A cellular complex of BACE1 and γ-secretase sequentially generates Aβ from its full-length precursor

Rhei Undulati Rhizoma attenuates memory decline and reduces amyloid-β induced neuritic dystrophy in 5xFAD mouse

BACKGROUND

Alzheimer's disease (AD) is a common type of dementia characterized by amyloid-β (Aβ) accumulation, lysosomal dysfunction, and tau hyperphosphorylation, leading to neurite dystrophy and memory loss. This study aimed to investigate whether Rhei Undulati Rhizoma (RUR), which has been reported to have anti-neuroinflammatory effect, attenuates Aβ-induced memory impairment, neuritic dystrophy, and tau hyperphosphorylation, and to reveal its mode of action.

METHODS

Five-month-old 5xFAD mice received RUR (50 mg/kg) orally for 2 months. The Y-maze test was used to assess working memory. After behavioral testing, brain tissue was analyzed using thioflavin S staining, western blotting, and immunofluorescence staining to investigate the mode of action of RUR. To confirm whether RUR directly reduces Aβ aggregation, a thioflavin T assay and dot blot were performed after incubating Aβ with RUR.

RESULTS

RUR administration attenuated the Aβ-induced memory impairment in 5xFAD mice. Furthermore, decreased accumulation of Aβ was observed in the hippocampus of the RUR-treated 5xFAD group compare to the vehicle-treated 5xFAD group. Moreover, RUR reduced the dystrophic neurites (DNs) that accumulate impaired endolysosomal organelles around Aβ. In particular, RUR treatment downregulated the expression of β-site amyloid precursor protein cleaving enzyme 1 and the hyperphosphorylation of tau within DNs. Additionally, RUR directly suppressed the aggregation of Aβ, and eliminated Aβ oligomers in vitro.

CONCLUSIONS

This study showed that RUR could attenuate Aβ-induced pathology and directly regulate the aggregation of Aβ. These results suggest that RUR could be an efficient material for AD treatment through Aβ regulation.

Multicolor, Cell-Impermeable, and High Affinity BACE1 Inhibitor Probes Enable Superior Endogenous Staining and Imaging of Single Molecules

The prevailing but not undisputed amyloid cascade hypothesis places the β-site of APP cleaving enzyme 1 (BACE1) center stage in Alzheimer's Disease pathogenesis. Here, we investigated functional properties of BACE1 with novel tag- and antibody-free labeling tools, which are conjugates of the BACE1-inhibitor IV (also referred to as C3) linked to different impermeable Alexa Fluor dyes. We show that these fluorescent small molecules bind specifically to BACE1, with a 1:1 labeling stoichiometry at their orthosteric site. This is a crucial property especially for single-molecule and super-resolution microscopy approaches, allowing characterization of the dyes' labeling capabilities in overexpressing cell systems and in native neuronal tissue. With multiple colors at hand, we evaluated BACE1-multimerization by Förster resonance energy transfer (FRET) acceptor-photobleaching and single-particle imaging of native BACE1. In summary, our novel fluorescent inhibitors, termed Alexa-C3, offer unprecedented insights into protein-protein interactions and diffusion behavior of BACE1 down to the single molecule level.

APP-BACE1 Interaction and Intracellular Localization Regulate Aβ Production in iPSC-Derived Cortical Neurons

Alzheimer's disease (AD) is characterized pathologically by amyloid β (Aβ)-containing plaques. Generation of Aβ from amyloid precursor protein (APP) by two enzymes, β- and γ-secretase, has therefore been in the AD research spotlight for decades. Despite this, how the physical interaction of APP with the secretases influences APP processing is not fully understood. Herein, we compared two genetically identical human iPSC-derived neuronal cell types: low Aβ-secreting neuroprogenitor cells (NPCs) and high Aβ-secreting mature neurons, as models of low versus high Aβ production. We investigated levels of substrate, enzymes and products of APP amyloidogenic processing and correlated them with the proximity of APP to β- and γ-secretase in endo-lysosomal organelles. In mature neurons, increased colocalization of full-length APP with the β-secretase BACE1 correlated with increased β-cleavage product sAPPβ. Increased flAPP/BACE1 colocalization was mainly found in early endosomes. In the same way, increased colocalization of APP-derived C-terminal fragment (CTF) with presenilin-1 (PSEN1), the catalytic subunit of γ-secretase, was seen in neurons as compared to NPCs. Furthermore, most of the interaction of APP with BACE1 in low Aβ-secreting NPCs seemed to derive from CTF, the remaining APP part after BACE1 cleavage, indicating a possible novel product-enzyme inhibition. In conclusion, our results suggest that interaction of APP and APP cleavage products with their secretases can regulate Aβ production both positively and negatively. β- and γ-Secretases are difficult targets for AD treatment due to their ubiquitous nature and wide range of substrates. Therefore, targeting APP-secretase interactions could be a novel treatment strategy for AD. Colocalization of APP species with BACE1 in a novel model of low- versus high-Aβ secretion-Two genetically identical human iPSC-derived neuronal cell types: low Aβ-secreting neuroprogenitor cells (NPCs) and high Aβ secreting mature neurons, were compared. Increased full-length APP (flAPP)/BACE1 colocalization in early endosomes was seen in neurons, while APP-CTF/BACE1 colocalization was much higher than flAPP/BACE1 colocalization in NPCs, although the cellular location was not determined.

The Amyloid Cascade Hypothesis in Alzheimer’s Disease: Should We Change Our Thinking?

Old age increases the risk of Alzheimer’s disease (AD), the most common neurodegenerative disease, a devastating disorder of the human mind and the leading cause of dementia. Worldwide, 50 million people have the disease, and it is estimated that there will be 150 million by 2050. Today, healthcare for AD patients consumes 1% of the global economy. According to the amyloid cascade hypothesis, AD begins in the brain by accumulating and aggregating Aβ peptides and forming β-amyloid fibrils (Aβ42). However, in clinical trials, reducing Aβ peptide production and amyloid formation in the brain did not slow cognitive decline or improve daily life in AD patients. Prevention studies in cognitively unimpaired people at high risk or genetically destined to develop AD also have not slowed cognitive decline. These observations argue against the amyloid hypothesis of AD etiology, its development, and disease mechanisms. Here, we look at other avenues in the research of AD, such as the presenilin hypothesis, synaptic glutamate signaling, and the role of astrocytes and the glutamate transporter EAAT2 in the development of AD.

Focus on Alzheimer's Disease: The Role of Fibroblast Growth Factor 21 and Autophagy

Alzheimer's disease (AD) is a disorder of the central nervous system that is typically marked by progressive cognitive impairment and memory loss. Amyloid β plaque deposition and neurofibrillary tangles with hyperphosphorylated tau are the two hallmark pathologies of AD. In mammalian cells, autophagy clears aberrant protein aggregates, thus maintaining proteostasis as well as neuronal health. Autophagy affects production and metabolism of amyloid β and accumulation of phosphorylated tau proteins, whose malfunction can lead to the progression of AD. On the other hand, defective autophagy has been found to induce the production of the neuroprotective factor fibroblast growth factor 21 (FGF21), although the underlying mechanism is unclear. In this review, we highlight the significance of aberrant autophagy in the pathogenesis of AD, discuss the possible mechanisms by which defective autophagy induces FGF21 production, and analyze the potential of FGF21 in the treatment of AD. The findings provide some insights into the potential role of FGF21 and autophagy in the pathogenesis of AD.

