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 β-amyloid peptide (Aβ) 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 Aβ. We show that endogenous Aβ as well as exogenously added synthetic Aβ 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. Aβ increased the activity of p38 MAPK, a kinase involved in long-term depression and inhibition of p38 MAPK abolished the loss of dendritic spines. Aβ-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 Aβ. We further show that treatment with the G protein inhibitor pertussis toxin (PTX) did not prevent dendritic spine loss in the presence of Aβ oligomers. Our data suggest that Aβ induces the activation of p38 MAPK and subsequent synaptic loss through Ca2+ flux- and G protein-independent mechanisms.

Mitochondrial dysfunction is a prominent feature of Alzheimer's disease (AD) and increased production of reactive oxygen species (ROS) has been described in postmortem brain samples and animal models. However, these observations were made at a late stage of disease and the inability to examine an early, presymptomatic phase in human neurons impeded our understanding of cause or consequence of mitochondrial dysfunction in AD. We used human induced pluripotent stem cell-derived neuronal cells (iN cells) from sporadic AD (SAD) patients and healthy control subjects (HCS) to show aberrant mitochondrial function in patient-derived cells. We observed that neuronal cultures from some patients produced more ROS and displayed higher levels of DNA damage. Furthermore, patient-derived cells showed increased levels of oxidative phosphorylation chain complexes, whereas mitochondrial fission and fusion proteins were not affected. Surprisingly, these effects neither correlated with Aβ nor phosphorylated and total tau levels. Synaptic protein levels were also unaffected in SAD iN cells. The results of this study give new insights into constitutional metabolic changes in neurons from subjects prone to develop Alzheimer's pathology. They suggest that increased ROS production may have an integral role in the development of sporadic AD prior to the appearance of amyloid and tau pathology.