Presynaptic localization of neprilysin contributes to efficient clearance of amyloid-beta peptide in mouse brain

Modelling Alzheimer's disease in a dish: dissecting amyloid-β metabolism in human neurons

This scientific commentary refers to 'Inhibition of insulin-degrading enzyme in human neurons promotes amyloid-β deposition' by Rowland et al. (https://doi.org/10.1042/NS20230016). Insulin-degrading enzyme (IDE) and neprilysin (NEP) have been proposed as two Aβ-degrading enzymes supported by human genetics and in vivo data. Rowland et al. provide complementary evidence of a key role for IDE in Aβ metabolism in human-induced pluripotent stem cell (iPSC)-derived cortical neurons.

Caffeoylquinic Acid Mitigates Neuronal Loss and Cognitive Decline in 5XFAD Mice Without Reducing the Amyloid-β Plaque Burden

Background

Caffeoylquinic acid (CQA), which is abundant in coffee beans and Centella asiatica, reportedly improves cognitive function in Alzheimer's disease (AD) model mice, but its effects on neuroinflammation, neuronal loss, and the amyloid-β (Aβ) plaque burden have remained unclear.

Objective

To assess the effects of a 16-week treatment with CQA on recognition memory, working memory, Aβ levels, neuronal loss, neuroinflammation, and gene expression in the brains of 5XFAD mice, a commonly used mouse model of familial AD.

Methods

5XFAD mice at 7 weeks of age were fed a 0.8% CQA-containing diet for 4 months and then underwent novel object recognition (NOR) and Y-maze tests. The Aβ levels and plaque burden were analyzed by enzyme-linked immunosorbent assay and immunofluorescent staining, respectively. Immunostaining of markers of mature neurons, synapses, and glial cells was analyzed. AmpliSeq transcriptome analysis and quantitative reverse-transcription-polymerase chain reaction were performed to assess the effect of CQA on gene expression levels in the cerebral cortex of the 5XFAD mice.

Results

CQA treatment for 4 months improved recognition memory and ameliorated the reduction of mature neurons and synaptic function-related gene mRNAs. The Aβ levels, plaque burden, and glial markers of neuroinflammation seemed unaffected.

Conclusions

These findings suggest that CQA treatment mitigates neuronal loss and improves cognitive function without reducing Aβ levels or neuroinflammation. Thus, CQA is a potential therapeutic compound for AD, improving cognitive function via as-yet unknown mechanisms independent of reductions in Aβ or neuroinflammation.

Neprilysin inhibitors and risk of Alzheimer's disease: A future perspective

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disease with multifaceted neuropathological disorders. AD is characterized by intracellular accumulation of phosphorylated tau proteins and extracellular deposition of amyloid beta (Aβ). Various protease enzymes, including neprilysin (NEP), are concerned with the degradation and clearance of Aβ. Indeed, a defective neuronal clearance pathway due to the dysfunction of degradation enzymes might be a possible mechanism for the accumulation of Aβ and subsequent progression of AD neuropathology. NEP is one of the most imperative metalloproteinase enzymes involved in the clearance of Aβ. This review aimed to highlight the possible role of NEP inhibitors in AD. The combination of sacubitril and valsartan which is called angiotensin receptor blocker and NEP inhibitor (ARNI) may produce beneficial and deleterious effects on AD neuropathology. NEP inhibitors might increase the risk of AD by the inhibition of Aβ clearance, and increase brain bradykinin (BK) and natriuretic peptides (NPs), which augment the pathogenesis of AD. These verdicts come from animal model studies, though they may not be applied to humans. However, clinical studies revealed promising safety findings regarding the use of ARNI. Moreover, NEP inhibition increases various neuroprotective peptides involved in inflammation, glucose homeostasis and nerve conduction. Also, NEP inhibitors may inhibit dipeptidyl peptidase 4 (DPP4) expression, ameliorating insulin and glucagon-like peptide 1 (GLP-1) levels. These findings proposed that NEP inhibitors may have a protective effect against AD development by increasing GLP-1, neuropeptide Y (NPY) and substance P, and deleterious effects by increasing brain BK. Preclinical and clinical studies are recommended in this regard.

Mistranslation-associated perturbations of proteostasis do not promote accumulation of amyloid beta and plaque deposition in aged mouse brain

A common perception in age-related neurodegenerative diseases posits that a decline in proteostasis is key to the accumulation of neuropathogenic proteins, such as amyloid beta (Aβ), and the development of sporadic Alzheimer's disease (AD). To experimentally challenge the role of protein homeostasis in the accumulation of Alzheimer's associated protein Aβ and levels of associated Tau phosphorylation, we disturbed proteostasis in single APP knock-in mouse models of AD building upon Rps9 D95N, a recently identified mammalian ram mutation which confers heightened levels of error-prone translation together with an increased propensity for random protein aggregation and which is associated with accelerated aging. We crossed the Rps9 D95N mutation into knock-in mice expressing humanized Aβ with different combinations of pathogenic mutations (wild-type, NL, NL-F, NL-G-F) causing a stepwise and quantifiable allele-dependent increase in the development of Aβ accumulation, levels of phosphorylated Tau, and neuropathology. Surprisingly, the misfolding-prone environment of the Rps9 D95N ram mutation did not affect Aβ accumulation and plaque formation, nor the level of phosphorylated Tau in any of the humanized APP knock-in lines. Our findings indicate that a misfolding-prone environment induced by error-prone translation with its inherent perturbations in protein homeostasis has little impact on the accumulation of pathogenic Aβ, plaque formation and associated phosphorylated Tau.

Apolipoprotein E genotype-dependent accumulation of amyloid β in APP-knock-in mouse model of Alzheimer's disease

Apolipoprotein E4 (APOE4), the strongest risk factor for late-onset Alzheimer's disease (AD), has been revealed to cause greater accumulation of extracellular amyloid β (Aβ) aggregates than does APOE3 in traditional transgenic mouse models of AD. However, concerns that the overexpression paradigm might have affected the phenotype remain. Amyloid precursor protein (APP)-knock-in (KI) mice, incorporating APP mutations associated with AD development, offer an alternative approach for overproducing pathogenic Aβ without needing overexpression of APP. Here, we present the results of comprehensive analyses of pathological and biochemical traits in the brains of APP-KI mice harboring APP-associated familial AD mutations (APPNL-G-F/NL-G-F mice) crossed with human APOE-KI mice. Immunohistochemical and biochemical analyses revealed the APOE genotype-dependent increase in Aβ pathology and glial activation, which was evident within 8 months in the mouse model. These results suggested that this mouse model may be valuable for investigating APOE pathobiology within a reasonable experimental time frame. Thus, this model can be considered in investigating the interaction between APOE and Aβ in vivo, which may not be addressed appropriately by using other transgenic mouse models.

Advances in Amyloid-β Clearance in the Brain and Periphery: Implications for Neurodegenerative Diseases

This review examines the role of impaired amyloid-β clearance in the accumulation of amyloid-β in the brain and the periphery, which is closely associated with Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The molecular mechanism underlying amyloid-β accumulation is largely unknown, but recent evidence suggests that impaired amyloid-β clearance plays a critical role in its accumulation. The review provides an overview of recent research and proposes strategies for efficient amyloid-β clearance in both the brain and periphery. The clearance of amyloid-β can occur through enzymatic or non-enzymatic pathways in the brain, including neuronal and glial cells, blood-brain barrier, interstitial fluid bulk flow, perivascular drainage, and cerebrospinal fluid absorption-mediated pathways. In the periphery, various mechanisms, including peripheral organs, immunomodulation/immune cells, enzymes, amyloid-β-binding proteins, and amyloid-β-binding cells, are involved in amyloid-β clearance. Although recent findings have shed light on amyloid-β clearance in both regions, opportunities remain in areas where limited data is available. Therefore, future strategies that enhance amyloid-β clearance in the brain and/or periphery, either through central or peripheral clearance approaches or in combination, are highly encouraged. These strategies will provide new insight into the disease pathogenesis at the molecular level and explore new targets for inhibiting amyloid-β deposition, which is central to the pathogenesis of sporadic AD (amyloid-β in parenchyma) and CAA (amyloid-β in blood vessels).

