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.

Aggregates of amyloid-beta (Aβ) and tau are hallmarks of Alzheimer’s disease (AD) leading to neurodegeneration and synaptic loss. While increasing evidence suggests that inhibition of N-methyl-D-aspartate receptors (NMDARs) may mitigate certain aspects of AD neuropathology, the precise role of different NMDAR subtypes for Aβ- and tau-mediated toxicity remains to be elucidated. Using mouse organotypic hippocampal slice cultures from arcAβ transgenic mice combined with Sindbis virus-mediated expression of human wild-type tau protein (hTau), we show that Aβ caused dendritic spine loss independently of tau. However, the presence of hTau was required for Aβ-induced cell death accompanied by increased hTau phosphorylation. Inhibition of NR2B-containing NMDARs abolished Aβ-induced hTau phosphorylation and toxicity by preventing GSK-3β activation but did not affect dendritic spine loss. Inversely, NR2A-containing NMDAR inhibition as well as NR2A-subunit knockout diminished dendritic spine loss but not the Aβ effect on hTau. Activation of extrasynaptic NMDARs in primary neurons caused degeneration of hTau-expressing neurons, which could be prevented by NR2B–NMDAR inhibition but not by NR2A knockout. Furthermore, caspase-3 activity was increased in arcAβ transgenic cultures. Activity was reduced by NR2A knockout but not by NR2B inhibition. Accordingly, caspase-3 inhibition abolished spine loss but not hTau-dependent toxicity in arcAβ transgenic slice cultures. Our data show that Aβ induces dendritic spine loss via a pathway involving NR2A-containing NMDARs and active caspase-3 whereas activation of eSyn NR2B-containing NMDARs is required for hTau-dependent neurodegeneration, independent of caspase-3.

Alzheimer's disease is characterized by synaptic alterations and neurodegeneration. Histopathological hallmarks represent amyloid plaques composed of amyloid-β (Aβ) and neurofibrillary tangles containing hyperphosphorylated tau. To determine whether synaptic changes and neurodegeneration share common pathways, we established anex vivomodel using organotypic hippocampal slice cultures from amyloid precursor protein transgenic mice combined with virus-mediated expression of EGFP-tagged tau constructs. Confocal high-resolution imaging, algorithm-based evaluation of spines, and live imaging were used to determine spine changes and neurodegeneration. We report that Aβ but not tau induces spine loss and shifts spine shape from mushroom to stubby through a mechanism involving NMDA receptor (NMDAR), calcineurin, and GSK-3β activation. In contrast, Aβ alone does not cause neurodegeneration but induces toxicity through phosphorylation of wild-type (wt) tau in an NMDAR-dependent pathway. We show that GSK-3β levels are elevated in APP transgenic cultures and that inhibiting GSK-3β activity or use of phosphorylation-blocking tau mutations prevented Aβ-induced toxicity of tau. FTDP-17 tau mutants are differentially affected by Aβ. While R406W tau shows increased toxicity in the presence of Aβ, no change is observed with P301L tau. While blocking NMDAR activity abolishes toxicity of both wt and R406W tau, the inhibition of GSK-3β only protects against toxicity of wt tau but not of R406W tau induced by Aβ. Tau aggregation does not correlate with toxicity. We propose that Aβ-induced spine pathology and tau-dependent neurodegeneration are mediated by divergent pathways downstream of NMDAR activation and suggest that Aβ affects wt and R406W tau toxicity by different pathways downstream of NMDAR activity.

The apolipoprotein E4 (APOE4) variant is the strongest genetic risk factor for Alzheimer disease (AD), while the APOE2 allele is protective. A major question is how different APOE genotypes affect the physiology of astrocytes, the main APOE-producing brain cells. Here, we differentiated human APOE-isogenic induced pluripotent stem cells (iPSCs) (APOE4, E3, E2, and APOE knockout [APOE-KO]) to functional “iAstrocytes”. Mass-spectrometry-based proteomic analysis showed genotype-dependent reductions of cholesterol and lipid metabolic and biosynthetic pathways (reduction: APOE4 > E3 > E2). Cholesterol efflux and biosynthesis were reduced in APOE4 iAstrocytes, while subcellular localization of cholesterol in lysosomes was elevated. An increase in immunoregulatory proteomic pathways (APOE4 > E3 > E2) was accompanied by elevated cytokine release in APOE4 cells (APOE4 > E3 > E2 > KO). Activation of iAstrocytes exacerbated proteomic changes and cytokine secretion mostly in APOE4 iAstrocytes, while APOE2 and APOE-KO iAstrocytes were least affected. Taken together, APOE4 iAstrocytes reveal a disease-relevant phenotype, causing dysregulated cholesterol/lipid homeostasis, increased inflammatory signaling, and reduced β-amyloid uptake, while APOE2 iAstrocytes show opposing effects.

Since their first description by Ramon y Cajal at the end of the 19th century, dendritic spines have been proposed as important sites of neuronal contacts and it has been suggested that changes in the activity of neurons directly affect spine morphology. In fact, since then it has been shown that about 90% of excitatory synapses end on spines. Recent data indicate that spines are highly dynamic structures and that spine shape correlates with the strength of synaptic transmission. Furthermore, several mental disorders including Alzheimers disease (AD) are associated with spine pathology suggesting that spine alterations play a central role in mental deficits. The aim of this review is to provide an overview about the current knowledge on spine morphology and function as well as about different experimental models to analyze spine changes and dynamics. The second part concentrates on disease-relevant factors that are associated with AD and which lead to spine alterations. In particular, data that provide evidence that Aβ oligomers or fibrillar Aβ deposits influence spine morphology and function will be presented and the contribution of tau pathology will be discussed. The review ends with the discussion of potential mechanisms how disease-relevant factors influence dendritic spines and whether and how spine changes could be therapeutically suppressed or reversed.

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.