Alzheimer's disease (AD) is the most prevalent cause of dementia in the elderly, characterized by the presence of amyloid-beta (Aβ) plaques, neurofibrillary tangles, neuroinflammation, synapse loss and neurodegeneration in the brain. The amyloid cascade hypothesis postulates that deposition of Aβ peptides is the causative agent of AD pathology, but we still lack comprehensive understanding of the molecular mechanisms connecting Aβ peptides to neuronal dysfunctions in AD. In this work, we investigate the early effects of Aβ peptide accumulation on the functional properties and gene expression profiles of human-induced neurons (hiNs). We show that hiNs acutely exposed to low concentrations of both cell-secreted Aβ peptides or synthetic Aβ1-42 exhibit alterations in the frequency of calcium transients suggestive of increased neuronal excitability. Using single-cell RNA sequencing, we also show that cell-secreted Aβ up-regulates the expression of several synapse-related genes and down-regulates the expression of genes associated with metabolic stress mainly in glutamatergic neurons and, to a lesser degree, in GABAergic neurons and astrocytes. These neuronal alterations correlate with activation of the SEMA5, EPHA and NECTIN signaling pathways, which are important regulators of synaptic plasticity. Altogether, our findings indicate that slight elevations in Aβ concentrations are sufficient to elicit transcriptional changes in human neurons, which can contribute to early alterations in neural network activity.

Background: Alzheimer’s disease (AD) is the most prevalent cause of dementia in the elderly, characterized by the presence of amyloid beta (Aβ) plaques, neurofibrillary tangles, neuroinflammation, synapse loss and neurodegeneration in the brain. The amyloid cascade hypothesis postulates that deposition of Aβ peptides is the causative agent of AD pathology, but we still lack comprehensive understanding about the molecular mechanisms connecting Aβ peptides to neuronal dysfunctions in AD. In this work, we investigated the early effects of Aβ peptides accumulation on the functional properties and gene expression profiles of human-induced neurons (hiNs). Methods We exposed 6-weeks-old hiNs to low concentrations of cell-secreted Aβ oligomers or synthetic Aβ and performed time-lapse time microscopy to detect fast calcium transients as an indirect readout of neuronal electrical function. Next, we used single-nucleus RNA sequencing (snRNA-seq) to probe early Aβ-mediated gene expression alterations in hiNs and human-induced astrocytes (hiAs). Lastly, we leveraged snRNA-seq data to identify patterns of intercellular communication modulated by Aβ oligomers. Results We show that hiNs acutely exposed to low concentrations of both cell-secreted Aβ peptides or synthetic Aβ 1−42 exhibit alterations in the frequency of calcium transients suggestive of increased neuronal excitability. We also show that cell-secreted Aβ up-regulates the expression of several synaptic-related genes and down-regulates the expression of genes associated with metabolic stress mainly in glutamatergic neurons and to a lesser degree in GABAergic neurons and astrocytes. These neuronal alterations correlate with activation of SEMA5, EPHA and NECTIN signaling pathways, which are important regulators of synaptic plasticity. Conclusions Our findings indicate that slight elevations in Aβ concentrations are sufficient to elicit transcriptional changes in human neurons with long lasting consequences to neural network activity and suggest that at least part of the effects of Aβ on synapses might be mediated by semaphorin, ephrin and nectin signaling pathways.

Bridging Integrator 1 ( BIN1 ) is the second most important Alzheimer’s disease (AD) risk gene after APOE , but its physiological roles and contribution to brain pathology are largely elusive. In this work, we tackled the short- and long-term effects of BIN1 deletion in human induced neurons (hiNs) grown in bi-dimensional cultures and in cerebral organoids. We show that BIN1 loss-of-function leads to specific transcriptional alterations in glutamatergic neurons involving mainly genes associated with calcium homeostasis, ion transport and synapse function. We also show that BIN1 regulates calcium transients and neuronal electrical activity through interaction with the L-type voltage-gated calcium channel Cav 1.2 and regulation of activity-dependent internalization of this channel. Treatment with the Cav 1.2 antagonist nifedipine partly rescues neuronal electrical alterations in BIN1 knockout hiNs. Together, our results indicate that BIN1 misexpression impairs calcium homeostasis in glutamatergic neurons, potentially contributing to the transcriptional changes and neural network dysfunctions observed in AD.

Recent meta-analyses of genome-wide association studies identified a number of genetic risk factors of Alzheimer's disease; however, little is known about the mechanisms by which they contribute to the pathological process. As synapse loss is observed at the earliest stage of Alzheimer's disease, deciphering the impact of Alzheimer's risk genes on synapse formation and maintenance is of great interest. In this article, we report a microfluidic co-culture device that physically isolates synapses from pre- and postsynaptic neurons and chronically exposes them to toxic amyloid β peptides secreted by model cell lines overexpressing wild-type or mutated (V717I) amyloid precursor protein. Co-culture with cells overexpressing mutated amyloid precursor protein exposed the synapses of primary hippocampal neurons to amyloid β1-42 molecules at nanomolar concentrations and induced a significant decrease in synaptic connectivity, as evidenced by distance-based assignment of postsynaptic puncta to presynaptic puncta. Treating the cells with antibodies that target different forms of amyloid β suggested that low molecular weight oligomers are the likely culprit. As proof of concept, we demonstrate that overexpression of protein tyrosine kinase 2 beta-an Alzheimer's disease genetic risk factor involved in synaptic plasticity and shown to decrease in Alzheimer's disease brains at gene expression and protein levels-selectively in postsynaptic neurons is protective against amyloid β1-42-induced synaptotoxicity. In summary, our lab-on-a-chip device provides a physiologically relevant model of Alzheimer's disease-related synaptotoxicity, optimal for assessing the impact of risk genes in pre- and postsynaptic compartments.
© The Author(s) (2020). Published by Oxford University Press on behalf of the Guarantors of Brain.

Trimethylamine-N-oxide (TMAO) is a gut microbial metabolite that promotes Alzheimer's disease (AD) progression. Given that probiotics can alleviate AD symptoms by inhibiting the synthesis of TMAO, here we investigated the correlation between TMAO and cognitive deterioration by measuring TMAO levels in the plasma of choline-treated APP/PS1 mice (an AD mouse model) with and without probiotic treatments. We found that declines in L.plantarum in the gut were associated with cognitive impairment. Moreover, 12-weeks of treatment with memantine plus L. plantarum ameliorated cognitive deterioration, decreased Αβ levels in the hippocampus, and protected neuronal integrity and plasticity. These effects were accompanied by reductions in TMAO synthesis and neuroinflammation. These experiments demonstrate that L. plantarum augments the beneficial therapeutic effects of memantine treatment in APP/PS1 mice by remodeling the intestinal microbiota, inhibiting the synthesis of TMAO, and reducing clusterin levels. Our results thus highlight intestinal microbiota as a potential therapeutic target to decrease the risk of AD.