Xanthorrhizol, a potential anticancer agent, from Curcuma xanthorrhiza Roxb

BACKGROUND

Xanthorrhizol (XTZ), a bisabolene sesquiterpenoid, is abundantly found in the plant Curcuma xanthorrhiza Roxb. Traditionally, C. xanthorrhiza is widely used for the treatment of different health conditions, including common fever, infection, lack of appetite, fatigue, liver complaints, and gastrointestinal disorders. XTZ exhibits wide-ranging pharmacological activities, including anticancer, antioxidative, anti-inflammatory, antimicrobial, and antidiabetic activities, in addition to a protective effect on multiple organs. The present review provides detailed findings on the anticancer activities of XTZ and the underlying cellular and molecular mechanisms.

METHODS

Literature was searched systematically in main databases following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, with keywords "tumor AND xanthorrhizol" or "cancer AND xanthorrhizol".

RESULTS

Studies show that XTZ has preventive and therapeutic activities against different types of cancer, including breast, cervical, colon, liver, lung, oral and esophageal, and skin cancers. XTZ regulates multiple signaling pathways that block carcinogenesis and proliferation. In vitro and in vivo studies showed that XTZ targets different kinases, inflammatory cytokines, apoptosis proteins, and transcription factors, leading to the suppression of angiogenesis, metastasis, and the activation of apoptosis and cell cycle arrest.

CONCLUSION

The potential anticancer benefits of XTZ recommend further in vivo studies against different types of cancer. Further, XTZ needs to be confirmed for its toxicity, bioavailability, protective, antifatigue, and energy booster activities. Future studies for the therapeutic development of XTZ may be directed to cancer-related fatigue.

Error-prone protein synthesis recapitulates early symptoms of Alzheimer disease in aging mice

Age-related neurodegenerative diseases (NDDs) are associated with the aggregation and propagation of specific pathogenic protein species (e.g., Aβ, α-synuclein). However, whether disruption of synaptic homeostasis results from protein misfolding per se rather than accumulation of a specific rogue protein is an unexplored question. Here, we show that error-prone translation, with its frequent outcome of random protein misfolding, is sufficient to recapitulate many early features of NDDs, including perturbed Ca²⁺ signaling, neuronal hyperexcitability, and mitochondrial dysfunction. Mice expressing the ribosomal ambiguity mutation Rps9 D95N exhibited disrupted synaptic homeostasis resulting in behavioral changes reminiscent of early Alzheimer disease (AD), such as learning and memory deficits, maladaptive emotional responses, epileptiform discharges, suppressed circadian rhythmicity, and sleep fragmentation, accompanied by hippocampal NPY expression and cerebral glucose hypometabolism. Collectively, our findings suggest that random protein misfolding may contribute to the pathogenesis of age-related NDDs, providing an alternative framework for understanding the initiation of AD.

Aβ/Amyloid Precursor Protein-Induced Hyperexcitability and Dysregulation of Homeostatic Synaptic Plasticity in Neuron Models of Alzheimer’s Disease

Alzheimer’s disease (AD) is increasingly seen as a disease of synapses and diverse evidence has implicated the amyloid-β peptide (Aβ) in synapse damage. The molecular and cellular mechanism(s) by which Aβ and/or its precursor protein, the amyloid precursor protein (APP) can affect synapses remains unclear. Interestingly, early hyperexcitability has been described in human AD and mouse models of AD, which precedes later hypoactivity. Here we show that neurons in culture with either elevated levels of Aβ or with human APP mutated to prevent Aβ generation can both induce hyperactivity as detected by elevated calcium transient frequency and amplitude. Since homeostatic synaptic plasticity (HSP) mechanisms normally maintain a setpoint of activity, we examined whether HSP was altered in AD transgenic neurons. Using methods known to induce HSP, we demonstrate that APP protein levels are regulated by chronic modulation of activity and that AD transgenic neurons have an impaired adaptation of calcium transients to global changes in activity. Further, AD transgenic compared to WT neurons failed to adjust the length of their axon initial segments (AIS), an adaptation known to alter excitability. Thus, we show that both APP and Aβ influence neuronal activity and that mechanisms of HSP are disrupted in primary neuron models of AD.

Physiological Roles of Monomeric Amyloid-β and Implications for Alzheimer's Disease Therapeutics

Alzheimer's disease (AD) progressively inflicts impairment of synaptic functions with notable deposition of amyloid-β (Aβ) as senile plaques within the extracellular space of the brain. Accordingly, therapeutic directions for AD have focused on clearing Aβ plaques or preventing amyloidogenesis based on the amyloid cascade hypothesis. However, the emerging evidence suggests that Aβ serves biological roles, which include suppressing microbial infections, regulating synaptic plasticity, promoting recovery after brain injury, sealing leaks in the blood-brain barrier, and possibly inhibiting the proliferation of cancer cells. More importantly, these functions were found in in vitro and in vivo investigations in a hormetic manner, that is to be neuroprotective at low concentrations and pathological at high concentrations. We herein summarize the physiological roles of monomeric Aβ and current Aβ-directed therapies in clinical trials. Based on the evidence, we propose that novel therapeutics targeting Aβ should selectively target Aβ in neurotoxic forms such as oligomers while retaining monomeric Aβ in order to preserve the physiological functions of Aβ monomers.

Recent Advances in the Modeling of Alzheimer's Disease

Since 1995, more than 100 transgenic (Tg) mouse models of Alzheimer's disease (AD) have been generated in which mutant amyloid precursor protein (APP) or APP/presenilin 1 (PS1) cDNA is overexpressed ( 1st generation models ). Although many of these models successfully recapitulate major pathological hallmarks of the disease such as amyloid β peptide (Aβ) deposition and neuroinflammation, they have suffered from artificial phenotypes in the form of overproduced or mislocalized APP/PS1 and their functional fragments, as well as calpastatin deficiency-induced early lethality, calpain activation, neuronal cell death without tau pathology, endoplasmic reticulum stresses, and inflammasome involvement. Such artifacts bring two important uncertainties into play, these being (1) why the artifacts arise, and (2) how they affect the interpretation of experimental results. In addition, destruction of endogenous gene loci in some Tg lines by transgenes has been reported. To overcome these concerns, single App knock-in mouse models harboring the Swedish and Beyreuther/Iberian mutations with or without the Arctic mutation (AppNL-G-F and AppNL-F mice) were developed ( 2nd generation models ). While these models are interesting given that they exhibit Aβ pathology, neuroinflammation, and cognitive impairment in an age-dependent manner, the model with the Artic mutation, which exhibits an extensive pathology as early as 6 months of age, is not suitable for investigating Aβ metabolism and clearance because the Aβ in this model is resistant to proteolytic degradation and is therefore prone to aggregation. Moreover, it cannot be used for preclinical immunotherapy studies owing to the discrete affinity it shows for anti-Aβ antibodies. The weakness of the latter model (without the Arctic mutation) is that the pathology may require up to 18 months before it becomes sufficiently apparent for experimental investigation. Nevertheless, this model was successfully applied to modulating Aβ pathology by genome editing, to revealing the differential roles of neprilysin and insulin-degrading enzyme in Aβ metabolism, and to identifying somatostatin receptor subtypes involved in Aβ degradation by neprilysin. In addition to discussing these issues, we also provide here a technical guide for the application of App knock-in mice to AD research. Subsequently, a new double knock-in line carrying the AppNL-F and Psen1 P117L/WT mutations was generated, the pathogenic effect of which was found to be synergistic. A characteristic of this 3rd generation model is that it exhibits more cored plaque pathology and neuroinflammation than the AppNL-G-F line, and thus is more suitable for preclinical studies of disease-modifying medications targeting Aβ. Furthermore, a derivative AppG-F line devoid of Swedish mutations which can be utilized for preclinical studies of β-secretase modifier(s) was recently created. In addition, we introduce a new model of cerebral amyloid angiopathy that may be useful for analyzing amyloid-related imaging abnormalities that can be caused by anti-Aβ immunotherapy. Use of the App knock-in mice also led to identification of the α-endosulfine-K ATP channel pathway as components of the somatostatin-evoked physiological mechanisms that reduce Aβ deposition via the activation of neprilysin. Such advances have provided new insights for the prevention and treatment of preclinical AD. Because tau pathology plays an essential role in AD pathogenesis, knock-in mice with human tau wherein the entire murine Mapt gene has been humanized were generated. Using these mice, the carboxy-terminal PDZ ligand of neuronal nitric oxide synthase (CAPON) was discovered as a mediator linking tau pathology to neurodegeneration and showed that tau humanization promoted pathological tau propagation. Finally, we describe and discuss the current status of mutant human tau knock-in mice and a non-human primate model of AD that we have successfully created.