The Binding of Different Substrate Molecules at the Docking Site and the Active Site of γ-Secretase Can Trigger Toxic Events in Sporadic and Familial Alzheimer's Disease

Pathogenic changes in γ-secretase activity, along with its response to different drugs, can be affected by changes in the saturation of γ-secretase with its substrate. We analyze the saturation of γ-secretase with its substrate using multiscale molecular dynamics studies. We found that an increase in the saturation of γ-secretase with its substrate could result in the parallel binding of different substrate molecules at the docking site and the active site. The C-terminal domain of the substrate bound at the docking site can interact with the most dynamic presenilin sites at the cytosolic end of the active site tunnel. Such interactions can inhibit the ongoing catalytic activity and increase the production of the longer, more hydrophobic, and more toxic Aβ proteins. Similar disruptions in dynamic presenilin structures can be observed with different drugs and disease-causing mutations. Both, C99-βCTF-APP substrate and its different Aβ products, can support the toxic aggregation. The aggregation depends on the substrate N-terminal domain. Thus, the C99-βCTF-APP substrate and β-secretase path can be more toxic than the C83-αCTF-APP substrate and α-secretase path. Nicastrin can control the toxic aggregation in the closed conformation. The binding of the C99-βCTF-APP substrate to γ-secretase can be controlled by substrate channeling between the nicastrin and β-secretase. We conclude that the presented two-substrate mechanism could explain the pathogenic changes in γ-secretase activity and Aβ metabolism in different sporadic and familial cases of Alzheimer's disease. Future drug-development efforts should target different cellular mechanisms that regulate the optimal balance between γ-secretase activity and amyloid metabolism.

Identification of the Aβ37/42 peptide ratio in CSF as an improved Aβ biomarker for Alzheimer's disease

INTRODUCTION

Identifying CSF-based biomarkers for the β-amyloidosis that initiates Alzheimer's disease (AD) could provide inexpensive and dynamic tests to distinguish AD from normal aging and predict future cognitive decline.

METHODS

We developed immunoassays specifically detecting all C-terminal variants of secreted amyloid β-protein and identified a novel biomarker, the Aβ 37/42 ratio, that outperforms the canonical Aβ42/40 ratio as a means to evaluate the γ-secretase activity and brain Aβ accumulation.

RESULTS

We show that Aβ 37/42 can distinguish physiological and pathological status in (1) presenilin-1 mutant vs wild-type cultured cells, (2) AD vs control brain tissue, and (3) AD versus cognitively normal (CN) subjects in CSF, where 37/42 (AUC 0.9622) outperformed 42/40 (AUC 0.8651) in distinguishing CN from AD.

DISCUSSION

We conclude that the Aβ 37/42 ratio sensitively detects presenilin/γ-secretase dysfunction and better distinguishes CN from AD than Aβ42/40 in CSF. Measuring this novel ratio alongside promising phospho-tau analytes may provide highly discriminatory fluid biomarkers for AD.

Circulating Small Extracellular Vesicle-Derived miR-342-5p Ameliorates Beta-Amyloid Formation via Targeting Beta-site APP Cleaving Enzyme 1 in Alzheimer's Disease

Alzheimer's disease (AD) is a common neurodegenerative disorder with progressive cognitive impairment in the elderly. Beta-amyloid (Aβ) formation and its accumulation in the brain constitute one of the pathological hallmarks of AD. Until now, how to modulate Aβ formation in hippocampal neurons remains a big challenge. Herein, we investigated whether the exosomal transfer of microRNA (miR) relates to amyloid pathology in the recipient neuron cells. We isolated circulating small extracellular vesicles (sEVs) from AD patients and healthy controls, determined the miR-342-5p level in the sEVs by RT-PCR, and evaluated its diagnostic performance in AD. Then, we took advantage of biomolecular assays to estimate the role of miR-342-5p in modulating the amyloid pathway, including amyloid precursor protein (APP), beta-site APP cleaving enzyme 1 (BACE1), and Aβ42. Furthermore, we subjected HT22 cells to the sEVs from the hippocampal tissues of transgenic APP mice (Exo-APP) or C57BL/6 littermates (Exo-CTL), and the Exo-APP enriched with miR-342-5p mimics or the control to assess the effect of the sEVs' delivery of miR-342-5p on Aβ formation. We observed a lower level of miR-342-5p in the circulating sEVs from AD patients compared with healthy controls. MiR-342-5p participated in Aβ formation by modulating BACE1 expression, specifically binding its 3'-untranslated region (UTR) sequence. Exo-APP distinctly promoted Aβ42 formation in the recipient cells compared to Exo-CTL. Intriguingly, miR-342-5p enrichment in Exo-APP ameliorated amyloid pathology in the recipient cells. Our study indicated that miR-342-5p was dysregulated in human circulating sEVs from AD patients; sEV transfer of miR-342-5p ameliorates Aβ formation by modulating BACE1 expression. These findings highlight the promising potential of exosomal miRNAs in AD clinical therapy.

Iron Chelation Remits Memory Deficits Caused by the High-Fat Diet in a Mouse Model of Alzheimer’s Disease

Background: Obesity is a worldwide health problem that has been implicated in many diseases, including Alzheimer’s disease (AD). AD is one of the most common neurodegenerative disorders and is characterized by two pathologies, including extracellular senior plaques composed of amyloid-β (Aβ) and intracellular neurofibrillary tangles (NFTs) consisting of abnormally hyperphosphorylated tau. According to current research, a high-fat diet (HFD) could exacerbate Aβ accumulation, oxidative damage, and cognitive defects in AD mice. However, the accurate role of HFD in the pathogenesis of AD is far more unclear. Objective: To explore the accurate role of HFD in the pathogenesis of AD. Methods: Open Field, Barns Maze, Elevated zero-maze, Contextual fear condition, Tail suspension test, western blotting, immunofluorescence, Fluoro-Jade C Labeling, Perls’ Prussian blue staining, and ELISA were used. Results: HFD caused nonheme iron overload in the brains of APPswe/PS1dE9 (APP/PS1) mice. Furthermore, the administration of M30 (0.5 mg/kg) for iron chelation once every 2 days per os (p.o.) for 1 month remitted memory deficits caused by HFD in APP/PS1 mice. Notably, a variety of hematological parameters in whole blood had no difference after iron chelation. In addition, iron chelation effectively reduced synaptic impairment in hippocampus and neuronal degeneration in cortex in the HFD-fed APP/PS1 mice. Meanwhile, iron chelation decreased Aβ 1–40 and Aβ 1–42 level as well as neuroinflammation in HFD-fed APP/PS1 mice. Conclusion: These data enhance our understanding of how HFD aggravates AD pathology and cognitive impairments and might shed light on future preclinical studies.

