significantly improve tau hyperphosphorylation, inhibiton of LTP, and decrease of synaptic proteins, but also reverse AGEs-induced memory impairment.(3) Early inhibiton of AGEs could revesrse memory deficit of AD model, which may be mediated by AGEs-induced decrease of A(3 production.We selected 6-m mice to treate AG. Then,6-m Tg2576 mice were used and subcutaneously injected with normal saline (NS) or aminoguanidine (AG) for 3 m. We found that AG treatment could effectively inhibit the production of AGEs, and compared with age-matched groups injected with NS, decrease of AGEs significantly reverse memory deficit of Tg2576 mice, with decrese of tau hyperphosphorylation at site of Thr231 and Ser396, and increase of synaptic proteins. Meanwhile, after treatment of AG, production of Aβwas declined with the decreae of AGEs. To explore the possible mechanism of decrease of Aβ, we used co-immunoprecipitation to study the interaction of AGEs and Aβ, and found that Aβcould be glycated by AGEs.Conclusion Our data reveal that AGEs can induce tau hyperphosphorylation and Aβoverproduction; AGEs cause impairments of spatial memory through RAGE-mediated GSK-3 activation; Early inhibiton of AGEs could reverse memory deficit in Tg2576 mice.