Somatostatin-evoked Aβ catabolism in the brain: Mechanistic involvement of α-endosulfine-KATP channel pathway

Alzheimer's disease (AD) is characterized by the deposition of amyloid β peptide (Aβ) in the brain. The neuropeptide somatostatin (SST) regulates Aβ catabolism by enhancing neprilysin (NEP)-catalyzed proteolytic degradation. However, the mechanism by which SST regulates NEP activity remains unclear. Here, we identified α-endosulfine (ENSA), an endogenous ligand of the ATP-sensitive potassium (KATP) channel, as a negative regulator of NEP downstream of SST signaling. The expression of ENSA is significantly increased in AD mouse models and in patients with AD. In addition, NEP directly contributes to the degradation of ENSA, suggesting a substrate-dependent feedback loop regulating NEP activity. We also discovered the specific KATP channel subtype that modulates NEP activity, resulting in the Aβ levels altered in the brain. Pharmacological intervention targeting the particular KATP channel attenuated Aβ deposition, with impaired memory function rescued via the NEP activation in our AD mouse model. Our findings provide a mechanism explaining the molecular link between KATP channel and NEP activation, and give new insights into alternative strategies to prevent AD.

Lysosomal gene Hexb displays haploinsufficiency in a knock-in mouse model of Alzheimer’s disease

Lysosomal network abnormalities are an increasingly recognised feature of Alzheimer’s disease (AD), which appear early and are progressive in nature. Sandhoff disease and Tay-Sachs disease (neurological lysosomal storage diseases caused by mutations in genes that code for critical subunits of β-hexosaminidase) result in accumulation of amyloid-β (Aβ) and related proteolytic fragments in the brain. However, experiments that determine whether mutations in genes that code for β-hexosaminidase are risk factors for AD are currently lacking. To determine the relationship between β-hexosaminidase and AD, we investigated whether a heterozygous deletion of Hexb, the gene that encodes the beta subunit of β-hexosaminidase, modifies the behavioural phenotype and appearance of disease lesions in App^{NL-G-F/NL-G-F}(App^KI/KI) mice. App^KI/KI and Hexb^+/- mice were crossed and evaluated in a behavioural test battery. Neuropathological hallmarks of AD and ganglioside levels in the brain were also examined. Heterozygosity of Hexb in App^KI/KI mice reduced learning flexibility during the Reversal Phase of the Morris water maze. Contrary to expectation, heterozygosity of Hexb caused a small but significant decrease in amyloid beta deposition and an increase in the microglial marker IBA1 that was region- and age-specific. Hexb heterozygosity caused detectable changes in the brain and in the behaviour of an AD model mouse, consistent with previous reports that described a biochemical relationship between HEXB and AD. This study reveals that the lysosomal enzyme gene Hexb is not haplosufficient in the mouse AD brain.

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The App NL-G-F Alzheimer mouse has lysosomal defects and stores ganglioside lipids.

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Heterozygous lysosomal Hexb did not drive amyloidosis in the App NL-G-F mouse.

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Heterozygous Hexb on an Alzheimer’s background reduced learning flexibility.

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Heterozygous Hexb on a wild-type mouse background produced hypoactivity.

Extracellular Vesicles Released from Neprilysin Gene-Modified Human Umbilical Cord-Derived Mesenchymal Stem Cell Enhance Therapeutic Effects in an Alzheimer's Disease Animal Model

Alzheimer's disease (AD) animal studies have reported that mesenchymal stem cells (MSCs) have therapeutic effects; however, clinical trial results are controversial. Neprilysin (NEP) is the main cleavage enzyme of β-amyloid (Aβ), which plays a major role in the pathology and etiology of AD. We evaluated whether transplantation of MSCs with NEP gene modification enhances the therapeutic effects in an AD animal model and then investigated these pathomechanisms. We manufactured NEP gene-enhanced human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) and intravenously transplanted them in Aβ_1-42-injected AD animal models. We compared the differences in behavioral tests and immunohistochemical assays between four groups: normal, Aβ_1-42 injection, naïve hUC-MSCs, and NEP-enhanced hUC-MSCs. Both naïve and NEP-enhanced hUC-MSC groups showed significant improvements in memory compared to the Aβ_1-42 injection group. There was no significant difference between naïve and NEP-enhanced hUC-MSC groups. There was a significant decrease in Congo red, BACE-1, GFAP, and Iba-1 and a significant increase in BDNF, NeuN, and NEP in both hUC-MSC groups compared to the Aβ_1-42 injection group. Among them, BDNF, NeuN, GFAP, Iba-1, and NEP showed more significant changes in the NEP-enhanced hUC-MSC group than in the naïve group. After stem cell injection, stem cells were not found. Extracellular vesicles (EVs) were equally observed in the hippocampus in the naïve and NEP-enhanced hUC-MSC groups. However, the EVs of NEP-enhanced hUC-MSCs contained higher amounts of NEP as compared to the EVs of naïve hUC-MSCs. Thus, hUC-MSCs affect AD animal models through stem cell-released EVs. Although there was no significant difference in cognitive function between the hUC-MSC groups, NEP-enhanced hUC-MSCs had superior neurogenesis and anti-inflammation properties compared to naïve hUC-MSCs due to increased NEP in the hippocampus by enriched NEP-possessing EVs. NEP gene-modified MSCs that release an increased amount of NEP within EVs may be a promising therapeutic option in AD treatment.

Genetic Therapies for Alzheimer's Disease: A Scoping Review

Effective, disease modifying therapies for Alzheimer's disease (AD) remain a quandary, following a panoply of expensive failures in human clinical trials. Given the stagnation in therapeutics, alternative approaches are needed. Recent successes of genetic therapies in other neurodegenerative diseases may highlight the way forward. This scoping review explores suggested targets of genetic therapy in AD, with a focus on vector-based approaches in pre-clinical and clinical trials. Putative targets of genetic therapies tested in pre-clinical trials include amyloid pathway intermediates and enzymes modulation, tau protein downregulation, APOE4 downregulation and APOE2 upregulation, neurotrophin expression (nerve growth factor (NGF) and brain-derived neurotrophic factor), and inflammatory cytokine alteration, among several other approaches. There have been three completed human clinical trials for genetic therapy in AD patients, all of which upregulated NGF in AD patients, showing some mixed evidence of benefit. Several impediments remain to be surpassed before genetic therapies can be successfully applied to AD, including the challenge of delivering monogenic genetic therapies for complex polygenic disorders, risks in the dominant delivery method (intracranial injection), stability of genetic therapies in vivo, poor translatability of pre-clinical AD models, and the expense of genetic therapy production. Genetic therapies represent an exciting opportunity within the world of AD therapeutics, but clinical applications likely remain a long term, rather than short term, possibility.

N-Truncated Aβ Starting at Position Four-Biochemical Features, Preclinical Models, and Potential as Drug Target in Alzheimer's Disease

The discussion of whether amyloid plaque Aβ is a valid drug target to fight Alzheimer’s disease (AD) has been a matter of scientific dispute for decades. This question can only be settled by successful clinical trials and the approval of disease-modifying drugs. However, many clinical trials with antibodies against different regions of the amyloid Aβ peptide have been discontinued, as they did not meet the clinical endpoints required. Recently, passive immunization of AD patients with Donanemab, an antibody directed against the N-terminus of pyroglutamate Aβ, showed beneficial effects in a phase II trial, supporting the concept that N-truncated Aβ is a relevant target for AD therapy. There is long-standing evidence that N-truncated Aβ variants are the main variants found in amyloid plaques besides full-length Aβ_1–42, t, therefore their role in triggering AD pathology and as targets for drug development are of interest. While the contribution of pyroglutamate Aβ_3–42 to AD pathology has been well studied in the past, the potential role of Aβ_4–42 has been largely neglected. The present review will therefore focus on Aβ_4–42 as a possible drug target based on human and mouse pathology, in vitro and in vivo toxicity, and anti-Aβ_4-X therapeutic effects in preclinical models.

Metallobiology and therapeutic chelation of biometals (copper, zinc and iron) in Alzheimer's disease: Limitations, and current and future perspectives

BACKGROUND

Alzheimer's disease (AD) is the most prevalent cause of cognitive impairment and dementia worldwide. The pathobiology of the disease has been studied in the form of several hypotheses, ranging from oxidative stress, amyloid-beta (Aβ) aggregation, accumulation of tau forming neurofibrillary tangles (NFT) through metal dysregulation and homeostasis, dysfunction of the cholinergic system, and to inflammatory and autophagic mechanism. However, none of these hypotheses has led to confirmed diagnostics or approved cure for the disease.

