Aβ Oligomers Alter NMDA Receptor Composition and Function in Early Stages of Alzheimer´s Disease

Abstract Background Amyloid beta (Aβ)-mediated synapse dysfunction is an early event in Alzheimer's disease (AD) pathogenesis and previous studies suggest that NMDA receptor (NMDAR) dysregulation may contribute to these pathological effects. Although Aβ peptides impair NMDAR expression and activity, the mechanisms mediating these alterations in early stages of AD are unclear. Here, we show that Aβ oligomers activate PKC, phosphorylate NR2B subunit and modulate its synaptic localization and function. Methods We isolated postsynaptic fractions (PSD) of AD prefrontal cortex and hippocampus of 6-month-old 3xTg-AD mice to quantify NR2B, PSD-95 and Aβ1-42 levels. To investigate the effects of Aβ oligomers on NR2B and PSD-95 expression, we use a range of techniques including mouse intrahippocampal injections of Aβ oligomers, isolation of protein membranes by cell-surface biotinylation, and synaptosomal fractionation as well as in vivo surface immunolabeling of EGFP-NR2B. Ca2+ imaging and PKC activity were monitored by fluorescent Ca2+ indicators and FRET analysis. Results We observed that NMDAR subunit NR2B and PSD-95 levels were aberrantly upregulated and correlated with Aβ42 load in human PSD fractions from early stages of AD patients as well as in hippocampus of 3xTg-AD mice. Importantly, NR2B and PSD95 dysregulation was revealed by an increased expression of both proteins in Aβ-injected mouse hippocampi. In cultured neurons, Aβ oligomers increased NR2B-containing NMDAR density and NMDA-induced synaptic Ca2+ influx in neuronal membranes in addition to colocalization in dendrites of NR2B subunit and PSD95. Mechanistically, Aβ oligomers required integrin β1 to promote synaptic location and function of NR2B-containing NMDARs and PSD95 by phosphorylation through classic PKCs. Conclusions These results provide evidence that Aβ oligomers modify the contribution of NR2B to NMDAR composition and function in early stages of AD through an integrin β1 and PKC-dependent pathway. These data reveal a novel role of Aβ oligomers in synaptic dysfunction that may be relevant to early-stage AD pathogenesis.