Meprin β knockout reduces brain Aβ levels and rescues learning and memory impairments in the APP/lon mouse model for Alzheimer's disease

β-Site amyloid precursor protein (APP) cleaving enzyme-1 (BACE1) is the major described β-secretase to generate Aβ peptides in Alzheimer's disease (AD). However, all therapeutic attempts to block BACE1 activity and to improve AD symptoms have so far failed. A potential candidate for alternative Aβ peptides generation is the metalloproteinase meprin β, which cleaves APP predominantly at alanine in p2 and in this study we can detect an increased meprin β expression in AD brain. Here, we report the generation of the transgenic APP/lon mouse model of AD lacking the functional Mep1b gene (APP/lon × Mep1b-/-). We examined levels of canonical and truncated Aβ species using urea-SDS-PAGE, ELISA and immunohistochemistry in brains of APP/lon mouse × Mep1b-/-. Additionally, we investigated the cognitive abilities of these mice during the Morris water maze task. Aβ1-40 and 1-42 levels are reduced in APP/lon mice when meprin β is absent. Immunohistochemical staining of mouse brain sections revealed that N-terminally truncated Aβ2-x peptide deposition is decreased in APP/lon × Mep1b-/- mice. Importantly, loss of meprin β improved cognitive abilities and rescued learning behavior impairments in APP/lon mice. These observations indicate an important role of meprin β within the amyloidogenic pathway and Aβ production in vivo.

Synaptic dysfunction in early phases of Alzheimer's Disease

The synapse is the locus of plasticity where short-term alterations in synaptic strength are converted to long-lasting memories. In addition to the presynaptic terminal and the postsynaptic compartment, a more holistic view of the synapse includes the astrocytes and the extracellular matrix to form a tetrapartite synapse. All these four elements contribute to synapse health and are crucial for synaptic plasticity events and, thereby, for learning and memory processes. Synaptic dysfunction is a common pathogenic trait of several brain disorders. In Alzheimer's Disease, the degeneration of synapses can be detected at the early stages of pathology progression before neuronal degeneration, supporting the hypothesis that synaptic failure is a major determinant of the disease. The synapse is the place where amyloid-β peptides are generated and is the target of the toxic amyloid-β oligomers. All the elements constituting the tetrapartite synapse are altered in Alzheimer's Disease and can synergistically contribute to synaptic dysfunction. Moreover, the two main hallmarks of Alzheimer's Disease, i.e., amyloid-β and tau, act in concert to cause synaptic deficits. Deciphering the mechanisms underlying synaptic dysfunction is relevant for the development of the next-generation therapeutic strategies aimed at modifying the disease progression.

Neurodegenerative Disorders of Alzheimer, Parkinsonism, Amyotrophic Lateral Sclerosis and Multiple Sclerosis: An Early Diagnostic Approach for Precision Treatment

Neurodegenerative diseases (NDs) are characterised by progressive dysfunction of synapses, neurons, glial cells and their networks. Neurodegenerative diseases can be classified according to primary clinical features (e.g., dementia, parkinsonism, or motor neuron disease), anatomic distribution of neurodegeneration (e.g., frontotemporal degenerations, extrapyramidal disorders, or spinocerebellar degenerations), or principal molecular abnormalities. The most common neurodegenerative disorders are amyloidosis, tauopathies, a-synucleinopathy, and TAR DNA-binding protein 43 (TDP-43) proteopathy. The protein abnormalities in these disorders have abnormal conformational properties along with altered cellular mechanisms, and they exhibit motor deficit, mitochondrial malfunction, dysfunctions in autophagic-lysosomal pathways, synaptic toxicity, and more emerging mechanisms such as the roles of stress granule pathways and liquid-phase transitions. Finally, for each ND, microglial cells have been reported to be implicated in neurodegeneration, in particular, because the microglial responses can shift from neuroprotective to a deleterious role. Growing experimental evidence suggests that abnormal protein conformers act as seed material for oligomerization, spreading from cell to cell through anatomically connected neuronal pathways, which may in part explain the specific anatomical patterns observed in brain autopsy sample. In this review, we mention the human pathology of select neurodegenerative disorders, focusing on how neurodegenerative disorders (i.e., Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis) represent a great healthcare problem worldwide and are becoming prevalent because of the increasing aged population. Despite many studies have focused on their etiopathology, the exact cause of these diseases is still largely unknown and until now with the only available option of symptomatic treatments. In this review, we aim to report the systematic and clinically correlated potential biomarker candidates. Although future studies are necessary for their use in early detection and progression in humans affected by NDs, the promising results obtained by several groups leads us to this idea that biomarkers could be used to design a potential therapeutic approach and preclinical clinical trials for the treatments of NDs.

EDEM1 Regulates Amyloid Precursor Protein (APP) Metabolism and Amyloid-β Production

Endoplasmic reticulum (ER) degradation-enhancing α-mannosidase-like protein 1 (EDEM1) is a quality control factor directly involved in the endoplasmic reticulum-associated degradation (ERAD) process. It recognizes terminally misfolded proteins and directs them to retrotranslocation which is followed by proteasomal degradation in the cytosol. The amyloid-β precursor protein (APP) is synthesized and N-glycosylated in the ER and transported to the Golgi for maturation before being delivered to the cell surface. The amyloidogenic cleavage pathway of APP leads to production of amyloid-β (Aβ), deposited in the brains of Alzheimer’s disease (AD) patients. Here, using biochemical methods applied to human embryonic kidney, HEK293, and SH-SY5Y neuroblastoma cells, we show that EDEM1 is an important regulatory factor involved in APP metabolism. We find that APP cellular levels are significantly reduced after EDEM1 overproduction and are increased in cells with downregulated EDEM1. We also report on EDEM1-dependent transport of APP from the ER to the cytosol that leads to proteasomal degradation of APP. EDEM1 directly interacts with APP. Furthermore, overproduction of EDEM1 results in decreased Aβ₄₀ and Aβ₄₂ secretion. These findings indicate that EDEM1 is a novel regulator of APP metabolism through ERAD.

Intermittent hypoxia treatment alleviates memory impairment in the 6-month-old APPswe/PS1dE9 mice and reduces amyloid beta accumulation and inflammation in the brain

Background

Alzheimer’s disease (AD) is a progressive, degenerative, and terminal disease without cure. There is an urgent need for a new strategy to treat AD. The aim of this study was to investigate the effects of intermittent hypoxic treatment (IHT) on cognitive functions in a mouse model of AD and unravel the mechanism of action of IHT.

Methods

Six-month-old APPswe/PS1dE9 (APP/PS1) male mice were exposed to hypoxic environment (14.3% O₂) 4 h/day for 14 days or 28 days. Cognitive functions were measured by Morris water maze test after either 14 days or 42 days of interval. Thereafter the distribution of amyloid plaque and microglial activation were determined by mouse brain immunohistochemistry, while the amyloid beta (Aβ) and inflammatory cytokines were measured by ELISA and Western Blot. Microarray was used for studying gene expressions in the hippocampus.

Results

IHT for 14 days or 28 days significantly improved the spatial memory ability of the 6-month-old APP/PS1 mice. The memory improvement by 14 days IHT lasted to 14 days, but not to 42 days. The level of Aβ plaques and neurofilament accumulations was reduced markedly after the IHT exposure. IHT reduced the pro-inflammatory cytokines IL-1β, IL-6 levels, and β-secretase cleavage of APP processing which implies reduced Aβ production. Microarray analysis revealed a large number of genes in the hippocampus were significantly altered which are known to be metabolism-regulated genes.