OBJECTIVE

This review is aimed as a basic and an encyclopedic short course into metals in AD and discusses the advances in chelation strategies and developments adopted in the treatment of the disease. Since there is accumulating evidence of the role of both biometal dyshomeostasis (iron (Fe), copper (Cu), and zinc (Zn)) and metal-amyloid interactions that lead to the pathogenesis of AD, this review focuses on unraveling therapeutic chelation strategies that have been considered in the treatment of the disease, aiming to sequester free and protein-bound metal ions and reducing cerebral metal burden. Promising compounds possessing chemically modified moieties evolving as multi-target ligands used as anti-AD drug candidates are also covered.

RESULTS AND CONCLUSION

Several multidirectional and multifaceted studies on metal chelation therapeutics show the need for improved synthesis, screening, and analysis of compounds to be able to effectively present chelating anti-AD drugs. Most drug candidates studied have limitations in their physicochemical properties; some enhance redistribution of metal ions, while others indirectly activate signaling pathways in AD. The metal chelation process in vivo still needs to be established and the design of potential anti-AD compounds that bi-functionally sequester metal ions as well as inhibit the Aβ aggregation by competing with the metal ions and reducing metal-induced oxidative damage and neurotoxicity may signal a bright end in chelation-based therapeutics of AD.

Lack of occlusal support did not impact amyloid β deposition in APP knock-in mice

PURPOSE

The lack of occlusal support is an epidemiological risk factor linked to Alzheimer's disease. This study sought to assess the relationship between amyloid β (Aβ) deposition and the lack of occlusal support in amyloid precursor protein (APP) knock-in mice.

METHODS

Sixteen experimental animals were divided into two groups. The upper molars were extracted in the extraction group (group E), and a sham operation was performed in the control group (group C). The Morris water maze test was performed 4 months after the tooth extraction. Aβ immunohistochemical staining and Nissl staining of the hippocampus were performed. Hippocampal plasma corticosterone and Aβ protein levels were measured.

RESULTS

In the maze task, the escape latency was significantly longer in group E than in group C. In the probe trials, the time elapsed in the target quadrant was significantly shorter in group E than in group C. The number of hippocampal neurons decreased in group E. There was no significant difference in the plasma corticosterone levels between the two groups, indicating that there was no effect of chronic stress on the behavioral results. Hippocampal Aβ40 and Aβ42 protein levels and Aβ deposition areas by immunohistochemical staining were not significantly different between the two groups.

CONCLUSIONS

Aβ deposition was not increased in the hippocampus of molarless APP knock-in mice. As such, it appears that cognitive impairment due to a lack of occlusal support was not related to Aβ deposition.

Redox-Active Metal Ions and Amyloid-Degrading Enzymes in Alzheimer's Disease

Redox-active metal ions, Cu(I/II) and Fe(II/III), are essential biological molecules for the normal functioning of the brain, including oxidative metabolism, synaptic plasticity, myelination, and generation of neurotransmitters. Dyshomeostasis of these redox-active metal ions in the brain could cause Alzheimer’s disease (AD). Thus, regulating the levels of Cu(I/II) and Fe(II/III) is necessary for normal brain function. To control the amounts of metal ions in the brain and understand the involvement of Cu(I/II) and Fe(II/III) in the pathogenesis of AD, many chemical agents have been developed. In addition, since toxic aggregates of amyloid-β (Aβ) have been proposed as one of the major causes of the disease, the mechanism of clearing Aβ is also required to be investigated to reveal the etiology of AD clearly. Multiple metalloenzymes (e.g., neprilysin, insulin-degrading enzyme, and ADAM10) have been reported to have an important role in the degradation of Aβ in the brain. These amyloid degrading enzymes (ADE) could interact with redox-active metal ions and affect the pathogenesis of AD. In this review, we introduce and summarize the roles, distributions, and transportations of Cu(I/II) and Fe(II/III), along with previously invented chelators, and the structures and functions of ADE in the brain, as well as their interrelationships.

Broadening COVID-19 Interventions to Drug Innovation: Neprilysin Pathway as a Friend, Foe, or Promising Molecular Target?

The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus is anticipated to transition to an endemic state as vaccines are providing relief in some, but not all, countries. Drug discovery for COVID-19 can offer another tool in the fight against the pandemic. Additionally, COVID-19 impacts multiple organs that call for a systems medicine approach to planetary health and therapeutics innovation. In this context, innovation for drugs that prevent and treat COVID-19 is timely and much needed. As the virus variants emerge under different ecological conditions and contexts in the long haul, a broad array of vaccine and drug options will be necessary. This expert review article argues for a need to expand the COVID-19 interventions, including and beyond vaccines, to stimulate discovery and development of novel medicines against SARS-CoV-2 infection. The Renin-Angiotensin-Aldosterone System (RAAS) is known to play a major role in SARS-CoV-2 infection. Neprilysin (NEP) and angiotensin-converting enzyme (ACE) have emerged as the pharmaceutical targets of interest in the search for therapeutic interventions against COVID-19. While the NEP/ACE inhibitors offer promise for repurposing against COVID-19, they may display a multitude of effects in different organ systems, some beneficial, and others adverse, in modulating the inflammation responses in the course of COVID-19. This expert review offers an analysis and discussion to deepen our present understanding of the pathophysiological function of neprilysin in multiple organs, and the possible effects of NEP inhibitor-induced inflammatory responses in COVID-19-infected patients.

Current Management and Therapeutic Strategies for Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA) is characterized by accumulation of amyloid β (Aβ) in walls of leptomeningeal vessels and cortical capillaries in the brain. The loss of integrity of these vessels caused by cerebrovascular Aβ deposits results in fragile vessels and lobar intracerebral hemorrhages. CAA also manifests with progressive cognitive impairment or transient focal neurological symptoms. Although development of therapeutics for CAA is urgently needed, the pathogenesis of CAA remains to be fully elucidated. In this review, we summarize the epidemiology, pathology, clinical and radiological features, and perspectives for future research directions in CAA therapeutics. Recent advances in mass spectrometric methodology combined with vascular isolation techniques have aided understanding of the cerebrovascular proteome. In this paper, we describe several potential key CAA-associated molecules that have been identified by proteomic analyses (apolipoprotein E, clusterin, SRPX1 (sushi repeat-containing protein X-linked 1), TIMP3 (tissue inhibitor of metalloproteinases 3), and HTRA1 (HtrA serine peptidase 1)), and their pivotal roles in Aβ cytotoxicity, Aβ fibril formation, and vessel wall remodeling. Understanding the interactions between cerebrovascular Aβ deposits and molecules that accumulate with Aβ may lead to discovery of effective CAA therapeutics and to the identification of biomarkers for early diagnosis.

A Nanoformulation-Mediated Multifunctional Stem Cell Therapy with Improved Beta-Amyloid Clearance and Neural Regeneration for Alzheimer's Disease

Alzheimer's disease (AD) is a common dementia that is currently incurable. The existing treatments can only moderately relieve the symptoms of AD to slow down its progress. How to achieve effective neural regeneration to ameliorate cognitive impairments is a major challenge for current AD treatment. Here, the therapeutic potential of a nanoformulation-mediated neural stem cell (NSC) therapy capable of simultaneous Aβ clearance and neural regeneration is investigated in a murine model. Genetically engineered NSCs capable of stably and continuously expressing neprilysin (NEP) are developed to enhance Aβ degradation and NSC survival in the brain. A PBAE-PLGA-Ag2 S-RA-siSOX9 (PPAR-siSOX9) nanoformulation with high gene/drug deliverability is synthesized to overcome AD microenvironment-associated adverse effects and to promote neuronal differentiation of the NEP-expressing NSCs. For achieving accurate stereotactic transplantation, Ag2 S quantum-dot-based fluorescence imaging is used to guide NSC transplantation in real time. This strategy shows numerous benefits, including efficient and long-lasting Aβ degradation, improved neural regeneration, and accurate cell transplantation. It is shown that a single administration of this therapy achieves long-term efficacy (6 months) with respect to memory reversal and improvement of learning deficits.

