Calcium flux-independent NMDA receptor activity is required for Aβ oligomer-induced synaptic loss

Synaptic loss is one of the major features of Alzheimer's disease (AD) and correlates with the degree of dementia. N-methyl-D-aspartate receptors (NMDARs) have been shown to mediate downstream effects of the beta-amyloid peptide (A beta) in AD models. NMDARs can trigger intracellular cascades via Ca2+ entry, however, also Ca2+-independent (metabotropic) functions of NMDARs have been described. We aimed to determine whether ionotropic or metabotropic NMDAR signaling is required for the induction of synaptic loss by A beta. We show that endogenous A beta as well as exogenously added synthetic A beta oligomers induced dendritic spine loss and reductions in pre- and postsynaptic protein levels in hippocampal slice cultures. Synaptic alterations were mitigated by blocking glutamate binding to NMDARs using NMDAR antagonist APV, but not by preventing ion flux with Ca2+ chelator BAPTA or open-channel blockers MK-801 or memantine. A beta increased the activity of p38 MAPK, a kinase involved in long-term depression and inhibition of p38 MAPK abolished the loss of dendritic spines. A beta-induced increase of p38 MAPK activity was prevented by APV but not by BAPTA, MK-801 or memantine treatment highlighting the role of glutamate binding to NMDARs but not Ca2+ flux for synaptic degeneration by A beta. We further show that treatment with the G protein inhibitor pertussis toxin (PTX) did not prevent dendritic spine loss in the presence of A beta oligomers. Our data suggest that A beta induces the activation of p38 MAPK and subsequent synaptic loss through Ca2+ flux-and G protein-independent mechanisms.