Conclusions

This study provides evidence of the beneficial effect of IHT on the progression of AD by alleviating memory impairment, reducing Aβ accumulation and inflammation in the brain. IHT can be developed as a novel measure to relieve the progression of AD by targeting multiple pathways in the AD pathogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-021-00935-z.

The Swedish dilemma - the almost exclusive use of APPswe-based mouse models impedes adequate evaluation of alternative β-secretases

Alzheimer's disease (AD) is the most common form of dementia, however incurable so far. It is widely accepted that aggregated amyloid β (Aβ) peptides play a crucial role for the pathogenesis of AD, as they cause neurotoxicity and deposit as so-called Aβ plaques in AD patient brains. Aβ peptides derive from the amyloid precursor protein (APP) upon consecutive cleavage at the β- and γ-secretase site. Hence, mutations in the APP gene are often associated with autosomal dominant inherited AD. Almost thirty years ago, two mutations at the β-secretase site were observed in two Swedish families (termed Swedish APP (APPswe) mutations), which led to early-onset AD. Consequently, APPswe was established in almost every common AD mouse model, as it contributes to early Aβ plaque formation and cognitive impairments. Analyzing these APPswe-based mouse models, the aspartyl protease BACE1 has been evolving as the prominent β-secretase responsible for Aβ release in AD and as the most important therapeutic target for AD treatment. However, with respect to β-secretase processing, the very rare occurring APPswe variant substantially differs from wild-type APP. BACE1 dominates APPswe processing resulting in the release of Aβ1-x, whereas N-terminally truncated Aβ forms are scarcely generated. However, these N-terminally truncated Aβ species such as Aβ2-x, Aβ3-x and Aβ4-x are elevated in AD patient brains and exhibit an increased potential to aggregate compared to Aβ1-x peptides. Proteases such as meprin β, cathepsin B and ADAMTS4 were identified as alternative β-secretases being capable of generating these N-terminally truncated Aβ species from wild-type APP. However, neither meprin β nor cathepsin B are capable of generating N-terminally truncated Aβ peptides from APPswe. Hence, the role of BACE1 for the Aβ formation during AD might be overrepresented through the excessive use of APPswe mouse models. In this review we critically discuss the consideration of BACE1 as the most promising therapeutic target. Shifting the focus of AD research towards alternative β secretases might unveil promising alternatives to BACE1 inhibitors constantly failing in clinical trials due to ineffectiveness and harmful side effects.

Nanoscale organization of Nicastrin, the substrate receptor of the γ-secretase complex, as independent molecular domains

Alterations in the canonical processing of Amyloid Precursor Protein generate proteoforms that contribute to the onset of Alzheimer’s Disease. Modified composition of γ-secretase or mutations in its subunits has been directly linked to altered generation of Amyloid beta. Despite biochemical evidence about the role of γ-secretase in the generation of APP, the molecular origin of how spatial heterogeneity in the generation of proteoforms arises is not well understood. Here, we evaluated the localization of Nicastrin, a γ-secretase subunit, at nanometer sized functional zones of the synapse. With the help of super resolution microscopy, we confirm that Nicastrin is organized into nanodomains of high molecular density within an excitatory synapse. A similar nanoorganization was also observed for APP and the catalytic subunit of γ-secretase, Presenilin 1, that were discretely associated with Nicastrin nanodomains. Though Nicastrin is a functional subunit of γ-secretase, the Nicastrin and Presenilin1 nanodomains were either colocalized or localized independent of each other. The Nicastrin and Presenilin domains highlight a potential independent regulation of these molecules different from their canonical secretase function. The collisions between secretases and substrate molecules decide the probability and rate of product formation for transmembrane proteolysis. Our observations of secretase nanodomains indicate a spatial difference in the confinement of substrate and secretases, affecting the local probability of product formation by increasing their molecular availability, resulting in differential generation of proteoforms even within single synapses.

An ultra-sensitive immunoassay detects and quantifies soluble Aβ oligomers in human plasma

INTRODUCTION

Evidence strongly suggests that soluble oligomers of amyloid beta protein (oAβ) help initiate the pathogenic cascade of Alzheimer's disease (AD). To date, there have been no validated assays specific for detecting and quantifying oAβ in human blood.

METHODS

We developed an ultrasensitive oAβ immunoassay using a novel capture antibody (71A1) with N-terminal antibody 3D6 for detection that specifically quantifies soluble oAβ in the human brain, cerebrospinal fluid (CSF), and plasma.

RESULTS

Two new antibodies (71A1; 1G5) are oAβ-selective, label Aβ plaques in non-fixed AD brain sections, and potently neutralize the synaptotoxicity of AD brain-derived oAβ. The 71A1/3D6 assay showed excellent dilution linearity in CSF and plasma without matrix effects, good spike recovery, and specific immunodepletion.

DISCUSSION

We have created a sensitive, high throughput, and inexpensive method to quantify synaptotoxic oAβ in human plasma for analyzing large cohorts of aged and AD subjects to assess the dynamics of this key pathogenic species and response to therapy.

High fat diet aggravates AD-related pathogenic processes in APP/PS1 mice

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disorder and negative life-style factors may contribute to its etiopathogenesis. Substantial evidence from humans and murine models reveal that Insulin Resistance (IR) associated with high fat diet (HFD) increase the risk of developing AD and age-related amyloidogenesis.

OBJECTIVE

To corroborate and clarify the influence of HFD on amyloidogenesis and cognitive deficits in AD model mice.

RESULTS

We here show that a four months HFD-feeding increases IR in both the periphery and brain of APP/PS1 mice, which are used as AD model. Meanwhile, long term HFD exacerbates cognitive defects and impairs dendritic integrity and expressions of synaptic proteins in APP/PS1 mice. Furthermore, HFD induces an increase in β-secretase (BACE1) expression and a decrease in insulin degrading enzyme (IDE) expression, resulting in β-amyloid (Aβ) accumulation.

CONCLUSION

Our data suggests that long term HFD, with the accompanying IR, promotes Aβ toxicity and cognitive deficits, indicating that modifiable life-style hazards such as HFD-induced IR might contribute to AD pathogenesis.

Molecular imaging of Alzheimer's disease-related gamma-secretase in mice and nonhuman primates

The pathogenesis of Alzheimer’s disease (AD) is primarily driven by brain accumulation of the amyloid-β-42 (Aβ₄₂) peptide generated from the amyloid-β precursor protein (APP) via cleavages by β- and γ-secretase. γ-Secretase is a prime drug target for AD; however, its brain regional expression and distribution remain largely unknown. Here, we are aimed at developing molecular imaging tools for visualizing γ-secretase. We used our recently developed γ-secretase modulators (GSMs) and synthesized our GSM-based imaging agent, [¹¹C]SGSM-15606. We subsequently performed molecular imaging in rodents, including AD transgenic animals, and macaques, which revealed that our probe displayed good brain uptake and selectivity, stable metabolism, and appropriate kinetics and distribution for imaging γ-secretase in the brain. Interestingly, rodents and macaques shared certain brain areas with high γ-secretase expression, suggesting a functional conservation of γ-secretase. Collectively, we have provided the first molecular brain imaging of γ-secretase, which may not only accelerate our drug discovery for AD but also advance our understanding of AD.