The role of neuropeptide somatostatin in the brain and its application in treating neurological disorders

Somatostatin (SST) is a well-known neuropeptide that is expressed throughout the brain. In the cortex, SST is expressed in a subset of GABAergic neurons and is known as a protein marker of inhibitory interneurons. Recent studies have identified the key functions of SST in modulating cortical circuits in the brain and cognitive function. Furthermore, reduced expression of SST is a hallmark of various neurological disorders, including Alzheimer's disease and depression. In this review, we summarize the current knowledge on SST expression and function in the brain. In particular, we describe the physiological roles of SST-positive interneurons in the cortex. We further describe the causal relationship between pathophysiological changes in SST function and various neurological disorders, such as Alzheimer's disease. Finally, we discuss potential treatments and possibility of novel drug developments for neurological disorders based on the current knowledge on the function of SST and SST analogs in the brain derived from experimental and clinical studies.

Enhanced neprilysin-mediated degradation of hippocampal Aβ42 with a somatostatin peptide that enters the brain

Background: Aggregation of the amyloid-beta (Aβ) peptide is one of the main neuropathological events in Alzheimer's disease (AD). Neprilysin is the major enzyme degrading Aβ, with its activity enhanced by the neuropeptide somatostatin (SST). SST levels are decreased in the brains of AD patients. The poor delivery of SST over the blood-brain barrier (BBB) and its extremely short half-life of only 3 min limit its therapeutic significance.

Methods: We recombinantly fused SST to a BBB transporter binding to the transferrin receptor. Using primary neuronal cultures and neuroblastoma cell lines, the ability of the formed fusion protein to activate neprilysin was studied. SST-scFv8D3 was administered to mice overexpressing the Aβ-precursor protein (AβPP) with the Swedish mutation (APPswe) as a single injection or as a course of three injections over a 72 h period. Levels of neprilysin and Aβ were quantified using an Enzyme-linked immunosorbent assay (ELISA). Distribution of SST-scFv8D3 in the brain, blood and peripheral organs was studied by radiolabeling with iodine-125.

Results: The construct, SST-scFv8D3, exhibited 120 times longer half-life than SST alone, reached the brain in high amounts when injected intravenously and significantly increased the brain concentration of neprilysin in APPswe mice. A significant decrease in the levels of membrane-bound Aβ42 was detected in the hippocampus and the adjacent cortical area after only three injections.

Conclusion: With intravenous injections of our BBB permeable SST peptide, we were able to significantly increase the levels neprilysin, an effect that was followed by a significant and selective degradation of membrane-bound Aβ42 in the hippocampus. Being that membrane-bound Aβ triggers neuronal toxicity and the hippocampus is the central brain area in the progression of AD, the study has illuminated a new potential treatment paradigm with a promising safety profile targeting only the disease affected areas.

Neprilysin expression and functions in development, ageing and disease

Neprilysin (NEP) is an integral membrane-bound metallopeptidase with a wide spectrum of substrates and physiological functions. It plays an important role in proteolytic processes in the kidney, cardiovascular regulation, immune response, cell proliferation, foetal development etc. It is an important neuropeptidase and amyloid-degrading enzyme which makes NEP a therapeutic target in Alzheimer's disease (AD). Moreover, it plays a preventive role in development of cancer, obesity and type-2 diabetes. Recently a role of NEP in COVID-19 pathogenesis has also been suggested. Despite intensive research into NEP structure and functions in different organisms, changes in its expression and regulation during brain development and ageing, especially in age-related pathologies, is still not fully understood. This prevents development of pharmacological treatments from various diseases in which NEP is implicated although recently a dual-acting drug sacubitril-valsartan (LCZ696) combining a NEP inhibitor and angiotensin receptor blocker has been approved for treatment of heart failure. Also, various natural compounds capable of upregulating NEP expression, including green tea (EGCG), have been proposed as a preventive medicine in prostate cancer and AD. This review summarizes the existing literature and our own research on the expression and activity of NEP in normal brain development, ageing and under pathological conditions.

Target Enzymes Considered for the Treatment of Alzheimer's Disease and Parkinson's Disease

Various amyloidogenic proteins have been suggested to be involved in the onset and progression of neurodegenerative diseases (ND) such as Alzheimer's disease (AD) and Parkinson's disease (PD). Particularly, the aggregation of misfolded amyloid-β and hyperphosphorylated tau and α-synuclein are linked to the pathogenesis of AD and PD, respectively. In order to care the diseases, multiple small molecules have been developed to regulate the aggregation pathways of these amyloid proteins. In addition to controlling the aggregation of amyloidogenic proteins, maintaining the levels of the proteins in the brain by amyloid degrading enzymes (ADE; neprilysin (NEP), insulin-degrading enzyme (IDE), asparagine endopeptidase (AEP), and ADAM10) is also essential to cure AD and PD. Therefore, numerous biological molecules and chemical agents have been investigated as either inducer or inhibitor against the levels and activities of ADE. Although the side effect of enhancing the activity of ADE could occur, the removal of amyloidogenic proteins could result in a relatively good strategy to treat AD and PD. Furthermore, since the causes of ND are diverse, various multifunctional (multitarget) chemical agents have been designed to control the actions of multiple risk factors of ND, including amyloidogenic proteins, metal ions, and reactive oxygen species. Many of them, however, were invented without considerations of regulating ADE levels and actions. Incorporation of previously created molecules with the chemical agents handling ADE could be a promising way to treat AD and PD. This review introduces the ADE and molecules capable of modulating the activity and expression of ADE.

Traumatic Injury Reduces Amyloid Plaque Burden in the Transgenic 5xFAD Alzheimer's Mouse Spinal Cord

BACKGROUND

Axonal injury has been implicated in the development of amyloid-β in experimental brain injuries and clinical cases. The anatomy of the spinal cord provides a tractable model for examining the effects of trauma on amyloid deposition.

OBJECTIVE

Our goal was to examine the effects of axonal injury on plaque formation and clearance using wild type and 5xFAD transgenic Alzheimer's disease mice.

METHODS

We contused the spinal cord at the T12 spinal level at 10 weeks, an age at which no amyloid plaques spontaneously accumulate in 5xFAD mice. We then explored plaque clearance by impacting spinal cords in 27-week-old 5xFAD mice where amyloid deposition is already well established. We also examined the cellular expression of one of the most prominent amyloid-β degradation enzymes, neprilysin, at the lesion site.

RESULTS

No plaques were found in wild type animals at any time points examined. Injury in 5xFAD prevented plaque deposition rostral and caudal to the lesion when the cords were examined at 2 and 4 months after the impact, whereas age-matched naïve 5xFAD mice showed extensive amyloid plaque deposition. A massive reduction in the number of plaques around the lesion was found as early as 7 days after the impact, preceded by neprilysin upregulation in astrocytes at 3 days after injury. At 7 days after injury, the majority of amyloid was found inside microglia/macrophages.

CONCLUSION

These observations suggest that the efficient amyloid clearance after injury in the cord may be driven by the orchestrated efforts of astroglial and immune cells.

N-Terminal Truncated Aβ4-42 Is a Substrate for Neprilysin Degradation in vitro and in vivo

In sporadic Alzheimer's disease (AD), an imbalance between production and clearance of amyloid-β (Aβ) peptides seems to account for enhanced Aβ accumulation. The metalloprotease neprilysin (NEP) is an important Aβ degrading enzyme as shown by a variety of in vitro and in vivo studies. While the degradation of full-length Aβ peptides such as Aβ1-40 and Aβ1-42 is well established, it is less clear whether NEP is also capable of degrading N-terminally truncated Aβ species such as the common variant Aβ4-42. In the present report, we confirmed the degradation of Aβ4-x species by neprilysin using in vitro digestion and subsequent analysis using gel-based assays and mass spectrometry. By crossing Tg4-42 mice expressing only Aβ4-42 peptides with homozygous NEP-knock-out mice (NEP-/-), we were able to demonstrate that NEP deficiency increased hippocampal intraneuronal Aβ levels and aggravated neuron loss in the Tg4-42 transgenic mouse model of AD.

Understanding the Pathophysiology of Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA), one of the main types of cerebral small vessel disease, is a major cause of spontaneous intracerebral haemorrhage and an important contributor to cognitive decline in elderly patients. Despite the number of experimental in vitro studies and animal models, the pathophysiology of CAA is still largely unknown. Although several pathogenic mechanisms including an unbalance between production and clearance of amyloid beta (Aβ) protein as well as ‘the prion hypothesis’ have been invoked as possible disease triggers, they do not explain completely the disease pathogenesis. This incomplete disease knowledge limits the implementation of treatments able to prevent or halt the clinical progression. The continuous increase of CAA patients makes imperative the development of suitable experimental in vitro or animal models to identify disease biomarkers and new pharmacological treatments that could be administered in the early disease stages to prevent irreversible changes and disease progression.

Using human induced pluripotent stem cells (hiPSCs) to investigate the mechanisms by which Apolipoprotein E (APOE) contributes to Alzheimer's disease (AD) risk

Although the biochemical and pathological hallmarks of Alzheimer's disease (AD), such as axonal transport defects, synaptic loss, and selective neuronal death, are well characterized, the underlying mechanisms that cause AD are largely unknown, thereby making it difficult to design effective therapeutic interventions. Genome-wide association studies (GWAS) studies have identified several factors associated with increased AD risk. Of these genetic factors, polymorphisms in the Apolipoprotein E (APOE) gene are the strongest and most prevalent. While it has been established that the ApoE protein modulates the formation of amyloid plaques and neurofibrillary tangles, the precise molecular mechanisms by which various ApoE isoforms enhance or mitigate AD onset and progression in aging adults are yet to be elucidated. Advances in cellular reprogramming to generate disease-in-a-dish models now provide a simplified and accessible system that complements animal and primary cell models to study ApoE in the context of AD. In this review, we will describe the use and manipulation of human induced pluripotent stem cells (hiPSCs) in dissecting the interaction between ApoE and AD. First, we will provide an overview of the proposed roles that ApoE plays in modulating pathophysiology of AD. Next, we will summarize the recent studies that have employed hiPSCs to model familial and sporadic AD. Lastly, we will speculate on how current advances in genome editing technologies and organoid culture systems can be used to improve hiPSC-based tools to investigate ApoE-dependent modulation of AD onset and progression.

Tryptophan metabolites modify brain Aβ peptide degradation: A role in Alzheimer's disease?

Among several processes, a decrease in amyloid-beta (Aβ) peptide elimination is thought to be one of the major pathophysiological factors in Alzheimer's disease (AD). Neprilysin (NEP) is a key metalloproteinase controlling the degradation and clearance of Aβ peptides in the brain. NEP is induced by several pharmacological substances, amyloid deposits and somatostatin, but the physiological regulation of its expression remains unclear. This situation hampers the exploitation of NEP regulatory factors/mechanisms to develop effective strategies against Aβ peptide accumulation-induced brain toxicity. Based on recent data aimed at elucidating this major question, the present paper addresses and critically discusses the role of 5-hydroxyindole-acetic acid (5-HIAA) and kynurenic acid (KYNA) in the regulation of NEP activity/expression in the brain. Both 5-HIAA and KYNA are endogenous metabolites of tryptophan, an essential amino-acid obtained through diet and gut microbiome. By interacting with the aryl hydrocarbon receptor, various tryptophan metabolites modulate several metalloproteinases regulating brain Aβ peptide levels under normal and pathological conditions such as AD. In particular, interesting data reviewed here show that 5-HIAA and KYNA stimulate NEP activity/expression to prevent Aβ peptide-induced neurotoxicity. These data open promising perspectives for the development of tryptophan metabolite-based therapies against AD.

Neutral Endopeptidase (Neprilysin) in Therapy and Diagnostics: Yin and Yang

Neprilysin (NEP) is a zinc-dependent metalloproteinase that exists in organisms in both transmembrane and soluble forms. NEP substrates are involved in regulating the cardiovascular and nervous systems. In this review, we discuss some of the biochemical characteristics and physiological functions of this enzyme with special emphasis on the use of NEP as a therapeutic target. The history and various physiological aspects of applying NEP inhibitors for treating heart failure and attempts to increase NEP activity when treating Alzheimer's disease using gene and cell therapies are described. Another important issue discussed is the role of NEP as a potential marker for predicting the risk of cardiovascular disease complications. The diagnostic and prognostic performance of soluble NEP in various types of heart failure is analyzed and presented. We also discuss the methods and approaches for measuring NEP activity for prognosis and diagnosis, as well as a possible new role of natriuretic peptides (NEP substrates) in cardiovascular diagnostics.

Alzheimer's Disease: From Firing Instability to Homeostasis Network Collapse

Alzheimer's disease (AD) starts from pure cognitive impairments and gradually progresses into degeneration of specific brain circuits. Although numerous factors initiating AD have been extensively studied, the common principles underlying the transition from cognitive deficits to neuronal loss remain unknown. Here we describe an evolutionarily conserved, integrated homeostatic network (IHN) that enables functional stability of central neural circuits and safeguards from neurodegeneration. We identify the critical modules comprising the IHN and propose a central role of neural firing in controlling the complex homeostatic network at different spatial scales. We hypothesize that firing instability and impaired synaptic plasticity at early AD stages trigger a vicious cycle, leading to dysregulation of the whole IHN. According to this hypothesis, the IHN collapse represents the major driving force of the transition from early memory impairments to neurodegeneration. Understanding the core elements of homeostatic control machinery, the reciprocal connections between distinct IHN modules, and the role of firing homeostasis in this hierarchy has important implications for physiology and should offer novel conceptual approaches for AD and other neurodegenerative disorders.

Humanization of the entire murine Mapt gene provides a murine model of pathological human tau propagation

In cortical regions of brains from individuals with preclinical or clinical Alzheimer's disease (AD), extracellular β-amyloid (Aβ) deposition precedes the aggregation of pathological intracellular tau (the product of the gene microtubule-associated protein tau (MAPT)). To our knowledge, current mouse models of tauopathy reconstitute tau pathology by overexpressing mutant human tau protein. Here, through a homologous recombination approach that replaced the entire murine Mapt gene with the human ortholog, we developed knock-in mice with humanized Mapt to create an in vivo platform for studying human tauopathy. Of note, the humanized Mapt expressed all six tau isoforms present in humans. We next cross-bred the MAPT knock-in mice with single amyloid precursor protein (App) knock-in mice to investigate the Aβ-tau axis in AD etiology. The double-knock-in mice exhibited higher tau phosphorylation than did single MAPT knock-in mice but initially lacked apparent tauopathy and neurodegeneration, as observed in the single App knock-in mice. We further observed that tau humanization significantly accelerates cell-to-cell propagation of AD brain-derived pathological tau both in the absence and presence of Aβ-amyloidosis. In the presence of Aβ-amyloidosis, tau accumulation was intensified and closely associated with dystrophic neurites, consistently showing that Aβ-amyloidosis affects tau pathology. Our results also indicated that the pathological human tau interacts better with human tau than with murine tau, suggesting species-specific differences between these orthologous pathogenic proteins. We propose that the MAPT knock-in mice will make it feasible to investigate the behaviors and characteristics of human tau in an animal model.

Associations of Neprilysin Activity in CSF with Biomarkers for Alzheimer’s Disease

Background: Neprilysin (NEP) cleaves amyloid-β 1–42 (Aβ42) in the brain. Hence, we aimed to elucidate the effect of NEP on Aβ42 in cerebrospinal fluid (CSF) and on in vivo brain amyloid load using amyloid positron emission tomography (PET) with [11C]PiB (Pittsburgh compound B). In addition, associations with the biomarkers for neuronal injury, CSF-tau and FDG-PET, were investigated. Methods: Associations were calculated using global and voxel-based (SPM8) linear regression analyses in the same cohort of 23 highly characterized Alzheimer’s disease patients. Results: CSF-NEP was significantly inversely associated with CSF-Aβ42 and positively with the extent of neuronal injury as measured by CSF-tau and FDG-PET. Conclusions: Our results on CSF-NEP are compatible with the assumption that local degradation, amongst other mechanisms of amyloid clearance, plays a role in the development of Alzheimer’s pathology. In addition, CSF-NEP is associated with the extent and the rate of neurodegeneration.

Critical review: involvement of endoplasmic reticulum stress in the aetiology of Alzheimer's disease

The endoplasmic reticulum (ER) stress response is regarded as an important process in the aetiology of Alzheimer's disease (AD). The accumulation of pathogenic misfolded proteins and the disruption of intracellular calcium (Ca²⁺) signalling are considered to be fundamental mechanisms that underlie the induction of ER stress, leading to neuronal cell death. Indeed, a number of studies have proposed molecular mechanisms linking ER stress to AD pathogenesis based on results from in vitro systems and AD mouse models. However, stress responsivity was largely different between each mouse model, even though all of these models display AD-related pathologies. While several reports have shown elevated ER stress responses in amyloid precursor protein (APP) and presenilin 1 (PS1) double-transgenic (Tg) AD mouse models, we and other groups, in contrast, observed no such ER stress response in APP-single-Tg or App-knockin mice. Therefore, it is debatable whether the ER stress observed in APP and PS1 double-Tg mice is due to AD pathology. From these findings, the roles of ER stress in AD pathogenesis needs to be carefully addressed in future studies. In this review, we summarize research detailing the relationship between ER stress and AD, and analyse the results in detail.