Amyloid Oligomers: A Joint Experimental/Computational Perspective on Alzheimer's Disease, Parkinson's Disease, Type II Diabetes, and Amyotrophic Lateral Sclerosis

Protein misfolding and aggregation is observed in many amyloidogenic diseases affecting either the central nervous system or a variety of peripheral tissues. Structural and dynamic characterization of all species along the pathways from monomers to fibrils is challenging by experimental and computational means because they involve intrinsically disordered proteins in most diseases. Yet understanding how amyloid species become toxic is the challenge in developing a treatment for these diseases. Here we review what computer, in vitro, in vivo, and pharmacological experiments tell us about the accumulation and deposition of the oligomers of the (Aβ, tau), α-synuclein, IAPP, and superoxide dismutase 1 proteins, which have been the mainstream concept underlying Alzheimer's disease (AD), Parkinson's disease (PD), type II diabetes (T2D), and amyotrophic lateral sclerosis (ALS) research, respectively, for many years.

Mind the Gap: Mitochondria and the Endoplasmic Reticulum in Neurodegenerative Diseases

The way organelles are viewed by cell biologists is quickly changing. For many years, these cellular entities were thought to be unique and singular structures that performed specific roles. However, in recent decades, researchers have discovered that organelles are dynamic and form physical contacts. In addition, organelle interactions modulate several vital biological functions, and the dysregulation of these contacts is involved in cell dysfunction and different pathologies, including neurodegenerative diseases. Mitochondria–ER contact sites (MERCS) are among the most extensively studied and understood juxtapositioned interorganelle structures. In this review, we summarise the major biological and ultrastructural dysfunctions of MERCS in neurodegeneration, with a particular focus on Alzheimer’s disease as well as Parkinson’s disease, amyotrophic lateral sclerosis and frontotemporal dementia. We also propose an updated version of the MERCS hypothesis in Alzheimer’s disease based on new findings. Finally, we discuss the possibility of MERCS being used as possible drug targets to halt cell death and neurodegeneration.

Hydrophilic loop 1 of Presenilin-1 and the APP GxxxG transmembrane motif regulate γ-secretase function in generating Alzheimer-causing Aβ peptides

γ-Secretase is responsible for the proteolysis of amyloid precursor protein (APP) into amyloid-beta (Aβ) peptides, which are centrally implicated in the pathogenesis of Alzheimer’s disease (AD). The biochemical mechanism of how processing by γ-secretase is regulated, especially as regards the interaction between enzyme and substrate, remains largely unknown. Here, mutagenesis reveals that the hydrophilic loop-1 (HL-1) of presenilin-1 (PS1) is critical for both γ-secretase step-wise cleavages (processivity) and its allosteric modulation by heterocyclic γ-modulatory compounds. Systematic mutagenesis of HL-1, including all of its familial AD mutations and additional engineered variants, and quantification of the resultant Aβ products show that HL-1 is necessary for proper sequential γ-secretase processivity. We identify Y106, L113, and Y115 in HL-1 as key targets for heterocyclic γ-secretase modulators (GSMs) to stimulate processing of pathogenic Aβ peptides. Further, we confirm that the GxxxG domain in the APP transmembrane region functions as a critical substrate motif for γ-secretase processivity: a G29A substitution in APP-C99 mimics the beneficial effects of GSMs. Together, these findings provide a molecular basis for the structural regulation of γ-processivity by enzyme and substrate, facilitating the rational design of new GSMs that lower AD-initiating amyloidogenic Aβ peptides.

Alteration in synaptic nanoscale organization dictates amyloidogenic processing in Alzheimer's disease

Despite intuitive insights into differential proteolysis of amyloid precursor protein (APP), the stochasticity behind local product formation through amyloidogenic pathway at individual synapses remain unclear. Here, we show that the major components of amyloidogenic machinery namely, APP and secretases are discretely organized into nanodomains of high local concentration compared to their immediate environment in functional zones of the synapse. Additionally, with the aid of multiple models of Alzheimer's disease (AD), we confirm that this discrete nanoscale chemical map of amyloidogenic machinery is altered at excitatory synapses. Furthermore, we provide realistic models of amyloidogenic processing in unitary vesicles originating from the endocytic zone of excitatory synapses. Thus, we show how an alteration in the stochasticity of synaptic nanoscale organization contributes to the dynamic range of C-terminal fragments β (CTFβ) production, defining the heterogeneity of amyloidogenic processing at individual synapses, leading to long-term synaptic deficits as seen in AD.

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Components of amyloidogenic machinery are organized into nanodomains

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Assembly of nanodomains differs between functional zones of the synapse

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Stochasticity of nanoscale organization dictates dynamic range of APP proteolysis

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Variability in composition of amyloidogenic machinery is associated with AD

A Curcumin Analog Reduces Levels of the Alzheimer's Disease-Associated Amyloid-β Protein by Modulating AβPP Processing and Autophagy

Alzheimer's disease (AD) is a devastating neurodegenerative disease with no cure currently available. A pathological hallmark of AD is accumulation and deposition of amyloid-β protein (Aβ), a ∼4 kDa peptide generated through serial cleavage of the amyloid-β protein precursor (AβPP) by β- and γ-secretases. Curcumin is a natural compound primarily found in the widely used culinary spice, turmeric, which displays therapeutic potential for AD. Recently, we reported the development of curcumin analogs and identified a lead compound, curcumin-like compound-R17 (CLC-R17), that significantly attenuates Aβ deposition in an AD transgenic mouse model. Here, we elucidated the mechanisms of this analog on Aβ levels and AβPP processing using cell models of AD. Using biochemical methods and our recently developed nanoplasmonic fiber tip probe technology, we showed that the lead compound potently lowers Aβ levels in conditioned media and reduces oligomeric amyloid levels in the cells. Furthermore, like curcumin, the lead compound attenuates the maturation of AβPP in the secretory pathway. Interestingly, it upregulated α-secretase processing of AβPP and inhibited β-secretase processing of AβPP by decreasing BACE1 protein levels. Collectively, our data reveal mechanisms of a promising curcumin analog in reducing Aβ levels, which strongly support its development as a potential therapeutic for AD.

LongShengZhi Capsule Attenuates Alzheimer-Like Pathology in APP/PS1 Double Transgenic Mice by Reducing Neuronal Oxidative Stress and Inflammation

Alzheimer’s disease (AD) is the most common form of dementia in the elderly. It may be caused by oxidative stress, inflammation, and cerebrovascular dysfunctions in the brain. LongShengZhi Capsule (LSZ), a traditional Chinese medicine, has been approved by the China Food and Drug Administration for treatment of patients with cardiovascular/cerebrovascular disease. LSZ contains several neuroprotective ingredients, including Hirudo, Astmgali Radix, Carthami Flos (Honghua), Persicae Semen (Taoren), Acori Tatarinowii Rhizoma (Shichangpu), and Acanthopanax Senticosus (Ciwujia). In this study, we aimed to determine the effect of LSZ on the AD process. Double transgenic mice expressing the amyloid-β precursor protein and mutant human presenilin 1 (APP/PS1) to model AD were treated with LSZ for 7 months starting at 2 months of age. LSZ significantly improved the cognition of the mice without adverse effects, indicating its high degree of safety and efficacy after a long-term treatment. LSZ reduced AD biomarker Aβ plaque accumulation by inhibiting β-secretase and γ-secretase gene expression. LSZ also reduced p-Tau expression, cell death, and inflammation in the brain. Consistently, in vitro, LSZ ethanol extract enhanced neuronal viability by reducing L-glutamic acid-induced oxidative stress and inflammation in HT-22 cells. LSZ exerted antioxidative effects by enhancing superoxide dismutase and glutathione peroxidase expression, reduced Aβ accumulation by inhibiting β-secretase and γ-secretase mRNA expression, and decreased p-Tau level by inhibiting NF-κB-mediated inflammation. It also demonstrated neuroprotective effects by regulating the Fas cell surface death receptor/B-cell lymphoma 2/p53 pathway. Taken together, our study demonstrates the antioxidative stress, anti-inflammatory, and neuroprotective effects of LSZ in the AD-like pathological process and suggests it could be a potential medicine for AD treatment.