Introduction of pathogenic mutations into the mouse Psen1 gene by Base Editor and Target-AID

Base Editor (BE) and Target-AID (activation-induced cytidine deaminase) are engineered genome-editing proteins composed of Cas9 and cytidine deaminases. These base-editing tools convert C:G base pairs to T:A at target sites. Here, we inject either BE or Target-AID mRNA together with identical single-guide RNAs (sgRNAs) into mouse zygotes, and compare the base-editing efficiencies of the two distinct tools in vivo. BE consistently show higher base-editing efficiency (10.0–62.8%) compared to that of Target-AID (3.4–29.8%). However, unexpected base substitutions and insertion/deletion formations are also more frequently observed in BE-injected mice or zygotes. We are able to generate multiple mouse lines harboring point mutations in the mouse presenilin 1 (Psen1) gene by injection of BE or Target-AID. These results demonstrate that BE and Target-AID are highly useful tools to generate mice harboring pathogenic point mutations and to analyze the functional consequences of the mutations in vivo.

CRISPR-guided cytidine deaminases, including BE3 (Base Editor 3) and Target-AID (activation-induced cytidine deaminase), can covert C:G base pairs to T:A at target site. Here, the authors generate missense mutations of mouse Psen1 gene and find BE3 has higher editing efficiency than Target-AID.

Targeting Alzheimer's disease with gene and cell therapies

Alzheimer's disease (AD) causes dementia in both young and old people affecting more than 40 million people worldwide. The two neuropathological hallmarks of the disease, amyloid beta (Aβ) plaques and neurofibrillary tangles consisting of protein tau are considered the major contributors to the disease. However, a more complete picture reveals significant neurodegeneration and decreased cell survival, neuroinflammation, changes in protein and energy homeostasis and alterations in lipid and cholesterol metabolism. In addition, gene and cell therapies for severe neurodegenerative disorders have recently improved technically in terms of safety and efficiency and have translated to the clinic showing encouraging results. Here, we review broadly current data within the field for potential targets that could modify AD through gene and cell therapy strategies. We envision that not only Aβ will be targeted in a disease-modifying treatment strategy but rather that a combination of treatments, possibly at different intervention times may prove beneficial in curing this devastating disease. These include decreased tau pathology, neuronal growth factors to support neurons and modulation of neuroinflammation for an appropriate immune response. Furthermore, cell based therapies may represent potential strategies in the future.

Degradation of amyloid β peptide by neprilysin expressed from Borna disease virus vector

Accumulation of amyloid β (Aβ40 and Aβ42) in the brain is a characteristic of Alzheimer's disease (AD). Because neprilysin (NEP) is a major Aβ-degrading enzyme, NEP delivery in the brain is a promising gene therapy for AD. Borna disease virus (BoDV) vector enables long-term transduction of foreign genes in the central nerve system. Here, we evaluated the proteolytic ability of NEP transduced by the BoDV vector and found that the amounts of Aβ40 and Aβ42 significantly decreased, which suggests that NEP expressed from the BoDV vector is functional to degrade Aβ.

Neuroprotective Effect of Alpha-Linolenic Acid against Aβ-Mediated Inflammatory Responses in C6 Glial Cell

Therapeutic approaches for neurodegeneration, such as Alzheimer's disease (AD), have been widely studied. One of the critical hallmarks of AD is accumulation of amyloid beta (Aβ). Aβ induces neurotoxicity and releases inflammatory mediators or cytokines through activation of glial cell, and these pathological features are observed in AD patient's brain. The purpose of this study is to investigate the protective effect of alpha-linolenic acid (ALA) on Aβ_25-35-induced neurotoxicity in C6 glial cells. Exposure of C6 glial cells to 50 μM Aβ_25-35 caused cell death, overproduction of nitric oxide (NO), and pro-inflammatory cytokines release [interleukin (IL)-6 and tumor necrosis factor-α], while treatment of ALA increased cell viability and markedly attenuated Aβ_25-35-induced excessive production of NO and those inflammatory cytokines. Inhibitory effect of ALA on generation of NO and cytokines was mediated by down-regulation of inducible nitric oxide synthase and cyclooxygenase-2 protein and mRNA expressions. In addition, ALA treatment inhibited reactive oxygen species generation induced by Aβ_25-35 through the enhancement of the nuclear factor-erythroid 2-related factor-2 (Nrf-2) protein levels and subsequent induction of heme-oxygenase-1 (HO-1) expression in C6 glial cells dose- and time-dependently. Furthermore, the levels of neprilysin and insulin-degrading enzyme protein expressions, which contribute to degradation of Aβ, were also increased by treatment of ALA compared to Aβ_25-35-treated control group. In conclusion, effects of ALA on Aβ degradation were shown to be mediated through inhibition of inflammatory responses and activation of antioxidative system, Nrf-2/HO-1 signaling pathway, in C6 glial cells. Our findings suggest that ALA might have the potential for therapeutics of AD.

Generation of App knock-in mice reveals deletion mutations protective against Alzheimer's disease-like pathology

Although, a number of pathogenic mutations have been found for Alzheimer’s disease (AD), only one protective mutation has been identified so far in humans. Here we identify possible protective deletion mutations in the 3′-UTR of the amyloid precursor protein (App) gene in mice. We use an App knock-in mouse model carrying a humanized Aβ sequence and three AD mutations in the endogenous App gene. Genome editing of the model zygotes using multiple combinations of CRISPR/Cas9 tools produces genetically mosaic animals with various App 3′-UTR deletions. Depending on the editing efficiency, the 3′-UTR disruption mitigates the Aβ pathology development through transcriptional and translational regulation of APP expression. Notably, an App knock-in mouse with a 34-bp deletion in a 52-bp regulatory element adjacent to the stop codon shows a substantial reduction in Aβ pathology. Further functional characterization of the identified element should provide deeper understanding of the pathogenic mechanisms of AD.

To date, only one mutation in the gene for amyloid-beta precursor protein APP has been suggested to be protective against Alzheimer’s disease. Here, authors found using gene editing of a mutant App knock-in mouse line that deletion of the 3’UTR region is protective against amyloid-β accumulation in vivo, and subsequently identify a 52-bp element in the 3’UTR region that is responsible for this effect.

Deletion of plasma Phospholipid Transfer Protein (PLTP) increases microglial phagocytosis and reduces cerebral amyloid-β deposition in the J20 mouse model of Alzheimer's disease

Plasma phospholipid transfer protein (PLTP) binds and transfers a number of amphipathic compounds, including phospholipids, cholesterol, diacylglycerides, tocopherols and lipopolysaccharides. PLTP functions are relevant for many pathophysiological alterations involved in neurodegenerative disorders (especially lipid metabolism, redox status, and immune reactions), and a significant increase in brain PLTP levels was observed in patients with Alzheimer's disease (AD) compared to controls. To date, it has not been reported whether PLTP can modulate the formation of amyloid plaques, i.e. one of the major histopathological hallmarks of AD.

We thus assessed the role of PLTP in the AD context by breeding PLTP-deficient mice with an established model of AD, the J20 mice. A phenotypic characterization of the amyloid pathology was conducted in J20 mice expressing or not PLTP. We showed that PLTP deletion is associated with a significant reduction of cerebral Aβ deposits and astrogliosis, which can be explained at least in part by a rise of Aβ clearance through an increase in the microglial phagocytic activity and the expression of the Aβ-degrading enzyme neprilysin.

PLTP arises as a negative determinant of plaque clearance and over the lifespan, elevated PLTP activity could lead to a higher Aβ load in the brain.

The antioxidant xanthorrhizol prevents amyloid-β-induced oxidative modification and inactivation of neprilysin

Activity of neprilysin (NEP), the major protease which cleaves amyloid-β peptide (Aβ), is reportedly reduced in the brains of patients with Alzheimer's disease (AD). Accumulation of Aβ generates reactive oxygen species (ROS) such as 4-hydroxynonenal (HNE), and then reduces activities of Aβ-degrading enzymes including NEP. Xanthorrhizol (Xan), a natural sesquiterpenoid, has been reported to possess antioxidant and anti-inflammatory properties. The present study examined the effects of Xan on HNE- or oligomeric Aβ₄₂-induced oxidative modification of NEP protein. Xan was added to the HNE- or oligomeric Aβ₄₂-treated SK-N-SH human neuroblastoma cells and then levels, oxidative modification and enzymatic activities of NEP protein were measured. Increased HNE levels on NEP proteins and reduced enzymatic activities of NEP were observed in the HNE- or oligomeric Aβ₄₂-treated cells. Xan reduced HNE levels on NEP proteins and preserved enzymatic activities of NEP in HNE- or oligomeric Aβ₄₂-treated cells. Xan reduced Aβ₄₂ accumulation and protected neurones against oligomeric Aβ₄₂-induced neurotoxicity through preservation of NEP activities. These findings indicate that Xan possesses therapeutic potential for the treatment of neurodegenerative diseases, including AD, and suggest a potential mechanism for the neuroprotective effects of antioxidants for the prevention of AD.