Brain transcriptome analysis reveals subtle effects on mitochondrial function and iron homeostasis of mutations in the SORL1 gene implicated in early onset familial Alzheimer's disease

To prevent or delay the onset of Alzheimer’s disease (AD), we must understand its molecular basis. The great majority of AD cases arise sporadically with a late onset after 65 years of age (LOAD). However, rare familial cases of AD can occur due to dominant mutations in a small number of genes that cause an early onset prior to 65 years of age (EOfAD). As EOfAD and LOAD share similar pathologies and disease progression, analysis of EOfAD genetic models may give insight into both subtypes of AD. Sortilin-related receptor 1 (SORL1) is genetically associated with both EOfAD and LOAD and provides a unique opportunity to investigate the relationships between both forms of AD. Currently, the role of SORL1 mutations in AD pathogenesis is unclear. To understand the molecular consequences of SORL1 mutation, we performed targeted mutagenesis of the orthologous gene in zebrafish. We generated an EOfAD-like mutation, V1482Afs, and a putatively null mutation, to investigate whether EOfAD-like mutations in sorl1 display haploinsufficiency by acting through loss-of-function mechanisms. We performed mRNA-sequencing on whole brains, comparing wild type fish with their siblings heterozygous for EOfAD-like or putatively loss-of-function mutations in sorl1, or transheterozygous for these mutations. Differential gene expression analysis identified a small number of differentially expressed genes due to the sorl1 genotypes. We also performed enrichment analysis on all detectable genes to obtain a more complete view on changes to gene expression by performing three methods of gene set enrichment analysis, then calculated an overall significance value using the harmonic mean p-value. This identified subtle effects on expression of genes involved in energy production, mRNA translation and mTORC1 signalling in both the EOfAD-like and null mutant brains, implying that these effects are due to sorl1 haploinsufficiency. Surprisingly, we also observed changes to expression of genes occurring only in the EOfAD-mutation carrier brains, suggesting gain-of-function effects. Transheterozygosity for the EOfAD-like and null mutations (i.e. lacking wild type sorl1), caused apparent effects on iron homeostasis and other transcriptome changes distinct from the single-mutation heterozygous fish. Our results provide insight into the possible early brain molecular effects of an EOfAD mutation in human SORL1. Differential effects of heterozygosity and complete loss of normal SORL1 expression are revealed.

The physiological roles of tau and Aβ: implications for Alzheimer's disease pathology and therapeutics

Tau and amyloid beta (Aβ) are the prime suspects for driving pathology in Alzheimer's disease (AD) and, as such, have become the focus of therapeutic development. Recent research, however, shows that these proteins have been highly conserved throughout evolution and may have crucial, physiological roles. Such functions may be lost during AD progression or be unintentionally disrupted by tau- or Aβ-targeting therapies. Tau has been revealed to be more than a simple stabiliser of microtubules, reported to play a role in a range of biological processes including myelination, glucose metabolism, axonal transport, microtubule dynamics, iron homeostasis, neurogenesis, motor function, learning and memory, neuronal excitability, and DNA protection. Aβ is similarly multifunctional, and is proposed to regulate learning and memory, angiogenesis, neurogenesis, repair leaks in the blood-brain barrier, promote recovery from injury, and act as an antimicrobial peptide and tumour suppressor. This review will discuss potential physiological roles of tau and Aβ, highlighting how changes to these functions may contribute to pathology, as well as the implications for therapeutic development. We propose that a balanced consideration of both the physiological and pathological roles of tau and Aβ will be essential for the design of safe and effective therapeutics.

Dysfunctional proteins in neuropsychiatric disorders: From neurodegeneration to autism spectrum disorders

Despite fundamental differences in disease course and outcomes, neurodevelopmental (autism spectrum disorders - ASD) and neurodegenerative disorders (Alzheimer's disease - AD and Parkinson's disease - PD) present surprising, common traits in their molecular pathomechanisms. Uncontrolled oligomerization and aggregation of amyloid β (Aβ), microtubule-associated protein (MAP) tau, or α-synuclein (α-syn) contribute to synaptic impairment and the ensuing neuronal death in both AD and PD. Likewise, the pathogenesis of ASD may be attributed, at least in part, to synaptic dysfunction; attention has also been recently paid to irregularities in the metabolism and function of the Aβ precursor protein (APP), tau, or α-syn. Commonly affected elements include signaling pathways that regulate cellular metabolism and survival such as insulin/insulin-like growth factor (IGF) - PI3 kinase - Akt - mammalian target of rapamycin (mTOR), and a number of key synaptic proteins critically involved in neuronal communication. Understanding how these shared pathomechanism elements operate in different conditions may help identify common targets and therapeutic approaches.

Amyloid β-protein and beyond: the path forward in Alzheimer's disease

Basic research on the biological mechanism of Alzheimer's disease has focused for decades on the age-related aggregation of the amyloid β-protein and its apparent downstream effects on microglia, astrocytes and neurons, including the posttranslational modification of the tau protein that seems necessary for symptom expression. Here, we discuss the highly challenging process of developing disease-modifying therapies and highlight several key areas of current research that are progressing in exciting directions. We conclude that further deep molecular analyses of the disease, including the mechanisms of β-amyloidosis, will enable more effective clinical trials and ultimately achieve the progress that our patients so deserve.

Super-resolution microscopy reveals majorly mono- and dimeric presenilin1/γ-secretase at the cell surface

γ-Secretase is a multi-subunit enzyme whose aberrant activity is associated with Alzheimer’s disease and cancer. While its structure is atomically resolved, γ-secretase localization in the membrane in situ relies mostly on biochemical data. Here, we combined fluorescent tagging of γ-secretase subunits with super-resolution microscopy in fibroblasts. Structured illumination microscopy revealed single γ-secretase complexes with a monodisperse distribution and in a 1:1 stoichiometry of PSEN1 and nicastrin subunits. In living cells, sptPALM revealed PSEN1/γ-secretase mainly with directed motility and frequenting ‘hotspots’ or high track-density areas that are sensitive to γ-secretase inhibitors. We visualized γ-secretase association with substrates like amyloid precursor protein and N-cadherin, but not with its sheddases ADAM10 or BACE1 at the cell surface, arguing against pre-formed megadalton complexes. Nonetheless, in living cells PSEN1/γ-secretase transiently visits ADAM10 hotspots. Our results highlight the power of super-resolution microscopy for the study of γ-secretase distribution and dynamics in the membrane.