DNA methylation level of the neprilysin promoter in Alzheimer's disease brains

Neprilysin (NEP), a membrane-bound metalloprotease, has been shown to play an essential role in the clearance of amyloid beta (Aβ) peptides. Previous studies have reported that NEP expression is downregulated in the normal aging brain as well as in the Alzheimer's disease (AD) brain, providing evidence that the downregulation of NEP expression contributes to the age-dependent deposition of Aβ-containing plaques, a pathological hallmark of AD. However, the mechanisms underlying the downregulation remain unclear. In this study, we explored the relationship between DNA methylation status of CpG islands in the NEP promoter and its expression level in AD brains. We performed pyrosequencing analyses to detect the DNA methylation level in 31 postmortem AD brains and 40 normal control brains. All 30 CpG sites showed no clear difference in methylation level. To further focus on methylation changes specific to neuronal cells, we performed methylation array experiments using neuronal nuclei from postmortem brains and found no clear difference in the methylation level between AD and normal control samples. Our detailed analyses, with a substantial number of brain samples, provide the first convincing evidence that DNA methylation of the NEP promoter is not involved in AD development and progression.

Suppression of Alzheimer's disease-related phenotypes by the heat shock protein 70 inducer, geranylgeranylacetone, in APP/PS1 transgenic mice via the ERK/p38 MAPK signaling pathway

HSP70 overexpression has a remedying effect in neurodegenerative diseases. In Alzheimer's disease (AD), the suppressive effects of HSP70 overexpression on AD-related phenotypes and the underlying mechanisms are unknown. In the current study, the effect of geranylgeranylacetone (GGA), a non-toxic inducer of heat shock protein (HSP)-70 expression, on cognitive function and other pathological phenotypes were evaluated in APP/PS1 mice. It was observed that all doses of orally administered GGA (200, 400, and 800 mg/kg/day) improved cognitive deficit (P<0.05) and lowered the levels of amyloid-β (Aβ) peptide (P<0.05) in APP/PS1 mice. GGA treatment also increased the levels of low density lipoprotein receptor-related protein 1 (LRP-1) (P<0.05), while the levels of p-glycoprotein and receptor for advanced glycation end products were unaltered. Significant decreases in the levels of inflammatory cytokines, namely tumor necrosis factor-α, interleukin-1β and cyclooxygenase-2, were also observed in the GGA-treated mice (P<0.05). Subsequent treatment with the HSP70 inhibitor quercetin caused significant decreases in the levels of phosphorylated (p)-p38 mitogen-activated protein kinase (p38 MAPK) and p-extracellular signal-regulated protein kinases (ERK; P<0.05), indicating that ERK/p38 MAPK signaling in AD-related phenotypes may be suppressed by oral administration of GGA. Finally, in APP/PS1 mice treated with GGA+SB-203580 (p38 inhibitor) and GGA+PD98059 (ERK inhibitor), it was observed that orally administered GGA led to the activation of ERK/p38 MAPK signaling (P<0.05) and increased LRP-1 expression (P<0.05), which subsequently aided the clearance of Aβ40 and Aβ42 (P<0.05) and alleviated AD-related phenotypes. These results indicate that oral administration of GGA in APP/PS1 mice alleviates AD-related phenotypes by regulation of the ERK/p38 MAPK signaling pathway. Thus, GGA may be a potential therapeutic for the treatment of AD.

Drosophila Neprilysins Are Involved in Middle-Term and Long-Term Memory

Neprilysins are type II metalloproteinases known to degrade and inactivate a number of small peptides. Neprilysins in particular are the major amyloid-β peptide-degrading enzymes. In mouse models of Alzheimer's disease, neprilysin overexpression improves learning and memory deficits, whereas neprilysin deficiency aggravates the behavioral phenotypes. However, whether these enzymes are involved in memory in nonpathological conditions is an open question. Drosophila melanogaster is a well suited model system with which to address this issue. Several memory phases have been characterized in this organism and the neuronal circuits involved are well described. The fly genome contains five neprilysin-encoding genes, four of which are expressed in the adult. Using conditional RNA interference, we show here that all four neprilysins are involved in middle-term and long-term memory. Strikingly, all four are required in a single pair of neurons, the dorsal paired medial (DPM) neurons that broadly innervate the mushroom bodies (MBs), the center of olfactory memory. Neprilysins are also required in the MB, reflecting the functional relationship between the DPM neurons and the MB, a circuit believed to stabilize memories. Together, our data establish a role for neprilysins in two specific memory phases and further show that DPM neurons play a critical role in the proper targeting of neuropeptides involved in these processes.

SIGNIFICANCE STATEMENT

Neprilysins are endopeptidases known to degrade a number of small peptides. Neprilysin research has essentially focused on their role in Alzheimer's disease and heart failure. Here, we use Drosophila melanogaster to study whether neprilysins are involved in memory. Drosophila can form several types of olfactory memory and the neuronal structures involved are well described. Four neprilysin genes are expressed in adult Drosophila Using conditional RNA interference, we show that all four are specifically involved in middle-term memory (MTM) and long-term memory (LTM) and that their expression is required in the mushroom bodies and also in a single pair of closely connected neurons. The data show that these two neurons play a critical role in targeting neuropeptides essential for MTM and LTM.

The role of amyloid beta clearance in cerebral amyloid angiopathy: more potential therapeutic targets

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of amyloid β-protein (Aβ) in the leptomeningeal and cortical blood vessels, which is an age-dependent risk factor for intracerebral hemorrhage (ICH), ischemic stroke and contributes to cerebrovascular dysfunction leading to cognitive impairment. However clinical prevention and treatment of the disease is very difficult because of its occult onset and severity of the symptoms. In recent years, many anti-amyloid β immunotherapies have not demonstrated clinical efficacy in subjects with Alzheimer’s disease (AD), and the failure may be due to the deposition of Aβ in the cerebrovascular export pathway resulting in further damage to blood vessels and aggravating CAA. So decreased clearance of Aβ in blood vessels plays a crucial role in the development of CAA and AD, and identification of the molecular pathways involved will provide new targets for treatment. In this review, we mainly describe the mechanisms of Aβ clearance through vessels, especially in terms of some proteins and receptors involved in this process.

Identification of Small Peptides in Human Cerebrospinal Fluid upon Amyloid-β Degradation

BACKGROUND

Amyloid-β (Aβ) degradation in brains of Alzheimer disease patients is a crucial focus for the clarification of disease pathogenesis. Nevertheless, the mechanisms underlying Aβ degradation in the human brain remain unclear.

OBJECTIVE

This study aimed to quantify the levels of small C-terminal Aβ fragments generated upon Aβ degradation in human cerebrospinal fluid (CSF).

METHODS

A fraction containing small peptides was isolated and purified from human CSF by high-pressure liquid chromatography. Degradation products of Aβ C termini were identified and measured by liquid chromatography-tandem mass spectrometry. The C-terminal fragments of Aβ in the conditioned medium of cultured cells transfected with the Swedish variant of βAPP (sw βAPP) were analyzed. These fragments in brains of PS1 I213T knock-in transgenic mice, overexpressing sw βAPP, were also analyzed.

RESULTS

The peptide fragments GGVV and GVV, produced by the cleavage of Aβ40, were identified in human CSF as well as in the brains of the transgenic mice and in the conditioned medium of the cultured cells. Relative to Aβ40 levels, GGVV and GVV levels were 7.6 ± 0.81 and 1.5 ± 0.18%, respectively, in human CSF. Levels of the GGVV fragment did not increase by the introduction of genes encoding neprilysin and insulin-degrading enzyme to the cultured cells.

CONCLUSION

Our results indicate that a substantial amount of Aβ40 in human brains is degraded via a neprilysin- or insulin-degrading enzyme-independent pathway.