β-amyloid and Oxidative Stress: Perspectives in Drug Development

Alzheimer's Disease (AD) is a slow-developing neurodegenerative disorder in which the main pathogenic role has been assigned to β-amyloid protein (Aβ) that accumulates in extracellular plaques. The mechanism of action of Aβ has been deeply analyzed and several membrane structures have been identified as potential mediators of its effect. The ability of Aβ to modify neuronal activity, receptor expression, signaling pathways, mitochondrial function, and involvement of glial cells have been analyzed. In addition, extensive literature deals with the involvement of oxidative stress in Aβ effects. Herein we focus more specifically on the reciprocal regulation of Aβ, that causes oxidative stress, that favors Aβ aggregation and toxicity and negatively affects the peptide clearance. Analysis of this strict interaction may offer novel opportunities for therapeutic intervention. Both common and new molecules endowed with antioxidant properties deserve attention in this regard.

Knock-in rats with homozygous PSEN1L435F Alzheimer mutation are viable and show selective γ-secretase activity loss causing low Aβ40/42 and high Aβ43

Familial forms of Alzheimer's disease (FAD) are caused by mutations in the gene encoding amyloid precursor protein, whose processing can result in formation of β-amyloid (Aβ). FAD can also result from mutations in the presenilin 1/2 (PSEN1/2) genes, whose protein products partially compose the γ-secretase complex that cleaves Aβ from amyloid precursor protein fragments. Psen1 KO mice and knock-in (KI) mice with homozygous FAD-associated L435F mutations (Psen1LF/LF ) are embryonic and perinatally lethal, precluding a more rigorous examination of the effect of Alzheimer's disease-causing Psen1 mutations on neurodegeneration. Given that the rat is a more suitable model organism with regard to surgical interventions and behavioral testing, we generated a rat KI model of the Psen1LF mutation. In this study, we focused on young Psen1LF rats to determine potential early pathogenic changes caused by this mutation. We found that, unlike Psen1LF/LF mice, Psen1LF/LF rats survive into adulthood despite loss of γ-secretase activity. Consistent with loss of γ-secretase function, Psen1LF/LF rats exhibited low levels of Aβ38, Aβ40, and Aβ42 peptides. In contrast, levels of Aβ43, a longer and potentially more amyloidogenic Aβ form, were significantly increased in Psen1LF/LF and Psen1LF/w rats. The longer survival of these KI rats affords the opportunity to examine the effect of homozygous Psen1 Alzheimer's disease-associated mutations on neurodegeneration in older animals.

Sorting Out the Role of the Sortilin-Related Receptor 1 in Alzheimer's Disease

Sortilin-related receptor 1 (SORL1) encodes a large, multi-domain containing, membrane-bound receptor involved in endosomal sorting of proteins between the trans-Golgi network, endosomes and the plasma membrane. It is genetically associated with Alzheimer's disease (AD), the most common form of dementia. SORL1 is a unique gene in AD, as it appears to show strong associations with the common, late-onset, sporadic form of AD and the rare, early-onset familial form of AD. Here, we review the genetics of SORL1 in AD and discuss potential roles it could play in AD pathogenesis.

Synapsin 1 promotes Aβ generation via BACE1 modulation

It has been revealed that β-amyloid (Aβ) is generated and released from the presynaptic terminals in activity-dependent manner. However, molecules modulating the presynaptic Aβ generation remain elusive. Here we test the hypothesis that Synapsin 1 (Syn1) may acts as a modulator of the Aβ production. Using biochemical and Förster resonance energy transfer (FRET)-based imaging approaches we have found that Syn1 knock down decreases, whereas (over)expression of Syn1 in cells increases the Aβ levels. Mechanistically, Syn1 does not seem to affect the activity of Presenilin 1 (PS1)/γ-secretase, PS1 conformation, or the proximity between PS1 and amyloid precursor protein (APP). However, we found that Syn1 is involved in up-regulation of the β-site APP cleaving enzyme 1 (BACE1)/β-secretase activity and increases the APP/BACE1 interaction. Therefore, we conclude that Syn1 may promote Aβ production via the modulation of BACE1.

Macromolecular complex in recognition and proteolysis of amyloid precursor protein in Alzheimer's disease

Proteolysis of amyloid precursor protein (APP), first extracellularly by β-secretase and then within the membrane by γ-secretase, produces β-amyloid peptides (Aβ). Aβ accumulates in the brain to form amyloid plaques, a hallmark of Alzheimer's disease (AD). Mutations in APP and presenilin (the catalytic subunit of γ-secretase) result in early onset of AD. Cryogenic electron microscopy (cryo-EM) structures of substrate-free and substrate-bound γ-secretase, determined at atomic resolutions, reveal the physical basis of distinct substrate specificity. These advances, together with the discovery and characterization of multiple proteins that interact with APP or presenilin, have given rise to an optimistic scenario for future mechanistic understanding of AD.

Alzheimer Disease: An Update on Pathobiology and Treatment Strategies

Alzheimer disease (AD) is a heterogeneous disease with a complex pathobiology. The presence of extracellular β-amyloid deposition as neuritic plaques and intracellular accumulation of hyperphosphorylated tau as neurofibrillary tangles remains the primary neuropathologic criteria for AD diagnosis. However, a number of recent fundamental discoveries highlight important pathological roles for other critical cellular and molecular processes. Despite this, no disease-modifying treatment currently exists, and numerous phase 3 clinical trials have failed to demonstrate benefits. Here, we review recent advances in our understanding of AD pathobiology and discuss current treatment strategies, highlighting recent clinical trials and opportunities for developing future disease-modifying therapies.

Emerging Alternative Proteinases in APP Metabolism and Alzheimer's Disease Pathogenesis: A Focus on MT1-MMP and MT5-MMP

Processing of amyloid beta precursor protein (APP) into amyloid-beta peptide (Aβ) by β-secretase and γ-secretase complex is at the heart of the pathogenesis of Alzheimer’s disease (AD). Targeting this proteolytic pathway effectively reduces/prevents pathology and cognitive decline in preclinical experimental models of the disease, but therapeutic strategies based on secretase activity modifying drugs have so far failed in clinical trials. Although this may raise some doubts on the relevance of β- and γ-secretases as targets, new APP-cleaving enzymes, including meprin-β, legumain (δ-secretase), rhomboid-like protein-4 (RHBDL4), caspases and membrane-type matrix metalloproteinases (MT-MMPs/η-secretases) have confirmed that APP processing remains a solid mechanism in AD pathophysiology. This review will discuss recent findings on the roles of all these proteinases in the nervous system, and in particular on the roles of MT-MMPs, which are at the crossroads of pathological events involving not only amyloidogenesis, but also inflammation and synaptic dysfunctions. Assessing the potential of these emerging proteinases in the Alzheimer’s field opens up new research prospects to improve our knowledge of fundamental mechanisms of the disease and help us establish new therapeutic strategies.

A Human Embryonic Stem Cell Model of Aβ-Dependent Chronic Progressive Neurodegeneration

We describe the construction and phenotypic analysis of a human embryonic stem cell model of progressive Aβ-dependent neurodegeneration (ND) with potential relevance to Alzheimer’s disease (AD). We modified one allele of the normal APP locus to directly express a secretory form of Aβ40 or Aβ42, enabling expression from this edited allele to bypass the normal amyloidogenic APP processing pathway. Following neuronal differentiation, edited cell lines specifically accumulate intracellular aggregated/oligomeric Aβ, exhibit a synaptic deficit, and have an abnormal accumulation of endolysosomal vesicles. Edited cultures progress to a stage of overt ND. All phenotypes appear at earlier culture times for Aβ42 relative to Aβ40. Whole transcriptome RNA-Seq analysis identified 23 up and 70 down regulated genes (differentially expressed genes) with similar directional fold change but larger absolute values in the Aβ42 samples suggesting common underlying pathogenic mechanisms. Pathway/annotation analysis suggested that down regulation of extracellular matrix and cilia functions is significantly overrepresented. This cellular model could be useful for uncovering mechanisms directly linking Aβ to neuronal death and as a tool to screen for new therapeutic agents that slow or prevent human ND.

Multiple BACE1 inhibitors abnormally increase the BACE1 protein level in neurons by prolonging its half-life

INTRODUCTION

There is keen interest in elucidating the biological mechanisms underlying recent failures of β-site amyloid precursor protein-cleaving enzyme-1 (BACE1) inhibitors in Alzheimer's disease trials.

METHODS

We developed a highly sensitive and specific immunoassay for BACE1 in cell lines and iPSC-derived human neurons to systematically analyze the effects of eight clinically relevant BACE1 inhibitors.

RESULTS

Seven of 8 inhibitors elevated BACE1 protein levels. Among protease inhibitors tested, the elevation was specific to BACE1 inhibitors. The inhibitors did not increase BACE1 transcription but extended the protein's half-life. BACE1 became elevated at concentrations below the IC₅₀ for amyloid β (Aβ).

DISCUSSION

Elevation of BACE1 by 7 of 8 BACE1 inhibitors raises new concerns about advancing such β-secretase inhibitors for AD. Chronic elevation could lead to intermittently uninhibited BACE1 when orally dosed inhibitors reach trough levels, abnormally increasing substrate processing. Compounds such as roburic acid that lower Aβ by dissociating β/γ secretase complexes are better candidates because they neither inhibit β- and γ-secretase nor increase BACE1 levels.

Regulation of the alternative β-secretase meprin β by ADAM-mediated shedding

Alzheimer's Disease (AD) is the sixth-leading cause of death in industrialized countries. Neurotoxic amyloid-β (Aβ) plaques are one of the pathological hallmarks in AD patient brains. Aβ accumulates in the brain upon sequential, proteolytic processing of the amyloid precursor protein (APP) by β- and γ-secretases. However, so far disease-modifying drugs targeting β- and γ-secretase pathways seeking a decrease in the production of toxic Aβ peptides have failed in clinics. It has been demonstrated that the metalloproteinase meprin β acts as an alternative β-secretase, capable of generating truncated Aβ2-x peptides that have been described to be increased in AD patients. This indicates an important β-site cleaving enzyme 1 (BACE-1)-independent contribution of the metalloprotease meprin β within the amyloidogenic pathway and may lead to novel drug targeting avenues. However, meprin β itself is embedded in a complex regulatory network. Remarkably, the anti-amyloidogenic α-secretase a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) is a direct competitor for APP at the cell surface, but also a sheddase of inactive pro-meprin β. Overall, we highlight the current cellular, molecular and structural understanding of meprin β as alternative β-secretase within the complex protease web, regulating APP processing in health and disease.

The vexing complexity of the amyloidogenic pathway

The role of the amyloidogenic pathway in the etiology of Alzheimer's disease (AD), particularly the common sporadic late onset forms of the disease, is controversial. To some degree, this is a consequence of the failure of drug and therapeutic antibody trials based either on targeting the proteases in this pathway or its amyloid end products. Here, we explore the formidable complexity of the biochemistry and cell biology associated with this pathway. For example, we review evidence that the immediate precursor of amyloid-β, the C99 domain of the amyloid precursor protein (APP), may itself be toxic. We also review important new results that appear to finally establish a direct genetic link between mutations in APP and the sporadic forms of AD. Based on the complexity of amyloidogenesis, it seems possible that a major contributor to the failure of related drug trials is that we have an incomplete understanding of this pathway and how it is linked to Alzheimer's pathogenesis. If so, this highlights a need for further characterization of this pathway, not its abandonment.

Tuning of Glutamate, But Not GABA, Release by an Intrasynaptic Vesicle APP Domain Whose Function Can Be Modulated by β- or α-Secretase Cleavage

APP, whose mutations cause familial Alzheimer's disease (FAD), modulates neurotransmission via interaction of its cytoplasmic tail with the synaptic release machinery. Here we identified an intravesicular domain of APP, called intraluminal SV-APP interacting domain (ISVAID), which interacts with glutamatergic, but not GABAergic, synaptic vesicle proteins. ISVAID contains the β- and α-secretase cleavage sites of APP: proteomic analysis of the interactome of ISVAID suggests that β- and α-secretase cleavage of APP cuts inside the interaction domain of ISVAID and destabilizes protein-protein interactions. We have tested the functional significance of the ISVAID and of β-/α-secretase-processing of APP using various ISVAID-derived peptides in competition experiments on both female and male mouse and rats hippocampal slices. A peptide encompassing the entire ISVAID facilitated glutamate, but not GABA, release acting as dominant negative inhibitor of the functions of this APP domain in acute hippocampal slices. In contrast, peptides representing the product of β-/α-secretase-processing of ISVAID did not alter excitatory neurotransmitter release. These findings suggest that cleavage of APP by either β- or α-secretase may inactivate the ISVAID, thereby enhancing glutamate release. Our present data support the notion that APP tunes glutamate release, likely through intravesicular and extravesicular interactions with synaptic vesicle proteins and the neurotransmitter release machinery, and that β-/α cleavage of APP facilitates the release of excitatory neurotransmitter.SIGNIFICANCE STATEMENT Alzheimer's disease has been linked to mutations in APP. However, the biological function of APP is poorly understood. Here we show that an intravesicular APP domain interacts with the proteins that control the release of glutamate, but not GABA. Interfering with the function of this domain promotes glutamate release. This APP domain contains the sites cleaved by β- and α-secretases: our data suggest that β-/α cleavage of APP inactivates this functional APP domain promoting excitatory neurotransmitter release.

Trafficking and proteolytic processing of amyloid precursor protein and secretases in Alzheimer's disease development: An up-to-date review

Alzheimer's disease (AD), which is predicted to affect 1 in 85 persons worldwide by 2050, results in progressive loss of neuronal functions, leading to impairments in memory and cognitive abilities. As being one of the major neuropathological hallmarks of AD, senile plaques mainly consist of amyloid-β (Aβ) peptides, which are derived from amyloid precursor protein (APP) via the sequential cleavage by β- and γ-secretases. Although the overproduction and accumulation of Aβ peptides are at the center of AD research, the new discoveries point out to the complexity of the disease development. In this respect, it is crucial to understand the processing and the trafficking of APP, the enzymes involved in its processing, the cleavage products and their therapeutic potentials. This review summarizes the salient features of APP processing focusing on APP, the canonical secretases as well as the novel secretases and the cleavage products with an update of the recent developments. We also discussed the intracellular trafficking of APP and secretases in addition to their potential in AD therapy.